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Nieto MA.. The Snail superfamily of zinc-finger transcription factors. Nat Rev Mol Cell Biol 3: 155-166

April 2002
Nature Reviews Molecular Cell Biology 3(3):155-66

April 2002
3(3):155-66

DOI:10.1038/nrm757

Source
PubMed

Authors:

M. Angela Nieto

Instituto de Neurociencias

The Snail superfamily of zinc-finger transcription factors is involved in processes that imply pronounced cell movements, both during embryonic development and in the acquisition of invasive and migratory properties during tumour progression. Different family members have also been implicated in the signalling cascade that confers left right identity, as well as in the formation of appendages, neural differentiation, cell division and cell survival.

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Assuming the veracity of Lewis Wolpert’s popular state-

ment

that it is not birth, marriage or death, but GASTRU-

LATION

, that is the most important event in the lifespan

of an individual, it seems almost trivial to mention that

the study of

MESODERM formation is a must for develop-

mental biologists. To put it in more conventional terms,

the formation of the third embryonic layer in

TRIPOBLASTIC

animals is, indeed, the time at which the embryonic

axes are coordinated and when the important morpho-

genetic movements that shape the embryo commence.

In this regard, the Snail family of zinc-finger tran-

scription factors occupies a central role in morphogene-

sis, as its members are essential for mesoderm forma-

tion in several organisms from flies to mammals

2–10

The analysis of different vertebrate Snail homologues

has highlighted their role not only in the development

of the mesoderm, but also in other processes that

require large-scale cell movements, such as the forma-

tion of the

NEURAL CREST

6,11–14

More recently, this role in promoting cell movement

has been extended and includes more generalized phe-

nomena such as the

EPITHELIAL–MESENCHYMAL TRANSITION

(EMT)

15,16

. EMT is the mechanism by which epithelial

cells that are generated in a particular region can disso-

ciate from the epithelium and migrate to reach different

locations

. As such, EMT is fundamental to both nor-

mal development and the progression of malignant

epithelial tumours

. In addition to triggering EMT,

Snail superfamily members have been implicated in

various important developmental processes, including

neural differentiation, cell fate and survival decisions,

and left–right identity

From an evolutionary point of view, the Snail fam-

ily provides a good model to study ancestry and the

acquisition of functions that are related to changes in

the

BODY PLAN. In this respect, this family is associated

with the appearance of the neural crest, which is essen-

tial for the formation of the vertebrate head

.The

recent identification of new family members and the

association of these members with new functions has

attracted researchers in many fields, from embryonic

pattern formation to cancer research. In this review, I

describe the diversity and organization of the Snail

superfamily, and then address the roles that have been

assigned to the different family members.

The Snail superfamily of repressors

The first member of the Snail family, snail, was

described in Drosophila melanogaster

20,21

, where it was

shown to be essential for the formation of the meso-

derm

. Subsequently, Snail homologues have been

found in many species including humans, other verte-

brates, non-vertebrate

CHORDATES (protochordates),

insects,

NEMATODES,ANNELIDS and molluscs (TABLE 1).

Snail family members encode transcription factors

of the zinc-finger type. They all share a similar organi-

zation, being composed of a highly conserved car-

boxy-terminal region, which contains from four to six

zinc fingers, and a much more divergent amino-termi-

nal region. The fingers correspond to the C

type

and function as sequence-specific DNA-binding

motifs. The fingers are structurally composed of two

β-strands followed by an α-helix, the amino-terminal

part of which binds to the major groove of the DNA.

THE SNAIL SUPERFAMILY OF ZINC-

FINGER TRANSCRIPTION FACTORS

M. Angela Nieto

The Snail superfamily of zinc-finger transcription factors is involved in processes that imply

pronounced cell movements, both during embryonic development and in the acquisition of

invasive and migratory properties during tumour progression. Different family members have

also been implicated in the signalling cascade that confers left–right identity, as well as in the

formation of appendages, neural differentiation, cell division and cell survival.

GASTRULATION

The morphogenetic movements

of the early embryo that lead to

the generation of the third

embryonic layer — the

mesoderm.

MESODERM

The third embryonic layer

generated during gastrulation,

which occupies an intermediate

position between the ectoderm

and the endoderm. It will give

rise to the skeleton, muscles and

connective tissue.

TRIPOBLAST

An animal that is composed of

three embryonic cell layers:

ectoderm, endoderm and

mesoderm.

NEURAL CREST

A cell population that originates

in the dorsal part of the neural

tube and gives rise to many

derivatives, including most of

the peripheral nervous system,

the cranio-facial skeleton and

pigmented cells of the body.

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 3 | MARCH 2002 | 155

Instituto Cajal, Doctor Arce,

37, 28002 Madrid, Spain.

e-mail: anieto@cajal.csic.es

DOI: 10.1038/nrm757

EPITHELIAL–MESENCHYMAL

TRANSITION

The transformation of an

epithelial cell into a

mesenchymal cell with

migratory and invasive

properties.

BODY PLAN

The organization of the

embryonic tissues to generate an

individual with specific

characters.

CHORDATE

An animal with a notochord.

These include ascidians,

amphioxus and all vertebrates.

NEMATODE

An unsegmented worm.

ANNELID

A segmented worm.

BASIC HELIX–LOOP–HELIX

PROTEIN

A transcription factor with a

basic domain that binds to a

hexanucleotide called the E box,

and a hydrophobic domain (the

helix–loop–helix) that allows the

formation of homo- and

heterodimers. They can also

have leucine repeats called a

leucine zipper.

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mammalian cells

. The SNAG domain is conserved in all

vertebrate Snail genes, and is also found in echinoderms,

cephalochordates

, in one of the limpet genes

and in

Drosophila scratch

. Its wide distribution might reflect an

early ancestry. This, in turn, would imply that it has been

lost in other Drosophila family members, Caenorhabditis

elegans and urochordates. Alternatively, the SNAG

domain might have been added independently in each of

the different species. The availability of complete coding

sequences from other groups will help to distinguish

between these two possibilities.

Despite the absence of a SNAG domain, Drosophila

snail also acts as a transcriptional repressor. This activity

is mediated through an interaction with a co-repressor,

CtBP (carboxy-terminal binding protein)

. Consensus

motifs for the binding of CtBP are present in other

Drosophila Snail family members (but not scratch) and a

partial consensus is found in several vertebrate family

The two conserved cysteines and histidines (C

)

coordinate the zinc ion. Both random selection and

transfection experiments with different promoters

have shown that the consensus binding site for Snail-

related genes contains a core of six bases,

CAGGTG

15,16,23–26

. This motif is identical to the so-

called E box, the consensus of the core binding site of

BASIC HELIX–LOOP–HELIX

(bHLH) transcription factors,

which indicates that Snail proteins might compete

with them for the same binding sequences

26–28

On binding to the E box, Snail family members are

thought to act as transcriptional repressors

9,14–16,24,26,27,29,30

The repressor activity depends not only on the finger

region, but also on at least two different motifs that are

found in the amino-terminal region. One of these is the

so-called SNAG (Snail/Gfi) domain, which was initially

described as a repressor domain in the zinc-finger protein

Gfi1

(REF. 31). This motif is important for repression in

Table 1 | Snail superfamily members

Species Common Gene Synonyms Accession no. Map References

name

Caenorhabditis elegans Nematode ces1* AAF01678 I:2.9 37

snail-like K02D7.2 T32983 IV:-26.1 36

scratch-like* C55C2.1 T15225 I:-9.3 36

Helobdella robusta Leech snail1 Hro-sna1 AF410864 43

snail2 Hro-sna2 AF410865 43

Patella vulgata Limpet snail1 Pv-sna1 AY049727 32

snail2 Pv-sna2 AY049791 32

Drosophila melanogaster Fruitfly snail S06222 35D2–3 20,100

escargot AAF12733 35D1 111

worniu S33639 35D2–3 95

scratch* AAA91035 64A2–3 33

scratch-like1* CG12605 AAF47818 64A1 36

scratch-like2* CG17181 AAF47394 61C7 36

Lytechinus variegatus Sea urchin Snail AAB67715 unpublished

Halocynthia roretzi Ascidia Snail BAA75811 8

Ciona intestinalis Ascidia Snail AAB61226 42

Branchiostoma floridae amphioxus Snail AAC35351 7

Takifugu rubripes Pufferfish Snail1 CAB54535 112

Snail2 CAB54536 112

Danio rerio Zebrafish snail1 CAA52795 5,49

snail2 AAA87196 11

slug AI722148 36

scratch* AI883776 36

Xenopus laevis African Snail Xsna P19382 113

clawed toad Slug

Xslu AF368041 78,114

Slug

Xslu

AF368043 78

Silurana tropicalis Western Slug Xslug AF368038 78

clawed frog

Gallus gallus Chicken Snail SnR CAA71033 50,84

Slug CAA54679 6

Mus musculus Mouse Snail Q02085 Chr.2-97.0 3,4

Slug Slugh AAB38365 Chr.16-9.4 50,73,85

Scratch* AY014997 35

Smuc Zfp293 NP038942 26

Homo sapiens Human SNAIL SNAIL1, SNAILH AF155233 20q13.1 115,116

SNAILP SNAI1P AF153502 2q34 115,116

SLUG SLUGH, SNAIL2 AAC34288 8q11 117

SCRATCH1* AY014996 8q24.3 35

SCRATCH2* AL121758 20p12.3–13 35

The Snail superfamily is subdivided into two families: Snail and Scratch (marked by an asterix). Accession numbers are from Entrez

(http://www.ncbi.nlm.nih.gov/Entrez).

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 3 | MARCH 2002 | 157

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This proposal is supported by the phylogenetic rela-

tionships that are established when the sequences of the

zinc-finger regions of all Snail superfamily members are

compared

. An updated version of such a phylogenetic

tree is shown in

FIG. 1, in which the Scratch genes are

closely grouped and the Snail genes are less tightly asso-

ciated, with several branches that emanate from the base

of the tree. The vertebrate Snail genes seem to be subdi-

vided into two subfamilies that have already been

described: Snail and Slug. The recently isolated mouse

gene Smuc

occupies a very unusual position in the

tree, which cannot be easily explained at present. It is

either a gene that originated very early, or it is only pre-

sent in the mouse and has undergone many changes.

Sequence comparisons have allowed the identifica-

tion of consensus sequences for the individual fingers,

both for the Snail and Scratch families as well as a com-

bined consensus for the zinc-finger region of the whole

superfamily

(FIG. 2). Signature domains have been

identified in the non-finger region that permit mem-

bers to be ascribed to the Scratch family and the Slug

subfamily

(FIG. 2). On the basis of this phylogenetic

analysis, a model for the evolution of this superfamily

that incorporates the gene duplication events that might

have led to the generation of the family from ancestral

genes is shown in

BOX 1.

Snail in mesoderm and neural-crest formation

In Drososphila embryos, snail is initially expressed in

the prospective mesoderm

(FIG. 3),

where it acts as a

repressor to inhibit the expression of neuroectodermal

genes such as rhomboid

and single-minded

.So,in

Drosophila, mesoderm specification is partly carried

out by the exclusion of alternative cell fates, and snail is

central to this process. The isolation of Snail homo-

logues in different species has confirmed a conserved

role for Snail in mesoderm specification in other

insects

, ascidians

8,42

and amphioxus

, and mesoderm

development in vertebrates (see below). However, the

expression pattern in the limpet

and leech

embryos

does not correlate with a role in mesoderm formation,

which indicates that this function cannot be extended

LOPHOTROCHOZOANS at present.

In addition to their function in the mesoderm,

vertebrate family members have also been linked

with the development of the neural crest. From an

evolutionary point of view, the appearance of this cell

population is extremely attractive, as, together with

the

EPIDERMAL PLACODES, the neural crest has been crucial

in the formation of the ‘new head’ of vertebrates

These two tissues differentiate vertebrates from the rest

of the chordates, and their origin correlates with the

shift to active predation and the appearance of paired

sense organs. Indeed, non-vertebrate chordates

(ascidians

8,42

and amphioxus

) do not have a neural

crest. However, these chordates do express Snail in

dorsal neural cells, just at the position in which the

neural crest forms in vertebrates

(FIG. 3).So,non-

vertebrate chordates could have the beginnings of a

genetic programme for neural-crest formation, and the

Snail-expressing cells could represent a neural-crest

members. Interestingly, urochordate snail genes, which

lack a SNAG motif, have CtBP consensus sites. So, it is

tempting to speculate that the repressor activity of Snail

proteins has been evolutionarily conserved, but could

use different mechanisms: CtBP co-repression or a

SNAG domain acting alone, or both in conjunction.

A new classification for the Snail family

Recently, new family members have been found in differ-

ent organisms. In particular, several new genes that have

been described in C. elegans, Drosophila, fish, mouse and

human

35,36

are much more similar to Drosophila

scratch

and the C. elegans cell death gene ces-1 (REF. 37)

than to any other Snail family member (TABLE 1).This

has led to the proposal that Snail is a superfamily that

can be subdivided into two related but independent

groups: the Snail and the Scratch families

Figure 1 | Phylogenetic tree of the Snail superfamily. The dark purple square engulfs all the

superfamily members. A light purple background groups the members of the Snail family and a

green background highlights the Scratch family members. The vertebrate Snail and Slug

subfamilies are shown with a light or heavy yellow hatching, respectively. The species shown

represent members of the lophotrochozoans: Pv, Patella vulgata (limpet); ecdysozoans:

Ce, Caenorhabditis elegans (nematode); Dm, Drosophila melanogaster (fruitfly); and

deuterostomes: Bf, Brachiostoma floridae (amphioxus); Ci, Ciona intestinalis (ascidion) and Hr,

Holocynthia roretzi (ascidians); Dr, Danio rerio (zebrafish); Gg, Gallus gallus (chicken), Hs,

Homo sapiens (human); Lv, Lytechinus variegatus (green sea urchin); Mm, Mus musculus

(mouse); Tr, Takifugu rubripes (pufferfish); and Xl, Xenopus laevis (African clawed toad). This is

an updated version of the tree published in REF. 36.

Ce ces1

Mm Smuc

Hr snail

Dm snail

Dm escargot

Dm worniu

Bf snail

snail

SNAIL

SNAILP

Snail

XI Slug

Gg Slug

Mm Slug

Hs Slug

Gg Snail

Pv sna2

Pv sna1

Lv Snail

snail1

Dr snail2

Tr snail2

Ci snail

Ce snail-like

Dm scratch

Dr scratch

Mm scratch

Hs SCRATCH1

Hs SCRATCH2

Dm scratch-like1

Dm scratch-like2

Ce scatch-like

Snail superfamily

Snail family

Snail subfamily

Vertebrates

Slug subfamily

Scratch family

snail1

LOPHOTROCHOZOAN

This group includes two

important animal groups, the

Lophophorata (brachiopods, flat

worms and nemerteans) and the

Trochozoa (molluscs and

annelids).

EPIDERMAL PLACODE

An epidermal thickening in the

embryonic head that

differentiates into neurons, as

well as into other cell types, at

the sites at which the sense

organs will form.

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in the presence of Slug expression

. So, in the chick, Slug

is involved in crest specification all along the anteroposte-

rior axis of the embryo and has an additional role in crest

migration in the head region. It is tempting to speculate

that Snail genes had an ancestral role in the specification

of tissues such as the mesoderm and the neural crest and

that the function in emigration might have been acquired

subsequently. In addition to these data on chick embryos

and the studies in Xenopus that show the role of Slug in

both specification and migration

9,14,45

, the expression pat-

terns of Slug and Snail in other vertebrate embryos, such

as in zebrafish

5,11,49

and mouse

3,4,50

, are also compatible

with their role in neural-crest development.

The role of Snail and Slug in triggering EMT is not

restricted to the mesoderm and neural crest. Snail

and/or Slug are also observed in other cells that undergo

EMT in the developing vertebrate embryo, such as dur-

ing the decondensation of somites

, formation of the

parietal endoderm

, formation of the heart cushions

and closure of the palate (C. Martinez and M.A.N.,

unpublished observations). Even in a mollusc (for exam-

ple the limpet embryo), Snail expression in the involuting

cells of the mantle tips is suggestive of a role in EMT

This indicates that EMT could be one of the ancestral

functions that are associated with the Snail family.

The function of Snail genes in mesoderm develop-

ment continues after EMT. In ascidians, snail has

been linked with the subdivision of the mesoderm in

precursor population

. With respect to vertebrates and

a bona fide neural crest, Slug (a Snail family member)

seems to be involved in neural-crest specification in

both the chick and Xenopus embryos

9,14,45,46

Having been specified, both the mesoderm and the

neural crest have to delaminate from the tissue in which

they originate — the

PRIMITIVE STREAK and the neural tube,

respectively — and migrate. Their migration pathways

are well defined, and this enables them to populate diverse

parts of the embryo and contribute to various structures.

Delamination is mediated by the triggering of EMT, and

converts the epithelial cells into mesenchymal cells, which

can migrate through the extracellular matrix

17,47

The first indication that the Snail family is involved in

EMT came from studies in the chick embryo. The incu-

bation of early chick embryos with antisense oligonu-

cleotides to Slug inhibited both neural crest and meso-

derm delamination

. Defects in crest migration and the

absence of specific crest derivatives have also been

described in Xenopus embryos after Slug antisense treat-

ment

or the expression of a dominant-negative Slug

construct

9,14

.Moreover,Slug gain of function leads to an

increase in neural-crest production in the chick embryo

Interestingly, this increase in the migratory population

was detected only in the head region. Therefore, different

mechanisms operate for neural-crest delamination in the

head and the trunk regions, explaining why inhibition of

neural-crest delamination could occur in the spinal cord

PRIMITIVE STREAK

A structure that is formed at the

posterior end of amniote

embryos at gastrulation stages.

An area of mesoderm

formation.

Figure 2 | Sequence comparison of the main conserved domains and consensus sequences for the individual zinc

fingers of the Snail superfamily. a | Composite of the overall structure of Snail superfamily members, which shows the relative

positions of the SNAG (Snail/Gfi) domain, the zinc fingers (I–V), and the Scratch- and Slug-specific boxes. b | Consensus sequences

of the different zinc fingers for the whole superfamily (dark purple) and the Snail (Sna; light purple) and Scratch (Scrt; green) families.

c, d | Sequence comparison of the specific domains that are present in the Slug or Scratch genes, respectively. Dm, Drosophila

melanogaster; Dr, Danio rerio; Gg, Gallus gallus; Hs, Homo sapiens; Mm, Mus musculus; Xl, Xenopus laevis. e | Sequences of the

SNAG domain that are present in representative members of the three big groups of bilateralians. Whereas the zinc-finger region

and the SNAG domain have been shown to be fundamental for protein function, the Slug and Scratch domains represent signature

domains that allow a gene to be unambiguously ascribed to the corresponding family (Scratch) or vertebrate subfamily (Slug).

Abbreviations as above and Bf, Brachiostoma floridae; Lv, Lytechinus variegatus; Pv, Patella vulgata.

--C--C-K-YST--GL-KH---H

--C-ECGK-YATSSNLSRHKQTH

--C--C-K-Y-T---L--H---H

F-CK-C-K-Y-SLGALKMHIRTH

K-CPTC-KAYVSMPALAMH-LTH

--C--C-K-Y-S--AL-MH--TH

C-C--CGKAFSRPWLLQGHIRTH

H-C-VCGK-FSRPWLLQGH-RSH

--C--CGK-FSRPWLLQGH-R-H

F-C-HC-RAFADRSNLRAHLQTH

F-C-HCGKAFADRSNLRAHMQTH

F-C-HC--AFADRSNLRAH-QTH

Y-C--C--TFSRMSLL-KH---G

--C-RC-K-FALKSYL-KH-ES-

--C--C---F---S-L-KH----

Sna I

Scrt I

consensus

Sna II

Scrt II

consensus

Sna III

Scrt III

consensus

Sna IV

Scrt IV

consensus

Sna V

Scrt V

consensus

SDTSS-KDHSGSESPISDEEERLQS-KLSD

SDTSS-KDHSGSESPISDEEERLQP-KLSD

SDTSS-KDHSGSESPISDEEERIQS-KLSD

SDTSS-KDHSGSESPISDEEERLQT-KLSD

SDTSS-KDLSGSESPISDEEERLHT-KLSD

SDTSSNKDHSGSESPRSDEEERIQSTKLSD

Hs SLUG

Mm Slug

Gg Slug

XI Slug α

XI Slug β

Dr slug

AVSEGYAADAFFITDGRSRR

AVTDSYSMDAFFISDGRSRR

AVSEGYAADAFFITDGRSRR

SLSEGYTMDAFFISDGRSRR

AKTVAYTYEAFFVSDGRSKR

Hs SCRATCH1

Hs SCRATCH2

Mm Scratch

Dr Scratch

Dm scratch

MPRSFLVKK

MPRSFLVRK

MPRSFLVKT

MPRSFLIKK

MPRAFLIKK

MPRCLIAKK

All vert. + Lv

Mm, Hs Snail

Mm Smuc

Bf snail

Pv snail 2

Dm scratch

SNAG

domain

Scratch

domain

Slug

domain

Zinc fingers

I II III IV

Zinc-finger consensus sequences

c Slug domain

d Scratch domain

e SNAG domain

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 3 | MARCH 2002 | 159

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However, the expression of E-cadherin in the meso-

derm of the Snail mutants is lower than that in the

ectoderm of the same embryos

, which indicates that

other cadherin repressors might act simultaneously

with Snail during gastrulation. Candidates include

bHLH-type transcription factors such as SIP1

(REF. 55)

and E47 (REF. 28), which have recently been found to

repress E-cadherin expression, and are also expressed in

the embryonic mesoderm.

A tight regulation of cadherin expression is funda-

mental for the emigration of the neural-crest cells

56,57

However, as the Snail-mutant mice die at gastrulation

stages, it has not been possible to address the conse-

quences of Snail loss of function in the neural crest.

Other targets. E-cadherin is the only direct target of

Snail described so far. However, genetic analysis and

overexpression experiments have generated a list of

candidate targets for direct or indirect regulation.

With regard to EMT, in addition to E-cadherin, Snail

transfectants downregulate other epithelial markers,

such as desmoplakin

, the epithelial mucin Muc-1

and cytokeratin-18 (

REF. 58; FIG. 4). Mesenchymal

markers such as vimentin and fibronectin are

upregulated and redistributed

. These changes can-

not be secondary to the loss of E-cadherin, as trans-

fection of E-cadherin is not enough to induce a

reversion to an epithelial morphology

. This indi-

cates that Snail must have additional targets that are

independent of E-cadherin.

different territories

— this is in agreement with a

new function described for Slug in Xenopus the pat-

terning of the dorsal mesoderm

. Furthermore, the

expression of the fish

5,11,49

, chick

and mouse

Snail

and Slug, and that of the mouse Smuc gene

, also

indicates a role of these proteins in mesodermal pat-

terning and differentiation.

EMT and Snail: target molecules

E-cadherin. The importance of Snail in triggering

EMT in mammals has been confirmed using two

independent approaches. First, Snail was shown to

convert otherwise normal epithelial cells into mes-

enchymal cells through the direct repression of

E-CADHERIN

expression

15,16

. More importantly, Snail knockout ani-

mals die at gastrulation stages and show defects in

EMT

. Mutant embryos form a mesodermal layer

that expresses some mesodermal markers, but is com-

posed of columnar cells with apical–basal polarity,

microvilli and

ADHERENS JUNCTIONS, which are all charac-

teristic of epithelial cells

. This indicates that they

have failed to undergo EMT. It is known that down-

regulation of E-cadherin is essential for ingression of

the mesodermal cells at gastrulation in mouse

embryos

, and in the Snail mutant these cells retain

E-cadherin expression. This is in agreement with Snail

acting as a repressor of E-cadherin expression

15,16

.The

phenotype is reminiscent of that shown by snail

mutants in Drosophila, which also fail to downregu-

late E-cadherin during gastrulation

METAZOA

The animal kingdom. Includes

sponges, diploblasts,

protostomes and deuterostomes.

PROTOSTOME

An animal in which the mouth

develops from the first opening

that develops in the embryo.

These include ecdysozoans and

lophotrochozoans.

DEUTEROSTOME

An animal in which the anus

develops from the first opening

of the embryo, and the mouth is

formed later. These include

echinoderms and chordates.

E-CADHERIN

The main cell–cell adhesion

molecule, which is central in

maintaining the integrity of

epithelial tissues, both in

physiology and pathology.

ADHERENS JUNCTION

A cell–cell and cell–extracellular

matrix adhesion complex that is

composed of integrins and

cadherins that are attached to

cytoplasmic actin filaments.

Box 1 | Proposed evolutionary history of the Snail gene superfamily

The duplication of a unique snail gene in the

METAZOAN ancestor would have given rise to two genes: snail and scratch.

Independent duplication events in

PROTOSTOMES and DEUTEROSTOMES gave rise to a different number of family members in

each group.

In Drosophila, intra-chromosomal duplications would give rise to three linked genes from each family. Non-vertebrate

chordates seem to have retained the early metazoan situation, with only one gene from each family. This assumes the

existence of a scratch gene that has not yet been isolated.

A whole-genome duplication event proposed to have occurred at the base of the vertebrate lineage

101

, or a massive gene

duplication

102

, would be responsible for the presence of two genes from each family in vertebrates: Snail and Slug on the

one hand, and Scratch1 and Scratch2 on the other hand. Again, an additional, nearly complete genome duplication

103

massive local duplications

104

in the teleost (bony fishes) lineage would explain the existence of two very closely related

snail genes (snail1 and snail2) in zebrafish and pufferfish. To distinguish them from ancestral genes, present genes are

shown in bold. Among the latter, the predicted genes are shown in purple.

snail1 snail2 scratch

(ces-1)

scratchscratch-

scratch-

like1

scratch-

like2

escargot snail worniu

snail

scratch

snail scratch snail-like scratch snail scratchsnail scratch Snail Scratch

Metazoan ancestor

Protostomes

Lophotrochozoans

Leech and limpet Ascidian and amphioxusC. elegans Drosophila

Ecdysozoans Vertebrates

(Teleosts)

Non-vertebrate chordates

Deuterostomes

snail1 snail2

Snail Slug Scratch1 Scratch2

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REVIEWS

the neural crest

by upregulating Slug

12,64,65

.In Xenopus

and zebrafish, the neural crest is induced at a threshold

concentration of BMP signalling. Higher BMP activity

gives rise to non-neural ectoderm, whereas low (or

null) activity generates neural plate

66,67

. Interestingly,

BMP has been proposed not only as a signal to induce

Slug, but also as a target of it, as overexpression of Slug

induces downregulation of BMPs

. Nevertheless, BMP

signalling alone is not sufficient for neural-crest induc-

tion, and studies in Xenopus, zebrafish and mouse have

indicated that members of the Wnt and fibroblast

growth factor (FGF) families are also needed to gener-

ate all the different premigratory precursors

45,68–70

.In

the chick embryo, FGF and BMP cooperate in the gen-

eration of the neural–non-neural boundary — the ter-

ritory of neural-crest specification

. So, the combina-

tion of BMP, Wnt and FGF signalling is needed for

neural-crest development.

Given their interactions with BMPs in neural-crest

development, could the FGF and/or Wnt signalling path-

ways induce the expression of Snail family members?

FGF induces Slug expression in extraembryonic epithe-

lial cells

and in the rat-bladder-carcinoma cell line

NBT-II

(REF. 73), and upregulates Snail and maintains Slug

expression during limb development in the chick

embryo

74–76

. In addition, mice that have a mutation for

one of the FGF receptors (FGFR1) fail to undergo EMT

at gastrulation, lose Snail expression and show ectopic

expression of E-cadherin

in the primitive streak. This

indicates that FGFR1 signalling is needed for the mainte-

nance of Snail expression in the domain of the primitive

streak that is fated to become embryonic mesoderm, and

promotes the downregulation of E-cadherin

With respect to Wnt signalling, the recent isolation of

Slug promoters in Xenopus has led to the characterization

of a functional binding site for the transcription factor

Lef-1, which regulates gene expression after activation of

Wnt signalling

. By contrast, Kwonseop et al.

did not

observe Snail or Slug upregulation after overexpression

of LEF in epithelial cells, nor was Snail regulated by LEF

in human colon carcinoma cells

. However, an interest-

ing relationship emerges between the FGF and Wnt sig-

nalling pathways through the role of Snail in repressing

E-cadherin expression. Activation of the canonical Wnt

signalling pathway stabilizes β-catenin in the cyto-

plasm, which makes it available to bind the TCF/LEF

transcription factors and together translocate to the

nucleus where they regulate gene expression

Conversely, high levels of E-cadherin sequester β-

catenin to form adhesion complexes at the cell mem-

brane. So, FGF signalling promotes Wnt signalling by

lowering the levels of E-cadherin through the mainte-

nance of Snail expression. This explains why FGFR1-

mutant mice have attenuated Wnt signalling that can

be reverted by disrupting E-cadherin function

Another factor that has been shown to induce Snail is

the parathyroid-hormone-related peptide, PTH(rP),

which is essential for triggering the EMT that leads to for-

mation of the

PARIETAL ENDODERM from the PRIMITIVE ENDODERM

and the VISCERAL ENDODERM

. This process occurs early in

mouse development, when implantation begins.

Also relevant to EMT is the upregulation of RhoB,

which is important for neural-crest development in

chick embryos

, and is ectopically expressed in the chick

neural tube after overexpression of Slug

. Regulation of

this small GTPase, which is involved in actin rearrange-

ments, links Snail and EMT with changes in cell shape

and, hence, with the morphogenetic movements that

occur during gastrulation and neural-crest delamina-

tion. Indeed, a Rho-mediated signalling cascade is crucial

for the morphogenetic changes during Drosophila gas-

trulation, a pathway that involves the exchange factor

RhoGEF2 in response to an extracellular signal called

folded gastrulation (Fog)

. Considering that, genetically,

Fog lies downstream of Snail

, it is tempting to speculate

that Rho GTPases might also be indirect targets of Snail

in the gastrulating fly.

EMT and Snail: inductive signals

Different signalling pathways have been linked with the

induction of Snail family members in the EMT

(FIG. 4).

Transforming growth factor (TGF)-β1 induces EMT

and Snail expression in hepatocytes

.TGF-β2 has been

proposed to be a signal for EMT and Slug induction in

heart development

; and signalling through other mem-

bers of the TGF-β superfamily — the bone morpho-

genetic proteins (BMPs) — participates in induction of

Figure 3 | Expression of Snail family members in Drosophila, amphioxus, chick and

mouse embryos. In Drosophila, snail is expressed (blue) in the precursors of the mesoderm, and

also later on, when these cells are involuting at gastrulation. In amphioxus, expression is detected

in the mesoderm and at the edges of the neural plate. In vertebrates, the two family members

Snail and Slug are differentially expressed in different species. Note the interchange in the patterns

between chick and mouse. Snail in the mouse and Slug in the chick are expressed in the

precursors of the mesoderm and the neural crest, and also in the migratory populations.

m, mesoderm; nc, neural crest; np, neural plate; pnc, premigratory neural crest; ps, primitive

streak. Photographs of Drosophila and amphioxus embryos have been kindly provided by Maria

Leptin and Jim Langeland, respectively.

snail snail

Snail Slug Snail Slug

Mesoderm Neural crest

Mouse

Chick

Drosophila Amphioxus

pnc

pnc pnc

pnc

PARIETAL ENDODERM

The extraembryonic tissue that

is derived from the primitive

endoderm and visceral

endoderm, and is composed of

motile cells that secrete high

amounts of extracellular matrix.

PRIMITIVE ENDODERM

The extraembryonic tissue that

gives rise to the visceral and

parietal endoderm.

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REVIEWS

has been explained by the demonstration of an inver-

sion in the expression patterns of Slug and Snail at sites

of EMT

. In the chick embryo, Slug is expressed in the

premigratory neural crest and the primitive streak, and

Snail is absent from these tissues; in the mouse embryo,

by contrast, Snail is expressed in these cells, which

undergo EMT

(FIG. 3). This led to the proposal that the

role of Slug in EMT in the chick should be carried out

by Snail in the mouse

. The transfection of Snail in

mammalian epithelial cells

15,16

, and the phenotype of

the Snail-mutant mice

discussed previously, con-

firmed this prediction. In other vertebrates, the situa-

tion seems more similar to that in the mouse. Indeed,

snail2 is expressed in the premigratory neural crest in

zebrafish

, and although both Snail and Slug are

expressed in the premigratory population in Xenopus,

Snail is the first family member to be transcribed

Experiments that are related to neural-crest develop-

ment in the frog have been carried out only for Slug,so

it will be interesting to analyse the effects of perturbing

Snail function.

The mechanism that is responsible for the observed

interchange is unknown. However, the inversion in

expression sites between chicks and mice is not com-

plete (some sites do not show this change), which indi-

cates that swapping of regulatory modules, differential

loss of tissue-specific cis-regulatory elements or differ-

ential availability of upstream regulators could occur.

Regardless of the mechanism, if Slug induces EMT in

the chick and Snail is responsible in the mouse, are they

functionally equivalent when ectopically expressed at the

appropriate sites? It would be interesting to determine

whether Slug can rescue the gastrulation phenotype

of the mouse Snail mutant. However, there is some

EMT processes also occur during the malignant con-

version of epithelial tumours, and pathological activa-

tion of Snail participates in this process

15,16

(BOX 2)

.The

same signalling molecules seem to operate for the

induction of Snail under these pathological circum-

stances. Indeed, TGF-β induces EMT in epithelial cells

and is necessary for acquisition of the invasive pheno-

type in carcinomas

82,83

. In addition, an integrin-linked

kinase (ILK)-dependent pathway has also been pro-

posed to activate Snail in colon carcinoma cells

(FIG. 4).

Different pathways converge in Snail to trigger EMT,

and this places Snail in a central position in this process.

Strict regulation of gene expression is therefore essential

for induction of EMT and maintenance of the migratory

phenotype — an indication of the cooperation that is

required between different signalling cascades. An inter-

esting model, which seems to be in keeping with the

results that have been obtained in different systems, is

that members of the TGF-β/BMP superfamily activate

Snail genes, the levels of which are maintained by FGF

signalling. Snail, in turn, maintains the downregulation

of E-cadherin, and this leaves the Wnt-signalling-medi-

ated, stabilized β-catenin available to bind TCF/LEF pro-

teins and activate gene expression in the nucleus.

Snail and Slug in chick and mouse

Differences in the sites of expression of Snail and Slug

between chick and mouse were the origin of some

confusion. Structural homologues were thought not to

be so, owing to the differences in the expression sites

— indeed, this was the case for the chick Snail-related

(SnR) protein

, which is the true Snail homologue

Studies of Slug-mutant mice showed that Slug is not

essential for mesoderm or neural-crest formation

. This

VISCERAL ENDODERM

The extraembryonic cell layer

that is involved in nutrient

uptake and transport.

Figure 4 | Snail genes occupy a central position in triggering EMT in physiological and pathological situations. Different

signalling molecules have been implicated in the activation of Snail genes in several processes that subsequently lead to the

conversion of epithelial cells into mesenchymal cells. Although the action of Snail in the epithelial–mesenchymal transition (EMT) as a

direct transcriptional regulator (repressor) has been shown only for E-cadherin, different in vitro and in vivo approaches point to a

series of target genes that are directly or indirectly regulated by these transcription factors. BMP, bone morphogenetic protein; FGF,

fibroblast growth factor; ILK, integrin-linked kinase; PTH(rP)R, parathyroid-hormone-related peptide receptor; TGF-β, transforming

growth factor-β.

FGF

Neural crest

Gastrulation

Epithelial tumour cells

Cytokeratin-18

Epithelial cells

Muc-1

Epithelial

cells

Desmoplakin

Epithelial

cells

E-cadherin

Parietal endoderm

Tumour progression

Hepatocytes

Mesoderm formation

Epithelial cells

Fibronectin

Epithelial cells

Vimentin

Epithelial cells

Rho GTPases

Neural-crest cells

Drosophila gastrulation

Wnt

Neural crest

BMP

Neural

crest

PTH(rP)R

Parietal

endoderm

Integrin

Tumour cells

TGF-β

Tumour progression

Hepatocytes

Heart development

Palatal fusion

Snail/Slug

ILK

Epithelial markers Mesenchymal markers Cytoskeletal changes

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REVIEWS

leukaemic factor; a putative CES-2 homologue) for the

E2A-positive transactivation domain. This leads to acti-

vation of a different family member, Slug, which in turn

represses a partial homologue of EGL-1 (BH3) and ren-

ders the anti-apoptotic BCL-X

protein active to pro-

mote survival, leading to leukaemia

(FIG. 5). So, both

Scratch and Slug seem to function as anti-apoptotic

agents, in agreement with data on the regulation of Slug

during limb development in the chick

75,76

.Here,Slug is

downregulated in the areas that are destined to die, and

is proposed to act as a survival factor that maintains the

undifferentiated mesenchymal phenotype.

Cell division and endoreduplication

During Drosophila gastrulation, the changes in cell

shape that are associated with formation of the ventral

furrow are accompanied by inhibition of mitosis. This

links morphogenesis with cell division. This inhibition

is mediated by Tribbles, a serine/threonine kinase that

counteracts String, the homologue of the

CDC25 phos-

phatase that is necessary for mitosis

87,88

. This inhibition

functional equivalence, at least during embryonic devel-

opment, both within and between species. Ectopic

expression of chick and mouse Snail in the chick hind-

brain induces an increase in neural-crest production, in

a similar way to that of the endogenous gene, Slug

.But

it is not clear whether this functional equivalence also

occurs during tumour progression, as Slug is expressed

in different carcinoma-derived cell lines regardless of

their phenotype in terms of

INVASIVENESS

Snail superfamily and cell survival

Several lines of evidence point to a role for Snail

superfamily members in regulating cell death or sur-

vival. In a particular population of C. elegans neurons,

the protein involved in cell death, CES-2, represses

CES-1 (scratch) function. This allows the cell-death

activator EGL-1 to repress the survival gene ced-9, and

allows the action of the cell-death proteins CED-4 and

CED-3 (

REF. 37; FIG. 5).

In some human leukaemias, a chromosomal translo-

cation swapped the repression domain of HLF (hepatic

INVASIVENESS

The ability to degrade and

migrate through the

extracellular matrix.

Box 2 | The epithelial–mesenchymal transition in tumour progression

The epithelial–mesenchymal

transition (EMT) occurs not

only during normal

embryonic development, but

also in pathological situations

such as acquisition of the

invasive phenotype in

epithelial tumours, in which it

constitutes the first step for

the formation of metastasis.

This pathological EMT has

been associated with the

downregulation of E-cadherin

expression and the acquisition

of migratory properties.

Indeed, the loss of E-cadherin

expression is crucial for the

progression from adenoma to

carcinoma

105

The idea that pathological

activation of Snail genes

could be involved in tumour

progression was proposed

several years ago

, and has

been shown recently. Snail is

a strong direct repressor of

E-cadherin expression, and

Snail transfection confers

tumorigenic, invasive and

migratory properties to

otherwise normal epithelial

cells

15,16

. An inverse

correlation has been found

between Snail and E-cadherin expression in mouse and human cell lines

15,16,106,107

. Furthermore, Snail is activated in vivo

at the invasive front of chemically induced mouse skin tumours

, and it is present in human breast carcinomas

108,109

,in

which it inversely correlates with the degree of differentiation and is associated with lymph-node metastasis

109

.As such,

Snail can be now considered a marker of malignancy; this paves the way for the design of anti-invasive therapies and

makes the search for endogenous or artificial regulators of exceptional interest.

Mesoderm formation

Invasive area in primary tumour

Neural-crest delamination

Early mesoderm

Primitive streak

Neural tube

Neural

crest

Epithelial cells

Mesenchymal cells

(invasive)

Snail

Invasive cells Tumour cells

Break point of

basement

membrane

Basement membrane

Snail-negative cells Snail-positive cells

Embryonic development

Snail transfection Tumour progression

Epithelial–mesenchymal transition

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REVIEWS

Snail in left–right asymmetry

A striking asymmetric and transient Snail expression

in the right-hand lateral mesoderm of the chick

embryo led Cooke and colleagues

to investigate a

possible role for this gene in the establishment of

LEFT–RIGHT ASYMMETRY. Incubation of early chick

embryos with antisense oligonucleotides to Snail led

to a randomization of the

HEART SITUS

. Further exper-

iments

93,94

established that Snail lies in the genetic

cascade that gives rise to bilateral body asymmetries.

Snail is downstream of the signal that is generated by

the TGF-β superfamily member Nodal, and

upstream of the transcription factor Pitx-2 — a

bicoid-type homeobox protein that is responsible for

activating the left-side-specific differentiation pro-

gramme

(FIG. 5). Inhibition of the BMP signal that

inactivates Nodal on the left side of the embryo leads

to the repression of Snail, which in turn cannot

repress Pitx-2. The left–right asymmetric expression

of Snail is also observed in the mouse embryo, which

constitutes one of the few sites of mesodermal

expression that have not been interchanged between

chick and mouse at these early stages

. The transient

nature of this asymmetric expression, particularly in

the mouse, might explain why it has not been

detected in Drosophila or other vertebrates. It would

be interesting to re-analyse other species, as a

left–right asymmetric expression has also been found

in the limpet Patella vulgata for one of the two Snail

genes isolated, sna2

(REF. 32). This conservation indi-

cates that this might be an ancient function that is

associated with the Snail family.

depends on Snail function

, which, therefore, might

act as a mitotic inhibitor. This is in agreement with

the low proliferation rate that is observed in Snail-

transfected epithelial cells compared with control cells

(S. Vega and M.A.N., unpublished observations). It

seems reasonable that cells that undergo massive

cytoskeletal reorganization associated with changes in

cell shape or active migration are prevented from

undergoing cell division.

Other members of the Snail family — including

mouse Snail itself — have been associated with mitosis

in two processes. Indeed, Escargot and mouse Snail are

involved in the control of polyploidy in several tissues,

including

IMAGINAL DISCS cells in Drosophila

24,89

, mouse

TROPHOBLAST cells

and human megakaryocytes

. Both

proteins inhibit

ENDOREDUPLICATION, and therefore induce

progression of the cell cycle to mitosis. The molecular

mechanism could be related to the activation of String,

as this protein is involved in the control of mitosis cou-

pled to the process of

ASYMMETRIC CELL DIVISION in

Drosophila

(FIG. 5). Certainly, a deficiency in the three

Snail-family members (snail, escargot and worniu) leads

to an inappropriate activation of Inscutable, which con-

trols the subcellular localization of Prospero, a key pro-

tein in determining the

GANGLION MOTHER CELL fate

91,92

.So,

depending on the cellular process, at least in Drosophila,

Snail seems to act as an inhibitor or an activator of

String. Snail transcription factors have been shown to

act as repressors

9,14–16,24,26,27,29,30

, which indicates that

Snail-mediated activation could be the result of an indi-

rect regulation. However, the possibility that they act as

activators cannot be excluded at the moment

CDC25

A family of protein phosphatases

that dephosphorylate cyclin-

dependent kinases during cell-

cycle progression.

IMAGINAL DISCS

The primordia of different adult

structures that are present in the

larvae of insects with complete

metamorphosis.

TROPHOBLAST

The extraembryonic epithelial

tissue that is crucial for

formation of the placenta.

ENDOREDUPLICATION

The process by which the cells

pass to rounds of DNA

duplication in the absence of a

mitotic division.

ASYMMETRIC CELL DIVISION

A process by which a cell gives

rise to two different descendants

after division.

GANGLION MOTHER CELL

One of the daughters of a

Drosophila neuroblast after

asymmetric cell division. It

divides once more to give rise to

two post-mitotic neurons.

LEFT–RIGHT ASYMMETRY

The differences along the

left–right axis of the body.

HEART SITUS

The position of the heart with

respect to the left–right axis of

the body.

Figure 5 | Different genetic pathways involving Snail function. In addition to triggering the epithelial–mesenchymal transition,

Snail function has been described in several genetic pathways that lead to a | cell death or survival, b | asymmetric cell division and

c | left–right (L/R) asymmetry. In all cases, arrows indicate the flow of the pathway, not direct transcriptional repression or activation.

Although function as transcriptional activators cannot be fully excluded, Snail proteins have been described as transcriptional

repressors in all the species analysed so far. To follow the sequence of active proteins in the corresponding pathway, genes that are

repressed or inactive are shown in red, and the inactive regulatory steps are shown as dotted lines. HLF, hepatic leukaemic factor;

BMP, bone morphogenetic protein; FGF, fibroblast growth factor.

C. elegans/human Human leukaemia Vertebrates

CES-2/HLF?

EGL-1/BH3

CED-9/BCL-X

CED-4/APAF-1

CED-3/caspase-9

Cell death

Left Right

CES-1/Slug

(Scratch)

Drosophila

Proneural genes

Inscutable String (cdc25)

Neuroblast

asymmetry

Neuroblast

division

Prospero asymmetric

localization

Ganglion mother-cell fate

Snail genes

(snail, escargot, worniu)

E2A-HLE (HLF con-

verted in activator)

BH3

BCL-X

Cell survival

(leukaemogenesis)

Slug

BMP

antagonist

Nodal

Snail

Pitx2

BMP

Nodal

Snail

Pitx2

BMP

FGF8

Cell survival L/R asymmetry

Neuroblast asymmetric

cell division

abc

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REVIEWS

However, this function is not unique for this family,

as vertebrate Snail and Slug are expressed in the ner-

vous system at later developmental stages (F. Marin

and M.A.N., unpublished observations), indicating

that, probably, neuronal differentiation might be a

function that is associated with both the Snail and

Scratch families

(BOX 3).

Cooperativity and antagonism

What is the relationship between different members of

the Snail superfamily when they act in the same biologi-

cal process or on similar targets? Interestingly, there are

examples of both cooperativity and antagonism.

With respect to cell differentiation, chick Slug

and mouse Snail and Slug have been proposed to

maintain the mesenchymal phenotype and repress dif-

ferentiation

15,75,76

. Similarly, Drosophila snail and escargot

also maintain the undifferentiated phenotype — they

antagonize neurogenesis by competing with bHLH pro-

teins

. So, they seem to antagonize the role of scratch in

promoting neural differentiation in Drosophila

, C. ele-

gans

and mouse

Chick and human Slug are associated with cell sur-

vival

25,75,76

, whereas chick Snail has been associated with

the apoptotic programme in the developing limb

With regard to target genes, Snail represses E-cadherin

expression during mesoderm formation in Drosophila

and mammals

15,16

; by contrast, escargot activates cad-

herin expression during tracheal development in the fly.

In some cases, different family members cooperate,

such as the three Drosophila snail genes in neurogene-

sis

and asymmetric cell division

91,92

.Moreover,snail

and escargot cooperate in wing development

in the fly,

and the vertebrate Snail and Slug genes might also coop-

erate in triggering EMT and maintaining the mesenchy-

mal phenotype during neural-crest development

13–15

Finally, a striking example is the regulation of

String by Drosophila snail, which seems to activate it

during asymmetric cell division

and inhibit it dur-

ing gastrulation

Perspectives

Although we now have invaluable information on the

different processes in which the Snail superfamily pro-

teins are involved — both during development and in

some pathological situations — we are a long way from

fully understanding their functions and mutual relation-

ships. Further work will take advantage of the completed

genomes and of the new imaging approaches that allow

cell movements to be followed in the living embryo.

As Snail-mutant mice die at gastrulation, spatio-

temporal, conditional Snail-mutant mice are needed to

study the participation of Snail in later processes such as

formation of the neural crest or differentiation of tissues

and organs, including the mesoderm. In terms of the

role of Snail in the appearance of the neural crest during

evolution, experiments that are similar to those carried

out for the Hox genes — in which regulatory sequences

from non-vertebrate chordates are introduced in trans-

genic mice

100

— will help to challenge the genetic pro-

gramme that is already present in the proposed precursor

The Snail superfamily in neural development

Although the four Drosophila genes that have been

analysed so far — snail, escargot, worniu and scratch —

are prominently expressed in the nervous system, indi-

vidual mutants do not show a strong neural phenotype.

However, double mutants of scratch and the HLH pro-

tein deadpan show loss of neurons

. Similarly, deletion

of snail, escargot and worniu leads to the loss of central

nervous system determinants

. The identification of

two additional scratch-related genes in Drosophila

indi-

cates that the three scratch genes could collaborate, as

the Snail members do, and a strong neural phenotype

might be expected for the triple mutant.

Interestingly, the C. elegans scratch homologue ces-1

(REF. 37) is essential for the formation of neurons, and

the mouse Scratch gene is neural specific and induces

neuronal differentiation in P19 embryonal carcinoma

cells

.In addition,a Scratch homologue is specifically

expressed in the primary neurons of the zebrafish

embryo (M. J. Blanco and M.A.N., unpublished obser-

vations), which indicates a neuronal-specific function

for both the invertebrate and vertebrate Scratch family.

Box 3 | Proposed ancestral and derived functions of the Snail superfamily

Phylogenetic and expression studies together with functional analyses in different model

organisms allow ancestral and acquired functions to be proposed for the different Snail

superfamily groups in metazoan evolution. An ancestral function in the development of

sensory and/or neuronal structures is proposed for the whole superfamily

,which

includes both the Snail and Scratch genes.An additional ancestral function in the control

of cell death/survival is also proposed

25,37,75,76

The role in epithelial–mesenchymal transitions (EMTs) seems to be exclusively

associated with the members of the Snail family, with representatives analysed in

Lophotrochozoans,

ECDYSOZOANS and deuterostomes. This role in EMT has been co-opted

for cell migration during mesoderm and neural-crest formation and tumour

progression, when these processes emerged

. In vertebrates, Snail and/or Slug proteins

participate in this process depending on the species. Further roles in the development of

appendages

74–76,99

and cell division

24,27,88–92

are associated with particular members of the

Snail family in different groups. Finally, a still-uncharacterized role in lens development

has been specifically proposed for Slug subfamily members, which seem to participate

neither in cell division nor in the definition of left–right (L/R) asymmetry. None of the

known functions has been specifically associated with the Scratch family or the

vertebrate Snail subfamily.

Wing/limb

development

L/R asymmetry

Mesoderm

specification/

development

Neural-crest

specification

Neural-crest

delamination

Lens developmentSlug

Snail family

(includes Slug

in vertebrates)

Cell division and EMT

Neural/sensory

development

Cell survival

Snail superfamily

(snail + scratch)

Metazoan

ancestor

Arthropods

and/or molluscs

Non-vertebrate

chordates

Vertebrates Mammals

Tumour

progression

ECDYSOZOANS

One of the important groups

within the animal kingdom, it

includes arthropods and

nematodes.

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REVIEWS

transcription factors for binding to E boxes will depend

on relative affinities that might need the participation

of different co-regulators, and cooperation with bHLH

proteins or other unidentified partners could provide

additional degrees of complexity for the patterning and

differentiation of specific cell types.

The description of Scratch as a new family offers

unexplored territory for the study of new functions in

the different species. And finally, the implication of the

Snail family in pathology challenges the use of

amenable systems to identify specific repressors that can

be used to develop new therapeutic strategies.

population

. Obviously, characterization of the regula-

tory sequences that drive specific spatio-temporal

expression of the different members in different tissues

and species is a long-term goal that has to be

approached systematically.

From a more biochemical point of view, we have lit-

tle information on the mechanism that is used by Snail

for transcriptional regulation. We do not know whether

Snail genes can act as activators, and have little infor-

mation on the proteins that induce or repress their

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Acknowledgements

I am very grateful to people in the lab for their work along the years

and for their encouraging discussions. Work in the lab is, at pre-

sent, supported by grants from the Spanish Ministries of Science

and Technology (DGESIC) and Health (FIS), and from the Local

Government (Comunidad Autónoma de Madrid).

Online links

DATABASES

The following terms in this article are linked online to:

FlyBase: http://flybase.bio.indiana.edu/

single-minded | String | snail | Tribbles | worniu

LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink

BCL-X

| ILK

Swiss-Prot: http://www.expasy.ch/

fibronectin | Gfi1 | HLF | Lef-1 | Muc-1 | Nodal | PTH(rP) | RhoB |

WormBase: http://www.wormbase.org/

ces-1 | CES-2 | CED-3 | CED-4 | ced-9 | EGL-1

Access to this interactive links box is free online.

Snail Transcriptionally Represses Brachyury to Promote the Mesenchymal-Epithelial Transition in Ascidian Notochord Cells

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Mar 2024
INT J MOL SCI

Mesenchymal-epithelial transition (MET) is a widely spread and evolutionarily conserved process across species during development. In Ciona embryogenesis, the notochord cells undergo the transition from the non-polarized mesenchymal state into the polarized endothelial-like state to initiate the lumen formation between adjacent cells. Based on previously screened MET-related transcription factors by ATAC-seq and Smart-Seq of notochord cells, Ciona robusta Snail (Ci-Snail) was selected for its high-level expression during this period. Our current knockout results demonstrated that Ci-Snail was required for notochord cell MET. Importantly, overexpression of the transcription factor Brachyury in notochord cells resulted in a similar phenotype with failure of lumen formation and MET. More interestingly, expression of Ci-Snail in the notochord cells at the late tailbud stage could partially rescue the MET defect caused by Brachyury-overexpression. These results indicated an inverse relationship between Ci-Snail and Brachyury during notochord cell MET, which was verified by RT-qPCR analysis. Moreover, the overexpression of Ci-Snail could significantly inhibit the transcription of Brachyury, and the CUT&Tag-qPCR analysis demonstrated that Ci-Snail is directly bound to the upstream region of Brachyury. In summary, we revealed that Ci-Snail promoted the notochord cell MET and was essential for lumen formation via transcriptionally repressing Brachyury.

Endothelial to Mesenchymal Transition in an HHT-like pediatric case of Multiple Pulmonary Arteriovenous Malformations

Preprint

Full-text available

Apr 2024

Pulmonary arteriovenous malformations (PAVMs) are vascular anomalies resulting in abnormal connection between pulmonary arteries and veins. In 80% of cases, PAVMs are present from birth, but clinical manifestations are rarely seen in childhood. These congenital malformations are typically associated with Hereditary Hemorrhagic Telangiectasia (HHT), a rare disease that affects 1 in 5,000/8,000 individuals. HHT disease is frequently caused by mutations in genes involved in the TGF-β pathway. However, approximately 15% of patients do not have a genetic diagnosis and, among the genetically diagnosed, more than 33% do not meet the Curaçao Criteria. This makes clinical diagnosis even more challenging in the pediatric age group. Here, we introduce an 8 years old patient bearing a severe phenotype of multiple diffuse PAVMs caused by an unknown mutation which ended in lung transplantation. Phenotypically, the case of study follows a molecular pattern HHT-like. Therefore, molecular biology and cellular functional analysis has been performed in primary endothelial cells (ECs) isolated from the explanted lung. The findings revealed a loss of functionality in lung endothelial tissue and a stimulation of endothelial to mesenchymal transition. Understanding the molecular basis of this transition could potentially offer new therapeutic strategies to delay lung transplantation in severe cases.

Escargot a Snail superfamily member and its multiple roles in Drosophila melanogaster development

Article

Full-text available

Apr 2024
J CELL PHYSIOL

The Snail superfamily of transcription factors plays a crucial role in metazoan development; one of the most important vertebrate members of this family is Snai1 which is orthologous to the Drosophila melanogaster esg gene. This review offers a comprehensive examination of the roles of the esg gene in Drosophila development, covering its expression pattern and downstream targets, and draws parallels between the vertebrate Snai1 family proteins on controlling the epithelial‐to‐mesenchymal transition and esg . This gene regulates stemness, ploidy, and pluripontency. esg is expressed in various tissues during development, including the gut, imaginal discs, and neuroblasts. The functions of the esg include the suppression of differentiation in intestinal stem cells and the preservation of diploidy in imaginal cells. In the nervous system development, esg expression also inhibits neuroblast differentiation, thus regulating the number of neurons and the moment in development of neuronal differentiation. Loss of esg function results in diverse developmental defects, including defects in intestinal stem cell maintenance and differentiation, and alters imaginal disc and nervous system development. Expression levels of esg also play a role in regulating longevity and metabolism in adult stages. This review provides an overview of the current understanding of esg's developmental role, emphasizing cellular and tissue effects that arise from its loss of function. The insights gained may contribute to a better understanding of evolutionary conserved developmental mechanisms and certain metabolic diseases.

Determination of In Vitro and In Vivo Effects of Taxifolin and Epirubicin on Epithelial-Mesenchymal Transition in Mouse Breast Cancer Cells

Article

Full-text available

Mar 2024
TECHNOL CANCER RES T

Background: One of the most significant characteristics of cancer is epithelial–mesenchymal transition and research on the relationship between phenolic compounds and anticancer medications and epithelial–mesenchymal transition is widespread. Methods: In order to investigate the potential effects of Taxifolin on enhancing the effectiveness of Epirubicin in treating breast cancer, specifically in 4T1 cells and an allograft BALB/c model, the effects of Taxifolin and Epirubicin, both individually and in combination, were examined. Cell viability assays and cytotoxicity assays in 4T1 cells were performed. In addition, 4T1 cells were implanted into female BALB/c mice to conduct in vivo studies and evaluate the therapeutic efficacy of Taxifolin and Epirubicin alone or in combination. Tumor volumes and histological analysis were also assessed in mice. To further understand the mechanisms involved, we examined the messenger RNA and protein levels of epithelial–mesenchymal transition-related genes, as well as active Caspase-3/7 levels, using quantitative real-time polymerase chain reaction, western blot, and enzyme-linked immunosorbent assays, respectively. Results: In vitro results demonstrated that the coadministration of Taxifolin and Epirubicin reduced cell viability and cytotoxicity in 4T1 cell lines. In vivo, coadministration of Taxifolin and Epirubicin suppressed tumor growth in BALB/c mice with 4T1 breast cancer cells. Additionally, this combination treatment significantly increased the levels of active caspase-3/7 and downregulated the messenger RNA and protein levels of N-cadherin, β-catenin, vimentin, snail, and slug, but upregulated the E-cadherin gene. It significantly decreased the messenger RNA levels of the Zeb1 and Zeb2 genes. Conclusion: The in vitro and in vivo results of our study indicate that the concurrent use of Epirubicin with Taxifolin has supportive effects on breast cancer treatment.

Expressed in HPV 16 Infected Epidermoid Carcinoma Cells, HIV‐1 Reverse Transcriptase Shifts Their Metabolism towards aerobic Glycolysis, Induces Hybrid E/M Cell Phenotype and Increases Expression of E6 Transcripts

Preprint

Full-text available

Dec 2023

High incidence of epithelial malignancies in HIV-1 infection is associated with co-infection with oncogenic viruses, such as high-risk human papillomaviruses (HR HPVs), most often HPV16. A collaboration between HIV-1 and HR HPVs in the malignant transformation of epithelial cells exposed to both viruses have long been anticipated. Here, we delineated the effects on in vitro and in vivo properties of HPV16-infected cervical cancer cells of HIV-1 reverse transcriptase. Epidermoid carcinoma Ca Ski cells were made to express RT of HIV-1 clade A (RT_A) or Green Fluorescence Protein (GFP) by lentiviral transduction, subclones with one or six copies of RT_A and six copies of GFP DNA were generated. Expression by subclones of GFP was assessed by flow cytometry, and of RT_A, by Western blotting. Cells were assessed for the levels of mRNA of the RT_A, E6 and its isoforms, and cellular factors characterizing oxidative stress, and EMT by the real-time PCR. Parameters of glycolysis and mitochondrial respiration were assessed by Seahorse technology. Subclones were surveyed for the changes in cell cycle, doubling time, migration capacity, clonogenic activity and for the capacity to form tumors in nude mice. Ca Ski RT_A produced 20-55 fg of RT_A per cell. Expression of RT_A caused a proportional increase in the expression of E6*I isoform. In Ca Ski, RT_A suppressed mitochondrial respiration and oxygen consumption, and increased glycolysis. Compared to parental cells and GFP control, transcription in Ca Ski RT_A of epithelial (E-CADHERIN) state marker was increased, and of mesenchymal (VIMENTIN) decreased, indicating acquisition by cells of the hybrid epithelial/mesenchymal (E/M) phenotype. While Ca Ski GFP and Ca Ski RT_A with one gene insert had reduced migration rate, decreased clonogenic activity in vitro, and diminished capacity to form tumors in nude mice, increase in RT_A copicity/expression mitigated these effects. Altogether, expression of HIV-1 RT_A gave Ca Ski cells the plasticity required to overcome negative effects of lentiviral transduction and potentially increased their tumorigenicity.

Tumor biomarkers for diagnosis, prognosis and targeted therapy

Article

Full-text available

May 2024

Tumor biomarkers, the substances which are produced by tumors or the body’s responses to tumors during tumorigenesis and progression, have been demonstrated to possess critical and encouraging value in screening and early diagnosis, prognosis prediction, recurrence detection, and therapeutic efficacy monitoring of cancers. Over the past decades, continuous progress has been made in exploring and discovering novel, sensitive, specific, and accurate tumor biomarkers, which has significantly promoted personalized medicine and improved the outcomes of cancer patients, especially advances in molecular biology technologies developed for the detection of tumor biomarkers. Herein, we summarize the discovery and development of tumor biomarkers, including the history of tumor biomarkers, the conventional and innovative technologies used for biomarker discovery and detection, the classification of tumor biomarkers based on tissue origins, and the application of tumor biomarkers in clinical cancer management. In particular, we highlight the recent advancements in biomarker-based anticancer-targeted therapies which are emerging as breakthroughs and promising cancer therapeutic strategies. We also discuss limitations and challenges that need to be addressed and provide insights and perspectives to turn challenges into opportunities in this field. Collectively, the discovery and application of multiple tumor biomarkers emphasized in this review may provide guidance on improved precision medicine, broaden horizons in future research directions, and expedite the clinical classification of cancer patients according to their molecular biomarkers rather than organs of origin.

Comprehensive molecular interaction map of TGFβ induced epithelial to mesenchymal transition in breast cancer

Article

Full-text available

May 2024

Breast cancer is one of the prevailing cancers globally, with a high mortality rate. Metastatic breast cancer (MBC) is an advanced stage of cancer, characterised by a highly nonlinear, heterogeneous process involving numerous singling pathways and regulatory interactions. Epithelial–mesenchymal transition (EMT) emerges as a key mechanism exploited by cancer cells. Transforming Growth Factor-β (TGFβ)-dependent signalling is attributed to promote EMT in advanced stages of breast cancer. A comprehensive regulatory map of TGFβ induced EMT was developed through an extensive literature survey. The network assembled comprises of 312 distinct species (proteins, genes, RNAs, complexes), and 426 reactions (state transitions, nuclear translocations, complex associations, and dissociations). The map was developed by following Systems Biology Graphical Notation (SBGN) using Cell Designer and made publicly available using MINERVA ( http://35.174.227.105:8080/minerva/?id=Metastatic_Breast_Cancer_1 ). While the complete molecular mechanism of MBC is still not known, the map captures the elaborate signalling interplay of TGFβ induced EMT-promoting MBC. Subsequently, the disease map assembled was translated into a Boolean model utilising CaSQ and analysed using Cell Collective. Simulations of these have captured the known experimental outcomes of TGFβ induced EMT in MBC. Hub regulators of the assembled map were identified, and their transcriptome-based analysis confirmed their role in cancer metastasis. Elaborate analysis of this map may help in gaining additional insights into the development and progression of metastatic breast cancer.

Brd4::Nutm1 fusion gene initiates NUT carcinoma in vivo

Article

Full-text available

May 2024

NUT carcinoma (NC) is an aggressive cancer with no effective treatment. About 70% of NUT carcinoma is associated with chromosome translocation events that lead to the formation of a BRD4::NUTM1 fusion gene. Because the BRD4::NUTM1 gene is unequivocally cytotoxic when ectopically expressed in cell lines, questions remain on whether the fusion gene can initiate NC. Here, we report the first genetically engineered mouse model for NUT carcinoma that recapitulates the human t(15;19) chromosome translocation in mice. We demonstrated that the mouse t(2;17) syntenic chromosome translocation, forming the Brd4::Nutm1 fusion gene, could induce aggressive carcinomas in mice. The tumors present histopathological and molecular features similar to human NC, with enrichment of undifferentiated cells. Similar to the reports of human NC incidence, Brd4::Nutm1 can induce NC from a broad range of tissues with a strong phenotypical variability. The consistent induction of poorly differentiated carcinoma demonstrated a strong reprogramming activity of BRD4::NUTM1. The new mouse model provided a critical preclinical model for NC that will lead to better understanding and therapy development for NC.

Chromatin Remodeling in Patient‐Derived Colorectal Cancer Models

Article

Full-text available

Feb 2024

Patient‐Derived Organoids (PDO) and Xenografts (PDX) are the current gold standards for patient‐derived models of cancer (PDMC). Nevertheless, how patient tumor cells evolve in these models and the impact on drug response remains unclear. Herein, the transcriptomic and chromatin accessibility landscapes of matched colorectal cancer (CRC) PDO, PDX, PDO‐derived PDX (PDOX), and original patient tumors (PT) are compared. Two major remodeling axes are discovered. The first axis delineates PDMC from PT, and the second axis distinguishes PDX and PDO. PDOX are more similar to PDX than PDO, indicating the growth environment is a driving force for chromatin adaptation. Transcription factors (TF) that differentially bind to open chromatins between matched PDO and PDOX are identified. Among them, KLF14 and EGR2 footprints are enriched in PDOX relative to matched PDO, and silencing of KLF14 or EGR2 promoted tumor growth. Furthermore, EPHA4, a shared downstream target gene of KLF14 and EGR2, altered tumor sensitivity to MEK inhibitor treatment. Altogether, patient‐derived CRC cells undergo both common and distinct chromatin remodeling in PDO and PDX/PDOX, driven largely by their respective microenvironments, which results in differences in growth and drug sensitivity and needs to be taken into consideration when interpreting their ability to predict clinical outcome.

UPP1 enhances bladder cancer progression and gemcitabine resistance through AKT

Article

Full-text available

Jan 2024
INT J BIOL SCI

UPP1, a crucial pyrimidine metabolism-related enzyme, catalyzes the reversible phosphorylation of uridine to uracil and ribose-1-phosphate. However, the effects of UPP1 in bladder cancer (BLCA) have not been elucidated. AKT, which is activated mainly through dual phosphorylation (Thr308 and Ser473), promotes tumorigenesis by phosphorylating downstream substrates. This study demonstrated that UPP1 promotes BLCA cell proliferation, migration, invasion, and gemcitabine resistance by activating the AKT signaling pathway in vitro and in vivo. Additionally, UPP1 promoted AKT activation by facilitating the binding of AKT to PDK1 and PDK2 and the recruitment of phosphatidylinositol 3,4,5-triphosphate to AKT. Moreover, the beneficial effects of UPP1 on BLCA tumorigenesis were mitigated upon UPP1 mutation with Arg94 or MK2206 treatment (AKT-specific inhibitor). AKT overexpression or SC79 (AKT-specific activator) treatment restored tumor malignancy and drug resistance. Thus, this study revealed that UPP1 is a crucial oncogene and a potential therapeutic target for BLCA and that UPP1 activates the AKT signaling pathway and enhances tumorigenesis and drug resistance to gemcitabine.

Control of imaginal cell development by the escargot gene of Drosophila

Article

Full-text available

May 1993

Mutations in the escargot (esg) locus, which codes for a zinc-finger-containing protein with similarity to the product of the snail gene, cause a variety of defects in adult structures such as loss of abdominal cuticle and malformation of the wings and legs. esg RNA is expressed in wing, haltere, leg and genital imaginal discs and in abdominal histoblast nests in the embryo. Expression in imaginal tissues is also found in third instar larvae. In esg mutant larvae, normally diploid abdominal histoblasts replicate their DNA without cell division and become similar in appearance to the polytene larval epidermal cells. A similar phenotype was also found in imaginal discs of larvae mutant for both esg and the Drosophila raf gene. These results suggest that one of the normal functions of esg may be the maintenance of diploidy in imaginal cells.

Vertebrate genome evolution and the zebrafish gene map (vol 18, pg 345, 1998)

Article

Full-text available

Jul 1998

Neural crest cell-cell adhesion controlled by sequential and subpopulation-specific expression of novel cadherins

Article

May 1995

We identified two cadherins, c-cad6B and c-cad7, expressed by neural crest cells at their premigratory and migratory stages, respectively, in chicken embryos. cDNA transfection experiments showed that both were homophilic adhesion molecules, endowing cells with specific adhesiveness. During development, c-cad6B appeared in the neural fold, localizing at the future neural crest area. This expression was maintained during neural tube closure, but disappeared after neural crest cells had left the neural tube, suggesting its role in neural fold fusion and/or in the formation and maintenance of the presumptive neural crest domain in the neural plate/tube. Crest cells emerging from the neural tube lost c-cad6B, and a subpopulation of them began to express c-cad7. This subpopulation-specific expression of c-cad7 persisted during their migration. The migrating c-cad7-positive cells clustered together, and eventually populated restricted regions including the dorsal and ventral roots but very little ganglia. The latter was populated with N-cadherin-positive crest cells. Migrating neural crest cells expressed alpha- and beta-catenin at cell-cell contacts, indicating that their cadherins are functioning. These results suggest that the migrating crest cells are grouped into subpopulations expressing different cadherins. The cadherin-mediated specific interaction between crest cells likely plays a role in intercellular signaling between homotypic cells as well as in sorting of heterotypic cells.

Genetic events and the role of TGFβ in epithelial tumour progression

Article

Jan 1999
J PATHOL

The mouse skin model of chemical carcinogenesis has been very well characterized with respect to epigenetic changes, which occur during tumour cell initiation, promotion and progression. The use of transgenic and gene knock‐out mice has contributed greatly to knowledge in this area. The H‐ras genetic locus has been shown to undergo multiple genetic changes, including mutagenic activation, amplification of the mutant gene, and loss of the normal allele. These different genetic events lead to thresholds of ras activity which contribute to different stages along the pathway to neoplasia. The genetic and epigenetic events which lead to tumour invasion and metastasis have been less well characterized than studies on tumour initiation and promotion, despite the fact that it is metastases which ultimately kill the animal/patient. In the mouse skin model, loss of p53 contributes to malignant conversion. Gene deletion of the INK4 locus is associated with transformation to a highly invasive spindle cell tumor phenotype. This spindle cell transformation can also be induced in vitro or in vivo by TGFβ 1, possible by synergizing with mutant H‐ras. TGFβ can have both positive and negative effects on tumourigenesis, acting early as a tumour suppresser, but later as a stimulator of tumour invasion. It is this latter effect which may be clinically more significant, since many human tumours overexpress TGFβ, yet the majority still retain the intracellular signaling systems necessary for the cell to respond to this growth factor. Copyright © 1999 John Wiley & Sons, Ltd.

Mutations affecting the pattern of the larval cuticle inDrosophila melanogaster : I. Zygotic loci on the second chromosome

Article

Sep 1984

In a search for embryonic lethal mutants on the second chromosome ofDrosophila melanogaster, 5764 balanced lines isogenic for an ethyl methane sulfonate (EMS)-treatedcn bw sp chromosome were established. Of these lines, 4217 carried one or more newly induced lethal mutations corresponding to a total of 7600 lethal hits. Eggs were collected from lethal-bearing lines and unhatched embryos from the lines in which 25% or more of the embryos did not hatch (2843 lines) were dechorionated, fixed, cleared and scored under the compound microscope for abnormalities of the larval cuticle. A total of 272 mutants were isolated with phenotypes unequivocally distinguishable from wild-type embryos on the basis of the cuticular pattern. In complementation tests performed between mutants with similar phenotype, 48 loci were identified by more than one allele, the average being 5.4 alleles per locus. Complementation of all other mutants was shown by 13 mutants. Members of the complementation groups were mapped by recombination analysis. No clustering of loci with similar phenotypes was apparent. From the distribution of the allele frequencies and the rate of discovery of new loci, it was estimated that the 61 loci represent the majority of embryonic lethal loci on the second chromosome yielding phenotypes recognizable in the larval cuticle.

Control of neural crest cell fate by the Wnt signaling pathway

Article

Jun 1998

The Slug Gene Is Not Essential for Mesoderm or Neural Crest Development in Mice

Article

Jun 1998

R Jiang

A novel Snail-related transcription factor Smuc regulates basic helix-loop-helix transcription factor activities via specific E-box motifs

Article

Jan 2000
NUCLEIC ACIDS RES

Hiroshi Kataoka

Snail family proteins are zinc finger transcriptional regulators first identified in Drosophila which play critical roles in cell fate determination. We identified a novel Snail-related gene from murine skeletal muscle cells designated Smuc. Northern blot analysis showed that Smuc was highly expressed in skeletal muscle and thymus. Smuc contains five putative DNA-binding zinc finger domains in its C-terminal half. In electrophoretic mobility shift assays, recombinant zinc finger domains of Smuc specifically bound to CAGGTG and CACCTG E-box motifs (CANNTG). Because basic helix–loop–helix transcription factors (bHLH) bind to the same E-box sequences, we examined whether Smuc competes with the myogenic bHLH factor MyoD for DNA binding. Smuc inhibited the binding of a MyoD–E12 complex to the CACCTG E-box sequence in a dose-dependent manner and suppressed the transcriptional activity of MyoD–E12. When heterologously targeted to the thymidine kinase promoter as fusion proteins with the GAL4 DNA-binding domain, the non-zinc finger domain of Smuc acted as a transcriptional repressor. Furthermore, overexpression of Smuc in myoblasts repressed transactivation of muscle differentiation marker Troponin T. Thus, Smuc might regulate bHLH transcription factors by zinc finger domains competing for E-box binding, and non-zinc finger repressor domains might also confer transcriptional repression to control differentiation processes.

The Inductive Properties of Mesoderm Suggest That the Neural Crest Cells Are Specified by a BMP Gradient

Article

Jun 1998

Lorena Marchant

Expression of snail2,a Second Member of the Zebrafish Snail Family, in Cephalic Mesendoderm and Presumptive Neural Crest of Wild-Type and spadetailMutant Embryos

Article

Nov 1995

Transcripts of a newly discovered gene calledsnail2,encoding a zinc finger protein of the Snail family, first appear in rows of cephalic mesendodermal cells in gastrulating zebrafish embryos. At the end of gastrulation,snail2RNA accumulates in a domain of ectodermal cells that mark the border between the epidermal epithelium and the neural plate and includes precursors of the neural crest. During somitogenesis,snail2expression becomes restricted to neural crest.snail2is thus one of the earliest genes yet known to be specifically expressed in neural crest in zebrafish embryos. Sincesnail2is expressed in mesendoderm, a tissue layer whose convergence in the trunk is known to be altered in embryos homozygous for thespadetailmutation, we examinedsnail2expression inspadetailembryos. In these mutants, the number of cephalic mesendodermal cells expressingsnail2is strongly reduced and the distribution of cells containingsnail2andno tailtranscripts in the axial mesoderm is much broader than normal. Moreover, the embryos are shorter than normal at the end of gastrulation. This shows that, in addition to the failure of paraxial mesoderm to converge normally in the trunk during gastrulation,spadetailalso affects the elongation of the embryo and the convergence of axial and lateral mesendoderm in both trunk and head.

Nieto MA.. The Snail superfamily of zinc-finger transcription factors. Nat Rev Mol Cell Biol 3: 155-166

Abstract

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