Identification of a Pax6-dependent epidermal growth factor family signaling source at the lateral edge of the embryonic cerebral cortex.
ABSTRACT In an emerging model, area patterning of the mammalian cerebral cortex is regulated in part by embryonic signaling centers. Two have been identified: an anterior telencephalic source of fibroblast growth factors and the cortical hem, a medial structure expressing winglessint (WNT) and bone morphogenetic proteins. We describe a third signaling source, positioned as a mirror image of the cortical hem, along the lateral margin of the cortical primordium. The cortical antihem is identified by gene expression for three epidermal growth factor (EGF) family members, Tgf(alpha), Neuregulin 1, and Neuregulin 3, as well as two other signaling molecules, Fgf7 and the secreted WNT antagonist Sfrp2. We find that the antihem is lost in mice homozygous for the Small eye (Pax6) mutation and suggest the loss of EGF signaling at least partially explains defects in cortical patterning and cell migration in Small eye mice.
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ABSTRACT: Three-dimensional reconstructions of the normal rat embryonic (E) neocortex on days E15, E17, E19, and E21, using Skandha (software designed by J. Prothero, University of Washington, Seattle), show that the neocortical ventricular zone shrinks rapidly in the medial direction during cortical morphogenesis. [3H]thymidine autoradiography indicates that the shrinkage of the ventricular zone occurs before neurons in lateral and ventrolateral parts of layers IV-II are generated. Consequently, most of these neurons originate 400-1000 microns medial to their settling sites in the cortical plate. Embryos killed at daily intervals up to E21 after a single injection of [3H]thymidine on either E17 or E18 revealed the presence of a prominent migratory path, the lateral cortical stream, used by neurons migrating to the lateral and ventrolateral cortical plate; neurons migrating to the dorsal cortical plate follow a direct radial path. Arrival times of neurons in the cortical plate depend on the migratory path and are proportional to the overall distance travelled. Neurons that migrate only radially arrive in the dorsal cortical plate in two days (shortest route). Neurons that migrate laterally arrive in the lateral cortical plate in 3 days (longer route) and in the ventrolateral cortical plate in 4 days (longest route). [3H]thymidine autoradiography also shows that cells generated in the neocortical ventricular zone migrate in the lateral cortical stream for 5 or more days and accumulate in a reservoir. Cells leave the reservoir to enter the piriform cortex and destinations (as yet undetermined) in the basal telencephalon. The lateral cortical stream is found wherever the neocortical primordium surrounds the basal ganglia and is absent behind the basal ganglia. A computer analysis of nuclear orientation in anterior and posterior parts of the intermediate zone in the dorsal neocortex between days E17 and E22 shows that horizontally oriented nuclei are more common anteriorly where many cells are migrating laterally than posteriorly where most cells are migrating radially.The Journal of Comparative Neurology 06/1991; 307(3):499-516. · 3.66 Impact Factor
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ABSTRACT: By homologous recombination of an internal ribosome entry site and Cre recombinase coding region into the 3'-untranslated region of the mouse Emx1 gene, we have generated a strain of mice, Emx1(IRES)cre, that expresses the Cre recombinase in a spatial and temporal pattern like that observed for Emx1. When mated to reporter strains, these mice are a sensitive means to fate-map the Emx1-expressing cells of the developing forebrain. Our results demonstrate that radial glia, Cajal-Retzius cells, glutamatergic neurons, astrocytes, and oligodendrocytes of most pallial structures originate from an Emx1-expressing lineage. On the other hand, most of the pallial GABAergic neurons arise outside the Emx1-expressing lineage. Structures that are located near the basal ganglia (e.g., the amygdala and endopiriform nuclei) are not uniformly derived from Emx1-expressing cells.Journal of Neuroscience 09/2002; 22(15):6309-14. · 6.91 Impact Factor
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ABSTRACT: The neocortex is divided into multiple areas with specific architecture, molecular identity and pattern of connectivity with the dorsal thalamus. Gradients of transcription factor expression in the cortical primordium regulate molecular regionalization and potentially the patterning of thalamic projections. We show that reduction of Fgf8 levels in hypomorphic mouse mutants shifts early gradients of gene expression rostrally, thereby modifying the molecular identity of rostral cortical progenitors. This shift correlates with a reduction in the size of a molecularly defined rostral neocortical domain and a corresponding rostral expansion of more caudal regions. Despite these molecular changes, the topography of projections between the dorsal thalamus and rostral neocortex in mutant neonates appears the same as the topography of wild-type littermates. Overall, our study demonstrates the role of endogenous Fgf8 in regulating early gradients of transcription factors in cortical progenitor cells and in molecular regionalization of the cortical plate.Development 06/2003; 130(9):1903-14. · 6.21 Impact Factor
the lateral margin of the cortical primordium. The cortical antihem is identified by gene expression for three epidermal growth factor
(EGF) family members, Tgf?, Neuregulin 1, and Neuregulin 3, as well as two other signaling molecules, Fgf7 and the secreted WNT
antagonist Sfrp2. We find that the antihem is lost in mice homozygous for the Small eye (Pax6) mutation and suggest the loss of EGF
Recent studies indicate that regional specification and growth
control of the cerebral cortex is initiated by signaling centers
operating on an originally homogeneous embryonic field
conferred by an anterior fibroblast growth factor (FGF) source
is regulated by the cortical hem, a medial signaling center en-
Grove, 2001; Hebert et al., 2002). Given the size and complexity
ing sources are almost certain to exist.
We hypothesized that the lateral edge of the cortical primor-
al., 2000; Kim et al., 2001). Sfrp2 expression is detected in this
and, by E12.5, describes a curve that laterally mirrors the medial
genes (see Fig. 1A). Together, these territories mark the lateral
and medial limits of the cortical ventricular zone (VZ) (see Fig.
1C,D). Two-color in situ hybridization experiments suggest a
the two territories are well separated in rostral telencephalon but
approach one another to meet in the caudal telencephalon (see
expected from studies of other cortical signaling centers to ex-
press multiple members of at least one secreted signaling mole-
growth factor (EGF) family members. First, a classic series of in
vitro experiments implicates EGF family members in the devel-
opment of cerebral cortical areas linked to the limbic system
(Ferri and Levitt, 1993; Levitt et al., 1997). The limbic system-
associated membrane protein LAMP is expressed in limbic cor-
tical areas (Levitt et al., 1997) and is upregulated in cells from
non-limbic cortical domains in response to EGF family ligands
(Ferri and Levitt, 1995). Second, EGF receptor-mediated signal-
ing controls dorsoventral neuronal specification in the develop-
ment of the Drosophila ventral nerve cord (Skeath, 1998; von
Ohlen and Doe, 2000). Two EGF ligands are involved in this
dorsoventral signaling: Spitz, a Tgf?-like molecule, and Vein,
which is similar in structure to the Neuregulin proteins, a sub-
family of vertebrate EGF ligands (Golembo et al., 1999).
Gene expression patterns were studied in CD-1 mice, Small eye (Sey, or
Pax6Sey-Neu) mice maintained on a C3H/He background, and Emx2-
targeted mice maintained on a C57BL/6 background. The day of plug
discovery was designated E0.5. In situ hybridization of E9.5–E18.5 fore-
brains and genotyping of the Sey and Emx2 mice were performed as
described previously (Pellegrini et al., 1996; Xu et al., 1997; Grove et al.,
This work was supported by a research grant from the National Alliance for Research on Schizophrenia and
Boncinelli, S. Dey, P. Godowski (Genentech, South San Francisco, CA), D. Lee, J. Nathans, R. Nusse, J. Pascall, J.
TheJournalofNeuroscience,July23,2003 • 23(16):6399–6403 • 6399
Amphiregulin (Das et al., 1995), Egf (Pascall and Brown, 1988), Hegfl/
Heparin-binding EGF-like growth factor/Diphtheria toxin receptor (IM-
AGE Consortium, GenBank accession number W80035), Fgf7/
Keratinocyte growth factor (IMAGE Consortium, GenBank accession
number BF159111), Nrg1 (IMAGE Consortium, GenBank accession
number AI197081), Nrg2 (IMAGE Consortium, GenBank accession
number AW476657), Nrg3 (Zhang et al., 1997), Nrg4 (IMAGE Consor-
tium, GenBank accession number AA238077), Sfrp2 (Rattner et al.,
1997), Tgfa/Tgf? (Vaughan et al., 1992; Kornblum et al., 1997), Tmeff1/
Transmembrane protein with EGF-like and two follistatin-like domains 1
(IMAGE Consortium, GenBank accession number BF147745) and
Tmeff2/Tomoregulin (IMAGE Consortium, GenBank accession number
AI098476), and a rat cDNA for Ereg/Epiregulin (Taylor et al., 1999).
We examined mRNA expression of 11 EGF family members: Egf
itself, Tgf?, Neuregulins 1–4, Amphiregulin, Epiregulin,
Heparin-binding EGF-like growth factor, Tmeff1, and Tmeff2.
Our screen disclosed that the lateral margin of the cortical pri-
mordium is enriched in expression of three Spitz and Vein ho-
mologs, Tgf?, Neuregulin1 (Nrg1), and Nrg3. At E12.5, Tgf? ex-
pression marks the antihem, which appears as a curve of strong
Tgf? expression when viewed from the lateral face of the cortical
primordium (Figs. 1,2A). Coronal sections show that Tgf? ex-
2B,C). All three genes display some graded expression in the
both Nrg genes, expression in the cortical primordium increased
as development proceeded (last age examined, E18.5). For all
three genes, however, the cortical antihem is the peak of expres-
sion in the cortical primordium.
Other EGF ligands, including Egf itself, were at least weakly
blum et al., 1997) but not concentrated in the antihem. At E10.5,
E12.5, and E14.5, Amphiregulin, Egf, Epiregulin, Hegf1, Nrg2,
Nrg4, Tmeff1, and Tmeff2 are expressed in the ventral telenceph-
alon, usually increasing in intensity with age. Most EGF family
members are expressed in the dorsal telencephalon from E12.5
onward, with the exception of Nrg2, which is detectable at E10.5.
No striking patterns of expression were detected except for Epi-
regulin and Tmeff1, which display expression gradients in far lat-
eral and medial embryonic cortex.
In a screen of Fgf gene expression in and near the cortical
primordium, we found that Fgf7 gene expression marks the an-
was noted previously in a lateral embryonic cortical region (Ma-
molecules are expressed along the lateral edge of the cortical
The cortical hem is part of the true cortical primordium as
characterized by progenitor cell behavior and gene expression
(Grove et al., 1998). Tgf? expression at the antihem also lies
within cortical primordium, defined by expression of Neuroge-
nin2 (Ngn2) (Fig. 3A,B; two-color in situ data not shown). Sfrp2
expression is within the Tgf? domain and marks the extreme
margin of the cortical primordium (Fig. 3G; Ngn2/Sfrp2 two-
color in situ data not shown). Fgf7 expression, which is strongest
in the posterior antihem, overlaps that of Sfrp2 (Fig. 3J). In con-
trast, expression of Nrg1 and Nrg3 is not restricted to the cortical
with two-color in situ hybridization. A, B, The “swoosh” of the medial cortical hem (A, blue
6400 • J.Neurosci.,July23,2003 • 23(16):6399–6403 Assimacopoulosetal.•EGFLigandsintheCorticalAntihem
The transcription factor genes Emx1 and Dlx2 are differen-
wedge clear of expression of either gene in the VZ of the lateral
margin of the cortical primordium (Fig. 3E) (Fernandez et al.,
1998). Two-color in situ hybridization shows that this wedge-
Tgf?, and Sfrp2 expression marks its ventral limit (Fig. 3D–G).
Although the cells that compose the antihem do not express
(Fernandez et al., 1998). Genetic fate mapping of Emx1-
expressing cells suggests that cells in this region give rise to por-
tions of the amygdala and lateral cortex (Gorski et al., 2002).
The antihem shows both similarities and differences with
other cortical signaling sources. For example, expression of EGF
family genes are not detectable in the antihem at E10.5, an age at
which Fgf, Wnt, and Bmp expression is prominent at other corti-
cal signaling sources. Nrg1 and Tgf? are only barely detectable in
the lateral cortical primordium at E11.5. Moreover, EGF gene
expression does not overlap as neatly as the expression of multi-
ple Wnt genes in the cortical hem or even of Fgf genes at the
anterior pole. Particularly for the Nrg genes, mRNA expression
sources in representing a peak of expression of several members
of a single signaling molecule family.
of the transcription factor Pax6 and displays impaired cortical
neurogenesis, cell migration, and patterning (Chapouton et al.,
defects in the antihem region, losing Sfrp2 expression (Wawersik
a complete absence of this signaling center in the homozygote
mutant, these mice and littermate controls were analyzed for
gene expression from E12.5 to E16.5 for Sfrp2 (n ? 9 homozy-
gotes; n ? 18 controls), Nrg1 (n ? 6 homozygotes; n ? 10 con-
Tgf? expression marks the antihem in controls but is eliminated
in homozygote mutants (Fig. 4A,B,E,F). Expression of Nrg1 in
the lateral cortical VZ is almost undetectable in the homozygote
Sey mouse, with the result that the gradient of expression is re-
versed, medial to lateral, with highest Nrg1 expression in the
hippocampal primordium (Fig. 4H, asterisk). Dense expression
(Fig. 4E), with a stronger gradient in the rest of the cortical pri-
mordium than in CD-1 mice (compare with Fig. 2B).
Mice lacking the transcription factor Emx2 also show wide-
spread defects in neurogenesis and patterning (Bishop et al.,
2000; Mallamaci et al., 2000) with concomitant defects in signal-
ing sources (Muzio et al., 2002). WNT signaling in the cortical
hem region is affected, as is anteroposterior gene expression of
the FGF receptor Fgfr3 (Muzio et al., 2002), suggesting that sig-
naling along both mediolateral and anteroposterior axes is ab-
is retained in the Emx2 homozygote mutant but appears dorsally
displaced (Fig. 4C,D; data not shown).
Several roles can be hypothesized for the cortical antihem. A
dorsal and ventral telencephalon, in both pattern formation and
cell migration. Localized Sfrp2 may limit the spread of WNT
signaling between dorsal and ventral telencephalon (Ragsdale et
within the cortical primordium, EGFs may antagonize BMP and
that EGF receptor-mediated signaling attenuates both Decapen-
taplegic (BMP) and Wingless (WNT) signals (O’Keefe et al.,
and ventral telencephalon and in regional patterning of the cere-
bral cortex. The latter role is supported by the ability of Tgf? to
convert nonlimbic to limbic cortex in vitro (Ferri and Levitt,
EGFs may also regulate cortical cell migration. In rodents, a
large proportion of cortical interneurons derived from the ven-
or Dlx2 (between arrows). This territory is filled by Tgf? expression (D), with Sfrp2 and Fgf7
of the cortical neuroepithelium. K, L, At E12.5, Egf (K) and Tmeff1 (L), like other EGF family
Location of the antihem relative to the transition between dorsal and ventral
Assimacopoulosetal.•EGFLigandsintheCorticalAntihemJ.Neurosci.,July23,2003 • 23(16):6399–6403 • 6401
oogenesis, the Tgf?-like ligand Gurken guides dorsal migration
EGF family ligands and misexpression of a constitutively acti-
vated form of the EGF receptor inhibit border cell migration
(Duchek and Rorth, 2001). Therefore, by analogy with fly devel-
family members, could promote migration of the correct ventral
telencephalic cells or inhibit the migration of incorrect cells. In
retroviral studies of rodent telencephalic development, Caric et
al. (2001) have found that increasing the EGF receptor levels in
neurons born in the antihem region, which are thought to mi-
be guided in part by the EGF ligands of the antihem.
the Sey/Sey mutant mouse, which lacks functional Pax6 and also
appears to lack an antihem. First, patterning defects in the pre-
sumptive area map are seen in Sey/Sey cerebral cortex just before
op et al., 2000). Second, gene expression patterns normally con-
to dorsal cell migration is enhanced in the mutant, suggesting
that incorrect cells are crossing the ventrodorsal telencephalic
boundary and that Pax6 is essential to allow the correct cells to
immigrate (Chapouton et al., 1999). We propose that Pax6 reg-
ulates development of the antihem and associated EGF ligand
gradients and that the patterning and migration defects in the
Sey/Sey mutant are at least in part mediated by the loss of the
tent, abnormality in this region. The antihem is present but
shifted dorsally (this report), and LAMP expression, marking
limbic cortex, is dorsally displaced in parallel (Mallamaci et al.,
The antihem differs from other signaling centers by express-
ing signaling molecules later than other centers and in a more
graded manner. However, these features may be significant to its
potential cortical patterning function. As the cortical primor-
dium grows larger, it becomes more difficult to explain how pat-
terning could occur according to the classic model of a morpho-
gen diffusing over an embryonic field with a width of 0.5 mm or
less (Wolpert, 1969; Gurdon et al., 1994). Yet, cortical pattern
remains labile relatively late in corticogenesis, when the embry-
onic cortex is larger than a typical embryonic field (Ragsdale and
sion with a peak at the antihem may directly set up a patterning
gradient of EGF proteins in the older and larger cortical primor-
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