DEVELOPMENT AND DISEASERESEARCH ARTICLE
In vertebrates, left-right (LR) asymmetry of the visceral situs is
established by Nodal signals from the posterior notochord (PNC),
also known as the ventral node in mammals (Brennan et al., 2002;
Levin et al., 1995), Kupffer’s vesicle in zebrafish, or the gastrocoel
roof plate (GRP) in Xenopus (Essner et al., 2002; Schweickert et
al., 2007). These structures extrude motile cilia, which propel a
leftward fluid flow to activate Nodal by an unknown mechanism
specifically on the left side (Nonaka et al., 2002; Nonaka et al.,
1998) (reviewed by Shiratori and Hamada, 2006). Scanning
electron microscopy analysis established that node cilia are tilted
towards the posterior pole, presumably because the basal bodies
are displaced from the center of the convex apical plasma
membrane to the posterior hemisphere (Cartwright et al., 2004;
Nonaka et al., 2005; Okada et al., 2005). Theoretical and
mechanical models suggest that the posterior tilt of the rotational
axes is indispensable to the coordination of effective strokes and
the generation of flow (Cartwright et al., 2004; Nonaka et al.,
2005; Okada et al., 2005). Confirming this prediction,
irregularities in the alignment of cilia in inv/inv mice carrying a
mutation in the ankyrin repeat protein inversin (Mochizuki et al.,
1998; Morgan et al., 1998; Yokoyama et al., 1993) are
accompanied by a drastic reduction in nodal flow (Okada et al.,
1999; Okada et al., 2005). Consistent with a role in cilia-driven
flow, inversin localizes to node cilia, and situs defects in inv/inv
embryos can be rescued in culture by administering an artificial
leftward flow (Watanabe et al., 2003). However, an inhibition of
flow in theory should randomize LR asymmetry, and it is unknown
why inv/inv mutants instead display situs inversions. To resolve
this conundrum, it is crucial to validate in independent models that
LR asymmetry and vectorial fluid flow in vivo are linked to the
planar orientation of cilia.
Besides perturbing LR asymmetry, mutations in inversin and
other ciliary proteins give rise to polycystic kidney disease (PKD),
and eventually to renal failure (Benzing and Walz, 2006; Fischer et
al., 2006). The hallmark of this diverse group of genetic disorders is
a progressive disruption of renal tubular morphology, preceded by
defects in apicobasal protein sorting and misoriented divisions of
renal epithelial cells (Benzing and Walz, 2006; Fischer et al., 2006;
Germino, 2005; Wilson, 2004). In the kidney, cilia act as
mechanosensors that stimulate Ca2+channels in response to urinary
flow (Praetorius and Spring, 2003). In addition, kidney cilia harbor
the atypical cadherin Fat4, a conserved regulator of planar cell
polarity (PCP) that is essential to orient renal cell divisions and
suppress cyst formation (Saburi et al., 2008). These findings directly
link cilia to PCP. However, the mechanisms by which cilia maintain
normal polarity and the tubular architecture of renal epithelial cells
remain poorly understood.
Several studies suggest that cilia and associated basal bodies
mediate PCP at the expense of canonical Wnt signaling (reviewed
by Gerdes et al., 2009), even though loss of cilia in mice lacking
intraflagellar transport proteins (IFT) other than Ift88 (Jones et al.,
2008) does not generally perturb classic readouts of PCP (reviewed
by Eggenschwiler and Anderson, 2007). In the canonical signaling
branch, Wnt proteins bind receptor complexes of frizzled and Lrp5
or Lrp6 that are endocytosed and recruit cytoplasmic dishevelled
(Dvl) to block a β-catenin destruction complex composed of axin,
Apc and Gsk3β. Alternatively, to activate the PCP branch,
complexes of Wnts and frizzled retain Dvl at the plasma membrane
and reorganize the actin cytoskeleton by stimulating the small
GTPases RhoA or Rac (reviewed by Kikuchi et al., 2009). The
propagation of PCP between cells in vertebrates relies on the core
PCP proteins frizzled (Fz3, Fz6 and Fz7), dishevelled (Dvl1, Dvl2
and Dvl3), prickle (Pk1 and Pk2), Van Gogh-like (Vangl1 and
Vangl2), Celsr1, diversin and possibly inversin (reviewed by
Simons and Mlodzik, 2008). A role for inversin is likely because
Bicaudal C, a novel regulator of Dvl signaling abutting
RNA-processing bodies, controls cilia orientation and
Charlotte Maisonneuve1,*, Isabelle Guilleret1,*, Philipp Vick2, Thomas Weber2, Philipp Andre2, Tina Beyer2,
Martin Blum2and Daniel B. Constam1,†
Polycystic diseases and left-right (LR) axis malformations are frequently linked to cilia defects. Renal cysts also arise in mice and frogs
lacking Bicaudal C (BicC), a conserved RNA-binding protein containing K-homology (KH) domains and a sterile alpha motif (SAM).
However, a role for BicC in cilia function has not been demonstrated. Here, we report that targeted inactivation of BicC randomizes
left-right (LR) asymmetry by disrupting the planar alignment of motile cilia required for cilia-driven fluid flow. Furthermore,
depending on its SAM domain, BicC can uncouple Dvl2 signaling from the canonical Wnt pathway, which has been implicated in
antagonizing planar cell polarity (PCP). The SAM domain concentrates BicC in cytoplasmic structures harboring RNA-processing
bodies (P-bodies) and Dvl2. These results suggest a model whereby BicC links the orientation of cilia with PCP, possibly by regulating
RNA silencing in P-bodies.
KEY WORDS: Polycystic kidney disease, PCP, Flow, Nodal, SAM domain, K-homology
Development 136, 3019-3030 (2009) doi:10.1242/dev.038174
1Ecole Polytechnique Fédérale de Lausanne (EPFL) SV ISREC, Station 19, CH-1015
Lausanne, Switzerland. 2University of Hohenheim, Institute of Zoology, D-70593
*These authors contributed equally to this work
†Author for correspondence (email@example.com)
Accepted 11 June 2009
depletion of inversin in Xenopus stabilizes cytoplasmic Dvl1 and
thereby hyperactivates β-catenin while inhibiting polarized
convergence extension movements during gastrulation (Simons et
al., 2005). Similarly, the inversin-related protein diversin, an
ortholog of the core PCP component Diego (Feiguin et al., 2001),
promotes PCP signaling of Dvl during gastrulation, at least in part,
by activating the β-catenin destruction complex (Moeller et al.,
2006; Schwarz-Romond et al., 2002). When delivered ectopically
to zebrafish pronephric duct, diversin can substitute for inversin to
suppress renal cyst formation (Simons et al., 2005). Together, these
observations led to the notion that both diversin and inversin
function as a molecular switch between PCP and canonical Dvl
signaling. β-catenin is also stabilized at the expense of PCP
signaling during zebrafish gastrulation upon suppression of the
ciliogenic kinesin Kif3a, or after depleting the basal body proteins
Bbs1, Bbs4 or Mkks (Bbs6) (Gerdes et al., 2007). Similarly,
mutations in ciliary (Kif3a, Ift88) or basal body proteins (Ofd)
enhance Wnt/β-catenin signaling in the mouse (Corbit et al., 2008).
However, the molecular machinery that links cilia to PCP remains
Apart from a potential role in promoting PCP, cilia themselves are
also subject to regulation by PCP. Evidence that cilia respond to PCP
comes from multiciliated Xenopus epithelial cells, in which docking
of the basal bodies at the apical plasma membrane is directed by Dvl2
(Park et al., 2008), Vangl2 (Mitchell et al., 2009) and by the PCP
effectors fuzzy and inturned (Park et al., 2006). It is plausible,
therefore, that PCP also mediates the polarizing effect of inversin on
node cilia. However, apart from inversin, no other core PCP genes are
directly implicated in establishing LR asymmetry. Possibly, this is due
to functional redundancy, as mild LR patterning defects of the heart,
such as transposition of the great arteries and double outlet right
ventricles, are observed in both Dvl3–/–mice and in Dvl2+/–;Dvl3+/–
compound heterozygotes (Etheridge et al., 2008). In addition,
redundant activities of Dvl1 and Dvl3, and Fz3 and Fz6 have been
shown to control the positioning of stereociliary bundles in the organ
of Corti in the inner ear (Wang et al., 2006a; Wang et al., 2006b).
Insights into the molecular link between Wnt and cilia signaling
pathways might be obtained by studying new animal models of
PKD. Previous analysis of the bpk and jcpk mouse models suggests
that autosomal recessive PKD (ARPKD) and autosomal dominant
PKD (ADPKD)-like disease can arise as a result of mutations in
Bicc1 (Cogswell et al., 2003), the homolog of Drosophila Bicaudal
C (BicC). BicC protein comprises three conserved K-homology
(KH) domains responsible for RNA binding, and a C-terminal sterile
alpha motif (SAM). Drosophila BicC can bind RNA in vitro and is
essential in the oocyte to downregulate oskar mRNA at the anterior
pole (Mahone et al., 1995; Saffman et al., 1998). In Drosophila
oocytes, loss of BicC also causes mislocalization of the Epidermal
Growth Factor Receptor ligand Grk, possibly because of
translational deregulation of the secretory pathway (Kugler et al.,
2009) or altered actin dynamics (Snee and Macdonald, 2009).
However, a role for mouse BicC during axis formation has remained
elusive (Cogswell et al., 2003).
Here, we assessed the function of BicC and its Xenopushomolog
xBicC by targeted inactivation using homologous recombination in
embryonic stem (ES) cells, or morpholino-mediated knockdown,
respectively. Inhibition of BicC during early development perturbs
the planar positioning of cilia and fluid flow in a manner reminiscent
of that observed in inv/inv embryos, but LR asymmetry is
randomized. Moreover, immunostaining of kidney cell lines
revealed that BicC, through its SAM domain, assembles
cytoplasmic scaffolds that harbor GFP-Dcp1a, a marker of RNA-
processing bodies (P-bodies). We found that these platforms can also
accommodate cytoplasmic Dvl2, and that BicC diminished the
ability of Dvl2 to induce TOPFLASH, a reporter of canonical Wnt
signals mediated by Gsk3βand β-catenin. Induction of TOPFLASH
by LiCl, an inhibitor of Gsk3β, was unaffected, suggesting that BicC
acts upstream of Gsk3β. Taken together, these results suggest that
BicC mediates the alignment of node cilia, possibly by regulating
the activities of Dvl and/or P-bodies. They also confirm for the first
time the prediction of the flow hypothesis that the misorientation of
cilia should randomize LR asymmetry.
MATERIALS AND METHODS
A 2.1-kb genomic fragment of Bicc1 comprising nucleotide 25 of exon 4
until intron 4, and a 6.0-kb Bicc13?homology arm (intron 11-15), were used
to flank a PGK-neomycin selection cassette in a targeting vector containing
a thymidine kinase cassette. The Bicc1 gene was disrupted by homologous
recombination in GS-1 embryonic stem cells (gift of Michel Aguet, EPFL
SV ISREC, Lausanne, Switzerland) of 129Sv/J mice. Correctly targeted ES
cells were identified by PCR and validated by Southern hybridization
analysis after HhaI digestion using 3? and 5? probes (Fig. S1A in the
supplementary material). Germ-line chimeras were produced by blastocyst
injection of selected G418-resistant clones. Mutant mice from three
independent ES cell clones were phenotypically identical, in a
129Sv/J?C57Bl/6 hybrid background, and also after serial backcrossing to
C56BL/6. Routine genotyping of mice was performed using the forward
primers 5?-CCCAACACGGCATCTTTAGTC-3?(complementary to intron
4) and 5?-CAGGGTCGCTCGGTGTTC-3? (specific for the neomycin
cassette), in conjunction with the reverse primer 5?-GCACGGAA -
GCAGGGTTATGTC-3? (complementary to exon 12).
A full-length BicC cDNA in a CMV-SPORT6 expression vector (EST
IMAGE clone 2655954) was obtained from Incyte Genomics (St Louis,
MO, USA). For epitope tagging, four concatemerized BstEII-HA linkers
composed of oligonucleotides GTGACTATCCATATGACG TCC CAG -
ATTACGCCG (sense) and ATAGGTATACTGCAGGGT CTAATGCG -
GCCACTG (antisense) were inserted in tandem into a unique BstEII site.
The ΔKH expression vector was generated by deleting a 1233-bp SfoI-StuI
fragment of HA-tagged BicC. The ΔSAM construct was obtained by EcoRI
digestion and re-ligation of the BicC-HA vector. For Xenopus injections,
untagged BicC cDNA was subcloned into the Not1-Sal1 sites of the pCS2+
Gene expression analysis
To monitor expression of the mutant Bicc1 allele, total RNA was extracted
using Trizol-LS (Invitrogen) and treated with DNase I. Both cDNA synthesis
and PCR were conducted in a single tube using the SUPERSCRIPT one-step
RT-PCR with Platinium Taq Kit (Invitrogen). For RT-PCR analysis, the
primers 5?-GAAGGAAGCCAAAGAAATGAT-3? and 5?-GGGC ACT -
CCAGACAGCAAAAT-3? were annealed at 55°C during 40 cycles.
Amplified fragments were sequenced by a commercial service (Fasteris SA,
Geneva, Switzerland). Whole-mount in situ hybridization and X-gal staining
of BAT-gal transgene expression were performed as described (Constam and
Robertson, 2000). An antisense probe complementary to the last 1741
nucleotides of the BicC cDNA was synthesized using T7 RNA polymerase.
Histology, scanning and transmission electron microscopy
Animals or embryos were sacrificed and organs were fixed in 4%
paraformaldehyde and embedded in paraffin. For scanning electron
microscopy, embryos were dissected 8.0 days post coitum in Hanks balanced
salt solution, fixed at room temperature for 2 hours in half Karnovsky’s
solution, rinsed for 10 minutes in 0.1 M Cacodylate buffer (pH 7.4) containing
10% sucrose, then incubated in ice-cold Cacodylate buffer containing 1%
OsO4. After osmification, embryos were rinsed four times in ice-cold water,
then dehydrated in graded EtOH. Before critical point drying in liquid CO2
and ion coating, embryos were soaked twice for 30 minutes in isoamyl acetate.
Xenopus dorsal explants were prepared for SEM analysis as described
Development 136 (17)
(Schweickert et al., 2007). For numerical analysis, the positioning of cilia was
counted in three or four embryos in which the ventral nodes were sufficiently
shallow to clearly see more than 75% of the anchoring points.
Video microscopy and image analysis
The flow of fluorescent beads (1000-fold dilution of FluoSpheres
carboxylate-modified microspheres, 0.2 μm, Invitrogen) in the cavity of the
PNC/ventral node was recorded for durations of 2 to 10 seconds as described
(Schweickert et al., 2007) at 20 to 50 frames per second (fps) and at 20?
magnification on an Axioplan 2 imaging microscope equipped with an
AxioCam HSm video camera. Flow at the GRP in dorsal explants of
Xenopus embryos was recorded as described (Schweickert et al., 2007),
except that bead solution was diluted 1:2500. Cilia movements in the murine
PNC/ventral node were recorded at 63-fold magnification for 2 seconds
at 100 fps. To visualize trajectories, 50 frames were analyzed using
ImageJ software in combination with the MTrackJ plugin
Cell culture and transfection
HEK293T, COS1 and MDCK cells were cultured in DMEM (Sigma)
supplemented with 10% fetal bovine serum (FBS, Sigma), glutamine 1%
(Invitrogen) and gentamycine 1% (Invitrogen). Polarized MDCK cells were
obtained after 10 days at confluence. HEK293T and MDCK cells were
transfected using calcium phosphate. COS1 cells were transfected using
DEAE-dextran. For stable transfection, HA-tagged BicC, ΔSAM and ΔKH
constructs were subcloned into a pEF-IRESpac plasmid (Hobbs et al., 1998),
and transfected into MDCK cells. Transfected cells were selected in
concentrations of up to 2.5 μg/ml puromycin and expanded for 3 weeks.
Luciferase assays using the Tcf/Lef-sensitive reporter TOPFLASH
HEK293T cells were plated into 96-well dishes in triplicate at a density of
7.5?104cells/well. After incubation for 24 hours, cells were transfected
with TOPFLASH (0.1 μg/well) (Korinek et al., 1997) together with an
identical amount of a lacZ expression plasmid using Lipofectamine 2000,
and then incubated for 48 hours in OptiMEM medium. To measure the levels
of luciferase, cell extracts were diluted 100-fold, and luminescent counts
were normalized to β-galactosidase activity.
Western blot and indirect immunofluorescence analysis
Western blot was performed on extracts of cells in Laemmli buffer. Equal
protein loading was confirmed by the presence of γ-tubulin. Antibodies were
from Roche (rat anti-HA) or from Sigma (anti-γ-tubulin).
For immunostaining, HEK293T, COS1 and MDCK cells were grown on
sterile coverslips. Expression vectors of HA-tagged BicC were transfected
with KDEL-GFP (a marker of endoplasmic reticulum structures, gift from E.
Snapp, Albert Einstein College of Medicine, NY, USA), Rab5-GFP or Rab7-
GFP (markers of endosomes and exosomes, respectively; gifts of Marino
Zerial, Max Planck Institute, Dresden, Germany), Dcp1a-GFP (a marker of
P-bodies, gift of B. Seraphin, Centre de Génétique Moléculaire, CNRS Gif-
sur-Yvette, France). For lysosome detection, coverslips were incubated for
30 minutes with 50 nM of LysoTracker Red (Cambrex), according to the
manufacturer’s protocol. Forty-eight hours after transfection, cells were fixed
for 15 minutes at room temperature in 4% paraformaldehyde, washed with
PBS, and permeabilized in 0.2% Triton X-100/PBS for 5 minutes. After two
washes in PBS, and a 30-minute blocking step in 1% BSA/PBS, primary
antibody was added for 1 hour. Rat anti-HA antibodies were purchased from
Roche; mouse anti-golgin-97 from Molecular Probes, and mouse anti-Flag
M2 and rabbit anti-HA from Sigma. After three washes in PBS, coverslips
were incubated for 1 hour with anti-mouse Cy5, rat Cy3, rat Cy5, Alexa 563
anti-rabbit, or biotinylated anti-mouse antibodies (Jackson). To detect
biotinylated secondary antibody, coverslips were rinsed three times with PBS,
then incubated with streptavidine APC for 40 minutes. Stained coverslips
were mounted and fluorescence was detected using a Leica Confocal Invert
or Zeiss LSM510 confocal microscope. To quantify the co-distribution of
cytoplasmic BicC, Dvl2 and Dcp1a, spots and surfaces were defined by peak
intensity detection and thresholding using Imaris software. Spots of FlagDvl2
were considered to contact P-bodies if their centers were located at a distance
of less than 0.5 μm from the Dcp1a-GFP surface that was reconstructed from
z-stacks using Imaris software. Similarly, spots of Dcp1a-GFP and FlagDvl2
were counted as being clustered with BicC if they were less than0.5 μm from
the BicC-HA surface.
Image processing and GTT analysis
Time-lapse movies were analyzed and transformed into trajectories by
ParticleTracker (Sbalzarini and Koumoutsakos, 2005)
http://rsb.info.nih.gov/ij/). Trajectories were processed via a custom-made
script for project-R (http://cran.r-project.org/): raw data were fitted to best
fit Bézier curves (iteration 10) to eliminate Brownian noise, then subjected
to a Rayleigh test of uniformity, to assess the significance of the mean
resultant length. Three criteria were employed to subtract trajectories
suffering from Brownian movements: (1) trajectories needed to be
significantly directed (Rayleigh test of uniformity); (2) trajectories had to be
present for a minimum of 10-20 frames to eliminate artifacts generated
during the tracking process; (3) mean velocities of beads had to exceed 2.5
μm/sec, the empirically estimated maximum speed of Brownian movement
under the conditions used (Schweickert et al., 2007). Gradient time trails
(GTTs) were generated as described (Schweickert et al., 2007).
Heterotaxia and randomization of asymmetric
Nodal signaling in Bicc1–/–mice
Using whole-mount in situ hybridization, we confirmed that mouse
Bicc1 is induced throughout the PNC/ventral node between
embryonic day (E) 7.5 and E8.5 (Wessely et al., 2001) (see Fig. S2
in the supplementary material). To determine the function of Bicc1
during development, we generated a targeted allele. Bicc1 resides
on mouse chromosome 10 B5.2 and gives rise to two splice variants
A and B, differing by the presence or absence of exon 21. The
sequence of the longer isoform is altered in the bpkallele by a frame
shift mutation in exon 22, leading to an ARPKD-like phenotype. By
contrast, the jcpk mutation, a model of ADPKD, carries a point
mutation in the splice acceptor of exon 3, such that exon 2 is
aberrantly spliced to exon 4 (Cogswell et al., 2003). Although the
resulting frame shift is expected to arrest translation, alternative
initiation in exon 4, or splicing of exon 2 to exon 5, would generate
a protein lacking only the KH1 domain (see Fig. S1A in the
supplementary material). Because the jcpk and bpk mutations had
provided no insight into potential functions of Bicc1 during early
embryogenesis (Guay-Woodford, 2003), we deleted exons 5-11 of
Bicc1 by homologous recombination in ES cells (see Fig. S1B,C in
Role of BicC during LR axis formation
Table 1. Analysis of situs defects in Bicc1 mutants at E13.5-E15.5
Li, liver; St, stomach; inv, inverted; n, normal; RLI, right lung isomerism; LLI, left lung isomerism.
the supplementary material). The deletion introduced a frameshift
after K131 in exon 12, resulting in premature termination of
translation. Analysis by RT-PCR and sequencing confirmed that the
mutant mRNA encodes a truncated protein that comprises KH1 but
terminates after K131 (see Fig. S1C in the supplementary material).
Although adult Bicc1+/–heterozygotes were healthy and fertile,
only 50% of the homozygotes developed to term (see Table S1 in the
supplementary material) and thereafter died within 2-15 days,
apparently as a result of renal failure (data not shown). Bicc1–/–
newborns frequently displayed complete situs inversions (53%,
n=18/34) or situs ambiguus (6%, n=2/34; see Fig. 1A,B). During
embryonic stages E13.5 to E15.5, we also detected ventricular septal
heart defects (n=7/13, Fig. 1B), and situs ambiguus was more frequent
(41%, n=11/27; Table 1), which is likely to account for the embryonic
lethality. Besides confirming a role for BicC in kidney morphogenesis,
targeted inactivation thus reveals a new function in LR axis formation.
The LR axis is patterned during early somite stages by the TGFβ
family member Nodal and its feedback inhibitors Lefty1 and Lefty2
(Shiratori and Hamada, 2006). Nodal signaling is confined to the left
side by a leftward fluid flow that is propelled by motile cilia in the
posterior notochord (PNC), also known as ventral node (Blum et al.,
2007; Hirokawa et al., 2006). Inhibition of ectopic Nodal signaling
on the right side is reliant on axial midline tissues (Shiratori and
Hamada, 2006). To test whether BicC is essential for midline
formation, we monitored the expression of Shh, brachyury and
Foxa2 mRNAs in axial mesoderm and the ventral neural tube. All
of these markers were expressed normally in Bicc1–/–embryos (Fig.
1C; data not shown). Nevertheless, only eight out of 47 (17%)
mutants expressed Nodal and its target genes Lefty1, Lefty2 and
Pitx2 asymmetrically on the left side. In the remaining Bicc1–/–
embryos, expression in lateral plate was bilateral (15/47), inverted
(10/47) or absent (14/47; see Fig. 1D,E). Interestingly, all embryos
Development 136 (17)
Fig. 1. LR patterning defects in Bicc1–/–mutants. (A)Frequency of complete situs inversions and heterotaxia (situs ambiguus) in Bicc1–/–mutants
at birth. (B)From top to bottom: Lung, heart, liver (li)/stomach (st)/spleen (sp) of control (left) and homozygous mutants (right) at E15.5 or after
birth (P2). A left-sided control stomach (st) and spleen (sp) are viewed dorsally, whereas a representative right-sided stomach of an asplenic mutant
is shown from the ventral side. The right lateral and caudate lobes of a control liver are highlighted by black and white dashed lines, respectively.
White arrow, absence of the caudate lobe in a left isomeric liver; open arrowhead, ventricular septal defect; black arrow, asplenia; ml, crl, al, cl, ll:
medial, cranial, anterior, caudal and left lung lobes; li, liver; sp, spleen; st, stomach. (C)Expression of Shh, and Brachyury mRNA in the axial midline
of wild-type and Bicc1–/–embryos at E8.5. n: numbers of embryos analyzed. (D)Nodal and its target gene Pitx2 are repressed on the right side by
Lefty-mediated remote feedback inhibition in the wild-type (Nakamura et al., 2006) (control). In mutants, expression of Nodal, Lefty1,2 and Pitx2
was frequently induced ectopically on the right side, but fails to normally extend in anterior direction. White arrowheads demarcate the position of
the PNC. (E)Summary of the expression patterns of LR markers in Bicc1 mutants. The numbers of embryos analyzed for each marker are indicated
in the histogram.
lacking Lefty2 mRNA in lateral plate mesoderm (n=7/17) instead
showed ectopic expression of Lefty1and/or Lefty2behind the PNC.
These results show that loss of BicC randomizes the sidedness of
Nodal signaling without disrupting midline formation.
BicC is necessary to correctly orient cilia and to
generate nodal flow
Next, we investigated whether laterality defects arose owing to
perturbations of cilia morphogenesis or flow. Scanning electron and
video microscopy analysis revealed no overt abnormalities in cilia
length or motility (Fig. 2A-J, see also Movies 1, 2 in the
supplementary material). However, although 82% (n=205/250) of
cilia emanated from the posterior hemisphere in control PNC cells
(Fig. 2D), this number was reduced to 38% (n=94/246) in BicC
mutants (Fig. 2H). Thus, the majority of cilia in the mutants failed to
become positioned correctly. Movies of beating cilia confirmed that
the rotational axes of control cilia were tilted towards the posterior
pole, whereas this polarity was perturbed in mutants (Fig. 2I,J). To
assess the ability of cilia to generate flow, we recorded the movement
of fluorescent beads in cultured PNC explants. Compared with the
leftward flow in control embryos, flow in BicC mutants was less
directed (see Movie 3 in the supplementary material)or, in a rare case
(1/31), was even diverted to the right (see Fig. S3 in the
supplementary material). Image analysis confirmed that trajectories
in wild-type embryos were directed strictly to the left (Fig. 2K). By
contrast, in BicC mutants, particles meandered considerably.
Role of BicC during LR axis formation
Fig. 2. Node cilia fail to respect the anteroposterior body axis in Bicc1–/–mutants. (A-D)Ventral node cells of control embryos. More than
80% of the cilia emanate from the three most posterior (p) octants (Nonaka et al., 2005; Okada et al., 2005). (E-H)In Bicc1 mutants, 33% of the
cilia are anchored in one of the three anterior octants. (A,E)Low magnifications (180?) in the inset show positioning of the embryos (a, anterior; p,
posterior; l, left; r, right); intermediate magnifications (1800?) show the shape of the PNC. (B,F)Boxed areas in A and E shown at higher
magnification (8000?). In B and F and the corresponding cartoons (C,G), cilia protruding posteriorly or from one of the three anterior octants are
marked at their anchoring points with red or blue dots, respectively. (D,H)Percentage of cilia found in each octant or at the cell center of control (D)
or mutant cells (H). The number of cilia (n) examined in three or four embryos (of a total of eight or ten) are indicated. (I)In control embryos, the
trajectories (colored lines) of the tips of motile cilia are displaced posteriorly (bottom) relative to their anchoring points (red dots). (J)Cilia with
misaligned rotational axes are marked with a blue dot at the anchoring points. Embryos in I and J were analyzed at the 2-somite stage. Numbers
indicate the ciliary beating frequency (Hz); white arrows indicate the orientation of the anterior (a)–posterior (p) axis. (K-N)Flow analysis in
representative control (K,M) and Bicc1 mutant embryos (L,N). (K,L)Flow displayed as gradient time trails (GTTs) of 2.1-second length (cf. color
gradient bar in L). Note the presence of left (1), right (2), circling (3) and posterior (4) GTTs in the Bicc1 mutant (L) compared with the predominantly
left GTTs in the control (K). (M,N)Frequency distribution of trajectory angles. Solid circles represent the 50% boundary, dashed circles mark
maximum frequency in histogram specified in percent. n, number of particles above threshold; ρ, quality of flow.
Individual trajectories pointed to the left, right, or were circling (Fig.
2L). The frequency distribution of trajectory angles revealed leftward
movement of about 50% of control beads (Fig. 2M), whereas beads
in BicC mutant embryos were transported in all directions (Fig. 2N).
As a measure of flow quality, we have used ρ to describe the
scattering of trajectory directions. The maximum value ρ=1 indicates
that all trajectories point in the same direction, whereas a ρ-value of
0 indicates uniform distribution in all directions. Control flow such
as the one displayed in Fig. 2K reached a ρ-value of 0.71, whereas
the BicC mutant flow was characterized by a ρ-value of 0.31 (Fig.
2L,N). Together, these results demonstrate that BicC is necessary to
align the tilt of PNC cilia with the anteroposterior body axis as a
prerequisite of leftward flow and LR axis formation.
A role for BicC in generating cilia-driven flow is
conserved in Xenopus
To quantitate flow dynamics, we depleted xBicC in Xenopus
embryos, which can be analyzed in large numbers using advanced
automated software tools (Schweickert et al., 2007). During flow
stages (stages 16-19), xBicC was expressed in the gastrocoel roof
plate (GRP), the floor plate, and the epithelial lining of the
circumblastoporal collar (Fig. 3A,B), consistent with a conserved
role for xBicC in regulating flow. To deplete xBicC in the GRP
and floor plate, the morpholino oligonucleotides xBicC MO1 and
MO2 (Tran et al., 2007) were injected into the marginal zone of
four-cell embryos (Blum et al., 2009). If xBicC MO1/2 was
injected on both sides (Fig. 3C,C?) or unilaterally (Fig. 3D,D?),
Development 136 (17)
Fig. 3. Knockdown of xBicC alters laterality, cilia polarization and leftward flow in Xenopus laevis. (A-B? ?)Expression of xBicC in the
gastrocoel roof plate (GRP; outlined by dotted lines in A and B), floor plate (arrow in A?,B?,B?) and epithelial lining of the circumblastoporal collar
(cbc) at flow stages 16 (A) and 18/19 (B), as shown by whole-mount in situ hybridization analysis of dorsal explants (ventral views). Levels of
sections in A?, A?, B?, B? are indicated by dashed lines. ar, archenteron roof; n, notochord; so, somite; black arrowhead, xBicC- positive somitic cells.
(C,C? ?) Neural tube closure defects in embryos injected with xBicC MO1/2 (C?) compared with Co-MO-injected specimen (C). Asterisks indicate
closed (C) and open (C?) neural tubes. (D,D? ?) Delayed closure of the neural tube in a unilaterally injected embryo with xBicC MO1/2 and lineage
tracer DsRed. Dorsal (D) and posterior (D?) view of superimposed fluorescence and bright-field images (midline indicated by dashed line, neural fold
by yellow dotted line, floor plate by white dotted line). bp, blastopore. (E)Summary of marker gene expression patterns (Xnr1, top; Pitx2, bottom).
Numbers indicate number of treated embryos. (F,J)Scanning electron microscopy analysis of GRP cells at stage 18 in Co-MO (F) and xBicC MO1/2
(J) injected embryos revealed predominantly posteriorly polarized cilia in control (F) and intermingled cilia orientation in xBicC morphant specimens.
Scale bar: 10μm. (G,K)Markedly reduced proportion of polarized cilia in xBicC MO1/2 (K) compared with Co-MO (G) injected embryos. The border
of the GRP area relevant for flow is indicated by dashed lines. (H,I,L,M) Flow analysis in Co-MO (H,I) and xBicC MO1/2 (L,M) injected embryos.
(H,L)Trajectories of beads displayed as GTTs of 25 second length (cf. color gradient bar in H). Red lines mark approximate limits of targeted areas, as
visualized by co-injected DsRed; white dashed lines indicate border of the GRP. Note the presence of left (1), right (2) and circling (3) GTTs in L.
(I,M)Frequency distributions of trajectory directionalities at the GRP of Co-MO (I) and xBicC MO1/2 (M) injected specimens. Solid circles represent
the 50% boundary, dashed circles mark maximum frequency in histogram specified in percent. n, number of particles above threshold; ρ, quality of
flow. Note the leftward direction of 60% of trajectories in (I) compared with the about equal distribution of trajectories in M, and the overall much
reduced number of particles above the threshold of 2.5μm/second in M. (N)Summary of flow analysis from 54 explants. Classification of embryos
with flow into categories I-IV based on ρ. Numbers indicate the number of analyzed explants.
neural tube closure between stages 12-24 was delayed. Therefore,
to analyze the left-sided marker genes Xnr1 and Pitx2, injected
embryos were cultured until stages 22-34. Embryos injected with
DsRed mRNA or a control morpholino oligonucleotide (Co-MO)
revealed normal left-sided gene expression patterns (Fig. 3E; see
also Table S2 in the supplementary material). By contrast,
injection of xBicC MO1/2 dose-dependently perturbed gene
expression patterns in 20-60% of cases (Fig. 3E; see also Table S2
and Fig. S4 in the supplementary material). To elucidate the cause
of LR asymmetry defects, xBicC morphants and controls were
analyzed by scanning electron microscopy at stage 17/18. In
representative control embryos, these stages were characterized
by about 75% (71/94) of cilia being tilted towards the posterior
pole in the flow-relevant areas (dotted lines) of wild-type GRPs
(Schweickert et al., 2007) (Fig. 3F,G). By contrast, in xBicC
morphants, posteriorly polarized cilia were reduced in number to
less than 50% (40/83), with the percentage of misaligned cilia
rising accordingly (Fig. 3J,K). These results suggest a conserved
role for xBicC in aligning the planar polarity of ciliated cells with
the anteroposterior body axis.
To monitor flow, dorsal explants of control- and MO-injected
embryos were analyzed by adding fluorescent beads (see Movie 4
in the supplementary material). For the evaluation of flow,
trajectories were selected that were directed and that exceeded the
empirically selected threshold of maximum Brownian movement.
Explants were grouped into four categories on the basis of flow
phenotype. Category I represented robust leftward flow (ρ=0.66-1);
category II (ρ=0.33-0.65) and category III (ρ<0.33) mildly and
severely affected flow, respectively (see Fig. S5 in the
supplementary material). If less than 25 particles per movie
displayed velocities of >2.5 μm/sec (i.e. above threshold), embryos
were classified as category IV (no flow). Trajectories of individual
beads were generated by automated computation of gradient time
trails (GTTs) (Schweickert et al., 2007). In representative control
specimens (category I), a robust leftward flow was evident by the
straight direction of trajectories towards the left (Fig. 3H). By
contrast, beads in the GRP of representative xBicC morphants
(category III) were frequently trapped by non-polarized cilia,
resulting in non-directional, meandering trails (Fig. 3L, see also
Movie 4 in the supplementary material). The frequency distribution
of trajectory angles above the GRP, and the ρ values for flow in
explants from xBicC morphants clearly differed from that in controls
(Fig. 3I,M,N). Furthermore, the effects of xBicC MO1/2 were dose
dependent, and a clear correlation between LR marker gene
expression patterns and flow categories was obvious (compare Fig.
3E with 3N), confirming that altered gene expression resulted from
The SAM domain recruits BicC to the periphery of
P-bodies and downregulates BicC protein levels in
polarized MDCK cells
Homologs of BicC in Drosophila and nematodes are implicated
in regulating the localization or translation of target mRNAs
(Chicoine et al., 2007; Eckmann et al., 2002; Mahone et al.,
1995). To determine how BicC might regulate target genes, we
visualized the subcellular localization of tagged BicC and
truncated mutant forms lacking the KH- or SAM domains.
Immunostaining of transfected HEK293T cells and confocal
imaging detected BicC in discrete cytoplasmic foci that do not
overlap with molecular markers of the ER, Golgi, endosomes or
lysosomes (see Fig. S6 in the supplementary material). BicC
formed similar structures in transfected COS1 cells (Fig. 4A).
Reconstruction of the 3D surface indicated that BicC foci formed
tube- and vesicle-like structures around or adjacent to P-bodies
that were marked by a GFP fusion of the decapping enzyme
Dcp1a (Cougot et al., 2004) (Fig. 4A). In representative cells
(n=10), 51±26% of the Dcp1a-GFP spots were at least partially
coated by BicC-HA. A similar distribution was observed for
mutant BicC lacking the KH domains (ΔKH, Fig. 4B), whereas
deletion of the SAM domain gave rise to a diffuse cytoplasmic
staining (Fig. 4C). Similarly in MDCK kidney epithelial cells,
BicC and ΔKH localized in cytoplasmic foci, whereas ΔSAM
staining was diffuse (Fig. 4D-G; data not shown). When MDCK
cells expressing BicC were differentiated into fully polarized,
ciliated epithelial cells (Fig. 4F,G, arrow), full-length BicC failed
to accumulate, indicating that the combination of the KH and
SAM domains negatively regulates BicC protein translation or
stability under the conditions examined (Fig. 4H). Altogether,
these experiments show that BicC is recruited to the periphery of
P-bodies in a SAM domain-dependent manner.
Role of BicC during LR axis formation
Fig. 4. The SAM domain assembles BicC in scaffolds
accommodating P-bodies. (A)Localization of HA-tagged BicC in
COS1 cells. To visualize how BicC (red) accommodates the P-body
marker Dcp1a-GFP (green), the red surface in A was reconstructed from
z-stacks. (B,C)Confocal imaging of truncated ΔKH reveals a similar
distribution (B), whereas ΔSAM stained diffusely. (D,E)Unpolarized
MDCK cells expressing BicC alone (D) or with Dcp1a-GFP. A
deconvoluted confocal section (inset in E) shows that Dcp1a-GFP is
sandwiched by BicC. (F,G)In polarized MDCK cells, neither ΔKH nor
ΔSAM (red) colocalize with acetylated tubulin (green) in cilia.
(H)Western blot analysis of unpolarized (u) or polarized (p) MDCK cells
stably transfected with full-length Bicc1, ΔKH or ΔSAM. Upon cell
polarization, the expression of full-length BicC drops below detectable
BicC inhibits Dvl signaling in the canonical Wnt
Dvl, a positive regulator of the canonical Wnt pathway has been
shown to multimerize in similar structures. (Bilic et al., 2007;
Schwarz-Romond et al., 2007a; Schwarz-Romond et al., 2005).
Prompted by our observations, we assessed whether Dvl and BicC
would colocalize in cytoplasmic punctae. Consistent with previous
reports, FlagDvl2 was detected in discrete puncta that arise by
multimerization via the DIX domain (Schwarz-Romond et al.,
2007a; Schwarz-Romond et al., 2007b). Interestingly, in Dvl2-
expressing cells, P-bodies formed irregularly shaped clusters and,
among 28±11 Dcp1a-GFP surfaces examined per cell (n=9), 7±3
(38±20%) abutted FlagDvl2 puncta (Fig. 5A). In cells co-expressing
FlagDvl2 and BicC-HA, P-bodies remained dispersed, and, of those
occupied by FlagDvl2 (40±9% of n=451 in seven cells), 60±7%
cuddled to the BicC-HA surface (Fig. 5B-D). Furthermore, 35±6%
of the FlagDvl2 puncta far from P-bodies (n=516/695 in seven cells)
were at the BicC-HA surface (Fig. 5C,D, red arrowheads). To
determine whether BicC directly interacts with Dvl2, transfected
COS1 cells were analyzed by co-immunoprecipitation. Neither
BicC nor ΔKH pulled down FlagDvl2, and their overexpression did
not deplete FlagDvl2 (see Fig. S7 in the supplementary material).
However, BicC dose-dependently inhibited FlagDvl2-mediated
induction of TOPFLASH, a reporter of canonical Wnt signaling
(Fig. 5E). This inhibitory effect was reduced upon deletion of the
SAM domain, indicating that concentration in cytoplasmic foci is
important. By contrast, the activity of ΔKH was similar to that of
wild-type BicC. Furthermore, BicC failed to diminish induction of
TOPFLASH by LiCl (Fig. 5F), indicating that BicC blocks signaling
of Dvl upstream of Gsk3β. Taken together, these results suggest that
BicC, via its SAM domain and independently of its KH domains, is
concentrated in cytoplasmic platforms that inhibit canonical Dvl2
To monitor the influence of BicC on the canonical Wnt pathway
in vivo, Bicc1 mutants were crossed with transgenic BAT-gal
reporter mice, which express a lacZ reporter of β-catenin/TCF
signaling (Maretto et al., 2003). Whole-mount staining of BAT-gal
embryos at E7.5-E8.0 showed that lacZexpression is reduced in the
PNC compared with the adjacent primitive streak region in Bicc1
wild-type embryos, and that this local down-modulation of BAT-gal
is impaired in the PNC of Bicc1–/–embryos (n=6/7; Fig. 6A),
indicating that BicC attenuates canonical Wnt activity in the
This study reveals a conserved role for BicC in directing the planar
orientation of cilia and leftward fluid flow during LR axis formation
that has not been described in bpk and jcpk mice. Thus, our new
Bicc1allele unequivocally confirms for the first time the prediction
of the flow hypothesis that misorientation of PNC cilia should
randomize LR asymmetry. Furthermore, we have shown that the
SAM domain concentrates BicC in cytoplasmic tube- and vesicle-
like structures harboring P-bodies and cytoplasmic Dvl2, and that
BicC inhibits Dvl2 signaling via the canonical β-catenin/TCF
Development 136 (17)
Fig. 5. Interactions of BicC with cytoplasmic Dvl2
signaling platforms. (A)Distribution of FlagDvl2 (blue)
and Dcp1a-GFP (green). Arrows in insets highlight areas
of overlap. (B-D)Triple fluorescence analysis of BiccHA
(red), FlagDvl2 (blue) and Dcp1a-GFP (green) in individual
and merged channels (B). Higher magnification of a
confocal section (C) combined with a 3D reconstruction
of the BiccHA surface (D) reveals clustering of FlagDvl2
with BicC (red arrowheads) or with Dcp1a-GFP (white
arrows), or with both (white arrowheads). For clarity, the
centers of green and blue staining in proximity (<0.5μm)
of the BiccHA surface in F are represented by spots.
(E,F)BicC dose-dependently inhibits induction of the
TOPFLASH reporter by Dvl2 in transfected HEK293T
kidney cells, and this inhibition was severely compromised
in ΔSAM, but not in ΔKH (E). By contrast, induction of
TOPFLASH by LiCl was unaffected or even slightly
enhanced by BicC (F). Similar results were obtained in
three independent experiments.
pathway. Based on these results, we propose that BicC links cilia
orientation to PCP signals, possibly by counteracting Dvl-induced
activation of β-catenin.
In bpk mice, a point mutation disrupting one of the two splice
variants of Bicc1 causes ARPKD, whereas jcpk mice carry a
dominant mutant allele associated with an ADPKD-like phenotype
(Cogswell et al., 2003). Our targeted mutation leads to a
randomization of visceral tissue positioning, combined with a cystic
phenotype in newborn kidney and pancreas that will be described
elsewhere (I.G., unpublished observation). Transcription of this
new allele might give rise to a truncated N-terminal peptide of 131
amino acids comprising the first KH domain. However, LR
patterning and kidney defects are not observed in heterozygotes,
suggesting that a gain-of-function is unlikely to account for LR
defects. Corroborating this conclusion, morpholino-mediated
knockdown confirmed that a homolog of BicC is also required for
LR axis formation in Xenopus.
LR asymmetry is specified by Nodal activity, which is
symmetrically induced in the PNC by the Notch pathway (Brennan
et al., 2002; Krebs et al., 2003; Raya et al., 2003), but is then biased
by leftward flow to signal preferentially on the left side (Nonaka et
al., 2002). Flow generation is thought to depend on the posterior
tilting of node cilia (Cartwright et al., 2004; Nonaka et al., 2005;
Okada et al., 2005). Consistent with this model, inhibition of flow
in Bicc1 mutants was accompanied by a decrease in the number of
correctly oriented cilia from 80% to 38%. A similar cilia
misorientation phenotype has only been described in inv/inv mice
(Okada et al., 2005). Even though cilia fail to align parallel to the
anteroposterior axis, they remain fully motile and thus counteract
the leftward fluid flow in both inv/inv and Bicc1–/–embryos.
However, the outcome differs between these mutants. Poor flow in
Bicc1 mutants leads to situs randomization. By contrast, cilia
malpositioning in inv/invembryos is associated with situs inversions
(Yokoyama et al., 1993). Although depletion of inversin can also
randomize heart positioning in zebrafish, other aspects of LR
asymmetry or cilia positioning have not been analyzed in that model
(Otto et al., 2003). The present observations thus indicate that the
inv/inv mutation probably has some additional function besides
perturbing the planar orientation of cilia.
In zebrafish pronephros, depletion of inversin leads to cystic
growth (Otto et al., 2003), which can be suppressed by injecting
diversin, an ortholog of the core PCP protein Diego (Simons et al.,
2005). Inversin selectively sequesters the cytoplasmic, but not the
membrane-bound pool of Dvl1, suggesting that it promotes PCP by
inhibiting the activity of Dvl in the Wnt/β-catenin pathway.
Consistent with this model, hyperactivation of β-catenin signaling
is sufficient to trigger cystic growth in the kidney of transgenic mice
(Qian et al., 2005; Saadi-Kheddouci et al., 2001). However, whether
an imbalance between PCP and Wnt/β-catenin signaling in inv/inv
mutants also accounts for the misorientation of node cilia during LR
axis formation is not known.
To assess whether BicC regulates Wnt signaling, we explored
interactions between BicC and Dvl. We have shown that BicC
downregulates expression of the BAT-gal reporter transgene in the
PNC/ventral node, and that it can inhibit β-catenin/TCF signaling
induced by Dvl2 in TOPFLASH reporter assays. By contrast, BicC
failed to inhibit the induction of TOPFLASH by LiCl.
Immunostaining showed that BicC forms cytoplasmic platforms,
which accommodate foci of Dvl2 previously associated with
canonical Wnt signaling (Bilic et al., 2007; Schwarz-Romond et al.,
2007a; Schwarz-Romond et al., 2005). These observations suggest
that BicC attenuates canonical Wnt signaling at the level of Dvl.
Furthermore, whereas inversin directly inhibits canonical Wnt
signals by depleting Dvl1 (Simons et al., 2005), BicC did not
significantly reduce the levels of Dvl2. Thus, BicC is likely to act in
parallel to inversin to align cilia in response to PCP cues (Fig. 6B).
PCP signaling is also required for neural tube closure (Wallingford
and Harland, 2002). Whereas neural tube closure is impaired in
xBicC-depleted Xenopus embryos, this and other readouts of PCP,
such as eyelid closure and hair follicle orientation in the skin, were
normal in Bicc1–/–mice. Also, convergence-extension movements
during gastrulation appeared normal, confirming that BicC is not a
core PCP protein. However, we do not rule out the possibility that a
subtle PCP defect contributes to the failure of Lefty1-expressing
cells in the midline to colonize the ventral neural tube (Brennan et
Dvl proteins must localize to the plasma membrane for PCP
signaling, whereas the canonical Wnt pathway relies on a
cytoplasmic pool of Dvl aggregating with axin during or after
endocytosis of Lrp6 (Bilic et al., 2007; Kikuchi et al., 2009;
Wallingford and Habas, 2005; Yamamoto et al., 2006). Although
the activities of these distinct pools of Dvl antagonize each other
(for a review, see Wallingford and Habas, 2005), the mechanisms
Role of BicC during LR axis formation
Fig. 6. Regulation of Dvl signaling by BicC. (A)Expression of the
BAT-gal reporter of β-catenin/TCF signaling in wild-type and Bicc1–/–
embryos at E7.75. Dashed lines in the high magnification images
indicate the periphery of the ventral node. (B)Dvl proteins in the
cytoplasm (c) or at the plasma membrane (p) transduce canonical and
non-canonical Wnt signals through β-catenin (βcat) or the small
GTPases (RhoA or Rac), respectively. (Red box, left) Dvlcinhibits Gsk3β
in the β–catenin destruction complex through an unknown mechanism
that is blocked by BicC. Unlike inversin (Inv), BicC inhibits Dvl without
triggering its degradation. Inhibition of Dvlc involves the SAM domain,
which concentrates BicC in structures abutting P-bodies. By interacting
with CCR4 deadenylase (Chicoine et al., 2007) and/or P-bodies, BicC
may stimulate degradation of an mRNA encoding a Gsk3β inhibitor (X).
Also Dvl2 can cluster with P-bodies, but its potential to regulate RNA
silencing (dashed red line) is unknown. (Green box, right) In addition,
BicC protein possibly sequesters associated mRNAs outside P-bodies
and delays their degradation after deadenylation (AAA); for example, to
promote the synthesis of an agonist (Y) of PCP signal transduction. The
proteins X and Y, and the interactions represented by the dashed lines
are hypothetical. Components that promote or antagonize PCP
signaling are depicted in green or red, respectively.
regulating pathway selection are poorly understood. Recent
studies in zebrafish revealed that Dvl interacts with seahorse, a
novel regulator of LR asymmetry and renal morphogenesis also
known as leucine rich repeat containing 6 (Lrrc6), and that
seahorse reduces the induction of Wnt/β-catenin target genes
during gastrulation (Kishimoto et al., 2008). In addition, a
genome-wide screen in Drosophila cells recently showed that
PCP signaling depends on an association of Dvl with negatively
charged phospholipids in the plasma membrane, and that this
interaction is facilitated by a reduction of the intracellular pH by
the sodium proton exchanger Nhe2 (Simons et al., 2009). It might
be interesting, therefore, to assess in future studies whether
canonical Wnt signals antagonize the expression or activity of
Here, we have directly demonstrated for the first time that BicC,
through its SAM domain, forms cytoplasmic structures that can
accommodate P-bodies and Dvl2. P-bodies are key regulators of
mRNA surveillance, degradation, translational repression and RNA-
mediated gene silencing (Cougot et al., 2004; Eulalio et al., 2007),
but to our knowledge, they are not implicated in transmitting
canonical Wnt signals. Our finding that the SAM domain promotes
both BicC localization and the inhibition of Dvl2 indicates that P-
bodies probably must at least communicate with BicC. In one
possible scenario, BicC could mediate translational silencing of a
Gsk3 inhibitor X by P-bodies (Fig. 6B, left box). The recent findings
that β-catenin can be released from Gsk3β by the RNA helicase
activity of p68 (Yang et al., 2006), and that p68 activates the let-7
miRNA precursor are consistent with such a model (Salzman et al.,
2007). Alternatively, or in addition, BicC might promote PCP by
blocking a direct inhibition of Gsk3β by Dvl2, the mechanism of
which is currently unknown.
In Drosophila oocytes, BicC confines Oskar expression to the
posterior pole to promote anterior cell fates (Mahone et al., 1995;
Saffman et al., 1998). Posterior localization of oskar mRNA also
depends on the Dcp1a homolog Dcp1 (Lin et al., 2006), which has
been detected in cytoplasmic foci of Drosophila oocytes and nurse
cells together with Maternal Expression at 31B (Me31B) (Lin et al.,
2006). Me31B, the homolog of the yeast decapping activator Dhh1p,
is localized to cytoplasmic foci by Drosophila BicC, apparently
through a direct interaction (Chicoine et al., 2007; Kugler et al., 2009).
DrosophilaBicC has also been shown to repress translation of its own
mRNA by recruiting the NOT3/5 component of the CCR4
deadenylase complex (Chicoine et al., 2007). CCR4 (also known as
Twin) shortens the poly A tail length of oskar mRNA (Benoit et al.,
2005), and in mammalian cells it stimulates the assembly of P-bodies
(Andrei et al., 2005; Cougot et al., 2004; Zheng et al., 2008). These
observations are consistent with our model that BicC regulates
translational silencing of target mRNAs in P-bodies. However, we do
not exclude that besides inhibiting cytoplasmic Dvl, BicC promotes
PCP through additional mechanisms (Fig. 6B, right box). It will be
interesting to determine in future studies the function of the novel P-
body microenvironment defined by BicC, and whether BicC is needed
to correctly canalize a response to Wnt signals at the level of RNA
We are grateful to Friedrich Beermann for blastocyst injections, to Olav Zilian and
Nadav Ben Haim for technical advice, and to Christophe Fuerer for critical
reading of the manuscript. GFP-Dvl2 and FlagDvl2 expression vectors were kindly
provided by Marianne Bienz, and Dcp1a-GFP by Bertrand Séraphin. BAT-gal
transgenic mice were a kind gift of Stefano Piccolo. We also thank Gisèle Ferrand
and her staff at the ISREC animal facility for animal care, Oliver Wessely for
sharing xBicC morpholinos and plasmids, Dr Nobutaka Hirokawa for the protocol
to fix cilia, Michel Bonin and Antonio Mucciolo for the SEM analysis of mouse
embryos, and Werner Amselgruber for access to his SEM in Hohenheim. This
project has been supported by Oncosuisse (grant SKL 1101-02-2001) and the
PKD Foundation (grant 151a2r) to D.B.C., and by DFG grants to M.B.
Competing interests statement
The authors declare that they have no competing financial interests.
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