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First published online January 26, 2007
doi: 10.1242/10.1242/dev.02772
Review |
1 Department of Molecular Biology and Genetics, Howard Hughes Medical Institute,
Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
2 Departments of Neuroscience and Ophthalmology, Johns Hopkins University School
of Medicine, Baltimore, MD 21205, USA.
e-mails: ywang{at}mail.jhmi.edu; jnathans{at}jhmi.edu
SUMMARY
This review focuses on the tissue/planar cell polarity (PCP) pathway and its role in generating spatial patterns in vertebrates. Current evidence suggests that PCP integrates both global and local signals to orient diverse structures with respect to the body axes. Interestingly, the system acts on both subcellular structures, such as hair bundles in auditory and vestibular sensory neurons, and multicellular structures, such as hair follicles. Recent work has shown that intriguing connections exist between the PCP-based orienting system and left-right asymmetry, as well as between the oriented cell movements required for neural tube closure and tubulogenesis. Studies in mice, frogs and zebrafish have revealed that similarities, as well as differences, exist between PCP in Drosophila and vertebrates.
Introduction
The genetic and molecular dissection of what is now referred to as planar
cell polarity (PCP) began 25 years ago with the realization by Gubb and
Garcia-Bellido (Gubb and Garcia-Bellido,
1982
) that a small set of genes controls the polarity of cuticular
hairs and bristles in Drosophila. Morphologists and embryologists had
long appreciated the precise orientation of cuticular structures with respect
to the body axes, but Gubb and Garcia-Bellido's work represented a conceptual
departure in that it suggested the existence of a genetically defined system
dedicated to coordinating these patterns. The genes that they studied are now
known to be players in a complex system of developmental regulation that
governs cell and tissue movements and patterns in both invertebrates and
vertebrates. Although this phenomenon is now commonly referred to as PCP, Gubb
and Garcia-Bellido's original and somewhat more general name `tissue polarity'
might ultimately prove more appropriate as its role is revealed in ever more
diverse developmental processes.
As is often the case in developmental biology, the vertebrate PCP field
owes a large debt to its Drosophila counterpart, which has served as
the source for many of the components and concepts in this system. However,
the numerous differences between vertebrates and invertebrates in anatomy,
tissue types and morphogenetic processes, together with the existence of a
number of distinct PCP components in vertebrates, have made the study of
vertebrate PCP uniquely interesting. In this review, we highlight recent work
on vertebrate PCP and discuss several developmental processes in which there
is suggestive, but still incomplete, evidence for PCP signaling or for the
activity of a subset of PCP components. We have not attempted to cover the PCP
field in a comprehensive manner because many excellent and detailed reviews
have recently been published in this area [reviews with an emphasis on
Drosophila PCP (Adler,
2002; Strutt,
2002; Tree et al.,
2002; Klein and Mlodzik,
2005; Strutt and Strutt,
2005); reviews on various aspects of vertebrate PCP
(Wallingford et al., 2002;
Barrow, 2006;
Karner et al., 2006;
Montcouquiol et al., 2006a); a
review on the very different mechanisms of planar polarity in plants
(Grebe, 2004)]. Instead, we
have focused on those areas that we think are the most exciting and that
address interesting unanswered questions. We hope that in the paragraphs that
follow we can convey some of this excitement.
PCP in Drosophila
In Drosophila, the eye and wing have been the favored tissues for studying PCP phenotypes (Fig. 1), and the wing has also been used in most studies of PCP protein localization. In the compound eye, each ommatidium is precisely oriented in a nearly crystalline lattice. Moreover, each ommatidium exhibits one of two possible chiralities, as defined by the asymmetric packing of the eight photoreceptors. In the wild-type (WT) eye, ommatidia of differing chirality are segregated into two mirror image zones by a transverse equator (see Fig. 1A,B). In PCP mutants, the ommatidia are variably oriented and the spatial segregation of ommatidial chirality is lost, with the result that individual ommatidia of the `wrong' chirality are found in each zone.
The surface of the wing is covered by a nearly crystalline epithelium of
hexagonal cells, each of which elaborates a single distally-directed
actin-filled protrusion (a wing hair). Both the orientation of the wing hairs
and the hexagonal shape and regular packing of the wing epithelial cells are
under PCP control (Classen et al.,
2005). In the wing, mutations in PCP genes generally do not cause
a complete randomization of hair orientation. Rather, hairs tend to be roughly
aligned with their immediate neighbors (see
Fig. 1D), leading to
large-scale patterns in which many hundreds of hairs create whorls and waves
that resemble the brushstrokes of an impressionist painting, inspiring the
mutant names Van Gogh [Vang; also known as
Strabismus or Stbm
(Taylor et al., 1998;
Wolff and Rubin, 1998)] and
starry night [stan; also known as flamingo or
fmi (Chae et al.,
1999; Usui et al.,
1999)]. As discussed more fully below, this propensity for local
order among neighboring polar structures in the context of global disorder
strongly suggests the existence of mechanistically distinct systems for
controlling global and local orientation.
The Drosophila wing has also revealed an interesting feature
referred to as `domineering non-autonomy' in which WT epithelial cells
adjacent to a clone of mutant cells exhibit a misoriented phenotype
(Vinson and Adler, 1987). In
many cases, the misorientation is only observed on one side of the mutant
tissue. For example, in the wing, domineering non-autonomy caused by a clone
of homozygous frizzled mutant cells generally affects only those WT
cells that reside distal to the patch of mutant tissue. In the
Drosophila abdominal epithelium, a wide variety of PCP gene
over-expression and loss-of-function clones have been studied in the context
of surrounding tissue that is either WT or one of various mutant backgrounds.
In these studies, bristles and hairs within the surrounding tissue either turn
toward or away from the clonal patch in a manner that is characteristic of the
mutant genotype of each clonal patch and the anterior or posterior location of
the surrounding tissue (Casal et al.,
2006). This asymmetry is most likely to reflect the asymmetric
propagation of a signaling molecule and/or the cooperative and asymmetric
assembly of cell-surface signaling complexes.
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; Boutros et al.,
1998; Wallingford and Habas,
2006). Genetic gain- and loss-of-function experiments support a
model in which Fz-dependent PCP signaling also utilizes the heterotrimeric
G-protein Galpha-O (Katanaev et al.,
2005), a signaling pathway first identified in zebrafish
(Slusarski et al., 1997). With
respect to signaling between cells, at present the identities of the global
signal that communicates the orientation of the body axes and the local signal
that communicates polarity between neighboring cells remain uncertain. One
attractive model posits that two atypical cadherins, Dachsous (Ds) and Fat
(Ft), together with a transmembrane Golgi complex protein, Four-jointed (Fj),
set up a global polarity signal, which is then sensed and propagated by the
asymmetric assembly of cell-surface complexes composed of Fz, Vang, Dsh, Stan
and an intracellular adaptor like protein, Diego (Dgo)
(Fig. 1E)
(Wong and Adler, 1993;
Adler et al., 1997;
Adler, 2002;
Strutt, 2002;
Yang et al., 2002;
Ma et al., 2003;
Uemura and Shimada, 2003;
Lawrence et al., 2004;
Venema et al., 2004;
Strutt and Strutt, 2005;
Klein and Mlodzik, 2005).
However, recent genetic mosaic experiments in the Drosophila abdomen
argue that these two systems may function in parallel rather than in series
(Casal et al., 2006).
Ironically, both the loss of function and the over-expression of Wnts, the
only known Fz ligands in Drosophila, have failed to implicate any Wnt
in PCP signaling (Klein and Mlodzik,
2005; Casal et al.,
2006). We note, however, that redundancy among Wnts might mask a
defect associated with single-gene, loss-of-function mutations, a situation
observed for frizzled and frizzled2 in the context of
embryonic patterning (Bhat,
1998; Kennerdell and Carthew,
1998; Bhanot et al.,
1999; Chen and Struhl,
1999). The recent identification of a non-Wnt ligand (Norrin) for
vertebrate Fz4 (Xu et al.,
2004) suggests that the field should be open to the possibility
that one or more non-Wnt Fz ligands might regulate PCP.
PCP processes and components in vertebrates
In vertebrates, the definition of what constitutes a PCP process is not
entirely clear. One rough operational definition is that PCP is any process
that affects cell polarity within an epithelial plane and involves one or more
of the core PCP genes (as defined by the PCP phenotype of the
Drosophila homolog). At present, the developmental processes that
meet these criteria are convergent extension, neural tube closure, eyelid
closure, hair bundle orientation in inner ear sensory cells, and hair follicle
orientation in the skin (Figs 2
and 3). At the edge of this
definition are some processes in both vertebrates and invertebrates that
involve PCP genes in cell or tissue patterning but which do not involve
epithelia. For example, the mutation of the core PCP gene stan in
Drosophila leads to aberrant pathfinding by photoreceptor axons and
defective dendritic morphologies in sensory neurons in the embryonic
peripheral nervous system (Gao et al.,
2000; Lee et al.,
2003; Senti et al.,
2003; Kimura et al.,
2006), and RNAi knockdown of Celsr2, one of three
vertebrate homologs of stan, in rat organotypic cerebellar and
cortical slice cultures leads to loss of dendrites
(Shima et al., 2004). Since
these non-planar and non-epithelial processes are potentially revealing of how
PCP components function, several of them are discussed below.
In addition to the core PCP proteins, there is a wider circle of proteins
essential for PCP but not solely devoted to it. Included in this group are:
proteins, such as Patj, that are involved in the localization of PCP proteins
to the apical edge of one or both lateral faces of the cell
(Djiane et al., 2005);
proteins, such as inversin, that appear to control the balance between
canonical and noncanonical Wnt signaling
(Simons et al., 2005); and
proteins, such as the c-Jun N-terminal kinase (JNK) (Basket - Flybase) and the
small GTPases RhoA (Rho 1- Flybase) and Rac1, that control cytoskeletal
dynamics. The recent observation in both Drosophila and mammals that
there are intracellular (most likely vesicular) pools of some PCP proteins -
perhaps serving as a reservoir for the plasma membrane population - suggests
that components of the vesicular transport machinery may also play a
supporting role in PCP (Shimada et al.,
2006; Wang et al.,
2006b).
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; Lu et al.,
2004). Also included are genes such as inturned
(in) and fuzzy (fy), which are considered to be PCP
effector genes in Drosophila and which have been found, by inhibition
or over-expression studies in frogs or fish embryos, to give a convergent
extension phenotype (Park et al.,
2006). Other, miscellaneous genes are included that either by
homology or function are likely to play a role in PCP or PCP-like
processes.
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Neurulation and its associated tissue movements are among the most ancient
of vertebrate embryological processes, and they have fascinated embryologists
for over a century (Wilson,
1925). In frogs and fish, the process begins with an elongation
and narrowing of the embryo, referred to as convergent extension (CE). The
elongated neural plate develops a central groove, and the dorsal margins of
the two walls of this U-shaped structure ultimately fuse to create the neural
tube. In humans, a failure to fuse the neural tube in its entirety occurs at a
frequency of 1 in 1000 live births, making it one of the most common
congenital defects (Copp et al.,
2003).
CE reflects the medial migration and intercalation of mesodermal cells,
movements that are directed by lamellipodia on the medial and lateral faces of
these cells and by the organized deposition of extracellular matrix (ECM)
fibrils, in particular fibronectin (Keller
et al., 2000; Wallingford et
al., 2002; Goto et al.,
2005). CE can be disrupted in zebrafish by mutation of
vangl2 (trilobite)
(Jessen et al., 2002),
prickle (Veeman et al.,
2003), or wnt11 (silberblick)
(Heisenberg et al., 2000) and
in Xenopus by interfering with any of several PCP proteins, for
example by overexpressing a dominant-negative Dishevelled variant that lacks
either the DEP or PDZ domains, which in Drosophila Dsh are essential
for PCP (Wallingford et al.,
2000; Wallingford et al.,
2002; Wallingford and Habas,
2006). In mice, loss of Vangl2
(Greene et al., 1998;
Kibar et al., 2001;
Murdoch et al., 2001),
Celsr1 (Curtin et al.,
2003), or Ptk7 (Lu et
al., 2004), or the simultaneous loss of two of the three
Dishevelled homologs (Dvl1 and Dvl2)
(Hamblet et al., 2002;
Wang, J. et al., 2006), or of
both Fz3 and Fz6 (Fzd3 and Fzd6 - Mouse
Genome Informatics) (Wang et al.,
2006b), all lead to a completely open neural tube and a shortened
embryo. It is interesting that many PCP mutants also show an eyelid closure
defect (Fig. 2B). Eyelid
closure normally occurs at about E16 in the mouse and, like neural tube
closure, involves a medial convergence of a pair of flanking epithelial
sheets.
A narrowing and lengthening analogous to CE also occurs during development
of the organ of Corti in the mammalian cochlea (see
Fig. 3A), and this shape change
fails to occur in Vangl2 mutants, Dvl1;Dvl2 double-mutants,
and in Fz3;Fz6 double-mutants, which all also have neural tube
closure defects (Montcouquiol et al.,
2003; Wang et al.,
2005; Wang et al.,
2006b). In the organ of Corti, CE-like movements occur after
sensorineural precursors have exited mitosis, indicating that this process
does not involve oriented cell division
(Wang, J. et al., 2006).
One of the earliest descriptions of the phenotypic consequence of genetic
disruption of Wnt-Fz signaling came from studies of the polarity of cleavage
planes during early cell divisions in C. elegans. Mutations in
mom-2 (a Wnt gene) or mom-5 (a Fz gene) misorient mitotic
spindles in several blastomeres (Rocheleau
et al., 1997; Thorpe et al.,
1997). Although the evolutionary divergence of Wnt-Fz signaling
makes it difficult to establish a clear one-to-one correlation between
vertebrate and C. elegans signaling pathways, it seems likely that
the pathway defined by the mom genes includes some elements of PCP
signaling (Park et al., 2004).
Gong et al. (Gong et al.,
2004) have extended this work to vertebrates by imaging zebrafish
that ubiquitously express histone H2B-GFP. In normal gastrulating zebrafish
embryos, dorsal epiblast cells in all layers tend to align their cell
divisions along the animal-vegetal axis, and this alignment depends on PCP
signaling as it is disrupted by the expression of dominant-negative Dvl
proteins or by injection of morpholino oligonucleotides that block the
synthesis of Vangl2. These observations suggest that, in some contexts, tissue
movements and tissue growth may be driven by oriented cell divisions
orchestrated by PCP.
New insights into the control of oriented cell division have recently come
from the study of mitotic spindle orientation in developing renal tubules
(Fischer et al., 2006). It has
been hypothesized that developing tubule elongation requires the preferential
displacement of pairs of daughter cells along the axis of the tubule, and that
cyst formation arises from an excess of transverse, rather than of
longitudinal, orientations of pairs of daughter cells
(Germino, 2005). Fischer et
al. (Fischer et al., 2006)
examined individual tubules from the developing kidney in WT controls and in
rat and mouse models of cystic kidney disease [polycystic kidney disease
(Pkd) mutant rats and hepatocyte nuclear factor-1 beta
(Hnf-1; Tcf2 - Mouse Genome Informatics) -deficient mice]
and found clear evidence supporting both hypotheses. The extent to which PCP
signaling plays a role in this process remains to be determined.
In contrast to the observations of Gong et al.
(Gong et al., 2004) on
zebrafish gastrulation, Ciruna et al.
(Ciruna et al., 2006) have
shown that coordinating polarized cell division is not the principal function
of PCP in zebrafish neural tube closure. Instead, PCP is required for the
reintegration of newly postmitotic cells into the neuroepithelium from which
they had been transiently extruded. Ciruna et al. observed that loss of
Vangl2 (trilobite) leads to an accumulation of apical
daughter cells from recent mitoses in the center of the U-shaped, and
incompletely closed, neural fold. A striking demonstration that the failure to
reintegrate these cells underlies the neural tube closure defect came from the
observation that pharmacologically blocking cell division in the
trilobite mutant late in gastrulation restores neural tube closure,
presumably because without cell division there are no extruded cells. By
contrast, mitotic inhibitors did not rescue the CE phenotype also caused by
the trilobite mutation.
The clearest example of PCP control of oriented cell movement is found in
the zebrafish hindbrain where facial motor neurons (which form the seventh
cranial nerve, nVII) migrate caudally from their birthplace in rhombomere four
to rhombomere six (Chandrasekhar et al.,
1997). This migration can be visualized in zebrafish that express
an islet1-GFP transgene, which is expressed selectively in this
subpopulation of hindbrain neurons
(Higashijima et al., 2000).
nVII motor neuron migration is impaired or abolished by mutations in the
zebrafish genes vangl2 (trilobite), prickle1 (also
known as pk1), scribble1/landlocked
(scrb1/llk), frizzled3a/off-limits
(fz3a/olt; one of two fz3 homologs in zebrafish),
and celsr2/off-road [ord; one of four stan
homologs in zebrafish (Bingham et al.,
2002; Jessen et al.,
2002; Carreira-Barbosa et al.,
2003; Wada et al.,
2005; Wada et al.,
2006)]. The fz3a and celsr2 genes were
identified in chemical mutagenesis screens for impaired nVII motor neuron
migration (Wada et al.,
2006). Genetic mosaic experiments show that each of these genes
promotes migration by mechanisms that include cell-nonautonomous components
(Jessen et al., 2002;
Wada et al., 2005;
Wada et al., 2006). The
principal role of the PCP system in promoting the caudal trajectory of the
nVII motor neurons appears to be to maintain these cells at the pial surface.
Loss of PCP gene function leads to the intercalation of the nVII motor neurons
into the underlying neuroepithelium with a concomitant switch from caudal to
radial migration.
Inner ear development
The vertebrate inner ear is an architectural tour-de-force in which bone,
vasculature, fluid-filled chambers, supporting cells, sensory neurons,
specialized extracellular deposits, and axons are all arranged with
extraordinary precision (Fig.
3A). Three types of sensory epithelia exist in the inner ear: the
organ of Corti, which detects airborne vibrations (i.e. sound) following its
conversion to a shearing motion of the structures within the central cavity of
the cochlea; the utricle and saccule, which detect linear acceleration by
sensing the inertial displacement of extracellular calcium-carbonate crystals
(otoliths); and the cristae, which detect angular acceleration by sensing the
inertial displacement of fluid in three microscopic gyroscopes called
semicircular canals. Given the complexities of the inner ear, it is perhaps
not surprising that most of the principal developmental signaling systems
known in vertebrates have been shown to play a role in its development,
including the retinoic acid, Hedgehog, Notch, Neurotrophin, BMP, Wnt and FGF
systems (Gao, 2003;
Kelley, 2003;
Wright and Mansour, 2003;
Barald and Kelley, 2004;
Fritzsch et al., 2004;
Romand et al., 2006).
The structural precision of the inner ear is reiterated subcellularly. In particular, each primary sensory neuron, the hair cell, elaborates on its apical face a set of actin-filled stereocilia adjacent to a single true cilium, the kinocilium (Fig. 3B). This mechanosensory structure, the sensory hair bundle, is precisely oriented with respect to the plane of the epithelium. Hair bundle orientation confers a directional selectivity on the mechanical response of the cell: hair bundle deflection toward the kinocilium opens plasma membrane cation channels; deflection away from the kinocilium closes the channels; and deflections to either side have no effect. When viewed face on, the stereocilia of cochlear hair cells are arranged in the shape of a chevron, with the kinocilium located at the apex of the V (Fig. 3). The stereocilia of vestibular hair cells (i.e. those in the utricule, saccule and cristae) are arranged in a dense cluster, with the kinocilium at one side of the cluster. In all sensory hair bundles, the stereocilia vary in length and are arranged in a precise step-wise manner with the longest stereocilia closest to the kinocilium and the shortest stereocilia furthest from the kinocilium (see Fig. 3B).
Nine years ago, Eaton proposed that PCP signaling orients stereociliary
hair bundles within the plane of inner ear sensory epithelia
(Eaton, 1997). Six years
later, Curtin et al. (Curtin et al.,
2003) and Montcouquiol et al.
(Montcouquiol et al., 2003)
simultaneously reported that mutations in Vangl2 [in the
Looptail (Lp) mouse mutant], Scribble [in the
Circletail (Crc) mouse mutant], and Celsr1 (in the
spin cycle and crash mouse mutants) cause precisely this
phenotype in the organ of Corti. Hair bundle orientation defects in the organ
of Corti have since been described in Ptk7 knockout and
Dvl1;Dvl2 and Fz3;Fz6 (Fzd6 - Mouse Genome
Informatics) double-knockout mice (Lu et
al., 2004; Wang et al.,
2005; Wang et al.,
2006b). Interestingly, the severity of defects and the subsets of
hair cells affected vary between mutants, and may also depend on the genetic
background. As an example of the latter, in one study, Vangl2
homozygotes were reported to have the single row of inner hair cells as well
as the outermost two of the three rows of outer hair cells severely
misorientated (Montcouquiol et al.,
2003), whereas the same Vangl2 allele was reported in
another study to cause severe misorientation defects in the outermost row of
outer hair cells and milder defects in all other rows of hair cells
(Wang et al., 2006b). In
contrast to both of these patterns, Fz3;Fz6 double-mutants have
severe orientation defects of the inner hair cells with only mild outer hair
cell defects. Some of these differences may reflect the participation of
additional and partially redundant family members: as seen in
Table 1, many mammalian PCP
genes are members of small, highly homologous gene families. Consistent with
this idea, Fz3 and Fz6 appear to be completely redundant in
inner ear development and are largely redundant in neural tube closure
(Wang et al., 2006b). In the
first analyses of PCP in the vestibular system, loss of Vangl2 was
found to randomize hair bundle orientation in both the utricle and cristae
(Montcouquiol et al., 2006b;
Wang, et al., 2006b).
Aside from its importance in the context of hearing and balance, the inner
ear sensory epithelium offers a powerful system for studying vertebrate PCP.
At present, it is the only place where a mammalian PCP phenotype can be
quantitatively scored at singlecell resolution. Moreover, the developing
sensory epithelium from the organ of Corti can be cultured in vitro for at
least one week, during which time hair bundles refine their orientations
(Dabdoub et al., 2003). In
this explant system, the application of Wnt7a or of soluble Wnt-binding
proteins leads to misoriented hair bundles, implicating Wnt ligands in the
orientation process. As described below, the inner ear has also provided a
useful system for determining the subcellular localization of vertebrate PCP
proteins and the effect of PCP gene mutation on protein localization.
The growth and guidance of axons and dendrites
The inclusion of a section on axon guidance and dendritic patterning in this review is not necessarily meant to imply that neurons and epithelial cells share the same PCP mechanisms. However, the discovery that PCP components function in the context of both epithelial and neuronal patterning suggests that at a molecular level these processes are at least partially related.
In mammals, one of the most dramatic axon growth and guidance phenotypes
identified to date is seen in mice that lack either Fz3 or
Celsr3, which are homologs of core PCP genes in Drosophila.
Loss of either of these genes eliminates the major axon tracts that connect
the thalamus and cortex, and causes spinal cord sensory axons to stall rather
than turn rostrally after midline crossing
(Fig. 4)
(Wang et al., 2002;
Wang et al., 2006a;
Lyuksyutova et al., 2003;
Tissir et al., 2005;
Price et al., 2006;
Bovolenta et al., 2006). In
both mutants, neuronal proliferation and migration in the forebrain appear to
be unaffected. In the Fz3-/- cortex, an analysis of cell
morphologies using genetically-directed cell labeling with alkaline
phosphatase or YFP shows that projection neurons send their axons into the
intermediate zone, the cortical layer in which corticothalamic axon bundles
would normally form, but these axons fail to extend and they eventually
degenerate (Wang et al.,
2006a). Preliminary data point to a similar outcome for
Celsr3-/- cortical axons
(Price et al., 2006). By
contrast, in both mutants thalamic axons extend and fasciculate but fail to
exit the thalamus. The near identity of the phenotypes observed in these two
mutants, and the known interactions between their Drosophila homologs
in the context of PCP, argues strongly that they function together in a common
axon guidance pathway.
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) observed that rostral turning in embryonic spinal cord
explants is lost if the explants are cut into narrow transverse strips,
suggesting that an orienting factor was being lost from the explant by
longitudinal diffusion. In testing a series of candidate factors produced by
COS cell aggregates cultured adjacent to the spinal cord explant, Lyuksyutva
et al. observed that several Wnts stimulate rostral turning. Intriguingly,
transcripts encoding Wnt4, a Wnt that promotes rostral turning, show a
rostral-caudal gradient of abundance along the spinal cord at midgestation at
the time when growing commissural axons cross the midline. Although these
data, together with the Fz3-/- rostral-turning defect,
suggest that Wnt4 promotes growth cone turning by activating Fz3, there is as
yet no direct biochemical evidence that these two proteins interact. However,
independent support for this general mechanism of axon guidance has come from
recent genetic studies in C. elegans: EGL-20 (a Wnt) appears to repel
axon growth along the anterior-posterior axis via its interaction with MIG-1
(a Fz) (Pan et al., 2006).
As noted above, the growth and maintenance of dendrites in both the
Drosophila and mammalian nervous systems involve Stan or its
mammalian homolog Celsr2, respectively
(Gao et al., 2000;
Shima et al., 2004;
Kimura et al., 2006). In the
WT Drosophila embryo, when dendrites of dendritic arborization (da)
neurons reach the dorsal midline, they avoid growing into regions that are
occupied by arbors from contralateral da neurons, thereby efficiently tiling
the surface of the embryo. In stan mutants, this dendritic growth
inhibition is lost. When expressed in cultured cells, Stan mediates homophilic
clustering and, in the Drosophila wing, localizes to both the
proximal and distal faces of epithelial cells. It is tempting to speculate
that homo- or heterophilic adhesion complexes that contain Stan may signal
directly or may localize signaling components in the context of both PCP and
dendrite development. Also consistent with a role for PCP-like signaling in
dendritic development, cultured hippocampal neurons from
Dvl1-/- mice show a decrease in dendritic growth and
arborization as compared with their WT counterparts, and the increase in
growth and arborization that is produced by the transfection of Dvl into WT
neurons is insensitive to the cotransfection of the canonical Wnt pathway
components glycogen synthase kinase-3 (Gsk-3; Gsk3a - Mouse Genome
Informatics) and ß-catenin, or to the pharmacological inhibition of Gsk-3
by treatment with lithium chloride or 6-bromoindirubin-3'-oxime
(Rosso et al., 2005).
Hair follicle orientation and hair patterning: separate global and local control systems
The mammalian PCP phenomenon that most closely resembles the oriented
patterning of hairs and bristles on the Drosophila cuticle is the
regular and locally parallel arrangement of hairs over the body surface. Hair
follicles make an acute angle with the skin, and therefore each follicle and
its associated hair has a defined orientation with respect to the body's axes.
A principal difference between mammalian hair follicles and
Drosophila hairs and bristles is one of scale: in
Drosophila, each wing epithelial cell makes a single actin-rich
protrusion (the hair), whereas each mammalian hair follicle is composed of
hundreds of cells and is separated from neighboring follicles by tens of cell
diameters. In general, the orientation of each follicle closely matches the
average orientation of its neighbors. This regular arrangement is defective in
Fz6-/- mice in the same distinctive manner in which
bristle and wing hair orientations are defective in Drosophila PCP
mutants: the pattern is globally disorganized but locally ordered, giving rise
to waves, whorls and tufts, each comprising dozens to hundreds of elements
(Figs 1 and
2)
(Guo et al., 2004).
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). This
analysis also shows that mammalian hair follicles, despite their large size,
possess an unexpected plasticity, reorienting in response to surrounding cues
in a matter of days. The data indicate that Fz6 normally functions early in
development to set the global orientation of hair follicles with respect to
the body axes. The subsequent reorientation is orchestrated by a
Fz6-independent system that aligns neighboring follicles. Thus, there appear
to be two distinct orienting systems, one that acts early in development and
globally, and a second that acts later and locally.
The existence of a local refinement mechanism permits the global signal to
produce no more than a rough alignment of immature follicles. In WT mice, the
local mechanism efficiently refines initially imperfect follicle orientations
to produce orientations that are almost perfectly parallel. As noted above,
two-stage models have also been proposed in the context of PCP signaling in
Drosophila, although it is not clear whether the Fz-dependent and
Fz-independent processes observed in Drosophila are analogous to the
ones defined by the Fz6-/- hair follicle phenotype.
Indeed, the general idea that a Ds, Ft and Fj system acts upstream to set up a
global orientation, and a Fz, Vang, Dsh and Stan system acts downstream to
refine that orientation, would appear to be at odds with the
Fz6-/- hair patterning phenotype. Two-stage mechanisms may
also exist in the context of other PCP processes in vertebrates. For example,
the progressive refinement of sensory hair bundle orientations within the
inner ear, a phenomenon observed in both mammals and birds
(Cotanche and Corwin, 1991;
Dabdoub et al., 2003), is
consistent with a two-stage process.
|
), PCP
patterning bears a strong conceptual resemblance to the patterning of electron
spins in a ferromagnet. As in PCP, there are both local and global effects:
local quantum mechanical interactions favor the alignment of adjacent electron
spins, and a global signal, in the form of the external magnetic field, biases
the alignment probabilities for all of the individual spins. Both
ferromagnetism and the progressive refinement of hair follicle orientations
can be modeled with a two-dimensional lattice of uniformly spaced vectors and
a local consensus `rule' (Fig.
6) (Wang et al.,
2006c). The rule is applied iteratively, and, with each iteration,
it subtly biases each vector's orientation in favor of the most recent average
of its neighbors' orientations. This simple lattice model captures the key
attributes of hair follicle patterning, including the growth by accretion of
hair patterns during development, the spread of oriented WT patterns into
adjacent Fz6-/- skin (in WT:Fz6-/-
chimeras), and the ability of a small initial bias in vector orientation to
rapidly dominate an otherwise randomly oriented population of follicles. PCP and cilia
A fascinating, but still poorly understood, connection has recently emerged
between PCP and nonmotile cila based on the observation that several genes
that affect vertebrate PCP also affect ciliary structure and/or function
(Bisgrove and Yost, 2006;
Davis et al., 2006;
Singla and Reiter, 2006). In
vertebrates, many, if not all, epithelial cells possess a single nonmotile
cilium (the primary cilium), which is typically located in the center of the
apical face of the cell. By contrast, in Drosophila and C.
elegans, nonmotile cilia have thus far only been found on subsets of
neurons. One link between PCP and cilia has come from the study of
Bardet-Beidl syndrome (BBS), a genetically heterogeneous human disorder with
pleiotropic manifestations including obesity, polydactyly, endocrine
dysfunction, cystic renal disease, progressive photoreceptor degeneration and
hearing loss (Bisgrove and Yost,
2006; Davis et al.,
2006). Bbs genes are structurally diverse but many share the
common feature that the encoded proteins localize to the cilium or its
cellular anchor, the basal body (Ansley et
al., 2003). Targeted disruption of Bbs1, Bbs4 or
Bbs6 (Mkks - Mouse Genome Informatics) in mice leads to
misorientation of inner ear sensory hair bundles
(Ross et al., 2005), and 14%
of Bbs4-/- mice display an open cephalic neural tube
(exencephaly). Moreover, both Bbs1 and Bbs6 alleles interact
genetically with Vangl2, and morpholino oligonucleotide knockdown of
Bbs4 in zebrafish leads to PCP phenotypes, including a failure of
embryonic CE. We note that some, and perhaps most, Bbs proteins may also
function in the wider context of cytoskeletal regulation - as indicated, for
example, by defects in melanosome transport that occur when Bbs genes are
knocked down in zebrafish (Yen et al.,
2006) - and therefore their effects on PCP may extend beyond their
roles in ciliary structure.
A second link between PCP and cilia has come from the identification of the
mouse inversin (Invs) gene, which encodes a large
adaptor-like protein with homology to the Drosophila PCP protein
Diego. Invs was discovered as the serendipitous target of a transgene
insertion event that produced a situs inversus phenotype, apparently
the result of a ciliary defect in the embryonic node
(Mochizuki et al., 1998;
Morgan et al., 1998;
Okada et al., 1999).
Invs mutations also cause cystic renal tubules and progressive renal
failure in both mice and humans (nephronophthisis type 2, NPHP2)
(Otto et al., 2003;
Bisgrove and Yost, 2006). The
subcellular localization of inversin is complex and dynamic, but includes the
basal bodies, primary cilia and, during metaphase and anaphase, the spindle
poles (Morgan et al., 2002;
Watanabe et al., 2003;
Eley et al., 2004;
Nurnberger, 2004). In
transfected cells, inversin binds Dvl and accelerates its degradation, and in
Xenopus embryos it is required for CE
(Simons et al., 2005). The
data suggest that inversin controls the balance between canonical and
noncanonical Wnt signaling, with higher inversin activity favoring
noncanonical (i.e. PCP) signaling and lower inversin activity favoring
canonical signaling and, with it, misregulated tubule growth and cyst
formation (Germino, 2005).
The most recent link between PCP and cilia comes from experiments with
Xenopus embryos in which homologs of the Drosophila PCP
genes fuzzy and inturned were unexpectedly found to be
required for Hedgehog signaling (Park et
al., 2006). The earlier discovery, in an unbiased chemical
mutagenesis screen in the mouse, that loss of various intraflagellar (i.e.
ciliary) transport proteins (IFTs) impairs Hedgehog signaling (reviewed by
Huangfu and Anderson, 2006;
Huangfu and Anderson, 2005)
suggests that Inturned and Fuzzy play a role in ciliary structure or function.
Consistent with this hypothesis, cilia in Inturned and Fuzzy
morphants in Xenopus are short and often misshapen, and the
underlying actin skeleton is of abnormally low density
(Park et al., 2006).
|
PCP protein localization
One of the most striking and mechanistically significant observations to
emerge from the study of PCP in Drosophila is that several PCP
proteins are asymmetrically distributed on the proximal or distal faces of
wing epithelial cells and on a subset of the lateral faces of R3 and R4
photoreceptors (Usui et al.,
1999; Axelrod,
2001; Shimada et al.,
2001; Strutt,
2001). In genetically mosaic pupal wings, Ds, Prickle (Pk), and
Vang localize to the proximal face of each cell; Dsh, Dgo, Ft and Fz localize
to the distal face; Stan localizes to both proximal and distal faces; and all
of these proteins are under-represented on the remaining (i.e. anterior and
posterior) faces (see Fig. 1E).
Further experiments examined the effect on PCP protein localization of
juxtaposing clones of mutant and WT cells and of mutating various PCP genes
(reviewed by Adler, 2002;
Strutt, 2002;
Klein and Mlodzik, 2005).
Together with the co-precipitation of overexpressed PCP proteins and protein
fragments, these studies have provided evidence that several of the
genetically defined PCP proteins interact with each other either directly or
indirectly at the surface of the same cell (i.e. in cis), or with PCP proteins
at the surface of the adjacent cell (in trans).
Analogous studies of PCP protein localization in vertebrates are only just
beginning. As in Drosophila, PCP protein complexes accumulate at the
apical edge of the lateral faces of epithelial cells. In the chicken inner
ear, Celsr1 localizes asymmetrically in both hair cells and supporting cells
in the sensory epithelium of the basilar papilla
(Davies et al., 2005). In the
mouse organ of Corti, Dvl2-EGFP expressed from a BAC transgene localizes
asymmetrically at the surface of hair cells and supporting cells, and this
localization is lost in a Vangl2 (Lp) mutant
(Wang et al., 2005;
Wang, J. et al., 2006).
Similarly, Fz3 and Fz6 colocalize asymmetrically at the surface of hair cells
and supporting cells in all inner ear sensory epithelia, and this localization
is also lost in the Vangl2 mutant
(Fig. 7A,B)
(Wang et al., 2006b). The
assembly of PCP protein complexes appears to be highly sensitive to the
orientation of the cell's sides with respect to the global axis of the
epithelium.
The spatial resolution of light microscopy does not permit a distinction to
be made between the localization of PCP proteins to one or the other (or both)
surfaces of neighboring cells. In the case of Fz6 localization in the inner
ear, it has been possible to make this distinction by immunostaining
WT:Fz6-/- chimeric tissue
(Fig. 7C-E)
(Wang et al., 2006b). Since
Fz3 is fully redundant with Fz6 in the inner ear, the asymmetric localization
of Fz6, where WT and Fz6-/- cells interface, takes place
in the context of phenotypically normal tissue. This analysis shows that Fz6
accumulates apically in both supporting cells and hair cells, and that the
polarity of Fz6 localization occurs with respect to the polarity of the
epithelium. Fz3, which has the same immunostaining pattern as Fz6 in
nonchimeric sensory epithelia and performs the same function in the inner ear,
is presumed to have the same subcellular localization.
Interestingly, in the inner ears of Fz3-/- mice, the
intensity of Fz6 immunostaining at the cell surface is increased relative to
WT. Fz3 immunoreactivity exhibits analogous behavior in the
Fz6-/- inner ear. These observations are consistent with
the genetic redundancy of Fz3 and Fz6, and they suggest the
existence of an intracellular pool of Fz (and perhaps other PCP proteins) that
is in equilibrium with the protein at the cell surface. Consistent with this
model, a complete absence of Fz3 and Fz6 at the cell surface in a
Vangl2 mutant is associated with little or no change in the total
abundance of the Fz3 and Fz6 proteins
(Wang et al., 2006b).
What form might the intracellular pools of Fz take? Careful inspection of
confocal images of sensory epthithelia immunostained for Fz3 or Fz6 reveals
numerous punctate immunoreactive structures, presumably vesicles, within the
cytoplasm. These puncta cannot be ascribed to background staining or other
artefacts because they are absent in tissue from which the corresponding
Fz gene has been deleted, as shown in
Fig. 7C-E. Polarized vesicular
transport of Fz has recently been described in Drosophila
(Shimada et al., 2006), and it
seems plausible in both Drosophila and mammals that the trafficking
of vesicular pools may regulate the abundance of PCP components at the cell
surface.
The immunolocalization of Vangl2 in the inner ear shows a pattern very much
like that of Fz3, whereas Scribble (a large cytosolic protein with multiple
PDZ domains) localizes uniformly along the circumference of the cell
(Montcouquiol et al., 2006b).
Binding experiments between Vangl2 and Scribble and between Vangl2 and Fz3
expressed in transfected cells indicate a direct interaction for each
(Montcouquiol et al., 2006b).
These data have been interpreted to imply that a fundamental difference exists
between Drosophila and mammalian PCP: in Drosophila, Fz and
Vang localize to opposite sides of wing epithelial cells, whereas in the mouse
inner ear Montcouquiol et al.
(Montcouquiol et al., 2006b)
suggest that Fz3 and Vangl2 colocalize.
Conclusions
The study of vertebrate PCP is still in its infancy, and there are currently many more questions than answers. What is the nature of the global orienting signal? How are local orienting signals sent and received? What are the compositions of cell-surface PCP signaling complexes? How are these complexes selectively localized to some but not other plasma membrane regions? How do Fz3 and Celsr3 function in axon guidance? Which other tissues or developmental processes utilize PCP signaling? Which human diseases arise from defects in PCP? Addressing these and other questions should make this an exciting area of inquiry for many years to come.
ACKNOWLEDGMENTS
The authors acknowledge the support of the Howard Hughes Medical Institute.
REFERENCES
Adler, P. N. (2002). Planar signaling and morphogenesis in Drosophila. Dev. Cell 2, 525-535.[CrossRef][Medline]
Adler, P. N., Krasnow, R. E. and Liu, J. (1997). Tissue polarity points from cells that have higher Frizzled levels towards cells that have lower Frizzled levels. Curr. Biol. 7,940 -949.[CrossRef][Medline]
Ansley, S. J., Badano, J. L., Blacque, O. E., Hill, J., Hoskins, B. E., Leitch, C. C., Kim, J. C., Ross, A. J., Eichers, E. R., Teslovich, T. M. et al. (2003). Basal body dysfunction is a likely cause of pleiotropic Bardet-Biedl syndrome. Nature 425,628 -633.[CrossRef][Medline]
Axelrod, J. D. (2001). Unipolar membrane
association of Dishevelled mediates Frizzled planar cell polarity signaling.
Genes Dev. 15,1182
-1187.
Barald, K. F. and Kelley, M. W. (2004). From
placode to polarization: new tunes in inner ear development.
Development 131,4119
-4130.
Barrow, J. R. (2006). Wnt/PCP signaling: a veritable polar star in establishing patterns of polarity in embryonic tissues. Semin. Cell Dev. Biol. 17,185 -193.[CrossRef][Medline]
Bhanot, P., Fish, M., Jemison, J. A., Nusse, R., Nathans, J. and Cadigan, K. M. (1999). Frizzled and Dfrizzled-2 function as redundant receptors for Wingless during Drosophila embryonic development. Development 126,4175 -4186.[Abstract]
Bhat, K. M. (1998). frizzled and frizzled 2 play a partially redundant role in wingless signaling and have similar requirements to wingless in neurogenesis. Cell 95,1027 -1036.[CrossRef][Medline]
Bingham, S., Higashijima, S., Okamoto, H. and Chandrasekhar, A. (2002). The Zebrafish trilobite gene is essential for tangential migration of branchiomotor neurons. Dev. Biol. 242,149 -160.[CrossRef][Medline]
Bisgrove, B. W. and Yost, H. J. (2006). The
roles of cilia in developmental disorders. Development
133,4131
-4143.
Boutros, M., Paricio, N., Strutt, D. I. and Mlodzik, M. (1998). Dishevelled activates JNK and discriminates between JNK pathways in planar polarity and wingless signaling. Cell 94,109 -118.[CrossRef][Medline]
Bovolenta, P., Rodriguez, J. and Esteve, P.
(2006). Frizzled/RYK mediated signalling in axon guidance.
Development 133,4399
-4408.
Carreira-Barbosa, F., Concha, M. L., Takeuchi, M., Ueno, N.,
Wilson, S. W. and Tada, M. (2003). Prickle 1 regulates cell
movements during gastrulation and neuronal migration in zebrafish.
Development 130,4037
-4046.
Casal, J., Lawrence, P. A. and Struhl, G.
(2006). Two separate molecular systems, Dachsous/Fat and Starry
night/Frizzled, act independently to confer planar cell polarity.
Development 133,4561
-4572.
Chae, J., Kim, M. J., Goo, J. H., Collier, S., Gubb, D., Charlton, J., Adler, P. N. and Park, W. J. (1999). The Drosophila tissue polarity gene starry night encodes a member of the protocadherin family. Development 126,5421 -5429.[Abstract]
Chandrasekhar, A., Moens, C. B., Warren, J. T., Jr, Kimmel, C. B. and Kuwada, J. Y. (1997). Development of branchiomotor neurons in zebrafish. Development 124,2633 -2644.[Abstract]
Chen, C. M. and Struhl, G. (1999). Wingless transduction by the Frizzled and Frizzled2 proteins of Drosophila. Development 126,5441 -5452.[Abstract]
Ciruna, B., Jenny, A., Lee, D., Mlodzik, M. and Schier, A. F. (2006). Planar cell polarity signalling couples cell division and morphogenesis during neurulation. Nature 439,220 -224.[CrossRef][Medline]
Classen, A. K., Anderson, K. I., Marois, E. and Eaton, S. (2005). Hexagonal packing of Drosophila wing epithelial cells by the planar cell polarity pathway. Dev. Cell 9, 805-817.[CrossRef][Medline]
Copp, A. J., Greene, N. D. and Murdoch, J. N. (2003). The genetic basis of mammalian neurulation. Nat. Rev. Genet. 4,784 -793.[CrossRef][Medline]
Cotanche, D. A. and Corwin, J. T. (1991). Stereociliary bundles reorient during hair cell development and regeneration in the chick cochlea. Hear. Res. 52,379 -402.[CrossRef][Medline]
Curtin, J. A., Quint, E., Tsipouri, V., Arkell, R. M., Cattanach, B., Copp, A. J., Henderson, D. J., Spurr, N., Stanier, P., Fisher, E. M. et al. (2003). Mutation of Celsr1 disrupts planar polarity of inner ear hair cells and causes severe neural tube defects in the mouse. Curr. Biol. 13,1129 -1133.[CrossRef][Medline]
Dabdoub, A., Donohue, M. J., Brennan, A., Wolf, V.,
Montcouquiol, M., Sassoon, D. A., Hseih, J. C., Rubin, J. S., Salinas, P. C.
and Kelley, M. W. (2003). Wnt signaling mediates
reorientation of outer hair cell stereociliary bundles in the mammalian
cochlea. Development
130,2375
-2384.
Davies, A., Formstone, C., Mason, I. and Lewis, J. (2005). Planar polarity of hair cells in the chick inner ear is correlated with polarized distribution of c-flamingo-1 protein. Dev. Dyn. 233,998 -1005.[CrossRef][Medline]
Davis, E. E., Brueckner, M. and Katsanis, N. (2006). The emerging complexity of the vertebrate cilium: new functional roles for an ancient organelle. Dev. Cell 11, 9-19.[CrossRef][Medline]
Denman-Johnson, K. and Forge, A. (1999). Establishment of hair bundle polarity and orientation in the developing vestibular system of the mouse. J. Neurocytol. 28,821 -835.[CrossRef][Medline]
Djiane, A., Yogev, S. and Mlodzik, M. (2005). The apical determinants aPKC and dPatj regulate Frizzled-dependent planar cell polarity in the Drosophila eye. Cell 121,621 -631.[CrossRef][Medline]
Eaton, S. (1997). Planar polarization of Drosophila and vertebrate epithelia. Curr. Opin. Cell Biol. 9,860 -866.[CrossRef][Medline]
Eley, L., Turnpenny, L., Yates, L. M., Craighead, A. S., Morgan, D., Whistler, C., Goodship, J. A. and Strachan, T. (2004). A perspective on inversin. Cell Biol. Int. 28,119 -124.[CrossRef][Medline]
Fischer, E., Legue, E., Doyen, A., Nato, F., Nicolas, J. F., Torres, V., Yaniv, M. and Pontoglio, M. (2006). Defective planar cell polarity in polycystic kidney disease. Nat. Genet. 38,21 -23.[CrossRef][Medline]
Fritzsch, B., Tessarollo, L., Coppola, E. and Reichardt, L. F. (2004). Neurotrophins in the ear: their roles in sensory neuron survival and fiber guidance. Prog. Brain Res. 146,265 -278.[Medline]
Gao, F. B., Kohwi, M., Brenman, J. E., Jan, L. Y. and Jan, Y. N. (2000). Control of dendritic field formation in Drosophila: the roles of flamingo and competition between homologous neurons. Neuron 28,91 -101.[CrossRef][Medline]
Gao, W. Q. (2003). Hair cell development in higher vertebrates. Curr. Top. Dev. Biol. 57,293 -319.[Medline]
Germino, G. G. (2005). Linking cilia to Wnts. Nat. Genet. 37,455 -457.[CrossRef][Medline]
Gong, Y., Mo, C. and Fraser, S. E. (2004). Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation. Nature 430,689 -693.[CrossRef][Medline]
Goto, T., Davidson, L., Asashima, M. and Keller, R. (2005). Planar cell polarity genes regulate polarized extracellular matrix deposition during frog gastrulation. Curr. Biol. 15,787 -793.[CrossRef][Medline]
Grebe, M. (2004). Ups and downs of tissue and planar polarity in plants. BioEssays 26,719 -729.[CrossRef][Medline]
Greene, N. D., Gerrelli, D., Van Straaten, H. W. and Copp, A. J. (1998). Abnormalities of floor plate, notochord and somite differentiation in the loop-tail (Lp) mouse: a model of severe neural tube defects. Mech. Dev. 73,59 -72.[CrossRef][Medline]
Gubb, D. and Garcia-Bellido, A. (1982). A genetic analysis of the determination of cuticular polarity during development in Drosophila melanogaster. J. Embryol. Exp. Morphol. 68, 37-57.[Medline]
Guo, N., Hawkins, C. and Nathans, J. (2004).
Frizzled6 controls hair patterning in mice. Proc. Natl. Acad. Sci.
USA 101,9277
-9281.
Hamblet, N. S., Lijam, N., Ruiz-Lozano, P., Wang, J., Yang, Y., Luo, Z., Mei, L., Chien, K. R., Sussman, D. J. and Wynshaw-Boris, A. (2002). Dishevelled 2 is essential for cardiac outflow tract development, somite segmentation and neural tube closure. Development 129,5827 -5838.[CrossRef][Medline]
Hannus, M., Feiguin, F., Heisenberg, C. P. and Eaton, S. (2002). Planar cell polarization requires Widerborst, a B' regulatory subunit of protein phosphatase 2A. Development 129,3493 -3503.[Medline]
Heisenberg, C. P., Tada, M., Rauch, G. J., Saude, L., Concha, M. L., Geisler, R., Stemple, D. L., Smith, J. C. and Wilson, S. W. (2000). Silberblick/Wnt11 mediates convergent extension movements during zebrafish gastrulation. Nature 405, 76-81.[CrossRef][Medline]
Higashijima, S., Hotta, Y. and Okamoto, H.
(2000). Visualization of cranial motor neurons in live transgenic
zebrafish expressing green fluorescent protein under the control of the
islet-1 promoter/enhancer. J. Neurosci.
20,206
-218.
Huangfu, D. and Anderson, K. V. (2005). Cilia
and Hedgehog responsiveness in the mouse. Proc. Natl. Acad. Sci.
USA 102,11325
-11330.
Huangfu, D. and Anderson, K. V. (2006).
Signaling from Smo to Ci/Gli: conservation and divergence of Hedgehog pathways
from Drosophila to vertebrates. Development
133, 3-14.
Jenny, A. and Mlodzik, M. (2006). Planar cell polarity signaling: a common mechanism for cellular polarization. Mt. Sinai J. Med. 73,738 -750.[Medline]
Jessen, J. R., Topczewski, J., Bingham, S., Sepich, D. S., Marlow, F., Chandrasekhar, A. and Solnica-Krezel, L. (2002). Zebrafish trilobite identifies new roles for Strabismus in gastrulation and neuronal movements. Nat. Cell Biol. 4, 610-615.[Medline]</
-45 to +45 degrees, a distribution
that narrows by P4. (C,D) Fz6-/- follicles at P0 have
random orientations, which by P4 have partially reoriented to produce spatial
domains with imperfect local order. Additional reorientation continues over
the next several days (see 
