Current Biology Vol 15 No 1
R14
Dispatches
Notochord Morphogenesis: A Prickly Subject for
Ascidians
Orchestrated cell movements marshalled by proper cell polarity in the
developing body axes are fundamental to the elongation of the
notochord during chordate embryogenesis. A recent study shows that,
in ascidians, the planar cell polarity gene prickle regulates sequential
establishment of cell polarity during two phases of notochord
morphogenesis.
Masazumi Tada
How cell polarity is established
and propagated within a large
population of cells is a key issue
to understanding the coordinated
and directional tissue movements
that shape the body axes of
animal embryos. One such
process is convergent extension,
best described in the presumptive
notochord of the Xenopus
embryo, by which medio-laterally
aligned cells converge on the
dorsal midline and intercalate
between one another, narrowing
the dorsal tissue medially and
simultaneously extending it along
the anterior–posterior axis of the
developing body [1]. This
fundamental process appears to
be conserved in other chordates,
including zebrafish and
ascidians [2,3].
Recently, a molecular picture of
the pathways involved in the
regulation of convergent
extension has emerged.
Components of the so-called
planar cell polarity pathway,
homologs of which are known to
play an essential role in the
establishment of cell polarity
within the plane of epithelial
tissues in Drosophila, have been
found to be key regulators of this
critical process [4]. Indeed, the
vertebrate planar cell polarity
pathway has also been implicated
in the regulation of a variety of
other coordinated morphogenetic
movements, such as neural tube
closure, cochlear hair cell
orientation and neuronal
migration [5,6].
In ascidian embryos, a mere 40
notochord cells are specified
before convergent extension takes
place by rearranging a four by ten
sheet of cells into a forty by one
stack of cells, thereby elongating
the notochord along the
anterior–posterior axis. Later,
individual notochord cells extend
along their anterior–posterior axis
as the tail elongates. Cellular
mechanisms underlying
convergent extension in ascidians
are similar to those in Xenopus in
that medio-lateral cell intercalation
is thought to be a cellautonomous driving force of this
process [3].
As reported in this issue of
Current Biology, by examining
this beautifully simple system,
Jiang and colleagues [7] have
identified the aimless locus,
which encodes a homolog of the
core planar cell polarity protein
Prickle, as a key regulator of
notochord morphogenesis in the
ascidian Ciona savignyi.
Notochord cells are specified
normally in larvae homozygous
for the aimless mutation, but fail
to undergo the normal
movements of convergent
extension. A shorter and thicker
notochord is generated,
reminiscent of those found in
mutant zebrafish or Xenopus
embryos defective for planar cell
polarity components such as
Prickle Strabismus or
Dishevelled [5,6,8].
This aimless mutant phenotype
is reminiscent of that seen in
Ciona embryos expressing a
dominant-negative form of
Dishevelled that specifically
disrupts its function in planar cell
polarity [9]. The defective
convergent extension phenotype
is consistent with expression of
prickle in the notochord. In
aimless/prickle embryos, the
notochord cell behavior that
underlies medio-lateral
intercalation is disrupted, such
that the orientation of
lamellipodia-like outgrowth is
randomised rather than having a
medio-lateral biased orientation
as in wild-type embryos.
How do planar cell polarity
proteins establish and maintain
cell polarity with respect to the
axis of the developing embryo?
This is best understood in the
case of the Drosophila wing
epithelium. Initially, all the
components are colocalised
symmetrically at the cell
membrane. The establishment of
cell polarity crucially involves
feedback amplification in which
proximally localised Prickle and
Strabismus suppress Frizzled and
Dishevelled on the proximal side,
thereby facilitating the
accumulation of Frizzled and
Dishevelled on the distal side
(Figure 1A) [4,10]. This localised
Frizzled/Dishevelled activity leads
to distal outgrowth of hairs.
In vertebrates, however, there is
no consensus as to where planar
cell polarity proteins are localised
with respect to the axis of
alignment of polarised cells. In
Xenopus dorsal marginal explants
undergoing convergent extension,
Dishevelled is variously reported
as localised to the membrane
uniformly [11] or in a medio-lateral
biased manner [12]. Colocalisation
or differential localisation of
Dishevelled and other planar cell
polarity components has not been
yet reported in cells undergoing
cell intercalations, though
Dishevelled and Prickle colocalise
to the membrane in the presence
of Fz7 in static Xenopus animal
pole cells [13,14], while Prickle
inhibits Fz7-mediated membrane
localisation of Dishevelled in static
zebrafish animal pole cells [15].
In ascidian notochord cells
actively undergoing convergent
Dispatch
R15
extension, Dishevelled
colocalises with Prickle to the
membrane uniformly without any
bias along the medio-lateral axis
(Figure 1B). This membrane
localisation of Dishevelled is
dependent on Prickle activity, as
Dishevelled is no longer localised
to the membrane in the
aimless/prickle mutant. It will be
intriguing to see if the ability of
Prickle to localise to the
membrane is also dependent on
Dishevelled, as an interdependency between Prickle and
Dishevelled is observed in the
initial localisation of planar cell
polarity proteins in establishing
cell polarity in the Drosophila
wing [10].
Interestingly, both Prickle and
Dishevelled disappear from the
cell membranes in contact with
the notochord–somite boundary
(Figure 1B). This asymmetric
localisation of the two proteins at
the notochord–somite boundary
suggests the presence of a cue
that influences the establishment
of cell polarity within the
notochord, consistent with the
observations of Munro and
Odell [3].
What molecular mechanism
might underlie this event? One
obvious candidate is the secreted
signaling molecule Wnt5, one of
the so-called non-canonical Wnts.
Wnt5 is secreted by the adjacent
muscle cells in Ciona [16] and
might help establish cell polarity
in notochord cells. This would be
consistent with the defective
convergent extension phenotype
of pipetail/wnt5 mutant
zebrafish [8].
Another candidate, possibly
expressed in the somites but not
in the notochord and implicated in
convergent extension, is paraxial
protocadherin (papc). In embryos
with compromised Papc activity,
convergence is impaired but
extension in the notochord is
relatively normal, and Papc
appears to act in part together
with the Fz7-mediated planar cell
polarity signal by activating the
RhoA small GTPase [17].
Importantly, protocadherins such
as Dachsous and Fat are also
implicated in propagation of the
Frizzled-mediated signal in
Drosophila planar cell polarity [4].
A
P
Pk
D
Dsh
Stbm
Fz
B
A
P
Somites
L
M
WT
L
Somites
Somites
aim
Somites
C
P
A
Somites
WT
Somites
Somites
aim
Somites
Current Biology
Figure 1. Sub-cellular localisation of planar cell polarity proteins during establishment
of polarity in the Drosophila wing and ascidian notochord.
(A) During establishment of planar cell polarity in the Drosophila wing, Prickle (Pk) and
Strabismus (Stbm) are localised to the proximal side of the cell, while Frizzled (Fz) and
Dishevelled (Dsh) are to the distal side. (B) During notochord morphogenesis in ascidians, Prickle and Dishevelled co-localise uniformly at the cell membranes, apart from at
the notochord–somite boundary in wild-type embryos. Dishevelled is no longer
localised to the cell membranes in aimless/prickle mutant embryos, which consequently
develop a shorter and thicker notochord. (C) As the nuclei position posteriorly within
the extending notochord cells, Prickle and Strabismus colocalise to the anterior edge
of the cells, whereas Dishevelled is laterally localised to the notochord–somite boundary in wild embryos. Note that the localisation of Dishevelled and Strabismus in
aimless/prickle embryos, in which the position of nuclei is randomised, is not reported.
Following convergent
extension, individual notochord
cells elongate along the
anterior–posterior axis and Prickle
and Strabismus become localised
to the anterior edge of cells, while
Dishevelled is found laterally at
the notochord–somite boundary
(Figure 1C). Correlated with the
anterior localisation of Prickle,
nuclei within the notochord
become localised to the posterior
edge of the cells. In
aimless/prickle mutant embryos,
the position of nuclei in notochord
cells is randomised with respect
to their anterior–posterior axis,
suggesting that Prickle might
establish or maintain
anterior–posterior polarity in the
later aspects of notochord
morphogenesis.
The biological significance of
this process is yet to be clarified,
but it will be interesting to
examine the position of nuclei, as
well as the anterior localisation of
Prickle, during this stage when
Dishevelled activity is
compromised. One planar cell
Current Biology Vol 15 No 1
R16
polarity-dependent process
occurring during zebrafish
gastrulation is directional cell
division aligned along the
anterior–posterior axis, which
contributes in part to the
elongation of the body axis [18].
William Smith’s group [19] has
previously reported that the chobi
mutant Ciona embryos show a
phenotype reminiscent of
aimless/prickle mutant embryos.
Further identification of Ciona
mutants that exhibit a shorter tail
phenotype with properly
differentiated notochord could
uncover novel members of the
planar cell polarity pathway, as
has been done for gastrulation
defects in zebrafish and for neural
tube defects in mice. Considering
that the extent of genome
redundancy in ascidians is similar
to that in Drosophila with respect
to planar cell polarity genes [20],
this approach might be more
successful in these species than
in other vertebrates.
4.
5.
6.
7.
8.
9.
10.
11.
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Circadian Biology: Fibroblast
Clocks Keep Ticking
Real-time cellular imaging of gene expression has revealed that
fibroblasts contain a robust, self-sustained and cell-autonomous
circadian oscillator, with a range of properties that both overlap and
contrast with those of the neural clock of the suprachiasmatic nuclei.
Michael H. Hastings
Circadian clocks enable
organisms to define biological
day and night, synchronising daily
rhythms of metabolism and
behaviour to the demands and
opportunities of the world [1].
Hence, clocks confer selective
advantage and are hard-wired
into our make-up. Our most
obvious circadian rhythm is that
of sleep and wakefulness, but
they range from mucosal cell
division through hormonal profiles
to susceptibility to cardiac arrest.
The circadian mantra used to be
easy; “there is but one true clock
and it sits in the brain”. By using
real-time cellular imaging, two
papers [2,3] have revealed a new
truth:, “clocks are all over the
body and all are equal, but some
are more equal than others”.
After early skepticism that
circadian rhythms are artefactual,
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(2003). The prickle-related gene in
vertebrates is essential for gastrulation
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Carreira-Barbosa, F., Concha, M.L.,
Takeuchi, M., Ueno, N., Wilson, S.W.,
and Tada, M. (2003). Prickle1 regulates
cell movements during gastrulation and
neuronal migration in zebrafish.
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Imai, K.S., Hino, K., Yagi, K., Satoh, N.,
and Satou, Y. (2004). Gene expression
profiles of transcription factors and
signaling molecules in the ascidian
embryo: towards a comprehensive
understanding of gene networks.
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Unterseher, F., Hefele, J.A., Giehl, K., De
Robertis, E.M., Wedlich, D., and
Schambony, A. (2004). Paraxial
protocadherin coordinates cell polarity
during convergent extension via Rho A
and JNK. EMBO J. 23, 3259–3269.
Gong, Y., Mo, C., and Fraser, S.E. (2004).
Planar cell polarity signalling controls
cell division orientation during zebrafish
gastrulation. Nature 430, 689–693.
Nakatani, Y., Moody, R., and Smith, W.C.
(1999). Mutations affecting tail and
notochord development in the ascidian
Ciona savignyi. Development 126,
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Hotta, K., Takahashi, H., Ueno, N., and
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phylogenetic comparisons of
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Department of Anatomy and
Developmental Biology, University
College London, Gower St, London
WC1E 6BT, UK. E-mail:
m.tada@ucl.ac.uk
DOI: 10.1016/j.cub.2005.36.25
the field was boosted in the 1980s
with the identification of the
hypothalamic suprachiasmatic
nuclei (SCN) as the body clock
controlling the sleep/wake and
endocrine cycles [4]. Further
respectability came in the nineties
with the discovery of genes
encoding the SCN clockwork [5].
Driven by complexes of the
transcription factors CLOCK and
BMAL, the ‘clock genes’ Period
and Cryptochrome encode
transcriptional inhibitors that
oppose CLOCK/BMAL activity,
thereby closing a negative
feedback loop that oscillates with
an approximately daily period.
Electrophysiological recordings of
dispersed cultures revealed that
the clockwork is cell-autonomous,
its activity within single SCN
neurons indicating that there must