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Dispatch
Vertebrate gastrulation: Calcium waves orchestrate cell movements
Masazumi Tada and Miguel L. Concha
A recent study reveals that the propagation of
intercellular calcium signals is closely associated with
the generation of convergent extension movements
during Xenopus gastrulation. Such signals provide a
mechanism whereby large populations of cells can
communicate to generate orchestrated cell movements.
Address: Department of Anatomy and Developmental Biology,
University College London, Gower Street, London, WC1E 6BT, UK.
E-mail: m.tada@ucl.ac.uk ; m.concha@ucl.ac.uk
Current Biology 2001, 11:R470–R472
0960-9822/01/$ – see front matter
© 2001 Elsevier Science Ltd. All rights reserved.
The basic body plan of vertebrate embryos is established
during gastrulation by a series of co-ordinated movements
of cell groups that gives rise to the germ layers and overtly
shapes the embryonic axis. One such movement, known as
convergent extension, is required for the antero-posterior
extension of dorsal tissues such as the notochord and
neural plate and is essential for the proper elongation of
the entire body axis. Convergent extension is the process
by which the co-ordinated intercalation of cells along the
mediolateral axis generates tissue extension along the
antero-posterior axis (Figure 1). The cellular basis of convergent extension has been well characterised in amphibian and teleost embryos [1–3], and in recent years we have
also begun to understand how this morphogenetic process
is regulated [4–7]. A recent study published in Current
Biology [8] sheds light on the presently obscure mechanism
by which the movements of such a large cell population
are co-ordinated in time and space.
The motility of cells undergoing convergent extension
becomes polarised along the medio-lateral axis of the
embryo and this directed movement may be the driving
force for subsequent medio-lateral cell intercalation and
axial extension. But how is this pattern of individual cell
motility translated into co-ordinated tissue rearrangement?
What signals orchestrate the complexity of these morphogenetic movements?
Signalling mediated by cell adhesion molecules such as
protocadherins [4], and secreted factors such as Wnts
(reviewed in [9]) are crucial in the regulation of morphogenetic movements. For example, inhibition and activation of Wnt ligands, Frizzled receptors and Dishevelled —
an intracellular signal component of the Wnt pathway —
can modulate convergent extension movements by regulating cell polarity [5–7]. Indeed, some Wnt ligands are
capable of stimulating intracellular calcium (Ca2+) release
and activating protein kinase C and calmodulin-dependent
kinase II (reviewed in [10]). These observations suggest
that Ca2+ signalling may contribute to the regulation of
convergent extension.
Ca2+ is a key regulator of a variety of biological processes
including cell locomotion (reviewed in [11]). For example,
the level of intracellular Ca2+ within the growth cone of a
developing neuron provides a directional clue for turning
Figure 1
Anterior
Animal
Vegetal
Medio-lateral
Posterior
Curent Biology
Model for convergent extension. Polarised
cells on the dorsal side of the gastrulating
Xenopus embryo intercalate in the mediolateral direction (arrows), to generate an
extension of the tissue along the anteriorposterior axis and a narrowing along the
medio-lateral axis (see [1]).
Dispatch
Figure 2
(a)
(a) Convergent extension and waves of Ca2+ signalling. In the
presence of intercellular Ca2+ waves, dorsal mesoderm cells within
Keller explants are able to undergo convergent extension. However,
when Ca2+ waves are abolished with pharmacological agents,
convergent extension movements fail to occur. Although the correlation
between the presence of the Ca2+ waves and the ability to undergo
convergent extension is strong, it remains unclear how Ca2+ facilitates
such movements and how Ca2+ waves influence, or are influenced by
other signalling pathways implicated in convergent extension. (b) A
possible scenario for the generation and propagation of intercellular
Ca2+ waves. The activity of a hypothetical extracellular factor X induces
the release of Ca2+ from the endoplasmic reticulum through the
activation of inositol triphosphate (IP3) signals via G-protein coupled
receptors. The increase in intracellular Ca2+ then triggers cellular
responses that mobilise IP3 or ATP signals to surrounding cells. IP3
may diffuse to adjacent cells through gap junctions to propagate the
Ca2+ wave across a large population of cells [15]. On the other hand,
ATP may be released into the extracellular space and activate adjacent
cells via purinergic receptors, which in turn transmit the Ca2+ wave
sequentially [14]. There is still the possibility that Ca2+ itself might
mobilise from cell to cell through gap junctions to act as a long-range
messenger. When Ca2+ release from the endoplasmic reticulum is
inhibited with cell-permeable inhibitors such as thapsigargin, the Ca2+
wave is abolished.
behaviour during axonal extension. How do cells subsequently translate changes in the level of intracellular Ca2+
into directed cell behaviour? Cells can generate signals that
mobilise Ca2+ molecules from either internal sources such
as the endoplasmic reticulum or from outside the cell via
influx channels. Such increases in intracellular Ca2+ are
often transient and are promptly restored to a resting state,
but are sufficient to trigger a series of Ca2+-sensitive cellular processes that can induce changes in the cytoskeleton.
Ca2+
Anterior
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Anterior
Ca2+
wave
Medio-lateral
No
wave
Posterior
Posterior
(b)
X
IP3
Ca2+ IP3
Ca2+
IP3
Ca2+
Ca2+
signalling, a role for
In addition to intracellular
in cell–cell communication is suggested by the presence of
intercellular Ca2+ waves which are coincident in time and
space with the morphogenetic movements of gastrulation
in both frog and zebrafish embryos [12,13]. Intercellular
Ca2+ signalling may therefore be a fundamental feature of
developmental tissues undergoing morphogenetic movements. In support of this view, a recent report by Wallingford et al. [8] reveals a link between intercellular Ca2+
waves and convergent extension thereby providing a mechanism by which large populations of cells could communicate to orchestrate such complex cell movements [8].
Wallingford et al. [8] show that intercellular Ca2+ waves
occur over distances of up to twenty cell diameters in cells
undergoing convergent extension in explants of Xenopus
gastrulae embryos. Waves are generated in 2–4 neighbouring cells, propagate to surrounding cells at a rate of
5 microns per second and at a frequency of 0.7 per hour;
thereafter Ca2+ levels rapidly return to the basal level.
Intercellular Ca2+ waves arise stochastically near the dorsal
lip of the mesoderm and are also observed to a lesser
ATP
Propagation
ATP
Curent Biology
extent in the neuroectoderm. The fact that Ca2+ waves are
absent in animal pole cells and in ventral marginal zone
explants, neither of which undergo pronounced convergent extension movements, indicates that these waves are
closely associated with cell populations undergoing convergent extension.
To test the possibility that such intercellular waves mobilise
Ca2+ from intracellular stores as in other systems, Wallingford et al. [8] made use of two pharmacological inhibitors
of the ATPase-dependent Ca2+ pump within the endoplasmic reticulum. Treatment of explants with either
thapsigargin or 2,5-di(t-butyl)-1,4-benzohydroquinione did
indeed suppress intercellular Ca2+ waves indicating a
requirement for intracellular release of Ca2+. Furthermore,
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these inhibitors inhibit, but do not completely block, convergent extension movements without changing cell fate
specification in both marginal zone explants and in whole
embryos (Figure 2a). Together, these data indicate a role
for Ca2+ signalling in regulating convergent extension
during gastrulation.
What might be responsible for initiating the intercellular
Ca2+ wave in the dorsal mesoderm? The fact that some
Wnt proteins play roles in both stimulating intracellular
Ca2+ release and co-ordinating convergent extension movements raises the possibility that the intercellular Ca2+
waves are dependent upon Wnt signals. To test this idea,
Wallingford et al. [8] showed that a dominant-negative
form of the Wnt receptor, Frizzled-8, suppressed convergent extension by inhibiting both canonical and noncanonical Wnt pathways. Surprisingly, intercellular Ca2+
waves are still observed in explants expressing dominantnegative Frizzled-8 although their frequency is slightly
reduced. This result argues either that Wnt signalling may
not be fully inhibited by the truncated Frizzled protein or
that the Wnt/Ca2+ pathway is not essential for either the
initiation or propagation of the Ca2+ wave. Further experiments are required to elucidate the relationship between
Wnt and Ca2+ signals, but for now the mechanisms underlying the generation of the Ca2+ waves remain unclear.
Once Ca2+ waves are initiated, their intercellular propagation may involve either the release of ATP to the surrounding cells via a relay mechanism [14] or the movement
of inositol triphosphate as a second messenger through gap
junctions [15] (Figure 2b). The latter possibility is supported by the fact that intercellular inositol triphosphate
waves have been observed in association with intercellular
Ca2+ waves in canine kidney cells [16]. Future studies are
needed to dissect between these two possibilities.
Perhaps most crucial of all is to understand how Ca2+
signals are able to influence cell behaviours. One candidate class of proteins that might be able to sense the propagation of Ca2+ signals between cells and thus modulate
convergent extension movements are the Ca2+ dependent
cell adhesion molecules, cadherins. These molecules have
been implicated in a variety of biological processes, and
most notably paraxial protocadherin has been shown to be
required for proper convergent extension in Xenopus
embryos [4]. Therefore one key experiment will be to
determine if the activity of paraxial protocadherin or other
protocadherins is dependent upon, or influenced by, the
intercellular Ca2+ waves.
It has long been a challenge for developmental biologists
to elucidate the mechanisms by which cells know where
they are and where they must move within a tissue. A
deeper understanding of the mechanisms involved in the
generation, propagation and interpretation of intercellular
Ca2+ signals within the embryo promises to help us to
understand how large cell population behaviours are coordinated to generate shape within the embryo.
Acknowledgements
We would like to thank Steve Wilson for encouraging us to write this article
and Olga Zolle for discussions on calcium signalling. MT is supported by an
MRC Career Development Award and MC is supported by a Wellcome
Trust Fellowship and research within our group is supported by grants from
the Wellcome Trust, BBSRC and MRC to MT, MC and Steve Wilson.
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