Congress Abstracts - Society for Developmental Biology

show that cell polarity is disrupted. This suggests that chem also participates as a negative regulator of proteins important for cell
polarity during development of the Drosophila embryo.
Program/Abstract # 192
A moving zone of actomyosin contractility drives epidermal zippering and neural tube closure in ascidian embryos
Hashimoto, Hidehiko; Robin, François; Sherrard, Kristin; Munro, Edwin (University of Chicago, USA)
Neural tube closure is a key morphogenetic event in chordate development, but its underlying mechanisms remain poorly understood.
We are investigating the cytomechanical basis for neural tube closure in ascidians - basal chordates that form a simple neural tube
with < 100 cells. Ascidian neurulation occurs by neuroectoderm (Ne) folding, followed by unidirectional "zippering" in which the
neural folds and adjacent epidermis (Epi) meet at the midline, then undergo junctional exchange (Ne/Epi -> Ne/Ne + Epi/Epi) in a
posterior-anterior progression to form a simple tube beneath a epidermal sheet. Combining time-lapse fluorescence microscopy and
immuno-staining of fixed embryos, we show that active non-muscle myosin II is highly enriched within a localized contractile zone
(CZ) just ahead of the moving zipper where individual Ne/Epi junctions undergo rapid shortening. Laser ablation experiments suggest
that junctional tension is highest in the CZ, and lower along newly formed Epi/Epi junctions just behind the zipper. Chemical
inhibiton of Rho kinase abolishes both myosin enrichment and the increased tension in the CZ, and prevents Ne/Epi junction
shortening and zipper progression. Kinetic analysis reveals that newly met epidermal cells remain transiently associated with the
zipper, become highly elongated as the zipper moves anteriorly, and then release from the zipper and relax towards more isodiametric
shapes. These data and computer simulations support a model in which localized actomyosin contractility ahead of the zipper provides
the driving force for zipper progression, while junctional release and cell shape relaxation behind the zipper creates an essential force
asymmetry to drive unidirectional zipper progression.
Program/Abstract # 193
The Claudin Family of Tight Junction Proteins Plays a Role in the Morphogenetic Movements that Drive Neural Tube Closure
in Chick
Baumholtz, Amanda; Simard, Annie; Collins, Michelle; Ryan, Aimee (McGill University, Canada)
Neural tube closure is dependent on the differentiation of ectoderm into neural and non-neural progenitors and the coordinated
morphogenetic movements of these populations of cells. Our lab has shown that members of the claudin family of tight junction
proteins are differentially expressed in the ectoderm prior to its differentiation into neural and non-neural progenitors and that these
expression patterns are maintained throughout neurulation. To test my hypothesis that claudins are important for differentiation and
for coordinating the morphogenetic movements of the ectoderm, I used the C-terminal domain of Clostridium perfringens enterotoxin
(C-CPE) to remove a subset of claudins from the ectoderm of chick embryos during neurulation. GST-treated control embryos
developed normally while GST-C-CPE-treated embryos had open neural tube defects (NTDs) that were classified according to their
location along the anterior-posterior axis and their similarity to human defects: 7% of the embryos had an open NTD at the anterior
end (anencephaly), 56% open at the posterior end (spina bifida) and 37% completely open (craniorachischisis). Whole mount in situ
hybridization analysis of GST and C-CPE-treated embryos showed that expression of genes in the ectoderm (Sox2 and AP2), genes
that demarcate the boundary between neural and non-neural ectoderm (Pax7), and genes differentially expressed along the anteriorposterior
neural ectoderm (Pax6 and Otx2) were normally expressed indicating that differentiation and anterior-posterior patterning of
the ectoderm occurred normally. These data suggest that claudins directly affect the morphogenetic movements required for neural
tube closure but not initial differentiation of cells in neural and non-neural ectoderm.
Program/Abstract # 194
Control of apical constriction by dynamic calcium signaling during Xenopus neural tube closure
Suzuki, Makoto, (National Institute for Basic Biology, Japan); Hara, Yusuke; Sato, Masanao; Nagai, Takeharu (The Institute of
Scientific and Industrial Research, Japan); Campbell, Robert (University of Alberta, Canada); Ueno, Naoto (National Institute for
Basic Biology, Japan)
During early development of the central nervous system (CNS), progenitor cells undergo a typical shape change, called apical
constriction, which makes the neural plate to bend mediolaterally to form the tubular structure. Actomyosin networks and their
regulators drive apical constriction, yet how dynamically it is controlled in time and space is not fully understood. In this study, we
investigated the possible role of calcium ion (Ca 2+ ) signaling using GFP/RFP-based Ca 2+ indicators G-GECO/R-GECO. We confirmed
that inhibition of Ca 2+ channels delayed Xenopus neural tube closure, suggesting that Ca 2+ signaling plays an important role(s) in
apical constriction. From the long-term time-lapse imaging, we found that dynamic intracellular Ca 2+ fluctuations occurred throughout
the neural plate at single-cell to whole-tissue levels. Spatio-temporal patterns of the Ca 2+ fluctuations appeared to be differentially
regulated by the Ca 2+ channel activity and the extracellular ATP, and the intracellular Ca 2+ increase temporally preceded the repeated
acceleration of the closing movements. Interestingly, the Ca 2+ increase also temporally correlated with apical constriction, and the
manipulation of cytoplasmic Ca 2+ by caged IP 3 caused cell shape change similar to apical constriction. These data suggest that
intracellular Ca 2+ is a positive regulator of apical constriction. Therefore, this dynamic Ca 2+ -dependent mechanism might act as a
compensatory system for Rho/ROCK-dependent apical constriction, and enable Xenopus embryos to ensure the primitive CNS
formation against environmental perturbations.
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