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Vertebrate neural tube formation involves two distinct morphogenetic events--convergent extension (CE) driven by mediolateral cell intercalation, and bending of the neural plate driven largely by cellular apical constriction. However, the cellular and molecular biomechanics of these processes are not understood. Here, using tissue-targeting techniques, we show that the myosin IIB motor protein complex is essential for both these processes, as well as for conferring resistance to deformation to the neural platetissue. We show that myosin IIB is required for actin-cytoskeletal organization in both superficial and deep layers of the Xenopus neural plate. In the superficial layer, myosin IIB is needed for apical actin accumulation, which underlies constriction of the neuroepithelial cells, and that ultimately drive neural plate bending, whereas in the deep neural cells myosin IIB organizes a cortical actin cytoskeleton, which we describe for the first time, and that is necessary for both normal neural cell cortical tension and shape and for autonomous CE of the neural tissue. We also show that myosin IIB is required for resistance to deformation ("stiffness") in the neural plate, indicating that the cytoskeleton-organizing roles of this protein translate in regulation of the biomechanical properties of the neural plate at the tissue-level.
Fig. S1. Targeting MHC-B MO to neural tissue does not alter neural cell fates. Stage 17 un-injected control embryos and neural targeted MHC-B morphants stained by in situ hybridization show similar patterns of expression of N-CAM, a pan neural marker, N-tubulin, a marker for prospective primary neuron, Slug, a neural crest marker and Rx1, an eye-field marker, indicating that neural patterning is not affected by MHC-B MO.
Fig. 3.
Anterior neural tube closure is asymmetrical in unilaterally neural plate targeted morphants. Movements of colored tracking dots on frames of a time-lapse recording of an anterior view of neural plate (A) show that points on the control (left) side move towards the midline faster than points on the morphant (right) side of the embryo. A transverse section through the prospective brain region of a stage 18 embryo stained with rhodamine phalloidin (B) shows strong apical actin accumulation on the control (left) side, but no actin accumulation on the morphant (right) side. Transverse sections through the prospective brain (C) and spinal cord (D) regions of a stage 17 embryo show background green fluorescent dextran used to determine cell shape, and red fluorescent dextran co-injected with the MHC-B MO used to identify morphant cells (right sides). Corresponding tracings of cell outlines (CD show that un-injected cells adopt typical bottle cell morphologies, with long thin necks and constricted apices, whereas morphant cells (low asterisk) are short and have broad apices.
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