The roles of Shroom family proteins during Xenopus development
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The Shroom family of proteins is currently comprised of four members, Shroom1, 2, 3 and 4. Since Shroom3 was shown to be a critical protein for neural tube closure, the other three proteins are also expected to play an important role for proper development. However, their functions during development were not clear. To address this, my study started with Shroom3 function in the neural plate. Shroom3 had been previously known to induce apical constriction by controlling actin filaments in neuroepithelial cells. My studies show that Shroom3 induces apico-basal cell heightening by controlling parallel microtubule assembly. Shroom3 is able to change the distribution of γ-tubulin, suggesting that Shroom3 controls apical constriction and apico-basal cell elongation via both actin filaments and microtubules. The ability to control γ-tubulin distribution is possessed not only by Shroom3, but also by all other Shroom proteins, although they can not induce apical constriction. In addition, they are expressed in tissues which contain apico-basally elongated cells. Data from functional assays with Shroom2 show that it induces cell elongation and is required for proper cell shape in deep layer neuroepithelial cells in Xenopus. These data suggest that Shroom family proteins control cell architecture during morphogenetic development. I have discovered another role for Shroom2. By comparative analysis with Xenopus and Physalaemus, which have different pigment patterns in eggs, I show that a high level of maternal Shroom2 mRNA is important for pigment polarity in Xenopus. Furthermore, Shroom2 controls the distribution of spectrin which plays a role in pigment granule movement. Thus, Shroom2 is suggested to be a key molecule to control the pigment polarity in amphibian eggs. Together all these data suggest that Shroom family proteins play a role in cell morphogenesis and polarization via controlling the cytoskeleton during Xenopus development.