Building the Posterior Lateral Line system In Zebrafish Embryos

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

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Abstract

BACKGROUND Formation of the Posterior Lateral Line system in zebrafish is pioneered by the posterior Lateral Line (pLL) primordium, a group of about 150 cells that forms near the ear. While leading cells in the pLL primordium have a relatively mesenchymal morphology, trailing cells are more epithelial; they have distinct apical basal polarity and they reorganize to sequentially form nascent neuromasts or protoneuromasts. The pLL primordium begins migration toward the tip of the tail at about 22 hours post fertilization (hpf). Proliferation adds to the growth of the primordium, nevertheless, as the primordium migrates, the length of the column of cells undergoing collective migration progressively shrinks as cells stop migrating and are deposited from the trailing end: cells that were incorporated into protoneuromasts are deposited as neuromasts, while cells that were not, are deposited between neuromasts as interneuromast cells. Eventually, the primordium ends its migration a day later after depositing 5-6 neuromasts and by resolving into 2-3 terminal neuromasts. Establishment of polarized Wnt and FGF signaling systems coordinates morphogenesis and migration of the primordium: Wnt signaling dominates at the leading end and is thought to determine the relatively mesenchymal morphology of leading cells, while FGF signaling dominates in the trailing end. There, FGF determines reorganization of groups of trailing cells to form rosettes as they constrict at their apical ends. Furthermore, FGF signaling determines the specification of a central cell in each rosette as a sensory hair cell progenitor and it helps determine collective migration of the pLL primordium cells. Wnt signaling promotes its own activity and at the same time drives expression of fgf3 and fgf10. However, leading cells do not respond to these FGF ligands because Wnt signaling simultaneously promotes expression of intracellular inhibitors of the FGF receptor. Instead, the FGFs activate FGF receptors and initiate FGF signaling at the trailing end of the primordium, where Wnt signaling is weakest. There, FGF signaling determines expression of the diffusible Wnt antagonist Dkk1b, which counteracts Wnt signaling to help establish stable FGF responsive centers. Once established, the trailing FGF signaling system coordinates morphogenesis of nascent neuromasts by simultaneously promoting the reorganization of cells into epithelial rosettes and by initiating expression of factors that help specify a sensory hair cell progenitor at the center of each forming neuromast. Over time, the leading domain with active Wnt signaling shrinks closer to the leading edge and additional FGF signaling centers form sequentially in its wake, each associated with formation of additional protoneuromasts. SoxB1 family members inhibit Wnt signaling to facilitate stable maturation and deposition of neuromasts by the migrating zebrafish Posterior Lateral Line primordium Greg Palardy, Sana Fatma, Abhishek Mukherjee, Chongmnin Wang and Ajay Chitnis Periodic formation of protoneuromasts within the migrating zebrafish Posterior Lateral Line primordium serves as a model for understanding steps that determine the self-organization of organ systems in development. Protoneuromast formation is initiated by Fgf signaling at the trailing zone of the migrating primordium in response to Fgfs produced by Wnt active cells in a leading zone. Progressive restriction of an initially broad Wnt signaling domain to a smaller leading zone allows new Fgf signaling-dependent protoneuromasts to form in the wake of the shrinking Wnt system. We show Sox2 and Sox3 are expressed in nascent and maturing protoneuromasts in a trailing part of the primordium in a pattern that is complementary to Wnt activity in a leading domain, where Sox1a, is expressed. Together, these SoxB1 factors help inhibit Wnt signaling to determine effective maturation of trailing protoneuromasts and the timely deposition of stable neuromasts. We can now define three steps in the periodic formation of neuromasts: first, a step that polarizes Wnt activity in the primordium, second, a pattern forming step that determines periodic formation of protoneuromasts in the context of polarized Wnt activity, and third, a step where inhibition of Wnt signaling is essential for stabilizing nascent neuromasts formed in the earlier pattern forming stage. Signaling and Mechanics influence the number and size of epithelial rosettes in the migrating zebrafish Posterior Lateral Line primordium Abhishek Mukherjee, Michael Hilzendeger, Arin Rinvelt, Sana Fatma, Megan Schupp, Damian Dalle Nogare and Ajay B Chitnis A prepattern of Fgf signaling triggers formation of epithelial rosettes as protoneuromasts form periodically in the migrating Posterior Lateral Line primordium. However, the number and size of epithelial rosettes is influenced by the balance of mechanical interactions that promote or oppose their formation. Selective slowing of leading cells in the primordium can result in the fusion of two rosettes to form one larger one, while slowing of trailing cells can result in splitting of a previously formed rosette to form two smaller ones. These observations can be accounted for by mechanics-based models, where local interactions associated with apical constriction and cell adhesion promote formation of rosettes, while tension along the length of the primordium, influenced by the relative efficacy of leading and trailing cell migration, opposes their formation. We describe computational models that illustrate how the relative speed of leading versus trailing cells, as well as changes in cell adhesion and mechanical coupling, can influence the pattern of protoneuromast formation and deposition by the migrating primordium. Our studies illustrate how signaling and mechanics together influence morphogenesis in the migrating primordium.