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 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, where it determines reorganization of groups of trailing cells into epithelial rosettes, 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. In past year our group has examined mechanisms that determine the dynamics of Wnt-FGF signaling in the pLL primordium, including those that determine its initial polarization. We have examined how progressive shrinking of the leading Wnt system and sequential formation of trailing FGF signaling centers in its wake accounts for the lineage and fate of cells in the primordium and how the rate at which the Wnt system shrinks helps predict the pace at which protoneuromasts form and are sequentially deposited by the migrating primordium. We have also collaborated with mathematicians at the University of British Columbia, Vancouver, to model dynamics of Wnt FGF signaling, the spontaneous polarization of these signaling systems and understand how it determines polarized migratory behavior of the primordium. A framework for understanding morphogenesis and migration of the zebrafish posterior Lateral Line primordium A description of zebrafish posterior Lateral Line (pLL) primordium development at single cell resolution together with the dynamics of Wnt, FGF, Notch and chemokine signaling in this system has allowed us to develop a framework to understand the self-organization of cell fate, morphogenesis and migration during its early development. The pLL primordium migrates under the skin, from near the ear to the tip of the tail, periodically depositing neuromasts. Nascent neuromasts, or protoneuromasts, form sequentially within the migrating primordium, mature, and are deposited from its trailing end. Initially broad Wnt signaling inhibits protoneuromast formation. However, protoneuromasts form sequentially in response to FGF signaling, starting from the trailing end, in the wake of a progressively shrinking Wnt system. While proliferation adds to the number of cells, the migrating primordium progressively shrinks as its trailing cells stop moving and are deposited. As it shrinks, the length of the migrating primordium correlates with the length of the leading Wnt system. Based on these observations we have shown how measuring the rate at which the Wnt system shrinks, the proliferation rate, the initial size of the primordium, its speed, and a few additional parameters allows us to predict the pattern of neuromast formation and deposition by the migrating primordium in both wild-type and mutant contexts. The mechanism that links the length of the leading Wnt system to that of the primordium remains unclear. However, FGFs, produced in response to Wnt signaling in leading cells, help determine collective migration of trailing cells, while a polarized response to a self-generated chemokine gradient serves as an efficient mechanism to steer primordium migration along its relatively long journey. In the context of these migratory mechanisms, we suggest that, as a source of diffusible factors required for collective migration of trailing cells, the length of the leading Wnt system determines at what point trailing cells eventually lose access to migratory cues produced by leading cells, stop migrating and are deposited. In this way, by determining when trailing cells can no longer participate in collective migration, the leading Wnt system determines the length of the column of cells in the migrating pLL primordium.
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