Atoh1a expression must be restricted by Notch signaling for effective morphogenesis of the posterior lateral line primordium in zebrafish Sensory nerves of the lateral line ganglion innervate the sensory hair cells in the neuromasts. Together they form a part of a sensory system on the surface of fish that detects water flow. The posterior lateral line primordium (pLLp) is a cohesive collection of about a hundred cells. It migrates caudally under the skin in the zebrafish trunk and tail periodically depositing neuromasts from it trailing end to establish the posterior lateral line system. The migrating primordium contains 3-4 proneuromasts at various stages of maturation. As they mature a sensory hair cell is specified at the center of the proneuromast. Support cells, which also serve as pool of progenitors, surround the hair cell. As they mature the proneuromasts also form center-oriented epithelial rosettes. Eventually, mature proneuromasts are deposited from the trailing end of the migrating pLLp and sensory nerves of the lateral line ganglion innervate the sensory hair cells. Understanding the self-organization of the pLLp system and how its developmental fails under specific experimental conditions provides an attractive context to understand the broader mechanisms regulating organogenesis in the developing embryo. It is proving to be an especially attractive system for understanding how interaction between distinct signaling pathways contributes to self-organization and how specification of cell fate and morphogenesis is integrated within a developing sensory organ. The neuromasts are archetypical sense organs whose development and morphogenesis has remarkable similarities to diverse sensory organs like ommatidia in the Drosophila eye and hair cells of the mammalian ear. A Wnt-dependent FGF signaling center at the leading end of the pLLp initiates formation of proneuromasts by facilitating the reorganization of cells into epithelial rosettes and by initiating atoh1a expression. Expression of atoh1a gives proneuromast cells the potential to become sensory hair cells. However, lateral inhibition mediated by Delta-Notch signaling restricts atoh1a expression and hair cell progenitor fate to a central cell and surrounding cells are allowed to become support cells instead. We have shown that as atoh1a expression becomes established in the central cell, it drives expression of fgf10 and the Notch ligand, deltaD, while it inhibits expression of fgfr1. As a source of FGF10, the central cell activates the FGF pathway in neighboring cells, ensuring that they form stable epithelial rosettes. At the same time DeltaD activates Notch in neighboring cells, inhibiting atoh1a expression and ensuring that they are specified as supporting cells. In mind bomb mutants, loss of Notch-mediated lateral inhibition prevents atoh1a expression from being restricted to a central cell in a maturing proneuromast. Instead atoh1a expression progressively expands to surrounding cells and they are specified as hair cell progenitors instead of support cells. Unregulated atoh1a expression reduces FGFR1 expression, eventually resulting in attenuated FGF signaling, which prevents effective maturation of epithelial rosettes in the pLLp. Surprisingly, the progressive expansion of atoh1a expression not only expands the number hair progenitors and reduces FGF signaling in the migrating primordium, it also eventually leads to catastrophic fragmentation of the pLLp. In the past year our studies have shown that atoh1a expressing cells have a relatively low level of e-cadherin expression. As a consequence expanding atoh1a expression reduces e-cadherin expression in the migrating primordium, while leaving n-cadherin expression slightly expanded or unchanged. The reduction of e-cadherin expression is likely to contribute to reduced cohesion and fragmentation of the pLLp. In the neuromasts central hair cell precursors expressing atoh1a are surrounded support cells that do not express atoh1a. Furthermore, a population of non-sensory cells surrounds the support cells. Our studies suggest that while the central atoh1a-expressing hair cell progenitors in maturing neuromasts express only n-cadherin, non-sensory cells express only e-cadherin. As support cells express both n-cadherin and e-cadherin they can form effective adhesive interactions with both the outer non-sensory cells that express only e-cadherin and inner hair cell precursors that express only n-cadherin. In this manner support cells serve as a glue holding cells expressing only e-cadherin and n-cadherin together in both maturing and deposited neuromasts. Failure of Notch signaling allows prospective support cells that surround the central atoh1a-expressing cell to start expressing atoh1a, which prevents them from expressing e-cadherin. This prevents the cells from effectively adhering to surrounding non-sensory cells that express only e-cadherin and this we believe also contributes to fragmentation of the pLLp when Notch signaling is lost. We have developed a simple computational model of the migrating pLLp to illustrate how replacement of prospective support cells that normally express both e-cadherin and n-cadherin with atoh1a-expressing cells expressing only n-cadherin eventally contributes to fragmentation of the pLLp.
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