Atoh1a expression must be restricted by Notch signaling for effective morphogenesis of the posterior lateral line primordium in zebrafish

Abstract

SUMMARYThe posterior lateral line primordium (pLLp) migrates caudally, depositing neuromasts to establish the posterior lateral line system in zebrafish. A Wnt-dependent FGF signaling center at the leading end of the pLLp initiates the 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, and lateral inhibition mediated by Delta-Notch signaling restricts atoh1a expression to a central cell. We show that as atoh1a expression becomes established in the central cell, it drives expression of fgf10 and of 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. When Notch signaling fails, unregulated atoh1a expression reduces Fgfr1 expression, eventually resulting in attenuated FGF signaling, which prevents effective maturation of epithelial rosettes in the pLLp. In addition, atoh1a inhibits e-cadherin expression, which is likely to reduce cohesion and contribute to fragmentation of the pLLp. Together, our observations reveal a genetic regulatory network that explains why atoh1a expression must be restricted by Notch signaling for effective morphogenesis of the pLLp.

Gene-expression analysis of hair cell regeneration in the zebrafish lateral line

“…Accordingly, between 3 and 5 h after neomycin treatment (Fig. 4 A and B), we observed renewed activation of the Notch target genes her4.1, notch3, and sox2 that likely is caused by the upregulation of dld (60).…”

“…The Fgf pathway initiates dld and subsequently Notch signaling in the mouse cochlea, chicken inner ear, and zebrafish lateral line neuromasts (23,60). Both pathways are active in 5-dpf untreated neuromasts (Fig.…”

“…The atoh1a target deltaD (dld) is upregulated between 3-5 h after neomycin treatment, reflecting the appearance of hair cell precursors (Fig. 4 A and B) (60). Notch signaling must be reinitiated for proper fate specification into hair and support cells and proper organ size control (9,60).…”

“…4 A and B) (60). Notch signaling must be reinitiated for proper fate specification into hair and support cells and proper organ size control (9,60). Accordingly, between 3 and 5 h after neomycin treatment (Fig.…”

“…Because Notch signaling needs to be reinitiated for fate specification of hair and support cells and control of organ size (9,60), the Notch target genes her4.1, notch3, and sox2 are again upregulated between 3 and 5 h after neomycin treatment (Fig. 4 A and B).…”