Integrin-α5 Coordinates Assembly of Posterior Cranial Placodes in Zebrafish and Enhances Fgf-Dependent Regulation of Otic/Epibranchial Cells

Neha Bhat,Bruce B Riley,Christoph Winkler

doi:10.1371/journal.pone.0027778

Abstract

Vertebrate sensory organs develop in part from cranial placodes, a series of ectodermal thickenings that coalesce from a common domain of preplacodal ectoderm. Mechanisms coordinating morphogenesis and differentiation of discrete placodes are still poorly understood. We have investigated whether placodal assembly in zebrafish requires Integrin- α5 (itga5), an extracellular matrix receptor initially expressed throughout the preplacodal ectoderm. Morpholino knockdown of itga5 had no detectable effect on anterior placodes (pituitary, nasal and lens), but posterior placodes developed abnormally, resulting in disorganization of trigeminal and epibranchial ganglia and reduction of the otic vesicle. Cell motion analysis in GFP-transgenic embryos showed that cell migration in itga5 morphants was highly erratic and unfocused, impairing convergence and blocking successive recruitment of new cells into these placodes. Further studies revealed genetic interactions between itga5 and Fgf signaling. First, itga5 morphants showed changes in gene expression mimicking modest reduction in Fgf signaling. Second, itga5 morphants showed elevated apoptosis in the otic/epibranchial domain, which was rescued by misexpression of Fgf8. Third, knockdown of the Fgf effector erm had no effect by itself but strongly enhanced defects in itga5 morphants. Finally, proper regulation of itga5 requires dlx3b/4b and pax8, which are themselves regulated by Fgf. These findings support a model in which itga5 coordinates cell migration into posterior placodes and augments Fgf signaling required for patterning of these tissues and cell survival in otic/epibranchial placodes.

Highlights

Development of cranial sensory organs in vertebrates requires essential contributions from transient embryonic structures termed cranial placodes
Proper regulation of itga5 requires dlx3b/4b and pax8, which are themselves regulated by Fgf. These findings support a model in which itga5 coordinates cell migration into posterior placodes and augments Fgf signaling required for patterning of these tissues and cell survival in otic/epibranchial placodes
We examined the effects of Fgf misexpression on cell migration in itga5 morphants

Summary

Introduction

Development of cranial sensory organs in vertebrates requires essential contributions from transient embryonic structures termed cranial placodes. The otic placode produces the entire inner ear, including the complex epithelial labyrinth, internal sensory epithelia, and all of its innervating neurons; and trigeminal and epibranchial placodes produce a segmental array of sensory ganglia that innervate much of the craniofacial and pharyngeal apparatus Despite their morphological and functional diversity, all cranial placodes arise from a common domain of preplacodal ectoderm that forms earlier around the anterior neural plate [2,3]. Specification of preplacodal ectoderm involves a sequence of signaling interactions that occur during blastula and gastrula stages, culminating in expression of a characteristic set of transcription factor genes near the end of gastrulation [4,5,6,7,8] This contiguous domain of gene expression subsequently breaks into discrete clusters of cells that generate the various diverse placodes. These data support a model in which itga coordinates directed cell migration into posterior placodes and augments Fgf signaling to promote cell survival and tissue patterning within the otic/epibranchial domain

Results

Discussion

Findings

Materials and Methods