Semicircular canal morphogenesis in the zebrafish inner ear requires the function ofgpr126(lauscher), an adhesion class G protein-coupled receptor gene

Abstract

Morphogenesis of the semicircular canal ducts in the vertebrate inner ear is a dramatic example of epithelial remodelling in the embryo, and failure of normal canal development results in vestibular dysfunction. In zebrafish and Xenopus, semicircular canal ducts develop when projections of epithelium, driven by extracellular matrix production, push into the otic vesicle and fuse to form pillars. We show that in the zebrafish, extracellular matrix gene expression is high during projection outgrowth and then rapidly downregulated after fusion. Enzymatic disruption of hyaluronan in the projections leads to their collapse and a failure to form pillars: as a result, the ears swell. We have cloned a zebrafish mutant, lauscher (lau), identified by its swollen ear phenotype. The primary defect in the ear is abnormal projection outgrowth and a failure of fusion to form the semicircular canal pillars. Otic expression of extracellular matrix components is highly disrupted: several genes fail to become downregulated and remain expressed at abnormally high levels into late larval stages. The lau mutations disrupt gpr126, an adhesion class G protein-coupled receptor gene. Expression of gpr126 is similar to that of sox10, an ear and neural crest marker, and is partially dependent on sox10 activity. Fusion of canal projections and downregulation of otic versican expression in a hypomorphic lau allele can be restored by cAMP agonists. We propose that Gpr126 acts through a cAMP-mediated pathway to control the outgrowth and adhesion of canal projections in the zebrafish ear via the regulation of extracellular matrix gene expression.

International Union of Basic and Clinical Pharmacology. XCIV. Adhesion G Protein–Coupled Receptors

“…Adgrc1 (Celsr1) mouse mutants exhibit defects in PCP of the inner ear sensory epithelium, revealed by misorientation of the outer but not inner hair cells (Curtin et al, 2003). Furthermore, zebrafish mutant for adgrg6 (gpr126) have semicircular canal vestibular morphogenesis defects and thus vestibular dysfunction of the inner ear (Geng et al, 2013).…”

Development of the inner ear

“…The generation of these canals -involving the topological conversion of the otic vesicle into a labyrinth of interconnected ducts and chambers -is a fascinating problem for the developmental biologist (Figure 1d and e). Work in the zebrafish has identified an adhesion class G protein-coupled receptor, Gpr126 that is required for the early fusion step in canal formation [44 ]. Possible transcriptional targets for the Gpr126 signalling pathway in the ear include genes coding for various extracellular matrix (ECM) components: dynamic and spatially restricted expression of several ECM genes accompanies outgrowth of the epithelial projections that form the canal system.…”

Glial Cell Development and Function in Zebrafish

“…This hypothesis was put forward to explain phenotypic differences between zebrafish and mouse gpr126 mutants. Most Gpr126 mutant mice die before birth with defects in trabeculation of the developing heart, whereas the available zebrafish nonsense mutants are viable with no apparent heart defects (Monk et al 2009(Monk et al , 2011Waller-Evans et al 2010;Geng et al 2013;Patra et al 2013;Paavola et al 2014). A possible explanation is that some functions of Gpr126 are species-specific.…”