Mutation of the atrophin2 gene in the zebrafish disrupts signaling by fibroblast growth factor during development of the inner ear

Abstract

The development of the vertebrate inner ear depends on the precise expression of fibroblast growth factors. In a mutagenesis screen for zebrafish with abnormalities of inner-ear development and behavior, we isolated a mutant line, ru622, whose phenotypic characteristics resembled those of null mutants for the gene encoding fibroblast growth factor 8 (Fgf8): an inconsistent startle response, circular swimming, fused otoliths, and abnormal semicircular canals. Positional cloning disclosed that the mutant gene encodes the transcriptional corepressor Atrophin2. Both the Fgf8 protein and zebrafish ''similar expression to fgf genes'' protein (Sef), an antagonist of fibroblast growth factors induced by Fgf8 itself, were found to be overexpressed in ru622 mutants. We therefore hypothesized that an excess of Sef eliminates Fgf8 signals and produces an fgf8 null phenotype in ru622 mutants. In support of this idea, we could rescue larvae whose atrophin2 expression had been diminished with morpholinos by reducing the expression of Sef as well. We propose that Atrophin2 plays a role in the feedback regulation of Fgf8 signaling. When mutation of the atrophin2 gene results in the overexpression of both Fgf8 and Sef, the excessive Sef inhibits Fgf8 signaling. The resultant imbalance of Fgf8 and Sef signals then underlies the abnormal aural development observed in ru622.auditory system ͉ hearing ͉ vestibular system T he complex structure of the inner ear is largely conserved across vertebrate species. In an adult animal, the inner ear contains several distinct sensory epithelia that are variously dedicated to the detection of linear acceleration, angular acceleration, and sound. Each sensory epithelium includes mechanosensory hair cells that are separated by nonsensory supporting cells and innervated by afferent terminals from the eighth cranial nerve (reviewed in ref.

Functional Architecture of Atrophins

“…Although the protein isoforms expressed by the zebrafish Atn2 homologues have not been described, the genomic structure is consistent with one or both of the loci being capable of expressing both Atr2L-like and Atn2S-like isoforms. There is also evidence suggesting that the function of Atn2 may be more tightly conserved between fish and mammals, as a role for Atn2 in the regulation of Fgf8 signaling has been described that is consistent with that in mice (15).…”

Rapid positional cloning of zebrafish mutations by linkage and homozygosity mapping using whole-genome sequencing

“…Zebrafish shares not only many of the same genes and genetic pathways with humans, but also similar cell types, tissue types, organs and developmental mechanisms that contribute to adult anatomy and physiology. For example, genetic screens in zebrafish identified mutations affecting the morphogenesis of the ear, regeneration of sensory hair cells and transduction of sound Malicki et al, 1996;Whitfield et al, 1996;Ernest et al, 2000;Sidi et al, 2003;Solomon et al, 2003;Kappler et al, 2004;Söllner et al, 2004;Starr et al, 2004;Nicolson, 2005;Seiler et al, 2005;Asai et al, 2006;López-Schier and Hudspeth, 2006;Schibler and Malicki, 2007;Obholzer et al, 2008;Abbas and Whitfield, 2009;Behra et al, 2009;Dutton et al, 2009;Gleason et al, 2009;Millimaki et al, 2010;Sweet et al, 2011). These discoveries increased our understanding and potentially our ability to treat human patients suffering from loss of hearing and balance.…”

Alcohol-Induced Morphological Deficits in the Development of Octavolateral Organs of the Zebrafish (Danio rerio)

“…40 One side of each anesthetized fish was examined at 225X to determine the presence or absence of individual ALL and PLL neuromasts using the Zeiss SteREO Discovery V20 fluorescence microscope with a GFP filter set. The intensity of fluorescent light stimulating embryos was calibrated with an X-Cite optical power measurement device (XR2100 and XP750, EXFO America, Richardson, TX) to ensure constant light intensity used for both control and experimental groups.…”