The genetics of hair-cell function in zebrafish

Abstract

Our ears are remarkable sensory organs, providing the important senses of balance and hearing. The complex structure of the inner ear, or 'labyrinth', along with the assorted neuroepithelia, have evolved to detect head movements and sounds with impressive sensitivity. The rub is that the inner ear is highly vulnerable to genetic lesions and environmental insults. According to National Institute of Health estimates, hearing loss is one of the most commonly inherited or acquired sensorineural diseases. To understand the causes of deafness and balance disorders, it is imperative to understand the underlying biology of the inner ear, especially the inner workings of the sensory receptors. These receptors, which are termed hair cells, are particularly susceptible to genetic mutations - more than two dozen genes are associated with defects in this cell type in humans. Over the past decade, a substantial amount of progress has been made in working out the molecular basis of hair-cell function using vertebrate animal models. Given the transparency of the inner ear and the genetic tools that are available, zebrafish have become an increasingly popular animal model for the study of deafness and vestibular dysfunction. Mutagenesis screens for larval defects in hearing and balance have been fruitful in finding key components, many of which have been implicated in human deafness. This review will focus on the genes that are required for hair-cell function in zebrafish, with a particular emphasis on mechanotransduction. In addition, the generation of new tools available for the characterization of zebrafish hair-cell mutants will be discussed.

Keywords: Hair cell; deafness gene; hair bundle; inner ear; lateral line organ; mechanotransduction; zebrafish.

Figure 1.

Genes required for hair-cell function in larval zebrafish. A, Dorsal view of the head region of a 5 dpf larva. The focal plane is centered on the two transparent inner ears, which contain two macular endorgans associated with otoliths, known as the anterior and posterior maculae (destined to become the utricle and saccule, respectively), and three developing semi-circular canals. Right panels, example of a cluster of hair cells in a crista positioned within a semi-circular canal, and a superficial neuromast of the lateral line organ. At this stage, approximately 50 neuromasts are present at the surface of the skin. B, Genes implicated in deafness and/or vestibular dysfunction in zebrafish. Mutations were generated by chemical (ethylnitrosourea) or retroviral mutagenesis for large-scale screens, or by gene editing technology. Three general categories are shown: genes that are required for (i) mechanotransduction, with protein products localized to the hair bundle (top group), or genes required for (ii) the function of the secretory pathway (middle group), and genes required for (iii) normal synaptic transmission (lower group). The excitatory axis of the hair bundle is indicated by the white arrow. k, kinocilium; hb, hair bundle (stereocilia); hc, hair cell; sc, supporting cell. Scale bars, 5 μm.

Figure 2.

Working model of the mechanotransduction machinery in zebrafish hair cells. A, Transmission electron micrograph of a tip link of a zebrafish hair cell (anterior macula). Note the tenting of the lower tip, suggesting that tension on the membrane was present. Also present is the upper density or plaque in the taller stereocilium. B, Schematic of the transduction components, including the presumptive orientation of the novel cadherins, Pcdh15a and Cdh23, and the position of the transduction channel at the lower link. C, Integral membrane proteins associated with Pcdh15a that are critical for mechanotransduction in zebrafish. The requirement for the Tmc proteins, which are currently the best candidates for the mechanotransduction channel in mammalian hair cells, has yet to be demonstrated in zebrafish. However, Pcdh15a physically associates with Tmc2a (see Maeda et al., 2014).