Deafness is one of the most common hereditary diseases, affecting 1 out of 1,000 children. Geneticists have made remarkable progress in identifying human genes responsible for deafness, yet in many cases, we do not clearly understand either the function of these genes or the pathology caused by the mutations. Moreover, our understanding of the sense of hearing at the molecular level lags behind our knowledge of other senses, such as vision, taste, and touch.
To gain insight into the molecular basis of mechanotransduction and synaptic transmission in sensory hair cells, we are taking advantage of both forward and reverse genetics in the model vertebrate organism, the zebrafish. We have identified more than two dozen genes required for larval auditory and vestibular function from large-scale mutagenesis screens, and we have cloned several zebrafish genes, including myosinVIIa, cadherin23, protocadherin15a, and myosinVIb. Orthologs of these genes are responsible for deafness in both humans and mice, demonstrating the relevance of our screen and a high degree of conservation of gene function.
Cadherin 23 and Protocadherin 15 are particularly interesting. Evidence from our laboratory and others supports the hypothesis that these two cadherins, which have unusually long extracellular domains, form a crucial part of the mechanotransduction apparatus—the tip link or extracellular filament that is thought to gate the transduction channel. We are currently exploring the role of these unusual cadherins in mediating the first mechanosensitive responses in developing hair cells.
In addition to components critical for transduction, we have also identified genes required for synaptic transmission, including the L-type calcium channel cav1.3a and the vesicular glutamate transporter vglut3 genes. Synaptic transmission in vertebrate hair cells occurs at highly specialized ribbon synapses that are capable of releasing hundreds of synaptic vesicles. Current models of ribbon function hypothesize that the ribbon body transports synaptic vesicles to the active zone and/or coordinates vesicle release. To understand the biogenesis and function of the hair-cell ribbon, we are creating tools such as transgenic lines that will allow us to elucidate its role in faithful transmission of auditory and vestibular signals to first-order neurons.
Defects in synaptic transmission in zebrafish hair cells can be assessed by recording from postsynaptic neurons such as the posterior lateral line neurons. These neurons synapse with sensory hair cells located at the surface of the skin of the animal. Superficial or lateral line sensory hair cells are important for detecting movement of water near the head and trunk of the fish. The cell bodies of the lateral line neurons also lie close to the surface of the animal, enabling us to record from whole, undissected larvae.
We have also established an additional technique of measuring hair-cell activity by using a hair-cell specific promoter to generate transgenic lines expressing genetic calcium indicators. These transgenic lines allow us to examine calcium transients in both the hair bundle and at synaptic ribbons in mutant hair cells, yielding new insights into gene function.
By combining genetic, behavioral, and electrophysiological data, we aim to gain a better understanding of the basic molecular mechanisms involved in transduction and transmission in hair cells, and to apply our findings to understanding the pathology of congenital deafness in humans.