Inner ear connexins: role in hearing acquisition and DFNB1 pathophysiology

Hereditary deafness is a genetic sensory disturb that represents 60% of congenital deafness. Mutations in genes that encode for proteins named connexins are the main cause of hereditary deafness. Connexins form nanoscopic channel, called gap junction channels, that connect cells to each other and transfer glucose and other key metabolites, signaling molecules and ions, such as calcium ion. The sense of hearing is formed within the delicate structures of the inner ear. A tiny organ there, named after Maquis Alfonso Corti, hosts specialized sensory receptor cells which convert sound stimuli into electrical signals that travel to the brain along eight nerve fibers. In order to mature and function properly, these sensory cells need to be sustained and nourished by non-sensory cells, which make the majority of the Corti organ. Inner ear non-sensory cells are all coupled to each other through nanoscopic channels of connexins. In the inner ear, gap junction channels are formed by two types of connexin proteins, called connexin 26 (Cx26) and connexin 30 (Cx30). Thanks to Telethon support, our laboratory has been active on connexins research for several years and has developed tools and methodologies which are promising in view of finding a cure for deafness of genetic origin due to connexins mutations. We uncovered key mechanisms by which cochlear non-sensory cells talk to each other even at relatively long distance by exchanging signals that are carried by waves of intracellular calcium concentration (Ca2+ waves). We have shown that the exchange of these signals is crucial for the correct development of the Corti organ. Failure to initiate and propagate intercellular Ca2+ waves among cochlear non-sensory cells of juvenile mice leads to deafness in the adult stage. Failure can be localized to specific regions of the organ, and in this case mice develop selective hearing loss for certain sound frequencies but may hear normally at other frequencies. Having understood all this, we are now trying to recover normal hearing in these transgenic mice by delivering healthy Cx26 or Cx30 genes to their inner ear before degeneration of the Corti organ takes place. Our NIH partners uncovered a particular type of virus that does just that, called bovine adeno-associate virus, or BAAV. We have shown that BAAV is excellent at infecting non-sensory cell in organotypic cultures of the inner ear. We have also delivered BAAV to the inner ear of live wild type mice, and found that it can express exogenous reporting genes without compromising hearing. In the course of the newly funded Telethon project, we will engineer new BAAVs that can sustain connexin protein expression also when delivered to the inner ear of newborn mice. In parallel, we will continue to study Ca2+ waves in order to understand how their suppression affects the normal development of the Corti organ. Finally, we will use sophisticated algorithms to model connexin channels down to the atomic scale in the memory of a supercomputer. Our goal, in this case, is to delineate with the highest possible precision how connexin mutations hamper the exchange of ions and signaling molecules through gap junction channels.