Mutations in the connexin26 (Cx26) gene (GJB2) are the predominant cause of inherited syndromic sensorineural deafness in humans. While most mutant connexins result in non-functional channels, some are able to form functional gap junction channels and to mediate electrical coupling (e.g., are permeable to K+ ions), yet they cause deafness. This suggests that the pathology is caused by defect(s) in intercellular movement of cytoplasmic substances other than potassium ions. It is now recognized that channels formed by each of the approximate 20 varieties of connexin differ from one another with regard to molecular permeability, and recent studies show that they can have dramatic and highly specific selectivity's among second messengers and other cytoplasmic molecules. Our hypothesis is that the deafness-causing yet functional mutants of Cx26 have altered permeability to cytoplasmic molecules, and that this defective permeability cannot be compensated for by the other connexins expressed in the inner ear. Our experimental plan is to determine the differences in selectivity of channels that contain WT and deafness-causing mutant Cx26 with regard to second messengers and other cytoplasmic molecules. In doing so, we hope to pinpoint, or narrow down the possibilities for, the specific defect in intercellular molecular movement that causes the deafness. In the inner ear almost all the channels that contain Cx26 also contain Cx30, so it is likely that the key difference in permeability is that between Cx26/Cx30 heteromeric channels that contain WT Cx26 or mutant Cx26. The proposed studies emphasize investigation studies of these heteromeric channels. The projects utilize a well-characterized reconstitution system to study native and heterologously-expressed connexin hemichannels to obtain information about connexin perm selectivity and its structural basis that has been long desired, and unavailable by other means. We will use the techniques that we have previously applied to study second messenger selectivity of other connexin channels. We anticipate that that our findings will elucidate the molecular mechanisms by which Cx26 mutants cause deafness, and identify modes of intercellular communication that are required for normal cochlear function. The information obtained will be informative regarding connexin channel function in other contexts, and will lead to insights about mechanisms of selectivity among cytoplasmic permeants.