Differing Properties of Human and Mouse Connexin26 Hemichannels and Gap Junctional Channels Expressed in Xenopus Oocytes

Abstract

Abstract: : Purpose: Gap junctional proteins (connexins) play an important role in signal transmission in sensory systems. Connexin26 (Cx26) hemichannels are implicated in the feedback mechanism by which horizontal cells regulate transmitter release from cone photoreceptors in teleost and turtle retinas. In addition, Cx26 is the major gap junctional protein found in the inner ear, and mutations in the Cx26–encoding gene is a leading cause of non–syndromic deafness. In this study we compare the electrophysiological characteristics of human Cx26 (hCx26), mouse Cx26 (mCx26), as well as mutants of both the human and mouse Cx26. Methods: Using the Xenopus oocyte expression system and the two–electrode voltage clamp technique, we studied the properties of hemichannels and gap junctional channels formed by hCx26, mCx26 and the mutated connexins. Results: Both hCx26– and mCx26–expressing cells produced constitutively open hemichannels with similar I–V relationships. However, there was a significant difference in the magnitude of the hemichannel current (currents in hCx26–expressing cells were 3–5 times larger). In paired oocytes, both hCx26 and mCx26–expressing cells formed gap junctional channels, but channel formation occurred less frequently with mCx26 (4 times less likely compared with hCx26–expressing cells). mCx26 also produced significantly smaller junctional conductances (G_j, ∼20 times smaller vs. hCx26–expressing cells). Additionally, G_j in both hCx26– and mCx26–expressing cells showed asymmetry in their gating, with a greater decay in G_j at positive V_j. Preliminary data suggests that there are differences also in the gating charge (∼2 in hCx26– and ∼3.5 in mCx26–expressing cells). Moreover, mutations in mCx26, e.g., by substituting phenylalanine for cysteine at residue 202 (mC202F), abolished the ability of the connexin to produce hemichannel currents or junctional conductance. Conclusions: Despite the high homology between hCx26 and mCx26, their behavior differs in terms of hemichannel current magnitude, G_j magnitude and gating at positive V_j. Mutating mCx26 at various loci, associated in the human Cx26 with non–syndromic deafness, may prevent the formation of functional hemichannels or gap junctional channels.

Keywords: gap junctions/coupling • retina