Two or more gap junction (GJ) proteins (connexins, pannexins, and innexins) are often coexpressed in the same cells. A poorly resolved question is how different GJ proteins contribute to electrical coupling of the same cells. Analyses of C. elegans body-wall muscle might shed light on this question because contributions of different GJ proteins may be evaluated by a combination of electrophysiological and genetic approaches. We previously showed that electrical coupling of the muscle cells is deficient but not absent in a null mutant of the innexin
unc-9, suggesting that UNC-9 and at least one additional innexin play roles in the coupling. To identify the remaining innexin(s) contributing to the coupling, we analyzed expression patterns for the remaining 24 members of the worm innexin family by expressing promoter::GFP transcriptional fusions. This analysis led to the identification of five more innexins expressed in body-wall muscle cells, including INX-8, INX-9, INX-11, INX-14 and INX-18. Comparisons of junctional currents (Ij), which are through intercellular channels, between wild-type and mutant worms suggest that INX-8, INX-9 and INX-11 do not contribute to the coupling. INX-14 also did not appear to play a role in the coupling because it was not localized to intercellular junctions. In contrast, Ij was significantly reduced in
inx-18 mutant and completely absent in
inx-18;
unc-9 double mutant, suggesting that INX-18 is an important contributor to the coupling. Immunohistochemistry with worms expressing epitope-tagged INX-18 and UNC-9 in muscle cells showed that INX-18 colocalized with UNC-9 as puncta at intercellular junctions. Nevertheless, INX-18 and UNC-9 localizations were independent, as determined by analyzing localization of one innexin in mutant of the other.
inx-18 mutant exhibited a significant increase in the bending amplitude of locomotion. Because
inx-18 expression was not detected in any neurons important to locomotion, this mutant phenotype suggests that the function of INX-18 in muscle cells is important to generating the normal locomotion waveform. Collectively, our observations suggest that INX-18 and UNC-9 are the most important contributors to electrical coupling of body-wall muscle cells.