The invertebrate innexins are non-homologous functional equivalents of the connexin family of vertebrate gap junction proteins. Recently the pannexins, a vertebrate family of proteins with some sequence similarity to innexins, have been shown to form gap junctions. Gap junctions are clusters of intercellular channels that allow small molecules to directly pass between cells and serve as electrical synapses between neurons. Channels are formed when a hemi-channel on one cell surface recognizes and docks with a hemi-channel on an apposing cell. Each hemi-channel consists of six gap junction protein subunits that may be homomeric or heteromeric. The process by which gap junction subunits associate with one another to form homomeric or heteromeric hemi-channels is poorly understood. Additionally, hemi-channels on apposing cells that dock with one another may be identical in sub-unit make-up (homotypic) or of mixed type (heterotypic). The recognition determinants for hemi-channel docking are also poorly understood.
unc-7 and
unc-9 mutants share identical uncoordinated forward and backward locomotion phenotypes, and both genes encode innexins. An
unc-9(
fc16)
daf-6 unc-7(
e5) mutant we have constructed exhibits the same uncoordination as
unc-9 or
unc-7 single mutants, supporting an interaction between their corresponding gene products. Using antibody specific to UNC-7 and an
unc-9::gfp construct, we have found that these innexins co-localize extensively throughout the ventral and dorsal nerve cords. Additionally we have found that the
unc-7 locus encodes at least 2 isoforms of UNC-7 that differ only in their amino termini. Expression of the smaller of these isoforms, UNC-7S, is capable of rescuing forward locomotion in an
unc-7(
e5) background, but does not rescue the uncoordinated backward movement. UNC-7S co-localizes with UNC-9::GFP in the vnc and can be visualized at what we believe to be gap junctions formed between the AVB interneurons controlling forward movement and the B class motor neurons implementing forward locomotion. When expressed in
unc-9 daf-6 unc-7 or
unc-9 animals, UNC-7S is mis-localized, suggesting the requirement of UNC-9 for proper localization. Currently we cannot distinguish between models whereby UNC-7 and UNC-9 interact in the same hemi-channel as heteromers, or in apposing heterotypic hemi-channels, or both. We are currently using heterologous promoters to address this question. We hope these studies will provide insight into how the specificity of electrical synapse formation in the nervous system is established.