Low Conductance Gap Junctions Mediate Specific Electrical Coupling in Body-wall Muscle Cells of Caenorhabditis elegans

Qiang Liu,Bojun Chen,Eric Gaier,Jaya Joshi,Zhao-Wen Wang

doi:10.1074/jbc.m512382200

Abstract

Invertebrate innexins and their mammalian homologues, the pannexins, are gap junction proteins. Although a large number of such proteins have been identified, few of the gap junctions that they form have been characterized to provide combined information of biophysical properties, coupling pattern, and molecular compositions. We adapted the dual whole cell voltage clamp technique to in situ analysis of electrical coupling in Caenorhabditis elegans body-wall muscle. We found that body-wall muscle cells were electrically coupled in a highly organized and specific pattern. The coupling was characterized by small (350 pS or less) junctional conductance (G(j)), which showed a bell-shaped relationship with junctional potential (V(j)) but was independent of membrane potential (V(m)). Injection of currents comparable to the junctional current (I(j)) into body-wall muscle cells caused significant depolarization, suggesting important functional relevance. The innexin UNC-9 appeared to be a key component of the gap junctions. Both Myc- and green fluorescent protein-tagged UNC-9 was localized to muscle intercellular junctions. G(j) was greatly inhibited in unc-9(fc16), a putative null mutant. Specific inhibition of UNC-9 function in muscle cells reduced locomotion velocity. Despite UNC-9 expression in both motor neurons and body-wall muscle cells, analyses of miniature and evoked postsynaptic currents in the unc-9 mutant showed normal neuromuscular transmission. These analyses provide a relatively detailed description of innexin-based gap junctions in a native tissue and suggest that innexin-based small conductance gap junctions can play an important role in processes such as locomotion.

Highlights

Channels [7] and hemichannels [9] when expressed in Xenopus oocytes
C. elegans body-wall muscle is potentially suitable for such analyses for several reasons: 1) C. elegans has only 95 body-wall muscle cells, which are organized in a regular pattern into four longitudinal strips called quadrants [24]; 2) gap junction-like structures have been identified in the muscle cells through ultrastructural analyses [25, 26]; and 3) innexin expression level in the muscle may be manipulated conveniently through genetic means in C. elegans
We found that low conductance gap junctions mediate cell-specific electrical coupling among body-wall muscle cells, the innexin UNC-9 is a major component of the gap junctions, and the electrical coupling is important to locomotion

Summary

Introduction

Channels [7] and hemichannels [9] when expressed in Xenopus oocytes. it has not been confirmed that mammalian pannexins form gap junctions in native tissues. It is yet to be confirmed whether any of the 25 innexins are expressed in C. elegans body-wall muscle cells to constitute the morphologically defined gap junctions [25, 26]. Two UNC-9 fusion proteins were expressed independently in body-wall muscle cells under the control of Pmyo-3.

Results

Conclusion