Background: ClC anion channels are ubiquitous and have been identified in organisms as diverse as bacteria and humans. Despite their widespread expression and likely physiological importance, the function and regulation of most ClCs are obscure. The nematode Caenorhabditis elegans offers significant experimental advantages for defining ClC biology. These advantages include a fully sequenced genome, cellular and molecular manipulability, and genetic tractability. Results: We show by patch clamp electrophysiology that C. elegans oocytes express a hyperpolarization- and swelling-activated Cl- current with biophysical characteristics strongly resembling those of mammalian ClC-2. Double-stranded RNA-mediated gene interference (RNAi) and single-oocyte RT-PCR demonstrated that the channel is encoded by clh-3, one of six C. elegans ClC genes. CLH-3 is inactive in immature oocytes but can be triggered by cell swelling. However, CLH-3 plays no apparent role in oocyte volume homeostasis. The physiological signal for channel activation is the induction of oocyte meiotic maturation. During meiotic maturation, the contractile activity of gonadal sheath cells, which surround oocytes and are coupled to them via gap junctions, increases dramatically. These ovulatory sheath cell contractions are initiated prematurely in animals in which CLH-3 expression is disrupted by RNAi. Conclusions: The inwardly rectifying Cl- current in C. elegans oocytes is due to the activity of a CIC channel encoded by clh-3. Functional and structural similarities suggest that CLH-3 and mammalian ClC-2 are orthologs. CLH-3 is activated during oocyte meiotic maturation and functions in part to modulate ovulatory contractions of gap junction-coupled gonadal sheath cells.
Bibliographical noteFunding Information:
We thank Lou DeFelice, David Miller, Luis Reuss, and Michael Welsh for helpful advice and critical reading of the manuscript. This work was supported by a National Institutes of Health National Research Service Award to E. R. (GM20764), a National Kidney Foundation postdoctoral fellowship to L. B., and National Institutes of Health Grants to D. G. (R01 GM57173), A. G. (R01 AR44506), and K. S. (R01 DK51610 and P01 DK58212).