Ca2+ permeability and Na+ conductance in cellular toxicity caused by hyperactive DEG/ENaC channels
Hyperactivated DEG/ENaC channels cause neuronal death mediated by intracellular Ca overload. Mammalian ASIC1a channels and MEC-4(d) neurotoxic channels in Caenorhabditis elegans both conduct Na and Ca, raising the possibility that direct Ca influx through these channels contributes to intracellular Ca overload. However, we showed that the homologous C. elegans DEG/ENaC channel UNC-8(d) is not Ca permeable, yet it is neurotoxic, suggesting that Na influx is sufficient to induce cell death. Interestingly, UNC-8(d) shows small currents due to extracellular Ca block in the Xenopus oocyte expression system. Thus, MEC-4(d) and UNC-8(d) differ both in current amplitude and Ca permeability. Given that these two channels show a striking difference in toxicity, we wondered how Na conductance vs. Ca permeability contributes to cell death. To address this question, we built an UNC-8/MEC-4 chimeric channel that retains the calcium permeability of MEC-4 and characterized its properties in Xenopus oocytes. Our data support the hypothesis that for Ca-permeable DEG/ENaC channels, both Ca permeability and Na conductance contribute to toxicity. However, for Ca-impermeable DEG/ENaCs (e.g., UNC-8), our evidence shows that constitutive Na conductance is sufficient to induce toxicity, and that this effect is enhanced as current amplitude increases. Our work further refines the contribution of different channel properties to cellular toxicity induced by hyperactive DEG/ENaC channels.