The neurotoxic MEC-4(d) DEG/ENaC sodium channel conducts calcium: implications for necrosis initiation

Abstract

Hyperactivation of the Caenorhabditis elegans MEC-4 Na(+) channel of the DEG/ENaC superfamily (MEC-4(d)) induces neuronal necrosis through an increase in intracellular Ca(2+) and calpain activation. How exacerbated Na(+) channel activity elicits a toxic rise in cytoplasmic Ca(2+), however, has remained unclear. We tested the hypothesis that MEC-4(d)-induced membrane depolarization activates voltage-gated Ca(2+) channels (VGCCs) to initiate a toxic Ca(2+) influx, and ruled out a critical requirement for VGCCs. Instead, we found that MEC-4(d) itself conducts Ca(2+) both when heterologously expressed in Xenopus oocytes and in vivo in C. elegans touch neurons. Data generated using the Ca(2+) sensor cameleon suggest that an induced release of endoplasmic reticulum (ER) Ca(2+) is crucial for progression through necrosis. We propose a refined molecular model of necrosis initiation in which Ca(2+) influx through the MEC-4(d) channel activates Ca(2+)-induced Ca(2+) release from the ER to promote neuronal death, a mechanism that may apply to neurotoxicity associated with activation of the ASIC1a channel in mammalian ischemia.

Activation of the Caenorhabditis elegans Degenerin Channel by Shear Stress Requires the MEC-10 Subunit

“…MEC-4 can form either a homomeric channel by itself or heterotrimeric channels with MEC-10, comprising two MEC-4 subunits and a MEC-10 subunit (7). Together, they form a channel that is permeable to Na ϩ and Ca 2ϩ (8,9) and sensitive to amiloride and specific amiloride derivatives (10).…”

A Method for Culturing Embryonic <em>C. elegans</em> Cells

“…Work done so far on cultured C. elegans embryonic cells supports that expression of GFP in specific cell types can be recapitulated in vitro 1,[18][19][20]27 (Figure 2A). Moreover the ratio of cells that express GFP in vitro is similar to the ratio found in vivo.…”

“…Cultured touch neurons also express the toxic mutant channel MEC-4(d), which causes death of the touch neurons in vivo 34 . Indeed, GFP expressing neurons prepared from a mec-4(d); pmec-4::GFP strain are initially present in culture but then degenerate 20 . Importantly, touch neurons prepared from mec-4(d); pmec-4::GFP behave in vitro similarly to how they behave in vivo.…”

Noncanonical cell death programs in the nematode Caenorhabditis elegans

“…77,[81][82][83] Genetic perturbations of other C. elegans channels, including those that reduce desensitization of the DEG-3 acetylcholine receptor 84,85 and expression of constitutively active Gas 86,87 (thought to induce glutamate excitotoxicity), can also initiate necrosis. Mutant ion channels that are all neurotoxic can conduct calcium, 83 and thus elevated calcium influx through plasma membrane ion channels appears to be a common necrosis-triggering event. In mammals, analogous glutamate excitotoxicity 88 and DEG/ENaC channel hyperactivation 89 are important in neuronal death consequent to stroke and ischemia.…”