NRA-2, a Nicalin Homolog, Regulates Neuronal Death by Controlling Surface Localization of Toxic Caenorhabditis elegans DEG/ENaC Channels

Background: Mechanisms that regulate plasma membrane expression of neuronally toxic DEG/ENaC channels are unclear. Results: Disruption of ER NRA-2, a Nicalin homolog, enhances C. elegans MEC-10(d) DEG/ENaC neurotoxicity. Immunocytochemistry, TIRF imaging, and electrophysiological assays support that NRA-2 controls relative MEC-10(d) distribution between ER and cell surface to regulate channel activity levels. Conclusion: NRA-2 regulates surface expression of a mutant DEG/ENaC channel. Significance: NRA-2 Nicalin can modulate DEG/ENaC pathophysiology. Hyperactivated DEG/ENaCs induce neuronal death through excessive cation influx and disruption of intracellular calcium homeostasis. Caenorhabditis elegans DEG/ENaC MEC-4 is hyperactivated by the (d) mutation and induces death of touch neurons. The analogous substitution in MEC-10 (MEC-10(d)) co-expressed in the same neurons is only mildly neurotoxic. We exploited the lower toxicity of MEC-10(d) to identify RNAi knockdowns that enhance neuronal death. We report here that knock-out of the C. elegans nicalin homolog NRA-2 enhances MEC-10(d)-induced neuronal death. Cell biological assays in C. elegans neurons show that NRA-2 controls the distribution of MEC-10(d) between the endoplasmic reticulum and the cell surface. Electrophysiological experiments in Xenopus oocytes support this notion and suggest that control of channel distribution by NRA-2 is dependent on the subunit composition. We propose that nicalin/NRA-2 functions in a quality control mechanism to retain mutant channels in the endoplasmic reticulum, influencing the extent of neuronal death. Mammalian nicalin may have a similar role in DEG/ENaC biology, therefore influencing pathological conditions like ischemia.