Sigma1 Receptor Inhibits TRPC1-Mediated Ca²⁺ Entry That Promotes Dopaminergic Cell Death

Regulation of Ca²⁺ homeostasis is essential for neuronal function and its survival. Recent data suggest that TRPC1 function as the endogenous store-mediated Ca²⁺ entry channel in dopaminergic cells, and loss of TRPC1 function leads to neurodegeneration; however, its regulation is not fully identified. Here we provide evidence that the sigma 1 receptor contributes to the loss of dopaminergic cells by blocking TRPC1-mediated Ca²⁺ entry. Importantly, downregulation of sigma 1 receptor expression significantly decreased neurotoxin-induced loss of dopaminergic cells as measured by MTT assays and caspase activity was also inhibited. Importantly, sigma 1 receptor inhibited TRPC1-mediated Ca²⁺ entry and silencing of sigma 1 receptor significantly restored store-dependent Ca²⁺ influx. Although co-immunoprecipitation failed to show an interaction between the TRPC1 and sigma 1 receptor, store depletion promoted a decrease in the sigma 1 receptor-STIM1 association. Neurotoxin-induced loss of Ca²⁺ entry was significantly restored in cells that had decreased sigma 1 receptor expression. Furthermore, TRPC1 or STIM1 silencing inhibited store-mediated Ca²⁺ entry, which was further increased upon the downregulation of the sigma 1 receptor expression. TRPC1 silencing prevented the increased neuroprotection and caspase activity observed upon the downregulation of sigma 1 receptor. Finally, sigma 1 receptor activation also significantly decreased TRPC1-mediated Ca²⁺ entry and lead to an increase in neurodegeneration. In contrast, addition of sigma 1 receptor antagonist prevented neurotoxin-induced neurodegeneration and facilitated TRPC1-mediated Ca²⁺ influx. Together these results suggest that the sigma 1 receptor is involved in the inhibition of TRPC1- mediated Ca²⁺ entry, which leads to the degeneration in the dopaminergic cells, and prevention of sigma 1 receptor function could protect neuronal cell death as observed in Parkinson's disease.