RADIATION LINKS TRPM2, MITOCHONDRIA, AND SOCE TO CAUSE LOSS OF SALIVARY GLAND FUNCTION
Salivary gland fluid secretion occurs as sequalae to neurotransmitter stimulation of PIP2 hydrolysis, IP3 generation, and a sustained increase in cytosolic [Ca2+] ([Ca2+]i) that is primarily dependent on Store-Operated Calcium Entry (SOCE). The latter is critically required for regulating sustained fluid secretion and is mediated by the channels Orai1 and TRPC1 that are activated by by the ER-Ca2+ sensor protein, STIM1. Radiation treatment of the head and neck region causes irreversible loss of salivary fluid secretion, severely affecting the oral health of patients. There is poor understanding of the mechanisms underlying the loss of salivary gland fluid secretion. Towards identifying this mechanism, we have examined the possible role of Transient Receptor Potential Melastatin-like 2 (TRPM2), a Ca2+-permeable nonselective cation channel that is activated by increase in ROS, in radiation-induced salivary gland dysfunction. Our studies demonstrate that TRPM2 is present in salivary gland cells and is activated in response to radiation treatment. Importantly, deletion of TRPM2, or suppression of activation, converted irreversible loss of salivary gland function to a transient loss of function in a mouse model. We have identified that radiation induces persistent defect in SOCE which underlies the loss of salivary fluid secretion. Our findings show that critical early effects of radiation include TRPM2-mediated Ca2+ entry, increase in the mitochondrial [Ca2+] and reactive oxygen species, decrease in mitochondrial membrane potential, and activation of caspase-3. The latter mediates cleavage of stromal interaction molecule 1 (STIM1), inducing loss of SOCE. We suggest that targeting the mechanisms underlying the loss of STIM1 would be a potentially useful approach for preserving salivary gland function.