Transient Receptor Potential Melastatin Channel-2 Mediates Microglial Activation and Migration After Injury

Abstract

TRPM7 and TRPM2 are non-selective cation channels of the Transient Receptor Potential channel superfamily and ‘melastatin’ subfamily. Each channel has gained attention for the role they play in mediating ischemic cell death in brain tissue, however their physiological expression and potential roles in the developing brain remained elusive. Real-time PCR assays revealed that TRPM7 mRNA expression peaks in the cortex 2-weeks after birth which correlates most closely with a period of rat brain development associated with neurite outgrowth. TRPM2 mRNA was most highly expressed in microglia and paralleled the perinatal expression timeline for microglial infiltration and maturation in the rat brain. These developmental expression profiles indicated a potential role for TRPM2 in regulating microglial activation. Since calcium regulation is important for cell function and migration, a specific role for TRPM2 in regulating cell migration after injury was investigated by time-lapse imaging of an in vitro model of cell migration and injury called ‘necrotaxis’. Primary microglial cells isolated from Trpm2-/- mice exhibited impaired migration, resulting in the loss of persistent directional migration. A high colocalization between Rab11 (recycling endosomes) and β1-integrin was also found in Trpm2-/- microglia, suggesting that TRPM2 affects the overall migration of microglial cells by mediating tail detachment via the trafficking or recycling of integrins. To determine whether the effects of knocking out TRPM2 on microglia persisted in an in vivo model of injury, Iba1+ immunoreactivity was assessed in wild-type and Trpm2-/- mice after inducing acute ischemic stroke. Immunohistochemistry analyses revealed a lower number of activated microglia in Trpm2-/- mice after acute ischemia compared to wild-type mice, suggesting that TRPM2 affects the recruitment of microglia to the site of injury. This study provides evidence that links TRPM2 to tail detachment via integrin trafficking/recycling which in turn affects the migration of microglial cells. TRPM2 may therefore be a good target to mitigate inflammation after stroke and other neurodegenerative diseases, when excessive microglial activation or proinflammatory processes may negatively impact brain recovery.