Receptor-selective diffusion barrier enhances sensitivity of astrocytic processes to metabotropic glutamate receptor stimulation

Metabotropic glutamate receptor (mGluR)- dependent calcium ion (Ca ²⁺) signaling in astrocytic processes regulates synaptic transmission and local blood flow essential for brain function. However, because of difficulties in imaging astrocytic processes, the subcellular spatial organization of mGluR-dependent Ca ²⁺ signaling is not well characterized and its regulatory mechanism remains unclear. Using genetically encoded Ca ²⁺ indicators, we showed that despite global stimulation by an mGluR agonist, astrocyte processes intrinsically exhibited a marked enrichment of Ca ²⁺ responses. Immunocytochemistry indicated that these polarized Ca ²⁺ responses could be attributed to increased density of surface mGluR5 on processes relative to the soma. Single-particle tracking of surface mGluR5 dynamics revealed a membrane barrier that blocked the movement of mGluR5 between the processes and the soma. Overexpression of mGluR or expression of its carboxyl terminus enabled diffusion of mGluR5 between the soma and the processes, disrupting the polarization of mGluR5 and of mGluR-dependent Ca ²⁺ signaling. Together, our results demonstrate an mGluR5-selective diffusion barrier between processes and soma that compartmentalized mGluR Ca ²⁺ signaling in astrocytes and may allow control of synaptic and vascular activity in specific subcellular domains.