ASTROCYTIC CANNABINOID SYSTEM: A NEW REGULATOR OF MITOCHONDRIAL -ENDOPLASMIC RETICULUM CONTACTS ARCHITECTURE

Regulation of astroglial Ca²⁺ signaling is one of the most powerful ways through which G protein-coupled receptors (GPCRs) control brain functions. While GPCRs are traditionally studied at plasma membranes, the type-1 cannabinoid receptor (CB1) can also function at mitochondrial membranes (mtCB1). Astroglial mtCB1 promotes mitochondrial Ca²⁺ uptake, ultimately impacting on brain functions and behavior. However, the molecular mechanisms underlying mtCB1-mediated regulation of calcium signaling remain unclear.
Ca²⁺ transfer from endoplasmic reticulum (ER) to mitochondria (MIT) occurs at specific ER-MIT contacts (MERCs) and it is optimal at the ER-MIT distance of 20nm. Electron microscopy pictures show mtCB1 at MERCs, suggesting that mtCB1 might modulate the architectural organization of these contact sites, creating optimal conditions for mitochondrial Ca²⁺ uptake.
Using a split GFP-based genetically-encoded sensor of molecular distance (SPLIC-20), we first observed that activation of mtCB1 receptors efficiently stabilizes astrocyte MERCS at the distance of 20nm, both in vitro and in vivo. This is accompanied with increased mitochondrial Ca²⁺ uptake, and both these effects depend on mitochondrial PKA (mtPKA) signaling. However, SPLIC-20 provides data in the hours range, limiting the temporal resolution of the observed effects. To overcome this limitation, we developed a new FLIM-based sensor, allowing to show that mtCB1 induces MERC remodeling in the seconds time scale. Finally, preliminary data indicate that specific mtCB1-dependent behavioral processes might possibly be determined by these mechanisms.
Altogether, these data identify mitochondrial cannabinoid signaling as a novel intracellular pathway that promotes dynamic MERCs reorganization and mitochondrial calcium uptake in astrocytes.