Localization of the cannabinoid type‐1 receptor in subcellular astrocyte compartments of mutant mouse hippocampus

Abstract

Astroglial type-1 cannabinoid (CB ) receptors are involved in synaptic transmission, plasticity and behavior by interfering with the so-called tripartite synapse formed by pre- and post-synaptic neuronal elements and surrounding astrocyte processes. However, little is known concerning the subcellular distribution of astroglial CB receptors. In particular, brain CB receptors are mostly localized at cells' plasmalemma, but recent evidence indicates their functional presence in mitochondrial membranes. Whether CB receptors are present in astroglial mitochondria has remained unknown. To investigate this issue, we included conditional knock-out mice lacking astroglial CB receptor expression specifically in glial fibrillary acidic protein (GFAP)-containing astrocytes (GFAP-CB -KO mice) and also generated genetic rescue mice to re-express CB receptors exclusively in astrocytes (GFAP-CB -RS). To better identify astroglial structures by immunoelectron microscopy, global CB knock-out (CB -KO) mice and wild-type (CB -WT) littermates were intra-hippocampally injected with an adeno-associated virus expressing humanized renilla green fluorescent protein (hrGFP) under the control of human GFAP promoter to generate GFAPhrGFP-CB -KO and -WT mice, respectively. Furthermore, double immunogold (for CB ) and immunoperoxidase (for GFAP or hrGFP) revealed that CB receptors are present in astroglial mitochondria from different hippocampal regions of CB -WT, GFAP-CB -RS and GFAPhrGFP-CB -WT mice. Only non-specific gold particles were detected in mouse hippocampi lacking CB receptors. Altogether, we demonstrated the existence of a precise molecular architecture of the CB receptor in astrocytes that will have to be taken into account in evaluating the functional activity of cannabinergic signaling at the tripartite synapse.

Role of the endocannabinoid system in drug addiction

“…The CB1 receptors (CB1R) are the most abundant G protein-coupled receptors in the brain [23]. Their expression in the central nervous system (CNS) is widespread and heterogeneous and have crucial roles in regulating brain function and pathophysiological processes [17,22,[24][25][26][27][28][29][30][31][32][33]. CB1R are abundant in the basal ganglia (substantia nigra reticulata, globus pallidus, striatum, entopeduncular nucleus), cortex, NAc, cerebellum and hippocampus [23,[34][35][36].…”

“…CB1R expression is very high in inhibitory GABAergic synaptic terminals mostly in cortical and hippocampal cholecystokinin (CCK)-positive GABAergic interneurons [17,26,35,[39][40][41][42], low in excitatory glutamatergic synapses [22,25,28,38,43,44] and very low in brain astrocytes [33,[45][46][47][48][49]. Brain CB1R are mostly localized in axon terminals and preterminals away from the presynaptic active zones and are also localized at mitochondria in neurons [50][51][52][53][54] and astrocytes [33]. Thus, CB1R were distributed in GABAergic terminals (~56%), glutamatergic terminals (~12%), astrocytes (~6%), mitochondria (~15%) and in other compartments (~11%) in the hippocampus [32,33].…”

“…Brain CB1R are mostly localized in axon terminals and preterminals away from the presynaptic active zones and are also localized at mitochondria in neurons [50][51][52][53][54] and astrocytes [33]. Thus, CB1R were distributed in GABAergic terminals (~56%), glutamatergic terminals (~12%), astrocytes (~6%), mitochondria (~15%) and in other compartments (~11%) in the hippocampus [32,33].…”

Expression of cannabinoid CB1R–GPR55 heteromers in neuronal subtypes of the Macaca fascicularis striatum

“…Interestingly, CB 1 R/GPR55Hets were identified in both neuronal cell surface and intracellular locations. Although there are a few studies related to the occurrence of GPCRs in positions other than the cell surface, the CB 1 R has been identified in mitochondria, where it regulates energy metabolism 39–42 . Despite technical challenges with detecting intracellular heteromers, the subcellular location of heteroreceptor complexes needs to be further investigated.…”

Signalisation mitochondriale des récepteurs couplés aux protéines G

“…Aucune différence de marquage n'a été observée selon que les mitochondries aient été observées dans les corps cellulaires ou dans les axones, excluant une polarisation de l'adressage des récepteurs vers des souspopulations de mitochondries. Une localisation mitochondriale similaire a été retrouvée en microscopie électronique dans les neurones hypothalamiques à pro-opiomélacortine (POMC) de souris (Koch et al, 2015) et des récepteurs mtCB 1 ont été identifiés en microscopie électronique dans les cellules de muscles striés squelettique et cardiaque (Mendizabal-Zubiaga et al, 2016) et dans les astrocytes (Gutierrez-Rodriguez et al, 2018). Il apparaît que le mécanisme d'adressage des récepteurs CB 1 vers les mitochondries n'est pas spécifique des neurones mais est présent dans de nombreux types cellulaires.…”