Constitutive activation drives compartment-selective endocytosis and axonal targeting of type 1 cannabinoid receptors.
The type 1 cannabinoid receptor (CB1R) is one of the most abundant G-protein-coupled receptors (GPCRs) in the brain, predominantly localized to axons of GABAergic neurons. Like several other neuronal GPCRs, CB1R displays significant in vitro constitutive activity (i.e., spontaneous activation in the absence of ligand). However, a clear biological role for constitutive GPCR activity is still lacking. This question was addressed by studying the consequences of constitutive activation on the intracellular trafficking of endogenous or transfected CB1Rs in cultured hippocampal neurons using optical and electron microscopy. We found that constitutive activity results in a permanent cycle of endocytosis and recycling, which is restricted to the somatodendritic compartment. Thus, CB1Rs are continuously removed by endocytosis from the plasma membrane in the somatodendritic compartment but not in axons, where CB1Rs accumulate on surface. Blocking constitutive activity by short-term incubation with inverse agonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281) results in sequestration of recycled CB1Rs on the somatodendritic plasma membrane. Long-term inhibition of endocytosis by cotransfection of dominant-negative proteins results in impaired axonal polarization of surface-bound CB1Rs. Kinetic analysis shows that the majority of newly synthesized CB1Rs arrive first to the somatodendritic plasma membrane, from where they are rapidly removed by AM281-sensitive constitutive endocytosis before being delivered to axons. Thus, constitutive-activity driven somatodendritic endocytosis is required for the proper axonal targeting of CB1R, representing a novel, conformation-dependent targeting mechanism for axonal GPCRs.
Laboratoire Neurobiologie et Diversité Cellulaire, Ecole Supérieure de Physique et de Chimie Industrielles-Centre, National de la Recherche Scientifique, Unité Mixte de Recherche 7637, 75013 Paris, France.
Christophe
C
Leterrier
Jeanne
J
Lainé
Michèle
M
Darmon
Hélène
H
Boudin
Jean
J
Rossier
Zsolt
Z
Lenkei
D000818:Animals; D001370:Axonal Transport / Q000187:drug effects / Q000502:physiology*; D001369:Axons / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D063385:Cannabinoid Receptor Modulators / Q000378:metabolism; D002451:Cell Compartmentation / Q000187:drug effects / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D003712:Dendrites / Q000378:metabolism / Q000648:ultrastructure; D004705:Endocytosis / Q000187:drug effects / Q000502:physiology*; D006624:Hippocampus / Q000378:metabolism* / Q000648:ultrastructure; D008024:Ligands; D046529:Microscopy, Electron, Transmission; D009025:Morpholines / Q000494:pharmacology; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology; D011720:Pyrazoles / Q000494:pharmacology; D051381:Rats; D043884:Receptor, Cannabinoid, CB1 / Q000187:drug effects / Q000378:metabolism*; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*
D063385:Cannabinoid Receptor Modulators; D008024:Ligands; D009025:Morpholines; D011720:Pyrazoles; D043884:Receptor, Cannabinoid, CB1; D043562:Receptors, G-Protein-Coupled; C109925:AM 281