Cell-type specific signalling of the CB1 cannabinoid receptor
Final Report Abstract
In the present project we have investigated how the endocannabinoid system is modifying intracellular processes in different types of neurons focussing on GABAergic and glutamatergic cells. To this end, we have utilized transgenic mice that specifically lack or specifically express CB1 in GABAergic or glutamatergic cells. We first investigated how loss of CB1 from forebrain GABAergic or cortical glutamatergic neurons modifies the influence of endocannabinoids on long term potentiation (LTP). We found that the lack of CB1 in glutamatergic cells strengthens LTP and leads to increased dendritic arborisation as well as increased dendritic spine number on pyramidal neurons in the hippocampus. At the same time; the lack of CB1 in GABAergic cells leads to opposite results: reduced LTP, reduced dendritic arborisation and reduced dendritic spine number. We also found that signalling through CB1 receptor is starting to influence LTP formation during the third week of life in mice. We have previously shown that in hippocampus and cortex, almost all pyramidal cells express low to moderate levels of CB1; simultaneously, a subpopulation of GABAergic neurons, cholecystokinin-positive interneurons express very high levels of CB1. Our data showed that in the hippocampus approximately one third of CB1 can be found in glutamatergic and two thirds of CB1 can be found in GABAergic neurons. We have now extended these studies to the frontal cortex and found, somewhat unexpectedly, that in the frontal cortex approximately half of CB1 can be found in glutamatergic and half in GABAergic neurons. We have also shown before that GABAergic and glutamatergic neurons differ not only in CB1 expression level but G-protein signalling intensity and pharmacological sensitivity too. We therefore were surprised to learn that ligand binding affinity and receptor kinetics is not significantly different between these two receptor populations. Next, we set out to study CB1 trafficking and polarization in cultured cells transfected with either a wild type or a mutated CB1, CB1F238L. More mutant CB1 was found in specialised membrane microdomains called lipid rafts, than wild type ones. Consequently, through increased lipid raft mediated constitutive endocytosis we saw a reduced surface expression of CB1F238L. Furthermore, CB1F238L showed increased axonal polarization in primary hippocampal neurons. We have also investigated in detail the exon-intron structure of the mouse Cnr1 gene and discovered two new splice variants with 39 and 62 amino acids shorter N-terminal domains. The two new mCB1 variants mCB1a and mCB1b showed increased internalization after agonist stimulation as well as enhanced inhibition of neurotransmission. CB1 receptor is present in many different neuronal populations and therefore is implicated in various physiological and pathological processes. Identifying the underlying molecular mechanisms of how the endocannabinoid system is modifying intracellular processes in different types of neurons might pave the way to new therapeutic approaches as well as may deepen our understanding of how Cannabis compounds may exert their medicinal effects. The results we have gathered during the years of the DFG-supported research will substantially contribute to expanding our knowledge about the cell type specific functioning of the endocannabinoid system.
Publications
- (2018) The F238L Point Mutation in the Cannabinoid Type 1 Receptor Enhances Basal Endocytosis via Lipid Rafts. Frontiers in molecular neuroscience 11 230
Wickert, Melanie; Hildick, Keri L.; Baillie, Gemma L.; Jelinek, Ruth; Aparisi Rey, Alejandro; Monory, Krisztina; Schneider, Miriam; Ross, Ruth A.; Henley, Jeremy M.; Lutz, Beat
(See online at https://doi.org/10.3389/fnmol.2018.00230) - (2015) Cannabinoid CB1 receptor calibrates excitatory synaptic balance in the mouse hippocampus. J Neurosci. 35: 3842-3850
Monory K, Polack M, Remus A, Lutz B, Korte M
(See online at https://doi.org/10.1523/JNEUROSCI.3167-14.2015) - (2017) Discovery and characterization of two novel CB1 receptor splice variants with modified N-termini in mouse. J Neurochem. 142: 521-533
Ruehle S, Wager-Miller J, Straiker A, Farnsworth J, Murphy MN, Loch S, Monory K, Mackie K, Lutz B
(See online at https://doi.org/10.1111/jnc.14099) - (2017) Singular Location and Signaling Profile of Adenosine A(2A)-Cannabinoid CB(1) Receptor Heteromers in the Dorsal Striatum. Neuropsychopharmacology
Moreno E, Chiarlone A, Medrano M, Puigdellívol M, Bibic L, Howell LA, Resel E, Puente N, Casarejos MJ, Perucho J, Botta J, Suelves N, Ciruela F, Ginés S, Galve-Roperh I, Casadó V, Grandes P, Lutz B, Monory K, Canela EI, Lluís C, McCormick PJ, Guzmán M
(See online at https://doi.org/10.1038/npp.2017.12)
