Cellular origin and function of brain 2-arachidonoylglycerol

点击翻译按钮获取中文摘要

The endocannabinoid system (ECS) is a lipid signalling system that modulates a wide range of physiological functions, many of which are important in the context of homeostatic regulation. At the core of this feedback mechanism are presynaptic G-protein-coupled cannabinoid receptors CB1 (CB1), which are also the target of the primary psychoactive component of marijuana (delta9-tetrahydrocannabinol, THC). A second cannabinoid receptor, CB2, is also weakly expressed by neurons, but mostly present on peripheral tissues and immune cells. The full CB1 and CB2 agonist 2-arachidonoyl glycerol (2-AG), one of the main endocannabinoids, is generated by the diacylglycerol lipase (DAGL) enzymes. The two isoforms DAGL-alpha and DAGL-beta have been described, which are encoded by the Dagla and Daglb genes, respectively. The second well-known endocannabinoid is arachidonoylethanolamide (AEA, anandamide). The brain level AEA is lower compared to 2-AG, and AEA also has a lower efficacy and functions as a partial agonist at CB1 and CB2 receptors.To date, all models of endocannabinoid signalling in the brain are based on the assumption that the postsynapse is the main source of 2-AG. However, our preliminary results with neuron-specific Dagla knockout (KO) mice show that this assumption is probably wrong, because we did not find any changes in 2-AG levels in these conditional KO mice - although Dagla expression was substantial reduced. However, constitutive Dagla KO animals show an 80% reduction of brain 2-AG levels with a concomitant phenotype of increased emotional or stress-related behaviours. This phenotype is entirely consistent with the notion that endocannabinoid signalling regulates affective behaviours and stress responses, and is involved in the pathology of depression. It is further known that besides neurons, astrocytes and microglia also express CB1 receptors and produce endocannabinoids in the brain. For microglia, the in vitro production of endocannabinoids is actually about 20-fold higher compared to neurons. Yet, the function of these endocannabinoids and their role in modulating behaviour is completely unclear. Hence, we aim to investigate the production of 2-AG by different brain cells (neurons, astrocytes and microglia) and their role in animal behaviours. Our focus will be on the modulation of anxiety-related behaviours and its influence on the affective states of animals, which is a documented function of the ECS in animals and humans. For this purpose we are planning to use conditional Dagla KO mice that we have recently described. The elucidation of the main cellular source of brain 2-AG will be of fundamental importance to the entire field of cannabinoid research. It will provide a basis for future studies, outside the scope of this proposal, involving physiology, cell and molecular biology.