Defining the molecular roles of peripheral CB1 and CB2 cannabinoid receptors in age-induced changes in energy and metabolic homeostasis.

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There is immense interest at present in targeting the action of a class of compounds, termed cannabinoids, in the treatment of obesity and metabolic-related disorders. Cannabinoids are present in cannabis, but our bodies naturally create cannabinoid-like chemicals, known as endocannabinoids that lock-on to protein molecules found on the surface of cells called cannabinoid receptors (i.e. CB1 and CB2). During obesity, diabetes and, as we have recently discovered, during ageing the CB1 is notably over-activated by endocannabinoids produced by the body resulting in impaired insulin action and dysregulation of energy balance in metabolically important tissues such as muscle, fat and liver. CB1 over-activation has been linked strongly with development of insulin resistance and increased adiposity. In contrast, emerging data indicates that CB2 may confer a protective physiological effect. In line with this idea, we find that CB2 inhibitors (antagonists) augment the insulin-desensitising effects of the endocannabinoid, anandamide (AEA) in muscle cells, whereas CB2 activators (agonists) ameliorate the loss in insulin signalling. Moreover, it is noteworthy that whilst CB1 expression is enhanced in ageing muscle that of CB2 is significantly decreased, consistent with the reduced insulin sensitivity that prevails in ageing skeletal muscle. Intriguingly, CB1 antagonists (e.g. rimonabant) promote glucose tolerance, stimulate energy expenditure and reduce body weight in obese animals by mechanisms independent of their appetite-reducing effect. Consistent with such findings, we find rimonabant ameliorates age-related tissue insulin resistance and fat mass gain in older animals. Precisely how CB1 antagonism elicits these beneficial effects is unclear, but our recent work indicates that CB1 blockade induces activation of AMPK - a molecule that not only senses cellular energy but promotes the breakdown and burning of fat in mitochondria (the cell's energy factory). The molecular events linking CB1 blockade to AMPK activation and the effect of the latter on enzymes involved in fat breakdown/burning and mitochondrial dynamics remain poorly understood. The proposed studies will utilise cultured muscle and fat cells to understand how CB1 inhibition or CB2 activation impacts upon molecules implicated in insulin action, energy balance and mitochondrial function and integrity. Our molecular analyses will involve biochemical and state-of-the-art imaging techniques for visualising mitochondrial staining in muscle and fat cells. These cell-based studies will be complemented with analysis of tissues from young and old mice genetically deficient in CB2 or experiments in young and aged mice administered a pharmacologically active dose of a CB1 antagonist or CB2 agonist for two weeks. During this period we will monitor food intake, glucose tolerance, energy expenditure, physical activity and fat mass before sampling blood/tissue for experimental analyses designed to dissect the mechanisms by which CB1 blockade or CB2 activation improves the metabolic status of aged animals. We also aim to test the effects of exercise in aged animals given that physical activity is known to help sustain tissue sensitivity to anabolic hormones such as insulin and preserve tissue functionality during aging. These studies will help unveil whether exercise curtails age-related changes in tissue CB1 and CB2 expression and, if so, whether these correlate favourably with measures of whole body energetics (i.e. body fat, glucose tolerance and energy expenditure). The proposed research will specifically expand our fundamental understanding of how modulating peripheral CB receptor activity influences energy balance and insulin action. The findings that will emerge will advance our knowledge of these key issues and prove invaluable in designing therapies that selectively target the peripheral ECS for treatment of age and obesity-related metabolic disorders.