Cannabinoid Receptor 2-Deficiency Ameliorates Disease Symptoms in a Mouse Model with Alzheimer's Disease-Like Pathology

It is widely accepted that the endocannabinoid system (ECS) is a modulator of neuroinflammation associated with neurodegenerative disorders, including Alzheimer's disease (AD). Thus, expression of the cannabinoid receptor 2 (CB2) is induced in plaque-associated microglia and astrocytes in brain tissues from AD patients and in genetic mouse models expressing pathogenic variants of the amyloid precursor protein (APP). However, the exact mechanism of CB2 signaling in this mouse model remains elusive, because the genetic deletion of CB2 and the pharmacological activation of CB2 both reduced neuroinflammation. Here, we demonstrate that CB2 deletion also improved cognitive and learning deficits in APP/PS1*CB2(-/-) mice. This was accompanied by reduced neuronal loss and decreased plaque levels and coincided with increased expression of A beta degrading enzymes. Interestingly, plaque-associated microglia in APP/PS1*CB2(-/-) mice showed a less activated morphology, while plaques were smaller and more condensed than in APP/PS1 mice. Taken together, these results indicate a beneficial effect of CB2-deficiency in APP transgenic mice. CB2 appears to be part of a protective system that may be detrimental when engaged continuously.