Several studies have indicated that the cannabinoid receptor 2 (CB2) plays an important role in neuroinflammation associated with Alzheimer's disease (AD) progression. The present study examined the role of CB2 in microglia activation in vitro as well as characterizing the neuroinflammatory process in a transgenic mouse model of AD (APP/PS1 mice). We demonstrate that microglia harvested from CB2−/− mice were less responsive to pro-inflammatory stimuli than CB2+/+ microglia, based on the cell surface expression of ICAM and CD40 and the release of chemokines and cytokines CCL2, IL-6, and TNFα. Transgenic APP/PS1 mice lacking CB2 showed reduced percentages of microglia and infiltrating macrophages. Furthermore, they showed lowered expression levels of pro-inflammatory chemokines and cytokines in the brain, as well as diminished concentrations of soluble Aβ 40/42. The reduction in neuroinflammation did not affect spatial learning and memory in APP/PS1*CB2−/− mice. These data suggest a role for the CB2 in Alzheimer's disease–associated neuroinflammation, independent of influencing Aβ-mediated pathology and cognitive impairment.
Alzheimer's disease (AD) is a neurodegenerative disorder and represents the most common type of dementia among the elderly population. Neuropathological hallmarks characteristic of AD include amyloid-β (Aβ) plaques and neurofibrillary tangles, accompanied by neuroinflammation characterized by astrocytosis and microgliosis. Microglia attracted to Aβ deposits represent the primary cellular component associated with AD neuroinflammation. In response to immune-stimulatory signals, they change from a resting to an activated state. Activated microglia express at least 2 phenotypes depending on their environmental stimulation: an M(IFNγ), formerly known as M1, phenotype associated with the production of proinflammatory mediators and an M(IL-4), formerly known as M2, alternatively activated phenotype characterized by anti-inflammatory qualities (Michelucci et al., 2009 and Murray et al., 2014). Several studies have shown that Aβ-mediated activation of microglia induces the production of various chemokines and cytokines, neurotoxic secretory products, free radical species, and NO intermediates (Heneka, et al., 2010). These proinflammatory mediators cause neuronal dysfunction and cell death, suggesting that activation of microglia plays a prominent role in neuroinflammation in the context of AD. In addition to activation of the pro-inflammatory cascade, microglia cells have also been shown to phagocytize Aβ plaques, thereby reducing the number of protein aggregates in an AD brain (Bolmont et al., 2008).
The endocannabinoid system (ECS) is a retrograde messenger system consisting of lipid signaling molecules that bind to at least 2 G-protein–coupled receptors. Cannabinoid receptor 1 (CB1) is mainly expressed in the central nervous system (CNS) but has also been detected in lung, kidney, and liver. In contrast, CB2 is primarily expressed on immune cells such as B-cells, T-cells, macrophages, dendritic cells, and microglia (Pacher and Mechoulam, 2011). Thus, the ECS affects both immune responses and cognition. In addition, studies have suggested that CB2 plays a role in the modulation of microglia activity relevant to AD.
Early studies already suggested a crucial role for the CB2 receptor in AD, based on findings that CB2 is overexpressed in Aβ plaque–associated microglia (Benito et al., 2003). Up-regulation of CB2 expression by microglia has also been described in Huntington disease, simian immunodeficiency virus–induced encephalitis, human immunodeficiency virus (HIV) encephalitis, and multiple sclerosis (Benito et al., 2003, Benito et al., 2005, Benito et al., 2008 and Ramírez et al., 2005).
To date however, most studies have examined the effects of pharmacological modulation of the ECS, showing that stimulation with specific CB2 agonists reduces the Aβ burden and neuroinflammation and rescues cognitive deficits in AD mouse models (Aso et al., 2013). Similar observations were also made in rats injected with amyloid fibrils in the hippocampal CA1 region to mimic AD pathology, followed by treatment with a CB2 agonist (Wu et al., 2013). In addition, extended oral treatment with cannabinoids reduced Aβ levels of APP2576 mice (Martín-Moreno et al., 2012) and decreased Aß plaque-deposition in 5xFAD APP transgenic mice (Chen et al., 2013). However, since the precise involvement of cannabinoid receptors remains unknown, we investigated here CB2 in AD using in vitro and in vivo models. Our results indicate an important function of CB2 in the modulation of AD-associated neuroinflammation.
