Accelerating Alzheimer’s pathogenesis by GRK5 deficiency via cholinergic dysfunction

G protein-coupled receptors (GPCRs) mediate a wide variety of physiological function. GPCR signaling is negatively regulated by the receptor desensitization, a procedure initiated by a group of kinases, including GPCR kinases (GRKs). Studies using genetargeted mice revealed that deficiency of a particular GRK member led to dysfunction of a highly selective group of GPCRs. In particular, for example, GRK5 deficiency specifically disrupts M2/M4-mediated muscarinic cholinergic function. Emerging evidence indicates that ?-amyloid accumulation may lead to GRK5 deficiency, while the latter impairs desensitization of M2/M4 receptors. Within memory circuits, M2 is primarily presynaptic autoreceptor serving as a negative feedback to inhibit acetylcholine release. The impaired desensitization of M2 receptor by GRK5 deficiency leads to hyperactive M2, which eventually suppresses acetylcholine release and results in an overall cholinergic hypofunctioning. Since the cholinergic hypofunctioning is known to cause ?-amyloid accumulation, the GRK5 deficiency appears to connect the cholinergic hypofunctioning and ?-amyloid accumulation together into a self-amplifying cycle, which accelerates both changes. Given that the ? -amyloid accumulation and the cholinergic hypofucntioning are the hallmark changes in the ?-amyloid hypothesis and the cholinergic hypothesis, respectively, the GRK5 deficiency appears to bring the two major hypotheses in Alzheimer’s disease together, whereas the GRK5 deficiency is the pivotal link. Therefore, any strategies that can break this cycle would be therapeutically beneficial for Alzheimer’s patients.