Cholinergic Regulation of Alzheimer's-Like Pathology
Abstract
Cholinergic dysfunction is a major contributor to Alzheimer’s Disease (AD), a neurodegenerative disorder characterized by cognitive impairment and loss of functional abilities. Prior studies from our group have shown that mice with a forebrain-specific deletion of the vesicular acetylcholine transporter (VAChT), a key protein for acetylcholine (ACh) release, exhibit pathological features associated with AD. However, whether the modulation of cholinergic signaling or its downstream targets could affect AD-like pathology remains unclear. In this dissertation, we assessed the effects of both upregulating and downregulating cholinergic tone on AD-like pathology in male and female mice.
To study the effects of enhancing cholinergic neurotransmission, we treated forebrain VAChT-knock-out (VAChTKO) mice with BQCA, a highly selective positive allosteric modulator, to specifically activate the M1 muscarinic ACh receptor. We quantified the levels of AD-associated proteins including amyloid β (Aβ), the splicing regulator hnRNPA2/B1 and the enzyme initiating Aβ synthesis, BACE1. Our results show that M1 activation modulates the abnormal levels of BACE1 and hnRNPA2/B1 in VAChT-deficient males compared with vehicle-treated mice, while no difference was found in Tris-soluble Aβ. Interestingly, BQCA-treated females did not show any difference in these proteins when compared to controls. Next, to specifically investigate the role of BACE1 in AD-like pathology, we generated a new mouse model by crossing VAChTKO mice with BACE1KO mice. Our results suggest that BACE1 signaling pathway modulates the levels of hnRNPA2/B1 and the synaptic marker PSD95 independently of the cholinergic tone. Moreover, we found that the absence of BACE1 does not rescue the deficiency of the non-amyloidogenic pathway identified in VAChTKO mice. Lastly, to study whether VAChT levels are causally associated with Aβ deposition, as well as the role of biological sex in this relationship, we generated new mouse lines by crossing mice expressing humanized amyloid precursor protein with mice either lacking or overexpressing VAChT. We found that VAChT levels have a bidirectional causal effect on amyloid pathology in male mice and ovariectomized female mice, but not in ovary-intact or estradiol-treated ovariectomized female mice. Together, these results show that cholinergic signaling modulates AD-like pathology in specific hormonal enviroments, highlighting a sexual dimorphism in this regulation.