In this study, we found that both genetical knockdown and pharmacological inhibition of α1-ARs alleviated behavioral deficits and pathological changes in an AD mouse model. Mechanistically, we suppose that inhibition of α1-ARs may exert neuroprotective effects against AD by inhibiting GSK3β/BACE1 signaling pathway, thus reducing Aβ production and subsequent neurodegeneration in the brain.
NE signaling plays a critical role in maintaining cognitive functions, but the noradrenergic system is suggested to be dysfunctional in the AD brain (3). There is profound neurodegeneration in the LC with AD progressing, but its pathological significance has not been thoroughly addressed to date. Enhancement in neuronal sprouting to different brain regions and NE production (4, 9, 15), and reduction in NE reuptake (16, 17) might compensate for LC neuronal loss to maintain NE levels in the AD brain. As a consequence of these compensatory changes, extracellular NE is suggested to be increased in the AD brain (18, 19). Furthermore, the increase in extracellular NE has been shown to be associated with the severity of AD (20, 21), with higher NE levels associating with more severe cognitive impairment. In addition, densities of α1-adrenertic receptors are suggested to be increased in the AD brain (4, 10). These findings may imply that although NE is important for maintaining cognitive functions, over-compensation of NE during the process of AD could be detrimental. This notion is supported by studies that have shown cognitive impairment in response to stress, because under stress conditions NE release is elevated (22, 23). Therefore, inhibition of the NE signaling pathway in the AD brain might be helpful for controlling the dysregulated NE system and aid in rescuing cognitive impairment. Previous studies have shown that signaling of α2 and β2-adrenergic receptors is detrimental, and inhibition of these adrenergic receptors improves behavioral deficits and AD-type pathologies in AD mouse models (6, 7). Although activation of the α1-ARs signaling pathway improves cognitive in non-AD animals (24–26), blockade of α1-ARs by prazosin also improves the cognitive deficits of an AD transgenic mouse model (27). In AD patients, activation of α1-ARs is suggested to contribute to the agitation behaviors (26), and prazosin can alleviate such symptoms (28). These results indicate that the activation of α1-ARs by NE may contribute to agitation and aggressive behaviors, which are commonly seen in AD patients. Mice overexpressing α1-ARs had enhanced expression of genes involved in apoptosis and neurodegeneration (29), further supporting the possible pathological effects of α1-ARs in the AD brain. In accordance with this hypothesis, we found in this study that genetical and pharmacological inhibition of α1-ARs alleviated behavioral cognitive deficits and attenuated AD-type pathologies, including Aβ deposition, Tau hyperphosphorylation, neuroinflammation and neuronal degeneration et al.
In this study, we found that terazosin inhibited GSK3β pathway in vitro, as reflected by increased pGSK3β expression after terazosin treatment. This finding is consistent with a previous study which found that doxazosin, another α1-ARs antagonist, also increased the levels of pGSK3β (30). It is well-recognized that activation of GSK-3β could increase the expression and activity of BACE1, thus promoting the amyloidogenic pathway (31, 32). Indeed, we found in this study that inhibition of α1-ARs reduced BACE1 expression both in vivo and in vitro, thus reduced Aβ production. Therefore, inhibition of the α1-AR pathway might attenuate Aβ pathologies through promoting ser9 phosphorylation of GSK3β. Interestingly, it is suggested that Aβ could in return activate α1-ARs (33), therefore, α1-ARs activation and Aβ accumulation during the pathogenesis of AD may form a vicious circle, which could be disrupted by inhibition of α1-ARs. Furthermore, a study found that terazosin rescues pathologies of Parkinson’s disease through enhancing glycolysis in the brain (34). Energy metabolism dysfunction and reduced ATP levels in the brain are common features of AD. Impaired glucose metabolism could significantly promote the expression of BACE1 (35). Therefore, we suppose that inhibition of α1-ARs signaling pathway may also reduce brain amyloid burden through improving energy metabolism.
In this study, both genetical and pharmacological inhibition of α1-ARs was found to reduce NFT formation and suppress Tau hyperphosphorylation at several sites. GSK3β activation also promotes Tau hyperphosphorylation, and ser9 phosphorylation of GSK3β inhibits this process (36), thus down-regulation of Tau hyperphosphorylation after inhibiting α1-ARs might be directly associated with its effect on promoting ser9 phosphorylation of GSK3β. Besides, amelioration of Aβ pathology might also contribute to reduced NFT formation and Tau hyperphosphorylation (37). Furthermore, other pathologies subsequent to Aβ or Tau, including cerebral amyloid angiopathy (CAA), overactivation of glia cells, down-regulation of synaptic proteins, dendritic damage and neuronal apoptosis et al. were also improved after inhibiting α1-ARs signaling pathways. We suppose that inhibition of α1-ARs may act on Aβ and suppress its toxicity of triggering subsequent pathological changes.