The increased expression and/or abnormal processing of the amyloid precursor protein (APP) is associated with the formation of amyloid plaques and cerebrovascular amyloid deposits, which are one of the major morphological hallmarks of Alzheimer's disease (AD). Among the processes regulating APP metabolism, the proteolytic cleavage of APP into amyloidogenic or nonamyloidogenic fragments is of special interest. The cleavage of the APP by the α-secretase within the β-amyloid sequence generates nonamyloidogenic C-terminal APP fragments and soluble APPsα, which has neurotrophic and neuroprotective activities. Proteolytic processing of APP by β-secretase, on the other hand, exposes the N-terminus of β-amyloid, which is liberated after γ-secretase cleavage at the variable amyloid C-terminus. The resulting 39-43 amino acid β-amyloid may be neurotoxic and disrupt neuronal connectivity after its accumulation in senile plaques. In this review, we discuss evidence derived from in vitro experiments, suggesting that the stimulation of protein kinase C (PKC)-coupled M1/M3 muscarinic acetylcholine receptors increases the nonamyloidogenic, secretory pathway of APP processing. It has also been shown in animal models that under conditions of reduced M1/M3 muscarinic acetylcholine receptor stimulation the secretory pathway of APP processing is inhibited and that constitutive upregulation of M1/M3-associated PKC increases APP secretion. Thus, the cortical cholinergic hypoactivity characteristic of AD may inhibit the nonamyloidogenic APP processing pathway and lead to increased β-amyloid generation. It has been shown in vitro that nerve growth factor (NGF)-associated signaling also influences the expression and catabolism of APP. Recent experiments with NGF- responsive cells revealed a specific role for the high-affinity NGF receptor, TrkA, in the increases in secretory APP processing and a role for the low- affinity neurotrophin receptor, p75(NTR), in the transcriptional regulation of APP. Therefore, treatments with NGF could ameliorate cortical cholinergic dysfunction in AD. These findings may influence the design of therapeutic strategies aimed at stimulating cholinergic function and at increasing nonamyloidogenic APP processing without elevating APP expression.
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