This project is designed to obtain biochemical support for developing treatments that prevent amyloid deposition in brains of AD patients. The overall goal of this project is to determine whether a pharmacological treatment affects levels of amyloid precursor protein (APP) derivatives in blood or cerebrospinal fluid (CSF) of patients with AD. The specific APP fragments are the N-terminal moities (APPs) and soluble Ab1-40 and Ab1-42. The pharmacological treatment is the investigational drug AF102B, which is a selective m1 receptor cholinergic agonist. The specific hypothesis is that AF102B administration will increase levels of APPs, and decrease levels of Ab1-42. Amyloid depositionis an early event in AD and characterizes much of its histopathology. Filmentous deposits of amyloid occur in the cortical neuropil as senile plaques Ab1-40 as congophilic angiopathy in AD. In some experimental systems, amyloid has neurotoxic effects which apparently derive from the intact Ab transmembrane fragment of a large amyloid precursor protein (APP). In most cases of AD, amyloid in AD is formed in the brain because of changes in proteolytic APP processing and not because of altered genetic or molecular properties. As information accrues regarding factors that influence APP processing, it should be possible to develop pharmacological treatments designed to decrease Ab deposition. If Ab deposition is an initiating event in causing neurotoxicity, then blocking Ab would be expected to prevent neuronal degeneration, and stop the evolution of dementia. There are two known pathways for APP processing: constitutive secretion, and internal processing within the endosomal-lysosomal system (1). In constitutive secretion, a membrane-associated endoprotease cleaves the Ab peptide into non-toxic fragments, whereas amyloidogenic fragments are likely to accumulate as a result of APP metabolism through the lysosomal pathway. Amino-terminal APP derivatives released by the constitutive se cretory pathway derive from proteolytic cleavage within the Ab segment; these compounds (APPs) are soluble and can be detected reeadily in human CSF and plasma (2). APP processing appears to be regulated by neurotransmitter-receptor coupled activation. In the resting state, the lysosomal pathway is dominant whereas activation of protein kinase C second meassenger systems preferentially favor APP processing by the secretory pathway. In studies performed in our laboratory, Nitsch et al. (3) demonstrated in cell culture that activation of the m1 and m3 muscarinic subtypes by the cholinergic agonist carbachol rapidly increased the release of soluble amino terminal APP derivatives into the medium. These results indicate that APP processing can be regulated by cell-surface neurotransmitter receptors. Moreover, electrical depolarization of hippocampal slices in vitro also caused a tetrodotoxin-sensitive enhacement of the release of APPs, implying a general role for neuronal activity in the regulation of APP processing (4). Application of m1 agonists, including AF102B, also increases APPs levels in hippocampal slice perfusates. Assuming that the release of amino-terminal APP derivatives preclude the internalization of full-length APP and subsequent lysosomal degradation to Ab amyloidogenic fragments, administration of selective m1 agonists such as AF102B would be expected to block amyloid deposition.
Showing the most recent 10 out of 945 publications
