|dc.description.abstract||Presenilin-1 (PS-1) is the catalytic core of the ã-secretase complex, which is best known for its role in the generation of the Alzheimer disease (AD)-related â-amyloid peptide. Mutated forms of PS-1 are known to be associated with particularly aggressive forms of AD. Changes in monoaminergic neurotransmitter systems, including the serotonin and norepinephrine systems, have long been associated with some of the earliest events in AD, whereas changes in the availability of these same monoamines have historically been associated with clinical depression. Therefore, it is not surprising that depression has now been proposed as a risk factor for developing AD and that pre-demented carriers of mutated forms of PS-1 are more prone to developing depression. MAO-A is historically associated with depression and is also a known risk factor for AD. Given this, I hypothesized that MAO-A represents a neurochemical link between depression and AD, and I chose to examine the influence of PS-1 mutations on MAO-A function in vivo/ex vivo and in vitro.
I first focused on the PS-1(M146V) knock-in mouse model of AD-related PS-1/ã-secretase function. I used a radioenzymatic assay to estimate MAO-A catalytic activity and western blot analysis to determine MAO-A protein expression, and found that MAO-A activity does not correlate with MAO-A expression in the cortex and cerebellum of the PS-1(M146V) mice. Furthermore, the potency of the MAO-A inhibitor clorgyline (CLG) is greater in both the cortex and cerebellum of the PS-1(M146V) mice compared to the potency of CLG in wildtype littermates. CLG dose-response curves suggest that there might be a change in cooperativity in the MAO-A protein from PS-1(M146V) cortex (which would suggest a change in conformation and/or access of the substrate to the catalytic pocket in MAO-A). High-pressure liquid chromatography was used to analyze monoamine levels in these same regions. The levels of monoamines (i.e. serotonin, dopamine and norepinephrine) suggest that PS-1 (M146V) inhibits MAO-A function in the cortex, but not in the cerebellum. Furthermore, CLG has no significant effect on amine levels in cortex, but tends to increase their accumulation in cerebellum.
The overexpression of PS-1 (M146V) in neuronal cultures reveals that this protein affects MAO-A activity and, more importantly, the PS-1(M146V) protein co-precipitates with MAO-A, thus suggesting a possibility for a direct protein-protein interaction. This is supported by the observation that MAO-A activity is increased in cell extracts incubated with the PS-1 substrate-competitor, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT). Preliminary studies have been undertaken to determine the motif in MAO-A that could be acting as a binding site/target site for PS-1.
These combined results support the hypothesis that PS-1 proteins can influence MAO-A function and, furthermore, that MAO-A is a novel interactor for PS-1/ã-secretase. This could well explain some of the ambiguous literature associated with both of these proteins in disorders as diverse as depression and AD.||en_US