Rivastigmine Lowers Aβ and Increases sAPPα Levels, Which Parallel Elevated Synaptic Markers and Metabolic Activity in Degenerating Primary Rat Neurons

Jason A Bailey,Nigel H Greig,Debomoy K Lahiri,Balmiki Ray,Mark P Mattson

doi:10.1371/journal.pone.0021954

Abstract

Overproduction of amyloid-β (Aβ) protein in the brain has been hypothesized as the primary toxic insult that, via numerous mechanisms, produces cognitive deficits in Alzheimer's disease (AD). Cholinesterase inhibition is a primary strategy for treatment of AD, and specific compounds of this class have previously been demonstrated to influence Aβ precursor protein (APP) processing and Aβ production. However, little information is available on the effects of rivastigmine, a dual acetylcholinesterase and butyrylcholinesterase inhibitor, on APP processing. As this drug is currently used to treat AD, characterization of its various activities is important to optimize its clinical utility. We have previously shown that rivastigmine can preserve or enhance neuronal and synaptic terminal markers in degenerating primary embryonic cerebrocortical cultures. Given previous reports on the effects of APP and Aβ on synapses, regulation of APP processing represents a plausible mechanism for the synaptic effects of rivastigmine. To test this hypothesis, we treated degenerating primary cultures with rivastigmine and measured secreted APP (sAPP) and Aβ. Rivastigmine treatment increased metabolic activity in these cultured cells, and elevated APP secretion. Analysis of the two major forms of APP secreted by these cultures, attributed to neurons or glia based on molecular weight showed that rivastigmine treatment significantly increased neuronal relative to glial secreted APP. Furthermore, rivastigmine treatment increased α-secretase cleaved sAPPα and decreased Aβ secretion, suggesting a therapeutic mechanism wherein rivastigmine alters the relative activities of the secretase pathways. Assessment of sAPP levels in rodent CSF following once daily rivastigmine administration for 21 days confirmed that elevated levels of APP in cell culture translated in vivo. Taken together, rivastigmine treatment enhances neuronal sAPP and shifts APP processing toward the α-secretase pathway in degenerating neuronal cultures, which mirrors the trend of synaptic proteins, and metabolic activity.

Highlights

Alzheimer’s disease (AD) is the most common cause of dementia in the elderly, afflicting approximately 5.4 million people in the United States [1] with steady increases projected to at least midcentury as the current population gradually ages [2]
A predominant single band corresponding to intracellular hAPP was observed in the lysates, and there was no change in hAPP observed as a result of rivastigmine treatment as compared to vehicle. These results suggest no change in total Ab precursor protein (APP) production in these cultures as a result of rivastigmine treatment, or perhaps a modest increase if APP holoprotein is combined with secreted APP (sAPP)
The currently available therapies based on cholinesterase inhibitors (ChEI) or NMDA receptor antagonist treatments have failed to adequately attain this goal

Summary

Introduction

Alzheimer’s disease (AD) is the most common cause of dementia in the elderly, afflicting approximately 5.4 million people in the United States [1] with steady increases projected to at least midcentury as the current population gradually ages [2]. Some of the potential causative or contributing factors implicated in AD include genetic and epigenetic factors [3,4], diet and physical activity [5], education [6], heavy metal exposure [7], and other gene-environment interactions [8]. AD is diagnosed post mortem based on the presence in brain of amyloid plaques, comprised mostly of amyloid-b (Ab) peptides, and neurofibrillary tangles (NFTs) of hyperphosphorylated microtubule associated protein tau (MAPT). Together with other proteolytic fragments of dysregulated amyloid precursor protein (APP) processing, have been implicated as major mediators of neurotoxicity in AD [10] and are known to induce toxic effects through a wide variety of mechanisms

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