Neuroprotective Sirtuin ratio reversed by ApoE4

Abstract

The canonical pathogenesis of Alzheimer's disease links the expression of apolipoprotein E e4 allele (ApoE) to amyloid precursor protein (APP) processing and Aβ peptide accumulation by a set of mechanisms that is incompletely defined. The development of a simple system that focuses not on a single variable but on multiple factors and pathways would be valuable both for dissecting the underlying mechanisms and for identifying candidate therapeutics. Here we show that, although both ApoE3 and ApoE4 associate with APP with nanomolar affinities, only ApoE4 significantly (i) reduces the ratio of soluble amyloid precursor protein alpha (sAPPα) to Aβ; (ii) reduces Sirtuin T1 (SirT1) expression, resulting in markedly differing ratios of neuroprotective SirT1 to neurotoxic SirT2; (iii) triggers Tau phosphorylation and APP phosphorylation; and (iv) induces programmed cell death. We describe a subset of drug candidates that interferes with the APP-ApoE interaction and returns the parameters noted above to normal. Our data support the hypothesis that neuronal connectivity, as reflected in the ratios of critical mediators such as sAPPα:Aβ, SirT1:SirT2, APP:phosphorylated (p)-APP, and Tau:p-Tau, is programmatically altered by ApoE4 and offer a simple system for the identification of program mediators and therapeutic candidates. A lzheimer's disease (AD) poses a health problem of pandemic proportions. It is estimated that by the year 2030 there will be greater than 60 million people worldwide with AD, and $375 billion will be spent annually in the United States to care for affected individuals. The tedious and expensive process of drug discovery in AD is complicated by the fact that the causes and underlying mechanisms of the disease are still incompletely defined, and presymptomatic diagnosis is not yet in routine clinical use (1).The apolipoprotein E e4 allele (ApoE; chromosomal locus 19q13) is the single most important genetic risk factor associated with AD. This allele confers increased risk for sporadic and familial AD (2). Individuals with two copies of the ApoE e4 allele have an approximately eightfold increased risk of AD and have a significantly lower age of onset compared with AD patients not carrying this allele (2). Recent data indicate that the greater risk of AD associated with the ApoE4 isoform might relate to ApoE's susceptibility to proteolysis and neurotoxicity or through its role in inhibiting Aβ clearance and/or stimulating Aβ deposition, leading to plaque formation (3, 4). Contrary to the previously reported work, one recent study provides evidence that ApoE and soluble Aβ have very minimal direct interaction, thus ApoE may influence soluble Aβ metabolism through its interactions with other receptors or transporters (5). Thus, despite knowing for over a decade that the ApoE e4 allele is somehow contributory to the disease process, the precise molecular mechanisms underlying ApoE and APP interactions, direct or indirect, resulting in ApoE4-mediated toxicity, remain unclear.The amyloid precursor protein, APP...

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“…As SirT1 has previously been shown to activate transcription of ADAM10 [52] and thus increase levels of neuroprotective sAPPα [53,54], one can conclude that a decrease in SirT1 expression as a result of the presence of ApoEε4 is likely to lead to decreased sAPPα levels. In further support of a role for ApoEε4 effects on SirT1 expression and ultimately sAPPα production, we showed that increasing SirT1 expression in the presence of ApoEε4 restores sAPPα levels in vitro by co-transfecting A172 cells with both ApoEε4 and SirT1 and identifying increases in sAPPα of 10% (1:1) or 20% (1:2) as compared to ApoEε4 transfection alone [47].As a result of these findings, we added SirT1 enhancement as a target for our in vitro screens to identify new small molecules for potential development as AD therapeutics. This resulted in our identification of brain-permeable small molecules that increase SirT1, sAPPα, and cell survival in vitro (manuscript in preparation).…”

“…Our recent studies have connected ApoEε4 expression with the major longevity determinants, sirtuins [47]. We showed that ApoEε4 -but not ApoEε3 -associates with APP with nanomolar affinity (Kd ~80 nM), and that only ApoEε4 significantly reduces the sAPPα/Aβ ratio.…”

The multi-functional drug tropisetron binds APP and normalizes cognition in a murine Alzheimer's model

“…Co-transfection of huAPPwt and ApoE ε4 has been shown to significantly decrease sAPPα secretion, and reduce the sAPPα/sAPP β or sAPPα/A β –1-40 ratio in several cell lines, and tropisetron – identified as “F03” - has been shown to reverse these reductions (Theendakara, 2013). Here we also show that tropisetron at 1μM significantly increased cell survival of both ApoE ε3- and ApoE ε4-transfected cells (Fig.…”

Reversal of cognitive decline: A novel therapeutic program

“…There are numerous physiological parameters that feed into this balance, such as hormones [28, 29], trophic factors [18], glucose metabolism [30], inflammatory mediators [31], ApoE genetic status [32] sleep-related factors [33], exercise-related factors [34], and many others; therefore, the therapeutic system is designed to reverse the self-reinforcing (i.e., prionic) signaling imbalance that we have hypothesized to mediate Alzheimer's disease pathophysiology [8]. …”