11 May 2012. With statins doing wonders for Alzheimer’s mice and apolipoprotein E as the top risk gene, links between cholesterol and Alzheimer’s presumably run deep. Much prior research has explored how cholesterol influences amyloid-β, the peptide that clogs the brains of Alzheimer’s disease (AD) patients. A study in this week’s Journal of Neuroscience proposes a mechanism for how amyloid-β, in turn, can regulate cholesterol homeostasis. From experiments in cultured rat neurons, Elena Posse de Chaves and colleagues at the University of Alberta, Canada, propose that Aβ42 oligomers promote cell death by blocking cleavage of a transcription factor that drives cholesterol synthesis. The team traced the cholesterol problem to a reduction in protein prenylation, and showed that treating neurons with intermediates of the cholesterol pathway can relieve Aβ-induced toxicity. The findings suggest that tampering with cholesterol production to prevent or treat AD may be even more complicated than previously believed.
The idea for this study came from the observation that neurons exposed to Aβ develop cholesterol transport defects closely resembling those seen in Niemann-Pick type C disease. “That’s when we started thinking Aβ may have an important role in cholesterol trafficking,” de Chaves told Alzforum. In support of that idea, her lab found that cholesterol synthesis is required for neurons to internalize Aβ in the absence of ApoE (Saavedra et al., 2007), and others reported that Aβ accumulates before cholesterol levels go up in APP/PS1 transgenic mice (ARF related news story on Fernández et al., 2009).
To see if Aβ triggers cholesterol trafficking problems, first author Amany Mohamed and colleagues prepared Aβ42 oligomers using a published protocol (Dahlgren et al., 2002), and added them at 20 μM to cultures of rat basal forebrain neurons. Confocal microscopy of cells stained with filipin—a fluorescent tag for non-esterified cholesterol—suggested cholesterol levels may be higher in neurons cultured with Aβ than in those without. But when they measured cholesterol more quantitatively, the scientists found that Aβ-treated cells did not have more of it—they just could not get it to “go where it’s supposed to go,” de Chaves said. Aβ42 oligomers slowed retrograde transport in the neurons, causing cholesterol to get stuck in late endosomes and at the Golgi apparatus.
When the researchers looked at cholesterol synthesis, they found it was down in neurons exposed to Aβ. Probing the mechanism, they focused on sterol regulatory element-binding protein (SREBP-2), because this transcription factor activates genes of the mevalonate pathway of cholesterol synthesis. Furthermore, SREBP-2 activation requires proper trafficking of the protein from the endoplasmic reticulum to the Golgi, where it gets cleaved to release the DNA-binding fragment. Indeed, Aβ42 did seem to inhibit SREBP-2 cleavage, as determined by Western blots showing full-length and cleaved forms.
The pathway activated by SREBP-2 produces not only cholesterol, but also isoprenoids, which are post-translationally added to proteins in a process known as prenylation. By inhibiting the mevalonate pathway, the authors figured, Aβ42 might also curb prenylation—and sure enough, they found this to be true for Rab proteins, which regulate vesicular trafficking, in cultured rat neurons and in brains of TgCRND8 mice. Some question whether the Aβ-induced inhibition SREBP-2 is robust enough to account for the prenylation effects (see Ben Wolozin comment below). Nevertheless, in cultured neurons, the scientists were able to offset Aβ42’s ill effects on cell metabolism and survival by supplying geranylgeranyl pyrophosphate (GGPP), an intermediate in the mevalonate pathway.
By suggesting reduction of prenylation as a possible means by which Aβ triggers cell death, the new data seem to fit with a prior study showing that blocking isoprenoid production leads to buildup of intracellular Aβ (ARF related news story on Cole et al., 2005). At the same time, the current findings appear at odds with other research linking decreased isoprenylation with beneficial outcomes—namely, reduced Aβ production (ARF related news story on Ostrowski et al., 2007) and enhanced α-processing of amyloid precursor protein (ARF related news story on Pedrini et al., 2005). The studies used different methods and looked at different subcellular compartments, suggesting that “more work is required to put the whole prenylation story together,” Sam Gandy of Mount Sinai School of Medicine, New York, wrote in an e-mail to Alzforum (see full comment below). Furthermore, proteins have different sensitivities to reduced prenylation, said De Chaves, noting that her team only saw effects in neurons that accumulate Aβ. “In the brain, not all cells may react the same way to Aβ,” she said.
The present finding sheds light on, yet also complicates, the situation with statins, which, despite causing remarkable improvements in AD mice, have yet to translate into benefits for people (see ARF related news story). “Our work indicates that statins and Aβ will have a synergistic effect because they both inhibit cholesterol synthesis,” de Chaves said. “If statins are provided early in the process of AD, they could decrease production of Aβ. But if they are given after Aβ has built up, you will have two agents causing the same effect—decreased cholesterol synthesis—with the additional impact of Aβ not only on cholesterol but on the whole [mevalonate] pathway. The implications of that could be detrimental.”—Esther Landhuis.
Mohamed A, Saavedra L, Di Pardo A, Sipione S, de Chaves EP. β-Amyloid Inhibits Protein Prenylation and Induces Cholesterol Sequestration by Impairing SREBP-2 Cleavage. J Neurosci. 9 May 2012;32(19):6490-6500. Abstract