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Mohamed A, Saavedra L, Di Pardo A, Sipione S, Posse de Chaves E. β-amyloid inhibits protein prenylation and induces cholesterol sequestration by impairing SREBP-2 cleavage. J Neurosci. 2012 May 9;32(19):6490-500. PubMed.
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Boston University School of Medicine
This manuscript presents some novel effects of oligomeric Aβ on neuronal biology. Mohamed et al. show that Aβ inhibits isoprenylation, and suggest that it does so by impairing cholesterol metabolism and SREBP-2 cleavage. The observations presented in the manuscript are quite convincing; however, I disagree with the interpretations.
The authors observe that cholesterol accumulates in the presence of Aβ despite treatment with pravastatin. They use this observation to pursue putative effects of Aβ on SREBP-2. Unfortunately, I'd caution that this assumption defies logic. If cholesterol accumulates despite pravastatin treatment, then the mechanism cannot be SREBP-mediated cholesterol synthesis, because pravastatin inhibits the rate-limiting step, which is HMG-CoA reductase, and this is what is controlled by SREBP. The authors then show that Aβ inhibits SREBP-2 by about 60 percent, and conclude that this accounts for the effects on isoprenylation. Again, I suspect this is not accurate because HMG-CoA reductase must be inhibited by more than 90 percent before one sees significant deficits in isoprenylation (this is because the affinity of HMG-CoA for its substrates is so strong).
Thus, while the observations are convincing and likely true, the interpretation is likely to be different than suggested in the manuscript. Unfortunately, I don't know the biology of SREBP-2 well enough to know whether it directly inhibits production of the isoprenyl synthetic enzymes/conjugases, such as geranylgeranyl synthetase. Such inhibition would better explain the observed phenomena. Hopefully, future studies will clarify the mechanism.
Icahn School of Medicine at Mount Sinai
The cholesterol/ApoE/Aβ connection is one of the most challenging areas in AD research. Studies of statins led to very dramatic benefits in mice that, so far, have not been translatable to humans. Elucidation of statins' effects led many labs, including our own, to investigate the isoprenoid pathway, where we implicated Rho kinase as a tonic physiological antagonist of the α-secretase pathway.
This new paper looks at a different step in the pathway. This group demonstrates that oligomeric Aβ inhibits protein prenylation (e.g., farnesylation, geranylgeranylation) in cultured neurons. This isn't immediately reconcilable with our earlier studies, where farnesylation or geranylgeranylation inhibitors seemed to be a good thing. To be fair, however, the new paper asks a question that is not really predicated on our results, and it is conceivable that both are right. In addition to using different protocols, our groups may be targeting different subcellular compartments. More work is required to put the whole prenylation story together.
Still, in this area, the more important questions relate to differential ApoE isoform action and the field's vexing inability to translate robust Aβ-lowering effects of statins into a meaningful therapeutic or prophylaxis. One is tempted to wonder whether a statin prevention trial might be worthwhile, but many colleagues in the area are convinced that there are enough statin data to exclude much possibility of success.
University of Alberta
We would like to respond to Dr. Wolozin on his disagreement with the interpretations of our results. His views focus mainly on cholesterol synthesis, when, in fact, our work suggests that changes in cholesterol synthesis are not responsible for the “cholesterol sequestration” phenotype observed in neurons challenged with Aβ during the experimental window. Although the finding that Aβ inhibited cholesterol synthesis seemed paradoxical to the intensive filipin staining, it is not unprecedented since the drug U18666A is a potent inhibitor of cholesterol synthesis and induces a similar pattern of cholesterol sequestration. Our rationale for examining SREBP-2 as the target for Aβ came from the observations that, although both Aβ and pravastatin significantly reduced cholesterol synthesis, pravastatin (at the concentration used in our study) did not cause cholesterol sequestration, nor did it cause apoptosis.
Moreover, in agreement with Dr. Wolozin’s concepts on HMGCoA and prenylation, we did not observe any significant change in protein prenylation in neurons treated with pravastatin. This suggests that the conditions of pravastatin treatment were insufficient to achieve enough HMGCoA inhibition, even though we did not measure HMGCoA reductase activity or levels. At no point in our work have we claimed that HMGCoA reductase was involved in the effects of Aβ, and we would not be confident to directly extrapolate levels of HMGCoA reductase from SREBP-2 levels; thus, we do not understand the remark about accuracy that has been made. Finally, as Dr. Wolozin suspected, SREBP-2 regulates many other enzymes of the mevalonate pathway including farnesyl diphosphate synthase, which catalyzes formation of farnesyl-PP; therefore, inhibition of SREBP-2 can be expected to have a higher impact on prenylation. We believe that our work presents strong evidence that SREBP-2 is a target of Aβ. We have identified one pathway affected by the decrease of nuclear SREBP-2, but we expect that other targets of SREBP-2 could also play important roles.
References:
Horton JD, Shah NA, Warrington JA, Anderson NN, Park SW, Brown MS, Goldstein JL. Combined analysis of oligonucleotide microarray data from transgenic and knockout mice identifies direct SREBP target genes. Proc Natl Acad Sci U S A. 2003 Oct 14;100(21):12027-32. PubMed.
Horton JD, Goldstein JL, Brown MS. SREBPs: activators of the complete program of cholesterol and fatty acid synthesis in the liver. J Clin Invest. 2002 May;109(9):1125-31. PubMed.
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