A cholesterol metabolite boosts accumulation of phosphorylated tau, according to a paper in the January 16 Cell Stem Cell. Researchers led by Lawrence Goldstein at the University of California, San Diego, reported that cholesteryl esters—which coalesce into lipid droplets for storage within the cell—boosted phosphorylated tau in neurons derived from people with Alzheimer’s disease. Reducing cholesteryl esters with efavirenz, a small-molecule drug that activates the enzyme 24-hydroxylase, reduced p-tau levels by ramping up its destruction by the proteasome.
- Cholesteryl esters boosted p-tau in human iPSC-derived neurons.
- Esters dampened proteasome activity.
- Efavirenz, which activates cholesterol hydroxylase, lowered p-tau.
Efavirenz is best known for inhibiting the reverse transcriptase of the HIV-1 virus, and is prescribed as an anti-AIDS medication. The drug has also been reported to reduce Aβ burden in animal models of amyloidosis. A Phase 1 clinical trial is underway to see if efavirenz safely boosts cholesterol hydroxylase activity in people with mild cognitive impairment. Goldstein and colleagues now propose that efavirenz could also lower p-tau in people with AD or other tauopathies, independent of its effect on APP.
“The study proposes that intraneuronal cholesterol dyshomeostasis could be an early trigger for both amyloidosis and tau pathological changes, albeit through distinct cascades of events,” commented Eloise Hudry of Massachusetts General Hospital in Boston.
Cholesteryl Esters Exacerbate p-tau, Aβ. Inhibitors of cholesterol synthesis lowered p-tau in neurons (top). Cholesteryl esters block p-tau degradation (middle). Statins and efavirenz lowered p-tau in human neurons by reducing ester concentrations (bottom). [Courtesy of van der Kant et al., 2019.]
The new findings emerged from an unbiased screen. Co-first authors Rik van der Kant and Vanessa Langness screened more than 1,600 FDA-approved or preclinical compounds for reduction of p-tau in neurons. Specifically, the researchers looked for changes in pThr231-tau/total tau ratios in induced pluripotent stem cell (iPSC)-derived neurons from a familial AD (FAD) patient carrying a duplication in the APP gene. Of 42 non-toxic hits that emerged, four were cholesterol-lowering statins: atorvastatin, simvastatin, fluvastatin, and rosuvastatin. Two lowered multiple isoforms of p-tau relative to total tau. The same happened in neurons from patients with late-onset sporadic AD and even in those from non-demented controls.
Statins squelch the first step in the cholesterol biosynthesis pathway by blocking the conversion of HMG-CoA into mevalonate. However, the researchers found that interfering with a much later step in the cholesterol pathway also reduced p-tau/tau ratio in the derived neurons. Specifically, the ratio dropped when the researchers dosed neurons with efavirenz, an allosteric activator of cholesterol 24-hydroxylase, aka CYP46A1. This enzyme, which is only expressed in the central nervous system, converts cholesterol into 24(S) hydroxycholesterol. Normally, neurons quickly export 24-OH-cholesterol, leaving less of it around to make cholesteryl esters. The neurons’ p-tau/tau ratio plummeted when the researchers overexpressed CYP46A1, or blocked Acyl-CoA cholesterol acyltransferase (ACAT), the enzyme that esterifies cholesterol, using the inhibitors avasimibe or K604. Conversely, the ratio shot up in response to exogenous cholesteryl esters. Together, these findings suggested that cholesteryl esters, not free cholesterol, elevated p-tau.
Why would this be? The scientists hypothesized that cholesteryl esters interfere with tau’s degradation. Though treatment with statins and efavirenz lowered the ratio of p-tau to tau, the absolute levels of both also dropped. However, when proteasome inhibitors were present, the drugs failed to lower tau. It’s not any type of degradation, though: Blockers of lysosomal/autophagosomal processing had no effect. Finally, the scientists found that statins or efavirenz increased levels of some proteasomal subunits, and revved up overall proteasome activity. Together, the data suggest that cholesterol esters obstruct the proteasome, letting tau build up. Goldstein does not know why this would change the p-tau/tau ratio.
Goldstein and others emphasized that cholesteryl esters enhance the production of tau independently of Aβ and not downstream of it. The details of proteasome inhibition remain to be worked out, and Goldstein acknowledged that efavirenz and similar drugs might enhance the degradation of many other proteins, as well.
Separately, the researchers extended to FAD neurons a previous observation made in mouse models, namely, that statin treatment lowers secretion of Aβ (Oct 2004 news). However, the tau effect seems separate, because statins still reduced p-tau in isogenic APP-null neurons made by knocking out the gene in iPSCs derived from the patient harboring the APP duplication. The researchers also confirmed that cholesterol’s sway over Aβ secretion was mediated by its binding to a previously identified domain in the β-CTF portion of APP and influencing APP processing (Jun 2012 news). It is unclear whether cholesterol or its esters mediate this effect.
“This excellent study provides evidence that enhancing the activity of CYP46A1 and increasing the conversion of cholesteryl esters to 24-hydroxy-cholesterol may alleviate tau pathology,” wrote Per Svenningsson of the Karolinska Institute in Stockholm. The findings agree with Svenningsson’s recent study, which correlated 24-hydroxy-cholesterol levels with reduced tau in neuronal cells and in CSF in several tauopathies (Björkhem et al., 2018). “The concept of using efavirenz to treat tauopathies is novel and exciting,” he added.
Hudry thinks that activating cholesterol 24-hydroxylase, or downregulating ACAT, the enzyme that esterifies cholesterol, are potential therapeutic strategies. “Because CYP46A1 is only expressed in neuronal cells, it is an especially well-suited drug target, offering a unique opportunity to alleviate early AD neuropathological changes without interfering with the maintenance of cholesterol homeostasis in other cells or tissues,” she wrote (see comment below).
Irina Pikuleva of Case Western Reserve University in Cleveland is heading the mild cognitive impairment trial of efavirenz (see clinicaltrials.gov). It aims to find a dose that safely boosts 24-hydroxy-cholesterol levels in serum. Because this metabolite is only produced by CYP46A1 in the CNS, even traces of it in serum reflect activity of the enzyme in the brain, Pikuleva explained. Efavirenz is toxic when used to treat HIV, but Pikuleva and Goldstein believe much lower doses will suffice to reduce cholesteryl esters in the brain. In fact, at high doses, efavirenz inhibits, rather than activates, CYP46A1.
Pikuleva sees therapeutic potential for efavirenz in AD. She previously reported that the drug lowered Ab plaque burden and lessened memory loss in in 5xFAD mice and, on the flip side, other scientists reported that lowering CYP46A1 in the APP23 mouse brain worsened their plaque burden, tau phosphorylation, and memory loss (Mast et al., 2017; Djelti et al., 2015).—Jessica Shugart
- ACAT and Mouse—Inhibiting Former Prevents AD-like Pathology in Latter
- Cholesterol Binds APP Fragment, May Direct It to Lipid Rafts
Research Models Citations
- Björkhem I, Patra K, Boxer AL, Svenningsson P. 24S-Hydroxycholesterol Correlates With Tau and Is Increased in Cerebrospinal Fluid in Parkinson's Disease and Corticobasal Syndrome. Front Neurol. 2018;9:756. Epub 2018 Sep 7 PubMed.
- Mast N, Saadane A, Valencia-Olvera A, Constans J, Maxfield E, Arakawa H, Li Y, Landreth G, Pikuleva IA. Cholesterol-metabolizing enzyme cytochrome P450 46A1 as a pharmacologic target for Alzheimer's disease. Neuropharmacology. 2017 Sep 1;123:465-476. Epub 2017 Jun 24 PubMed.
- Djelti F, Braudeau J, Hudry E, Dhenain M, Varin J, Bièche I, Marquer C, Chali F, Ayciriex S, Auzeil N, Alves S, Langui D, Potier MC, Laprevote O, Vidaud M, Duyckaerts C, Miles R, Aubourg P, Cartier N. CYP46A1 inhibition, brain cholesterol accumulation and neurodegeneration pave the way for Alzheimer's disease. Brain. 2015 Aug;138(Pt 8):2383-98. Epub 2015 Jul 2 PubMed.
- van der Kant R, Langness VF, Herrera CM, Williams DA, Fong LK, Leestemaker Y, Steenvoorden E, Rynearson KD, Brouwers JF, Helms JB, Ovaa H, Giera M, Wagner SL, Bang AG, Goldstein LS. Cholesterol Metabolism Is a Druggable Axis that Independently Regulates Tau and Amyloid-β in iPSC-Derived Alzheimer's Disease Neurons. Cell Stem Cell. 2019 Jan 16; PubMed.