Cholesterol, Apoe4, and Alzheimer’s disease

Alzheimer’s Afternoons Seminar Series (April 2)

This seminar was presented by Dr. Rik van der Kant, a Research Associate in both the Faculty of Science, (Functional Genomics) and Amsterdam Neuroscience (Neurodegeneration) at Vrije Universiteit Amsterdam.  He is an expert on cholesterol metabolism and is studying how apoe4 contributes to LOAD.  The title of his seminar was “Cholesterol metabolism as a dual driver of neuronal AD pathology”.  He described his recent work that was published (open access) here:

van der Kant, R.; Langness, V. F.; Herrera, C. M.; Williams, D. A.; Fong, L. K.; Leestemaker, Y.; Steenvoorden, E.; Rynearson, K. D.; Brouwers, J. F.; Helms, J. B.; Ovaa, H.; Giera, M.; Wagner, S. L.; Bang, A. G.; Goldstein, L. S. B. Cholesterol Metabolism Is a Druggable Axis That Independently Regulates Tau and Amyloid-β in IPSC-Derived Alzheimer’s Disease Neurons. Cell Stem Cell 2019, 24 (3), 363-375.e9.

Dr. van der Kant began by reviewing how human cells can be converted in stem cells, then into almost any other type of cells.  This is useful for making populations of neurons, astrocyctes, and microglia carrying apoe4 genes.  This work is described in another recent article, found here:

van der Kant, R.; Goldstein, L. S. B.; Ossenkoppele, R. Amyloid-β-Independent Regulators of Tau Pathology in Alzheimer Disease. Nat. Rev. Neurosci. 2020, 21 (1), 21–35.

He explained that when such cells are created from patients with LOAD they produce high levels of amyloid beta (Aβ40) and hyperphosphorylated tau (p-tau) – hallmarks of the disease.

Recall that amyloid beta is a peptide fragment of the large amyloid precursor protein (APP), which occurs on many types of cells. APP is cleaved by the enzymes beta secretase and gamma secretase to yield Aβ.  Amyloid beta fragments can be 36 to 43 amino acids long, with Aβ40 and Aβ42 the mostly commonly studied forms. These fragments can exist as a single monomer unit, or combine to form larger oligomers, fibrils and plaques.  Misfolded oligomer fragments can act as “seeds” inducing other amyloid beta fragments to misfold and aggregate, a process akin to a prionic infection.

(This is easily imagined by recalling the Slinky, a once-popular children’s toy – essentially a coil of wire which “walked down stairs, alone or in pairs” until becoming a tangled mess.  Imagine placing a tangled Slinky (a “seed”) into the drawer of new, perfectly-coiled Slinkys.  The inevitable outcome is a drawer full of “misfolded and aggregated” toys.)

Tau proteins are essential components of cell cytoskeletons, specifically the assembly of microtubules.  However, tau proteins that have become hyperphosphorylated – covered in extra phosphate groups – also tend to misfold and stick together, becoming insoluble aggregates called neurofibrillary tangles.

Dr. van der Kant’s group used these populations of human cells grown in culture to test 1684 potential drugs to determine if they could lower either Aβ40 or p-tau.  They identified 160 promising compounds, including 42 of which might be safe and/or effective in patients.

Interestingly, all statin drugs they tested were effective.  This is explained more fully in their article published in the journal Cell Stem Cell. He mentioned that in previous studies statins have been shown to reduce AD risk. In their cell cultures, several statins consistently reduced p-tau accumulation in a dosage dependent manner.  He noted that atorvastatin stood out as being just a bit better than the others in this regard.

This has also been noted in previous studies, including:

“Results of this analysis revealed that statin users had better cognitive scores than nonusers; and this effect was somewhat more evident with the use of some lipophilic statins (atorvastatin and lovastatin).”  Geifman, N., Brinton, R.D., Kennedy, R.E. et al. Evidence for benefit of statins to modify cognitive decline and risk in Alzheimer’s disease. Alz Res Therapy 9, 10 (2017).

However, other studies reporting that statins reduced the risk of developing LOAD found no clear differences between the various hydrophilic and lipophilic statins; for example:

Haag, M. D. M.; Hofman, A.; Koudstaal, P. J.; Stricker, B. H. C.; Breteler, M. M. B. Statins Are Associated with a Reduced Risk of Alzheimer Disease Regardless of Lipophilicity. The Rotterdam Study. Journal of Neurology, Neurosurgery & Psychiatry 2009, 80 (1), 13–17.

Probing the system further Dr. van der Kant’s group found that cholesterol itself was not the critical link to Aβ and p-tau accumulation in the brain.  Rather they found that levels of cholesterol esters (CEs), a storage and transport form of the molecule, was key.

Cholesterol is an essential building block of animal cell membranes, and can be obtained from the diet or synthesized by cells.  It is required for the synthesis of steroid hormones and vitamin D.  It is not a water-soluble molecule and it is transported in blood within bubble-like lipoprotein particles.  The brain is a cholesterol hog.  It possesses 20% of the bodies cholesterol, by weight.

When cholesterol is bound to a fatty acid chain by an ester bond, cholesterol esters are formed.  Most cholesterol in our diets and in circulating lipoprotein particles are in this form.

These researchers found that lowering CE concentrations using statins – again in cells cultures from AD patients – lowers Aβ and p-tau levels.  These two effects seem to occur separately, by independent mechanisms.

Statin drugs inhibit the HMG CoA reductase enzyme, the rate limiting step in cholesterol production.

One interesting finding is that the amyloid precursor protein (APP; the cell surface protein that can be cleaved to form Aβ) has a molecular site that binds strongly to cholesterol.  Dr. van der Kant pointed out that eliminating this binding site prevented statins from lowering Aβ levels but that in this case p-tau was still reduced.  This is more solid evidence that statins reduce Aβ and p-tau in different ways, but at the same time.

Here Dr. van der Kant made an important point. He stressed that despite their ability to reduce LOAD incidence, to a modest degree, in large population studies, statins don’t generally work to treat LOAD in individual patients.  He suggested this is because statins either don’t reach the brain in high enough concentrations or because statins can be toxic to astrocytes, brain cells which nurture and protect neurons. He shared data to demonstrate this occurring in cell culture.

Similar statin toxicity has been observed previously.  For example:

“However, there are data indicating that statins can have toxic effects on brain neural cells….In general, what distinguishes the studies on protective effects of statins versus toxic effects are the drug concentrations used in the different studies.”  Wood, W. G.; Eckert, G. P.; Igbavboa, U.; Muller, W. E. Statins and Neuroprotection: A Prescription to Move the Field Forward. Annals of the New York Academy of Sciences 2010, 1199 (1), 69–76.

A review of the positive and negative effects of statins for LOAD is:

Schultz, B. G.; Patten, D. K.; Berlau, D. J. The Role of Statins in Both Cognitive Impairment and Protection against Dementia: A Tale of Two Mechanisms. Translational neurodegeneration 2018, 7 (1), 5–11.

In short, while large population studies sometimes find that long-term statin is associated with modest reduction in the incidence of LOAD, suggesting a potential preventative role, statins do not cure individual patients with Alzheimer’s disease and at high levels can be toxic to important brain cell types.

Dr. van der Kant’s group identified a different drug that seems to be effective, without harming astrocyctes.

Efavirnez, sold under the brand name Sustiva (among others), is an antiretroviral medication used to treat and prevent HIV/AIDS.  The researchers found that it crosses the blood brain barrier and binds specifically to an enzyme found primarily in neurons to effectively reduce CE levels.  This is an encouraging dataset.

He pointed out that accumulation of “lipid droplets” is a characteristic of AD first observed by Dr. Alois Alzheimer and suggested that these droplets could have been accumulations of excess CEs in the brains of LOAD patients.  This observation seems to fit with the theory that reducing CE over-production could reduce the accumulation of Aβ and p-tau.

Recall that the apoe4 risk gene seems to increase lipid droplet accumulation.

For more information about lipid droplets, apoe4, and LOAD, see the summary of Dr. Lance Johnson’s seminar from March 26.

Dr. van der Kant’s group – and others – have tested Efavirnez in mice and found it effective.  He suggested these previous, related studies focusing on this drug:

Mast, N.; Li, Y.; Linger, M.; Clark, M.; Wiseman, J.; Pikuleva, I. A. Pharmacologic Stimulation of Cytochrome P450 46A1 and Cerebral Cholesterol Turnover in Mice. J. Biol. Chem. 2014, 289 (6), 3529–3538.

Petrov, A. M.; Lam, M.; Mast, N.; Moon, J.; Li, Y.; Maxfield, E.; Pikuleva, I. A. CYP46A1 Activation by Efavirenz Leads to Behavioral Improvement without Significant Changes in Amyloid Plaque Load in the Brain of 5XFAD Mice. Neurotherapeutics 2019, 16 (3), 710–724.

In concluding, Dr. van der Kant hypothesized that therapeutics such as Efavirnez, which lower CE over-production, could be most beneficial for individuals carrying one or two copies of the apoe4 risk gene.  His group is hoping to continue on to clinical trials with this already-FDA approved drug. He cautions that the drug has some side effects, of course, and it remains to be seen if it is well tolerated.

NOTE: the original list of drugs with potential to lower Aβ and p-tau levels, presented by Dr. van der Kant, included some SSRIs.  There are some interesting links between SSRIs, statins, and cognition – and some suggestion that they work better together to treat mood disorders.  For example, see: Köhler, O.; Gasse, C.; Petersen, L.; Ingstrup, K. G.; Nierenberg, A. A.; Mors, O.; Østergaard, S. D. The Effect of Concomitant Treatment With SSRIs and Statins: A Population-Based Study. American Journal of Psychiatry 2016, 173 (8), 807–815.