Having the ApoE4 gene variant is one of the strongest risk factors for developing late-onset Alzheimer's disease. Now, scientists have found out why. ApoE4 is less able to control production of the inflammatory cytokine cyclophilin A than other variants of the ApoE gene, and high levels of cyclophilin A set off inflammatory processes that ultimately damage the blood-brain barrier and the brain vasculature.
The process that senior author Berislav Zlokovic and his team have described is independent of amyloid-beta, a major target of drug developers but one that has yielded disappointing results in the clinic. Their paper, which appeared in the May 16, 2012, advance online edition of Nature, "describes an A-beta independent pathway," Zlokovic told BioWorld Today, which is consistent with scientific "consensus that many of the ApoE4 effects are A-beta independent."
Previous studies had shown that mice that are ApoE knockouts, or have the human variant ApoE4, end up with damage to their blood-brain barrier. And so Zlokovic, who is director of the Center for Neurodegeneration and Regeneration at the University of Southern California's Zilkha Neurogenetic Institute, and his team set out to understand how.
In humans, ApoE comes in three forms, ApoE 2, 3 and 4. The scientists compared mice with each of the three ApoE alleles, as well as ApoE knockouts. In earlier studies, cyclophilin A was responsible for vasculature damage in ApoE knockouts with aneurysm and atherosclerosis, and so the team tested whether inhibiting cyclophilin A, or knocking it out, could mitigate some of the problems of knockouts and animals with ApoE4.
The reason ApoE4 can't control cyclophilin A lies in its binding – or lack thereof – to another protein: low-density lipoprotein receptor protein (LRP1). LRP1, Zlokovic explained, "controls the transcription of cyclophilin A" when it is bound to ApoE.
Evolutionarily speaking, ApoE4 is by far the oldest of the ApoE variants. E2 and E3 "came much later, and have a different structure" – a structure that allows them to bind strongly to LRP1, and so keep cyclophilin subdued. "But ApoE4 cannot do it," Zlokovic said. "It doesn't have much affinity for LRP1 at all," and so cyclophilin is expressed at higher levels in individuals with the ApoE4 variant.
High levels of cyclophilin, in turn, lead to the activation of other proteins – specifically, NF-kappaB and matrix metalloproteinase (MMP9). MMP9 is an enzyme that chews up the extracellular matrix proteins, and its activation leads, among other things, to a leaky blood-brain barrier. That, in turn, allows any number of substances to cross into the brain that the barrier is designed to keep out – with the net effect, Zlokovic said, that "neurons just accumulate tons of these toxins, which then block their function."
The blood-brain barrier breakdown also had another effect. It damaged the minuscule blood vessels that are extremely important in order to keep neurons – which use up far more energy than other cell types – supplied with oxygen and nutrients.
The results have several different implications for how to possibly target Alzheimer's disease. They suggested the inhibition of cyclophilin A as one obvious possibility. But Zlokovic noted that "the pathway has been very clearly elucidated," beyond cyclophilin A, and that therapeutically speaking, downstream components NF-kappaB and MMP9 are also worth a look.
His team is also developing a brain imaging test to see whether the blood supply of the hippocampus, which is important for learning and memory, varies according to which ApoE variant a person has.
Last of all, Zlokovic and his team want to see whether activated protein C might be useful for mitigating the effects of ApoE4 – for now, in animals. Recombinant activated protein C was approved as a sepsis treatment in 2001. Zlokovic and his team have also looked at its effects on amyotrophic lateral sclerosis. (See BioWorld Today, Oct. 20, 2009.)
Maker Eli Lilly & Co. withdrew recombinant activated protein C, which it had sold under the name Xigris, in 2011, after a study showed it was no more effective than placebo for the treatment of severe sepsis. But Zlokovic said that given its effects, the drug may be worth looking at for Alzheimer's disease. "It tightens the blood barrier, and is vasculoprotective," he said – the same reason his team looked at the drug for ALS.