Although the ApoE4 allele represents the primary genetic risk factor for late-onset Alzheimer’s disease, few researchers have attempted to target it therapeutically. Exactly how the allele promotes AD is only gradually becoming clearer, and the literature conflicts on such basic questions as whether raising or lowering ApoE would be the way to go. Two new papers in the May 21 Journal of Neuroscience strengthen the case for targeting ApoE, although the authors employ quite different strategies. Researchers led by David Holtzman at Washington University in St. Louis report that an antibody directed against mouse ApoE can clear amyloid deposits in AD model mice, while improving brain connectivity and learning. Meanwhile, researchers led by Daniel Michaelson at Tel Aviv University in Israel used the cancer drug bexarotene to increase lipidation of human ApoE4 in transgenic mice. This treatment reversed many of the pathologies associated with ApoE4, suppressing Aβ and phosphorylated tau while sharpening cognition. This finding supports the idea that the state of ApoE, rather than its level, may be crucial in determining its effects.
“The two papers provide further evidence that ApoE plays central roles in AD pathogenesis and is a legitimate target for therapy,” Gary Landreth at Case Western Reserve University, Cleveland, told Alzforum.
ApoE comes in three varieties: the harmful ApoE4 allele, the common ApoE3 allele, and ApoE2, which protects against Alzheimer’s disease (see Nov 2013 news story). ApoE4 has been most prominently linked to poor clearance of Aβ and greater amyloid accumulation (see Jul 2013 news story), though other mechanisms exist as well. Genetic studies by Holtzman’s group and others have found that halving the amount of ApoE3 or ApoE4 slows amyloid buildup (see Dec 2011 news story). They further showed that giving young AD mice an antibody to endogenous ApoE prevented Aβ deposits from forming (see Kim et al., 2012).
This raised the question of whether the same strategy would work in mice that already had amyloid deposits. To test this, first author Fan Liao injected the anti-ApoE antibody HJ6.3 into 7-month-old APP/PS1 mice once per week. At this age, the mice actively deposit amyloid and have plaques. After five months of treatment, plaque load nudged down by 20 percent in the cerebral cortex and 40 percent in the thalamus. Curiously, plaques in the hippocampus stayed put. The authors saw no change in soluble Aβ40 in the brain, but the peptide jumped up by 50 percent in plasma. This might indicate enhanced clearance from brain, they suggest.
To take a closer look, the authors followed the fate of individual plaques after exposing the surface of the brain to the HJ6.3 antibody for two weeks. Those plaques that contained ApoE and bound the antibody shrank or even disappeared over the course of treatment (see image). While there were fewer plaques, the overall effect on amyloid deposits was modest, as plaque load per square millimeter did not change, the authors noted. This contrasts with the antibody’s strong effect on preventing plaque formation in younger mice, Holtzman said.
It is unclear whether the antibody directly breaks up plaques by binding to ApoE, or stimulates microglia to gobble up plaques, Holtzman said. Intriguingly, the treatment only slightly lowered ApoE levels in the cerebral cortex. Soluble ApoE in the brain did not change, while the amount extractable by detergent dropped by about a quarter. This fraction might represent ApoE that resides in cell membranes or binds to the extracellular matrix, Holtzman said.
The authors then investigated the clinical benefits of treatment. Treated mice learned better than their untreated littermates in the Morris water maze, and also showed improved brain connectivity by functional connectivity optical intrinsic signal imaging. This technique is similar to functional MRI, except that blood flow is measured by shining different wavelengths of light into the brain and observing how they are absorbed or reflected by oxygen through detectors attached to the skull. In the treated mice, brain regions synchronized their activity better than they did in the untreated animals.
Commentators praised the study and said it supports further research into ApoE-based therapeutics. “The results from this study are very encouraging, and if they hold true, it would mean that only a slight reduction of ApoE in a specific ApoE pool is needed to get beneficial effects,” Henrietta Nielsen at the Mayo Clinic in Jacksonville, Florida, wrote to Alzforum (see full comment below).
This study, however, did not examine how human ApoE would behave. Holtzman plans to address this in future work by crossing transgenic mice carrying human ApoE3 or ApoE4 with AD model mice.
In the other paper, Michaelson and colleagues further studied the effect of bexarotene on human ApoE variants. Landreth’s lab had reported that this cancer drug lowers soluble Aβ and improves cognition in mice, although other researchers could not replicate these results (see Feb 2012 news story; May 2013 news story). Bexarotene stimulates retinoid X receptor (RXR) transcription factors, and in mouse models pumps up the expression of endogenous ApoE and two enzymes that load lipids onto proteins, ABCA1 and ABCG1 (see Feb 2012 conference story). Previous research indicates that human ApoE4 lacks lipids compared to ApoE3.
Michaelson and first author Anat Boehm-Cagan fed bexarotene for 10 days to transgenic mice in which endogenous ApoE had been replaced by either human ApoE3 or ApoE4. The treatment amplified expression of ABCA1 and ABCG1, as expected, but surprisingly, not that of ApoE. Michaelson noted that this may represent a difference between human and mouse protein, as other researchers have told him they found similar results. In treated human ApoE4 knock-ins, the amount of high-molecular weight ApoE increased to the levels seen in the ApoE3 knock-ins. This indicates a boost in ApoE4 lipidation, Michaelson said. Since ApoE4 levels were comparable to levels in untreated mice, this experiment allowed the authors to examine only the effects of increased lipidation.
They report that treatment normalized many aspects of the E4 knock-ins’ physiology. Levels of Aβ42 and phosphorylated tau in the hippocampus both dropped to levels seen in ApoE3 knock-ins. Meanwhile, levels of the presynaptic marker vesicular glutamatergic transporter 1 recovered. Treated ApoE4 knock-ins did better in the Morris water maze and novel object recognition tests, matching the performance of the E3 animals.
“The findings confirm the effect of bexarotene on cognitive performance. The lowering of phosphorylated tau is new and interesting, and should be explored further,” Radosveta Koldamova at the University of Pittsburgh told Alzforum. She was not involved in the research.
The data support increasing ApoE4 lipidation as a therapeutic strategy, Michaelson said. He is collaborating with biotech company Artery Therapeutics, Inc., of San Francisco to develop ABCA1 and ABCG1 agonists. The company will present preliminary data on this at the Alzheimer’s Association International Conference in July in Copenhagen.
Commentators agreed that this approach looks promising. “These data provide compelling evidence that increasing lipoprotein lipidation in the central nervous system may reverse the loss of function associated with ApoE4,” wrote Mary Jo LaDu and Leon Tai at the University of Illinois at Chicago (see full comment below).—Madolyn Bowman Rogers.
- Averting a Late-life Crisis: Midlife ApoE2 Clears Plaques in Mice
- A Genetic Approach to the ApoE4 Puzzle
- Lowering ApoE Brings Down Amyloid in Mice
- Upping Brain ApoE, Drug Treats Alzheimer's Mice
- Bexarotene Revisited: Improves Mouse Memory But No Effect on Plaques
- San Francisco: Tweaking Brain ApoE Reduces Aβ, Symptoms
Research Models Citations
- Kim J, Eltorai AE, Jiang H, Liao F, Verghese PB, Stewart FR, Basak JM, Holtzman DM. Anti-apoE immunotherapy inhibits amyloid accumulation in a transgenic mouse model of Aβ amyloidosis. J Exp Med. 2012 Nov 19;209(12):2149-56. PubMed.
- Liao F, Hori Y, Hudry E, Bauer AQ, Jiang H, Mahan TE, Lefton KB, Zhang TJ, Dearborn JT, Kim J, Culver JP, Betensky R, Wozniak DF, Hyman BT, Holtzman DM. Anti-ApoE antibody given after plaque onset decreases Aβ accumulation and improves brain function in a mouse model of Aβ amyloidosis. J Neurosci. 2014 May 21;34(21):7281-92. PubMed.
- Boehm-Cagan A, Michaelson DM. Reversal of apoE4-driven brain pathology and behavioral deficits by bexarotene. J Neurosci. 2014 May 21;34(21):7293-301. PubMed.