Past research efforts, focused on Alzheimer's disease (AD) pathology and the genetics of familial AD, led to the discovery of important AD-related proteins, including amyloid-beta (Aβ) and tau. However, efforts to develop AD therapies based on targeting Aβ and tau have largely failed.
Years of disappointing trials have brought increasing demand for new AD targets and alternative treatment approaches. Using mice expressing apolipoprotein E4 (apoE4), a human protein that has long been associated with AD, Tel Aviv University scientists have recently made a compelling case for apoE4 as an important and feasible target for investigating novel Alzheimer's disease therapies.
- Disappointing trials over the years have brought increasing demand for new Alzheimer's disease targets and alternative treatment approaches.
- apoE4 has long been associated with Alzheimer's disease, but has been understudied as a therapeutic target.
- Recent research in transgenic mice have described several promising strategies to target apoE4.
APOE Alleles in Alzheimer's Disease
apoE is a class of apolipoprotein that associates with chylomicrons and intermediate-density lipoproteins to regulate peripheral cholesterol metabolism. In the brain, apoE has an important role, functioning as the principal cholesterol carrier.
Three polymorphic APOE alleles occur in the human population (APOE2, APOE3, and APOE4), differing by only a few amino acids. Despite their seemingly minor sequence differences, the APOE variants deviate significantly in their structural and functional features. This can result in pronounced effects on organismal biology.
The APOE4 allele has a frequency of roughly 14% and, among other pathological associations, is the largest known genetic risk factor for late-onset sporadic AD. Although the association between APOE4 and AD was established decades ago, apoE4 has not been a major target for AD therapy investigations. Research has focused predominantly on Aβ and, to a lesser extent, Tau proteins.
In comments to Taconic, Professor Daniel Michaelson, with the Department of Neurobiochemistry at Tel Aviv University, suggested that AD research focus has been too narrow: "The field was acting on the, not often stated openly, assumption that a single 'magic bullet' treatment for all Alzheimer's patients can be developed. There is a growing body of evidence that Alzheimer's is heterogeneous and, therefore, a single, fit-for-all 'magic bullet' will not work".
Dr. Michaelson believes that the AD field is ready for a paradigm shift and apoE4 may be the target to follow. Supporting this idea, he suggests the field "should focus on specific subpopulations of patients whose common denominator provides a therapeutic target that these patients are more likely to benefit from".
Alzheimer's Disease Therapies Targeting apoE4
In recent investigations, Professor Michaelson and colleagues explored counteracting the effects of apoE4. "In principle, the pathological effects of apoE4 may be due to a gain of bad function and could thus be reversed by removal/counteracting the effects of apoE4, or, alternatively, to a loss of a feature of the good form of the gene, in which case the therapeutic approach should be based on reversing a structural/biochemical feature of apoE4, rendering it similar to the good apoE."
Using transgenic mice harboring human APOE variants (APOE3 or APOE4) in place of the endogenous mouse ApoE, Professor Michaelson and his team investigated several distinct approaches to counteracting the effects of apoE4. Their recent studies suggest correcting a structural deficit in apoE4, counteracting aberrant downstream signaling associated with apoE4, and removal of apoE4 may all be viable approaches for future APOE4-related Alzheimer's disease therapies.
Correcting a Structural Deficit in APOE4
A structural distinction in apoE4, which has been suggested to contribute to AD pathology, is decreased lipidation compared to its corresponding AD-benign form, apoE3. Lipidation of apoE is mediated by ATP-binding cassette transporters A1 and G1 (ABCA1 and ABCG1, respectively) with transcriptional expression controlled by the retinoid X receptor (RXR) system.
In a recent study, Dr. Michaelson's team described the effects of bexarotene, an RXR agonist, on apoE4-driven pathological phenotypes in APOE4 vs. APOE3 targeted replacement mice. Bexarotene increased the mRNA and protein levels of ABCA1 and ABCG1 in hippocampal neurons, without affecting levels of apoE.
Bexarotene treatment was also associated with reversal of apoE4 lipidation deficiency and reversal of some cognitive impairments in APOE4 mice. Furthermore, bexarotene treatment reduced the accumulation of Aβ42 and hyperphosphorylated tau (AT8) in hippocampal neurons to levels comparable to APOE3 mice.
In a following study, Dr. Michaelson's team demonstrated that similar results could be obtained by substituting an ABCA1 agonist in place of bexarotene. Cumulatively, these data suggest the possibility that RXR or ABCA1 activation could be useful in the treatment of human APOE4 carriers.
Counteracting Downstream APOE4 Driven Signaling
APOE4 is associated with increased neurodegeneration and vascular impairments. Related to these pathologies, Dr. Michaelson's team observed decreased expression of vascular endothelial growth factor (VEGF), a key angiogenic factor, in the brains of APOE4 vs. APOE3 mice.
In their 2016 publication, Dr. Michaelson's team described the effects of increasing VEGF levels in the brains of APOE4 and APOE3 mice. Following intra-hippocampal injections of a VEGF-expressing lentivirus (LV-VEGF) into the brains of APOE3 and APOE4 mice, increased brain VEGF expression was associated with reversed apoE4-driven cognitive impairments. However, in APOE3 mice, LV-VEGF treatment resulted in increased Aβ42 and AT8, which confounded the interpretation of effects in APOE4 mice.
In following experiments, the researchers changed their VEGF-delivery system to a VEGF-expressing adeno-associated-virus (AAV-VEGF), which localized VEGF expression to astrocytes. This modification prevented increased Aβ42 and AT8 in APOE3 mice, and reversed apoE4-driven accumulation of Aβ42 and AT8.
These results suggest that apoE4-driven pathologies involve a downstream VEGF-dependent pathway. Thus, novel therapies that correct VEGF expression in specific CNS cells may prove effective for treating APOE4 carriers in AD.
Removal of APOE4
In a 2016 Current Alzheimer Research report, Dr. Michaelson's team studied the extent to which the pathological effects of apoE4 can be counteracted with an apoE4-specific monoclonal antibody (mAb). The research team reported intracerebroventricular administration of the mAb prevented apoE4-driven accumulation of Aβ in hippocampal neurons, providing initial validation for their approach.
In subsequent experiments, Dr. Michaelson's team delivered their monoclonal antibody to mice using intraperitoneal (IP) injections. These treatments resulted in specific apoE4/IgG complexes that were associated with reversal of the cognitive impairments and reversal of key apoE4-driven β42 and AT8 accumulation.
The results from this study suggest anti-apoE4 immunotherapy can counteract the deleterious effects of apoE4. Importantly, IP administration of anti-apoE4 mAbs proved effective in APOE4 mice, suggesting therapeutic antibody levels can cross the blood-brain-barrier. Such an approach could be adapted in the clinic to benefit human APOE4 carriers.