APOE2 Heterozygosity Reduces Hippocampal Soluble Amyloid-β42 Levels in Non-Hyperlipidemic Mice

APOE2 lowers Alzheimer's disease (AD) risk; unfortunately, the mechanism remains poorly understood and the use of mice models is problematic as APOE2 homozygosity is associated with hyperlipidemia. In this study, we developed mice that are heterozygous for APOE2 and APOE3 or APOE4 and overexpress amyloid-β peptide (Aβ) (EFAD) to evaluate the effect of APOE2 dosage on Aβ pathology. We found that heterozygous mice do not exhibit hyperlipidemia. Hippocampal but not cortical levels of soluble Aβ42 followed the order E2/2FAD > E2/3FAD≤E3/3FAD and E2/2FAD > E2/4FAD < E4/4FAD without an effect on insoluble Aβ42. These findings offer initial insights on the impact of APOE2 on Aβ pathology.

A novel apoE-mimetic increases brain apoE levels, reduces Aβ pathology and improves memory when treated before onset of pathology in male mice that express APOE3

BACKGROUND

Alzheimer's disease (AD) is characterized by cognitive dysfunction and amyloid plaques composed of the amyloid-beta peptide (Aβ). APOE is the greatest genetic risk for AD with APOE4 increasing risk up to ~ 15-fold compared to APOE3. Evidence suggests that levels and lipidation of the apoE protein could regulate AD progression. In glia, apoE is lipidated via cholesterol efflux from intracellular pools, primarily by the ATP-binding cassette transporter A1 (ABCA1). Therefore, increasing ABCA1 activity is suggested to be a therapeutic approach for AD. CS-6253 (CS) is a novel apoE mimetic peptide that was developed to bind and stabilize ABCA1 and maintain its localization into the plasma membrane therefore promoting cholesterol efflux. The goal of this study was to determine whether CS could modulate apoE levels and lipidation, Aβ pathology, and behavior in a model that expresses human APOE and overproduce Aβ.

METHODS

In vitro, APOE3-glia or APOE4-glia were treated with CS. In vivo, male and female, E3FAD (5xFAD+/-/APOE3+/+) and E4FAD (5xFAD+/-/APOE4+/+) mice were treated with CS via intraperitoneal injection at early (from 4 to 8 months of age) and late ages (from 8 to 10 months of age). ApoE levels, ABCA1 levels and, apoE lipidation were measured by western blot and ELISA. Aβ and amyloid levels were assessed by histochemistry and ELISA. Learning and memory were tested by Morris Water Maze and synaptic proteins were measured by Western blot.

RESULTS

CS treatment increased apoE levels and cholesterol efflux in primary glial cultures. In young male E3FAD mice, CS treatment increased soluble apoE and lipid-associated apoE, reduced soluble oAβ and insoluble Aβ levels as well as Aβ and amyloid deposition, and improved memory and synaptic protein levels. CS treatment did not induce any therapeutic benefits in young female E3FAD and E4FAD mice or in any groups when treatment was started at later ages.

CONCLUSIONS

CS treatment reduced Aβ pathology and improved memory only in young male E3FAD, the cohort with the least AD pathology. Therefore, the degree of Aβ pathology or Aβ overproduction may impact the ability of targeting ABCA1 to be an effective AD therapeutic. This suggests that ABCA1-stabilizing treatment by CS-6253 works best in conditions of modest Aβ levels.

APOE genotype and sex modulate Alzheimer's disease pathology in aged EFAD transgenic mice

Increasing evidence supports that age, APOE and sex interact to modulate Alzheimer's disease (AD) risk, however the underlying pathways are unclear. One way that AD risk factors may modulate cognition is by impacting amyloid beta (Aβ) accumulation as plaques, and/or neuroinflammation Therefore, the goal of the present study was to evaluate the extent to which age, APOE and sex modulate Aβ pathology, neuroinflammation and behavior in vivo. To achieve this goal, we utilized the EFAD mice, which express human APOE3 or APOE4 and have five familial AD mutations (FAD) that result in Aβ42 overproduction. We assessed Aβ levels, reactive glia and Morris water maze performance in 6-, 10-, 14-, and 18-month-old EFAD mice. Female APOE4 mice had the highest Aβ deposition, fibrillar amyloid deposits and neuroinflammation as well as earlier behavior deficits. Interestingly, we found that female APOE3 mice and male APOE4 mice had similar levels of pathology. Collectively our data support that the combination of APOE4 and female sex is the most detrimental combination for AD, and that at older ages, female sex may be equivalent to APOE4 genotype.

A small-molecule TLR4 antagonist reduced neuroinflammation in female E4FAD mice

BACKGROUND

APOE genotype is the greatest genetic risk factor for sporadic Alzheimer's disease (AD). APOE4 increases AD risk up to 12-fold compared to APOE3, an effect that is greater in females. Evidence suggests that one-way APOE could modulate AD risk and progression through neuroinflammation. Indeed, APOE4 is associated with higher glial activation and cytokine levels in AD patients and mice. Therefore, identifying pathways that contribute to APOE4-associated neuroinflammation is an important approach for understanding and treating AD. Human and in vivo evidence suggests that TLR4, one of the key receptors involved in the innate immune system, could be involved in APOE-modulated neuroinflammation. Consistent with that idea, we previously demonstrated that the TLR4 antagonist IAXO-101 can reduce LPS- and Aβ-induced cytokine secretion in APOE4 glial cultures. Therefore, the goal of this study was to advance these findings and determine whether IAXO-101 can modulate neuroinflammation, Aβ pathology, and behavior in mice that express APOE4.

METHODS

We used mice that express five familial AD mutations and human APOE3 (E3FAD) or APOE4 (E4FAD). Female and male E4FAD mice and female E3FAD mice were treated with vehicle or IAXO-101 in two treatment paradigms: prevention from 4 to 6 months of age or reversal from 6 to 7 months of age. Learning and memory were assessed by modified Morris water maze. Aβ deposition, fibrillar amyloid deposition, astrogliosis, and microgliosis were assessed by immunohistochemistry. Soluble levels of Aβ and apoE, insoluble levels of apoE and Aβ, and IL-1β were measured by ELISA.

RESULTS

IAXO-101 treatment resulted in lower Iba-1 coverage, lower number of reactive microglia, and improved memory in female E4FAD mice in both prevention and reversal paradigms. IAXO-101-treated male E4FAD mice also had lower Iba-1 coverage and reactivity in the RVS paradigm, but there was no effect on behavior. There was also no effect of IAXO-101 treatment on neuroinflammation and behavior in female E3FAD mice.

CONCLUSION

Our data supports that TLR4 is a potential mechanistic therapeutic target for modulating neuroinflammation and cognition in APOE4 females.

Inhibition of ACAT as a Therapeutic Target for Alzheimer's Disease Is Independent of ApoE4 Lipidation

APOE4, encoding apolipoprotein E4 (apoE4), is the greatest genetic risk factor for Alzheimer's disease (AD), compared to the common APOE3. While the mechanism(s) underlying APOE4-induced AD risk remains unclear, increasing the lipidation of apoE4 is an important therapeutic target as apoE4-lipoproteins are poorly lipidated compared to apoE3-lipoproteins. ACAT (acyl-CoA: cholesterol-acyltransferase) catalyzes the formation of intracellular cholesteryl-ester droplets, reducing the intracellular free cholesterol (FC) pool. Thus, inhibiting ACAT increases the FC pool and facilitates lipid secretion to extracellular apoE-containing lipoproteins. Previous studies using commercial ACAT inhibitors, including avasimibe (AVAS), as well as ACAT-knock out (KO) mice, exhibit reduced AD-like pathology and amyloid precursor protein (APP) processing in familial AD (FAD)-transgenic (Tg) mice. However, the effects of AVAS with human apoE4 remain unknown. In vitro, AVAS induced apoE efflux at concentrations of AVAS measured in the brains of treated mice. AVAS treatment of male E4FAD-Tg mice (5xFAD+/-APOE4+/+) at 6-8 months had no effect on plasma cholesterol levels or distribution, the original mechanism for AVAS treatment of CVD. In the CNS, AVAS reduced intracellular lipid droplets, indirectly demonstrating target engagement. Surrogate efficacy was demonstrated by an increase in Morris water maze measures of memory and postsynaptic protein levels. Amyloid-beta peptide (Aβ) solubility/deposition and neuroinflammation were reduced, critical components of APOE4-modulated pathology. However, there was no increase in apoE4 levels or apoE4 lipidation, while amyloidogenic and non-amyloidogenic processing of APP were significantly reduced. This suggests that the AVAS-induced reduction in Aβ via reduced APP processing was sufficient to reduce AD pathology, as apoE4-lipoproteins remained poorly lipidated.

Role of estrogen in women's Alzheimer's disease risk as modified by APOE

Alzheimer's disease (AD) is characterized by numerous sexual dimorphisms that impact the development, progression, and probably the strategies to prevent and treat the most common form of dementia. In this review, we consider this topic from a female perspective with a specific focus on how women's vulnerability to the disease is affected by the individual and interactive effects of estrogens and apolipoprotein E (APOE) genotype. Importantly, APOE appears to modulate systemic and neural outcomes of both menopause and estrogen-based hormone therapy. In the brain, dementia risk is greater in APOE4 carriers, and the impacts of hormone therapy on cognitive decline and dementia risk vary according to both outcome measure and APOE genotype. Beyond the CNS, estrogen and APOE genotype affect vulnerability to menopause-associated bone loss, dyslipidemia and cardiovascular disease risk. An emerging concept that may link these relationships is the possibility that the effects of APOE in women interact with estrogen status by mechanisms that may include modulation of estrogen responsiveness. This review highlights the need to consider the key AD risk factors of advancing age in a sex-specific manner to optimize development of therapeutic approaches for AD, a view aligned with the principle of personalized medicine.