The APOEε3/ε4 Genotype Drives Distinct Gene Signatures in the Cortex of Young Mice

Abstract

IntroductionRestrictions on existing APOE mouse models have impacted research toward understanding the strongest genetic risk factor contributing to Alzheimer’s disease (AD) and dementia, APOEε4, by hindering observation of a key, common genotype in humans – APOEε3/ε4. Human studies are typically underpowered to address APOEε4 allele risk as the APOEε4/ε4 genotype is rare, which leaves human and mouse research unsupported to evaluate the APOEε3/ε4 genotype on molecular and pathological risk for AD and dementia.MethodsAs a part of MODEL-AD, we created and validated new versions of humanized APOEε3/ε3 and APOEε4/ε4 mouse strains that, due to unrestricted breeding, allow for the evaluation of the APOEε3/ε4 genotype. As biometric measures are often translatable between mouse and human, we profiled circulating lipid concentrations. We also performed transcriptional profiling of the cerebral cortex at 2 and 4 months (mos), comparing APOEε3/ε4 and APOEε4/ε4 to the reference APOEε3/ε3 using linear modeling and WGCNA. Further, APOE mice were exercised and compared to litter-matched sedentary controls, to evaluate the interaction between APOEε4 and exercise at a young age.ResultsExpression of human APOE isoforms were confirmed in APOEε3/ε3, APOEε3/ε4 and APOEε4/ε4 mouse brains. At two mos, cholesterol composition was influenced by sex, but not APOE genotype. Results show that the APOEε3/ε4 and APOEε4/ε4 genotype exert differential effects on cortical gene expression. APOEε3/ε4 uniquely impacts ‘hormone regulation’ and ‘insulin signaling,’ terms absent in APOEε4/ε4 data. At four mos, cholesterol and triglyceride levels were affected by sex and activity, with only triglyceride levels influenced by APOE genotype. Linear modeling revealed APOEε3/ε4, but not APOEε4/ε4, affected ‘extracellular matrix’ and ‘blood coagulation’ related terms. We confirmed these results using WGCNA, indicating robust, yet subtle, transcriptional patterns. While there was little evidence of APOE genotype by exercise interaction on the cortical transcriptome at this young age, running was predicted to affect myelination and gliogenesis, independent of APOE genotype with few APOE genotype-specific affects identified.DiscussionAPOEε4 allele dosage-specific effects were observed in circulating lipid levels and cortical transcriptional profiles. Future studies are needed to establish how these data may contribute to therapeutic development in APOEε3/ε4 and APOEε4/ε4 dementia patients.

Independent APOE4 knock‐in mouse models display reduced brain APOE protein, altered neuroinflammation, and simplification of dendritic spines

“…Although APOE ‐targeted replacement mice have been a critical tool to study APOE isoform‐specific effects on brain structure, activity, and neurochemistry, studying the effects of the APOE genotype in a different mouse model allows the identification of which phenotypes are the strongest and most reproducible. In the current study, we compared measurements associated with APOE risk in a previously used human APOE ‐targeted replacement model ( APOE3 ‐TR and APOE4 ‐TR) (Sullivan et al, 1997) with a model developed more recently (Foley et al, 2022) and distributed by Jackson Laboratory (JAX‐E3 and JAX‐E4). We used the JAX‐APOE mice to measure the levels and distribution of APOE protein in the brain and the levels of pro‐inflammatory cytokines.…”

APOE ε4 and exercise interact in a sex‐specific manner to modulate dementia risk factors

“…Novel APOE mouse strains were created on C57BL/6J (B6) and maintained at The Jackson Laboratory as previously described 34 . Mice were kept in a 12/12‐hour light/dark cycle (06:00–18:00 light) and fed ad libitum 6% kcal fat standard mouse chow.…”

“…We performed RNA sequencing on six brains per group (sex/genotype/activity) at 12 months. RNA extraction, library construction, RNA sequencing, and seq quality control were performed as described previously 34,38 . Genes were then filtered by (1) removing all genes that did not vary in expression (gene count change across all samples was 0) and (2) removing all genes that did not have at least five reads in 50% of the samples.…”

“…Novel APOE mouse strains were created on C57BL/6J (B6) and maintained at The Jackson Laboratory as previously described. 34 Mice were…”

Apolipoprotein E intersects with amyloid-β within neurons

“…Cellular trafficking of ApoE might however differ depending on the lipidation of ApoE. Therefore, we next studied the cellular localization of ApoE under more physiological conditions by using ApoE particles obtained from humanized ApoE3-knock-in (KI) or ApoE4-KI primary astrocytes 36 . Astrocytes are the main source of ApoE in the brain 37 and astrocyte-derived ApoE is lipidated 38 and transported to other cell types including neurons.…”