Relationship of Muscle Apolipoprotein E Expression with Markers of Cellular Stress, Metabolism, and Blood Biomarkers in Cognitively Healthy and Impaired Older Adults.

BACKGROUND: Individuals with mild cognitive impairment (MCI) have reduced lipid-stimulated mitochondrial respiration in skeletal muscle. A major risk factor for Alzheimer's disease (AD), the apolipoprotein E4 (APOE4) allele, is implicated in lipid metabolism and is associated with metabolic and oxidative stress that can result from dysfunctional mitochondria. Heat shock protein 72 (Hsp72) is protective against these stressors and is elevated in the AD brain. OBJECTIVE: Our goal was to characterize skeletal muscle ApoE and Hsp72 protein expression in APOE4 carriers in relationship to cognitive status, muscle mitochondrial respiration and AD biomarkers. METHODS: We analyzed previously collected skeletal muscle tissue from 24 APOE4 carriers (60y+) who were cognitively healthy (CH, n = 9) or MCI (n = 15). We measured ApoE and Hsp72 protein levels in muscle and phosphorylated tau181 (pTau181) levels in plasma, and leveraged previously collected data on APOE genotype, mitochondrial respiration during lipid oxidation, and VO2 max. RESULTS: Muscle ApoE (p = 0.013) and plasma pTau181 levels (p < 0.001) were higher in MCI APOE4 carriers. Muscle ApoE positively correlated with plasma pTau181 in all APOE4 carriers (R2 = 0.338, p = 0.003). Hsp72 expression negatively correlated with ADP (R2 = 0.775, p = <0.001) and succinate-stimulated respiration (R2 = 0.405, p = 0.003) in skeletal muscle of MCI APOE4 carriers. Plasma pTau181 negatively tracked with VO2 max in all APOE4 carriers (R2 = 0.389, p = 0.003). Analyses were controlled for age. CONCLUSION: This work supports a relationship between cellular stress in skeletal muscle and cognitive status in APOE4 carriers.