Basic Science and Pathogenesis

Background: Apolipoprotein ε4 (APOE4) is a major risk factor for Alzheimer's disease (AD). APOE4 carriers display altered whole-body metabolism, including increased blood glucose and inuslin. Although conditions affecting whole-body metabolism like obesity and diabetes are AD risk factors, knowledge regarding the contribution of peripheral tissues to this effect is minimal. Skeletal muscle is an important regulator of whole-body metabolism as a primary site for nutrient oxidation, yet it is unclear if APOE4 affects muscle metabolism. We sought to determine the impact of APOE4 on muscle bioenergetic function.

Method: Male and female APOE3 and APOE4 targeted-replacement mice on a C57BL/6 background (n = 61) from Taconic were fed a high (HFD, 45% fat) or low-fat diet (LFD, 10% fat) for 4 months before sacrifice. APOE3 mice served as controls because APOE3 does not modify AD risk. At 8-9 months old, carbohydrate-driven oxygen consumption in isolated quadriceps mitochondria was measured at basal, state 3 (ADP), state 3 + glutamate, and state 3S (succinate) on the Oroboros Oxygraph-2k and the gastrocnemius proteome was analyzed on a timsTOF HT mass spectrometer. Mitochondrial-related metabolic proteins were identified using MitoCarta3.0. Serum lactate was measured using an Abcam kit.

Result: APOE4 associated with differential expression of 49 proteins compared to APOE3 in LFD females only (q-value <0.05), including reduced levels of proteins involved in oxidative phosphorylation. APOE4 did not affect mitochondrial respiration. HFD altered the expression of 469 proteins in APOE4 females and between 80-101 proteins in other groups (q-value <0.05). HFD upregulated proteins involved in lipid metabolism in all groups. HFD upregulated proteins involved in carbohydrate metabolism and increased mitochondrial respiration (basal, p = 0.062; state 3, p = 0.009; state 3 +glutamate, p = 0.032; state 3S, p = 0.17) in APOE4 females only. HFD reduced serum lactate in APOE3 males (p = 0.041), APOE4 males (p = 0.081) and APOE3 females (p = 0.112).

Conclusion: HFD-driven upregulation of proteins involved in carbohydrate metabolism in APOE4 female muscle only likely supports increased carbohydrate oxidative capacity and may explain the lack of reduction in serum lactate in this group. Proteomic analysis of human muscle is ongoing to determine if this is reflected in humans.