Impaired Mitochondrial Folate Metabolism Reduces Mitochondrial Respiration in Mouse Muscle Progenitor Cells

Mitochondrial dysfunction is a central contributor to aging. Mitochondrial DNA mutations increase with aging and are associated with low serum folate. Furthermore, both increased age and low serum folate are suggested to increase skeletal muscle fiber loss and impair muscle function. However, no research has focused on the metabolic pathway linking the causal relationship. Data from our lab indicate that folate-mediated one-carbon metabolism may play a role, as loss of the mitochondrial specific isozyme (serine hydroxymethyltransferase 2, SHMT2) and folate depletion both result in mitochondrial DNA instability and impaired mitochondrial function. Furthermore, serum folate and SHMT2 gene expression levels decrease with aging, potentially providing a causal mechanism for age-associated mitochondrial DNA mutations and skeletal muscle impairments. To date, little is known about the relationship between loss of SHMT2 enzyme expression and folate status in skeletal muscles. Tibialis anterior muscles were harvested from 10-week-old wild-type (Shmt2+/+) and heterozygous (Shmt2+/−) mice that were weaned onto either a folate-sufficient diet or a modified diet lacking folic acid. Skeletal muscles used to isolate primary muscle progenitor cells (MPCs) were isolated from 20-week-old Shmt2+/+ and Shmt2+/− mouse hindlimbs. MPCs were cultured to 80–90% confluency and then differentiated in low-serum medium for 7–10 days. Skeletal muscles were examined for mitochondrial DNA content and MPCs were examined for mitochondrial function. Heterozygous loss of Shmt2 resulted in an 80% reduction in SHMT2 protein levels in mouse tibialis anterior muscle. Mice fed a folate-depleted diet for 7 weeks exhibited a 25% reduction in mitochondrial DNA content compared to the folate-sufficient group. In MPCs, reduced Shmt2 expression impaired basal and maximal respiration. Furthermore, protein levels of mitochondrial NADH dehydrogenase 3 were reduced and protein levels of PGC1α and mitofusin 2 were elevated with Shmt2 heterozygosity in MPCs. Our results demonstrate that folate depletion reduced mitochondrial DNA content in tibialis anterior skeletal muscle and loss of SHMT2 impairs MPCs mitochondrial respiration and may influence mitochondrial biogenesis and dynamics. President's Council of Cornell Women Award.