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Skeletal muscle (SM) is essential for movement, stability, and overall body function, and it readily adapts to changes in energy demand. Myogenesis is energy-intensive and involves complex molecular and cellular events. We recently demonstrated that the absence of lysosomal acid lipase (LAL) in vivo significantly impacts the SM phenotype, primarily by disrupting energy homeostasis and reducing ATP production. As systemic LAL deficiency affects multiple organs, we hypothesized that the altered SM phenotype resulted from systemic rather than SM-specific loss of LAL activity.To distinguish between systemic and cell-intrinsic effects, we used primary myoblasts isolated from Lal-deficient (-/-) mice as well as C2C12 cells treated with the pharmacological inhibitor of LAL, Lalistat-2. We found a significant accumulation of cholesteryl esters in both models studied, highlighting the central role of LAL in lipid catabolism in the SM. However, lipid accumulation was absent under lipoprotein-deficient culture conditions. Neither genetic loss nor pharmacological inhibition of LAL affected myofiber formation or mitochondrial function in vitro, in contrast to what we observed in SM isolated from Lal-/- mice. Tracing [13C6]-labeled glucose in both cell culture models revealed only minor changes in tricarboxylic acid cycle metabolites. These results suggest that although LAL plays an essential role in lipid metabolism, its impact on the processes involved in muscle differentiation and cellular energy production is minor. We conclude that the cell-intrinsic effects of Lal-/- SM are unlikely to drive the SM phenotype observed in vivo.

Objective: Lysosomal acid lipase (LAL) is the only enzyme known to hydrolyze cholesteryl esters (CE) and triacylglycerols in lysosomes at an acidic pH. Despite the importance of lysosomal hydrolysis in skeletal muscle (SM), research in this area is limited. We hypothesized that LAL may play an important role in SM development, function, and metabolism as a result of lipid and/or carbohydrate metabolism disruptions. Results: Mice with systemic LAL deficiency (Lal−/−) had markedly lower SM mass, cross-sectional area, and Feret diameter despite unchanged proteolysis or protein synthesis markers in all SM examined. In addition, Lal−/− SM showed increased total cholesterol and CE concentrations, especially during fasting and maturation. Regardless of increased glucose uptake, expression of the slow oxidative fiber marker MYH7 was markedly increased in Lal−/−SM, indicating a fiber switch from glycolytic, fast-twitch fibers to oxidative, slow-twitch fibers. Proteomic analysis of the oxidative and glycolytic parts of the SM confirmed the transition between fast- and slow-twitch fibers, consistent with the decreased Lal−/− muscle size due to the “fiber paradox”. Decreased oxidative capacity and ATP concentration were associated with reduced mitochondrial function of Lal−/− SM, particularly affecting oxidative phosphorylation, despite unchanged structure and number of mitochondria. Impairment in muscle function was reflected by increased exhaustion in the treadmill peak effort test in vivo. Conclusion: We conclude that whole-body loss of LAL is associated with a profound remodeling of the muscular phenotype, manifested by fiber type switch and a decline in muscle mass, most likely due to dysfunctional mitochondria and impaired energy metabolism, at least in mice.

Abstract Due to intact reactive oxygen species homeostasis and glucose metabolism, C57BL/6NRj mice are especially suitable to study cellular alterations in metabolism. We applied Nuclear Magnetic resonance spectroscopy to analyze five different tissues of this mouse strain during aging and included female and male mice aged 3, 6, 12, and 24 months. Metabolite signatures allowed separation between the age groups in all tissues, and we identified the most prominently changing metabolites in female and male tissues. A refined analysis of individual metabolite levels during aging revealed an early onset of age-related changes at 6 months, sex-specific differences in the liver, and a biphasic pattern for various metabolites in the brain, heart, liver, and lung. In contrast, a linear decrease of amino acids was apparent in muscle tissues. Based on these results, we assume that age-related metabolic alterations happen at a comparably early aging state and are potentially associated with a metabolic switch. Moreover, identified differences between female and male tissues stress the importance of distinguishing between sexes when studying age-related changes and developing new treatment approaches. Besides, metabolomic features seem to be highly dependent on the genetic background of mouse strains.