Sialylation and muscle performance: sialic acid is a marker of muscle ageing

Abstract

Sialic acids (Sia) are widely expressed as terminal monosaccharides on eukaryotic glycoconjugates. They are involved in many cellular functions, such as cell-cell interaction and signal recognition. The key enzyme of sialic acid biosynthesis is the bifunctional UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE), which catalyses the first two steps of Sia biosynthesis in the cytosol. In this study we analysed sialylation of muscles in wild type (C57Bl/6 GNE (+/+)) and heterozygous GNE-deficient (C57Bl/6 GNE (+/-)) mice. We measured a significantly lower performance in the initial weeks of a treadmill exercise in C57Bl/6 GNE (+/-) mice compared to wild type C57Bl/6 GNE (+/+) animals. Membrane bound Sia of C57Bl/6 GNE (+/-) mice were reduced by 33-53% at week 24 and by 12-15% at week 80 in comparison to C57Bl/6 GNE (+/+) mice. Interestingly, membrane bound Sia concentration increased with age of the mice by 16-46% in C57Bl/6 GNE (+/+), but by 87-207% in C57Bl/6 GNE (+/-). Furthermore we could identify specific morphological changes in aged muscles. Here we propose that increased Sia concentrations in muscles are a characteristic feature of ageing and could be used as a marker for age-related changes in muscle.

Figure 1. Treadmill exercise in the cohort of the wild type C57Bl/6 GNE ^+/+ (black) and the C57Bl/6 GNE ^+/− (grey)

(A). Electron microscopic findings of 80-week old female mice: depicts subsarcolemmal nucleus and perinuclear vacuole containing abundant dense granular material (unlikely to be lipofuscin) in the wild type mouse. Adjacent to the large vacuole are round structures (arrowhead) of sarcoplasmatic tubules, which are confined by double membranes and contain dark granules and in the centre a corpuscule of medium densitiy (gastrocnemius muscle at 16.700×magnification) (B). The perinuclear vacuole is surrounded by a single membrane in the heterozygous mouse and displays similar features. Note the two rounded structures in the vicinity containing either membranous structures or dense, granular material (arrowhead) (quadriceps muscle at 16.700×magnification) (C). Quantification of sialylation from membrane bound sialic acid in in anterior tibial, gastrocnemic, and quadriceps femoral muscle in wild type C57Bl/6 GNE ^+/+ (black bars) and the C57Bl/6 GNE ^+/− (gray bars). Values represent means ± 1 SD of three independent experiments (D). Relative sialic acid concentrations in different muscles (anterior tibial, gastrocnemic and quadriceps femoral muscle). Sialic acid concentration after 180 days was set to 100% and all other values were expressed in percent of this value. Note that maximal sialic acid concentration was reached after 180 days and no further increase of sialic acid concentration was observed after that time (E).