Lectin-based proteomic profiling of aged skeletal muscle: Decreased pyruvate kinase isozyme M1 exhibits drastically increased levels of N-glycosylation

Abstract

Since various neuromuscular diseases are associated with abnormal glycosylation, it was of interest to determine whether this key post-translational modification is also altered in aged skeletal muscle. Lectins represent highly versatile carbohydrate-binding proteins that are routinely employed for the characterization of glycoproteins. Here, we used the lectin wheat germ agglutinin (WGA) for the proteomic profiling of senescent fibers. WGA labeling of the soluble proteome from 3-month- versus 30-month-old rat gastrocnemius muscle, following two-dimensional gel electrophoretic separation, resulted in the identification of 13 distinct protein species. Analysis of WGA binding levels, in conjunction with mass spectrometric fingerprinting, revealed that one isoform of a major metabolic muscle protein exhibited a drastic alteration in the content of sialic acid and N-acetylglucosaminyl sugar residues. Pyruvate kinase isoform M1 with protein accession number gi|16757994|, exhibiting a pI of 6.6 and an apparent molecular mass of 57.8 kDa, showed a six fold increase in N-glycosylation and a three fold decrease in protein expression. In contrast to comparable levels of N-glycosylated proteins in young adult versus senescent muscle, as judged by fluorescein-conjugated WGA labeling of transverse muscle cryosections, staining with antibodies to the M1 isoform of pyruvate kinase showed reduced expression of this cytosolic element. Furthermore, activity assays demonstrated a reduced activity of this glycolytic enzyme in senescent muscle. This agrees with the idea that abnormal post-translational modifications in key metabolic enzymes may be involved in the conversion of aged muscle to slower twitch patterns and a drastic shift to more aerobic-oxidative metabolism.