Hyposialylation of neprilysin possibly affects its expression and enzymatic activity in hereditary inclusion‐body myopathy muscle

Abstract

Autosomal recessive hereditary inclusion‐body myopathy (h‐IBM) is caused by mutations of the UDP‐N‐acetylglucosamine 2‐epimerase/N‐acetylmannosamine kinase gene, a rate‐limiting enzyme in the sialic acid metabolic pathway. Previous studies have demonstrated an abnormal sialylation of glycoproteins in h‐IBM. h‐IBM muscle shows the abnormal accumulation of proteins including amyloid‐β (Aβ). Neprilysin (NEP), a metallopeptidase that cleaves Aβ, is characterized by the presence of several N‐glycosylation sites, and changes in these sugar moieties affect its stability and enzymatic activity. In the present study, we found that NEP is hyposialylated and its expression and enzymatic activity reduced in all h‐IBM muscles analyzed. In vitro, the experimental removal of sialic acid by Vibrio Cholerae neuraminidase in cultured myotubes resulted in reduced expression of NEP. This was most likely because of a post‐translational modification consisting in an abnormal sialylation of the protein that leads to its reduced stability. Moreover, treatment with Vibrio Cholerae neuraminidase was associated with an increased immunoreactivity for Aβ mainly in the form of distinct cytoplasmic foci within myotubes. We hypothesize that, in h‐IBM muscle, hyposialylated NEP has a role in hampering the cellular Aβ clearing system, thus contributing to its abnormal accumulation within vulnerable fibers and possibly promoting muscle degeneration.

Peracetylated N-Acetylmannosamine, a Synthetic Sugar Molecule, Efficiently Rescues Muscle Phenotype and Biochemical Defects in Mouse Model of Sialic Acid-deficient Myopathy

“…Again, the sialic acid levels of all organs recovered after oral Ac 4 ManNAc treatment were higher than those after ManNAc and NeuAc treatment (19). 4 ManNAc Treatment-Hyposialylation of some muscle glycoproteins has been reported in DMRV/hIBM muscles (26,30,31). In this study, we evaluated the sialylation status of ␣SG, ␤DG, and NEP by two-dimensional PAGE analysis of skeletal muscle membrane proteins of non-treated littermates and DMRV/hIBM mice who were given either placebo, Ac4MN-LD, or Ac4MN-HD (Fig.…”

“…In certain muscle glycoproteins, including ␣-dystroglycan, increased reactivity to peanut agglutinin, a lectin that is reactive to desialylated forms of O-glycans, was observed (30); thus, it was proposed that the stability of such glycoproteins could be influenced by sialylation, and therefore these proteins could be involved in the pathomechanism of DMRV/hIBM. Likewise, it was also hypothesized that hyposialylation of neural cell adhesion molecule and NEP (26,32) may contribute to the symptomatology of disease. In this study, we showed that hyposialylation of transferrin in blood, ␣SG and NEP in skeletal muscle, and podocalyxin in both kidney and skeletal muscles of DMRV/hIBM mouse were almost completely recovered after Ac 4 ManNAc treatment, although until now, the relevance of changes in specific glycoproteins was still being clarified.…”

“…In this study, we showed that hyposialylation of transferrin in blood, ␣SG and NEP in skeletal muscle, and podocalyxin in both kidney and skeletal muscles of DMRV/hIBM mouse were almost completely recovered after Ac 4 ManNAc treatment, although until now, the relevance of changes in specific glycoproteins was still being clarified. However, hyposialylation of NEP, a catabolic enzyme for A␤ peptides, has been hypothesized to cause disturbance of amyloid metabolism at least in DMRV/hIBM (26). As amyloid deposits within myofibers precede the occurrence of rimmed vacuoles and muscle degeneration in DMRV/hIBM mice (18) and amyloid has been shown to exert toxicity in muscle cells (44), the NEP hypothesis may be a reasonable platform to explain the pathomechanism of DMRV/hIBM on the basis of hyposialylation and its effect on the progression of the disease.…”

“…After treatment, the A␤1-40 and A␤1-42 levels were reduced in both Ac4MN-LD and Ac4MN-HD. To understand the mechanism by which A␤ peptides are generated in DMRV/hIBM, we measured ␤-secretase (generating enzyme) and NEP (catabolizing enzyme) activities, which are hypothesized to up-regulate A␤ levels (26,33). ␤-Secretase activity was not changed in DMRV/hIBM mouse muscle (data not shown), whereas the activity of NEP was decreased (Fig.…”

Genetics of GNE myopathy in the non-Jewish Persian population

“…It has been shown that normal sialylation is crucial for the stabilization and function of skeletal muscle glycoproteins and that modifications in the sialylation of cell surface glycoproteins can influence cell adhesion and signal transduction and cause myofibrillar degeneration, resulting in loss of normal muscle function. [16][17][18] Studies have demonstrated that some skeletal muscle proteins, such as neprilysin, 19 alphadystroglycan, 16 O-linked glycans, 20 and neural cell adhesion molecule (NCAM) 17 are hyposialylated in GNE myopathy, a distinctive feature in comparison with other myopathies with similar clinical manifestations. The suggested impact of GNE variants on sialylation remains controversial and under study.…”