肌肉素 Musclin is a muscle-derived secretory factor

Musclin, a novel skeletal muscle-derived secretory factor

Skeletal muscle is involved in the homeostasis of glucose and lipid metabolism. We hypothesized that the skeletal muscle produces and secretes bioactive factor(s), similar to adipocytokines secreted by fat tissue. Here, we report the identification of a novel secretory factor, musclin, by signal sequence trap (SST) of mouse skeletal muscle cDNAs. Musclin cDNA encoded 130 amino acids, including N-terminal 30-amino acid signal sequence. Musclin protein contained a region homologous to natriuretic peptide (NP) family, and KKKR, a putative serine protease cleavage site, similar to NP family. Full-length musclin protein and KKKR-dependent cleaved form were secreted in media of musclin cDNA-transfected mammalian cell cultures. Musclin mRNA was expressed almost exclusively in the skeletal muscle of mice and rats. Musclin mRNA levels in skeletal muscle were markedly low in fasted, increased upon re-feeding, and were low in streptozotocin-treated insulin-deficient mice. Musclin mRNA expression was induced at late stage in the differentiation of C2C12 myocytes. In myocytes, insulin increased while epinephrine, isoproterenol and forskolin reduced musclin mRNA, all of which are known to increase the cellular content of cyclic AMP, a counter regulator to insulin. Pathologically, overexpression of musclin mRNA was noted in the muscles of obese insulin-resistant KKAy and db/db mice. Functionally, recombinant musclin significantly attenuated insulin-stimulated glucose uptake in myocytes. In conclusion, we identified musclin, a novel skeletal muscle-derived secretory factor. Musclin expression level is tightly regulated by nutritional changes and its physiological role could be linked to glucose metabolism.

Nishizawa H, et al. J Biol Chem. 2004 Mar 24 [Epub ahead of print]

Antibody for Musclin Immunohistochemistry

mouse musclin bar

musclin sequence

Alignments of musclin amino acid sequences for mouse, rat and human. Predicted signal sequence; shaded box. The putative serine protease cleavage site, KKKR; open box. The region homologous to mouse ANP, BNP and CNP; hatched box. C, Secretion of musclin protein. The cell lysates and culture medium from C2C12 myocytes infected by retrovirus carrying Flag-tagged mouse musclin or GFP were immunoprecipitated, and subjected to western blot analysis, using anti-Flag antibody. Mus; musclin. D, Secretion of wild type and mutant musclin from HEK 293 cells. The culture medium from HEK 293 cells transfected with pEF-BOS-WT musclin Flag (left lane) or pEFBOS-Mut musclin Flag (76KKKR⁷⁹-⁷⁶A) (right lane) was subjected to immunoprecipitation followed by western blotting using anti-Flag antibody.

musclin mRNA

In vivo expression of musclin mRNA. A, Northern blot analysis of musclin expression in mice. B, Quantitative RT-PCR analysis of musclin mRNA expression in mice. C, Regulation of musclin mRNA expression by nutritional status. Gastrocnemius muscles of mice fed ad libitum, fasted for 48 h, or fasted for 48 h and refed for 24 h (n=5, each) were used for real-time RT-PCR. Inset: representative Northern blotting (n=2, each). D, Musclin mRNA expression in insulin-deficient STZ mice. Gastrocnemius muscles of STZ and control mice (n=5, each) were subjected to real-time RT-PCR. The values were expressed relative to the level of 36B4 mRNA. E, Musclin mRNA expression in KKAy mice. Gastrocnemius muscles of 13-wk-old KKAy mice and age-matched C57BL/6J mice (n=8, each) were used for real-time RT-PCR. The values were normalized to the level of cyclophilin mRNA. Data are mean +- SEM values. *p< 0.05, **p< 0.01.

Regulation of musclin mRNA expression by various hormones, and effects of musclin on insulin-stimulated glucose uptake and glycogen synthesis in C2C12 myocytes. A, Musclin mRNA expression during myocyte differentiation. C2C12 cells were harvested at indicated days after differentiation-induction for real-time PCR. B-D, Regulation of musclin mRNA expression by insulin (INS), IGF-1 (IGF), epinephrine (Epi), isoproterenol (Iso), and forskolin (FSK) (real-time PCR). The expression level was expressed relative to that of cyclophlin mRNA (n=3, each). E-F, Effect of recombinant musclin on 2-DG uptake and glycogen synthesis. After 5-h pre-treatment with Flag-peptide (0.5 ug/ml) as control or Flagtagged musclin (0.5 ug/ml), 2-DG uptake (E) and glycogen synthesis (F) were determined as described in Experimental Procedures. Data are mean+- SEM values (n=6). **p<0.01. Similar results were obtained in two other independent experiments.

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