Musclin, a Novel Skeletal Muscle-derived Secretory Factor

Abstract

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 of mouse skeletal muscle cDNAs. Musclin cDNA encoded 130 amino acids, including NH 2 -terminal 30-amino acid signal sequence. Musclin protein contained a region homologous to natriuretic peptide family, and KKKR, a putative serine protease cleavage site, similar to the natriuretic peptide 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. 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 mice. Functionally, recombinant musclin significantly attenuated insulin-stimulated glucose uptake and glycogen synthesis 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.Adipose tissue has been shown to produce secretory factors conceptualized adipocytekines (1, 2).Muscle-specific glucose transporter GLUT4 1 (3) and peroxisome proliferator-activated receptor-␥ (4, 5) knock-out mice exhibited the alterations in insulin sensitivity in fat and liver. Muscle-specific insulin receptor knockout mice showed adipocyte hyperproliferation (6). These findings suggest that the skeletal muscle may release bioactive factors (myokines), like adipocytokines, to target fat, liver, and potentially skeletal muscle itself.In the present study, we attempted to identify skeletal muscle-derived secretory factors using an efficient signal sequence trap (SST) method (7). Here, we report a novel skeletal musclederived secretory factor, musclin, whose mRNA was dynamically regulated by nutrition and hormonal factors. EXPERIMENTAL PROCEDURES Cloning of Mouse Musclin cDNA-Poly(A)ϩ RNAs were extracted from gastrocnemius muscles of 10-week-old male C57BL/6J mice under ad libitum, 24-h fasting or 24-h refeeding after 24-h fasting condition and ad libitum db/db mice. Equal amounts of poly(A) ϩ RNA from each group were pooled to synthesize cDNA. To selectively clone the genes that possess signal sequence at the NH 2 -terminal end of cDNAs, SST-REX system (signal sequence trap by retrovirus-mediated expressio...

Musclin is an activity-stimulated myokine that enhances physical endurance

“…This study is focused on the recently discovered myokine musclin (4,5). Two groups initially identified this peptide: one as bone-derived osteocrin (5) and the second as muscle-secreted musclin (4).…”

“…Musclin mRNA expression has been linked to insulin-induced activation of protein kinase B (Akt) that phosphorylates forkhead box O1 transcription factor (FOXO1), causing it to be exported from the nucleus and thus releasing the musclin-encoding gene from transcriptional inhibition (4,6). This pathway has been demonstrated to regulate musclin transcription in both cell culture and skeletal muscles (4,6). Musclin contains two KKKR putative serine protease cleavage sites and a region homologous to members of the natriuretic peptide (NP) family (4,5).…”

“…Two groups initially identified this peptide: one as bone-derived osteocrin (5) and the second as muscle-secreted musclin (4). Musclin mRNA expression has been linked to insulin-induced activation of protein kinase B (Akt) that phosphorylates forkhead box O1 transcription factor (FOXO1), causing it to be exported from the nucleus and thus releasing the musclin-encoding gene from transcriptional inhibition (4,6). This pathway has been demonstrated to regulate musclin transcription in both cell culture and skeletal muscles (4,6).…”

L’ostéocrine, une nouvelle molécule de signalisation osseuse

“…Parmi ceux-ci, on compte plusieurs protéines sécrétées et leurs récepteurs respectifs dont les plus nuent d'être décelés, notamment l'ostéocrine, une protéine que notre groupe a découverte lors d'un criblage visant l'identification du sécrétome des cellules de l'os [1]. Le gène de l'ostéocrine a également été cloné à partir du muscle, et nommée muscline [2]. Le gène codant l'ostéocrine est exprimé presque exclu- NOUVELLES MAGAZINE sivement par les ostéoblastes [1] et les cellules musculaires [2][3][4].…”

“…Le gène de l'ostéocrine a également été cloné à partir du muscle, et nommée muscline [2]. Le gène codant l'ostéocrine est exprimé presque exclu- NOUVELLES MAGAZINE sivement par les ostéoblastes [1] et les cellules musculaires [2][3][4]. L'ostéocrine est une protéine hautement conservée entre les espèces (Figure 1) et l'examen de sa structure primaire met en évidence des motifs rappelant ceux entrant dans la composition de plusieurs prohormones [5].…”