Myostatin reduces Akt/TORC1/p70S6K signaling, inhibiting myoblast differentiation and myotube size

Abstract

Myostatin is a negative regulator of skeletal muscle size, previously shown to inhibit muscle cell differentiation. Myostatin requires both Smad2 and Smad3 downstream of the activin receptor II (ActRII)/activin receptor-like kinase (ALK) receptor complex. Other transforming growth factor-beta (TGF-beta)-like molecules can also block differentiation, including TGF-beta(1), growth differentiation factor 11 (GDF-11), activins, bone morphogenetic protein 2 (BMP-2) and BMP-7. Myostatin inhibits activation of the Akt/mammalian target of rapamycin (mTOR)/p70S6 protein synthesis pathway, which mediates both differentiation in myoblasts and hypertrophy in myotubes. Blockade of the Akt/mTOR pathway, using small interfering RNA to regulatory-associated protein of mTOR (RAPTOR), a component of TOR signaling complex 1 (TORC1), increases myostatin-induced phosphorylation of Smad2, establishing a myostatin signaling-amplification role for blockade of Akt. Blockade of RAPTOR also facilitates myostatin's inhibition of muscle differentiation. Inhibition of TORC2, via rapamycin-insensitive companion of mTOR (RICTOR), is sufficient to inhibit differentiation on its own. Furthermore, myostatin decreases the diameter of postdifferentiated myotubes. However, rather than causing upregulation of the E3 ubiquitin ligases muscle RING-finger 1 (MuRF1) and muscle atrophy F-box (MAFbx), previously shown to mediate skeletal muscle atrophy, myostatin decreases expression of these atrophy markers in differentiated myotubes, as well as other genes normally upregulated during differentiation. These findings demonstrate that myostatin signaling acts by blocking genes induced during differentiation, even in a myotube, as opposed to activating the distinct "atrophy program." In vivo, inhibition of myostatin increases muscle creatine kinase activity, coincident with an increase in muscle size, demonstrating that this in vitro differentiation measure is also upregulated in vivo.

Association of the K153R polymorphism in the myostatin gene and extreme longevity

“…For instance, under starvation conditions, TORC1 is responsible for up-regulating autophagy, a process that generates energy by degrading portions of the cytoplasm (Neufeld 2010;Ravikumar et al 2010). A number of studies have demonstrated that myostatin acts as a negative regulator of mTOR-directed signalling, which is consistent with its inhibitory effect on protein synthesis (Amirouche et al 2009;Langley et al 2002;Lipina et al 2010;McFarlane et al 2006;Rios et al 2004;Sartori et al 2009;Trendelenburg et al 2009) and more specifically on protein translation in skeletal muscle (Trendelenburg et al 2009). On the other hand, myostatin deletion is beneficial for bone density, insulin sensitivity and heart function in senescent mice (Morissette et al 2009), and inhibition of ActRIIB, the cell membrane receptor for which myostatin has the highest affinity (Sakuma and Yamaguchi 2012), increases survival (by 17 %) in myotubularin-deficient mice, due to a delay in the point at which animals experience weight loss or complete hind limb paralysis (Lawlor et al 2011) (X-linked myotubular myopathy is a severe form of congenital myopathy which most often manifests with severe perinatal weakness and respiratory failure).…”

“…The mTOR pathway is an evolutionarily conserved nutrient-sensing pathway that adjusts metabolism and growth to amino acid availability, growth factors, energy status and stress (Fenton and Gout 2011;Sengupta et al 2010). A number of studies have demonstrated that myostatin acts as a negative regulator of mTOR-directed signalling (Amirouche et al 2009;Langley et al 2002;Lipina et al 2010;McFarlane et al 2006;Rios et al 2004;Sartori et al 2009;Trendelenburg et al 2009), which provides support for a potential role of myostatin inhibition in longevity. Myostatin deletion is in fact beneficial for bone density, insulin sensitivity and heart function in senescent mice (Morissette et al 2009).…”

Characterization of GHRs, IGFs and MSTNs, and analysis of their expression relationships in blunt snout bream, Megalobrama amblycephala

“…Many levels of interaction between MSTNs and IGFs attributed this delicate relationship: 1. The differential regulation of intracellular signaling pathways (Amirouche et al, 2009;Trendelenburg et al, 2009); 2. Through the expression of IGF binding protein, which was partly mediated by the inhibitory effects of MSTN on myoblast proliferation (Dayton and White, 2008;Williams et al, 2011); 3.…”

Impaired hypertrophy in myoblasts is improved with testosterone administration

“…Following aspiration, 2 ml methanol and acetone ( (left end, middle, right end) and determining the mean of the 3 values as previously described [48,49]. An indicator of myonuclear accretion (fusion index) was calculated by dividing myotubes into two classes; myotubes which expressed 3-4 nuclei or myotubes which expressed 5+ nuclei, with the data expressed as percentages.…”