Absence of Hyperplasia inGasp-1Overexpressing Mice is Dependent on Myostatin Up-Regulation

Abstract

Background/Aims: Overexpression of Gasp-1, an inhibitor of myostatin, leads to a hypermuscular phenotype due to hypertrophy rather than hyperplasia in mice. However to date, the cellular and molecular mechanisms underlying this phenotype are not investigated. Methods: Skeletal muscles of overexpressing Gasp-1 mice, called Tg(Gasp-1) mice, were analyzed by histological methods. Satellite cell-derived myoblasts from these mice were used to investigate the molecular mechanisms. Results: We demonstrated that hypertrophy in Tg(Gasp-1) mice was related to a myonuclear accretion during the first 3 postnatal weeks and an activation of the pro-hypertrophic Akt/mTORC/p70S6K signaling. In accordance with these results, we showed that overexpressing Gasp-1 primary myoblasts proliferated faster and myonuclei average per myotube was increased during differentiation. Molecular analysis revealed that Gasp-1 overexpression resulted in increased myostatin expression related to its auto-regulation. Despite its inhibition, myostatin led to Pax7 deregulation through its non-canonical Erk1/2 signaling pathway. Consistent with this, inhibition of Erk1/2 signaling pathway as well as neutralization of secreted myostatin rescue the Pax7 expression in overexpressing Gasp-1 myoblasts. Conclusion: Our study shows that myostatin is able to act independently of its canonical pathway to regulate the Pax7 expression. Altogether, our results indicate that myostatin could regulate muscle development despite its protein inhibition.

Alterations in Adiposity and Glucose Homeostasis in Adult Gasp-1 Overexpressing Mice

“…Our previous molecular analysis performed on young Tg(Gasp-1) mice revealed an increase of myostatin expression responsible for the absence of hyperplasia [31]. We hypothesized that this upregulation of myostatin could also be implicated in the insulin resistance phenotype observed in old mutant mice.…”

“…Recent studies have shown that Mstn -/- mice leads to an increase of insulin sensitivity, a decrease of adiposity and a resistance to obesity, demonstrating that myostatin can also impact on metabolism [14, 15]. In previous studies, we have shown that transgenic mice overexpressing Gasp-1 , a myostatin inhibitor, present a muscle hypertrophy without hyperplasia but no loss of fat mass unlike Mstn -/- mice [27, 31]. To explore in more details the role of Gasp-1 on energetic metabolism, we characterized young and old overexpressing Gasp-1 mice phenotypes in comparison to young and old WT mice.…”

“…Myostatin and insulin concentrations for mouse plasmas were determined in a sandwich ELISA (GDF8/Myostatin ELISA kit, R&D Systems) and (RAT/Mouse Insulin Elisa, Millipore) as previously described [31]. All measurements were performed in triplicate and data for the standard curve were fitted to a logistic plot with the MARS Data Analysis Software (BMG Labtech) to determine the levels of myostatin and insulin.…”

“…Altogether, these data would suggest that transgenic mice for which myostatin is inhibited or reduced present a resistance to diet induced obesity and are protected against the development of insulin resistance. But compared to these models, overexpressing Gasp-1 -Tg(Gasp-1) -mice present only a myofiber hypertrophy without hyperplasia and without a switch of fiber types [27, 31]. Using protein arrays on satellite cell derived myoblasts from Tg(Gasp-1) mice, we have shown an upregulation of myostatin associated with an activation ERK1/2 signaling pathway and a decrease of Pax7 expression, which could explain the absence of hyperplasia in our model [31].…”

“…But compared to these models, overexpressing Gasp-1 -Tg(Gasp-1) -mice present only a myofiber hypertrophy without hyperplasia and without a switch of fiber types [27, 31]. Using protein arrays on satellite cell derived myoblasts from Tg(Gasp-1) mice, we have shown an upregulation of myostatin associated with an activation ERK1/2 signaling pathway and a decrease of Pax7 expression, which could explain the absence of hyperplasia in our model [31]. Moreover, no significant loss of fat mass was observed in Tg(Gasp-1) [27].…”