Limits...
Denervation atrophy is independent from Akt and mTOR activation and is not rescued by myostatin inhibition.

MacDonald EM, Andres-Mateos E, Mejias R, Simmers JL, Mi R, Park JS, Ying S, Hoke A, Lee SJ, Cohn RD - Dis Model Mech (2014)

Bottom Line: Our results suggest that this protection from disuse atrophy is regulated by serum and glucocorticoid-induced kinase (SGK) rather than by Akt.Denervation atrophy, however, was not protected by ActRIIB treatment, yet resulted in an upregulation of the pro-growth factors Akt, SGK and components of the mTOR pathway.We then treated the denervated mice with the mTOR inhibitor rapamycin and found that, despite a reduction in mTOR activation, there is no alteration of the atrophy phenotype.

View Article: PubMed Central - PubMed

Affiliation: McKusick-Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.

ABSTRACT
The purpose of our study was to compare two acquired muscle atrophies and the use of myostatin inhibition for their treatment. Myostatin naturally inhibits skeletal muscle growth by binding to ActRIIB, a receptor on the cell surface of myofibers. Because blocking myostatin in an adult wild-type mouse induces profound muscle hypertrophy, we applied a soluble ActRIIB receptor to models of disuse (limb immobilization) and denervation (sciatic nerve resection) atrophy. We found that treatment of immobilized mice with ActRIIB prevented the loss of muscle mass observed in placebo-treated mice. Our results suggest that this protection from disuse atrophy is regulated by serum and glucocorticoid-induced kinase (SGK) rather than by Akt. Denervation atrophy, however, was not protected by ActRIIB treatment, yet resulted in an upregulation of the pro-growth factors Akt, SGK and components of the mTOR pathway. We then treated the denervated mice with the mTOR inhibitor rapamycin and found that, despite a reduction in mTOR activation, there is no alteration of the atrophy phenotype. Additionally, rapamycin prevented the denervation-induced upregulation of the mTORC2 substrates Akt and SGK. Thus, our studies show that denervation atrophy is not only independent from Akt, SGK and mTOR activation but also has a different underlying pathophysiological mechanism than disuse atrophy.

Show MeSH

Related in: MedlinePlus

Dysregulation of mTOR signaling in both disuse and denervation atrophy. Western blot analysis of TA muscle protein lysates. (A) Expression levels of the components of the mTOR complexes, including p-mTOR, total mTOR, raptor and rictor, were not different between control (‘C’), immobilized (‘I’) and ActRIIB-treated immobilized (‘I+A’) mice. Denervation (‘D’) and denervation with ActRIIB treatment (‘D+A’) led to a substantial increase in p-mTOR, total mTOR, raptor and rictor when compared with sham-operated controls (‘S’). (B) Immobilization resulted in a decrease of p70S6k expression but not when the mice were treated with ActRIIB. Denervation led to a decrease in active phosphorylation of p70S6k with no loss of total protein expression and this was not prevented by treatment with ActRIIB. Quantitative analysis of blots is displayed in the graphs (right) with arbitrary units of mean ± s.e.m. *P<5.0×10−2 with respect to controls. Lines indicate where intervening lanes have been removed from a single image to show the most representative band for that treatment group. Arrows in B indicate the correct size of p70S6k.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3974457&req=5

f5-0070471: Dysregulation of mTOR signaling in both disuse and denervation atrophy. Western blot analysis of TA muscle protein lysates. (A) Expression levels of the components of the mTOR complexes, including p-mTOR, total mTOR, raptor and rictor, were not different between control (‘C’), immobilized (‘I’) and ActRIIB-treated immobilized (‘I+A’) mice. Denervation (‘D’) and denervation with ActRIIB treatment (‘D+A’) led to a substantial increase in p-mTOR, total mTOR, raptor and rictor when compared with sham-operated controls (‘S’). (B) Immobilization resulted in a decrease of p70S6k expression but not when the mice were treated with ActRIIB. Denervation led to a decrease in active phosphorylation of p70S6k with no loss of total protein expression and this was not prevented by treatment with ActRIIB. Quantitative analysis of blots is displayed in the graphs (right) with arbitrary units of mean ± s.e.m. *P<5.0×10−2 with respect to controls. Lines indicate where intervening lanes have been removed from a single image to show the most representative band for that treatment group. Arrows in B indicate the correct size of p70S6k.

Mentions: We found no change in the mTOR complex components p-mTOR, total mTOR, raptor and rictor in immobilized mice with or without ActRIIB treatment (Fig. 5A). However, a scaffold protein common between both mTOR complexes, GβL, was downregulated with immobilization, but was maintained at control levels in ActRIIB-treated immobilized mice (supplementary material Fig. S2). A substrate of mTORC1, p70S6k, and a second scaffold protein, eIF3f, which facilitates the interaction between mTORC1 and p70S6k, also showed reduced expression in immobilized muscle (Fig. 5B and supplementary material Fig. S2). The loss of p70S6k and eIF3f expression was not seen in immobilized mice treated with ActRIIB (Fig. 5B). A second mTORC1 substrate, 4E-BP1, did not show any change in phosphorylation or total abundance in placebo-or ActRIIB-treated immobilized mice compared with controls (supplementary material Fig. S2).


Denervation atrophy is independent from Akt and mTOR activation and is not rescued by myostatin inhibition.

MacDonald EM, Andres-Mateos E, Mejias R, Simmers JL, Mi R, Park JS, Ying S, Hoke A, Lee SJ, Cohn RD - Dis Model Mech (2014)

Dysregulation of mTOR signaling in both disuse and denervation atrophy. Western blot analysis of TA muscle protein lysates. (A) Expression levels of the components of the mTOR complexes, including p-mTOR, total mTOR, raptor and rictor, were not different between control (‘C’), immobilized (‘I’) and ActRIIB-treated immobilized (‘I+A’) mice. Denervation (‘D’) and denervation with ActRIIB treatment (‘D+A’) led to a substantial increase in p-mTOR, total mTOR, raptor and rictor when compared with sham-operated controls (‘S’). (B) Immobilization resulted in a decrease of p70S6k expression but not when the mice were treated with ActRIIB. Denervation led to a decrease in active phosphorylation of p70S6k with no loss of total protein expression and this was not prevented by treatment with ActRIIB. Quantitative analysis of blots is displayed in the graphs (right) with arbitrary units of mean ± s.e.m. *P<5.0×10−2 with respect to controls. Lines indicate where intervening lanes have been removed from a single image to show the most representative band for that treatment group. Arrows in B indicate the correct size of p70S6k.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3974457&req=5

f5-0070471: Dysregulation of mTOR signaling in both disuse and denervation atrophy. Western blot analysis of TA muscle protein lysates. (A) Expression levels of the components of the mTOR complexes, including p-mTOR, total mTOR, raptor and rictor, were not different between control (‘C’), immobilized (‘I’) and ActRIIB-treated immobilized (‘I+A’) mice. Denervation (‘D’) and denervation with ActRIIB treatment (‘D+A’) led to a substantial increase in p-mTOR, total mTOR, raptor and rictor when compared with sham-operated controls (‘S’). (B) Immobilization resulted in a decrease of p70S6k expression but not when the mice were treated with ActRIIB. Denervation led to a decrease in active phosphorylation of p70S6k with no loss of total protein expression and this was not prevented by treatment with ActRIIB. Quantitative analysis of blots is displayed in the graphs (right) with arbitrary units of mean ± s.e.m. *P<5.0×10−2 with respect to controls. Lines indicate where intervening lanes have been removed from a single image to show the most representative band for that treatment group. Arrows in B indicate the correct size of p70S6k.
Mentions: We found no change in the mTOR complex components p-mTOR, total mTOR, raptor and rictor in immobilized mice with or without ActRIIB treatment (Fig. 5A). However, a scaffold protein common between both mTOR complexes, GβL, was downregulated with immobilization, but was maintained at control levels in ActRIIB-treated immobilized mice (supplementary material Fig. S2). A substrate of mTORC1, p70S6k, and a second scaffold protein, eIF3f, which facilitates the interaction between mTORC1 and p70S6k, also showed reduced expression in immobilized muscle (Fig. 5B and supplementary material Fig. S2). The loss of p70S6k and eIF3f expression was not seen in immobilized mice treated with ActRIIB (Fig. 5B). A second mTORC1 substrate, 4E-BP1, did not show any change in phosphorylation or total abundance in placebo-or ActRIIB-treated immobilized mice compared with controls (supplementary material Fig. S2).

Bottom Line: Our results suggest that this protection from disuse atrophy is regulated by serum and glucocorticoid-induced kinase (SGK) rather than by Akt.Denervation atrophy, however, was not protected by ActRIIB treatment, yet resulted in an upregulation of the pro-growth factors Akt, SGK and components of the mTOR pathway.We then treated the denervated mice with the mTOR inhibitor rapamycin and found that, despite a reduction in mTOR activation, there is no alteration of the atrophy phenotype.

View Article: PubMed Central - PubMed

Affiliation: McKusick-Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.

ABSTRACT
The purpose of our study was to compare two acquired muscle atrophies and the use of myostatin inhibition for their treatment. Myostatin naturally inhibits skeletal muscle growth by binding to ActRIIB, a receptor on the cell surface of myofibers. Because blocking myostatin in an adult wild-type mouse induces profound muscle hypertrophy, we applied a soluble ActRIIB receptor to models of disuse (limb immobilization) and denervation (sciatic nerve resection) atrophy. We found that treatment of immobilized mice with ActRIIB prevented the loss of muscle mass observed in placebo-treated mice. Our results suggest that this protection from disuse atrophy is regulated by serum and glucocorticoid-induced kinase (SGK) rather than by Akt. Denervation atrophy, however, was not protected by ActRIIB treatment, yet resulted in an upregulation of the pro-growth factors Akt, SGK and components of the mTOR pathway. We then treated the denervated mice with the mTOR inhibitor rapamycin and found that, despite a reduction in mTOR activation, there is no alteration of the atrophy phenotype. Additionally, rapamycin prevented the denervation-induced upregulation of the mTORC2 substrates Akt and SGK. Thus, our studies show that denervation atrophy is not only independent from Akt, SGK and mTOR activation but also has a different underlying pathophysiological mechanism than disuse atrophy.

Show MeSH
Related in: MedlinePlus