CNR - Institute of Neuroscience CNR
Institute of Neuroscience
 

Project

Signaling pathways controlling skeletal muscle hypertrophy and atrophy

Muscle hypertrophy in an inducible Akt transgenic model

 

A better understanding of the signaling pathways that control muscle growth is required to identify appropriate countermeasures to prevent or reverse the loss of muscle mass and force induced by aging, disuse, or neuromuscular diseases. However, two major issues in this field have not yet been fully addressed. The first concerns the pathways involved in leading to physiological changes in muscle size. Muscle hypertrophy based on perturbations of specific signaling pathways is either characterized by impaired force generation, e.g., myostatin knockout, or incompletely studied from the physiological point of view, e.g., IGF-1 overexpression. A second issue is whether satellite cell proliferation and incorporation into growing muscle fibers is required for a functional hypertrophy. To address these issues, we used an inducible transgenic model of muscle hypertrophy by short-term Akt activation in adult skeletal muscle (Fig. 1). In this model, Akt activation for 3 wk was followed by marked hypertrophy (approximately 50% of muscle mass) and by increased force generation, as determined in vivoby ankle plantar flexor stimulation, ex vivo in intact isolated diaphragm strips, and in single-skinned muscle fibers. No changes in fiber-type distribution and resistance to fatigue were detectable. Bromodeoxyuridine incorporation experiments showed that Akt-dependent muscle hypertrophy was accompanied by proliferation of interstitial cells (Fig. 2). In contrast, proliferation of satellite cell and their incorporation into myofibers was not observed, indicating that i) myofiber hypertrophy induced by Akt activation is accompanied by an increase in myonuclear domain size, and ii) in this model of muscle hypertrophy myonuclear domain can increase without compromising muscle performance (Blaauw et al, FASEB J 2009).

Akt activation prevents the force drop induced by eccentric contractions in dystrophin-deficient skeletal muscle

 

Skeletal muscles of the mdx mouse, a model of Duchenne Muscular Dystrophy, show an excessive reduction in the maximal tetanic force following eccentric contractions. This specific sign of the susceptibility of dystrophin-deficient muscles to mechanical stress can be used as a quantitative test to measure the efficacy of therapeutic interventions. To determine whether muscle hypertrophy can affect this effect, we cossed mdx mice with an inducible AKT transgenic line (see above). We found that the force drop induced by eccentric contractions in mdx mice becomes similar to that of wild-type mice when Akt activity is increased (Fig. 3). However, this effect is not correlated with muscle hypertrophy and is not blocked by rapamycin treatment. The force drop induced by eccentric contractions is similar in skinned muscle fibers from mdx and Akt- mdx mice when stretch is applied directly to skinned fibers. However, skinned fibers isolated from mdx muscles exposed to eccentric contractions in vivo develop less isometric force than wild-type fibers and this force depression is completely prevented by Akt activation. These experiments indicate that the myofibrillar-cytoskeletal system of dystrophin-deficient muscle is highly susceptible to a damage caused by eccentric contraction when elongation is applied in vivo, and this damage can be prevented by Akt activation. Microarray and PCR analyses indicate that Akt activation induces up-regulation of genes coding for proteins associated with Z-disks and costameres, and for proteins with anti-oxidant or chaperone function. The protein levels of utrophin and dysferlin are also increased by Akt activation (Blaauw et al, Hum Mol Genet 2008).

Muscle atrophy: role of FoxO in the control of the ubiquitin-proteasome and autophagy-lysosome pathways

In collaboration with Prof. A. Goldberg, Harvard Medical School, we have carried out a series of studies that support a major role of the transcription factors of the FoxO family in muscle protein degradation. We first showed that the upregulation of the muscle-specific ubiquitin ligases atrogin-1 (MAFbx1) and MURF1, which are crucial for muscle atrophy in different settings, is controlled by FoxO3 (Sandri et al, 2004). We subsequently found that this transcription factor is also required for the upregulation of autophagy genes, such as LC3, and the formation of autophagosomes induced by starvation and denervation (Mammucari et al, 2008; Zhao et al, 2008). Thus FoxO3, by controlling the activation of the ubiquitin-proteasome and autophagy-lysosome pathways, emerges as a master regulator of muscle protein degradation leading to muscle atrophy. To determine the role of autophagy in skeletal muscle, we have now generated two muscle-specific knockout models for the gene Atg7, which is essential for autophagy, by crossing Atg7 floxed mice with a muscle-specific Cre recombinase line, and with an inducible muscle-specific Cre recombinase line. Autophagy inhibition resulted in muscle atrophy and decrease in muscle force, and exacerbated muscle loss during denervation and fasting. Morphological analysis showed accumulation of abnormal mitochondria, sarcoplasmic reticulum distension, and formation of aberrant concentric membranous structures. Thus maintenance of autophagy flux is important to preserve muscle mass and to maintain myofiber integrity (Masiero et al, submitted).

Publications

  • Blaauw B, Canato M, Agatea L, Toniolo L, Mammucari C, Masiero E, Abraham R, Sandri M, Schiaffino S, Reggiani C (2009) Inducible activation of Akt increases skeletal muscle mass and force without satellite cell activation. FASEB J. 23:3896-905.
  • Blaauw B, Mammucari C, Toniolo L, Agatea L, Abraham R, Sandri M, Reggiani C, Schiaffino S (2008) Akt activation prevents the force drop induced by eccentric contractions in dystrophin-deficient skeletal muscle. Hum. Mol. Genet. 17:3686-96.
  • Mammucari C, Schiaffino S, Sandri M (2008) Downstream of Akt: FoxO3 and mTOR in the regulation of autophagy in skeletal muscle. Autophagy 4:524-6.
  • Zhao J, Brault JJ, Schild A, Cao P, Sandri M, Schiaffino S, Lecker SH, Goldberg AL (2007) FoxO3 coordinately activates protein degradation by the autophagic/lysosomal and proteasomal pathways in atrophying muscle cells. Cell Metab. 6:472-83.
  • Mammucari C, Milan G, Romanello V, Masiero E, Rudolf R, Del Piccolo P, Burden SJ, Di Lisi R, Sandri C, Zhao J, Goldberg AL, Schiaffino S, Sandri M (2007) FoxO3 controls autophagy in skeletal muscle in vivo. Cell Metab. 6:458-71.
  • McCullagh KJ, Calabria E, Pallafacchina G, Ciciliot S, Serrano AL, Argentini C, Kalhovde JM, Lømo T, Schiaffino S (2004) NFAT is a nerve activity sensor in skeletal muscle and controls activity-dependent myosin switching. Proc. Natl. Acad. Sci. U.S.A. 101:10590-5.

Grants

European Commission, VI Programme: EXGENESIS (Integrated Project) "Health benefits of exercise: identification of genes and signalling pathways involved in effects of exercise on insulin resistance, obesity and the metabolic syndrome". (5yrs: 2005-2009). € 436.000,00

European Commission, FP VII Programme: MYOAGE (Integrated Project) "Understanding and combating human age-related muscle weakness" (4 yrs: 2009-2012). € 632.482,00

ASI Progetto OSMA: "Preventing muscle atrophy during space flights by interfering with signaling pathways that control muscle protein degradation". (3 yrs: 2006-09). € 180.000,00

Collaborations

  • Marco Sandri, Department of Biomedical Sciences, University of Padova, Italy.
  • Alfred L. Goldberg, Department of Cell Biology, Harvard Medical School, Boston.
  • Carlo Reggiani, Department of Human Anatomy and Physiology, University of Padova.

 

PI photo

Stefano Schiaffino

Contact information

email  E-mail

email  049 7923232

Participating staff

Cristina Mammucari
Assistant Professor

Bert Blaauw
Postdoc

Eva Masiero
PhD student

Lisa Agatea
Technician