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Sarcospan-dependent Akt activation is required for utrophin expression and muscle regeneration.

ABSTRACT: Utrophin is normally confined to the neuromuscular junction (NMJ) in adult muscle and partially compensates for the loss of dystrophin in mdx mice. We show that Akt signaling and utrophin levels were diminished in sarcospan (SSPN)-deficient muscle. By creating several transgenic and knockout mice, we demonstrate that SSPN regulates Akt signaling to control utrophin expression. SSPN determined ?-dystroglycan (?-DG) glycosylation by affecting levels of the NMJ-specific glycosyltransferase Galgt2. After cardiotoxin (CTX) injury, regenerating myofibers express utrophin and Galgt2-modified ?-DG around the sarcolemma. SSPN-null mice displayed delayed differentiation after CTX injury caused by loss of utrophin and Akt signaling. Treatment of SSPN-null mice with viral Akt increased utrophin and restored muscle repair after injury, revealing an important role for the SSPN-Akt-utrophin signaling axis in regeneration. SSPN improved cell surface expression of utrophin by increasing transportation of utrophin and DG from endoplasmic reticulum/Golgi membranes. Our experiments reveal functions of utrophin in regeneration and new pathways that regulate utrophin expression at the cell surface.

SUBMITTER: Marshall JL

PROVIDER: S-EPMC3384411 | biostudies-literature | 2012 Jun

REPOSITORIES: biostudies-literature

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Sarcospan-dependent Akt activation is required for utrophin expression and muscle regeneration.

Marshall Jamie L JL Holmberg Johan J Chou Eric E Ocampo Amber C AC Oh Jennifer J Lee Joy J Peter Angela K AK Martin Paul T PT Crosbie-Watson Rachelle H RH

The Journal of cell biology 20120601 7

Utrophin is normally confined to the neuromuscular junction (NMJ) in adult muscle and partially compensates for the loss of dystrophin in mdx mice. We show that Akt signaling and utrophin levels were diminished in sarcospan (SSPN)-deficient muscle. By creating several transgenic and knockout mice, we demonstrate that SSPN regulates Akt signaling to control utrophin expression. SSPN determined α-dystroglycan (α-DG) glycosylation by affecting levels of the NMJ-specific glycosyltransferase Galgt2. ...[more]

PMID: 22734004

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Project description:BackgroundCancer-related muscle wasting occurs in most cancer patients. An important regulator of adult muscle mass and function is the Akt-mTORC1 pathway. While Akt-mTORC1 signalling is important for adult muscle homeostasis, it is also a major target of numerous cancer treatments. Which role Akt-mTORC1 signalling plays during cancer cachexia in muscle is currently not known. Here, we aimed to determine how activation or inactivation of the pathway affects skeletal muscle during cancer cachexia.MethodsWe used inducible, muscle-specific Raptor ko (mTORC1) mice to determine the effect of reduced mTOR signalling during cancer cachexia. On the contrary, in order to understand if skeletal muscles maintain their anabolic capacity and if activation of Akt-mTORC1 signalling can reverse cancer cachexia, we generated mice in which we can inducibly activate Akt specifically in skeletal muscles.ResultsWe found that mTORC1 signalling is impaired during cancer cachexia, using the Lewis lung carcinoma and C26 colon cancer model, and is accompanied by a reduction in protein synthesis rates of 57% (P < 0.01). Further reduction of mTOR signalling, as seen in Raptor ko animals, leads to a 1.5-fold increase in autophagic flux (P > 0.001), but does not further increase muscle wasting. On the other hand, activation of Akt-mTORC1 signalling in already cachectic animals completely reverses the 15-20% loss in muscle mass and force (P < 0.001). Interestingly, Akt activation only in skeletal muscle completely normalizes the transcriptional deregulation observed in cachectic muscle, despite having no effect on tumour size or spleen mass. In addition to stimulating muscle growth, it is also sufficient to prevent the increase in protein degradation normally observed in muscles from tumour-bearing animals.ConclusionsHere, we show that activation of Akt-mTORC1 signalling is sufficient to completely revert cancer-dependent muscle wasting. Intriguingly, these results show that skeletal muscle maintains its anabolic capacities also during cancer cachexia, possibly giving a rationale behind some of the beneficial effects observed in exercise in cancer patients.

Project description:Duchenne muscular dystrophy is a severe and currently incurable progressive neuromuscular condition, caused by mutations in the DMD gene that result in the inability to produce dystrophin. Lack of dystrophin leads to loss of muscle fibres and a reduction in muscle mass and function. There is evidence from dystrophin-deficient mouse models that increasing levels of utrophin at the muscle fibre sarcolemma by genetic or pharmacological means significantly reduces the muscular dystrophy pathology. In order to determine the efficacy of utrophin modulators in clinical trials, it is necessary to accurately measure utrophin levels and other biomarkers on a fibre by fibre basis within a biopsy section. Our aim was to develop robust and reproducible staining and imaging protocols to quantify sarcolemmal utrophin levels, sarcolemmal dystrophin complex members and numbers of regenerating fibres within a biopsy section. We quantified sarcolemmal utrophin in mature and regenerating fibres and the percentage of regenerating muscle fibres, in muscle biopsies from Duchenne, the milder Becker muscular dystrophy and controls. Fluorescent immunostaining followed by image analysis was performed to quantify utrophin intensity and ?-dystrogylcan and ? -sarcoglycan intensity at the sarcolemma. Antibodies to fetal and developmental myosins were used to identify regenerating muscle fibres allowing the accurate calculation of percentage regeneration fibres in the biopsy. Our results indicate that muscle biopsies from Becker muscular dystrophy patients have fewer numbers of regenerating fibres and reduced utrophin intensity compared to muscle biopsies from Duchenne muscular dystrophy patients. Of particular interest, we show for the first time that the percentage of regenerating muscle fibres within the muscle biopsy correlate with the clinical severity of Becker and Duchenne muscular dystrophy patients from whom the biopsy was taken. The ongoing development of these tools to quantify sarcolemmal utrophin and muscle regeneration in muscle biopsies will be invaluable for assessing utrophin modulator activity in future clinical trials.

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Sarcospan-dependent Akt activation is required for utrophin expression and muscle regeneration.

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Sarcospan-dependent Akt activation is required for utrophin expression and muscle regeneration.

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