Cyclosporin A treatment upregulates Id1 and Smad3 expression and delays skeletal muscle regeneration

Implications of pre-transplant sarcopenia and frailty in patients with non-alcoholic steatohepatitis and alcoholic liver disease

Frailty manifesting as sarcopenia is an independent risk factor for mortality in cirrhosis, and often presents in low model for end-stage liver disease (MELD) patients. Its etiology is multifactorial, but key physiologic changes culminate in altered energy utilization in the fasting state, preferentially utilizing muscle amino acids for gluconeogenesis thereby promoting sarcopenia. Hyperammonemia alters the circulating amino acid profile, diminishing pro-muscle branched-chain amino acids like leucine. The metabolic syndrome worsens sarcopenia through multi-tissue insulin resistance. Alcohol also exacerbates sarcopenia as a direct muscle toxin and inhibitor of growth signaling. Therapy is aimed at alcohol cessation, frequent high-protein meals, branched-chain amino acid supplementation, and diminished time spent fasting. Moderate exercise can improve muscle mass and muscle quality, though precise exercise regimens have not yet been explicitly determined. Studies are ongoing into the effects of myostatin antagonists and insulin sensitizers. The Liver Frailty Index can predict patients most at risk of poor outcome and should be considered in the management of all cirrhotic patients. Specialty testing like dual-energy X-ray absorptiometry (DEXA) scanning and cross-sectional estimates of muscle mass are areas of active research and may play a future role in clinical risk-stratification.

Frailty in CKD and Transplantation

The population is aging. Although older adults have higher rates of comorbidities and adverse health events, they represent a heterogeneous group with different health trajectories. Frailty, a clinical syndrome of decreased physiological reserve and increased susceptibility to illness and death, has emerged as a potential risk stratification tool in older patients with chronic kidney disease (CKD). Frailty is commonly observed in patients with CKD and associated with numerous adverse outcomes, including falls, decreased quality of life, hospitalizations, and death. Multiple pathologic factors contribute to the development of frailty in patients with CKD, including biological mechanisms of aging and physiological dysregulation. Current interventions to reduce frailty are promising, but additional investigations are needed to determine whether optimizing frailty measures improves renal and overall health outcomes. This review of frailty in CKD examines frailty definitions, the impact of frailty on health outcomes across the CKD spectrum, mechanisms of frailty, and antifrailty interventions (e.g., exercise or senescent cell clearance) tested in CKD patients. In addition, existing knowledge gaps, limitations of current frailty definitions in CKD, and challenges surrounding effective antifrailty strategies in CKD are considered.

Striated Preferentially Expressed Protein Kinase (SPEG)-Deficient Skeletal Muscles Display Fewer Satellite Cells with Reduced Proliferation and Delayed Differentiation

Centronuclear myopathies (CNMs) are a subtype of congenital myopathies characterized by skeletal muscle weakness and an increase in the number of central myonuclei. SPEG (striated preferentially expressed protein kinase) has been identified as the sixth gene associated with CNM, and it has been shown that striated muscle-specific Speg-knockout (KO) mice have defective triad formation, abnormal excitation-contraction coupling, and calcium mishandling. The impact of SPEG deficiency on the survival and function of myogenic cells remains to be deciphered. In this study, the authors examined the overall population, proliferation, and differentiation of myogenic cells obtained from striated muscle-specific Speg-KO mice and compared them with wild-type (WT) controls. SPEG-deficient skeletal muscles contained fewer myogenic cells, which on further study demonstrated reduced proliferation and delayed differentiation compared with those from WT muscles. Regenerative response to skeletal muscle injury in Speg-KO mice was compared with that of WT mice, leading to the identification of similar abnormalities including fewer satellite cells, fewer dividing cells, and an increase in apoptotic cells in KO mice. Overall, these results reveal specific abnormalities in myogenic cell number and behavior associated with SPEG deficiency. Similar satellite cell defects have been reported in mouse models of MTM1- and DNM2-associated CNM, suggestive of shared underlying pathways.

Muscle Mass and Mortality After Cardiac Transplantation

BACKGROUND

Frailty assessment is recommended to evaluate the candidacy of adults referred for orthotopic heart transplantation (OHT). Psoas muscle area (PMA) is an easily measured biomarker for frailty. There has yet to be a study examining the prognostic impact of PMA in OHT patients.

METHODS

In this retrospective study, preoperative and postoperative computed tomography (CT) scans were retrieved for adults transplanted between 2000 and 2015 at a tertiary care hospital. Psoas muscle area was measured on a single axial image. Outcomes of interest were all-cause mortality over 6 years and a composite of in-hospital mortality or major morbidity (prolonged ventilation, stroke, dialysis, mediastinitis, or reoperation).

RESULTS

Of 161 adult patients transplanted, 82 had at least 1 abdominal CT scan. At baseline, mean PMA was 25.7 ± 5.8 cm in men and 16.0 ± 3.6 cm in women, and decreased by 8% from the first to the last available CT scan. Adjusting for age, sex, body mass index, and cardiomyopathy etiology, every 1-cm increase in PMA was found to be associated with a 9% reduction in long-term mortality (hazard ratio, 0.91; 95% confidence interval [CI], 0.83-0.99; P = 0.031) and a 17% reduction in in-hospital mortality or major morbidity (odds ratio, 0.83; 95% CI, 0.72-0.96; P = 0.014). When PMA was smaller than the sex-specific median, the risk of mortality or major morbidity increased fourfold (odds ratio, 4.29; 95% CI, 1.19-15.46; P = 0.026).

CONCLUSIONS

Muscle mass is an independent predictor of mortality and major morbidity after OHT. Further research is needed to determine whether frail OHT patients with low PMA may benefit from muscle-building interventions to improve outcomes.

Myostatin and other musculoskeletal markers in lung transplant recipients

Recipients of lung transplantation (LuTx) may experience impaired muscle function and bone metabolism even after rehabilitation. We investigated the potential use of musculoskeletal markers in identifying the impairment of muscle function and bone function in these patients. Biochemical parameters, bodily functions, and lung function of 37 LuTx recipients were evaluated at the time of their discharge from the hospital stay and about 6 months later. The biomarkers were also assessed in 30 healthy age and gender distribution-matched controls. Compared to controls, the negative muscle regulator myostatin was elevated in LuTx recipients at baseline and follow-up, whereas its opponent follistatin only showed a group-specific difference at follow-up. LuTx recipients had reduced serum levels of sclerostin and increased levels of dickkopf 1 and periostin. Lung function and physical function were improved during follow-up. The change in lung function was correlated with the change in chair-rising time and the 6-min walking test. At follow-up, all musculoskeletal markers of LuTx recipients differed from those of controls, thus reflecting their still reduced lung function and bodily functions. Among the tested biomarkers, myostatin, sclerostin, dickkopf 1, and periostin were useful to detect impaired musculoskeletal function in LuTx recipients. Myostatin may serve as a target of treatment in the future.

A 5-Year Follow-Up of The Benefits of an Exercise Training Program in Liver Recipients Transplanted Due to Familial Amyloidotic Polyneuropathy

BACKGROUND

Supervised (SE) and home-based exercise (HBE) training regimes are effective on reconditioning patients with familial amyloidotic polyneuropathy (FAP) after liver transplantation, but research of the long-term retention of the benefits attained in patients with FAP has not yet been conducted.

PURPOSE

In this 5-year follow-up study, we aimed to determine whether the exercise training gains in body composition, physical activity, and function promoted by a 24-week SE or HBE training regimes are retained in patients with FAP who resume normal activity.

METHODOLOGY

Sixteen liver-transplanted patients with FAP were reassessed for body composition (dual X-ray absorptiometry), physical activity (questionnaire), and function (handgrip strength and 6-minute walk test).

RESULTS

Total body fat increased with both exercise regimes during follow-up ( P < .05; η² = 0.432-0.625) as well as femoral neck bone density ( P = .048; η² = 0.119). However, gains in upper limbs muscle quality during follow-up ( P < .001; η² = 0.597) were only found in the SE group ( P = .042; η² = 0.245). Both exercise regimes showed retaining aptitudes in walking capacity ( P < .05; η² = 0.329-0.460) and muscle mass ( P = .05; η² = 0.245). Still, none could retain the physical activity levels.

CONCLUSION

Long-term resumption of normal activity following a 24-week SE or HBE regime in patients with FAP resulted in loss of exercise induced increases in physical activity but counterweighted postoperative losses in bone mineral density and substantially retained the benefits in walking capacity, muscle mass, and quality, in particular, in the SE group.

Vascular Delivery of Allogeneic MuStem Cells in Dystrophic Dogs Requires Only Short-Term Immunosuppression to Avoid Host Immunity and Generate Clinical/Tissue Benefits

Growing demonstrations of regenerative potential for some stem cells led recently to promising therapeutic proposals for neuromuscular diseases. We have shown that allogeneic MuStem cell transplantation into Golden Retriever muscular dystrophy (GRMD) dogs under continuous immunosuppression (IS) leads to persistent clinical stabilization and muscle repair. However, long-term IS in medical practice is associated with adverse effects raising safety concerns. Here, we investigate whether the IS removal or its restriction to the transplantation period could be considered. Dogs aged 4-5 months old received vascular infusions of allogeneic MuStem cells without IS (GRMDMU/no-IS) or under transient IS (GRMDMU/tr-IS). At 5 months post-infusion, persisting clinical status improvement of the GRMDMU/tr-IS dogs was observed while GRMDMU/no-IS dogs exhibited no benefit. Histologically, only 9-month-old GRMDMU/tr-IS dogs showed an increased muscle regenerative activity. A mixed cell reaction with the host peripheral blood mononucleated cells (PBMCs) and corresponding donor cells revealed undetectable to weak lymphocyte proliferation in GRMDMU/tr-IS dogs compared with a significant proliferation in GRMDMU/no-IS dogs. Importantly, any dog group showed neither cellular nor humoral anti-dystrophin responses. Our results show that transient IS is necessary and sufficient to sustain allogeneic MuStem cell transplantation benefits and prevent host immunity. These findings provide useful critical insight to designing therapeutic strategies.

Transmission of Porcine Endogenous Retrovirus Produced from Different Recipient Cells In Vivo

Humanized pigs have been developed to reduce the incidence of immune rejection in xenotransplantation, but significant concerns remain, such as transmission of viral zoonosis. Porcine endogenous retroviruses (PERV), which exist in the genome of pigs, are produced as infectious virions from all porcine cells and cause zoonosis. Here, we examined the possibility of zoonosis of hosts under conditions of immune suppression or xenotransplantation of cells producing host-adapted viruses. Upon transplantation of PERV-producing porcine cells into mice, no transmission of PERV was detected, whereas, transmission of PERV from mice transplanted with mouse-adapted PERV-producing cells was detected. In addition, the frequency of PERV transmission was increased in CsA treated mice transplanted with PERV-producing murine cells, compared with PERV-producing porcine cells. Transmission of PERV to host animals did not affect weight but immune responses, in particular, the number of T cells from PERV-transmitted mice, were notably reduced. The observed risk of PERV zoonosis highlights the requirement for thorough evaluation of viral zoonosis under particular host conditions, such as immunosuppressive treatment and transplantation with host-adapted virus-producing cells.

The Effect of SERCA1b Silencing on the Differentiation and Calcium Homeostasis of C2C12 Skeletal Muscle Cells

The sarcoplasmic/endoplasmic reticulum Ca2+ATPases (SERCAs) are the main Ca2+ pumps which decrease the intracellular Ca2+ level by reaccumulating Ca2+ into the sarcoplasmic reticulum. The neonatal SERCA1b is the major Ca2+ pump in myotubes and young muscle fibers. To understand its role during skeletal muscle differentiation its synthesis has been interfered with specific shRNA sequence. Stably transfected clones showing significantly decreased SERCA1b expression (cloneC1) were selected for experiments. The expression of the regulatory proteins of skeletal muscle differentiation was examined either by Western-blot at the protein level for MyoD, STIM1, calsequestrin (CSQ), and calcineurin (CaN) or by RT-PCR for myostatin and MCIP1.4. Quantitative analysis revealed significant alterations in CSQ, STIM1, and CaN expression in cloneC1 as compared to control cells. To examine the functional consequences of the decreased expression of SERCA1b, repeated Ca2+-transients were evoked by applications of 120 mM KCl. The significantly higher [Ca2+]i measured at the 20th and 40th seconds after the beginning of KCl application (112±3 and 110±3 nM vs. 150±7 and 135±5 nM, in control and in cloneC1 cells, respectively) indicated a decreased Ca2+-uptake capability which was quantified by extracting the maximal pump rate (454±41 μM/s vs. 144±24 μM/s, in control and in cloneC1 cells). Furthermore, the rate of calcium release from the SR (610±60 vs. 377±64 μM/s) and the amount of calcium released (843±75 μM vs. 576±80 μM) were also significantly suppressed. These changes were also accompanied by a reduced activity of CaN in cells with decreased SERCA1b. In parallel, cloneC1 cells showed inhibited cell proliferation and decreased myotube nuclear numbers. Moreover, while cyclosporineA treatment suppressed the proliferation of parental cultures it had no effect on cloneC1 cells. SERCA1b is thus considered to play an essential role in the regulation of [Ca2+]i and its ab ovo gene silencing results in decreased skeletal muscle differentiation.

Detection of the mechanism of immunotoxicity of cyclosporine A in murine in vitro and in vivo models

Transcriptomics in combination with in vitro cell systems is a powerful approach to unravel modes of action of toxicants. An important question is to which extent the modes of action as revealed by transcriptomics depend on cell type, species and study type (in vitro or in vivo). To acquire more insight into this, we assessed the transcriptomic effects of the immunosuppressive drug cyclosporine A (CsA) upon 6 h of exposure of the mouse cytotoxic T cell line CTLL-2, the thymoma EL-4 and primary splenocytes and compared these to the effects in spleens of mice orally treated with CsA for 7 days. EL-4 and CTLL-2 cells showed the highest similarities in response. CsA affected many genes in primary splenocytes that were not affected in EL-4 or CTLL-2. Pathway analysis demonstrated that CsA upregulated the unfolded protein response, endoplasmic reticulum stress and NRF2 activation in EL-4 cells, CTLL-2 cells and primary mouse splenocytes but not in mouse spleen in vivo. As expected, CsA downregulated cell cycle and immune response in splenocytes in vitro, spleens in vivo as well as CTLL-2 in vitro. Genes up- and downregulated in human Jurkat, HepG2 and renal proximal tubular cells were similarly affected in CTLL-2, EL-4 and primary splenocytes in vitro. In conclusion, of the models tested in this study, the known mechanism of immunotoxicity of CsA is best represented in the mouse cytotoxic T cell line CTLL-2. This is likely due to the fact that this cell line is cultured in the presence of a T cell activation stimulant (IL-2) making it more suitable to detect inhibitory effects on T cell activation.

Cyclosporin A Promotes in vivo Myogenic Response in Collagen VI-Deficient Myopathic Mice

Mutations of genes encoding for collagen VI cause various muscle diseases in humans, including Bethlem myopathy and Ullrich congenital muscular dystrophy. Collagen VI null (Col6a1 (-/-)) mice are affected by a myopathic phenotype with mitochondrial dysfunction, spontaneous apoptosis of muscle fibers, and defective autophagy. Moreover, Col6a1 (-/-) mice display impaired muscle regeneration and defective self-renewal of satellite cells after injury. Treatment with cyclosporin A (CsA) is effective in normalizing the mitochondrial, apoptotic, and autophagic defects of myofibers in Col6a1 (-/-) mice. A pilot clinical trial with CsA in Ullrich patients suggested that CsA may increase the number of regenerating myofibers. Here, we report the effects of CsA administration at 5 mg/kg body weight every 12 h in Col6a1 (-/-) mice on muscle regeneration under physiological conditions and after cardiotoxin (CdTx)-induced muscle injury. Our findings indicate that CsA influences satellite cell activity and triggers the formation of regenerating fibers in Col6a1 (-/-) mice. Data obtained on injured muscles show that under appropriate administration, regimens CsA is able to stimulate myogenesis in Col6a1 (-/-) mice by significantly increasing the number of myogenin (MyoG)-positive cells and of regenerating myofibers at the early stages of muscle regeneration. CsA is also able to ameliorate muscle regeneration of Col6a1 (-/-) mice subjected to multiple CdTx injuries, with a concurrent maintenance of the satellite cell pool. Our data show that CsA is beneficial for muscle regeneration in Col6a1 (-/-) mice.

Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates

OPINION STATEMENT

Liver transplantation is the definitive therapy for cirrhosis, and malnutrition is the most frequent complication in these patients. Sarcopenia or loss of muscle mass is the major component of malnutrition in cirrhotics and adversely affects their outcome. In addition to the metabolic consequences, functional consequences of sarcopenia include reduced muscle strength and deconditioning. Despite nearly universal occurrence of sarcopenia and its attendant complications, there are no established therapies to prevent or reverse the same. Major reasons for this deficiency include the lack of established standardized definitions or measures to quantify muscle mass, as well as paucity of mechanistic studies or identified molecular targets to develop specific therapeutic interventions. Anthropometric evaluation, bioelectrical impedance analysis, and DEXA scans are relatively imprecise measures of muscle mass, and recent data on imaging measures to determine muscle mass accurately are likely to allow well-defined outcome responses to treatments. Resurgence of interest in the mechanisms of muscle loss in liver disease has been directly related to the rapid advances in the field of muscle biology. Metabolic tracer studies on whole body kinetics have been complemented by direct studies on the skeletal muscle of cirrhotics. Hypermetabolism and anabolic resistance contribute to sarcopenia. Reduced protein synthesis and increased autophagy have been reported in cirrhotic skeletal muscle, while the contribution of the ubiquitin-proteasome pathway is controversial. Increased plasma concentration and skeletal muscle expression of myostatin, a TGFβ superfamily member that causes reduction in muscle mass, have been reported in cirrhosis. Hyperammonemia and TNFα have been reported to increase myostatin expression and may be responsible for sarcopenia in cirrhosis. Nutriceutical interventions with leucine enriched amino acid mixtures, myostatin antagonists and physical activity hold promise as measures to reverse sarcopenia. There is even less data on muscle function and deconditioning in cirrhosis and studies in this area are urgently needed. Even though macronutrient replacement is a major therapeutic goal, micronutrient supplementation, specifically vitamin D, is expected to improve outcomes.

Exercise limitation following transplantation

Organ transplantation is one of the medical miracles or the 20th century. It has the capacity to substantially improve exercise performance and quality of life in patients who are severely limited with chronic organ failure. We focus on the most commonly performed solid-organ transplants and describe peak exercise performance following recovery from transplantation. Across all of the common transplants, evaluated significant reduction in VO2peak is seen (typically renal and liver 65%-80% with heart and/or lung 50%-60% of predicted). Those with the lowest VO2peak pretransplant have the lowest VO2peak posttransplant. Overall very few patients have a VO2peak in the normal range. Investigation of the cause of the reduction of VO2peak has identified many factors pre- and posttransplant that may contribute. These include organ-specific factors in the otherwise well-functioning allograft (e.g., chronotropic incompetence in heart transplantation) as well as allograft dysfunction itself (e.g., chronic lung allograft dysfunction). However, looking across all transplants, a pattern emerges. A low muscle mass with qualitative change in large exercising skeletal muscle groups is seen pretransplant. Many factor posttransplant aggravate these changes or prevent them recovering, especially calcineurin antagonist drugs which are key immunosuppressing agents. This results in the reduction of VO2peak despite restoration of near normal function of the initially failing organ system. As such organ transplantation has provided an experiment of nature that has focused our attention on an important confounder of chronic organ failure-skeletal muscle dysfunction.

Posttransplant sarcopenia: an underrecognized early consequence of liver transplantation

Liver transplantation is believed to reverse the clinical and metabolic abnormalities of cirrhosis. Reduced skeletal muscle mass or sarcopenia contributes to increased mortality and adverse consequences of cirrhosis. Failure of reversal of sarcopenia of cirrhosis after liver transplantation is not well recognized. Six temporally, geographically, and methodologically distinct follow-up studies in 304 cirrhotics reported conflicting data on changes in indirect measures of skeletal muscle mass after transplantation. Distinct measures of body composition but not skeletal muscle mass were used and did not focus on the clinical consequences of sarcopenia after transplantation. A number of studies reported an initial rapid postoperative loss of lean mass followed by incomplete recovery with a maximum follow-up of 2 years. Posttransplant sarcopenia may be responsible for metabolic syndrome and impaired quality of life after liver transplantation. Potential reasons for failure to reverse sarcopenia after liver transplantation include use of immunosuppressive agents [mammalian target of rapamycin (mTOR) and calcineurin inhibitors] that impair skeletal muscle growth and protein accretion. Repeated hospitalizations, posttransplant infections, and renal failure also contribute to posttransplant sarcopenia. Finally, recovery from muscle deconditioning is limited by lack of systematic nutritional and physical-activity-based interventions to improve muscle mass. Despite the compelling data on sarcopenia before liver transplantation, the impact of posttransplant sarcopenia on clinical outcomes is not known. There is a compelling need for studies to examine the mechanisms and consequences of sarcopenia post liver transplantation to permit development of therapies to prevent and reverse this disorder.

Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses

Malnutrition is the most common, reversible complication of cirrhosis that adversely affects survival, response to other complications, and quality of life. Sarcopenia, or loss of skeletal muscle mass, and loss of adipose tissue and altered substrate use as a source of energy are the 2 major components of malnutrition in cirrhosis. Current therapies include high protein supplementation especially as a late evening snack. Exercise protocols have the potential of aggravating hyperammonemia and portal hypertension. Recent advances in understanding the molecular regulation of muscle mass has helped identify potential novel therapeutic targets including myostatin antagonists, and mTOR resistance.

The functional role of calcineurin in hypertrophy, regeneration, and disorders of skeletal muscle

Skeletal muscle uses calcium as a second messenger to respond and adapt to environmental stimuli. Elevations in intracellular calcium levels activate calcineurin, a serine/threonine phosphatase, resulting in the expression of a set of genes involved in the maintenance, growth, and remodeling of skeletal muscle. In this review, we discuss the effects of calcineurin activity on hypertrophy, regeneration, and disorders of skeletal muscle. Calcineurin is a potent regulator of muscle remodeling, enhancing the differentiation through upregulation of myogenin or MEF2A and downregulation of the Id1 family and myostatin. Foxo may also be a downstream candidate for a calcineurin signaling molecule during muscle regeneration. The strategy of controlling the amount of calcineurin may be effective for the treatment of muscular disorders such as DMD, UCMD, and LGMD. Activation of calcineurin produces muscular hypertrophy of the slow-twitch soleus muscle but not fast-twitch muscles.

Presenilin-1 acts via Id1 to regulate the function of muscle satellite cells in a gamma-secretase-independent manner

Muscle satellite cells are the resident stem cells of adult skeletal muscle. Here, we have examined the role of the multifunctional protein presenilin-1 (PS1) in satellite cell function. PS1 acts as a crucial component of the gamma-secretase complex, which is required to cleave single-pass transmembrane proteins such as Notch and amyloid-beta precursor protein. PS1, however, also functions through gamma-secretase-independent pathways. Activation of satellite cells was accompanied by induction of PS1, with PS1 knockdown enhancing their myogenic differentiation, but reducing their self-renewal. Transfection with siRNA against PS1 led to accelerated myogenic differentiation during muscle regeneration in vivo. Conversely, constitutive expression of PS1 resulted in the suppression of myogenic differentiation and promotion of the self-renewal phenotype. Importantly, we found that PS1 also acts independently of its role in gamma-secretase activity in controlling myogenesis, which is mediated in part by Id1 (inhibitor of DNA binding 1), a negative regulator of the myogenic regulatory factor MyoD. PS1 can control Id1, which affects satellite cell fate by regulating the transcriptional activity of MyoD. Taken together, our observations show that PS1 is a key player in the choice of satellite cell fate, acting through both gamma-secretase-dependent and gamma-secretase-independent mechanisms.

Ca2+/calmodulin-dependent transcriptional pathways: potential mediators of skeletal muscle growth and development

The loss of muscle mass with age and disuse has a significant impact on the physiological and social well-being of the aged; this is an increasingly important problem as the population becomes skewed towards older age. Exercise has psychological benefits but it also impacts on muscle protein synthesis and degradation, increasing muscle tissue volume in both young and older individuals. Skeletal muscle hypertrophy involves an increase in muscle mass and cross-sectional area and associated increased myofibrillar protein content. Attempts to understand the molecular mechanisms that underlie muscle growth, development and maintenance, have focused on characterising the molecular pathways that initiate, maintain and regenerate skeletal muscle. Such understanding may aid in improving targeted interventional therapies for age-related muscle loss and muscle wasting associated with diseases. Two major routes through which skeletal muscle development and growth are regulated are insulin-like growth factor I (IGF-I) and Ca(2+)/calmodulin-dependent transcriptional pathways. Many reviews have focused on understanding the signalling pathways of IGF-I and its receptor, which govern skeletal muscle hypertrophy. However, alternative molecular signalling pathways such as the Ca(2+)/calmodulin-dependent transcriptional pathways should also be considered as potential mediators of muscle growth. These latter pathways have received relatively little attention and the purpose herein is to highlight the progress being made in the understanding of these pathways and associated molecules: calmodulin, calmodulin kinases (CaMKs), calcineurin and nuclear factor of activated T-cell (NFAT), which are involved in skeletal muscle regulation. We describe: (1) how conformational changes in the Ca(2+) sensor calmodulin result in the exposure of binding pockets for the target proteins (CaMKs and calcineurin). (2) How Calmodulin consequently activates either the Ca(2+)/calmodulin-dependent kinases pathways (via CaMKs) or calmodulin-dependent serine/threonine phosphatases (via calcineurin). (3) How calmodulin kinases alter transcription in the nucleus through the phosphorylation, deactivation and translocation of histone deacetylase 4 (HDAC4) from the nucleus to the cytoplasm. (4) How calcineurin transmits signals to the nucleus through the dephosphorylation and translocation of NFAT from the cytoplasm to the nucleus.

Cyclosporin A modulates cellular localization of MEF2C protein and blocks fiber hypertrophy in the overloaded soleus muscle of mice

The molecular signaling pathway linked to hypertrophy of the anti-gravity/postural soleus muscle after mechanical overloading has not been identified. Using reverse transcription-polymerase chain reaction (RT-PCR), Western blot, and immunohistochemical analyses, we investigated whether the amounts of myocyte enhancer factor (MEF)2C, MEF2D, and myogenin change in the mechanically overloaded soleus muscle after treatment with the calcineurin inhibitor cyclosporine A (CsA). Adult male ICR mice were subjected to a surgical ablation of the gastrocnemius muscle and treated with either CsA (25 mg/kg) or vehicle, once daily. They were killed at 2, 4, 7, 10, and 14 days post-injury. Mechanical overloading resulted in a significant increase in the wet weight and the cross-sectional area of slow and fast fibers of the soleus muscle in placebo-treated mice but not CsA-treated mice. RT-PCR analysis did not show a marked difference in MEF2C and MEF2D mRNA levels in the overloaded soleus muscle in placebo- or CsA-administered mice. After 2 days of mechanical overloading, we observed co-localization of MEF2C and myogenin in several mononuclear cells under both conditions. These MEF2C-positive mononuclear cells also possessed immunoreactivity for c-Met, a satellite cell marker. At 4 days, mechanical overloading induced marked expression of MEF2C but not MEF2D in the subsarcolemmal region in a group of myotubes and/or myofibers. Such a MEF2C-positive region emerged less often in the hypertrophied soleus muscle subjected to the treatment with CsA. At 7 days, we observed many mononuclear cells possessing both MEF2C and myogenin protein in mice treated with CsA, but not the placebo. Our results demonstrated that CsA treatment modulates the amount and cellular localization of MEF2C protein. The modulation of MEF2C by CsA treatment may inhibit the hypertrophic process in the soleus muscle after mechanical overloading.

Stimulation of calcineurin Aalpha activity attenuates muscle pathophysiology in mdx dystrophic mice

Calcineurin activation ameliorates the dystrophic pathology of hindlimb muscles in mdx mice and decreases their susceptibility to contraction damage. In mdx mice, the diaphragm is more severely affected than hindlimb muscles and more representative of Duchenne muscular dystrophy. The constitutively active calcineurin Aalpha transgene (CnAalpha) was overexpressed in skeletal muscles of mdx (mdx CnAalpha*) mice to test whether muscle morphology and function would be improved. Contractile function of diaphragm strips and extensor digitorum longus and soleus muscles from adult mdx CnAalpha* and mdx mice was examined in vitro. Hindlimb muscles from mdx CnAalpha* mice had a prolonged twitch time course and were more resistant to fatigue. Because of a slower phenotype and a decrease in fiber cross-sectional area, normalized force was lower in fast- and slow-twitch muscles of mdx CnAalpha* than mdx mice. In the diaphragm, despite a slower phenotype and a approximately 35% reduction in fiber size, normalized force was preserved. This was likely mediated by the reduction in the area of the diaphragm undergoing degeneration (i.e., mononuclear cell and connective and adipose tissue infiltration). The proportion of centrally nucleated fibers was reduced in mdx CnAalpha* compared with mdx mice, indicative of improved myofiber viability. In hindlimb muscles of mdx mice, calcineurin activation increased expression of markers of regeneration, particularly developmental myosin heavy chain isoform and myocyte enhancer factor 2A. Thus activation of the calcineurin signal transduction pathway has potential to ameliorate the mdx pathophysiology, especially in the diaphragm, through its effects on muscle degeneration and regeneration and endurance capacity.

Differential calcineurin signalling activity and regeneration efficacy in diaphragm and limb muscles of dystrophic mdx mice

Calcineurin activity is essential for successful skeletal muscle regeneration in young mdx mice and in wild type mice following myotoxic injury and cryodamage. In mature myofibres of adult mdx mice, calcineurin stimulation can ameliorate the dystrophic pathology. The aim of this study was to test the hypothesis that the more severe dystrophic pathology of the diaphragm compared with hindlimb muscles of mdx mice could be attributed to aberrant calcineurin signalling and that due to ongoing regeneration calcineurin activity would be greater in muscles of adult mdx than wild type mice. Differences in markers of regeneration between tibialis anterior and diaphragm muscles were also characterised, to determine whether there was an association between regeneration efficacy and calcineurin activity in dystrophic muscles. In diaphragm muscles of adult mdx mice, the proportion of centrally nucleated fibres and developmental myosin heavy chain protein expression was lower and myogenin protein expression was higher than in tibialis anterior muscles. Calcineurin and activated NFATc1 protein content and calcineurin phosphatase activity were higher in muscles from mdx than wild type mice and calcineurin activation was greater in diaphragm than tibialis anterior muscles of mdx mice. Thus, despite greater calcineurin activity in diaphragm compared to hindlimb muscles, regeneration events downstream of myoblast differentiation and mediated by the injured myofibre were severely compromised.

Satellite cells from dystrophic (mdx) mice display accelerated differentiation in primary cultures and in isolated myofibers

In the dystrophic (mdx) mouse, skeletal muscle undergoes cycles of degeneration and regeneration, and myogenic progenitors (satellite cells) show ongoing proliferation and differentiation at a time when counterpart cells in normal healthy muscle enter quiescence. However, it remains unclear whether this enhanced satellite cell activity is triggered solely by the muscle environment or is also governed by factors inherent in satellite cells. To obtain a better picture of myogenesis in dystrophic muscle, a direct cell-by-cell analysis was performed to compare satellite cell dynamics from mdx and normal (C57Bl/10) mice in two cell culture models. In one model, the kinetics of satellite cell differentiation was quantified in primary cell cultures from diaphragm and limb muscles by immunodetection of MyoD, myogenin, and MEF2. In mdx cell cultures, myogenin protein was expressed earlier than normal and was followed more rapidly by dual myogenin/MEF2A expression and myotube formation. In the second model, the dynamics of satellite cell myogenesis were investigated in cultured myofibers isolated from flexor digitorum brevis (FDB) muscle, which retain satellite cells in the native position. Consistent with primary cultures, satellite cells in mdx myofibers displayed earlier myogenin expression, as well as an enhanced number of myogenin-expressing satellite cells per myofiber compared to normal. The addition of fibroblast growth factor 2 (FGF2) led to an increase in the number of satellite cells expressing myogenin in normal and mdx myofibers. However, the extent of the FGF effect was more robust in mdx myofibers. Notably, many myonuclei in mdx myofibers were centralized, evidence of segmental regeneration; all central nuclei and many peripheral nuclei in mdx myofibers were positive for MEF2A. Results indicated that myogenic cells in dystrophic muscle display accelerated differentiation. Furthermore, the study demonstrated that FDB myofibers are an excellent model of the in vivo state of muscle, as they accurately represented the dystrophic phenotype.