Insulin-like Growth Factor-II Delays Early but Enhances Late Regeneration of Skeletal Muscle

Per1/Per2-Igf2 axis-mediated circadian regulation of myogenic differentiation

Circadian rhythms regulate cell proliferation and differentiation, but circadian control of tissue regeneration remains elusive at the molecular level. Here, we show that proper myoblast differentiation and muscle regeneration are regulated by the circadian master regulators Per1 and Per2. Depletion of Per1 or Per2 suppressed myoblast differentiation in vitro and muscle regeneration in vivo, demonstrating their nonredundant functions. Both Per1 and Per2 were required for the activation of Igf2, an autocrine promoter of myoblast differentiation, accompanied by Per-dependent recruitment of RNA polymerase II, dynamic histone modifications at the Igf2 promoter and enhancer, and the promoter–enhancer interaction. This circadian epigenetic priming created a preferred time window for initiating myoblast differentiation. Consistently, muscle regeneration was faster if initiated at night, when Per1, Per2, and Igf2 were highly expressed compared with morning. This study reveals the circadian timing as a significant factor for effective muscle cell differentiation and regeneration.

Marginal zinc deficiency negatively affects recovery from muscle injury in mice

The aim of the present study was to elucidate whether the recovery from muscle injury is impaired in marginal zinc deficiency. C57BL/6 male mice were fed a marginally zinc-deficient diet (MZD: 8 mg Zn/kg diet), a zinc-adequate diet (ZA: zinc 35 mg Zn/kg diet), and a zinc-high diet (ZH: 190 mg Zn/kg diet) for 4 weeks. Muscle injury was induced in the gastrocnemius muscles using cardiotoxin. The gastrocnemius muscles of these mice were harvested at 3, 5, 7, 10, 14, and 20 days after injury. We evaluated the regeneration of the skeletal muscle with hematoxylin and eosin staining and developmental myosin heavy-chain (dMHC: implicated in regeneration) immunostaining. The rate of dMHC-positive cells was significantly low in MZD mice compared with ZA mice at 3 days after cardiotoxin injection. The peak dMHC expression was found at 3 days after injection in ZA mice, 5 days in ZH mice, and 7 days in MZD mice. These results suggest that recovery from muscle injury might be partly impaired and delayed in MZD mice. Therefore, we strongly suggest the appropriate zinc intake to prevent the impairment of skeletal muscle regeneration.

Key signalling factors and pathways in the molecular determination of skeletal muscle phenotype

The molecular basis and control of the biochemical and biophysical properties of skeletal muscle, regarded as muscle phenotype, are examined in terms of fibre number, fibre size and fibre types. A host of external factors or stimuli, such as ligand binding and contractile activity, are transduced in muscle into signalling pathways that lead to protein modifications and changes in gene expression which ultimately result in the establishment of the specified phenotype. In skeletal muscle, the key signalling cascades include the Ras-extracellular signal regulated kinase-mitogen activated protein kinase (Erk-MAPK), the phosphatidylinositol 3'-kinase (PI3K)-Akt1, p38 MAPK, and calcineurin pathways. The molecular effects of external factors on these pathways revealed complex interactions and functional overlap. A major challenge in the manipulation of muscle of farm animals lies in the identification of regulatory and target genes that could effect defined and desirable changes in muscle quality and quantity. To this end, recent advances in functional genomics that involve the use of micro-array technology and proteomics are increasingly breaking new ground in furthering our understanding of the molecular determinants of muscle phenotype.

Physiological and histological changes in skeletal muscle following in vivo gene transfer by electroporation

Electroporation (EP) is used to transfect skeletal muscle fibers in vivo, but its effects on the structure and function of skeletal muscle tissue have not yet been documented in detail. We studied the changes in contractile function and histology after EP and the influence of the individual steps involved to determine the mechanism of recovery, the extent of myofiber damage, and the efficiency of expression of a green fluorescent protein (GFP) transgene in the tibialis anterior (TA) muscle of adult male C57Bl/6J mice. Immediately after EP, contractile torque decreased by ∼80% from pre-EP levels. Within 3 h, torque recovered to ∼50% but stayed low until day 3. Functional recovery progressed slowly and was complete at day 28. In muscles that were depleted of satellite cells by X-irradiation, torque remained low after day 3, suggesting that myogenesis is necessary for complete recovery. In unirradiated muscle, myogenic activity after EP was confirmed by an increase in fibers with central nuclei or developmental myosin. Damage after EP was confirmed by the presence of necrotic myofibers infiltrated by CD68+ macrophages, which persisted in electroporated muscle for 42 days. Expression of GFP was detected at day 3 after EP and peaked on day 7, with ∼25% of fibers transfected. The number of fibers expressing green fluorescent protein (GFP), the distribution of GFP+ fibers, and the intensity of fluorescence in GFP+ fibers were highly variable. After intramuscular injection alone, or application of the electroporating current without injection, torque decreased by ∼20% and ∼70%, respectively, but secondary damage at D3 and later was minimal. We conclude that EP of murine TA muscles produces variable and modest levels of transgene expression, causes myofiber damage due to the interaction of intramuscular injection with the permeabilizing current, and that full recovery requires myogenesis.

Therapeutic approaches for muscle wasting disorders

Muscle wasting and weakness are common in many disease states and conditions including aging, cancer cachexia, sepsis, denervation, disuse, inactivity, burns, HIV-acquired immunodeficiency syndrome (AIDS), chronic kidney or heart failure, unloading/microgravity, and muscular dystrophies. Although the maintenance of muscle mass is generally regarded as a simple balance between protein synthesis and protein degradation, these mechanisms are not strictly independent, but in fact they are coordinated by a number of different and sometimes complementary signaling pathways. Clearer details are now emerging about these different molecular pathways and the extent to which these pathways contribute to the etiology of various muscle wasting disorders. Therapeutic strategies for attenuating muscle wasting and improving muscle function vary in efficacy. Exercise and nutritional interventions have merit for slowing the rate of muscle atrophy in some muscle wasting conditions, but in most cases they cannot halt or reverse the wasting process. Hormonal and/or other drug strategies that can target key steps in the molecular pathways that regulate protein synthesis and protein degradation are needed. This review describes the signaling pathways that maintain muscle mass and provides an overview of some of the major conditions where muscle wasting and weakness are indicated. The review provides details on some therapeutic strategies that could potentially attenuate muscle atrophy, promote muscle growth, and ultimately improve muscle function. The emphasis is on therapies that can increase muscle mass and improve functional outcomes that will ultimately lead to improvement in the quality of life for affected patients.

Sequence of IGF-I, IGF-II, and HGF expression in regenerating skeletal muscle

Various cytokines are thought to play a role in muscle regeneration, however, the interaction and mechanisms of action of these cytokines remains largely unknown. In this study, we investigated the role of HGF, IGF-I, and IGF-II during myogenesis using the regeneration model of skeletal muscle as well as myoblast culture. RT-PCR analysis revealed that HGF and IGF-I expressions were markedly upregulated, in regenerating muscle. In contrast, there was no significant difference in IGF-II expression between normal and regenerating muscle. Immunohistochemical analysis demonstrated that HGF was expressed mostly by myocytes during the early stages of muscle regeneration. Additionally, HGF inhibited the formation of myotubes by myoblasts, but promoted cellular proliferation. Otherwise, IGF-I and IGF-II were expressed by myocytes through the early to middle stages of muscle regeneration. The addition of HGF to myoblast growing in vitro significantly increased the number of cells. These findings indicate that these three cytokines have pleiotropic effects in regenerating skeletal muscle.