The absence of MyoD in regenerating skeletal muscle affects the expression pattern of basement membrane, interstitial matrix and integrin molecules that is consistent with delayed myotube formation

Nystagmus Associated With the Absence of MYOD Expression Across the Lifespan in Extraocular and Limb Muscles

Purpose

The extraocular muscles (EOMs) undergo significant levels of continuous myonuclear turnover and myofiber remodeling throughout life, in contrast to limb skeletal muscles. Activation of the myogenic pathway in muscle precursor cells is controlled by myogenic transcription factors, such as MYOD. Limb muscles from MyoD-/- mice develop normally but have a regeneration defect, and these mice develop nystagmus. We examined MyoD-/- mice to determine if they have an aging phenotype.

Methods

Eye movements of aging MyoD-/- mice and littermate controls (wild type) were examined using optokinetic nystagmus (OKN). We assessed limb muscle function, changes to myofiber number, mean cross-sectional area, and abundance of the PAX7 and PITX2 populations of myogenic precursor cells.

Results

Aging did not significantly affect limb muscle function despite decreased mean cross-sectional areas at 18+ months. Aging wild type mice had normal OKN responses; all aging MyoD-/- mice had nystagmus. With OKN stimulus present, the MyoD-/- mice at all ages had shorter slow phase durations compared to wild type age matched controls. In the dark, the MyoD-/- mice had a shorter slow phase duration with age. This correlated with significantly decreased fiber numbers and cross-sectional areas. The EOM in MyoD-/- mice had increased numbers of PAX7-positive satellite cells and significantly decreased PITX2-positive myonuclei.

Conclusions

The absence of MYOD expression in aging mice causes a decrease in on-going myofiber remodeling, EOM fiber size, and number, and is associated with the development of spontaneous nystagmus. These results suggest that muscle-specific mutations can result in nystagmus, with increasing aging-related changes in the MyoD-/- EOM.

miRNA mediated downregulation of cyclase-associated protein 1 (CAP1) is required for myoblast fusion

Myoblast fusion is essential for the formation, growth, and regeneration of skeletal muscle, but the molecular mechanisms that govern fusion and myofiber formation remain poorly understood. Past studies have shown an important role of the actin cytoskeleton and actin regulators in myoblast fusion. The Cyclase-Associated Proteins (CAP) 1 and 2 recently emerged as critical regulators of actin treadmilling in higher eukaryotes including mammals. Whilst the role of CAP2 in skeletal muscle development and function is well characterized, involvement of CAP1 in this process remains elusive. Here we report that CAP1, plays a critical role in cytoskeletal remodeling during myoblast fusion and formation of myotubes. Cap1 mRNA and protein are expressed in both murine C2C12 and human LHCN-M2 myoblasts, but their abundance decreases during myogenic differentiation. Perturbing the temporally controlled expression of CAP1 by overexpression or CRISPR-Cas9 mediated knockout impaired actin rearrangement, myoblast alignment, expression of profusion molecules, differentiation into multinucleated myotubes, and myosin heavy chain expression. Endogenous Cap1 expression is post-transcriptionally downregulated during differentiation by canonical myomiRs miR-1, miR-133, and miR-206, which have conserved binding sites at the 3′ UTR of the Cap1 mRNA. Deletion of the endogenous 3′ UTR by CRISPR-Cas9 in C2C12 cells phenocopies overexpression of CAP1 by inhibiting myotube formation. Our findings implicates Cap1 and its myomiR-mediated downregulation in the myoblast fusion process and the generation of skeletal muscle.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Decellularized extracellular matrices derived from cultured cells at stepwise myogenic stages for the regulation of myotube formation

The regulation of stem cell differentiation is key for muscle tissue engineering and regenerative medicine. To this end, various substrates mimicking the native extracellular matrix (ECM) have been developed with consideration of the mechanical, topological, and biochemical properties. However, mimicking the biochemical properties of the native ECM is difficult due to its compositional complexity. To develop substrates that mimic the native ECM and its biochemical properties, decellularization is typically used. Here, substrates mimicking the native ECM at each myogenic stage are prepared as stepwise myogenesis-mimicking matrices via the in vitro myogenic culture of C2C12 myoblasts and decellularization. Cells adhered to the stepwise myogenesis-mimicking matrices at similar levels. However, the matrices derived from cells at the myogenic early stage suppressed cell growth and promoted myogenesis. This promotion of myogenesis was potentially due to the suppression of the activation of endogenous BMP signaling following the suppression of the expression of the myogenic-inhibitory factors, Id2 and Id3. Our stepwise myogenesis-mimicking matrices will be suitable ECM models for basic biological research and myogenesis of stem cells. Further, these matrices will provide insights that improve the efficacy of decellularized ECM for muscle repair.

Linker proteins restore basement membrane and correct LAMA2-related muscular dystrophy in mice

LAMA2-related muscular dystrophy (LAMA2 MD or MDC1A) is the most frequent form of early-onset, fatal congenital muscular dystrophies. It is caused by mutations in LAMA2, the gene encoding laminin-α2, the long arm of the heterotrimeric (α2, β1, and γ1) basement membrane protein laminin-211 (Lm-211). We establish that despite compensatory expression of laminin-α4, giving rise to Lm-411 (α4, β1, and γ1), muscle basement membrane is labile in LAMA2 MD biopsies. Consistent with this deficit, recombinant Lm-411 polymerized and bound to cultured myotubes only weakly. Polymerization and cell binding of Lm-411 were enhanced by addition of two specifically designed linker proteins. One, called αLNNd, consists of the N-terminal part of laminin-α1 and the laminin-binding site of nidogen-1. The second, called mini-agrin (mag), contains binding sites for laminins and α-dystroglycan. Transgenic expression of mag and αLNNd in a mouse model for LAMA2 MD fully restored basement membrane stability, recovered muscle force and size, increased overall body weight, and extended life span more than five times to a maximum survival beyond 2 years. These findings provide a mechanistic understanding of LAMA2 MD and establish a strong basis for a potential treatment.

Function of the myogenic regulatory factors Myf5, MyoD, Myogenin and MRF4 in skeletal muscle, satellite cells and regenerative myogenesis

Discovery of the myogenic regulatory factor family of transcription factors MYF5, MYOD, Myogenin and MRF4 was a seminal step in understanding specification of the skeletal muscle lineage and control of myogenic differentiation during development. These factors are also involved in specification of the muscle satellite cell lineage, which becomes the resident stem cell compartment inadult skeletal muscle. While MYF5, MYOD, Myogenin and MRF4 have subtle roles in mature muscle, they again play a crucial role in directing satellite cell function to regenerate skeletal muscle: linking the genetic control of developmental and regenerative myogenesis. Here, I review the role of the myogenic regulatory factors in developing and mature skeletal muscle, satellite cell specification and muscle regeneration.

Carbon Dioxide Water Bathing Enhances Myogenin but Not MyoD Protein Expression after Skeletal Muscle Injury

[Purpose] We reported that carbon dioxide (CO₂) water bathing accelerates skeletal muscle regeneration; however, the underlying mechanism was unclear. MyoD and myogenin play roles in muscle regeneration, and the purpose of this study was to determine changes in MyoD and myogenin caused by CO₂ water bathing after injury. [Subjects] Sixteen female Wistar rats (n = 4 per group) were used. [Methods] The rats were divided into four groups: no-injury (NI), injury (IC), injury + tap water bathing (ITW), and injury + CO₂ water bathing (ICO₂). Muscle injury was induced by injection of bupivacaine hydrochloride into the left tibial anterior (TA) muscles. Tap water and CO₂ (1,000 ppm) water bathing were performed at 37 °C for 30 minutes once a day. The left TA muscles were removed 4 days after injury, and the expressions of MyoD and myogenin were measured. [Results] MyoD and myogenin were increased in the IC, ITW, and ICO₂ groups compared with the NI group. Although the MyoD level was similar in the IC, ITW, and ICO₂ groups, myogenin increased more in the ICO₂ group than in the IC and ITW groups. [Conclusion] CO₂ water bathing after muscle injury appears to induce an increase in the expression of myogenin.

Extracellular matrix metalloproteinase inducer (EMMPRIN/CD147) as a novel regulator of myogenic cell differentiation

Matrix metalloproteinases (MMPs) are thought to play an important role in skeletal muscle cell growth and differentiation. In view of the MMP inducing function of EMMPRIN/CD147, its role in myogenic cell differentiation was investigated. EMMPRIN level increased during differentiation of both rat primary myoblasts derived from satellite cells and mouse C2.7 myogenic cells and was associated with an alteration in its molecular forms. In parallel, expression of pro-MMP-9 gradually decreased and that of pro-MMP-2 and active MMP-2 increased. While small interfering RNA (siRNA) inhibition of EMMPRIN expression accelerated cell differentiation, exogenously added recombinant EMMPRIN inhibited differentiation by an MMP-mediated mechanism, as the MMP inhibitor marimastat abrogated EMMPRIN's effect. Our results further suggest that EMMPRIN regulates differentiation through an MMP activation of transforming growth factor beta (TGFβ), a known inhibitor of myoblast's differentiation, as the increased activation and signaling of TGFβ by EMMPRIN was attenuated in the presence of marimastat. EMMPRIN inhibition may thus represent a novel strategy in the treatment of muscular degenerative disorders.

A transitional extracellular matrix instructs cell behavior during muscle regeneration

Urodele amphibians regenerate appendages through the recruitment of progenitor cells into a blastema that rebuilds the lost tissue. Blastemal formation is accompanied by extensive remodeling of the extracellular matrix. Although this remodeling process is important for appendage regeneration, it is not known whether the remodeled matrix directly influences the generation and behavior of blastemal progenitor cells. By integrating in vivo 3-dimensional spatiotemporal matrix maps with in vitro functional time-lapse imaging, we show that key components of this dynamic matrix, hyaluronic acid, tenascin-C and fibronectin, differentially direct cellular behaviors including DNA synthesis, migration, myotube fragmentation and myoblast fusion. These data indicate that both satellite cells and fragmenting myofibers contribute to the regeneration blastema and that the local extracellular environment provides instructive cues for the regenerative process. The fact that amphibian and mammalian myoblasts exhibit similar responses to various matrices suggests that the ability to sense and respond to regenerative signals is evolutionarily conserved.

Heat stress increases myonuclear number and fiber size via satellite cell activation in rat regenerating soleus fibers

To investigate the effects of heat stress (hyperthermia) on muscle degeneration-regeneration, the soleus muscles of adult male Wistar rats were injected bilaterally with a single injection of bupivacaine. The rats were assigned to a sedentary control (Con), heat stress (Heat), bupivacaine-injected (BPVC), or bupivacaine-injected plus heat stress (BPVC+Heat) group. Heat stress was induced in the Heat and BPVC+Heat groups by immersion of the lower half of the body into water maintained at 42 ± 1°C for 30 min 48 h after the injection of bupivacaine and every other day during the following 1 or 2 wk. The soleus muscles in all groups were excised 24 h after the final bout of heat stress. Mean muscle weight, fiber cross-sectional area, myonuclear number, and heat shock protein 72 (Hsp72) and calcineurin protein levels were lower in the BPVC than in the Con or Heat groups at both time points. In contrast, several of these parameters in the BPVC+Heat group were not different or higher than in the Con or Heat groups at the 1- and/or 2-wk time points. The number of total and activated satellite cells, estimated by analyses of Pax7-negative, M-cadherin-negative, and MyoD-positive nuclei, was greater in BPVC+Heat than in all other groups. Combined, the results indicate that heat stress-related activation of satellite cells and upregulation of Hsp72 and calcineurin expression played important roles in the regeneration of the soleus fibers after bupivacaine injection.

Alpha v beta 3 and alpha v beta 5 integrins and their role in muscle precursor cell adhesion

BACKGROUND INFORMATION

Functional adaptation of skeletal muscle is a requirement for different muscle groups (e.g. craniofacial, ocular and limb) to undergo site-specific changes. Such tissue remodelling depends on dynamic interactions between muscle cells and their extracellular matrix, via participation of multifunctional molecules such as integrins. In view of data suggesting a role in fundamental muscle biology and muscle development in other systems, the present study has focused on expression and function of alpha v integrins, in cultured adult human craniofacial muscle (masseter) precursor cells and myotubes, and the predominantly fibroblastic IC (interstitial cells) population.

RESULTS AND CONCLUSIONS

Flow-cytometric phenotyping and immunofluorescence phenotyping show that alpha v, alpha v beta 3 and alpha v beta 5 are expressed in all mononuclear cells (muscle precursors and IC) seeded on muscle extracellular molecules such as gelatin, VN (vitronectin) and FN (fibronectin). In this system, blockade of alpha v activity using a function-perturbing antibody abrogates cell migration on VN and FN. alpha v integrins act predominantly as VN receptors as cell-substrate attachment is diminished when alpha v neutralizing agents are introduced into cultures seeded on VN, and this inhibition is reversible; these integrins also appear to be minor FN receptors. These results demonstrate that the alpha v subset of integrins present on both myogenic precursors and IC is an essential cohort of VN and, to a lesser extent, FN receptors mediating cell adhesion and, either directly or indirectly, arbiters of cell motility.

Increased survival of muscle stem cells lacking the MyoD gene after transplantation into regenerating skeletal muscle

MyoD is a myogenic master transcription factor that plays an essential role in muscle satellite cell (muscle stem cell) differentiation. To further investigate the function of MyoD in satellite cells, we examined the transplantation of satellite cell-derived myoblasts lacking the MyoD gene into regenerating skeletal muscle. After injection into injured muscle, MyoD(-/-) myoblasts engrafted with significantly higher efficiency compared with wild-type myoblasts. In addition, MyoD(-/-) myoblast-derived satellite cells were detected underneath the basal lamina of muscle fibers, indicating the self-renewal property of MyoD(-/-) myoblasts. To gain insights into MyoD gene deficiency in muscle stem cells, we investigated the pathways regulated by MyoD by GeneChip microarray analysis of gene expression in wild-type and MyoD(-/-) myoblasts. MyoD deficiency led to down-regulation of many muscle-specific genes and up-regulation of some stem cell markers. Importantly, in MyoD(-/-) myoblasts, many antiapoptotic genes were up-regulated, whereas genes known to execute apoptosis were down-regulated. Consistent with these gene expression profiles, MyoD(-/-) myoblasts were revealed to possess remarkable resistance to apoptosis and increased survival compared with wild-type myoblasts. Forced expression of MyoD or the proapoptotic protein Puma increased cell death in MyoD(-/-) myoblasts. Therefore, MyoD(-/-) myoblasts may preserve stem cell characteristics, including their resistance to apoptosis, expression of stem cell markers, and efficient engraftment and contribution to satellite cells after transplantation. Furthermore, our data offer evidence for improved therapeutic stem cell transplantation for muscular dystrophy, in which suppression of MyoD in myogenic progenitors would be beneficial to therapy by providing a selective advantage for the expansion of stem cells.

Foxc2 induces expression of MyoD and differentiation of the mouse myoblast cell line C2C12

The Fox family of transcription factors is expressed in various organs and tissues during development, and is involved in a variety of developmental and cellular differentiation processes. Foxc2 mRNA is strongly expressed in mesoderm-derived tissues in the embryo, but the molecular mechanism of Foxc2-induced cellular differentiation and the physiological role of Foxc2 are unclear. In mouse myoblast C2C12 cells, over-expression of Foxc2 increased the expression of desmin, the muscle-specific member of the intermediate filament family of proteins, and induced the synthesis of myotubes. Transient expression of Foxc2 increased MyoD mRNA and protein levels, as assessed by real-time PCR and Western blot, respectively. Chromatin immunoprecipitation (ChIP) analysis showed that Foxc2 does not bind to the promoter region of the MyoD gene, which indicated that Foxc2 does not directly activate MyoD. In contrast to reports that Foxc2 regulates the production of basement membrane components in endothelial cells, we found no evidence of Foxc2-mediated regulation of Collagen type IV alpha 1 (Col4a1) or Col4a2 in myoblast cells. Taken together, these results indicate that Foxc2 plays an important role in the development of the mesenchyme through the regulation of MyoD gene expression.

Effect of phase limited inhibition of MyoD expression on the terminal differentiation of bovine myoblasts: no alteration of Myf5 or myogenin expression

To investigate the roles played by MyoD in the terminal differentiation of satellite cell-derived myoblasts, the effect of antisense inhibition of MyoD expression was examined in bovine adult myoblast culture, in which inhibition treatment was limited to the terminal differentiation phase. MyoD antisense oligonucleotide DNA (AS-mD) suppressed the formation of multinucleated myotubes in the cell culture. Myotube formation was suppressed even when AS-mD treatment was limited to the period preceding the onset of myotube formation. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that treatment with AS-mD suppressed the expression of myosin heavy chain embryonic isoform and troponin T isoforms at 4 days after the induction of differentiation. AS-mD also suppressed the expression of MRF4, but did not alter the expression of either Myf5 or myogenin, in contrast to previous results using mouse cells possessing MyoD(-/-) genetic background. These findings suggest that MyoD controls myogenesis but not Myf5 or myogenin mRNA expression during the terminal differentiation phase. Furthermore, among the alpha4, alpha5, alpha6, and alpha7 integrins, alpha4, alpha5, and alpha7 integrin expression was suppressed by AS-mD treatment, in parallel with the suppression of myotube formation, which suggests that MyoD controls myotube formation by regulating the expression of alpha4, alpha5, and alpha7 integrins.

Muscle regeneration and myogenic differentiation defects in mice lacking TIS7

The tetradecanoyl phorbol acetate-induced sequence 7 gene (tis7) is regulated during cell fate processes and functions as a transcriptional coregulator. Here, we describe the generation and analysis of mice lacking the tis7 gene. Surprisingly, TIS7 knockout mice show no gross histological abnormalities and are fertile. Disruption of the tis7 gene by homologous recombination delayed muscle regeneration and altered the isometric contractile properties of skeletal muscles after muscle crush damage in TIS7(-/-) mice. Cultured primary myogenic satellite cells (MSCs) from TIS7(-/-) mice displayed marked reductions in differentiation potential and fusion index in a strictly cell-autonomous fashion. Loss of TIS7 caused the down-regulation of muscle-specific genes, such as those for MyoD, myogenin, and laminin-alpha2. Fusion potential in TIS7(-/-) MSCs could be rescued by TIS7 expression or laminin supplementation. Therefore, TIS7 is not essential for mouse development but plays a novel regulatory role during adult muscle regeneration.

Critical Role for Lysine 133 in the Nuclear Ubiquitin-mediated Degradation of MyoD

The ubiquitin-proteasome system is responsible for the regulation and turnover of the nuclear transcription factor MyoD. The degradation of MyoD can occur via an NH2 terminus-dependent pathway or a lysine-dependent pathway, suggesting that MyoD ubiquitination may be driven by different mechanisms. To understand this process, deletion analysis was used to identify the region of MyoD that is required for rapid proteolysis in the lysine-dependent pathway. Here we report that the basic helix-loop-helix domain is required for ubiquitination and lysine-dependent degradation of MyoD in the nucleus. Site-directed mutagenesis in MyoD revealed that lysine 133 is the major internal lysine of ubiquitination. The half-life of the MyoD K133R mutant protein was longer than that of wild type MyoD, substantiating the implication of lysine 133 in the turnover of MyoD in myoblasts. In addition, the MyoD K133R mutant displayed activity 2-3-fold higher than the wild type in transactivation muscle-specific gene and myogenic conversion of 10T1/2 cells. Taken together, our data demonstrate that lysine 133 is targeted for ubiquitination and rapid degradation of MyoD in the lysine-dependent pathway and plays an integral role in compromising MyoD activity in the nucleus.

Reloading of atrophied rat soleus muscle induces tenascin-C expression around damaged muscle fibers

The hypothesis was tested that mechanical loading, induced by hindlimb suspension and subsequent reloading, affects expression of the basement membrane components tenascin-C and fibronectin in the belly portion of rat soleus muscle. One day of reloading, but not the previous 14 days of hindlimb suspension, led to ectopic accumulation of tenascin-C and an increase of fibronectin in the endomysium of a proportion (8 and 15%) of muscle fibers. Large increases of tenascin-C (40-fold) and fibronectin (7-fold) mRNA within 1 day of reloading indicates the involvement of pretranslational mechanisms in tenascin-C and fibronectin accumulation. The endomysial accumulation of tenascin-C was maintained up to 14 days of reloading and was strongly associated with centrally nucleated fibers. The observations demonstrate that an unaccustomed increase of rat soleus muscle loading causes modification of the basement membrane of damaged muscle fibers through ectopic endomysial expression of tenascin-C.