Regenerating Myofibers after an Acute Muscle Injury: What Do We Really Know about Them?

Current methodologies for inducing aligned myofibers in tissue constructs for skeletal muscle tissue regeneration

SIGNIFICANCE

Volumetric muscle loss (VML) results in the loss of large amounts of tissue that inhibits muscle regeneration. Existing therapies, such as autologous muscle transfer and physical therapy, are incapable of returning full function and force production to injured muscle.

RECENT ADVANCES

Skeletal muscle tissue constructs may provide an alternative to existing therapies currently used to treat VML. Unlike autologous muscle transplants, muscle constructs can be cultured in vitro and are not reliant on intact muscle tissue. Skeletal muscle constructs can be generated from small muscle biopsies and could be used to generate skeletal muscle tissue constructs to replace injured tissues.

CRITICAL ISSUES

To serve as effective therapies, muscle constructs must be capable of generating contractile forces that can assist the function of host skeletal muscle. The contractile force of native muscle arises in part as a consequence of the highly aligned, bundled architecture of myofibers. Attempts to induce similar alignment include: applications of tension/strain across hydrogels, inducing aligned architectures within scaffolds, casting tissues in straited molds, and 3D printing. While all these methods have demonstrated efficacy towards inducing myofiber alignment, the extent of myofiber alignment, tissue formation, and force production varies. This manuscript critically reviews the advantages and limitations of these methods, and specifically discusses their ability to impart mechanical and architectural cues to induce alignment within constructs.

FUTURE DIRECTIONS

As tissue synthesizing techniques continue to improve, muscle constructs must include more cell types than simply myoblasts, such as the addition of neuronal and endothelial cells. Higher level tissue organization is critical to the success of these constructs. Many of these technologies have yet to be implanted into host tissue to understand engraftment and how they can contribute to traumatic injury, and as such continued collaboration between surgeons and tissue engineers is necessary to ultimately result in clinical translation.

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

miR-27b-3p reduces muscle fibrosis during chronic skeletal muscle injury by targeting TGF-βR1/Smad pathway

BACKGROUND

Fibrosis is a significant pathological feature of chronic skeletal muscle injury, profoundly affecting muscle regeneration. Fibro-adipogenic progenitors (FAPs) have the ability to differentiate into myofibroblasts, acting as a primary source of extracellular matrix (ECM). the process by which FAPs differentiate into myofibroblasts during chronic skeletal muscle injury remains inadequately explored.

METHOD

mouse model with sciatic nerve denervated was constructed and miRNA expression profiles between the mouse model and uninjured mouse were analyzed. qRT/PCR and immunofluorescence elucidated the effect of miR-27b-3p on fibrosis in vivo and in vitro. Dual-luciferase reporter identified the target gene of miR-27b-3p, and finally knocked down or overexpressed the target gene and phosphorylation inhibition of Smad verified the influence of downstream molecules on the abundance of miR-27b-3p and fibrogenic differentiation of FAPs.

RESULT

FAPs derived from a mouse model with sciatic nerves denervated exhibited a progressively worsening fibrotic phenotype over time. Introducing agomiR-27b-3p effectively suppressed fibrosis both in vitro and in vivo. MiR-27b-3p targeted Transforming Growth Factor Beta Receptor 1 (TGF-βR1) and the abundance of miR-27b-3p was negatively regulated by TGF-βR1/Smad.

CONCLUSION

miR-27b-3p targeting the TGF-βR1/Smad pathway is a novel mechanism for regulating fibrogenic differentiation of FAPs. Increasing abundance of miR-27b-3p, suppressing expression of TGF-βR1 and inhibiting phosphorylation of smad3 presented potential strategies for treating fibrosis in chronic skeletal muscle injury.

The roles of miRNAs in adult skeletal muscle satellite cells

Satellite cells are bona fide muscle stem cells that are indispensable for successful post-natal muscle growth and regeneration after severe injury. These cells also participate in adult muscle adaptation in several capacities. MicroRNAs (miRNAs) are post-transcriptional regulators of mRNA that are implicated in several aspects of stem cell function. There is evidence to suggest that miRNAs affect satellite cell behavior in vivo during development and myogenic progenitor behavior in vitro, but the role of miRNAs in adult skeletal muscle satellite cells is less studied. In this review, we provide evidence for how miRNAs control satellite cell function with emphasis on satellite cells of adult skeletal muscle in vivo. We first outline how miRNAs are indispensable for satellite cell viability and control the phases of myogenesis. Next, we discuss the interplay between miRNAs and myogenic cell redox status, senescence, and communication to other muscle-resident cells during muscle adaptation. Results from recent satellite cell miRNA profiling studies are also summarized. In vitro experiments in primary myogenic cells and cell lines have been invaluable for exploring the influence of miRNAs, but we identify a need for novel genetic tools to further interrogate how miRNAs control satellite cell behavior in adult skeletal muscle in vivo.