Inhibitors of tyrosine phosphatases and apoptosis reprogram lineage-marked differentiated muscle to myogenic progenitor cells

Restoring the Cell Cycle and Proliferation Competence in Terminally Differentiated Skeletal Muscle Myotubes

Terminal differentiation is an ill-defined, insufficiently characterized, nonproliferation state. Although it has been classically deemed irreversible, it is now clear that at least several terminally differentiated (TD) cell types can be brought back into the cell cycle. We are striving to uncover the molecular bases of terminal differentiation, whose fundamental understanding is a goal in itself. In addition, the field has sought to acquire the ability to make TD cells proliferate. Attaining this end would probe the very molecular mechanisms we are trying to understand. Equally important, it would be invaluable in regenerative medicine, for tissues depending on TD cells and devoid of significant self-repair capabilities. The skeletal muscle has long been used as a model system to investigate the molecular foundations of terminal differentiation. Here, we summarize more than 50 years of studies in this field.

Twist2 amplification in rhabdomyosarcoma represses myogenesis and promotes oncogenesis by redirecting MyoD DNA binding

Li et al. show that TWIST2 shapes the epigenetic landscape to drive chromatin opening at oncogenic loci and chromatin closing at myogenic loci. These epigenetic changes redirect MyoD binding from myogenic genes toward oncogenic, metabolic, and growth genes.

Rhabdomyosarcoma (RMS) is an aggressive pediatric cancer composed of myoblast-like cells. Recently, we discovered a unique muscle progenitor marked by the expression of the Twist2 transcription factor. Genomic analyses of 258 RMS patient tumors uncovered prevalent copy number amplification events and increased expression of TWIST2 in fusion-negative RMS. Knockdown of TWIST2 in RMS cells results in up-regulation of MYOGENIN and a decrease in proliferation, implicating TWIST2 as an oncogene in RMS. Through an inducible Twist2 expression system, we identified Twist2 as a reversible inhibitor of myogenic differentiation with the remarkable ability to promote myotube dedifferentiation in vitro. Integrated analysis of genome-wide ChIP-seq and RNA-seq data revealed the first dynamic chromatin and transcriptional landscape of Twist2 binding during myogenic differentiation. During differentiation, Twist2 competes with MyoD at shared DNA motifs to direct global gene transcription and repression of the myogenic program. Additionally, Twist2 shapes the epigenetic landscape to drive chromatin opening at oncogenic loci and chromatin closing at myogenic loci. These epigenetic changes redirect MyoD binding from myogenic genes toward oncogenic, metabolic, and growth genes. Our study reveals the dynamic interplay between two opposing transcriptional regulators that control the fate of RMS and provides insight into the molecular etiology of this aggressive form of cancer.

Chemical modulation of cell fates: in situ regeneration

Chemical modulation of cell fates has been widely used to promote tissue and organ regeneration. Small molecules can target the self-renewal, expansion, differentiation, and survival of endogenous stem cells for enhancing their regenerative power or induce dedifferentiation or transdifferentiation of mature cells into proliferative progenitors or specialized cell types needed for regeneration. Here, we discuss current progress and potential using small molecules to promote in vivo regenerative processes by regulating the cell fate. Current studies of small molecules in regeneration will provide insights into developing safe and efficient chemical approaches for in situ tissue repair and regeneration.

Application of bio-orthogonal proteome labeling to cell transplantation and heterochronic parabiosis

Studies of heterochronic parabiosis demonstrated that with age, the composition of the circulatory milieu changes in ways that broadly inhibit tissue regenerative capacity. In addition, local tissue niches have age-specific influences on their resident stem cells. Here we use bio-orthogonal proteome labeling for detecting in vivo proteins present only in transplanted myoblasts, but not in host tissue, and proteins exclusive to one young mouse and transferred during parabiosis to its old partner. We use a transgenic mouse strain that ubiquitously expresses a modified tRNA methionine synthase, metRS, which preferentially incorporates the methionine surrogate azido-nor-leucine (ANL) into newly generated proteins. Using click chemistry and a modified antibody array to detect ANL-labeled proteins, we identify several ‘young’ systemic factors in old regenerating muscle of the heterochronic parabiotic partners. Our approach enables the selective profiling of mammalian proteomes in mixed biological environments such as cell and tissue transplantation, apheresis or parabiosis.

Clarifying the source of proteins in mixed biological environments, such as after transplantation or parabiosis, remains a challenge. Here, the authors address this need with a mouse strain that incorporates a methionine derivate into proteins, allowing for their detection using click chemistry and antibody arrays.

Lessons from the swamp: developing small molecules that confer salamander muscle cellularization in mammals

The ability of salamanders, such as newts, to regenerate damaged tissues has been studied for centuries. A prominent example of this regenerative power is the ability to re-grow entire amputated limbs. One important step in this regeneration process is skeletal muscle cellularization, in which the muscle fibers break down into dedifferentiated, mononuclear cells that proliferate and form new muscle in the replacement limb. In contrast, mammalian skeletal muscle does not undergo cellularization after injury. A significant proportion of research about tissue regeneration in salamanders aims to characterize regulatory genes that may have mammalian homologs. A less mainstream approach is to develop small molecule compounds that induce regeneration-related mechanisms in mammals. In this commentary, we discuss progress in discovering small molecules that induce cellularization in mammalian muscle. New research findings using these compounds has also shed light on cellular processes that regulate cellularization, such as apoptotic signaling. Although formidable technical hurdles remain, this progress increases our understanding of tissue regeneration and provide opportunities for developing small molecules that may enhance tissue repair in humans.

The Molecular and Cellular Choreography of Appendage Regeneration

Recent advances in limb regeneration are revealing the molecular events that integrate growth control, cell fate programming, and positional information to yield the exquisite replacement of the amputated limb. Parallel progress in several invertebrate and vertebrate models has provided a broader context for understanding the mechanisms and the evolution of regeneration. Together, these discoveries provide a foundation for describing the principles underlying regeneration of complex, multi-tissue structures. As such these findings should provide a wealth of ideas for engineers seeking to reconstitute regeneration from constituent parts or to elicit full regeneration from partial regeneration events.

Phosphotyrosine phosphatase inhibitor bisperoxovanadium endows myogenic cells with enhanced muscle stem cell functions via epigenetic modulation of Sca-1 and Pw1 promoters

Understanding the regulation of the stem cell fate is fundamental for designing novel regenerative medicine strategies. Previous studies have suggested that pharmacological treatments with small molecules provide a robust and reversible regulation of the stem cell program. Previously, we showed that treatment with a vanadium compound influences muscle cell fatein vitro In this study, we demonstrate that treatment with the phosphotyrosine phosphatase inhibitor bisperoxovanadium (BpV) drives primary muscle cells to a poised stem cell stage, with enhanced function in muscle regenerationin vivofollowing transplantation into injured muscles. Importantly, BpV-treated cells displayed increased self-renewal potentialin vivoand replenished the niche in both satellite and interstitial cell compartments. Moreover, we found that BpV treatment induces specific activating chromatin modifications at the promoter regions of genes associated with stem cell fate, includingSca-1andPw1 Thus, our findings indicate that BpV resets the cell fate program by specific epigenetic regulations, such that the committed myogenic cell fate is redirected to an earlier progenitor cell fate stage, which leads to an enhanced regenerative stem cell potential.-Smeriglio, P., Alonso-Martin, S., Masciarelli, S., Madaro, L., Iosue, I., Marrocco, V., Relaix, F., Fazi, F., Marazzi, G., Sassoon, D. A., Bouché, M. Phosphotyrosine phosphatase inhibitor bisperoxovanadium endows myogenic cells with enhanced muscle stem cell functionsviaepigenetic modulation of Sca-1 and Pw1 promoters.

Mononuclear cells from dedifferentiation of mouse myotubes display remarkable regenerative capability

Certain lower organisms achieve organ regeneration by reverting differentiated cells into tissue-specific progenitors that re-enter embryonic programs. During muscle regeneration in the urodele amphibian, postmitotic multinucleated skeletal myofibers transform into mononucleated proliferating cells upon injury, and a transcription factor-msx1 plays a role in their reprograming. Whether this powerful regeneration strategy can be leveraged in mammals remains unknown, as it has not been demonstrated that the dedifferentiated progenitor cells arising from muscle cells overexpressing Msx1 are lineage-specific and possess the same potent regenerative capability as their amphibian counterparts. Here, we show that ectopic expression of Msx1 reprograms postmitotic, multinucleated, primary mouse myotubes to become proliferating mononuclear cells. These dedifferentiated cells reactivate genes expressed by embryonic muscle progenitor cells and generate only muscle tissue in vivo both in an ectopic location and inside existing muscle. More importantly, distinct from adult muscle satellite cells, these cells appear both to fuse with existing fibers and to regenerate myofibers in a robust and time-dependent manner. Upon transplantation into a degenerating muscle, these dedifferentiated cells generated a large number of myofibers that increased over time and replenished almost half of the cross-sectional area of the muscle in only 12 weeks. Our study demonstrates that mammals can harness a muscle regeneration strategy used by lower organisms when the same molecular pathway is activated.

Turning terminally differentiated skeletal muscle cells into regenerative progenitors

The ability to repeatedly regenerate limbs during the entire lifespan of an animal is restricted to certain salamander species among vertebrates. This ability involves dedifferentiation of post-mitotic cells into progenitors that in turn form new structures. A long-term enigma has been how injury leads to dedifferentiation. Here we show that skeletal muscle dedifferentiation during newt limb regeneration depends on a programmed cell death response by myofibres. We find that programmed cell death-induced muscle fragmentation produces a population of ‘undead' intermediate cells, which have the capacity to resume proliferation and contribute to muscle regeneration. We demonstrate the derivation of proliferating progeny from differentiated, multinucleated muscle cells by first inducing and subsequently intercepting a programmed cell death response. We conclude that cell survival may be manifested by the production of a dedifferentiated cell with broader potential and that the diversion of a programmed cell death response is an instrument to achieve dedifferentiation.

Newts can regenerate amputated limbs via unknown mechanism involving dedifferentiation of cells in the stump into progenitors that contribute to the new appendages. Here the authors show that skeletal muscle dedifferentiation in regenerating newt limbs relies on a diverted programmed cell death response by myofibers.

Stemistry: the control of stem cells in situ using chemistry

A new paradigm for drug research has emerged, namely the deliberate search for molecules able to selectively affect the proliferation, differentiation, and migration of adult stem cells within the tissues in which they exist. Recently, there has been significant interest in medicinal chemistry toward the discovery and design of low molecular weight molecules that affect stem cells and thus have novel therapeutic activity. We believe that a successful agent from such a discover program would have profound effects on the treatment of many long-term degenerative disorders. Among these conditions are examples such as cardiovascular decay, neurological disorders including Alzheimer's disease, and macular degeneration, all of which have significant unmet medical needs. This perspective will review evidence from the literature that indicates that discovery of such agents is achievable and represents a worthwhile pursuit for the skills of the medicinal chemist.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Age dependent increase in the levels of osteopontin inhibits skeletal muscle regeneration

Skeletal muscle regeneration following injury is accompanied by rapid infiltration of macrophages, which play a positive role in muscle repair. Increased chronic inflammation inhibits the regeneration of dystrophic muscle, but the properties of inflammatory cells are not well understood in the context of normal muscle aging. This work uncovers pronounced age-specific changes in the expression of osteopontin (OPN) in CD11b+ macrophages present in the injured old muscle as well as in the blood serum of old injured mice and in the basement membrane surrounding old injured muscle fibers. Furthermore, young CD11b+ macrophages enhance regenerative capacity of old muscle stem cells even when old myofibers and old sera are present; and neutralization of OPN similarly rejuvenates the myogenic responses of old satellite cells in vitro and notably, in vivo. This study highlights potential mechanisms by which age related inflammatory responses become counter-productive for muscle regeneration and suggests new strategies for enhancing muscle repair in the old.

Reawakening atlas: chemical approaches to repair or replace dysfunctional musculature

Muscle diseases are major health concerns. For example, ischemic heart disease is the third most common cause of death. Cell therapy is an attractive approach for treating muscle diseases, although this is hampered by the need to generate large numbers of functional muscle cells. Small molecules have become established as attractive tools for modulating cell behavior and, in this review, we discuss the recent, rapid research advances made in the development of small molecule methods to facilitate the production of functional cardiac, skeletal, and smooth muscle cells. We also describe how new developments in small molecule strategies for muscle disease aim to induce repair and remodelling of the damaged tissues in situ. Recent progress has been made in developing small molecule cocktails that induce skeletal muscle regeneration, and these are discussed in a broader context, because a similar phenomenon occurs in the early stages of salamander appendage regeneration. Although formidable technical hurdles still remain, these new advances in small molecule-based methodologies should provide hope that cell therapies for patients suffering from muscle disease can be developed in the near future.

Stem cells in the hood: the skeletal muscle niche

It is generally accepted that the principal resident progenitor underlying regenerative capacity in skeletal muscle is the satellite cell. Satellite cells are present throughout life even though regenerative capacity declines with age and disease. Recently, other stem cell populations have been identified that can participate in muscle growth and regeneration. These cells may provide therapeutically useful sources of muscle stem cells as an alternative to satellite cells; however, the roles of these nonsatellite cell populations during muscle homeostasis, regeneration, and aging are unclear. Here, we discuss how the stem cell neighborhood influences satellite cell behavior and bring together recent discoveries pertaining to a wide variety of adult stem cells, including muscle stem cells and their niche.