Skeletal muscle formation is a complex process that requires tight spatiotemporal control of key myogenic factors. Emerging evidence suggests that RNA processing is crucial for the regulation of ...these factors, and that multiple post-transcriptional regulatory pathways work dependently and independently of one another to enable precise control of transcripts throughout muscle development and repair. Moreover, disruption of these pathways is implicated in neuromuscular disease, and the recent development of RNA-mediated therapies shows enormous promise in the treatment of these disorders. We discuss the overlapping post-transcriptional regulatory pathways that mediate muscle development, how these pathways are disrupted in neuromuscular disorders, and advances in RNA-mediated therapies that present a novel approach to the treatment of these diseases.
RNA processing is crucial for muscle development and repair.Myogenic transcripts are regulated by overlapping post-transcriptional processes.Disruption of one or more of these processes contributes to neuromuscular disease.Recent breakthroughs in RNA-mediated therapies show promise in the treatment of spinal muscular atrophy and amyotrophic lateral sclerosis.Leveraging other post-transcriptional regulatory pathways may reveal alternative therapeutic approaches for other neuromuscular disorders.
In response to muscle injury, satellite cells activate the p38α/β MAPK pathway to exit quiescence, then proliferate, repair skeletal muscle, and self-renew, replenishing the quiescent satellite cell ...pool. Although satellite cells are capable of asymmetric division, the mechanisms regulating satellite cell self-renewal are not understood. We found that satellite cells, once activated, enter the cell cycle and a subset undergoes asymmetric division, renewing the satellite cell pool. Asymmetric localization of the Par complex activates p38α/β MAPK in only one daughter cell, inducing MyoD, which permits cell cycle entry and generates a proliferating myoblast. The absence of p38α/β MAPK signaling in the other daughter cell prevents MyoD induction, renewing the quiescent satellite cell. Thus, satellite cells employ a mechanism to generate distinct daughter cells, coupling the Par complex and p38α/β MAPK signaling to link the response to muscle injury with satellite cell self-renewal.
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► All satellite cells divide upon explant and a subset divide asymmetrically ► Asymmetric division generates quiescent satellite-SP cells ► Asymmetric activation/sequestration of p38α/β MAPK produces a myoblast daughter ► A PAR-3/PKCλ complex asymmetrically activates/sequesters p38α/β MAPK
Satellite cells activate p38α/β MAPK to divide asymmetrically in response to muscle injury. A Par-3/PKCλ complex localizes and sequesters p38α/β MAPK in one of the daughter cells, which becomes a myoblast, while the other daughter cell returns to quiescence.
Skeletal muscle aging results in a gradual loss of skeletal muscle mass, skeletal muscle function and regenerative capacity, which can lead to sarcopenia and increased mortality. Although the ...mechanisms underlying sarcopenia remain unclear, the skeletal muscle stem cell, or satellite cell, is required for muscle regeneration. Therefore, identification of signaling pathways affecting satellite cell function during aging may provide insights into therapeutic targets for combating sarcopenia. Here, we show that a cell-autonomous loss in self-renewal occurs via alterations in fibroblast growth factor receptor-1, p38α and p38β mitogen-activated protein kinase signaling in satellite cells from aged mice. We further demonstrate that pharmacological manipulation of these pathways can ameliorate age-associated self-renewal defects. Thus, our data highlight an age-associated deregulation of a satellite cell homeostatic network and reveal potential therapeutic opportunities for the treatment of progressive muscle wasting.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
A dominant histopathological feature in neuromuscular diseases, including amyotrophic lateral sclerosis and inclusion body myopathy, is cytoplasmic aggregation of the RNA-binding protein TDP-43. ...Although rare mutations in TARDBP-the gene that encodes TDP-43-that lead to protein misfolding often cause protein aggregation, most patients do not have any mutations in TARDBP. Therefore, aggregates of wild-type TDP-43 arise in most patients by an unknown mechanism. Here we show that TDP-43 is an essential protein for normal skeletal muscle formation that unexpectedly forms cytoplasmic, amyloid-like oligomeric assemblies, which we call myo-granules, during regeneration of skeletal muscle in mice and humans. Myo-granules bind to mRNAs that encode sarcomeric proteins and are cleared as myofibres mature. Although myo-granules occur during normal skeletal-muscle regeneration, myo-granules can seed TDP-43 amyloid fibrils in vitro and are increased in a mouse model of inclusion body myopathy. Therefore, increased assembly or decreased clearance of functionally normal myo-granules could be the source of cytoplasmic TDP-43 aggregates that commonly occur in neuromuscular disease.
Satellite cells are myogenic precursors responsible for skeletal muscle regeneration. Satellite cells are absent in the
Pax-7
−/− mouse, suggesting that this transcription factor is crucial for ...satellite cell specification Seale, P., Sabourin, L.A., Girgis-Gabardo, A., Mansouri, A., Gruss, P., Rudnicki, M.A., 2000. Pax7 is required for the specification of myogenic satellite cells. Cell 102, 777–786. Analysis of Pax-7 expression in activated satellite cells unexpectedly revealed substantial heterogeneity within individual clones. Further analyses show that Pax-7 and myogenin expression are mutually exclusive during differentiation, where Pax-7 appears to be up-regulated in cells that escape differentiation and exit the cell cycle, suggesting a regulatory relationship between these two transcription factors. Indeed, overexpression of Pax-7 down-regulates MyoD, prevents myogenin induction, and blocks MyoD-induced myogenic conversion of 10T1/2 cells. Overexpression of Pax-7 also promotes cell cycle exit even in proliferation conditions. Together, these results suggest that Pax-7 may play a crucial role in allowing activated satellite cells to reacquire a quiescent, undifferentiated state. These data support the concept that satellite cell self-renewal may be a primary mechanism for replenishment of the satellite cell compartment during skeletal muscle regeneration.
This “Controversies in Cardiovascular Research” article evaluates the evidence for and against the hypothesis that the circulating blood level of growth differentiation factor 11 (GDF11) decreases in ...old age and that restoring normal GDF11 levels in old animals rejuvenates their skeletal muscle and reverses pathological cardiac hypertrophy and cardiac dysfunction. Studies supporting the original GDF11 hypothesis in skeletal and cardiac muscle have not been validated by several independent groups. These new studies have either found no effects of restoring normal GDF11 levels on cardiac structure and function or have shown that increasing GDF11 or its closely related family member growth differentiation factor 8 actually impairs skeletal muscle repair in old animals. One possible explanation for what seems to be mutually exclusive findings is that the original reagent used to measure GDF11 levels also detected many other molecules so that age-dependent changes in GDF11 are still not well known. The more important issue is whether increasing blood GDF11 repairs old skeletal muscle and reverses age-related cardiac pathologies. There are substantial new and existing data showing that GDF8/11 can exacerbate rather than rejuvenate skeletal muscle injury in old animals. There is also new evidence disputing the idea that there is pathological hypertrophy in old C57bl6 mice and that GDF11 therapy can reverse cardiac pathologies. Finally, high GDF11 causes reductions in body and heart weight in both young and old animals, suggestive of a cachexia effect. Our conclusion is that elevating blood levels of GDF11 in the aged might cause more harm than good.
Postnatal growth and regeneration of skeletal muscle requires a population of resident myogenic precursors named satellite cells. The transcription factor Pax7 is critical for satellite cell ...biogenesis and survival and has been also implicated in satellite cell self-renewal; however, the underlying molecular mechanisms remain unclear. Previously, we showed that Pax7 overexpression in adult primary myoblasts down-regulates MyoD and prevents myogenin induction, inhibiting myogenesis. We show that Pax7 prevents muscle differentiation independently of its transcriptional activity, affecting MyoD function. Conversely, myogenin directly affects Pax7 expression and may be critical for Pax7 down-regulation in differentiating cells. Our results provide evidence for a cross-inhibitory interaction between Pax7 and members of the muscle regulatory factor family. This could represent an additional mechanism for the control of satellite cell fate decisions resulting in proliferation, differentiation, and self-renewal, necessary for skeletal muscle maintenance and repair.
In this issue of Cell Stem Cell, Chan et al. (2018) report that in vivo differentiation of pluripotent stem cells in induced teratomas produces functional embryonic-like muscle stem cells. These ...purified muscle stem cells engraft with high efficiency and regenerate serially injured muscle.
In this issue of Cell Stem Cell, Chan et al. (2018) report that in vivo differentiation of pluripotent stem cells in induced teratomas produces functional embryonic-like muscle stem cells. These purified muscle stem cells engraft with high efficiency and regenerate serially injured muscle.
Adult skeletal muscle adapts to functional needs, maintaining consistent numbers of myonuclei and stem cells. Although resident muscle stem cells or satellite cells are required for muscle growth and ...repair, in uninjured muscle, these cells appear quiescent and metabolically inactive. To investigate the satellite cell contribution to myofibers in adult uninjured skeletal muscle, we labeled satellite cells by inducing a recombination of LSL-tdTomato in Pax7(CreER) mice and scoring tdTomato+ myofibers as an indicator of satellite cell fusion.
Satellite cell fusion into myofibers plateaus postnatally between 8 and 12 weeks of age, reaching a steady state in hindlimb muscles, but in extra ocular or diaphragm muscles, satellite cell fusion is maintained at postnatal levels irrespective of the age assayed. Upon recombination and following a 2-week chase in 6-month-old mice, tdTomato-labeled satellite cells fused into myofibers as 20, 50, and 80 % of hindlimb, extra ocular, and diaphragm myofibers, respectively, were tdTomato+. Satellite cells contribute to uninjured myofibers either following a cell division or directly without an intervening cell division.
The frequency of satellite cell fusion into the skeletal muscle fibers is greater than previously estimated, suggesting an important functional role for satellite cell fusion into adult myofibers and a requirement for active maintenance of satellite cell numbers in uninjured skeletal muscle.
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