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Abstract Fibro-adipogenic progenitors (FAPs) are resident mesenchymal progenitors in adult skeletal muscle that support muscle repair, but also give rise to fibrous and adipose infiltration in response to disease and chronic injury. FAPs are identified using cell surface markers that do not distinguish between quiescent FAPs and FAPs actively engaged in the regenerative process. We have shown previously that FAPs are derived from cells that express the transcription factor Osr1 during development. Here we show that adult FAPs express Osr1 at low levels and frequency, however upon acute injury FAPs reactivate Osr1 expression in the injured tissue. Osr1+ FAPs are enriched in proliferating and apoptotic cells demonstrating that Osr1 identifies activated FAPs. In vivo genetic lineage tracing shows that Osr1+ activated FAPs return to the resident FAP pool after regeneration as well as contribute to adipocytes after glycerol-induced fatty degeneration. In conclusion, reporter LacZ or eGFP-CreERt2 expression from the endogenous Osr1 locus serves as marker for FACS isolation and tamoxifen-induced manipulation of activated FAPs. Highlights • Odd skipped related 1 (Osr1) marks adult FAPs activated by acute injury • Osr1 is expressed in FAPs during phases of active myogenesis • Osr1 can be used to mark, access and lineage-trace adult injury-activated FAPs [ABSTRACT FROM AUTHOR]

Degenerative myopathies typically display a decline in satellite cells coupled with a replacement of muscle fibers by fat and fibrosis. During this pathological remodeling, satellite cells are present at lower numbers and do not display a proper regenerative function. Whether a decline in satellite cells directly contributes to disease progression or is a secondary result is unknown. In order to dissect these processes, we used a genetic model to reduce the satellite cell population by ~70-80% which leads to a nearly complete loss of regenerative potential. We observe that while no overt tissue damage is observed following satellite cell depletion, muscle fibers atrophy accompanied by changes in the stem cell niche cellular composition. Treatment of these mice with an Activin receptor type-2B (AcvR2B) pathway blocker reverses muscle fiber atrophy as expected, but also restores regenerative potential of the remaining satellite cells. These findings demonstrate that in addition to controlling fiber size, the AcvR2B pathway acts to regulate the muscle stem cell niche providing a more favorable environment for muscle regeneration. [ABSTRACT FROM AUTHOR]

The satellite cell is the major tissue-resident stem cell underlying muscle regeneration; however, multiple non-satellite myogenic progenitors as well as non-myogenic populations that support the muscle regenerative process have been identified. PW1 is expressed in satellite cells as well as in a subset of interstitial cells with myogenic potential termed PICs (PW1+ interstitial cells). Microarray profiling revealed that PICs express a broad range of genes common to mesenchymal stem cells, whereas satellite cells express genes consistent with a committed myogenic progenitor. Isolated PICs from both young and adult muscles can differentiate into smooth and skeletal muscle and fat whereas satellite cells are restricted to a skeletal muscle fate. We demonstrate that the adipogenic potential of PICs corresponds to a subpopulation that expresses platelet derived growth factor receptor alpha (PDGFRα) and overlaps with the recently described interstitial adipogenic progenitors. By contrast, PICs with myogenic potential do not express PDGFRα. Moreover, we observe a discrete and transient population of juvenile PICs based upon SCA1 expression that disappears by 3 weeks of postnatal development coincident with a switch in the cellular and genetic mechanisms underlying postnatal muscle growth. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Muscle wasting is a major cause of morbidity in the elderly. Ku80 is required for DNA double strand repair and is implicated in telomere maintenance. Complete loss-of-function leads to reduced post-natal growth and severe progeria in mice. We examined the role of Ku80 in age-related skeletal muscle atrophy. While complete loss of Ku80 leads to pronounced aging in muscle as expected, accompanied by accumulation of DNA damage, loss of a single allele is sufficient to accelerate aging in skeletal muscle although post-natal growth is normal. Ku80 heterozygous muscle shows no DNA damage accumulation but undergoes premature telomere shortening that alters stem cell self-renewal through stress response pathways including p53. These data reveal an unexpected requirement for both Ku80 alleles for optimal progenitor function and prevention of early onset aging in muscle, as well as providing a useful model for therapeutic approaches. [ABSTRACT FROM AUTHOR]

Abstract: PW1 is a mediator of p53 and TNFα signaling pathways previously identified in a screen to isolate muscle stem cell regulators. We generated transgenic mice carrying a C-terminal deleted form of PW1 (ΔPW1) which blocks p53-mediated cell death and TNFα-mediated NFκB activation fused to the myogenin promoter. Embryonic/fetal muscle development appears normal during transgene expression, however, postnatal transgenic pups display severe phenotypes including runtism, reduced muscle mass and fiber diameters resembling atrophy. Atrogin-1, a marker of skeletal muscle atrophy, is expressed postnatally in transgenic mice. Electron microscopic analyses of transgenic muscle reveal a marked decrease in quiescent muscle satellite cells suggesting a deregulation of postnatal stem cells. Furthermore, transgenic primary myoblasts show a resistance to the effects of TNFα upon differentiation. Taken together, our data support a role for PW1 and related stress pathways in mediating skeletal muscle stem cell behavior which in turn is critical for postnatal muscle growth and homeostasis. In addition, these data reveal that postnatal stem cell behavior is likely specified during early muscle development. [Copyright &y& Elsevier]

Muscle wasting (cachexia) is an incurable complication associated with chronic infection and cancers that leads to an overall poor prognosis for recovery. Tumor necrosis factor-α (TNFα) is a key inflammatory cytokine associated with cachexia. TNFα inhibits myogenic differentiation and skeletal muscle regeneration through downstream effectors of the p53 cell death pathway including PW1/Peg3, bax, and caspases. We report that p53 is required for the TNFα-mediated inhibition of myogenesis in vitro and contributes to muscle wasting in response to tumor load in vivo. We further demonstrate that PW1 and p53 participate in a positive feedback regulatory loop in vitro. Consistent with this observation, we find that the number of PW1-expressing stem cells in skeletal muscle declines significantly in p53 nullizygous mice. Furthermore, gene transfer of a dominant-negative form of PW1 into muscle tissue in vivo blocks myofiber atrophy in response to tumor load. Taken together, these results show a novel role for p53 in mediating muscle stem cell behavior and muscle atrophy, and point to new targets for the therapeutic treatment of muscle wasting. [ABSTRACT FROM AUTHOR]