David Sassoon

Identification and characterization of a non-satellite cell muscle resident progenitor during postnatal development

Satellite cells are resident myogenic progenitors in postnatal skeletal muscle involved in muscle postnatal growth and adult regenerative capacity. Here, we identify and describe a population of muscle-resident stem cells, which are located in the interstitium, that express the cell stress mediator PW1 but do not express other markers of muscle stem cells such as Pax7. PW1+/Pax7− interstitial cells (PICs) are myogenic in vitro and efficiently contribute to skeletal muscle regeneration in vivo as well as generating satellite cells and PICs. Whereas Pax7 mutant satellite cells show robust myogenic potential, Pax7 mutant PICs are unable to participate in myogenesis and accumulate during postnatal growth. Furthermore, we found that PICs are not derived from a satellite cell lineage. Taken together, our findings uncover a new and anatomically identifiable population of muscle progenitors and define a key role for Pax7 in a non-satellite cell population during postnatal muscle growth.

Fibroadipogenic progenitors mediate the ability of HDAC inhibitors to promote regeneration in dystrophic muscles of young, but not old Mdx mice

HDAC inhibitors (HDACi) exert beneficial effects in mdx mice, by promoting endogenous regeneration; however, the cellular determinants of HDACi activity on dystrophic muscles have not been determined. We show that fibroadipogenic progenitors (FAP) influence the regeneration potential of satellite cells during disease progression in mdx mice and mediate HDACi ability to selectively promote regeneration at early stages of disease. FAPs from young mdx mice promote, while FAPs from old mdx mice repress, satellite cell‐mediated formation of myotubes. In young mdx mice HDACi inhibited FAP adipogenic potential, while enhancing their ability to promote differentiation of adjacent satellite cells, through upregulation of the soluble factor follistatin. By contrast, FAPs from old mdx mice were resistant to HDACi‐mediated inhibition of adipogenesis and constitutively repressed satellite cell‐mediated formation of myotubes. We show that transplantation of FAPs from regenerating young muscles restored HDACi ability to increase myofibre size in old mdx mice. These results reveal that FAPs are key cellular determinants of disease progression in mdx mice and mediate a previously unappreciated stage‐specific beneficial effect of HDACi in dystrophic muscles.

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.

Tumor Necrosis Factor-α Inhibition of Skeletal Muscle Regeneration Is Mediated by a Caspase-Dependent Stem Cell Response

Skeletal muscle is susceptible to injury following trauma, neurological dysfunction, and genetic diseases. Skeletal muscle homeostasis is maintained by a pronounced regenerative capacity, which includes the recruitment of stem cells. Chronic exposure to tumor necrosis factor‐α (TNF) triggers a muscle wasting reminiscent of cachexia. To better understand the effects of TNF upon muscle homeostasis and stem cells, we exposed injured muscle to TNF at specific time points during regeneration. TNF exposure delayed the appearance of regenerating fibers, without exacerbating fiber death following the initial trauma. We observed modest cellular caspase activation during regeneration, which was markedly increased in response to TNF exposure concomitant with an inhibition in regeneration. Caspase activation did not lead to apoptosis and did not involve caspase‐3. Inhibition of caspase activity improved muscle regeneration in either the absence or the presence of TNF, revealing a nonapoptotic role for this pathway in the myogenic program. Caspase activity was localized to the interstitial cells, which also express Sca‐1, CD34, and PW1. Perturbation of PW1 activity blocked caspase activation and improved regeneration. The restricted localization of Sca‐1+, CD34+, PW1+ cells to a subset of interstitial cells with caspase activity reveals a critical regulatory role for this population during myogenesis, which may directly contribute to resident muscle stem cells or indirectly regulate stem cells through cell‐cell interactions.

N-WASP is required for Amphiphysin-2/BIN1-dependent nuclear positioning and triad organization in skeletal muscle and is involved in the pathophysiology of centronuclear myopathy

Mutations in amphiphysin‐2/BIN1, dynamin 2, and myotubularin are associated with centronuclear myopathy (CNM), a muscle disorder characterized by myofibers with atypical central nuclear positioning and abnormal triads. Mis‐splicing of amphiphysin‐2/BIN1 is also associated with myotonic dystrophy that shares histopathological hallmarks with CNM. How amphiphysin‐2 orchestrates nuclear positioning and triad organization and how CNM‐associated mutations lead to muscle dysfunction remains elusive. We find that N‐WASP interacts with amphiphysin‐2 in myofibers and that this interaction and N‐WASP distribution are disrupted by amphiphysin‐2 CNM mutations. We establish that N‐WASP functions downstream of amphiphysin‐2 to drive peripheral nuclear positioning and triad organization during myofiber formation. Peripheral nuclear positioning requires microtubule/Map7/Kif5b‐dependent distribution of nuclei along the myofiber and is driven by actin and nesprins. In adult myofibers, N‐WASP and amphiphysin‐2 are only involved in the maintenance of triad organization but not in the maintenance of peripheral nuclear positioning. Importantly, we confirmed that N‐WASP distribution is disrupted in CNM and myotonic dystrophy patients. Our results support a role for N‐WASP in amphiphysin‐2‐dependent nuclear positioning and triad organization and in CNM and myotonic dystrophy pathophysiology.

Loss of a single allele for Ku80 leads to progenitor dysfunction and accelerated aging in skeletal muscle

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.

0074 : Resident PW1+ progenitor cells participate in vascular remodeling during pulmonary arterial hypertension

Rationale Pulmonary arterial hypertension (PAH) is characterized by vascular remodeling and neomuscularization. PW1+ progenitor cells can differentiate into smooth muscle cells (SMC) in vitro. Objective To determine the role of pulmonary PW1+ progenitor cells in vascular remodeling characteristic of PAH. Methods and results We investigated their contribution during chronic hypoxia (CH)-induced vascular remodeling in Pw1nLacZ+/– mouse expressing β-galactosidase in PW1+ cells and in differentiated cells derived from PW1+ cells. PW1+ progenitor cells are present in the perivascular zone in rodent and human control lungs. Using progenitor markers, three distinct myogenic PW1+ cell populations were isolated from the mouse lung of which two were significantly increased after 4 days of CH. The number of proliferating pulmonary PW1+ cells and the proportion of β-gal+ vascular SMC were increased, indicating a recruitment of PW1+ cells and their differentiation into vascular SMC during early CH-induced neomuscularization. CXCR4 inhibition using AMD3100 prevented PW1+ cells differentiation into SMC but did not inhibit their proliferation. Bone marrow transplantation experiments showed that the newly formed β-gal+ SMC were not derived from circulating bone marrow-derived PW1+ progenitor cells, confirming a resident origin of the recruited PW1+ cells. The number of pulmonary PW1+ cells was also increased in rats after monocrotaline (MCT) injection. In the human PAH lung, PW1-expressing cells were observed in large numbers in remodeled vascular structures. Conclusions These results demonstrate the existence of a novel population of resident SMC progenitor cells expressing PW1 and participating in PAHassociated vascular remodeling. The author hereby declares no conflict of interest