Macrophage-released ADAMTS1 promotes muscle stem cell activation.

Coordinated activation of muscle stem cells (known as satellite cells) is critical for postnatal muscle growth and regeneration. The muscle stem cell niche is central for regulating the activation state of satellite cells, but the specific extracellular signals that coordinate this regulation are poorly understood. Here we show that macrophages at sites of muscle injury induce activation of satellite cells via expression of Adamts1. Overexpression of Adamts1 in macrophages in vivo is sufficient to increase satellite cell activation and improve muscle regeneration in young mice. We demonstrate that NOTCH1 is a target of ADAMTS1 metalloproteinase activity, which reduces Notch signaling, leading to increased satellite cell activation. These results identify Adamts1 as a potent extracellular regulator of satellite cell activation and have significant implications for understanding the regulation of satellite cell activity and regeneration after muscle injury.Satellite cells are crucial for growth and regeneration of skeletal muscle. Here the authors show that in response to muscle injury, macrophages secrete Adamts1, which induces satellite cell activation by modulating Notch1 signaling

Investigating the Cellular Origins of Heterotopic Ossification and Assessing the Plasticity of Tissue Resident Progenitor Cells

In regenerative biology, the overarching goal is to rebuild degenerating or absent tissue with a patient\u27s own cells. While this ultimate aim is rather ambitious, the motivation to achieve these ideal clinical tissues has driven the field of stem and progenitor cell biology to unprecedented levels of scientific and public involvement. Additionally, many pathological ailments, such as cancer and heterotopic ossification, are believed to be the consequence of abnormal progenitor cell behavior. Here, we focus on identifying and characterizing tissue resident progenitors. These primitive-like cells hold promise as an autologous cell source for regenerative treatments and importantly, afford us the opportunity to begin to understand seemingly normal cellular processes in pathological contexts. In this regard, we have concentrated on the tissue resident adult progenitors in skeletal muscle and specifically, their role in soft tissue associated heterotopic ossification. We use extensively, the Cre/loxP lineage tracing system to label putative plastic cell types and assess the inherent plasticity of these cells in biologically relevant mouse-chick chimeric models of developmental potency and bioassays of heterotopic ossification. ^ Skeletal muscle, a known and plentiful source of progenitor cells is commonly associated with extraskeletal pathologies such as muscular dystrophy and heterotopic ossification. Two groups of resident progenitors, the skeletal muscle satellite cells (SCs) and vascular endothelium (VE)/vascular endothelial progenitors (VEPs), have been considered to be putative cell sources for ectopic lesions of heterotopic ossification due to their reported multipotent characteristics. However, we find that SCs and VE/VEPs remain committed to their respective lineages in biologically relevant tests of osteogenicity, contradicting much of the published work in regards to their alleged skeletogenic potential. Instead, we identify a novel mesenchymal cell type located in the interstitial space of skeletal muscle that harnesses robust in vivo chondrogenic, osteogenic and adipogenic capabilities. Furthermore, we decisively prove through in vitro clonal assays that the majority of these mesenchymal progenitors are capable of multilineage differentiation, contributing to spontaneously forming adipogenic and fibrocyte-like cells, and to BMP-induced osteogenic cells. We use in parallel many technologies that have been considerably refined to identify, isolate, and characterize, specific and practically pure cell populations, critically allowing us to attribute functional outcomes of potency tests to unique cell types. In conclusion, these data decisively establish degrees of potency for skeletal muscle resident progenitors and identify the putative cell-of-origin for the ectopic skeletal anlagen of heterotopic ossification.

Investigating the Cellular Origins of Heterotopic Ossification and Assessing the Plasticity of Tissue Resident Progenitor Cells

In regenerative biology, the overarching goal is to rebuild degenerating or absent tissue with a patient\u27s own cells. While this ultimate aim is rather ambitious, the motivation to achieve these ideal clinical tissues has driven the field of stem and progenitor cell biology to unprecedented levels of scientific and public involvement. Additionally, many pathological ailments, such as cancer and heterotopic ossification, are believed to be the consequence of abnormal progenitor cell behavior. Here, we focus on identifying and characterizing tissue resident progenitors. These primitive-like cells hold promise as an autologous cell source for regenerative treatments and importantly, afford us the opportunity to begin to understand seemingly normal cellular processes in pathological contexts. In this regard, we have concentrated on the tissue resident adult progenitors in skeletal muscle and specifically, their role in soft tissue associated heterotopic ossification. We use extensively, the Cre/loxP lineage tracing system to label putative plastic cell types and assess the inherent plasticity of these cells in biologically relevant mouse-chick chimeric models of developmental potency and bioassays of heterotopic ossification. ^ Skeletal muscle, a known and plentiful source of progenitor cells is commonly associated with extraskeletal pathologies such as muscular dystrophy and heterotopic ossification. Two groups of resident progenitors, the skeletal muscle satellite cells (SCs) and vascular endothelium (VE)/vascular endothelial progenitors (VEPs), have been considered to be putative cell sources for ectopic lesions of heterotopic ossification due to their reported multipotent characteristics. However, we find that SCs and VE/VEPs remain committed to their respective lineages in biologically relevant tests of osteogenicity, contradicting much of the published work in regards to their alleged skeletogenic potential. Instead, we identify a novel mesenchymal cell type located in the interstitial space of skeletal muscle that harnesses robust in vivo chondrogenic, osteogenic and adipogenic capabilities. Furthermore, we decisively prove through in vitro clonal assays that the majority of these mesenchymal progenitors are capable of multilineage differentiation, contributing to spontaneously forming adipogenic and fibrocyte-like cells, and to BMP-induced osteogenic cells. We use in parallel many technologies that have been considerably refined to identify, isolate, and characterize, specific and practically pure cell populations, critically allowing us to attribute functional outcomes of potency tests to unique cell types. In conclusion, these data decisively establish degrees of potency for skeletal muscle resident progenitors and identify the putative cell-of-origin for the ectopic skeletal anlagen of heterotopic ossification.

Macrophage-released ADAMTS1 promotes muscle stem cell activation.

Coordinated activation of muscle stem cells (known as satellite cells) is critical for postnatal muscle growth and regeneration. The muscle stem cell niche is central for regulating the activation state of satellite cells, but the specific extracellular signals that coordinate this regulation are poorly understood. Here we show that macrophages at sites of muscle injury induce activation of satellite cells via expression of Adamts1. Overexpression of Adamts1 in macrophages in vivo is sufficient to increase satellite cell activation and improve muscle regeneration in young mice. We demonstrate that NOTCH1 is a target of ADAMTS1 metalloproteinase activity, which reduces Notch signaling, leading to increased satellite cell activation. These results identify Adamts1 as a potent extracellular regulator of satellite cell activation and have significant implications for understanding the regulation of satellite cell activity and regeneration after muscle injury.Satellite cells are crucial for growth and regeneration of skeletal muscle. Here the authors show that in response to muscle injury, macrophages secrete Adamts1, which induces satellite cell activation by modulating Notch1 signaling

Mesenchymal Stromal Cells Are Required for Regeneration and Homeostatic Maintenance of Skeletal Muscle

Summary: The necessity of mesenchymal stromal cells, called fibroadipogenic progenitors (FAPs), in skeletal muscle regeneration and maintenance remains unestablished. We report the generation of a PDGFRαCreER knockin mouse model that provides a specific means of labeling and targeting FAPs. Depletion of FAPs using Cre-dependent diphtheria toxin expression results in loss of expansion of muscle stem cells (MuSCs) and CD45+ hematopoietic cells after injury and impaired skeletal muscle regeneration. Furthermore, FAP-depleted mice under homeostatic conditions exhibit muscle atrophy and loss of MuSCs, revealing that FAPs are required for the maintenance of both skeletal muscle and the MuSC pool. We also report that local tamoxifen metabolite delivery to target CreER activity in a single muscle, removing potentially confounding systemic effects of ablating PDGFRα+ cells distantly, also causes muscle atrophy. These data establish a critical role of FAPs in skeletal muscle regeneration and maintenance. : Wosczyna et al. develop genetic models to target and deplete fibroadipogenic progenitors (FAPs) in skeletal muscle (SkM). Following injury of FAP-depleted SkM, muscle stem cell (MuSC) expansion is impaired, leading to a regenerative deficit. Under homeostatic conditions, FAP-depleted SkM undergoes muscle fiber atrophy, and MuSC numbers decline. Keywords: Mesenchymal, stromal, fibroadipogenic progenitor, FAP, muscle, stem cell, satellite cell, niche, PDGFRα, local recombinatio

Identification of Progenitor Cells That Contribute to Heterotopic Skeletogenesis

Background: Individuals who have fibrodysplasia ossificans progressiva develop an ectopic skeleton because of genetic dysregulation of bone morphogenetic protein (BMP) signaling in the presence of inflammatory triggers. The identity of progenitor cells that contribute to various stages of BMP-induced heterotopic ossification relevant to fibrodysplasia ossificans progressiva and related disorders is unknown. An understanding of the cellular basis of heterotopic ossification will aid in the development of targeted, cell-specific therapies for the treatment and prevention of heterotopic ossification

Bioengineered Viral Platform for Intramuscular Passive Vaccine Delivery to Human Skeletal Muscle

Skeletal muscle is ideal for passive vaccine administration as it is easily accessible by intramuscular injection. Recombinant adeno-associated virus (rAAV) vectors are in consideration for passive vaccination clinical trials for HIV and influenza. However, greater human skeletal muscle transduction is needed for therapeutic efficacy than is possible with existing serotypes. To bioengineer capsids with therapeutic levels of transduction, we utilized a directed evolution approach to screen libraries of shuffled AAV capsids in pools of surgically resected human skeletal muscle cells from five patients. Six rounds of evolution were performed in various muscle cell types, and evolved variants were validated against existing muscle-tropic serotypes rAAV1, 6, and 8. We found that evolved variants NP22 and NP66 had significantly increased primary human and rhesus skeletal muscle fiber transduction from surgical explants ex vivo and in various primary and immortalized myogenic lines in vitro. Importantly, we demonstrated reduced seroreactivity compared to existing serotypes against normal human serum from 50 adult donors. These capsids represent powerful tools for human skeletal muscle expression and secretion of antibodies from passive vaccines. Keywords: AAV, passive vaccine, directed evolution, skeletal muscle, human, screen, functional transductio