The mammalian musculature provides mechanical force for locomotion. This system undergoes remodeling throughout lifetime and displays remarkable ability to regenerate from reservoirs of muscle stem cells. However, the muscle regenerative capacity declines with age. This proposal focuses on defining the origin(s) and roles of muscle stem cells during the life span of a mouse. We propose to use a knock-in allele replacing Pax7 with the Cre-ERT2 fusion gene (the Pax7CE allele) for tamoxifen (tmx) inducible cell lineage tracing in following aims:
Aim 1 : Pax7 lineage during embryogenesis: We will use the Pax7CE allele to determine: 1) When and where do Pax7+ embryonic cells become fate-restricted to the muscle lineage? 2) When do Pax7+ embryonic cells take on a satellite cell fate? 3) Do embryonically derived Pax7 descendant cells give rise to adult satellite cells capable of muscle regeneration? Aim 2: Pax7 lineage in adult muscles: We will further ask: 1) After forced exercise, do Pax7+ cells become activated and fuse into existing myofibers? 2) When Pax7+ cells are genetically ablated by diptheria toxin (DTA) in adulthood, can muscle maintain homeostasis in mass and/or regenerate after acute injury? Aim 3: Pax7 lineage in aged muscles: Aged (>2 year old for a mouse) muscles are poor in regeneration and accumulate fibrosis and adipocytes. We design experiments to ask: 1) How quiescent are adult satellite cells? 2) Are aged satellite cells a different population of stem cells from those of young adults? 3) Do aged Pax7+ satellite cells trans-fate into fibroblasts or adipocytes after injury?

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Developmental and in vitro studies in the mouse have implicated Pax7 as a master regulator for satellite cells, the presumed muscle stem cell. We plan to use newly developed genetic tools to define the lineage of Pax7 expressing cells during mouse embryogenesis, in the adult muscle after exercise, and in the aging muscle after injury. Muscle stem cells have the therapeutic potential in ameliorating muscular dystrophies. Our research will provide complimentary knowledge to understand the basic biology of satellite cells in vivo.

National Institute of Health (NIH)
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
Research Project (R01)
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Special Emphasis Panel (ZRG1-MOSS-K (03))
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Boyce, Amanda T
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Carnegie Institution of Washington, D.C.
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Li, Lydia; Fan, Chen-Ming (2017) A CREB-MPP7-AMOT Regulatory Axis Controls Muscle Stem Cell Expansion and Self-Renewal Competence. Cell Rep 21:1253-1266
Webster, Micah T; Harvey, Tyler; Fan, Chen-Ming (2016) Quantitative 3D Time Lapse Imaging of Muscle Progenitors in Skeletal Muscle of Live Mice. Bio Protoc 6:
Lukjanenko, Laura; Jung, M Juliane; Hegde, Nagabhooshan et al. (2016) Loss of fibronectin from the aged stem cell niche affects the regenerative capacity of skeletal muscle in mice. Nat Med 22:897-905
Webster, Micah T; Manor, Uri; Lippincott-Schwartz, Jennifer et al. (2016) Intravital Imaging Reveals Ghost Fibers as Architectural Units Guiding Myogenic Progenitors during Regeneration. Cell Stem Cell 18:243-52
Rozo, Michelle; Li, Liangji; Fan, Chen-Ming (2016) Targeting ?1-integrin signaling enhances regeneration in aged and dystrophic muscle in mice. Nat Med 22:889-96
Van Mater, David; Añó, Leonor; Blum, Jordan M et al. (2015) Acute tissue injury activates satellite cells and promotes sarcoma formation via the HGF/c-MET signaling pathway. Cancer Res 75:605-14
Choi, Moon-Chang; Ryu, Soyoung; Hao, Rui et al. (2014) HDAC4 promotes Pax7-dependent satellite cell activation and muscle regeneration. EMBO Rep 15:1175-83
Günther, Stefan; Kim, Johnny; Kostin, Sawa et al. (2013) Myf5-positive satellite cells contribute to Pax7-dependent long-term maintenance of adult muscle stem cells. Cell Stem Cell 13:590-601
Webster, Micah T; Fan, Chen-Ming (2013) c-MET regulates myoblast motility and myocyte fusion during adult skeletal muscle regeneration. PLoS One 8:e81757
Blum, Jordan M; Añó, Leonor; Li, Zhizhong et al. (2013) Distinct and overlapping sarcoma subtypes initiated from muscle stem and progenitor cells. Cell Rep 5:933-40

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