A Twist2 Expressing Progenitor Cell Type Population Exists In Human Skeletal Muscle

Abstract

INTRODUCTION. Satellite cells are muscle stem cells that function to support long-term muscle homeostasis, repair and exercise adaptations. Recent evidence in rodents has revealed the existence of an additional muscle progenitor cell population with the capacity to specifically regulate the repair and maintenance of type-IIb skeletal muscle fibres. These cells are typified by the expression of the transcription factor Twist2 (Tw2) and represent a district, non-satellite cell population found within the myofibre interstitium. However, the presence and function of Tw2-positive cells within human skeletal muscle is currently unknown. Therefore, the PURPOSE of this investigation was to identify and characterize Tw2-positive cells within skeletal muscle under basal conditions. METHODS: Muscle biopsy samples were obtained from the hamstrings muscle of young healthy males and females undergoing anterior cruciate ligament repair (n=8, 3 male, 4 female, mean age ~25years), for immunohistochemical (IHC) analysis of muscle cross-sections and immunocytochemical (ICC)analysis of cytospun mononuclear cells enzymatically digested from muscle biopsy samples. RESULTS: ICC staining revealed numerous TW2-positive cells in the isolated mononuclear cell fraction suggesting they originated from the myofibre interstitium. This was confirmed through IHC staining for Tw2 and laminin in tissue cross-sections which revealed that Tw2 expression was localized to a population of cells outside the myofibre membrane at a density of 0.014/mm2. In agreement with previous reports, Tw2 protein expression was localized within both the cytosol and the nucleus ofTw2-positive cells. Importantly, IHC analysis of the satellite cell marker pax7 and Tw2 demonstrated that cells expressing these markers were mutually exclusive demonstrated that Tw2-positive cells represent a unique cell type, independent of satellite cells. Ongoing analysis is examining the response to Tw2-positive cells to acute and chronic exercise stimuli. CONCLUSION: These findings identify a novel non-satellite cell population typified by Twist-2 expression in human skeletal muscle, the function of which currently remains unknown.