The Function Of Necroptosis In The Wound Healing Process Of Skeletal Muscles

The balances among cell proliferation,cell differentiation,and cell death are crucial for the development and homeostasis of multicellular organisms.In the traditional points of view,apoptosis was the only way of programmed cell death,while necrosis was a passive and uncontrolled form of cell death that happened only in extreme conditions.However,recent studies discovered a tightly regulated kind of cell death with necrotic morphology under pathological conditions,termed programmed necrosis.MLKL dependent necroptosis is the most well-defined signal pathway of programmed necrosis.The key event of necroptosis happening is RIPK3 activation by upstream signals,then RIPK3 activates MLKL by phosphorylation.As the executioner of cell death,phospho-MLKL transports into plasma membranes and cellular membranes,causing membrane breakdown and subsequent release of cellular components into the extracellular spaces.Although RIPK3 and MLKL are dispensable for normal development,they display important roles under a variety of pathological conditions.Many works on the pro-inflammatory roles of necroptosis have established the detrimental nature of necroptotic cell-released factors.However,whether these factors have functions besides triggering inflammation is not clear.Skeletal muscle has a remarkable capacity for regeneration after injury.The repair process dramatically relies on the activation,proliferation,and differentiation of adult muscle stem cells(Mu SCs,also called muscle satellite cells).The complex behavior of Mu SCs during skeletal muscle regeneration is tightly regulated by the dynamic interplays between intrinsic factors within Mu SCs and the extrinsic factors constituting the Mu SC niche/microenvironment.Questions about how damaged myofibers die after injury and the relationship between injured myofibers with the niche factors for Mu SCs have not been explored so far.Here we report the finding that upon acute muscle injury,myofibers committed necroptosis,the normally undetectable RIPK3 and MLKL are drastically upregulated in injured myofibers,and MLKL was highly phosphorylated,indicating the activation of necroptosis.Mice that lack MLKL display muscle regeneration defects during repair due to the failure of proper Mu SCs proliferation after injury.The MCK-Cre;Mlkl^f/fmice that specifically deleted Mlkl in mature myofibers phenocopied the defects of Mlkl^-/-mice in muscle regeneration.Further studies found the necroptotic myofibers release cellular factors to promote Mu SCs proliferation for regeneration.The major factor was purified from necroptosis conditioned medium and identified as Tenascin-C(TNC),which directly activating EGF receptor(EGFR)with its EGF-like domain to promote Mu SCs proliferation.TNC was not detected in healthy myofibers while highly upregulated in damaged myofibers,a process that was also dependent on necroptosis happening.Thus,via upregulating and releasing TNC,necroptotic myofibers provide a conditional niche for adult stem cell proliferation upon injury,which constitutes a fundamental physiological role of necroptosis in tissue regeneration.