Dynamic membrane repair and remodelling is an elemental process that maintains cell integrity and mediates efficient cellular function. Here we report that MG53, a muscle-specific tripartite motif family protein (TRIM72), is a component of the sarcolemmal membrane-repair machinery. MG53 interacts with phosphatidylserine to associate with intracellular vesicles that traffic to and fuse with sarcolemmal membranes. Mice null for MG53 show progressive myopathy and reduced exercise capability, associated with defective membrane-repair capacity. Injury of the sarcolemmal membrane leads to entry of the extracellular oxidative environment and MG53 oligomerization, resulting in recruitment of MG53-containing vesicles to the injury site. After vesicle translocation, entry of extracellular Ca 2+ facilitates vesicle fusion to reseal the membrane. Our data indicate that intracellular vesicle translocation and Ca 2+ -dependent membrane fusion are distinct steps involved in the repair of membrane damage and that MG53 may initiate the assembly of the membrane repair machinery in an oxidation-dependent manner.To maintain cellular homeostasis, eukaryotic cells must conserve the integrity of their plasma membrane through active recycling and repair in response to various sources of damage 1 . Defects in the intrinsic membrane repair response have been linked to numerous disease states, including muscular dystrophy, heart failure and neurodegeneration [2][3][4][5] . Repair of plasma membrane damage requires recruitment of intracellular vesicles to injury sites 6,7 . One protein that has been linked to membrane repair in skeletal muscle is dysferlin [8][9][10] , which is thought to act as a fusogen that participates in restoration of sarcolemmal membrane integrity following muscle injury. Evidence for this role of dysferlin comes, in part, from studies showing that ablation of dysferlin in mice results in muscular dystrophy 8 .Repair of damage to the plasma membrane is an active and dynamic process that requires several steps, including participation of molecular sensor(s) that can detect acute injury to 6 Correspondence should be addressed to J.M. or H.T. (maj2@umdnj.edu; takeshim@pharm.kyoto-u.ac.jp).Note: Supplementary Information is available on the Nature Cell Biology website. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests. NIH Public Access Author ManuscriptNat Cell Biol. Author manuscript; available in PMC 2010 November 23. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript the plasma membrane, nucleation of intracellular vesicles at the injury site and vesicle fusion to enable membrane patch formation. It is well demonstrated that entry of extracellular Ca 2+ is involved in the fusion of intracellular vesicles to reseal the injured plasma membrane 6,11,12 , whereas the molecular machinery involved in sensing the damaged membrane signal and the nucleation process for repair-patch formation have not been fully resolved.We have previously established an immunopr...

Recombinant human MG53 protein can increase membrane repair after injury in cells and can reduce pathology in animal models of muscle injury and muscular dystrophy.

Cell signalling requires efficient Ca2+ mobilization from intracellular stores through Ca2+ release channels, as well as predicted counter-movement of ions across the sarcoplasmic/endoplasmic reticulum membrane to balance the transient negative potential generated by Ca2+ release. Ca2+ release channels were cloned more than 15 years ago, whereas the molecular identity of putative counter-ion channels remains unknown. Here we report two TRIC (trimeric intracellular cation) channel subtypes that are differentially expressed on intracellular stores in animal cell types. TRIC subtypes contain three proposed transmembrane segments, and form homo-trimers with a bullet-like structure. Electrophysiological measurements with purified TRIC preparations identify a monovalent cation-selective channel. In TRIC-knockout mice suffering embryonic cardiac failure, mutant cardiac myocytes show severe dysfunction in intracellular Ca2+ handling. The TRIC-deficient skeletal muscle sarcoplasmic reticulum shows reduced K+ permeability, as well as altered Ca2+ 'spark' signalling and voltage-induced Ca2+ release. Therefore, TRIC channels are likely to act as counter-ion channels that function in synchronization with Ca2+ release from intracellular stores.

Most excitable cells maintain tight control of intracellular Ca(2+) through coordinated interaction between plasma membrane and endoplasmic or sarcoplasmic reticulum. Quiescent sarcoplasmic reticulum Ca(2+) release machinery is essential for the survival and normal function of skeletal muscle. Here we show that subtle membrane deformations induce Ca(2+) sparks in intact mammalian skeletal muscle. Spontaneous Ca(2+) sparks can be reversibly induced by osmotic shock, and participate in a normal physiological response to exercise. In dystrophic muscle with fragile membrane integrity, stress-induced Ca(2+) sparks are essentially irreversible. Moreover, moderate exercise in mdx muscle alters the Ca(2+) spark response. Thus, membrane-deformation-induced Ca(2+) sparks have an important role in physiological and pathophysiological regulation of Ca(2+) signalling, and uncontrolled Ca(2+) spark activity in connection with chronic activation of store-operated Ca(2+) entry may function as a dystrophic signal in mammalian skeletal muscle.

Biomacromolecules with poor mechanical properties cannot satisfy the stringent requirement for load‐bearing as bioscaffolds. Herein, a biodegradable high‐strength supramolecular polymer strengthened hydrogel composed of cleavable poly( N ‐acryloyl 2‐glycine) (PACG) and methacrylated gelatin (GelMA) (PACG‐GelMA) is successfully constructed by photo‐initiated polymerization. Introducing hydrogen bond‐strengthened PACG contributes to a significant increase in the mechanical strengths of gelatin hydrogel with a high tensile strength (up to 1.1 MPa), outstanding compressive strength (up to 12.4 MPa), large Young's modulus (up to 320 kPa), and high compression modulus (up to 837 kPa). In turn, the GelMA chemical crosslinking could stabilize the temporary PACG network, showing tunable biodegradability by adjusting ACG/GelMA ratios. Further, a biohybrid gradient scaffold consisting of top layer of PACG‐GelMA hydrogel‐Mn 2+ and bottom layer of PACG‐GelMA hydrogel‐bioactive glass is fabricated for repair of osteochondral defects by a 3D printing technique. In vitro biological experiments demonstrate that the biohybrid gradient hydrogel scaffold not only supports cell attachment and spreading but also enhances gene expression of chondrogenic‐related and osteogenic‐related differentiation of human bone marrow stem cells. Around 12 weeks after in vivo implantation, the biohybrid gradient hydrogel scaffold significantly facilitates concurrent regeneration of cartilage and subchondral bone in a rat model.