In muscle and other mechanically active tissue, cell membranes are constantly injured and their repair depends on the injury induced increase in cytosolic calcium. Here we show that injury-triggered Ca2+ increase results in assembly of ESCRTIII and accessory proteins at the site of repair. This process is initiated by the calcium binding protein - Apoptosis Linked Gene (ALG)-2. ALG-2 facilitates accumulation of ALG-2 interacting protein X (ALIX), ESCRT III, and Vps4 complex at the injured cell membrane, which in turn results in cleavage and shedding of the damaged part of the cell membrane. Lack of ALG-2, ALIX, or Vps4B each prevents shedding, and repair of the injured cell membrane. These results demonstrate Ca2+-dependent accumulation of ESCRTIII-Vps4 complex following large focal injury to the cell membrane and identify the role of ALG-2 as the initiator of sequential ESCRTIII-Vps4 complex assembly that facilitates scission and repair of the injured cell membrane.

Strain and physical trauma to mechanically active cells, such as skeletal muscle myofibers, injures their plasma membranes, and mitochondrial function is required for their repair. Here we found that mitochondrial function was also needed for plasma membrane repair in myoblasts as well as nonmuscle cells, which depended on mitochondrial uptake of calcium through the mitochondrial calcium uniporter (MCU). Calcium uptake transiently increased the production of mitochondrial reactive oxygen species (ROS), which locally activated the GTPase RhoA, triggering F-actin accumulation at the site of injury and facilitating membrane repair. Blocking mitochondrial calcium uptake or ROS production prevented injury-triggered RhoA activation, actin polymerization, and plasma membrane repair. This repair mechanism was shared between myoblasts, nonmuscle cells, and mature skeletal myofibers. Quenching mitochondrial ROS in myofibers during eccentric exercise ex vivo caused increased damage to myofibers, resulting in a greater loss of muscle force. These results suggest a physiological role for mitochondria in plasma membrane repair in injured cells, a role that highlights a beneficial effect of ROS.

Mutations of the DYSF gene leading to reduced dysferlin protein level causes limb girdle muscular dystrophy type 2B (LGMD2B). Dysferlin facilitates sarcolemmal membrane repair in healthy myofibers, thus its deficit compromises myofiber repair and leads to chronic muscle inflammation. An experimental therapeutic approach for LGMD2B is to protect damage or improve repair of myofiber sarcolemma. Here, we compared the effects of prednisolone and vamorolone (a dissociative steroid; VBP15) on dysferlin-deficient myofiber repair. Vamorolone, but not prednisolone, stabilized dysferlin-deficient muscle cell membrane and improved repair of dysferlin-deficient mouse (B6A/J) myofibers injured by focal sarcolemmal damage, eccentric contraction-induced injury or injury due to spontaneous in vivo activity. Vamorolone decreased sarcolemmal lipid mobility, increased muscle strength, and decreased late-stage myofiber loss due to adipogenic infiltration. In contrast, the conventional glucocorticoid prednisolone failed to stabilize dysferlin deficient muscle cell membrane or improve repair of dysferlinopathic patient myoblasts and mouse myofibers. Instead, prednisolone treatment increased muscle weakness and myofiber atrophy in B6A/J mice-findings that correlate with reports of prednisolone worsening symptoms of LGMD2B patients. Our findings showing improved cellular and pre-clinical efficacy of vamorolone compared to prednisolone and better safety profile of vamorolone indicates the suitability of vamorolone for clinical trials in LGMD2B.

Mitochondrial complex‐I dysfunction has been observed in patients of Huntington’s disease (HD). We assessed whether such a defect is present in the 3‐nitropropionic acid (3‐NP) model of HD. Rats treated with 3‐NP (10–20 mg/kg i.p., for 4 days) exhibited weight loss, gait abnormalities, and striatal lesions with increased glial fibrillary acidic protein immunostaining on fifth and ninth days, while increase in striatal dopamine and loss of tyrosine hydroxylase immunoreactivity were observed on fifth day following treatment. We report for the first time a dose‐dependent reduction in complex‐I activity in the cerebral cortex when analyzed spectrophotometrically and by blue native‐polyacrylamide gel electrophoresis following 3‐NP treatment. The citrate synthase normalized activities of mitochondrial complex‐I, ‐II, ‐(I + III) and ‐IV were decreased in the cortex of 3‐NP treated rats. In addition, succinate driven State 3 respiration was also significantly inhibited in vivo and in the isolated mitochondria. These findings taken together with the observation of a significant decrease in vivo but not in vitro of State 3 respiration with NAD+‐linked substrates, suggest complex‐I dysfunction in addition to irreversible inhibition of complex‐II and succinate dehydrogenase activity as a contributing factor in 3‐NP‐induced cortico‐striatal lesion.

The ability of injured cells to heal is a fundamental cellular process, but cellular and molecular mechanisms involved in healing injured cells are poorly understood. Here assays are described to monitor the ability and kinetics of healing of cultured cells following localized injury. The first protocol describes an end point based approach to simultaneously assess cell membrane repair ability of hundreds of cells. The second protocol describes a real time imaging approach to monitor the kinetics of cell membrane repair in individual cells following localized injury with a pulsed laser. As healing injured cells involves trafficking of specific proteins and subcellular compartments to the site of injury, the third protocol describes the use of above end point based approach to assess one such trafficking event (lysosomal exocytosis) in hundreds of cells injured simultaneously and the last protocol describes the use of pulsed laser injury together with TIRF microscopy to monitor the dynamics of individual subcellular compartments in injured cells at high spatial and temporal resolution. While the protocols here describe the use of these approaches to study the link between cell membrane repair and lysosomal exocytosis in cultured muscle cells, they can be applied as such for any other adherent cultured cell and subcellular compartment of choice.