Three signalling molecules cause increases in intracellular Ca2+ levels by triggering release of Ca2+ from intracellular stores due to their action on specific Ca2+-permeable receptors: inositol-1,4,5-trisphosphate binds to and opens the sarcoplasmic reticulum InsP3 receptor; cyclic ADP ribose activates the endoplasmic reticulum ryanodine receptor; but the molecular identity and location of the nicotinic acid adenine dinucleotide phosphate (NAADP) receptor is unknown. Here Calcraft et al. show that the lysosomal two-pore channel, TPC2, is the molecular target of NAADP. Ca2+ mobilization from intracellular stores represents an important cell signalling process that is regulated, in mammalian cells, by inositol-1,4,5-trisphosphate (InsP3), cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). While the nature of the receptors for InsP3 and cyclic ADP ribose are known, here the lysosomal two-pore channel, TPC2, is shown to be the molecular target of NAADP. Ca2+ mobilization from intracellular stores represents an important cell signalling process1 that is regulated, in mammalian cells, by inositol-1,4,5-trisphosphate (InsP3), cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). InsP3 and cyclic ADP ribose cause the release of Ca2+ from sarcoplasmic/endoplasmic reticulum stores by the activation of InsP3 and ryanodine receptors (InsP3Rs and RyRs). In contrast, the nature of the intracellular stores targeted by NAADP and the molecular identity of the NAADP receptors remain controversial1,2, although evidence indicates that NAADP mobilizes Ca2+ from lysosome-related acidic compartments3,4. Here we show that two-pore channels (TPCs) comprise a family of NAADP receptors, with human TPC1 (also known as TPCN1) and chicken TPC3 (TPCN3) being expressed on endosomal membranes, and human TPC2 (TPCN2) on lysosomal membranes when expressed in HEK293 cells. Membranes enriched with TPC2 show high affinity NAADP binding, and TPC2 underpins NAADP-induced Ca2+ release from lysosome-related stores that is subsequently amplified by Ca2+-induced Ca2+ release by InsP3Rs. Responses to NAADP were abolished by disrupting the lysosomal proton gradient and by ablating TPC2 expression, but were only attenuated by depleting endoplasmic reticulum Ca2+ stores or by blocking InsP3Rs. Thus, TPCs form NAADP receptors that release Ca2+ from acidic organelles, which can trigger further Ca2+ signals via sarcoplasmic/endoplasmic reticulum. TPCs therefore provide new insights into the regulation and organization of Ca2+ signals in animal cells, and will advance our understanding of the physiological role of NAADP.

Wound care is a major healthcare expenditure. Treatment of burns, surgical and trauma wounds, diabetic lower limb ulcers and skin wounds is a major medical challenge with current therapies largely focused on supportive care measures. Successful wound repair requires a series of tightly coordinated steps including coagulation, inflammation, angiogenesis, new tissue formation and extracellular matrix remodelling. Zinc is an essential trace element (micronutrient) which plays important roles in human physiology. Zinc is a cofactor for many metalloenzymes required for cell membrane repair, cell proliferation, growth and immune system function. The pathological effects of zinc deficiency include the occurrence of skin lesions, growth retardation, impaired immune function and compromised would healing. Here, we discuss investigations on the cellular and molecular mechanisms of zinc in modulating the wound healing process. Knowledge gained from this body of research will help to translate these findings into future clinical management of wound healing.

Mitsugumin 53 (MG53), a muscle-specific TRIM family protein, is an essential component of the cell membrane repair machinery. Here, we examined the translational value of targeting MG53 function in tissue repair and regenerative medicine. Although native MG53 protein is principally restricted to skeletal and cardiac muscle tissues, beneficial effects that protect against cellular injuries are present in nonmuscle cells with overexpression of MG53. In addition to the intracellular action of MG53, injury to the cell membrane exposes a signal that can be detected by MG53, allowing recombinant MG53 protein to repair membrane damage when provided in the extracellular space. Recombinant human MG53 (rhMG53) protein purified from Escherichia coli fermentation provided dose-dependent protection against chemical, mechanical, or ultraviolet-induced damage to both muscle and nonmuscle cells. Injection of rhMG53 through multiple routes decreased muscle pathology in the mdx dystrophic mouse model. Our data support the concept of targeted cell membrane repair in regenerative medicine, and present MG53 protein as an attractive biological reagent for restoration of membrane repair defects in human diseases.

Mitsugumin 53 (MG53) negatively regulates skeletal myogenesis by targeting insulin receptor substrate 1 (IRS-1). Here, we show that MG53 is an ubiquitin E3 ligase that induces IRS-1 ubiquitination with the help of an E2-conjugating enzyme, UBE2H. Molecular manipulations that disrupt the E3-ligase function of MG53 abolish IRS-1 ubiquitination and enhance skeletal myogenesis. Skeletal muscles derived from the MG53^−/− mice show an elevated IRS-1 level with enhanced insulin signalling, which protects the MG53^−/− mice from developing insulin resistance when challenged with a high-fat/high-sucrose diet. Muscle samples derived from human diabetic patients and mice with insulin resistance show normal expression of MG53, indicating that altered MG53 expression does not serve as a causative factor for the development of metabolic disorders. Thus, therapeutic interventions that target the interaction between MG53 and IRS-1 may be a novel approach for the treatment of metabolic diseases that are associated with insulin resistance.