Intracellular Ca2+ transients in delta-sarcoglycan knockout mouse skeletal muscle.

BACKGROUND: delta-Sarcoglycan (delta-SG) knockout (KO) mice develop skeletal muscle histopathological alterations similar to those in humans with limb muscular dystrophy. Membrane fragility and increased Ca(2+) permeability have been linked to muscle degeneration. However, little is known about the mechanisms by which genetic defects lead to disease. METHODS: Isolated skeletal muscle fibers of wild-type and delta-SG KO mice were used to investigate whether the absence of delta-SG alters the increase in intracellular Ca(2+) during single twitches and tetani or during repeated stimulation. Immunolabeling, electrical field stimulation and Ca(2+) transient recording techniques with fluorescent indicators were used. RESULTS: Ca(2+) transients during single twitches and tetani generated by muscle fibers of delta-SG KO mice are similar to those of wild-type mice, but their amplitude is greatly decreased during protracted stimulation in KO compared to wild-type fibers. This impairment is independent of extracellular Ca(2+) and is mimicked in wild-type fibers by blocking store-operated calcium channels with 2-aminoethoxydiphenyl borate (2-APB). Also, immunolabeling indicates the localization of a delta-SG isoform in the sarcoplasmic reticulum of the isolated skeletal muscle fibers of wild-type animals, which may be related to the functional differences between wild-type and KO muscles. CONCLUSIONS: delta-SG has a role in calcium homeostasis in skeletal muscle fibers. GENERAL SIGNIFICANCE: These results support a possible role of delta-SG on calcium homeostasis. The alterations caused by the absence of delta-SG may be related to the pathogenesis of muscular dystrophy.

Activated dendritic cells modulate proliferation and differentiation of human myoblasts.

Idiopathic Inflammatory Myopathies (IIMs) are a heterogeneous group of autoimmune diseases affecting skeletal muscle tissue homeostasis. They are characterized by muscle weakness and inflammatory infiltration with tissue damage. Amongst the cells in the muscle inflammatory infiltration, dendritic cells (DCs) are potent antigen-presenting and key components in autoimmunity exhibiting an increased activation in inflamed tissues. Since, the IIMs are characterized by the focal necrosis/regeneration and muscle atrophy, we hypothesized that DCs may play a role in these processes. Due to the absence of a reliable in vivo model for IIMs, we first performed co-culture experiments with immature DCs (iDC) or LPS-activated DCs (actDC) and proliferating myoblasts or differentiating myotubes. We demonstrated that both iDC or actDCs tightly interact with myoblasts and myotubes, increased myoblast proliferation and migration, but inhibited myotube differentiation. We also observed that actDCs increased HLA-ABC, HLA-DR, VLA-5, and VLA-6 expression and induced cytokine secretion on myoblasts. In an in vivo regeneration model, the co-injection of human myoblasts and DCs enhanced human myoblast migration, whereas the absolute number of human myofibres was unchanged. In conclusion, we suggest that in the early stages of myositis, DCs may play a crucial role in inducing muscle-damage through cell-cell contact and inflammatory cytokine secretion, leading to muscle regeneration impairment.

Nanospan, an alternatively spliced isoform of sarcospan, localizes to the sarcoplasmic reticulum in skeletal muscle and is absent in limb girdle muscular dystrophy 2F.

BACKGROUND: Sarcospan (SSPN) is a transmembrane protein that interacts with the sarcoglycans (SGs) to form a tight subcomplex within the dystrophin-glycoprotein complex that spans the sarcolemma and interacts with laminin in the extracellular matrix. Overexpression of SSPN ameliorates Duchenne muscular dystrophy in murine models. METHODS: Standard cloning approaches were used to identify nanospan, and nanospan-specific polyclonal antibodies were generated and validated. Biochemical isolation of skeletal muscle membranes and two-photon laser scanning microscopy were used to analyze nanospan localization in muscle from multiple murine models. Duchenne muscular dystrophy biopsies were analyzed by immunoblot analysis of protein lysates as well as indirect immunofluorescence analysis of muscle cryosections. RESULTS: Nanospan is an alternatively spliced isoform of sarcospan. While SSPN has four transmembrane domains and is a core component of the sarcolemmal dystrophin-glycoprotein complex, nanospan is a type II transmembrane protein that does not associate with the dystrophin-glycoprotein complex. We demonstrate that nanospan is enriched in the sarcoplasmic reticulum (SR) fractions and is not present in the T-tubules. SR fractions contain membranes from three distinct structural regions: a region flanking the T-tubules (triadic SR), a SR region across the Z-line (ZSR), and a longitudinal SR region across the M-line (LSR). Analysis of isolated murine muscles reveals that nanospan is mostly associated with the ZSR and triadic SR, and only minimally with the LSR. Furthermore, nanospan is absent from the SR of δ-SG-null (Sgcd-/-) skeletal muscle, a murine model for limb girdle muscular dystrophy 2F. Analysis of skeletal muscle biopsies from Duchenne muscular dystrophy patients reveals that nanospan is preferentially expressed in type I (slow) fibers in both control and Duchenne samples. Furthermore, nanospan is significantly reduced in Duchenne biopsies. CONCLUSIONS: Alternative splicing of proteins from the SG-SSPN complex produces δ-SG3, microspan, and nanospan that localize to the ZSR and the triadic SR, where they may play a role in regulating resting calcium levels as supported by previous studies (Estrada et al., Biochem Biophys Res Commun 340:865-71, 2006). Thus, alternative splicing of SSPN mRNA generates three protein isoforms (SSPN, microspan, and nanospan) that differ in the number of transmembrane domains affecting subcellular membrane association into distinct protein complexes.

Phosphorus-free membrane lipids of Sinorhizobium meliloti are not required for the symbiosis with alfalfa but contribute to increased cell yields under phosphorus-limiting conditions of growth.

The microsymbiont of alfalfa, Sinorhizobium meliloti, possesses phosphatidylglycerol, cardiolipin, phosphatidylethanolamine, and phosphatidylcholine as major membrane phospholipids, when grown in the presence of sufficient accessible phosphorus sources. Under phosphate-limiting conditions of growth, S. meliloti replaces its phospholipids by membrane lipids that do not contain any phosphorus in their molecular structure and, in S. meliloti, these phosphorus-free membrane lipids are sulphoquinovosyl diacylglycerols (SL), ornithine-containing lipids (OL), and diacylglyceryl-N,N,N-trimethylhomoserines (DGTS). In earlier work, we demonstrated that neither SL nor OL are required for establishing a nitrogen-fixing root nodule symbiosis with alfalfa. We now report the identification of the two structural genes btaA and btaB from S. meliloti required for DGTS biosynthesis. When the sinorhizobial btaA and btaB genes are expressed in Escherichia coli, they cause the formation of DGTS in this latter organism. A btaA-deficient mutant of S. meliloti is unable to form DGTS but can form nitrogen-fixing root nodules on alfalfa, demonstrating that sinorhizobial DGTS is not required for establishing a successful symbiosis with the host plant. Even a triple mutant of S. meliloti, unable to form any of the phosphorus-free membrane lipids SL, OL, or DGTS is equally competitive for nodule occupancy as the wild type. Only under growth-limiting concentrations of phosphate in culture media did mutants that could form neither OL nor DGTS grow to lesser cell densities.

Altered calcium pump and secondary deficiency of gamma-sarcoglycan and microspan in sarcoplasmic reticulum membranes isolated from delta-sarcoglycan knockout mice.

Sarcoglycans (SGs) and sarcospan (SSPN) are transmembrane proteins of the dystrophin-glycoprotein complex. Mutations in the genes encoding SGs cause many inherited forms of muscular dystrophy. In this study, using purified membranes of wild-type (WT) and delta-SG knockout (KO) mice, we found the specific localization of the SG-SSPN isoforms in transverse tubules (TT) and sarcoplasmic reticulum (SR) membranes. Immunoblotting revealed that the absence of delta-SG isoforms in TT and SR results in a secondary deficiency of gamma-SG and microSPN. Our results showed augmented ATP hydrolytic activity, ATP-dependent calcium uptake and passive calcium efflux, probably through SERCA1 in KO compared to WT mice. Furthermore, we found a conformational change in SERCA1 isolated from KO muscle as demonstrated by calorimetric analysis. Following these alterations with mechanical properties, we found an increase in force in KO muscle with the same rate of fatigue but with a decreased fatigue recovery compared to WT. Together our observations suggest, for the first time, that the delta-SG isoforms may stabilize the expression of gamma-SG and microSPN in the TT and SR membranes and that this possible complex may play a role in the maintenance of a stable level of resting cytosolic calcium concentration in skeletal muscle.