Abnormal Features in Skeletal Muscle from Mice Lacking Mitsugumin29

Physiological roles of the members of the synaptophysin family, carrying four transmembrane segments and being basically distributed on intracellular membranes including synaptic vesicles, have not been established yet. Recently, mitsugumin29 (MG29) was identified as a novel member of the synaptophysin family from skeletal muscle. MG29 is expressed in the junctional membrane complex between the cell surface transverse (T) tubule and the sarcoplasmic reticulum (SR), called the triad junction, where the depolarization signal is converted to Ca2+ release from the SR. In this study, we examined biological functions of MG29 by generating knockout mice. The MG29-deficient mice exhibited normal health and reproduction but were slightly reduced in body weight. Ultrastructural abnormalities of the membranes around the triad junction were detected in skeletal muscle from the mutant mice, i.e., swollen T tubules, irregular SR structures, and partial misformation of triad junctions. In the mutant muscle, apparently normal tetanus tension was observed, whereas twitch tension was significantly reduced. Moreover, the mutant muscle showed faster decrease of twitch tension under Ca2+-free conditions. The morphological and functional abnormalities of the mutant muscle seem to be related to each other and indicate that MG29 is essential for both refinement of the membrane structures and effective excitation-contraction coupling in the skeletal muscle triad junction. Our results further imply a role of MG29 as a synaptophysin family member in the accurate formation of junctional complexes between the cell surface and intracellular membranes.

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The exocyst complex regulates insulin-stimulated glucose uptake of skeletal muscle cells

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00109.2019 ◽

2019 ◽

Vol 317 (6) ◽

pp. E957-E972

Author(s):

Brent A. Fujimoto ◽

Madison Young ◽

Lamar Carter ◽

Alina P. S. Pang ◽

Michael J. Corley ◽

...

Keyword(s):

Skeletal Muscle ◽

Plasma Membrane ◽

Cell Surface ◽

Glucose Uptake ◽

Glucose Transporter ◽

Muscle Cells ◽

Skeletal Muscle Cells ◽

Cell Surface Receptor ◽

Exocyst Complex

Skeletal muscle handles ~80–90% of the insulin-induced glucose uptake. In skeletal muscle, insulin binding to its cell surface receptor triggers redistribution of intracellular glucose transporter GLUT4 protein to the cell surface, enabling facilitated glucose uptake. In adipocytes, the eight-protein exocyst complex is an indispensable constituent in insulin-induced glucose uptake, as it is responsible for the targeted trafficking and plasma membrane-delivery of GLUT4. However, the role of the exocyst in skeletal muscle glucose uptake has never been investigated. Here we demonstrate that the exocyst is a necessary factor in insulin-induced glucose uptake in skeletal muscle cells as well. The exocyst complex colocalizes with GLUT4 storage vesicles in L6-GLUT4myc myoblasts at a basal state and associates with these vesicles during their translocation to the plasma membrane after insulin signaling. Moreover, we show that the exocyst inhibitor endosidin-2 and a heterozygous knockout of Exoc5 in skeletal myoblast cells both lead to impaired GLUT4 trafficking to the plasma membrane and hinder glucose uptake in response to an insulin stimulus. Our research is the first to establish that the exocyst complex regulates insulin-induced GLUT4 exocytosis and glucose metabolism in muscle cells. A deeper knowledge of the role of the exocyst complex in skeletal muscle tissue may help our understanding of insulin resistance in type 2 diabetes.

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Comparison of glucose-transporter-containing vesicles from rat fat and muscle tissues: evidence for a unique endosomal compartment

Biochemical Journal ◽

10.1042/bj3070383 ◽

1995 ◽

Vol 307 (2) ◽

pp. 383-390 ◽

Cited By ~ 77

Author(s):

K V Kandror ◽

L Coderre ◽

A V Pushkin ◽

P F Pilch

Keyword(s):

Skeletal Muscle ◽

Cell Surface ◽

Glucose Transporter ◽

Simple Procedure ◽

Protein Composition ◽

Muscle Cells ◽

Membrane Structures ◽

Rat Adipocytes ◽

Endosomal Compartment ◽

Biochemical Comparison

Insulin-sensitive tissues (fat and muscle) express a specific isoform of glucose-transporter protein, GLUT4, which normally resides in intracellular vesicular structures and is translocated to the cell surface in response to insulin. Here we provide a biochemical comparison of GLUT4-containing structures from fat and muscle cells. We demonstrate that, in spite of totally different protocols for cell homogenization and fractionation used for adipocytes as compared with skeletal-muscle tissue, GLUT4-containing vesicles from both sources have identical buoyant densities, sedimentation coefficients, and a very similar, if not identical, protein composition. The individual proteins first identified in GLUT4-containing vesicles from adipocytes (GTV3/SCAMPs proteins and aminopeptidase gp160) are also present in the analogous vesicles from muscle. Intracellular microsomes from rat adipocytes also contain GLUT1, a ubiquitously expressed glucose-transporter isoform. GLUT1 has not been detected in intracellular vesicular pool(s) from skeletal-muscle cells, probably because of its low abundance there. GLUT1 in adipocytes is excluded from GLUT4-containing vesicles, but is found in membrane structures which are indistinguishable from the former by all methods tested and demonstrate the same type of regulation by insulin. That is, the GLUT1- and GLUT4-containing vesicles have identical densities and sedimentation coefficients in sucrose gradients, and translocate to the cell surface in response to hormonal exposure. Also, we describe a simple procedure for the purification of native glucose-transporter vesicles from rat adipocytes. Overall, our data suggest the existence of a unique endosomal compartment in fat and muscle cells which is functionally and compositionally different from other microsomal vesicles and which is responsible for insulin-sensitive glucose transport in these tissues.

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Cryo-Electron Tomography of Isolated Triad Junctions from Skeletal Muscle

Microscopy and Microanalysis ◽

10.1017/s1431927600026544 ◽

2001 ◽

Vol 7 (S2) ◽

pp. 94-95 ◽

Cited By ~ 1

Author(s):

C.-E. Hsieh ◽

M. Marko ◽

B.K. Rath ◽

S. Fleischer ◽

T. Wagenknecht

Keyword(s):

Skeletal Muscle ◽

Plasma Membrane ◽

Electron Tomography ◽

Mass Density ◽

Tomographic Reconstruction ◽

Three Dimensional ◽

Cryo Electron Tomography ◽

Junctional Complexes ◽

Triad Junction ◽

Triad Junctions

In skeletal muscle, depolarization of the plasma membrane, which is initiated at the neuromuscular junction, is transduced to a rise in cytoplasmic calcium at specialized structures known as triad junctions (TJs). TJs occur in the myofiber’s interior at regions near the z-lines, where transversely oriented tubular invaginations of the plasma membrane (T-tubules) form junctions with two elements of the sarcoplasmic reticulum (SR). Isolation of membrane fractions that are enriched in junctional complexes and which retain function has been reported.Figure 1 shows a region of an electron micrograph containing an isolated TJ in the frozen-hydrated state. in the orientation shown, two SR-derived vesicles sandwich a flattened vesicle derived from the T-tubule. The junctional regions contain a complex distribution of density, presumably due to proteins that are known to be present in TJs. Electron tomography offers the means to determine the three-dimensional mass density from such micrographs, which would greatly aid in their interpretation. Only recently has the automated data collection technology for determining tomograms of non-stained, frozen-hydrated specimens become available. Here we describe the first tomographic reconstruction of a frozen-hydrated triad junction by automated electron tomography.

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Functional role of arginine 425 in the mammalian Na+/H+ exchanger

Biochemistry and Cell Biology ◽

10.1139/bcb-2014-0070 ◽

2014 ◽

Vol 92 (6) ◽

pp. 541-546 ◽

Cited By ~ 3

Author(s):

Xiuju Li ◽

Yike Ma ◽

Larry Fliegel

Keyword(s):

Cell Surface ◽

Mammalian Cells ◽

Transmembrane Helices ◽

Basic Residue ◽

Mutant Proteins ◽

Transmembrane Segments ◽

Nhe1 Activity ◽

And Function ◽

Extracellular Sodium

Na+/H+ exchanger isoform 1 (NHE1) is the principal plasma membrane Na+/H+ exchanger of mammalian cells and functions by exchanging one intracellular proton for one extracellular sodium ion. Critical transmembrane segments of Na+/H+ exchangers have discontinuous transmembrane helices, which result in a dipole within the membrane. Amino acid R425 has been suggested to play an important role in neutralizing one such helix dipole. To investigate this hypothesis, R425 was mutated to alanine, glutamine, histidine, or lysine and the mutant NHE1 proteins were expressed and characterized in NHE1-deficient cells. The R425A and R425E mutants exhibited complete loss of expression of mature, fully glycosylated NHE1, reduced expression overall, and greatly reduced cell surface targeting. The cell surface targeting, expression, and activity of the R425H and R425K mutant proteins were also impaired, though residual NHE1 activity remained. When reduced targeting and expression were accounted for, the R425H and R425K mutant proteins had activity similar to that of the wild-type protein. The results suggest that R425 is critical for NHE1 expression, targeting, and activity and that replacement with another basic residue can rescue activity. The findings are consistent with a role for R425 in both neutralizing a helix dipole and maintaining NHE1 structure and function.

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585 Increased expression of MafBx ubiquitin ligase mRNA in skeletal muscle after induction of heart failure: role of inflammatory cytokines

European Journal of Heart Failure Supplements ◽

10.1016/s1567-4215(05)80360-1 ◽

2005 ◽

Vol 4 (1) ◽

pp. 131-131

Keyword(s):

Heart Failure ◽

Skeletal Muscle ◽

Inflammatory Cytokines ◽

Ubiquitin Ligase

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The Role of the Interaction between Fe(III) and Cell Surface in the Accumulation of 67Ga by Tumor Cells

Nuklearmedizin ◽

10.1055/s-0038-1629590 ◽

1991 ◽

Vol 30 (06) ◽

pp. 290-293 ◽

Cited By ~ 1

Author(s):

P. Maleki ◽

A. Martinezi ◽

M. C. Crone-Escanye ◽

J. Robert ◽

L. J. Anghileri

Keyword(s):

Cell Surface ◽

Tumor Cells ◽

Ionic Composition ◽

Iron Complex ◽

Iron Binding ◽

Complex Time ◽

Interaction Product ◽

Thermodynamic Constant ◽

Number Of Cells

The study of the interaction between complexed iron and tumor cells in the presence of 67Ga-citrate indicates that a phenomenon of iron-binding related to the thermodynamic constant of stability of the iron complex, and a hydrolysis (or anion penetration) of the interaction product determine the uptake of 67Ga. The effects of various parameters such as ionic composition of the medium, nature of the iron complex, time of incubation and number of cells are discussed.

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