Albumin Is a Major Protein Component of Transverse Tubule Vesicles Isolated from Skeletal Muscle

Raptor ablation in skeletal muscle decreases Cav1.1 expression and affects the function of the excitation-contraction coupling supramolecular complex

The protein mammalian target of rapamycin (mTOR) is a serine/threonine kinase regulating a number of biochemical pathways controlling cell growth. mTOR exists in two complexes termed mTORC1 and mTORC2. Regulatory associated protein of mTOR (raptor) is associated with mTORC1 and is essential for its function. Ablation of raptor in skeletal muscle results in several phenotypic changes including decreased life expectancy, increased glycogen deposits and alterations of the twitch kinetics of slow fibres. In the present paper, we show that in muscle-specific raptor knockout (RamKO), the bulk of glycogen phosphorylase (GP) is mainly associated in its cAMP-non-stimulated form with sarcoplasmic reticulum (SR) membranes. In addition, 3[H]-ryanodine and 3[H]-PN200-110 equilibrium binding show a ryanodine to dihydropyridine receptors (DHPRs) ratio of 0.79 and 1.35 for wild-type (WT) and raptor KO skeletal muscle membranes respectively. Peak amplitude and time to peak of the global calcium transients evoked by supramaximal field stimulation were not different between WT and raptor KO. However, the increase in the voltage sensor-uncoupled RyRs leads to an increase of both frequency and mass of elementary calcium release events (ECRE) induced by hyper-osmotic shock in flexor digitorum brevis (FDB) fibres from raptor KO. The present study shows that the protein composition and function of the molecular machinery involved in skeletal muscle excitation-contraction (E-C) coupling is affected by mTORC1 signalling.

In vivo imaging of GLUT4 translocationThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

In skeletal muscle, both insulin and muscle contractions mediate translocation of glucose transporter GLUT4 to the plasma membrane proper, the sarcolemma, and the specialized membrane channel network, the transverse (t)-tubules. Despite the fact that skeletal muscle glucose uptake plays a major role in normal conditions, in insulin resistance, and type II diabetes, the details of GLUT4 translocation and the intracellular signalling involved have not been fully described. A main reason is that the majority of experiments have been carried out in muscle cultures in vitro. In vitro cultured muscle is not fully differentiated and, therefore, diverges from real muscle, in that it has lower expression of GLUT4, an underdeveloped or nonexistent t-tubule network, and a reduced or nonexistent response to insulin. Thus, experiments carried out in cultured muscle cell systems might give misleading results on how GLUT4 translocation and the signalling involved takes place. To address this problem, a confocal imaging technique has been developed that allows delineation of the spartial and spatial distribution of GFP-tagged GLUT4 (GLUT4-GFP) translocation in living muscle fibers in situ in anesthetized mice. The effects of stimuli with insulin or in situ muscle contractions in fully differentiated muscle fibers can now be studied before, during, and after applying stimuli. Initial analysis of insulin-stimulated GLUT4-GFP translocation showed a delay in maximal translocation between the sarcolemma and t-tubules. Corresponding to the delay, we found that fluorescent tagged insulin reaches the sarcolemma first and then, with a delay, diffuses into the t-tubule system, enabling interaction with local insulin receptors and, in turn, triggering local insulin signalling and local GLUT4 translocation. In parallel, we showed that the majority of GLUT4 depot vesicles do not move long distances but are depleted locally in the sarcolemma or t-tubule regions. Analysis of GLUT4 translocation in insulin-resistant muscle showed that, primarily, GLUT4 recruitment in the t-tubule region is affected. We have now analysed the kinetics of contraction-mediated GLUT4 translocation and reinternalization, as well as dilineated some of the key signalling points involved in these processes.

Novel role of calpain-3 in the triad-associated protein complex regulating calcium release in skeletal muscle

Calpain-3 (CAPN3) is a non-lysosomal cysteine protease that is necessary for normal muscle function, as mutations in CAPN3 result in an autosomal recessive form of limb girdle muscular dystrophy type 2A. To elucidate the biological roles of CAPN3 in skeletal muscle, we performed a search for potential substrates and interacting partners. By yeast-two-hybrid analysis we identified the glycolytic enzyme aldolase A (AldoA) as a binding partner of CAPN3. In co-expression studies CAPN3 degraded AldoA; however, no accumulation of AldoA was observed in total extracts from CAPN3-deficient muscles suggesting that AldoA is not an in vivo substrate of CAPN3. Instead, we found CAPN3 to be necessary for recruitment of AldoA to one specific location, namely the triads, which are structural components of muscle responsible for calcium transport and excitation-contraction coupling. Both aldolase and CAPN3 are present in the triad-enriched fraction and are able to interact with ryanodine receptors (RyR) that form major calcium release channels. Levels of triad-associated AldoA and RyR were decreased in CAPN3-deficient muscles compared with wild-type. Consistent with these observations we found calcium release to be significantly reduced in fibers from CAPN3-deficient muscles. Together, these data suggest that CAPN3 is necessary for the structural integrity of the triad-associated protein complex and that impairment of calcium transport is a phenotypic feature of CAPN3-deficient muscle.

Hypertrophy of mature Xenopus muscle fibres in culture induced by synergy of albumin and insulin

The aim of this study was to investigate effects of albumin and insulin separately as well as in combination on mature muscle fibres during long-term culture. Single muscle fibres were dissected from m. iliofibularis of Xenopus laevis and attached to a force transducer in a culture chamber. Fibres were cultured in a serum-free medium at slack length (mean sarcomere length 2.3 mum) for 8 to 22 days. The medium was supplemented with (final concentrations): (1) bovine insulin (6 nmol/L or 200-600 nmol/L), (2) 0.2% bovine albumin or (3) 0.2% bovine albumin in combination with insulin (120 nmol/L). In culture medium with insulin, 50% of the muscle fibres became in-excitable within 7-12 days, whereas the other 50% were stable. Caffeine contractures of in-excitable muscle fibres produced 80.4 +/- 2.4% of initial peak tetanic force, indicating impaired excitation-contraction (E-C) coupling in in-excitable fibres. In the presence of albumin, all cultured muscle fibres were stable for at least 10 days. Muscle fibres cultured in medium with insulin or albumin exclusively did not hypertrophy or change the number of sarcomeres in series. In contrast, muscle fibres cultured with both albumin and insulin showed an increase in tetanic force and fibre cross-sectional area of 19.6 +/- 2.8% and 32.5 +/- 4.9%, respectively, (means +/- SEM.; P = 0.007) after 16.3 +/- 1.7 days, whereas the number of sarcomeres in series remained unchanged. We conclude that albumin prevents muscle fibre damage and preserves E-C coupling in culture. Furthermore, albumin is important in regulating muscle fibre adaptation by a synergistic action with growth factors like insulin.

Over-expression of Microspan, a novel component of the sarcoplasmic reticulum, causes severe muscle pathology with triad abnormalities

Sarcospan (SSPN) is a core component of the dystrophin-glycoprotein complex (DGC). Multiple SSPN transcripts are ubiquitously expressed and SSPN splicing is disrupted in many lung tumors, suggesting the importance of SSPN-related mRNAs. We describe the isolation of an alternatively spliced isoform of SSPN, which we designate 'microspan' based on its small size relative to SSPN. Microspan has two transmembrane domains and a novel C-terminus. We demonstrate that microspan is not an integral component of the DGC and is not perturbed by the loss of dystrophin. Microspan protein is detected at the sarcoplasmic reticulum (SR) using indirect immunofluorescence and immunoelectron microscopy. Furthermore, microspan purifies with skeletal muscle SR membranes and not transverse tubules. Mice engineered to over-express microspan display severe kyphosis and die at approximately 8 weeks of age. Levels of ryanodine receptor, dihydropyridine receptor, and SERCA-1 are greatly reduced in microspan transgenic muscle. Furthermore, electron microscopy reveals that microspan over-expression causes a dramatic perturbation in triad structure. Our findings suggest that microspan is an important component of the SR and may contribute to excitation-contraction coupling.

Imaging of insulin signaling in skeletal muscle of living mice shows major role of T-tubules

Insulin stimulates glucose transport in skeletal muscle by glucose transporter GLUT4 translocation to sarcolemma and membrane invaginations, the t-tubules. Although muscle glucose uptake plays a key role in insulin resistance and type 2 diabetes, the dynamics of GLUT4 translocation and the signaling involved are not well described. We have now developed a confocal imaging technique to follow trafficking of green fluorescent protein-labeled proteins in living muscle fibers in situ in anesthetized mice. Using this technique, by imaging the dynamics of GLUT4 translocation and phosphatidylinositol 3,4,5 P(3) (PIP(3)) production in response to insulin, here, for the first time, we delineate the temporal and spatial distribution of these processes in a living animal. We find a 10-min delay of maximal GLUT4 recruitment and translocation to t-tubules compared with sarcolemma. Time-lapse imaging of a fluorescent dye after intravenous injection shows that this delay is similar to the time needed for insulin diffusion into the t-tubule system. Correspondingly, immunostaining of muscle fibers shows that insulin receptors are present throughout the t-tubule system. Finally, PIP(3) production, an early event in insulin signaling, progresses slowly along the t-tubules with a 10-min delay between maximal PIP(3) production at sarcolemma compared with deep t-tubules following the appearance of dye-labeled insulin. Our findings in living mice indicate a major role of the t-tubules in insulin signaling in skeletal muscle and show a diffusion-associated delay in insulin action between sarcolemma and inner t-tubules.

Serum albumin as a potential carrier for the apocrine secretion of proteins in the rat coagulating gland

A protein of 66k was purified to homogeneity from the total secretion of rat coagulating gland. Its close structural relationship to serum albumin was demonstrated by N-terminal amino acid sequence analysis, proteolytic fingerprinting and Western blotting studies using polyclonal antibodies raised against the 66k protein and rat serum albumin. Immunofluorescence staining showed that the 66k protein was localised in the cytoplasm of coagulating gland epithelial cells from which it is released via apocrine blebs. Performing immunoelectron microscopy, the 66k protein was by no means detectable in the endoplasmic reticulum and the Golgi apparatus. Reverse transcription-PCR, Northern blotting studies and in situ hybridisation experiments demonstrated that mRNA of albumin is not expressed by coagulating gland epithelial cells. Therefore, intravascular albumin should be transferred into the epithelial cells of the rat coagulating gland followed by secretion via aposomes. Furthermore, overlay blots proved that the 66k protein binds to the apocrine proteins carbonic anhydrase II and secretory transglutaminase and vice versa. In contrast, no binding was evident to the merocrine 115k protein and to cytoplasmic resident proteins e.g. lactate dehydrogenase. These findings point to the assumption that serum albumin taken up from extracellular sources could function as a selective carrier for cytoplasmic proteins destined for apocrine secretion.

Caveolin-3 associates with developing T-tubules during muscle differentiation

Caveolae, flask-shaped invaginations of the plasma membrane, are particularly abundant in muscle cells. We have recently cloned a muscle-specific caveolin, termed caveolin-3, which is expressed in differentiated muscle cells. Specific antibodies to caveolin-3 were generated and used to characterize the distribution of caveolin-3 in adult and differentiating muscle. In fully differentiated skeletal muscle, caveolin-3 was shown to be associated exclusively with sarcolemmal caveolae. Localization of caveolin-3 during differentiation of primary cultured muscle cells and development of mouse skeletal muscle in vivo suggested that caveolin-3 is transiently associated with an internal membrane system. These elements were identified as developing transverse-(T)-tubules by double-labeling with antibodies to the alpha 1 subunit of the dihydropyridine receptor in C2C12 cells. Ultrastructural analysis of the caveolin-3-labeled elements showed an association of caveolin-3 with elaborate networks of interconnected caveolae, which penetrated the depths of the muscle fibers. These elements, which formed regular reticular structures, were shown to be surface-connected by labeling with cholera toxin conjugates. The results suggest that caveolin-3 transiently associates with T-tubules during development and may be involved in the early development of the T-tubule system in muscle.

Purification, amino-terminal sequence and functional properties of a 64 kDa cytosolic protein from heart muscle capable of modulating calcium transport across the sarcoplasmic reticulum in vitro

In previous studies we have described the inhibitory action of a cytosolic protein fraction from heart muscle on ATP-dependent Ca2+ uptake by the sarcoplasmic reticulum (SR); further this inhibition was shown to be blocked by an inhibitor antagonist, also derived from the cytosol (Narayanan et al., Biochim. Biophys. Acta. 735: 53-66, 1983; Can. J. Physiol. Pharmacol. 67: 999-1006, 1989). Here we report the complete purification of the antagonist protein (AP) and characterization of its functional properties. AP was purified to homogeneity from rabbit heart cytosol using two procedures, one utilizing sequential DE52-cellulose and hydroxylapatite chromatography, and the other utilizing anion exchange chromatography on Mono Q HR 5/5 column in a Pharmacia FPLC system. The purified AP has an apparent molecular weight of 64 kDa; it is made up of about 43% hydrophobic and 57% hydrophilic residues with the following amino-terminal sequence: E-A-H-K-S-E-I-A-H-R-F-N-D-V-G-E-E-H-F-I-G-L-V-L-I-T-F-S-Q-Y-L-Q-K-X-P-Y- E-E-H-A . This partial amino acid sequence data indicate strong sequence homology to serum albumin (sequence homology: 85% to rat serum albumin and 74% to sheep and bovine serum albumin). The purified AP caused concentration-dependent-blockade of the inhibition of Ca2+ uptake by SR observed in the presence of the cytosolic Ca2+ uptake inhibitor protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of transverse tubules in insulin stimulated muscle glucose transport

Although the strongest evidence for recruitment of glucose transporters in response to insulin comes from studies with adipocytes, studies in muscle seem in general to confirm that glucose transporters are also translocated to the cell membrane in muscle in response to insulin. However, the observation that transverse tubule (T-tubule) membranes contain approximately five times more glucose transporter than sarcolemma raised a question as to where glucose transport occurs in muscle. The T-tubule membrane system is continuous with the surface sarcolemma and is a tubule system in which extracellular fluid is in proximity with the interior of the muscle fiber. The purpose of this Prospects article is to evaluate the possibility that the T-tubule membrane may represent a major site of glucose transport in skeletal muscle. Using immunocytochemical techniques we have located GLUT4 glucose transporters on the T-tubule membrane and in vesicles near T-tubules. Since T-tubules form channels into the interior of the muscle fiber, glucose could diffuse or be moved by some peristaltic-like pumping action into the transverse tubules and then be transported across the membrane deep into the interior of the muscle fiber. This mode of transport directly into the interior of the cell would be advantageous over transport across the sarcolemma and subsequent diffusion around the myofibrils to reach the interior of the muscle. Thus, in addition to the role of the T-tubule in ion fluxes and contraction, this unique membrane system can also provide a pathway for the delivery of substrates into the center of the muscle cell where many glycolytic enzymes and glycogen deposits are located.

Tooth 'enamelins' identified mainly as serum proteins. Major 'enamelin' is albumin

The major enamelin protein component present in EDTA or EDTA/guanidine hydrochloride extracts of developing bovine enamel has a molecular mass of about 67 kDa; it has an amino acid composition similar to that of bovine serum albumin and reacts with polyclonal and monoclonal antibodies to albumin. Two-dimensional separation of the components in the enamelin extract by isoelectric focusing and SDS/PAGE reveal that the major approximately 67-kDa component and almost all of the minor Coomassie-staining protein components of approximately 67 kDa, as well as many of the other minor components with different molecular masses, also react with polyclonal and monoclonal antialbumin. The approximately 67-kDa band eluted after SDS/PAGE, as well as the major approximately 67-kDa spots eluted after two-dimensional separation, were found to have N-terminal amino acid sequences identical to that of bovine serum albumin. Albumin accounted for at least 70-80% of the total protein content of the enamelin extract and was essentially the only protein in the approximately 67-kDa component. The serum proteins alpha-2 HS glycoprotein, gamma-globulin and fetuin, and the proline-rich salivary protein termed P-B were also identified in the enamelin extract. The serum proteins and the salivary protein account for greater than 95% of the proteins in the enamelin extracts. Of the remaining very small amounts of non-serum or salivary protein isolated from the enamelin extracts, three minor components were isolated which had N-terminal amino acid sequences which were not similar to any known protein in the protein sequence data base and could therefore conceivably be true 'enamelins' synthesized by ameloblasts. One additional protein had the first five N-terminal amino acids and residue 8 of amelogenin, residues 6 and 7 being different from those of amelogenin. Two other very minor protein components had amino acid compositions distinct from the amelogenins and the serum proteins, but were N-terminally blocked on attempted sequencing. None of the components in the neutral soluble low-ionic-strength extract or in the 4 M guanidine hydrochloride extract, both of which consist principally of amelogenins, immunoreacted with anti-albumin or with any of the antibodies to other serum proteins and fetuin, despite the fact that the amelogenin extracts also contain non-amelogenin proteins. On the basis of the data presented, studies employing antibodies to the so-called enamelin proteins and hypotheses as to their molecular conformation, their roles as evolutionary markers, or their positive role in mineralization should be reconsidered and reviewed.