Phosphoinositides in membranes that build up the triads of rabbit skeletal muscle

Excess membrane cholesterol is an early contributing reversible aspect of skeletal muscle insulin resistance in C57BL/6NJ mice fed a Western-style high-fat diet

Skeletal muscle insulin resistance manifests shortly after high-fat feeding, yet mechanisms are not known. Here we set out to determine whether excess skeletal muscle membrane cholesterol and cytoskeletal derangement known to compromise glucose transporter (GLUT)4 regulation occurs early after high-fat feeding. We fed 6-wk-old male C57BL/6NJ mice either a low-fat (LF, 10% kcal) or a high-fat (HF, 45% kcal) diet for 1 wk. This HF feeding challenge was associated with an increase, albeit slight, in body mass, glucose intolerance, and hyperinsulinemia. Liver analyses did not reveal signs of hepatic insulin resistance; however, skeletal muscle immunoblots of triad-enriched regions containing transverse tubule membrane showed a marked loss of stimulated GLUT4 recruitment. An increase in cholesterol was also found in these fractions from HF-fed mice. These derangements were associated with a marked loss of cortical filamentous actin (F-actin) that is essential for GLUT4 regulation and known to be compromised by increases in membrane cholesterol. Both the withdrawal of the HF diet and two subcutaneous injections of the cholesterol-lowering agent methyl-β-cyclodextrin at 3 and 6 days during the 1-wk HF feeding intervention completely mitigated cholesterol accumulation, cortical F-actin loss, and GLUT4 dysregulation. Moreover, these beneficial membrane/cytoskeletal changes occurred concomitant with a full restoration of metabolic responses. These results identify skeletal muscle membrane cholesterol accumulation as an early, reversible, feature of insulin resistance and suggest cortical F-actin loss as an early derangement of skeletal muscle insulin resistance.

Dysferlin mediates membrane tubulation and links T-tubule biogenesis to muscular dystrophy

The multi-C2 domain protein dysferlin localizes to the plasma membrane and the T-tubule system in skeletal muscle; however, its physiological mode of action is unknown. Mutations in the DYSF gene lead to autosomal recessive limb-girdle muscular dystrophy type 2B and Miyoshi myopathy. Here, we show that dysferlin has membrane tubulating capacity and that it shapes the T-tubule system. Dysferlin tubulates liposomes, generates a T-tubule-like membrane system in non-muscle cells, and links the recruitment of phosphatidylinositol 4,5-bisphosphate to the biogenesis of the T-tubule system. Pathogenic mutant forms interfere with all of these functions, indicating that muscular wasting and dystrophy are caused by the dysferlin mutants' inability to form a functional T-tubule membrane system.

Characterization of lipid binding specificities of dysferlin C2 domains reveals novel interactions with phosphoinositides

Dysferlin is a type II transmembrane protein implicated in Ca(2+)-dependent sarcolemmal membrane repair. Dysferlin has seven C2 domains, which are lipid and protein binding modules. In this study, we sought to characterize the lipid binding specificity of dysferlin's seven C2 domains. Dysferlin's C2A domain was able to bind to phosphatidylserine (PS), phosphatidylinositol 4-phosphate [PtdIns(4)P], and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] in a Ca(2+)-dependent fashion. The remainder of the C2 domains exhibited weaker and Ca(2+)-independent binding to PS and no significant binding to phosphoinositides.

Isolation of T-tubules from skeletal muscle

The transverse tubules (T-tubules) of mammalian cardiac and skeletal muscles are invaginations of the sarcolemma. They play a crucial role in excitation-contraction coupling as well as in intracellular signaling and in regulation of glucose transport. The biochemical purification of T-tubule membranes is a difficult task, and membrane fractions enriched in transverse tubules are usually contaminated with other cell-surface and intracellular membranes. This unit includes methods that permit the isolation and purification of T-tubules from skeletal muscle.

Membrane electrical activity elicits inositol 1,4,5-trisphosphate-dependent slow Ca2+ signals through a Gbetagamma/phosphatidylinositol 3-kinase gamma pathway in skeletal myotubes

Tetanic electrical stimulation of myotubes evokes a ryanodine receptor-related fast calcium signal, during the stimulation, followed by a phospholipase C/inositol 1,4,5-trisphosphate-dependent slow calcium signal few seconds after stimulus end. L-type calcium channels (Cav 1.1, dihydropyridine receptors) acting as voltage sensors activate an unknown signaling pathway involved in phospholipase C activation. We demonstrated that both G protein and phosphatidylinositol 3-kinase were activated by electrical stimulation, and both the inositol 1,4,5-trisphosphate rise and slow calcium signal induced by electrical stimulation were blocked by pertussis toxin, by a Gbetagamma scavenger peptide, and by phosphatidylinositol 3-kinase inhibitors. Immunofluorescence using anti-phosphatidylinositol 3-kinase gamma antibodies showed a clear location in striations within the cytoplasm, consistent with a position near the I band region of the sarcomere. The time course of phosphatidylinositol 3-kinase activation, monitored in single living cells using a pleckstrin homology domain fused to green fluorescent protein, was compatible with sequential phospholipase Cgamma1 activation as confirmed by phosphorylation assays for the enzyme. Co-transfection of a dominant negative form of phosphatidylinositol 3-kinase gamma inhibited the phosphatidylinositol 3-kinase activity as well as the slow calcium signal. We conclude that Gbetagamma/phosphatidylinositol 3-kinase gamma signaling pathway is involved in phospholipase C activation and the generation of the slow calcium signal induced by tetanic stimulation. We postulate that membrane potential fluctuations in skeletal muscle cells can activate a pertussis toxin-sensitive G protein, phosphatidylinositol 3-kinase, phospholipase C pathway toward modulation of long term, activity-dependent plastic changes.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Interaction of phosphatidic acid and phosphatidylserine with the Ca2+-ATPase of sarcoplasmic reticulum and the mechanism of inhibition

The sarcoplasmic reticulum of skeletal muscle contains anionic phospholipids as well as the zwitterionic phosphatidylcholine and phosphatidylethanolamine. Here we study the effects of anionic phospholipids on the activity of the Ca2+-ATPase purified from the membrane. Reconstitution of the Ca2+-ATPase into dioleoylphosphatidylserine [di(C18:1)PS] or dioleoylphosphatidic acid [di(C18:1)PA] leads to a decrease in ATPase activity. Measurements of the quenching of the tryptophan fluorescence of the ATPase by brominated phospholipids give a relative binding constant for the anionic lipids compared with dioleoylphosphatidylcholine close to 1 and suggest that phosphatidic acid only binds to the ATPase at the bulk lipid sites around the ATPase. Addition of di(C18:1)PS or di(C18:1)PA to the ATPase in the short-chain dimyristoleoylphosphatidylcholine [di(C14:1)PC] reverse the effects of the short-chain lipid on ATPase activity and on Ca2+ binding, as revealed by the response of tryptophan fluorescence intensity to Ca2+ binding. It is concluded that the lipid headgroup and lipid fatty acyl chains have separate effects on the function of the ATPase. The anionic phospholipids have no significant effect on Ca2+ binding to the ATPase; the level of Ca2+ binding to the ATPase, the affinity of binding and the rate of dissociation of Ca2+ are unchanged by reconstitution into di(C18:1)PA. The major effect of the anionic lipids is a reduction in the maximal level of binding of MgATP. This is attributed to the formation of oligomers of the Ca2+-ATPase, in which only one molecule of the ATPase can bind MgATP dimers in di(C18:1)PS and trimers or tetramers in di(C18:1)PA. The rates of phosphorylation and dephosphorylation for the proportion of the ATPase still able to bind ATP are unaffected by reconstitution. Larger changes were observed in the level of phosphorylation of the ATPase by Pi, which became very low in the anionic phospholipids. The fluorescence response to Mg2+ for the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin was also changed in di(C18:1)PS and di(C18:1)PA, so that effects of Mg2+ became comparable with those seen on phosphorylation for the unreconstituted ATPase. The anionic phospholipids could induce a conformational change in the ATPase on binding Mg2+ equivalent to that normally induced by phosphorylation or by binding inhibitors such as thapsigargin.

Cloning of a phospholipase C-delta 1 of rabbit skeletal muscle

The phospholipase C isoform responsible for the increase in the total myoplasmic inositol 1,4,5-trisphosphate concentration during tetanic contraction of isolated skeletal muscle and its mechanism of activation is not known. We have cloned and sequenced a phospholipase C cDNA of rabbit skeletal muscle coding for a protein of 745 amino acids with a molecular mass of 84,440 kDa. The deduced amino acid sequence exhibits the phospholipase C-specific domains X and Y which according to current knowledge very likely represent the catalytic centre of the enzyme. An overall sequence homology of 88% to the phospholipase C-delta 1 of rat brain suggests that the encoded protein represents a phospholipase C-delta 1 isoform of rabbit skeletal muscle. Northern blot analysis shows, that this phospholipase C-delta is dominantly expressed in skeletal muscle, less strongly in smooth muscle (uterus) and lung and weakly in heart, kidney and brain. In the N-terminal part of the primary structure a consensus sequence for a canonical EF-hand Ca2+ binding domain can be identified together with a short positively charged motif which recently has been suggested to be essential for the binding of phosphatidylinositol 4,5-bisphosphate. If these two domains which are unique for phospholipase C-delta are sufficient in establishing a mechanism for the activation of the enzyme, inositol 1,4,5-trisphosphate formation in skeletal muscle could be the consequence of an increase in myoplasmic Ca2+.

The T-tubule is a cell-surface target for insulin-regulated recycling of membrane proteins in skeletal muscle

(1) In this study we have determined the distribution of various membrane proteins involved in insulin-activated glucose transport in T-tubules and in sarcolemma from rat skeletal muscle. Two independent experimental approaches were used to determine the presence of membrane proteins in T-tubules: (i) the purification of T-tubules free from sarcolemmal membranes by lectin agglutination, and (ii) T-tubule vesicle immunoadsorption. These methods confirmed that T-tubules from rat skeletal muscle were enriched with dihydropyridine receptors and tt28 protein and did not contain the sarcolemmal markers dystrophin or beta 1-integrin. Both types of experiments revealed an abundant content of GLUT4 glucose carriers, insulin receptors and SCAMPs (secretory carrier membrane proteins) in T-tubule membranes. (2) Acute administration in vivo of insulin caused an increased abundance of GLUT4 in T-tubules and sarcolemma. On the contrary, insulin led to a 50% reduction in insulin receptors present in T-tubules and in sarcolemma, demonstrating that insulin-induced insulin receptor internalization affects T-tubules in the muscle fibre. The alteration in the content of GLUT4 and insulin receptors in T-tubules was a consequence of insulin-induced redistribution of these proteins. SCAMPs also redistributed in muscle membranes in response to insulin. They were recruited by insulin from intracellular high-density fractions to intracellular lighter-density fractions and to the cell surface, showing a pattern of insulin-induced cellular redistribution distinct from those of GLUT4 and the insulin receptor. (3) In conclusion, the T-tubule is a cell-surface target for membrane proteins involved in recycling such as SCAMPs or for membrane proteins that acutely redistribute in response to insulin such as GLUT4 or insulin receptors.

Lipid structure and Ca(2+)-ATPase function

Effects of lipid structure on the function of the Ca(2+)-ATPase of skeletal muscle of sarcoplasmic reticulum are reviewed. Binding of phospholipids to the ATPase shows little specificity. Phosphatidylcholines with short (C14) or long (C24) fatty acyl chains have marked effects on the activity of the ATPase, including a change in the stoichiometry of Ca binding. Low ATPase activity in gel phase lipid follows from low rate of phosphorylation. Phosphatidylinositol 4-phosphate increases ATPase activity by increasing the rate of dephosphorylation of the phosphorylated ATPase. Stimulation is not seen with other anionic phospholipids; phosphatidic acid decreases ATPase activity in a Mg(2-)-dependent manner.

Phosphatidylinositol 4-phosphate increases the rate of dephosphorylation of the phosphorylated Ca(2+)-ATPase

Incubation of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum with ATP in the absence of Ca2+ leads to phosphorylation of phosphatidylinositol (PtdIns) to phosphatidylinositol 4-phosphate (PtdIns-4P) and to a doubling of ATPase activity. Similarly, reconstitution of the ATPase with mixtures of dioleoylphosphatidylcholine and PtdIns-4P also led to a doubling of activity; ATPase activity increased with increasing PtdIns-4P content, up to 10% beyond which no further increase was observed. Reconstitution with PtdIns had a much smaller effect on activity. Changes in the Ca2+ affinity of the ATPase following incubation with ATP or reconstitution with PtdIns-4P were small. The rates of phosphorylation of the ATPase by ATP and of the Ca2+ transport step were unaffected, but the rate of dephosphorylation of the phosphorylated ATPase increased by a factor of 2 either following incubation with ATP or following reconstitution with PtdIns-4P. Activation of the ATPase led to a decrease in the level of phosphorylation of the ATPase by Pi corresponding to a 10-fold decrease in the equilibrium constant E2PMg/E2PiMg.