Increased activity of calcium leak channels in myotubes of Duchenne human and mdx mouse origin

Dystrophin and the cardiomyocyte membrane cytoskeleton in the healthy and failing heart

The cardiomyocyte membrane cytoskeleton consists of the costameric proteins that mediate force transduction from the cell to the extracellular matrix, and a sub-membrane network composed of dystrophin and associated proteins. Studies of the precise cellular distribution of dystrophin and of the consequences of genetic mutations leading to abnormal expression of the dystrophin molecule, as occurs in Duchenne and Becker's muscular dystrophies, highlight potential functional roles of this sub-membrane protein complex in cardiomyocytes. Detailed investigation of dystrophin distribution using the complementary cell imaging techniques of immunoconfocal microscopy and freeze-fracture cytochemistry at the electron-microscopical level show that, in contrast to rat cardiomyocytes, the dystrophin network in human cardiomyocytes is locally enriched at costameres. Thus located, the dystrophin network appears to have a mechanical role, involving stabilization of the peripheral plasma membrane during the repetitive distortion associated with cardiac contraction and, in the human myocyte, contributing to lateral force-transduction. Evidence from animal models of muscular dystrophy and from investigation of the interactions of the sub-membrane cytoskeleton with other membrane-associated proteins including ion channels, receptors and enzymes, further suggests a role for dystrophin in organization and regulation of membrane domains. The relative preservation of the membrane cytoskeleton in non-dystrophic dilated cardiomyopathy and in ischemic cardiomyopathy, conditions in which the myocyte contractile apparatus and internal desmin-based cytoskeleton are commonly disrupted, emphasizes the vital role of the membrane cytoskeleton in cell survival. Continued cardiomyocyte survival despite loss of contractile protein organization has implications in the potential for reversibility of left ventricular remodeling that can be achieved in the clinical setting.

Mechanisms of stretch-induced muscle damage in normal and dystrophic muscle: role of ionic changes

Muscle damage, characterized by prolonged weakness and delayed onset of stiffness and soreness, is common following contractions in which the muscles are stretched. Stretch-induced damage of this sort is more pronounced in the muscular dystrophies and the profound muscle damage observed in these conditions may involve similar pathways. It has been known for many years that damaged muscles accumulate calcium and that elevating calcium in normal muscles simulates many aspects of muscle damage. The changes in intracellular calcium, sodium and pH following stretched contractions are reviewed and the various pathways which have been proposed to allow ion entry are discussed. One possibility is that TRPC1 (transient receptor potential, canonical), a protein which seems to form both a stretch-activated channel and a store-operated channel, is the main source of Ca(2+) entry. The mechanisms by which the changes in intracellular ions contribute to reduced force production, to increased protein breakdown and to increased membrane permeability are considered. A hypothetical scheme for muscle damage which incorporates these ideas is presented.

Protective effects of Ca2+ handling drugs against abnormal Ca2+ homeostasis and cell damage in myopathic skeletal muscle cells

Deficiency of delta-sarcoglycan (delta-SG), a component of the dystrophin-glycoprotein complex (DGC), causes skeletal muscular dystrophy and cardiomyopathy in BIO14.6 hamsters. Here, we studied the involvement of abnormal Ca2+ homeostasis in muscle degeneration and the protective effect of drugs against Ca2+ handling proteins in vivo as well as in vitro. First, we characterized the properties of cultured myotubes from muscles of normal and BIO14.6 hamsters (30-60 days old). While there were no apparent differences in the levels of expression of various Ca2+ handling proteins (L-type Ca2+ channel, ryanodine receptor, SR-Ca2+ ATPase, and Na+/Ca2+ exchanger), muscle-specific proteins (contractile actin and acetylcholine receptor), or DGC member proteins except SGs, BIO14.6 myotubes showed a high degree of susceptibility to mechanical stressors, such as cyclic stretching and hypo-osmotic stress as compared to normal myotubes, as evidenced by marked increases in creatine phosphokinase (CK) release and bleb formation. BIO14.6 myotubes showed abnormal Ca2+ homeostasis characterized by elevated cytosolic Ca2+ concentration, frequent Ca2+ oscillation, and increased 45Ca2+ uptake. These abnormal Ca2+ events and CK release were significantly prevented by Ca2+ handling drugs, tranilast, diltiazem, and FK506. The calpain inhibitor E64 prevented CK release, but not 45Ca2+ uptake. Some of these drugs (tranilast, diltiazem, and FK506) also exerted a significant protective effect for muscle degeneration in BIO14.6 hamsters and mdx mice in vivo. These observations suggest that elevated Ca2+ entry through sarcolemmal Ca2+ channels predominantly contributes to muscle degeneration and that the drugs tested here may have novel therapeutic potential against muscular dystrophy.

Glycopeptides related to beta-endorphin adopt helical amphipathic conformations in the presence of lipid bilayers

A series of glycosylated endorphin analogues designed to penetrate the blood-brain barrier (BBB) have been studied by circular dichroism and by 2D-NMR in the presence of water; TFE/water; SDS micelles; and in the presence of both neutral and anionic bicelles. In water, the glycopeptides showed only nascent helix behavior and random coil conformations. Chemical shift indices and nuclear Overhauser effects (NOE) confirmed helices in the presence of membrane mimics. NOE volumes provided distance constraints for molecular dynamics calculations used to provide detailed backbone conformations. In all cases, the glycopeptides were largely helical in the presence of membrane bilayer models (micelles or bicelles). Plasmon waveguide resonance (PWR) studies showed hen egg phosphatidyl choline (PC) bilayers produce amphipathic helices laying parallel to the membrane surface, with dissociation constants (K(D)) in the low nanomolar to micromolar concentration range. Two low-energy states are suggested for the glycosylated endorphin analogues, a flexible aqueous state and a restricted membrane bound state. Strong interactions between the glycopeptide amphipaths and membranes are crucial for penetration of the BBB via an endocytotic mechanism (transcytosis).

Molecular, cellular, and pharmacological therapies for Duchenne/Becker muscular dystrophies

Although the molecular defect causing Duchenne/Becker muscular dystrophy (DMD/BMD) was identified nearly 20 years ago, the development of effective therapeutic strategies has nonetheless remained a daunting challenge. Over the years, a variety of different approaches have been explored in an effort to compensate for the lack of the DMD gene product called dystrophin. This review not only presents some of the most promising molecular, cellular, and pharmacological strategies but also highlights some issues that need to be addressed before considering their implementation. Specifically, we describe current strategies being developed to exogenously deliver healthy copies of the dystrophin gene to dystrophic muscles. We present the findings of several studies that have focused on repairing the mutant dystrophin gene using various approaches. We include a discussion of cell-based therapies that capitalize on the use of myoblast or stem cell transfer. Finally, we summarize the results of several studies that may eventually lead to the development of appropriate drug-based therapies. In this context, we review our current knowledge of the mechanisms regulating expression of utrophin, the autosomal homologue of dystrophin. Given the complexity associated with the dystrophic phenotype, it appears likely that a combinatorial approach involving different therapeutic strategies will be necessary for the appropriate management and eventual treatment of this devastating neuromuscular disease.

Role of basal extracellular Ca2+ entry during 5-HT-induced vasoconstriction of canine pulmonary arteries

1. Measurements of artery contraction, cytosolic [Ca(2+)], and Ca(2+) permeability were made to examine contractile and cytosolic [Ca(2+)] responses of canine pulmonary arteries and isolated cells to 5-hydroxytryptamine (5-HT), and to determine the roles of intracellular Ca(2+) release and extracellular Ca(2+) entry in 5-HT responses. 2. The EC(50) for 5-HT-mediated contractions and cytosolic [Ca(2+)] increases was approximately 10(-7) M and responses were inhibited by ketanserin, a 5-HT(2A)-receptor antagonist. 3. 5-HT induced cytosolic [Ca(2+)] increases were blocked by 20 microM Xestospongin-C and by 2-APB (IC(50)=32 microM inhibitors of InsP(3) receptor activation. 4. 5-HT-mediated contractions were reliant on release of InsP(3) but not ryanodine-sensitive Ca(2+) stores. 5. 5-HT-mediated contractions and cytosolic [Ca(2+)] increases were partially inhibited by 10 microM nisoldipine, a voltage-dependent Ca(2+) channel blocker. 6. Extracellular Ca(2+) removal reduced 5-HT-mediated contractions further than nisoldipine and ablated cytosolic [Ca(2+)] increases and [Ca(2+)] oscillations. Similar to Ca(2+) removal, Ni(2+) reduced cytosolic [Ca(2+)] and [Ca(2+)] oscillations. 7. Mn(2+) quench of fura-2 and voltage-clamp experiments showed that 5-HT failed to activate any significant voltage-independent Ca(2+) entry pathways, including store-operated and receptor-activated nonselective cation channels. Ni(2+) but not nisoldipine or Gd(3+) blocked basal Mn(2+) entry. 8. Voltage-clamp experiments showed that simultaneous depletion of both InsP(3) and ryanodine-sensitive intracellular Ca(2+) stores activates a current with linear voltage dependence and a reversal potential consistent with it being a nonselective cation channel. 5-HT did not activate this current. 9. Basal Ca(2+) entry, rather than CCE, is important to maintain 5-HT-induced cytosolic [Ca(2+)] responses and contraction in canine pulmonary artery.

Relationship between side chain structure and 14-helix stability of beta3-peptides in water

Folded polymers are used in Nature for virtually every vital process. Nonnatural folded polymers, or foldamers, have the potential for similar versatility, and the design and refinement of such molecules is of considerable current interest. Here we report a complete and systematic analysis of the relationship between side chain structure and the 14-helicity of a well-studied class of foldamers, beta(3)-peptides, in water. Our experimental results (1) verify the importance of macrodipole stabilization for maintaining 14-helix structure, (2) provide comprehensive evidence that beta(3)-amino acids branched at the first side chain carbon are 14-helix-stabilizing, (3) suggest a novel role for side chain hydrogen bonding as an additional stabilizing force in beta(3)-peptides containing beta(3)-homoserine or beta(3)-homothreonine, and (4) demonstrate that diverse functionality can be incorporated into a stable 14-helix. Gas- and solution-phase calculations and Monte Carlo simulations recapitulate the experimental trends only in the context of oligomers, yielding insight into the mechanisms behind 14-helix folding. The 14-helix propensities of beta(3)-amino acids differ starkly from the alpha-helix propensities of analogous alpha-amino acids. This contrast informs current models for alpha-helix folding, and suggests that 14-helix folding is governed by different biophysical forces than is alpha-helix folding. The ability to modulate 14-helix structure through side chain choice will assist rational design of 14-helical beta-peptide ligands for macromolecular targets.

Uncontrolled calcium sparks act as a dystrophic signal for mammalian skeletal muscle

Most excitable cells maintain tight control of intracellular Ca(2+) through coordinated interaction between plasma membrane and endoplasmic or sarcoplasmic reticulum. Quiescent sarcoplasmic reticulum Ca(2+) release machinery is essential for the survival and normal function of skeletal muscle. Here we show that subtle membrane deformations induce Ca(2+) sparks in intact mammalian skeletal muscle. Spontaneous Ca(2+) sparks can be reversibly induced by osmotic shock, and participate in a normal physiological response to exercise. In dystrophic muscle with fragile membrane integrity, stress-induced Ca(2+) sparks are essentially irreversible. Moreover, moderate exercise in mdx muscle alters the Ca(2+) spark response. Thus, membrane-deformation-induced Ca(2+) sparks have an important role in physiological and pathophysiological regulation of Ca(2+) signalling, and uncontrolled Ca(2+) spark activity in connection with chronic activation of store-operated Ca(2+) entry may function as a dystrophic signal in mammalian skeletal muscle.

gamma-Sarcoglycan deficiency increases cell contractility, apoptosis and MAPK pathway activation but does not affect adhesion

The functions of gamma-sarcoglycan (gammaSG) in normal myotubes are largely unknown, however gammaSG is known to assemble into a key membrane complex with dystroglycan and its deficiency is one known cause of limb-girdle muscular dystrophy. Previous findings of apoptosis from gammaSG-deficient mice are extended here to cell culture where apoptosis is seen to increase more than tenfold in gammaSG-deficient myotubes compared with normal cells. The deficient myotubes also exhibit an increased contractile prestress that results in greater shortening and widening when the cells are either lightly detached or self-detached. However, micropipette-forced peeling of single myotubes revealed no significant difference in cell adhesion. Consistent with a more contractile phenotype, acto-myosin striations were more prominent in gammaSG-deficient myotubes than in normal cells. An initial phosphoscreen of more than 12 signaling proteins revealed a number of differences between normal and gammaSG(-/-) muscle, both before and after stretching. MAPK-pathway proteins displayed the largest changes in activation, although significant phosphorylation also appeared for other proteins linked to hypertension. We conclude that gammaSG normally moderates contractile prestress in skeletal muscle, and we propose a role for gammaSG in membrane-based signaling of the effects of prestress and sarcomerogenesis.

Structure, Assembly, and Topology of the G185R Mutant of the Fourth Transmembrane Domain of Divalent Metal Transporter

The mammalian iron transporter, divalent metal transporter (DMT1), is a 12-transmembrane domain integral protein, responsible for dietary iron uptake in the duodenum and iron acquisition from transferrin in peripheral tissues. Two disease-causing mutants in animals have been found and attributed to the same missense mutation (G185R), which occurs within the putative transmembrane domain 4 (TM4) of DMT1. We have characterized a synthetic 24-mer peptide, corresponding to the sequence of the TM4 of DMT1 with G185R mutation using circular dichroism (CD) and NMR spectroscopy and show that the G185R peptide assumes mainly alpha-helical conformations in various membrane-mimetic environments. Solution structures derived from NMR and molecular dynamics/simulated annealing calculations demonstrate that the peptide exhibits a highly defined alpha-helix in its middle portion, flanked by a highly flexible N-terminus and a relatively ordered C-terminus. Both the folding and location of the C-terminus in SDS micelles are regulated by pH values. Paramagnetic broadening on peptide NMR signals by spin-labeled 5- and 16-doxylstearic acids and Mn(2+) ion suggests that both the N-terminus and the helical region of the peptide are embedded in SDS micelles. Surprisingly, self-association of the peptides for both the wild type and the G185R mutant studied by CD, electrospray ionization mass spectrometry, and NMR diffusion-ordered spectroscopy demonstrated that mutation of the Gly185 to a bulky and positively charged arginine causes a different self-assembly of the peptide, e.g., from a trimer to a hexamer, which implies that the quaternary structure of integral DMT1 may be crucial for its function in vivo.

Effects of stretch-activated channel blockers on [Ca2+]i and muscle damage in the mdx mouse

The mdx mouse lacks dystrophin and is a model of human Duchenne muscular dystrophy. Single mdx muscle fibres were isolated and subjected to a series of stretched (eccentric) contractions while measuring intracellular calcium concentration ([Ca(2+)](i)) with fluo-3 and confocal microscopy. Following the stretched contractions there was a slow rise in resting [Ca(2+)](i) and after 30 min both the [Ca(2+)](i) during a tetanus (tetanic [Ca(2+)](i)) and the tetanic force were reduced. Two blockers of stretch-activated channels, streptomycin and the spider venom toxin GsMTx4, prevented the rise of resting [Ca(2+)](i) and partially prevented the decline of tetanic [Ca(2+)](i) and force. Reducing extracellular calcium to zero also prevented the rise in resting [Ca(2+)](i) and prevented some of the decline in tetanic [Ca(2+)](i) and force. Patch-clamping experiments identified a stretch-activated channel in both wild-type and mdx myotubes which was blocked by GsMTx4. These data suggest that blockers of stretch-activated channels can ameliorate the force reduction following stretched contractions by reducing the influx of Ca(2+) into the muscle. We therefore tested whether in intact mdx mice streptomycin, added to the drinking water, was capable of reducing muscle damage. mdx mice show a period of muscle damage from 20 to 40 days of life and fibres which regenerate from this damage display central nuclei. We measured the frequency of central nuclei in control mdx mice compared to streptomycin-treated mdx mice and showed that the incidence of central nuclei was significantly reduced by streptomycin treatment. This result suggests that blockers of stretch-activated channels may protect against muscle damage in the intact mdx mouse.

Laminin-alpha1 globular domains 3 and 4 induce heterotrimeric G protein binding to alpha-syntrophin's PDZ domain and alter intracellular Ca2+ in muscle

Alpha-syntrophin is a component of the dystrophin glycoprotein complex (DGC). It is firmly attached to the dystrophin cytoskeleton via a unique COOH-terminal domain and is associated indirectly with alpha-dystroglycan, which binds to extracellular matrix laminin. Syntrophin contains two pleckstrin homology (PH) domains and one PDZ domain. Because PH domains of other proteins are known to bind the betagamma-subunits of the heterotrimeric G proteins, whether this is also a property of syntrophin was investigated. Isolated syntrophin from rabbit skeletal muscle binds bovine brain Gbetagamma-subunits in gel blot overlay experiments. Laminin-1-Sepharose or specific antibodies against syntrophin, alpha- and beta-dystroglycan, or dystrophin precipitate a complex with Gbetagamma from crude skeletal muscle microsomes. Bacterially expressed syntrophin fusion proteins and truncation mutants allowed mapping of Gbetagamma binding to syntrophin's PDZ domain; this is a novel function for PDZ domains. When laminin-1 is bound, maximal binding of Gsalpha and Gbetagamma occurs and active Gsalpha, measured as GTP-gamma35S bound, decreases. Because intracellular Ca2+ is elevated in Duchenne muscular dystrophy and Gsalpha is known to activate the dihydropyridine receptor Ca2+ channel, whether laminin also altered intracellular Ca2+ was investigated. Laminin-1 decreases active (GTP-gammaS-bound) Gsalpha, and the Ca2+ channel is inhibited by laminin-1. The laminin alpha1-chain globular domains 4 and 5 region, the region bound by DGC alpha-dystroglycan, is sufficient to cause an effect, and an antibody that specifically blocks laminin binding to alpha-dystroglycan inhibits Gbeta binding by syntrophin in C2C12 myotubes. These observations suggest that DGC is a matrix laminin, G protein-coupled receptor.

Improvement of calcium handling and changes in calcium-release properties after mini- or full-length dystrophin forced expression in cultured skeletal myotubes

Dystrophin is a cytoskeletal protein normally expressed underneath the sarcolemma of muscle fibers. The lack of dystrophin in Duchenne muscular Dystrophy (DMD) muscles results in fiber necrosis, which was proposed to be mediated by chronic calcium mishandling. The extensive comparison of dystrophic cells from human or mdx mice with normal muscles have suggested that the lack of dystrophin may alter the resting calcium permeability and steady-state levels of calcium, but this latter observation remains controversial. It is also not clear, whether calcium mishandling is resulting from the dystrophic process or if dystrophin can directly regulate calcium handling in muscle cells. This prompted us to determine if transfection of full-length dystrophin or Becker Muscular Dystrophy (BMD) minidystrophin, a candidate for viral-mediated gene therapy, could change calcium handling properties. We took advantage of specific properties of Sol8 cell line showing the absence of dystrophin expression together with a drastic calcium mishandling. Here, we show that full-length dystrophin allowed the recovery of a low resting intracellular-free calcium concentration together with lower calcium transients. We also show for the first time that stable expression of minidystrophin was able to restore normal calcium handling in Sol8 myotubes through a better control of steady-state levels, calcium transients, and subcellular calcium events. It suggests that dystrophin could play a regulatory role on calcium homeostasis apparatus and that functional links exist between calcium signaling and cytoskeleton.

Cloning of cDNA encoding a regeneration-associated muscle protease whose expression is attenuated in cell lines derived from Duchenne muscular dystrophy patients

In the dystrophin-mutant mdx mouse, an animal model for Duchenne muscular dystrophy (DMD), damaged skeletal muscles are efficiently regenerated and thus the animals thrive. The phenotypic differences between DMD patients and the mdx mice suggest the existence of factors that modulate the muscle wasting in the mdx mice. To identify these factors, we searched for mRNAs affected by the mdx mutation by using cDNA microarrays with newly established skeletal muscle cell lines from mdx and normal mice. We found that in the mdx muscle cell line, 12 genes, including L-arginine:glycine amidinotransferase and thymosin beta4, are up-regulated, whereas 7 genes, including selenoprotein P and a novel regeneration-associated muscle protease (RAMP), are down-regulated. Northern blot analysis and in situ hybridization revealed that RAMP mRNA is predominantly expressed in normal skeletal muscle and brain, and its production is enhanced in the regenerating area of injured skeletal muscle in mice. RAMP expression was much lower in individual muscle cell lines derived from biopsies of six DMD patients compared to a normal muscle cell line. These results suggest that RAMP may play a role in the regeneration of skeletal muscle and that its down-regulation could be involved in the progression of DMD in humans.

Effect of Side Chains on Turns and Helices in Peptides of β3-Aminoxy Acids

We have investigated, using NMR, IR, and CD spectroscopy and X-ray crystallography, the conformational properties of peptides 1-10 of beta(3)-aminoxy acids (NH(2)OCHRCH(2)COOH) having different side chains on the beta carbon atom (e.g., R = Me, Et, COOBn, CH(2)CH(2)CH=CH(2), i-Bu, i-Pr). The beta N-O turns and beta N-O helices that involve a nine-membered-ring intramolecular hydrogen bond between NH(i)(+2) and CO(i), which have been found previously in peptides of beta(2,2)-aminoxy acids (NH(2)OCH(2)CMe(2)COOH), are also present in those beta(3)-aminoxy peptides. X-ray crystal structures and NMR spectral analysis reveal that, in the beta N-O turns and beta N-O helices induced by beta(3)-aminoxy acids, the N-O bond could be either anti or gauche to the C(alpha)-C(beta) bond depending on the size of the side chain; in contrast, only the anti conformation was found in beta(2,2)-aminoxy peptides. Both diamide 1 and triamide 9 exist in different conformations in solution and in the solid state: parallel sheet structures in the solid state and predominantly beta N-O turn and beta N-O helix conformations in nonpolar solvents. Theoretical studies on a series of model diamides rationalize very well the experimentally observed conformational features of these beta(3)-aminoxy peptides.

Mini-dystrophin restores L-type calcium currents in skeletal muscle of transgenic mdx mice

L-type calcium currents (iCa) were recorded using the two-microelectrode voltage-clamp technique in single short toe muscle fibres of three different mouse strains: (i) C57/SV129 wild-type mice (wt); (ii) mdx mice (an animal model for Duchenne muscular dystrophy; and (iii) transgenically engineered mini-dystrophin (MinD)-expressing mdx mice. The activation and inactivation properties of iCa were examined in 2- to 18-month-old animals. Ca2+ current densities at 0 mV in mdx fibres increased with age, but were always significantly smaller compared to age-matched wild-type fibres. Time-to-peak (TTP) of iCa was prolonged in mdx fibres compared to wt fibres. MinD fibres always showed similar TTP and current amplitudes compared to age-matched wt fibres. In all three genotypes, the voltage-dependent inactivation and deactivation of iCa were similar. Intracellular resting calcium concentration ([Ca2+]i) and the distribution of dihydropyridine binding sites were also not different in young animals of all three genotypes, whereas iCa was markedly reduced in mdx fibres. We conclude, that dystrophin influences L-type Ca2+ channels via a direct or indirect linkage which may be disrupted in mdx mice and may be crucial for proper excitation-contraction coupling initiating Ca2+ release from the sarcoplasmic reticulum. This linkage seems to be fully restored in the presence of mini-dystrophin.

Targeting the Immune System to Improve Ventilatory Function in Muscular Dystrophy

Skeletal muscle is a unique tissue whose function is dependent in great part on its ultrastructure. Repeated intense muscular contractions, especially those resulting in muscle lengthening, can lead to alterations in muscle structure (i.e., muscle damage) and subsequent decline in contractile force. The damage-induced decline in contractile force can have a significant impact on exercise performance during an athletic performance. In some disease conditions such as Duchenne muscular dystrophy (DMD), the muscles are more vulnerable to contraction-induced damage than normal muscle. In the case of the respiratory muscles, for example, the diaphragm, the consequences of muscle weakness secondary to damage are profound in that respiratory failure leading to premature death often ensues. In normal skeletal muscle, damage is followed by an inflammatory response involving multiple cell types that subsides after several days. This transient inflammatory response is a normal homeostatic reaction to muscle damage. In contrast, a persistent inflammatory response is observed in dystrophic skeletal muscle that leads to an altered extracellular environment, including an increased presence of inflammatory cells (e.g., macrophages) and elevated levels of various inflammatory cytokines (e.g., TNF-alpha, TGF-beta). The signals that lead to successful muscle repair in healthy muscle may promote muscle wasting and fibrosis in dystrophic muscle. Preliminary data indicate that immunosuppression in dystrophic (mdx) mice has beneficial effects on some indices of muscle dysfunction, thereby indicating that targeted immunosuppression may offer some promise in delaying the pathological progression of this insidious muscular disease.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Extrajunctional resting Ca2+ influx is not increased in a severely dystrophic expiratory muscle (triangularis sterni) of the mdx mouse

Freshly isolated adult mdx and nondystrophic (C57B110SnJ) muscle fibers were used to examine the potential role of resting Ca2+ influx in the pathogenesis of Duchenne and related dystrophies. Microfluorimetric determinations of resting divalent cation influx were obtained from undissociated intact muscle fibers in the triangularis sterni (TS), a thin expiratory muscle. Morphological evidence indicated severe dystrophic alterations in the mdx TS at 5 months, and a pronounced loss of fibers with connective tissue infiltration in older animals. To examine resting Ca2+ influx, fibers were loaded with FURA PE3 and the rate of quenching of intracellular signal following the extracellular addition of Mn2+ was determined from extrajunctional regions. There was no significant difference in quench rate between nondystrophic and mdx TS fibers. These results indicate that severe dystrophic pathology in the absence of dystrophin is not due to generalized increases in resting Ca2+ influx.

A novel mechanism of myocyte degeneration involving the Ca2+-permeable growth factor-regulated channel

Disruption of the dystrophin-glycoprotein complex caused by genetic defects of dystrophin or sarcoglycans results in muscular dystrophy and/or cardiomyopathy in humans and animal models. However, the key early molecular events leading to myocyte degeneration remain elusive. Here, we observed that the growth factor-regulated channel (GRC), which belongs to the transient receptor potential channel family, is elevated in the sarcolemma of skeletal and/or cardiac muscle in dystrophic human patients and animal models deficient in dystrophin or delta-sarcoglycan. However, total cell GRC does not differ markedly between normal and dystrophic muscles. Analysis of the properties of myotubes prepared from delta-sarcoglycan-deficient BIO14.6 hamsters revealed that GRC is activated in response to myocyte stretch and is responsible for enhanced Ca2+ influx and resultant cell damage as measured by creatine phosphokinase efflux. We found that cell stretch increases GRC translocation to the sarcolemma, which requires entry of external Ca2+. Consistent with these findings, cardiac-specific expression of GRC in a transgenic mouse model produced cardiomyopathy due to Ca2+ overloading, with disease expression roughly parallel to sarcolemmal GRC levels. The results suggest that GRC is a key player in the pathogenesis of myocyte degeneration caused by dystrophin-glycoprotein complex disruption.

Pharmacological strategies for muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal, genetic disorder whose relentless progression underscores the urgency for developing a cure. Although Duchenne initiated clinical trials roughly 150 years ago, therapies for DMD remain supportive rather than curative. A paradigm shift towards developing rational therapeutic strategies occurred with identification of the DMD gene. Gene- and cell-based therapies designed to replace the missing gene and/or dystrophin protein have achieved varying degrees of success. However, pharmacological strategies not designed to replace dystrophin per se appear promising, and can circumvent many hurdles hampering gene- and cell-based therapy. Here, we will review present pharmacological strategies, in particular those dealing with functional substitution of dystrophin by utrophin and enhancing muscle progenitor commitment by myostatin blockade, with a view toward facilitating drug discovery for DMD.

The energy landscape of unsolvated peptides: the role of context in the stability of alanine/glycine helices

Ion mobility measurements have been used to examine the conformations present for unsolvated Ac-(AG)(7)A+H(+) and (AG)(7)A+H(+) peptides (Ac = acetyl, A = alanine, and G = glycine) over a broad temperature range (100-410 K). The results are compared to those recently reported for Ac-A(4)G(7)A(4)+H(+) and A(4)G(7)A(4)+H(+), which have the same compositions but different sequences. Ac-(AG)(7)A+H(+) shows less conformational diversity than Ac-A(4)G(7)A(4)+H(+); it is much less helical than Ac-A(4)G(7)A(4)+H(+) at the upper end of the temperature range studied, and at low temperatures, one of the two Ac-A(4)G(7)A(4)+H(+) features assigned to helical conformations is missing for Ac-(AG)(7)A+H(+). Molecular dynamics simulations suggest that the different conformational preferences are not due to differences in the stabilities of the helical states, but differences in the nonhelical states: it appears that Ac-(AG)(7)A+H(+) is more flexible and able to adopt lower energy globular conformations (compact random looking three-dimensional structures) than Ac-A(4)G(7)A(4)+H(+). The helix to globule transition that occurs for Ac-(AG)(7)A+H(+) at around 250-350 K is not a direct (two-state) process, but a creeping transition that takes place through at least one and probably several intermediates.

Dual wavelength parametric test of two-state models for circular dichroism spectra of helical polypeptides: anomalous dichroic properties of alanine-rich peptides

A two-state helix-coil model underlies all calculations of fractional helicities FH from CD spectra of helical polypeptides. The presence of an isodichroic point near 203 nm is widely assumed to validate this model, but is shown here to provide inadequate validation for alanine-rich peptides. A parametric correlation with constant slope B between CD ellipticities at a pair of wavelengths is introduced as a more rigorous two-state test. Correlations of temperature-dependent [theta](222) vs [theta](208) values are reported for a variety of peptides. Constant slopes B are observed for literature CD data obtained from fragments of helical proteins and dimeric helical coiled coils, but parametric correlations of CD data for alanine-rich peptides consistently exhibit anomalous concave upward curvature, characterized by local slopes that are linearly temperature dependent. Low-temperature CD studies together with parametric correlations at a series of wavelengths demonstrate that the curvature anomaly is maximal at 222 nm and localized in the 215-230 nm wavelength region. Precedented structural variation of the phi, psi dihedral angles of the alpha-helix is suggested as a possible explanation. For the important case of alanine-rich peptides, experiments are proposed that may yield temperature corrections for [theta](222) and permit reliable calculations of FH from [theta](222) values.

Dihydropyridines as inhibitors of capacitative calcium entry in leukemic HL-60 cells

A series of 1,4-dihydropyridines (DHPs) were investigated as inhibitors of capacitative calcium influx through store-operated calcium (SOC) channels. Such channels activate after ATP-elicited release of inositol trisphosphate (IP(3))-sensitive calcium stores in leukemia HL-60 cells. The most potent DHPs were those containing a 4-phenyl group with an electron-withdrawing substituent, such as m- or p-nitro- or m-trifluoromethyl (IC(50) values: 3-6 microM). Benzyl esters, corresponding to the usual ethyl/methyl esters of the DHPs developed as L-type calcium channel blockers, retained potency at SOC channels, as did N-substituted DHPs. N-Methylation reduced by orders of magnitude the potency at L-type channels resulting in DHPs nearly equipotent at SOC and L-type channels. DHPs with N-ethyl, N-allyl, and N-propargyl groups also had similar potencies at SOC and L-type channels. Replacement of the usual 6-methyl group of DHPs with larger groups, such as cyclobutyl or phenyl, eliminated activity at the SOC channels; such DHPs instead elicited formation of inositol phosphates and release of IP(3)-sensitive calcium stores. Other DHPs also caused a release of calcium stores, but usually at significantly higher concentrations than those required for the inhibition of capacitative calcium influx. Certain DHPs appeared to cause an incomplete blockade of SOC channel-dependent elevations of calcium, suggesting the presence of more than one class of such channels in HL-60 cells. N-Methylnitrendipine (IC(50) 2.6 microM, MRS 1844) and N-propargylnifrendipine (IC(50) 1.7 microM, MRS 1845) represent possible lead compounds for the development of selective SOC channel inhibitors.

Skeletal muscle metabolism in Duchenne muscular dystrophy (DMD): an in-vitro proton NMR spectroscopy study

The metabolic differences in the skeletal muscle of patients with Duchenne muscular dystrophy (DMD) and normal subjects (controls) were investigated using in-vitro high-resolution proton NMR spectroscopy. In all, 56 metabolites were unambiguously identified in the perchloric acid extract of muscle tissue using one- and two-dimensional NMR. The concentrations of glycolytic substrate glucose (Glc; p < 0.05), gluconeogenic amino acids such as glutamine (Gln; p < 0.05) and alanine (Ala; p < 0.05) and the glycolytic product lactate (Lac; p < 0.05) were statistically significantly lower in DMD patients as compared to controls. A significant reduction in the concentrations of total creatine (TCr; p < 0.05), glycerophosphoryl choline + phosphoryl choline + carnitine (GPC/PC/Car; p < 0.05), choline (Cho; p < 0.05) and acetate (Ace; p < 0.05) was also observed in these patients. Decrease in the level of glucose may be attributed to the reduction in the concentrations of gluconeogenic substrates or membrane abnormalities in degenerated muscle of DMD patients. Lower levels of choline containing compounds indicate membrane abnormalities. Decrease in the concentration of lactate in the muscle of DMD patients may be due to the reduction in anaerobic glycolytic activity or lower substrate concentration. The decrease in the concentration of acetate may reflect reduced transport of fatty acids into mitochondria due to decreased concentration of carnitine in DMD patients. Kreb's cycle intermediate alpha-ketoglutarate was observed only in the diseased muscle, which is suggestive of predominant oxidative metabolism for energy generation.

Molecular pathophysiology of myofiber injury in deficiencies of the dystrophin-glycoprotein complex

Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin, a 427 kd protein normally found at the cytoplasmic face of the sarcolemma. In normal muscle, dystrophin is associated with a multimolecular glycoprotein complex. Primary mutations in the genes encoding members of this glycoprotein complex are also associated with muscular dystrophy. The dystrophin-glycoprotein complex provides a physical linkage between the internal cytoskeleton of myofibers and the extracellular matrix, but the precise functions of the dystrophin-glycoprotein complex remain uncertain. In this review, five potential pathogenetic mechanisms implicated in the initiation of myofiber injury in dystrophin-glycoprotein complex deficiencies are discussed: (1) mechanical weakening of the sarcolemma, (2) inappropriate calcium influx, (3) aberrant cell signaling, (4) increased oxidative stress, and (5) recurrent muscle ischemia. Particular emphasis is placed on the multifunctional nature of the dystrophin-glycoprotein complex and the fact that the above mechanisms are in no way mutually exclusive and may interact with one another to a significant degree.

Pharmacological control of cellular calcium handling in dystrophic skeletal muscle

Duchenne muscular dystrophy arises due to the lack of the cytoskeletal protein dystrophin. In Duchenne muscular dystrophy muscle, the lack of dystrophin is accompanied by alterations in the dystrophin-glycoprotein complex. We and others have found that the absence of dystrophin in cells of the Duchenne muscular dystrophy animal model, the mdx mouse, leads to elevated Ca(2+) influx and cytosolic Ca(2+) concentrations when exposed to stress. We have also shown that alpha-methylprednisolone, the only drug used successfully in the therapy of Duchenne muscular dystrophy, and creatine lowered cytosolic Ca(2+) levels in mdx myotubes. It is likely that chronic elevation of [Ca(2+)] in the cytosol in response to stress is an initiating event for apoptosis and/or necrosis in Duchenne muscular dystrophy or mdx muscle and that alterations in mitochondrial function and metabolism are involved. Other cellular signalling pathways (e.g. nitric oxide) might also be affected.

Ca(2+) regulation in differentiating lens cells in culture

In the ocular lens, cataract formation is associated with an elevated intracellular Ca(2+) concentration (Ca(2+)(i)) resulting from the loss of lens cell Ca(2+) regulation. The mechanisms regulating Ca(2+)(i) have been characterized previously in lens epithelial cells, but have not been well characterized in the more differentiated lens fiber cells. The mechanisms regulating Ca(2+)(i) in clusters of fiber-like cells (lentoids) in a sheep lens primary cell culture system in which the epithelial cells differentiate into enlarged fiber-like cells were investigated. Only approximately 50% of the lentoids responded to thapsigargin and/or agonists (ATP and epinephrine), compared to>95% of the epithelial cells. Remarkably, most (90%) lentoids exhibited a resting cytosolic Ca(2+)(i) that was approximately three-fold greater than that in epithelial cells (approximately 100n M). This elevated resting cytosolic Ca(2+)(i) was not affected by thapsigargin treatment, but decreased upon removal of extracellular Ca(2+) or addition of the Ca(2+) channel blocker Gd(3+) (5mM ). These results suggest that a plasma membrane Ca(2+) channel is more active in lentoids than in epithelial cells. Indeed, when plasma membrane cation channel activity was monitored by Mn(2+) influx and quenching of fura-2 fluorescence, quenching was faster in lentoids than epithelial cells. Following thapsigargin treatment, capacitative Ca(2+) entry was activated in epithelial cells but not lentoids. In conclusion, during differentiation in primary cell culture, lens cells lose their ability to respond to agonists and exhibit an elevated resting Ca(2+)(i) that was dependent on the activation of a Ca(2+) influx pathway. The results of this study support the possibility that a sustained elevation in resting Ca(2+)(i) is one of the factors controlling lens cell differentiation, possibly by triggering events such as calpain activation.

Hypotonic medium increases calcium permeant channels activity in human normal and dystrophic myotubes

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin and an elevated intracellular calcium level. Single-channel recordings were performed with the cell-attached configuration of the patch-clamp technique. The present study shows, on human co-cultured normal and dystrophic muscle cells, the evidence for an increased activity of calcium permeant cationic mechano-sensitive channels under hypotonic medium stimulation. This activity was particularly enhanced in DMD cells. The hypotonic medium induced drastic changes in the single-channel activity characteristics, which are: a large increase of the calcium over potassium permeability ratio; and a great enhancement of the quantity of current crossing through these channels. These channels could contribute to a significant calcium entry, which could participate in the abnormal calcium homeostasis observed in DMD muscle.

The energy landscape of unsolvated peptides: helix formation and cold denaturation in Ac-A4G7A4 + H+

Ion mobility measurements and molecular dynamics simulations were performed for unsolvated A4G7A4 + H+ and Ac-A4G7A4 + H+ (Ac = acetyl, A = alanine, G = glycine) peptides. As expected, A4G7A4 + H+ adopts a globular conformation (a compact, random-looking, three-dimensional structure) over the entire temperature range examined (100-410 K). Ac-A4G7A4 + H+ on the other hand is designed to have a flat energy landscape with a marginally stable helical state. This peptide shows at least four different conformations at low temperatures (<230 K). The two conformations with the largest cross sections are attributed to - and partial -helices, while the one with the smallest cross section is globular. The other main conformation may be partially helical. Ac-A4G7A4 + H+ becomes predominantly globular at intermediate temperatures and then becomes more helical as the temperature is raised further. This unexpected behavior may be due to the helix having a higher vibrational entropy than the globular state, as predicted by some recent calculations (Ma, B.; Tsai, C.-J.; Nussinov, R. Biophys. J. 2000, 79, 2739-2753).

Function and genetics of dystrophin and dystrophin-related proteins in muscle

The X-linked muscle-wasting disease Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin. There is currently no effective treatment for the disease; however, the complex molecular pathology of this disorder is now being unravelled. Dystrophin is located at the muscle sarcolemma in a membrane-spanning protein complex that connects the cytoskeleton to the basal lamina. Mutations in many components of the dystrophin protein complex cause other forms of autosomally inherited muscular dystrophy, indicating the importance of this complex in normal muscle function. Although the precise function of dystrophin is unknown, the lack of protein causes membrane destabilization and the activation of multiple pathophysiological processes, many of which converge on alterations in intracellular calcium handling. Dystrophin is also the prototype of a family of dystrophin-related proteins, many of which are found in muscle. This family includes utrophin and alpha-dystrobrevin, which are involved in the maintenance of the neuromuscular junction architecture and in muscle homeostasis. New insights into the pathophysiology of dystrophic muscle, the identification of compensating proteins, and the discovery of new binding partners are paving the way for novel therapeutic strategies to treat this fatal muscle disease. This review discusses the role of the dystrophin complex and protein family in muscle and describes the physiological processes that are affected in Duchenne muscular dystrophy.

Mdx myotubes have normal excitability but show reduced contraction-relaxation dynamics

The pathogenesis of Duchenne muscular dystrophy (DMD), characterised by lack of the cytoskeletal protein dystrophin, is not completely understood. An early event in the degenerative process of DMD muscle could be a rise in cytosolic calcium concentration. In order to investigate whether this leads to alterations of contractile behaviour, we studied the excitability and contractile properties of cultured myotubes from control (C57BL/10) and mdx mice, an animal model for DMD. The myotubes were stimulated electrically and their motion was recorded photometrically. No significant differences were found between control and mdx myotubes with respect to the following parameters: chronaxy and rheobase (0.33 +/- 0.03 ms and 23 +/- 4 V vs. 0.39 +/- 0.07 ms and 22 +/- 2 V for C57 and mdx myotubes, respectively), tetanisation frequency (a similar distribution pattern was found between 5 and 30 Hz), fatigue during tetanus (found in 35% of both types of myotubes) and post-tetanic contracture. In contrast, contraction and relaxation times were longer (P < 0.005) in mdx (36 +/- 2 and 142 +/- 13 ms, respectively) than in control myotubes (26 +/- 1 and 85 +/- 9 ms, respectively). Together with our earlier findings, these results suggest a decreased capacity for calcium removal in mdx cells leading, in particular, to alterations of muscle relaxation.

Drastic reduction of calsequestrin-like proteins and impaired calcium binding in dystrophic mdx muscle

Although the reduction in dystrophin-associated glycoproteins is the primary pathophysiological consequence of the deficiency in dystrophin, little is known about the secondary abnormalities leading to x-linked muscular dystrophy. As abnormal Ca(2+) handling may be involved in myonecrosis, we investigated the fate of key Ca(2+) regulatory membrane proteins in dystrophic mdx skeletal muscle membranes. Whereas the expression of the ryanodine receptor, the dihydropyridine receptor, the Ca(2+)-ATPase, and calsequestrin was not affected, a drastic decline in calsequestrin-like proteins of 150-220 kDa was observed in dystrophic microsomes using one-dimensional immunoblotting, two-dimensional immunoblotting with isoelectric focusing, diagonal two-dimensional blotting technique, and immunoprecipitation. In analogy, overall Ca(2+) binding was reduced in the sarcoplasmic reticulum of dystrophic muscle. The reduction in Ca(2+) binding proteins might be directly involved in triggering impaired Ca(2+) sequestration within the lumen of the sarcoplasmic reticulum. Thus disturbed sarcolemmal Ca(2+) fluxes seem to influence overall Ca(2+) homeostasis, resulting in distinct changes in the expression profile of a subset of Ca(2+) handling proteins, which might be an important factor in the progressive functional decline of dystrophic muscle fibers.

Velocity of actomyosin sliding in vitro is reduced in dystrophic mouse diaphragm

It has recently been suggested that dystrophin deficiency in mdx diaphragm muscle is associated with quantitative changes in the myosin molecular motor. In vitro motility assays were used to study the kinetics of actomyosin interactions between purified actin filaments and myosin molecules. Monomeric myosin was obtained from the diaphragm and limb (semitendinosus) muscles of 9-mo-old male mdx (mdx) and age-matched control mice. The sliding velocity (vo, microm/s) of fluorescent-labeled actin filaments moving over a myosin-coated surface (40 microg/ml) was measured. In diaphragm, vo was significantly slower in mdx than in control mice (1.2 +/- 0.1 microm s(-1) versus 1.9 +/- 0.1 microm s(-1), p < 0.001). Conversely, there was no significant difference in vo between control and mdx semitendinous muscles (2.4 +/- 0.1 microm s(-1) versus 2.5 +/- 0.1 micro(-1)). As compared with control mice, mdx diaphragm exhibited a shift from IIX-MHC to IIA-MHC (p < 0.001) and a reduction in IIB-MHC (p < 0.01). Semitendinous muscle from control and mdx mice contained almost exclusively type IIB MHC. Our results are in good agreement with the proposal that myosin is altered in dystrophic mouse diaphragm.

The dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies

Mutations of different components of the dystrophin-glycoprotein complex (DGC) cause muscular dystrophies that vary in terms of severity, age of onset, and selective involvement of muscle groups. Although the primary pathogenetic processes in the muscular dystrophies have clearly been identified as apoptotic and necrotic muscle cell death, the pathogenetic mechanisms that lead to cell death remain to be determined. Studies of components of the DGC in muscle and in nonmuscle tissues have revealed that the DGC is undoubtedly a multifunctional complex and a highly dynamic structure, in contrast to the unidimensional concept of the DGC as a mechanical component in the cell. Analysis of the DGC reveals compelling analogies to two other membrane-associated protein complexes, namely integrins and caveolins. Each of these complexes mediates signal transduction cascades in the cell, and disruption of each complex causes muscular dystrophies. The signal transduction cascades associated with the DGC, like those associated with integrins and caveolins, play important roles in cell survival signaling, cellular defense mechanisms, and regulation of the balance between cell survival and cell death. This review focuses on the functional components of the DGC, highlighting the evidence of their participation in cellular signaling processes important for cell survival. Elucidating the link between these functional components and the pathogenetic processes leading to cell death is the foremost challenge to understanding the mechanisms of disease expression in the muscular dystrophies due to defects in the DGC.

Depletion of Ca2+ in the sarcoplasmic reticulum stimulates Ca2+ entry into mouse skeletal muscle fibres

To examine whether a capacitative Ca2+ entry pathway is present in skeletal muscle, thin muscle fibre bundles were isolated from extensor digitorum longus (EDL) muscle of adult mice, and isometric tension and fura-2 signals were simultaneously measured. The sarcoplasmic reticulum (SR) in the muscle fibres was successfully depleted of Ca2+ by repetitive treatments with high-K+ solutions, initially in the absence and then in the presence of a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor. Depletion of the SR of Ca2+ enabled us for the first time to show convincingly that the vast majority of the voltage-sensitive Ca2+ store overlaps the caffeine-sensitive Ca2+ store in intact fibres from mouse EDL muscle. This conclusion was based on the observation that both high-K+ solution and caffeine failed to cause a contracture in the depleted muscle fibres. The existence of a Ca2+ influx pathway active enough to refill the depleted SR within several minutes was shown in skeletal muscle fibres. Ca2+ entry was sensitive to Ni2+, but resistant to nifedipine and was suppressed by plasma membrane depolarisation. Evidence for store-operated Ca2+ entry was provided by measurements of Mn2+ entry. Significant acceleration of Mn2+ entry was observed only when the SR was severely depleted of Ca2+. The Mn2+ influx, which was blocked by Ni2+ but not by nifedipine, was inwardly rectifying, as is the case with the Ca2+ entry. These results indicate that the store-operated Ca2+ entry is similar to the Ca2+ release-activated Ca2+ channel (CRAC) current described in other preparations.

Stretch-activated cation channels in skeletal muscle myotubes from sarcoglycan-deficient hamsters

Deficiency of delta-sarcoglycan (delta-SG), a component of the dystrophin-glycoprotein complex, causes cardiomyopathy and skeletal muscle dystrophy in Bio14.6 hamsters. Using cultured myotubes prepared from skeletal muscle of normal and Bio14.6 hamsters (J2N-k strain), we investigated the possibility that the delta-SG deficiency may lead to alterations in ionic conductances, which may ultimately lead to myocyte damage. In cell-attached patches (with Ba(2+) as the charge carrier), an approximately 20-pS channel was observed in both control and Bio14.6 myotubes. This channel is also permeable to K(+) and Na(+) but not to Cl(-). Channel activity was increased by pressure-induced stretch and was reduced by GdCl(3) (>5 microM). The basal open probability of this channel was fourfold higher in Bio14.6 myotubes, with longer open and shorter closed times. This was mimicked by depolymerization of the actin cytoskeleton. In intact Bio14.6 myotubes, the unidirectional basal Ca(2+) influx was enhanced compared with control. This Ca(2+) influx was sensitive to GdCl(3), signifying that stretch-activated cation channels may have been responsible for Ca(2+) influx in Bio14.6 hamster myotubes. These results suggest a possible mechanism by which cell damage might occur in this animal model of muscular dystrophy.

Helix unfolding in unsolvated peptides

The conformations of unsolvated Ac-K(AGG)(5)+H(+) and Ac-(AGG)(5)K+H(+) peptides (Ac = acetyl, A = alanine, G = glycine, and K = lysine) have been examined by ion mobility measurements over a wide temperature range (150-410 K). The Ac-K(AGG)(5)+H(+) peptide remains a globule (a compact, roughly spherical structure) over the entire temperature range, while both an alpha-helix and a globule are found for Ac-(AGG)(5)K+H(+) at low temperature. As the temperature is raised the alpha-helix unfolds. Rate constants for loss of the helix (on a millisecond time scale) have been determined as a function of temperature and yield an Arrhenius activation energy and preexponential factor of 38.2 +/- 1.0 kJ mol(-1) and 6.5 +/- 3.7 x 10(9) s(-1), respectively. The alpha-helix apparently does not unfold directly into the globule, but first converts into a long-lived intermediate which survives to a significantly higher temperature before converting. According to molecular dynamics simulations, there is a partially untwisted helical conformation that has both a low energy and a well-defined geometry. This special structure lies between the helix and globule and may be the long-lived intermediate.

Increased calcium influx is responsible for the sustained mechanical tone in colon from dystrophic (mdx) mice

BACKGROUND & AIMS

Proximal colon from dystrophic mice develops spontaneous tone increment, but the mechanisms involved in its development have not been investigated. This study examined whether alterations in the properties of cell membrane calcium channels and/or sarcoplasmic reticular (SR) Ca2+-adenosine triphosphatase (ATPase) contribute to tone development.

METHODS

Effects of calcium-free solution, nifedipine, pinaverium (calcium channel blockers), and cyclopiazonic acid (CPA; SR Ca2+-ATPase inhibitor) on the contractile activity of colon from mdx and control mice were determined.

RESULTS

Calcium-free solution abolished spontaneous contractions in both preparations, but decreased the tone only in mdx mice. Nifedipine or pinaverium abolished phasic contractions, acting with different sensitivities on the 2 preparations. They also decreased the tone in colons of mdx mice, and Ca2+-free solution did not cause any further loss of tone. CPA, after an early contractile effect, abolished spontaneous contractions in control animals. It did not suppress the contractile activity in mdx mice. CPA inhibited the repletion of intracellular calcium stores in both tissues to the same degree.

CONCLUSIONS

Increased Ca2+ influx through L-type voltage-dependent Ca2+ channels seems to be responsible for the sustained mechanical tone of proximal colon from mdx mice. The mechanisms for sequestering calcium appear to be unaltered.

Identification of altered gene expression in skeletal muscles from Duchenne muscular dystrophy patients

Mutations in the dystrophin gene lead to dystrophin deficiency, which is the cause of Duchenne muscular dystrophy (DMD). This important discovery more than 10 years ago opened a new field for very productive investigations. However, the exact functions of dystrophin are still not fully understood and the complex process leading to subsequent muscle fiber necrosis has not been clearly described; hence there has not yet been any marked improvement in patient treatment. To decipher the molecular mechanisms induced by a lack of dystrophin, we started identifying genes whose expression is altered in DMD skeletal muscles. The approach was based on differential screening of a human muscle cDNA array. Nine genes were found to be up- or downregulated. Our results indicate expression alterations in mitochondrial genes, titin, a muscle transcription factor and three novel genes. First characterizations of these novel genes indicated that two of them have striated muscle tissue specificity.

Alteration in calcium handling at the subcellular level in mdx myotubes

In this study, we have tested the hypothesis that augmented [Ca(2+)] in subcellular regions or organelles, which are known to play a key role in cell survival, is the missing link between Ca(2+) homeostasis alterations and muscular degeneration associated with muscular dystrophy. To this end, different targeted chimeras of the Ca(2+)-sensitive photoprotein aequorin have been transiently expressed in subcellular compartments of skeletal myotubes of mdx mice, the animal model of Duchenne muscular dystrophy. Direct measurements of the [Ca(2+)] in the sarcoplasmic reticulum, [Ca(2+)](sr), show a higher steady state level at rest and a larger drop after KCl-induced depolarization in mdx compared with control myotubes. The peaks in [Ca(2+)] occurring in the mitochondrial matrix of mdx myotubes are significantly larger than in controls upon KCl-induced depolarization or caffeine application. The augmented response of mitochondria precedes the alterations in the Ca(2+) responses of the cytosol and of the cytoplasmic region beneath the membrane, which become significant only at a later stage of myotube differentiation. Taking into account the key role played by mitochondria Ca(2+) handling in the control of cell death, our data suggest that mitochondria are potential targets of impaired Ca(2+) homeostasis in muscular dystrophy.

Calcium homeostasis and cell death in Sol8 dystrophin-deficient cell line in culture

Abnormalities of calcium homeostasis are involved in the process of cell injuries such as Duchenne muscular dystrophy characterized by the absence of the protein dystrophin. But how the absence of dystrophin leads to cytosolic calcium overload is as yet poorly understood. This question has been addressed with skeletal muscle cells from human DMD muscles or mdx mice. Although easier to obtain than human muscles, mdx muscle cells have provided controversial data concerning the resting intracellular calcium level ([Ca2+](i)). This work describes the culture of Sol8 cell line that expresses neither dystrophin nor adhalin, a dystrophin-associated protein. The [Ca2+](i)and intracellular calcium transients induced by different stimuli (acetylcholine, caffeine and high potassium) are normal during the first days of culture. At later stages, calcium homeostasis exhibits drastic alterations with a breaking down of the calcium responses and a large [Ca2+](i)elevation. Concomitantly, Sol8 cells exhibit morphological signs of cell death like cytoplasmic shrinkage and incorporation of propidium iodide. Cell death could be significantly reduced by blocking the activity of calpains, a type of calcium-regulated proteases. These results suggest that Sol8 cell line provides an alternative model of dystrophin-deficient skeletal muscle cells for which a clear disturbance of the calcium homeostasis is observed in culture in association with calpain-dependent cell death. It is shown that transfection with a plasmid cDNA permits the forced expression of dystrophin in Sol8 myotubes as well as a correct sorting of the protein. This approach could be used to explore possible interactions between dystrophin deficiency, calcium homeostasis alteration, and dystrophic cell death.

Cationic channels in normal and dystrophic human myotubes

Human skeletal muscle cells obtained from normal and Duchenne muscular dystrophy patients were cocultured with explants of rat dorsal root ganglions. Single-channel recordings were performed with the cell-attached configuration of the patch-clamp technique and negative pressure was applied via the patch-pipette in order to mechanically stimulate the membrane patch. Inward elementary current activity was recorded under control or negative pressure conditions. Its occurrence and mean open probability were higher in Duchenne muscular dystrophy. Amplitude histograms reveal that these channels have a small unitary conductance of around 10 pS in 110 mM Ca2+ and could be inhibited in a dose-dependent manner by gadolinium. Results show that the membrane stress favoured calcium permeation through these channels. Taken together these data provide arguments for the involvement of such channels in calcium overload previously observed in cocultured dystrophic human (Duchenne muscular dystrophy) muscle cells.

Dynamic restoration of dystrophin to dystrophin-deficient myotubes

Dystrophin domains are observed in myoblast transplantation experiments and in muscle fibers after somatic reversion in human Duchenne and mdx mouse muscular dystrophy. However, the formation and evolution of dystrophin-positive domains are not well established. Using a muscle satellite cell coculture system, we examined the dynamic restoration of dystrophin expression in dystrophin-deficient myotubes. The dystrophin-positive domains around source nuclei were clearly identified in hybrid myotubes. The occurrence of dystrophin domains was higher in myotubes differentiated from cocultures with a low concentration of normal wild-type satellite cells in relation to dystrophin-deficient satellite cells. At higher seeding ratios, the domain feature of dystrophin expression was more transitory and decreased as myotubes differentiated over time in culture. The average domain size initially increased with the addition of new nuclei by fusion early after differentiation of cocultures. However, separating dystrophin-positive domains according to their number of dystrophin-expressing contributory nuclei showed that diffusion of dystrophin contributed to domain elongation, even in early myotubes and later without fusion of additional nuclei. Diffusion occurred for all domains of one to six wild-type nuclei, and the diffusion rate was higher in domains with larger numbers of nuclei. This dynamic domain feature of dystrophin expression was also related to restoring the organization of dystrophin-associated proteins and acetylcholine receptors to hybrid myotubes. Factors regulating domain formation and diffusion therefore are important considerations in the design of strategies for both myoblast transplantation and gene therapy of Duchenne muscular dystrophy.