Decreased expression of brain beta-dystroglycan in Duchenne muscular dystrophy but not in the mdx animal model

Abnormalities in the muscle dystrophin-glycoprotein complex are implicated in the molecular pathogenesis of various neuromuscular disorders. Weakening of the trans-sarcolemmal linkage between the actin membrane-cytoskeleton and the extracellular matrix appears to trigger destabilization of the muscle cell periphery. In addition to muscular weakness, one-third of patients suffering from Duchenne muscular dystrophy exhibit mental retardation. Since little is known about the pathophysiology of brain abnormalities in these patients, we investigated the fate of the most abundant dystrophin-associated protein, beta-dystroglycan, in the central nervous system. It was found to be present throughout all normal brain regions studied. In contrast, this glycoprotein was greatly reduced in brain microsomes derived from Duchenne specimens, while it is of normal abundance in the brain from the dystrophic animal model mdx. Deficiency in brain beta-dystroglycan might render nervous tissue more susceptible to cellular disturbances and this may result in cognitive impairment in some Duchenne patients.

Complex formation between calsequestrin and the ryanodine receptor in fast- and slow-twitch rabbit skeletal muscle

Linkage between the high-capacity Ca2+-binding protein calsequestrin and the ryanodine receptor is proposed to be essential for proper Ca2+-release during skeletal muscle excitation-contraction coupling. However, no direct biochemical evidence exists showing a connection between these high-molecular-mass complexes in native skeletal muscle membranes. Here, using immunoblot analysis of chemically crosslinked membrane vesicles enriched in triad junctions, we have demonstrated that a very close neighborhood relationship exists between calsequestrin and the ryanodine receptor in both main fiber types. Hence, the luminal Ca2+-reservoir complex appears to be directly coupled to the membrane Ca2+-release complex and oligomerization seems to be of functional importance.

Excitation-contraction-relaxation cycle: role of Ca2+-regulatory membrane proteins in normal, stimulated and pathological skeletal muscle (review)

Extremely large protein complexes involved in the Ca2+-regulatory system of the excitation-contraction-relaxation cycle have been identified in skeletal muscle, i.e. clusters of the Ca2+-binding protein calsequestrin, apparent tetramers of Ca2+-ATPase pump units and complexes between the transverse-tubular alpha1-dihydropyridine receptor and ryanodine receptor Ca2+-release channel tetramers of the sarcoplasmic reticulum. While receptor interactions appear to be crucial for signal transduction during excitation-contraction coupling, avoidance of passive disintegration of junctional complexes and stabilization of receptor interactions may be mediated by disulfide-bonded clusters of triadin. Oligomerization of Ca2+-release, Ca2+-sequestration and Ca2+-uptake complexes appear to be an intrinsic property of these muscle membrane proteins. During chronic low-frequency stimulation, the expression of triad receptors is decreased while conditioning has only a marginal effect on Ca2+-binding proteins. In contrast, muscle stimulation induces a switch from the fast-twitch Ca2+-ATPase to its slow-twitch/cardiac isoform. These alterations in Ca2+-handling might reflect early functional adaptations to electrical stimulation. Studying Ca2+-homeostasis in transformed muscles is important regarding the evaluation of new clinical applications such as dynamic cardiomyoplasty. Studies of Ca2+-handling in skeletal muscle fibers have not only increased our understanding of muscle regulation, but have given important insights into the molecular pathogenesis of malignant hyperthermia, hypokalemic periodic paralysis and Brody disease.

Oligomerization of beta-dystroglycan in rabbit diaphragm and brain as revealed by chemical crosslinking

The surface component beta-dystroglycan is a member of the dystrophin-glycoprotein complex providing a trans-sarcolemmal linkage between the actin membrane cytoskeleton and the extracellular matrix component laminin-alpha2. Although abnormalities in this complex are involved in the pathophysiology of various neuromuscular disorders, little is known about the organization of dystrophin-associated glycoproteins in diaphragm and brain. We therefore investigated the oligomerization of beta-dystroglycan and its connection with the most abundant dystrophin homologues in these two tissues. Employing detergent solubilization and alkaline extraction procedures of native membranes, it was confirmed that beta-dystroglycan behaves like an integral surface molecule as predicted by its cDNA sequence. Immunoblot analysis following chemical crosslinking of native membranes showed that beta-dystroglycan has a tendency to form high-molecular-mass complexes. Within these crosslinkable complexes, immuno-reactive overlaps were observed between beta-dystroglycan, alpha-dystroglycan, laminin and 427 kDa dystrophin in diaphragm and skeletal muscle. In synaptosomes, the major brain dystrophin isoform Dp116 also exhibited an immuno-reactive overlap with members of the dystroglycan complex. These findings demonstrate that beta-dystroglycan does not exist as a monomer in native membranes and imply that certain dystrophin isoforms and dystrophin-associated components interact with this surface protein in diaphragm and brain as has been previously shown for skeletal and heart muscle.

Oligomerization is an intrinsic property of calsequestrin in normal and transformed skeletal muscle

In skeletal muscle fibers, the high-capacity medium-affinity Ca(2+)-binding protein calsequestrin functions as the major Ca(2+)-reservoir of the sarcoplasmic reticulum. To determine the oligomeric status of calsequestrin, immunoblotting of microsomal proteins following chemical crosslinking was performed. Diagonal non-reducing/reducing two-dimensional gel electrophoresis was employed to unequivocally differentiate between cross-linked species of 63 kDa calsequestrin and calsequestrin-like proteins of higher relative molecular mass. Since chronic low-frequency stimulation has a profound effect on the expression of many muscle-specific protein isoforms, we investigated normal and conditioned muscle fibers. Calsequestrin was found to exist in a wide range of high-molecular-mass clusters in normal and chronically stimulated skeletal muscle fibers. Hence, oligomerization is an intrinsic property of this important Ca(2+)-binding protein and does not appear to be influenced by the fast-to-slow transformation process. Although fiber-type specific differences exist in the physiology of the skeletal muscle Ca(2+)-regulatory system, oligomerization of calsequestrin seems to be essential for proper functioning.

Early functional and biochemical adaptations to low-frequency stimulation of rabbit fast-twitch muscle

To examine mechanisms underlying force reduction after the onset of chronic low-frequency (10 Hz) stimulation (CLFS), we exposed rabbit tibialis anterior muscles to various durations of CLFS. To follow changes in isometric contractile properties and electromyographic (EMG) activity, we studied stimulated and contralateral muscles during a terminal test at 10 Hz for 10 min. In addition, activities and protein amounts of the sarcoplasmic reticulum Ca(2+)-ATPase, content of Na(+)-K(+)-ATPase, and expression patterns of triad junction components were examined. Force output and EMG amplitude declined abruptly soon after the onset of stimulation, suggesting refractoriness of a large fiber population. Although twitch force and to a lesser extent EMG activity gradually recovered after stimulation for 6 days and longer, the muscles exhibited profoundly altered properties, i.e., enhanced fatigue resistance, absence of twitch potentiation, and prolonged contraction and relaxation times. These changes were associated with significant increases in Na(+)-K(+)-ATPase concentration and significant decreases in Ca(2+)-ATPase, ryanodine receptor, dihydropyridine receptor, and triadin concentrations over the course of the 20 days of stimulation. Alterations in excitability, Ca2+ handling, and excitation-contraction coupling prior to changes in myofibrillar protein isoforms may thus be responsible for early functional alterations.

Cross-linking analysis of the ryanodine receptor and alpha1-dihydropyridine receptor in rabbit skeletal muscle triads

In mature skeletal muscle, excitation-contraction (EC) coupling is thought to be mediated by direct physical interactions between the transverse tubular, voltage-sensing dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR) Ca2+ release channel of the sarcoplasmic reticulum (SR). Although previous attempts at demonstrating interactions between purified RyR and alpha1-DHPR have failed, the cross-linking analysis shown here indicates low-level complex formation between the SR RyR and the junctional alpha1-DHPR. After cross-linking of membranes highly enriched in triads with dithiobis-succinimidyl propionate, distinct complexes of more than 3000 kDa were detected. This agrees with numerous physiological and electron-microscopic findings, as well as co-immunoprecipitation experiments with triad receptors and receptor domain-binding studies. However, a distinct overlap of immunoreactivity between receptors was not observed in crude microsomal preparations, indicating that the triad complex is probably of low abundance. Disulphide-bonded, high-molecular-mass clusters of triadin, the junctional protein proposed to mediate interactions in triads, were confirmed to be linked to the RyR. Calsequestrin and the SR Ca2+-ATPase were not found in cross-linked complexes of the RyR and alpha1-DHPR. Thus, whereas recent studies indicate that the two receptors exhibit temporal differences in their developmental inductions and that receptor interactions are not essential for the formation and maintenance of triads, this study supports the signal transduction hypothesis of direct physical interactions between triad receptors in adult skeletal muscle.

Rabbit brain and muscle isoforms containing the carboxy-terminal domain of 427 kDa skeletal muscle dystrophin exhibit similar biochemical properties

The membrane cytoskeletal component dystrophin, which is the protein product of the human Duchenne muscular dystrophy gene, exists in manifold isoforms. Using immunoblot analysis with an antibody to the carboxy-terminal domain of the 427 kDa skeletal muscle dystrophin, we investigated the membrane cytoskeletal properties of dystrophin isoforms from rabbit skeletal muscle, heart and brain. All isoforms identified, including the abundant brain isoforms Dp116 which lacks the amino-terminal actin-binding domain of 427 kDa dystrophin, exhibited similar biochemical properties, i.e. insolubility in non-ionic detergent but extraction from the membrane with alkaline solutions. In muscle, beta-dystroglycan was found to be more tightly associated with dystrophin than alpha-sarcoglycan. These findings agree with the proposed structure of identified muscle and brain dystrophin isoforms and are also consistent with the current model of the dystrophin-glycoprotein complex.

[Molecular pathogenesis of muscular diseases]

Recent advances in the field of molecular myology have provided significant insight into the pathological mechanisms underlying a variety of neuromuscular disorders. Genetic abnormalities can now be linked to primary and secondary pathophysiological changes in muscle fibres which compromise structural, metabolic, regulatory or contractile mechanisms. Ion channel myopathies such as paramyotonia congenita, hyper- and hypokalaemic periodic paralysis, myotonia congenita, episodic ataxia and malignant hyperthermia were established as linked to mutations in genes encoding the sodium channel, dihydropyridine receptor, chloride channel, potassium channel and the ryanodine receptor calcium release channel, respectively. Metabolic disorders affecting skeletal muscle were found to be due to deficiencies in a variety of enzymes. Identification of defects in components belonging to the gigantic dystrophin-glycoprotein complex led to the discovery of the molecular pathogenesis of Duchenne muscular dystrophy and related disorders. Based on these molecular findings, it is now feasible to design and evaluate new techniques such as gene and myoblast transfer therapy in order to replace defective components in diseased muscle fibres.

Oligomerization of sarcoplasmic reticulum Ca2+-ATPase from rabbit skeletal muscle

Although the primary structure and catalytic cycle of the sarcoplasmic reticulum Ca2+-ATPase has been revealed, it is not well understood whether functional Ca2+ pump proteins exist in a monomeric or an oligomeric state in native skeletal muscle membranes. Here, we show that the Ca2+-ATPase tends to form high molecular weight complexes, estimated to be dimers and tetramers using immunoblotting of two-dimensionally separated microsomal membranes following crosslinking. This agrees with both electron microscopical and biochemical findings which demonstrate that Ca2+-ATPase clusters are the predominant molecular species in native membranes and that oligomerization may play a role in cooperative kinetics and enzyme stabilization.

Characterisation of the dystrophin-related protein utrophin in highly purified skeletal muscle sarcolemma vesicles

Due to its restricted localisation to the neuromuscular junction and based on sequence homology to cytoskeletal proteins, the dystrophin-related protein utrophin is thought to be an important constituent of the membrane cytoskeleton of the postsynaptic muscle membrane and may be involved in the clustering of acetylcholine receptors at the neuromuscular junction. However, due to the low density of utrophin in microsomal muscle membranes, it is difficult to analyse the biochemical properties of the skeletal muscle isoform of utrophin. To overcome these technical difficulties, we used here immunoblot analysis of highly purified muscle surface membranes enriched even in sarcolemma markers of very low density such as ecto-5' nucleotidase and the calmodulin-sensitive Ca(2+)-ATPase. This enabled us to analyse the membrane biochemical properties of this dystrophin isoform of extremely low abundance. Since alkaline treatment released utrophin from the bilayer while it stayed associated with the insoluble pellet following detergent extraction, utrophin exhibits biochemical properties typical of a membrane cytoskeletal protein. Therefore, utrophin appears to be a specialised isoform which performs the membrane cytoskeletal function(s) of dystrophin at the postsynaptic membrane of the neuromuscular junction.

Towards an understanding of the dystrophin-glycoprotein complex: linkage between the extracellular matrix and the membrane cytoskeleton in muscle fibers

In muscle, the large membrane cytoskeletal protein dystrophin is tightly associated with several sarcolemmal proteins. It is now well established that the extracellular 156 kDa dystrophin-associated glycoprotein alpha-dystroglycan is a laminin binding protein which is linked to the sarcolemma via an integral glycoprotein complex. Beta-dystroglycan of 43 kDa, together with the 50 kDa protein adhalin and three other dystrophin-associated glycoproteins of 25, 35 and 43 kDa form this sarcolemmal complex which binds to the cysteine-rich domain of dystrophin. While the carboxy-terminal domain of dystrophin is associated with the syntrophin triplet of apparent 59 kDa, the amino-terminal dystrophin domain binds to the subsarcolemmal actin cytoskeleton. Numerous studies into the molecular pathogenesis of muscular dystrophies and cardiomyopathies suggest that primary defects in components of the dystrophin-glycoprotein complex are responsible for certain forms of these muscular disorders. Disturbances in the composition of the dystrophin-glycoprotein complex in skeletal and cardiac muscle seem to disrupt the linkage between the extracellular matrix and the actin membrane cytoskeleton which might render muscle fibers more susceptible to necrosis. In normal muscle, the vital function of the dystrophin-glycoprotein complex appears to be the stabilization of the muscle cell periphery during contraction.

Role of the sea urchin egg receptor for sperm in gamete interactions

Embryonic development is initiated by a multi-step fertilization process involving induction of the acrosome reaction in sperm, sperm-egg binding, gamete membrane fusion and egg activation. In sea urchins, acrosome-reacted sperm interact, presumably via the sperm protein bindin, with a highly glycosylated receptor on the egg surface. This article highlights the recent advances in the molecular structure of the sea urchin sperm receptor and discusses its possible role in egg activation.

Developmental expression of the sea urchin egg receptor for sperm

Little is known about the biochemical changes underlying the morphological differentiation of the sea urchin egg during oogenesis. Because of this and the essential role of gamete recognition in fertilization, we studied the developmental expression of the recently identified egg surface receptor for sperm during oogenesis. Consecutive stages of ovaries undergoing oogenesis over a 4-month time course were examined morphologically and assessed with respect to content of sperm receptor mRNA, as well as the content and subcellular distribution of the sperm receptor glycoprotein. Although in early oocyte stages neither mRNA encoding for the receptor nor receptor glycoprotein was detectable, at the last two stages of development the level of receptor mRNA accumulation increased dramatically. This finding correlated well with immunoblot analyses which established that sperm receptor protein was only detectable at the last two stages of egg maturation. Interestingly, immunocytochemistry showed that the formation of the receptor correlated temporally and spatially with the formation of cortical granules. In the earlier of these two stages of maturation, the receptor population identified by immunoblotting was found by immunocytochemistry to be restricted to the cortical granules and small vesicles in the cytoplasm. In contrast, at the last stage of egg maturation, sperm receptor was also detected at the surface of the oocyte, localized predominantly to the microvilli. Two receptor populations appear to exist, one in cortical granules and a second at the cell surface that may be formed via secretory vesicles. The late appearance of the receptor on the plasma membrane during oogenesis is consistent with its biological role in binding sperm to the mature egg cell surface.

The biologically active form of the sea urchin egg receptor for sperm is a disulfide-bonded homo-multimer

Since many cell surface receptors exist in their active form as oligomeric complexes, we have investigated the subunit composition of the biologically active sperm receptor in egg plasma membranes from Strongylocentrotus purpuratus. Electrophoretic analysis of the receptor without prior reduction of disulfide bonds revealed that the surface receptor exists in the form of a disulfide-bonded multimer, estimated to be a tetramer. These findings are in excellent agreement with the fact that the NH2-terminus of the extracellular domain of the sperm receptor is rich in cysteine residues. Studies with cross-linking agents of various length and hydrophobicity suggest that no other major protein is tightly associated with the receptor. Given the multimeric structure of the receptor, we investigated the effect of disulfide bond reduction on its biological activity. Because in quantitative bioassays fertilization was found to be inhibited by treatment of eggs with 5 mM dithiothreitol, we undertook more direct studies of the effect of reduction on properties of the receptor. First, we studied the effect of addition of isolated, pure receptor on fertilization. Whereas the non-reduced, native receptor complex inhibited fertilization in a dose-dependent manner, the reduced and alkylated receptor was inactive. Second, we tested the ability of the isolated receptor to mediate binding of acrosome-reacted sperm to polystyrene beads. Whereas beads coated with native receptor bound sperm, those containing reduced and alkylated receptor did not. Thus, these results demonstrate that the biologically active form of the sea urchin sperm receptor consists only of 350 kD subunits and that these must be linked as a multimer via disulfide bonds to produce a complex that is functional in sperm recognition and binding.

The role of the dystrophin-glycoprotein complex in the molecular pathogenesis of muscular dystrophies

The dystrophin-glycoprotein complex is considered to be a major trans-sarcolemmal structure which provides a linkage between the subsarcolemmal actin cytoskeleton and the extracellular matrix component laminin. Recently, deficiency of the dystrophin-associated proteins has been shown to play an important role in the molecular pathogenesis of several forms of muscular dystrophy. These include Duchenne muscular dystrophy (DMD), symptomatic DMD carriers, Becker muscular dystrophy and severe childhood autosomal recessive muscular dystrophy with DMD-like phenotype prevalent in North Africa. In Fukuyama-type congenital muscular dystrophy (FCMD), the finding of abnormal expression of the dystrophin-associated proteins may provide a clue to its molecular pathogenesis. These recent findings indicate that the linkage between the subsarcolemmal cytoskeleton and extracellular matrix via the dystrophin-glycoprotein complex is critical for maintaining the integrity of muscle cell function.

The sea urchin egg receptor for sperm: isolation and characterization of the intact, biologically active receptor

The species-specific binding of sea urchin sperm to the egg is mediated by an egg cell surface receptor. Although earlier studies have resulted in the cloning and sequencing of the receptor, structure/function studies require knowledge of the structure of the mature cell surface protein. In this study, we report the purification of this glycoprotein to homogeneity from a cell surface complex of Strongylocentrotus purpuratus eggs using lectin and ion exchange chromatography. Based on the yield of receptor it can be calculated that each egg contains approximately 1.25 x 10(6) receptor molecules on its surface. The receptor, which has an apparent M(r) of 350 kD, is a highly glycosylated transmembrane protein composed of approximately 70% carbohydrate. Because earlier studies on the partially purified receptor and on a pure, extracellular fragment of the receptor indicated that the carbohydrate chains were important in sperm binding, we undertook compositional analysis of the carbohydrate in the intact receptor. These analyses and lectin binding studies revealed that the oligosaccharide chains of the receptor are sulfated and that both N- and O-linked chains are present. Functional analyses revealed that the purified receptor retained biological activity; it inhibited fertilization in a species-specific and dose-dependent manner, and polystyrene beads coated with it bound to acrosome-reacted sperm in a species-specific manner. The availability of biochemical quantities of this novel cell recognition molecule opens new avenues to studying the interaction of complementary cell surface ligands in fertilization.

Disruption of the dystrophin-glycoprotein complex in the cardiomyopathic hamster

Cardiomyopathies are a diverse group of primary cardiac diseases, most of which have a poorly understood etiology. One type of hereditary cardiomyopathy is caused by defects in the dystrophin gene in Duchenne and Becker muscular dystrophy patients. Our laboratory has identified a complex of dystrophin-associated proteins in skeletal and cardiac muscle which span the sarcolemma, linking the subsarcolemmal cytoskeleton to the extracellular matrix. The absence of dystrophin in Duchenne muscular dystrophy patients leads to the loss of dystrophin-associated proteins in both skeletal and cardiac muscle, suggesting that a primary loss of one or more dystrophin-associated proteins might lead to other forms of cardiomyopathy. Here we report the specific deficiency of the 50-kDa dystrophin-associated glycoprotein in cardiac and skeletal muscles of the BIO 14.6 strain of cardiomyopathic hamsters, which experience both autosomal recessive cardiomyopathy and myopathy. Other dystrophin-associated proteins are well preserved in myopathic hamster skeletal muscle, but the link between dystrophin and dystroglycan is disrupted. All dystrophin-associated proteins are decreased in abundance in the cardiomyopathic hamster heart, perhaps explaining why the cardiomyopathy is more severe than the myopathy. Thus, the disruption of the dystrophin-glycoprotein complex may play a role in skeletal and cardiac myocyte necrosis of the cardiomyopathic hamster.

Duchenne muscular dystrophy: deficiency of dystrophin-associated proteins in the sarcolemma

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is a major component of the subsarcolemmal cytoskeleton and exists in a large oligomeric complex tightly associated with several sarcolemmal glycoproteins which provide a linkage to the extracellular matrix protein, laminin. In the present study, we investigated the status of the dystrophin-associated proteins in the skeletal muscle from 17 DMD patients of various ages. The results revealed a dramatic reduction in all of the dystrophin-associated proteins in the sarcolemma of DMD muscle compared with normal muscle and muscle from a variety of other neuromuscular diseases. This abnormality was common in all 17 DMD patients, irrespective of age. Our results indicate that the absence of dystrophin leads to the loss in all of the dystrophin-associated proteins, which renders DMD muscle fibers susceptible to necrosis. The analysis of dystrophin-associated proteins is important in the assessment of experimental therapies that attempt to replace dystrophin in DMD muscle.