Calcium transport and phosphoenzyme formation in dystrophic mouse sarcoplasmic reticulum

Cholesterol alterations in young dystrophic mice

Cholesterol and cholesteryl ester concentrations and cholesteryl ester fatty acid substituents have been measured during the first 10 weeks of life in tissues of normal and dystrophic mice. In normal Swiss and 129ReJ(+/?) mice the concentrations of both cholesterol and cholesteryl esters remain essentially constant in liver, increase in brain and fall sharply in both thigh (mixed fiber type muscles) and chest muscles (predominantly slow oxidative muscles) over this period. In all cases the concentration of free cholesterol exceeds that of esterified cholesterol. In dystrophic mice, similar patterns are found in brain and liver. In both thigh and chest muscles, however, the developmental pattern is significantly different. After an initial decrease the concentrations of cholesterol and cholesteryl esters increase rapidly with the largest increase occurring in the concentration of cholesteryl esters which by 10 weeks of age exceeds the concentration of cholesterol in chest muscle. During the same period the pattern of esterified fatty acids changes gradually in dystrophic tissues towards an increasing ratio of unsaturated/saturated fatty acids. By 10 weeks of age this ratio is significantly higher in dystrophic tissues than normal in all tissues tested.

Early over-expression of low-affinity [3H]ryanodine receptor sites in heavy sarcoplasmic reticulum fraction from dystrophic chicken pectoralis major

Heavy sarcoplasmic reticulum (SR) membranes enriched in [3H]ryanodine receptor have been isolated from pectoralis major (PM) of normal line 412 and dystrophic line 413 chickens paired at various stages during post-hatch development. Normal PM 2 days ex ovo yields 17% lower protein recovery in the heavy SR subfractions compared to preparations from paired dystrophic PM (0.80 vs. 0.96 mg/g PM, respectively). By 2 weeks ex ovo, protein recovery in normal SR subfractions decreases over 3-fold to 0.24 mg/g PM, whereas yields from dystrophic PM increase to 1.67 mg/g PM. Dystrophic preparations consistently give 7-9-fold higher recoveries of protein in heavy SR subfractions compared to normal PM when examined at 2, 4, and 5.5 weeks ex ovo. [3H]Ryanodine binding to normal SR from PM 2 days ex ovo exhibits nonlinear Scatchard plots which resolve into high-(Kd,app = 18 nM; Bmax = 1.7 pmol/mg protein) and low-(Kd,app = 532 nM; Bmax = 2.6 pmol/mg protein) affinity binding sites, whereas dystrophic preparations exhibit only high-affinity [3H]ryanodine binding sites (Kd,app = 31 nM; Bmax = 2.1 pmol/mg protein). Both normal and dystrophic PM have similar capacities to bind [3H]ryanodine (2.6 vs. 2.0 pmol/g PM, respectively) at 2 days ex ovo. However, at 2, 4, and 5.5 weeks ex ovo the density of high-affinity [3H]ryanodine binding sites in normal SR drops dramatically to 3.5, 1.2, and 0.4 pmol/mg protein, respectively, and corresponds with disappearance of the low-affinity binding sites. In marked contrast, heavy SR membranes from dystrophic PM 2, 4, and 5.5 weeks ex ovo, maintain their high-affinity binding sites for [3H]ryanodine and exhibit high densities of low-affinity binding sites (Kd,app = 725-4500 nM; Bmax = 15.4-25.1 pmol/mg protein). Early developmental over-expression of [3H]ryanodine binding capacity in dystrophic PM ranges from 34-fold to 388-fold that of normal PM at 2 weeks and 5.5 weeks, respectively, and correspond to the intensity with which high molecular weight doublets of Mr 340,000 and 320,000 stain on SDS-PAGE. Low-affinity [3H]ryanodine binding sites of dystrophic SR exhibit 4-6-fold higher sensitivity to activation by Ca2+ and altered sensitivity to activation by caffeine and adenine nucleotides. These results demonstrate that over-expression of junctional SR and [3H]ryanodine receptors having altered radioligand binding properties is a very early event in the post-hatch development of dystrophic PM.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

Postnatal development of Ca2+-sequestration by the sarcoplasmic reticulum of fast and slow muscles in normal and dystrophic mice

Ca2+-uptake activities of the sarcoplasmic reticulum (SR) were determined with a Ca2+-sensitive electrode in homogenates from fast- and slow-twitch muscles from both normal and dystrophic mice (C57BL/6J strain) of different ages. Immunochemical quantification of tissue Ca2+-ATPase content allowed determination of the specific Ca2+-transport activity of the enzyme. In 3-week-old mice of the dystrophic strain specific Ca2+ transport was already significantly lower than in the normal strain. It progressively decreased with maturation and reached only 40-50% and 30-50% of the normal values in fast- and slow-twitch muscles of adult dystrophic animals, respectively. Tissue contents of calsequestrin were reduced in both types of muscle leading to an increased Ca2+-ATPase to calsequestrin protein ratio. Equal amounts of the Ca2+-ATPase protein (detected by Coomassie blue staining of polyacrylamide gels) were present in SR vesicles isolated by Ca2+-oxalate loading from adult normal and dystrophic fast-twitch muscles. However, the specific ATP-hydrolysing activity of the enzyme was approximately 50% lower in dystrophic than in normal SR. The reduced ATP-hydrolysing activity was correlated with decreased Ca2+-transport activity, phosphoprotein formation and fluorescein isothiocyanate labeling as determined in total microsomal and heavy SR fractions. Although the Ca2+ and ATP affinities of the enzyme were unaltered, its ATPase activity was reduced at all levels of ATP in the dystrophic SR. Taken together, these findings point to a markedly impaired function of the SR and an increase in the population of inactive SR Ca2+-ATPase molecules in murine muscular dystrophy.

Free radicals: a potential pathogenic mechanism in inherited muscular dystrophy

Despite years of intensive work, the biochemical defect responsible for the pathogenesis of inherited muscular dystrophy has not been identified either in humans or animal models. This review examines evidence in support of the hypothesis that free radicals may be responsible for muscle degeneration in this disorder. A variety of cellular abnormalities noted in dystrophic muscles can be accounted for by free radical mediated damage. In addition, chemical by-products associated with free radical damage are found in dystrophic muscle tissue from humans and animals with this disease. Various enzymatic antioxidant systems can be enhanced as a normal cellular response to oxidative stress, and such changes are seen both in dystrophic muscle cells and certain other tissues of dystrophic animals. An increased level of free radical damage would follow from either: enhanced production of free radical species, or a deficient component of the cellular antioxidant system, such as vitamin E. The free radical hypothesis of muscular dystrophy can account for data supporting several alternative theories of the pathogenesis of this disease, as well as other observations which have not previously been explained.

Protein profiles of sarcoplasmic reticulum from normal and dystrophic mouse muscle

Sarcoplasmic reticulum (SR) was isolated from skeletal muscle of dystrophic (C57BL/6J dy2J/dy2J) mice and the protein composition analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Densitometric analysis of dystrophic SR preparations indicated a decrease in the Ca2+-ATPase and calsequestrin, and the appearance of a protein with molecular weight 72 000. These differences in the protein profiles between normal and dystrophic SR became more apparent as the disease progressed. The observations are discussed in relation to secondary changes in the dystrophic process such as changes in fibre type and the presence of immature fibres.

Impaired biosynthesis of highly unsaturated phosphatidylcholines: a hypothesis on the molecular etiology of some muscular dystrophies

A brief review of the literature concerning the synthesis of phosphatidylcholine and phosphatidylethanolamine in muscle suggests that the cytidine pathways are replaced by the recently proposed acyl-specific de novo and salvage glycerolphosphodiester pathways (Infante, 1984) in fully differentiated muscle. An analysis of published data suggests an impaired synthesis of 4,7,10,13,16,19-docosahexaenoic phosphatidylcholine, at the level of de novo sn-3-glycerolphosphorylcholine synthesis, as the primary defect in Duchenne and (dy) murine muscular dystrophies. This phosphatidylcholine species is postulated to be required for optimum sarcoplasmic Ca2+ transport activity. It is proposed that this impairment initiates the secondary series of events which lead to the observed pathology of these diseases. Based on some predictions of the hypothesis, potential diagnosis and treatments are suggested.

Isolation and characterization of transverse tubule from normal and dystrophic mice

I have recently reported the isolation and characterization of sarcoplasmic reticulum from normal and dystrophic mice. These sarcoplasmic reticulum fractions were similar in calcium pump function, calcium release properties, and lipid composition. In this report, I describe the isolation of mouse muscle transverse tubule membranes using a calcium phosphate-loading technique. When the relative purity of normal and dystrophic preparations was considered, transverse tubule from normal and dystrophic mice were similar in calcium-insensitive ATPase activity, cholesterol content, and membrane microviscosity (as estimated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene); transverse tubule yield from dystrophic muscle, however, was twice that from normal muscle, while sarcoplasmic reticulum yield from these same dystrophic muscles was only 60% that from normal muscle. This result may reflect a difference in the relative quantities of these membranes in situ.

Membrane crystals of Ca2+-ATPase in sarcoplasmic reticulum of normal and dystrophic muscle

Two-dimensional crystalline arrays of the Ca2+ transport ATPase develop after treatment of sarcoplasmic reticulum vesicles with Na3VO4. The dimensions of the crystal lattice are similar in sarcoplasmic reticulum membranes isolated from normal and genetically dystrophic human, mouse, and chicken muscles. These observations indicate similar requirements for ATPase-ATPase interactions in sarcoplasmic reticulum of normal and dystrophic muscles and lessen the likelihood of a molecular defect of the Ca2+ pump in the various forms of genetic muscular dystrophies.

Slowing of twitch of dystrophic mouse muscle in partially due to altered activity pattern

Fast-twitch muscles of the hindlimb of dystrophic (dy2J) mice show a prolongation of both the contraction and relaxation phases of the isometric twitch. Comparable muscles of the forelimb of these mice exhibit relatively little increase in time to peak tension but time to half-relaxation is as severely affected as in the hindlimb. When examined with an immunohistochemical technique to demonstrate the presence of "slow" myosin it was apparent that there were no fibers containing the "slow" isoenzyme in either hindlimb or forelimb muscles of 6-month control mice. In dy2J mice hindlimb muscles contained many fibers with "slow" myosin whereas forelimb muscles did not. It is suggested that the spontaneous twitching activity produced in the hindlimbs, as a result of amyelination of the spinal roots, induces synthesis of "slow" myosin, which in turn leads to prolongation of time to peak twitch tension.

Normal function in sarcoplasmic reticulum from mice with muscular dystrophy

A rapid, gentle technique has been developed for the isolation of sarcoplasmic reticulum (SR) from small amounts of skeletal muscle from normal and dystrophic mice. Assays for mitochondrial and surface membrane marker enzymes revealed only low levels of contamination in the isolated fractions. A small amount of calcium-insensitive ("basal") ATPase in the normal preparation and a higher value in the dystrophic were shown to be due to contamination by a lighter membrane fraction of probable surface membrane origin. Isolated SR from normal and dystrophic mice were indistinguishable by thin section and freeze-fracture electron microscopy. Only small differences in calcium loading rates and capacity, and in calcium-stimulated ATPase activity, were present. These were attributable to small differences in purity. We conclude that there is no difference in SR from normal and dystrophic mice in the properties measured.

The influence of atherogenic diet on electromechanical coupling in rabbit myocardium

To characterize the role of the SR in the electromechanical coupling process in rabbit myocardium after a 12-week-long atherogenic diet containing cholesterol and oil, the influence of changes in Ca2e+ on isometric force, relaxation rate, and resting potentiation of isolated trabeculae were compared with 45Ca2+ binding and uptake by the SR. The diet produces a more strongly expressed resting potentiation of force at 1.25 and 2.5 mM Ca2e+ and an increase in the Vmax equivalent, the contractility index [(dF/dt)max/F'] and the relaxation index [-(dF/dt)max/F]. A higher Ca2e+-accumulating activity of heart microsomes from animals was observed after the diet. It is concluded that changes of SR function are involved in the mechanism responsible for modification of electromechanical coupling induced by cholesterol- and oil-containing diet.

Incorporation of amino acids into soluble and membrane protein fractions of dystrophic hamsters

The incorporation of labelled leucine was measured in protein fractions of muscle in intact control and dystrophic female hamsters and also in cell-free preparations obtained from these animals. The labelling of the soluble sarcoplasmic protein fraction, the microsomal protein fraction and the sarcolemma protein fraction was increased in the dystrophic hindleg muscle. The specific radioactivities of the sarcolemma protein fraction and other fractions were increased markedly relative to that of free leucine in the dystrophic muscle. In cell-free preparations where ribonuclease effects were avoided, the dystrophic muscle exhibited an increased synthesis of peptide bonds.

Characterization of ATPase in sarcoplasmic reticulum from two strains of dystrophic mice

The ATPase activities and phosphoenzyme levels have been determined in sarcoplasmic reticulum (SR) membranes prepared from two animal models of muscular dystrophy, myodystrophic (myd/myd) and strain 129 dystrophic (129 dy/dy) mice. In both myd/myd and 129 dy/dy SR membranes, the basal ATPase activities are elevated above control levels, while the Ca-dependent ATPase activities are normal. The addition of 0.1% Triton X-100 not only lowers the basal ATPase activity of myodystrophic control SR membranes by 60%, but also lowers the elevated basal ATPase activity of myd/myd SR membranes to a similar level. The Ca-dependent ATPase activities of myodystrophic control and myd/myd SR membranes are increased approximately threefold by the addition of Triton. The addition of 0.1% Triton X-100 lowers the basal ATPase activities of 129 control and 129 dy/dy SR membranes to similar levels, but stimulates the CA-dependent ATPase activity of 129 dy/dy SR membranes to a level that is only 60% of that of 129 control SR membranes. The level of phosphoenzyme intermediate is decreased approximately 15% in myd/myd SR membranes and approximately 30% in 129 dy/dy SR membranes.

Rapid kinetics of calcium ion transport and ATPase activity in the sarcoplasmic reticulum of dystrophic muscle

Vesicular fragments of sarcoplasmic reticulum were isolated from pectoralis muscle of normal and dystrophic chicken. Purification of both preparations was equally satisfactory, as shown by a prominent ATPase band in electrophoresis gels. Measurements of ATPase phosphorylation, Ca2+ transport and Pi cleavage by rapid quench methods revealed a lower specific activity of the dystrophic vesicles with respect to all of these functions. On the other hand, Ca2+-independent ATPase activity was found to be increased in dystrophic vesicles. It is suggested that a fraction of ATPase units of dystrophic sarcoplasmic reticulum is not activated by Ca2+, owing to an altered protein assembly within the membrane bilayer. In fact, when the membrane structure is perturbed by detergents normal and dystropic preparations acquire an equally high Ca2+-dependent ATPase.

Role of intracellular calcium in promoting muscle damage: a strategy for controlling the dystrophic condition

It is suggested that various muscle diseases and examples of experimentally-induced muscle damage arise because of a high calcium level in the myoplasm. When [Ca2+]i is raised experimentally in amphibian or mammaliam muscle by treatment with A23187 or caffeine, myofilament degradation follows quickly. Such a rapid action suggests the involvement of a sequence of proteolytic activity that is stimulated by a rise in [Ca2+]i. Ca2+ might either trigger protease activity directly or indirectly, or promote the release of lysosomal enzymes. A high [Ca2+]i in dystrophic muscle is believed to be the resultant of a sequence of events that is summarized in the figure. Suggestions are presented for different ways in which the steady-state position of [Ca2+]i might ultimately be controlled for the clinical amelioration of some dystrophic conditions.