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PubMed1982

Evidence for membrane microheterogeneity in the sarcoplasmic reticulum of fast twitch skeletal muscle.

Sarcoplasmic reticulum (SR) from rabbit back muscles can be readily subfractionated into two morphologically and compositionally different vesicular populations, SRH (heavy) and SRL (light) derived from terminal cisternae and longitudinal SR, respectively. Polyacrylamide gels indicate that SRH contains most of the calsequestrin. Quantitation of freeze-fractured isolated preparations reveals that, while differences in vesicular dimensions are seen in SRH and SRL, the intramembrane particle (Ca2+ ATPase) density is identical. Phospholipid headgroup composition is the same in SRH and SRL, but fatty acyl moieties show significant differences in the ratio of saturated to unsaturated phospholipids in the two fractions. The vesicular dimensions of the purified Ca2+-ATPases, SRHP and SRLP, from the two fractions are identical, but the freeze-fracture particle density is higher in the SRLP fraction. The phospholipid composition remains similar after purification, but the differences in phospholipid fatty acyl composition of the preparations are maintained. SRH and SRHP contain almost twice as much of the unsaturated species as compared to SRL and SRLP. Differences in intramembrane particle density in purified fractions, thermotropic segregation of particles in freeze-fractured purified fractions, as well as differences in turnover of the acyl phosphate, appear to reflect the differences in fatty acyl chain composition of the two SR fractions and provide evidence of microheterogeneity in lipid-protein environment of the SR.

Animals↗
PubMed1981

Increased turnover of proteins from the sarcoplasmic reticulum of dystrophic chicken muscle cells in tissue culture.

Chicken myoblasts were cultured from the pectoralis muscles of dystrophic and normal 11-day-old embryos. Cells were allowed to grow to fusion (differentiation) and exposed to [35S]methionine for a short period. Subsequently, the decay of labeled proteins in the presence of cycloheximide was measured for various cellular fractions as well as individual proteins isolated from the sarcoplasmic reticulum and separated by gel electrophoresis. Some dystrophic material showed an increased decay when compared to normal material. The most significant (p less than 0.005) difference was found in a Mr = 65,000 component of the sarcoplasmic reticulum. This same component accumulates label at an accelerated rate in the presence of the protease inhibitor leupeptin. Increased turnover of this protein, possibly calsequestrin, may be a manifestation of the genetic disease.

Animals↗
PubMed1980

Structure of sarcoplasmic reticulum.

A classification of the sarcoplasmic reticulum (SR) membrane into areas devoted to different functions is described. The junctional (j) SR is covered by feet, it faces towards the transverse tubules, and it is probably devoted to receiving a signal from them. The free (f) SR contains the calcium pump. Isolated SR forms two fractions: light and heavy SR. The heavy fraction is composed of the lateral sacs of the triad, containing calsequestrin, and its membrane comprises both f and j SR. The light fraction is entirely composed of fSR membrane. It is proposed that the feet join jSR membrane to particles contained within the transverse tubules and that they play a direct role in excitation-contraction coupling.

Animals↗
PubMed1994

Enzymatic properties, metal composition and SH-group reactivity of the light and heavy sarcoplasmic reticulum vesicles.

Light (LT) and heavy (TTC) microsomes were isolated from the fragmented sarcoplasmic reticulum (FSR) of rabbit skeletal muscle by sucrose gradient centrifugation. The amount of the protein components (ATPase, "feet proteins", calsequestrine) showed substantial differences between the light and heavy fractions. The amounts of calcium, magnesium and zinc were about 2-4 times higher in the TTC fraction, then those of the LT fraction. The activities of the Ca2+ + Mg2+ activated para-nitrophenyl-phosphatase and acetylcholin-esterase were (about 1.5 times) also higher in the TTC fraction compared to the LT fraction. The ratio of the Ca-transport vs. ATP was 2.1 in case of the LT and 0.6 in case of the TTC fraction. The number of titerable SH-groups of the LT fraction, measured in EGTA-medium containing Ca2+, was higher than those measured in the absence of Ca2+, while for the TTC fraction this number was higher when determined in the absence of Ca2+. We suppose that due to the higher amount of Ca2+ and Zn2+ in the TTC fraction those SH-groups which were present as Ca- or Zn-thiolates became titerable in EGTA-medium in the absence of Ca2+.

4-Nitrophenylphosphatase↗
PubMed1993

Cloning and characterization of a putative Ca2+/H+ antiporter gene from Escherichia coli upon functional complementation of Na+/H+ antiporter-deficient strains by the overexpressed gene.

DNA libraries from alkaliphilic Bacillus firmus OF4 had been screened earlier (Ivey, D.M., Guffanti, A.A., Bossewitch, J. S., Padan, E., and Krulwich, T. A. (1991) J. Biol. Chem. 266, 23483-23489) for clones that would functionally complement a strain of Escherichia coli (NM81) with a deletion in one of its Na+/H+ antiporter genes. During those studies, an alkaliphile antiporter gene was hypothesized to have been incorporated into the chromosome of strain NM81, producing Na(+)-resistant NM8191. After introduction of a deletion in the second known E. coli Na+/H+ antiporter gene, libraries were prepared from NM8191 and screened for complementation of Na+/H+ antiporter-deficient mutants of E. coli. Instead of retrieving an alkaliphile gene, an unexpected E. coli gene was identified on the basis of its ability to restore Na+ resistance and membrane Na+/H+ antiporter activity to such mutant strains. The active open reading frame in the clone maps at 27 min on the E. coli chromosome and is identical in sequence to a wild type counterpart. It would be predicted to encode an extremely hydrophobic protein with multiple membrane-spanning regions and a molecular weight of 39,200. A region in one of the predicted hydrophilic loops in the gene product structure possesses striking sequence similarity to calsequestrin. The Ca2+/H+ antiporter activity of membranes from an E. coli transformant with a clone possessing only this open reading frame was indeed found to have enhanced pH-independent Ca2+/H+ antiporter activity. The Ca2+/H+ and Na+/H+ antiporter activities conferred by the clone were both inhibited by Mg2+. The gene was designated chaA and is proposed to be the structural gene for a Ca2+/H+ antiporter whose overexpression leads to resistance to growth inhibition by both calcium and sodium.

Amino Acid Sequence↗
PubMed1996

Regulation of endoplasmic reticulum-Ca-ATPase by glycolysis in eukaryotic cells.

This paper reviews work by our and other laboratories that explores the coupling between glycolysis and endoplasmic reticulum (ER)-Ca-ATPases in regulating Ca2+ homeostasis in several cell types. Changes in intracellular Ca2+ [(Ca2+]in) link interaction between hormones and cell surface receptors with the initiation of specific cellular functions. Thus, changes in [Ca2+]in mediate signal transduction mechanisms that modulate many physiological functions including cell growth, muscle cell contractility, and exocytosis in secretory cells. In most eukaryotic cells, total cellular Ca2+ is in the millimolar range, yet only a fraction (i.e., nanomolar) is free in the cytosol. Cells use both active and 'passive' mechanisms to maintain [Ca2+]in within a narrow range. Active mechanisms include plasma membrane and endoplasmic/sarcoplasmic reticulum (ER/SR)-Ca-ATPases, Ca2+ channels (inositol trisphosphate- and voltage-sensitive), and Na+/Ca2+ exchangers. 'Passive' mechanisms include Ca(2+)-binding proteins (e.g., calsequestrin, calmodulin, calreticulin). The relative contribution of active and 'passive' mechanisms to [Ca2+]in homeostasis in a given cell is not known. Ca2+ might move among several intracellular compartments, including the ER/SR, mitochondria, nucleus, Golgi apparatus, endosomes and lysosomes. The ubiquitous distribution of ER-Ca-ATPases in these intracellular organelles suggests a major role of this pump in Ca2+ homeostasis, but the importance of intracellular compartments to [Ca2+]in homeostasis is not well understood. Glucose has been suggested to have a role in regulating some of these ion transport processes. Thus, the increased cell metabolism that follows glucose stimulation is associated with altered [Ca2+]in homeostasis. The precise mechanisms by which glucose or its metabolites modulate [Ca2+]in homeostasis are unknown but might involve regulation of ER-Ca-ATPases.

Calcium↗
PubMed2002

Nucleolin is a calcium-binding protein.

We have purified a prominent 110-kDa protein (p110) from 1.6 M NaCl extracts of rat liver nuclei that appears to bind Ca2+. p110 was originally identified by prominent blue staining with 'Stains-All' in sodium dodecyl sulfate-polyacrylamide gels and was observed to specifically bind ruthenium red and 45Ca2+ in nitrocellulose blot overlays. In spin-dialysis studies, purified p110 saturably bound approximately 75 nmol Ca2+/mg protein at a concentration of 1 mM total Ca2+ with half-maximal binding observed at 105 microM Ca2+. With purification, p110 became increasingly susceptible to proteolytic (likely autolytic) fragmentation, although most intermediary peptides between 40 and 90 kDa retained "Stains-All", ruthenium red, and 45Ca2+ binding. N-terminal sequencing of intact p110 and a 70-kDa autolytic peptide fragment revealed a strong homology to nucleolin. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/IEF revealed autolysis produced increasingly acidic peptide fragments ranging in apparent pI's from 5.5 for intact p110 to 3.5 for a 40 kDa peptide fragment. Intact p110 and several peptide fragments were immunostained with a highly specific anti-nucleolin antibody, R2D2, thus confirming the identity of this protein with nucleolin. These annexin-like Ca2+-binding characteristics of nucleolin are likely contributed by its highly acidic argyrophilic N-terminus with autolysis apparently resulting in largely selective removal of its basic C-terminal domain. Although the Ca2+-dependent functions of nucleolin are unknown, we discuss the possibility that like the structurally analogous HMG-1, its Ca2+-dependent actions may regulate chromatin structure, possibly during apoptosis.

Animals↗
PubMed1996

Skeletal muscle of patients with Duchenne's muscular dystrophy: evidence of a mitochondrial proteolytic factor responsible for calmitine deficiency.

We studied the effect of mitochondrial extracts of skeletal muscle obtained from patients with Duchenne's muscular dystrophy (DMD) on calmitine of the mitochondrial matrix isolated from skeletal muscle of control mice. Our results in vitro clearly show that calmitine of the mitochondrial matrix of control muscle was degraded in the presence of mitochondrial extracts of muscle from DMD patients. The diseased muscle apparently contains an abnormal calmitine-specific proteolytic factor responsible for the calmitine deficiency previously observed in this tissue. As calmitine binds calcium and probably plays a role in regulating the balance of bound and free calcium within mitochondria, a calmitine deficiency could result in an overload of mitochondrial free calcium. Certain enzymes involved in ATP synthesis would be inhibited, resulting in the muscular degeneration characteristic of this myopathy. Our results suggest the cause of mitochondrial calcium overload and the events leading to muscular degeneration in this disease model. Abnormal protease activity could be the factor triggering all of these processes in the DMD patient. These findings suggest that it may now feasible to search for an efficient pharmacologic treatment for DMD.

Animals↗
PubMed1996

Absence of a calmitine-specific protease inhibitor in skeletal muscle mitochondria of patients with Duchenne's muscular dystrophy.

We studied the effect of mitochondrial extracts from skeletal muscle of patients with Duchenne's muscular dystrophy (DMD) on calmitine from the skeletal muscle of normal mice and control subjects. Our results clearly show the existence of an abnormal proteolytic activity of mitochondria from patients with DMD on calmitine from the normal mouse. This proteolytic activity was not found on calmitine from the control subject. Overall, our observations suggest that calmitine concentration in the muscle of the control subject remains elevated because of the presence of a calmitine-specific protease and an inhibitor of this protease which regulates and/or suppresses the activity of the enzyme according to the requirements of the muscle cell. Conversely, the calmitine deficiency observed in the muscle of patients with DMD would be due to the absence of this inhibitor. This would account for the continual activity of the enzyme in degrading calmitine as soon as it is synthesized. The identification of this inhibitor is currently being investigated in our laboratory.

Animals↗
PubMed1997

A drug inhibits the mitochondrial protease inducing calmitine deficiency in skeletal muscle of patients with Duchenne's muscular dystrophy and dy/dy dystrophic mice.

This study demonstrates that the cause of calmitine deficiency in dy/dy dystrophic mice and patients with Duchenne's muscular dystrophy (DMD) is the same; i.e., the absence of an inhibitor of calmitine-specific mitochondrial protease. This inhibitor, which is present in control mice and control subjects, prevented degradation of the protein. It is also shown that a drug (IP96) was capable in vitro of inhibiting calmitine-specific mitochondrial protease from muscle of DMD patients and dy/dy mice. This drug was also active in vivo in an experimental model of myopathy created in the normal mouse by a single injection of chlorpromazine, a myotoxic drug, which induced temporary calmitine degradation. Thus, it seems quite likely that IP96 prevents calmitine degradation by inhibiting the specific protease.

Animals↗
PubMed2002

Altered excitation-contraction coupling in myocytes from remodeled myocardium after chronic myocardial infarction.

Following myocardial infarction (MI), the left ventricle undergoes progressive dilatation and eccentric hypertrophy, i.e., remodeling, which is greater in the adjacent than the remote region. The cellular mechanisms underlying these regional differences were studied. One (n=5) and 8 weeks (n=8) after anteroapical MI in sheep, cardiac myocytes were isolated from the adjacent and remote regions. At 8 weeks after MI, myocyte function in the remote region was not different from values either in sham controls (n=3) or animals 1 week after MI. At 8 weeks after MI, myocyte contractile function (% contraction) was decreased, P<0.01, in the adjacent region (6.4+/-0.4%), as compared with the remote region (8.8+/-0.5%) and was associated with decreased amplitude of Ca(2+)transients (adjacent, 0.69+/-0.09 v remote, 1.08+/-0.20, P<0.05) and L-type Ca(2+)current density (adjacent, 3.6+/-0.2 v remote, 4.8+/-0.2 pA/pF, P<0.05). Relaxation was also impaired significantly in myocytes from the adjacent region, associated with decreased protein levels of SERCA2a. The myocytes were hypertrophied more in the adjacent region than the remote region. Furthermore, focal areas of central myofibrillar lysis and increased glycogen deposition were observed in the adjacent region. These results indicate that impaired excitation-contraction coupling underlies dysfunction of myocytes from the adjacent non-infarcted myocardium after chronic MI, even in the absence of heart failure. Hypertrophy is implicated as the mechanism, since these changes were noted at 8 weeks, but not at 1 week after MI.

Animals↗
PubMed1998

Contributions of electron microscopy and single-particle techniques to the determination of the ryanodine receptor three-dimensional structure.

The ryanodine receptor is the main intracellular calcium release channel from the sarcoplasmic reticulum in striated muscle. It is the largest ion channel known, composed of four identical major subunits of 565 kDa and four smaller 12-kDa subunits, identified as FK-506 binding protein. The successful isolation of the ryanodine receptor together with the development of cryoelectron microscopy and single-particle image processing techniques have enabled major progress to be made in the determination of the receptor's structure over the past decade. Three-dimensional reconstruction shows the receptor to be composed of two main parts, a large square shaped cytoplasmic assembly and a smaller transmembrane assembly. The cytoplasmic assembly has an unusual architecture in which about 10 domain-like structures are interconnected in a loosely packed manner. Subsequent studies have started to reveal conformational changes associated with channel gating and the localization of binding sites for some proteins with which the receptor interacts (calmodulin, and FK-506 binding protein). It is becoming clear that long-range induced conformational changes must be involved in the mechanisms of modulation of the receptor's gating properties.

Animals↗
PubMed1991

Contractile and calcium regulating capacities of myocardia of different sized mammals scale with resting heart rate.

The purpose of this study was to determine if selected biochemical parameters representing the contractile and calcium regulating systems of cardiac muscle scaled among mammals having inherently different resting heart rates (RHR). Eight mammalian species with RHR ranging from 51 to 475 beats per minute (bpm) were studied. The oxidative capacity of the myocardium is highly correlated with the RHR. The hypothesis of the present study was that the capacities of the energy utilizing processes of contraction and calcium regulation would also be correlated to the functional demand imposed on the muscle as represented by the RHR. Myosin (M) and myofibrillar (MF) ATPase activities, myosin isoenzyme distribution and sarcoplasmic reticulum (SR) ATPase activity were determined. Animals with RHR above 300 bpm express V1 myosin while animals with lower RHR express primarily V3. M and MF ATPase activities correlated with RHR, but the major difference in activities occurred at the 'threshold' RHR of about 300 bpm at which the switch from V3 to V1 appears to occur. SR ATPase activity per mg of microsomal protein was for the most part constant among different mammals, but the SR ATPase activity per g of heart tissue was significantly correlated with RHR as slower beating hearts tended to yield less SR protein per unit mass. We conclude that both the contractile and calcium regulating systems are scaled to the functional parameter of RHR among different mammals. The contractile system uses a slow myosin ATPase isoform at low resting heart rates whereas above the postulated threshold RHR of about 300 bpm a switch in gene expression to a fast myosin ATPase isoform occurs.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗
PubMed1995

Muscle regeneration and mitochondrial calmitine increase in the dystrophic dy/dy mouse after intramuscular chlorpromazine injection.

We studied the effect of chlorpromazine injection on the gastrocnemius muscles of C57BL/6J dy/dy dystrophic mice. Changes in mitochondrial calmitine concentrations and differences in microscopy studies, fibre typing and morphometry were compared in gastrocnemius muscles of dystrophic and control mice before and 2 and 21 days after injection. In both cases, calmitine reduction associated with muscle degeneration was observed 2 days after drug injection. Calmitine then increased, reaching a level at day 21 nearly identical to that of controls before injection. This increase was associated with muscle regeneration. These results clearly indicate that dystrophic mouse muscle can regenerate calmitine after drug-induced damage.

Animals↗
PubMed1982

Transitions in membrane composition during postnatal development of rabbit fast muscle.

Early postnatal changes (4-5 days to 15 days after birth) in the biochemical composition of microsomes were investigated in rabbit skeletal muscles destined to become fast-twitch muscles. During this period, a steady decrease in the microsomal content of cholesterol and of ouabain-sensitive Na + /K + -ATPase activity, as well as a decrease in protein electrophoretic components in the 80 000-70 000 molecular weight range, were observed. These changes are probably due to a diminishing yield of microsomal membranes derived from T-tubules, as the age of the animals increases, and are indicated from a knowledge of the mixed composition of muscle microsomes and previous biochemical data on isolated T-tubules. The content of cytochrome b5, which was found to be high in muscle microsomes of newborn animals, decreased strikingly as the amount of membrane-bound Ca2 + -ATPase protein increased, with a crossing-over point at about 7-10 days after birth. These changes, possibly corresponding to a transition from precursor sarcoplasmic reticulum (SR) to mature SR, were found to be temporally correlated with changes in [3H] alpha-tocopherol binding ability of the microsomes and in the mitochondrial content of glycerol phosphate dehydrogenase. At the same critical periods, coincident with the onset of motile activity, the immunological cross-reactivity of the Ca2 + -ATPase protein of microsomal vesicles, with antibody specific for the Ca2 + -ATPase of adult fast SR, was found to increase markedly, as tested by competitive enzyme-linked immunosorbent assay (ELISA). The immunological data are consistent with data in the literature demonstrating an increase in the concentration of Ca2 + -ATPase molecules in the SR membranes during ontogenic development. Both these data and catalytic data, however, suggest that the Ca2 + -ATPase protein is present in the same form in the SR of immature and of adult fast muscle and, in an antigenically different form, in slow muscle SR.

Animals↗