Biosynthesis of intrinsic sarcoplasmic reticulum proteins during differentiation of the myogenic cell line L6

Sequential expression during postnatal development of specific markers of junctional and free sarcoplasmic reticulum in chicken pectoralis muscle

Skeletal muscle sarcoplasmic reticulum comprises two distinct membrane domains, i.e., the Ca(2+)-pump membrane, corresponding mainly to longitudinal tubules, and the junctional membrane of the terminal cisternae containing the ryanodine receptor/Ca(2+)-release channel. Additional minor proteins previously shown in rabbit fast-twitch skeletal muscle to fractionate selectively to each membrane domain comprise 160- and 53-kDa glycoproteins and 170-kDa low-density lipoprotein (LDL)-binding protein, respectively (Damiani and Margreth, 1991, Biochem. J. 277, 825-832). We report evidence in chicken pectoralis, a predominantly fast muscle, on two closely immunologically related glycoproteins, a minor component of 130-kDa and a major 53-kDa protein. In contrast to the seemingly highly conserved structure of this protein, our results show marked differences in mobilities for chicken 125I-LDL that were detected as a 130- to 116-kDa protein doublet after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, although being otherwise indistinguishable from rabbit 170-kDa protein in LDL-binding characteristics, as well as for preferential association to junctional terminal cisternae. Chicken Ca(2+)-ATPase, although being extensively homologous to rabbit Ca(2+)-ATPase, is shown to be less active and to differ slightly in electrophoretic properties. We have investigated the time course of expression of the specific protein components of longitudinal and of junctional sarcoplasmic reticulum in chick pectoralis muscle from late embryonic development up to 2 months after hatching. Coincident with the posthatching increase in membrane density of high-affinity [3H]ryanodine-binding sites in muscle, both calsequestrin and the species-specific LDL-binding protein(s) are detected in increasing amounts, using ligand blot techniques. In contrast, the appearance and steady accumulation in muscle of Ca(2+)-ATPase, like the time-correlated increase of sarcoplasmic reticulum glycoproteins, are relatively delayed, the most striking changes occurring from 1 week after hatching onward. The sequential expression in chick developing muscle of proteins selectively associated with the junctional terminal cisternae and with longitudinal sarcoplasmic reticulum, respectively, argues for a similar morphogenetic program in avian and mammalian species and, to account for that, for the existence of common epigenetic differentiating influences on the expression of sarcoplasmic reticulum protein genes.

Influence of the 53 kDa glycoprotein on the cooperativity of the Ca(2+)-ATPase of the sarcoplasmic reticulum

Previous results from this laboratory suggest that the 53 kDa glycoprotein (GP-53) of rabbit skeletal muscle sarcoplasmic reticulum membrane (SR) may influence coupling between Ca2+ transport and ATP hydrolysis by the Ca(2+)-ATPase. Here we report evidence that GP-53 may influence the cooperative behavior of the Ca(2+)-ATPase. The ATPase activity of the Ca(2+)-ATPase displays negative cooperative dependence (Hill coefficient n less than 1) on [MgATP] and has positive cooperative dependence (n greater than 1) on [Ca2+]free. We have determined the degree of cooperativity for native SR vesicles, SR preincubated with antiserum against GP-53 or preimmune serum, and SR partially extracted with KCl-cholate. Our results show that SR preincubated with preimmune serum or SR treated with cholate in 50 mM KCl (yielding membranes rich in GP-53) demonstrate a cooperative dependence of Ca(2+)-ATPase activity on both [ATP] and [Ca2+] similar to that of untreated SR. SR preincubated with anti-GP-53 antiserum (which causes an uncoupling of Ca2+ transport from ATP hydrolysis) or SR extracted with cholate in 1 M KCl (yielding membranes depleted of GP-53) displays decreased positive cooperative dependence on [Ca2+] and decreased negative cooperative dependence on [ATP]. The results are consistent with the interpretation that GP-53 may influence the cooperative behavior of the Ca(2+)-ATPase.

Differentiation-dependent expression of apolipoprotein A-I in chicken myogenic cells in culture

Northern blot hybridization experiments showed that Apolipoprotein A-I (Apo A-I) mRNA is present at high concentration in chicken myotubes cultured in vitro, while it is virtually absent in fibroblasts and myoblasts. Myotubes are also capable of translating and secreting in the culture medium a protein which is specifically immunoprecipitated by anti-Apo A-I antibodies and has the same electrophoretic mobility as Apo A-I purified from circulating high-density lipoproteins. The appearance of Apo A-I mRNA in myotubes depends on the transcriptional activation of the corresponding gene, as it was shown by hybridizing 32P-labeled RNA synthesized in isolated nuclei to Apo A-I cDNA. The activation of the Apo A-I gene is regulated by the muscle cell coordinately with muscle-specific genes. In fact, treatment with TPA, a powerful inhibitor of differentiation, efficiently prevents myoblasts from producing Apo A-I mRNA, as well as muscle actin mRNA, and causes myotubes to quickly cease Apo A-I mRNA synthesis. The existence of a strict relationship between Apo A-I mRNA concentration and myogenic cell differentiation was also confirmed by experiments with quail myoblasts transformed with a temperature-sensitive mutant of the Rous Sarcoma Virus. Cells raised at the permissive temperature (undifferentiated phenotype) do not contain Apo A-I as well as alpha-actin mRNAs, while shifting to the nonpermissive temperature (differentiated phenotype) causes a rapid increase in Apo A-I and alpha-actin mRNA concentration.

Calcium transport by sarcoplasmic reticulum of skeletal muscle is inhibited by antibodies against the 53-kilodalton glycoprotein of the sarcoplasmic reticulum membrane

The effects of an antiserum against the 53-kDa glycoprotein (GP-53) of the sarcoplasmic reticulum (SR) and of monoclonal antibodies against GP-53 on Ca2+ transport and ATP hydrolysis by SR of rabbit skeletal muscle have been investigated. Preincubation of SR with an antiserum against GP-53 resulted in decreased ATP-driven Ca2+ transport by the SR but had no effect on Ca2+-stimulated ATP hydrolysis. Preincubation of SR with preimmune serum had no significant effect on either Ca2+ transport or Ca2+-ATPase activity. The effect of anti-GP-53 serum was time and concentration dependent. Preincubation of SR with two monoclonal antibodies against GP-53 had no effect on Ca2+ transport or on Ca2+-stimulated ATP hydrolysis. However, preincubation of SR with either monoclonal antibody against GP-53 together with a monoclonal antibody against the Ca2+-ATPase (at levels which had little effect alone) resulted in markedly decreased rates of Ca2+ uptake and ATP hydrolysis. Preincubation of SR with anti-GP-53-serum or with monoclonal antibodies, under the same conditions that inhibited Ca2+ uptake, did not increase the passive permeability of the SR membrane to Ca2+, did not decrease the permeability of the SR to oxalate, and did not cause significant proteolysis of the Ca2+-ATPase. Our results are consistent with the interpretation that GP-53 may modulate the function of the Ca2+-ATPase of the SR membrane.

The Duchenne muscular dystrophy gene product is localized in sarcolemma of human skeletal muscle

Duchenne muscular dystrophy (DMD) and its milder form, Becker muscular dystrophy (BMD), are allelic X-linked muscle disorders in man. The gene responsible for the disease has been cloned from knowledge of its map location at band Xp21 on the short arm of the X chromosome. The product of the DMD gene, a protein of relative molecular mass 400,000 (Mr 400K) recently named dystrophin, has been reported to co-purify with triads of mouse and rabbit skeletal muscle when assayed using polyclonal antibodies raised against fusion proteins encoded by regions of mouse DMD complementary DNA. Here we show that antibodies directed against synthetic peptides and fusion proteins derived from the N-terminal region of human DMD cDNA strongly react with an antigen present in skeletal muscle sarcolemma on cryostat sections of normal human muscle biopsies. This immunoreactivity is reduced or absent in muscle fibres from DMD patients but appears normal in muscle fibres from patients with other myopathic diseases. The same antibodies specifically react with a 400K protein in sodium dodecyl sulphate (SDS) extracts of normal human muscle subjected to Western blot analysis. We conclude that the product of the DMD gene is associated with the sarcolemma rather than with the triads and speculate that it strengthens the sarcolemma by anchoring elements of the internal cytoskeleton to the surface membrane.

Monoclonal antibodies to the Ca2+ + Mg2+-dependent ATPase of sarcoplasmic reticulum identify polymorphic forms of the enzyme and indicate the presence in the enzyme of a classical high-affinity Ca2+ binding site

In order to determine whether polymorphic forms of the Ca2+ + Mg2+-dependent ATPase exist, we have examined the cross-reactivity of five monoclonal antibodies prepared against the rabbit skeletal muscle sarcoplasmic reticulum enzyme with proteins from microsomal fractions isolated from a variety of muscle and nonmuscle tissues. All of the monoclonal antibodies cross-reacted in immunoblots against rat skeletal muscle Ca2+ + Mg2+-dependent ATPase but they cross-reacted differentially with the enzyme from chicken skeletal muscle. No cross-reactivity was observed with the Ca2+ + Mg2+-dependent ATPase of lobster skeletal muscle. The pattern of antibody cross-reactivity with a 100,000 dalton protein from sarcoplasmic reticulum and microsomes isolated from various muscle and nonmuscle tissues of rabbit demonstrated the presence of common epitopes in multiple polymorphic forms of the Ca2+ + Mg2+-dependent ATPase. One of the monoclonal antibodies prepared against the purified Ca2+ + Mg2+-dependent ATPase of rabbit skeletal muscle sarcoplasmic reticulum was found to cross-react with calsequestrin and with a series of other Ca2+-binding proteins and their proteolytic fragments. Its cross-reactivity was enhanced in the presence of EGTA and diminished in the presence of Ca2+. Its lack of cross-reactivity with proteins that do not bind Ca2+ suggests that it has specificity for antigenic determinants that make up the Ca2+-binding sites in several Ca2+-binding proteins including the Ca2+ + Mg2+-dependent ATPase.