Alterations of housekeeping proteins in human aged and diseased hearts.

Pathological remodeling includes alterations of ion channel function and calcium homeostasis and ultimately cardiac maladaptive function during the process of disease development. Biochemical assays are important approaches for assessing protein abundance and post-translational modification of ion channels. Several housekeeping proteins are commonly used as internal controls to minimize loading variabilities in immunoblotting protein assays. Yet, emerging evidence suggests that some housekeeping proteins may be abnormally altered under certain pathological conditions. However, alterations of housekeeping proteins in aged and diseased human hearts remain unclear. In the current study, immunoblotting was applied to measure three commonly used housekeeping proteins (ß-actin, calsequestrin, and GAPDH) in well-procured human right atria (RA) and left ventricles (LV) from diabetic, heart failure, and aged human organ donors. Linear regression analysis suggested that the amounts of linearly loaded total proteins and quantified intensity of total proteins from either Ponceau S (PS) blot-stained or Coomassie Blue (CB) gel-stained images were highly correlated. Thus, all immunoblotting data were normalized with quantitative CB or PS data to calibrate potential loading variabilities. In the human heart, ß-actin was reduced in diabetic RA and LV, while GAPDH was altered in aged and diabetic RA but not LV. Calsequestrin, an important Ca2+ regulatory protein, was significantly changed in aged, diabetic, and ischemic failing hearts. Intriguingly, expression levels of all three proteins were unchanged in non-ischemic failing human LV. Overall, alterations of human housekeeping proteins are heart chamber specific and disease context dependent. The choice of immunoblotting loading controls should be carefully evaluated. Usage of CB or PS total protein analysis could be a viable alternative approach for some complicated pathological specimens.

Changes in calsequestrin, TNF-α, TGF-ß and MyoD levels during the progression of skeletal muscle dystrophy in mdx mice: a comparative analysis of the quadriceps, diaphragm and intrinsic laryngeal muscles.

In Duchenne muscular dystrophy (DMD), the search for new biomarkers to follow the evolution of the disease is of fundamental importance in the light of the evolving gene and pharmacological therapies. In addition to the lack of dystrophin, secondary events including changes in calcium levels, inflammation and fibrosis greatly contribute to DMD progression and the molecules involved in these events may represent potential biomarkers. In this study, we performed a comparative evaluation of the progression of dystrophy within muscles that are differently affected by dystrophy (diaphragm; DIA and quadriceps; QDR) or spared (intrinsic laryngeal muscles) using the mdx mice model of DMD. We assessed muscle levels of calsequestrin (calcium-related protein), tumour necrosis factor (TNF-α; pro-inflammatory cytokine), tumour growth factor (TGF-ß; pro-fibrotic factor) and MyoD (muscle proliferation) vs. histopathology at early (1 and 4 months of age) and late (9 months of age) stages of dystrophy. Fibrosis was the primary feature in the DIA of mdx mice (9 months: 32% fibrosis), which was greater than in the QDR (9 months: 0.6% fibrosis). Muscle regeneration was the primary feature in the QDR (9 months: 90% of centrally nucleated fibres areas vs. 33% in the DIA). The QDR expressed higher levels of calsequestrin than the DIA. Laryngeal muscles showed normal levels of TNF-α, TGF-ß and MyoD. A positive correlation between histopathology and cytokine levels was observed only in the diaphragm, suggesting that TNF-α and TGF-ß serve as markers of dystrophy primarily for the diaphragm.

Proteomic profiling of x-linked muscular dystrophy.

Progressive x-linked muscular dystrophy represents the most commonly inherited neuromuscular disorder in humans. Although the disintegration of the dystrophin-associated glycoprotein complex triggers the initial pathogenesis of Duchenne muscular dystrophy, secondary alterations in metabolic pathways, cellular signaling and the regulation of ion homeostasis are probably crucial factors that cause end-stage fibre degeneration. The application of mass spectrometry-based proteomics for the global cataloguing of muscle biomarkers has recently been applied to the analysis of the mdx animal model of muscular dystrophy and the biochemical evaluation of experimental exon skipping therapy. The fluorescence difference in-gel electrophoretic analysis of normal versus mdx diaphragm muscle revealed changed expression levels of proteins involved in nucleotide metabolism, Ca 2+-handling, the cellular stress response and key bioenergetic processes. The swift up-regulation of small heat shock proteins, such as cvHsp, seems to form an integral part of the repair mechanisms in dystrophic fibres and may be exploitable as a new option to treat inherited muscle degeneration. Importantly, the mass spectrometry-based profiling of mdx muscle following the specific removal of exon 23 in the mutated dystrophin gene transcript showed a partial reversal of important secondary changes. Experimental exon skipping restored the expression of the dystrophin isoform Dp427, its associated glycoprotein beta-dystroglycan, neuronal nitric oxide synthase, calsequestrin, adenylate kinase and the muscle-specific stress protein cvHsp. In the future, a well defined set of signature molecules could be used to improve diagnosis, monitor disease progression, identify new therapeutic pathways, and validate the effects of novel drugs or experimental treatments such as gene therapy.

Defective glycosylation of calsequestrin in heart failure.

OBJECTIVE: Levels of Ca2+ regulatory proteins have been extensively analyzed in cardiomyopathies as possible indices of change in sarcoplasmic reticulum (SR) structure and function. Measures of calsequestrin (CSQ), however, a critical protein component of the Ca2+ release complex in junctional sarcoplasmic reticulum, have provided little or no evidence of underlying dysfunction. We previously reported that calsequestrin isolated from heart tissue exists in a variety of glycoforms and phosphoforms reflecting mannose trimming of N-linked glycans and phosphorylation and dephosphorylation on protein kinase CK2-sensitive sites. METHODS: Here, we tested whether the distribution of molecular forms changes in heart failure (HF) reflecting possible remodeling of diseased tissue. Canine hearts were paced (220 beats/min) for 6-8 weeks to induce heart failure. Calsequestrin was purified from heart failure and sham-operated (control) treated canine ventricles and analyzed by electrospray mass spectrometry. RESULTS: The results showed striking changes in the mass distribution of calsequestrin molecules present in tissue from heart failure (five animals) compared with control (five animals). In heart failure, calsequestrin contained glycan structures that were uncharacteristic of normal junctional sarcoplasmic reticulum, consistent with altered metabolism or altered trafficking through secretory compartments. Glycoforms containing Man8,9, expected for a phenotype less muscle-like, were more than doubled in heart failure hearts, and molecules were also phosphorylated to a higher level. CONCLUSIONS: These data reveal in tachycardia-induced heart failure a new and potentially important change in the mannose content of calsequestrin glycans, perhaps indicative of defective junctional SR trafficking and Ca2+ release complex assembly.

Depletion of T-tubules and specific subcellular changes in sarcolemmal proteins in tachycardia-induced heart failure.

OBJECTIVE: The T-tubule membrane network is integrally involved in excitation-contraction coupling in ventricular myocytes. Ventricular myocytes from canine hearts with tachycardia-induced dilated cardiomyopathy exhibit a decrease in accessible T-tubules to the membrane-impermeant dye, di8-ANNEPs. The present study investigated the mechanism of loss of T-tubule staining and examined for changes in the subcellular distribution of membrane proteins essential for excitation-contraction coupling. METHODS: Isolated ventricular myocytes from canine hearts with and without tachycardia-induced heart failure were studied using fluorescence confocal microscopy and membrane fractionation techniques using a variety of markers specific for sarcolemmal and sarcoplasmic reticulum proteins. RESULTS: Probes for surface glycoproteins, Na/K ATPase, Na/Ca exchanger and Ca(v)1.2 demonstrated a prominent but heterogeneous reduction in T-tubule labeling in both intact and permeabilised failing myocytes, indicating a true depletion of T-tubules and associated membrane proteins. Membrane fractionation studies showed reductions in L-type Ca(2+) channels and beta-adrenergic receptors but increased levels of Na/Ca exchanger protein in both surface sarcolemma and T-tubular sarcolemma-enriched fractions; however, the membrane fraction enriched in junctional complexes of sarcolemma and junctional sarcoplasmic reticulum demonstrated no significant changes in the density of any sarcolemmal protein or sarcoplasmic reticulum protein assayed. CONCLUSION: Failing canine ventricular myocytes exhibit prominent depletion of T-tubules and changes in the density of a variety of proteins in both surface and T-tubular sarcolemma but with preservation of the protein composition of junctional complexes. This subcellular remodeling contributes to abnormal excitation-contraction coupling in heart failure.

Hypertrophy and functional alterations in hyperdynamic phospholamban-knockout mouse hearts under chronic aortic stenosis.

OBJECTIVE: To determine whether the hyperdynamic phospholamban-knockout hearts are capable of withstanding a chronic aortic stenosis. METHODS: The transverse section of the aorta was banded in phospholamban-knockout and their isogenic wild-type mice, which were followed with echocardiography in parallel, along with sham-operated mice, before and at 2.5, 5 and 10 weeks after surgery. RESULTS: Cardiac decompensation was evidenced by the presence of lung congestion in some banded knockouts and wild-types, giving rise to a subset of non-failing and failing hearts within each group. The incidence of heart failure was not genotype-dependent but rather associated with higher heart rates before surgery. The development of left ventricular hypertrophy was similar between knockouts and wild-types and longitudinal assessment of end-diastolic dimension indicated progressive increases after banding, with a greater dilation in failing mice. Fractional shortening was reduced in failing knockouts and wild-types to a similar degree, with an earlier onset in the knockouts. In addition, fractional shortening was decreased in non-failing knockouts but not wild-types. Ejection times shortened after aortic banding particularly for failing hearts. Assessment of the SR Ca(2+)-ATPase protein levels indicated similar downregulation for failing knockouts and wild-types, while the phospholamban levels were not significantly altered in wild-types. CONCLUSION: The hyperdynamic phospholamban-knockout hearts are able to compensate against a sustained aortic stenosis similar to wild-types.

Defective mitochondrial oxidative phosphorylation in myopathies with tubular aggregates originating from sarcoplasmic reticulum.

Abnormalities of the sarcotubular system presenting as tubular aggregates (TAs) have been described in a variety of neuromuscular disorders. Here, we report on immunohistochemical and biochemical findings in 7 patients (2 familial and 5 sporadic cases) suffering from myopathies with TAs. In muscle biopsy specimens from 5 of the 7 patients, TAs were immunopositive for the ryanodine receptor (RYR 1) of the sarcoplasmic reticulum (SR), the SR Ca2+ pump (SERCA2-ATPase), and the intraluminal SR Ca2+ binding protein calsequestrin, indicating an SR origin of these aggregates. Furthermore, these 5 cases showed decreased respiratory chain enzyme activities (NADH:CoQ oxidoreductase. complex I and cytochrome c oxidase [COX], complex IV), while the remaining 2 patients exhibited normal values. Our findings indicate a functional link between mitochondrial dysfunction and the presence of TAs originating from the sarcoplasmic reticulum.

Caveolae from canine airway smooth muscle contain the necessary components for a role in Ca(2+) handling.

To explain that bronchial smooth muscle undergoes sustained agonist-induced contractions in a Ca(2+)-free medium, we hypothesized that caveolae in the plasma membrane (PM) contain protected Ca(2+). We isolated caveolae from canine tracheal smooth muscle by detergent treatment of PM-derived microsomes. Detergent-resistant membranes were enriched in caveolin-1, a specific marker for caveolae as well as for L-type Ca(2+) channels and Ca(2+) binding proteins (calsequestrin and calreticulin) as determined by Western blotting. Also, the PM Ca(2+) pump was present but not connexin 43 (a noncaveolae PM protein), the sarcoplasmic reticulum (SR) Ca(2+) pump, or the type 1 inositol 1,4, 5-trisphosphate receptor, supporting the idea that SR-derived membranes were not present. Antibodies to caveolin coimmunoprecipitated caveolin with calsequestrin or calreticulin. Thus some of the cellular calsequestrin and calreticulin associated with caveolin on the cytoplasmic face of each caveola. Immunohistochemistry of tracheal smooth muscle crysosections confirmed the localization of caveolin and the PM Ca(2+) pump to the cell periphery, whereas the SR Ca(2+) pump was located deeper in the cell. The presence of L-type Ca(2+) channels, the PM Ca(2+) pump, and the Ca(2+) bindng proteins calsequestrin and calreticulin in caveolin-enriched membranes supports caveola involvement in airway smooth muscle Ca(2+) handling.

Sprint training normalizes Ca(2+) transients and SR function in postinfarction rat myocytes.

Previous studies have shown that myocytes isolated from sedentary (Sed) rat hearts 3 wk after myocardial infarction (MI) undergo hypertrophy, exhibit altered intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics and abnormal contraction, and impaired sarcoplasmic reticulum (SR) function manifested as prolonged half-time of [Ca(2+)](i) decline. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca(2+) regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would restore [Ca(2+)](i) dynamics and SR function in MI myocytes toward normal. In MI rats, HIST ameliorated myocyte hypertrophy as indicated by significant (P

Oligomeric status of the dihydropyridine receptor in aged skeletal muscle.

A prominent feature of aging is represented by a decrease in muscle mass and strength. Abnormalities in Ca2+ -regulatory membrane complexes are involved in many muscular disorders. In analogy, we determined potential age-related changes in a key component of excitation-contraction coupling, the dihydropyridine receptor. Immunoblotting of the microsomal fraction from aged rabbit muscle revealed a drastic decline in the voltage-sensing alpha1-subunit of this transverse-tubular receptor, but only marginally altered expression of its auxiliary alpha(2)-subunit and the Na+/K+ -ATPase. A shift to slower fibre type characteristics was indicated by an age-related increase in the slow calsequestrin isoform. Chemical crosslinking analysis showed that the triad receptor complex has a comparable tendency of protein-protein interactions in young and aged muscles. Hence, a reduced expression and not modified oligomerization of the principal dihydropyridine receptor subunit might be involved in triggering impaired triadic signal transduction and abnormal Ca2+ -homeostasis resulting in a progressive functional decline of skeletal muscles.

Heterogeneous transmural gene expression of calcium-handling proteins and natriuretic peptides in the failing human heart.

OBJECTIVE: Human heart failure is associated with a disturbed intracellular calcium (Ca2+) homeostasis. In this regard, ventricular wall stress is considered to be a determinant for expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a). In the present study, we analyzed the transmural protein and/or mRNA levels of SERCA2a, other Ca(2+)-handling proteins, and of atrial and brain natriuretic peptides (ANP and BNP) in the human heart. METHODS: Subepicardial (epi), midmyocardial (mid), and subendocardial (endo) sections of the left ventricular free wall from end-stage failing (n = 17) and nonfailing (n = 5) human hearts were analyzed by Western blot for immunoreactive protein levels of SERCA2a, phospholamban (PLN), and calsequestrin (CS). Subepi- and subendocardial sections were analyzed by Northern blot for steady-state mRNA levels of SERCA2a, Na(+)-Ca2+ exchanger (NCX1), ANP, and BNP. RESULTS: SERCA2a protein and mRNA levels were reduced by 40 +/- 5% (P < 0.01) and 25 +/- 7% (P < 0.05) in endo compared to epi in the failing heart and by 27 +/- 14% and 16 +/- 12% (non-significant) in the nonfailing heart, respectively. PLN protein levels were reduced by 23 +/- 6% (P < 0.05) in endo compared to epi in the failing heart and by 17 +/- 25% (non-significant) in the nonfailing heart, whereas CS protein levels and NCX1 mRNA levels were similar across the left ventricular wall. Strikingly, in the failing heart, both BNP and ANP mRNA levels were upregulated predominantly in endo. CONCLUSIONS: In the failing human heart, SERCA2a and PLN, as well as natriuretic peptides but not CS and NCX1 are differentially expressed across the left ventricular wall, implicating (1) different susceptibility of subendocardium and subepicardium to factors affecting expression of these proteins and (2) differences in regulation of the distinct calcium-cycling proteins.

Frequency dependent force generation correlates with sarcoplasmic calcium ATPase activity in human myocardium.

OBJECTIVE: In congestive heart failure both a decreased function of the sarcoplasmic Ca(2+)-ATPase and a negative force-frequency relationship have been shown. This study aimed to investigate a possible relationship between frequency potentiation, sarcoplasmic Ca(2+)-ATPase activity, and SERCA2 protein expression in human myocardium. METHODS: Frequency potentiation was studied in electrically stimulated, isometric, left ventricular papillary muscle strip preparations (37 degrees C, 0.5-3.0 Hz) from terminally failing (NYHA i.v.; n = 5, dilated cardiomyopathy) and nonfailing (donor hearts, n = 5) human myocardium. In the identical samples the Ca(2+)-ATPase activity (NADH coupled assay) and the protein expression of sarcoplasmic Ca(2+)-ATPase (SERCA2), phospholamban, and calsequestrin (western blot) were determined. The frequency dependent change in the force of contraction and Vmax of the Ca(2+)-ATPase activity and the protein expression of SERCA2 were correlated with each other. RESULTS: In terminally failing myocardium the force-frequency relationship was negative (2.0 Hz vs. 0.5 Hz: -0.2 +/- 0.1 delta mN) contrasting a positive rate dependent potentiation of force in nonfailing tissue (2.0 Hz vs. 0.5 Hz: +0.8 +/- 0.2 delta mN; p < 0.01). In failing myocardium the corresponding maximal sarcoplasmic Ca(2+)-ATPase activity (Vmax) was reduced significantly compared to nonfailing myocardium (174 +/- 24 vs. 296 +/- 31 nmol ATP/mg.min, p < 0.01). The protein expression of SERCA2, phospholamban, and calsequestrin remained unchanged in failing myocardium. The maximal Ca(2+)-ATPase activity significantly correlated with the frequency dependent change in force of contraction (2 Hz vs. 0.5 Hz: r = 0.88, p = 0.001; 3 Hz vs. 0.5 Hz: r = 0.84, p = 0.004). No correlation between protein expression of SERCA2 and Ca(2+)-ATPase activity or change in force of contraction was observed. CONCLUSION: Due to a significant correlation between sarcoplasmic Ca(2+)-ATPase activity and frequency potentiation, the negative rate dependent force potentiation in human heart failure could be at least in part be attributed to decreased function of the sarcoplasmic Ca(2+)-ATPase.

Human SERCA2a levels correlate inversely with age in senescent human myocardium.

OBJECTIVES: This study sought to characterize functional impairment after simulated ischemia-reperfusion (I/R) or Ca2+ bolus in senescent human myocardium and to determine if age-related alterations in myocardial concentrations of SERCA2a, phospholamban, or calsequestrin participate in senescent myocardial dysfunction. BACKGROUND: Candidates for elective cardiac interventions are aging, and an association between age and impairment of relaxation has been reported in experimental animals. Function of the sarcoplasmic reticulum resulting in diastolic dysfunction could be dysregulated at the level of cytosolic Ca2+ uptake by SERCA2a, its inhibitory subunit (phospholamban), or at the level of Ca2+ binding by calsequestrin. METHODS: Human atrial trabeculae from 17 patients (45-75 years old) were suspended in organ baths, field simulated at 1 Hz, and force development was recorded during I/R (45/120 min). Trabeculae from an additional 12 patients (53-73 years old) were exposed to Ca2+ bolus (2-3 mmol/L bath concentration). Maximum +/- dF/dt and the time constant of force decay (tau) were measured before and after I/R or Ca2+ bolus and related to age. SERCA2a, phospholamban, and calsequestrin from 12 patients (39-77 years old) were assessed by immunoblot. RESULTS: Functional results indicated that maximum +/-dF/dt and tau were prolonged in senescent (>60 years) human myocardium after I/R (p < 0.05). Calcium bolus increased the maximum +/-dF/dt and decreased tau in younger, but not older patients (p < 0.05). SERCA2a and the ratio of SERCA2a to either phospholamban or calsequestrin were decreased in senescent human myocardium (p < 0.05). CONCLUSIONS: Senescent human myocardium exhibits decreased myocardial SERCA2a content with age, which may, in part, explain impaired myocardial function after either I/R or Ca2+ exposure.

Expression of calcium regulatory proteins in short-term hibernation and stunning in the in situ porcine heart.

BACKGROUND: Myocardial hibernation and stunning are characterised by a reversible contractile dysfunction during and after ischaemia, respectively. Calcium homeostasis might be disturbed in hibernation and stunning due to altered expression of cardiac proteins involved in calcium handling. METHODS: In enflurane-anaesthetised swine the coronary blood flow through the left anterior descending coronary artery was decreased to reduce regional contractile function (microsonometry) by approximately 50%. In transmural biopsies obtained during ischaemia and reperfusion creatine phosphate as well as the expression of sarcoplasmic reticulum calcium ATPase (SERCA), phospholamban (PLB), calsequestrin (CSQ), and troponin inhibitor (TnI) were determined. RESULTS: During ischaemia creatine phosphate, after an initial reduction, recovered back to control values, and necrosis was absent (hibernation). After 90 min of ischaemia the myocardium was reperfused for 120 min but regional contractile function continued to be depressed (stunning). PLB, SERCA, CSQ, and TnI proteins were unchanged during ischaemia as well as reperfusion. Likewise, levels of PLB and SERCA mRNAs were unchanged. CONCLUSION: It is concluded that other mechanisms than altered expression of these regulating proteins underlie the contractile dysfunction observed during acute ischaemia, short-term hibernation and stunning.

Altered turnover of calcium regulatory proteins of the sarcoplasmic reticulum in aged skeletal muscle.

We have measured the in vivo protein turnover for the major calcium regulatory proteins of the sarcoplasmic reticulum from the skeletal muscle of young adult (7 months) and aged (28 months) Fischer 344 rats. From the time course of the incorporation and decay of protein-associated radioactivity after a pulse injection of [14C]leucine and correcting for leucine reutilization, in young rats, the apparent half-lives for calsequestrin, the 53-kDa glycoprotein, and ryanodine receptor are 5.4 +/- 0.4, 6.3 +/- 1.3, and 8.3 +/- 1.3 days, respectively. A half-life of 14.5 +/- 2.5 days was estimated for the Ca-ATPase isolated from young muscle. Differences in protein turnover associated with aging were determined using sequential injection of two different isotopic labels ([14C]leucine and [3H]leucine) to provide an estimate of protein synthesis and degradation within the same animal. The Ca-ATPase and ryanodine receptor isolated from aged muscle exhibits 27 +/- 5% and 25 +/- 3% slower protein turnover, respectively, relative to that from young muscle. In contrast, the 53-kDa glycoprotein exhibits a 25 +/- 5% more rapid turnover in aged SR, while calsequestrin exhibits no age-dependent alteration in turnover. Statistical analysis comparing the sensitivity of various methods for discriminating different rates of protein turnover validates the approach used in this study and demonstrates that the use of two isotopic labels provides at least a 6-fold more sensitive means to detect age-related differences in protein turnover relative to other methods.

Calreticulin, a potential vascular regulatory protein, reduces intimal hyperplasia after arterial injury.

Both thrombotic and inflammatory responses to arterial injury have been implicated in atherosclerotic plaque growth. Calreticulin is a ubiquitous calcium-binding protein with antithrombotic activity and, in addition, is associated with leukocyte activation. We are investigating calreticulin as a potential vascular regulatory protein. The development of intimal hyperplasia was studied at sites of balloon injury in iliofemoral arteries from 91 rats. Calreticulin was infused directly into the artery immediately before balloon injury, and plaque growth was then assessed at 4 weeks' follow-up. Parallel studies of the effects of each calreticulin domain as well as a related calcium-binding protein, calsequestrin, were examined. The effects of calreticulin on platelet activation, clot formation, and mononuclear cell migration were also studied. When infused before balloon injury in rat iliofemoral arteries, calreticulin, or its high-capacity Ca(2+)-binding C domain, significantly reduces plaque development, whereas calsequestrin, a related calcium-binding protein that lacks the multifunctional nature of calreticulin, does not decrease plaque area (saline: 0.037 +/- 0.007 mm2, calsequestrin: 0.042 +/- 0.021 mm2, calreticulin: 0.003 +/- 0.002 mm2, n = 46, P < .04). The N domain and more specifically the P domain, a low-capacity, high-affinity calcium-binding domain in calreticulin, do not reduce intimal hyperplasia (N + P domain: 0.038 +/- 0.012 mm2, C domain: 0.003 +/- 0.002 mm2, n = 45 rats, P < .0001). Calreticulin reduces macrophage and T cell staining in the arterial wall after injury but has no direct effect on monocyte migration in vitro (percent medial area staining positive for macrophage 24 hours after injury (N + P: 4.06 +/- 1.42, calreticulin: 0.273 +/- 0.02; n = 26, P < .009). Calreticulin does, however, reduce platelet-dependent whole blood clotting time, in vitro (baseline: 78.23 +/- 2.04 seconds, calreticulin: 113.5 +/- 1.95 seconds; n = 5, P < .002). We conclude that calreticulin significantly reduces intimal hyperplasia after arterial injury, potentially acting as a vascular regulatory protein.

Differential distribution of calcium stores in paramecium cells. Occurrence of a subplasmalemmal store with a calsequestrin-like protein.

We have analyzed in Paramecium cells the occurrence and intracellular distribution of the high capacity/low affinity calcium-binding proteins, calsequestrin (CS) and calreticulin (CR) using antibodies against CS from rat skeletal muscle and against CR from rat liver, respectively. As revealed by Western blots, a CS-like protein isolated by affinity chromatography from Paramecium cells comigrated with CS isolated from rat skeletal muscle. The immunoreactivity of this 53 kDa protein band was blocked when the antibodies had been preadsorbed with purified rat CS. A band of identical molecular size was shown to bind 45Ca in overlays. By immunofluorescence and immunogold labeling this CS-like protein was localized selectively to the extended subplasmalemmal calcium stores, the "alveolar sacs", which cover almost the entire cell surface. Concomitantly the 53 kDa 45Ca-binding band became increasingly intense in overlays as we increasingly enriched alveolar sacs. Antibodies against rat CR react with a 61 kDa band but do not cross-react with CS-like protein in Paramecium. These antibodies selectively stained intracellular reticular structures, identified bona fide as endoplasmic reticulum.

Regulation of calcium binding proteins calreticulin and calsequestrin during differentiation in the myogenic cell line L6.

In this report we defined the structural and temporal limits within which calreticulin and calsequestrin participate in the muscle cell phenotype, in the L6 model myogenic system. Calreticulin and calsequestrin are two Ca2+ binding proteins thought to participate in intracellular Ca2+ homeostasis. We show that calsequestrin protein and mRNA were expressed when L6 cells were induced to differentiate, during which time the level of expression of calreticulin protein did not change appreciably. Calreticulin mRNA levels, however, were constant throughout L6 cell differentiation except for slight decline in the mRNA levels at the very late stages of L6 differentiation (day 11-12). We also show that the two Ca2+ binding proteins are coexpressed in differentiated L6 cells. Based on its mobility in SDS-PAGE, L6 rat skeletal muscle cells in culture expressed cardiac isoform of calsequestrin. In the mature rat skeletal muscle, calreticulin and calsequestrin were localized to sarcoplasmic reticulum (SR). Calreticulun, but not calsequestrin, staining was also observed in the perinuclear region. These data suggest that expression of calreticulin and calsequestrin may be under different control during myogenesis in rat L6 cells in culture.

Evidence that spinach leaves express calreticulin but not calsequestrin.

The presence of either calreticulin (CR) or calsequestrin (CS-like proteins in spinach (Spinacia oleracea L.) leaves has been previously described. Here we report the purification from spinach leaves of two highly acidic (isoelectric point 5.2) Ca(2+)-binding proteins of 56 and 54 kD by means of DEAE-cellulose chromatography followed by phenyl-Sepharose chromatography in the presence of Zn(2+) (i.e., under experimental conditions that allowed the purification of CR from human liver). On the other hand, we failed to identify any protein sharing with animal CS the ability to bind to phenyl-Sepharose in the absence of Ca(2+). Based on the N-terminal amino acid sequence, the 56- and 54-kD spinach Ca(2+)-binding proteins were identified as two distinct isoforms of CR. Therefore, we conclude that CR, and not CS, is expressed in spinach leaves. The 56-kD spinach CR isoform was found to be glycosylated, as judged by ligand blot techniques with concanavalin A and affinity chromatography with concanavalin A-Sepharose. Furthermore, the 56-kD CR was found to differ from rabbit liver CR in amino acid sequence, peptide mapping after partial digestion with Staphylococcus aureus V8 protease, pH-dependent shift of electrophoretic mobility, and immunological cross-reactivity with an antiserum raised to spinach CR, indicating a low degree of structural homology with animal CRs.

Characterization of the sarcoplasmic reticulum proteins in the thermogenic muscles of fish.

Marlins, sailfish, spearfishes, and swordfish have extraocular muscles that are modified into thermogenic organs beneath the brain. The modified muscle cells, called heater cells, lack organized myofibrils and are densely packed with sarcoplasmic reticulum (SR), transverse (T) tubules, and mitochondria. Thermogenesis in the modified extraocular muscle fibers is hypothesized to be associated with increased energy turnover due to Ca2+ cycling at the SR. In this study, the proteins associated with sequestering and releasing Ca2+ from the SR (ryanodine receptor, Ca2+ ATPase, calsequestrin) of striated muscle cells were characterized in the heater SR using immunoblot and immunofluorescent techniques. Immunoblot analysis with a monoclonal antibody that recognizes both isoforms of nonmammalian RYRs indicates that the fish heater cells express only the alpha RYR isoform. The calcium dependency of [3H]ryanodine binding to the RYR isoform expressed in heater indicates functional identity with the non-mammalian alpha RYR isoform. Fluorescent labeling demonstrates that the RYR is localized in an anastomosing network throughout the heater cell cytoplasm. Measurements of oxalate supported 45Ca2+ uptake, Ca2+ ATPase activity, and [32P]phosphoenzyme formation demonstrate that the SR contains a high capacity for Ca2+ uptake via an ATP dependent enzyme. Immunoblot analysis of calsequestrin revealed a significant amount of the Ca2+ binding protein in the heater cell SR. The present study provides the first direct evidence that the heater SR system contains the proteins necessary for Ca2+ release, re-uptake and sequestration, thus supporting the hypothesis that thermogenesis in the modified muscle cells is achieved via an ATP-dependent cycling of Ca2+ between the SR and cytosolic compartments.