Phosphorylation and dephosphorylation of calsequestrin on CK2-sensitive sites in heart

Calsequestrin (CSQ) concentrates in junctional sarcoplasmic reticulum (SR) where it functions in regulation of Ca2+ release. When purified from heart tissue, cardiac CSQ contains phosphate on a cluster of C-terminal serine residues, but little is known about the cellular site of kinase action, and the identity of the kinase remains uncertain. To determine basic features of the phosphorylation, we examined the reaction in canine heart preparations. CSQ phosphorylation was observed in [32P]metabolically-labeled heart cells after adenoviral overexpression, and its constitutive phosphorylation was limited to a CK2-sensitive C-terminal serine cluster. The CSQ kinase was oriented intralumenally, as was CSQ, inside membrane vesicles, such that exposure to each required detergent permeabilization. Yet even after detergent permeabilization, CSQ was phosphorylated much less efficiently by protein kinase CK2 in cardiac microsomes than was purified CSQ. Reduced phosphorylation was strongly dependent upon protein concentration, and phosphorylation time courses revealed a phosphatase activity that occurred constitutively as phosphorylated substrate accumulates. Evidence of selective dephosphorylation of CSQ glycoforms in heart homogenates was also seen by mass spectrometry analysis. Molecules with greater mannose content, a feature of early secretory pathway compartments, were more highly phosphorylated, while greater dephosphorylation was apparent in more distal compartments. Taken together, the analyses of CSQ phosphorylation in heart suggest that a constitutive process of phosphate turnover occurs for cardiac CSQ perhaps associated with its intracellular transport.

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.

Mass spectrometry of cardiac calsequestrin characterizes microheterogeneity unique to heart and indicative of complex intracellular transit

Cardiac calsequestrin concentrates in junctional sarcoplasmic reticulum in heart and skeletal muscle cells by an undefined mechanism. During transit through the secretory pathway, it undergoes an as yet uncharacterized glycosylation and acquires phosphate on CK2-sensitive sites. In this study, we have shown that active calsequestrin phosphorylation occurred in nonmuscle cells as well as muscle cells, reflecting a widespread cellular process. To characterize this post-translational modification and resolve individual molecular mass species, we subjected purified calsequestrin to mass spectrometry using electrospray ionization. Mass spectra showed that calsequestrin glycan structure in nonmuscle cells was that expected for an endoplasmic reticulum-localized glycoprotein and showed that each glycoform existed as four mass peaks representing molecules that also had 0-3 phosphorylation sites occupied. In heart, mass peaks indicated carbohydrate modifications characteristic of transit through Golgi compartments. Phosphorylation did not occur on every glycoform present, suggesting a far more complex movement of calsequestrin molecules in heart cells. Significant amounts of calsequestrin contained glycan with only a single mannose residue, indicative of a novel post-endoplasmic reticulum mannosidase activity. In conclusion, glyco- and phosphoforms of calsequestrin chart a complex cellular transport in heart, with calsequestrin following trafficking pathways not present or not accessible to the same molecules in nonmuscle.

Novel competitive inhibitor of NAD(P)H oxidase assembly attenuates vascular O(2)(-) and systolic blood pressure in mice

We previously reported enhanced expression of the p67(phox) and gp91(phox) components of NAD(P)H oxidase in angiotensin (Ang) II-induced hypertension, suggesting de novo assembly in response to Ang II. To examine the direct involvement of NAD(P)H oxidases in Ang II-induced O(2)(-) production, we designed a chimeric peptide that inhibits p47(phox) association with gp91(phox) in NAD(P)H oxidase (gp91ds-tat). This was achieved by linking a 9-amino acid peptide (aa) derived from HIV-coat protein (tat) to a 9-aa sequence of gp91(phox) (known to interact with p47(phox)). As a control, we constructed a chimera containing tat and a scrambled gp91 sequence (scramb-tat). We found that gp91ds-tat decreased O(2)(-) levels in aortic rings treated with Ang II (10 pmol/L) but had no effect on either the O(2)(-)-generating enzyme xanthine oxidase or potassium superoxide-generated O(2)(-). We infused vehicle, Ang II (0.75 mg. kg(-1). d(-1)), Ang II+gp91ds-tat (10 mg. kg(-1). d(-1)), or Ang II+scramb-tat intraperitoneally in C57Bl/6 mice and measured systolic blood pressure (SBP) on days 0, 3, 5, and 7 of infusion. SBP increased by day 3 in mice given Ang II and Ang II+scramb-tat but was significantly lower with Ang II+gp91-tat. On day 7, SBP was still significantly inhibited in mice given Ang II+gp91ds-tat, whereas Ang II-induced O(2)(-) production was inhibited throughout the aorta as detected by dihydroethidium staining, consistent with the ability of this inhibitor to block the various vascular NAD(P)H oxidase isoforms. These data support the hypothesis that inhibition of the interaction of p47(phox) and gp91(phox) (or its homologues) can block O(2)(-) production and attenuate blood pressure elevation in mice.

Upregulated Expression of Rat Heart Intercellular Adhesion Molecule-1 in Angiotensin II- but Not Phenylephrine- Induced Hypertension

-Intercellular adhesion molecule-1 (ICAM-1), part of an immunoglobulin-like superfamily of adhesion molecules, is involved in several cardiovascular diseases. We investigated whether in vivo angiotensin II (Ang II) increases ICAM-1 in rats. Sprague-Dawley rats were infused with vehicle or Ang II (750 µg. kg(-1). d(-1) SC) for 7 days. The contribution of Ang II receptors to ICAM-1 expression was investigated with a nonpeptide Ang II type 1 (AT(1)) receptor antagonist losartan (30 mg. kg(-1). d(-1) in drinking water). Systolic blood pressure was elevated in Ang II-treated animals compared with sham-treated controls, and losartan blocked this increase. Tumor necrosis factor (TNF)-alpha (5 µg/kg IP bolus), a prototype inducer of ICAM-1, was administered as a positive control for ICAM-1 expression. After treatment, hearts were frozen in liquid nitrogen; homogenates were subjected to SDS-PAGE and immunoblotted with an anti-rat ICAM-1 monoclonal antibody. We detected a predominantly high-molecular-weight band in homogenates from non-TNF-alpha-treated rats, which was enhanced by 80+/-5% in TNF-alpha-treated rats. This band measured approximately 200 kDa, which is the molecular weight of ICAM-1 in its native dimer form. The same band was detected in homogenates from sham and Ang II-treated rats, with the latter showing a 150+/-10% increase in ICAM-1 versus sham controls. Immunoprecipitation of rat heart homogenates with anti-rat ICAM-1 antibody resulted in a dominant band of the same molecular weight as samples not treated with antibody. Losartan prevented enhanced expression of ICAM-1 in the presence of Ang II but had no effect on basal ICAM-1 expression. Phenylephrine, an alpha-agonist (3 mg. kg(-1). d(-1) ), was infused for 1 week but had no effect on ICAM-1 expression, even though systolic blood pressure was elevated to the same level as in rats treated with Ang II. Thus, heart ICAM-1 expression is enhanced via AT(1) receptor activation independent of hypertension. Ang II-induced ICAM-1 expression was time and dose dependent, with maximal expression occurring within 5 to 7 days at 100 to 750 µg/kg Ang II. Immunohistochemical staining demonstrated markedly increased ICAM-1 levels in the perivascular area in Ang II-infused rats. Monocyte/macrophage accumulation was significantly greater in Ang II-treated rat hearts than in sham-treated hearts (10+/-1; P:<0.001; n=5). Thus during inflammation, overexpression of ICAM-1 may contribute to cardiovascular damage in diseases characterized by increased activity of the renin-angiotensin system.

Glycosphingolipid receptor function is modified by fatty acid content. Verotoxin 1 and verotoxin 2c preferentially recognize different globotriaosyl ceramide fatty acid homologues

Verotoxins (VT) are a family of Escherichia coli-derived toxins which have been associated with hemolytic uremic syndrome, the leading cause of acute pediatric renal failure, and hemorrhagic colitis. Verotoxins (VT1 and VT2c) both show terminal gal alpha 1-4gal-dependent binding to globotriaosylceramide (Gal alpha 1-4Gal beta 1-4Glc-Cer; Gb3), yet VT2c shows a thousandfold lower specific cytotoxic activity in vitro. Our previous studies have shown this discrepancy is a function of the receptor binding B subunit and that VT1/Gb3 binding in a lipid matrix is affected by heterogeneity in the ceramide fatty acid chain length. The influence of the fatty acid composition of Gb3 on the binding of VT1 and VT2c has now been compared using 14 homogeneous semisynthetic Gb3 molecular species of differing fatty acid chain length and degree of saturation from C12 to C24. The binding of verotoxin was quantitated by Scatchard analysis using a solid-phase binding assay in the presence of auxiliary lipids, which may in some respects approximate to receptor function within the plasma membrane of sensitive cells. Differential binding was observed for several of these species in the lipid matrix, indicating that the fatty acid moiety of Gb3 is important for VT binding under such conditions. The short chain fatty acid containing Gb3 (C12 and C14) showed minimal binding. Middle and long chain fatty acid Gb3 homologues (C16, C18, C20, C22, and C24) were effectively recognized by VTs. The presence of an unsaturated fatty acid in Gb3 significantly increased VT binding in all cases. C20:0 and C22:1 containing Gb3 had the greatest capacity to bind VT1. In contrast, C18:0 and C18:1 homologues showed the greatest capacity for VT2c binding (higher than VT1). These results were, in general, reflected in cell cytotoxicity in that receptor-deficient cells reconstituted with C22:1Gb3 were maximally sensitive to VT1 in vitro whereas cells reconstituted with C18:1Gb3 were maximally sensitive to VT2c. VT2c was an ineffective inhibitor of 125I-VT1 binding to C22:1 Gb3 but in contrast, more effective than VT1 to compete binding to C18:1 Gb3. Similarly, VT1 was less effective than VT2c to compete binding of 125I-VT2c to C18:1 but more effective than VT2c to compete for C22:1 Gb3 binding. These results suggest that VT1 and VT2c bind selectively to different but overlapping carbohydrate epitopes on the Gb3 molecule which are differentially available in these Gb3 fatty acid homologues in a lipid environment.(ABSTRACT TRUNCATED AT 400 WORDS)