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.