Matrix protein Osteonectin (SPARC) reduces inflammation and mortality during viral myocarditis

Abstract

Rationale: Acute viral myocarditis (VM) is one of the leading causes of heart failure in young adults, lacking specific therapy. Osteonection (SPARC) is a non-structural matrix protein modulating cell-matrix interaction in the heart, and has been implicated in cardiac remodeling. Whether SPARC may regulate cardiac inflammation and necrosis in VM through its matri-cellular function is unknown. Methods and results: To address the implication of SPARC in VM, a human coxsackie B3 (CVB3) murine model of VM was performed in SPARC-null and WT animals. SPARC transcript- and protein levels significantly increased by 100% percent 1 week after CVB3 injection compared to control injection. Furthermore, absence of SPARC increased the mortality (4 out of 6 in null vs. 3 out of 7 in WT at 15 days), which was associated with increased cardiac inflammation and necrosis (9% ± 3.03 in KO vs. 1.3% ± 0.24 in WT). Concordantly, Adenoviral SPARC (adSPARC) over-expression, prior to CVB3 injection, improved the survival in both SPARC-null and WT mice, and reduced cardiac inflammation and necrosis by 55 percent in the SPARC null mice. Most importantly, adSPARC over-expression in WT animals resulted in improved cardiac function after viral injury as compared to control vector (fractional shortening was 28,2% ± 0,6 vs. 21,2% ± 0,8 respectively). Intra-vital microscopy of the cremaster muscle allowed us to study leukocyte behavior and permeability in vivo. This demonstrated significantly increased permeability with 450% and decreased leukocyte velocities in SPARC null mice upon TNF-α stimulation, in line with increased leukocyte extravasation. Moreover we performed in vivo measurement of the endothelial glycocalyx, a gel like structure at the luminal side of endothelial cells which is important in endothelial barrier properties. We found glycocalyx quality to be significantly diminished in SPARC null mice. VCAM-1 was further identified as a possible mediator of proper leukocyte recruitment through increased SPARC, since VCAM-1 staining clearly co-localized with SPARC at the endothelial cell level. Conclusion: Here, we demonstrated a protective role of SPARC in VM by improving cardiac function and reducing cardiac inflammation, necrosis and overall mortality. This protective role can be attributed to SPARC by improving endothelial barrier properties and thereby prohibiting exaggerated leukocyte recruitment. Clinical Relevance: As therapy for acute myocarditis is lacking and mainly supportive, we believe SPARC may be a novel target for the treatment of VM.