| Summary: | Viral myocarditis, the inflammation of myocardium initiated by viral infection, is an important cause of mortality in neonates and children. In addition, it is a precursor to dilated cardiomyopathy (DCM). To date, no effective therapy is available for viral myocarditis/DCM. Coxsackievirus B3 (CVB3) is an important human pathogen of viral myocarditis. Extensive research efforts on CVB3 have broadened our understanding of the virus-host protein interactions. However, the pathogenesis of coxsackievirus-induced myocarditis is not fully understood. The objective of this dissertation is to explore the role of host protein manipulation in coxsackieviral replication and pathogenicity. My hypotheses are that (1) coxsackievirus hijacks host’s cellular autophagy mechanism to facilitate its own replication; and (2) the serum response factor (SRF) is cleaved by viral protease 2A during coxsackievirus infection and contributes to impaired myocardial function and progression to DCM. For project 1, I demonstrated that CVB3 manipulates the host autophagy pathway to supplement viral replication. Autophagy is an evolutionary conserved homeostatic mechanism in eukaryotes that degrades and recycles long-lived cytoplasmic proteins, as well as damaged organelles. The hallmark of autophagy is the formation of double-membrane vesicles known as autophagosomes. I provided the initial evidence that CVB3 infection induces the formation of autophagosomes. Up-regulation of autophagosome formation enhances CVB3 replication, whereas downregulation of autophagy pathway reduces CVB3 replication. My results help clarify the nature of the intracellular membranes previously shown to be required for viral replication. For project 2, I demonstrated that CVB3 manipulates SRF expression via protein cleavage. SRF is a transcription factor vital for the expression of cardiac contractile/regulator genes, as well as gene silencing microRNAs. Cardiac-specific knockout of SRF in adult transgenic mice results in disruption of cardiac gene expression and development of severe DCM. I showed that SRF is cleaved in CVB3-infected mouse hearts and cardiomyocytes. Further studies revealed that SRF is cleaved at the 327 amino acids position by CVB3-encoded protease 2A. I demonstrated that SRF cleavage contributes to DCM by abolishing the transactivation property of SRF and generating dominant-negative SRF-truncates. Taken together, these novel viral strategies bridged existing knowledge and may serve as therapeutic targets for viral myocarditis/DCM.
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