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Previous studies have shown that GSIV induces apoptotic cell death through upregulation of the pro-apoptotic genes Bax and Bak in Grouper fin cells (GF-1 cells). However, the role of viral genome-encoded protein(s) in this death process remains unknown. In this study, we demonstrated that the Giant seaperch iridovirus (GSIV) genome encoded a serine/threonine kinase (ST kinase) protein, and induced apoptotic cell death via a p53-mediated Bax upregulation approach and a downregulation of Bcl-2 in fish cells. The ST kinase expression profile was identified through Western blot analyses, which indicated that expression started at day 1 h post-infection (PI), increased up to day 3, and then decreased by day 5 PI. This profile indicated the role of ST kinase expression during the early and middle phases of viral replication. We then cloned the ST kinase gene and tested its function in fish cells. The ST kinase was transiently expressed and used to investigate possible novel protein functions. The transient expression of ST kinase in GF-1 cells resulted in apoptotic cell features, as revealed with Terminal deoxynucleotidyl transferase biotin-dUTP nick-end labeling (TUNEL) assays and Hoechst 33258 staining at 24 h (37 %) and 48 h post-transfection (PT) (49 %). Then, through studies on the mechanism of cell death, we found that ST kinase overexpression could upregulate the anti-stress gene p53 and the pro-apoptotic gene Bax at 48 h PT. Interestingly, this upregulation of p53 and Bax also correlated to alterations in the mitochondria function that induced loss of mitochondrial membrane potential (MMP) and activated the initiator caspase-9 and the effector caspase-3 in the downstream. Moreover, when the p53-dependent transcriptional downstream gene was blocked by a specific transcriptional inhibitor, it was found that pifithrin-α not only reduced Bax expression, but also averted cell death in GF-1 cells during the ST kinase overexpression. Taken altogether, these results suggested that aquatic GSIV ST kinase could induce apoptosis via upregulation of p53 and Bax expression, resulting in mitochondrial disruption, which activated a downstream caspases-mediated cell death pathway.

Mitochondria are organelles critical for cell survival because they produce ATP and modulate programmed cell death (PCD) pathways. PCD pathways are important in many clinical disorders, such as ischemia/reperfusion injuries, trauma, and toxic/metabolic syndromes, as well as in chronic neurodegenerative conditions, such as amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, and Huntington's disease. Moreover, many viruses and other pathogens target the mitochondria. Viruses induce the production of various proteins in their hosts that have proapoptotic and anti-apoptotic activities, depending on the cellular environment. More specifically, many viruses that target mitochondria regulate the balance between the anti-and proapoptotic Bcl-2 family proteins and thereby increase their own survival within the host cell. Recent studies indicated that mitochondria centralize several critical innate immune responses based on the presence of several important signaling proteins within the mitochondria: mitochondrial antiviral signaling (MAVS), stimulation of interferon genes (STING), and NLR family member X1. Therefore, mitochondria are not only vital because they regulate cell survival and death but also they have broad roles in the control of cell functions following pathogen invasion. This chapter highlights the tight interplay between viral infection and mitochondria.