Apoptosis in spotted sea bass brain and astroglia-like cell line implies a possible pathogenesis of Lateolabrax maculatus iridovirus infection.

Brain undergoing apoptosis often results in serious dysfunction, especially when subjected to viral infection. Our previous findings revealed that Lateolabrax maculatus iridovirus (LMIV), genetically closely related to Red Sea Bream Iridovirus (RSIV), was able to invade the brain tissue of L. maculatus. The analysis of consequent brain injury may bring us explanations for the pathogenesis of LMIV infection. In the present study, viral loads in different brain segments (telencephalon, optic tectum, cerebellum, hypothalamus, medulla oblongata, optic nerve) of L. maculatus were measured by qPCR. Then, injury and apoptosis in abovementioned brain divisions were compared by HE staining and TUNEL staining in histological slides, respectively. In vitro assays were further performed using an astroglia-like cell line, LMB-L, developed from brain of L. maculatus. In addition, LMIV-induced apoptotic LMB-L cells were examined utilizing classic methods such as Annexin V/PI staining, JC-1 staining, detection of caspase activity as well as qRT-PCR. We found that LMIV exhibited tropism for different brain regions, of which cerebellum and optic nerve containing the highest viral loads, varying from 188-fold to 115-fold compared with medulla oblongata. Correspondingly, LMIV infection led to serious apoptosis in cerebellum, optic nerve, and optic tectum showing plenty of TUNEL-positive cells. This phenomenon seemed to be congruent with obvious morphological changes in cerebellum and optic nerve with decrease of cells and loose tissue. The apoptosis rates of virus-infected LMB-L cells dramatically increased from 13.43 to 42.32%. Apoptosis in LMB-L cells coincided with depolarization of mitochondrial membrane potential, significant activation of caspase 9, upregulation of the p53 (22.3-fold), Bax (62-fold), and downregulation of Mcl-1, which suggested that the apoptosis was probably mediated by mitochondrial signaling pathway. Moreover, it was also observed that transcription of Fas gene and activity of caspase 8 were evoked by virus infection in LMB-L cells. Thus, death receptor pathway may also be involved in the LMIV-induced apoptosis. Our findings provide new insights into the understanding about the pathogenesis of LMIV infection and apoptosis mechanisms mediated by virus.