Porcine epidemic diarrhea virus S1 protein is the critical inducer of apoptosis

Porcine Epidemic Diarrhea (PED) is an acute and highly contagious enteric disease caused by PED virus (PEDV), characterized by vomitting, watery diarrhea and fatal dehydration with high mortality in sucking piglets of one week of age. Although PEDV induced cell apoptosis has been established in vitro and in vivo, the functional protein that contributes to this event remains unclear.

The activation or cleavage of main apoptosis-associated molecular such as AIFM1, caspase-3, caspase-8, caspase-9 and PARP in PEDV infected host cells were analyzed by western blotting. The nuclear change of infected cell was monitored by confocal immunofluorescence assay. The overexpressing plasmids of 16 non-structural proteins (Nsp1–16) and 6 structural proteins (M, N, E, ORF3, S1 and S2) were constructed by cloning. Cell apoptosis induced by PEDV or overexpression non-structural or structural proteins was measured by the flow cytometry assay.

PEDV could infect various host cells including Vero, Vero-E6 and Marc-145 and cause obvious cytopathic effects, including roundup, cell fusion, cell membrane vacuolation, syncytium formation and cause apparent apoptosis. In infected cells, PEDV-induced apoptosis is accompanied by nuclear concentration and fragmentation as a result of caspase-3 and caspase-8 activation and AIFM1 and PARP cleavage. Overexpression of S1 Spike protein of PEDV SM98 strain effectively induced host cell apoptosis, while the expression of the other non-structure proteins (Nsp1–16) and structural proteins (M, N, E, S2 and ORF3) has no or less effect on cell apoptosis. Similarly, expression of S1 protein from wild-type strain BJ2011 or cell-adapted strain CV777, also induce apoptosis in transfected cells. Finally, we demonstrated that the S1 proteins from various coronavirus family members such as TGEV, IBV, CCoV, SARS and MERS could also induce Vero-E6 cells apoptosis.

S1 Spike protein is one of the most critical functional proteins that contribute to cell apoptosis. Expression of S1 proteins of the coronavirus tested in this study could all induce cell apoptosis suggesting S1 maybe is an effective inducer in Coronavirus-induced cell apoptosis and targeting S1 protein expression probably is a promising strategy to inhibit coronavirus infection and thus mediated apoptosis on host cells.