Bracovirus-mediated innexin hemichannel closure in cell disassembly

Abstract

Summary Cell-cell communication is necessary for cellular immune response. Hemichannel closure disrupts communication between intracellular and extracellular environments during polydnavirus-induced immunosuppression in invertebrates. However, the effects of hemichannel closure on cellular immune response are unclear. Here, we examined apoptotic body formation triggered by hemichannel closure in hemocytes of Spodoptera litura infected with bracovirus from the parasitic wasp, Microplitis bicoloratus . We showed that Microplitis bicoloratus bracovirus (MbBV) induced apoptotic cell disassembly, accompanied by hemichannel closure. Hemocyte apoptotic body formation was caused by the dysregulation of the innexins (Inxs), Inx1, Inx2, Inx3, and Inx4, during the MbBV-mediated inhibition of pI3K/AKT signaling and activation of caspase-3, which cleaved gap junction Inx proteins. Our results showed that hemichannel opening or closure in response to various stimuli, which induces the modulation of Inx levels, could inhibit or activate apoptotic body formation, respectively. Therefore, the “hemichannel open and close” model may regulate the cellular immune response.

Bracovirus Sneaks Into Apoptotic Bodies Transmitting Immunosuppressive Signaling Driven by Integration-Mediated eIF5A Hypusination

“…We have previously shown that MbBV induces AB formation by shutting down hemocyte hemichannels to activate caspase-3 and cleave Innexins (Inxs), triggering apoptotic cell disassembly ( 4 ). However, the biological role of MbBV-induced ABs in immunosuppression remains poorly understood ( Figure 1A ).…”

“…However, the biological role of MbBV-induced ABs in immunosuppression remains poorly understood ( Figure 1A ). To determine whether MbBV suppresses the immune response via AB phagocytosis, we used ABs-induced by reBac-Inx3 ( 4 , 31 , 32 ) as a positive control, detectable using 6 × His tag ( Figure S1A ) to allow us to determine whether ABs can be phagocytosed by Sf9 cells. The ABs induced by reBac-Inx3 were divided equally ( Figure S1B ) for direct protein quantification from the ABs, while the other was incubated with Sf9 cells ( Figures 1B–D ).…”

“…Next, we investigated the biological role of viral fragment reintegration. We have previously found that blocking the PI3K/AKT signaling pathway is essential for MbBV gene products to perform immunosuppressive function ( 4 ). To demonstrate that the virus genes are responsible for the physiological and biochemical changes in recipient cells, we examined the activation of the PI3K/AKT signaling pathway in both Sf9 and High Five recipient cells.…”

“…Bracovirus induce apoptosis in granulocytes and loss of adhesion in plasmatocyte ( 49 , 57 ), and further disassembly of apoptotic cells ( 4 ). In our research, Sf9 cells, which have low–level of activated caspase-3, were characterized to be like of granulocytes, while High Five cells, which have inactive caspase-3, were characterized to be more like plasmatocyte ( 58 ).…”

“…Microplitis bicoloratus bracovirus (MbBV), a member of the PDV family, plays an essential role in suppressing immune response of lepidopteran hosts to protect the growth and development of parasitoid larvae in the host hemocoel ( 2 , 3 ). Our recent studies have found that MbBV inhibition of the PI3K/AKT pathway induced hemichannel shutdown, triggering apoptosis and promoting the formation of apoptotic bodies (ABs) ( 4 ).…”