Bombyx mori β-1,3-Glucan Recognition Protein 4 (BmβGRP4) Could Inhibit the Proliferation of B. mori Nucleopolyhedrovirus through Promoting Apoptosis

Abstract

β-1,3-glucan recognition proteins (βGRPs) as pattern recognition receptors (PRRs) play an important role in recognizing various pathogens and trigger complicated signaling pathways in insects. In this study, we identified a Bombyx mori β-1,3-glucan recognition protein gene named BmβGRP4, which showed differential expression, from a previous transcriptome database. The full-length cDNA sequence was 1244 bp, containing an open reading frame (ORF) of 1128 bp encoding 375 amino acids. BmβGRP4 was strongly expressed in the larval stages and highly expressed in the midgut of B. mori larvae in particular. After BmNPV infection, the expression of BmβGRP4 was reduced significantly in the midgut. Furthermore, a significant increase in the copy number of BmNPV was observed after the knockdown of BmβGRP4 in 5th instar larvae, while the overexpression of BmβGRP4 suppressed the proliferation of BmNPV in BmN cells. Subsequently, the expression analysis of several apoptosis-related genes and observation of the apoptosis morphology demonstrated that overexpression of BmβGRP4 facilitated apoptosis induced by BmNPV in BmN cells. Moreover, BmβGRP4 positively regulated the phosphatase and tensin homolog gene (BmPTEN), while expression of the inhibitor of apoptosis gene (BmIAP) was negatively regulated by BmβGRP4. Hence, we hypothesize that BmNPV infection might suppress BmPTEN and facilitate BmIAP to inhibit cell apoptosis by downregulating the expression of BmβGRP4 to escape host antiviral defense. Taken together, these results show that BmβGRP4 may play a role in B. mori response to BmNPV infection and lay a foundation for studying its functions.

Identification of Host Molecules Involved in the Proliferation of Nucleopolyhedrovirus in Bombyx mori

“…Several molecules were identified to inhibit or promote the proliferation of BmNPV in the silkworm. , Here, we showed that the genes BmSBSPON , BmPallc , BmGART , and BmppyR , whose transcription was upregulated by BmNPV, promoted the proliferation of BmNPV, while BmCLIP3 and BmCCE-un1 , which were downregulated by BmNPV, inhibited it, suggesting a positive role of these transcriptional variations for viral proliferation in the fat body of P50 larvae. The functions of BmSBSPON and BmppyR , as well as their homologues, in animals still need to be investigated: here, they were both identified to be associated with viral proliferation for the first time.…”

“…mori serine protease, BmSINAL10, red fluorescent protein (RFP), BmTCP-1β protein, B. mori proteinase lipase member H-A, and BmβGRP4 . Notably, BmAtlastin-n enhances the resistance of silkworms to both BmNPV and Nosema bombycis .…”

Bombyx mori C-Type Lectin (BmIML-2) Inhibits the Proliferation of B. mori Nucleopolyhedrovirus (BmNPV) through Involvement in Apoptosis

“…Interestingly, its transcript level significantly increased at 24 h after silkworms received BmNPV infection, while it was dramatically downregulated 48 h and 72 h after viral infection compared with the control group. Previous studies suggested that the expression level of B. mori βGRP4 dramatically decreased from 12 h to 72 h after BmNPV infection [ 25 ]. It was likely that more BmIML-2 were induced and were secreted to recognize the invading BmNPV within 24 h of infection, and BmNPV might suppress the host’s own immune defense mechanisms after 48 h to 72 h. We considered that BmIML-2 might respond to the BmNPV infection and possess multiple functions in regulating innate immune responses.…”

“…Conversely, the increased proliferation of BmNPV was observed after the knockdown of BmIML-2 via siRNA. Moreover, several other PRRs identified previously have also shown that up- or downregulation of these genes would significantly modulate the proliferation of BmNPV, such as BmβGRP4 [ 25 ] and BmPGRP2-2 [ 14 ]. Thus, we conceived that BmNPV, acting as a typical PPR, was provided with the effective capacity to inhibit the infection and replication of BmNPV.…”

“…For example, Bombyx mori peptidoglycan recognition protein BmPGRP2-2 negatively regulated the expression of BmPTEN and then inhibited the apoptosis through repressing PTEN-phosphoinositide 3-kinase (PI3K)/Akt signaling [ 14 ]. Additionally, another important PRR, BmβGRP4, could positively regulate the transcript level of BmPTEN and repress the expression of an apoptosis suppressor gene named BmIAP [ 25 ].…”