Nucleoprotein of influenza A virus negatively impacts antiapoptotic protein API5 to enhance E2F1-dependent apoptosis and virus replication.

A K Mayank,H Nailwal,S Sharma,S K Lal

doi:10.1038/cddis.2015.360

Abstract

Apoptosis of host cells profoundly influences virus propagation and dissemination, events that are integral to influenza A virus (IAV) pathogenesis. The trigger for activation of apoptosis is regulated by an intricate interplay between cellular and viral proteins, with a strong bearing on IAV replication. Though the knowledge of viral proteins and mechanisms employed by IAV to induce apoptosis has advanced considerably of late, we know relatively little about the repertoire of host factors targeted by viral proteins. Thus, identification of cellular proteins that are hijacked by the virus will help us not only to understand the molecular underpinnings of IAV-induced apoptosis, but also to design future antiviral therapies. Here we show that the nucleoprotein (NP) of IAV directly interacts with and suppresses the expression of API5, a host antiapoptotic protein that antagonizes E2F1-dependent apoptosis. siRNA-mediated depletion of API5, in NP-overexpressed as well as IAV-infected cells, leads to upregulation of apoptotic protease activating factor 1 (APAF1), a downstream modulator of E2F1-mediated apoptosis, and cleavage of caspases 9 and 3, although a reciprocal pattern of these events was observed on ectopic overexpression of API5. In concordance with these observations, annexin V and 7AAD staining assays exhibit downregulation of early and late apoptosis in IAV-infected or NP-transfected cells on overexpression of API5. Most significantly, while overexpression of API5 decreases viral titers, cellular NP protein as well as mRNA levels in IAV-infected A549 cells, silencing of API5 expression causes a steep rise in the same parameters. From the data reported in this manuscript, we propose a proapoptotic role for NP in IAV pathogenesis, whereby it suppresses expression of antiapoptotic factor API5, thus potentiating the E2F1-dependent apoptotic pathway and ensuring viral replication.

Highlights

Actively elicit apoptotic response upon infection, as a mechanism of cell death and virus propagation.[4,5,6,7,8,9] apoptosis plays a pivotal role in the influenza life cycle
From the data reported in this manuscript, we propose a proapoptotic role for NP in influenza A virus (IAV) pathogenesis, whereby it suppresses expression of antiapoptotic factor apoptosis inhibitor protein 5 (API5), potentiating the E2F1-dependent apoptotic pathway and ensuring viral replication
The co-transformants tested positive for histidine prototrophy and β-galactosidase activity (Supplementary Figure S1), confirming that IAV NP interacted with full-length human API5 protein

Summary

Introduction

Actively elicit apoptotic response upon infection, as a mechanism of cell death and virus propagation.[4,5,6,7,8,9] apoptosis plays a pivotal role in the influenza life cycle. Besides having crucial roles in the viral life cycle, NP functions as a key adapter molecule mediating the cross talk between virus and host cell processes, owing to its ability to interact with a wide variety of both viral as well as cellular factors.[22] In the present study, we show that NP of IAV induces apoptosis in host cells by repressing apoptosis inhibitor protein 5 (API5).

Methods

Results

Conclusion