BH3-only sensors Bad, Noxa and Puma are Key Regulators of Tacaribe virus-induced Apoptosis.

Julia Holzerland,Julia E Hölper,Allison Groseth,Logan Banadyga,Michael R Knittler,Lucie Fénéant,Jens H Kuhn

doi:10.1371/journal.ppat.1008948

Julia Holzerland, Julia E Hölper + Show 5 more

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https://doi.org/10.1371/journal.ppat.1008948

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Abstract

Pathogenicity often differs dramatically among even closely related arenavirus species. For instance, Junín virus (JUNV), the causative agent of Argentine hemorrhagic fever (AHF), is closely related to Tacaribe virus (TCRV), which is normally avirulent in humans. While little is known about how host cell pathways are regulated in response to arenavirus infection, or how this contributes to virulence, these two viruses have been found to differ markedly in their ability to induce apoptosis. However, details of the mechanism(s) governing the apoptotic response to arenavirus infections are unknown. Here we confirm that TCRV-induced apoptosis is mitochondria-regulated, with associated canonical hallmarks of the intrinsic apoptotic pathway, and go on to identify the pro- and anti-apoptotic Bcl-2 factors responsible for regulating this process. In particular, levels of the pro-apoptotic BH3-only proteins Noxa and Puma, as well as their canonical transcription factor p53, were strongly increased. Interestingly, TCRV infection also led to the accumulation of the inactive phosphorylated form of another pro-apoptotic BH3-only protein, Bad (i.e. as phospho-Bad). Knockout of Noxa or Puma suppressed apoptosis in response to TCRV infection, whereas silencing of Bad increased apoptosis, confirming that these factors are key regulators of apoptosis induction in response to TCRV infection. Further, we found that while the highly pathogenic JUNV does not induce caspase activation, it still activated upstream pro-apoptotic factors, consistent with current models suggesting that JUNV evades apoptosis by interfering with caspase activation through a nucleoprotein-mediated decoy function. This new mechanistic insight into the role that individual BH3-only proteins and their regulation play in controlling apoptotic fate in arenavirus-infected cells provides an important experimental framework for future studies aimed at dissecting differences in the apoptotic responses between arenaviruses, their connection to other cell signaling events and ultimately the relationship of these processes to pathogenesis.

Highlights

The arenavirus family is made up mainly of rodent-borne viruses, exclusively reptileassociated viruses have recently been identified [1]
While some virus species cause severe disease, resulting in hemorrhagic fever and/ or neurological symptoms, other closely related species exhibit little or no pathogenicity. The basis for these dramatically different outcomes is insufficiently understood, but investigations of host cell responses have suggested that apoptosis, i.e. non-inflammatory programmed cell death, is regulated differently between pathogenic and apathogenic arenaviruses
We demonstrate that apoptosis induced by the avirulent Tacaribe virus (TCRV), proceeds via the mitochondria, and is regulated by a combination of factors that appear to balance activation (i.e. Noxa and Puma) and inactivation (i.e. Bad-P) of this cascade

Summary

Introduction

The arenavirus family is made up mainly of rodent-borne viruses, exclusively reptileassociated viruses have recently been identified [1]. The classical mammalian arenaviruses are further sub-classified according to their antigenic properties, geographical distribution and phylogeny into the Old World (OW) and New World (NW) arenaviruses [2, 3] Regardless, they all share a common structure consisting of an enveloped particle containing a bi-segmented negative strand RNA genome, with both segments encoding two open reading frames (ORFs) in an ambisense coding arrangement [4]. The small genome segment (S segment) encodes the glycoprotein (GP) and the nucleoprotein (NP), while the large genome segment (L segment) encodes the polymerase (L) and the matrix protein (Z) [5] Arenaviruses possess such a limited coding capacity, they include a number of notable human pathogens. There is evidence that regulation of apoptosis by arenaviruses may play an important role in infection and be linked to pathogenesis (reviewed in [12])

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