Antiviral functions of bone morphogenetic proteins and the activins
Keywords: HCV
Innate Immunity
Activin
Antiviral
BMP

Previous work in the Drakesmith lab has revealed a novel anti-HCV function of bone morphogenetic protein 6 (BMP6), a TGFβ-superfamily cytokine unrelated to type I IFN. Recombinant BMP6 is antiviral against both replication-competent HCV and a full-length genomic replicon model. Data presented in this thesis demonstrate that an anti-HCV effect extends to multiple BMPs and segregates with ability to ligate the type I BMP receptor. Canonically, the type I BMP receptor signals intracellularly via phosphorylation of SMAD1/5/8 transcription factors.
Prior work in the lab shows that BMP6 exerts both type I IFN-dependent and type I IFN-independent antiviral effects. In terms of delineating mechanistic basis for the latter, we have formulated a model whereby BMP6 induces cell cycle arrest in phases characterized by reduced cytosolic nucleotide availability, and which are therefore less permissive to viral replication.
A recent report indicates that another TGFβ-type cytokine, activin B, is able to signal through a nonclassical type I BMP receptor dependent mechanism. Activin A and B have multiple established roles in innate immunity and inflammatory responses. However, no direct link between activin A and B and the early response to viral infection has been described. Given their "immune precedent" within the literature, and their high level of structural and phylogenetic homology to the BMPs, both activin A and B represented promising candidates to explore for an antiviral effect.
Our data indicate that activin A mRNA, encoded by the INHBA gene, is induced upon activation of RIG-I, MDA5 and TLR7/8 viral nucleic acid sensors in vitro, across multiple cell lines and also in PBMCs. In vitro infection of A549 lung adenocarcinoma-derived cells and Huh7 hepatoma-derived cells with the murine paramyxovirus Sendai Virus also elicits robust INHBA induction. In vitro dengue virus infection also elicits INHBA upregulation by Huh7.5 hepatoma cells. In vivo, infection of mice with influenza A PR8 also elicits induction of activin A message within the lung.
Treatment of Huh7 cells with activin A increases transcription of multiple type I IFN transduction elements; moreover, co-incubation of Huh7 cells with IFNα and either activin A or B augments transcriptional induction of key anti-HCV enzymes. This boosting of type I IFN extends to a functional enhancement: activin A elicits a synergistic, dose-dependent enhancement of both type I and type III IFN’s antiviral effect against a full-length HCV genomic replicon. In a full-length genomic replicon model of HCV, both activin A and B alone exert a potent, dose-dependent antiviral effect that is contingent upon signalling via type I BMP receptor. A component of the activins' antiviral effect does not require intact type I IFN signalling. A small-molecule inhibitor of signalling downstream of type I IFN receptor blocks the anti-HCV effect of IFNα but does not impair the antiviral effects of activin A. Both BMP6 and activin A exert dose-dependent antiviral effects against Hepatitis B Virus infection in vitro.
Of note, SMAD1/5/8-binding sites have been identified in the promoter sequences of multiple antiviral Interferon Stimulated Genes (ISG), providing a possible route for the enhancement of ISG induction by the SMAD1/5/8 axis. Furthermore, strong topological homology exists between of the transactivation domains of the SMADs and Interferon Response Factors (IRF), which postulated to have diverged from a common ancestor in early metazoans. Preliminary bioinformatic analyses reveal striking parallels between the genome-wide binding profiles of activated SMAD1 and IRF1, including proximal to genes encoding antiviral effectors.
The observations presented in this study may represent the first characterization of a non-IFN intracellular antiviral response in human cells, with implications for the development of targeted therapies against diverse viral diseases. Moreover, these data reveal a novel facet of activin biology, in addition to in part elucidating the nature of the genomic interactions between BMP-SMAD and IFNIRF signalling.