Cell cycle independent role of cyclin D3 in host restriction of influenza virus infection

Abstract

Co-evolution and adaptation of virus-host interactions can be described as a zero-sum biological arms race. Emerging evidence indicates that pathogens share a complex functional interface with their hosts. As obligate intracellular parasites, viruses must usurp host machinery to accomplish their life cycle, and influenza A virus (IAV) is no exception. Although effort has been put into the identification of cellular partners of IAV, there is a surprising lack of convergence among identified hits. Moreover, mechanistic understanding of such interactions is few and far between. My thesis project, while providing insights into fundamental mechanisms of IAV infection, also paves the way for development of novel therapeutics.

Amongst the IAV proteins, M2 has received scant attention with respect to its role in exploiting host factors. To discover novel targets of M2, we performed a yeast two-hybrid screen using the cytoplasmic tail of M2 from the highly pathogenic H5N1 strain. The screen revealed a high-score interaction with cyclin D3, a key regulator of cell cycle early G1 phase. M2–cyclin D3 interaction was validated through GST pull-down and recapitulated in influenza A/WSN/33-infected cells. Knockdown of Ccnd3 by small interfering RNA significantly enhanced virus progeny titers in cell culture supernatants. Interestingly, the increase in virus production was due to cyclin D3 deficiency per se, and not merely to deregulation of the cell cycle. A combined knockdown of Ccnd3 and Rb1, which rescued cell cycle progression into the S phase, failed to normalize virus production.

Infection by IAV triggered redistribution of cyclin D3 from the nucleus to the cytoplasm followed by its proteasomal degradation. IAV-mediated reduction of cyclin D3 led to inhibited phosphorylation of retinoblastoma protein and a subsequent arrest in cell cycle progression The intracellular levels of viral proteins were not modified upon the removal of cyclin D3 through RNA interference, suggesting cyclin D3 modulates the late stage of virus life cycle. Instead, when over-expressed in HEK 293T cells cyclin D3 impaired binding of M2 with M1, which is essential for proper assembly of progeny virions, lending further support to its role as a putative restriction factor.

Data acquired in this study are significant for several reasons. Identification of host factors involved in the assembly and release of IAV remains a challenge, evident from the widely divergent data sets acquired from genome-wide siRNA screens. There is no published evidence on the role of cyclin D3 in influenza infection. Our data unambiguously show that deficiency of cyclin D3 significantly enhances virus progeny titers, supported by the use of multiple strains to rule out any strain specific effect. We have performed detailed analyses to decouple the role of cyclin D3 as a cell-cycle regulator from its restrictive effect during influenza infection. Cyclin D3 perturbs M1–M2 interaction, which is a crucial step in virus assembly and release. This places cyclin D3 amongst the various host restriction factors that function to inhibit infection at multiple steps in the intracellular virus life cycle and provides valuable insights in charting out host pathways that are often hijacked by viruses.