Antagonizing the SAMD9 pathway is key to myxoma virus host shut-off and immune evasion.

Poxviruses have life cycles exclusively in the cytoplasm. However, these viruses can have profound impact to host transcription. One possible mechanism is through viral manipulation of host protein synthesis and such ability is critical for viral immune evasion. Many mammalian poxviruses encode more than one viral protein to interact with the host SAMD9 protein. In myxoma virus (MYXV), a rabbit specific poxvirus and non-pathogenic for other species, viral M062 protein is the lone inhibitor to SAMD9 with broad species specificity and loss of M062R in viral genome (Δ M062R mutant) leads to profound infection defect. We previously found Δ M062R remodeled transcriptomic landscape in monocytes/macrophages that is associated with the crosstalk between the SAMD9 pathway and cGAS/STING/IRF3 DNA sensing pathway. In this study we completed the characterization of Δ M062R infection. We observed that although this replication-defective virus preserved intact early protein synthesis, it failed to conduct host shutoff. Despite a defect in viral DNA replication, Δ M062R infection retained intact intermediate protein synthesis comparable to the wildtype virus. Using time course dual RNAseq analyses we found that the overall viral gene transcription profile was mostly indistinguishable from that of the wildtype MYXV. However, the slightly attenuated late RNA synthesis along with the block at viral protein synthesis led to its infection defect. Infection by Δ M062R in macrophages potentiated the antiviral responses to new danger signals. We provided an initial characterization of such a state in which host antiviral protein synthesis may be promoted leading to the immunological consequence. Importance: Poxviruses utilize multi-faceted strategies to evade and manipulate host immunity. Through targeted gene deletion, we generated useful tools of mutant poxviruses to investigate specific crosstalk between host defense mechanisms. Through studying MYXV M062 protein function, we previously identified SAMD9 as one host target of poxvirus C7L superfamily, in which family M062R is one member. However, what kind of cellular outcome caused by Δ M062R infection remained unknown. The infection defect of Δ M062R caused the induction of host inflammation program is likely due to the activation of the host pathway governed by SAMD9. Because little is known about SAMD9 cellular function and the pathways it regulates, which are important for cellular homeostasis and immune regulation, this study on Δ M062R induced effect in host cells will provide new insight on how SAMD9 affects cellular protein synthesis and immunological responses.

Myxoma virus lacking the host range determinant M062 stimulates cGAS-dependent type 1 interferon response and unique transcriptomic changes in human monocytes/macrophages.

The evolutionarily successful poxviruses possess effective and diverse strategies to circumvent or overcome host defense mechanisms. Poxviruses encode many immunoregulatory proteins to evade host immunity to establish a productive infection and have unique means of inhibiting DNA sensing-dependent type 1 interferon (IFN-I) responses, a necessity given their dsDNA genome and exclusively cytoplasmic life cycle. We found that the key DNA sensing inhibition by poxvirus infection was dominant during the early stage of poxvirus infection before DNA replication. In an effort to identify the poxvirus gene products which subdue the antiviral proinflammatory responses (e.g., IFN-I response), we investigated the function of one early gene that is the known host range determinant from the highly conserved poxvirus host range C7L superfamily, myxoma virus (MYXV) M062. Host range factors are unique features of poxviruses that determine the species and cell type tropism. Almost all sequenced mammalian poxviruses retain at least one homologue of the poxvirus host range C7L superfamily. In MYXV, a rabbit-specific poxvirus, the dominant and broad-spectrum host range determinant of the C7L superfamily is the M062R gene. The M062R gene product is essential for MYXV infection in almost all cells tested from different mammalian species and specifically inhibits the function of host Sterile α Motif Domain-containing 9 (SAMD9), as M062R-null (ΔM062R) MYXV causes abortive infection in a SAMD9-dependent manner. In this study we investigated the immunostimulatory property of the ΔM062R. We found that the replication-defective ΔM062R activated host DNA sensing pathway during infection in a cGAS-dependent fashion and that knocking down SAMD9 expression attenuated proinflammatory responses. Moreover, transcriptomic analyses showed a unique feature of the host gene expression landscape that is different from the dsDNA alone-stimulated inflammatory state. This study establishes a link between the anti-neoplastic function of SAMD9 and the regulation of innate immune responses.

Insights into the NF-κB-DNA Interaction through NMR Spectroscopy.

Transcription factors bind specifically to their target elements in the genome, eliciting specific gene expression programs. The nuclear factor-κB (NF-κB) system is a family of proteins comprising inducible transcription activators, which play a critical role in inflammation and cancer. The NF-κB members function as dimers with each monomeric unit binding the κB-DNA. Despite the available structures of the various NF-κB dimers in complex with the DNA, the structural features of these dimers in the nucleic acid-free form are not well-characterized. Using solution NMR spectroscopy, we characterize the structural features of 73.1 kDa p50 subunit of the NF-κB homodimer in the DNA-free form and compare it with the κB DNA-bound form of the protein. The study further reveals that in the nucleic acid-free form, the two constituent domains of p50, the N-terminal and the dimerization domains, are structurally independent of each other. However, in a complex with the κB DNA, both the domains of p50 act as a single unit. The study also provides insights into the mechanism of κB DNA recognition by the p50 subunit of NF-κB.

Domain Stability Regulated through the Dimer Interface Controls the Formation Kinetics of a Specific NF-κB Dimer.

The NF-κB family of transcription factors is a key regulator of the immune response in the vertebrates. The family comprises five proteins that function as dimers formed in various combinations among the members, with the RelA-p50 dimer being physiologically the most abundant. While most of the 15 possible dimers are scarcely present in the cell with some remaining experimentally undetected to date, there are specific gene sets that are only activated by certain sparsely populated NF-κB dimers. The mechanism of transcription activation of such specific genes that are activated only by specific NF-κB dimers remains unclear. Here we show that the dimer interfacial residues control the stabilization of the global hydrogen bond network of the NF-κB dimerization domain, which, in turn, controls the thermodynamic stabilization of different NF-κB dimers. The relatively low thermodynamic stability of the RelA-RelA homodimer is critical as it facilitates the formation of the more stable RelA-p50 heterodimer. Through the modulation of the thermodynamic stability of the RelA-RelA homodimer, the kinetics of the RelA-p50 heterodimer formation can be regulated. This phenomenon provides an insight into the mechanism of RelA-RelA specific target gene regulation in physiology.

Super-Resolved Nuclear Magnetic Resonance Spectroscopy.

We present a novel method that breaks the resolution barrier in nuclear magnetic resonance (NMR) spectroscopy, allowing one to accurately estimate the chemical shift values of highly overlapping or broadened peaks. This problem is routinely encountered in NMR when peaks have large linewidths due to rapidly decaying signals, hindering its application. We address this problem based on the notion of finite-rate-of-innovation (FRI) sampling, which is based on the premise that signals such as the NMR signal, can be accurately reconstructed using fewer measurements than that required by existing approaches. The FRI approach leads to super-resolution, beyond the limits of contemporary NMR techniques. Using this method, we could measure for the first time small changes in chemical shifts during the formation of a Gold nanorod-protein complex, facilitating the quantification of the strength of such interactions. The method thus opens up new possibilities for the application and acceleration of multidimensional NMR spectroscopy across a wide range of systems.