Specific interaction between hnRNP H and HPV16 L1 proteins: implications for late gene auto-regulation enabling rapid viral capsid protein production

Diverse roles of heterogeneous nuclear ribonucleoproteins in viral life cycle

Understanding the host-virus interactions helps to decipher the viral replication strategies and pathogenesis. Viruses have limited genetic content and rely significantly on their host cell to establish a successful infection. Viruses depend on the host for a broad spectrum of cellular RNA-binding proteins (RBPs) throughout their life cycle. One of the major RBP families is the heterogeneous nuclear ribonucleoproteins (hnRNPs) family. hnRNPs are typically localized in the nucleus, where they are forming complexes with pre-mRNAs and contribute to many aspects of nucleic acid metabolism. hnRNPs contain RNA binding motifs and frequently function as RNA chaperones involved in pre-mRNA processing, RNA splicing, and export. Many hnRNPs shuttle between the nucleus and the cytoplasm and influence cytoplasmic processes such as mRNA stability, localization, and translation. The interactions between the hnRNPs and viral components are well-known. They are critical for processing viral nucleic acids and proteins and, therefore, impact the success of the viral infection. This review discusses the molecular mechanisms by which hnRNPs interact with and regulate each stage of the viral life cycle, such as replication, splicing, translation, and assembly of virus progeny. In addition, we expand on the role of hnRNPs in the antiviral response and as potential targets for antiviral drug research and development.

Newcastle disease virus V protein interacts with hnRNP H1 to promote viral replication

The interactions between host cellular proteins and viral proteins are important for successful infection by viruses. Previous studies from our group have identified various host cellular proteins that can interact with the Newcastle disease virus V protein (Chu et al., 2018a), but their function in NDV replication has not been fully determined. The present study reports that heterogenous nuclear ribonucleoprotein H1 (hnRNP H1) can interact with NDV V protein in yeast. The immunofluorescence results showed that hnRNP H1 and V protein could colocalize in the cytoplasm of a chicken embryo fibroblast cell line (DF-1 cells). Co-immunoprecipitation assays further verified the interaction of these two proteins. The effects of overexpression and knockdown of hnRNP H1 on NDV replication were evaluated in DF-1 cells through real time quantitative PCR (RT-qPCR) and plaque assays. The regulation of V protein on hnRNP H1 expression was also examined. The results indicated that overexpression of hnRNP H1 facilitated NDV replication, while knockdown of hnRNP H1 decreased NDV replication. It was also shown that V protein could regulate hnRNP H1 expression at the protein level instead of the transcription level. The effect of V protein and hnRNP H1 on the DF-1 cell cycle was also tested and the results revealed that V protein may regulate cell proliferation by controlling the expression of hnRNP H1. Taken together, these results suggest that NDV V protein could promote viral replication by interacting with hnRNP H1.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

The Role of RNA Splicing Factors in Cancer: Regulation of Viral and Human Gene Expression in Human Papillomavirus-Related Cervical Cancer

The spliceosomal complex components, together with the heterogeneous nuclear ribonucleoproteins (hnRNPs) and serine/arginine-rich (SR) proteins, regulate the process of constitutive and alternative splicing, the latter leading to the production of mRNA isoforms coding multiple proteins from a single pre-mRNA molecule. The expression of splicing factors is frequently deregulated in different cancer types causing the generation of oncogenic proteins involved in cancer hallmarks. Cervical cancer is caused by persistent infection with oncogenic human papillomaviruses (HPVs) and constitutive expression of viral oncogenes. The aberrant activity of hnRNPs and SR proteins in cervical neoplasia has been shown to trigger the production of oncoproteins through the processing of pre-mRNA transcripts either derived from human genes or HPV genomes. Indeed, hnRNP and SR splicing factors have been shown to regulate the production of viral oncoprotein isoforms necessary for the completion of viral life cycle and for cell transformation. Target-therapy strategies against hnRNPs and SR proteins, causing simultaneous reduction of oncogenic factors and inhibition of HPV replication, are under development. In this review, we describe the current knowledge of the functional link between RNA splicing factors and deregulated cellular as well as viral RNA maturation in cervical cancer and the opportunity of new therapeutic strategies.

Characterization of an RNA aptamer against HPV-16 L1 virus-like particles

The human papillomavirus (HPV) capsid is mainly composed of the L1 protein that can self-assemble into virus-like particles (VLPs) that are structurally and immunologically similar to the infectious virions. We report here the characterization of RNA aptamers that recognize baculovirus-produced HPV-16 L1 VLPs. Interaction and slot-blot binding assays showed that all isolated aptamers efficiently bound HPV-16 VLPs, although the Sc5-c3 aptamer showed the highest specificity and affinity (Kd=0.05 pM). Sc5-c3 secondary structure consisted of a hairpin with a symmetric bubble and an unstructured 3'end. Biochemical and genetic analyses showed that the Sc5-c3 main loop is directly involved on VLPs binding. In particular, binding specificity appeared mediated by five non-consecutive nucleotide positions. Experiments using bacterial-produced HPV-16 L1 resulted in low Sc5-c3 binding, suggesting that recognition of HPV-16 L1 VLPs relies on quaternary structure features not present in bacteria-produced L1 protein. Sc5-c3 produced specific and stable binding to HPV-16 L1 VLPs even in biofluid protein mixes and thus it may provide a potential diagnostic tool for active HPV infection.

Papillomavirus transcripts and posttranscriptional regulation

Papillomavirus gene expression is strictly linked to the differentiation state of the infected cell and is highly regulated at the level of transcription and RNA processing. All papillomaviruses make extensive use of alternative mRNA polyadenylation and splicing to control gene expression. This chapter contains a compilation of all known alternatively spliced papillomavirus mRNAs and it summarizes our current knowledge of viral RNA elements, and viral and cellular factors that control papillomavirus mRNA processing.