Dual-Role Ubiquitination Regulation Shuttling the Entire Life Cycle of the Flaviviridae

A single-cell atlas of the Culex tarsalis midgut during West Nile virus infection

The mosquito midgut functions as a key interface between pathogen and vector. However, studies of midgut physiology and virus infection dynamics are scarce, and in Culex tarsalis - an extremely efficient vector of West Nile virus (WNV) - nonexistent. We performed single-cell RNA sequencing on Cx. tarsalis midguts, defined multiple cell types, and determined whether specific cell types are more permissive to WNV infection. We identified 20 cell states comprising 8 distinct cell types, consistent with existing descriptions of Drosophila and Aedes aegypti midgut physiology. Most midgut cell populations were permissive to WNV infection. However, there were higher levels of WNV RNA (vRNA) in enteroendocrine cells, suggesting enhanced replication in this population. In contrast, proliferating intestinal stem cells (ISC) had the lowest levels of vRNA, a finding consistent with studies suggesting ISC proliferation in the midgut is involved in infection control. ISCs were also found to have a strong transcriptional response to WNV infection; genes involved in ribosome structure and biogenesis, and translation were significantly downregulated in WNV-infected ISC populations. Notably, we did not detect significant WNV-infection induced upregulation of canonical mosquito antiviral immune genes (e.g., AGO2, R2D2, etc.) at the whole-midgut level. Rather, we observed a significant positive correlation between immune gene expression levels and vRNA load in individual cells, suggesting that within midgut cells, high levels of vRNA may trigger antiviral responses. Our findings establish a Cx. tarsalis midgut cell atlas, and provide insight into midgut infection dynamics of WNV by characterizing cell-type specific enhancement/restriction of, and immune response to, infection at the single-cell level.

Host-Driven Ubiquitination Events in Vector-Transmitted RNA Virus Infections as Options for Broad-Spectrum Therapeutic Intervention Strategies

Many vector-borne viruses are re-emerging as public health threats, yet our understanding of the virus-host interactions critical for productive infection remains limited. The ubiquitination of proteins, including host- and pathogen-derived proteins is a highly prominent and consistent post-translational modification that regulates protein function through signaling and degradation. Viral proteins are documented to hijack the host ubiquitination machinery to modulate multiple host processes including antiviral defense mechanisms. The engagement of the host ubiquitination machinery in the post-translational modification of viral proteins to support aspects of the viral life cycle including assembly and egress is also well documented. Exploring the role ubiquitination plays in the life cycle of vector-transmitted viral pathogens will increase the knowledge base pertinent to the impact of host-enabled ubiquitination of viral and host proteins and the consequences on viral pathogenesis. In this review, we explore E3 ligase-regulated ubiquitination pathways functioning as proviral and viral restriction factors in the context of acutely infectious, vector-transmitted viral pathogens and the potential for therapeutically targeting them for countermeasures development.

Ubiquitination in viral entry and replication: Mechanisms and implications

The ubiquitination process is a reversible posttranslational modification involved in many essential cellular functions, such as innate immunity, cell signaling, trafficking, protein stability, and protein degradation. Viruses can use the ubiquitin system to efficiently enter host cells, replicate and evade host immunity, ultimately enhancing viral pathogenesis. Emerging evidence indicates that enveloped viruses can carry free (unanchored) ubiquitin or covalently ubiquitinated viral structural proteins that can increase the efficiency of viral entry into host cells. Furthermore, viruses continuously evolve and adapt to take advantage of the host ubiquitin machinery, highlighting its importance during virus infection. This review discusses the battle between viruses and hosts, focusing on how viruses hijack the ubiquitination process at different steps of the replication cycle, with a specific emphasis on viral entry. We discuss how ubiquitination of viral proteins may affect tropism and explore emerging therapeutics strategies targeting the ubiquitin system for antiviral drug discovery.

The Dynamic Landscape of Capsid Proteins and Viral RNA Interactions in Flavivirus Genome Packaging and Virus Assembly

The Flavivirus genus of the Flaviviridae family of enveloped single-stranded RNA viruses encompasses more than 70 members, many of which cause significant disease in humans and livestock. Packaging and assembly of the flavivirus RNA genome is essential for the formation of virions, which requires intricate coordination of genomic RNA, viral structural, and nonstructural proteins in association with virus-induced, modified endoplasmic reticulum (ER) membrane structures. The capsid (C) protein, a small but versatile RNA-binding protein, and the positive single-stranded RNA genome are at the heart of the elusive flavivirus assembly process. The nucleocapsid core, consisting of the genomic RNA encapsidated by C proteins, buds through the ER membrane, which contains viral glycoproteins prM and E organized as trimeric spikes into the lumen, forming an immature virus. During the maturation process, which involves the low pH-mediated structural rearrangement of prM and E and furin cleavage of prM in the secretory pathway, the spiky immature virus with a partially ordered nucleocapsid core becomes a smooth, mature virus with no discernible nucleocapsid. This review focuses on the mechanisms of genome packaging and assembly by examining the structural and functional aspects of C protein and viral RNA. We review the current lexicon of critical C protein features and evaluate interactions between C and genomic RNA in the context of assembly and throughout the life cycle.

Protein Quality Control Systems and ER Stress as Key Players in SARS-CoV-2-Induced Neurodegeneration

The COVID-19 pandemic has brought to the forefront the intricate relationship between SARS-CoV-2 and its impact on neurological complications, including potential links to neurodegenerative processes, characterized by a dysfunction of the protein quality control systems and ER stress. This review article explores the role of protein quality control systems, such as the Unfolded Protein Response (UPR), the Endoplasmic Reticulum-Associated Degradation (ERAD), the Ubiquitin-Proteasome System (UPS), autophagy and the molecular chaperones, in SARS-CoV-2 infection. Our hypothesis suggests that SARS-CoV-2 produces ER stress and exploits the protein quality control systems, leading to a disruption in proteostasis that cannot be solved by the host cell. This disruption culminates in cell death and may represent a link between SARS-CoV-2 and neurodegeneration.