Roles of the Endogenous Lunapark Protein during Flavivirus Replication

Functional Roles and Host Interactions of Orthoflavivirus Non-Structural Proteins During Replication

Orthoflavivirus, a genus encompassing arthropod-borne, positive-sense, single-stranded RNA viruses in the Flaviviridae family, represents clinically relevant viruses that pose significant threats to human and animal health worldwide. With warming climates and persistent urbanization, arthropod vectors and the viruses they transmit continue to widen their geographic distribution, expanding endemic zones. Flaviviruses such as dengue virus, Zika virus, West Nile virus, and tick-borne encephalitis virus cause debilitating and fatal infections globally. In 2024, the World Health Organization and the Pan American Health Organization declared the current dengue situation a Multi-Country Grade 3 Outbreak, the highest level. FDA-approved treatment options for diseases caused by flaviviruses are limited or non-existent, and vaccines are suboptimal for many flaviviruses. Understanding the molecular characteristics of the flavivirus life cycle, virus-host interactions, and resulting pathogenesis in various cells and model systems is critical for developing effective therapeutic intervention strategies. This review will focus on the virus-host interactions of mosquito- and tick-borne flaviviruses from the virus replication and assembly perspective, emphasizing the interplay between viral non-structural proteins and host pathways that are hijacked for their advantage. Highlighting interaction pathways, including innate immunity, intracellular movement, and membrane modification, emphasizes the need for rigorous and targeted antiviral research and development against these re-emerging viruses.

Microcephaly protein ANKLE2 promotes Zika virus replication

Orthoflaviviruses are positive-sense single-stranded RNA viruses that hijack host proteins to promote their own replication. Zika virus (ZIKV) is infamous among orthoflaviviruses for its association with severe congenital birth defects, notably microcephaly. We previously mapped ZIKV-host protein interactions and identified the interaction between ZIKV non-structural protein 4A (NS4A) and host microcephaly protein ankyrin repeat and LEM domain-containing 2 (ANKLE2). Using a fruit fly model, we showed that NS4A induced microcephaly in an ANKLE2-dependent manner. Here, we explore the role of ANKLE2 in ZIKV replication to understand the biological significance of the interaction from a viral perspective. We observe that ANKLE2 localization is drastically shifted to sites of NS4A accumulation during infection and that knockout of ANKLE2 reduces ZIKV replication in multiple human cell lines. This decrease in virus replication is coupled with a moderate increase in innate immune activation. Using microscopy, we observe dysregulated formation of virus-induced endoplasmic reticulum rearrangements in ANKLE2 knockout cells. Knockdown of the ANKLE2 ortholog in Aedes aegypti cells also decreases virus replication, suggesting ANKLE2 is a beneficial replication factor across hosts. Finally, we show that NS4A from four other orthoflaviviruses physically interacts with ANKLE2 and is also beneficial to their replication. Thus, ANKLE2 likely promotes orthoflavivirus replication by regulating membrane rearrangements that serve to accelerate viral genome replication and protect viral dsRNA from immune detection. Taken together with our previous results, our findings indicate that ZIKV and other orthoflaviviruses hijack ANKLE2 for a conserved role in replication, and this drives unique pathogenesis for ZIKV since ANKLE2 has essential roles in developing tissues.IMPORTANCEZIKV is a major concern due to its association with birth defects, including microcephaly. We previously identified a physical interaction between ZIKV NS4A and host microcephaly protein ANKLE2. Mutations in ANKLE2 cause congenital microcephaly, and NS4A induces microcephaly in an ANKLE2-dependent manner. Here, we establish the role of ANKLE2 in ZIKV replication. Depletion of ANKLE2 from cells significantly reduces ZIKV replication and disrupts virus-induced membrane rearrangements. ANKLE2's ability to promote ZIKV replication is conserved in mosquito cells and for other related mosquito-borne orthoflaviviruses. Our data point to an overall model in which ANKLE2 regulates virus-induced membrane rearrangements to accelerate orthoflavivirus replication and avoid immune detection. However, ANKLE2's unique role in ZIKV NS4A-induced microcephaly is a consequence of ZIKV infection of important developing tissues in which ANKLE2 has essential roles.

How host ER membrane chaperones and morphogenic proteins support virus infection

The multi-functional endoplasmic reticulum (ER) is exploited by viruses to cause infection. Morphologically, this organelle is a highly interconnected membranous network consisting of sheets and tubules whose levels are dynamic, changing in response to cellular conditions. Functionally, the ER is responsible for protein synthesis, folding, secretion and degradation, as well as Ca2+ homeostasis and lipid biosynthesis, with each event catalyzed by defined ER factors. Strikingly, these ER host factors are hijacked by viruses to support different infection steps, including entry, translation, replication, assembly and egress. Although the full repertoire of these ER factors that are hijacked is unknown, recent studies have uncovered several ER membrane machineries that are exploited by viruses - ranging from polyomavirus to flavivirus and coronavirus - to facilitate different steps of their life cycle. These discoveries should provide better understanding of virus infection mechanisms, potentially leading to the development of more effective anti-viral therapies.

Let's Get Physical: Flavivirus-Host Protein-Protein Interactions in Replication and Pathogenesis

Flaviviruses comprise a genus of viruses that pose a significant burden on human health worldwide. Transmission by both mosquito and tick vectors, and broad host tropism contribute to the presence of flaviviruses globally. Like all viruses, they require utilization of host molecular machinery to facilitate their replication through physical interactions. Their RNA genomes are translated using host ribosomes, synthesizing viral proteins that cooperate with each other and host proteins to reshape the host cell into a factory for virus replication. Thus, dissecting the physical interactions between viral proteins and their host protein targets is essential in our comprehension of how flaviviruses replicate and how they alter host cell behavior. Beyond replication, even single interactions can contribute to immune evasion and pathogenesis, providing potential avenues for therapeutic intervention. Here, we review protein interactions between flavivirus and host proteins that contribute to virus replication, immune evasion, and disease.

Enhanced Seroconversion to West Nile Virus Proteins in Mice by West Nile Kunjin Replicon Virus-like Particles Expressing Glycoproteins from Crimean–Congo Hemorrhagic Fever Virus

Removal of genes coding for major parts of capsid (C), premembrane (prM), and envelope (E) proteins on the flavivirus genome aborts the production of infectious virus particles where the remaining genome forms a replicon that retains replicability in host cells. The C-prM-E proteins can also be expressed in trans with the flavivirus replicons to generate single-round infectious replicon virus-like particles (RVPs). In this study, we characterized the use of RVPs based on the Kunjin strain of WNV (WNV_KUN) as a putative WNV vaccine candidate. In addition, the WNV_KUN C-prM-E genes were substituted with the Crimean–Congo hemorrhagic fever virus (CCHFV) genes encoding the glycoproteins Gn and Gc to generate a WNV_KUN replicon expressing the CCHFV proteins. To generate RVPs, the WNV_KUN replicon was transfected into a cell line expressing the WNV_KUN C-prM-E. Using immunoblotting and immunofluorescence assays, we showed that the replicon can express the CCHFV Gn and Gc proteins and the RVPs can transduce cells to express WNV_KUN proteins and the CCHFV Gn and Gc proteins. Our study also revealed that these RVPs have potential as a vaccine platform with low risk of recombination as it infects cells only in one cycle. The immunization of mice with the RVPs resulted in high seroconversion to both WNV E and NS1 but limited seroconversion to CCHFV Gn and Gc proteins. Interestingly, we found that there was enhanced production of WNV E, NS1 antibodies, and neutralizing antibodies by the inclusion of CCHFV Gc and Gn into WNV_KUN RVPs. Thus, this study indicates a complementary effect of the CCHFV Gn and Gc proteins on the immunogenicity by WNV_KUN RVPs, which may be applied to develop a future vaccine against the WNV.

Roles of ESCRT proteins (ALIX and CHIMP4A) and their interplay with ISG15 during tick-borne flavivirus infection

Flaviviruses are usually transmitted to humans via mosquito or tick bites. During infection, virus replication and assembly, whose cellular sites are relatively close, are controlled by virus proteins and a diverse range of host proteins. By siRNA-mediated gene silencing, we showed that ALIX and CHMP4A, two members of the host endosomal sorting complex required for transport (ESCRT) protein machinery, are required during flavivirus infection. Using cell lines expressing subgenomic replicons and replicon virus-like particles, we demonstrated specific roles for ALIX and CHMP4A in viral replication and assembly, respectively. Employing biochemical and imaging methodology, we showed that the ESCRT proteins are recruited by a putative specific late (L) domain motif LYXLA within the NS3 protein of tick-borne flaviviruses. Furthermore, to counteract the recruitment of ESCRT proteins, the host cells may elicit defense mechanisms. We found that ectopic expression of the interferon-stimulated gene 15 (ISG15) or the E3 ISG15-protein ligase (HERC5) reduced virus replication by suppressing the positive effects of ALIX and CHMP4A. Collectively, these results have provided new insights into flavivirus-host cell interactions that function as checkpoints, including the NS3 and the ESCRT proteins, the ISG15 and the ESCRT proteins, at essential stages of the virus life cycle.

IMPORTANCE Flaviviruses are important zoonotic viruses with high fatality rates worldwide. Here, we report that during infection, the virus employs members of ESCRT proteins for virus replication and assembly. Among the ESCRT proteins, ALIX acts during virus replication, while CHMP4A is required during virus assembly. Another important ESCRT protein, TSG101, is not required for virus production. The ESCRT, complex, ALIX-CHMP4A, is recruited to NS3 through their interactions with the putative L domain motif of NS3, while CHMP4A is recruited to E. In addition, we demonstrate the antiviral mechanism of ISG15 and HERC5, which degrades ALIX and CHIMP4A, indirectly targets virus infection. In summary, we reveal host-dependency factors supporting flavivirus infection, but these factors may also be targeted by antiviral host effector mechanisms.