VPS29 Exerts Opposing Effects on Endocytic Viral Entry

Inhibition of early EHDV2-Ibaraki infection steps in bovine cells by endosome alkalinization or ikarugamycin, but not by blockage of individual endocytic pathways

The Epizootic hemorrhagic disease virus (EHDV), an orbivirus, is the etiological factor of a fatal hemorrhagic disease of wild ruminants. A subset of EHDV serotypes, including the Ibaraki strain of EHDV2 (EHDV2-Ibaraki), infect and cause disease in cattle, thus posing a potential threat to livestock. As a member of the Sedoreoviridae family, the EHDV particle is devoid of a membrane envelope and is predicted to employ endocytic pathways for infection. However, the degree of dependence of EHDV2-Ibaraki on specific internalization pathways while infecting bovine cells (its natural host) is unknown. The endosome alkalinizing agent ammonium chloride blocked EHDV2-Ibaraki infection of Madin-Darby Bovine Kidney (MDBK) cells with dependence on its time of addition, suggesting the criticality of endosomal pH for the completion of early stages of infection. Treatment of cells within the alkalinization-sensitive window (i.e., before endosomal processing) with inhibitors of actin polymerization, macropinocytosis (amiloride), or dynamin GTPase activity (dynasore or dynole), or with the cholesterol-depleting agent methyl-β-cyclodextrin, failed to reduce EHDV2-Ibaraki infection. In contrast, in this same treatment time frame, ikarugamycin potently inhibited infection. Moreover, ikarugamycin inhibited interferon induction in infected cells and induced the accumulation of enlarged Rab7- and lamtor4-decorated vacuoles, suggesting its ability to block viral processing and modify late-endosome compartments. Notably, ikarugamycin treatment at initial infection stages, augmented the infection of MDBK cells with the vesicular stomatitis virus while inhibiting infection with bluetongue virus serotype 8. Together, our results point to differential antiviral effects of ikarugamycin on viruses dependent on distinct sets of endosomes for entry/processing.

A review of virus host factor discovery using CRISPR screening

The emergence of genome-scale forward genetic screening techniques, such as Haploid Genetic screen and clustered regularly interspaced short palindromic repeats (CRISPR) knockout screen has opened new horizons in our understanding of virus infection biology. CRISPR screening has become a popular tool for the discovery of novel host factors for several viruses due to its specificity and efficiency in genome editing. Here, we review how CRISPR screening has revolutionized our understanding of virus-host interactions from scientific and technological viewpoints. A summary of the published screens conducted thus far to uncover virus host factors is presented, highlighting their experimental design and significant findings. We will outline relevant methods for customizing the CRISPR screening process to answer more specific hypotheses and compile a glossary of conducted CRISPR screens to show their design aspects. Furthermore, using flaviviruses and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as examples, we hope to offer a broad-based perspective on the capabilities of CRISPR screening to serve as a reference point to guide future unbiased discovery of virus host factors.

Single-cell image-based genetic screens systematically identify regulators of Ebola virus subcellular infection dynamics

Ebola virus (EBOV) is a high-consequence filovirus that gives rise to frequent epidemics with high case fatality rates and few therapeutic options. Here, we applied image-based screening of a genome-wide CRISPR library to systematically identify host cell regulators of Ebola virus infection in 39,085,093 million single cells. Measuring viral RNA and protein levels together with their localization in cells identified over 998 related host factors and provided detailed information about the role of each gene across the virus replication cycle. We trained a deep learning model on single-cell images to associate each host factor with predicted replication steps, and confirmed the predicted relationship for select host factors. Among the findings, we showed that the mitochondrial complex III subunit UQCRB is a post-entry regulator of Ebola virus RNA replication, and demonstrated that UQCRB inhibition with a small molecule reduced overall Ebola virus infection with an IC50 of 5 μM. Using a random forest model, we also identified perturbations that reduced infection by disrupting the equilibrium between viral RNA and protein. One such protein, STRAP, is a spliceosome-associated factor that was found to be closely associated with VP35, a viral protein required for RNA processing. Loss of STRAP expression resulted in a reduction in full-length viral genome production and subsequent production of non-infectious virus particles. Overall, the data produced in this genome-wide high-content single-cell screen and secondary screens in additional cell lines and related filoviruses (MARV and SUDV) revealed new insights about the role of host factors in virus replication and potential new targets for therapeutic intervention.

Spike mutations contributing to the altered entry preference of SARS-CoV-2 omicron BA.1 and BA.2

SARS-CoV-2 B.1.1.529.1 (Omicron BA.1) emerged in November 2021 and quickly became the predominant circulating SARS-CoV-2 variant globally. Omicron BA.1 contains more than 30 mutations in the spike protein, which contribute to its altered virological features when compared to the ancestral SARS-CoV-2 or previous SARS-CoV-2 variants. Recent studies by us and others demonstrated that Omicron BA.1 is less dependent on transmembrane serine protease 2 (TMPRSS2), less efficient in spike cleavage, less fusogenic, and adopts an altered propensity to utilize the plasma membrane and endosomal pathways for virus entry. Ongoing studies suggest that these virological features of Omicron BA.1 are in part retained by the subsequent Omicron sublineages. However, the exact spike determinants that contribute to these altered features of Omicron remain incompletely understood. In this study, we investigated the spike determinants for the observed virological characteristics of Omicron. By screening for the individual changes on Omicron BA.1 and BA.2 spike, we identify that 69-70 deletion, E484A, and H655Y contribute to the reduced TMPRSS2 usage while 25-27 deletion, S375F, and T376A result in less efficient spike cleavage. Among the shared spike mutations of BA.1 and BA.2, S375F and H655Y reduce spike-mediated fusogenicity. Interestingly, the H655Y change consistently reduces serine protease usage while increases the use of endosomal proteases. In keeping with these findings, the H655Y substitution alone reduces plasma membrane entry and facilitates endosomal entry when compared to SARS-CoV-2 WT. Overall, our study identifies key changes in Omicron spike that contributes to our understanding on the virological determinant and pathogenicity of Omicron.

A snapshot of protein trafficking in SARS-CoV-2 infection

SARS-CoV-2 is a human pathogenic virus responsible for the COVID-19 (coronavirus disease 2019) pandemic. The infection cycle of SARS-CoV-2 involves several related steps, including virus entry, gene expression, RNA replication, assembly of infectious virions and their egress. For all of these steps, the virus relies on and exploits host cell factors, cellular organelles, and processes such as endocytosis, nuclear transport, protein secretion, metabolite transport at membrane contact sites (MSC) and exocytotic pathways. To do this, SARS-CoV-2 has evolved multifunctional viral proteins that hijack cellular factors and modulate their function by unique strategies. In this Review, we highlight cellular trafficking factors, processes, and organelles of relevance to the SARS-CoV-2 infection cycle and how viral proteins make use of and perturb cellular transport during the viral infection cycle.

Many Keys Unlock the Doors for Virus Entry

To successfully infect, viruses must respond to cues that promote their genome delivery into host cells. These keys to virus entry frequently reside inside endocytic vesicles. In a recent mBio article, Poston et al. (D. Poston, Y. Weisblum, A. Hobbs, and P. D. Bieniasz, mBio 13:e0300221, 2022, https://doi.org/10.1128/mbio.03002-21) identified and characterized protein complexes generating endocytic environments favorable for virus entry. These included retromer-associated vacuolar protein sorting 29 (VPS29) proteins. Without VPS29, endosomes lacked cathepsin activities, making them incapable of supporting those viruses in which endosomal proteolysis triggers entry. These protease-dependent viruses encompass several zoonotic filoviruses and coronaviruses, including recent SARS-CoV-2 variants of concern. The valuable findings of Poston et al. reveal retromer complexes as master keys for select endosomal virus entry processes and raise the possibility that threatening coronaviruses might be resisted through targeted inactivation of components controlling endosome structure and function.