Endocytic pathways involved in filovirus entry: advances, implications and future directions

Prevention and post-exposure management of occupational exposure to Ebola virus

There have been significant advances in the prevention and management of Ebola virus disease (EVD) caused by Zaire Ebola virus (ZEBOV), including the development of two effective vaccines, rVSV-ZEBOV and Ad26.ZEBOV/MVA-BN-Filo. In addition, ZEBOV monoclonal antibodies have become first-line therapy for EVD. However, the 2022-23 outbreak of Sudan Ebola virus (SUDV) in Uganda has highlighted the gap in current therapies and vaccines, whose efficacy is uncertain against non-ZEBOV species. Health-care and laboratory staff working in EVD treatment centres or Ebola virus diagnostic and research laboratories face unique risks relating to potential occupational exposure to Ebola viruses. Given the substantial morbidity and mortality associated with EVD, facilities should have strategies in place to manage occupational exposures, including consideration of post-exposure therapies. In this Review, we discuss currently available evidence for prevention and post-exposure prophylaxis of EVD, including therapies currently under evaluation for SUDV.

Mechanisms of Immune Evasion by Ebola Virus

The 2013-2016 Ebola virus epidemic in West Africa, which also spread to the USA, UK and Europe, was the largest reported outbreak till date (World Health Organization. 2016. https://apps.who.int/iris/bitstream/handle/10665/208883/ebolasitrep_10Jun2016_eng.pdf;jsessionid=8B7D74BC9D82D2BE1B110BAFFAD3A6E6?sequence=1 ). The recent Ebola outbreak in the Democratic Republic of the Congo has raised immense global concern on this severe and often fatal infection. Although sporadic, the severity and lethality of Ebola virus disease outbreaks has led to extensive research worldwide on this virus. Vaccine (World Health Organization. 2016. https://www.who.int/en/news-room/detail/23-12-2016-final-trial-results-confirm-ebola-vaccine-provides-high-protection-against-disease ; Henao-Restrepo et al. Lancet 389:505-518, 2017) and drug (Hayden. Nature, 557, 475-476, 2018; Dyall et al. J Infect Dis 218(suppl_5), S672-S678, 2018) development efforts against Ebola virus are research hotspots, and a few approved therapeutics are currently available (Centers for Disease Control and Prevention. 2021. https://www.cdc.gov/vhf/ebola/clinicians/vaccine/index.html; Centers for Disease Control and Prevention. 2021. https://www.cdc.gov/vhf/ebola/treatment/index.html). Ebola virus has evolved several mechanisms of host immune evasion, which facilitate its replication and pathogenesis. This chapter describes the Ebola virus morphology, genome, entry, replication, pathogenesis and viral proteins involved in host immune evasion. Further understanding of the underlying molecular mechanisms of immune evasion may facilitate development of additional novel and sustainable strategies against this deadly virus.

Targeting Ebola virus replication through pharmaceutical intervention

Introduction. The consistent emergence/reemergence of filoviruses into a world that previously lacked an approved pharmaceutical intervention parallels an experience repeatedly played-out for most other emerging pathogenic zoonotic viruses. Investment to preemptively develop effective and low-cost prophylactic and therapeutic interventions against viruses that have high potential for emergence and societal impact should be a priority.Areas covered. Candidate drugs can be characterized into those that interfere with cellular processes required for Ebola virus (EBOV) replication (host-directed), and those that directly target virally encoded functions (direct-acting). We discuss strategies to identify pharmaceutical interventions for EBOV infections. PubMed/Web of Science databases were searched to establish a detailed catalog of these interventions.Expert opinion. Many drug candidates show promising in vitro inhibitory activity, but experience with EBOV shows the general lack of translation to in vivo efficacy for host-directed repurposed drugs. Better translation is seen for direct-acting antivirals, in particular monoclonal antibodies. The FDA-approved monoclonal antibody treatment, Inmazeb™ is a success story that could be improved in terms of impact on EBOV-associated disease and mortality, possibly by combination with other direct-acting agents targeting distinct aspects of the viral replication cycle. Costs need to be addressed given EBOV emergence primarily in under-resourced countries.

A carbohydrate mimetic peptide modified size-shrinkable micelle nanocluster for anti-tumor targeting and penetrating drug delivery

Purpose

To deliver the chemotherapeutics through the nanoparticles, the delivery system should accumulate at the tumor site first and then penetrate through the interstitium into the interior. The specific tumor-targeting pathway mediated via the receptor-ligand binding could achieve the desirable accumulation of nanoparticles, and the nanoparticles with smaller sizes were required for penetration.

Methods and materials

We constructed a size-shrinkable nanocluster modified with a tumor-targeting motif IF-7 (IF-7-MNC) based on a pH-sensitive framework which could be disintegrated in an acid environment to release the micelles aggregated inside. The micelles were constructed by amphiphilic block copolymers PEG−PLA to encapsulate paclitaxel (PTX), while the cross-linked framework consisting of TPGS-PEI was used as a net to gather and release micelles. This nanoplatform could specifically bind with the tumor receptor Annexin A1 through the ligand IF-7 and then shrunk into small micelles with a desirable size for penetration.

Conclusion

IF-7-MNC of 112.27±6.81 nm could shrink into micelles in PBS (0.01 M, pH 5.0) with sizes of 14.89±0.32 nm. The cellular-uptake results showed that IF-7-MNC could be significantly internalized by A549 cells and HUVEC cells, while the penetration of IF-7-MNC could be more prominent into the 3D-tumor spheroids compared with that of MNC. The biodistribution results displayed that the fluorescence of IF-7-MNC in the tumor site at 24 hrs was 4.5-fold stronger than that of MNC. The results of anti-tumor growth demonstrated that IF-7-MNC was more favorable for the tumor therapy than MNC, where the inhibitory rate of tumor growth was 88.29% in the PTX-loaded IF-7-MNC (IF-7-PMNC) treated group, significantly greater than PMNC treatment group (p<0.05).

Identification of a coumarin-based antihistamine-like small molecule as an anti-filoviral entry inhibitor

Filoviruses, consisting of Ebola virus, Marburg virus and Cuevavirus, cause severe hemorrhagic fevers in humans with high mortality rates up to 90%. Currently, there is no approved vaccine or therapy available for the prevention and treatment of filovirus infection in humans. The recent 2013-2015 West African Ebola epidemic underscores the urgency to develop antiviral therapeutics against these infectious diseases. Our previous study showed that GPCR antagonists, particularly histamine receptor antagonists (antihistamines) inhibit Ebola and Marburg virus entry. In this study, we screened a library of 1220 small molecules with predicted antihistamine activity, identified multiple compounds with potent inhibitory activity against entry of both Ebola and Marburg viruses in human cancer cell lines, and confirmed their anti-Ebola activity in human primary cells. These small molecules target a late-stage of Ebola virus entry. Further structure-activity relationship studies around one compound (cp19) reveal the importance of the coumarin fused ring structure, especially the hydrophobic substituents at positions 3 and/or 4, for its antiviral activity, and this identified scaffold represents a favorable starting point for the rapid development of anti-filovirus therapeutic agents.

Anticancer kinase inhibitors impair intracellular viral trafficking and exert broad-spectrum antiviral effects

Global health is threatened by emerging viral infections, which largely lack effective vaccines or therapies. Targeting host pathways that are exploited by multiple viruses could offer broad-spectrum solutions. We previously reported that AAK1 and GAK, kinase regulators of the host adaptor proteins AP1 and AP2, are essential for hepatitis C virus (HCV) infection, but the underlying mechanism and relevance to other viruses or in vivo infections remained unknown. Here, we have discovered that AP1 and AP2 cotraffic with HCV particles in live cells. Moreover, we found that multiple viruses, including dengue and Ebola, exploit AAK1 and GAK during entry and infectious virus production. In cultured cells, treatment with sunitinib and erlotinib, approved anticancer drugs that inhibit AAK1 or GAK activity, or with more selective compounds inhibited intracellular trafficking of HCV and multiple unrelated RNA viruses with a high barrier to resistance. In murine models of dengue and Ebola infection, sunitinib/erlotinib combination protected against morbidity and mortality. We validated sunitinib- and erlotinib-mediated inhibition of AAK1 and GAK activity as an important mechanism of antiviral action. Additionally, we revealed potential roles for additional kinase targets. These findings advance our understanding of virus-host interactions and establish a proof of principle for a repurposed, host-targeted approach to combat emerging viruses.

Reverse Genetics of Filoviruses

Reverse genetics systems are used for the generation of recombinant viruses. For filoviruses, this technology has been available for more than 15 years and has been used to investigate questions regarding the molecular biology, pathogenicity, and host adaptation determinants of these viruses. Further, reporter-expressing, recombinant viruses are increasingly used as tools for screening for and characterization of candidate medical countermeasures. Thus, reverse genetics systems represent powerful research tools. Here we provide an overview of available reverse genetics systems for the generation of recombinant filoviruses, potential applications, and the achievements that have been made using these systems.

Ebola virus entry into host cells: identifying therapeutic strategies

Filoviruses cause severe hemorrhagic fever in humans. The archetypal virus of this group, Ebola virus, is responsible for the current filovirus epidemic in West Africa. Filoviruses infect most mammalian cells, resulting in broad species tropism and likely contributing to rapid spread of virus throughout the body. A thorough understanding of filovirus entry events will facilitate the development of therapeutics against these critical steps in the viral life cycle. This review summarizes the current understanding of filovirus entry and discusses some of the recent advancements in therapeutic strategies that target entry.

Marburg virus inclusions: A virus-induced microcompartment and interface to multivesicular bodies and the late endosomal compartment

Filovirus infection of target cells leads to the formation of virally induced cytoplasmic inclusions that contain viral nucleocapsids at different stages of maturation. While the role of the inclusions has been unclear since the identification of Marburg and Ebola viruses, it recently became clear that the inclusions are the sites of viral replication, nucleocapsid formation and maturation. Live cell imaging analyses revealed that mature nucleocapsids are transported from inclusions to the filopodia, which represent the major budding sites. Moreover, inclusions recruit cellular proteins that have been shown to support the transport of nucleocapsids. For example, the tumor susceptibility gene 101 protein (Tsg101) interacts with a late domain motif in the nucleocapsid protein NP and recruits the actin-nucleation factor IQGAP1. Complexes of nucleocapsids together with Tsg101 and IQGAP1 are then co-transported along actin filaments. We detected additional proteins (Alix, Nedd4 and the AAA-type ATPase VPS4) of the endosomal sorting complex required for transport (ESCRT) that are recruited into inclusions. Together, the results suggest that nucleocapsids recruit the machinery that enhances viral budding at the plasma membrane. Furthermore, we identified Lamp1 as a marker of the late endosomal compartment in inclusions, while ER, Golgi, TGN and early endosomal markers were absent. In addition, we observed that LC3, a marker of autophagosomal membranes, was present in inclusions. The 3D structures of inclusions show an intricate structure that seems to accommodate an intimate cooperation between cellular and viral components with the intention to support viral transport and budding.

Structural and Functional Studies on the Marburg Virus GP2 Fusion Loop

Marburg virus (MARV) and the ebolaviruses belong to the family Filoviridae (the members of which are filoviruses) that cause severe hemorrhagic fever. Infection requires fusion of the host and viral membranes, a process that occurs in the host cell endosomal compartment and is facilitated by the envelope glycoprotein fusion subunit, GP2. The N-terminal fusion loop (FL) of GP2 is a hydrophobic disulfide-bonded loop that is postulated to insert and disrupt the host endosomal membrane during fusion. Here, we describe the first structural and functional studies of a protein corresponding to the MARV GP2 FL. We found that this protein undergoes a pH-dependent conformational change, as monitored by circular dichroism and nuclear magnetic resonance. Furthermore, we report that, under low pH conditions, the MARV GP2 FL can induce content leakage from liposomes. The general aspects of this pH-dependent structure and lipid-perturbing behavior are consistent with previous reports on Ebola virus GP2 FL. However, nuclear magnetic resonance studies in lipid bicelles and mutational analysis indicate differences in structure exist between MARV and Ebola virus GP2 FL. These results provide new insight into the mechanism of MARV GP2-mediated cell entry.

Tissue and cellular tropism, pathology and pathogenesis of Ebola and Marburg viruses

Ebola viruses and Marburg viruses include some of the most virulent and fatal pathogens known to humans. These viruses cause severe haemorrhagic fevers, with case fatality rates in the range 25-90%. The diagnosis of filovirus using formalin-fixed tissues from fatal cases poses a significant challenge. The most characteristic histopathological findings are seen in the liver; however, the findings overlap with many other viral and non-viral haemorrhagic diseases. The need to distinguish filovirus infections from other haemorrhagic fevers, particularly in areas with multiple endemic viral haemorrhagic agents, is of paramount importance. In this review we discuss the current state of knowledge of filovirus infections and their pathogenesis, including histopathological findings, epidemiology, modes of transmission and filovirus entry and spread within host organisms. The pathogenesis of filovirus infections is complex and involves activation of the mononuclear phagocytic system, with release of pro-inflammatory cytokines, chemokines and growth factors, endothelial dysfunction, alterations of the innate and adaptive immune systems, direct organ and endothelial damage from unrestricted viral replication late in infection, and coagulopathy. Although our understanding of the pathogenesis of filovirus infections has rapidly increased in the past few years, many questions remain unanswered.