Coronavirus infection of polarized epithelial cells

Neurological manifestations of coronavirus infections, before and after COVID-19: a review of animal studies

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This virus, which was first identified in December 2019 in China, has resulted in a yet ongoing viral pandemic. Coronaviridae could potentially cause several disorders in a wide range of hosts such as birds and mammals. Although infections caused by this family of viruses are predominantly limited to the respiratory tract, Betacoronaviruses are potentially able to invade the central nervous system (CNS) as well as many other organs, thereby inducing neurological damage ranging from mild to lethal in both animals and humans. Over the past two decades, three novel CoVs, SARS-CoV-1, MERS-CoV, and SARS-CoV-2, emerging from animal reservoirs have exhibited neurotropic properties causing severe and even fatal neurological diseases. The pathobiology of these neuroinvasive viruses has yet to be fully known. Both clinical features of the previous CoV epidemics (SARS-CoV-1 and MERS-CoV) and lessons from animal models used in studying neurotropic CoVs, especially SARS and MERS, constitute beneficial tools in comprehending the exact mechanisms of virus implantation and in illustrating pathogenesis and virus dissemination pathways in the CNS. Here, we review the animal research which assessed CNS infections with previous more studied neurotropic CoVs to demonstrate how experimental studies with appliable animal models can provide scientists with a roadmap in the CNS impacts of SARS-CoV-2. Indeed, animal studies can finally help us discover the underlying mechanisms of damage to the nervous system in COVID-19 patients and find novel therapeutic agents in order to reduce mortality and morbidity associated with neurological complications of SARS-CoV-2 infection.

Assembly and Cellular Exit of Coronaviruses: Hijacking an Unconventional Secretory Pathway from the Pre-Golgi Intermediate Compartment via the Golgi Ribbon to the Extracellular Space

Coronaviruses (CoVs) assemble by budding into the lumen of the intermediate compartment (IC) at the endoplasmic reticulum (ER)-Golgi interface. However, why CoVs have chosen the IC as their intracellular site of assembly and how progeny viruses are delivered from this compartment to the extracellular space has remained unclear. Here we address these enigmatic late events of the CoV life cycle in light of recently described properties of the IC. Of particular interest are the emerging spatial and functional connections between IC elements and recycling endosomes (REs), defined by the GTPases Rab1 and Rab11, respectively. The establishment of IC-RE links at the cell periphery, around the centrosome and evidently also at the noncompact zones of the Golgi ribbon indicates that—besides traditional ER-Golgi communication—the IC also promotes a secretory process that bypasses the Golgi stacks, but involves its direct connection with the endocytic recycling system. The initial confinement of CoVs to the lumen of IC-derived large transport carriers and their preferential absence from Golgi stacks is consistent with the idea that they exit cells following such an unconventional route. In fact, CoVs may share this pathway with other intracellularly budding viruses, lipoproteins, procollagen, and/or protein aggregates experimentally introduced into the IC lumen.

CRFK and Primary Macrophages Transcriptomes in Response to Feline Coronavirus Infection Differ Significantly

Coronaviruses are highly infectious and common in many species, including in humans, and agricultural and domestic animals. Host responses play an important role in viral entry, replication, assembly, and pathogenesis, although much is still to be understood, particularly host-virus interactions. Feline coronavirus is highly contagious, and ubiquitous in virtually all cat populations. Host-pathogen interactions have not been studied extensively due to the complex pathogenesis and development of clinical disease. Few studies have investigated cellular host responses to feline coronavirus infection, particularly at early time points. Transcriptome studies based on next-generation sequencing have the potential to elucidate the early responses of cells after viral infection and, consequently, give further insight into the pathogenesis of viruses. The current study aims to characterize and compare the viral- and immune-related differentially expressed genes in response to the coronavirus FIPV across different time points in a cell line which is permissive for productive replication versus primary cells implicated in pathogenesis. When comparing host responses in Crandell-Rees Feline Kidney (CRFK) cells to primary macrophages, many differences were observed with regards to expressed genes and their enrichments for both KEGG pathways and GO terms. CRFK cells which are permissive for productive replication of feline infectious peritonitis virus, showed induction of a large network of immunological and virally induced pathways. In contrast, Macrophages did not show similar host responses, with stronger pathway enrichment in downregulated transcripts. This study provides insights to better understand gene transcription in immune cells compared to epithelial cells discerning pathways relevant to pathogenesis in the early stages of infection.

Influenza A Virus M2 Protein Apical Targeting Is Required for Efficient Virus Replication

The influenza A virus (IAV) M2 protein is a multifunctional protein with critical roles in virion entry, assembly, and budding. M2 is targeted to the apical plasma membrane of polarized epithelial cells, and the interaction of the viral proteins M2, M1, HA, and NA near glycolipid rafts in the apical plasma membrane is hypothesized to coordinate the assembly of infectious virus particles. To determine the role of M2 protein apical targeting in IAV replication, a panel of M2 proteins with basolateral plasma membrane (M2-Baso) or endoplasmic reticulum (M2-ER) targeting sequences was generated. MDCK II cells stably expressing M2-Baso, but not M2-ER, complemented the replication of M2-stop viruses. However, in primary human nasal epithelial cell (hNEC) cultures, viruses encoding M2-Baso and M2-ER replicated to negligible titers compared to those of wild-type virus. M2-Baso replication was negatively correlated with cell polarization. These results demonstrate that M2 apical targeting is essential for IAV replication: targeting M2 to the ER results in a strong, cell type-independent inhibition of virus replication, and targeting M2 to the basolateral membrane has greater effects in hNECs than in MDCK cells.IMPORTANCE Influenza A virus assembly and particle release occur at the apical membrane of polarized epithelial cells. The integral membrane proteins encoded by the virus, HA, NA, and M2, are all targeted to the apical membrane and believed to recruit the other structural proteins to sites of virus assembly. By targeting M2 to the basolateral or endoplasmic reticulum membranes, influenza A virus replication was significantly reduced. Basolateral targeting of M2 reduced the infectious virus titers with minimal effects on virus particle release, while targeting to the endoplasmic reticulum resulted in reduced infectious and total virus particle release. Therefore, altering the expression and the intracellular targeting of M2 has major effects on virus replication.

Biology and pathogenesis of cytomegalovirus in periodontal disease

Human periodontitis is associated with a wide range of bacteria and viruses and with complex innate and adaptive immune responses. Porphyromonas gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, Treponema denticola, cytomegalovirus and other herpesviruses are major suspected pathogens of periodontitis, and a combined herpesvirus–bacterial periodontal infection can potentially explain major clinical features of the disease. Cytomegalovirus infects periodontal macrophages and T‐cells and elicits a release of interleukin‐1β and tumor necrosis factor‐α. These proinflammatory cytokines play an important role in the host defense against the virus, but they also have the potential to induce alveolar bone resorption and loss of periodontal ligament. Gingival fibroblasts infected with cytomegalovirus also exhibit diminished collagen production and release of an increased level of matrix metalloproteinases. This article reviews innate and adaptive immunity to cytomegalovirus and suggests that immune responses towards cytomegalovirus can play roles in controlling, as well as in exacerbating, destructive periodontal disease.

Influenza H5N1 virus infection of polarized human alveolar epithelial cells and lung microvascular endothelial cells

Background

Highly pathogenic avian influenza (HPAI) H5N1 virus is entrenched in poultry in Asia and Africa and continues to infect humans zoonotically causing acute respiratory disease syndrome and death. There is evidence that the virus may sometimes spread beyond respiratory tract to cause disseminated infection. The primary target cell for HPAI H5N1 virus in human lung is the alveolar epithelial cell. Alveolar epithelium and its adjacent lung microvascular endothelium form host barriers to the initiation of infection and dissemination of influenza H5N1 infection in humans. These are polarized cells and the polarity of influenza virus entry and egress as well as the secretion of cytokines and chemokines from the virus infected cells are likely to be central to the pathogenesis of human H5N1 disease.

Aim

To study influenza A (H5N1) virus replication and host innate immune responses in polarized primary human alveolar epithelial cells and lung microvascular endothelial cells and its relevance to the pathogenesis of human H5N1 disease.

Methods

We use an in vitro model of polarized primary human alveolar epithelial cells and lung microvascular endothelial cells grown in transwell culture inserts to compare infection with influenza A subtype H1N1 and H5N1 viruses via the apical or basolateral surfaces.

Results

We demonstrate that both influenza H1N1 and H5N1 viruses efficiently infect alveolar epithelial cells from both apical and basolateral surface of the epithelium but release of newly formed virus is mainly from the apical side of the epithelium. In contrast, influenza H5N1 virus, but not H1N1 virus, efficiently infected polarized microvascular endothelial cells from both apical and basolateral aspects. This provides a mechanistic explanation for how H5N1 virus may infect the lung from systemic circulation. Epidemiological evidence has implicated ingestion of virus-contaminated foods as the source of infection in some instances and our data suggests that viremia, secondary to, for example, gastro-intestinal infection, can potentially lead to infection of the lung. HPAI H5N1 virus was a more potent inducer of cytokines (e.g. IP-10, RANTES, IL-6) in comparison to H1N1 virus in alveolar epithelial cells, and these virus-induced chemokines were secreted onto both the apical and basolateral aspects of the polarized alveolar epithelium.

Conclusion

The predilection of viruses for different routes of entry and egress from the infected cell is important in understanding the pathogenesis of influenza H5N1 infection and may help unravel the pathogenesis of human H5N1 disease.

Molecular interactions in the assembly of coronaviruses

This chapter describes the interactions between the different structural components of the viruses and discusses their relevance for the process of virion formation. Two key factors determine the efficiency of the assembly process: intracellular transport and molecular interactions. Many viruses have evolved elaborate strategies to ensure the swift and accurate delivery of the virion components to the cellular compartment(s) where they must meet and form (sub) structures. Assembly of viruses starts in the nucleus by the encapsidation of viral DNA, using cytoplasmically synthesized capsid proteins; nucleocapsids then migrate to the cytosol, by budding at the inner nuclear membrane followed by deenvelopment, to pick up the tegument proteins.

Entry of viruses through the epithelial barrier: pathogenic trickery

Mucosal surfaces--such as the lining of the gut or the reproductive tract--are the main point of entry for viruses into the body. As such, almost all viruses interact with epithelial cells, and make use of the normal epithelial signalling and trafficking pathways of the host cell. In addition to protein receptors, carbohydrate chains of proteoglycans and epithelial-membrane glycosphingolipids have emerged as a new class of receptors for viral attachment to the host cell.

The molecular dynamics of feline coronaviruses

Feline coronaviruses are widespread and come in different flavors. There are two main serotypes both of which occur in two pathotypes, the avirulent enteric viruses and the virulent, usually fatal peritonitis viruses, the latter in turn occurring either in a 'wet' or exudative form or in a 'dry' or proliferative form. In this paper a concise overview is given of the molecular features of these viruses. Special attention is given to the genetic dynamics of the viruses as these now allow us to begin to understand the origin of the different phenotypes, in particular the genesis of virulence during persistent infection. As discussed, the surprising new insights obtained over the last few years call for a critical reevaluation of strategies for protection.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.