Morphogenesis of avian infectious bronchitis virus in primary chick kidney cells

Neurological manifestations of SARS-CoV-2: complexity, mechanism and associated disorders

BACKGROUND

Coronaviruses such as Severe Acute Respiratory Syndrome coronavirus (SARS), Middle Eastern Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) are associated with critical illnesses, including severe respiratory disorders. SARS-CoV-2 is the causative agent of the deadly COVID-19 illness, which has spread globally as a pandemic. SARS-CoV-2 may enter the human body through olfactory lobes and interact with the angiotensin-converting enzyme2 (ACE2) receptor, further facilitating cell binding and entry into the cells. Reports have shown that the virus can pass through the blood-brain barrier (BBB) and enter the central nervous system (CNS), resulting in various disorders. Cell entry by SARS-CoV-2 largely relies on TMPRSS2 and cathepsin L, which activate S protein. TMPRSS2 is found on the cell surface of respiratory, gastrointestinal and urogenital epithelium, while cathepsin-L is a part of endosomes.

AIM

The current review aims to provide information on how SARS-CoV-2 infection affects brain function.. Furthermore, CNS disorders associated with SARS-CoV-2 infection, including ischemic stroke, cerebral venous thrombosis, Guillain-Barré syndrome, multiple sclerosis, meningitis, and encephalitis, are discussed. The many probable mechanisms and paths involved in developing cerebrovascular problems in COVID patients are thoroughly detailed.

MAIN BODY

There have been reports that the SARS-CoV-2 virus can cross the blood-brain barrier (BBB) and enter the central nervous system (CNS), where it could cause a various illnesses. Patients suffering from COVID-19 experience a range of neurological complications, including sleep disorders, viral encephalitis, headaches, dysgeusia, and cognitive impairment. The presence of SARS-CoV-2 in the cerebrospinal fluid (CSF) of COVID-19 patients has been reported. Health experts also reported its presence in cortical neurons and human brain organoids. The possible mechanism of virus infiltration into the brain can be neurotropic, direct infiltration and cytokine storm-based pathways. The olfactory lobes could also be the primary pathway for the entrance of SARS-CoV-2 into the brain.

CONCLUSIONS

SARS-CoV-2 can lead to neurological complications, such as cerebrovascular manifestations, motor movement complications, and cognitive decline. COVID-19 infection can result in cerebrovascular symptoms and diseases, such as strokes and thrombosis. The virus can affect the neural system, disrupt cognitive function and cause neurological disorders. To combat the epidemic, it is crucial to repurpose drugs currently in use quickly and develop novel therapeutics.

Cognitive decline following acute viral infections: literature review and projections for post-COVID-19

Recently, much attention has been drawn to the importance of the impact of infectious disease on human cognition. Several theories have been proposed, to explain the cognitive decline following an infection as well as to understand better the pathogenesis of human dementia, especially Alzheimer's disease. This article aims to review the state of the art regarding the knowledge about the impact of acute viral infections on human cognition, laying a foundation to explore the possible cognitive decline followed coronavirus disease 2019 (COVID-19). To reach this goal, we conducted a narrative review systematizing six acute viral infections as well as the current knowledge about COVID-19 and its impact on human cognition. Recent findings suggest probable short- and long-term COVID-19 impacts in cognition, even in asymptomatic individuals, which could be accounted for by direct and indirect pathways to brain dysfunction. Understanding this scenario might help clinicians and health leaders to deal better with a wave of neuropsychiatric issues that may arise following COVID-19 pandemic as well as with other acute viral infections, to alleviate the cognitive sequelae of these infections around the world.

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.

The impact of Parkinson's disease on manifestations and outcomes of Covid-19 patients: A systematic review and meta-analysis

Parkinson's disease (PD) patients who contracted Coronavirus disease 2019 (Covid‐19) had a decline in motor functions; nevertheless, there is limited evidence on whether PD patients have a higher risk for contracting Covid‐19 or have worse outcomes. This is the first systematic review and meta‐analysis to review the impact of PD on the prognosis of Covid‐19 patients. We performed a systematic search through seven electronic databases under the recommendations of the Preferred Reporting Items for Systematic Review and Meta‐analyses statement (PRISMA) guidelines. The R software version 4.0.2 was used to calculate pooled sample sizes and their associated confidence intervals (95%CI). Finally, we included 13 papers in this study. The pooled prevalence rate of Covid‐19 was 2.12% (95%CI: 0.75–5.98). Fever, cough, fatigue and anorexia were the most common symptoms with a rate of 72.72% (95% CI: 57.3 ‐ 92.29), 66.99% (95% CI: 49.08–91.42), 61.58% (95% CI: 46.69–81.21) and 52.55% (95% CI: 35.09–78.68), respectively. The pooled rates were 39.89% (95% CI: 27.09–58.73) for hospitalisation, 4.7% (95% CI: 1.56–14.16) for ICU admission and 25.1% (95%CI: 16.37–38.49) for mortality. On further comparison of hospitalisation and mortality rates among Covid‐19 patients with and without PD, there were no significant differences. In conclusion, the prevalence and prognosis of Covid‐19 patients seem comparable in patients with PD and those without it. The increased hospitalisation and mortality may be attributed to old age and co‐morbidities.

Possible Link between SARS-CoV-2 Infection and Parkinson's Disease: The Role of Toll-Like Receptor 4

Parkinson's disease (PD) is the most common neurodegenerative motor disorder characterized by selective degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) of the midbrain, depletion of dopamine (DA), and impaired nigrostriatal pathway. The pathological hallmark of PD includes the aggregation and accumulation α-synuclein (α-SYN). Although the precise mechanisms underlying the pathogenesis of PD are still unknown, the activation of toll-like receptors (TLRs), mainly TLR4 and subsequent neuroinflammatory immune response, seem to play a significant role. Mounting evidence suggests that viral infection can concur with the precipitation of PD or parkinsonism. The recently identified coronavirus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of ongoing pandemic coronavirus disease 2019 (COVID-19), responsible for 160 million cases that led to the death of more than three million individuals worldwide. Studies have reported that many patients with COVID-19 display several neurological manifestations, including acute cerebrovascular diseases, conscious disturbance, and typical motor and non-motor symptoms accompanying PD. In this review, the neurotropic potential of SARS-CoV-2 and its possible involvement in the pathogenesis of PD are discussed. Specifically, the involvement of the TLR4 signaling pathway in mediating the virus entry, as well as the massive immune and inflammatory response in COVID-19 patients is explored. The binding of SARS-CoV-2 spike (S) protein to TLR4 and the possible interaction between SARS-CoV-2 and α-SYN as contributing factors to neuronal death are also considered.

SARS-CoV-2 Infection in the Central and Peripheral Nervous System-Associated Morbidities and Their Potential Mechanism

The recent outbreak of SARS-CoV-2 infections that causes coronavirus-induced disease of 2019 (COVID-19) is the defining and unprecedented global health crisis of our time in both the scale and magnitude. Although the respiratory tract is the primary target of SARS-CoV-2, accumulating evidence suggests that the virus may also invade both the central nervous system (CNS) and the peripheral nervous system (PNS) leading to numerous neurological issues including some serious complications such as seizures, encephalitis, and loss of consciousness. Here, we present a comprehensive review of the currently known role of SARS-CoV-2 and identify all the neurological problems reported among the COVID-19 case reports throughout the world. The virus might gain entry into the CNS either through the trans-synaptic route via the olfactory neurons or through the damaged endothelium in the brain microvasculature using the ACE2 receptor potentiated by neuropilin-1 (NRP-1). The most critical of all symptoms appear to be the spontaneous loss of breathing in some COVID-19 patients. This might be indicative of a dysfunction within the cardiopulmonary regulatory centers in the brainstem. These pioneering studies, thus, lay a strong foundation for more in-depth basic and clinical research required to confirm the role of SARS-CoV-2 infection in neurodegeneration of critical brain regulatory centers.

Coronavirus disease 2019 (COVID-19) pathogenesis: a concise narrative review

SARS-CoV-2 is the third zoonotic coronavirus. Since December 2019, it has spread through the globe and infects more than four million patients (as of May 10^th, 2020). The disease was named coronavirus disease 2019 (COVID-19) by the World Health Organization (WHO). It involves many organs and systems in the human organism. We aimed to describe the pathogenesis of the COVID-19.

Severe acute respiratory syndrome coronavirus 2 may be an underappreciated pathogen of the central nervous system

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes a highly contagious respiratory disease referred to as COVID-19. However, emerging evidence indicates that a small but growing number of COVID-19 patients also manifest neurological symptoms, suggesting that SARS-CoV-2 may infect the nervous system under some circumstances. SARS-CoV-2 primarily enters the body through the epithelial lining of the respiratory and gastrointestinal tracts, but under certain conditions this pleiotropic virus may also infect peripheral nerves and gain entry into the central nervous system (CNS). The brain is shielded by various anatomical and physiological barriers, most notably the blood-brain barrier (BBB) which functions to prevent harmful substances, including pathogens and pro-inflammatory mediators, from entering the brain. The BBB is composed of highly specialized endothelial cells, pericytes, mast cells and astrocytes that form the neurovascular unit, which regulates BBB permeability and maintains the integrity of the CNS. In this review, potential routes of viral entry and the possible mechanisms utilized by SARS-CoV-2 to penetrate the CNS, either by disrupting the BBB or infecting the peripheral nerves and using the neuronal network to initiate neuroinflammation, are briefly discussed. Furthermore, the long-term effects of SARS-CoV-2 infection on the brain and in the progression of neurodegenerative diseases known to be associated with other human coronaviruses are considered. Although the mechanisms of SARS-CoV-2 entry into the CNS and neurovirulence are currently unknown, the potential pathways described here might pave the way for future research in this area and enable the development of better therapeutic strategies.

COVID-19 and possible links with Parkinson's disease and parkinsonism: from bench to bedside

This Viewpoint discusses insights from basic science and clinical perspectives on coronavirus disease 2019 (COVID-19)/severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection in the brain, with a particular focus on Parkinson's disease. Major points include that neuropathology studies have not answered the central issue of whether the virus enters central nervous system neurons, astrocytes or microglia, and the brain vascular cell types that express virus have not yet been identified. Currently, there is no clear evidence for human neuronal or astrocyte expression of angiotensin-converting enzyme 2 (ACE2), the major receptor for viral entry, but ACE2 expression may be activated by inflammation, and a comparison of healthy and infected brains is important. In contrast to the 1918 influenza pandemic and avian flu, reports of encephalopathy in COVID-19 have been slow to emerge, and there are so far no documented reports of parkinsonism apart from a single case report. We recommend consensus guidelines for the clinical treatment of Parkinson's patients with COVID-19. While a role for the virus in causing or exacerbating Parkinson's disease appears unlikely at this time, aggravation of specific motor and non-motor symptoms has been reported, and it will be important to monitor subjects after recovery, particularly for those with persisting hyposmia.

The potential role of the carotid body in COVID-19

The carotid body (CB) plays a contributory role in the pathogenesis of various respiratory, cardiovascular, renal, and metabolic diseases through reflex changes in ventilation and sympathetic output. On the basis of available data about peripheral arterial chemoreception and severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2), a potential involvement in the coronavirus disease 2019 (COVID-19) may be hypothesized through different mechanisms. The CB could be a site of SARS-CoV-2 invasion, due to local expression of its receptor [angiotensin-converting enzyme (ACE) 2] and an alternative route of nervous system invasion, through retrograde transport along the carotid sinus nerve. The CB function could be affected by COVID-19-induced inflammatory/immune reactions and/or ACE1/ACE2 imbalance, both at local or systemic level. Increased peripheral arterial chemosensitivity and reflex sympatho-activation may contribute to the increased morbidity and mortality in COVID-19 patients with respiratory, cardiovascular, renal, or metabolic comorbidities.

Potential neurological effects of severe COVID-19 infection

Coronaviruses (CoVs) are large positive stranded enveloped RNA viruses that generally cause enteric and respiratory diseases in humans and in animals. Most human CoVs have recently attracted global attention to their lethal potential and great infectious capacity. A highly pathogenic CoV, called COVID-19 or SARS-CoV-2, dramatically emerged in December 2019 in Wuhan, China. This new CoV has caused severe pneumonia in China and rapidly spreads around the world, the COVID-19 pandemic. Growing evidence pieces show that viruses, such as CoVs, can enter the central nervous system from different pathways and inducing neurotoxicity. Therefore, it is urgent to make clear whether SARS-CoV-2 has access to the central nervous system and can cause direct neuronal effects. Moreover, a brain-lung-brain axis is been proposed from the scientific community where severe neurological dysfunction and injury are associated with lung injury, and vice versa. In this axis, virus-induced inflammation and oxidative stress could be the common mechanisms responsible for CoV neurological symptoms. Therefore, is important to make clear whether SARS-CoV-2 lung damage can cause direct or indirect neuronal effects.

Sinus Node Dysfunction in a Young Patient With COVID-19

A 34-year-old man was admitted with acute lung injury and COVID-19 pneumonia. In the intensive care unit, he experienced episodes of prolonged asystole accompanied by hypotension without loss of consciousness. Once reversible causes were excluded, symptoms were related to dysfunction of the sinus node, and the patient underwent implantation of a pacemaker. (Level of Difficulty: Beginner.)

The neuroinvasive potential of SARS-CoV2 may play a role in the respiratory failure of COVID-19 patients

Following the severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), another highly pathogenic coronavirus named SARS-CoV-2 (previously known as 2019-nCoV) emerged in December 2019 in Wuhan, China, and rapidly spreads around the world. This virus shares highly homological sequence with SARS-CoV, and causes acute, highly lethal pneumonia coronavirus disease 2019 (COVID-19) with clinical symptoms similar to those reported for SARS-CoV and MERS-CoV. The most characteristic symptom of patients with COVID-19 is respiratory distress, and most of the patients admitted to the intensive care could not breathe spontaneously. Additionally, some patients with COVID-19 also showed neurologic signs, such as headache, nausea, and vomiting. Increasing evidence shows that coronaviruses are not always confined to the respiratory tract and that they may also invade the central nervous system inducing neurological diseases. The infection of SARS-CoV has been reported in the brains from both patients and experimental animals, where the brainstem was heavily infected. Furthermore, some coronaviruses have been demonstrated able to spread via a synapse-connected route to the medullary cardiorespiratory center from the mechanoreceptors and chemoreceptors in the lung and lower respiratory airways. Considering the high similarity between SARS-CoV and SARS-CoV2, it remains to make clear whether the potential invasion of SARS-CoV2 is partially responsible for the acute respiratory failure of patients with COVID-19. Awareness of this may have a guiding significance for the prevention and treatment of the SARS-CoV-2-induced respiratory failure.

The emergence of a novel coronavirus (SARS-CoV-2) disease and their neuroinvasive propensity may affect in COVID-19 patients

An outbreak of a novel coronavirus (SARS‐CoV‐2) infection has recently emerged and rapidly spreading in humans causing a significant threat to international health and the economy. Rapid assessment and warning are crucial for an outbreak analysis in response to serious public health. SARS‐CoV‐2 shares highly homological sequences with SARS‐CoVs causing highly lethal pneumonia with respiratory distress and clinical symptoms similar to those reported for SARS‐CoV and MERS‐CoV infections. Notably, some COVID‐19 patients also expressed neurologic signs like nausea, headache, and vomiting. Several studies have reported that coronaviruses are not only causing respiratory illness but also invade the central nervous system through a synapse‐connected route. SARS‐CoV infections are reported in both patients and experimental animals' brains. Interestingly, some COVID‐19 patients have shown the presence of SARS‐CoV‐2 virus in their cerebrospinal fluid. Considering the similarities between SARS‐CoV and SARS‐CoV‐2 in various aspects, it remains to clarify whether the potent invasion of SARS‐CoV‐2 may affect in COVID‐19 patients. All these indicate that more detailed criteria are needed for the treatment and the prevention of SARS‐CoV‐2 infected patients. In the absence of potential interventions for COVID‐19, there is an urgent need for an alternative strategy to control the spread of this disease.

Neurotropic virus tracing suggests a membranous-coating-mediated mechanism for transsynaptic communication

Swine hemagglutinating encephalomyelitis virus (HEV) has been shown to have a capability to propagate via neural circuits to the central nervous system after peripheral inoculation, resulting in acute deadly encephalomyelitis in natural host piglets as well as in experimental younger rodents. This study has systematically examined the assembly and dissemination of HEV 67N in the primary motor cortex of infected rats and provides additional evidence indicating that membranous‐coating‐mediated endo‐/exocytosis can be used by HEV for its transsynaptic transfer. In addition, our results suggested that this transsynaptic pathway could adapted for larger granular materials, such as viruses. These findings should help in understanding the mechanisms underlying coronavirus infections as well as the intercellular exchanges occurring at the synaptic junctions. J. Comp. Neurol. 521:203–212, 2013. © 2012 Wiley Periodicals, Inc.

Coronavirus infection of rat dorsal root ganglia: ultrastructural characterization of viral replication, transfer, and the early response of satellite cells

Swine hemagglutinating encephalomyelitis virus (HEV) has been shown to have a capability to gain access to the cell bodies of sensory neurons after peripheral inoculation, resulting in ganglionic infection. It is not clearly understood how this virus is replicated within and released from the sensory neurons, and it remains to know how satellite cells response to the HEV invasion. By ultrastructurally examining HEV-infected rat dorsal root ganglia, we found that HEV in the cell bodies of infected neurons budded from endoplasmic reticulum–Golgi intermediate compartments, and were assembled either individually within small vesicles or in groups within large vesicles. The progeny virions were released from the sensory neurons mainly by smooth-surfaced vesicle-mediated secretory pathway, which occurred predominantly at the perikaryal projections and infoldings of sensory neurons. Released HEV particles were subsequently taken up by the adjacent satellite cells. Almost all virus particles in the cytoplasm of satellite cells were contained in groups within vesicles and lysosome-like structures, suggesting that these glial cells may restrict the local diffusion of HEV. These observations give some insights into the pathogenesis of coronavirus infection and are thought to help understand the interactions between sensory neurons and their satellite cells.

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.

Four proteins processed from the replicase gene polyprotein of mouse hepatitis virus colocalize in the cell periphery and adjacent to sites of virion assembly

The replicase gene (gene 1) of the coronavirus mouse hepatitis virus (MHV) encodes two co-amino-terminal polyproteins presumed to incorporate all the virus-encoded proteins necessary for viral RNA synthesis. The polyproteins are cotranslationally processed by viral proteinases into at least 15 mature proteins, including four predicted cleavage products of less than 25 kDa that together would comprise the final 59 kDa of protein translated from open reading frame 1a. Monospecific antibodies directed against the four distinct domains detected proteins of 10, 12, and 15 kDa (p1a-10, p1a-12, and p1a-15) in MHV-A59-infected DBT cells, in addition to a previously identified 22-kDa protein (p1a-22). When infected cells were probed by immunofluorescence laser confocal microscopy, p1a-10, -22, -12, and -15 were detected in discrete foci that were prominent in the perinuclear region but were widely distributed throughout the cytoplasm as well. Dual-labeling experiments demonstrated colocalization of the majority of p1a-22 in replication complexes with the helicase, nucleocapsid, and 3C-like proteinase, as well as with p1a-10, -12, and -15. p1a-22 was also detected in separate foci adjacent to the replication complexes. The majority of complexes containing the gene 1 proteins were distinct from sites of accumulation of the M assembly protein. However, in perinuclear regions the gene 1 proteins and nucleocapsid were intercalated with sites of M protein localization. These results demonstrate that the complexes known to be involved in RNA synthesis contain multiple gene 1 proteins and are closely associated with structural proteins at presumed sites of virion assembly.

Ultrastructure of human nasal epithelium during an episode of coronavirus infection

The nasal epithelium from a young girl was examined by electron microscopy and found to be infected by coronavirus. Virions are seen within and outside the ciliated cells, but not outside or within the goblet cells or other cells of the nasal mucosa. Some virions are located near the microvilli, others in pockets in the apical cell membrane. The cytoplasm contains many small vesicles with a single virion, large apical vesicles containing hundreds of virions, and lysosome-like cytosomes with a moderate number of virions. Some virus-like particles devoid of an electron-dense interior are seen both in the cytosomes and extracellularly. Virus budding was observed in the Golgi apparatus but nowhere else in the cell. The ciliated cells seem not to be destroyed by the viruses, although in many cases the cilia are withdrawn into the cell body. The loss of cilia is likely to cause rhinorrhoea.

Coronavirus IBV-induced membrane fusion occurs at near-neutral pH

The lysosomotropic agent NH4Cl caused a reduction of 80-95% in the number of chick kidney (CK) cells and Vero cells infected by infectious bronchitis virus (IBV) strain Beaudette, as determined by immunofluorescence at the end of the first replication cycle. Inhibition only occurred when NH4Cl was present during the first 2 h after infection. Syncytium formation was studied during replication of IBV-Beaudette in Vero cells. Some cell-cell fusion occurred at pH 7.0 and pH 6.5 but it was optimal at pH 6.7. IBV strain UK/123/82 did not replicate in Vero cells and was studied in CK cells in which it grew well but without forming syncytia. In contrast to IBV-Beaudette, NH4Cl had virtually no effect on the replication of UK/123/82. The results show that the IBV spike glycoprotein induces membrane fusion at near neutral pH although some IBV strains may require a mildly acidic environment for the efficient uncoating of the virion RNA.

Differentiation of acid-pH-dependent and -nondependent entry pathways for mouse hepatitis virus

Early events of infection of MHV were studied in comparison with those of VSV, which is known to enter cells by an endocytic pathway. Treatment of mouse L-2 fibroblasts with ammonium chloride, chloroquine, or dansylcadaverine inhibited infection of MHV to a much lesser degree than that of VSV, suggesting a relatively minor role for the endocytic pathway and functional endosomes in MHV infection. Endocytosis of MHV and VSV into L-2 cells was assayed by the recovery of infectious (i.e., not uncoated) viruses from homogenates of cells harvested within the first few minutes of infection (and treated with protease to remove surface-bound virus). The results thus suggest that while a small proportion of the MHV inoculum is internalized by endocytosis, productive infection does not depend on functional endocytosis as utilized by VSV. Studies on direct virion-mediated cell fusion showed that MHV can induce fusion at pH 7.4, whereas VSV causes fusion at pH 5.0. Taken together, the above results suggest that MHV enters L-2 cells predominantly by membrane fusion with a non-acidified compartment such as the plasma membrane, endocytic vesicles, or endosomes (prior to their acidification). Results obtained from cell lines which differed in permissiveness to MHV infection suggested that the ability to support MHV infection does not correlate with endocytosis. Rather, nonpermissive cells, such as rat astrocytoma (C-6) and Vero cells, showed higher levels of recoverable internalized MHV than did fully permissive L-2 cells. Cells which are normally nonpermissive to MHV, could be rendered MHV-susceptible by PEG-induced fusion of cell surface-bound virus. Such PEG-mediated susceptibility to MHV infection was insensitive to inhibition by ammonium chloride, supporting the idea that host cell restriction of MHV infection in C-6 and Vero cells may be due to a block in nonendosomal membrane fusion. Thus endocytic internalization of MHV, which clearly occurs in a variety of cells, does not guarantee productive infection.

Initial events in bovine coronavirus infection: analysis through immunogold probes and lysosomotropic inhibitors

The early events in the infection of human rectal tumor cells by bovine coronavirus were investigated by colloidal gold-mediated immunoelectron microscopy and by analysis of the effect of lysosomotropic weak bases on virus yield. Electron microscopic studies revealed sites of fusion between the virus envelope and the plasmalemma but fusion events along intracellular membranes were not observed despite extensive searches. Virion-antibody-colloidal gold complexes were, in fact, endocytosed by synchronously infected cells. These complexes were apparently non-infectious, and they accumulated in vacuoles that resembled secondary lysosomes. Exposure of cells to ammonium chloride or to methylamine during the first hour of infection had little inhibitory effect on the production of infectious virus. Chloroquine treatments were inhibitory but this effect depended on relatively late events in the infectious process. The chloroquine inhibitory step blocked infection of virus absorbed to cells that were exposed to buffers in the pH range of 4.4 to 8.4. These findings indicate that BCV penetrates its host cell by direct fusion with the plasmalemma and does not require an acidic intracellular compartment for infectious entry.

Comparative features of a coronavirus isolated from a cheetah with feline infectious peritonitis

A coronavirus which was isolated from a cheetah (Acinonyx jubatus) that succumbed to feline infectious peritonitis was characterized in vitro. The virus was determined to be highly cell-associated with Crandell feline kidney (CrFK) cells and was routinely maintained as a persistent infection (CrFK 83-4497). The cheetah coronavirus was compared with other members of the feline coronavirus group including the feline enteric coronavirus (FECV) 79-1683 and the feline infectious peritonitis viruses (FIPV), 79-1146, and UCD-1. The cheetah coronavirus was demonstrated to have a restricted host-cell range with limited cytopathic effect. Indirect immunofluorescence with antisera to FIPV UCD-1 revealed the concentration of viral antigens in the perinuclear region of cells infected with the cheetah coronavirus. Ultrastructural studies of the cheetah coronavirus indicated a limited number of immature viral particles within cytoplasmic vesicles and at the cell surface. This was in contrast to electron microscopy results of FECV 79-1683 and FIPV 79-1146, which had numerous mature virus particles within the cytoplasmic vesicles, as well as at the cell surface. The cheetah coronavirus was tentatively placed in the feline coronavirus family based upon its antigenic reactivity by immunofluorescence; however, the possibility that it represents a unique coronavirus of cheetahs should not be dismissed without further analyses at the host and genomic levels.

Ultrastructure and protein A-gold immunolabelling of HRT-18 cells infected with turkey enteric coronavirus

The Minnesota strain of turkey enteric coronavirus (TCV) was propagated in HRT-18 cells, a cell line derived from human rectum adenocarcinoma. A productive non-cytopathic infection was established, without a previous adaptation, in these cells as shown by the specific hemagglutinating activity in cell culture supernatants. A post-embedding immunochemical technique, using specific antiserum directed against the original egg-adapted virus and colloidal-gold-labelled protein A as the electron-dense marker, was used for the identification of the virus and related antigens in the cells by electron microscopy. Budding of typical coronavirus particles, through intracytoplasmic membranes and accumulation of complete virus within cytoplasmic vesicles or the lumen of rough endoplasmic reticulum, were the main features of the viral morphogenesis. Late in infection, numerous progeny viral particles were shown at the outer surface of infected cells, but budding could not be demonstrated at this level. Two different types of surface projections were observed on the extracellular particles of this avian coronavirus. These morphological characteristics have been thus far described only for mammalian hemagglutinating coronaviruses.

Pathogenesis of infectious bronchitis nephritis. 1. Morphometric analysis of kidney proximal tubular epithelium in chickens

The ultrastructure of the proximal tubular epithelial cells in chicken kidneys was examined throughout the course of an experimental infection with infectious bronchitis virus. A quantitative assessment of the structural changes in the cells was related to these in normal cells. Significant alterations were detected in the membrane structures and in the mitochondria. There was a reduction in surface area of the microvillus membrane, the basolateral membrane and the apical tubular membrane. There were alterations in the shape of mitochondrial profiles and a decrease in the volume density of mitochondria. Vesicular structures, which are a possible site of viral release, were observed in the lateral surface of cells. These changes in the functional components of the cells indicate impaired transport of ions and water. The results demonstrate the value of stereological methods for the study of viral-host cell interactions in the pathogenesis of viral disease.

Attenuation of murine coronavirus infection by ammonium chloride

Ammonium chloride at a concentration of 20 mM delayed by 4-5 hr the production of virus progeny in mouse L-2 cells infected at high multiplicity with mouse hepatitis virus (MHV). This delay was seen in the production of both intracellular and extracellular virus. However, the final titers were similar to those produced by MHV-infected cells maintained in normal medium. The manifestation of virus-induced cell fusion was similarly found to be delayed, but not otherwise decreased in severity, when ammonium chloride was present in the culture medium. Ammonium chloride caused similar delays in production of virus-specific, positive-sense RNAs and of viral polypeptides. The relative proportions and apparent molecular weights of viral RNAs and polypeptides were similar to those found in MHV-infected cells cultured in normal medium. In vitro translation of endogenously produced viral RNAs in cell extracts, prepared from MHV-infected cells, was not inhibited by ammonium chloride. Thus, ammonium chloride has no specific, inhibitory effect on viral protein synthesis. Ammonium chloride did not reduce the number of virus-infected cells in culture, as monitored by infectious center assay. Analysis of early events in MHV infection showed that ammonium chloride did not affect adsorption or internalization of MHV by L-2 cells. However, the subsequent eclipse phase, as monitored by decline in infectivity of internalized virus inoculum proceeded less efficiently in the presence of ammonium chloride. On the basis of the known inhibitory effects of ammonium chloride on lysosomal/endosomal functions, the results suggest an endosomal mechanism of MHV uncoating. Thus the primary effect of ammonium chloride on MHV infection of L-2 cells is to attenuate virus uncoating, thereby chronologically displacing all subsequent virus-encoded functions.

The molecular biology of coronaviruses

Coronaviruses have recently emerged as an important group of animal and human pathogens that share a distinctive replicative cycle. Some of the unique characteristics in the replication of coronaviruses include generation of a 3' coterminal-nested set of five or six subgenomic mRNAs, each of which appears to direct the synthesis of one protein. Two virus-specific RNA polymerase activities have been identified. Many of the distinctive features of coronavirus infection and coronavirus-induced diseases may result from the properties of the two coronavirus glycoproteins. The intracellular budding site, which may be important in the establishment and maintenance of persistent infections, appears to be due to the restricted intracytoplasmic migration of the E1 glycoprotein, which acts as a matrix-like transmembrane glycoprotein. E1 also exhibits distinctive behavior by self-aggregating on heating at 100°C in sodium dodecyl sulfate (SDS) and by its interaction with RNA in the viral nucleocapsid. The E1 of mouse hepatitis virus (MHV) is an O -linked glycoprotein, unlike most other viral glycoproteins. Thus, the coronavirus system may be a useful model for the study of synthesis, glycosylation, and transport of O -linked cellular glycoproteins.

In vivo and in vitro models of demyelinating diseases. V. Comparison of the assembly of mouse hepatitis virus, strain JHM, in two murine cell lines

The developmental sequence of a neurotropic strain (JHM) of mouse hepatitis virus was examined by transmission electron microscopy and immunocytology. The nucleoprotein core of this coronavirus, which contains RNA of positive polarity and is helical in configuration, becomes incorporated into enveloped particles in the same manner as the nucleocapsids of the orthomyxo- and paramyxoviruses. However, JHM virus is assembled intracellularly by budding at surfaces of smooth membranous vacuoles. A comparison of JHM virus replication in L2 and 17Cl-1 cell lines revealed that L2 cells undergo more rapid cytopathology and cease virus production much sooner than 17Cl-l cells. In L2 cells the accumulation of core material appears to continue after the abrupt cessation of virus assembly. This is evident by the massive cytoplasmic accumulation of structure resembling nucleocapsids, which react with hybridoma antibody to the nucleocapsid antigen as demonstrated by the immunoperoxidase procedure. The current findings are consistent with our previously published demonstration, using cells of neural and other deviation, of the fundamental role of the host cell type in regulating the replication and expression of coronaviruses.

Morphological and biological properties of a new coronavirus associated with diarrhea in infant mice

Biological and morphological properties of a virus, isolated from the intestine of infant mice with clinical signs of diarrhea and designated as diarrhea virus of infant mice (DVIM), were examined. The first infective virus was detected on the cells 4 hours post infection, followed by rapid release of the virus into the culture fluids. Virus-induced syncytia in BALB/c-3T3 cell cultures caused hemadsorption at 4 degrees C and viral antigens were shown to be located in the cytoplasm of the syncytia by immunofluorescent techniques. By scanning electron-microscopy, budding virus-like particles were detected on the surface of virus-induced syncytia. Morphologically the virus was shown to be enveloped and approximately 100 nm in diameter. Two types of projections were demonstrated, one type of projection was club-shaped, 20 nm in length and the other type was small, granular, 5 nm in length. The latter type of projection might be the basal part of the club-shaped type and related to the hemagglutinating activity.

On the entry of Semliki forest virus into BHK-21 cells

The pathway by which semliki forest virus (SFV), a membrane-containing animal virus, enters BHK-21 cells was studied morphologically and biochemically. After attaching to the cell surface, the majority of viruses was rapidly trapped into coated pits, internalized by endocytosis in coated vesicles, and sequestered into intracellular vacuoles and lysosomes. Direct penetration of viruses through the plasma membrane was never observed. To assess the possible involvement of lysosomes in the release of the genome into the cytoplasm, the effect of five lysosomotropic agents, known to increase the lysosomal pH, was tested. All of these agents inhibited SFV infectivity and one, chloroquine (the agent studied in most detail), inhibited a very early step in the infection but had no effect on binding, endocytosis, or intracellular distribution of SFV. Thus, the inhibitory effect was concluded to be either on penetration of the nucelocapsid into the cytoplasm or on uncoating of the viral RNA. Possible mechanisms for the penetration of the genome into the cytoplasm were studied in vitro, using phospholipids-cholesterol liposomes and isolated SFV. When the pH was 6.0 or lower, efficient fusion of the viral membranes and the liposomal membranes occurred, resulting in the transfer of the nucleocapsid into the liposomes. Infection of cells could also be induced by brief low pH treatment of cells with bound SFV under conditions where the normal infection route was blocked. The results suggest that the penetration of the viral genome into the cytosol takes place intracellularly through fusion between the limiting membrane of intracellular vacuoles and the membrane of viruses contained within them. The low pH required for fusion together with the inhibitory effect of lysosomotropic agents implicate lysosomes, or other intracellular vacuoles with sufficiently low pH, as the main sites of penetration.