Massive transient damage of the olfactory epithelium associated with infection of sustentacular cells by SARS-CoV-2 in golden Syrian hamsters

Ocular Symptoms Associated with COVID-19 Are Correlated with the Expression Profile of Mouse SARS-CoV-2 Binding Sites

The COVID-19 pandemic has engendered significant scientific efforts in the understanding of its infectious agent SARS-CoV-2 and of its associated symptoms. A peculiar characteristic of this virus lies in its ability to challenge our senses, as its infection can lead to anosmia and ageusia. While ocular symptoms, such as conjunctivitis, optic neuritis or dry eyes, are also reported after viral infection, they have lower frequencies and severities, and their functional development is still elusive. Here, using combined technical approaches based on histological and gene profiling methods, we characterized the expression of SARS-CoV-2 binding sites (Ace2/Tmprss2) in the mouse eye. We found that ACE2 was ectopically expressed in subtissular ocular regions, such as in the optic nerve and in the Harderian/intraorbital lacrimal glands. Moreover, we observed an important variation of Ace2/Tmprss2 expression that is not only dependent on the age and sex of the animal, but also highly heterogenous between individuals. Our results thus give new insight into the expression of SARS-CoV-2 binding sites in the mouse eye and propose an interpretation of the human ocular-associated symptoms linked to SARS-CoV-2.

Neurons, Nose, and Neurodegenerative Diseases: Olfactory Function and Cognitive Impairment

Olfactory capacity declines with aging, but increasing evidence shows that smell dysfunction is one of the early signs of prodromal neurodegenerative diseases such as Alzheimer's and Parkinson's disease. The study of olfactory ability and its role in neurodegenerative diseases arouses much interest in the scientific community. In neurology, olfactory impairment is a potential early marker for the onset of neurodegenerative diseases, but the underlying mechanism is poorly understood. The loss of smell is considered a clinical sign of early-stage disease and a marker of the disease's progression and cognitive impairment. Highlighting the importance of biological bases of smell and molecular pathways could be fundamental to improve neuroprotective and therapeutic strategies. We focused on the review articles and meta-analyses on olfactory and cognitive impairment. We depicted the neurobiology of olfaction and the most common olfactory tests in neurodegenerative diseases. In addition, we underlined the close relationship between the olfactory and cognitive deficit due to nasal neuroepithelium, which is a direct extension of the CNS in communication with the external environment. Neurons, Nose, and Neurodegenerative diseases highlights the role of olfactory dysfunction as a clinical marker for early stages of neurodegenerative diseases when it is associated with molecular, clinical, and neuropathological correlations.

Technique of flat-mount immunostaining for mapping the olfactory epithelium and counting the olfactory sensory neurons

The pathophysiology underlying olfactory dysfunction is still poorly understood, and more efficient biomolecular tools are necessary to explore this aspect. Immunohistochemistry (IHC) on cross sections is one of the major tools to study the olfactory epithelium (OE), but does not allow reliable counting of olfactory sensory neurons (OSNs) or cartography of the OE. In this study, we want to present an easy immunostaining technique to compensate for these defects of IHC. Using the rat model, we first validated and pre-screened the key OSN markers by IHC on cross sections of the OE. Tuj-1, OMP, DCX, PGP9.5, and N-cadherin were selected for immunostaining on flat-mounted OE because of their staining of OSN dendrites. A simple technique for immunostaining on flat-mounted septal OE was developed: fixation of the isolated septum mucosa in 0.5% paraformaldehyde (PFA) preceded by pretreatment of the rat head in 1% PFA for 1 hour. This technique allowed us to correctly reveal the olfactory areas using all the 5 selected markers on septum mucosa. By combining the mature OSN marker (OMP) and an immature OSN marker (Tuj-1), we quantified the mature (OMP+, Tuj-1-), immature (OMP-, Tuj-1+), transitory (OMP+, Tuj-1+) and total OSN density on septal OE. They were respectively 42080 ± 11820, 49384 ± 7134, 14448 ± 5865 and 105912 ± 13899 cells per mm2 (mean ± SD). Finally, the same immunostaining technique described above was performed with Tuj-1 for OE cartography on ethmoid turbinates without flat-mount.

Olfactory system measurements in COVID-19: a systematic review and meta-analysis

PURPOSE

The neurotropism of SARS-CoV-2 and the consequential damage to the olfactory system have been proposed as one of the possible underlying causes of olfactory dysfunction in COVID-19. We aimed to aggregate the results of the studies which reported imaging of the olfactory system of patients with COVID-19 versus controls.

METHODS

PubMed and EMBASE were searched to identify relevant literature reporting the structural imaging characteristics of the olfactory bulb (OB), olfactory cleft, olfactory sulcus (OS), or olfactory tract in COVID-19 patients. Hedge's g and weighted mean difference were used as a measure of effect size. Quality assessment, subgroup analyses, meta-regression, and sensitivity analysis were also conducted.

RESULTS

Ten studies were included in the qualitative synthesis, out of which seven studies with 183 cases with COVID-19 and 308 controls without COVID-19 were enrolled in the quantitative synthesis. No significant differences were detected in analyses of right OB volume and left OB volume. Likewise, right OS depth and left OS depth were also not significantly different in COVID-19 cases compared to non-COVID-19 controls. Also, we performed subgroup analysis, meta-regression, and sensitivity analysis to investigate the potential effect of confounding moderators.

CONCLUSION

The findings of this review did not confirm alterations in structural imaging of the olfactory system, including OB volume and OS depth by Covid-19 which is consistent with the results of recent histopathological evaluations.

Pathophysiology of SARS-CoV-2 Infection of Nasal Respiratory and Olfactory Epithelia and Its Clinical Impact

PURPOSE OF REVIEW

While the predominant cause for morbidity and mortality with SARS-CoV-2 infection is the lower respiratory tract manifestations of the disease, the effects of SARS-CoV-2 infection on the sinonasal tract have also come to the forefront especially with the increased recognition of olfactory symptom. This review presents a comprehensive summary of the mechanisms of action of the SARS-CoV-2 virus, sinonasal pathophysiology of COVID-19, and the correlation with the clinical and epidemiological impact on olfactory dysfunction.

RECENT FINDINGS

ACE2 and TMPRSS2 receptors are key players in the mechanism of infection of SARS-CoV-2. They are present within both the nasal respiratory as well as olfactory epithelia. There are however differences in susceptibility between different groups of individuals, as well as between the different SARS-CoV-2 variants. The sinonasal cavity is an important route for SARS-CoV-2 infection. While the mechanism of infection of SARS-CoV-2 in nasal respiratory and olfactory epithelia is similar, there exist small but significant differences in the susceptibility of these epithelia and consequently clinical manifestations of the disease. Understanding the differences and nuances in sinonasal pathophysiology in COVID-19 would allow the clinician to predict and counsel patients suffering from COVID-19. Future research into molecular pathways and cytokine responses at different stages of infection and different variants of SARS-CoV-2 would evaluate the individual clinical phenotype, prognosis, and possibly response to vaccines and therapeutics.

Olfactory dysfunction in COVID-19: new insights into the underlying mechanisms

The mechanisms of olfactory dysfunction in COVID-19 are still unclear. In this review, we examine potential mechanisms that may explain why the sense of smell is lost or altered. Among the current hypotheses, the most plausible is that death of infected support cells in the olfactory epithelium causes, besides altered composition of the mucus, retraction of the cilia on olfactory receptor neurons, possibly because of the lack of support cell-derived glucose in the mucus, which powers olfactory signal transduction within the cilia. This mechanism is consistent with the rapid loss of smell with COVID-19, and its rapid recovery after the regeneration of support cells. Host immune responses that cause downregulation of genes involved in olfactory signal transduction occur too late to trigger anosmia, but may contribute to the duration of the olfactory dysfunction.

[COVID-19-associated anosmia]

The article is a systematic review of the literature data summarizes to date on the issue of COVID-19-associated anosmia. We mainly used full-text and abstract electronic databases (PubMed, Scopus and Web of Science). The paper discusses hypothetical mechanisms of development, clinical features, as well as methods of diagnosis and treatment of COVID-19-associated anosmia.

SARS-CoV-2 infection of sustentacular cells disrupts olfactory signaling pathways

Loss of olfactory function has been commonly reported in SARS-CoV-2 infections. Recovery from anosmia is not well understood. Previous studies showed that sustentacular cells, and occasionally olfactory sensory neurons (OSNs) in the olfactory epithelium (OE), are infected in SARS-CoV-2-infected patients and experimental animals. Here, we show that SARS-CoV-2 infection of sustentacular cells induces inflammation characterized by infiltration of myeloid cells to the olfactory epithelium and variably increased expression of proinflammatory cytokines. We observed widespread damage to, and loss of cilia on, OSNs, accompanied by downregulation of olfactory receptors and signal transduction molecules involved in olfaction. A consequence of OSN dysfunction was a reduction in the number of neurons in the olfactory bulb expressing tyrosine hydroxylase, consistent with reduced synaptic input. Resolution of the infection, inflammation, and olfactory dysfunction occurred over 3-4 weeks following infection in most but not all animals. We also observed similar patterns of OE infection and anosmia/hyposmia in mice infected with other human coronaviruses such as SARS-CoV and MERS-CoV. Together, these results define the downstream effects of sustentacular cell infection and provide insight into olfactory dysfunction in COVID-19-associated anosmia.

Persistent post-COVID-19 smell loss is associated with immune cell infiltration and altered gene expression in olfactory epithelium

SARS-CoV-2 causes profound changes in the sense of smell, including total smell loss. Although these alterations are often transient, many patients with COVID-19 exhibit olfactory dysfunction that lasts months to years. Although animal and human autopsy studies have suggested mechanisms driving acute anosmia, it remains unclear how SARS-CoV-2 causes persistent smell loss in a subset of patients. To address this question, we analyzed olfactory epithelial samples collected from 24 biopsies, including from nine patients with objectively quantified long-term smell loss after COVID-19. This biopsy-based approach revealed a diffuse infiltrate of T cells expressing interferon-γ and a shift in myeloid cell population composition, including enrichment of CD207+ dendritic cells and depletion of anti-inflammatory M2 macrophages. Despite the absence of detectable SARS-CoV-2 RNA or protein, gene expression in the barrier supporting cells of the olfactory epithelium, termed sustentacular cells, appeared to reflect a response to ongoing inflammatory signaling, which was accompanied by a reduction in the number of olfactory sensory neurons relative to olfactory epithelial sustentacular cells. These findings indicate that T cell-mediated inflammation persists in the olfactory epithelium long after SARS-CoV-2 has been eliminated from the tissue, suggesting a mechanism for long-term post-COVID-19 smell loss.

COVID-19 induces CNS cytokine expression and loss of hippocampal neurogenesis

Infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is associated with acute and postacute cognitive and neuropsychiatric symptoms including impaired memory, concentration, attention, sleep and affect. Mechanisms underlying these brain symptoms remain understudied. Here we report that SARS-CoV-2-infected hamsters exhibit a lack of viral neuroinvasion despite aberrant blood-brain barrier permeability. Hamsters and patients deceased from coronavirus disease 2019 (COVID-19) also exhibit microglial activation and expression of interleukin (IL)-1β and IL-6, especially within the hippocampus and the medulla oblongata, when compared with non-COVID control hamsters and humans who died from other infections, cardiovascular disease, uraemia or trauma. In the hippocampal dentate gyrus of both COVID-19 hamsters and humans, we observed fewer neuroblasts and immature neurons. Protracted inflammation, blood-brain barrier disruption and microglia activation may result in altered neurotransmission, neurogenesis and neuronal damage, explaining neuropsychiatric presentations of COVID-19. The involvement of the hippocampus may explain learning, memory and executive dysfunctions in COVID-19 patients.

Bioinformatics and systems biology approaches to identify the effects of COVID-19 on neurodegenerative diseases: A review

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing coronavirus disease (COVID-19), has been devastated by COVID-19 in an increasing number of countries and health care systems around the world since its announcement of a global pandemic on 11 March 2020. During the pandemic, emerging novel viral mutant variants have caused multiple outbreaks of COVID-19 around the world and are prone to genetic evolution, causing serious damage to human health. As confirmed cases of COVID-19 spread rapidly, there is evidence that SARS-CoV-2 infection involves the central nervous system (CNS) and peripheral nervous system (PNS), directly or indirectly damaging neurons and further leading to neurodegenerative diseases (ND), but the molecular mechanisms of ND and CVOID-19 are unknown. We employed transcriptomic profiling to detect several major diseases of ND: Alzheimer 's disease (AD), Parkinson' s disease (PD), and multiple sclerosis (MS) common pathways and molecular biomarkers in association with COVID-19, helping to understand the link between ND and COVID-19. There were 14, 30 and 19 differentially expressed genes (DEGs) between COVID-19 and Alzheimer 's disease (AD), Parkinson' s disease (PD) and multiple sclerosis (MS), respectively; enrichment analysis showed that MAPK, IL-17, PI3K-Akt and other signaling pathways were significantly expressed; the hub genes (HGs) of DEGs between ND and COVID-19 were CRH, SST, TAC1, SLC32A1, GAD2, GAD1, VIP and SYP. Analysis of transcriptome data suggests multiple co-morbid mechanisms between COVID-19 and AD, PD, and MS, providing new ideas and therapeutic strategies for clinical prevention and treatment of COVID-19 and ND.

Neutrophils play a major role in the destruction of the olfactory epithelium during SARS-CoV-2 infection in hamsters

The loss of smell (anosmia) related to SARS-CoV-2 infection is one of the most common symptoms of COVID-19. Olfaction starts in the olfactory epithelium mainly composed of olfactory sensory neurons surrounded by supporting cells called sustentacular cells. It is now clear that the loss of smell is related to the massive infection by SARS-CoV-2 of the sustentacular cells in the olfactory epithelium leading to its desquamation. However, the molecular mechanism behind the destabilization of the olfactory epithelium is less clear. Using golden Syrian hamsters infected with an early circulating SARS-CoV-2 strain harboring the D614G mutation in the spike protein; we show here that rather than being related to a first wave of apoptosis as proposed in previous studies, the innate immune cells play a major role in the destruction of the olfactory epithelium. We observed that while apoptosis remains at a low level in the damaged area of the infected epithelium, the latter is invaded by Iba1+ cells, neutrophils and macrophages. By depleting the neutrophil population or blocking the activity of neutrophil elastase-like proteinases, we could reduce the damage induced by the SARS-CoV-2 infection. Surprisingly, the impairment of neutrophil activity led to a decrease in SARS-CoV-2 infection levels in the olfactory epithelium. Our results indicate a counterproductive role of neutrophils leading to the release of infected cells in the lumen of the nasal cavity and thereby enhanced spreading of the virus in the early phase of the SARS-CoV-2 infection.

Glial regulators of ions and solutes required for specific chemosensory functions in Caenorhabditis elegans

Glia and accessory cells regulate the microenvironment around neurons and primary sensory cells. However, the impact of specific glial regulators of ions and solutes on functionally diverse primary cells is poorly understood. Here, we systemically investigate the requirement of ion channels and transporters enriched in Caenorhabditis elegans Amsh glia for the function of chemosensory neurons. Although Amsh glia ablated worms show reduced function of ASH, AWC, AWA, and ASE neurons, we show that the loss of glial enriched ion channels and transporters impacts these neurons differently, with nociceptor ASH being the most affected. Furthermore, our analysis underscores the importance of K⁺, Cl^-, and nucleoside homeostasis in the Amphid sensory organ and uncovers the contribution of glial genes implicated in neurological disorders. Our findings build a unique fingerprint of each glial enriched ion channel and transporter and may provide insights into the function of supporting cells of mammalian sensory organs.

Olfactory Cleft Length: A Possible Risk Factor for Persistent Post-COVID-19 Olfactory Dysfunction

INTRODUCTION

To date, little is known about predisposing factors for persistent COVID-19-induced olfactory dysfunction (pCIOD). The objective was to determine whether olfactory cleft (OC) measurements associate with pCIOD risk.

MATERIAL AND METHODS

Three subgroups were recruited: group A included patients with pCIOD, group B included patients without olfactory dysfunction following SARS-CoV-2 infection (ntCIOD), and group C consisted in controls without past history of SARS-CoV-2 infection (noCOVID-19). Olfactory perception threshold (OPT) and visual analog scale for olfactory impairment (VAS-olf) were obtained. OC measurements were obtained through computed tomography scans. Results were subsequently compared.

RESULTS

A total of 55 patients with a mean age of 39 ± 10 years were included. OPT was significantly lower in pCIOD patients (group A: 4.2 ± 2.1 vs. group B: 12.3 ± 1.8 and group C: 12.2 ± 1.5, p < 0.001). VAS-olf was significantly higher in pCIOD (group A: 6 ± 2.6 vs. group B: 1.7 ± 1.6 and group C: 1.6 ± 1.5, p < 0.001). OC length was significantly higher in group A (42.8 ± 4.6) compared to group B (39.7 ± 3.4, p = 0.047) and C (39.8 ± 4, p = 0.037). The odd of pCIOD occurring after COVID-19 infection increased by 21% (95% CI [0.981, 1.495]) for a one unit (mm) increase in OC length. The odd of pCIOD occurring was 6.9 times higher when OC length >40 mm.

CONCLUSION

Longer OC may be a predisposing factor for pCIOD. This study is expected to encourage further research on OC morphology and its impact on olfactory disorders.

Evidence for the spread of SARS-CoV-2 and olfactory cell lineage impairment in close-contact infection Syrian hamster models

Objectives

Close contact with patients with COVID-19 is speculated to be the most common cause of viral transmission, but the pathogenesis of COVID-19 by close contact remains to be elucidated. In addition, despite olfactory impairment being a unique complication of COVID-19, the impact of SARS-CoV-2 on the olfactory cell lineage has not been fully validated. This study aimed to elucidate close-contact viral transmission to the nose and lungs and to investigate the temporal damage in the olfactory receptor neuron (ORN) lineage caused by SARS-CoV-2.

Methods

Syrian hamsters were orally administered SARS-CoV-2 nonvariant nCoV-19/JPN/TY/WK521/2020 as direct-infection models. On day 3 after inoculation, infected and uninfected hamsters were housed in the same cage for 30 minutes. These uninfected hamsters were subsequently assigned to a close-contact group. First, viral presence in the nose and lungs was verified in the infection and close-contact groups at several time points. Next, the impacts on the olfactory epithelium, including olfactory progenitors, immature ORNs, and mature ORNs were examined histologically. Then, the viral transmission status and chronological changes in tissue damage were compared between the direct-infection and close-contact groups.

Results

In the close-contact group, viral presence could not be detected in both the nose and lungs on day 3, and the virus was identified in both tissues on day 7. In the direct-infection group, the viral load was highest in the nose and lungs on day 3, decreased on day 7, and was no longer detectable on day 14. Histologically, in the direct-infection group, mature ORNs were most depleted on day 3 (p <0.001) and showed a recovery trend on day 14, with similar trends for olfactory progenitors and immature ORNs. In the close-contact group, there was no obvious tissue damage on day 3, but on day 7, the number of all ORN lineage cells significantly decreased (p <0.001).

Conclusion

SARS-CoV-2 was transmitted even after brief contact and subsequent olfactory epithelium and lung damage occurred more than 3 days after the trigger of infection. The present study also indicated that SARS-CoV-2 damages all ORN lineage cells, but this damage can begin to recover approximately 14 days post infection.

Olfactory training - Thirteen years of research reviewed

The sense of smell is interrelated with psychosocial functioning. Olfactory disorders often decrease quality of life but treatment options for people with olfactory loss are limited. Additionally, olfactory loss accompanies and precedes psychiatric and neurodegenerative diseases. Regular, systematic exposure to a set of odors, i.e., olfactory training (OT) has been offered for rehabilitation of the sense of smell in clinical practice. As signals from the olfactory bulb are directly projected to the limbic system it has been also debated whether OT might benefit psychological functioning, i.e., mitigate cognitive deterioration or improve emotional processing. In this review we synthesize key findings on OT utility in the clinical practice and highlight the molecular, cellular, and neuroanatomical changes accompanying olfactory recovery in people with smell loss as well as in experimental animal models. We discuss how OT and its modifications have been used in interventions aiming to support cognitive functions and improve well-being. We delineate main methodological challenges in research on OT and suggest areas requiring further scientific attention.

Molecular mechanisms involved in anosmia induced by SARS-CoV-2, with a focus on the transmembrane serine protease TMPRSS2

Since 2020, SARS-CoV-2 has caused a pandemic virus that has posed many challenges worldwide. Infection with this virus can result in a number of symptoms, one of which is anosmia. Olfactory dysfunction can be a temporary or long-term viral complication caused by a disorder of the olfactory neuroepithelium. Processes such as inflammation, apoptosis, and neuronal damage are involved in the development of SARS-CoV-2-induced anosmia. One of the receptors that play a key role in the entry of SARS-CoV-2 into the host cell is the transmembrane serine protease TMPRSS2, which facilitates this process by cleaving the viral S protein. The gene encoding TMPRSS2 is located on chromosome 21. It contains 15 exons and has many genetic variations, some of which increase the risk of disease. Delta strains have been shown to be more dependent on TMPRSS2 for cell entry than Omicron strains. Blockade of this receptor by serine protease inhibitors such as camostat and nafamostat can be helpful for treating SARS-CoV-2 symptoms, including anosmia. Proper understanding of the different functional aspects of this serine protease can help to overcome the therapeutic challenges of SARS-CoV-2 symptoms, including anosmia. In this review, we describe the cellular and molecular events involved in anosmia induced by SARS-CoV-2 with a focus on the function of the TMPRSS2 receptor.

Olfactory dysfunction in COVID-19, new insights from a cohort of 353 patients: The ANOSVID study

Olfactory disorders (OD) pathogenesis, underlying conditions, and prognostic in coronavirus disease 2019 (COVID-19) remain partially described. ANOSVID is a retrospective study in Nord Franche-Comté Hospital (France) that included COVID-19 patients from March 1 2020 to May 31 2020. The aim was to compare COVID-19 patients with OD (OD group) and patients without OD (no-OD group). A second analysis compared patients with anosmia (high OD group) and patients with hyposmia or no OD (low or no-OD group). The OD group presented less cardiovascular and other respiratory diseases compared to the no-OD group (odds ratio [OR] = 0.536 [0.293-0.981], p = 0.041 and OR = 0.222 [0.056-0.874], p = 0.037 respectively). Moreover, history of malignancy was less present in the high OD group compared with the low or no-OD group (OR = 0.170 [0.064-0.455], p < 0.001). The main associated symptoms (OR > 5) with OD were loss of taste (OR = 24.059 [13.474-42.959], p = 0.000) and cacosmia (OR = 5.821 [2.246-15.085], p < 0.001). Most of all ORs decreased in the second analysis, especially for general, digestive, and ENT symptoms. Only two ORs increased: headache (OR = 2.697 [1.746-4.167], p < 0.001) and facial pain (OR = 2.901 [1.441-5.842], p = 0.002). The high OD group had a higher creatinine clearance CKD than the low or no-OD group (89.0 ± 21.1 vs. 81.0 ± 20.5, p = 0.040). No significant difference was found concerning the virological, radiological, and severity criteria. OD patients seem to have less comorbidity, especially better cardiovascular and renal function. Associated symptoms with OD were mostly neurological symptoms. We did not find a significant relationship between OD and less severity in COVID-19 possibly due to methodological bias.

Mechanistic Understanding of the Olfactory Neuroepithelium Involvement Leading to Short-Term Anosmia in COVID-19 Using the Adverse Outcome Pathway Framework

Loss of the sense of smell (anosmia) has been included as a COVID-19 symptom by the World Health Organization. The majority of patients recover the sense of smell within a few weeks postinfection (short-term anosmia), while others report persistent anosmia. Several studies have investigated the mechanisms leading to anosmia in COVID-19; however, the evidence is scattered, and the mechanisms remain poorly understood. Based on a comprehensive review of the literature, we aim here to evaluate the current knowledge and uncertainties regarding the mechanisms leading to short-term anosmia following SARS-CoV-2 infection. We applied an adverse outcome pathway (AOP) framework, well established in toxicology, to propose a sequence of measurable key events (KEs) leading to short-term anosmia in COVID-19. Those KEs are (1) SARS-CoV-2 Spike proteins binding to ACE-2 expressed by the sustentacular (SUS) cells in the olfactory epithelium (OE); (2) viral entry into SUS cells; (3) viral replication in the SUS cells; (4) SUS cell death; (5) damage to the olfactory sensory neurons and the olfactory epithelium (OE). This AOP-aligned approach allows for the identification of gaps where more research should be conducted and where therapeutic intervention could act. Finally, this AOP gives a frame to explain several disease features and can be linked to specific factors that lead to interindividual differences in response to SARS-CoV-2 infection.

COVID-19 and cognitive impairment: neuroinvasive and blood‒brain barrier dysfunction

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to a global pandemic. Although COVID-19 was initially described as a respiratory disease, there is growing evidence that SARS-CoV-2 is able to invade the brains of COVID-19 patients and cause cognitive impairment. It has been reported that SARS-CoV-2 may have invasive effects on a variety of cranial nerves, including the olfactory, trigeminal, optic, and vagus nerves, and may spread to other brain regions via infected nerve endings, retrograde transport, and transsynaptic transmission. In addition, the blood-brain barrier (BBB), composed of neurovascular units (NVUs) lining the brain microvasculature, acts as a physical barrier between nerve cells and circulating cells of the immune system and is able to regulate the transfer of substances between the blood and brain parenchyma. Therefore, the BBB may be an important structure for the direct and indirect interaction of SARS-CoV-2 with the brain via the blood circulation. In this review, we assessed the potential involvement of neuroinvasion under the SARS-CoV-2 infection, and the potential impact of BBB disorder under SARS-CoV-2 infection on cognitive impairment.

Biosynthetic proteins targeting the SARS-CoV-2 spike as anti-virals

The binding of the SARS-CoV-2 spike to angiotensin-converting enzyme 2 (ACE2) promotes virus entry into the cell. Targeting this interaction represents a promising strategy to generate antivirals. By screening a phage-display library of biosynthetic protein sequences build on a rigid alpha-helicoidal HEAT-like scaffold (named αReps), we selected candidates recognizing the spike receptor binding domain (RBD). Two of them (F9 and C2) bind the RBD with affinities in the nM range, displaying neutralisation activity in vitro and recognizing distinct sites, F9 overlapping the ACE2 binding motif. The F9-C2 fusion protein and a trivalent αRep form (C2-foldon) display 0.1 nM affinities and EC₅₀ of 8–18 nM for neutralization of SARS-CoV-2. In hamsters, F9-C2 instillation in the nasal cavity before or during infections effectively reduced the replication of a SARS-CoV-2 strain harbouring the D614G mutation in the nasal epithelium. Furthermore, F9-C2 and/or C2-foldon effectively neutralized SARS-CoV-2 variants (including delta and omicron variants) with EC₅₀ values ranging from 13 to 32 nM. With their high stability and their high potency against SARS-CoV-2 variants, αReps provide a promising tool for SARS-CoV-2 therapeutics to target the nasal cavity and mitigate virus dissemination in the proximal environment.

The entry of SARS-CoV-2 in permissive cells is mediated by the binding of its spike to angiotensin-converting enzyme 2 (ACE2) on the cell surface. To select ligands able to block this interaction, we screened a library of phages encoding biosynthetic proteins (named αReps) for binding to its receptor binding domain (RBD). Two of them were able to bind the RBD with high affinity and block efficiently the virus entry in cultured cells. Assembled αReps through covalent or non-covalent linkages blocked virus entry at lower concentration than their precursors (with around 20-fold activity increase for a trimeric αRep). These αReps derivates neutralize efficiently SARS-CoV-2 β, γ, δ and Omicron virus variants. Instillation of an αRep dimer in the nasal cavity effectively reduced virus replication in the hamster model of SARS-CoV-2 and pathogenicity.

In vitro and in vivo differences in neurovirulence between D614G, Delta And Omicron BA.1 SARS-CoV-2 variants

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is associated with various neurological complications. Although the mechanism is not fully understood, several studies have shown that neuroinflammation occurs in the acute and post-acute phase. As these studies have predominantly been performed with isolates from 2020, it is unknown if there are differences among SARS-CoV-2 variants in their ability to cause neuroinflammation. Here, we compared the neuroinvasiveness, neurotropism and neurovirulence of the SARS-CoV-2 ancestral strain D614G, the Delta (B.1.617.2) and Omicron BA.1 (B.1.1.529) variants using in vitro and in vivo models. The Omicron BA.1 variant showed reduced neurotropism and neurovirulence compared to Delta and D614G in human induced pluripotent stem cell (hiPSC)-derived cortical neurons co-cultured with astrocytes. Similar differences were obtained in Syrian hamsters inoculated with D614G, Delta and the Omicron BA.1 variant 5 days post infection. Replication in the olfactory mucosa was observed in all hamsters, but most prominently in D614G inoculated hamsters. Furthermore, neuroinvasion into the CNS via the olfactory nerve was observed in D614G, but not Delta or Omicron BA.1 inoculated hamsters. Furthermore, neuroinvasion was associated with neuroinflammation in the olfactory bulb of hamsters inoculated with D614G. Altogether, our findings suggest differences in the neuroinvasive, neurotropic and neurovirulent potential between SARS-CoV-2 variants using in vitro hiPSC-derived neural cultures and in vivo in hamsters during the acute phase of the infection.

After the virus has cleared-Can preclinical models be employed for Long COVID research?

Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2) can cause the life-threatening acute respiratory disease called COVID-19 (Coronavirus Disease 2019) as well as debilitating multiorgan dysfunction that persists after the initial viral phase has resolved. Long COVID or Post-Acute Sequelae of COVID-19 (PASC) is manifested by a variety of symptoms, including fatigue, dyspnea, arthralgia, myalgia, heart palpitations, and memory issues sometimes affecting between 30% and 75% of recovering COVID-19 patients. However, little is known about the mechanisms causing Long COVID and there are no widely accepted treatments or therapeutics. After introducing the clinical aspects of acute COVID-19 and Long COVID in humans, we summarize the work in animals (mice, Syrian hamsters, ferrets, and nonhuman primates (NHPs)) to model human COVID-19. The virology, pathology, immune responses, and multiorgan involvement are explored. Additionally, any studies investigating time points longer than 14 days post infection (pi) are highlighted for insight into possible long-term disease characteristics. Finally, we discuss how the models can be leveraged for treatment evaluation, including pharmacological agents that are currently in human clinical trials for treating Long COVID. The establishment of a recognized Long COVID preclinical model representing the human condition would allow the identification of mechanisms causing disease as well as serve as a vehicle for evaluating potential therapeutics.

COVID-19, sens chimiques et pathologies métaboliques

Une réduction importante de l’odorat, indépendamment de l’obstruction nasale, et du goût, a été signalée comme un des symptômes majeurs suite à l’infection par la COVID-19. Cette réduction est si fréquente qu’elle a été proposée comme un des prédicteurs le plus relevant pour diagnostiquer l’infection. Différents mécanismes par lesquels les virus affectent l’odorat et le goût ont été proposés. L’ACE2 (enzyme de conversion de l’angiotensine 2) a été caractérisé comme le principal récepteur d’entrée du virus SARS-CoV-2 qui interagit avec les protéines « spikes » du virus, ce qui permet à ce dernier d’entrer dans la cellule hôte par un domaine de fusion. Il est principalement exprimé dans la partie supérieure des voies respiratoires, et la plus forte densité de ces protéines se trouve dans les épithéliums olfactif et gustatif. Les données actuellement disponibles indiquent que la cause la plus probable de l’anosmie pendant la COVID-19 est une altération de la fonction des neurones sensoriels olfactifs, associée à l’infection et à la mort des cellules microvillaires, et des péricytes vasculaires. Les mécanismes généraux sont les mêmes en ce qui concerne le goût. La pathogenèse des troubles olfactifs et gustatifs dans la COVID-19 peut entraîner des altérations diverses, dont des modifications de la prise alimentaire et du métabolisme énergétique. Les individus porteurs de pathologies métaboliques ayant une plus forte susceptibilité à la COVID-19 sont, de ce fait, plus exposés aux perturbations des sens chimiques et à leurs conséquences. De plus, des études récentes montrent que la COVID-19 augmenterait la susceptibilité au diabète en s’attaquant directement aux cellules β-pancréatiques.

Sociodemographic Characteristics and Comorbidities of Patients With Long COVID and Persistent Olfactory Dysfunction

IMPORTANCE

Determining the characteristics, type, and severity of olfactory dysfunction in patients with long COVID is important for the prognosis and potential treatment of the affected population.

OBJECTIVE

To describe the sociodemographic and clinical features of patients with long COVID who develop persistent olfactory dysfunction.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study, conducted at a rehabilitation center at a public university in the Amazon region of Brazil between September 9, 2020, and October 20, 2021, comprised 219 patients with long COVID and self-reported neurologic symptoms. Of these 219 patients, 139 received a diagnosis of chronic olfactory dysfunction, as confirmed by the Connecticut Chemosensory Clinical Research Center (CCCRC) test.

EXPOSURE

Clinical diagnosis of long COVID.

MAIN OUTCOMES AND MEASURES

Electronic case report forms were prepared for the collection of sociodemographic and clinical data. Patients' sense of smell was evaluated via a CCCRC test, and the association of olfactory dysfunction with aspects of daily life was recorded using a questionnaire.

RESULTS

Of the 219 patients included in the study, 164 (74.9%) were women, 194 (88.6%) were between 18 and 59 years of age (mean [SD] age, 43.2 [12.9] years), 206 (94.1%) had more than 9 years of education, and 115 (52.5%) had a monthly income of up to US $192.00. In the study group, 139 patients (63.5%) had some degree of olfactory dysfunction, whereas 80 patients (36.5%) had normosmia. Patients with olfactory dysfunction had a significantly longer duration of long COVID symptoms than those in the normosmia group (mean [SD], 242.7 [101.9] vs 221.0 [97.5] days; P = .01). Among patients with anosmia, there was a significant association between olfactory dysfunction and daily activities, especially in terms of impairment in hazard detection (21 of 31 patients [67.7%]), personal hygiene (21 of 31 patients [67.7%]), and food intake (21 of 31 patients [67.7%]). Univariable logistic regression analyses found that ageusia symptoms were associated with the occurrence of olfactory dysfunction (odds ratio [OR], 11.14 [95% CI, 4.76-26.07]; P < .001), whereas headache (OR, 0.41 [95% CI, 0.22-0.76]; P < .001) and sleep disorders (OR, 0.48 [95% CI, 0.26-0.92]; P = .02) showed an inverse association with the occurrence of olfactory dysfunction.

CONCLUSIONS AND RELEVANCE

Olfactory dysfunction is one of the most important long-term neurologic symptoms of COVID-19, with the highest prevalence seen among women, adults, and outpatients. Patients with olfactory dysfunction may experience persistent severe hyposmia or anosmia more than 1 year from the onset of symptoms, suggesting the possibility of the condition becoming a permanent sequela.

ELF5 is a potential respiratory epithelial cell-specific risk gene for severe COVID-19

Despite two years of intense global research activity, host genetic factors that predispose to a poorer prognosis of COVID-19 infection remain poorly understood. Here, we prioritise eight robust (e.g., ELF5) or suggestive but unreported (e.g., RAB2A) candidate protein mediators of COVID-19 outcomes by integrating results from the COVID-19 Host Genetics Initiative with population-based plasma proteomics using statistical colocalisation. The transcription factor ELF5 (ELF5) shows robust and directionally consistent associations across different outcome definitions, including a >4-fold higher risk (odds ratio: 4.88; 95%-CI: 2.47-9.63; p-value < 5.0 × 10-6) for severe COVID-19 per 1 s.d. higher genetically predicted plasma ELF5. We show that ELF5 is specifically expressed in epithelial cells of the respiratory system, such as secretory and alveolar type 2 cells, using single-cell RNA sequencing and immunohistochemistry. These cells are also likely targets of SARS-CoV-2 by colocalisation with key host factors, including ACE2 and TMPRSS2. In summary, large-scale human genetic studies together with gene expression at single-cell resolution highlight ELF5 as a risk gene for severe COVID-19, supporting a role of epithelial cells of the respiratory system in the adverse host response to SARS-CoV-2.

The residual effect of coronovirus disease 2019 on olfactory acuity and mucociliary clearance time: a cross-sectional, controlled study

Objective

This study evaluated the olfactory, sinonasal and mucociliary functions of patients with post-coronavirus disease 2019 long-term persistent olfactory dysfunction.

Method

Three groups of 30 patients each were formed: patients with a history of coronavirus disease 2019 infection with self-reported, persistent, sudden-onset olfactory dysfunction (group 1), patients with a history of coronavirus disease 2019 infection without any self-reported olfactory dysfunction (group 2) and healthy controls with no history of coronavirus disease 2019 infection (group 3). Saccharin time, Sniffin’ Sticks, Turkish Nasal Obstruction Symptom Evaluation and Sino-Nasal Outcome Test 22 scores were compared.

Results

Turkish Nasal Obstruction Symptom Evaluation scores were similar between groups (p = 0.252). Sino-Nasal Outcome Test-22 scores were higher in group 1 than groups 2 and 3 (p < 0.01 and p < 0.001, respectively). Saccharin time was significantly longer in group 1 than groups 2 and 3 (p < 0.05 and p < 0.01, respectively). Group 1 had lower olfactory scores than groups 2 and 3 (p < 0.001 and p < 0.001, respectively).

Conclusion

Mucociliary clearance time was significantly prolonged in patients with post-coronavirus disease 2019 persistent olfactory dysfunction. Coronavirus disease 2019 infection was likely to cause asymptomatic olfactory dysfunction.

SARS-CoV-2 and the Nucleus

The ongoing COVID-19 pandemic is caused by an RNA virus, SARS-CoV-2. The genome of SARS-CoV-2 lacks a nuclear phase in its life cycle and is replicated in the cytoplasm. However, interfering with nuclear trafficking using pharmacological inhibitors greatly reduces virus infection and virus replication of other coronaviruses is blocked in enucleated cells, suggesting a critical role of the nucleus in virus infection. Here, we summarize the alternations of nuclear pathways caused by SARS-CoV-2, including nuclear translocation pathways, innate immune responses, mRNA metabolism, epigenetic mechanisms, DNA damage response, cytoskeleton regulation, and nuclear rupture. We consider how these alternations contribute to virus replication and discuss therapeutic treatments that target these pathways, focusing on small molecule drugs that are being used in clinical studies.

Revealing the mystery of persistent smell loss in Long COVID patients

COVID-19 is hopefully approaching its end in many countries as herd immunity develops and weaker strains of SARS-CoV-2 dominate. However, a new concern occurs over the long-term effects of COVID-19, collectively called "Long COVID", as some symptoms of the nervous system last even after patients recover from COVID-19. This review focuses on studies of anosmia, i.e., impairment of smell, which is the most common sensory defect during the disease course and is caused by olfactory dysfunctions. It remains mysterious how the olfactory functions are affected since the virus can't invade olfactory receptor neurons. We describe several leading hypotheses about the mystery in hope to provide insights into the pathophysiology and treatment strategies for anosmia.

A Systematic Review on Neurological Aspects of COVID-19: Exploring the Relationship Between COVID-19-Related Olfactory Dysfunction and Neuroinvasion

Objectives

To identify neurological aspects of Coronavirus disease 2019 (COVID-19) and to investigate COVID-19 infected patients with and without olfactory dysfunction in relation to polymerase chain reaction (PCR) assay results for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) in the cerebrospinal fluid (CSF).

Methods

PubMed and EMBASE databases were searched until March 26, 2021, for observational studies with COVID-19 patients that had performed CSF PCR assay due to the neurologic symptom and reported anosmia status.

Results

Initially, 2,387 studies were identified;167 studies performed SARS-CoV-2 CSF PCR assay, of which our review comprised 45 observational studies that conducted CSF PCR assay for SARS-CoV-2 in 101 patients and reported anosmia status in 55 of 101 patients. Central and peripheral neurological manifestations observed in COVID-19 patients were diverse. The most common neurological diagnoses were Guillain-Barré syndrome (GBS) and its variants (24%), followed by encephalopathy (21%). The SARS-CoV-2 PCR assay was positive in only four CSF samples, of which two patients had olfactory dysfunction while the others did not.

Conclusions

The neurological spectrum of COVID-19 is diverse, and direct neuroinvasion of SARS-CoV-2 is rare. The neuroprotection against SARS-CoV-2 in COVID-19 patients with anosmia is controversial, as an equal number of patients with and without olfactory dysfunction had positive CSF PCR results for SARS-CoV-2 in our study, and further studies are required to provide more insight into this topic.

Tailored Approach for Persistent Olfactory Dysfunction After SARS-CoV-2 Infection: A Pilot Study

OBJECTIVE

So far, no original studies explored non-randomized, standardized protocols for COVID-19 associated olfactory dysfunction. The main objective was to determine the efficacy of a new protocol for post-COVID olfactopathy while assessing the benefit of adding adjuvant therapies to olfactory training.

METHODS

Patients suffering from long-lasting post-COVID-19 olfactory dysfunction were evaluated. A non-randomized protocol based on individual nasal endoscopy findings and patient's preferences was applied. Patients were assigned for olfactory training alone or olfactory training + adjuvant therapy. Participants performed olfactory objective and subjective evaluations at first consultation and 3 months after treatment, and results were compared.

RESULTS

A total of 47 patients were enrolled. All groups showed significant improvement in olfactory thresholds at 3-month follow-up suggesting protocol effectiveness (olfactory training group alone showed a mean threshold difference of 2.9, P < .001; Olfactory training + Topical Corticosteroid showed a mean threshold difference of 4, P = .006; Olfactory training + Topical Corticosteroid + Vitamin B complex showed a mean threshold difference of 4.4, P = .006; Olfactory training + Intranasal Vitamin A and E showed a mean threshold difference of 4.4, P < .001). Olfactory training alone showed lower mean olfactory threshold improvement, when compared to patients undergoing olfactory training + adjuvant therapy (olfactory training alone mean improvement 2.9 ± 2.3 vs olfactory training + adjuvants mean improvement 4.3 ± 2.458, P = .03).

CONCLUSIONS

This is one of the first studies to demonstrate results in the treatment of post-COVID-19 persistent olfactory impairment. A customized approach based on endoscopy findings and patient's preferences may be a valid option for the management of persistent post-COVID-19 olfactory disorder. Adjuvant therapy could be considered in addition to olfactory training, but further studies are needed in order to confirm their effectiveness in this setting.

LEVEL OF EVIDENCE

2c (outcomes research).

Chemosensory loss in COVID ‐19

The COVID‐19 pandemic caused by SARS‐CoV‐2 virus quickly spread globally, infecting over half a billion individuals, and killing over 6 million*. One of the more unusual symptoms was patients' complaints of sudden loss of smell and/or taste, a symptom that has become more apparent as the virus mutated into different variants. Anosmia and ageusia, the loss of smell and taste, respectively, seem to be transient for some individuals, but for others persists even after recovery from the infection. Causes for COVID‐19‐associated chemosensory loss have undergone several hypotheses. These include non‐functional or destroyed olfactory neurons and gustatory receptors or of their supporting cells, disruption of the signaling protein Neuropilin‐1, and disruption in the interaction with semaphorins, key molecules in the gustatory and olfactory axon guidance. The current paper will review these hypotheses and chart out potential therapeutic avenues.

Comparative pathology of the nasal epithelium in K18-hACE2 Tg mice, hACE2 Tg mice, and hamsters infected with SARS-CoV-2

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) causes severe viral pneumonia and is associated with a high fatality rate. A substantial proportion of patients infected by SARS-CoV-2 suffer from mild hyposmia to complete loss of olfactory function, resulting in anosmia. However, the pathogenesis of the olfactory dysfunction and comparative pathology of upper respiratory infections with SARS-CoV-2 are unknown. We describe the histopathological, immunohistochemical, and in situ hybridization findings from rodent models of SARS-CoV-2 infection. The main histopathological findings in the olfactory epithelia of K8-hACE2 Tg mice, hACE2 Tg mice, and hamsters were varying degrees of inflammatory lesions, including disordered arrangement, necrosis, exfoliation, and macrophage infiltration of the olfactory epithelia, and inflammatory exudation. On the basis of these observations, the nasal epithelia of these rodent models appeared to develop moderate, mild, and severe rhinitis, respectively. Correspondingly, SARS-CoV-2 viral RNA and antigen were mainly identified in the olfactory epithelia and lamina propria. Moreover, viral RNA was abundant in the cerebrum of K18-hACE2 Tg mice, including the olfactory bulb. The K8-hACE2 Tg mouse, hACE2 Tg mouse, and hamster models could be used to investigate the pathology of SARS-CoV-2 infection in the upper respiratory tract and central nervous system. These models could help to provide a better understanding of the pathogenic process of this virus and to develop effective medications and prophylactic treatments.

Hamster models of COVID-19 pneumonia reviewed: How human can they be?

The dramatic global consequences of the coronavirus disease 2019 (COVID-19) pandemic soon fueled quests for a suitable model that would facilitate the development and testing of therapies and vaccines. In contrast to other rodents, hamsters are naturally susceptible to infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and the Syrian hamster (Mesocricetus auratus) rapidly developed into a popular model. It recapitulates many characteristic features as seen in patients with a moderate, self-limiting course of the disease such as specific patterns of respiratory tract inflammation, vascular endothelialitis, and age dependence. Among 4 other hamster species examined, the Roborovski dwarf hamster (Phodopus roborovskii) more closely mimics the disease in highly susceptible patients with frequent lethal outcome, including devastating diffuse alveolar damage and coagulopathy. Thus, different hamster species are available to mimic different courses of the wide spectrum of COVID-19 manifestations in humans. On the other hand, fewer diagnostic tools and information on immune functions and molecular pathways are available than in mice, which limits mechanistic studies and inference to humans in several aspects. Still, under pandemic conditions with high pressure on progress in both basic and clinically oriented research, the Syrian hamster has turned into the leading non-transgenic model at an unprecedented pace, currently used in innumerable studies that all aim to combat the impact of the virus with its new variants of concern. As in other models, its strength rests upon a solid understanding of its similarities to and differences from the human disease, which we review here.

The nervous system during COVID-19: Caught in the crossfire. Immunol Rev 2022;311:90-111. [PMID: 35770683 PMCID: PMC9350403 DOI: 10.1111/imr.13114]

SARS‐CoV‐2, the virus that causes coronavirus disease (COVID)‐19, has become a persistent global health threat. Individuals who are symptomatic for COVID‐19 frequently exhibit respiratory illness, which is often accompanied by neurological symptoms of anosmia and fatigue. Mounting clinical data also indicate that many COVID‐19 patients display long‐term neurological disorders postinfection such as cognitive decline, which emphasizes the need to further elucidate the effects of COVID‐19 on the central nervous system. In this review article, we summarize an emerging body of literature describing the impact of SARS‐CoV‐2 infection on central nervous system (CNS) health and highlight important areas of future investigation.

SARS-CoV-2 Omicron variant causes mild pathology in the upper and lower respiratory tract of hamsters

Since its discovery in 2019, multiple variants of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) have been identified. This study investigates virus spread and associated pathology in the upper and lower respiratory tracts of Syrian golden hamsters at 4 days post intranasal SARS-CoV-2 Omicron infection, in comparison to infection with variants of concern (VOCs) Gamma and Delta as well as ancestral strain 614 G. Pathological changes in the upper and lower respiratory tract of VOC Omicron infected hamsters are milder than those caused by other investigated strains. VOC Omicron infection causes a mild rhinitis with little involvement of the olfactory epithelium and minimal lesions in the lung, with frequent sparing of the alveolar compartment. Similarly, viral antigen, RNA and infectious virus titers are lower in respiratory tissues of VOC Omicron infected hamsters. These findings demonstrate that the variant has a decreased pathogenicity for the upper and lower respiratory tract of hamsters.

Since the emergence of SARS-CoV-2 several variants of concerns have been identified, with altered disease progression and transmission dynamics. Here, Armando et al. compare virus spread and pathology in the upper and lower respiratory tracts of Syrian golden hamster after 4 days post infection for VOCs Gamma, Delta and Omicron and find milder pathology for Omicron.

Olfactory and gustatory disorders in COVID-19

Loss of olfaction is one of the symptoms most commonly reported by patients with coronavirus disease 2019 (COVID-19). Although the spontaneous recovery rate is high, recent studies have shown that up to 7% of patients remain anosmic for more than 12 months after the onset of infection, leaving millions of people worldwide suffering from severe olfactory impairment. Olfactory training remains the first recommended treatment. With the continued lack of approved drug treatments, new therapeutic options are being explored. This article reviews the current state of science on COVID-19-related olfactory disorders, focusing on epidemiology, pathophysiology, cure rates, currently available treatment options, and research on new treatments.

Potential mechanism of SARS-CoV-2-associated central and peripheral nervous system impairment

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is more than merely a respiratory disease, as it also presents with various neurological symptoms. SARS‐CoV‐2 may infect the central nervous system (CNS) and thus is neurotropic. However, the pathophysiological mechanism of coronavirus disease 2019 (COVID‐19)‐associated neuropathy remains unclear. Many studies have reported that SARS‐CoV‐2 enters the CNS through the hematogenous and neuronal routes, as well as through the main host neurological immune responses and cells involved in these responses. The neurological immune responses to COVID‐19 and potential mechanisms of the extensive neuroinflammation induced by SARS‐CoV‐2 have been investigated. Although CNS infection with SARS‐CoV‐2 was shown to lead to neuronal impairment, certain aspects of this mechanism remain controversial and require further analysis. In this review, we discussed the pathway and mechanisms of SARS‐CoV‐2 invasion in the CNS, and associated clinical manifestations, such as anosmia, headache, and hyposmia. Moreover, the mechanism of neurological damage caused by SARS‐CoV‐2 may provide potential treatment methods for patients presenting with SARS‐CoV‐2‐associated neuropathy.

Oral SARS-CoV-2 Inoculation Causes Nasal Viral Infection Leading to Olfactory Bulb Infection: An Experimental Study

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections can cause long-lasting anosmia, but the impact of SARS-CoV-2 infection, which can spread to the nasal cavity via the oral route, on the olfactory receptor neuron (ORN) lineage and olfactory bulb (OB) remains undetermined. Using Syrian hamsters, we explored whether oral SARS-CoV-2 inoculation can lead to nasal viral infection, examined how SARS-CoV-2 affects the ORN lineage by site, and investigated whether SARS-CoV-2 infection can spread to the OB and induce inflammation. On post-inoculation day 7, SARS-CoV-2 presence was confirmed in the lateral area (OCAM-positive) but not the nasal septum of NQO1-positive and OCAM-positive areas. The virus was observed partially infiltrating the olfactory epithelium, and ORN progenitor cells, immature ORNs, and mature ORNs were fewer than in controls. The virus was found in the olfactory nerve bundles to the OB, suggesting the nasal cavity as a route for SARS-CoV-2 brain infection. We demonstrated that transoral SARS-CoV-2 infection can spread from the nasal cavity to the central nervous system and the possibility of central olfactory dysfunction due to SARS-CoV-2 infection. The virus was localized at the infection site and could damage all ORN-lineage cells.

The Prevalence and Pathophysiology of Chemical Sense Disorder Caused by the Novel Coronavirus

Emerging viral infections are a ceaseless challenge and remain a global public health concern. The world has not yet come back to normal from the devastating effects of the highly contagious and pathogenic novel coronavirus, or Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Olfactory and taste dysfunction is common in patients infected by the novel coronavirus. In light of the emergence of different coronavirus variants, it is important to update the prevalence and pathophysiology of these side effects. In this review, articles published on the prevalence of olfactory and taste dysfunction from coronavirus disease (COVID-19) and their possible pathophysiologic mechanisms have been reviewed and reported. The modulatory role of different SARS-CoV-2 variants on the chemical senses is then described. The clinical relevance of chemical sense disorder and its long-term morbidity and management is also discussed.

Olfactory dysfunction and COVID-19

Here, we provide an overview of olfactory dysfunction associated with COVID-19. We provide background regarding the organization and function of the peripheral olfactory system. A review of the relevant literature on anosmia and parosmia due to infection with SARS-CoV-2, the virus causing COVID-19, is provided. Specific attention is focused on possible mechanisms by which the virus may interact with and damage the cell populations of peripheral olfactory system. Evidence from human studies as well as animal models is considered. Finally, we discuss current recommendations for evaluation and management of patients with persistent post-COVID olfactory dysfunction, as well as possible future research directions.

Mechanisms of coronavirus infectious disease 2019-related neurologic diseases

PURPOSE OF REVIEW

As of January 8, 2022, a global pandemic caused by infection with severe acute respiratory syndrome coronavirus (SARS-CoV)-2, a new RNA virus, has resulted in 304,896,785 cases in over 222 countries and regions, with over 5,500,683 deaths (www.worldometers.info/coronavirus/). Reports of neurological and psychiatric symptoms in the context of coronavirus infectious disease 2019 (COVID-19) range from headache, anosmia, and dysgeusia, to depression, fatigue, psychosis, seizures, delirium, suicide, meningitis, encephalitis, inflammatory demyelination, infarction, and acute hemorrhagic necrotizing encephalopathy. Moreover, 30-50% of COVID-19 survivors develop long-lasting neurologic symptoms, including a dysexecutive syndrome, with inattention and disorientation, and/or poor movement coordination. Detection of SARS-CoV-2 RNA within the central nervous system (CNS) of patients is rare, and mechanisms of neurological damage and ongoing neurologic diseases in COVID-19 patients are unknown. However, studies demonstrating viral glycoprotein effects on coagulation and cerebral vasculature, and hypoxia- and cytokine-mediated coagulopathy and CNS immunopathology suggest both virus-specific and neuroimmune responses may be involved. This review explores potential mechanistic insights that could contribute to COVID-19-related neurologic disease.

RECENT FINDINGS

While the development of neurologic diseases during acute COVID-19 is rarely associated with evidence of viral neuroinvasion, new evidence suggests SARS-CoV-2 Spike (S) protein exhibits direct inflammatory and pro-coagulation effects. This, in conjunction with immune dysregulation resulting in cytokine release syndrome (CRS) may result in acute cerebrovascular or neuroinflammatory diseases. Additionally, CRS-mediated loss of blood-brain barrier integrity in specific brain regions may contribute to the expression of proinflammatory mediators by neural cells that may impact brain function long after resolution of acute infection. Importantly, host co-morbid diseases that affect vascular, pulmonary, or CNS function may contribute to the type of neurologic disease triggered by SARS-COV-2 infection.

SUMMARY

Distinct effects of SARS-CoV-2 S protein and CNS compartment- and region-specific responses to CRS may underlie acute and chronic neuroinflammatory diseases associated with COVID-19.

Postmortem Assessment of Olfactory Tissue Degeneration and Microvasculopathy in Patients With COVID-19

Importance

Loss of smell is an early and common presentation of COVID-19 infection. Although it has been speculated that viral infection of olfactory neurons may be the culprit, it is unclear whether viral infection causes injuries in the olfactory bulb region.

Objective

To characterize the olfactory pathology associated with COVID-19 infection in a postmortem study.

Design, Setting, and Participants

This multicenter postmortem cohort study was conducted from April 7, 2020, to September 11, 2021. Deceased patients with COVID-19 and control individuals were included in the cohort. One infant with congenital anomalies was excluded. Olfactory bulb and tract tissue was collected from deceased patients with COVID-19 and appropriate controls. Histopathology, electron microscopy, droplet digital polymerase chain reaction, and immunofluorescence/immunohistochemistry studies were performed. Data analysis was conducted from February 7 to October 19, 2021.

Main Outcomes and Measures

(1) Severity of degeneration, (2) losses of olfactory axons, and (3) severity of microvasculopathy in olfactory tissue.

Results

Olfactory tissue from 23 deceased patients with COVID-19 (median [IQR] age, 62 [49-69] years; 14 men [60.9%]) and 14 control individuals (median [IQR] age, 53.5 [33.25-65] years; 7 men [50%]) was included in the analysis. The mean (SD) axon pathology score (range, 1-3) was 1.921 (0.569) in patients with COVID-19 and 1.198 (0.208) in controls (P < .001), whereas axon density was 2.973 (0.963) × 104/mm2 in patients with COVID-19 and 3.867 (0.670) × 104/mm2 in controls (P = .002). Concomitant endothelial injury of the microvasculature was also noted in olfactory tissue. The mean (SD) microvasculopathy score (range, 1-3) was 1.907 (0.490) in patients with COVID-19 and 1.405 (0.233) in control individuals (P < .001). Both the axon and microvascular pathology was worse in patients with COVID-19 with smell alterations than those with intact smell (mean [SD] axon pathology score, 2.260 [0.457] vs 1.63 [0.426]; P = .002; mean [SD] microvasculopathy score, 2.154 [0.528] vs 1.694 [0.329]; P = .02) but was not associated with clinical severity, timing of infection, or presence of virus.

Conclusions and Relevance

This study found that COVID-19 infection is associated with axon injuries and microvasculopathy in olfactory tissue. The striking axonal pathology in some cases indicates that olfactory dysfunction in COVID-19 infection may be severe and permanent.

Neuroinvasion and Neurotropism by SARS-CoV-2 Variants in the K18-hACE2 Mouse

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) not only affects the respiratory tract but also causes neurological symptoms such as loss of smell and taste, headache, fatigue or severe cerebrovascular complications. Using transgenic mice expressing human angiotensin-converting enzyme 2 (hACE2), we investigated the spatiotemporal distribution and pathomorphological features in the CNS following intranasal infection with SARS-CoV-2 variants, as well as after prior influenza A virus infection. Apart from Omicron, we found all variants to frequently spread to and within the CNS. Infection was restricted to neurons and appeared to spread from the olfactory bulb mainly in basally oriented regions in the brain and into the spinal cord, independent of ACE2 expression and without evidence of neuronal cell death, axonal damage or demyelination. However, microglial activation, microgliosis and a mild macrophage and T cell dominated inflammatory response was consistently observed, accompanied by apoptotic death of endothelial, microglial and immune cells, without their apparent infection. Microgliosis and immune cell apoptosis indicate a potential role of microglia for pathogenesis and viral effect in COVID-19 and the possible impairment of neurological functions, especially in long COVID. These data may also be informative for the selection of therapeutic candidates and broadly support the investigation of agents with adequate penetration into relevant regions of the CNS.

Microgliosis and neuronal proteinopathy in brain persist beyond viral clearance in SARS-CoV-2 hamster model

BACKGROUND

Neurological symptoms such as cognitive decline and depression contribute substantially to post-COVID-19 syndrome, defined as lasting symptoms several weeks after initial SARS-CoV-2 infection. The pathogenesis is still elusive, which hampers appropriate treatment. Neuroinflammatory responses and neurodegenerative processes may occur in absence of overt neuroinvasion.

METHODS

Here we determined whether intranasal SARS-CoV-2 infection in male and female syrian golden hamsters results in persistent brain pathology. Brains 3 (symptomatic) or 14 days (viral clearance) post infection versus mock (n = 10 each) were immunohistochemically analyzed for viral protein, neuroinflammatory response and accumulation of tau, hyperphosphorylated tau and alpha-synuclein protein.

FINDINGS

Viral protein in the nasal cavity led to pronounced microglia activation in the olfactory bulb beyond viral clearance. Cortical but not hippocampal neurons accumulated hyperphosphorylated tau and alpha-synuclein, in the absence of overt inflammation and neurodegeneration. Importantly, not all brain regions were affected, which is in line with selective vulnerability.

INTERPRETATION

Thus, despite the absence of virus in brain, neurons develop signatures of proteinopathies that may contribute to progressive neuronal dysfunction. Further in depth analysis of this important mechanism is required.

FUNDING

Federal Ministry of Health (BMG; ZMV I 1-2520COR501), Federal Ministry of Education and Research (BMBF 01KI1723G), Ministry of Science and Culture of Lower Saxony in Germany (14 - 76103-184 CORONA-15/20), German Research Foundation (DFG; 398066876/GRK 2485/1), Luxemburgish National Research Fund (FNR, Project Reference: 15686728, EU SC1-PHE-CORONAVIRUS-2020 MANCO, no > 101003651).

The neuroinvasiveness, neurotropism, and neurovirulence of SARS-CoV-2

Severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) infection is associated with a diverse spectrum of neurological complications during the acute and postacute stages. The pathogenesis of these complications is complex and dependent on many factors. For accurate and consistent interpretation of experimental data in this fast-growing field of research, it is essential to use terminology consistently. In this article, we outline the distinctions between neuroinvasiveness, neurotropism, and neurovirulence. Additionally, we discuss current knowledge of these distinct features underlying the pathogenesis of SARS-CoV-2-associated neurological complications. Lastly, we briefly discuss the advantages and limitations of different experimental models, and how these approaches can further be leveraged to advance the field.

Intranasal delivery of SARS-CoV-2 spike protein is sufficient to cause olfactory damage, inflammation and olfactory dysfunction in zebrafish

Anosmia, loss of smell, is a prevalent symptom of SARS-CoV-2 infection. Anosmia may be explained by several mechanisms driven by infection of non-neuronal cells and damage in the nasal epithelium rather than direct infection of olfactory sensory neurons (OSNs). Previously, we showed that viral proteins are sufficient to cause neuroimmune responses in the teleost olfactory organ (OO). We hypothesize that SARS-CoV-2 spike (S) protein is sufficient to cause olfactory damage and olfactory dysfunction. Using an adult zebrafish model, we report that intranasally delivered SARS-CoV-2 S RBD mostly binds to the non-sensory epithelium of the olfactory organ and causes severe olfactory histopathology characterized by loss of cilia, hemorrhages and edema. Electrophysiological recordings reveal impaired olfactory function to both food and bile odorants in animals treated intranasally with SARS-CoV-2 S RBD. However, no loss of behavioral preference for food was detected in SARS-CoV-2 S RBD treated fish. Single cell RNA-Seq of the adult zebrafish olfactory organ indicated widespread loss of olfactory receptor expression and inflammatory responses in sustentacular, endothelial, and myeloid cell clusters along with reduced numbers of T_regs. Combined, our results demonstrate that intranasal SARS-CoV-2 S RBD is sufficient to cause structural and functional damage to the zebrafish olfactory system. These findings may have implications for intranasally delivered vaccines against SARS-CoV-2.

Not Only COVID-19: Involvement of Multiple Chemosensory Systems in Human Diseases

Chemosensory systems are deemed marginal in human pathology. In appraising their role, we aim at suggesting a paradigm shift based on the available clinical and experimental data that will be discussed. Taste and olfaction are polymodal sensory systems, providing inputs to many brain structures that regulate crucial visceral functions, including metabolism but also endocrine, cardiovascular, respiratory, and immune systems. Moreover, other visceral chemosensory systems monitor different essential chemical parameters of “milieu intérieur,” transmitting their data to the brain areas receiving taste and olfactory inputs; hence, they participate in regulating the same vital functions. These chemosensory cells share many molecular features with olfactory or taste receptor cells, thus they may be affected by the same pathological events. In most COVID-19 patients, taste and olfaction are disturbed. This may represent only a small portion of a broadly diffuse chemosensory incapacitation. Indeed, many COVID-19 peculiar symptoms may be explained by the impairment of visceral chemosensory systems, for example, silent hypoxia, diarrhea, and the “cytokine storm”. Dysregulation of chemosensory systems may underlie the much higher mortality rate of COVID-19 Acute Respiratory Distress Syndrome (ARDS) compared to ARDSs of different origins. In chronic non-infectious diseases like hypertension, diabetes, or cancer, the impairment of taste and/or olfaction has been consistently reported. This may signal diffuse chemosensory failure, possibly worsening the prognosis of these patients. Incapacitation of one or few chemosensory systems has negligible effects on survival under ordinary life conditions but, under stress, like metabolic imbalance or COVID-19 pneumonia, the impairment of multiple chemosensory systems may lead to dire consequences during the course of the disease.

Animal models for studying covid-19, prevention, and therapy: Pathology and disease phenotypes

Translational models have played an important role in the rapid development of safe and effective vaccines and therapeutic agents for the ongoing coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Animal models recapitulating the clinical and underlying pathological manifestations of COVID-19 have been vital for identification and rational design of safe and effective vaccines and therapies. This manuscript provides an overview of commonly used COVID-19 animal models and the pathologic features of SARS-CoV-2 infection in these models in relation to their clinical presentation in humans. Also discussed are considerations for selecting appropriate animal models for infectious diseases such as COVID-19, the host determinants that can influence species-specific susceptibility to SARS-CoV-2, and the pathogenesis of COVID-19. Finally, the limitations of currently available COVID-19 animal models are highlighted.