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

Do SARS-CoV-2 Variants Differ in Their Neuropathogenicity?

Neurological complications associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections are a huge societal problem. Although the neuropathogenicity of SARS-CoV-2 is not yet fully understood, there is evidence that SARS-CoV-2 can invade and infect cells of the central nervous system. Kong et al. (https://doi.org/10.1128/mbio.02308-22) shows that the mechanism of virus entry into astrocytes in brain organoids and primary astrocytes differs from entry into respiratory epithelial cells. However, how SARS-CoV-2 enters susceptible CNS cells and whether there are differences among SARS-CoV-2 variants is still unclear. In vivo and in vitro models are useful to study these important questions and may reveal important differences among SARS-CoV-2 variants in their neuroinvasive, neurotropic, and neurovirulent potential. In this commentary we address how this study contributes to the understanding of the neuropathology of SARS-CoV-2 and its variants.

Chemosensory Ability and Sensitivity in Health and Disease: Epigenetic Regulation and COVID-19

Throughout the animal kingdom, our two chemical senses, olfaction and gustation, are defined by two primary factors: genomic architecture of the organisms and their living environment. During the past three years of the global COVID-19 pandemic, these two sensory modalities have drawn much attention at the basic science and clinical levels because of the strong association of olfactory and gustatory dysfunction with viral infection. Loss of our sense of smell alone, or together with a loss of taste, has emerged as a reliable indicator of COVID-19 infection. Previously, similar dysfunctions have been detected in a large cohort of patients with chronic conditions. The research focus remains on understanding the persistence of olfactory and gustatory disturbances in the post-infection phase, especially in cases with long-term effect of infection (long COVID). Also, both sensory modalities show consistent age-related decline in studies aimed to understand the pathology of neurodegenerative conditions. Some studies using classical model organisms show an impact on neural structure and behavior in offspring as an outcome of parental olfactory experience. The methylation status of specific odorant receptors, activated in parents, is passed on to the offspring. Furthermore, experimental evidence indicates an inverse correlation of gustatory and olfactory abilities with obesity. Such diverse lines of evidence emerging from basic and clinical research studies indicate a complex interplay of genetic factors, evolutionary forces, and epigenetic alterations. Environmental factors that regulate gustation and olfaction could induce epigenetic modulation. However, in turn, such modulation leads to variable effects depending on genetic makeup and physiological status. Therefore, a layered regulatory hierarchy remains active and is passed on to multiple generations. In the present review, we attempt to understand the experimental evidence that indicates variable regulatory mechanisms through multilayered and cross-reacting pathways. Our analytical approach will add to enhancement of prevailing therapeutic interventions and bring to the forefront the significance of chemosensory modalities for the evaluation and maintenance of long-term health.

Human Nasal Epithelium Damage as the Probable Mechanism Involved in the Development of Post-COVID-19 Parosmia

Purpose

Since the beginning of the COVID-19 pandemic, understanding the physiopathological mechanisms of its manifestations has been crucial to understand the disease and its implications. As the disease evolved, post-infection complications have arisen such as olfactory dysfunctions including parosmia in which odourants are perceived in a distorted or an unpleasant way.

Methods

In this article, we attempt to clarify these mechanisms and the role of human nasal epithelium in the development of post-COVID-19 parosmia.

Results

The mechanisms by which SARS-CoV-2 generates olfactory dysfunction have not been elucidated, and multiple theories have been proposed pointing to the sustentacular cells of the olfactory epithelium as the main probable target of the virus.

Conclusion

Establishing the main physiopathological mechanism of post-COVID-19 parosmia will set a path for further investigations and determine treatment and preventive options for patients who have been reported to be extensively affected in multiple aspects of their lives such as eating habits and mental health.

In Vitro and In Vivo Characterization of H5N8 High-Pathogenicity Avian Influenza Virus Neurotropism in Ducks and Chickens

H5N8 high-pathogenicity avian influenza virus (HPAIV) of clade 2.3.4.4B, which circulated during the 2016 epizootics in Europe, was notable for causing different clinical signs in ducks and chickens. The clinical signs preceding death were predominantly neurological in ducks versus respiratory in chickens. To investigate the determinants for the predominant neurological signs observed in ducks, we infected duck and chicken primary cortical neurons. Viral replication was identical in neuronal cultures from both species. In addition, we did not detect any major difference in the immune and inflammatory responses. These results suggest that the predominant neurological involvement of H5N8 HPAIV infection in ducks could not be recapitulated in primary neuronal cultures. In vivo, H5N8 HPAIV replication in ducks peaked soon after infection and led to an early colonization of the central nervous system. In contrast, viral replication was delayed in chickens but ultimately burst in the lungs of chickens, and the chickens died of respiratory distress before brain damage became significant. Consequently, the immune and inflammatory responses in the brain were significantly higher in duck brains than those in chickens. Our study thus suggests that early colonization of the central nervous system associated with prolonged survival after the onset of virus replication is the likely primary cause of the sustained inflammatory response and subsequent neurological disorders observed in H5N8 HPAIV-infected ducks. IMPORTANCE The severity of high-pathogenicity avian influenza virus (HPAIV) infection has been linked to its ability to replicate systemically and cause lesions in a variety of tissues. However, the symptomatology depends on the host species. The H5N8 virus of clade 2.3.4.4B had a pronounced neurotropism in ducks, leading to severe neurological disorders. In contrast, neurological signs were rarely observed in chickens, which suffered mostly from respiratory distress. Here, we investigated the determinants of H5N8 HPAIV neurotropism. We provide evidence that the difference in clinical signs was not due to a difference in neurotropism. Our results rather indicate that chickens died of respiratory distress due to intense viral replication in the lungs before viral replication in the brain could produce significant lesions. In contrast, ducks better controlled virus replication in the lungs, thus allowing the virus to replicate for a sufficient duration in the brain, to reach high levels, and to cause significant lesions.

Warmer ambient air temperatures reduce nasal turbinate and brain infection, but increase lung inflammation in the K18-hACE2 mouse model of COVID-19

Warmer climatic conditions have been associated with fewer COVID-19 cases. Herein we infected K18-hACE2 mice housed at the standard animal house temperature of ∼22 °C, or at ∼31 °C, which is considered to be thermoneutral for mice. On day 2 post infection, RNA-Seq analyses showed no significant differential gene expression lung in lungs of mice housed at the two temperatures, with almost identical viral loads and type I interferon responses. There was also no significant difference in viral loads in lungs on day 5, but RNA-Seq and histology analyses showed clearly elevated inflammatory signatures and infiltrates. Thermoneutrality thus promoted lung inflammation. On day 2 post infection mice housed at 31 °C showed reduced viral loads in nasal turbinates, consistent with increased mucociliary clearance at the warmer ambient temperature. These mice also had reduced virus levels in the brain, and an ensuing amelioration of weight loss and a delay in mortality. Warmer air temperatures may thus reduce infection of the upper respiratory track and the olfactory epithelium, resulting in reduced brain infection. Potential relevance for anosmia and neurological sequelae in COVID-19 patients is discussed.

Autoimmunity, COVID-19 Omicron Variant, and Olfactory Dysfunction: A Literature Review

Smelling is a critical sense utilized daily. Consequently, smelling impairment or anosmia may lead to a reduction in life quality. Systemic diseases and particular autoimmune conditions can impair olfactory function; among others are Systemic Lupus Erythematosus, Sjögren Syndrome, and Rheumatoid Arthritis. Interactions between the olfactory process and the immune systems cause this phenomenon. Alongside autoimmune conditions, in the recent COVID-19 pandemic, anosmia was also described as a prevalent infection symptom. Nevertheless, the occurrence of anosmia is significantly less common in Omicron-infected patients. Several theories have been proposed to explain this phenomenon. One possibility is that the Omicron variant preferentially enters host cells via endocytosis, rather than plasma cell membrane fusion. This endosomal pathway is less dependent on the activation of Transmembrane serine protease 2 (TMPRSS2), expressed at the olfactory epithelium. As a result, the Omicron variant may have reduced efficiency in penetrating the olfactory epithelium, leading to a lower prevalence of anosmia. Furthermore, olfactory changes are known to be associated with inflammatory conditions. The Omicron variant elicits a less robust autoimmune and inflammatory response, believed to reduce the probability of anosmia. This review elaborates on the commonalities and differences in autoimmune and COVID-19 omicron-associated anosmia.

[Olfaction and respiratory viruses… A relationship revealed by Covid-19]

The sense of smell has been underestimated for a long time in humans. It has been brought to the fore by its sudden disappearance during the Covid-19 pandemic of which anosmia (complete loss of smell) is one of the major symptoms. However, respiratory viruses have long been associated with smell disorders, 25% of which are linked to a viral infection. Olfaction begins in the nose within the olfactory epithelium which has the particularity of containing neurons in direct contact with the environment. Several respiratory viruses are known for their replicative capacity within this epithelium. This is particularly the case for the flu virus (influenza) and bronchiolitis (respiratory syncytial virus) but their tropism for this tissue is much lower than SARS-CoV-2. The understanding of the SARS-CoV-2 pathophysiology in the nasal cavity makes it possible to reveal part of the links between viral infection and olfactory disorders.

Animal Models for the Study of Neurologic Manifestations Of COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the worldwide coronavirus (COVID-19) pandemic, has infected an estimated 525 million people with over 6 million deaths. Although COVID-19 is primarily a respiratory disease, an escalating number of neurologic symptoms have been reported in humans. Some neurologic symptoms, such as loss of smell or taste, are mild. However, other symptoms, such as meningoencephalitis or stroke, are potentially fatal. Along with surveys and postmortem evaluations on humans, scientists worked with several animal species to try to elucidate the causes of neurologic symptoms. Neurologic sequelae remain challenging to study due to the complexity of the nervous system and difficulties in identification and quantification of neurologic signs. We reviewed animal models used in the study of neurologic COVID-19, specifically research in mice, hamsters, ferrets, and nonhuman primates. We summarized findings on the presence and pathologic effects of SARS-CoV-2 on the nervous system. Given the need to increase understanding of COVID-19 and its effects on the nervous system, scientists must strive to obtain new information from animals to reduce mortality and morbidity with neurologic complications in humans.

Olfactory impairment in patients with coronavirus disease 2019 self-perceived as asymptomatic

BACKGROUND

Olfactory impairment may be present among patients with coronavirus disease 2019 self-perceived as asymptomatic. This study aimed to assess olfactory function among these individuals.

METHODS

A cross-sectional study involving patients with coronavirus disease 2019 self-perceived as asymptomatic was conducted. Assessments included the subjective Malaysian Smell and Taste Questionnaire and the culturally adapted Malaysian version of the objective Sniffin' Sticks Identification smell test.

RESULTS

In 81 participants (mean age of 31.59 ± 12.04 years), with mean time from diagnosis to smell test of 7.47 ± 3.79 days, subjective assessment showed that 80.2 per cent were asymptomatic (questionnaire score of 6) and 19 per cent had mild symptoms (questionnaire score of 7 and 8). The mean objective smell test score was 10.89 ± 2.11. The prevalence of olfactory impairment was 76.6 per cent among patients with coronavirus disease 2019 self-perceived as asymptomatic. There was no association between the questionnaire and the smell test scores (p = 0.25). There was a correlation between the smell test score and the duration from diagnosis to smell test (p = 0.04).

CONCLUSION

The objective assessment demonstrated that coronavirus disease 2019 patients who perceived themselves as asymptomatic showed olfactory impairment.

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.

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.

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.

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.

The nervous system during COVID-19: Caught in the crossfire

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.

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.

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.

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.

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.

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.