Olfactory epithelium: Cells, clinical disorders, and insights from an adult stem cell niche

A Dry Powder Platform for Nose-to-Brain Delivery of Dexamethasone: Formulation Development and Nasal Deposition Studies

Nasal route of administration offers a unique opportunity of brain targeted drug delivery via olfactory and trigeminal pathway, providing effective CNS concentrations at lower doses and lower risk for adverse reactions compared to systemic drug administration. Therefore, it has been recently proposed as a route of choice for glucocorticoids to control neuroinflammation processes in patients with severe Covid-19. However, appropriate delivery systems tailored to enhance their efficacy yet need to emerge. In this work we present the development of sprayable brain targeting powder delivery platform of dexamethasone sodium phosphate (DSP). DSP-loaded microspheres, optimised employing Quality-by-Design approach, were blended with soluble inert carriers (mannitol or lactose monohydrate). Powder blends were characterized in terms of homogeneity, flow properties, sprayability, in vitro biocompatibility, permeability and mucoadhesion. Nasal deposition studies were performed using 3D printed nasal cavity model. Mannitol provided better powder blend flow properties compared to lactose. Microspheres blended with mannitol retained or enlarged their mucoadhesive properties and enhanced DSP permeability across epithelial model barrier. DSP dose fraction deposited in the olfactory region reached 17.0% revealing the potential of developed powder platform for targeted olfactory delivery. The observed impact of nasal cavity asymmetry highlighted the importance of individual approach when aiming olfactory region.

Viral infection and smell loss: The case of COVID-19

Olfactory disorders have been increasingly reported in individuals infected with SARS-CoV-2, the virus causing the coronavirus disease 2019 (COVID-19). Losing the sense of smell has a strong impact on the quality of life, since it may lead to malnutrition, weight loss, food poisoning, depression, and exposure to dangerous chemicals. Individuals who suffer from anosmia (inability to smell) also cannot sense the flavor of food, which is a combination of taste and smell. Interestingly, infected individuals have reported sudden loss of smell with no congested nose, as is frequently observed in common colds or other upper respiratory tract infections. These observations suggest that SARS-CoV-2 infection leads to olfactory loss through a distinct mechanism, which is still unclear. This article provides an overview of olfactory loss and the recent findings relating to COVID-19. Possible mechanisms of SARS-CoV-2-induced olfactory loss are also discussed.

Role of the SphK-S1P-S1PRs pathway in invasion of the nervous system by SARS-CoV-2 infection

Global spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still ongoing. Before an effective vaccine is available, the development of potential treatments for resultant coronavirus disease 2019 (COVID-19) is crucial. One of the disease hallmarks is hyper-inflammatory responses, which usually leads to a severe lung disease. Patients with COVID-19 also frequently suffer from neurological symptoms such as acute diffuse encephalomyelitis, brain injury and psychiatric complications. The metabolic pathway of sphingosine-1-phosphate (S1P) is a dynamic regulator of various cell types and disease processes, including the nervous system. It has been demonstrated that S1P and its metabolic enzymes, regulating neuroinflammation and neurogenesis, exhibit important functions during viral infection. S1P receptor 1 (S1PR1) analogues including AAL-R and RP-002 inhibit pathophysiological responses at the early stage of H1N1 virus infection and then play a protective role. Fingolimod (FTY720) is an S1P receptor modulator and is being tested for treating COVID-19. Our review provides an overview of SARS-CoV-2 infection and critical role of the SphK-S1P-SIPR pathway in invasion of SARS-CoV-2 infection, particularly in the central nervous system (CNS). This may help design therapeutic strategies based on the S1P-mediated signal transduction, and the adjuvant therapeutic effects of S1P analogues to limit or prevent the interaction between the host and SARS-CoV-2, block the spread of the SARS-CoV-2, and consequently treat related complications in the CNS.

COVID-19 as a worldwide selective event and bitter taste receptor polymorphisms: An ecological correlational study

Background

Given the observed olfactory and gustatory dysfunctions in patients with COVID-19 and recent findings on taste receptors possible important activities in the immune system, we elected to estimate the correlation between COVID-19 mortality and polymorphism of a particular type of bitter taste receptor gene called TAS2R38, in a worldwide epidemiological point of view.

Methods

Pooled rate of each of the rs713598, rs1726866, rs10246939, and PAV/AVI polymorphisms of the TAS2R38 gene was obtained in different countries using a systematic review methodology and its relationship with the mortality of COVID-19. Data were analyzed by the comprehensive meta-analysis software and SPSS.

Results

There was only a significant reverse Pearson correlation in death counts and PAV/AVI ratio, p = 0.047, r = −0.503. Also, a significant reverse correlation of PAV/AVI ratio and death rate was seen, r = −0.572 p = 0.021. rs10246939 ratio had a significant positive correlation with death rate, r = 0.851 p = 0.031. Further analysis was not significant. Our results showed that the higher presence of PAV allele than AVI, and a higher rate of G allele than A in rs10246939 polymorphism in a country, could be associated with lower COVID-19 mortality. While assessing all three polymorphisms showed a huge diversity worldwide.

Conclusion

Due to extraoral activities of bitter taste receptor genes, especially in mucosal immunity, this gene seems to be a good candidate for future studies on COVID-19 pathophysiology. Also, the high worldwide diversity of TAS2R38 genes polymorphism and its possible assassination with mortality raises concerns about the efficiency of vaccine projects in different ethnicities.

Evidence of SARS-CoV2 Entry Protein ACE2 in the Human Nose and Olfactory Bulb

Usually, pandemic COVID-19 disease, caused by SARS-CoV2, presents with mild respiratory symptoms such as fever, cough, but frequently also with anosmia and neurological symptoms. Virus-cell fusion is mediated by angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) with their organ expression pattern determining viral tropism. Clinical presentation suggests rapid viral dissemination to the central nervous system leading frequently to severe symptoms including viral meningitis. Here, we provide a comprehensive expression landscape of ACE2 and TMPRSS2 proteins across human postmortem nasal and olfactory tissue. Sagittal sections through the human nose complemented with immunolabelling of respective cell types represent different anatomically defined regions including olfactory epithelium, respiratory epithelium of the nasal conchae and the paranasal sinuses along with the hardly accessible human olfactory bulb. ACE2 can be detected in the olfactory epithelium as well as in the respiratory epithelium of the nasal septum, the nasal conchae, and the paranasal sinuses. ACE2 is located in the sustentacular cells and in the glandular cells in the olfactory epithelium as well as in the basal cells, glandular cells, and epithelial cells of the respiratory epithelium. Intriguingly, ACE2 is not expressed in mature or immature olfactory receptor neurons and basal cells in the olfactory epithelium. Similarly, ACE2 is not localized in the olfactory receptor neurons albeit the olfactory bulb is positive. Vice versa, TMPRSS2 can also be detected in the sustentacular cells and the glandular cells of the olfactory epithelium. Our findings provide the basic anatomical evidence for the expression of ACE2 and TMPRSS2 in the human nose, olfactory epithelium, and olfactory bulb. Thus, they are substantial for future studies that aim to elucidate the symptom of SARS-CoV2 induced anosmia via the olfactory pathway.

Neurological Complications of the COVID-19 Pandemic: What Have We Got So Far?

The recently emerged coronavirus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causal agent of COVID-19, is the newest threat to human health. It has already infected more than 54.5 million people worldwide, currently leading to more than 1.3 million deaths. Although it causes a mild flu-like disease in most patients, lethality may increase to more than 20% in elderly subjects, especially in those with comorbidities, like hypertension, diabetes, or lung and cardiac disease, and the mechanisms are still elusive. Common symptoms at the onset of illness are fever, cough, myalgia or fatigue, headache, and diarrhea or constipation. Interestingly, respiratory viruses have also placed themselves as relevant agents for central nervous system (CNS) pathologies. Conversely, SARS-CoV-2 has already been detected in the cerebrospinal fluid. Here, we discuss several clinical features related to CNS infection during COVID-19. Patients may progress from headaches and migraines to encephalitis, stroke, and seizures with leptomeningitis. However, the pathway used by the virus to reach the brain is still unknown. It may infect the olfactory bulb by retrograde neuronal transportation from olfactory epithelium, or it could be transported by the blood. Either way, neurological complications of COVID-19 add greatly to the complex pathophysiology of the disease. Neurological signs and symptoms must alert physicians not only to worst outcomes but also to future possible degenerative diseases.

The importance of smell and taste in everyday life: Dysfunction in COVID-19 patients

Human-to-human transmission of coronavirus (SARS-CoV-2) - COVID-19 (corona virus disease 2019) - is characterized by a pandemic exponential rate and the patients with mild to moderate infection have odor and taste problems that represent a new atypical disease. A new viral syndrome of acute anosmia or "new loss of taste or smell" without rhinitis and nasal obstruction or rhinorrhea has been placed on the list of symptoms that may occur 2 to 14 days after exposure to the COVID-19 virus. Two months after declaring the COVID-19 pandemic in May 2020, the World Health Organization (WHO) has recognized changes in the perception of smell and taste as symptoms of this disease. The described cardinal symptoms are more common in the population of young patients and able-bodied people which facilitates the spread of disease. Significantly higher prevalence of patients with COVID-19 who have lost their taste and smell is treated at home (rare hospitalization), lung damage is rare, as well as oxygen therapy with mild lymphopenia. Different scenarios of SARS-CoV-2 viral infection can be assumed: it is probable that the virus does not enter directly into olfactory sensory neurons (they do not have ACE2 and TMPRSS2 receptors), but it is localized to vascular pericytes and causes inflammatory processes and vasculopathies. On the other hand, direct infection of non-neuronal cells which contain said receptors is possible. Those are specific cell types in the olfactory epithelium such as sustentacular, horizontal basal cells, as well as Bowman's glands, which leads to massive degeneration and loss of olfactory neurons. The sense of taste is a complex sensation that is the result of the interaction of smell, taste, temperature and texture of food. The virus damages cranial nerves, epithelial receptors and blood vessels leading to taste damage (ageusia or dysgeusia). A multidisciplinary approach with epidemiological, clinical and basic research is needed to elucidate the mechanism of sensorineural odor and taste loss caused by coronavirus.

A Comparison of the Primary Sensory Neurons Used in Olfaction and Vision

Vision, hearing, smell, taste, and touch are the tools used to perceive and navigate the world. They enable us to obtain essential resources such as food and highly desired resources such as mates. Thanks to the investments in biomedical research the molecular unpinning’s of human sensation are rivaled only by our knowledge of sensation in the laboratory mouse. Humans rely heavily on vision whereas mice use smell as their dominant sense. Both modalities have many features in common, starting with signal detection by highly specialized primary sensory neurons—rod and cone photoreceptors (PR) for vision, and olfactory sensory neurons (OSN) for the smell. In this chapter, we provide an overview of how these two types of primary sensory neurons operate while highlighting the similarities and distinctions.

Involvement of the nervous system in COVID-19: The bell should toll in the brain

The world is fuming at SARS-CoV-2 for being the culprit for causing the devastating COVID-19, claiming millions of lives across the globe in the form of respiratory disorders. But lesser known are its effects on the CNS that are slowly surfacing in the worldwide population. Our review illustrates findings that claim SARS-CoV-2's arrival onto the ACE2 receptors of neuronal and glial cells mainly via CSF, olfactory nerve, trigeminal nerve, neuronal dissemination, and hematogenous pathways. The role of SARS-CoV-2 structural proteins in its smooth viral infectivity of the host cannot be ignored, especially the spike proteins that mediate spike attachment and host membrane fusion. Worth mentioning the nucleocapsid, envelope, and membrane proteins make the proliferation of SARS-CoV-2 much simpler than expected in spreading infection. This has led to catastrophic conditions like seizures, Guillain-Barré syndrome, viral encephalitis, meningoencephalitis, acute cerebrovascular disease, and respiratory failures. Placing a magnifying lens on the lesser-explored CNS consequences of COVID-19, we attempt to shift the focus of our readers onto the new supporting threats to which further studies are needed.

On the in vivo origin of human nasal mesenchymal stem cell cultures

OBJECTIVES

Mesenchymal stem cells (MSCs), classically expanded in culture from bone marrow, are of broad interest to the regenerative medicine community. Human nasal turbinate mesenchymal-like stem cell cultures have also been described, defined by an in vitro phenotype similar to bone marrow MSCs. Nonetheless, the identity in vivo of the cells that give rise to nasal MSC-like cultures remains unclear, and these cells are often suggested to be related to olfactory lineages. Here, we sought to define the in vivo phenotype of human nasal MSC-like cells.

METHODS

Human turbinate tissue samples were used for RNA and immunohistochemical analysis. We also analyzed a recently published single cell RNA-sequencing dataset from adult human olfactory and respiratory mucosa samples from our lab, to focus on cell populations expressing MSC markers. Immunochemistry was performed to stain turbinate sections and nasal MSC cultures for selected markers.

RESULTS

While there is no single MSC-specific gene, we identified a human nasal mucosal cell population in vivo that uniquely expressed transcripts characteristic of typical MSC cultures, including ENG (CD105), NES, and CD34, and lacked expression of other transcripts associated with surface epithelia. The expression of transcription factors such as SOX17, EBF1, and FOXP1 suggests cells in the MSC-like cluster maintain an ability to direct cell fate, consistent with the behavior of nasal MSC-like cells in vitro. SOX17 was found to be uniformly expressed by nasal MSC cultures, consistent with the in vivo data. Immunohistochemistry of human nasal tissue samples indicated that ENG, CD34, and SOX17 expression localized selectively to cells surrounding blood vessels in the lamina propria.

CONCLUSION

Our findings provide evidence that the in vivo origin of nasal MSC-like cultures is likely a vascular or pericyte population, rather than cells related to the olfactory neuronal lineage.

LEVEL OF EVIDENCE

NA.

Olfactory and taste disorders in COVID-19: a systematic review

INTRODUCTION

The SARS-CoV-2 virus causes COVID-19, and it is responsible for the largest pandemic since the 1918 H1N1 influenza outbreak. The classic symptoms of the disease have been well defined by the World Health Organization; however, olfactory/gustatory disorders have been reported in some studies, but there are still several missing points in the understanding and in the consensus about the clinical management of these cases.

OBJECTIVE

To identify evidence in the scientific literature about olfactory/gustatory disorders, their clinical presentation, prevalence and possible specific treatments associated with COVID-19.

METHODS

A systematic review of articles published up to April 25, 2020 was performed in Medline, Cochrane Clinical Trials, ScienceDirect, Lilacs, Scopus and Google Schoolar, OpenGrey.eu, DissOnline, The New York Academy of Medicine and Reasearch Gate.

INCLUSION CRITERIA

(1) Studies on patients with COVID-19; (2) Records of COVID-19 signs/symptoms, and olfactory/gustatory functions.

EXCLUSION CRITERIA

(1) Studies on non-human coronavirus; (2) Review articles; (3) Experimental studies (in animals or in vitro); (4) Olfactory/gustatory disorders initiated prior to SARS-CoV-2 infection. The risk assessment of bias of the selected studies was performed using the Newcastle-Ottawa scale.

RESULTS

Six articles from the 1788 records met the inclusion criteria and were analyzed. A total of 1457 patients of different ethnicities were assessed; of them, 885 (60.7%) and 822 (56.4%) had smell and taste disorders, respectively, with women being most often affected. There were olfactory/gustatory disorders even without nasal obstruction/rhinorrhea and beginning even before the signs/symptoms of COVID-19; the recovery of smell/taste, when it occurs, usually happened in the first two weeks after COVID-19 resolution. There is evidence that olfactory/gustatory disorders are strong predictors of infection by SARS-CoV-2, and it is possible to recommend patient isolation, as early as of the medical consultation, preventing the spread of the virus. No scientific evidence has been identified for effective treatments for any of the disorders.

CONCLUSION

Olfactory/gustatory disorders may occur at varying intensities and prior to the general symptoms of COVID-19 and should be considered as part of the clinical features of COVID-19, even in mild cases. There is still no scientific evidence of specific treatments for such disorders in COVID-19 disease.

A new perspective on imaging of olfactory dysfunction: Does size matter?

PURPOSE

This study assesses the diagnostic utility of olfactory nerve and bulb morphologies in addition to volumetric analysis in classification of different olfactory dysfunction etiologies.

METHODS

106 patients presenting with olfactory loss and 17 control subjects were included. Based on detailed anamnesis, smell test and ear-nose-throat examination; patients were categorized into four groups as post-viral, post-traumatic, idiopathic, and obstructive olfactory dysfunction. Olfactory region was imaged with paranasal sinus CT and MRI dedicated to olfactory nerve. Olfactory bulb volume and olfactory sulcus depths were calculated on MRI. The olfactory bulb was assessed for morphology, contour lobulations and T2-signal intensity; and olfactory nerve for uniformity and clumping.

RESULTS

Volumetric analysis showed decreased olfactory bulb volume in idiopathic and obstructive group compared to control subjects. Olfactory sulci were shallower in post-viral, post-traumatic, idiopathic, and obstructive group compared to the control group. In post-viral group; olfactory bulbs had lobulated contour and focal T2-hyperintense regions in 67 % of cases, and olfactory nerves had a clumped and thickened appearance in 66 % of cases. In idiopathic group, olfactory bulbs were rectangular shaped with minimally deformed contours, and olfactory nerves were thin and hard to delineate. No specific olfactory bulb or nerve pattern was identified in obstructive and post-traumatic groups, however closed olfactory cleft and siderotic frontobasal changes were helpful clues in obstructive and post-traumatic groups, respectively.

CONCLUSION

In addition to olfactory cleft patency, olfactory sulcus depth and olfactory bulb volume; bulb and nerve morphologies may provide diagnostic information on different etiologies of olfactory dysfunction.

Is loss of sense of smell a diagnostic marker in COVID-19: A systematic review and meta-analysis

Aims

To systematically review the currently available evidence investigating the association between olfactory dysfunction (OD) and the novel coronavirus (COVID‐19). To analyse the prevalence of OD in patients who have tested positive on polymerase chain reaction (PCR) for COVID‐19. To perform a meta‐analysis of patients presenting with olfactory dysfunction, during the pandemic, and to investigate the positive predictive value for a COVID‐19‐positive result in this population. To assess whether olfactory dysfunction could be used as a diagnostic marker for COVID‐19 positivity and aid public health approaches in tackling the current outbreak.

Methods

We systematically searched MedLine (PubMed), Embase, Health Management Information Consortium (HMIC), Medrxiv, the Cochrane Library, the Cochrane COVID‐19 Study Register, NIHR Dissemination centre, Clinical Evidence, National Health Service Evidence and the National Institute of Clinical Excellence to identify the current published evidence which associates coronaviridae or similar RNA viruses with anosmia. The initial search identified 157 articles. A total of 145 papers were excluded following application of our exclusion criteria. The 12 remaining articles that presented evidence on the association between COVID‐19 and olfactory dysfunction were critically analysed.

Results

Olfactory dysfunction has been shown to be the strongest predictor of COVID‐19 positivity when compared to other symptoms in logistic regression analysis. In patients who had tested positive for COVID‐19, there was a prevalence of 62% of OD. In populations of patients who are currently reporting OD, there is a positive predictive value of 61% for a positive COVID‐19 result.

Conclusion

Our review has shown that there is already significant evidence which demonstrates an association between OD and the novel coronavirus—COVID‐19. It is unclear if this finding is unique to this coronavirus as individual viral phenotypes rarely present in such concentrated large numbers. We have demonstrated that OD is comparatively more predictive for COVID‐19 positivity compared to other associated symptoms. We recommend that people who develop OD during the pandemic should be self‐isolate and this guidance should be adopted internationally to prevent transmission.

SARS-CoV-2 Infection: A Role for S1P/S1P Receptor Signaling in the Nervous System?

The recent coronavirus disease (COVID-19) is still spreading worldwide. The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the virus responsible for COVID-19, binds to its receptor angiotensin-converting enzyme 2 (ACE2), and replicates within the cells of the nasal cavity, then spreads along the airway tracts, causing mild clinical manifestations, and, in a majority of patients, a persisting loss of smell. In some individuals, SARS-CoV-2 reaches and infects several organs, including the lung, leading to severe pulmonary disease. SARS-CoV-2 induces neurological symptoms, likely contributing to morbidity and mortality through unknown mechanisms. Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid with pleiotropic properties and functions in many tissues, including the nervous system. S1P regulates neurogenesis and inflammation and it is implicated in multiple sclerosis (MS). Notably, Fingolimod (FTY720), a modulator of S1P receptors, has been approved for the treatment of MS and is being tested for COVID-19. Here, we discuss the putative role of S1P on viral infection and in the modulation of inflammation and survival in the stem cell niche of the olfactory epithelium. This could help to design therapeutic strategies based on S1P-mediated signaling to limit or overcome the host-virus interaction, virus propagation and the pathogenesis and complications involving the nervous system.

Heterogeneity of Antiviral Responses in the Upper Respiratory Tract Mediates Differential Non-lytic Clearance of Influenza Viruses

Influenza viruses initiate infection in the upper respiratory tract (URT), but early viral tropism and the importance of cell-type-specific antiviral responses in this tissue remain incompletely understood. By infecting transgenic lox-stop-lox reporter mice with a Cre-recombinase-expressing influenza B virus, we identify olfactory sensory neurons (OSNs) as a major viral cell target in the URT. These cells become infected, then eliminate the virus and survive in the host post-resolution of infection. OSN responses to infection are characterized by a strong induction of interferon-stimulated genes and more rapid clearance of viral protein relative to other cells in the epithelium. We speculate that this cell-type-specific response likely serves to protect the central nervous system from infection. More broadly, these results highlight the importance of evaluating antiviral responses across different cell types, even those within the same tissue, to more fully understand the mechanisms of viral disease.

Smell dysfunction: a biomarker for COVID-19

BACKGROUND

Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19), is responsible for the largest pandemic since the 1918 influenza A virus subtype H1N1 influenza outbreak. The symptoms presently recognized by the World Health Organization are cough, fever, tiredness, and difficulty breathing. Patient-reported smell and taste loss has been associated with COVID-19 infection, yet no empirical olfactory testing on a cohort of COVID-19 patients has been performed.

METHODS

The University of Pennsylvania Smell Identification Test (UPSIT), a well-validated 40-odorant test, was administered to 60 confirmed COVID-19 inpatients and 60 age- and sex-matched controls to assess the magnitude and frequency of their olfactory dysfunction. A mixed effects analysis of variance determined whether meaningful differences in test scores existed between the 2 groups and if the test scores were differentially influenced by sex.

RESULTS

Fifty-nine (98%) of the 60 patients exhibited some smell dysfunction (mean [95% CI] UPSIT score: 20.98 [19.47, 22.48]; controls: 34.10 [33.31, 34.88]; p < 0.0001). Thirty-five of the 60 patients (58%) were either anosmic (15/60; 25%) or severely microsmic (20/60; 33%); 16 exhibited moderate microsmia (16/60; 27%), 8 mild microsmia (8/60; 13%), and 1 normosmia (1/60; 2%). Deficits were evident for all 40 UPSIT odorants. No meaningful relationships between the test scores and sex, disease severity, or comorbidities were found.

CONCLUSION

Quantitative smell testing demonstrates that decreased smell function, but not always anosmia, is a major marker for SARS-CoV-2 infection and suggests the possibility that smell testing may help, in some cases, to identify COVID-19 patients in need of early treatment or quarantine.

Epigenome-450K-wide methylation signatures of active cigarette smoking: The Young Finns Study

Smoking as a major risk factor for morbidity affects numerous regulatory systems of the human body including DNA methylation. Most of the previous studies with genome-wide methylation data are based on conventional association analysis and earliest threshold-based gene set analysis that lacks sensitivity to be able to reveal all the relevant effects of smoking. The aim of the present study was to investigate the impact of active smoking on DNA methylation at three biological levels: 5'-C-phosphate-G-3' (CpG) sites, genes and functionally related genes (gene sets). Gene set analysis was done with mGSZ, a modern threshold-free method previously developed by us that utilizes all the genes in the experiment and their differential methylation scores. Application of such method in DNA methylation study is novel. Epigenome-wide methylation levels were profiled from Young Finns Study (YFS) participants' whole blood from 2011 follow-up using Illumina Infinium HumanMethylation450 BeadChips. We identified three novel smoking related CpG sites and replicated 57 of the previously identified ones. We found that smoking is associated with hypomethylation in shore (genomic regions 0-2 kilobases from CpG island). We identified smoking related methylation changes in 13 gene sets with false discovery rate (FDR) ≤ 0.05, among which is olfactory receptor activity, the flagship novel finding of the present study. Overall, we extended the current knowledge by identifying: (i) three novel smoking related CpG sites, (ii) similar effects as aging on average methylation in shore, and (iii) a novel finding that olfactory receptor activity pathway responds to tobacco smoke and toxin exposure through epigenetic mechanisms.

Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia

Altered olfactory function is a common symptom of COVID-19, but its etiology is unknown. A key question is whether SARS-CoV-2 (CoV-2) - the causal agent in COVID-19 - affects olfaction directly, by infecting olfactory sensory neurons or their targets in the olfactory bulb, or indirectly, through perturbation of supporting cells. Here we identify cell types in the olfactory epithelium and olfactory bulb that express SARS-CoV-2 cell entry molecules. Bulk sequencing demonstrated that mouse, non-human primate and human olfactory mucosa expresses two key genes involved in CoV-2 entry, ACE2 and TMPRSS2. However, single cell sequencing revealed that ACE2 is expressed in support cells, stem cells, and perivascular cells, rather than in neurons. Immunostaining confirmed these results and revealed pervasive expression of ACE2 protein in dorsally-located olfactory epithelial sustentacular cells and olfactory bulb pericytes in the mouse. These findings suggest that CoV-2 infection of non-neuronal cell types leads to anosmia and related disturbances in odor perception in COVID-19 patients.

COVID-19 and the Chemical Senses: Supporting Players Take Center Stage

The main neurological manifestation of COVID-19 is loss of smell or taste. The high incidence of smell loss without significant rhinorrhea or nasal congestion suggests that SARS-CoV-2 targets the chemical senses through mechanisms distinct from those used by endemic coronaviruses or other common cold-causing agents. Here we review recently developed hypotheses about how SARS-CoV-2 might alter the cells and circuits involved in chemosensory processing and thereby change perception. Given our limited understanding of SARS-CoV-2 pathogenesis, we propose future experiments to elucidate disease mechanisms and highlight the relevance of this ongoing work to understanding how the virus might alter brain function more broadly.

Olfactory Dysfunction in Traumatic Brain Injury: the Role of Neurogenesis

PURPOSE OF REVIEW

Olfactory functioning disturbances are common following traumatic brain injury (TBI) having a significant impact on quality of life. A spontaneous recovery of the olfactory function over time may occur in TBI patients. Although there is no standard treatment for patients with posttraumatic olfactory loss, olfactory training (OT) has shown some promise beneficial effects. However, the mechanisms underlying spontaneous recovery and olfactory improvement induced by OT are not completely known.

RECENT FINDINGS

The spontaneous recovery of the olfactory function and the improvement of olfactory function after OT have recently been associated with an increase in subventricular (SVZ) neurogenesis and an increase in olfactory bulb (OB) glomerular dopaminergic (DAergic) interneurons. In addition, after OT, an increase in electrophysiological responses at the olfactory epithelium (OE) level has been reported, indicating that recovery of olfactory function not only affects olfactory processing at the central level, but also at peripheral level. However, the role of OE stem cells in the spontaneous recovery and in the improvement of olfactory function after OT in TBI is still unknown. In this review, we describe the physiology of the olfactory system, and the olfactory dysfunction after TBI. We highlight the possible role for the SVZ neurogenesis and DAergic OB interneurons in the recovery of the olfactory function. In addition, we point out the relevance of the OE neurogenesis process as a future target for the research in the pathophysiological mechanisms involved in the olfactory dysfunction in TBI. The potential of basal stem cells as a promising candidate for replacement therapies is also described.

Age-Related Olfactory Dysfunction: Epidemiology, Pathophysiology, and Clinical Management

Like other sensory systems, olfactory function deteriorates with age. Epidemiological studies have revealed that the incidence of olfactory dysfunction increases at the age of 60 and older and males are more affected than females. Moreover, smoking, heavy alcohol use, sinonasal diseases, and Down’s syndrome are associated with an increased incidence of olfactory dysfunction. Although the pathophysiology of olfactory dysfunction in humans remains largely unknown, studies in laboratory animals have demonstrated that both the peripheral and central olfactory nervous systems are affected by aging. Aged olfactory neuroepithelium in the nasal cavity shows the loss of mature olfactory neurons, replacement of olfactory neuroepithelium by respiratory epithelium, and a decrease in basal cell proliferation both in the normal state and after injury. In the central olfactory pathway, a decrease in the turnover of interneurons in the olfactory bulb (OB) and reduced activity in the olfactory cortex under olfactory stimulation is observed. Recently, the association between olfactory impairment and neurodegenerative diseases, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), has gained attention. Evidence-based pharmacotherapy to suppress or improve age-related olfactory dysfunction has not yet been established, but preliminary results suggest that olfactory training using odorants may be useful to improve some aspects of age-related olfactory impairment.

Smell dysfunction: a biomarker for COVID-19

Background

Severe acute respiratory syndrome‐coronavirus‐2 (SARS‐CoV‐2), the virus that causes coronavirus disease 2019 (COVID‐19), is responsible for the largest pandemic since the 1918 influenza A virus subtype H1N1 influenza outbreak. The symptoms presently recognized by the World Health Organization are cough, fever, tiredness, and difficulty breathing. Patient‐reported smell and taste loss has been associated with COVID‐19 infection, yet no empirical olfactory testing on a cohort of COVID‐19 patients has been performed.

Methods

The University of Pennsylvania Smell Identification Test (UPSIT), a well‐validated 40‐odorant test, was administered to 60 confirmed COVID‐19 inpatients and 60 age‐ and sex‐matched controls to assess the magnitude and frequency of their olfactory dysfunction. A mixed effects analysis of variance determined whether meaningful differences in test scores existed between the 2 groups and if the test scores were differentially influenced by sex.

Results

Fifty‐nine (98%) of the 60 patients exhibited some smell dysfunction (mean [95% CI] UPSIT score: 20.98 [19.47, 22.48]; controls: 34.10 [33.31, 34.88]; p < 0.0001). Thirty‐five of the 60 patients (58%) were either anosmic (15/60; 25%) or severely microsmic (20/60; 33%); 16 exhibited moderate microsmia (16/60; 27%), 8 mild microsmia (8/60; 13%), and 1 normosmia (1/60; 2%). Deficits were evident for all 40 UPSIT odorants. No meaningful relationships between the test scores and sex, disease severity, or comorbidities were found.

Conclusion

Quantitative smell testing demonstrates that decreased smell function, but not always anosmia, is a major marker for SARS‐CoV‐2 infection and suggests the possibility that smell testing may help, in some cases, to identify COVID‐19 patients in need of early treatment or quarantine.

Cellular mechanisms and treatments for chemobrain: insight from aging and neurodegenerative diseases

Chemotherapy is a life-saving treatment for cancer patients, but also causes long-term cognitive impairment, or "chemobrain", in survivors. However, several challenges, including imprecise diagnosis criteria, multiple confounding factors, and unclear and heterogeneous molecular mechanisms, impede effective investigation of preventions and treatments for chemobrain. With the rapid increase in the number of cancer survivors, chemobrain is an urgent but unmet clinical need. Here, we leverage the extensive knowledge in various fields of neuroscience to gain insights into the mechanisms for chemobrain. We start by outlining why the post-mitotic adult brain is particularly vulnerable to chemotherapy. Next, through drawing comparisons with normal aging, Alzheimer's disease, and traumatic brain injury, we identify universal cellular mechanisms that may underlie the cognitive deficits in chemobrain. We further identify existing neurological drugs targeting these cellular mechanisms that can be repurposed as treatments for chemobrain, some of which were already shown to be effective in animal models. Finally, we briefly describe future steps to further advance our understanding of chemobrain and facilitate the development of effective preventions and treatments.

Cyclophosphamide has Long-Term Effects on Proliferation in Olfactory Epithelia

Chemotherapy patients often experience chemosensory changes during and after drug therapy. The chemotherapy drug, cyclophosphamide (CYP), has known cytotoxic effects on sensory and proliferating cells of the taste system. Like the taste system, cells in the olfactory epithelia undergo continuous renewal. Therefore, we asked if a single injection of 75 mg/kg CYP would affect cell proliferation in the anterior dorsomedial region of the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) from 0 to 125 days after injection. Both epithelia showed a decrease in Ki67-labeled cells compared to controls at day 1 and no Ki67+ cells at day 2 postinjection. In the sensory layer of the MOE, cell proliferation began to recover 4 days after CYP injection and by 6 days, the rate of proliferation was significantly greater than controls. Ki67+ cells peaked 30 days postinjection, then declined to control levels at day 45. Similar temporal sequences of initial CYP-induced suppression of cell proliferation followed by elevated rates peaking 30-45 days postinjection were seen in the sustentacular layer of the MOE and all 3 areas (sensory, sustentacular, marginal) of the VNO. CYP affected proliferation in the sensory layer of the MOE more than the sustentacular layer and all 3 areas of the VNO. These findings suggest that chemotherapy involving CYP is capable of affecting cell renewal of the olfactory system and likely contributes to clinical loss of function during and after chemotherapy.

Olfactory Dysfunction and Chronic Rhinosinusitis

Olfactory dysfunction (OD) is one of the cardinal symptoms of chronic rhinosinusitis (CRS), and its prevalence ranges from 60% to 80% in patients with CRS. It is much more common in CRS with nasal polyposis patients compared to CRS without nasal polyposis. Decreased olfactory function is associated with significant decreases in patient-reported quality of life (QOL), and notably, depression and the enjoyment of food. Objective measures can help detail the degree of OD, whereas subjective measures can help to determine in the impact on patient. There is variable treatment response to OD with both medical and surgical therapies.

Olfactory dysfunction in the pathophysiological continuum of dementia

Sensory capacities like smell, taste, hearing, vision decline with aging, but increasing evidence show that sensory dysfunctions are one of the early signs diagnosing the conversion from physiological to pathological brain state. Smell loss represents the best characterized sense in clinical practice and is considered as one of the first preclinical signs of Alzheimer's and Parkinson's disease, occurring a decade or more before the onset of cognitive and motor symptoms. Despite the numerous scientific reports and the adoption in clinical practice, the etiology of sensory damage as prodromal of dementia remains largely unexplored and more studies are needed to resolve the mechanisms underlying sensory network dysfunction. Although both cognitive and sensory domains are progressively affected, loss of sensory experience in early stages plays a major role in reducing the autonomy of demented people in their daily tasks or even possibly contributing to their cognitive decline. Interestingly, the chemosensory circuitry is devoid of a blood brain barrier, representing a vulnerable port of entry for neurotoxic species that can spread to the brain. Furthermore, the exposure of the olfactory system to the external environment make it more susceptible to mechanical injury and trauma, which can cause degenerative neuroinflammation. In this review, we will summarize several findings about chemosensory impairment signing the conversion from healthy to pathological brain aging and we will try to connect those observations to the promising research linking environmental influences to sporadic dementia. The scientific body of knowledge will support the use of chemosensory diagnostics in the presymptomatic stages of AD and other biomarkers with the scope of finding treatment strategies before the onset of the disease.

Loss of BMI1 in mature olfactory sensory neurons leads to increased olfactory basal cell proliferation

BACKGROUND

Damage to olfactory sensory neurons (OSNs), situated within the neuroepithelium of the olfactory cleft, may be associated with anosmia. Although their direct contact with the nasal airspace make OSNs vulnerable to injury and death, multiple mechanisms maintain epithelium integrity and olfactory function. We hypothesized that BMI1, a polycomb protein found to be enriched in OSNs, may function in neuroprotection. Here, we explored BMI1 function in a mouse model.

METHODS

Utilizing a mouse genetic approach to delete Bmi1 selectively in mature OSNs, we investigated changes in OE homeostasis by performing immunohistochemical, biochemical, and functional assays. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR), immunostaining, and electro-olfactograms were used to compare gene expression, cell composition, and olfactory function in OSN-specific BMI1 knockout mice (n = 3 to 5) and controls. Chromatin studies were also performed to identify protein-DNA interactions between BMI1 and its target genes (n = 3).

RESULTS

OSN-specific BMI1 knockout led to increased neuron death and basal cell activation. Chromatin studies suggested a mechanism of increased neurodegeneration due to de-repression of a pro-apoptosis gene, p19ARF. Despite the increased turnover, we found that olfactory neuroepithelium thickness and olfactory function remained intact. Our studies also revealed the presence of additional polycomb group proteins that may compensate for the loss of BMI1 in mature OSNs.

CONCLUSION

The olfactory neuroepithelium employs multiple mechanisms to maintain epithelial homeostasis. Our findings provide evidence that in a mouse model of BMI1 deletion, the overall integrity and function of the olfactory neuroepithelium are not compromised, despite increased neuronal turnover, reflecting a remarkable reparative capacity to sustain a critical sensory system.

Distinct progenitor populations mediate regeneration in the zebrafish lateral line

Mechanosensory hair cells of the zebrafish lateral line regenerate rapidly following damage. These renewed hair cells arise from the proliferation of surrounding support cells, which undergo symmetric division to produce two hair cell daughters. Given the continued regenerative capacity of the lateral line, support cells presumably have the ability to replenish themselves. Utilizing novel transgenic lines, we identified support cell populations with distinct progenitor identities. These populations show differences in their ability to generate new hair cells during homeostasis and regeneration. Targeted ablation of support cells reduced the number of regenerated hair cells. Furthermore, progenitors regenerated after targeted support cell ablation in the absence of hair cell damage. We also determined that distinct support cell populations are independently regulated by Notch signaling. The existence of independent progenitor populations could provide flexibility for the continued generation of new hair cells under a variety of conditions throughout the life of the animal.

Acute N-Acetylcysteine Administration Ameliorates Loss of Olfactory Neurons Following Experimental Injury In Vivo

The olfactory epithelium (OE) is the peripheral organ for the sense of smell, housing primary sensory neurons that project axons from the nose to the brain. Due to the presence of a basal stem cell niche, the adult mammalian OE is a dynamic tissue capable of replacing neurons following their loss. Nonetheless, certain conditions, such as blunt head trauma, can result in persistent olfactory loss, thought to be due to shearing of olfactory nerve filaments at the skull base, degeneration, and failures in proper regeneration/reinnervation. The identification of new treatment strategies aimed at preventing degeneration of olfactory neurons is, therefore, needed. In considering potential therapies, we have focused on N-acetylcysteine (NAC), a glutathione substrate shown to be neuroprotective, with a record of safe clinical use. Here, we have tested the use of NAC in an animal model of olfactory degeneration. Administered acutely, we found that NAC (100 mg/kg, twice daily) resulted in a reduction of olfactory neuronal loss from the OE of the nose following surgical ablation of the olfactory bulb. At 1 week postlesion, we identified 54 ± 8.1 mature neurons per 0.5 mm epithelium in NAC-treated animals vs. 28 ± 4.2 in vehicle-treated controls (P = 0.02). Furthermore, in an olfactory cell culture model, we have identified significant alterations in the expression of several genes involved in oxidative stress pathways following NAC exposure. Our results provide evidence supporting the potential therapeutic utility for NAC acutely following head trauma-induced olfactory loss. Anat Rec, 303:626-633, 2020. © 2019 American Association for Anatomy.

Induction of smell through transethmoid electrical stimulation of the olfactory bulb

BACKGROUND

Anosmia has an estimated prevalence of 5% of the general population. Outside of inflammatory causes, therapeutic options are limited despite research advances. Bypassing peripheral neuronal damage through central stimulation is a potential therapeutic option that has shown success in other sensory systems, most notably with hearing. We performed a pilot study to determine the feasibility of inducing smell through artificial electrical stimulation of the olfactory bulbs in humans.

METHODS

Subjects with a history of sinus surgery, including total ethmoidectomy, with intact ability to smell were enrolled. The ability to smell was confirmed with a 40-item smell identification test. Awake subjects underwent nasal endoscopy and either a monopolar or bipolar electrode was positioned at 3 areas along the lateral lamella of the cribriform plate within the ethmoid sinus cavity. A graded stimulation current of 1-20 mA at 3.17 Hz was administered while cortical evoked potential (CEP) recordings were collected. Subjective responses of perceived smell along with reports of discomfort were recorded. Subjects with artificially induced smell underwent repeat stimulation after medically induced anosmia.

RESULTS

Five subjects (age, 43-72 years) were enrolled. Three subjects reported smell perception smell with electrical stimulation. This was reproducible after inducing anosmia, but CEP recordings could not provide objective support. All subjects tolerated the study with minimal discomfort.

CONCLUSION

This is the first report of induced smell through transethmoid electrical stimulation of the olfactory bulb. These results provide a proof of concept for efforts in development of an olfactory implant system.