Damage to Olfactory Progenitor Cells Is Involved in Cigarette Smoke-Induced Olfactory Dysfunction in Mice

Olfactory dysfunction and the role of stem cells in the regeneration of olfactory neurons

The prevalence of COVID-19 has drawn increasing attention to olfactory dysfunction among researchers. Olfactory dysfunction manifests in various clinical types, influenced by numerous pathogenic factors. Despite this diversity, the underlying pathogenesis remains largely elusive, contributing to a lack of standardized treatment approaches. However, the potential regeneration of olfactory neurons within the nasal cavity presents a promising avenue for addressing olfactory dysfunction effectively. Our review aims to delve into the current research landscape and treatment modalities concerning olfactory dysfunction, emphasizing etiology, pathogenesis, clinical interventions, and the role of stem cells in regenerating olfactory nerves. Through this comprehensive examination, we aim to provide valuable insights into understanding the onset, progression, and treatment of olfactory dysfunction diseases.

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.

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.

Squamous and Respiratory Metaplasia After Olfactory Mucosal Resection

Resection of the olfactory mucosa (OM) is sometimes unavoidable during surgery; however, it is not known whether the OM can completely recover thereafter. The aim of this study was to uncover whether the OM fully recovers after mucosal resection and describe the process of OM regeneration. 8-week-old male Sprague–Dawley rats (n = 18) were subjected to OM resection at the nasal septum; six rats were euthanized for histological examination 0, 30, and 90 days after surgery. Immunohistochemistry was performed to identify olfactory receptor neuron (ORN) lineage cells [mature and immature ORNs and ORN progenitors, and olfactory ensheathing cells (OECs)], as well as dividing and apoptotic cells. Squamous and respiratory metaplasia and inflammatory cell infiltration were also assessed. On day 30 after resection, the mucosa had regenerated, and mainly contained thin nerve bundles, basal cells, and immature ORNs, with a few mature ORNs and OECs. On day 90, the repaired nasal mucosa had degenerated into stratified squamous or ciliated pseudostratified columnar epithelia, with reducing ORNs. The lamina propria contained numerous macrophages. Partial regeneration was observed within 1 month after OM resection, whereas subsequent degeneration into squamous and respiratory epithelia occurred within 3 months. Given the poor persistence of ORNs and OECs, OM resection is likely to result in olfactory impairment. Overall, surgeons should be cautious not to injure the OM during surgery.

Exposure to Cadmium, Lead, and Tobacco Smoke and the 10-Year Cumulative Incidence of Olfactory Impairment: The Beaver Dam Offspring Study

Importance

Olfactory impairment is common in older adults. Identification of modifiable risk factors for olfactory impairment at midlife has the potential to reduce the burden of olfactory impairment at older ages.

Objective

To determine the 10-year cumulative incidence of olfactory impairment and evaluate potentially modifiable risk factors for impairment including exposure to cadmium, lead, and tobacco smoke.

Design, Setting, and Participants

Data from the Beaver Dam Offspring Study, a longitudinal cohort study of sensory health and aging in a general population, were available from examinations at baseline (2005-2008), 5 years (2010-2013), and 10 (2015-2017) years. A total of 2312 participants without olfactory impairment at baseline and with olfaction data available at the 5- and/or 10-year examination were included. The present study was conducted from April 28, 2020, to January 8, 2021.

Main Outcomes and Measures

Olfactory impairment was measured by the San Diego Odor Identification Test. Cox discrete-time proportional hazards analyses were used to model associations between baseline covariates, including blood cadmium and lead levels and tobacco smoke exposure, and the 10-year cumulative incidence of olfactory impairment.

Results

Of the 2312 participants, 1269 (54.9%) were women; mean age was 49 years (range, 22-84 years) at baseline. The 10-year cumulative incidence of olfactory impairment was 4.6% (95% CI, 3.7%-5.6%) and increased with age. Because of high collinearity, cadmium and tobacco smoke exposure were modeled separately. In a multivariable adjusted model, higher blood cadmium level (hazard ratio [HR], 1.70; 95% CI, 1.05-2.74) was associated with the 10-year cumulative incidence of olfactory impairment. Substituting tobacco smoke exposure for cadmium in the model, high exposure to tobacco smoke as a current smoker (HR, 2.94; 95% CI, 1.63-5.29, smoker vs no exposure) or from environmental tobacco smoke (HR, 2.65; 95% CI, 1.24-5.63, high vs no exposure) was also associated with an increased risk for developing olfactory impairment. Blood lead levels were not associated with olfactory impairment.

Conclusions and Relevance

Results of this longitudinal cohort study suggest that modifiable environmental exposures may contribute to olfactory impairment that occurs with aging. Identification of modifiable risk factors for olfactory impairment may lead to prevention strategies that have the potential to reduce the burden of olfactory impairment at older ages.

Exposure to Cigarette Smoke Enhances Pneumococcal Transmission Among Littermates in an Infant Mouse Model

Streptococcus pneumoniae, one of the most common commensal pathogens among children, is spread by close contact in daycare centers or within a family. Host innate immune responses and bacterial virulence factors promote pneumococcal transmission. However, investigations into the effects of environmental factors on transmission have been limited. Passive smoking, a great concern for children's health, has been reported to exacerbate pneumococcal diseases. Here, we describe the effect of cigarette smoke exposure on an infant mouse model of pneumococcal transmission. Our findings reveal that the effect of cigarette smoke exposure significantly promotes pneumococcal transmission by enhancing bacterial shedding from the colonized host and by increasing susceptibility to pneumococcal colonization in the new host, both of which are critical steps of transmission. Local inflammation, followed by mucosal changes (such as mucus hypersecretion and disruption of the mucosal barrier), are important underlying mechanisms for promotion of transmission by smoke exposure. These effects were attributable to the constituents of cigarette smoke rather than smoke itself. These findings provide the first experimental evidence of the impact of environmental factors on pneumococcal transmission and the mechanism of pathogenesis.

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.

Chronic Cigarette Smoking Associates Directly and Indirectly with Self-Reported Olfactory Alterations: Analysis of the 2011-2014 National Health and Nutrition Examination Survey

INTRODUCTION

Population-based studies show inconsistent effects of cigarette smoking on olfactory function. We aimed to identify direct and indirect associations between measures of smoking exposure/nicotine dependence and altered olfaction in a nationally representative sample of adults.

METHODS

NHANES 2011-2014 (n = 7418) participants (mean age = 57.8 ± 12.2 years) self-reported olfaction and related health and demographic risks. Affirmative answers to three questions defined altered olfaction (olfactory problems in the past years; worse ability since age 25; phantom smells). Smoking (never, former, current) was self-reported by chronicity (pack years, PY) and dependency (time to first cigarette upon waking) and verified by serum cotinine. Associations were tested with logistic regression, reporting odds ratios (ORs) and 95% confidence intervals (CIs), and mediation models.

RESULTS

Estimated prevalence of altered olfaction was 22.3%, with age-related increases. Nearly half of the sample were former/current smokers (47.4%). Controlling for olfactory-related risks, ≥10 PY smokers had significantly greater odds of altered olfaction versus never smokers (OR 1.36, CI: 1.06-1.74). The odds of altered olfaction were heightened among current smokers (≥10 PY) who also had high nicotine dependence (smoked ≤30 min of waking) (OR 1.41, CI: 1.01-1.99). Light smokers (≤10 PY smokers) did not show increased odds versus never smokers. Current smokers who also were heavy drinkers (≥4 drinks/day) had the highest odds for altered olfaction (OR 1.96, CI: 1.20-3.19). Olfactory-related pathologies (sinonasal problems, serious head injury, tonsillectomy, xerostomia) partially mediated the association between smoking and altered olfaction.

CONCLUSIONS

Chronic cigarette smoking was associated with increased odds of self-reported olfactory alterations, directly and indirectly via olfactory-related pathologies.

IMPLICATIONS

Analysis of the US nationally representative data revealed significant positive associations between chronic smoking and alterations in the sense of smell. Rates of smell alteration (self-reported problems in the past year, losses with aging, and phantom smells) increased from 23% among adults to 33% for chronic smokers and 38% for chronic smokers who also reported heavy drinking. Chronic smoking showed associations with smell alteration that were direct and indirect through exposure to olfactory-related pathologies (naso-sinus problems, dry mouth, head/facial injury). Smell alteration can impact smokers' quality of life by challenging the ability to sense warning odors, food flavor, and olfactory-stimulated emotions and memories.

TRPM5-expressing Microvillous Cells Regulate Region-specific Cell Proliferation and Apoptosis During Chemical Exposure

The mammalian main olfactory epithelium (MOE) is exposed to a wide spectrum of external chemicals during respiration and relies on adaptive plasticity to maintain its structural and functional integrity. We previously reported that the chemo-responsive and cholinergic transient receptor potential channel M5 (TRPM5)-expressing-microvillous cells (MCs) in the MOE are required for maintaining odor-evoked electrophysiological responses and olfactory-guided behavior during two-week exposure to an inhaled chemical mixture. Here, we investigated the underlying factors by assessing the potential modulatory effects of TRPM5-MCs on MOE morphology and cell proliferation and apoptosis, which are important for MOE maintenance. In the posterior MOE of TRPM5-GFP mice, we found that two-week chemical exposure induced a significant increase in Ki67-expressing proliferating basal stem cells without a significant reduction in the thickness of the whole epithelium or mature olfactory sensory neuron (OSN) layer. This adaptive increase in stem cell proliferation was missing in chemical-exposed transcription factor Skn-1a knockout (Skn-1a^-/-) mice lacking TRPM5-MCs. In addition, a greater number of isolated OSNs from chemical-exposed Skn-1a^-/- mice displayed unhealthily high levels of resting intracellular Ca²⁺. Intriguingly, in the anterior MOE where we found a higher density of TRPM5-MCs, chemical-exposed TRPM5-GFP mice exhibited a time-dependent increase in apoptosis and a loss of mature OSNs without a significant increase in proliferation or neurogenesis to compensate for OSN loss. Together, our data suggest that TRPM5-MC-dependent region-specific upregulation of cell proliferation in the majority of the MOE during chemical exposure contributes to the adaptive maintenance of OSNs and olfactory function.

Effects of Cigarette Smoke on the Nasal Respiratory and Olfactory Mucosa in Allergic Rhinitis Mice

Objective

Cigarette smoke (CS) exposure reportedly enhances allergic airway inflammation. However, some studies have shown an association between current cigarette smoke exposure and a low risk for allergic rhinitis. Thus, the impact of CS exposure on allergic rhinitis remains poorly understood. The purpose of this study was to investigate the effects of CS on the respiratory mucosa (RM) and the olfactory epithelium (OE) of mice with allergic rhinitis, as the effects may differ depending on the nasal histological compartments.

Methods

Eight-week-old male BALB/c mice were used for this study. We developed a mouse model of smoking by intranasally administering 10 doses of a CS solution (CSS), and a mouse model of allergic rhinitis by sensitization with intraperitoneal ovalbumin (OVA) injection and intranasal challenge with OVA. We examined the effects of CS on the nasal RM and OE in mice with or without allergic rhinitis using histological, serum, and genetic analyses. First, we examine whether CSS exposure induces allergic responses and then, examined allergic responses in the OVA-sensitized allergic rhinitis mice with or without CSS exposure.

Results

Short-term CSS administration intensified allergic responses including increased infiltration of eosinophils and inflammatory cells and upregulation of interleukin-5 expression in the nasal RM of OVA-immunized mice, although only CSS induced neither allergic responses nor impairment of the RM and OE. Notably, repetitive OVA-immunization partially impaired the OE in the upper-lateral area, but CSS administration did not reinforce this impairment in OVA-induced allergic mice.

Conclusion

Short-term CSS exposure strengthened allergic responses in the nasal RM and did not change the structure of the OE. These results suggest that patients with allergic rhinitis could experience exacerbation of allergic symptoms after CS exposure.

Age-Related Dysfunctions: Evidence and Relationship with Some Risk Factors and Protective Drugs

The theories interpreting senescence as a phenomenon favored by natural selection require the existence of specific, genetically determined and regulated mechanisms that cause a progressive age-related increase in mortality. The mechanisms defined in the subtelomere-telomere theory suggest that progressive slackening of cell turnover and decline in cellular functions are determined by the subtelomere-telomere-telomerase system, which causes a progressive "atrophic syndrome" in all organs and tissues. If the mechanisms underlying aging-related dysfunctions are similar and having the same origin, it could be hypothesized that equal interventions could produce similar effects. This article reviews the consequences of some factors (diabetes, obesity/dyslipidemia, hypertension, smoking, moderate use and abuse of alcohol) and classes of drugs [statins, angiotensin-converting enzyme (ACE) inhibitors, sartans] in accelerating and anticipating or in counteracting the process of aging. The evidence is compatible with the programmed aging paradigm and the mechanisms defined by the subtelomere-telomere theory but it has no obvious discriminating value against the theories of non-programmed aging paradigm. However, the existence of mechanisms, determined by the subtelomere-telomere-telomerase system and causing a progressive age-related decline in fitness through gradual cell senescence and cell senescence, is not justifiable without an evolutionary motivation. Their existence is expected by the programmed aging paradigm, while is incompatible with the opposite paradigm.

Construction of an irreversible allergic rhinitis-induced olfactory loss mouse model

Clinical data show that part of patients with sinonasal diseases suffered from olfactory dysfunction, especially with allergic rhinitis (AR) and chronic rhinosinusitis (CRS). However, the mechanisms responsible for AR-induced olfactory loss are still largely unknown. Because of the difficulty to obtain human olfactory mucosa, an AR-induced olfactory loss animal model needs to be constructed to clarify the mechanism. The AR mouse model was induced by intraperitoneal sensitizing with ovalbumin (OVA) followed by intranasal challenge lasted from 1 to 12 weeks. For groups with recovery, mice were housed for another 4-week long without any treatment after the last intranasal challenge. Olfactory function, olfactory receptor neurons (ORNs) density and leukocytes infiltration were examined at different time points. Olfactory loss occurs immediately after 1-week intranasal challenge and deteriorates almost to anosmia after 8th week, and after that olfactory loss become irreversible. Nasal inflammation induces significant infiltration of leukocytes into olfactory epithelium (OE), which negatively correlated with the density of ORNs and mouse olfaction in a time dependent manner. The neutrophilic subtype dominates in number amongst the total infiltrated leukocytes, indicating its pivotal role in nasal inflammation-induced olfactory dysfunction. In this study, we constructed a persistent AR-induced olfactory loss mouse model, losing the ability to recover from dysfunction if the disease duration more than eight weeks, which implies that timely treatments are necessary. Meanwhile, this mouse model could provide an easy and reliable system to clarify the mechanisms of AR-induced irreversible olfactory dysfunction.

Toxic exposures and the senses of taste and smell

This review addresses the adverse influences of neurotoxic exposures on the ability to smell and taste. These chemical senses largely determine the flavor of foods and beverages, impact food intake, and ultimately nutrition, and provide a warning for spoiled or poisonous food, leaking natural gas, smoke, airborne pollutants, and other hazards. Hence, toxicants that damage these senses have a significant impact on everyday function. As noted in detail, a large number of toxicants encountered in urban and industrial air pollution, including smoke, solvents, metals, and particulate matter can alter the ability to smell. Their influence on taste, i.e., sweet, sour, bitter, salty, and savory (umami) sensations, is not well documented. Given the rather direct exposure of olfactory receptors to the outside environment, olfaction is particularly vulnerable to damage from toxicants. Some toxicants, such as nanoparticles, have the potential to damage not only the olfactory receptor cells, but also the central nervous system structures by their entrance into the brain through the olfactory mucosa.

Heightened olfactory dysfunction and oral irritation among chronic smokers and heightened propylthiouracil (PROP) bitterness among menthol smokers

Chronic cigarette smoking may influence chemosensory function, which in turn, may affect cigarette usage. Because menthol in cigarettes can attenuate nicotine bitterness, choice of menthol/nonmenthol cigarettes may be influenced by ability to perceive bitterness. We examined chemosensory function of chronic smokers, hypothesizing they would show altered function in comparison to non-smokers and by menthol cigarette preference. In laboratory-based measures, chronic smokers (N = 135; 84 menthol smokers) self-reported their chemosensory function and participated in smell (identification task with perceived intensity) and taste (quinine and NaCl intensity on tongue-tip and whole mouth) testing. A taste genetics probe (propylthiouracil (PROP) bitterness) also was assessed. Self-reported and measured chemosensory function were compared with nationally-representative 2013-2014 National Health and Nutrition Examination Survey (NHANES) data generated with similar measures. The taste measures also were compared between smokers and age- and sex-matched non-smokers from a laboratory database. Frequencies of self-reported smell and taste alterations among smokers exceeded NHANES prevalence estimates for non-smokers. The rate of measured smell dysfunction also exceeded NHANES prevalence for hyposmia. Compared to non-smokers, smokers reported elevated tongue-tip and whole mouth intensities from 1 M NaCl, with no significant differences in whole mouth quinine or 0.32 M NaCl. Inconsistent with previous hypotheses, smokers were not more likely to report depressed PROP bitterness than non-smokers. However, as expected, menthol smokers reported greater PROP bitterness than non-menthol smokers. In conclusion, chemosensory alterations were more frequent among chronic smokers, including hyposmia and heightened intensity from NaCl at an oral-irritant concentration. PROP supertasters were most likely to prefer mentholated cigarettes.

Dose-Dependent Effects of Insulin-Like Growth Factor 1 in the Aged Olfactory Epithelium

Background: Olfaction is known to be impaired by aging. We hypothesized that insulin-like growth factor-1 (IGF-1) administered at an appropriate dose could prevent age-induced negative effects on olfactory receptor neurons (ORNs). We explored the effects of low- and high-dose administration of IGF-1 on the ORN cell system in aged mice and investigated the involvement of the cellular mechanisms of IGF-1 in the regeneration of ORNs in aged mice. Methods: We subcutaneously administered recombinant human IGF-1 (rhIGF-1) to 16-month-old male mice over 56 days, and then examined the histological effects of rhGF-1 on cellular composition, cell proliferation, and cell death in the aged olfactory epithelium (OE), by comparing among saline-treated and low- and high-dose rhIGF-1-treated mice. Results: Low-dose rhIGF-1 administration increased the numbers of olfactory progenitors, immature ORNs, and mature ORNs in the OE, despite an increase in Cas3+ apoptotic cells. Notably, high-dose rhIGF-1 administration increased the numbers of only immature ORNs, not olfactory progenitors and mature ORNs, with a concurrent increase in apoptotic cells. Conclusion: Our data suggest that in aged mice, IGF-1 administered at an appropriate dose could increase the number of mature ORNs and further human studies may contribute to the development of treatments for aging-related olfactory impairment.

Cigarette Smoke-Induced Cell Death Causes Persistent Olfactory Dysfunction in Aged Mice

Introduction: Exposure to cigarette smoke is a cause of olfactory dysfunction. We previously reported that in young mice, cigarette smoke damaged olfactory progenitors and decreased mature olfactory receptor neurons (ORNs), then, mature ORNs gradually recovered after smoking cessation. However, in aged populations, the target cells in ORNs by cigarette smoke, the underlying molecular mechanisms by which cigarette smoke impairs the regenerative ORNs, and the degree of ORN regeneration after smoking cessation remain unclear.

Objectives: To explore the effects of cigarette smoke on the ORN cell system using an aged mouse model of smoking, and to investigate the extent to which smoke-induced damage to ORNs recovers following cessation of exposure to cigarette smoke in aged mice.

Methods: We intranasally administered a cigarette smoke solution (CSS) to 16-month-old male mice over 24 days, then examined ORN existence, cell survival, changes of inflammatory cytokines in the olfactory epithelium (OE), and olfaction using histological analyses, gene analyses and olfactory habituation/dishabituation tests.

Results: CSS administration reduced the number of mature ORNs in the OE and induced olfactory dysfunction. These changes coincided with an increase in the number of apoptotic cells and Tumor necrosis factor (TNF) expression and a decrease in Il6 expression. Notably, the reduction in mature ORNs did not recover even on day 28 after cessation of treatment with CSS, resulting in persistent olfactory dysfunction.

Conclusion: In aged mice, by increasing ORN death, CSS exposure could eventually overwhelm the regenerative capacity of the OE, resulting in continued reduction in the number of mature ORNs and olfactory dysfunction.

Reduction of Proliferating Olfactory Cells and Low Expression of Extracellular Matrix Genes Are Hallmarks of the Aged Olfactory Mucosa

Background: The incidence of olfactory impairment increases with age; however, the detailed molecular and cellular mechanisms underlying this increase are yet to be determined. Methods: We examined the influence of aging on olfactory receptor neurons (ORNs), which are maintained by a unique stem cell system, from olfactory progenitor cells to mature ORNs, by histological comparisons of the physiological status of the olfactory epithelium between young adult and aged mice. Furthermore, we clarified the expression of genes encoding inflammatory cytokines, neurotrophins, growth factors, and extracellular matrix proteins to reveal the molecular mechanisms underlying olfactory impairment caused by aging. Results: The numbers of mature and immature ORNs, but not olfactory progenitors, decreased in the aged olfactory epithelium, with a concurrent reduction in Ki-67-positive proliferating cells. Transcriptome analyses revealed an increase in Il6, encoding a component of senescence-associated secretary phenotypes (SASP), and a decrease in Igf1, encoding a growth factor for ORNs, in the aged nasal mucosa. Interestingly, expression levels of several extracellular matrix genes, including Col1a2, decreased in the aged nasal mucosa. Consistent with the transcriptional changes, the number of Col1a2-GFP-positive cells decreased in the aged lamina propria. Conclusions: Our data suggest that reduction in ORN number and cell proliferation, reduced extracellular matrix gene expression, and increased SASP contribute to olfactory impairment during aging.

Morphological evaluation using MRI of the olfactory filaments (fila) in a post-traumatic olfactory rat model

OBJECTIVE

To investigate whether magnetic resonance imaging (MRI) can be used to directly assess olfactory bulb (OB) lesions and quantify the associated morphological changes of olfactory filaments (OF), also known as fila, in an in vivo OB-lesion rat model of the brain.

METHODS

A surgical group (n = 5) of male Sprague-Dawley rats was subjected to the unilateral damage of the OB by a steel needle. The control group (n = 5) did not receive surgery. To assess olfactory system injury in vivo, T2-weighted MRI images were acquired in an oblique plane at a 30° angle from transverse plane one day after surgery. These brain regions were also assessed in the controls. The olfactory function was evaluated using the buried food pellet test (BFPT) 5 days before and after surgery.

RESULTS

The OF could be clearly observed on the MRI images from all animals. The left and right OF mean lengths (mm) were similar in the control group (0.81 ± 0.18 vs 0.89 ± 0.17, P > 0.05). In the surgical group, the OB was partially injured in all rats. These rats did not show differences in OF length between left- and right-side (0.83 ± 0.18 vs 0.93 ± 0.24, P > 0.05) at the time of measurement. The time (sec) required to find the food pellets in the BFPT was longer after than before the surgery (83.80 ± 34.37 vs 231.44 ± 53.23, P < 0.05).

CONCLUSIONS

MicroMRI may be a feasible tool to evaluate the OF and OBs in rat models. The unilateral partial OB lesion model appears to be an effective post-traumatic olfactory dysfunction model.

Lipopolysaccharide-initiated persistent rhinitis causes gliosis and synaptic loss in the olfactory bulb

The olfactory mucosa (OM) is exposed to environmental agents and therefore vulnerable to inflammation. To examine the effects of environmental toxin-initiated OM inflammation on the olfactory bulb (OB), we induced persistent rhinitis in mice and analyzed the spatial and temporal patterns of histopathological changes in the OM and OB. Mice received unilateral intranasal administration of lipopolysaccharide (LPS) or saline three times per week, and were immunohistologically analyzed at 1, 3, 7, 14 and 21 days after the first administration. LPS administration induced an inflammatory response in the OM, including the infiltration of Ly-6G-, CD11b-, Iba-1- and CD3-positive cells, the production of interleukin-1β by CD11b- and Iba-1-positive cells, and loss of olfactory sensory neurons (OSNs). In the OB, we observed activation of microglia and astrocytes and decreased expression of tyrosine hydroxylase in periglomerular cells, vesicular glutamate transporter 1, a presynaptic protein, in mitral and tufted projection neurons, and 5T4 in granule cells. Thus, the OM inflammation exerted a detrimental effect, not only on OSNs, but also on OB neurons, which might lead to neurodegeneration.

Evaluation of the effect of cigarette smoking on the olfactory neuroepithelium of New Zealand white rabbit, using scanning electron microscope

To detect ultra-structural changes of Rabbit's olfactory neuro-epithelium using scanning electron microscope after exposure to cigarette smoking. Sixty six rabbits (Pathogen free New Zealand white rabbits weighing 1-1.5 kg included in the study were randomly assigned into one of three groups: control group did not expose to cigarette smoking, study group 1 was exposed to cigarette smoking for 3 months and study group 2 was exposed to cigarette smoking 3 months and then stopped for 2 months. Olfactory neuro-epithelium from all rabbits were dissected and examined under Philips XL-30 scanning electron microscope. Changes that were found in the rabbits of study group 1 in comparison to control group were loss of microvilli of sustentacular cells (p = 0.016) and decreases in distribution of specialized cilia of olfactory receptor cells (p = 0.046). Also respiratory metaplasia was detected. These changes were reversible in study group 2. Cigarette smoking causes ultra-structural changes in olfactory neuro-epithelium which may explain why smell was affected in cigarette smokers. Most of these changes were reversible after 45 days of cessation of cigarette smoking to the rabbits.

Laryngeal mucus hypersecretion is exacerbated after smoking cessation and ameliorated by glucocorticoid administration

INTRODUCTION

The mechanisms underlying the effects of cigarette smoke and smoking cessation on respiratory secretion, especially in the larynx, remain unclear.

OBJECTIVES

The aims of this study were to determine the effects of cigarette smoke and smoking cessation on laryngeal mucus secretion and inflammation, and to investigate the effects of glucocorticoid administration.

METHODS

We administered cigarette smoke solution (CSS) to eight-week-old male Sprague Dawley rats for four weeks, then examined laryngeal mucus secretion and inflammatory cytokine expression on days 1, 28 and 90 after smoking cessation. We also investigated the effects of the glucocorticoid triamcinolone acetonide when administered on day 1 after smoking cessation.

RESULTS

Exposure to CSS resulted in an increase in laryngeal mucus secretion that was further excacerbated following smoking cessation. This change coincided with an increase in the expression of mRNA for the inflammatory cytokines tumor necrosis factor and interleukin-6, as well as mRNA for MUC5AC, which is involved in mucin production. Triamcinolone suppressed CSS-induced laryngeal mucus hypersecretion and pro-inflammatory cytokine production.

CONCLUSION

Cigarette smoke-associated inflammation may contribute to the exacerbated laryngeal mucus hypersecretion that occurs following smoking cessation. The inflammatory response represents a promising target for the treatment of cigarette smoke-associated mucus hypersecretion.

Cigarette Smoke Delays Regeneration of the Olfactory Epithelium in Mice

The olfactory system is a unique part of the mammalian nervous system due to its capacity for neurogenesis and the replacement of degenerating receptor neurons. Cigarette smoking is a major cause of olfactory dysfunction. However, the mechanisms by which cigarette smoke impairs the regenerative olfactory receptor neurons (ORNs) remain unclear. Here, we investigated the influence of cigarette smoke on ORN regeneration following methimazole-induced ORN injury. Administration of methimazole caused detachment of the olfactory epithelium from the basement membrane and induced olfactory dysfunction, thus enabling us to analyze the process of ORN regeneration. We found that intranasal administration of cigarette smoke solution (CSS) suppressed the recovery of ORNs and olfaction following ORN injury. Defective ORN recovery in CSS-treated mice was not associated with any change in the number of SOX2(+) ORN progenitor cells in the basal layer of the OE, but was associated with impaired recovery of GAP43(+) immature ORNs. In the nasal mucosa, mRNA expression levels of neurotrophic factors such as brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-5, glial cell-derived neurotrophic factor, and insulin-like growth factor-1 (IGF-1) were increased following OE injury, whereas CSS administration decreased the ORN injury-induced IGF-1 expression. Administration of recombinant human IGF-1 prevented the CSS-induced suppression of ORN recovery following injury. These results suggest that CSS impairs regeneration of ORNs by suppressing the development of immature ORNs from ORN progenitors, at least partly by reducing IGF-1 in the nasal mucosa.