Macrophage-mediated neuroprotection and neurogenesis in the olfactory epithelium

Early corticosteroid treatment enhances recovery from SARS-CoV-2 induced loss of smell in hamster

Among the numerous long COVID symptoms, olfactory dysfunction persists in ∼10 % of patients suffering from SARS-CoV-2 induced anosmia. Among the few potential therapies, corticoid treatment has been used for its anti-inflammatory effect with mixed success in patients. In this study, we explored its impact using hamster as an animal model. SARS-CoV-2 infected hamsters lose their smell abilities and this loss is correlated with damage of the olfactory epithelium and persistent presence of innate immunity cells. We started a dexamethasone treatment 2 days post infection, when olfaction was already impacted, until 11 days post infection when it started to recover. We observed an improvement of olfactory capacities in the animals treated with corticoid compared to those treated with vehicle. This recovery was not related to differences in the remaining damage to the olfactory epithelium, which was similar in both groups. This improvement was however correlated with a reduced inflammation in the olfactory epithelium with a local increase of the mature olfactory neuron population. Surprisingly, at 11 days post infection, we observed an increased and disorganized presence of immature olfactory neurons, especially in persistent inflammatory zones of the epithelium. This unusual population of immature olfactory neurons coincided with a strong increase of olfactory epithelium proliferation in both groups. Our results indicate that persistent inflammation of the olfactory epithelium following SARS-CoV-2 infection may alter the extent and speed of regeneration of the olfactory neuron population, and that corticoid treatment is effective to limit inflammation and improve olfaction recovery following SARS-CoV-2 infection.

The immune mechanism of the nasal epithelium in COVID-19-related olfactory dysfunction

During the first waves of the coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, olfactory dysfunction (OD) was reported as a frequent clinical sign. The nasal epithelium is one of the front-line protections against viral infections, and the immune responses of the nasal mucosa may be associated with OD. Two mechanisms underlying OD occurrence in COVID-19 have been proposed: the infection of sustentacular cells and the inflammatory reaction of the nasal epithelium. The former triggers OD and the latter likely prolongs OD. These two alternative mechanisms may act in parallel; the infection of sustentacular cells is more important for OD occurrence because sustentacular cells are more likely to be the entry point of SARS-CoV-2 than olfactory neurons and more susceptible to early injury. Furthermore, sustentacular cells abundantly express transmembrane protease, serine 2 (TMPRSS2) and play a major role in the olfactory epithelium. OD occurrence in COVID-19 has revealed crucial roles of sustentacular cells. This review aims to elucidate how immune responses of the nasal epithelium contribute to COVID-19-related OD. Understanding the underlying immune mechanisms of the nasal epithelium in OD may aid in the development of improved medical treatments for COVID-19-related OD.

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.

Recent Advancements in Understanding Primary Cytomegalovirus Infection in a Mouse Model

Animal models that mimic human infections provide insights in virus–host interplay; knowledge that in vitro approaches cannot readily predict, nor easily reproduce. Human cytomegalovirus (HCMV) infections are acquired asymptomatically, and primary infections are difficult to capture. The gap in our knowledge of the early events of HCMV colonization and spread limits rational design of HCMV antivirals and vaccines. Studies of natural infection with mouse cytomegalovirus (MCMV) have demonstrated the olfactory epithelium as the site of natural colonization. Systemic spread from the olfactory epithelium is facilitated by infected dendritic cells (DC); tracking dissemination uncovered previously unappreciated DC trafficking pathways. The olfactory epithelium also provides a unique niche that supports efficient MCMV superinfection and virus recombination. In this review, we summarize recent advances to our understanding of MCMV infection and spread and the tissue-specific mechanisms utilized by MCMV to modulate DC trafficking. As these mechanisms are likely conserved with HCMV, they may inform new approaches for preventing HCMV infections in humans.

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

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

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Modulation of olfactory signal detection in the olfactory epithelium: focus on the internal and external environment, and the emerging role of the immune system

Detection and discrimination of odorants by the olfactory system plays a pivotal role in animal survival. Olfactory-based behaviors must be adapted to an ever-changing environment. Part of these adaptations includes changes of odorant detection by olfactory sensory neurons localized in the olfactory epithelium. It is now well established that internal signals such as hormones, neurotransmitters, or paracrine signals directly affect the electric activity of olfactory neurons. Furthermore, recent data have shown that activity-dependent survival of olfactory neurons is important in the olfactory epithelium. Finally, as olfactory neurons are directly exposed to environmental toxicants and pathogens, the olfactory epithelium also interacts closely with the immune system leading to neuroimmune modulations. Here, we review how detection of odorants can be modulated in the vertebrate olfactory epithelium. We choose to focus on three cellular types of the olfactory epithelium (the olfactory sensory neuron, the sustentacular and microvillar cells) to present the diversity of modulation of the detection of odorant in the olfactory epithelium. We also present some of the growing literature on the importance of immune cells in the functioning of the olfactory epithelium, although their impact on odorant detection is only just beginning to be unravelled.

Functional Role of the Rodlet Cell and Macrophage in Neural Protection of the Olfactory Neuroepithelium in a Teleostean: Gobiid (Pseudapocryptes lanceolatus [Bloch and Schneider, 1801]): An Ultrastructural Study

Background:

Neural protection of the olfactory epithelium against pathogenic invasion is still hardly addressed in fish chemosensory research.

Aims and Objectives:

The ultrastructural detail on the rodlet cell and macrophage has been studied within the olfactory neuroepithelium of Pseudapocryptes lanceolatus to correlate their role in the neural protection of the chemosensory system.

Materials and Methods:

The cellular structures were examined under light microscope (LM: Primo Star; Carl Zeiss Microscopy, GmbH, Germany) and transmission electron microscope (Morgagni 268D).

Results:

Three distinct stages of the rodlet cell (viz., immature, mature, and degenerative) and macrophages have been characterized at the various depths of the olfactory neuroepithelium in P. lanceolatus. The cytoarchitecture of degenerative rodlet cell indicates holocrine mode of secretion against pathogenic invasion into the nasal cavity. Macrophages possess prominent pseudopodia, extending toward invading pathogens. The interaction between macrophage and invading pathogens implicates the role of macrophage as a scavenger to eliminate the pathogens by phagocytosis from the neuroepithelial system.

Conclusion:

This study denotes a significant difference in the mode of action of rodlet cell and macrophages, but they are commonly involved in cell-mediated nonspecific immune response against the invading pathogens.

IL-17c is involved in olfactory mucosa responses to Poly(I:C) mimicking virus presence

At the interface of the environment and the nervous system, the olfactory mucosa (OM) is a privileged pathway for environmental toxicants and pathogens towards the central nervous system. The OM is known to produce antimicrobial and immunological components but the mechanisms of action of the immune system on the OM remain poorly explored. IL-17c is a potent mediator of respiratory epithelial innate immune responses, whose receptors are highly expressed in the OM of mice. We first characterized the presence of the IL-17c and its receptors in the OM. While IL-17c was weakly expressed in the control condition, it was strongly expressed in vivo after intranasal administration of polyinosinic-polycytidylic (Poly I:C), a Toll Like Receptor 3 agonist, mimicking a viral infection. Using calcium imaging and electrophysiological recordings, we found that IL-17c can effectively activate OM cells through the release of ATP. In the longer term, intranasal chronic instillations of IL-17c increased the cellular dynamics of the epithelium and promoted immune cells infiltrations. Finally, IL-17c decreased cell death induced by Poly(I:C) in an OM primary culture. The OM is thus a tissue highly responsive to immune mediators, proving its central role as a barrier against airway pathogens.

Reversible deafferentation of the zebrafish olfactory bulb with wax plug insertion

BACKGROUND

Deafferentation of the zebrafish olfactory bulb allows investigation of neuroplasticity in a particularly dynamic brain region of a popular model animal known for its regenerative abilities. Current methods to remove sensory input to the zebrafish olfactory bulb differ in the extent of deafferentation and potential for recovery.

NEW METHOD

We present a novel method of olfactory bulb deafferentation using continuous wax plug insertions into the nasal cavity of zebrafish. Wax plugs were placed in the nasal cavity and replaced if needed over 1wk or 3wk survival periods. Wax plugs were removed from fish after 1wk of occlusion to analyze the potential recovery of the olfactory organ and bulb.

RESULTS

Wax plug insertions caused a dramatic reduction in olfactory organ size and structure and significantly reduced afferent input to the olfactory bulb after 1wk and 3wk. Removal of the wax plugs after 1wk allowed for recovery of the olfactory organ and subsequent reinnervation of the olfactory bulb.

COMPARISONS WITH EXISTING METHODS

Chemical ablation with detergent causes partial, temporary deafferentation of the olfactory bulb. Cautery ablation causes complete, permanent deafferentation of the olfactory bulb. Wax plug insertions cause nearly complete, temporary deafferentation, allowing both significant deafferentation and the potential for reinnervation of the olfactory bulb.

CONCLUSIONS

The wax plug insertion method of deafferentation described here is unique in that it destroys almost completely the structure of the olfactory organ and removes almost completely sensory input to the olfactory bulb, yet the organ returns to its typical morphology and afferent innervation returns.

Brain-peripheral cell crosstalk in white matter damage and repair

White matter damage is an important part of cerebrovascular disease and may be a significant contributing factor in vascular mechanisms of cognitive dysfunction and dementia. It is well accepted that white matter homeostasis involves multifactorial interactions between all cells in the axon-glia-vascular unit. But more recently, it has been proposed that beyond cell-cell signaling within the brain per se, dynamic crosstalk between brain and systemic responses such as circulating immune cells and stem/progenitor cells may also be important. In this review, we explore the hypothesis that peripheral cells contribute to damage and repair after white matter damage. Depending on timing, phenotype and context, monocyte/macrophage can possess both detrimental and beneficial effects on oligodendrogenesis and white matter remodeling. Endothelial progenitor cells (EPCs) can be activated after CNS injury and the response may also influence white matter repair process. These emerging findings support the hypothesis that peripheral-derived cells can be both detrimental or beneficial in white matter pathology in cerebrovascular disease. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia, edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Hepatoma-Derived Growth Factor: Its Possible Involvement in the Progression of Hepatocellular Carcinoma

The development of hepatocellular carcinoma (HCC) is an important complication of viral infection induced by hepatitis virus C, and our major research theme is to identify a new growth factor related to the progression of HCC. HDGF (hepatoma-derived growth factor) is a novel growth factor that belongs to a new gene family. HDGF was initially purified from the conditioned medium of a hepatoma cell line. HDGF promotes cellular proliferation as a DNA binding nuclear factor and a secreted protein acting via a receptor-mediated pathway. HDGF is a unique multi-functional protein that can function as a growth factor, angiogenic factor and anti-apoptotic factor and it participates in the development and progression of various malignant diseases. The expression level of HDGF may be an independent prognostic factor for predicting the disease-free and overall survival in patients with various malignancies, including HCC. Furthermore, the overexpression of HDGF promotes the proliferation of HCC cells, while a reduction in the HDGF expression inhibits the proliferation of HCC cells. This article provides an overview of the characteristics of HDGF and describes the potential role of HDGF as a growth-promoting factor for HCC.

The influences of age on olfaction: a review

Decreased olfactory function is very common in the older population, being present in over half of those between the ages of 65 and 80 years and in over three quarters of those over the age of 80 years. Such dysfunction significantly influences physical well-being and quality of life, nutrition, the enjoyment of food, as well as everyday safety. Indeed a disproportionate number of the elderly die in accident gas poisonings each year. As described in this review, multiple factors contribute to such age-related loss, including altered nasal engorgement, increased propensity for nasal disease, cumulative damage to the olfactory epithelium from viral and other environmental insults, decrements in mucosal metabolizing enzymes, ossification of cribriform plate foramina, loss of selectivity of receptor cells to odorants, changes in neurotransmitter and neuromodulator systems, and neuronal expression of aberrant proteins associated with neurodegenerative disease. It is now well established that decreased smell loss can be an early sign of such neurodegenerative diseases as Alzheimer's disease and sporadic Parkinson's disease. In this review we provide an overview of the anatomy and physiology of the aging olfactory system, how this system is clinically evaluated, and the multiple pathophysiological factors that are associated with its dysfunction.

Short-term intermittent administration of CXCR4 antagonist AMD3100 facilitates myocardial repair in experimental myocardial infarction

The binding of the stromal cell-derived factor-1α (SDF-1α) to the cysteine (C)-X-C motif chemokine receptor 4 (CXCR4) has emerged as a key signal for stem and progenitor cells trafficking to the circulation from the bone marrow. Our aim was to investigate the role of daily intermittent administration of AMD3100 (a specific reversible CXCR4 receptor antagonist) during the healing process after myocardial infarction (MI). Wistar rats were subjected to MI and AMD3100 was injected intraperitoneally after surgery. SDF-1α mRNA expression was measured by real-time polymerase chain reaction. Histology changes were analyzed with immunofluorescence, Masson's trichrome staining, and wheat germ agglutinin. The number of leukocytes in peripheral blood was measured by complete blood cell count analysis. The activities of matrix metalloproteinase-2/9 (MMP-2/9) were determined by gelatin zymography. The expression level of SDF-1α mRNA in the infarcted tissue was enhanced rapidly (6 h), peaked at 24 h, and then declined to the normal level at 7 days post-MI. AMD3100 further enhanced the increase of SDF-1α in infarct area. Increased leukocytes were observed in AMD3100-treated groups. The mobilization of c-kit(+) stem/progenitor cells and enhanced neovascularization were augmented by AMD3100. Additionally, AMD3100 improved ventricular remodeling, which was revealed by the decrease of infarct size, viable cardiomyocyte cross-sectional area and left ventricle (LV) expansion index, and the increase of LV free wall thickness. The activities of MMP-2/9 were up-regulated by AMD3100. In conclusion, short-term intermittent administration of AMD3100 could accelerate the wound healing process in experimental MI and be a potential therapy for the treatment of MI.

CX₃CR1 deficiency exacerbates neuronal loss and impairs early regenerative responses in the target-ablated olfactory epithelium

The olfactory epithelium is a site of sustained adult neurogenesis where olfactory sensory neurons are continuously replaced from endogenous stem/progenitor cells. Epithelial macrophages have been implicated in the phagocytosis of degenerating cells but the molecular mechanisms allowing for their recruitment and activation while maintaining a neurogenic microenvironment are poorly understood. We have previously shown that the chemokine fractalkine (CX₃CL1) is expressed by olfactory sensory neurons and ensheathing cells in the olfactory epithelium. In turn, the fractalkine receptor, CX₃CR1, is expressed on macrophages and dendritic cells within the olfactory epithelium. We report that a selective cell death of olfactory sensory neurons in the epithelium of CX₃CR1-deficient mice via target ablation (i.e. olfactory bulbectomy) results in an exacerbated loss of olfactory sensory neurons compared to wild-type mice. In addition, reduced proliferation of intraepithelial stem/progenitor cells was observed in lesioned CX₃CR1-deficient mice, suggesting an impaired regenerative response. Importantly, a lack of CX₃CL1-signaling caused increased recruitment of macrophages into the olfactory epithelium, which in turn contained higher levels of pro-inflammatory cytokines (e.g. TNF-α and IL-6) as determined by qPCR. We also present novel data showing that, relative to wild-type, CX₃CR1-deficient macrophages have diminished phagocytic activity following stimulation with CX₃CL1. Collectively, our data indicate that signaling through the CX₃CR1 receptor modulates macrophage activity, resulting in an environment conducive to olfactory sensory neuron clearance and targeted replacement from endogenous stem/progenitor cells.

Bone marrow chimeric mice reveal a role for CX₃CR1 in maintenance of the monocyte-derived cell population in the olfactory neuroepithelium

Macrophages in the olfactory neuroepithelium are thought to play major roles in tissue homeostasis and repair. However, little information is available at present about possible heterogeneity of these monocyte-derived cells, their turnover rates, and the role of chemokine receptors in this process. To start addressing these issues, this study used Cx₃cr1(gfp) mice, in which the gene sequence for eGFP was knocked into the CX₃CR1 gene locus in the mutant allele. Using neuroepithelial whole-mounts from Cx₃cr1(gfp/+) mice, we show that eGFP(+) cells of monocytic origin are distributed in a loose network throughout this tissue and can be subdivided further into two immunophenotypically distinct subsets based on MHC-II glycoprotein expression. BM chimeric mice were created using Cx₃cr1(gfp/+) donors to investigate turnover of macrophages (and other monocyte-derived cells) in the olfactory neuroepithelium. Our data indicate that the monocyte-derived cell population in the olfactory neuroepithelium is actively replenished by circulating monocytes and under the experimental conditions, completely turned over within 6 months. Transplantation of Cx₃cr1(gfp/gfp) (i.e., CX₃CR1-deficient) BM partially impaired the replenishment process and resulted in an overall decline of the total monocyte-derived cell number in the olfactory epithelium. Interestingly, replenishment of the CD68(low)MHC-II(+) subset appeared minimally affected by CX₃CR1 deficiency. Taken together, the established baseline data about heterogeneity of monocyte-derived cells, their replenishment rates, and the role of CX₃CR1 provide a solid basis to further examine the importance of different monocyte subsets for neuroregeneration at this unique frontier with the external environment.

Autoimmune pathology accounts for common manifestations in a wide range of neuro-psychiatric disorders: the olfactory and immune system interrelationship

Smell has traditionally been considered a less important sense when compared to sight or hearing, but recent research has unraveled important features inherent to the sense of smell. Once considered just a chemical sensor for sampling the environment, data from animal models and human studies currently imply numerous and complex effects of smell on behavior, mood, and on the immune response. In this review we discuss a possible inter-relationship between olfactory impairment, autoimmunity and neurological/psychiatric symptoms in several diseases affecting the central nervous system (CNS) such as Parkinson, Alzheimer's disease, autism, schizophrenia, multiple sclerosis and neuropsychiatric lupus erythematosus. We suggest that common manifestations are not mere coincidences. Current data from animal models show that neuropsychiatric manifestations are intimately associated with smell impairment, and autoimmune dysregulation, via autoantibodies (anti-NMDAR, anti-ribosomal P) or other mechanisms. From clues of pathological manifestations, we propose a novel approach to the understanding of the interactions between the CNS, the smell and the immune system.