Multipotency of purified, transplanted globose basal cells in olfactory epithelium

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.

HB-EGF promotes progenitor cell proliferation and sensory neuron regeneration in the zebrafish olfactory epithelium

Maintenance and regeneration of the zebrafish olfactory epithelium (OE) are supported by two distinct progenitor cell populations that occupy spatially discrete stem cell niches and respond to different tissue conditions. Globose basal cells (GBCs) reside at the inner and peripheral margins of the sensory OE and are constitutively active to replace sporadically dying olfactory sensory neurons (OSNs). In contrast, horizontal basal cells (HBCs) are uniformly distributed across the sensory tissue and are selectively activated by acute injury conditions. Here we show that expression of the heparin-binding epidermal growth factor-like growth factor (HB-EGF) is strongly and transiently upregulated in response to OE injury and signals through the EGF receptor (EGFR), which is expressed by HBCs. Exogenous stimulation of the OE with recombinant HB-EGF promotes HBC expansion and OSN neurogenesis in a pattern that resembles the tissue response to injury. In contrast, pharmacological inhibition of HB-EGF membrane shedding, HB-EGF availability, and EGFR signaling strongly attenuate or delay injury-induced HBC activity and OSN restoration without affecting maintenance neurogenesis by GBCs. Thus, HB-EGF/EGFR signaling appears to be a critical component of the signaling network that controls HBC activity and, consequently, repair neurogenesis in the zebrafish OE.

Olfactory Loss in Rhinosinusitis: Mechanisms of Loss and Recovery

Chronic rhinosinusitis (CRS) is a highly prevalent disease and up to 83% of CRS patients suffer from olfactory dysfunction (OD). Because OD is specifically seen in those CRS patients that present with a type 2 eosinophilic inflammation, it is believed that type 2 inflammatory mediators at the level of the olfactory epithelium are involved in the development of this olfactory loss. However, due to the difficulties in obtaining tissue from the olfactory epithelium, little is known about the true mechanisms of inflammatory OD. Thanks to the COVID-19 pandemic, interest in olfaction has been growing rapidly and several studies have been focusing on disease mechanisms of OD in inflammatory conditions. In this paper, we summarize the most recent data exploring the pathophysiological mechanisms underlying OD in CRS. We also review what is known about the potential capacity of olfactory recovery of the currently available treatments in those patients.

Multimodal spatiotemporal monitoring of basal stem cell-derived organoids reveals progression of olfactory dysfunction in Alzheimer's disease

Olfactory dysfunction (OD) is a highly prevalent symptom and an early sign of neurodegenerative diseases in humans. However, the roles of peripheral olfactory system in disease progression and the mechanisms behind neurodegeneration remain to be studied. Olfactory epithelium (OE) organoid is an ideal model to study pathophysiology in vitro, yet the reliance on 3D culture condition limits continual in situ monitoring of organoid development. Here, we combined impedance biosensors and live imaging for real-time spatiotemporal analysis of OE organoids morphological and physiological features during Alzheimer's disease (AD) progression. The impedance measurements showed that organoids generated from basal stem cells of APP/PS1 transgenic mice had lower proliferation rate than that from wild-type mice. In concert with the biosensor measurements, live imaging enabled to visualize the spatial and temporal dynamics of organoid morphology. Abnormal protein aggregation and accumulation, including amyloid plaques and neurofibrillary tangles, was found in AD organoids and increased as disease progressed. This multimodal in situ bioelectrical measurement and imaging provide a new platform for investigating onset mechanisms of OD, which would shed new light on early diagnosis and treatment of neurodegenerative disease.

Molecular Evidence for Olfactory Neuroblastoma as a Tumor of Malignant Globose Basal Cells

Olfactory neuroblastoma (ONB, esthesioneuroblastoma) is a sinonasal cancer with an underdeveloped diagnostic toolkit, and is the subject of many incidents of tumor misclassification throughout the literature. Despite its name, connections between the cancer and normal cells of the olfactory epithelium have not been systematically explored and markers of olfactory epithelial cell types are not deployed in clinical practice. Here, we utilize an integrated human-mouse single-cell atlas of the nasal mucosa, including the olfactory epithelium, to identify transcriptomic programs that link ONB to a specific population of stem/progenitor cells known as olfactory epithelial globose basal cells (GBCs). Expression of a GBC transcription factor NEUROD1 distinguishes both low- and high-grade ONB from sinonasal undifferentiated carcinoma, a potential histologic mimic with a distinctly unfavorable prognosis. Furthermore, we identify a reproducible subpopulation of highly proliferative ONB cells expressing the GBC stemness marker EZH2, suggesting that EZH2 inhibition may play a role in the targeted treatment of ONB. Finally, we study the cellular states comprising ONB parenchyma using single-cell transcriptomics and identify evidence of a conserved GBC transcriptional regulatory circuit that governs divergent neuronal-versus-sustentacular differentiation. These results link ONB to a specific cell type for the first time and identify conserved developmental pathways within ONB that inform diagnostic, prognostic, and mechanistic investigation.

Pseudotime analysis reveals novel regulatory factors for multigenic onset and monogenic transition of odorant receptor expression

During their maturation from horizontal basal stem cells, olfactory sensory neurons (OSNs) are known to select exactly one out of hundreds of olfactory receptors (ORs) and express it on their surface, a process called monogenic selection. Monogenic expression is preceded by a multigenic phase during which several OR genes are expressed in a single OSN. Here, we perform pseudotime analysis of a single cell RNA-Seq dataset of murine olfactory epithelium to precisely align the multigenic and monogenic expression phases with the cell types occurring during OSN differentiation. In combination with motif analysis of OR gene cluster-associated enhancer regions, we identify known and novel transcription (co-)factors (Ebf1, Lhx2, Ldb1, Fos and Ssbp2) and chromatin remodelers (Kdm1a, Eed and Zmynd8) associated with OR expression. The inferred temporal order of their activity suggests novel mechanisms contributing to multigenic OR expression and monogenic selection.

Revealing the mystery of persistent smell loss in Long COVID patients

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

Restoration of olfactory dysfunctions by nanomaterials and stem cells-based therapies: Current status and future perspectives

Dysfunction in the olfactory system of a person can have adverse effects on their health and quality of life. It can even increase mortality among individuals. Olfactory dysfunction is related to many factors, including post-viral upper respiratory infection, head trauma, and neurodegenerative disorders. Although some clinical therapies such as steroids and olfactory training are already available, their effectiveness is limited and controversial. Recent research in the field of therapeutic nanoparticles and stem cells has shown the regeneration of dysfunctional olfactory systems. Thus, we are motivated to highlight these regenerative approaches. For this, we first introduce the anatomical characteristics of the olfactory pathway, then detail various pathological factors related to olfactory dysfunctions and current treatments, and then finally discuss the recent regenerative endeavors, with particular focus on nanoparticle-based drug delivery systems and stem cells. This review offers insights into the development of future therapeutic approaches to restore and regenerate dysfunctional olfactory systems.

Modification of the Peripheral Olfactory System by Electronic Cigarettes

Electronic cigarettes (e-cigs) are used by millions of adolescents and adults worldwide. Commercial e-liquids typically contain flavorants, propylene glycol, and vegetable glycerin with or without nicotine. These chemical constituents are detected and evaluated by chemosensory systems to guide and modulate vaping behavior and product choices of e-cig users. The flavorants in e-liquids are marketing tools. They evoke sensory percepts of appealing flavors through activation of chemical sensory systems to promote the initiation and sustained use of e-cigs. The vast majority of flavorants in e-liquids are volatile odorants, and as such, the olfactory system plays a dominant role in perceiving these molecules that enter the nasal cavity either orthonasally or retronasally during vaping. In addition to flavorants, e-cig aerosol contains a variety of by-products generated through heating the e-liquids, including odorous irritants, toxicants, and heavy metals. These harmful substances can directly and adversely impact the main olfactory epithelium (MOE). In this article, we first discuss the olfactory contribution to e-cig flavor perception. We then provide information on MOE cell types and their major functions in olfaction and epithelial maintenance. Olfactory detection of flavorants, nicotine, and odorous irritants and toxicants are also discussed. Finally, we discuss the cumulated data on modification of the MOE by flavorant exposure and toxicological impacts of formaldehyde, acrolein, and heavy metals. Together, the information presented in this overview may provide insight into how e-cig exposure may modify the olfactory system and adversely impact human health through the alteration of the chemosensory factor driving e-cig use behavior and product selections. © 2021 American Physiological Society. Compr Physiol 11:2621-2644, 2021.

Analyzing Olfactory Neuron Precursors Non-Invasively Isolated through NADH FLIM as a Potential Tool to Study Oxidative Stress in Alzheimer's Disease

Among all the proposed pathogenic mechanisms to understand the etiology of Alzheimer’s disease (AD), increased oxidative stress seems to be a robust and early disease feature where many of those hypotheses converge. However, despite the significant lines of evidence accumulated, an effective diagnosis and treatment of AD are not yet available. This limitation might be partially explained by the use of cellular and animal models that recapitulate partial aspects of the disease and do not account for the particular biology of patients. As such, cultures of patient-derived cells of peripheral origin may provide a convenient solution for this problem. Peripheral cells of neuronal lineage such as olfactory neuronal precursors (ONPs) can be easily cultured through non-invasive isolation, reproducing AD-related oxidative stress. Interestingly, the autofluorescence of key metabolic cofactors such as reduced nicotinamide adenine dinucleotide (NADH) can be highly correlated with the oxidative state and antioxidant capacity of cells in a non-destructive and label-free manner. In particular, imaging NADH through fluorescence lifetime imaging microscopy (FLIM) has greatly improved the sensitivity in detecting oxidative shifts with minimal intervention to cell physiology. Here, we discuss the translational potential of analyzing patient-derived ONPs non-invasively isolated through NADH FLIM to reveal AD-related oxidative stress. We believe this approach may potentially accelerate the discovery of effective antioxidant therapies and contribute to early diagnosis and personalized monitoring of this devastating disease.

Transient Effects of Cyclophosphamide on Basal Cell Proliferation of Olfactory Epithelia

Cancer is often treated with broad-spectrum cytotoxic drugs that not only eradicate cancerous cells but also have detrimental side effects. One of these side effects, disruption of the olfactory system, impedes a patient's ability to smell, perceive flavor, and ultimately may interfere with their nutritional intake and recovery from cancer. Recent studies reported that the chemotherapy drug, cyclophosphamide (CYP), can damage gustatory epithelia and disrupt cell proliferation in olfactory epithelia. In this study, we asked if CYP altered globose and horizontal basal cell proliferation in the murine main olfactory epithelium (MOE) and vomeronasal organ (VNO). We used antibodies for Ki67, a marker strictly associated with cell proliferation, and Keratin 5, a marker for the cytoskeleton of horizontal basal cells. Our results revealed a significant CYP-induced decrease in the number of proliferative cells in both epithelia, especially globose basal cells in the MOE, within the first 1-2 days postinjection. Recovery of cell renewal was apparent 6 days after injection. The immunohistochemical markers showed significantly higher levels of globose and horizontal basal cell proliferation in CYP-injected mice at 14 and 30 days postinjection compared with control mice. The prolonged proliferative activation of globose and horizontal basal cells suggests that, besides altering proliferation of olfactory epithelia, the epithelial substrate needed for successful cell renewal may be adversely affected by CYP.

Renewal and Differentiation of GCD Necklace Olfactory Sensory Neurons

Both canonical olfactory sensory neurons (OSNs) and sensory neurons belonging to the guanylate cyclase D (GCD) "necklace" subsystem are housed in the main olfactory epithelium, which is continuously bombarded by toxins, pathogens, and debris from the outside world. Canonical OSNs address this challenge, in part, by undergoing renewal through neurogenesis; however, it is not clear whether GCD OSNs also continuously regenerate and, if so, whether newborn GCD precursors follow a similar developmental trajectory to that taken by canonical OSNs. Here, we demonstrate that GCD OSNs are born throughout adulthood and can persist in the epithelium for several months. Phosphodiesterase 2A is upregulated early in the differentiation process, followed by the sequential downregulation of β-tubulin and the upregulation of CART protein. The GCD and MS4A receptors that confer sensory responses upon GCD neurons are initially expressed midway through this process but become most highly expressed once CART levels are maximal late in GCD OSN development. GCD OSN maturation is accompanied by a horizontal migration of neurons toward the central, curved portions of the cul-de-sac regions where necklace cells are concentrated. These findings demonstrate that-like their canonical counterparts-GCD OSNs undergo continuous renewal and define a GCD-specific developmental trajectory linking neurogenesis, maturation, and migration.

Neuroimmune regulatory networks of the airway mucosa in allergic inflammatory disease

Communication between the nervous and immune systems serves a key role in host‐protective immunity at mucosal barrier sites including the respiratory tract. In these tissues, neuroimmune interactions operate in bidirectional circuits that can sense and respond to mechanical, chemical, and biologic stimuli. Allergen‐ or helminth‐induced products can produce airway inflammation by direct action on nociceptive afferents and adjacent tissues. The activity of nociceptive afferents can regulate innate and adaptive immune responses via neuropeptides and neurotransmitter signaling. This review will summarize recent work investigating the role of neuropeptides CGRP, VIP, neuromedins, substance P, and neurotransmitters dopamine and the B2‐adrenoceptor agonists epinepherine/norepinepherine, each of which influence type 2 immunity by instructing mast cell, innate lymphoid cell type 2, dendritic cell, and T cell responses, both in the airway and the draining lymph node. Afferents in the airway also contain receptors for alarmins and cytokines, allowing their activity to be modulated by immune cell secreted products, particularly those secreted by mast cells. Taken together, we propose that further investigation of how immunoregulatory neuropeptides shape respiratory inflammation in experimental systems may reveal novel therapeutic targets for addressing the increasing prevalence of chronic airway disease in humans.

Expansion of murine and human olfactory epithelium/mucosa colonies and generation of mature olfactory sensory neurons under chemically defined conditions

Olfactory dysfunctions, including hyposmia and anosmia, affect ~100 million people around the world and the underlying causes are not fully understood. Degeneration of olfactory sensory neurons and incapacity of globose basal cells to generate olfactory sensory neurons are found in elder people and patients with smell disorders. Thus, olfactory stem cell may function as a promising tool to replace inactivated globose basal cells and to generate sensory neurons. Methods: We established clonal expansion of cells from the murine olfactory epithelium as well as colony growth from human olfactory mucosa using Matrigel-based three-dimensional system. These colonies were characterized by immunostaining against olfactory epithelium cellular markers and by calcium imaging of responses to odors. Chemical addition was optimized to promote Lgr5 expression, colony growth and sensory neuron generation, tested by quantitative PCR and immunostaining against progenitor and neuronal markers. The differential transcriptomes in multiple signaling pathways between colonies under different base media and chemical cocktails were determined by RNA-Seq. Results: In defined culture media, we found that VPA and CHIR99021 induced the highest Lgr5 expression level, while LY411575 resulted in the most abundant yield of OMP⁺ mature sensory neurons in murine colonies. Different base culture media with drug cocktails led to apparent morphological alteration from filled to cystic appearance, accompanied with massive transcriptional changes in multiple signaling pathways. Generation of sensory neurons in human colonies was affected through TGF-β signaling, while Lgr5 expression and cell proliferation was regulated by VPA. Conclusion: Our findings suggest that targeting expansion of olfactory epithelium/mucosa colonies in vitro potentially results in discovery of new source to cell replacement-based therapy against smell loss.

Age-Dependent Activation and Neuronal Differentiation of Lgr5+ Basal Cells in Injured Olfactory Epithelium via Notch Signaling Pathway

Aging is an important factor affecting function of smell, leading to the degeneration of mature olfactory sensory neurons and inducing the occurrence of smell loss. The mammalian olfactory epithelium (OE) can regenerate when subjected to chemical assaults. However, this capacity is not limitless. Inactivation of globose basal cells and failure to generate sensory neurons are the main obstacles to prevent the OE regeneration. Here, we found the significant attenuation in mature sensory neuronal generation and apparent transcriptional alternation in the OE from aged mice compared with young ones. The recruitment of leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5)-positive cells in injured OE was weakened in aged mice, and more Lgr5+ cells remained quiescence in aged OE postinjury. Lineage-traced progenies from Lgr5+ cells were significantly fewer in the OE with aging. Moreover, Notch activation enhanced the neuronal regeneration in aged OE, making the regenerative capacity of aged OE comparable with that of young animals after injury. The growth and morphology of three-dimensional (3D)-cultured organoids from the OE of young and aged mice varied and was modulated by small molecules regulating the Notch signaling pathway. Thus, we concluded that activation of Lgr5+ cells in injured OE was age dependent and Notch activation could enhance the capacity of neuronal generation from Lgr5+ cells in aged OE after injury.

Maturation of the Olfactory Sensory Neuron and Its Cilia

Olfactory sensory neurons (OSNs) are bipolar neurons, unusual because they turn over continuously and have a multiciliated dendrite. The extensive changes in gene expression accompanying OSN differentiation in mice are largely known, especially the transcriptional regulators responsible for altering gene expression, revealing much about how differentiation proceeds. Basal progenitor cells of the olfactory epithelium transition into nascent OSNs marked by Cxcr4 expression and the initial extension of basal and apical neurites. Nascent OSNs become immature OSNs within 24-48 h. Immature OSN differentiation requires about a week and at least 2 stages. Early-stage immature OSNs initiate expression of genes encoding key transcriptional regulators and structural proteins necessary for further neuritogenesis. Late-stage immature OSNs begin expressing genes encoding proteins important for energy production and neuronal homeostasis that carry over into mature OSNs. The transition to maturity depends on massive expression of one allele of one odorant receptor gene, and this results in expression of the last 8% of genes expressed by mature OSNs. Many of these genes encode proteins necessary for mature function of axons and synapses or for completing the elaboration of non-motile cilia, which began extending from the newly formed dendritic knobs of immature OSNs. The cilia from adjoining OSNs form a meshwork in the olfactory mucus and are the site of olfactory transduction. Immature OSNs also have a primary cilium, but its role is unknown, unlike the critical role in proliferation and differentiation played by the primary cilium of the olfactory epithelium's horizontal basal cell.

Preparation of Adhesion Culture of Neural Stem/Progenitor Cells of the Olfactory Mucosa for the Treatment of Spinal Cord Injuries

In this work, an optimal protocol was developed for obtaining adhesion culture of neural stem/progenitor cells (NSPC) of rat olfactory mucosa. During the development of the protocol, the conditions for cell culturing on adhesion substrates fibronectin and laminin in DMEM/F-12 and neurobasal media with the same culture additives were compared. Cell proliferation was maximum during culturing on both substrates in the neurobasal medium. Using the immunofluorescence method, we found that culturing on fibronectin in the neurobasal medium ensured maximum (52.22%) content of nestin-positive cells in comparison with other culturing conditions. The highest percentage of βIII-tubulin-positive cells was detected in cultures growing on fibronectin in the neurobasal medium and in DMEM/F-12 (79.11 and 83.52%, respectively). Culturing in adhesion cultures in the neurobasal medium on fibronectin allowed obtaining cultures enriched with NSPC and neurons differentiating from them in a quantity sufficient for further transplantation. The developed protocol can be recommended for obtaining NPSC from human olfactory mucosa for the treatment of spinal cord injuries.

Inhibition of focal adhesion kinase increases adult olfactory stem cell self-renewal and neuroregeneration through ciliary neurotrophic factor

Constant neuroregeneration in adult olfactory epithelium maintains olfactory function by basal stem cell proliferation and differentiation to replace lost olfactory sensory neurons (OSNs). Understanding the mechanisms regulating this process could reveal potential therapeutic targets for stimulating adult olfactory neurogenesis under pathological conditions and aging. Ciliary neurotrophic factor (CNTF) in astrocytes promotes forebrain neurogenesis but its function in the olfactory system is unknown. Here, we show in mouse olfactory epithelium that CNTF is expressed in horizontal basal cells, olfactory ensheathing cells (OECs) and a small subpopulation of OSNs. CNTF receptor alpha was expressed in Mash1-positive globose basal cells (GBCs) and OECs. Thus, CNTF may affect GBCs in a paracrine manner. CNTF−/− mice did not display altered GBC proliferation or olfactory function, suggesting that CNTF is not involved in basal olfactory renewal or that they developed compensatory mechanisms. Therefore, we tested the effect of increased CNTF in wild type mice. Intranasal instillation of a focal adhesion kinase (FAK) inhibitor, FAK14, upregulated CNTF expression. FAK14 also promoted GBC proliferation, neuronal differentiation and basal stem cell self-renewal but had no effective in CNTF−/− mice, suggesting that FAK inhibition promotes olfactory neuroregeneration through CNTF, making them potential targets to treat sensorineural anosmia due to OSN loss.

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.

The Phenomenon of Compensatory Cell Proliferation in Olfactory Epithelium in Fish Caused by Prolonged Exposure to Natural Odorants

It was previously shown that activation of the processes of neurogenesis in the olfactory epithelium (OE) can be caused after intranasal administration of toxic or neurotrophic factors, after axon transection, or as a result of bulbectomy. Our study showed for the first time that a significant increase in olfactory cell renewal can also occur in animals due to periodic chemostimulation with natural odorants (amino acids and peptides) for 15 days. Using electron and laser confocal microscopy in fish (Paracottus knerii (Cottidae), Dybowski, 1874) from Lake Baikal, we showed that periodic stimulation of aquatic organisms with a water-soluble mixture of amino acids and peptides causes stress in OE, which leads to programmed death cells and compensatory intensification of their renewal. We estimated the level of reactive oxygen species, number of functionally active mitochondria, intensity of apoptosis processes, and mitosis activity of cells in the OE of fish in the control group and after periodic natural odorants exposure. This study showed that new stem cells are activated during enhanced odor stimulation and subsequent degenerative changes in the cells of the sensory apparatus. Those new activated stem cells are located in previously proliferatively inactive regions of OE that become involved in compensatory processes for the formation of new cells.

Cranial Nerve Stimulation for Olfaction (Cranial Nerve 1)

Like sensory maps in other systems, the sense of smell has an organizational structure based on converging projections of olfactory receptor neurons containing unique odorant receptors onto the olfactory bulb in synaptic aggregations termed glomeruli. This organizational structure provides the potential for electrical stimulation and restoration of smell. Prior animal and human studies support the feasibility of an olfactory stimulation device, encouraging ongoing work in development of olfactory implants.

Olfactory Loss and Dysfunction in Ciliopathies: Molecular Mechanisms and Potential Therapies

BACKGROUND

Ciliopathies are a class of inherited pleiotropic genetic disorders in which alterations in cilia assembly, maintenance, and/or function exhibit penetrance in the multiple organ systems. Olfactory dysfunction is one such clinical manifestation that has been shown in both patients and model organisms. Existing therapies for ciliopathies are limited to the treatment or management of symptoms. The last decade has seen an increase in potential curative therapeutic options including small molecules and biologics. Recent work in multiciliated olfactory sensory neurons has demonstrated the capacity of targeted gene therapy to restore ciliation in terminally differentiated cells and rescue olfactory function. This review will discuss the current understanding of the penetrance of ciliopathies in the olfactory system. Importantly, it will highlight both pharmacological and biological approaches, and their potential therapeutic value in the olfactory system and other ciliated tissues.

METHODS

We undertook a structured and comprehensive search of peer-reviewed research literature encompassing in vitro, in vivo, model organism, and clinical studies. From these publications, we describe the olfactory system, and discuss the penetrance of ciliopathies and impact of cilia loss on olfactory function. In addition, we outlined the developing therapies for ciliopathies across different organ and cell culture systems, and discussed their potential therapeutic application to the mammalian olfactory system.

RESULTS

One-hundred sixty-one manuscripts were included in the review, centering on the understanding of olfactory penetrance of ciliopathies, and discussing the potential therapeutic options for ciliopathies in the context of the mammalian olfactory system. Forty-four manuscripts were used to generate a table listing the known congenital causes of olfactory dysfunction, with the first ten listed are linked to ciliopathies. Twenty-three manuscripts were used to outline the potential of small molecules for the olfactory system. Emphasis was placed on HDAC6 inhibitors and lithium, both of which were shown to stabilize microtubule structures, contributing to ciliogenesis and cilia lengthening. Seventy-five manuscripts were used to describe gene therapy and gene therapeutic strategies. Included were the implementation of adenoviral, adeno-associated virus (AAV), and lentiviral vectors to treat ciliopathies across different organ systems and application toward the olfactory system. Thus far, adenoviral and AAVmeditated ciliary restoration demonstrated successful proof-of-principle preclinical studies. In addition, gene editing, ex vivo gene therapy, and transplantation could serve as alternative therapeutic and long-term approaches. But for all approaches, additional assessment of vector immunogenicity, specificity, and efficacy need further investigation. Currently, ciliopathy treatments are limited to symptomatic management with no curative options. However, the accessibility and amenability of the olfactory system to treatment would facilitate development and advancement of a viable therapy.

CONCLUSION

The findings of this review highlight the contribution of ciliopathies to a growing list of congenial olfactory dysfunctions. Promising results from other organ systems imply the feasibility of biologics, with results from gene therapies proving to be a viable therapeutic option for ciliopathies and olfactory dysfunction.

Zebrafish as a translational regeneration model to study the activation of neural stem cells and role of their environment

The review is an overview of the current knowledge of neuronal regeneration properties in mammals and fish. The ability to regenerate the damaged parts of the nervous tissue has been demonstrated in all vertebrates. Notably, fish and amphibians have the highest capacity for neurogenesis, whereas reptiles and birds are able to only regenerate specific regions of the brain, while mammals have reduced capacity for neurogenesis. Zebrafish (Danio rerio) is a promising model of study because lesions in the brain or complete cross-section of the spinal cord are followed by an effective neuro-regeneration that successfully restores the motor function. In the brain and the spinal cord of zebrafish, stem cell activity is always able to re-activate the molecular programs required for central nervous system regeneration. In mammals, traumatic brain injuries are followed by reduced neurogenesis and poor axonal regeneration, often insufficient to functionally restore the nervous tissue, while spinal injuries are not repaired at all. The environment that surrounds the stem cell niche constituted by connective tissue and stimulating factors, including pro-inflammation molecules, seems to be a determinant in triggering stem cell proliferation and/or the trans-differentiation of connective elements (mainly fibroblasts). Investigating and comparing the neuronal regeneration in zebrafish and mammals may lead to a better understanding of the mechanisms behind neurogenesis, and the failure of the regenerative response in mammals, first of all, the role of inflammation, considered the main inhibitor of the neuronal regeneration.

Cell-Based Therapy Restores Olfactory Function in an Inducible Model of Hyposmia

Stem cell-based therapies have been proposed as a strategy to replace damaged tissues, especially in the nervous system. A primary sensory modality, olfaction, is impaired in 12% of the US population, but lacks treatment options. We report here the development of a novel mouse model of inducible hyposmia and demonstrate that purified tissue-specific stem cells delivered intranasally engraft to produce olfactory neurons, achieving recovery of function. Adult mice were rendered hyposmic by conditional deletion of the ciliopathy-related IFT88 gene in the olfactory sensory neuron lineage and following experimentally induced olfactory injury, received either vehicle or stem cell infusion intranasally. Engraftment-derived olfactory neurons were identified histologically, and functional improvements were measured via electrophysiology and behavioral assay. We further explored mechanisms in culture that promote expansion of engraftment-competent adult olfactory basal progenitor cells. These findings provide a basis for translational research on propagating adult tissue-specific sensory progenitor cells and testing their therapeutic potential.

•

A novel mouse model of inducible olfactory loss was used to test stem cell therapy

•

Purified adult tissue-specific stem cells can engraft and restore olfaction

•

Culture expansion of engraftment-competent stem cells was examined via RNA-seq

Activating a Reserve Neural Stem Cell Population In Vitro Enables Engraftment and Multipotency after Transplantation

The olfactory epithelium (OE) regenerates after injury via two types of tissue stem cells: active globose cells (GBCs) and dormant horizontal basal cells (HBCs). HBCs are roused to activated status by OE injury when P63 levels fall. However, an in-depth understanding of activation requires a system for culturing them that maintains both their self-renewal and multipotency while preventing spontaneous differentiation. Here, we demonstrate that mouse, rat, and human HBCs can be cultured and passaged as P63+ multipotent cells. HBCs in vitro closely resemble HBCs in vivo based on immunocytochemical and transcriptomic comparisons. Genetic lineage analysis demonstrates that HBCs in culture arise from both tissue-derived HBCs and multipotent GBCs. Treatment with retinoic acid induces neuronal and non-neuronal differentiation and primes cultured HBCs for transplantation into the lesioned OE. Engrafted HBCs generate all OE cell types, including olfactory sensory neurons, confirming that HBC multipotency and neurocompetency are maintained in culture.

Spatial Determination of Neuronal Diversification in the Olfactory Epithelium

Neurons in the murine olfactory epithelium (OE) differ by the olfactory receptor they express as well as other molecular phenotypes that are regionally restricted. These patterns can be precisely regenerated following epithelial injury, suggesting that spatial cues within the tissue can direct neuronal diversification. Nonetheless, the permanency and mechanism of this spatial patterning remain subject to debate. Via transplantation of stem and progenitor cells from dorsal OE into ventral OE, we demonstrate that, in mice of both sexes, nonautonomous spatial cues can direct the spatially circumscribed differentiation of olfactory sensory neurons. The vast majority of dorsal transplant-derived neurons express the ventral marker OCAM (NCAM2) and lose expression of NQO1 to match their new location. Single-cell analysis also demonstrates that OSNs adopt a fate defined by their new position following progenitor cell transplant, such that a ventral olfactory receptor is expressed after stem and progenitor cell engraftment. Thus, spatially constrained differentiation of olfactory sensory neurons is plastic, and any bias toward an epigenetic memory of place can be overcome.SIGNIFICANCE STATEMENT Spatially restricted differentiation of olfactory sensory neurons is both key to normal olfactory function and a challenging example of biological specificity. That the stem cells of the olfactory epithelium reproduce the organization of the olfactory periphery to a very close approximation during lesion-induced regeneration begs the question of whether stem cell-autonomous genomic architecture or environmental cues are responsible. The plasticity demonstrated after transfer to a novel location suggests that cues external to the transplanted stem and progenitor cells confer neuronal identity. Thus, a necessary prerequisite is satisfied for using engraftment of olfactory stem and progenitor cells as a cellular therapeutic intervention to reinvigorate neurogenesis whose exhaustion contributes to the waning of olfaction with age.

The olfactory mucosa: a potential source of stem cells for hearing regeneration

The olfactory mucosa contains cells that enable it to generate new neurons and other supporting cells throughout life, allowing it to replace cells of the mucosa that have been damaged by exposure to various insults. In this article, we discuss the different types of stem cell found within the olfactory mucosa and their properties. In particular, the mesenchymal-like cells found within the lamina propria will be reviewed in detail. In addition, we discuss potential applications of olfactory-derived stem cells toward hearing regeneration secondary to either inner hair cell loss or primary or secondary auditory nerve degeneration.

The Neuroregenerative Capacity of Olfactory Stem Cells Is Not Limitless: Implications for Aging

The olfactory epithelium (OE) of vertebrates is a highly regenerative neuroepithelium that is maintained under normal conditions by a population of stem and progenitor cells, globose basal cells (GBCs), which also contribute to epithelial reconstitution after injury. However, aging of the OE often leads to neurogenic exhaustion, the disappearance of both GBCs and olfactory sensory neurons (OSNs). Aneuronal tissue may remain as olfactory, with an uninterrupted sheet of apically arrayed microvillar-capped sustentacular cell, or may undergo respiratory metaplasia. We have generated a transgenic mouse model for neurogenic exhaustion using olfactory marker protein-driven Tet-off regulation of the A subunit of Diphtheria toxin such that the death of mature OSNs is accelerated. At as early as 2 months of age, the epithelium of transgenic mice, regardless of sex, recapitulates what is seen in the aged OE of humans and rodents. Areas of the epithelium completely lack neurons and GBCs; whereas the horizontal basal cells, a reserve stem cell population, show no evidence of activation. Surprisingly, other areas that were olfactory undergo respiratory metaplasia. The impact of accelerated neuronal death and reduced innervation on the olfactory bulb (OB) was also examined. Constant neuronal turnover leaves glomeruli shrunken and affects the dopaminergic interneurons in the periglomerular layer. Moreover, the acceleration of OSN death can be reversed in those areas where some GBCs persist. However, the projection onto the OB recovers incompletely and the reinnervated glomeruli are markedly altered. Therefore, the capacity for OE regeneration is tempered when GBCs disappear.SIGNIFICANCE STATEMENT A large percentage of humans lose or suffer a significant decline in olfactory function as they age. Therefore, quality of life suffers and safety and nutritional status are put at risk. With age, the OE apparently becomes incapable of fully maintaining the neuronal population of the epithelium despite its well known capacity for recovering from most forms of injury when younger. Efforts to identify the mechanism by which olfactory neurogenesis becomes exhausted with age require a powerful model for accelerating age-related tissue pathology. The current OMP-tTA;TetO-DTA transgenic mouse model, in which olfactory neurons die when they reach maturity and accelerated death can be aborted to assess the capacity for structural recovery, satisfies that need.

Notch Signaling Regulates Lgr5+ Olfactory Epithelium Progenitor/Stem Cell Turnover and Mediates Recovery of Lesioned Olfactory Epithelium in Mouse Model

The Notch signaling pathway regulates stem cell proliferation and differentiation in multiple tissues and organs, and is required for tissue maintenance. However, the role of Notch in regulation of olfactory epithelium (OE) progenitor/stem cells to maintain tissue function is still not clear. A recent study reported that leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5) is expressed in globose basal cells (GBCs) localized in OE. Through lineage tracing in vivo, we found that Lgr5⁺ cells act as progenitor/stem cells in OE. The generation of daughter cells from Lgr5⁺ progenitor/stem cells is delicately regulated by the Notch signaling pathway, which not only controls the proliferation of Lgr5⁺ cells and their immediate progenies but also affects their subsequent terminal differentiation. In conditionally cultured OE organoids in vitro, inhibition of Notch signaling promotes neuronal differentiation. Besides, OE lesion through methimazole administration in mice induces generation of more Notch1⁺ cells in the horizontal basal cell (HBC) layer, and organoids derived from lesioned OE possesses more proliferative Notch1⁺ HBCs. In summary, we concluded that Notch signaling regulates Lgr5⁺ GBCs by controlling cellular proliferation and differentiation as well as maintaining epithelial cell homeostasis in normal OE. Meanwhile, Notch1 also marks HBCs in lesioned OE and Notch1⁺ HBCs are transiently present in OE after injury. This implies that Notch1⁺ cells in OE may have dual roles, functioning as GBCs in early development of OE and HBCs in restoring the lesioned OE. Stem Cells 2018;36:1259-1272.

Canonical Notch Signaling Directs the Fate of Differentiating Neurocompetent Progenitors in the Mammalian Olfactory Epithelium

The adult olfactory epithelium (OE) has the remarkable capacity to regenerate fully both neurosensory and non-neuronal cell types after severe epithelial injury. Lifelong persistence of two stem cell populations supports OE regeneration when damaged: the horizontal basal cells (HBCs), dormant and held in reserve; and globose basal cells, a heterogeneous population most of which are actively dividing. Both populations regenerate all cell types of the OE after injury, but the mechanisms underlying neuronal versus non-neuronal lineage commitment after recruitment of the stem cell pools remains unknown. We used both retroviral transduction and mouse lines that permit conditional cell-specific genetic manipulation as well as the tracing of progeny to study the role of canonical Notch signaling in the determination of neuronal versus non-neuronal lineages in the regenerating adult OE. Excision of either Notch1 or Notch2 genes alone in HBCs did not alter progenitor fate during recovery from epithelial injury, whereas conditional knock-out of both Notch1 and Notch2 together, retroviral transduction of progenitors with a dominant-negative form of MAML (mastermind-like), or excision of the downstream cofactor RBPJ caused progeny to adopt a neuronal fate exclusively. Conversely, we show that overexpressing the Notch1-intracellular domain (N1ICD) either genetically or by transduction blocks neuronal differentiation completely. However, N1ICD overexpression requires both alleles of the canonical cofactor RBPJ to specify downstream lineage. Together, our results suggest that canonical RBPJ-dependent Notch signaling through redundant Notch1 and Notch2 receptors is both necessary and sufficient for determining neuronal versus non-neuronal differentiation in the regenerating adult OE.SIGNIFICANCE STATEMENT Despite the substantial reconstitution of the olfactory epithelium and its population of sensory neurons after injury, disruption and exhaustion of neurogenesis is a consequence of aging and a cause of olfactory dysfunction. Understanding the mechanisms underlying the generation of replacement neurons and non-neuronal cells is critical to any therapeutic strategy aimed at rebuilding a functional neuroepithelium. The results shown here demonstrate that canonical Notch signaling determines the balance between neurons and non-neuronal cells during restoration of the epithelium after injury. Moreover, the complexities of the multiple Notch pathways impinging on that decision are dissected in detail. Finally, RBPJ, the canonical Notch transcriptional cofactor, exhibits a heretofore unreported haploinsufficiency in setting the balance among the regenerating populations.

Transcriptional and Epigenetic Control of Mammalian Olfactory Epithelium Development

The postnatal mammalian olfactory epithelium (OE) represents a major aspect of the peripheral olfactory system. It is a pseudostratified tissue that originates from the olfactory placode and is composed of diverse cells, some of which are specialized receptor neurons capable of transducing odorant stimuli to afford the perception of smell (olfaction). The OE is known to offer a tractable miniature model for studying the systematic generation of neurons and glia that typify neural tissue development. During OE development, stem/progenitor cells that will become olfactory sensory neurons and/or non-neuronal cell types display fine spatiotemporal expression of neuronal and non-neuronal genes that ensures their proper proliferation, differentiation, survival, and regeneration. Many factors, including transcription and epigenetic factors, have been identified as key regulators of the expression of such requisite genes to permit normal OE morphogenesis. Typically, specific interactive regulatory networks established between transcription and epigenetic factors/cofactors orchestrate histogenesis in the embryonic and adult OE. Hence, investigation of these regulatory networks critical for OE development promises to disclose strategies that may be employed in manipulating the stepwise transition of olfactory precursor cells to become fully differentiated and functional neuronal and non-neuronal cell types. Such strategies potentially offer formidable means of replacing injured or degenerated neural cells as therapeutics for nervous system perturbations. This review recapitulates the developmental cellular diversity of the olfactory neuroepithelium and discusses findings on how the precise and cooperative molecular control by transcriptional and epigenetic machinery is indispensable for OE ontogeny.

The Therapeutic Effects after Transplantation of Whole-Layer Olfactory Mucosa in Rats with Optic Nerve Injury

Background

Existing evidence suggests the potential therapy of transplanting olfactory ensheathing cells (OEC) either alone or in combination with neurotrophic factors or other cell types in optic nerve injury (ONI). However, clinical use of autologous OEC in the acute stages of ONI is not possible. On the other hand, acute application of heterologous transplantation may bring the issue of immune rejection. The olfactory mucosa (OM) with OEC in the lamina propria layer is located in the upper region of the nasal cavity and is easy to dissect under nasal endoscopy, which makes it a candidate as autograft material in acute stages of ONI. To investigate the potential of the OM on the protection of injured neurons and on the promotion of axonal regeneration, we developed a transplantation of syngenic OM in rats with ONI model.

Methods

After the right optic nerve was crushed in Lewis rats, pieces of syngenic whole-layer OM were transplanted into the lesion. Rats undergoing phosphate buffered saline (PBS) injection were used as negative controls (NC). The authors evaluated the regeneration of retinal ganglion cells (RGCs) and axons for 3, 7, 14, and 28 days after transplantation. Obtained retinas and optic nerves were analyzed histologically.

Results

Transplantations of OM significantly promoted the survival of retinal ganglion cells (RGCs) and axonal growth of RGCs compared with PBS alone. Moreover, OM group was associated with higher expression of GAP-43 in comparison with the PBS group. In addition to the potential effects on RGCs, transplantations of OM significantly decreased the expression of GFAP in the retinas, suggesting inhibiting astrocyte activation.

Conclusions

Transplantation of whole-layer OM in rats contributes to the neuronal survival and axon regeneration after ONI.

Injury Induces Endogenous Reprogramming and Dedifferentiation of Neuronal Progenitors to Multipotency

Adult neurogenesis in the olfactory epithelium is often depicted as a unidirectional pathway during homeostasis and repair. We challenge the unidirectionality of this model by showing that epithelial injury unlocks the potential for Ascl1+ progenitors and Neurog1+ specified neuronal precursors to dedifferentiate into multipotent stem/progenitor cells that contribute significantly to tissue regeneration in the murine olfactory epithelium (OE). We characterize these dedifferentiating cells using several lineage-tracing strains and single-cell mRNA-seq, and we show that Sox2 is required for initiating dedifferentiation and that inhibition of Ezh2 promotes multipotent progenitor expansion. These results suggest that the apparent hierarchy of neuronal differentiation is not irreversible and that lineage commitment can be overridden following severe tissue injury. We elucidate a previously unappreciated pathway for endogenous tissue repair by a highly regenerative neuroepithelium and introduce a system to study the mechanisms underlying plasticity in the OE that can be adapted for other tissues.

Dissecting LSD1-Dependent Neuronal Maturation in the Olfactory Epithelium

Neurons in the olfactory epithelium (OE) each express a single dominant olfactory receptor (OR) allele from among roughly 1,000 different OR genes. While monogenic and monoallelic OR expression has been appreciated for over two decades, regulators of this process are still being described; most recently, epigenetic modifiers have been of high interest as silent OR genes are decorated with transcriptionally repressive trimethylated histone 3 lysine 9 (H3K9me3) whereas active OR genes are decorated with transcriptionally activating trimethylated histone 3 lysine 4 (H3K4me3). The lysine specific demethylase 1 (LSD1) demethylates at both of these lysine residues and has been shown to disrupt neuronal maturation and OR expression in the developing embryonic OE. Despite the growing literature on LSD1 expression in the OE, a complete characterization of the timing of LSD1 expression relative to neuronal maturation and of the function of LSD1 in the adult OE have yet to be reported. To fill this gap, the present study determined that LSD1 (1) is expressed in early dividing cells before OR expression and neuronal maturation and decreases at the time of OR stabilization; (2) colocalizes with the repressor CoREST (also known as RCOR1) and histone deacetylase 2 in these early dividing cells; and (3) is required for neuronal maturation during a distinct time window between activating reserve stem cells (horizontal basal cells) and Neurogenin1 (+) immediate neuronal precursors. Thus, this study clarifies the role of LSD1 in olfactory neuronal maturation.

Notch1 maintains dormancy of olfactory horizontal basal cells, a reserve neural stem cell

The remarkable capacity of the adult olfactory epithelium (OE) to regenerate fully both neurosensory and nonneuronal cell types after severe epithelial injury depends on life-long persistence of two stem cell populations: the horizontal basal cells (HBCs), which are quiescent and held in reserve, and mitotically active globose basal cells. It has recently been demonstrated that down-regulation of the ΔN form of the transcription factor p63 is both necessary and sufficient to release HBCs from dormancy. However, the mechanisms by which p63 is down-regulated after acute OE injury remain unknown. To identify the cellular source of potential signaling mechanisms, we assessed HBC activation after neuron-only and sustentacular cell death. We found that ablation of sustentacular cells is sufficient for HBC activation to multipotency. By expression analysis, next-generation sequencing, and immunohistochemical examination, down-regulation of Notch pathway signaling is coincident with HBC activation. Therefore, using HBC-specific conditional knockout of Notch receptors and overexpression of N1ICD, we show that Notch signaling maintains p63 levels and HBC dormancy, in contrast to its suppression of p63 expression in other tissues. Additionally, Notch1, but not Notch2, is required to maintain HBC dormancy after selective neuronal degeneration. Taken together, our data indicate that the activation of HBCs observed after tissue injury or sustentacular cell ablation is caused by the reduction/elimination of Notch signaling on HBCs; elimination of Jagged1 expressed by sustentacular cells may be the ligand responsible.

Deconstructing Olfactory Stem Cell Trajectories at Single-Cell Resolution

A detailed understanding of the paths that stem cells traverse to generate mature progeny is vital for elucidating the mechanisms governing cell fate decisions and tissue homeostasis. Adult stem cells maintain and regenerate multiple mature cell lineages in the olfactory epithelium. Here we integrate single-cell RNA sequencing and robust statistical analyses with in vivo lineage tracing to define a detailed map of the postnatal olfactory epithelium, revealing cell fate potentials and branchpoints in olfactory stem cell lineage trajectories. Olfactory stem cells produce support cells via direct fate conversion in the absence of cell division, and their multipotency at the population level reflects collective unipotent cell fate decisions by single stem cells. We further demonstrate that Wnt signaling regulates stem cell fate by promoting neuronal fate choices. This integrated approach reveals the mechanisms guiding olfactory lineage trajectories and provides a model for deconstructing similar hierarchies in other stem cell niches.

Isolation, culture optimization and functional characterization of stem cell neurospheres from mouse neonatal olfactory bulb and epithelium

The olfactory epithelium contains basal cells with stem cell characteristics, which have the capacity to differentiate throughout life into olfactory receptor neurons (ORNs). Here we investigate the in vitro characteristics of stem cells taken from the olfactory bulb (OB) and the olfactory epithelium (OE) of neonatal TIS21 knock-in mice. The major aim of the study was the generation of olfactory neurospheres (ONS) derived from OB and OE of neonatal mice as a tool to further analyze the elementary processes of ORN development. Our data showed that the presence of epidermal growth factor (EGF) and fibroblast growth factor (FGF) leads to a significant increase in number of ONS derived from OB but not from OE. The differentiation of ONSs led to the formation of different neuronal cell types, in particular to bipolar-shaped cells as well as putative pyramidal-neurons, astrocytes and oligodendrocytes. Immunohistochemical staining confirmed the presence of astrocytes and neurons in both types of ONSs. In order to investigate the functionality of the neurons we performed calcium imaging and patch-clamp experiments. Calcium imaging experiments revealed that the application of high potassium concentration provokes calcium transients. No excitable properties, neither sodium currents nor action potentials, were observed for the bipolar-shaped cells derived from OB and OE neurospheres, which means that these types of cells morphologically defined as putative neuronal cells, were not physiologically active. Interestingly, patch-clamp recordings performed in the pyramidal-shaped cells of OB neurospheres showed sodium and potassium currents as well as action potentials. Our study will help to establish further models in the field of olfactology.

Anosmia: Differential diagnosis, evaluation, and management

The ability to scrutinize our surroundings remains heavily dependent on the sense of smell. From the ability to detect dangerous situations such as fires to the recollection of a fond memory triggered by an odor, the advantages of an intact olfactory system cannot be overstated. Outcomes studies have highlighted the profound negative impact of anosmia and parosmia on the overall quality of life. The National Institute on Deafness and Other Communication Disorders estimates that ∼1.4% of the United States population experiences chronic olfactory dysfunction and smell loss. Efforts have focused on improving both the diagnosis of olfactory dysfunction through olfactory testing and improved reporting of treatment outcomes of olfactory training. The purpose of this article was to review the differential diagnosis, workup, and current treatment strategies of anosmia and smell disorders.

Globose basal cells for spinal cord regeneration

Spinal cord injury (SCI) is a devastating condition with loss of motor and sensory functions below the injury level. Cell based therapies are experimented in pre-clinical studies around the world. Neural stem cells are located intra-cranially in subventricular zone and hippocampus which are highly invasive sources. The olfactory epithelium is a neurogenic tissue where neurogenesis takes place throughout the adult life by a population of stem/progenitor cells. Easily accessible olfactory neuroepithelial stem/progenitor cells are an attractive cell source for transplantation in SCI. Globose basal cells (GBCs) were isolated from rat olfactory epithelium, characterized by flow cytometry and immunohistochemically. These cells were further studied for neurosphere formation and neuronal induction. T10 laminectomy was done to create drop-weight SCI in rats. On the 9^th day following SCI, 5 × 10⁵ cells were transplanted into injured rat spinal cord. The outcome of transplantation was assessed by the Basso, Beattie and Bresnahan (BBB) locomotor rating scale, motor evoked potential and histological observation. GBCs expressed neural stem cell markers nestin, SOX2, NCAM and also mesenchymal stem cell markers (CD29, CD54, CD90, CD73, CD105). These cells formed neurosphere, a culture characteristics of NSCs and on induction, differentiated cells expressed neuronal markers βIII tubulin, microtubule-associated protein 2, neuronal nuclei, and neurofilament. GBCs transplanted rats exhibited hindlimb motor recovery as confirmed by BBB score and gastrocnemius muscle electromyography amplitude was increased compared to controls. Green fluorescent protein labelled GBCs survived around the injury epicenter and differentiated into βIII tubulin-immunoreactive neuron-like cells. GBCs could be an alternative to NSCs from an accessible source for autologous neurotransplantation after SCI without ethical issues.

Contribution of Polycomb group proteins to olfactory basal stem cell self-renewal in a novel c-KIT+ culture model and in vivo

Olfactory epithelium (OE) has a lifelong capacity for neurogenesis due to the presence of basal stem cells. Despite the ability to generate short-term cultures, the successful in vitro expansion of purified stem cells from adult OE has not been reported. We sought to establish expansion-competent OE stem cell cultures to facilitate further study of the mechanisms and cell populations important in OE renewal. Successful cultures were prepared using adult mouse basal cells selected for expression of c-KIT. We show that c-KIT signaling regulates self-renewal capacity and prevents neurodifferentiation in culture. Inhibition of TGFβ family signaling, a known negative regulator of embryonic basal cells, is also necessary for maintenance of the proliferative, undifferentiated state in vitro Characterizing successful cultures, we identified expression of BMI1 and other Polycomb proteins not previously identified in olfactory basal cells but known to be essential for self-renewal in other stem cell populations. Inducible fate mapping demonstrates that BMI1 is expressed in vivo by multipotent OE progenitors, validating our culture model. These findings provide mechanistic insights into the renewal and potency of olfactory stem cells.

Intranasal Location and Immunohistochemical Characterization of the Equine Olfactory Epithelium

The olfactory epithelium (OE) is the only body site where neurons contact directly the environment and are therefore exposed to a broad variation of substances and insults. It can serve as portal of entry for neurotropic viruses which spread via the olfactory pathway to the central nervous system. For horses, it has been proposed and concluded mainly from rodent studies that different viruses, e.g., Borna disease virus, equine herpesvirus 1 (EHV-1), hendra virus, influenza virus, rabies virus, vesicular stomatitis virus can use this route. However, little is yet known about cytoarchitecture, protein expression and the intranasal location of the equine OE. Revealing differences in cytoarchitecture or protein expression pattern in comparison to rodents, canines, or humans might help to explain varying susceptibility to certain intranasal virus infections. On the other hand, disclosing similarities especially between rodents and other species, e.g., horses would help to underscore transferability of rodent models. Analysis of the complete noses of five adult horses revealed that in the equine OE two epithelial subtypes with distinct marker expression exist, designated as types a and b which resemble those previously described in dogs. Detailed statistical analysis was carried out to confirm the results obtained on the descriptive level. The equine OE was predominantly located in caudodorsal areas of the nasal turbinates with a significant decline in rostroventral direction, especially for type a. Immunohistochemically, olfactory marker protein and doublecortin (DCX) expression was found in more cells of OE type a, whereas expression of proliferating cell nuclear antigen and tropomyosin receptor kinase A was present in more cells of type b. Accordingly, type a resembles the mature epithelium, in contrast to the more juvenile type b. Protein expression profile was comparable to canine and rodent OE but equine types a and b were located differently within the nose and revealed differences in its cytoarchitecture when compared to canine OE. Equine OE type a closely resembles rat OE. Whether the observed differences contribute to species-specific susceptibility to intranasal insults such as virus infections has to be further investigated.

Stem and progenitor cells of the mammalian olfactory epithelium: Taking poietic license

The capacity of the olfactory epithelium (OE) for lifelong neurogenesis and regeneration depends on the persistence of neurocompetent stem cells, which self-renew as well as generating all of the cell types found within the nasal epithelium. This Review focuses on the types of stem and progenitor cells in the epithelium and their regulation. Both horizontal basal cells (HBCs) and some among the population of globose basal cells (GBCs) are stem cells, but the two types plays vastly different roles. The GBC population includes the basal cells that proliferate in the uninjured OE and is heterogeneous with respect to transcription factor expression. From upstream in the hierarchy to downstream, GBCs encompass 1) Sox2⁺ /Pax6⁺ stem-like cells that are totipotent and self-renew over the long term, 2) Ascl1⁺ transit-amplifying progenitors with a limited capacity for expansive proliferation, and 3) Neurog1⁺ /NeuroD1⁺ immediate precursor cells that make neurons directly. In contrast, the normally quiescent HBCs are activated to multipotency and proliferate when sustentacular cells are killed, but not when only OSNs die, indicating that HBCs are reserve stem cells that respond to severe epithelial injury. The master regulator of HBC activation is the ΔN isoform of the transcription factor p63; eliminating ΔNp63 unleashes HBC multipotency. Notch signaling, via Jagged1 ligand on Sus cells and Notch1 and Notch2 receptors on HBCs, is likely to play a major role in setting the level of p63 expression. Thus, ΔNp63 becomes a potential therapeutic target for reversing the neurogenic exhaustion characteristic of the aged OE. J. Comp. Neurol. 525:1034-1054, 2017. © 2016 Wiley Periodicals, Inc.

Transcriptional regulatory network during development in the olfactory epithelium

Regeneration, a process of reconstitution of the entire tissue, occurs throughout life in the olfactory epithelium (OE). Regeneration of OE consists of several stages: proliferation of progenitors, cell fate determination between neuronal and non-neuronal lineages, their differentiation and maturation. How the differentiated cell types that comprise the OE are regenerated, is one of the central questions in olfactory developmental neurobiology. The past decade has witnessed considerable progress regarding the regulation of transcription factors (TFs) involved in the remarkable regenerative potential of OE. Here, we review current state of knowledge of the transcriptional regulatory networks that are powerful modulators of the acquisition and maintenance of developmental stages during regeneration in the OE. Advance in our understanding of regeneration will not only shed light on the basic principles of adult plasticity of cell identity, but may also lead to new approaches for using stem cells and reprogramming after injury or degenerative neurological diseases.

Regeneration and rewiring of rodent olfactory sensory neurons

The olfactory sensory neurons are the only neurons in the mammalian nervous system that not only regenerate naturally and in response to injury, but also project to specific targets in the brain. The stem cells in the olfactory epithelium commit to both neuronal and non-neuronal lineages depending on the environmental conditions. They provide a continuous supply of new neurons. A newly generated neuron must express a specific odorant receptor gene and project to a central target consist of axons expressing the same receptor type. Recent studies have provided insights into this highly regulated, complex process. However, the molecular mechanisms that determine the regenerative capacity of stem cells, and the ability of newly generated neurons in directing their axons toward specific targets, remain elusive. Here we review progresses and controversies in the field and offer testable models.

Sox2 and Pax6 Play Counteracting Roles in Regulating Neurogenesis within the Murine Olfactory Epithelium

In the adult olfactory epithelium, the transcription factors Pax6 and Sox2 are co-expressed in sustentacular cells, horizontal basal cells (HBCs), and less-differentiated globose basal cells (GBCs)–both multipotent and transit amplifying categories—but are absent from immediate neuronal precursor GBCs and olfactory sensory neurons (OSNs). We used retroviral-vector transduction to over-express Pax6 and Sox2 individually and together during post-lesion recovery to determine how they regulate neuronal differentiation. Both Pax6 and Sox2, separately and together, can suppress the production of OSNs, as fewer clones contain neurons than with empty vector (EV), although this effect is not absolute. In this regard, Pax6 has the strongest effect when acting alone. In clones where neurons form, Pax6 reduces neuron numbers by comparison with EV, while Sox2 expands their numbers. Co-transduction with Pax6 and Sox2 produces an intermediate result. The increased production of OSNs driven by Sox2 is due to the expansion of neuronal progenitors, since proliferation and the numbers of Ascl1, Neurog1, and NeuroD1-expressing GBCs are increased. Conversely, Pax6 seems to accelerate neuronal differentiation, since Ascl1 labeling is reduced, while Neurog1- and NeuroD1-labeled GBCs are enriched. As a complement to the over-expression experiments, elimination of Sox2 in spared cells of floxed Sox2 mice, by retroviral Cre or by K5-driven CreER^T2, reduces the production of OSNs and non-neuronal cells during OE regeneration. These data suggest that Pax6 and Sox2 have counteracting roles in regulating neurogenesis, in which Pax6 accelerates neuronal production, while Sox2 retards it and expands the pool of neuronal progenitors.

Aging of perennial cells and organ parts according to the programmed aging paradigm

If aging is a physiological phenomenon-as maintained by the programmed aging paradigm-it must be caused by specific genetically determined and regulated mechanisms, which must be confirmed by evidence. Within the programmed aging paradigm, a complete proposal starts from the observation that cells, tissues, and organs show continuous turnover: As telomere shortening determines both limits to cell replication and a progressive impairment of cellular functions, a progressive decline in age-related fitness decline (i.e., aging) is a clear consequence. Against this hypothesis, a critic might argue that there are cells (most types of neurons) and organ parts (crystalline core and tooth enamel) that have no turnover and are subject to wear or manifest alterations similar to those of cells with turnover. In this review, it is shown how cell types without turnover appear to be strictly dependent on cells subjected to turnover. The loss or weakening of the functions fulfilled by these cells with turnover, due to telomere shortening and turnover slowing, compromises the vitality of the served cells without turnover. This determines well-known clinical manifestations, which in their early forms are described as distinct diseases (e.g., Alzheimer's disease, Parkinson's disease, age-related macular degeneration, etc.). Moreover, for the two organ parts (crystalline core and tooth enamel) without viable cells or any cell turnover, it is discussed how this is entirely compatible with the programmed aging paradigm.

Human olfactory stem cells for injured facial nerve reconstruction in a rat model

BACKGROUND

The purpose of this study was to show the efficacy of olfactory stem cells for injured facial nerve reconstruction in a rat model.

METHODS

Olfactory stem cells were isolated from the olfactory mucosa of human participants. A 2-mm excision was performed on the right facial nerve of all rats. Reconstruction was performed with a conduit in group 1 (n = 9); a conduit and phosphate-buffered saline in group 2 (n = 9); and a conduit and labeled olfactory stem cell in group 3 (n = 9). Rats were followed for whisker movements and electroneuronography (ENoG) analyses.

RESULTS

The whisker-movement scores for group 3 were significantly different from other groups (p < .001). ENoG showed that the amplitude values for group 3 were significantly different from group 1 and group 2 (p = .030; p < .001). Group 3 showed marked olfactory stem cell under a fluorescence microscope.

CONCLUSION

This study suggests that olfactory stem cells may be used as a potent cellular therapy for accelerating the regeneration of peripheral nerve injuries. © 2016 Wiley Periodicals, Inc. Head Neck 38: E2011-E2020, 2016.

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

Exposure to cigarette smoke is a major cause of olfactory dysfunction. However, the underlying mechanisms by which cigarette smoke interferes with the highly regenerative olfactory nerve system remain unclear. To investigate whether cigarette smoke induces olfactory dysfunction by disrupting cell proliferation and cell survival in the olfactory epithelium (OE), we developed a mouse model of smoking that involved intranasal administration of a cigarette smoke solution (CSS). Immunohistological analyses and behavioral testing showed that CSS administration during a period of 24 days reduced the number of olfactory marker protein-positive mature olfactory receptor neurons (ORNs) in the OE and induced olfactory dysfunction. These changes coincided with a reduction in the number of SOX2(+) ORN progenitors and Ki-67(+) proliferating cells in the basal layer of the OE, an increase in the number of caspase-3(+) apoptotic cells, and an increase in the expression of mRNA for the inflammatory cytokines IL-1β and IL-6. Notably, the proliferating ORN progenitor population recovered after cessation of treatment with CSS, resulting in the subsequent restoration of mature ORN numbers and olfaction. These results suggest that SOX2(+) ORN progenitors are targets of CSS-induced impairment of the OE, and that by damaging the ORN progenitor population and increasing ORN death, CSS exposure eventually overwhelms the regenerative capacity of the epithelium, resulting in reduced numbers of mature ORNs and olfactory dysfunction.

Posttraumatic olfactory dysfunction

Impairment of smell may occur following injury to any portion of the olfactory tract, from nasal cavity to brain. A thorough understanding of the anatomy and pathophysiology combined with comprehensively obtained history, physical exam, olfactory testing, and neuroimaging may help to identify the mechanism of dysfunction and suggest possible treatments. Although most olfactory deficits are neuronal mediated and therefore currently unable to be corrected, promising technology may provide novel treatment options for those most affected. Until that day, patient counseling with compensatory strategies and reassurance is essential for the maintenance of safety and QoL in this unique and challenging patient population.