Spatiotemporal dynamics exhibited by horizontal basal cells reveal a pro-neurogenic pathway during injury-induced olfactory epithelium regeneration.

Horizontal basal cells (HBCs) mediate olfactory epithelium (OE) regeneration following severe tissue injury. The dynamism of the post-injury environment is well illustrated by in silico modeling of RNA sequencing data that demonstrate an evolving HBC transcriptome. Unfortunately, spatiotemporally dynamic processes occurring within HBCs in situ remain poorly understood. Here, we show that HBCs at 24 h post-OE injury spatially redistribute a constellation of proteins, which, in turn, helped to nominate Rac1 as a regulator of HBC differentiation during OE regeneration. Using our primary culture model to activate HBCs pharmacologically, we demonstrate that concurrent Rac1 inhibition attenuates HBC differentiation potential. This in vitro functional impairment manifested in vivo as decreased HBC differentiation into olfactory sensory neurons following HBC-specific Rac1 conditional knockout. Taken together, our data potentiate the design of hyposmia-alleviating therapies and highlight aspects of in situ HBC spatiotemporal dynamics that deserve further investigation.

Horizontal basal cells self-govern their neurogenic potential during injury-induced regeneration of the olfactory epithelium.

Horizontal basal cells (HBCs) residing within severely damaged olfactory epithelium (OE) mediate OE regeneration by differentiating into odorant-detecting olfactory sensory neurons (OSNs) and other tissue supporting non-neuronal cell types. Depending on both tissue type and integrity, the Notch signaling pathway can either positively or negatively regulate resident stem cell activity. Although Notch1 specifies HBC dormancy in the uninjured OE, little is known about how HBCs are influenced by the Notch pathway following OE injury. Here, we show that HBCs depend on a functional inversion of the Notch pathway to appropriately mediate OE regeneration. At 24 h post-injury, HBCs enhance Notch1-mediated signaling. Moreover, at 3 days post-injury when the regenerating OE is composed of multiple cell layers, HBCs enrich both Notch1 and the Notch ligand, Dll1. Notably, HBC-specific Notch1 knockout increases HBC quiescence and impairs HBC differentiation into neuronal progenitors and OSNs. Interestingly, complete HBC knockout of Dll1 only decreases differentiation of HBC-derived OSNs. These data underscore the context-dependent nature of Notch signaling. Furthermore, they reveal that HBCs regulate their own neurogenic potential after OE injury.

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.

An in vitro model of acute horizontal basal cell activation reveals dynamic gene regulatory networks underlying the acute activation phase.

Horizontal basal cells (HBCs) activate only in response to severe olfactory epithelium (OE) injury. This activation is mediated by the loss of the transcription factor TP63. Using the compound phorbol 12-myristate 13-acetate (PMA), we find that we can model the process of acute HBC activation. First, we find that PMA treatment induces a rapid loss in TP63 protein and induces the expression of HOPX and the nuclear translocation of RELA, previously identified to mediate HBC activation. Using bulk RNA sequencing, we find that PMA-treated HBCs pass through various stages of acute activation identifiable by transcriptional regulatory signatures that mimic stages identified in vivo . These temporal stages are associated with varying degrees of engraftment and differentiation potential in transplantation assays. Together, this data shows that our model can model physiologically relevant features of HBC activation and identifies new candidates for mechanistic testing.

Rapid fluorescent vital imaging of olfactory epithelium.

Olfactory epithelium (OE) undergoes degeneration in disorders such as age-related and post-viral olfactory dysfunction. However, methods for real-time in vivo detection of OE and assessment of total extent within the nasal cavity are currently unavailable. We identified two fluorescence probes for rapidly detecting and evaluating the entire extent of mice OE with topical application. Taking advantage of the differential expression of the enzymes cytochrome p450 (CYP) and γ-glutamyltranspeptidase (GGT) in OE relative to respiratory epithelium, we utilized the conversion of coumarin (a substrate of various CYP subtypes) and gGlu-HRMG (a substrate of GGT) by these enzymes to form metabolites with fluorescent emissions in the duct cells and sustentacular cells of neuron-containing OE. In depleted and regenerated OE model, the emission of these probes remained absent in respiratory metaplasia but appeared in regenerated OE. These substrates could be used to monitor OE degeneration and follow regenerative response to therapeutic interventions.

Lifespan of mature olfactory sensory neurons varies with location in the mouse olfactory epithelium and age of the animal.

The olfactory sensory neurons (OSNs) of the olfactory epithelium (OE) exhibit a remarkable regenerative capability, which protects the population against environmental insult and enables adjustment to new odors. The lifespan of OSNs is still open to question, with estimates ranging from 1 month to at least 1 year. However, the estimates come with some caveats, including low labeling efficiency and a focus solely on newborn neurons. We revisited the issue via the use of OMP-tTA; TetO-Cre; Rosa26-fl(stop)-Tdtomato (OMP-tTA;TdT) mice, which allowed us to selectively label ∼95% of the OMP(+) OSN population that reach maturity by a given time and, by switching to doxycycline chow, to "chase" this preexisting OSN population. Two loading protocols were used: conception to 2 months old and conception to 4.5 months old. Surviving OSNs were common up to 6 months chase time in both groups, but more neurons survived when loading for 4.5 months as compared with 2 months. A spatial difference was evident: higher percentages of OSNs survived in the dorsomedial OE as compared with ventrolateral and in posterior versus anterior OE regions. Finally, proliferation rates anticorrelated with the spatial differences in OSN survival; higher proliferation rates were observed ventrally. Together, these results demonstrate spatial and temporal differences in OSN survival, highlighting it as a dynamic system that can be studied for factors affecting neuronal survival.

Integrated age-related immunohistological changes occur in human olfactory epithelium and olfactory bulb.

Olfactory epithelium (OE) is capable of lifelong regeneration due to presence of basal progenitor cells that respond to injury or neuronal loss with increased activity. However, this capability diminishes with advancing age and a decrease in odor perception in older individuals is well established. To characterize changes associated with age in the peripheral olfactory system, an in-depth analysis of the OE and its neuronal projections onto the olfactory bulb (OB) as a function of age was performed. Human olfactory tissue autopsy samples from 36 subjects with an average age of 74.1 years were analyzed. Established cell type-specific antibodies were used to identify OE component cells in whole mucosal sheets and epithelial sections as well as glomeruli and periglomerular structures in OB sections. With age, a reduction in OE area occurs across the mucosa progressing in a posterior-dorsal direction. Deterioration of the olfactory system is accompanied with diminution of neuron-containing OE, mature olfactory sensory neurons (OSNs) and OB innervation. On an individual level, the neuronal density within the epithelium appears to predict synapse density within the OB. The innervation of the OB is uneven with higher density at the ventral half that decreases with age as opposed to stable innervation at the dorsal half. Respiratory metaplasia, submucosal cysts, and neuromata, were commonly identified in aged OE. The finding of respiratory metaplasia and aneuronal epithelium with reduction in global basal cells suggests a progression of stem cell quiescence as an underlying pathophysiology of age-related smell loss in humans. KEY POINTS: A gradual loss of olfactory sensory neurons with age in human olfactory epithelium is also reflected in a reduction in glomeruli within the olfactory bulb. This gradual loss of neurons and synaptic connections with age occurs in a specific, spatially inhomogeneous manner. Decreasing mitotically active olfactory epithelium basal cells may contribute to age-related neuronal decline and smell loss in humans.

International consensus statement on allergy and rhinology: Olfaction.

BACKGROUND: The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O). METHODS: Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus. RESULTS: The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies. CONCLUSION: This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Identifying Treatments for Taste and Smell Disorders: Gaps and Opportunities.

The chemical senses of taste and smell play a vital role in conveying information about ourselves and our environment. Tastes and smells can warn against danger and also contribute to the daily enjoyment of food, friends and family, and our surroundings. Over 12% of the US population is estimated to experience taste and smell (chemosensory) dysfunction. Yet, despite this high prevalence, long-term, effective treatments for these disorders have been largely elusive. Clinical successes in other sensory systems, including hearing and vision, have led to new hope for developments in the treatment of chemosensory disorders. To accelerate cures, we convened the "Identifying Treatments for Taste and Smell Disorders" conference, bringing together basic and translational sensory scientists, health care professionals, and patients to identify gaps in our current understanding of chemosensory dysfunction and next steps in a broad-based research strategy. Their suggestions for high-yield next steps were focused in 3 areas: increasing awareness and research capacity (e.g., patient advocacy), developing and enhancing clinical measures of taste and smell, and supporting new avenues of research into cellular and therapeutic approaches (e.g., developing human chemosensory cell lines, stem cells, and gene therapy approaches). These long-term strategies led to specific suggestions for immediate research priorities that focus on expanding our understanding of specific responses of chemosensory cells and developing valuable assays to identify and document cell development, regeneration, and function. Addressing these high-priority areas should accelerate the development of novel and effective treatments for taste and smell disorders.

A Group of Olfactory Receptor Alleles that Encode Full Length Proteins are Down-Regulated as Olfactory Sensory Neurons Mature.

The family of olfactory receptors (ORs) subserves the sense of smell and includes both functional alleles and pseudogenes, the latter identified by mutations resulting in frame shift or premature truncation. During neuronal differentiation, nonfunctional ORs are expressed initially but then are switched out, and/or the olfactory sensory neurons (OSNs) expressing them die. We carried out a transcriptomic analysis of FACS-isolated cells from ΔSox2-eGFP, Neurog1-eGFP BAC and ΔOMP-eGFP strains of uninjured and olfactory bulbectomized transgenic mice that correspond to distinct stages in the progression from globose basal cell stem cells to fully mature OSNs. We analyzed the expression pattern of 1094 unique receptors across this progression and found that the vast majority were characterized by a typical and expected pattern of expression; i.e., levels of OR mRNA peaking in mature OSNs. However, 43 ORs, including several known pseudogenes, were different, such that mRNA expression declined in the mature OSNs relative to earlier stages. Protein and promoter sequence analysis of the atypical group did not uncover any obvious differences between them and more typical ORs. Nonetheless, the pattern of expression suggests that atypical ORs may be non-functional despite the lack of any obvious abnormality in the sequence analyses.

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 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.

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.

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.

Replication of JC Virus DNA in the G144 Oligodendrocyte Cell Line Is Dependent Upon Akt.

Progressive multifocal leukoencephalopathy (PML) is an often-fatal demyelinating disease of the central nervous system. PML results when oligodendrocytes within immunocompromised individuals are infected with the human JC virus (JCV). We have identified an oligodendrocyte precursor cell line, termed G144, that supports robust levels of JCV DNA replication, a central part of the JCV life cycle. In addition, we have determined that JC virus readily infects G144 cells. Furthermore, we have determined that JCV DNA replication in G144 cells is stimulated by myristoylated (i.e., constitutively active) Akt and reduced by the Akt-specific inhibitor MK2206. Thus, this oligodendrocyte-based model system will be useful for a number of purposes, such as studies of JCV infection, establishing key pathways needed for the regulation of JCV DNA replication, and identifying inhibitors of this process.IMPORTANCE The disease progressive multifocal leukoencephalopathy (PML) is caused by the infection of particular brain cells, termed oligodendrocytes, by the JC virus. Studies of PML, however, have been hampered by the lack of an immortalized human cell line derived from oligodendrocytes. Here, we report that the G144 oligodendrocyte cell line supports both infection by JC virus and robust levels of JCV DNA replication. Moreover, we have established that the Akt pathway regulates JCV DNA replication and that JCV DNA replication can be inhibited by MK2206, a compound that is specific for Akt. These and related findings suggest that we have established a powerful oligodendrocyte-based model system for studies of JCV-dependent PML.

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.

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.

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.