Olfactory epithelium progenitors: insights from transgenic mice and in vitro biology

Role and Mechanism of Olfactory Stem Cells in the Treatment of Olfactory Disorders

Olfactory dysfunction is one of the most prevalent diseases in otorhinolaryngology, particularly since the coronavirus 2019 (COVID-19) pandemic, with a potential impact on daily life. Several etiological factors can contribute to olfactory dysfunction owing to the complexity and specificity of the olfactory transmission pathway. However, current treatments for olfactory dysfunction are limited and their efficacy is unsatisfactory. Olfactory stem cells are multifunctional stem cells in the olfactory mucosa that comprise both horizontal and global basal stem cells (HBCs and GBCs, respectively). These cells can differentiate into various cell types in response to different stimuli with distinct characteristics. The aim of the study was to discuss the mechanisms and functions of stem cells and their application in the treatment of olfactory dysfunction.

The great migration: how glial cells could regulate GnRH neuron development and shape adult reproductive life

In mammals, reproductive function is under the control of hypothalamic neurons named Gonadotropin-Releasing Hormone (GnRH) neurons. These neurons migrate from the olfactory placode to the brain, during embryonic development. For the past 40 years, these neurons have been considered an example of tangential migration, i.e., dependent on the olfactory/vomeronasal/terminal nerves. Numerous studies have highlighted the factors involved in the migration of these neurons but thus far overlooked the cellular microenvironment that produces them. Many of these factors are dysregulated in hypogonadotropic hypogonadism, resulting in subfertility/infertility. Nevertheless, over the past ten years, several papers have reported the influence of glial cells (named olfactory ensheathing cells [OECs]) in the migration and differentiation of GnRH neurons. This review will describe the atypical origins, migration, and differentiation of these neurons, focusing on the latest discoveries. There will be a more specific discussion on the involvement of OECs in the development of GnRH neurons, during embryonic and perinatal life; as well as on their potential implication in the development of congenital or idiopathic hypogonadotropic hypogonadism (such as Kallmann syndrome).

Epithelial morphometric alterations and mucosecretory responses in the nasal cavity of mice chronically exposed to hydrothermal emissions

Air pollutants (either of natural or anthropogenic origin) represent a considerable environmental risk to human health by affecting the respiratory system and causing respiratory disorders. In this study, we investigate the effects of chronic exposure to hydrothermal emissions on the nasal cavity of mice since it is the first and the most exposed region of the respiratory system. This study, carried in S. Miguel Island, Azores-Portugal, used Mus musculus as a bioindicator species. Mice were captured in an area with non-eruptive active volcanism (Furnas Village) and another area without volcanism (Rabo de Peixe, reference site). The hydrothermal emissions present at Furnas Village are characterized by the continuous release of several gases (CO₂, H₂S, ²²²Rn) along with metals (e.g. Hg, Cd, Zn, Al) and particulate matter into the environment. We test the hypothesis whether chronic exposure to this specific type of pollution causes epithelial morphometric, mucosecretory and neuronal alterations on the nasal cavity. Thickness measurements were taken in the squamous, respiratory and olfactory epithelia. The relative density of cell types (basal, support and neurons) was also assessed in the olfactory epithelium and the mucosecretory activity was determined in the lateral nasal glands, Bowman's gland and goblet cells. Mice chronically exposed to hydrothermal emissions presented thinner olfactory epithelia and lesser mucous production, which could result in loss of olfactory capabilities as well as a decrease in the protective function provided by the mucous to the lower respiratory tract. For the first time, it is demonstrated that, in mice, this specific type of non-eruptive active volcanism causes epithelial and mucosecretory alterations, leading to the loss of olfactory capabilities.

Study of the glial cytoarchitecture of the developing olfactory bulb of a shark using immunochemical markers of radial glia

During development of the olfactory bulb (OB), glial cells play key roles in axonal guiding/targeting, glomerular formation and synaptic plasticity. Studies in mammals have shown that radial glial cells and peripheral olfactory glia (olfactory ensheathing cells, OECs) are involved in the development of the OB. Most studies about the OB glia were carried out in mammals, but data are lacking in most non-mammalian vertebrates. In the present work, we studied the development of the OB glial system in the cartilaginous fish Scyliorhinus canicula (catshark) using antibodies against glial markers, such as glial fibrillary acidic protein (GFAP), brain lipid-binding protein (BLBP), and glutamine synthase (GS). These glial markers were expressed in cells with radial morphology lining the OB ventricle of embryos and this expression continues in ependymal cells (tanycytes) in early juveniles. Astrocyte-like cells were also observed in the granular layer and surrounding glomeruli. Numerous GS-positive cells were present in the primary olfactory pathway of embryos. In the developmental stages analysed, the olfactory nerve layer and the glomerular layer were the regions with higher GFAP, BLBP and GS immuno-reactivity. In addition, numerous BLBP-expressing cells (a marker of mammalian OECs) showing proliferative activity were present in the olfactory nerve layer. Our findings suggest that glial cells of peripheral and central origin coexist in the OB of catshark embryos and early juveniles. These results open the path for future studies about the differential roles of glial cells in the catshark OB during embryonic development and in adulthood.

Olfactory ensheathing cells: Unique glial cells promising for treatments of spinal cord injury

Spinal cord injury (SCI) is generally the consequence of physical damage, which may result in devastating consequences such as paraplegia or paralysis. Some certain candidates for SCI repair are olfactory ensheathing cells (OECs), which are unique glial cells located in the transition region of the peripheral nervous system and central nervous system and perform neuron regeneration in the olfactory system throughout life. Culture studies have clarified many properties of OECs, but their mechanisms of actions are not fully understood. Successful results achieved in animal models showcased that SCI treatment with OEC transplants is suitable for clinical trials. However, clinical trials are limited by difficulties like cell acquisition for autograft transplantation. Despite the improvements in both animal and clinical studies so far, there is still insufficient information about the mechanism of actions, adverse effects, proper application methods, effective subtypes, and sources of cells. This review summarizes pre-clinical and clinical literature focused on the cellular characterization of both OECs in vitro and post-transplantation. We highlight the roles and effects of OECs on (a) the injury-induced glial milieu, (b) neuronal growth/regeneration, and (c) functional recovery after injury. Due to the shown benefits of OECs with in vitro and animal studies and a limited number of clinical trials, where safety and effectivity were shown, it is necessary to conduct more studies on OECs to obtain effective and feasible treatment methods.

Structural and ultrastructural studies on the developing vomeronasal sensory epithelium in the grass snake Natrix natrix (Squamata: Colubroidea)

The sensory olfactory epithelium and the vomeronasal sensory epithelium (VSE) are characterized by continuous turnover of the receptor cells during postnatal life and are capable of regeneration after injury. The VSE, like the entire vomeronasal organ, is generally well developed in squamates and is crucial for detection of pheromones and prey odors. Despite the numerous studies on embryonic development of the VSE in squamates, especially in snakes, an ultrastructural analysis, as far as we know, has never been performed. Therefore, we investigated the embryology of the VSE of the grass snake (Natrix natrix) using electron microscopy (SEM and TEM) and light microscopy. As was shown for adult snakes, the hypertrophied ophidian VSE may provide great resolution of changes in neuron morphology located at various epithelial levels. The results of this study suggest that different populations of stem/progenitor cells occur at the base of the ophidian VSE during embryonic development. One of them may be radial glia-like cells, described previously in mouse. The various structure and ultrastructure of neurons located at different parts of the VSE provide evidence for neuronal maturation and aging. Based on these results, a few nonmutually exclusive hypotheses explaining the formation of the peculiar columnar organization of the VSE in snakes were proposed.

Human olfactory mesenchymal stromal cells co-expressing horizontal basal and ensheathing cell proteins in culture

Introduction:

The olfactory neuro-epithelium has an intrinsic capability of renewal during lifetime provided by the existence of globose and horizontal olfactory precursor cells. Additionally, mesenchymal stromal olfactory cells also support the homeostasis of the olfactory mucosa cell population. Under in vitro culture conditions with Dulbecco modified eagle/F12 medium supplemented with 10% fetal bovine serum, tissue biopsies from upper turbinate have generated an adherent population of cells expressing mainly mesenchymal stromal phenotypic markers. A closer examination of these cells has also found co-expression of olfactory precursors and ensheathing cell phenotypic markers. These results were suggestive of a unique property of olfactory mesenchymal stromal cells as potentially olfactory progenitor cells.

Objective:

To study whether the expression of these proteins in mesenchymal stromal cells is modulated upon neuronal differentiation.

Materials and methods:

We observed the phenotype of olfactory stromal cells under DMEM/F12 plus 10% fetal bovine serum in comparison to cells from spheres induced by serum-free medium plus growth factors inducers of neural progenitors.

Results:

The expression of mesenchymal stromal (CD29+, CD73+, CD90+, CD45-), horizontal basal (ICAM-1/CD54+, p63+, p75NGFr+), and ensheathing progenitor cell (nestin+, GFAP+) proteins was determined in the cultured population by flow cytometry. The determination of Oct 3/4, Sox-2, and Mash-1 transcription factors, as well as the neurotrophins BDNF, NT3, and NT4 by RT-PCR in cells, was indicative of functional heterogeneity of the olfactory mucosa tissue sample.

Conclusions:

Mesenchymal and olfactory precursor proteins were downregulated by serum-free medium and promoted differentiation of mesenchymal stromal cells into neurons and astroglial cells.

Olig2 regulates terminal differentiation and maturation of peripheral olfactory sensory neurons

The bHLH transcription factor Olig2 is required for sequential cell fate determination of both motor neurons and oligodendrocytes and for progenitor proliferation in the central nervous system. However, the role of Olig2 in peripheral sensory neurogenesis remains unknown. We report that Olig2 is transiently expressed in the newly differentiated olfactory sensory neurons (OSNs) and is down-regulated in the mature OSNs in mice from early gestation to adulthood. Genetic fate mapping demonstrates that Olig2-expressing cells solely give rise to OSNs in the peripheral olfactory system. Olig2 depletion does not affect the proliferation of peripheral olfactory progenitors and the fate determination of OSNs, sustentacular cells, and the olfactory ensheathing cells. However, the terminal differentiation and maturation of OSNs are compromised in either Olig2 single or Olig1/Olig2 double knockout mice, associated with significantly diminished expression of multiple OSN maturation and odorant signaling genes, including Omp, Gnal, Adcy3, and Olfr15. We further demonstrate that Olig2 binds to the E-box in the Omp promoter region to regulate its expression. Taken together, our results reveal a distinctly novel function of Olig2 in the periphery nervous system to regulate the terminal differentiation and maturation of olfactory sensory neurons.

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.

Anatomy of the olfactory mucosa

The classic notion that humans are microsmatic animals was born from comparative anatomy studies showing the reduction in the size of both the olfactory bulbs and the limbic brain relative to the whole brain. However, the human olfactory system contains a number of neurons comparable to that of most other mammals, and humans have exquisite olfactory abilities. Major advances in molecular and genetic research have resulted in the identification of extremely large gene families that express receptors for sensing odors. Such advances have led to a renaissance of studies focused on both human and nonhuman aspects of olfactory physiology and function. Evidence that olfactory dysfunction is among the earliest signs of a number of neurodegenerative and neuropsychiatric disorders has led to considerable interest in the use of olfactory epithelial biopsies for potentially identifying such disorders. Moreover, the unique features of the olfactory ensheathing cells have made the olfactory mucosa a promising and unexpected source of cells for treating spinal cord injuries and other neural injuries in which cell guidance is critical. The olfactory system of humans and other primates differs in many ways from that of other species. In this chapter we provide an overview of the anatomy of not only the human olfactory mucosa but of mucosae from a range of mammals from which more detailed information is available. Basic information regarding the general organization of the olfactory mucosa, including its receptor cells and the large number of other cell types critical for their maintenance and function, is provided. Cross-species comparisons are made when appropriate. The polemic issue of the human vomeronasal organ in both the adult and fetus is discussed, along with recent findings regarding olfactory subsystems within the nose of a number of mammals (e.g., the septal organ and Grüneberg ganglion).

FGF20-Expressing, Wnt-Responsive Olfactory Epithelial Progenitors Regulate Underlying Turbinate Growth to Optimize Surface Area

The olfactory epithelium (OE) is a neurosensory organ required for the sense of smell. Turbinates, bony projections from the nasal cavity wall, increase the surface area within the nasal cavity lined by the OE. Here, we use engineered fibroblast growth factor 20 (Fgf20) knockin alleles to identify a population of OE progenitor cells that expand horizontally during development to populate all lineages of the mature OE. We show that these Fgf20-positive epithelium-spanning progenitor (FEP) cells are responsive to Wnt/β-Catenin signaling. Wnt signaling suppresses FEP cell differentiation into OE basal progenitors and their progeny and positively regulates Fgf20 expression. We further show that FGF20 signals to the underlying mesenchyme to regulate the growth of turbinates. These studies thus identify a population of OE progenitor cells that function to scale OE surface area with the underlying turbinates.

Potential role of CBX7 in regulating pluripotency of adult human pluripotent-like olfactory stem cells in stroke model

The adult olfactory mucosa, a highly regenerative tissue with unique life-long neurogenesis ability, is thought to harbor a naïve yet tightly controlled stem cell population. It will provide unique benefits in various stem cell-based therapies, such as stroke treatment. Here, we identified a subpopulation of adult pluripotent-like olfactory stem cells (APOSCs), which were modulated by an epigenetic repressor of CBX7. APOSCs form a floating sphere, express pluripotency markers Nanog, Oct-4, Sox-2, and SSEA-4 and show alkaline phosphatase activity. In addition, APOSCs display self-renewal and a pluripotent potential to differentiate into all three germ layers. Moreover, APOSCs coexpress pluripotency markers with CBX7. Within their natural niche, APOSCs from CBX7^+/+ mice responded promptly to either spontaneous or injury-induced tissue regeneration. However, APOSCs from CBX7^−/− mice manifested an impaired self-renewal and differentiation potential. Similarly, in vitro-cultivated CBX7^−/− APOSCs underwent premature senescence, whereas CBX7^+/+ APOSCs still actively divided, indicating that CBX7 is required for the self-renewal of APOSCs. Intracerebral implantation of APOSCs improved the stroke-mediated neurological dysfunction in rodents. These findings indicate that CBX7 plays a critical role in the regenerative properties of APOSCs and indicate the safety and feasibility of implantation of autologous APOSCs in stroke treatment.

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.

Rostro-caudal maturation of glial cells in the accessory olfactory system during development: involvement in outgrowth of GnRH neurites

During mammalian embryonic development, GnRH neurones differentiate from the nasal placode and migrate through the nasal septum towards the forebrain. We previously showed that a category of glial cells, the olfactory ensheathing cells (OEC), forms the microenvironment of migrating GnRH neurones. Here, to characterize the quantitative and qualitative importance of this glial, we investigated the spatiotemporal maturation of glial cells in situ and the role of maturing glia in GnRH neurones development ex vivo. More than 90% of migrating GnRH neurones were found to be associated with glial cells. There was no change in the cellular microenvironment of GnRH neurones in the regions crossed during embryonic development as glial cells formed the main microenvironment of these neurones (53.4%). However, the phenotype of OEC associated with GnRH neurones changed across regions. The OEC progenitors immunoreactive to brain lipid binding protein formed the microenvironment of migrating GnRH neurones from the vomeronasal organ to the telencephalon and were also present in the diencephalon. However, during GnRH neurone migration, maturation of OEC to [GFAP+] state (glial fibrillary acid protein) was only observed in the nasal septum. Inducing depletion of OEC in maturation, using transgenic mice expressing herpes simplex virus thymidine kinase driven by the GFAP promoter, had no impact on neurogenesis or on triggering GnRH neurones migration in nasal explant culture. Nevertheless, depletion of [GFAP+] cells decreased GnRH neurites outgrowth by 57.4%. This study suggests that specific maturation of OEC in the nasal septum plays a role in morphological differentiation of GnRH neurones.

Canonical Wnt Signaling Drives Tumor-Like Lesions from Sox2-Positive Precursors of the Murine Olfactory Epithelium

Canonical Wnt signaling is known to promote proliferation of olfactory stem cells. In order to investigate the effects of a constitutive activation of Wnt signaling in Sox2-positive precursor cells of the olfactory epithelium, we used transgenic mice that allowed an inducible deletion of exon 3 of the Ctnnb1 gene, which is responsible for the phosphorylation and degradation of Ctnnb1 protein. After induction of aberrant Wnt activation by Ctnnb1 deletion at embryonic day 14, such mice developed tumor-like lesions in upper parts of the nasal cavity. We still observed areas of epithelial hyperplasia within the olfactory epithelium following early postnatal Wnt activation, but the olfactory epithelial architecture remained unaffected in most parts when Wnt was activated at postnatal day 21 or later. In summary, our results suggest an age-dependent tumorigenic potential of aberrant Wnt signaling in the olfactory epithelium of mice.

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.

Evaluation of Nestin Expression in the Developing and Adult Mouse Inner Ear

Adult stem cells are undifferentiated cells with the capacity to proliferate and form mature tissue-specific cell types. Nestin is an intermediate filament protein used to identify cells with stem cell characteristics. Its expression has been observed in a population of cells in developing and adult cochleae. In vitro studies using rodent cochlear tissue have documented the potential of nestin-expressing cells to proliferate and form hair and supporting cells. In this study, nestin coupled to green fluorescent protein (GFP) transgenic mice were used to provide a more complete characterization of the spatial and temporal expression of nestin in the inner ear, from organogenesis to adulthood. During development, nestin is expressed in the spiral ganglion cell region and in multiple cell types in the organ of Corti, including nascent hair and supporting cells. In adulthood, its expression is reduced but persists in the spiral ganglion, in a cell population medial to and below the inner hair cells, and in Deiters' cells in the cochlear apex. Moreover, nestin-expressing cells can proliferate in restricted regions of the inner ear during development shown by coexpression with Ki67 and MCM2 and by 5-ethynyl-2'-deoxyuridine incorporation. Results suggest that nestin may label progenitor cells during inner ear development and may not be a stem cell marker in the mature organ of Corti; however, nestin-positive cells in the spiral ganglion exhibit some stem cell characteristics. Future studies are necessary to determine if these cells possess any latent stem cell-like qualities that may be targeted as a regenerative approach to treat neuronal forms of hearing loss.

mSWI/SNF (BAF) Complexes Are Indispensable for the Neurogenesis and Development of Embryonic Olfactory Epithelium

Neurogenesis is a key developmental event through which neurons are generated from neural stem/progenitor cells. Chromatin remodeling BAF (mSWI/SNF) complexes have been reported to play essential roles in the neurogenesis of the central nervous system. However, whether BAF complexes are required for neuron generation in the olfactory system is unknown. Here, we identified onscBAF and ornBAF complexes, which are specifically present in olfactory neural stem cells (oNSCs) and olfactory receptor neurons (ORNs), respectively. We demonstrated that BAF155 subunit is highly expressed in both oNSCs and ORNs, whereas high expression of BAF170 subunit is observed only in ORNs. We report that conditional deletion of BAF155, a core subunit in both onscBAF and ornBAF complexes, causes impaired proliferation of oNSCs as well as defective maturation and axonogenesis of ORNs in the developing olfactory epithelium (OE), while the high expression of BAF170 is important for maturation of ORNs. Interestingly, in the absence of BAF complexes in BAF155/BAF170 double-conditional knockout mice (dcKO), OE is not specified. Mechanistically, BAF complex is required for normal activation of Pax6-dependent transcriptional activity in stem cells/progenitors of the OE. Our findings unveil a novel mechanism mediated by the mSWI/SNF complex in OE neurogenesis and development.

Evidence for a Notch1-mediated transition during olfactory ensheathing cell development

Olfactory ensheathing cells (OECs) are a unique glial population found in both the peripheral and central nervous system: they ensheath bundles of unmyelinated olfactory axons from their peripheral origin in the olfactory epithelium to their central synaptic targets in the glomerular layer of the olfactory bulb. Like all other peripheral glia (Schwann cells, satellite glia, enteric glia), OECs are derived from the embryonic neural crest. However, in contrast to Schwann cells, whose development has been extensively characterised, relatively little is known about their normal development in vivo. In the Schwann cell lineage, the transition from multipotent Schwann cell precursor to immature Schwann cell is promoted by canonical Notch signalling. Here, in situ hybridisation and immunohistochemistry data from chicken, mouse and human embryos are presented that suggest a canonical Notch-mediated transition also occurs during OEC development.

Expression of homeobox genes in the mouse olfactory epithelium

Homeobox genes constitute a large family of genes widely studied because of their role in the establishment of the body pattern. However, they are also involved in many other events during development and adulthood. The main olfactory epithelium (MOE) is an excellent model to study neurogenesis in the adult nervous system. Analyses of homeobox genes during development show that some of these genes are involved in the formation and establishment of cell diversity in the MOE. Moreover, the mechanisms of expression of odorant receptors (ORs) constitute one of the biggest enigmas in the field. Analyses of OR promoters revealed the presence of homeodomain binding sites in their sequences. Here we characterize the expression patterns of a set of 49 homeobox genes in the MOE with in situ hybridization. We found that seven of them (Dlx3, Dlx5, Dlx6, Msx1, Meis1, Isl1, and Pitx1) are zonally expressed. The homeobox gene Emx1 is expressed in three guanylate cyclase(+) populations, two located in the MOE and the third one in an olfactory subsystem known as Grüneberg ganglion located at the entrance of the nasal cavity. The homeobox gene Tshz1 is expressed in a unique patchy pattern across the MOE. Our findings provide new insights to guide functional studies that aim to understand the complexity of transcription factor expression and gene regulation in the MOE. J. Comp. Neurol. 524:2713-2739, 2016. © 2016 Wiley Periodicals, Inc.

Primary Cilia on Horizontal Basal Cells Regulate Regeneration of the Olfactory Epithelium

The olfactory epithelium (OE) is one of the few tissues to undergo constitutive neurogenesis throughout the mammalian lifespan. It is composed of multiple cell types including olfactory sensory neurons (OSNs) that are readily replaced by two populations of basal stem cells, frequently dividing globose basal cells and quiescent horizontal basal cells (HBCs). However, the precise mechanisms by which these cells mediate OE regeneration are unclear. Here, we show for the first time that the HBC subpopulation of basal stem cells uniquely possesses primary cilia that are aligned in an apical orientation in direct apposition to sustentacular cell end feet. The positioning of these cilia suggests that they function in the detection of growth signals and/or differentiation cues. To test this idea, we generated an inducible, cell type-specific Ift88 knock-out mouse line (K5rtTA;tetOCre;Ift88(fl/fl)) to disrupt cilia formation and maintenance specifically in HBCs. Surprisingly, the loss of HBC cilia did not affect the maintenance of the adult OE but dramatically impaired the regeneration of OSNs following lesion. Furthermore, the loss of cilia during development resulted in a region-specific decrease in neurogenesis, implicating HBCs in the establishment of the OE. Together, these results suggest a novel role for primary cilia in HBC activation, proliferation, and differentiation.

SIGNIFICANCE STATEMENT

We show for the first time the presence of primary cilia on a quiescent population of basal stem cells, the horizontal basal cells (HBCs), in the olfactory epithelium (OE). Importantly, our data demonstrate that cilia on HBCs are necessary for regeneration of the OE following injury. Moreover, the disruption of HBC cilia alters neurogenesis during the development of the OE, providing evidence that HBCs participate in the establishment of this tissue. These data suggest that the mechanisms of penetrance for ciliopathies in the OE extend beyond that of defects in olfactory sensory neurons and may include alterations in OE maintenance and regeneration.

Insm1 promotes neurogenic proliferation in delaminated otic progenitors

INSM1 is a zinc-finger protein expressed throughout the developing nervous system in late neuronal progenitors and nascent neurons. In the embryonic cortex and olfactory epithelium, Insm1 may promote the transition of progenitors from apical, proliferative, and uncommitted to basal, terminally-dividing and neuron producing. In the otocyst, delaminating and delaminated progenitors express Insm1, whereas apically-dividing progenitors do not. This expression pattern is analogous to that in embryonic olfactory epithelium and cortex (basal/subventricular progenitors). Lineage analysis confirms that auditory and vestibular neurons originate from Insm1-expressing cells. In the absence of Insm1, otic ganglia are smaller, with 40% fewer neurons. Accounting for the decrease in neurons, delaminated progenitors undergo fewer mitoses, but there is no change in apoptosis. We conclude that in the embryonic inner ear, Insm1 promotes proliferation of delaminated neuronal progenitors and hence the production of neurons, a similar function to that in other embryonic neural epithelia. Unexpectedly, we also found that differentiating, but not mature, outer hair cells express Insm1, whereas inner hair cells do not. Insm1 is the earliest known gene expressed in outer versus inner hair cells, demonstrating that nascent outer hair cells initiate a unique differentiation program in the embryo, much earlier than previously believed.

Injury in aged animals robustly activates quiescent olfactory neural stem cells

While the capacity of the olfactory epithelium (OE) to generate sensory neurons continues into middle age in mice, it is presumed that this regenerative potential is present throughout all developmental stages. However, little experimental evidence exists to support the idea that this regenerative capacity remains in late adulthood, and questions about the functionality of neurons born at these late stages remain unanswered. Here, we extend our previous work in the VNO to investigate basal rates of proliferation in the OE, as well as after olfactory bulbectomy (OBX), a commonly used surgical lesion. In addition, we show that the neural stem cell retains its capacity to generate mature olfactory sensory neurons in aged animals. Finally, we demonstrate that regardless of age, a stem cell in the OE, the horizontal basal cell (HBC), exhibits a morphological switch from a flattened, quiescent phenotype to a pyramidal, proliferative phenotype following chemical lesion in aged animals. These findings provide new insights into determining whether an HBC is active or quiescent based on a structural feature as opposed to a biochemical one. More importantly, it suggests that neural stem cells in aged mice are responsive to the same signals triggering proliferation as those observed in young mice.

Primary Cilia on Horizontal Basal Cells Regulate Regeneration of the Olfactory Epithelium

The olfactory epithelium (OE) is one of the few tissues to undergo constitutive neurogenesis throughout the mammalian lifespan. It is composed of multiple cell types including olfactory sensory neurons (OSNs) that are readily replaced by two populations of basal stem cells, frequently dividing globose basal cells and quiescent horizontal basal cells (HBCs). However, the precise mechanisms by which these cells mediate OE regeneration are unclear. Here, we show for the first time that the HBC subpopulation of basal stem cells uniquely possesses primary cilia that are aligned in an apical orientation in direct apposition to sustentacular cell end feet. The positioning of these cilia suggests that they function in the detection of growth signals and/or differentiation cues. To test this idea, we generated an inducible, cell type-specific Ift88 knock-out mouse line (K5rtTA;tetOCre;Ift88(fl/fl)) to disrupt cilia formation and maintenance specifically in HBCs. Surprisingly, the loss of HBC cilia did not affect the maintenance of the adult OE but dramatically impaired the regeneration of OSNs following lesion. Furthermore, the loss of cilia during development resulted in a region-specific decrease in neurogenesis, implicating HBCs in the establishment of the OE. Together, these results suggest a novel role for primary cilia in HBC activation, proliferation, and differentiation.

SIGNIFICANCE STATEMENT

We show for the first time the presence of primary cilia on a quiescent population of basal stem cells, the horizontal basal cells (HBCs), in the olfactory epithelium (OE). Importantly, our data demonstrate that cilia on HBCs are necessary for regeneration of the OE following injury. Moreover, the disruption of HBC cilia alters neurogenesis during the development of the OE, providing evidence that HBCs participate in the establishment of this tissue. These data suggest that the mechanisms of penetrance for ciliopathies in the OE extend beyond that of defects in olfactory sensory neurons and may include alterations in OE maintenance and regeneration.

Horizontal Basal Cell-Specific Deletion of Pax6 Impedes Recovery of the Olfactory Neuroepithelium Following Severe Injury

In the mammalian olfactory epithelium (OE), olfactory receptor neurons (ORNs) are continuously regenerated throughout the animal's lifetime. Horizontal basal cells (HBCs) in the OE express the epithelial marker keratin 5 (K5) and the stem cell marker Pax6 and are considered relatively quiescent tissue stem cells in the OE. Pax6 is a key regulator of several developmental processes in the central nervous system and in sensory organs. Although Pax6 is expressed in the OE, its precise role remains unknown, particularly with respect to stem cell-like HBCs. To investigate the function of Pax6 in the developmental and regenerative processes in the OE, we generated conditional Pax6-knockout mice carrying a loxP-floxed Pax6 gene. Homozygous Pax6-floxed mice were crossed with K5-Cre transgenic mice to generate HBC-specific Pax6-knockout (Pax6-cKO) mice. We confirmed that the deletion of Pax6 expression in HBCs was sufficiently achieved in zone 1 of the OE in Pax6-cKO mice 3 days after methimazole-induced severe damage. In this condition, regeneration of the OE was dramatically impaired; both OE thickness and the number of ORNs were significantly decreased in the regenerated OE of Pax6-cKO mice. These results suggest that Pax6 expression is essential for HBCs to differentiate into neuronal cells during the regeneration process following severe injury.

Odorant receptors regulate the final glomerular coalescence of olfactory sensory neuron axons

Odorant receptors (OR) are strongly implicated in coalescence of olfactory sensory neuron (OSN) axons and the formation of olfactory bulb (OB) glomeruli. However, when ORs are first expressed relative to basal cell division and OSN axon extension is unknown. We developed an in vivo fate-mapping strategy that enabled us to follow OSN maturation and axon extension beginning at basal cell division. In parallel, we mapped the molecular development of OSNs beginning at basal cell division, including the onset of OR expression. Our data show that ORs are first expressed around 4 d following basal cell division, 24 h after OSN axons have reached the OB. Over the next 6+ days the OSN axons navigate the OB nerve layer and ultimately coalesce in glomeruli. These data provide a previously unidentified perspective on the role of ORs in homophilic OSN axon adhesion and lead us to propose a new model dividing axon extension into two phases. Phase I is OR-independent and accounts for up to 50% of the time during which axons approach the OB and begin navigating the olfactory nerve layer. Phase II is OR-dependent and concludes as OSN axons coalesce in glomeruli.

Differing phagocytic capacities of accessory and main olfactory ensheathing cells and the implication for olfactory glia transplantation therapies

The rodent olfactory systems comprise the main olfactory system for the detection of odours and the accessory olfactory system which detects pheromones. In both systems, olfactory axon fascicles are ensheathed by olfactory glia, termed olfactory ensheathing cells (OECs), which are crucial for the growth and maintenance of the olfactory nerve. The growth-promoting and phagocytic characteristics of OECs make them potential candidates for neural repair therapies such as transplantation to repair the injured spinal cord. However, transplanting mixed populations of glia with unknown properties may lead to variations in outcomes for neural repair. As the phagocytic capacity of the accessory OECs has not yet been determined, we compared the phagocytic capacity of accessory and main OECs in vivo and in vitro. In normal healthy animals, the accessory OECs accumulated considerably less axon debris than main OECs in vivo. Analysis of freshly dissected OECs showed that accessory OECs contained 20% less fluorescent axon debris than main OECs. However, when assayed in vitro with exogenous axon debris added to the culture, the accessory OECs phagocytosed almost 20% more debris than main OECs. After surgical removal of one olfactory bulb which induced the degradation of main and accessory olfactory sensory axons, the accessory OECs responded by phagocytosing the axon debris. We conclude that while accessory OECs have the capacity to phagocytose axon debris, there are distinct differences in their phagocytic capacity compared to main OECs. These distinct differences may be of importance when preparing OECs for neural transplant repair therapies.

Translational potential of olfactory mucosa for the study of neuropsychiatric illness

The olfactory mucosa (OM) is a unique source of regenerative neural tissue that is readily obtainable from living human subjects and thus affords opportunities for the study of psychiatric illnesses. OM tissues can be used, either as ex vivo OM tissue or in vitro OM-derived neural cells, to explore parameters that have been difficult to assess in the brain of living individuals with psychiatric illness. As OM tissues are distinct from brain tissues, an understanding of the neurobiology of the OM is needed to relate findings in these tissues to those of the brain as well as to design and interpret ex vivo or in vitro OM studies. To that end, we discuss the molecular, cellular and functional characteristics of cell types within the olfactory mucosa, describe the organization of the OM and highlight its role in the olfactory neurocircuitry. In addition, we discuss various approaches to in vitro culture of OM-derived cells and their characterization, focusing on the extent to which they reflect the in vivo neurobiology of the OM. Finally, we review studies of ex vivo OM tissues and in vitro OM-derived cells from individuals with psychiatric, neurodegenerative and neurodevelopmental disorders. In particular, we discuss the concordance of this work with postmortem brain studies and highlight possible future approaches, which may offer distinct strengths in comparison to in vitro paradigms based on genomic reprogramming.

Radial glia phagocytose axonal debris from degenerating overextending axons in the developing olfactory bulb

Axon targeting during the development of the olfactory system is not always accurate, and numerous axons overextend past the target layer into the deeper layers of the olfactory bulb. To date, the fate of the mis-targeted axons has not been determined. We hypothesized that following overextension, the axons degenerate, and cells within the deeper layers of the olfactory bulb phagocytose the axonal debris. We utilized a line of transgenic mice that expresses ZsGreen fluorescent protein in primary olfactory axons. We found that overextending axons closely followed the filaments of radial glia present in the olfactory bulb during embryonic development. Following overextension into deeper layers of the olfactory bulb, axons degenerated and radial glia responded by phagocytosing the resulting debris. We used in vitro analysis to confirm that the radial glia had phagocytosed debris from olfactory axons. We also investigated whether the fate of overextending axons was altered when the development of the olfactory bulb was perturbed. In mice that lacked Sox10, a transcription factor essential for normal olfactory bulb development, we observed a disruption to the morphology and positioning of radial glia and an accumulation of olfactory axon debris within the bulb. Our results demonstrate that during early development of the olfactory system, radial glia play an important role in removing overextended axons from the deeper layers of the olfactory bulb.

Wnt-responsive Lgr5⁺ globose basal cells function as multipotent olfactory epithelium progenitor cells

Persistent neurogenesis in the olfactory epithelium provides a unique model to study neural stem cell self-renewal and fate determination. In the olfactory neuroepithelium, globose basal cells (GBCs) are considered to be the direct progenitors of olfactory neurons. However, the study of neurogenesis from GBCs has been impeded by the paucity of GBC-specific markers. Here we report that Lgr5, a recently discovered adult stem cell marker, is exclusively expressed in GBCs in neonatal and adult mice. Lgr5(+) cells display characteristics of cycling stem cells, including Ki67 expression and EdU incorporation. Lineage tracing analysis demonstrates that Lgr5(+) GBCs regenerate multiple cell types under normal turnover condition or after olfactory lesion. Furthermore, upregulation or downregulation of Wnt signaling in vivo indicates a key role of Wnt signaling not only in maintaining Lgr5(+) cell proliferation and promoting neuroregeneration, but also in delaying sensory neuron maturation. Together, our observations provided new insights into the dynamics of neurogenesis in the olfactory epithelium.

Cellular and molecular mechanisms regulating embryonic neurogenesis in the rodent olfactory epithelium

Mechanisms that regulate cellular differentiation in developing embryos are maintained across multiple physiological systems, including the nervous system where neurons and glia are generated. The olfactory epithelium, which arises from the olfactory pit, is a stratified tissue in which the stepwise generation of neurons and support cells can easily be assessed and followed during embryogenesis and throughout adulthood. During olfactory epithelium morphogenesis, progenitor cells respond to factors that control their proliferation, survival, and differentiation in order to generate olfactory receptor neurons that detect odorants in the environment and glia-like sustentacular cells. The tight temporal regulation of expression of proneural genes in dividing progenitor cells, including Mash-1, Neurogenin-1, and NeuroD1, plays a central role in the production of olfactory receptor neurons. Multiple factors that either positively or negatively affect the generation of olfactory receptor neurons have been identified and shown to impinge on the transcriptional regulatory network in dividing progenitor cells. Several growth factors, such as FGF-8, act to promote neurogenesis by ensuring survival of progenitor cells that will give rise to olfactory receptor neurons. In contrast, other molecules, including members of the large TGF-β family of proteins, have negative impacts on neurogenesis by restricting progenitor cell proliferation and stalling their differentiation. Since recent reviews have focused on neurogenesis in the regenerating adult olfactory epithelium, this review describes neurogenesis at embryonic stages of olfactory epithelium development and summarizes our current understanding of how both cell intrinsic and extrinsic factors control this process.

Wnt-responsive Lgr5⁺ globose basal cells function as multipotent olfactory epithelium progenitor cells

Persistent neurogenesis in the olfactory epithelium provides a unique model to study neural stem cell self-renewal and fate determination. In the olfactory neuroepithelium, globose basal cells (GBCs) are considered to be the direct progenitors of olfactory neurons. However, the study of neurogenesis from GBCs has been impeded by the paucity of GBC-specific markers. Here we report that Lgr5, a recently discovered adult stem cell marker, is exclusively expressed in GBCs in neonatal and adult mice. Lgr5(+) cells display characteristics of cycling stem cells, including Ki67 expression and EdU incorporation. Lineage tracing analysis demonstrates that Lgr5(+) GBCs regenerate multiple cell types under normal turnover condition or after olfactory lesion. Furthermore, upregulation or downregulation of Wnt signaling in vivo indicates a key role of Wnt signaling not only in maintaining Lgr5(+) cell proliferation and promoting neuroregeneration, but also in delaying sensory neuron maturation. Together, our observations provided new insights into the dynamics of neurogenesis in the olfactory epithelium.

Progressive effects of N-myc deficiency on proliferation, neurogenesis, and morphogenesis in the olfactory epithelium

N-myc belongs to the myc proto-oncogene family, which is involved in numerous cellular processes such as proliferation, growth, apoptosis, and differentiation. Conditional deletion of N-myc in the mouse nervous system disrupted brain development, indicating that N-myc plays an essential role during neural development. How the development of the olfactory epithelium and neurogenesis within are affected by the loss of N-myc has, however, not been determined. To address these issues, we examined an N-myc^Foxg1Cre conditional mouse line, in which N-myc is depleted in the olfactory epithelium. First changes in N-myc mutants were detected at E11.5, with reduced proliferation and neurogenesis in a slightly smaller olfactory epithelium. The phenotype was more pronounced at E13.5, with a complete lack of Hes5-positive progenitor cells, decreased proliferation, and neurogenesis. In addition, stereological analyses revealed reduced cell size of post-mitotic neurons in the olfactory epithelium, which contributed to a smaller olfactory pit. Furthermore, we observed diminished proliferation and neurogenesis also in the vomeronasal organ, which likewise was reduced in size. In addition, the generation of gonadotropin-releasing hormone neurons was severely reduced in N-myc mutants. Thus, diminished neurogenesis and proliferation in combination with smaller neurons might explain the morphological defects in the N-myc depleted olfactory structures. Moreover, our results suggest an important role for N-myc in regulating ongoing neurogenesis, in part by maintaining the Hes5-positive progenitor pool. In summary, our results provide evidence that N-myc deficiency in the olfactory epithelium progressively diminishes proliferation and neurogenesis with negative consequences at structural and cellular levels. © 2013 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 74: 643–656, 2014

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.

Time course of structural and functional maturation of human olfactory epithelial cells in vitro

The unique ability of olfactory neurons to regenerate in vitro has allowed their use for the study of olfactory function, regeneration, and neurodegenerative disorders; thus, characterization of their properties is important. This present study attempts to establish the timeline of structural (protein expression) and functional (odorant sensitivity) maturation of human olfactory epithelial cells (hOE) in vitro using biopsy-derived cultured tissue. Cells were grown for 7 days; on each day, cells were tested for odorant sensitivity using calcium imaging techniques and then protein expression of each cell was tested using immunocytochemistry for proteins typically used for characterizing olfactory cells. Previous studies have shown that mature olfactory neurons in vitro attain a unique "phase-bright" morphology and express the olfactory marker protein (OMP). By day 3 in vitro, a variety of cells were odorant-sensitive, including both "phase-bright" and "phase-dark" cells that have previously been considered glial-like cells. The functional maturation of these hOEs appears to take place within 4 days. Interestingly, the emergence of an odorant sensitivity profile of both phase-bright and phase-dark cells preceded the expression of marker protein expression for OMP (which is expressed only by mature neurons in vivo). This structural maturation took 5 days, suggesting that the development of odorant sensitivity is not coincident with the expression of marker molecules that are hallmarks of structural maturation. These results have important implications for the use of hOEs as in vitro models of olfactory and neuronal function.

Olfactory ensheathing glia are required for embryonic olfactory axon targeting and the migration of gonadotropin-releasing hormone neurons

Kallmann's syndrome is caused by the failure of olfactory axons and gonadotropin-releasing hormone (GnRH) neurons to enter the embryonic forebrain, resulting in anosmia and sterility. Sox10 mutations have been associated with Kallmann's syndrome phenotypes, but their effect on olfactory system development is unknown. We recently showed that Sox10 is expressed by neural crest-derived olfactory ensheathing cells (OECs). Here, we demonstrate that in homozygous Sox10(lacZ/lacZ) mouse embryos, OEC differentiation is disrupted; olfactory axons accumulate in the ventromedial olfactory nerve layer and fewer olfactory receptor neurons express the maturation marker OMP (most likely owing to the failure of axonal targeting). Furthermore, GnRH neurons clump together in the periphery and a smaller proportion enters the forebrain. Our data suggest that human Sox10 mutations cause Kallmann's syndrome by disrupting the differentiation of OECs, which promote embryonic olfactory axon targeting and hence olfactory receptor neuron maturation, and GnRH neuron migration to the forebrain.

Critical role of GFRα1 in the development and function of the main olfactory system

Glial cell line-derived neurotrophic factor (GDNF) and its receptor GFRα1 are prominently expressed in the olfactory epithelium (OE) and olfactory bulb (OB), but their importance for olfactory system development is completely unknown. We have investigated the consequences of GFRα1 deficiency for mouse olfactory system development and function. In the OE, GFRα1 was expressed in basal precursors, immature olfactory sensory neurons (OSNs), and olfactory ensheathing cells (OECs), but was excluded from mature OSNs. The OE of newborn Gfra1 knock-out mice was thinner and contained fewer OSNs, but more dividing precursors, suggesting deficient neurogenesis. Immature OSN axon bundles were enlarged and associated OECs increased, indicating impaired migration of OECs and OSN axons. In the OB, GFRα1 was expressed in immature OSN axons and OECs of the nerve layer, as well as mitral and tufted cells, but was excluded from GABAergic interneurons. In newborn knock-outs, the nerve layer was dramatically reduced, exhibiting fewer axons and OECs. Bulbs were smaller and presented fewer and disorganized glomeruli and a significant reduction in mitral cells. Numbers of tyrosine hydroxylase-, calbindin-, and calretinin-expressing interneurons were also reduced in newborn mice lacking Gfra1. At birth, the OE and OB of Gdnf knock-out mice displayed comparable phenotypes. Similar deficits were also found in adult heterozygous Gfra1(+/-) mutants, which in addition displayed diminished responses in behavioral tests of olfactory function. We conclude that GFRα1 is critical for the development and function of the main olfactory system, contributing to the development and allocation of all major classes of neurons and glial cells.

Olfactory ensheathing cells form the microenvironment of migrating GnRH-1 neurons during mouse development

During development, GnRH-1 neurons differentiate extracerebraly from the nasal placode and migrate from the vomeronasal organ to the forebrain along vomeronasal and terminal nerves. Numerous studies have described the influence of different molecules on the migration of GnRH-1 neurons, however, the role of microenvironment cells remains poorly understood. This study used GFAP-GFP transgenic mice to detect glial cells at early developmental stages. Using nasal explant cultures, the comigration of glial cells with GnRH-1 neurons was clearly demonstrated. This in vitro approach showed that glial cells began migrating from the explants before GnRH-1 neurons. They remained ahead of the GnRH-1 migratory front and stopped migrating after the GnRH-1 neurons. The association of these glial cells with the axons combined with gene expression analysis of GFAP-GFP sorted cells enabled them to be identified as olfactory ensheathing cells (OEC). Immunohistochemical analysis revealed the presence of multiple glial cell-type markers showing several OEC subpopulations surrounding GnRH-1 neurons. Moreover, these OEC expressed genes whose products are involved in the migration of GnRH-1 neurons, such as Nelf and Semaphorin 4. In situ data confirmed that the majority of the GnRH-1 neurons were associated with glial cells along the vomeronasal axons in nasal septum and terminal nerves in the nasal forebrain junction as early as E12.5. Overall, these data demonstrate an OEC microenvironment for migrating GnRH-1 neurons during mouse development. The fact that this glial cell type precedes GnRH-1 neurons and encodes for molecules involved in their nasal migration suggests that it participates in the GnRH-1 system ontogenesis.

Fibroblast growth factor signaling regulates neurogenesis at multiple stages in the embryonic olfactory epithelium

Lifelong neurogenesis in the mouse olfactory epithelium (OE) is regulated by the response of stem/progenitor cells to local signals, but embryonic and adult OE progenitors appear to be quite different--with potentially different mechanisms of regulation. A recently identified progenitor unique to embryonic OE--the nestin+ radial glial-like progenitor--precedes some Mash1+ progenitors in the olfactory receptor neuron (ORN) lineage, which then gives rise to immediate neuronal precursors and immature ORNs. Neurogenic drive at each stage is governed largely by exogenous factors. Fibroblast growth factor 2 (FGF2) is believed to increase cell proliferation in both presumptive OE stem cells and immediate neuronal precursors in explants, but whether FGF2 directly acts on different target progenitors or stages in the embryonic OE is not known. Here we show that fibroblast growth factor receptor (FGFR)1 and FGFR2 are found in a variety of embryonic olfactory cells, including olfactory ensheathing cells and their precursors, and neuronal nestin+ and Mash1+ progenitors. Combining gain and loss of function for FGF2 activity in a novel in vitro clonal progenitor assay, we reveal that different colony phenotypes are formed by presumably different OE progenitors. FGF2 is essential for the survival and expansion of colony-forming cells of the OE, and also enhances the proliferation of embryonic Mash1+ progenitors, leading to long-lived enhancement of neurogenesis. Our data suggest that distinct OE progenitors yield different in vitro phenotypes with different potentials, that colony-forming activity is profoundly altered by laminin and collagen, that multiple ORNs can be produced from single colony-forming progenitors, and demonstrate a broader progenitor range of FGF action in the embryonic OE than previously demonstrated.

Odor memory stability after reinnervation of the olfactory bulb

The olfactory system, particularly the olfactory epithelium, presents a unique opportunity to study the regenerative capabilities of the brain, because of its ability to recover after damage. In this study, we ablated olfactory sensory neurons with methimazole and followed the anatomical and functional recovery of circuits expressing genetic markers for I7 and M72 receptors (M72-IRES-tau-LacZ and I7-IRES-tau-GFP). Our results show that 45 days after methimazole-induced lesion, axonal projections to the bulb of M72 and I7 populations are largely reestablished. Furthermore, regenerated glomeruli are re-formed within the same areas as those of control, unexposed mice. This anatomical regeneration correlates with functional recovery of a previously learned odorant-discrimination task, dependent on the cognate ligands for M72 and I7. Following regeneration, mice also recover innate responsiveness to TMT and urine. Our findings show that regeneration of neuronal circuits in the olfactory system can be achieved with remarkable precision and underscore the importance of glomerular organization to evoke memory traces stored in the brain.

Neural crest and olfactory system: new prospective

Sensory neurons in vertebrates are derived from two embryonic transient cell sources: neural crest (NC) and ectodermal placodes. The placodes are thickenings of ectodermal tissue that are responsible for the formation of cranial ganglia as well as complex sensory organs that include the lens, inner ear, and olfactory epithelium. The NC cells have been indicated to arise at the edges of the neural plate/dorsal neural tube, from both the neural plate and the epidermis in response to reciprocal interactions Moury and Jacobson (Dev Biol 141:243-253, 1990). NC cells migrate throughout the organism and give rise to a multitude of cell types that include melanocytes, cartilage and connective tissue of the head, components of the cranial nerves, the dorsal root ganglia, and Schwann cells. The embryonic definition of these two transient populations and their relative contribution to the formation of sensory organs has been investigated and debated for several decades (Basch and Bronner-Fraser, Adv Exp Med Biol 589:24-31, 2006; Basch et al., Nature 441:218-222, 2006) review (Baker and Bronner-Fraser, Dev Biol 232:1-61, 2001). Historically, all placodes have been described as exclusively derived from non-neural ectodermal progenitors. Recent genetic fate-mapping studies suggested a NC contribution to the olfactory placodes (OP) as well as the otic (auditory) placodes in rodents (Murdoch and Roskams, J Neurosci Off J Soc Neurosci 28:4271-4282, 2008; Murdoch et al., J Neurosci 30:9523-9532, 2010; Forni et al., J Neurosci Off J Soc Neurosci 31:6915-6927, 2011b; Freyer et al., Development 138:5403-5414, 2011; Katoh et al., Mol Brain 4:34, 2011). This review analyzes and discusses some recent developmental studies on the OP, placodal derivatives, and olfactory system.

From chemical neuroanatomy to an understanding of the olfactory system

The olfactory system is the appropriate model for studying several aspects of neuronal physiology spanning from the developmental stage to neural network remodelling in the adult brain. Both the morphological and physiological understanding of this system were strongly supported by classical histochemistry. It is emblematic the case of the Olfactory Marker Protein (OMP) staining, the first, powerful marker for fully differentiated olfactory receptor neurons and a key tool to investigate the dynamic relations between peripheral sensory epithelia and central relay regions given its presence within olfactory fibers reaching the olfactory bulb (OB). Similarly, the use of thymidine analogues was able to show neurogenesis in an adult mammalian brain far before modern virus labelling and lipophilic tracers based methods. Nowadays, a wealth of new histochemical techniques combining cell and molecular biology approaches is available, giving stance to move from the analysis of the chemically identified circuitries to functional research. The study of adult neurogenesis is indeed one of the best explanatory examples of this statement. After defining the cell types involved and the basic physiology of this phenomenon in the OB plasticity, we can now analyze the role of neurogenesis in well testable behaviours related to socio-chemical communication in rodents.

The dual origin of the peripheral olfactory system: placode and neural crest

BACKGROUND

The olfactory epithelium (OE) has a unique capacity for continuous neurogenesis, extending axons to the olfactory bulb with the assistance of olfactory ensheathing cells (OECs). The OE and OECs have been believed to develop solely from the olfactory placode, while the neural crest (NC) cells have been believed to contribute only the underlying structural elements of the olfactory system. In order to further elucidate the role of NC cells in olfactory development, we examined the olfactory system in the transgenic mice Wnt1-Cre/Floxed-EGFP and P0-Cre/Floxed-EGFP, in which migrating NC cells and its descendents permanently express GFP, and conducted transposon-mediated cell lineage tracing studies in chick embryos.

RESULTS

Examination of these transgenic mice revealed GFP-positive cells in the OE, demonstrating that NC-derived cells give rise to OE cells with morphologic and antigenic properties identical to placode-derived cells. OECs were also positive for GFP, confirming their NC origin. Cell lineage tracing studies performed in chick embryos confirmed the migration of NC cells into the OE. Furthermore, spheres cultured from the dissociated cells of the olfactory mucosa demonstrated self-renewal and trilineage differentiation capacities (neurons, glial cells, and myofibroblasts), demonstrating the presence of NC progenitors in the olfactory mucosa.

CONCLUSION

Our data demonstrates that the NC plays a larger role in the development of the olfactory system than previously believed, and suggests that NC-derived cells may in part be responsible for the remarkable capacity of the OE for neurogenesis and regeneration.

Composition of the migratory mass during development of the olfactory nerve

The embryonic development of the olfactory nerve includes the differentiation of cells within the olfactory placode, migration of cells into the mesenchyme from the placode, and extension of axons by the olfactory sensory neurons (OSNs). The coalition of both placode-derived migratory cells and OSN axons within the mesenchyme is collectively termed the "migratory mass." Here we address the sequence and coordination of the events that give rise to the migratory mass. Using neuronal and developmental markers, we show subpopulations of neurons emerging from the placode by embryonic day (E)10, a time at which the migratory mass is largely cellular and only a few isolated OSN axons are seen, prior to the first appearance of OSN axon fascicles at E11. These neurons also precede the emergence of the gonadotropin-releasing hormone neurons and ensheathing glia which are also resident in the mesenchyme as part of the migratory mass beginning at about E11. The data reported here begin to establish a spatiotemporal framework for the migration of molecularly heterogeneous placode-derived cells in the mesenchyme. The precocious emigration of the early arriving neurons in the mesenchyme suggests they may serve as "guidepost cells" that contribute to the establishment of a scaffold for the extension and coalescence of the OSN axons.

Neural crest and ectodermal cells intermix in the nasal placode to give rise to GnRH-1 neurons, sensory neurons, and olfactory ensheathing cells

The origin of GnRH-1 cells and olfactory ensheathing cells has been controversial. Genetic Cre-lox lineage tracing of the neural crest (NC) versus ectodermal contribution to the developing nasal placode was performed using two complementary mouse models, the NC-specific Wnt1Cre mouse line and an ectodermal-specific Crect mouse line. Using these lines we prove that the NC give rise to the olfactory ensheathing cells and subpopulations of GnRH-1 neurons, olfactory and vomeronasal cells. These data demonstrate that Schwann cells and olfactory ensheathing cells share a common developmental origin. Furthermore, the results indicate that certain conditions that impact olfaction and sexual development, such as Kallmann syndrome, may be in part neurocristopathies.

Canonical Wnt signaling promotes the proliferation and neurogenesis of peripheral olfactory stem cells during postnatal development and adult regeneration

The mammalian olfactory epithelium (OE) has a unique stem cell or progenitor niche, which is responsible for the constant peripheral neurogenesis throughout the lifespan of the animal. However, neither the signals that regulate the behavior of these cells nor the lineage properties of the OE stem cells are well understood. Multiple Wnt signaling components exhibit dynamic expression patterns in the developing OE. We generated Wnt signaling reporter TOPeGFP transgenic mice and found TOPeGFP activation predominantly in proliferating Sox2(+) OE basal cells during early postnatal development. FACS-isolated TOPeGFP(+) OE basal cells are required, but are not sufficient, for formation of spheres. Wnt3a significantly promotes the proliferation of the Sox2(+) OE sphere cells. Wnt-stimulated OE sphere cells maintain their multipotency and can differentiate into most types of neuronal and non-neuronal epithelial cells. Also, Wnt activators shift the production of differentiated cells toward olfactory sensory neurons. Moreover, TOPeGFP(+) cells are robustly increased in the adult OE after injury. In vivo administration of Wnt modulators significantly alters the regeneration potential. This study demonstrates the role of the canonical Wnt signaling pathway in the regulation of OE stem cells or progenitors during development and regeneration.

In sickness and in health: the role of methyl-CpG binding protein 2 in the central nervous system

The array of specialized neuronal and glial cell types that characterize the adult central nervous system originates from neuroepithelial proliferating precursor cells. The transition from proliferating neuroepithelial precursor cells to neuronal lineages is accompanied by rapid global changes in gene expression in coordination with epigenetic modifications at the level of the chromatin structure. A number of genetic studies have begun to reveal how epigenetic deregulation results in neurodevelopmental disorders such as mental retardation, autism, Rubinstein-Taybi syndrome and Rett syndrome. In this review we focus on the role of the methyl-CpG binding protein 2 (MeCP2) during development of the central nervous system and its involvement in Rett syndrome. First, we present recent findings that indicate a previously unconsidered role of glial cells in the development of Rett syndrome. Next, we discuss evidence of how MeCP2 deficiency or loss of function results in aberrant gene expression leading to Rett syndrome. We also discuss MeCP2's function as a repressor and activator of gene expression and the role of its different target genes, including microRNAs, during neuronal development. Finally, we address different signaling pathways that regulate MeCP2 expression at both the post-transcriptional and post-translational level, and discuss how mutations in MeCP2 may result in lack of responsiveness to environmental signals.