Mash1 and Ngn1 control distinct steps of determination and differentiation in the olfactory sensory neuron lineage

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.

Molecular mechanisms of differentiation and class choice of olfactory sensory neurons

The sense of smell is intricately linked to essential animal behaviors necessary for individual survival and species preservation. During vertebrate evolution, odorant receptors (ORs), responsible for detecting odor molecules, have evolved to adapt to changing environments, transitioning from aquatic to terrestrial habitats and accommodating increasing complex chemical environments. These evolutionary pressures have given rise to the largest gene family in vertebrate genomes. Vertebrate ORs are phylogenetically divided into two major classes; class I and class II. Class I OR genes, initially identified in fish and frog, have persisted across vertebrate species. On the other hand, class II OR genes are unique to terrestrial animals, accounting for ~90% of mammalian OR genes. In mice, each olfactory sensory neuron (OSN) expresses a single functional allele of a single OR gene from either the class I or class II OR repertoire. This one neuron-one receptor rule is established through two sequential steps: specification of OR class and subsequent exclusive OR expression from the corresponding OR class. Consequently, OSNs acquire diverse neuronal identities during the process of OSN differentiation, enabling animals to detect a wide array of odor molecules. This review provides an overview of the OSN differentiation process through which OSN diversity is achieved, primarily using the mouse as a model animal.

Following the p63/Keratin5 basal cells in the sensory and non-sensory epithelia of the vomeronasal organ

The vomeronasal organ (VNO) is a part of the accessory olfactory system, which detects pheromones and chemical factors that trigger a spectrum of sexual and social behaviors. The vomeronasal epithelium (VNE) shares several features with the epithelium of the main olfactory epithelium (MOE). However, it is a distinct neuroepithelium populated by chemosensory neurons that differ from the olfactory sensory neurons in cellular structure, receptor expression, and connectivity. The vomeronasal organ of rodents comprises a sensory epithelium (SE) and a thin non-sensory epithelium (NSE) that morphologically resembles the respiratory epithelium. Sox2-positive cells have been previously identified as the stem cell population that gives rise to neuronal progenitors in MOE and VNE. In addition, the MOE also comprises p63 positive horizontal basal cells, a second pool of quiescent stem cells that become active in response to injury. Immunolabeling against the transcription factor p63, Keratin-5 (Krt5), Krt14, NrCAM, and Krt5Cre tracing experiments highlighted the existence of horizontal basal cells distributed along the basal lamina of SE of the VNO. Single cell sequencing and genetic lineage tracing suggest that the vomeronasal horizontal basal cells arise from basal progenitors at the boundary between the SE and NSE proximal to the marginal zones. Moreover, our experiments revealed that the NSE of rodents is, like the respiratory epithelium, a stratified epithelium where the p63/Krt5+ basal progenitor cells self-replicate and give rise to the apical columnar cells facing the lumen of the VNO.

Relationship between inflammatory markers in human olfactory neural progenitor cells and antidepressant response

Response to antidepressants is related to hippocampal neurogenesis integrity, a process mediated by neurotrophins, such as Brain Derived Neurotrophic Factor (BDNF). In turn, pro-inflammatory state appears to reduce neurogenesis, and has been associated with refractory depressive states. We propose to analyze the human neural progenitor cells derived from the olfactory epithelium (HNPCs-OE) as an indicator of neurogenesis in humans. Therefore, we compared the number and content of HNPCs-OE in depressed patients taking antidepressants, according to response to treatment. Twenty depressed patients were followed during eight weeks after antidepressant treatment was prescribed. At the end evaluation they were divided in two groups according to Hamilton depression rating scale (HDRS) scores: responders and non-responders. We compared the number and components of HNPCs-OE between groups and observed an elevation of interleukine-8 in those patients who do not achieve response to treatment, BDNF levels were no related to antidepressant response.

Diversity within olfactory sensory derivatives revealed by the contribution of Dbx1 lineages

In vertebrates, the embryonic olfactory epithelium contains progenitors that will give rise to distinct classes of neurons, including olfactory sensory neurons (OSNs; involved in odor detection), vomeronasal sensory neurons (VSNs; responsible for pheromone sensing), and gonadotropin-releasing hormone (GnRH) neurons that control the hypothalamic-pituitary-gonadal axis. Currently, these three neuronal lineages are usually believed to emerge from uniform pools of progenitors. Here, we found that the homeodomain transcription factor Dbx1 is expressed by neurogenic progenitors in the developing and adult mouse olfactory epithelium. We demonstrate that Dbx1 itself is dispensable for neuronal fate specification and global organization of the olfactory sensory system. Using lineage tracing, we characterize the contribution of Dbx1 lineages to OSN, VSN, and GnRH neuron populations and reveal an unexpected degree of diversity. Furthermore, we demonstrate that Dbx1-expressing progenitors remain neurogenic in the absence of the proneural gene Ascl1. Our work therefore points to the existence of distinct neurogenic programs in Dbx1-derived and other olfactory lineages.

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.

SOX2-Sensing: Insights into the Role of SOX2 in the Generation of Sensory Cell Types in Vertebrates

The SOX2 transcription factor is a key regulator of nervous system development, and its mutation in humans leads to a rare disease characterized by severe eye defects, cognitive defects, hearing defects, abnormalities of the CNS and motor control problems. SOX2 has an essential role in neural stem cell maintenance in specific regions of the brain, and it is one of the master genes required for the generation of induced pluripotent stem cells. Sox2 is expressed in sensory organs, and this review will illustrate how it regulates the differentiation of sensory cell types required for hearing, touching, tasting and smelling in vertebrates and, in particular, in mice.

Making a head: Neural crest and ectodermal placodes in cranial sensory development

During development of the vertebrate sensory system, many important components like the sense organs and cranial sensory ganglia arise within the head and neck. Two progenitor populations, the neural crest, and cranial ectodermal placodes, contribute to these developing vertebrate peripheral sensory structures. The interactions and contributions of these cell populations to the development of the lens, olfactory, otic, pituitary gland, and cranial ganglia are vital for appropriate peripheral nervous system development. Here, we review the origins of both neural crest and placode cells at the neural plate border of the early vertebrate embryo and investigate the molecular and environmental signals that influence specification of different sensory regions. Finally, we discuss the underlying molecular pathways contributing to the complex vertebrate sensory system from an evolutionary perspective, from basal vertebrates to amniotes.

Defective jagged-1 signaling affects GnRH development and contributes to congenital hypogonadotropic hypogonadism

In vertebrate species, fertility is controlled by gonadotropin-releasing hormone (GnRH) neurons. GnRH cells arise outside the central nervous system, in the developing olfactory pit, and migrate along olfactory/vomeronasal/terminal nerve axons into the forebrain during embryonic development. Congenital hypogonadotropic hypogonadism (CHH) and Kallmann syndrome are rare genetic disorders characterized by infertility, and they are associated with defects in GnRH neuron migration and/or altered GnRH secretion and signaling. Here, we documented the expression of the jagged-1/Notch signaling pathway in GnRH neurons and along the GnRH neuron migratory route both in zebrafish embryos and in human fetuses. Genetic knockdown of the zebrafish ortholog of JAG1 (jag1b) resulted in altered GnRH migration and olfactory axonal projections to the olfactory bulbs. Next-generation sequencing was performed in 467 CHH unrelated probands, leading to the identification of heterozygous rare variants in JAG1. Functional in vitro validation of JAG1 mutants revealed that 7 out of the 9 studied variants exhibited reduced protein levels and altered subcellular localization. Together our data provide compelling evidence that Jag1/Notch signaling plays a prominent role in the development of GnRH neurons, and we propose that JAG1 insufficiency may contribute to the pathogenesis of CHH in humans.

Identity, lineage and fates of a temporally distinct progenitor population in the embryonic olfactory epithelium

We defined a temporally and transcriptionally divergent precursor cohort in the medial olfactory epithelium (OE) shortly after it differentiates as a distinct tissue at mid-gestation in the mouse. This temporally distinct population of Ascl1+ cells in the dorsomedial OE is segregated from Meis1+/Pax7+ progenitors in the lateral OE, and does not appear to be generated by Pax7+ lateral OE precursors. The medial Ascl1+ precursors do not yield a substantial number of early-generated ORNs. Instead, they first generate additional proliferative precursors as well as a distinct population of frontonasal mesenchymal cells associated with the migratory mass that surrounds the nascent olfactory nerve. Parallel to these in vivo distinctions, isolated medial versus lateral OE precursors in vitro retain distinct proliferative capacities and modes of division that reflect their in vivo identities. At later fetal stages, these early dorsomedial Ascl1+ precursors cells generate spatially restricted subsets of ORNs as well as other non-neuronal cell classes. Accordingly, the initial compliment of ORNs and other OE cell types is derived from at least two distinct early precursor populations: lateral Meis1/Pax7+ precursors that generate primarily early ORNs, and a temporally, spatially, and transcriptionally distinct subset of medial Ascl1+ precursors that initially generate additional OE progenitors and apparent migratory mass cells before yielding a subset of ORNs and likely supporting cell classes.

Sociosexual behavior requires both activating and repressive roles of Tfap2e/AP-2ε in vomeronasal sensory neurons

Neuronal identity dictates the position in an epithelium, and the ability to detect, process, and transmit specific signals to specified targets. Transcription factors (TFs) determine cellular identity via direct modulation of genetic transcription and recruiting chromatin modifiers. However, our understanding of the mechanisms that define neuronal identity and their magnitude remain a critical barrier to elucidate the etiology of congenital and neurodegenerative disorders. The rodent vomeronasal organ provides a unique system to examine in detail the molecular mechanisms underlying the differentiation and maturation of chemosensory neurons. Here, we demonstrated that the identity of postmitotic/maturing vomeronasal sensory neurons (VSNs), and vomeronasal-dependent behaviors can be reprogrammed through the rescue of Tfap2e/AP-2ε expression in the Tfap2eNull mice, and partially reprogrammed by inducing ectopic Tfap2e expression in mature apical VSNs. We suggest that the TF Tfap2e can reprogram VSNs bypassing cellular plasticity restrictions, and that it directly controls the expression of batteries of vomeronasal genes.

Revealing the mystery of persistent smell loss in Long COVID patients

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

Notch signaling determines cell-fate specification of the two main types of vomeronasal neurons of rodents

The ability of terrestrial vertebrates to find food and mating partners, and to avoid predators, relies on the detection of chemosensory information. Semiochemicals responsible for social and sexual behaviors are detected by chemosensory neurons of the vomeronasal organ (VNO), which transmits information to the accessory olfactory bulb. The vomeronasal sensory epithelium of most mammalian species contains a uniform vomeronasal system; however, rodents and marsupials have developed a more complex binary vomeronasal system, containing vomeronasal sensory neurons (VSNs) expressing receptors of either the V1R or V2R family. In rodents, V1R/apical and V2R/basal VSNs originate from a common pool of progenitors. Using single cell RNA-sequencing, we identified differential expression of Notch1 receptor and Dll4 ligand between the neuronal precursors at the VSN differentiation dichotomy. Our experiments show that Notch signaling is required for effective differentiation of V2R/basal VSNs. In fact, Notch1 loss of function in neuronal progenitors diverts them to the V1R/apical fate, whereas Notch1 gain of function redirects precursors to V2R/basal. Our results indicate that Notch signaling plays a pivotal role in triggering the binary differentiation dichotomy in the VNO of rodents.

SyNPL: Synthetic Notch pluripotent cell lines to monitor and manipulate cell interactions in vitro and in vivo

Cell-cell interactions govern differentiation and cell competition in pluripotent cells during early development, but the investigation of such processes is hindered by a lack of efficient analysis tools. Here, we introduce SyNPL: clonal pluripotent stem cell lines that employ optimised Synthetic Notch (SynNotch) technology to report cell-cell interactions between engineered ‘sender’ and ‘receiver’ cells in cultured pluripotent cells and chimaeric mouse embryos. A modular design makes it straightforward to adapt the system for programming differentiation decisions non-cell-autonomously in receiver cells in response to direct contact with sender cells. We demonstrate the utility of this system by enforcing neuronal differentiation at the boundary between two cell populations. In summary, we provide a new adaptation of SynNotch technology that could be used to identify cell interactions and to profile changes in gene or protein expression that result from direct cell-cell contact with defined cell populations in culture and in early embryos, and that can be customised to generate synthetic patterning of cell fate decisions.

Summary: Optimised Synthetic Notch circuitry in mouse pluripotent stem cells provides a modular tool with which to monitor cell-cell interactions and program synthetic patterning of cell fates in culture and in embryos.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Insm1-expressing neurons and secretory cells develop from a common pool of progenitors in the sea anemone Nematostella vectensis

Neurons are highly specialized cells present in nearly all animals, but their evolutionary origin and relationship to other cell types are not well understood. We use here the sea anemone Nematostella vectensis as a model system for early-branching animals to gain fresh insights into the evolutionary history of neurons. We generated a transgenic reporter line to show that the transcription factor NvInsm1 is expressed in postmitotic cells that give rise to various types of neurons and secretory cells. Expression analyses, double transgenics, and gene knockdown experiments show that the NvInsm1-expressing neurons and secretory cells derive from a common pool of NvSoxB(2)-positive progenitor cells. These findings, together with the requirement for Insm1 for the development of neurons and endocrine cells in vertebrates, support a close evolutionary relationship of neurons and secretory cells.

Sox2 regulates globose basal cell regeneration in the olfactory epithelium

BACKGROUND

In the olfactory epithelium, mitotically active globose basal cells (GBCs) continuously replenish olfactory sensory neurons (OSNs) lost throughout life. Although an essential role of the transcription factor Sox2 in expanding olfactory progenitors/stem cells has been shown, its precise role in olfactory GBCs remain incompletely understood.

METHODS

We characterized the Sox2 expression in olfactory GBCs in normal conditions and in a lesion-regeneration model using a Lgr5EGFP-IRES-creERT2 strain. During GBC-mediated regeneration, genetic deletion of sox2 and lineage tracing experiments were performed to examine the function of Sox2 in the progeny of Lgr5-EGFP⁺ GBCs.

RESULTS

Over 95% of Lgr5-EGFP⁺ GBCs express Sox2 in normal or regeneration conditions. Loss of Sox2 dramatically reduces the cell number in each lineage traced cluster. In the progeny of Lgr5-EGFP⁺ GBCs, loss of Sox2 significantly decreased the portion of OMP⁺ OSNs. However, the generation of sustentacular cells was unchanged.

CONCLUSIONS

Our observations support an essential role of Sox2 in adult olfactory regeneration, likely acting on neuronal-lineage GBCs.

HES1 protein oscillations are necessary for neural stem cells to exit from quiescence

Quiescence is a dynamic process of reversible cell cycle arrest. High-level persistent expression of the HES1 transcriptional repressor, which oscillates with an ultradian periodicity in proliferative neural stem cells (NSCs), is thought to mediate quiescence. However, it is not known whether this is due to a change in levels or dynamics. Here, we induce quiescence in embryonic NSCs with BMP4, which does not increase HES1 level, and we find that HES1 continues to oscillate. To assess the role of HES1 dynamics, we express persistent HES1 under a moderate strength promoter, which overrides the endogenous oscillations while maintaining the total HES1 level within physiological range. We find that persistent HES1 does not affect proliferation or entry into quiescence; however, exit from quiescence is impeded. Thus, oscillatory expression of HES1 is specifically required for NSCs to exit quiescence, a finding of potential importance for controlling reactivation of stem cells in tissue regeneration and cancer.

Mechanisms underlying pre- and postnatal development of the vomeronasal organ

The vomeronasal organ (VNO) is sensory organ located in the ventral region of the nasal cavity in rodents. The VNO develops from the olfactory placode during the secondary invagination of olfactory pit. The embryonic vomeronasal structure appears as a neurogenic area where migratory neuronal populations like endocrine gonadotropin-releasing hormone-1 (GnRH-1) neurons form. Even though embryonic vomeronasal structures are conserved across most vertebrate species, many species including humans do not have a functional VNO after birth. The vomeronasal epithelium (VNE) of rodents is composed of two major types of vomeronasal sensory neurons (VSNs): (1) VSNs distributed in the apical VNE regions that express vomeronasal type-1 receptors (V1Rs) and the G protein subunit Gαi2, and (2) VSNs in the basal territories of the VNE that express vomeronasal type-2 receptors (V2Rs) and the G subunit Gαo. Recent studies identified a third subclass of Gαi2 and Gαo VSNs that express the formyl peptide receptor family. VSNs expressing V1Rs or V2Rs send their axons to distinct regions of the accessory olfactory bulb (AOB). Together, VNO and AOB form the accessory olfactory system (AOS), an olfactory subsystem that coordinates the social and sexual behaviors of many vertebrate species. In this review, we summarize our current understanding of cellular and molecular mechanisms that underlie VNO development. We also discuss open questions for study, which we suggest will further enhance our understanding of VNO morphogenesis at embryonic and postnatal stages.

Bcl11b is required for proper odorant receptor expression in the mouse septal organ

Individual olfactory sensory neurons (OSNs) in the mouse main olfactory epithelium express a single odorant receptor (OR) gene from the repertoire of either class I or class II ORs. The transcription factor Bcl11b determines the OR class to be expressed in OSNs. The septal organ (SO), a small neuroepithelium located at the ventral base of the nasal septum, is considered as an olfactory subsystem because it expresses a specific subset of ORs. However, the mechanisms underlying the generation and differentiation of SO-OSN remain unknown. In the present study, we show that the generation and differentiation of SO-OSN employ the same genetic pathway as in the OSN lineage, which is initiated by the neuronal fate determinant factor Ascl1. Additionally, the key role of Bcl11b in the SO is demonstrated by the abnormal phenotypes of Bcl11b-deficient mice: significant reduction in the expression of OR genes and in the number of mature SO-OSNs. Although SO-OSNs are specified to express a subset of class II OR genes in wild-type mice, the Bcl11b deletion led to the expression of class I OR genes, while the expression of class II OR genes was significantly decreased, with one exception of Olfr15. These results indicate that Bcl11b is necessary for proper OR expression in SO-OSNs.

Maturation of the Olfactory Sensory Neuron and Its Cilia

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

Investigating cellular and molecular mechanisms of neurogenesis in Capitella teleta sheds light on the ancestor of Annelida

BACKGROUND

Diverse architectures of nervous systems (NSs) such as a plexus in cnidarians or a more centralized nervous system (CNS) in insects and vertebrates are present across Metazoa, but it is unclear what selection pressures drove evolution and diversification of NSs. One underlying aspect of this diversity lies in the cellular and molecular mechanisms driving neurogenesis, i.e. generation of neurons from neural precursor cells (NPCs). In cnidarians, vertebrates, and arthropods, homologs of SoxB and bHLH proneural genes control different steps of neurogenesis, suggesting that some neurogenic mechanisms may be conserved. However, data are lacking for spiralian taxa.

RESULTS

To that end, we characterized NPCs and their daughters at different stages of neurogenesis in the spiralian annelid Capitella teleta. We assessed cellular division patterns in the neuroectoderm using static and pulse-chase labeling with thymidine analogs (EdU and BrdU), which enabled identification of NPCs that underwent multiple rounds of division. Actively-dividing brain NPCs were found to be apically-localized, whereas actively-dividing NPCs for the ventral nerve cord (VNC) were found apically, basally, and closer to the ventral midline. We used lineage tracing to characterize the changing boundary of the trunk neuroectoderm. Finally, to start to generate a genetic hierarchy, we performed double-fluorescent in-situ hybridization (FISH) and single-FISH plus EdU labeling for neurogenic gene homologs. In the brain and VNC, Ct-soxB1 and Ct-neurogenin were expressed in a large proportion of apically-localized, EdU+ NPCs. In contrast, Ct-ash1 was expressed in a small subset of apically-localized, EdU+ NPCs and subsurface, EdU- cells, but not in Ct-neuroD+ or Ct-elav1+ cells, which also were subsurface.

CONCLUSIONS

Our data suggest a putative genetic hierarchy with Ct-soxB1 and Ct-neurogenin at the top, followed by Ct-ash1, then Ct-neuroD, and finally Ct-elav1. Comparison of our data with that from Platynereis dumerilii revealed expression of neurogenin homologs in proliferating NPCs in annelids, which appears different than the expression of vertebrate neurogenin homologs in cells that are exiting the cell cycle. Furthermore, differences between neurogenesis in the head versus trunk of C. teleta suggest that these two tissues may be independent developmental modules, possibly with differing evolutionary trajectories.

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.

Comparing Sensory Organs to Define the Path for Hair Cell Regeneration

Deafness or hearing deficits are debilitating conditions. They are often caused by loss of sensory hair cells or defects in their function. In contrast to mammals, nonmammalian vertebrates robustly regenerate hair cells after injury. Studying the molecular and cellular basis of nonmammalian vertebrate hair cell regeneration provides valuable insights into developing cures for human deafness. In this review, we discuss the current literature on hair cell regeneration in the context of other models for sensory cell regeneration, such as the retina and the olfactory epithelium. This comparison reveals commonalities with, as well as differences between, the different regenerating systems, which begin to define a cellular and molecular blueprint of regeneration. In addition, we propose how new technical advances can address outstanding questions in the field.

Nasal Placode Development, GnRH Neuronal Migration and Kallmann Syndrome

The development of Gonadotropin releasing hormone-1 (GnRH) neurons is important for a functional reproduction system in vertebrates. Disruption of GnRH results in hypogonadism and if accompanied by anosmia is termed Kallmann Syndrome (KS). From their origin in the nasal placode, GnRH neurons migrate along the olfactory-derived vomeronasal axons to the nasal forebrain junction and then turn caudally into the developing forebrain. Although research on the origin of GnRH neurons, their migration and genes associated with KS has identified multiple factors that influence development of this system, several aspects still remain unclear. This review discusses development of the olfactory system, factors that regulate GnRH neuron formation and development of the olfactory system, migration of the GnRH neurons from the nose into the brain, and mutations in humans with KS that result from disruption of normal GnRH/olfactory systems development.

A Short History of Nearly Every Sense-The Evolutionary History of Vertebrate Sensory Cell Types

Evolving from filter feeding chordate ancestors, vertebrates adopted a more active life style. These ecological and behavioral changes went along with an elaboration of the vertebrate head including novel complex paired sense organs such as the eyes, inner ears, and olfactory epithelia. However, the photoreceptors, mechanoreceptors, and chemoreceptors used in these sense organs have a long evolutionary history and homologous cell types can be recognized in many other bilaterians or even cnidarians. After briefly introducing some of the major sensory cell types found in vertebrates, this review summarizes the phylogenetic distribution of sensory cell types in metazoans and presents a scenario for the evolutionary history of various sensory cell types involving several cell type diversification and fusion events. It is proposed that the evolution of novel cranial sense organs in vertebrates involved the redeployment of evolutionarily ancient sensory cell types for building larger and more complex sense organs.

A non-canonical role for the proneural gene Neurog1 as a negative regulator of neocortical neurogenesis

Neural progenitors undergo temporal identity transitions to sequentially generate the neuronal and glial cells that make up the mature brain. Proneural genes have well-characterised roles in promoting neural cell differentiation and subtype specification, but they also regulate the timing of identity transitions through poorly understood mechanisms. Here, we investigated how the highly related proneural genes Neurog1 and Neurog2 interact to control the timing of neocortical neurogenesis. We found that Neurog1 acts in an atypical fashion as it is required to suppress rather than promote neuronal differentiation in early corticogenesis. In Neurog1-/- neocortices, early born neurons differentiate in excess, whereas, in vitro, Neurog1-/- progenitors have a decreased propensity to proliferate and form neurospheres. Instead, Neurog1-/- progenitors preferentially generate neurons, a phenotype restricted to the Neurog2+ progenitor pool. Mechanistically, Neurog1 and Neurog2 heterodimerise, and while Neurog1 and Neurog2 individually promote neurogenesis, misexpression together blocks this effect. Finally, Neurog1 is also required to induce the expression of neurogenic factors (Dll1 and Hes5) and to repress the expression of neuronal differentiation genes (Fezf2 and Neurod6). Neurog1 thus employs different mechanisms to temper the pace of early neocortical neurogenesis.

The transcription factor Tfap2e/AP-2ε plays a pivotal role in maintaining the identity of basal vomeronasal sensory neurons

The identity of individual neuronal cell types is defined and maintained by the expression of specific combinations of transcriptional regulators that control cell type-specific genetic programs. The epithelium of the vomeronasal organ of mice contains two major types of vomeronasal sensory neurons (VSNs): 1) the apical VSNs which express vomeronasal 1 receptors (V1r) and the G-protein subunit Gαi2 and; 2) the basal VSNs which express vomeronasal 2 receptors (V2r) and the G-protein subunit Gαo. Both cell types originate from a common pool of progenitors and eventually acquire apical or basal identity through largely unknown mechanisms. The transcription factor AP-2ε, encoded by the Tfap2e gene, plays a role in controlling the development of GABAergic interneurons in the main and accessory olfactory bulb (AOB), moreover AP-2ε has been previously described to be expressed in the basal VSNs. Here we show that AP-2ε is expressed in post-mitotic VSNs after they commit to the basal differentiation program. Loss of AP-2ε function resulted in reduced number of basal VSNs and in an increased number of neurons expressing markers of the apical lineage. Our work suggests that AP-2ε, which is expressed in late phases of differentiation, is not needed to initiate the apical-basal differentiation dichotomy but for maintaining the basal VSNs' identity. In AP-2ε mutants we observed a large number of cells that entered the basal program can express apical genes, our data suggest that differentiated VSNs of mice retain a notable level of plasticity.

Development of Transgenic Mouse Models for the Study of Human Olfactory Dysfunction

Background

Olfactory loss is a significant health problem that remains incompletely understood. The development of suitable animal models is essential to the progress of human olfactory loss research. Recent advancements in transgenic technology allow the creation of model systems to address causes of olfactory neuron dysfunction.

Methods

This review describes two transgenic mouse models with potential usefulness in the study of olfactory loss and highlights the molecular techniques that underlie the development of such systems.

Results

Genetic constructs generated using standard molecular biological techniques are introduced into mouse germ lines either by homologous recombination or by random integration. One construct (UbI7) places the olfactory receptor I7 under control of the olfactory marker protein promoter. The other two constructs (TI) act together to direct expression of cytokines in the olfactory epithelium, creating a novel approach to the study of inflammatory olfactory loss.

Conclusions

Powerful scientific tools now exist to develop animal models useful to the study of human olfactory disease. Transgenic olfactory neurons from the UbI7 mouse strain will respond to known odorants, facilitating experiments that examine in vitro modulation of olfactory neuron function. The ability to express specific genes in the olfactory mucosa of the TI mouse has great potential in elucidating the role of cytokines in the development of olfactory dysfunction in vivo.

Role of neuropeptide Y (NPY) in the differentiation of Trpm-5-positive olfactory microvillar cells

The mouse olfactory neuroepithelium (ON) is comprised of anatomically distinct populations of cells in separate regions; apical (sustentacular and microvillar), neuronal (olfactory sensory neurons) and basal (horizontal and globose basal cells). The existence of microvillar cells (MVCs) is well documented but their nature and function remains unclear. An important transcription factor for the differentiation of MVCs is Skn-1a, with loss of function of Skn-1a in mice resulting in a complete loss of Trpm-5 expressing MVCs, while olfactory sensory neuron differentiation is normal. Our previous research has shown that neuropeptide Y (NPY) is expressed in MVCs and is important in the neuroproliferation of olfactory precursors. This study showed that following X-ray irradiation of the snout of wildtype mice, which decreases the proliferation of basal precursor cells, the numbers of Trpm-5-positive MVCs is increased at 2 and 5 weeks post-irradiation compared to controls. Skn-1a expression in the ON following X-ray irradiation also increases at 2 weeks post-irradiation in a regionally specific manner matching the expression pattern of Trpm-5-positive MVCs. In parallel, NPYCre knock-in mice were used to examine the expression of Skn-1a following activation of NPY unilaterally in the ON (unilateral nasal irrigation of AAV-NPY-FLEX). These experiments demonstrated that Skn-1a is only expressed when NPY is activated in MVCs. Therefore the expression of NPY is necessary for the transcription factor-mediated differentiation of olfactory MVCs.

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.

Sox2 is required for olfactory pit formation and olfactory neurogenesis through BMP restriction and Hes5 upregulation

The transcription factor Sox2 is necessary to maintain pluripotency of embryonic stem cells, and to regulate neural development. Neurogenesis in the vertebrate olfactory epithelium persists from embryonic stages through adulthood. The role Sox2 plays for the development of the olfactory epithelium and neurogenesis within has, however, not been determined. Here, by analysing Sox2 conditional knockout mouse embryos and chick embryos deprived of Sox2 in the olfactory epithelium using CRISPR-Cas9, we show that Sox2 activity is crucial for the induction of the neural progenitor gene Hes5 and for subsequent differentiation of the neuronal lineage. Our results also suggest that Sox2 activity promotes the neurogenic domain in the nasal epithelium by restricting Bmp4 expression. The Sox2-deficient olfactory epithelium displays diminished cell cycle progression and proliferation, a dramatic increase in apoptosis and finally olfactory pit atrophy. Moreover, chromatin immunoprecipitation data show that Sox2 directly binds to the Hes5 promoter in both the PNS and CNS. Taken together, our results indicate that Sox2 is essential to establish, maintain and expand the neuronal progenitor pool by suppressing Bmp4 and upregulating Hes5 expression.

Summary: Analysis of Sox2 mutant mouse and Sox2 CRISPR-targeted chick embryos reveals that Sox2 controls the establishment of sensory progenitors in the olfactory epithelium by suppressing Bmp4 and upregulating Hes5 expression.

RGMB and neogenin control cell differentiation in the developing olfactory epithelium

Cellular interactions are key for the differentiation of distinct cell types within developing epithelia, yet the molecular mechanisms engaged in these interactions remain poorly understood. In the developing olfactory epithelium (OE), neural stem/progenitor cells give rise to odorant-detecting olfactory receptor neurons (ORNs) and glial-like sustentacular (SUS) cells. Here, we show in mice that the transmembrane receptor neogenin (NEO1) and its membrane-bound ligand RGMB control the balance of neurons and glial cells produced in the OE. In this layered epithelium, neogenin is expressed in progenitor cells, while RGMB is restricted to adjacent newly born ORNs. Ablation of Rgmb via gene-targeting increases the number of dividing progenitor cells in the OE and leads to supernumerary SUS cells. Neogenin loss-of-function phenocopies these effects observed in Rgmb(-/-) mice, supporting the proposal that RGMB-neogenin signaling regulates progenitor cell numbers and SUS cell production. Interestingly, Neo1(-/-) mice also exhibit increased apoptosis of ORNs, implicating additional ligands in the neogenin-dependent survival of ORNs. Thus, our results indicate that RGMB-neogenin-mediated cell-cell interactions between newly born neurons and progenitor cells control the ratio of glia and neurons produced in the OE.

Acute inflammation regulates neuroregeneration through the NF-κB pathway in olfactory epithelium

Adult neural stem cells/progenitor cells residing in the basal layer of the olfactory epithelium are capable of reconstituting the neuroepithelium even after severe damage. The molecular events underlying this regenerative capacity remain elusive. Here we show that the repair of neuroepithelium after lesioning is accompanied by an acute, but self-limited, inflammatory process. Attenuation of inflammatory cell recruitment and cytokine production by dexamethasone impairs proliferation of progenitor horizontal basal cells (HBCs) and subsequent neuronal differentiation. Using TNF-α receptor-deficient mice, we identify TNF-α signaling as an important contributor to this inflammatory and reparative process, mainly through TNF-α receptor 1. HBC-selective genetic ablation of RelA (p65), the transcriptional activator of the NF-κB pathway, retards inflammation and impedes proliferation at the early stages of regeneration and suggests HBCs directly participate in cross-talk between immune response and neurogenesis. Loss of RelA in the regenerating neuroepithelium perturbs the homeostasis between proliferation and apoptosis while enhancing JNK signaling. Together, our results support a model in which acute inflammation after injury initiates important regenerative signals in part through NF-κB-mediated signaling that activates neural stem cells to reconstitute the olfactory epithelium.

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.

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.

Transcriptional regulatory network during development in the olfactory epithelium

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

Regeneration and rewiring of rodent olfactory sensory neurons

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

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

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

A cellular and molecular mosaic establishes growth and differentiation states for cranial sensory neurons

We compared apparent origins, cellular diversity and regulation of initial axon growth for differentiating cranial sensory neurons. We assessed the molecular and cellular composition of the developing olfactory and otic placodes, and cranial sensory ganglia to evaluate contributions of ectodermal placode versus neural crest at each site. Special sensory neuron populations-the olfactory and otic placodes, as well as those in vestibulo-acoustic ganglion- are entirely populated with cells expressing cranial placode-associated, rather than neural crest-associated markers. The remaining cranial sensory ganglia are a mosaic of cells that express placode-associated as well as neural crest-associated markers. We found two distinct populations of neural crest in the cranial ganglia: the first, as expected, is labeled by Wnt1:Cre mediated recombination. The second is not labeled by Wnt1:Cre recombination, and expresses both Sox10 and FoxD3. These populations-Wnt1:Cre recombined, and Sox10/Foxd3-expressing- are proliferatively distinct from one another. Together, the two neural crest-associated populations are substantially more proliferative than their placode-associated counterparts. Nevertheless, the apparently placode- and neural crest-associated populations are similarly sensitive to altered signaling that compromises cranial morphogenesis and differentiation. Acute disruption of either Fibroblast growth factor (Fgf) or Retinoic acid (RA) signaling alters axon growth and cell death, but does not preferentially target any of the three distinct populations. Apparently, mosaic derivation and diversity of precursors and early differentiating neurons, modulated uniformly by local signals, supports early cranial sensory neuron differentiation and growth.

Patterning, morphogenesis, and neurogenesis of zebrafish cranial sensory placodes

Peripheral sensory organs and ganglia found in the vertebrate head arise during embryonic development from distinct ectodermal thickenings, called cranial sensory placodes (adenohypophyseal, olfactory, lens, trigeminal, epibranchial, and otic). A series of patterning events leads to the establishment of these placodes. Subsequently, these placodes undergo specific morphogenetic movements and cell-type specification in order to shape the final placodal derivatives and to produce differentiated cell types necessary for their function. In this chapter, we will focus on recent studies in the zebrafish that have advanced our understanding of cranial sensory placode development. We will summarize the signaling events and their molecular effectors guiding the formation of the so-called preplacodal region, and the subsequent subdivision of this region along the anteroposterior axis that gives rise to specific placode identities as well as those controlling morphogenesis and neurogenesis. Finally, we will highlight the approaches used in zebrafish that have been established to precisely label cell populations, to follow their development, and/or to characterize cell fates within a specific placode.

Hes1 inhibitor isolated by target protein oriented natural products isolation (TPO-NAPI) of differentiation activators of neural stem cells

Hes1 dimer inhibitor, agalloside (2), which can accelerate the differentiation of neural stem cells was isolated by using Hes1-immobilized beads.

The Hes1 dimer inhibitor, agalloside (2), which can accelerate the differentiation of neural stem cells is described. Six natural products, including one new natural product, which bind to Hes1 were rapidly isolated by a developed “target protein oriented natural products isolation” (TPO-NAPI) method using Hes1-immobilized beads. Of the six compounds, 2 inhibited Hes1 dimer formation at both the protein- and cellular level. Neural stem cells treated with 2 differentiated to neurons with longer neurites than cells treated with varproic acid or retinoic acid. Moreover, 2 exhibited specificity for neurons. This promotion of differentiation was supported by an increase in the mRNA expression of the proneural genes, Mash1 and Ngn2, which were inhibited by Hes1.

Transcription factor p63 controls the reserve status but not the stemness of horizontal basal cells in the olfactory epithelium

Adult tissue stem cells can serve two broad functions: to participate actively in the maintenance and regeneration of a tissue or to wait in reserve and participate only when activated from a dormant state. The adult olfactory epithelium, a site for ongoing, life-long, robust neurogenesis, contains both of these functional stem cell types. Globose basal cells (GBCs) act as the active stem cell population and can give rise to all the differentiated cells found in the normal tissue. Horizontal basal cells (HBCs) act as reserve stem cells and remain dormant unless activated by tissue injury. Here we show that HBC activation following injury by the olfactotoxic gas methyl bromide is coincident with the down-regulation of protein 63 (p63) but anticipates HBC proliferation. Gain- and loss-of-function studies show that this down-regulation of p63 is necessary and sufficient for HBC activation. Moreover, activated HBCs give rise to GBCs that persist for months and continue to act as bona fide stem cells by participating in tissue maintenance and regeneration over the long term. Our analysis provides mechanistic insight into the dynamics between tissue stem cell subtypes and demonstrates that p63 regulates the reserve state but not the stem cell status of HBCs.

PPARβ/δ and PPARγ maintain undifferentiated phenotypes of mouse adult neural precursor cells from the subventricular zone

The subventricular zone (SVZ) is one of the main niches of neural stem cells in the adult mammalian brain. Stem and precursor cells in this region are the source for neurogenesis and oligodendrogesis, mainly in the olfactory bulb and corpus callosum, respectively. The identification of the molecular components regulating the decision of these cells to differentiate or maintain an undifferentiated state is important in order to understand the modulation of neurogenic processes in physiological and pathological conditions. PPARs are a group of transcription factors, activated by lipid ligands, with important functions in cellular differentiation and proliferation in several tissues. In this work, we demonstrate that mouse adult neural precursor cells (NPCs), in situ and in vitro, express PPARβ/δ and PPARγ. Pharmacological activation of both PPARs isoforms induces proliferation and maintenance of the undifferentiated phenotype. Congruently, inhibition of PPARβ/δ and PPARγ results in a decrease of proliferation and loss of the undifferentiated phenotype. Interestingly, PPARγ regulates the level of EGFR in adult NPCs, concurrent with it is function described in embryonic NPCs. Furthermore, we describe for the first time that PPARβ/δ regulates SOX2 level in adult NPCs, probably through a direct transcriptional regulation, as we identified two putative PPAR response elements in the promoter region of Sox2. EGFR and SOX2 are key players in neural stem/precursor cells self-renewal. Finally, rosiglitazone, a PPARγ ligand, increases PPARβ/δ level, suggesting a possible cooperation between these two PPARs in the control of cell fate behavior. Our work contributes to the understanding of the molecular mechanisms associated to neural cell fate decision and places PPARβ/δ and PPARγ as interesting new targets of modulation of mammalian brain homeostasis.