Target-independent pattern specification in the olfactory epithelium

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.

Developing and maintaining a nose-to-brain map of odorant identity

Olfactory sensory neurons (OSNs) in the olfactory epithelium of the nose transduce chemical odorant stimuli into electrical signals. These signals are then sent to the OSNs' target structure in the brain, the main olfactory bulb (OB), which performs the initial stages of sensory processing in olfaction. The projection of OSNs to the OB is highly organized in a chemospatial map, whereby axon terminals from OSNs expressing the same odorant receptor (OR) coalesce into individual spherical structures known as glomeruli. This nose-to-brain map of odorant identity is built from late embryonic development to early postnatal life, through a complex combination of genetically encoded, OR-dependent and activity-dependent mechanisms. It must then be actively maintained throughout adulthood as OSNs experience turnover due to external insult and ongoing neurogenesis. Our review describes and discusses these two distinct and crucial processes in olfaction, focusing on the known mechanisms that first establish and then maintain chemospatial order in the mammalian OSN-to-OB projection.

Identification and characterization of olfactory receptor genes and olfactory perception in rapa whelk Rapana venosa (Valenciennes, 1846) during larval settlement and metamorphosis

The rapa whelk Rapana venosa, an economically important marine fishery resource in China but a major invader all over the world, changes from a phytophagous to a carnivorous form following settlement and metamorphosis. However, the low settlement and metamorphosis rates (<1%) of larvae limit the abundance of R. venosa. This critical step (settlement and metamorphosis) remains poorly characterized but may be related to how larvae perceive the presence of shellfish, their new source of food. Here, we report that larvae may use olfactory perception to sense shellfish. Olfactory receptor (OR) genes are involved in odor sensing in animals. We identified a total of 463 OR genes, which could be grouped into nine clades based on phylogenetic analysis. When assessing the attraction of larvae at different developmental stages to oyster odor, R. venosa showed active settlement and metamorphosis behavior only at the J4 stage (competent larva, 1000-1500 μm shell length) and in the presence of shellfish odor at the same time. Expression of OR gene family members differed between stage 2 (four-spiral whorl stage) and stage 1 (single- to three-spiral whorl stage), indicating significant changes in the olfactory system during larval development. These findings broaden our understanding of olfactory perception, settlement, and metamorphosis in gastropods and can be used to improve R. venosa harvesting, as well as the sustainable development and utilization of this resource.

Differences in olfactory habituation between orthonasal and retronasal pathways

The odorant arrives at nasal olfactory epithelium ortho- and retronasally. This experiment aimed to study the potential different olfactory habituation in orthonasal and retronasal pathways. 68 subjects were stimulated by constant airflow with an odor (50% phenethyl alcohol, PEA or 5% n-butyl acetate, BA) presented ortho- or retronasally. Participants rated the perceived odor intensity (0-10 points) per minute until the odor sensation disappeared. We also investigated the cross-habituation: when the subjects achieved full habituation, continue to rate odor intensity in a different pathway after instantly switching the odor stimulation pathway. The olfactory habituation curve was drawn. The differences of ratings between the orthonasal and retronasal olfaction at different time points and between male and female subjects were analyzed. The two odor intensity ratings decreased as the time extended, share the same "fast followed by slow" type. The ratings of orthonasal olfaction decreased faster than that of retronasal. The intensity rating of PEA of male retronasal approach was lower than that of female at the 5th min (p = 0.018). When orthonasal full habituation achieved, there was significant difference between the intensity ratings and the initial ratings of the retronasal stimulation pathway (p < 0.0001), and vice versa. We found obvious habituation as well as cross-habituation in both orthonasal and retronasal olfaction. The habituation of orthonasal olfaction was faster than that of retronasal olfaction. These different habituations were related to the gender.

Renewal and Differentiation of GCD Necklace Olfactory Sensory Neurons

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

Why Does the Face Predict the Brain? Neural Crest Induction, Craniofacial Morphogenesis, and Neural Circuit Development

Mesenchephalic and rhombencephalic neural crest cells generate the craniofacial skeleton, special sensory organs, and subsets of cranial sensory receptor neurons. They do so while preserving the anterior-posterior (A-P) identity of their neural tube origins. This organizational principle is paralleled by central nervous system circuits that receive and process information from facial structures whose A-P identity is in register with that in the brain. Prior to morphogenesis of the face and its circuits, however, neural crest cells act as "inductive ambassadors" from distinct regions of the neural tube to induce differentiation of target craniofacial domains and establish an initial interface between the brain and face. At every site of bilateral, non-axial secondary induction, neural crest constitutes all or some of the mesenchymal compartment for non-axial mesenchymal/epithelial (M/E) interactions. Thus, for epithelial domains in the craniofacial primordia, aortic arches, limbs, the spinal cord, and the forebrain (Fb), neural crest-derived mesenchymal cells establish local sources of inductive signaling molecules that drive morphogenesis and cellular differentiation. This common mechanism for building brains, faces, limbs, and hearts, A-P axis specified, neural crest-mediated M/E induction, coordinates differentiation of distal structures, peripheral neurons that provide their sensory or autonomic innervation in some cases, and central neural circuits that regulate their behavioral functions. The essential role of this neural crest-mediated mechanism identifies it as a prime target for pathogenesis in a broad range of neurodevelopmental disorders. Thus, the face and the brain "predict" one another, and this mutual developmental relationship provides a key target for disruption by developmental pathology.

Developmental regulation of olfactory circuit formation in mice

In mammals, odorants induce various behavioral responses that are critical to the survival of the individual and species. Binding signals of odorants to odorant receptors (ORs) expressed in the olfactory epithelia are converted to an odor map, a pattern of activated glomeruli, in the olfactory bulb (OB). This topographic map is used to identify odorants for memory‐based learned decisions. In the embryo, a coarse olfactory map is generated in the OB by a combination of dorsal‐ventral and anterior‐posterior targeting of olfactory sensory neurons (OSNs), using specific sets of axon‐guidance molecules. During the process of OSN projection, odor signals are sorted into distinct odor qualities in separate functional domains in the OB. Odor information is then conveyed by the projection neurons, mitral/tufted cells, to various regions in the olfactory cortex, particularly to the amygdala for innate olfactory decisions. Although the basic architecture of hard‐wired circuits is generated by a genetic program, innate olfactory responses are modified by neonatal odor experience in an activity‐dependent manner. Stimulus‐driven OR activity promotes post‐synaptic events and dendrite selection in the responding glomeruli making them larger. As a result, enhanced odor inputs in neonates establish imprinted olfactory memory that induces attractive responses in adults, even when the odor quality is innately aversive. In this paper, I will provide an overview of the recent progress made in the olfactory circuit formation in mice.

The strengths of the genetic approach to understanding neural systems development and function: Ray Guillery's synthesis

The organization and function of sensory systems, especially the mammalian visual system, has been the focus of philosophers and scientists for centuries-from Descartes and Newton onward. Nevertheless, the utility of understanding development and its genetic foundations for deeper insight into neural function has been debated: Do you need to know how something is assembled-a car, for example-to understand how it works or how to use it-to turn on the ignition and drive? This review addresses this issue for sensory pathways. The pioneering work of the late Rainer W. (Ray) Guillery provides an unequivocal answer to this central question: Using genetics for mechanistic exploration of sensory system development yields essential knowledge of organization and function. Ray truly built the foundation for this now accepted tenet of modern neuroscience. His work on the development and reorganization of visual pathways in albino mammals-all with primary genetic mutations in genes for pigmentation-defined the genetic approach to neural systems development, function and plasticity. The work that followed his lead in a variety of sensory systems, including my own work in the developing olfactory system, proceeds directly from Ray's fundamental contributions.

Differential timing of neurogenesis underlies dorsal-ventral topographic projection of olfactory sensory neurons

Background

The mammalian primary olfactory system has a spatially-ordered projection in which olfactory sensory neurons (OSNs) located in the dorsomedial (DM) and ventrolateral (VL) region of the olfactory epithelium (OE) send their axons to the dorsal and ventral region of the olfactory bulb (OB), respectively. We previously found that OSN axonal projections occur sequentially, from the DM to the VL region of the OE. The differential timing of axonal projections is important for olfactory map formation because early-arriving OSN axons secrete guidance cues at the OB to help navigate late-arriving OSN axons. We hypothesized that the differential timing of axonal projections is regulated by the timing of OSN neurogenesis. To test this idea, we investigated spatiotemporal patterns of OSN neurogenesis during olfactory development.

Methods and results

To determine the time of OSN origin, we used two thymidine analogs, BrdU and EdU, which can be incorporated into cells in the S-phase of the cell-cycle. We injected these two analogs at different developmental time points and analyzed distribution patterns of labeled OSNs. We found that OSNs with different dates of origin were differentially distributed in the OE. The majority of OSNs generated at the early stage of development were located in the DM region of the OE, whereas OSNs generated at the later stage of development were preferentially located in the VL region of the OE.

Conclusions

These results indicate that the number of OSNs is sequentially increased from the DM to the VL axis of the OE. Moreover, the temporal sequence of OSN proliferation correlates with that of axonal extension and emergence of glomerular structures in the OB. Thus, we propose that the timing of OSN neurogenesis regulates that of OSN axonal projection and thereby helps preserve the topographic order of the olfactory glomerular map along the dorsal–ventral axis of the OB.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0079-0) contains supplementary material, which is available to authorized users.

Lysine-specific demethylase-1 (LSD1) is compartmentalized at nuclear chromocenters in early post-mitotic cells of the olfactory sensory neuronal lineage

Mammalian olfaction depends on the development of specialized olfactory sensory neurons (OSNs) that each express one odorant receptor (OR) protein from a large family of OR genes encoded in the genome. The lysine-specific demethylase-1 (LSD1) protein removes activating H3K4 or silencing H3K9 methylation marks at gene promoters and is required for proper OR regulation. We show that LSD1 protein exhibits variable organization within nuclei of developing OSNs, and tends to consolidate into a single dominant compartment at the edges of chromocenters within nuclei of early post-mitotic cells of the mouse olfactory epithelium (MOE). Using an immortalized cell line derived from developing olfactory placode, we show that consolidation of LSD1 appears to be cell-cycle regulated, with a peak occurrence in early G1. LSD1 co-compartmentalizes with CoREST, a protein known to collaborate with LSD1 to carry out a variety of chromatin-modifying functions. We show that LSD1 compartments co-localize with 1-3 OR loci at the exclusion of most OR genes, and commonly associate with Lhx2, a transcription factor involved in OR regulation. Together, our data suggests that LSD1 is sequestered into a distinct nuclear space that might restrict a histone-modifying function to a narrow developmental time window and/or range of OR gene targets.

Neural map formation in the mouse olfactory system

In the mouse olfactory system, odorants are detected by ~1,000 different odorant receptors (ORs) produced by olfactory sensory neurons (OSNs). Each OSN expresses only one functional OR species, which is referred to as the “one neuron–one receptor” rule. Furthermore, OSN axons bearing the same OR converge to a specific projection site in the olfactory bulb (OB) forming a glomerular structure, i.e., the “one glomerulus–one receptor” rule. Based on these basic rules, binding signals of odorants detected by OSNs are converted to topographic information of activated glomeruli in the OB. During development, the glomerular map is formed by the combination of two genetically programmed processes: one is OR-independent projection along the dorsal–ventral axis, and the other is OR-dependent projection along the anterior-posterior axis. The map is further refined in an activity-dependent manner during the neonatal period. Here, we summarize recent progress of neural map formation in the mouse olfactory system.

Developmental expression of the calcium-activated chloride channels TMEM16A and TMEM16B in the mouse olfactory epithelium

Calcium-activated chloride channels are involved in several physiological processes including olfactory perception. TMEM16A and TMEM16B, members of the transmembrane protein 16 family (TMEM16), are responsible for calcium-activated chloride currents in several cells. Both are present in the olfactory epithelium of adult mice, but little is known about their expression during embryonic development. Using immunohistochemistry we studied their expression in the mouse olfactory epithelium at various stages of prenatal development from embryonic day (E) 12.5 to E18.5 as well as in postnatal mice. At E12.5, TMEM16A immunoreactivity was present at the apical surface of the entire olfactory epithelium, but from E16.5 became restricted to a region near the transition zone with the respiratory epithelium, where localized at the apical part of supporting cells and in their microvilli. In contrast, TMEM16B immunoreactivity was present at E14.5 at the apical surface of the entire olfactory epithelium, increased in subsequent days, and localized to the cilia of mature olfactory sensory neurons. These data suggest different functional roles for TMEM16A and TMEM16B in the developing as well as in the postnatal olfactory epithelium. The presence of TMEM16A at the apical part and in microvilli of supporting cells is consistent with a role in the regulation of the chloride ionic composition of the mucus covering the apical surface of the olfactory epithelium, whereas the localization of TMEM16B to the cilia of mature olfactory sensory neurons is consistent with a role in olfactory signal transduction.

Molecular events in the cell types of the olfactory epithelium during adult neurogenesis

Background

Adult neurogenesis, fundamental for cellular homeostasis in the mammalian olfactory epithelium, requires major shifts in gene expression to produce mature olfactory sensory neurons (OSNs) from multipotent progenitor cells. To understand these dynamic events requires identifying not only the genes involved but also the cell types that express each gene. Only then can the interrelationships of the encoded proteins reveal the sequences of molecular events that control the plasticity of the adult olfactory epithelium.

Results

Of 4,057 differentially abundant mRNAs at 5 days after lesion-induced OSN replacement in adult mice, 2,334 were decreased mRNAs expressed by mature OSNs. Of the 1,723 increased mRNAs, many were expressed by cell types other than OSNs and encoded proteins involved in cell proliferation and transcriptional regulation, consistent with increased basal cell proliferation. Others encoded fatty acid metabolism and lysosomal proteins expressed by infiltrating macrophages that help scavenge debris from the apoptosis of mature OSNs. The mRNAs of immature OSNs behaved dichotomously, increasing if they supported early events in OSN differentiation (axon initiation, vesicular trafficking, cytoskeletal organization and focal adhesions) but decreasing if they supported homeostatic processes that carry over into mature OSNs (energy production, axon maintenance and protein catabolism). The complexity of shifts in gene expression responsible for converting basal cells into neurons was evident in the increased abundance of 203 transcriptional regulators expressed by basal cells and immature OSNs.

Conclusions

Many of the molecular changes evoked during adult neurogenesis can now be ascribed to specific cellular events in the OSN cell lineage, thereby defining new stages in the development of these neurons. Most notably, the patterns of gene expression in immature OSNs changed in a characteristic fashion as these neurons differentiated. Initial patterns were consistent with the transition into a neuronal morphology (neuritogenesis) and later patterns with neuronal homeostasis. Overall, gene expression patterns during adult olfactory neurogenesis showed substantial similarity to those of embryonic brain.

Expression profile of G-protein βγ subunit gene transcripts in the mouse olfactory sensory epithelia

Heterotrimeric G-proteins mediate a variety of cellular functions, including signal transduction in sensory neurons of the olfactory system. Whereas the Gα subunits in these neurons are well characterized, the gene transcript expression profile of Gβγ subunits is largely missing. Here we report our comprehensive expression analysis to identify Gβ and Gγ subunit gene transcripts in the mouse main olfactory epithelium (MOE) and the vomeronasal organ (VNO). Our reverse transcriptase PCR (RT-PCR) and realtime qPCR analyses of all known Gβ (β_1,2,3,4,5) and Gγ (γ_{1,2,2t,3,4,5,7,8,10,11,12,13}) subunits indicate presence of multiple Gβ and Gγ subunit gene transcripts in the MOE and the VNO at various expression levels. These results are supported by our RNA in situ hybridization (RISH) experiments, which reveal the expression patterns of two Gβ subunits and four Gγ subunits in the MOE as well as one Gβ and four Gγ subunits in the VNO. Using double-probe fluorescence RISH and line intensity scan analysis of the RISH signals of two dominant Gβγ subunits, we show that Gγ₁₃ is expressed in mature olfactory sensory neurons (OSNs), while Gβ₁ is present in both mature and immature OSNs. Interestingly, we also found Gβ₁ to be the dominant Gβ subunit in the VNO and present throughout the sensory epithelium. In contrast, we found diverse expression of Gγ subunit gene transcripts with Gγ₂, Gγ₃, and Gγ₁₃ in the Gα_i2-expressing neuronal population, while Gγ₈ is expressed in both layers. Further, we determined the expression of these Gβγ gene transcripts in three post-natal developmental stages (p0, 7, and 14) and found their cell-type specific expression remains largely unchanged, except the transient expression of Gγ₂ in a single basal layer of cells in the MOE during P7 and P14. Taken together, our comprehensive expression analyses reveal cell-type specific gene expression of multiple Gβ and Gγ in sensory neurons of the olfactory system.

Odorant responsiveness of embryonic mouse olfactory sensory neurons expressing the odorant receptors S1 or MOR23

The mammalian olfactory system has developed some functionality by the time of birth. There is behavioral and limited electrophysiological evidence for prenatal olfaction in various mammalian species. However, there have been no reports, in any mammalian species, of recordings from prenatal olfactory sensory neurons (OSNs) that express a given odorant receptor (OR) gene. Here we have performed patch-clamp recordings from mouse OSNs that express the OR gene S1 or MOR23, using the odorous ligands 2-phenylethyl alcohol or lyral, respectively. We found that, out of a combined total of 20 OSNs from embryos of these two strains at embryonic day (E)16.5 or later, all responded to a cognate odorous ligand. By contrast, none of six OSNs responded to the ligand at E14.5 or E15.5. The kinetics of the odorant-evoked electrophysiological responses of prenatal OSNs are similar to those of postnatal OSNs. The S1 and MOR23 glomeruli in the olfactory bulb are formed postnatally, but the axon terminals of OSNs expressing these OR genes may be synaptically active in the olfactory bulb at embryonic stages. The upper limit of the acquisition of odorant responsiveness for S1 and MOR23 OSNs at E16.5 is consistent with the developmental expression patterns of components of the olfactory signaling pathway.

Getting neural circuits into shape with semaphorins

Semaphorins are key players in the control of neural circuit development. Recent studies have uncovered several exciting and novel aspects of neuronal semaphorin signalling in various cellular processes--including neuronal polarization, topographical mapping and axon sorting--that are crucial for the assembly of functional neuronal connections. This progress is important for further understanding the many neuronal and non-neuronal functions of semaphorins and for gaining insight into their emerging roles in the perturbed neural connectivity that is observed in some diseases. This Review discusses recent advances in semaphorin research, focusing on novel aspects of neuronal semaphorin receptor regulation and previously unexplored cellular functions of semaphorins in the nervous system.

Respiratory and olfactory cytotoxicity of inhaled 2,3-pentanedione in Sprague-Dawley rats

Flavorings-related lung disease is a potentially disabling disease of food industry workers associated with exposure to the α-diketone butter flavoring, diacetyl (2,3-butanedione). To investigate the hypothesis that another α-diketone flavoring, 2,3-pentanedione, would cause airway damage, rats that inhaled air, 2,3-pentanedione (112, 241, 318, or 354 ppm), or diacetyl (240 ppm) for 6 hours were sacrificed the following day. Rats inhaling 2,3-pentanedione developed necrotizing rhinitis, tracheitis, and bronchitis comparable to diacetyl-induced injury. To investigate delayed toxicity, additional rats inhaled 318 (range, 317.9-318.9) ppm 2,3-pentanedione for 6 hours and were sacrificed 0 to 2, 12 to 14, or 18 to 20 hours after exposure. Respiratory epithelial injury in the upper nose involved both apoptosis and necrosis, which progressed through 12 to 14 hours after exposure. Olfactory neuroepithelial injury included loss of olfactory neurons that showed reduced expression of the 2,3-pentanedione-metabolizing enzyme, dicarbonyl/L-xylulose reductase, relative to sustentacular cells. Caspase 3 activation occasionally involved olfactory nerve bundles that synapse in the olfactory bulb (OB). An additional group of rats inhaling 270 ppm 2,3-pentanedione for 6 hours 41 minutes showed increased expression of IL-6 and nitric oxide synthase-2 and decreased expression of vascular endothelial growth factor A in the OB, striatum, hippocampus, and cerebellum using real-time PCR. Claudin-1 expression increased in the OB and striatum. We conclude that 2,3-pentanedione is a respiratory hazard that can also alter gene expression in the brain.

Wired for behaviors: from development to function of innate limbic system circuitry

The limbic system of the brain regulates a number of behaviors that are essential for the survival of all vertebrate species including humans. The limbic system predominantly controls appropriate responses to stimuli with social, emotional, or motivational salience, which includes innate behaviors such as mating, aggression, and defense. Activation of circuits regulating these innate behaviors begins in the periphery with sensory stimulation (primarily via the olfactory system in rodents), and is then processed in the brain by a set of delineated structures that primarily includes the amygdala and hypothalamus. While the basic neuroanatomy of these connections is well-established, much remains unknown about how information is processed within innate circuits and how genetic hierarchies regulate development and function of these circuits. Utilizing innovative technologies including channel rhodopsin-based circuit manipulation and genetic manipulation in rodents, recent studies have begun to answer these central questions. In this article we review the current understanding of how limbic circuits regulate sexually dimorphic behaviors and how these circuits are established and shaped during pre- and post-natal development. We also discuss how understanding developmental processes of innate circuit formation may inform behavioral alterations observed in neurodevelopmental disorders, such as autism spectrum disorders, which are characterized by limbic system dysfunction.

Axon-axon interactions in neuronal circuit assembly: lessons from olfactory map formation

During the development of the nervous system, neurons often connect axons and dendrites over long distances, which are navigated by chemical cues. During the past few decades, studies on axon guidance have focused on chemical cues provided by the axonal target or intermediate target. However, recent studies have shed light on the roles and mechanisms underlying axon-axon interactions during neuronal circuit assembly. The roles of axon-axon interactions are best exemplified in recent studies on olfactory map formation in vertebrates. Pioneer-follower interaction is essential for the axonal pathfinding process. Pre-target axon sorting establishes the anterior-posterior map order. The temporal order of axonal projection is converted to dorsal-ventral topography with the aid of secreted molecules provided by early-arriving axons. An activity-dependent process to form a discrete map also depends on axon sorting. Thus, an emerging principle of olfactory map formation is the 'self-organisation' of axons rather than the 'lock and key' matching between axons and targets. In this review, we discuss how axon-axon interactions contribute to neuronal circuit assembly.

How is the olfactory map formed and interpreted in the mammalian brain?

Odor signals received by odorant receptors (ORs) expressed by olfactory sensory neurons (OSNs) in the olfactory epithelium (OE) are represented as an odor map in the olfactory bulb (OB). In the mouse, there are ~1,000 different OR species, and each OSN expresses only one functional OR gene in a monoallelic manner. Furthermore, OSN axons expressing the same type of OR converge on a specific target site in the OB, forming a glomerular structure. Because each glomerulus represents a single OR species, and a single odorant can interact with multiple OR species, odor signals received in the OE are converted into a topographic map of multiple glomeruli activated with varying magnitudes. Here we review recent progress in the study of the mammalian olfactory system, focusing on the formation of the olfactory map and the transmission of topographical information in the OB to the olfactory cortex to elicit various behaviors.

Bcl11b/Ctip2 controls the differentiation of vomeronasal sensory neurons in mice

The transcription factor Bcl11b/Ctip2 plays critical roles in the development of several systems and organs, including the immune system, CNS, skin, and teeth. Here, we show that Bcl11b/Ctip2 is highly expressed in the developing vomeronasal system in mice and is required for its proper development. Bcl11b/Ctip2 is expressed in postmitotic vomeronasal sensory neurons (VSNs) in the vomeronasal epithelium (VNE) as well as projection neurons and GABAergic interneurons in the accessory olfactory bulb (AOB). In the absence of Bcl11b, these neurons are born in the correct number, but VSNs selectively die by apoptosis. The critical role of Bcl11b in vomeronasal system development is demonstrated by the abnormal phenotypes of Bcl11b-deficient mice: disorganization of layer formation of the AOB, impaired axonal projections of VSNs, a significant reduction in the expression of vomeronasal receptor genes, and defective mature differentiation of VSNs. VSNs can be classified into two major types of neurons, vomeronasal 1 receptor (V1r)/Gα(i2)-positive and vomeronasal 2 receptor (V2r)/Gα(o)-positive VSNs. We found that all Gα(i2)-positive cells coexpressed Gα(o) during embryogenesis. This coexpression is also observed in newly differentiated neurons in the adult VNE. Interestingly, loss of Bcl11b function resulted in an increased number of V1r/Gα(i2)-type VSNs and a decreased number of V2r/Gα(o)-type VSNs, suggesting that Bcl11b regulates the fate choice between these two VSN types. These results indicate that Bcl11b/Ctip2 is an essential regulator of the differentiation and dichotomy of VSNs.

Neural map formation in the mouse olfactory system

Odor signals received by odorant receptors (ORs) in the olfactory epithelium are represented as an odor map of activated glomeruli in the olfactory bulb. In the mouse olfactory system, it appears that much of axon pathfinding and sorting occurs autonomously by olfactory neuron axons. Here, we review the recent progress on the study of olfactory map formation in rodents. We will discuss how neuronal identity is represented at axon termini and how the OR-instructed axonal projection is regulated.

Chromosomal location-dependent nonstochastic onset of odor receptor expression

As odorant receptors (ORs) are thought to be critical determinants of olfactory sensory neuron (OSN) axon targeting and organization, we examined the spatiotemporal onset of mice ORs expression from the differentiation of OSNs in the olfactory placode to an aging olfactory epithelium. ORs were first detected in the placode at embryonic day 9 (E9), at the onset of OSN differentiation but before axon extension. By E13, 22 of 23 ORs were expressed. Onset of individual OR expression was diverse; levels and patterns of expression were unique for each OR. Regional distribution of ORs within zones of the olfactory epithelium appeared stable across development; adult-like patterns were observed by E13. Finally, analysis of OR expression and chromosomal location suggests that ORs are not stochastically expressed; they show evidence of coordinated expression. Collectively, these studies demonstrate that ORs are not equally represented in the "olfactome" across an animal's lifespan.

Early expression of odorant receptors distorts the olfactory circuitry

The odor response properties of a mammalian olfactory sensory neuron (OSN) are determined by the tightly regulated expression of a single member of a very large family of odorant receptors (ORs). The OR also plays an important role in focusing the central projections of all OSNs expressing that particular receptor to a pair of stereotypic locations (glomeruli) in each olfactory bulb (OB), thus creating a spatial map of odor responses in the brain. Here we show that when initiated late in neural development, transgenic expression of one OR in almost all OSNs has little influence on the architecture of the OB in mice. In contrast, early OR-transgene expression (mediated by the Ggamma8-promoter) in 50-70% of OSNs grossly distorts the morphology of glomeruli and alters the projection patterns of many residual OSNs not expressing the transgene. Interestingly, this disruption of targeting persists in adult animals despite the downregulation of Ggamma8 and transgenic OR expression that occurs as olfactory neurogenesis declines. Indeed, functional imaging studies reveal a dramatic decrease in the complexity of responses to odorants in adult Ggamma8-transgenic OR mice. Thus, we show that initiation of transgenic OR expression early in the development of OSNs, rather than just the extent of transgene expression, determines its effectiveness at modifying OB anatomy and function. Together, these data imply that OR-expression timing needs to be very tightly controlled to achieve the precise wiring and function of the mammalian olfactory system.

Achieving singularity in mammalian odorant receptor gene choice

Odor discrimination requires differential expression of odor detectors. In fact, olfactory input to the brain is organized in units (glomeruli) innervated only by olfactory sensory neurons that express the same odorant receptor (OR). Therefore, discriminatory capacity is maximized if each sensory neuron expresses only one allele of a single OR gene, a postulate sometimes canonized as the "one neuron-one receptor rule." OR gene choice appears to result from a hierarchy of processes: differential availability of the alleles of each OR gene, zonal exclusion (or selection), OR gene switching during the initiation of OR gene transcription, and OR-dependent feedback to solidify the choice of one OR gene. The mechanisms underlying these processes are poorly understood, though a few elements are known or suspected. For example, the mechanism of activation of OR gene transcription appears to work in part through a few homeobox transcription factors (Emx2, and perhaps Lhx2) and the Ebf family of transcription factors. Further insights will probably come from several directions, but a promising hypothesis is that epigenetic mechanisms contribute to all levels of the hierarchical control of OR gene expression, especially the repressive events that seem to be necessary to achieve the singularity of OR gene choice.

Onset of odorant receptors

Odorant receptors are thought to be critical determinants of olfactory sensory neuron axon targeting and organization. Nonetheless, a systematic characterization of the onset of odorant receptor expression has not yet been done in the main olfactory epithelium. Here, we briefly review our current understanding regarding the onset of odorant receptor expression in the main olfactory epithelium and identify some of those questions which we believe must be of high priority for future study.

Sensing odorants and pheromones with chemosensory receptors

Olfaction is a critical sensory modality that allows living things to acquire chemical information from the external world. The olfactory system processes two major classes of stimuli: (a) general odorants, small molecules derived from food or the environment that signal the presence of food, fire, or predators, and (b) pheromones, molecules released from individuals of the same species that convey social or sexual cues. Chemosensory receptors are broadly classified, by the ligands that activate them, into odorant or pheromone receptors. Peripheral sensory neurons expressing either odorant or pheromone receptors send signals to separate odor- and pheromone-processing centers in the brain to elicit distinct behavioral and neuroendocrinological outputs. General odorants activate receptors in a combinatorial fashion, whereas pheromones activate narrowly tuned receptors that activate sexually dimorphic neural circuits in the brain. We review recent progress on chemosensory receptor structure, function, and circuitry in vertebrates and invertebrates from the point of view of the molecular biology and physiology of these sensory systems.

Genomics of olfactory receptors

In many species, the sense of smell plays important roles in locating food, detecting predators, navigating, and communicating social information. The olfactory system has evolved complex repertoires of odor receptors (ORs) to fulfill these functions. Through computational data mining, OR repertoires of multiple species were identified, revealing a surprisingly large OR gene family in rodents and evolutionary fluctuation among different organisms. Characteristics of OR genes were explored through computational and experimental methods, showing a complicated gene structure and special genomic distribution. Utilizing high-throughput OR microarrays, expression profiles of the mouse and human OR repertoire were examined, their olfactory functions verified, and their zonal, ectopic and developmental expression determined. Variation in human smelling abilities results from different functional OR repertoires, variable expressional levels and polymorphisms in the copy number of the OR genes. These genomic approaches have both provided new data and generated new questions.

Odorant and pheromone receptors in insects

Since the emergence of the first living cells, survival has hinged on the ability to detect and localize chemicals in the environment. Modern animal species ranging from insects to mammals express large odorant receptor repertoires to detect the structurally diverse array of volatile molecules important for survival. Despite the essential nature of chemical detection, there is surprising diversity in the signaling mechanisms that different species use for odorant detection. In vertebrates, odorant receptors are classical G-protein coupled, seven transmembrane receptors that activate downstream effector enzymes that, in turn, produce second messengers that open ion channels. However, recent work reveals that insects have adopted different strategies to detect volatile chemicals. In Drosophila, the odorant receptors, predicted to have seven transmembrane domains, have reversed membrane topology compared to classical G-protein coupled receptors. Furthermore, insect odorant receptors appear to form odorant-gated ion channels. Pheromone detection in insects is even more unusual, utilizing soluble, extracellular receptors that undergo conformational activation. These alternate olfactory signaling strategies are discussed in terms of receptor design principles.

Emx2 stimulates odorant receptor gene expression

The mechanisms selecting a single odorant receptor (OR) gene for expression in each olfactory sensory neuron (OSN) establish an OR expression pattern critical for odor discrimination. These mechanisms are largely unknown, but putative OR promoters contain homeodomain-like sites, implicating homeobox transcription factors such as Emx2. At embryonic day 18.5, expression of 49-76% of ORs was decreased in mice lacking Emx2, depending on the metric used. The decreases were due to fewer OSNs expressing each OR. Affected ORs showed changes that were disproportionately greater than the 42% reduction in mature neurons and similar decreases in unrelated olfactory neuron-enriched messenger RNAs in Emx2(-/-) mice. Both Class I and Class II ORs decreased, as did ORs expressed in both the dorsal and ventral regions of the epithelium. Conversely, 7% of Class II ORs tested were expressed more frequently, suggesting that some ORs are independent of Emx2. Emx2 helps stimulate transcription for many OR genes, which we hypothesize is through direct action at OR promoters, but Emx2 appears to have no significant role in regulating other aspects of OR gene expression, including the zonal patterns, OR gene cluster selection mechanisms, and singularity of OR gene choice.

Expressing exogenous functional odorant receptors in cultured olfactory sensory neurons

BACKGROUND

Olfactory discrimination depends on the large numbers of odorant receptor genes and differential ligand-receptor signaling among neurons expressing different receptors. In this study, we describe an in vitro system that enables the expression of exogenous odorant receptors in cultured olfactory sensory neurons. Olfactory sensory neurons in the culture express characteristic signaling molecules and, therefore, provide a system to study receptor function within its intrinsic cellular environment.

RESULTS

We demonstrate that cultured olfactory sensory neurons express endogenous odorant receptors. Lentiviral vector-mediated gene transfer enables successful ectopic expression of odorant receptors. We show that the ectopically expressed mouse I7 is functional in the cultured olfactory sensory neurons. When two different odorant receptors are ectopically expressed simultaneously, both receptor proteins co-localized in the same olfactory sensory neurons up to 10 days in vitro.

CONCLUSION

This culture technique provided an efficient method to culture olfactory sensory neurons whose morphology, molecular characteristics and maturation progression resembled those observed in vivo. Using this system, regulation of odorant receptor expression and its ligand specificity can be studied in its intrinsic cellular environment.

Sall3 is required for the terminal maturation of olfactory glomerular interneurons

Sall3 is a zinc finger containing putative transcription factor and a member of the Sall gene family. Members of the Sall gene family are highly expressed during development. Sall3-deficient mice die in the perinatal period because of dehydration and display alterations in palate formation and cranial nerve formation (Parrish et al. [2004] Mol Cell Biol 24:7102-7112). We examined the role of Sall3 in the development of the olfactory system. We determined that Sall3 is expressed by cells in the olfactory epithelium and olfactory bulb. Sall3 deficiency specifically alters formation of the glomerular layer. The glomerular layer was hypocellular, because of a decrease in the number of interneurons. The lateral ganglionic eminence and rostral migratory stream developed normally in Sall3-deficient animals, which suggests that Sall3 is not required for the initial specification of olfactory bulb interneurons. Fewer GAD65/67-, Pax6-, calretinin-, and calbindin-positive cells were detected in the glomerular layer, accompanied by an increase in cells positive for these markers in the granule cell layer. In addition, a complete absence of tyrosine hydroxylase expression was observed in the olfactory bulb in the absence of Sall3. However, expression of Nurr1, a marker of dopaminergic precursors, was maintained, indicating that dopaminergic precursors were present. Our data suggest that Sall3 is required for the terminal maturation of neurons destined for the glomerular layer.

Differential development of odorant receptor expression patterns in the olfactory epithelium: a quantitative analysis in the mouse septal organ

The rodent olfactory epithelium expresses more than 1000 odorant receptors (ORs) with distinct patterns, yet it is unclear how such patterns are established during development. In the current study, we investigated development of the expression patterns of different ORs in the septal organ, a small patch of olfactory epithelium predominantly expressing nine identified ORs. The presumptive septal organ first appears at about embryonic day 16 (E16) and it completely separates from the main olfactory epithelium (MOE) at about postnatal day 7 (P7). Using in situ hybridization, we quantified the densities of the septal organ neurons labeled by specific RNA probes of the nine abundant OR genes from E16 to postnatal 3 months. The results indicate that olfactory sensory neurons (OSNs) expressing different ORs have asynchronous temporal onsets. For instance, MOR256-17 and MOR236-1 cells are present in the septal organ at E16; however, MOR0-2 cells do not appear until P0. In addition, OSNs expressing different ORs show distinct developmental courses and reach their maximum densities at different stages ranging from E16 (e.g. MOR256-17) to 1 month (e.g. MOR256-3 and MOR235-1). Furthermore, early onset does not correlate with high abundance in adult. This study reveals a dynamic composition of the OSNs expressing different ORs in the developing olfactory epithelium.

Members of the miRNA-200 family regulate olfactory neurogenesis

MicroRNAs (miRNAs) are highly expressed in vertebrate neural tissues, but the contribution of specific miRNAs to the development and function of different neuronal populations is still largely unknown. We report that miRNAs are required for terminal differentiation of olfactory precursors in both mouse and zebrafish but are dispensable for proper function of mature olfactory neurons. The repertoire of miRNAs expressed in olfactory tissues contains over 100 distinct miRNAs. A subset, including the miR-200 family, shows high olfactory enrichment and expression patterns consistent with a role during olfactory neurogenesis. Loss of function of the miR-200 family phenocopies the terminal differentiation defect observed in absence of all miRNA activity in olfactory progenitors. Our data support the notion that vertebrate tissue differentiation is controlled by conserved subsets of organ-specific miRNAs in both mouse and zebrafish and provide insights into control mechanisms underlying olfactory differentiation in vertebrates.

The protocadherin-alpha family is involved in axonal coalescence of olfactory sensory neurons into glomeruli of the olfactory bulb in mouse

Olfactory sensory neurons (OSNs) that express the same odorant receptor project their axons to specific glomeruli in the main olfactory bulb. Protocadherin-alpha (Pcdha) proteins, diverse cadherin-related molecules that are encoded as a gene cluster, are highly concentrated in OSN axons and olfactory glomeruli. Here, we describe Pcdha mutant mice, in which the constant region of the Pcdha gene cluster has been deleted by gene targeting. The mutant mice show abnormal sorting of OSN axons into glomeruli. There are multiple, small, extraneous glomeruli for the odorant receptors M71 and MOR23. These abnormal patterns of M71 and MOR23 glomeruli persist until adulthood. Many M71 glomeruli, but apparently not MOR23 glomeruli, are heterogeneous in axonal innervation. Thus, Pcdha molecules are involved in coalescence of OSN axons into OR-specific glomeruli of the olfactory bulb.

Sall1 regulates mitral cell development and olfactory nerve extension in the developing olfactory bulb

Sall1 is a zinc finger containing transcription factor that is highly expressed during mammalian embryogenesis. In humans, the developmental disorder Townes Brocks Syndrome is associated with mutations in the SALL1 gene. Sall1-deficient animals die at birth due to kidney deficits; however, its function in the nervous system has not been characterized. We examined the role of Sall1 in the developing olfactory system. We demonstrate that Sall1 is expressed by cells in the olfactory epithelium and olfactory bulb (OB). Sall1-deficient OBs are reduced in size and exhibit alterations in neurogenesis and mitral cell production. In addition, the olfactory nerve failed to extend past the ventral-medial region of the OB in Sall1-deficient animals. We observed intrinsic patterns of neurogenesis during olfactory development in control animals. In Sall1-mutant animals, these patterns of neurogenesis were disrupted. These findings suggest a role for Sall1 in regulating neuronal differentiation and maturation in developing neural structures.

Molecular biology of peptide pheromone production and reception in mice

Intraspecies communication via pheromones plays an important role in social and sexual behaviors, which are critical for survival and reproduction in many animal species. In mice, pheromonal signals are processed by the parallel action of two olfactory systems: the main olfactory system and the vomeronasal pathway. Pheromones are recognized by chemosensory receptors expressed in the main olfactory epithelium and by V1R- and V2R-type receptors expressed in the vomeronasal organ (VNO). Mice take advantage of the chemical properties of both types of pheromones (i.e., volatile/nonvolatile) to precisely control the spatial and temporal transmission of their individual signals. The recent discovery of the exocrine gland-secreting peptide (ESP) family, which appears to encode a VNO-specific ligand repertoire, should open a new avenue to understanding peptide pheromone-mediated communication via the vomeronasal pathway in mice. In this chapter, I will review the current knowledge on genetic and molecular aspects of peptide pheromones and their receptors, by focusing primarily on the mouse VNO system. It is also an intriguing aspect to discuss peptide pheromones in the context of the evolutionary importance of species-specific chemical communication.

Axonal wiring in the mouse olfactory system

The main olfactory epithelium of the mouse is a mosaic of 2000 populations of olfactory sensory neurons (OSNs). Each population expresses one allele of one of the 1000 intact odorant receptor (OR) genes. An OSN projects a single unbranched axon to a single glomerulus, from an array of 1600-1800 glomeruli in the main olfactory bulb. Within a glomerulus the OSN axon synapses with the dendrites of second-order neurons and interneurons. Axons of OSNs that express the same OR project to the same glomeruli-typically one glomerulus per half-bulb and thus four glomeruli per mouse. These glomeruli are located at characteristic positions within the glomerular layer of the bulb. ORs determine both the odorant response profile of the OSN and the projection of its axon to a specific glomerulus. I focus on genetic approaches to the axonal wiring problem, particularly on how ORs may function in axonal wiring.

Expression of pheromone receptor gene families during olfactory development in the mouse: expression of a V1 receptor in the main olfactory epithelium

In the mouse, two large gene families, V1R and V2R, encoding putative pheromone receptors have been described. Studies have suggested a homotypic recognition role for V1Rs and V2Rs during development in the targeting of vomeronasal axons to specific sets of glomeruli in the accessory olfactory bulb (AOB). Analysis of the onset of expression of the V1R and V2R gene families in developing vomeronasal neurons using polymerase chain reaction and in situ hybridization now suggests that a role for these receptors in the organization of axon projections is only likely at the final stages of targeting within the AOB. Surprisingly, our studies reveal expression of a V1Rd receptor in scattered cells within the main olfactory epithelium, suggesting that limited pheromone detection may also take place in this structure. The pheromone sensory neurons of the vomeronasal system and the neuroendocrine gonadotrophin-releasing hormone (GnRH) neurons that regulate fertility both arise from progenitor cells of the nasal placode. The development of these two cell types is intimately linked, and the GnRH neuron population migrates into the forebrain during embryogenesis in close association with a subset of vomeronasal sensory axons; how GnRH neurons recognize this axon subset is unknown. We report selective expression of a V1Ra gene in the clonal NLT GnRH cell line, raising the possibility of a similar role for V1Rs or V2Rs in the directed migration of GnRH neurons. However, no expression of this gene or of other V1Rs and V2Rs is detectable at the cellular level in migrating GnRH neurons in the mouse.

Induced and constitutive heat shock protein expression in the olfactory system—A review, new findings, and some perspectives

Heat shock, or stress, proteins (HSPs) are cellular proteins induced in response to conditions that cause protein denaturation, and their induction is essential for survival of such conditions. In the olfactory system we have found intense HSP expression occurs during normal processing of environmental odorants/inhalants as well as following hyperthermia and drug exposure. The HSPs involved include ubiquitin, HSP70, HSC70, and HSP25. Responses are both cell type- and stress-specific, occurring primarily in olfactory supporting cells and to some extent in Bowman's gland acinar cells. Responses to these stresses are not seen in olfactory sensory neurons. This article reviews those studies and the significance of their findings. It also discusses a distinct subpopulation of rat olfactory sensory neurons (OSNs), the 2A4(+)OSNs, found to be constitutively reactive with HSP70, the predominantly stress-inducible isoform of the 70 kD HSP family. Their high HSP70 expression appears to confer on the 2A4(+)OSNs an enhanced ability to survive damage-induced OSN turnover. New findings are also presented on HSP25-specific changes following olfactory bulbectomy. All data are discussed in the context of the overall olfactory and bioprotective functions of the olfactory mucosa.

Transcriptional Regulation of Neurogenesis in the Olfactory Epithelium

1. The olfactory epithelium (OE) is a simple structure that gives rise to olfactory sensory neurons (OSNs) throughout life. 2. Numerous transcription factors (TFs) are expressed in regions of the OE which contain progenitor cells and OSNs. The function of some of these TFs in OSN development has been elucidated with the aide of transgenic knockout mice. 3. We review here the current state of knowledge on the role of TFs in OE neurogenesis and relate the expression of these TFs, where possible, to the well-documented phenotype of the cells as they progress through the OSN lineage from progenitor cells to mature neurons.

Retinoic acid receptor-dependent survival of olfactory sensory neurons in postnatal and adult mice

To address the hypothesis that retinoids produced by synthesizing enzymes present in the primary olfactory system influence the mouse olfactory sensory map, we expressed a dominant-negative retinoic acid receptor selectively in olfactory sensory neurons. We show that neurons deficient in nuclear retinoid signaling are responsive to odors and form correct odorant receptor-specific axonal projections to target neurons in the olfactory bulb of the brain. Subsequent to the formation of the map, the neurons die prematurely by retrograde-driven caspase-3 activation, which resembles the previously described mechanism of neural death after olfactory bulb ablation. This neurodegenerative event is initiated the second postnatal week and occurs in the adult animal without a compensatory increase of progenitor cell proliferation. In addition, we find that nuclear retinoid signaling is required for the expression of a retinoic acid-degrading enzyme, Cyp26B1, in a small fraction of mature neurons. Collectively, the results provide evidence for a role of locally regulated retinoid metabolism in neuroprotection and in determining population size of neurons at a late stage of neural circuit formation.

Noses and neurons: induction, morphogenesis, and neuronal differentiation in the peripheral olfactory pathway

Non-axial mesenchymal/epithelial (M/E) induction guides peripheral olfactory pathway differentiation using cellular and molecular mechanisms similar to those in the developing limbs, aortic arches, and branchial arches. At each of these bilaterally symmetric sites off the midline axis, a thickened ectodermal epithelium is apposed to a specialized mesenchyme derived largely, but not exclusively, from the neural crest. The capacity of M/E interaction in the olfactory primordia (the combined olfactory placodal epithelium and adjacent mesenchyme) to induce a distinct class of sensory receptor neurons-olfactory receptor neurons-suggests that this mechanism has been modified to accommodate neurogenesis, neurite outgrowth, and axon guidance, in addition to musculoskeletal differentiation, chondrogenesis, and vasculogenesis. Accordingly, although the olfactory primordia share signaling molecules and transcriptional regulators with other bilaterally symmetric, non-axial sites such as limb buds, their activity may be adapted to mediate distinct aspects of cellular differentiation and process outgrowth during the initial assembly of a sensory pathway-the primary olfactory pathway-during early forebrain development.

Once and again: Retinoic acid signaling in the developing and regenerating olfactory pathway

Retinoic acid (RA), a member of the steroid/thyroid superfamily of signaling molecules, is an essential regulator of morphogenesis, differentiation, and regeneration in the mammalian olfactory pathway. RA-mediated teratogenesis dramatically alters olfactory pathway development, presumably by disrupting retinoid-mediated inductive signaling that influences initial olfactory epithelium (OE) and bulb (OB) morphogenesis. Subsequently, RA modulates the genesis, growth, or stability of subsets of OE cells and OB interneurons. RA receptors, cofactors, and synthetic enzymes are expressed in the OE, OB, and anterior subventricular zone (SVZ), the site of neural precursors that generate new OB interneurons throughout adulthood. Their expression apparently accommodates RA signaling in OE cells, OB interneurons, and slowly dividing SVZ neural precursors. Deficiency of vitamin A, the dietary metabolic RA precursor, leads to cytological changes in the OE, as well as olfactory sensory deficits. Vitamin A therapy in animals with olfactory system damage can accelerate functional recovery. RA-related pathology as well as its potential therapeutic activity may reflect endogenous retinoid regulation of neuronal differentiation, stability, or regeneration in the olfactory pathway from embryogenesis through adulthood. These influences may be in register with retinoid effects on immune responses, metabolism, and modulation of food intake.

Fgf8 expression defines a morphogenetic center required for olfactory neurogenesis and nasal cavity development in the mouse

In vertebrate olfactory epithelium (OE), neurogenesis proceeds continuously, suggesting that endogenous signals support survival and proliferation of stem and progenitor cells. We used a genetic approach to test the hypothesis that Fgf8 plays such a role in developing OE. In young embryos, Fgf8 RNA is expressed in the rim of the invaginating nasal pit (NP), in a small domain of cells that overlaps partially with that of putative OE neural stem cells later in gestation. In mutant mice in which the Fgf8 gene is inactivated in anterior neural structures, FGF-mediated signaling is strongly downregulated in both OE proper and underlying mesenchyme by day 10 of gestation. Mutants survive gestation but die at birth, lacking OE, vomeronasal organ (VNO), nasal cavity, forebrain, lower jaw, eyelids and pinnae. Analysis of mutants indicates that although initial NP formation is grossly normal, cells in the Fgf8-expressing domain undergo high levels of apoptosis, resulting in cessation of nasal cavity invagination and loss of virtually all OE neuronal cell types. These findings demonstrate that Fgf8 is crucial for proper development of the OE, nasal cavity and VNO, as well as maintenance of OE neurogenesis during prenatal development. The data suggest a model in which Fgf8 expression defines an anterior morphogenetic center, which is required not only for the sustenance and continued production of primary olfactory (OE and VNO) neural stem and progenitor cells, but also for proper morphogenesis of the entire nasal cavity.

Arx homeobox gene is essential for development of mouse olfactory system

The olfactory system provides an excellent model in which to study cell proliferation, migration, differentiation, axon guidance, dendritic morphogenesis, and synapse formation. We report here crucial roles of the Arx homeobox gene in the developing olfactory system by analyzing its mutant phenotypes. Arx protein was expressed strongly in the interneurons and weakly in the radial glia of the olfactory bulb, but in neither the olfactory sensory neurons nor bulbar projection neurons. Arx-deficient mice showed severe anatomical abnormalities in the developing olfactory system: (1) size reduction of the olfactory bulb, (2) reduced proliferation and impaired entry into the olfactory bulb of interneuron progenitors, (3) loss of tyrosine hydroxylase-positive periglomerular cells, (4) disorganization of the layer structure of the olfactory bulb, and (5) abnormal axonal termination of olfactory sensory neurons in an unusual axon-tangled structure, the fibrocellular mass. Thus, Arx is required for not only the proper developmental processes of Arx-expressing interneurons, but also the establishment of functional olfactory neural circuitry by affecting Arx-non-expressing sensory neurons and projection neurons. These findings suggest a likely role of Arx in regulating the expression of putative instructive signals produced in the olfactory bulb for the proper innervation of olfactory sensory axons.

The Lim homeobox gene Lhx2 is required for olfactory sensory neuron identity

Progenitor cells in the mouse olfactory epithelium generate over a thousand subpopulations of neurons, each expressing a unique odorant receptor (OR) gene. This event is under the control of spatial cues, since neurons in different epithelial regions are restricted to express region-specific subsets of OR genes. We show that progenitors and neurons express the LIM-homeobox gene Lhx2 and that neurons in Lhx2-null mutant embryos do not diversify into subpopulations expressing different OR genes and other region-restricted genes such as Nqo1 and Ncam2. Lhx2-/- embryos have, however, a normal distribution of Mash1-positive and neurogenin 1-positive neuronal progenitors that leave the cell cycle, acquire pan-neuronal traits and form axon bundles. Increased cell death in combination with increased expression of the early differentiation marker Neurod1, as well as reduced expression of late differentiation markers (Galphaolf and Omp), suggests that neuronal differentiation in the absence of Lhx2 is primarily inhibited at, or immediate prior to, onset of OR expression. Aberrant regional expression of early and late differentiation markers, taken together with unaltered region-restricted expression of the Msx1 homeobox gene in the progenitor cell layer of Lhx2-/- embryos, shows that Lhx2 function is not required for all aspects of regional specification of progenitors and neurons. Thus, these results indicate that a cell-autonomous function of Lhx2 is required for differentiation of progenitors into a heterogeneous population of individually and regionally specified mature olfactory sensory neurons.

High-throughput microarray detection of olfactory receptor gene expression in the mouse

The large number of olfactory receptor genes necessitates high throughput methods to analyze their expression patterns. We have therefore designed a high-density oligonucleotide array containing all known mouse olfactory receptor (OR) and V1R vomeronasal receptor genes. This custom array detected a large number of receptor genes, demonstrating specific expression in the olfactory sensory epithelium for approximately 800 OR genes previously designated as ORs based solely on genomic sequences. The array also enabled us to monitor the spatial and temporal distribution of gene expression for the entire OR family. Interestingly, OR genes showing spatially segregated expression patterns were also segregated on the chromosomes. This correlation between genomic location and spatial expression provides unique insights about the regulation of this large family of genes.

Gene switching and the stability of odorant receptor gene choice

Individual olfactory sensory neurons express only a single odorant receptor from a large family of genes, and this singularity is an essential feature in models of olfactory perception. We have devised a genetic strategy to examine the stability of receptor choice. We observe that immature olfactory sensory neurons that express a given odorant receptor can switch receptor expression, albeit at low frequency. Neurons that express a mutant receptor gene switch receptor transcription with significantly greater probability, suggesting that the expression of a functional odorant receptor elicits a feedback signal that terminates switching. This process of receptor gene switching assures that a neuron will ultimately express a functional receptor and that the choice of this receptor will remain stable for the life of the cell.

The LIM-homeodomain protein Lhx2 is required for complete development of mouse olfactory sensory neurons

In mice, approximately 1,000 odorant receptor (OR) genes are expressed in olfactory sensory neurons (OSNs). Homeodomain sites can be recognized in the promoter and upstream regions of several OR genes. Here, using the yeast one-hybrid system and electrophoretic mobility shift assay, we report that Lhx2, a LIM-homeodomain protein, binds to the homeodomain site in the mouse M71 OR promoter region. In Lhx2-deficient mice, the morphology of the olfactory epithelium is grossly normal. However, expression of OMP is abolished and that of GAP43 is severely reduced, indicating that no mature and few immature OSNs are produced. M71 and other OR genes also are not expressed. OSN development appears to be arrested between the terminal differentiation into neurons and the transition to immature neurons. Thus, Lhx2 is required for complete development of OSNs in mice.