Selective targeting of zebrafish olfactory receptor neurons by the endogenous OMP promoter

The forkhead transcription factor Foxj1 controls vertebrate olfactory cilia biogenesis and sensory neuron differentiation

In vertebrates, olfactory receptors localize on multiple cilia elaborated on dendritic knobs of olfactory sensory neurons (OSNs). Although olfactory cilia dysfunction can cause anosmia, how their differentiation is programmed at the transcriptional level has remained largely unexplored. We discovered in zebrafish and mice that Foxj1, a forkhead domain-containing transcription factor traditionally linked with motile cilia biogenesis, is expressed in OSNs and required for olfactory epithelium (OE) formation. In keeping with the immotile nature of olfactory cilia, we observed that ciliary motility genes are repressed in zebrafish, mouse, and human OSNs. Strikingly, we also found that besides ciliogenesis, Foxj1 controls the differentiation of the OSNs themselves by regulating their cell type-specific gene expression, such as that of olfactory marker protein (omp) involved in odor-evoked signal transduction. In line with this, response to bile acids, odors detected by OMP-positive OSNs, was significantly diminished in foxj1 mutant zebrafish. Taken together, our findings establish how the canonical Foxj1-mediated motile ciliogenic transcriptional program has been repurposed for the biogenesis of immotile olfactory cilia, as well as for the development of the OSNs.

Damage to Olfactory Organs of Adult Zebrafish Induced by Diesel Particulate Matter

Particulate matter (PM) is an environmental hazard that is associated with various human health risks. The olfactory system is directly exposed to PM; therefore, the influence of PM exposure on olfactory function must be investigated. In this study, we propose a zebrafish olfactory model to evaluate the effects of exposure to diesel particulate matter (DPM), which was labeled Korean diesel particulate matter (KDP20). KDP20 comprises heavy metals and polycyclic aromatic hydrocarbons (PAHs). KDP20 exposed olfactory organs exhibited reduced cilia and damaged epithelium. Olfactory dysfunction was confirmed using an odor-mediated behavior test. Furthermore, the olfactory damage was analyzed using Alcian blue and anti-calretinin staining. KDP20 exposed olfactory organs exhibited histological damages, such as increased goblet cells, decreased cell density, and calretinin level. Quantitative real-time polymerase chain reaction (qRT-PCR) revealed that PAHs exposure related genes (AHR2 and CYP1A) were upregulated. Reactive oxidation stress (ROS) (CAT) and inflammation (IL-1B) related genes were upregulated. Furthermore, olfactory sensory neuron (OSN) related genes (OMP and S100) were downregulated. In conclusion, KDP20 exposure induced dysfunction of the olfactory system. Additionally, the zebrafish olfactory system exhibited a regenerative capacity with recovery conditions. Thus, this model may be used in future investigating PM-related diseases.

Diving into the streams and waves of constitutive and regenerative olfactory neurogenesis: insights from zebrafish

The olfactory system is renowned for its functional and structural plasticity, with both peripheral and central structures displaying persistent neurogenesis throughout life and exhibiting remarkable capacity for regenerative neurogenesis after damage. In general, fish are known for their extensive neurogenic ability, and the zebrafish in particular presents an attractive model to study plasticity and adult neurogenesis in the olfactory system because of its conserved structure, relative simplicity, rapid cell turnover, and preponderance of neurogenic niches. In this review, we present an overview of the anatomy of zebrafish olfactory structures, with a focus on the neurogenic niches in the olfactory epithelium, olfactory bulb, and ventral telencephalon. Constitutive and regenerative neurogenesis in both the peripheral olfactory organ and central olfactory bulb of zebrafish is reviewed in detail, and a summary of current knowledge about the cellular origin and molecular signals involved in regulating these processes is presented. While some features of physiologic and injury-induced neurogenic responses are similar, there are differences that indicate that regeneration is not simply a reiteration of the constitutive proliferation process. We provide comparisons to mammalian neurogenesis that reveal similarities and differences between species. Finally, we present a number of open questions that remain to be answered.

Tributyltin impaired spermatogenesis and reproductive behavior in male zebrafish

Tributyltin (TBT) was reported to affect sexual behavior and gametogenesis in fish. However, the modes of action involved are largely unclear. In order to elucidate the toxicological mechanisms of TBT in reproduction, zebrafish (Danio rerio) males were exposed to TBT at concentrations of 100 and 500 ng/L for 28 days. After exposure, the sperm count of the treated fish was sharply decreased though the testis weight and gonadosomatic index remained unchanged. Moreover, reduced number of spermatogonia and spermatozoa and increased spermatocytes were observed in TBT-treated fish by histological observation and PCNA-immunostaining. Increased number of apoptotic-positive spermatocytes was also present in TBT-treated fish, indicating an enhanced apoptosis in these cells. Consistent to decreased number of spermatogonia, down-regulated expressions of genes responsible for germ cell proliferation (cyclind1 and pcna) were observed in TBT-treated fish. In contrast, TBT elevated the expressions of genes involved in meiotic entry and maintenance (aldhla2, sycp3 and dmc1) while suppressed the mRNA level of gene responsible for terminus of meiotic entry (cyp26a1), in agreement with arrested meiosis and reduced sperm count. Furthermore, TBT significantly elevated the ratios of bax/bcl-2 and tnfrsf1a/tnfrsf1b in testis, which are markers for intrinsic- and extrinsic-apoptotic pathways, consistent with the enhanced TUNEL positive signals in spermatocytes. Moreover, TBT also significantly affected the parameter of reproductive behaviors in treated fish (reflected by decreased frequency of meeting, visits and time spent in spawning area). Consistently, the expressions of genes responsible for the modulation of reproductive behaviors in brain (such as cyp19a1b, kiss2, gnrh3 and ompb) were significantly down-regulated in treated-fish. Interestingly, disrupted reproductive behaviors of untreated female fish were also observed in the present study. The present study indicated that TBT might affect the reproduction of zebrafish male by disrupting the spermatogenesis and reproductive behavior of the fish.

Purinergic signalling selectively modulates maintenance but not repair neurogenesis in the zebrafish olfactory epithelium

Olfactory sensory neurons (OSNs) of the vertebrate olfactory epithelium (OE) undergo continuous turnover but also regenerate efficiently when the OE is acutely damaged by traumatic injury. Two distinct pools of neuronal stem/progenitor cells, the globose (GBCs), and horizontal basal cells (HBCs) have been shown to selectively contribute to intrinsic OSN turnover and damage-induced OE regeneration, respectively. For both types of progenitors, their rate of cell divisions and OSN production must match the actual loss of cells to maintain or to re-establish sensory function. However, signals that communicate between neurons or glia cells of the OE and resident neurogenic progenitors remain largely elusive. Here, we investigate the effect of purinergic signaling on cell proliferation and OSN neurogenesis in the zebrafish OE. Purine stimulation elicits transient Ca²⁺ signals in OSNs and distinct non-neuronal cell populations, which are located exclusively in the basal OE and stain positive for the neuronal stem cell marker Sox2. The more apical population of Sox2-positive cells comprises evenly distributed glia-like sustentacular cells (SCs) and spatially restricted GBC-like cells, whereas the more basal population expresses the HBC markers keratin 5 and tumor protein 63 and lines the entire sensory OE. Importantly, exogenous purine stimulation promotes P2 receptor-dependent mitotic activity and OSN generation from sites where GBCs are located but not from HBCs. We hypothesize that purine compounds released from dying OSNs modulate GBC progenitor cell cycling in a dose-dependent manner that is proportional to the number of dying OSNs and, thereby, ensures a constant pool of sensory neurons over time.

Olfactory receptor neurons express olfactory marker protein but not calpain 5 from the same genomic locus

Gene expression is highly regulated to functionally diversify cells. Genes that cooperate in the same physiological processes occasionally reside within nearby regions in a chromosome. Olfactory marker protein (OMP) is highly expressed in mature olfactory receptor neurons (ORNs), but its physiological roles are not fully understood. According to the genomic map, the OMP gene is located within an intron of the calcium-dependent protease, calpain 5 (CAPN5); in other words, the OMP gene is a nested intronic gene. Thus, we attempted to investigate the gene expression and protein distribution of CAPN5 in the olfactory epithelium compared with that in the central nervous system (CNS). By performing reverse-transcriptase PCR and in situ hybridization, we confirmed that CAPN5 mRNA was expressed in the olfactory epithelium. We then performed immunohistological investigations using sliced preparations obtained from mice expressing GFP under OMP promoter activity. The detected GFP fluorescence was restricted to the knob, soma and axon bundles of the ORNs, while CAPN5 immunoreactivity (CAPN5-IR) was ubiquitously detected in the olfactory epithelial layer and lamina propria; signals were strongly detected in the supporting cells within the epithelium. In the CNS, CAPN5 signals were widely detected and were especially strong in the hippocampal formation and the piriform cortex as previously indicated. Therefore, these data indicate that ORNs express OMP but not CAPN5 from CAPN5 gene expression even though they are localized in the same genomic locus. The mechanisms by which the OMP promoter is regulated require detailed investigations.

Re-evaluating the functional landscape of the cardiovascular system during development

The cardiovascular system facilitates body-wide distribution of oxygen, a vital process for the development and survival of virtually all vertebrates. However, the zebrafish, a vertebrate model organism, appears to form organs and survive mid-larval periods without a functional cardiovascular system. Despite such dispensability, it is the first organ to develop. Such enigma prompted us to hypothesize other cardiovascular functions that are important for developmental and/or physiological processes. Hence, systematic cellular ablations and functional perturbations were performed on the zebrafish cardiovascular system to gain comprehensive and body-wide understanding of such functions and to elucidate the underlying mechanisms. This approach identifies a set of organ-specific genes, each implicated for important functions. The study also unveils distinct cardiovascular mechanisms, each differentially regulating their expressions in organ-specific and oxygen-independent manners. Such mechanisms are mediated by organ-vessel interactions, circulation-dependent signals, and circulation-independent beating-heart-derived signals. A comprehensive and body-wide functional landscape of the cardiovascular system reported herein may provide clues as to why it is the first organ to develop. Furthermore, these data could serve as a resource for the study of organ development and function.

Summary: The body-wide landscape of the cardiovascular functions during development is reported. Such landscape may provide clues as to why the cardiovascular system is the first organ to develop.

Patterned Arrangements of Olfactory Receptor Gene Expression in Zebrafish are Established by Radial Movement of Specified Olfactory Sensory Neurons

Spatial restriction of olfactory receptor (OR) gene expression in peripheral sense organs is a common phenomenon across species, suggesting that zonal OR expression somehow contributes to olfactory function. In zebrafish OR expression patterns reminiscent of zones occur as concentric domains with preferred diameters for different ORs. However, the function and the developmental origin of the pattern are unknown. Here we investigate olfactory sensory neuron (OSN) neurogenesis in the adult zebrafish olfactory epithelium (OE) to understand how the zonally organized OR pattern is established and maintained during the lifetime of the animal. We find that OSNs are generated from two discontinuous proliferation zones located at the central and peripheral edge of the sensory OE. OSNs turn on OR expression soon after they exit mitosis and invade the sensory tissue, approaching each other from both ends of the OE. Biased generation of OSN subpopulations at both neurogenic sites and elimination of OSNs along their route across the OE generates the impression of OR-specific expression domains. We formulated a simple mathematical model based on exact parameters derived from our analysis of OSN neurogenesis, which accurately generates OR-like distributions without the need to invoke molecular signals to pattern the OE.

Attractant and repellent cues cooperate in guiding a subset of olfactory sensory axons to a well-defined protoglomerular target

Olfactory sensory axons target well-defined intermediate targets in the zebrafish olfactory bulb called protoglomeruli well before they form odorant receptor-specific glomeruli. A subset of olfactory sensory neurons are labeled by expression of the or111-7:IRES:GAL4 transgene whose axons terminate in the central zone (CZ) protoglomerulus. Previous work has shown that some of these axons misproject to the more dorsal and anterior dorsal zone (DZ) protoglomerulus in the absence of Netrin 1/Dcc signaling. In search of additional cues that guide these axons to the CZ, we found that Semaphorin 3D (Sema3D) is expressed in the anterior bulb and acts as a repellent that pushes them towards the CZ. Further analysis indicates that Sema3D signaling is mediated through Nrp1a, while Nrp2b also promotes CZ targeting but in a Sema3D-independent manner. nrp1a, nrp2b and dcc transcripts are detected in or111-7 transgene-expressing neurons early in development and both Nrp1a and Dcc act cell-autonomously in sensory neurons to promote accurate targeting to the CZ. dcc and nrp1a double mutants have significantly more DZ misprojections than either single mutant, suggesting that the two signaling systems act independently and in parallel to direct a specific subset of sensory axons to their initial protoglomerular target.

Distinct functions of two olfactory marker protein genes derived from teleost-specific whole genome duplication

BACKGROUND

Whole genome duplications (WGDs) have been proposed to have made a significant impact on vertebrate evolution. Two rounds of WGD (1R and 2R) occurred in the common ancestor of Gnathostomata and Cyclostomata, followed by the third-round WGD (3R) in a common ancestor of all modern teleosts. The 3R-derived paralogs are good models for understanding the evolution of genes after WGD, which have the potential to facilitate phenotypic diversification. However, the recent studies of 3R-derived paralogs tend to be based on in silico analyses. Here we analyzed the paralogs encoding teleost olfactory marker protein (OMP), which was shown to be specifically expressed in mature olfactory sensory neurons and is expected to be involved in olfactory transduction.

RESULTS

Our genome database search identified two OMPs (OMP1 and OMP2) in teleosts, whereas only one was present in other vertebrates. Phylogenetic and synteny analyses suggested that OMP1 and 2 were derived from 3R. Both OMPs showed distinct expression patterns in zebrafish; OMP1 was expressed in the deep layer of the olfactory epithelium (OE), which is consistent with previous studies of mice and zebrafish, whereas OMP2 was sporadically expressed in the superficial layer. Interestingly, OMP2 was expressed in a very restricted region of the retina as well as in the OE. In addition, the analysis of transcriptome data of spotted gar, a non-teleost fish, revealed that single OMP gene was expressed in the eyes.

CONCLUSION

We found distinct expression patterns of zebrafish OMP1 and 2 at the tissue and cellular level. These differences in expression patterns may be explained by subfunctionalization as the model of molecular evolution. Namely, single OMP gene was speculated to be originally expressed in the OE and the eyes in the common ancestor of all Osteichthyes (bony fish including tetrapods). Then, two OMP gene paralogs derived from 3R-WGD reduced and specialized the expression patterns. This study provides a good example for analyzing a functional subdivision of the teleost OE and eyes as revealed by 3R-derived paralogs of OMPs.

Silencing of odorant receptor genes by G protein βγ signaling ensures the expression of one odorant receptor per olfactory sensory neuron

Olfactory sensory neurons express just one out of a possible ∼ 1,000 odorant receptor genes, reflecting an exquisite mode of gene regulation. In one model, once an odorant receptor is chosen for expression, other receptor genes are suppressed by a negative feedback mechanism, ensuring a stable functional identity of the sensory neuron for the lifetime of the cell. The signal transduction mechanism subserving odorant receptor gene silencing remains obscure, however. Here, we demonstrate in the zebrafish that odorant receptor gene silencing is dependent on receptor activity. Moreover, we show that signaling through G protein βγ subunits is both necessary and sufficient to suppress the expression of odorant receptor genes and likely acts through histone methylation to maintain the silenced odorant receptor genes in transcriptionally inactive heterochromatin. These results link receptor activity with the epigenetic mechanisms responsible for ensuring the expression of one odorant receptor per olfactory sensory neuron.

Role of a ubiquitously expressed receptor in the vertebrate olfactory system

Odorant cues are recognized by receptors expressed on olfactory sensory neurons, the primary sensory neurons of the olfactory epithelium. Odorant receptors typically obey the "one receptor, one neuron" rule, in which the receptive field of the olfactory neuron is determined by the singular odorant receptor that it expresses. Odor-evoked receptor activity across the population of olfactory neurons is then interpreted by the brain to identify the molecular nature of the odorant stimulus. In the present study, we characterized the properties of a C family G-protein-coupled receptor that, unlike most other odorant receptors, is expressed in a large population of microvillous sensory neurons in the zebrafish olfactory epithelium and the mouse vomeronasal organ. We found that this receptor, OlfCc1 in zebrafish and its murine ortholog Vmn2r1, is a calcium-dependent, low-sensitivity receptor specific for the hydrophobic amino acids isoleucine, leucine, and valine. Loss-of-function experiments in zebrafish embryos demonstrate that OlfCc1 is required for olfactory responses to a diverse mixture of polar, nonpolar, acidic, and basic amino acids. OlfCc1 was also found to promote localization of other OlfC receptor family members to the plasma membrane in heterologous cells. Together, these results suggest that the broadly expressed OlfCc1 is required for amino acid detection by the olfactory system and suggest that it plays a role in the function and/or intracellular trafficking of other olfactory and vomeronasal receptors with which it is coexpressed.

Distribution and functional organization of glomeruli in the olfactory bulbs of zebrafish (Danio rerio)

Odor molecules are transduced by thousands of olfactory sensory neurons (OSNs) located in the nasal cavity. Each OSN expresses a single functional odorant receptor protein and projects an axon from the sensory epithelia to an olfactory bulb glomerulus, which is selectively innervated by only one or a few OSN types. We used whole-mount immunocytochemistry to study the neurochemistry and anatomical organization of glomeruli in the zebrafish olfactory system. By employing combinations of antibodies against G-protein α subunits, calcium-binding proteins, and general neuronal markers, we selectively labeled various OSN types, their axonal projections to glomeruli, and the detailed anatomical distributions of individual glomeruli in different regions of the olfactory bulb. In this way we identified ≈140 glomeruli in each olfactory bulb of mature zebrafish. A small subset (27) of these glomeruli was unambiguously identifiable in nearly all animals examined. These units were large and, located mainly in the medial olfactory bulbs. Most glomeruli, however, were comparatively small, anatomically indistinguishable, and located in coarsely circumscribed regions; almost all of these latter glomeruli were innervated by OSNs that were labeled with anti-G(α s/olf) and/or anti-calretinin antibodies. Collectively, our results provide a uniquely detailed description of a vertebrate olfactory system and highlight anatomically distinct parallel neural pathways that mediate early aspects of olfactory processing in the zebrafish.

Netrin/DCC signaling guides olfactory sensory axons to their correct location in the olfactory bulb

Olfactory sensory neurons expressing particular olfactory receptors project to specific reproducible locations within the bulb. The axonal guidance cues that organize this precise projection pattern are only beginning to be identified. To aid in their identification and characterization, we generated a transgenic zebrafish line, OR111-7:IRES:Gal4, in which a small subset of olfactory sensory neurons is labeled. Most sensory neurons expressing the OR111-7 transgene project to a specific location within the bulb, the central zone protoglomerulus, while a smaller number project to the lateral glomerulus 1 protoglomerulus. Inhibiting Netrin/DCC (deleted in colorectal cancer) signaling perturbs the ability of OR111-7-expressing axons to enter the olfactory bulb and alters their patterns of termination within the bulb. The Netrin receptor DCC is expressed in olfactory sensory neurons around the time that they elaborate their axons, netrin1a is expressed near the medial-most margin of the olfactory bulb, and netrin1b is expressed within the ventral region of the bulb. Loss of Netrin/DCC signaling components causes some OR111-7-expressing sensory axons to wander posteriorly after exiting the olfactory pit, away from netrin-expressing areas in the bulb. OR111-7-expressing axons that enter the bulb target the central zone less precisely than normal, spreading away from netrin-expressing regions. These pathfinding errors can be corrected by the reexpression of DCC within OR111-7 transgene-expressing neurons in DCC morphant embryos. These findings implicate Netrins as the only known attractants for olfactory sensory neurons, first drawing OR111-7-expressing axons into the bulb and then into the ventromedially positioned central zone protoglomerulus.

Partially redundant proneural function reveals the importance of timing during zebrafish olfactory neurogenesis

Little is known about proneural gene function during olfactory neurogenesis in zebrafish. Here, we show that the zebrafish Atonal genes neurogenin1 (neurog1) and neurod4 are redundantly required for development of both early-born olfactory neurons (EONs) and later-born olfactory sensory neurons (OSNs). We show that neurod4 expression is initially absent in neurog1 mutant embryos but recovers and is sufficient for the delayed development of OSN. By contrast, EON numbers are significantly reduced in neurog1 mutant embryos despite the recovery of neurod4 expression. Our results suggest that a shortened time window for EON development causes this reduction; the last S-phase of EON is delayed in neurog1 mutant embryos but mutant EONs are all post-mitotic at the same stage as EONs in wild-type embryos. Finally, we show that expression of certain genes, such as robo2, is never detected in neurog1 mutant EONs. Failure of robo2 expression to recover correlates with defects in the fasciculation of neurog1 mutant olfactory axonal projections and in the organisation of proto-glomeruli because projections arrive at the olfactory bulb that are reminiscent of those in robo2 mutant embryos. We conclude that the duration of proneural expression in EON progenitors is crucial for correct development of the zebrafish olfactory system.

Onset and dynamic expression of S100 proteins in the olfactory organ and the lateral line system in zebrafish development

In the zebrafish olfactory epithelium, three morphologically distinct olfactory neurons express different marker proteins. We utilize this feature to access developmental dynamics of one of the neuron types, the crypt cells, to determine whether they are differentiated at a stage similar to other olfactory neurons. Immunohistochemical studies using an S100 antibody that specifically recognizes crypt cells showed that S100-positive cells appear in olfactory rosettes as early as at 2day postfertilization (dpf). However, some of the rosettes did not have any S100-positive cells until 4 dpf. The number of S100-positive cells in individual rosettes increased steadily over the next 3days before it decreased significantly. There were 7.8 S100-positive cells per rosettes on average in larvae at 7 dpf. The number reduced to 2.2 at 9 dpf. A recovery to a pre-reduction level was detected in 12 dpf larvae. We also observed S100-positive cells in neuromasts of the lateral line system in 2 dpf larvae, suggesting that the crypt cells and sensory cells in the neuromasts have similar onsets of differentiation. Our data have provided a time line of differentiation of crypt cells in development of the olfactory system and demonstrated that this type of cell is differentiated at a stage similar to ciliated and microvillous olfactory neurons. A nonlinear growth trajectory of the crypt cell population in the first nine days of zebrafish development implicates a possible functional significance of crypt cells in early life stages of zebrafish.

Selective neuronal requirement for huntingtin in the developing zebrafish

Huntington's disease shares a common molecular basis with eight other neurodegenerative diseases, expansion of an existing polyglutamine tract. In each case, this repeat tract occurs within otherwise unrelated proteins. These proteins show widespread and overlapping patterns of expression in the brain and yet the diseases are distinguished by neurodegeneration in a specific subset of neurons that are most sensitive to the mutation. It has therefore been proposed that expansion of the polyglutamine region in these genes may result in perturbation of the normal function of the respective proteins, and that this perturbation in some way contributes to the neuronal specificity of these diseases. The normal functions of these proteins have therefore become a focus for investigation as potential pathogenic pathways. We have used synthetic antisense morpholinos to inhibit the translation of huntingtin mRNA during early zebrafish development and have previously reported the effects of huntingtin reduction on iron transport and homeostasis. Here we report an analysis of the effects of huntingtin loss-of-function on the developing nervous system, observing distinct defects in morphology of neuromasts, olfactory placode and branchial arches. The potential common origins of these defects were explored, revealing impaired formation of the anterior-most region of the neural plate as indicated by reduced pre-placodal and telencephalic gene expression with no effect on mid- or hindbrain formation. These investigations demonstrate a specific 'rate-limiting' role for huntingtin in formation of the telencephalon and the pre-placodal region, and differing levels of requirement for huntingtin function in specific nerve cell types.

Molecular characterization and histochemical demonstration of salmon olfactory marker protein in the olfactory epithelium of lacustrine sockeye salmon (Oncorhynchus nerka)

Despite the importance of olfactory receptor neurons (ORNs) for homing migration, the expression of olfactory marker protein (OMP) is not well understood in ORNs of Pacific salmon (genus Oncorhynchus). In this study, salmon OMP was characterized in the olfactory epithelia of lacustrine sockeye salmon (O. nerka) by molecular biological and histochemical techniques. Two cDNAs encoding salmon OMP were isolated and sequenced. These cDNAs both contained a coding region encoding 173 amino acid residues, and the molecular mass of the two proteins was calculated to be 19,581.17 and 19,387.11Da, respectively. Both amino acid sequences showed marked homology (90%). The protein and nucleotide sequencing demonstrates the existence of high-level homology between salmon OMPs and those of other teleosts. By in situ hybridization using a digoxigenin-labeled salmon OMP cRNA probe, signals for salmon OMP mRNA were observed preferentially in the perinuclear regions of the ORNs. By immunohistochemistry using a specific antibody to salmon OMP, OMP-immunoreactivities were noted in the cytosol of those neurons. The present study is the first to describe cDNA cloning of OMP in salmon olfactory epithelium, and indicate that OMP is a useful molecular marker for the detection of the ORNs in Pacific salmon.

Foxg1 is required for development of the vertebrate olfactory system

Illuminating the molecular identity and regulation of early progenitor cells in the olfactory sensory epithelium represents an important challenge in the field of neural development. We show in both mouse and zebrafish that the winged helix transcription factor Foxg1 is expressed in an early progenitor population of the olfactory placode. In the mouse, Foxg1 is first expressed throughout the olfactory placode but later becomes restricted to the ventrolateral olfactory epithelium. The essential role of Foxg1 in olfactory development is demonstrated by the strikingly severe phenotype of Foxg1 knock-out mice: older embryos have no recognizable olfactory structures, including epithelium, bulb, or vomeronasal organs. Initially, a small number of olfactory progenitors are specified but show defects in both proliferation and differentiation. Similarly, antisense RNA knockdown of Foxg1 expression in the zebrafish shows a reduction in the number of neurons and mitotic cells in olfactory rosettes, mirroring the phenotype seen in the mouse Foxg1 null mutant. Using mosaic analysis in the zebrafish, we show that Foxg1 is required cell-autonomously for the production of mature olfactory receptor neurons. Therefore, we identified an evolutionarily conserved requirement for Foxg1 in the development of the vertebrate olfactory system.

Appearance of crypt neurons in the olfactory epithelium of the skate Raja clavata during development

Crypt neurons are olfactory receptor cells located in the olfactory epithelium of fishes. They exhibit a peculiar and well-recognizable morphology, although their odorant specificity is still unknown. Data on their appearance during development are few and far between. This study set out to identify the time at which crypt neurons appeared in the skate, Raja clavata, using histological and immunohistochemical methods. For this purpose, embryos and juveniles at different stages of development, from 13 weeks after laying (11 weeks before hatching) to 24 weeks after hatching, were examined. The crypt neurons were identified on a morphological basis. An anti-alpha-tubulin antibody and two lectins (wheat germ agglutinin and peanut agglutinin) were used to highlight morphological details. The olfactory marker protein was detected by immunohistochemistry, because this protein is a marker of neuronal maturity in vertebrates. The crypt neurons could be detected by their morphology at 15 weeks after laying and became strongly olfactory marker protein immunoreactive 22 weeks after laying. Although involvement of crypt neurons in reproductive behavior has been inferred in various studies on bony fishes, their early presence in skate embryos and juveniles may suggest that they are not exclusively involved in sexual behavior.

Olfactory imprinting is correlated with changes in gene expression in the olfactory epithelia of the zebrafish

Odors experienced as juveniles can have significant effects on the behavior of mature organisms. A dramatic example of this occurs in salmon, where the odors experienced by developing fish determine the river to which they return as adults. Further examples of olfactory memories are found in many animals including vertebrates and invertebrates. Yet, the cellular and molecular bases underlying the formation of olfactory memory are poorly understood. We have devised a series of experiments to determine whether zebrafish can form olfactory memories much like those observed in salmonids. Here we show for the first time that zebrafish form and retain olfactory memories of an artificial odorant, phenylethyl alcohol (PEA), experienced as juveniles. Furthermore, we demonstrate that exposure to PEA results in changes in gene expression within the olfactory sensory system. These changes are evident by in situ hybridization in the olfactory epithelium of the developing zebrafish. Strikingly, our analysis by in situ hybridization demonstrates that the transcription factor, otx2, is up regulated in the olfactory sensory epithelia in response to PEA. This increase is evident at 2-3 days postfertilization and is maintained in the adult animals. We propose that the changes in otx2 gene expression are manifest as an increase in the number of neuronal precursors in the cells olfactory epithelium of the odor-exposed fish. Thus, our results reveal a role for the environment in controlling gene expression in the developing peripheral nervous system.

First detection of olfactory marker protein (OMP) immunoreactivity in the olfactory epithelium of a cartilaginous fish

Olfactory marker protein (OMP) is a protein expressed in the mature olfactory and vomeronasal neurons of many vertebrates, such as mammals, amphibians and bony fishes. Aim of this work was to investigate the OMP expression in the olfactory epithelium of the shark Scyliorhinus canicula (Linnaeus, 1758), by immunohistochemistry (IHC). Immunoreactivity was detected in the olfactory receptor neurons, in the crypt neurons and in the nerve fibers below the epithelium. Although very little is known about the OMP's function, its involvement in synaptogenesis, transduction cascade, neurogenesis and development of olfactory system has been suggested. The present work shows for the first time OMP's presence in a cartilaginous fish.

Mutually exclusive glomerular innervation by two distinct types of olfactory sensory neurons revealed in transgenic zebrafish

The olfactory epithelium of fish contains two major types of olfactory sensory neurons (OSNs) that are distinct morphologically (ciliated vs microvillous) and possibly functionally. Here, we found that these OSNs express different sets of signal transduction machineries: the ciliated OSNs express OR-type odorant receptors, cyclic nucleotide-gated channel A2 subunit, and olfactory marker protein (OMP), whereas the microvillous OSNs express V2R-type receptors and transient receptor potential channel C2 (TRPC2). To visualize patterns of axonal projection from the two types of OSNs to the olfactory bulb (OB), we generated transgenic zebrafish in which spectrally distinct fluorescent proteins are expressed in the ciliated and microvillous OSNs under the control of OMP and TRPC2 gene promoters, respectively. An observation of whole-mount OB in adult double-transgenic zebrafish revealed that the ciliated OSNs project axons mostly to the dorsal and medial regions of the OB, whereas the microvillous OSNs project axons to the lateral region of the OB. A careful histological examination of OB sections clarified that the axons from the two distinct types of OSNs target different glomeruli in a mutually exclusive manner. This segregation is already established at very early developmental stages in zebrafish embryos. These findings clearly demonstrate the relationships among cell morphology, molecular signatures, and axonal terminations of the two distinct types of OSNs and suggest that the two segregated neural pathways are responsible for coding and processing of different types of odor information in the zebrafish olfactory system.

Robo2 is required for establishment of a precise glomerular map in the zebrafish olfactory system

Olfactory sensory neurons (OSNs) expressing a given odorant receptor project their axons to specific glomeruli, creating a topographic odor map in the olfactory bulb (OB). The mechanisms underlying axonal pathfinding of OSNs to their precise targets are not fully understood. Here, we demonstrate that Robo2/Slit signaling functions to guide nascent olfactory axons to the OB primordium in zebrafish. robo2 is transiently expressed in the olfactory placode during the initial phase of olfactory axon pathfinding. In the robo2 mutant, astray (ast), early growing olfactory axons misroute ventromedially or posteriorly, and often penetrate into the diencephalon without reaching the OB primordium. Four zebrafish Slit homologs are expressed in regions adjacent to the olfactory axon trajectory,consistent with their role as repulsive ligands for Robo2. Masking of endogenous Slit gradients by ubiquitous misexpression of Slit2 in transgenic fish causes posterior pathfinding errors that resemble the astphenotype. We also found that the spatial arrangement of glomeruli in OB is perturbed in ast adults, suggesting an essential role for the initial olfactory axon scaffold in determining a topographic glomerular map. These data provide functional evidence for Robo2/Slit signaling in the establishment of olfactory neural circuitry in zebrafish.

The interaction of Bex and OMP reveals a dimer of OMP with a short half-life

Olfactory marker protein (OMP) participates in the olfactory signal transduction pathway. This is evident from the behavioral and electrophysiological deficits of OMP-null mice, which can be reversed by intranasal infection of olfactory sensory neurons with an OMP-expressing adenovirus. Bex, brain expressed X-linked protein, has been identified as a protein that interacts with OMP. We have now further characterized the interaction of OMP and Bex1/2 by in vitro binding assays and by immuno-coprecipitation experiments. OMP is a 19 kDa protein but these immunoprecipitation studies have revealed the unexpected presence of a 38 kDa band in addition to the expected 19 kDa band. Furthermore, the 38 kDa form was preferentially co-immunoprecipitated with Bex from cell extracts. In-gel tryptic digestion, mass spectrometry, and two-dimensional gel electrophoresis indicate that the 38 kDa protein behaves as a covalently cross-linked OMP-homodimer. The 38 kDa band was also identified in western blots of olfactory epithelium demonstrating its presence in vivo. The stabilities and subcellular localizations of the OMP-monomer and -dimer were studied in transfected cells. These results demonstrated that the OMP-dimer is much less stable than the monomer, and that while the monomer is present both in the nuclear and cytosolic compartments, the dimer is preferentially located in a Triton X-100 insoluble cytoskeletal fraction. These novel observations led us to hypothesize that regulation of the level of the rapidly turning-over OMP-dimer and its interaction with Bex1/2 is critical for OMP function in sensory transduction.

Cloning of an olfactory sensory neuron-specific protein in the land snail (Eobania vermiculata)

We have isolated a gene encoding for an olfactory sensory neuron (OSN)-specific protein in an invertebrate, the land snail Eobania vermiculata (GenBank accession number AY147909). Using in situ hybridization, we detected expression of its mRNA in the dendrite, cell body and axon of OSNs. By neural tracing, using the lipophilic tracer DiI and in situ hybridization, we have revealed the organization of OSNs and their connections with olfactory glomeruli in the land snail. Sequence and expression pattern analogy of land snail protein with olfactory marker protein (OMP) from vertebrates suggest that the land snail protein is an OMP-like protein. This protein could represent a plesiomorphic character in the evolution of olfactory proteins.

Identification of 24 genes and two pseudogenes coding for olfactory receptors in Japanese loach, classified into four subfamilies: a putative evolutionary process for fish olfactory receptor genes by comprehensive phylogenetic analysis

Twenty-four olfactory receptor (OR) genes and two pseudogenes have been identified in the genome of Japanese loach (Misgurnus anguillicaudatus). The genes were classified into four subfamilies according to the similarity of the amino acid sequences. In each subfamily, members showed high sequence similarity not only to each other but also to orthologues of other fish species. The number of members in each OR subfamily was roughly estimated to be from 3 to 10 by genomic Southern blot analysis. The genes of all four OR subfamilies were shown to express on olfactory neurons of the olfactory epithelium by in situ hybridization analysis. Two major features of fish OR genes were found by comprehensive and comparative analyses on OR genes of Japanese loach and other fish species including catfish, zebrafish and pufferfish. First, the phylogenetic tree comprising of representative subfamily members suggests the existence of several prototype genes common to the genomes of many fish species. Second, when all members of orthologous subfamilies identified in each clade of the tree are integrated, the members of a single species comprise a monophyletic group. This means that 'intraspecies' sequence homology, that is, homology among paralogous genes of the same subfamily in a species, is higher than 'interspecies' homology, that is, homology between orthologous genes of different species. This suggests that the subfamily members of a species have evolved recently. Taken together, fish OR genes have evolved from a limited number of prototype genes common to most fish species, and several genes in a subfamily have diversely evolved in each species from each prototype.

Identification of members of the Bex gene family as olfactory marker protein (OMP) binding partners

Olfactory marker protein (OMP) expression is a hallmark of mature vertebrate olfactory receptor neurons (ORNs). Evidence for OMP function derives from altered behavioral and electrophysiological activities of OMP-KO mice. The molecular basis for the altered phenotype following the deletion of OMP is still unclear. Recent structural studies predict the involvement of OMP in protein-protein interaction. Here we report the identification of an OMP partner, Bex2, by phage-display screening of an olfactory mucosal cDNA-library. In situ hybridization demonstrates cellular co-localization of OMP mRNA with mRNAs for Bex1, Bex2, and Bex3 in ORNs of olfactory tissue of the mouse. The OMP/Bex interaction has been confirmed by demonstrating the chemical cross-linking of recombinant rat OMP with a synthetic peptide derived from the Bex amino acid sequence. The subcellular localization of Bex and OMP proteins was evaluated in transfected HEK293 cells. Bex is visualized in the nucleus and cytoplasm. Following co-transfection we observed the unexpected presence of some OMP in the nucleus along with Bex. Together, these data argue convincingly that we have identified Bex as an OMP partner whose further characterization will provide insight to the role of OMP and to the mechanism of the OMP/Bex interaction in ORN differentiation and function.

Analysis of penetrance and expressivity during ontogenesis supports a stochastic choice of zebrafish odorant receptors from predetermined groups of receptor genes

Olfactory receptor neurons select a single odourant receptor gene for expression out of a large gene family. The mechanisms of this extreme selectivity are largely unknown. We have determined in detail the developmental expression dynamics of a representative subset of the zebrafish odourant receptor repertoire, using in situ hybridization analysis. We have thus generated a dataset, which allows us to test hypotheses of odourant receptor gene regulation. The receptors chosen belong to four different groups with respect to ontogenetic onset of expression (onset groups). Statistical analysis of the data supports a model in which the final choice of an individual odourant receptor gene occurs stochastically from within a group of genes sharing a deterministically defined onset of expression. Genomic mapping revealed a pronounced correlation of onset of expression with genomic neighbourhood. During a protracted juvenile developmental period individual regulatory influences seem to modify the expression of odourant receptor genes, a notable example being a transient decrease in expressivity of two odourant receptor genes.