Projection Of Olfactory Sensory Neurons Research Articles

A novel function for α-synuclein as a regulator of NCK2 in olfactory bulb: implications for its role in olfaction.

To investigate physiological function of α-synuclein is important for understanding its pathophysiological mechanism in synucleinopathies including Parkinson's disease. Employing knockout mice, we found that Snac/α-synuclein deletion induced aberrant projection of olfactory sensory neurons and hyposmia. We identified 9 axon guidance associated differentially expressed proteins using iTRAQbased Liquid Chromatograph Mass Spectrometer. NCK2 is most significantly down-regulated protein among them. We further found that either α-synuclein deletion or NCK2 deficiency induced Eph A4 inactivation. Re-expressing Snac/α-synuclein in its knockout neurons reversed the down-regulation of NCK2, as well as the inactivation of EphA4. Overexpression of Snac/α-synuclein in α-synuclein deleted mice reversed the down-regulation of NCK2 and pEphA4, and improved the olfactory impairment of mice. Correlation analysis showed that there is a significant correlation between the protein level of α-synuclein, NCK2, and pEphA4, respectively. Nonetheless, immunoprecipitation analysis showed that NCK2 was associated with both EphA4 and Rho A, suggesting that NCK2 as a scaffolding protein to modulate Eph A4/Rho A pathway. Moreover, Rho A activity was significantly lower in α-synuclein deficient mice. Thus, α-synuclein regulates olfactory neurons projection through NCK2 dependent EphA4/Rho A pathway. Malfunction of α-synuclein because of deletion may cause aberrant olfactory neurons projection. This extended our knowledge of α-synuclein functions, which may explain why olfaction is usually impaired in some synucleinopathies.

Pcdh19 mediates olfactory sensory neuron coalescence during postnatal stages and regeneration

The mouse olfactory system regenerates constantly throughout life. While genes critical for the initial projection of olfactory sensory neurons (OSNs) to the olfactory bulb have been identified, what genes are important for maintaining the olfactory map during regeneration are still unknown. Here we show a mutation in Protocadherin 19 (Pcdh19), a cell adhesion molecule and member of the cadherin superfamily, leads to defects in OSN coalescence during regeneration. Surprisingly, lateral glomeruli were more affected and males in particular showed a more severe phenotype. Single cell analysis unexpectedly showed OSNs expressing the MOR28 odorant receptor could be subdivided into two major clusters. We showed that at least one protocadherin is differentially expressed between OSNs coalescing on the medial and lateral glomeruli. Moreover, females expressed a slightly different complement of genes from males. These features may explain the differential effects of mutating Pcdh19 on medial and lateral glomeruli in males and females.

MRI tractography reveals the human olfactory nerve map connecting the olfactory epithelium and olfactory bulb

The olfactory nerve map describes the topographical neural connections between the olfactory epithelium in the nasal cavity and the olfactory bulb. Previous studies have constructed the olfactory nerve maps of rodents using histological analyses or transgenic animal models to investigate olfactory nerve pathways. However, the human olfactory nerve map remains unknown. Here, we demonstrate that high-field magnetic resonance imaging and diffusion tensor tractography can be used to visualize olfactory sensory neurons while maintaining their three-dimensional structures. This technique allowed us to evaluate the olfactory sensory neuron projections from the nasal cavities to the olfactory bulbs and visualize the olfactory nerve maps of humans, marmosets and mice. The olfactory nerve maps revealed that the dorsal-ventral and medial-lateral axes were preserved between the olfactory epithelium and olfactory bulb in all three species. Further development of this technique might allow it to be used clinically to facilitate the diagnosis of olfactory dysfunction.

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.

Acquisition of innate odor preference depends on spontaneous and experiential activities during critical period.

Animals possess an inborn ability to recognize certain odors to avoid predators, seek food, and find mates. Innate odor preference is thought to be genetically hardwired. Here we report that acquisition of innate odor recognition requires spontaneous neural activity and is influenced by sensory experience during early postnatal development. Genetic silencing of mouse olfactory sensory neurons during the critical period has little impact on odor sensitivity, discrimination, and recognition later in life. However, it abolishes innate odor preference and alters the patterns of activation in brain centers. Exposure to innately recognized odors during the critical period abolishes the associated valence in adulthood in an odor-specific manner. The changes are associated with broadened projection of olfactory sensory neurons and expression of axon guidance molecules. Thus, a delicate balance of neural activity is needed during the critical period in establishing innate odor preference and convergent axon input is required to encode innate odor valence.

Plant-associated odor perception and processing in two parasitoid species with different degrees of host specificity: Implications for host location strategies

Microplitis croceipes and Cotesia marginiventris (Hymenoptera: Braconidae) are parasitoids of lepidopteran larvae with different degrees of host specificity. Both parasitoid species rely on host-related plant volatiles as odor cues to locate their herbivore hosts. To better understand mechanisms of odor processing in parasitoids, we tested responses of olfactory sensory neurons (OSNs) in the antennal sensilla placodea of female parasitoids to select plant volatiles and mixtures. The compounds tested include two green leaf volatiles (i.e., cis-3-hexenol and hexanal) and three herbivore-induced plant volatiles (i.e., cis-3-hexenyl butyrate, cis-3-hexenyl acetate and linalool). Single-sensillum recording showed that the test compounds elicited activity in large and small amplitude neurons housed in the short sensilla placodea of both parasitoid species. In general, C. marginiventris showed greater OSN responses to a low dose while M. croceipes showed greater responses to a high dose of test compounds. Binary mixtures of cis-3-hexenol and linalool inhibited OSN activity in M. croceipes, but not in C. marginiventris. These differences may have implications for odor discrimination in the two parasitoid species. In addition, anterograde neurobiotin stainings were performed to map glomerular projections of OSNs in the antennal lobe of the parasitoids. In M. croceipes, a mixture of cis-3-hexenol and linalool inhibited activity of the glomerulus activated by cis-3-hexenol alone. In C. marginiventris, a mixture of cis-3-hexenol and cis-3-hexenyl acetate showed intense labeling in their respective glomeruli, possibly suggesting a synergistic interaction. These differences in detection and coding of single compounds and mixtures may impact host location strategies in the two parasitoid species.

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

BackgroundThe 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 resultsTo 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.ConclusionsThese 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.

Homotypic and Heterotypic Adhesion Induced by Odorant Receptors and the β2-Adrenergic Receptor

In the mouse olfactory system regulated expression of a large family of G Protein-Coupled Receptors (GPCRs), the Odorant Receptors (ORs), provides each sensory neuron with a single OR identity. In the wiring of the olfactory sensory neuron projections, a complex axon sorting process ensures the segregation of >1,000 subpopulations of axons of the same OR identity into homogeneously innervated glomeruli. ORs are critical determinants in axon sorting, and their presence on olfactory axons raises the intriguing possibility that they may participate in axonal wiring through direct or indirect trans-interactions mediating adhesion or repulsion between axons. In the present work, we used a biophysical assay to test the capacity of ORs to induce adhesion of cell doublets overexpressing these receptors. We also tested the β2 Adrenergic Receptor, a non-OR GPCR known to recapitulate the functions of ORs in olfactory axon sorting. We report here the first evidence for homo- and heterotypic adhesion between cells overexpressing the ORs MOR256-17 or M71, supporting the hypothesis that ORs may contribute to olfactory axon sorting by mediating differential adhesion between axons.

Olfactory receptors and the mechanism of odor perception

The olfactory system plays one of the key roles in the lives of humans and animals. It can detect thousands of different odor molecules through a large family of olfactory receptors (ORs), of a diverse protein sequence, which are located in olfactory sensory neurons (OSNs) in the olfactory epithelium in the nose of humans, and in the vomeronasal organ in animals. The OR family is comprised of 172 subfamilies, whose members have related protein sequences and are encoded by a single chromosomal locus. The human receptor gene family includes 339 intact receptor genes and 297 receptor pseudogenes, unevenly distributed among 51 different loci on 21 human chromosomes. Different parts of the genome may be involved in the detection of different types of structural odorants. Odor detection is mediated by odorant receptors. Signals generated in OSNs in response to odorants are transmitted to the olfactory bulb (OB) of the brain, i.e., the first relay station in the control of the olfactory system in mammals. Then, signals are transmitted to the olfactory brain cortex, which has a cortical structure with distinct layers and numerous glomerular modules, and forms a map of olfactory axon terminals. The axonal projection of OSNs is precisely organized with a few topographically fixed glomeruli. The purpose of this paper is to present the recent literature in the field of the undertaken subject. The review of articles is devoted to olfactory receptors and the mechanism of odor perception. In the first part, this review summarizes the mammalian protein ORs that are encoded by genes, as well as the location and structure of these receptors. Extensive studies reveal that mammals can have up to 1000 different OR genes, which constitute approximately 1% of the genomic complement of genes. The analysis of the entire receptor family has shown that it is comprised of 172 subfamilies, whose members are 60% identical in protein amino acid sequence and can recognize odorants with related structures. The second part of this review presents the opinions of many authors concerning the olfactory perception that initiates in the olfactory epithelium. Odor signals are further transmitted to the OB, i.e., the first relay station of the central olfactory system in the mammalian brain. Then such signals ultimately reach higher cortical areas involved in the conscious perception of odor. At the review's conclusion some diseases associated with smell disorders are discussed. The olfactory cortex still remains an unexplored and unexplained area in terms of the processing of odorant information. This subject requires further study.

β-Site Amyloid Precursor Protein (APP)-cleaving Enzyme 1 (BACE1)-deficient Mice Exhibit a Close Homolog of L1 (CHL1) Loss-of-function Phenotype Involving Axon Guidance Defects

BACE1 is the β-secretase enzyme that initiates production of the β-amyloid peptide involved in Alzheimer disease. However, little is known about the functions of BACE1. BACE1-deficient mice exhibit mild but complex neurological phenotypes suggesting therapeutic BACE1 inhibition may not be completely free of mechanism-based side effects. Recently, we have reported that BACE1 null mice have axon guidance defects in olfactory sensory neuron projections to glomeruli in the olfactory bulb. Here, we show that BACE1 deficiency also causes an axon guidance defect in the hippocampus, a shortened and disorganized infrapyramidal bundle of the mossy fiber projection from the dentate gyrus to CA3. Although we observed that a classical axon guidance molecule, EphA4, was cleaved by BACE1 when co-expressed with BACE1 in HEK293 cells, we could find no evidence of BACE1 processing of EphA4 in the brain. Remarkably, we discovered that the axon guidance defects of BACE1(-/-) mice were strikingly similar to those of mice deficient in a recently identified BACE1 substrate, the neural cell adhesion molecule close homolog of L1 (CHL1) that is involved in neurite outgrowth. CHL1 undergoes BACE1-dependent processing in BACE1(+/+), but not BACE1(-/-), hippocampus, and olfactory bulb, indicating that CHL1 is a BACE1 substrate in vivo. Finally, BACE1 and CHL1 co-localize in the terminals of hippocampal mossy fibers, olfactory sensory neuron axons, and growth cones of primary hippocampal neurons. We conclude that BACE1(-/-) axon guidance defects are likely the result of abrogated BACE1 processing of CHL1 and that BACE1 deficiency produces a CHL1 loss-of-function phenotype. Our results imply the possibility that axon mis-targeting may occur in adult neurogenic and/or regenerating neurons as a result of chronic BACE1 inhibition and add a note of caution to BACE1 inhibitor development.

Constitutively expressed Protocadherin-α regulates the coalescence and elimination of homotypic olfactory axons through its cytoplasmic region

Olfactory sensory neuron (OSN) axons coalesce into specific glomeruli in the olfactory bulb (OB) according to their odorant receptor (OR) expression. Several guidance molecules enhance the coalescence of homotypic OSN projections, in an OR-specific- and neural-activity-dependent manner. However, the mechanism by which homotypic OSN axons are organized into glomeruli is unsolved. We previously reported that the clustered protocadherin-α (Pcdh-α) family of diverse cadherin-related molecules plays roles in the coalescence and elimination of homotypic OSN axons throughout development. Here we showed that the elimination of small ectopic homotypic glomeruli required the constitutive expression of a Pcdh-α isoform and Pcdh-α's cytoplasmic region, but not OR specificity or neural activity. These results suggest that Pcdh-α proteins provide a cytoplasmic signal to regulate repulsive activity for homotypic OSN axons independently of OR expression and neural activity. The counterbalancing effect of Pcdh-α proteins for the axonal coalescence mechanisms mediated by other olfactory guidance molecules indicate a possible mechanism for the organization of homotypic OSN axons into glomeruli during development.

Formation and patterning of the forebrain and olfactory system by zinc‐finger genes Fezf1 and Fezf2

The zinc finger genes Fezf1 (Fez) and Fezf2 (Fez-like, Fezl, Zfp312) were initially identified as anterior neuroectoderm-specific genes in Xenopus and zebrafish. They encode transcriptional regulators containing an Engrailed homology 1 (Eh1) repressor motif, which is known to interact with Groucho/TLE (Transducin-Like Enhancer of Split)-type transcriptional co-repressors. Both Fezf1 and Fezf2 are expressed in the prospective forebrain region during early embryogenesis, and they subsequently show both overlapping and distinct expression domains in the olfactory epithelium and forebrain. Loss-of-function studies in mouse and zebrafish revealed roles for Fezf1 and Fezf2 in the development of the olfactory system and forebrain. In mice, Fezf1, expressed in olfactory sensory neurons, is required for the axonal projection of olfactory sensory neurons, and controls the layer formation of the olfactory bulb in a non-cell autonomous manner. Fezf2 is involved in the differentiation of subplate neurons and the formation of the fimbria and fornix. Fezf2 is also essential for specification of the subcerebral projection neurons in the neocortex. Fezf1 and Fezf2 control the rostro-caudal patterning of the diencephalon by repressing the caudal diencephalon fate in the rostral diencephalon in mice and zebrafish. In zebrafish, fezf2 is also required for the development of monoaminergic (dopaminergic and serotonergic) neurons in the basal forebrain.

Lactosamine differentially affects olfactory sensory neuron projections to the olfactory bulb

During embryonic development, olfactory sensory neurons extend axons that form synapses with the dendrites of projection neurons in glomeruli of the olfactory bulb (OB). The glycosyltransferase beta3GnT1 regulates the expression of 1B2-reactive lactosamine glycans that are mosaically distributed among glomeruli. In newborn beta3GnT1-/- mice, lactosamine expression is lost, and many glomeruli fail to form. To determine the role of lactosamine in OB targeting, we analyzed the trajectories of specific OR axon populations and their reactivity with 1B2 in beta3GnT1-/- mice. mI7 axons and P2 axons, both of which are weakly 1B2+ in wild-type mice, fail to grow to their normal positions in the glomerular layer during early postnatal development and never recover in adult mutant mice. In contrast, many M72 axons, which are always lactosamine negative in wild-type mice, survive but are misguided to the extreme anterior OB in neonatal mutant mice and persist as heterotypic glomeruli, even in adult null mice. These results show that the loss of lactosamine differentially affects each OR population. Those that lose their normal expression of lactosamine fail to form stable connections with mitral and tufted cells in the OB, disappear during early postnatal development, and do not recover in adults. Neurons that are normally lactosamine negative, survive early postnatal degeneration in beta3GnT1-/- mice but extend axons that converge on inappropriate targets in the mutant OB.

Hierarchical regulation of odorant receptor gene choice and subsequent axonal projection of olfactory sensory neurons in zebrafish.

In both Drosophila and mice, olfactory sensory neurons (OSNs) expressing a given odorant receptor (OR) project axons to specific glomeruli in the antennal lobe or olfactory bulb (OB), developing a topographic odor map. To gain insights into the modes of OR expression and axonal projection in zebrafish, we generated a bacterial artificial chromosome transgenic line carrying an OR gene cluster in which two OR-coding sequences, OR111-7 and OR103-1, were replaced with yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP), respectively. In the transgenic embryos, YFP and CFP signals appear in small populations of OSNs at an early stage of development when OR expression is first observed. Time-lapse imaging of living embryos revealed that both YFP- and CFP-expressing OSNs project axons to the medial portion of the OB. This pattern of axonal projection is maintained in the adult transgenic fish, in which fluorescently labeled OSN axons target a topographically fixed cluster of glomeruli in the medial OB. Because the OR-coding sequences were replaced with fluorescent reporter genes, we examined which OR genes are expressed in YFP/CFP-expressing OSNs and found that the OR choice is mostly restricted to OR members within the same subfamily of the cluster. Furthermore, we found that the one receptor-one neuron rule is not always applicable to zebrafish OSNs and that multiple receptors-one neuron is true for a subpopulation of OSNs in both wild-type and transgenic fish. These data demonstrate two distinct modes of OR expression and suggest a model of the hierarchical regulation of OR gene choice and subsequent axonal projection in the zebrafish olfactory system.

Disrupted compartmental organization of axons and dendrites within olfactory glomeruli of mice deficient in the olfactory cell adhesion molecule, OCAM

There is an overall topographic connectivity in the axonal projections of olfactory sensory neurons from the olfactory epithelium (OE) to the olfactory bulb (OB). The molecular determinants of this overall topographic OE–OB connectivity are not known. For 20 years, the intriguing expression pattern of the olfactory cell adhesion molecule (OCAM) has made it the leading candidate as determinant of overall topographic OE–OB connectivity. Here, we have generated a strain of OCAM knockout mice by gene targeting. There were no obvious alterations in the distribution of olfactory sensory neurons within the OE or in the coalescence of axons into specific glomeruli. However, the compartmental organization of dendrites and axons within the glomeruli was disrupted. Surprisingly, the mutant mice exhibited an increase in olfactory acuity; they appeared to have a better sense of smell. Thus, despite its striking expression pattern, OCAM is not essential for overall topographic OE–OB connectivity. Instead, OCAM is required for establishing or maintaining the compartmental organization and the segregation of axodendritic and dendrodendritic synapses within glomeruli.

Negative Feedback Regulation Ensures the One Neuron-One Receptor Rule in the Mouse Olfactory System

In the mouse olfactory system, there are ~1500 odorant receptor (OR) genes clustered at ~50 different loci which are scattered among most of the chromosomes (Buck and Axel, 1991; Zhang and Firestein, 2002; Godfrey et al., 2004). Like the antigen receptor genes in lymphocytes, the mammalian OR genes are expressed in a mutually exclusive (Malnic et al., 1999; Serizawa et al., 2000) and monoallelic (Chess et al., 1994; Ishii et al., 2001) manner in olfactory sensory neurons (OSNs). DNA rearrangement has long been thought of as a possible mechanism for the allelic exclusion of the OR genes. However, mice cloned with OSN nuclei excluded the possibility of irreversible gene translocation as a mechanism to activate a single OR gene in each OSN (Eggan et al., 2004; Li et al., 2004). How is it, then, that allelic exclusion is achieved in the olfactory system? Since OSNs expressing a given OR gene converge their axons to a specific set of projection sites (glomeruli) on the olfactory bulb (OB) (Ressler et al., 1994; Vassar et al., 1994; Mombaerts et al., 1996; Feinstein and Mombaerts, 2004), odorous stimuli received in the olfactory epithelium (OE) are converted to a topographic map of activated glomeruli on the OB (Mori et al., 1999). Thus, the one neuron–one receptor rule forms the genetic basis for OR-instructed axonal projection of OSNs. Here, we discuss possible mechanisms that regulate single OR gene expression in each OSN in mouse.

Differential requirements for semaphorin 3F and Slit-1 in axonal targeting, fasciculation, and segregation of olfactory sensory neuron projections.

The formation of precise stereotypic connections in sensory systems is critical for defining accurate internal representations of the external world; however, the molecular mechanisms underlying the formation of sensory maps are poorly understood. Here, we examine the roles of two structurally unrelated repulsive guidance cues, semaphorin 3F (Sema3F) and Slit-1, in olfactory sensory axon fasciculation, targeting, and segregation. Using sema3F-/- mice, we show that Sema3F is critical for vomeronasal sensory neuron axonal fasciculation and for segregation of these sensory afferents from the main olfactory system; however, Sema3F plays only a minor role in targeting of apical vomeronasal neuron axons to the anterior accessory olfactory bulb (AOB). In addition, we show that Sema3F is required for lamina-specific targeting of olfactory sensory axons within the main olfactory system. In contrast to Sema3F, Slit-1 is dispensable for fasciculation of basal vomeronasal neuron axons but is critical for targeting these axons to the posterior AOB. These results reveal discrete and complementary roles for secreted semaphorins and slits in axonal targeting, fasciculation, and segregation of olfactory sensory neuron projections.

Assessment of neuronal maturation and acquisition of functional competence in the developing zebrafish olfactory system.

Olfactory coding at the level of the olfactory bulb is thought to depend upon an ensemble response of mitral cells receiving input from chemotopically-organized projections of olfactory sensory neurons and regulated by lateral inhibitory circuits. Immunocytochemical methods are described to metabolically classify neurons in the developing zebrafish olfactory system based on the relative concentrations of taurine, glutamate, GABA (and potentially other small biogenic amines) and a small guanidium-based cation, agmatine, which labels NMDA-sensitive cells by permeating through active ionotropic glutamate receptor channels. Using metabolic profiling in conjunction with activity dependent labeling we demonstrate that neuronal differentiation in the developing olfactory bulb, as assessed by acquisition of a mature neurochemical profile, and sensitivity to an ionotropic glutamate receptor agonist, NMDA, occurs during the second day of development. This experimental approach is likely to be useful in studies concerned with the development of glutamatergic signaling pathways.

Axonal projection of olfactory sensory neurons during the developmental and regeneration processes

We have studied the projection of olfactory sensory neurons (OSNs), during the developmental and regeneration processes, using the transgenic mouse carrying the differently tagged odorant receptor genes, MOR28. We have found that the axon terminals of the two sets of MOR28-positive OSNs, one expressing the lacZ tag and the other expressing the green fluorescent protein gene, are dispersed and intermingled at early developmental or regeneration stages. Projection areas become more distinct and separated at later stages, however, two sets of axon fibers are not typically bundled or segregated during pathfinding. It appears that segregation of axons mainly occurs when they target at the olfactory bulb to form the glomerular structure.

Axonal projection of olfactory sensory neurons during development and regeneration

Axonal projection of olfactory sensory neurons during development and regeneration