ABSTRACT
In the mouse olfactory system, the anatomical locations of olfactory sensory neurons (OSNs) roughly correlate with their axonal projection sites along the dorsal-ventral (D-V) axis of the olfactory bulb (OB). Here we report that an axon guidance receptor, Neuropilin-2 (Nrp2), and its repulsive ligand, Semaphorin-3F (Sema3F), are expressed by OSNs in a complementary manner that is important for establishing olfactory map topography. Sema3F is secreted by early-arriving axons of OSNs and is deposited at the anterodorsal OB to repel Nrp2-positive axons that arrive later. Sequential arrival of OSN axons as well as the graded and complementary expression of Nrp2 and Sema3F by OSNs help to form the topographic order along the D-V axis.
Subject(s)
Axons/metabolism , Membrane Proteins/metabolism , Nerve Tissue Proteins/metabolism , Olfactory Bulb/metabolism , Animals , Gene Expression , Membrane Proteins/genetics , Mice , Mice, Knockout , Nerve Tissue Proteins/genetics , Neuropilin-2/metabolism , Receptors, Cell Surface/metabolism , X Chromosome InactivationABSTRACT
NB-2 is a neuronal cell recognition molecule that is preferentially expressed in auditory pathways. Mice deficient in the NB-2 gene exhibit aberrant responses to acoustic stimuli. Here we examined the expression and localization of NB-2 in the auditory brainstem during development in the rat. NB-2 was strongly expressed in the ventral cochlear nucleus (VCN), ventral acoustic stria, lateral and medial superior olivary complex (SOC), superior paraolivary nucleus, medial nucleus of the trapezoid body (MNTB), ventrolateral lemniscus, and central nucleus of the inferior colliculus (CIC). In the VCN and CIC, NB-2 was expressed in the regions that are known to respond to high frequencies. In situ hybridization combined with immunohistochemistry suggested that NB-2 is expressed only in neurons. NB-2 was colocalized with glutamatergic elements in the neuropil and the calyces of Held but not with glycinergic or GABAergic elements. NB-2 expression in the SOC was first detected at embryonic day (E)19, reached a maximum level at postnatal day (P)7, and declined thereafter. Immunolabeling with VGLUT1 and VGLUT2, markers for mature and premature glutamatergic synapses, respectively, in combination with NB-2 immunolabeling revealed that NB-2 is expressed at glutamatergic synapses. Collectively, our findings suggest that NB-2 plays a key role in maturation of glutamatergic synapses in the brainstem during the final stages of auditory development.
Subject(s)
Auditory Pathways/metabolism , Brain Stem/metabolism , Cell Adhesion Molecules, Neuronal/metabolism , Glutamic Acid/metabolism , Neurons/metabolism , Animals , Auditory Pathways/embryology , Auditory Pathways/growth & development , Blotting, Western , Brain Stem/embryology , Brain Stem/growth & development , Contactins , Glutamate Decarboxylase/metabolism , Immunohistochemistry , In Situ Hybridization , Mice , Mice, Inbred C57BL , Neurons/ultrastructure , Rats , Rats, Wistar , Synapses/metabolism , Vesicular Glutamate Transport Protein 1/metabolism , Vesicular Glutamate Transport Protein 2/metabolismABSTRACT
In the mouse, olfactory sensory neurons (OSNs) expressing the same odorant receptor (OR) converge their axons to a specific set of glomeruli in the olfactory bulb. To study how OR-instructed axonal fasciculation is controlled, we searched for genes whose expression profiles are correlated with the expressed ORs. Using the transgenic mouse in which the majority of OSNs express a particular OR, we identified such genes coding for the homophilic adhesive molecules Kirrel2/Kirrel3 and repulsive molecules ephrin-A5/EphA5. In the CNGA2 knockout mouse, where the odor-evoked cation influx is disrupted, Kirrel2 and EphA5 were downregulated, while Kirrel3 and ephrin-A5 were upregulated, indicating that these genes are transcribed in an activity-dependent manner. Mosaic analysis demonstrated that gain of function of these genes generates duplicated glomeruli. We propose that a specific set of adhesive/repulsive molecules, whose expression levels are determined by OR molecules, regulate the axonal fasciculation of OSNs during the process of glomerular map formation.
Subject(s)
Axons/metabolism , Neurons, Afferent/cytology , Neurons, Afferent/metabolism , Receptors, Odorant/metabolism , Animals , Carrier Proteins/chemistry , Carrier Proteins/genetics , Cell Adhesion Molecules/metabolism , Ephrin-A5/genetics , Ephrin-A5/metabolism , Membrane Proteins/chemistry , Membrane Proteins/genetics , Mice , Mice, Transgenic , Models, Biological , Olfactory Bulb/cytology , Protein Binding , Receptor, EphA5/metabolism , Receptors, Odorant/geneticsABSTRACT
In mammals, olfactory signals received by odorant receptors (ORs) in the olfactory epithelium (OE) are converted to a topographical map of activated glomeruli in the olfactory bulb (OB). It has been reported that the OE can be divided into four topographically distinct zones and that olfactory sensory neurons (OSNs) expressing a particular OR gene are randomly distributed within one zone. Here, we analyzed 80 different class II OR genes for their expression patterns in the OE by in situ hybridization. It was found that the expression area in the OE does not always fit into one of the four conventional zones. Expression areas are specific to each OR gene and are arranged in an overlapping and continuous manner in the OE. We also analyzed a spatial relationship between the OE and the OB for OSN projection. Our transgenic as well as DiI retrograde staining experiments demonstrated that the dorsal/ventral arrangement of glomeruli in the OB is correlated with the expression areas of corresponding ORs along the dorsomedial/ventrolateral axis in the OE. The present study indicates that the OR gene choice may be more restricted by the OSN location in the OE than what has been thought.
Subject(s)
Olfactory Bulb/cytology , Olfactory Mucosa/cytology , Olfactory Mucosa/metabolism , Olfactory Receptor Neurons/metabolism , Receptors, Odorant/genetics , Animals , Axons/metabolism , Carbocyanines/metabolism , Green Fluorescent Proteins/biosynthesis , Green Fluorescent Proteins/genetics , In Situ Hybridization/methods , Mice , Mice, Inbred C57BL , Mice, Transgenic , Molecular Biology/methods , Nerve Growth Factors/genetics , Olfactory Bulb/metabolism , Olfactory Pathways/anatomy & histology , Olfactory Pathways/metabolism , Receptors, Odorant/classification , Receptors, Odorant/metabolism , Wheat Germ Agglutinins/biosynthesis , Wheat Germ Agglutinins/genetics , tau Proteins/biosynthesis , tau Proteins/geneticsSubject(s)
Gene Expression Regulation , Neurons/metabolism , Olfactory Pathways , Smell/physiology , Animals , Feedback , Humans , Mice , Models, Biological , Models, Neurological , Multigene Family , Nervous System , TransgenesABSTRACT
Similar to the expression of antigen receptor genes in lymphocytes, the mammalian odorant receptor (OR) genes are expressed in a mutually exclusive and monoallelic manner in olfactory sensory neurons (OSNs). DNA rearrangement has long been regarded as a possible mechanism for the allelic exclusion of the OR genes. However, mice cloned from mature OSN nuclei expressed the full repertoire of ORs, and the possibility of irreversible gene translocation was excluded as a mechanism to activate a single OR gene in each OSN. How is allelic exclusion achieved in the olfactory system? Recent transgenic experiments indicated an inhibitory role of the OR protein in preventing further activation of other OR genes. Stochastic activation of an OR gene and negative-feedback regulation by the OR gene product might ensure the maintenance of the one neuron-one receptor rule in the mammalian olfactory system.
Subject(s)
Receptors, Odorant/genetics , Animals , Gene Expression Regulation , Mice , Multigene Family , Olfactory Receptor Neurons/metabolismSubject(s)
Alleles , Gene Expression Regulation , Receptors, Odorant/genetics , Animals , DNA/genetics , Feedback, Physiological/genetics , Frameshift Mutation , Gene Conversion , Gene Rearrangement , Genetic Code/genetics , Humans , Locus Control Region , Multigene Family/genetics , Pseudogenes/genetics , Recombination, Genetic , TransgenesABSTRACT
In the mouse olfactory system, each olfactory sensory neuron (OSN) expresses only one odorant receptor (OR) gene in a monoallelic and mutually exclusive manner. Such expression forms the genetic basis for OR-instructed axonal projection of OSNs to the olfactory bulb of the brain during development. Here, we identify an upstream cis-acting DNA region that activates the OR gene cluster in mouse and allows the expression of only one OR gene within the cluster. Deletion of the coding region of the expressed OR gene or a naturally occurring frame-shift mutation allows a second OR gene to be expressed. We propose that stochastic activation of only one OR gene within the cluster and negative feedback regulation by that OR gene product are necessary to ensure the one receptor-one neuron rule.
Subject(s)
Feedback, Physiological , Gene Expression Regulation , Locus Control Region , Olfactory Receptor Neurons/metabolism , Receptors, Odorant/genetics , Alleles , Animals , Axons/physiology , Chromosomes, Artificial, Yeast , Conserved Sequence , Frameshift Mutation , Gene Silencing , In Situ Hybridization , Mice , Mice, Transgenic , Multigene Family , Olfactory Bulb/cytology , Promoter Regions, Genetic , Pseudogenes , Receptors, Odorant/metabolism , Recombinant Fusion Proteins/metabolism , Signal Transduction , TransgenesABSTRACT
In rodents, olfactory receptor (OR) genes are expressed in one of four zones in the olfactory epithelium (OE), and olfactory sensory neurons (OSNs) expressing the same OR project their axons to a specific set of glomeruli on the olfactory bulb (OB). Using the yeast artificial chromosome (YAC) transgenic system, we have analysed the expression of the murine OR gene MOR29A of the MOR28 cluster located on chromosome 14. Although expression of the endogenous MOR29A was restricted to the most dorsomedial zone, the transgenic MOR29A (Tg MOR29A) was expressed in all four zones of the OE. When the OB of the transgenic mouse was analysed, the axons of the OSNs expressing Tg MOR29A were found to project not only to the dorsal side but also to the ventral side of the OB as well. The ectopic projection sites on the ventral side gradually disappear during postnatal development. Naris occlusion prevents this elimination, suggesting that odorant stimulation is involved in eliminating the ectopic projection sites.
Subject(s)
Olfactory Bulb/growth & development , Olfactory Mucosa/growth & development , Olfactory Mucosa/metabolism , Olfactory Nerve/growth & development , Receptors, Odorant/metabolism , Animals , Immunohistochemistry , In Situ Hybridization , Mice , Mice, Transgenic , Olfactory Receptor Neurons/growth & development , Olfactory Receptor Neurons/metabolism , Receptors, Odorant/genetics , Sensory DeprivationABSTRACT
Genomic analysis was performed for the murine odorant receptor (OR) genes. The MOR28 cluster on chromosome 14 was extensively studied. It contains six OR genes, MOR28, 10, 83, 29A, 29B and 30. The human homolog of this cluster is located on the human chromosome 14, and contains five OR genes, HOR28/10, 83, 29A, 29B and 30. Sequence comparison of these OR gene paralogs and orthologs suggests that the coding homologies are accounted for not only by recent gene duplication, but also by gene conversion among the coding sequences within the cluster. A possible role of gene conversion in the olfactory system is discussed in the context of the olfactory map.
