Aberrant sensory innervation of the olfactory bulb in neuropilin-2 mutant mice

Protocols for two- and three-color fluorescent RNA in situ hybridization of the main and accessory olfactory epithelia in mouse

The main and accessory olfactory epithelia of the mouse are composed of many cell populations. Each sensory neuron is thought to express one allele of one of the approximately 1000 odorant or approximately 300 vomeronasal receptor genes. Sensory neurons die and are replaced by new neurons that differentiate from precursor cells throughout the lifetime of the individual. Neuronal replacement is asynchronous, resulting in the co-existence of cells at various stages of differentiation. Receptor gene diversity and ongoing neuronal differentiation produce complex mosaics of gene expression within these epithelia. Accurate description of gene expression patterns will facilitate the understanding of mechanisms of gene choice and differentiation. Here we report a detailed protocol for two- and three-color fluorescent RNA in situ hybridization (ISH) and its combination with immunohistochemistry, or detection of bromodeoxyuridine (BrdU)-incorporated DNA after labeling. The protocol is applied to cryosections of the main and accessory olfactory epithelia in mouse.

Molecular gradients and development of retinotopic maps

Gradients of axon guidance molecules have long been postulated to control the development of the organization of neural connections into topographic maps. We review progress in identifying molecules required for mapping and the mechanisms by which they act, focusing on the visual system, the predominant model for map development. The Eph family of receptor tyrosine kinases and their ligands, the ephrins, remain the only molecules that meet all criteria for graded topographic guidance molecules, although others fulfill some criteria. Recent reports further define their modes of action and new roles for them, including EphB/ephrin-B control of dorsal-ventral mapping, bidirectional signaling of EphAs/ephrin-As, bifunctional action of ephrins as attractants or repellents in a context-dependent manner, and complex interactions between multiple guidance molecules. In addition, spontaneous patterned neural activity has recently been shown to be required for map refinement during a brief critical period. We speculate on additional activities required for map development and suggest a synthesis of molecular and cellular mechanisms within the context of the complexities of map development.

Olfactory marker protein modulates primary olfactory axon overshooting in the olfactory bulb

Olfactory marker protein (OMP) is expressed by mature primary olfactory sensory neurons during development and in adult mice. In mice that lack OMP, olfactory sensory neurons have perturbed electrophysiological activity, and the mice exhibit altered responses and behavior to odor stimulation. To date, defects in axon guidance in mice that lack OMP have not been investigated. During development of the olfactory system in mouse, primary olfactory axons often overshoot their target glomerular layer and project into the deeper external plexiform layer. These aberrant axonal projections are normally detected within the external plexiform layer up to postnatal day 12. We have examined the projections of primary olfactory axons in OMP-tau:LacZ mice and OMP-GFP mice, two independent lines in which the OMP coding region has been replaced by reporter molecules. We found that axons overshoot their target layer and grow into the external plexiform layer in these OMP null mice as they do in wild-type animals. However, in the absence of OMP, overshooting axons are more persistent and remain prominent until 5 weeks postnatally, after which their numbers decrease. Overshooting axons are still present in these mice even at 8 months of age. In heterozygous mice, axons also overshoot into the external plexiform layer; however, there are fewer axons, and they project for shorter distances, compared with those in a homozygous environment. Our results suggest that perturbed electrophysiological responses, caused by loss of OMP in primary olfactory neurons, reduce the ability of primary olfactory axons to recognize their glomerular target.

Differentially expressed transcripts from phenotypically identified olfactory sensory neurons

In comparing purified mouse olfactory sensory neurons (OSNs) with neighboring cells, we identified 54 differentially expressed transcripts. One-third of the transcripts encode proteins with no known function, but the others have functions that correlate with challenges faced by OSNs. The OSNs expressed a diversity of signaling protein genes, including stomatin (Epb7.2), S100A5, Ddit3, Sirt2, CD81, Sdc2, Omp, and Ptpla. The elaboration of dendrites, cilia, and axons that places OSNs in contact with diverse cell types and signals presumably also requires large investments in cytoskeletal-associated proteins, lipid biosynthesis, and energy production. Several of the genes encode proteins that participate in these biological processes, including ATP5g3, Ndufa9, Sqrdl, Mdh1, Got1, beta-2 tubulin, Capza1, Bin3, Tom1, Acl6, and similar to O-MACS. Three transcripts had restricted expression patterns. Similar to O-MACS and Gstm2 had zonally restricted expression patterns in OSNs and sustentacular cells but not in Bowman's glands, suggesting that zonality can be differentially regulated by cell type. The mosaic expression pattern of S100A5 in approximately 70% of OSNs predicts that it is coexpressed with a subset of odorant receptors. We captured four abundant transcripts, Cyp2a4, similar to Cyp2g1, Gstm2, and Cbr2, that encode xenobiotic metabolizing enzymes expressed by sustentacular cells or Bowman's glands, reinforcing the interpretation that clearance of xenobiotic compounds is a major function of these cells. Within the olfactory epithelium, Cbr2 is a new anatomical marker for sustentacular cells. We also discovered that Reg3g is a marker for respiratory epithelium.

Target tissue influences the peripheral trajectory of mouse primary sensory olfactory axons

Primary olfactory neurons situated in the nasal septum project axons within fascicles along a highly stereotypical trajectory en route to the olfactory bulb. The ventral fascicles make a distinct dorsovental turn at the rear of the septum so as to reach the olfactory bulb. In the present study we have used a brain and nasal septum coculture system to examine the role of target tissue on the peripheral trajectory of olfactory sensory axons. In cultures of isolated embryonic nasal septa, olfactory axons form numerous parallel fascicles that project caudally in the submucosa, as they do in vivo. The ventral axon fascicles in the septum, however, often fail to turn, and do not project dorsally towards the roof of the nasal cavity. The presence of olfactory bulb, cortical, or tectal tissue apposed to the caudal end of the septum rescued this phenotype, causing the ventral fascicles to follow a normal in vivo-like trajectory. Ectopic placements of the explants revealed that brain tissue is not tropic for olfactory axons but appears to maintain the peripheral trajectory of growing axons in the nasal septum. Although primary olfactory axons are able to penetrate into olfactory bulb in vitro, they only superficially enter cortical tissue, whereas they do not grow into tectal explants. The ability of axons to differentially grow into different brain regions was shown to be unrelated to the migratory behavior of olfactory ensheathing cells, indicating that olfactory axons are directly responsive to guidance cues in the brain.

Mechanisms of axon guidance in the developing nervous system

The human brain assembles an incredible network of over a billion neurons. Understanding how these connections form during development in order for the brain to function properly is a fundamental question in biology. Much of this wiring takes place during embryonic development. Neurons are generated in the ventricular zone, migrate out, and begin to differentiate. However, neurons are often born in locations some distance from the target cells with which they will ultimately form connections. To form connections, neurons project long axons tipped with a specialized sensing device called a growth cone. The growing axons interact directly with molecules within the environment through which they grow. In order to find their targets, axonal growth cones use guidance molecules that can either attract or repel them. Understanding what these guidance cues are, where they are expressed, and how the growth cone is able to transduce their signal in a directionally specific manner is essential to understanding how the functional brain is constructed. In this chapter, we review what is known about the mechanisms involved in axonal guidance. We discuss how the growth cone is able to sense and respond to its environment and how it is guided by pioneering cells and axons. As examples, we discuss current models for the development of the spinal cord, the cerebral cortex, and the visual and olfactory systems.

Expression and putative role of lactoseries carbohydrates present on NCAM in the rat primary olfactory pathway

Primary olfactory neurons project axons from the olfactory neuroepithelium lining the nasal cavity to the olfactory bulb in the brain. These axons grow within large mixed bundles in the olfactory nerve and then sort out into homotypic fascicles in the nerve fiber layer of the olfactory bulb before terminating in topographically fixed glomeruli. Carbohydrates expressed on the cell surface have been implicated in axon sorting within the nerve fiber layer. We have identified two novel subpopulations of primary olfactory neurons that express distinct alpha-extended lactoseries carbohydrates recognised by monoclonal antibodies LA4 and KH10. Both carbohydrate epitopes are present on novel glycoforms of the neural cell adhesion molecule, which we have named NOC-7 and NOC-8. Primary axon fasciculation is disrupted in vitro when interactions between these cell surface lactoseries carbohydrates and their endogenous binding molecules are inhibited by the LA4 and KH10 antibodies or lactosamine sugars. We report the expression of multiple members of the lactoseries binding galectin family in the primary olfactory system. In particular, galectin-3 is expressed by ensheathing cells surrounding nerve fascicles in the submucosa and nerve fiber layer, where it may mediate cross-linking of axons. Galectin-4, -7, and -8 are expressed by the primary olfactory axons as they grow from the nasal cavity to the olfactory bulb. A putative role for NOC-7 and NOC-8 in axon fasciculation and the expression of multiple galectins in the developing olfactory nerve suggest that these molecules may be involved in the formation of this pathway, particularly in the sorting of axons as they converge towards their target.

A contextual model for axonal sorting into glomeruli in the mouse olfactory system

No models fully account for how odorant receptors (ORs) function in the guidance of axons of olfactory sensory neurons (OSNs) to glomeruli in the olfactory bulb. Here, we use gene targeting in mice to demonstrate that the OR amino acid sequence imparts OSN axons with an identity that allows them to coalesce into glomeruli. Replacements between the coding regions of the M71 and M72 OR genes reroute axons to their respective glomeruli. A series of M71-M72 hybrid ORs uncover a spectrum of glomerular phenotypes, leading to the concept that the identity of OSN axons is revealed depending on what other axons are present. Naturally occurring amino acid polymorphisms in other ORs also produce distinct axonal identities. These critical amino acid residues are distributed throughout the protein and reside predominantly within transmembrane domains. We propose a contextual model for axon guidance in which ORs mediate homotypic interactions between like axons.

Sensory deficits in mice hypomorphic for a mammalian homologue of unc-53

The migration of cells and the extension of cellular processes along pathways to their defined destinations are crucial in the development of higher organisms. Caenorhabditis elegans unc-53 plays an important role in cell migration and the outgrowth of cellular processes such as axons. To gain further insight into the biological function of unc53H2, a recently identified mammalian homologue of unc-53, we have generated mice carrying a mutation of unc53H2 and provide evidence that unc53H2 is involved in neuronal development and, more specifically, the development of different sensory systems. The unc53H2 hypomorphic mouse showed a general impaired acuity of several sensory systems (olfactory, auditory, visual and pain sensation) which in case of the visual system was corroborated by the morphological observation of hypoplasia of the optic nerve. We hypothesize that in analogy with its C. elegans homologue, unc53H2 may play a role in the processes of cellular outgrowth and migration.

NQO1 activity in the main and the accessory olfactory systems correlates with the zonal topography of projection maps

The mouse olfactory epithelium (OE) is divided into spatial zones, each containing neurons expressing zone-specific subsets of odorant receptor genes. Likewise, the vomeronasal (VN) organ is organized into apical and basal subpopulations of neurons expressing different VN receptor gene families. Axons projecting from the different OE zones and VN subpopulations form synapses within circumscribed regions in the glomerular layer of the olfactory bulb (OB) and accessory olfactory bulb (AOB), respectively. We here show that mature neurons in one defined zone selectively express NADPH:quinone oxidoreductase (NQO1), an enzyme that catalyses reduction of quinones. Immunohistochemistry and in situ hybridization analyses show non-overlapping expression of NQO1 and the Rb8 neural cell adhesion molecule (RNCAM/OCAM) in OE and axon terminals within glomeruli of the OB. In addition, NQO1 immunoreactivity reveals selective, zone-specific axon fasciculation in the olfactory nerve. VN subpopulations do not show complementary patterns of RNCAM and NQO1 immunoreactivity, instead both genes are co-expressed in apical VN neurons that project to the rostral AOB. These results indicate that one division of both the accessory and the main olfactory projection maps are composed of sensory neurons that are specialized to reduce environmental and/or endogenously produced quinones via an NQO1-dependent mechanism. The role of NQO1 in bioactivation of quinoidal drugs also points to a connection between zone-specific NQO1 expression and zone-specific toxicity of certain olfactory toxins.

In vitro analyses of interactions between olfactory receptor growth cones and glial cells that mediate axon sorting and glomerulus formation

During development, the axons of olfactory receptor neurons project to the CNS and converge on glomerular targets. For vertebrate and invertebrate olfactory systems, neuron-glia interactions have been hypothesized to regulate the sorting and targeting of olfactory receptor axons and the development of glomeruli. In the moth Manduca sexta, glial reduction experiments have directly implicated two types of central olfactory glia, the sorting zone- and neuropil-associated glia, in key events in olfactory development, including axon sorting and glomerulus stabilization. By using cocultures containing central olfactory glial cells and explants of olfactory receptor epithelium, we show that olfactory receptor growth cones elaborate extensively and cease advancement following contact with sorting zone- and neuropil-associated glial cells. These effects on growth cone behavior were specific to central olfactory glia; peripheral glial cells of the olfactory nerve failed to elicit similar responses in olfactory receptor growth cones. We propose that sorting zone- and neuropil-associated glial cells similarly modify axon behavior in vitro by altering the adhesive properties and cytoskeleton of olfactory receptor growth cones and that these in vitro changes may underlie functionally relevant changes in growth cone behavior in vivo.

Semaphorin 3A-mediated axon guidance regulates convergence and targeting of P2 odorant receptor axons

Semaphorins are known to play an important role in axon guidance of vertebrate olfactory sensory neurons to their targets in specific glomeruli of the olfactory bulb (OB). However, it is not clear how semaphorin-mediated guidance contributes to a systematic hierarchy of cues that govern the organization of this system. Because of the putative role that odorant receptor molecules such as P2 could play in establishing appropriate glomerular destinations for growing olfactory axons, we have also determined the spatial organization of P2 glomeruli in semaphorin 3A (Sema3A) mutant mice. First, in the postnatal OB of control and Sema3A(-/-) mice, we analysed the trajectories of olfactory axons that express the Sema3A receptor, neuropilin-1 (npn-1) and the positions of npn-1(+) glomeruli. Sema3A at the ventral OB midline guides npn-1(+) axons to targets in the lateral and medial OB. Absence of Sema3A permits many npn-1 axons to terminate aberrantly in the rostral and ventral OB. Second, in Sema3A(-/-) mice, many P2 axons are abnormally distributed throughout the ventral OB nerve layer and converge in atypical locations compared with littermate controls where P2 axons converge on stereotypically located lateral and medial glomeruli. In addition to their radically altered spatial distribution, P2 glomeruli in Sema3A(-/-) mice are significantly smaller and more numerous than in heterozygote littermates. These data show that Sema3A is an important repulsive olfactory guidance cue that establishes restricted npn-1(+) subcompartments in the olfactory bulb. Furthermore, Sema3A plays a key role in the convergence of axons expressing the odorant receptor P2 onto their appropriate targets.

Cell surface carbohydrates and glomerular targeting of olfactory sensory neuron axons in the mouse

Cell surface carbohydrates have been implicated in axon guidance and targeting throughout the nervous system. We have begun to test the hypothesis that, in the olfactory system, a differential distribution of cell surface carbohydrates may influence olfactory sensory neuron (OSN) axon targeting. Specifically, we have examined the spatial distribution of two different plant lectins, Ulex europaeus agglutinin (UEA) and Dolichos biflorus agglutinin (DBA), to determine whether they exhibit differential and reproducible projections onto the main olfactory bulb. Each lectin exhibited a unique spatial domain of glomerular labeling that was consistent across animals. UEA labeling was strongest in the ventral aspect of the olfactory bulb; DBA labeling was strongest in the dorsal aspect of the olfactory bulb. Some evidence for colocalization was present where these two borders intersected. Large areas of the glomerular layer were not labeled by either lectin. To determine whether patterns of lectin labeling were reproducible at the level of individual glomeruli, UEA labeling was assessed relative to M72-IRES-taulacZ- and P2-IRES-taulacZ-labeled axons. Although glomeruli neighboring these two identified glomeruli were consistently labeled with UEA, none of the lacZ positive axons was lectin labeled. Labeling of vomeronasal sensory neuron axons in the accessory olfactory bulb was more uniform for the two lectins. These data are the first to show a differential distribution of UEA vs. DBA labeling in the main olfactory bulb and are consistent with the hypothesis that a differential distribution of cell surface carbohydrates, a glycocode, may contribute to the targeting of OSN axons.

Cranial expression of class 3 secreted semaphorins and their neuropilin receptors

The semaphorin family of chemorepellents and their receptors the neuropilins are implicated in a variety of cellular processes, including axon guidance and cell migration. Semaphorins may bind more than one neuropilin or a heterodimer of both, thus a detailed knowledge of their expression patterns may reveal possible cases of redundancy or mutual antagonism. To assess their involvement in cranial development, we cloned fragments of the chick orthologues of Sema3B and Sema3F. We then carried out mRNA in situ hybridisation of all six class 3 semaphorins and both neuropilins in the embryonic chick head. We present evidence for spatiotemporal regulation of these molecules in the brainstem and developing head, including the eye, ear, and branchial arches. These expression patterns provide a basis for functional analysis of semaphorins and neuropilins in the development of axon projections and the morphogenesis of cranial structures.

On the topographic targeting of basal vomeronasal axons through Slit-mediated chemorepulsion

The vomeronasal projection conveys information provided by pheromones and detected by neurones in the vomeronasal organ (VNO) to the accessory olfactory bulb (AOB) and thence to other regions of the brain such as the amygdala. The VNO-AOB projection is topographically organised such that axons from apical and basal parts of the VNO terminate in the anterior and posterior AOB respectively. We provide evidence that the Slit family of axon guidance molecules and their Robo receptors contribute to the topographic targeting of basal vomeronasal axons. Robo receptor expression is confined largely to basal VNO axons, while Slits are differentially expressed in the AOB with a higher concentration in the anterior part, which basal axons do not invade. Immunohistochemistry using a Robo-specific antibody reveals a zone-specific targeting of VNO axons in the AOB well before cell bodies of these neurones in the VNO acquire their final zonal position. In vitro assays show that Slit1-Slit3 chemorepel VNO axons, suggesting that basal axons are guided to the posterior AOB due to chemorepulsive activity of Slits in the anterior AOB. These data in combination with recently obtained other data suggest a model for the topographic targeting in the vomeronasal projection where ephrin-As and neuropilins guide apical VNO axons, while Robo/Slit interactions are important components in the targeting of basal VNO axons.

A gene expression atlas of the central nervous system based on bacterial artificial chromosomes

The mammalian central nervous system (CNS) contains a remarkable array of neural cells, each with a complex pattern of connections that together generate perceptions and higher brain functions. Here we describe a large-scale screen to create an atlas of CNS gene expression at the cellular level, and to provide a library of verified bacterial artificial chromosome (BAC) vectors and transgenic mouse lines that offer experimental access to CNS regions, cell classes and pathways. We illustrate the use of this atlas to derive novel insights into gene function in neural cells, and into principal steps of CNS development. The atlas, library of BAC vectors and BAC transgenic mice generated in this screen provide a rich resource that allows a broad array of investigations not previously available to the neuroscience community.

Sorting and convergence of primary olfactory axons are independent of the olfactory bulb

Primary olfactory axons expressing the same odorant receptor gene sort out and converge to fixed sites in the olfactory bulb. We examined the guidance of axons expressing the P2 odorant receptor when they were challenged with different cellular environments in vivo. In the mutant extratoes mouse, the olfactory bulb is lacking and is replaced by a fibrocellular mass. In these animals, primary olfactory axons form glomerular-like loci despite the absence of normal postsynaptic targets. P2 axons are able to sort out from other axons in this fibrocellular mass and converge to form loci of like axons. The sites of these loci along mediolateral and ventrodorsal axes were highly variable. Similar convergence was observed for larger subpopulations of axons expressing the same cell surface carbohydrates. The sorting out and convergence of like axons also occurred during regeneration following bulbectomy. Olfactory axon behaviour in these models demonstrates that sorting and convergence of axons are independent of the target, which instead provides distinct topographic cues for guidance.

Axonal ephrin-As and odorant receptors: coordinate determination of the olfactory sensory map

Olfactory sensory neurons expressing a given odorant receptor (OR) project with precision to specific glomeruli in the olfactory bulb, generating a topographic map. In this study, we demonstrate that neurons expressing different ORs express different levels of ephrin-A protein on their axons. Moreover, alterations in the level of ephrin-A alter the glomerular map. Deletion of the ephrin-A5 and ephrin-A3 genes posteriorizes the glomerular locations for neurons expressing either the P2 or SR1 receptor, whereas overexpression of ephrin-A5 in P2 neurons results in an anterior shift in their glomeruli. Thus the ephrin-As are differentially expressed in distinct subpopulations of neurons and are likely to participate, along with the ORs, as one of a complement of guidance receptors governing the targeting of like axons to precise locations in the olfactory bulb.

The Dlx5 homeodomain gene is essential for olfactory development and connectivity in the mouse

The distalless-related homeogene Dlx5 is expressed in the olfactory placodes and derived tissues and in the anterior-basal forebrain. We investigated the role of Dlx5 in olfactory development. In Dlx5(-/-) mice, the olfactory bulbs (OBs) lack glomeruli, exhibit disorganized cellular layers, and show reduced numbers of TH- and GAD67-positive neurons. The olfactory epithelium in Dlx5(-/-) mice is composed of olfactory receptor neurons (ORNs) that appear identical to wild-type ORNs, but their axons fail to contact the OBs. We transplanted Dlx5(-/-) OBs into a wild-type newborn mouse; wild-type ORN axons enter the mutant OB and form glomeruli, but cannot rescue the lamination defect or the expression of TH and GAD67. Thus, the absence of Dlx5 in the OB does not per se prevent ORN axon ingrowth. In conclusion, Dlx5 plays major roles in the connectivity of ORN axons and in the differentiation of OB interneurons.