Inverse expression of olfactory cell adhesion molecule in a subset of olfactory axons and a subset of mitral/tufted cells in the developing rat main olfactory bulb

Nrp2 is sufficient to instruct circuit formation of mitral-cells to mediate odour-induced attractive social responses

Odour information induces various innate responses that are critical to the survival of the individual and for the species. An axon guidance molecule, Neuropilin 2 (Nrp2), is known to mediate targeting of olfactory sensory neurons (primary neurons), to the posteroventral main olfactory bulb (PV MOB) in mice. Here we report that Nrp2-positive (Nrp2⁺) mitral cells (MCs, second-order neurons) play crucial roles in transmitting attractive social signals from the PV MOB to the anterior part of medial amygdala (MeA). Semaphorin 3F, a repulsive ligand to Nrp2, regulates both migration of Nrp2⁺ MCs to the PV MOB and their axonal projection to the anterior MeA. In the MC-specific Nrp2 knockout mice, circuit formation of Nrp2⁺ MCs and odour-induced attractive social responses are impaired. In utero, electroporation demonstrates that activation of the Nrp2 gene in MCs is sufficient to instruct their circuit formation from the PV MOB to the anterior MeA.

Cell type-dependent axonal localization of translational regulators and mRNA in mouse peripheral olfactory neurons

Local protein synthesis in mature axons may play a role in synaptic plasticity, axonal arborization, or functional diversity of the circuit. To gain insight into this question, we investigated the axonal localization of translational regulators and associated mRNAs in five parallel olfactory circuits, four in the main olfactory bulb and one in the accessory olfactory bulb. Axons in all four main olfactory bulb circuits exhibited axonal localization of Fragile X granules (FXGs), structures that comprise ribosomes, mRNA, and RNA binding proteins including Fragile X mental retardation protein (FMRP) and the related protein FXR2P. In contrast, FXGs were not seen in axons innervating the accessory olfactory bulb. Similarly, axons innervating the main olfactory bulb, but not the accessory olfactory bulb, contained the FXG-associated mRNA Omp (olfactory marker protein). This differential localization was not explained by circuit-dependent differences in expression of FXG components or Omp, suggesting that other factors must regulate their axonal transport. The specificity of this transport was highlighted by the absence from olfactory axons of the calmodulin transcript Calm1, which is highly expressed in peripheral olfactory neurons at levels equivalent to Omp. Regulation of axonal translation by FMRP may shape the structure and function of the axonal arbor in mature sensory neurons in the main olfactory system but not in the accessory olfactory system.

Tag1 deficiency results in olfactory dysfunction through impaired migration of mitral cells

The olfactory system provides mammals with the abilities to investigate, communicate and interact with their environment. These functions are achieved through a finely organized circuit starting from the nasal cavity, passing through the olfactory bulb and ending in various cortical areas. We show that the absence of transient axonal glycoprotein-1 (Tag1)/contactin-2 (Cntn2) in mice results in a significant and selective defect in the number of the main projection neurons in the olfactory bulb, namely the mitral cells. A subpopulation of these projection neurons is reduced in Tag1-deficient mice as a result of impaired migration. We demonstrate that the detected alterations in the number of mitral cells are well correlated with diminished odor discrimination ability and social long-term memory formation. Reduced neuronal activation in the olfactory bulb and the corresponding olfactory cortex suggest that Tag1 is crucial for the olfactory circuit formation in mice. Our results underpin the significance of a numerical defect in the mitral cell layer in the processing and integration of odorant information and subsequently in animal behavior.

The E3 ubiquitin ligase Mycbp2 genetically interacts with Robo2 to modulate axon guidance in the mouse olfactory system

The E3 ubiquitin ligase Mycbp2 and it homologues play an important role in axon guidance and synaptogenesis in Drosophila, Caenorhabditis elegans, zebrafish and mouse. Despite this conserved function, the molecular and cellular basis of Mycbp2-dependent axon guidance remains largely unclear. We have examined here the effect of the loss-of-MYCBP2 function on the topography of the olfactory sensory neuron projection from the nasal cavity to the olfactory bulb in mice. A subpopulation of olfactory sensory axons failed to project to the dorsal surface of the olfactory bulb causing abnormal topography in this neural pathway. These defects were similar to the olfactory bulb phenotype in loss-of-ROBO2 function mice. While mice heterozygous for either Mycbp2 or Robo2 were normal, mice double heterozygous for these two genes produced severe defects in the olfactory system. Therefore, Mycbp2 and Robo2 were found to cooperate within a genetic network that has profound effects on axon guidance. The Mycbp2 phenotype could be partly explained by aberrant patterning of olfactory sensory neurons residing in the dorsal compartment of the nasal cavity. Some of these neurons fail to appropriately express Robo2 which is consistent with their aberrant projection to the ventral olfactory bulb. These results provide the first evidence linking an ubiquitin ligase to an axon guidance receptor during pathfinding in the developing mammalian nervous system.

Timing of neurogenesis is a determinant of olfactory circuitry

An odorant receptor map in mammals that is constructed by the glomerular coalescence of sensory neuron axons in the olfactory bulb is essential for proper odor information processing. How this map is linked with olfactory cortex is unknown. Using a battery of methods, including various markers of cell division in combination with tracers of neuronal connections and time-lapse live imaging, we found that early- and late-generated mouse mitral cells became differentially distributed in the dorsal and ventral subdivisions of the odorant receptor map. In addition, the late-generated mitral cells extended substantially stronger projections to the olfactory tubercle than did the early-generated cells. Together, these data indicate that the odorant receptor map is developmentally linked to the olfactory cortices in part by the birthdate of mitral cells. Thus, different olfactory cortical regions become involved in processing information from distinct regions of the odorant receptor map.

Loss of olfactory cell adhesion molecule reduces the synchrony of mitral cell activity in olfactory glomeruli

Odours generate activity in olfactory receptor neurons, whose axons contact the dendritic tufts of mitral cells within olfactory bulb glomeruli. These axodendritic synapses are anatomically separated from dendrodendritic synapses within each glomerulus. Mitral cells within a glomerulus show highly synchronized activity as assessed with whole-cell recording from pairs of mitral cells. We examined glomerular activity in mice lacking the olfactory cell adhesion molecule (OCAM). Glomeruli in mice lacking OCAM show a redistribution of synaptic subcompartments, but the total area occupied by axonal inputs was similar to wild-type mice. Stimulation of olfactory nerve bundles showed that excitatory synaptic input to mitral cells as well as dendrodendritic inhibition was unaffected in the knockout. However, correlated spiking in mitral cells was significantly reduced, as was electrical coupling between apical dendrites. To analyse slow network dynamics we induced slow oscillations with a glutamate uptake blocker. Evoked and spontaneous slow oscillations in mitral cells and external tufted cells were broader and had multiple peaks in OCAM knockout mice, indicating that synchrony of slow glomerular activity was also reduced. To assess the degree of shared activity between mitral cells under physiological conditions, we analysed spontaneous sub-threshold voltage oscillations using coherence analysis. Coherent activity was markedly reduced in cells from OCAM knockout mice across a broad range of frequencies consistent with a decrease in tightly time-locked activity. We suggest that synchronous activity within each glomerulus is dependent on segregation of synaptic subcompartments.

Sequential arrival and graded secretion of Sema3F by olfactory neuron axons specify map topography at the bulb

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.

Wiring Olfaction: The Cellular and Molecular Mechanisms that Guide the Development of Synaptic Connections from the Nose to the Cortex

Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.

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

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

WITHDRAWN: The Neural Cell Adhesion Molecule NCAM2/OCAM/RNCAM, a Close Relative to NCAM

Cell adhesion molecules (CAMs) constitute a large class of plasma membrane-anchored proteins that mediate attachment between neighboring cells and between cells and the surrounding extracellular matrix (ECM). However, CAMs are more than simple mediators of cell adhesion. The neural cell adhesion molecule (NCAM) is a well characterized, ubiquitously expressed CAM that is highly expressed in the nervous system. In addition to mediating cell adhesion, NCAM participates in a multitude of cellular events, including survival, migration, and differentiation of cells, outgrowth of neurites, and formation and plasticity of synapses. NCAM shares an overall sequence identity of approximately 44% with the neural cell adhesion molecule 2 (NCAM2), a protein also known as olfactory cell adhesion molecule (OCAM) and Rb-8 neural cell adhesion molecule (RNCAM), and the region-for-region sequence homology between the two proteins suggests that they are transcribed from paralogous genes. However, very little is known about the function of NCAM2, although it originally was described more than 20 years ago. In this review we summarize the known properties and functions of NCAM2 and describe some of the differences and similarities between NCAM and NCAM2.

Unusual patch-matrix organization in the retrosplenial cortex of the reeler mouse and Shaking rat Kawasaki

The rat granular retrosplenial cortex (GRS) is a simplified cortex, with distinct stratification and, in the uppermost layers, distinct modularity. Thalamic and cortical inputs are segregated by layers and in layer 1 colocalize, respectively, with apical dendritic bundles originating from neurons in layers 2 or 5. To further investigate this organization, we turned to reelin-deficient reeler mouse and Shaking rat Kawasaki. We found that the disrupted lamination, evident in Nissl stains in these rodents, is in fact a patch-matrix mosaic of segregated afferents and dendrites. Patches consist of thalamocortical connections, visualized by vesicular glutamate transporter 2 (VGluT2) or AChE. The surrounding matrix consists of corticocortical terminations, visualized by VGluT1 or zinc. Dendrites concentrate in the matrix or patches, depending on whether they are OCAM positive (matrix) or negative (patches). In wild-type rodents and, presumably, mutants, OCAM(+) structures originate from layer 5 neurons. By double labeling for dendrites (filled by Lucifer yellow in fixed slice) and OCAM immunofluorescence, we ascertained 2 populations in reeler: dendritic branches either preferred (putative layer 5 neurons) or avoided (putative supragranular neurons) the OCAM(+) matrix. We conclude that input-target relationships are largely preserved in the mutant GRS and that dendrite-dendrite interactions involving OCAM influence the formation of the mosaic configuration.

Novel subdomains of the mouse olfactory bulb defined by molecular heterogeneity in the nascent external plexiform and glomerular layers

Background

In the mouse olfactory system, the role of the olfactory bulb in guiding olfactory sensory neuron (OSN) axons to their targets is poorly understood. What cell types within the bulb are necessary for targeting is unknown. What genes are important for this process is also unknown. Although projection neurons are not required, other cell-types within the external plexiform and glomerular layers also form synapses with OSNs. We hypothesized that these cells are important for targeting, and express spatially differentially expressed guidance cues that act to guide OSN axons within the bulb.

Results

We used laser microdissection and microarray analysis to find genes that are differentially expressed along the dorsal-ventral, medial-lateral, and anterior-posterior axes of the bulb. The expression patterns of these genes divide the bulb into previously unrecognized subdomains. Interestingly, some genes are expressed in both the medial and lateral bulb, showing for the first time the existence of symmetric expression along this axis. We use a regeneration paradigm to show that several of these genes are altered in expression in response to deafferentation, consistent with the interpretation that they are expressed in cells that interact with OSNs.

Conclusion

We demonstrate that the nascent external plexiform and glomerular layers of the bulb can be divided into multiple domains based on the expression of these genes, several of which are known to function in axon guidance, synaptogenesis, and angiogenesis. These genes represent candidate guidance cues that may act to guide OSN axons within the bulb during targeting.

Axon behavior in the olfactory nerve reflects the involvement of catenin-cadherin mediated adhesion

The projection of olfactory sensory neuron (OSN) axons to the olfactory bulb (OB) is a complex but well-regulated process. Although odorant receptor proteins, and other molecules, are implicated in this process, our understanding remains incomplete. We demonstrate that axons remain restricted to the outer olfactory nerve layer (ONLo) until they are proximal to their target glomeruli, where they enter the inner ONL (ONLi), dividing the ONL into extension and sorting zones. Sorting is likely contingent on cell:cell interactions mediated in part by cell adhesion molecules. The cadherins are a large family of adhesion molecules whose function is contingent on their intracellular binding partners, the catenins, which in turn link to the cytoskeleton. We previously demonstrated that the organization of the cytoskeleton changed as olfactory sensory neuron axons moved from the ONLo to the ONLi. To further assess the role of cadherin mediated adhesion in the developing mouse ONL, we localized alpha-, beta-, gamma-, delta-, and p120-catenins as well as neural cadherin (N-cadherin; CDH2) in the OB. alpha- and beta-catenins are found throughout the OB and are uniform throughout the ONL. In contrast, gamma-catenin and CDH2 are expressed predominantly in the ONLo during perinatal development, but are uniform across the ONL beginning at P7 and into adulthood. Finally, p120- and delta-catenins are expressed in nonoverlapping patterns by olfactory axons and OB neuronal dendrites, respectively. We conclude that gamma-catenin-mediated CDH2 adhesion may influence OSN targeting by restricting axons to the ONLo until they reach the appropriate domain of the OB.

Axonal wiring in the mouse olfactory system

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

Time frame of mitral cell development in the mice olfactory bulb

Along with tufted cells, mitral cells are the principal projection neurons in the olfactory bulb (OB). During the development of the OB, mitral cells migrate from the ventricular zone to the intermediate zone, where they begin to send axons along the lateral olfactory tract (LOT) to the cortical olfactory zones. Subsequently, they lose their tangential orientation, enabling them to make contact with the axons of the olfactory sensory neurons (OSN) that innervate the whole OB. Here, we investigated the distinct morphological features displayed by developing mitral cells and analyzed the relationship between the changes undertaken by these neurons and the arrival of the OSN axons. Immunostaining for specific markers of developing axons and dendrites, coupled with the use of fluorescent tracers, revealed the morphological changes, the continuous reorientation, and the final refinement that these cells undergo. We found that some of these changes are dependent on the arrival of the OSN axons. Indeed, we identified three main chronological events: 1) newly generated neurons become established in the intermediate zone and project to the LOT; 2) the cells reorient and spread their dendrites at the same time as OSN axons penetrate the OB (this is a sensitive period between embryonic day (E)15-16, in which the arrival of afferents establishes a spatial and temporal gradient that facilitates protoglomerulus and glomerulus formation); and 3) final refinement of the radially orientated cells to adopt a mature morphology. These results suggest that both afferent inputs and intrinsic factors participate to produce the well-defined sensory system.

Immunohistochemical expression of two members of the GDNF family of growth factors and their receptors in the olfactory system

The glial cell line-derived (GDNF) family of trophic factors, GDNF, neurturin, persephin and artemin, are known to support the survival and regulate differentiation of many neuronal populations, including peripheral autonomic, enteric and sensory neurons. Members of this family of related ligands bind to specific GDNF family receptor (GFR) proteins, which complex and signal through the Ret receptor tyrosine kinase. We showed previously that GDNF protein was detectable in olfactory sensory neurons (OSNs) in the olfactory neuroepithelium (ON). In this immunohistochemical study, we localized GDNF, neurturin, GFRalpha1, GFRalpha2 and Ret in the adult rat ON and olfactory bulb. We found that GDNF and Ret were widely expressed by immature and mature OSNs, while neurturin was selectively expressed in a subpopulation of OSNs zonally restricted in the ON. The GFRs had differential expression, with mature OSNs and their axons preferentially expressing GFRalpha1, whereas progenitors and immature neurons more avidly expressed GFRalpha2. In the bulb, GDNF was highly expressed by the mitral and tufted cells, and by periglomerular cells, and its distribution generally resembled that of Ret, with the exception that Ret was far more predominant on fibers than cell bodies. Neurturin, in contrast, was present at lower levels and was more restricted in its expression to the axonal compartment. GFRalpha2 appeared to be the dominant accessory protein in the bulb. These data are supportive of two members of this neurotrophic family, GDNF and neurturin, playing different physiological roles in the olfactory neuronal system.

Activation of epidermal growth factor receptor mediates receptor axon sorting and extension in the developing olfactory system of the moth Manduca sexta

During development of the adult olfactory system of the moth Manduca sexta, olfactory receptor neurons extend axons from the olfactory epithelium in the antenna into the brain. As they arrive at the brain, interactions with centrally derived glial cells cause axons to sort and fasciculate with other axons destined to innervate the same glomeruli. Here we report studies indicating that activation of the epidermal growth factor receptor (EGFR) is involved in axon ingrowth and targeting. Blocking the EGFR kinase domain pharmacologically leads to stalling of many axons in the sorting zone and nerve layer as well as abnormal axonal fasciculation in the sorting zone. We also find that neuroglian, an IgCAM known to activate the EGFR through homophilic interactions in other systems, is transiently present on olfactory receptor neuron axons and on glia during the critical stages of the sorting process. The neuroglian is resistant to extraction with Triton X-100 in the sorting zone and nerve layer, possibly indicating its stabilization by homophilic binding in these regions. Our results suggest a mechanism whereby neuroglian molecules on axons and possibly sorting zone glia bind homophilically, leading to activation of EGFRs, with subsequent effects on axon sorting, pathfinding, and extension, and glomerulus development.

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.

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.

Laminar organization of the developing lateral olfactory tract revealed by differential expression of cell recognition molecules

The projection neurons in the olfactory bulb (mitral and tufted cells) send axons through the lateral olfactory tract (LOT) onto several structures of the olfactory cortex. However, little is known of the molecular and cellular mechanisms underlying establishment of functional connectivity from the bulb to the cortex. Here, we investigated the developmental process of LOT formation by observing expression patterns of cell recognition molecules in embryonic mice. We immunohistochemically identified a dozen molecules expressed in the developing LOT and some of them were localized to subsets of mitral cell axons. Combinatorial immunostaining for these molecules revealed that the developing LOT consists of three laminas: superficial, middle, and deep. Detailed immunohistochemical, in situ hybridization, and 5-bromodeoxyuridine labeling analyses suggested that the laminar organization reflects: 1) the segregated pathways from the accessory and main olfactory bulbs, and 2) the different maturity of mitral cell axons. Mitral cell axons of the accessory olfactory bulb were localized to the deep lamina, segregated from those of the main olfactory bulb. In the main olfactory pathway, axons of mature mitral cells, whose somata is located in the apical sublayer of the mitral cell layer, were localized to the middle lamina within LOT, while those of immature mitral cells that located in the basal sublayer were complementarily localized to the superficial lamina. These results suggest that newly generated immature axons are added to the most superficial lamina of LOT successively, leading to the formation of piled laminas with different maturational stages of the mitral cell axons.

Differential expression of the mammalian homologue of fasciclin II during olfactory development in vivo and in vitro

Developing olfactory sensory neurons are guided to the glomeruli of the olfactory bulb by an increasingly stringent process that is influenced by expression of odorant receptors, cell adhesion molecules (CAMs), and other kinds of signaling cascades. A fundamental feature of the projection is the connecting of broad zones in the epithelium to broad zones in the bulb, also termed rhinotopy. One molecule that parallels and may aid neurons in establishing rhinotopy is the mammalian homologue of fasciclin II (OCAM/mamFas II; also known as RNCAM and NCAM-2), an immunoglobulin superfamily CAM that is differentially expressed in the developing and mature olfactory epithelium (OE): Axons elaborated by ventral and lateral epithelium express the protein at high levels, whereas dorsomedial axons express little or no OCAM/mamFas II. Our investigation has demonstrated that OCAM/mamFas II is detectable early in the development of the rat OE. mRNA is evident on RT-PCR and in situ hybridization by E12.5, and protein is apparent by immunohistochemistry by E13.5. By using a tissue culture system that separates ventral septal epithelium (OCAM/mamFas II-positive) from dorsal (OCAM/mamFas II-negative), we find that explants maintain protein expression levels in vitro that are characteristic of the phenotype at the original location in vivo. At least some neurons are born in culture, suggesting that any cues that direct differential expression are also maintained in vitro. Finally, high OCAM/mamFas II expression correlates with increased growth and fasciculation of olfactory axons in vitro. These data and the similarity between OCAM/mamFas II, on the one hand, and fasciclin II and NCAM, on the other, suggest that OCAM/mamFas II might play a role in growth and fasciculation of primary olfactory axons during development of the projection.

Differential expression of Homer family proteins in the developing mouse brain

Homer acts as a postsynaptic adaptor protein that links multiple targets, such as proteins involved in glutamate receptor signaling. We report the differential expression of the long form of Homer proteins produced from three distinctive genes during postnatal development of the mouse brain. Homer 1b/c and Cupidin/Homer 2a/b are widespread throughout the developing brain and are down-regulated in hindbrain-origin regions, such as the cerebellum, pons, and medulla oblongata. In contrast, Homer 3a/b is restricted to the cerebellum, hippocampus, and neonatal olfactory bulb. In the cerebellum, Homer 1b/c and Cupidin/Homer 2a/b predominate in the postsynapses of developing granule cells, whereas Homer 3a/b is concentrated in the dendritic spines of Purkinje cells and their axons. The down-regulation of Homer 1b/c and Cupidin/Homer 2a/b is in marked contrast to the up-regulation of Homer 3a/b between the first and the second postnatal weeks. In the hippocampus, Homer 1b/c and Cupidin/Homer 2a/b are largely located in the CA1 region and the CA1-CA2 region, respectively, whereas Homer 3a/b is largely distributed in the CA2-CA3 region and peaks around the third postnatal week. In hippocampal cell cultures, Homer 1b/c and Cupidin/Homer 2a/b are expressed in inhibitory and excitatory neurons, whereas Homer 3a/b is largely expressed in excitatory neurons but not in inhibitory neurons. In the developing olfactory bulb, Homer 1b/c and Cupidin/Homer 2a/b are up-regulated in the granular, external plexiform, and glomerular layers, whereas Homer 3a/b drastically decreases in these regions within the first postnatal week. Cupidin/Homer 2a/b is also expressed in olfactory sensory neurons within a distinct olfactory epithelial zone and is then widely distributed to both the axons in the olfactory nerve layer and the cilia in the olfactory epithelium. These results demonstrate that Homer family members have distinct regional, cellular, and subcellular distributions in time and space during postnatal brain development.

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.

Developmental study of dendritic bundles in layer 1 of the rat granular retrosplenial cortex with special reference to a cell adhesion molecule, OCAM

In the granular retrosplenial cortex (GRS) of adult rats, callosally projecting pyramidal neurons in layer 2 form dendritic bundles, 30-100 micro m wide, in layer 1. The distinctness of these bundles makes the GRS an attractive model system for investigating the developmental, microcircuitry, and basic organizational features related to dendritic modularity. In this report, we investigate the developmental time course of the dendritic bundles, visualized by immunohistochemistry for microtubule-associated protein 2 (MAP2) and glutamate receptor subunits 2/3 (GluR2/3). Bundles in layer 1 are apparent as early as postnatal day 5, first with GluR2/3 and then, from postnatal day 14, with MAP2. As a step toward understanding the mechanisms of dendritic aggregation, we further investigated the ontogeny of expression of the cell adhesion molecule OCAM. OCAM exhibits a patchy distribution in layer 1 from postnatal day 3 to adult, and the regions of weak OCAM immunoreactivity selectively correspond to the dendritic bundles (in both GluR2/3 and MAP2). The periodic geometry of OCAM-immunoreactive regions, the time course of their appearance and the distinct localization complementary to the bundles support the possibility that this molecule is one contributor to the establishment and maintenance of dendritic modules. The interdigitating relationship between regions of high OCAM immunoreactivity and the dendritic bundles in layer 1 suggests that OCAM may have a repellent influence on the formation of these bundles.