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

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.

Mechanisms of retinotopic map development: Ephs, ephrins, and spontaneous correlated retinal activity

This chapter summarizes mechanisms that control the development of retinotopic maps in the brain, focusing on work from our laboratory using as models the projection of retinal ganglion cells (RGCs) to the chick optic tectum (OT) or rodent superior colliculus (SC). The formation of a retinotopic map involves the establishment of an initial, very coarse map that subsequently undergoes large-scale remodeling to generate a refined map. All arbors are formed by interstitial branches that form in a topographically biased manner along RGC axons that overshoot their correct termination zone (TZ) along the anterior-posterior (A-P) axis of the OT/SC. The interstitial branches exhibit directed growth along the lateral-medial (L-M) axis of the OT/SC to position the branch at the topographically correct location, where it arborizes to form the TZ. EphA receptors and ephrin-A ligands control in part RGC axon mapping along the A-P axis by inhibiting branching and arborization posterior to the correct TZ. Ephrin-B1 acts bifunctionally through EphB forward signaling to direct branches along the L-M axis of the OT/SC to their topographically correct site. Computational modeling indicates that multiple graded activities are required along each axis to generate a retinotopic map, and makes several predictions, including: the progressive addition of ephrin-As within the OT/SC, due to its expression on RGC axon branches and arbors, is required to increase topographic specificity in branching and arborization as well as eliminate the initial axon overshoot, and that interactions amongst RGC axons that resemble correlated neural activity are required to drive retinotopic refinement. Analyses of mutant mice that lack early spontaneous retinal waves that correlate activity amongst neighboring RGCs, confirm this modeling prediction and show that correlated activity during an early brief critical period is required to drive the large-scale remodeling of the initially topographically coarse projection into a refined one. In summary, multiple graded guidance molecules, retinal waves and correlated spontaneous RGC activity cooperate to generate retinotopic maps.

Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly

Eph receptor tyrosine kinases and ephrins have key roles in regulation of the migration and adhesion of cells required to form and stabilize patterns of cell organization during development. Activation of Eph receptors or ephrins can lead either to cell repulsion or to cell adhesion and invasion, and recent work has found that cells can switch between these distinct responses. This review will discuss biochemical mechanisms and developmental roles of the diverse cell responses controlled by Eph receptors and ephrins.

Theoretical consideration of olfactory axon projection with an activity-dependent neural network model

Olfactory sensory neurons (OSNs) that express a given odorant receptor (OR) target their axons onto a few specific glomeruli in the olfactory bulb. Although the odorant receptor plays an indispensable role in olfactory axon targeting, the mechanisms underlying this guidance are largely unknown. In particular, there is much controversy regarding the involvement of activity-dependent mechanism in the targeting process. In this study, we developed an activity-dependent self-organization model of the glomerular layer in the olfactory bulb and simulated the targeting of olfactory axons onto the layer. Our model successfully constructed discrete glomeruli that received olfactory axons expressing a common odorant receptor through odorant-evoked neural activities. Furthermore, our model explained the perplexing experimental results that have been reported in olfactory axon targeting. For example, dispersion of olfactory axons in knockout mice for the odorant receptor gene was reasonably reproduced in the simulation. The segregated projection of the axons that express the same odorant receptor transcribed from the different alleles was also successfully simulated if the genetically modified allele was assumed to express a smaller amount of the receptor protein. The activity-dependent model even explained the inconsistent effects of disruption of the activity-evoking ion channel on axons expressing different odorant receptors, although some of these results were regarded as evidence for activity-independency of the olfactory targeting. Taken together, the activity-dependent targeting of olfactory axons seems to be a simple probable mechanism that can provide a unified explanation of glomerular formation.

Eph/ephrin signaling in morphogenesis, neural development and plasticity

Ephrins are cell-surface-tethered ligands for Eph receptors, the largest family of receptor tyrosine kinases. During development, the Eph/ephrin cell communication system appears to influence cell behavior such as attraction/repulsion, adhesion/de-adhesion and migration, thereby influencing cell fate, morphogenesis and organogenesis. During adulthood, the Eph/ephrin system continues to play roles in tissue plasticity, for example in shaping dendritic spines during neuronal plasticity. Mechanistically, Eph-ephrin repulsive behavior appears to require ligand-receptor internalization and signaling to Rho GTPases.

A relative signalling model for the formation of a topographic neural map

The highly ordered wiring of retinal ganglion cell (RGC) neurons in the eye to their synaptic targets in the superior colliculus of the midbrain has long served as the dominant experimental system for the analysis of topographic neural maps. Here we describe a quantitative model for the development of one arm of this map--the wiring of the nasal-temporal axis of the retina to the caudal-rostral axis of the superior colliculus. The model is based on RGC-RGC competition that is governed by comparisons of EphA receptor signalling intensity, which are made using ratios of, rather than absolute differences in, EphA signalling between RGCs. Molecular genetic experiments, exploiting a combinatorial series of EphA receptor knock-in and knockout mice, confirm the salient predictions of the model, and show that it both describes and predicts topographic mapping.

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.

Spatial patterns of gene expression in the olfactory bulb

How olfactory sensory neurons converge on spatially invariant glomeruli in the olfactory bulb is largely unknown. In one model, olfactory sensory neurons interact with spatially restricted guidance cues in the bulb that orient and guide them to their target. Identifying differentially expressed molecules in the olfactory bulb has been extremely difficult, however, hindering a molecular analysis of convergence. Here, we describe several such genes that have been identified in a screen that compiled microarray data to create a three-dimensional model of gene expression within the mouse olfactory bulb. The expression patterns of these identified genes form the basis of a nascent spatial map of differential gene expression in the bulb.

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.

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.

Loss-of-function analysis of EphA receptors in retinotectal mapping

EphA tyrosine kinases are thought to act as topographically specific receptors in the well-characterized projection map from the retina to the tectum. Here, we describe a loss-of-function analysis of EphA receptors in retinotectal mapping. Expressing patches of a cytoplasmically truncated EphA3 receptor in chick retina caused temporal axons to have reduced responsiveness to posterior tectal repellent activity in vitro and to shift more posteriorly within the map in vivo. A gene disruption of mouse EphA5, replacing the intracellular domain with beta-galactosidase, reduced in vitro responsiveness of temporal axons to posterior target membranes. It also caused map abnormalities in vivo, with temporal axons shifted posteriorly and nasal axons anteriorly, but with the entire target still filled by retinal axons. The anterior shift of nasal axons was not accompanied by increased responsiveness to tectal repellent activity, in contrast to the comparable anterior shift in ephrin-A knock-outs, helping to resolve a previous ambiguity in interpreting the ephrin gene knock-outs. The results show the functional requirement for endogenous EphA receptors in retinotectal mapping, show that the receptor intracellular domain is required for a forward signaling response to topographic cues, and provide new evidence for a role of axon competition in topographic mapping.

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.

Computational modeling of retinotopic map development to define contributions of EphA-ephrinA gradients, axon-axon interactions, and patterned activity

The topographic projection of retinal ganglion cell (RGC) axons to mouse superior colliculus (SC) or chick optic tectum (OT) is formed in three phases: RGC axons overshoot their termination zone (TZ); they exhibit interstitial branching along the axon that is topographically biased for the correct location of their future TZ; and branches arborize preferentially at the TZ and the initial exuberant projection refines through axon and branch elimination to generate a precise retinotopic map. We present a computational model of map development that demonstrates that the countergradients of EphAs and ephrinAs in retina and the OT/SC and bidirectional repellent signaling between RGC axons and OT/SC cells are sufficient to direct an initial topographic bias in RGC axon branching. Our model also suggests that a proposed repellent action of EphAs/ephrinAs present on RGC branches and arbors added to that of EphAs/ephrinAs expressed by OT/SC cells is required to progressively restrict branching and arborization to topographically correct locations and eliminate axon overshoot. Simulations show that this molecular framework alone can develop considerable topographic order and refinement, including axon elimination, a feature not programmed into the model. Generating a refined map with a condensed TZ as in vivo requires an additional parameter that enhances branch formation along an RGC axon near sites that it has a higher branch density, and resembles an assumed role for patterned neural activity. The same computational model generates the phenotypes reported in ephrinA deficient mice and Isl2-EphA3 knockin mice. This modeling suggests that gradients of counter-repellents can establish a substantial degree of topographic order in the OT/SC, and that repellents present on RGC axon branches and arbors make a substantial contribution to map refinement. However, competitive interactions between RGC axons that enhance the probability of continued local branching are required to generate precise retinotopy.

Positioning sensory terminals in the olfactory lobe of Drosophila by Robo signaling

Olfactory receptor neurons and the interneurons of the olfactory lobe are organized in distinct units called glomeruli. We have used expression patterns and genetic analysis to demonstrate that a combinatorial code of Roundabout (Robo) receptors act to position sensory terminals within the olfactory lobe. Groups of sensory neurons possess distinct blends of Robo and Robo3 and disruption of levels by loss-of-function or ectopic expression results in aberrant targeting. In the wild type, most of the neurons send collateral branches to the contralateral lobe. Our data suggests that guidance of axons across brain hemispheres is mediated by Slit-dependent Robo2 signaling. The location of sensory arbors at distinct positions within the lobe allows short-range interactions with projection neurons leading to formation of the glomeruli.

Spontaneous Neural Activity Is Required for the Establishment and Maintenance of the Olfactory Sensory Map

We have developed a genetic approach to examine the role of spontaneous activity and synaptic release in the establishment and maintenance of an olfactory sensory map. Conditional expression of tetanus toxin light chain, a molecule that inhibits synaptic release, does not perturb targeting during development, but neurons that express this molecule in a competitive environment fail to maintain appropriate synaptic connections and disappear. Overexpression of the inward rectifying potassium channel, Kir2.1, diminishes the excitability of sensory neurons and more severely disrupts the formation of an olfactory map. These studies suggest that spontaneous neural activity is required for the establishment and maintenance of the precise connectivity inherent in an olfactory sensory map.

Repelling class discrimination: ephrin-A5 binds to and activates EphB2 receptor signaling

The interactions between Eph receptor tyrosine kinases and their ephrin ligands regulate cell migration and axon pathfinding. The EphA receptors are generally thought to become activated by ephrin-A ligands, whereas the EphB receptors interact with ephrin-B ligands. Here we show that two of the most widely studied of these molecules, EphB2 and ephrin-A5, which have never been described to interact with each other, do in fact bind one another with high affinity. Exposure of EphB2-expressing cells to ephrin-A5 leads to receptor clustering, autophosphorylation and initiation of downstream signaling. Ephrin-A5 induces EphB2-mediated growth cone collapse and neurite retraction in a model system. We further show, using X-ray crystallography, that the ephrin-A5-EphB2 complex is a heterodimer and is architecturally distinct from the tetrameric EphB2-ephrin-B2 structure. The structural data reveal the molecular basis for EphB2-ephrin-A5 signaling and provide a framework for understanding the complexities of functional interactions and crosstalk between A- and B-subclass Eph receptors and ephrins.

After the holy grail: establishing a molecular basis for Mammalian olfaction

The quest to identify mammalian odorant receptors was a triumph of molecular biology. The characterization of these molecules has provided extraordinary insight into the strategy used by one neuronal system to organize sensory structures and code complex information. The odorant receptor genes have also served as powerful tools in understanding genomic organization and gene regulation.

Interaxonal Eph-ephrin signaling may mediate sorting of olfactory sensory axons in Manduca sexta

We have investigated possible roles of the Eph family receptor tyrosine kinases and their ligand ephrins in the developing primary olfactory nerve pathway in the moth Manduca sexta. The Manduca homologs of the Eph receptor (MsEph) and ephrin ligand (MsEphrin) are most closely related to Drosophila Eph and ephrin, respectively. In situ labeling with Fc-fusion probes, in which IgG Fc was linked to the extracellular domain of MsEph (Eph-Fc) or MsEphrin (ephrin-Fc), reveals that both Eph receptors and ephrins are expressed on axons of olfactory receptor cells (ORCs) during their ingrowth to the primary center, the antennal lobe (AL). Interestingly, Eph receptors and ephrins are differentially distributed among identifiable glomeruli such that glomeruli with high receptor staining show little or no ligand staining, and vice versa, suggesting a complementary Eph-ephrin expression by subsets of ORC axons innervating a particular set of glomeruli. In contrast, neither Eph receptors nor ephrins are detectable in intrinsic components of the AL. In vitro, ephrin-Fc and Eph-Fc, when present homogeneously in the substratum, inhibit neurite outgrowth from olfactory epithelial explants. Moreover, in patterned substratum, neurites growing on the standard substratum turn or stop after encountering the test substratum containing ephrin-Fc. These in vitro observations indicate that MsEphrin can act as an inhibitor/repulsive cue for ORC axons. Based on results from in situ and in vitro experiments, we hypothesize that Eph receptors and ephrins mediate axon sorting and fasciculation through repulsive axon-axon interactions.

Fiber order of the normal and regenerated optic tract of the frog (Rana pipiens)

In the normal frog, axons from the peripheral retina arising at the temporal pole course superficially in the middle stream of the diencephalic optic tract. Axons from the nasal pole course in two streams running in the opposite margins of the tract, dorsonasal axons ventrally, ventronasal axons dorsally. Axons from the dorsal and ventral poles of the retina occupy the intervals between the aforementioned middle and marginal streams. Axons from more central regions of the retina tend to occupy deeper levels of the optic tract. The regenerated optic tract does not regain its normal organization, e.g., axons of peripheral nasal origin are spread out widely over the entire width of the tract. However, axons from the temporal pole of the retina do return approximately to their original location in the middle stream. The concentration of temporal axons in the middle stream of the optic tract after regeneration may now be understood in terms of the expression pattern of the ephrin-A class of receptor tyrosine kinase ligands in the cellular matrix of the optic tract. The ephrin-As, which have a repellent effect on growing temporal retinal axons, are concentrated in and along the margins of the diencephalic optic tract and essentially absent from its middle stream. It is proposed here that peripheral temporal axons may be forced into this middle region by their avoidance of the higher levels of ephrin-A expression in the tract margins. In contrast, the growth pattern of regenerating peripheral nasal axons would not be affected by the ephrin-A gradient in the optic tract.