Development of continuous and discrete neural maps

Differentially expressed Drl and Drl-2 play opposing roles in Wnt5 signaling during Drosophila olfactory system development

In Drosophila, odor information received by olfactory receptor neurons (ORNs) is processed by glomeruli, which are organized in a stereotypic manner in the antennal lobe (AL). This glomerular organization is regulated by Wnt5 signaling. In the embryonic CNS, Wnt5 signaling is transduced by the Drl receptor, a member of the Ryk family. During development of the olfactory system, however, it is antagonized by Drl. Here, we identify Drl-2 as a receptor mediating Wnt5 signaling. Drl is found in the neurites of brain cells in the AL and specific glia, whereas Drl-2 is predominantly found in subsets of growing ORN axons. A drl-2 mutation produces only mild deficits in glomerular patterning, but when it is combined with a drl mutation, the phenotype is exacerbated and more closely resembles the Wnt5 phenotype. Wnt5 overexpression in ORNs induces aberrant glomeruli positioning. This phenotype is ameliorated in the drl-2 mutant background, indicating that Drl-2 mediates Wnt5 signaling. In contrast, forced expression of Drl-2 in the glia of drl mutants rescues the glomerular phenotype caused by the loss of antagonistic Drl function. Therefore, Drl-2 can also antagonize Wnt5 signaling. Additionally, our genetic data suggest that Drl localized to developing glomeruli mediates Wnt5 signaling. Thus, these two members of the Ryk family are capable of carrying out a similar molecular function, but they can play opposing roles in Wnt5 signaling, depending on the type of cells in which they are expressed. These molecules work cooperatively to establish the olfactory circuitry in Drosophila.

Odorant receptor-mediated signaling in the mouse

In the mouse olfactory system, there are approximately 1000 types of odorant receptors (ORs), which perform multiple functions in olfactory sensory neurons (OSNs). In addition to detecting odors, the functional OR protein ensures the singular gene choice of the OR by negative-feedback regulation. ORs also direct the axonal projection of OSNs both globally and locally by modulating the transcriptional levels of axon-guidance and axon-sorting molecules. In these latter processes, the second messenger, cAMP, plays differential roles in the fasciculation and targeting of axons. In this review, we will discuss how ORs differentially regulate intracellular signals for distinct functions.

Specification of motor axon trajectory by ephrin-B:EphB signaling: symmetrical control of axonal patterning in the developing limb

Studies of the innervation of limb muscles by spinal motor neurons have helped to define mechanisms by which axons establish trajectories to their targets. Related motor axons select dorsal or ventral pathways at the base of the limb, raising the question of how these alternate trajectories are specified. EphA signaling has been proposed to control the dorsal trajectory of motor axons in conjunction with other signaling systems, although the respective contributions of each system to motor axon guidance are unclear. We show that the expression of EphB receptors by motor axons, and ephrin-B ligands by limb mesenchymal cells, directs the ventral trajectory of motor axons. Our findings reveal symmetry in the molecular strategies that establish this aspect of nerve-muscle connectivity. The involvement of ephrin:Eph signaling in guiding both sets of motor axons raises the possibility that other signaling systems function primarily to refine or modulate a core Eph signaling program.

Key roles of Ephs and ephrins in retinotectal topographic map formation

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.

Subcellular topography of visually driven dendritic activity in the vertebrate visual system

Neural pathways projecting from sensory organs to higher brain centers form topographic maps in which neighbor relationships are preserved from a sending to a receiving neural population. Sensory input can generate compartmentalized electrical and biochemical activity in the dendrites of a receiving neuron. Here, we show that in the developing retinotectal projection of young Xenopus tadpoles, visually driven Ca2+ signals are topographically organized at the subcellular, dendritic scale. Functional in vivo two-photon Ca2+ imaging revealed that the sensitivity of dendritic Ca2+ signals to stimulus location in visual space is correlated with their anatomical position within the dendritic tree of individual neurons. This topographic distribution was dependent on NMDAR activation, whereas global Ca2+ signals were mediated by Ca2+ influx through dendritic, voltage-dependent Ca2+ channels. These findings suggest a framework for plasticity models that invoke local dendritic Ca2+ signaling in the elaboration of neural connectivity and dendrite-specific information storage.

Secreted TARSH regulates olfactory mitral cell dendritic complexity

Olfactory sensory neurons synapse with mitral cells to form stereotyped connections in the olfactory bulb (OB). Mitral cell apical dendrites receive input from olfactory sensory neurons expressing the same odorant receptor. During development, this restricted dendritic targeting of mitral cells is achieved through eliminating elaborated dendritic trees to a single apical dendrite. Through a genome-wide microarray screen, we identified TARSH (Target of NESH SH3) as a transiently expressed molecule in mitral cells during the dendritic refinement period. TARSH expression is restricted to pyramidal neurons along the main olfactory pathway, including the anterior olfactory nucleus and piriform cortex. The dynamic TARSH expression is not altered when odor-evoked activity is blocked by naris closure or in AC3 knockout mice. We also demonstrate that TARSH is a secreted protein. In dissociated OB cultures, secreted TARSH promotes the reduction of mitral cell dendritic complexity and restricts dendritic branching and outgrowth of interneurons. Dendritic morphological changes were also observed in mitral cells overexpressing TARSH themselves. We propose that TARSH is part of the genetic program that regulates mitral cell dendritic refinement.

STOP proteins contribute to the maturation of the olfactory system

Regulation of microtubule dynamics is crucial for axon growth and guidance as well as for the establishment of synaptic connections. STOPs (Stable Tubule Only Polypeptides) are microtubule-associated proteins that regulate microtubule stabilization but are also able to interact with actin or Golgi membranes. Here, we have investigated the involvement of STOPs during the development of the olfactory system. We first describe the spatio-temporal expression patterns of N- and E-STOP, the two neuronal-specific isoforms of STOP. E- and N-STOP are expressed in the axonal compartment of olfactory sensory neurons, but are differentially regulated during development. Interestingly, each neuronal isoform displays a specific gradient distribution within the olfactory nerve layer. Then, we have examined the development of the olfactory system in the absence of STOPs. Olfactory axons display a normal outgrowth and targeting in STOP-null mice, but maturation of the synapses in the glomerular neuropil is altered.

Alpha cells take off first

Using a transgenic mouse line in which GFP is expressed in a single population of retinal ganglion cells (RGCs), Huberman and colleagues report in this issue of Neuron that the axon terminals of RGCs exhibit an orderly pattern of distribution in the higher visual centers. This pattern undergoes a developmental refinement, and synchronous activity in the retina regulates columnar but not laminar formation.

Architecture and activity-mediated refinement of axonal projections from a mosaic of genetically identified retinal ganglion cells

Our understanding of how mammalian sensory circuits are organized and develop has long been hindered by the lack of genetic markers of neurons with discrete functions. Here, we report a transgenic mouse selectively expressing GFP in a complete mosaic of transient OFF-alpha retinal ganglion cells (tOFF-alphaRGCs). This enabled us to relate the mosaic spacing, dendritic anatomy, and electrophysiology of these RGCs to their complete map of projections in the brain. We find that tOFF-alphaRGCs project exclusively to the superior colliculus (SC) and dorsal lateral geniculate nucleus and are restricted to a specific laminar depth within each of these targets. The axons of tOFF-alphaRGC are also organized into columns in the SC. Both laminar and columnar specificity develop through axon refinement. Disruption of cholinergic retinal waves prevents the emergence of columnar- but not laminar-specific tOFF-alphaRGC connections. Our findings reveal that in a genetically identified sensory map, spontaneous activity promotes synaptic specificity by segregating axons arising from RGCs of the same subtype.

Small molecules can selectively inhibit ephrin binding to the EphA4 and EphA2 receptors

The erythropoietin-producing hepatocellular (Eph) family of receptor tyrosine kinases regulates a multitude of physiological and pathological processes. Despite the numerous possible research and therapeutic applications of agents capable of modulating Eph receptor function, no small molecule inhibitors targeting the extracellular domain of these receptors have been identified. We have performed a high throughput screen to search for small molecules that inhibit ligand binding to the extracellular domain of the EphA4 receptor. This yielded a 2,5-dimethylpyrrolyl benzoic acid derivative able to inhibit the interaction of EphA4 with a peptide ligand as well as the natural ephrin ligands. Evaluation of a series of analogs identified an isomer with similar inhibitory properties and other less potent compounds. The two isomeric compounds act as competitive inhibitors, suggesting that they target the high affinity ligand-binding pocket of EphA4 and inhibit ephrin-A5 binding to EphA4 with K(i) values of 7 and 9 mum in enzyme-linked immunosorbent assays. Interestingly, despite the ability of each ephrin ligand to promiscuously bind many Eph receptors, the two compounds selectively target EphA4 and the closely related EphA2 receptor. The compounds also inhibit ephrin-induced phosphorylation of EphA4 and EphA2 in cells, without affecting cell viability or the phosphorylation of other receptor tyrosine kinases. Furthermore, the compounds inhibit EphA4-mediated growth cone collapse in retinal explants and EphA2-dependent retraction of the cell periphery in prostate cancer cells. These data demonstrate that the Eph receptor-ephrin interface can be targeted by inhibitory small molecules and suggest that the two compounds identified will be useful to discriminate the activities of EphA4 and EphA2 from those of other co-expressed Eph receptors that are activated by the same ephrin ligands. Furthermore, the newly identified inhibitors represent possible leads for the development of therapies to treat pathologies in which EphA4 and EphA2 are involved, including nerve injuries and cancer.

Crystal structure and NMR binding reveal that two small molecule antagonists target the high affinity ephrin-binding channel of the EphA4 receptor

The Eph receptor tyrosine kinases regulate a variety of physiological and pathological processes not only during development but also in adult organs, and therefore they represent a promising class of drug targets. The EphA4 receptor plays important roles in the inhibition of the regeneration of injured axons, synaptic plasticity, platelet aggregation, and likely in certain types of cancer. Here we report the first crystal structure of the EphA4 ligand-binding domain, which adopts the same jellyroll beta-sandwich architecture as shown previously for EphB2 and EphB4. The similarity with EphB receptors is high in the core beta-stranded regions, whereas large variations exist in the loops, particularly the D-E and J-K loops, which form the high affinity ephrin binding channel. We also used isothermal titration calorimetry, NMR spectroscopy, and computational docking to characterize the binding to EphA4 of two small molecules, 4- and 5-(2,5 dimethyl-pyrrol-1-yl)-2-hydroxybenzoic acid which antagonize ephrin-induced effects in EphA4-expressing cells. We show that the two molecules bind to the EphA4 ligand-binding domain with K(d) values of 20.4 and 26.4 microm, respectively. NMR heteronuclear single quantum coherence titrations revealed that upon binding, both molecules significantly perturb EphA4 residues Ile(31)-Met(32) in the D-E loop, Gln(43) in the E beta-strand, and Ile(131)-Gly(132) in the J-K loop. Molecular docking shows that they can occupy a cavity in the high affinity ephrin binding channel of EphA4 in a similar manner, by interacting mainly with the EphA4 residues in the E strand and D-E and J-K loops. However, many of the interactions observed in Eph receptor-ephrin complexes are absent, which is consistent with the small size of the two molecules and may account for their relatively weak binding affinity. Thus, our studies provide the first published structure of the ligand-binding domain of an EphA receptor of the A subclass. Furthermore, the results demonstrate that the high affinity ephrin binding channel of the Eph receptors is amenable to targeting with small molecule antagonists and suggest avenues for further optimization.

Bone morphogenetic proteins, eye patterning, and retinocollicular map formation in the mouse

Patterning events during early eye formation determine retinal cell fate and can dictate the behavior of retinal ganglion cell (RGC) axons as they navigate toward central brain targets. The temporally and spatially regulated expression of bone morphogenetic proteins (BMPs) and their receptors in the retina are thought to play a key role in this process, initiating gene expression cascades that distinguish different regions of the retina, particularly along the dorsoventral axis. Here, we examine the role of BMP and a potential downstream effector, EphB, in retinotopic map formation in the lateral geniculate nucleus (LGN) and superior colliculus (SC). RGC axon behaviors during retinotopic map formation in wild-type mice are compared with those in several strains of mice with engineered defects of BMP and EphB signaling. Normal RGC axon sorting produces axon order in the optic tract that reflects the dorsoventral position of the parent RGCs in the eye. A dramatic consequence of disrupting BMP signaling is a missorting of RGC axons as they exit the optic chiasm. This sorting is not dependent on EphB. When BMP signaling in the developing eye is genetically modified, RGC order in the optic tract and targeting in the LGN and SC are correspondingly disrupted. These experiments show that BMP signaling regulates dorsoventral RGC cell fate, RGC axon behavior in the ascending optic tract, and retinotopic map formation in the LGN and SC through mechanisms that are in part distinct from EphB signaling in the LGN and SC.

Topography of thalamic projections requires attractive and repulsive functions of Netrin-1 in the ventral telencephalon

Recent studies have demonstrated that the topography of thalamocortical (TC) axon projections is initiated before they reach the cortex, in the ventral telencephalon (VTel). However, at this point, the molecular mechanisms patterning the topography of TC projections in the VTel remains poorly understood. Here, we show that a long-range, high-rostral to low-caudal gradient of Netrin-1 in the VTel is required in vivo for the topographic sorting of TC axons to distinct cortical domains. We demonstrate that Netrin-1 is a chemoattractant for rostral thalamic axons but functions as a chemorepulsive cue for caudal thalamic axons. In accordance with this model, DCC is expressed in a high-rostromedial to low-caudolateral gradient in the dorsal thalamus (DTh), whereas three Unc5 receptors (Unc5A–C) show graded expression in the reverse orientation. Finally, we show that DCC is required for the attraction of rostromedial thalamic axons to the Netrin-1–rich, anterior part of the VTel, whereas DCC and Unc5A/C receptors are required for the repulsion of caudolateral TC axons from the same Netrin-1–rich region of the VTel. Our results demonstrate that a long-range gradient of Netrin-1 acts as a counteracting force from ephrin-A5 to control the topography of TC projections before they enter the cortex.

The functional properties of each structure in the central nervous system are critically dependent on the precision of neuronal connectivity. The cerebral cortex in particular is a highly organized structure divided into many distinct cortical areas underlying important sensory, motor, and cognitive functions in the brain. Each primary cortical area receives its synaptic inputs from the periphery via the dorsal thalamus. The main relay station for sensory information to the cortex, the thalamus, can be divided into specific nuclei projecting topographically to individual cortical areas. How is the complex topography of thalamic axon projection to individual cortical areas specified during development? Recent evidence demonstrated that thalamic axons are routed to different cortical domains before they enter the cortex, by putative axon guidance cues present in the ventral forebrain. In the present study, we provide evidence that a secreted axon guidance cue, Netrin-1, expressed in a long-range gradient in the ventral forebrain, plays a critical role in the establishment of the topography of thalamic projections by directing different subsets of axons to specific cortical domains. These results provide important insights into the molecular mechanisms responsible for shaping the topographical patterns of thalamocortical axon projections in mammals.

A long-range gradient of Netrin-1 plays a critical role in the specification of the topography of thalamocortical projections in the ventral telencephalon. The function of Netrin-1 requires both its attractive and repulsive functions to guide different subsets of thalamic axons to specific cortical domains.

Precise circuitry links bilaterally symmetric olfactory maps

Olfactory sensory neurons expressing a common receptor gene converge onto one or a few glomeruli with stereotyped positions within the mouse main olfactory bulb (MOB), producing a map of approximately 1800 olfactory columns representing approximately 1000 odorant receptors. Here, we report that this precise olfactory map is maintained over several synapses that ultimately cross MOB hemispheres to link bilateral isofunctional olfactory columns. Focal injection of tracer into genetically identified glomeruli revealed an exquisite topography that involves a bilateral connection via the anterior olfactory nucleus pars externa (AONpE) that links isofunctional olfactory columns in the contralateral MOB. Physiological and behavioral assays revealed an important role for the AONpE in bilateral exchange of odorant-specific information. These results indicate that the interbulbar link through the AONpE integrates bilateral olfactory sensory maps and exchanges olfactory information, positioning it as a unique model system for studying interhemispheric connections.

Zic2 regulates retinal ganglion cell axon avoidance of ephrinB2 through inducing expression of the guidance receptor EphB1

The navigation of retinal axons to ipsilateral and contralateral targets in the brain depends on the decision to cross or avoid the midline at the optic chiasm, a critical guidance maneuver that establishes the binocular visual pathway. Previous work has identified a specific guidance receptor, EphB1, that mediates the repulsion of uncrossed axons away from its ligand, ephrinB2, at the optic chiasm midline (Williams et al., 2003), and a transcription factor Zic2, that, like EphB1, is required for formation of the ipsilateral retinal projection (Herrera et al., 2003). Although the reported similarities in localization implicated that Zic2 regulates EphB1 (Herrera et al., 2003; Williams et al., 2003; Pak et al., 2004), whether Zic2 drives expression of EphB1 protein has not been elucidated. Here we show that EphB1 protein is expressed in the growth cones of axons from ventrotemporal (VT) retina that project ipsilaterally and that repulsion by ephrinB2 is determined by the presence of this receptor on growth cones. Moreover, ectopic delivery of Zic2 into explants from non-VT retina induces expression of EphB1 mRNA and protein. The upregulated EphB1 receptor protein is localized to growth cones and is functional, because it is sufficient to change retinal ganglion cell axon behavior from extension onto, to avoidance of, ephrinB2 substrates. Our results demonstrate that Zic2 upregulates EphB1 expression and define a link between a transcription factor and expression of a guidance receptor protein essential for axon guidance at the vertebrate midline.

Iroquois complex genes induce co-expression of rhodopsins in Drosophila

The Drosophila eye is a mosaic that results from the stochastic distribution of two ommatidial subtypes. Pale and yellow ommatidia can be distinguished by the expression of distinct rhodopsins and other pigments in their inner photoreceptors (R7 and R8), which are implicated in color vision. The pale subtype contains ultraviolet (UV)-absorbing Rh3 in R7 and blue-absorbing Rh5 in R8. The yellow subtype contains UV-absorbing Rh4 in R7 and green-absorbing Rh6 in R8. The exclusive expression of one rhodopsin per photoreceptor is a widespread phenomenon, although exceptions exist. The mechanisms leading to the exclusive expression or to co-expression of sensory receptors are currently not known. We describe a new class of ommatidia that co-express rh3 and rh4 in R7, but maintain normal exclusion between rh5 and rh6 in R8. These ommatidia, which are localized in the dorsal eye, result from the expansion of rh3 into the yellow-R7 subtype. Genes from the Iroquois Complex (Iro-C) are necessary and sufficient to induce co-expression in yR7. Iro-C genes allow photoreceptors to break the "one receptor-one neuron" rule, leading to a novel subtype of broad-spectrum UV- and green-sensitive ommatidia.

Eph-ephrin bidirectional signaling in physiology and disease

Receptor tyrosine kinases of the Eph family bind to cell surface-associated ephrin ligands on neighboring cells. The ensuing bidirectional signals have emerged as a major form of contact-dependent communication between cells. New findings reveal that Eph receptors and ephrins coordinate not only developmental processes but also the normal physiology and homeostasis of many adult organs. Imbalance of Eph/ephrin function may therefore contribute to a variety of diseases. The challenge now is to better understand the complex and seemingly paradoxical signaling mechanisms of Eph receptors and ephrins, which will enable effective strategies to target these proteins in the treatment of diseases such as diabetes and cancer.

Segregation of axial motor and sensory pathways via heterotypic trans-axonal signaling

Execution of motor behaviors relies on circuitries effectively integrating immediate sensory feedback to efferent pathways controlling muscle activity. It remains unclear how, during neuromuscular circuit assembly, sensory and motor projections become incorporated into tightly coordinated, yet functionally separate pathways. We report that, within axial nerves, establishment of discrete afferent and efferent pathways depends on coordinate signaling between coextending sensory and motor projections. These heterotypic axon-axon interactions require motor axonal EphA3/EphA4 receptor tyrosine kinases activated by cognate sensory axonal ephrin-A ligands. Genetic elimination of trans-axonal ephrin-A --> EphA signaling in mice triggers drastic motor-sensory miswiring, culminating in functional efferents within proximal afferent pathways. Effective assembly of a key circuit underlying motor behaviors thus critically depends on trans-axonal signaling interactions resolving motor and sensory projections into discrete pathways.

Odorant receptor gene choice and axonal projection in the mouse olfactory system

In the mouse olfactory system, each olfactory sensory neuron (OSN) expresses a single type of odorant receptor (OR) out of approximately 1,000 in a monoallelic manner. Furthermore, OSNs expressing the same OR converge their axons to a specific set of glomeruli on the olfactory bulb. These two basic principles are fundamental to the peripheral olfactory system, and are regulated by the expressed OR protein itself. Singular OR gene choice is ensured by the combination of stochastic enhancer-promoter interaction and negative-feedback regulation by OR proteins. In the axonal projection, OR-derived cyclic AMP signals and neuronal activity determine the expression levels of axon guidance/sorting molecules, and thereby direct glomerular positioning and axon sorting.