The hippocampus and septum play central roles in one of the most important spheres of brain function: learning and memory. Although their topographic connections have been known for two decades and topography may be critical for cognitive functions, the basis for hippocamposeptal topographic projection is unknown. We now report for the first time that Elf-1, a membrane-bound eph family ligand, is a candidate molecular tag for the genesis of the hippocamposeptal topographic projection. Elf-i is expressed in an increasing gradient from dorsal to ventral septum. Furthermore, Elf-i selectively allows growth of neurites from topographically appropriate lateral hippocampal neurons, while inhibiting neurite outgrowth by medial hippocampal neurons.Complementary to the expression of Elf-1, an eph family receptor, Bsk, is expressed in the hippocampus in a lateral to medial gradient, consistent with a function as a receptor for Elf-i. Further, Elf-1 specifically bound Bsk, eliciting tyrosine kinase activity. We conclude that the Elf-1/Bsk ligandreceptor pair exhibits traits of a chemoaffinity system for the organization of hippocamposeptal topographic projections.Topographic projection is a general feature of brain architecture, and appears to be critical for appropriate coding and processing of information (1). Nevertheless, little is known about the mechanisms that govern topographic organization. Among the many regions exhibiting topographic relations, the hippocampus and septum have been the focus of intense interest, since these structures play central roles in learning and memory (2-5). The hippocampus projects to the lateral septum and receives afferents from the medial septum (6-8). Moreover, hippocampal projections to the lateral septum are arranged in a precise order. Axons from the medial hippocampus project to the dorsal lateral septum, whereas axons from the lateral hippocampus project to the ventral lateral septum (6-8). Molecular mechanisms underlying these topographic projections are unknown.The development of topographic projections is thought to require both long-range signals to guide axons to the general target area and local cues to specify individual targets precisely for each axon terminal (9-11). Long-range signals are likely to be diffusible, such as netrins, which attract selected, distant growth cones, while repelling others (12)(13)(14)(15). In contrast, local guidance cues must match axon terminals and specific cellular targets, a requirement accommodated by matching fixed tags on afferents and corresponding targets. Complementarity of molecular tags on afferents and targets was first postulated by Sperry in his chemoaffinity hypothesis more than 50 years ago (16,17). Only recently have specific candidate molecules been identified (18)(19)(20)(21)(22). The eph family ligand and receptor, Elf-1 and Mek4, are expressed as complementary gradients in the tectum and retina, respectively (18). The repulsive axon guidance signal, a molecule closely related to Elf-1, repels the growth of retina...

The cerebral cortex is parcellated into different functional domains that receive distinct inputs from other cortical and subcortical regions. The molecular mechanisms underlying the specificity of connections of cortical afferents remain unclear. We report here that the Eph family tyrosine kinase receptor EphA5 and the ligand ephrin-A5 may play a key role in the exclusion of the limbic thalamic afferents from the sensorimotor cortex by mediating repulsive interactions. In situ hybridization shows that the EphA5 transcript is expressed at high levels in both cortical and subcortical limbic regions, including the frontal cortex, the subiculum, and the medial thalamic nuclei. In contrast, ephrin-A5 is transcribed abundantly in the sensorimotor cortex. Consistent with the complementary expression, the ligand inhibited dramatically the growth of neurites from neurons isolated from the medial thalamus but was permissive for the growth of neurites from lateral thalamic neurons, which is primarily nonlimbic. Similarly, the growth of neurites from Eph-A5-expressing neurons isolated from the subiculum was inhibited by ephrin-A5. Our studies suggest that the Eph family ligand ephrin-A5 serves as a general inhibitor of axonal growth from limbic neurons, which may serve to prevent innervation of inappropriate primary sensorimotor regions, thus contributing to the generation of specificity of thalamic cortical afferents.The cerebral cortex is parcellated into multiple domains that subserve different functions (1-3). The medial cortical areas, which include prefrontal, cingulate, and retrosplenial cortices, are components of limbic circuits, in contrast to nonlimbic sensory and motor cortices (3, 4). The limbic cortices receive projections from nuclei in the medial and anterior thalamus and from other limbic areas, including the hippocampal formation (5). In contrast, the sensorimotor cortex is innervated specifically by primary sensory and motor thalamic nuclei located in the ventrolateral and posterior thalamus (5). This topographic arrangement of mature functional pathways in the adult brain is paralleled by an early specificity of thalamocortical projections during development (2, 6, 7). This specificity suggests the presence of guidance mechanisms that facilitate the formation of distinct projection patterns.In the classic model of topographic map formation, Sperry (8, 9) proposed that molecular tags form gradients in projecting and target fields and interact to guide axons to appropriate regions. Such guidance cues have been identified recently and have been shown to be receptors and ligands of the Eph family of tyrosine kinases (10-13). In the retinotectal topographic map, the Eph family receptor EphA3 is expressed by retinal ganglion neurons in a nasal (low) to temporal (high) gradient whereas two ligands, ephrin-A2 and -A5, are distributed in a complementary anterior (low) to posterior (high) gradient in the tectum (10, 14). In the hippocamposeptal projection, the Eph family receptor EphA5 is expressed in a l...

Neuronal connections are arranged topographically such that the spatial organization of neurons is preserved by their termini in the targets. During the development of topographic projections, axons initially explore areas much wider than the final targets, and mistargeted axons are pruned later. The molecules regulating these processes are not known. We report here that the ligands of the Eph family tyrosine kinase receptors may regulate both the initial outgrowth and the subsequent pruning of axons. In the presence of ephrins, the outgrowth and branching of the receptor-positive hippocampal axons are enhanced. However, these axons are induced later to degenerate. These observations suggest that the ephrins and their receptors may regulate topographic map formation by stimulating axonal arborization and by pruning mistargeted axons.Topographic neuronal projections are regulated, at least in part, by complementary gradients of the Eph family tyrosine kinase receptors and ligands expressed in the pre-and postsynaptic neurons (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). During the development of hippocamposeptal topographic projections, the Eph family receptor EphA5 is expressed in a lateral (low) to medial (high) gradient in the hippocampus (13), whereas three ligands of the receptor, ephrin-A2, A3, and A5, are transcribed in the major subcortical hippocampal target, the lateral septum, in a dorsomedialto-ventrolateral gradient (13,14). In the mature hippocamposeptal topographic map, medial hippocampal neurons, which express high levels of EphA5, project to the ligand-poor dorsomedial target. In contrast, the lateral hippocampal neurons, which express low levels of the Eph receptor, send axons to the ligand-rich ventrolateral target (13-16). The opposing gradients of the receptor and ligand expression suggest that the receptor-ligand interaction contributes to the development of the hippocamposeptal topographic map by negatively regulating axonal growth of receptor-positive medial hippocampal neurons. Similar ligand-receptor gradients also exist in retinotectal system and are proposed to map retinal ganglion axons along the rostrocaudal and dorsoventral axes of the tectum (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(17)(18)(19)(20).In vitro studies using coculture assays in which hippocampal neurons are exposed to the ephrin-expressing cells showed that ephrin-A2 selectively inhibits neurite outgrowth of receptorrich medial hippocampal neurons but has little effects on the growth of the lateral neurites (13). In addition, temporal retina axons, which express high levels of EphA3, a different member of the Eph family, are repelled by ephrin-A2-and A5-containing membrane extracts in stripe assays, in which the extracts with or without the ephrins are laid side by side in narrow stripes (8,9). In contrast, the ephrins have a much weaker repulsive activity against the nasal retina axons, which do not express high levels of the receptor. These in vitro analyses, together with in vivo expression patterns, indica...

Increasing evidence indicates that the eph family of ligands and receptors guides the formation of topographic maps in the brain through repulsive interactions. For example, we have recently found that in the hippocamposeptal system, the ligand ephrin-A2, which is expressed in an increasing gradient from dorsal to ventral septum, selectively induces pruning of topographically inappropriate medial hippocampal axons. The recent detection of ephrins A3 and A5, as well as A2, in the septum raised critical functional questions. Do the ligands act combinatorially, ensuring appropriate three-dimensional spatiotemporal projection, or do they exert entirely distinct actions in addition to guidance mechanisms? To approach these alternatives, we cloned mouse ephrin-A2 and compared the activities of the three ligands. Here, we show that these ligands reduce the number of hippocampal neurites in a similar fashion. The effect was regionally specific; medial hippocampal neurites were reduced 1.5- to 1.8-fold, whereas lateral hippocampal neurites were not significantly affected, conforming to topographic projection in vivo. Furthermore, we found that ephrins regulated neurite number in a stage-specific fashion, affecting E19 hippocampal neurites more than E16 neurites. Our observations suggest that all three septal ephrins, A2, A3, and A5, play spatiotemporally specific roles in guiding topographic projections from the hippocampus.