INTRODUCTION A central process during development of the nervous system is the guidance of axons from their site of origin to their target area. A large number of molecules have been shown to be involved in this process, such as the netrins and their receptors (Dcc and Unc5), the semaphorins and the neuropilin and plexin receptors, the Slits and Robos, as well as the ephrins, which interact with receptor tyrosine kinases of the Eph family (Müller, 1999; Tessier-Lavigne and Goodman, 1996). This latter family comprises 14 different receptors and eight ligands so far, all of which are widely expressed in the central and peripheral nervous systems during development and in adult. Individual members of the Eph family have been implicated in axon guidance as well as in a number of other (possibly related) processes, such as cell migration, boundary formation through restricting cell intermingling, axonal pathfinding, layer-specific arborisations in the target area and angiogenesis (for reviews, see Flanagan and Vanderhaeghen, 1998; Frisen et al., 1999; Holder and Klein, 1999; O’Leary and Wilkinson, 1999; Wilkinson, 2000). At present, an overwhelming number of reports indicate that the Eph family exerts its function by a repellent mechanism, although there are some reports of an adhesive or permissive function of Eph receptors and ligands (Daniel et al., 1996; Davy et al., 2000; Davy et al., 1999; Holash et al., 1997; Holmberg et al., 2000; Huai and Drescher, 2001; Huynh-Do et al., 1999; Pandey et al., 1995; Stein et al., 1998). The Eph family can be subdivided into two classes: EphA receptors interact with glycosylphosphatidylinositol (GPI)- anchored ephrin-A ligands and the EphB receptors interact with ephrin-B ligands, which are transmembrane anchored. This subdivision is strictly followed, the only exception being the EphA4 receptor, which interacts with both ephrin-A and ephrin-B molecules. Within both families, bi-directional signalling has been observed, such that interaction of the respective axons or cells leads to an activation of specific signalling pathways in the ‘receptor’-expressing cell as well as in the ‘ligand’ expressing cell (Brückner et al., 1997; Davy et al., 2000; Davy et al., 1999; Henkemeyer et al., 1996; Holland et al., 1996; Huai and Drescher, 2001). This has been shown, for example, in case of the anterior commissure, where axons expressing ephrin-B ligands are guided by Eph receptors located in the immediate vicinity of migrating axons (Brückner and Klein, 1998; Holland et al., 1998), as well as in the processes leading to the segmental patterning in the hindbrain (Mellitzer et al., 1999; Xu et al., 1999). For the EphA family, such a role has been observed so far for fibroblast cells, where binding of EphA receptors to ephrin-A-expressing cells leads to an β1-integrin-dependent upregulation of the adhesiveness of these cells (Davy et al., 2000; Davy et al., 1999; Huai and Drescher, 2001). Members of the Eph family are involved in the development 895 Development 128, 895-906 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV1632 We have investigated the role of the Eph family of receptor tyrosine kinases and their ligands in the establishment of the vomeronasal projection in the mouse. Our data show intriguing differential expression patterns of ephrin-A5 on vomeronasal axons and of EphA6 in the accessory olfactory bulb (AOB), such that axons with high ligand concentration project onto regions of the AOB with high receptor concentration and vice versa. These data suggest a mechanism for development of this projection that is the opposite of the repellent interaction between Eph receptors and ligands observed in other systems. In support of this idea, when given the choice of whether to grow on lanes containing EphA-F c /laminin or F c /laminin protein (in the stripe assay), vomeronasal axons prefer to grow on EphA- F c /laminin. Analysis of ephrin-A5 mutant mice revealed a disturbance of the topographic targeting of vomeronasal axons to the AOB. In summary, these data, which are derived from in vitro and in vivo experiments, indicate an important role of the EphA family in setting up the vomeronasal projection. Key words: Eph, Topographic projection, Mouse, Ephrin, Vomeronasal, Mouse SUMMARY A role for the EphA family in the topographic targeting of vomeronasal axons Bernd Knöll 1, *, Konstantinos Zarbalis 2,3, *, Wolfgang Wurst 2,3,§ and Uwe Drescher 1,‡,§ 1 Department of Physical Biology, Max-Planck-Institute for Developmental Biology, Spemannstrasse 35 / I, 72076 Tübingen, Germany 2 Clinical Neurogenetics, Max-Planck-Institute of Psychiatry, Kraepelinstr. 2, 80804 Munich, Germany 3 GSF-Research Centre, Institute for Mammalian Genetics, Ingolstaedter Landstr. 1, 85764 Neuherberg, Germany *These authors contributed equally to this work ‡ Present address: MRC Centre for Developmental Neurobiology, King’s College London, 4th Floor, New Hunt’s House, Guy’s Campus, London SE1 1UL, UK § Authors for correspondence (e-mail: uwe.drescher@kcl.ac.uk and wurst@mpipsykl.mpg.de) Accepted 18 December 2000; published on WWW 26 February 2001