Analysis of Eph receptors and ephrin ligands in the development of hippocamposeptal projections

Axons travel frequently in bundles to reach their target. Upon arriving at the target, axon terminals defasciculate, migrate to topographically defined positions, and form synapses with appropriate target neurons. Molecular signals expressed in the target tissues are likely to play critical roles in regulating these processes. However, these signals have not been well characterized. Here we present evidence that the EphB-type receptors and a ligand, ephrin-B3, regulate hippocampal axon defasciculation. In wild type mice, hippocampal bundles enter the septum at embryonic day 16, begin to defasciculate and terminate in the lateral septum at E17. During development, two Eph B receptors, EphB2 and B3, are expressed uniformly along the medial-lateral axis of the hippocampus. The ligand, ephrin-B3, is transcribed in the lateral septum, the major subcortical target of hippocampal neurons. Ephrin-B3 reduces fasciculation of hippocampal axons in vitro, and the loss of the receptor EphB2 abrogates the effects of ephrin-B3. Analysis of mice deficient in EphB2 and B3 receptors reveals that hippocampal axons fail to defasciculate in the septal target. These observations indicate that the receptors EphB2 and B3 together with the ligand, ephrin-B3, regulate hippocampal axon defasciculation at the septal target.