ABSTRACT
Cortical interneurons are born in the germinative zones of the ganglionic eminences in the subpallium, and migrate tangentially in spatially and temporally well-defined corridors into the neocortex. Because ephrin-A5 is expressed in the ventricular zone (VZ) of the ganglionic eminences at these developmental stages, we examined the possible effects of this molecule on interneuron migration. Double-immunocytochemistry of dissociated neurons from the medial ganglionic eminences (MGE) revealed that calbindin-positive cells express the EphA4-receptor. In situ, EphA4 is strongly expressed in the subventricular zone of the ganglionic eminences. Using different in vitro assays, we found that ephrin-A5 acts as a repellent cue for MGE neurons. We then examined interneuron migration in slice overlay experiments, where MGE-derived explants from enhanced green fluorescent protein-expressing transgenic mice were homotopically grafted into host slices from wild-type littermate embryos. In these in vitro preparations, interneurons recapitulated in vivo cell migration in several respects. However, interneurons in brain slices also migrated in the VZ of the ganglionic eminences, a region that is strictly avoided in vivo. In situ hybridizations revealed that ephrin-A5 became downregulated in the VZ in vitro. When recombinant ephrin-A5-Fc was added to the slices, it preferentially bound to the VZ, and migrating MGE neurons avoided the VZ as in vivo. The restoration of the normal migration pathway in slices required ephrin-A5 clustering and signalling of Src family kinases. Together, these experiments suggest that ephrin-A5 acts as an inhibitory flank that contributes to define the pathway of migrating interneurons.
Subject(s)
Cell Movement/physiology , Ephrin-A5/metabolism , Interneurons/physiology , Animals , Brain/anatomy & histology , Brain/metabolism , Coculture Techniques , Cues , Ephrin-A5/genetics , Female , Fibroblasts/cytology , Fibroblasts/physiology , Green Fluorescent Proteins/genetics , Green Fluorescent Proteins/metabolism , Interneurons/cytology , Mice , Mice, Inbred C57BL , Mice, Transgenic , NIH 3T3 Cells , Receptor, EphA4/metabolismABSTRACT
The complex task of wiring up the brain during embryonic development is achieved by a multitude of guidance signals acting in complex combinations to drive growing axons to their proper targets. The somatosensory system provides an extensively studied model system featuring many universal mechanisms of neural development. In rodents, it constitutes an important model to study how precise topographic connections are achieved. Recent evidence suggests that the Eph/ephrin family of guidance molecules is of pivotal importance for the development of the somatosensory system. Members of Eph/ephrin family are thought to be involved in the global presorting of thalamic axons projecting to the cortex, in labeling specific cortical areas for innervation, in providing topographic cues within the target area, and in distinguishing cortical layers for intracortical wiring. The Eph/ephrin system also seems to contribute to the formation of specific corticothalamic feedback projections. So far, the functions of only a few members of the Eph/ephrin family have been examined, but expression analysis indicates complex combinatorial effects of these signaling molecules. Understanding the Eph/ephrin wiring code is expected to yield new insights into the development and plasticity of brain circuits involved in higher functions.
