Ephrin-A5 modulates the topographic mapping and connectivity of commissural axons in murine hippocampus.

Entorhinal and commissural/associational projections show a non-overlapping distribution in the hippocampus proper and the dentate gyrus. The expression of Ephrins and their Eph receptors in the developing hippocampus indicates that this family of axonal guidance molecules may modulate the formation of these connections. Here we focused on the role of the ephrin-A5 ligand in the development of the main hippocampal afferents. In situ hybridization showed that ephrin-A5 mRNA was detected mainly in the principal cells of the hippocampus proper and in the dentate gyrus throughout postnatal development. Immunocytochemical analyses revealed prominent expression of the EphA3 receptor, a putative receptor for ephrin-A5, in the main cells and the neuropil of the developing hippocampus. Tracing experiments in ephrin-A5(-/-) mice showed that commissural projections were transiently altered in the hippocampus proper at P5, but they were mistargeted throughout the postnatal development in the dentate gyrus. Immunocytochemistry with anti-calbindin antibodies revealed that the dentate mossy fiber projection was not altered in ephrin-A5(-/-) mice. Electron microscopy studies showed alterations in the density of synapses and spines in commissural/associational layers, but not in entorhinal layers, and in the mossy fibers in these animals. Taken together, these findings indicate that ephrin-A5 signaling is involved in the formation and maturation of synapses in the hippocampus.

Disruption of ephrin-A/EphA binding alters synaptogenesis and neural connectivity in the hippocampus.

Ephrins are guidance cues that modulate axonal growth and the subsequent axonal topographic maps in many regions of the CNS. Here we studied the functional roles of ephrin-A/EphA interactions in the layer-specific pattern of axonal projections in the hippocampus by disrupting the ephrin-A signaling by over-expression of a soluble EphA receptor. Tracing experiments in EphA5-Fc over-expressing mice revealed that reduction of ephrin-A/EphA interactions did not affect the proper distribution of the main hippocampal afferents, i.e. entorhinal and commissural projections. However, further ultrastructural analyses showed a reduction in the density of synaptic terminals in the entorhinal and commissural termination layers in these mice. In addition, using anti-calbindin antibodies, we analyzed the dentate mossy fiber projections following disruption of ephrin-A/EphA interactions throughout developing hippocampus. While the main mossy fiber bundle appeared normal, the infrapyramidal bundle formed longer projections that established ectopic contacts in these transgenic mice. Later, the expected specific pruning of the infrapyramidal bundle was not observed at adult stages. Ultrastructural analyses confirmed a higher number of mossy fiber terminals in the infrapyramidal bundle in adult EphA5-Fc transgenic mice and showed that these terminals were larger and established a greater number of contacts than in controls. Our results demonstrate that ephrin-A/EphA interactions regulate the synaptogenesis of hippocampal afferents and the proper development and refinement of granule cell projections.

[Ephrins, neuronal development and plasticity]. / Efrinas, desarrollo neuronal y plasticidad.

AIMS: In this work we review the main characteristics of ephrins and their Eph receptors (ER), as well as descriptions that have been published to date of the different functions the ephrin/Eph system (EES) performs in neuronal development. DEVELOPMENT: ER constitute the largest group of tyrosine kinase receptors and are found in many different types of cells during development and in mature tissues. Their ligands, the ephrins, are membrane anchored proteins that are divided into class A ephrins, with a glycosylphosphatidylinositol bond, and class B ephrins, with a hydrophobic transmembrane region and a cytoplasmic domain. The EES is the only one that involves bidirectional signalling. Thus, the ephrin Eph interaction both activates the tyrosine kinase domain of the ER, with the resulting signal transduction in the cell that expresses Eph, and produces a reverse signal in the cells that contain the ligands. Over the last decade a number of studies have been conducted that establish the involvement of the EES in neuronal development. Although the classic function of this system is that of establishing patterns of both cellular and axonal organisation, recent reports describe how the ER and their ephrin ligands regulate synaptogenesis and the maturation of terminals during development, as well as the plasticity of the adult brain. CONCLUSIONS: Recent findings open up new expectations regarding the possible functions carried out by the interaction of ephrin and Eph. They also confirm the crucial role played by this system in all the processes involved in allowing neuronal development to take place in a correct fashion.

Efrinas, desarrollo neuronal y plasticidad / Ephrins, neuronal development and plasticity

Objetivo. En este trabajo se revisan las principales características de la efrinas y de sus receptores Eph (RE), así como las diferentes funciones descritas hasta ahora del sistema efrina/Eph (SEE) en el desarrollo neuronal. Desarrollo. Los RE comprenden el mayor grupo de receptores tirosinacuinasas y se encuentran en gran variedad de tipos celulares durante el desarrollo y en tejidos maduros. Sus ligandos, las efrinas, son proteínas ancladas a membrana, que se dividen en efrinas de clase A, con una unión glicosilfosfatidilinositol, y efrinas de clase B, con una región transmembrana hidrofóbica y un dominio citoplasmático. El SEE es el único que implica una señalización bidireccional. Así, la interacción efrina-Eph activa tanto el dominio tirosina cinasa del RE, con la consecuente transducción de señal en la célula que expresa Eph, como produce una señalización reversa en las células que contienen los ligandos. En la última década se han realizado numerosos estudios que involucran al SEE con el desarrollo neuronal. Si bien la función clásica de este sistema es la del establecimiento de patrones de organización, tanto celulares como axonales, recientemente se ha descrito una regulación, por parte de los RE y sus ligandos efrinas, de la sinaptogénesis y maduración de terminales durante el desarrollo, así como de la plasticidad en el cerebro adulto. Conclusiones. Los recientes resultados abren nuevas expectativas sobre las posibles funciones de la interacción efrina/Eph y confirman el papel crucial de este sistema en todos los procesos que permiten un correcto desarrollo neuronal (AU)

TrkB and TrkC signaling are required for maturation and synaptogenesis of hippocampal connections.

Recent studies have suggested a role for neurotrophins in the growth and refinement of neural connections, in dendritic growth, and in activity-dependent adult plasticity. To unravel the role of endogenous neurotrophins in the development of neural connections in the CNS, we studied the ontogeny of hippocampal afferents in trkB (-/-) and trkC (-/-) mice. Injections of lipophilic tracers in the entorhinal cortex and hippocampus of newborn mutant mice showed that the ingrowth of entorhinal and commissural/associational afferents to the hippocampus was not affected by these mutations. Similarly, injections of biocytin in postnatal mutant mice (P10-P16) did not reveal major differences in the topographic patterns of hippocampal connections. In contrast, quantification of biocytin-filled axons showed that commissural and entorhinal afferents have a reduced number of axon collaterals (21-49%) and decreased densities of axonal varicosities (8-17%) in both trkB (-/-) and trkC (-/-) mice. In addition, electron microscopic analyses showed that trkB (-/-) and trkC (-/-) mice have lower densities of synaptic contacts and important structural alterations of presynaptic boutons, such as decreased density of synaptic vesicles. Finally, immunocytochemical studies revealed a reduced expression of the synaptic-associated proteins responsible for synaptic vesicle exocytosis and neurotransmitter release (v-SNAREs and t-SNAREs), especially in trkB (-/-) mice. We conclude that neither trkB nor trkC genes are essential for the ingrowth or layer-specific targeting of hippocampal connections, although the lack of these receptors results in reduced axonal arborization and synaptic density, which indicates a role for TrkB and TrkC receptors in the developmental regulation of synaptic inputs in the CNS in vivo. The data also suggest that the genes encoding for synaptic proteins may be targets of TrkB and TrkC signaling pathways.