Assuntos
Neurologia/tendências , Doenças do Sistema Nervoso Periférico/epidemiologia , Comunismo , Cuba , Educação Médica , História do Século XX , Humanos , Neurologia/educação , Neurocirurgia/economia , Neurocirurgia/tendências , Doenças do Sistema Nervoso Periférico/diagnóstico , Degenerações Espinocerebelares/epidemiologiaRESUMO
Toward understanding topographically specific branching of retinal axons in their target area, we have studied the interaction between neurotrophin receptors and members of the Eph family. TrkB and its ligand BDNF are uniformly expressed in the retina and tectum, respectively, and exert a branch-promoting activity, whereas EphAs and ephrinAs are expressed in gradients in retina and tectum and can mediate a suppression of axonal branching. We have identified a novel cis interaction between ephrinA5 and TrkB on retinal ganglion cell axons. TrkB interacts with ephrinA5 via its second cysteine-rich domain (CC2), which is necessary and sufficient for binding to ephrinA5. Their functional interaction is twofold: ephrinA5 augments BDNF-promoted retinal axon branching in the absence of its activator EphA7-Fc, whereas EphA7-Fc application abolishes branching in a local and concentration-dependent manner. The importance of TrkB in this process is shown by the fact that overexpression of an isolated TrkB-CC2 domain interfering with the ephrinA/TrkB interaction abolishes this regulatory interplay, whereas knockdown of TrkB via RNA interference diminishes the ephrinA5-evoked increase in branching. The ephrinA/Trk interaction is neurotrophin induced and specifically augments the PI-3 kinase/Akt pathway generally known to be involved in the promotion of branching. In addition, ephrinAs/TrkB modulate axon branching and also synapse formation of hippocampal neurons. Our findings uncover molecular mechanisms of how spatially restricted axon branching can be achieved by linking globally expressed branch-promoting with differentially expressed branch-suppressing activities. In addition, our data suggest that growth factors and the EphA-ephrinA system interact in a way that affects axon branching and synapse development.
Assuntos
Axônios/metabolismo , Neurogênese/fisiologia , Receptor trkB/metabolismo , Receptores da Família Eph/metabolismo , Retina/embriologia , Células Ganglionares da Retina/metabolismo , Animais , Axônios/ultraestrutura , Fator Neurotrófico Derivado do Encéfalo/metabolismo , Células Cultivadas , Embrião de Galinha , Efrina-A5/química , Efrina-A5/genética , Efrina-A5/metabolismo , Hipocampo/citologia , Hipocampo/crescimento & desenvolvimento , Hipocampo/metabolismo , Camundongos , Camundongos Knockout , Técnicas de Cultura de Órgãos , Células PC12 , Fosfatidilinositol 3-Quinases/metabolismo , Estrutura Terciária de Proteína/fisiologia , RNA/metabolismo , Ratos , Receptor trkB/genética , Receptores da Família Eph/química , Receptores da Família Eph/genética , Retina/citologia , Células Ganglionares da Retina/citologia , Transdução de Sinais/fisiologia , Colículos Superiores/citologia , Colículos Superiores/embriologia , Colículos Superiores/metabolismo , Sinapses/metabolismoRESUMO
EphAs and ephrinAs are expressed in multiple areas of the developing brain in overlapping countergradients, notably in the retina and tectum. Here they are involved in targeting retinal axons to their correct topographic position in the tectum. We have used truncated versions of EphA3, single-amino acid point mutants of ephrinA5 and fluorescence resonance energy transfer technology to uncover a cis interaction between EphA3 and ephrinA5 that is independent of the established ligand-binding domain of EphA3. This cis interaction abolishes the induction of tyrosine phosphorylation of EphA3 and results in a loss of sensitivity of retinal axons to ephrinAs in trans. Our data suggest that formation of this complex transforms the uniform expression of EphAs in the nasal part of the retina into a gradient of functional EphAs and has a key role in controlling retinotectal mapping.