RESUMO
Coordinated migration of neurons in the developing and adult brain is essential for its proper function. The secreted glycoprotein Reelin (also known as RELN) guides migration of neurons by binding to two lipoprotein receptors, the very-low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2, also known as LRP8). Loss of Reelin function in humans results in the severe developmental disorder lissencephaly and it has also been associated with other neurological disorders such as epilepsy, schizophrenia and Alzheimer's disease. The molecular mechanisms by which Reelin activates its receptors and controls cellular functions are largely unknown. Here we show that the neuronal guidance cues ephrin B proteins are essential for Reelin signalling during the development of laminated structures in the brain. We show that ephrin Bs genetically interact with Reelin. Notably, compound mouse mutants (Reln(+/-); Efnb3(-/-) or Reln(+/-); Efnb2(-/-)) and triple ephrin B1, B2, B3 knockouts show neuronal migration defects that recapitulate the ones observed in the neocortex, hippocampus and cerebellum of the reeler mouse. Mechanistically, we show that Reelin binds to the extracellular domain of ephrin Bs, which associate at the membrane with VLDLR and ApoER2 in neurons. Clustering of ephrin Bs leads to the recruitment and phosphorylation of Dab1 which is necessary for Reelin signalling. Conversely, loss of function of ephrin Bs severely impairs Reelin-induced Dab1 phosphorylation. Importantly, activation of ephrin Bs can rescue the reeler neuronal migration defects in the absence of Reelin protein. Together, our results identify ephrin Bs as essential components of the Reelin receptor/signalling pathway to control neuronal migration during the development of the nervous system.
Assuntos
Moléculas de Adesão Celular Neuronais/metabolismo , Movimento Celular , Córtex Cerebral/citologia , Efrinas/metabolismo , Proteínas da Matriz Extracelular/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Neurônios/citologia , Neurônios/metabolismo , Serina Endopeptidases/metabolismo , Transdução de Sinais , Animais , Moléculas de Adesão Celular Neuronais/genética , Córtex Cerebral/embriologia , Córtex Cerebral/metabolismo , Efrina-B1/deficiência , Efrina-B1/genética , Efrina-B1/metabolismo , Efrina-B2/deficiência , Efrina-B2/genética , Efrina-B2/metabolismo , Efrina-B3/deficiência , Efrina-B3/genética , Efrina-B3/metabolismo , Efrinas/deficiência , Efrinas/genética , Proteínas da Matriz Extracelular/genética , Feminino , Proteínas Relacionadas a Receptor de LDL/metabolismo , Ligantes , Masculino , Camundongos , Camundongos Knockout , Proteínas do Tecido Nervoso/genética , Fenótipo , Fosforilação , Ligação Proteica , Receptores de LDL/metabolismo , Proteína Reelina , Serina Endopeptidases/genéticaRESUMO
The axon initial segment (AIS) is the site of action potential generation and a locus of activity-dependent homeostatic plasticity. A multimeric complex of sodium channels, linked via a cytoskeletal scaffold of ankyrin G and beta IV spectrin to submembranous actin rings, mediates these functions. The mechanisms that specify the AIS complex to the proximal axon and underlie its plasticity remain poorly understood. Here we show phosphorylated myosin light chain (pMLC), an activator of contractile myosin II, is highly enriched in the assembling and mature AIS, where it associates with actin rings. MLC phosphorylation and myosin II contractile activity are required for AIS assembly, and they regulate the distribution of AIS components along the axon. pMLC is rapidly lost during depolarization, destabilizing actin and thereby providing a mechanism for activity-dependent structural plasticity of the AIS. Together, these results identify pMLC/myosin II activity as a common link between AIS assembly and plasticity.
Assuntos
Actinas/metabolismo , Segmento Inicial do Axônio/metabolismo , Cadeias Leves de Miosina/metabolismo , Miosina Tipo II/metabolismo , Citoesqueleto de Actina/metabolismo , Animais , Córtex Cerebral/metabolismo , Feminino , Hipocampo/metabolismo , Masculino , Camundongos Endogâmicos C57BL , Camundongos Knockout , Fosfatase de Miosina-de-Cadeia-Leve/genética , Fosforilação , Cultura Primária de Células , Ratos Sprague-DawleyRESUMO
Regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor trafficking in response to neuronal activity is critical for synaptic function and plasticity. Here, we show that neuronal activity induces the binding of ephrinB2 and ApoER2 receptors at the postsynapse to regulate de novo insertion of AMPA receptors. Mechanistically, the multi-PDZ adaptor glutamate-receptor-interacting protein 1 (GRIP1) binds ApoER2 and bridges a complex including ApoER2, ephrinB2, and AMPA receptors. Phosphorylation of ephrinB2 in a serine residue (Ser-9) is essential for the stability of such a complex. In vivo, a mutation on ephrinB2 Ser-9 in mice results in a complete disruption of the complex, absence of ApoER2 downstream signaling, and impaired activity-induced and ApoER2-mediated AMPA receptor insertion. Using compound genetics, we show the requirement of this complex for long-term potentiation (LTP). Together, our findings uncover a cooperative ephrinB2 and ApoER2 signaling at the synapse, which serves to modulate activity-dependent AMPA receptor dynamic changes during synaptic plasticity.