RESUMO
Thalamocortical (TC) connectivity is reorganized by thalamic inputs during postnatal development; however, the dynamic characteristics of TC reorganization and the underlying mechanisms remain unexplored. We addressed this question using dendritic refinement of layer 4 (L4) stellate neurons in mouse barrel cortex (barrel cells) as a model; dendritic refinement of L4 neurons is a critical component of TC reorganization through which postsynaptic L4 neurons acquire their dendritic orientation toward presynaptic TC axon termini. Simultaneous labeling of TC axons and individual barrel cell dendrites allowed in vivo time-lapse imaging of dendritic refinement in the neonatal cortex. The barrel cells reinforced the dendritic orientation toward TC axons by dynamically moving their branches. In N-methyl-D-aspartate receptor (NMDAR)-deficient barrel cells, this dendritic motility was enhanced, and the orientation bias was not reinforced. Our data suggest that L4 neurons have "fluctuating" dendrites during TC reorganization and that NMDARs cell autonomously regulate these dynamics to establish fine-tuned circuits.
Assuntos
Córtex Cerebral/citologia , Dendritos/fisiologia , Proteínas do Tecido Nervoso/metabolismo , Neurônios/citologia , Receptores de N-Metil-D-Aspartato/metabolismo , Tálamo/fisiologia , Animais , Animais Recém-Nascidos , Córtex Cerebral/fisiologia , Dendritos/efeitos dos fármacos , Agonistas de Aminoácidos Excitatórios/farmacologia , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Potenciais Pós-Sinápticos Excitadores/genética , Feminino , Regulação da Expressão Gênica no Desenvolvimento/fisiologia , Técnicas In Vitro , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Modelos Biológicos , N-Metilaspartato/farmacologia , Proteínas do Tecido Nervoso/genética , Vias Neurais/crescimento & desenvolvimento , Vias Neurais/metabolismo , Neurônios/efeitos dos fármacos , Receptores de N-Metil-D-Aspartato/genética , Ácido alfa-Amino-3-hidroxi-5-metil-4-isoxazol Propiônico/farmacologiaRESUMO
The ephrin/Eph system plays a central role in neuronal circuit formation; however, its downstream effectors are poorly understood. Here we show that alpha-chimerin Rac GTPase-activating protein mediates ephrinB3/EphA4 forward signaling. We discovered a spontaneous mouse mutation, miffy (mfy), which results in a rabbit-like hopping gait, impaired corticospinal axon guidance, and abnormal spinal central pattern generators. Using positional cloning, transgene rescue, and gene targeting, we demonstrated that loss of alpha-chimerin leads to mfy phenotypes similar to those of EphA4(-/-) and ephrinB3(-/-) mice. alpha-chimerin interacts with EphA4 and, in response to ephrinB3/EphA4 signaling, inactivates Rac, which is a positive regulator of process outgrowth. Moreover, downregulation of alpha-chimerin suppresses ephrinB3-induced growth cone collapse in cultured neurons. Our findings indicate that ephrinB3/EphA4 signaling prevents growth cone extension in motor circuit formation via alpha-chimerin-induced inactivation of Rac. They also highlight the role of a Rho family GTPase-activating protein as a key mediator of ephrin/Eph signaling.
