Eliminação de neurônios infragranulares afeta a especificação de neurônios granulares e supragranulares do córtex cerebral em desenvolvimento

Abstract

The cerebral cortex of mammals is histologically organized into in different layers of excitatory neurons that have distinct patterns of connections with cortical or subcortical targets. During development, these cortical layers are sequentially established through an intricate combination of neuronal specification and migration in a radial pattern known as "inside-out": deep-layer neurons are generated prior to upper-layer neurons. In the last few decades, several genes encoding transcription factors involved in the specification of neurons destined to different cortical layers have been identified. However, the influence of early-generated neurons in to the specification of subsequent neuronal cohorts remains unclear. To investigate the possible effects early born neurons ablation on the specification of late born neurons, we induced the selective death of cortical neurons from layers V and VI neurons before the generation of neurons destined to layers II, III and IV. Our data shows that oneday after ablation, progenitors resumed generation of layer VI neurons expressing the transcription factor TBR1, whereas virtually no TBR1-expressing neuron was generated at the same developmental stage in age-matched controls. Interestingly, many TBR1-positive neurons generated after deep-layer ablation settled within superficial cortical layers, as expected for upper-layer neurons generated at that stage, suggesting that migration post-mitotic neurons is independent of fate-specification. Furthermore, we observed an increase in layer V neurons expressing CTIP2 and cortico-cortical neurons expressing SATB2 at the expense of layer IV neurons in P0 animals. When these animals became young adults (P30) the increase os SATB2 and CTIP2 neurons is no longer observed, however these neurons are distributed in a different way in somatosensory areas from ablated animals. In vitro experiments show that the laminar cytoarchitectural organization of the cortex is necessary to regenerate the previously deleted TBR1 + neurons. In addition, in vitro experiments indicate that in a condition of low cell density the neurons phnotype is altered, they express several transcription factors at the same time. Together, our data indicate the existence of feedback mechanism either from early-generated neurons to progenitors involved in the generation of upper-layer neurons or from deep-layer neurons to postmitotic neurons generated subsequently. This mechanism could help to control the number of neurons in different layers and contribute to the establishment of different cortical areas.