Characterization of the earliest thalamocortical interactions in vivo and in vitro

The degree to which intrinsic versus extrinsic factors control the development of the cerebral cortex has been the subject of sustained research. While there is a large body of evidence for an early control of patterning that is intrinsic to the neocortex, a major extrinsic source of patterning is provided by the thalamo-cortical afferents (TCA) that reach to the developing cortex at very early stages, before the peak of neurogenesis and neuronal migration take place. We are interested in this complex array of external signals that regulate region-specific cortical development, and their interplay with the mechanisms intrinsic to cortical progenitors and neurons. We are particularly interested in how area-specific TCA begin to influence corticogenesis in the germinal zone and start to influence initial circuit formation at very early stages, when this extrinsic contribution might underlie major differences in the region-specific developmental programme of cortical progenitors and early postmytotic neurons in higher-order mammals. The first compartment of the developing cortex reached by the early-incoming TCAs is the subplate (SP) zone, a transient embryonic cortical compartment which is greatly expanded in size and complexity during primate evolution, culminating in humans. This structure contains a heterogeneous population of cells and it contributes to the guidance and areal targeting of TCA at early stages of corticogenesis. Intriguingly, SP abnormalities have been implicated in the pathogenesis of human developmental disorders including cerebral palsy, childhood epilepsy, schizophrenia and autism. Moreover, recent observations in our laboratory suggest that the outer subventricular zone (oSVZ), a unique germinal zone present specifically in higher-order mammals, might represent another initial target of TCA during the initial phases of corticogenesis. The establishment of such connection would provide a better explanation to the dramatic reduction in neuronal numbers in primary visual cortex reported in anophtalmic human and enucleated non-human primate models, thus further confirming a crucial role of TCA in shaping the earliest compartments of the developing neocortex. Together these observations raise the hypothesis of an unexpected major role of TCA in early areal specification -especially in primates-, and the SP and oSVZ might represent the anatomical correlates of the interaction between intrinsic and extrinsic regulatory events. We will address the large expansion of the SP and oSVZ in concomitance with TCA early-arrival in the developing cortex, by investigating the cell types as well as the specific molecular mechanisms involved in the establishment of a functional interaction among these components. To this aim, we will perform tracing analysis and immunohistochemical staining in human post-mortem foetal tissues, embryonic macaque tissue (provided by our collaborators), as well as in mouse models, in order to cover a wide range of time-points throughout early cortical development and compare the main processes involved in different species. Once we will characterise the cellular subtypes involved in this interaction, we will further dissect the molecular pathways underlying its effect, and validate the data obtained ex-vivo/in-vivo in an in-vitro setting. The latter approach would entail modelling a simplified version of human (and mouse) early corticogenesis in a dish by organotipic cultures in the first place, and by more complex 3D cellular models (cerebral organoids fused with thalamic organoids) at a later stage of the project. This would allow us not only to confirm the data we will obtained in post-mortem human tissues (and ex-vivo murine samples for direct comparison) but it would also provide a novel optimized in-vitro model of human early thalamo-cortical interaction where further analyses could be done in the future, by testing genetic manipulations and/or pharmacological treatments on this system.