During cortical development, the identity of major classes of long-distance projection neurons is established by the expression of molecular determinants, which become gradually restricted and mutually exclusive. However, the mechanisms by which projection neurons acquire their final properties during postnatal stages are still poorly understood. In this study, we show that the number of neurons co-expressing Ctip2 and Satb2, respectively involved in the early specification of subcerebral and callosal projection neurons, progressively increases after birth in the somatosensory cortex. Ctip2/Satb2 postnatal co-localization defines two distinct neuronal subclasses projecting either to the contralateral cortex or to the brainstem suggesting that Ctip2/Satb2 co-expression may refine their properties rather than determine their identity. Gain- and loss-of-function approaches reveal that the transcriptional adaptor Lmo4 drives this maturation program through modulation of epigenetic mechanisms in a time- and area-specific manner, thereby indicating that a previously unknown genetic program postnatally promotes the acquisition of final subtype-specific features.

The mammalian neocortex is subdivided into cytoarchitectural areas with distinct connectivity, gene expression and neural functions. Areal identity is initially specified by rostrocaudal and mediolateral gene expression gradients in neuroepithelial and radial glial progenitors (the ‘protomap’). On further differentiation, distinct sets of gene expression gradients arise in intermediate progenitors and postmitotic neurons, and are necessary to implement areal specification. However, it is still unknown whether postmitotic gene expression gradients can determine areal identity independently of protomap gradients. Here we show, by cell type-restricted genetic loss- and gain-of-function, that high levels of postmitotic COUP-TFI (Nr2f1) expression are necessary and sufficient for the development of sensory (caudal) areal identity. Our data indicate a crucial role for postmitotic patterning genes in areal specification and reveal an unexpected plasticity in this process, which may account for complex and evolutionarily novel structures characteristic of the mammalian neocortex.

Mutations in theSHANK3gene have been recognized as a genetic risk factor for Autism Spectrum Disorder (ASD), a neurodevelopmental disease characterized by social deficits and repetitive behaviors. While heterozygousSHANK3mutations are usually the types of mutations associated with idiopathic autism in patients, heterozygous deletion ofShank3gene in mice does not commonly induce ASD-related behavioral deficit. Here, we used in-vivo and ex-vivo approaches to demonstrate that region-specific neonatal downregulation ofShank3in the Nucleus Accumbens promotes D1R-medium spiny neurons (D1R-MSNs) hyperexcitability and upregulates Transient Receptor Potential Vanilloid 4 (Trpv4)to impair social behavior. Interestingly, genetically vulnerableShank3^+/−mice, when challenged with Lipopolysaccharide to induce an acute inflammatory response, showed similar circuit and behavioral alterations that were rescued by acute Trpv4 inhibition. Altogether our data demonstrate shared molecular and circuit mechanisms between ASD-relevant genetic alterations and environmental insults, which ultimately lead to sociability dysfunctions.

The formation of functional cortical maps in the cerebral cortex results from a timely regulated interaction between intrinsic genetic mechanisms and electrical activity. To understand how transcriptional regulation influences network activity and neuronal excitability within the neocortex, we used mice deficient for Nr2f1 (also known as COUP-TFI), a key determinant of primary somatosensory (S1) area specification during development. We found that the cortical loss of Nr2f1 impacts on spontaneous network activity and synchronization of S1 cortex at perinatal stages. In addition, we observed alterations in the intrinsic excitability and morphological features of layer V pyramidal neurons. Accordingly, we identified distinct voltage-gated ion channels regulated by Nr2f1 that might directly influence intrinsic bioelectrical properties during critical time windows of S1 cortex specification. Altogether, our data suggest a tight link between Nr2f1 and neuronal excitability in the developmental sequence that ultimately sculpts the emergence of cortical network activity within the immature neocortex.

Lipid droplets (LDs) are dynamic lipid storage organelles. They are tightly linked to metabolism and can exert protective functions, making them important players in health and disease. Most LD studies in vivo rely on staining methods, providing only a snapshot. We therefore developed a LD-reporter mouse by labelling the endogenous LD coat protein perilipin 2 (PLIN2) with tdTomato, enabling staining-free fluorescent LD visualisation in living and fixed tissues and cells. Here we validate this model under standard and high-fat diet conditions and demonstrate that LDs are highly abundant in various cell types in the healthy brain, including neurons, astrocytes, ependymal cells, neural stem/progenitor cells and microglia. Furthermore, we also show that LDs are abundant during brain development and can be visualized using live imaging of embryonic slices. Taken together, our tdTom-Plin2 mouse serves as a novel tool to study LDs and their dynamics under both physiological and diseased conditions in all tissues expressing Plin2.