Neocortical Projection Neurons Research Articles

Life after transcription: control of the fate of neurons

The neocortex is a complex structure responsible for higher-order cognitive abilities in mammals. It consists of six cell layers, each comprised of various subtypes of excitatory and inhibitory neurons. These neurons project their axons to specific targets within each layer. All neocortical projection neurons are generated by neural progenitor cells located in the proliferative ventricular zone. In recent decades, significant strides have been made in comprehending the transcriptional programs governing neuronal cell fate specification. Nonetheless, posttranscriptional mechanisms involved in this process remain largely unexplored. Recently, we found that TrkC-T1, an isoform of the TrkC receptor lacking the kinase domain, determines the fate of corticofugal projection neurons (CFuPN). Our study reveals that the balance between TrkC-T1 and TrkC-TK+, a more widely recognized isoform containing the kinase domain, depends on the type of cell within the developing cortex. Additionally, we demonstrate that two RNA-binding proteins, Srsf1 and Elavl1, work in opposition to establish this balance. Additionally, our data suggests that Srsf1 stimulates the CFuPN fate while Elavl1 stimulates the callosal projection neuron (CPN) fate in vivo by regulating the different TrkC-T1 to TrkC-TK+ ratios. In this study, we identified a protein translation-dependent mechanism that governs the cell fate switch in the developing neocortex. Our results demonstrate that Ire1α, the Inositol-Requiring Enzyme 1α, regulates global translation rates in the developing neocortex through its dynamic interaction with the ribosome, as well as the regulation of expression of translation elongation factors eIF4A1 and eEF-2. Inactivation of Ire1α leads to decreased protein synthesis rates that are associated with stalled ribosomes and a reduced number of sites for translation initiation. We demonstrate the distinctive sensitivity of neurons determined for the upper layer to translation rates. While eEF-2 is necessary for cortical lamination, eIF4A1 regulates the attainment of upper layer fate in a mechanism of translational control that is dependent on structural elements embedded in the 5’UTR of genes responsible for determining fate downstream of Ire1α. Our findings reveal the developmental control of ribosome dynamics as post-transcriptional mechanisms that coordinate the establishment of neuronal diversity and the assembly of cortical layers.

The characteristics of excitatory lineage differentiation and the developmental conservation in Reeler neocortex

AbstractThe majority of neocortical projection neurons are generated indirectly from radial glial cells (RGCs) mediated by intermediate progenitor cells (IPCs) in mice. IPCs are thought to be a great breakthrough in the evolutionary expansion of the mammalian neocortex. However, the precise ratio of neuron production from IPCs and characteristics of RGC differentiation process are still unclear. Our study revealed that direct neurogenesis was seldom observed and increased slightly at late embryonic stage. Besides, we conducted retrovirus sparse labelling combined carboxyfluorescein diacetate succinimide ester (CFSE) and Tbr2‐CreER strain to reconstruct individual lineage tree in situ. The lineage trees simulated the output of RGCs at per round of division in sequence with high temporal, spatial and cellular resolution at P7. We then demonstrated that only 1.90% of neurons emanated from RGCs directly in mouse cerebral neocortex and 79.33% of RGCs contributed to the whole clones through IPCs. The contribution of indirect neurogenesis was underestimated previously because approximately a quarter of IPC‐derived neurons underwent apoptosis. Here, we also showed that abundant IPCs from first‐generation underwent self‐renewing division and generated four neurons ultimately. We confirmed that the intermediate proliferative progenitors expressed higher Cux2 characteristically at early embryonic stage. Finally, we validated that the characteristics of neurogenetic process in lineages and developmental fate of neurons were conserved in Reeler mice. This study contributes to further understanding of neurogenesis in neocortical development.

Transcriptional regulation in the development and dysfunction of neocortical projection neurons.

Glutamatergic projection neurons generate sophisticated excitatory circuits to integrate and transmit information among different cortical areas, and between the neocortex and other regions of the brain and spinal cord. Appropriate development of cortical projection neurons is regulated by certain essential events such as neural fate determination, proliferation, specification, differentiation, migration, survival, axonogenesis, and synaptogenesis. These processes are precisely regulated in a tempo-spatial manner by intrinsic factors, extrinsic signals, and neural activities. The generation of correct subtypes and precise connections of projection neurons is imperative not only to support the basic cortical functions (such as sensory information integration, motor coordination, and cognition) but also to prevent the onset and progression of neurodevelopmental disorders (such as intellectual disability, autism spectrum disorders, anxiety, and depression). This review mainly focuses on the recent progress of transcriptional regulations on the development and diversity of neocortical projection neurons and the clinical relevance of the failure of transcriptional modulations.

Vitamin D modulates cortical transcriptome and behavioral phenotypes in an Mecp2 heterozygous Rett syndrome mouse model

Rett syndrome (RTT) is an X-linked neurological disorder caused by mutations in the transcriptional regulator MECP2. Mecp2 loss-of-function leads to the disruption of many cellular pathways, including aberrant activation of the NF-κB pathway. Genetically attenuating the NF-κB pathway in Mecp2-null mice ameliorates hallmark phenotypes of RTT, including reduced dendritic complexity, raising the question of whether NF-κB pathway inhibitors could provide a therapeutic avenue for RTT. Vitamin D is a known inhibitor of NF-κB signaling; further, vitamin D deficiency is prevalent in RTT patients and male Mecp2-null mice. We previously demonstrated that vitamin D rescues the aberrant NF-κB activity and reduced neurite outgrowth of Mecp2-knockdown cortical neurons in vitro, and that dietary vitamin D supplementation rescues decreased dendritic complexity and soma size of neocortical projection neurons in both male hemizygous Mecp2-null and female heterozygous mice in vivo. Here, we have identified over 200 genes whose dysregulated expression in the Mecp2+/− cortex is modulated by dietary vitamin D. Genes normalized with vitamin D supplementation are involved in dendritic complexity, synapses, and neuronal projections, suggesting that the rescue of their expression could underpin the rescue of neuronal morphology. Further, there is a disruption in the homeostasis of the vitamin D synthesis pathway in Mecp2+/− mice, and motor and anxiety-like behavioral phenotypes in Mecp2+/− mice correlate with circulating vitamin D levels. Thus, our data indicate that vitamin D modulates RTT pathology and its supplementation could provide a simple and cost-effective partial therapeutic for RTT.

Cellular transcriptomics reveals evolutionary identities of songbird vocal circuits.

Birds display advanced behaviors, including vocal learning and problem-solving, yet lack a layered neocortex, a structure associated with complex behavior in mammals. To determine whether these behavioral similarities result from shared or distinct neural circuits, we used single-cell RNA sequencing to characterize the neuronal repertoire of the songbird song motor pathway. Glutamatergic vocal neurons had considerable transcriptional similarity to neocortical projection neurons; however, they displayed regulatory gene expression patterns more closely related to neurons in the ventral pallium. Moreover, while γ-aminobutyric acid-releasing neurons in this pathway appeared homologous to those in mammals and other amniotes, the most abundant avian class is largely absent in the neocortex. These data suggest that songbird vocal circuits and the mammalian neocortex have distinct developmental origins yet contain transcriptionally similar neurons.

Optimized Immunostaining of Embryonic and Early Postnatal Mouse Brain Sections.

The mammalian neocortex, the outer layer of the cerebrum and most recently evolved brain region, is characterized by its unique areal and laminar organization. Distinct cortical layers and areas can be identified by the protein expression of graded transcription factors and molecular determinants that define the identity of different projection neurons. Thus, specific detection and visualization of protein expression is crucial for assessing the identity of neocortical neurons and, more broadly, for understanding early and late developmental mechanisms and function of this complex system. Several immunostaining/immunofluorescence methods exist to detect protein expression. Published protocols vary with regard to subtle details, which may impact the final outcome of the immunofluorescence. Here, we provide a detailed protocol, suitable for both thin cryostat sections and thick vibratome sections, which has successfully worked for a wide range of antibodies directed against key molecular players of neocortical development. Ranging from early technical steps of brains collection down to image analysis and statistics, we include every detail concerning sample inclusion and sectioning, slide storage and optimal antibody dilutions aimed at reducing non-specific background. Routinely used in the lab, our background-optimized immunostaining protocol allows efficient detection of area- and layer- specific molecular determinants of distinct neocortical projection neurons. Graphic abstract: Workflow chart for the optimized immunostaining protocol of mouse brain sections. A. A flow chart for different steps of the optimized immunostaining protocol on both thin cryostat and thick vibratome sections. B. Example for immunostaining against Satb2 and Ctip2 on a thin coronal section (20 μm) at the level of the somatosensory cortex. The first column to the left shows the binning system where 6 bins can be overlaid on the image. On the bottom, an example of counting analysis showing the percentage of marker-positive cells normalized to the total number of DAPI or Hoechst-positive cells. C. Example for immunostaining against Satb2 and Ctip2 on a GFP+ thick vibratome section (200 μm). Images are taken at low magnification (10x, left) and high magnification (40x, right). The graph shows a counting of the percentage of Ctip2-positive neurons normalized to the total number of GFP-electroporated neurons on high-magnification images. Images on B and C are modified from Harb et al. (2016).

Non-Cell-Autonomous Mechanisms in Radial Projection Neuron Migration in the Developing Cerebral Cortex.

Concerted radial migration of newly born cortical projection neurons, from their birthplace to their final target lamina, is a key step in the assembly of the cerebral cortex. The cellular and molecular mechanisms regulating the specific sequential steps of radial neuronal migration in vivo are however still unclear, let alone the effects and interactions with the extracellular environment. In any in vivo context, cells will always be exposed to a complex extracellular environment consisting of (1) secreted factors acting as potential signaling cues, (2) the extracellular matrix, and (3) other cells providing cell–cell interaction through receptors and/or direct physical stimuli. Most studies so far have described and focused mainly on intrinsic cell-autonomous gene functions in neuronal migration but there is accumulating evidence that non-cell-autonomous-, local-, systemic-, and/or whole tissue-wide effects substantially contribute to the regulation of radial neuronal migration. These non-cell-autonomous effects may differentially affect cortical neuron migration in distinct cellular environments. However, the cellular and molecular natures of such non-cell-autonomous mechanisms are mostly unknown. Furthermore, physical forces due to collective migration and/or community effects (i.e., interactions with surrounding cells) may play important roles in neocortical projection neuron migration. In this concise review, we first outline distinct models of non-cell-autonomous interactions of cortical projection neurons along their radial migration trajectory during development. We then summarize experimental assays and platforms that can be utilized to visualize and potentially probe non-cell-autonomous mechanisms. Lastly, we define key questions to address in the future.

Loss of non-canonical KCC2 functions promotes developmental apoptosis of cortical projection neurons.

KCC2, encoded in humans by the SLC12A5 gene, is a multifunctional neuron-specific protein initially identified as the chloride (Cl- ) extruder critical for hyperpolarizing GABAA receptor currents. Independently of its canonical function as a K-Cl cotransporter, KCC2 regulates the actin cytoskeleton via molecular interactions mediated through its large intracellular C-terminal domain (CTD). Contrary to the common assumption that embryonic neocortical projection neurons express KCC2 at non-significant levels, here we show that loss of KCC2 enhances apoptosis of late-born upper-layer cortical projection neurons in the embryonic brain. In utero electroporation of plasmids encoding truncated, transport-dead KCC2 constructs retaining the CTD was as efficient as of that encoding full-length KCC2 in preventing elimination of migrating projection neurons upon conditional deletion of KCC2. This was in contrast to the effect of a full-length KCC2 construct bearing a CTD missense mutation (KCC2R952H ), which disrupts cytoskeletal interactions and has been found in patients with neurological and psychiatric disorders, notably seizures and epilepsy. Together, our findings indicate ion transport-independent, CTD-mediated regulation of developmental apoptosis by KCC2 in migrating cortical projection neurons.

N-cadherin-regulated FGFR ubiquitination and degradation control mammalian neocortical projection neuron migration.

The functions of FGF receptors (FGFRs) in early development of the cerebral cortex are well established. Their functions in the migration of neocortical projection neurons, however, are unclear. We have found that FGFRs regulate multipolar neuron orientation and the morphological change into bipolar cells necessary to enter the cortical plate. Mechanistically, our results suggest that FGFRs are activated by N-Cadherin. N-Cadherin cell-autonomously binds FGFRs and inhibits FGFR K27- and K29-linked polyubiquitination and lysosomal degradation. Accordingly, FGFRs accumulate and stimulate prolonged Erk1/2 phosphorylation. Neurons inhibited for Erk1/2 are stalled in the multipolar zone. Moreover, Reelin, a secreted protein regulating neuronal positioning, prevents FGFR degradation through N-Cadherin, causing Erk1/2 phosphorylation. These findings reveal novel functions for FGFRs in cortical projection neuron migration, suggest a physiological role for FGFR and N-Cadherin interaction in vivo and identify Reelin as an extracellular upstream regulator and Erk1/2 as downstream effectors of FGFRs during neuron migration.

Intraventricular Transplantation of Engineered Neuronal Precursors for In Vivo Neuroarchitecture Studies.

Gene control of neuronal cytoarchitecture is currently the subject of intensive investigation. Described here is a simple method developed to study in vivo gene control of neocortical projection neuron morphology. This method is based on (1) in vitro lentiviral engineering of neuronal precursors as "test" and "control" cells, (2) their co-transplantation into wild-type brains, and (3) paired morphometric evaluation of their neuronal derivatives. Specifically, E12.5 pallial precursors from panneuronal, genetically labeled donors, are employed for this purpose. They are engineered to take advantage of selected promoters and tetON/OFF technology, and they are free-hand transplanted into neonatal lateral ventricles. Later, upon immunofluorescence profiling of recipient brains, silhouettes of transplanted neurons are fed into NeurphologyJ open source software, their morphometric parameters are extracted, and average length and branching index are calculated. Compared to other methods, this one offers three main advantages: it permits achieving of fine control of transgene expression at affordable costs, it only requires basic surgical skills, and it provides statistically reliable results upon analysis of a limited number of animals. Because of its design, however, it is not adequate to address non cell-autonomous control of neuroarchitecture. Moreover, it should be preferably used to investigate neurite morphology control after completion of neuronal migration. In its present formulation, this method is exquisitely tuned to investigate gene control of glutamatergic neocortical neuron architecture. Taking advantage of transgenic lines expressing EGFP in other specific neural cell types, it can be re-purposed to address gene control of their architecture.

Maternal hyperglycemia disturbs neocortical neurogenesis via epigenetic regulation in C57BL/6J mice

Offspring of mothers with hyperglycemia during pregnancy have a higher incidence of long-term neuropsychiatric disorders than offspring from a normal pregnancy, indicating that neocortical neurogenesis might be affected by maternal hyperglycemia. A paucity of study evaluating the effects of hyperglycemia on neocortical neurogenetic differentiation of neural stem cells, and the mechanism remains unclear. We sought to investigate the the roles and possible molecular mechanism of maternal hyperglycemia on neocortical neurogenetic differentiation of neural stem cells. We established a mouse model of a hyperglycemic pregnancy to study effects of intrauterine exposure to maternal hyperglycemia on neocortical neurogenesis. We observed morphological changes in the neocortex and detected the neurogenetic differentiation of neural stem cells in offspring affected by high glucose levels. We investigated the regulatory network between epigenetic modification and transcription factors in differentiated neural stem cells under hyperglycemic conditions. Maternal hyperglycemia disturbs neocortical lamination in some non-malformed offspring. Our results suggested that hyperglycemia altered the early-born neuron fate and the distribution of newborn neurons in deep layers by promoting the earlier differentiation of neural stem cells. Altered histone acetylation and its regulation on the transcription of proneural genes might be correlated to the disrupted differentiation of neural stem cells and altered distribution of newborn projection neurons in the neocortex. Our data raised the possibility that maternal hyperglycemia in pregnancy disturbs the laminar distribution of neocortical projection neurons in some non-malformed offspring via epigenetic regulation on neural stem cell differentiation and the birthdate of neocortical neurons.

Differential gene regulatory plasticity between upper and lower layer cortical excitatory neurons

Neocortical projection neurons consist of intracortical connected upper layer (UL, layer II–IV) neurons and subcortical connected lower layer (LL, layer V–VI) neurons. Afferent activity from the thalamus regulates layer-specific gene expression during postnatal development, which is critical for the formation of proper neocortical cytoarchitecture. Here, we show that activity-dependent gene regulation is confined to UL cortical neurons, but not LL neurons, and that this distinction is likely due to epigenetic modifications of chromatin. We found that the immediate early genes (IEGs), EGR1 and c-FOS, are downregulated in all cortical laminar layers in the absence of afferent activity in vivo. Transcriptional assays demonstrated that EGR1 and c-FOS are able to bind to the promoters of UL- and LL-specific genes to induce transcription. Furthermore, we discovered that LL neurons express higher levels of heterochromatin markers, such as H3K9m3 and H4K20m3, compared to UL neurons. Our results suggest that differential epigenetic modifications of chromatin is an intrinsic mechanism that underlies the different sensitivities of cortical neurons to activity-dependent gene regulation.

Zbtb20 Regulates Developmental Neurogenesis in the Olfactory Bulb and Gliogenesis After Adult Brain Injury.

The transcription factor (TF) Zbtb20 is important for the hippocampal specification and the regulation of neurogenesis of neocortical projection neurons. Herein, we show a critical involvement of theTF Zbtb20 in the neurogenesis of both projection neurons and interneurons of the olfactory bulb during embryonic stages. Our data indicate that the lack of Zbtb20 significantly diminishes the generation of a set of early-born Tbr2+ neurons during embryogenesis. Furthermore, we provide evidence that Zbtb20 regulates the transition between neurogenesis to gliogenesis in cortical radial glial progenitor cells at the perinatal (E18.5) stage. In the adult mammalian brain, Zbtb20 is expressed by GFAP+ neural progenitor cells (NPCs) located in the forebrain neurogenic niche, i.e.,the subventricular zone (SVZ) of the lateral ventricles. Upon induction of cerebral ischemia, we found that Zbtb20 expression is upregulated in astrocytic-like cells, whereas diminishing the expression levels of Zbtb20 significantly reduces the ischemia-induced astrocytic reaction as observed in heterozygous Zbtb20 loss-of-function mice. Altogether, these results highlight the important role of the TF Zbtb20 as a temporal regulator of neurogenesis or gliogenesis, depending on the developmental context.

Foxg1 Overexpression in Neocortical Pyramids Stimulates Dendrite Elongation Via Hes1 and pCreb1 Upregulation.

The architecture of neocortical projection neurons is subject of a complex gene control. Here we demonstrated that Foxg1, a transcription factor gene which patterns the early rostral brain and sets the pace of telencephalic neuronogenesis, specifically stimulates dendrite elongation. This phenomenon occurs in vivo like in vitro, and it is detectable even upon moderate changes of Foxg1 expression levels. We found that Foxg1 acts by stimulating Hes1, which in turn upregulates pCreb1, a well-known pro-dendritogenic effector, and downregulates Syt and Ndr1, namely two established antagonizers of dendrite elongation. Moreover, Foxg1-driven pCreb1 upregulation requires PKA and AKT, and correlates with reduced PP1 and PP2A phosphatase activity. These findings contribute to clarify normal neurodevelopmental and activity-related regulation of neuritogenesis. They further suggest that an abnormal sizing of the dendritic tree of neocortical projection neurons may occur in West and Rett syndrome patients with anomalous FOXG1 allele dosages and contribute to their neurolopathological profiles.

Variation in Activity State, Axonal Projection, and Position Define the Transcriptional Identity of Individual Neocortical Projection Neurons

Single-cell RNA sequencing has generated catalogs of transcriptionally defined neuronal subtypes of the brain. However, the cellular processes that contribute to neuronal subtype specification and transcriptional heterogeneity remain unclear. By comparing the gene expression profiles of single layer 6 corticothalamic neurons in somatosensory cortex, we show that transcriptional subtypes primarily reflect axonal projection pattern, laminar position within the cortex, and neuronal activity state. Pseudotemporal ordering of 1,023 cellular responses to sensory manipulation demonstrates that changes in expression of activity-induced genes both reinforced cell-type identity and contributed to increased transcriptional heterogeneity within each cell type. This is due to cell-type biased choices oftranscriptional states following manipulation of neuronal activity. These results reveal that axonal projection pattern, laminar position, and activity state define significant axes of variation that contribute both to the transcriptional identity of individual neurons and to the transcriptional heterogeneity within each neuronal subtype.

Satb2Cre/+ mouse as a tool to investigate cell fate determination in the developing neocortex

BackgroundGeneration of different neuronal subtypes during neocortical development is the most important step in the establishment of cortical cytoarchitecture. The transcription factor Satb2 is expressed in neocortical projection neurons that send their axons intracortically as opposed to Satb2-negative neurons that preferentially project to subcortical targets. New methodIn this report, we present a novel method to carry out large scale screening for molecules that control cell fate in the developing neocortex. It is based on a Satb2Cre/+ mouse strain that expresses Cre recombinase from the Satb2 locus. ResultsBy transfecting neuronould determine the proportion of cells that become al progenitors with a Cre-inducible reporter construct by nucleofection or in utero electroporation, we cSatb2-positive. Comparison with existing methodsCompared to genetic tracing or lineage analysis, this method offers a fast, easy-to-perform, and reliable way of determining cell fate of newly born neurons. ConclusionsWe demonstrate that the Satb2Cre/+ mouse can be applied to study factors, such as small molecule inhibitors, sh-RNAs or overexpression constructs, that can alter the proportion of Satb2-positive cells and thus play key roles in differentiation and acquisition of cell fate.

Enhancing Neuronogenesis and Counteracting Neuropathogenic Gene Haploinsufficiencies by RNA Gene Activation.

Small activating RNAs (saRNAs), targeting endogenous genes and stimulating their transcription, are a promising tool for implementing a variety of neurotherapeutic strategies. Among these there is the stimulation of select histogenetic subroutines for purposes of cell-based brain repair, as well as the therapeutic treatment of gene expression deficits underlying severe neurological disorders.We employed RNA activation (RNAa) to transactivate the Emx2 transcription factor gene in embryonic cortico-cerebral precursor cells. This led to enhanced self-renewal, delayed differentiation, and reduced death of neuronally committed precursors, resulting in a remarkable expansion of the neuronogenic precursors pool. These results are of paramount interest for purposes of gene-promoted brain repair. As such, RNAa makes therapeutic stimulation of neuronogenesis via Emx2 overexpression a feasible goal, preventing the drawbacks of exogenous gene copies introduction.Moreover, we employed RNAa to achieve a gentle transactivation of the Foxg1 transcription factor gene, specifically in cortico-cerebral cells. This manipulation led to an appreciable biological outcome, while complying with endogenous gene tuning linked to early central nervous system regionalization and late activity of neocortical projection neurons. Foxg1-activating miRNAs stimulated RNApolII recruitment, possibly via Ago1. One of them worked promisingly in vivo. As such, RNAa can be a valuable approach for therapeutic treatment of the FOXG1-haploinsufficiency-linked variant of the Rett syndrome. Remarkably, hemizygosity for specific genes and polygenic chromosomal segments underlies a huge number of neuropathological entities for which no cure is presently available. Based on the results reported above, RNAa might be a simple and scalable approach for fixing this class of problems.

The RNA binding protein HuR determines the differential translation of autism-associated FoxP subfamily members in the developing neocortex.

Forkhead-box domain (Fox) containing family members are known to play a role in neocorticogenesis and have also been associated with disorders on the autism spectrum. Here we show that a single RNA-binding protein, Hu antigen R (HuR), dictates translation specificity of bound mRNAs and is sufficient to define distinct Foxp-characterized subpopulations of neocortical projection neurons. Furthermore, distinct phosphorylation states of HuR differentially regulate translation of Foxp mRNAs in vitro. This demonstrates the importance of RNA binding proteins within the framework of the developing brain and further confirms the role of mRNA translation in autism pathogenesis.

Target-specific membrane potential dynamics of neocortical projection neurons during goal-directed behavior.

Goal-directed behavior involves distributed neuronal circuits in the mammalian brain, including diverse regions of neocortex. However, the cellular basis of long-range cortico-cortical signaling during goal-directed behavior is poorly understood. Here, we recorded membrane potential of excitatory layer 2/3 pyramidal neurons in primary somatosensory barrel cortex (S1) projecting to either primary motor cortex (M1) or secondary somatosensory cortex (S2) during a whisker detection task, in which thirsty mice learn to lick for water reward in response to a whisker deflection. Whisker stimulation in 'Good performer' mice, but not 'Naive' mice, evoked long-lasting biphasic depolarization correlated with task performance in S2-projecting (S2-p) neurons, but not M1-projecting (M1-p) neurons. Furthermore, S2-p neurons, but not M1-p neurons, became excited during spontaneous unrewarded licking in 'Good performer' mice, but not in 'Naive' mice. Thus, a learning-induced, projection-specific signal from S1 to S2 may contribute to goal-directed sensorimotor transformation of whisker sensation into licking motor output.

Ctip1 Regulates the Balance between Specification of Distinct Projection Neuron Subtypes in Deep Cortical Layers.

The molecular linkage between neocortical projection neuron subtype and area development, which enables the establishment of functional areas by projection neuron populations appropriate for specific sensory and motor functions, is poorly understood. Here, we report that Ctip1 controls precision of neocortical development by regulating subtype identity in deep-layer projection neurons. Ctip1 is expressed by postmitotic callosal and corticothalamic projection neurons but is excluded over embryonic development from corticospinal motor neurons, which instead express its close relative, Ctip2. Loss of Ctip1 function results in a striking bias in favor of subcerebral projection neuron development in sensory cortex at the expense of corticothalamic and deep-layer callosal development, while misexpression of Ctip1 invivo represses subcerebral gene expression and projections. As we report in a paired paper, Ctip1 also controls acquisition of sensory area identity. Therefore, Ctip1 couples subtype and area specification, enabling specific functional areas to organize precise ratios of appropriate output projections.