Mutations in Human Accelerated Regions Disrupt Cognition and Social Behavior.

Comparative analyses have identified genomic regions potentially involved in human evolution but do not directly assess function. Human accelerated regions (HARs) represent conserved genomic loci with elevated divergence in humans. If some HARs regulate human-specific social and behavioral traits, then mutations would likely impact cognitive and social disorders. Strikingly, rare biallelic point mutations-identified by whole-genome and targeted "HAR-ome" sequencing-showed a significant excess in individuals with ASD whose parents share common ancestry compared to familial controls, suggesting a contribution in 5% of consanguineous ASD cases. Using chromatin interaction sequencing, massively parallel reporter assays (MPRA), and transgenic mice, we identified disease-linked, biallelic HAR mutations in active enhancers for CUX1, PTBP2, GPC4, CDKL5, and other genes implicated in neural function, ASD, or both. Our data provide genetic evidence that specific HARs are essential for normal development, consistent with suggestions that their evolutionary changes may have altered social and/or cognitive behavior. PAPERCLIP.

R-Ras1 and R-Ras2 Expression in Anatomical Regions and Cell Types of the Central Nervous System.

Since the optic nerve is one of the most myelinated tracts in the central nervous system (CNS), many myelin diseases affect the visual system. In this sense, our laboratory has recently reported that the GTPases R-Ras1 and R-Ras2 are essential for oligodendrocyte survival and maturation. Hypomyelination produced by the absence of one or both proteins triggers axonal degeneration and loss of visual and motor function. However, little is known about R-Ras specificity and other possible roles that they could play in the CNS. In this work, we describe how a lack of R-Ras1 and/or R-Ras2 could not be compensated by increased expression of the closely related R-Ras3 or classical Ras. We further studied R-Ras1 and R-Ras2 expression within different CNS anatomical regions, finding that both were more abundant in less-myelinated regions, suggesting their expression in non-oligodendroglial cells. Finally, using confocal immunostaining colocalization, we report for the first time that R-Ras2 is specifically expressed in neurons. Neither microglia nor astrocytes expressed R-Ras1 or R-Ras2. These results open a new avenue for the study of neuronal R-Ras2's contribution to the process of myelination.

Absence of R-Ras1 and R-Ras2 causes mitochondrial alterations that trigger axonal degeneration in a hypomyelinating disease model.

Fast synaptic transmission in vertebrates is critically dependent on myelin for insulation and metabolic support. Myelin is produced by oligodendrocytes (OLs) that maintain multilayered membrane compartments that wrap around axonal fibers. Alterations in myelination can therefore lead to severe pathologies such as multiple sclerosis. Given that hypomyelination disorders have complex etiologies, reproducing clinical symptoms of myelin diseases from a neurological perspective in animal models has been difficult. We recently reported that R-Ras1-/- and/or R-Ras2-/- mice, which lack GTPases essential for OL survival and differentiation processes, present different degrees of hypomyelination in the central nervous system with a compounded hypomyelination in double knockout (DKO) mice. Here, we discovered that the loss of R-Ras1 and/or R-Ras2 function is associated with aberrant myelinated axons with increased numbers of mitochondria, and a disrupted mitochondrial respiration that leads to increased reactive oxygen species levels. Consequently, aberrant myelinated axons are thinner with cytoskeletal phosphorylation patterns typical of axonal degeneration processes, characteristic of myelin diseases. Although we observed different levels of hypomyelination in a single mutant mouse, the combined loss of function in DKO mice lead to a compromised axonal integrity, triggering the loss of visual function. Our findings demonstrate that the loss of R-Ras function reproduces several characteristics of hypomyelinating diseases, and we therefore propose that R-Ras1-/- and R-Ras2-/- neurological models are valuable approaches for the study of these myelin pathologies.

Essential function for the GTPase TC21 in homeostatic antigen receptor signaling.

T cell antigen receptors (TCRs) and B cell antigen receptors (BCRs) transmit low-grade signals necessary for the survival and maintenance of mature cell pools. We show here that TC21, a small GTPase encoded by Rras2, interacted constitutively with both kinds of receptors. Expression of a dominant negative TC21 mutant in T cells produced a rapid decrease in cell viability, and Rras2(-/-) mice were lymphopenic, possibly as a result of diminished homeostatic proliferation and impaired T cell and B cell survival. In contrast, TC21 was overexpressed in several human lymphoid malignancies. Finally, the p110delta catalytic subunit of phosphatidylinositol-3-OH kinase (PI(3)K) was recruited to the TCR and BCR in a TC21-dependent way. Consequently, we propose TC21 directly links antigen receptors to PI(3)K-mediated survival pathways.

R-Ras GTPases Signaling Role in Myelin Neurodegenerative Diseases.

Myelination is required for fast and efficient synaptic transmission in vertebrates. In the central nervous system, oligodendrocytes are responsible for creating myelin sheaths that isolate and protect axons, even throughout adulthood. However, when myelin is lost, the failure of remyelination mechanisms can cause neurodegenerative myelin-associated pathologies. From oligodendrocyte progenitor cells to mature myelinating oligodendrocytes, myelination is a highly complex process that involves many elements of cellular signaling, yet many of the mechanisms that coordinate it, remain unknown. In this review, we will focus on the three major pathways involved in myelination (PI3K/Akt/mTOR, ERK1/2-MAPK, and Wnt/ß-catenin) and recent advances describing the crosstalk elements which help to regulate them. In addition, we will review the tight relation between Ras GTPases and myelination processes and discuss its potential as novel elements of crosstalk between the pathways. A better understanding of the crosstalk elements orchestrating myelination mechanisms is essential to identify new potential targets to mitigate neurodegeneration.

R-Ras1 and R-Ras2 Are Essential for Oligodendrocyte Differentiation and Survival for Correct Myelination in the Central Nervous System.

Rapid and effective neural transmission of information requires correct axonal myelination. Modifications in myelination alter axonal capacity to transmit electric impulses and enable pathological conditions. In the CNS, oligodendrocytes (OLs) myelinate axons, a complex process involving various cellular interactions. However, we know little about the mechanisms that orchestrate correct myelination. Here, we demonstrate that OLs express R-Ras1 and R-Ras2. Using female and male mutant mice to delete these proteins, we found that activation of the PI3K/Akt and Erk1/2-MAPK pathways was weaker in mice lacking one or both of these GTPases, suggesting that both proteins coordinate the activity of these two pathways. Loss of R-Ras1 and/or R-Ras2 diminishes the number of OLs in major myelinated CNS tracts and increases the proportion of immature OLs. In R-Ras1-/- and R-Ras2-/--null mice, OLs show aberrant morphologies and fail to differentiate correctly into myelin-forming phenotypes. The smaller OL population and abnormal OL maturation induce severe hypomyelination, with shorter nodes of Ranvier in R-Ras1-/- and/or R-Ras2-/- mice. These defects explain the slower conduction velocity of myelinated axons that we observed in the absence of R-Ras1 and R-Ras2. Together, these results suggest that R-Ras1 and R-Ras2 are upstream elements that regulate the survival and differentiation of progenitors into OLs through the PI3K/Akt and Erk1/2-MAPK pathways for proper myelination.SIGNIFICANCE STATEMENT In this study, we show that R-Ras1 and R-Ras2 play essential roles in regulating myelination in vivo and control fundamental aspects of oligodendrocyte (OL) survival and differentiation through synergistic activation of PI3K/Akt and Erk1/2-MAPK signaling. Mice lacking R-Ras1 and/or R-Ras2 show a diminished OL population with a higher proportion of immature OLs, explaining the observed hypomyelination in main CNS tracts. In vivo electrophysiology recordings demonstrate a slower conduction velocity of nerve impulses in the absence of R-Ras1 and R-Ras2. Therefore, R-Ras1 and R-Ras2 are essential for proper axonal myelination and accurate neural transmission.

CXCR4/CXCR7 molecular involvement in neuronal and neural progenitor migration: focus in CNS repair.

In the adult brain, neural progenitor cells (NPCs) reside in the subventricular zone (SVZ) of the lateral ventricles, the dentate gyrus and the olfactory bulb. Following CNS insult, NPCs from the SVZ can migrate along the rostral migratory stream (RMS), a migration of NPCs that is directed by proinflammatory cytokines. Cells expressing CXCR4 follow a homing signal that ultimately leads to neuronal integration and CNS repair, although such molecules can also promote NPC quiescence. The ligand, SDF1 alpha (or CXCL12) is one of the chemokines secreted at sites of injury that it is known to attract NSC-derived neuroblasts, cells that express CXCR4. In function of its concentration, CXCL12 can induce different responses, promoting NPC migration at low concentrations while favoring cell adhesion via EGF and the alpha 6 integrin at high CXCL12 concentrations. However, the preclinical effectiveness of chemokines and their relationship with NPC mobilization requires further study, particularly with respect to CNS repair. NPC migration may also be affected by the release of cytokines or chemokines induced by local inflammation, through autocrine or paracrine mechanisms, as well as through erythropoietin (EPO) or nitric oxide (NO) release. CXCL12 activity requires G-coupled proteins and the availability of its ligand may be modulated by its binding to CXCR7, for which it shows a stronger affinity than for CXCR4.

R-RAS2 overexpression in tumors of the human central nervous system.

Malignant tumors of the central nervous system (CNS) are the 10th most frequent cause of cancer mortality. Despite the strong malignancy of some such tumors, oncogenic mutations are rarely found in classic members of the RAS family of small GTPases. This raises the question as to whether other RAS family members may be affected in CNS tumors, excessively activating RAS pathways. The RAS-related subfamily of GTPases is that which is most closely related to classical Ras and it currently contains 3 members: RRAS, RRAS2 and RRAS3. While R-RAS and R-RAS2 are expressed ubiquitously, R-RAS3 expression is restricted to the CNS. Significantly, both wild type and mutated RRAS2 (also known as TC21) are overexpressed in human carcinomas of the oral cavity, esophagus, stomach, skin and breast, as well as in lymphomas. Hence, we analyzed the expression of R-RAS2 mRNA and protein in a wide variety of human CNS tumors and we found the R-RAS2 protein to be overexpressed in all of the 90 CNS cancer samples studied, including glioblastomas, astrocytomas and oligodendrogliomas. However, R-Ras2 was more strongly expressed in low grade (World Health Organization grades I-II) rather than high grade (grades III-IV) tumors, suggesting that R-RAS2 is overexpressed in the early stages of malignancy. Indeed, R-RAS2 overexpression was evident in pre-malignant hyperplasias, both at the mRNA and protein levels. Nevertheless, such dramatic changes in expression were not evident for the other two subfamily members, which implies that RRAS2 is the main factor triggering neural transformation.

Pathways Involved in Remyelination after Cerebral Ischemia.

Brain ischemia, also known as ischemic stroke, occurs when there is a lack of blood supply into the brain. When an ischemic insult appears, both neurons and glial cells can react in several ways that will determine the severity and prognosis. This high heterogeneity of responses has been a major obstacle in developing effective treatments or preventive methods for stroke. Although white matter pathophysiology has not been deeply assessed in stroke, its remodelling can greatly influence the clinical outcome and the disability degree. Oligodendrocytes, the unique cell type implied in CNS myelination, are sensible to ischemic damage. Loss of myelin sheaths can compromise axon survival, so new Oligodendrocyte Precursor Cells are required to restore brain function. Stroke can, therefore, enhance oligodendrogenesis to regenerate those new oligodendrocytes that will ensheath the damaged axons. Given that myelination is a highly complex process that requires coordination of multiple pathways such as Sonic Hedgehog, RTKs or Wnt/ß-catenin, we will analyse new research highlighting their importance after brain ischemia. In addition, oligodendrocytes are not isolated cells inside the brain, but rather form part of a dynamic environment of interactions between neurons and glial cells. For this reason, we will put some context into how microglia and astrocytes react against stroke and influence oligodendrogenesis to highlight the relevance of remyelination in the ischemic brain. This will help to guide future studies to develop treatments focused on potentiating the ability of the brain to repair the damage.

Cux-2 controls the proliferation of neuronal intermediate precursors of the cortical subventricular zone.

Whereas neurons of the lower layers (VI-V) of the cerebral cortex are first born from dividing precursors at the ventricular zone, upper layer neurons (II-IV) subsequently arise from divisions of intermediate neuronal precursors at the subventricular zone (SVZ). Little is known about mechanisms that control the proliferation of SVZ neuronal precursors. We herein report that the restricted expression of the homeodomain transcription factor Cux-2 in the SVZ regulates the proliferation of intermediate neuronal precursors and the number of upper layer neurons. In Cux-2-deficient mice (Cux-2-/-), there is excessive number of upper layer neurons and selective expansion of SVZ neuronal precursors. Double-labeling experiments demonstrate that Cux-2-/- upper layer precursors reenter the cell cycle in a higher frequency than wild-type precursors. Overexpression studies indicate that Cux-2 controls cell cycle exit in a cell-autonomous manner. Analysis of Cux-1-/-; Cux-2-/- double mutant revealed that Cux-2 controls SVZ proliferation independently of Cux-1, demonstrating that this is a unique function of Cux-2, not redundant with Cux-1 activities. Our results point to Cux-2 as a key element in the control of the proliferation rates of the SVZ precursors and the number of upper cortical neurons, without altering the number of deep cortical layers.

Mechanisms of endoplasmic-reticulum export of glycine transporter-1 (GLYT1).

The GLYT1 (glycine transporter-1) regulates both glycinergic and glutamatergic neurotransmission by controlling the reuptake of glycine at synapses. Trafficking to the cell surface of GLYT1 is critical for its function. In the present paper, by using mutational analysis of the GLYT1 C-terminal domain, we identified the evolutionarily conserved motif R(575)L(576)(X(8))D(585) as being necessary for ER (endoplasmic reticulum) export. This is probably due to its capacity to bind Sec24D, a component of the COPII (coatomer coat protein II) complex. This ER export motif was active when introduced into the related GLYT2 transporter but not in the unrelated VSVG (vesicular-stomatitis virus glycoprotein)-GLYT1 protein in which this motif was mutated but was not transported to the plasma membrane, although this effect was rescued by co-expressing these mutants with wild-type GLYT1. This behaviour suggests that GLYT1 might form oligomers along the trafficking pathway. Cross-linking assays performed in rat brain synaptosomes and FRET (fluorescence resonance energy transfer) microscopy in living cells confirmed the existence of GLYT1 oligomers. In summary, we have identified a motif involved in the ER exit of GLYT1 and, in analysing the influence of this motif, we have found evidence that oligomerization is important for the trafficking of GLYT1 to the cell surface. Because this motif is conserved in the NSS (sodium- and chloride-dependent neurotransmitter transporter) family, it is possible that this finding could be extrapolated to other related transporters.

The glutamate transporter GLT1b interacts with the scaffold protein PSD-95.

The glutamate transporter (GLT1) regulates glutamate concentrations in glutamatergic synapses and it is expressed in at least two isoforms, GLT1a and GLT1b. In this work, we show that the C-terminus of GLT1b is able to interact with the PDZ domains of a number of proteins. Notably, one of them might be the scaffold protein post-synaptic density (PSD-95). GLT1b formed co-immunoprecipitable complexes with PSD-95 in solubilizated rat brain extracts, complexes that also contained NMDA receptors. Co-transfection of GLT1b, PSD-95, and NMDA receptor subunits in heterologous expression systems recapitulated in vitro the interactions among these proteins that had been observed in the rat brain extracts and revealed the importance of the GLT1b C-terminal PDZ binding motif in tethering this transporter to PSD-95. Significantly, co-expression of GLT1b and PSD-95 increased the V(max) of the transporter by decreasing the rate of GLT1b endocytosis. Moreover, GLT1b transfected into primary cultured neurons or glia formed protein clusters that co-localized with co-transfected PSD-95, clusters that in these neurons accumulated preferentially in dendritic spines. We hypothesize that the GLT1b/PSD-95 interaction, characterized here in vitro, might anchor this transporter close to the post-synaptic glutamate receptors, thereby permitting the fine regulation of glutamate concentrations in this microenvironment. This tight association might also facilitate the regulation of GLT1b through the signaling pathways initiated by the activation of glutamate receptors.

Target and untargeted GC-MS based metabolomic study of mouse optic nerve and its potential in the study of neurological visual diseases.

The optic nerve is made of highly specialized neurons and the energetic supply to their axons is crucial due to their great demand. The energy comes basically through the oxidative phosphorylation in the mitochondria, supported by glial cells metabolism. Mitochondrial dysfunction is a shared feature encountered within the optic neuropathies, including Leber's Hereditary Optic Neuropathy, Leigh's Syndrome, or Kjer's syndrome. In an effort to investigate the metabolic alterations produced within the optic nerve in a mutant mouse model of Neurological Visual Disease (NVD), a rapid, robust, and efficient one-single phase extraction methodology has been developed and validated for the GC-MS platform. Once the method was successfully validated for lactic acid and pyruvic acid as markers of an adequate optic nerve function, the protocol was applied to unveil the metabolomic signature of the wild-type mouse optic nerve. Along the chromatographic profile of the optic nerve, 94 peaks were identified and, to our knowledge, for the first time. Afterwards, a targeted metabolomics analysis was performed to quantify lactic acid and pyruvic acid in the NDV mice group (n = 8) and its corresponding wild-type (n = 8). Finally, an untargeted metabolomic study was carried out and univariate and multivariate data analyses showed 34 compounds modified in the optic nerve of the mouse with NVD mutation. Then, the metabolic reaction network of the identified metabolites highlighted alterations in the catabolism of proteins, TCA cycle, and urea cycle, reflecting a mitochondrial energetic dysfunction. Taken together, this metabolomic study has proven to be suited for the study of optic neuropathies.

The glycine transporter GLYT1 interacts with Sec3, a component of the exocyst complex.

Evidence is accumulating that the glycine transporter GLYT1 regulates NMDA receptor function by modulating the glycine concentration in glutamatergic synapses. In this article, we describe a physical and functional interaction between GLYT1 and the exocyst complex. Through a yeast two-hybrid screen to search for proteins capable of interacting with the intracellular C-terminal tail of GLYT1, we identified a protein that is highly homologous to the human and mouse Sec3 protein, a component of the exocyst complex. Pull-down and immunoprecipitation assays confirmed the physical interaction between the C-terminus of GLYT1 and Sec3. Subsequently, immunofluorescence experiments indicated that Sec3-GFP was partially recruited to the plasma membrane upon coexpression with GLYT1. The interaction of GLYT1 with exocyst components was also observed in the native rat brain since complexes immunoprecipitated from brain extracts with anti-GLYT1 antibodies contained both Sec6 and Sec8. Functional assays revealed that Sec3 increased the transporter capacity of GLYT1, suggesting that the exocyst favors insertion of GLYT1 into the plasma membrane.

Cux1 and Cux2 selectively target basal and apical dendritic compartments of layer II-III cortical neurons.

A number of recent reports implicate the differential regulation of apical and basal dendrites in autism disorders and in the higher functions of the human brain. They show that apical and basal dendrites are functionally specialized and that mechanisms regulating their development have important consequences for neuron function. The molecular identity of layer II-III neurons of the cerebral cortex is determined by the overlapping expression of Cux1 and Cux2. We previously showed that both Cux1 and Cux2 are necessary and nonredundant for normal dendrite development of layer II-III neurons. Loss of function of either gene reduced dendrite arbors, while overexpression increased dendritic complexity and suggested additive functions. We herein characterize the function of Cux1 and Cux2 in the development of apical and basal dendrites. By in vivo loss and gain of function analysis, we show that while the expression level of either Cux1 or Cux2 influences both apical and basal dendrites, they have distinct effects. Changes in Cux1 result in a marked effect on the development of the basal compartment whereas modulation of Cux2 has a stronger influence on the apical compartment. These distinct effects of Cux genes might account for the functional diversification of layer II-III neurons into different subpopulations, possibly with distinct connectivity patterns and modes of neuron response. Our data suggest that by their differential effects on basal and apical dendrites, Cux1 and Cux2 can promote the integration of layer II-III neurons in the intracortical networks in highly specific ways.

Localization of the glycine transporter GLYT1 in glutamatergic synaptic vesicles.

We have previously shown the presence of the glycine transporter GLYT1 in glutamatergic terminals of the rat brain. In this study we present immunohistochemical and biochemical evidence indicating that GLYT1 is expressed not only at the plasma membrane of glutamatergic neurons, but also at synaptic vesicles. Confocal microscopy, immunoblots analysis of a highly purified synaptic vesicle fraction and immunoisolation of synaptic vesicles with anti-synaptophysin antibodies strongly suggested the presence of GLYT1 in synaptic vesicles. Moreover, direct observation with the electron microscope of purified vesicles immunoreacted with anti-GLYT1 and colloidal gold demonstrated that about 40% of the small vesicles of the purified vesicle fraction contained GLYT1. Double labeling for GLYT1 and synaptophysin of this vesicular fraction revealed that more of ninety percent of them were synaptic vesicles. Moreover, a significant part of the GLYT1 containing vesicles (86%) also contained the vesicular glutamate transporter vGLUT1, suggesting a functional role of GLYT1 in a subpopulation of glutamatergic vesicles.

Enrichment of conserved synaptic activity-responsive element in neuronal genes predicts a coordinated response of MEF2, CREB and SRF.

A unique synaptic activity-responsive element (SARE) sequence, composed of the consensus binding sites for SRF, MEF2 and CREB, is necessary for control of transcriptional upregulation of the Arc gene in response to synaptic activity. We hypothesize that this sequence is a broad mechanism that regulates gene expression in response to synaptic activation and during plasticity; and that analysis of SARE-containing genes could identify molecular mechanisms involved in brain disorders. To search for conserved SARE sequences in the mammalian genome, we used the SynoR in silico tool, and found the SARE cluster predominantly in the regulatory regions of genes expressed specifically in the nervous system; most were related to neural development and homeostatic maintenance. Two of these SARE sequences were tested in luciferase assays and proved to promote transcription in response to neuronal activation. Supporting the predictive capacity of our candidate list, up-regulation of several SARE containing genes in response to neuronal activity was validated using external data and also experimentally using primary cortical neurons and quantitative real time RT-PCR. The list of SARE-containing genes includes several linked to mental retardation and cognitive disorders, and is significantly enriched in genes that encode mRNA targeted by FMRP (fragile X mental retardation protein). Our study thus supports the idea that SARE sequences are relevant transcriptional regulatory elements that participate in plasticity. In addition, it offers a comprehensive view of how activity-responsive transcription factors coordinate their actions and increase the selectivity of their targets. Our data suggest that analysis of SARE-containing genes will reveal yet-undescribed pathways of synaptic plasticity and additional candidate genes disrupted in mental disease.

Intrinsic programs regulating dendrites and synapses in the upper layer neurons of the cortex.

Dendrites and spines are key regulators of neuronal function often affected in cognitive disorders. Neuronal subclasses are characterized by a wide range of dendritic morphologies that aid their specific functions. However, how subclass-specific dendritic trees arise during vertebrate development remains largely unknown. We have recently reported that the restricted expression of Cux1 and Cux2 genes in the upper layers of the cerebral cortex determines the specific morphology of dendrites and spines and the function of these neurons. Since Cux genes are the vertebrate homologs of Drosophila Cut, which specifies the dendritic morphologies of certain sensory neuron populations, our findings suggest that mechanisms of dendrite differentiation are conserved between Drosophila and mammals, which had yet to be demonstrated. Importantly, we found that Cux genes not only modulate dendritic branching, but also dendritic spine morphogenesis, the functional synapse and cognition. Dendritic spine stabilization was partly mediated by direct repression of genes of the Xlr family, previously implicated in cognitive defects in a model of Turner syndrome. Hence, our work indicates that neuronal subclass specific determinants may intrinsically affect synaptic activity beyond expected. The functions of Cux1 and Cux2 were additive and complement each other to establish the final pattern of the dendritic tree and the number and strength of the synapses. This work unravels novel mechanisms of dendritogenesis and synaptogenesis and illustrates how regulating dendritic structures contributes to the specialization of upper layer neurons. It will be interesting to dissect how these mechanisms regulate cortical activity, area specialization and cognitive functions.

Cux1 and Cux2 regulate dendritic branching, spine morphology, and synapses of the upper layer neurons of the cortex.

Dendrite branching and spine formation determines the function of morphologically distinct and specialized neuronal subclasses. However, little is known about the programs instructing specific branching patterns in vertebrate neurons and whether such programs influence dendritic spines and synapses. Using knockout and knockdown studies combined with morphological, molecular, and electrophysiological analysis, we show that the homeobox Cux1 and Cux2 are intrinsic and complementary regulators of dendrite branching, spine development, and synapse formation in layer II-III neurons of the cerebral cortex. Cux genes control the number and maturation of dendritic spines partly through direct regulation of the expression of Xlr3b and Xlr4b, chromatin remodeling genes previously implicated in cognitive defects. Accordingly, abnormal dendrites and synapses in Cux2(-/-) mice correlate with reduced synaptic function and defects in working memory. These demonstrate critical roles of Cux in dendritogenesis and highlight subclass-specific mechanisms of synapse regulation that contribute to the establishment of cognitive circuits.

Cux-1 and Cux-2 control the development of Reelin expressing cortical interneurons.

Homeodomain transcription factors play important roles in the specification and differentiation of neuronal subpopulations. In the cerebral cortex, the expression patterns of Cux-1 and Cux-2 in the medial ganglionic eminence (MGE) suggest a role for these transcription factors in the development of interneurons, a heterogeneous neuronal population. In this report, we describe expression of Cux-1 and Cux-2 proteins in Reelin-secreting interneurons of the cortical plate, but not in calretinin or parvalbumin subpopulations. The role of Cux genes in the development of Reelin positive neurons was studied using Cux-1 and Cux-2 knockout mice. These experiments demonstrate that Cux-1-/-; Cux-2-/- double mutation is embryonically lethal. Although this phenotype is highly penetrant, a small proportion of mice develop to birth (P0). Analysis of these animals demonstrate that expression of Reelin is completely absent in layers II-IV of Cux-1-/-; Cux-2-/- double mutant mice, but it is not affected in the cortex of Cux-1-/- or Cux-2-/- single mutants. No Cux-1-/-; Cux-2-/- double-mutant were collected after P0. Since, GABA-ergic populations mature at late postnatal stages, this did not allow us to analyze the expression of subclass specific markers and define the affected interneuron subpopulations. Our analysis of Cux-1-/-; Cux-2-/- double mutant thus demonstrates essential yet redundant roles for Cux-1 and Cux-2 in specifying Reelin expressing cortical interneurons.