Reelin regulates cadherin function via Dab1/Rap1 to control neuronal migration and lamination in the neocortex

Molecular mechanisms of cell polarity in a range of model systems and in migrating neurons

Cell polarity is defined as the asymmetric distribution of cellular components along an axis. Most cells, from the simplest single-cell organisms to highly specialized mammalian cells, are polarized and use similar mechanisms to generate and maintain polarity. Cell polarity is important for cells to migrate, form tissues, and coordinate activities. During development of the mammalian cerebral cortex, cell polarity is essential for neurogenesis and for the migration of newborn but as-yet undifferentiated neurons. These oriented migrations include both the radial migration of excitatory projection neurons and the tangential migration of inhibitory interneurons. In this review, I will first describe the development of the cerebral cortex, as revealed at the cellular level. I will then define the core molecular mechanisms - the Par/Crb/Scrib polarity complexes, small GTPases, the actin and microtubule cytoskeletons, and phosphoinositides/PI3K signaling - that are required for asymmetric cell division, apico-basal and front-rear polarity in model systems, including C elegans zygote, Drosophila embryos and cultured mammalian cells. As I go through each core mechanism I will explain what is known about its importance in radial and tangential migration in the developing mammalian cerebral cortex.

Pathogenic DDX3X Mutations Impair RNA Metabolism and Neurogenesis during Fetal Cortical Development

De novo germline mutations in the RNA helicase DDX3X account for 1%-3% of unexplained intellectual disability (ID) cases in females and are associated with autism, brain malformations, and epilepsy. Yet, the developmental and molecular mechanisms by which DDX3X mutations impair brain function are unknown. Here, we use human and mouse genetics and cell biological and biochemical approaches to elucidate mechanisms by which pathogenic DDX3X variants disrupt brain development. We report the largest clinical cohort to date with DDX3X mutations (n = 107), demonstrating a striking correlation between recurrent dominant missense mutations, polymicrogyria, and the most severe clinical outcomes. We show that Ddx3x controls cortical development by regulating neuron generation. Severe DDX3X missense mutations profoundly disrupt RNA helicase activity, induce ectopic RNA-protein granules in neural progenitors and neurons, and impair translation. Together, these results uncover key mechanisms underlying DDX3X syndrome and highlight aberrant RNA metabolism in the pathogenesis of neurodevelopmental disease.

Expression and localization of DAB1 and Reelin during normal human kidney development

Aim

To explore the spatial and temporal expression patterns of DAB1 and Reelin in the developing and postnatal healthy human kidneys as potential determinants of kidney development.

Methods

Paraffin-embedded fetal kidney tissue between the 13/14th and 38th developmental weeks (dw) and postnatal tissue at 1.5 and 7 years were stained with DAB1 and Reelin antibodies by double immunofluorescence.

Results

During the fetal kidney development and postnatal period, DAB1 and Reelin showed specific spatial expression pattern and diverse fluorescence intensity. During the fetal period, DAB1 was strongly expressed in the distal convoluted tubules (DCT), with strong reactivity, and diversely in the proximal convoluted tubules (PCT) and glomeruli. In the postnatal period, DAB1 expression decreased. The strongest Reelin expression in early fetal stages was observed in the PCT. In the postnatal period, Reelin expression decreased dramatically in all observed structures. These two markers were colocalized during early developmental stages, mostly in PCT, DCT, and podocytes.

Conclusion

The appearance of DAB1 and Reelin during fetal kidney development confirms their potential significant role in the formation of kidney structure or function. High DAB1 expression in the DCT implies its regulatory role in tubular formation or function maintenance during development. Reelin was highly expressed in human kidneys at early fetal stages, mostly in the PCT, while at later fetal stages and postnatal period its expression decreased.

N-cadherin (Cdh2) Maintains Migration and Postmitotic Survival of Cortical Interneuron Precursors in a Cell-Type-Specific Manner

The multiplex role of cadherin-based adhesion complexes during development of pallial excitatory neurons has been thoroughly characterized. In contrast, much less is known about their function during interneuron development. Here, we report that conditional removal of N-cadherin (Cdh2) from postmitotic neuroblasts of the subpallium results in a decreased number of Gad65-GFP-positive interneurons in the adult cortex. We also found that interneuron precursor migration into the pallium was already delayed at E14. Using immunohistochemistry and TUNEL assay in the embryonic subpallium, we excluded decreased mitosis and elevated cell death as possible sources of this defect. Moreover, by analyzing the interneuron composition of the adult somatosensory cortex, we uncovered an unexpected interneuron-type-specific defect caused by Cdh2-loss. This was not due to a fate-switch between interneuron populations or altered target selection during migration. Instead, potentially due to the migration delay, part of the precursors failed to enter the cortical plate and consequently got eliminated at early postnatal stages. In summary, our results indicate that Cdh2-mediated interactions are necessary for migration and survival during the postmitotic phase of interneuron development. Furthermore, we also propose that unlike in pallial glutamatergic cells, Cdh2 is not universal, rather a cell type-specific factor during this process.

Invaders Exposed: Understanding and Targeting Tumor Cell Invasion in Diffuse Intrinsic Pontine Glioma

Diffuse Intrinsic Pontine Glioma (DIPG) is a rare, highly aggressive pediatric brain tumor that originates in the pons. DIPG is untreatable and universally fatal, with a median life expectancy of less than a year. Resection is not an option, due to the anatomical location of the tumor, radiotherapy has limited effect and no chemotherapeutic or targeted treatment approach has proven to be successful. This poor prognosis is partly attributed to the tumor's highly infiltrative diffuse and invasive spread. Thus, targeting the invasive behavior of DIPG has the potential to be of therapeutic value. In order to target DIPG invasion successfully, detailed mechanistic knowledge on the underlying drivers is required. Here, we review both DIPG tumor cell's intrinsic molecular processes and extrinsic environmental factors contributing to DIPG invasion. Importantly, DIPG represents a heterogenous disease and through advances in whole-genome sequencing, different subtypes of disease based on underlying driver mutations are now being recognized. Recent evidence also demonstrates intra-tumor heterogeneity in terms of invasiveness and implies that highly infiltrative tumor subclones can enhance the migratory behavior of neighboring cells. This might partially be mediated by “tumor microtubes,” long membranous extensions through which tumor cells connect and communicate, as well as through the secretion of extracellular vesicles. Some of the described processes involved in invasion are already being targeted in clinical trials. However, more research into the mechanisms of DIPG invasion is urgently needed and might result in the development of an effective therapy for children suffering from this devastating disease. We discuss the implications of newly discovered invasive mechanisms for therapeutic targeting and the challenges therapy development face in light of disease in the developing brain.

Two Novel Loci of RELN Associated With Antipsychotics Response in Chinese Han Population

Background

There are great individual differences in the drug responses; however, there are few prognostic drug response biomarkers available. RELN is one of the more extensively examined schizophrenia candidate genes. The purpose of this study was to determine whether RELN can affect antipsychotics response in the Chinese population. This may lead to the discovery of relevant novel drug response markers.

Methods

The unrelated 260 Chinese Han inpatients with schizophrenia were enrolled in the present study. The enrolled subjects have been prescribed antipsychotic medication during the study. A total of 15 SNPs of RELN were genotyped by MassARRAY^® platform. The association of the RELN gene with therapeutic response to antipsychotics was analyzed based on sex and age at onset.

Results

Two novel SNPs of RELN were found to be associated with antipsychotic treatment response (rs155333, p = 0.010 and rs6465938, p = 0.049) at nominal significance threshold, but not after multiple correction. Our study also revealed highly significant association of a haplotype consisting of three SNPs (rs362814-rs362626-rs2237628) with antipsychotic treatment response. Even after permutation, the p-value indicated significant association (rs362814-rs362626-rs2237628: ACT, χ² = 6.353, p = 0.0117, permuted p = 0.04). Furthermore, a novel SNP, rs2535764, was found to be associated with antipsychotic response under overdominant genetic model at a marginal significant level of 0.046 (C/T vs. C/C + T/T: p = 0.046, AIC = 314.7, BIC = 321.6).

Conclusion

Our data indicated that RELN can affect antipsychotic treatment outcomes in the Chinese population. SNPs of RELN could be used as predictive biomarkers for future personalized medicine of antipsychotic drug treatment. However, none of the three novel SNPs (rs155333, rs6465938, and rs2535764) remained significant after Bonferroni correction. Therefore, validation is needed in larger pharmacogenetic studies.

Neurogenesis, neuronal migration, and axon guidance

Development of the central nervous system (CNS) is a complex, dynamic process that involves a precisely orchestrated sequence of genetic, environmental, biochemical, and physical factors from early embryonic stages to postnatal life. Duringthe past decade, great strides have been made to unravel mechanisms underlying human CNS development through the employment of modern genetic techniques and experimental approaches. In this chapter, we review the current knowledge regarding the main developmental processes and signaling mechanisms of (i) neurogenesis, (ii) neuronal migration, and (iii) axon guidance. We discuss mechanisms related to neural stem cells proliferation, migration, terminal translocation of neuronal progenitors, and axon guidance and pathfinding. For each section, we also provide a comprehensive overview of the underlying regulatory processes, including transcriptional, posttranscriptional, and epigenetic factors, and a myriad of signaling pathways that are pivotal to determine the fate of neuronal progenitors and newly formed migrating neurons. We further highlight how impairment of this complex regulating system, such as mutations in its core components, may cause cortical malformation, epilepsy, intellectual disability, and autism in humans. A thorough understanding of normal human CNS development is thus crucial to decipher mechanisms responsible for neurodevelopmental disorders and in turn guide the development of effective and targeted therapeutic strategies.

Terminal neuron localization to the upper cortical plate is controlled by the transcription factor NEUROD2

Excitatory neurons of the mammalian cerebral cortex are organized into six functional layers characterized by unique patterns of connectivity, as well as distinctive physiological and morphological properties. Cortical layers appear after a highly regulated migration process in which cells move from the deeper, proliferative zone toward the superficial layers. Importantly, defects in this radial migration process have been implicated in neurodevelopmental and psychiatric diseases. Here we report that during the final stages of migration, transcription factor Neurogenic Differentiation 2 (Neurod2) contributes to terminal cellular localization within the cortical plate. In mice, in utero knockdown of Neurod2 resulted in reduced numbers of neurons localized to the uppermost region of the developing cortex, also termed the primitive cortical zone. Our ChIP-Seq and RNA-Seq analyses of genes regulated by NEUROD2 in the developing cortex identified a number of key target genes with known roles in Reelin signaling, a critical regulator of neuronal migration. Our focused analysis of regulation of the Reln gene, encoding the extracellular ligand REELIN, uncovered NEUROD2 binding to conserved E-box elements in multiple introns. Furthermore, we demonstrate that knockdown of NEUROD2 in primary cortical neurons resulted in a strong increase in Reln gene expression at the mRNA level, as well as a slight upregulation at the protein level. These data reveal a new role for NEUROD2 during the late stages of neuronal migration, and our analysis of its genomic targets offers new genes with potential roles in cortical lamination.

MCF2 is linked to a complex perisylvian syndrome and affects cortical lamination

The combination of congenital bilateral perisylvian syndrome (CBPS) with lower motor neuron dysfunction remains unusual and suggests a potential common genetic insult affecting basic neurodevelopmental processes. Here we identify a putatively pathogenic missense mutation in the MCF2 gene in a boy with CBPS. Using in utero electroporation to genetically manipulate cortical neurons during corticogenesis, we demonstrate that the mouse Mcf2 gene controls the embryonic migration of cortical projection neurons. Strikingly, we find that the CBPS-associated MCF2 mutation impairs cortical laminar positioning, supporting the hypothesis that alterations in the process of embryonic neuronal migration can lead to rare cases of CBPS.

Loss of Dmrt5 Affects the Formation of the Subplate and Early Corticogenesis

Dmrt5 (Dmrta2) and Dmrt3 are key regulators of cortical patterning and progenitor proliferation and differentiation. In this study, we show an altered apical to intermediate progenitor transition, with a delay in SP neurogenesis and premature birth of Ctip2⁺ cortical neurons in Dmrt5^−/− mice. In addition to the cortical progenitors, DMRT5 protein appears present in postmitotic subplate (SP) and marginal zone neurons together with some migrating cortical neurons. We observed the altered split of preplate and the reduced SP and disturbed radial migration of cortical neurons into cortical plate in Dmrt5^−/− brains and demonstrated an increase in the proportion of multipolar cells in primary neuronal cultures from Dmrt5^−/− embryonic brains. Dmrt5 affects cortical development with specific time sensitivity that we described in two conditional mice with slightly different deletion time. We only observed a transient SP phenotype at E15.5, but not by E18.5 after early (Dmrt5^lox/lox;Emx1^Cre), but not late (Dmrt5^lox/lox;Nestin^Cre) deletion of Dmrt5. SP was less disturbed in Dmrt5^lox/lox;Emx1^Cre and Dmrt3^−/− brains than in Dmrt5^−/− and affects dorsomedial cortex more than lateral and caudal cortex. Our study demonstrates a novel function of Dmrt5 in the regulation of early SP formation and radial cortical neuron migration.

Summary Statement

Our study demonstrates a novel function of Dmrt5 in regulating marginal zone and subplate formation and migration of cortical neurons to cortical plate.

Semaphorin 6A-Plexin A2/A4 Interactions with Radial Glia Regulate Migration Termination of Superficial Layer Cortical Neurons

Precise regulation of neuronal migration termination is crucial for the establishment of brain cytoarchitectures. However, little is known about how neurons terminate migration. Here we focused on interactions between migrating cortical neurons and their substrates, radial glial (RG) cells, and analyzed the role of Plexin A2 and A4 (PlxnA2/A4) receptors and their repulsive ligand, Semaphorin 6A (Sema6A), for this process. In both PlxnA2/A4 double-knockout and Sema6A mutant mice, the outermost cortical plate neurons ectopically invade layer 1 at a stage when they should reach their destinations. PlxnA2/A4 proteins are abundantly expressed on their leading processes, whereas Sema6A mRNA is enriched in RG cell somata. Cell-targeted gene expression and conditional knockouts indicate critical roles for these molecules. We hypothesize that the timely appearance of repulsive signaling mediated by Sema6A-PlxnA2/A4 weakens migrating neuron-RG cell interactions, leading to migration termination.

Alternative Splicing of Disabled-1 Controls Multipolar-to-Bipolar Transition of Migrating Neurons in the Neocortex

Multipolar-to-bipolar transition (MBT) is crucial for the neuronal migration and positioning in the neocortex. Reelin-Disabled-1 (Dab1) signaling plays a pivotal role in neuronal migration, yet how Dab1 coordinatively regulates downstream molecules to affect MBT remains unclear. We have previously found that alternative splicing produces multiple Dab1 isoforms with different tyrosine motifs and differential ability to recruit downstream effectors. Here, we report that splicing of Dab1 exons 7 and 8 and 9bc dynamically regulates the inclusion and activities of Dab1 tyrosine motifs in the neocortex. By in utero electroporation, we show that expression of Dab1 isoforms missing exons 7 and 8 or retaining exons 9bc in WT neurons resulted in neuronal migration defects with attenuated Dab1 tyrosine phosphorylation, disrupted leading process extension, and disorientated multipolar neurons in the multipolar accumulation zone. Introducing the canonical Dab1 form, but not those missing exons 7 and 8 or retaining exons 9bc, into Dab1-deficient neurons promoted MBT and rescued neuronal migration defects, suggesting that alternative splicing of Dab1 modulates the tyrosine motif switch and mediates MBT of cortical neurons. Our study reveals a critical mechanism by which Dab1 alternative splicing coordinately controls MBT and neuronal migration in a spatiotemporal manner.

DCC-Mediated Dab1 Phosphorylation Participates in the Multipolar-to-Bipolar Transition of Migrating Neurons

Newborn neurons undergo inside-out migration to their final destinations during neocortical development. Reelin-induced tyrosine phosphorylation of disabled 1 (Dab1) is a critical mechanism controlling cortical neuron migration. However, the roles of Reelin-independent phosphorylation of Dab1 remain unclear. Here, we report that deleted in colorectal carcinoma (DCC) interacts with Dab1 via its P3 domain. Netrin 1, a DCC ligand, induces Dab1 phosphorylation at Y220 and Y232. Interestingly, knockdown of DCC or truncation of its P3 domain dramatically delays neuronal migration and impairs the multipolar-to-bipolar transition of migrating neurons. Notably, the migration delay and morphological transition defects are rescued by the expression of a phospho-mimetic Dab1 or a constitutively active form of Fyn proto-oncogene (Fyn), a member of the Src-family tyrosine kinases that effectively induces Dab1 phosphorylation. Collectively, these findings illustrate a DCC-Dab1 interaction that ensures proper neuronal migration during neocortical development.

Enhanced cell-cell contact stability and decreased N-cadherin-mediated migration upon fibroblast growth factor receptor-N-cadherin cross talk

N-cadherin adhesion has been reported to enhance cancer and neuronal cell migration either by mediating actomyosin-based force transduction or initiating fibroblast growth factor receptor (FGFR)-dependent biochemical signalling. Here we show that FGFR1 reduces N-cadherin-mediated cell migration. Both proteins are co-stabilised at cell-cell contacts through direct interaction. As a consequence, cell adhesion is strengthened, limiting the migration of cells on N-cadherin. Both the inhibition of migration and the stabilisation of cell adhesions require the FGFR activity stimulated by N-cadherin engagement. FGFR1 stabilises N-cadherin at the cell membrane through a pathway involving Src and p120. Moreover, FGFR1 stimulates the anchoring of N-cadherin to actin. We found that the migratory behaviour of cells depends on an optimum balance between FGFR-regulated N-cadherin adhesion and actin dynamics. Based on these findings we propose a positive feed-back loop between N-cadherin and FGFR at adhesion sites limiting N-cadherin-based single-cell migration.

Unravelling the genetic basis of schizophrenia and bipolar disorder with GWAS: A systematic review

OBJECTIVES

To systematically review findings of GWAS in schizophrenia (SZ) and in bipolar disorder (BD); and to interpret findings, with a focus on identifying independent replications.

METHOD

PubMed search, selection and review of all independent GWAS in SZ or BD, published since March 2011, i.e. studies using non-overlapping samples within each article, between articles, and with those of the previous review (Li et al., 2012).

RESULTS

From the 22 GWAS included in this review, the genetic associations surviving standard GWAS-significance were for genetic markers in the regions of ACSL3/KCNE4, ADCY2, AMBRA1, ANK3, BRP44, DTL, FBLN1, HHAT, INTS7, LOC392301, LOC645434/NMBR, LOC729457, LRRFIP1, LSM1, MDM1, MHC, MIR2113/POU3F2, NDST3, NKAPL, ODZ4, PGBD1, RENBP, TRANK1, TSPAN18, TWIST2, UGT1A1/HJURP, WHSC1L1/FGFR1 and ZKSCAN4. All genes implicated across both reviews are discussed in terms of their function and implication in neuropsychiatry.

CONCLUSION

Taking all GWAS to date into account, AMBRA1, ANK3, ARNTL, CDH13, EFHD1 (albeit with different alleles), MHC, PLXNA2 and UGT1A1 have been implicated in either disorder in at least two reportedly non-overlapping samples. Additionally, evidence for a SZ/BD common genetic basis is most strongly supported by the implication of ANK3, NDST3, and PLXNA2.

Spatiotemporal Regulation of Rho GTPases in Neuronal Migration

Neuronal migration is essential for the orchestration of brain development and involves several contiguous steps: interkinetic nuclear movement (INM), multipolar–bipolar transition, locomotion, and translocation. Growing evidence suggests that Rho GTPases, including RhoA, Rac, Cdc42, and the atypical Rnd members, play critical roles in neuronal migration by regulating both actin and microtubule cytoskeletal components. This review focuses on the spatiotemporal-specific regulation of Rho GTPases as well as their regulators and effectors in distinct steps during the neuronal migration process. Their roles in bridging extracellular signals and cytoskeletal dynamics to provide optimal structural support to the migrating neurons will also be discussed.

Expression of the guanine nucleotide exchange factor, RAPGEF5, during mouse and human embryogenesis

Rap GTPases mediate fundamental cellular processes, including cell adhesion, migration and intracellular signal transduction. The subcellular activity of these GTPases is regulated by dedicated activators (guanine nucleotide exchange factors, GEFs) and deactivators (GTPase-activating proteins, GAPs). RAPGEF5 is a potent activator of Rap proteins and mutations in RAPGEF5 have been linked to both neurological disorders and congenital heart disease. In the frog model, Xenopus tropicalis, Rapgef5 is a critical regulator of the canonical Wnt signalling pathway and is required for normal gastrulation and correct establishment of the left-right body axis. However, requirements for RAPGEF5 in other developmental contexts, and in mammalian embryogenesis in particular, remain undefined. Here, we describe RAPGEF5 mRNA expression patterns during mouse (E9.5 - E16.5) and human (Carnegie stage 21) development, as an initial step towards better understanding its developmental functions.

Csmd2 Is a Synaptic Transmembrane Protein that Interacts with PSD-95 and Is Required for Neuronal Maturation

Mutations and copy number variants of the CUB and Sushi multiple domains 2 (CSMD2) gene are associated with neuropsychiatric disease. CSMD2 encodes a single-pass transmembrane protein with a large extracellular domain comprising repeats of CUB and Sushi domains. High expression of CSMD2 in the developing and mature brain suggests possible roles in neuron development or function, but the cellular functions of CSMD2 are not known. In this study, we show that mouse Csmd2 is expressed in excitatory and inhibitory neurons in the forebrain. Csmd2 protein exhibits a somatodendritic localization in the neocortex and hippocampus, with smaller puncta localizing to the neuropil. Using immunohistochemical and biochemical methods, we demonstrate that Csmd2 localizes to dendritic spines and is enriched in the postsynaptic density (PSD). Accordingly, we show that the cytoplasmic tail domain of Csmd2 interacts with synaptic scaffolding proteins of the membrane-associated guanylate kinase (MAGUK) family. The association between Csmd2 and MAGUK member PSD-95 is dependent on a PDZ-binding domain on the Csmd2 tail, which is also required for synaptic targeting of Csmd2. Finally, we show that knock-down of Csmd2 expression in hippocampal neuron cultures results in reduced complexity of dendritic arbors and deficits in dendritic spine density. Knock-down of Csmd2 in immature developing neurons results in reduced filopodia density, whereas Csmd2 knock-down in mature neurons causes significant reductions in dendritic spine density and dendrite complexity. Together, these results point toward a function for Csmd2 in development and maintenance of dendrites and synapses, which may account for its association with certain psychiatric disorders.

Cdk5 regulates N-cadherin-dependent neuronal migration during cortical development

Cyclin-dependent kinase 5 (Cdk5) controls neuronal migration in the developing cortex when multipolar newborn neurons transform to become bipolar. However, by which mechanisms Cdk5 controls cell adhesion in migrating neurons are not fully understood. In this study, we examined the functional interaction between Cdk5 and N-cadherin (Ncad) in newborn neurons when they undergo the multipolar to bipolar transition in the intermediate zone (IZ). Detailed expression analysis revealed that both Cdk5 and Ncad were present in GFP-electroporated migrating neurons in the IZ. Misexpression of dominant negative Cdk5 into the embryonic brains stalled neuronal locomotion in the lower IZ in which arrested cells were round or multipolar. When Ncad was co-introduced with Cdk5DN, however, cells continue to migrate into the cortical plate (CP) and migrating neurons acquired typical bipolar morphology with a pia-directed leading process. Similarly, downregulation of CDK5 resulted in lesser aggregation ability, reversed by the expression of Ncad in vitro. Down-regulation of activity or protein level of CDK5 did not alter the total amount of NCAD proteins but lowered its surface expression in cells. Lastly, expression of CDK5 and NCAD overlapped in the IZ of the human fetal cortex, indicating that the role of Cdk5 and Ncad in neuronal migration is evolutionarily conserved.

Prenatal and early life diesel exhaust exposure disrupts cortical lamina organization: Evidence for a reelin-related pathogenic pathway induced by interleukin-6

Several epidemiological studies have shown associations between developmental exposure to traffic-related air pollution and increased risk for autism spectrum disorders (ASD), a spectrum of neurodevelopmental disorders with increasing prevalence rate in the United States. Though animal studies have provided support for these associations, little is known regarding possible underlying mechanisms. In a previous study we found that exposure of C57BL/6J mice of both sexes to environmentally relevant levels (250-300 µg/m³) of diesel exhaust (DE) from embryonic day 0 to postnatal day 21 (E0 to PND21) caused significant changes in all three characteristic behavioral domains of ASD in the offspring. In the present study we investigated a potential mechanistic pathway that may be of relevance for ASD-like changes associated with developmental DE exposure. Using the same DE exposure protocol (250-300 µg/m³ DE from E0 to PND21) several molecular markers were examined in the brains of male and female mice at PND3, 21, and 60. Exposure to DE as above increased levels of interleukin-6 (IL-6) in placenta and in neonatal brain. The JAK2/STAT3 pathway, a target for IL-6, was activated by STAT3 phosphorylation, and the expression of DNA methyltransferase 1 (DNMT1), a STAT3 target gene, was increased in DE-exposed neonatal brain. DNMT1 has been reported to down-regulate expression of reelin (RELN), an extracellular matrix glycoprotein important in regulating the processes of neuronal migration. RELN is considered an important modulator for ASD, since there are several polymorphisms in this gene linked to the disease, and since lower levels of RELN have been reported in brains of ASD patients. We observed decreased RELN expression in brains of the DE-exposed mice at PND3. Since disorganized patches in the prefrontal cortex have been reported in ASD patients and disrupted cortical organization has been found in RELN-deficient mice, we also assessed cortical organization, by labeling cells expressing the lamina-specific-markers RELN and calretinin. In DE-exposed mice we found increased cell density in deeper cortex (lamina layers VI-IV) for cells expressing either RELN or calretinin. These findings demonstrate that developmental DE exposure is associated with subtle disorganization of the cerebral cortex at PND60, and suggest a pathway involving IL-6, STAT3, and DNMT1 leading to downregulation of RELN expression that could be contributing to this long-lasting disruption in cortical laminar organization.

Fetal exposure to high levels of maternal glucocorticoids alters reelin signaling in the prefrontal cortex of rat pups

Maternal stress (MS) is associated with various neuropsychiatric disorders and cognitive impairment in the offspring. However, it is unclear how early life stress alters the pup's brain development and how it contributes to the pathology of neuropsychiatric disorders later in life. Reelin is a large extracellular matrix glycoprotein that plays essential roles in early brain development such as neural migration, synaptic development, and maturation. Dysregulation of reelin and its signaling proteins is associated with the emergence of neuropsychiatric disorders in adulthood. This study examined the effect of repeated maternal Carbenoxolone (CBX) injection during late gestation on reelin signaling in the prefrontal cortex (PFC) of rat pups. CBX is a selective 11β-HSD2 enzyme inhibitor that promotes the direct transfer of maternal corticosteroids (CORT) to the fetus. Therefore, treatment with CBX can mimic the animal model of early life exposure to high levels of maternal stress hormone. In this study, pregnant rats were injected daily with either saline or CBX during gestation day (GD) 14-21, and the levels of reelin and its signaling proteins were examined in the PFC of rat pups at different postnatal age from P0-P21. The main result of this study is the repeated maternal CBX injections during GD14-21 acutely increase reln mRNA and protein expression in the PFC of rat pups at birth (P0) and follow by a significant decrease during P7-P14. The treatment also causes long term decreases in the amount of VLDLR and Dab1 which are the downstream signaling proteins for the reelin pathway, at least until P21. Our results indicated that fetal exposure to high levels of maternal CORT interferes with reelin signaling which might have profound effects on cortical development associated with neuropsychiatric disorders later in life.

The Mechanism of Rap1 Regulates N-cadherin to Control Neuronal Migration

Rap1 and N-cadherin regulate glia-independent translocation of cortical neurons. It remains unclear how Rap1 regulates N-cadherin-mediated neuronal migration. Here, we overexpressed Rap1gap in mouse brains (embryonic day 16) to inactivate Rap1, and observed that neurons did not migrate to the outer layer. We confirmed that Rap1 was involved in the regulation of late neurons in vivo. Rap1gap overexpression and Rap1 suppression in CHO cells decreased the expression of cytoskeletal proteins such as tubulin. Changes in the expression of cell morphology regulators, such as N-cadherin and β-catenin, were also observed. Inhibition of N-cadherin in mouse brains prevented neuronal migration to the outer layer. The morphology of CHO cells was changed after overexpression of Rap1gap. We propose that Rap1 regulates the expression of N-cadherin during embryonic development, which affects β-catenin expression. Beta-catenin in turn regulates cytoskeletal protein expression, ultimately affecting neuronal morphology and migration.

Lmx1a drives Cux2 expression in the cortical hem through activation of a conserved intronic enhancer

During neocortical development, neurons are produced by a diverse pool of neural progenitors. A subset of progenitors express the Cux2 gene and are fate restricted to produce certain neuronal subtypes; however, the upstream pathways that specify these progenitor fates remain unknown. To uncover the transcriptional networks that regulate Cux2 expression in the forebrain, we characterized a conserved Cux2 enhancer that recapitulates Cux2 expression specifically in the cortical hem. Using a bioinformatic approach, we identified putative transcription factor (TF)-binding sites for cortical hem-patterning TFs. We found that the homeobox TF Lmx1a can activate the Cux2 enhancer in vitro Furthermore, we showed that Lmx1a-binding sites were required for enhancer activity in the cortical hem in vivo Mis-expression of Lmx1a in hippocampal progenitors caused an increase in Cux2 enhancer activity outside the cortical hem. Finally, we compared several human enhancers with cortical hem-restricted activity and found that recurrent Lmx1a-binding sites are a top shared feature. Uncovering the network of TFs involved in regulating Cux2 expression will increase our understanding of the mechanisms pivotal in establishing Cux2 lineage fates in the developing forebrain.

Differential Action of Reelin on Oligomerization of ApoER2 and VLDL Receptor in HEK293 Cells Assessed by Time-Resolved Anisotropy and Fluorescence Lifetime Imaging Microscopy

The canonical Reelin signaling cascade regulates correct neuronal layering during embryonic brain development. Details of this pathway are still not fully understood since the participating components are highly variable and create a complex mixture of interacting molecules. Reelin is proteolytically processed resulting in five different fragments some of which carrying the binding site for two different but highly homologous receptors, apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR). The receptors are expressed in different variants in different areas of the developing brain. Binding of Reelin and its central fragment to the receptors results in phosphorylation of the intracellular adapter disabled-1 (Dab1) in neurons. Here, we studied the changes of the arrangement of the receptors upon Reelin binding and its central fragment at the molecular level in human embryonic kidney 293 (HEK293) cells by time-resolved anisotropy and fluorescence lifetime imaging microscopy (FLIM). In the off-state of the pathway ApoER2 and VLDLR form homo or hetero-di/oligomers. Upon binding of full length Reelin ApoER2 and VLDLR homo-oligomers are rearranged to higher order receptor clusters which leads to Dab1 phosphorylation. When the central fragment of Reelin binds to the receptors the cluster size of homo-oligomers is not affected and Dab1 is not phosphorylated. Hetero-oligomerization, however, can be induced, but does not lead to Dab1 phosphorylation. Cells expressing only ApoER2 or VLDLR change their shape when stimulated with the central fragment. Cells expressing ApoER2 produce filopodia/lamellipodia and cell size increases, whereas VLDLR-expressing cells decrease in size. These findings demonstrate that the primary event in the canonical Reelin pathway is the rearrangement of preformed receptor homo-oligomers to higher order clusters. In addition the possibility of yet another signaling mechanism which is mediated by the central Reelin fragment independent of Dab1 phosphorylation became apparent.

Correct setup of the substantia nigra requires Reelin-mediated fast, laterally-directed migration of dopaminergic neurons

Midbrain dopaminergic (mDA) neurons migrate to form the laterally-located substantia nigra pars compacta (SN) and medially-located ventral tegmental area (VTA), but little is known about the underlying cellular and molecular processes. Here we visualize the dynamic cell morphologies of tangentially migrating SN-mDA neurons in 3D and identify two distinct migration modes. Slow migration is the default mode in SN-mDA neurons, while fast, laterally-directed migration occurs infrequently and is strongly associated with bipolar cell morphology. Tangential migration of SN-mDA neurons is altered in absence of Reelin signaling, but it is unclear whether Reelin acts directly on migrating SN-mDA neurons and how it affects their cell morphology and migratory behavior. By specifically inactivating Reelin signaling in mDA neurons we demonstrate its direct role in SN-mDA tangential migration. Reelin promotes laterally-biased movements in mDA neurons during their slow migration mode, stabilizes leading process morphology and increases the probability of fast, laterally-directed migration.

Distinct roles for dynein light intermediate chains in neurogenesis, migration, and terminal somal translocation

Cytoplasmic dynein participates in multiple aspects of neocortical development. These include neural progenitor proliferation, morphogenesis, and neuronal migration. The cytoplasmic dynein light intermediate chains (LICs) 1 and 2 are cargo-binding subunits, though their relative roles are not well understood. Here, we used in utero electroporation of shRNAs or LIC functional domains to determine the relative contributions of the two LICs in the developing rat brain. We find that LIC1, through BicD2, is required for apical nuclear migration in neural progenitors. In newborn neurons, we observe specific roles for LIC1 in the multipolar to bipolar transition and glial-guided neuronal migration. In contrast, LIC2 contributes to a novel dynein role in the little-studied mode of migration, terminal somal translocation. Together, our results provide novel insight into the LICs' unique functions during brain development and dynein regulation overall.

Drebrin-like (Dbnl) Controls Neuronal Migration via Regulating N-Cadherin Expression in the Developing Cerebral Cortex

The actin cytoskeleton is crucial for neuronal migration in the mammalian developing cerebral cortex. The adaptor protein Drebrin-like (Dbnl) plays important roles in reorganization of the actin cytoskeleton, dendrite formation, and endocytosis by interacting with F-actin, cobl, and dynamin. Although Dbnl is known to be expressed in the brain, the functions of this molecule during brain development are largely unknown. In this study, to examine the roles of Dbnl in the developing cerebral cortex, we conducted experiments using mice of both sexes with knockdown of Dbnl, effected by in utero electroporation, in the migrating neurons of the embryonic cortex. Time-lapse imaging of the Dbnl-knockdown neurons revealed that the presence of Dbnl is a prerequisite for appropriate formation of processes in the multipolar neurons in the multipolar cell accumulation zone or the deep part of the subventricular zone, and for neuronal polarization and entry into the cortical plate. We found that Dbnl knockdown decreased the amount of N-cadherin protein expressed on the plasma membrane of the cortical neurons. The defect in neuronal migration caused by Dbnl knockdown was rescued by moderate overexpression of N-cadherin and αN-catenin or by transfection of the phospho-mimic form (Y337E, Y347E), but not the phospho-resistant form (Y337F, Y347F), of Dbnl. These results suggest that Dbnl controls neuronal migration, neuronal multipolar morphology, and cell polarity in the developing cerebral cortex via regulating N-cadherin expression.SIGNIFICANCE STATEMENT Disruption of neuronal migration can cause neuronal disorders, such as lissencephaly and subcortical band heterotopia. During cerebral cortical development, the actin cytoskeleton plays a key role in neuronal migration; however, the mechanisms of regulation of neuronal migration by the actin cytoskeleton still remain unclear. Herein, we report that the novel protein Dbnl, an actin-binding protein, controls multiple events during neuronal migration in the developing mouse cerebral cortex. We also showed that this regulation is mediated by phosphorylation of Dbnl at tyrosine residues 337 and 347 and αN-catenin/N-cadherin, suggesting that the Dbnl-αN-catenin/N-cadherin pathway is important for neuronal migration in the developing cortex.

FOXG1 Orchestrates Neocortical Organization and Cortico-Cortical Connections

The hallmarks of FOXG1 syndrome, which results from mutations in a single FOXG1 allele, include cortical atrophy and corpus callosum agenesis. However, the etiology for these structural deficits and the role of FOXG1 in cortical projection neurons remain unclear. Here we demonstrate that Foxg1 in pyramidal neurons plays essential roles in establishing cortical layers and the identity and axon trajectory of callosal projection neurons. The neuron-specific actions of Foxg1 are achieved by forming a transcription complex with Rp58. The Foxg1-Rp58 complex directly binds and represses Robo1, Slit3, and Reelin genes, the key regulators of callosal axon guidance and neuronal migration. We also found that inactivation of one Foxg1 allele specifically in cortical neurons was sufficient to cause cerebral cortical hypoplasia and corpus callosum agenesis. Together, this study reveals a novel gene regulatory pathway that specifies neuronal characteristics during cerebral cortex development and sheds light on the etiology of FOXG1 syndrome. VIDEO ABSTRACT.

Endothelial Dab1 signaling orchestrates neuro-glia-vessel communication in the central nervous system

The architecture of the neurovascular unit (NVU) is controlled by the communication of neurons, glia, and vascular cells. We found that the neuronal guidance cue reelin possesses proangiogenic activities that ensure the communication of endothelial cells (ECs) with the glia to control neuronal migration and the establishment of the blood-brain barrier in the mouse brain. Apolipoprotein E receptor 2 (ApoER2) and Disabled1 (Dab1) expressed in ECs are required for vascularization of the retina and the cerebral cortex. Deletion of Dab1 in ECs leads to a reduced secretion of laminin-α4 and decreased activation of integrin-β1 in glial cells, which in turn control neuronal migration and barrier properties of the NVU. Thus, reelin signaling in the endothelium is an instructive and integrative cue essential for neuro-glia-vascular communication.

The Sema3A receptor Plexin-A1 suppresses supernumerary axons through Rap1 GTPases

The highly conserved Rap1 GTPases perform essential functions during neuronal development. They are required for the polarity of neuronal progenitors and neurons as well as for neuronal migration in the embryonic brain. Neuronal polarization and axon formation depend on the precise temporal and spatial regulation of Rap1 activity by guanine nucleotide exchange factors (GEFs) and GTPases-activating proteins (GAPs). Several Rap1 GEFs have been identified that direct the formation of axons during cortical and hippocampal development in vivo and in cultured neurons. However little is known about the GAPs that limit the activity of Rap1 GTPases during neuronal development. Here we investigate the function of Sema3A and Plexin-A1 as a regulator of Rap1 GTPases during the polarization of hippocampal neurons. Sema3A was shown to suppress axon formation when neurons are cultured on a patterned substrate. Plexin-A1 functions as the signal-transducing subunit of receptors for Sema3A and displays GAP activity for Rap1 GTPases. We show that Sema3A and Plexin-A1 suppress the formation of supernumerary axons in cultured neurons, which depends on Rap1 GTPases.

N-cadherin provides a cis and trans ligand for astrotactin that functions in glial-guided neuronal migration

Prior studies demonstrate that astrotactin (ASTN1) provides a neuronal receptor for glial-guided CNS migration. Here we report that ASTN1 binds N-cadherin (CDH2) and that the ASTN1:CDH2 interaction supports cell-cell adhesion. To test the function of ASTN1:CDH2 binding in glial-guided neuronal migration, we generated a conditional loss of Cdh2 in cerebellar granule cells and in glia. Granule cell migration was slowed in cerebellar slice cultures after a conditional loss of neuronal Cdh2, and more severe migration defects occurred after a conditional loss of glial Cdh2 Expression in granule cells of a mutant form of ASTN1 that does not bind CDH2 also slowed migration. Moreover, in vitro chimeras of granule cells and glia showed impaired neuron-glia attachment in the absence of glial, but not neuronal, Cdh2 Thus, cis and trans bindings of ASTN1 to neuronal and glial CDH2 form an asymmetric neuron-glial bridge complex that promotes glial-guided neuronal migration.

Effects of N-cadherin on neuronal migration during chicken optic tectum development

N-cadherin, a member of the cadherin family, plays an important role in neural development. In addition, N-cadherin has been reported to be crucial in neuronal migration, axonal outgrowth, and axonal path-finding. However, the mechanism underlying the effects of N-cadherin in neuronal migration is not entirely clear. In this study, we investigated the overexpression or knockdown of N-cadherin in the optic tectum during chicken embryo development, and then analyzed the effect of N-cadherin on neuronal migration. The results showed that compared with the control group, in the N-cadherin knockdown group, the neuronal migration of the optic tectum was significantly affected and could not arrive at destination. The stratum griseum central layer of the optic tectum mainly includes multipolar neurons, which could not be formed after the knockdown of N-cadherin, and more neurons form the bipolar or monopolar neurons compared with the control group. Compared with the control group, more cells stayed in the neuroepithelium layer. The axonal length in the optic tectum was significantly (P < 0.001) shorter in the N-cadherin knockdown group than in the control group. These results reveal that the knockdown of N-cadherin mainly affects the length of axons and formation of multipolar neurons in the development of the chicken optic tectum, which eventually results in the inhibition of neuronal migration.

Adult neurogenesis and its role in brain injury and psychiatric diseases

In the adult mammalian brain, neural stem cells (NSCs) reside in two neurogenic regions, the walls of the lateral ventricles, and the subgranular zone of the hippocampus, which generate new neurons for the olfactory bulb and dentate gyrus, respectively. These adult NSCs retain their self-renewal ability and capacity to differentiate into neurons and glia as demonstrated by in vitro studies. However, their contribution to tissue repair in disease and injury is limited, lending credence to the claim by prominent neuropathologist Ramón y Cajal that 'once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably'. However, recent progress toward understanding the fundamental biology of adult NSCs and their role in pathological conditions has provided new insight into the potential therapeutic utility of endogenous NSCs. In this short review, we highlight two topics: the altered behavior of NSCs after brain damage and the dysfunction of NSCs and oligodendrocyte precursor cells, another type of undifferentiated cell in the adult brain, in mood affective disorders.

ApoER2 Controls Not Only Neuronal Migration in the Intermediate Zone But Also Termination of Migration in the Developing Cerebral Cortex

Neuronal migration contributes to the establishment of mammalian brain. The extracellular protein Reelin sends signals to various downstream molecules by binding to its receptors, the apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor and exerts essential roles in the neuronal migration and formation of the layered neocortex. However, the cellular and molecular functions of Reelin signaling in the cortical development are not yet fully understood. Here, to gain insight into the role of Reelin signaling during cortical development, we examined the migratory behavior of Apoer2-deficient neurons in the developing brain. Stage-specific labeling of newborn neurons revealed that the neurons ectopically invaded the marginal zone (MZ) and that neuronal migration of both early- and late-born neurons was disrupted in the intermediate zone (IZ) in the Apoer2 KO mice. Rescue experiments showed that ApoER2 functions both in cell-autonomous and noncell-autonomous manners, that Rap1, integrin, and Akt are involved in the termination of migration beneath the MZ, and that Akt also controls neuronal migration in the IZ downstream of ApoER2. These data indicate that ApoER2 controls multiple processes in neuronal migration, including the early stage of radial migration and termination of migration beneath the MZ in the developing neocortex.

Coactosin-like protein 1 inhibits neuronal migration during mouse corticogenesis

Coactosin-like protein 1 (Cotl1), a member of the actin-depolymerizing factor (ADF)/cofilin family, was first purified from a soluble fraction of Dictyostelium discoideum cells. Neuronal migration requires cytoskeletal remodeling and actin regulation. Although Cotl1 strongly binds to F-actin, the role of Cotl1 in neuronal migration remains undescribed. In this study, we revealed that Cotl1 overexpression impaired migrationof both early- and late-born neurons during mouse corticogenesis. Moreover, Cotl1 overexpression delayed, rather than blocked, neuronal migration in late-born neurons. Cotl1 expression disturbed the morphology of migrating neurons, lengthening the leading processes. This study is the first to investigate the function of Cotl1, and the results indicate that Cotl1 is involved in the regulation of neuronal migration and morphogenesis.

Rare RELN variants affect Reelin-DAB1 signal transduction in autism spectrum disorder

The Reelin-DAB1 signaling pathway plays a crucial role in regulating neuronal migration and synapse function. Although many rare heterozygous variants in the Reelin gene (RELN) have been identified in patients with autism spectrum disorder (ASD), most variants are still of unknown clinical significance. Also, genetic data suggest that heterozygous variants in RELN alone appear to be insufficient to cause ASD. Here, we describe the identification and functional characterization of rare compound heterozygous missense variants in RELN in a patient with ASD in whom we have previously reported hyperfunctional mTORC1 signaling of yet unknown etiology. Using iPSC-derived neural progenitor cells (NPCs) from this patient, we provide experimental evidence that the identified variants are deleterious and lead to diminished Reelin secretion and impaired Reelin-DAB1 signal transduction. Also, our results suggest that mTORC1 pathway overactivation may function as a second hit event contributing to downregulation of the Reelin-DAB1 cascade in patient-derived NPCs, and that inhibition of mTORC1 by rapamycin attenuates Reelin-DAB1 signaling impairment. Taken together, our findings point to an abnormal interplay between Reelin-DAB1 and mTORC1 networks in nonsyndromic ASD.

Perturbed Developmental Serotonin Signaling Affects Prefrontal Catecholaminergic Innervation and Cortical Integrity

Proper development of the medial prefrontal cortex (mPFC), crucial for correct cognitive functioning, requires projections from, among others, the serotonergic (5-HT) and catecholaminergic systems, but it is unclear how these systems influence each other during development. Here, we describe the parallel development of the 5-HT and catecholaminergic prefrontal projection systems in rat and demonstrate a close engagement of both systems in the proximity of Cajal-Retzius cells. We further show that in the absence of the 5-HT transporter (5-HTT), not only the developing 5-HT but also the catecholaminergic system, including their projections towards the mPFC, are affected. In addition, the layer identity of the mPFC neurons and reelin-positive interneuron number and integration are altered in the absence of the 5-HTT. Together, our data demonstrate a functional interplay between the developing mPFC 5-HT and catecholaminergic systems, and call for a holistic approach in studying neurotransmitter systems-specific developmental consequences for adult behavior, to eventually allow the design of better treatment strategies for neuropsychiatric disorders.

Estradiol and the Development of the Cerebral Cortex: An Unexpected Role?

The cerebral cortex undergoes rapid folding in an "inside-outside" manner during embryonic development resulting in the establishment of six discrete cortical layers. This unique cytoarchitecture occurs via the coordinated processes of neurogenesis and cell migration. In addition, these processes are fine-tuned by a number of extracellular cues, which exert their effects by regulating intracellular signaling pathways. Interestingly, multiple brain regions have been shown to develop in a sexually dimorphic manner. In many cases, estrogens have been demonstrated to play an integral role in mediating these sexual dimorphisms in both males and females. Indeed, 17β-estradiol, the main biologically active estrogen, plays a critical organizational role during early brain development and has been shown to be pivotal in the sexually dimorphic development and regulation of the neural circuitry underlying sex-typical and socio-aggressive behaviors in males and females. However, whether and how estrogens, and 17β-estradiol in particular, regulate the development of the cerebral cortex is less well understood. In this review, we outline the evidence that estrogens are not only present but are engaged and regulate molecular machinery required for the fine-tuning of processes central to the cortex. We discuss how estrogens are thought to regulate the function of key molecular players and signaling pathways involved in corticogenesis, and where possible, highlight if these processes are sexually dimorphic. Collectively, we hope this review highlights the need to consider how estrogens may influence the development of brain regions directly involved in the sex-typical and socio-aggressive behaviors as well as development of sexually dimorphic regions such as the cerebral cortex.

Comprehensive behavioral analysis of mice deficient in Rapgef2 and Rapgef6, a subfamily of guanine nucleotide exchange factors for Rap small GTPases possessing the Ras/Rap-associating domain

Rapgef2 and Rapgef6 define a subfamily of guanine nucleotide exchange factors for Rap small GTPases, characterized by the possession of the Ras/Rap-associating domain. Previous genomic analyses suggested their possible involvement in the etiology of schizophrenia. We recently demonstrated the development of an ectopic cortical mass (ECM), which resembles the human subcortical band heterotopia, in the dorsal telencephalon-specific Rapgef2 conditional knockout (Rapgef2-cKO) brains. Additional knockout of Rapgef6 in Rapgef2-cKO mice resulted in gross enlargement of the ECM whereas knockout of Rapgef6 alone (Rapgef6-KO) had no discernible effect on the brain morphology. Here, we performed a battery of behavioral tests to examine the effects of Rapgef2 or Rapgef6 deficiency on higher brain functions. Rapgef2-cKO mice exhibited hyperlocomotion phenotypes. They showed decreased anxiety-like behavior in the elevated plus maze and the open-field tests as well as increased depression-like behavior in the Porsolt forced swim and tail suspension tests. They also exhibited increased sociability especially in novel environments. They showed defects in cognitive function as evidenced by reduced learning ability in the Barnes circular maze test and by impaired working memory in the T maze tests. In contrast, although Rapgef6 and Rapgef2 share similarities in biochemical roles, Rapgef6-KO mice exhibited mild behavioral abnormalities detected with a number of behavioral tests, such as hyperlocomotion phenotype in the open-field test and the social interaction test with a novel environment and working-memory defects in the T-maze test. In conclusion, although there were differences in their brain morphology and the magnitude of the behavioral abnormalities, Rapgef2-cKO mice and Rapgef6-KO mice exhibited hyperlocomotion phenotype and working-memory defect, both of which could be recognized as schizophrenia-like behavior.

The Dorsal Wave of Neocortical Oligodendrogenesis Begins Embryonically and Requires Multiple Sources of Sonic Hedgehog

Neural progenitor cells in the developing dorsal forebrain give rise to excitatory neurons, astrocytes, and oligodendrocytes for the neocortex. While we are starting to gain a better understanding about the mechanisms that direct the formation of neocortical neurons and astrocytes, far less is known about the molecular mechanisms that instruct dorsal forebrain progenitors to make oligodendrocytes. In this study, we show that Sonic hedgehog (Shh) signaling is required in dorsal progenitors for their late embryonic transition to oligodendrogenesis. Using genetic lineage-tracing in mice of both sexes, we demonstrate that most oligodendrocytes in the embryonic neocortex derive from Emx1⁺ dorsal forebrain progenitors. Deletion of the Shh signaling effector Smo specifically in Emx1⁺ progenitors led to significantly decreased oligodendrocyte numbers in the embryonic neocortex. Conversely, knock-out of the Shh antagonist Sufu was sufficient to increase neocortical oligodendrogenesis. Using conditional knock-out strategies, we found that Shh ligand is supplied to dorsal progenitors through multiple sources. Loss of Shh from Dlx5/6⁺ interneurons caused a significant reduction in oligodendrocytes in the embryonic neocortex. This phenotype was identical to that observed upon Shh deletion from the entire CNS using Nestin-Cre, indicating that interneurons migrating into the neocortex from the subpallium are the primary neural source of Shh for dorsal oligodendrogenesis. Additionally, deletion of Shh from migrating interneurons together with the choroid plexus epithelium led to a more severe loss of oligodendrocytes, suggesting that the choroid plexus is an important non-neural source of Shh ligand. Together, our studies demonstrate that the dorsal wave of neocortical oligodendrogenesis occurs earlier than previously appreciated and requires highly regulated Shh signaling from multiple embryonic sources.SIGNIFICANCE STATEMENT Most neocortical oligodendrocytes are made by neural progenitors in the dorsal forebrain, but the mechanisms that specify this fate are poorly understood. This study identifies Sonic hedgehog (Shh) signaling as a critical pathway in the transition from neurogenesis to oligodendrogenesis in dorsal forebrain progenitors during late embryonic development. The timing of this neuron-to-glia "switch" coincides with the arrival of migrating interneurons into the dorsal germinal zone, which we identify as a critical source of Shh ligand, which drives oligodendrogenesis. Our data provide evidence for a new model in which Shh signaling increases in the dorsal forebrain late in embryonic development to provide a temporally regulated mechanism that initiates the third wave of neocortical oligodendrogenesis.

Detachment of Chain-Forming Neuroblasts by Fyn-Mediated Control of cell-cell Adhesion in the Postnatal Brain

In the rodent olfactory system, neuroblasts produced in the ventricular-subventricular zone of the postnatal brain migrate tangentially in chain-like cell aggregates toward the olfactory bulb (OB) through the rostral migratory stream (RMS). After reaching the OB, the chains are dissociated and the neuroblasts migrate individually and radially toward their final destination. The cellular and molecular mechanisms controlling cell-cell adhesion during this detachment remain unclear. Here we report that Fyn, a nonreceptor tyrosine kinase, regulates the detachment of neuroblasts from chains in the male and female mouse OB. By performing chemical screening and in vivo loss-of-function and gain-of-function experiments, we found that Fyn promotes somal disengagement from the chains and is involved in neuronal migration from the RMS into the granule cell layer of the OB. Fyn knockdown or Dab1 (disabled-1) deficiency caused p120-catenin to accumulate and adherens junction-like structures to be sustained at the contact sites between neuroblasts. Moreover, a Fyn and N-cadherin double-knockdown experiment indicated that Fyn regulates the N-cadherin-mediated cell adhesion between neuroblasts. These results suggest that the Fyn-mediated control of cell-cell adhesion is critical for the detachment of chain-forming neuroblasts in the postnatal OB.SIGNIFICANCE STATEMENT In the postnatal brain, newly born neurons (neuroblasts) migrate in chain-like cell aggregates toward their destination, where they are dissociated into individual cells and mature. The cellular and molecular mechanisms controlling the detachment of neuroblasts from chains are not understood. Here we show that Fyn, a nonreceptor tyrosine kinase, promotes the somal detachment of neuroblasts from chains, and that this regulation is critical for the efficient migration of neuroblasts to their destination. We further show that Fyn and Dab1 (disabled-1) decrease the cell-cell adhesion between chain-forming neuroblasts, which involves adherens junction-like structures. Our results suggest that Fyn-mediated regulation of the cell-cell adhesion of neuroblasts is critical for their detachment from chains in the postnatal brain.

The Candidate Schizophrenia Risk Gene DGCR2 Regulates Early Steps of Corticogenesis

BACKGROUND

Alterations in early steps of cortical circuit assembly are thought to play a critical role in vulnerability to schizophrenia (SZ), but the pathogenic impact of SZ-risk mutations on corticogenesis remains to be determined. DiGeorge syndrome critical region 2 (DGCR2) is located in the 22q11.2 locus, whose deletion is a major risk factor for SZ. Moreover, exome sequencing of individuals with idiopathic SZ identified a rare missense mutation in DGCR2, further suggesting that DGCR2 is involved in SZ.

METHODS

Here we investigated the function of Dgcr2 and the pathogenic impact of the SZ-risk DGCR2 mutation in mouse corticogenesis using in utero electroporation targeted to projection neurons.

RESULTS

Dgcr2 knockdown impaired radial locomotion and final translocation of projection neurons, leading to persistent laminar positioning alterations. The DGCR2 missense SZ-risk mutation had a pathogenic impact on projection neuron laminar allocation by reducing protein expression. Mechanistically, we identified Dgcr2 as a novel member of the Reelin complex, regulating the phosphorylation of Reelin-dependent substrates and the expression of Reelin-dependent transcriptional targets.

CONCLUSIONS

Overall, this study provides biological evidence that the SZ-risk gene DGCR2 regulates critical steps of early corticogenesis possibly through a Reelin-dependent mechanism. Additionally, we found that the SZ-risk mutation in DGCR2 has a pathogenic impact on cortical formation by reducing protein expression level, suggesting a functional role for DGCR2 haploinsufficiency in the 22q11.2 deletion syndrome.

Promoting Myelin Repair through In Vivo Neuroblast Reprogramming

Demyelination is frequently observed in a variety of CNS insults and neurodegenerative diseases. In rodents, adult neural stem cells can generate oligodendrocytes and participate to myelin repair. However, these cells mainly produce migratory neuroblasts that differentiate in the olfactory bulb. Here, we show that, in the demyelination context, a small subset of these neuroblasts can spontaneously convert into myelinating oligodendrocytes. Furthermore, we demonstrate that the contribution of neuroblasts to myelin repair can be improved by in vivo forced expression of two transcription factors: OLIG2 and SOX10. These factors promote directed fate conversion of endogenous subventricular zone neuroblasts into mature functional oligodendrocytes, leading to enhanced remyelination in a cuprizone-induced mouse model of demyelination. These findings highlight the unexpected plasticity of committed neuroblasts and provide proof of concept that they could be targeted for the treatment of demyelinated lesions in the adult brain.

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Sox10 and Olig2 convert endogenous neuroblasts into myelinating oligodendrocytes

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Converted cells migrate to the corpus callosum, striatum, and cortex

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Converted cells produce myelin and participate to the formation of node of Ranvier

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Forced neuroblast conversion improves myelin regeneration

Rap1 GTPases Are Master Regulators of Neural Cell Polarity in the Developing Neocortex

During the development of the mammalian neocortex, the generation of neurons by neural progenitors and their migration to the final position are closely coordinated. The highly polarized radial glial cells (RGCs) serve both as progenitor cells to generate neurons and as support for the migration of these neurons. After their generation, neurons transiently assume a multipolar morphology before they polarize and begin their migration along the RGCs. Here, we show that Rap1 GTPases perform essential functions for cortical organization as master regulators of cell polarity. Conditional deletion of Rap1 GTPases leads to a complete loss of cortical lamination. In RGCs, Rap1 GTPases are required to maintain their polarized organization. In newborn neurons, the loss of Rap1 GTPases prevents the formation of axons and leading processes and thereby interferes with radial migration. Taken together, the loss of RGC and neuronal polarity results in the disruption of cortical organization.

Multisensory cortical processing and dysfunction across the neuropsychiatric spectrum

Sensory processing is affected in multiple neuropsychiatric disorders like schizophrenia and autism spectrum disorders. Genetic and environmental factors guide the formation and fine-tuning of brain circuitry necessary to receive, organize, and respond to sensory input in order to behave in a meaningful and consistent manner. During certain developmental stages the brain is sensitive to intrinsic and external factors. For example, disturbed expression levels of certain risk genes during critical neurodevelopmental periods may lead to exaggerated brain plasticity processes within the sensory circuits, and sensory stimulation immediately after birth contributes to fine-tuning of these circuits. Here, the neurodevelopmental trajectory of sensory circuit development will be described and related to some example risk gene mutations that are found in neuropsychiatric disorders. Subsequently, the flow of sensory information through these circuits and the relationship to synaptic plasticity will be described. Research focusing on the combined analyses of neural circuit development and functioning are necessary to expand our understanding of sensory processing and behavioral deficits that are relevant across the neuropsychiatric spectrum.

Transient Cell-intrinsic Activity Regulates the Migration and Laminar Positioning of Cortical Projection Neurons

Neocortical microcircuits are built during development and require the coordinated assembly of excitatory glutamatergic projection neurons (PNs) into functional networks. Neuronal migration is an essential step in this process. In addition to cell-intrinsic mechanisms, external cues including neurotransmitters regulate cortical neuron migration, suggesting that early activity could influence this process. Here, we aimed to investigate the role of cell-intrinsic activity in migrating PNs in vivo using a designer receptor exclusively activated by a designer drug (DREADD) chemogenetic approach. In utero electroporation was used to specifically express the human M3 muscarinic cholinergic Gq-coupled receptor (hM3Dq) in PNs and calcium activity, migratory dynamics, gene expression, and laminar positioning of PNs were assessed following embryonic DREADD activation. We found that transient embryonic DREADD activation induced premature branching and transcriptional changes in migrating PNs leading to a persistent laminar mispositioning of superficial layer PNs into deep cortical layers without affecting expression of layer-specific molecular identity markers. In addition, live imaging approaches indicated that embryonic DREADD activation increased calcium transients in migrating PNs and altered their migratory dynamics by increasing their pausing time. Taken together, these results support the idea that increased cell-intrinsic activity during migration acts as a stop signal for migrating cortical PNs.