The Emerging Roles of the Cephalic Neural Crest in Brain Development and Developmental Encephalopathies.

The neural crest, a unique cell population originating from the primitive neural field, has a multi-systemic and structural contribution to vertebrate development. At the cephalic level, the neural crest generates most of the skeletal tissues encasing the developing forebrain and provides the prosencephalon with functional vasculature and meninges. Over the last decade, we have demonstrated that the cephalic neural crest (CNC) exerts an autonomous and prominent control on the development of the forebrain and sense organs. The present paper reviews the primary mechanisms by which CNC can orchestrate vertebrate encephalization. Demonstrating the role of the CNC as an exogenous source of patterning for the forebrain provides a novel conceptual framework with profound implications for understanding neurodevelopment. From a biomedical standpoint, these data suggest that the spectrum of neurocristopathies is broader than expected and that some neurological disorders may stem from CNC dysfunctions.

Three foxg1 paralogues in lampreys and gnathostomes-brothers or cousins?

Foxg1 is a key regulator of the early development of the vertebrate forebrain and sensory organs. In this study, we describe for the first time three foxg1 paralogues in lamprey, representative of one of two basally diverged lineages of vertebrates-the agnathans. We also first describe three foxg1 genes in sterlet-representative of one of the evolutionarily ancient clades of gnathostomes. According to the analysis of local genomic synteny, three foxg1 genes of agnathans and gnathostomes have a common origin as a result of two rounds of genomic duplications in the early evolution of vertebrates. At the same time, it is difficult to reliably establish pairwise orthology between foxg1 genes of agnathans and gnathostomes based on the analysis of phylogeny and local genomic synteny, as well as our studies of the spatiotemporal expression of foxg1 genes in the river lamprey Lampetra fluviatilis and the sterlet Acipenser ruthenus. Thus, the appearance of three foxg1 paralogues in agnathans and gnathostomes could have occurred either as a result of two rounds of duplication of the vertebrate common ancestor genome (2R hypothesis) or as a result of the first common round followed by subsequent independent polyploidizations in two evolutionary lineages (1R hypothesis).

17q12 deletion syndrome mouse model shows defects in craniofacial, brain and kidney development, and glucose homeostasis.

17q12 deletion (17q12Del) syndrome is a copy number variant (CNV) disorder associated with neurodevelopmental disorders and renal cysts and diabetes syndrome (RCAD). Using CRISPR/Cas9 genome editing, we generated a mouse model of 17q12Del syndrome on both inbred (C57BL/6N) and outbred (CD-1) genetic backgrounds. On C57BL/6N, the 17q12Del mice had severe head development defects, potentially mediated by haploinsufficiency of Lhx1, a gene within the interval that controls head development. Phenotypes included brain malformations, particularly disruption of the telencephalon and craniofacial defects. On the CD-1 background, the 17q12Del mice survived to adulthood and showed milder craniofacial and brain abnormalities. We report postnatal brain defects using automated magnetic resonance imaging-based morphometry. In addition, we demonstrate renal and blood glucose abnormalities relevant to RCAD. On both genetic backgrounds, we found sex-specific presentations, with male 17q12Del mice exhibiting higher penetrance and more severe phenotypes. Results from these experiments pinpoint specific developmental defects and pathways that guide clinical studies and a mechanistic understanding of the human 17q12Del syndrome. This mouse mutant represents the first and only experimental model to date for the 17q12 CNV disorder. This article has an associated First Person interview with the first author of the paper.

Identification of disease-relevant modulators of the SHH pathway in the developing brain.

Pathogenic gene variants in humans that affect the sonic hedgehog (SHH) pathway lead to severe brain malformations with variable penetrance due to unknown modifier genes. To identify such modifiers, we established novel congenic mouse models. LRP2-deficient C57BL/6N mice suffer from heart outflow tract defects and holoprosencephaly caused by impaired SHH activity. These defects are fully rescued on a FVB/N background, indicating a strong influence of modifier genes. Applying comparative transcriptomics, we identified Pttg1 and Ulk4 as candidate modifiers upregulated in the rescue strain. Functional analyses showed that ULK4 and PTTG1, both microtubule-associated proteins, are positive regulators of SHH signaling, rendering the pathway more resilient to disturbances. In addition, we characterized ULK4 and PTTG1 as previously unidentified components of primary cilia in the neuroepithelium. The identification of genes that powerfully modulate the penetrance of genetic disturbances affecting the brain and heart is likely relevant to understanding the variability in human congenital disorders.

Sox2-Evf2 lncRNA-mediated mechanisms of chromosome topological control in developing forebrain.

The Evf2 long non-coding RNA directs Dlx5/6 ultraconserved enhancer(UCE)-intrachromosomal interactions, regulating genes across a 27 Mb region on chromosome 6 in mouse developing forebrain. Here, we show that Evf2 long-range gene repression occurs through multi-step mechanisms involving the transcription factor Sox2. Evf2 directly interacts with Sox2, antagonizing Sox2 activation of Dlx5/6UCE, and recruits Sox2 to the Dlx5/6eii shadow enhancer and key Dlx5/6UCE interaction sites. Sox2 directly interacts with Dlx1 and Smarca4, as part of the Evf2 ribonucleoprotein complex, forming spherical subnuclear domains (protein pools, PPs). Evf2 targets Sox2 PPs to one long-range repressed target gene (Rbm28), at the expense of another (Akr1b8). Evf2 and Sox2 shift Dlx5/6UCE interactions towards Rbm28, linking Evf2/Sox2 co-regulated topological control and gene repression. We propose a model that distinguishes Evf2 gene repression mechanisms at Rbm28 (Dlx5/6UCE position) and Akr1b8 (limited Sox2 availability). Genome-wide control of RNPs (Sox2, Dlx and Smarca4) shows that co-recruitment influences Sox2 DNA binding. Together, these data suggest that Evf2 organizes a Sox2 PP subnuclear domain and, through Sox2-RNP sequestration and recruitment, regulates chromosome 6 long-range UCE targeting and activity with genome-wide consequences.

A novel telencephalon-opto-hypothalamic morphogenetic domain coexpressing Foxg1 and Otp produces most of the glutamatergic neurons of the medial extended amygdala.

Deficits in social cognition and behavior are a hallmark of many psychiatric disorders. The medial extended amygdala, including the medial amygdala and the medial bed nucleus of the stria terminalis, is a key component of functional networks involved in sociality. However, this nuclear complex is highly heterogeneous and contains numerous GABAergic and glutamatergic neuron subpopulations. Deciphering the connections of different neurons is essential in order to understand how this structure regulates different aspects of sociality, and it is necessary to evaluate their differential implication in distinct mental disorders. Developmental studies in different vertebrates are offering new venues to understand neuronal diversity of the medial extended amygdala and are helping to establish a relation between the embryonic origin and molecular signature of distinct neurons with the functional subcircuits in which they are engaged. These studies have provided many details on the distinct GABAergic neurons of the medial extended amygdala, but information on the glutamatergic neurons is still scarce. Using an Otp-eGFP transgenic mouse and multiple fluorescent labeling, we show that most glutamatergic neurons of the medial extended amygdala originate in a distinct telencephalon-opto-hypothalamic embryonic domain (TOH), located at the transition between telencephalon and hypothalamus, which produces Otp-lineage neurons expressing the telencephalic marker Foxg1 but not Nkx2.1 during development. These glutamatergic cells include a subpopulation of projection neurons of the medial amygdala, which activation has been previously shown to promote autistic-like behavior. Our data open new venues for studying the implication of this neuron subtype in neurodevelopmental disorders producing social deficits.

Profiling of lincRNAs in human pluripotent stem cell derived forebrain neural progenitor cells.

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) can be differentiated into many different cell types of the central nervous system. One challenge when using pluripotent stem cells is to develop robust and efficient differentiation protocols that result in homogenous cultures of the desired cell type. Here, we have utilized the SMAD-inhibitors SB431542 and Noggin in a fully defined monolayer culture model to differentiate human pluripotent cells into homogenous forebrain neural progenitors. Temporal fate analysis revealed that this protocol results in forebrain-patterned neural progenitor cells that start to express early neuronal markers after two weeks of differentiation, allowing for the analysis of gene expression changes during neurogenesis. Using this system, we were able to identify many previously uncharacterized long intergenic non-coding RNAs that display dynamic expression during human forebrain neurogenesis.

Novel heterozygous variants in KMT2D associated with holoprosencephaly.

Lysine methyltransferase 2D (KMT2D; OMIM 602113) encodes a histone methyltransferase involved in transcriptional regulation of the beta-globin and estrogen receptor as part of a large protein complex known as activating signal cointegrator-2-containing complex (ASCOM). Heterozygous germline mutations in the KMT2D gene are known to cause Kabuki syndrome (OMIM 147920), a developmental multisystem disorder. Neither holoprosencephaly nor other defects in human forebrain development have been previously associated with Kabuki syndrome. Here we report two patients diagnosed with alobar holoprosencephaly in their antenatal period with de novo monoallelic KMT2D variants identified by trio-based exome sequencing. The first patient was found to have a stop-gain variant c.12565G>T (p.Gly4189*), while the second patient had a missense variant c.5A>G (p.Asp2Gly). Phenotyping of each patient did not reveal any age-related feature of Kabuki syndrome. These two cases represent the first report on association between KMT2D and holoprosencephaly.

Syndromes associated with holoprosencephaly.

Holoprosencephaly (HPE) is partial or complete failure of the forebrain to divide into hemispheres and can be an isolated finding or associated with a syndrome. Most cases of HPE are associated with a syndrome and roughly 40%-60% of fetuses with HPE have trisomy 13 which is the most common etiology of HPE. Other syndromes associated with HPE include additional aneuploidies like trisomy 18 and single gene disorders such as Smith-Lemli-Opitz syndrome. There are a number of syndromes such as pseudotrisomy 13 which do not have a known molecular etiology; therefore, this review has two parts: syndromes with a molecular diagnosis and syndromes where the etiology is yet to be found. As most HPE is syndromic, this review provides a comprehensive list and description of syndromes associated with HPE that may be used as a differential diagnosis and starting point for evaluating individuals with HPE.

Tangential migration of corridor guidepost neurons contributes to anxiety circuits.

In mammals, thalamic axons are guided internally toward their neocortical target by corridor (Co) neurons that act as axonal guideposts. The existence of Co-like neurons in non-mammalian species, in which thalamic axons do not grow internally, raised the possibility that Co cells might have an ancestral role. Here, we investigated the contribution of corridor (Co) cells to mature brain circuits using a combination of genetic fate-mapping and assays in mice. We unexpectedly found that Co neurons contribute to striatal-like projection neurons in the central extended amygdala. In particular, Co-like neurons participate in specific nuclei of the bed nucleus of the stria terminalis, which plays essential roles in anxiety circuits. Our study shows that Co neurons possess an evolutionary conserved role in anxiety circuits independently from an acquired guidepost function. It furthermore highlights that neurons can have multiple sequential functions during brain wiring and supports a general role of tangential migration in the building of subpallial circuits.

The role of ISWI chromatin remodeling complexes in brain development and neurodevelopmental disorders.

The mammalian ISWI (Imitation Switch) genes SMARCA1 and SMARCA5 encode the ATP-dependent chromatin remodeling proteins SNF2L and SNF2H. The ISWI proteins interact with BAZ (bromodomain adjacent to PHD zinc finger) domain containing proteins to generate eight distinct remodeling complexes. ISWI complex-mediated nucleosome positioning within genes and gene regulatory elements is proving important for the transition from a committed progenitor state to a differentiated cell state. Genetic studies have implicated the involvement of many ATP-dependent chromatin remodeling proteins in neurodevelopmental disorders (NDDs), including SMARCA1. Here we review the characterization of mice inactivated for ISWI and their interacting proteins, as it pertains to brain development and disease. A better understanding of chromatin dynamics during neural development is a prerequisite to understanding disease pathologies and the development of therapeutics for these complex disorders.

GABAergic Interneuron Differentiation in the Basal Forebrain Is Mediated through Direct Regulation of Glutamic Acid Decarboxylase Isoforms by Dlx Homeobox Transcription Factors.

GABA is the key inhibitory neurotransmitter in the cortex but regulation of its synthesis during forebrain development is poorly understood. In the telencephalon, members of the distal-less (Dlx) homeobox gene family are expressed in, and regulate the development of, the basal ganglia primodia from which many GABAergic neurons originate and migrate to other forebrain regions. The Dlx1/Dlx2 double knock-out mice die at birth with abnormal cortical development, including loss of tangential migration of GABAergic inhibitory interneurons to the neocortex (Anderson et al., 1997a). We have discovered that specific promoter regulatory elements of glutamic acid decarboxylase isoforms (Gad1 and Gad2), which regulate GABA synthesis from the excitatory neurotransmitter glutamate, are direct transcriptional targets of both DLX1 and DLX2 homeoproteins in vivo Further gain- and loss-of-function studies in vitro and in vivo demonstrated that both DLX1 and DLX2 are necessary and sufficient for Gad gene expression. DLX1 and/or DLX2 activated the transcription of both Gad genes, and defects in Dlx function disrupted the differentiation of GABAergic interneurons with global reduction in GABA levels in the forebrains of the Dlx1/Dlx2 double knock-out mouse in vivo Identification of Gad genes as direct Dlx transcriptional targets is significant; it extends our understanding of Dlx gene function in the developing forebrain beyond the regulation of tangential interneuron migration to the differentiation of GABAergic interneurons arising from the basal telencephalon, and may help to unravel the pathogenesis of several developmental brain disorders.SIGNIFICANCE STATEMENT GABA is the major inhibitory neurotransmitter in the brain. We show that Dlx1/Dlx2 homeobox genes regulate GABA synthesis during forebrain development through direct activation of glutamic acid decarboxylase enzyme isoforms that convert glutamate to GABA. This discovery helps explain how Dlx mutations result in abnormal forebrain development, due to defective differentiation, in addition to the loss of tangential migration of GABAergic inhibitory interneurons to the neocortex. Reduced numbers or function of cortical GABAergic neurons may lead to hyperactivity states such as seizures (Cobos et al., 2005) or contribute to the pathogenesis of some autism spectrum disorders. GABAergic dysfunction in the basal ganglia could disrupt the learning and development of complex motor and cognitive behaviors (Rubenstein and Merzenich, 2003).

Persistent Expression of VCAM1 in Radial Glial Cells Is Required for the Embryonic Origin of Postnatal Neural Stem Cells.

During development, neural stem cells (NSCs) undergo transitions from neuroepithelial cells to radial glial cells (RGCs), and later, a subpopulation of slowly dividing RGCs gives rise to the quiescent adult NSCs that populate the ventricular-subventricular zone (V-SVZ). Here we show that VCAM1, a transmembrane protein previously found in quiescent adult NSCs, is expressed by a subpopulation of embryonic RGCs, in a temporal and region-specific manner. Loss of VCAM1 reduced the number of active embryonic RGCs by stimulating their premature neuronal differentiation while preventing quiescence in the slowly dividing RGCs. This in turn diminished the embryonic origin of postnatal NSCs, resulting in loss of adult NSCs and defective V-SVZ regeneration. VCAM1 affects the NSC fate by signaling through its intracellular domain to regulate ß-catenin signaling in a context-dependent manner. Our findings provide new insight on how stem cells in the embryo are preserved to meet the need for growth and regeneration.

Endocytic receptor LRP2/megalin-of holoprosencephaly and renal Fanconi syndrome.

Megalin (or LRP2) is an endocytic receptor that plays a central role in embryonic development and adult tissue homeostasis. Loss of this receptor in congenital or acquired diseases results in multiple organ dysfunctions, including forebrain malformation (holoprosencephaly) and renal reabsorption defects (renal Fanconi syndrome). Here, we describe current concepts of the mode of receptor action that include co-receptors and a repertoire of different ligands, and we discuss how these interactions govern functional integrity of the kidney and the brain, and cause disease when defective.

LRP2, an auxiliary receptor that controls sonic hedgehog signaling in development and disease.

To fulfill their multiple roles in organ development and adult tissue homeostasis, hedgehog (HH) morphogens act through their receptor Patched (PTCH) on target cells. However, HH actions also require HH binding proteins, auxiliary cell surface receptors that agonize or antagonize morphogen signaling in a context-dependent manner. Here, we discuss recent findings on the LDL receptor-related protein 2 (LRP2), an exemplary HH binding protein that modulates sonic hedgehog activities in stem and progenitor cell niches in embryonic and adult tissues. LRP2 functions are crucial for developmental processes in a number of tissues, including the brain, the eye, and the heart, and defects in this receptor pathway are the cause of devastating congenital diseases in humans. Developmental Dynamics 245:569-579, 2016. © 2016 Wiley Periodicals, Inc.

ARX regulates cortical intermediate progenitor cell expansion and upper layer neuron formation through repression of Cdkn1c.

Mutations in the Aristaless-related homeobox (ARX) gene are found in a spectrum of epilepsy and X-linked intellectual disability disorders. During development Arx is expressed in pallial ventricular zone (VZ) progenitor cells where the excitatory projection neurons of the cortex are born. Arx(-/Y) mice were shown to have decreased proliferation in the cortical VZ resulting in smaller brains; however, the basis for this reduced proliferation was not established. To determine the role of ARX on cell cycle dynamics in cortical progenitor cells, we generated cerebral cortex-specific Arx mouse mutants (cKO). The loss of pallial Arx resulted in the reduction of cortical progenitor cells, particularly the proliferation of intermediate progenitor cells (IPCs) was affected. Later in development and postnatally cKO brains showed a reduction of upper layer but not deeper layer neurons consistent with the IPC defect. Transcriptional profile analysis of E14.5 Arx-ablated cortices compared with control revealed that CDKN1C, an inhibitor of cell cycle progression, is overexpressed in the cortical VZ and SVZ of Arx KOs throughout corticogenesis. We also identified ARX as a direct regulator of Cdkn1c transcription. Together these data support a model where ARX regulates the expansion of cortical progenitor cells through repression of Cdkn1c.

Fezf2 promotes neuronal differentiation through localised activation of Wnt/ß-catenin signalling during forebrain development.

Brain regionalisation, neuronal subtype diversification and circuit connectivity are crucial events in the establishment of higher cognitive functions. Here we report the requirement for the transcriptional repressor Fezf2 for proper differentiation of neural progenitor cells during the development of the Xenopus forebrain. Depletion of Fezf2 induces apoptosis in postmitotic neural progenitors, with concomitant reduction in forebrain size and neuronal differentiation. Mechanistically, we found that Fezf2 stimulates neuronal differentiation by promoting Wnt/ß-catenin signalling in the developing forebrain. In addition, we show that Fezf2 promotes activation of Wnt/ß-catenin signalling by repressing the expression of two negative regulators of Wnt signalling, namely lhx2 and lhx9. Our findings suggest that Fezf2 plays an essential role in controlling when and where neuronal differentiation occurs within the developing forebrain and that it does so by promoting local Wnt/ß-catenin signalling via a double-repressor model.

How cell-autonomous is neuronal migration in the forebrain? Molecular cross-talk at the cell membrane.

In the adult brain, different cell types communicate with each other through cell-cell contacts and brain activity is regulated at the cell membrane. But long before the brain is fully functional, different excitatory and inhibitory cell types generated at distinct places migrate through the developing brain to their final position. The elements guiding these migrating neurons, either structural axonal scaffolds or chemical guidance factors, are relatively well described. However, the molecules involved in the individual short-timed membrane contacts migrating cells make with other cells during their migration process are less well understood. This update focuses on recent novel insights into the molecular nature of these cell-cell contacts and the cross-talk taking place at the cell membrane.

Transgenic increase of Wnt7b in neural progenitor cells decreases expression of T-domain transcription factors and impairs neuronal differentiation.

Wnt/beta-catenin signaling plays an important role in neural development, instructing both progenitor cell division and differentiation. During early corticogenesis, Wnt7b is expressed in a restricted expression pattern in the ventricular zone progenitor cells. However, its influence on progenitor cell behavior has not been fully studied. We report that transgenic overexpression of Wnt7b in neural progenitor cells impairs neuronal differentiation and the development of forebrain structures at embryonic day 10.5 (E10.5). This was accompanied by a decreased expression of T-domain transcription factors Tbr1 and Tbr2, in both progenitor cells and post-mitotic neurons. However, proliferation, apoptosis and the overall proportion of pax6(+) neural progenitor cells were similar to wild-type litter mates. These results suggest that Wnt signaling may affect early neural progenitor differentiation by regulating the expression of pro-neural transcription factors.

Ras-dva1 small GTPase regulates telencephalon development in Xenopus laevis embryos by controlling Fgf8 and Agr signaling at the anterior border of the neural plate.

We previously found that the small GTPase Ras-dva1 is essential for the telencephalic development in Xenopus laevis because Ras-dva1 controls the Fgf8-mediated induction of FoxG1 expression, a key telencephalic regulator. In this report, we show, however, that Ras-dva1 and FoxG1 are expressed in different groups of cells; whereas Ras-dva1 is expressed in the outer layer of the anterior neural fold, FoxG1 and Fgf8 are activated in the inner layer from which the telencephalon is derived. We resolve this paradox by demonstrating that Ras-dva1 is involved in the transduction of Fgf8 signal received by cells in the outer layer, which in turn send a feedback signal that stimulates FoxG1 expression in the inner layer. We show that this feedback signal is transmitted by secreted Agr proteins, the expression of which is activated in the outer layer by mediation of Ras-dva1 and the homeodomain transcription factor Otx2. In turn, Agrs are essential for maintaining Fgf8 and FoxG1 expression in cells at the anterior neural plate border. Our finding reveals a novel feedback loop mechanism based on the exchange of Fgf8 and Agr signaling between neural and non-neural compartments at the anterior margin of the neural plate and demonstrates a key role of Ras-dva1 in this mechanism.