Regionalization within the mammalian telencephalon is mediated by changes in responsiveness to Sonic Hedgehog

Shh activation restores interneurons and cognitive function in newborns with intraventricular haemorrhage

Premature infants with germinal matrix haemorrhage-intraventricular haemorrhage (GMH-IVH) suffer from neurobehavioural deficits as they enter childhood and adolescence. Yet the underlying mechanisms remain unclear. Impaired development and function of interneurons contribute to neuropsychiatric disorders. Therefore, we hypothesized that the occurrence of IVH would reduce interneuron neurogenesis in the medial ganglionic eminence and diminish the population of parvalbumin+ and somatostatin+ cortical interneurons. Because Sonic Hedgehog promotes the production of cortical interneurons, we also postulated that the activation of Sonic Hedgehog signalling might restore neurogenesis, cortical interneuron population, and neurobehavioural function in premature newborns with IVH. These hypotheses were tested in a preterm rabbit model of IVH and autopsy samples from human preterm infants. We compared premature newborns with and without IVH for intraneuronal progenitors, cortical interneurons, transcription factors regulating neurogenesis, single-cell transcriptome of medial ganglionic eminence and neurobehavioural functions. We treated premature rabbit kits with adenovirus expressing Sonic Hedgehog (Ad-Shh) or green fluorescence protein gene to determine the effect of Sonic Hedgehog activation on the interneuron production, cortical interneuron population and neurobehaviour. We discovered that IVH reduced the number of Nkx2.1+ and Dlx2+ progenitors in the medial ganglionic eminence of both humans and rabbits by attenuating their proliferation and inducing apoptosis. Moreover, IVH decreased the population of parvalbumin+ and somatostatin+ neurons in the frontal cortex of both preterm infants and kits relative to controls. Sonic Hedgehog expression and the downstream transcription factors, including Nkx2.1, Mash1, Lhx6 and Sox6, were also reduced in kits with IVH. Consistent with these findings, single-cell transcriptomic analyses of medial ganglionic eminence identified a distinct subpopulation of cells exhibiting perturbation in genes regulating neurogenesis, ciliogenesis, mitochondrial function and MAPK signalling in rabbits with IVH. More importantly, restoration of Sonic Hedgehog level by Ad-Shh treatment ameliorated neurogenesis, cortical interneuron population and neurobehavioural function in kits with IVH. Additionally, Sonic Hedgehog activation alleviated IVH-induced inflammation and several transcriptomic changes in the medial ganglionic eminence. Taken together, IVH reduced intraneuronal production and cortical interneuron population by downregulating Sonic Hedgehog signalling in both preterm rabbits and humans. Notably, activation of Sonic Hedgehog signalling restored interneuron neurogenesis, cortical interneurons and cognitive function in rabbit kits with IVH. These findings highlight disruption in cortical interneurons in IVH and identify a novel therapeutic strategy to restore cortical interneurons and cognitive function in infants with IVH. These studies can accelerate the development of new therapies to enhance the neurodevelopmental outcome of survivors with IVH.

Establishing Hedgehog Gradients during Neural Development

A morphogen is a signaling molecule that induces specific cellular responses depending on its local concentration. The concept of morphogenic gradients has been a central paradigm of developmental biology for decades. Sonic Hedgehog (Shh) is one of the most important morphogens that displays pleiotropic functions during embryonic development, ranging from neuronal patterning to axon guidance. It is commonly accepted that Shh is distributed in a gradient in several tissues from different origins during development; however, how these gradients are formed and maintained at the cellular and molecular levels is still the center of a great deal of research. In this review, we first explored all of the different sources of Shh during the development of the nervous system. Then, we detailed how these sources can distribute Shh in the surrounding tissues via a variety of mechanisms. Finally, we addressed how disrupting Shh distribution and gradients can induce severe neurodevelopmental disorders and cancers. Although the concept of gradient has been central in the field of neurodevelopment since the fifties, we also describe how contemporary leading-edge techniques, such as organoids, can revisit this classical model.

Development, Diversity, and Death of MGE-Derived Cortical Interneurons

In the mammalian brain, cortical interneurons (INs) are a highly diverse group of cells. A key neurophysiological question concerns how each class of INs contributes to cortical circuit function and whether specific roles can be attributed to a selective cell type. To address this question, researchers are integrating knowledge derived from transcriptomic, histological, electrophysiological, developmental, and functional experiments to extensively characterise the different classes of INs. Our hope is that such knowledge permits the selective targeting of cell types for therapeutic endeavours. This review will focus on two of the main types of INs, namely the parvalbumin (PV⁺) or somatostatin (SOM⁺)-containing cells, and summarise the research to date on these classes.

Role of SHH in Patterning Human Pluripotent Cells towards Ventral Forebrain Fates

The complexities of human neurodevelopment have historically been challenging to decipher but continue to be of great interest in the contexts of healthy neurobiology and disease. The classic animal models and monolayer in vitro systems have limited the types of questions scientists can strive to answer in addition to the technical ability to answer them. However, the tridimensional human stem cell-derived organoid system provides the unique opportunity to model human development and mimic the diverse cellular composition of human organs. This strategy is adaptable and malleable, and these neural organoids possess the morphogenic sensitivity to be patterned in various ways to generate the different regions of the human brain. Furthermore, recapitulating human development provides a platform for disease modeling. One master regulator of human neurodevelopment in many regions of the human brain is sonic hedgehog (SHH), whose expression gradient and pathway activation are responsible for conferring ventral identity and shaping cellular phenotypes throughout the neural axis. This review first discusses the benefits, challenges, and limitations of using organoids for studying human neurodevelopment and disease, comparing advantages and disadvantages with other in vivo and in vitro model systems. Next, we explore the range of control that SHH exhibits on human neurodevelopment, and the application of SHH to various stem cell methodologies, including organoids, to expand our understanding of human development and disease. We outline how this strategy will eventually bring us much closer to uncovering the intricacies of human neurodevelopment and biology.

Sonic hedgehog expands neural stem cells in the neocortical region leading to an expanded and wrinkled neocortical surface

An expanded and folded neocortex is characteristic of higher mammals, including humans and other primates. The neocortical surface area was dramatically enlarged during the course of mammalian brain evolution from lissencephalic to gyrencephalic mammals, and this bestowed higher cognitive functions especially to primates, including humans. In this study, we generated transgenic (Tg) mice in which the expression of Sonic hedgehog (Shh) could be controlled in neural stem cells (NSCs) and neural progenitors by using the Tet-on system. Shh overexpression during embryogenesis promoted the symmetric proliferative division of NSCs in the neocortical region, leading to the expansion of lateral ventricles and tangential extension of the ventricular zone. Moreover, Shh-overexpressing Tg mice showed dramatic expansion of the neocortical surface area and exhibited a wrinkled brain when overexpression was commenced at early stages of neural development. These results indicate that Shh is able to increase the neocortical NSCs and contribute to expansion of the neocortex.

Non-Syndromic Intellectual Disability and Its Pathways: A Long Noncoding RNA Perspective

Non-syndromic intellectual disability (NS-ID or idiopathic) is a complex neurodevelopmental disorder that represents a global health issue. Although many efforts have been made to characterize it and distinguish it from syndromic intellectual disability (S-ID), the highly heterogeneous aspect of this disorder makes it difficult to understand its etiology. Long noncoding RNAs (lncRNAs) comprise a large group of transcripts that can act through various mechanisms and be involved in important neurodevelopmental processes. In this sense, comprehending the roles they play in this intricate context is a valuable way of getting new insights about how NS-ID can arise and develop. In this review, we attempt to bring together knowledge available in the literature about lncRNAs involved with molecular and cellular pathways already described in intellectual disability and neural function, to better understand their relevance in NS-ID and the regulatory complexity of this disorder.

New Insights Into the Intricacies of Proneural Gene Regulation in the Embryonic and Adult Cerebral Cortex

Historically, the mammalian brain was thought to lack stem cells as no new neurons were found to be made in adulthood. That dogma changed ∼25 years ago with the identification of neural stem cells (NSCs) in the adult rodent forebrain. However, unlike rapidly self-renewing mature tissues (e.g., blood, intestinal crypts, skin), the majority of adult NSCs are quiescent, and those that become 'activated' are restricted to a few neurogenic zones that repopulate specific brain regions. Conversely, embryonic NSCs are actively proliferating and neurogenic. Investigations into the molecular control of the quiescence-to-proliferation-to-differentiation continuum in the embryonic and adult brain have identified proneural genes encoding basic-helix-loop-helix (bHLH) transcription factors (TFs) as critical regulators. These bHLH TFs initiate genetic programs that remove NSCs from quiescence and drive daughter neural progenitor cells (NPCs) to differentiate into specific neural cell subtypes, thereby contributing to the enormous cellular diversity of the adult brain. However, new insights have revealed that proneural gene activities are context-dependent and tightly regulated. Here we review how proneural bHLH TFs are regulated, with a focus on the murine cerebral cortex, drawing parallels where appropriate to other organisms and neural tissues. We discuss upstream regulatory events, post-translational modifications (phosphorylation, ubiquitinylation), protein-protein interactions, epigenetic and metabolic mechanisms that govern bHLH TF expression, stability, localization, and consequent transactivation of downstream target genes. These tight regulatory controls help to explain paradoxical findings of changes to bHLH activity in different cellular contexts.

Examining the developmental toxicity of piperonyl butoxide as a Sonic hedgehog pathway inhibitor

Piperonyl butoxide (PBO) is a semisynthetic chemical present in hundreds of pesticide formulations used in agricultural, commercial, and residential settings. PBO acts as a pesticide synergist by inhibiting insect cytochrome P450 enzymes and is often present at much higher concentrations than active insecticidal ingredients. PBO was recently discovered to also inhibit Sonic hedgehog (Shh) signaling, a key molecular pathway in embryonic development and in brain and face morphogenesis. Recent animal model studies have shown that in utero PBO exposure can cause overt craniofacial malformations or more subtle neurodevelopmental abnormalities. Related adverse developmental outcomes in humans are etiologically heterogeneous, and, while studies are limited, PBO exposure during pregnancy has been linked to neurodevelopmental deficits. Contextualized in PBO's newly recognized mechanism as a Shh signaling inhibitor, these findings support more rigorous examination of the developmental toxicity of PBO and its potential contribution to etiologically complex human birth defects. In this review, we highlight environmental sources of human PBO exposure and summarize existing animal studies examining the developmental impact of prenatal PBO exposure. Also presented are critical knowledge gaps in our understanding of PBO's pharmacokinetics and potential role in gene-environment and environment-environment interactions that should be addressed to better understand the human health impact of environmental PBO exposure.

Developmental Toxicity Assessment of Piperonyl Butoxide Exposure Targeting Sonic Hedgehog Signaling and Forebrain and Face Morphogenesis in the Mouse: An in Vitro and in Vivo Study

BACKGROUND

Piperonyl butoxide (PBO) is a pesticide synergist used in residential, commercial, and agricultural settings. PBO was recently found to inhibit Sonic hedgehog (Shh) signaling, a key developmental regulatory pathway. Disruption of Shh signaling is linked to birth defects, including holoprosencephaly (HPE), a malformation of the forebrain and face thought to result from complex gene-environment interactions.

OBJECTIVES

The impact of PBO on Shh signaling in vitro and forebrain and face development in vivo was examined.

METHODS

The influence of PBO on Shh pathway transduction was assayed in mouse and human cell lines. To examine its teratogenic potential, a single dose of PBO (22-1,800mg/kg) was administered by oral gavage to C57BL/6J mice at gestational day 7.75, targeting the critical period for HPE. Gene-environment interactions were investigated using Shh+/- mice, which model human HPE-associated genetic mutations.

RESULTS

PBO attenuated Shh signaling in vitro through a mechanism similar to that of the known teratogen cyclopamine. In utero PBO exposure caused characteristic HPE facial dysmorphology including dose-dependent midface hypoplasia and hypotelorism, with a lowest observable effect level of 67mg/kg. Median forebrain deficiency characteristic of HPE was observed in severely affected animals, whereas all effective doses disrupted development of Shh-dependent transient forebrain structures that generate cortical interneurons. Normally silent heterozygous Shh null mutations exacerbated PBO teratogenicity at all doses tested, including 33mg/kg.

DISCUSSION

These findings demonstrate that prenatal PBO exposure can cause overt forebrain and face malformations or neurodevelopmental disruptions with subtle or no craniofacial dysmorphology in mice. By targeting Shh signaling as a sensitive mechanism of action and examining gene-environment interactions, this study defined a lowest observable effect level for PBO developmental toxicity in mice more than 30-fold lower than previously recognized. Human exposure to PBO and its potential contribution to etiologically complex birth defects should be rigorously examined. https://doi.org/10.1289/EHP5260.

The Evf2 Ultraconserved Enhancer lncRNA Functionally and Spatially Organizes Megabase Distant Genes in the Developing Forebrain

Gene regulation requires selective targeting of DNA regulatory enhancers over megabase distances. Here we show that Evf2, a cloud-forming Dlx5/6 ultraconserved enhancer (UCE) lncRNA, simultaneously localizes to activated (Umad1, 1.6 Mb distant) and repressed (Akr1b8, 27 Mb distant) chr6 target genes, precisely regulating UCE-gene distances and cohesin binding in mouse embryonic forebrain GABAergic interneurons (INs). Transgene expression of Evf2 activates Lsm8 (12 Mb distant) but fails to repress Akr1b8, supporting trans activation and long-range cis repression. Through both short-range (Dlx6 antisense) and long-range (Akr1b8) repression, the Evf2-5'UCE links homeodomain and mevalonate pathway-regulated enhancers to IN diversity. The Evf2-3' end is required for long-range activation but dispensable for RNA cloud localization, functionally dividing the RNA into 3'-activator and 5'UCE repressor and targeting regions. Together, these results support that Evf2 selectively regulates UCE interactions with multi-megabase distant genes through complex effects on chromosome topology, linking lncRNA-dependent topological and transcriptional control with interneuron diversity and seizure susceptibility.

Understanding the Role of lncRNAs in Nervous System Development

The diversity of lncRNAs has expanded within mammals in tandem with the evolution of increased brain complexity, suggesting that lncRNAs play an integral role in this process. In this chapter, we will highlight the identification and characterization of lncRNAs in nervous system development. We discuss the potential role of lncRNAs in nervous system and brain evolution, along with efforts to create comprehensive catalogues that analyze spatial and temporal changes in lncRNA expression during nervous system development. Additionally, we focus on recent endeavors that attempt to assign function to lncRNAs during nervous system development. We highlight discrepancies that have been observed between in vitro and in vivo studies of lncRNA function and the challenges facing researchers in conducting mechanistic analyses of lncRNAs in the developing nervous system. Altogether, this chapter highlights the emerging role of lncRNAs in the developing brain and sheds light on novel, RNA-mediated mechanisms by which nervous system development is controlled.

SHH E176/E177-Zn2+ conformation is required for signaling at endogenous sites

Sonic hedgehog (SHH) is a master developmental regulator. In 1995, the SHH crystal structure predicted that SHH-E176 (human)/E177 (mouse) regulates signaling through a Zn2+-dependent mechanism. While Zn2+ is known to be required for SHH protein stability, a regulatory role for SHH-E176 or Zn2+ has not been described. Here, we show that SHH-E176/177 modulates Zn2+-dependent cross-linking in vitro and is required for endogenous signaling, in vivo. While ectopically expressed SHH-E176A is highly active, mice expressing SHH-E177A at endogenous sites (ShhE177A/-) are morphologically indistinguishable from mice lacking SHH (Shh-/-), with patterning defects in both embryonic spinal cord and forebrain. SHH-E177A distribution along the embryonic spinal cord ventricle is unaltered, suggesting that E177 does not control long-range transport. While SHH-E177A association with cilia basal bodies increases in embryonic ventral spinal cord, diffusely distributed SHH-E177A is not detected. Together, these results reveal a novel role for E177-Zn2+ in regulating SHH signaling that may involve critical, cilia basal-body localized changes in cross-linking and/or conformation.

Development of the Neuroendocrine Hypothalamus

The neuroendocrine hypothalamus is composed of the tuberal and anterodorsal hypothalamus, together with the median eminence/neurohypophysis. It centrally governs wide-ranging physiological processes, including homeostasis of energy balance, circadian rhythms and stress responses, as well as growth and reproductive behaviours. Homeostasis is maintained by integrating sensory inputs and effecting responses via autonomic, endocrine and behavioural outputs, over diverse time-scales and throughout the lifecourse of an individual. Here, we summarize studies that begin to reveal how different territories and cell types within the neuroendocrine hypothalamus are assembled in an integrated manner to enable function, thus supporting the organism's ability to survive and thrive. We discuss how signaling pathways and transcription factors dictate the appearance and regionalization of the hypothalamic primordium, the maintenance of progenitor cells, and their specification and differentiation into neurons. We comment on recent studies that harness such programmes for the directed differentiation of human ES/iPS cells. We summarize how developmental plasticity is maintained even into adulthood and how integration between the hypothalamus and peripheral body is established in the median eminence and neurohypophysis. Analysis of model organisms, including mouse, chick and zebrafish, provides a picture of how complex, yet elegantly coordinated, developmental programmes build glial and neuronal cells around the third ventricle of the brain. Such conserved processes enable the hypothalamus to mediate its function as a central integrating and response-control mediator for the homeostatic processes that are critical to life. Early indications suggest that deregulation of these events may underlie multifaceted pathological conditions and dysfunctional physiology in humans, such as obesity.

Hedgehog Signaling Modulates the Release of Gliotransmitters from Cultured Cerebellar Astrocytes

Sonic hedgehog (Shh), a member of the Hedgehog (Hh) family, plays essential roles in the development of the central nervous system. Recent studies suggest that the Hh signaling pathway also functions in mature astrocytes under physiological conditions. We first examined the expression of genes encoding Hh signaling molecules in the adult mouse cerebellum by in situ hybridization histochemistry. mRNA for Patched homolog 1 (Ptch1), a receptor for Hh family members, was expressed in S100β-positive astrocytes and Shh mRNA was expressed in HuC/D-positive neurons, implying that the Hh signaling pathway contributes to neuro-glial interactions. To test this hypothesis, we next examined the effects of recombinant SHH N-terminal protein (rSHH-N) on the functions of cultured cerebellar astrocytes. rSHH-N up-regulated Hh signal target genes such as Ptch1 and Gli-1, a key transcription factor of the Hh signaling pathway. Although activation of Hh signaling by rSHH-N or purmorphamine influenced neither glutamate uptake nor gliotransmitters release, inhibition of the Hh signaling pathway by cyclopamine, neutralizing antibody against SHH or intracellular Ca(2+) chelation decreased glutamate and ATP release from cultured cerebellar astrocytes. On the other hand, cyclopamine, neutralizing antibody against SHH or Ca(2+) chelator hardly affected D-serine secretion. Various kinase inhibitors attenuated glutamate and ATP release, while only U0126 reduced D-serine secretion from the astrocytes. These results suggested that the Hh signaling pathway sustains the release of glutamate and ATP and participates in neuro-glial interactions in the adult mouse brain. We also propose that signaling pathways distinct from the Hh pathway govern D-serine secretion from adult cerebellar astrocytes.

iPSC-based drug screening for Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by an expansion of the CAG repeat in exon 1 of the huntingtin gene. The disease generally manifests in middle age with both physical and mental symptoms. There are no effective treatments or cures and death usually occurs 10-20 years after initial symptoms. Since the original identification of the Huntington disease associated gene, in 1993, a variety of models have been created and used to advance our understanding of HD. The most recent advances have utilized stem cell models derived from HD-patient induced pluripotent stem cells (iPSCs) offering a variety of screening and model options that were not previously available. The discovery and advancement of technology to make human iPSCs has allowed for a more thorough characterization of human HD on a cellular and developmental level. The interaction between the genome editing and the stem cell fields promises to further expand the variety of HD cellular models available for researchers. In this review, we will discuss the history of Huntington's disease models, common screening assays, currently available models and future directions for modeling HD using iPSCs-derived from HD patients. This article is part of a Special Issue entitled SI: PSC and the brain.

Evf2 lncRNA/BRG1/DLX1 interactions reveal RNA-dependent inhibition of chromatin remodeling

Transcription-regulating long non-coding RNAs (lncRNAs) have the potential to control the site-specific expression of thousands of target genes. Previously, we showed that Evf2, the first described ultraconserved lncRNA, increases the association of transcriptional activators (DLX homeodomain proteins) with key DNA enhancers but represses gene expression. In this report, mass spectrometry shows that the Evf2-DLX1 ribonucleoprotein (RNP) contains the SWI/SNF-related chromatin remodelers Brahma-related gene 1 (BRG1, SMARCA4) and Brahma-associated factor (BAF170, SMARCC2) in the developing mouse forebrain. Evf2 RNA colocalizes with BRG1 in nuclear clouds and increases BRG1 association with key DNA regulatory enhancers in the developing forebrain. While BRG1 directly interacts with DLX1 and Evf2 through distinct binding sites, Evf2 directly inhibits BRG1 ATPase and chromatin remodeling activities. In vitro studies show that both RNA-BRG1 binding and RNA inhibition of BRG1 ATPase/remodeling activity are promiscuous, suggesting that context is a crucial factor in RNA-dependent chromatin remodeling inhibition. Together, these experiments support a model in which RNAs convert an active enhancer to a repressed enhancer by directly inhibiting chromatin remodeling activity, and address the apparent paradox of RNA-mediated stabilization of transcriptional activators at enhancers with a repressive outcome. The importance of BRG1/RNA and BRG1/homeodomain interactions in neurodevelopmental disorders is underscored by the finding that mutations in Coffin-Siris syndrome, a human intellectual disability disorder, localize to the BRG1 RNA-binding and DLX1-binding domains.

Emergence of neuronal diversity from patterning of telencephalic progenitors

During central nervous system (CNS) development, hundreds of distinct neuronal subtypes are generated from a single layer of multipotent neuroepithelial progenitor cells. Within the rostral CNS, initial regionalization of the telencephalon marks the territories where the cerebral cortex and the basal ganglia originate. Subsequent refinement of the primary structures determines the formation of domains of differential gene expression, where distinct fate-restricted progenitors are located. To understand how diversification of neural progenitors and neurons is achieved in the telencephalon, it is important to address early and late patterning events in this context. In particular, important questions include: How does the telencephalon become specified and regionalized along the major spatial axes? Within each region, are the differences in neuronal subtypes established at the progenitor level or at the postmitotic stage? If distinct progenitors exist that are committed to subtype-specific neuronal lineages, how does the diversification emerge? What is the contribution of positional and temporal cues and how is this information integrated into the intrinsic programs of cell identity? WIREs For further resources related to this article, please visit the WIREs website.

The Role of Sonic Hedgehog in the Specification of Human Cortical Progenitors In Vitro

Impaired sonic hedgehog (Shh) signaling is involved in the pathology of cortical formation found in neuropsychiatric disorders. However, its role in the specification of human cortical progenitors is not known. Here, we report that Shh is expressed in the human developing cortex at mid-gestation by radial glia cells (RGCs) and cortical neurons. We used RGC cultures, established from the dorsal (cortical) telencephalon of human brain at mid-gestation to study the effect of Shh signaling. Cortical RGCs in vitro maintained their regional characteristics, expressed components of Shh signaling, and differentiated into Nkx2.1, Lhx6, and calretinin-positive (CalR(+)) cells, potential cortical interneuron progenitors. Treatment with exogenous Shh increased the pool of Nkx2.1(+) progenitors, decreased Lhx6 expression, and suppressed the generation of CalR(+) cells. The blockade of endogenous Shh signaling increased the number of CalR(+) cells, but did not affect Nkx2.1 expression, implying the existence of parallel Shh-independent pathways for cortical Nkx2.1 regulation. These results support the idea that, during human brain development, Shh plays an important role in the specification of cortical progenitors. Since direct functional studies in humans are limited, the in vitro system that we established here could be of great interest for modeling the development of human cortical progenitors.

The complexity of the calretinin-expressing progenitors in the human cerebral cortex

The complex structure and function of the cerebral cortex critically depend on the balance of excitation and inhibition provided by the pyramidal projection neurons and GABAergic interneurons, respectively. The calretinin-expressing (CalR⁺) cell is a subtype of GABAergic cortical interneurons that is more prevalent in humans than in rodents. In rodents, CalR⁺ interneurons originate in the caudal ganglionic eminence (CGE) from Gsx2⁺ progenitors, but in humans it has been suggested that a subpopulation of CalR⁺ cells can also be generated in the cortical ventricular/subventricular zone (VZ/SVZ). The progenitors for cortically generated CalR⁺ subpopulation in primates are not yet characterized. Hence, the aim of this study was to identify patterns of expression of the transcription factors (TFs) that commit cortical stem cells to the CalR fate, with a focus on Gsx2. First, we studied the expression of Gsx2 and its downstream effectors, Ascl1 and Sp8 in the cortical regions of the fetal human forebrain at midgestation. Next, we established that a subpopulation of cells expressing these TFs are proliferating in the cortical SVZ, and can be co-labeled with CalR. The presence and proliferation of Gsx2⁺ cells, not only in the ventral telencephalon (GE) as previously reported, but also in the cerebral cortex suggests cortical origin of a subpopulation of CalR⁺ neurons in humans. In vitro treatment of human cortical progenitors with Sonic hedgehog (Shh), an important morphogen in the specification of interneurons, decreased levels of Ascl1 and Sp8 proteins, but did not affect Gsx2 levels. Taken together, our ex-vivo and in vitro results on human fetal brain suggest complex endogenous and exogenous regulation of TFs implied in the specification of different subtypes of CalR⁺ cortical interneurons.

Evf2 (Dlx6as) lncRNA regulates ultraconserved enhancer methylation and the differential transcriptional control of adjacent genes

Several lines of evidence suggest that long non-coding RNA (lncRNA)-dependent mechanisms regulate transcription and CpG DNA methylation. Whereas CpG island methylation has been studied in detail, the significance of enhancer DNA methylation and its relationship with lncRNAs is relatively unexplored. Previous experiments proposed that the ultraconserved lncRNA Evf2 represses transcription through Dlx6 antisense (Dlx6as) transcription and methyl-CpG binding protein (MECP2) recruitment to the Dlx5/6 ultraconserved DNA regulatory enhancer (Dlx5/6ei) in embryonic day 13.5 medial ganglionic eminence (E13.5 MGE). Here, genetic epistasis experiments show that MECP2 transcriptional repression of Evf2 and Dlx5, but not Dlx6, occurs through antagonism of DLX1/2 in E13.5 MGE. Analysis of E13.5 MGE from mice lacking Evf2 and of partially rescued Evf2 transgenic mice shows that Evf2 prevents site-specific CpG DNA methylation of Dlx5/6ei in trans, without altering Dlx5/6 expression. Dlx1/2 loss increases CpG DNA methylation, whereas Mecp2 loss does not affect Dlx5/6ei methylation. Based on these studies, we propose a model in which Evf2 inhibits enhancer DNA methylation, effectively modulating competition between the DLX1/2 activator and MECP2 repressor. Evf2 antisense transcription and Evf2-dependent balanced recruitment of activator and repressor proteins enables differential transcriptional control of adjacent genes with shared DNA regulatory elements.

Modeling Huntington's disease with induced pluripotent stem cells

Huntington's disease (HD) causes severe motor dysfunction, behavioral abnormalities, cognitive impairment and death. Investigations into its molecular pathology have primarily relied on murine tissues; however, the recent discovery of induced pluripotent stem cells (iPSCs) has opened new possibilities to model neurodegenerative disease using cells derived directly from patients, and therefore may provide a human-cell-based platform for unique insights into the pathogenesis of HD. Here, we will examine the practical implementation of iPSCs to study HD, such as approaches to differentiate embryonic stem cells (ESCs) or iPSCs into medium spiny neurons, the cell type most susceptible in HD. We will explore the HD-related phenotypes identified in iPSCs and ESCs and review how brain development and neurogenesis may actually be altered early, before the onset of HD symptoms, which could inform the search for drugs that delay disease onset. Finally, we will speculate on the exciting possibility that ESCs or iPSCs might be used as therapeutics to restore or replace dying neurons in HD brains.

Developmentally coordinated extrinsic signals drive human pluripotent stem cell differentiation toward authentic DARPP-32+ medium-sized spiny neurons

Medium-sized spiny neurons (MSNs) are the only neostriatum projection neurons, and their degeneration underlies some of the clinical features of Huntington's disease. Using knowledge of human developmental biology and exposure to key neurodevelopmental molecules, human pluripotent stem (hPS) cells were induced to differentiate into MSNs. In a feeder-free adherent culture, ventral telencephalic specification is induced by BMP/TGFβ inhibition and subsequent SHH/DKK1 treatment. The emerging FOXG1(+)/GSX2(+) telencephalic progenitors are then terminally differentiated, resulting in the systematic line-independent generation of FOXP1(+)/FOXP2(+)/CTIP2(+)/calbindin(+)/DARPP-32(+) MSNs. Similar to mature MSNs, these neurons carry dopamine and A2a receptors, elicit a typical firing pattern and show inhibitory postsynaptic currents, as well as dopamine neuromodulation and synaptic integration ability in vivo. When transplanted into the striatum of quinolinic acid-lesioned rats, hPS-derived neurons survive and differentiate into DARPP-32(+) neurons, leading to a restoration of apomorphine-induced rotation behavior. In summary, hPS cells can be efficiently driven to acquire a functional striatal fate using an ontogeny-recapitulating stepwise method that represents a platform for in vitro human developmental neurobiology studies and drug screening approaches.

Six3 cooperates with Hedgehog signaling to specify ventral telencephalon by promoting early expression of Foxg1a and repressing Wnt signaling

Six3 exerts multiple functions in the development of anterior neural tissue of vertebrate embryos. Whereas complete loss of Six3 function in the mouse results in failure of forebrain formation, its hypomorphic mutations in human and mouse can promote holoprosencephaly (HPE), a forebrain malformation that results, at least in part, from abnormal telencephalon development. However, the roles of Six3 in telencephalon patterning and differentiation are not well understood. To address the role of Six3 in telencephalon development, we analyzed zebrafish embryos deficient in two out of three Six3-related genes, six3b and six7, representing a partial loss of Six3 function. We found that telencephalon forms in six3b;six7-deficient embryos; however, ventral telencephalic domains are smaller and dorsal domains are larger. Decreased cell proliferation or excess apoptosis cannot account for the ventral deficiency. Instead, six3b and six7 are required during early segmentation for specification of ventral progenitors, similar to the role of Hedgehog (Hh) signaling in telencephalon development. Unlike in mice, we observe that Hh signaling is not disrupted in embryos with reduced Six3 function. Furthermore, six3b overexpression is sufficient to compensate for loss of Hh signaling in isl1- but not nkx2.1b-positive cells, suggesting a novel Hh-independent role for Six3 in telencephalon patterning. We further find that Six3 promotes ventral telencephalic fates through transient regulation of foxg1a expression and repression of the Wnt/β-catenin pathway.

Molecular regulation of striatal development: a review

The central nervous system is composed of the brain and the spinal cord. The brain is a complex organ that processes and coordinates activities of the body in bilaterian, higher-order animals. The development of the brain mirrors its complex function as it requires intricate genetic signalling at specific times, and deviations from this can lead to brain malformations such as anencephaly. Research into how the CNS is specified and patterned has been studied extensively in chick, fish, frog, and mice, but findings from the latter will be emphasised here as higher-order mammals show most similarity to the human brain. Specifically, we will focus on the embryonic development of an important forebrain structure, the striatum (also known as the dorsal striatum or neostriatum). Over the past decade, research on striatal development in mice has led to an influx of new information about the genes involved, but the precise orchestration between the genes, signalling molecules, and transcription factors remains unanswered. We aim to summarise what is known to date about the tightly controlled network of interacting genes that control striatal development. This paper will discuss early telencephalon patterning and dorsal ventral patterning with specific reference to the genes involved in striatal development.

Deriving excitatory neurons of the neocortex from pluripotent stem cells

The human cerebral cortex is an immensely complex structure that subserves critical functions that can be disrupted in developmental and degenerative disorders. Recent innovations in cellular reprogramming and differentiation techniques have provided new ways to study the cellular components of the cerebral cortex. Here, we discuss approaches to generate specific subtypes of excitatory cortical neurons from pluripotent stem cells. We review spatial and temporal aspects of cortical neuron specification that can guide efforts to produce excitatory neuron subtypes with increased resolution. Finally, we discuss distinguishing features of human cortical development and their translational ramifications for cortical stem cell technologies.

Purmorphamine increases DARPP-32 differentiation in human striatal neural stem cells through the Hedgehog pathway

Transplantation of neural stem cells (NSCs) is a promising therapeutic approach for Huntington's disease (HD). HD is characterized by a progressive loss of medium-sized spiny neurons (MSNs) in the striatum. DARPP-32 (dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa) is expressed in 98% of these MSNs. To establish an effective cell therapy for HD, the differentiation of human NSCs into MSNs is essential. Enhancing differentiation of NSCs is therefore an important aspect to optimize transplant efficacy. A comparison of 5 differentiation protocols indicated that the Hedgehog agonist purmorphamine (1 μM) most significantly increased the neuronal differentiation of a human striatal NSC line (STROC05). This 3-fold increase in neurons was associated with a dramatic reduction in proliferation as well as a decrease in astrocytic differentiation. A synergistic effect between purmorphamine and cell density even further increased neuronal differentiation from 20% to 30% within 7 days. Upon long-term differentiation (21 days), this combined differentiation protocol tripled the number of DARPP-32 cells (7%) and almost doubled the proportion of calbindin cells. However, there was no effect on calretinin cells. Differential expression of positional specification markers (DLX2, MASH1, MEIS2, GSH2, and NKX2.1) further confirmed the striatal identity of these differentiated cells. Purmorphamine resulted in a significant upregulation of the Hedgehog (Hh) signaling pathway (GLI1 expression). Cyclopamine, an Hh inhibitor, blocked this effect, indicating that purmorphamine specifically acts through this pathway to increase neuronal differentiation. These results demonstrate that small synthetic molecules can play a pivotal role in directing the differentiation of NSCs to optimize their therapeutic potential in HD.

Midbrain and forebrain patterning delivers immunocytochemically and functionally similar populations of neuropeptide Y containing GABAergic neurons

Neurons differentiated in vitro from embryonic stem cells (ESCs) have the potential to serve both as models of disease states and in drug discovery programs. In this study, we use sonic hedgehog (SHH) and fibroblast growth factor 8 (FGF-8) to enrich for forebrain and midbrain phenotypes from mouse ESCs. We then investigate, using Ca(2+) imaging and [(3)H]-GABA release studies, whether the GABAergic neurons produced exhibit distinct functional phenotypes. At day 24 of differentiation, reverse transcriptase-PCR showed the presence of both forebrain (Bf-1, Hesx1, Pgc-1α, Six3) and midbrain (GATA2, GATA3) selective mRNA markers in developing forebrain-enriched cultures. All markers were present in midbrain cultures except for Bf-1 and Pgc-1α. Irrespective of culture conditions all GABA immunoreactive neurons were also immunoreactive to neuropeptide Y (NPY) antibodies. Forebrain and midbrain GABAergic neurons responded to ATP (1 mM), L-glutamate (30 μM), noradrenaline (30 μM), acetylcholine (30 μM) and dopamine (30 μM), with similar elevations of intracellular Ca(2+)([Ca(2+)](i)). The presence of GABA(A) and GABA(B) antagonists, bicuculline (30 μM) and CGP55845 (1 μM), increased the elevation of [Ca(2+)](i) in response to dopamine (30 μM) in midbrain, but not forebrain GABAergic neurons. All agonists, except dopamine, elicited similar [(3)H]-GABA release from forebrain and midbrain cultures. Dopamine (30 μM) did not stimulate significant [(3)H]-GABA release in midbrain cultures, although it was effective in forebrain cultures. This study shows that differentiating neurons toward a midbrain fate restricts the expression of forebrain markers. Forebrain differentiation results in the expression of forebrain and midbrain markers. All GABA(+) neurons contain NPY, and show similar agonist-induced elevations of [Ca(2+)](i) and [(3)H]-GABA release. This study indicates that the pharmacological phenotype of these particular neurons may be independent of the addition of the patterning factors that direct neurons toward forebrain and midbrain fates.

Long non-coding RNAs and enhancers

Long non-coding RNAs (ncRNAs) are emerging as important regulatory factors in mammalian genomics. A number of reports within the last 2 years have identified thousands of actively expressed long ncRNA transcripts with distinct properties. The long ncRNAs show differential expression patterns and regulation in a wide variety of cells and tissues, adding significant complexity to the understanding of their biological role. Furthermore, genome-wide studies of transcriptional enhancers based on chromatin modifications and enhancer binding proteins have led to the identification of putative enhancers and provided insight into their tissue-specific regulation of gene expression. In an exciting turn of events, new evidence is indicating that long ncRNAs are associated with enhancer regions and that such non-coding transcription correlate with the increased activity of the neighboring genes. Moreover, additional experiments suggest that enhancer-function can be mediated through a transcribed long ncRNA and that this might be a common function for long ncRNAs. Here, we review recent advances made both in the genome-wide characterization of enhancers and in the identification of new classes of long ncRNAs, and discuss the functional overlap of these two classes of regulatory elements.

Balanced Shh signaling is required for proper formation and maintenance of dorsal telencephalic midline structures

BACKGROUND

The rostral telencephalic dorsal midline is an organizing center critical for the formation of the future cortex and hippocampus. While the intersection of WNTs, BMPs, and FGFs establishes boundaries within this critical center, a direct role of Shh signaling in this region remains controversial. In this paper we show that both increased and decreased Shh signaling directly affects boundary formation within the telencephalic dorsal midline.

RESULTS

Viral over-expression of Shh in the embryonic telencephalon prevents formation of the cortical hem and choroid plexus, while expanding the roof plate. In a transgenic model where cholesterol-lacking ShhN is expressed from one allele (ShhN/+), genes expressed in all three domains, cortical hem, choroid plexus and roof plate expand. In Gli1/2 -/- mutant brains, where Shh signaling is reduced, the roof plate expands, again at the expense of cortical hem and plexus. Cell autonomous activation of Shh signaling in the dorsal midline through Gdf7-driven activated Smoothened expression results in expansion of the Wnt3a-expressing cortical hem into the plexus domain. In addition, developmental stage determines dorsal midline responsiveness to Shh.

CONCLUSIONS

Together, these data demonstrate that balanced Shh signaling is critical for maintaining regional boundaries within the dorsal midline telencephalic organizing center.

Ptch1-mediated dosage-dependent action of Shh signaling regulates neural progenitor development at late gestational stages

Sonic hedgehog (Shh) signaling regulates cell differentiation and proliferation during brain development. However, the role of Shh in neurogenesis during late gestation (embryonic day 13.5-18.5) remains unclear. Herein, we used a genetic approach and in utero electroporation to investigate the role of mouse Shh and patched homolog 1 (Ptch1), the putative receptor for Shh. Proliferating cortical intermediate (basal) progenitor cells (IPCs) were severely reduced in Shh mutant mice, suggesting that endogenous Shh signaling could play an essential role in cortical IPC development. During cortical neurogenesis, strong upregulation of Shh signaling enhanced the transition from ventricular zone (VZ) progenitors to ventralized IPCs, while low levels of signaling enhanced the generation and proliferation of cortical IPCs in the subventricular zone. The effects of Shh upregulation in this study were consistent with a phenotype of conditional loss of function of Ptch1, and the phenotype of a hypomorphic allele of Ptch1, respectively. These data indicated that endogenous Ptch1 mediates the broad effects of Shh on the transition from VZ progenitors to IPCs and activation of proliferation of the IPCs in the cortex during late gestational stages.

Proliferation and cell death in an experimental model of brain tissue heterotopia in the lung

PURPOSE

To investigate the proliferation and neuronal death in brain tissue heterotopia in the lung in an experimental model during both fetal and neonatal periods.

METHODS

Twenty four pregnant female Swiss mice were used to induce brain tissue heterotopia on the 15th gestational day. Briefly, the brain of one fetus of each dam was extracted, disaggregated and injected into the right hemithorax of siblings. Six of these fetuses with pulmonary brain tissue implantation (PBI) were collected on the 18th gestational day (group E18) and six other on the 8th postnatal day (group P8). Immunohistochemical staining for PCNA and Bcl2 were used to assess proliferation and cell death.

RESULTS

PCNA Labelling Index (LI) in heterotopic brain tissue was greater in fetal than postnatal period (E18 > P8) (p<0.05) and the immunostaining with Bcl2 antibody did not show difference.

CONCLUSION

Cell proliferation is maintained in brain tissue heterotopia, although apoptosis is also observed.

Regulatory RNAs in brain function and disorders

Regulatory RNAs are being increasingly investigated in neurons, and important roles in brain function have been revealed. Regulatory RNAs are non-protein-coding RNAs (npcRNAs) that comprise a heterogeneous group of molecules, varying in size and mechanism of action. Regulatory RNAs often exert post-transcriptional control of gene expression, resulting in gene silencing or gene expression stimulation. Here, we review evidence that regulatory RNAs are implicated in neuronal development, differentiation, and plasticity. We will also discuss npcRNA dysregulation that may be involved in pathological states of the brain such as neurodevelopmental disorders, neurodegeneration, and epilepsy.

Sonic hedgehog functions through dynamic changes in temporal competence in the developing forebrain

Morphogens act during development to provide graded spatial information that controls patterning and cell lineage specification in the nervous system. The role of morphogen signaling in instructing the expression of downstream effector transcription factors has been well established. However, a key requirement for morphogen signaling is the existence of functional intracellular machinery able to mediate the appropriate response in target cells. Here we suggest that dynamic changes in the temporal responses to Shh in the developing ventral telencephalon occur through alterations in progenitor competence. We suggest these developmental changes in competence are mediated by a transcriptional mechanism that intrinsically integrates information from the distinct signaling pathways that act to pattern the telencephalic neuroepithelium.

The transcription factor Foxg1 regulates the competence of telencephalic cells to adopt subpallial fates in mice

Foxg1 is required for development of the ventral telencephalon in the embryonic mammalian forebrain. Although one existing hypothesis suggests that failed ventral telencephalic development in the absence of Foxg1 is due to reduced production of the morphogens sonic hedgehog (Shh) and fibroblast growth factor 8 (Fgf8), the possibility that telencephalic cells lacking Foxg1 are intrinsically incompetent to generate the ventral telencephalon has remained untested. We examined the ability of Foxg1(-/-) telencephalic cells to respond to Shh and Fgf8 by examining the expression of genes whose activation requires Shh or Fgf8 in vivo and by testing their responses to Shh and Fgf8 in culture. We found that many elements of the Shh and Fgf8 signalling pathways continue to function in the absence of Foxg1 but, nevertheless, we were unable to elicit normal responses of key ventral telencephalic marker genes in Foxg1(-/-) telencephalic tissue following a range of in vivo and in vitro manipulations. We explored the development of Foxg1(-/-) cells in Foxg1(-/-) Foxg1(+/+) chimeric embryos that contained ventral telencephalon created by normally patterned wild-type cells. We found that Foxg1(-/-) cells contributed to the chimeric ventral telencephalon, but that they retained abnormal specification, expressing dorsal rather than ventral telencephalic markers. These findings indicate that, in addition to regulating the production of ventralising signals, Foxg1 acts cell-autonomously in the telencephalon to ensure that cells develop the competence to adopt ventral identities.

Patterning of ventral telencephalon requires positive function of Gli transcription factors

The ability of neuroepithelial cells to generate a diverse array of neurons is influenced by locally secreted signals. In the spinal cord, Sonic Hedgehog (Shh) is known to induce distinct cell fates in a concentration-dependent manner by regulating the activities of the three Gli transcription factors in neural precursors. However, whether Gli-mediated Shh signaling is also required to induce different cell types in the ventral telencephalon has been controversial. In particular, loss of Shh has little effect on dorsoventral patterning of the telencephalon when Gli3 is also removed. Furthermore, no ventral telencephalic phenotypes have been found in individual Gli mutants. To address this issue, we first characterized Shh-responding ventral telencephalic progenitors between E9.5 and E12.5 and found that they produce neurons migrating to different layers of the cortex. We also discovered a loss of Nkx2.1 and Nkx6.2 expression in two subgroups of progenitors in embryos lacking major Gli activators. Finally, we analyzed the telencephalic phenotypes of embryos lacking all Gli genes and found that the ventral telencephalon was highly disorganized with intermingling of distinct neuronal cell types. Together, these studies unravel a role for Gli transcription factors in mediating Shh signaling to control specification, differentiation and positioning of ventral telencephalic neurons.

Balanced gene regulation by an embryonic brain ncRNA is critical for adult hippocampal GABA circuitry

Genomic studies demonstrate that while the majority of the mammalian genome is transcribed, only about 2% of these transcripts are protein coding. We have been investigating how the long, polyadenylated Evf2 non-coding RNA regulates transcription of homeodomain transcription factors DLX5 and DLX6 in the developing mouse forebrain. Here we show that in developing ventral forebrain, Evf2 recruits DLX and MECP2 transcription factors to key DNA regulatory elements in the Dlx 5/6 intergenic region and controls Dlx5, Dlx6, and GAD67 expression through trans and cis-acting mechanisms. Evf2 mouse mutants have reduced numbers of GABAergic interneurons in early post-natal hippocampus and dentate gyrus. Although the numbers of GABAergic interneurons and GAD67 RNA levels return to normal in Evf2 mutant adult hippocampus, reduced synaptic inhibition occurs. These results suggest that non-coding RNA-dependent balanced gene regulation in embryonic brain is critical for proper formation of GABA-dependent neuronal circuitry in adult brain.

Purifying the hedgehog protein and its variants

The purification of recombinant versions of the N-terminal signaling fragment of Sonic hedgehog (ShhN) from E. coli, Hi-5 insect cells, yeast, and mammalian cell sources reveals diverse post-translational modifications that affect the potency of the purified protein. Modifications to the N-terminal cysteine with fatty acyl groups results in significant increases in potency, up to 100-fold, when compared with the unmodified protein. Proteolytic clipping at sites near the N-terminus results in inactivation of signaling activity. The ShhN protein is particularly sensitive to metal ion-induced oxidation, and the methods described here were developed to minimize this oxidation. The purification methods developed for ShhN were applicable to human Indian and Desert hedgehog N-terminal signaling proteins, and therefore should be useful for hedgehog proteins from other species.

The requirement of Nkx2-1 in the temporal specification of cortical interneuron subtypes

Previous work has demonstrated that the character of mouse cortical interneuron subtypes can be directly related to their embryonic temporal and spatial origins. The relationship between embryonic origin and the character of mature interneurons is likely reflected by the developmental expression of genes that direct cell fate. However, a thorough understanding of the early genetic events that specify subtype identity has been hampered by the perinatal lethality resulting from the loss of genes implicated in the determination of cortical interneurons. Here, we employ a conditional loss-of-function approach to demonstrate that the transcription factor Nkx2-1 is required for the proper specification of specific interneuron subtypes. Removal of this gene at distinct neurogenic time points results in a switch in the subtypes of neurons observed at more mature ages. Our strategy reveals a causal link between the embryonic genetic specification by Nkx2-1 in progenitors and the functional attributes of their neuronal progeny in the mature nervous system.

Mash1 and neurogenin 2 enhance survival and differentiation of neural precursor cells after transplantation to rat brains via distinct modes of action

Neural precursor cells (NPCs) are regarded as a promising source of donor cells in transplantation-based therapies for neurodegenerative disorders. However, poor survival and limited neuronal differentiation of the transplanted NPCs remain critical limitations for developing therapeutic strategies. In this study, we investigated the effects of the proneural basic helix-loop-helix (bHLH) transcription factors Mash1 and Neurogenin 2 (Ngn2) in neuronal differentiation and survival of NPCs. Induction of Mash1 or Ngn2 expression strikingly enhanced neuronal differentiation of cultured NPCs in vitro. Ngn2-transduced NPCs underwent a rapid cell cycle arrest, which often accompanies differentiation. In contrast, cells continuously expanded upon Mash1 expression during NPC differentiation. Notably, sonic hedgehog (SHH) was upregulated by Mash1 and mediated the proliferative and survival effects of Mash1 on NPCs. Upon transplantation into adult rat brains, Mash1-expressing NPCs yielded large grafts enriched with neurons compared to control LacZ-transduced NPCs. Interestingly, enhancements in neuronal yield, as well as in donor cell survival, were also achieved by transplanting Ngn2-transduced NPCs. We show that a differentiation stage- and cell density-dependent survival effect of Ngn2 involves neurotrophin3 (NT3)/TrkC-mediated signaling. Together, these findings suggest potential benefits of bHLH gene manipulation to develop successful transplantation strategies for brain disorders.

FGF8 signaling patterns the telencephalic midline by regulating putative key factors of midline development

FGF8 has been reported to act as a primary regulator of neocortical patterning along the anteroposterior (AP) axis in the mouse telencephalon, and disruption of FGF signaling causes distortion of molecular arealization along the AP axis. Since hypoplasia of midline structures is observed in Fgf8 mutant mice, FGF8 is also postulated to be involved in telencephalic midline development. In this study we analyzed the role of FGF8 in midline development by means of gain-of-function and loss-of-function experiments. The results showed that FGF8 up-regulates the expression of transcription factor (TF) genes, including putative key factors involved in midline development. Although FGF8 had been thought to act downstream of SHH signaling, ectopic FGF8 up-regulates the expression of midline TF genes in Shh null mice, suggesting that FGF signaling acts as an upstream positive regulator of midline TFs during midline development independently of SHH.

Expression patterns of the three Teashirt-related genes define specific boundaries in the developing and postnatal mouse forebrain

We compare the expression patterns of the three mouse Teashirt (mTsh) genes during development of the forebrain and at a postnatal stage. During development, mTsh genes are expressed in domains that are restricted both dorsoventrally and rostrocaudally, with major changes in expression level coinciding with compartment boundaries. Striking complementarities in the distribution of mTsh transcripts were observed in the developing diencephalon, telencephalon, and olfactory bulb (OB). A mTsh1-positive cell population is part of the DLX-positive population localized in the dorsalmost portion of the lateral ganglionic eminence (dLGE). Comparison of the mTsh1 expression domain with the domains of Er81 and Islet1, which mark two distinct progenitor populations in the subventricular zone of the LGE, suggests that mTsh1 marks OB interneuron progenitors. Furthermore, the distinct expression patterns of mTsh1 and mTsh2 in the ventral LGE and the dLGE highlight the differential contributions of these structures to the striatum and the amydaloid complex. For Sey/Sey mutants, we show that Pax6 function is critical for the correct specification of the mTsh1+ population in the dLGE during embryogenesis. At postnatal stages in the OB, mTsh1 is expressed in granule and periglomerular cells, which originate from the subpallium during development. Furthermore, mTsh1+ cells line the walls of the anterior lateral ventricle, a region that gives rise to the interneurons that migrate in the rostral migratory streams and populate the OB postnatally. Our results suggest a role for mTsh genes in the establishment of regional identity and specification of cell types in the developing and adult forebrain.

Co-option of signaling mechanisms from neural induction to telencephalic patterning

This article provides an overview of signaling processes during early specification of the anterior neural tube, with special emphasis on the telencephalon. A series of signaling systems based on the action of distinct morphogens acts at different developmental stages, specifying interacting developmental fields that define axes of differentiation in the rostrocaudal and the dorsoventral domains. Interestingly, many of these signaling systems are co-opted for several differentiation processes. This strategy provides a simple and efficient mechanism to generate novel structures in evolution, and may have been especially important in the origin of the telencephalon and the mammalian cerebral cortex. For example, the action of fibroblast growth factor (FGF) secreted in early stages from the anterior neural ridge, but in later stages from the dorsal anterior forebrain, may have been a key factor in the early differentiation of the ventral telencephalon and in the eventual expansion of the mammalian neocortex. Likewise, bone morphogenetic proteins (BMPs) participate at several stages in neural patterning, even if early neural induction consists of the inhibition of the BMP pathway. BMPs, secreted dorsally, interact with FGFs in the frontal aspect of the hemispheres, and with PAX6-dependent signaling sources located laterally, to pattern the dorsal telencephalon. The actions of other morphogens are also described in this context, such as the ventralizing factor SHH, the dorsalizing element GLI3, and other factors related to the dorsomedial telencephalon such as WNTs and EMXs. The main conclusion we draw from this review is the well-known phylogenetic and developmental conservatism of signaling pathways, which in evolution have been applied in different embryological contexts, generating novel interactions between morphogenetic fields and leading to the generation of new morphological structures.

Ectopic sonic hedgehog signaling impairs telencephalic dorsal midline development: implication for human holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental anomaly of the human forebrain, and in its severe form, the cerebral hemispheres fail to completely separate into two distinct halves. Although disruption of ventral forebrain induction is thought to underlie most HPE cases, a subset of HPE patients exhibits preferential dysgenesis of forebrain dorsal midline structures with unknown etiology. In this study, we show that Sonic hedgehog (Shh) lacking cholesterol moiety in one allele (ShhN/+) in mice can elicit ectopic Shh signaling in early telencephalon to induce ventral progenitor marker expression in the cortical region and impair telencephalic dorsal midline development. Prolonged ectopic ShhN signaling impaired Bmp and Wnt signaling from the dorsal patterning center through upregulation of Fgf8, leading to augmented cell proliferation, decreased cell death and impaired roof plate morphogenesis. Accordingly, ShhN/+ mutant telencephalic dorsal midline structures, including cortical hem, hippocampus and choroid plexus, either failed to form or were hypoplastic. Strikingly, ShhN/+ mutants displayed a spectrum of phenotypic features such as failure of anterior cerebral hemisphere to divide, hydrocephalus and cleft palate which have been observed in a human patient with milder HPE predicted to produce SHHN protein due to a truncation mutation in one SHH allele. We propose that elevated ectopic Shh signaling can impair dorsal telencephalic midline morphogenesis, and lead to non-cleavage of midline structures mimicking human HPE with dorsal midline defects.

Perlecan controls neurogenesis in the developing telencephalon

Background

Perlecan is a proteoglycan expressed in the basal lamina of the neuroepithelium during development. Perlecan absence does not impair basal lamina assembly, although in the 55% of the mutants early disruptions of this lamina conducts to exencephaly, impairing brain development. The rest of perlecan-null brains complete its prenatal development, maintain basal lamina continuity interrupted by some isolated ectopias, and are microcephalic. Microcephaly consists of thinner cerebral walls and underdeveloped ganglionic eminences. We have studied the mechanisms that generate brain atrophy in telencephalic areas where basal lamina is intact.

Results

Brain atrophy in the absence of perlecan started in the ventral forebrain and extended to lateral and dorsal parts of the cortex in the following stages. First, the subpallial forebrain developed poorly in early perlecan-null embryos, because of a reduced cell proliferation: the number of cells in mitosis decreased since the early stages of development. This reduction resulted in a decreased tangential migration of interneurons to the cerebral cortex. Concomitant with the early hypoplasia observed in the medial ganglionic eminences, Sonic Hedgehog signal decreased in the perlecan-null floor plate basal lamina at E12.5. Second, neurogenesis in the pallial neuroepithelium was affected in perlecan deficient embryos. We found reductions of nearly 50% in the number of cells exiting the cell cycle at E12–E13. The labeling index, which was normal at this age, significantly decreased with advancing corticogenesis. Moreover, nestin⁺ or PCNA⁺ progenitors increased since E14.5, reaching up to about 150% of the proportion of PCNA⁺ cells in the wild-type at E17.5. Thus, labeling index reduction together with increased progenitor population, suggests that atrophy is the result of altered cell cycle progression in the cortical progenitors. Accordingly, less neurons populated the cortical plate and subplate of perlecan-null neocortex, as seen with the neuronal markers β-tubulin and Tbr1.

Conclusion

As a component of the basal lamina, perlecan both maintains this structure and controls the response of the neuroepithelium to growth factors. Less mitotic cells in the early medial ganglionic eminences, and impaired cell cycle progression in the late neocortex, suggests insufficient recruitment and signaling by neurogenic morphogens, such as SHH or FGF2.

Isolation of rat telencephalic neural explants to assay Shh GABAergic interneuron differentiation-inducing activity

Multiple assays for Shh activity using cell lines, primary cultures, and explanted tissue have been described. We first described the use of E11.5 rat dorsal telencephalic explants to assay Shh ventralizing and differentiation-inducing activity in Kohtz et al. (1). Using this assay, we subsequently showed that N-lipid modification is critical for Shh activity in the telencephalon (2). In vivo assays for lipid-modified Shh support the results of our E11.5 telencephalic neural explant assay (2). More recently, the method of isolating telencephaic explants was improved by an intraocular grid, increasing both its accuracy and reproducibility (3). Shh induces the expression of the following ventral telencephalic markers: MASH-1, the Dlx's, and Islet 1/2. Therefore, this assay for Shh induction of GABAergic interneurons defines a competent, but naïve region within the E11.5 dorsal telencephalon, allowing the study of GABAergic interneuron induction and differentiation from an unspecified progenitor population.

Genetic patterning of the mammalian telencephalon by morphogenetic molecules and transcription factors

Patterning centers that produce gradients of morphogenetic molecules, including fibroblast growth factor (FGF), bone morphogenetic proteins (BMP), Wnt, Sonic hedgehog (Shh), and retinoic acid (RA), are located in telencephalic anlage during early stages of development. Genetic evidence based on loss-of-function and gain-of-function studies indicate that they are involved in regional specification of the dorsal, ventral, and lateral telencephalon. For patterning of the dorsal telencephalon, FGF8 controls the anteroposterior patterning, while BMP and Wnt molecules regulate the mediolateral patterning. Shh and retinoic acid regulate patterning of the ventral and the lateral telencephalon. The regionalization of telencephalon is accompanied by expression of region-specific codes of transcription factors, which in turn regulate different phases of neuronal development to generate different cell types in each brain region. Therefore, bioactive signals of morphogenetic molecules are translated into transcription factor codes for regional specification, which subsequently leads to neurogenesis of the diversity of cell types in different regions of the telencephalon.