Retinoic acid signalling specifies intermediate character in the developing telencephalon

The retinoic acid family-like nuclear receptor SmRAR identified by single-cell transcriptomics of ovarian cells controls oocyte differentiation in Schistosoma mansoni

Studies on transcription regulation in platyhelminth development are scarce, especially for parasitic flatworms. Here, we employed single-cell transcriptomics to identify genes involved in reproductive development in the trematode model Schistosoma mansoni. This parasite causes schistosomiasis, a major neglected infectious disease affecting >240 million people worldwide. The pathology of schistosomiasis is closely associated with schistosome eggs deposited in host organs including the liver. Unlike other trematodes, schistosomes exhibit distinct sexes, with egg production reliant on the pairing-dependent maturation of female reproductive organs. Despite this significance, the molecular mechanisms underlying ovary development and oocyte differentiation remain largely unexplored. Utilizing an organ isolation approach for S. mansoni, we extracted ovaries of paired females followed by single-cell RNA sequencing (RNA-seq) with disassociated oocytes. A total of 1967 oocytes expressing 7872 genes passed quality control (QC) filtering. Unsupervised clustering revealed four distinct cell clusters: somatic, germ cells and progeny, intermediate and late germ cells. Among distinct marker genes for each cluster, we identified a hitherto uncharacterized transcription factor of the retinoic acid receptor family, SmRAR. Functional analyses of SmRAR and associated genes like Smmeiob (meiosis-specific, oligonucleotide/oligosaccharide binding motif (OB) domain-containing) demonstrated their pairing-dependent and ovary-preferential expression and their decisive roles in oocyte differentiation of S. mansoni.

Spatial enhancer activation influences inhibitory neuron identity during mouse embryonic development

The mammalian telencephalon contains distinct GABAergic projection neuron and interneuron types, originating in the germinal zone of the embryonic basal ganglia. How genetic information in the germinal zone determines cell types is unclear. Here we use a combination of in vivo CRISPR perturbation, lineage tracing and ChIP-sequencing analyses and show that the transcription factor MEIS2 favors the development of projection neurons by binding enhancer regions in projection-neuron-specific genes during mouse embryonic development. MEIS2 requires the presence of the homeodomain transcription factor DLX5 to direct its functional activity toward the appropriate binding sites. In interneuron precursors, the transcription factor LHX6 represses the MEIS2-DLX5-dependent activation of projection-neuron-specific enhancers. Mutations of Meis2 result in decreased activation of regulatory enhancers, affecting GABAergic differentiation. We propose a differential binding model where the binding of transcription factors at cis-regulatory elements determines differential gene expression programs regulating cell fate specification in the mouse ganglionic eminence.

Understanding and modeling regional specification of the human ganglionic eminence

Inhibitory neurons originating from the ventral forebrain are associated with several neurological conditions. Distinct ventral forebrain subpopulations are generated from topographically defined zones; lateral-, medial- and caudal ganglionic eminences (LGE, MGE and CGE), yet key specification factors often span across developing zones contributing to difficulty in defining unique LGE, MGE or CGE profiles. Here we use human pluripotent stem cell (hPSC) reporter lines (NKX2.1-GFP and MEIS2-mCherry) and manipulation of morphogen gradients to gain greater insight into regional specification of these distinct zones. We identified Sonic hedgehog (SHH)-WNT crosstalk in regulating LGE and MGE fate and uncovered a role for retinoic acid signaling in CGE development. Unraveling the influence of these signaling pathways permitted development of fully defined protocols that favored generation of the three GE domains. These findings provide insight into the context-dependent role of morphogens in human GE specification and are of value for in vitro disease modeling and advancement of new therapies.

Self-organization of human dorsal-ventral forebrain structures by light induced SHH

Organizing centers secrete morphogens that specify the emergence of germ layers and the establishment of the body's axes during embryogenesis. While traditional experimental embryology tools have been instrumental in dissecting the molecular aspects of organizers in model systems, they are impractical in human in-vitro model systems to dissect the relationships between signaling and fate along embryonic coordinates. To systematically study human embryonic organizer centers, we devised a collection of optogenetic ePiggyBac vectors to express a photoactivatable Cre-loxP recombinase, that allows the systematic induction of organizer structures by shining blue-light on human embryonic stem cells (hESCs). We used a light stimulus to geometrically confine SHH expression in neuralizing hESCs. This led to the self-organization of mediolateral neural patterns. scRNA-seq analysis established that these structures represent the dorsal-ventral forebrain, at the end of the first month of development. Here, we show that morphogen light-stimulation is a scalable tool that induces self-organizing centers.

Regulation of otocyst patterning by Tbx2 and Tbx3 is required for inner ear morphogenesis in the mouse

All epithelial components of the inner ear, including sensory hair cells and innervating afferent neurons, arise by patterning and differentiation of epithelial progenitors residing in a simple sphere, the otocyst. Here, we identify the transcriptional repressors TBX2 and TBX3 as novel regulators of these processes in the mouse. Ablation of Tbx2 from the otocyst led to cochlear hypoplasia, whereas loss of Tbx3 was associated with vestibular malformations. The loss of function of both genes (Tbx2/3cDKO) prevented inner ear morphogenesis at midgestation, resulting in indiscernible cochlear and vestibular structures at birth. Morphogenetic impairment occurred concomitantly with increased apoptosis in ventral and lateral regions of Tbx2/3cDKO otocysts around E10.5. Expression analyses revealed partly disturbed regionalisation, and a posterior-ventral expansion of the neurogenic domain in Tbx2/3cDKO otocysts at this stage. We provide evidence that repression of FGF signalling by TBX2 is important to restrict neurogenesis to the anterior-ventral otocyst and implicate another T-box factor, TBX1, as a crucial mediator in this regulatory network.

Retinoic acid signaling regulates proliferation and lamina formation in the developing chick optic tectum

The retinotectal system has been extensively studied for investigating the mechanism(s) for topographic map formation. The optic tectum, which is composed of multiple laminae, is the major retino recipient structure in the developing avian brain. Laminar development of the tectum results from cell proliferation, differentiation and migration, coordinated in strict temporal and spatial patterns. However, the molecular mechanisms that orchestrate these complex developmental events, have not been fully elucidated. In this study, we have identified the presence of differential retinoic acid (RA) signaling along the rostro-caudal and dorsoventral axis of the tectum. We show for the first time that loss of RA signaling in the anterior optic tectum, leads to an increase in cell proliferation and gross changes in the morphology manifested as defects in lamination. Detailed analysis points to delayed migration of cells as the plausible cause for the defects in lamina formation. Thus, we conclude that in the optic tectum, RA signaling is involved in maintaining cell proliferation and in regulating the formation of the tectal laminae.

Probing disrupted neurodevelopment in autism using human stem cell-derived neurons and organoids: An outlook into future diagnostics and drug development

Autism spectrum disorders (ASDs) represent a spectrum of neurodevelopmental disorders characterized by impaired social interaction, repetitive or restrictive behaviors, and problems with speech. According to a recent report by the Centers for Disease Control and Prevention, one in 68 children in the US is diagnosed with ASDs. Although ASD-related diagnostics and the knowledge of ASD-associated genetic abnormalities have improved in recent years, our understanding of the cellular and molecular pathways disrupted in ASD remains very limited. As a result, no specific therapies or medications are available for individuals with ASDs. In this review, we describe the neurodevelopmental processes that are likely affected in the brains of individuals with ASDs and discuss how patient-specific stem cell-derived neurons and organoids can be used for investigating these processes at the cellular and molecular levels. Finally, we propose a discovery pipeline to be used in the future for identifying the cellular and molecular deficits and developing novel personalized therapies for individuals with idiopathic ASDs.

Retinoic acid regulates avian lung branching through a molecular network

Retinoic acid (RA) is of major importance during vertebrate embryonic development and its levels need to be strictly regulated otherwise congenital malformations will develop. Through the action of specific nuclear receptors, named RAR/RXR, RA regulates the expression of genes that eventually influence proliferation and tissue patterning. RA has been described as crucial for different stages of mammalian lung morphogenesis, and as part of a complex molecular network that contributes to precise organogenesis; nonetheless, nothing is known about its role in avian lung development. The current report characterizes, for the first time, the expression pattern of RA signaling members (stra6, raldh2, raldh3, cyp26a1, rarα, and rarβ) and potential RA downstream targets (sox2, sox9, meis1, meis2, tgfβ2, and id2) by in situ hybridization. In the attempt of unveiling the role of RA in chick lung branching, in vitro lung explants were performed. Supplementation studies revealed that RA stimulates lung branching in a dose-dependent manner. Moreover, the expression levels of cyp26a1, sox2, sox9, rarβ, meis2, hoxb5, tgfβ2, id2, fgf10, fgfr2, and shh were evaluated after RA treatment to disclose a putative molecular network underlying RA effect. In situ hybridization analysis showed that RA is able to alter cyp26a1, sox9, tgfβ2, and id2 spatial distribution; to increase rarβ, meis2, and hoxb5 expression levels; and has a very modest effect on sox2, fgf10, fgfr2, and shh expression levels. Overall, these findings support a role for RA in the proximal-distal patterning and branching morphogenesis of the avian lung and reveal intricate molecular interactions that ultimately orchestrate branching morphogenesis.

Retinoic acid signaling regulates development of the dorsal forebrain midline and the choroid plexus in the chick

The developing forebrain roof plate (RP) contains a transient signaling center, perturbations in which have been linked to holoprosencephaly (HPE). Here, we describe a novel domain of retinoic acid (RA) signaling that is specific to the chick RP and demonstrate that RA signaling is sufficient for inducing characteristics of the RP in ectopic locations. We further demonstrate that, unlike what has been observed in the mouse, RA signaling is essential for invagination of the RP in chick, failure of which leads to an HPE-like phenotype. In addition, we found that RA exerts a negative influence on choroid plexus differentiation. Thus, our findings identify RA as a novel regulator of chick forebrain RP development.

RA and FGF signalling are required in the zebrafish otic vesicle to pattern and maintain ventral otic identities

During development of the zebrafish inner ear, regional patterning in the ventral half of the otic vesicle establishes zones of gene expression that correspond to neurogenic, sensory and non-neural cell fates. FGF and Retinoic acid (RA) signalling from surrounding tissues are known to have an early role in otic placode induction and otic axial patterning, but how external signalling cues are translated into intrinsic patterning during otic vesicle (OV) stages is not yet understood. FGF and RA signalling pathway members are expressed in and around the OV, suggesting important roles in later patterning or maintenance events. We have analysed the temporal requirement of FGF and RA signalling for otic development at stages after initial anteroposterior patterning has occurred. We show that high level FGF signalling acts to restrict sensory fates, whereas low levels favour sensory hair cell development; in addition, FGF is both required and sufficient to promote the expression of the non-neural marker otx1b in the OV. RA signalling has opposite roles: it promotes sensory fates, and restricts otx1b expression and the development of non-neural fates. This is surprisingly different from the earlier requirement for RA signalling in specification of non-neural fates via tbx1 expression, and highlights the shift in regulation that takes place between otic placode and vesicle stages in zebrafish. Both FGF and RA signalling are required for the development of the otic neurogenic domain and the generation of otic neuroblasts. In addition, our results indicate that FGF and RA signalling act in a feedback loop in the anterior OV, crucial for pattern refinement.

The vertebrate inner ear is a complex three-dimensional structure with hearing and balance functions. To form a functional ear in the embryo, it is crucial that the right cells develop at the right time and in the right place. These cells include the sensory hair cells that detect sound and movement, neurons that relay sensory information to the brain, and structural cells. We have investigated patterning and maintenance events in the developing ear of the zebrafish embryo. We show that two signalling pathways, FGF and Retinoic Acid (RA), act in an antagonistic manner to regulate the numbers of sensory hair cells that develop, together with the expression of a key gene, otx1b, required for the development of structural cells. However, the two signalling pathways act in concert to regulate the emergence of neuronal cells. Our data also indicate that FGF and RA signalling form a feedback loop, placing them at the heart of the regulatory network that ensures correct patterning is maintained in the ear. Both FGF and RA signalling are employed to generate hair cells and neurons for replacement therapies to treat hearing loss. Understanding the roles of FGF and RA signalling underpins the development of such therapies.

Retinoic acid and human olfactory ensheathing cells cooperate to promote neural induction from human bone marrow stromal stem cells

The generation of induced neuronal cells from human bone marrow stromal stem cells (hBMSCs) provides new avenues for basic research and potential transplantation therapies for nerve injury and neurological disorders. However, clinical application must seriously consider the risk of tumor formation by hBMSCs, neural differentiation efficiency and biofunctions resembling neurons. Here, we co-cultured hBMSCs exposed to retinoic acid (RA) with human olfactory ensheathing cells (hOECs) to stimulate its differentiation into neural cells, and found that hBMSCs following 1 and 2 weeks of stimulation promptly lost their immunophenotypical profiles, and gradually acquired neural cell characteristics, as shown by a remarkable up-regulation of expression of neural-specific markers (Tuj-1, GFAP and Galc) and down-regulation of typical hBMSCs markers (CD44 and CD90), as well as a rapid morphological change. Concomitantly, in addition to a drastic decrease in the number of BrdU incorporated cells, there was a more elevated synapse formation (a hallmark for functional neurons) in the differentiated hBMSCs. Compared with OECs alone, this specific combination of RA and hOECs was significantly potentiated neuronal differentiation of hBMSCs. The results suggest that RA can enhance and orchestrate hOECs to neural differentiation of hBMSCs. Therefore, these findings may provide an alternative strategy for the repair of traumatic nerve injury and neurological diseases with application of the optimal combination of RA and OECs for neuronal differentiation of hBMSCs.

Molecular mechanisms of inner ear development

The inner ear is a structurally complex vertebrate organ built to encode sound, motion, and orientation in space. Given its complexity, it is not surprising that inner ear dysfunction is a relatively common consequence of human genetic mutation. Studies in model organisms suggest that many genes currently known to be associated with human hearing impairment are active during embryogenesis. Hence, the study of inner ear development provides a rich context for understanding the functions of genes implicated in hearing loss. This chapter focuses on molecular mechanisms of inner ear development derived from studies of model organisms.

Retinoic acid signalling during development

Retinoic acid (RA) is a vitamin A-derived, non-peptidic, small lipophilic molecule that acts as ligand for nuclear RA receptors (RARs), converting them from transcriptional repressors to activators. The distribution and levels of RA in embryonic tissues are tightly controlled by regulated synthesis through the action of specific retinol and retinaldehyde dehydrogenases and by degradation via specific cytochrome P450s (CYP26s). Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations. Here, we provide an overview of the RA biosynthesis, degradation and signalling pathways and review the main functions of this molecule during embryogenesis.

Cortical neurogenesis and morphogens: diversity of cues, sources and functions

The cerebral cortex is composed of hundreds of different types of neurons, which underlie its ability to perform highly complex neural processes. How cortical neurons are generated during development constitutes a major challenge in developmental neurosciences with important implications for brain repair and diseases. Cortical neurogenesis is dependent on intrinsic and extrinsic cues, which interplay to generate cortical neurons at the right number, time and place. While the role of intrinsic factors such as proneural and Notch genes has been well established, recent evidence indicate that most classical morphogens, produced by various neural and non-neural sources throughout embryonic development, contribute to the master control and fine tuning of cortical neurogenesis. Here we review some recent advances in the dissection of the molecular logic underlying neurogenesis in the cortex, with special emphasis on the roles of morphogenic cues in this process.

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.

Modulation of Wnt and Hedgehog signaling pathways is linked to retinoic acid-induced amelioration of chronic allograft dysfunction

Chronic renal allograft damage (CAD) is manifested by a smoldering inflammatory process that leads to transplant glomerulopathy, diffuse interstitial fibrosis and tubular atrophy with loss of tubular structures. Using a Fischer 344 (RT1lvl) to Lewis (RT1l) rat renal allograft model, transcriptomic profiling and pathway mapping, we have previously shown that dynamic dysregulation of the Wnt signaling pathways may underlie progressive CAD. Retinoic acid, an important regulator of differentiation during vertebrate embryogenesis, can moderate the damage observed in this experimental model of CAD. We show here that subsets of the Hedgehog (Hh) and canonical Wnt signaling pathways are linked to the pathophysiology of progressive fibrosis, loss of cilia in epithelia and chronic dysfunction. Oral treatment with 13cis retinoic acid (13cRA) was found to selectively ameliorate the dysregulation of the Hh and canonical Wnt pathways associated with CAD, and lead to a general preservation of cilial structures. Interplay between these pathways helps explain the therapeutic effects of retinoic acid treatment in CAD, and suggests future targets for moderating chronic fibrosing organ damage.

Vitamin A in reproduction and development

The requirement for vitamin A in reproduction was first recognized in the early 1900's, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.

Meis gene expression patterns in the developing chicken inner ear

We are interested in stable gene network activities operating sequentially during inner ear specification. The implementation of this patterning process is a key event in the generation of functional subdivisions of the otic vesicle during early embryonic development. The vertebrate inner ear is a complex sensory structure that is a good model system for characterization of developmental mechanisms controlling patterning and specification. Meis genes, belonging to the TALE family, encode homodomain-containing transcription factors remarkably conserved during evolution, which play a role in normal and neoplastic development. To gain understanding of the possible role of homeobox Meis genes in the developing chick inner ear, we comprehensively analyzed their spatiotemporal expression patterns from early otic specification stages onwards. In the invaginating otic placode, Meis1/2 transcripts were observed in the borders of the otic cup, being absent in the portion of otic epithelium closest to the hindbrain. As development proceeds, Meis1 and Meis2 expressions became restricted to the dorsomedial otic epithelium. Both genes were strongly expressed in the entire presumptive domain of the semicircular canals, and more weakly in all associated cristae. The endolymphatic apparatus was labeled in part by Meis1/2. Meis1 was also expressed in the lateral wall of the growing cochlear duct, while Meis2 expression was detected in a few cells of the developing acoustic-vestibular ganglion. Our results suggest a possible role of Meis assigning regional identity in the morphogenesis, patterning, and specification of the developing inner ear.

Milestones of directed differentiation of mouse and human embryonic stem cells into telencephalic neurons based on neural development in vivo

Understanding the normal development of individual neural subtypes provides an essential framework for the design of rational approaches to embryonic stem cell differentiation for in vitro studies and cell replacement therapies. Of particular interest and a particular challenge are the cells that build-up the telencephalon. Recent research has unraveled key developmental mechanisms contributing to the generation of specific telencephalic cells. We focus on morphogens and transcription factors known to regulate distinct developmental processes. These include early anterior/posterior patterning, dorsal/ventral patterning, and generation of progenitor domains and neuronal specification into major classes of telencephalic cells: glutamatergic projection neurons, different subtypes of γ-aminobutyric acid-ergic interneurons and projection neurons, as well as cholinergic interneurons and projection neurons. Based on a comparison with in vivo telencephalic neurogenesis, we propose that the specific combinations of transcription factors expressed during development can serve as milestones for the in vitro differentiation of embryonic stem cells toward specific telencephalic neurons.

Nolz1 promotes striatal neurogenesis through the regulation of retinoic acid signaling

Background

Nolz1 is a zinc finger transcription factor whose expression is enriched in the lateral ganglionic eminence (LGE), although its function is still unknown.

Results

Here we analyze the role of Nolz1 during LGE development. We show that Nolz1 expression is high in proliferating neural progenitor cells (NPCs) of the LGE subventricular zone. In addition, low levels of Nolz1 are detected in the mantle zone, as well as in the adult striatum. Similarly, Nolz1 is highly expressed in proliferating LGE-derived NPC cultures, but its levels rapidly decrease upon cell differentiation, pointing to a role of Nolz1 in the control of NPC proliferation and/or differentiation. In agreement with this hypothesis, we find that Nolz1 over-expression promotes cell cycle exit of NPCs in neurosphere cultures and negatively regulates proliferation in telencephalic organotypic cultures. Within LGE primary cultures, Nolz1 over-expression promotes the acquisition of a neuronal phenotype, since it increases the number of β-III tubulin (Tuj1)- and microtubule-associated protein (MAP)2-positive neurons, and inhibits astrocyte generation and/or differentiation. Retinoic acid (RA) is one of the most important morphogens involved in striatal neurogenesis, and regulates Nolz1 expression in different systems. Here we show that Nolz1 also responds to this morphogen in E12.5 LGE-derived cell cultures. However, Nolz1 expression is not regulated by RA in E14.5 LGE-derived cell cultures, nor is it affected during LGE development in mouse models that present decreased RA levels. Interestingly, we find that Gsx2, which is necessary for normal RA signaling during LGE development, is also required for Nolz1 expression, which is lost in Gsx2 knockout mice. These findings suggest that Nolz1 might act downstream of Gsx2 to regulate RA-induced neurogenesis. Keeping with this hypothesis, we show that Nolz1 induces the selective expression of the RA receptor (RAR)β without altering RARα or RARγ. In addition, Nozl1 over-expression increases RA signaling since it stimulates the RA response element. This RA signaling is essential for Nolz1-induced neurogenesis, which is impaired in a RA-free environment or in the presence of a RAR inverse agonist. It has been proposed that Drosophila Gsx2 and Nolz1 homologues could cooperate with the transcriptional co-repressors Groucho-TLE to regulate cell proliferation. In agreement with this view, we show that Nolz1 could act in collaboration with TLE-4, as they are expressed at the same time in NPC cultures and during mouse development.

Conclusions

Nolz1 promotes RA signaling in the LGE, contributing to the striatal neurogenesis during development.

Arx acts as a regional key selector gene in the ventral telencephalon mainly through its transcriptional repression activity

The homeobox-containing gene Arx is expressed during ventral telencephalon development and required for correct GABAergic interneuron tangential migration from the ganglionic eminences to the olfactory bulbs, cerebral cortex and striatum. Its human ortholog is associated with a variety of neurological clinical manifestations whose symptoms are compatible with the loss of cortical interneurons and altered basal ganglia-related activities. Herein, we report the identification of a number of genes whose expression is consistently altered in Arx mutant ganglionic eminences. Our analyses revealed a striking ectopic expression in the ganglionic eminences of several of these genes normally at most marginally expressed in the ventral telencephalon. Among them, Ebf3 was functionally analyzed. Thus, its ectopic expression in ventral telencephalon was found to prevent neuronal tangential migration. Further, we showed that Arx is sufficient to repress Ebf3 endogenous expression and that its silencing in Arx mutant tissues partially rescues tangential cell movement. Together, these data provide new insights into the molecular pathways regulated by Arx during telencephalon development.

FGF signaling is strictly required to maintain early telencephalic precursor cell survival

The FGF family of extracellular signaling factors has been proposed to play multiple roles in patterning the telencephalon, the precursor to the cerebrum. In this study, unlike previous ones, we effectively abolish FGF signaling in the anterior neural plate via deletion of three FGF receptor (FGFR) genes. Triple FGFR mutant mice exhibit a complete loss of the telencephalon, except the dorsal midline. Disruption of FGF signaling prior to and coincident with telencephalic induction reveals that FGFs promote telencephalic character and are strictly required to keep telencephalic cells alive. Moreover, progressively more severe truncations of the telencephalon are observed in FGFR single, double and triple mutants. Together with previous gain-of-function studies showing induction of Foxg1 expression and mirror-image duplications of the cortex by exogenous FGF8, our loss-of-function results suggest that, rather than independently patterning different areas, FGF ligands and receptors act in concert to mediate organizer activity for the whole telencephalon.

Cerebral cortex development: From progenitors patterning to neocortical size during evolution

The central nervous system is composed of thousands of distinct neurons that are assembled in a highly organized structure. In order to form functional neuronal networks, distinct classes of cells have to be generated in a precise number, in a spatial and temporal hierarchy and to be positioned at specific coordinates. An exquisite coordination of appropriate growth of competent territories and their patterning is required for regionalization and neurogenesis along both the anterior-posterior and dorso-ventral axis of the developing nervous system. The neocortex represents the brain territory that has undergone a major increase in its relative size during the course of mammalian evolution. In this review we will discuss how the fine tuning of growth and cell fate patterning plays a crucial role in the achievement of the final size of central nervous system structures and how divergence might have contributed to the surface increase of the cerebral cortex in mammals. In particular, we will describe how lack of precision might have been instrumental to neocortical evolution.

Analysis of early ventral telencephalic defects in mice lacking functional Gli3 protein

The transcription factor Gli3 is expressed throughout developing telencephalon. Previous studies have focused on Gli3's role in dorsal telencephalon, which is greatly reduced in size in Gli3(Xt/Xt) mutants. We examined the effects of loss of Gli3 on early development of ventral telencephalon. Ventral telencephalon was defined in both wildtypes and Gli3(Xt/Xt) mutants on the basis of its expression of Olig2, Nkx2.1, Mash1, and Foxg1 and its lack of expression of Pax6. We found that at embryonic day (E)10.5 the volume of the ventral telencephalon is about 50% greater in Gli3(Xt/Xt) mutants than in wildtypes. By E12.5, however, the volume of the ventral telencephalon is about 20% lower in Gli3(Xt/Xt) mutants than in wildtypes. We observed a significant increase in the number of both apoptotic cells and newly differentiated neurons in the E10.5 Gli3(Xt/Xt) ventral telencephalon, suggesting that increased cell death and withdrawal of cells from the cell cycle might account for the failure of the Gli3(Xt/Xt) ventral telencephalon to grow normally by E12.5. We found no changes in the lengths of the cell cycles of proliferating ventral telencephalic cells at E10.5. We used marker analysis and optical projection tomography to assess the Gli3(Xt/Xt) forebrain in three dimensions and found that the Gli3(Xt/Xt) diencephalon is shifted relatively rostrally. We conclude that in the absence of Gli3 an abnormally large portion of the newly formed telencephalon is specified to a ventral fate but this then suffers impaired growth, due to defects of cell differentiation and death, contributing to severe distortion of the forebrain.

FGF signals induce Caprin2 expression in the vertebrate lens

The lens of the eye is derived from the non-neural ectoderm situated next to the optic vesicle. Fibroblast growth factor (FGF) signals play a major role at various stages of vertebrate lens development ranging from induction and proliferation to differentiation. Less is however known about the identity of genes that are induced by FGF activity within the lens. We have isolated and characterized mouse cytoplasmic activation/proliferation-associated protein-2 (Caprin2), with domains belonging to both the Caprin family and the C1q and tumour necrosis factor (TNF) super-family. Here we show that Caprin2 is expressed in the developing vertebrate lens in mouse and chick, and that Caprin2 expression is up-regulated in primary lens fiber cells, after the induction of crystallins the earliest known markers for differentiated lens fiber cells. Caprin2 is subsequently down-regulated in the centre of the lens at the time and at the position of the first fiber cell denucleation and terminal differentiation. In vitro analyses of lens fiber cell differentiation provide evidence that FGF activity emanating from neighboring prospective retinal cells is required and that FGF8 activity is sufficient to induce Caprin2 in lens fiber cells. These results not only provide evidence that FGF signals induce the newly characterized protein Caprin2 in the lens, but also support the general idea that FGF signals are required for lens fiber cell differentiation.

All-trans retinol and retinol-binding protein from embryonic cerebrospinal fluid exhibit dynamic behaviour during early central nervous system development

Embryonic cerebrospinal fluid (E-CSF) is involved in the regulation of survival, proliferation and neurogenesis of neuroectodermal progenitor cells, as well as in the control of mesencephalic gene expression in collaboration with the isthmic organizer. Recently, we showed the presence of retinol-binding protein (RBP) within the E-CSF proteome. RBP is an all-trans retinol carrier, a molecule that can be metabolized into retinoic acid, a morphogen involved in central nervous system (CNS) morphogenesis and patterning. Here we demonstrate the presence of all-trans retinol within the E-CSF and analyse the dynamics of RBP and all-trans retinol within this fluid, as well as the expression of retinoic acid-synthesizing enzymes during early CNS development. Our results suggest a relationship between the dynamics of these molecules and the early events of CNS patterning.

The genetics of early telencephalon patterning: some assembly required

The immense range of human behaviours is rooted in the complex neural networks of the cerebrum. The creation of these networks depends on the precise integration of specific neuronal subtypes that are born in different regions of the telencephalon. Here, using the mouse as a model system, we review how these proliferative zones are established. Moreover, we discuss how these regions can be traced back in development to the function of a few key genes, including those that encode fibroblast growth factors (FGFs), sonic hedgehog (SHH), bone morphogenetic proteins (BMPs), forkhead box G1 (FOXG1), paired box 6 (PAX6) and LIM homeobox protein 2 (LHX2), that pattern the early telencephalon.

Expression patterns of Shh, Ptc2, Raldh3, Pitx2, Isl1, Lim3 and Pax6 in the developing chick hypophyseal placode and Rathke's pouch

The adenohypophysis is derived from a structure called the Rathke's pouch, which is an invagination of the hypophyseal placode. Hedgehog (Hh) and retinoic acid (RA) signals as well as several transcription factors have been suggested to play a role in the development of the adenohypophysis. We have therefore examined the expression pattern of Sonic hedgehog (Shh), the hedgehog receptor Patched2 (Ptc2), the retinoic acid producing enzyme Retinaldehyde dehydrogenase3 (Raldh3) and four transcription factors, Pitx2, Isl1, Lim3 and Pax6 in chick embryos from head fold stage to embryonic day (E) 4.5. We show that already at the head fold stage, Ptc2 is expressed in prospective hypophyseal placodal cells and that Shh is expressed in the underlying mesoderm. Moreover, Shh continues to be expressed in tissues surrounding the prospective adenohypophysis, and Ptc2 is expressed in prospective hypophyseal cells. Lim3 and Pax6 are expressed from stage 10 in the prospective hypophyseal placode, whereas Pitx2 starts to be expressed before stage 10. Pitx2 is together with Pax6 expressed in the entire domain of the Rathke's pouch. Raldh3 is detected at the 20 somite stage and is together with Lim3 expressed in the anterior part of the Rathke's pouch. Isl1 is expressed in the most posterior part of the hypophyseal ectoderm in a complementary pattern to Raldh3 and Lim3.

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.

Modular patterning of structure and function of the striatum by retinoid receptor signaling

Retinoid signaling plays a crucial role in patterning rhombomeres in the hindbrain and motor neurons in the spinal cord during development. A fundamentally interesting question is whether retinoids can pattern functional organization in the forebrain that generates a high order of cognitive behavior. The striatum contains a compartmental structure of striosome (or "patch") and intervening matrix. How this highly complex mosaic design is patterned by the genetic programs during development remains elusive. We report a developmental mechanism by which retinoid receptor signaling controls compartmental formation in the striatum. We analyzed RARbeta(-/-) mutant mice and found a selective loss of striosomal compartmentalization in the rostral mutant striatum. The loss of RARbeta signaling in the mutant mice resulted in reduction of cyclin E2, a cell cycle protein regulating transition from G(1) to S phase, and also reduction of the proneural gene Mash1, which led to defective neurogenesis of late-born striosomal cells. Importantly, during striatal neurogenesis, endogenous levels of retinoic acid were spatiotemporally regulated such that transduction of high levels of retinoic acid through RARbeta selectively expanded the population of late-born striosomal progenitors, which evolved into a highly elaborate compartment in the rostral striatum. RARbeta(-/-) mutant mice, which lacked such enlarged compartment, displayed complex alternations of dopamine agonist-induced stereotypic motor behavior, including exaggeration of head bobbing movement and reduction of rearing activity. RARbeta signaling thus plays a crucial role in setting up striatal compartments that may engage in neural circuits of psychomotor control.

Early development of the central and peripheral nervous systems is coordinated by Wnt and BMP signals

The formation of functional neural circuits that process sensory information requires coordinated development of the central and peripheral nervous systems derived from neural plate and neural plate border cells, respectively. Neural plate, neural crest and rostral placodal cells are all specified at the late gastrula stage. How the early development of the central and peripheral nervous systems are coordinated remains, however, poorly understood. Previous results have provided evidence that at the late gastrula stage, graded Wnt signals impose rostrocaudal character on neural plate cells, and Bone Morphogenetic Protein (BMP) signals specify olfactory and lens placodal cells at rostral forebrain levels. By using in vitro assays of neural crest and placodal cell differentiation, we now provide evidence that Wnt signals impose caudal character on neural plate border cells at the late gastrula stage, and that under these conditions, BMP signals induce neural crest instead of rostral placodal cells. We also provide evidence that both caudal neural and caudal neural plate border cells become independent of further exposure to Wnt signals at the head fold stage. Thus, the status of Wnt signaling in ectodermal cells at the late gastrula stage regulates the rostrocaudal patterning of both neural plate and neural plate border, providing a coordinated spatial and temporal control of the early development of the central and peripheral nervous systems.

Expression of the dominant negative retinoid receptor, RAR403, alters telencephalic progenitor proliferation, survival, and cell fate specification

Retinoic acid (RA) signaling plays critical roles in diverse cellular processes during nervous system development. In mouse models, the roles for RA signals in telencephalic development remain unclear, partly because of the ambiguity of RA telencephalic sources after E8.75. Here, we have developed a genetic approach that utilizes Cre-lox technology to conditionally express a potent dominant negative retinoid receptor, RAR403, in vivo. This approach blocks RA signaling pathways at the receptor level, enabling the disruption of RA signals in contexts in which the RA source is unknown. RAR403 expression throughout the developing telencephalon causes pronounced hypoplasia resulting from defective proliferation in dorsal telencephalic progenitors and extensive cell death. Furthermore, Nkx2.1(+) progenitors in the medial ganglionic eminence (MGE) are misspecified such that they acquire a subset of lateral ganglionic eminence (LGE)-specific properties at the expense of MGE fates. This genetic approach reveals new roles for RA signaling in telencephalic proliferation, survival and fate specification, and underscores its utility in investigating the function of retinoid signaling pathways throughout peri- and postnatal development.

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.

Cross-regulatory interactions between Fgf8 and Shh in the avian frontonasal prominence

The frontonasal prominence of the developing avian embryo contains an organizing center, defined by juxtaposition of the Sonic hedgehog (Shh) and Fibroblast growth factor 8 (Fgf8) expression domains. This molecular interface presages any detectable growth of the frontonasal prominence, and experiments involving transplantation of this boundary epithelium have demonstrated it is a source of dorsal-ventral and rostral-caudal patterning information for the neural crest-derived mesenchyme of the upper beak. We explored the ontogeny of this organizing center by mapping the expression domains of both genes and their receptors and downstream targets. We tested the extent to which Shh and Fgf8 regulate each other's expression in this frontonasal organizer by either blocking or ectopically activating these pathways. Our experiments revealed mutual antagonism between the two molecules, which aids in establishing and maintaining a molecular boundary that subsequently influences patterning and growth of the middle and upper face.

Role of retinoic acid during forebrain development begins late when Raldh3 generates retinoic acid in the ventral subventricular zone

Retinoic acid (RA) synthesized by Raldh3 in the frontonasal surface ectoderm of chick embryos has been suggested to function in early forebrain patterning by regulating Fgf8, Shh, and Meis2 expression. Similar expression of Raldh3 exists in E8.75 mouse embryos, but Raldh2 is also expressed in the optic vesicle at this stage suggesting that both genes may play a role in early forebrain patterning. Furthermore, Raldh3 is expressed later in the forebrain itself (lateral ganglionic eminence; LGE) starting at E12.5, suggesting a later role in forebrain neurogenesis. Here we have analyzed mouse embryos carrying single or double null mutations in Raldh2 and Raldh3 for defects in forebrain development. Raldh2(-/-);Raldh3(-/-) embryos completely lacked RA signaling activity in the early forebrain, but exhibited relatively normal expression of Fgf8, Shh, and Meis2 in the forebrain. Thus, we find no clear requirement for RA in controlling expression of these important forebrain patterning genes, but Raldh3 expression in the frontonasal surface ectoderm was found to be needed for normal Fgf8 expression in the olfactory pit. Our studies revealed that later expression of Raldh3 in the subventricular zone of the LGE is required for RA signaling activity in the ventral forebrain. Importantly, expression of dopamine receptor D2 in E18.5 Raldh3(-/-) embryos was essentially eliminated in the developing nucleus accumbens, a tissue lying close to the source of RA provided by Raldh3. Our results suggest that the role of RA during forebrain development begins late when Raldh3 expression initiates in the ventral subventricular zone.

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.

Sonic hedgehog signaling in forebrain development and its interactions with pathways that modify its effects

During the development of the nervous system and other organs in the embryo, a limited set of master signaling pathways are used repeatedly for induction, patterning and growth. Among these, the Sonic hedgehog (Shh) pathway is crucial for the development of many structures in the brain. How the context-specific interplay between these various signaling pathways produces distinct temporal and spatial outcomes is not clear. Resolving this problem is a major goal in the study of cell and organ development. Here, we focus on signaling events during dorso-ventral patterning of the embryonic forebrain in vertebrates. In particular, we discuss the role of the Shh pathway in this process and on its interactions with the FGF, retinoic acid and Nodal pathways and other information cascades that modify its effects.

FGF signalling generates ventral telencephalic cells independently of SHH

Sonic hedgehog (SHH) is required to generate ventral cell types throughout the central nervous system. Its role in directly specifying ventral cells,however, has recently been questioned because loss of the Shh gene has little effect on ventral development if the Gli3 gene is also mutant. Consequently, another ventral determinant must exist. Here, genetic evidence establishes that FGFs are required for ventral telencephalon development. First, simultaneous deletion of Fgfr1 and Fgfr3specifically in the telencephalon results in the loss of differentiated ventromedial cells; and second, in the Fgfr1;Fgfr2 double mutant, ventral precursor cells are lost, mimicking the phenotype obtained previously with a loss of SHH signalling. Yet, in the Fgfr1;Fgfr2 mutant, Shh remains expressed, as does Gli1, the transcription of which depends on SHH activity, suggesting that FGF signalling acts independently of SHH to generate ventral precursors. Moreover, the Fgfr1;Fgfr2 phenotype, unlike the Shhphenotype, is not rescued by loss of Gli3, further indicating that FGFs act downstream of Shh and Gli3 to generate ventral telencephalic cell types.

Mechanisms of ventral patterning in the vertebrate nervous system

Dorsoventral patterning of the neural tube has a crucial role in shaping the functional organization of the CNS. It is well established that hedgehog signalling plays a key role in specifying ventral cell types throughout the neuroectoderm, and major progress has been made in elucidating how hedgehog signalling works in this ventral specification. In addition, other molecular pathways, including nodal, retinoic acid and fibroblast growth factor signalling, have been identified as important molecular cues for ventral patterning of the spinal cord, telencephalon and eye. Here, we discuss recent advances in this field, highlighting the emerging interplay of these signalling pathways in the molecular specification of ventral patterning at different rostrocaudal levels of the CNS.

Retinaldehyde dehydrogenase 2 (RALDH2)-mediated retinoic acid synthesis regulates early mouse embryonic forebrain development by controlling FGF and sonic hedgehog signaling

Although retinoic acid (RA) has been implicated as one of the diffusible signals regulating forebrain development, patterning of the forebrain has not been analyzed in detail in knockout mouse mutants deficient in embryonic RA synthesis. We show that the retinaldehyde dehydrogenase 2 (RALDH2) enzyme is responsible for RA synthesis in the mouse craniofacial region and forebrain between the 8- and 15-somite stages. Raldh2-/- knockout embryos exhibit defective morphogenesis of various forebrain derivatives, including the ventral diencephalon, the optic and telencephalic vesicles. These defects are preceded by regionally decreased cell proliferation in the neuroepithelium, correlating with abnormally low D-cyclin gene expression. Increases in cell death also contribute to the morphological deficiencies at later stages. Molecular analyses reveal abnormally low levels of FGF signaling in the craniofacial region, and impaired sonic hedgehog signaling in the ventral diencephalon. Expression levels of several regulators of diencephalic, telencephalic and optic development therefore cannot be maintained. These results unveil crucial roles of RA during early mouse forebrain development, which may involve the regulation of the expansion of neural progenitor cells through a crosstalk with FGF and sonic hedgehog signaling pathways.

Retinoic acid signaling identifies a distinct precursor population in the developing and adult forebrain

We asked whether retinoic acid (RA), an established transcriptional regulator in regenerating and developing tissues, acts directly on distinct cell classes in the mature or embryonic forebrain. We identified a subset of slowly dividing precursors in the adult subventricular zone (SVZ) that is transcriptionally activated by RA. Most of these cells express glial fibrillary acidic protein, a smaller subset expresses the epidermal growth factor receptor, a few are terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling positive, and they can be mitotically labeled by sustained rather than acute bromodeoxyuridine exposure. RA activation in similar cells in SVZ-derived neurospheres depends on retinoid synthesis from the premetabolite retinol. The apparent influence of RA on precursors in vitro is consistent with key properties of RA activation in the SVZ; in neurospheres, altered retinoid signaling elicits neither cell death nor an acute increase in cell proliferation. There is apparent continuity of RA signaling in the forebrain throughout life. RA-activated, proliferative precursors with radial glial characteristics are found in the dorsal lateral ganglionic eminence and ventrolateral palliumembryonic rudiments of the SVZ. Thus, endogenous RA signaling distinguishes subsets of neural precursors with glial characteristics in a consistent region of the adult and developing forebrain.

Gene networks controlling early cerebral cortex arealization

Early thalamus-independent steps in the process of cortical arealization take place on the basis of information intrinsic to the cortical primordium, as proposed by Rakic in his classical protomap hypothesis [Rakic, P. (1988)Science, 241, 170-176]. These steps depend on a dense network of molecular interactions, involving genes encoding for diffusible ligands which are released around the borders of the cortical field, and transcription factor genes which are expressed in graded ways throughout this field. In recent years, several labs worldwide have put considerable effort into identifying members of this network and disentangling its topology. In this respect, a considerable amount of knowledge has accumulated and a first, provisional description of the network can be delineated. The aim of this review is to provide an organic synthesis of our current knowledge of molecular genetics of early cortical arealization, i.e. to summarise the mechanisms by which secreted ligands and graded transcription factor genes elaborate positional information and trigger the activation of distinctive area-specific morphogenetic programs.