Identification of the zebrafish homologues of IMPG2, a retinal proteoglycan

Photoreceptor outer segments are surrounded by a carbohydrate-rich matrix, the interphotoreceptor matrix, necessary for physiological retinal function. Few roles for molecules characterizing the interphotoreceptor matrix have been clearly defined. Recent studies have found the presence of nonsense mutations in the interphotoreceptor matrix proteoglycan 2 (IMPG2) gene in patients affected by retinal dystrophies. IMPG2 encodes for a proteoglycan synthesized by photoreceptors and secreted in the interphotoreceptor matrix. Little is known about the structure and function of this protein, we thus decided to characterize zebrafish impg2. In zebrafish there are two Impg2 proteins, Impg2a and Impg2b. We generated a phylogenetic tree based on IMPG2 protein sequence similarity among vertebrates, showing a significant similarity between humans and teleosts. The human and zebrafish proteins share conserved domains, as also shown by homology models. Expression analyses of impg2a and impg2b show a continued expression in the photoreceptor layer starting from developmental stages and continuing through adulthood. Between 1 and 6 months post-fertilization, there is a significant shift of Impg2 expression toward the outer segment region, suggesting an increase in secretion. This raises intriguing hypotheses about its possible role(s) during retinal maturation, laying the groundwork for the generation of most needed models for the study of IMPG2-related inherited retinal dystrophies.

Exon Skipping Through Chimeric Antisense U1 snRNAs to Correct Retinitis Pigmentosa GTPase-Regulator (RPGR) Splice Defect

Inherited retinal dystrophies are caused by mutations in more than 250 genes, each of them carrying several types of mutations that can lead to different clinical phenotypes. Mutations in Retinitis Pigmentosa GTPase-Regulator (RPGR) cause X-linked Retinitis pigmentosa (RP). A nucleotide substitution in intron 9 of RPGR causes the increase of an alternatively spliced isoform of the mature mRNA, bearing exon 9a (E9a). This introduces a stop codon, leading to truncation of the protein. Aiming at restoring impaired gene expression, we developed an antisense RNA-based therapeutic approach for the skipping of RPGR E9a. We designed a set of specific U1 antisense snRNAs (U1_asRNAs) and tested their efficacy in vitro, upon transient cotransfection with RPGR minigene reporter systems in HEK-293T, 661W, and PC-12 cell lines.

Generation of hiPSC-Derived Functional Dopaminergic Neurons in Alginate-Based 3D Culture

Human induced pluripotent stem cells (hiPSCs) represent an unlimited cell source for the generation of patient-specific dopaminergic (DA) neurons, overcoming the hurdle of restricted accessibility to disease-affected tissue for mechanistic studies on Parkinson's disease (PD). However, the complexity of the human brain is not fully recapitulated by existing monolayer culture methods. Neurons differentiated in a three dimensional (3D) in vitro culture system might better mimic the in vivo cellular environment for basic mechanistic studies and represent better predictors of drug responses in vivo. In this work we established a new in vitro cell culture system based on the microencapsulation of hiPSCs in small alginate/fibronectin beads and their differentiation to DA neurons. Optimization of hydrogel matrix concentrations and composition allowed a high viability of embedded hiPSCs. Neural differentiation competence and efficiency of DA neuronal generation were increased in the 3D cultures compared to a conventional 2D culture methodology. Additionally, electrophysiological parameters and metabolic switching profile confirmed increased functionality and an anticipated metabolic resetting of neurons grown in alginate scaffolds with respect to their 2D counterpart neurons. We also report long-term maintenance of neuronal cultures and preservation of the mature functional properties. Furthermore, our findings indicate that our 3D model system can recapitulate mitochondrial superoxide production as an important mitochondrial phenotype observed in neurons derived from PD patients, and that this phenotype might be detectable earlier during neuronal differentiation. Taken together, these results indicate that our alginate-based 3D culture system offers an advantageous strategy for the reliable and rapid derivation of mature and functional DA neurons from hiPSCs.

Vertically-Aligned Functionalized Silicon Micropillars for 3D Culture of Human Pluripotent Stem Cell-Derived Cortical Progenitors

Silicon is a promising material for tissue engineering since it allows to produce micropatterned scaffolding structures resembling biological tissues. Using specific fabrication methods, it is possible to build aligned 3D network-like structures. In the present study, we exploited vertically-aligned silicon micropillar arrays as culture systems for human iPSC-derived cortical progenitors. In particular, our aim was to mimic the radially-oriented cortical radial glia fibres that during embryonic development play key roles in controlling the expansion, radial migration and differentiation of cortical progenitors, which are, in turn, pivotal to the establishment of the correct multilayered cerebral cortex structure. Here we show that silicon vertical micropillar arrays efficiently promote expansion and stemness preservation of human cortical progenitors when compared to standard monolayer growth conditions. Furthermore, the vertically-oriented micropillars allow the radial migration distinctive of cortical progenitors in vivo. These results indicate that vertical silicon micropillar arrays can offer an optimal system for human cortical progenitors' growth and migration. Furthermore, similar structures present an attractive platform for cortical tissue engineering.

AAV-miR-204 Protects from Retinal Degeneration by Attenuation of Microglia Activation and Photoreceptor Cell Death

Inherited retinal diseases (IRDs) represent a frequent cause of genetic blindness. Their high genetic heterogeneity hinders the application of gene-specific therapies to the vast majority of patients. We recently demonstrated that the microRNA miR-204 is essential for retinal function, although the underlying molecular mechanisms remain poorly understood. Here, we investigated the therapeutic potential of miR-204 in IRDs. We subretinally delivered an adeno-associated viral (AAV) vector carrying the miR-204 precursor to two genetically different IRD mouse models. The administration of AAV-miR-204 preserved retinal function in a mouse model for a dominant form of retinitis pigmentosa (RHO-P347S). This was associated with a reduction of apoptotic photoreceptor cells and with a better preservation of photoreceptor marker expression. Transcriptome analysis showed that miR-204 shifts expression profiles of transgenic retinas toward those of healthy retinas by the downregulation of microglia activation and photoreceptor cell death. Delivery of miR-204 exerted neuroprotective effects also in a mouse model of Leber congenital amaurosis, due to mutations of the Aipl1 gene. Our study highlights the mutation-independent therapeutic potential of AAV-miR204 in slowing down retinal degeneration in IRDs and unveils the previously unreported role of this miRNA in attenuating microglia activation and photoreceptor cell death.

An Eye on the Wnt Inhibitory Factor Wif1

The coordinated interplay between extrinsic activating and repressing cell signaling molecules is pivotal for embryonic development and subsequent tissue homeostasis. This is well exemplified by studies on the evolutionarily conserved Wnt signaling pathways. Tight temporal and spatial regulation of Wnt signaling activity is required throughout lifetime, from maternal stages before gastrulation until and throughout adulthood. Outside cells, the action of numerous Wnt ligands is counteracted and fine-tuned by only a handful of well characterized secreted inhibitors, such as for instance Dickkopf, secreted Frizzled Related Proteins and Cerberus. Here, we give an overview of our current understanding of another secreted Wnt signaling antagonist, the Wnt inhibitory factor Wif1. Wif1 can directly interact with various Wnt ligands and inhibits their binding to membrane bound receptors. Epigenetic promoter methylation of Wif1, leading to silencing of its transcription and concomitant up-regulation of Wnt signaling, is a common feature during cancer progression. Furthermore, an increasing number of reports describe Wif1 involvement in regulating processes during embryonic development, which so far has not received as much attention. We will summarize our knowledge on Wif1 function and its mode of action with a particular focus on the zebrafish (Danio rerio). In addition, we highlight the potential of Wif1 research to understand and possibly influence mechanisms underlying eye diseases and regeneration.

Noggin 1 overexpression in retinal progenitors affects bipolar cell generation

Waves of Bone Morphogenetic Proteins (BMPs) and their antagonists are present during initial eye development, but their possible roles in retinogenesis are still unknown. We have recently shown that noggin 1, a BMP antagonist, renders pluripotent cells able to differentiate into retinal precursors, and might be involved in the maintenance of retinal structures in the adult vertebrate eye. Here, we report that noggin 1, differently from noggin 2 and noggin 4, is expressed during all phases of Xenopus laevis retinal development. Gain-of-function experiments by electroporation in the optic vesicle show that overexpression of noggin 1 significantly decreases the number of bipolar cells in the inner nuclear layer of the retina, without significantly affecting the generation of the other retinal cell types. Our data suggest that BMP signaling could be involved in the differentiation of retinal progenitors into specific retinal subtypes during late phases of vertebrate retinal development.

Neurobiological bases of autism-epilepsy comorbidity: a focus on excitation/inhibition imbalance

Autism spectrum disorders (ASD) and epilepsy are common neurological diseases of childhood, with an estimated incidence of approximately 0.5-1% of the worldwide population. Several genetic, neuroimaging and neuropathological studies clearly showed that both ASD and epilepsy have developmental origins and a substantial degree of heritability. Most importantly, ASD and epilepsy frequently coexist in the same individual, suggesting a common neurodevelopmental basis for these disorders. Genome-wide association studies recently allowed for the identification of a substantial number of genes involved in ASD and epilepsy, some of which are mutated in syndromes presenting both ASD and epilepsy clinical features. At the cellular level, both preclinical and clinical studies indicate that the different genetic causes of ASD and epilepsy may converge to perturb the excitation/inhibition (E/I) balance, due to the dysfunction of excitatory and inhibitory circuits in various brain regions. Metabolic and immune dysfunctions, as well as environmental causes also contribute to ASD pathogenesis. Thus, an E/I imbalance resulting from neurodevelopmental deficits of multiple origins might represent a common pathogenic mechanism for both diseases. Here, we will review the most significant studies supporting these hypotheses. A deeper understanding of the molecular and cellular determinants of autism-epilepsy comorbidity will pave the way to the development of novel therapeutic strategies.

Brain-derived neurotrophic factor signaling is altered in the forebrain of Engrailed-2 knockout mice

Engrailed-2 (En2), a homeodomain transcription factor involved in regionalization and patterning of the midbrain and hindbrain regions has been associated to autism spectrum disorders (ASDs). En2 knockout (En2(-/-)) mice show ASD-like features accompanied by a significant loss of GABAergic subpopulations in the hippocampus and neocortex. Brain-derived neurotrophic factor (BDNF) is a crucial factor for the postnatal development of forebrain GABAergic neurons, and altered GABA signaling has been hypothesized to underlie the symptoms of ASD. Here we sought to determine whether interneuron loss in the En2(-/-) forebrain might be related to altered expression of BDNF and its signaling receptors. We first evaluated the expression of different BDNF mRNA isoforms in the neocortex and hippocampus of wild-type (WT) and En2(-/-) mice. Quantitative RT-PCR showed a marked down-regulation of several splicing variants of BDNF mRNA in the neocortex but not hippocampus of adult En2(-/-) mice, as compared to WT controls. Accordingly, levels of mature BDNF protein were lower in the neocortex but not hippocampus of En2(-/-) mice, as compared to WT. Increased levels of phosphorylated TrkB and decreased levels of p75 receptor were also detected in the neocortex of mutant mice. Accordingly, the expression of low density lipoprotein receptor (LDLR) and RhoA, two genes regulated via p75 was significantly altered in forebrain areas of mutant mice. These data indicate that BDNF signaling alterations might be involved in the anatomical changes observed in the En2(-/-) forebrain and suggest a pathogenic role of altered BDNF signaling in this mouse model of ASD.

TMEM16A is associated with voltage-gated calcium channels in mouse retina and its function is disrupted upon mutation of the auxiliary α2δ4 subunit

Photoreceptors rely upon highly specialized synapses to efficiently transmit signals to multiple postsynaptic targets. Calcium influx in the presynaptic terminal is mediated by voltage-gated calcium channels (VGCC). This event triggers neurotransmitter release, but also gates calcium-activated chloride channels (TMEM), which in turn regulate VGCC activity. In order to investigate the relationship between VGCC and TMEM channels, we analyzed the retina of wild type (WT) and Cacna2d4 mutant mice, in which the VGCC auxiliary α2δ4 subunit carries a nonsense mutation, disrupting the normal channel function. Synaptic terminals of mutant photoreceptors are disarranged and synaptic proteins as well as TMEM16A channels lose their characteristic localization. In parallel, calcium-activated chloride currents are impaired in rods, despite unaltered TMEM16A protein levels. Co-immunoprecipitation revealed the interaction between VGCC and TMEM16A channels in the retina. Heterologous expression of these channels in tsA-201 cells showed that TMEM16A associates with the CaV1.4 subunit, and the association persists upon expression of the mutant α2δ4 subunit. Collectively, our experiments show association between TMEM16A and the α1 subunit of VGCC. Close proximity of these channels allows optimal function of the photoreceptor synaptic terminal under physiological conditions, but also makes TMEM16A channels susceptible to changes occurring to calcium channels.

Activin/Nodal Signaling Supports Retinal Progenitor Specification in a Narrow Time Window during Pluripotent Stem Cell Neuralization

Retinal progenitors are initially found in the anterior neural plate region known as the eye field, whereas neighboring areas undertake telencephalic or hypothalamic development. Eye field cells become specified by switching on a network of eye field transcription factors, but the extracellular cues activating this network remain unclear. In this study, we used chemically defined media to induce in vitro differentiation of mouse embryonic stem cells (ESCs) toward eye field fates. Inhibition of Wnt/β-catenin signaling was sufficient to drive ESCs to telencephalic, but not retinal, fates. Instead, retinal progenitors could be generated from competent differentiating mouse ESCs by activation of Activin/Nodal signaling within a narrow temporal window corresponding to the emergence of primitive anterior neural progenitors. Activin also promoted eye field gene expression in differentiating human ESCs. Our results reveal insights into the mechanisms of eye field specification and open new avenues toward the generation of retinal progenitors for translational medicine.

Noggin-Mediated Retinal Induction Reveals a Novel Interplay Between Bone Morphogenetic Protein Inhibition, Transforming Growth Factor β, and Sonic Hedgehog Signaling

It has long been known that the depletion of bone morphogenetic protein (BMP) is one of the key factors necessary for the development of anterior neuroectodermal structures. However, the precise molecular mechanisms that underlie forebrain regionalization are still not completely understood. Here, we show that Noggin1 is involved in the regionalization of anterior neural structures in a dose-dependent manner. Low doses of Noggin1 expand prosencephalic territories, while higher doses specify diencephalic and retinal regions at the expense of telencephalic areas. A similar dose-dependent mechanism determines the ability of Noggin1 to convert pluripotent cells in prosencephalic or diencephalic/retinal precursors, as shown by transplant experiments and molecular analyses. At a molecular level, the strong inhibition of BMP signaling exerted by high doses of Noggin1 reinforces the Nodal/transforming growth factor (TGF)β signaling pathway, leading to activation of Gli1 and Gli2 and subsequent activation of Sonic Hedgehog (SHH) signaling. We propose a new role for Noggin1 in determining specific anterior neural structures by the modulation of TGFβ and SHH signaling.

Neural stem cells: ready for therapeutic applications?

Neural stem cells (NSCs) offer a unique and powerful tool for basic research and regenerative medicine. However, the challenges that scientists face in the comprehension of the biology and physiological function of these cells are still many. Deciphering NSCs fundamental biological aspects represents indeed a crucial step to control NSCs fate and functional integration following transplantation, and is essential for a safe and appropriate use of NSCs in injury/disease conditions. In this review, we focus on the biological properties of NSCs and discuss how these cells may be exploited to provide effective therapies for neurological disorders. We also review and discuss ongoing NSC-based clinical trials for these diseases.

Neural stem cells: ready for therapeutic applications?

Neural stem cells (NSCs) offer a unique and powerful tool for basic research and regenerative medicine. However, the challenges that scientists face in the comprehension of the biology and physiological function of these cells are still many. Deciphering NSCs fundamental biological aspects represents indeed a crucial step to control NSCs fate and functional integration following transplantation, and is essential for a safe and appropriate use of NSCs in injury/disease conditions. In this review, we focus on the biological properties of NSCs and discuss how these cells may be exploited to provide effective therapies for neurological disorders. We also review and discuss ongoing NSC-based clinical trials for these diseases.

Sorting of Sox1-GFP Mouse Embryonic Stem Cells Enhances Neuronal Identity Acquisition upon Factor-Free Monolayer Differentiation

Embryonic stem cells (ESCs) can give rise to all the differentiated cell types of the organism, including neurons. However, the efficiency and specificity of neural differentiation protocols still needs to be improved in order to plan their use in cell replacement therapies. In this study, we modified a monolayer differentiation protocol by selecting green fluorescent protein (GFP) positive neural precursors with fluorescence-activated cell sorting (FACS). The enhancement of neural differentiation was obtained by positively selecting for neural precursors, while specific neuronal subtypes spontaneously differentiated without additional cues; a comparable but delayed behavior was also observed in the GFP negative population, indicating that sorting settings per se eliminated nonneural and undifferentiated ESCs. This highly reproducible approach could be applied as a strategy to enhance neuronal differentiation and could be the first step toward the selection of pure populations of neurons, to be generated by the administration of specific factors in high throughput screening assays.

Splicing-correcting therapeutic approaches for retinal dystrophies: where endogenous gene regulation and specificity matter

Splicing is an important and highly regulated step in gene expression. The ability to modulate it can offer a therapeutic option for many genetic disorders. Antisense-mediated splicing-correction approaches have recently been successfully exploited for some genetic diseases, and are currently demonstrating safety and efficacy in different clinical trials. Their application for the treatment of retinal dystrophies could potentially solve a vast panel of cases, as illustrated by the abundance of mutations that could be targeted and the versatility of the technique. In this review, we will give an insight of the different therapeutic strategies, focusing on the current status of their application for retinal dystrophies.

Noggin Expression in the Adult Retina Suggests a Conserved Role during Vertebrate Evolution

Vertebrates share common mechanisms in the control of development and in the maintenance of neural and retinal function. The secreted factor Noggin, a BMP inhibitor, plays a crucial role in neural induction during embryonic development. Moreover, we have shown its involvement in retinal differentiation of pluripotent cells. Here we show Noggin expression in the adult retina in three vertebrate species. Four Noggin genes are present in zebrafish (Danio rerio; ZbNog1, 2, 3, 5), three in frog (Xenopus laevis; XenNog1, 2 and 4), and one in mouse (Mus musculus; mNog). Quantitative RT-PCR experiments show the presence of ZbNog3 and ZbNog5 mRNAs, but not ZbNog1 and ZbNog2, in the adult zebrafish retina. All three genes are expressed in the frog retina, and mNog in the mouse. Immunohistochemistry data show that Noggin proteins are predominantly localized in the Golgi apparatus of photoreceptors and in the fibers of the outer plexiform layer. Lower expression levels are also found in inner plexiform layer fibers, in ganglion cells, in the ciliary marginal zone, and in retinal pigmented epithelium. Our results show that Noggin has a specific cellular and sub-cellular expression in the adult vertebrate retina, which is conserved during evolution. In addition to its established role during embryonic development, we postulate that Noggin also exerts a functional role in the adult retina.

Transcriptome profiling in engrailed-2 mutant mice reveals common molecular pathways associated with autism spectrum disorders

BACKGROUND

Transcriptome analysis has been used in autism spectrum disorder (ASD) to unravel common pathogenic pathways based on the assumption that distinct rare genetic variants or epigenetic modifications affect common biological pathways. To unravel recurrent ASD-related neuropathological mechanisms, we took advantage of the En2-/- mouse model and performed transcriptome profiling on cerebellar and hippocampal adult tissues.

METHODS

Cerebellar and hippocampal tissue samples from three En2-/- and wild type (WT) littermate mice were assessed for differential gene expression using microarray hybridization followed by RankProd analysis. To identify functional categories overrepresented in the differentially expressed genes, we used integrated gene-network analysis, gene ontology enrichment and mouse phenotype ontology analysis. Furthermore, we performed direct enrichment analysis of ASD-associated genes from the SFARI repository in our differentially expressed genes.

RESULTS

Given the limited number of animals used in the study, we used permissive criteria and identified 842 differentially expressed genes in En2-/- cerebellum and 862 in the En2-/- hippocampus. Our functional analysis revealed that the molecular signature of En2-/- cerebellum and hippocampus shares convergent pathological pathways with ASD, including abnormal synaptic transmission, altered developmental processes and increased immune response. Furthermore, when directly compared to the repository of the SFARI database, our differentially expressed genes in the hippocampus showed enrichment of ASD-associated genes significantly higher than previously reported. qPCR was performed for representative genes to confirm relative transcript levels compared to those detected in microarrays.

CONCLUSIONS

Despite the limited number of animals used in the study, our bioinformatic analysis indicates the En2-/- mouse is a valuable tool for investigating molecular alterations related to ASD.

Epilepsy as a neurodevelopmental disorder

Epilepsy is characterized by spontaneous recurrent seizures and comprises a diverse group of syndromes with different etiologies. Epileptogenesis refers to the process whereby the brain becomes epileptic and can be related to several factors, such as acquired structural brain lesions, inborn brain malformations, alterations in neuronal signaling, and defects in maturation and plasticity of neuronal networks. In this review, we will focus on alterations of brain development that lead to an hyperexcitability phenotype in adulthood, providing examples from both animal and human studies. Malformations of cortical development (including focal cortical dysplasia, lissencephaly, heterotopia, and polymicrogyria) are frequently epileptogenic and result from defects in cell proliferation in the germinal zone and/or impaired neuronal migration and differentiation. Delayed or reduced arrival of inhibitory interneurons into the cortical plate is another possible cause of epileptogenesis. GABAergic neurons are generated during early development in the ganglionic eminences, and failure to pursue migration toward the cortex alters the excitatory/inhibitory balance resulting in aberrant network hyperexcitability. More subtle defects in the developmental assembly of excitatory and inhibitory synapses are also involved in epilepsy. For example, mutations in the presynaptic proteins synapsins and SNAP-25 cause derangements of synaptic transmission and plasticity which underlie appearance of an epileptic phenotype. Finally, there is evidence that defects in synapse elimination and remodeling during early "critical periods" can trigger hyperexcitability later in life. Further clarification of the developmental pathways to epilepsy has important implications for disease prevention and therapy.

Noggin elicits retinal fate in Xenopus animal cap embryonic stem cells

Driving specific differentiation pathways in multipotent stem cells is a main goal of cell therapy. Here we exploited the differentiating potential of Xenopus animal cap embryonic stem (ACES) cells to investigate the factors necessary to drive multipotent stem cells toward retinal fates. ACES cells are multipotent, and can be diverged from their default ectodermal fate to give rise to cell types from all three germ layers. We found that a single secreted molecule, Noggin, is sufficient to elicit retinal fates in ACES cells. Reverse-transcription polymerase chain reaction, immunohistochemistry, and in situ hybridization experiments showed that high doses of Noggin are able to support the expression of terminal differentiation markers of the neural retina in ACES cells in vitro. Following in vivo transplantation, ACES cells expressing high Noggin doses form eyes, both in the presumptive eye field region and in ectopic posterior locations. The eyes originating from the transplants in the eye field region are functionally equivalent to normal eyes, as seen by electrophysiology and c-fos expression in response to light. Our data show that in Xenopus embryos, proper doses of a single molecule, Noggin, can drive ACES cells toward retinal cell differentiation without additional cues. This makes Xenopus ACES cells a suitable model system to direct differentiation of stem cells toward retinal fates and encourages further studies on the role of Noggin in the retinal differentiation of mammalian stem cells.

Increased susceptibility to kainic acid-induced seizures in Engrailed-2 knockout mice

The En2 gene, coding for the homeobox-containing transcription factor Engrailed-2 (EN2), has been associated to autism spectrum disorder (ASD). Due to neuroanatomical and behavioral abnormalities, which partly resemble those observed in ASD patients, En2 knockout (En2(-/-)) mice have been proposed as a model for ASD. In the mouse embryo, En2 is involved in the specification of midbrain/hindbrain regions, being predominantly expressed in the developing cerebellum and ventral midbrain, and its expression is maintained in these structures until adulthood. Here we show that in the adult mouse brain, En2 mRNA is expressed also in the hippocampus and cerebral cortex. Hippocampal En2 mRNA content decreased after seizures induced by kainic acid (KA). This suggests that En2 might also influence the functioning of forebrain areas during adulthood and in response to seizures. Indeed, a reduced expression of parvalbumin and somatostatin was detected in the hippocampus of En2(-/-) mice as compared to wild-type (WT) mice, indicating an altered GABAergic innervation of limbic circuits in En2(-/-) mice. In keeping with these results, En2(-/-) mice displayed an increased susceptibility to KA-induced seizures. KA (20 mg/kg) determined more severe and prolonged generalized seizures in En2(-/-) mice, when compared to WT animals. Seizures were accompanied by a widespread c-fos and c-jun mRNA induction in the brain of En2(-/-) but not WT mice. Long-term histopathological changes (CA1 cell loss, upregulation of neuropeptide Y) also occurred in the hippocampus of KA-treated En2(-/-) but not WT mice. These findings suggest that En2(-/-) mice might be used as a novel tool to study the link between epilepsy and ASD.

Expression of cytosolic branched chain aminotransferase (BCATc) mRNA in the developing mouse brain

Branched-chain aminotransferase (BCAT) catalyzes the transamination of essential branched-chain amino acids (BCAAs: leucine, isoleucine and valine) with alpha-ketoglutarate. Through this reaction, BCAAs provide nitrogen for the synthesis of glutamate, the predominant excitatory neurotransmitter. Two BCAT isoforms have been identified: one cytosolic (BCATc) and one mitochondrial (BCATm). In adult rodents, BCATc is expressed in a wide variety of structures of the central nervous system (CNS), in neurons. So far, no data were available about the expression of BCATc in the developing CNS. Here, we analyse the expression profile of BCATc mRNA in the mouse brain from embryonic day 12.5 to adult age. BCATc mRNA gradually appears in different brain regions starting from early stages of neural development, and is maintained until adulthood. BCATc mRNA is predominantly present in the cerebral cortex, hippocampus, thalamus, ventral midbrain, raphe, cerebellum and precerebellar system. This study represents the first detailed analysis of BCATc mRNA expression in the developing mouse brain.

Cloning and developmental expression of the Xenopus homeobox gene Xvsx1

In contrast to the high degree of evolutionary conservation of the Vsx2/Chx10 gene family, vertebrate orthologues of Vsx1 display more divergent sequences and spatio-temporal expression patterns. Here, we report the cloning and expression pattern of Xenopus laevis Vsx1. Differently from the mouse and zebrafish orthologues, Xvsx1 transcription is activated at early neurula both in the evaginating eye vesicles and in the presumptive spinal cord. Compared to other retinal homeobox genes, such as Xrx1, Xsix3 and Xpax6, Xvsx1 is activated at a later stage; in addition, its anterior expression appears to be more specifically restricted to the retina. At tail bud stage, Xvsx1 expression in retinal progenitors persists, and its neural tube expression, which in the spinal cord corresponds to interneurons, progressively expands anteriorly reaching the midbrain-hindbrain boundary. During retinal neurogenesis, Xvsx1 expression is maintained in retinal progenitors and in a peripheral region of the ciliary marginal zone, while in the central retina, it becomes restricted to differentiated bipolar cells.

Genetic analysis of metamorphic and premetamorphic Xenopus ciliary marginal zone

A major event affecting the eye during amphibian metamorphosis is an asymmetrical growth of the ventrotemporal portion of the retina compared with its dorsonasal counterpart. This event is due to an increased proliferation of the precursors of the ventral ciliary marginal zone (CMZ). Here, we analyze the expression patterns of several key homeobox genes implicated in eye development (Xrx1, Xvax2, Xsix3, Xpax6, Xchx10, Xotx2) to understand whether they are active at the time in which the metamorphic changes of the retina occur. We also analyze their expression patterns in the ventral and dorsal CMZ and compare them with bromodeoxyuridine incorporation in the CMZ. Our results suggest that the metamorphic CMZ maintains the functional subdivisions described during embryonic development. Moreover, we find that genes involved in proliferation and cell type determination of the embryonic retina are actively transcribed in the proliferating CMZ, thus indicating a potential regulatory role for these genes in the metamorphic retina.

Xrx1 controls proliferation and neurogenesis in Xenopus anterior neural plate

In Xenopus neuroectoderm, posterior cells start differentiating at the end of gastrulation, while anterior cells display an extended proliferative period and undergo neurogenesis only at tailbud stage. Recent studies have identified several important components of the molecular pathways controlling posterior neurogenesis, but little is known about those controlling the timing and positioning of anterior neurogenesis. We investigate the role of Xrx1, a homeobox gene required for eye and anterior brain development, in the control of proliferation and neurogenesis of the anterior neural plate. Xrx1 is expressed in the entire proliferative region of the anterior neural plate delimited by cells expressing the neuronal determination gene X-ngnr-1, the neurogenic gene X-Delta-1, and the cell cycle inhibitor p27Xic1. Positive and negative signals position Xrx1 expression to this region. Xrx1 is activated by chordin and Hedgehog gene signaling, which induce anterior and proliferative fate, and is repressed by the differentiation-promoting activity of neurogenin and retinoic acid. Xrx1 is required for anterior neural plate proliferation and, when overexpressed, induces proliferation, inhibits X-ngnr-1, X-Delta-1 and N-tubulin and counteracts X-ngnr-1- and retinoic acid-mediated differentiation. We find that Xrx1 does not act by increasing lateral inhibition but by inducing the antineurogenic transcriptional repressors Xhairy2 and Zic2, and by repressing p27Xic1. The effects of Xrx1 on proliferation, neurogenesis and gene expression are restricted to the most rostral region of the embryo, implicating this gene as an anterior regulator of neurogenesis.

Xrx1 controls proliferation and multipotency of retinal progenitors

We investigated the function of Xrx1 during Xenopus retinogenesis. Xrx1 overexpression lengthens mitotic activity and ectopically activates the expression of markers of undifferentiated progenitors in the developing retina. We assayed Xrx1 ability to support proliferation with a cell-autonomous mechanism by in vivo lipofection of single retinal progenitors. Xrx1 overexpression increases clonal proliferation while Xrx1 functional inactivation exerts the opposite effect. We also compared the effects of Xrx1 with those of the cyclin-dependent kinase cdk2, a strong mitotic promoter. Despite the similar increase in clonal proliferation displayed by both factors, Xrx1 and cdk2 act differently on retinal cell fate determination. cdk2/cyclinA2 lipofected retinas show a decrease in early-born cell types as ganglion cells and cones and an increase in late-born types such as bipolar neurons. On the contrary, Xrx1 lipofected retinas show no changes in the proportions of the different cell types, thus suggesting a role in supporting multipotency of retinal progenitors.

Mash1 and Ngn1 control distinct steps of determination and differentiation in the olfactory sensory neuron lineage

bHLH transcription factors are expressed sequentially during the development of neural lineages, suggesting that they operate in genetic cascades. In the olfactory epithelium, the proneural genes Mash1 and neurogenin1 are expressed at distinct steps in the same olfactory sensory neuron lineage. Here, we show by loss-of-function analysis that both genes are required for the generation of olfactory sensory neurons. However, their mutant phenotypes are strikingly different, indicating that they have divergent functions. In Mash1 null mutant mice, olfactory progenitors are not produced and the Notch signalling pathway is not activated, establishing Mash1 as a determination gene for olfactory sensory neurons. In neurogenin1 null mutant mice, olfactory progenitors are generated but they express only a subset of their normal repertoire of regulatory molecules and their differentiation is blocked. Thus neurogenin1 is required for the activation of one of several parallel genetic programs functioning downstream of Mash1 in the differentiation of olfactory sensory neurons. These results illustrate the versatility of neural bHLH genes which adopt either a determination or a differentiation function, depending primarily on the timing of their expression in neural progenitors.

Hes genes regulate sequential stages of neurogenesis in the olfactory epithelium

We have characterised the functions of the bHLH transcriptional repressors HES1 and HES5 in neurogenesis, using the development of the olfactory placodes in mouse embryos as a model. Hes1 and Hes5 are expressed with distinct patterns in the olfactory placodes and are subject to different regulatory mechanisms. Hes1 is expressed in a broad placodal domain, which is maintained in absence of the neural determination gene Mash1. In contrast, expression of Hes5 is restricted to clusters of neural progenitor cells and requires Mash1 function. Mutations in Hes1 and Hes5 also have distinct consequences on olfactory placode neurogenesis. Loss of Hes1 function leads both to expression of Mash1 outside of the normal domain of neurogenesis and to increased density of MASH1-positive progenitors within this domain, and results in an excess of neurons after a delay. A mutation in Hes5 does not produce any apparent defect. However, olfactory placodes that are double mutant for Hes1 and Hes5 upregulate Ngn1, a neural bHLH gene activated downstream of Mash1, and show a strong and rapid increase in neuronal density. Together, our results suggest that Hes1 regulates Mash1 transcription in the olfactory placode in two different contexts, initially as a prepattern gene defining the placodal domain undergoing neurogenesis and, subsequently, as a neurogenic gene controlling the density of neural progenitors in this domain. Hes5 synergizes with Hes1 and regulates neurogenesis at the level of Ngn1 expression. Therefore, the olfactory sensory neuron lineage is regulated at several steps by negative signals acting through different Hes genes and targeting the expression of different proneural gene homologs.

A role for neural determination genes in specifying the dorsoventral identity of telencephalic neurons

Neurogenin1 (Ngn1), Neurogenin2(Ngn2), and Mash1 encode bHLH transcription factors with neuronal determination functions. In the telencephalon, theNgns and Mash1 are expressed at high levels in complementary dorsal and ventral domains, respectively. We found thatNgn function is required to maintain these two separate expression domains, as Mash1 expression is up-regulated in the dorsal telencephalon of Ngn mutant embryos. We have taken advantage of the replacement of the Ngns by Mash1 in dorsal progenitors to address the role of the neural determination genes in neuronal-type specification in the telencephalon. InNgn2 single and Ngn1; Ngn2 double mutants, a population of early born cortical neurons lose expression of dorsal-specific markers and ectopically express a subset of ventral telencephalic-specific markers. Analysis of Mash1; Ngn2double mutant embryos and of embryos carrying a Ngn2 toMash1 replacement mutation demonstrated that ectopic expression of Mash1 is required and sufficient to confer these ventral characteristics to cortical neurons. Our results indicate that in addition to acting as neuronal determinants, Mash1 andNgns play a role in the specification of dorsal-ventral neuronal identity, directly linking pathways of neurogenesis and regional patterning in the forebrain.

A role for neural determination genes in specifying the dorsoventral identity of telencephalic neurons

Neurogenin1 (Ngn1), Neurogenin2 (Ngn2), and Mash1 encode bHLH transcription factors with neuronal determination functions. In the telencephalon, the Ngns and Mash1 are expressed at high levels in complementary dorsal and ventral domains, respectively. We found that Ngn function is required to maintain these two separate expression domains, as Mash1 expression is up-regulated in the dorsal telencephalon of Ngn mutant embryos. We have taken advantage of the replacement of the Ngns by Mash1 in dorsal progenitors to address the role of the neural determination genes in neuronal-type specification in the telencephalon. In Ngn2 single and Ngn1; Ngn2 double mutants, a population of early born cortical neurons lose expression of dorsal-specific markers and ectopically express a subset of ventral telencephalic-specific markers. Analysis of Mash1; Ngn2 double mutant embryos and of embryos carrying a Ngn2 to Mash1 replacement mutation demonstrated that ectopic expression of Mash1 is required and sufficient to confer these ventral characteristics to cortical neurons. Our results indicate that in addition to acting as neuronal determinants, Mash1 and Ngns play a role in the specification of dorsal-ventral neuronal identity, directly linking pathways of neurogenesis and regional patterning in the forebrain.

Transcription factors Mash-1 and Prox-1 delineate early steps in differentiation of neural stem cells in the developing central nervous system

Like other tissues and organs in vertebrates, multipotential stem cells serve as the origin of diverse cell types during genesis of the mammalian central nervous system (CNS). During early development, stem cells self-renew and increase their total cell numbers without overt differentiation. At later stages, the cells withdraw from this self-renewal mode, and are fated to differentiate into neurons and glia in a spatially and temporally regulated manner. However, the molecular mechanisms underlying this important step in cell differentiation remain poorly understood. In this study, we present evidence that the expression and function of the neural-specific transcription factors Mash-1 and Prox-1 are involved in this process. In vivo, Mash-1- and Prox-1-expressing cells were defined as a transient proliferating population that was molecularly distinct from self-renewing stem cells. By taking advantage of in vitro culture systems, we showed that induction of Mash-1 and Prox-1 coincided with an initial step of differentiation of stem cells. Furthermore, forced expression of Mash-1 led to the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells, and up-regulation of Prox-1, suggesting that Mash-1 positively regulates cell differentiation. In support of these observations in vitro, we found specific defects in cellular differentiation and loss of expression of Prox-1 in the developing brain of Mash-1 mutant mice in vivo. Thus, we propose that induction of Mash-1 and Prox-1 is one of the critical molecular events that control early development of the CNS.

Mash1 regulates neurogenesis in the ventral telencephalon

Previous studies have shown that mice mutant for the gene Mash1 display severe neuronal losses in the olfactory epithelium and ganglia of the autonomic nervous system, demonstrating a role for Mash1 in development of neuronal lineages in the peripheral nervous system. Here, we have begun to analyse Mash1 function in the central nervous system, focusing our studies on the ventral telencephalon where it is expressed at high levels during neurogenesis. Mash1 mutant mice present a severe loss of progenitors, particularly of neuronal precursors in the subventricular zone of the medial ganglionic eminence. Discrete neuronal populations of the basal ganglia and cerebral cortex are subsequently missing. An analysis of candidate effectors of Mash1 function revealed that the Notch ligands Dll1 and Dll3, and the target of Notch signaling Hes5, fail to be expressed in Mash1 mutant ventral telencephalon. In the lateral ganglionic eminence, loss of Notch signaling activity correlates with premature expression of a number of subventricular zone markers by ventricular zone cells. Therefore, Mash1 is an important regulator of neurogenesis in the ventral telencephalon, where it is required both to specify neuronal precursors and to control the timing of their production.

Xrx1, a novel Xenopus homeobox gene expressed during eye and pineal gland development

We have isolated a novel Xenopus homeobox gene, Xrx1, belonging to the paired-like class of homeobox genes. Xrx1 is expressed in the anterior neural plate, and subsequently in the neural structures of the developing eye (neural retina and pigmented epithelium), and in other forebrain structures deriving from the anterior neural plate: in the pineal gland, throughout its development, in the diencephalon floor and in the hypophysis. Its rostral limit of expression corresponds to the chiasmatic ridge, which some authors consider as the anteriormost limit of the neural tube: thus, Xrx1 may represent one of the most anteriorly expressed homeobox genes reported to date. Moreover, its expression in organs implicated in the establishment of circadian rhythms, may suggest for Xrx1 a role in the genetic control of this function. Finally, analysis of Xrx1 expression in embryos subjected to various treatments, or microinjected with different dorsalizing agents (noggin, Xwnt-8), suggests that vertical inductive signals leading to head morphogenesis are required to activate Xrx1.