Regionalization of the optic tectum: combinations of gene expression that define the tectum

The Oct4-related PouV gene, pou5f3, mediates isthmus development in zebrafish by directly and dynamically regulating pax2a

Using a transgenic zebrafish line harboring a heat-inducible dominant-interference pou5f3 gene (en-pou5f3), we reported that this PouV gene is involved in isthmus development at the midbrain-hindbrain boundary (MHB), which patterns the midbrain and cerebellum. Importantly, the functions of pou5f3 reportedly differ before and after the end of gastrulation. In the present study, we examined in detail the effects of en-pou5f3 induction on isthmus development during embryogenesis. When en-pou5f3 was induced around the end of gastrulation (bud stage), the isthmus was abrogated or deformed by the end of somitogenesis (24 hours post-fertilization). At this stage, the expression of MHB markers -- such as pax2a, fgf8a, wnt1, and gbx2 -- was absent in embryos lacking the isthmus structure, whereas it was present, although severely distorted, in embryos with a deformed isthmus. We further found that, after en-pou5f3 induction at late gastrulation, pax2a, fgf8a, and wnt1 were immediately and irreversibly downregulated, whereas the expression of en2a and gbx2 was reduced only weakly and slowly. Induction of en-pou5f3 at early somite stages also immediately downregulated MHB genes, particularly pax2a, but their expression was restored later. Overall, the data suggested that pou5f3 directly upregulates at least pax2a and possibly fgf8a and wnt1, which function in parallel in establishing the MHB, and that the role of pou5f3 dynamically changes around the end of gastrulation. We next examined the transcriptional regulation of pax2a using both in vitro and in vivo reporter analyses; the results showed that two upstream 1.0-kb regions with sequences conserved among vertebrates specifically drove transcription at the MHB. These reporter analyses confirmed that development of the isthmic organizer is regulated by PouV through direct regulation of pax2/pax2a in vertebrate embryos.

Involvement of nr2f genes in brain regionalization and eye development during early zebrafish development

Nuclear receptor subfamily 2 group F (Nr2f) proteins are essential for brain development in mice, but little is known about their precise roles and their evolutionary diversification. In the present study, the expression patterns of major nr2f genes (nr2f1a, nr2f1b, and nr2f2) during early brain development were investigated in zebrafish. Comparisons of their expression patterns revealed similar but temporally and spatially distinct patterns after early somite stages in the brain. Frameshift mutations in the three nr2f genes, achieved using the CRISPR/Cas9 method, resulted in a smaller telencephalon and smaller eyes in the nr2f1a mutants; milder forms of those defects were present in the nr2f1b and nr2f2 mutants. Acridine orange staining revealed enhanced cell death in the brain and/or eyes in all nr2f homozygous mutants. The expression of regional markers in the brain did not suggest global defects in brain regionalization; however, shha expression in the preoptic area and hypothalamus, as well as fgf8a expression in the anterior telencephalon, was disturbed in nr2f1a and nr2f1b mutants, potentially leading to a defective telencephalon. Specification of the retina and optic stalk was also significantly affected. The overexpression of nr2f1b by injection of mRNA disrupted the anterior brain at a high dose, and the expression of pax6a in the eyes and fgf8a in the telencephalon at a low dose. The results of these loss- and gain-of-function approaches showed that nr2f genes regulate the development of the telencephalon and eyes in zebrafish embryos.

Propensity of Patient-Derived iPSCs for Retinal Differentiation: Implications for Autologous Cell Replacement

Prior to use, newly generated induced pluripotent stem cells (iPSC) should be thoroughly validated. While excellent validation and release testing assays designed to evaluate potency, genetic integrity, and sterility exist, they do not have the ability to predict cell type-specific differentiation capacity. Selection of iPSC lines that have limited capacity to produce high-quality transplantable cells, places significant strain on valuable clinical manufacturing resources. The purpose of this study was to determine the degree and root cause of variability in retinal differentiation capacity between cGMP-derived patient iPSC lines. In turn, our goal was to develop a release testing assay that could be used to augment the widely used ScoreCard panel. IPSCs were generated from 15 patients (14-76 years old), differentiated into retinal organoids, and scored based on their retinal differentiation capacity. Despite significant differences in retinal differentiation propensity, RNA-sequencing revealed remarkable similarity between patient-derived iPSC lines prior to differentiation. At 7 days of differentiation, significant differences in gene expression could be detected. Ingenuity pathway analysis revealed perturbations in pathways associated with pluripotency and early cell fate commitment. For example, good and poor producers had noticeably different expressions of OCT4 and SOX2 effector genes. QPCR assays targeting genes identified via RNA sequencing were developed and validated in a masked fashion using iPSCs from 8 independent patients. A subset of 14 genes, which include the retinal cell fate markers RAX, LHX2, VSX2, and SIX6 (all elevated in the good producers), were found to be predictive of retinal differentiation propensity.

The neurodevelopmental gene MSANTD2 belongs to a gene family formed by recurrent molecular domestication of Harbinger transposons at the base of vertebrates

The formation of new genes is a major source of organism evolutionary innovation. Beyond their mutational effects, transposable elements can be co-opted by host genomes to form different types of sequences including novel genes, through a mechanism named molecular domestication.We report the formation of four genes through molecular domestication of Harbinger transposons, three in a common ancestor of jawed vertebrates about 500 million years ago and one in sarcopterygians approx. 430 million years ago. Additionally, one processed pseudogene arose approx. 60 million years ago in simians. In zebrafish, Harbinger-derived genes are expressed during early development but also in adult tissues, and predominantly co-expressed in male brain. In human, expression was detected in multiple organs, with major expression in the brain particularly during fetal development. We used CRISPR/Cas9 with direct gene knock-out in the F0 generation and the morpholino antisense oligonucleotide knock-down technique to study in zebrafish the function of one of these genes called MSANTD2, which has been suggested to be associated to neuro-developmental diseases such as autism spectrum disorders and schizophrenia in human. MSANTD2 inactivation led to developmental delays including tail and nervous system malformation at one day post fertilization. Affected embryos showed dead cell accumulation, major anatomical defects characterized by impaired brain ventricle formation and alterations in expression of some characteristic genes involved in vertebrate nervous system development. Hence, the characterization of MSANTD2 and other Harbinger-derived genes might contribute to a better understanding of the genetic innovations having driven the early evolution of the vertebrate nervous system.

From Causal Networks to Adverse Outcome Pathways: A Developmental Neurotoxicity Case Study

The last decade has seen the adverse outcome pathways (AOP) framework become one of the most powerful tools in chemical risk assessment, but the development of new AOPs remains a slow and manually intensive process. Here, we present a faster approach for AOP generation, based on manually curated causal toxicological networks. As a case study, we took a recently published zebrafish developmental neurotoxicity network, which contains causally connected molecular events leading to neuropathologies, and developed two new adverse outcome pathways: Inhibition of Fyna (Src family tyrosine kinase A) leading to increased mortality via decreased eye size (AOP 399 on AOP-Wiki) and GSK3beta (Glycogen synthase kinase 3 beta) inactivation leading to increased mortality via defects in developing inner ear (AOP 410). The approach consists of an automatic separation of the toxicological network into candidate AOPs, filtering the AOPs according to available evidence and length as well as manual development of new AOPs and weight-of-evidence evaluation. The semiautomatic approach described here provides a new opportunity for fast and straightforward AOP development based on large network resources.

H2O2 and Engrailed 2 paracrine activity synergize to shape the zebrafish optic tectum

Although a physiological role for redox signaling is now clearly established, the processes sensitive to redox signaling remains to be identified. Ratiometric probes selective for H₂O₂ have revealed its complex spatiotemporal dynamics during neural development and adult regeneration and perturbations of H₂O₂ levels disturb cell plasticity and morphogenesis. Here we ask whether endogenous H₂O₂ could participate in the patterning of the embryo. We find that perturbations of endogenous H₂O₂ levels impact on the distribution of the Engrailed homeoprotein, a strong determinant of midbrain patterning. Engrailed 2 is secreted from cells with high H₂O₂ levels and taken up by cells with low H₂O₂ levels where it leads to increased H₂O₂ production, steering the directional spread of the Engrailed gradient. These results illustrate the interplay between protein signaling pathways and metabolic processes during morphogenetic events.

NCBP3/SNHG6 inhibits GBX2 transcription in a histone modification manner to facilitate the malignant biological behaviour of glioma cells

RNA-binding proteins (RBPs) are significantly dysregulated in glioma. In this study, we demonstrated the upregulation of Nuclear cap-binding subunit 3 (NCBP3) in glioma tissues and cells. Further, knockdown of NCBP3 inhibited the malignant progression of glioma. NCBP3 directly bound to small nucleolar RNA host gene 6 (SNHG6) and stabilized SNHG6 expression. In contrast, the gastrulation brain homeobox 2 (GBX2) transcription factor was downregulated in glioma tissues and cells. SNHG6 inhibited GBX2 transcription by mediating the H3K27me3 modification induced by polycomb repressive complex 2 (PRC2). Moreover, GBX2 decreased the promoter activities and downregulated the expression of the flotillin protein family 1 (FLOT1) oncogene. In conclusion, NCBP3/SNHG6 inhibits GBX2 transcription in a PRC2-dependent manner to facilitate the malignant progression of gliomas.

Change in the developmental fate of the chick optic vesicle from the neural retina to the telencephalon

The forebrain develops into the telencephalon, diencephalon, and optic vesicle (OV). The OV further develops into the optic cup, the inner and outer layers of which develop into the neural retina and retinal pigmented epithelium (RPE), respectively. We studied the change in fate of the OV by using embryonic transplantation and explant culture methods. OVs excised from 10-somite stage chick embryos were freed from surrounding tissues (the surface ectoderm and mesenchyme) and were transplanted back to their original position in host embryos. Expression of neural retina-specific genes, such as Rax and Vsx2 (Chx10), was downregulated in the transplants. Instead, expression of the telencephalon-specific gene Emx1 emerged in the proximal region of the transplants, and in the distal part of the transplants close to the epidermis, expression of an RPE-specific gene Mitf was observed. Explant culture studies showed that when OVs were cultured alone, Rax was continuously expressed regardless of surrounding tissues (mesenchyme and epidermis). When OVs without surrounding tissues were cultured in close contact with the anterior forebrain, Rax expression became downregulated in the explants, and Emx1 expression became upregulated. These findings indicate that chick OVs at stage 10 are bi-potential with respect to their developmental fates, either for the neural retina or for the telencephalon, and that the surrounding tissues have a pivotal role in their actual fates. An in vitro tissue culture model suggests that under the influence of the anterior forebrain and/or its surrounding tissues, the OV changes its fate from the retina to the telencephalon.

A Conserved Developmental Mechanism Builds Complex Visual Systems in Insects and Vertebrates

The visual systems of vertebrates and many other bilaterian clades consist of complex neural structures guiding a wide spectrum of behaviors. Homologies at the level of cell types and even discrete neural circuits have been proposed, but many questions of how the architecture of visual neuropils evolved among different phyla remain open. In this review we argue that the profound conservation of genetic and developmental steps generating the eye and its target neuropils in fish and fruit flies supports a homology between some core elements of bilaterian visual circuitries. Fish retina and tectum, and fly optic lobe, develop from a partitioned, unidirectionally proliferating neurectodermal domain that combines slowly dividing neuroepithelial stem cells and rapidly amplifying progenitors with shared genetic signatures to generate large numbers and different types of neurons in a temporally ordered way. This peculiar 'conveyor belt neurogenesis' could play an essential role in generating the topographically ordered circuitry of the visual system.

Regional expression of Pax7 in the brain of Xenopus laevis during embryonic and larval development

Pax7 is a member of the highly conserved Pax gene family that is expressed in restricted zones of the central nervous system (CNS) during development, being involved in early brain regionalization and the maintenance of the regional identity. Using sensitive immunohistochemical techniques we have analyzed the spatiotemporal pattern of Pax7 expression in the brain of the anuran amphibian Xenopus laevis, during development. Pax7 expression was first detected in early embryos in the basal plate of prosomere 3, roof and alar plates of prosomere 1 and mesencephalon, and the alar plate of rhombomere 1. As development proceeded, Pax7 cells were observed in the hypothalamus close to the catecholaminergic population of the mammillary region. In the diencephalon, Pax7 was intensely expressed in a portion of the basal plate of prosomere 3, in the roof plate and in scattered cells of the thalamus in prosomere 2, throughout the roof of prosomere 1, and in the commissural and juxtacommissural domains of the pretectum. In the mesencephalon, Pax7 cells were localized in the optic tectum and, to a lesser extent, in the torus semicircularis. The rostral portion of the alar part of rhombomere 1, including the ventricular layer of the cerebellum, expressed Pax7 and, gradually, some of these dorsal cells were observed to populate ventrally the interpeduncular nucleus and the isthmus (rhombomere 0). Additionally, Pax7 positive cells were found in the ventricular zone of the ventral part of the alar plate along the rhombencephalon and the spinal cord. The findings show that the strongly conserved features of Pax7 expression through development shared by amniote vertebrates are also present in the anamniote amphibians as a common characteristic of the brain organization of tetrapods.

Otx2 cell-autonomously determines dorsal mesencephalon versus cerebellum fate independently of isthmic organizing activity

During embryonic development, the rostral neuroectoderm is regionalized into broad areas that are subsequently subdivided into progenitor compartments with specialized identity and fate. These events are controlled by signals emitted by organizing centers and interpreted by target progenitors, which activate superimposing waves of intrinsic factors restricting their identity and fate. The transcription factor Otx2 plays a crucial role in mesencephalic development by positioning the midbrain-hindbrain boundary (MHB) and its organizing activity. Here, we investigated whether Otx2 is cell-autonomously required to control identity and fate of dorsal mesencephalic progenitors. With this aim, we have inactivated Otx2 in the Pax7(+) dorsal mesencephalic domain, previously named m1, without affecting MHB integrity. We found that the Pax7(+) m1 domain can be further subdivided into a dorsal Zic1(+) m1a and a ventral Zic1(-) m1b sub-domain. Loss of Otx2 in the m1a (Pax7(+) Zic1(+)) sub-domain impairs the identity and fate of progenitors, which undergo a full switch into a coordinated cerebellum differentiation program. By contrast, in the m1b sub-domain (Pax7(+) Zic1(-)) Otx2 is prevalently required for post-mitotic transition of mesencephalic GABAergic precursors. Moreover, genetic cell fate, BrdU cell labeling and Otx2 conditional inactivation experiments indicate that in Otx2 mutants all ectopic cerebellar cell types, including external granule cell layer (EGL) precursors, originate from the m1a progenitor sub-domain and that reprogramming of mesencephalic precursors into EGL or cerebellar GABAergic progenitors depends on temporal sensitivity to Otx2 ablation. Together, these findings indicate that Otx2 intrinsically controls different aspects of dorsal mesencephalic neurogenesis. In this context, Otx2 is cell-autonomously required in the m1a sub-domain to suppress cerebellar fate and promote mesencephalic differentiation independently of the MHB organizing activity.

Electroporation: past, present and future

Gene transfer by electroporation has become an indispensable method for the study of developmental biology. The technique is applied not only in chick embryos but also in mice and other organisms. Here, a short history and perspectives of electroporation for gene transfer in vertebrates are described.

Neuronal and glial differentiation during lizard (Gallotia galloti) visual system ontogeny

We studied the histogenesis of the lizard visual system (E30 to adulthood) by using a selection of immunohistochemical markers that had proved relevant for other vertebrates. By E30, the Pax6(+) pseudostratified retinal epithelium shows few newborn retinal ganglion cells (RGCs) in the centrodorsal region expressing neuron- and synaptic-specific markers such as betaIII-tubulin (Tuj1), synaptic vesicle protein-2 (SV2), and vesicular glutamate transporter-1 (VGLUT1). Concurrently, pioneer RGC axons run among the Pax2(+) astroglia in the optic nerve and reach the superficial optic tectum. Between E30 and E35, the optic chiasm and optic tract remain acellular, but the latter contains radial processes with subpial endfeet expressing vimentin (Vim). From E35, neuron- and synaptic-specific stainings spread in the retina and optic tectum, whereas retinal Pax6, and Tuj1/SV2 in RGC axons decrease. Müller glia and abundant optic nerve glia express a variety of glia-specific markers until adulthood. Subpopulations of optic nerve glia are also VGLUT1(+) and cluster differentiation-44 (CD44)-positive but cytokeratin-negative, unlike the case in other regeneration-competent species. Specifically, coexpression of CD44/Vim and glutamine synthetase (GS)/VGLUT1 reflects glial specialization, insofar as most CD44(+) glia are GS(-). In the adult optic tract and tectum, radial glia and free astroglia coexist. The latter show different immunocharacterization (Pax2(-)/CD44(-) /Vim(-)) compared with that in the optic nerve. We conclude that upregulation of Tuj1 and SV2 is required for axonal outgrowth and search for appropriate targets, whereas Pax2(+) optic nerve astroglia and Vim(+) radial glia may aid in early axonal guidance. Spontaneous axonal regrowth seems to succeed despite the heterogeneous mammalian-like glial environment in the lizard optic nerve.

Identification and analysis of conserved cis-regulatory regions of the MEIS1 gene

Meis1, a conserved transcription factor of the TALE-homeodomain class, is expressed in a wide variety of tissues during development. Its complex expression pattern is likely to be controlled by an equally complex regulatory landscape. Here we have scanned the Meis1 locus for regulatory elements and found 13 non-coding regions, highly conserved between humans and teleost fishes, that have enhancer activity in stable transgenic zebrafish lines. All these regions are syntenic in most vertebrates. The composite expression of all these enhancer elements recapitulate most of Meis1 expression during early embryogenesis, indicating they comprise a basic set of regulatory elements of the Meis1 gene. Using bioinformatic tools, we identify a number of potential binding sites for transcription factors that are compatible with the regulation of these enhancers. Specifically, HHc2:066650, which is expressed in the developing retina and optic tectum, harbors several predicted Pax6 sites. Biochemical, functional and transgenic assays indicate that pax6 genes directly regulate HHc2:066650 activity.

Nuclear translocation of FGF8 and its implication to induce Sprouty2

Fibroblast growth factor 8 (FGF8) functions as a local organizing signal for the tectum and cerebellum. FGF8 activates Ras-ERK signaling pathway to induce cerebellar development. We paid attention to the difference in the expression pattern of the molecules that are induced by FGF8 in the mid-hind brain region during normal development and after FGF8 misexpression; some are expressed in the area corresponding to the ERK activation domain but the others are expressed corresponding to the Fgf8 expression domain. Since some of the FGF family members are localized in the nucleus, we wondered if FGF8 could localize in the nuclei and function in the nucleus. We first show that in cultured NIH3T3 cells transfected FGF8b could localize in the nucleus. Transfected FGF8b could also localize in the nucleus of the cells in the chick neural tube. In mouse embryonic neural tube, we detected endogenous FGF8 in the nuclei. Implantation of an FGF8b-soaked bead showed that exogenous FGF8b could be translocated to the nuclei in the isthmus. Furthermore, signal-peptide-deletion mutant of FGF8b mainly localized in the nuclei, and induced Sprouty2 without activating ERK in the mesencephalon. Signal-peptide-deletion mutant of FGF8b could not induce Pax2 expression. Taken together, we concluded that FGF8b could be translocated to the nuclei, and that the nuclear FGF8 could function as transcriptional regulator to induce Sprouty2 in the isthmus.

Midbrain-hindbrain boundary patterning and morphogenesis are regulated by diverse grainy head-like 2-dependent pathways

The isthmic organiser located at the midbrain-hindbrain boundary (MHB) is the crucial developmental signalling centre responsible for patterning mesencephalic and metencephalic regions of the vertebrate brain. Formation and maintenance of the MHB is characterised by a hierarchical program of gene expression initiated by fibroblast growth factor 8 (Fgf8), coupled with cellular morphogenesis, culminating in the formation of the tectal-isthmo-cerebellar structures. Here, we show in zebrafish that one orthologue of the transcription factor grainy head-like 2 (Grhl2), zebrafish grhl2b plays a central role in both MHB maintenance and folding by regulating two distinct, non-linear pathways. Loss of grhl2b expression induces neural apoptosis and extinction of MHB markers, which are rescued by re-expression of engrailed 2a (eng2a), an evolutionarily conserved target of the Grhl family. Co-injection of sub-phenotypic doses of grhl2b and eng2a morpholinos reproduces the apoptosis and MHB marker loss, but fails to substantially disrupt formation of the isthmic constriction. By contrast, a novel direct grhl2b target, spec1, identified by phylogenetic analysis and confirmed by ChIP, functionally cooperates with grhl2b to induce MHB morphogenesis, but plays no role in apoptosis or maintenance of MHB markers. Collectively, these data show that MHB maintenance and morphogenesis are dissociable events regulated by grhl2b through diverse transcriptional targets.

PAX Genes in Cancer; Friends or Foes?

PAX genes have been shown to be critically required for the development of specific tissues and organs during embryogenesis. In addition, PAX genes are expressed in a handful of adult tissues where they are thought to play important roles, usually different from those in embryogenesis. A common theme in adult tissues is a requirement for PAX gene expression in adult stem cell maintenance or tissue regeneration. The connections between adult stem cell PAX gene expression and cancer are intriguing, and the literature is replete with examples of PAX gene expression in either situation. Here we systematically review the literature and present an overview of postnatal PAX gene expression in normal and cancerous tissue. We discuss the potential link between PAX gene expression in adult tissue and cancer. In addition, we discuss whether persistent PAX gene expression in cancer is favorable or unfavorable.

Evo-devo and brain scaling: candidate developmental mechanisms for variation and constancy in vertebrate brain evolution

Biologists have long been interested in both the regularities and the deviations in the relationship between brain, development, ecology, and behavior between taxa. We first examine some basic information about the observed ranges of fundamental changes in developmental parameters (i.e. neurogenesis timing, cell cycle rates, and gene expression patterns) between taxa. Next, we review what is known about the relative importance of different kinds of developmental mechanisms in producing brain change, focusing on mechanisms of segmentation, local and general features of neurogenesis, and cell cycle kinetics. We suggest that a limited set of developmental alterations of the vertebrate nervous system typically occur and that each kind of developmental change may entail unique anatomical, functional, and behavioral consequences for the organism. Thus, neuroecologists who posit a direct mapping of brain size to behavior should consider that not any change in brain anatomy is possible.

Role of transcription factors in cell replacement therapies for neurodegenerative conditions

Parkinson's disease is the second most common neurological condition, behind dementia, for which there is currently no cure. A promising curative treatment approach is cell replacement therapy, which involves the introduction of new dopaminergic cells into a degenerative Parkinson's disease brain. The future progression of this field into a clinically viable treatment option is reliant on generating replacement dopaminergic cells. Furthermore, as the ability of transplanted dopaminergic neurons to form connections with host tissue is dependent on where the cells are derived from, the replacement dopaminergic cells will need to be phenotypically similar to substantia nigra dopaminergic neurons. This article focuses on how developmental transcription factors have been utilized to assist the progression of stem cell therapies for Parkinson's disease. Key transcription factor-mediated stages of substantia nigra dopaminergic neuronal development is described in the belief that a comprehensive understanding of this specific dopaminergic differentiation pathway is necessary for the progression of successful cell therapies for Parkinson's disease.

Species differences in early patterning of the avian brain

The telencephalon is proportionately larger in parrots than in galliformes (chicken-like birds), whereas the midbrain tectum is proportionately smaller. We here test the hypothesis that the adult species difference in midbrain proportion is due to an evolutionary change in early brain patterning. In particular, we compare the size of the early embryonic midbrain between parakeets (Melopsittacus undulatus) and bobwhite quail (Colinus virgianus) by examining the expression domains of transcription factors Pax6 and Gbx2, which are expressed in the forebrain and hindbrain, respectively. Because these expression domains form rostral and caudal borders with the presumptive midbrain when this region is specified (Hamburger-Hamilton stages 9-11), they allow us to measure and compare the sizes of a molecularly defined presumptive midbrain in the two species. Based on published data from older embryos, we predicted that the molecularly defined midbrain territory is significantly larger in quail than parakeets. Indeed, our data show that normalized midbrain length is 33% greater in quail and that the midbrain to forebrain ratio is 28% greater. This is strong evidence of a significant species difference in early brain patterning.

Phylogenetic origins of early alterations in brain region proportions

Adult galliform birds (e.g. chickens) exhibit a relatively small telencephalon and a proportionately large optic tectum compared with parrots and songbirds. We previously examined the embryonic origins of these adult species differences and found that the optic tectum is larger in quail than in parakeets and songbirds at early stages of development, prior to tectal neurogenesis onset. The aim of this study was to determine whether a proportionately large presumptive tectum is a primitive condition within birds or a derived feature of quail and other galliform birds. To this end, we examined embryonic brains of several avian species (emus, parrots, songbirds, waterfowl, galliform birds), reptiles (3 lizard species, alligators, turtles) and a monotreme (platypuses). Brain region volumes were estimated from serial Nissl-stained sections. We found that the embryos of galliform birds and lizards exhibit a proportionally larger presumptive tectum than all the other examined species. The presumptive tectum of the platypus is unusually small. The most parsimonious interpretation of these data is that the expanded embryonic tectum of lizards and galliform birds is a derived feature in both of these taxonomic groups.

Activation of Ras-ERK pathway by Fgf8 and its downregulation by Sprouty2 for the isthmus organizing activity

In the previous studies, we showed that strong Fgf8 signaling activates the Ras-ERK pathway to induce cerebellum. Here, we show importance of negative regulation following activation of this pathway for proper regionalization of mesencephalon and metencephalon in chick embryos. 'Prolonged' activation of ERK by misexpression of Fgf8b and dominant-negative Sprouty2 (dnSprouty2) did not change the fate of the mesencephalic alar plate. Downregulation of ERK activity using an MEK inhibitor, U0126, or by tetracycline-dependent Tet-off system after co-expression of Fgf8b and dnSprouty2 forced the mesencephalic alar plate to differentiate into cerebellum. We then paid attention to Mkp3. After misexpression of dnMkp3 and Fgf8b, slight downregulation of ERK activity occurred, which may be due to Sprouty2, and the mesencephalon transformed to the isthmus-like structure. The results indicate that ERK must be once upregulated and then be downregulated for cerebellar differentiation and that differential ERK activity level established by negative regulators receiving Fgf8 signal may determine regional specificity of mesencephalon and metencephalon.

The duration of Fgf8 isthmic organizer expression is key to patterning different tectal-isthmo-cerebellum structures

The isthmic organizer and its key effector molecule, fibroblast growth factor 8 (Fgf8), have been cornerstones in studies of how organizing centers differentially pattern tissues. Studies have implicated different levels of Fgf8 signaling from the mid/hindbrain boundary (isthmus) as being responsible for induction of different structures within the tectal-isthmo-cerebellum region. However, the role of Fgf8 signaling for different durations in patterning tissues has not been studied. To address this, we conditionally ablated Fgf8 in the isthmus and uncovered that prolonged expression of Fgf8 is required for the structures found progressively closer to the isthmus to form. We found that cell death cannot be the main factor accounting for the loss of brain structures near the isthmus, and instead demonstrate that tissue transformation underlies the observed phenotypes. We suggest that the remaining Fgf8 and Fgf17 signaling in our temporal Fgf8 conditional mutants is sufficient to ensure survival of most midbrain/hindbrain cells near the isthmus. One crucial role for sustained Fgf8 function is in repressing Otx2 in the hindbrain, thereby allowing the isthmus and cerebellum to form. A second requirement for sustained Fgf8 signaling is to induce formation of a posterior tectum. Finally, Fgf8 is also required to maintain the borders of expression of a number of key genes involved in tectal-isthmo-cerebellum development. Thus, the duration as well as the strength of Fgf8 signaling is key to patterning of the mid/hindbrain region. By extrapolation, the length of Fgf8 expression could be crucial to Fgf8 function in other embryonic organizers.

Developmental origins of mosaic brain evolution: Morphometric analysis of the developing zebra finch brain

In adult zebra finches (Taeniopygia guttata), the telencephalon occupies 64% of the entire brain. This fraction is similar to what is seen in parrots, but many other birds possess a significantly smaller telencephalon. The aim of the present study was to determine the developmental time course and cellular basis of telencephalic enlargement in zebra finches, and then to compare these findings with what is known about telencephalic enlargement in other birds. To this end we estimated the volumes of all major brain regions from serial sections in embryonic and post-hatching zebra finches. We also labeled proliferating cells with antibodies against proliferating cell nuclear antigen and phosphorylated histone H3. An important finding to emerge from this work is that the telencephalon of zebra finches at hatching contains a thick proliferative subventricular zone (SVZ) that extends from the subpallium into the dorsal pallium. The data also show that the onset and offset of telencephalic neurogenesis are both delayed in zebra finches relative to quail (Galliformes). This delay in neurogenesis, in conjunction with the expanded SVZ, probably accounts for most of the telencephalic enlargement in passerines such as the zebra finch. In addition, passerines enlarged their telencephalon by decreasing the proportional size of their midbrain tectum. Because the presumptive tectum is proportionally smaller in zebra finches than quail before neurogenesis begins, this difference in tectum size cannot be due to evolutionary alterations in neurogenesis timing. Collectively these findings indicate that several different developmental mechanisms underlie the evolution of a large telencephalon in passerines.

BMP7 and SHH regulate Pax2 in mouse retinal astrocytes by relieving TLX repression

Pax2 is essential for development of the neural tube, urogenital system, optic vesicle, optic cup and optic tract. In the eye, Pax2 deficiency is associated with coloboma, a loss of astrocytes in the optic nerve and retina, and abnormal axonal pathfinding of the ganglion cell axons at the optic chiasm. Thus, appropriate expression of Pax2 is essential for astrocyte determination and differentiation. Although BMP7 and SHH have been shown to regulate Pax2 expression, the molecular mechanism by which this regulation occurs is not well understood. In this study, we determined that BMP7 and SHH activate Pax2 expression in mouse retinal astrocyte precursors in vitro. SHH appeared to play a dual role in Pax2 regulation; 1) SHH may regulate BMP7 expression, and 2) the SHH pathway cooperates with the BMP pathway to regulate Pax2 expression. BMP and SHH pathway members can interact separately or together with TLX, a repressor protein in the tailless transcription factor family. Here we show that the interaction of both pathways with TLX relieves the repression of Pax2 expression in mouse retinal astrocytes. Together these data reveal a new mechanism for the cooperative actions of signaling pathways in astrocyte determination and differentiation and suggest interactions of regulatory pathways that are applicable to other developmental programs.

Autoregulatory loop and retinoic acid repression regulate pou2/pou5f1 gene expression in the zebrafish embryonic brain

Zebrafish pou5f1, also known as pou2, encodes a POU-family transcription factor that is transiently expressed in the prospective midbrain and anterior hindbrain during gastrulation, governing brain development. In the present study, we found that the main regulatory elements reside in the proximal upstream DNA sequence from -2.2 to -0.12 kb (the -2.2/-0.1 region). The electrophoretic gel mobility shift assay (EMSA) revealed four functional octamer sequences that can associate with zebrafish Pou2/Pou5f1. The expression of mutated reporter constructs, as well as EMSA, suggested that these four octamer sequences operate in a cooperative manner to drive expression in the mid/hindbrain. We also identified a retinoic acid (RA)-responsive element in this proximal region, which was required to repress transcription in the posterior part of the embryo. These data provide a scheme wherein pou2/pou5f1 expression in zebrafish embryos is regulated by both an autoregulatory loop and repression by RA emanating from the posterior mesoderm.

Ectopic Pax2 expression in chick ventral optic cup phenocopies loss of Pax2 expression

Pax2 is essential for the development of the urogenital system, neural tube, otic vesicle, optic cup and optic tract [Dressler, G.R., Deutsch, U., et al., 1990. PAX2, a new murine paired-box-containing gene and its expression in the developing excretory system. Development 109 (4), 787-795; Nornes, H.O., Dressler, G.R., et al., 1990. Spatially and temporally restricted expression of Pax2 during murine neurogenesis. Development 109 (4), 797-809; Eccles, M.R., Wallis, L.J., et al., 1992. Expression of the PAX2 gene in human fetal kidney and Wilms' tumor. Cell Growth Differ 3 (5), 279-289]. Within the visual system, a loss-of-function leads to lack of choroid fissure closure (known as a coloboma), a loss of optic nerve astrocytes, and anomalous axonal pathfinding at the optic chiasm [Favor, J., Sandulache, R., et al., 1996. The mouse Pax2(1Neu) mutation is identical to a human PAX2 mutation in a family with renal-coloboma syndrome and results in developmental defects of the brain, ear, eye, and kidney. Proc. Natl. Acad. Sci. U. S. A. 93 (24), 13870-13875; Torres, M., Gomez-Pardo, E., et al., 1996. Pax2 contributes to inner ear patterning and optic nerve trajectory. Development 122 (11), 3381-3391]. This study is directed at determining the effects of ectopic Pax2 expression in the chick ventral optic cup past the normal developmental period when Pax2 is found. In ovo electroporation of Pax2 into the chick ventral optic cup results in the formation of colobomas, a condition typically associated with a loss of Pax2 expression. While the overexpression of Pax2 appears to phenocopy a loss of Pax2, the mechanism of the failure of choroid fissure closure is associated with a cell fate switch from ventral retina and retinal pigmented epithelium (RPE) to an astrocyte fate. Further, ectopic expression of Pax2 in RPE appears to have non-cell autonomous effects on adjacent RPE, creating an ectopic neural retina in place of the RPE.

Transcription of fgf8 is regulated by activating and repressive cis-elements at the midbrain-hindbrain boundary in zebrafish embryos

Fgf8 is expressed in the isthmic region of the developing brain, serving an organizing function in vertebrate embryos. We previously identified S4.2 downstream to the zebrafish fgf8 gene as a regulatory region that drives transcription in the anterior hindbrain. Here, we investigated the mechanism of fgf8 regulation by the S4.2 region during development. Reporter analyses in embryos revealed that S4.2 closely recapitulates fgf8 expression in the anteriormost hindbrain during somitogenesis. This region contains a sequence highly conserved in fgf8 of diverse vertebrates. Further analyses of S4.2 revealed a 342-bp core region composed of three subregions (#2, #3, and #4). Regions #3 and #4 drove expression broadly in the brain from the midbrain to r5 of the hindbrain, whereas a 28-bp sequence in #2 repressed ectopic expression in the midbrain and in r2 to r5. The enhancer function of S4.2 was absent in pax2a mutant embryos, while it was activated ectopically by pax2a misexpression in the hindbrain. We identified two sites in the core region that are bound by Pax2a in vitro and in vivo, the disruption of which abrogated the S4.2 activity. Thus, fgf8 expression in the anteriormost hindbrain involves activation and repression, with Pax2a as a pivotal regulator.

Regulation of EphA8 gene expression by TALE homeobox transcription factors during development of the mesencephalon

The mouse ephA8 gene is expressed in a rostral-to-caudal gradient in the developing superior colliculus, and these EphA gradients may contribute to the proper development of the retinocollicular projection. Thus, it is of considerable interest to elucidate how the ephA8 gene expression is controlled by upstream regulators during the development of the mesencephalon. In this study, we employed in vivo expression analysis in transgenic mouse embryos to dissect the cis-acting DNA regulatory region, leading to the identification of a CGGTCA sequence critical for the ephA8 enhancer activity. Using this element as the target in a yeast one-hybrid system, we identified a Meis homeobox transcription factor. Significantly, DNA binding sites for Pbx, another TALE homeobox transcription factor, were also identified in the ephA8 enhancer region. Meis2 and Pbx1/2 are specifically expressed in the entire region of the dorsal mesencephalon, where specific colocalization of EphA8 and Meis is restricted to a subset of cells. Meis2 and Pbx2 synergistically bind the ephA8 regulatory sequence in vitro, and this interaction is critical for the transcriptional activation of a reporter construct bearing the ephA8 regulatory region in the presence of histone deacetylase inhibitor. More importantly, when expressed in the embryonic midbrain, the dominant-negative form of Meis down-regulates endogenous ephA8. Interestingly, we found that both Meis2 and Pbx2 are constitutively bound in the ephA8 regulatory region in the dorsal mesencephalon. These studies strongly suggest that Meis and Pbx homeobox transcription factors tightly associate with the ephA8 regulatory sequence and require an additional unidentified regulator to ensure the specific activation of ephA8.

Pax-5 expression in nonhematopoietic tissues

We evaluated 123 formalin-fixed, paraffin-embedded samples, including neuroendocrine tumors, adult brain, mesonephric tissues, and from various other sites. A pre-B lymphoma cell line, Daudi, and a small cell carcinoma cell line, NCL-H128, were evaluated by Western blot. All tissues were immunostained by mouse monoclonal anti-Pax-5 antibody by using standard, synthetic polymer-based detection methods. Our study describes for the first time distribution of Pax-5 in adult brain tissue, including periaqueductal gray matter of the midbrain, area postrema of the medulla oblongata, and occasional cells of the spinal trigeminal nucleus (caudal nucleus). We confirm that Pax-5 is expressed regularly in poorly differentiated neuroendocrine tumors but never in well-differentiated classic carcinoid tumors. Pax-5 expression also was found readily in benign and malignant mesonephric tissues and focally in müllerian duct-derived tissues and tumors. Expression of Pax-5 in the Daudi and NCL-H128 cell lines was confirmed by Western blot. Together, these results are important for correct interpretation of results in immunophenotyping of undifferentiated tumors, for diagnosis of mesonephric carcinoma, and, potentially, for correct classification of neuroendocrine tumors in small biopsy samples.

Repulsive guidance molecule plays multiple roles in neuronal differentiation and axon guidance

Repulsive guidance molecule (RGM) is a membrane-bound protein originally isolated as a guidance molecule for retinal axons. Three RGM isoforms (RGMa-RGMc) exist in vertebrates. We showed previously that RGMa is a cell-survival factor in the neuroepithelium of chick embryos that suppresses the proapoptotic activity of its receptor neogenin. In the present study, we performed gain- and loss-of-function analysis of RGMa in chick embryos to further investigate RGMa function. We found that RGMa overexpression promotes neuronal differentiation, whereas RGMa small interference RNA represses it. Similar experiments conducted at later developmental stages using retroviral vectors reveal that perturbation of RGMa expression disturbs the retinotectal projection. Our work provides the first evidence for a role for RGMs in axon guidance in vivo. In addition, these results suggest that RGMa exerts multiple functions during neural development.

Human neurospheres derived from the fetal central nervous system are regionally and temporally specified but are not committed

Proliferating single cells were isolated from various CNS regions (telencephalon, diencephalon, midbrain, cerebellum, pons and medulla, and spinal cord) of human fetal cadavers at 13 weeks of gestation and grown as neurospheres in long-term cultures. We investigated whether neural stem cells (NSCs) or progenitors within spheres have specific regional or temporal characteristics with regard to growth, differentiation, and region-specific gene expression, and whether these molecular specifications are reversible. Regardless of regional origin, all of the neurospheres were found to contain cells of different subtypes, which suggests that multipotent NSCs, progenitors or radial glial cells co-exist with restricted neuronal or glial progenitors within the neurospheres. Neurospheres from the forebrain grew faster and gave rise to significantly more neurons than did those from either the midbrain or hindbrain, and regional differences in neuronal differentiation appeared to be sustained during long-term passage of neurospheres in culture. There was also a trend towards a reduction in neuronal emergence from the respective neurospheres over time in culture, although the percentages of neurons generated from cerebellum-derived neurospheres increased dramatically. These results suggest that differences in neuronal differentiation for the various neurospheres are spatially and temporally determined. In addition, the properties of glial fibrillary acidic protein (GFAP)-, glutamate-, and gamma-aminobutyric acid (GABA)-expressing cells derived from neurospheres of the respective CNS regions appear to be regionally and temporally different. Isolated human neurospheres from different CNS compartments expressed distinctive molecular markers of regional identity and maintained these patterns of region-specific gene expression during long-term passage in vitro. To determine the potential of human neurospheres for regional fate plasticity, single spheres from the respective regions were co-cultured with embryonic day 16.5 (E16.5 d) mouse brain slices. Specific cues from the developing mouse brain tissues induced the human neurospheres to express different marker genes of regional identity and to suppress the expression of their original marker genes. Thus, even the early regional identities of human neurospheres may not be irreversible and may be altered by local inductive cues. These findings have important implications for understanding the characteristics of growth, differentiation, and molecular specification of human neurospheres derived from the developing CNS, as well as the therapeutic potential for neural repair.

FGF regulated gene-expression and neuronal differentiation in the developing midbrain-hindbrain region

The neuroectodermal tissue close to the midbrain-hindbrain boundary (MHB) is an important secondary organizer in the developing neural tube. This so-called isthmic organizer (IsO) secretes signaling molecules, such as fibroblast growth factors (FGFs), which regulate cellular survival, patterning and proliferation in the midbrain and rhombomere 1 (R1) of the hindbrain. We have previously shown that FGF-receptor 1 (FGFR1) is required for the normal development of this brain region in the mouse embryo. Here, we have compared the gene expression profiles of midbrain-R1 tissues from wild-type embryos and conditional Fgfr1 mutants, in which FGFR1 is inactivated in the midbrain and R1. Loss of Fgfr1 results in the downregulation of several genes expressed close to the midbrain-hindbrain boundary and in the disappearance of gene expression gradients in the midbrain and anterior hindbrain. Our screen identified several previously uncharacterized genes which may participate in the development of midbrain-R1 region. Our results also show altered neurogenesis in the midbrain and R1 of the Fgfr1 mutants. Interestingly, the neuronal progenitors in midbrain and R1 show different responses to the loss of signaling through FGFR1.

A distinct preisthmic histogenetic domain is defined by overlap of Otx2 and Pax2 gene expression in the avian caudal midbrain

Correlative in situ hybridization of Otx2, Pax2, Gbx2, and Fgf8 mRNA probes in adjacent serial sections through the chicken midbrain and isthmus at early to intermediate stages of development served to map in detail the area of overlap of Otx2 and Pax2 transcripts in the caudal midbrain. The neuronal populations developing within this preisthmic domain made up a caudal part of the midbrain reticular formation, the interfascicular nucleus, and the magnocellular (pre)isthmic nucleus, plus the corresponding part of the periaqueductal gray. The torus semicircularis-the inferior colliculus homolog-expressed Otx2 in its ventricular lining exclusively, but it never expressed Pax2. The parvicellular isthmic nucleus, although placed inside the midbrain lobe, never expressed Otx2, and its cells rapidly down-regulated an early transient Pax2 signal; this pattern is consistent with its reported isthmic origin and forward tangential translocation. This analysis reveals the existence of four distinct midbrain histogenetic domains along the longitudinal axis, at least for the alar plate. These presumably result from step-like isthmic organizer effects on Otx2-expressing midbrain neuroepithelium at different distances from a caudal FGF8 morphogen source (isthmic Fgf8-positive domain). The final phenotypes of these domains are histologically diverse and make up the griseum tectale (rostrally), the optic tectum, the torus semicircularis, and the presently characterized preisthmic domain (lying closest to the isthmic organizer). Available comparative data for reptiles and mammals suggest the general validity of this scheme.

Isthmus organizer for midbrain and hindbrain development

Classical transplantation studies showed that the isthmus has an organizing activity upon the tectum and cerebellum. Since Fgf8 is expressed in the isthmus and mimics functionally isthmic grafts, it is accepted that Fgf8 plays pivotal role in the isthmic organizing activity. The fate of brain vesicles is determined by the combinations of transcription factors. The neural tube region where Otx2, Pax2, and En1 are expressed early on acquires midbrain identity. Pax3/7 forces the midbrain to differentiate into tectum. En1/2, Pax2/5, and Fgf8 form a positive feedback loop for their expression, thus misexpression of one of these molecules turns on the loop and forces presumptive diencephalon to differentiate into tectum. The isthmic organizer signal, Fgf8, stabilizes or changes the expression of the transcription factors in mid/hindbrain region. A strong Fgf8 signal activates the Ras-ERK signaling pathway, which in turn activates Irx2 in a rostrodorsal part of the hindbrain, and forces this tissue to differentiate into cerebellum.

How does Fgf signaling from the isthmic organizer induce midbrain and cerebellum development?

The mesencephalic/rhombomere 1 border (isthmus) is an organizing center for early development of midbrain and cerebellum. In this review, we summarize recent progress in studies of Fgf signaling in the isthmus and discuss how the isthmus instructs the differentiation of the midbrain versus cerebellum. Fgf8 is shown to play a pivotal role in isthmic organizer activity. Only a strong Fgf signal mediated by Fgf8b activates the Ras-extracellular signal-regulated kinase (ERK) pathway, and this is sufficient to induce cerebellar development. A lower level of signaling transduced by Fgf8a, Fgf17 and Fgf18 induce midbrain development. Numerous feedback loops then maintain appropriate mesencephalon/rhombomere1 and organizer gene expression.

Identification of Pax2-regulated genes by expression profiling of the mid-hindbrain organizer region

The paired domain transcription factor Pax2 is required for the formation of the isthmic organizer (IsO) at the midbrain-hindbrain boundary, where it initiates expression of the IsO signal Fgf8. To gain further insight into the role of Pax2 in mid-hindbrain patterning, we searched for novel Pax2-regulated genes by cDNA microarray analysis of FACS-sorted GFP+ mid-hindbrain cells from wild-type and Pax2-/- embryos carrying a Pax2(GFP) BAC transgene. Here, we report the identification of five genes that depend on Pax2 function for their expression in the mid-hindbrain boundary region. These genes code for the transcription factors En2 and Brn1 (Pou3f3), the intracellular signaling modifiers Sef and Tapp1, and the non-coding RNA Ncrms. The Brn1 gene was further identified as a direct target of Pax2, as two functional Pax2-binding sites in the promoter and in an upstream regulatory element of Brn1 were essential for lacZ transgene expression at the mid-hindbrain boundary. Moreover, ectopic expression of a dominant-negative Brn1 protein in chick embryos implicated Brn1 in Fgf8 gene regulation. Together, these data defined novel functions of Pax2 in the establishment of distinct transcriptional programs and in the control of intracellular signaling during mid-hindbrain development.

Evolution of the brain developmental plan: Insights from agnathans

In vertebrate evolution, the brain exhibits both conserved and unique morphological features in each animal group. Thus, the molecular program of nervous system development is expected to have experienced various changes through evolution. In this review, we discuss recent data from the agnathan lamprey (jawless vertebrate) together with available information from amphioxus and speculate the sequence of changes during chordate evolution that have been brought into the brain developmental plan to yield the current variety of the gnathostome (jawed vertebrate) brains.

Exploring alternative models of rostral-caudal patterning in the zebrafish neurectoderm with computer simulations

The StarLogo and NetLogo programming environments allow developmental biologists to build computer models of cell-cell interactions in an epithelium and visualize emergent properties of hypothetical genetic regulatory networks operating in the cells. These environments were used to explore alternative models that show how a posteriorizing morphogen gradient might define gene-expression domains along the rostral-caudal axis in the zebrafish neurectoderm. The models illustrate how a hypothetical genetic network based on auto-activation and cross-repression could lead to establishment of discrete non-overlapping gene-expression domains.

Fgfr1-dependent boundary cells between developing mid- and hindbrain

Signaling molecules regulating development of the midbrain and anterior hindbrain are expressed in distinct bands of cells around the midbrain-hindbrain boundary. Very little is known about the mechanisms responsible for the coherence of this signaling center. One of the fibroblast growth factor (FGF) receptors, Fgfr1, is required for establishment of a straight border between developing mid- and hindbrain. Here we show that the cells close to the border have unique features. Unlike the cells further away, these cells express Fgfr1 but not the other FGF receptors. The cells next to the midbrain-hindbrain boundary express distinct cell cycle regulators and proliferate less rapidly than the surrounding cells. In Fgfr1 mutants, these cells fail to form a coherent band at the boundary. The slowly proliferating boundary cells are necessary for development of the characteristic isthmic constriction. They may also contribute to compartmentalization of this brain region.