Expression of an engrailed-like gene during development of the early embryonic chick nervous system

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Engrailed homeoproteins in visual system development

Engrailed is a homeoprotein transcription factor. This family of transcription factors is characterized by their DNA-binding homeodomain and some members, including Engrailed, can transfer between cells and regulate protein translation in addition to gene transcription. Engrailed is intimately involved in the development of the vertebrate visual system. Early expression of Engrailed in dorsal mesencephalon contributes to the development and organization of a visual structure, the optic tectum/superior colliculus. This structure is an important target for retinal ganglion cell axons that carry visual information from the retina. Engrailed regulates the expression of Ephrin axon guidance cues in the tectum/superior colliculus. More recently it has been reported that Engrailed itself acts as an axon guidance cue in synergy with the Ephrin system and is proposed to enhance retinal topographic precision.

Neuronal maturation and laminar formation in the chicken optic tectum are accompanied by the transition of phosphorylated cofilin from cytoplasm to nucleus

Laminar formation in the chicken optic tectum requires processes that coordinate proliferation, migration and differentiation of neurons, in which the dynamics of actin filaments are crucial. Cofilin plays pivotal roles in regulating actin arrangement via its phosphorylation on Ser3. Given poor studies on the profile of phosphorylated cofilin (p-cofilin) in the developing tectum, we investigated its expression pattern. As determined by immunofluorescence histochemistry and western blotting, p-cofilin could be detected in most tectal layers except for the neural epithelium. In addition, we found p-cofilin was expressed both in the cytoplasm and the nucleus. During development, the expression of the cytoplasmic p-cofilin was decreasing and the nuclear p-cofilin was gradually increasing, but the total level of p-cofilin was down regulated. Double-labeling experiments revealed that the nuclear p-cofilin could be labeled in mature neurons but undetected in immature neurons. Furthermore, the number of cells co-stained with nuclear p-cofilin and NeuN was up-regulated during lamination and 60% cells were detected to be mature neurons that can express nuclear p-cofilin just at the first appearance of completed laminae. Our results demonstrate that the maturation of neurons is accompanied by this cytoplasm-to-nucleus transition of p-cofilin, and the nuclear p-cofilin can work effectively as a marker in the laminar formation of the chicken optic tectum.

Key roles of Ephs and ephrins in retinotectal topographic map formation

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.

Formation and patterning of the avian neuraxis: one dozen hypotheses

Formation of the neuraxis is dependent on cell-cell interactions and cell movements beginning during stages of gastrulation. Cell movements bring together new combinations of cells, allowing sequential inductive interactions to occur and leading to the specification of the neural plate and to its ultimate mediolateral (subsequently dorsoventral) and rostrocaudal patterning. Formation of the neural plate involves changes in the shape of its constituent cells and the first appearance of neural-specific cell markers. Shortly after the neural plate forms it undergoes 'shaping', in which the pseudostratified columnar epithelium constituting the neural plate thickens apicobasally, narrows transversely and extends longitudinally. Shaping is driven by three principal intrinsic types of cell behaviour: changes in cell shape, position and number. The next stage of neurulation begins while shaping is underway--bending of the neural plate. Bending involves two main processes, furrowing and folding. Furrowing of the neural plate is associated with the formation of the hinge points; these are localized, longitudinal areas where the neuroepithelium is attached to adjacent tissues and where wedging of neuroepithelial cells occurs. Cell wedging in the median hinge point occurs as a result of inductive interactions with the notochord; such wedging drives furrowing, thereby facilitating subsequent folding. Folding of the neural plate requires extrinsic forces generated largely by the surface ectoderm. Types of cell behaviour that could provide such forces include changes in cell shape, position and number. As a result of shaping and bending of the neural plate, the neural folds are brought into apposition in the dorsal midline. Final closure of the neural groove is mediated by cell surface glycoconjugates coating the apical surfaces of the neural folds. Patterning of the neuraxis begins during shaping of the neural plate and continues throughout stages of neurulation and into early postneurula stages. Patterning probably involves inductive interactions with adjacent tissues and the expression of putative positional identity genes such as homeobox-containing genes.

A model of early molecular regionalization in the chicken embryonic pretectum

The pretectal region of the brain is visualized as a dorsal region of prosomere 1 in the caudal diencephalon, including derivatives from both the roof and alar plates. Its neuronal derivatives in the adult brain are known as pretectal nuclei. The literature is inconsistent about the precise anteroposterior delimitation of this region and on the number of specific histogenetic domains and subdomains that it contains. We performed a cross-correlated gene-expression map of this brain area in chicken embryos, with the aim of identifying differently fated pretectal domains on the basis of combinatorial gene expression patterns. We examined in detail Pax3, Pax6, Pax7, Tcf4, Meis1, Meis2, Nkx2.2, Lim1, Dmbx1, Dbx1, Six3, FoxP2, Zic1, Ebf1, and Shh mRNA expression, as well as PAX3 and PAX7 immunoreaction, between stages HH11 and HH28. The patterns analyzed serve to fix the cephalic and caudal boundaries of the pretectum and to define three molecularly distinct anteroposterior pretectal domains (precommissural, juxtacommissural, and commissural) and several dorsoventral subdomains. These molecular specification patterns are established step by step between stages HH10 and HH18, largely before neurogenesis begins. This set of gene-architectonic data constitutes a useful scaffold for correlations with fate maps and other experimental embryologic results and may serve as well for inquiries on homologies in this part of the brain.

Genetic basis of Joubert syndrome and related disorders of cerebellar development

Over three decades have passed since Marie Joubert described the original proband for Joubert syndrome, a rare neurological disorder featuring absence of the cerebellar vermis (i.e. midline). Efforts at deciphering the molecular basis for this disease have been complicated by the clinical and genetic heterogeneity as well as extensive phenotypic overlap with other syndromes. However, progress has been made in recent years with the mapping of three genetic loci and the identification of mutations in two genes, AHI1 and NPHP1. These genes encode proteins with some shared functional domains, but their role in brain development is unclear. Clues may come from studies of related syndromes, including Bardet-Biedl syndrome and nephronophthisis, for which all of the encoded proteins localize to primary cilia. The data suggest a tantalizing connection between intraflagellar transport in cilia and brain development.

The chick/quail transplantation model: Discovery of the isthmic organizer center

This paper summarizes chick/quail transplantation experiments performed in the INSERM U106 by Alvarado-Mallart's group from 1989 to 2002. First, it will present the various steps leading us to demonstrate that, at stage 10 of Hamburger and Hamilton, the avian neuroepithelium is still competent to change its fate influenced by environmental inductive factors and that these factors emanate from the cerebellar neuroepithelium; then, it will be briefly reported, experiments aimed to characterize the genetic cascade involved in the formation of the midbrain/hindbrain boundary and the specification of the meso-isthmic-cerebellar domain.

Factors involved in the determination of the neurotransmitter phenotype of developing neurons of the CNS: Applications in cell replacement treatment for Parkinson's disease

The developmental stages involved in the conversion of stem cells to fully functional neurons of specific neurotransmitter phenotype are complex and not fully understood. Over the past decade many studies have been published that demonstrate that in vitro manipulation of the epigenetic environment of the stem cells allows experimental control of final neuronal phenotypic choice. This review presents the evidence for the involvement of a number of endogenous neurobiochemicals, which have been reported to potently influence DAergic (and other neurotransmitter) phenotype expression in vitro. They act at different stages on the pathway to neurotransmitter phenotype determination, and in different ways. Many are better known for their involvement in other aspects of development, and in other biochemical roles. Their proper place, and precise roles, in neurotransmitter phenotype determination in vivo will no doubt be determined in the future. Meanwhile, considerable medical benefits are offered from producing large, long-term, viable cryostores of self-regenerating multipotential neural precursor cells (i.e., brain stem cells), which can be used for cell replacement therapies in the treatment of degenerative brain diseases, such as Parkinson's disease.

Engrailed genes control developmental fate of serotonergic and noradrenergic neurons in mid- and hindbrain in a gene dose-dependent manner

In vertebrates and insects, the homeobox transcription factors of the engrailed family have a dual function. They take part in regionalization during early embryogenesis and later in neuronal specification. In mammals, two engrailed homologues exist, engrailed-1 and engrailed-2, which are expressed in a broad band around the isthmus at an age when the serotonergic and noradrenergic neurons in mid/hindbrain are generated. The analysis of engrailed-1 and -2 double mutant mice revealed a specific, redundant, and gene dose-dependent requirement of the two transcription factors for the development of the serotonergic dorsal raphe nucleus and the noradrenergic locus caeruleus. Both nuclei are lost in engrailed double mutant mice; however, directly adjacent nuclei of the same neurotransmitter phenotype are not affected. An almost identical phenotype is found in mutant mice null for Wnt1, indicating that the engrailed genes provide essential positional information for the development of the two nuclei during early embryogenesis.

Development of the dopaminergic neurons in the rodent brainstem

The loss of dopaminergic (DA) neurons in the ventral midbrain is the principal cause of Parkinson's disease. The search for candidate molecules that promote the genesis and survival capacities of DA neurons is a major area of investigation and hope. A better characterization of the developmental pathways that govern the specification, differentiation, and survival of these neurons will be essential in devising therapies aimed to rescue or replace midbrain DA neurons in Parkinson's patients. In this brief review, we will discuss the major steps in the normal development of midbrain DA neurons.

Specification of the meso-isthmo-cerebellar region: the Otx2/Gbx2 boundary

The midbrain/hindbrain (MH) territory containing the mesencephalic and isthmocerebellar primordial is characterized by the expression of several families of regulatory genes including transcription factors (Otx, Gbx, En, and Pax) and signaling molecules (Fgf and Wnt). At earlier stages of avian neural tube, those genes present a dynamic expression pattern and only at HH18-20 onwards, when the mesencephalic/metencephalic constriction is coincident with the Otx2/Gbx2 boundary, their expression domains become more defined. This review summarizes experimental data concerning the genetic mechanisms involved in the specification of the midbrain/hindbrain territory emphasizing the chick/quail chimeric experiments leading to the discovery of a secondary isthmic organizer. Otx2 and Gbx2 co-regulation could determine the precise location of the MH boundary and involved in the inductive events characteristic of the isthmic organizer center.

Expression patterns of engrailed-like proteins in the chick embryo

The protein products of both of the identified chick engrailed-like (En) genes, chick En-1 and chick En-2, are localized in cells of the developing brain, mandibular arch, spinal cord, dermatome, and ventral limb bud ectoderm, as demonstrated by labeling with the polyclonal antiserum alpha Enhb-1 developed by Davis et al. (Development 111:281-298, 1991). A subpopulation of cephalic neural crest cells is also En-protein-positive. The monoclonal antibody 4D9 recognizes the chick En-2 gene product exclusively (Patel et al.: Cell 58:955-968, 1989; Davis et al., 1991) and colocalizes with chick En-2 mRNA in the developing head region of the chick embryo as shown by in situ hybridization (Gardner et al.: J. Neurosci. Res. 21:426-437, 1988). In the present study we examine the pattern of alpha Enhb-1 and 4D9 localization throughout the chick embryo from the first appearance of antibody (Ab)-positive cells at stage 8 (Hamburger and Hamilton: J. Morphol. 88:49-92, 1951) through stage 28 (1-5.5 days). We compare the localization patterns of the two Abs to each other, as well as to the localization of the monoclonal Ab, HNK-1, which recognizes many neural crest cells, using double- and triple-label fluorescence immunohistochemistry. Most En protein-positive cells in the path of neural crest cell migration are not HNK-1 positive. In detailed examination of alpha Enhb-1 and 4D9 localization, we find previously undetected patterns of En protein localization in the prechordal plate, hindbrain, myotome, ventral body-wall mesoderm, and extraembryonic membranes. Based upon these observations we propose: 1) that En expression in the mesoderm may be induced through interaction with En expressing cells in the neuroectoderm; 2) that En expression in the head mesenchyme is associated with somitomere 4; and 3) that En expression may be involved in epithelial-mesenchymal cell transformations.

Possible role of Hes5 for the rostrocaudal polarity formation of the tectum

The alar plate of the mesencephalon differentiates into the optic tectum. Retinal fibers project to the tectum topographically in a retinotopic manner. Engrailed (En) is responsible for the tectum polarity formation and regionalization. Former study indicated the presence of the molecule whose expression is repressed by En and that represses the isthmus-related gene expression. To isolate such molecules, we constructed a subtracted library between cDNA population of the normal rostral mesencephalon and of the rostral mesencephalon that misexpresses En2. From the library, we isolated cHes5, a chicken homolog of Drosophila hairy/Enhancer of split. cHes5 begins to be expressed in the rostral part of the E2 mesencephalon, and spreads to caudal mesencephalon by E3. To our expectation, cHes5 expression was repressed by En2. Furthermore, misexpression of cHes5 in the mesencephalon inhibited expression of ephrinA2, a marker of caudal mesencephalon. An active repressor form of Hes5, which is a chimeric molecule of Hes5 and repressor domain of En2, showed a similar but more severe phenotype. The results indicate that Hes5 is regulated by En and is responsible for rostral identity of mesencephalon by repressing ephrinA2.

Gene silencing in chick embryos with a vector-based small interfering RNA system

In this paper, the use of vector-based RNA interference (RNAi) to specifically interfere with gene expression in chick embryos is reported. In ovo electroporation was carried out to transfer a small interfering RNA (siRNA) expression vector into chick embryos. En2 was chosen for the target gene because the family gene, En1, is expressed in a similar pattern. Four sets of 19-mer sequences were designed with the En2 open reading frame region connected to a sequence of short hairpin RNA (shRNA), which exerts siRNA effects after being transcribed, and inserted into pSilencer U6-1.0 vector. En2 and En1 expression were suppressed by the siRNA whose sequence completely matched En2 and En1. Suppression occurred when the siRNA sequence differed by up to two nucleotides from the target sequence. The sequence that differed by four nucleotides from the target gene did not show siRNA effects. One set that completely matched the En2 target did not show siRNA effects, which may be due to location of the siRNA in the target gene. Thus, multiple sets of shRNA must be prepared if we are to consider. This system will greatly contribute to the analysis of function of genes of interest, because the target gene can be silenced in a locally and temporally desired manner.

The role of Grg4 in tectal laminar formation

Mature chick optic tecta consist of 16 laminae and receive retinal fiber projections in a precise retinotopic manner. Retinal axons arborize in laminae a-f of the SGFS, but do not cross the border between lamina f and g. In order to elucidate molecular mechanisms of tectal laminar formation, we first looked at the migration of tectal postmitotic cells. We found that the migration pattern of postmitotic cells changes around E5 and that late migratory cells intervened laminae that were formed by early migratory cells. The coincident appearance of Grg4 expression in the tectal ventricular layer and the change in migration pattern suggested an important role for Grg4. Clonal misexpression of Grg4 resulted in cells migrating to laminae h-j of the SGFS. Massive misexpression of Grg4 resulted in disruption of laminae that were formed by early migratory cells, in particular lamina g of the SGFS. Application of Grg4 morpholino antisense oligonucleotide or the misexpression of a dominant-negative form of Grg4 exerted the opposite effect. We concluded that Grg4 may direct tectal postmitotic cells to follow a late migratory pathway.

Fate map of the chicken neural plate at stage 4

A detailed fate map was obtained for the early chick neural plate (stages 3d/4). Numerous overlapping plug grafts were performed upon New-cultured chick embryos, using fixable carboxyfluorescein diacetate succinimidyl ester to label donor chick tissue. The specimens were harvested 24 hours after grafting and reached in most cases stages 9-11 (early neural tube). The label was detected immunocytochemically in wholemounts, and cross-sections were later obtained. The positions of the graft-derived cells were classified first into sets of purely neural, purely non-neural and mixed grafts. Comparisons between these sets established the neural plate boundary at stages 3d/4. Further analysis categorized graft contributions to anteroposterior and dorsoventral subdivisions of the early neural tube, including data on the floor plate and the eye field. The rostral boundary of the neural plate was contained within the earliest expression domain of the Ganf gene, and the overall shape of the neural plate was contrasted and discussed with regard to the expression patterns of the genes Plato, Sox2, Otx2 and Dlx5 (and others reported in the literature) at stages 3d/4.

Cellular and cis-regulation of En-2 expression in the mandibular arch

Investigations into early muscle development have focused primarily on somite derived cells. Cranial mesoderm does not undergo somitogenesis, and muscle formation in this region is less well understood. In the present study, we have focused upon the expression of engrailed in mandibular arch myoblasts. We demonstrate that En-2 is expressed in mandibular arch myoblasts of the mouse. The activity of the En-2 enhancer is maintained in several functionally related muscles that arise from the first arch. Through the use of reporter transgenics, we demonstrate that local cell-cell interactions are important in maintaining En-2 expression in the mandibular arch cells. En-2 enhancer activity in the first arch requires a combination of cis-acting sequences that includes a motif which is identical to one found in the Otx2 enhancer and which is sufficient to direct expression in the first arch. These data support the notion that cranial muscle development is regulated by local cell-cell interactions which distinguish distinct anatomical and functional muscle groups.

Regionalisation and acquisition of polarity in the optic tectum

The optic tectum differentiates from the alar plate of the mesencephalon and receives retinal fibres in a precise retinotopic manner. Here, mechanisms of tectum polarisation and regionalisation are reviewed. Misexpression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to that of the tectum. Ephrin A2 and A5 are expressed in a gradient in the tectum, caudal high and rostral low, and may play important roles in the formation of a precise retinotectal projection map. Retinal fibres that express receptors for these ligands, and which come from the temporal retina, are repulsed by the ligands and do not invade the caudal tectum. Both En1 and En2 can regulate posterior characteristics in the tectum by inducing ephrin A2 and A5. Transplantation experiments in chick have indicated that the mes/metencephalic boundary works as an organiser for the tectum and the cerebellum. Fgf8 is a candidate signalling molecule in the organiser. Pax2/5, En, and Fgf8 are in a positive feedback loop for their expression such that misexpression of one of these genes in the diencephalon turns on the feedback loop and can result in induction of an optic tectum. Otx2 and Gbx2 appear to repress each other's expression and contribute to defining the posterior border of the tectum. Misexpression of Otx2 in the metencephalon can change the fate of its alar plate to a tectum, and misexpression of Gbx2 in the mesencephalon can cause anterior shifting of the caudal limit of the tectum. The anterior border of the tectum may be determined as a result of repressive interactions between Pax6 and En1/Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to ventralize the tissue; that is, Shh can change the fate of the presumptive tectum to that of the tegmentum that is the ventral structure. It is proposed that the brain vesicle that expresses Otx2, Pax2, and En1 may differentiate into the tectum.

Identification of ephrin-A3 and novel genes specific to the midbrain-MHB in embryonic zebrafish by ordered differential display

Development of the tectum and the cerebellum is induced by a reciprocal inductive signaling between their respective primordia, the midbrain and the midbrain/hindbrain boundary (MHB). We set out to identify molecules that function in and downstream of this reciprocal signaling. Overexpression of LIM domain of the transcription factor Islet-3 (LIM(Isl-3)) leads to inhibition of this reciprocal signaling and to resultant defects in tectal and cerebellar development. We therefore searched for genes that may be either up- or down-regulated by overexpression of LIM(Isl-3) by comparing the gene expression profiles in the midbrain and the MHB of normal embryos and embryos in which Islet-3 function was repressed, using a combination of ordered differential display and whole-mount in situ hybridization. Among genes identified in this search, two cDNA fragments encoded Wnt1 and FGF8, which are already known to be essential for the reciprocal signaling between the midbrain and the MHB, confirming the effectiveness of our strategy. We identified four other partial cDNA clones that were specifically expressed around the MHB, ten cDNAs specifically expressed in the tectum, and three cDNAs expressed in neural crest cells including those derived from the midbrain level. The ephrin-A3 gene was specifically expressed in posterior tectum in a gradient that decreased anteriorly. Although ephrin-A2 and ephrin-A5 have been reported to be expressed in the corresponding region in mouse embryos, the superior/inferior colliculi, mouse ephrin-A3 is not expressed prominently in this region, suggesting that the role of ephrin-A3 in brain development may have been altered in the process of brain evolution.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Fate of midbrain dopaminergic neurons controlled by the engrailed genes

Deficiencies in neurotransmitter-specific cell groups in the midbrain result in prominent neural disorders, including Parkinson's disease, which is caused by the loss of dopaminergic neurons of the substantia nigra. We have investigated in mice the role of the engrailed homeodomain transcription factors, En-1 and En-2, in controlling the developmental fate of midbrain dopaminergic neurons. En-1 is highly expressed by essentially all dopaminergic neurons in the substantia nigra and ventral tegmentum, whereas En-2 is highly expressed by a subset of them. These neurons are generated and differentiate their dopaminergic phenotype in En-1/En-2 double null mutants, but disappear soon thereafter. Use of an En-1/tau-LacZ knock-in mouse as an autonomous marker for these neurons indicates that they are lost, rather than that they change their neurotransmitter phenotype. A single allele of En-1 on an En-2 null background is sufficient to produce a wild type-like substantia nigra and ventral tegmentum, whereas in contrast a single allele of En-2 on an En-1 null background results in the survival of only a small proportion of these dopaminergic neurons, a finding that relates to the differential expression of En-1 and En-2. Additional findings indicate that En-1 and En-2 regulate expression of alpha-synuclein, a gene that is genetically linked to Parkinson's disease. These findings show that the engrailed genes are expressed by midbrain dopaminergic neurons from their generation to adulthood but are not required for their specification. However, the engrailed genes control the survival of midbrain dopaminergic neurons in a gene dose-dependent manner. Our findings also suggest a link between engrailed and Parkinson's disease.

Two members of the IgLON family are expressed in a restricted region of the developing chick brain and neural crest

The precise expression patterns of two IgLON genes, CEPU-1 and limbic system-associated membrane protein (LAMP), were studied during early embryogenesis. It was found that expression of both was localized to restricted regions of the brain and neural crest. In the developing neural tube, CEPU-1 was expressed in the isthmus and a restricted region of the hindbrain, whereas LAMP was expressed in the anterior midbrain. Most neural crest cells expressed LAMP, whereas CEPU-1 expression was limited to crest cells derived from the hindbrain. These results suggest that members of the IgLON family have important roles during embryogenesis, particularly in brain formation and differentiation.

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

The optic tectum differentiates from the alar plate of the mesencephalon. Here, the molecular mechanisms for differentiation of the tectum are reviewed. Mis-expression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to become the tectum. En, Fgf8, Pax2 and Pax5, exist in a positive feedback loop for their expression so that mis-expression of any of these genes acts on the feedback loop resulting in induction of the optic tectum in the diencephalon. Otx2 and Gbx2 can repress the expression of each other and contribute to the formation of the posterior border of the tectum. Mis-expression of Otx2 in the metencephalon changed the fate of its alar plate to the tectum. The anterior border of the tectum might be determined as a result of repressive interaction of Pax6 with En1 and Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to the ventralization of the tissue, that is, the area affected by Shh differentiates into the tegmentum. It is proposed that the brain vesicle that expresses Otx2, Pax2 and En1 might differentiate into the tectum.

Topographical and physiological characterization of interneurons that express engrailed-1 in the embryonic chick spinal cord

A number of homeodomain transcription factors have been implicated in controlling the differentiation of various types of neurons including spinal motoneurons. Some of these proteins are also expressed in spinal interneurons, but their function is unknown. Progress in understanding the role of transcription factors in interneuronal development has been slow because the synaptic connections of interneurons, which in part define their identity, are difficult to establish. Using whole cell recording in the isolated spinal cord of chick embryos, we assessed the synaptic connections of lumbosacral interneurons expressing the Engrailed-1 (En1) transcription factor. Specifically we established whether En1-expressing interneurons made direct connections with motoneurons and whether they constitute a single interneuron class. Cells were labeled with biocytin and subsequently processed for En1 immunoreactivity. Our findings indicate that the connections of En1-expressing cells with motoneurons and with sensory afferents were diverse, suggesting that the population was heterogeneous. In addition, the synaptic connections we tested were similar in interneurons that expressed the En1 protein and in many that did not. The majority of sampled En1 cells did, however, exhibit a direct synaptic connection to motoneurons that is likely to be GABAergic. Because our physiological methods underestimate the number of direct connections with motoneurons, it is possible that the great majority, perhaps all, En1-expressing cells make direct synaptic connections with motoneurons. Our results raise the possibility that En1 could be involved in interneuron-motoneuron connectivity but that its expression is not restricted to a distinct functional subclass of ventral interneuron. These findings constrain hypotheses about the role of En-1 in interneuron development and function.

Pattern formation in the cerebellar cortex

The cerebellar cortex is subdivided rostrocaudally and mediolaterally into a reproducible array of zones and stripes. This makes the cerebellum a valuable model for studying pattern formation in the vertebrate central nervous system. The structure of the adult mouse cerebellar cortex and the series of embryological events that generate the topography are reviewed.Key words: zebrin, Hsp25, Purkinje cells.

Alteration of the retinotectal projection map by the graft of mesencephalic floor plate or sonic hedgehog

The floor plate plays crucial roles in the specification and differentiation of neurons along the dorsal-ventral (DV) axis of the neural tube. The transplantation of the mesecephalic floor plate (mfp) into the dorsal mesencephalon in chick embryos alters the fate of the mesencephalon adjacent to the transplant from the tectum to the tegmentum, a ventral tissue of the mesencephalon. In this study, to test whether the mfp is involved in the specification of the DV polarity of the tectum and affects the projection patterns of retinal fibers to the tectum along the DV axis, we transplanted quail mfp into the dorsal mesencephalon of chick embryos, and analyzed projection patterns of dorsal and ventral retinal fibers to the tectum. In the embryos with the mfp graft, dorsal retinal fibers grew into the dorsal part of the tectum which is the original target for ventral but not dorsal retinal fibers and formed tight focuses there. In contrast, ventral retinal fibers did not terminate at any part of the tectum. Transplantation of Sonic hedgehog (Shh)-secreting quail fibroblasts into the dorsal mesencephalon also induced the ectopic tegmentum and altered the retinotectal projection along the DV axis, as the mfp graft did. These results suggest that some factors from the mesencephalic floor plate or the tegmentum, or Shh itself, play a crucial role in the establishment of the DV polarity of the tectum and the retinotectal projection map along the DV axis.

Engrailed defines the position of dorsal di-mesencephalic boundary by repressing diencephalic fate

Regionalization of a simple neural tube is a fundamental event during the development of central nervous system. To analyze in vivo the molecular mechanisms underlying the development of mesencephalon, we ectopically expressed Engrailed, which is expressed in developing mesencephalon, in the brain of chick embryos by in ovo electroporation. Misexpression of Engrailed caused a rostral shift of the di-mesencephalic boundary, and caused transformation of dorsal diencephalon into tectum, a derivative of dorsal mesencephalon. Ectopic Engrailed rapidly repressed Pax-6, a marker for diencephalon, which preceded the induction of mesencephalon-related genes such as Pax-2, Pax-5, Fgf8, Wnt-1 and EphrinA2. In contrast, a mutant Engrailed, En-2(F51rE), bearing mutation in EH1 domain, which has been shown to interact with a co-repressor, Groucho, did not show the phenotype induced by wild-type Engrailed. Furthermore, VP16-Engrailed chimeric protein, the dominant positive form of Engrailed, caused caudal shift of di-mesencephalic boundary and ectopic Pax-6 expression in mesencephalon. These data suggest that (1) Engrailed defines the position of dorsal di-mesencephalic boundary by directly repressing diencephalic fate, and (2) Engrailed positively regulates the expression of mesencephalon-related genes by repressing the expression of their negative regulator(s).

Regulation of epidermal bullous pemphigoid antigen 1 (BPAG1) synthesis by homeoprotein transcription factors

In a recent gene-trap screen, we identified the gene coding for Epidermal Bullous Pemphigoid Antigen 1 (BPAG1) as a putative transcriptional target of Engrailed and of other homeoproteins with a glutamine in position 50 of their homeodomain. We now show that the nuclear addressing of the homeodomains of Engrailed (EnHD) and Antennapedia (AntpHD) upregulates BPAG1e transcription in immortalized human keratinocytes (GMA24FIA) expressing En1. This upregulation is not observed with AntpHD-Q50A, a variant of AntpHD in which a single mutation abolishes its high-affinity binding to target DNA, thus strongly suggesting that BPAG1e upregulation homeodomains reflects their specific recognition of homeoprotein-binding sites in the BPAG1e locus. This is further confirmed by DNase I footprinting and electrophoretic mobility shift assays that reveal, within the cloned BPAG1e promoter, several sites of direct interaction with EnHD and Engrailed. Co-transfection experiments in GMA24FIA human keratinocytes, COS-7 simian fibroblasts, and CHP-100 human neuroepithelial cells show that Engrailed, Hoxa-5, and Hoxc-8 regulate BPAG1e promoter activity and that this regulation is context-dependent. Finally, using a mouse line with LacZ inserted within the En1 locus, we identify the keratinocytes of the ventral paws, including the epithelial cells of the eccrine tubules, as a strong site of En1 expression throughout adulthood. We therefore propose that BPAG1e, a 230 kDa keratin-binding protein expressed in keratinocytes and participating in the maintenance of hemidesmosomes at the dermis-epidermis border, is directly regulated by homeoprotein transcription factors.

Roles of Pax-2 in initiation of the chick tectal development

Transplantation experiments have shown that the mes-metencephalic boundary (isthmus) acts as an organizer for the development of the optic tectum. We have cloned Pax-2 which is expressed in the isthmus. Previously it was shown that Pax-5, a member of the same Pax subfamily as Pax-2, transformed the diencephalon into a tectum-like structure and induced isthmus- and tectum-related genes both in the mesencephalon and in the diencephalon. In order to define the distinct roles between Pax-2 and Pax-5 in development of the tectum, we expressed Pax-2 ectopically in the mesencephalon and the diencephalon of E2 chick embryos by in ovo electroporation. Histological observation demonstrated that Pax-2 transformed the diencephalon into a tectum-like structure. In Pax-2, transfected embryos the expression of isthmus- and tectum-related genes such as Fgf8 and En-2 was induced in the diencephalon. However, neither Fgf8 nor En-2 expression was induced in the mesencephalon, making a striking contrast with the result of Pax-5 misexpression. In E2 chick embryos, the mesencephalon is committed of its fate to differentiate into the tectum, but the diencephalon has plasticity on its fate. Moreover, Pax-2 expression in the isthmus precedes Pax-5 expression. Taking these results into consideration, it is suggested that Pax-2 plays a crucial role in initiation of the tectal development, and that Pax-5 functions to maintain the state of tectal differentiation.

Fgf8 and Gbx2 induction concomitant with Otx2 repression is correlated with midbrain-hindbrain fate of caudal prosencephalon

Chick/quail transplantation experiments were performed to analyse possible factors involved in the regionalisation of the midbrain-hindbrain domain. The caudal prosomeres, expressing Otx2, were transplanted at stage HH10 into rostrocaudal levels of the midbrain-hindbrain domain, either straddling the intra-metencephalic constriction (type 1 grafts), or at rostral and medial levels of pro-rhombomere A1 (type 2 and 3 grafts, respectively); thus, in all situations, one border of the graft was in contact with the host Gbx2- and Fgf8-expressing domains. The area containing the graft, recognised by QCPN immunohistochemistry, was first analysed 48 hours after transplantation for Otx2, Gbx2, En2 and Fgf8. Although in all three situations, a large part of the graft maintained Otx2 expression, another part became Otx2 negative and was induced to express Gbx2 and Fgf8. These inductive events occurred exclusively at the interface between the Otx2-positive transplanted domain and the ipsilateral host Gbx2-positive rhombomere 1, creating a new Otx2-Gbx2 boundary within the grafted territory. In type 1 and 2 grafts, the induced Fgf8 domain is in continuity with the host Fgf8 isthmic domain, whereas for type 3 grafts, these two domains are separate. High levels of En2 expression were also induced in the area expressing Gbx2 and Fgf8, and Wnt1 and Pax2 expressions, analysed in type 3 grafts, were induced at the intragraft Otx2-Gbx2 new boundary. Moreover, at later embryonic stages, the graft developed meso-isthmo-cerebellar structures. Thus, gene expressions induced in the grafted prosencephalon not only mimicked the pattern observed in the normal midbrain-hindbrain domain, but is followed by midbrain-hindbrain cytodifferentiation, indicating that not only Fgf8 but also confrontation of Otx2 and Gbx2 may play an essential role during midbrian-hindbrain regionalisation.

FGF8 induces formation of an ectopic isthmic organizer and isthmocerebellar development via a repressive effect on Otx2 expression

Beads containing recombinant FGF8 (FGF8-beads) were implanted in the prospective caudal diencephalon or midbrain of chick embryos at stages 9–12. This induced the neuroepithelium rostral and caudal to the FGF8-bead to form two ectopic, mirror-image midbrains. Furthermore, cells in direct contact with the bead formed an outgrowth that protruded laterally from the neural tube. Tissue within such lateral outgrowths developed proximally into isthmic nuclei and distally into a cerebellum-like structure. These morphogenetic effects were apparently due to FGF8-mediated changes in gene expression in the vicinity of the bead, including a repressive effect on Otx2 and an inductive effect on En1, Fgf8 and Wnt1 expression. The ectopic Fgf8 and Wnt1 expression domains formed nearly complete concentric rings around the FGF8-bead, with the Wnt1 ring outermost. These observations suggest that FGF8 induces the formation of a ring-like ectopic signaling center (organizer) in the lateral wall of the brain, similar to the one that normally encircles the neural tube at the isthmic constriction, which is located at the boundary between the prospective midbrain and hindbrain. This ectopic isthmic organizer apparently sends long-range patterning signals both rostrally and caudally, resulting in the development of the two ectopic midbrains. Interestingly, our data suggest that these inductive signals spread readily in a caudal direction, but are inhibited from spreading rostrally across diencephalic neuromere boundaries. These results provide insights into the mechanism by which FGF8 induces an ectopic organizer and suggest that a negative feedback loop between Fgf8 and Otx2 plays a key role in patterning the midbrain and anterior hindbrain.

Sequential roles for Fgf4, En1 and Fgf8 in specification and regionalisation of the midbrain

Experiments involving tissue recombinations have implicated both early vertical and later planar signals in the specification and polarisation of the midbrain. Here we investigate the role of fibroblast growth factors in regulating these processes in the avian embryo. We show that Fgf4 is expressed in the notochord anterior to Hensen's node before transcripts for the earliest molecular marker of midbrain tissue in the avian embryo, En1, are detected. The presence of notochord is required for the expression of En1 in neural plate explants in vitro and FGF4 mimics this effect of notochord tissue. Subsequently, a second member of the fibroblast growth factor family, Fgf8, is expressed in the isthmus in a manner consistent with it providing a polarising signal for the developing midbrain. Using a retroviral vector to express En1 ectopically, we show that En1 can induce Fgf8 expression in midbrain and posterior diencephalon. Results of the introduction of FGF8 protein into the anterior midbrain or posterior diencephalon are consistent with it being at least part of the isthmic activity which can repolarise the former tissue and respecify the latter to a midbrain fate. However, the ability of FGF8 to induce expression of genes which have earlier onsets of expression than Fgf8 itself, namely En1 and Pax2, strongly suggests that the normal function of FGF8 is in maintaining patterns of gene expression in posterior midbrain. Finally, we provide evidence that FGF8 also provides mitogenic stimulation during avian midbrain development.

Role of Pax-5 in the regulation of a mid-hindbrain organizer's activity

The mes-metencephalic boundary (isthmus) has been suggested to act as an organizer in the development of the optic tectum. Pax-5 was cloned as a candidate for regulator of the organizing center. Isthmus-specific expression of Pax-5 and analogy with the genetic cascade in Drosophila suggest that Pax-5 may be at a higher hierarchical position in the gene regulation cascade of tectum development. To examine this possibility, a gain-of-function experiment on Pax-5 was carried out. In ovo electroporation on E2 chick brain with the eucaryotic expression vector that encodes chick Pax-5 cDNA was used. Not only was a considerable amount of Pax-5 expressed ectopically in the transfected brain, but irregular bulging of the neuroepithelium was induced in the diencephalon and mesencephalon. At Pax-5 misexpressing sites, uptake of BrdU was increased. Histological examination of E7 transfected brain revealed that Pax-5 caused transdifferentiation of diencephalon into the tectum-like structure. In the bulges of the E7 mesencephalon, differentiation of laminar structure was repressed when compared to the normal side. In transfected embryos, En-2, Wnt-1 and Fgf8 were up-regulated ectopically, and Otx2 was down-regulated in the diencephalon to mesencephalon. Moreover, Ephrin-A2, which is expressed specifically in the tectum with a gradient highest at the caudal end, is suggested to be involved in pathfinding of the retinal fibers, and was induced in the bulges. When the mouse Fgf8 expression vector was electroporated, Pax-5 and chick Fgf8 were also induced ectopically. These results suggest that Pax-5, together with Fgf8, hold a higher position in the genetic hierarchy of the isthmus organizing center and regulate its activity.

Getting organized: new insights into the organizer of higher vertebrates

The Organizer of higher vertebrates (e.g., Hensen's node in birds and the node in mammals) functions much like the Organizer of lower vertebrates (e.g., embryonic shield in fish and dorsal lip of the blastopore in amphibians). In all classes of vertebrates, the Organizer displays a number of unique properties including the fate, migratory patterns, morphogenetic movements, and the level of commitment of its cells; its pattern of gene expression; its ability to induce neural differentiation; and its ability to organize and regionalize a secondary embryo when grafted ectopically. The importance of Organizer activity to formation of the neuraxis is highlighted by results from studies in which the Organizer is eliminated experimentally. Such studies demonstrate that an auxiliary system is present that can generate a reconstituted Organizer, which completely mimics the activity of the original Organizer. For almost 50 years after the discovery of Spemann's Organizer, the molecular nature of Organizer activity was virtually unknown. However, recent progress in identifying the morphoregulatory molecules underlying Organizer activity has been substantial, and a full understanding of the molecular basis of this activity is imminent. Thus, the intriguing question of how the Organizer organizes, raised by the seminal experiments of Spemann and Mangold, is finally being answered in this exciting renaissance of developmental biology driven by new molecular and genetic approaches.

Identification of a signal sequence necessary for the unconventional secretion of Engrailed homeoprotein

BACKGROUND

Engrailed-1 and Engrailed-2 are homeoproteins--transcription factors implicated in the morphogenesis of discrete structures. Engrailed proteins have a role in patterning the midbrain-hindbrain region and are expressed in the nuclei of rat embryo midbrain-hindbrain cells. We have previously found that both endogenous and exogenously expressed Engrailed proteins also associate with membrane regions implicated in signal transduction and secretion. Within total membrane fractions, a small proportion of Engrailed--about 5%--is protected against proteinase K proteolysis, suggesting that Engrailed has access to a luminal compartment. Together with our finding that homeodomains and homeoproteins can be internalized by live cells, these observations suggest that Engrailed might act as a polypeptidic messenger. In order to investigate this possibility, we looked to see if Engrailed could be secreted.

RESULTS

Engrailed expressed in COS cells can be recovered in abutting primary neurons and this is dependent on a short sequence in its homeodomain distinct from 'classical' secretion signals. This sequence, which overlaps with the sequence necessary for Engrailed internalization and which is highly conserved among homeoproteins, is the first example of an 'unconventional' sequence necessary for secretion. Less than 50% of total intracellular Engrailed is secreted and there is a correlation between secretion and access to the membrane compartment where the protein is protected against proteinase K.

CONCLUSIONS

Our results lend weight to the proposal that Engrailed, and possibly other homeoproteins, might act as intercellular polypeptidic messengers.

Signalling interactions during facial development

The development of the vertebrate face is a dynamic multi-step process which starts with the formation of neural crest cells in the developing brain and their subsequent migration to form, together with mesodermal cells, the facial primordia. Signalling interactions co-ordinate the outgrowth of the facial primordia from buds of undifferentiated mesenchyme into the intricate series of bones and cartilage structures that, together with muscle and other tissues, form the adult face. Some of the molecules that are thought to be involved have been identified through the use of mouse mutants, data from human craniofacial syndromes and by expression studies of signalling molecules during facial development. However, the way that these molecules control the epithelial-mesenchymal interactions which mediate facial outgrowth and morphogenesis is unclear. The role of neural crest cells in these processes has also not yet been well defined. In this review we discuss the complex interaction of all these processes during face development and describe the candidate signalling molecules and their possible target genes.

The Eph family receptors and ligands

The Eph family is the largest of all known tyrosine kinase receptor-ligand systems. They are expressed in distinct, but overlapping, spatial and temporal patterns during embryonic development and postnatal life, and function in a variety of morphogenic events. The best known function is their role in the guidance of migration of axons and cells in the nervous system through repulsive interactions. They may also play a role in angiogenesis, tissue patterning, and tumor formation.

Patterning of the chick forebrain anlage by the prechordal plate

We analysed the role of the prechordal plate in forebrain development of chick embryos in vivo. After transplantation to uncommitted ectoderm a prechordal plate induces an ectopic, dorsoventrally patterned, forebrain-like vesicle. Grafting laterally under the anterior neural plate causes ventralization of the lateral side of the forebrain, as indicated by a second expression domain of the homeobox gene NKX2.1. Such a lateral ventralization cannot be induced by the secreted factor Sonic Hedgehog alone, as this is only able to distort the ventral forebrain medially. Removal of the prechordal plate does not reduce the rostrocaudal extent of the anterior neural tube, but leads to significant narrowing and cyclopia. Excision of the head process results in the caudal expansion of the NKX2.1 expression in the ventral part of the anterior neural tube, while PAX6 expression in the dorsal part remains unchanged. We suggest that there are three essential steps in early forebrain patterning, which culminate in the ventralization of the forebrain. First, anterior neuralization occurs at the primitive streak stage, when BMP-4-antagonizing factors emanate from the node and spread in a planar fashion to induce anterior neural ectoderm. Second, the anterior translocation of organizer-derived cells shifts the source of neuralizing factors anteriorly, where the relative concentration of BMP-4-antagonists is thus elevated, and the medial part of the prospective forebrain becomes competent to respond to ventralizing factors. Third, the forebrain anlage is ventralized by signals including Sonic Hedgehog, thereby creating a new identity, the prospective hypothalamus, which splits the eye anlage into two lateral domains.

Gastrulation and homeobox genes in chick embryos

We review the early stages of chick embryogenesis, in particular the formation of the hypoblast, and the ingression of endoderm and mesoderm through the primitive streak. The formation of a trilaminar embryo during gastrulation is accompanied by the specification of body axes. The first axis is already present in the unfertilized egg and runs from the cytoplasmatic animal to the yolk rich vegetal pole. Already within the uterus a second axis conveys bilateral symmetry to the embryo. It extends from a dorsal/anterior to a ventral/posterior position. These axial poles segregate during gastrulation to form the classical coordinates, a dorsal-ventral and an anterior-posterior axis. The establishment of axes is accompanied by the expression of specific combinations of homeobox genes during gastrulation in the chick, as in other metazoa. We review the avian specific information and compare it with findings in other species. A combinatorial homeobox code for the specification of identities during development is discussed.

Immortalized cell lines from embryonic avian and murine otocysts: tools for molecular studies of the developing inner ear

Recently, our studies have focused on genes expressed at the earliest stages of inner ear development. Our aim is to identify and characterize genes that are involved in determining the axes of the semicircular canals, in otic crest delamination and in early innervation of the inner ear. Many elegant studies of auditory development have been done in animal models. However, the need for large amounts of well-characterized embryonic material for molecular studies makes the development of otocyst cell lines with different genetic repertoires attractive. We have therefore derived immortalized otocyst cells from two of the most widely used animal models of ear development: avians and mice. Avian cell isolates were produced from quail otocysts (embryonic stage 19) that were transformed with temperature-sensitive variants of the Rous sarcoma virus (RSV). Among the individual transformed cells are those that produce neuron-like derivatives in response to treatment with 10(-9) M retinoic acid. Mammalian cell isolates were derived from otocysts, of 9 day (post coitus) embryos of the H2kbtsA58 transgenic mouse (Immortomouse), which carries a temperature-sensitive variant of the Simian Virus 40 Tumor antigen. The vast majority of cells of the Immortomouse are capable of being immortalized at 33 degrees C, the permissive temperature for transgene expression, in the presence of gamma-interferon. Several putative clones et these cells differentiated into neuron-like cells after temperature shift and withdrawal of gamma-interferon; another isolate of cells assumed a neuron-like morphology on exposure to brain-derived neurotrophic factor even at the permissive temperature. We describe also a cell isolate that expresses the Pax-2 protein product and two putative cell lines that express the protein product of the chicken equivalent of the Drosophila segmentation gene engrailed. These genes and their protein products are expressed in specific subpopulation of otocyst cells at early stages. Both mouse and quail immortalized cell lines will be used to study inner ear development at the molecular level.

Vertical induction of engrailed-2 and other region-specific markers in the early chick embryo

We investigated the role of vertical signals in the regulation of Engrailed-2, a regionally restricted (mesencephalon/metencephalon) neuroectodermal marker, using epiblast grafted from prospective neuroectoderm or prospective trunk mesoderm at mid-stage 3 in the gastrulating chick embryo. Grafts that were isolated from the rostral (prospective neuroectodermal) epiblast and placed rostral to or at the future mesencephalon/metencephalon level, between the endoderm and epiblast of stage 3d to stage 8 host embryos, expressed Engrailed-2 after 24 hr in culture, whereas these same grafts failed to express this marker when placed at a more caudal level. Grafts from caudal = (prospective trunk mesodermal) epiblast, which would ordinarily not express Engrailed-2, also expressed this marker when placed at the mesencephalon/metencephalon level, and failed to express it when grafted more caudally. The expression of four other markers, L5, Fgf8, Wnt-1, and paraxis, were also evaluated. Collectively, our results show that regionally restricted vertical signals are capable of inducing neuroectoderm from naive tissue, and of patterning epiblast to express some but not all mesencephalon/metencephalon isthmus markers. Experiments using grafts taken from older embryos indicated that the competence of prospective neuroectoderm to become regionally patterned by vertical signals is gradually lost between stage 3c and stage 7. Similarly, prospective mesoderm from the caudal epiblast becomes unable to respond to vertical, neural-inductive signals at these stages. These observations support a role for vertical signals in the induction and patterning of the neuroectoderm at gastrula and early neurula stages.

Evidence that FGF8 signalling from the midbrain-hindbrain junction regulates growth and polarity in the developing midbrain

The developing vertebrate mesencephalon shows a rostrocaudal gradient in the expression of a number of molecular markers and in the cytoarchitectonic differentiation of the tectum, where cells cease proliferating and differentiate in a rostral to caudal progression. Tissue grafting experiments have implicated cell signalling by the mesencephalic-metencephalic (mid-hindbrain) junction (or isthmus) in orchestrating these events. We have explored the role of Wnt-1 and FGF8 signalling in the regulation of mesencephalic polarity. Wnt-1 is expressed in the caudal mesencephalon and Fgf8 in the most rostral metencephalon. Wnt-1 regulates Fgf8 expression in the adjacent metencephalon, most likely via a secondary mesencephalic signal. Ectopic expression of Fgf8 in the mesencephalon is sufficient to activate expression of Engrailed-2 (En-2) and ELF-1, two genes normally expressed in a decreasing caudal to rostral gradient in the posterior mesencephalon. Ectopic expression of Engrailed-1 (En-1), a functionally equivalent homologue of En-2 is sufficient to activate ELF-1 expression by itself. These results indicate the existence of a molecular hierarchy in which FGF8 signalling establishes the graded expression of En-2 within the tectum. This in turn may act to specify other aspects of A-P polarity such as graded ELF-1 expression. Our studies also reveal that FGF8 is a potent mitogen within the mesencephalon: when ectopically expressed, neural precursors continue to proliferate and neurogenesis is prevented. Taken together our results suggest that FGF8 signalling from the isthmus has a key role in coordinately regulating growth and polarity in the developing mesencephalon.

En-2 regulates the expression of the ligands for Eph type tyrosine kinases in chick embryonic tectum

The retinotectal projection map is organized in a precise retinotopic order, so that the temporo-nasal axis of the retina corresponds to the rostro-caudal axis of the tectum. en-1 and en-2, homologues of the Drosophila segment polarity gene engrailed, are expressed in a gradient along the rostro-caudal axis of the tectal anlage, and are suggested to confer caudal characteristics as the results of transplantation and ectopic engrailed (en) expression. Recently the ligands for Eph type receptor tyrosine kinases have been shown to be expressed strongly at the caudal tectum and play a role in retinotectal map formation by repulsing the temporal retinal fibers. Using the system of replication competent retroviral vector, en-2 RCAS (A/B), we misexpressed en-2 on the tectum. Elf-1 or RAGS was induced at the ectopic En-2 sites. The present results have shown that En-2 can regulate expression of both Elf-1 and RAGS. This suggests that the cells which express en at the early stage of tectum development acquire positional specificity as 'caudal' tectum, and these cells may later express the ligands for Eph type receptor tyrosine kinases. Therefore the temporal retinal fibers which have the receptors are repelled when they meet the ligands on the tectum.

Embryonic neural chimeras in the study of vertebrate brain and head development

Construction of neural chimeras between quail and chick embryos has been employed since 1969 when the unique nucleolar structure of the quail nucleus and its use to devise a cell marking technique by associating quail and chick cells in ovo were described in the "Bulletin Biologique de la France et de la Belgique." This method was first applied to the ontogeny of the neural crest, a structure whose development involves extensive cell migration, and, since 1984, to that of the central nervous system (CNS). This chapter highlights some of the most significant findings provided by this approach concerning the CNS, such as (i) demonstration of the common origin of the floor plate and notochord from a group of cells localized in the "organizer", i.e., Hensen's node, and the way in which these two structures become positioned respectively within and under the neural tube during gastrulation and neurulation in Amniotes; (ii) the neural crest origin of the skull vault and the facial and hypobranchial skeleton. This means that the mesodermal contribution to the skull is limited to the occipital and otic regions and extends only to the rostral limit of the notochord. A correlation can be drawn between the development of the telencephalon and the mesectodermally derived skull in the vertebrate phylum; (iii) demonstration that the midbrain-hindbrain junction, at the stage of the encephalic vesicles, acts as an organizing center for tectal and cerebellar structures. This function was correlated with the activity of several developmental genes, thus providing insight into their function during neurogenesis; (iv) the pattern of morphogenetic movements and cell migration taking place in defined brain-to-be areas, as well as the origin of various cell types of nervous tissues; and (v) a new avenue for studying brain localization of either behavioral traits or genetically encoded brain disorders.

Developmental changes in expression of the ATBF1 transcription factor gene

The expression of the ATBF1 gene in developing brain was analyzed in mice from 13 days of gestation to 28 days after birth using RNase protection and in situ hybridization methods. The level of ATBF1 transcripts was the highest on embryonic day 13-15 and then progressively decreased to a hardly detectable level on postnatal day 28. Throughout the period examined, ATBF1 mRNA was expressed consistently in the basal telencephalon, diencephalon, and mesencephalon, with the highest levels in the inferior colliculus and thalamus. On the other hand, no significant expression was observed in cerebellum, neocortex, hippocampus, and olfactory bulb. These results illustrate significant regional differences in the ATBF1 expression and suggest a role of the ATBF1 gene in the formation of some specific cell populations in developing central nervous system.

Retroviral misexpression of engrailed genes in the chick optic tectum perturbs the topographic targeting of retinal axons

We have investigated the role of the homeodomain transcription factor genes En-1 and En-2, homologs of the Drosophila segment polarity gene engrailed, in regulating the development of the retinotopic map in the chick optic tectum. The En proteins are distributed in a gradient along the rostral-caudal axis of the developing tectum, with highest amounts found caudally. Previous evidence suggests that En-1 and En-2 may regulate the polarity of the rostral-caudal axis of the tectum and the subsequent topographic mapping of retinal axons. We have tested this hypothesis by using a recombinant replication-competent retrovirus to overexpress the En-1 or En-2 genes in the developing tectum. Anterograde labeling with the axon tracer Dil was used to analyze the topographic mapping of retinal axons after the time that the retinotectal projection is normally topographically organized. Overexpression of either En-1 or En-2 perturbed the topographic targeting of retinal axons. In En-infected tecta, nasal retinal axons form an abnormally diffuse projection with numerous aberrant axons, branches, and arbors found at topographically incorrect locations, colocalized with domains of viral infection. In contrast, temporal axons did not form a diffuse projection or discrete aberrant arbors; however, many temporal axons were stunted and ended aberrantly rostral to their appropriate TZ, or in other cases either did not enter the tectum or formed a dense termination at its extreme rostral edge. These findings indicate that En-1 and En-2 are involved in regulating the development of the retinotopic map in the tectum. Furthermore, they support the hypothesis that En genes regulate the polarity of the rostral-caudal axis of the tectum, most likely by controlling the expression of retinal axon guidance molecules.

The behavior of optic axons on substrate gradients of retinal basal lamina proteins and merosin

To study the behavior of optic axons to continuously changing concentrations of their substrate, explants from embryonic retina were placed across gradients of retinal basal lamina proteins and merosin. The following growth patterns of axons in response to the substrate gradients were found: (1) Axons that grew up gradients, i.e., from low to high substrate concentrations, became longer and less fasciculated with increasing concentration of the substrate. On shallow basal lamina gradients, the axons also showed a directional response that resulted in guidance to higher substrate concentrations. (2) Axons that grew down gradients, i.e., from high to low substrate concentrations, became shorter and more fasciculated with decreasing concentrations of the substrate. On gradients of merosin, a significant alteration in the axonal growth direction toward higher substrate concentrations was detected. Axons heading down gradients never U turned to higher substrate concentrations. (3) Axons confronted with discontinuous substrates were confined to the borders of the substrate exclusively, whereas axons confronted with substrate gradients were able to cross into the territory beyond the substrate. (4) The growth patterns of axons on substrate gradients of basal lamina proteins and merosin were similar but not identical, indicating that axons may respond to substrate gradients dependent on its chemical composition. The present results show that substrate gradients can regulate length and fasciculation of neurites and have a limited capability to direct axons to higher substrate concentrations.