A small population of anterior cells patterns the forebrain during zebrafish gastrulation

Getting a-head of the organizer: anterior-posterior patterning of the forebrain

The molecular mechanisms that drive the development of embryonic tissues are being uncovered rapidly. One such fascinating example is the development of the forebrain, the most anterior part of the nervous system. In this review, we will discuss the mechanisms that induce the formation of the forebrain in multiple vertebrate systems, placing emphasis on a recent article published by Grinblat et al. ((1)) Using zebrafish as a model system, these authors combine elegant embryological manipulations with the use of early markers of the presumptive forebrain, to show that initial induction and patterning of this tissue occurs near the onset of gastrulation. In addition, their results confirm observations made in other systems that planar signals, those traveling in the plane of the ectoderm, are involved in forebrain induction and patterning.

The anterior margin of the mammalian gastrula: comparative and phylogenetic aspects of its role in axis formation and head induction

Recent findings on morphology and gene expression in several mammalian embryos suggest that there is a new landmark and possibly a center with organizer activity in the anterior margin of the embryo at the onset of gastrulation. This review compiles morphological variations and similarities found among mammals during gastrulation stages and, at the same time, stresses the common aspects, at the morphological and the molecular level, of setting up the body plan with regard to axis formation and head induction. Both morphological and functional aspects are then used to draw comparisons with equivalent developmental stages in lower vertebrate species, such as birds, amphibia, and bony fish. Finally, a suggestion is made as to how gastrulation may have evolved in the vertebrate phylum.

The specification of dorsal cell fates in the vertebrate central nervous system

The generation of distinct classes of neurons at defined positions within the developing vertebrate nervous system depends on inductive signals provided by local cell groups that act as organizing centers. Genetic and embryological studies have begun to elucidate the processes that control the pattern and identity of neuronal cell types. Here we discuss the cellular interactions and molecular mechanisms that direct neuronal cell fates in the dorsal half of the vertebrate central nervous system. The specification of dorsal neuronal cell fates appears to depend on a cascade of inductive signals initiated by cells of the epidermal ectoderm that flank the neural plate and propagated by roof plate cells within the neural tube. Members of the transforming growth factor-beta (TGF beta) family of secreted proteins have a prominent role in mediating these dorsalizing signals. Additional signals involving members of the Wnt and fibroblast growth factor (FGF) families may also contribute to the proliferation and differentiation of dorsal neuronal cell types.

Functional equivalency between Otx2 and Otx1 in development of the rostral head

Mice have two Otx genes, Otx1 and Otx2. Prior to gastrulation, Otx2 is expressed in the epiblast and visceral endoderm. As the primitive streak forms, Otx2 expression is restricted to the anterior parts of all three germ layers. Otx1 expression begins at the 1 to 3 somite stage in the anterior neuroectoderm. Otx2 is also expressed in cephalic mesenchyme. Otx2 homozygous mutants fail to develop structures anterior to rhombomere 3 (r3), and Otx2 heterozygotes exhibit craniofacial defects. Otx1 homozygous mutants do not show apparent defects in early brain development. In Otx1 and Otx2 double heterozygotes, rostral neuroectoderm is induced normally, but development of the mes/diencephalic domain is impaired starting at around the 3 to 6 somite stage, suggesting cooperative interactions between the two genes in brain regionalization. To determine whether Otx1 and Otx2 genes are functionally equivalent, we generated knock-in mice in which Otx2 was replaced by Otx1. In homozygous mutants, gastrulation occurred normally, and rostral neuroectoderm was induced at 7.5 days postcoitus (7.5 dpc), but the rostral brain failed to develop. Anterior structures such as eyes and the anterior neural ridge were lost by 8.5 dpc, but the isthmus and r1 and r2 were formed. In regionalization of the rostral neuroectoderm, the cooperative interaction of Otx2 with Otx1 revealed by the phenotype of Otx2 and Otx1 double heterozygotes was substitutable by Otx1. The otocephalic phenotype indicative of Otx2 haploinsufficiency was also largely restored by knocked-in Otx1. Thus most Otx2 functions were replaceable by Otx1, but the requirement for Otx2 in the anterior neuroectoderm prior to onset of Otx1 expression was not. These data indicate that Otx2 may have evolved new functions required for establishment of anterior neuroectoderm that Otx1 cannot perform.

Otx genes and the genetic control of brain morphogenesis

Understanding the genetic mechanisms that control brain patterning in vertebrates represents a major challenge for developmental neurobiology. The cloning of genes likely to be involved in the organization of the brain and an analysis of their roles have revealed insights into the molecular pathways leading to neural induction, tissue specification, and regionalization of the brain. Among these genes, both Otx1 and Otx2, two murine homologs of the Drosophila orthodenticle (otd) gene, contribute to several steps in brain morphogenesis. Recent findings have demonstrated that Otx2 plays a major role in gastrulation and in the early specification of the anterior neural plate while Otx1 is mainly involved in corticogenesis, and Otx1 and Otx2 genes cooperate in such a way that a minimal level of OTX proteins are required for proper regionalization and subsequent patterning of the developing brain. Finally, experiments have shown functional equivalence between Drosophila otd and vertebrate Otx genes, suggesting a surprising conservation of function required in brain development throughout evolution.

In vivo analysis using variants of zebrafish BMPR-IA: range of action and involvement of BMP in ectoderm patterning

It has been an intriguing problem whether the polypeptide growth factors belonging to the transforming growth factor-beta (TGF-beta) superfamily function as direct and long-range signaling molecules in pattern formation of the early embryo. In this study, we examined the mechanism of signal propagation of bone morphogenetic protein (BMP) in the ectodermal patterning of zebrafish embryos, in which BMP functions as an epidermal inducer and a neural inhibitor. To estimate the effective range of zbmp-2, we first performed whole-mount in situ hybridization analysis. The zbmp-2-expressing domain and the neuroectoderm, marked by otx-2 expression, were complementary, suggesting that BMP has a short-range effect in vivo. Moreover, mosaic experiments using a constitutively active form of a zebrafish BMP type I receptor (CA-BRIA) demonstrated that the cell-fate conversion, revealed by ectopic expression of gata-3 and repression of otx-2, occurred in a cell-autonomous manner, denying the involvement of the relay mechanism. We also found that zbmp-2 was induced cell autonomously within the transplanted cells in the host ectoderm, suggesting that BMP cannot influence even the neighboring cells. This result is consistent with the observation that there is no gap between the expression domains of zbmp-2 and otx-2. Taken together, we propose that, in ectodermal patterning, BMP exerts a direct and cell-autonomous effect to fate uncommitted ectodermal cells to become epidermis.

Visceral endoderm-restricted translation of Otx1 mediates recovery of Otx2 requirements for specification of anterior neural plate and normal gastrulation

Otx1 and Otx2, two murine homologs of the Drosophila orthodenticle (otd) gene, contribute to brain morphogenesis. In particular Otx1 null mice are viable and show spontaneous epileptic seizures and abnormalities affecting the dorsal telencephalic cortex. Otx2 null mice die early in development and fail in specification of the rostral neuroectoderm and proper gastrulation. In order to determine whether Otx1(−/−)and Otx2(−/−) highly divergent phenotypes reflect differences in temporal expression or biochemical activity of OTX1 and OTX2 proteins, the Otx2-coding sequence was replaced by a human Otx1 full-coding cDNA. Homozygous mutant embryos recovered anterior neural plate and proper gastrulation but failed to maintain forebrain-midbrain identities, displaying a headless phenotype from 9 days post coitum (d.p.c.) onwards. Unexpectedly, in spite of the RNA distribution in both visceral endoderm (VE) and epiblast, the hOTX1 protein was synthesized only in the VE. This VE-restricted translation was sufficient to recover Otx2 requirements for specification of the anterior neural plate and proper organization of the primitive streak, thus providing evidence that the difference between Otx1 and Otx2 null mice phenotypes originates from their divergent expression patterns. Moreover, our data lead us to hypothesize that the differential post-transcriptional control existing between VE and epiblast cells may potentially contribute to fundamental regulatory mechanisms required for head specification.

Otx1 and Otx2 in the development and evolution of the mammalian brain

In the last decade, a number of genes related to the induction, specification and regionalization of the brain were isolated and their functional properties currently are being dissected. Among these, Otx1 and Otx2 play a pivotal role in several processes of brain morphogenesis. Findings from several groups now confirm the importance of Otx2 in the early specification of neuroectoderm destined to become fore-midbrain, the existence of an Otx gene dosage-dependent mechanism in patterning the developing brain, and the involvement of Otx1 in corticogenesis. Some of these properties appear particularly fascinating when considered in evolutionary terms and highlight the central role of Otx genes in the establishment of the genetic program defining the complexity of a vertebrate brain. This review deals with the major aspects related to the roles played by Otx1 and Otx2 in the development and evolution of the mammalian brain.

Determination of the zebrafish forebrain: induction and patterning

We report an analysis of forebrain determination and patterning in the zebrafish Danio rerio. In order to study these events, we isolated zebrafish homologs of two neural markers, odd-paired-like (opl), which encodes a zinc finger protein, and fkh5, which encodes a forkhead domain protein. At mid-gastrula, expression of these genes defines a very early pattern in the presumptive neurectoderm, with opl later expressed in the telencephalon, and fkh5 in the diencephalon and more posterior neurectoderm. Using in vitro explant assays, we show that forebrain induction has occurred even earlier, by the onset of gastrulation (shield stage). Signaling from the early gastrula shield, previously shown to be an organizing center, is sufficient for activation of opl expression in vitro. In order to determine whether the organizer is required for opl regulation, we removed from late blastula stage embryos either the presumptive prechordal plate, marked by goosecoid (gsc) expression, or the entire organizer, marked by chordin (chd) expression. opl was correctly expressed after removal of the presumptive prechordal plate and consistently, opl was correctly expressed in one-eyed pinhead (oep) mutant embryos, where the prechordal plate fails to form. However, after removal of the entire organizer, no opl expression was observed, indicating that this region is crucial for forebrain induction. We further show that continued organizer function is required for forebrain induction, since beads of BMP4, which promotes ventral fates, also prevented opl expression when implanted during gastrulation. Our data show that forebrain specification begins early during gastrulation, and that a wide area of dorsal mesendoderm is required for its patterning.

An early phase of embryonic Dlx5 expression defines the rostral boundary of the neural plate

Relatively little is known about the molecular events that specify the rostrocaudal axis of the neural plate. Here we show that a member of the Distal-less (Dlx) homeobox gene family, Dlx5, is one of the earliest known markers for the most rostral ectoderm, before the formation of an overt neural plate. During late gastrulation Dlx5 expression becomes localized to the anterior neural ridge, which defines the rostral boundary of the neural plate, and also extends caudolaterally, marking the region of the presumptive neural crest. Subsequently, Dlx5 is expressed in tissues (olfactory epithelium, ventral cephalic epithelium) that are believed to derive from the anterior neural ridge, based on the avian fate map. The early phase of Dlx5 expression in the anterior neural ridge and its derivatives is distinct from a later phase of expression in the ventral telencephalon and diencephalon and also appears to be unique for Dlx5 among members of the Dlx family. Another distinctive feature of Dlx5 expression is the occurrence of an alternative transcript (deltaDlx5), which encodes a truncated protein lacking the homeodomain, and represents a significant fraction of total Dlx5 transcripts at all embryonic stages that were examined. In contrast with full-length DLX5, the deltaDLX5 truncated protein is deficient in DNA-binding activity and does not interact with the homeoprotein partner MSX1. Taken together, our findings suggest that Dlx5 activity may be regulated via the expression of an alternative transcript and demonstrate that Dlx5 marks the anterior boundary of the neural plate.

cyclops encodes a nodal-related factor involved in midline signaling

Ventral structures in the central nervous system are patterned by signals emanating from the underlying mesoderm as well as originating within the neuroectoderm. Mutations in the zebrafish, Danio rerio, are proving instrumental in dissecting these midline signals. The cyclops mutation leads to a loss of medial floor plate and to severe deficits in ventral forebrain development, leading to cyclopia. Here, we report that the cyclops locus encodes the nodal-related protein Ndr2, a member of the transforming growth factor type beta superfamily of factors. The evidence includes identification of a missense mutation in the initiation codon and rescue of the cyclops phenotype by expression of ndr2 RNA, here renamed "cyclops." Thus, in interaction with other molecules implicated in these processes such as sonic hedgehog and one-eyed-pinhead, cyclops is required for ventral midline patterning of the embryonic central nervous system.

The zebrafish organizer

Signals from the organizer play a crucial role in patterning the vertebrate embryo. Recent molecular analysis of zebrafish mutations has established an essential role for BMP2 and chordin in organizer function and has identified one-eyed pinhead as a novel EGF-like gene involved in prechordal plate and endoderm formation. In addition, embryological studies have suggested that the zebrafish organizer is induced by extraembryonic cues and have defined two novel organizing centers that pattern the nervous system along the anterior-posterior axes.

Overexpression of the forebrain-specific homeobox gene six3 induces rostral forebrain enlargement in zebrafish

The Drosophila homeobox gene sine oculis is expressed in the rostral region of the embryo in early development and is essential for eye and brain formation. Its murine homolog, Six3, is expressed in the anterior neural plate and eye anlage, and may have crucial functions in eye and brain development. In this study, we describe the cloning and expression of zebrafish six3, the apparent ortholog of the mouse Six3 gene. Zebrafish six3 transcripts are first seen in hypoblast cells in early gastrula embryos and are found in the anterior axial mesendoderm through gastrulation. six3 expression in the head ectoderm begins at late gastrula. Throughout the segmentation period, six3 is expressed in the rostral region of the prospective forebrain. Overexpression of six3 in zebrafish embryos induced enlargement of the rostral forebrain, enhanced expression of pax2 in the optic stalk and led to a general disorganization of the brain. Disruption of either the Six domain or the homeodomain abolish these effects, implying that these domains are essential for six3 gene function. Our results suggest that the vertebrate Six3 genes are involved in the formation of the rostral forebrain.

Vax1 is a novel homeobox-containing gene expressed in the developing anterior ventral forebrain

The vertebrate forebrain is formed at the rostral end of the neural plate under the regulation of local and specific signals emanating from both the endomesoderm and neuroectoderm. The development of the rostral and ventral forebrain in particular was difficult to study, mainly because no specific markers are available to date. Here, we report the identification of Vax1, a novel homeobox-containing gene identified in mouse, Xenopus and human. It is closely related to members of the Not and Emx gene families, all of which are required for the formation of structures where they are expressed. In mouse and Xenopus, Vax1 expression first occurs in the rostral neural plate, in the medial anterior neural ridge and adjacent ectoderm. Later, at midgestation in the mouse and tadpole stage in Xenopus, the expression remains confined in the derivatives of this territory which differentiate into rostromedial olfactory placode, optic nerve and disc, and anterior ventral forebrain. Together, these observations suggest that Vax1 could have an early evolutionary origin and could participate in the specification and formation of the rostral and ventral forebrain in vertebrates. Comparison of the limits of the expression territory of Vax1 with that of Dlx1, Pax6 and Emx1 indicates that the corticostriatal ridge is a complex structure with distinct identifiable genetic compartments. Besides, the study of Vax1 expression in Pax6-deficient homozygous brains indicates that its regulation is independent of Pax6, although the expression patterns of these two genes appear complementary in wild-type animals. Vax1 chromosomal location is mapped at the distal end of the mouse chromosome 19, linked with that of Emx2. These two genes may have arisen by tandem duplication. The Vax1 gene is thus an interesting new tool to study the rostral ventral forebrain patterning, morphogenesis and evolution as well as the terminal differentiation of the forebrain in mouse and Xenopus.

Anterior patterning in mouse

The anteroposterior axis of the vertebrate embryo becomes explicit during gastrulation, the process that converts a relatively featureless embryonic precursor population into new tissues assembled into a recognisable body pattern. Vertebrate embryos arrive at gastrulation in very different states in terms of their size, cell number and reliance on factors inherited from the unfertilized egg. However, they emerge from gastrulation looking very similar, and there is now extensive molecular genetic evidence to indicate that the bare essentials of the gastrulation process have been well conserved during evolution. Here, we review recent findings in the mouse that suggest that anterior identity is, in fact, established before gastrulation starts. They suggest that it is first manifest in extraembryonic tissue and that this tissue is essential for the embryo to develop normal anterior structures, such as the forebrain. We also argue that this precocious anterior pattern could have a counterpart in other non-mammalian vertebrates.

FGF and Shh signals control dopaminergic and serotonergic cell fate in the anterior neural plate

During development, distinct classes of neurons are specified in precise locations along the dorso-ventral and anterior-posterior axes of the neural tube. We provide evidence that intersections of Shh, which is expressed along the ventral neural tube, and FGF8, which is locally produced at the mid/hindbrain boundary and in the rostral forebrain, create induction sites for dopaminergic neurons in the midbrain and forebrain. The same intersection, when preceded by a third signal, FGF4, which is expressed in the primitive streak, defines an inductive center for hindbrain 5-HT neurons. These findings illustrate that cell patterning in the neural plate is a multistep process in which early inducers, which initially divide the neural plate into crude compartments, are replaced by multiple local organizing centers, which specify distinct neuronal cell types within these compartments.