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

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.

Comparative review of adult midbrain and striatum neurogenesis with classical neurogenesis

Parkinson's Disease (PD) motor symptoms are caused by loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) of the midbrain. Dopamine cell replacement therapy (DA CRT), either by cell transplantation or endogenous repair, has been a potential treatment to replace dead cells and improve PD motor symptoms. Adult midbrain and striatum have been studied for many years to find evidence of neurogenesis. Although the literature is controversial, recent research has revived the possibility of neurogenesis here. This paper aims to review the process of neurogenesis (by focusing on gene expression patterns) in the adult midbrain/striatum and compare it with classical neurogenesis that occurs in developing midbrain, Sub Ventricular Zone (SVZ) and Sub Granular Zone (SGZ) of the adult brain.

Expression of segment polarity genes in brachiopods supports a non-segmental ancestral role of engrailed for bilaterians

The diverse and complex developmental mechanisms of segmentation have been more thoroughly studied in arthropods, vertebrates and annelids-distantly related animals considered to be segmented. Far less is known about the role of "segmentation genes" in organisms that lack a segmented body. Here we investigate the expression of the arthropod segment polarity genes engrailed, wnt1 and hedgehog in the development of brachiopods-marine invertebrates without a subdivided trunk but closely related to the segmented annelids. We found that a stripe of engrailed expression demarcates the ectodermal boundary that delimits the anterior region of Terebratalia transversa and Novocrania anomala embryos. In T. transversa, this engrailed domain is abutted by a stripe of wnt1 expression in a pattern similar to the parasegment boundaries of insects-except for the expression of hedgehog, which is restricted to endodermal tissues of the brachiopod embryos. We found that pax6 and pax2/5/8, putative regulators of engrailed, also demarcate the anterior boundary in the two species, indicating these genes might be involved in the anterior patterning of brachiopod larvae. In a comparative phylogenetic context, these findings suggest that bilaterians might share an ancestral, non-segmental domain of engrailed expression during early embryogenesis.

Fgf8 signaling for development of the midbrain and hindbrain

In this paper, we review how midbrain and hindbrain are specified. Otx2 and Gbx2 are expressed from the early phase of development, and their expression abuts at the midbrain hindbrain boundary (MHB), where Fgf8 expression is induced, and functions as an organizing molecule for the midbrain and hindbrain. Fgf8 induces En1 and Pax2 expression at the region where Otx2 is expressed to specify midbrain. Fgf8 activates Ras-ERK pathway to specify hindbrain. Downstream of ERK, Pea3 specifies isthmus (rhombomere 0, r0), and Irx2 may specify r1, where the cerebellum is formed.

Lhx1 in the proximal region of the optic vesicle permits neural retina development in the chicken

How the eye forms has been one of the fundamental issues in developmental biology. The retinal anlage first appears as the optic vesicle (OV) evaginating from the forebrain. Subsequently, its distal portion invaginates to form the two-walled optic cup, which develops into the outer pigmented and inner neurosensory layers of the retina. Recent work has shown that this optic-cup morphogenesis proceeds as a self-organizing activity without any extrinsic molecules. However, intrinsic factors that regulate this process have not been elucidated. Here we show that a LIM-homeobox gene, Lhx1, normally expressed in the proximal region of the nascent OV, induces a second neurosensory retina formation from the outer pigmented retina when overexpressed in the chicken OV. Lhx2, another LIM-homeobox gene supposed to be involved in early OV formation, could not substitute this function of Lhx1, while Lhx5, closely related to Lhx1, could replace it. Conversely, knockdown of Lhx1 expression by RNA interference resulted in the formation of a small or pigmented vesicle. These results suggest that the proximal region demarcated by Lhx1 expression permits OV development, eventually dividing the two retinal domains.

Nuclear translocation of FGF8 and its implication to induce Sprouty2

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

Role of transcription factors in cell replacement therapies for neurodegenerative conditions

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

The PAX258 gene subfamily: a comparative perspective

Whole genome duplication events are thought to have substantially contributed to organismal complexity, largely via divergent transcriptional regulation. Members of the vertebrate PAX2, PAX5 and PAX8 gene subfamily derived from an ancient class of paired box genes and arose from such whole genome duplication events. These genes are critical in establishing the midbrain-hindbrain boundary, specifying interneuron populations and for eye, ear and kidney development. Also PAX2 has adopted a unique role in pancreas development, whilst PAX5 is essential for early B-cell differentiation. The contribution of PAX258 genes to their collective role has diverged across paralogues and the animal lineages, resulting in a complex wealth of literature. It is now timely to provide a comprehensive comparative overview of these genes and their ancient and divergent roles. We also discuss their fundamental place within gene regulatory networks and the likely influence of cis-regulatory elements over their differential roles during early animal development.

Coordinated regulation of dorsal bone morphogenetic protein 4 and ventral Sonic hedgehog signaling specifies the dorso-ventral polarity in the optic vesicle and governs ocular morphogenesis through fibroblast growth factor 8 upregulation

Dorsal and ventral specification in the early optic vesicle plays a crucial role in vertebrate ocular morphogenesis, and proper dorsal-ventral polarity in the optic vesicle ensures that distinct structures develop in separate domains within the eye primordium. The polarity is determined progressively during development by coordinated regulation of extraocular dorsal and ventral factors. In the present study, we cultured discrete portions of embryonic chick brains by preparing anterior cephalon, anterior dorsal cephalon and anterior ventral cephalon, and clearly demonstrate that bone morphogenetic protein 4 (BMP4) and Sonic hedgehog (Shh) constitute a dorsal-ventral signaling system together with fibroblast growth factor 8 (FGF8). BMP4 and Shh upregulate Tbx5 and Pax2, as reported previously, and at the same time Shh downregulates Tbx5, while BMP4 affects Pax2 expression to downregulate similarly. Shh induces Fgf8 expression in the ventral optic vesicle. This, in turn, determines the distinct boundary of the retinal pigmented epithelium and the neural retina by suppressing Mitf expression. The lens develops only when signals from both the dorsal and ventral regions come across together. Inverted deposition of Shh and BMP4 signals in organ-cultured optic vesicle completely re-organized ocular structures to be inverted. Based on these observations we propose a novel model in which the two signals govern the whole of ocular development when they encounter each other in the ocular morphogenic domain.

Meis2 competes with the Groucho co-repressor Tle4 for binding to Otx2 and specifies tectal fate without induction of a secondary midbrain-hindbrain boundary organizer

The transcription factor Otx2 is expressed throughout the anterior neuroectoderm and is required for the formation of all forebrain- and midbrain-derived structures. The molecular determinants that cooperate with Otx2 to subdivide its expression domain into distinct functional units are, however, poorly understood at present. We show here that the TALE-homeodomain protein Meis2 is expressed in the chick tectal anlage and is both necessary and sufficient for tectal development. Unlike known tectum-inducing genes, the ability of Meis2 to initiate tectal development does not involve the formation of a secondary midbrain-hindbrain boundary organizer, but instead requires direct interaction with Otx2. Using an Otx2-dependent reporter assay we demonstrate that Meis2 competes with the Groucho co-repressor Tle4 (Grg4) for binding to Otx2 and thereby restores Otx2 transcriptional activator function. Together, our data suggest a model in which the balance between a co-repressor and a co-activator, which compete for binding to Otx2 in the mesencephalic vesicle, provides spatial and temporal control over tectal development. Controlled formation of Meis2-containing higher order protein complexes might thus serve as a general mechanism to achieve subdivision of the anterior neuroectoderm into distinct functional units during embryogenesis.

EphrinB2 coordinates the formation of a morphological boundary and cell epithelialization during somite segmentation

During early morphogenesis, tissue segregation is often accompanied by changes in cell shape. To understand how such coordination is regulated, somitogenesis was used as a model. When a somite forms in the anterior end of the presomitic mesoderm, an intersomitic boundary (gap) emerges, and it is rapidly followed by cell epithelialization at this border. It has been known that the gap formation is regulated by intercellular signals. We here demonstrate that cMeso-1, the chicken homolog of mouse Mesp2, up-regulates EphA4 in the cells located posteriorly to a forming boundary. This in turn activates EphrinB2-reverse signals in the anteriorly juxtaposed cells, where the EphrinB2 signal is sufficient to cause a gap formation and cell epithelialization cell-autonomously. During these processes, Cdc42 needs to be repressed via tyrosine phosphorylation of EphrinB2. This is the first demonstration that Ephrin-reverse signal acts as a platform that couples distinct morphogenetic changes in cell polarity and tissue shape.

Mitf functions as an in ovo regulator for cell differentiation and proliferation during development of the chick RPE

Mitf has been reported to play a crucial role in regulating the differentiation of pigment cells in homeothermal animals, i.e. the melanocytes and the retinal pigment epithelium (RPE). However, less is known about the functions of Mitf in the developing RPE. To elucidate such functions, we introduced wild-type and dominant-negative Mitf expression vectors into chick optic vesicles by electroporation. Over-expression of wild-type Mitf altered neural retina cells to become RPE-like and repressed the expression of neural retina markers in vivo. In contrast, dominant-negative Mitf inhibited pigmentation in the RPE. The percentage of BrdU-positive cells decreased during normal RPE development, which was followed by Mitf protein expression. The percentage of BrdU-positive cells decreased in the wild-type Mitf-transfected neural retina, but increased in the dominant-negative Mitf-transfected RPE. p27(kip1), one of the cyclin-dependent kinase inhibitors, begins to be expressed in the proximal region of the RPE at stage 16. Transfection of wild-type Mitf induced expression of p27(kip1), while transfection of dominant-negative Mitf inhibited p27(kip1) expression. We found that Mitf was associated with the endogenous p27(kip1) 5' flanking region. These results demonstrate for the first time "in vivo" that Mitf uniquely regulates both differentiation and cell proliferation in the developing RPE.

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

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

Improved translation efficiency of injected mRNA during early embryonic development

Injection techniques are a powerful approach to study gene function in fish and frog model systems. In particular, in vitro transcribed mRNA is broadly used for such misexpression experiments. Sequence elements flanking the coding region, such as untranslated repeats and polyadenylation sequences, are known to affect the stability and the translation efficiency of mRNA. Here we show that in early embryos, poly(A) signals strongly contribute to the activity of the injected mRNA. Of interest, they only marginally affect mRNA stability, whereas the translation efficiency is dramatically enhanced. Combination of a poly(A) tail and an SV40 late poly(A) signal leads to highly synergistic effects of the two elements for injected mRNA. Compared with established vector systems, we detected a 20-fold improvement for mRNA derived from the novel transcription vector pMC.

Tet-on inducible system combined with in ovo electroporation dissects multiple roles of genes in somitogenesis of chicken embryos

The in ovo electroporation technique in chicken embryos has enabled investigators to uncover the functions of numerous developmental genes. In this technique, the ubiquitous promoter, CAGGS (CMV base), has often been used for overexpression experiments. However, if a given gene plays a role in multiple steps of development and if overexpression of this gene causes fatal consequences at the time of electroporation, its roles in later steps of development would be overlooked. Thus, a technique with which expression of an electroporated DNA can be controlled in a stage-specific manner needs to be formulated. Here we show for the first time that the tetracycline-controlled expression method, "tet-on" and "tet-off", works efficiently to regulate gene expression in electroporated chicken embryos. We demonstrate that the onset or termination of expression of an electroporated DNA can be precisely controlled by timing the administration of tetracycline into an egg. Furthermore, with this technique we have revealed previously unknown roles of RhoA, cMeso-1 and Pax2 in early somitogenesis. In particular, cMeso-1 appears to be involved in cell condensation of a newly forming somite by regulating Pax2 and NCAM expression. Thus, the novel molecular technique in chickens proposed in this study provides a useful tool to investigate stage-specific roles of developmental genes.

Three enhancer regions regulate gbx2 gene expression in the isthmic region during zebrafish development

In vertebrate embryos, positioning of the boundary between the midbrain and hindbrain (MHB) and subsequent isthmus formation are dependent upon the interaction between the Otx2 and Gbx genes. In zebrafish, sequential expression of gbx1 and gbx2 in the anterior hindbrain contributes to this process, whereas in mouse embryos, a single Gbx gene (Gbx2) is responsible for MHB development. In the present study, to investigate the regulatory mechanism of gbx2 in the MHB/isthmic region of zebrafish embryos, we cloned the gene and showed that its organization is conserved among different vertebrates. Promoter analyses revealed three enhancers that direct reporter gene expression after the end of epiboly in the anterior-most hindbrain, which is a feature of the zebrafish gbx2 gene. One of the enhancers is located upstream of gbx2 (AMH1), while the other two enhancers are located downstream of gbx2 (AMH2 and AMH3). Detailed analysis of the AMH1 enhancer showed that it directs expression in the rhombomere 1 (r1) region and the dorsal thalamus, as has been shown for gbx2, whereas no expression was induced by the AMH1 enhancer in other embryonic regions in which gbx2 is expressed. The AMH1 enhancer is composed of multiple regulatory subregions that share the same spatial specificity. The most active of the regulatory subregions is a 291-bp region that contains at least two Pax2-binding sites, both of which are necessary for the function of the main component (PB1-A region) of the AMH1 enhancer. In accordance with these results, enhancer activity in the PB1-A region, as well as gbx2 expression in r1, was missing in no isthmus mutant embryos that lacked functional pax2a. In addition, we identified an upstream conserved sequence of 227bp that suppresses the enhancer activity of AMH1. Taken together, these findings suggest that gbx2 expression during the somitogenesis stage in zebrafish is regulated by a complex mechanism involving Pax2 as well as activators and suppressors in the regions flanking the gene.

Positional regulation of Pax2 expression pattern in mesencephalic and diencephalic alar plate

Regionalization of the neural tube is a fundamental event in the development of the CNS. The isthmic organizer controls the development of the mesencephalic-rhombencephalic junction by means of fibroblast growth factor 8 (Fgf8) expression. The transcription factor paired box 2 (Pax2) is expressed early in the midbrain and is later progressively restricted to the mid-hindbrain region, playing an important role in the development of the mesencephalon and the cerebellum. In this study, by implanting Fgf8-beads in the chick neural tube, we show that Fgf8 induces a heterogeneous pattern of Pax2 expression in the diencephalon. To explore the mechanisms controlling this asymmetric induction and the down-regulation of Pax2 in the anterior mesencephalon we performed antero-posterior inversions of diencephalic and/or mesencephalic neuroepithelium, with or without Fgf8-bead implantation. We show that anterior factors do not play a role in Pax2 regionalization, while posterior factors could explain these expression patterns. We conclude that the anterior mesencephalon and the diencephalon are able to retain or activate Pax2 expression under caudal influences, and to develop as a tectal structure at later developmental stages.

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

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

Isthmus organizer for midbrain and hindbrain development

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

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.

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.

The Fgf8 signal causes cerebellar differentiation by activating the Ras-ERK signaling pathway

The mes/metencephalic boundary (isthmus) is an organizing center for the optic tectum and cerebellum. Fgf8 is accepted as a crucial organizing signal. Previously, we reported that Fgf8b could induce cerebellum in the mesencephalon, while Fgf8a transformed the presumptive diencephalon into mesencephalon. Since lower doses of Fgf8b exerted similar effects to those of Fgf8a, the type difference could be attributed to the difference in the strength of the signal. It is of great interest to uncover mechanisms of signal transduction pathways downstream of the Fgf8 signal in tectal and cerebellar development, and in this report we have concentrated on the Ras-ERK pathway. In normal embryos,extracellular-signal-regulated kinase (ERK) is activated at the site where Fgf8 mRNA is expressed. Fgf8b activated ERK while Fgf8a or a lower dose of Fgf8b did not activate ERK in the mes/metencephalon. Disruption of the Ras-ERK signaling pathway by a dominant negative form of Ras (RasS17N) changed the fate of the metencephalic alar plate from cerebellum to tectum. RasS17N canceled the effects of Fgf8b, while co-transfection of Fgf8a and RasS17N exerted additive effects. Disruption of Fgf8b, not Fgf8a, by siRNA resulted in posterior extension of the Otx2 expression domain. Our results indicate that the presumptive metencephalon receives a strong Fgf8 signal that activates the Ras-ERK pathway and differentiates into the cerebellum.

Microbubble-enhanced sonoporation: efficient gene transduction technique for chick embryos

The gene transduction technique is a useful method to study gene functions that underlie vertebrate embryogenesis. In this study, a new gene transduction technique is reported using microbubble-enhanced sonoporation (hereafter referred to as sonoporation) to achieve ectopic and transient gene expression for several embryonic organs including embryonic chick limb bud mesenchymes. The technique proposed in this study has the advantages of 1) relatively simple gene transduction procedures, and 2) efficient exogenous gene transduction and expression with lower damages to embryos. Green fluorescent protein (GFP) or LacZ was misexpressed in limb bud mesenchymes by sonoporation, with the introduced expression transiently detected in the injected sites. Most of the transduced chick embryos survived without showing significant embryonic abnormalities or cell death after sonoporation. To demonstrate its efficacy for assessing the effect of transient gene transduction, the Shh (sonic hedgehog) was transduced into the developing chick limb bud. The transduced limb bud displayed limb malformations including partial digit duplication. Advantages and possible future applications in relation to this method are discussed.

Potential role of Pax-2 in retinal axon navigation through the chick optic nerve stalk and optic chiasm

The degree of fiber decussation at the optic chiasm differs between species, ranging from complete crossing in lower vertebrates to highly complex patterns of intermingling of the fibers from the two eyes seen in mammals and birds. Understanding the genetic control of fiber guidance through the chiasm is therefore important to unravel the developmental mechanisms within the visual system. Here we first report on early stages of chiasm formation, with pioneering axons from the left eye consistently arriving earlier than their counterparts from the right eye. This initial left-right asymmetry is transient and no functional significance is assigned to it yet. Secondly, we examined formation of the chiasm in relation with the expression of the transcription factor Pax-2 along the ventral eye cup and optic nerve stalk. Finally, in order to examine causal involvement of Pax-2 in chiasm formation, the gene was overexpressed along the neuraxis and in the eye cup at embryonic stages preceding the exit of axons from the eye, and hence arrival of axons at the chiasm. When studied with neuroanatomical tracing, Pax-2 overexpression resulted in visibly anomalous decussation of axons at the chiasm. A likely consequence of this perturbation was erroneous arrival of axons at the tectum, as observed by anterograde staining from the retina. These data suggest that balanced expression of Pax-2 results in the correct formation of the chick chiasm at early stages by imposing accurate pathfinding within the optic stalk and the midline.

Enhanced in vitro midbrain dopamine neuron differentiation, dopaminergic function, neurite outgrowth, and 1-methyl-4-phenylpyridium resistance in mouse embryonic stem cells overexpressing Bcl-XL

Embryonic stem (ES) cells provide a potentially unlimited source of specialized cells for regenerative medicine. The ease of inducing stable genetic modifications in ES cells allows for in vitro manipulations to enhance differentiation into specific cell types and to optimize in vivo function of differentiated progeny in animal models of disease. We have generated mouse ES cells that constitutively express Bcl-XL, an antiapoptotic protein of Bcl-2 family. In vitro differentiation of Bcl-XL overexpressing ES (Bcl-ES) cells resulted in higher expression of genes related to midbrain dopamine (DA) neuron development and increased the number of ES-derived neurons expressing midbrain DA markers compared with differentiation of wild-type ES cells. Moreover, DA neurons derived from Bcl-ES cells were less susceptible to 1-methyl-4-phenylpyridium, a neurotoxin for DA neurons. On transplantation into parkinsonian rats, the Bcl-ES-derived DA neurons exhibited more extensive fiber outgrowth and led to a more pronounced reversal of behavioral symptoms than wild-type ES-derived DA neurons. These data suggest a role for Bcl-XL during in vitro midbrain DA neuron differentiation and provide an improved system for cell transplantation in a preclinical animal model of Parkinson's disease.

Polarity and laminar formation of the optic tectum in relation to retinal projection

The mes-metencephalic boundary (isthmus) works as an organizer for the tectum, and the organizing molecule may be Fgf8. The region where Otx2, En1, and Pax2 are expressed overlappingly may differentiate into the mesencephalon. The di-mesencephalic and mes-metencephalic boundaries are determined by repressive interaction of Pax6 and En1/Pax2 and of Otx2 and Gbx2, respectively. The optic tectum is a visual center in lower vertebrates. The tectum and the retina should be regionalized and be positionally specialized for the proper retinotopic projection. Gradient of En2 plays a crucial role in rostrocaudal polarity formation of the tectum. En2 confers caudal characteristics of the retina by inducing ephrinA2 and A5, which are the repellant molecules for the growth cones of temporal retinal ganglion cells. Grg4 antagonizes the isthmus-related genes, and is involved in the formation of di-mesencephalic boundary and tectal polarity formation at an early phase of development. Then, Grg4 plays a role in tectal laminar formation by controlling the migration pathway. Migration pathway of tectal postmitotic cells changes after E5. The late migratory cells split the early migratory neurons to form laminae h-j of SGFS. Grg4 is expressed in the ventricular layer after E5, and forces postmitotic cells to follow the late migratory pathway, though retinal fibers terminate at laminae a-f of SGFS. Misexpression of Grg4 disrupts the lamina g, and in such tecta retinal arbors invade deep into the tectal layer, indicating that lamina g is a nonpermissive lamina for the retinal arbors.

Neuroepithelial secondary organizers and cell fate specification in the developing brain

In vertebrates, elaborate cellular interactions regulate the establishment of the complex structural pattern of the developing central nervous system. Distinct neural and glial identities are acquired by neuroepithelial cells, through progressive restriction of histogenetic potential under the influence of local environmental signals. The localization of the sources of such morphogenetic signals in discrete domains of the developing neural primordium has led to the concept of secondary organizers which refine the identity and polarity of neighboring neuroepithelial regions. Thus, these organizers, secondary to those that operate throughout the embryo during gastrulation, act to pattern the anterior neural plate and tube giving rise to the forebrain, midbrain and hindbrain vesicles. Important progress has recently been made in understanding their genesis and function.

Pax 2 expression in mesodermal segmentation and its relationship with EphA4 and Lunatic-fringe during chicken somitogenesis

In the Pax gene family, which encodes DNA-binding proteins, Pax 2 has been known to play important roles in the formation of the midbrain/hindbrain boundary, eye, inner ear and kidney in vertebrates (Bioessays 19 (1997) 755). In this article, we report a segmentally regulated pattern of Pax 2 expression during chicken somitogenesis. Pax 2 mRNA is localized in the rostral end of the unsegmented presomitic mesoderm (PSM), abutting anteriorly on a prospective segmentation border. This pattern repeats every segmentation cycle (90 min) observed in ovo and also in the half embryo culture assay in which one half of PSM along the midline is fixed immediately while the other half is cultured for a given period. We also determined the sequence of changes in Pax 2 expression during a segmentation cycle by comparing the pattern of Pax 2 with that of Lunatic-fringe (L-fringe), known to cycle periodically in posterior PSM. A systematic comparison of the expression patterns between Pax 2, L-fringe and EphA4 further highlighted a close relationship between EphA4 and Pax 2 during a segmentation cycle. Lastly, Pax 2 is not segmentally expressed in mouse PSM, suggestive of species (avian)-specific mechanisms underlying somitic segmentation.

Early subdivisions in the neural plate define distinct competence for inductive signals

Regionalization of the embryonic brain is achieved through multi-step processes that operate sequentially and/or simultaneously. Localized sources of various signaling molecules act as organizing centers that pattern neighboring fields to create molecularly distinct domains. We investigated the mechanisms underlying the regionally distinct competence for two such organizing signals, Fibroblast growth factor 8 (Fgf8) and Sonic hedgehog (Shh), using chick embryos. First, we demonstrated that FGF receptor 1 (Fgfr1) and Fgfr3, expressed differentially in the developing brain, possess an equivalent potential to induce the regionally distinct Fgf8-responsive genes, depending on the anterior-posterior dimension of the brain. Next we found that homeodomain transcription factors Six3 and Irx3 can alter the regional responses to both Fgf8 and Shh in the forebrain. Six3 confers the ability to express Bf1, a gene essential for the telencephalon and eye development, and Nkx2.1, which is required for development of the hypothalamus. In contrast, Irx3 confers the ability to express En2 and Nkx6.1 in response to Fgf8 and Shh, respectively. Furthermore, an alteration in the region-specific response to Fgf8 upon misexpression of Irx3 resulted in transformation of diencephalic and possibly telencephalic tissues into the optic tectum. Finally, we demonstrated that Six3 and Irx3 can mutually repress their expression, which may contribute to the establishment of their complementary expression domains in the neural plate. These repressive interactions are specific, as Six3 did not repress Gbx2, and Irx3 did not disturb Otx2 expression. These findings provide evidence that the early embryonic forebrain is demarcated into two domains with distinct genetic programs, which argues against the authentic telendiencephalic subdivision.

Temporal sequence of gene expression leading caudal prosencephalon to develop a midbrain/hindbrain phenotype

Transplantation of prosomeres 1-2 into the cerebellar plate were used, by using chick/quail chimeras, to analyse the temporal sequence of the genetic cascade leading the graft to develop a midbrain/hindbrain phenotype. Our results show that (1) at Hamburger and Hamilton (HH) stage 13, Pax2 and En2 are already induced within the graft, before all other genes of the cascade, whereas misexpression of Fgf8 is also observed within the contiguous host cerebellar plate; (2) within the graft, Otx2 repression and Gbx2 induction (see Hidalgo-Sánchez et al. [1999] Development 126:3191-3203) are secondary events that affect, from stages HH14-15, the areas in contact with the host Gbx2/Fgf8-expressing cerebellar plate; (3) at these stages, the repressed Otx2 territory extends beyond the areas induced to express Gbx2, with the two territories not abutting before HH17-18; (4) Fgf8 expression becomes progressively induced within the Otx2-repressed/Gbx2-induced territory, starting at HH15-16. Our results support the hypothesis that the host-Gbx2/graft-Otx2 interface could trigger the genetic cascade induced within the graft and that the Gbx2-induced domain could play a key role during the establishment of the induced intragraft midbrain/hindbrain boundary.

Early patterning of the prospective midbrain-hindbrain boundary by the HES-related gene XHR1 in Xenopus embryos

The molecular mechanisms that govern early patterning of anterior neuroectoderm (ANE) for the prospective brain region in vertebrates are largely unknown. Screening a cDNA library of Xenopus ANE led to the isolation of a Hairy and Enhancer of split- (HES)-related transcriptional repressor gene, Xenopus HES-related 1 (XHR1). XHR1 is specifically expressed in the midbrain-hindbrain boundary (MHB) region at the tailbud stage. The localized expression of XHR1 was detected as early as the early gastrula stage in the presumptive MHB region, an area just anterior to the involuting dorsal mesoderm that is demarcated by the expression of the gene Xbra. Expression of XHR1 was detected much earlier than that of other known MHB genes, XPax-2 and En-2, and also before the formation of the expression boundary between Xotx2 and Xgbx-2, suggesting that the early patterning of the presumptive MHB is independent of Xotx2 and Xgbx-2. Instead, the location of XHR1 expression appears to be determined in relation to the Xbra expression domain, since reduced or ectopic expression of Xbra altered the XHR1 expression domain according to the location of Xbra expression. In functional assays using mRNA injection, overexpression of dominant-negative forms of XHR1 in the MHB region led to marked reduction of XPax-2 and En-2 expression, and this phenotype was rescued by coexpression of wild-type XHR1. Furthermore, ectopically expressed wild-type XHR1 near the MHB region enhanced En-2 expression only in the MHB region but not in the region outside the MHB. These data suggest that XHR1 is required, but not sufficient by itself, to initiate MHB marker gene expression. Based on these data, we propose that XHR1 demarcates the prospective MHB region in the neuroectoderm in Xenopus early gastrulae.

Distinct regulators control the expression of the mid-hindbrain organizer signal FGF8

Local expression of FGF8 at the mid/hindbrain boundary (MHB) governs the development of multiple neurons and support cells. Here we show that the paired-domain protein Pax2 is necessary and sufficient for the induction of FGF8 in part by regulating the expression of Pax5&8. A network of transcription and secreted factors, including En1, Otx2, Gbx2, Grg4 and Wnt1&4, that is established independently of Pax2, further refines the expression domain and level of FGF8 at the MHB through opposing effects on Pax2 activity. Our results indicate that the expression of local organizing factors is controlled by combinatorial interaction between inductive and modulatory factors.

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.

Regionalisation of anterior neuroectoderm and its competence in responding to forebrain and midbrain inducing activities depend on mutual antagonism between OTX2 and GBX2

The anterior neural ridge (ANR), and the isthmic organiser (IsO) represent two signalling centres possessing organising properties necessary for forebrain (ANR) as well as midbrain and rostral hindbrain (IsO) development. An important mediator of ANR and IsO organising property is the signalling molecule FGF8. Previous work has indicated that correct positioning of the IsO and Fgf8 expression in this domain is controlled by the transcription factors Otx2 and Gbx2. In order to provide novel insights into the roles of Otx2 and Gbx2, we have studied mutant embryos carrying different dosages of Otx2, Otx1 and Gbx2. Embryos deficient for both OTX2 and GBX2 proteins (hOtx12/hOtx12; Gbx2–/–) show abnormal patterning of the anterior neural tissue, which is evident at the presomite-early somite stage prior to the onset of Fgf8 neuroectodermal expression. Indeed, hOtx12/hOtx12; Gbx2–/– embryos exhibit broad co-expression of early forebrain, midbrain and rostral hindbrain markers such as hOtx1, Gbx2, Pax2, En1 and Wnt1 and subsequently fail to activate forebrain and midbrain-specific gene expression. In this genetic context, Fgf8 is expressed throughout the entire anterior neural plate, thus indicating that its activation is independent of both OTX2 and GBX2 function. Analysis of hOtx12/hOtx12; Gbx2–/– and Otx1+/–; Otx2+/– mutant embryos also suggests that FGF8 cannot repress Otx2 without the participation of GBX2. Finally, we report that embryos carrying a single strong hypomorphic Otx2 allele (Otx2λ) in an Otx2 and Gbx2 null background (Otx2λ/–; Gbx2–/–) recover both the headless phenotype exhibited by Otx2λ/– embryos and forebrain- and midbrain-specific gene expression that is not observed in hOtx12/hOtx12; Gbx2–/– mutants. Together, these data provide novel genetic evidence indicating that OTX2 and GBX2 are required for proper segregation of early regional identities anterior and posterior to the mid-hindbrain boundary (MHB) and for conferring competence to the anterior neuroectoderm in responding to forebrain-, midbrain- and rostral hindbrain-inducing activities.

spiel ohne grenzen/pou2is required during establishment of the zebrafish midbrain-hindbrain boundary organizer

The vertebrate midbrain-hindbrain boundary (MHB) organizes patterning and neuronal differentiation in the midbrain and anterior hindbrain. Formation of this organizing center involves multiple steps, including positioning of the MHB within the neural plate, establishment of the organizer and maintenance of its regional identity and signaling activities. Juxtaposition of the Otx2 and Gbx2 expression domains positions the MHB. How the positional information is translated into activation of Pax2, Wnt1 and Fgf8 expression during MHB establishment remains unclear. In zebrafish spiel ohne grenzen (spg) mutants, the MHB is not established, neither isthmus nor cerebellum form, the midbrain is reduced in size and patterning abnormalities develop within the hindbrain. In spg mutants, despite apparently normal expression of otx2, gbx1 and fgf8 during late gastrula stages, the initial expression of pax2.1, wnt1 and eng2, as well as later expression of fgf8 in the MHB primordium are reduced. We show that spg mutants have lesions in pou2, which encodes a POU-domain transcription factor. Maternal pou2 transcripts are distributed evenly in the blastula, and zygotic expression domains include the midbrain and hindbrain primordia during late gastrulation. Microinjection of pou2 mRNA can rescue pax2.1 and wnt1 expression in the MHB of spg/pou2 mutants without inducing ectopic expression. This indicates an essential but permissive role for pou2 during MHB establishment. pou2 is expressed normally in noi/pax2.1 and ace/fgf8 zebrafish mutants, which also form no MHB. Thus, expression of pou2 does not depend on fgf8 and pax2.1. Our data suggest that pou2 is required for the establishment of the normal expression domains of wnt1 and pax2.1 in the MHB primordium.

Role of Pax3/7 in the tectum regionalization

Pax3/7 is expressed in the alar plate of the mesencephalon. The optic tectum differentiates from the alar plate of the mesencephalon, and expression of Pax3/7 is well correlated to the tectum development. To explore the function of Pax3 and Pax7 in the tectum development, we misexpressed Pax3 and Pax7 in the diencephalon and ventral mesencephalon. Morphological and molecular marker gene analysis indicated that Pax3 and Pax7 misexpression caused fate change of the alar plate of the presumptive diencephalon to that of the mesencephalon, that is, a tectum and a torus semicircularis were formed ectopically. Ectopic tectum in the diencephalon appeared to be generated through sequential induction of Fgf8, En2 and Pax3/7. In ventral mesencephalon, which expresses En but does not differentiate to the tectum in normal development, Pax3 and Pax7 misexpression induced ectopic tectum. In normal development, Pax3 and Pax7 expression in the mesencephalon commences after Otx2, En and Pax2/5 expression. In addition, expression domain of Pax3 and Pax7 is well consistent with presumptive tectum region in a dorsoventral axis. Taken together with normal expression pattern of Pax3 and Pax7, results of misexpression experiments suggest that Pax3 and Pax7 define the tectum region subsequent to the function of Otx2 and En.

Early anterior/posterior patterning of the midbrain and cerebellum

Transplantation studies performed in chicken embryos indicated that early anterior/posterior patterning of the vertebrate midbrain and cerebellum might be regulated by an organizing center at the junction between the midbrain and hindbrain. More than a decade of molecular and genetic studies have shown that such an organizer is indeed central to development of the midbrain and anterior hindbrain. Furthermore, a complicated molecular network that includes multiple positive and negative feedback loops underlies the establishment and refinement of a mid/hindbrain organizer, as well as the subsequent function of the organizer. In this review, we first introduce the expression patterns of the genes known to be involved in this patterning process and the quail-chick transplantation experiments that have provided the foundation for understanding the genetic pathways regulating mid/hindbrain patterning. Subsequently, we discuss the molecular genetic studies that have revealed the roles for many genes in normal early patterning of this region. Finally, some of the remaining questions and future directions are discussed.

Inductive signal and tissue responsiveness defining the tectum and the cerebellum

The mes/metencephalic boundary (isthmus) has an organizing activity for mesencephalon and metencephalon. The candidate signaling molecule is Fgf8 whose mRNA is localized in the region where the cerebellum differentiates. Responding to this signal, the cerebellum differentiates in the metencephalon and the tectum differentiates in the mesencephalon. Based on the assumption that strong Fgf8 signal induces the cerebellum and that the Fgf8b signal is stronger than that of Fgf8a, we carried out experiments to misexpress Fgf8b and Fgf8a in chick embryos. Fgf8a did not affect the expression pattern of Otx2, Gbx2 or Irx2. En2 expression was upregulated in the mesencephalon and in the diencephalon by Fgf8a. Consequently, Fgf8a misexpression resulted in the transformation of the presumptive diencephalon to the fate of the mesencephalon. In contrast, Fgf8b repressed Otx2 expression, but upregulated Gbx2 and Irx2 expression in the mesencephalon. As a result, Fgf8b completely changed the fate of the mesencephalic alar plate to cerebellum. Quantitative analysis showed that Fgf8b signal is 100 times stronger than Fgf8a signal. Co-transfection of Fgf8b with Otx2 indicates that Otx2 is a key molecule in mesencephalic generation. We have shown by RT-PCR that both Fgf8a and Fgf8b are expressed, Fgf8b expression prevailing in the isthmic region. The results all support our working hypothesis that the strong Fgf8 signal induces the neural tissue around the isthmus to differentiate into the cerebellum.

A gene encoding a new ONECUT class homeodomain protein in the ascidian Halocynthia roretzi functions in the differentiation and specification of neural cells in ascidian embryogenesis

Genes encoding a novel group of homeodomain transcription factors, ONECUT class homeodomain proteins, have previously been isolated from vertebrate and insect. Among them, vertebrate HNF-6 is expressed in hepatocytes and the central nervous system during embryogenesis. Although the functions of HNF-6 in hepatocytes have been well studied, the functions of HNF-6 in the central nervous system remain unknown. In this study, we isolated HrHNF-6, which encodes a new ONECUT class homeodomain protein, from an ascidian, Halocynthia roretzi. HrHNF-6 mRNA was expressed exclusively in neural cells, just posterior to the expression of Hroth during embryogenesis. One of the functions of HrHNF-6 in neural cells is the activation of the expression of HrTBB2, the ascidian beta-tubulin gene. Another is the restriction of the expression of HrPax-258 (which is expressed in the neural tube), suggesting that HrHNF-6 functions in the specification of the neural tube. These results indicate that HrHNF-6 functions in the differentiation and regional specification of neural cells during ascidian embryogenesis.

Sparking new frontiers: using in vivo electroporation for genetic manipulations

In vivo electroporation is a fascinating new approach by which gene expression, regulation, and function can be studied in developmental systems. This technique offers new opportunities for manipulations in animal models that lack genetic approaches, including avians. Furthermore, this approach is applicable to other embryo populations including mice, ascidians, zebrafish, Xenopus, and Drosophila. In this review, we discuss technical aspects of in vivo electroporation, review recent studies where this approach has been utilized successfully, and identify future directions.

Neuroepithelial co-expression of Gbx2 and Otx2 precedes Fgf8 expression in the isthmic organizer

The most studied secondary neural organizer is the isthmic organizer, which is localized at the mid-hindbrain transition of the neural tube and controls the anterior hindbrain and midbrain regionalization. Otx2 and Gbx2 expressions are fundamental for positioning the organizer and the establishment of molecular interactions that induce Fgf8. We present here evidences demonstrating that Otx2 and Gbx2 have an overlapping expression in the isthmic region. This area is the transversal domain where expression of Fgf8 is induced. The Fgf8 protein produced in the isthmus stabilizes and up-regulates Gbx2 expression, which, in turn, down-regulates Otx2 expression. The inductive effect of the Gbx2/Otx2 limit keeps Fgf8 expression stable and thus maintains its positive role in the expression of Pax2, En1,2 and Wnt1.

The midbrain--hindbrain boundary organizer

Cell fate in the cephalic neural primordium is controlled by an organizer located at the midbrain-hindbrain boundary. Studies in chick, mouse and zebrafish converge to show that mutually repressive interactions between homeodomain transcription factors of the Otx and Gbx class position this organizer in the neural primordium. Once positioned, independent signaling pathways converge in their activity to drive organizer function. Fibroblast growth factors secreted from the organizer are necessary for, and sufficient to mimic, organizer activity in patterning the midbrain and anterior hindbrain, and are tightly controlled by feedback inhibition.

Neural plate patterning: upstream and downstream of the isthmic organizer

Two organizing centres operate at long-range distances within the anterior neural plate to pattern the forebrain, midbrain and hindbrain. Important progress has been made in understanding the formation and function of one of these organizing centres, the isthmic organizer, which controls the development of the midbrain and anterior hindbrain. Here we review our current knowledge on the identity, localization and maintenance of the isthmic organizer, as well as on the molecular cascades that underlie the activity of this organizing centre.

EN and GBX2 play essential roles downstream of FGF8 in patterning the mouse mid/hindbrain region

Fgf8, which is expressed at the embryonic mid/hindbrain junction, is required for and sufficient to induce the formation of midbrain and cerebellar structures. To address through what genetic pathways FGF8 acts, we examined the epistatic relationships of mid/hindbrain genes that respond to FGF8, using a novel mouse brain explant culture system. We found that En2 and Gbx2 are the first genes to be induced by FGF8 in wild-type E9.5 diencephalic and midbrain explants treated with FGF8-soaked beads. By examining gene expression in En1/2 double mutant mouse embryos, we found that Fgf8, Wnt1 and Pax5 do not require the En genes for initiation of expression, but do for their maintenance, and Pax6 expression is expanded caudally into the midbrain in the absence of EN function. Since E9.5 En1/2 double mutants lack the mid/hindbrain region, forebrain mutant explants were treated with FGF8 and, significantly, the EN transcription factors were found to be required for induction of Pax5. Thus, FGF8-regulated expression of Pax5 is dependent on EN proteins, and a factor other than FGF8 could be involved in initiating normal Pax5 expression in the mesencephalon/metencephalon. The En genes also play an important, but not absolute, role in repression of Pax6 in forebrain explants by FGF8. Previous Gbx2 gain-of-function studies have shown that misexpression of Gbx2 in the midbrain can lead to repression of Otx2. However, in the absence of Gbx2, FGF8 can nevertheless repress Otx2 expression in midbrain explants. In contrast, Wnt1 is initially broadly induced in Gbx2 mutant explants, as in wild-type explants, but not subsequently repressed in cells near FGF8 that normally express Gbx2. Thus GBX2 acts upstream of, or parallel to, FGF8 in repressing Otx2, and acts downstream of FGF8 in repression of Wnt1. This is the first such epistatic study performed in mouse that combines gain-of-function and loss-of-function approaches to reveal aspects of mouse gene regulation in the mesencephalon/metencephalon that have been difficult to address using either approach alone.

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.