Expression of the winged helix genes fkh-4 and fkh-5 defines domains in the central nervous system

Regulation of Wnt Signaling by FOX Transcription Factors in Cancer

Simple Summary

Cancer is caused by a breakdown of cell-to-cell communication, which results in the unrestricted expansion of cells within a tissue. In many cases, tumor growth is maintained by the continuous activation of cell signaling programs that normally drive embryonic development and wound repair. In this review article, I discuss how one of the largest human protein families, namely FOX proteins, controls the activity of the Wnt pathway, a major regulatory signaling cascade in developing organisms and adult stem cells. Evidence suggests that there is considerable crosstalk between FOX proteins and the Wnt pathway, which contributes to cancer initiation and progression. A better understanding of FOX biology may therefore lead to the development of new targeted treatments for many types of cancer.

Abstract

Aberrant activation of the oncogenic Wnt signaling pathway is a hallmark of numerous types of cancer. However, in many cases, it is unclear how a chronically high Wnt signaling tone is maintained in the absence of activating pathway mutations. Forkhead box (FOX) family transcription factors are key regulators of embryonic development and tissue homeostasis, and there is mounting evidence that they act in part by fine-tuning the Wnt signaling output in a tissue-specific and context-dependent manner. Here, I review the diverse ways in which FOX transcription factors interact with the Wnt pathway, and how the ectopic reactivation of FOX proteins may affect Wnt signaling activity in various types of cancer. Many FOX transcription factors are partially functionally redundant and exhibit a highly restricted expression pattern, especially in adults. Thus, precision targeting of individual FOX proteins may lead to safe treatment options for Wnt-dependent cancers.

Cellular taxonomy and spatial organization of the murine ventral posterior hypothalamus

The ventral posterior hypothalamus (VPH) is an anatomically complex brain region implicated in arousal, reproduction, energy balance, and memory processing. However, neuronal cell type diversity within the VPH is poorly understood, an impediment to deconstructing the roles of distinct VPH circuits in physiology and behavior. To address this question, we employed a droplet-based single-cell RNA sequencing (scRNA-seq) approach to systematically classify molecularly distinct cell populations in the mouse VPH. Analysis of >16,000 single cells revealed 20 neuronal and 18 non-neuronal cell populations, defined by suites of discriminatory markers. We validated differentially expressed genes in selected neuronal populations through fluorescence in situ hybridization (FISH). Focusing on the mammillary bodies (MB), we discovered transcriptionally-distinct clusters that exhibit neuroanatomical parcellation within MB subdivisions and topographic projections to the thalamus. This single-cell transcriptomic atlas of VPH cell types provides a resource for interrogating the circuit-level mechanisms underlying the diverse functions of VPH circuits.

Wnt activator FOXB2 drives the neuroendocrine differentiation of prostate cancer

Aberrant activation of the homeostatic Wnt signaling pathway is a hallmark of various types of cancer. In many cases, it is unclear how elevated Wnt levels are maintained in the absence of activating pathway mutations. Here we find that the uncharacterized transcription factor FOXB2, whose expression is usually restricted to the developing brain, is induced in aggressive prostate cancer. FOXB2 strongly activates Wnt signaling via the induction of multiple pathway agonists, particularly the neurogenic ligand WNT7B. Accordingly, our analyses suggest that FOXB2 imposes a neuronal differentiation program on prostate cancer cells, which is associated with treatment failure and poor prognosis. Thus, our work identifies FOXB2 as a tissue-specific Wnt activator that may play a role in prostate cancer progression.

The Wnt signaling pathway is of paramount importance for development and disease. However, the tissue-specific regulation of Wnt pathway activity remains incompletely understood. Here we identify FOXB2, an uncharacterized forkhead box family transcription factor, as a potent activator of Wnt signaling in normal and cancer cells. Mechanistically, FOXB2 induces multiple Wnt ligands, including WNT7B, which increases TCF/LEF-dependent transcription without activating Wnt coreceptor LRP6 or β-catenin. Proximity ligation and functional complementation assays identified several transcription regulators, including YY1, JUN, and DDX5, as cofactors required for FOXB2-dependent pathway activation. Although FOXB2 expression is limited in adults, it is induced in select cancers, particularly advanced prostate cancer. RNA-seq data analysis suggests that FOXB2/WNT7B expression in prostate cancer is associated with a transcriptional program that favors neuronal differentiation and decreases recurrence-free survival. Consistently, FOXB2 controls Wnt signaling and neuroendocrine differentiation of prostate cancer cell lines. Our results suggest that FOXB2 is a tissue-specific Wnt activator that promotes the malignant transformation of prostate cancer.

Foxb1 Regulates Negatively the Proliferation of Oligodendrocyte Progenitors

Oligodendrocyte precursor cells (OPC), neurons and astrocytes share a neural progenitor cell (NPC) in the early ventricular zone (VZ) of the embryonic neuroepithelium. Both switch to produce either of the three cell types and the generation of the right number of them undergo complex genetic regulation. The components of these regulatory cascades vary between brain regions giving rise to the unique morphological and functional heterogeneity of this organ. Forkhead b1 (Foxb1) is a transcription factor gene expressed by NPCs in specific regions of the embryonic neuroepithelium. We used the mutant mouse line Foxb1-Cre to analyze the cell types derived from Fobx1-expressing NPCs (the Foxb1 cell lineage) from two restricted regions, the medulla oblongata (MO; hindbrain) and the thalamus (forebrain), of normal and Foxb1-deficient mice. Foxb1 cell lineage derivatives appear as clusters in restricted regions, including the MO (hindbrain) and the thalamus (forebrain). Foxb1-expressing NPCs produce mostly oligodendrocytes (OL), some neurons and few astrocytes. Foxb1-deficient NPCs generate mostly OPC and immature OL to the detriment of neurons, astrocytes and mature OL. The axonal G-ratio however is not changed. We reveal Foxb1 as a novel modulator of neuronal and OL generation in certain restricted CNS regions. Foxb1 biases NPCs towards neuronal generation and inhibits OPC proliferation while promoting their differentiation.

Lhx5 controls mamillary differentiation in the developing hypothalamus of the mouse

Acquisition of specific neuronal identity by individual brain nuclei is a key step in brain development. However, how the mechanisms that confer neuronal identity are integrated with upstream regional specification networks is still mysterious. Expression of Sonic hedgehog (Shh), is required for hypothalamic specification and is later downregulated by Tbx3 to allow for the differentiation of the tubero-mamillary region. In this region, the mamillary body (MBO), is a large neuronal aggregate essential for memory formation. To clarify how MBO identity is acquired after regional specification, we investigated Lhx5, a transcription factor with restricted MBO expression. We first generated a hypomorph allele of Lhx5—in homozygotes, the MBO disappears after initial specification. Intriguingly, in these mutants, Tbx3 was downregulated and the Shh expression domain abnormally extended. Microarray analysis and chromatin immunoprecipitation indicated that Lhx5 appears to be involved in Shh downregulation through Tbx3 and activates several MBO-specific regulator and effector genes. Finally, by tracing the caudal hypothalamic cell lineage we show that, in the Lhx5 mutant, at least some MBO cells are present but lack characteristic marker expression. Our work shows how the Lhx5 locus contributes to integrate regional specification pathways with downstream acquisition of neuronal identity in the MBO.

Cadherins mediate sequential roles through a hierarchy of mechanisms in the developing mammillary body

Expression of intricate combinations of cadherins (a family of adhesive membrane proteins) is common in the developing central nervous system. On this basis, a combinatorial cadherin code has long been proposed to underlie neuronal sorting and to be ultimately responsible for the layers, columns and nuclei of the brain. However, experimental proof of this particular function of cadherins has proven difficult to obtain and the question is still not clear. Alternatively, non-specific, non-combinatorial, purely quantitative adhesive differentials have been proposed to explain neuronal sorting in the brain. Do cadherin combinations underlie brain cytoarchitecture? We approached this question using as model a well-defined forebrain nucleus, the mammillary body (MBO), which shows strong, homogeneous expression of one single cadherin (Cdh11) and patterned, combinatorial expression of Cdh6, −8 and −10. We found that, besides the known combinatorial Cdh pattern, MBO cells are organized into a second, non-overlapping pattern grouping neurons with the same date of neurogenesis. We report that, in the Foxb1 mouse mutant, Cdh11 expression fails to be maintained during MBO development. This disrupted the combination-based as well as the birthdate-based sorting in the mutant MBO. In utero RNA interference (RNAi) experiments knocking down Cdh11 in MBO-fated migrating neurons at one specific age showed that Cdh11 expression is required for chronological entrance in the MBO. Our results suggest that neuronal sorting in the developing MBO is caused by adhesion-based, non-combinatorial mechanisms that keep neurons sorted according to birthdate information (possibly matching them to target neurons chronologically sorted in the same manner). Non-specific adhesion mechanisms would also prevent cadherin combinations from altering the birthdate-based sorting. Cadherin combinations would presumably act later to support specific synaptogenesis through specific axonal fasciculation and final target recognition.

The forkhead transcription factor FoxB1 regulates the dorsal-ventral and anterior-posterior patterning of the ectoderm during early Xenopus embryogenesis

The formation of the dorsal-ventral (DV) and anterior-posterior (AP) axes, fundamental to the body plan of animals, is regulated by several groups of polypeptide growth factors including the TGF-β, FGF, and Wnt families. In order to ensure the establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated. However, the molecular mechanisms responsible for these interactions remain unclear. Here, we demonstrate that the forkhead box transcription factor FoxB1, which is upregulated by the neuralizing factor Oct-25, plays an important role in the formation of the DV and AP axes. Overexpression of FoxB1 promoted neural induction and inhibited BMP-dependent epidermal differentiation in ectodermal explants, thereby regulating the DV patterning of the ectoderm. In addition, FoxB1 was also found to promote the formation of posterior neural tissue in both ectodermal explants and whole embryos, suggesting its involvement in embryonic AP patterning. Using knockdown analysis, we found that FoxB1 is required for the formation of posterior neural tissues, acting in concert with the Wnt and FGF pathways. Consistent with this, FoxB1 suppressed the formation of anterior structures via a process requiring the function of XWnt-8 and eFGF. Interestingly, while downregulation of FoxB1 had little effect on neural induction, we found that it functionally interacted with its upstream factor Oct-25 and plays a supportive role in the induction and/or maintenance of neural tissue. Our results suggest that FoxB1 is part of a mechanism that fine-tunes, and leads to the coordinated formation of, the DV and AP axes during early development.

Interaction between axons and specific populations of surrounding cells is indispensable for collateral formation in the mammillary system

BACKGROUND

An essential phenomenon during brain development is the extension of long collateral branches by axons. How the local cellular environment contributes to the initial sprouting of these branches in specific points of an axonal shaft remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

The principal mammillary tract (pm) is a landmark axonal bundle connecting ventral diencephalon to brainstem (through the mammillotegmental tract, mtg). Late in development, the axons of the principal mammillary tract sprout collateral branches at a very specific point forming a large bundle whose target is the thalamus. Inspection of this model showed a number of distinct, identified cell populations originated in the dorsal and the ventral diencephalon and migrating during development to arrange themselves into several discrete groups around the branching point. Further analysis of this system in several mouse lines carrying mutant alleles of genes expressed in defined subpopulations (including Pax6, Foxb1, Lrp6 and Gbx2) together with the use of an unambiguous genetic marker of mammillary axons revealed: 1) a specific group of Pax6-expressing cells in close apposition with the prospective branching point is indispensable to elicit axonal branching in this system; and 2) cooperation of transcription factors Foxb1 and Pax6 to differentially regulate navigation and fasciculation of distinct branches of the principal mammillary tract.

CONCLUSIONS/SIGNIFICANCE

Our results define for the first time a model system where interaction of the axonal shaft with a specific group of surrounding cells is essential to promote branching. Additionally, we provide insight on the cooperative transcriptional regulation necessary to promote and organize an intricate axonal tree.

Fetal and neonatal exposure to the endocrine disruptor methoxychlor causes epigenetic alterations in adult ovarian genes

Exposure to endocrine-disrupting chemicals during development could alter the epigenetic programming of the genome and result in adult-onset disease. Methoxychlor (MXC) and its metabolites possess estrogenic, antiestrogenic, and antiandrogenic activities. Previous studies showed that fetal/neonatal exposure to MXC caused adult ovarian dysfunction due to altered expression of key ovarian genes including estrogen receptor (ER)-beta, which was down-regulated, whereas ERalpha was unaffected. The objective of the current study was to evaluate changes in global and gene-specific methylation patterns in adult ovaries associated with the observed defects. Rats were exposed to MXC (20 microg/kgxd or 100 mg/kg.d) between embryonic d 19 and postnatal d 7. We performed DNA methylation analysis of the known promoters of ERalpha and ERbeta genes in postnatal d 50-60 ovaries using bisulfite sequencing and methylation-specific PCRs. Developmental exposure to MXC led to significant hypermethylation in the ERbeta promoter regions (P < 0.05), whereas the ERalpha promoter was unaffected. We assessed global DNA methylation changes using methylation-sensitive arbitrarily primed PCR and identified 10 genes that were hypermethylated in ovaries from exposed rats. To determine whether the MXC-induced methylation changes were associated with increased DNA methyltransferase (DNMT) levels, we measured the expression levels of Dnmt3a, Dnmt3b, and Dnmt3l using semiquantitative RT-PCR. Whereas Dnmt3a and Dnmt3l were unchanged, Dnmt3b expression was stimulated in ovaries of the 100 mg/kg MXC group (P < 0.05), suggesting that increased DNMT3B may cause DNA hypermethylation in the ovary. Overall, these data suggest that transient exposure to MXC during fetal and neonatal development affects adult ovarian function via altered methylation patterns.

Aberrant axonal projections from mammillary bodies in Pax6 mutant mice: possible roles of Netrin-1 and Slit 2 in mammillary projections

Early events in the axonal tract formation from mammillary bodies remain poorly understood. In the present study, we reported an aberrant pattern of axonal projections from mammillary bodies to the dorsal thalamus in mice lacking the transcription factor Pax6. We found that Netrin-1 was ectopically up-regulated and that both Slit1 and Slit2 were down-regulated in the presumptive dorsal thalamus of Pax6 mutant mice. We then examined the effects of Netrin-1 and Slit2 on the mammillary axons by in utero electroporation techniques. Netrin-1 had an attractive action toward the mammillary axons. Moreover, mammillary trajectories were disorganized in Netrin-1-deficient mice. On the other hand, Slit2 had a repulsive effect on the mammillary axons. These findings suggest that the combination of Netrin and Slit may be involved in proper axonal projection from the mammillary bodies and that their misexpression in the diencephalon may cause the misrouting of these axons in Pax6 mutant mice.

Genetic mapping of Foxb1-cell lineage shows migration from caudal diencephalon to telencephalon and lateral hypothalamus

The hypothalamus is a brain region with vital functions, and alterations in its development can cause human disease. However, we still do not have a complete description of how this complex structure is put together during embryonic and early postnatal stages. Radially oriented, outside-in migration of cells is prevalent in the developing hypothalamus. In spite of this, cell contingents from outside the hypothalamus as well as tangential hypothalamic migrations also have an important role. Here we study migrations in the hypothalamic primordium by genetically labeling the Foxb1 diencephalic lineage. Foxb1 is a transcription factor gene expressed in the neuroepithelium of the developing neural tube with a rostral expression boundary between caudal and rostral diencephalon, and therefore appropriate for marking migrations from caudal levels into the hypothalamus. We have found a large, longitudinally oriented migration stream apparently originating in the thalamic region and following an axonal bundle to end in the anterior portion of the lateral hypothalamic area. Additionally, we have mapped a specific expansion of the neuroepithelium into the rostral diencephalon. The expanded neuroepithelium generates abundant neurons for the medial hypothalamus at the tuberal level. Finally, we have uncovered novel diencephalon-to-telencephalon migrations into septum, piriform cortex and amygdala.

Expression of Foxp4 in the developing and adult rat forebrain

Many members of the Fox family are transcription factors that regulate the morphogenesis of various organs. In the present study, we examined the expression pattern of Foxp4, a member of the Foxp subfamily, and compared its pattern with the patterns of Foxp2 and Foxp1 in the developing rat brain. In general, these three Foxp genes shared partially overlapping and yet differentially regulated expression patterns in the striatum, the cerebral cortex, and the thalamus during development. In the developing dorsal telencephalon, a mediolateral gradient of Foxp4 was present in the cortical primordium, with high levels in the ventricular zone of the medial cortex. By contrast, no gradient of Foxp2 and Foxp1 was detected in the dorsal telencephalon. At later stages, Foxp4 was expressed throughout all cortical layers as opposed to the layer-specific expression of Foxp2 and Foxp1. In the developing striatum, the pattern of Foxp4 expression was distinct from the patterns of Foxp2 and Foxp1. The spatial expression pattern of Foxp4 was similar to that of Foxp2 during the early embryonic stage. However, from the late embryonic stage to postnatal day 4, Foxp4 was expressed in a mediolateral gradient and decreased in the striosomal compartment, in contrast to the striosomal expression of Foxp2 and homogeneous expression of Foxp1. Foxp4 was developmentally down-regulated such that Foxp4 was undetectable in the forebrain after postnatal day 14, whereas Foxp2 and Foxp1 were persistently expressed in adulthood. Given that Foxp4, Foxp2, and Foxp1 are capable of heterodimerization for transcriptional regulation, the partially overlapping expression patterns of Foxp4, Foxp2, and Foxp1 in different domains of the developing forebrain suggest that each member and/or different combinatory actions of the Foxp subfamily may play a pivotal role in regulating forebrain development.

Identification of forkhead transcription factors in cortical and dopaminergic areas of the adult murine brain

The murine forkhead family of transcription factors consists of over 30 members, the vast majority of which is important in embryonic development. Implicated in processes such as proliferation, differentiation and survival, forkhead factors show highly restricted expression patterns. In search for forkhead genes expressed in specific neural systems, we identified multiple family members. We performed a detailed expression analysis for Foxj2, Foxk1 and the murine orthologue of the human ILF1 gene, which show a remarkable preference for complex cortical structures. In addition, a comprehensive examination of forkhead gene expression in dopamine neurons of the ventral tegmental area and substantia nigra pars compacta, revealed Ilf1 as a novel transcriptional regulator in midbrain dopamine neurons. These forkhead transcription factors may play a role in maintenance and survival of developing and adult neurons.

Regulatory gene expressions in the ascidian ventral sensory vesicle: evolutionary relationships with the vertebrate hypothalamus

In extant chordates, the overall patterning along the anteroposterior and dorsoventral axes of the neural tube is remarkably conserved. It has thus been proposed that four domains corresponding to the vertebrate presumptive forebrain, midbrain-hindbrain transition, hindbrain, and spinal cord were already present in the common chordate ancestor. To obtain insights on the evolution of the patterning of the anterior neural tube, we performed a study aimed at characterizing the expression of regulatory genes in the sensory vesicle of Ciona intestinalis, the anteriormost part of the central nervous system (CNS) related to the vertebrate forebrain, at tailbud stages. Selected genes encoded primarily for homologues of transcription factors involved in vertebrate forebrain patterning. Seven of these genes were expressed in the ventral sensory vesicle. A prominent feature of these ascidian genes is their restricted and complementary domains of expression at tailbud stages. These patterning markers thus refine the map of the developing sensory vesicle. Furthermore, they allow us to propose that a large part of the ventral and lateral sensory vesicle consists in a patterning domain corresponding to the vertebrate presumptive hypothalamus.

Genetic ablation of the mammillary bodies in the Foxb1 mutant mouse leads to selective deficit of spatial working memory

Mammillary bodies and the mammillothalamic tract are parts of a classic neural circuitry that has been implicated in severe memory disturbances accompanying Korsakoff's syndrome. However, the specific role of mammillary bodies in memory functions remains controversial, often being considered as just an extension of the hippocampal memory system. To study this issue we used mutant mice with a targeted mutation in the transcription factor gene Foxb1. These mice suffer perinatal degeneration of the medial and most of the lateral mammillary nuclei, as well as of the mammillothalamic bundle. Foxb1 mutant mice showed no deficits in such hippocampal-dependent tasks as contextual fear conditioning and social transmission of food preference. They were also not impaired in the spatial reference memory test in the radial arm maze. However, Foxb1 mutants showed deficits in the task for spatial navigation within the Barnes maze. Furthermore, they showed impairments in spatial working memory tasks such as the spontaneous alternation and the working memory test in the radial arm maze. Thus, our behavioural analysis of Foxb1 mutants suggests that the medial mammillary nuclei and mammillothalamic tract play a role in a specific subset of spatial tasks, which require combined use of both spatial and working memory functions. Therefore, the mammillary bodies and the mammillothalamic tract may form an important route through which the working memory circuitry receives spatial information from the hippocampus.

Pitx2 distinguishes subtypes of terminally differentiated neurons in the developing mouse neuroepithelium

Pitx2, a homeodomain transcription factor, is essential for normal development of pituitary, eyes, heart, and teeth. In the developing mouse brain, Pitx2 (Rieg, Ptx2, Otlx2, Brx1) mRNA is expressed in discrete regions of the diencephalon, mesencephalon, and rhombencephalon. While prior reports have provided an overview of the temporal and regional specificity of Pitx2 mRNA expression in the brain, the precise cell types that express PITX2 are not known. In this study, we analyzed Pitx2 mRNA and PITX2 protein expression in individual cells of the developing e10.5-e14.5 mouse CNS using multiple markers of cellular proliferation and differentiation. We identified Pitx2 expression in nestin-positive neural progenitors and in postmitotic, developing neurons. In the diencephalon, PITX2 is expressed in neurons of the zona limitans intrathalamica and mammillary region and in gamma-aminobutyric acid (GABA)-producing neurons of the zona incerta. In the mesencephalon, PITX2-labeled nuclei also appear in differentiated neurons, some of which are GABAergic and destined to occupy superior colliculus. Our results suggest that PITX2 expression in postmitotic neurons may contribute to development of GABAergic and other differentiated neuronal phenotypes.

The evolution of chordate neural segmentation

Amphioxus is the closest relative to vertebrates but lacks key vertebrate characters, like rhombomeres, neural crest cells, and the cartilaginous endoskeleton. This reflects major differences in the developmental patterning of neural and mesodermal structures between basal chordates and vertebrates. Here, we analyse the expression pattern of an amphioxus FoxB ortholog and an amphioxus single-minded ortholog to gain insight into the evolution of vertebrate neural segmentation. AmphiFoxB expression shows cryptic segmentation of the cerebral vesicle and hindbrain, suggesting that neuromeric segmentation of the chordate neural tube arose before the origin of the vertebrates. In the forebrain, AmphiFoxB expression combined with AmphiSim and other amphioxus gene expression patterns shows that the cerebral vesicle is divided into several distinct domains: we propose homology between these domains and the subdivided diencephalon and midbrain of vertebrates. In the Hox-expressing region of the amphioxus neural tube that is homologous to the vertebrate hindbrain, AmphiFoxB shows the presence of repeated blocks of cells along the anterior-posterior axis, each aligned with a somite. This and other data lead us to propose a model for the evolution of vertebrate rhombomeric segmentation, in which rhombomere evolution involved the transfer of mechanisms regulating neural segmentation from vertical induction by underlying segmented mesoderm to horizontal induction by graded retinoic acid signalling. A consequence of this would have been that segmentation of vertebrate head mesoderm would no longer have been required, paving the way for the evolution of the unsegmented head mesoderm seen in living vertebrates.

Sequence and expression of FoxB2 (XFD-5) and FoxI1c (XFD-10) in Xenopus embryogenesis

We report on the temporal and spatial expression pattern of two novel genes of the Xenopus fork head/winged helix family, xFoxB2 and xFoxI1c. xFoxB2 is activated at the late blastula stage and first expressed within the dorsolateral ectoderm except for the organiser territory. During gastrulation, xFoxB2 is found in two ectodermal stripes adjacent to the dorsal midline. Expression is completely down-regulated during neurulation. However, two distinct sets of cells expressing xFoxB2 re-appear in the rhombencephalon of swimming tadpoles. xFoxI1c is initially expressed at the early neurula stage in an epidermal ring around the neural field. Subsequent expression is found to be increased, and is exclusively localised to placodal precursor cells. The placodal expression remains until stage 40, when it is restricted to a distinct region in the lateral body wall behind the gills.

Early anteroposterior division of the presumptive neurectoderm in Xenopus

We analyze the timing of neural patterning in Xenopus and the mechanism by which the early pattern is generated. With regard to timing, we show that by early gastrula, two domains of the anteroposterior (A/P) pattern exist in the presumptive neurectoderm, since the opl gene is expressed throughout the future neural plate, while the fkh5 gene is expressed only in more posterior ectoderm. By mid-gastrula, this pattern has become more elaborate, with an anterior domain defined by expression of opl and otx2, a middle domain defined by expression of opl and fkh5, and a posterior domain defined by expression of opl, fkh5 and HoxD1. Explant assays indicate that the late blastula dorsal ectoderm is specified as the anterior domain, but is not yet specified as middle or posterior domains. With regard to the mechanism by which the A/P pattern is generated, gain and loss of function assays indicate that quantitatively and qualitatively different factors may be involved in inducing the early A/P neural pattern. These data show that neural patterning occurs early in Xenopus and suggest a molecular basis for initiating this pattern.

The winged helix gene, Foxb1, controls development of mammary glands and regions of the CNS that regulate the milk-ejection reflex

The ability to lactate is a process restricted to mammals and is necessary for the survival of nonhuman mammals. Female mice carrying a null mutation in the winged helix transcription factor Foxb1 (previously Mf3/Fkh5/TWH) have lactation defects on inbred genetic backgrounds. To determine the cellular basis of the Foxb1 lactation defect we have inserted a tau-lacZ lineage marker into the locus to follow the fate of Foxb1 expressing cells. This approach has revealed that Foxb1 is expressed in epithelial cells of developing and adult mammary glands as well as previously described regions of the central nervous system. Mammary glands from C57BL/6 Foxb1-/- mice have incomplete lobuloalveolar development. In addition, the tau-lacZ lineage label was used to determine that the mammillothalamic tract was lost in all Foxb1-/- mice. Finally, morphological defects in the inferior colliculi of the midbrain in Foxb1-/- mice correlate with the inability to lactate, suggesting that the midbrain defect, but not the loss of the mammillothalamic tract, may be responsible for the lactation defect.

Winged helix transcription factor Foxb1 is essential for access of mammillothalamic axons to the thalamus

Our aim was to study the mechanisms of brain histogenesis. As a model, we have used the role of winged helix transcription factor gene Foxb1 in the emergence of a very specific morphological trait of the diencephalon, the mammillary axonal complex. Foxb1 is expressed in a large hypothalamic neuronal group (the mammillary body), which gives origin to a major axonal bundle with branches to thalamus, tectum and tegmentum. We have generated mice carrying a targeted mutation of Foxb1 plus the tau-lacZ reporter. In these mutants, a subpopulation of dorsal thalamic ventricular cells “thalamic palisade” show abnormal persistence of Foxb1 transcriptional activity; the thalamic branch of the mammillary axonal complex is not able to grow past these cells and enter the thalamus. The other two branches of the mammillary axonal complex (to tectum and tegmentum) are unaffected by the mutation. Most of the neurons that originate the mammillothalamic axons suffer apoptosis after navigational failure. Analysis of chimeric brains with wild-type and Foxb1 mutant cells suggests that correct expression of Foxb1 in the thalamic palisade is sufficient to rescue the normal phenotype. Our results indicate that Foxb1 is essential for diencephalic histogenesis and that it exerts its effects by controlling access to the target by one particular axonal branch.

Minimal phenotype of mice homozygous for a null mutation in the forkhead/winged helix gene, Mf2

Mf2 (mesoderm/mesenchyme forkhead 2) encodes a forkhead/winged helix transcription factor expressed in numerous tissues of the mouse embryo, including paraxial mesoderm, somites, branchial arches, vibrissae, developing central nervous system, and developing kidney. We have generated mice homozygous for a null mutation in the Mf2 gene (Mf2(lacZ)) to examine its role during embryonic development. The lacZ allele also allows monitoring of Mf2 gene expression. Homozygous null mutants are viable and fertile and have no major developmental defects. Some mutants show renal abnormalities, including kidney hypoplasia and hydroureter, but the penetrance of this phenotype is only 40% or lower, depending on the genetic background. These data suggest that Mf2 can play a unique role in kidney development, but there is functional redundancy in this organ and other tissues with other forkhead/winged helix genes.

The fork head transcription factor Fkh5/Mf3 is a developmental marker gene for superior colliculus layers and derivatives of the hindbrain somatic afferent zone

Fork head-5 (Fkh5; also known as Mf3 and TWH) is a transcription factor of the winged helix family. As part of an extended project to understand the function of this protein in the developing mouse brain, in the present work we have used Fkh5/Mf3 expression as a marker to study the development of the midbrain and hindbrain. In the midbrain, Fkh5/Mf3 is expressed in the superior colliculus, in the ventricular layer of the inferior colliculus and in the isthmus. In the superior colliculus, Fkh5/Mf3 is expressed by cells of layers 4a and 4c since early in development. In the hindbrain, Fkh5/Mf3 is a longitudinal marker (as opposed to a transverse or rhombomeric one), since it labels nuclei belonging to the somatic afferent zone (ventral cochlear nucleus, cuneate and external cuneate nuclei, principal and spinal nuclei of the trigeminal). In addition, Fkh5/Mf3 is expressed by the developing endopiriform nucleus and by the olivary pretectal nucleus. The results suggest that Fkh5/Mf3 has an early role in the lamination of the tectum and in the longitudinal differentiation of the hindbrain.

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.

The mouse Fkh1/Mf1 gene: cDNA sequence, chromosomal localization and expression in adult tissues

The 'winged helix' or 'forkhead' transcription factor gene family is defined by a common 100-amino-acid DNA-binding motif. Here, we describe the chromosomal position, start site of transcription, sequence and adult expression pattern of the mouse Fkh1/Mf1 (Forkhead homologue 1/mesoderm/mesenchyme forkhead 1) gene. This gene contains one exon and encodes a protein of 553 amino acids that is highly related to the mouse MFH1 protein. The Fkh1/Mf1 mRNA is expressed widely in adult tissues. Linkage analysis showed that the Fkh1/Mf1 gene is localized to chromosome 13 at 17.02cM from the centromer, in close proximity to Bmp6 and Hfh1, another distinct member of the winged helix gene family.

The winged helix transcription factor Hfh2 is expressed in neural crest and spinal cord during mouse development

The embryonic neural crest is a unique group of cells that gives rise to the peripheral nervous system as well as many other cell types. Most of the work describing the behavior and specification of these cells has been done in the avian system, a system amenable to experimental manipulations such as tissue grafting, cell transplantation and lineage tracing. This work has been greatly facilitated by the use of molecular markers of neural crest cells such as HNK-1 and slug, markers that are not yet available for the study of mouse neural crest. Here we demonstrate that Hfh2 (HNF3 forkhead homologue 2), a member of the 'winged helix' or 'forkhead' transcription factor gene family, is expressed in premigratory and migrating neural crest cells in the early mouse embryo and in motorneuron progenitors in the developing spinal cord. Using linkage analysis we have localized the Hfh2 gene to chromosome 4 at 44.91 cM from the centromere.

Expression of the mouse Fkh1/Mf1 and Mfh1 genes in late gestation embryos is restricted to mesoderm derivatives

We have compared the expression patterns of the mouse Forkhead homologue 1/ mesoderm/mesenchyme forkhead 1 (Fkh1/Mf1) gene with that of the highly related winged helix gene Mfh1 in late gestation mouse embryos. Transcripts for both genes are restricted to derivatives of the mesoderm. Co-expression was found in cartilage primordia of the head, ribs, vertebra and bones. However, in several structures analyzed, Fkh1/Mf1 signals are lower in the inner layers of the developing cartilage than those of Mfh1.

Mouse Mesenchyme forkhead 2 (Mf2): expression, DNA binding and induction by sonic hedgehog during somitogenesis

Cloning and sequencing of mouse Mf2 (mesoderm/mesenchyme forkhead 2) cDNAs revealed an open reading frame encoding a putative protein of 492 amino acids which, after in vitro translation, binds to a DNA consensus sequence. Mf2 is expressed at high levels in the ventral region of newly formed somites, in sclerotomal derivatives, in lateral plate and cephalic mesoderm and in the first and second branchial arches. Other regions of mesodermal expression include the developing tongue, meninges, nose, whiskers, kidney, genital tubercule and limb joints. In the nervous system Mf2 is transcribed in restricted regions of the mid- and forebrain. In several tissues, including the early somite, Mf2 is expressed in cell populations adjacent to regions expressing sonic hedgehog (Shh) and in explant cultures of presomitic mesoderm Mf2 is induced by Shh secreted by COS cells. These results suggest that Mf2, like other murine forkhead genes, has multiple roles in embryogenesis, possibly mediating the response of cells to signaling molecules such as SHH.

Fkh5-deficient mice show dysgenesis in the caudal midbrain and hypothalamic mammillary body

The murine winged helix gene Fkh5 is specifically expressed in the developing central nervous system (CNS). Early embryonic Fkh5 expression is restricted to the mammiliary body region of the caudal hypothalamus, midbrain, hindbrain and spinal cord. Postnatally, signals persist in specific nuclei of the mammillary body and in the midbrain. We generated Fkh5 deficient mice by homologous recombination to assess its in vivo function. At birth, Fkh5-deficient mice are viable and indistinguishable from wild-type and Fkh5 heterozygous littermates. However, about one third die within the first two days and another fifth before weaning. Surviving Fkh5-deficient mice become growth retarded within the first week and remain smaller throughout their whole life span. Fkh5-deficient females on 129Sv × C57BL/6 genetic background are fertile, but do not nurture their pups. More detailed analysis of Fkh5-deficient brains reveals distinct alterations in the CNS. In the midbrain, mutant mice exhibit reduced inferior colliculi and an overgrown anterior cerebellum. Furthermore, the hypothalamic mammillary body of Fkh5-deficient brains lacks the medial mammillary nucleus. These results suggest that Fkh5 plays a major role during CNS development.

Expression patterns of Brx1 (Rieg gene), Sonic hedgehog, Nkx2.2, Dlx1 and Arx during zona limitans intrathalamica and embryonic ventral lateral geniculate nuclear formation

The Brx1 homeobox gene has been isolated and shown to be expressed in the zona limitans intrathalamica (ZLI) of the mouse embryo. Brx1 is a member of the Brx gene family and comprises the genes for Brx1a and Brx1b, which differ in the sequence in the region located on the 5'-terminal side of the homeobox. The complete amino acid sequences of the open reading frame of Brx1a and Brx1b were determined and each was found to be similar to that of Rgs, the mouse homologue of the Rieger syndrome associated human RIEG gene (RGS), to the extent that the sequence of Rgs has been clarified. Brx1 was strongly expressed in the mammillary area as well as in the ZLI of the mouse embryonic brain. Homologues of Brx1a and Brx1b were isolated in chick in which the expression of Brx1 in the ventral diencephalon was well conserved. The expression of Brx1 along with that of Sonic hedgehog (Shh), Nkx2.2, Dlx1 and Arx was examined at the time of the formation of ZLI in mouse embryos. The expression of Shh was initially noted in the ventricular zone of the presumptive ZLI and was then replaced by that of Brx1 at the time of radial migration of the neuroepithelial cells. Nkx2.2 was widely expressed in the ventricular zone of presumptive ZLI and also as a narrow band in the mantle zone. The expression of Dlx1 and Arx in the presumptive ventral thalamus extended as far as ZLI and overlapped with that of Brx1. The Dlx1- and Arx-expressing cells in ZLI, which extended towards the lateral (pial) surface of the diencephalic wall, differed from those expressing Nkx2.2 and Brx1. The embryonic ventral lateral geniculate nucleus present in the visual pathway was eventually formed from these cells. Each homeobox gene was also expressed regionally in the nucleus, suggesting that the nucleus is comprised of subdivisions.

The winged helix transcription factor MFH1 is required for proliferation and patterning of paraxial mesoderm in the mouse embryo

The gene mfh1, encoding a winged helix/forkhead domain transcription factor, is expressed in a dynamic pattern in paraxial and presomitic mesoderm and developing somites during mouse embryogenesis. Expression later becomes restricted to condensing mesenchyme of the vertebrae, head, limbs, and kidney. A targeted disruption of the gene was generated by homologous recombination in embryonic stem cells. Most homozygous mfh1 null embryos die prenatally but some survive to birth, with multiple craniofacial and vertebral column defects. Using molecular markers, we show that the initial formation and patterning of somites occurs normally in mutants. Differentiation of sclerotome-derived cells also appears unaffected, although a reduction of the level of some markers [e.g., mtwist, mf1, scleraxis, and alpha1(II) collagen] is seen in the anterior of homozygous mutants. The most significant difference, however, is a marked reduction in the proliferation of sclerotome-derived cells, as judged by BrdU incorporation. This proliferation defect was also seen in micromass cultures of somite-derived cells treated with transforming growth factor beta1 and fibroblast growth factors. Our findings establish a requirement for a winged helix/forkhead domain transcription factor in the development of the paraxial mesoderm. A model is proposed for the role of mfh1 in regulating the proliferation and differentiation of cell lineages giving rise to the axial skeleton and skull.

The winged helix gene, Mf3, is required for normal development of the diencephalon and midbrain, postnatal growth and the milk-ejection reflex

The mouse Mf3 gene, also known as Fkh5 and HFH-e5.1, encodes a winged helix/forkhead transcription factor. In the early embryo, transcripts for Mf3 are restricted to the presomitic mesoderm and anterior neurectoderm and mesoderm. By 9.5 days post coitum, expression in the nervous system is predominantly in the diencephalon, midbrain and neural tube. After midgestation, the highest level of mRNA is in the mammillary bodies, the posterior-most part of the hypothalamus. Mice homozygous for a deletion of the mf3 locus on a [129 × Black Swiss] background display variable phenotypes consistent with a requirement for the gene at several stages of embryonic and postnatal development. Approximately six percent of the mf3−/− embryos show an open neural tube in the diencephalon and midbrain region, and another five percent show a severe reduction of the posterior body axis; both these classes of affected embryos die in utero. Surviving homozygotes have an apparently normal phenotype at birth. Postnatally, however, mf3−/− pups are severely growth retarded and approximately one third die before weaning. This growth defect is not a direct result of lack of circulating growth hormone or thyrotropin. Mice that survive to weaning are healthy, but they show an abnormal clasping of the hindfeet when suspended by the tail. Although much smaller than normal, the mice are fertile. However, mf3−/− females cannot eject their milk supply to feed their pups. This nursing defect can be corrected with interperitoneal injections of oxytocin. These results provide evidence that Mf3 is required for normal hypothalamus development and suggest that Mf3 may play a role in postnatal growth and lactation.

TWH regulates the development of subsets of spinal cord neurons

Thymocyte winged helix (TWH) is a putative transcription factor expressed in the developing neural tube. At midgestation, TWH expression identifies subsets of spinal cord motor neurons and interneurons. TWH-expressing motor neurons were restricted to specific spinal cord levels, distinguishing motor neurons at lumbar from those at cervical levels. To understand the developmental role of TWH, we replaced the TWH gene with the lacZ reporter gene and generated mice with a homozygous disruption of the TWH gene. TWH(-/-) mutant mice had increased perinatal mortality, retarded postnatal growth, and motor weakness. The TWH(-/-) mutation resulted in alterations in the sizes and position of different neuronal populations. Our results demonstrate that TWH plays a critical role in neuronal development and suggest that TWH regulates the early differentiation of neural progenitors.