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

FOXC1 and FOXC2 regulate growth plate chondrocyte maturation towards hypertrophy in the embryonic mouse limb skeleton

The Forkhead box transcription factors FOXC1 and FOXC2 are expressed in condensing mesenchyme cells at the onset of endochondral ossification. We used the Prx1-cre mouse to ablate Foxc1 and Foxc2 in limb skeletal progenitor cells. Prx1-cre;Foxc1Δ/Δ;Foxc2Δ/Δ limbs were shorter than controls, with worsening phenotypes in distal structures. Cartilage formation and mineralization was severely disrupted in the paws. The radius and tibia were malformed, whereas the fibula and ulna remained unmineralized. Chondrocyte maturation was delayed, with fewer Indian hedgehog-expressing, prehypertrophic chondrocytes forming and a smaller hypertrophic chondrocyte zone. Later, progression out of chondrocyte hypertrophy was slowed, leading to an accumulation of COLX-expressing hypertrophic chondrocytes and formation of a smaller primary ossification center with fewer osteoblast progenitor cells populating this region. Targeting Foxc1 and Foxc2 in hypertrophic chondrocytes with Col10a1-cre also resulted in an expanded hypertrophic chondrocyte zone and smaller primary ossification center. Our findings suggest that FOXC1 and FOXC2 direct chondrocyte maturation towards hypertrophic chondrocyte formation. At later stages, FOXC1 and FOXC2 regulate function in hypertrophic chondrocyte remodeling to allow primary ossification center formation and osteoblast recruitment.

Foxc1 and Foxc2 function in osteochondral progenitors for the progression through chondrocyte hypertrophy and mineralization of the primary ossification center

The forkhead box transcription factor genes Foxc1 and Foxc2 are expressed in the condensing mesenchyme of the developing skeleton prior to the onset of chondrocyte differentiation. To determine the roles of these transcription factors in limb development we deleted both Foxc1 and Foxc2 in lateral plate mesoderm using the Prx1-cre mouse line. Resulting compound homozygous mice died shortly after birth with exencephaly, and malformations to this sternum and limb skeleton. Notably distal limb structures were preferentially affected, with the autopods displaying reduced or absent mineralization. The radius and tibia bowed and the ulna and fibula were reduced to an unmineralized rudimentary structure. Molecular analysis revealed reduced expression of Ihh leading to reduced proliferation and delayed chondrocyte hypertrophy at E14.5. At later ages, Prx1-cre;Foxc1Δ/ Δ;Foxc2 Δ / Δ embryos exhibited restored Ihh expression and an expanded COLX-positive hypertrophic chondrocyte region, indicating a delayed exit and impaired remodeling of the hypertrophic chondrocytes. Osteoblast differentiation and mineralization were disrupted at the osteochondral junction and in the primary ossification center (POC). Levels of OSTEOPONTIN were elevated in the POC of compound homozygous mutants, while expression of Phex was reduced, indicating that impaired OPN processing by PHEX may underlie the mineralization defect we observe. Together our findings suggest that Foxc1 and Foxc2 act at different stages of endochondral ossification. Initially these genes act during the onset of chondrogenesis leading to the formation of hypertrophic chondrocytes. At later stages Foxc1 and Foxc2 are required for remodeling of HC and for Phex expression required for mineralization of the POC.

Loss of Foxc1 and Foxc2 function in chondroprogenitor cells disrupts endochondral ossification

Endochondral ossification initiates the growth of the majority of the mammalian skeleton and is tightly controlled through gene regulatory networks. The forkhead box transcription factors Foxc1 and Foxc2 regulate aspects of osteoblast function in the formation of the skeleton, but their roles in chondrocytes to control endochondral ossification are less clear. Here, we demonstrate that Foxc1 expression is directly regulated by the activity of SRY (sex-determining region Y)-box 9, one of the earliest transcription factors to specify the chondrocyte lineage. Moreover, we demonstrate that elevated expression of Foxc1 promotes chondrocyte differentiation in mouse embryonic stem cells and loss of Foxc1 function inhibits chondrogenesis in vitro. Using chondrocyte-targeted deletion of Foxc1 and Foxc2 in mice, we reveal a role for these factors in chondrocyte differentiation in vivo. Loss of both Foxc1 and Foxc2 caused a general skeletal dysplasia predominantly affecting the vertebral column. The long bones of the limbs were smaller, mineralization was reduced, and organization of the growth plate was disrupted; in particular, the stacked columnar organization of the proliferative chondrocyte layer was reduced in size and cell proliferation was decreased. Differential gene expression analysis indicated disrupted expression patterns of chondrogenesis and ossification genes throughout the entire process of endochondral ossification in chondrocyte-specific Foxc1/Foxc2 KO embryos. Our results suggest that Foxc1 and Foxc2 are required for normal chondrocyte differentiation and function, as loss of both genes results in disorganization of the growth plate, reduced chondrocyte proliferation, and delays in chondrocyte hypertrophy that prevents ossification of the skeleton.

Somite Compartments in Amphioxus and Its Implications on the Evolution of the Vertebrate Skeletal Tissues

Mineralized skeletal tissues of vertebrates are an evolutionary novelty within the chordate lineage. While the progenitor cells that contribute to vertebrate skeletal tissues are known to have two embryonic origins, the mesoderm and neural crest, the evolutionary origin of their developmental process remains unclear. Using cephalochordate amphioxus as our model, we found that cells at the lateral wall of the amphioxus somite express SPARC (a crucial gene for tissue mineralization) and various collagen genes. During development, some of these cells expand medially to surround the axial structures, including the neural tube, notochord and gut, while others expand laterally and ventrally to underlie the epidermis. Eventually these cell populations are found closely associated with the collagenous matrix around the neural tube, notochord, and dorsal aorta, and also with the dense collagen sheets underneath the epidermis. Using known genetic markers for distinct vertebrate somite compartments, we showed that the lateral wall of amphioxus somite likely corresponds to the vertebrate dermomyotome and lateral plate mesoderm. Furthermore, we demonstrated a conserved role for BMP signaling pathway in somite patterning of both amphioxus and vertebrates. These results suggest that compartmentalized somites and their contribution to primitive skeletal tissues are ancient traits that date back to the chordate common ancestor. The finding of SPARC-expressing skeletal scaffold in amphioxus further supports previous hypothesis regarding SPARC gene family expansion in the elaboration of the vertebrate mineralized skeleton.

Cell lineage- and expression-based inference of the roles of forkhead box transcription factor Foxc2 in craniofacial development

BACKGROUND

Foxc2 is a member of the winged helix/forkhead (Fox) box family of transcription factors. Loss of function of Foxc2 causes craniofacial abnormalities such as cleft palate and deformed cranial base, but its role during craniofacial development remains to be elucidated.

RESULTS

The contributions of Foxc2-positive and its descendant cells to the craniofacial structure at E18.5 were examined using a tamoxifen-inducible Cre driver mouse (Foxc2-CreERT2) crossed with the R26R-LacZ reporter mouse. Foxc2 expression at E8.5 is restricted to the cranial mesenchyme, contributing to specific components including the cranial base, sensory capsule, tongue, upper incisor, and middle ear. Expression at E10.5 was still positively regulated in most of those regions. In situ hybridization analysis of Foxc2 and its closely related gene, Foxc1, revealed that expression domains of these genes largely overlap in the cephalic mesenchyme. Meanwhile, the tongue expressed Foxc2 but not Foxc1, and its development was affected by the neural crest-specific deletion of Foxc2 in mice (Wnt1-Cre; Foxc2^fl/fl ).

CONCLUSIONS

Foxc2 is expressed in cranial mesenchyme that contributes to specific craniofacial tissue components from an early stage, and it seems to be involved in their development in cooperation with Foxc1. Foxc2 also has its own role in tongue development.

Role of FOXC2 and PITX2 rare variants associated with mild functional alterations as modifier factors in congenital glaucoma

Congenital glaucoma (CG) is a severe and inherited childhood optical neuropathy that leads to irreversible visual loss and blindness in children. CG pathogenesis remains largely unexplained in most patients. Herein we have extended our previous studies to evaluate the role of FOXC2 and PITX2 variants in CG. Variants of the proximal promoter and transcribed sequence of these two genes were analyzed by Sanger sequencing in a cohort of 133 CG families. To investigate possible oligogenic inheritance involving FOXC2 or PITX2 and CYP1B1, we also analyzed FOXC2 and PITX2 variants in a group of 25 CG cases who were known to carry CYP1B1 glaucoma-associated genotypes. The functional effect of three identified variants was assessed by transactivation luciferase reporter assays, protein stability and subcellular localization analyses. We found eight probands (6.0%) who carried four rare FOXC2 variants in the heterozygous state. In addition, we found an elevated frequency (8%) of heterozygous and rare PITX2 variants in the group of CG cases who were known to carry CYP1B1 glaucoma-associated genotypes, and one of these PITX2 variants arose de novo. To the best of our knowledge, two of the identified variants (FOXC2: c.1183C>A, p.(H395N); and PITX2: c.535C>A, p.(P179T)) have not been previously identified. Examination of the genotype-phenotype correlation in this group suggests that the presence of the infrequent PITX2 variants increase the severity of the phenotype. Transactivation reporter analyses showed partial functional alteration of three identified amino acid substitutions (FOXC2: p.(C498R) and p.(H395N); PITX2: p.(P179T)). In summary, the increased frequency in PCG patients of rare FOXC2 and PITX2 variants with mild functional alterations, suggests they play a role as putative modifier factors in this disease further supporting that CG is not a simple monogenic disease and provides novel insights into the complex pathological mechanisms that underlie CG.

Foxc2CreERT2 knock-in mice mark stage-specific Foxc2-expressing cells during mouse organogenesis

Foxc2, a member of the winged helix transcription factor family, is essential for eye, calvarial bone, cardiovascular and kidney development in mice. Nevertheless, how Foxc2-expressing cells and their descendent cells contribute to the development of these tissues and organs has not been elucidated. Here, we generated a Foxc2 knock-in (Foxc2^CreERT2 ) mouse, in which administration of estrogen receptor antagonist tamoxifen induces nuclear translocation of Cre recombinase in Foxc2-expressing cells. By crossing with ROSA-LacZ reporter mice (Foxc2^CreERT2 ; R26R), the fate of Foxc2 positive (Foxc2⁺ ) cells was analyzed through LacZ staining at various embryonic stages. We found Foxc2⁺ cell descendants in the supraoccipital and exoccipital bone in E18.5 embryos, when tamoxifen was administered at embryonic day (E) 8.5. Furthermore, Foxc2⁺ descendant cranial neural crest cells at E8-10 were restricted to the corneal mesenchyme, while Foxc2⁺ cell derived cardiac neural crest cells at E6-12 were found in the aorta, pulmonary trunk and valves, and endocardial cushions. Foxc2⁺ cell descendant contributions to the glomerular podocytes in the kidney were also observed following E6.5 tamoxifen treatment. Our results are consistent with previous reports of Foxc2 expression during early embryogenesis and the Foxc2^CreERT2 mouse provides a tool to investigate spatiotemporal roles of Foxc2 and contributions of Foxc2⁺ expressing cells during mouse embryogenesis.

Difference in apical and basal growth of the frontal bone primordium in Foxc1ch/ch mice

The frontal and parietal bones form the major part of the calvarium and their primordia appear at the basolateral region of the head and grow apically. A spontaneous loss of Foxc1 function mutant mouse, congenital hydrocephalus (Foxc1(ch/ch)), results in congenital hydrocephalus accompanied by defects in the apical part of the skull vault. We found that during the initiation stage of apical growth of the frontal bone primordium in the Foxc1(ch/ch) mouse, the Runx2 expression domain extended only to the basal side and bone sialoprotein (Bsp) and N-cadherin expression domains appeared only in the basal region. Fluorescent dye (DiI) labeling of the frontal primordium by ex-utero surgery confirmed that apical extension of the frontal bone primordium of the mouse was severely retarded, while extension to the basal side underneath the brain was largely unaffected. Consistent with this observation, decreased cell proliferation activity was seen at the apical tip but not the basal tip of the frontal bone primordium as determined by double detection of Runx2 transcripts and BrdU incorporation. Furthermore, expression of the osteogenic-related genes Bmp4 and-7 was observed only in the basal part of the meninges during the initiation period of primordium growth. These results suggest that a loss of Foxc1 function affects skull bone formation of the apical region and that Bmp expression in the meninges might influence the growth of the calvarial bone primordium.

Generation of conditional alleles for Foxc1 and Foxc2 in mice

The Forkhead box transcription factors, Foxc1 and Foxc2, are crucial for development of the eye, cardiovascular network, and other physiological systems, but their cell-type specific and postdevelopmental functions are unknown, in part because conventional (i.e., whole-organism) homozygous-null mutations of either factor result in perinatal death. Here, we describe the generation of mice with conditional-null Foxc1(flox) and Foxc2(flox) mutations that are induced via Cre-mediated recombination. Mice homozygous for the unrecombined alleles are viable and fertile, indicating that the conditional alleles retain their wild-type function. The embryos of Foxc1(flox) or Foxc2(flox) mice crossed with Cre-deleter mice that are homozygous for the recombined allele (i.e., Foxc1(Δ/Δ) or Foxc2(Δ/Δ) embryos) lack expression of the corresponding gene and show the same developmental defects observed in conventional homozygous mutant embryos. We expect these conditional mutations to enable characterization of the cell-type specific functions of Foxc1 and Foxc2 in development, disease, and adult animals.

Gene expression signatures defining fundamental biological processes in pluripotent, early, and late differentiated embryonic stem cells

Investigating the molecular mechanisms controlling the in vivo developmental program postembryogenesis is challenging and time consuming. However, the developmental program can be partly recapitulated in vitro by the use of cultured embryonic stem cells (ESCs). Similar to the totipotent cells of the inner cell mass, gene expression and morphological changes in cultured ESCs occur hierarchically during their differentiation, with epiblast cells developing first, followed by germ layers and finally somatic cells. Combination of high throughput -omics technologies with murine ESCs offers an alternative approach for studying developmental processes toward organ-specific cell phenotypes. We have made an attempt to understand differentiation networks controlling embryogenesis in vivo using a time kinetic, by identifying molecules defining fundamental biological processes in the pluripotent state as well as in early and the late differentiation stages of ESCs. Our microarray data of the differentiation of the ESCs clearly demonstrate that the most critical early differentiation processes occur at days 2 and 3 of differentiation. Besides monitoring well-annotated markers pertinent to both self-renewal and potency (capacity to differentiate to different cell lineage), we have identified candidate molecules for relevant signaling pathways. These molecules can be further investigated in gain and loss-of-function studies to elucidate their role for pluripotency and differentiation. As an example, siRNA knockdown of MageB16, a gene highly expressed in the pluripotent state, has proven its influence in inducing differentiation when its function is repressed.

Comparative gene expression analysis and fate mapping studies suggest an early segregation of cardiogenic lineages in Xenopus laevis

Retrospective clonal analysis in mice suggested that the vertebrate heart develops from two sources of cells called first and second lineages, respectively. Cells of the first lineage enter the linear heart tube and initiate terminal differentiation earlier than cells of the second lineage. It is thought that both heart lineages arise from a common progenitor cell population prior to the cardiac crescent stage (E7.5 of mouse development). The timing of segregation of different lineages as well as the molecular mechanisms underlying this process is not yet known. Furthermore, gene expression data for those lineages are very limited. Here we provide the first comparative study of cardiac marker gene expression during Xenopus laevis embryogenesis complemented by single cell RT-PCR analysis. In addition we provide fate mapping data of cardiac progenitor cells at different stages of development. Our analysis indicates an early segregation of cardiac lineages and a fairly complex heterogeneity of gene expression in the cardiac progenitor cells. Furthermore, this study sets a reference for all further studies analyzing cardiac development in X. laevis.

Expression of FoxC, FoxF, FoxL1, and FoxQ1 genes in the dogfish Scyliorhinus canicula defines ancient and derived roles for Fox genes in vertebrate development

In the human genome, members of the FoxC, FoxF, FoxL1, and FoxQ1 gene families are found in two paralagous clusters. Here we characterize all four gene families in the dogfish Scyliorhinus canicula, a member of the cartilaginous fish lineage that diverged before the radiation of osteichthyan vertebrates. We identify two FoxC genes, two FoxF genes, and single FoxQ1 and FoxL1 genes, demonstrating cluster duplication preceded the radiation of gnathostomes. The expression of all six genes was analyzed by in situ hybridization. The results show conserved expression of FoxL1, FoxF, and FoxC genes in different compartments of the mesoderm and of FoxQ1 in pharyngeal endoderm and its derivatives, confirming these as ancient sites of Fox gene expression, and also illustrate multiple cases of lineage-specific expression domains. Comparison to invertebrate chordates shows that the majority of conserved vertebrate expression domains mark tissues that are part of the primitive chordate body plan.

Indian hedgehog signaling from endothelial cells is required for sclera and retinal pigment epithelium development in the mouse eye

The development of extraocular orbital structures, in particular the choroid and sclera, is regulated by a complex series of interactions between neuroectoderm, neural crest and mesoderm derivatives, although in many instances the signals that mediate these interactions are not known. In this study we have investigated the function of Indian hedgehog (Ihh) in the developing mammalian eye. We show that Ihh is expressed in a population of non-pigmented cells located in the developing choroid adjacent to the RPE. The analysis of Hh mutant mice demonstrates that the RPE and developing scleral mesenchyme are direct targets of Ihh signaling and that Ihh is required for the normal pigmentation pattern of the RPE and the condensation of mesenchymal cells to form the sclera. Our findings also indicate that Ihh signals indirectly to promote proliferation and photoreceptor specification in the neural retina. This study identifies Ihh as a novel choroid-derived signal that regulates RPE, sclera and neural retina development.

Mesenchyme Forkhead 1 (FOXC2) plays a key role in metastasis and is associated with aggressive basal-like breast cancers

The metastatic spread of epithelial cancer cells from the primary tumor to distant organs mimics the cell migrations that occur during embryogenesis. Using gene expression profiling, we have found that the FOXC2 transcription factor, which is involved in specifying mesenchymal cell fate during embryogenesis, is associated with the metastatic capabilities of cancer cells. FOXC2 expression is required for the ability of murine mammary carcinoma cells to metastasize to the lung, and overexpression of FOXC2 enhances the metastatic ability of mouse mammary carcinoma cells. We show that FOXC2 expression is induced in cells undergoing epithelial-mesenchymal transitions (EMTs) triggered by a number of signals, including TGF-beta1 and several EMT-inducing transcription factors, such as Snail, Twist, and Goosecoid. FOXC2 specifically promotes mesenchymal differentiation during an EMT and may serve as a key mediator to orchestrate the mesenchymal component of the EMT program. Expression of FOXC2 is significantly correlated with the highly aggressive basal-like subtype of human breast cancers. These observations indicate that FOXC2 plays a central role in promoting invasion and metastasis and that it may prove to be a highly specific molecular marker for human basal-like breast cancers.

Identification of Tgf beta1i4 as a downstream target of Foxc1

Craniofacial development is severely affected by null mutations in Foxc1, indicating a multifunctional role for Foxc1 in ocular, maxilla and mandible, skull and facial gland development. To delineate signaling pathways in which Foxc1 is involved we compared the transcriptomes of whole heads of Foxc1+/+ and Foxc1-/- embryos using a candidate cDNA array comprising genes expressed in the head mesenchyme and ocular region, and a 7K oligo array. Absence of Foxc1 led to downregulation of Stat1 and Galnt4, and upregulation of Tgf beta1i4 at embryonic day 13.5 in the developing head mesenchyme. Comparative analyses revealed differences in the expression pattern of Tgf beta1i4 in the head mesenchyme of Foxc1-/- and Foxc1+/+ embryos. In the ocular regions of Foxc1-/- embryos, Tgf beta1i4 was expressed in higher levels in the conjunctival epithelium and in the condensing mesenchyme on the nasal aspect of the developing eye while in wild-type embryos more intense expression was seen in the mesenchyme on the temporal aspect of the eye. Such data indicate that Foxc1 regulation of Tgf beta1i4 is complex and may be cell-type dependent. Analysis of the regulation of Tgf beta1i4 by Foxc1 in a more homogenous cell population, mesenchymal cells isolated from the periocular region revealed that, in these cells, Foxc1 negatively regulated Tgf beta1i4 expression, presumably via secreted factors such as TGF-beta1. Since Foxc1 expression is essential for normal craniofacial development, it is possible that its downstream targets play a role in the development of the phenotypes associated with null mutations in Foxc1.

Forkhead transcription factors, Foxc1 and Foxc2, are required for the morphogenesis of the cardiac outflow tract

Previous studies have shown that Foxc1 and Foxc2, closely related Fox transcription factors, have interactive roles in cardiovascular development. However, little is known about their functional overlap during early heart morphogenesis. Here, we show that Foxc genes are coexpressed in a novel heart field, the second heart field, as well as the cardiac neural crest cells (NCCs), endocardium, and proepicardium. Notably, compound Foxc1; Foxc2 mutants have a wide spectrum of cardiac abnormalities, including hypoplasia or lack of the outflow tract (OFT) and right ventricle as well as the inflow tract, dysplasia of the OFT and atrioventricular cushions, and abnormal formation of the epicardium, in a dose-dependent manner. Most importantly, in the second heart field, compound mutants exhibit significant downregulation of Tbx1 and Fgf8/10 and a reduction in cell proliferation. Moreover, NCCs in compound mutants show extensive apoptosis during migration, leading to a failure of the OFT septation. Taken together, our results demonstrate that Foxc1 and Foxc2 play pivotal roles in the early processes of heart development, especially acting upstream of the Tbx1-FGF cascade during the morphogenesis of the OFT.

The forkhead transcription factors, Foxc1 and Foxc2, are required for arterial specification and lymphatic sprouting during vascular development

Accumulating evidence suggests that in the vertebrate embryo, acquisition of arterial and venous identity is established early by genetic mechanisms, including those regulated by vascular endothelial growth factor (VEGF) and Notch signaling. However, although the COUP-TFII nuclear receptor has recently been shown to regulate vein identity, very little is known about the molecular mechanisms of transcriptional regulation in arterial specification. Here, we show that mouse embryos compound mutant for Foxc1 and Foxc2, two closely related Fox transcription factors, exhibit arteriovenous malformations and lack of induction of arterial markers whereas venous markers such as COUP-TFII are normally expressed, suggesting that mutant endothelial cells fail to acquire an arterial fate. Notably, consistent with this observation, overexpression of Foxc genes in vitro induces expression of arterial markers such as Notch1 and its ligand Delta-like 4 (Dll4), and Foxc1 and Foxc2 directly activate the Dll4 promoter via a Foxc-binding site. Moreover, compound Foxc mutants show a defect in sprouting of lymphatic endothelial cells from veins in early lymphatic development, due to reduced expression of VEGF-C. Taken together, our results demonstrate that Foxc transcription factors are novel regulators of arterial cell specification upstream of Notch signaling and lymphatic sprouting during embryonic development.

Foxc1 integrates Fgf and Bmp signalling independently of twist or noggin during calvarial bone development

Calvarial bone and suture development is under complex regulation where bone morphogenetic protein (Bmp) and fibroblast growth factor (Fgf) signalling interact with Msx2/Twist and Noggin and regulate frontal bone primordia proliferation and suture fusion, respectively. We have shown previously that the winged helix transcription factor Foxc1, which is necessary for calvarial bone development, is required for the Bmp regulation of Msx2. We now show that FGF2 regulates the expression of Foxc1, indicating that Foxc1 integrates Bmp and Fgf signalling pathways. We also show that Foxc1 is not needed for the acquisition of osteogenic potential or for the differentiation of osteoblasts. The expression of Fgf receptors and Twist were normal in Foxc1-deficient calvarial mesenchyme, and ectopic FGF2 was able to induce the expression Osteopontin. Furthermore, we demonstrate that Foxc1 does not participate in the regulation of Noggin expression. Our findings indicate that Foxc1 integrates the Bmp and Fgf signalling pathways independently of Twist or Noggin. This signalling network is essential for the correct patterning and growth of calvarial bones.

Foxc2 is expressed in developing lymphatic vessels and other tissues associated with lymphedema–distichiasis syndrome

The molecular events involved in lymphatic development are poorly understood. Hence, the genes responsible for hereditary lymphedema are of great interest due to the potential for providing insights into the mechanisms of lymphatic development, the diagnosis, prevention and treatment of lymphedema, and lymphangiogenesis during tumor growth. Mutations in the FOXC2 transcription factor cause a major form of hereditary lymphedema, the lymphedema-distichiasis syndrome. We have conducted a study of Foxc2 expression during mouse development using immunohistochemistry, and examined its expression in lymphatics compared to its paralog Foxc1 and to Vegfr-3, Prox1 and other lymphatic and blood vascular proteins. We have found that Foxc2 is expressed in lymphatic primordia, jugular lymph sacs, lymphatic collectors and capillaries, as well as in podocytes, developing eyelids and other tissues associated with abnormalities in lymphedema-distichiasis syndrome.

Progression of calvarial bone development requires Foxc1 regulation of Msx2 and Alx4

Calvarial bones form by direct ossification of mesenchyme. This requires condensation of mesenchymal cells which then proliferate and differentiate into osteoblasts. Congenital hydrocephalus (ch) mutant mice lack the forkhead/winged helix transcription factor Foxc1. In ch mutant mice, calvarial bones remain rudimentary at the sites of initial osteogenic condensations. In this study, we have localized the ossification defect in ch mutants to the calvarial mesenchyme, which lacks the expression of transcription factors Msx2 and Alx4. This lack of expression is associated with a reduction in the proliferation of osteoprogenitor cells. We have previously shown that BMP induces Msx2 in calvarial mesenchyme (Development 125, 1241-1251, 1998). Here, we show that BMP also induces Alx4 in this tissue. We also show that BMP-induced expression of Msx2 and Alx4 requires Foxc1. We therefore suggest that Foxc1 regulates BMP-mediated osteoprogenitor proliferation and that this regulation is required for the progression of osteogenesis beyond the initial condensations in calvarial bone development.

Bone morphogenetic protein regulation of forkhead/winged helix transcription factor Foxc2 (Mfh1) in a murine mesodermal cell line C1 and in skeletal precursor cells

Mfh1/Foxc2 is a member of forkhead/winged helix transcription factor family in which its members serve as key regulators in embryogenesis and cell differentiation in various species. Mutant mice null for Mfh1 show defects in axial and cranial skeletogenesis, suggesting requirement of Mfh1 for skeletal tissue development. However, the roles of Mfh1 and its regulation during early skeletogenesis have not been understood fully yet. In this study, we investigated developmental regulation of Mfh1 expression during embryonic skeletogenesis in vivo and in vitro chondrogenic cell differentiation using a mesodermal progenitor-like cell line C1. We first examined expression patterns of Mfh1 in relation to the cartilage phenotype-related molecules including bone morphogenetic proteins (BMPs) during mouse embryogenesis by in situ hybridization. In 10.5 days postcoitum (dpc) mouse limb, Mfh1 messenger RNA (mRNA) was expressed in the mesenchymal cells in the tissues that later give rise to skeleton. In 11.5 dpc embryos, Mfh1 transcripts were expressed in the cell condensation of skeletal blastemas. BMP2 transcripts were expressed in the cell condensation proximal to the Mfh1-expressing cells in the limbs and those of BMP-7 were expressed in the mesenchymal tissue surrounding the Mfh1-positive cell condensation. In 12.5 dpc and 13.5 dpc embryos, the expression of Mfh1 was localized to the perichondrium, which surrounds cells that express noggin and SOX9 mRNA. BMP-2 expression was overlapped with that of Mfh1 in the peripheral layer of 12.5 dpc and 13.5 dpc limb skeletal blastemas. Mfh1 expression persisted in the perichondrium of 15.5 dpc embryos though its level was reduced. We then examined the expression of Mfh1 in the mouse mesodermal cell line C1 that differentiates into chondrocytes in vitro. Mfh1 mRNA was expressed constitutively at low levels in C1 cells before the induction of its differentiation. On the differentiation of C1 cells into chondrocytes by the treatment with dexamethasone (Dex), Mfh1 expression was increased and peaked on day 4 of Dex treatment. Treatment with BMP-4/7 and BMP-7 protein also enhanced Mfh1 expression in C1 cells. To further examine the causative relationship between BMP and Mfh1 in mesenchymal tissue, we performed a mouse limb bud organ culture to implant BMP proteins with carriers into the mesenchymal tissue of the limb bud. Implantation of BMP-7 protein in the limb bud of 11.5 dpc embryos induced Mfh1 expression, suggesting that BMP regulates Mfh1 expression in limb mesenchyme. These results indicate that Mfh1 expression is associated with the early stage of chondrogenic differentiation both in vivo and in vitro and that BMPs regulate Mfh1 expression in skeletal precursor cells.

The murine winged helix transcription factors, Foxc1 and Foxc2, are both required for cardiovascular development and somitogenesis

The murine Foxc1/Mf1 and Foxc2/Mfh1 genes encode closely related forkhead/winged helix transcription factors with overlapping expression in the forming somites and head mesoderm and endothelial and mesenchymal cells of the developing heart and blood vessels. Embryos lacking either Foxc1 or Foxc2, and most compound heterozygotes, die pre- or perinatally with similar abnormal phenotypes, including defects in the axial skeleton and cardiovascular system. However, somites and major blood vessels do form. This suggested that the genes have similar, dose-dependent functions, and compensate for each other in the early development of the heart, blood vessels, and somites. In support of this hypothesis, we show here that compound Foxc1; Foxc2 homozygotes die earlier and with much more severe defects than single homozygotes alone. Significantly, they have profound abnormalities in the first and second branchial arches, and the early remodeling of blood vessels. Moreover, they show a complete absence of segmented paraxial mesoderm, including anterior somites. Analysis of compound homozygotes shows that Foxc1 and Foxc2 are both required for transcription in the anterior presomitic mesoderm of paraxis, Mesp1, Mesp2, Hes5, and Notch1, and for the formation of sharp boundaries of Dll1, Lfng, and ephrinB2 expression. We propose that the two genes interact with the Notch signaling pathway and are required for the prepatterning of anterior and posterior domains in the presumptive somites through a putative Notch/Delta/Mesp regulatory loop.

Molecular determinants of neural crest migration

Normal septation of the cardiac outflow tract requires migration of neural crest cells from the posterior rhombencephalon to the branchial arches and developing conotruncal endocardial cushions. Proper migration of these cells is mediated by a variety of molecular cues. Adhesion molecules, such as integrins, are involved in the interaction of neural crest cells with the extracellular matrix, while cadherins allow neural crest cells to interact with each other during their migration. Pax3 appears to be important for proliferation of neural crest precursors, and connexin-43-mediated gap junction communication influences the rate of migration. Endothelin and its receptors are required for normal postmigratory differentiation. Platelet-derived growth factor and retinoic acid have roles in neural crest migration and differentiation as well. Finally, the similarity between the cardiovascular malformations seen in the DiGeorge and 22q11 deletion syndromes and animal models of neural crest deficiency has led to the examination of the role of genes located near or within the DiGeorge critical region in neural crest migration.

Analyses of the effects that disease-causing missense mutations have on the structure and function of the winged-helix protein FOXC1

Five missense mutations of the winged-helix FOXC1 transcription factor, found in patients with Axenfeld-Rieger (AR) malformations, were investigated for their effects on FOXC1 structure and function. Molecular modeling of the FOXC1 forkhead domain predicted that the missense mutations did not alter FOXC1 structure. Biochemical analyses indicated that, whereas all mutant proteins correctly localize to the cell nucleus, the I87M mutation reduced FOXC1-protein levels. DNA-binding experiments revealed that, although the S82T and S131L mutations decreased DNA binding, the F112S and I126M mutations did not. However, the F112S and I126M mutations decrease the transactivation ability of FOXC1. All the FOXC1 mutations had the net effect of reducing FOXC1 transactivation ability. These results indicate that the FOXC1 forkhead domain contains separable DNA-binding and transactivation functions. In addition, these findings demonstrate that reduced stability, DNA binding, or transactivation, all causing a decrease in the ability of FOXC1 to transactivate genes, can underlie AR malformations.

Cross-talk in kidney development

As in most organs, the emerging theme in kidney development is the importance of cross-talk between several tissues and cell lineages to allow morphogenesis to proceed in a complex but highly regulated way. Over the past few years, knock-out and transgenic analyses in mice and evolutionary comparison with non-mammalian species have been particularly instrumental in identifying molecules with crucial functions for tissue-tissue interactions. The transcription factors Wt1 and Eya1, the signalling molecules Gdnf and LIF and the receptors c-Ret and GdnfRalpha have been demonstrated to fulfil fundamental roles in the first step of metanephric induction, the outgrowth of the ureter. Signalling by members of the Wnt, BMP and FGF families, regulated by transcription factors such as Pax2, mediates nephrogenesis by adjusting the balance between the ureteric bud epithelium, stromal and nephrogenic tissues. The stromal tissue, neglected for many years, has been shown to serve important functions in regulating the growth of nephrons. Finally, we have also begun to gain insight into the molecular events underlying patterning of the nephron into distinct functional units including glomerulus, proximal and distal tubule.

Murine forkhead/winged helix genes Foxc1 (Mf1) and Foxc2 (Mfh1) are required for the early organogenesis of the kidney and urinary tract

The murine genes, Foxc1 and Foxc2 (previously, Mf1 and Mfh1), encode forkhead/winged helix transcription factors with virtually identical DNA-binding domains and overlapping expression patterns in various embryonic tissues. Foxc1/Mf1 is disrupted in the mutant, congenital hydrocephalus (Foxc1/Mf1(ch)), which has multiple developmental defects. We show here that, depending on the genetic background, most Foxc1 homozygous mutants are born with abnormalities of the metanephric kidney, including duplex kidneys and double ureters, one of which is a hydroureter. Analysis of embryos reveals that Foxc1 homozygotes have ectopic mesonephric tubules and ectopic anterior ureteric buds. Moreover, expression in the intermediate mesoderm of Glial cell-derived neurotrophic factor (Gdnf), a primary inducer of the ureteric bud, is expanded more anteriorly in Foxc1 homozygous mutants compared with wild type. These findings support the hypothesis of Mackie and Stephens concerning the etiology of duplex kidney and hydroureter in human infants with congenital kidney abnormalities (Mackie, G. G. and Stephens, F. G. (1975) J. Urol. 114, 274–280). Previous studies established that most Foxc1(lacZ)Foxc2(tm1) compound heterozygotes have the same spectrum of cardiovascular defects as single homozygous null mutants, demonstrating interaction between the two genes in the cardiovascular system. Here, we show that most compound heterozygotes have hypoplastic kidneys and a single hydroureter, while all heterozygotes are normal. This provides evidence that the two genes interact in kidney as well as heart development.

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.

Roles for the winged helix transcription factors MF1 and MFH1 in cardiovascular development revealed by nonallelic noncomplementation of null alleles

The murine Mf1 and Mfh1 genes have overlapping patterns of expression in the embryo and encode forkhead/winged helix transcription factors with virtually identical DNA binding domains. Previous studies have shown that Mfh1 null mutants have severe cardiovascular defects, including interruptions and coarctations of the aortic arch and ventricular septal defects (Iida et al., Development 124, 4627-4638, 1997). Here, we show that Mf1(lacZ) homozygous null mutants also have a similar spectrum of cardiovascular abnormalities. Moreover, most embryos doubly heterozygous for Mfh1(tm1) and Mf1(lacZ) die before birth with interruptions and coarctations of the aortic arch, dysgenesis of the aortic and pulmonary valves, ventricular septal defects, and other cardiac anomalies. This nonallelic noncomplementation and the similar patterns of expression of the two genes in the mesenchyme and endothelial cells of the branchial arches, outflow tract, and heart suggest that Mf1 and Mfh1 play interactive roles in the morphogenesis of the cardiovascular system. Implications for the development of human congenital heart defects are discussed.

Expression of the Mf1 gene in developing mouse hearts: implication in the development of human congenital heart defects

The transcription factor FKHL7 gene has recently been associated with the anterior segment dysgenesis disorder of the eye known as Axenfeld-Rieger anomaly (ARA). A growing body of evidence indicates that mutations in FKHL7 cause not only defects in the anterior segment of the eye but defects in the heart valves and septa as well. In order to evaluate its contribution to normal heart septation and valve formation, expression of the mouse homologue Mf1 in embryonic hearts was analyzed by in situ hybridization. A weak but significant level of Mf1 expression could be detected in the endocardium of mouse embryos as early as day 8.5 post-conception (p.c.). Mf1 expression was undetectable in the hearts of day 9.5 p.c. embryos, but by day 10.5-11 p.c., Mf1 transcripts could be found again in the endocardium of both the atrium and ventricle and a relatively strong signal was observed in the dorsal portion of the septum primum, in what appeared to be the spinal vestibule. At day 13 p.c. when aortic and pulmonary trunks are separated, relatively more Mf1 transcripts were detected in the leaflets of aortic, pulmonary, and venous valves, the ventral portion of the septum primum, as well as in the single layer of cells on the edges of the atrioventricular cushion tissues. Surprisingly, there was no signal detected in the developing interventricular septum. At day 15 p.c., overall Mf1 signals were greatly decreased. However, significant levels of expression could still be observed in the atrial septum, the tricuspid valve, the mitral valve, and in the venous valve but not in the interventricular septum. The temporal and spatial expression patterns of the Mf1 gene in developing mouse hearts suggest that Mf1 may play a critical role in the formation of valves and septa with the exception of the interventricular septum. This is further supported by our studies showing that mutations in the FKHL7 gene were associated with defects in the anterior segment of the eye as well as atrial septal defects or mitral valve defects. Dev Dyn 1999;216:16-27.

Expression pattern of the winged helix factor XFD-11 during Xenopus embryogenesis

We have identified a cDNA encoding a novel Xenopus winged helix transcription factor termed as XFD-11 (Xenopus fork head domain). The DNA binding domain is most closely related to those of human or murine FREAC-3 (FKHL7/MF-1/FKH-1) proteins. The XFD-11 gene is activated at late blastula/early gastrula and transcription proceeds throughout embryogenesis. Early expression is found in ventral and lateral but not in dorsal mesoderm. At neurula stages, transcripts are found in posterior mesoderm except for the dorsal midline, and the gene is also transcribed at the lateral border of the neural plate and within anterior neuroectoderm. At later stages of development, transcripts are detected within the pronephros, the heart, within neural crest cells surrounding the eye, in the mandibular, hyoid and branchial arches, and within the tail.