Opposite phenotypes of hypomorphic and Y766 phosphorylation site mutations reveal a function for Fgfr1 in anteroposterior patterning of mouse embryos

Fibroblast growth factor receptor signaling is essential for normal mammary gland development and stem cell function

Fibroblast growth factor (FGF) signaling plays an important role in embryonic stem cells and adult tissue homeostasis, but the function of FGFs in mammary gland stem cells is less well defined. Both FGFR1 and FGFR2 are expressed in basal and luminal mammary epithelial cells (MECs), suggesting that together they might play a role in mammary gland development and stem cell dynamics. Previous studies have demonstrated that the deletion of FGFR2 resulted only in transient developmental defects in branching morphogenesis. Using a conditional deletion strategy, we investigated the consequences of FGFR1 deletion alone and then the simultaneous deletion of both FGFR1 and FGFR2 in the mammary epithelium. FGFR1 deletion using a keratin 14 promoter-driven Cre-recombinase resulted in an early, yet transient delay in development. However, no reduction in functional outgrowth potential was observed following limiting dilution transplantation analysis. In contrast, a significant reduction in outgrowth potential was observed upon the deletion of both FGFR1 and FGFR2 in MECs using adenovirus-Cre. Additionally, using a fluorescent reporter mouse model to monitor Cre-mediated recombination, we observed a competitive disadvantage following transplantation of both FGFR1/R2-null MECs, most prominently in the basal epithelial cells. This correlated with the complete loss of the mammary stem cell repopulating population in the FGFR1/R2-attenuated epithelium. FGFR1/R2-null MECs were partially rescued in chimeric outgrowths containing wild-type MECs, suggesting the potential importance of paracrine mechanisms involved in the maintenance of the basal epithelial stem cells. These studies document the requirement for functional FGFR signaling in mammary stem cells during development.

Signals and switches in Mammalian neural crest cell differentiation

Neural crest cells (NCCs) comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types during vertebrate development. These include cartilage and bone, tendons, and connective tissue, as well as neurons, glia, melanocytes, and endocrine and adipose cells; this remarkable lineage potential persists into adult life. Taken together with a limited capacity for self-renewal, neural crest cells bear the hallmarks of stem and progenitor cells and are considered to be synonymous with vertebrate evolution. The neural crest has provided a system for exploring the mechanisms that govern developmental processes such as morphogenetic induction, cell migration, and fate determination. Today, much of the focus on neural crest cells revolves around their stem cell-like characteristics and potential for use in regenerative medicine. A thorough understanding of the signals and switches that govern mammalian neural crest patterning is central to potential therapeutic application of these cells and better appreciation of the role that neural crest cells play in vertebrate evolution, development, and disease.

LoxP-FRT Trap (LOFT): a simple and flexible system for conventional and reversible gene targeting

Background

Conditional gene knockout (cKO) mediated by the Cre/LoxP system is indispensable for exploring gene functions in mice. However, a major limitation of this method is that gene KO is not reversible. A number of methods have been developed to overcome this, but each method has its own limitations.

Results

We describe a simple method we have named LOFT [LoxP-flippase (FLP) recognition target (FRT) Trap], which is capable of reversible cKO and free of the limitations associated with existing techniques. This method involves two alleles of a target gene: a standard floxed allele, and a multi-functional allele bearing an FRT-flanked gene-trap cassette, which inactivates the target gene while reporting its expression with green fluorescent protein (GFP); the trapped allele is thus a null and GFP reporter by default, but is convertible into a wild-type allele. The floxed and trapped alleles can typically be generated using a single construct bearing a gene-trap cassette doubly flanked by LoxP and FRT sites, and can be used independently to achieve conditional and constitutive gene KO, respectively. More importantly, in mice bearing both alleles and also expressing the Cre and FLP recombinases, sequential function of the two enzymes should lead to deletion of the target gene, followed by restoration of its expression, thus achieving reversible cKO. LOFT should be generally applicable to mouse genes, including the growing numbers of genes already floxed; in the latter case, only the trapped alleles need to be generated to confer reversibility to the pre-existing cKO models. LOFT has other applications, including the creation and reversal of hypomorphic mutations. In this study we proved the principle of LOFT in the context of T-cell development, at a hypomorphic allele of Baf57/Smarce1 encoding a subunit of the chromatin-remodeling Brg/Brahma-associated factor (BAF) complex. Interestingly, the FLP used in the current work caused efficient reversal in peripheral T cells but not thymocytes, which is advantageous for studying developmental epigenetic programming of T-cell functions, a fundamental issue in immunology.

Conclusions

LOFT combines well-established basic genetic methods into a simple and reliable method for reversible gene targeting, with the flexibility of achieving traditional constitutive and conditional KO.

Signaling by FGF4 and FGF8 is required for axial elongation of the mouse embryo

Fibroblast growth factor (FGF) signaling has been shown to play critical roles in vertebrate segmentation and elongation of the embryonic axis. Neither the exact roles of FGF signaling, nor the identity of the FGF ligands involved in these processes, has been conclusively determined. Fgf8 is required for cell migration away from the primitive streak when gastrulation initiates, but previous studies have shown that drastically reducing the level of FGF8 later in gastrulation has no apparent effect on somitogenesis or elongation of the embryo. In this study, we demonstrate that loss of both Fgf8 and Fgf4 expression during late gastrulation resulted in a dramatic skeletal phenotype. Thoracic vertebrae and ribs had abnormal morphology, lumbar and sacral vertebrae were malformed or completely absent, and no tail vertebrae were present. The expression of Wnt3a in the tail and the amount of nascent mesoderm expressing Brachyury were both severely reduced. Expression of genes in the NOTCH signaling pathway involved in segmentation was significantly affected, and somite formation ceased after the production of about 15-20 somites. Defects seen in the mutants appear to result from a failure to produce sufficient paraxial mesoderm, rather than a failure of mesoderm precursors to migrate away from the primitive streak. Although the epiblast prematurely decreases in size, we did not detect evidence of a change in the proliferation rate of cells in the tail region or excessive apoptosis of epiblast or mesoderm cells. We propose that FGF4 and FGF8 are required to maintain a population of progenitor cells in the epiblast that generates mesoderm and contributes to the stem cell population that is incorporated in the tailbud and required for axial elongation of the mouse embryo after gastrulation.

Ndrg2 regulates vertebral specification in differentiating somites

It is generally thought that vertebral patterning and identity are globally determined prior to somite formation. Relatively little is known about the regulators of vertebral specification after somite segmentation. Here, we demonstrated that Ndrg2, a tumor suppressor gene, was dynamically expressed in the presomitic mesoderm (PSM) and at early stage of differentiating somites. Loss of Ndrg2 in mice resulted in vertebral homeotic transformations in thoracic/lumbar and lumbar/sacral transitional regions in a dose-dependent manner. Interestingly, the inactivation of Ndrg2 in osteoblasts or chondrocytes caused defects resembling those observed in Ndrg2(-/-) mice, with a lower penetrance. In addition, forced overexpression of Ndrg2 in osteoblasts or chondrocytes also conferred vertebral defects, which were distinct from those in Ndrg2(-/-) mice. These genetic analyses revealed that Ndrg2 modulates vertebral identity in segmented somites rather than in the PSM. At the molecular level, combinatory alterations of the amount of Hoxc8-11 gene transcripts were detected in the differentiating somites of Ndrg2(-/-) embryos, which may partially account for the vertebral defects in Ndrg2 mutants. Nevertheless, Bmp/Smad signaling activity was elevated in the differentiating somites of Ndrg2(-/-) embryos. Collectively, our findings unveiled Ndrg2 as a novel regulator of vertebral specification in differentiating somites.

Fibroblast growth factor signaling deficiencies impact female reproduction and kisspeptin neurons in mice

Fibroblast growth factor (FGF) signaling is essential for the development of the gonadotropin-releasing hormone (GnRH) system. Mice harboring deficiencies in Fgf8 or Fgf receptor 1 (Fgfr1) suffer a significant loss of GnRH neurons, but their reproductive phenotypes have not been examined. This study examined if female mice hypomorphic for Fgf8, Fgfr1, or both (compound hypomorphs) exhibited altered parameters of pubertal onset, estrous cyclicity, and fertility. Further, we examined the number of kisspeptin (KP)-immunoreactive (ir) neurons in the anteroventral periventricular/periventricular nuclei (AVPV/PeV) of these mice to assess if changes in the KP system, which stimulates the GnRH system, could contribute to the reproductive phenotypes. Single hypomorphs (Fgfr1(+/-) or Fgf8(+/-)) had normal timing for vaginal opening (VO) but delayed first estrus. However, after achieving the first estrus, they underwent normal expression of estrous cycles. In contrast, the compound hypomorphs underwent early VO and normal first estrus, but had disorganized estrous cycles that subsequently reduced their fertility. KP immunohistochemistry on Postnatal Day 15, 30, and 60 transgenic female mice revealed that female compound hypomorphs had significantly more KP-ir neurons in the AVPV/PeV compared to their wild-type littermates, suggesting increased KP-ir neurons may drive early VO but could not maintain the cyclic changes in GnRH neuronal activity required for female fertility. Overall, these data suggest that Fgf signaling deficiencies differentially alter the parameters of female pubertal onset and cyclicity. Further, these deficiencies led to changes in the AVPV/PeV KP-ir neurons that may have contributed to the accelerated VO in the compound hypomorphs.

Cell-autonomous FGF signaling regulates anteroposterior patterning and neuronal differentiation in the mesodiencephalic dopaminergic progenitor domain

The structure and projection patterns of adult mesodiencephalic dopaminergic (DA) neurons are one of the best characterized systems in the vertebrate brain. However, the early organization and development of these nuclei remain poorly understood. The induction of midbrain DA neurons requires sonic hedgehog (Shh) from the floor plate and fibroblast growth factor 8 (FGF8) from the isthmic organizer, but the way in which FGF8 regulates DA neuron development is unclear. We show that, during early embryogenesis, mesodiencephalic neurons consist of two distinct populations: a diencephalic domain, which is probably independent of isthmic FGFs; and a midbrain domain, which is dependent on FGFs. Within these domains, DA progenitors and precursors use partly different genetic programs. Furthermore, the diencephalic DA domain forms a distinct cell population, which also contains non-DA Pou4f1(+) cells. FGF signaling operates in proliferative midbrain DA progenitors, but is absent in postmitotic DA precursors. The loss of FGFR1/2-mediated signaling results in a maturation failure of the midbrain DA neurons and altered patterning of the midbrain floor. In FGFR mutants, the DA domain adopts characteristics that are typical for embryonic diencephalon, including the presence of Pou4f1(+) cells among TH(+) cells, and downregulation of genes typical of midbrain DA precursors. Finally, analyses of chimeric embryos indicate that FGF signaling regulates the development of the ventral midbrain cell autonomously.

Microtia: epidemiology and genetics

Microtia is a congenital anomaly of the ear that ranges in severity from mild structural abnormalities to complete absence of the ear, and can occur as an isolated birth defect or as part of a spectrum of anomalies or a syndrome. Microtia is often associated with hearing loss and patients typically require treatment for hearing impairment and surgical ear reconstruction. The reported prevalence varies among regions, from 0.83 to 17.4 per 10,000 births, and the prevalence is considered to be higher in Hispanics, Asians, Native Americans, and Andeans. The etiology of microtia and the cause of this wide variability in prevalence are poorly understood. Strong evidence supports the role of environmental and genetic causes for microtia. Although some studies have identified candidate genetic variants for microtia, no causal genetic mutation has been confirmed. The application of novel strategies in developmental biology and genetics has facilitated elucidation of mechanisms controlling craniofacial development. In this paper we review current knowledge of the epidemiology and genetics of microtia, including potential candidate genes supported by evidence from human syndromes and animal models. We also discuss the possible etiopathogenesis in light of the hypotheses formulated to date: Neural crest cells disturbance, vascular disruption, and altitude.

Alternative Splicing of Fibroblast Growth Factor Receptor IgIII Loops in Cancer

Alternative splicing of the IgIII loop of fibroblast growth factor receptors (FGFRs) 1–3 produces b- and c-variants of the receptors with distinctly different biological impact based on their distinct ligand-binding spectrum. Tissue-specific expression of these splice variants regulates interactions in embryonic development, tissue maintenance and repair, and cancer. Alterations in FGFR2 splicing are involved in epithelial mesenchymal transition that produces invasive, metastatic features during tumor progression. Recent research has elucidated regulatory factors that determine the splice choice both on the level of exogenous signaling events and on the RNA-protein interaction level. Moreover, methodology has been developed that will enable the in depth analysis of splicing events during tumorigenesis and provide further insight on the role of FGFR 1–3 IIIb and IIIc in the pathophysiology of various malignancies. This paper aims to summarize expression patterns in various tumor types and outlines possibilities for further analysis and application.

FGFs: Neurodevelopment's Jack-of-all-Trades - How Do They Do it?

From neurulation to postnatal processes, the requirements for FGF signaling in many aspects of neural precursor cell biology have been well documented. However, identifying a requirement for FGFs in a particular neurogenic process provides only an initial and superficial understanding of what FGF signaling is doing. How FGFs specify cell types in one instance, yet promote cell survival, proliferation, migration, or differentiation in other instances remains largely unknown and is key to understanding how they function. This review describes what we have learned primarily from in vivo vertebrate studies about the roles of FGF signaling in neurulation, anterior–posterior patterning of the neural plate, brain patterning from local signaling centers, and finally neocortex development as an example of continued roles for FGFs within the same brain area. The potential explanations for the diverse functions of FGFs through differential interactions with cell intrinsic and extrinsic factors is then discussed with an emphasis on how little we know about the modulation of FGF signaling in vivo. A clearer picture of the mechanisms involved is nevertheless essential to understand the behavior of neural precursor cells and to potentially guide their fates for therapeutic purposes.

Vitamin A: a multifunctional tool for development

Extensive research carried out over the last 100 years has established that the fat-soluble organic compound vitamin A plays crucial roles in early development, organogenesis, cell proliferation, differentiation and apoptosis as well as in tissue homeostasis. Given its importance during development, the delivery of vitamin A to the embryo is very tightly regulated with perturbations leading to severe malformations. This review discusses the roles of vitamin A during human development and the molecular mechanisms controlling its biological effects, hence bridging the gap between human development and molecular genetic work carried out in animal models. Vitamin A delivery during pregnancy and its developmental teratology in humans are thus discussed alongside work on model organisms, such as chicken or mice, revealing the molecular layout and functions of vitamin A metabolism and signaling. We conclude that, during development, vitamin A-derived signals are very tightly controlled in time and space and that this complex regulation is achieved by elaborate autoregulatory loops and by sophisticated interactions with other signaling cascades.

Breaking evolutionary and pleiotropic constraints in mammals: On sloths, manatees and homeotic mutations

Background

Mammals as a rule have seven cervical vertebrae, except for sloths and manatees. Bateson proposed that the change in the number of cervical vertebrae in sloths is due to homeotic transformations. A recent hypothesis proposes that the number of cervical vertebrae in sloths is unchanged and that instead the derived pattern is due to abnormal primaxial/abaxial patterning.

Results

We test the detailed predictions derived from both hypotheses for the skeletal patterns in sloths and manatees for both hypotheses. We find strong support for Bateson's homeosis hypothesis. The observed vertebral and rib patterns cannot be explained by changes in primaxial/abaxial patterning. Vertebral patterns in sloths and manatees are similar to those in mice and humans with abnormal numbers of cervical vertebrae: incomplete and asymmetric homeotic transformations are common and associated with skeletal abnormalities. In sloths the homeotic vertebral shift involves a large part of the vertebral column. As such, similarity is greatest with mice mutant for genes upstream of Hox.

Conclusions

We found no skeletal abnormalities in specimens of sister taxa with a normal number of cervical vertebrae. However, we always found such abnormalities in conspecifics with an abnormal number, as in many of the investigated dugongs. These findings strongly support the hypothesis that the evolutionary constraints on changes of the number of cervical vertebrae in mammals is due to deleterious pleitropic effects. We hypothesize that in sloths and manatees low metabolic and activity rates severely reduce the usual stabilizing selection, allowing the breaking of the pleiotropic constraints. This probably also applies to dugongs, although to a lesser extent.

A missense mutation in Fgfr1 causes ear and skull defects in hush puppy mice

The hush puppy mouse mutant has been shown previously to have skull and outer, middle, and inner ear defects, and an increase in hearing threshold. The fibroblast growth factor receptor 1 (Fgfr1) gene is located in the region of chromosome 8 containing the mutation. Sequencing of the gene in hush puppy heterozygotes revealed a missense mutation in the kinase domain of the protein (W691R). Homozygotes were found to die during development, at approximately embryonic day 8.5, and displayed a phenotype similar to null mutants. Reverse transcription PCR indicated a decrease in Fgfr1 transcript in heterozygotes and homozygotes. Generation of a construct containing the mutation allowed the function of the mutated receptor to be studied. Immunocytochemistry showed that the mutant receptor protein was present at the cell membrane, suggesting normal expression and trafficking. Measurements of changes in intracellular calcium concentration showed that the mutated receptor could not activate the IP₃ pathway, in contrast to the wild-type receptor, nor could it initiate activation of the Ras/MAP kinase pathway. Thus, the hush puppy mutation in fibroblast growth factor receptor 1 appears to cause a loss of receptor function. The mutant protein appears to have a dominant negative effect, which could be due to it dimerising with the wild-type protein and inhibiting its activity, thus further reducing the levels of functional protein. A dominant modifier, Mhspy, which reduces the effect of the hush puppy mutation on pinna and stapes development, has been mapped to the distal end of chromosome 7 and may show imprinting.

Signaling networks in RUNX2-dependent bone development

RUNX2 is an essential transcription factor for osteoblast differentiation and chondrocyte maturation. SP7, another transcription factor, is required for osteoblast differentiation. Major signaling pathways, including FGF, Wnt, and IHH, also play important roles in skeletal development. RUNX2 regulates Sp7 expression at an early stage of osteoblast differentiation. FGF2 upregulates Runx2 expression and activates RUNX2, and gain-of-function mutations of FGFRs cause craniosynostosis and limb defect with upregulation of Runx2 expression. Wnt signaling upregulates Runx2 expression and activates RUNX2, and RUNX2 induces Tcf7 expression. IHH is required for Runx2 expression in osteoprogenitor cells during endochondral bone development, and RUNX2 directly regulates Ihh expression in chondrocytes. Thus, RUNX2 regulates osteoblast differentiation and chondrocyte maturation through the network with SP7 and with FGF, Wnt, and IHH signaling pathways during skeletal development.

FGF signaling gradient maintains symmetrical proliferative divisions of midbrain neuronal progenitors

For the correct development of the central nervous system, the balance between self-renewing and differentiating divisions of the neuronal progenitors must be tightly regulated. To maintain their self-renewing identity, the progenitors need to retain both apical and basal interfaces. However, the identities of fate-determining signals which cells receive via these connections, and the exact mechanism of their action, are poorly understood. The conditional inactivation of Fibroblast growth factor (FGF) receptors 1 and 2 in the embryonic mouse midbrain-hindbrain area results in premature neuronal differentiation. Here, we aim to elucidate the connection between FGF signaling and neuronal progenitor maintenance. Our results reveal that the loss of FGF signaling leads to downregulation of Hes1 and upregulation of Ngn2, Dll1, and p57 in the ventricular zone (VZ) cells, and that this increased neurogenesis occurs cell-autonomously. Yet the cell cycle progression, apico-basal-polarity, cell-cell connections, and the positioning of mitotic spindle in the mutant VZ appear unaltered. Interestingly, FGF8-protein is highly concentrated in the basal lamina. Thus, FGFs may act through basal processes of neuronal progenitors to maintain their progenitor status. Indeed, midbrain neuronal progenitors deprived in vitro of FGFs switched from symmetrical proliferative towards symmetrical neurogenic divisions. We suggest that FGF signaling in the midbrain VZ is cell-autonomously required for the maintenance of symmetrical proliferative divisions via Hes1-mediated repression of neurogenic genes.

FGF signalling: diverse roles during early vertebrate embryogenesis

Fibroblast growth factor (FGF) signalling has been implicated during several phases of early embryogenesis, including the patterning of the embryonic axes, the induction and/or maintenance of several cell lineages and the coordination of morphogenetic movements. Here, we summarise our current understanding of the regulation and roles of FGF signalling during early vertebrate development.

Signaling gradients during paraxial mesoderm development

The sequential formation of somites along the anterior-posterior axis is under control of multiple signaling gradients involving the Wnt, FGF, and retinoic acid (RA) pathways. These pathways show graded distribution of signaling activity within the paraxial mesoderm of vertebrate embryos. Although Wnt and FGF signaling show highest activity in the posterior, unsegmented paraxial mesoderm (presomitic mesoderm [PSM]), RA signaling establishes a countergradient with the highest activity in the somites. The generation of these graded activities relies both on classical source-sink mechanisms (for RA signaling) and on an RNA decay mechanism (for FGF signaling). Numerous studies reveal the tight interconnection among Wnt, FGF, and RA signaling in controlling paraxial mesoderm differentiation and in defining the somite-forming unit. In particular, the relationship to a molecular oscillator acting in somite precursors in the PSM-called the segmentation clock-has been recently addressed. These studies indicate that high levels of Wnt and FGF signaling are required for the segmentation clock activity. Furthermore, we discuss how these signaling gradients act in a dose-dependent manner in the progenitors of the paraxial mesoderm, partly by regulating cell movements during gastrulation. Finally, links between the process of axial specification of vertebral segments and Hox gene expression are discussed.

Cdx mutant axial progenitor cells are rescued by grafting to a wild type environment

Cdx transcription factors are required for axial extension. Cdx genes are expressed in the posterior growth zone, a region that supplies new cells for axial elongation. Cdx2(+/-)Cdx4(-/-) (Cdx2/4) mutant embryos show abnormalities in axis elongation from E8.5, culminating in axial truncation at E10.5. These data raised the possibility that the long-term axial progenitors of Cdx mutants are intrinsically impaired in their ability to contribute to posterior growth. We investigated whether we could identify cell-autonomous defects of the axial progenitor cells by grafting mutant cells into a wild type growth zone environment. We compared the contribution of GFP labeled mutant and wild type progenitors grafted to unlabeled wild type recipients subsequently cultured over the period during which Cdx2/4 defects emerge. Descendants of grafted cells were scored for their contribution to differentiated tissues in the elongating axis and to the posterior growth zone. No difference between the contribution of descendants from wild type and mutant grafted progenitors was detected, indicating that rescue of the Cdx mutant progenitors by the wild type recipient growth zone is provided non-cell autonomously. Recently, we showed that premature axial termination of Cdx mutants can be partly rescued by stimulating canonical Wnt signaling in the posterior growth zone. Taken together with the data shown here, this suggests that Cdx genes function to maintain a signaling-dependent niche for the posterior axial progenitors.

Compound deficiencies in multiple fibroblast growth factor signalling components differentially impact the murine gonadotrophin-releasing hormone system

Gonadotrophin-releasing hormone (GnRH) neurones control the onset and maintenance of fertility. Aberrant development of the GnRH system underlies infertility in Kallmann syndrome [KS; idiopathic hypogonadotropic hypogonadism (IHH) and anosmia]. Some KS patients harbour mutations in the fibroblast growth factor receptor 1 (Fgfr1) and Fgf8 genes. The biological significance of these two genes in GnRH neuronal development was corroborated by the observation that GnRH neurones were severely reduced in newborn transgenic mice deficient in either gene. In the present study, we hypothesised that the compound deficiency of Fgf8 and its cognate receptors, Fgfr1 and Fgfr3, may lead to more deleterious effects on the GnRH system, thereby resulting in a more severe reproductive phenotype in patients harbouring these mutations. This hypothesis was tested by counting the number of GnRH neurones in adult transgenic mice with digenic heterozygous mutations in Fgfr1/Fgf8, Fgfr3/Fgf8 or Fgfr1/Fgfr3. Monogenic heterozygous mutations in Fgfr1, Fgf8 or Fgfr3 caused a 30-50% decrease in the total number of GnRH neurones. Interestingly, mice with digenic mutations in Fgfr1/Fgf8 showed a greater decrease in GnRH neurones compared to mice with a heterozygous defect in the Fgfr1 or Fgf8 alone. This compounding effect was not detected in mice with digenic heterozygous mutations in Fgfr3/Fgf8 or Fgfr1/Fgfr3. These results support the hypothesis that IHH/KS patients with digenic mutations in Fgfr1/Fgf8 may have a further reduction in the GnRH neuronal population compared to patients harbouring monogenic haploid mutations in Fgfr1 or Fgf8. Because only Fgfr1/Fgf8 compound deficiency leads to greater GnRH system defect, this also suggests that these fibroblast growth factor signalling components interact in a highly specific fashion to support GnRH neuronal development.

Role of fibroblast growth factor signaling in gonadotropin-releasing hormone neuronal system development

There is growing evidence demonstrating that fibroblast growth factor (FGF) signaling is important for the development of the gonadotropin-releasing hormone (GnRH) neuronal system. In humans, loss-of-function mutations in FGF receptor 1 (Fgfr1) and Fgf8 lead to hypogonadotropic hypogonadism (HH) with or without anosmia. Insights into how FGF signaling deficiency disrupts the GnRH system in humans are beginning to emerge from studies using transgenic mouse models. In this review, we summarize GnRH system defects in several lines of FGF signaling-deficient mice. We showed that FGF signaling is critically required for olfactory placode induction, differentiation, and GnRH neuronal fate specification and postnatal maintenance. Extrapolating from these transgenic mouse data, we suggest that idiopathic HH in patients harboring loss-of-function Fgfr1 and/or Fgf8 mutations is not merely a result of defective GnRH neuronal migration, but also insults accumulated in the GnRH system during fate specification and postnatal development.

A case of Kallmann syndrome carrying a missense mutation in alternatively spliced exon 8A encoding the immunoglobulin-like domain IIIb of fibroblast growth factor receptor 1

Fibroblast growth factor receptor 1 (FGFR1) is one of the causative genes for Kallmann syndrome (KS), which is characterized by isolated hypogonadotropic hypogonadism with anosmia/hyposmia. The third immunoglobulin-like domain (D3) of FGFR1 has the isoforms FGFR1-IIIb and FGFR1-IIIc, which are generated by alternative splicing of exons 8A and 8B, respectively. To date, the only mutations to have been identified in D3 of FGFR1 are in exon 8B. We performed mutation analysis of FGFR1 in a 23-year-old female patient with KS and found a missense mutation (c.1072C>T) in exon 8A of FGFR1. The c.1072C>T mutation was not detected in her family members or in 220 normal Japanese and 100 Caucasian female controls. No mutation in other KS genes, KS 1, prokineticin-2, prokineticin receptor-2 and FGF-8 was detected in the affected patient or in her family members. Therefore, this is the first case of KS carrying a de novo missense mutation in FGFR1 exon 8A, suggesting that isoform FGFR1-IIIb, as well as isoform FGFR1-IIIc, plays a crucial role in the pathogenesis of KS.

Hox genes and segmentation of the hindbrain and axial skeleton

Segmentation is an important process that is frequently used during development to segregate groups of cells with distinct features. Segmental compartments provide a mechanism for generating and organizing regional properties along an embryonic axis and within tissues. In vertebrates the development of two major systems, the hindbrain and the paraxial mesoderm, displays overt signs of compartmentalization and depends on the process of segmentation for their functional organization. The hindbrain plays a key role in regulating head development, and it is a complex coordination center for motor activity, breathing rhythms, and many unconscious functions. The paraxial mesoderm generates somites, which give rise to the axial skeleton. The cellular processes of segmentation in these two systems depend on ordered patterns of Hox gene expression as a mechanism for generating a combinatorial code that specifies unique identities of the segments and their derivatives. In this review, we compare and contrast the signaling inputs and transcriptional mechanisms by which Hox gene regulatory networks are established during segmentation in these two different systems.

Redefining the progression of lineage segregations during mammalian embryogenesis by clonal analysis

Clonal lineage information is fundamental in revealing cell fate choices. Using genetic single-cell labeling in utero, we investigated lineage segregations during anteroposterior axis formation in mouse. We show that while endoderm and surface ectoderm segregate during gastrulation, neural ectoderm and mesoderm share a common progenitor persisting through all stages of axis elongation. These data challenge the paradigm that the three germ layers, formed by gastrulation, constitute the primary branchpoints in differentiation of the pluripotent epiblast toward tissue-specific precursors. Bipotent neuromesodermal progenitors show self-renewing characteristics and may represent the cellular substrate coupling sustained axial elongation and coordinated differentiation of these tissues. These findings have important implications for the interpretation of the phenotypic defects of several mouse mutants and the directed differentiation of embryonic stem (ES) cells in vitro.

Temporal and spatial expression of FGF ligands and receptors during Xenopus development

Fibroblast growth factor (FGF) signalling plays a major role during early vertebrate development. It is involved in the specification of the mesoderm, control of morphogenetic movements, patterning of the anterior-posterior axis, and neural induction. In mammals, 22 FGF ligands have been identified, which can be grouped into seven subfamilies according to their sequence homology and function. We have cloned 17 fgf genes from Xenopus tropicalis and have analysed their temporal expression by RT-PCR and spatial expression by whole mount in situ hybridisation at key developmental stages. It reveals the diverse expression pattern of fgf genes during early embryonic development. Furthermore, our analysis shows the transient nature of expression of several fgfs in a number of embryonic tissues. This study constitutes the most comprehensive description of the temporal and spatial expression pattern of fgf ligands and receptors during vertebrate development to date. Developmental Dynamics 238:1467-1479, 2009. (c) 2009 Wiley-Liss, Inc.

Stem cells, signals and vertebrate body axis extension

The progressive generation of chick and mouse axial tissues - the spinal cord, skeleton and musculature of the body - has long been proposed to depend on the activity of multipotent stem cells. Here, we evaluate evidence for the existence and multipotency of axial stem cells. We show that although the data strongly support their existence, there is little definitive information about their multipotency or extent of contribution to the axis. We also review the location and molecular characteristics of these putative stem cells, along with their evolutionary conservation in vertebrates and the signalling mechanisms that regulate and arrest axis extension.

Evidence that Fgf10 contributes to the skeletal and visceral defects of an Apert syndrome mouse model

Apert syndrome (AS) is a severe congenital disease caused by mutations in fibroblast growth factor receptor-2 (FGFR2), and characterised by craniofacial, limb, visceral, and neural abnormalities. AS-type FGFR2 molecules exert a gain-of-function effect in a ligand-dependent manner, but the causative FGFs and their relative contribution to each of the abnormalities observed in AS remains unknown. We have generated mice that harbour an AS mutation but are deficient in or heterozygous for Fgf10. The genetic knockdown of Fgf10 can rescue the skeletal as well as some of the visceral defects observed in this AS model, and restore a near normal level of FgfR2 signaling involving an apparent switch between ERK(p44/p42) and p38 phosphorylation. Surprisingly, it can also yield de novo cleft palate and blind colon in a subset of the compound mutants. These findings strongly suggest that Fgf10 contributes to AS-like pathologies and highlight a complexity of Fgf10 function in different tissues.

Induction of fibroblast growth factor 10 (FGF10) in the ileal crypt epithelium after massive small bowel resection suggests a role for FGF10 in gut adaptation

We have previously reported that fibroblast growth factor 10 (FGF10) is crucial for the survival and proliferation of progenitor cells during embryonic gastrointestinal development. We sought to characterize the potential role of FGF10 signaling in the adaptive response following small bowel resection. Adult wild-type and Fgf10(LacZ) mice underwent 50% small bowel resection (SBR) or sham operation. Tissues were harvested 24 or 48 hr after surgery for histology, immunohistochemistry, and in situ hybridization. After SBR, Fgf10 expression was demonstrated in the epithelium at the base of the crypts. Moreover, there was a statistically significant increase in proliferating cells and goblet cells after SBR. In vitro studies using rat intestinal epithelial crypt (IEC-6) cells exposed to medium with or without recombinant FGF10 showed increased proliferation and phosphorylation of Raf and AKT with the addition of FGF10. Our results suggest that FGF10 may play a therapeutic role in diseases involving intestinal failure.

Fibroblast growth factor 9 signaling inhibits airway smooth muscle differentiation in mouse lung

In mammalian lungs, airway smooth muscle cells (airway SMCs) are present in the proximal lung adjacent to bronchi and bronchioles, but are absent in the distal lung adjacent to terminal sacs that expand during gas exchange. Evidence suggests that this distribution is essential for the formation of a functional respiratory tree, but the underlying genetic mechanism has not been elucidated. In this study, we test the hypothesis that fibroblast growth factor 9 (Fgf9) signaling is essential to restrict SMC differentiation to the proximal lung. We show that loss of Fgf9 or conditional inactivation of Fgf receptors (Fgfr) 1 and 2 in mouse lung mesenchyme results in ectopic SMCs. Our data support a model where FGF9 maintains a SMC progenitor population by suppressing differentiation and promoting growth. This model also represents our findings on the genetic relationship between FGF9 and sonic hedgehog (SHH) in the establishment of airway SMC pattern.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

Retinoic acid affects craniofacial patterning by changing Fgf8 expression in the pharyngeal ectoderm

Retinoic acid signaling plays important roles in establishing normal patterning and cellular differentiation during embryonic development. In this study, we show that single administration of retinoic acid at embryonic day 8.5 causes homeotic transformation of the lower jaw into upper jaw-like structures. This homeosis was preceded by downregulation of Fgf8 and Sprouty expression in the proximal domain of the first pharyngeal arch. Downregulation of mesenchymal genes such as Dlx5, Hand2, Tbx1 and Pitx2 was also observed. The oropharynx in retinoic acid-treated embryos was severely constricted. Consistent with this observation, Patched expression in the arch endoderm and mesenchyme was downregulated. Thus, retinoic acid affects the expression of subsets of epithelial and mesenchymal genes, possibly disrupting the regional identity of the pharyngeal arch.

Establishment of Hox vertebral identities in the embryonic spine precursors

The vertebrate spine exhibits two striking characteristics. The first one is the periodic arrangement of its elements-the vertebrae-along the anteroposterior axis. This segmented organization is the result of somitogenesis, which takes place during organogenesis. The segmentation machinery involves a molecular oscillator-the segmentation clock-which delivers a periodic signal controlling somite production. During embryonic axis elongation, this signal is displaced posteriorly by a system of traveling signaling gradients-the wavefront-which depends on the Wnt, FGF, and retinoic acid pathways. The other characteristic feature of the spine is the subdivision of groups of vertebrae into anatomical domains, such as the cervical, thoracic, lumbar, sacral, and caudal regions. This axial regionalization is controlled by a set of transcription factors called Hox genes. Hox genes exhibit nested expression domains in the somites which reflect their linear arrangement along the chromosomes-a property termed colinearity. The colinear disposition of Hox genes expression domains provides a blueprint for the regionalization of the future vertebral territories of the spine. In amniotes, Hox genes are activated in the somite precursors of the epiblast in a temporal colinear sequence and they were proposed to control their progressive ingression into the nascent paraxial mesoderm. Consequently, the positioning of the expression domains of Hox genes along the anteroposterior axis is largely controlled by the timing of Hox activation during gastrulation. Positioning of the somitic Hox domains is subsequently refined through a crosstalk with the segmentation machinery in the presomitic mesoderm. In this review, we focus on our current understanding of the embryonic mechanisms that establish vertebral identities during vertebrate development.

Fibroblast growth factor 8 signaling through fibroblast growth factor receptor 1 is required for the emergence of gonadotropin-releasing hormone neurons

GnRH neurons are essential for the onset and maintenance of reproduction. Mutations in both fibroblast growth factor receptor (Fgfr1) and Fgf8 have been shown to cause Kallmann syndrome, a disease characterized by hypogonadotropic hypogonadism and anosmia, indicating that FGF signaling is indispensable for the formation of a functional GnRH system. Presently it is unclear which stage of GnRH neuronal development is most impacted by FGF signaling deficiency. GnRH neurons express both FGFR1 and -3; thus, it is also unclear whether FGFR1 or FGFR3 contributes directly to GnRH system development. In this study, we examined the developing GnRH system in mice deficient in FGF8, FGFR1, or FGFR3 to elucidate the individual contribution of these FGF signaling components. Our results show that the early emergence of GnRH neurons from the embryonic olfactory placode requires FGF8 signaling, which is mediated through FGFR1, not FGFR3. These data provide compelling evidence that the developing GnRH system is exquisitely sensitive to reduced levels of FGF signaling. Furthermore, Kallmann syndrome stemming from FGF signaling deficiency may be due primarily to defects in early GnRH neuronal development prior to their migration into the forebrain.

Exon III splicing of fibroblast growth factor receptor 1 is modulated by growth factors and cyclin D1

OBJECTIVES

Fibroblast growth factor receptor 1 (FGFR1) isoform IIIc enhances and FGFR1-IIIb inhibits pancreatic cancer cell growth. Nothing is presently known about the expression and regulation of human FGFR1-III isoforms. The aim of this study was to identify regulators modulating the specific expression of human FGFR1-IIIb and FGFR1-IIIc.

METHODS

Parental cells, cells overexpressing FGFR1-III isoforms, and cells harboring a tetracycline-inducible cyclin D1 antisense expression vector system were used as model systems.

RESULTS

FGFR1-IIIb and -IIIc were coexpressed in human pancreatic cancer cells, with FGFR1-IIIc being the predominant isoform. FGFR1-IIIb mRNA expression decreased at higher cell density, whereas FGFR1-IIIc expression remained constant. Insulinlike growth factor I and epidermal growth factor induced expression of FGFR1-IIIc without altering FGFR1-IIIb. In contrast, fibroblast growth factor (FGF)1, FGF2, and FGF5 induced FGFR1-IIIc and reduced the expression of FGFR1-IIIb. Overexpression of one isoform did not alter the expression of the corresponding FGFR1-III isoform. Inhibition of cyclin D1, known to be induced by insulinlike growth factor I, epidermal growth factor, and FGF2, resulted in an inhibition of FGFR1-IIIc expression, whereas FGFR1-IIIb expression was enhanced.

CONCLUSIONS

This study demonstrated for the first time that FGFR1-IIIb and FGFR1-IIIc are coexpressed and that the FGFR1-III isoformsare differentially regulated by growth factors and cyclin D1.

Increased Cdx protein dose effects upon axial patterning in transgenic lines of mice

To investigate the link between Cdx protein concentration and axial patterning in embryos, we made lines of mice OE1, OE2 and OE4 that overexpress each of the Cdx genes Cdx1, Cdx2 and Cdx4, respectively. The lines carry Cdx transgenes under the transcriptional control of their own promoter/enhancer elements. Transgenic embryos show Cdx transcription at 8.5 to 8.7 days within normal spatial domains for Cdx expression (primitive streak/tailbud), yet, overall, they contain elevated levels of Cdx proteins. Increased doses of Cdx proteins result in homeotic shifts in vertebral types along most of the vertebral column, with transformations being most obvious within the cervical region. Most of the shifts are anterior-to-posterior transformations and the anterior limits of these are commonly skull/vertebra 1 (v1) for OE1, v1/v2 for OE2 and v7 for OE4. OE embryos display anterior shifts in the expression of a Hoxa7/lacZ reporter within neural, paraxial and lateral plate mesoderm tissues. Hoxa7/lacZ expression commences at the normal time in OE1 and OE4 embryos. OE2 embryos display a forward shift in the gradient of Cdx2 protein along the axis, suggesting that a Cdx morphogen gradient model could account, at least in part, for the homeotic shifts in vertebral types. OE mice display additional defects: forelimb deficiencies in OE1, multiple tail axes, vertebral mis-alignments and axial truncations in OE2.

Expression of ERK signaling inhibitors Dusp6, Dusp7, and Dusp9 during mouse ear development

The levels of fibroblast growth factor (FGF) signaling play important roles in coordinating development of the mouse inner, middle, and outer ears. Extracellular signal-regulated kinases (ERKs) are among the effectors that transduce the FGF signal to the nucleus and other cellular compartments. Attenuation of ERK activity by dephosphorylation is necessary to modulate the magnitude and duration of the FGF signal. Recently, we showed that inactivation of the ERK phosphatase, dual specificity phosphatase 6 (DUSP6), causes partially penetrant postnatal lethality, hearing loss and skeletal malformations. To determine whether other Dusps may function redundantly with Dusp6 during otic development, we surveyed the expression domains of the three ERK-specific DUSP transcripts, Dusp6, Dusp7, and Dusp9, in the embryonic mouse ear. We show that each is expressed in partially overlapping patterns that correspond to regions of active FGF signaling, suggesting combinatorial roles in negative regulation of this pathway during ear development.

Epithelial fibroblast growth factor receptor 1 regulates enamel formation

The interaction between epithelial and mesenchymal tissues plays a critical role in the development of organs such as teeth, lungs, and hair. During tooth development, fibroblast growth factor (FGF) signaling is critical for regulating reciprocal epithelial and mesenchymal interactions. FGF signaling requires FGF ligands and their receptors (FGFRs). In this study, we investigated the role of epithelial FGF signaling in tooth development, using the Cre-loxp system to create tissue-specific inactivation of Fgfr1 in mice. In K14-Cre;Fgfr1(fl/fl) mice, the apical sides of enamel-secreting ameloblasts failed to adhere properly to each other, although ameloblast differentiation was unaffected at early stages. Prior to eruption, enamel structure was compromised in the K14-Cre;Fgfr1(fl/fl) mice and displayed severe enamel defects that mimic amelogenesis imperfecta (AI), with a rough, irregular enamel surface. These results suggest that there is a cell-autonomous requirement for FGF signaling in the dental epithelium during enamel formation. Loss of Fgfr1 affects ameloblast organization at the enamel-secretory stage and, hence, the formation of enamel.

Fibroblast growth factor receptor 2 phosphorylation on serine 779 couples to 14-3-3 and regulates cell survival and proliferation

The fibroblast growth factors (FGFs) exert their diverse (or pleiotropic) biological responses through the binding and activation of specific cell surface receptors (FGFRs). While FGFRs are known to initiate intracellular signaling through receptor tyrosine phosphorylation, the precise mechanisms by which the FGFRs regulate pleiotropic biological responses remain unclear. We now identify a new mechanism by which FGFR2 is able to regulate intracellular signaling and cellular responses. We show that FGFR2 is phosphorylated on serine 779 (S779) in response to FGF2. S779, which lies adjacent to the phospholipase Cgamma binding site at Y766, provides a docking site for the 14-3-3 phosphoserine-binding proteins and is essential for the full activation of the phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase pathways. Furthermore, S779 signaling is essential for promoting cell survival and proliferation in both Ba/F3 cells and BALB/c 3T3 fibroblasts. This new mode of FGFR2 phosphoserine signaling via the 14-3-3 proteins may provide an increased repertoire of signaling outputs to allow the regulation of pleiotropic biological responses. In this regard, we have identified conserved putative phosphotyrosine/phosphoserine motifs in the cytoplasmic domains of diverse cell surface receptors, suggesting that they may perform important functional roles beyond the FGFRs.

Diversity in fibroblast growth factor receptor 1 regulation: learning from the investigation of Kallmann syndrome

The unravelling of the genetic basis of the hypogonadotrophic hypogonadal disorders, including Kallmann syndrome (KS), has led to renewed interest into the developmental biology of gonadotrophin-releasing hormone (GnRH) neurones and, more generally, into the molecular mechanisms of reproduction. KS is characterised by the association of GnRH deficiency with diminished olfaction. Until recently, only two KS-associated genes were known: KAL1 and KAL2. KAL1 encodes the cell membrane and extracellular matrix-associated secreted protein anosmin-1 which is implicated in the X-linked form of KS. Anosmin-1 shows high affinity binding to heparan sulphate (HS) and its function remains the focus of ongoing investigation, although a role in axonal guidance and neuronal migration, which are processes essential for normal GnRH ontogeny and olfactory bulb histogenesis, has been suggested. KAL2, identified as the fibroblast growth factor receptor 1 (FGFR1) gene, has now been recognised to be the underlying genetic defect for an autosomal dominant form of KS. The diverse signalling pathways initiated upon FGFR activation can elicit pleiotropic cellular responses depending on the cellular context. Signalling through FGFR requires HS for receptor dimerisation and ligand binding. Current evidence supports a HS-dependent interaction between anosmin-1 and FGFR1, where anosmin-1 serves as a co-ligand activator enhancing the signal activity, the finer details of whose mechanism remain the subject of intense investigation. Recently, mutations in the genes encoding prokineticin 2 (PK2) and prokineticin receptor 2 (PKR2) were reported in a cohort of KS patients, further reinforcing the view of KS as a multigenic trait involving divergent pathways. Here, we review the historical and current understandings of KS and discuss the latest findings from the molecular and cellular studies of the KS-associated proteins, and describe the evidence that suggests convergence of several of these pathways during normal GnRH and olfactory neuronal ontogeny.

Two distinct sources for a population of maturing axial progenitors

In mammals, the primitive streak region and its descendant, the tail bud,are the source of nascent mesoderm and spinal cord throughout axial elongation. A localised population of long-term axial progenitors has been identified in a region of the tail bud, the chordoneural hinge, but the localisation of such progenitors at earlier stages is so far untested. By studying gene expression, we have shown that a specific topological arrangement of domains persists from the streak to the tail bud, and includes an area (the node-streak border) in which ectoderm that expresses primitive streak markers overlies the prospective notochord. This arrangement persists in the chordoneural hinge. Homotopic grafts show that, as in other vertebrates, cells in the streak and node predominantly produce mesoderm,whereas those in the node-streak border and lateral to the streak additionally produce neurectoderm. Node-streak border descendants populate not only neurectoderm, somites and notochord throughout the axis, but also the chordoneural hinge. Ectoderm lateral to the embryonic day (E)8.5 streak is later recruited to the midline, where it produces somites and chordoneural hinge cells, the position of which overlaps that of border-derived cells. Therefore, the E8.5 axial progenitors that will make the tail comprise cells from two distinct sources: the border and lateral ectoderm. Furthermore,heterotopic grafts of cells from outside the border to this region also populate the chordoneural hinge. Expression of several streak- and tail bud-specific genes declines well before elongation ends, even though this late population can be successfully transplanted into earlier embryos. Therefore,at least some aspects of progenitor status are conferred by the environment and are not an intrinsic property of the cells.

Human fibroblast growth factor receptor 1-IIIb is a functional fibroblast growth factor receptor expressed in the pancreas and involved in proliferation and movement of pancreatic ductal cells

OBJECTIVES

The possible functions of the human IIIb-messenger RNA splice variant of fibroblast growth factor (FGF) receptor 1 (FGFR-1 IIIb) are yet to be delineated. In this study, the expression and functionality of the human FGFR-1 IIIb were characterized in the pancreas.

METHODS

In situ hybridization with a specific FGFR-1 IIIb probe in human pancreatic tissues demonstrated that FGFR-1 IIIb localized in normal pancreatic acinar and in ductal-like pancreatic cancer cells. To further assess the potential role of this receptor, a full-length human FGFR-1 IIIb was stably expressed in TAKA-1 pancreatic ductal cells not expressing endogenous FGFR-1.

RESULTS

The FGFR-1 IIIb-expressing TAKA-1 cells synthesized a glycosylated 110-kd protein capable of inducing proliferation on incubation with exogenous FGF-1, -2, and -4. These effects were paralleled by tyrosine phosphorylation of FGFR substrate 2 and association of FGFR substrate 2 with FGFR-1 IIIb. The FGF-1, -2, and -10 induced the activation of p44/42 mitogen-activated protein kinase (MAPK), p38 MAPK, and c-Jun N-terminal kinase. Pharmacological inhibition revealed that FGF-induced proliferation was dependent on the concomitant activation of p44/42 MAPK and c-Jun N-terminal kinase. The FGFR-1 IIIb expression enhanced single-cell movement and plating efficacy.

CONCLUSIONS

Our results demonstrate that the human FGFR-1 IIIb variant is a functional FGFR expressed in the pancreas that can alter pancreatic functions that regulate proliferation, adhesion, and movement.

Osteogenic differentiation of murine embryonic stem cells is mediated by fibroblast growth factor receptors

The mechanisms involved in the control of embryonic stem (ES) cell differentiation are yet to be fully elucidated. However, it has become clear that the family of fibroblast growth factors (FGFs) are centrally involved. In this study we examined the role of the FGF receptors (FGFRs 1-4) during osteogenesis in murine ES cells. Single cells were obtained after the formation of embryoid bodies, cultured on gelatin-coated plates, and coaxed to differentiate along the osteogenic lineage. Upregulation of genes was analyzed at both the transcript and protein levels using gene array, relative-quantitative PCR (RQ-PCR), and Western blotting. Deposition of a mineralized matrix was evaluated with Alizarin Red staining. An FGFR1-specific antibody was generated and used to block FGFR1 activity in mES cells during osteogenic differentiation. Upon induction of osteogenic differentiation in mES cells, all four FGFRs were clearly upregulated at both the transcript and protein levels with a number of genes known to be involved in osteogenic differentiation including bone morphogenetic proteins (BMPs), collagen I, and Runx2. Cells were also capable of depositing a mineralized matrix, confirming the commitment of these cells to the osteogenic lineage. When FGFR1 activity was blocked, a reduction in cell proliferation and a coincident upregulation of Runx2 with enhanced mineralization of cultures was observed. These results indicate that FGFRs play critical roles in cell recruitment and differentiation during the process of osteogenesis in mES cells. In particular, the data indicate that FGFR1 plays a pivotal role in osteoblast lineage determination.

HOX deregulation in acute myeloid leukemia

The deregulation of homeobox (HOX) genes in acute myeloid leukemia (AML) and the potential for these master regulators to perturb normal hematopoiesis is well established. To date, overexpression of HOX genes in AML has been attributed to specific chromosomal aberrations and abnormalities involving mixed-lineage leukemia (MLL), an upstream regulator of HOX genes. The finding reported in this issue of the JCI by Scholl et al. that caudal-type homeobox transcription factor 2 (CDX2), which is capable of affecting HOX gene expression during embryogenesis, is overexpressed in 90% of patients with AML and induces a transplantable AML in murine models provides an alternative mechanism for HOX-induced leukemogenesis and yields important insights into the hierarchy of HOX gene regulation in AML (see the related article beginning on page 1037).

FGF signalling pathways in development of the midbrain and anterior hindbrain

Development of the central nervous system is coordinated by intercellular signalling centres established within the neural tube. The isthmic organizer (IsO), located between the midbrain and anterior hindbrain, is one such centre. Important signal molecules secreted by the IsO include members of the fibroblast growth factor and Wnt families. These signals are integrated with dorsally and ventrally derived signals to regulate development of the midbrain and rhombomere 1 of the hindbrain. The IsO is operational for a remarkably long period of time. Depending on the developmental stage, it controls a variety of processes such as cell survival, cell identity, neural precursor proliferation, neuronal differentiation and axon guidance. This review focuses on the fibroblast growth factor signalling, its novel molecular regulatory mechanisms and how this pathway regulates multiple aspects of cell behaviour in the developing midbrain and anterior hindbrain.

Signaling and transcriptional regulation in osteoblast commitment and differentiation

The major event that triggers osteogenesis is the transition of mesenchymal stem cells into bone forming, differentiating osteoblast cells. Osteoblast differentiation is the primary component of bone formation, exemplified by the synthesis, deposition and mineralization of extracellular matrix. Although not well understood, osteoblast differentiation from mesenchymal stem cells is a well-orchestrated process. Recent advances in molecular and genetic studies using gene targeting in mouse enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. Osteoblast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. We review Wnt signaling pathway and Runx2 regulation network, which are critical for osteoblast differentiation. Many other factors and signaling pathways have been implicated in regulation of osteoblast differentiation in a network manner, such as the factors Osterix, ATF4, and SATB2 and the TGF-beta, Hedgehog, FGF, ephrin, and sympathetic signaling pathways. This review summarizes the recent advances in the studies of signaling transduction pathways and transcriptional regulation of osteoblast cell lineage commitment and differentiation. The knowledge of osteoblast commitment and differentiation should be applied towards the development of new diagnostic and therapeutic alternatives for human bone diseases.

Skeletal overgrowth is mediated by deficiency in a specific isoform of fibroblast growth factor receptor 3

Fibroblast growth factor receptor 3 (FGFR3) plays an important role in the control of chondrocyte proliferation and differentiation, a process critical for normal development of the skeleton. To reveal the contributions of the epithelial Fgfr3b isoform and the mesenchymal Fgfr3c isoform to skeletal overgrowth seen in mice, in which both isoforms have been inactivated (Fgf3c(-/-) mice), we have generated mice in which each of the two Fgfr3 isoforms has been selectively inactivated. Whereas no apparent phenotype was detected in Fgfr3b(-/-) mice, strong stimulation of chondrocyte proliferation in the growth plates of Fgf3c(-/-) mice caused dramatic skeletal overgrowth and other skeletal abnormalities resembling the phenotype of mice deficient in both Fgfr3 isoforms. In addition, Fgfr3c(-/-) mice exhibited decreased bone mineral density in the cortical and trabecular bone, whereas the bone mineral density of Fgfr3b(-/-) mice resembled that of WT mice. These experiments demonstrated that the mesenchymal Fgfr3c isoform is responsible for controlling chondrocyte proliferation and differentiation that mediate normal skeletal development, whereas the epithelial Fgfr3b isoform does not contribute toward this process.

FGF2 posttranscriptionally down-regulates expression of SDF1 in bone marrow stromal cells through FGFR1 IIIc

The chemokine stromal cell-derived factor-1 (SDF-1) is constitutively expressed by bone marrow stromal cells and plays key roles in hematopoiesis. Fibroblast growth factor 2 (FGF2), a member of the FGF family that plays important roles in developmental morphogenic processes, is abnormally elevated in the bone marrow from patients with clonal myeloid disorders and other disorders where normal hematopoiesis is impaired. Here, we report that FGF2 reduces SDF-1 secretion and protein content in bone marrow stromal cells. By inhibiting SDF-1 production, FGF2 compromises stromal cell support of hematopoietic progenitor cells. Reverse-transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that bone marrow stromal cells express 5 FGF receptors (FGFRs) among the 7 known FGFR subtypes. Blocking experiments identified FGFR1 IIIc as the receptor mediating FGF2 inhibition of SDF-1 expression in bone marrow stromal cells. Analysis of the mechanisms underlying FGF2 inhibition of SDF-1 production in bone marrow stromal cells revealed that FGF2 reduces the SDF-1 mRNA content by posttranscriptionally accelerating SDF-1 mRNA decay. Thus, we identify FGF2 as an inhibitor of SDF-1 production in bone marrow stromal cells and a regulator of stromal cell supportive functions for hematopoietic progenitor cells.

Chemotactic cell movement during Dictyostelium development and gastrulation

Many developmental processes involve chemotactic cell movement up or down dynamic chemical gradients. Studies of the molecular mechanisms of chemotactic movement of Dictyostelium amoebae up cAMP gradients highlight the importance of PIP3 signaling in the control of cAMP-dependent actin polymerization, which drives the protrusion of lamellipodia and filopodia at the leading edge of the cell, but also emphasize the need for myosin thick filament assembly and motor activation for the contraction of the back of the cell. These process become even more important during the multicellular stages of development, when propagating waves of cAMP coordinate the chemotactic movement of tens of thousands of cells, resulting in multicellular morphogenesis. Recent experiments show that chemotaxis, especially in response to members of the FGF, PDGF and VEGF families of growth factors, plays a key role in the guidance of mesoderm cells during gastrulation in chick, mouse and frog embryos. The molecular mechanisms of signal detection and signaling to the actin-myosin cytoskeleton remain to be elucidated.

Molecular regulation of receptor tyrosine kinases in hematopoietic malignancies

Dysregulation of receptor tyrosine kinase (RTK) activity has been implicated in the progression of a variety of human leukemias. Most notably, mutations and chromosomal translocations affecting regulation of tyrosine kinase activity in the Kit receptor, the Flt3 receptor, and the PDGFbeta/FGF1 receptors have been demonstrated in mast cell leukemia, acute myeloid leukemia (AML), and chronic myelogenous leukemias (CML), respectively. In addition, critical but non-overlapping roles for the Ron and Kit receptor tyrosine kinases in the progression of animal models of erythroleukemia have been demonstrated [Persons, D., Paulson, R., Loyd, M., Herley, M., Bodner, S., Bernstein, A., Correll, P. and Ney, P., 1999. Fv2 encodes a truncated form of the Stk receptor tyrosine kinase. Nat. Gen. 23, 159-165.; Subramanian, A., Teal, H.E., Correll, P.H. and Paulson, R.F., 2005. Resistance to friend virus-induced erythroleukemia in W/Wv mice is caused by a spleen-specific defect which results in a severe reduction in target cells and a lack of Sf-Stk expression. J. Virol. 79 (23), 14586-14594.]. The various classes of RTKs implicated in the progression of leukemia have been recently reviewed [Reilly, J., 2003. Receptor tyrosine kinases in normal and malignant haematopoiesis. Blood Rev. 17 (4), 241-248.]. Here, we will discuss the mechanism by which alterations in these receptors result in transformation of hematopoietic cells, in the context of what is known about the molecular regulation of RTK activity, with a focus on our recent studies of the Ron receptor tyrosine kinase.