The roles of FGFs in the early development of vertebrate limbs

Sef is synexpressed with FGFs during chick embryogenesis and its expression is differentially regulated by FGFs in the developing limb

The signaling pathways leading to growth and patterning of various organs are tightly controlled during the development of any organism. These control mechanisms usually involve the utilization of feedback- and pathway-specific antagonists where the pathway induces the expression of its own antagonist. Sef is a feedback antagonist of fibroblast growth factor (FGF) signaling, which has been identified recently in zebrafish and mammals. Here, we report the isolation of chicken Sef (cSef) and demonstrate the conserved nature of the regulatory relationship with FGF signaling. In chick embryos, Sef is expressed in a pattern that coincides with many known sites of FGF signaling. In the developing limb, cSef is expressed in the mesoderm underlying the apical ectodermal ridge (AER) in the region known as the progress zone. cSef message first appeared after limb budding and AER formation. Expression was intense at stages of rapid limb outgrowth, and gradually decreased to almost undetectable levels when differentiation was clearly apparent. Gain- and loss-of-function experiments showed that FGFs differentially regulate the expression of cSef in various tissues. Thus, removal of the AER down-regulated cSef expression, and FGF2 but not FGF4 or FGF8 beads substituted for the AER in maintaining cSef expression. At sites where cSef is not normally expressed, FGF4 and FGF2, but not FGF8 beads, induced cSef expression. Our results demonstrate the complexity of cSef regulation by FGFs and point to FGF2 as a prime candidate in regulating cSef expression during normal limb development. The spatiotemporal pattern of cSef expression during limb development suggests a role for cSef in regulating limb outgrowth but not limb initiation.

Modulation of Fgf8 activity during vertebrate brain development

In recent years much emphasis has been placed on investigation of the precise control of FGF signaling during brain development. Such control is achieved in part by regulatory elements that determine the domains and levels of expression of genes coding for the diverse FGF ligands via specific molecular signaling pathways. There is new knowledge on the operation of such mechanisms in regions of the neural tube involved in the correct patterning of adjacent territories (known as secondary organizers of neural tube pattern). In the present minireview we intend to summarize recent evidence and emerging conclusions on potent modulators that govern the activity of Fgf8 signals in the developing vertebrate brain, focusing our attention on the best known secondary organizer, the isthmic organizer.

Inactivation of FGF8 in early mesoderm reveals an essential role in kidney development

To bypass the essential gastrulation function of Fgf8 and study its role in lineages of the primitive streak, we have used a new mouse line,T-Cre, to generate mouse embryos with pan-mesodermal loss of Fgf8expression. Surprisingly, despite previous models in which Fgf8 has been assigned a pivotal role in segmentation/somite differentiation, Fgf8 is not required for these processes. However, mutant neonates display severe renal hypoplasia with deficient nephron formation. In mutant kidneys, aberrant cell death occurs within the metanephric mesenchyme (MM),particularly in the cortical nephrogenic zone, which provides the progenitors for recurring rounds of nephron formation. Prior to mutant morphological changes, Wnt4 and Lim1 expression, which is essential for nephrogenesis, is absent in MM. Furthermore, comparative analysis of Wnt4-null homozygotes reveals concomitant downregulation of Lim1 and diminished tubule formation. Our data support a model whereby FGF8 and WNT4 function in concert to induce the expression of Lim1 for MM survival and tubulogenesis.

Structural basis for fibroblast growth factor receptor activation

FGF signaling plays a ubiquitous role in human biology as a regulator of embryonic development, homeostasis and regenerative processes. In addition, aberrant FGF signaling leads to diverse human pathologies including skeletal, olfactory, and metabolic disorders as well as cancer. FGFs execute their pleiotropic biological actions by binding, dimerizing and activating cell surface FGF receptors (FGFRs). Proper regulation of FGF-FGFR binding specificity is essential for the regulation of FGF signaling and is achieved through primary sequence variations among the 18 FGFs and seven FGFRs. The severity of human skeletal syndromes arising from mutations that violate FGF-FGFR specificity is a testament to the importance of maintaining precision in FGF-FGFR specificity. The discovery that heparin/heparan sulfate (HS) proteoglycans are required for FGF signaling led to numerous models for FGFR dimerization and heralded one of the most controversial issues in FGF signaling. Recent crystallographic analyses have led to two fundamentally different models for FGFR dimerization. These models differ in both the stoichiometry and minimal length of heparin required for dimerization, the quaternary arrangement of FGF, FGFR and heparin in the dimer, and in the mechanism of 1:1 FGF-FGFR recognition and specificity. In this review, we provide an overview of recent structural and biochemical studies used to differentiate between the two crystallographic models. Interestingly, the structural and biophysical analyses of naturally occurring pathogenic FGFR mutations have provided the most compelling and unbiased evidences for the correct mechanisms for FGF-FGFR dimerization and binding specificity. The structural analyses of different FGF-FGFR complexes have also shed light on the intricate mechanisms determining FGF-FGFR binding specificity and promiscuity and also provide a plausible explanation for the molecular basis of a large number craniosynostosis mutations.

Fibroblast growth factor receptor 3 inhibition by short hairpin RNAs leads to apoptosis in multiple myeloma

The presence of t(4;14)(p16.3;q32.3) in multiple myeloma cells results in dysregulated expression of the fibroblast growth factor receptor 3 (FGFR3). FGFR3 acts as an oncogene to promote multiple myeloma cell proliferation and antiapoptosis. These encourage the clinical development of FGFR3-specific inhibitors. Three short hairpin RNAs (shRNA) targeting different sites of FGFR3 were selected and subsequently transfected into KMS-11, OPM-2, and NCI-H929 human myeloma cell lines, all of which are characterized by t(4;14) and FGFR3 over expression. The combination of these three shRNAs can effectively inhibit FGFR3 expression in all three cell lines. Sequential immunocytochemistry/fluorescence in situ hybridization was employed to validate that the shRNAs specifically inhibited FGFR3 expression in OPM-2 cells. Decreased expression of B-cell chronic lymphocytic leukemia/lymphoma 2 (BCL2) and myeloid cell leukemia sequence 1 (MCL1) proteins and increased staining of Annexin V-positive cells showed that inhibition of FGFR3 induces apoptosis. After confirming down-regulation of FGFR3 by real-time PCR, HU-133 plus 2.0 array was employed to compare the gene expression profile of shRNA-treated sample with that of the control. Besides the down-regulation of FGFR3, expression of the antiapoptotic genes CFLAR, BCL2, MCL1, and some members of NF-kappaB family decreased, whereas expression of the proapoptotic genes CYC, BID, CASP2, and CASP6 increased. Microarray results also revealed changes in genes previously implicated in multiple myeloma pathogenesis (RAS, RAF, IL-6R, and VEGF), as well as others (TLR4, KLF4, and GADD45A) not previously linked to multiple myeloma. Our observations indicate that shRNAs can specifically and effectively inhibit FGFR3 expression. This targeted approach may be worth testing in multiple myeloma patients with t(4;14) and FGFR3 overexpression in the future.

A novel isoform of Vinexin, Vinexin gamma, regulates Sox9 gene expression through activation of MAPK cascade in mouse fetal gonad

Recent loss-of-function and gain-of-function studies have revealed that transcription factor Sox9 is required for testis formation by governing Sertoli cell differentiation, and thereafter regulating transcription of Sertoli marker genes. In the present study, we identified a novel isoform of Vinexin, which is expressed in somatic cells but not germ cells of sexually indifferent stages of fetal gonads. After the sex is determined, the expression continues in testicular Sertoli cells. Immunohistochemical analyses with a specific antibody to Vinexin indicated that Vinexin gamma is localized in the cytoplasm. Functional studies with C3H10T1/2 cells showed that Vinexin gamma acted as a scaffold protein to activate MEK and ERK through interaction with c-Raf and ERK. Ultimately, Sox9 transcription was induced by Vinexin gamma. This up-regulation of Sox9 expression disappeared when the cells were treated with a specific MEK inhibitor, U0126. To determine the role of Vinexin gamma during gonad formation, the gene was disrupted by targeted mutagenesis. The phenotype displayed by the mice indicated that ERK activation was decreased in the Vinexin gamma(-/-) XY gonads, and Sox9 expression was down-regulated. Thus, Vinexin gamma seems to be implicated in regulation of Sox9 gene expression by modulating MAPK cascade in mouse fetal gonads.

Function and regulation of Alx4 in limb development: complex genetic interactions with Gli3 and Shh

The role of the aristaless-related homeobox gene Alx4 in antero-posterior (AP-) patterning of the developing vertebrate limb has remained somewhat elusive. Polydactyly of Alx4 mutant mice is known to be accompanied by ectopic anterior expression of genes like Shh, Fgf4 and 5'Hoxd. We reported previously that polydactyly in Alx4 mutant mice requires SHH signaling, but we now show that in early Alx4-/- limb buds the anterior ectopic expression of Fgf4 and Hoxd13, and therefore disruption of AP-patterning, occurs independently of SHH signaling. To better understand how Alx4 functions in the pathways that regulate AP-patterning, we also studied genomic regulatory sequences that are capable of directing expression of a reporter gene in a pattern corresponding to endogenous Alx4 expression in anterior limb bud mesenchyme. We observed, as expected for authentic Alx4 expression, expansion of reporter construct expression in a Shh-/- background. Total lack of reporter expression in a Gli3-/- background confirms the existence of Gli3-dependent and -independent Alx4 expression in the limb bud. Apparently, these two modules of Alx4 expression are linked to dissimilar functions.

Differentiation of the vertebrate retina is coordinated by an FGF signaling center

In vertebrates, midline-derived sonic hedgehog and nodal are crucial for the initial proximal-distal patterning of the eye. The establishment of the distal optic stalk is in turn a prerequisite to initiate retinogenesis. However, the signal that activates this process is unknown. Here, we demonstrate that in both chick and fish, the initiation of retinal differentiation is triggered by a species-specific localized Fgf signaling center that acts as mediator of the midline signals. The concerted activity of Fgf8 and Fgf3 is both necessary and sufficient to coordinate retinal differentiation independent of the connecting optic stalk.

Analysis of Msx1; Msx2 double mutants reveals multiple roles for Msx genes in limb development

The homeobox-containing genes Msx1 and Msx2 are highly expressed in the limb field from the earliest stages of limb formation and, subsequently, in both the apical ectodermal ridge and underlying mesenchyme. However, mice homozygous for a null mutation in either Msx1 or Msx2 do not display abnormalities in limb development. By contrast, Msx1; Msx2 double mutants exhibit a severe limb phenotype. Our analysis indicates that these genes play a role in crucial processes during limb morphogenesis along all three axes. Double mutant limbs are shorter and lack anterior skeletal elements (radius/tibia, thumb/hallux). Gene expression analysis confirms that there is no formation of regions with anterior identity. This correlates with the absence of dorsoventral boundary specification in the anterior ectoderm, which precludes apical ectodermal ridge formation anteriorly. As a result, anterior mesenchyme is not maintained, leading to oligodactyly. Paradoxically, polydactyly is also frequent and appears to be associated with extended Fgf activity in the apical ectodermal ridge, which is maintained up to 14.5 dpc. This results in a major outgrowth of the mesenchyme anteriorly, which nevertheless maintains a posterior identity, and leads to formation of extra digits. These defects are interpreted in the context of an impairment of Bmp signalling.

FGF8 initiates inner ear induction in chick and mouse

In both chick and mouse, the otic placode, the rudiment of the inner ear, is induced by at least two signals, one from the cephalic paraxial mesoderm and the other from the neural ectoderm. In chick, the mesodermal signal, FGF19, induces neural ectoderm to express additional signals, including WNT8c and FGF3, resulting in induction of the otic placode. In mouse, mesodermal Fgf10 acting redundantly with neural Fgf3 is required for induction of the placode. To determine how the mesodermal inducers of the otic placode are localized, we took advantage of the unique strengths of the two model organisms. We show that endoderm is necessary for otic induction in the chick and that Fgf8, expressed in the chick endoderm subjacent to Fgf19, is both sufficient and necessary for the expression of Fgf19 in the mesoderm. In the mouse, Fgf8 is also expressed in endoderm as well as in other germ layers in the periotic placode region. We show that otic induction fails in embryos null for Fgf3 and hypomorphic for Fgf8 and expression of mesodermal Fgf10 is reduced. Thus, Fgf8 plays a critical upstream role in an FGF signaling cascade required for otic induction in chick and mouse.

Analysis of mutations in fibroblast growth factor (FGF) and a pathogenic mutation in FGF receptor (FGFR) provides direct evidence for the symmetric two-end model for FGFR dimerization

Two competing models for fibroblast growth factor (FGF) receptor (FGFR) dimerization have recently emerged based on ternary FGF-FGFR-heparin crystal structures. In the symmetric two-end model, heparin promotes dimerization of two FGF-FGFR complexes by stabilizing bivalent interactions of the ligand and receptor through primary and secondary sites and by stabilizing direct receptor-receptor contacts. In the asymmetric model, there are no protein-protein contacts between the two FGF-FGFR complexes, which are bridged solely by heparin. To identify the correct mode of FGFR dimerization, we abolished interactions at the secondary ligand-receptor interaction site, which are observed only in the symmetric two-end model, using site-directed mutagenesis. Cellular studies and real-time binding assays, as well as matrix-assisted laser desorption ionization-time of flight analysis, demonstrate that loss of secondary ligand-receptor interactions results in diminished FGFR activation due to decreased dimerization without affecting FGF-FGFR binding. Additionally, structural and biochemical analysis of an activating FGFR2 mutation resulting in Pfeiffer syndrome confirms the physiological significance of receptor-receptor contacts in the symmetric two-end model and provides a novel mechanism for FGFR gain of function in human skeletal disorders. Taken together, the data validate the symmetric two-end model of FGFR dimerization and argue against the asymmetric model of FGFR dimerization.

FGFR2b signaling regulates ex vivo submandibular gland epithelial cell proliferation and branching morphogenesis

Branching morphogenesis of mouse submandibular glands is regulated by multiple growth factors. Here, we report that ex vivo branching of intact submandibular glands decreases when either FGFR2 expression is downregulated or soluble recombinant FGFR2b competes out the endogenous growth factors. However, a combination of neutralizing antibodies to FGF1, FGF7 and FGF10 is required to inhibit branching in the intact gland, suggesting that multiple FGF isoforms are required for branching. Exogenous FGFs added to submandibular epithelial rudiments cultured without mesenchyme induce distinct morphologies. FGF7 induces epithelial budding, whereas FGF10 induces duct elongation, and both are inhibited by FGFR or ERK1/2 signaling inhibitors. However, a PI3-kinase inhibitor also decreases FGF7-mediated epithelial budding, suggesting that multiple signaling pathways exist. We immunolocalized FGF receptors and analyzed changes in FGFR, FGF and MMP gene expression to identify the mechanisms of FGF-mediated morphogenesis. FGFR1b and FGFR2b are present throughout the epithelium,although FGFR1b is more highly expressed around the periphery of the buds and the duct tips. FGF7 signaling increases FGFR1b and FGF1expression, and MMP2 activity, when compared with FGF10, resulting in increased cell proliferation and expansion of the epithelial bud, whereas FGF10 stimulates localized proliferation at the tip of the duct. FGF7- and FGF10-mediated morphogenesis is inhibited by an MMP inhibitor and a neutralizing antibody to FGF1, suggesting that both FGF1 and MMPs are essential downstream mediators of epithelial morphogenesis. Taken together,our data suggests that FGFR2b signaling involves a regulatory network of FGFR1b/FGF1/MMP2 expression that mediates budding and duct elongation during branching morphogenesis.

Mechanisms underlying differential responses to FGF signaling

Fibroblast growth factors (FGFs) are key regulators of several developmental processes in which cell fate and differentiation to various tissue lineages are determined. The importance of the proper spatial and temporal regulation of FGF signals is evident from human and mouse genetic studies which show that mutations leading to the dysregulation of FGF signals cause a variety of developmental disorders including dominant skeletal diseases and cancer. The FGF ligands signal via a family of receptor tyrosine kinases and, depending on the cell type or stage of maturation, produce diverse biological responses that include proliferation, growth arrest, differentiation or apoptosis. A central issue in FGF biology is to understand how these diverse cellular responses are determined and how similar signaling inputs can generate distinct patterns of gene expression that govern the specificity of the cellular response. In this review we draw upon studies from the past fifteen years and attempt to construct a molecular picture of the different levels of regulation by which such specific cellular responses could be achieved by FGF signals. We discuss whether specificity could lie in the nature of the ligand, the particular receptor, the signal transduction pathways utilized, or the transcriptional regulation of specific genes. Finally, we also discuss how the interplay of FGF signals with other signaling systems could contribute to the cellular response. In particular we focus on the interaction with the Wnt pathway since FGF/Wnt cross-talk is emerging as an important nexus in regulating a variety of biological processes.

Cytokine therapy for craniosynostosis

The birth prevalence of craniosynostosis (premature suture fusion) is 300-500 per 1,000,000 live births. Surgical management involves the release of the synostosed suture. In many cases, however, the suturectomy site rapidly reossifies, further restricts the growing brain and alters craniofacial growth. This resynostosis requires additional surgery, which increases patient morbidity and mortality. New findings in bone biology and molecular pathways involved with suture fusion, combined with novel tissue engineering techniques, may allow the design of targeted and complementary therapies to decrease complications inherent in high-risk surgical procedures. This paper selectively reviews recent advances in i) identifying genetic mutations and the aetiopathogenesis of a number of craniosynostotic conditions; ii) cranial suture biology and molecular biochemical pathways involved in suture fusion; and iii) the design, development and application of various vehicles and tissue engineered constructs to deliver cytokines and genes to cranial sutures. Such biologically based therapies may be used as surgical adjuncts to rescue fusing sutures or help manage postoperative resynostosis.

The contribution of chicken embryology to the understanding of vertebrate limb development

The chicken is an excellent model organism for studying vertebrate limb development, mainly because of the ease of manipulating the developing limb in vivo. Classical chicken embryology has provided fate maps and elucidated the cell-cell interactions that specify limb pattern. The first defined chemical that can mimic one of these interactions was discovered by experiments on developing chick limbs and, over the last 15 years or so, the role of an increasing number of developmentally important genes has been uncovered. The principles that underlie limb development in chickens are applicable to other vertebrates and there are growing links with clinical genetics. The sequence of the chicken genome, together with other recently assembled chicken genomic resources, will present new opportunities for exploiting the ease of manipulating the limb.

Conserved cross-interactions inDrosophilaandXenopusbetween Ras/MAPK signaling and the dual-specificity phosphatase MKP3

The extracellular signal-regulated kinase (ERK) is a key transducer of the epidermal growth factor receptor (EGFR) and fibroblast growth factor receptor (FGFR) signaling pathways, and its function is required in multiple processes during animal development. The activity of ERK depends on the phosphorylation state of conserved threonine and tyrosine residues, and this state is regulated by different kinases and phosphatases. A family of phosphatases with specificity toward both threonine and tyrosine residues in ERK (dual-specificity phosphatases) play a conserved role in its dephosphorylation and consequent inactivation. Here, we characterize the function of the dual-specificity phosphatase MKP3 in Drosophila EGFR and Xenopus FGFR signaling. The function of MKP3 is required during Drosophila wing vein formation and Xenopus anteroposterior neural patterning. We find that the expression of the MKP3 gene is localized in places of high EGFR and FGFR signaling. Furthermore, this restricted expression depends on ERK function both in Drosophila and Xenopus, suggesting that MKP3 constitutes a conserved negative feedback loop on the activity of the Ras/ERK signaling pathway.

Fibroblast growth factor signaling during early vertebrate development

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

Effects of FGF-2/-9 in calvarial bone cell cultures: differentiation stage-dependent mitogenic effect, inverse regulation of BMP-2 and noggin, and enhancement of osteogenic potential

Systemically administered fibroblast growth factors (FGFs) show anabolic effects on bone formation in animals, whereas in vitro cell culture studies have demonstrated that FGFs block mineralized bone nodule formation. These apparently contradictory outcomes indicate that the nature of FGF action is complex and that the biological effect of FGFs may depend on the differentiation stage of osteoblasts, interaction with other cytokines, or the length and mode of exposure to factors. Thus, we have utilized primary calvarial bone cell populations at different maturation phases to determine their responses to 2, FGF-9, and BMP-2, the factors expressed in bone. FGF-2 and FGF-9 stimulated proliferation of the cell populations consisting of more mature osteoblasts, but not those with undifferentiated precursor cells. Continuous treatment with FGF-2/-9 inhibited expression of several osteoblast marker genes and mineralization. However, brief pretreatment with FGF-2/-9 or sequential treatment with FGF-2/-9 followed by BMP-2 led to marked stimulation of mineralization, suggesting that FGFs enhance the intrinsic osteogenic potential. Furthermore, FGF-2 and FGF-9 increased expression of other osteogenic factors BMP-2 and TGFbeta-1. Meanwhile, blocking endogenous FGF signaling, using a virally transduced dominant-negative FGF receptor (FgfR), resulted in drastically reduced expression of the BMP-2 gene, demonstrating for the first time that endogenous FGF/FgfR signaling is a positive upstream regulator of the BMP-2 gene in calvarial osteoblasts. In contrast, expression of a BMP antagonist noggin was inhibited by FGF-2 and FGF-9. Thus, collective data from this study suggest that FGF/FgfR signaling enhances the intrinsic osteogenic potential by selectively expanding committed osteogenic cell populations as well as inversely regulating BMP-2 and noggin gene expression.

The pre-Mendelian, pre-Darwinian world: Shifting relations between genetic and epigenetic mechanisms in early multicellular evolution

The reliable dependence of many features of contemporary organisms on changes in gene content and activity is tied to the processes of Mendelian inheritance and Darwinian evolution. With regard to morphological characters, however, Mendelian inheritance is the exception rather than the rule, and neo-Darwinian mechanisms in any case do not account for the origination (as opposed to the inherited variation) of such characters. It is proposed, therefore, that multicellular organisms passed through a pre-Mendelian, pre-Darwinian phase, whereby cells, genes and gene products constituted complex systems with context-dependent, self-organizing morphogenetic capabilities. An example is provided of a plausible 'core' mechanism for the development of the vertebrate limb that is both inherently pattern forming and morphogenetically plastic. It is suggested that most complex multicellular structures originated from such systems. The notion that genes are privileged determinants of biological characters can only be sustained by neglecting questions of evolutionary origination and the evolution of developmental mechanisms.

The developing limb and the control of the number of digits

Congenital malformations of the limbs are among the most frequent congenital anomalies found in humans, and they preferentially affect the distal part--the hand or foot. The presence of extra digits, a condition called polydactyly, is the most common limb deformity of the human hand and is the consequence of disturbances in the normal program of limb development. However, despite the extensive use of the developing limb as a classical developmental model, the cellular and genetic mechanisms that control the number and identity of the digits are not completely understood. The aim of this review is to introduce the reader to the current state of knowledge in limb development and to provide the necessary background for an understanding of how deviations from the normal developmental program may lead to polydactyly.

Levels of Gli3 repressor correlate with Bmp4 expression and apoptosis during limb development

Removal of the posterior wing bud leads to massive apoptosis of the remaining anterior wing bud mesoderm. We show here that this finding correlates with an increase in the level of the repressor form of the Gli3 protein, due to the absence of the Sonic hedgehog (Shh) protein signaling. Therefore, we used the anterior wing bud mesoderm as a model system to analyze the relationship between the repressor form of Gli3 and apoptosis in the developing limb. With increased Gli3R levels, we demonstrate a concomitant increase in Bmp4 expression and signaling in the anterior mesoderm deprived of Shh signaling. Several experimental approaches show that the apoptosis can be prevented by exogenous Noggin, indicating that Bmp signaling mediates it. The analysis of Bmp4 expression in several mouse and chick mutations with defects in either expression or processing of Gli3 indicates a correlation between the level of the repressor form of Gli3 and Bmp4 expression in the distal mesoderm. Our analysis adds new insights into the way Shh differentially controls the processing of Gli3 and how, subsequently, BMP4 expression may mediate cell survival or cell death in the developing limb bud in a position-dependent manner.

Mkp3 is a negative feedback modulator of Fgf8 signaling in the mammalian isthmic organizer

The pivotal mechanisms that govern the correct patterning and regionalization of the distinct areas of the mammalian CNS are driven by key molecules that emanate from the so-called secondary organizers at neural plate and tube stages. FGF8 is the candidate morphogenetic molecule to pattern the mesencephalon and rhombencephalon in the isthmic organizer (IsO). Recognizable relevance has been given to the intracellular pathways by which Fgf8 is regulated and modulated. In chick limb bud development, a dual mitogen-activated protein kinase phosphatase-3 (Mkp3) plays a role as a negative feedback modulator of Fgf8 signaling. We have investigated the role of Mkp3 and its functional relationship with the Fgf8 signaling pathway in the mouse IsO using gene transfer microelectroporation assays and protein-soaked bead experiments. Here, we demonstrate that MKP3 has a negative feedback action on the MAPK/ERK-mediated FGF8 pathway in the mouse neuroepithelium.

Regulation of endocytosis, nuclear translocation, and signaling of fibroblast growth factor receptor 1 by E-cadherin

Fibroblast growth factor (FGF) receptors (FGFRs) signal to modulate diverse cellular functions, including epithelial cell morphogenesis. In epithelial cells, E-cadherin plays a key role in cell-cell adhesion, and its function can be regulated through endocytic trafficking. In this study, we investigated the location, trafficking, and function of FGFR1 and E-cadherin and report a novel mechanism, based on endocytic trafficking, for the coregulation of E-cadherin and signaling from FGFR1. FGF induces the internalization of surface FGFR1 and surface E-cadherin, followed by nuclear translocation of FGFR1. The internalization of both proteins is regulated by common endocytic machinery, resulting in cointernalization of FGFR1 and E-cadherin into early endosomes. By blocking endocytosis, we show that this is a requisite, initial step for the nuclear translocation of FGFR1. Overexpression of E-cadherin blocks both the coendocytosis of E-cadherin and FGFR1, the nuclear translocation of FGFR1 and FGF-induced signaling to the mitogen-activated protein kinase pathway. Furthermore, stabilization of surface adhesive E-cadherin, by overexpressing p120ctn, also blocks internalization and nuclear translocation of FGFR1. These data reveal that conjoint endocytosis and trafficking is a novel mechanism for the coregulation of E-cadherin and FGFR1 during cell signaling and morphogenesis.

Limb chondrogenesis of the seepage salamander,Desmognathus aeneus (Amphibia: Plethodontidae)

Salamanders are infrequently mentioned in analyses of tetrapod limb formation, as their development varies considerably from that of amniotes. However, urodeles provide an opportunity to study how limb ontogeny varies with major differences in life history. Here we assess limb development in Desmognathus aeneus, a direct-developing salamander, and compare it to patterns seen in salamanders with larval stages (e.g., Ambystoma mexicanum). Both modes of development result in a limb that is morphologically indistinct from an amniote limb. Developmental series of A. mexicanum and D. aeneus were investigated using Type II collagen immunochemistry, Alcian Blue staining, and whole-mount TUNEL staining. In A. mexicanum, as each digit bud extends from the limb palette Type II collagen and proteoglycan secretion occur almost simultaneously with mesenchyme condensation. Conversely, collagen and proteoglycan secretion in digits of D. aeneus occur only after the formation of an amniote-like paddle. Within each species, Type II collagen expression patterns resemble those of proteoglycans. In both, distal structures form before more proximal structures. This observation is contrary to the proximodistal developmental pattern of other tetrapods and may be unique to urodeles. In support of previous findings, no cell death was observed during limb development in A. mexicanum. However, apoptotic cells that may play a role in digit ontogeny occur in the limbs of D. aeneus, thereby suggesting that programmed cell death has evolved as a developmental mechanism at least twice in tetrapod limb evolution.

Tbx5 and Tbx4 Are Not Sufficient to Determine Limb-Specific Morphologies but Have Common Roles in Initiating Limb Outgrowth

Morphological differences between forelimbs and hindlimbs are thought to be regulated by Tbx5 expressed in the forelimb and Tbx4 and Pitx1 expressed in the hindlimb. Gene deletion and misexpression experiments have suggested that these factors have two distinct functions during limb development: the initiation and/or maintenance of limb outgrowth and the specification of limb-specific morphologies. Using genetic methods in the mouse, we have investigated the roles of Tbx5, Tbx4, and Pitx1 in both processes. Our results support a role for Tbx5 and Tbx4, but not for Pitx1, in initiation of limb outgrowth. In contrast to conclusions from gene misexpression experiments in the chick, our results demonstrate that Tbx5 and Tbx4 do not determine limb-specific morphologies. However, our results support a role for Pitx1 in the specification of hindlimb-specific morphology. We propose a model in which positional codes, such as Pitx1 and Hox genes in the lateral plate mesoderm, dictate limb-specific morphologies.

Origination and innovation in the vertebrate limb skeleton: an epigenetic perspective

The vertebrate limb has provided evolutionary and developmental biologists with grist for theory and experiment for at least a century. Its most salient features are its pattern of discrete skeletal elements, the general proximodistal increase in element number as development proceeds, and the individualization of size and shape of the elements in line with functional requirements. Despite increased knowledge of molecular changes during limb development, however, the mechanisms for origination and innovation of the vertebrate limb pattern are still uncertain. We suggest that the bauplan of the limb is based on an interplay of genetic and epigenetic processes; in particular, the self-organizing properties of precartilage mesenchymal tissue are proposed to provide the basis for its ability to generate regularly spaced nodules and rods of cartilage. We provide an experimentally based "core" set of cellular and molecular processes in limb mesenchyme that, under realistic conditions, exhibit the requisite self-organizing behavior for pattern origination. We describe simulations that show that under limb bud-like geometries the core mechanism gives rise to skeletons with authentic proximodistal spatiotemporal organization. Finally, we propose that evolution refines skeletal templates generated by this process by mobilizing accessory molecular and biomechanical regulatory processes to shape the developing limb and its individual elements. Morphological innovation may take place when such modulatory processes exceed a threshold defined by the dynamics of the skeletogenic system and elements are added or lost.

Different thresholds of fibroblast growth factors pattern the ventral foregut into liver and lung

Cell fate and morphogenesis within the embryo is dependent upon secreted molecules that transduce signals between neighboring tissues. Reciprocal mesenchymal-epithelial interactions have proven essential during branching morphogenesis and cell differentiation within the lung; however, the interactions that result in lung specification from the foregut endoderm, prior to lung bud formation, are poorly understood. In this study, we investigate the tissue requirements and signals necessary for specification of a pulmonary cell fate using embryo tissue explants. We show that NKX2.1, an early transcription factor crucial for lung development, is expressed in the ventral foregut endoderm shortly after albumin and Pdx1, early markers of the liver and pancreas lineages, respectively. Similar to hepatic specification, direct contact of cardiac mesoderm with ventral endoderm is required to induce in vitro expression of NKX2.1 and downstream lung target genes including surfactant protein C and Clara cell secretory protein. In the absence of cardiac mesoderm, ventral foregut endoderm explants respond to exogenous fibroblast growth factor (FGF) 1 and FGF2 in a dose-dependent manner, with lower concentrations activating liver specific genes and higher concentrations activating lung specific genes. This signaling appears to be instructive, as the prospective dorsal midgut endoderm, which predominantly gives rise to the intestinal tract, is competent to respond to FGFs by inducing NKX2.1. Furthermore, the temporal expression and selective inhibition of FGF receptors 1 and 4 present within the endoderm implies that signaling through FGFR4 is involved in specifying lung versus liver. Together, the findings suggest that a concentration threshold of FGFs emanating from the cardiac mesoderm are involved in patterning the foregut endoderm.

Vertebrate limb development: from Harrison's limb disk transplantations to targeted disruption of Hox genes

Various animal organs have long been used to investigate the cellular and molecular nature of embryonic growth and morphogenesis. Among those organs, the tetrapod limb has been preferentially used as a model system for elucidating general patterning mechanisms. At the appropriate time during the embryonic period, the limb territories are first determined at the right positions along the cephalocaudal axis of the animal body, and soon the limb buds grow out from the flanks as mesenchymal cell masses covered by simple ectoderm. The position, number, and identity of the limbs depend on the expression of specific Hox genes. Limb morphogenesis occurs along three axes, which become gradually fixed: first the anteroposterior axis, then the dorsoventral, and finally the proximodistal axis, along which the bulk of limb growth occurs. Growth of the limb in amniotes depends on the formation of the apical ectodermal ridge, which, by secreting many members of the fibroblast growth factors family, attracts lateral plate and somitic mesodermal cells, keeps these cells in the progress zone proliferating, and prevents their differentiation until an appropriate time period. Mutual interactions between mesoderm and ectoderm are important in the growth process, and signaling regions have been identified, such as the zone of polarizing activity, the dorsal limb ectoderm, and the apical ectodermal ridge. Several molecules have been found to play leading roles in various biological processes relevant to morphogenesis. Besides its intrinsic merit as a model for unraveling the mechanisms of development, the limb deserves considerable clinical interest because defects of limb development are the most common single category of congenital abnormalities.

Tbx5 and Tbx4 transcription factors interact with a new chicken PDZ-LIM protein in limb and heart development

The T-domain transcription factors, Tbx5 and Tbx4, play important roles in vertebrate limb and heart development. To identify interacting and potential Tbx-regulating proteins, we performed a yeast two-hybrid screen with the C-terminal domain of Tbx5 as bait. We identified a new PDZ-LIM protein composed of one N-terminal PDZ and three C-terminal LIM domains, which we named chicken LMP-4. Among the Tbx2, 3, 4, 5 subfamily, we observed exclusive interaction with Tbx5 and Tbx4 proteins. Tbx3 nor Tbx2 can substitute for LMP-4 binding. While chicken LMP-4 associates with Tbx5 or Tbx4, it uses distinct LIM domains to bind to the individual proteins. Subcellular co-localization of LMP-4 and Tbx proteins supports the protein interaction and reveals interference of LMP-4 with Tbx protein distribution, tethering the transcription factors to the cytoskeleton. The protein-protein interaction indicates regulation of Tbx function at the level of transcription factor nuclear localization. During chicken limb and heart development, Tbx5/LMP-4 and Tbx4/LMP-4 are tightly co-expressed in a temporal and spatial manner, suggesting that they operate in the same pathway. Surprisingly, chicken LMP-4 expression domains outside those of Tbx5 in the heart led to the discovery of Tbx4 expression in the outflow tract and the right ventricle of this organ. The Tbx4-expressing cells coincide with those of the recently discovered secondary anterior heart-forming field. The discrete posterior or anterior expression domains in the heart and the exclusive fore- or hindlimb expression of Tbx5 and Tbx4, respectively, suggest common pathways in the heart and limbs. The identification of a new Tbx5/4-specific binding factor further suggests a novel mechanism for Tbx transcription factor regulation in development and disease.

The roles of Fgf4 and Fgf8 in limb bud initiation and outgrowth

Although numerous molecules required for limb bud formation have recently been identified, the molecular pathways that initiate this process and ensure that limb formation occurs at specific axial positions have yet to be fully elucidated. Based on experiments in the chick, Fgf8 expression in the intermediate mesoderm (IM) has been proposed to play a critical role in the initiation of limb bud outgrowth via restriction of Fgf10 expression to the appropriate region of the lateral plate mesoderm. Contrary to the outcome predicted by this model, ablation of Fgf8 expression in the intermediate mesoderm before limb bud initiation had no effect on initial limb bud outgrowth or on the formation of normal limbs. When their expression patterns were first elucidated, both Fgf4 and Fgf8 were proposed to mediate critical functions of the apical ectodermal ridge (AER), which is required for proper limb bud outgrowth. Although mice lacking Fgf4 in the AER have normal limbs, limb development is severely affected in Fgf8 mutants and certain skeletal elements are not produced. By creating mice lacking both Fgf4 and Fgf8 function in the forelimb AER, we show that limb bud mesenchyme fails to survive in the absence of both FGF family members. Thus, Fgf4 is responsible for the partial compensation of distal limb development in the absence of Fgf8. A prolonged period of increased apoptosis, beginning at 10 days of gestation in a proximal-dorsal region of the limb bud, leads to the elimination of enough mesenchymal cells to preclude formation of distal limb structures. Expression of Shh and Fgf10 is nearly abolished in double mutant limb buds. By using a CRE driver expressed in both forelimb and hindlimb ectoderm to inactivate Fgf4 and Fgf8, we have produced mice lacking all limbs, allowing a direct comparison of FGF requirements in the two locations.

Biology of developmental and regenerative skeletogenesis

Embryonic skeletal development involves the recruitment, commitment, differentiation, and maturation of mesenchymal cells into those in the skeletal tissue lineage, specifically cartilage and bone along the intramembranous and endochondral ossification pathways. The exquisite control of skeletal development is regulated at the level of gene transcription, cellular signaling, cell-cell and cell-matrix interactions, as well as systemic modulation. Mediators include transcription factors, growth factors, cytokines, metabolites, hormones, and environmentally derived influences. Understanding the mechanisms underlying developmental skeletogenesis is crucial to harnessing the inherent regenerative potential of skeletal tissues for wound healing and repair, as well as for functional skeletal tissue engineering. In this review, a number of key issues are discussed concerning the current and future challenges of the scientific investigation of developmental skeletogenesis in the embryo, specifically limb cartilage development, and how these challenges relate to regenerative or reparative skeletogenesis in the adult. Specifically, a more complete understanding the biology of skeletogenic progenitor cells and the cellular and molecular mechanisms governing tissue patterning and morphogenesis should greatly facilitate the development of regenerative approaches to cartilage repair.

Sp8 and Sp9, two closely related buttonhead-like transcription factors, regulate Fgf8expression and limb outgrowth in vertebrate embryos

Initiation and maintenance of signaling centers is a key issue during embryonic development. The apical ectodermal ridge, a specialized epithelial structure and source of Fgf8, is a pivotal signaling center for limb outgrowth. We show that two closely related buttonhead-like zinc-finger transcription factors, Sp8 and Sp9, are expressed in the AER, and regulate Fgf8 expression and limb outgrowth. Embryological and genetic analyses have revealed that Sp8and Sp9 are ectodermal targets of Fgf10 signaling from the mesenchyme. We also found that Wnt/β-catenin signaling positively regulates Sp8, but not Sp9. Overexpression functional analyses in chick unveiled their role as positive regulators of Fgf8expression. Moreover, a dominant-negative approach in chick and knockdown analysis with morpholinos in zebrafish revealed their requirement for Fgf8 expression and limb outgrowth, and further indicate that they have a coordinated action on Fgf8 expression. Our study demonstrates that Sp8 and Sp9, via Fgf8, are involved in mediating the actions of Fgf10 and Wnt/β-catenin signaling during vertebrate limb outgrowth.

Dynamical mechanisms for skeletal pattern formation in the vertebrate limb

We describe a 'reactor-diffusion' mechanism for precartilage condensation based on recent experiments on chondrogenesis in the early vertebrate limb and additional hypotheses. Cellular differentiation of mesenchymal cells into subtypes with different fibroblast growth factor (FGF) receptors occurs in the presence of spatio-temporal variations of FGFs and transforming growth factor-betas (TGF-betas). One class of differentiated cells produces elevated quantities of the extracellular matrix protein fibronectin, which initiates adhesion-mediated preskeletal mesenchymal condensation. The same class of cells also produces an FGF-dependent laterally acting inhibitor that keeps condensations from expanding beyond a critical size. We show that this 'reactor-diffusion' mechanism leads naturally to patterning consistent with skeletal form, and describe simulations of spatio-temporal distribution of these differentiated cell types and the TGF-beta and inhibitor concentrations in the developing limb bud.

Gene therapy approaches for bone regeneration

Gene therapy represents a promising approach for delivering regenerative molecules to specific tissues including bone. Several laboratories have shown that virus-based BMP expression vectors can stimulate osteoblast differentiation and bone formation in vivo. Both in vivo and ex vivo transduction of cells can induce bone formation at ectopic and orthotopic sites. Adenovirus and direct DNA delivery of genes encoding regenerative molecules can heal critical-sized defects of cranial and long bones. Although osteogenic activity can be demonstrated for individual BMP vectors, substantial synergies may be achieved using combinatorial gene therapy to express complimentary osteogenic signals including specific combinations of BMPs or BMPs and transcription factors. Further control of the bone regeneration process may also be achieved through the use of inducible promoters that can be used to control the timing and magnitude of expression for a particular gene. Using these types of approaches, it should be possible to mimic natural processes of bone development and fracture repair and, in so doing, be able to precisely control both the amount and type of bone regenerated.

Chick Dach1 interacts with the Smad complex and Sin3a to control AER formation and limb development along the proximodistal axis

Based on recent data, a new view is emerging that vertebrate Dachshund(Dach) proteins are components of Six1/6 transcription factor-dependent signaling cascades. Although Drosophila data strongly suggest a tight link between Dpp signaling and the Dachshund gene, a functional relationship between vertebrate Dach and BMP signaling remains undemonstrated. We report that chick Dach1 interacts with the Smad complex and the corepressor mouse Sin3a, thereby acting as a repressor of BMP-mediated transcriptional control. In the limb, this antagonistic action regulates the formation of the apical ectodermal ridge (AER) in both the mesenchyme and the AER itself, and also controls pattern formation along the proximodistal axis of the limb. Our data introduce a new paradigm of BMP antagonism during limb development mediated by Dach1, which is now proven to function in different signaling cascades with distinct interacting partners.

Disruption of Fgf10/Fgfr2b-coordinated epithelial-mesenchymal interactions causes cleft palate

Classical research has suggested that early palate formation develops via epithelial-mesenchymal interactions, and in this study we reveal which signals control this process. Using Fgf10-/-, FGF receptor 2b-/- (Fgfr2b-/-), and Sonic hedgehog (Shh) mutant mice, which all exhibit cleft palate, we show that Shh is a downstream target of Fgf10/Fgfr2b signaling. Our results demonstrate that mesenchymal Fgf10 regulates the epithelial expression of Shh, which in turn signals back to the mesenchyme. This was confirmed by demonstrating that cell proliferation is decreased not only in the palatal epithelium but also in the mesenchyme of Fgfr2b-/- mice. These results reveal a new role for Fgf signaling in mammalian palate development. We show that coordinated epithelial-mesenchymal interactions are essential during the initial stages of palate development and require an Fgf-Shh signaling network.

Gremlin-mediated BMP antagonism induces the epithelial-mesenchymal feedback signaling controlling metanephric kidney and limb organogenesis

Epithelial-mesenchymal feedback signaling is the key to diverse organogenetic processes such as limb bud development and branching morphogenesis in kidney and lung rudiments. This study establishes that the BMP antagonist gremlin (Grem1) is essential to initiate these epithelial-mesenchymal signaling interactions during limb and metanephric kidney organogenesis. A Grem1 null mutation in the mouse generated by gene targeting causes neonatal lethality because of the lack of kidneys and lung septation defects. In early limb buds, mesenchymal Grem1 is required to establish a functional apical ectodermal ridge and the epithelial-mesenchymal feedback signaling that propagates the sonic hedgehog morphogen. Furthermore, Grem1-mediated BMP antagonism is essential to induce metanephric kidney development as initiation of ureter growth,branching and establishment of RET/GDNF feedback signaling are disrupted in Grem1-deficient embryos. As a consequence, the metanephric mesenchyme is eliminated by apoptosis, in the same way as the core mesenchymal cells of the limb bud.

Mutations affecting liver development and function in Medaka, Oryzias latipes, screened by multiple criteria

We report here mutations affecting various aspects of liver development and function identified by multiple assays in a systematic mutagenesis screen in Medaka. The 22 identified recessive mutations assigned to 19 complementation groups fell into five phenotypic groups. Group 1, showing defective liver morphogenesis, comprises mutations in four genes, which may be involved in the regulation of growth or patterning of the gut endoderm. Group 2 comprises mutations in three genes that affect the laterality of the liver; in kendama mutants of this group, the laterality of the heart and liver is uncoupled and randomized. Group 3 includes mutations in three genes altering bile color, indicative of defects in hemoglobin-bilirubin metabolism and globin synthesis. Group 4 consists of mutations in three genes, characterized by a decrease in the accumulation of fluorescent metabolite of a phospholipase A(2) substrate, PED6, in the gall bladder. Lipid metabolism or the transport of lipid metabolites may be affected by these mutations. Mutations in Groups 3 and 4 may provide animal models for relevant human diseases. Group 5 mutations in six genes affect the formation of endoderm, endodermal rods and hepatic bud from which the liver develops. These Medaka mutations, identified by morphological and metabolite marker screens, should provide clues to understanding molecular mechanisms underlying formation of a functional liver.

Fgf10 is an oncogene activated by MMTV insertional mutagenesis in mouse mammary tumors and overexpressed in a subset of human breast carcinomas

Mouse mammary tumor virus (MMTV) infection causes a high incidence of murine mammary carcinomas by insertion of its proviral DNA in the genome of mammary epithelial cells. Retroviral insertion can activate flanking proto-oncogenes by a process called insertional mutagenesis. By sequencing the DNA adjacent to MMTV proviral insertions in mammary tumors from BALB/c mice infected with C3H-MMTV, we have found a common MMTV insertion site in the Fgf10 locus. RT-PCR studies showed that Fgf10 is expressed only in those tumors harboring a MMTV proviral insertion in this locus, suggesting that Fgf10 is a proto-oncogene. The oncogenicity of Fgf10 was evaluated in vivo by subcutaneous transplantation of retrovirally transduced HC11 mammary epithelial cells into BALB/c mice. Highly vascularized invasive subcutaneous tumors developed indicating that Fgf10 can act as an oncogene. A survey of primary human breast carcinomas revealed strongly elevated Fgf10 mRNA levels in approximately 10% of the tumors tested, suggesting that Fgf10 may also be involved in oncogenicity of a subset of human breast cancers.

IkappaB kinase-alpha acts in the epidermis to control skeletal and craniofacial morphogenesis

IkappaB kinase-alpha (IKK-alpha) exhibits protein-kinase-dependent and -independent functions. Its kinase activity is required for lymphoid organogenesis and mammary gland development, whereas a kinase-independent activity is required for epidermal keratinocyte differentiation. In addition to failed epidermal differentiation, IKK-alpha-deficient mice exhibit abnormal skeletal and craniofacial morphogenesis. As similar defects are not exhibited by mice that experience systemic inhibition of NF-kappaB, we postulated that the morphogenetic defects in IKK-alpha-deficient mice are not caused by reduced NF-kappaB activity but instead are due to failed epidermal differentiation that disrupts proper epidermal-mesodermal interactions. We tested this hypothesis by introducing an epidermal-specific Ikka (also known as Chuk) transgene into IKK-alpha-deficient mice. Mice lacking IKK-alpha in all cell types including bone and cartilage, but not in basal epidermal keratinocytes, exhibit normal epidermal differentiation and skeletal morphology. Thus, epidermal differentiation is required for proper morphogenesis of mesodermally derived skeletal elements. One way by which IKK-alpha controls skeletal and craniofacial morphogenesis is by repressing expression of fibroblast growth factor (FGF) family members, such as FGF8, whose expression is specifically elevated in the limb bud ectoderm of IKK-alpha-deficient mice.

SOX7 and GATA-4 are competitive activators of Fgf-3 transcription

Fgf-3 is expressed in a dynamic and complex spatiotemporal pattern during mouse development. Previous studies identified GATA-4 as a transcription factor that binds the key regulatory element PS4A of the Fgf-3 promoter and stimulates transcription. Here we show that members of the SOX family of transcription factors also bind PS4A and differentially modulate transcription. At least five SOX genes, Sox2, Sox6, Sox7, Sox13, and Sox17, were expressed in F9 cells, and of these, Sox7 and Sox17 were dramatically induced in parallel with Fgf-3 following differentiation into parietal endoderm-like cells with retinoic acid and dibutyryl cAMP. Complexes could be detected on PS4A with SOX2, SOX7, and SOX17 by using nuclear extracts from differentiated F9 cells. However, only Sox7 expression markedly activated the Fgf-3 promoter in these cells. By contrast, SOX2 was a poor activator of Fgf-3 transcription, and when Sox2 was coexpressed with Gata4, it negatively modulated the strong activation mediated by GATA-4. More detailed analyses showed that SOX7 competes with GATA-4 for PS4A occupancy and to activate the Fgf-3 promoter. In situ hybridization analysis showed that Sox7 is co-expressed with Fgf-3 and Gata4 in the parietal endoderm of E7.5 mouse embryos. In culture, GATA-4-deficient embryonal stem cells were shown to express Fgf-3 upon differentiation into embryoid bodies, although at lower levels than were found in wild type embryonal stem cells. This Fgf-3 expression was virtually abolished when Sox7 expression was suppressed by RNA interference. These results show that SOX7 is a potent activator of Fgf-3 transcription.

Promotion and attenuation of FGF signaling through the Ras-MAPK pathway

The fibroblast growth factors (FGFs) represent a large family of ligands that activate signal transduction pathways leading to diverse biological responses, including many involved in various processes during development. Here, we discuss the discovery of a subset of conserved FGF target genes that encode feedback regulators of FGF signaling itself. Members of the Sprouty, Sef, and mitogen-activated protein kinase phosphatase families are negative modulators of FGF signaling, whereas positive factors that promote FGF signaling include the ETS transcription factors ERM and PEA3 and the transmembrane protein XFLRT3. These molecules affect the FGF signaling cascade at different levels to regulate the final output of the pathway. This multilayered regulation suggests that precise adjustment of FGF signaling is critical in development.

Effects of ethanol and hydrogen peroxide on mouse limb bud mesenchyme differentiation and cell death

Many of the morphological defects associated with embryonic alcohol exposure are a result of cell death. During limb development, ethanol administration produces cell death in the limb and digital defects, including postaxial ectrodactyly. Because an accumulation of reactive oxygen species (ROS) is produced in adult and embryonic tissues by ethanol exposure, this investigation examines the possibility that ethanol-induced cell death in the limb is a result of ROS. Using an in vitro primary culture of limb mesenchyme, the effects of hydrogen peroxide (H2O2) and ethanol on cell death and differentiation were examined. In addition, a dichlorofluorescein diacetate assay was performed to determine the relative intracellular ROS levels after exposure to several concentrations of ethanol and H2O2. Exposure of 1 to 100 microM H2O2 resulted in a 1.08-1.21 times control increase in cartilage matrix accumulation. Cell death was increased 1.69-2.76 times the untreated control value. Production of ROS ranged from 1.25-1.51 times untreated controls. Ethanol exposure of 0.25 to 1.00% (v/v) did not affect cartilage matrix accumulation but resulted in an increase of cell death (1.45-2.31 times untreated control). Intracellular ROS levels after ethanol exposure increased 1.08-1.15 times control but were lower than that produced by 1 microM H2O2. On the basis of the correlation between ROS level produced by H2O2, it was concluded that ethanol-induced cell death in limb mesenchyme is a result of a non-ROS-mediated mechanism. Therefore, in addition to ethanol-induced cell death mediated by ROS reported in the literature, ethanol-induced cell death can be induced in limb mesenchyme by mechanisms that are not dependent upon ROS.

Homozygous WNT3 mutation causes tetra-amelia in a large consanguineous family

Tetra-amelia is a rare human genetic disorder characterized by complete absence of all four limbs and other anomalies. We studied a consanguineous family with four affected fetuses displaying autosomal recessive tetra-amelia and craniofacial and urogenital defects. By homozygosity mapping, the disease locus was assigned to chromosome 17q21, with a maximum multipoint LOD score of 2.9 at markers D17S931, D17S1785, D17SS1827, and D17S1868. Further fine mapping defined a critical interval of approximately 8.9 Mb between D17S1299 and D17S797. We identified a homozygous nonsense mutation (Q83X) in the WNT3 gene in affected fetuses of the family. WNT3, a human homologue of the Drosophila wingless gene, encodes a member of the WNT family known to play key roles in embryonic development. The Q83X mutation truncates WNT3 at its amino terminus, suggesting that loss of function is the most likely cause of the disorder. Our findings contrast with the observation of early lethality in mice homozygous for null alleles of Wnt3. To our knowledge, this is the first report of a mutation in a WNT gene associated with a Mendelian disorder. The identification of a WNT3 mutation in tetra-amelia indicates that WNT3 is required at the earliest stages of human limb formation and for craniofacial and urogenital development.

Neurotrophin-independent attraction of growing sensory and motor axons towards developing Xenopus limb buds in vitro

The mechanisms for directing axons to their targets in developing limbs remain largely unknown though recent studies in mice have demonstrated the importance of neurotrophins in this process. We now report that in co-cultures of larval Xenopus laevis limb buds with spinal cords and dorsal root ganglia of Xenopus and axolotl (Ambystoma mexicanum) axons grow directly to the limb buds over distances of up to 800 microm and in particular to sheets of epidermal cells which migrate away from the limb buds and also tail segments in culture. This directed axonal growth persists in the presence of trk-IgG chimeras, which sequester neurotrophins, and k252a, which blocks their actions mediated via trk receptors. These findings indicate that developing limb buds in Xenopus release diffusible factors other than neurotrophins, able to attract growth of sensory and motor axons over long distances.

Distinctive Expression Patterns of Heparan Sulfate O-Sulfotransferases and Regional Differences in Heparan Sulfate Structure in Chick Limb Buds

The skeletal tissue development and patterning in chick limb buds are known to be under the spacio-temporal control of various heparin-binding cell growth factors such as fibroblast growth factors and bone morphogenetic proteins. Different structural regions on heparan sulfate (HS) chains of proteoglycans could be implicated in regional differences in the binding capacities of these cell growth factors, by which they could selectively interact with targeted cells and regulate their signaling in those processes. In this study we first demonstrated by cDNA cloning that one heparan sulfate 2-O-sulfotransferase (HS2ST) and two isoforms of heparan sulfate 6-O-sulfotransferase (HS6ST-1 and -2) occurred in chick embryos and had different substrate specificities each other. We next showed by whole mount in situ hybridization that the HS6ST-1 and HS6ST-2 transcripts were preferentially localized to the anterior proximal region and at the posterior proximal region of the limb bud, respectively, whereas the HS2ST transcript was distributed rather uniformly throughout the bud. Analyses of the structures of HS from different regions of the wing buds have shown variation in that 6-O-sulfated residues are more abundant in the proximal than distal region, whereas iduronosyl 6-O-sulfated residues are abundant in the anterior proximal region and glucuronosyl 6-O-sulfated residues in the posterior proximal region. These results suggest that HS with different sulfation patterns created with multiple sulfotransferase activities provides an appropriate extracellular environment for morphogenetic signal transduction.

A novel hypothesis for thalidomide-induced limb teratogenesis: redox misregulation of the NF-kappaB pathway

Several hypotheses have been proposed to explain the mechanisms of thalidomide teratogenesis, although none adequately accounts for the observed malformations and explains the basis for species specificity. Recent observations that thalidomide increases the production of free radicals and elicits oxidative stress, coupled with new insights into the redox regulation of nuclear transcription factors, lead to the suggestion that thalidomide may act through redox misregulation of the limb outgrowth pathways. Oxidative stress, as marked by glutathione depletion/oxidation and a shift in intracellular redox potential toward the positive, occurs preferentially in limbs of thalidomide-sensitive rabbits, but not in resistant rats. DNA binding of nuclear factor kappa-B (NF-kappaB), a redox-sensitive transcription factor and key regulator of limb outgrowth, was shown to be significantly attenuated in rabbit limb cells and could be restored following the addition of a free radical spin-trapping agent, phenyl N-tert-butyl nitrone. The inability of NF-kappaB to bind to its DNA promoter results in the failure of limb cells to express fibroblast growth factor (FGF)-10 and twist in the limb progress zone (PZ) mesenchyme, which in turn attenuates expression of FGF-8 in the apical ectodermal ridge (AER). Failure to establish an FGF-10/FGF-8 feedback loop between the PZ and AER results in the truncation of limb outgrowth. We hypothesize that species-selective alterations in redox microenvironment caused by free radical production from thalidomide results in attenuation of the NF-kappaB-mediated gene expression that is responsible for limb outgrowth.

Skeletal development is regulated by fibroblast growth factor receptor 1 signalling dynamics

Ligand-dependent signalling pathways have been characterised as having morphogen properties where there is a quantitative relationship between receptor activation and response, or threshold characteristics in which there is a binary switch in response at a fixed level of receptor activation. Here we report the use of a bacterial artificial chromosome (BAC)-based transgenic system in which a hypermorphic mutation has been introduced into the murine Fgfr1 gene. These mice exhibit cranial suture and sternal fusions that are exacerbated when the BAC copy number is increased. Surprisingly,increasing mutant BAC copy number also leads to the de novo appearance of digit I polydactyly in the hind limb and transformations of the vertebrae. Polydactyly is accompanied by a reduction of programmed cell death in the developing hind limb. Candidate gene analysis reveals downregulation of Dkk1 in the digit I field and upregulation of Wnt5a and Hoxd13. These findings show that Fgfr1-mediated developmental pathways exhibit differing signalling dynamics, whereby development of the cranial sutures and sternum follows a morphogen mode, whereas development of the vertebral column and the hind limbs has threshold signalling properties.