FGF-4 regulates expression of Evx-1 in the developing mouse limb

Relaxed selection in evolution of genes regulating limb development gives clue to variation in forelimb morphology of cetaceans and other mammals

Cetaceans have evolved unique limb structures, such as flippers, due to genetic changes during their transition to aquatic life. However, the full understanding of the genetic and evolutionary mechanisms behind these changes is still developing. By examining 25 limb-related protein-coding genes across various mammalian species, we compared genetic changes between aquatic mammals, like whales, and other mammals with unique limb structures such as bats, rodents and elephants. Our findings revealed significant modifications in limb-related genes, including variations in the Hox, GDF5 and Evx genes. Notably, a relaxed selection in several key genes was observed, suggesting a lifting of developmental constraints, which might have facilitated the emergence of morphological innovations in cetacean limb morphology. We also uncovered non-synonymous changes, insertions and deletions in these genes, particularly in the polyalanine tract of HOXD13, which are distinctive to cetaceans or convergent with other aquatic mammals. These genetic variations correlated with the diverse and specialized limb structures observed in cetaceans, indicating a complex interplay of relaxed selection and specific mutations in mammalian limb evolution.

Integrated live imaging and molecular profiling of embryoid bodies reveals a synchronized progression of early differentiation

Embryonic stem cells can spontaneously differentiate into cell types of all germ layers within embryoid bodies (EBs) in a highly variable manner. Whether there exists an intrinsic differentiation program common to all EBs is unknown. Here, we present a novel combination of high-throughput live two-photon imaging and gene expression profiling to study early differentiation dynamics spontaneously occurring within developing EBs. Onset timing of Brachyury-GFP was highly variable across EBs, while the spatial patterns as well as the dynamics of mesendodermal progression following onset were remarkably similar. We therefore defined a 'developmental clock' using the Brachyury-GFP signal onset timing. Mapping snapshot gene expression measurements to this clock revealed their temporal trends, indicating that loss of pluripotency, formation of primitive streak and mesodermal lineage progression are synchronized in EBs. Exogenous activation of Wnt or BMP signaling accelerated the intrinsic clock. CHIR down-regulated Wnt3, allowing insights into dependency mechanisms between canonical Wnt signaling and multiple genes. Our findings reveal a developmental clock characteristic of an early differentiation program common to all EBs, further establishing them as an in vitro developmental model.

Polymodal Transient Receptor Potential Vanilloid (TRPV) Ion Channels in Chondrogenic Cells

Mature and developing chondrocytes exist in a microenvironment where mechanical load, changes of temperature, osmolarity and acidic pH may influence cellular metabolism. Polymodal Transient Receptor Potential Vanilloid (TRPV) receptors are environmental sensors mediating responses through activation of linked intracellular signalling pathways. In chondrogenic high density cultures established from limb buds of chicken and mouse embryos, we identified TRPV1, TRPV2, TRPV3, TRPV4 and TRPV6 mRNA expression with RT-PCR. In both cultures, a switch in the expression pattern of TRPVs was observed during cartilage formation. The inhibition of TRPVs with the non-selective calcium channel blocker ruthenium red diminished chondrogenesis and caused significant inhibition of proliferation. Incubating cell cultures at 41 °C elevated the expression of TRPV1, and increased cartilage matrix production. When chondrogenic cells were exposed to mechanical load at the time of their differentiation into matrix producing chondrocytes, we detected increased mRNA levels of TRPV3. Our results demonstrate that developing chondrocytes express a full palette of TRPV channels and the switch in the expression pattern suggests differentiation stage-dependent roles of TRPVs during cartilage formation. As TRPV1 and TRPV3 expression was altered by thermal and mechanical stimuli, respectively, these are candidate channels that contribute to the transduction of environmental stimuli in chondrogenic cells.

Quantitative analysis of tissue deformation dynamics reveals three characteristic growth modes and globally aligned anisotropic tissue deformation during chick limb development

Tissue-level characterization of deformation dynamics is crucial for understanding organ morphogenetic mechanisms, especially the interhierarchical links among molecular activities, cellular behaviors and tissue/organ morphogenetic processes. Limb development is a well-studied topic in vertebrate organogenesis. Nevertheless, there is still little understanding of tissue-level deformation relative to molecular and cellular dynamics. This is mainly because live recording of detailed cell behaviors in whole tissues is technically difficult. To overcome this limitation, by applying a recently developed Bayesian approach, we here constructed tissue deformation maps for chick limb development with high precision, based on snapshot lineage tracing using dye injection. The precision of the constructed maps was validated with a clear statistical criterion. From the geometrical analysis of the map, we identified three characteristic tissue growth modes in the limb and showed that they are consistent with local growth factor activity and cell cycle length. In particular, we report that SHH signaling activity changes dynamically with developmental stage and strongly correlates with the dynamic shift in the tissue growth mode. We also found anisotropic tissue deformation along the proximal-distal axis. Morphogenetic simulation and experimental studies suggested that this directional tissue elongation, and not local growth, has the greatest impact on limb shaping. This result was supported by the novel finding that anisotropic tissue elongation along the proximal-distal axis occurs independently of cell proliferation. Our study marks a pivotal point for multi-scale system understanding in vertebrate development.

Summary: Chick limb morphogenesis occurs through issue elongation independently of cell proliferation, with three tissue growth modes correlated with SHH expression.

Differential gene expression of the intermediate and outer interzone layers of developing articular cartilage in murine embryos

Nascent embryonic joints, interzones, contain a distinct cohort of progenitor cells responsible for the formation of the majority of articular tissues. However, to date the interzone has largely been studied using in situ analysis for candidate genes in the context of the embryo rather than using an unbiased genome-wide expression analysis on isolated interzone cells, leaving significant controversy regarding the exact role of the intermediate and outer interzone layers in joint formation. Therefore, in this study, using laser capture microdissection (three biological replicates), we selectively harvested the intermediate and outer interzones of mouse embryos at gestational age 15.5 days, just prior to cavitation, when the differences between the layers should be most profound. Microarray analysis (Agilent Whole Mouse Genome Oligo Microarrays) was performed and the differential gene expression between the intermediate interzone cells and outer interzone cells was examined by performing a two-sided paired Student's t-test and pathway analysis. One hundred ninety-seven genes were differentially expressed (≥ 2-fold) between the intermediate interzone and the outer interzone with a P-value ≤ 0.01. Of these, 91 genes showed higher expression levels in the intermediate interzone and 106 were expressed higher in the outer interzone. Pathway analysis of differentially expressed genes suggests an important role for inflammatory processes in the interzone layers, especially in the intermediate interzone, and hence in joint and articular cartilage development. The high representation of genes relevant to chondrocyte hypertrophy and endochondral ossification in the outer interzone suggests that it undergoes endochondral ossification.

Evx1 is required for joint formation in zebrafish fin dermoskeleton

The transcription factor Evx1 is expressed in the joints between individual lepidotrichia (bony ray) segments and at the distal tips of the lepidotrichia in developing zebrafish fins. It is also expressed in the apical growth zone in regenerating fins. However, so far there is no functional evidence that addresses whether Evx1 is required for any aspect of fin development or regeneration. In this study, we use a novel mutation in evx1 to address this. We find that Evx1 is not required for either fin outgrowth or regeneration. All of the fins form normally in evx1 mutants, and there are no significant changes in fin length. In contrast, Evx1 is required for lepidotrichia joint formation during both fin development and regeneration. This is a very specific phenotype as both lepidotrichia hemisegment separations and lepidotrichia bifurcations still form normally in evx1 mutant fins, as do joints in the more proximal endoskeletal radials.

The role of spatially controlled cell proliferation in limb bud morphogenesis

Although the vertebrate limb bud has been studied for decades as a model system for spatial pattern formation and cell specification, the cellular basis of its distally oriented elongation has been a relatively neglected topic by comparison. The conventional view is that a gradient of isotropic proliferation exists along the limb, with high proliferation rates at the distal tip and lower rates towards the body, and that this gradient is the driving force behind outgrowth. Here we test this hypothesis by combining quantitative empirical data sets with computer modelling to assess the potential role of spatially controlled proliferation rates in the process of directional limb bud outgrowth. In particular, we generate two new empirical data sets for the mouse hind limb--a numerical description of shape change and a quantitative 3D map of cell cycle times--and combine these with a new 3D finite element model of tissue growth. By developing a parameter optimization approach (which explores spatial patterns of tissue growth) our computer simulations reveal that the observed distribution of proliferation rates plays no significant role in controlling the distally extending limb shape, and suggests that directional cell activities are likely to be the driving force behind limb bud outgrowth. This theoretical prediction prompted us to search for evidence of directional cell orientations in the limb bud mesenchyme, and we thus discovered a striking highly branched and extended cell shape composed of dynamically extending and retracting filopodia, a distally oriented bias in Golgi position, and also a bias in the orientation of cell division. We therefore provide both theoretical and empirical evidence that limb bud elongation is achieved by directional cell activities, rather than a PD gradient of proliferation rates.

Fgf-dependent Etv4/5 activity is required for posterior restriction of Sonic Hedgehog and promoting outgrowth of the vertebrate limb

Crosstalk between the fibroblast growth factor (FGF) and Sonic Hedgehog (Shh) pathways is critical for proper patterning and growth of the developing limb bud. Here, we show that FGF-dependent activation of the ETS transcription factors Etv4 and Etv5 contributes to proximal-distal limb outgrowth. Surprisingly, blockage of Etv activity in early distal mesenchyme also resulted in ectopic, anterior expansion of Shh, leading to a polydactylous phenotype. These data indicate an unexpected function for an FGF/Etv pathway in anterior-posterior patterning. FGF activity in the limb is not only responsible for maintaining posterior-specific Shh expression, but it also acts via Etvs to prevent inappropriate anterior expansion of Shh. This study identifies another level of genetic interaction between the orthogonal axes during limb development.

TGF-beta3 inhibits chondrogenesis of cultured chick leg bud mesenchymal cells via downregulation of connexin 43 and integrin beta4

Transforming growth factor beta (TGF-beta) is a multifunctional cytokine that regulates a number of biological responses including chemotaxis, cell cycle progression, differentiation, and apoptosis of cells. Even though temporal and spatial expression of TGF-beta3 suggests its role in chick limb development, it is not well characterized how TGF-beta3 regulates chondrogenic differentiation of limb bud mesenchymal cells. In this study, differential display polymerase chain reaction (DD-PCR) screening and reverse transcription PCR analysis revealed that the mRNA expression of the gap junction protein, connexin 43 (Cx43), was significantly decreased during the first treatment of TGF-beta3 for 24 h in cultured chick leg bud mesenchymal cells. Treatment of these cells with lindane, a general gap junction blocker, or expression of dominant negative Cx43 increased apoptotic cell death and decreased the level of integrin beta4 protein, in a manner similar to that observed when these cells were exposed to TGF-beta3. Similarly, exposure of cultured leg chondroblasts to a functional blocking antibody against integrin-beta4 induced an increase in apoptosis. Treatment of cells with TGF-beta3 decreased the membrane translocation of PKC-alpha, leading to activation of ERK. The increase in apoptotic cell death triggered by TGF-beta3 and dominant negative Cx43 was blocked by inhibition of ERK but increased by inhibition of PKC. Collectively, these data indicate that, in cultured chick leg bud mesenchyme cells, TGF-beta3 treatment downregulates Cx43 and induces apoptotic cell death via downregulation of integrin beta4, activation of ERK and suppression of PKC-alpha activation.

Expression of the short stature homeobox gene Shox is restricted by proximal and distal signals in chick limb buds and affects the length of skeletal elements

SHOX is a homeobox-containing gene, highly conserved among species as diverse as fish, chicken and humans. SHOX gene mutations have been shown to cause idiopathic short stature and skeletal malformations frequently observed in human patients with Turner, Leri-Weill and Langer syndromes. We cloned the chicken orthologue of SHOX, studied its expression pattern and compared this with expression of the highly related Shox2. Shox is expressed in central regions of early chick limb buds and proximal two thirds of later limbs, whereas Shox2 is expressed more posteriorly in the proximal third of the limb bud. Shox expression is inhibited distally by signals from the apical ectodermal ridge, both Fgfs and Bmps, and proximally by retinoic acid signaling. We tested Shox functions by overexpression in embryos and micromass cultures. Shox-infected chick limbs had normal proximo-distal patterning but the length of skeletal elements was consistently increased. Primary chick limb bud cell cultures infected with Shox showed an initial increase in cartilage nodules but these did not enlarge. These results fit well with the proposed role of Shox in cartilage and bone differentiation and suggest chick embryos as a useful model to study further the role of Shox in limb development.

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.

Fibroblast growth factors lead to increased Msx2 expression and fusion in calvarial sutures

Craniosynostosis, the premature fusion of the skull bones at the sutures, represents a disruption to the coordinated growth and development of the expanding brain and calvarial vault and is the second most common birth defect that affects the craniofacial complex. Mutations in the human homeobox-containing gene, Msx2, have been shown to cause Boston type craniosynostosis, and we have shown that overexpression of Msx2 leads to craniosynostosis in mice. Activating mutations in fibroblast growth factor (FGF) receptors are thought to cause craniosynostosis in Crouzon, Apert, Jackson-Weiss, Beare-Stevenson, and Muenke syndromes. To mimic activated signaling by mutated FGF receptors, we used heparin acrylic beads to deliver FGF ligands to mouse calvaria and demonstrated increased Msx2, Runx2, Bsp, and Osteocalcin gene expression, decreased cell proliferation, and suture obliteration and fusion. FGF2 elicited the greatest increase in Msx2 expression, and FGF1 was most likely to cause suture obliteration and fusion. Of the three sutures studied, the coronal suture exhibited the greatest increase in Msx2 expression and was the most likely to undergo obliteration and fusion. These results are intriguing because the coronal suture is the most commonly affected suture in syndromic craniosynostosis. These results suggest that Msx2 is a downstream target of FGF receptor signaling and that increased FGF signaling leads to osteogenic differentiation by sutural mesenchyme in mouse calvaria. These results are consistent with the hypotheses that increased Msx2 expression and activated signaling by mutated FGF receptors lead to craniosynostosis.

Inhibition of BMP activity by the FGF signal promotes posterior neural development in zebrafish

The expression patterns of region-specific neuroectodermal genes and fate-map analyses in zebrafish gastrulae suggest that posterior neural development is initiated by nonaxial signals, distinct from organizer-derived secreted bone morphogenetic protein (BMP) antagonists. This notion is further supported by the misexpression of a constitutively active form of zebrafish BMP type IA receptor (CA-BRIA) in the zebrafish embryos. It effectively suppressed the anterior neural marker, otx2, but not the posterior marker, hoxb1b. Furthermore, we demonstrated that the cells in the presumptive posterior neural region lose their neural fate only when CA-BRIA and Xenopus dominant-negative fibroblast growth factor (FGF) receptors (XFD) are coexpressed. The indications are that FGF signaling is involved in the formation of the posterior neural region, counteracting the BMP signaling pathway within the target cells. We then examined the functions of Fgf3 in posterior neural development. Zebrafish fgf3 is expressed in the correct place (dorsolateral margin) and at the correct time (late blastula to early gastrula stages), the same point that the most precocious posterior neural marker, hoxb1b, is first activated. Unlike other members of the FGF family, Fgf3 had little mesoderm-inducing activity. When ectopically expressed, Fgf3 expands the neural region with suppression of anterior neural fate. However, this effect was mediated by Chordino (zebrafish Chordin), because Fgf3 induces chordino expression in the epiblast and Fgf3-induced neural expansion was substantially suppressed in dino mutants with mutated chordino genes. The results obtained in the present study reveal multiple actions of the FGF signal on neural development: it antagonizes BMP signaling within posterior neural cells, induces the expression of secreted BMP antagonists, and suppresses anterior neural fate.

Multiple functions of fibroblast growth factor-8 (FGF-8) in chick eye development

Fibroblast growth factor-8 (FGF-8) is an important signaling molecule in the generation and patterning of the midbrain, tooth, and limb. In this study we show that it is also involved in eye development. In the chick, Fgf-8 transcripts first appear in the distal optic vesicle when it contacts the head ectoderm. Subsequently Fgf-8 expression increases and becomes localized to the central area of the presumptive neural retina (NR) only. Application of FGF-8 has two main effects on the eye. First, it converts presumptive retinal pigment epithelium (RPE) into NR. This is apparent by the failure to express Bmp-7 and Mitf (a marker gene for the RPE) in the outer layer of the optic cup, coupled with the induction of NR genes, such as Rx, Sgx-1 and Fgf-8 itself. The induced retina displays the typical multilayered cytoarchitecture and expresses late neuronal differentiation markers such as synaptotagmin and islet-1. The second effect of FGF-8 exposure is the induction of both lens formation and lens fiber differentiation. This is apparent by the expression of a lens specific marker, L-Maf, and by morphological changes of lens cells. These results suggest that FGF-8 plays a role in the initiation and differentiation of neural retina and lens.

Nerve dependency in scarless fetal wound healing

The human fetus is capable of healing cutaneous wounds without scar up to the third trimester of development This process of tissue repair is more akin to newt limb regeneration than classic adult scar forming wound repair. Regeneration of the newt limb is dependent on neural input in its early stages. This study was an attempt to determine whether a similar dependence on neural input exists for mammalian fetal wounds to heal without scar. The left hind limb of six fetal lambs was denervated during the early second trimester of development (day 55; term = 145 days). Two weeks after denervation, the animals were again exposed to create bilateral incisional and 6-mm-diameter excisional wounds on their innervated right and denervated left lower extremities. Five days after creation of these defects, the wounds were examined for alterations in repair. Four fetal lambs survived, and three were suitable for evaluation. There were marked alterations in wound healing seen after denervation. Excisional wounds on the innervated side contracted and decreased their surface area by 14 percent. In contrast, the denervated wounds not only failed to contract, but increased in size by 60 percent. Changes in the incisional wounds were equally distinctive. Innervated incisional wounds healed completely without scar and had a wound breaking strength comparable to that of normal skin (Table I). In contrast, two of the three denervated incisional wounds dehisced and failed to heal, even in the regions where the skin was approximated by suture. The third denervated incisional wound did heal but with a significant amount of scar. Electron microscopy confirmed this finding by clearly demonstrating thickened and irregular collagen deposition in the extracellular matrix of all the denervated incisional specimens. In summary, like the regenerating newt limb, scarless fetal skin wound repair requires neural stimulation for tissue regeneration to occur. Therefore, in the mammal, the primary regulator for this unique type of tissue repair may have a central neural, rather than a local, tissue origin.

Delta-1 negatively regulates the transition from prehypertrophic to hypertrophic chondrocytes during cartilage formation

Endochondral bone development begins with the formation of a cartilage template. Chondrocytes within this template undergo a progressive program of maturation from proliferative to prehypertrophic chondrocytes to hypertrophic chondrocytes. The progression of cells through these steps of differentiation must be carefully controlled to ensure coordinated growth. Because the Delta/Notch signaling system is known to regulate cell fate choices, we sought to determine if these molecules might be involved in the progressive cell fate decisions that chondocytes undergo. Here we demonstrate in the chick that Delta/Notch signaling negatively regulates progression from the prehypertrophic to hypertrophic state of differentiation. Delta-1 is expressed specifically in the hypertrophic chondrocytes while Notch-2 is expressed in chondrocytes at all stages. Misexpression of Delta-1 using a replication-competent retrovirus blocks chondrocyte maturation. Prehypertrophic cells form normally but do not undergo differentiation to hypertrophic cells, resulting in shortened skeletal elements that lack ossification. We conclude that Delta-1 acts during chondrogenesis to inhibit the transition from prehypertrophic chondrocytes to hypertrophic chondrocytes, thus defining a novel mechanism for the regulation of the chondrocyte maturation program. In addition, these results reveal a new role for Delta/Notch signaling in regulating the progression to a terminally differentiated state.

Targeted disruption of fibroblast growth factor (FGF) receptor 2 suggests a role for FGF signaling in pregastrulation mammalian development

We disrupted the fibroblast growth factor (FGF) receptor 2 (FGFR2) gene by introducing a neo cassette into the IIIc ligand binding exon and by deleting a genomic DNA fragment encoding its transmembrane domain and part of its kinase I domain. A recessive embryonic lethal mutation was obtained. Preimplantation development was normal until the blastocyst stage. Homozygous mutant embryos died a few hours after implantation at a random position in the uterine crypt, with collapsed yolk cavity. Mutant blastocysts hatched, adhered, and formed a layer of trophoblast giant cells in vitro, but after prolonged culture, the growth of the inner cell mass stopped, no visceral endoderm formed, and finally the egg cylinder disintegrated. It follows that FGFR2 is required for early postimplantation development between implantation and the formation of the egg cylinder. We suggest that FGFR2 contributes to the outgrowth, differentiation, and maintenance of the inner cell mass and raise the possibility that this activity is mediated by FGF4 signals transmitted by FGFR2. The role of early FGF signaling in pregastrulation development as a possible adaptation to mammalian (amniote) embryogenesis is discussed.

Analysis of the genetic pathway leading to formation of ectopic apical ectodermal ridges in mouse Engrailed-1 mutant limbs

The apical ectodermal ridge (AER), a rim of thickened ectodermal cells at the interface between the dorsal and ventral domains of the limb bud, is required for limb outgrowth and patterning. We have previously shown that the limbs of En1 mutant mice display dorsal-ventral and proximal-distal abnormalities, the latter being reflected in the appearance of a broadened AER and formation of ectopic ventral digits. A detailed genetic analysis of wild-type, En1 and Wnt7a mutant limb buds during AER development has delineated a role for En1 in normal AER formation. Our studies support previous suggestions that AER maturation involves the compression of an early broad ventral domain of limb ectoderm into a narrow rim at the tip and further show that En1 plays a critical role in the compaction phase. Loss of En1 leads to a delay in the distal shift and stratification of cells in the ventral half of the AER. At later stages, this often leads to development of a secondary ventral AER, which can promote formation of an ectopic digit. The second AER forms at the juxtaposition of the ventral border of the broadened mutant AER and the distal border of an ectopic Lmx1b expression domain. Analysis of En1/Wnt7a double mutants demonstrates that the dorsalizing gene Wnt7a is required for the formation of the ectopic AERs in En1 mutants and for ectopic expression of Lmx1b in the ventral mesenchyme. We suggest a model whereby, in En1 mutants, ectopic ventral Wnt7a and/or Lmx1b expression leads to the transformation of ventral cells in the broadened AER to a more dorsal phenotype. This leads to induction of a second zone of compaction ventrally, which in some cases goes on to form an autonomous secondary AER.

Analysis of the developing Xenopus tail bud reveals separate phases of gene expression during determination and outgrowth

We have studied Xenopus tail development from the end of gastrulation to the commencement of outgrowth at the tail bud stage. We show that an early group of genes are expressed at the stage of tail bud determination, at the end of gastrulation, and a late group are expressed at around stage 27 just before tail bud outgrowth. Together, these genes define seven distinct regions of the tail bud as outgrowth commences. We have previously shown that formation of a tail bud depends on the interaction of three tissue regions, called N, M and C, at stage 13. Here we show that expression of the late group of genes is dependent on this NMC interaction. We describe molecular correlates of two of these regions, M and C, which were formerly unobservable and whose existence was inferred from embryological experiments.

Transcriptional regulation of the HOX4C gene by basic fibroblast growth factor on rheumatoid synovial fibroblasts

OBJECTIVE

To examine the expression of genes of the HOX D cluster in the synovial tissue of patients with rheumatoid arthritis (RA), and to determine whether basic fibroblast growth factor (bFGF) influences the expression and transcriptional regulation of the gene.

METHODS

The expression of genes of the HOX D cluster, including HOX4C, HOX4D, HOX4H, and HOX4I, was determined in the synovium of 4 patients with RA and 4 with osteoarthritis (OA) by in situ reverse transcription (RT) and RT-polymerase chain reaction (RT-PCR). The induction of HOX4C messenger RNA (mRNA) by bFGF was determined by RT-PCR. The binding activity of a transcriptional regulator of the HOX4C gene, C2, was analyzed by the mobility shift assay. NIH-3T3 cells transfected with a construct containing C2 binding sequence were incubated with bFGF, and the activity of the reporter was measured by luciferase assay.

RESULTS

Using an in situ RT assay, specific expression of HOX4C mRNA was detected in 3 of 4 RA synovial samples, whereas none of the OA synovia expressed HOX4C. HOX4D, HOX4H, and HOX4I genes were expressed in all synovial samples from RA and OA patients. The presence of HOX4C mRNA was also confirmed by RT-PCR and Southern blotting. Treatment with bFGF increased the expression of HOX4C mRNA in RA fibroblasts. The mobility shift assay and luciferase assay showed that bFGF enhanced C2 binding activity and significantly increased the transcriptional activity on RA fibroblasts.

CONCLUSION

Our findings suggest that HOX4C is involved in synovial hyperplasia, and that the transcriptional regulation of HOX4C genes by bFGF may play a crucial role in the pathogenesis of RA.

Antagonistic interactions between FGF and BMP signaling pathways: a mechanism for positioning the sites of tooth formation

Vertebrate organogenesis is initiated at sites that are often morphologically indistinguishable from the surrounding region. Here we have identified Pax9 as a marker for prospective tooth mesenchyme prior to the first morphological manifestation of odontogenesis. We provide evidence that the sites of Pax9 expression in the mandibular arch are positioned by the combined activity of two signals, one (FGF8) that induces Pax9 expression and the other (BMP2 and BMP4) that prevents this induction. Thus it appears that the position of the teeth is determined by a combination of two different types of signaling molecules produced in wide but overlapping domains rather than by a single localized inducer. We suggest that a similar mechanism may be used for specifying the sites of development of other organs.

Limb deformity proteins: role in mesodermal induction of the apical ectodermal ridge

During early limb development, distal tip ectoderm is induced by the underlying mesenchyme to form the apical ectodermal ridge. Subsequent limb growth and patterning depend on reciprocal signaling between the mesenchyme and ridge. Mice that are homozygous for mutations at the limb deformity (ld) locus do not form a proper ridge and the anteroposterior axis of the limb is shortened. Skeletal analyses reveal shortened limbs that involve loss and fusion of distal bones and digits, defects in both anteroposterior and proximodistal patterning. Using molecular markers and mouse-chick chimeras we examined the ridge-mesenchymal interactions to determine the origin of the ld patterning defects. In the ld ridge, fibroblast growth factor 8 (Fgf8) RNA is decreased and Fgf4 RNA is not detected. In the ld mesenchyme, Sonic hedgehog (Shh), Evx1 and Wnt5a expression is decreased. In chimeras between ld ectoderm and wild-type mesenchyme, a ridge of normal morphology and function is restored, Fgf8 and Shh are expressed normally, Fgf4 is induced and a normal skeletal pattern arises. These results suggest that the ld mesenchyme is unable to induce the formation of a completely functional ridge. This primary defect causes a disruption of ridge function and subsequently leads to the patterning defects observed in ld limbs. We propose a model in which ridge induction requires at least two phases: an early competence phase, which includes induction of Fgf8 expression, and a later differentiation phase in which Fgf4 is induced and a morphological ridge is formed. Ld proteins appear to act during the differentiation phase.

Expression and regulation of Cek-8, a cell to cell signalling receptor in developing chick limb buds

The Eph-related receptor tyrosine kinase gene, Cek-8, is expressed in mesenchyme at the tip of chick limb buds, with high levels of transcripts posteriorly and apically but fading out anteriorly. Expression of Cek-8 in distal mesenchyme is regulated by apical ridge- and FGF-polarising signals and retinoic acid, and is uniform across the anteroposterior axis in talpid3 mutants. These data indicate that Cek-8 expression responds to regulatory signals during limb patterning and suggest that this receptor tyrosine kinase may have a role in coordinating responses to signals in the progress zone of early buds. Later on in limb development, Cek-8 expression is associated with cell condensations that form tendons and their attachments to cartilage rudiments and then in developing feather buds.

'Regeneration' of wing bud stumps of chick embryos and reactivation of Msx-1 and Shh expression in response to FGF-4 and ridge signals

We examined systematically the ability of chick limb bud stumps to regenerate distal structures when fibroblast growth factor (FGF)-4 is applied. When amputations were made within 600 mu m of the tip and FGF-4 applied either posteriorly or both apically and posteriorly, outgrowth of stump tissues occurred and a virtually complete skeleton developed. 'Regeneration' of distal structures was correlated with reactivation of Msx-1 and Shh expression. At proximal amputation levels where FGF-4 did not lead to 'regeneration', neither Msx-1 nor Shh expression was induced. We also grafted cells from progressively more proximal levels of mouse limb buds to chick wing bud tips beneath the apical ridge and the pattern of reactivation of Msx-1 expression along the proximal distal axis of the limb buds was similar to that found in chick limb buds.

Polydactyly in the Strong's luxoid mouse is suppressed by limb deformity alleles

The study of limb development has provided insight into pattern formation during vertebrate embryogenesis. Genetic approaches offer powerful ways to identify the critical molecules and their pathways of action required to execute a complex morphogenetic program. We have applied genetic analysis to the process of limb development by studying two mouse mutants, limb deformity (ld) and Strong's luxoid (lst). These mutations confer contrasting phenotypic alterations to the anteroposterior limb pattern. The six mutant ld alleles are fully recessive and result in oligosyndactyly of all four limbs. By contrast, the two mutant lst alleles result in a mirror-image polydactylous limb phenotype inherited in a semidominant fashion. Morphological and molecular analysis of embryonic limbs has shown that the ld and lst alleles affect the extent and distribution of two key signaling centers differentially: the apical ectodermal ridge and the zone of polarizing activity. Molecular characterization of the ld gene has defined a new family of evolutionarily conserved proteins termed the formins. The underlying molecular defect in the lst mutation has not been identified; however, both loci are tightly linked on mouse chromosome 2, suggesting the possibility that they may be allelic. In this study, we have used genetic analysis to examine the epistatic and allelic relationships of ld and lst. We observed that in + ld/lst + double heterozygotes, a single mutant ld allele is able to suppress the semi-dominant polydactylous lst limb phenotype. By segregating the lst and ld loci in a backcross, we observed that these loci recombine and are separated by a genetic distance of approximately 6 cM. Therefore, while our observations demonstrate a genetic interaction between ld and lst, it is probable that ld and lst are not allelic. Instead, lst and ld may be operating either in a linear or in a parallel (bypass) genetic pathway to affect the limb signaling centers.

Formin isoforms are differentially expressed in the mouse embryo and are required for normal expression of fgf-4 and shh in the limb bud

Mice homozygous for the recessive limb deformity (ld) mutation display both limb and renal defects. The limb defects, oligodactyly and syndactyly, have been traced to improper differentiation of the apical ectodermal ridge (AER) and shortening of the anteroposterior limb axis. The renal defects, usually aplasia, are thought to result from failure of ureteric bud outgrowth. Since the ld locus gives rise to multiple RNA isoforms encoding several different proteins (termed formins), we wished to understand their role in the formation of these organs. Therefore, we first examined the embryonic expression patterns of the four major ld mRNA isoforms. Isoforms I, II and III (all containing a basic amino terminus) are expressed in dorsal root ganglia, cranial ganglia and the developing kidney including the ureteric bud. Isoform IV (containing an acidic amino terminus) is expressed in the notochord, the somites, the apical ectodermal ridge (AER) of the limb bud and the developing kidney including the ureteric bud. Using a lacZ reporter assay in transgenic mice, we show that this differential expression of isoform IV results from distinct regulatory sequences upstream of its first exon. These expression patterns suggest that all four isoforms may be involved in ureteric bud outgrowth, while isoform IV may be involved in AER differentiation. To define further the developmental consequences of the ld limb defect, we analyzed the expression of a number of genes thought to play a role in limb development. Most significantly, we find that although the AERs of ld limb buds express several AER markers, they do not express detectable levels of fibroblast growth factor 4 (fgf-4), which has been proposed to be the AER signal to the mesoderm. Thus we conclude that one or more formins are necessary to initiate and/or maintain fgf-4 production in the distal limb. Since ld limbs form distal structures such as digits, we further conclude that while fgf-4 is capable of supporting distal limb outgrowth in manipulated limbs, it is not essential for distal outgrowth in normal limb development. In addition, ld limbs show a severe decrease in the expression of several mesodermal markers, including sonic hedgehog (shh), a marker for the polarizing region and Hoxd-12, a marker for posterior mesoderm. We propose that incomplete differentiation of the AER in ld limb buds leads to reduction of polarizing activity and defects along the anteroposterior axis.

Homeobox genes and growth factors in regulation of craniofacial and tooth morphogenesis

Homeobox genes encode a special group of transcription factors that regulate gene expression in the developing embryo. The so-called Hox-cluster genes were first discovered in the Drosophila (fruit fly). They specify the identity of body segments and their patterning along the anteroposterior axis. Other homeobox-containing genes appear to regulate patterning of the head and face. The function of the Msx-1 homeobox gene has been shown to be necessary for tooth development. In general, it is thought that special combinations of homeobox genes specify the patterning of individual structures. Bone morphogenetic proteins (BMPs) are growth factors belonging to the family of transforming growth factor-beta (TGF-beta). BMPs regulate bone and cartilage development, and individual BMPs have been shown to contribute to the shaping of various skeletal elements. BMPs regulate bone and dentin formation also postnatally, and they have therapeutic potential in reparative osteogenesis and odontogenesis. BMPs also act as inductive signals between tissue layers in the embryo, and they regulate the expression of several transcription factors, including homeobox-containing genes. BMP-4 has been identified as an epithelial inductive signal in tooth development. As it is produced by early dental epithelium and regulates tooth-specific gene expression in the dental mesenchyme, including Msx-1 expression, it may be an important signal for the initiation of tooth development.

The mouse Fgf8 gene encodes a family of polypeptides and is expressed in regions that direct outgrowth and patterning in the developing embryo

Evidence is accumulating that members of the FGF gene family provide signals that act locally to regulate growth and patterning in vertebrate embryos. In this report, we provide a detailed analysis of the mouse Fgf8 gene. We have mapped the Fgf8 locus to the distal region of mouse chromosome 19, and sequenced the 5′ coding region of the gene. Our data identify a new coding exon, and locate multiple splice donor and splice acceptor sites that can be used to produce at least seven transcripts encoding a family of secreted FGF8 proteins with different N termini. From these results, it appears that Fgf8 is structurally the most complex member of the FGF family described to date. In the embryo, many of the regions in which Fgf8 RNA is localized are known to direct outgrowth and patterning, including the apical ectodermal ridge of the limb bud, the primitive streak and tail bud, the surface ectoderm overlying the facial primorida and the midbrain-hindbrain junction, suggesting that FGF8 may be a component of the regulatory signals that emanate from these regions.

Requirement of FGF-4 for postimplantation mouse development

Fibroblast growth factors (FGFs) are thought to influence many processes in vertebrate development because of their diverse sites of expression and wide range of biological activities in in vitro culture systems. As a means of elucidating embryonic functions of FGF-4, gene targeting was used to generate mice harboring a disrupted Fgf4 gene. Embryos homozygous for the null allele underwent uterine implantation and induced uterine decidualization but did not develop substantially thereafter. As was consistent with their behavior in vivo, Fgf4 null embryos cultured in vitro displayed severely impaired proliferation of the inner cell mass, whereas growth and differentiation of the inner cell mass were rescued when null embryos were cultured in the presence of FGF-4 protein.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Comparative analysis of the tissue distribution of three fibroblast growth factor receptor mRNAs during amphibian morphogenesis

We have used in situ hybridization to survey the expression pattern of three fibroblast growth factor receptor (FGFR) mRNAs (PFR-1, PFR-3 and PFR-4, which we previously identified as the amphibian Pleurodeles waltl homologs of human FGFR-1, FGFR-3 and FGFR-4, respectively) during morphogenesis. Previous work suggests that these FGFR mRNAs exhibit a distinct pattern of expression at early developmental stages. In the present study we have tested the functional activity of these receptors and shown that both FGF-1 (acidic FGF) and FGF-2 (basic FGF), but not FGF-7 (keratinocyte growth factor), can lead to their activation, suggesting that the three cDNAs encode functional receptors. Results from in situ hybridization indicate that various FGFRs are involved in various developmental events. Their involvement in these processes is both overlapping and distinct. During the differentiation of the central nervous system (CNS), PFR-1 and PFR-4 mRNAs show high levels of redundant expression, while the sites of expression of PFR-3 mRNA correlate with regions, such as the diencephalon and the rhombencephalon, undergoing important anatomic changes. The three FGFR mRNAs are distinctly expressed in the cranial ganglia, the pigmented epithelia of retina and the otic vesicles. Most significantly, we found that they are strongly expressed at cranial and branchial mesenchymal condensation sites. PFR-3 mRNA is expressed earlier in this process than PFR-1 and PFR-4 mRNAs. Furthermore PFR-3 mRNA is detected in the mesenchyme of the limb bud, while PFR-1 and PFR-4 mRNAs are found in the primordia of the skeletal elements. In addition, PFR-1 mRNA is expressed in axial mesenchyme and PFR-4 mRNA is detected in the melanophores, xanthophores and in the pronephros. These results suggest that various FGFRs may be involved in distinct developmental events including cell proliferation and differentiation. We also discuss the functional redundancy of the FGFR system during amphibian morphogenesis.

Function of FGF-4 in limb development

The apical ectodermal ridge plays a central role in limb development through its interactions with the underlying mesenchyme. Removal of the AER results in cessation of limb outgrowth and leads to truncation of the limb along the proximo-distal axis. The many functions attributed to the ridge include maintenance of the progress zone mesenchyme. Here, cells are stimulated to proliferate, are maintained in an undifferentiated state, and are assigned progressively more distal positional values as the limb grows. The AER also functions to maintain the activity of the polarizing region, a region of mesenchyme which is thought to provide the primary signal for patterning along the antero-posterior axis. We have begun to explore the function of fibroblast growth factor-4 (FGF-4) during limb development. FGF-4, which encodes an efficiently secreted protein, is expressed in the AER. We have previously demonstrated that FGF-4 protein can stimulate limb mesenchyme proliferation and can induce the expression of a downstream homeobox gene, Evx-1 (homologue of the Drosophila even-skipped gene), that is normally regulated by a signal from the AER. To determine to what extent FGF-4 protein can substitute for the AER to allow normal limb outgrowth, we performed experiments on the developing chick limb in ovo. Remarkably, we find that after AER removal, the FGF-4 protein can provide all the signals required for virtually normal outgrowth and patterning of the limb.(ABSTRACT TRUNCATED AT 250 WORDS)

Targeted disruption of the even-skipped gene, evx1, causes early postimplantation lethality of the mouse conceptus

Implantation within the mammalian uterus elicits dramatic changes in the growth, differentiation, and morphogenesis of the conceptus. This process is interrupted in mice carrying a targeted disruption of the murine evx1 gene, a homolog of the Drosophila even-skipped (eve) gene. Upon implantation, presumptive evx1- homozygotes elicit a decidual response, invade the uterine epithelium, and attach to the basement membrane between uterine stroma and epithelium, but fail to differentiate extraembryonic tissues or to form egg cylinders prior to resorption. Retrograde analysis of embryo genotypes demonstrates that homozygotes could be isolated as free-floating blastocysts but not as gastrulating egg cylinders. Homozygous mutant blastocysts appeared normal and, when grown in vitro, attach, proliferate, and form trophoblastic giant cells surrounding a growing inner cell mass before rapidly degenerating. In situ hybridization analysis demonstrates evx1 gene expression within the visceral endoderm after implantation and prior to gastrulation, at a time in which the mutant phenotype is first detected.

Mechanisms of limb patterning

The development of the vertebrate limb requires the coordinated action of multiple signals to achieve the proper arrangement of adult tissues. Recently, several molecules have been identified which play central roles in patterning of the limb bud. Sonic hedgehog, a homolog of the Drosophila segment polarity gene hedgehog, is likely to regulate anterior/posterior pattern formation. FGF-2 and FGF-4, members of the fibroblast growth factor family, have been shown to provide important signals for limb bud outgrowth and to indirectly regulate proximal/distal patterning. Some candidate effectors of the activity of Sonic hedgehog and of FGFs are known, including members of the clustered Hox genes.

Expression and function of FGF-4 in peri-implantation development in mouse embryos

One of the earliest events in mammalian embryogenesis is the formation of the inner cell mass (ICM) and the subsequent delamination of primitive endoderm. We have found that mRNA for fibroblast growth factor (FGF)-4, but not FGF-3, is expressed in preimplantation mouse blastocysts and that the FGF-4 polypeptide is present in ICM cells. ICM-like embryonal carcinoma cells and embryonic stem cells also express FGF-4. Conversely, differentiated embryonal carcinoma cells in the endoderm lineage express FGF-3, but not FGF-4 mRNA. Although mouse embryos expressed FGF-4 mRNA from the 1-cell stage, embryos cultured from the 2-cell through the blastocyst stage in the presence of recombinant FGF-4 did not respond mitogenically. However, when ICMs that were isolated by immunosurgery were cultured with FGF-4, the number of morphologically distinct, differentiated parietal endoderm cells growing out onto the coverslip increased, without an increase in the number of undifferentiated ICM cells. ICM outgrowths cultured with FGF-4 increased their secretion of 92 × 10(3) M(r) gelatinase and tissue plasminogen activator, a hallmark of migrating cells. Receptors for FGF-4 (FGFR-3 and FGFR-4) are expressed in all cells of the mouse blastocyst. These findings indicate that FGF-4 produced by undifferentiated ICM cells acts in the peri-implantation period of embryogenesis to influence the production and behavior of endoderm cells derived from them.

Msx1 (Hox-7.1) in the adult mouse uterus: cellular interactions underlying regulation of expression

We report here that Msx1 (formerly Hox-7.1) is expressed at high levels in uterine epithelial cells of the non-pregnant adult. These cells undergo pronounced changes in morphology in response to embryo implantation and show a concomitant decrease in Msx1 levels. While Msx1 is restricted to the uterus in adulthood, we observe Msx1 expression throughout the entire perinatal Mullerian duct epithelium in the prospective uterus, cervix and vagina. Through analysis of tissue recombinants, the expression of Msx1 in the epithelium was shown to be dependent upon an interaction with the underlying mesenchyme of uterine origin. The capacity of uterine mesenchyme to support or induce Msx1 expression in Mullerian epithelium is correlated with mesenchymal expression of Wnt-5a. Whereas Msx1 expression in the epithelium results from interaction with uterine mesenchyme, Wnt-5a expression is an intrinsic property of the uterine mesenchyme and does not depend upon the epithelium. The observation that Msx1 is expressed in the adult uterine epithelium and that conversion of the presumptive vaginal epithelium to uterine epithelium can be elicited only during the first week of postnatal development when Msx1 expression is detected suggests that, in addition to regulating various aspects of uterine epithelial morphology and function (e.g. gestation), this homeobox-containing gene plays a role in maintaining the uterus in a morphogenic and developmentally responsive state prerequisite for its unique function.