Fibroblast growth factor receptor 3 is a negative regulator of bone growth

Cripto is required for correct orientation of the anterior-posterior axis in the mouse embryo

The anterior-posterior axis of the mouse embryo is established by two distinct organizing centres in the anterior visceral endoderm and the distal primitive streak. These organizers induce and pattern the head and trunk respectively, and have been proposed to be localized through coordinate cell movements that rotate a pre-existing proximal-distal axis. Here we show that correct localization of both head- and trunk-organizing centres requires Cripto, a putative signalling molecule that is a member of the EGF-CFC gene family. Before gastrulation, Cripto is asymmetrically expressed in a proximal-distal gradient in the epiblast, and subsequently is expressed in the primitive streak and newly formed embryonic mesoderm. A Cripto null mutation generated by targeted gene disruption results in homozygous Cripto-/- embryos that mostly consist of anterior neuroectoderm and lack posterior structures, thus resembling a head without a trunk. Notably, markers of the head organizer are located at the distal end of the embryo, whereas markers of the primitive streak are absent or localized to the proximal side. Our results indicate that Cripto signalling is essential for the conversion of a proximal-distal asymmetry into an orthogonal anterior-posterior axis.

Terminal differentiation of chondrocytes is arrested at distinct stages identified by their expression repertoire of marker genes

During endochondral bone formation, cells in the emerging cartilaginous model transit through a cascade of several chondrocyte differentiation stages, each characterized by a specific expression repertoire of matrix macromolecules, until, as a final step, the hypertrophic cartilage is replaced by bone. In many permanent cartilage tissues, however, late differentiation of chondrocytes does not occur, due to negative regulation by the environment of the cells. Here, addressing the reason for the difference between chondrocyte fates in the chicken embryo sternum, cells from the caudal and cranial part were cultured separately in serum-free agarose gels with complements defined earlier that either permit or prevent hypertrophic development. Total RNA was extracted using a novel protocol adapted to agarose cultures, and the temporal changes in developmental stage-specific mRNA expression were monitored by Northern hybridization and phosphor image analysis. Kinetic studies of the mRNA accumulation not only showed significant differences between the expression patterns of cranial and caudal cultures after recovery, but also revealed two checkpoints of chondrocyte differentiation in keeping with cartilage development in vivo. Terminal differentiation of caudal chondrocytes is blocked at the late proliferative stage (stage Ib), while the cranial cells can undergo hypertrophic development spontaneously. The differentiation of cranial chondrocytes is reversible, since they can re-assume an early proliferative (stage Ia) phenotype under the influence of insulin, fibroblast growth factor-2 and transforming growth factor-beta in combination. Thus, the expression pattern in the latter culture resembles that of articular chondrocytes. We also provide evidence that the capacities of caudal and sternal chondrocytes to progress from the late proliferative (stage Ib) to hypertrophic stage (stage II) correlate with their differing abilities to express the Indian hedgehog gene.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

FGFR-3 and FGFR-4 function cooperatively to direct alveogenesis in the murine lung

Mammalian lungs begin as an outpocket of the foregut, and depend on multiple stages of branching morphogenesis and alveogenesis to reach their final form. An examination of fgf receptor gene expression indicated that all four receptors (fgfr-1 to fgfr-4) are expressed in postnatal lungs at varying levels. We show that mice homozygous for a targeted mutation of fgfr-4 exhibited no overt abnormalities in the lungs or any other organ. However, mice doubly homozygous for disruptions of the fgfr-3 and fgfr-4 genes display novel phenotypes not present in either single mutant, which include pronounced dwarfism and lung abnormalities. Lungs of fgfr-3(−/−)fgfr-4(−/−)animals, which are normal at birth, are completely blocked in alveogenesis and do not form secondary septae to delimit alveoli. Consequently, air spaces in the lung are expanded and no alveoli can be seen. The mutant lungs failed to downregulate postnatal elastin deposition despite their normal levels of surfactant expression and cell proliferation. These data revealed a cooperative function of FGFR-3 and FGFR-4 to promote the formation of alveoli during postnatal lung development.

Fibroblast growth factor downregulates expression of a basic helix-loop-helix-type transcription factor, scleraxis, in a chondrocyte-like cell line, TC6

Scleraxis is a basic helix-loop-helix-type transcription factor that is expressed in sclerotome. Fibroblast growth factor (FGF) is one of the cytokines produced by the cells in skeletal tissues and is a potent modulator of skeletogenesis. The aim of this study was to examine the effects of FGF on the expression of scleraxis in chondrocyte-like cells, TC6. In these cells, scleraxis mRNA was constitutively expressed as a 1 .2 kb message at a high level in contrast to its low levels of expression in fibroblast-like cells or osteoblast-like cells. Upon treatment with FGF, scleraxis mRNA level was decreased within 12 h. This effect was at its nadir at 24 h and the scleraxis mRNA level returned to its base line level by 48 h. The FGF effect was maximal at 1 ng/ml. FGF effects on scleraxis were blocked by actinomycin D but not by cycloheximide, suggesting the involvement of transcriptional events that do not require new protein synthesis. The FGF effects on scleraxis were blocked by genistein, suggesting the involvement of tyrosine kinase in the post-receptor signaling. TGFbeta treatment of TC6 cells enhanced scleraxis mRNA expression; however, combination of the saturation doses of FGF and TGFbeta resulted in suppression of scleraxis mRNA level. BMP2 also suppressed scleraxis mRNA expression in TC6 cells and no further suppression was observed in combination with FGF. These results indicate that scleraxis is expressed in chondrocyte-like TC6 cells and it is one of the targets of FGF action in these cells.

Spatio-temporal expression of FGFR 1, 2 and 3 genes during human embryo-fetal ossification

Mutations in FGFR 1-3 genes account for various human craniosynostosis syndromes, while dwarfism syndromes have been ascribed exclusively to FGFR 3 mutations. However, the exact role of FGFR 1-3 genes in human skeletal development is not understood. Here we describe the expression pattern of FGFR 1-3 genes during human embryonic and fetal endochondral and membranous ossification. In the limb bud, FGFR 1 and FGFR 2 are initially expressed in the mesenchyme and in epidermal cells, respectively, but FGFR 3 is undetectable. At later stages, FGFR 2 appears as the first marker of prechondrogenic condensations. In the growing long bones, FGFR 1 and FGFR 2 transcripts are restricted to the perichondrium and periosteum, while FGFR 3 is mainly expressed in mature chondrocytes of the cartilage growth plate. Marked FGFR 2 expression is also observed in the periarticular cartilage. Finally, membranous ossification of the skull vault is characterized by co-expression of the FGFR 1-3 genes in preosteoblasts and osteoblasts. In summary, the simultaneous expression of FGFR 1-3 genes in cranial sutures might explain their involvement in craniosynostosis syndromes, whereas the specific expression of FGFR 3 in chondrocytes does correlate with the involvement of FGFR 3 mutations in inherited defective growth of human long bones.

Overlapping effects of different members of the FGF family on lens fiber differentiation in transgenic mice

Fibroblast growth factors (FGFs), such as FGF-1, have been shown to induce differentiation of lens epithelial cells both in tissue culture and in transgenic mice. In the present study, using the alpha A-crystallin promoter, we generated transgenic mice that express different FGFs (FGF-4, FGF-7, FGF-8, FGF-9) specifically in the lens. All four FGFs induced changes in ocular development. Microphthalmic eyes were evident in transgenic mice expressing FGF-8, FGF-9 and some lines expressing FGF-4. A developmental study of the microphthalmic eyes revealed that, by embryonic day 15, expression of these FGFs induced lens epithelial cells to undergo premature fiber differentiation. In less severely affected lines expressing FGF-4 or FGF-7, the lens epithelial cells exhibited a premature exit from the cell cycle and underwent a fiber differentiation response later in development, leading to cataract formation. The responsiveness of lens cells to different FGFs indicates that these proteins stimulate the same or overlapping downstream signalling pathway(s). These overlapping effects of different FGFs on a common cell type indicate that the normal developmental roles for these genes are determined by the temporal and spatial regulation of their expression patterns. The fact that any of these FGFs can induce ocular defects and loss of lens transparency implies that it is essential for the normal eye to maintain very specific spatial control over FGF expression in order to prevent cataract induction.

Failure of egg cylinder elongation and mesoderm induction in mouse embryos lacking the tumor suppressor smad2

smad genes constitute a family of nine members whose products serve as intracellular mediators of transforming growth factor beta signals. SMAD2, which is a tumor suppressor involved in colorectal and lung cancer, has been shown to induce dorsal mesoderm in Xenopus laevis in response to transforming growth factor beta and activins. The smad2 gene is expressed ubiquitously during murine embryogenesis and in many adult mouse tissues. Animals that lacked smad2 died before 8.5 days of development (E8.5). E6.5 homozygous mutants were smaller than controls, lacked the extraembryonic portion of the egg cylinder, and appeared strikingly similar to E6.5 smad4 mutants. This similarity was no longer evident at E7.5, however, because the smad2 mutants contained embryonic ectoderm within their interiors. Molecular analysis showed that smad2 mutant embryos did not undergo gastrulation or make mesoderm. The results demonstrate that smad2 is required for egg cylinder elongation, gastrulation, and mesoderm induction.

Graded reduction of Pafah1b1 (Lis1) activity results in neuronal migration defects and early embryonic lethality

Heterozygous mutation or deletion of the beta subunit of platelet-activating factor acetylhydrolase (PAFAH1B1, also known as LIS1) in humans is associated with type I lissencephaly, a severe developmental brain disorder thought to result from abnormal neuronal migration. To further understand the function of PAFAH1B1, we produced three different mutant alleles in mouse Pafah1b1. Homozygous null mice die early in embryogenesis soon after implantation. Mice with one inactive allele display cortical, hippocampal and olfactory bulb disorganization resulting from delayed neuronal migration by a cell-autonomous neuronal pathway. Mice with further reduction of Pafah1b1 activity display more severe brain disorganization as well as cerebellar defects. Our results demonstrate an essential, dosage-sensitive neuronal-specific role for Pafah1b1 in neuronal migration throughout the brain, and an essential role in early embryonic development. The phenotypes observed are distinct from those of other mouse mutants with neuronal migration defects, suggesting that Pafah1b1 participates in a novel pathway for neuronal migration.

Identification of the cytoplasmic regions of fibroblast growth factor (FGF) receptor 1 which play important roles in induction of neurite outgrowth in PC12 cells by FGF-1

Fibroblast growth factor 1 (FGF-1) induces neurite outgrowth in PC12 cells. Recently, we have shown that the FGF receptor 1 (FGFR-1) is much more potent than FGFR-3 in induction of neurite outgrowth. To identify the cytoplasmic regions of FGFR-1 that are responsible for the induction of neurite outgrowth in PC12 cells, we took advantage of this difference and prepared receptor chimeras containing different regions of the FGFR-1 introduced into the FGFR-3 protein. The chimeric receptors were introduced into FGF-nonresponsive variant PC12 cells (fnr-PC12 cells), and their ability to mediate FGF-stimulated neurite outgrowth of the cells was assessed. The juxtamembrane (JM) and carboxy-terminal (COOH) regions of FGFR-1 were identified as conferring robust and moderate abilities, respectively, for induction of neurite outgrowth to FGFR-3. Analysis of FGF-stimulated activation of signal transduction revealed that the JM region of FGFR-1 conferred strong and sustained tyrosine phosphorylation of several cellular proteins and activation of MAP kinase. The SNT/FRS2 protein was demonstrated to be one of the cellular substrates preferentially phosphorylated by chimeras containing the JM domain of FGFR-1. SNT/FRS2 links FGF signaling to the MAP kinase pathway. Thus, the ability of FGFR-1 JM domain chimeras to induce strong sustained phosphorylation of this protein would explain the ability of these chimeras to activate MAP kinase and hence neurite outgrowth. The role of the COOH region of FGFR-1 in induction of neurite outgrowth involved the tyrosine residue at amino acid position 764, a site required for phospholipase C gamma binding and activation, whereas the JM region functioned primarily through a non-phosphotyrosine-dependent mechanism. In contrast, assessment of the chimeras in the pre-B lymphoid cell line BaF3 for FGF-1-induced mitogenesis revealed that the JM region did not play a role in this cell type. These data indicate that FGFR signaling can be regulated at the level of intracellular interactions and that signaling pathways for neurite outgrowth and mitogenesis use different regions of the FGFR.

New insights into heparin-induced FGF oligomerization

Fibroblast growth factors (FGFs) play important roles in a variety of developmental processes in mammals. The dependence of their activity on heparin binding has been a puzzle that, in recent years, has been the subject of active investigation. Recent structural analyses on complexes of FGFs with heparin fragments or heparin analogs have unveiled the extreme complexity of these systems.

Wiskott-Aldrich syndrome protein-deficient mice reveal a role for WASP in T but not B cell activation

The Wiskott-Aldrich syndrome (WAS) is a human X-linked immunodeficiency resulting from mutations in a gene (WASP) encoding a cytoplasmic protein implicated in regulating the actin cytoskeleton. To elucidate WASP function, we disrupted the WASP gene in mice by gene-targeted mutation. WASP-deficient mice showed apparently normal lymphocyte development, normal serum immunoglobulin levels, and the capacity to respond to both T-dependent and T-independent type II antigens. However, these mice did have decreased peripheral blood lymphocyte and platelet numbers and developed chronic colitis. Moreover, purified WASP-deficient T cells showed markedly impaired proliferation and antigen receptor cap formation in response to anti-CD3epsilon stimulation. Yet, purified WASP-deficient B cells showed normal responses to anti-Ig stimulation. We discuss the implications of our findings regarding WASP function in receptor signaling and cytoskeletal reorganization in T and B cells and compare the effects of WASP deficiency in mice and humans.

Effects of fibroblast growth factor-2 on longitudinal bone growth

In vivo, fibroblast growth factor-2 (FGF-2) inhibits longitudinal bone growth. Similarly, activating FGF receptor 3 mutations impair growth in achondroplasia and thanatophoric dysplasia. To investigate the underlying mechanisms, we chose a fetal rat metatarsal organ culture system that would maintain growth plate histological architecture. Addition of FGF-2 to the serum-free medium inhibited longitudinal growth. We next assessed each major component of longitudinal growth: proliferation, cellular hypertrophy, and cartilage matrix synthesis. Surprisingly, FGF-2 stimulated proliferation, as assessed by [3H]thymidine incorporation. However, autoradiographic studies demonstrated that this increased proliferation occurred only in the perichondrium, whereas decreased labeling was seen in the proliferative and epiphyseal chondrocytes. FGF-2 also caused a marked decrease in the number of hypertrophic chondrocytes. To assess cartilage matrix synthesis, we measured 35SO4 incorporation into newly synthesized glycosaminoglycans. Low concentrations (10 ng/ml) of FGF-2 stimulated cartilage matrix production, but high concentrations (1000 ng/ml) inhibited matrix production. We conclude that FGF-2 inhibits longitudinal bone growth by three mechanisms: decreased growth plate chondrocyte proliferation, decreased cellular hypertrophy, and, at high concentrations, decreased cartilage matrix production. These effects may explain the impaired growth seen in patients with achondroplasia and related skeletal dysplasias.

Bone morphogenetic proteins and bFGF exert opposing regulatory effects on PTHrP expression and inorganic pyrophosphate elaboration in immortalized murine endochondral hypertrophic chondrocytes (MCT cells)

A fundamental question in endochondral development is why the expression of parathyroid hormone-related protein (PTHrP), which inhibits chondrocyte maturation and mineralization, becomes attenuated at the stage of chondrocyte hypertrophy. To address this question, we used clonal, phenotypically stable SV40-immortalized murine endochondral chondrocytes that express a growth-arrested hypertrophic phenotype in culture (MCT cells). Addition of individual cytokines to the medium of MCT cells revealed that bone morphogenetic protein (BMP)-6, which commits chondrocytes to hypertrophy, markedly inhibited PTHrP production. This activity was shared by three other osteogenic bone morphogenetic proteins (BMP-2, BMP-4, and BMP-7) and by transforming growth factor beta (TGF-beta), which all inhibited the level of PTHrP mRNA. In contrast, basic fibroblast growth factor (bFGF), an inhibitor of chondrocyte maturation to hypertrophy, induced PTHrP in MCT cells and antagonized the effects of BMP-2, BMP-4, BMP-6, and BMP-7 and TGF-beta on PTHrP expression. Opposing effects of bFGF and BMPs also were exerted on the elaboration of inorganic pyrophosphatase (PPi), which regulates the ability of hypertrophic chondrocytes to mineralize the matrix. Specifically, BMP-2 and BMP-4, but not BMP-6 and BMP-7, shared the ability of TGF-beta to induce PPi release, and this activity was inhibited by bFGF in MCT cells. Our results suggest that effects on PTHrP expression could contribute to the ability of BMP-6 to promote chondrocyte maturation. BMPs and bFGF exert opposing effects on more than one function in immortalized hypertrophic chondrocytes. Thus, the normal decrease in bFGF responsiveness that accompanies chondrocyte hypertrophy may function in part by removing the potential for bFGF to induce PTHrP expression and to oppose the effects of BMPs. MCT cells may be useful in further understanding the mechanisms regulating the differentiation and function of hypertrophic chondrocytes.

Fibroblast growth factor receptor 3 mutations promote apoptosis but do not alter chondrocyte proliferation in thanatophoric dysplasia

Thanatophoric dysplasia (TD) is a lethal skeletal disorder caused by recurrent mutations in the fibroblast growth factor receptor 3 (FGFR 3) gene. The mitogenic response of fetal TD I chondrocytes in primary cultures upon stimulation by either FGF 2 or FGF 9 did not significantly differ from controls. Although the levels of FGFR 3 mRNAs in cultured TD chondrocytes were similar to controls, an abundant immunoreactive material was observed at the perinuclear level using an anti-FGFR 3 antibody in TD cells. Transduction signaling via the mitogen-activated protein kinase pathway was assessed by measuring extracellular signal-regulated kinase activity (ERK 1 and ERK 2). Early ERKs activation following FGF 9 supplementation was observed in TD chondrocytes (2 min) as compared with controls (5 min) but no signal was detected in the absence of ligand. By contrast ligand-independent activation of the STAT signaling pathway was demonstrated in cultured TD cells and confirmed by immunodetection of Stat 1 in the nuclei of hypertrophic TD chondrocytes. Moreover, the presence of an increased number of apoptotic chondrocytes in TD fetuses was associated with a higher expression of Bax and the simultaneous decrease of Bcl-2 levels. Taken together, these results indicate that FGFR 3 mutations in TD I fetuses do not hamper chondrocyte proliferation but rather alter their differentiation by triggering premature apoptosis through activation of the STAT signaling pathway.

Functional overlap between murine Inpp5b and Ocrl1 may explain why deficiency of the murine ortholog for OCRL1 does not cause Lowe syndrome in mice

The oculocerebrorenal syndrome of Lowe (OCRL) is an X-linked human genetic disorder characterized by mental retardation, congenital cataracts, and renal tubular dysfunction. The Lowe syndrome gene, OCRL1, encodes a phosphatidylinositol 4,5-bisphosphate 5-phosphatase in the Golgi complex. The pathogenesis of Lowe syndrome due to deficiency of a phosphatidylinositol 4,5-bisphosphate 5-phosphatase in the Golgi complex is unknown. We have used targeted disruption in embryonic stem cells to make mice deficient in Ocrl1, the mouse homologue for OCRL1, as an animal model for the disease. Surprisingly, mice deficient in Ocrl1 do not develop the congenital cataracts, renal Fanconi syndrome, or neurological abnormalities seen in the human disorder. We hypothesized that Ocrl1 deficiency is complemented in mice by inositol polyphosphate 5-phosphatase (Inpp5b), an autosomal gene that encodes a phosphatidylinositol bisphosphate 5-phosphatase highly homologous to Ocrl1. We created mice deficient in Inpp5b; the mice were viable and fertile without phenotype except for testicular degeneration in males beginning after sexual maturation. We crossed mice deficient in Ocrl1 to mice deficient in Inpp5b. No liveborn mice or embryos lacking both enzymes were found, demonstrating that Ocrl1 and Inpp5b have overlapping functions in mice and suggesting that the lack of phenotype in Ocrl1-deficient mice may be due to compensating Inpp5b function.

Morphological examination of bone synthesis via direct administration of basic fibroblast growth factor into rat bone marrow

Woven bone induced by direct injection of basic fibroblast growth factor (bFGF) into rat bone marrow was examined. On the first day after injection, fibrous tissues formed in the treated region of the bone marrow. Tissue-nonspecific alkaline phosphatase (TNAPase)-immunopositive osteoblastic cells and osteopontin immunopositive-extracellular matrices were observed in the fibrous tissues, indicating bone induction. On the fifth day, the bFGF-induced bone was found broadly in the bone marrow. In the originally existing bone, osteopontin-immunoreactivity was observed at cement lines, but not in the fully calcified matrix, whereas the woven bone displayed immunoreactivity throughout the matrix. Numerous TRAPase-positive osteoclasts were present on the surfaces of the woven bone, but no obvious cement line was observed. Therefore, both bone formation and resorption appeared highly active, without normal cellular coupling equilibrated between bone formation and resorption performed by osteoblasts and osteoclasts. On the tenth day, the bFGF-induced bone was almost replaced by bone marrow. Thus, bone formation actively occurred in the first half of the experimental period, whereas bone resorption came to be predominant thereafter. This study demonstrated that bFGF stimulates bone formation, which, however, is subject to subsequent resorption, probably due in part to the absence of coordinated cellular coupling between osteoclasts and osteoblasts.

Neuronal defects and delayed wound healing in mice lacking fibroblast growth factor 2

Basic fibroblast growth factor (FGF2) is a wide-spectrum mitogenic, angiogenic, and neurotrophic factor that is expressed at low levels in many tissues and cell types and reaches high concentrations in brain and pituitary. FGF2 has been implicated in a multitude of physiological and pathological processes, including limb development, angiogenesis, wound healing, and tumor growth, but its physiological role is still unclear. To determine the function of FGF2 in vivo, we have generated FGF2 knockout mice, lacking all three FGF2 isoforms, by homologous recombination in embryonic stem cells. FGF2(-/-) mice are viable, fertile and phenotypically indistinguishable from FGF2(+/+) littermates by gross examination. However, abnormalities in the cytoarchitecture of the neocortex, most pronounced in the frontal motor-sensory area, can be detected by histological and immunohistochemical methods. A significant reduction in neuronal density is observed in most layers of the motor cortex in the FGF2(-/-) mice, with layer V being the most affected. Cell density is normal in other regions of the brain such as the striatum and the hippocampus. In addition, the healing of excisional skin wounds is delayed in mice lacking FGF2. These results indicate that FGF2, although not essential for embryonic development, plays a specific role in cortical neurogenesis and skin wound healing in mice, which, in spite of the apparent redundancy of FGF signaling, cannot be carried out by other FGF family members.

Natriuretic peptide regulation of endochondral ossification. Evidence for possible roles of the C-type natriuretic peptide/guanylyl cyclase-B pathway

The natriuretic peptide family consists of three structurally related endogenous ligands: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). The biological actions of natriuretic peptides are thought to be mediated through the activation of two guanylyl cyclase (GC)-coupled receptor subtypes (GC-A and GC-B). In this study, we examined the effects of ANP and CNP, which are endogenous ligands for GC-A and GC-B, respectively, on bone growth using an organ culture of fetal mouse tibias, an in vitro model of endochondral ossification. CNP increased the cGMP production much more potently than ANP, thereby resulting in an increase in the total longitudinal bone length. Histological examination revealed an increase in the height of the proliferative and hypertrophic chondrocyte zones in fetal mouse tibias treated with CNP. The natriuretic peptide stimulation of bone growth, which was mimicked by 8-bromo-cGMP, was inhibited by HS-142-1, a non-peptide GC-coupled natriuretic peptide receptor antagonist. The spontaneous increase in the total longitudinal bone growth and cGMP production was also inhibited significantly by HS-142-1. CNP mRNA was expressed abundantly in fetal mouse tibias, where no significant amounts of ANP and BNP mRNAs were detected. A considerable amount of GC-B mRNA was present in fetal mouse tibias. This study suggests the physiologic significance of the CNP/GC-B pathway in the process of endochondral ossification.

Limb development: molecular dysmorphology is at hand!

We present a review of limb development integrating current molecular information and selected genetic disorders to illustrate the advances made in this field over the last few years. With this knowledge, clinical geneticists can now begin to consider molecular mechanisms and pathways when investigating patients with limb malformation syndromes.

MMP-9/gelatinase B is a key regulator of growth plate angiogenesis and apoptosis of hypertrophic chondrocytes

Homozygous mice with a null mutation in the MMP-9/gelatinase B gene exhibit an abnormal pattern of skeletal growth plate vascularization and ossification. Although hypertrophic chondrocytes develop normally, apoptosis, vascularization, and ossification are delayed, resulting in progressive lengthening of the growth plate to about eight times normal. After 3 weeks postnatal, aberrant apoptosis, vascularization, and ossification compensate to remodel the enlarged growth plate and ultimately produce an axial skeleton of normal appearance. Transplantation of wild-type bone marrow cells rescues vascularization and ossification in gelatinase B-null growth plates, indicating that these processes are mediated by gelatinase B-expressing cells of bone marrow origin, designated chondroclasts. Growth plates from gelatinase B-null mice in culture show a delayed release of an angiogenic activator, establishing a role for this proteinase in controlling angiogenesis.

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.

Effects of parathyroid hormone (PTH) and PTH-related peptide on expressions of matrix metalloproteinase-2, -3, and -9 in growth plate chondrocyte cultures

The roles of PTH and PTH-related peptide (PTH-rp) in the expression of matrix metalloproteinases (MMPs) during endochondral bone formation were investigated, using various cartilages obtained from young rabbits and rabbit chondrocyte cultures. Immunohistochemical, immunoblotting, zymographical, and/or Northern blot analyses showed that MMP-2 and -9 levels were much higher in the growth plate than in permanent cartilage in vivo. In growth plate chondrocyte cultures, PTH, PTH-rp, and (Bu)2cAMP increased the amount of MMP-2 present in the culture medium, as revealed by zymograms and immunoblots, whereas the other tested growth factors or cytokines, including bone morphogenetic protein-2 and interleukin-1, did not increase the MMP-2 level. PTH also increased the MMP-2 messenger RNA level within 24 h. In addition, PTH increased MMP-3 and -9 levels in the growth plate chondrocyte cultures. However, in articular chondrocyte cultures, PTH had little effect on the levels of MMP-2, -3, and -9. In contrast to PTH, interleukin-1 induced MMP-3 and -9, but not MMP-2, in growth plate and articular chondrocytes. These findings suggest that in ossifying cartilage, PTH/PTH-rp plays a pivotal role in the induction of various MMPs, including MMP-2 (which is considered to be a constitutive enzyme), and that PTH/PTH-rp is involved in the control of cartilage-matrix degradation during endochondral bone formation.

Renal microvascular assembly and repair: power and promise of molecular definition

Developmental assembly of the renal microcirculation is a precise and coordinated process now accessible to experimental scrutiny. Although definition of the cellular and molecular determinants is incomplete, recent findings have reframed concepts and questions about the origins of vascular cells in the glomerulus and the molecules that direct cell recruitment, specialization and morphogenesis. New findings illustrate principles that may be applied to defining critical steps in microvascular repair following glomerular injury. Developmental assembly of endothelial, mesangial and epithelial cells into glomerular capillaries requires that a coordinated, temporally defined series of steps occur in an anatomically ordered sequence. Recent evidence shows that both vasculogenic and angiogenic processes participate. Local signals direct cell migration, proliferation, differentiation, cell-cell recognition, formation of intercellular connections, and morphogenesis. Growth factor receptor tyrosine kinases on vascular cells are important mediators of many of these events. Cultured cell systems have suggested that basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), and vascular endothelial growth factor (VEGF) promote endothelial cell proliferation, migration or morphogenesis, while genetic deletion experiments have defined an important role for PDGF beta receptors and platelet-derived growth factor (PDGF) B in glomerular development. Receptor tyrosine kinases that convey non-proliferative signals also contribute in kidney and other sites. The EphB1 receptor, one of a diverse class of Eph receptors implicated in neural cell targeting, directs renal endothelial migration, cell-cell recognition and assembly, and is expressed with its ligand in developing glomeruli. Endothelial TIE2 receptors bind angiopoietins (1 and 2), the products of adjacent supportive cells, to signals direct capillary maturation in a sequence that defines cooperative roles for cells of different lineages. Ultimately, definition of the cellular steps and molecular sequence that direct microvascular cell assembly promises to identify therapeutic targets for repair and adaptive remodeling of injured glomeruli.

New overgrowth syndrome and FGFR3 dosage effect

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Gene disruption in mice: models of development and disease

Gene targeting technology in mice by homologous recombination has become an important method to generate loss-of-function of genes in a predetermined locus. Although the inactivation is limited to irreversible alteration of chromosomal DNA and a surprising variety of genes have given unexpected and disappointing results, modification of the basic technology now provides additional choices for a more specific and variety of manipulations of the mouse genome. This includes conditional cell-type specific gene targeting, knockin technique and the induction of the specific balanced chromosomal translocations. In the past decade this technique not only generated a wealth of knowledge concerning the roles of growth factors, oncogenes, hormone receptors and Hox genes but also helped to produce animal models for several human genetic disorders. In the future it may provide more powerful and necessary tools to dissect the psychiatric disorders, understanding the complex central nervous system and to correct the inherited disorders.

The tumor suppressor SMAD4/DPC4 is essential for epiblast proliferation and mesoderm induction in mice

Members of the transforming growth factor (TGF)-beta superfamily have been shown to play a variety of important roles in embryogenesis, including dorsal and ventral mesoderm induction. The tumor suppressor SMAD4, also known as DPC4, is believed to be an essential factor that mediates TGF-beta signals. To explore functions of SMAD4 in development, we have mutated it by truncating its functional C-domain. We show that Smad4 is expressed ubiquitously during murine embryogenesis. Mice heterozygous for the Smad4(ex8/+) mutation are developmentally normal, whereas homozygotes die between embryonic day 6.5 (E6.5) and 8.5. All Smad4(ex8/ex8) mutants are developmentally delayed at E6 and show little or no elongation in the extraembryonic portion of late egg cylinder stage embryos. Consistent with this, cultured Smad4(ex8/ex8) blastocyst outgrowths suffer cellular proliferation defects and fail to undergo endoderm differentiation. Although a portion of mutant embryos at E8.5 show an increase in the embryonic ectoderm and endoderm, morphological and molecular analyses indicate that they do not form mesoderm. Altogether, these data demonstrate that SMAD4-mediated signals are required for epiblast proliferation, egg cylinder formation, and mesoderm induction.

Soluble dominant-negative receptor uncovers essential roles for fibroblast growth factors in multi-organ induction and patterning

Despite a wealth of experimental data implicating fibroblast growth factor (FGF) signaling in various developmental processes, genetic inactivation of individual genes encoding specific FGFs or their receptors (FGFRs) has generally failed to demonstrate their role in vertebrate organogenesis due to early embryonic lethality or functional redundancy. Here we show that broad mid-gestational expression of a novel secreted kinase-deficient receptor, specific for a defined subset of the FGF superfamily, caused agenesis or severe dysgenesis of kidney, lung, specific cutaneous structures, exocrine and endocrine glands, and craniofacial and limb abnormalities reminiscent of human skeletal disorders associated with FGFR mutations. Analysis of diagnostic molecular markers revealed that this soluble dominant-negative mutant disrupted early inductive signaling in affected tissues, indicating that FGF signaling is required for growth and patterning in a broad array of organs and in limbs. In contrast, transgenic mice expressing a membrane-tethered kinase-deficient FGFR were viable. Our results demonstrate that secreted FGFR mutants are uniquely effective as dominant-negative agents in vivo, and suggest that related soluble receptor isoforms expressed in wild-type mouse embryos may help regulate FGF activity during normal development.

Regulation of growth region cartilage proliferation and differentiation by perichondrium

Endochondral bone formation in vertebrates requires precise coordination between proliferation and differentiation of the participating chondrocytes. We examined the role of perichondrium in this process using an organ culture system of chicken embryonic tibiotarsi. A monoclonal antibody against chicken collagen type X, specifically expressed by hypertrophic chondrocytes, was utilized to monitor the terminal differentiation of chondrocytes. Proliferation of chondrocytes was examined by a BrdU-labeling procedure. The absence of perichondrium is correlated with an extended zone of cartilage expressing collagen type X, suggesting that the perichondrium regulates chondrocyte hypertrophy in a negative manner. Removal of perichondrium, in addition, resulted in an extended zone of chondrocytes incorporating BrdU, indicating that the perichondrium also negatively regulates the proliferation of chondrocytes. Partial removal of perichondrium from one side of the tibiotarsus led to expansion of both the collagen type X-positive domain and the BrdU-positive zone at the site of removal but not where the perichondrium remained intact. This suggests that both types of regulation by the perichondrium are local effects. Furthermore, addition of bovine parathyroid hormone (PTH) to perichondrium-free cultures reversed the expansion of the collagen type X-positive domain but not that of the proliferative zone. This suggests that the regulation of differentiation is dependent upon the PTH/PTHrP receptor and that the regulation of proliferation is likely independent of it. Taken together, these results are consistent with a model where perichondrium regulates both the exit of chondrocytes from the cell cycle, and their subsequent differentiation.

Regulation of the fibroblast growth factor receptor 3 promoter and intron I enhancer by Sp1 family transcription factors

Fibroblast growth factor receptor 3 (FGFR3) has a complex spatial and temporal pattern of expression and is essential for the normal development of a diverse set of tissues. Recently, mutations have been identified in FGFR3 that result in constitutive tyrosine kinase activity and cause a number of different human skeletal disorders. To examine the regulatory mechanisms governing FGFR3 expression, the promoter for the FGFR3 gene was identified and characterized. It resides in a CpG island, which encompasses the 5' end of the FGFR3 gene and lacks classical cis-regulatory motifs. As little as 100 base pairs of sequence 5' to the initiation site can confer a 20-40-fold increase in transcriptional activity upon a promoter-less vector. The transcriptional activity of these cis-regulatory sequences is further stimulated by elements found within the first intron. Mapping of the enhancer activity found within intron I identified two purine-rich sequence motifs between +340 and +395. Electrophoretic mobility shift assays demonstrated that sequences within this region bind members of the Sp1 family of transcription factors. In a background lacking Sp1-like activity, we demonstrate that Sp1 can enhance transcription of the minimal promoter (which contains five classical Sp1 sites), whereas both Sp1 and Sp3 can enhance transcription through the elements found in intron I. Although these transcription factors are ubiquitously expressed, we demonstrate that the sequences between -220 and +609 of the FGFR3 gene are sufficient to promote the tissue-specific expression of a reporter gene in transgenic mice.

Fibroblast growth factor receptor 2 (FGFR2)-mediated reciprocal regulation loop between FGF8 and FGF10 is essential for limb induction

FGFR2 is a membrane-spanning tyrosine kinase that serves as a high affinity receptor for several members of the fibroblast growth factor (FGF) family. To explore functions of FGF/FGFR2 signals in development, we have mutated FGFR2 by deleting the entire immunoglobin-like domain III of the receptor. We showed that murine FGFR2 is essential for chorioallantoic fusion and placenta trophoblast cell proliferation. Fgfr2(DeltaIgIII/DeltaIgIII) embryos displayed two distinct defects that resulted in failures in formation of a functional placenta. About one third of the mutants failed to form the chorioallantoic fusion junction and the remaining mutants did not have the labyrinthine portion of the placenta. Consequently, all mutants died at 10–11 days of gestation. Interestingly, Fgfr2(DeltaIgIII/DeltaIgIII) embryos do not form limb buds. Consistent with this defect, the expression of Fgf8, an apical ectodermal factor, is absent in the mutant presumptive limb ectoderm, and the expression of Fgf10, a mesenchymally expressed limb bud initiator, is down regulated in the underlying mesoderm. These findings provide direct genetic evidence that FGF/FGFR2 signals are absolutely required for vertebrate limb induction and that an FGFR2 signal is essential for the reciprocal regulation loop between FGF8 and FGF10 during limb induction.

Mammalian DNA topoisomerase IIIalpha is essential in early embryogenesis

Targeted disruption of the mouse TOP3alpha gene encoding DNA topoisomerase IIIalpha was carried out to study the physiological functions of the mammalian type IA DNA topoisomerase. Whereas heterozygous top3alpha+/- mutant mice were found to resemble phenotypically their TOP3alpha+/+ litermates, no viable top3alpha-/- homozygotes were found among over 100 progeny of top3alpha+/- intercrosses. Examination of embryos dissected from decidual swellings and in vitro culturing of blastocysts from top3alpha+/- intercrosses showed that implantation of top3alpha-/- embryos and the induction of decidualization could occur, but viability of these embryos was severely compromised at an early stage of development. The requirement of mouse DNA topoisomerase IIIalpha during early embryogenesis is discussed in terms of its plausible role in chromosome replication and its interaction with the RecQ/SGS1 family of DNA helicases, whose members include the Bloom's syndrome and the Werner's syndrome gene products.

Antagonistic effects of FGF4 on BMP induction of apoptosis and chondrogenesis in the chick limb bud

In an effort to define the roles of bone morphogenic proteins (BMPs) and fibroblast growth factors (FGFs) during chick limb development more closely, we have implanted beads impregnated with these growth factors into chick limb buds between stages 20 and 26. Embryos were sacrificed at the time the bone chondrocyte condensations first appear (stages 27-28). Implantation of beads containing BMPs at the earlier stages (20-22) caused apoptosis to occur, in the most severe cases leading to complete limb degeneration. Application of FGF4, either in the same, or in a different bead, prevented the BMP-induced apoptosis. We argue that the apoptosis observed on removal of the AER prior to stage 23 of development could be brought about by BMPs. The action of epithelial FGF in preventing BMP-mediated apoptosis in the mesenchyme would define a novel aspect of epithelial-mesenchymal interactions. Implanting the BMP4 beads into the core of the limb bud a day later (stages 25-26) caused intense chondrogenesis rather than apoptosis. FGF4 could again nullify this effect and by itself caused a reduction in bone size. This is the reverse of the functional relationship these growth factors have in mouse tooth specification (where it is BMP4 that inhibits the FGF8 function), and suggests that the balance between the effects of FGFs and BMPs could control the size of the chondrocyte precursor cell pool. In this way members of these two growth factor families could control the size of appendages when they are initially formed.

Role of the subchondral vascular system in endochondral ossification: endothelial cells specifically derepress late differentiation in resting chondrocytes in vitro

Endochondral ossification in growth plates proceeds through several consecutive steps of late cartilage differentiation leading to chondrocyte hypertrophy, vascular invasion, and, eventually, to replacement of the tissue by bone. It is well established that the subchondral vascular system is pivotal in the regulation of this process. Cells of subchondral blood vessels act as a source of vascular invasion and, in addition, release factors influencing growth and differentiation of chondrocytes in the avascular growth plate. To elucidate the paracrine contribution of endothelial cells we studied the hypertrophic development of resting chondrocytes from the caudal third of chick embryo sterna in co-culture with endothelial cells. The design of the experiments prevented cell-to-cell contact but allowed paracrine communication between endothelial cells and chondrocytes. Under these conditions, chondrocytes rapidly became hypertrophied in vitro and expressed the stage-specific markers collagen X and alkaline phosphatase. This development also required signaling by thyroid hormone in synergy. Conditioned media could replace the endothelial cells, indicating that diffusible factors mediated this process. By contrast, smooth muscle cells, fibroblasts, or hypertrophic chondrocytes did not secrete this activity, suggesting that the factors were specific for endothelial cells. We conclude that endochondral ossification is under the control of a mutual communication between chondrocytes and endothelial cells. A finely tuned balance between chondrocyte-derived signals repressing cartilage maturation and endothelial signals promoting late differentiation of chondrocytes is essential for normal endochondral ossification during development, growth, and repair of bone. A dysregulation of this balance in permanent joint cartilage also may be responsible for the initiation of pathological cartilage degeneration in joint diseases.

Inactivation of the FGF-4 gene in embryonic stem cells alters the growth and/or the survival of their early differentiated progeny

Previous studies have shown that early mouse embryos with both FGF-4 alleles inactivated are developmentally arrested shortly after implantation. To understand the roles of FGF-4 during early development, we prepared genetically engineered embryonic stem (ES) cells, which are unable to produce FGF-4. Specifically, we describe the isolation and characterization of ES cells with both FGF-4 alleles inactivated. The FGF-4-/- ES cells do not require FGF-4 to proliferate in vitro, and addition of FGF-4 to the medium has little or no effect on their growth. Thus, FGF-4 does not appear to act as an autocrine growth factor for cultured ES cells. We also demonstrate that FGF-4-/- ES cells, like their unmodified counterparts, are capable of forming highly complex tumors in syngeneic mice composed of a wide range of differentiated cells types, including neural tissue, glandular epithelium, and muscle. In addition, we demonstrate that the FGF-4-/- ES cells can differentiate in vitro after exposure to retinoic acid; however, the growth and/or survival of the differentiated cells is severely compromised. Importantly, addition of FGF-4 to the culture medium dramatically increases the number of differentiated cells derived from the FGF-4-/- ES cells, in particular cells with many of the properties of parietal extraembryonic endoderm. Finally, we demonstrate that there are differences in the RNA profiles expressed by the differentiated progeny formed in vitro from FGF-4-/- ES cells and FGF-4+/+ ES cells when they are cultured with FGF-4. Taken together, the studies described in this report indicate that certain lineages formed in vitro are affected by the inactivation of the FGF-4 gene, in particular specific cells that form during the initial stage of ES cell differentiation. Thus, ES cells with both FGF-4 alleles inactivated should shed light on the important roles of FGF-4 during the early stages of mammalian development and help determine why FGF-4-/- embryos die shortly after implantation.

Impaired granulopoiesis, myelodysplasia, and early lethality in CCAAT/enhancer binding protein epsilon-deficient mice

Polymorphonuclear leukocytes are essential for host defense to infectious diseases. CCAAT/enhancer binding protein epsilon (C/EBP epsilon) is preferentially expressed in granulocytes and lymphoid cells. Mice with a null mutation in C/EBP epsilon develop normally and are fertile but fail to generate functional neutrophils and eosinophils. Opportunistic infections and tissue destruction lead to death by 3-5 months of age. Furthermore, end-stage mice develop myelodysplasia, characterized by proliferation of atypical granulocytes that efface the bone marrow and result in severe tissue destruction. Thus, C/EBP epsilon is essential for terminal differentiation and functional maturation of committed granulocyte progenitor cells.

FGF-2 converts mature oligodendrocytes to a novel phenotype

Fibroblast growth factor (FGF)-2 differentially regulates oligodendrocyte progenitor proliferation and differentiation in culture, and modulates gene expression of its own receptors, in a developmental and receptor type-specific manner (Bansal et al., 1996a,b). Three FGF receptors (types 1, 2, 3) are expressed in postmitotic, terminally differentiating oligodendrocytes. Exposure of such cells to FGF-2 results in: (a) the down-regulation of myelin-specific gene expression (e.g., ceramide galactosyltransferase, 2',3'-cyclic nucleotide 3'-phosphohydrolase, myelin basic protein, proteolipid protein), (b) dramatic increases in the length of cellular processes in a time- and dose-dependent manner, (c) re-entrance into the cell cycle without accompanying mitosis, and (d) the alteration of the expression of both low- and high-affinity FGF receptors. Compared to oligodendrocyte progenitors, the differentiated oligodendrocytes treated with FGF-2 incorporate BrdU at a slower rates, exhibit different patterns of both FGF high- and low-affinity (syndecans) receptors, and are morphologically very different. In addition, they do not re-express the progenitor markers A2B5, NG2 or PDGFalpha receptor. Therefore, although the FGF-treated cells lose their differentiated OL/myelin markers, they do not revert to progenitors and clearly represent a different, apparently novel, phenotype both morphologically and biochemically, which we have termed NOLs. These data indicate that terminally differentiated oligodendrocytes retain the plasticity to reprogram their differentiation fate under the influence of environmental factors. The possible significance of this response to FGF relative to normal and pathological physiology is discussed. In particular, on the basis of these data we predict the appearance of cells in and around multiple sclerosis plaques with the phenotype O4+, NG2-, A2B5-, O1-, MBP-.

A role for fibroblast growth factor signaling in the lobuloalveolar development of the mammary gland

The inappropriate expression of growth factors, or activating mutations of their receptors, have been implicated as causative factors in mouse and human mammary cancer. For example, it has been known for some time that three members of the fibroblast growth factor (FGF) family behave like oncogenes in virally induced mammary cancer of mice. In normal circumstances, signaling via FGF receptors is known to mediate growth, differentiation, and patterning, during embryogenesis and fetal development. A powerful approach to dissecting the roles for these signaling pathways is to determine the developmental consequences of abrogating their function in transgenic mice. In this review, we describe the use of dominant negative FGF receptors to evaluate the contribution of specific FGF signals in normal mammary gland development. These studies have revealed that normal lobuloalveolar development requires FGF signaling to the mammary epithelium, a function that is presumably usurped by MMTV in mouse mammary tumorigenesis.

Abnormal FGFR 3 expression in cartilage of thanatophoric dysplasia fetuses

Thanatophoric dysplasia (TD), the commonest lethal skeletal dysplasia in humans, is accounted for by recurrent mutations in the fibroblast growth factor receptor 3 gene (FGFR 3), causing its constitutive activation in vitro. Taking advantage of medical abortion of 18 TD fetuses, cartilage sections were studied for FGFR 3 gene expression by in situ hybridization and immunohistochemistry. Specific antibodies revealed high amounts of FGFR 3 in cartilage of TD fetuses with no increased level of the corresponding mRNA. The specific signal was mainly detected in the nucleus of proliferative and hypertrophic chondrocytes. Based on this observation and the abnormal expression of collagen type X in hypertrophic TD chondrocytes, we suggest that constitutive activation of the receptor through formation of a stable dimer increases its stability and promotes its translocation into the nucleus, where it might interfere with terminal chondrocyte differentiation.

A homeo-interaction sequence in the ectodomain of the fibroblast growth factor receptor

Interaction of fibroblast growth factor receptors (FGFR) sufficient for a trans-phosphorylation event in which one intracellular domain is substrate for the other is essential for signal transduction. By analysis of the direct interaction of recombinant constructions co-expressed in baculoviral-infected insect cells, we identified a 17-amino acid sequence that is required for the stable interaction between ectodomains of FGFR. The sequence 160ERSPHRPILQAGLPANK176 (Glu160-Lys176) connects immunoglobulin modules II and III. In insect cells, the interaction between Glu160-Lys176 domains occurs independently of intact heparin or FGF binding domains. The sequence is not required for the binding of heparin or FGF-1, but is essential for mitogenic activity of the FGFR kinase in mammalian cells. The results support a model in which the homeo-interaction between Glu160-Lys176 in the ectodomain contributes to the interaction between intracellular domains in mammalian cell membranes (Kan, M., Wang, F., Kan, M., To, B., Gabriel, J. L., and McKeehan, W. L. (1996) J. Biol. Chem. 271, 26143-26148). We propose that the Glu160-Lys176 domain plays a pivotal role in restriction of the interaction between kinases by pericellular matrix heparan sulfate proteoglycan and divalent cations. Restrictions are overcome by FGF or constitutively by diverse gain of function mutations which cause skeletal and craniofacial abnormalities.

Social interaction and sensorimotor gating abnormalities in mice lacking Dvl1

Mice completely deficient for Dvl1, one of three mouse homologs of the Drosophila segment polarity gene Dishevelled, were created by gene targeting. Dvl1-deficient mice are viable, fertile, and structurally normal. Surprisingly, these mice exhibited reduced social interaction, including differences in whisker trimming, deficits in nest-building, less huddling contact during home cage sleeping, and subordinate responses in a social dominance test. Sensorimotor gating was abnormal, as measured by deficits in prepulse inhibition of acoustic and tactile startle. Thus, Dvl1 mutants may provide a model for aspects of several human psychiatric disorders. These results are consistent with an interpretation that common genetic mechanisms underlie abnormal social behavior and sensorimotor gating deficits and implicate Dvl1 in processes underlying complex behaviors.

Expression of fibroblast growth factor receptors (FGFR1, FGFR2, FGFR3) in the developing head and face

Fibroblast growth factors may play an important role in the differential growth of the skull, brain, and facial prominences. In order to understand the role of FGFs in vivo, we have analyzed the competency of head mesenchyme to respond to FGFs via expression of the high affinity receptors FGFR1, 2, and 3. Receptor transcripts, especially those of FGFR2 and FGFR3, were localized to specific regions of the head. We raise the possibilities of particular receptor-ligand combinations and the possible functions of these interactions in the morphogenesis of the head, face, and brain. Finally, we discuss the relationship between FGF receptor expression in the chicken and the phenotypes of FGF receptor mutations in humans.

Translocations involving 4p16.3 in three families: deletion causing the Pitt-Rogers-Danks syndrome and duplication resulting in a new overgrowth syndrome

Three families are reported who have a translocation involving 4p16.3. Nine subjects are described with the clinical features of the Pitt-Rogers-Danks (PRD) syndrome confirming pre- and postnatal growth failure, microcephaly, severe mental retardation, seizures, and a distinctive facial appearance; a deletion of 4p16.3 was seen in all eight patients studied with fluorescence in situ hybridisation (FISH). Eleven subjects had a new syndrome with physical overgrowth, heavy facial features, and mild to moderate mental handicap; a duplication of the chromosome region 4p16.3 was found in the four subjects studied. It is suggested that the growth abnormalities in these two families may be explained by a dosage effect of the fibroblast growth factor receptor gene 3 (FGFR3), which is located at 4p16.3, that is, a single dose leads to growth failure and a triple dose to physical overgrowth. We describe the molecular mapping of the translocation breakpoint and define it to within locus D4S43.

Fgfr2 and osteopontin domains in the developing skull vault are mutually exclusive and can be altered by locally applied FGF2

Mutations in the human fibroblast growth factor receptor type 2 (FGFR2) gene cause craniosynostosis, particularly affecting the coronal suture. We show here that, in the fetal mouse skull vault, Fgfr2 transcripts are most abundant at the periphery of the membrane bones; they are mutually exclusive with those of osteopontin (an early marker of osteogenic differentiation) but coincide with sites of rapid cell proliferation. Fibroblast growth factor type 2 (FGF2) protein, which has a high affinity for the FGFR2 splice variant associated with craniosynostosis, is locally abundant; immunohistochemical detection showed it to be present at low levels in Fgfr2 expression domains and at high levels in differentiated areas. Implantation of FGF2-soaked beads onto the fetal coronal suture by ex utero surgery resulted in ectopic osteopontin expression, encircled by Fgfr2 expression, after 48 hours. We suggest that increased FGF/FGFR signalling in the developing skull, whether due to FGFR2 mutation or to ectopic FGF2, shifts the cell proliferation/differentiation balance towards differentiation by enhancing the normal paracrine down-regulation of Fgfr2.

Articular chondrocytes produce factors that inhibit maturation of sternal chondrocytes in serum-free agarose cultures: a TGF-beta independent process

Under normal conditions, articular chondrocytes persist throughout postnatal life, whereas "transient" chondrocytes, which constitute the bulk of prenatal and early postnatal cartilaginous skeleton, undergo maturation, hypertrophy, and replacement by bone cells. The mechanisms regulating the markedly different behavior and fate of articular and transient chondrocytes are largely unclear. In the present study, we asked whether articular chondrocytes possess dominant antimaturation properties which may subtend their ability to persist throughout life. Adult chicken articular chondrocytes and transient maturing chondrocytes from the core region of day 17, chick embryo cephalic sternum were cultured or cocultured in serum-free agarose conditions. When the sternal cells were grown by themselves, they quickly developed into hypertrophic type X collagen-synthesizing cells; however, when they were cocultured with as few as 10% articular chondrocytes or fed with articular chondrocyte-conditioned medium, their maturation was markedly impaired, as revealed by a sharp drop in type X collagen synthesis. A similar, albeit less potent, antimaturation activity characterized resting and proliferating immature chondrocytes isolated from other regions of embryonic sternum. Transforming growth factor-beta 2 (TGF-beta 2) was previously suggested to be an inhibitor of chondrocyte maturation. We found, however, that treatment with a neutralizing antiserum to TGF-beta did not counteract the inhibition of maturation in cocultures of articular and maturing core sternal chondrocytes. Indeed, articular chondrocytes produced and accumulated relatively low levels of TGF-beta in their culture medium, about 15 ng/ml/48 h, of which over 90% was latent; surprisingly, maturing sternal core chondrocytes accumulated over 10-fold more TGF-beta in the medium, about 150 ng/ml/48 h, of which over 20% was endogenously active. These results indicate that articular chondrocytes do possess dominant antimaturation properties which appear to be TGF-beta independent. The TGF-beta s may thus have a more prominent role in the terminal phases of chondrocyte maturation, as indicated by their abundance and greater activity in hypertrophic chondrocytes.

Receptor protein tyrosine kinases in perinatal developing rat kidney

We have identified receptor protein tyrosine kinases (PTKs) that are expressed and/or activated during kidney development. mRNA from fetal rat kidneys in late gestation (embryonic day 21), was used to prepare a cDNA template for polymerase chain reaction amplification with primers based on conserved regions of PTKs, and products were subcloned and sequenced. Among 346 clones, we identified epidermal growth factor receptor (EGF-R), Tie-2, platelet-derived growth factor receptor (PDGF-R)-alpha, PDGF-R beta, Flk-1, Flt-4, fibroblast growth factor receptor (FGF-R)-1, FGF-R3, FGF-R4, Met, and RYK/Nbtk-1. PTK expression was studied by immunoprecipitation and immunoblotting of kidney membrane proteins with specific antibodies. EGF-R, PDGF-R alpha, FGF-R1, FGF-R3, Met, and in some cases Tie-2 protein expression was greater in fetal kidneys, as compared with kidneys from 12-week-old adult rats (controls). Flk-1, PDGF-R beta, and FGF-R4 proteins were expressed comparably, however, Flt-4 was not detected. As a reflection of receptor PTK activity, we assessed endogenous tyrosine phosphorylation, and in vitro autophosphorylation. EGF-R and PDGF-R alpha displayed activity in fetal, but not adult kidneys. FGF-R3 and Flk-1 were active in some fetal kidneys, and the other PTKs were not active. Thus, in late gestational rat kidney, there are distinct patterns of receptor PTK expression and activity. EGF-R, PDGF-R alpha, FGF-R3 and Flk-1 are among the PTKs that are activated, and they may mediate perinatal development of renal epithelial, interstitial, or vascular structures.

Achondroplasia: recent advances in diagnosis and treatment

Achondroplasia (ACH) is the most common form of chondrodysplasia in humans. This disorder is inherited as an autosomal dominant trait, though most cases are sporadic. Recent advances in molecular biology have revealed its genetic defect in fibroblast growth factor-3 gene. This may introduce a new diagnostic tool and the classification of ACH and related disorders. Recent molecular engineering techniques have made it possible to provide large amounts of the various kinds of biofactors, such as erythropoietin, granulocyte colony stimulating factor and human growth hormone (GH), for clinical use. In fact, GH has been widely used to treat non-GH-deficient forms of short stature, such as Turner's syndrome, skeletal dysplasia, intrauterine growth retardation, chronic illness and idiopathic short stature, with beneficial effects. This may also be introduced into the medical management of ACH.