Abnormal lung development and cleft palate in mice lacking TGF-beta 3 indicates defects of epithelial-mesenchymal interaction

Testing candidate genes for non-syndromic oral clefts using a case-parent trio design

Markers in five candidate genes were examined on 269 case-parent trios ascertained through a child with an isolated, non-syndromic oral cleft (cleft lip, CL; cleft palate, CP; or cleft lip and palate, CLP). Cases and their parents were ascertained through treatment centers in Maryland. Markers at two of the five candidate genes, transforming growth factor beta3 (TGFbeta3) and MSX1, showed consistent evidence of linkage and disequilibrium due to linkage using several statistical tests (e.g., the global chi-square for TGFbeta3 was 21.1 with 12 df, P = 0.03; that for MSX1 was 8.7 with 3 df, P = 0.03). There was little evidence of heterogeneity in the role of TGFbeta3 between different types of oral clefts, but MSX1 did yield marginal evidence for such heterogeneity. MSX1 also showed evidence for interaction between infant's genotype and maternal smoking, giving a likelihood ratio test for heterogeneity between smoker and non-smoker mothers of 7.16 (2 df, P = 0.03). Using a conditional logistic model to test for gene-gene interaction showed no evidence of interaction between TGFbeta3 and MSX1, with both seeming to contribute independently to risk of isolated, non-syndromic oral clefts.

Beyond Mitomycin: TGF-beta and wound healing

The introduction of the anti-cancer drugs Mitomycin and 5-fluorouracil as anti-scarring agents within the last decade, has greatly improved surgical results of glaucoma filtration surgery. However, a number of problems associated with their use have emerged. At the same time, the transforming growth factor-beta (TGF-beta) has been identified as an important component of wound healing, particularly in the conjunctival scarring response. Recent developments in molecular therapy offer exciting prospects for the modulation of wound healing, specifically those targeting TGF-beta. As TGF-beta is such a potent stimulant of scarring, this review examines its biology and role in ocular wound healing and repair, and discusses promising new approaches to modifying its activity.

TGFbeta2 in corneal morphogenesis during mouse embryonic development

To examine the roles of TGFbeta isoforms on corneal morphogenesis, the eyes of mice that lack TGFbetas were analyzed at different developmental stages for cell proliferation, migration and apoptosis, and for expression patterns of keratin 12, lumican, keratocan and collagen I. Among the three Tgfb(-/-) mice, only Tgfb2(-/-) mice have abnormal ocular morphogenesis characterized by thin corneal stroma, absence of corneal endothelium, fusion of cornea to lens (a Peters'-like anomaly phenotype), and accumulation of hyaline cells in vitreous. In Tgfb2(-/-) mice, fewer keratocytes were found in stroma that has a decreased accumulation of ECM; for example, lumican, keratocan and collagen I were greatly diminished. The absence of TGFbeta2 did not compromise cell proliferation, nor enhance apoptosis. The thinner stroma resulting from decreased ECM synthesis may account for the decreased cell number in the stroma of Tgfb2 null mice. Keratin 12 expression was not altered in Tgfb2(-/-) mice, implicating normal corneal type epithelial differentiation. Delayed appearance of macrophages in ocular tissues was observed in Tgfb2(-/-) mice. Malfunctioning macrophages may account for accumulation of cell mass in vitreous of Tgfb2 null mice.

Antagonistic effects of Smad2 versus Smad7 are sensitive to their expression level during tooth development

Members of the transforming growth factor-beta (TGF-beta) superfamily regulate cell proliferation, differentiation, and apoptosis, controlling the development and maintenance of most tissues. TGF-beta signal is transmitted through the phosphorylation of Smad proteins by TGF-beta receptor serine/threonine kinase. During early tooth development, TGF-beta inhibits proliferation of enamel organ epithelial cells but the underlying molecular mechanisms are largely unknown. Here we tested the hypothesis that antagonistic effects between Smad2 and Smad7 regulate TGF-beta signaling during tooth development. Attenuation of Smad2 gene expression resulted in significant advancement of embryonic tooth development with increased proliferation of enamel organ epithelial cells, while attenuation of Smad7 resulted in significant inhibition of embryonic tooth development with increased apoptotic activity within enamel organ epithelium. These findings suggest that different Smads may have differential activities in regulating TGF-beta-mediated cell proliferation and death. Furthermore, functional haploinsufficiency of Smad2, but not Smad3, altered TGF-beta-mediated tooth development. The results indicate that Smads are critical factors in orchestrating TGF-beta-mediated gene regulation during embryonic tooth development. The effectiveness of TGF-beta signaling is highly sensitive to the level of Smad gene expression.

Abnormal function of astroglia lacking Abr and Bcr RacGAPs

Experiments in cultured cells have implicated the molecular switch Rac in a wide variety of cellular functions. Here we demonstrate that the simultaneous disruption of two negative regulators of Rac, Abr and Bcr, in mice leads to specific abnormalities in postnatal cerebellar development. Mutants exhibit granule cell ectopia concomitant with foliation defects. We provide evidence that this phenotype is causally related to functional and structural abnormalities of glial cells. Bergmann glial processes are abnormal and GFAP-positive astroglia were aberrantly present on the pial surface. Older Abr;Bcr-deficient mice show spontaneous mid-brain glial hypertrophy, which can further be markedly enhanced by kainic acid. Double null mutant astroglia are hyper-responsive to stimulation with epidermal growth factor and lipopolysaccharide and exhibit constitutively increased phosphorylation of p38 mitogen-activated protein kinase, which is regulated by Rac. These combined data demonstrate a prominent role for Abr and Bcr in the regulation of glial cell morphology and reactivity, and consequently in granule cell migration during postnatal cerebellar development in mammals.

Removal of the floxed neo gene from a conditional knockout allele by the adenoviral Cre recombinase in vivo

Conditional and tissue specific gene targeting using the Cre-loxP recombination system in combination with established ES cell techniques has become a standard for in vivo loss of function studies. In a typical flox and delete gene targeting strategy, the loxP-neo-loxP cassette is inserted into an intron and an additional loxP site is located in one of the homology arms so that loxP sites surround a functionally essential part of the gene. The neo cassette in usually removed by transient expression of the Cre recombinase in ES cells to avoid selection gene interference and genetic ambiquity. However, this causes a significant increase in manipulation of ES cells and often compromises ES cell pluripotency. Here we describe a method in which the floxed neo gene is removed from a knockout allele by infecting 16-cell-stage morulae by the recombinant Cre adenovirus. This virus provides only transient Cre expression and does not integrate into the mouse genome. Produced mosaic mice transmitted the desired allele without the neo cassette with high frequency to their offspring. This method is rapid and easy and does not require any special equipment. Moreover, because superovulated mice can be used as donors, this method does not necessitate a large number of mice.

Requirement for TGFβ receptor signaling during terminal lens fiber differentiation

Several families of growth factors have been identified as regulators of cell fate in the developing lens. Members of the fibroblast growth factor family are potent inducers of lens fiber differentiation. Members of the transforming growth factor β (TGFβ) family, particularly bone morphogenetic proteins, have also been implicated in various stages of lens and ocular development, including lens induction and lens placode formation. However, at later stages of lens development, TGFβ family members have been shown to induce pathological changes in lens epithelial cells similar to those seen in forms of human subcapsular cataract. Previous studies have shown that type I and type II TGFβ receptors, in addition to being expressed in the epithelium, are also expressed in patterns consistent with a role in lens fiber differentiation. In this study we have investigated the consequences of disrupting TGFβ signaling during lens fiber differentiation by using the mouse αΑ-crystallin promoter to overexpress mutant (kinase deficient), dominant-negative forms of either type I or type II TGFβ receptors in the lens fibers of transgenic mice. Mice expressing these transgenes had pronounced bilateral nuclear cataracts. The phenotype was characterized by attenuated lens fiber elongation in the cortex and disruption of fiber differentiation, culminating in fiber cell apoptosis and degeneration in the lens nucleus. Inhibition of TGFβ signaling resulted in altered expression patterns of the fiber-specific proteins, α-crystallin, filensin, phakinin and MIP. In addition, in an in vitro assay of cell migration, explanted lens cells from transgenic mice showed impaired migration on laminin and a lack of actin filament assembly, compared with cells from wild-type mice. These results indicate that TGFβ signaling is a key event during fiber differentiation and is required for completion of terminal differentiation.

Activins as regulators of branching morphogenesis

Development of glandular organs such as the kidney, lung, and prostate involves the process of branching morphogenesis. The developing organ begins as an epithelial bud that invades the surrounding mesenchyme, projecting dividing epithelial cords or tubes away from the site of initiation. This is a tightly regulated process that requires complex epithelial-mesenchymal interactions, resulting in a three-dimensional treelike structure. We propose that activins are key growth and differentiation factors during this process. The purpose of this review is to examine the direct, indirect, and correlative lines of evidence to support this hypothesis. The expression of activins is reviewed together with the effect of activins and follistatins in the development of branched organs. We demonstrate that activin has both negative and positive effects on cell growth during branching morphogenesis, highlighting the complex nature of activin in the regulation of proliferation and differentiation. We propose potential mechanisms for the way in which activins modify branching and address the issue of whether activin is a regulator of branching morphogenesis.

TGF-beta signaling in tumor suppression and cancer progression

Epithelial and hematopoietic cells have a high turnover and their progenitor cells divide continuously, making them prime targets for genetic and epigenetic changes that lead to cell transformation and tumorigenesis. The consequent changes in cell behavior and responsiveness result not only from genetic alterations such as activation of oncogenes or inactivation of tumor suppressor genes, but also from altered production of, or responsiveness to, stimulatory or inhibitory growth and differentiation factors. Among these, transforming growth factor beta (TGF-beta) and its signaling effectors act as key determinants of carcinoma cell behavior. The autocrine and paracrine effects of TGF-beta on tumor cells and the tumor micro-environment exert both positive and negative influences on cancer development. Accordingly, the TGF-beta signaling pathway has been considered as both a tumor suppressor pathway and a promoter of tumor progression and invasion. Here we evaluate the role of TGF-beta in tumor development and attempt to reconcile the positive and negative effects of TGF-beta in carcinogenesis.

The transforming growth factor-beta 3 knock-out mouse: an animal model for cleft palate

The recent report of a transforming growth factor-beta 3 (TGF-beta 3) knock-out mouse in which 100 percent of the homozygous pups have cleft palate raised the question as to the potential usefulness of these animals as a model for cleft palate research. The specific aim in this study was to carefully document the anatomy of the cleft palate in the TGF-beta 3 knock-out mice as compared with wild type controls. Special attention was paid to the levator veli palatini muscle, the tensor veli palatini muscle, and their respective innervation. Because the TGF-beta 3 knock-out is lethal in the early perinatal period and because the heterozygotes are phenotypically normal, polymerase chain reaction was required to genotype the animals before mating. Time-mated pregnancies between proven heterozygotes were then delivered by cesarean section at gestational day 18.5 to prevent maternal cannibalism of homozygote pups. All delivered pups were killed and their tails processed by polymerase chain reaction to verify genotype. The heads were then fixed and sectioned in axial, coronal, or sagittal planes. Sections were stained with hematoxylin and eosin or processed for immunohistochemistry with nerve specific protein gene product 9.5 and calcitonin gene-related peptide antibodies. Sections were analyzed in a serial fashion. Nine wild type control animals were analyzed along with nine TGF-beta 3 knock-out homozygotes. Time matings between proven heterozygotes yielded wild type pups, heterozygote pups, and homozygote knock-out pups in the expected mendelian ratios (28 percent to 46 percent to 26 percent; n = 43). The results demonstrated 100 percent clefting in the homozygous TGF-beta 3 knock-out pups. Complete clefting of the secondary palate was seen in four of nine and incomplete clefting was seen in five of nine. The levator veli palatini and tensor veli palatini muscles were demonstrated coursing parallel to the cleft margin in all cleft mice. The orientation of these muscles differs from the normal transverse sling of the levator veli palatini muscle and the normal palatine aponeurosis of the tensor veli palatini muscle at the soft palate in control animals. Innervation of the levator veli palatini muscle by cranial nerve IX and the tensor veli palatini muscle by cranial nerve V were demonstrated in both cleft and control animals by use of immunohistochemistry with nerve-specific antibodies. Demonstration of a teratogen-free, reproducible animal model of clefting of the palate with a known, single-gene etiology is an important step in the systematic understanding of a congenital defect whose multifactorial etiology has hampered previous research efforts. This study presents a detailed anatomic description of such a model, including a description of the muscular anatomy and the innervation of the muscles of the palate. Because of early perinatal mortality, this model has limited applications for postnatal studies.

TGF-beta3-induced palatogenesis requires matrix metalloproteinases

Cleft lip and palate syndromes are among the most common congenital malformations in humans. Mammalian palatogenesis is a complex process involving highly regulated interactions between epithelial and mesenchymal cells of the palate to permit correct positioning of the palatal shelves, the remodeling of the extracellular matrix (ECM), and subsequent fusion of the palatal shelves. Here we show that several matrix metalloproteinases (MMPs), including a cell membrane-associated MMP (MT1-MMP) and tissue inhibitor of metalloproteinase-2 (TIMP-2) were highly expressed by the medial edge epithelium (MEE). MMP-13 was expressed both in MEE and in adjacent mesenchyme, whereas gelatinase A (MMP-2) was expressed by mesenchymal cells neighboring the MEE. Transforming growth factor (TGF)-beta3-deficient mice, which suffer from clefting of the secondary palate, showed complete absence of TIMP-2 in the midline and expressed significantly lower levels of MMP-13 and slightly reduced levels of MMP-2. In concordance with these findings, MMP-13 expression was strongly induced by TGF-beta3 in palatal fibroblasts. Finally, palatal shelves from prefusion wild-type mouse embryos cultured in the presence of a synthetic inhibitor of MMPs or excess of TIMP-2 failed to fuse and MEE cells did not transdifferentiate, phenocopying the defect of the TGF-beta3-deficient mice. Our observations indicate for the first time that the proteolytic degradation of the ECM by MMPs is a necessary step for palatal fusion.

The transcriptional repressor CDP (Cutl1) is essential for epithelial cell differentiation of the lung and the hair follicle

The mammalian Cutl1 gene codes for the CCAAT displacement protein (CDP), which has been implicated as a transcriptional repressor in diverse processes such as terminal differentiation, cell cycle progression, and the control of nuclear matrix attachment regions. To investigate the in vivo function of Cutl1, we have replaced the C-terminal Cut repeat 3 and homeodomain exons with an in-frame lacZ gene by targeted mutagenesis in the mouse. The CDP-lacZ fusion protein is retained in the cytoplasm and fails to repress gene transcription, indicating that the Cutl1(lacZ) allele corresponds to a null mutation. Cutl1 mutant mice on inbred genetic backgrounds are born at Mendelian frequency, but die shortly after birth because of retarded differentiation of the lung epithelia, which indicates an essential role of CDP in lung maturation. A less pronounced delay in lung development allows Cutl1 mutant mice on an outbred background to survive beyond birth. These mice are growth-retarded and develop an abnormal pelage because of disrupted hair follicle morphogenesis. The inner root sheath (IRS) is reduced, and the transcription of Sonic hedgehog and IRS-specific genes is deregulated in Cutl1 mutant hair follicles, consistent with the specific expression of Cutl1 in the progenitors and cell lineages of the IRS. These data implicate CDP in cell-lineage specification during hair follicle morphogenesis, which resembles the role of the related Cut protein in specifying cell fates during Drosophila development.

Elevated TGFbeta signaling inhibits ocular vascular development

Alterations in the ocular vasculature are associated with retinal diseases such as retinopathy of prematurity and diabetic retinopathy. Vascular endothelial growth factor (VEGF) as a potent stimulator for normal and abnormal vascular growth has been extensively studied. However, little is known about secreted factors that negatively regulate vascular growth in ocular tissues. We now report that expression of a self-activating TGFbeta1 in the ocular lens of transgenic mice results in inhibition of retinal angiogenesis followed by retinal degeneration. Transgenic TGFbeta1 can rescue the hyperplasic hyaloid tissue and reverse the corneal deficiency in TGFbeta2-null embryos. These results demonstrate that TGFbeta signaling modulates development of ocular vasculature and cornea in a dosage-dependent manner and that TGFbeta1 can substitute for TGFbeta2 in ocular tissues.

Transforming growth factor-beta signal transduction in epithelial cells

Transforming growth factor (TGF)-beta is a natural and potent growth inhibitor of a variety of cell types, including epithelial, endothelial, and hematopoietic cells. The ability of TGF-beta to potently inhibit the growth of many solid tumors of epithelial origin, including breast and colon carcinomas, is of particular interest. However, many solid tumor cells become refractory to the growth inhibitory effects of TGF-beta due to defects in TGF-beta signaling pathways. In addition, TGF-beta may stimulate the invasiveness of tumor cells via the paracrine effects of TGF-beta. Accordingly, in order to develop more effective anticancer therapeutics, it is necessary to determine the TGF-beta signal transduction pathways underlying the growth inhibitory effects and other cellular effects of TGF-beta in normal epithelial cells. Thus far, two primary signaling cascades downstream of the TGF-beta receptors have been elucidated, the Sma and mothers against decapentaplegic homologues and the Ras/mitogen-activated protein kinase pathways. The major objective of this review is to summarize TGF-beta signaling in epithelial cells, focusing on recent advances involving the Sma and mothers against decapentaplegic homologues and Ras/mitogen-activated protein kinase pathways. This review is particularly timely in that it provides a comprehensive summary of both signal transduction mechanisms and the cell cycle effects of TGF-beta.

TGF-beta1 perturbs vascular development and inhibits epithelial differentiation in fetal lung in vivo

Members of the transforming growth factor beta (TGF-beta) family of polypeptides have been implicated in morphogenesis and differentiation in numerous tissues, including the lung. In order to further define effects of TGF-beta signaling in lung morphogenesis, a constitutively active form of TGF-beta1 was expressed in respiratory epithelial cells of the fetal mouse lung in vivo. Expression of TGF-beta1 arrested lung morphogenesis in the pseudoglandular stage of development, inhibiting synthesis of differentiation-dependent proteins, SP-B, SP-C, and CCSP, and maintaining embryonic patterns of staining for thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor-3beta (HNF-3beta). The pulmonary mesenchyme was thickened and vascular density was increased by TGF-beta1. TGF-beta1 decreased expression of vascular endothelial growth factor-A (VEGF-A) mRNA and protein, and the abundance of Flk-1 mRNA in the lung mesenchyme. Distribution of platelet-endothelial cell adhesion molecule (PECAM)-1, a marker of pulmonary blood vessels, was altered, and ultrastructural studies demonstrated that TGF-beta1 inhibited vascular development in the fetal lung. TGF-beta1 perturbed both epithelial cell differentiation and formation of the pulmonary vasculature, supporting the concept that precise control of signaling via the TGF-beta receptor pathway is critical for normal lung morphogenesis.

Functional analysis of secreted and transmembrane proteins critical to mouse development

We describe the successful application of a modified gene-trap approach, the secretory trap, to systematically analyze the functions in vivo of large numbers of genes encoding secreted and membrane proteins. Secretory-trap insertions in embryonic stem cells can be transmitted to the germ line of mice with high efficiency and effectively mutate the target gene. Of 60 insertions analyzed in mice, one-third cause recessive lethal phenotypes affecting various stages of embryonic and postnatal development. Thus, secretory-trap mutagenesis can be used for a genome-wide functional analysis of cell signaling pathways that are critical for normal mammalian development and physiology.

Lefty inhibits receptor-regulated Smad phosphorylation induced by the activated transforming growth factor-beta receptor

Transforming growth factor-beta (TGF-beta) is a pleiotropic cytokine that regulates growth and differentiation of diverse types of cells. TGF-beta actions are directed by ligand-induced activation of TGF-beta receptors with intrinsic serine/threonine kinase activity that trigger phosphorylation of receptor-regulated Smad (R-Smad) protein. Phosphorylated R-Smad proteins bind to Smad4, and the complexes formed move into the nucleus, where they act as components of a transcriptional complex. Here, we show that TGF-beta signaling is inhibited by lefty, a novel member of the TGF-beta superfamily. Lefty perturbed TGF-beta signaling by inhibiting the phosphorylation of Smad2 following activation of the TGF-beta receptor. Moreover, lefty inhibited the events that lie downstream from R-Smad phosphorylation, including heterodimerization of R-Smad proteins with Smad4 and nuclear translocation of the R-Smad.Smad4 complex. Lefty repressed TGF-beta-induced expression of reporter genes for the p21, cdc25, and connective tissue growth factor promoters and of a reporter gene driven by the Smad-binding element. Similarly, lefty inhibited both BMP-mediated Smad5 phosphorylation and gene transcription. The action of lefty does not appear to depend on protein synthesis, including synthesis of inhibitory Smad proteins. Thus, lefty provides a repressed state of TGF-beta- or BMP-responsive genes and participates in negative modulation of TGF-beta and BMP signaling by inhibition of phosphorylation of R-Smad proteins.

Lung hypoplasia and neonatal death inFgf9-null mice identify this gene as an essential regulator of lung mesenchyme

Mammalian lung develops as an evagination of ventral gut endoderm into the underlying mesenchyme. Iterative epithelial branching, regulated by the surrounding mesenchyme, generates an elaborate network of airways from the initial lung bud. Fibroblast growth factors (FGFs) often mediate epithelial-mesenchymal interactions and mesenchymal Fgf10 is essential for epithelial branching in the developing lung. However, no FGF has been shown to regulate lung mesenchyme. In embryonic lung, Fgf9 is detected in airway epithelium and visceral pleura at E10.5, but is restricted to the pleura by E12.5. We report that mice homozygous for a targeted disruption of Fgf9 exhibit lung hypoplasia and early postnatal death. Fgf9−/− lungs exhibit reduced mesenchyme and decreased branching of airways, but show significant distal airspace formation and pneumocyte differentiation. Our results suggest that Fgf9 affects lung size by stimulating mesenchymal proliferation. The reduction in the amount of mesenchyme in Fgf9−/− lungs limits expression of mesenchymal Fgf10. We suggest a model whereby FGF9 signaling from the epithelium and reciprocal FGF10 signaling from the mesenchyme coordinately regulate epithelial airway branching and organ size during lung embryogenesis.

Genetics of craniofacial development and malformation

The head is anatomically the most sophisticated part of the body and its evolution was fundamental to the origin of vertebrates; understanding its development is a formidable problem in biology. A synthesis of embryology, evolution and mouse genetics is shaping our understanding of head development and in this review we discuss its application to studies of human craniofacial malformations. Many of these disorders have their origins in specific embryological processes, including abnormalities of brain patterning, of the migration and fusion of tissues in the face, and of bone differentiation in the skull vault.

Changes in extracellular matrix and in transforming growth factor beta isoforms after coronary artery ligation in rats

Extensive myocardial remodeling occurs after transmural myocardial infarction (MI). The infarcted myocardium is being replaced by scar tissue after gradual resorption of the necrotic tissue. The remodeling process involves both synthesis and degradation of collagens as major components of the extracellular matrix (ECM). In the present study we have analyzed the time-dependent changes of the processes related to this fibrosis in the infarct area and in the non-infarcted left ventricle (LV) six hours to 82 days after occlusion of the left anterior descending coronary artery (LAD) in rats. We also examined whether changes occurred in the expression pattern of the transforming growth factor (TGF) beta isoforms, since this cytokine is known as powerful inductor of fibrosis. Elevation in colligin expression preceded the pronounced increase in mRNA expression of both type I and type III collagen after MI from day three onwards. The maximal increase in colligin protein in the infarct area coincided with the most pronounced expression of collagen I and collagen III mRNA expression. Also, the expression and activity of matrix metalloproteinases (MMPs) and of tissue inhibitor of matrix metalloproteinase (TIMP)-2 mRNA were increased predominantly in the infarct area. TGF beta(1)and TGF-beta(2)expression increased within the first days after MI, whereas TGF-beta(3)expression was elevated predominantly in the infarct area. This pronounced increase in TGF-beta(3)persisted up to 82 days and correlated positively with the parameters of ECM metabolism. Thus, the scar formation is an ongoing dynamic process in which TGF-beta(3)seems to play an active role in the complex ventricular remodeling.

Isolation and characterization of the murine transforming growth factor-beta2 promoter

This report describes the isolation and characterization of the 5' flanking region of the murine transforming growth factor beta-2 (TGF-beta2) gene. A genomic clone containing the promoter region of the gene was isolated after screening a bacteriophage P1 genomic library. The resulting clone was sequenced and compared to promoters for the human and chicken TGF-beta2 genes. The sequence located near the transcription start site is highly conserved. It includes a TATA box, an E-box, and a largely conserved CRE/ATF site. A series of murine TGF-beta2 promoter/reporter constructs was generated to identify regulatory regions of the gene. As in the case of the human TGF-beta2 gene, sequences just upstream of the TATA box, including the CRE/ATF site, actively stimulate the murine TGF-beta2 promoter. However, unlike the human TGF-beta2 gene, the 5' flanking region of the murine TGF-beta2 gene contains a long alternating purine/pyrimidine repeat that unexpectedly exerts a strong positive effect on its promoter. This is of particular interest since alternating purine/pyrimidine repeats in other promoters have been observed to be inhibitory.

Role of keratinocyte growth factor in the control of surfactant synthesis by fetal lung mesenchyme

Fetal lung maturation is regulated by mesenchymal-epithelial cell communication, which plays a major role in the control of surfactant synthesis by alveolar type II cells. We have recently shown that keratinocyte growth factor (KGF), also called fibroblast growth factor-7, enhances the maturation of fetal alveolar epithelial type II cells. Here, we investigated, among the factors produced by lung mesenchyme, the part attributable to KGF in the control of surfactant synthesis. Using a KGF-neutralizing antibody, we assessed surfactant phospholipid synthesis by measuring choline incorporation into disaturated phosphatidylcholine of isolated fetal type II cells. We found that KGF accounts for about half of the stimulating activity present in fetal lung fibroblast-conditioned medium (FCM). By contrast, the use of an epidermal growth factor-neutralizing antibody did not alter the FCM-stimulating activity. To further delineate KGF properties as a mesenchymal mediator, we wondered about its possibility to relay glucocorticoid-stimulating activity on the synthesis of the phospholipid moiety of surfactant in fetal lung fibroblasts. A 24-h exposure to dexamethasone led us to detect a 50% increase in the level of KGF messenger RNA (mRNA) in isolated fetal lung fibroblasts. Moreover, anti-KGF antibody totally abolished the further increase of FCM-stimulating activity induced by dexamethasone. Thus, KGF seems to be a major player in mediating glucocorticoid stimulation of fetal lung maturation.

Abnormal angiogenesis but intact hematopoietic potential in TGF-beta type I receptor-deficient mice

Deletion of the transforming growth factor beta1 (TGF-beta1) gene in mice has previously suggested that it regulates both hematopoiesis and angiogenesis. To define the function of TGF-beta more precisely, we inactivated the TGF-beta type I receptor (TbetaRI) gene by gene targeting. Mice lacking TbetaRI die at midgestation, exhibiting severe defects in vascular development of the yolk sac and placenta, and an absence of circulating red blood cells. However, despite obvious anemia in the TbetaRI(-/-) yolk sacs, clonogenic assays on yolk sac-derived hematopoietic precursors in vitro revealed that TbetaRI(-/-) mice exhibit normal hematopoietic potential compared with wild-type and heterozygous siblings. Endothelial cells derived from TbetaRI-deficient embryos show enhanced cell proliferation, improper migratory behavior and impaired fibronectin production in vitro, defects that are associated with the vascular defects seen in vivo. We thus demonstrate here that, while TbetaRI is crucial for the function of TGF-beta during vascular development and can not be compensated for by the activin receptor-like kinase-1 (ALK-1), functional hematopoiesis and development of hematopoietic progenitors is not dependent on TGF-beta signaling via TbetaRI.

Genetic and teratogenic approaches to craniofacial development

Craniofacial malformations are the most common birth defects that occur in humans, with facial clefting representing the majority of these defects. Facial clefts can arise at any stage of development due to perturbations that alter the extracellular matrix as well as affect the patterning, migration, proliferation, and differentiation of cells. In this review, we focus on recent advances in the understanding of the developmental basis for facial clefting through the analysis of the effects of gene disruption experiments and treatments with teratogens in both chickens and mice. Specifically, we analyze the results of disruptions to genes such as Sonic hedgehog (Shh), epidermal growth factor receptor (EGFR), Distal-less (Dlx), and transforming growth factor beta 3 (TGFbeta3). We also describe the effects that teratogens such as retinoic acid, jervine, and cyclopamine have on facial clefting and discuss mechanisms for their action. In addition to providing insight into the bases for abnormal craniofacial growth, genetic and teratogenic techniques are powerful tools for understanding the normal developmental processes that generate and pattern the face.

Malignant glioma biology: role for TGF-beta in growth, motility, angiogenesis, and immune escape

Characteristics of human malignant glioma are excessive proliferation, infiltrative growth, angiogenesis and suppression of anti-tumor immune surveillance. Transforming growth factor-beta (TGF-beta), a versatile cytokine, is intimately involved in the regulation of these processes. Here, we discuss the interactions of TGF-beta with growth factors, such as basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) and platelet derived growth factor (PDGF), metalloproteinases (MMP-2, MMP-9) and their inhibitor, plasmin activator inhibitor-1 (PAI-1), and immune cells, like natural killer cells, T-cells and microglia. The differential effects of TGF-beta in glioma biology are outlined with emphasis on the induction of a survival advantage for glioma cells by enforced cell growth, migration, invasion, angiogenesis and immune paralysis. By virtue of its growth regulatory and immunomodulatory properties, TGF-beta promises to become a novel target for the experimental therapy of human malignant glioma.

Transforming growth factor beta and mouse development

Transforming growth factor beta has many biological effects including the control of cellular growth, differentiation, migration and extracellular matrix production; these are all processes essential for normal development. Although mice first generated more than eight years ago, bearing mutations in TGF beta ligands demonstrated the importance of TGF beta-induced signal transduction pathways for development in mammals but complete functional analysis is still lacking. Here, the current state-of-the-art in mouse development is reviewed. As a basis for understanding function, the principle features of mouse development over the 21 days of pregnancy are described and illustrated, from fertilization of the egg to mid-gestation when organogenesis is largely complete. This is completed with a description of when and where TGF beta ligands, receptors and downstream signalling molecules are expressed as the mouse embryo develops. The functions of TGF beta in preimplantation development, in implantation of the embryos in the uterine wall and in postimplantation development are then described through a review of the literature on gene ablation of the ligands, receptors and downstream molecules, or the ectopic expression of dominant negative forms of the receptors in vivo, which interfere with normal signal transduction. The evidence confirms multifunctional roles at all stages of development.

Type III TGF-beta receptor-independent signalling of TGF-beta2 via TbetaRII-B, an alternatively spliced TGF-beta type II receptor

Transforming growth factor-beta (TGF-beta) signals through membrane-bound serine/threonine kinase receptors, which upon stimulation phosphorylate Smad proteins and thereby trigger their nuclear translocation and transcriptional activity. Although the three mammalian isoforms of TGF-beta are highly homologous at the level of sequence, analysis of their in vivo function by gene knockouts revealed striking differences, suggesting no significant functional redundancy between TGF-beta1, -2 and -3. While signal transduction by TGF-beta1 has been well characterized, receptor binding and activation by the TGF-beta2 isoform is less well understood. Here, we show that TbetaRII-B, an alternatively spliced variant of the TGF-beta type II receptor, is a TGF-beta2 binding receptor, which mediates signalling via the Smad pathway in the absence of any TGF-beta type III receptor (TbetaRIII). L6 cells lacking endogenous TbetaRIII as well as TbetaRII-B do not respond to TGF-beta2. Transfection of these cells with TbetaRII-B restores TGF-beta2 sensitivity. The expression of TbetaRII-B is restricted to cells originating from tissues such as bone where the isoform TGF-beta2 has a predominant role. This reflects the importance of this receptor in TGF-beta isoform-specific signalling.

Isolation and characterization of a transforming growth factor-beta Type II receptor cDNA from Xenopus laevis

Transforming Growth Factor-beta (TGF-beta) and their receptors have been characterized from many organisms. Two TGF-beta signaling receptors called Type I and II have been described for various ligands of the superfamily from organisms ranging from Drosophila to humans. In Xenopus laevis, TGF-beta2 and 5 have been reported and presumably, play important roles during early development. Several Type I and type II receptors for many ligands of the TGF-beta superfamily except TGF-beta type II receptor (TbetaIIR), have been characterized in Xenopus laevis. A chemical cross linking experiment using iodinated TGF-beta1 and -beta5, revealed four specific binding proteins on XTC cells. In order to understand the TGF-beta involvement during Xenopus development, a TGF-beta type II receptor (XTbetaIIR) has been isolated from a XTC cDNA library. XTbetaIIR was a partial cDNA lacking a portion of the signal peptide. The sequence analysis and homology comparison with the human TbetaIIR revealed 67% amino acid similarity in the extra cellular domain, 60% similarity in the transmembrane domain and 87% similarity in the cytoplasmic kinase domain, suggesting that XTbetaIIR is a putative TGF-beta type II receptor. In addition, the consensus amino acid motif for serine threonine receptor kinases was also present. Further, a dominant negative expression construct lacking the cytoplasmic kinase domain (engineered with the signal peptide from human TGF-beta type II receptor), was able to abolish TGF-beta mediated induction of a luciferase reporter plasmid, in a transient cell transfection assay. This substantiates the notion that XTbetaIIR cDNA can act as a receptor for TGF-beta. RT-PCR analysis using RNA isolated from various developmental stages of Xenopus laevis revealed expression of this gene in all the early stages of development and in the adult organs, except in stages 46/48.

Revelations of the RYK receptor

Significant progress has been made over the last decade in elucidating the mechanisms employed by receptor protein tyrosine kinases (RTKs) in transducing extracellular signals critical for the regulation of diverse cellular activities. Nevertheless, revealing the biological significance of a subset of the RTKs that contain catalytically inactive protein tyrosine kinase domains has proven more elusive. ErbB3 has served as the prototype for models of catalytically inactive RTK function, performing the role of signal diversification in heterodimeric receptor complexes with other ErbB subfamily members. The receptor related to tyrosine kinases (RYK) is unique amongst the catalytically inactive RTKs. Based on structural or functional properties of the extracellular domain, RYK cannot be classified into an existing RTK subfamily. Recent genetic analyses of mouse Ryk and its Drosophila orthologue derailed have defined a role for this novel subfamily of receptors in the control of craniofacial development and neuronal pathway selection, respectively. Recent biochemical data lead us to propose a model that involves RYK in signal crosstalk and scaffold assembly with Eph receptors. This model is consistent with the established roles of Eph receptors and ephrins in craniofacial and nervous system morphogenesis. BioEssays 23:34-45, 2001.

Placental cell fates are regulated in vivo by HIF-mediated hypoxia responses

Placental development is profoundly influenced by oxygen (O2) tension. Human cytotrophoblasts proliferate in vitro under low O2 conditions but differentiate at higher O2 levels, mimicking the developmental transition they undergo as they invade the placental bed to establish the maternal–fetal circulation in vivo. Hypoxia-inducible factor-1 (HIF-1), consisting of HIF-1α and ARNT subunits, activates many genes involved in the cellular and organismal response to O2deprivation. Analysis of Arnt−/− placentas reveals an aberrant cellular architecture due to altered cell fate determination of Arnt−/− trophoblasts. Specifically, Arnt−/− placentas show greatly reduced labyrinthine and spongiotrophoblast layers, and increased numbers of giant cells. We further show that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells. Our results clearly establish that O2 levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation. The unique placental phenotype of Arnt−/− animals also provides an important tool for studying the disease of preeclampsia. Interestingly, aggregation of Arnt−/− embryonic stem (ES) cells with tetraploid wild-type embryos rescues their placental defects; however, these embryos still die from yolk sac vascular and cardiac defects.

Placental cell fates are regulated in vivo by HIF-mediated hypoxia responses

Placental development is profoundly influenced by oxygen (O(2)) tension. Human cytotrophoblasts proliferate in vitro under low O(2) conditions but differentiate at higher O(2) levels, mimicking the developmental transition they undergo as they invade the placental bed to establish the maternal-fetal circulation in vivo. Hypoxia-inducible factor-1 (HIF-1), consisting of HIF-1alpha and ARNT subunits, activates many genes involved in the cellular and organismal response to O(2) deprivation. Analysis of Arnt(-/-) placentas reveals an aberrant cellular architecture due to altered cell fate determination of Arnt(-/-) trophoblasts. Specifically, Arnt(-/-) placentas show greatly reduced labyrinthine and spongiotrophoblast layers, and increased numbers of giant cells. We further show that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells. Our results clearly establish that O(2) levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation. The unique placental phenotype of Arnt(-/-) animals also provides an important tool for studying the disease of preeclampsia. Interestingly, aggregation of Arnt(-/-) embryonic stem (ES) cells with tetraploid wild-type embryos rescues their placental defects; however, these embryos still die from yolk sac vascular and cardiac defects.

Epithelial-mesenchymal transformation is the mechanism for fusion of the craniofacial primordia involved in morphogenesis of the chicken lip

We have previously demonstrated that epithelial-mesenchymal transformation (EMT) brings about TGF beta 3-induced confluence of craniofacial primordia that derive from the maxillary processes and give rise to the avian palate. The upper lip of the chick embryo forms by confluence of primordia also derived from the maxillary processes, but in this case, they fuse with the intermaxillary segment of the nasofrontal process. Here, we ask whether the bilateral epithelial seams formed when these primordia contact each other in vivo are removed by apoptosis (as formerly was believed to occur in developing palate) or by EMT. We found that, as is the case in the palate, the periderm of the two-layered embryonic epithelium begins to slough shortly before these primordia fuse, bringing the basal epithelial cells into close contact. We show by TUNEL staining and confirm by TEM that apoptosis occurs only in periderm. TEM reveals that basal epithelial cells contacting each other to form the midline seam produce numerous desmosomes with each other. Then, basement membrane begins to disappear, numerous filopodia extend from the basal surfaces of epithelial cells, the space between them enlarges, and the seam breaks apart, leaving mesenchymal cells in its wake. Transformation of the carboxyfluorescein (CCFSE)-labeled epithelial seam is demonstrated in vivo by detection of CCFSE bodies in mesenchymal cells that replace it. This demonstration of EMT in avian lip development lays important groundwork for understanding the causes of human cleft lip and analyzing the mechanism of action of growth factors, such as SHH and BMPs, that have been shown (J. A. Helms et al., 1997, Dev. Biol. 187, 25-35) to be involved in avian lip confluence.

Targeted mutations of transforming growth factor-beta genes reveal important roles in mouse development and adult homeostasis

Transforming growth factors-beta (TGF-beta) are multifunctional molecules with profound biological effects in many developmental processes including regulation of cell proliferation, differentiation, cell adhesion, skeletal development, haematopoiesis, inflammatory responses, and wound healing. To learn about the role of TGF-beta in vivo, phenotypes of targeted mutations of molecules within the TGF-beta signalling pathway, TGF-beta1, -beta2, -beta3, TGF-beta receptor (TbetaR-II) and the signalling molecules SMAD2, SMAD3 and SMAD4, are discussed in this review. The three individual TGF-beta mutants show distinct and only partially overlapping phenotypes. In mice, targeted disruption of the TGF-beta1 gene results in diffuse and lethal inflammation about 3 weeks after birth, suggesting a prominent role of TGF-beta in the regulation of immune cell proliferation and extravasation into tissues. However, just half of the TGF-beta1 (-/-) conceptuses actually reach partuition due to defective haematopoiesis and endothelial differentiation. Targeted disruption of both TGF-beta2 and TGF-beta3 genes results in perinatal lethality. TGF-beta2 null mice exhibit a broad range of developmental defects, including cardiac, lung, craniofacial, limb, eye, ear and urogenital defects, whereas TGF-beta3 gene ablation results exclusively in defective palatogenesis and delayed pulmonary development. The TbetaR-II null phenotype closely resembles that of TGF-beta1 (-/-) conceptuses, which die in utero by E10.5. Loss of SMAD2 or SMAD4 results in related phenotypes: the mutants fail to form an organized egg cylinder, lack mesoderm required for gastrulation and die prior to E8.5. Together, gene ablation within the TGF-beta signalling pathway supports the notion of a prominent role of TGF-beta during development.

The latent transforming growth factor-beta-binding protein-1 promotes in vitro differentiation of embryonic stem cells into endothelium

The latent transforming growth factor-beta-binding protein-1 (LTBP-1) belongs to a family of extracellular glycoproteins that includes three additional isoforms (LTBP-2, -3, and -4) and the matrix proteins fibrillin-1 and -2. Originally described as a TGF-beta-masking protein, LTBP-1 is involved both in the sequestration of latent TGF-beta in the extracellular matrix and the regulation of its activation in the extracellular environment. Whereas the expression of LTBP-1 has been analyzed in normal and malignant cells and rodent and human tissues, little is known about LTBP-1 in embryonic development. To address this question, we used murine embryonic stem (ES) cells to analyze the appearance and role of LTBP-1 during ES cell differentiation. In vitro, ES cells aggregate to form embryoid bodies (EBs), which differentiate into multiple cell lineages. We analyzed LTBP-1 gene expression and LTBP-1 fiber appearance with respect to the emergence and distribution of cell types in differentiating EBs. LTBP-1 expression increased during the first 12 d in culture, appeared to remain constant between d 12 and 24, and declined thereafter. By immunostaining, fibrillar LTBP-1 was observed in those regions of the culture containing endothelial, smooth muscle, and epithelial cells. We found that inclusion of a polyclonal antibody to LTBP-1 during EB differentiation suppressed the expression of the endothelial specific genes ICAM-2 and von Willebrand factor and delayed the organization of differentiated endothelial cells into cord-like structures within the growing EBs. The same effect was observed when cultures were treated with either antibodies to TGF-beta or the latency associated peptide, which neutralize TGF-beta. Conversely, the organization of endothelial cells was enhanced by incubation with TGF-beta 1. These results suggest that during differentiation of ES cells LTBP-1 facilitates endothelial cell organization via a TGF-beta-dependent mechanism.

Coordinated expression of Hoxb genes and signaling molecules during development of the chick respiratory tract

To elucidate the molecular mechanism for regulating the region-specific morphogenesis of the chicken respiratory tract, we analyzed the spatiotemporal expression patterns of the Hoxb genes, Bmp-2, Bmp-4, Wnt-5a, and Wnt-11 in the developing respiratory tract. We found region-specific expression of these genes in the mesenchymal layer of the respiratory tract. Before bronchial branching proceeds, Hoxb genes show nested expression patterns around the ventral-distal tip of the lung bud. As morphogenesis proceeds, these expression domains correspond to the morphological subdivisions of the chick respiratory tract. Hoxb-5 and Hoxb-6 expression domains demarcate the trachea, bronchial tree, and air sacs. Particularly the expression domains of Hoxb-6 to -9 correspond to the morphological subdivisions of the air sacs along the proximodistal axis. Bmp-4 and Bmp-2 are expressed throughout the entire pulmonary mesenchyme and its dorsal half, respectively. Wnt-5a and Wnt-11 are expressed in the tracheal mesenchyme. Interestingly, the expression domain of Bmp-2 is complementary to the Hoxb-6 domain. The respiratory mesenchyme influences the process of epithelial branching during morphogenesis. By tissue recombination experiments, we found that the dorsal and the ventral pulmonary mesenchyme, demarcated by Hoxb-6 expression, have different inductive capacities toward the tracheal epithelium. These observations suggest the possibility that Hoxb genes are involved in the system specifying regional differences in morphogenesis and cytodifferentiation of respiratory tract. In addition, it is possible that BMPs and WNTs mediate region-specific epithelial-mesenchymal interaction in this system.

Activation of latent transforming growth factor-beta1 is induced by mannose 6-phosphate/insulin-like growth factor-II receptor

This study was conducted to further explore the mechanism of transforming growth factor beta1 (TGF-beta1) activation, which plays a critical role in many physiological and pathological conditions. We have previously shown that the large (270 kDa), but not small (40 kDa), mannose 6-phosphate receptors facilitate the cellular response to latent TGF-beta1 released from genetically modified cells. In this study, we explored the role of cell membrane associated transglutaminase and plasmin in mannose 6-phosphate receptor induced latent TGF-beta activation using MS and MS-9 cells bearing either no receptors or the 270 kDa mannose 6-phosphate/insulin-like growth factor II receptors, respectively. As a source of latent TGF-beta1, PA317 cells were transfected with either pLin-TGF-beta1 vector or pLin retroviral vector with no TGF-beta1 insert using calcium phosphate precipitation. The latency and bioactivity of TGF-beta1 in conditioned medium derived from transfected PA317 cells were evaluated by enzyme-linked immunosorbent assay and mink lung epithelial cell growth inhibition assay, respectively. The level of latent TGF-beta1 was 13-fold higher (20.1 +/- 0.4 vs. 1.5 +/- 0.03 ng/ml) in conditioned medium from pLin-TGF-beta1 transfected cells than that of control. The latency and bioactivity of TGF-beta1 released from pLin-TGF-beta1 transfected cells were confirmed by evaluation of 3H-thymidine incorporation in Mv1Lu epithelial cells treated with non- and heat-activated 10% conditioned medium. The results showed a significantly lower 3H-thymidine incorporation in Mv1Lu epithelial cells treated with heat-activated PA317 conditioned medium (4% of control) relative to those treated with either control or nonheated conditioned medium. This inhibition was abrogated by addition of 40 microg/ml of TGF-beta1 neutralizing antibody. The level of 3H-thymidine incorporation was then evaluated in MS-9 cells receiving Dulbecco's modified Eagle medium containing either 0% 10%, 30% or 50% volumes of nonactivated PA317 conditioned medium for 24 hours. The results showed a markedly lower proliferation in response to 30% and 50% conditioned medium used in MS-9 cells. Under similar experimental conditions, addition of only mannose 6-phosphate, but not fructose 6-phosphate or mannose 1-phosphate, at 1 mM concentration restored the MS-9 cell proliferative response to latent TGF-beta1. The inhibitory effects of latent TGF-beta1 on MS-9 cell proliferation were restored by addition of either TGF-beta1 neutralizing antibody or cystamine, a transglutaminase inhibitor. In contrast, addition of aprotinin, a plasmin inhibitor, had a marginal influence on inhibitory effects of latent TGF-beta1 on MS-9 cell proliferation. Interestingly, a mixture of latent TGF-beta1 + MS-9 cell membranes, but not MS cell membranes, also inhibited the mink lung epithelial cell proliferation (34% of control). These findings indicate that mannose 6-phosphate/insulin-like growth factor II receptors are involved in latent TGF-beta activation and that is at least partly dependent on cell membrane associated transglutaminase, but not on plasmin.

Transforming growth factor-beta in renal disease

An extensive number of animal and clinical studies indicate that transforming growth factor-beta (TGF-beta s) play an important role in inflammatory and fibrotic diseases, including renal fibrosis. Recent mouse models harboring genetically engineered alterations in TGF-beta pathways reveal complicated mechanisms of regulation of TGF-beta activity in vivo. The purpose of this review is to present recent advances relevant to our understanding of the TGF-beta-signaling system in renal physiology and pathophysiology.

Role of transcription factors in fetal lung development and surfactant protein gene expression

Branching morphogenesis of the lung and differentiation of specialized cell populations is dependent upon reciprocal interactions between epithelial cells derived from endoderm of embryonic foregut and surrounding mesenchymal cells. These interactions are mediated by elaboration and concerted actions of a variety of growth and differentiation factors binding to specific receptors. Such factors include members of the fibroblast growth factor family, sonic hedgehog, members of the transforming growth factor-beta family, epidermal growth factor, and members of the platelet-derived growth factor family. Hormones that increase cyclic AMP formation, glucocorticoids, and retinoids also play important roles in branching morphogenesis, alveolar development, and cellular differentiation. Expression of the genes encoding these morphogens and their receptors is controlled by a variety of transcription factors that also are highly regulated. Several of these transcription factors serve dual roles as regulators of genes involved in early lung development and in specialized functions of differentiated cells. Targeted null mutations of genes encoding many of these morphogens and transcription factors have provided important insight into their function during lung development. In this chapter, the cellular and molecular mechanisms that control lung development are considered, as well as those that regulate expression of the genes encoding the surfactant proteins.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Abnormal gastrointestinal development in PDGF-A and PDGFR-(alpha) deficient mice implicates a novel mesenchymal structure with putative instructive properties in villus morphogenesis

Development of the gastrointestinal (GI) tract depends on reciprocal epithelial-mesenchymal cell signaling. Here, we demonstrate a role for platelet-derived growth factor-A (PDGF-A) and its receptor, PDGFR-(alpha), in this process. Mice lacking PDGF-A or PDGFR-(alpha) were found to develop an abnormal GI mucosal lining, including fewer and misshapen villi and loss of pericryptal mesenchyme. Onset of villus morphogenesis correlated with the formation of clusters of PDGFR-(alpha) positive cells, ‘villus clusters’, which remained located at the tip of the mesenchymal core of the growing villus. Lack of PDGF-A or PDGFR-(alpha) resulted in progressive depletion of PDGFR-(alpha) positive mesenchymal cells, the formation of fewer villus clusters, and premature expression of smooth muscle actin (SMA) in the villus mesenchyme. We found that the villus clusters were postmitotic, expressed BMP-2 and BMP-4, and that their formation correlated with downregulated DNA synthesis in adjacent intestinal epithelium. We propose a model in which villus morphogenesis is initiated as a result of aggregation of PDGFR-(α) positive cells into cell clusters that subsequently function as mesenchymal centers of signaling to the epithelium. The role of PDGF-A seems to be to secure renewal of PDGFR-(alpha) positive cells when they are consumed in the initial rounds of cluster formation.

The expression and structure of TGF-beta2 transcripts in rat muscles

The transforming growth factor-beta2 (TGF-beta2) transcripts expressed in various tissues of rat were characterised by RT-PCR and the nucleotide sequence of the cDNAs determined. A transcript with an 84-nucleotide insert in the latency-associated peptide region, the long form, was found. The long form of TGF-beta2 was detected in the aorta, primary bronchus, uterus, heart, skeletal muscle, sciatic nerve and spinal cord but not in the intestine. The 3' untranslated region of TGF-beta2 contained several putative AU-rich elements and multiple polyadenylation sites, indicating post-transcriptional regulation of TGF-beta2 synthesis. The levels of TGF-beta2 transcripts were estimated using semi-quantitative RT-PCR. They were down-regulated during muscle development and up-regulated after denervation. The long form constituted approximately 6% of the total TGF-beta2 messages in skeletal muscle.

Metabolic consequences of adenosine deaminase deficiency in mice are associated with defects in alveogenesis, pulmonary inflammation, and airway obstruction

Adenosine deaminase (ADA) is a purine catabolic enzyme that manages levels of the biologically active purines adenosine and 2'-deoxyadenosine in tissues and cells. ADA-deficient mice die at 3 wk of age from severe respiratory distress. This phenotype is progressive and is linked to perturbations in pulmonary purine metabolism. The inflammatory changes found in the lungs of ADA-deficient mice included an accumulation of activated alveolar macrophages and eosinophils. These changes were accompanied by a pronounced enlargement of alveolar spaces and increases in mucus production in the bronchial airways. The alveolar enlargement was found to be due in part to abnormal alveogenesis. Lowering adenosine and 2'-deoxyadenosine levels using ADA enzyme therapy decreased the pulmonary eosinophilia and resolved many of the lung histopathologies. In addition, genetically restoring ADA to the forestomach of otherwise ADA-deficient mice prevented adenine metabolic disturbances as well as lung inflammation and damage. These data suggest that disturbances in purinergic signaling mediate the lung inflammation and damage seen in ADA-deficient mice.

Delayed wound healing in immunodeficient TGF-beta 1 knockout mice

Previous studies showed that full-thickness wounds in transforming growth factor-beta1-deficient mice initially heal normally. Unfortunately, transforming growth factor-beta1 deficiency leads to a multifocal inflammatory disease affecting most organs of the body, which ultimately interferes with later stages of wound healing in these mice. As this inflammatory disease is eliminated in transforming growth factor-beta1-deficient mice lacking T and B cells (Tgfb1-/- Scid-/- mice), we hypothesized that wound repair in the latter would proceed normally, even at later stages of healing. Unexpectedly, Tgfb1-/- Scid-/- mice demonstrate a major delay of approximately 1 wk in each of the major phases of wound healing: inflammation, proliferation, and maturation. Immuno- deficient Scid-/- mice that have the wild-type Tgfb1 allele do not experience this delay in wound healing. One interpretation of these findings is that lymphocytes and transforming growth factor-beta1 affect compensatory pathways in wound healing. An alternative interpretation is that the delayed expression of Tgfb2 and Tgfb3 that occurs in the absence of transforming growth factor-beta1 results in the delayed wound healing, suggesting that transforming growth factor-beta2 and/or transforming growth factor-beta3 play important parts in wound healing.