The PDGF alpha receptor is required for neural crest cell development and for normal patterning of the somites

Blockade of platelet-derived growth factor or its receptors transiently delays but does not prevent fibrous cap formation in ApoE null mice

Platelet-derived growth factor (PDGF) is a potent stimulant of smooth muscle cell migration and proliferation in culture. To test the role of PDGF in the accumulation of smooth muscle cells in vivo, we evaluated ApoE -/- mice that develop complex lesions of atherosclerosis. Fetal liver cells from PDGF-B-deficient embryos were used to replace the circulating cells of lethally irradiated ApoE -/- mice. One month after transplant, all monocytes in PDGF-B -/- chimeras are of donor origin (lack PDGF), and no PDGF-BB is detected in circulating platelets, primary sources of PDGF in lesions. Although lesion volumes are comparable in the PDGF-B +/+ and -/- chimeras at 35 weeks, lesions in PDGF-B -/- chimeras contain mostly macrophages, appear less mature, and have a reduced frequency of fibrous cap formation as compared with PDGF-B +/+ chimeras. However, after 45 weeks, smooth muscle cell accumulation in fibrous caps is indistinguishable in the two groups. Comparison of elicited peritoneal macrophages by RNase protection assay shows an altered cytokine and cytokine receptor profile in PDGF-B -/- chimeras. ApoE -/- mice were also treated for up to 50 weeks with a PDGF receptor antagonist that blocks all three PDGF receptor dimers. Blockade of the PDGF receptors similarly delays, but does not prevent, accumulation of smooth muscle and fibrous cap formation. Thus, elimination of PDGF-B from circulating cells or blockade of PDGF receptors does not appear sufficient to prevent smooth muscle accumulation in advanced lesions of atherosclerosis.

Angiogenesis stimulated by PDGF-CC, a novel member in the PDGF family, involves activation of PDGFR-alphaalpha and -alphabeta receptors

A newly discovered PDGF isoform, PDGF-CC, is expressed in actively angiogenic tissues such as placenta, some embryonic tissues, and tumors. We test the possibility that PDGF-CC promotes angiogenesis in vivo. The core domain (mature form) of human PDGF-CC is sufficiently potent to stimulate neovascularization in the mouse cornea. The corneal angiogenic response induced by PDGF-CC is robust although the area of neovascularization is smaller than those of FGF-2- and VEGF-stimulated angiogenesis. Similarly, PDGF-BB and PDGF-AB induce angiogenic responses virtually indistinguishable from PDGF-CC-stimulated vessels. In contrast, PDGF-AA displays only a weak angiogenic response in the mouse cornea. Although there was no significant difference in incorporation of mural cells to the newly formed blood vessels induced by PDGF-BB and -CC, the percentage of mural cell positive vessels induced by PDGF-AA was greater than those induced by FGF-2, PDGF-BB, and PDGF-CC. In the developing chick embryo, PDGF-CC induced branch sprouts from established blood vessels. In PDGF receptor-transfected endothelial cells, PDGF-CC activated the PDGF receptor alpha subunit (PDGFR-alpha). PDGF-CC, but not PDGF-AA, was able to activate PDGFR-beta receptor in endothelial cells that coexpress both alpha and beta forms of receptors. Thus, the PDGF-CC-mediated angiogenic response is most likely transduced by PDGF-alphaalpha and -alphabeta receptors. These data demonstrate that the PDGF family is a complex and important group of proangiogenic factors.

Bone growth factors in maxillofacial skeletal reconstruction

A literature review was performed to survey the available information on the potential of bone growth factors in skeletal reconstruction in the maxillofacial area. The aim of this review was to characterize the biological and developmental nature of the growth factors considered, their molecular level of activity and their osteogenic potential in craniofacial bone repair and reconstruction. A total of 231 references were selected for evaluation by the content of the abstracts. All growth factors considered have a fundamental role in growth and development. In postnatal skeletal regeneration, PDGF plays an important role in inducing proliferation of undifferentiated mesenchymal cells. It is an important mediator for bone healing and remodelling during trauma and infection. It can enhance bone regeneration in conjunction with other growth factors but is unlikely to provide entirely osteogenic properties itself. IGFs have an important role in general growth and maintenance of the body skeleton. The effect of local application of IGFs alone in craniofacial skeletal defects has not yet shown a clear potential for enhancement of bone regeneration in the reported dosages. The combination of IGF-I with PDGF has been effective in promoting bone regeneration in dentoalveolar defects around implants or after periodontal bone loss. TGFbeta alone in skeletal reconstruction appears to be associated with uncertain results. The presence of committed cells is required for enhancement of bone formation by TGFbeta. It has a biphasic effect, which suppresses proliferation and osteoblastic differentiation at high concentrations. BMPs, BMP2, BMP4 and BMP7 in particular, appear to be the most effective growth factors in terms of osteogenesis and osseous defect repair. Efficacy of BMPs for defect repair is strongly dependent on the type of carrier and has been subject to unknown factors in clinical feasibility trials resulting in ambiguous results. The current lack of clinical data may prolong the period until this factor is introduced into routine clinical application. PRP is supposed to increase proliferation of undifferentiated mesenchymal cells and to enhance angiogenesis. There is little scientific evidence about the benefit of PRP in skeletal reconstructive and preprosthetic surgery yet and it is unlikely that peri-implant bone healing or regeneration of local bone into alloplastic material by the application of PRP alone will be significantly enhanced.

Gene expression profile analysis of rheumatoid synovial fibroblast cultures revealing the overexpression of genes responsible for tumor-like growth of rheumatoid synovium

To elucidate the aberrant growth properties of rheumatoid synoviocytes, we have examined the gene expression profile of rheumatoid synovial fibroblasts (RSFs) and compared with that of normal synovial fibroblasts (NSF). Gene expression profile analysis was conducted with synoviocyte cultures obtained from five rheumatoid arthritis (RA) patients and five control cases using a commercial cDNA array containing the defined 588 cancer-related genes. The results were confirmed by real-time RT-PCR. Gene expression levels for the platelet-derived growth factor receptor alpha (PDGFRalpha), plasminogen activator inhibitor-1 (PAI-1), and stromal cell derived factor 1A (SDF1A) are constitutively augmented in RSF compared with NSF. The mRNA levels of PDGFRalpha, PAI-1, and SDF1A in RSF over NSF were 4.6-, 14-, and 2.8-fold, respectively, by real-time RT-PCR. In fact, we found that RSFs showed greater sensitivity to the cell proliferative effect of PDGF. T his aberrant gene expression profile suggests that RSF may have retained the premature phenotype of primordial synoviocytes.

Mutually regulated expression of Pax6 and Six3 and its implications for the Pax6 haploinsufficient lens phenotype

Pax6 is a key regulator of eye development in vertebrates and invertebrates, and heterozygous loss-of-function mutations of the mouse Pax6 gene result in the Small eye phenotype, in which a small lens is a constant feature. To provide an understanding of the mechanisms underlying this haploinsufficient phenotype, we evaluated in Pax6 heterozygous mice the effects of reduced Pax6 gene dosage on the activity of other transcription factors regulating eye formation. We found that Six3 expression was specifically reduced in lenses of Pax6 heterozygous mouse embryos. Interactions between orthologous genes from the Pax and Six families have been identified in Drosophila and vertebrate species, and we examined the control of Pax6 and Six3 gene expression in the developing mouse lens. Using in vitro and transgenic approaches, we found that either transcription factor binds regulatory sequences from the counterpart gene and that both genes mutually activate their expression. These studies define a functional relationship in the lens in which Six3 expression is dosage-dependent on Pax6 and where, conversely, Six3 activates Pax6. Accordingly, we show a rescue of the Pax6 haploinsufficient lens phenotype after lens-specific expression of Six3 in transgenic mice. This phenotypic rescue was accompanied by cell proliferation and activation of the platelet-derived growth factor alpha-R/cyclin D1 signaling pathway. Our findings thus provide a mechanism implicating gene regulatory interactions between Pax6 and Six3 in the tissue-specific defects found in Pax6 heterozygous mice.

Cooperation of Hoxa5 and Pax1 genes during formation of the pectoral girdle

Hox and Pax transcription factors are master regulators of skeletal and organ morphogenesis. Some skeletal malformations encountered in Hoxa5 mutants are shared by the undulated (un) mice, which bear a point mutation in the Pax1 gene. To investigate whether Hoxa5 and Pax1 act in common pathways during skeletal development, we analyzed Hoxa5;un compound mutants. Our genetic studies show that Hoxa5 and Pax1 cooperate in the vertebral patterning of the cervicothoracic transition region and in acromion morphogenesis. The dynamics of expression of Hoxa5 and Pax1 in the pectoral girdle region suggest that both genes function in a complementary fashion during acromion formation. Whereas Pax1 is required for the recruitment of acromion precursor cells, Hoxa5 may provide regional cues essential for the correct formation of the acromion by ensuring Pax1 expression at the proper time and position during morphogenesis of the pectoral girdle. Hoxa5 also has a distinctive role in specifying the fate of perichondrial and chondrogenic cell lineages in a Sox9-dependent way.

PDGF receptor-alpha deficiency in glomerular mesangial cells of tenascin-C knockout mice

Tenascin-C (TNC) knockout (TNKO) mice showed reduced proliferation of mesangial cells and abnormal restoration after habu-snake venom (HSV)-induced glomerulonephritis. In this study, we examined the relationship of TNC and platelet-derived growth factor receptor (PDGFR) in glomerular mesangial cells. TNC and PDGFR-alpha and -beta transcriptions were up-regulated in wild type (WT) mice after HSV injection, but in TNKO mice PDGFR-alpha transcription was not up-regulated. Immunohistochemistry showed that PDGFR-alpha was found in mesangial areas of colocalized alpha-smooth muscle actin, but in TNKO mice it was not detectable. In vitro studies showed that the expressions of PDGFR-alpha and -beta mRNA and protein in cultured glomerular mesangial cells (GMC) of TNKO mice were lower than those in WT GMC. These results suggest that failures of both TNC and PDGFR-alpha are a candidate for abnormal restoration of TNKO mice.

Perspectives on the development of a molecularly targeted agent

STI571 (Gleevec, imatinib mesylate) exemplifies the successful development of a rationally designed, molecularly targeted therapy for the treatment of a specific cancer. This article reviews the identification of Bcr-Abl as a therapeutic target in chronic myelogenous leukemia and the steps in the development of an agent to specifically inactivate this abnormality. Issues related to clinical trials of molecularly targeted agents are discussed, including dose and patient selection, as are possible mechanisms of resistance to STI571. Lastly, the potential use of STI571 in other malignancies and the translation of this paradigm to other malignancies is explored.

Vertebrate somitogenesis

In vertebrates, the paraxial mesoderm corresponds to the bilateral strips of mesodermal tissue flanking the notochord and neural tube and which are delimited laterally by the intermediate mesoderm and the lateral plate. The paraxial mesoderm comprises the head or cephalic mesoderm anteriorly and the somitic region throughout the trunk and the tail of the vertebrates. Soon after gastrulation, the somitic region of vertebrates starts to become segmented into paired blocks of mesoderm, termed somites. This process lasts until the number of somites characteristic of the species is reached. The somites later give rise to all skeletal muscles of the body, the axial skeleton, and part of the dermis. In this review I discuss the processes involved in the formation of the paraxial mesoderm and its segmentation into somites in vertebrates.

PDGF and the testis

Testicular development is controlled by a complex hierarchy of gene regulatory proteins, growth factors, cell adhesion molecules, signaling molecules and hormones that interact, often acting within short time windows, via reciprocal control relationships. The identification in the testis of platelet-derived growth factor (PDGF), a key regulator of connective tissue cells in embryogenesis and pathogenesis, has focused attention on the role of this growth factor in testicular pathophysiology. This review summarizes recent advances in the study of the actions of PDGF in the male gonad, and attempts to incorporate complex in vitro and in vivo experimental data into a model that might clarify the role played by PDGF in the mammalian testis.

An allelic series at the PDGFalphaR locus indicates unequal contributions of distinct signaling pathways during development

A central issue in signal transduction is the physiological contribution of different growth factor-initiated signaling pathways. We have generated knockin mice harboring mutations in the PDGFalpha receptor (PDGFalphaR) that selectively eliminate its capacity to activate PI3 kinase (alpha(PI3K)) or Src family kinases (alpha(Src)). The alpha(PI3K) mutation leads to neonatal lethality due to impaired signaling in many cell types, but the alpha(Src) mutation only affects oligodendrocyte development. A third knockin line containing mutations that eliminate multiple docking sites does not increase the severity of the alpha(PI3K) mutation. However, embryos with mutations in the PI3K binding sites of both PDGFRs (alpha and beta) recapitulate the PDGFalphaR null phenotype. Our results indicate that PI3K has a predominant role in PDGFalphaR signaling in vivo and that RTK-activated signaling pathways execute both specific and overlapping functions during mammalian development.

PDGF-A/PDGF alpha-receptor signaling is required for lung growth and the formation of alveoli but not for early lung branching morphogenesis

Platelet-derived growth factors (PDGF) constitute a family of four gene products (PDGF-A-D) acting by means of two receptor tyrosine kinases, PDGFR alpha and beta. Three of the ligands (PDGF-A, -B, and -C) bind to PDGFR alpha with high affinity. Knockout of pdgf-a in mice has demonstrated a role for PDGF-A in the recruitment of smooth muscle cells to the alveolar sacs and their further compartmentalization into alveoli. Although this is a late, postnatal step in lung development, pdgf-a antisense oligonucleotides were previously shown to inhibit epithelial branching in rat lung explants in vitro, which reflects an early embryonic process. These conflicting results may be explained by substitution of genetic loss of pdgf-a by maternal transfer of PDGF-A to the knockout embryo or the presence of other PDGFR alpha agonists (PDGF-B and -C) in vivo, potentially masking an effect of PDGF-A on branching morphogenesis. Alternatively, the administration of pdgf-a antisense oligonucleotides affected other processes than the intended. To discriminate between these opposing possibilities, we have analyzed lung development in pdgfr alpha -/- embryos and lung primordia grown in vitro. Our analysis shows that, while the pdgfr alpha -/- lungs and explanted lung rudiments were smaller than normal, branching morphogenesis appears qualitatively intact and proceeds until at least embryonic day 15.5, generating both prospective conducting and respiratory airways. We conclude that, although PDGF-AA signaling over PDGFR alpha may have direct or indirect roles in overall lung growth, it does not specifically control early branching of the lung epithelium.

Srg3, a mouse homolog of yeast SWI3, is essential for early embryogenesis and involved in brain development

Srg3 (SWI3-related gene product) is a mouse homolog of yeast SWI3, Drosophila melanogaster MOIRA (also named MOR/BAP155), and human BAF155 and is known as a core subunit of SWI/SNF complex. This complex is involved in the chromatin remodeling required for the regulation of transcriptional processes associated with development, cellular differentiation, and proliferation. We generated mice with a null mutation in the Srg3 locus to examine its function in vivo. Homozygous mutants develop in the early implantation stage but undergo rapid degeneration thereafter. An in vitro outgrowth study revealed that mutant blastocysts hatch, adhere, and form a layer of trophoblast giant cells, but the inner cell mass degenerates after prolonged culture. Interestingly, about 20% of heterozygous mutant embryos display defects in brain development with abnormal organization of the brain, a condition known as exencephaly. Histological examination suggests that exencephaly is caused by the failure in neural fold elevation, resulting in severe brain malformation. Our findings demonstrate that Srg3 is essential for early embryogenesis and plays an important role in the brain development of mice.

Role of growth factors and their receptors in gastric ulcer healing

The repair of gastric ulcers requires the reconstitution of epithelial structures and the underlying connective tissue, including vessels and muscle layers. Several growth factors have been implicated in this process, since they are able to regulate important cell functions, such as cell proliferation, migration, differentiation, secretion, and degradation of extracellular matrix, all of which are essential during tissue healing. Epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), hepatocyte growth factor (HGF), and trefoil factors (TFFs) are mainly involved in the reconstitution of the epithelial structures. Platelet derived growth factor (PDGF), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), and transforming growth factor-beta (TGF-beta) play a major role in the reconstitution of connective tissue, including vessels and smooth muscle cells, and provide the extracellular matrix substrate for cell migration and differentiation. The expression of these growth factors and their receptors is increased during ulcer healing and, in some cases, intracellular signaling related to receptor binding and transduction has been demonstrated. EGF, TGF-alpha and TFFs are normally present either in the gastric juice or in the mucosa, and may exert their effects immediately after damage, before newly synthesized EGF and TFFs are released from the ulcer margin. The inhibition of their effects by neutralizing antibodies may result in delayed ulcer healing, while the administration of recombinant or natural analogues may improve ulcer repair. In this review, we will summarize the basic molecular characteristics of some of these growth factors, and will discuss available evidence supporting their role in the ulcer repair process.

Functional blockade of platelet-derived growth factor receptor-beta but not of receptor-alpha prevents vascular smooth muscle cell accumulation in fibrous cap lesions in apolipoprotein E-deficient mice

BACKGROUND

The vascular smooth muscle cell (VSMC) is the central cell component involved in the fibroproliferative response in atherogenesis. As the lesion advances, VSMCs migrate from the media into the subendothelial space, thereby forming fibrous plaque lesions. Platelet-derived growth factor (PDGF) has been known to be a potent chemoattractant and mitogen for SMCs, but the pathophysiological role of the 2 PDGF receptors, receptor-alpha (PDGFR-alpha) and receptor-beta (PDGFR-beta) in atherogenesis is poorly understood. To clarify this problem, we prepared antagonistic rat monoclonal antibodies, APA5 and APB5, against murine PDGFR-alpha and PDGFR-beta, respectively.

METHODS AND RESULTS

Apolipoprotein E-deficient mice were fed a high-fat diet containing 0.3% cholesterol from 6 weeks of age and subjected to injection with 1 mg/d IP of either antibody from 12 to 18 weeks every other day. In the mice injected with APB5, the aortic atherosclerotic lesion size and the number of intimal VSMCs were reduced by 67% and 80%, respectively, compared with the control mice injected with irrelevant rat IgG. In contrast, the mice that received APA5 showed only minimal reduction of lesion size, and a large number of VSMCs were observed in the intima. In the intima of advanced lesions, APB5 immunolabeled VSMCs, whereas APA5 could detect VSMCs mainly in the media.

CONCLUSIONS

These results indicate that PDGFR-beta plays a significant role in formation of fibrous atherosclerotic lesions and that regulation of the signal transduction through PDGFR-beta could affect atherogenesis in mice.

Platelet-derived growth factor receptor-alpha in ventricular zone cells and in developing neurons

Cells in the early neuroepithelium differentiate and give rise to all cells in the central nervous system (CNS). The ways from a multipotent CNS stem cell to specialized neurons and glia are not fully understood. Using immunohistochemistry we found that neuroepithelial cells express the platelet-derived growth factor receptor-alpha (PDGFR-alpha) in the neural plate at embryonic day 8.5 and onwards in the neural tube. The protein was polarized to ventricular endfeet. Furthermore, PDGFR-alpha expression was localized to cells undergoing early neuronal development. We also found PDGFR-alpha expression in developing granule cells in the postnatal cerebellum, in Purkinje cells in the adult cerebellum and on processes of developing dorsal root ganglion cells. Previous reports mainly describe PDGFR-alpha expression in oligodendrocyte precursors and glial cells. We believe, in line with a few previous reports, that the PDGFR-alpha in addition marks a pool of undifferentiated cells, which are able to differentiate into neurons.

Developmental roles of platelet-derived growth factors

Platelet-derived growth factor (PDGF) was originally identified in platelets and in serum as a mitogen for fibroblasts, smooth muscle cells (SMC) and glia cells in culture. PDGF has since expanded to a family of dimers of at least four gene products, whose biological actions are mediated through two receptor tyrosine kinases, PDGFRs. The present review summarizes and discusses the biological functions of PDGFs and PDGFRs in developmental processes, mainly as revealed through genetic analysis in mice. Such studies have demonstrated multiple critical roles of PDGFs and PDGFRs in embryonic and postnatal development. PDGFs seem to act upon specific populations of progenitor cells that give rise to several different cell types with distinct functions in a variety of developmental processes. Analogies are seen between the cell functions and the developmental processes controlled by PDGFs. This suggests that ancestral PDGF and PDGFR expression patterns and functions may have been iterated in related sets of morphogenetic processes in the course of evolution.

Cellular interactions in vascular growth and differentiation

In nature, mammalian cells do not exist in isolation, but rather are involved in interactions with other cells and matrix. In this review, several aspects of cellular interactions that are important in vascular growth and development will be highlighted. The cardiovascular system is the earliest to develop in the embryo. A number of growth factors and their receptors mediate the complex stages of migration, assembly, organization, and stabilization of developing vessels. In the adult organism, normal angiogenesis is restricted primarily to tissue growth (such as muscle and fat), the wound healing process and the female reproductive system. However, pathological angiogenesis, such as with tumor growth, diabetic retinopathy, and arthritis, is of great concern. The identification and/or development of exogenous and endogenous angiogenesis inhibitors has added to the understanding of these pathological processes. In addition to cellular interactions via ligands and receptors, cells also interact directly through physical contacts. These interactions facilitate anchorage, communication, and permeability. Since vessels serve as non-leaky conduits for blood flow as well as interfaces for molecular diffusion, the physical interactions between the cells that make up vessels must be specific for the function at hand. Permeability is a specialized function of vessels and is mediated by intracellular mechanisms and intercellular interactions. Cells also interact with the surrounding extracellular matrix. Integrin-matrix interaction is a two-way exchange critical for angiogenesis. Matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases play major roles in embryonic remodeling, adult injury, and pathological conditions. Several experimental model systems have been useful in our understanding of cellular interactions. These in vitro models incorporate heterotypic cell-cell interactions and/or allow cell-matrix interactions to occur.

Chromosomal location, exon structure, and vascular expression patterns of the human PDGFC and PDGFD genes

BACKGROUND

Platelet-derived growth factor (PDGF), which is a major mitogen for vascular smooth muscle cells and has been implicated in the pathogenesis of arteriosclerosis, is composed of dimers of PDGF-A and PDGF-B polypeptide chains, encoded by different genes. Here, we have analyzed the chromosomal localization, structure, and expression of 2 newly identified human genes of the PDGF family, called PDGFC and PDGFD.

METHODS AND RESULTS

We used fluorescence in situ hybridization to locate PDGFC and PDGFD in chromosomes 4q32 and 11q22.3 to 23.2, respectively. Exon structures of PDGFC and PDGFD were determined by sequencing from genomic DNA clones. The coding region of PDGFC consists of 6 and PDGFD of 7 exons, of which the last 2 encode the C-terminal PDGF cystine knot growth factor homology domain. An N-terminal CUB domain is encoded by exons 2 and 3 of both genes, and a region of proteolytic cleavage involved in releasing and activating the growth factor domain is located in exon 4 in PDGFC and exon 5 in PDGFD. PDGF-C was expressed predominantly in smooth muscle cells and PDGF-D in fibroblastic adventitial cells, and both genes were active in cultured endothelial cells and in a variety of tumor cell lines. Both PDGF-C and PDGF-D also stimulated human coronary artery smooth muscle cells.

CONCLUSIONS

PDGFC and PDGFD have similar genomic structures, which resemble those of the PDGFA and PDGFB genes. Their expression in the arterial wall and cultured vascular cells suggests that they can transduce proliferation/migration signals to pericytes and smooth muscle cells.

PDGF-D, a new protease-activated growth factor

Platelet-derived growth factor (PDGF) has been directly implicated in developmental and physiological processes, as well as in human cancer, fibrotic diseases and arteriosclerosis. The PDGF family currently consists of at least three gene products, PDGF-A, PDGF-B and PDGF-C, which selectively signal through two PDGF receptors (PDGFRs) to regulate diverse cellular functions. After two decades of searching, PDGF-A and B were the only ligands identified for PDGFRs. Recently, however, database mining has resulted in the discovery of a third member of the PDGF family, PDGF-C, a functional analogue of PDGF-A that requires proteolytic activation. PDGF-A and PDGF-C selectively activate PDGFR-alpha, whereas PDGF-B activates both PDGFR-alpha and PDGFR-beta. Here we identify and characterize a new member of the PDGF family, PDGF D, which also requires proteolytic activation. Recombinant, purified PDGF-D induces DNA synthesis and growth in cells expressing PDGFRs. In cells expressing individual PDGFRs, PDGF-D binds to and activates PDGFR-beta but not PDGFR-alpha. However, in cells expressing both PDGFRs, PDGF-D activates both receptors. This indicates that PDGFR-alpha activation may result from PDGFR-alpha/beta heterodimerization.

Retinoic acid inhibits cardiac neural crest migration by blocking c-Jun N-terminal kinase activation

Retinoic acid (RA), a potent teratogen, produces a characteristic set of embryonic cardiovascular malformations similar to those observed in neural crest ablated avians. While the effects of RA on neural crest are well described, the molecular mechanism(s) of RA action on these cells is less clear. The present study examines the relationship between RA and mitogen-activated protein kinase signaling in neural crest cells and demonstrates that c-Jun N-terminal kinase (JNK) activation is severely repressed by RA. RA suppressed migration and proliferation of primary cultures of mouse neural crest cells treated in vitro as well as from animals treated in vivo. On Western blots, JNK activation/phosphorylation in neural crest cultures was reduced, while neither extracellular signal-regulated kinase (ERK) nor p38 pathways were affected. Both the dose-dependent stimulation of neural crest outgrowth and JNK phosphorylation by platelet-derived growth factor AA, which promotes outgrowth but not proliferation of neural crest cultures, were completely abrogated by RA. To establish the relevance of the JNK signaling pathway to cardiac neural crest migration, dominant negative adenoviral constructs were used to inhibit upstream activation of JNK or c-Jun downstream responses. Both adenoviral constructs markedly reduced neural crest cell outgrowth, while a dominant negative inhibitor of the p38 pathway had no effect. These data demonstrate that the JNK signaling pathway and c-Jun activation are critical for cardiac neural crest outgrowth and are potential targets for the action of RA.

Involvement of platelet-derived growth factor in disease: development of specific antagonists

Platelet-derived growth factor (PDGF) is a family of dimeric isoforms that stimulates, e.g., growth, chemotaxis and cell shape changes of various connective tissue cell types and certain other cells. The cellular effects of PDGF isoforms are exerted through binding to two structurally related tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation. This enables a number of SH2 domain containing signal transduction molecules to bind to the receptors, thereby initiating various signaling pathways. PDGF isoforms have important roles during the embryonic development, particularly in the formation of connective tissue in various organs. In the adult, PDGF stimulates wound healing. Overactivity of PDGF has been implicated in certain disorders, including fibrotic conditions, atherosclerosis, and malignancies. Different kinds of PDGF antagonists are currently being developed and evaluated in different animal disease models, as well as in clinical trials.

The two PDGF receptors maintain conserved signaling in vivo despite divergent embryological functions

Gene targeting studies have indicated that the two receptors for PDGF, alpha and beta, direct unique functions during development. Distinct ligand affinities, patterns of gene expression, and/or mechanisms of signal relay may account for functional specificity of the two PDGF receptor isoforms. To distinguish between these factors, we have created two complementary lines of knockin mice in which the intracellular signaling domains of one PDGFR have been removed and replaced by those of the other PDGFR. While both lines demonstrated substantial rescue of normal development, substitution of the PDGFbetaR signaling domains with those of the PDGFalphaR resulted in varying degrees of vascular disease. This observation provides a framework for discussing the evolution of receptor tyrosine kinase functional specificity.

Promoter haplotype combinations of the platelet-derived growth factor alpha-receptor gene predispose to human neural tube defects

Neural tube defects (NTDs), including anencephaly and spina bifida, are multifactorial diseases that occur with an incidence of 1 in 300 births in the United Kingdom. Mouse models have indicated that deregulated expression of the gene encoding the platelet-derived growth factor alpha-receptor (Pdgfra) causes congenital NTDs (refs. 2-4), whereas mutant forms of Pax-1 that have been associated with NTDs cause deregulated activation of the human PDGFRA promoter. There is an increasing awareness that genetic polymorphisms may have an important role in the susceptibility for NTDs (ref. 6). Here we identify five different haplotypes in the human PDGFRA promoter, of which the two most abundant ones, designated H1 and H2 alpha, differ in at least six polymorphic sites. In a transient transfection assay in human bone cells, the five haplotypes differ strongly in their ability to enhance reporter gene activity. In a group of patients with sporadic spina bifida, haplotypes with low transcriptional activity, including H1, were under-represented, whereas those with high transcriptional activity, including H2 alpha, were over-represented. When testing for haplotype combinations, H1 homozygotes were fully absent from the group of sporadic patients, whereas H1/H2 alpha heterozygotes were over-represented in the groups of both sporadic and familial spina bifida patients, but strongly under-represented in unrelated controls. Our data indicate that specific combinations of naturally occurring PDGFRA promoter haplotypes strongly affect NTD genesis.

Osteoprotegerin, a crucial regulator of bone metabolism, also regulates B cell development and function

Osteoprotegerin (OPG) is a CD40-regulated gene in B cells and dendritic cells (DCs). We investigated the role of OPG in the immune system by generating opg(-/-) mice. Like its role as a regulator of bone metabolism, OPG also influences processes in the immune system, notably in B cell development. Ex vivo, opg(-/-) pro-B cells have enhanced proliferation to IL-7, and in opg(-/-) spleen, there is an accumulation of type 1 transitional B cells. Furthermore, opg(-/-) bone marrow-derived DCs are more effective in stimulating allogeneic T cells than control DCs. When challenged with a T-dependent Ag, opg(-/-) mice had a compromised ability to sustain an IgG3 Ag-specific response. Thus, in the immune system, OPG regulates B cell maturation and development of efficient Ab responses.

The evolution of the pectoral girdle

The pectoral girdle articulates the forelimb with the axial skeleton in all vertebrates with paired anterior appendages. The structure of the pectoral girdle and its position along the axial skeleton has changed significantly during vertebrate evolution. These morphological changes have been well described, but there is little comparative embryology to indicate how these changes may have occurred. It is equally obscure how the muscles that connect the head with the pectoral girdle have maintained appropriate attachments even though these 2 structures have become separated. Here I review the changes in the pectoral girdle across different vertebrate taxa, indicating, where known, the developmental mechanisms underlying these changes. I also suggest how the muscular connections between the head and pectoral girdle have been maintained between these once adjacent bones, displaced during vertebrate evolution.

Retention of PDGFR-beta function in mice in the absence of phosphatidylinositol 3'-kinase and phospholipase Cgamma signaling pathways

Signal transduction by the platelet-derived growth-factor receptor beta (PDGFR-beta) tyrosine kinase is required for proper formation of vascular smooth muscle cells (VSMC). However, the importance of individual PDGFR-beta signal transduction pathways in vivo is not known. To investigate the role of two of the pathways believed to be critical for PDGF signal transduction, we have generated mice that bear a PDGFR-beta that can no longer activate PI3kinase or PLCgamma. Although these mutant mice have normal vasculature, we provide multiple lines of evidence in vivo and from cells derived from the mutant mice that suggest that the mutant PDGFR-beta operates at suboptimal levels. Our observations indicate that although loss of these pathways can lead to attenuated PDGF-dependent cellular function, certain PDGFR-beta-induced signal cascades are not essential for survival in mice.

Early myotome specification regulates PDGFA expression and axial skeleton development

Reciprocal defects in signaling between the myotome and the sclerotome compartments of the somites in PDGFRalpha and Myf5 mutant embryos lead to alterations in the formation of the vertebrae and the ribs. To investigate the significance of these observations, we have examined the role of PDGF signaling in the developing somite. PDGFA ligand expression was not detected in the myotome of Myf5 null mutant embryos and PDGFA promoter activity was regulated by Myf5 in vitro. PDGFA stimulated chondrogenesis in somite micromass cultures as well as in embryos when PDGFA was knocked into the Myf5 locus, resulting in increased vertebral and rib development. PDGFA expression in the myotome was fully restored in embryos in which MyoD has been introduced at the Myf5 locus but to a lesser extent in similar myogenin knock-in embryos. These results underscore the importance of growth factor signaling within the developing somite and suggest an important role for myogenic determination factors in orchestrating normal development of the axial skeleton.

Dorsal spinal cord inhibits oligodendrocyte development

Oligodendrocytes are the myelinating cells of the mammalian central nervous system. In the mouse spinal cord, oligodendrocytes are generated from strictly restricted regions of the ventral ventricular zone. To investigate how they originate from these specific regions, we used an explant culture system of the E12 mouse cervical spinal cord and hindbrain. In this culture system O4(+) cells were first detected along the ventral midline of the explant and were subsequently expanded to the dorsal region similar to in vivo. When we cultured the ventral and dorsal spinal cords separately, a robust increase in the number of O4(+) cells was observed in the ventral fragment. The number of both progenitor cells and mature cells also increased in the ventral fragment. This phenomenon suggests the presence of inhibitory factor for oligodendrocyte development from dorsal spinal cord. BMP4, a strong candidate for this factor that is secreted from the dorsal spinal cord, did not affect oligodendrocyte development. Previous studies demonstrated that signals from the notochord and ventral spinal cord, such as sonic hedgehog and neuregulin, promote the ventral region-specific development of oligodendrocytes. Our present study demonstrates that the dorsal spinal cord negatively regulates oligodendrocyte development.

A human YAC transgene rescues craniofacial and neural tube development in PDGFRalpha knockout mice and uncovers a role for PDGFRalpha in prenatal lung growth

The platelet-derived growth factor alpha-receptor (PDGFRalpha) plays a vital role in the development of vertebrate embryos, since mice lacking PDGFRalpha die in mid-gestation. PDGFRalpha is expressed in several types of migratory progenitor cells in the embryo including cranial neural crest cells, lung smooth muscle progenitors and oligodendrocyte progenitors. To study PDGFRalpha gene regulation and function during development, we generated transgenic mice by pronuclear injection of a 380 kb yeast artificial chromosome (YAC) containing the human PDGFRalpha gene. The YAC transgene was expressed in neural crest cells, rescued the profound craniofacial abnormalities and spina bifida observed in PDGFRalpha knockout mice and prolonged survival until birth. The ultimate cause of death was respiratory failure due to a defect in lung growth, stemming from failure of the transgene to be expressed correctly in lung smooth muscle progenitors. However, the YAC transgene was expressed faithfully in oligodendrocyte progenitors, which was not previously observed with plasmid-based transgenes containing only upstream PDGFRalpha control sequences. Our data illustrate the complexity of PDGFRalpha genetic control, provide clues to the location of critical regulatory elements and reveal a requirement for PDGF signalling in prenatal lung growth, which is distinct from the known requirement in postnatal alveogenesis. In addition, we found that the YAC transgene did not prolong survival of Patch mutant mice, indicating that genetic defects outside the PDGFRalpha locus contribute to the early embryonic lethality of Patch mice.

Protein tyrosine kinases in malignant melanoma

Protein tyrosyl phosphorylation is an essential component in intracellular signalling, with diverse and crucial functions including mediation of cell proliferation, survival, death, differentiation, migration and attachment. It is regulated by the balance between the activities of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases. A number of PTKs are encoded by proto-oncogenes or viral oncogenes, and are thus strongly implicated in cancer. While a role for PTKs in human melanoma is less firmly established, human melanomas or melanoma cells have been reported to contain more tyrosine phosphate than normal melanocytes, and some receptor PTKs (EPH-A2/ ECK and EPH-B3) are overexpressed in over 90% of melanoma cell lines. Other specific PTKs are also frequently overexpressed, including KDR and fibroblast growth factor receptor-4 (FGF-R4), while, interestingly, yet others, such as KIT and FES, are consistently downregulated in melanoma cell lines. All of these differentially expressed PTKs are candidates for gene products important in melanoma development. In addition, PTKs expressed in significant amounts in both benign and malignant melanocytes, such as insulin-like growth factor-1 receptor (IGF1-R), FGF-R1, HER2/NEU and FAK, are likely to play a role in melanoma genesis and progression.

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.

Thoracic skeletal defects in myogenin- and MRF4-deficient mice correlate with early defects in myotome and intercostal musculature

Myogenin and MRF4 are skeletal muscle-specific bHLH transcription factors critical for muscle development. In addition to a variety of skeletal muscle defects, embryos homozygous for mutations in myogenin or MRF4 display phenotypes in the thoracic skeleton, including rib fusions and sternal defects. These skeletal defects are likely to be secondary because myogenin and MRF4 are not expressed in the rib cartilage or sternum. In this study, the requirement for myogenin and MRF4 in thoracic skeletal development was further examined. When a hypomorphic allele of myogenin and an MRF4-null mutation were placed together, the severity of the thoracic skeletal defects was greatly increased and included extensive rib cartilage fusion and fused sternebrae. Additionally, new rib defects were observed in myogenin/MRF4 compound mutants, including a failure of the rib cartilage to contact the sternum. These results suggested that myogenin and MRF4 share overlapping functions in thoracic skeletal formation. Spatial expression patterns of skeletal muscle-specific markers in myogenin- and MRF4-mutant embryos revealed early skeletal muscle defects not previously reported. MRF4-/- mice displayed abnormal intercostal muscle morphology, including bifurcation and fusion of adjacent intercostals. myogenin/MRF4-mutant combinations displayed ventral myotome defects, including a failure to express normal levels of myf5. The results suggested that the early muscle defects observed in myogenin and MRF4 mutants may cause subsequent thoracic skeletal defects, and that myogenin and MRF4 have overlapping functions in ventral myotome differentiation and intercostal muscle morphogenesis.

Vertebrate mesendoderm induction and patterning

Many of the key molecular events underlying the induction and patterning of the vertebrate mesoderm and endoderm have recently been elucidated. T-box transcription factors and TGF-beta and Wnt signaling pathways play crucial roles in the initial induction of the mesendoderm and the subdivision of the posterior mesoderm into rostral and caudal domains.

Platelet-derived growth factor (PDGF) receptor-alpha activates c-Jun NH2-terminal kinase-1 and antagonizes PDGF receptor-beta -induced phenotypic transformation

Platelet-derived growth factor (PDGF) is a potent mitogen for mesenchymal cells. The PDGF B-chain (c-sis proto-oncogene) homodimer (PDGF BB) and v-sis, its viral counterpart, activate both alpha- and beta-receptor subunits (alpha-PDGFR and beta-PDGFR) and mediate anchorage-independent growth in NIH3T3 cells. In contrast, the PDGF A chain homodimer (PDGF AA) activates alpha-PDGFR only and fails to induce phenotypic transformation. In the present study, we investigated alpha- and beta-PDGFR specific signaling pathways that are responsible for the differences between the transforming ability of PDGF AA and BB. To study PDGF BB activation of beta-PDGFR, we established NIH3T3 clones in which alpha-PDGFR signaling is inhibited by a dominant-negative alpha-PDGFR, or an antisense construct of alpha-PDGFR. Here, we demonstrate that beta-PDGFR activation alone is sufficient for PDGF BB-mediated anchorage-independent cell growth. More importantly, inhibition of alpha-PDGFR signaling enhanced PDGF BB-mediated phenotypic transformation, suggesting that alpha-PDGFR antagonizes beta-PDGFR-induced transformation. While both alpha- and beta-receptors effectively activate ERKs, alpha-PDGFR, but not beta-PDGFR, activates stress-activated protein kinase-1/c-Jun NH(2)-terminal kinase-1 (JNK-1). Inhibition of JNK-1 activity using a dominant-negative JNK-1 mutant markedly enhanced PDGF BB-mediated anchorage-independent cell growth, demonstrating an antagonistic role for JNK-1 in PDGF-induced transformation. Consistently, overexpression of wild-type JNK-1 reduced PDGF BB-mediated transformation. Taken together, the present study showed that alpha- and beta-PDGFRs differentially regulate Ras-mitogen-activated protein kinase pathways critical for regulation of cell transformation, and transformation suppressing activity of alpha-PDGFR involves JNK-1 activation.

Platelet-derived growth factor A modulates limb chondrogenesis both in vivo and in vitro

Cartilage formation in the chick limb follows rapid proliferation, condensation and differentiation of limb mesenchyme. The control of these early events is poorly understood. Platelet-derived growth factor receptor alpha (PDGFR-alpha) is present throughout the mesenchyme of early chick limb buds, while its ligand, PDGF-A, is expressed in the surrounding epithelium. PDGFR-alpha is down-regulated in areas that will not give rise to cartilage and is then lost from cartilage forming areas after they begin to differentiate. PDGF-A increases chondrogenesis in micromass cultures of stage-20-24 limb buds, but not stage 25, where it inhibits chondrogenesis. Ectopic PDGF-A in the chick wing can lead to either a localized increase in cartilage formation, or an inhibition. Inhibition of PDGF signalling in the chick limb results in the loss of cartilage. These data demonstrate that PDGF-A functions to promote chondrogenesis at early stages of limb development and suggest that it inhibits chondrogenesis at later stages.

Platelet-derived growth factor-BB supports the survival of cultured rat Schwann cell precursors in synergy with neurotrophin-3

To understand the intimate axon-Schwann cell relationship required for the accurate development and regeneration of the peripheral nervous system (PNS), it is important to elucidate the repertoire of growth factors involved in this tightly regulated bi-directional dialogue. We focused on the identification and functional characterization of receptor tyrosine kinases (RTKs) in Schwann cells to gain insights into the corresponding growth factor ligands, which may be regulating the highly controlled differentiation of the Schwann cell lineage. Using an RT-PCR based differential display approach, we have identified 17 tyrosine kinases in embryonic rat sciatic nerves during the crucial transition from Schwann cell precursors to early Schwann cells. In this study, we have examined the expression and function of TrkC and the platelet-derived growth factor (PDGF) receptors alpha and beta on Schwann cell precursor cells. These receptors are expressed on freshly isolated Schwann cell precursors, and we show that PDGF-BB is able to rescue a subpopulation of these cells from apoptotic cell death in vitro. Furthermore, the TrkC-ligand neurotrophin-3 (NT-3) can act synergistically to potentiate this effect. However, PDGF-BB and NT-3 do not induce Schwann cell precursor proliferation or differentiation. Our data are consistent with a model suggesting that a combination of growth factors that include PDGF-BB and NT-3 are acting in concert and in synergy to regulate early Schwann cell development.

PDGF-C is a new protease-activated ligand for the PDGF alpha-receptor

Platelet-derived growth factors (PDGFs) are important in many types of mesenchymal cell. Here we identify a new PDGF, PDGF-C, which binds to and activates the PDGF alpha-receptor. PDGF-C is activated by proteolysis and induces proliferation of fibroblasts when overexpressed in transgenic mice. In situ hybridization analysis in the murine embryonic kidney shows preferential expression of PDGF-C messenger RNA in the metanephric mesenchyme during epithelial conversion. Analysis of kidneys lacking the PDGF alpha-receptor shows selective loss of mesenchymal cells adjacent to sites of expression of PDGF-C mRNA; this is not found in kidneys from animals lacking PDGF-A or both PDGF-A and PDGF-B, indicating that PDGF-C may have a unique function.

Genetics of muscle determination and development

Skeletal muscles in vertebrates develop from somites as the result of patterning and cell type specification events. Here, we review the current knowledge of genes and signals implicated in these processes. We discuss in particular the role of the myogenic determination genes as deduced from targeted gene disruptions in mice and how their expression may be controlled. We also refer to other transcription factors which collaborate with the myogenic regulators in positive or negative ways to control myogenesis. Moreover, we review experiments that demonstrate the influence of tissues surrounding the somites on the process of muscle formation and provide model views on the underlying mechanisms. Finally, we present recent evidence on genes that play a role in regeneration of muscle in adult organisms.

Signaling pathways crucial for craniofacial development revealed by endothelin-A receptor-deficient mice

Most of the bone and cartilage in the craniofacial region is derived from cephalic neural crest cells, which undergo three primary developmental events: migration from the rhombomeric neuroectoderm to the pharyngeal arches, proliferation as the ectomesenchyme within the arches, and differentiation into terminal structures. Interactions between the ectomesenchymal cells and surrounding cells are required in these processes, in which defects can lead to craniofacial malformation. We have previously shown that the G-protein-coupled endothelin-A receptor (ET(A)) is expressed in the neural crest-derived ectomesenchyme, whereas the cognate ligand for ET(A), endothelin-1 (ET-1), is expressed in arch epithelium and the paraxial mesoderm-derived arch core; absence of either ET(A) or ET-1 results in numerous craniofacial defects. In this study we have attempted to define the point at which cephalic neural crest development is disrupted in ET(A)-deficient embryos. We find that, while neural crest cell migration in the head of ET(A)(-/-) embryos appears normal, expression of a number of transcription factors in the arch ectomesenchymal cells is either absent or significantly reduced. These ET(A)-dependent factors include the transcription factors goosecoid, Dlx-2, Dlx-3, dHAND, eHAND, and Barx1, but not MHox, Hoxa-2, CRABP1, or Ufd1. In addition, the size of the arches in E10.5 to E11.5 ET(A)(-/-) embryos is smaller and an increase in ectomesenchymal apoptosis is observed. Thus, ET(A) signaling in ectomesenchymal cells appears to coordinate specific aspects of arch development by inducing expression of transcription factors in the postmigratory ectomesenchyme. Absence of these signals results in retarded arch growth, defects in proper differentiation, and, in some mesenchymal cells, apoptosis. In particular, this developmental pathway appears distinct from the pathway that includes UFD1L, implicated as a causative gene in CATCH 22 patients, and suggests parallel complementary pathways mediating craniofacial development.

Insulin-like growth factor I is the key growth factor in serum that protects neuroblastoma cells from hyperosmotic-induced apoptosis

Neuroblastoma is a childhood tumor of the peripheral nervous system that remains largely uncurable by conventional methods. Mannitol induces apoptosis in neuroblastoma cell types and insulin-like growth factor I (IGF-I) protects these cells from hyperosmotic-induced apoptosis by affecting apoptosis-regulatory proteins. In the current study, we investigate factors that enable SH-SY5Y neuroblastoma cells to survive in the presence of an apoptotic stimulus. When SH-SY5Y cells are exposed to high mannitol concentrations, more than 60% of the cells are apoptotic within 48 h. Normal CS prevents hyperosmotic-induced apoptosis in a dose-dependent manner, with 0.6% CS protecting 50% of the cells, and 3% CS rescuing more than 70% of the cells from apoptosis. Serum also delays the commitment point for SH-SY5Y cells from 9 h to 35 h. A survey of several growth factors, including epidermal growth factor (EGF), platelet-derived growth factor (PDGF), nerve growth factor (NGF), fibroblast growth factor (FGF), and IGF-I reveals that IGF-I is a component of serum necessary for protection of neuroblastoma cells from death. Mitochondrial membrane depolarization occurs in greater than 40% of the cells after mannitol exposure and caspase-3 activation is increased in high mannitol conditions after 9 h. IGF-I blocks both the mitochondrial membrane depolarization and caspase-3 activation normally induced by hyperosmotic treatment in neuroblastoma cells. Our results suggest that (1) IGF-I is a key factor in serum necessary for protection from death and (2) IGF-I acts upstream from the mitochondria and the caspases to prevent apoptosis in human neuroblastoma.

Growth factor signaling pathways in vascular development

Recent research on the formation and maintenance of the vasculature in the embryo and in the adult has provided a greater understanding of the cellular signals involved in these processes. With this understanding comes the potential means of controlling vascularization in pathological situations such as tumorigenesis and wounding. For the purpose of this review, we will discuss the key receptor tyrosine kinases involved in vascular function and the molecules which relay signals downstream of receptor activation. The receptor tyrosine kinases discussed include the vascular endothelial cell growth factor receptors, Eph receptors, Tie1, and Tie2, all of which are expressed on vascular endothelial cells. We also discuss the roles of the platelet derived growth factor receptors which are expressed on vascular smooth muscle cells. While all of these receptor tyrosine kinases activate many similar effector molecules, some of the signals initiated appear to be distinct. This may explain, at least in part, how different receptor tyrosine kinases expressed in overlapping patterns on the developing vasculature, direct unique biological functions.

Early specification of oligodendrocytes in the chick embryonic brain

Oligodendrocytes are the myelin-forming cells in the central nervous system of vertebrates. In the rodent embryo, these cells have been shown to emerge from restricted territories of the neuroepithelium. However, a comprehensive view of the development of oligodendroglial populations from their ventricular sources remains to be established. As a first step toward this aim, we have examined in vivo the spatiotemporal emergence of oligodendrocytes in the chick embryonic brain. We have detailed the patterns of expression of three early markers of the oligodendroglial lineage: the plp/dm-20 and PDGFRalpha transcripts and the O4-reactive antigen. During embryonic development, these molecules showed a similar segmental pattern of expression. However, plp/dm-20(+) cells were already observed, in the ventricular layer, at E2.5, i.e., 2 days before the appearance of O4(+) and PDGFRalpha(+) cells, suggesting that oligodendrocyte precursors arise nearly simultaneously with neurons. In the chick embryonic brain, the onset of expression of plp/dm-20 appears therefore to be the earliest event indicative of oligodendroglial specification and we propose, based on the expression of plp/dm-20 transcript, a ventricular map of the foci at which oligodendrocytes originate. In addition, we document the precocious segregation, from E5, of plp/dm-20(+) and PDGFRalpha(+) oligodendroglial cells in the subventricular and mantle layers of the brain.

Zic1 regulates the patterning of vertebral arches in cooperation with Gli3

Skeletal abnormalities are described that appeared in Zic1-deficient mice. These mice show multiple abnormalities in the axial skeleton. The deformities are severe in the dorsal parts of the vertebrae, vertebral arches, but less so in the vertebral bodies (spina bifida occulta). The proximal ribs are deformed having ectopic processes. The abnormalities found in the vertebral arches can be traced back to disturbed segmental patterns of dorsal sclerotome. The Zic1/Gli3 double mutants showed severe abnormalities of vertebral arches not found in single mutants. The abnormalities in the vertebral arches were less severe in Zic1/Pax1 mutants than Zic1/Gli3 mutants, but significantly more pronounced than in Zic1 single mutants. The three genes may act synergistically in the development of the vertebral arches.

Tyrosinase-related protein 2 promoter targets transgene expression to ocular and neural crest-derived tissues

In an effort to identify a promoter suitable for studying early ocular development, we generated transgenic mice carrying the lacZ reporter gene linked to the tyrosinase-related protein 2 (TRP2) promoter. TRP2-lacZ was expressed in early retinal pigment epithelium (RPE) and early neural crest cells in embryos. The promoter activity was robust and consistent in independent transgenic lines. The transgene was also expressed in the optic nerve and neural crest-derived neuronal cells in which the endogenous TRP2 gene is not expressed. This suggests that repressor elements may be missing in the promoter used in this study. To test whether this promoter can be used to study melanocyte development, we cross-mated TRP2-lacZ transgenic mice with mice heterozygous for the Patch (Ph) mutation. The pattern of beta-galactosidase activity in the embryos correlates well with the pigmentation phenotype in postnatal and adult Ph/+ mice. We also generated transgenic mice expressing fibroblast growth factor 9 (FGF9) directed by the TRP2 promoter and examined the effect on ocular development. Ectopic expression of FGF9 in the early embryonic RPE switched its differentiation pathway to a neuronal fate, resulting in formation of a duplicated neural retina in transgenic mice. These studies demonstrate that the TRP2 promoter is valuable for transgenic studies of ocular differentiation and development of neural crest cells.