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

Inactivation of the polycomb group protein Ring1B unveils an antiproliferative role in hematopoietic cell expansion and cooperation with tumorigenesis associated with Ink4a deletion

Polycomb group (PcG) proteins act as positive regulators of cell proliferation. Ring1B is a PcG gene essential for embryonic development, but its contribution to cell turnover in regenerating tissues in not known. Here, we have generated a conditional mouse mutant line to study the Ring1B role in adult hematopoiesis. Mutant mice developed a hypocellular bone marrow that paradoxically contained an enlarged, hyperproliferating compartment of immature cells, with an intact differentiation potential. These alterations were associated with differential upregulation of cyclin D2, which occurred in all mutant bone marrow cells, and of p16(Ink4a), observed only in the differentiated compartment. Concurrent inactivation of Ink4a rescued the defective proliferation of maturing cells but did not affect the hyperproliferative activity of progenitors and resulted in a shortening of the onset of lymphomas induced by Ink4a inactivation. These data show that Ring1B restricts the progenitors' proliferation and promotes the proliferation of their maturing progeny by selectively altering the expression pattern of cell cycle regulators along hematopoietic differentiation. The novel antiproliferative role of Ring1B's downregulation of a cell cycle activator may play an important role in the tight control of hematopoietic cell turnover.

Identification of genes differentially expressed during prenatal development of skeletal muscle in two pig breeds differing in muscularity

Background

Postnatal muscle growth is largely depending on the number and size of muscle fibers. The number of myofibers and to a large extent their metabolic and contractile properties, which also influence their size, are determined prenatally during the process of myogenesis. Hence identification of genes and their networks governing prenatal development of skeletal muscles will provide insight into the control of muscle growth and facilitate finding the source of its variation. So far most of the genes involved in myogenesis were identified by in vitro studies using gene targeting and transgenesis. Profiling of transcriptome changes during the myogenesis in vivo promises to obtain a more complete picture. In order to address this, we performed transcriptome profiling of prenatal skeletal muscle using differential display RT-PCR as on open system with the potential to detect novel transcripts. Seven key stages of myogenesis (days 14, 21, 35, 49, 63, 77 and 91 post conception) were studied in two breeds, Pietrain and Duroc, differing markedly in muscularity and muscle structure.

Results

Eighty prominent cDNA fragments were sequenced, 43 showing stage-associated and 37 showing breed-associated differences in the expression, respectively. Out of the resulting 85 unique expressed sequence tags, EST, 52 could be assigned to known genes. The most frequent functional categories represented genes encoding myofibrillar proteins (8), genes involved in cell adhesion, cell-cell signaling and extracellular matrix synthesis/remodeling (8), genes regulating gene expression (8), and metabolism genes (8). Some of the EST that showed no identity to any known transcripts in the databases are located in introns of known genes and most likely represent novel exons (e.g. HMGA2). Expression of thirteen transcripts along with five reference genes was further analyzed by means of real-time quantitative PCR. Nine of the target transcripts showed higher than twofold differences in the expression between the two breeds (GATA3, HMGA2, NRAP, SMC6L1, SPP1, RAB6IP2, TJP1 and two EST).

Conclusion

The present study revealed several genes and novel transcripts not previously associated with myogenesis and expands our knowledge of genetic factors operating during myogenesis. Genes that exhibited differences between the divergent breeds represent candidate genes for muscle growth and structure.

Mesenchymal stem cells and neovascularization: role of platelet-derived growth factor receptors

There is now accumulating evidence that bone marrow-derived mesenchymal stem cells (MSCs) make an important contribution to postnatal vasculogenesis, especially during tissue ischaemia and tumour vascularization. Identifying mechanisms which regulate the role of MSCs in vasculogenesis is a key therapeutic objective, since while increased neovascularization can be advantageous during tissue ischaemia, it is deleterious during tumourigenesis. The potent angiogenic stimulant vascular endothelial growth factor (VEGF) is known to regulate MSC mobilization and recruitment to sites of neovascularization, as well as directing the differentiation of MSCs to a vascular cell fate. Despite the fact that MSCs did not express VEGF receptors, we have recently identified that VEGF-A can stimulate platelet-derived growth factor (PDGF) receptors, which regulates MSC migration and proliferation. This review focuses on the role of PDGF receptors in regulating the vascular cell fate of MSCs, with emphasis on the function of the novel VEGF-A/PDGF receptor signalling mechanism.

Neural crest contribution to the cardiovascular system

Normal cardiovascular development requires complex remodeling of the outflow tract and pharyngeal arch arteries to create the separate pulmonic and systemic circulations. During remodeling, the outflow tract is septated to form the ascending aorta and the pulmonary trunk. The initially symmetrical pharyngeal arch arteries are remodeled to form the aortic arch, subclavian and carotid arteries. Remodeling is mediated by a population of neural crest cells arising between the mid-otic placode and somite four called the cardiac neural crest. Cardiac neural crest cells form smooth muscle and pericytes in the great arteries, and the neurons of cardiac innervation. In addition to the physical contribution of smooth muscle to the cardiovascular system, cardiac neural crest cells also provide signals required for the maintenance and differentiation of the other cell layers in the pharyngeal apparatus. Reciprocal signaling between the cardiac neural crest cells and cardiogenic mesoderm of the secondary heart field is required for elaboration of the conotruncus and disruption in this signaling results in primary myocardial dysfunction. Cardiovascular defects attributed to the cardiac neural crest cells may reflect either cell autonomous defects in the neural crest or defects in signaling between the neural crest and adjacent cell layers.

Coexpression of platelet-derived growth factor receptor alpha and fetal liver kinase 1 enhances cardiogenic potential in embryonic stem cell differentiation in vitro

Nascent mesodermal cells derived from EB5 embryonic stem (ES) cells were sorted in terms of cardiogenic potential on the basis of their expression levels of platelet-derived growth factor receptor alpha (PDGFRalpha) and fetal liver kinase 1 (Flk-1). The sorted cells were cocultured with OP9 stromal cells to induce terminal differentiation into contractile cardiac colonies. A significant number of cardiac colonies were found in the Flk-1+/PDGFRalpha+ fraction. The enrichment double-positive fraction produced approximately fivefold more cardiac colonies than the Flk-1+/PDGFRalpha- fraction and 10-fold more than the Flk-1-/PDGFRalpha+ fraction. To investigate the involvement of these markers in embryonic cardiogenesis, the cells that disseminated from the E7.5-7.75 embryos were fractionated and seeded on OP9 cells. The cardiogenic potential was markedly enhanced in the Flk-1+/PDGFRalpha+ fraction. These results suggest that some of the precursor cells coexpressing these markers are selectively involved in cardiogenic events, and that the identification of ES-cell-derived precursors with these markers will contribute to the effective production of cardiomyocytes for cell therapies.

A hypermorphic mouse Gli3 allele results in a polydactylous limb phenotype

Gli3 protein processing to generate the Gli3 repressor is mediated by proteasome and inhibited by Hedgehog signaling. The Gli3 repressor concentration is graded along the anterior-posterior axis of the developing vertebrate limb due to posteriorly restricted Sonic hedgehog expression. In this study, we created a small deletion at the Gli3 locus (Gli3(Delta68)), which causes a half reduction in the Gli3 repressor levels and a slightly increased activity of full-length mutant protein in the limb. Mice homozygous for Gli3(Delta68) develop one to two extra partial digits in the anterior of the limb, while mice carrying one copy of the Gli3(Delta68) allele die soon after birth and display seven digits. These phenotypes are more severe than those found in mice lacking one wild-type Gli3 allele. The expression of dHand, Hoxd12, and Hoxd13 is anteriorly expanded in the limb, even though no up-regulation of Gli1 and Ptc RNA expression is detected. These findings suggest that a decrease in the Gli3 repressor level in combination with an increase in Gli3 full-length activity results in more severe digit patterning abnormalities than those caused by a loss of one wild-type Gli3 allele.

The neutrophil serine protease inhibitor serpinb1 preserves lung defense functions in Pseudomonas aeruginosa infection

Neutrophil serine proteases (NSPs; elastase, cathepsin G, and proteinase-3) directly kill invading microbes. However, excess NSPs in the lungs play a central role in the pathology of inflammatory pulmonary disease. We show that serpinb1, an efficient inhibitor of the three NSPs, preserves cell and molecular components responsible for host defense against Pseudomonas aeruginosa. On infection, wild-type (WT) and serpinb1-deficient mice mount similar early responses, including robust production of cytokines and chemokines, recruitment of neutrophils, and initial containment of bacteria. However, serpinb1^−/− mice have considerably increased mortality relative to WT mice in association with late-onset failed bacterial clearance. We found that serpinb1-deficient neutrophils recruited to the lungs have an intrinsic defect in survival accompanied by release of neutrophil protease activity, sustained inflammatory cytokine production, and proteolysis of the collectin surfactant protein–D (SP-D). Coadministration of recombinant SERPINB1 with the P. aeruginosa inoculum normalized bacterial clearance in serpinb1^−/− mice. Thus, regulation of pulmonary innate immunity by serpinb1 is nonredundant and is required to protect two key components, the neutrophil and SP-D, from NSP damage during the host response to infection.

Mesenchymal cells are required for functional development of thymic epithelial cells

Epithelial-mesenchymal interactions have essential roles in thymus organogenesis. Mesenchymal cells are known to be required for epithelial cell proliferation. However, the contribution of mesenchymal cells to thymic epithelial cell differentiation is still unclear. In the present study, we have investigated the roles of mesenchymal cells in functional development of epithelial cells in the thymus anlage in patch (ph) mutant mice, which have a primarily defect in mesenchymal cells caused by the absence of platelet-derived growth factor receptor alpha expression. In the ph/ph thymus anlage, T cell progenitors migrate normally among the epithelial cells, however, they are severely impaired to proliferate and differentiate to CD25-positive cells. Epithelial cells of the ph/ph thymus anlage show severely impaired proliferation and expression of functional molecules, such as SCF, Delta-like 4 and MHC class II, which have crucial roles in T cell development. Moreover, the cultured ph/ph thymus anlage fails to develop into a mature organ supporting full T cell development. Addition of intact thymic mesenchymal cells to organ culture induces development of the ph/ph thymus anlage. In the cultured lobes, added mesenchymal cells contribute to form not only the capsule but also the meshwork structure mingled with epithelial cells. Our present results strongly suggest the roles of mesenchymal cells in functional development of epithelial cells in thymus organogenesis. In addition, our data suggest that mesenchymal cells are required to create the thymic microenvironment and to maintain epithelial architecture and function.

Cooperation between the PDGF receptors in cardiac neural crest cell migration

Neural crest cells (NCCs) are essential components of the sympathetic nervous system, skin, craniofacial skeleton, and aortic arch. It has been known for many years that perturbation of migration, proliferation, and/or differentiation of these cells leads to birth defects such as cleft palate and persistent truncus arteriosus (PTA). Previously, we had shown that disruption of the platelet-derived growth factor receptor (PDGFR) alpha in NCCs resulted in defects in craniofacial and aortic arch development, the latter with variable penetrance. Because we observed ventricular septal defects in embryos that are null for the PDGFRbeta, we hypothesized that both PDGF receptors are involved in NCC formation. Here, we show that both receptors are expressed in cardiac NCCs and that the combined loss of the PDGFRalpha and PDGFRbeta in NCCs resulted in NCC-related heart abnormalities, including PTA and a ventricular septal defect (VSD). Using NCC lineage tracing, we observed that loss of PDGF receptor signaling resulted in reduced NCCs in the conotruncus region, leading to defects in aortic arch septation. These results indicate that while PDGFRalpha plays a predominant role in NCC development, the PDGFRbeta is expressed by and functions in cardiac NCCs. Combined PDGF receptor signaling is required for sufficient recruitment of cardiac NCCs into the conotruncal region and for formation of the aortico-pulmonary and ventricular septum.

Candidate genes for congenital diaphragmatic hernia from animal models: sequencing of FOG2 and PDGFRα reveals rare variants in diaphragmatic hernia patients

Congenital diaphragmatic hernia (CDH) is a common, life threatening birth defect. Although there is strong evidence implicating genetic factors in its pathogenesis, few causative genes have been identified, and in isolated CDH, only one de novo, nonsense mutation has been reported in FOG2 in a female with posterior diaphragmatic eventration. We report here that the homozygous null mouse for the Pdgfralpha gene has posterolateral diaphragmatic defects and thus is a model for human CDH. We hypothesized that mutations in this gene could cause human CDH. We sequenced PDGFRalpha and FOG2 in 96 patients with CDH, of which 53 had isolated CDH (55.2%), 36 had CDH and additional anomalies (37.5%), and 7 had CDH and known chromosome aberrations (7.3%). For FOG2, we identified novel sequence alterations predicting p.M703L and p.T843A in two patients with isolated CDH that were absent in 526 and 564 control chromosomes respectively. These altered amino acids were highly conserved. However, due to the lack of available parental DNA samples we were not able to determine if the sequence alterations were de novo. For PDGFRalpha, we found a single variant predicting p.L967V in a patient with CDH and multiple anomalies that was absent in 768 control chromosomes. This patient also had one cell with trisomy 15 on skin fibroblast culture, a finding of uncertain significance. Although our study identified sequence variants in FOG2 and PDGFRalpha, we have not definitively established the variants as mutations and we found no evidence that CDH commonly results from mutations in these genes.

The genetics and embryology of zebrafish metamerism

Somites are the most obvious metameric structures in the vertebrate embryo. They are mesodermal segments that form in bilateral pairs flanking the notochord and are created sequentially in an anterior to posterior sequence concomitant with the posterior growth of the trunk and tail. Zebrafish somitogenesis is regulated by a clock that causes cells in the presomitic mesoderm (PSM) to undergo cyclical activation and repression of several notch pathway genes. Coordinated oscillation among neighboring cells manifests as stripes of gene expression that pass through the cells of the PSM in a posterior to anterior direction. As axial growth continually adds new cells to the posterior tail bud, cells of the PSM become relatively less posterior. This gradual assumption of a more anterior position occurs over developmental time and constitutes part of a maturation process that governs morphological segmentation in conjunction with the clock. Segment morphogenesis involves a mesenchymal to epithelial transition as prospective border cells at the anterior end of the mesenchymal PSM adopt a polarized, columnar morphology and surround a mesenchymal core of cells. The segmental pattern influences the development of the somite derivatives such as the myotome, and the myotome reciprocates to affect the formation of segment boundaries. While somites appear to be serially homologous, there may be variation in the segmentation mechanism along the body axis. Moreover, whereas the genetic architecture of the zebrafish, mouse, and chick segmentation clocks shares many common elements, there is evidence that the gene networks have undergone independent modification during evolution.

Platelet-derived growth factor C plays a role in the branchial arch malformations induced by retinoic acid

BACKGROUND

All-trans-retinoic acid (RA) can produce branchial arch abnormalities in postimplantation rodent embryos cultured in vitro. Platelet-derived growth factor C (PDGF-C) was recently identified as a member of the PDGF ligand family. Many members of the PDGF family are essential for branchial arch morphogenesis and can be regulated by RA. The roles of PDGF-C in branchial arch malformations induced by RA and possible mechanisms were investigated.

METHODS

In whole embryo culture (WEC), mouse embryos were exposed to RA at 0, 0.1, 0.4, 1.0, or 10.0 microM, PDGF-C at 25, 50, or 75 ng/mL, or PDGF-C at 25, 50, or 75 ng/mL containing 0.4 microM RA. After 48 h of culture, mouse embryos were examined for dysmorphogenesis, and whole-mount immunohistochemistry was applied to PDGF-C. In explant cultures, explants were exposed to the same doses of RA and PDGF-C as WEC. Semiquantitative RT-PCR, zymography, and reverse zymography were used to evaluate the expressions and activities of matrix metalloproteinase (MMP)-2, MMP-14, and tissue inhibitor of metalloproteinase (TIMP)-2.

RESULTS

PDGF-C was reduced by RA, and exogenous PDGF-C rescued the branchial arch malformations induced by RA. Moreover, PDGF-C prevented RA-induced inhibition of the migratory ability of mesenchymal cells in the first branchial arch, by regulating the expressions of MMP-2, MMP-14, and TIPM-2.

CONCLUSIONS

Our results suggest that RA exposure reduces the expression of PDGF-C. The branchial arch malformations resulting from fetal RA exposure are caused at least partially by loss of PDGF-C and subsequent misregulations of the expressions of MMP-2, MMP-14, and TIMP-2.

The Shh-independent activator function of the full-length Gli3 protein and its role in vertebrate limb digit patterning

Anterior-posterior (A/P) limb patterning in vertebrates is determined by the counteraction between the Sonic Hedgehog (Shh) and the Gli3 transcription factor. Shh exerts its effect on Gli3 by regulating the full-length Gli3 protein processing to generate a Gli3 repressor gradient along the A/P axis of the limb. However, it is not clear whether the full-length Gli3 is an activator in vivo and plays any role in the limb patterning. Here we show that mouse limbs expressing only a Gli3 repressor form exhibit mild polysyndactyly and a partial loss of digit identity, while limbs expressing only a full-length Gli3 protein display severe polysyndactyly and a complete loss of digit identity. Interestingly, when the full-length Gli3 and the repressor are equally expressed in the limb, the digit patterning is overall normal except for an extra anterior digit. Furthermore, in the presence of one Gli3 wild type allele, a Gli3 mutant allele that expresses only the full-length form can rescue the Shh mutant digit phenotype to a great extent. The full-length Gli3 protein can also activate Shh target gene expression without Shh. Thus, our data indicate that the full-length Gli3 protein is an activator in vivo and that the ratio of the Gli3 activator to repressor, but neither the Gli3 repressor gradient nor the Gli3 activator/repressor ratio gradient, determines limb digit patterning.

Isolation and culture of rat and mouse oligodendrocyte precursor cells

The ability to isolate oligodendroglial precursor cells (OPCs) provides a powerful means to characterize their differentiation, properties and potential for myelin repair. Although much knowledge is available for isolation of OPCs from the rat central nervous system, preparation and maintenance of mouse OPCs has been until recently a challenge owing to difficulties in obtaining a sufficient quantity of purified OPCs. Here, we describe protocols to prepare highly enriched rat OPCs and nearly homogenous mouse OPCs. The mouse method generates predominantly OPCs from cortical neural progenitor cells as clonal aggregates called "oligospheres" by taking advantage of molecular genetic tools. Isolated OPCs can be further differentiated into oligodendrocytes. Collectively, we describe simple and efficient methods for the preparation and in vitro maintenance of enriched OPCs from rats and mice. Isolation and culture of a large, homogenous population of rodent OPCs should significantly facilitate studies on OPC lineage progression and their utility in myelin repair after injury.

Thoracic skeletal defects and cardiac malformations: a common epigenetic link?

Congenital heart defects (CHDs) are the most common birth defects in humans. In addition, cardiac malformations represent the most frequently identified anomaly in teratogenicity experiments with laboratory animals. To explore the mechanisms of these drug-induced defects, we developed a model in which pregnant rats are treated with dimethadione, resulting in a high incidence of heart malformations. Interestingly, these heart defects were accompanied by thoracic skeletal malformations (cleft sternum, fused ribs, extra or missing ribs, and/or wavy ribs), which are characteristic of anterior-posterior (A/P) homeotic transformations and/or disruptions at one or more stages in somite development. A review of other teratogenicity studies suggests that the co-occurrence of these two disparate malformations is not unique to dimethadione, rather it may be a more general phenomenon caused by various structurally unrelated agents. The coexistence of cardiac and thoracic skeletal malformations has also presented clinically, suggesting a mechanistic link between cardiogenesis and skeletal development. Evidence from genetically modified mice reveals that several genes are common to heart development and to formation of the axial skeleton. Some of these genes are important in regulating chromatin architecture, while others are tightly controlled by chromatin-modifying proteins. This review focuses on the role of these epigenetic factors in development of the heart and axial skeleton, and examines the hypothesis that posttranslational modifications of core histones may be altered by some developmental toxicants.

Immunohistochemical detection of insulin-like growth factors, platelet-derived growth factor, and their receptors in ameloblastic tumors

BACKGROUND

To evaluate the roles of growth factors in oncogenesis and cytodifferentiation of odontogenic tumors, expression of insulin-like growth factors (IGFs), platelet-derived growth factor (PDGF), and their receptors was analyzed in ameloblastic tumors as well as in tooth germs.

METHODS

Tissue specimens of 10 tooth germs, 47 ameloblastomas, and five malignant ameloblastic tumors were examined immunohistochemically with the use of antibodies against IGF-I, IGF-II, IGF-I receptor (IGF-IR), PDGF A-chain, PDGF B-chain, PDGF alpha-receptor, and PDGF beta-receptor.

RESULTS

Immunohistochemical reactivity for IGFs, PDGF chains, and their receptors was detected predominantly in odontogenic epithelial cells near the basement membrane in tooth germs and in benign and malignant ameloblastic tumors. The expression levels of IGF-II and PDGF chains were significantly higher in ameloblastic tumors than in tooth germs. Malignant ameloblastic tumors showed higher reactivity for PDGF chains than benign ameloblastomas and higher reactivity for platelet-derived growth factor receptors than tooth germs. The expression levels of PDGF chains were significantly higher in follicular ameloblastomas than in plexiform ameloblastomas. Desmoplastic ameloblastomas showed higher expression of IGFs and IGF-IR when compared with other ameloblastoma subtypes.

CONCLUSION

Expression of IGFs, PDGF, and their receptors in tooth germs and ameloblastic tumors suggests that these growth factor signals contribute to cell proliferation or survival in both normal and neoplastic odontogenic tissues. Expression of these molecules in odontogenic tissues possibly affects interactions with the bone microenvironment during tooth development and intraosseous progression of ameloblastic tumors. Altered expression of the ligands and receptors in ameloblastic tumors may be involved in oncogenesis, malignant potential, and tumor cell differentiation.

An adenosine triphosphatase of the sucrose nonfermenting 2 family, androgen receptor-interacting protein 4, is essential for mouse embryonic development and cell proliferation

An adenosine triphosphatase of the sucrose nonfermenting 2 protein family, androgen receptor-interacting protein 4 (ARIP4), modulates androgen receptor activity. To elucidate receptor-dependent and -independent functions of ARIP4, we have analyzed Arip4 gene-targeted mice. Heterozygous Arip4 mutants were normal. Arip4 is expressed mainly in the neural tube and limb buds during early embryonic development. Arip4-/- embryos were abnormal already at embryonic d 9.5 (E9.5) and died by E11.5. At E9.5 and E10.5, almost all major tissues of Arip4-null embryos were proportionally smaller than those of wild-type embryos, and the neural tube was shrunk in some Arip4-/- embryos. Dramatically reduced cell proliferation and increased apoptosis were observed in E9.5 and E10.5 Arip4-null embryos. Mouse embryonic fibroblasts (MEFs) isolated from Arip4-/- embryos ceased to grow after two to three passages and exhibited increased apoptosis and decreased DNA synthesis compared with wild-type MEFs. Comparison of gene expression profiles of Arip4-/- and wild-type MEFs at E9.5 revealed that putative ARIP4 target genes are involved in cell growth and proliferation, apoptosis, cell death, DNA replication and repair, and development. Collectively, ARIP4 plays an essential role in mouse embryonic development and cell proliferation, and it appears to coordinate multiple essential biological processes, possibly through a complex chromatin remodeling system.

Human cytomegalovirus downregulates expression of receptors for platelet-derived growth factor by smooth muscle cells

Infection by human cytomegalovirus (HCMV) is associated with the development of vascular diseases and may cause severe brain damage in infected fetuses. Platelet-derived growth factor receptors alpha and beta (PDGFR-alpha and -beta) control important cellular processes associated with atherosclerosis and fetal development. In the present investigation, our goal was to determine whether infection by HCMV can influence the expression of PDGFR-alpha and -beta in human smooth muscle cells (SMCs). In connection with HCMV infection in vitro the levels of PDGFR-alpha and -beta at the cell surface and in the total cellular protein of SMCs were reduced in parallel with decreases in the levels of the corresponding mRNAs. These effects were dependent on immediate-early (IE) or early (E) HCMV gene products, since inhibition of late genes did not prevent HCMV from affecting the expression of PDGFR-alpha and -beta. The downregulation of PDGFR caused by HCMV was dose dependent. Furthermore, confocal microscopy revealed that the localization of PDGFR-beta was altered in HCMV-infected cells, in which this protein colocalized with proteins associated with endosomes (Rab4 and -5) and lysosomes (Lamp1 and -2), indicating entrance into pathways for protein degradation. Altogether these observations indicate that an IE and/or E HCMV protein(s) downregulates the expression of PDGFR-alpha and -beta in SMCs. This phenomenon may disrupt cellular processes of importance in connection with cellular differentiation, migration, and/or proliferation. These observations may explain why congenital infection with HCMV can cause fetal brain damage.

[Known gene interactions as implicated in craniofacial development]

Many genes intervening in development, morphogenesis and craniofacial growth have been identified, primarily by the use of mice mutants. We can distinguish two families: the signalling factors and the transcription factors. The latter interact with DNA to activate or to inhibit the expression of other genes. Some of the transcription factors are called homeogenes because they interact with DNA by a sequence of amino acids known as homeobox that has been carefully conserved throughout the course of evolution. Those factors interact, and signalling cascades have been described. Current research projects seek to discern the exact role of each of these genes in craniofacial growth and to develop a better understanding of the interactions between them.

Platelet-derived growth factor-beta receptor activation is essential for fibroblast and pericyte recruitment during cutaneous wound healing

Connective tissue remodeling provides mammals with a rapid mechanism to repair wounds after injury. Inappropriate activation of this reparative process leads to scarring and fibrosis. Here, we studied the effects of platelet-derived growth factor receptor-beta blockade in vivo using the platelet-derived growth factor receptor (PDGFR)-beta inhibitor imatinib mesylate on tissue repair. After 7 days, healing of wounds was delayed with significantly reduced wound closure and concomitant reduction in myofibroblast frequency, expression of fibronectin ED-A, and collagen type I. Using a collagen type I transgenic reporter mouse, we showed that inhibiting PDGFR-beta activation restricted the distribution of collagen-synthesizing cells to wound margins and dramatically reduced cell proliferation in vivo. By 14 days, treated wounds were fully closed. Blocking PDGFR-beta signaling did not prevent the differentiation of myofibroblasts in vitro but potently inhibited fibroblast proliferation and migration. In addition, PDGFR-beta inhibition in vivo was accompanied by abnormal microvascular morphogenesis reminiscent of that observed in PDGFR-beta-/- mice with significantly reduced immunostaining of the pericyte marker NG2. Imatinib treatment also inhibited pericyte proliferation and migration in vitro. This study highlights the significance of PDGFR-beta signaling for the recruitment, proliferation, and functional activities of fibro-blasts and pericytes during the early phases of wound healing.

Molecular Mechanism and Evolutional Significance of Epithelial–Mesenchymal Interactions in the Body‐ and Tail‐Dependent Metamorphic Transformation of Anuran Larval Skin

The epidermis of an anuran larva is composed of apical and skein cells that are both mitotically active and self-renewed through larval life. In contrast, the epidermis of an adult frog, with typical stratified squamous epithelium composed of germinative basal, spinous, granular, and cornified cells, is histologically identical to the mammalian epidermis. Two important issues have not yet been addressed in the study of the development of anuran skin. One is the origin of adult basal cells in the larval epidermis and the other is the mechanism by which larval basal cells are transformed into adult basal cells in a region- (body- and tail-) dependent manner. The cell lineage relationship between the larval and adult epidermal cells was determined by examining the expression profiles of several genes that are expressed specifically in larval and/or adult epidermal cells and differentiation profiles of larval basal cells cultured in the presence of thyroid hormone (TH). Histological analyses using several markers led to the identification of the skin transformation center (STC) where the conversion of larval skin to the adult counterpart is taking place. The STC emerges at a specific place in the body skin and at a specific stage of larval development. The STC progressively "moves" into and "invades" the adjacent larval region of the trunk skin as a larva develops, converting the larval skin into the preadult skin, but never into the tail region. The larva to preadult skin conversion requires an epidermal-mesenchymal interaction. The genesis of preadult basal cells is suppressed in the tail epidermis due to the influence of underlying mesenchyme in the tail region. PDGF signaling is one of the molecular cues of epidermal-mesenchymal interactions. In addition, a unique feature of anuran skin metamorphosis is presented referring to the skin of other vertebrates. Histological comparisons of the skin among vertebrate species strongly suggested a similarity between the anuran larval skin and the teleost fish adult skin and between the anuran adult skin and the adult skin of other tetrapod species. Based on these similarities, the evolutional significance of anuran skin metamorphosis is proposed. Finally, studies are reviewed that reveal the molecular mechanism of anuran metamorphosis in relation to TH-TR-TRE signaling. The results of these studies suggest new aspects of the biological significance of TH, and also enable us to envision concerted regulations of the expression of a gene in the frame of the gene network responsible for metamorphic remodeling of larval tissues. The present review will contribute to an understanding of the molecular mechanism of region-dependent skin development of anurans from not only a metamorphic but also from an evolutional point of view, and will provide a new way to understand the biological significance of TH in anurans.

PDGF Receptors as Targets in Tumor Treatment

Signaling through platelet-derived growth factor (PDGF) receptors contributes to multiple tumor-associated processes. The recent introduction of clinically useful PDGF inhibitors have the last years validated PDGF receptors in malignant and stromal cells as relevant cancer drug targets. Mutational activation of PDGF receptor signaling in malignant cells has been described in some rare tumor types such as dermatofibrosarcoma protuberans, a subset of GISTs, and some hematologic malignancies. Furthermore, expression of PDGF receptors on pericytes is a common characteristic of solid tumors. The clinical efficacy of novel multikinase inhibitors, such as sunitinib and sorafenib, most likely involves targeting of PDGF receptor-dependent pericytes. Preclinical studies suggest that targeting of stromal PDGF receptors might also constitute a novel strategy to enhance tumor drug uptake. Finally, recent studies have implied both pro- and antimetastatic effects of PDGF receptors on malignant and stromal cells. The studies on the roles of PDGF receptors in cancer signaling are thus presently in a dynamic phase where collaborations between oncologists, pathologists, and tumor biologists are predicted to be highly productive.

Cilia and developmental signaling

Recent studies have revealed unexpected connections between the mammalian Hedgehog (Hh) signal transduction pathway and the primary cilium, a microtubule-based organelle that protrudes from the surface of most vertebrate cells. Intraflagellar transport proteins, which are required for the construction of cilia, are essential for all responses to mammalian Hh proteins, and proteins required for Hh signal transduction are enriched in primary cilia. The phenotypes of different mouse mutants that affect ciliary proteins suggest that cilia may act as processive machines that organize sequential steps in the Hh signal transduction pathway. Cilia on vertebrate cells are likely to be important in additional developmental signaling pathways and are required for PDGF receptor alpha signaling in cultured fibroblasts. Cilia are not essential for either canonical or noncanonical Wnt signaling, although cell-type-specific modulation of cilia components may link cilia and Wnt signaling in some tissues. Because ciliogenesis in invertebrates is limited to a very small number of specialized cell types, the role of cilia in developmental signaling pathways is likely a uniquely vertebrate phenomenon.

PDGF signaling specificity is mediated through multiple immediate early genes

Growth factor signaling leads to the induction or repression of immediate early genes, but how these genes act collectively as effectors of downstream processes remains unresolved. We have used gene trap-coupled microarray analysis to identify and mutate multiple platelet-derived growth factor (PDGF) intermediate early genes in mice. Mutations in these genes lead to a high frequency of phenotypes that affect the same cell types and processes as those controlled by the PDGF pathway. We conclude that these genes form a network that controls specific processes downstream of PDGF signaling.

PI3-kinase/Akt-dependent antiapoptotic signaling by the PDGF alpha receptor is negatively regulated by Src family kinases

Regulation of growth factor dependent cell survival is crucial for development and disease progression. Here, we report a novel function of Src kinases as a negative regulator of platelet-derived growth factor (PDGF) dependent cell survival. We characterized a series of PDGF alpha receptor (PDGFRA) mutants, which lack the binding sites for Src, phosphatidylinositol 3'-kinase (PI3K), SHP-2 or phospholipase C-gamma. We found that PDGFRA-dependent cell survival was mainly mediated through activation of PI3K, and was negatively regulated by Src. Characterization of the downstream signaling events revealed that PI3K activates the protein kinase Akt, which in turn phosphorylates and thus inactivates proapoptotic Forkhead transcription factors. Src phosphorylates the ubiquitin-ligase c-Cbl, which is required for degradation of the activated receptor. Consequently, overexpression of c-Cbl prevented PDGFRA-mediated cell survival, whereas it did not affect this response, when Src was unable to associate with the receptor. This novel function of Src in antiapoptotic signaling introduces Src kinases as an interesting therapeutic target in apoptosis related diseases.

Cell cycle regulation in the developing lens

Regulation of cell proliferation is a critical aspect of the development of multicellular organisms. The ocular lens is an excellent model system in which to unravel the mechanisms controlling cell proliferation during development. In recent years, several cell cycle regulators have been shown to be essential for maintaining normal patterns of lens cell proliferation. Additionally, many growth factor signaling pathways and cell adhesion factors have been shown to have the capacity to regulate lens cell proliferation. Given this complexity, understanding the cross talk between these many signaling pathways and how they are coordinated are important directions for the future.

Cadmium-induced gene expression changes in the mouse embryo, and the influence of pretreatment with zinc

Cadmium (Cd) administered to female C57BL/6 mice on gestation day 8 induces a high incidence of anterior neural tube defects (exencephaly). This adverse effect can be attenuated by maternal pretreatment with zinc (Zn). In this study we used replicated microarray analysis and real-time PCR to investigate gene expression changes induced in the embryo 5 and 10h after maternal Cd exposure in the absence or presence of Zn pretreatment. We report nine genes with a transcriptional response induced by Cd, none of which was influenced by Zn pretreatment, and two genes induced only by combined maternal Cd exposure and Zn pretreatment. We discuss the results in relation to the possibility that Cd is largely excluded from the embryo, that the teratogenic effects of Cd may be secondary to toxicity in extraembryonic tissues, and that the primary protective role of Zn may not be to reverse Cd-induced transcription in the embryo.

Platelet-derived growth factor receptor-alpha is a key determinant of smooth muscle alpha-actin filaments in bone marrow-derived mesenchymal stem cells

Smooth muscle alpha-actin filaments are a defining feature of mesenchymal stem cells, and of mesenchyme-derived contractile smooth muscle cells, pericytes and myofibroblasts. Here, we show that adult bone marrow-derived mesenchymal stem cells express abundant cell surface platelet-derived growth factor receptor-alpha, having a high ratio to platelet-derived growth factor receptor-beta. Signaling through platelet-derived growth factor receptor-alpha increases smooth muscle alpha-actin filaments by activating RhoA, which results in Rho-associated kinase (ROCK)-dependent cofilin phosphorylation, enhancing smooth muscle alpha-actin filament polymerization, and also upregulates smooth muscle alpha-actin expression. In contrast, platelet-derived growth factor receptor-beta signaling strongly upregulates RhoE, which inhibits ROCK activity, promoting smooth muscle alpha-actin filament depolymerization. This study thus provides new insights into the distinct roles of platelet-derived growth factor receptor-alpha and -beta signaling in regulating the adult mesenchymal stem cell contractile cytoskeleton.

The primary cilium in cell signaling and cancer

The primary cilium is a microtubule-based antenna-like structure that emanates from the surface of virtually all cells in the mammalian body. It is anchored to the cell by the basal body, which develops from the mother centriole of the centrosome in a manner that is coordinately regulated with the cell cycle. The primary cilium is a sensory organelle that receives both mechanical and chemical signals from other cells and the environment, and transmits these signals to the nucleus to elicit a cellular response. Recent studies revealed that multiple components of the Sonic hedgehog and platelet-derived growth factor receptor-alpha signal transduction pathways localize to the primary cilium, and that loss of the cilium blocks ligand-induced signaling by both pathways. In light of the major role that these pathways play in numerous types of cancer, we anticipate that the emerging discoveries being made about the function of the primary cilium in signaling pathways that are critical for embryonic development and tissue homeostasis in adults will also provide novel insights into the molecular mechanisms of carcinogenesis.

Multiple repressor pathways contribute to phenotypic switching of vascular smooth muscle cells

Smooth muscle cell (SMC) differentiation is an essential component of vascular development and these cells perform biosynthetic, proliferative, and contractile roles in the vessel wall. SMCs are not terminally differentiated and possess the ability to modulate their phenotype in response to changing local environmental cues. The focus of this review is to provide an overview of the current state of knowledge of molecular mechanisms involved in controlling phenotypic switching of SMC with particular focus on examination of processes that contribute to the repression of SMC marker genes. We discuss the environmental cues which actively regulate SMC phenotypic switching, such as platelet-derived growth factor-BB, as well as several important regulatory mechanisms required for suppressing expression of SMC-specific/selective marker genes in vivo, including those dependent on conserved G/C-repressive elements, and/or highly conserved degenerate CArG elements found in the promoters of many of these marker genes. Finally, we present evidence indicating that SMC phenotypic switching involves multiple active repressor pathways, including Krüppel-like zinc finger type 4, HERP, and ERK-dependent phosphorylation of Elk-1 that act in a complementary fashion.

Potential of dental mesenchymal cells in developing teeth

The tooth, composed of dentin and enamel, develops through epithelium-mesenchyme interactions. Neural crest (NC) cells contribute to the dental mesenchyme in the developing tooth and differentiate into dentin-secreting odontoblasts. NC cells are known to differentiate into chondrocytes and osteoblasts in the craniofacial region. However, it is not clear whether the dental mesenchymal cells in the developing tooth possess the potential to differentiate into a lineage(s) other than the odontoblast lineage. In this study, we prepared mesenchymal cells from E13.5 tooth germ cells and assessed their potential for differentiation in culture. They differentiated into odontoblasts, chondrocyte-like cells, and osteoblast-like cells. Their derivation was confirmed by tracing NC-derived cells as LacZ(+) cells using P0-Cre/Rosa26R mice. Using the flow cytometry-fluorescent di-beta-D-galactosidase system, which makes it possible to detect LacZ(+) cells as living cells, cell surface molecules of dental mesenchymal cells were characterized. Large number of LacZ(+) NC-derived cells expressed platelet-derived growth factor receptor alpha and integrins. Taken together, these results suggest that NC-derived cells with the potential to differentiate into chondrocyte-like and osteoblast-like cells are present in the developing tooth, and these cells may contribute to tooth organogenesis.

Homology of the reptilian coracoid and a reappraisal of the evolution and development of the amniote pectoral apparatus

As in monotreme mammals, the pectoral apparatus of basal (fossil) amniotes includes two coracoid elements, the procoracoid and metacoracoid. Among extant reptiles the metacoracoid has long been assumed lost; this notion is herein challenged. A comprehensive review of data from numerous sources, including the fossil record, experimental embryology, genetic manipulations and an analysis of morphology at the level cell condensations, supports the conclusion that the metacoracoid gives rise to the majority of the reptilian coracoid. By contrast, the reptilian procoracoid remains as a rudiment that is incorporated as a process of the (meta)coracoid and/or the glenoid region of the scapula early during development, prior to skeletogenesis. Application of this integrated approach corroborates and enhances previous work describing the evolution of the pectoral apparatus in mammals. A revised scenario of amniote coracoid evolution is presented emphasizing the importance of considering cell condensations when evaluating the homology of a skeletal complex.

Molecular control of secondary palate development

Compared with the embryonic development of other organs, development of the secondary palate is seemingly simple. However, each step of palatogenesis, from initiation until completion, is subject to a tight molecular control that is governed by epithelial-mesenchymal interactions. The importance of a rigorous molecular regulation of palatogenesis is reflected when loss of function of a single protein generates cleft palate, a frequent malformation with a complex etiology. Genetic studies in humans and targeted mutations in mice have identified numerous factors that play key roles during palatogenesis. This review highlights the current understanding of the molecular and cellular mechanisms involved in normal and abnormal palate development with special respect to recent advances derived from studies of mouse models.

Over-expression of PDGF-C using a lung specific promoter results in abnormal lung development

PDGF isoforms are a family of polypeptides that bind to cell surface receptors and induce fibroblast proliferation and chemotaxis. PDGF-A and -B chain isoforms have previously been shown to be involved in murine lung development. A new PDGF polypeptide, PDGF-C, was recently recognized and differs from the PDGF-A and -B isoforms in that it requires proteolytic cleavage before it can bind and activate the PDGF alpha receptor. In these studies PDGF-C was over-expressed during embryogenesis using the lung specific surfactant protein C promoter. PDGF-C transgenic pups died from respiratory insufficiency within minutes following birth. At E18.5, nontransgenic lungs exhibited lung morphology consistent with the saccular stage of lung development. In contrast, E18.5 transgenic lungs retained many features of the canalicular stage of lung development and had abundant numbers of large poorly differentiated mesenchymal cells. These results suggest that PDGF-C is activated during lung development and is a potent growth factor for mesenchymal cells in vivo.

Ras signaling is essential for lens cell proliferation and lens growth during development

The vertebrate ocular lens is a simple and continuously growing tissue. Growth factor-mediated receptor tyrosine kinases (RTKs) are believed to be required for lens cell proliferation, differentiation and survival. The signaling pathways downstream of the RTKs remain to be elucidated. Here, we demonstrate the important role of Ras in lens development by expressing a dominant-negative form of Ras (dn-Ras) in the lens of transgenic mice. We show that lens in the transgenic mice was smaller and lens growth was severely inhibited as compared to the wild-type lens. However, the lens shape, polarity and transparency appeared normal in the transgenic mice. Further analysis showed that cell proliferation is inhibited in the dn-Ras lens. For example, the percentage of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in epithelial layer was about 2- to 3-fold lower in the transgenic lens than in the wild-type lens, implying that Ras activity is required for normal cell proliferation during lens development. We also found a small number of apoptotic cells in both epithelial and fiber compartment of the transgenic lens, suggesting that Ras also plays a role in cell survival. Interestingly, although there was a delay in primary fiber cell differentiation, secondary fiber cell differentiation was not significantly affected in the transgenic mice. For example, the expression of beta- and gamma-crystallins, the marker proteins for fiber differentiation, was not changed in the transgenic mice. Biochemical analysis indicated that ERK activity, but not Akt activity, was significantly reduced in the dn-Ras transgenic lenses. Overall, our data imply that the RTK-Ras-ERK signaling pathway is essential for cell proliferation and, to a lesser extent, for cell survival, but not for crystallin gene expression during fiber differentiation. Thus, some of the fiber differentiation processes are likely mediated by RTK-dependent but Ras-independent pathways.

Development of the upper lip: morphogenetic and molecular mechanisms

The vertebrate upper lip forms from initially freely projecting maxillary, medial nasal, and lateral nasal prominences at the rostral and lateral boundaries of the primitive oral cavity. These facial prominences arise during early embryogenesis from ventrally migrating neural crest cells in combination with the head ectoderm and mesoderm and undergo directed growth and expansion around the nasal pits to actively fuse with each other. Initial fusion is between lateral and medial nasal processes and is followed by fusion between maxillary and medial nasal processes. Fusion between these prominences involves active epithelial filopodial and adhering interactions as well as programmed cell death. Slight defects in growth and patterning of the facial mesenchyme or epithelial fusion result in cleft lip with or without cleft palate, the most common and disfiguring craniofacial birth defect. Recent studies of craniofacial development in animal models have identified components of several major signaling pathways, including Bmp, Fgf, Shh, and Wnt signaling, that are critical for proper midfacial morphogenesis and/or lip fusion. There is also accumulating evidence that these signaling pathways cross-regulate genetically as well as crosstalk intracellularly to control cell proliferation and tissue patterning. This review will summarize the current understanding of the basic morphogenetic processes and molecular mechanisms underlying upper lip development and discuss the complex interactions of the various signaling pathways and challenges for understanding cleft lip pathogenesis.

Haplotype-dependent binding of nuclear proteins to the promoter of the neural tube defects-associated platelet-derived growth factor alpha-receptor gene

We have previously shown that polymorphisms in the promoter of the human platelet-derived growth factor alpha-receptor (PDGFRA) gene can be grouped into five distinct haplotypes, designated H1, H 2 alpha, H 2 beta, H 2 gamma and H 2 delta, and that specific combinations of these promoter haplotypes predispose to neural tube defects (NTDs). These promoter haplotypes differ strongly in their ability to drive reporter gene expression in various human cell lines, with highest activity for H 2 alpha and H 2 beta. Here, we show that the haplotype-linked PDGFRA promoter region extends to 3.6 kb upstream from the transcription start site, and contains a total of ten polymorphic sites. For two of these polymorphic sites, i.e. -909 C/A and +68 GAins/del, we observed differential binding of nuclear proteins from human osteosarcoma (HOS) cells. The protein complex binding specifically to -909 C, which is present in all haplotypes except the low activity haplotype H 2 gamma, contained members of the upstream stimulatory factor (USF) family of transcription factors. Furthermore, we identified a protein complex of 125 kDa which bound specifically to the low activity haplotype H1 at position +68 GAdel and may represent an H1-specific PDGFRA transcriptional repressor. The current identification of cis-acting elements in the PDGFRA promoter and the transcription factors that bind them, provides a new strategy for the identification of genes that are potentially involved in neural tube defects.

Microarray analysis of PDGFR alpha+ populations in ES cell differentiation culture identifies genes involved in differentiation of mesoderm and mesenchyme including ARID3b that is essential for development of embryonic mesenchymal cells

An inherent difficulty in using DNA microarray technology on the early mouse embryo is its relatively small size. In this study, we investigated whether use of ES cell differentiation culture, which has no theoretical limit in the number of cells that can be generated, can improve this situation. Seven distinct ES-cell-derived populations were analyzed by DNA microarray and examined for genes whose distribution patterns are similar to those of PDGFRalpha, a gene implicated in differentiation of mesoderm/mesenchymal lineages. Using software developed in our laboratory, we formed a group of 30 genes which showed the highest similarity to PDGFRalpha, 18 of these genes were shown to be involved in development of either mesodermal, mesenchymal or neural crest cells. This list also contains several genes whose role in embryogenesis has not yet been fully identified. One such molecule is mARID3b. The mARID3b expression is found in the paraxial mesoderm and cranial mesenchyme. mARID3b-null mouse showed early embryonic lethality, and most phenotypes of this mutant appear to develop from a failure to generate a sufficient number of cranial mesenchymal cells. These results demonstrate the potential use of ES cell differentiation culture in identifying novel genes playing an indispensable role in embryogenesis.

Rhabdomyosarcomas utilize developmental, myogenic growth factors for disease advantage: a report from the Children's Oncology Group

BACKGROUND

Unresectable or metastatic disease represents the greatest obstacle to cure for children with rhabdomyosarcoma. In this study we sought to identify gene expression signatures of advanced stage and progressive disease.

PROCEDURE

Using oligonucleotide gene expression analysis for a focused set of 60 genes, we analyzed the myogenic expression profiles of 89 rhabdomyosarcomas from the Intergroup Rhabdomyosarcoma Study-IV.

RESULTS

While the expression profile of rhabdomyosarcomas closely paralleled gene expression profiles of normal embryonic myogenic progenitors, growth factors were most closely associated with disease progression. Specifically, we identified platelet-derived growth factor (PDGF) receptors and insulin-like growth factor as strongly correlated with decreased failure-free survival. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) of an independent data set suggested that autocrine growth signaling, if present, is not regulated in a simple manner at the transcriptional level.

CONCLUSIONS

Increased transcriptional levels of PDGF receptors and insulin-like growth factor are associated with decreased survival in rhabdomyosarcomas. Dual blockade of these growth-factor-signaling pathways may be a valuable strategy in preclinical therapeutic studies.

Targeted disruption of the DM domain containing transcription factor Dmrt2 reveals an essential role in somite patterning

Dmrt2 is expressed in the dermomyotome of developing vertebrate somites. To determine the role of Dmrt2 during mouse embryonic development, we generated a null mutation of Dmrt2 via homologous recombination in embryonic stem cells. Dmrt2 heterozygous mice derived from these cells are phenotypically normal. However, Dmrt2 homozygotes die soon after birth. The cause of death is likely due to abnormal rib and sternal development, leading to an inability to breathe. Loss of Dmrt2 leads to embryonic somite patterning defects, first evidenced at embryonic day (E) 10.5 and more pronounced by E11.5. Notably, both the dermomyotome and myotome fail to adopt a normal epithelial morphology in the absence of Dmrt2. Accompanying these morphological defects are alterations in the expression patterns of dermomyotomal and myotomal transcription factors including Pax3, Paraxis, Myf5, myogenin, Mrf4 and MyoD. Despite these defects, embryos harvested from E13.5 onwards exhibited relatively normal muscle pattern and mass, suggesting that early myotomal defects are corrected by a Dmrt2-independent mechanism. Taken together, our results define an essential function for Dmrt2 in somite development and provide evidence that DM domain genes have been co-opted into other critical developmental pathways distinct from that of sex determination or differentiation.

Promotor genotype of the platelet-derived growth factor receptor-alpha gene shows population stratification but not association with spina bifida meningomyelocele

Neural tube defects (NTDs) constitute a major group of congenital malformations with an overall incidence of approximately 1-2 in 1,000 live births in the United States. Hispanic Americans have a 2.5 times higher risk than the Caucasian population. Spina bifida meningomyelocele (SBMM) is a major clinical presentation of NTDs resulting from lack of closure of the spinal cord caudal to the head. In a previous study of spina bifida (SB) patients of European Caucasian descent, it was suggested that specific haplotypes of the platelet-derived growth factor receptor-alpha (PDGFRA) gene P1 promoter strongly affected the rate of NTD genesis. In our study, we evaluated the association of PDGFRA P1 in a group of 407 parent-child triads (167 Caucasian, 240 Hispanics) and 164 unrelated controls (89 Caucasian, 75 Hispanic). To fully evaluate the association of PDGFRA P1, we performed both transmission-disequilibrium test (TDT) and association analyses to test the hypotheses that PDGFRA P1 was (1) transmitted preferentially in SBMM affected children and (2) associated with the condition of SBMM comparing affected children to unaffected controls. We did find that there was a different allelic and genotypic distribution of PDGFRA P1 when comparing Hispanics and Caucasians. However, neither ethnic group showed strong association between SBMM and the PDGFRA P1 region. These findings suggest that PDGFRA P1 does not have a major role in the development of SBMM.

Platelet-derived growth factors in the developing avian heart and maturating coronary vasculature

Platelet-derived growth factors (PDGFs) are important in embryonic development. To elucidate their role in avian heart and coronary development, we investigated protein expression patterns of PDGF-A, PDGF-B, and the receptors PDGFR-alpha and PDGFR-beta using immunohistochemistry on sections of pro-epicardial quail-chicken chimeras of Hamburger and Hamilton (HH) 28-HH35. PDGF-A and PDGFR-alpha were expressed in the atrial septum, sinus venosus, and throughout the myocardium, with PDGFR-alpha retreating to the trabeculae at later stages. Additionally, PDGF-A and PDGFR-alpha were present in outflow tract cushion mesenchyme and myocardium, respectively. Small cardiac nerves and (sub)epicardial cells expressed PDGF-B and PDGFR-beta. Furthermore, endothelial cells expressed PDGF-B, while vascular smooth muscle cells and interstitial epicardium-derived cells expressed PDGFR-beta, indicating a role in coronary maturation. PDGF-B is also present in ventricular septal development, in the absence of any PDGFR. Epicardium-derived cells in the atrioventricular cushions expressed PDGFR-beta. We conclude that all four proteins are involved in myocardial development, whereas PDGF-B and PDGFR-beta are specifically important in coronary maturation.

Role of morphogens in neural crest cell determination

The neural crest is a transient, migratory cell population found in all vertebrate embryos that generate a diverse range of cell and tissue derivatives including, but not limited, to the neurons and glia of the peripheral nervous system, smooth muscle, connective tissue, melanocytes, craniofacial cartilage, and bone. Over the past few years, many studies have provided tremendous insights into understanding the mechanisms regulating the induction and migration of neural crest cell development. This review highlights the surprising and perhaps unexpected roles for morphogens in these distinct processes. A comparison of studies performed in several different vertebrates emphasizes the requirement for coordination between multiple signaling pathways in the induction and migration of neural crest cells in the developing embryo.

Activation of AKT kinases in cancer: implications for therapeutic targeting

The AKT1, AKT2, and AKT3 kinases have emerged as critical mediators of signal transduction pathways downstream of activated tyrosine kinases and phosphatidylinositol 3-kinase. An ever-increasing list of AKT substrates has precisely defined the multiple functions of this kinase family in normal physiology and disease states. Cellular processes regulated by AKT include cell proliferation and survival, cell size and response to nutrient availability, intermediary metabolism, angiogenesis, and tissue invasion. All these processes represent hallmarks of cancer, and a burgeoning literature has defined the importance of AKT alterations in human cancer and experimental models of tumorigenesis, continuing the legacy represented by the original identification of v-Akt as the transforming oncogene of a murine retrovirus. Many oncoproteins and tumor suppressors intersect in the AKT pathway, finely regulating cellular functions at the interface of signal transduction and classical metabolic regulation. This careful balance is altered in human cancer by a variety of activating and inactivating mechanisms that target both AKT and interrelated proteins. Reprogramming of this altered circuitry by pharmacologic modulation of the AKT pathway represents a powerful strategy for rational cancer therapy. In this review, we summarize a large body of data, from many types of cancer, indicating that AKT activation is one of the most common molecular alterations in human malignancy. We also review mechanisms of activation of AKT kinases, examples of therapeutic modulation of the AKT pathway in animal models, and the current status of efforts to target molecular components of the AKT pathway for cancer therapy and, possibly, cancer prevention.

[Vascularization of the head and neck during development]

One of the earliest priorities of the embryonic vascular system is to ensure the metabolic needs of the head. This review covers some of the principles that govern the cellular assembly and localization of blood vessels in the head. In order to understand the development and organization of the cephalic vascular tree, one needs to recall the morphogenetic movements underlying vertebrate head formation and giving rise to the constituent cells of the vascular system. Some of the major signaling molecules involved in vascular development are discussed, including the angiopoietins, the endothelins, the FGFs, the Notch receptors, the PDGFs, Sonic hedgehog, the TGF family and the VEGFs, in order to underline similarities between embryonic and postnatal vascular development, even in the context of increasingly divergent form.

PDGF and FGF2 pathways regulate distinct oligodendrocyte lineage responses in experimental demyelination with spontaneous remyelination

Repair of myelin damage in the adult CNS requires oligodendrocyte progenitor (OP) proliferation and subsequent differentiation into remyelinating oligodendrocytes. Platelet-derived growth factor (PDGF) and fibroblast growth factor-2 (FGF2) have been predicted to act individually and/or cooperatively to generate remyelinating oligodendrocytes. Analysis of PDGF alpha receptor (PDGF alpha R) heterozygous (+/-) mice indicates that PDGF alpha R expression modulates oligodendrocyte density in non-lesioned adult CNS. Analysis of cuprizone demyelination and recovery in PDGF alpha R+/- mice, FGF2 knockout (-/-) mice, and PDGF alpha R+/- FGF2-/- mice demonstrated that: (1) OP proliferation and oligodendrocyte regeneration is impaired in PDGF alpha R heterozygotes, (2) PDGF alpha R+/- and FGF2-/- deletions do not act cooperatively to impair OP amplification, (3) oligodendrocyte differentiation is more frequent in FGF2-/- mice, and (4) FGF2 deletion in combination with the PDGF alpha R+/- genotype rescues impaired oligodendrocyte regeneration of PDGF alpha R heterozygotes. These findings demonstrate distinct roles for PDGF and FGF2 in vivo in the context of a demyelinating disease with spontaneous remyelination.

Targeting the platelet-derived growth factor receptor alpha with a neutralizing human monoclonal antibody inhibits the growth of tumor xenografts: implications as a potential therapeutic target

Platelet-derived growth factor receptor alpha (PDGFRalpha) is a type III receptor tyrosine kinase that is expressed on a variety of tumor types. A neutralizing monoclonal antibody to human PDGFRalpha, which did not cross-react with the beta form of the receptor, was generated. The fully human antibody, termed 3G3, has a Kd of 40 pmol/L and blocks both PDGF-AA and PDGF-BB ligands from binding to PDGFRalpha. In addition to blocking ligand-induced cell mitogenesis and receptor autophosphorylation, 3G3 inhibited phosphorylation of the downstream signaling molecules Akt and mitogen-activated protein kinase. This inhibition was seen in both transfected and tumor cell lines expressing PDGFRalpha. The in vivo antitumor activity of 3G3 was tested in human glioblastoma (U118) and leiomyosarcoma (SKLMS-1) xenograft tumor models in athymic nude mice. Antibody 3G3 significantly inhibited the growth of U118 (P=0.0004) and SKLMS-1 (P <0.0001) tumors relative to control. These data suggest that 3G3 may be useful for the treatment of tumors that express PDGFRalpha.

PDGFR-alpha signaling is critical for tooth cusp and palate morphogenesis

Platelet-derived growth factor receptor alpha (PDGFR-alpha) and PDGF ligands are key regulators for embryonic development. Although Pdgfralpha is spatially expressed in the cranial neural crest (CNC)-derived odontogenic mesenchyme, mice deficient for Pdgfralpha are embryonic lethal, making it impossible to investigate the functional significance of PDGF signaling in regulating the fate of CNC cells during tooth morphogenesis. Taking advantage of the kidney capsule assay, we investigated the biological function of PDGF signaling in regulating tooth morphogenesis. Pdgfralpha and Pdgfa are specifically and consistently expressed in the CNC-derived odontogenic mesenchyme and the dental epithelium, respectively, throughout all stages of tooth development, suggesting a paracrine function of PDGF signaling in regulating tooth morphogenesis. Highly concentrated expression patterns of Pdgfralpha and Pdgfa are associated with the developing dental cusp, suggesting possible functional importance of PDGF signaling in regulating cusp formation. Loss of the Pdgfralpha gene does not affect proper odontoblasts proliferation and differentiation in the CNC-derived odontogenic mesenchyme but perturbs the formation of extracellular matrix and the organization of odontoblast cells at the forming cusp area, resulting in dental cusp growth defect. Pdgfralpha-/- mice have complete cleft palate. We show that the cleft palate in Pdgfralpha mutant mice results from an extracellular matrix defect within the CNC-derived palatal mesenchyme. The midline epithelium of the mutant palatal shelf remains functionally competent to mediate palatal fusion once the palatal shelves are placed in close contact in vitro. Collectively, our data suggests that PDGFRalpha and PDGFA are critical regulators for the continued epithelial-mesenchymal interaction during tooth and palate morphogenesis. Disruption of PDGFRalpha signaling disturbs the growth of dental cusp and interferes with the critical extension of palatal shelf during craniofacial development.

New insights into the regulation of mammalian sex determination and male sex differentiation

In mammals, sex development is a genetically and hormonally controlled process that begins with the establishment of chromosomal or genetic sex (XY or XX) at conception. At approximately 6 to 7 weeks of human gestation or embryonic day e11.5 in the mouse, expression of the Y chromosome-linked sex determining gene called SRY (described in detail in this chapter) then initiates gonadal differentiation, which is the formation of either a testis (male) or an ovary (female). Male sex differentiation (development of internal and external reproductive organs and acquisition of male secondary sex characteristics) is then controlled by three principal hormones produced by the testis: Mullerian inhibiting substance (MIS) or anti-Mullerian hormone (AMH), testosterone, and insulin-like factor 3 (INSL3). In the absence of these critical testicular hormones, female sex differentiation ensues. This sequential, three-step process of mammalian sex development is also known as the Jost paradigm. With the advent of modern biotechnologies over the past decade, such as transgenics, array-based gene profiling, and proteomics, the field of mammalian sex determination has witnessed a remarkable boost in the understanding of the genetics and complex molecular mechanisms that regulate this fundamental biological event. Consequently, a number of excellent reviews have been devoted to this topic. The purpose of the present chapter is to provide an overview of selected aspects of mammalian sex determination and differentiation with an emphasis on studies that have marked this field of study.