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

Alternative splicing regulates Prdm1/Blimp-1 DNA binding activities and corepressor interactions

Prdm1/Blimp-1 is a master regulator of gene expression in diverse tissues of the developing embryo and adult organism. Its C-terminal zinc finger domain mediates nuclear import, DNA binding, and recruitment of the corepressors G9a and HDAC1/2. Alternatively spliced transcripts lacking exon 7 sequences encode a structurally divergent isoform (Blimp-1Δexon7) predicted to have distinct functions. Here we demonstrate that the short Blimp-1Δexon7 isoform lacks DNA binding activity and fails to bind G9a or HDAC1/2 but retains the ability to interact with PRMT5. To investigate functional roles of alternative splicing in vivo, we engineered novel mouse strains via embryonic stem (ES) cell technology. Like null mutants, embryos carrying a targeted deletion of exon 7 and exclusively expressing Blimp-1Δexon7 die at around embryonic day 10.5 (E10.5) due to placental defects. In heterozygous Δexon7 mice, there is no evidence of dominant-negative effects. Mice carrying a knock-in allele with an exon 6-exon 7 fusion express full-length Blimp-1 only, develop normally, are healthy and fertile as adults, and efficiently generate mature plasma cells. These findings strongly suggest that the short Blimp-1Δexon7 isoform is dispensable. We propose that developmentally regulated alternative splicing is influenced by chromatin structure at the locus and fine-tunes Blimp-1's functional capabilities.

Stromal platelet-derived growth factor receptor α (PDGFRα) provides a therapeutic target independent of tumor cell PDGFRα expression in lung cancer xenografts

In lung cancer, platelet-derived growth factor receptor α (PDGFRα) is expressed frequently by tumor-associated stromal cells and by cancer cells in a subset of tumors. We sought to determine the effect of targeting stromal PDGFRα in preclinical lung tumor xenograft models (human tumor, mouse stroma). Effects of anti-human (IMC-3G3) and anti-mouse (1E10) PDGFRα monoclonal antibodies (mAb) on proliferation and PDGFRα signaling were evaluated in lung cancer cell lines and mouse fibroblasts. Therapy studies were conducted using established PDGFRα-positive H1703 cells and PDGFRα-negative Calu-6, H1993, and A549 subcutaneous tumors in immunocompromised mice treated with vehicle, anti-PDGFRα mAbs, chemotherapy, or combination therapy. Tumors were analyzed for growth and levels of growth factors. IMC-3G3 inhibited PDGFRα activation and the growth of H1703 cells in vitro and tumor growth in vivo, but had no effect on PDGFRα-negative cell lines or mouse fibroblasts. 1E10 inhibited growth and PDGFRα activation of mouse fibroblasts, but had no effect on human cancer cell lines in vitro. In vivo, 1E10-targeted inhibition of murine PDGFRα reduced tumor growth as single-agent therapy in Calu-6 cells and enhanced the effect of chemotherapy in xenografts derived from A549 cells. We also identified that low expression cancer cell expression of VEGF-A and elevated expression of PDGF-AA were associated with response to stromal PDGFRα targeting. We conclude that stromal PDGFRα inhibition represents a means for enhancing control of lung cancer growth in some cases, independent of tumor cell PDGFRα expression.

Phf14, a novel regulator of mesenchyme growth via platelet-derived growth factor (PDGF) receptor-α

The regulation of mesenchymal cell growth by signaling molecules plays an important role in maintaining tissue functions. Aberrant mesenchymal cell proliferation caused by disruption of this regulatory process leads to pathogenetic events such as fibrosis. In the current study we have identified a novel nuclear factor, Phf14, which controls the proliferation of mesenchymal cells by regulating PDGFRα expression. Phf14-null mice died just after birth due to respiratory failure. Histological analyses of the lungs of these mice showed interstitial hyperplasia with an increased number of PDGFRα(+) mesenchymal cells. PDGFRα expression was elevated in Phf14-null mesenchymal fibroblasts, resulting in increased proliferation. We demonstrated that Phf14 acts as a transcription factor that directly represses PDGFRα expression. Based on these results, we used an antibody against PDGFRα to successfully treat mouse lung fibrosis. This study shows that Phf14 acts as a negative regulator of PDGFRα expression in mesenchymal cells undergoing normal and abnormal proliferation, and is a potential target for new treatments of lung fibrosis.

PDGF-α stimulates intestinal epithelial cell turnover after massive small bowel resection in a rat

Numerous cytokines have been shown to affect epithelial cell differentiation and proliferation through epithelial-mesenchymal interaction. Growing evidence suggests that platelet-derived growth factor (PDGF) signaling is an important mediator of these interactions. The purpose of this study was to evaluate the effect of PDGF-α on enterocyte turnover in a rat model of short bowel syndrome (SBS). Male rats were divided into four groups: Sham rats underwent bowel transection, Sham-PDGF-α rats underwent bowel transection and were treated with PDGF-α, SBS rats underwent a 75% bowel resection, and SBS-PDGF-α rats underwent bowel resection and were treated with PDGF-α. Parameters of intestinal adaptation, enterocyte proliferation and apoptosis were determined at euthanasia. Illumina's Digital Gene Expression analysis was used to determine PDGF-related gene expression profiling. PDGF-α and PDGF-α receptor (PDGFR-α) expression was determined by real-time PCR. Western blotting was used to determine p-ERK, Akt1/2/3, bax, and bcl-2 protein levels. SBS rats demonstrated a significant increase in PDGF-α and PDGFR-α expression in jejunum and ileum compared with sham animals. SBS-PDGF-α rats demonstrated a significant increase in bowel and mucosal weight, villus height, and crypt depth in jejunum and ileum compared with SBS animals. PDGF-α receptor expression in crypts increased in SBS rats (vs. sham) and was accompanied by an increased cell proliferation following PDGF-α administration. A significant decrease in cell apoptosis in this group was correlated with lower bax protein levels. In conclusion, in a rat model of SBS, PDGF-α stimulates enterocyte turnover, which is correlated with upregulated PDGF-α receptor expression in the remaining small intestine.

Dynamic regulation of platelet-derived growth factor receptor α expression in alveolar fibroblasts during realveolarization

Although the importance of platelet-derived growth factor receptor (PDGFR)-α signaling during normal alveogenesis is known, it is unclear whether this signaling pathway can regulate realveolarization in the adult lung. During alveolar development, PDGFR-α-expressing cells induce α smooth muscle actin (α-SMA) and differentiate to interstitial myofibroblasts. Fibroblast growth factor (FGF) signaling regulates myofibroblast differentiation during alveolarization, whereas peroxisome proliferator-activated receptor (PPAR)-γ activation antagonizes myofibroblast differentiation in lung fibrosis. Using left lung pneumonectomy, the roles of FGF and PPAR-γ signaling in differentiation of myofibroblasts from PDGFR-α-positive precursors during compensatory lung growth were assessed. FGF receptor (FGFR) signaling was inhibited by conditionally activating a soluble dominant-negative FGFR2 transgene. PPAR-γ signaling was activated by administration of rosiglitazone. Changes in α-SMA and PDGFR-α protein expression were assessed in PDGFR-α-green fluorescent protein (GFP) reporter mice using immunohistochemistry, flow cytometry, and real-time PCR. Immunohistochemistry and flow cytometry demonstrated that the cell ratio and expression levels of PDGFR-α-GFP changed dynamically during alveolar regeneration and that α-SMA expression was induced in a subset of PDGFR-α-GFP cells. Expression of a dominant-negative FGFR2 and administration of rosiglitazone inhibited induction of α-SMA in PDGFR-α-positive fibroblasts and formation of new septae. Changes in gene expression of epithelial and mesenchymal signaling molecules were assessed after left lobe pneumonectomy, and results demonstrated that inhibition of FGFR2 signaling and increase in PPAR-γ signaling altered the expression of Shh, FGF, Wnt, and Bmp4, genes that are also important for epithelial-mesenchymal crosstalk during early lung development. Our data demonstrate for the first time that a comparable epithelial-mesenchymal crosstalk regulates fibroblast phenotypes during alveolar septation.

Examination of a palatogenic gene program in zebrafish

Human palatal clefting is debilitating and difficult to rectify surgically. Animal models enhance our understanding of palatogenesis and are essential in strategies designed to ameliorate palatal malformations in humans. Recent studies have shown that the zebrafish palate, or anterior neurocranium, is under similar genetic control to the amniote palatal skeleton. We extensively analyzed palatogenesis in zebrafish to determine the similarity of gene expression and function across vertebrates. By 36 hours postfertilization (hpf) palatogenic cranial neural crest cells reside in homologous regions of the developing face compared with amniote species. Transcription factors and signaling molecules regulating mouse palatogenesis are expressed in similar domains during palatogenesis in zebrafish. Functional investigation of a subset of these genes, fgf10a, tgfb2, pax9, and smad5 revealed their necessity in zebrafish palatogenesis. Collectively, these results suggest that the gene regulatory networks regulating palatogenesis may be conserved across vertebrate species, demonstrating the utility of zebrafish as a model for palatogenesis.

Blood vessels as targets in tumor therapy

The landmark papers published by Judah Folkman in the early 1970s on tumor angiogenesis and therapeutic implications promoted the rapid development of a very dynamic field where basic scientists, oncologists, and pharmaceutical industry joined forces to determine the molecular mechanisms in blood vessel formation and find means to exploit this knowledge in suppressing tumor vascularization and growth. A wealth of information has been collected on angiogenic growth factors, and in 2004 the first specific blood vessel-targeted cancer therapy was introduced: a neutralizing antibody against vascular endothelial growth factor (VEGF). Now (2011) we know that suppression of tumor angiogenesis may be a double-edged sword and that the therapy needs to be further refined and individualized. This review describes the hallmarks of tumor vessels, how different angiogenic growth factors exert their function, and the perspectives for future development of anti-angiogenic therapy.

PDGF and PDGF receptors in glioma

The family of platelet-derived growth factors (PDGFs) plays a number of critical roles in normal embryonic development, cellular differentiation, and response to tissue damage. Not surprisingly, as it is a multi-faceted regulatory system, numerous pathological conditions are associated with aberrant activity of the PDGFs and their receptors. As we and others have shown, human gliomas, especially glioblastoma, express all PDGF ligands and both the two cell surface receptors, PDGFR-α and -β. The cellular distribution of these proteins in tumors indicates that glial tumor cells are stimulated via PDGF/PDGFR-α autocrine and paracrine loops, while tumor vessels are stimulated via the PDGFR-β. Here we summarize the initial discoveries on the role of PDGF and PDGF receptors in gliomas and provide a brief overview of what is known in this field.

CXCL1 regulation of oligodendrocyte progenitor cell migration is independent of calcium signaling

Cell migration is an indispensable aspect of tissue patterning during embryonic development. Oligodendrocytes, the myelinating cells of the central nervous system, migrate significantly during development of the brain. Several growth factors have been identified as being critical regulators of oligodendrocyte progenitor migration, including platelet derived growth factor-A (PDGFA), and fibroblast growth factor-2 (FGF2). Further, the chemokine CXCL1 has been shown to play a critical role in regulating the dispersal of oligodendrocyte progenitors during development, although the mechanisms underlying this regulation are unknown. Previous studies have also shown that calcium flux is required for oligodendrocyte progenitor migration. CXCL1 induces calcium flux in cells; therefore, we hypothesized that CXCL1 inhibition of oligodendrocyte progenitor migration is regulated via changes in intracellular calcium flux. The current study shows that CXCL1 inhibition of oligodendrocyte progenitor migration is independent of calcium signaling. Further, we show that CXCL1 inhibition of oligodendrocyte progenitor migration is specific to PDGFA induced migration. Finally, we show that CXCL1 inhibition of oligodendrocyte progenitor migration is independent of activation of the cell cycle. Our results provide intriguing results relevant to specific aspects of patterning of white matter tracts in the central nervous system, and may further the understanding of tissue remodeling seen during disease-related processes.

PDGFRa mutations in humans with isolated cleft palate

Isolated cleft palate (CP) is common in humans and has complex genetic etiologies. Many genes have been found to contribute to CP, but the full spectrum of genes remains unknown. PCR-sequencing of the entire coding regions and the 3' untranslated region (UTR) of the platelet-derived growth factor receptor alpha (PDGFRa) and the microRNA (miR), miR-140 identified seven novel single base-pair substitutions in the PDGFRa in 9/102 patients with CP (8.8%), compared with 5/500 ethnic-matched unaffected controls (1%) (the two-tailed P-value<0.0001). Of these seven, four were missense mutations in the coding regions and three in the 3'UTR. Frequencies of four changes (three in coding, one in 3'UTR) were statistically different from those of controls (P-value<0.05). The c.*34G>A was identified in 1/102 cases and 0/500 controls. This position is conserved in primates and located 10 bp away from a predicted binding site for the miR-140. Luciferase assay revealed that, in the presence of miR-140, the c.*34G>A significantly repressed luciferase activity compared with that of the wild type, suggesting functional significance of this variant. This is the first study providing evidence supporting a role of PDGFRa in human CP.

Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development

Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.

An essential role for a mammalian SWI/SNF chromatin-remodeling complex during male meiosis

Germ cell development and gametogenesis require genome-wide transitions in epigenetic modifications and chromatin structure. These changes include covalent modifications to the DNA and histones as well as remodeling activities. Here, we explore the role of the mammalian SWI/SNF chromatin-remodeling complex during spermatogenesis using a conditional allele of the ATPase subunit, brahma-related gene 1 (Brg1, or Smarca4). Not only do BRG1 levels peak during the early stages of meiosis, genetic ablation of Brg1 in murine embryonic gonocytes results in arrest during prophase of meiosis I. Coincident with the timing of meiotic arrest, mutant spermatocytes accumulate unrepaired DNA and fail to complete synapsis. Furthermore, mutant spermatocytes show global alterations to histone modifications and chromatin structure indicative of a more heterochromatic genome. Together, these data demonstrate a requirement for BRG1 activity in spermatogenesis, and suggest a role for the mammalian SWI/SNF complex in programmed recombination and repair events that take place during meiosis.

Repression of PDGF-R-α after cellular injury involves TNF-α, formation of a c-Fos-YY1 complex, and negative regulation by HDAC

Wound healing is a complex dynamic process involving a variety of cell types, including fibroblasts that express and respond to cytokines and growth factors in the local microenvironment. The mechanisms controlling gene expression after injury at a transcriptional level are poorly understood. Here we show that decreased expression of a key receptor, PDGF-receptor (R)-α, after fibroblast injury is due to the release and paracrine activity of TNF-α. TNF-α inhibits PDGF-R-α expression and this involves formation of a c-Fos-Yin Yang 1 (YY1) complex and histone deacetylase (HDAC) activity. c-Fos, induced by TNF-α, negatively regulates PDGF-R-α transcription. Small interfering RNA (siRNA) targeting c-Fos or the zinc finger transcription factor YY1 inhibits TNF-α suppression of PDGF-R-α expression. Coimmunoprecipitation studies show that TNF-α stimulates the formation of a complex between c-Fos with YY1. Furthermore, chromatin immunoprecipitation (ChIP) analysis reveals the enrichment of c-Fos, YY1, and HDAC-1 at the PDGF-R-α promoter in cells exposed to TNF-α. With suberoylanilide hydroxamic acid (SAHA) and HDAC-1 siRNA, we demonstrate that HDAC mediates TNF-α repression of PDGF-R-α. These findings demonstrate that transcriptional repression of PDGF-R-α after fibroblast injury involves paracrine activity of endogenous TNF-α, the formation of a c-Fos-YY1 complex, and negative regulatory activity by HDAC.

The function of platelet-derived growth factor in the differentiation of mouse tongue striated muscle

OBJECTIVE

To determine the function of platelet-derived growth factor (PDGF) in the final differentiation phase of tongue striated muscle cells.

MATERIALS AND METHODS

We analyzed the expressions of PDGF-A, -B, platelet-derived growth factor receptor (PDGFR)-α, and PDGFR-β in mouse tongues between embryonic days (E) 11 and 15. Furthermore, we examined the effects of human recombinant PDGF-AB and the peptide antagonist for PDGFRs using an organ culture system of mouse embryonic tongue. Mouse tongues at E12 were cultured in BGJb medium containing human recombinant PDGF-AB for 4 days or the peptide antagonist for PDGF receptors for 8 days.

RESULTS

PDGF-A, -B, PDGFR-α, and -β were expressed in the differentiating muscle cells between E11 and 15. The human recombinant PDGF-AB induced increases in the mRNA expressions of myogenin and muscle creatine kinase (MCK) and the number of fast myosin heavy chain (fMHC)-positive cells, markers for the differentiation of muscle cells. On the other hand, the peptide antagonist for PDGFRs induced suppressions in the mRNA expressions of myogenin and MCK, and the number of fMHC-positive cells. Both the PDGF-AB and the antagonist failed to affect the expressions of cell proliferation markers.

CONCLUSION

These results suggest that PDGF functions as a positive regulator in the final differentiation phase of tongue muscle cells in mouse embryos.

Expression of Neurog1 instead of Atoh1 can partially rescue organ of Corti cell survival

In the mammalian inner ear neurosensory cell fate depends on three closely related transcription factors, Atoh1 for hair cells and Neurog1 and Neurod1 for neurons. We have previously shown that neuronal cell fate can be altered towards hair cell fate by eliminating Neurod1 mediated repression of Atoh1 expression in neurons. To test whether a similar plasticity is present in hair cell fate commitment, we have generated a knockin (KI) mouse line (Atoh1^KINeurog1) in which Atoh1 is replaced by Neurog1. Expression of Neurog1 under Atoh1 promoter control alters the cellular gene expression pattern, differentiation and survival of hair cell precursors in both heterozygous (Atoh1^+/KINeurog1) and homozygous (Atoh1^{KINeurog1/KINeurog1}) KI mice. Homozygous KI mice develop patches of organ of Corti precursor cells that express Neurog1, Neurod1, several prosensory genes and neurotrophins. In addition, these patches of cells receive afferent and efferent processes. Some cells among these patches form multiple microvilli but no stereocilia. Importantly, Neurog1 expressing mutants differ from Atoh1 null mutants, as they have intermittent formation of organ of Corti-like patches, opposed to a complete ‘flat epithelium’ in the absence of Atoh1. In heterozygous KI mice co-expression of Atoh1 and Neurog1 results in change in fate and patterning of some hair cells and supporting cells in addition to the abnormal hair cell polarity in the later stages of development. This differs from haploinsufficiency of Atoh1 (Pax2cre; Atoh1^f/+), indicating the effect of Neurog1 expression in developing hair cells. Our data suggest that Atoh1^KINeurog1 can provide some degree of functional support for survival of organ of Corti cells. In contrast to the previously demonstrated fate plasticity of neurons to differentiate as hair cells, hair cell precursors can be maintained for a limited time by Neurog1 but do not transdifferentiate as neurons.

Neural crest delamination and migration: from epithelium-to-mesenchyme transition to collective cell migration

After induction and specification in the ectoderm, at the border of the neural plate, the neural crest (NC) population leaves its original territory through a delamination process. Soon afterwards, the NC cells migrate throughout the embryo and colonize a myriad of tissues and organs where they settle and differentiate. The delamination involves a partial or complete epithelium-to-mesenchyme transition (EMT) regulated by a complex network of transcription factors including several proto-oncogenes. Studying the relationship between these genes at the time of emigration, and their individual or collective impact on cell behavior, provides valuable information about their role in EMT in other contexts such as cancer metastasis. During migration, NC cells are exposed to large number of positive and negative regulators that control where they go by generating permissive and restricted areas and by modulating their motility and directionality. In addition, as most NC cells migrate collectively, cell-cell interactions play a crucial role in polarizing the cells and interpreting external cues. Cell cooperation eventually generates an overall polarity to the population, leading to directional collective cell migration. This review will summarize our current knowledge on delamination, EMT and migration of NC cells using key examples from chicken, Xenopus, zebrafish and mouse embryos. Given the similarities between neural crest migration and cancer invasion, these cells may represent a useful model for understanding the mechanisms of metastasis.

Platelet-derived growth factor C deficiency in C57BL/6 mice leads to abnormal cerebral vascularization, loss of neuroependymal integrity, and ventricular abnormalities

Platelet-derived growth factors (PDGFs) and their tyrosine kinase receptors (PDGFRs) are known to play important roles during development of the lungs, central nervous system (CNS), and skeleton and in several diseases. PDGF-C is a ligand for the tyrosine kinase receptor PDGFRα. Mutations in the gene encoding PDGF-C have been linked to clefts of the lip and/or palate in humans, and ablation of PDGF-C in 129/Sv background mice results in death during the perinatal period. In this study, we report that ablation of PDGF-C in C57BL/6 mice results in a milder phenotype than in 129/Sv mice, and we present a phenotypic characterization of PDGF-C deficiency in the adult murine CNS. Multiple congenital defects were observed in the CNS of PDGF-C-null C57BL/6 mice, including cerebral vascular abnormalities with abnormal vascular smooth muscle cell coverage. In vivo imaging of mice deficient in PDGF-C also revealed cerebral ventricular abnormalities, such as asymmetry of the lateral ventricles and hypoplasia of the septum, reminiscent of cavum septum pellucidum in humans. We further noted that PDGF-C-deficient mice displayed a distorted ependymal lining of the lateral ventricles, and we found evidence of misplaced neurons in the ventricular lining. We conclude that PDGF-C plays a critical role in the development of normal cerebral ventricles and neuroependymal integrity as well as in normal cerebral vascularization.

Shared molecular targets in pediatric gliomas and ependymomas

BACKGROUND

Recent advances in multidisciplinary treatment approaches have improved the overall prognosis of pediatric brain tumors, but some patients remain refractory to treatment and do poorly. Several molecularly targeted therapies are under development for the treatment of brain tumors, and high-grade gliomas in adults are a particular area of study.

PROCEDURE

To better understand if these new therapies can be used in pediatric populations, we examined the expression of the following seven marker genes involved in signaling pathways targeted by new therapies: β-catenin, suppressor of fused (SUFU), erythroblastic leukemia viral oncogene homolog (ERBB) 2, platelet-derived growth factor receptorα (PDGFRα), proliferating cell nuclear antigen (PCNA), secreted protein acid and rich in cysteine (SPARC), and granulocyte colony-stimulating factor receptor (G-CSFR). Samples from 27 patients with the primitive neuroectodermal tumor (PNET)/medulloblastomas (MBs) (n = 8), ependymomas (n = 5), or gliomas (n = 14) were assessed by quantitative real-time PCR. [Correction made here after initial online publication]. We assigned an EXP score to compare across samples and determined the levels of gene expression among tumor cell types.

RESULTS

Gene expression varied among the different tumors, but, within a tumor type, clear expression patterns were seen. The expression of SUFU, ERBB2, and PCNA in metastatic MBs were greater than that seen in non-metastatic MBs. Most glioma cases highly expressed PDGFRα and G-CSFR. Additionally, the expression patterns of gliomas and ependymomas were similar (r = 0.77, P = 0.04), but PNET/MBs substantially differed from gliomas (r = -0.37, P = 0.41) or ependymomas (r = 0.23, P = 0.62).

CONCLUSIONS

The development of new drugs targeting up-regulated pathways may be useful for the treatment of pediatric brain tumors. As new drugs are developed, gliomas and ependymomas may be treated with similar compounds.

Mechanisms of thymus organogenesis and morphogenesis

The thymus is the primary organ responsible for generating functional T cells in vertebrates. Although T cell differentiation within the thymus has been an area of intense investigation, the study of thymus organogenesis has made slower progress. The past decade, however, has seen a renewed interest in thymus organogenesis, with the aim of understanding how the thymus develops to form a microenvironment that supports T cell maturation and regeneration. This has prompted modern revisits to classical experiments and has driven additional genetic approaches in mice. These studies are making significant progress in identifying the molecular and cellular mechanisms that control specification, early organogenesis and morphogenesis of the thymus.

Pdgfrα and Flk1 are direct target genes of Mixl1 in differentiating embryonic stem cells

The Mixl1 homeodomain protein plays a key role in mesendoderm patterning during embryogenesis, but its target genes remain to be identified. We compared gene expression in differentiating heterozygous Mixl1(GFP/w) and homozygous null Mixl1(GFP/Hygro) mouse embryonic stem cells to identify potential downstream transcriptional targets of Mixl1. Candidate Mixl1 regulated genes whose expression was reduced in GFP+ cells isolated from differentiating Mixl1(GFP/Hygro) embryoid bodies included Pdgfrα and Flk1. Mixl1 bound to ATTA sequences located in the Pdgfrα and Flk1 promoters and chromatin immunoprecipitation assays confirmed Mixl1 occupancy of these promoters in vivo. Furthermore, Mixl1 transactivated the Pdgfrα and Flk1 promoters through ATTA sequences in a DNA binding dependent manner. These data support the hypothesis that Mixl1 directly regulates Pdgfrα and Flk1 gene expression and strengthens the position of Mixl1 as a key regulator of mesendoderm development during mammalian gastrulation.

Hepatocyte growth factor is crucial for development of the carapace in turtles

Turtles are characterized by their shell, composed of a dorsal carapace and a ventral plastron. The carapace first appears as the turtle-specific carapacial ridge (CR) on the lateral aspect of the embryonic flank. Accompanying the acquisition of the shell, unlike in other amniotes, hypaxial muscles in turtle embryos appear as thin threads of fibrous tissue. To understand carapacial evolution from the perspective of muscle development, we compared the development of the muscle plate, the anlage of hypaxial muscles, between the Chinese soft-shelled turtle, Pelodiscus sinensis, and chicken embryos. We found that the ventrolateral lip (VLL) of the thoracic dermomyotome of P. sinensis delaminates early and produces sparse muscle plate in the lateral body wall. Expression patterns of the regulatory genes for myotome differentiation, such as Myf5, myogenin, Pax3, and Pax7 have been conserved among amniotes, including turtles. However, in P. sinensis embryos, the gene hepatocyte growth factor (HGF), encoding a regulatory factor for delamination of the dermomyotomal VLL, was uniquely expressed in sclerotome and the lateral body wall at the interlimb level. Implantation of COS-7 cells expressing a HGF antagonist into the turtle embryo inhibited CR formation. We conclude that the de novo expression of HGF in the turtle mesoderm would have played an innovative role resulting in the acquisition of the turtle-specific body plan.

A boundary element between Tsix and Xist binds the chromatin insulator Ctcf and contributes to initiation of X-chromosome inactivation

In mammals, X-chromosome inactivation (XCI) equalizes X-linked gene expression between XY males and XX females and is controlled by a specialized region known as the X-inactivation center (Xic). The Xic harbors two chromatin interaction domains, one centered around the noncoding Xist gene and the other around the antisense Tsix counterpart. Previous work demonstrated the existence of a chromatin transitional zone between the two domains. Here, we investigate the region and discover a conserved element, RS14, that presents a strong binding site for Ctcf protein. RS14 possesses an insulatory function suggestive of a boundary element and is crucial for cell differentiation and growth. Knocking out RS14 results in compromised Xist induction and aberrant XCI in female cells. These data demonstrate that a junction element between Tsix and Xist contributes to the initiation of XCI.

PDGF function in diverse neural crest cell populations

Activation of platelet derived growth factor (PDGF) receptors causes context-dependent cellular responses, including proliferation and migration, and studies in model organisms have demonstrated that this receptor family (PDGFRα and PDGFRβ) is required in many mesenchymal and migratory cell populations during embryonic development. One of these migratory cell populations is the neural crest, which forms cranial bone and mesenchyme, sympathetic neurons and ganglia, melanocytes, and smooth muscle. Mice with disruption of PDGF signaling exhibit defects in some of these neural crest derivatives including the palate, aortic arch, salivary gland, and thymus. Although many of these neural crest defects were identified many years ago, the mechanism of action of PDGF in neural crest remains controversial. In this review, we examine the current knowledge of PDGF function during neural crest cell (NCC) development, focusing on its role in the formation of different neural crest-derived tissues and the implications for PDGF receptors in NCC-related human birth defects.

Differential effects of growth factors on oligodendrocyte progenitor migration

Oligodendrocytes are myelinating cells of the CNS that originate as progenitor cells (OP) in discrete areas of the developing brain. During brain development, OP migrate significant distances prior to proliferating and myelinating the axons of the putative white matter tracts. Growth factors play a major regulatory role in the behavior of OP. Specifically, platelet-derived growth factor A (PDGF-A) and fibroblast growth factor 2 (FGF2) are two of the most well characterized regulators of OP development. Both growth factors interact with tyrosine kinase receptors, activating various intracellular signaling pathways. The current study advances our earlier research by comparing the effects of both PDGF-A and FGF2 on OP migration. Our results show that activation of ERK is required for OP migration. These findings correlate well with our previous demonstration of the ERK pathway mediating PDGF-A induced OP migration. We also demonstrate the significance of threshold levels of growth factors and temporal regulation for OP migration. In addition, ERK activation alone is not sufficient to induce OP migration. The current research supports the involvement of the non-ERK mediated signaling pathway in OP migration.

Notochordal and foregut abnormalities correlate with elevated neural crest apoptosis in Patch embryos

Although Patch mutants show severe abnormalities in many neural crest-derived structures including the face and the heart, there is a paucity of information characterizing the mechanisms underlying these congenital defects. Via manipulating the genetic background to circumvent early embryonic lethality, our results revealed that Patch phenotypes are most likely due to a significant decrease in migratory neural crest lineage due to diminished neural crest survival and elevated apoptosis. Homozygous mutant neural crest precursors can undergo typical expansion within the neural tube, epithelial-to-mesenchymal transformation, and initiate normal neural crest emigration. Moreover, in vitro explant culture demonstrated that when isolated from the surrounding mesenchyme, Patch mutant neural crest cells (NCCs) can migrate appropriately. Additionally, Patch foregut, notochord and somitic morphogenesis, and Sonic hedgehog expression profiles were all perturbed. Significantly, the timing of lethality and extent of apoptosis correlated with the degree of severity of Patch mutant foregut, notochord, and somite dysfunction. Finally, analysis of Balb/c-enriched surviving Patch mutants revealed that not all the neural crest subpopulations are affected and that Patch mutant neural crest-derived sympathetic ganglia and dorsal root ganglia were unaffected. We hypothesize that loss of normal coordinated signaling from the notochord, foregut, and somites underlies the diminished survival of the neural crest lineage within Patch mutants resulting in subsequent neural crest-deficient phenotypes.

Genetics of nonsyndromic orofacial clefts

With an average worldwide prevalence of approximately 1.2/1000 live births, orofacial clefts are the most common craniofacial birth defects in humans. Like other complex disorders, these birth defects are thought to result from the complex interplay of multiple genes and environmental factors. Significant progress in the identification of underlying genes and pathways has benefited from large populations available for study, increased international collaboration, rapid advances in genotyping technology, and major improvements in analytic approaches. Here we review recent advances in genetic epidemiological approaches to complex traits and their applications to studies of nonsyndromic orofacial clefts. Our main aim is to bring together a discussion of new and previously identified candidate genes to create a more cohesive picture of interacting pathways that shape the human craniofacial region. In future directions, we highlight the need to search for copy number variants that affect gene dosage and rare variants that are possibly associated with a higher disease penetrance. In addition, sequencing of protein-coding regions in candidate genes and screening for genetic variation in noncoding regulatory elements will help advance this important area of research.

Epicardial-derived cell epithelial-to-mesenchymal transition and fate specification require PDGF receptor signaling

RATIONALE

In early heart development, platelet-derived growth factor (PDGF) receptor expression in the heart ventricles is restricted to the epicardium. Previously, we showed that PDGFRβ is required for coronary vascular smooth muscle cell (cVSMC) development, but a role for PDGFRα has not been identified. Therefore, we investigated the combined and independent roles of these receptors in epicardial development.

OBJECTIVE

To understand the contribution of PDGF receptors in epicardial development and epicardial-derived cell fate determination.

METHODS AND RESULTS

By generating mice with epicardial-specific deletion of the PDGF receptors, we found that epicardial epithelial-to-mesenchymal transition (EMT) was defective. Sox9, an SRY-related transcription factor, was reduced in PDGF receptor-deficient epicardial cells, and overexpression of Sox9 restored epicardial migration, actin reorganization, and EMT gene expression profiles. The failure of epicardial EMT resulted in hearts that lacked epicardial-derived cardiac fibroblasts and cVSMC. Loss of PDGFRα resulted in a specific disruption of cardiac fibroblast development, whereas cVSMC development was unperturbed.

CONCLUSIONS

Signaling through both PDGF receptors is necessary for epicardial EMT and formation of epicardial-mesenchymal derivatives. PDGF receptors also have independent functions in the development of specific epicardial-derived cell fates.

The emerging face of primary cilia

Primary cilia are microtubule-based organelles that serve as hubs for the transduction of various developmental signaling pathways including Hedgehog, Wnt, FGF, and PDGF. Ciliary dysfunction contributes to a range of disorders, collectively known as the ciliopathies. Recently, interest has grown in these syndromes, particularly among craniofacial biologists, as many known and putative ciliopathies have severe craniofacial defects. Herein we discuss the current understanding of ciliary biology and craniofacial development in an attempt to gain insight into the molecular etiology for craniofacial ciliopathies, and uncover a characteristic ciliopathic craniofacial gestalt.

Gsk3β is required in the epithelium for palatal elevation in mice

In Wnt/β-catenin signaling pathway, Gsk3β functions to facilitate β-catenin degradation. Inactivation of Gsk3β in mice causes a cleft palate formation, suggesting an involvement of Wnt/β-catenin signaling during palatogenesis. In this study, we have investigated the expression pattern, tissue-specific requirement and function of Gsk3β during mouse palatogenesis. We showed that Gsk3β is primarily expressed in the palatal epithelium, particularly in the medial edge epithelium overlapping with β-catenin. Tissue-specific gene inactivation studies demonstrated an essential role for Gsk3β in the epithelium for palate elevation, and disruption of which contributes to cleft palate phenotype in Gsk3β mutant. We observed that expression of Aixn2, a direct target gene of Wnt/β-catenin signaling, is ectopically activated in the mutant tongue, but not in the palate. Our results indicate that Gsk3β is an intrinsic regulator required in the epithelium for palate elevation, and could act through a pathway independent of Wnt/β-catenin signaling to regulate palate development.

Increased proteolytic processing of full-length Gli2 transcription factor reduces the hedgehog pathway activity in vivo

The proteolytic processing of Gli2 and Gli3 full-length transcription factors into repressors is a key step of the regulation in Hedgehog (Hh) signaling. The differential Gli2 and Gli3 processing is controlled by the processing determinant domain or PDD, but its significance is not clear. We generated a Gli2 mutant allele, Gli2(3PDD) , in which the Gli3PDD substitutes for the Gli2PDD. As expected, Gli2(3PDD) is processed more efficiently and at a different position as compared to Gli2, indicating that PDD also determines the extent and site of Gli2 and Gli3 processing in vivo. The increase in levels of the Gli2 repressor in Gli2(3PDD) mutant reduces the Hh pathway activity. Gli2(3PDD) processing is still regulated by Hh signaling. These results indicate that the proper balance between the Gli2 full-length activator and repressor is essential for Hh signaling.

PDGF-A controls mesoderm cell orientation and radial intercalation during Xenopus gastrulation

Radial intercalation is a common, yet poorly understood, morphogenetic process in the developing embryo. By analyzing cell rearrangement in the prechordal mesoderm during Xenopus gastrulation, we have identified a mechanism for radial intercalation. It involves cell orientation in response to a long-range signal mediated by platelet-derived growth factor (PDGF-A) and directional intercellular migration. When PDGF-A signaling is inhibited, prechordal mesoderm cells fail to orient towards the ectoderm, the endogenous source of PDGF-A, and no longer migrate towards it. Consequently, the prechordal mesoderm fails to spread during gastrulation. Orientation and directional migration can be rescued specifically by the expression of a short splicing isoform of PDGF-A, but not by a long matrix-binding isoform, consistent with a requirement for long-range signaling.

The CNS microvascular pericyte: pericyte-astrocyte crosstalk in the regulation of tissue survival

The French scientist Charles Benjamin Rouget identified the pericyte nearly 140 years ago. Since that time the role of the pericyte in vascular function has been difficult to elucidate. It was not until the development of techniques to isolate and culture pericytes that scientists have begun to understand the true impact of this unique cell in the maintenance of tissue homeostasis. In the brain the pericyte is an integral cellular component of the blood-brain barrier and, together with other cells of the neurovascular unit (endothelial cells, astrocytes and neurons) the pericyte makes fine-tuned regulatory adjustments and adaptations to promote tissue survival. These regulatory changes involve trans-cellular communication networks between cells. In this review we consider evidence for cell-to-cell crosstalk between pericytes and astrocytes during development and in adult brain.

Targeting vector construction through recombineering

Gene targeting in mouse embryonic stem cells is an essential, yet still very expensive and highly time-consuming, tool and method to study gene function at the organismal level or to create mouse models of human diseases. Conventional cloning-based methods have been largely used for generating targeting vectors, but are hampered by a number of limiting factors, including the variety and location of restriction enzymes in the gene locus of interest, the specific PCR amplification of repetitive DNA sequences, and cloning of large DNA fragments. Recombineering is a technique that exploits the highly efficient homologous recombination function encoded by λ phage in Escherichia coli. Bacteriophage-based recombination can recombine homologous sequences as short as 30-50 bases, allowing manipulations such as insertion, deletion, or mutation of virtually any genomic region. The large availability of mouse genomic bacterial artificial chromosome (BAC) libraries covering most of the genome facilitates the retrieval of genomic DNA sequences from the bacterial chromosomes through recombineering. This chapter describes a successfully applied protocol and aims to be a detailed guide through the steps of generation of targeting vectors through recombineering.

Evaluation of 64 candidate single nucleotide polymorphisms as risk factors for neural tube defects in a large Irish study population

Individual studies of the genetics of neural tube defects (NTDs) contain results on a small number of genes in each report. To identify genetic risk factors for NTDs, we evaluated potentially functional single nucleotide polymorphisms (SNPs) that are biologically plausible risk factors for NTDs but that have never been investigated for an association with NTDs, examined SNPs that previously showed no association with NTDs in published studies, and tried to confirm previously reported associations in folate-related and non-folate-related genes. We investigated 64 SNPs in 34 genes for association with spina bifida in up to 558 case families (520 cases, 507 mothers, 457 fathers) and 994 controls in Ireland. Case-control and mother-control comparisons of genotype frequencies, tests of transmission disequilibrium, and log-linear regression models were used to calculate effect estimates. Spina bifida was associated with over-transmission of the LEPR (leptin receptor) rs1805134 minor C allele [genotype relative risk (GRR): 1.5; 95% confidence interval (CI): 1.0-2.1; P = 0.0264] and the COMT (catechol-O-methyltransferase) rs737865 major T allele (GRR: 1.4; 95% CI: 1.1-2.0; P = 0.0206). After correcting for multiple comparisons, these individual test P-values exceeded 0.05. Consistent with previous reports, spina bifida was associated with MTHFR 677C>T, T (Brachyury) rs3127334, LEPR K109R, and PDGFRA promoter haplotype combinations. The associations between LEPR SNPs and spina bifida suggest a possible mechanism for the finding that obesity is a NTD risk factor. The association between a variant in COMT and spina bifida implicates methylation and epigenetics in NTDs.

Time-specific blockade of PDGFR with Imatinib (Glivec®) causes cataract and disruption of lens fiber cells in neonatal mice

This study aimed at investigating the response of lens epithelial cells in postnatal mice to Imatinib (Glivec®, a potent inhibitor of platelet-derived growth factor receptor (PDGFR)) treatment. Mouse eyes were sampled 10 days after administration of Imatinib (0.5 mg·g(-1)·day(-1)) for 3 days, at either 7, 14, or 21 days postpartum. Structural changes of lens were revealed by routine H.E. staining. Levels of proliferation and apoptosis were revealed by BrdU incorporation and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, respectively, and immunofluorescent staining with anti-PDGFRα antibody was carried out on the sections of eyeball. PDGFRα and p-PDGFRαprotein levels were evaluated by Western blot. Our results indicated that administration of Imatinib led to blockade of PDGFR signaling. Formation of cataracts was found only in those mice where treatment started from 7 days postpartum (P7), but was not observed in those samples from P14 nor P21. Fiber cells were disorganized in cataract lens core as observed histologically, and migration of epithelial cells was also inhibited. No apoptosis was detected with the TUNEL method. Our results indicated blockade of PDGFR at the neonatal stage (P7) would lead to cataracts and lens fiber cells disorganization, suggesting that PDGFR signaling plays a time-specific and crucial role in the postnatal development of lens in the mouse, and also may provide a new approach to produce a congenital cataract animal model.

Role of platelet-derived growth factors in the testis

Normal development and function of the testis are controlled by endocrine and paracrine signaling pathways. Platelet-derived growth factors (PDGFs) are growth factors that mediate epithelial-mesenchymal interactions in various tissues during normal and abnormal processes such as embryo development, wound healing, tissue fibrosis, vascular disorders, and cancer. PDGFs and their receptors (PDGFRs) have emerged as key players in the regulation of embryonic and postnatal development of the male gonad. Cells that express PDGFs and PDGFRs are found in the testis of mammals, birds, and reptiles, and their distribution, regulation, and function vary across species. Testicular PDGFs and PDGFRs appear after the process of sex determination in animals that use either genetic sex determination or environmental sex determination. Sertoli cells are the main PDGF-producing cells during the entire period of prenatal and postnatal testis development. Fetal Leydig cells and their precursors, adult Leydig cells and their stem cell precursors, peritubular myoid cells, cells of the blood vessels, and gonocytes are the testicular cell types expressing PDGFRs. Genetically targeted deletions of PDGFs, PDGFRs, PDGFR target genes or pharmacological silencing of PDGF signaling produce profound damage on the target cells that, depending on the developmental period, are under direct or indirect control of PDGF. PDGF signaling may also serve diverse functions outside of the realm of testis development, including testicular tumors. In this review, we provide a framework of the current knowledge to clarify the useful information regarding how PDGFs function in individual cells of the testis.

Cardiac malformations in Pdgfralpha mutant embryos are associated with increased expression of WT1 and Nkx2.5 in the second heart field

Platelet-derived growth factor receptor alpha (Pdgfralpha) identifies cardiac progenitor cells in the posterior part of the second heart field. We aim to elucidate the role of Pdgfralpha in this region. Hearts of Pdgfralpha-deficient mouse embryos (E9.5-E14.5) showed cardiac malformations consisting of atrial and sinus venosus myocardium hypoplasia, including venous valves and sinoatrial node. In vivo staining for Nkx2.5 showed increased myocardial expression in Pdgfralpha mutants, confirmed by Western blot analysis. Due to hypoplasia of the primary atrial septum, mesenchymal cap, and dorsal mesenchymal protrusion, the atrioventricular septal complex failed to fuse. Impaired epicardial development and severe blebbing coincided with diminished migration of epicardium-derived cells and myocardial thinning, which could be linked to increased WT1 and altered alpha4-integrin expression. Our data provide novel insight for a possible role for Pdgfralpha in transduction pathways that lead to repression of Nkx2.5 and WT1 during development of posterior heart field-derived cardiac structures.

Zebrafish wnt9a is expressed in pharyngeal ectoderm and is required for palate and lower jaw development

Wnt activity is critical in craniofacial morphogenesis. Dysregulation of Wnt/β-catenin signaling results in significant alterations in the facial form, and has been implicated in cleft palate phenotypes in mouse and man. In zebrafish, we show that wnt9a is expressed in the pharyngeal arch, oropharyngeal epithelium that circumscribes the ethmoid plate, and ectodermal cells superficial to the lower jaw structures. Alcian blue staining of morpholino-mediated knockdown of wnt9a results in loss of the ethmoid plate, absence of lateral and posterior parachordals, and significant abrogation of the lower jaw structures. Analysis of cranial neural crest cells in the sox10:eGFP transgenic demonstrates that the wnt9a is required early during pharyngeal development, and confirms that the absence of Alcian blue staining is due to absence of neural crest derived chondrocytes. Molecular analysis of genes regulating cranial neural crest migration and chondrogenic differentiation suggest that wnt9a is dispensable for early cranial neural crest migration, but is required for chondrogenic development of major craniofacial structures. Taken together, these data corroborate the central role for Wnt signaling in vertebrate craniofacial development, and reveal that wnt9a provides the signal from the pharyngeal epithelium to support craniofacial chondrogenic morphogenesis in zebrafish.

A role for PDGF signaling in expansion of the extra-embryonic endoderm lineage of the mouse blastocyst

The inner cell mass (ICM) of the implanting mammalian blastocyst comprises two lineages: the pluripotent epiblast (EPI) and primitive endoderm (PrE). We have identified platelet-derived growth factor receptor alpha (PDGFRα) as an early marker of the PrE lineage and its derivatives in both mouse embryos and ex vivo paradigms of extra-embryonic endoderm (ExEn). By combining live imaging of embryos and embryo-derived stem cells expressing a histone H2B-GFP fusion reporter under the control of Pdgfra regulatory elements with the analysis of lineage-specific markers, we found that Pdgfra expression coincides with that of GATA6, the earliest expressed transcriptional regulator of the PrE lineage. We show that GATA6 is required for the activation of Pdgfra expression. Using pharmacological inhibition and genetic inactivation we addressed the role of the PDGF pathway in the PrE lineage. Our results demonstrate that PDGF signaling is essential for the establishment, and plays a role in the proliferation, of XEN cells, which are isolated from mouse blastocyst stage embryos and represent the PrE lineage. Implanting Pdgfra mutant blastocysts exhibited a reduced number of PrE cells, an effect that was exacerbated by delaying implantation. Surprisingly, we also noted an increase in the number of EPI cells in implantation-delayed Pdgfra-null mutants. Taken together, our data suggest a role for PDGF signaling in the expansion of the ExEn lineage. Our observations also uncover a possible role for the PrE in regulating the size of the pluripotent EPI compartment.

Distribution and possible role of PDGF-AA and PDGFR-alpha in the gastrointestinal tract of adult guinea pigs

It was reported that a signaling pathway of platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) played a critical role in the developing gut of mice. Overexpression of the PDGFR-alpha gene in gastrointestinal stromal tumors (GISTs) indicated that parts of tumor cells originated from PDGFR-alpha-positive cells, but a more detailed distribution of PDGFR-alpha and possible role in the adult mammalian gut are still unclear. In the present study, we examined the expression of both PDGF-AA and its receptor PDGFR-alpha in the gastrointestinal (GI) tract of adult guinea pigs using western blotting and immunohistochemistry. PDGF-AA-immunoreactive cells were mainly distributed in the mucosal epithelium of the stomach, small intestine, and large intestine. Only a few PDGF-AA-positive cells were seen in the longitudinal muscle layer of the large intestine. In contrast, PDGFR-alpha-positive cells were widely distributed throughout the GI tract, including the lamina propria, muscular layer, and subserosa. Double staining showed that the distribution of the PDGFR-alpha-positive cells in the muscular layer were similar to those of the interstitial cells of Cajal (ICCs), and they were associated with ICCs and enteric nerves, but no double-labeled cells were observed by anti-PDGFR or Kit antibody. It was noted that PDGFR-alpha-positive cells were also stained with a vimentin monoclonal antibody. Based on the double staining and morphological features, we consider the PDGFR-alpha-positive cells belong to a subtype of fibroblast. Our results not only provide a roadmap for understanding the function of the PDGF/PDGFR signaling pathway in both normal adult mammals and during gut injury and repair but also might help in understanding the growth and development of GISTs in the clinic.

Inactivation of Six2 in mouse identifies a novel genetic mechanism controlling development and growth of the cranial base

The cranial base is essential for integrated craniofacial development and growth. It develops as a cartilaginous template that is replaced by bone through the process of endochondral ossification. Here, we describe a novel and specific role for the homeoprotein Six2 in the growth and elongation of the cranial base. Six2-null newborn mice display premature fusion of the bones in the cranial base. Chondrocyte differentiation is abnormal in the Six2-null cranial base, with reduced proliferation and increased terminal differentiation. Gain-of-function experiments indicate that Six2 promotes cartilage development and growth in other body areas and appears therefore to control general regulators of chondrocyte differentiation. Our data indicate that the main factors restricting Six2 function to the cranial base are tissue-specific transcription of the gene and compensatory effects of other Six family members. The comparable expression during human embryogenesis and the high protein conservation from mouse to human implicate SIX2 loss-of-function as a potential congenital cause of anterior cranial base defects in humans.

Hox genes and regional patterning of the vertebrate body plan

Several decades have passed since the discovery of Hox genes in the fruit fly Drosophila melanogaster. Their unique ability to regulate morphologies along the anteroposterior (AP) axis (Lewis, 1978) earned them well-deserved attention as important regulators of embryonic development. Phenotypes due to loss- and gain-of-function mutations in mouse Hox genes have revealed that the spatio-temporally controlled expression of these genes is critical for the correct morphogenesis of embryonic axial structures. Here, we review recent novel insight into the modalities of Hox protein function in imparting specific identity to anatomical regions of the vertebral column, and in controlling the emergence of these tissues concomitantly with providing them with axial identity. The control of these functions must have been intimately linked to the shaping of the body plan during evolution.

Evidence for a myotomal Hox/Myf cascade governing nonautonomous control of rib specification within global vertebral domains

Hox genes are essential for the patterning of the axial skeleton. Hox group 10 has been shown to specify the lumbar domain by setting a rib-inhibiting program in the presomitic mesoderm (PSM). We have now produced mice with ribs in every vertebra by ectopically expressing Hox group 6 in the PSM, indicating that Hox genes are also able to specify the thoracic domain. We show that the information provided by Hox genes to specify rib-containing and rib-less areas is first interpreted in the myotome through the regional-specific control of Myf5 and Myf6 expression. This information is then transmitted to the sclerotome by a system that includes FGF and PDGF signaling to produce vertebrae with or without ribs at different axial levels. Our findings offer a new perspective of how Hox genes produce global patterns in the axial skeleton and support a redundant nonmyogenic role of Myf5 and Myf6 in rib formation.

The primary cilium: a signalling centre during vertebrate development

The primary cilium has recently stepped into the spotlight, as a flood of data show that this organelle has crucial roles in vertebrate development and human genetic diseases. Cilia are required for the response to developmental signals, and evidence is accumulating that the primary cilium is specialized for hedgehog signal transduction. The formation of cilia, in turn, is regulated by other signalling pathways, possibly including the planar cell polarity pathway. The cilium therefore represents a nexus for signalling pathways during development. The connections between cilia and developmental signalling have begun to clarify the basis of human diseases associated with ciliary dysfunction.

Rationale for the development of IMC-3G3, a fully human immunoglobulin G subclass 1 monoclonal antibody targeting the platelet-derived growth factor receptor alpha

A large body of evidence suggests that the platelet-derived growth factor (PDGF) family and associated receptors are potential targets in oncology therapeutic development because of their critical roles in the proliferation and survival of various cancers and in the regulation and growth of the tumor stroma and blood vessels. Several small molecules that nonspecifically target the PDGF signaling axis are in current use or development as anticancer therapies. However, for the majority of these agents, PDGF and its receptors are neither the primary targets nor the principal mediators of anticancer activity. IMC-3G3, a fully human monoclonal antibody of the immunoglobulin G subclass 1, specifically binds to the human PDGF receptor alpha (PDGFRalpha) with high affinity and blocks PDGF ligand binding and PDGFRalpha activation. The results of preclinical studies and the frequent expression of PDGFRalpha in many types of cancer and in cancer-associated stroma support a rationale for the clinical development of IMC-3G3. Currently, IMC-3G3 is being evaluated in early clinical development for patients with several types of solid malignancies.

PDGFs regulate tooth germ proliferation and ameloblast differentiation

OBJECTIVE

The purpose of this study was to elucidate the effects of platelet-derived growth factors (PDGFs) during tooth development, as well as the mechanisms underlying the interactions of growth factors with PDGF signalling during odontogenesis.

DESIGN

We used an ex vivo tooth germ organ culture system and two dental cell lines, SF2 cells and mDP cells, as models of odontogenesis. AG17, a tyrosine kinase inhibitor, was utilised for blocking PDGF receptor signalling. To analyse the expressions of PDGFs, reverse transcriptase (RT)-PCR and immunohistochemistry were performed. Proliferation was examined using a BrdU incorporation assay for the organ cultures and a cell counting kit for the cell lines. The expressions of Fgf2 and ameloblastin were analysed by real-time RT-PCR.

RESULTS

The PDGF ligands PDGF-A and PDGF-B, and their receptors, PDGFRalpha and PDGFRbeta, were expressed throughout the initial stages of tooth development. In the tooth germ organ cultures, PDGF-AA, but not PDGF-BB, accelerated cusp formation. Conversely, AG17 suppressed both growth and cusp formation of tooth germs. Exogenous PDGF-BB promoted mDP cell proliferation. Furthermore, PDGF-AA decreased Fgf2 expression and increased that of ameloblastin, a marker of differentiated ameloblasts.

CONCLUSION

Our results indicate that PDGFs are involved in initial tooth development and regulate tooth size and shape, as well as ameloblast differentiation.

Neuropilin-1 regulates platelet-derived growth factor receptor signalling in mesenchymal stem cells

Using human MSCs (mesenchymal stem cells) lacking VEGF (vascular endothelial growth factor) receptors, we show that the pro-angiogenic receptor neuropilin-1 associates with phosphorylated PDGFRs [PDGF (platelet-derived growth factor) receptors], thereby regulating cell signalling, migration, proliferation and network assembly. Neuropilin-1 co-immunoprecipitated and co-localized with phosphorylated PDGFRs in the presence of growth factors. Neuropilin-1 knockdown blocked PDGF-AA-induced PDGFRalpha phosphorylation and migration, reduced PDGF-BB-induced PDGFRbeta activation and migration, blocked VEGF-A activation of both PDGFRs, and attenuated proliferation. Neuropilin-1 prominently co-localized with both PDGFRs within MSC networks assembled in Matrigel and in the chorioallantoic membrane vasculature microenvironment, and its knockdown grossly disrupted network assembly and decreased PDGFR signalling. Thus neuropilin-1 regulates MSCs by forming ligand-specific receptor complexes that direct PDGFR signalling, especially the PDGFRalpha homodimer. This receptor cross-talk may control the mobilization of MSCs in neovascularization and tissue remodelling.

CML in pregnancy and childhood

With the improved survivals offered by the tyrosine kinase inhibitors has come the necessity to address issues relating to quality of life and one such area is that of fertility and parenting. Animal data suggest that imatinib at standard dosages is unlikely to impair fertility in either adult males or females but human data remain limited. Children born to men who are actively taking imatinib at the time of conception appear healthy and current advice is not to discontinue treatment. In contrast the data relating to children born to women exposed to imatinib during pregnancy are less encouraging. Although numbers are small there has been a disturbing cluster of rare congenital malformations such that imatinib cannot be safely recommended, particularly during the period of organogenesis. The appropriate management of children with CML has also been radically changed by the advent of imatinib. The features of the disease at presentation, the natural history and the response to therapy seem to be identical in children to that seen in adults. Now that imatinib has been in clinical use for almost ten years without severe long-term side effects, most physicians are now comfortable advising a trial of imatinib prior to consideration of transplant. Data relating to the efficacy and safety of second generation tyrosine kinase inhibitors in childhood is entirely absent and transplant remains the first choice for patients failing imatinib and perhaps also for young patients with sub-optimal responses.