Selective expression of PDGF A and its receptor during early mouse embryogenesis

The uterine secretory cycle: recurring physiology of endometrial outputs that setup the uterine luminal microenvironment

Conserved in female reproduction across all mammalian species is the estrous cycle and its regulation by the hypothalamic-pituitary-gonadal (HPG) axis, a collective of intersected hormonal events that are crucial for ensuring uterine fertility. Nonetheless, knowledge of the direct mediators that synchronously shape the uterine microenvironment for successive yet distinct events, such as the transit of sperm and support for progressive stages of preimplantation embryo development, remain principally deficient. Toward understanding the timed endometrial outputs that permit luminal events as directed by the estrous cycle, we used Bovidae as a model system to uniquely surface sample and study temporal shifts to in vivo endometrial transcripts that encode for proteins destined to be secreted. The results revealed the full quantitative profile of endometrial components that shape the uterine luminal microenvironment at distinct phases of the estrous cycle (estrus, metestrus, diestrus, and proestrus). In interpreting this comprehensive log of stage-specific endometrial secretions, we define the "uterine secretory cycle" and extract a predictive understanding of recurring physiological actions regulated within the uterine lumen in anticipation of sperm and preimplantation embryonic stages. This repetitive microenvironmental preparedness to sequentially provide operative support was a stable intrinsic framework, with only limited responses to sperm or embryos if encountered in the lumen within the cyclic time period. In uncovering the secretory cycle and unraveling realistic biological processes, we present novel foundational knowledge of terminal effectors controlled by the HPG axis to direct a recurring sequence of vital functions within the uterine lumen.NEW & NOTEWORTHY This study unravels the recurring sequence of changes within the uterus that supports vital functions (sperm transit and development of preimplantation embryonic stages) during the reproductive cycle in female Ruminantia. These data present new systems knowledge in uterine reproductive physiology crucial for setting up in vitro biomimicry and artificial environments for assisted reproduction technologies for a range of mammalian species.

Platelet-derived growth factor PDGF-AA upregulates connexin 43 expression and promotes gap junction formations in osteoblast cells through p-Akt signaling

Gap junctions, which are mainly composed of connexin units, play an indispensable role in cell morphogenesis, proliferation, migration, adhesion and differentiation of osteoblast lineage cells, and thus mediate bone development, homeostasis and disease occurrence. Platelet-derived growth factor-AA (PDGF-AA) is proved to have a great influence on osteoblast cell lines and is widely applied in the field of bone defect and wound healing. However, the role of PDGF-AA on gap junction formation in the osteoblast lineage remains elusive. In the current study, we aimed to investigate the impact of PDGF-AA on gap junction formation and cell-to-cell communication in the osteoblast lineage and explore its underlying biomechanism. We first found that PDGF-AA promoted cell proliferation and thus increased gap junction formations in living primary osteoblasts and MC3T3-E1 cells through scrape loading and dye transfer (SL/DT) assay. We then confirmed that PDGF-AA enhanced gap junction formations through up-regulation of connexin 43 (Cx43). We next detected the activation of p-Akt signaling in primary osteoblasts and MC3T3-E1 cells that were induced by PDGF-AA. Through inhibitory experiments, we further confirmed that PDGF-AA-mediated gap junction formation occurred via the activation of PI3K/Akt signaling. Taking together, our results provided evidences that PDGF-AA promoted gap junction formation in the osteoblast lineage through p-Akt signaling, which helped to understand the role of PDGF-AA in bone regeneration and diseases.

Cell migration in the Xenopus gastrula

Xenopus gastrulation movements are in large part based on the rearrangement of cells by differential cell-on-cell migration within multilayered tissues. Different patterns of migration-based cell intercalation drive endoderm and mesoderm internalization and their positioning along their prospective body axes. C-cadherin, fibronectin, integrins, and focal contact components are expressed in all gastrula cells and play putative roles in cell-on-cell migration, but their actual functions in this respect are not yet understood. The gastrula can be subdivided into two motility domains, and in the vegetal, migratory domain, two modes of cell migration are discerned. Vegetal endoderm cells show ingression-type migration, a variant of amoeboid migration characterized by the lack of locomotory protrusions and by macropinocytosis as a mechanism of trailing edge resorption. Mesendoderm and prechordal mesoderm cells use lamellipodia in a mesenchymal mode of migration. Gastrula cell motility can be dissected into traits, such as cell polarity, adhesion, mobility, or protrusive activity, which are controlled separately yet in complex, combinatorial ways. Cells can instantaneously switch between different combinations of traits, showing plasticity as they respond to substratum properties. This article is categorized under: Early Embryonic Development > Gastrulation and Neurulation.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Platelet-derived growth factor (PDGF) signaling directs cardiomyocyte movement toward the midline during heart tube assembly

Communication between neighboring tissues plays a central role in guiding organ morphogenesis. During heart tube assembly, interactions with the adjacent endoderm control the medial movement of cardiomyocytes, a process referred to as cardiac fusion. However, the molecular underpinnings of this endodermal-myocardial relationship remain unclear. Here, we show an essential role for platelet-derived growth factor receptor alpha (Pdgfra) in directing cardiac fusion. Mutation of pdgfra disrupts heart tube assembly in both zebrafish and mouse. Timelapse analysis of individual cardiomyocyte trajectories reveals misdirected cells in zebrafish pdgfra mutants, suggesting that PDGF signaling steers cardiomyocytes toward the midline during cardiac fusion. Intriguingly, the ligand pdgfaa is expressed in the endoderm medial to the pdgfra-expressing myocardial precursors. Ectopic expression of pdgfaa interferes with cardiac fusion, consistent with an instructive role for PDGF signaling. Together, these data uncover a novel mechanism through which endodermal-myocardial communication can guide the cell movements that initiate cardiac morphogenesis.

DOI:http://dx.doi.org/10.7554/eLife.21172.001

In the growing embryo, the heart initially develops in the form of a simple tube. Its outer layer is made up of muscular cells, called myocardial cells, that pump blood through the tube. Before the heart tube develops, two groups of myocardial cells exist – one on each side of the embryo. To assemble the heart, these two populations of cells must move as a group to the middle of the embryo, where they meet and merge through a process called cardiac fusion. This movement of myocardial cells toward the middle of the embryo depends upon interactions with a neighboring tissue called the endoderm. How the endoderm directs the movement of the myocardial cells was not well understood.

The PDGF signaling pathway guides the movement of several different types of cells in the body, but it had not been previously linked to the early stages of heart tube assembly. In this pathway, a molecule called platelet-derived growth factor (PDGF) binds to PDGF receptors that sit on the surface of cells. Using microscopy and genetic analysis to study zebrafish and mouse embryos, Bloomekatz et al. now show that embryos that carry mutations in a gene that encodes a PDGF receptor suffer from defects in heart tube assembly. Further examination of the mutant zebrafish embryos revealed that the myocardial cells were not properly directed toward the middle of the embryo. In fact, many of these cells appeared to move away from the midline.

Bloomekatz et al. also observed that, in normal embryos, the endoderm cells that lie adjacent to the myocardial cells produce PDGF. Therefore, it appears that PDGF produced by the endoderm could interact with PDGF receptors on the myocardial cells to direct these cells toward the middle of the embryo. The next step will be to figure out how this signaling influences the machinery inside the myocardial cells that controls their movement. Ultimately, this knowledge could lead to new ways to identify and treat congenital heart diseases.

DOI:http://dx.doi.org/10.7554/eLife.21172.002

Role of retinoic acid and platelet-derived growth factor receptor cross talk in the regulation of neonatal gonocyte and embryonal carcinoma cell differentiation

Neonatal gonocytes are direct precursors of spermatogonial stem cells, the cell pool that supports spermatogenesis. Although unipotent in vivo, gonocytes express pluripotency genes common with embryonic stem cells. Previously, we found that all-trans retinoic acid (RA) induced the expression of differentiation markers and a truncated form of platelet-derived growth factor receptor (PDGFR)β in rat gonocytes, as well as in F9 mouse embryonal carcinoma cells, an embryonic stem cell-surrogate that expresses somatic lineage markers in response to RA. The present study is focused on identifying the signaling pathways involved in RA-induced gonocyte and F9 cell differentiation. Mitogen-activated protein kinase kinase (MEK) 1/2 activation was required during F9 cell differentiation towards somatic lineage, whereas its inhibition potentiated RA-induced Stra8 expression, suggesting that MEK1/2 acts as a lineage specification switch in F9 cells. In both cell types, RA increased the expression of the spermatogonial/premeiotic marker Stra8, which is in line with F9 cells being at a stage before somatic-germline lineage specification. Inhibiting PDGFR kinase activity reduced RA-induced Stra8 expression. Interestingly, RA increased the expression of PDGFRα variant forms in both cell types. Together, these results suggest a potential cross talk between RA and PDGFR signaling pathways in cell differentiation. RA receptor-α inhibition partially reduced RA effects on Stra8 in gonocytes, indicating that RA acts in part via RA receptor-α. RA-induced gonocyte differentiation was significantly reduced by inhibiting SRC (v-src avian sarcoma [Schmidt-Ruppin A-2] viral oncogene) and JAK2/STAT5 (Janus kinase 2/signal transducer and activator of transcription 5) activities, implying that these signaling molecules play a role in gonocyte differentiation. These results suggest that gonocyte and F9 cell differentiation is regulated via cross talk between RA and PDGFRs using different downstream pathways.

Proprotein convertase 5/6 cleaves platelet-derived growth factor A in the human endometrium in preparation for embryo implantation

Establishment of endometrial receptivity is vital for successful embryo implantation. Proprotein convertase 5/6 (referred to as PC6) is up-regulated in the human endometrium specifically at the time of epithelial receptivity. PC6, a serine protease of the proprotein convertase family, plays an important role in converting precursor proteins into their active forms through specific proteolysis. The proform of platelet-derived growth factor A (pro-PDGFA) requires PC cleavage to convert to the active-PDGFA. We investigated the PC6-mediated activation of PDGFA in the human endometrium during the establishment of receptivity. Proteomic analysis identified that the pro-PDGFA was increased in the conditioned medium of HEC1A cells in which PC6 was stably knocked down by small interfering RNA (PC6-siRNA). Western blot analysis demonstrated an accumulation of the pro-PDGFA but a reduction in the active-PDGFA in PC6-siRNA cell lysates and medium compared with control. PC6 cleavage of pro-PDGFA was further confirmed in vitro by incubation of recombinant pro-PDGFA with PC6. Immunohistochemistry revealed cycle-stage-specific localization of the active-PDGFA in the human endometrium. During the non-receptive phase, the active-PDGFA was barely detectable. In contrast, it was localized specifically to the apical surface of the luminal and glandular epithelium in the receptive phase. Furthermore, the active-PDGFA was detected in uterine lavage with levels being significantly higher in the receptive than the non-receptive phase. We thus identified that the secreted PDGFA may serve as a biomarker for endometrial receptivity. This is also the first study demonstrating that the active-PDGFA localizes to the apical surface of the endometrium during receptivity.

Effect of concentration of platelet-derived growth factor on proliferative activity of human fibroblasts

Serum concentration of platelet-derived growth factor was measured in patients with trophic ulcers and proliferative activity of human fibroblasts was measured in wells with different content of added serum platelet-derived growth factor. In parallel, morphofunctional analysis of platelets from the whole blood of these patients was performed. A close correlation was found between morphofunctional parameters of blood platelets and serum concentration of platelet-derived growth factor. Platelet-derived growth factor in concentrations below 150 pg per well stimulated fibroblast proliferation and preserved their viability. Platelet-derived growth factor in concentrations >200 pg per well suppressed fibroblast proliferation and impaired their viability.

The roles of PDGF in development and during neurogenesis in the normal and diseased nervous system

The four platelet-derived growth factor (PDGF) ligands and PDGF receptors (PDGFRs), α and β (PDGFRA, PDGFRB), are essential proteins that are expressed during embryonic and mature nervous systems, i.e., in neural progenitors, neurons, astrocytes, oligodendrocytes, and vascular cells. PDGF exerts essential roles from the gastrulation period to adult neuronal maintenance by contributing to the regulation of development of preplacodal progenitors, placodal ectoderm, and neural crest cells to adult neural progenitors, in coordinating with other factors. In adulthood, PDGF plays critical roles for maintenance of many specific cell types in the nervous system together with vascular cells through controlling the blood brain barrier homeostasis. At injury or various stresses, PDGF modulates neuronal excitability through adjusting various ion channels, and affecting synaptic plasticity and function. Furthermore, PDGF stimulates survival signals, majorly PI3-K/Akt pathway but also other ways, rescuing cells from apoptosis. Studies imply an involvement of PDGF in dendrite spine morphology, being critical for memory in the developing brain. Recent studies suggest association of PDGF genes with neuropsychiatric disorders. In this review, we will describe the roles of PDGF in the nervous system, from the discovery to recent findings, in order to understand the broad spectrum of PDGF in the nervous system. Recent development of pharmacological and replacement therapies targeting the PDGF system is discussed.

GFAP promoter driven transgenic expression of PDGFB in the mouse brain leads to glioblastoma in a Trp53 null background

Glioblastomas are the most common and malignant astrocytic brain tumors in human adults. The tumor suppressor gene TP53 is commonly mutated and/or lost in astrocytic brain tumors and the TP53 alterations are often found in combination with excessive growth factor signaling via PDGF/PDGFRalpha. Here, we have generated transgenic mice over-expressing human PDGFB in brain, under control of the human GFAP promoter. These mice showed no phenotype, but on a Trp53 null background a majority of them developed brain tumors. This occurred at 2-6 months of age and tumors displayed human glioblastoma-like features with integrated development of Pdgfralpha+ tumor cells and Pdgfrbeta+/Nestin+ vasculature. The transgene was expressed in subependymal astrocytic cells, in glia limitans, and in astrocytes throughout the brain substance, and subsequently, microscopic tumor lesions were initiated equally in all these areas. With tumor size, there was an increase in Nestin positivity and variability in lineage markers. These results indicate an unexpected plasticity of all astrocytic cells in the adult brain, not only of SVZ cells. The results also indicate a contribution of widely distributed Pdgfralpha+ precursor cells in the tumorigenic process.

PDGF signalling controls the migration of mesoderm cells during chick gastrulation by regulating N-cadherin expression

In the early chick embryo, Pdgfa is expressed in the epiblast,outlining the migration route that mesoderm cells expressing the receptor, Pdgfrα, follow to form somites. Both expression of a dominant-negative PDGFRα and depletion of endogenous PDGFRαligands through injection of PDGFRα-Fc fragments, inhibit the migration of mesoderm cells after their ingression through the primitive streak. siRNA-mediated downregulation of Pdgfa expression in the epiblast on one side of the streak strongly blocks the migration of mesoderm cells into that side. Beads soaked in PDGFA elicit a directional attractive movement response in mesoderm cells, showing that PDGFA can provide directional information. Surprisingly, however, PDGF signalling is also required for directional movement towards other attractants, such as FGF4. PDGF signalling controls N-cadherin expression on mesoderm cells, which is required for efficient migration. PDGF signalling activates the PI3 kinase signalling pathway in vivo and activation of this pathway is required for proper N-cadherin expression.

Platelet-derived growth factor receptors direct vascular development independent of vascular smooth muscle cell function

Complete loss of platelet-derived growth factor (PDGF) receptor signaling results in embryonic lethality around embryonic day 9.5, but the cause of this lethality has not been identified. Because cardiovascular failure often results in embryonic lethality at this time point, we hypothesized that a failure in cardiovascular development could be the cause. To assess the combined role of PDGF receptor alpha (PDGFRalpha) and PDGFRbeta, we generated embryos that lacked these receptors in cardiomyocytes and vascular smooth muscle cells (VSMC) using conditional gene ablation. Deletion of either PDGFRalpha or PDGFRbeta caused no overt vascular defects, but loss of both receptors using an SM22alpha-Cre transgenic mouse line led to a disruption in yolk sac blood vessel development. The cell population responsible for this vascular defect was the yolk sac mesothelial cells, not the cardiomyocytes or the VSMC. Coincident with loss of PDGF receptor signaling, we found a reduction in collagen deposition and an increase in MMP-2 activity. Finally, in vitro allantois cultures demonstrated a requirement for PDGF signaling in vessel growth. Together, these data demonstrate that PDGF receptors cooperate in the yolk sac mesothelium to direct blood vessel maturation and suggest that these effects are independent of their role in VSMC development.

Role of platelet-derived growth factors in physiology and medicine

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) have served as prototypes for growth factor and receptor tyrosine kinase function for more than 25 years. Studies of PDGFs and PDGFRs in animal development have revealed roles for PDGFR-alpha signaling in gastrulation and in the development of the cranial and cardiac neural crest, gonads, lung, intestine, skin, CNS, and skeleton. Similarly, roles for PDGFR-beta signaling have been established in blood vessel formation and early hematopoiesis. PDGF signaling is implicated in a range of diseases. Autocrine activation of PDGF signaling pathways is involved in certain gliomas, sarcomas, and leukemias. Paracrine PDGF signaling is commonly observed in epithelial cancers, where it triggers stromal recruitment and may be involved in epithelial-mesenchymal transition, thereby affecting tumor growth, angiogenesis, invasion, and metastasis. PDGFs drive pathological mesenchymal responses in vascular disorders such as atherosclerosis, restenosis, pulmonary hypertension, and retinal diseases, as well as in fibrotic diseases, including pulmonary fibrosis, liver cirrhosis, scleroderma, glomerulosclerosis, and cardiac fibrosis. We review basic aspects of the PDGF ligands and receptors, their developmental and pathological functions, principles of their pharmacological inhibition, and results using PDGF pathway-inhibitory or stimulatory drugs in preclinical and clinical contexts.

Requirement for Foxd3 in the maintenance of neural crest progenitors

Understanding the molecular mechanisms of stem cell maintenance is crucial for the ultimate goal of manipulating stem cells for the treatment of disease. Foxd3 is required early in mouse embryogenesis; Foxd3(-/-) embryos fail around the time of implantation, cells of the inner cell mass cannot be maintained in vitro, and blastocyst-derived stem cell lines cannot be established. Here, we report that Foxd3 is required for maintenance of the multipotent mammalian neural crest. Using tissue-specific deletion of Foxd3 in the neural crest, we show that Foxd3(flox/-); Wnt1-Cre mice die perinatally with a catastrophic loss of neural crest-derived structures. Cranial neural crest tissues are either missing or severely reduced in size, the peripheral nervous system consists of reduced dorsal root ganglia and cranial nerves, and the entire gastrointestinal tract is devoid of neural crest derivatives. These results demonstrate a global role for this transcriptional repressor in all aspects of neural crest maintenance along the anterior-posterior axis, and establish an unprecedented molecular link between multiple divergent progenitor lineages of the mammalian embryo.

All-trans retinoic acid inhibited angiotensin II-induced increase in cell growth and collagen secretion of neonatal cardiac fibroblasts

AIM

To determine whether all-trans retinoic acid (atRA) acts to modulate angiotensin II (Ang II)-induced cardiac fibroblast cell growth and collagen secretion.

METHODS

Cultured neonatal rat cardiac fibroblasts (CF) were used in the experiment. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay was used to detect cell growth of the CF; and immunocytochemistry and Western blotting were used to measure the production and secretion of collagen and the expression of transforming growth factor-beta1 (TGF-beta1) by the CF.

RESULTS

atRA (10(-7) to 10(-5) mol/L) inhibited the Ang II-induced increase in cell growth of CF (P<0.05). Ang II stimulated the secretion of collagen types I and III by the CF. This effect was blocked by AT(1) receptor antagonist losartan (10(-6) mol/L), but not by AT2 receptor antagonist PD123319 (up to 10(-6) mol/L). Exposure of CF to atRA (10(-5) mol/L) attenuated the Ang II-induced increase in the secretion of collagen types I and III (P<0.05). atRA (10(-5) mol/L) also blocked the Ang II-induced increase in the expression of TGF-beta1.

CONCLUSION

atRA inhibits the Ang II-induced increase in cell growth and collagen secretion of neonatal rat CF. The effect of atRA is possibly mediated by lowering the TGF-beta1 level. These observations support the notion that atRA is a potential candidate for the prevention and therapy of cardiac remodeling.

Regulation of the stepwise proteolytic cleavage and secretion of PDGF-B by the proprotein convertases

Platelet-derived growth factor-B (PDGF-B) is important for normal tissue growth and maintenance and its overexpression has been linked to several diseases, including cancer, fibrotic disease and atherosclerosis. Here, we show that synthesized as a precursor, proPDGF-B is converted to a mature form by proteolytic cleavage at two sites and its N-terminal cleavage is a prerequisite for processing at its C-terminus. The first cleavage occurs at residues RGRR81/, and the second cleavage close to residues ARPVT190, just before the C-terminal amino-acid sequence crucial for PDGF-B retention to cell surface. Cotransfection of a Furin-deficient cell line LoVo-C5 with proPDGF-B and different PC members revealed that Furin, PACE4, PC5, and PC7 are candidate proPDGF-B convertases. This finding is consistent with the in vitro digestions of a synthetic peptide mimicking the cleavage site of proPDGF-B. The processing of proPDGF-B is blocked by site-directed mutagenesis of the RGRR81/ sequence and by various PC inhibitors. Mutation of the PDGF-A and/or PDGF-B convertase sites, revealed that processing of both A and B chains is required for the formation of mature PDGF-B dimers and that the processing of the B chain controls the level of secreted and matrix-bound PDGF-BB forms. Our findings emphasize the importance of the convertase-directed processing of proPDGF-B at the RGRR81/ sequence for PDGF-B maturation and secretion.

Vascular Cells

Embryonic stem (ES) cells are cells derived from the inner cell mass of a blastocyst stage embryo. These self-renewing multipotent cells are able to differentiate to the three embryonic germ layers, the endoderm, ectoderm, and mesoderm, and are thus able to produce virtually all cell types. The ES cell capacity to generate various cell types has been studied extensively, and exploitation of ES cell characteristics allowed the production of several differentiated cell types of multiple tissues. Moreover, the process of ES cell differentiation provides a unique opportunity to observe early embryonic developmental events that are unattainable in the embryo itself. This chapter addresses the in vitro differentiation procedure of endothelial and vascular smooth muscle cells from human ES cells, with reference to similar studies performed in mouse and nonhuman primate ES cells, and provides several tools for the detailed characterization of differentiated cells.

Imprinted X-inactivation in extra-embryonic endoderm cell lines from mouse blastocysts

The extra-embryonic endoderm lineage plays a major role in the nutritive support of the embryo and is required for several inductive events, such as anterior patterning and blood island formation. Blastocyst-derived embryonic stem (ES) and trophoblast stem (TS) cell lines provide good models with which to study the development of the epiblast and trophoblast lineages,respectively. We describe the derivation and characterization of cell lines that are representative of the third lineage of the blastocyst –extra-embryonic endoderm. Extra-embryonic endoderm (XEN) cell lines can be reproducibly derived from mouse blastocysts and passaged without any evidence of senescence. XEN cells express markers typical of extra-embryonic endoderm derivatives, but not those of the epiblast or trophoblast. Chimeras generated by injection of XEN cells into blastocysts showed exclusive contribution to extra-embryonic endoderm cell types. We used female XEN cells to investigate the mechanism of X chromosome inactivation in this lineage. We observed paternally imprinted X-inactivation, consistent with observations in vivo. Based on gene expression analysis, chimera studies and imprinted X-inactivation, XEN cell lines are representative of extra-embryonic endoderm and provide a new cell culture model of an early mammalian lineage.

Gli1 is not required for Pdgfrα expression during mouse embryonic development

Pdgfra is expressed in the mesenchyme of multiple organs during embryonic development and Pdgfralpha is involved in cell proliferation, differentiation, migration, and apoptosis in many tissues. A fine-tuned regulation of gene transcription is required to achieve these effects. To investigate if the Shh signaling pathway is involved in the tightly regulated Pdgfra expression during embryogenesis, we systematically compared Gli1 and Pdgfralpha mRNA expression patterns in vivo from mouse embryonic day 9.5 to 14.5. We found that an initial partly overlapping expression of Gli1 and Pdgfralpha in the mesenchyme of foregut and somites was changed to different expression patterns when the mesenchyme differentiated into specialized structures such as intestinal villi and chondrocytes. Gli1 and Pdgfra were also expressed differently in the developing lung, heart, central nervous system, skin, tooth, and eye. Importantly, neither Pdgfralpha mRNA patterns nor levels were altered in Ihh mutant embryos although Gli1 and Ptc mRNA levels were dramatically reduced. Our results demonstrate that Gli1 is not required to induce Pdgfra expression during embryonic bone development, and are consistent with previous findings that Pdgfralpha and Hh pathways serve different functions in, e.g., bone, gut, and lung development. However, we cannot exclude the possibility that Glis can have more complex regulatory effects on Pdgfra gene activity, nor can we exclude such effects in pathological conditions.

Forced expression of platelet-derived growth factor B in the mouse cerebellar primordium changes cell migration during midline fusion and causes cerebellar ectopia

The platelet-derived growth factor (PDGF) and receptors are expressed in the developing central nervous system and in brain tumors. To investigate the role of PDGF during normal cerebellar development, we created transgenic mice where PDGF-B was introduced into the endogenous Engrailed1 locus (En1). These mice expressed PDGF-B in all types of cells that constitute the developing cerebellum, with localized high expression in the ventral midline of the cerebellar anlage. This affected cell migration in the midline during fusion of the cerebellar anlage and caused misplacement of midline structures. PDGFR-alpha- and laminin alpha1-positive meningeal cells migrated inwards, attracted by the ectopic transgene expression in the ventral neuroepithelium. Other cells followed the meningeal cells and in the adult mouse, cells from all cortical cell layers were found misplaced in the midline. Moreover, the transgene caused an enhancement of capillary vessels. The findings indicate that normal PDGF signaling is important for proper neural tube fusion. It also illustrates that meningeal structures can influence the process.

Neural crest and the origin of ectomesenchyme: neural fold heterogeneity suggests an alternative hypothesis

The striking similarity between mesodermally derived fibroblasts and ectomesenchyme cells, which are thought to be derivatives of the neural crest, has long been a source of interest and controversy. In mice, the gene encoding the alpha subunit of the platelet-derived growth factor receptor (PDGFRalpha) is expressed both by mesodermally derived mesenchymal cells and by ectomesenchyme. Whole-mount immunostaining previously revealed that PDGFRalpha is present in the cephalic neural fold epithelium of early murine embryos (Takakura et al. [1997] J Histochem Cytochem 45:883-893). We now show that, within the neural fold, a sharp boundary exists between E-cadherin-expressing non-neural epithelium and the neural epithelium of the dorsal ridge. In addition, we found that cells coexpressing E-cadherin and PDGFRalpha are present in the non-neural epithelium of the neural folds. These observations raise the possibility that at least some PDGFRalpha(+) ectomesenchyme originates from the lateral non-neural domain of neural fold epithelium. This inference is consistent with previous reports (Nichols [ 1981] J Embryol Exp Morphol 64:105-120; Nichols [ 1986] Am J Anat 176:221-231) that mesenchymal cells emerge precociously from an epithelial neural fold domain resembling the primitive streak in the early embryonic epiblast. Therefore, we propose the name "metablast" for this non-neural epithelial domain to indicate that it is the site of a delayed local delamination of mesenchyme similar to involution of mesoderm during gastrulation. We further propose the testable hypothesis that neural crest and ectomesenchyme are developmentally distinct progenitor populations and that at least some ectomesenchyme is metablast-derived rather than neural crest-derived tissue. Developmental Dynamics 229:118-130, 2004.

C/EBP is an essential component of PDGFRA transcription in MG-63 cells

Interleukin-1beta (IL-1beta) is a potent inhibitor of platelet-derived growth factor alpha receptor (PDGFRalpha) expression in MG-63 cells. Its effect is mediated at the transcriptional level, but the transcription factors involved in this process are unknown. In the current study, we found that IL-1beta could inhibit the PDGFRalpha gene promoter activity, and this effect was strongly correlated with increased binding of CCAAT/enhancer-binding protein (C/EBP) to the responsive promoter region. In addition, forced expression of C/EBPbeta could mimic the IL-1beta effect on the promoter activity, but subsequent mutation analysis of the C/EBP binding sites indicated that direct C/EBP binding to the promoter is not required for the IL-1beta response. However, our data clearly demonstrated that the C/EBP binding site at position-162 relative to the transcriptional start site is essential for high basal level PDGFRalpha promoter activity.

The renal retinoid system: time-dependent activation in experimental glomerulonephritis

Retinoids reduce renal damage in rat experimental glomerulonephritis. It is unknown, however, how local and systemic retinoid pathways respond to renal injury. We used a rat model of artificially induced acute anti-Thy1.1-nephritis (THY-GN). We examined the extrarenal and glomerular expression of the retinol (RoDH) and retinal (RalDH) dehydrogenases 1 and 2 as well as the expression of the retinoic acid (RAR) and retinoid X (RXR) receptor subtypes α, β, and γ. Furthermore, we investigated serum and glomerular retinoid concentration patterns. On days 3, 7, and 14, we compared nonnephritic rats (control group; CON) to THY-GN rats with respect to systolic blood pressure and glomerular cell count per cross section. Systolic blood pressure and glomerular cell count were significantly higher in THY-GN rats on days 7 and 14 ( P < 0.001). We found a 60% reduction in expression levels for retinoid receptors and dehydrogenases in nephritic glomeruli on day 3, but a threefold increase on day 7 ( P < 0.001 vs. CON). The same applies to RARα protein. Hepatic expression of retinoid receptors was not influenced. On day 14, glomerular expression levels for retinoid receptors and retinoid-metabolizing enzymes had returned to a normal level, glomerular cell count being still increased. Administering 13- cis retinoic acid (isotretinoin) lowered blood pressure and glomerular cell count in nephritic rats but failed to influence the glomerular expression of retinoid receptors or retinoid-metabolizing enzymes. Our data document a stimulation of glomerular retinoid-synthesizing enzymes and expression of retinoid receptors in the early repair phase of THY-GN, suggesting activation of this system in acute renal disease.

Combinatorial function of the homeodomain proteins Nkx2.1 and Gsh2 in ventral telencephalic patterning

Regional patterning of the mammalian telencephalon requires the function of three homeodomain-containing transcription factors, Pax6, Gsh2 and Nkx2.1. These factors are required for the development of the dorsal, lateral and medial domains of the telencephalon, respectively. Previous work has indicated that two of the genes encoding these factors, Pax6 and Gsh2, cross-repress one another in the formation of the border between dorsal and lateral region of the telencephalon. Here, we examine whether similar interactions are responsible for the establishment of other boundaries of telencephalic gene expression. Surprisingly, despite the fact that, at specific times in development, both Pax6 and Gsh2 maintain a complementary pattern of expression with Nkx2.1, in neither case are these boundaries maintained through a similar cross-repressive mechanism. Rather, as revealed by analysis of double-mutant mice, Nkx2.1 and Gsh2 act cooperatively in many aspects to pattern the ventral telencephalon. By contrast, as indicated by both loss- and gain-of-function analysis, Gsh2 expression in the medial ganglionic eminence after E10.5 may negatively regulate Nkx2.1 dependent specification of oligodendrocytes. Therefore, both integrative and antagonistic interactions between homeodomain-containing transcription factors contribute to the patterning of the telencephalon.

Sonic hedgehog is required for progenitor cell maintenance in telencephalic stem cell niches

To directly test the requirement for hedgehog signaling in the telencephalon from early neurogenesis, we examined conditional null alleles of both the Sonic hedgehog and Smoothened genes. While the removal of Shh signaling in these animals resulted in only minor patterning abnormalities, the number of neural progenitors in both the postnatal subventricular zone and hippocampus was dramatically reduced. In the subventricular zone, this was partially attributable to a marked increase in programmed cell death. Consistent with Hedgehog signaling being required for the maintenance of stem cell niches in the adult brain, progenitors from the subventricular zone of floxed Smo animals formed significantly fewer neurospheres. The loss of hedgehog signaling also resulted in abnormalities in the dentate gyrus and olfactory bulb. Furthermore, stimulation of the hedgehog pathway in the mature brain resulted in elevated proliferation in telencephalic progenitors. These results suggest that hedgehog signaling is required to maintain progenitor cells in the postnatal telencephalon.

PDGF and FGF2 regulate oligodendrocyte progenitor responses to demyelination

Acute demyelination of adult CNS, resulting from trauma or disease, is initially followed by remyelination. However, chronic lesions with subsequent functional impairment result from eventual failure of the remyelination process, as seen in multiple sclerosis. Studies using animal models of successful remyelination delineate a progression of events facilitating remyelination. A universal feature of this repair process is extensive proliferation of oligodendrocyte progenitor cells (OPs) in response to demyelination. To investigate signals that regulate OP proliferation in response to demyelination we used murine hepatitis virus-A59 (MHV-A59) infection of adult mice to induce focal demyelination throughout the spinal cord followed by spontaneous remyelination. We cultured glial cells directly from demyelinating and remyelinating spinal cords using conditions that maintain the dramatically enhanced OP proliferative response prior to CNS remyelination. We identify PDGF and FGF2 as significant mitogens regulating this proliferative response. Furthermore, we demonstrate endogenous PDGF and FGF2 activity in these glial cultures isolated from demyelinated CNS tissue. These findings correlate well with our previous demonstration of increased in vivo expression of PDGF and FGF2 ligand and corresponding receptors in MHV-A59 lesions. Together these studies support the potential of these pathways to function in vivo as critical factors in regulating remyelination.

Oligodendrocyte precursor hypercellularity and abnormal retina development in mice overexpressing PDGF-B in myelinating tracts

Platelet-derived growth factor (PDGF) influences the generation of neurons and glia during embryogenesis and in early postnatal life. In an attempt to determine the consequences of an overexpression of PDGF-B during the first weeks of life, we targeted transgenic expression of a human PDGF-B cDNA to myelinating tracts using the promoter region of the myelin basic protein (MBP) gene. Transgenic mRNA and protein were expressed in the brain and the expression profile of the human PDGF-B during early postnatal development closely paralleled that of the endogenous mouse MBP gene. The gross morphological appearance of transgenic brains was normal but at the cellular level several phenotypic alterations could be identified. In white matter tracts such as the corpus callosum and cerebellar medulla, there was a marked hypercellularity. The number of oligodendrocyte precursors was increased and astrocytes were more abundant. In adult mice carrying the MBP-PDGF-B transgene, however, myelination appeared normal and the amount of oligodendrocytes was similar to that of control littermates. In addition to the phenotypic alterations in the brain, investigation of eye structure revealed a striking disorganization of retinal architecture. The retina was folded with cells collected in papillar or follicular-like structures. Retinal whole mount preparations after India ink perfusion revealed capillary disorganization with large-caliber vessels supporting only a few fine branches. Our observations strengthen the notion that PDGF is an important effector molecule in postnatal CNS development.

Analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced gene expression profile in vivo using pathway-specific cDNA arrays

In the current study, we used pathway-specific cDNA arrays to detect the transcriptional signature induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in vivo by studying simultaneously the expression profiles of 83 genes involved in apoptosis, cytokine production and angiogenesis. To this end, C57BL/6 mice were injected i.p. with 50 microg/kg body weight of TCDD and 1 or 3 days later, the thymus was analyzed for gene expression profiles. In the thymus, 23 out of 37 apoptotic genes screened were up-regulated by TCDD by a factor of two or more when compared to the vehicle-treated controls. In contrast, in the spleen, 20 out of 22 and in the liver, 16 out of 37 apoptotic genes were up-regulated. In the thymus, several genes encoding caspases, and members of the TNF family, including Fas ligand, were induced. Also, in the thymus, eight out of 23, and in the spleen, six out of 23 cytokine genes were up-regulated. In the liver and to a lesser extent in the thymus, certain angiogenesis genes were induced while others were repressed. When mice were injected with 0.1, 1, 10 or 50 microg/kg body weight of TCDD and the thymus was analyzed for apoptotic genes 1 day later, a dose-dependent response was not seen with most apoptotic genes. However, certain apoptotic genes were induced in the thymus even at low doses of 0.1 microg/kg body weight of TCDD. These data demonstrate that TCDD alters the expression of a large array of genes involved in apoptosis, cytokine production and angiogenesis. Thus, pathway-specific cDNA arrays may help in the identification of specific gene expression profiles induced by xenobiotics and to delineate the molecular mechanisms of toxicity.

Increased bone morphogenetic protein-6 expression in mouse long bones after estrogen administration

High-dose estrogen administration is known to induce new bone formation in mouse long bones. To study the role of regulatory proteins in this response, we examined associated changes in femoral messenger RNA (mRNA) for candidate factors. 17beta-estradiol (E2) 0.5 mg was administered to intact female mice by weekly injection, and Northern blot analysis was performed 1, 2, 4, 8, 12, and 16 days after the first injection. In contrast to other factors, an increase was observed in mRNA for bone morphogenetic protein-6 (BMP-6), which reached significance at day 8 and subsequent time-points. Estrogen-induced changes in BMP-6 protein expression were assessed by immunocytochemistry in longitudinal femoral sections. In untreated animals, BMP-6 was expressed by a significant proportion of growth plate chondrocytes and a subpopulation of bone marrow cells. In contrast, osteoblasts were consistently BMP-6 negative. From as early as 4 days after starting estrogen, clusters of slightly elongated BMP-6-positive cells were observed within the marrow cavity; the majority were close to active bone formation surfaces. Double immunolabeling studies revealed that only approximately 10% of BMP-6-positive bone marrow cells co-expressed the osteoblast transcription factor Cbfa1 suggesting that they are largely distinct from the osteoblast precursor population generated concurrently. BMP-6-positive cells expressed neither leukocyte nor erythroid markers (CD45 and TER-119, respectively), consistent with a stromal origin. We conclude that estrogen-induced osteogenesis in female mice is associated with increased levels of BMP-6 mRNA in mouse femurs, which seems to reflect the emergence of clusters of BMP-6 positive stromal cells adjacent to active bone formation surfaces. These findings raise the possibility that BMP-6 serves as a paracrine mediator of estrogen's osteogenic action in mice.

Retinoid receptor-specific agonists alleviate experimental glomerulonephritis

Retinoids are potent antiproliferative and anti-inflammatory compounds. We previously demonstrated that the natural pan-agonists all-trans retinoic acid (RA) and 13-cis RA efficiently preserve renal structure and function in rat mesangioproliferative glomerulonephritis. We examine effects of synthetic retinoid receptor-specific agonists 1) to identify common and receptor subtype-specific pathways in this model and 2) to characterize effects of retinoids on the renal endothelin (ET) system. Vehicle-injected control rats were compared with rats treated with daily subcutaneous injections of agonists specific for retinoid A (Ro-137410) and retinoid X (Ro-257386) receptors and the complex anti-activator protein-1 active retinoid BMS-453 7 days after induction of anti-Thy1.1 nephritis (n = 7-9/group). The different retinoids lowered glomerular ET-1 and ET type A and B receptor gene expression in control and nephritic rats with comparable efficacy. Reduction of glomerular c-Fos and GATA-2 mRNA expression levels suggests downregulation of transcription factors required for ET expression. The different retinoids were similar in their action on the glomerular capillary occlusion score, number of total glomerular cells, and glomerular infiltrating macrophage count. They differed in their ability to normalize blood pressure (Ro-257386 > BMS-453 > arotinoid), albuminuria (BMS-453 > Ro-257386 > arotinoid), and creatinine clearance (arotinoid > BMS-453 > Ro-257386). No signs of toxicity were observed. We conclude that all retinoid agonists with different subtype specificity are highly efficient in reducing renal damage and proliferation of mesangial cells. Retinoid X and A receptor-specific pathways are apparently involved in the antiproliferative, anti-inflammatory, and anti-ET action. Further studies are indicated to define the potential use of retinoid agonists in inflammatory renal disease.

Cloning and characterization of a chicken platelet-derived growth factor B-chain cDNA

Avian thrombocytes are nucleated blood cells homologous in function to mammalian platelets. In the present study, we obtained a cDNA from chicken thrombocyte polyadenylated RNA [Poly(A)+RNA], which coded for the chicken PDGF-B chain. The sequence was 1083-bp long and had an open reading frame (ORF) of 753-bp. At the amino acid level, the predicted mature protein showed 69% homology with the processed coding region of human PDGF-B. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed that PDGF-B mRNA was expressed at high levels in thrombocytes and in the lung. The expression of PDGF-B chain mRNA in thrombocytes reached its maximum level 12h following type 1 collagen treatment. These results suggest that chicken PDGF-B chain may play an important role in the vascular system and in healing wounded tissue.

The antiangiogenic agents SU5416 and SU6668 increase the antitumor effects of fractionated irradiation

Angiogenesis is critical for tumor development, growth and metastasis. The vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF) and platelet-derived growth factor (PDGF) and their tyrosine kinase receptors are major regulators of angiogenesis. Radiation induces the production of VEGF, FGF and PDGF in many tumor cells. We hypothesized that inhibition of the function of these growth factors could inhibit tumor angiogenesis and thereby enhance the efficacy of radiation therapy. To test this hypothesis, we used the small molecule inhibitors SU5416 (an inhibitor for Vegf receptor) and SU6668 (an inhibitor for Vegf, Fgf and Pdgf receptors) alone and in combination with fractionated irradiation to treat C3H mice bearing SCC VII carcinomas. The SCC VII tumors express Vegf, Fgf2 (also known as bFGF), Pdgf and their associated receptors. Animals were given either SU5416 or SU6668 daily before or after irradiation (2 Gy per fraction per day for 5 days). The results from these experiments demonstrate that administration of either SU5416 or SU6668 without radiation delayed tumor growth. Administration of SU5416 at a dose of 25 mg/kg per day (the maximum tolerated effective dose) inhibited tumor growth by 17.9% on day 7 (P < 0.05 compared to untreated control mice) and produced an average tumor growth delay time of 0.5-2.0 days. When combined with fractionated irradiation, administration of SU5416 increased the inhibition of tumor growth to 50-53% on day 7 and the tumor growth delay time to 5.7-6.5 days (P < 0.001 compared with SU5416 alone; P < or = 0.05 compared with radiation alone). SU6668 alone inhibited tumor growth in a dose-dependent manner. Administration of SU6668 at a dose of 75 mg/kg per day (a suboptimal dose) inhibited tumor growth by 36% on day 7 and produced an average tumor growth delay time of 3.3 +/- 1.4 days. The combination of SU6668 with fractionated radiation increased inhibition of tumor growth to 66-70% and the tumor growth delay time from 3.3 days to 11.9 days (P < or = 0.001 compared with either radiation alone or SU6668 alone). Administration of these agents before or after irradiation produced similar results (P = 0.40 for SU5416; P = 0.98 for SU6668). SU5416 or SU6668 alone or in combination with radiation was very well tolerated with little or no toxicity. These results suggest that inhibition of Vegf, Fgf and Pdgf receptor function by SU5416 and SU6668 can enhance the efficacy of irradiation. The targeting of multiple tyrosine kinase receptors by SU6668 is more effective than inhibition of the Vegf receptor alone by SU5416 for the enhancement of tumor cell killing by fractionated irradiation.

Effects of all-trans retinoic acid on renin-angiotensin system in rats with experimental nephritis

We previously demonstrated that all-trans retinoic acid (RA) preserves glomerular structure and function in anti-Thy1.1 nephritis (Wagner J, Dechow C, Morath C, Lehrke I, Amann K, Floege J, and Ritz E. J Am Soc Nephrol 11: 1479-1489, 2000). Because the renin-angiotensin system (RAS) contributes to renal damage, we 1) studied retinoid-specific effects on its components and 2) compared the effects of all-trans-RA with those of the AT(1)-receptor blocker candesartan. Rats were pretreated for 3 days before injection of the OX-7 antibody and continued with treatment with either vehicle or daily injections of 10 mg/kg all-trans-RA only (study 1) or 10 mg/kg body wt all-trans-RA, 1 mg/kg candesartan, or both (study 2) for an additional 7 days. The blood pressure increase observed in anti-Thy1.1 nephritic rats was equally normalized by all-trans-RA and candesartan (P < 0.05). In nephritic rats, mRNAs of angiotensinogen and angiotensin-converting enzyme (ACE) in the kidney were unchanged, but renin mRNA was lower (P < 0.01). Renal and glomerular AT(1)-receptor gene and protein expression levels were higher in anti-Thy1.1 nephritic rats (P < 0.05). In the renal cortex of nephritic rats, pretreatment with all-trans-RA significantly reduced mRNAs of all the examined RAS components, but in the glomeruli it increased ACE gene and protein expression (P < 0.01). In nephritic rats, candesartan reduced the number of glomerular cells and mitoses (P < 0.05) less efficiently than all-trans-RA (P < 0.01). Both substances reduced cellular proliferation (proliferating cell nuclear antigen) significantly (P < 0.05). No additive effects were noted when both compounds were combined. In conclusion, all-trans-RA influences the renal RAS in anti-Thy1.1 nephritis by decreasing ANG II synthesis and receptor expression. The beneficial effect of retinoids may be explained, at least in part, by reduction of RAS activity.

Sonic hedgehog is required during an early phase of oligodendrocyte development in mammalian brain

Oligodendrocyte precursor development in the embryonic spinal cord is thought to be regulated by the secreted signal, Sonic hedgehog (Shh). Such precursors can be identified by the expression of Olig genes, encoding basic helix-loop-helix factors, in the spinal cord and brain. However, the signaling pathways that govern oligodendrocyte precursor (OLP) development in the rostral central nervous system are poorly understood. Here, we show that Shh is required for oligodendrocyte development in the mouse forebrain and spinal cord, and that Shh proteins are both necessary and sufficient for OLP production in cortical neuroepithelial cultures. Moreover, adenovirus-mediated Olig1 ectopic expression can promote OLP formation independent of Shh activity. Our results demonstrate essential functions for Shh during early phases of oligodendrocyte development in the mammalian central nervous system. They further suggest that a key role of Shh signaling is activation of Olig genes.

TNF-alpha, PDGF, and TGF-beta(1) expression by primary mouse bronchiolar-alveolar epithelial and mesenchymal cells: tnf-alpha induces TGF-beta(1)

The bronchiolar-alveolar epithelium (BAE) is a primary target site for inhaled agents that cause lung injury. These cells, consequently, release a broad range of mediators that influence other cell populations, including interstitial lung fibroblasts that are central to the development of interstitial pulmonary fibrosis (IPF). A number of peptide growth factors (GF) have been postulated to be essential in the pathogenesis of IPF. We demonstrate here that primary populations of mouse BAE and mesenchymal cells, maintained in culture, synthesize four potent GF. These are platelet-derived growth factor isoforms (PDGF) A and B, transforming growth factor beta-1 (TGF-beta(1)), and tumor necrosis factor alpha (TNF-alpha). A mouse lung epithelial cell isolation technique pioneered in this laboratory has been used to purify the BAE cells to greater than 85% (80 +/- 5.6% alveolar type II and 9 +/- 2.3% Clara cells) in culture. Northern analysis, RNase protection assay, and immunocytochemistry (ICC) were used to establish mRNA and protein expression of the GF over time in the cultured BAE and mesenchymal cells. We show for the first time in these primary mouse lung cells that treatment of both cell types with TNF-alpha upregulates expression of TGF-beta(1). The four GF are produced by both epithelial and mesenchymal cells but with different temporal patterns. TGF-beta(1) is expressed constitutively by BAE and mesenchymal cells, whereas TNF-alpha expression wanes over time. The findings by ICC were consistent with levels of mRNA expression in both cell types. As genetically defined and altered mouse strains are becoming increasingly valuable for modeling lung disease, studying the gene expression patterns of target cells from these animals in vitro would be useful in sorting out the complex responses by individual cell types of the lung and the interactions among the multitude of mediators that are released during lung cell injury.

Characterization and expression of three forms of cDNA encoding chicken platelet-derived growth factor-A chain

Platelet-derived growth factor (PDGF) affects cell proliferation and differentiation during mammalian embryogenesis. In a number of avian species, PDGF-alpha receptors and PDGF-A chain (PDGF-A) are present during chicken limb and lens development. However, little is understood about the chicken PDGF-A gene. The present study identified short form type 1 (S1), long form (L) and short form type 2 (S2) cDNA clones encoding chicken PDGF-A chain (PDGF-A). These clones were isolated from a chicken hepatoma cell line (LMH) mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR) and cDNA library cloning. Genomic sequencing and Southern blotting revealed that these forms were generated by alternative splicing. The mRNAs of S1 and L contained two transcription start sites on one exon. At the amino acid level, the mature protein encoded by the L clone showed 90 and 85% homology with the processed coding regions of the long form of human and Xenopus PDGF-A, respectively. The putative mature peptides of all forms of chicken PDGF-A encompassed the eight cysteine residues conserved in all known forms of PDGF. We examined the expression of the three forms in chicken tissues and cells using RT-PCR. Expression of these forms varied among tissues and cells. Levels of PDGF mRNAs were very low in chicken thrombocytes, which are analogous to mammalian platelets. However, the level of PDGF-A chain mRNA expression in chicken thrombocytes peaked 4 h after exposure to type 1 collagen or thrombin, and then decreased gradually with continued incubation. These results suggest that chicken PDGF in thrombocytes plays an important role in the vascular system and in healing damaged tissue.

Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon

In the caudal neural tube, oligodendrocyte progenitors (OLPs) originate in the ventral neuroepithelium under the influence of Sonic hedgehog (SHH), then migrate throughout the spinal cord and brainstem before differentiating into myelin-forming cells. We present evidence that oligodendrogenesis in the anterior neural tube follows a similar pattern. We show that OLPs in the embryonic mouse forebrain express platelet-derived growth factor alpha-receptors (PDGFRA), as they do in more caudal regions. They first appear within a region of anterior hypothalamic neuroepithelium that co-expresses mRNA encoding SHH, its receptor PTC1 (PTCH) and the transcription factors OLIG1, OLIG2 and SOX10. Pdgfra-positive progenitors later spread through the forebrain into areas where Shh is not expressed, including the cerebral cortex. Cyclopamine inhibited OLP development in cultures of mouse basal forebrain, suggesting that hedgehog (HH) signalling is obligatory for oligodendrogenesis in the ventral telencephalon. Moreover, Pdgfra-positive progenitors did not appear on schedule in the ventral forebrains of Nkx2.1 null mice, which lack the telencephalic domain of Shh expression. However, OLPs did develop in cultures of Nkx2.1−/− basal forebrain and this was blocked by cyclopamine. OLPs also developed in neocortical cultures, even though Shh transcripts could not be detected in the embryonic cortex. Here, too, the appearance of OLPs was suppressed by cyclopamine. In keeping with these findings, we detected mRNA encoding SHH and Indian hedgehog (IHH) in both Nkx2.1−/− basal forebrain cultures and neocortical cultures. Overall, the data are consistent with the idea that OLPs in the telencephalon, possibly even some of those in the cortex, develop under the influence of SHH in the ventral forebrain.

Developmental roles of platelet-derived growth factors

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

Sonic hedgehog contributes to oligodendrocyte specification in the mammalian forebrain

This study addresses the role of Sonic hedgehog (Shh) in promoting the generation of oligodendrocytes in the mouse telencephalon. We show that in the forebrain, expression of the early oligodendrocyte markers Olig2, plp/dm20 and PDGFR(alpha) corresponds to regions of Shh expression. To directly test if Shh can induce the development of oligodendrocytes within the telencephalon, we use retroviral vectors to ectopically express Shh within the mouse embryonic telencephalon. We find that infections with Shh-expressing retrovirus at embryonic day 9.5, result in ectopic Olig2 and PDGFR(alpha) expression by mid-embryogenesis. By postnatal day 21, cells expressing ectopic Shh overwhelmingly adopt an oligodendrocyte identity. To determine if the loss of telencephalic Shh correspondingly results in the loss of oligodendrocyte production, we studied Nkx2.1 mutant mice in which telencephalic expression of Shh is selectively lost. In accordance with Shh playing a role in oligodendrogenesis, within the medial ganglionic eminence of Nkx2.1 mutants, the early expression of PDGFR(alpha) is absent and the level of Olig2 expression is diminished in this region. In addition, in these same mutants, expression of both Shh and plp/dm20 is lost in the hypothalamus. Notably, in the prospective amygdala region where Shh expression persists in the Nkx2.1 mutant, the presence of plp/dm20 is unperturbed. Further supporting the idea that Shh is required for the in vivo establishment of early oligodendrocyte populations, expression of PDGFR(alpha) can be partially rescued by virally mediated expression of Shh in the Nkx2.1 mutant telencephalon. Interestingly, despite the apparent requirement for Shh for oligodendrocyte specification in vivo, all regions of either wild-type or Nkx2.1 mutant telencephalon are competent to produce oligodendrocytes in vitro. Furthermore, analysis of CNS tissue from Shh null animals definitively shows that, in vitro, Shh is not required for the generation of oligodendrocytes. We propose that oligodendrocyte specification is negatively regulated in vivo and that Shh generates oligodendrocytes by overcoming this inhibition. Furthermore, it appears that a Shh-independent pathway for generating oligodendrocytes exists.

Early myotome specification regulates PDGFA expression and axial skeleton development

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

Platelet-derived growth factor is constitutively secreted from neuronal cell bodies but not from axons

Neurons synthesise and secrete many growth and survival factors but it is not usually clear whether they are released locally at the cell body or further afield from axons or axon terminals. Without this information, we cannot predict the site(s) of action or the biological functions of many neuron-derived factors. For example, can neuronal platelet-derived growth factor (PDGF) be secreted from axons and reach glial cells in nerve-fibre (white-matter) tracts? To address this question, we expressed PDGF-A in retinal ganglion neurons in transgenic mice and tested for release of PDGF from cell bodies in the retina and from axons in the optic nerve. In both the retina and optic nerve, there are glial cells that express PDGF receptor alpha (PDGFR alpha) [1] and divide in response to PDGF [2-5], so we could detect functional PDGF indirectly through the mitogenic response of glia at both locations. Expressing PDGF-A in neurons under the control of the neuron-specific enolase promoter (NSE-PDGF-A) resulted in a striking hyperplasia of retinal astrocytes, demonstrating that PDGF is secreted from the cell bodies of neurons in the retina [4]. In contrast, glial proliferation in the optic nerve was unaffected, indicating that PDGF is not released from axons. When PDGF was expressed directly in the optic nerve under the control of an astrocyte-specific promoter (GFAP-PDGF-A), oligodendrocyte progenitors hyperproliferated, resulting in a hypertrophic optic nerve. We conclude that PDGF is constitutively secreted from neuronal cell bodies in vivo, but not from axons in white-matter tracts.

Photoreceptor-specific expression of platelet-derived growth factor-B results in traction retinal detachment

Expression of platelet-derived growth factor (PDGF)-A and PDGF-B is increased in patients with proliferative retinopathies in which traction retinal detachments occur. Previous studies have demonstrated that increased expression of PDGF-A in the retina of transgenic mice results in retinal gliosis due to proliferation of astrocytes with different retinal phenotypes based on the time of onset and location of the PDGF-A production. In this study, we investigated the effects of PDGF-B in the retina using gain-of-function transgenic mice that express PDGF-B in photoreceptors. These mice show proliferation of astrocytes, pericytes, and, to a lesser extent, endothelial cells, resulting in ectopic cells on the surface and extending into the retina. The sheets of cells exert traction on the retina resulting in traction retinal detachments similar to those seen in humans with proliferative retinopathies. These studies suggest that PDGF-B has more dramatic effects in the retina than PDGF-A, because it acts on additional cell types, in particular on pericytes, which have a highly developed contractile apparatus. These studies in the retina suggest a means that could be used in other tissues throughout the body to achieve graded PDGF effects. They also provide a new model of traction retinal detachment that can be used to investigate new treatments for patients with proliferative retinopathies.

Retinoic acid reduces glomerular injury in a rat model of glomerular damage

ABSTRACT.: In the reaction of kidneys to injury, cytokine-driven proliferation plays an important role and precedes the development of glomerulosclerosis. There is great interest in agents that may interfere with such proliferation. Therefore, a rat model of mesangioproliferative glomerulonephritis (induced by anti-Thy1.1) was studied, and the effects of all-trans-retinoic acid (all-trans-RA) and isotretinoin, powerful antiproliferative and anti-inflammatory substances, on glomerular damage and cell proliferation were examined. Vehicle-injected control rats were compared with rats treated with daily subcutaneous injections of 10 mg/kg body wt all-trans-RA or 40 mg/kg body wt isotretinoin (n = 9 to 11 per group), using either a pretreatment (days -2 through 8) or posttreatment (days +3 through +8) protocol, i.e., starting before or after the induction of anti-Thy1.1 nephritis, respectively. All-trans-RA prevented the BP increase evoked by anti-Thy1.1 (anti-Thy1.1/vehicle, 112.2 +/- 4.8 mmHg; anti-Thy1.1/RA, 87.5 +/- 2. 5 mmHg; P < 0.001). Treatment with all-trans-RA or isotretinoin produced a 70% decrease in the urinary albumin excretion rate (P < 0. 02). Periodic acid-Schiff staining of saline-perfused kidneys (day 8) revealed significantly fewer glomerular cells in RA-treated nephritic rats (anti-Thy1.1/vehicle, 97 +/- 3.1 cells/glomerulus; anti-Thy1.1/RA, 80 +/- 4.4; P < 0.02; control/vehicle, 69 +/- 1.2). No difference was observed between all-trans-RA and isotretinoin treatment. The capillary occlusion scores were significantly lower for the anti-Thy1.1/RA-treated group (1.9 +/- 0.1) than for the anti-Thy1.1/vehicle-treated group (2.9 +/- 0.5, P < 0.001). In the anti-Thy1.1/vehicle-treated group, 11.9 +/- 1.1 glomerular cells were proliferating cell nuclear antigen-positive; however, in the anti-Thy1.1/RA-treated group, only 5.3 +/- 0.8 cells were proliferating cell nuclear antigen-positive (P < 0.002; control, 2.2 +/- 0.2). Glomerular mitoses were reduced by 67% in the anti-Thy1. 1/RA-treated group, compared with the anti-Thy1.1/control group (P < 0.002). Glomerular staining for platelet-derived growth factor B-chain was significantly reduced in anti-Thy1.1-treated nephritic rats in the presence of isotretinoin or all-trans-RA, compared with the vehicle-treated group (P < 0.001). It is concluded that all-trans-RA limits glomerular proliferation, glomerular lesions, and albuminuria in an established model of renal damage. The findings point to retinoids as potential novel modulators of glomerular injury.

Retinoic acid stimulates immature lung fibroblast growth via a PDGF-mediated autocrine mechanism

All trans-retinoic acid (RA) enhances alveolarization in neonates and reinitiates alveolarization in emphysematous adult rat lungs, suggesting that RA may stimulate cell proliferation by upregulating growth factor ligand and/or receptor expression either indirectly or directly by acting on RA-responsive genes encoding growth factors. We report that RA and 1,25-dihydroxyvitamin D(3) (Vit D), alone and in combination, significantly increase [(3)H]thymidine incorporation in cultured fetal and postnatal rat lung fibroblasts (P < 0.05). The greatest increase (11-fold) was seen in 4-day cells treated with the two agents in combination (P < 0.0001). [(3)H]thymidine incorporation was age dependent. The greatest response to RA occurred in 4-day fibroblasts (P < 0.01), whereas the response to Vit D was greatest in embryonic day 20 fibroblasts (P < 0.001). Neutralizing antibody to platelet-derived growth factor (PDGF)-AB decreased [(3)H]thymidine incorporation in response to RA alone or in combination with Vit D, indicating a role for PDGF. Expression of mRNAs for PDGF-A and PDGF receptor (PDGFR)-alpha and -beta was upregulated at the transcriptional level in an age- and treatment-dependent manner. Thus exogenous RA may influence alveolarization by stimulating fibroblast proliferation through a PDGF-mediated autocrine mechanism, which is enhanced when RA and Vit D are administered in combination.

Expression and regulation of histidine decarboxylase mRNA expression in the uterus during pregnancy in the mouse

It has been hypothesized that hormonally regulated histamine production plays a role in preparation of the uterus for implantation. Histidine decarboxylase (HDC) is the rate-limiting enzyme for histamine production. The current study was designed to determine intrauterine expression of HDC mRNA expression during pregnancy in the mouse. High levels of HDC mRNA expression were observed in the preimplantation mouse uterus with peak expression occurring on day 4. High levels of HDC mRNA expression were also detected in the post-implantation uterus. In an effort to determine whether HDC mRNA is regulated by pro-inflammatory cytokines, the HDC mRNA pattern was compared to intrauterine expression of mRNA's for interleukin-1alpha (IL-1alpha), IL-1beta, macrophage chemotactic protein-1 (MCP-1) and RANTES (regulated on activation, normal T expressed and secreted) during the peri-implantation period. IL-1beta, MCP-1 and RANTES mRNA levels were increased in the uterus on days 1-2 and on days 4-5. Increased expression of IL-1alpha mRNA was observed on days 1-2 and days 5-7. There was no clear relationship between HDC mRNA expression and cytokine/chemokine mRNA expression. Progesterone-stimulated intrauterine expression of HDC mRNA. Intrauterine cytokine/chemokine mRNA was also hormonally regulated. This data allowed the possibility that one or more of these pro-inflammatory cytokines could be involved in regulating intrauterine HDC mRNA production. Recombinant IL-1alpha, IL-1beta, MCP-1 and RANTES all failed to induce HDC mRNA expression in the preimplantation uterus in a mouse pseudopregnancy model. At the same time, IL-1beta induced the expression of mRNA for each of the four cytokines/chemokines. Despite the fact that these were also produced in the uterus during pregnancy and were hormonally regulated, none of these cytokines induced intrauterine HDC mRNA expression. The data suggest that progesterone is involved in the regulation of HDC mRNA expression in the preimplantation uterus, but IL-1alpha/beta, MCP-1 and RANTES, which have been reported to regulate histamine synthesis during inflammatory processes, do not appear to play a role.

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

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