Selective expression of the chicken platelet-derived growth factor alpha (PDGF alpha) receptor during limb bud development

VEGF coordinates interaction of pericytes and endothelial cells during vasculogenesis and experimental angiogenesis

Biological activities of vascular endothelial growth factor (VEGF) have been studied extensively in endothelial cells (ECs), but few data are available regarding its effects on pericytes. In murine embryoid body cultures, VEGF-induced expression of desmin and alpha-smooth muscle actin (alpha-SMA) in CD-31+ cells. The number of CD-31+/desmin+ vascular chords increased with VEGF treatment time and peaked during a differentiation window between 6 and 9 days after plating. In vivo, VEGF-induced elongation and migration of desmin-positive pericytes and coverage of angiogenic capillaries, as revealed by analysis of Sambucus nigra lectin-stained vascular beds of the chick chorioallantoic membrane. VEGF also caused significant decrease of intercapillary spaces, an indicator for intussusceptive vascular growth. These VEGF-mediated effects point at a more intricate interaction between ECs and pericytes cells than previously demonstrated and suggest that pericytes may be derived from EC progenitors in vitro and not only stabilize capillaries but also participate in vascular remodeling in vivo.

Effects of platelet-derived growth factor-AA and -BB on embryonic cardiac development

Several studies have shown that disruption of the normal expression patterns of platelet-derived growth factor (PDGF) ligands and receptors during development results in gross cardiac defects and embryonic or neonatal death. However, little is known about the specific role that PDGF plays in the differentiation of cardiac myocytes. In experiments complementing studies that utilized naturally-occurring Patch mice lacking the PDGFr alpha, or knockout animals lacking a PDGF ligand or receptor, we used rat and mouse whole-embryo culture (WEC) techniques to increase the exposure of embryos to the PDGF-AA or -BB ligands. Following a 48-hr culture period, we analyzed heart growth and cardiac myocyte differentiation. Exposure of rat embryos to 50 ng/ml of PDGF-AA resulted in a 42% increase in total protein levels in the heart, but did not result in a significant increase in heart growth, as determined by measurements of the atrioventricular length and the left ventricular length and width. Exposure of embryos to 50 ng/ml of PDGF-BB resulted in a 77% increase in total protein levels and a significant (P < 0.05) 8-15% increase in the measured heart parameters. Although a comparison of control and PDGF-AA-treated embryos showed no increase in the overall size of the heart, confocal microscopy showed an increase in the size and number of myofibrillar bundles in the developing myocardium. In addition, transmission electron microscopy (TEM) revealed an increase in the presence of sarcomeres, indicating that myofibrils were more highly differentiated in these areas of the treated embryos. In PDGF-BB-treated embryos, the compact zone of the myocardium was thicker and, as shown by confocal microscopy and TEM, f-actin and well-developed sarcomeres were more prevalent, indicating that the myofibrils were more differentiated in the treated embryos than in the control embryos. These studies indicate that increased exposure of embryonic hearts to PDGF-AA or -BB increases the rate of myocardial development.

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.

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.

Activation of the Jak-STAT-signaling pathway in embryonic lens cells

Previous studies showed that lens epithelial cells proliferate rapidly in the embryo and that a lens mitogen, most likely derived from the blood, is present in the anterior chamber of the embryonic eye (Hyatt, G. A., and Beebe, D. C., Development 117, 701-709, 1993). Messenger RNAs for several growth factor receptors have been identified in embryonic lens epithelial cells. We tested several growth factors that are ligands for these receptors for their ability to maintain lens cell proliferation. Embryo serum, PDGF, GM-CSF, and G-CSF maintained lens cell proliferation, but NGF, VEGF, and HGF did not. This and a previous study (Potts, J. D., Harocopos, G. J., and Beebe, D. C., Curr. Eye Res. 12, 759-763, 1993) detected members of the Janus kinase family (Jaks) in the developing lens. Because Jaks are central players in the Jak-STAT-signaling pathway, we identified STAT proteins in the lens and tested whether they were phosphorylated in response to mitogens. STAT1 and STAT3, but not STAT 5 were detected in chicken embryo lens epithelial cells. Only STAT3 was found in terminally differentiated lens fiber cells. STAT1 and STAT3 were phosphorylated in lens cells analyzed immediately after removal from the embryo and when lens epithelial explants were treated with embryo serum, PDGF, or GM-CSF, but not with NGF. Chicken embryo vitreous humor or IGF-1, factors that stimulate lens cell differentiation, but not proliferation, did not cause STAT phosphorylation. When lens epithelial cells were cultured for 4 h in unsupplemented medium, STAT1 and STAT3 declined to nearly undetectable levels. Treatment with PDGF or embryo serum for an additional 15 min restored STAT1 and -3 levels. This recovery was blocked by cycloheximide, but not actinomycin D, suggesting that STAT levels are regulated at the level of translation. STAT levels were maintained in epithelial explants by lens mitogens, but not by factors that stimulated lens fiber differentiation. Both factors that stimulated lens cell proliferation and those that caused fiber differentiation protected cultured lens epithelial cells from apoptosis. These data suggest that the factor(s) responsible for lens cell proliferation in vivo activates the Jak-STAT-signaling pathway. They also indicate that growth factors maintain STAT protein levels in lens epithelial cells by promoting the translation of STAT mRNA, an aspect of STAT regulation that has not been described previously. Signaling by most of the growth factors and cytokines known to activate the Jak-STAT pathway has been disrupted in mice by mutation or targeted deletion. Consideration of the phenotypes of these mice suggests that the factor responsible for lens cell proliferation in vivo may be a growth factor or cytokine that has not yet been described.

The control of cell death in the early chick embryo wing bud

Developmentally programmed cell death occurs in several regions of the chick wing bud. We have studied the nature and control of this cell death in vitro in tissues from two of these regions, the posterior necrotic zone (PNZ) and the opaque patch (OP). When tissue from these regions is excised prior to normal cell death and placed into organ culture, cell death ensues. Under these conditions, cell death in tissue from both of these regions is inhibited by fibroblast growth factor-2 (FGF-2). The only other growth factor we have found to have this function is insulin-like growth factor-II. Cell death in tissue from the OP and PNZ occurs by apoptosis, as indicated by the internucleosomal degradation of DNA and the inhibition of cell death by cycloheximide, an inhibitor of protein synthesis. If cell death is inhibited by FGF-2 and then the growth factor is washed away, a compensatory burst of cell death occurs in the PNZ tissue but not the OP tissue. This finding may indicate that in the PNZ, a death program progresses in the face of FGF-2 inhibition, resulting in more cells on the brink of death when the growth factor is removed.

Distribution and functions of platelet-derived growth factors and their receptors during embryogenesis

Platelet-derived growth factors (PDGFs) are soluble proteins that mediate intercellular signaling via receptor tyrosine kinases. The patterns of PDGF and PDGF receptor expression during embryogenesis are complex and dynamic and suggest that signaling can be autocrine or paracrine, depending on the particular tissue and the stage of development. Mesenchymal cells throughout the embryo and within some developing organs produce PDGF receptors, whereas their ligands are often produced by adjacent epithelial or endothelial cells. Disruption of PDGF signaling in the embryo leads to morphogenetic defects and embryonic or perinatal lethality. Tissues that are particularly susceptible to the absence of PDGF signaling are migrating mesoderm cells during gastrulation, nonneuronal neural crest cell derivatives, and kidney mesangial cells. These tissues share the common feature of undergoing epithelial-mesenchymal transitions. We review current knowledge of the distribution of PDGF ligands and receptors and discuss how this distribution may relate to several roles for PDGF during embryogenesis, particularly the regulation of mesenchymal cell behavior.

Expression of transforming growth factor beta in the embryonic avian lens coincides with the presence of mitochondria

During their maturation, lens cells lose all membrane bound organelles, including mitochondria. In chicken embryos this process begins in the central lens fibers beginning around embryonic day 12 (E12). Transforming growth factor beta (TGF beta) is a multipotent growth modulator thought to play a role in numerous developmental processes. TGF beta 1 has been localized to mitochondria in rat liver cells and muscle cells. In the present study, we examined the expression of TGF beta isoform mRNAs and proteins during chicken embryonic lens development. PCR analysis demonstrated TGF beta 2 and TGF beta 3 transcripts in the lens epithelium and fibers throughout pre- and post-hatching development. TGF beta isoforms were detected throughout the lens epithelium and fibers early in development (E6). However by E19, the distribution of TGF beta 2 and TGF beta 3 transcripts and proteins coincided with regions of the lens that contained mitochondria. In addition, intense TGF beta staining was observed in the basal portions of the equatorial epithelial cells, a region with abundant mitochondria. Transcripts for TGF beta 1 and TGF beta 4 were not detected in any tissue or time frame examined. Similarly, no immunostaining for TGF beta 1 was observed.