Chimaeric analysis reveals role of Pdgf receptors in all muscle lineages.

Blood vessels originate as simple endothelial cell tubes. It has been proposed that platelet-derived growth factor B polypeptide (Pdgfb) secreted by these endothelial cells drives the formation of the surrounding muscular wall by recruiting nearby mesenchymal cells. However, targetted inactivation of the Pdgfb gene or the Pdgf receptor beta (Pdgfrb) gene, by homologous recombination, does not prevent the development of apparently normal large arteries and connective tissue. We have used an in vivo competition assay in which we prepared chimaeric blastocysts, composed of a mixture of wild-type (Pdgfrb[+/+]) and Pdgfrb(+/-) or wild-type and Pdgfrb(-/-) cells, and quantified the relative success of cells of the two component genotypes in competing for representation in different cell lineages as the chimaeric embryos developed. This study revealed that the participation of Pdgfrb(-/-) cells in all muscle lineages (smooth, cardiac, skeletal and pericyte) was reduced by eightfold compared with Pdgfrb(+/+) cells, and that participation of Pdgfrb(+/-) cells was reduced by twofold (eightfold for pericytes). Pdgfrb inactivation did not affect cell contribution to non-muscle mesodermal lineages, including fibroblasts and endothelial cells. Chimaera competition is therefore a sensitive, quantitative method for determining developmental roles of specific genes, even when those roles are not apparent from analysis of purebred mutants; most likely because they are masked by homeostatic mechanisms.

Fas/FADD-mediated activation of a specific program of inflammatory gene expression in vascular smooth muscle cells.

Apoptosis of smooth muscle cells is a common feature of vascular lesions but its pathophysiological significance is not known. We demonstrate that signals initiated by regulated Fas-associated death domain protein overexpression in rat vascular smooth muscle cells in the carotid artery induce expression of monocyte-chemoattractant protein-1 and interleukin-8, and cause massive immigration of macrophages in vivo. These chemokines, and a specific set of other pro-inflammatory genes, are also upregulated in human vascular smooth muscle cells during Fas-induced apoptosis, in part through a process that requires interleukin-1alpha activation. Induction of a pro-inflammatory program by apoptotic vascular smooth muscle cells may thus contribute to the pathogenesis of vascular disease.

Two classes of PDGF receptor recognize different isoforms of PDGF.

Previous studies involving platelet-derived growth factor (PDGF) have been based on the premise that a single cell-surface receptor binds all three isoforms of PDGF (AA, BB, and AB). It is now shown that two populations of PDGF receptor exist and can be distinguished by their ligand binding specificity. The B receptor binds only the BB dimer, whereas the A/B receptor binds AA, BB, and AB dimers. Human dermal fibroblasts appear to express seven times as much B receptor as A/B receptor. The B receptor is responsible for most PDGF receptor phosphorylation.

Regional expression of the platelet-derived growth factor and its receptors in a primate graft model of vessel wall assembly.

Healing baboon polytetrafluoroethylene grafts express PDGF mRNA in the neointima. Perfusates of graft segments also contain PDGF-like mitogenic activity. To extend these findings, we studied the expression and regional distribution of the PDGF protein isoforms and their receptors in this prosthetic graft model. By immunohistochemistry, as well as ELISA and Western blot analysis of tissue extracts, both PDGF-A and PDGF-B were identified in macrophages within the interstices of the synthetic material. In contrast, the neointima contained predominantly PDGF-A localized to the endothelial surface and the immediate subjacent smooth muscle cell layers. Tissue extracts of neointima and graft material were mitogenic for baboon aortic smooth muscle cells in culture; nearly all of this proliferative activity was blocked by a neutralizing anti-PDGF antibody. PDGF receptor beta-subunit mRNA and protein were easily detectable in the neointima and graft material. PDGF receptor alpha-subunit mRNA was also observed in the graft matrix and at lower levels in the neointima. This pattern of ligand and receptor expression further implicates locally produced PDGF as a regulator of neointimal smooth muscle cell growth in this model. The coexpression of ligand and receptor in the macrophage-rich matrix also suggests that PDGF may participate in the foreign body response.

Endothelial cells of hematopoietic origin make a significant contribution to adult blood vessel formation.

Granulation tissue formation is an example of new tissue development in an adult. Its rich vascular network has been thought to derive via angiogenic sprouting and extension of preexisting vessels from the surrounding tissue. The possibility that circulating cells of hematopoietic origin can differentiate into vascular endothelial cells (ECs) in areas of vascular remodeling has recently gained credibility. However, no quantitative data have placed the magnitude of this contribution into a physiological perspective. We have used hematopoietic chimeras to determine that 0.2% to 1.4% of ECs in vessels in control tissues derived from hematopoietic progenitors during the 4 months after irradiation and hematopoietic recovery. By contrast, 8.3% to 11.2% of ECs in vessels that developed in sponge-induced granulation tissue during 1 month derived from circulating hematopoietic progenitors. This recruitment of circulating progenitors to newly forming vessels would be difficult to observe in standard histological studies, but it is large enough to be encouraging for attempts to manipulate this contribution for therapeutic gain.

Association of increased platelet-derived growth factor secretion and retroviral expression in transformed mouse and human cells.

The activation of platelet-derived growth factor (PDGF) production by transformed cells is often observed but not well understood. We have examined cell lines that showed "spontaneous" increases in PDGF secretion, i.e., in which the increase was not in response to intentional intervention. In one case the increase was associated with an obvious change in morphology and mitogen requirements accompanying spontaneous transformation of Swiss 3T3 cells. In the other case the increase occurred during growth of a human tumor cell in a nude mouse and was not associated with an alteration in the morphology or growth properties of the cells. Rates of PDGF secretion did not correlate with specific changes in the pattern of expression of PDGF mRNA. In the human tumor system PDGF A- and B-chain transcripts were present at similar levels before and after transplantation in the nude mouse. In the 3T3 cell system, B-chain transcripts were detected only after transformation, and there was no change in the low basal expression of A-chain. A change which did consistently correlate with the increased secretion of PDGF was that both the spontaneously transformed murine cells and the transplanted human cells expressed murine leukemia virus transcripts and synthesized retroviral envelope glycoproteins, while their original counterparts did not.

A receptor-like inositol lipid phosphatase is required for the maturation of developing cochlear hair bundles.

A screen for protein tyrosine phosphatases (PTPs) expressed in the chick inner ear yielded a high proportion of clones encoding an avian ortholog of protein tyrosine phosphatase receptor Q (Ptprq), a receptor-like PTP. Ptprq was first identified as a transcript upregulated in rat kidney in response to glomerular nephritis and has recently been shown to be active against inositol phospholipids. An antibody to the intracellular domain of Ptprq, anti-Ptprq, stains hair bundles in mice and chicks. In the chick ear, the distribution of Ptprq is almost identical to that of the 275 kDa hair-cell antigen (HCA), a component of hair-bundle shaft connectors recognized by a monoclonal antibody (mAb) that stains inner-ear hair bundles and kidney glomeruli. Furthermore, anti-Ptprq immunoblots a 275 kDa polypeptide immunoprecipitated by the anti-HCA mAb from the avian inner ear, indicating that the HCA and Ptprq are likely to be the same molecule. In two transgenic mouse strains with different mutations in Ptprq, anti-Ptprq immunoreactivity cannot be detected in the ear. Shaft connectors are absent from mutant vestibular hair bundles, but the stereocilia forming the hair bundle are not splayed, indicating that shaft connectors are not necessary to hold the stereocilia together; however, the mice show rapid postnatal deterioration in cochlear hair-bundle structure, associated with smaller than normal transducer currents with otherwise normal adaptation properties, a progressive loss of basal-coil cochlear hair cells, and deafness. These results reveal that Ptprq is required for formation of the shaft connectors of the hair bundle, the normal maturation of cochlear hair bundles, and the long-term survival of high-frequency auditory hair cells.

Proliferation of chicken myoblasts is regulated by specific isoforms of platelet-derived growth factor: evidence for differences between myoblasts from mid and late stages of embryogenesis.

In the present study we measured the level of PDGF receptor expression by chicken myoblasts and the effect of the three different PDGF isoforms (AA, AB, BB) on DNA synthesis by myoblasts. We examined PDGF receptor expression and function on clonally derived myoblasts in order to eliminate contaminating fibroblasts which are present in myogenic cultures and which bind PDGF. Furthermore, since we have previously shown that fetal myoblasts are replaced with adult myoblasts during late chicken embryogenesis, we compared PDGF receptor expression and function on myoblasts from Embryonic Day 10 (E10, mid development) and from Embryonic Day 19 (E19, late development). We found that all myogenic clones from late embryos (E19) express many receptors for PDGF-BB, far fewer receptors for PDGF-AB, and even fewer, if any, receptors for PDGF-AA. Myoblast clones derived from E10 were more heterogeneous in their PDGF binding pattern ranging from clones similar to E19 clones to clones having very few PDGF binding sites. We also found that both PDGF-AB and PDGF-BB can promote DNA synthesis by clonally derived chicken myoblasts maintained in 2.5% fetal bovine serum whereas PDGF-AA has no detectable effect. Finally, we observed that primary myogenic cultures from E10 and E19 differ strikingly in levels of PDGF binding; E19 cultures bind much more PDGF than do E10 cultures. We conclude that PDGF can enhance the proliferation of chicken myoblasts and that myoblasts responsive to PDGF are more frequent in late than in mid stages of development. We propose that PDGF may be a modulator of myogenesis of adult but not fetal myoblasts.

Locally acting growth factors for vascular smooth muscle cells: endogenous synthesis and release from platelets.

Release of platelet-derived growth factor (PDGF) from platelets has been postulated to stimulate at least some of the cell proliferation seen at sites of tissue damage, both beneficially (wound healing) and perniciously (during formation of atherosclerotic lesions). Two other growth factors have been localized to the platelet: epidermal growth factor and transforming growth factor. These factors may function synergistically with PDGF in promoting smooth muscle cell proliferation in the injured vessel wall. PDGF-like molecules (PDGF-c) that bind to the PDGF receptor and are at least partially recognized by antiserum against PDGF may also be synthesized by vessel wall cells themselves under certain circumstances. Arterial endothelial cells secrete several mitogens, one of which is a PDGF-c. Release is greatly stimulated by exposure of the cells to physiologic concentrations of thrombin. Also, aortic smooth muscle cells from 2-week-old rats secrete mitogenic levels of PDGF-c. In this case, PDGF-c accounts for all the mitogenic activity in conditioned medium (when assayed on 3T3 cells). Smooth muscle cells obtained from adult rat aortae secrete 150-fold less PDGF-c. In a third example, when adult rat carotid arteries are damaged with a balloon catheter, smooth muscle cells migrate into the intima of the artery and proliferate. By 2 weeks, the number of smooth muscle cells in the artery has doubled. When these intimal smooth muscle cells are cultured, they are found to secrete PDGF-c. These findings suggest that activation of endogenous synthesis of PDGF-c may contribute to the smooth muscle cell proliferation seen in response to vascular injury.

Synthesis, phosphorylation, and degradation of multiple forms of the platelet-derived growth factor receptor studied using a monoclonal antibody.

We have developed a monoclonal antibody, designated PR7212 (IgG1), which specifically recognizes the platelet-derived growth factor receptor (PDGFR) of primate cells. The antibody recognizes an extracellular epitope of the receptor, demonstrated by its ability to bind to intact cells. Using this antibody, we have detected three forms of PDGFR of approximately 180, 164, and 130 kDa. All three of the forms were detected by Western blot analysis of human dermal fibroblasts. Immunoprecipitates of 32P-labeled membrane extracts of human dermal fibroblasts demonstrate that phosphorylation of all three forms of the receptor is stimulated by PDGF. In addition, several smaller molecules were detected, ranging in size from 113 to 49 kDa, which are also phosphorylated in response to PDGF addition. These smaller molecules may be either PDGFR kinase substrates or partially degraded PDGFR. Only the 180- and the 164-kDa forms of the receptor are detectable from immunoprecipitates of soluble extracts of 35S-metabolically labeled cells. Pulse-chase experiments demonstrate that the 164-kDa form is a precursor of the 180-kDa molecule. After PDGF binding at 37 degrees C, the 180-kDa form disappears from the cell surface in parallel with a decrease in 125I-PDGF binding, providing evidence that occupation results in internalization of PDGFR rather than inactivation.

PDGF-AB requires PDGF receptor alpha-subunits for high-affinity, but not for low-affinity, binding and signal transduction.

There are two PDGF receptor proteins (PDGFR alpha and PDGFR beta) which are proposed to function as subunits to form a high-affinity dimeric PDGF receptor. One aspect of this model about which there is still disagreement is whether PDGF-AB can bind to cells that express only PDGFR beta and, if so, whether PDGF-AB can act as an agonist or an antagonist. To address this question, we derived 3T3 cell lines from Patch mutant mouse embryos in which the PDGFR alpha gene is deleted but which express normal levels of PDGFR beta. Comparison between the binding and response properties of mutant and wild type 3T3 cell lines allowed us to define the contribution that PDGFR alpha makes to the ability of a cell to bind, and respond to, PDGF-AB. We found that PDGF-AB binds to PDGFR alpha-negative 3T3 cells and can induce DNA synthesis, PDGFR beta dimerization, and phosphorylation on tyrosine. In addition we found that PDGF-AB binding and stimulation of these activities is strongly temperature-dependent, whereas PDGF-AB binding and activation of PDGFR beta in the presence of PDGFR alpha is not. However, 3T3 cells that do not express PDGFR alpha require for activation PDGF-AB concentrations that were nearly 100-fold greater than for cells that do express PDGFR alpha. These results suggest that neither PDGF-AA nor PDGF-AB are likely to be physiologically significant activators of cells unless the cells express PDGFR alpha.

Differential regulation of expression of two platelet-derived growth factor receptor subunits by transforming growth factor-beta.

The binding of three radiolabeled isoforms of platelet-derived growth factor (PDGF), 125I-PDGF-AA, 125I-PDGF-AB, and 125I-PDGF-BB, is differentially affected by exposure of quiescent 3T3 cells to transforming growth factor-beta (TGF-beta). By 24 h after exposure to TGF-beta, binding of 125I-PDGF-AA and 125I-PDGF-AB is almost completely lost, whereas binding of 125I-PDGF-BB is reduced by only 40%. The loss of PDGF-binding sites caused by TGF-beta is time- and concentration-dependent and reflects a change in the pattern of expression of receptor subunits; the number of alpha-subunits decreases, and the number of beta-subunits increases. The loss of binding sites for PDGF-AA is accompanied by a decreased mitogenic response to PDGF-AA but not to PDGF-AB or PDGF-BB. These results suggest that TGF-beta may differentially regulate the expression of PDGF-binding sites and the mitogenic responsiveness toward the three PDGF isoforms. TGF-beta did not stimulate synthesis of PDGF A-chain mRNA or PDGF-AA protein, and PDGF-AA receptors could not be restored by the presence of suramin, suggesting that the loss of binding sites may result from direct effects on receptor expression rather than autocrine down-regulation by PDGF-AA.

Sera and conditioned media contain different isoforms of platelet-derived growth factor (PDGF) which bind to different classes of PDGF receptor.

Platelet-derived growth factor (PDGF) is encoded by separate genes for two possible subunit chains (A-chain and B-chain) which can form three possible dimers (AA, AB, and BB). We have recently presented evidence that multiple forms of PDGF receptor exist which distinguish between these isoforms (Hart, C. H., Forstrom, J. W., Kelley, J. D., Smith, R. A., Ross, R., Murray, M. J., and Bowen-Pope, D. F. (1988) Science 240, 1529-1531). We used this specificity to determine the amount of PDGF from different sources which is able to bind to each class of receptor and found that each source had a characteristic isoform composition. Levels of total PDGF activity in sera from different species ranged more than 15-fold, from less than 1 ng/ml in dog, chicken, pig, and calf, to greater than 13 ng/ml in mouse and human. Despite these differences in PDGF content, the total mitogenic activities of the sera were comparable indicating that the relative importance of PDGF as a serum mitogen may vary considerably between species. Analysis of the total PDGF into the amounts of each isoform revealed great differences in composition. PDGF-BB constitutes only about 15% of the total binding activity in human PDGF purified by the method of Raines and Ross (Raines, E. W., and Ross, R. (1982) J. Biol. Chem. 257, 5154-5160) but is the predominant isoform in whole blood serum from all other species. In contrast to serum, medium conditioned by cultured PDGF-secreting cell types contained no detectable PDGF-BB except in two cases: medium conditioned by vascular endothelial cells and by cells transformed by simian sarcoma virus. The existence of isoform-specific PDGF receptors and the large variation in PDGF isoform composition dependent upon source may provide an important mechanism through which the effects of PDGF can be targeted to different cell types and/or toward eliciting different cell responses.

Regulation of platelet-derived growth factor receptor expression by cell context overrides regulation by cytokines.

Immunocytochemical data has indicated that platelet-derived growth factor receptor beta-subunit (PDGFR beta) expression by connective tissue cells is up-regulated in many disease states. To investigate potential causes of this up-regulation, we have evaluated conditions that regulate PDGF receptor transcript levels in cultured diploid human fibroblast model systems. We found combinations of soluble mediators and cell "context," which can regulate receptor transcripts (and receptor protein) over a 50-fold range, with cell context factors being far more potent regulators than soluble mediators. For cells grown under standard monolayer conditions on plastic, levels of both PDGFR beta and PDGFR alpha increase 10-fold as culture density increases. Cells grown in suspension or in three-dimensional gels express 10- to 20-fold higher transcript levels than cells plated on plastic at comparable density and serum concentration. The soluble mediators tested, including 14 cytokines and conditioned medium from activated lymphocytes, have only modest effects on transcript levels. Lymph decreases PDGFR beta transcript expression 4-fold, suggesting that a component of interstitial fluid contributes to maintenance of the low basal level of expression in normal tissues. The mitogenic responsiveness of cells cultured at different densities parallels the level of PDGFR beta expression. Blocking anti-PDGF receptor antibodies decrease receptor availability and mitogenic responsiveness in parallel. In both cases, the striking overlap between the PDGF-BB binding and mitogenesis dose-response curves suggests that the level of PDGF receptor expression can limit responsiveness to PDGF. Overall, these results suggest that the up-regulation of PDGF receptor expression seen under pathological conditions may be due to disruption of the cell's normal environment/context/cell shape/cell attachment and that this could serve to ensure that a proliferative response to PDGF would occur only under conditions in which there had been significant tissue damage.

Developmentally regulated production of platelet-derived growth factor-like molecules.

Platelet-derived growth factor (PDGF) is thought to mediate the proliferation of smooth muscle cells in injured arteries, and may be involved in the pathogenesis of atherosclerosis. PDGF-like molecules from non-platelet sources may also play a role in the regulation of cell activity in other circumstances. Transformation of cells by a wide range of oncongenic agents appears to activate a cellular gene encoding a PDGF-like molecule, possibly accounting for the ability of transformed cells to grow without addition of exogenous mitogens. We show here that a molecule (PDGF-c) which can compete with 125I-PDGF for binding to PDGF receptors is secreted by cultured rat aortic smooth muscle cells (rASMC) isolated from 13 to 18-day-old rats (pups) but not from three-month-old animals (adults). Thus, production of PDGF-c appears to be developmentally regulated and may be a factor in the more rapid proliferation of rASMC and synthesis of connective tissue components which occurs during growth of the aorta in vivo.

Protein-tyrosine phosphatases in the vessel wall: differential expression after acute arterial injury.

Many protein-tyrosine phosphatases (PTPases) have now been identified, but little is known about PTPase expression and regulation in vascular tissue and in vascular disease. Polymerase chain reaction (PCR) amplification and cDNA fingerprinting of PTPase catalytic domains, combined with random sequencing of PCR product libraries, identified 18 (8 receptor-like and 10 cytosolic) PTPases in the rat carotid artery and revealed differential expression of 5 of these PTPases during neointima formation after balloon catheter injury. In situ hybridization was used to localize mRNA expression in vessel cross sections for the 5 differentially expressed PTPases. This revealed that for 3 PTPases (SHP1, CD45, and PTPbeta), differential transcript abundance was due to appearance/loss of the cell types by which they were expressed (leukocytes for SHP1 and CD45, endothelial cells for PTPbeta). However, mRNA expression of 2 PTPases (PTPL1 and PTP1B) was specifically upregulated by proliferating and migrating smooth muscle cells (SMCs) in characteristic temporal and regional patterns in response to vessel damage. Quantitative PCR analysis showed that PTP1B and PTPL1 were induced approximately 30-fold and approximately 60-fold, respectively, by 2 weeks after injury in the damaged vessels compared with the uninjured vessels. PTP1B was rapidly upregulated in the media after vessel injury and remained highly expressed in the developing neointima. By contrast, PTPL1 expression did not increase dramatically until the SMCs had migrated into the intima. The differential expression of PTP1B and PTPL1 by SMCs after injury suggests roles for these PTPases in the regulation of vessel wall remodeling.

Chimera analysis reveals that fibroblasts and endothelial cells require platelet-derived growth factor receptorbeta expression for participation in reactive connective tissue formation in adults but not during development.

The hypothesis that platelet-derived growth factor (PDGF) plays an important role in repair of connective tissue has been difficult to test experimentally, in part because the disruption of any of the PDGF ligand and receptor genes is embryonic lethal. We have developed a method that circumvents the embryonic lethality of the PDGF receptor (R)beta-/- genotype and minimizes the tendency of compensatory processes to mask the phenotype of gene disruption by comparing the behavior of wild-type and PDGFRbeta-/- cells within individual chimeric mice. This quantitative chimera analysis method has revealed that during development PDGFRbeta expression is important for all muscle lineages but not for fibroblast or endothelial lineages. Here we report that fibroblasts and endothelial cells, but not leukocytes, are dependent on PDGFRbeta expression during the formation of new connective tissue in and around sponges implanted under the skin. Even the 50% reduction in PDGFRbeta gene dosage in PDGFRbeta+/- cells reduces fibroblast and endothelial cell participation by 85%. These results demonstrate that the PDGFRbeta/PDGF B-chain system plays an important direct role in driving both fibroblast and endothelial cell participation in connective tissue repair, that cell behavior can be regulated by relatively small changes in PDGFRbeta expression, and that the functions served by PDGF in wound healing are different from the roles served during development.

Expression of platelet-derived growth factor and its receptors in the developing and adult mouse kidney.

BACKGROUND: Experimental analysis of gene function is increasingly being accomplished using mouse models. Glomerular malformations occur in mice in which the platelet-derived growth factor (PDGF) B-chain gene or the PDGF receptor beta-subunit gene have been deleted. To understand potential PDGF signaling pathways in the kidney, we determined the expression pattern of PDGF ligand and receptor genes in mouse kidney during development and in the mature adult kidney. METHODS: We used in situ hybridization to map the expression of transcripts encoding the PDGF ligands (A-chain and B-chain) and PDGF receptors (PDGFRalpha and PDGFRbeta) in the developing and mature kidney of the mouse. RESULTS: PDGF A-chain transcripts are expressed by epithelial cells (especially in what appear to be the loop of Henle) and possibly in vascular smooth muscle cells. Its receptor, PDGFRalpha, is expressed by interstitial cells. PDGF B-chain transcripts are most highly expressed by vascular endothelial cells of developing and adult kidney and minimally by visceral epithelia of immature glomeruli. PDGFRbeta transcripts are expressed by fetal blastemal cells, interstitial cells, mesangial cells, and vascular smooth muscle cells and by adult mesangial and interstitial cells. PDGFRalpha and PDGFRbeta expression is especially prominent in lipid-laden interstitial cells in the adult kidney. CONCLUSIONS: These patterns of expression are similar, but not identical, to those observed in rat and human and suggest that paracrine interactions mediated by the PDGF/PDGF receptor system may coordinate the development of the tubular, vascular, and interstitial components during kidney development and disease.

PDGF-receptor localizes to mesangial, parietal epithelial, and interstitial cells in human and primate kidneys.

There is evidence that platelet derived growth factor (PDGF) is a mediator of proliferative changes in renal arteries and mesangium in human disease, in the mesangium in experimental mesangial proliferative glomerulonephritis, and in the interstitium in a rodent model of angiotensin II mediated hypertension. We utilized a monoclonal antibody to the beta-subunit of the PDGF-receptor to localize constitutive expression of this receptor in human and nonhuman primate tissues. Tissues were fixed in cold 2 or 4% paraformaldehyde, and immunohistochemical techniques both at the light microscopic level and immunoelectron microscopy were employed. In the glomerulus, there is widespread expression of this molecule by mesangial cells, and there is frequent expression on the apical and lateral surface of parietal epithelial cells. There is also widespread expression of this molecule by cortical and medullary peritubular interstitial cells, but not by glomerular or peritubular capillary endothelium or other renal parenchymal structures. The identification of receptors capable of binding PDGF B-chain at each of these sites: (1) provides a basis for PDGF mediated mesangial proliferation in human disease; (2) provides a basis for PDGF mediated interstitial cell migration and/or proliferation and/or activation at sites of tubulointerstitial injury; and (3) suggests that glomerular parietal epithelial cells may be responsive to stimulation by PDGF.

Developmental patterns of PDGF B-chain, PDGF-receptor, and alpha-actin expression in human glomerulogenesis.

Expression of PDGF B-chain and the PDGF receptor beta-subunit (PDGFR beta) is detected immunocytochemically during the development of glomeruli in human kidneys of 54 to 105 days gestational age. During the early stages (vesicular, comma-shape and S-shape) of glomerulogenesis, PDGF B-chain is localized to differentiating epithelium of the glomerular vesicle, while PDGFR beta is expressed in the undifferentiated metanephric blastema, vascular structures, and interstitial cells. During this stage PDGF may be acting as a paracrine growth factor and as a chemoattractant acting to recruit mesangial progenitor cells into the developing glomerulus. As the glomerular tuft forms, both PDGF B-chain and PDGFR beta can be detected in an arboreal pattern radiating from the hilus of the glomerular tuft. Immunocytochemical studies using markers specific to endothelium (Ulex europaeus I lectin, Factor VIII related antigen), and smooth muscle (alpha-smooth muscle actin), indicate that the PDGF B-chain and PDGFR beta are both expressed primarily by mesangial cells. During this stage, PDGF may be acting primarily to provide an autocrine factor to mediate further mesangial cell proliferation. Glomerular expression of alpha-smooth muscle actin is limited to later stages of glomerulogenesis; at these stages the pattern of expression is similar to that of PDGF-B chain and PDGFR beta. The upregulation of mesangial PDGF, PDGFR beta, and alpha-smooth muscle actin expression that has been identified in some disease states in both humans and experimental animals appears to represent a recapitulation of this normal developmental process.