Platelet-derived growth factor B-chain of hematopoietic origin is not necessary for granulation tissue formation and its absence enhances vascularization

The role of endothelial cell-pericyte interactions in vascularization and diseases

BACKGROUND

Endothelial cells (ECs) and pericytes (PCs) are crucial components of the vascular system, with ECs lining the inner layer of blood vessels and PCs surrounding capillaries to regulate blood flow and angiogenesis. Intercellular communication between ECs and PCs is vital for the formation, stability, and function of blood vessels. Various signaling pathways, such as the vascular endothelial growth factor/vascular endothelial growth factor receptor pathway and the platelet-derived growth factor-B/platelet-derived growth factor receptor-β pathway, play roles in communication between ECs and PCs. Dysfunctional communication between these cells is associated with various diseases, including vascular diseases, central nervous system disorders, and certain types of cancers.

AIM OF REVIEW

This review aimed to explore the diverse roles of ECs and PCs in the formation and reshaping of blood vessels. This review focused on the essential signaling pathways that facilitate communication between these cells and investigated how disruptions in these pathways may contribute to disease. Additionally, the review explored potential therapeutic targets, future research directions, and innovative approaches, such as investigating the impact of EC-PCs in novel systemic diseases, addressing resistance to antiangiogenic drugs, and developing novel antiangiogenic medications to enhance therapeutic efficacy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Disordered EC-PC intercellular signaling plays a role in abnormal blood vessel formation, thus contributing to the progression of various diseases and the development of resistance to antiangiogenic drugs. Therefore, studies on EC-PC intercellular interactions have high clinical relevance.

Design of a composite biomaterial system for tissue engineering applications

Biomaterials that regulate vascularized tissue formation have the potential to contribute to new methods of tissue replacement and reconstruction. The goal of this study was to develop a porous, degradable tissue engineering scaffold that could deliver multiple growth factors and regulate vessel assembly within the porous structure of the material. Porous hydrogels of poly(ethylene glycol)-co-(L-lactic acid) (PEG-PLLA) were prepared via salt leaching. The degradation time of the hydrogels could be controlled between 1 and 7 weeks, based on hydrogel composition. Fibrin was incorporated into the interconnected pores of the hydrogels to promote neovascularization and as a reservoir for rapid (<5 days) growth factor delivery. Poly(lactic-co-glycolic acid) (PLGA) microspheres were incorporated into the degradable polymeric hydrogel scaffold to allow sustained (>30 days) growth factor delivery. Fibroblast growth factor-1 (FGF-1) and platelet-derived growth factor-BB (PDGF-BB) were delivered from the system owing to their roles in the promotion of angiogenesis and vascular stabilization, respectively. Hydrogels tested in vivo with a subcutaneous implantation model were selected based on the results from in vitro degradation and growth factor release kinetics. Dual growth factor delivery promoted significantly more tissue ingrowth in the scaffold compared with blank or single growth factor delivery. The sequential delivery of FGF-1 following PDGF-BB promoted more persistent and mature blood vessels. In conclusion, a biomaterials system was developed to provide structural support for tissue regeneration, as well as delivery of growth factors that stimulate neovascularization within the structure prior to complete degradation.

The brain tumor microenvironment

High-grade brain tumors are heterogeneous with respect to the composition of bona fide tumor cells and with respect to a range of intermingling parenchymal cells. Glioblastomas harbor multiple cell types, some with increased tumorigenicity and stem cell-like capacity. The stem-like cells maybe the cells of origin for tumor relapse. However, the tumor-associated parenchymal cells such as vascular cells,microglia, peripheral immune cells, and neural precursor cells also play a vital role in controlling the course of pathology.In this review, we describe the multiple interactions of bulk glioma cells and glioma stem cells with parenchymal cell populations and highlight the pathological impact as well as signaling pathways known for these types of cell-cell communication. The tumor-vasculature not only nourishes glioblastomas, but also provides a specialized niche for these stem-like cells. In addition, microglial cells,which can contribute up to 30% of a brain tumor mass,play a role in glioblastoma cell invasion. Moreover, non-neoplastic astrocytes can be converted into a reactive phenotype by the glioma microenvironment and can then secrete a number of factors which influences tumor biology. The young brain may have the capacity to inhibit gliomagenesis by the endogenous neural precursor cells, which secrete tumor suppressive factors. The factors, pathways, and interactions described in this review provide a new prospective on the cell biology of primary brain tumors, which may ultimately generate new treatment modalities. However, our picture of the multiple interactions between parenchymal and tumor cells is still incomplete.

The brain tumor microenvironment

High-grade brain tumors are heterogeneous with respect to the composition of bona fide tumor cells and with respect to a range of intermingling parenchymal cells. Glioblastomas harbor multiple cell types, some with increased tumorigenicity and stem cell-like capacity. The stem-like cells maybe the cells of origin for tumor relapse. However, the tumor-associated parenchymal cells such as vascular cells,microglia, peripheral immune cells, and neural precursor cells also play a vital role in controlling the course of pathology.In this review, we describe the multiple interactions of bulk glioma cells and glioma stem cells with parenchymal cell populations and highlight the pathological impact as well as signaling pathways known for these types of cell-cell communication. The tumor-vasculature not only nourishes glioblastomas, but also provides a specialized niche for these stem-like cells. In addition, microglial cells,which can contribute up to 30% of a brain tumor mass,play a role in glioblastoma cell invasion. Moreover, non-neoplastic astrocytes can be converted into a reactive phenotype by the glioma microenvironment and can then secrete a number of factors which influences tumor biology. The young brain may have the capacity to inhibit gliomagenesis by the endogenous neural precursor cells, which secrete tumor suppressive factors. The factors, pathways, and interactions described in this review provide a new prospective on the cell biology of primary brain tumors, which may ultimately generate new treatment modalities. However, our picture of the multiple interactions between parenchymal and tumor cells is still incomplete.

PDGF-C: a new performer in the neurovascular interplay

The importance of neurovascular crosstalk in development, normal physiology, and pathologies is increasingly being recognized. Although vascular endothelial growth factor (VEGF), a prototypic regulator of neurovascular interaction, has been studied intensively, defining other important regulators in this process is warranted. Recent studies have shown that platelet-derived growth factor C (PDGF-C) is both angiogenic and a neuronal survival factor, and it appears to be an important component of neurovascular crosstalk. Importantly, the expression pattern and functional properties of PDGF-C and its receptors differ from those of VEGF, and thus the PDGF-C-mediated neurovascular interaction may represent a new paradigm of neurovascular crosstalk.

Platelet-derived growth factors and their receptors in normal and malignant hematopoiesis

Platelet-derived growth factors (PDGF) bind to two closely related receptor tyrosine kinases, PDGF receptor α and β, which are encoded by the PDGFRA and PDGFRB genes. Aberrant activation of PDGF receptors occurs in myeloid malignancies associated with hypereosinophilia, due to chromosomal alterations that produce fusion genes, such as ETV6-PDGFRB or FIP1L1-PDGFRA. Most patients are males and respond to low dose imatinib, which is particularly effective against PDGF receptor kinase activity. Recently, activating point mutations in PDGFRA were also described in hypereosinophilia. In addition, autocrine loops have been identified in large granular lymphocyte leukemia and HTLV-transformed lymphocytes, suggesting new possible indications for tyrosine kinase inhibitor therapy. Although PDGF was initially purified from platelets more than 30 years ago, its physiological role in the hematopoietic system remains unclear. Hematopoietic defects in PDGF-deficient mice have been reported but appear to be secondary to cardiovascular and placental abnormalities. Nevertheless, PDGF acts directly on several hematopoietic cell types in vitro, such as megakaryocytes, platelets, activated macrophages and, possibly, certain lymphocyte subsets and eosinophils. The relevance of these observations for normal human hematopoiesis remains to be established.

Paradoxical effects of PDGF-BB overexpression in endothelial cells on engineered blood vessels in vivo

Therapeutic revascularization with either exogenous angiogenic growth factors or vascular cells has yet to demonstrate efficacy in the clinic. Injection of angiogenic growth factors often produces unstable and abnormal blood vessels. Blood vascular networks derived from implanted endothelial cells persist only transiently due to the insufficient recruitment of perivascular cells. We hypothesize that a combination of the two approaches may act synergistically to yield a better result. To enhance the recruitment of perivascular cells, human umbilical vein endothelial cells were genetically modified to overexpress platelet-derived growth factor (PDGF)-BB. PDGF-BB overexpression promoted both proliferation and migration of perivascular precursor cells (10T1/2 cells) in vitro. When mock-infected endothelial cells were implanted alone in vivo, they formed transient blood vascular networks that regressed by day 30. PDGF-BB overexpression enhanced the survival of endothelial cells in vivo. However, the PDGF-BB-expressing vessel network failed to establish patent blood flow. Co-implantation of PDGF-BB-overexpressing endothelial cells with 10T1/2 cells paradoxically resulted in the rapid regression of the vascular networks in vivo. PDGF-BB stimulated the expression of both chemokine (C-C motif) ligand 2 (CCL2) and CCL7 in 10T1/2 cells and led to the increased accumulation of macrophages in vivo. These results suggest a potential negative interaction between angiogenic growth factors and vascular cells; their use in combination should be carefully tested in vivo for such opposing effects.

Overview of murine thrombosis models

There are a myriad of options on where and how to perform thrombosis studies in mice. Models have been developed for systemic thrombosis, larger and smaller vessels of both the arterial and venous systems as well as several different microvascular beds. However, there are important differences between the models and investigators need to be careful and thoughtful when they choose which model to use.

The role of platelet-derived growth factor signaling in healing myocardial infarcts

OBJECTIVES

This study sought to examine the role of platelet-derived growth factor (PDGF) signaling in healing myocardial infarcts.

BACKGROUND

Platelet-derived growth factor isoforms exert potent fibrogenic effects through interactions with PDGF receptor (PDGFR)-alpha and PDGFR-beta. In addition, PDGFR-beta signaling mediates coating of developing vessels with mural cells, leading to the formation of a mature vasculature. We hypothesized that PDGFR activation may regulate fibrosis and vascular maturation in healing myocardial infarcts.

METHODS

Mice undergoing reperfused infarction protocols were injected daily with a neutralizing anti-PDGFR-beta antibody (APB5), an anti-PDGFR-alpha antibody (APA5), or control immunoglobulin G, and were killed after 7 days of reperfusion.

RESULTS

The PDGF-B, PDGFR-alpha, and PDGFR-beta mRNA expression was induced in reperfused mouse infarcts. Perivascular cells expressing phosphorylated PDGFR-beta were identified in the infarct after 7 days of reperfusion, indicating activation of the PDGF-BB/PDGFR-beta pathway. The PDGFR-beta blockade resulted in impaired maturation of the infarct vasculature, enhanced capillary density, and formation of dilated uncoated vessels. Defective vascular maturation in antibody-treated mice was associated with increased and prolonged extravasation of red blood cells and monocyte/macrophages, suggesting increased permeability. These defects resulted in decreased collagen content in the healing infarct. In contrast, PDGFR-alpha inhibition did not affect vascular maturation, but significantly decreased collagen deposition in the infarct.

CONCLUSIONS

Platelet-derived growth factor signaling critically regulates postinfarction repair. Both PDGFR-beta- and PDGFR-alpha-mediated pathways promote collagen deposition in the infarct. Activation of PDGF-B/PDGFR-beta is also involved in recruitment of mural cells by neovessels, regulating maturation of the infarct vasculature. Acquisition of a mural coat and maturation of the vasculature promotes resolution of inflammation and stabilization of the scar.

Endothelial/pericyte interactions

Interactions between endothelial cells and mural cells (pericytes and vascular smooth muscle cells) in the blood vessel wall have recently come into focus as central processes in the regulation of vascular formation, stabilization, remodeling, and function. Failure of the interactions between the 2 cell types, as seen in numerous genetic mouse models, results in severe and often lethal cardiovascular defects. Abnormal interactions between the 2 cell types are also implicated in a number of human pathological conditions, including tumor angiogenesis, diabetic microangiopathy, ectopic tissue calcification, and stroke and dementia syndrome CADASIL. In the present review, we summarize current knowledge concerning the identity, characteristics, diversity, ontogeny, and plasticity of pericytes. We focus on the advancement in recent years of the understanding of intercellular communication between endothelial and mural cells with a focus on transforming growth factor beta, angiopoietins, platelet-derived growth factor, spingosine-1-phosphate, and Notch ligands and their respective receptors. We finally highlight recent important data contributing to the understanding of the role of pericytes in tumor angiogenesis, diabetic retinopathy, and hereditary lymphedema.

Wound-healing studies in transgenic and knockout mice

Injury to the skin initiates a cascade of events including inflammation, new tissue formation, and tissue remodeling, that finally lead to at least partial reconstruction of the original tissue. Historically, animal models of repair have taught us much about how this repair process is orchestrated and, over recent years, the use of genetically modified mice has helped define the roles of many key molecules. Aside from conventional knockout technology, many ingenious approaches have been adopted, allowing researchers to circumvent such problems as embryonic lethality, or to affect gene function in a tissue- or temporal-specific manner. Together, these studies provide us with a growing source of information describing, to date, the in vivo function of nearly 100 proteins in the context of wound repair. This article focuses on the studies in which genetically modified mouse models have helped elucidate the roles that many soluble mediators play during wound repair, encompassing the fibroblast growth factor (FGF) and transforming growth factor-beta (TGF-beta) families and also data on cytokines and chemokines. Finally, we include a table summarizing all of the currently published data in this rapidly growing field. For a regularly updated web archive of studies, we have constructed a Compendium of Published Wound Healing Studies on Genetically Modified Mice which is avaialble at http://icbxs.ethz.ch/members/grose/woundtransgenic/home.html.

PDGF and cardiovascular disease

Platelet-derived growth factor (PDGF) was identified in a search for serum factors that stimulate smooth muscle cell (SMC) proliferation. During the development of lesions of atherosclerosis that can ultimately lead to vessel occlusion, SMC are stimulated by inflammatory factors to migrate from their normal location in the media. They accumulate within the forming lesion where they contribute to lesion expansion by proliferation and deposition of extracellular matrix. Different genetic manipulations in vascular cells combined with various inhibitory strategies have provided strong evidence for PDGF playing a prominent role in the migration of SMC into the neointima following acute injury and in atherosclerosis. Other activities of PDGF identified in vivo suggest additional functions for PDGF in the pathogenesis of cardiovascular disease.

PDGF-D induces macrophage recruitment, increased interstitial pressure, and blood vessel maturation during angiogenesis

Platelet-derived growth factor-D (PDGF-D) is a recently characterized member of the PDGF family with unknown in vivo functions. We investigated the effects of PDGF-D in transgenic mice by expressing it in basal epidermal cells and then analyzed skin histology, interstitial fluid pressure, and wound healing. When compared with control mice, PDGF-D transgenic mice displayed increased numbers of macrophages and elevated interstitial fluid pressure in the dermis. Wound healing in the transgenic mice was characterized by increased cell density and enhanced recruitment of macrophages. Macrophage recruitment was also the characteristic response when PDGF-D was expressed in skeletal muscle or ear by an adeno-associated virus vector. Combined expression of PDGF-D with vascular endothelial growth factor-E (VEGF-E) led to increased pericyte/smooth muscle cell coating of the VEGF-E-induced vessels and inhibition of the vascular leakiness that accompanies VEGF-E-induced angiogenesis. These results show that full-length PDGF-D is activated in tissues and is capable of increasing interstitial fluid pressure and macrophage recruitment and the maturation of blood vessels in angiogenic processes.

Chimera analysis supports a predominant role of PDGFRbeta in promoting smooth-muscle cell chemotaxis after arterial injury

The carotid artery shows a common response to many forms of injury, including a rapid activation of smooth muscle cell (SMC) proliferation in the media and migration of SMCs into the intima to form a neointima. Platelet-derived growth factor (PDGF) is believed to play a role in this response to injury, but it has proven difficult to distinguish whether it is stimulating cell migration or cell proliferation, and whether the action is direct or indirect. To determine this, we created chimeric mice composed of both wild-type (WT) and marked PDGF receptor beta (PDGFRbeta)-deficient cells, and determined the consequences of PDGFRbeta expression for SMC participation in response to ligation of the left common carotid artery. The proportion of PDGFRbeta-/- SMCs increased 4.5-fold in the media and decreased 1.8-fold during formation of the neointima, consistent with migration of WT SMCs out of the media and into the intima, leaving the PDGFRbeta-/- cells behind. The fibrotic reaction in the adventitia, which does not involve cell migration, did not result in any change in relative abundance of WT and PDGFRbeta-deficient fibroblasts. We conclude that the most significant direct role of PDGFRbeta is to mediate responses that involve cell migration rather than proliferation.

Regulation of wound healing by growth factors and cytokines

Cutaneous wound healing is a complex process involving blood clotting, inflammation, new tissue formation, and finally tissue remodeling. It is well described at the histological level, but the genes that regulate skin repair have only partially been identified. Many experimental and clinical studies have demonstrated varied, but in most cases beneficial, effects of exogenous growth factors on the healing process. However, the roles played by endogenous growth factors have remained largely unclear. Initial approaches at addressing this question focused on the expression analysis of various growth factors, cytokines, and their receptors in different wound models, with first functional data being obtained by applying neutralizing antibodies to wounds. During the past few years, the availability of genetically modified mice has allowed elucidation of the function of various genes in the healing process, and these studies have shed light onto the role of growth factors, cytokines, and their downstream effectors in wound repair. This review summarizes the results of expression studies that have been performed in rodents, pigs, and humans to localize growth factors and their receptors in skin wounds. Most importantly, we also report on genetic studies addressing the functions of endogenous growth factors in the wound repair process.

Blockade of platelet-derived growth factor or its receptors transiently delays but does not prevent fibrous cap formation in ApoE null mice

Platelet-derived growth factor (PDGF) is a potent stimulant of smooth muscle cell migration and proliferation in culture. To test the role of PDGF in the accumulation of smooth muscle cells in vivo, we evaluated ApoE -/- mice that develop complex lesions of atherosclerosis. Fetal liver cells from PDGF-B-deficient embryos were used to replace the circulating cells of lethally irradiated ApoE -/- mice. One month after transplant, all monocytes in PDGF-B -/- chimeras are of donor origin (lack PDGF), and no PDGF-BB is detected in circulating platelets, primary sources of PDGF in lesions. Although lesion volumes are comparable in the PDGF-B +/+ and -/- chimeras at 35 weeks, lesions in PDGF-B -/- chimeras contain mostly macrophages, appear less mature, and have a reduced frequency of fibrous cap formation as compared with PDGF-B +/+ chimeras. However, after 45 weeks, smooth muscle cell accumulation in fibrous caps is indistinguishable in the two groups. Comparison of elicited peritoneal macrophages by RNase protection assay shows an altered cytokine and cytokine receptor profile in PDGF-B -/- chimeras. ApoE -/- mice were also treated for up to 50 weeks with a PDGF receptor antagonist that blocks all three PDGF receptor dimers. Blockade of the PDGF receptors similarly delays, but does not prevent, accumulation of smooth muscle and fibrous cap formation. Thus, elimination of PDGF-B from circulating cells or blockade of PDGF receptors does not appear sufficient to prevent smooth muscle accumulation in advanced lesions of atherosclerosis.