Exploring the role of PDGF-D in health and disease

Abstract: The platelet-derived growth factors (PDGF) and their receptors (PDGFRs) regulate growth and migration in cell populations of mesenchymal origin. The PDGF signaling system is vital for development, in neural crest formation, in organogenesis, and in maturation of the microvasculature. In the adult, PDGFs are needed to maintain homeostasis. They are also released in response to tissue injury, where they promote wound healing and neovascularization. In the adult, high expression of PDGFs is also seen in atherosclerosis, fibrosis and in malignant conditions. The PDGF family consists of four ligands that are present as dimers (PDGF-AA, PDGF-BB, PDGF-CC and PDGF-DD) and two tyrosine kinase receptors (PDGFRα and PDGFRβ). Upon ligand binding, receptor dimerization and auto-phosphorylation is induced. Downstream signaling leads to immediate effects in receptor-expressing cells, but also prolonged effects through modulation of transcription are seen. PDGF-D is the most recently found ligand and its biological function is still unclear, although its signaling receptor PDGFRβ is mainly expressed in vascular smooth muscle cells, thus indicating a vascular role also for PDGF-D. PDGF-B, the other PDGFRβ ligand, also binds to PDGFRα, thus making PDGF-D the only ligand that signals exclusively through PDGFRβ. Moreover, PDGF-D expression is uncoupled from its signaling as it is released in a latent, full-length form requiring proteolytic cleavage for receptor binding. In contrast, the other PDGFRβ ligand, PDGF-B, is active already upon release. PDGF signaling has been studied through a multitude of genetically modified animals, and these studies have contributed greatly to the understanding of PDGF function. In the work included in this thesis, we present the PDGF-D knockout mouse strain, and characterize the expression and function of PDGF-D in vivo, in both physiological conditions and in the tumor setting. We confirm that PDGF-D has a vascular expression pattern, and show that it is mainly expressed in arteries and in the endothelium, but it can also be expressed in vSMCs. We show that targeted deletion of PDGF-D affected an NG2-expressing pericyte population in the heart, and that animals lacking PDGF-D have slightly elevated blood pressure. Furthermore, we present evidence that paracrine PDGF-D signaling from the vasculature induces the production of factors from a rare PDGFRβ-expressing tumor cell subpopulation, thereby contributing to tumor growth. We also define a possible role for a co-receptor in this process. Finally, we present NRP1 as co-receptor for PDGF-D in PDGFRβ signaling, and thereby also suggest a mechanistic basis for PDGF-D-specific PDGFRβ-NRP1 complex formation and signaling. The addition of NRP1-mediated modulation adds complexity to the current model of PDGF-D/PDGFRβ signaling. Ultimately, these findings will lead to a better understanding of the role(s) of PDGF-D signaling, and thereby to improved development of tailored therapeutics for conditions where PDGF-D signaling might be dysregulated, such as atherosclerosis and cancer.