Conditional targeting of the PDGF-B gene

Abstract: Platelet derived growth factors (PDGF) are known as mitogens for fibroblasts, smooth muscle cells (SMC) and glia cells in culture. Previous in vivo analyses have shown that PDGF-B is necessary for normal embryonic development and life after birth, but the relative importance of the various cellular sources of PDGF-B during development has not been clarified. In order to evaluate the role of PDGF-B from different cell types in embryonic as well as postnatal development, we have used the Cre/loxP system to enable tissue specific ablation of the PDGF-B gene. In this work we have studied the ablation of PDGF-B in blood vascular endothelium and in neurons. The endothelium-specific PDGF-B knockout leads to phenotypes similar to those seen in the total knockout, including loss of mesangial cells in the kidney glomeruli and loss of pericytes in the brain, leading to dilated capillaries. The severity of the phenotypes varies, depending on a variable chimeric situation caused by incomplete recombination of the PDGF-B gene. Most of the endothelium-specific mutants survive and reach adulthood, enabling studies of the retina, which develops postnatally. In these mice we found a situation similar to diabetic retinopathy, with pericyte loss leading to microangiopathy. However the chimeric situation led to an intraindividual variation of the pericyte amount, that correlated to the type of retinopathy. When the pericyte amount was below 52% of the normal situation, the retina showed signs of proliferative retinopathy, which was never seen above this threshold. We could also correlate the pericyte loss to regression of vessels. These findings indicate that endothelium derived PDGF-B is the critical source for the recruitment of pericytes to blood vessels and mesangial cells to the kidney glomeruli. Our data also suggest a causal link between pericyte loss and microangiopathy as well as proliferative retinopathy.The ablation of PDGF-B in neurons on the other hand did not lead to any obvious phenotypes. The animals develop into adulthood without apparent neuro-anatomical or behavioural defects. To evaluate a possible role of PDGF-B, following CNS trauma, we exposed these mice to electrical brain injury. This led to a normal healing process, showing reactive gliosis and neovascularization of the injured area.These results indicate that PDGF-B does not contribute to normal CNS development or reactive gliosis and hence the function of neural-derived PDGF-B is still unknown.