Discovery of microvascular miRNAs using public gene expression data: miR-145 is expressed in pericytes and is a regulator of Fli1

International audienceBACKGROUND: A function for the microRNA (miRNA) pathway in vascular development and angiogenesis has been firmly established. miRNAs with selective expression in the vasculature are attractive as possible targets in miRNA-based therapies. However, little is known about the expression of miRNAs in microvessels in vivo. Here, we identified candidate microvascular-selective miRNAs by screening public miRNA expression datasets. METHODS: Bioinformatics predictions of microvascular-selective expression were validated with real-time quantitative reverse transcription PCR on purified microvascular fragments from mouse. Pericyte expression was shown with in situ hybridization on tissue sections. Target sites were identified with 3' UTR luciferase assays, and migration was tested in a microfluid chemotaxis chamber. RESULTS: miR-145, miR-126, miR-24, and miR-23a were selectively expressed in microvascular fragments isolated from a range of tissues. In situ hybridization and analysis of Pdgfb retention motif mutant mice demonstrated predominant expression of miR-145 in pericytes. We identified the Ets transcription factor Friend leukemia virus integration 1 (Fli1) as a miR-145 target, and showed that elevated levels of miR-145 reduced migration of microvascular cells in response to growth factor gradients in vitro. CONCLUSIONS: miR-126, miR-24 and miR-23a are selectively expressed in microvascular endothelial cells in vivo, whereas miR-145 is expressed in pericytes. miR-145 targets the hematopoietic transcription factor Fli1 and blocks migration in response to growth factor gradients. Our findings have implications for vascular disease and provide necessary information for future drug design against miRNAs with selective expression in the microvasculature

A functional genomics approach to PDGF and pericyte biology

The rapid expansion of genome sequence data demands for increased efforts and speed concerning functional analysis of gene products. Two approaches to such analysis, namely transgenic technologies and analysis of gene expression have successfully been used to study functions of gene products in complex organisms, such as mammals. In this thesis I present functional analyses of platelet-derived growth factors (PDGFs) during development using these two approaches. The PDGF family has four members, PDGF-A-D, and their functions in vivo have been extensively studied with transgenic techniques. Mice lacking the expression of PDGF-A exhibit defects in multiple tissues, including lung, gut and brain. This thesis presents novel findings demonstrating the importance of PDGF-A also in the development of skin and testis. It gives further evidence for PDGF-A as an important regulator of the proliferation of mesenchymal progenitor cells. Mice lacking PDGF-A have a reduced number of such cells in both the skin and in the testis, leading eventually to abnormalities in skin and hair and to testicular defects including a loss of Leydig cells and arrest of spermatogenesis. Mice lacking PDGF-B or its receptor PDGF-Rb show abnormal development of the vascular system resulting in microaneurysms and bleedings. These defects are due to the lack of vascular smooth muscle cells and pericytes surrounding the endothelial tube in blood vessels. Pericytes are found in capillaries and their functions have long been debated. We have used mice lacking PDGF-B or PDGF-Rb to identify genes specifically expressed in pericytes. Analysing the transcriptome of pericytes will reveal information about the intracellular signalling in pericytes and about their communication with other cells. Such analyses may lead to the identification of new drug targets in diseases involving pericytes and blood vessel formation

PDGF-A and PDGF-B induces cardiac fibrosis in transgenic mice

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) contribute to normal heart development. Deficient or abnormal expression of Pdgf and Pdgfr genes have a negative impact on cardiac development and function. The cellular effects of PDGFs in the hearts of Pdgf/Pdgfr mutants and the pathogenesis of the resulting abnormalities are poorly understood, but different PDGF isoforms induce varying effects. Here, we generated three new transgenic mouse types which complete a set of studies, where all different PDGF ligands have been expressed under the same heart specific alpha-myosin heavy chain promoter. Transgenic expression of the natural isoforms of Pdgfa and Pdgfb resulted in isoform specific fibrotic reactions and cardiac hypertrophy. Pdgfa overexpression resulted in a severe fibrotic reaction with up to 8-fold increase in cardiac size, leading to lethal cardiac failure within a few weeks after birth. In contrast, Pdgfb overexpression led to focal fibrosis and moderate cardiac hypertrophy. As PDGF-A and PDGF-B have different affinity for the two PDGF receptors, we analyzed the expression of the receptors and the histology of the fibrotic hearts. Our data suggest that the stronger fibrotic effect generated by Pdgfa overexpression was mediated by Pdgfra in cardiac interstitial mesenchymal cells, i.e. the likely source of extracellular matrix depostion and fibrotic reaction. The apparent sensitivity of the heart to ectopic PDGFR alpha agonists supports a role for endogenous PDGFRa agonists in the pathogenesis of cardiac fibrosis

Large-scale identification of genes implicated in kidney glomerulus development and function

To advance our understanding of development, function and diseases in the kidney glomerulus, we have established and large-scale sequenced cDNA libraries from mouse glomeruli at different stages of development, resulting in a catalogue of 6053 different genes. The glomerular cDNA clones were arrayed and hybridized against a series of labeled targets from isolated glomeruli, non-glomerular kidney tissue, FACS-sorted podocytes and brain capillaries, which identified over 300 glomerular cell-enriched transcripts, some of which were further sublocalized to podocytes, mesangial cells and juxtaglomerular cells by in situ hybridization. For the earliest podocyte marker identified, Foxc2, knockout mice were used to analyze the role of this protein during glomerular development. We show that Foxc2 controls the expression of a distinct set of podocyte genes involved in podocyte differentiation and glomerular basement membrane maturation. The primary podocyte defects also cause abnormal differentiation and organization of the glomerular vascular cells. We surmise that studies on the other novel glomerulus-enriched transcripts identified in this study will provide new insight into glomerular development and pathomechanisms of disease