Studies on pericytes in health and disease

Pericytes are the perivascular mural cells of microvessels. They are intimately associated with endothelial cells and communicate with them via direct physical contact or through paracrine signaling pathways. These interactions are important for blood vessel maturation, remodeling, and maintenance. Pericytes are versatile and their varying morphological characteristics and distribution make them difficult to study. The lack of universal pericyte markers is a major problem. Recently, regulator of Gprotein signaling 5 was discovered to be a novel pericyte gene. In paper I we studied the role of RGS5 in vivo by analyzing transgenic mice in which the gene has been knocked out. The vasculature of these mice appeared to develop normally with proper pericyte coverage, and the gene inactivation seemed to have no major influence on pathological angiogenesis. However, a significant decrease in blood pressure was observed, indicating a functional effect of RGS5 on the regulation of vascular tone. A number of different functions have been attributed to pericytes, and in some organs they have more specific roles. In the brain, for instance, pericytes are part of the neurovascular unit together with specialized endothelial cells and astrocytic end-feet. Brain blood vessels have unique characteristics that restrict the passage of molecules between the brain and the blood (the blood-brain barrier), and this is critical for proper neuronal function. It has been postulated that endothelial tight junctions and astrocytic end-feet are the main players in maintaining blood-brain barrier integrity, but in paper III, we show, for the first time in vivo, the important contribution of pericytes. In pericyte-deficient mouse models, we show that blood-brain barrier function is impaired, as injected tracers extravasate via macromolecular transcytosis. The role of pericytes in tumor vessels is debated, but a common belief is that pericytes contribute to stability, and that they might protect the vessels from antiangiogenic therapy such as VEGF-A targeting. In paper II, however, we show that the absence of pericytes in experimental tumor models does not increase tumor sensitivity to VEGF-A withdrawal. This finding has consequences for the design of antiangiogenic therapy, and for some of the anticancer strategies in use today. PDGF-B and –Rβ mutant mice display a severe reduction in pericytes due to a failure in their recruitment, and they suffer from a wide range of defects, ultimately leading to death in utero. The cardiac abnormalities of these animals are studied in detail in paper IV. Several of the malformations observed point towards an involvement of PDGF-B/-Rβ signaling in the contribution of epicardium-derived cells and cardiac neural crest cells to the primitive heart. Studies included in this thesis have investigated the role of the pericytespecific gene RGS5 in vivo, and also used various pericyte deficient mouse models to study the role of pericytes in the blood-brain barrier and in tumor vasculature. The cardiac defects in PDGF-B/-Rβ mutants were also analyzed in depth. To summarize, these studies have revealed novel functions for pericytes and confirmed the multifaceted nature of these cells. LIST OF PUBLICATIONS This thesis is based on the following papers, which will be referred to in the text by their Roman numerals: I. Generation and characterization of rgs5 mutant mice. Nisancioglu MH, Mahoney WM Jr, Kimmel DD, Schwartz SM, Betsholtz C, Genove G. Mol Cell Biol. 2008 Apr;28(7):2324-31. II. The absence of pericytes does not increase the sensitivity of tumor vasculature to VEGF-A blockade. Nisancioglu MH, Betsholtz C, Genove G. Cancer Res. In press. III. Pericytes regulate the blood-brain barrier. Armulik A, Genove G, Mae M, Nisancioglu MH, Wallgard E, He L, Norlin J, Lindblom P, Strittmatter K, Johansson B.R. , Betsholtz C. Submitted. IV. PDGF-B signaling is important for murine cardiac development: its role in developing atrioventricular valves, coronaries, and cardiac innervation. Van den Akker NM, Winkel LC, Nisancioglu MH, Maas S, Wisse LJ, Armulik A, Poelmann RE, Lie-Venema H, Betsholtz C, Gittenberger-de Groot AC. Dev.Dyn. 2008 Feb;237(2):494-503. Other publications not discussed in the thesis: Peripheral mural cell recruitment requires cell-autonomous heparan sulphate. Stenzel D, Nye E, Nisancioglu MH, Adams RH, Yamaguchi Y, Gerhardt H. Blood. 2009 Jul 23;114(4):915-24. Identification of a core set of 58 gene transcripts with broad and specific expression in the microvasculature. Wallgard E, Larsson E, He L, Hellstrom M, Armulik A, Nisancioglu MH, Genove G, Lindahl P, Betsholtz C. Arterioscler Thromb Vasc Biol. 2008 Aug;28(8):1469-76.