Maya H. Nisancioglu

The blood–brain barrier is a gatekeeper between the central nervous system and the rest of the body, and is made up of vascular endothelial cells. Previous work upheld the notion that the barrier was formed postnatally as a result of signalling from non-neuronal cells called astrocytes to endothelial cells. Now, two independent studies demonstrate that the barrier is in fact formed during embryogenesis, with the critical factor being the interaction between blood-vessel-surrounding cells called pericytes and epithelial cells. A better understanding of the tight relationship between pericytes, neuroendothelial cells and astrocytes in blood–brain barrier function will contribute to our understanding of the breakdown of the barrier during central nervous system injury and disease. The blood–brain barrier (BBB) is made up of vascular endothelial cells and was thought to have formed postnatally from astrocytes. Two independent studies demonstrate that this barrier forms during embryogenesis, with pericyte/endothelial cell interactions being critical to regulate the BBB during development. A better understanding of the relationship among pericytes, neuroendothelial cells and astrocytes in BBB function will contribute to our understanding of BBB breakdown during central nervous system injury and disease. The blood–brain barrier (BBB) consists of specific physical barriers, enzymes and transporters, which together maintain the necessary extracellular environment of the central nervous system (CNS)1. The main physical barrier is found in the CNS endothelial cell, and depends on continuous complexes of tight junctions combined with reduced vesicular transport2. Other possible constituents of the BBB include extracellular matrix, astrocytes and pericytes3, but the relative contribution of these different components to the BBB remains largely unknown1,3. Here we demonstrate a direct role of pericytes at the BBB in vivo. Using a set of adult viable pericyte-deficient mouse mutants we show that pericyte deficiency increases the permeability of the BBB to water and a range of low-molecular-mass and high-molecular-mass tracers. The increased permeability occurs by endothelial transcytosis, a process that is rapidly arrested by the drug imatinib. Furthermore, we show that pericytes function at the BBB in at least two ways: by regulating BBB-specific gene expression patterns in endothelial cells, and by inducing polarization of astrocyte end-feet surrounding CNS blood vessels. Our results indicate a novel and critical role for pericytes in the integration of endothelial and astrocyte functions at the neurovascular unit, and in the regulation of the BBB.

OBJECTIVE: Pathological angiogenesis is an integral component of many diseases. Antiangiogenesis and vascular targeting are therefore promising new therapeutic principles. However, few endothelial-specific putative drug targets have been identified, and information is still limited about endothelial-specific molecular processes. Here we aimed at determining the endothelial cell-specific core transcriptome in vivo. METHODS AND RESULTS: Analysis of publicly available microarray data identified a mixed vascular/lung cluster of 132 genes that correlated with known endothelial markers. Filtering against kidney glomerular/nonglomerular and brain vascular/nonvascular microarray profiles separated contaminating lung markers, leaving 58 genes with broad and specific microvascular expression. More than half of these have not previously been linked to endothelial functions or studied in detail before. The endothelial cell-specific expression of a selected subset of these, Eltd1, Gpr116, Ramp2, Slc9a3r2, Slc43a3, Rasip1, and NM_023516, was confirmed by real-time quantitative polymerase chain reaction and/or immunohistochemistry. CONCLUSIONS: We have used a combination of publicly available and own microarray data to identify 58 gene transcripts with broad yet specific expression in microvascular endothelium. Most of these have unknown functions, but many of them are predicted to be cell surface expressed or implicated in cell signaling processes and should therefore be explored as putative microvascular drug targets.

We hypothesized that PDGF-B/PDGFR-beta-signaling is important in the cardiac contribution of epicardium-derived cells and cardiac neural crest, cell lineages crucial for heart development. We analyzed hearts of different embryonic stages of both Pdgf-b-/- and Pdgfr-beta-/- mouse embryos for structural aberrations with an established causal relation to defective contribution of these cell lineages. Immunohistochemical staining for alphaSMA, periostin, ephrinB2, EphB4, VEGFR-2, Dll1, and NCAM was performed on wild-type and knockout embryos. We observed that knockout embryos showed perimembranous and muscular ventricular septal defects, maldevelopment of the atrioventricular cushions and valves, impaired coronary arteriogenesis, and hypoplasia of the myocardium and cardiac nerves. The abnormalities correspond with models in which epicardial development is impaired and with neuronal neural crest-related innervation deficits. This implies a role for PDGF-B/PDGFR-beta-signaling specifically in the contribution of these cell lineages to cardiac development.

Regulators of G-protein signaling (RGS) are involved in a wide variety of functions, including olfaction, vision, and cell migration. RGS5 has a perivascular expression pattern and was recently identified as a marker for brain pericytes. This suggests a role for RGS5 in vascular development and pericyte biology. We have created a mouse line which lacks the rgs5 gene and replaced it with a green fluorescent protein (GFP) reporter (rgs5(GFP/GFP)). The mice are viable and fertile and display no obvious developmental defects, and the vasculature appears to develop normally with proper pericyte coverage. Also, no differences were observed in the vasculature under pathological conditions, such as tumor growth and oxygen-induced retinopathy. The GFP expression in pericytes of rgs5(GFP) mice allows detection and sorting of these cells, thereby providing a valuable novel tool for pericyte research.

Recent progress with therapies targeting endothelial cells has drawn attention also to the pericytes as potential target cells for antiangiogenic therapy. Published data suggest that pericytes might confer resistance to vascular endothelial growth factor (VEGF) withdrawal in tumors. This hypothesis has been supported by experiments using tumors with reversible transgenic expression of VEGF-A as well as by individual pharmacologically targeting VEGF and platelet-derived growth factor receptor signaling in endothelial cells and pericytes using receptor tyrosine kinase (RTK) inhibitors with different specificities. However, the RTK inhibitors applied thus far are not entirely specific to the mentioned pathways, and therefore, the effects putatively attributed to pericyte targeting might reflect other antitumor effects. Here, we have reinvestigated the putative benefits of doubly targeting endothelial cells and pericytes in the treatment of experimental tumors. For this purpose, we used two highly specific tools, the pericyte-deficient pdgfb(ret/ret) mouse and the recently developed specific anti-VEGF-A antibody G6-31, which neutralizes both murine and human VEGF-A. We generated B16, Lewis lung carcinoma, and T241 subcutaneous tumors in both pdgfb(ret/ret) and control mice and treated these mice with G6-31. Our results fail to show any improved effect of VEGF inhibition, as measured by tumor growth or decrease in vascular density, in pericyte-deficient tumors compared with controls. Our observations suggest that additional targeting of pericytes does not increase the antitumor effect already generated by anti-VEGF drugs.