Disorders of Vascular Permeability

Abstract

The endothelial barrier maintains vascular and tissue homeostasis and modulates many physiological processes, such as angiogenesis. Vascular barrier integrity can be disrupted by a variety of soluble permeability factors, and changes in barrier function can exacerbate tissue damage during disease progression. Understanding endothelial barrier function is critical for vascular homeostasis. Many of the signaling pathways promoting vascular permeability can also be triggered during disease, resulting in prolonged or uncontrolled vascular leak. It is believed that recovery of the normal vasculature requires diminishing this hyperpermeable state. Although the molecular mechanisms governing vascular leak have been studied over the last few decades, recent advances have identified new therapeutic targets that have begun to show preclinical and clinical promise. These approaches have been successfully applied to an increasing number of disease conditions. New perspectives regarding how vascular leak impacts the progression of various diseases are highlighted in this review.

Keywords: angiogenesis; barrier; endothelium; junctions; permeability.

Figure 1

Transcellular and paracellular pathways in endothelial cells. The passage of macromolecules, fluids, and cells through the endothelial barrier can occur through transcellular (vesiculo-vacuolar organelles) or paracellular (tight and adherens junctions) pathways. Abbreviations: JAM, junctional adhesion molecule; VVO, vesiculo-vacuolar organelle; ZO, zona occludens.

Figure 2

Pericyte loss in the retinal microvasculature is associated with microvascular permeability. iDTR;M-Cre mice were generated by breeding Cre-inducible DT receptor–transgenic mice [C57BL/6-Gt(ROSA)26Sor^{tm1(HBEGF)Awai}/J (annotated as iDTR)] with mice expressing tamoxifen-inducible Cre under the control of the SMMHC promoter (annotated as M-Cre). DT-treated iDTR;M-Cre mice were perfused with fluorescein dextran (2 × 10⁶ mW) before retinal imaging by fluorescence microscopy. Arrows indicate sites of increased microvascular permeability in the retinal microvasculature of mice with inducible mural cell loss. Scale bar: 250 μm. Abbreviations: DT, diphtheria toxin; SMMHC, smooth muscle myosin heavy chain. Unpublished photo by C. Valdez, J. Arboleda, and P.A. D’Amore.

Figure 3

Astrocyte-endothelial cell cocultures and time course of TER. (a–i) Brain astrocytes were isolated from rat pups at postnatal day 1, plated onto tissue culture flasks, and allowed to grow for 7–10 days. Cells were cultured on tissue culture plastic, in Matrigel or on Transwell membranes for morphological comparison. (a,b) Astrocytes immunolabeled with GFAP were cultured (a) for 24 h alone or (b) in coculture with rat microvascular ECs on tissue culture plastic. Arrows indicate ECs next to astrocytes. Astrocytes and bovine microvascular ECs were grown alone and in coculture in a three-dimensional Matrigel assay for 24 h. (c) Astrocytes were labeled green and (d) ECs were labeled red to distinguish the cell populations. (e,f) Astrocytes and ECs were grown in Matrigel coculture. (g–i) Astrocytes and bovine microvascular ECs were grown on opposite sides of a Transwell membrane and identified with GFAP labeling (red) or simplicifolia lectin (green), respectively, to distinguish the cell types. The arrows indicate areas of astrocyte contact with ECs, and asterisks indicate areas shown in higher magnification. (j) ECs were plated onto 12 well Transwell inserts alone (green triangles), cocultured with astrocytes on the underside of the well (orange circles), or cocultured with BAE on the underside of the well (blue squares). TER was measured daily with a Millipore electrical resistance unit. TER from an empty Transwell was used as background and was subtracted from EC readings. The average of triplicate measures from each well was used to calculate TER/cm². Readings were measured from triplicate wells. Scale bar: 20 μm in panels a–e and g–i and 40 μm in panel f. Abbreviations: BAE, bovine aortic endothelium; EC, endothelial cell; GFAP, glial fibrillary acidic protein; TER, transendothelial electrical resistance. Figure adapted from Reference with permission from Elsevier Limited.

Figure 4

Miles permeability assay. Different concentrations of VEGF-A were injected subcutaneously into mice, followed immediately by an intravenous injection of Evans blue dye. HBSS was injected as a control. Abbreviations: HBSS, Hanks’ buffered saline solution; VEGF-A, vascular endothelial growth factor-A. Image courtesy of Janice Nagy and Harold Dvorak (Department of Pathology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02115).