R. O. Schlingemann

Vascular endothelial growth factors and angiogenesis in eye disease

The vascular endothelial growth factor (VEGF) family of growth factors controls pathological angiogenesis and increased vascular permeability in important eye diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). The purpose of this review is to develop new insights into the cell biology of VEGFs and vascular cells in angiogenesis and vascular leakage in general, and to provide the rationale and possible pitfalls of inhibition of VEGFs as a therapy for ocular disease. From the literature it is clear that overexpression of VEGFs and their receptors VEGFR-1, VEGFR-2 and VEGFR-3 is causing increased microvascular permeability and angiogenesis in eye conditions such as DR and AMD. When we focus on the VEGF receptors, recent findings suggest a role of VEGFR-1 as a functional receptor for placenta growth factor (PlGF) and vascular endothelial growth factor-A (VEGF)-A in pericytes and vascular smooth muscle cells in vivo rather than in endothelial cells, and strongly suggest involvement of pericytes in early phases of angiogenesis. In addition, the evidence pointing to distinct functions of VEGFs in physiology in and outside the vasculature is reviewed. The cellular distribution of VEGFR-1, VEGFR-2 and VEGFR-3 suggests various specific functions of the VEGF family in normal retina, both in the retinal vasculature and in neuronal elements. Furthermore, we focus on recent findings that VEGFs secreted by epithelia, including the retinal pigment epithelium (RPE), are likely to mediate paracrine vascular survival signals for adjacent endothelia. In the choroid, derailment of this paracrine relation and overexpression of VEGF-A by RPE may explain the pathogenesis of subretinal neovascularisation in AMD. On the other hand, this paracrine relation and other physiological functions of VEGFs may be endangered by therapeutic VEGF inhibition, as is currently used in several clinical trials in DR and AMD.

Increased expression of endothelial antigen PAL-E in human diabetic retinopathy correlates with microvascular leakage

Aims/hypothesis. The Pathologische Anatomie Leiden-Endothelium (PAL-E) antigen is a marker for loss of the blood-brain barrier function in brain tumours. It is endothelium specific and is associated with the endothelial plasmalemmal vesicles (caveolae) involved in transcellular transport. To test whether blood-retinal barrier loss in diabetic retinopathy is associated with cellular changes in the endothelium, the expression of antigen PAL-E in relation to microvascular leakage in human diabetic retinopathy was investigated. Methods. Immunohistochemical staining of frozen tissue sections of postmortem eyes obtained from 30 persons without and 41 persons with diabetes mellitus was carried out with monoclonal antibodies against PAL-E and CD31 and with antibodies against endogenous fibrinogen, albumin and IgG as indicators of vascular leakage. Results. Patchy or uniform microvascular PAL-E staining was observed in the retina of 17 of the 41 eyes of diabetic patients and in 2 of the 30 normal control eyes. In the diabetic eyes, PAL-E staining co-localized with microvascular staining for endogenous fibrinogen, albumin and IgG. Strong staining for PAL-E was observed in sites without blood-tissue barriers, like the choroid. Conclusions/interpretation. In microvessels with an intact blood-retina barrier the endothelial antigen PAL-E is absent. Its expression is increased in retinal vessels of patients with diabetic retinopathy and correlates with microvascular leakage of plasma proteins. This phenotypic shift involving an antigen associated with caveolae suggests that dysfunction of the endothelium forms the cellular basis for microvascular leakage in diabetic retinopathy, rather than passive endothelial damage. [Diabetologia (1999) 42: 596–602]

Role of VEGF-A in endothelial phenotypic shift in human diabetic retinopathy and VEGF-A-induced retinopathy in monkeys.

The endothelium-specific antigen PAL-E, associated with transport vesicles in non-barrier endothelium, is almost absent from barrier capillaries in the normal brain and retina. We have recently demonstrated that only leaking retinal capillaries in diabetic retinopathy (DR) in humans characteristically express PAL-E. Here we investigated the relation between the expression of the PAL-E antigen and vascular endothelial growth factor-A (VEGF) in human post-mortem eyes of individuals with diabetes mellitus (DM) and in experimental VEGF-induced retinopathy in cynomolgus monkeys. Cryosections were cut of eyes of 41 individuals with and 30 individuals without DM and eyes of 2 cynomolgus monkeys who received 4 injections of 0.5 µg VEGF in the vitreous of one eye and PBS in the other. The sections were stained with antibodies against VEGF, PAL-E and endogenous markers for microvascular leakage. Specific retinal vascular staining for VEGF was only observed in 10 out of the 41 cases with DM. These 10 cases also had marked uniform PAL-E staining and widespread vascular leakage. In contrast, diabetic patients without microvascular leakage and controls were negative for VEGF and PAL-E. Likewise, PAL-E was found only in the leaky retinal vessels of monkey eyes injected with VEGF. These results indicate that increased expression of the PAL-E antigen in retinal endothelium in conditions with microvascular leakage is related to VEGF and suggest that VEGF directly or indirectly induces PAL-E. PAL-E expression may reflect important endothelial changes involved in the disturbance of the blood-retina barrier in DR.