Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function.

Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes.

Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses.

During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema.

Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.     

Diabetic retinopathy. Pathogenesis and the role of retina-derived growth factor in angiogenesis

Diabetic retinopathy results from a combination of systemic and ocular abnormalities. Vasodilation, basement membrane pathology, microaneurysms, abnormal blood flow and tissue oxygenation, connective tissue abnormalities, and retinal ischemia are all components of early diabetic retinopathy. The pathogenesis of neovascularization is discussed with respect to the effects of vasodilation, vascular leakage, vitreous changes, and retinal ischemia. The evidence supporting Michaelson's hypothesis that a chemical messenger from the retina provides the stimulus for neovascularization is cited. The sequence of events involved in angiogenesis are cellular and basement membrane changes, endothelial cell migration, endothelial cell proliferation, and vessel formation. The experimental evidence in support of a role for retina-derived growth factor as a mediator of these cellular events is reviewed.     

糖尿病视网膜病变血-视网膜屏障的损伤机制

糖尿病严重影响了视网膜微循环,从而引起一系列组织结构的病理改变.这些改变最终可导致内皮细胞过度增生,血管通透性改变,异常的视网膜血管形成并发视力减退.本文阐述了高血糖对视网膜微血管系统的影响,从细胞功能和分子生物学的角度阐明高血糖所导致的细胞损伤效应.高血糖可通过影响细胞连接功能、诱导细胞凋亡以及改变细胞间的相互作用等方式对视网膜血管细胞造成损伤.这将为人们了解糖尿病视网膜病变发生发展中的分子及细胞缺陷提供新思路.     

Retinal vessels: proliferation of endothelium in vitro

Tissue culture of retinal vessels from fetal calf eyes produced colonies of endothelium, pericytes, and smooth muscle. Identification of endothelial cells was based upon culture morphology, [3H]thymidine labeling of isolated vessels, and factor VIII immunofluorescence. Thimerosal added to the culture medium destroyed pericytes and muscle cells, leaving only endothelial cell colonies in the primary cultures. This tissue culture system may be useful in the study of retinal vascular cell biochemistry and pathophysiology.     

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.     

Regulation of cell-mediated collagen gel contraction in human retinal pigment epithelium cells by vascular endothelial growth factor compared with transforming growth factor-β2

Background: To explore the potential role of vascular endothelial growth factor compared with transforming growth factor‐β2 in the regulation of human retinal pigment epithelium cell‐mediated collagen gel contraction. Methods: The retinal pigment epithelium cell mediated type I collagen gel contraction assay was performed to evaluate and compare the effect of vascular endothelial growth factor and transforming growth factor‐β2. The number of viable retinal pigment epithelium cells in the gel and the expression of α‐smooth muscle actin were analysed. Results: Both vascular endothelial growth factor and transforming growth factor‐β2 caused a time dependent gel contraction, associated with over expression of α‐smooth muscle actin in retinal pigment epithelium cells undergoing a fibroblast like transformation. The decrease in volume of the collagen gel and increase in α‐smooth muscle actin expression were more significant in the transforming growth factor‐β2‐treated group than in vascular endothelial growth factor‐treated group beginning at day 2, and the growth of retinal pigment epithelium cells was significantly more inhibited in the transforming growth factor‐β2‐treated group compared with the vascular endothelial growth factor‐treated group after day 1 (P < 0.05). Transforming growth factor‐β2 stimulation increased both vascular endothelial growth factor mRNA expression and secretion. The α‐smooth muscle actin expression and the change in volume of collagen gel were significantly positively correlated in both experimental groups. Conclusions: Both vascular endothelial growth factor and transforming growth factor‐β2 can cause induction of retinal pigment epithelium cell‐mediated collagen gel contraction in vitro via partial upregulation of α‐smooth muscle actin expression.     

Are endothelin-1 and neuropeptide Y involved in the pathogenesis of glaucoma?

Vasospasm in the vessels, supporting the optic disc, is nowadays considered one of the possible etiological factors leading to development of glaucomatous neuropathy. This article describes physiological regulatory mechanisms of blood circulation in these vessels, including influence of autonomic nervous system and blood flow autoregulation; it also explains why vasospasm may disturb autoregulation. Special attention is paid to the role of vascular endothelial mediators, which are responsible for autoregulation. Observations indicating that vasospasm may be a risk factor of glaucomatous damage are also presented. The article describes data concerning vasospastic effects of two mediators: endothelin-1 (ET-1), released by vascular endothelium, and neuropeptide Y (NPY), a sympathetic nervous system neurotransmitter, on the vessels supporting the optic disc and their possible role in producing blood flow disturbances in these vessels. Investigation results indicating that endothelial dysfunction, connected with increased ET-1 plasma levels and sympathetic nervous system disorders, may be involved in the pathogenesis of normal-tension glaucoma, are presented.     

Susceptibility of retinal vascular endothelium to infection with Toxoplasma gondii tachyzoites

PURPOSE: Retinochoroidal infection with the protozoan parasite Toxoplasma gondii is the most common cause of posterior uveitis worldwide. Tachyzoites spread throughout the body through the blood stream and lymphatics, but preferentially encyst in the eye and other parts of the central nervous system (CNS). It is unknown whether T. gondii penetrates the CNS selectively or whether these sites of immune privilege have limited capacity to eradicate the parasite. METHODS: Human vascular endothelial cell lines, including retinal (three lines from three different donors), aortic, umbilical vein, and dermal microvascular endothelium, as well as human foreskin fibroblasts, were grown to confluence in 24-well plates. Cells were incubated with RH-strain T. gondii tachyzoites in the presence of [(3)H]-uracil. Trichloroacetic acid-insoluble radioactivity was measured as an index of T. gondii proliferation, because tachyzoites, but not human cells, incorporate uracil directly through pyrimidine salvage. RESULTS: Tachyzoites showed higher [(3)H]-uracil incorporation after incubation with retinal vascular endothelial cells in comparison with aortic (55% more), umbilical vein (33% more) and dermal (34% more) endothelial cells. In eight separate assays, significantly greater radioactivity was measured for tachyzoites cultured with retinal versus other cell subtypes (P < 0.05), except for one assay in which differences reached only borderline significance (P 相似文献   

Endothelial dysfunction in glaucoma

Glaucoma is a group of ocular diseases characterized by optic neuropathy associated with loss of the retinal nerve fibre layer and re‐modelling of the optic nerve head, and a subsequent particular pattern of visual field loss. Increased intraocular pressure is the most important risk factor for the disease, but the pathogenesis of glaucoma is not monofactorial. Among other factors, ischaemia and vascular dysregulation have been implicated in the mechanisms underlying glaucoma. The vascular endothelium plays an important role in the regulation of ocular blood flow and pathological alterations of vascular endothelial cells may induce ischaemia and dysregulation. The present review summarizes our current evidence of endothelial dysfunction in glaucoma. This is of interest because endothelial dysfunction is a good prognostic factor for progression in several diseases. Although such data are lacking for glaucoma, endothelial dysfunction may provide an attractive target for therapeutic intervention in open‐angle glaucoma and other vascular disorders of the eye.     

Role of the endothelium in acetylcholine-induced relaxation and spontaneous tone of bovine isolated retinal small arteries

Acetylcholine induced a variable concentration-dependent relaxation of bovine isolated retinal small arteries contracted with PGF2 alpha. The acetylcholine-mediated relaxation was linearly related to the sodium nitroprusside-induced relaxation, suggesting that the endothelium is well preserved in the vessels and that the variable effect of acetylcholine is due to variations in the soluble guanylate-cyclase enzyme activity in the smooth muscle. The vessels became desensitized to acetylcholine by repeated exposures. L-arginine and indomethacin did not abolish the desensitization. The vessels also became desensitized to the direct smooth muscle relaxing effect of sodium nitroprusside, indicating that desensitization to the endothelium-dependent relaxation by acetylcholine is related primarily to the vascular smooth muscle cells. Atropine, methylene blue and removal of endothelium abolished the acetylcholine-induced relaxation completely, whereas indomethacin had no inhibitory action on the acetylcholine-induced relaxation, suggesting that acetylcholine mediates release of EDRF (nitric oxide: NO) through stimulation of muscarinic receptors. Methylene blue contracted endothelium intact retinal arteries but a spontaneous tone was not present in endothelial denuded arteries. This may indicate a basal release of both a contractile factor, e.g. endothelin, and a relaxing factor. NO, form the retinal endothelium. The results demonstrate that endothelial-derived factors may participate in normal as well as pathophysiological regulation of retinal vascular smooth muscle tone.     

Quantitative enumeration of vascular smooth muscle cells and endothelial cells derived from bone marrow precursors in experimental choroidal neovascularization

Choroidal neovascularization (CNV) is characterized by the subretinal invasion of a pathologic new vessel complex from the choriocapillaris. Although CNV is traditionally considered to consist of endothelial cells, the cellular population of CNV is likely more complex in nature, comprising several different cell types. In addition, recent studies suggest that the CNV cell population has a dual origin (circulating versus resident populations). In this study we sought to determine the contribution and origin of different cell types in experimental CNV. Laser-induced CNV was performed on chimeric mice generated by reconstituting C57BL/6 mice with bone marrow from green fluorescent protein (GFP)-transgenic mice. In these mice, bone marrow-derived cells are GFP-labeled. Immunofluorescence staining was used to examine both flatmount preparations of the choroid and cross sections of the posterior pole for macrophages, endothelial cells, vascular smooth muscle cells, retinal pigment epithelial (RPE) cells, lymphocytes, or neutrophils at day 3, 7, 14 and 28 post-laser (n=5 per group). Cell types present in CNV included macrophages (20% of the cells in CNV), endothelial cells (25%), vascular smooth muscle cells (11%), RPE cells (12%) and non-labeled cells (32%). The macrophage population was mostly derived from circulating monocytes at all timepoints studied (70% were GFP labeled), while endothelial and vascular smooth muscle cells were partly bone marrow derived (50-60% were GFP labeled), and RPE cells appeared to be entirely derived from preexisting tissue resident cells. These results demonstrate that bone marrow-derived progenitor cells contribute significantly to the vascular and inflammatory components of CNV. Knowledge of the cellular composition and origin might help understand the pathogenic mechanisms controlling CNV severity as well as indicate potential targets for therapeutic intervention.     

Caveolar and non-Caveolar Caveolin-1 in ocular homeostasis and disease

Caveolae, specialized plasma membrane invaginations present in most cell types, play important roles in multiple cellular processes including cell signaling, lipid uptake and metabolism, endocytosis and mechanotransduction. They are found in almost all cell types but most abundant in endothelial cells, adipocytes and fibroblasts. Caveolin-1 (Cav1), the signature structural protein of caveolae was the first protein associated with caveolae, and in association with Cavin1/PTRF is required for caveolae formation. Genetic ablation of either Cav1 or Cavin1/PTRF downregulates expression of the other resulting in loss of caveolae. Studies using Cav1-deficient mouse models have implicated caveolae with human diseases such as cardiomyopathies, lipodystrophies, diabetes and muscular dystrophies. While caveolins and caveolae are extensively studied in extra-ocular settings, their contributions to ocular function and disease pathogenesis are just beginning to be appreciated. Several putative caveolin/caveolae functions are relevant to the eye and Cav1 is highly expressed in retinal vascular and choroidal endothelium, Müller glia, the retinal pigment epithelium (RPE), and the Schlemm's canal endothelium and trabecular meshwork cells. Variants at the CAV1/2 gene locus are associated with risk of primary open angle glaucoma and the high risk HTRA1 variant for age-related macular degeneration is thought to exert its effect through regulation of Cav1 expression. Caveolins also play important roles in modulating retinal neuroinflammation and blood retinal barrier permeability. In this article, we describe the current state of caveolin/caveolae research in the context of ocular function and pathophysiology. Finally, we discuss new evidence showing that retinal Cav1 exists and functions outside caveolae.     

Pathogenesis of diabetic macular oedema

Hyperglycaemia causes breakdown of the blood retina barrier leading to formation of macular oedema and consecutive visual loss. Three major mechanisms are involved in barrier breakdown: increased paracellular permeability of vascular endothelium due to disruption of cell junctions, loss of endothelial cell layer integrity due to cell destruction, and increased transcellular transport through the endothelium. This review focuses on the molecular basis of these mechanisms and discusses the role of cytokines and cellular interactions in blood retina barrier breakdown.     

The inner blood-retinal barrier in diabetes

The inner blood-retinal barrier is highly complex, dependent on a normally functioning retinal vascular endothelium and possibly adjacent perivascular cells. There are undoubtedly many different grades of barrier failure, and in the early stages an inbuilt compensatory mechanism may prevent significant fluid accumulation and delay associated structural and functional changes. In diabetes, early barrier failure may be a function of an increase in the rate of endothelial cell death and turnover, probably exaggerated by the early and widespread loss of the supporting cells and dilatation of the small blood vessels.     

Functional and morphological characteristics of the retinal and choroidal vasculature

This review is about vascular endothelial phenotype heterogeneity in the retinal and choroidal circulations. It is becoming increasingly clear that the functional and structural heterogeneity is present in the retinal and choroidal circulations. Differential responses of the vessels to vasoactive substances have been shown with intraluminal and extraluminal delivery and in different regions of the same vascular bed. Vascular endothelial phenotype is highly heterogenic and site-specific, particularly in the retinal and choroidal veins. Updated information of such heterogeneity may help us to further understand the control mechanisms of the retinal and choroidal circulations which are important in compensating for the physiological and pathological challenges faced by these vascular beds. The site-specific changes of vascular endothelial phenotype may be linked with endothelium dysfunction, and site-specific diseases such as central and branch retinal vein occlusion. Endothelial dysfunction has been recognized as an initial step for many vascular diseases. Endothelial cells are a strategic and valid target for therapeutic intervention. Fundamentally important questions regarding the role of vascular endothelial cell function in the eye are discussed.     

Phosphoinositide metabolism and prostacyclin formation in retinal microvascular endothelium: stimulation by adenine nucleotides

Phosphoinositide lipid metabolism and prostacyclin production are implicated in endothelium dependent vascular relaxation in large blood vessels. To determine if these biochemical pathways might be involved in the regulation of microvascular tone in the retina, we measured the formation of 6-keto-prostaglandin-F1 alpha, the stable end product of prostacyclin, and inositol phosphates from 3H-labeled phosphoinositide lipids, in endothelial cells prepared from bovine retinal microvessels and maintained in long-term culture. We found that adenosine 5'-triphosphate and adenosine 5'-diphosphate both stimulated a dose-dependent accumulation of inositol phosphates and of 6-keto-prostaglandin-F1 alpha in these cells. The agonist specificity of the responses, with stimulation by adenosine 5'-triphosphate and adenosine 5'-diphosphate, and inactivity of adenosine 5'-monophosphate and adenosine, suggest that they are mediated through P2 purinergic receptors. The similar early time courses of 6-keto-prostaglandin-F1 alpha and inositol triphosphate production support the hypothesis that prostacyclin formation could result from the mobilization of intracellular calcium by inositol triphosphate, which activates phospholipase A, and thereby releases arachidonic acid to form prostacyclin. These findings point to a role for these cells in the regulation of normal retinal vascular tone. Because phosphoinositide lipid metabolism is altered in diabetes, dysfunction of these biochemical pathways in retinal endothelium could underlie the pathophysiology of diabetic retinopathy.     

Endothelium-dependent vasoactive modulation in the ophthalmic circulation

The vascular endothelium is strategically located between the circulating blood and the vascular smooth muscle cells. Different agonists or stimuli transported by the circulating blood can trigger the endothelium to release potent relaxing (nitric oxide, prostacyclin, endothelium-derived hyperpolarizing factor) or contracting factors (endothelin, cycloxygenase products). These endothelium-derived vasoactive factors can modulate blood flow locally. Heterogeneity exists from one vascular bed to the other, or even between vessels, in the agonists able to stimulate the release of endothelium-derived vasoactive factors. In the ophthalmic circulation, nitric oxide and endothelin are strong vasoactive modulators. In many vascular diseases that are of importance in ophthalmology (hypercholesterolemia, arteriosclerosis, hypertension, diabetes, vasospastic syndrome, ischemia and reperfusion, etc) the function of the endothelium can be impaired. There exist different drugs that can modulate the vasoactive function of the vascular endothelium. In other words, it appears that the vascular endothelium plays an important role in both the physiology and pathophysiology of the regulation of blood flow. The modulation of this regulatory system by different drugs might open new therapeutical approaches to treat vascular disorders in ophthalmology.