Blood–retinal barrier in hypoxic ischaemic conditions: Basic concepts, clinical features and management
Introduction
The presence of an intact blood–retinal barrier (BRB) is essential for the structural and functional integrity of the retina and in clinical conditions where BRB breakdown occurs vision may be seriously and adversely affected.
The BRB consists of inner and outer components and plays an important role in the homeostatic regulation of the retinal microenvironment. The BRB, like the blood– brain barrier (BBB), controls fluid and molecular movement between the ocular vascular beds and the retinal tissues and prevents leakage into the retina of macromolecules and other potentially harmful agents. The inner BRB (iBRB) is formed by the tight junctions (TJ) between neighbouring capillary endothelial cells (Shakib and Cunha-Vaz 1966) which rest on a basal lamina (Fig. 1) covered by the foot processes of astrocytes and Müller cells (Ashton, 1965, Reichenbach et al., 2007). Pericytes, separated from the endothelial cells by the basal lamina, are the third cell type of relevance to the iBRB. Astrocytes, Müller cells and pericytes are thought to contribute to the proper functioning of the iBRB.
The outer BRB (oBRB) is formed by TJ between cells of the retinal pigment epithelium (RPE) (Cuhna-Vaz, 1976) (Fig.2). The RPE resting upon the underlying Bruch's membrane separates the neural retina from the fenestrated choriocapillaris (Fig. 3) and plays an important role in transporting nutrients from the blood to the outer retina.
The apical surface of the RPE shows long projecting processes that partially envelop the outer segments of the photoreceptors, longer villi ensheathing cone outer segments and shorter villi rod outer segments (Steinberg et al., 1977) (Fig. 3). These villi are involved in the phagocytosis of outer segment discs (Steinberg et al., 1977). The metabolic interaction between these apical villi and the photoreceptors, e.g. exchange of metabolites such as vitamin A (Dowling, 1961) and amino acids (Miller and Steinberg, 1976), is considered to be critical for the maintenance of visual function (Futter, 2006). There are no structural attachments between the RPE and the outer retina but neural cell adhesion molecules expressed on the apical surface of the RPE cells create adhesion between the retina and RPE (Gundersen et al., 1993) although functional factors may be more important (Foulds, 1975, Foulds, 1985, Marmor, 2006). Mechanical separation of the retina from the RPE occurs more readily after death than in the living eye (Zauberman and DeGuillebon, 1972). The strength of the adhesion between RPE apical villi and the outer segments of the retinal photoreceptors is demonstrated in the experimental animal eye in which a partial retinal detachment has been induced. In the detached area, photoreceptor outer segments torn from their inner segments remain embedded within the apical villi of the RPE (Fig. 4).
While inter-RPE cell TJ are important in the control of paracellular movement of fluids and molecules between the choroid and retina, the polarised distribution of membrane proteins in the RPE is also important (Anderson and Van Itallie, 1995, Rizzolo, 1997).
Retinal ischaemia is an important cause of visual impairment and blindness. It has been stressed (Osborne et al., 2004) that at the cellular level ischaemia results in a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and glutamatergic excitation.
As the integrity of the BRB is dependent upon the normal functioning of the cells that determine its structure, it is no surprise that hypoxia–ischaemia is associated with BRB breakdown.
Section snippets
Capillary endothelial cells
The blood vessels in the retina are distributed as two plexuses: the inner or superficial plexus located in the nerve fibre and ganglion cell layers, and the outer or deep plexus located at the junction of inner nuclear and outer plexiform layers. In the human there are also radial peripapillary capillaries lying in the superficial nerve fibre layer (Henkind and De Oliveira, 1967).
The endothelial cells of retinal capillaries are not fenestrated (Bernstein and Hollenberg, 1965) and have a
Outer blood–retinal barrier
In health molecular movement across the RPE is both transcellular and paracellular. The outward movement of molecules from the subretinal space involves hydrostatic and osmotic forces largely acting on small molecules travelling through the paracellular inter- RPE cellular clefts and by active transport through the transcellular route (Pederson, 2006). Paracellular movement of larger molecules is restricted by the TJ between neighbouring RPE cells. As with TJ elsewhere, occludin-1, claudins,
Supplementary barriers to molecular movement
Molecular movement from the retinal and choroidal blood vascular systems into, out of and across the retina is complex and limited by a number of relative or absolute barriers to free diffusion. There is a continuous molecular movement of small molecules (mainly water) from the vitreous cavity and inner retina across the retina and RPE to the choroid (Moseley et al., 1984, Alm, 1992). A major proportion of aqueous humour secreted by the ciliary body from its rich vascular supply provides a bulk
Retinal energy requirements
The retina consumes oxygen more rapidly than any other tissue (Cohen and Noell, 1965). Oxidative enzymatic activity is highest in the inner segments of the retinal photoreceptors and in the RPE (Pearse, 1961, Hansson, 1970). The high energy requirements of the RPE stem from synthesis of cellular constituents required for their constant renewal and in the photoreceptors for the replacement of outer segment discs that undergo a rapid daily turnover (Young and Bok, 1969). In both the RPE and in
Hypoxia–ischaemia and the inner blood–retinal barrier
Retinal hypoxia occurs acutely in central retinal artery occlusion (CRAO) and more chronically in ischaemic central retinal vein occlusion. Tissue hypoxia and hyperglycaemia are regarded as principal stimulants of diabetic retinal pathology (Wilkinson-Berka, 2004). Systemic causes of retinal hypoxia include the cardiovascular effects of chronic obstructive airways disease, the ocular ischaemic syndrome associated with arterial obstructive conditions such as carotid artery stenosis (Brown and
Hypoxia–ischaemia and outer blood–retinal barrier
The RPE appears to be resistant to hypoxic–ischaemic insults and even post-mortem the RPE maintains its morphological characteristics for some time after autolysis has destroyed other retinal elements (Johnson and Grierson, 1976).
In a hypoxic adult rat model in which a severe disruption of the iBRB is induced by two hours of breathing a 93%/7% nitrogen/oxygen mixture, no disruption of the oBRB was seen (Kaur et al., 2007). The intercellular spaces between RPE cells were widened but TJ remained
Hypoxia–ischaemia, oxidative stress and blood–retinal barrier
Generation of reactive oxygen species (ROS), resulting in oxidative stress, occurs in many tissues in hypoxia–ischaemia. In ocular pathologies, ROS have been correlated with neovascularisation in diabetic eyes in humans (Augustin et al., 1993) and animals (Armstrong and al-Awadi, 1991), and in eyes with retinopathy of prematurity (Saugstad and Rognum, 1988).
Oxidative stress has also been reported to affect the integrity of the iBRB and oBRB as it is associated with a redistribution of occludin
Hypoxia–ischaemia and tight junctions
Hypoxia has been reported to produce alteration in the TJ proteins claudin-3, ZO-1 and ZO-2, correlating with increased permeability (Witt et al., 2003). A recent study has reported that there was no significant change in the distribution of occludin, ZO-1 and ZO-2 under hypoxic/reoxygenation stress (Torii et al., 2007). In vitro and in vivo experiments have indicated that claudin-5 is a target molecule in hypoxia leading to disruption of the barrier function of neural vasculature (Koto et al.,
Hypoxia–ischaemia, inflammation and blood–retinal barrier
Inflammation occurs in the retina in hypoxic–ischaemic injuries. Retinal hypoxia–ischaemia has been shown to induce the gene expression of monocyte chemoattractant protein (MCP)-1 to attract macrophages to the hypoxic area. The hypoxia-activated macrophagic/microglial release of TNF-α has been reported as a triggering factor activating the production of interleukin (IL)-8, VEGF, basic fibroblast growth factor or MCP-1 in retinal vascular cells and/or glial cells adjacent to microvessels (
Ageing and the blood–retinal barrier
In the ageing eye there is a marked loss of both retinal capillary endothelial cells and pericytes, many of the capillaries being converted into acellular tubes (Fig. 16). There is also a loss of capillaries including those in the central retina so that there is an enlargement of the foveal avascular zone in the elderly eye (Wu et al., 1985). Even in old age, however, loss of retinal capillary endothelial cells and pericytes is not accompanied by retinal oedema or other evidence of iBRB
Clinical features and significance of hypoxia-induced blood–retinal barrier breakdown
As the blood–retinal barriers subserve important functions in the eye it is no surprise that their failure has adverse effects upon the eye and on vision. The functions of the BRBs that are clinically important can be summarised as follows:
- 1.
Control of the delivery of nutrients and the removal of waste products of metabolism to and from the retina
- 2.
The exclusion of large molecules from the transparent tissues of the eye (especially vitreous) so as to maintain optical transparency
- 3.
The maintenance of
Diabetes
There is good evidence that hypoxia plays a significant role in the iBRB breakdown that can occur in the diabetic eye. Breakdown of the iBRB in the diabetic eye can occur early and can be detected by vitreous fluorophotometry before there is any overt sign of diabetic retinopathy (Cunha-Vaz et al., 1978).
The commonest manifestation of iBRB breakdown in the diabetic eye is an increase in retinal volume from an accumulation of extracellular fluid within the retina. Extracellular fluid in the
Outer blood–retinal barrier breakdown related to hypoxia
Where there is a breakdown in the oBRB the commonest clinical manifestation is a localised or more generalised serous retinal detachment the extent of which is dependent upon the extent of oBRB breakdown. As the oBRB is largely constituted by the TJ between neighbouring RPE cells and as the closely related choroidal circulation is greatly in excess of metabolic requirements, hypoxic damage to the oBRB is uncommon unless, as has been suggested, hypoxia plays a role in the development of the
Management of hypoxic–ischaemic blood retinal breakdown
As hypoxia induced BRB breakdown is largely restricted to the iBRB, its management is essentially the management of retinal oedema and the adverse effects this has upon vision.
Apart from measures to improve control of blood sugar levels in diabetes or to correct underlying aetiological factors in retinal vein occlusion the mainstay of treatment for both diabetic macular oedema (DMO) and for the macular oedema that may follow retinal vein occlusion has been laser photocoagulation. This includes
Anti-vascular endothelial growth factor treatment for inner blood–retinal barrier breakdown
It is now accepted that VEGF is a major factor in the genesis of the angiogenesis that characterises CNV in ARMD, the retinal new vessel formation of proliferative diabetic retinopathy or ischaemic retinal vein occlusion and the destructive neovascularisation of the retinopathy of prematurity.
VEGF is undoubtedly the Factor X postulated originally by Michaelson as aetiological in pathological neovascularisation (Michaelson, 1948). When the pivotal role of VEGF in neovascularisation became
Laser treatment of inner blood–retinal barrier breakdown
The mainstay of treatment of CSMO until recently was focal laser photocoagulation in the case of focal iBRB breakdown, and grid laser treatment as originally suggested by Dobree (1970) for diffuse macular oedema.
In one study (ETDRS (Early Treatment of Diabetic Retinopathy Study Research Group), 1987) either focal or grid laser treatment reduced visual loss associated with DMO from 24% in untreated cases to 12% in those treated by laser. Grid laser treatment tended to improve reading vision more
Corticosteroids for hypoxia-induced inner blood–retinal barrier breakdown
Corticosteroids have a specific effect on reducing vascular permeability (Nussenblatt et al., 1996). Steroids have anti-angiogenic, anti-inflammatory, anti-apoptotic, anti-proliferative and anti-oedematous activity (Cunningham et al., 2008). Hypoxia induces an inflammatory response in the retina with increased levels of inflammatory cytokines and macrophage attractants. As corticosteroids are anti-inflammatory, the actions of corticosteroids in conditions of hypoxia induced iBRB breakdown
Summary
In many clinically important conditions including diabetic retinopathy, central or BRVO, and some respiratory diseases retinal hypoxia results in a breakdown in the BRB. Disruption of the iBRB with increased vascular permeability causes vasogenic retinal oedema and tissue damage, with consequent adverse effects upon vision. Factors such as enhanced production of VEGF, NO, oxidative stress and inflammation underlie the increased permeability of the iBRB and inhibition of these factors may be
Abbreviations
AQ Aquaporin
ARMD Age-related macular degeneration
ATP Adenosine triphosphate
BBB Blood–brain barrier
BRB Blood–retinal barrier
iBRB Inner BRB
BRVO Branch retinal vein occlusion
CMO Cystoid macular oedema
CNV Choroidal neovascularisation
CRAO Central retinal artery occlusion
CRT Central retinal thickness
CRVO Central retinal vein occlusion
CSMO Clinically significant macular oedema
DMO Diabetic macular oedema
2,3-DPG 2,3-Diphosphoglycerate
ELM External limiting membrane
eNOS Endothelial nitric oxide synthase
Acknowledgements
This study was supported by a research grant (R-181-000-098-112) from the National University of Singapore and a research grant (05/1/35/19/422) from the Singapore Biomedical Research Council. The help provided by Dr Viswanathan Sivakumar in the preparation of the manuscript is gratefully acknowledged. Figs. 3 and 7A are reproduced with permission of the Nature Publishing Group in respect of earlier publication in the Transactions of the Ophthalmological Societies of the United Kingdom or in Eye
References (320)
- et al.
Purtscher's and Purtscher-like retinopathies: a review
Surv. Ophthalmol
(2006) - et al.
Tight junctions: molecular architecture and function
Int. Rev. Cytol
(2006) - et al.
Role of rheological factors in patients with acute central retinal vein occlusion
Ophthalmology
(1996) - et al.
Lipid peroxidation and retinopathy in streptozotocin-induced diabetes
Free Radic. Biol. Med
(1991) - et al.
Roles of nitric oxide in brain hypoxia-ischemia
Biochim. Biophys. Acta
(1999) - et al.
Meta-analysis, of plasma homocysteine, serum folate, serum vitamin B, and thermolabile MTHFR as risk factors for retinal vascular occlusive disease
Am. J. Ophthalmol
(2003) - et al.
Ranibizumab for macular edema due to retinal vein occlusions: implication of VEGF as a critical stimulator
Mol. Ther
(2008) Retinoids in ocular tissues: binding proteins, transport, and mechanisms of action
- et al.
Upregulation of vascular endothelial growth factor by H2O2 in rat heart endothelial cells
Free Radic. Biol. Med.
(1998) - et al.
VEGF-mediated inflammation precedes angiogenesis in adult brain
Exp. Neurol
(2004)