Effect of glycated opticin on angiogenesis: mechanism & relevance to proliferative diabetic retinopathy

Benjama, Ahmeda Ibrahim (2014) Effect of glycated opticin on angiogenesis: mechanism & relevance to proliferative diabetic retinopathy. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Diabetic retinopathy is one of the major microvascular complications of diabetes and considered as the common cause of blindness worldwide. Various studies have indicated that increased AGE formation during diabetes might play an essential role in the development and progression of retinopathy. Preretinal neovascularisation is responsible for the onset of diabetic retinopathy and ultimately blindness. CollagenIV of the vitreous humour provides an essential substrate for this process. The blindness is normally associated with pathologies including non proliferative diabetic retinopathy (NPDR), retinopathy of prematurity (ROP), and retinal vein occlusion. It is characterised by the angiogenic growth of neovessels from the preexisting inner retinal vasculature into the vitreous humour. Visual loss is caused by vitreous haemorrhage and/or tractional retinal detachment induced by the contraction of fibrous tissue associated with these new blood vessels which are immature, with a low pericyte and smooth muscle cell covering, and often abnormally shaped. Opticin is an extracellular matrix glycoprotein and a member of the small leucine– rich repeat protein/proteoglycan family. It is highly expressed in the eye throughout life and localises to the vitreous humour. It has been previously shown to be a potent endogenous inhibitor of angiogenesis, although the signalling mechanisms responsible for its action have not been determined. Here, it is demonstrated that opticin significantly inhibited migration and tube-like structure formation of bovine aortic endothelial cells (BAEC) and human retinal microvessel endothelial cells (HREC) in matrigel, induced by FGF-2. Regarding cellular signalling, opticin also inhibited FGF- 2-induced phosphorylation of MEK1/2, p38 and JNK2, confirmed by Western blotting, which could explain its anti-angiogenic properties. We also observed increasing in Akt expression. Protein glycation is also a feature of chronic diabetes and can result in modified cellular function. Here we show that exposure of opticin to high methyalglyoxal produces glycation that inhibited its ability to reduce endothelial cell migration and tube-formation, and reduced its ability to inhibit cell signalling through ERK/MEK and JNK. This may be one mechanism through which the angiogenic switch is altered in pro-angiogenic proliferative retinopathy. Here it was observed that when opticin became glycated, it was less able to bind to collagen, the collagen, thereby inhibiting α2β1 integrin binding to collagen affecting the angiogenesis inhibiting properties of opticin. AGEs are localized in retinal vessels and neuroglia of diabetic patients where they exert a range of deleterious effects on cell function. In vivo and in vitro studies suggest that elevated AGE level occurring in diabetes may be an important factor in retinopathy initiation and progression. Our result shows the distributions of the opticin mostly in front of the eye with a small amount in retina and vitreous. Furthermore, the diabetic mouse has more AGEs than the non-diabetic mouse, and the increased co-localization of AGEs with opticin was very clear in diabetic mouse compared with non-diabetic mouse. We can conclude the opticin become glycated if exposed to high sugar level and this reduces its ability to bind to collagen and impair angiogenic signalling. The accumulation of AGEs in retina may play a causative role in the development of corneal epithelial disorders of diabetic patients. Glycation of opticin could one of mechanisms allowing venel growth into the vitreous during PDR.