Adhesion Failures Determine the Pattern of Choroidal Neovascularization in the Eye: A Computer Simulation Study

Choroidal neovascularization (CNV) of the macular area of the retina is the major cause of severe vision loss in adults. In CNV, after choriocapillaries initially penetrate Bruch's membrane (BrM), invading vessels may regress or expand (CNV initiation). Next, during Early and Late CNV, the expanding vasculature usually spreads in one of three distinct patterns: in a layer between BrM and the retinal pigment epithelium (sub-RPE or Type 1 CNV), in a layer between the RPE and the photoreceptors (sub-retinal or Type 2 CNV) or in both loci simultaneously (combined pattern or Type 3 CNV). While most studies hypothesize that CNV primarily results from growth-factor effects or holes in BrM, our three-dimensional simulations of multi-cell model of the normal and pathological maculae recapitulate the three growth patterns, under the hypothesis that CNV results from combinations of impairment of: 1) RPE-RPE epithelial junctional adhesion, 2) Adhesion of the RPE basement membrane complex to BrM (RPE-BrM adhesion), and 3) Adhesion of the RPE to the photoreceptor outer segments (RPE-POS adhesion). Our key findings are that when an endothelial tip cell penetrates BrM: 1) RPE with normal epithelial junctions, basal attachment to BrM and apical attachment to POS resists CNV. 2) Small holes in BrM do not, by themselves, initiate CNV. 3) RPE with normal epithelial junctions and normal apical RPE-POS adhesion, but weak adhesion to BrM (e.g. due to lipid accumulation in BrM) results in Early sub-RPE CNV. 4) Normal adhesion of RBaM to BrM, but reduced apical RPE-POS or epithelial RPE-RPE adhesion (e.g. due to inflammation) results in Early sub-retinal CNV. 5) Simultaneous reduction in RPE-RPE epithelial binding and RPE-BrM adhesion results in either sub-RPE or sub-retinal CNV which often progresses to combined pattern CNV. These findings suggest that defects in adhesion dominate CNV initiation and progression

Human retinal oximetry using hyperspectral imaging

The aim of the work reported in this thesis was to investigate the possibility of measuring human retinal oxygen saturation using hyperspectral imaging. A direct non-invasive quantitative mapping of retinal oxygen saturation is enabled by hyperspectral imaging whereby the absorption spectra of oxygenated and deoxygenated haemoglobin are recorded and analysed. Implementation of spectral retinal imaging thus requires ophthalmic instrumentation capable of efficiently recording the requisite spectral data cube. For this purpose, a spectral retinal imager was developed for the first time by integrating a liquid crystal tuneable filter into the illumination system of a conventional fundus camera to enable the recording of narrow-band spectral images in time sequence from 400nm to 700nm. Postprocessing algorithms were developed to enable accurate exploitation of spectral retinal images and overcome the confounding problems associated with this technique due to the erratic eye motion and illumination variation. Several algorithms were developed to provide semi-quantitative and quantitative oxygen saturation measurements. Accurate quantitative measurements necessitated an optical model of light propagation into the retina that takes into account the absorption and scattering of light by red blood cells. To validate the oxygen saturation measurements and algorithms, a model eye was constructed and measurements were compared with gold-standard measurements obtained by a Co-Oximeter. The accuracy of the oxygen saturation measurements was (3.31%± 2.19) for oxygenated blood samples. Clinical trials from healthy and diseased subjects were analysed and oxygen saturation measurements were compared to establish a merit of certain retinal diseases. Oxygen saturation measurements were in agreement with clinician expectations in both veins (48%±9) and arteries (96%±5). We also present in this thesis the development of novel clinical instrument based on IRIS to perform retinal oximetry.Al-baath University, Syri

Association Between Age-Related Macular Degeneration and Sleep-Disordered Breathing

Age-related macular degeneration (AMD) is a chronic, irreversible disease that robs individuals of vision, quality of life, and independence. It is the leading cause of blindness in industrialized countries. Sleep-disordered breathing (SDB) is a condition characterized by repeated episodes of apnea and/or hypopnea, insomnia, short sleep duration, and/or sleep disturbances (snoring, gasping, etc.). Because SDB has been shown to cause chronic hypoxia resulting in oxidative stress on the retina, it has been proposed that SDB may be associated with AMD. Based on the life course theory of chronic disease, this quantitative, cross-sectional study used data from the 2005-2008 National Health and Nutrition Examination Survey to study whether there was an association between SDB and AMD, including neovascular AMD and geographic atrophy in adults 40 years and older. Descriptive statistics and logistic regression analyses were used. The results suggest that AMD is associated with diagnosed sleep disorders, including sleep apnea and insomnia, as well as sleep apnea symptoms of gasping snoring, snorting, and stopping breathing. The findings of this study highlight the importance of diagnostic screening and therapeutic intervention to treat SDB. Early diagnosis and therapy for SDB could address not only the comorbidities associated with SDB, but could also prevent or slow the progression of AMD. In turn, this would yield lower rates of vision loss, reduced comorbidities associated with vision loss, and reduced impact of AMD on the health care system and social and financial costs to society

Phototherapeutic Devices for the Treatment of Diabetic Retinopathy

Diabetic retinopathy is a microvascular disease of the retina and a leading cause of vision loss worldwide. In the non-proliferative phase, diabetes-induced degradation of the retinal blood supply leads to edema and progressive tissue hypoxia. In response, the retinal tissue expresses proangiogenic growth factors (e.g. vascular endothelial growth factor), which drive proliferation of aberrant blood vessels within the eye. These poorly formed vessels leak fluid and blood cells into the eye and grow into the vitreous, which puts traction on the retina and leads to detachment. Given the hypoxic etiology, retinal oxygen tension and metabolism have received considerable attention. Dark-adapted conditions drive the retina to a significantly lower oxygen tension compared to light- adapted conditions as rod cells consume more energy in order to boost sensitivity. While tolerable in the healthy retina, it has been hypothesized that increased nightly metabolism overwhelms the compromised oxygen supply in the diabetic retina, leading to hypoxia and pathological vascular endothelial growth factor expression. This thesis develops ocular devices that shine light onto the retina to modulate rod metabolism, reducing oxygen demand and mitigating nightly hypoxia. The phototherapeutic effect is characterized through mathematical modeling of retinal metabolism and in vivo testing. Implantable phototherapy devices are designed, fabricated, and evaluated. This thesis also develops overnight phototherapeutic contact lenses utilizing radioluminescence, chemiluminescence, and electroluminescence approaches. Phototherapy holds promise as a non-invasive, preventative therapy for the treatment of hypoxic retinal diseases such as diabetic retinopathy.</p

Two-component Hydrogel as Biomimetic Vitreous Substitutes

The vitreous humor of the eye is a biological hydrogel principally composed of fibrillary collagen interspersed with semi-flexible polyelectrolyte, hyaluronic acid (HA). Certain pathological conditions necessitate its removal and replacement. Current vitreous substitutes, such as silicone oils and perfluorocarbons, are hydrophobic, do not resemble the properties of the vitreous, and have known complications. An ideal vitreous substitute should have properties of the natural vitreous, perform its functions, and be biocompatible in the eye. Inspired by the structure and composition of the natural vitreous, we used bio-mimicry to develop an injectable two-component hydrogel. The hydrogel is composed of a fibrillary gellan, an analogue of collagen, and a semi-flexible polyelectrolyte poly[methacrylamide-co-(methacrylic acid)], an analogue of hyaluronic acid, both endowed with thiol cross-linkers for reversible covalent linkage. The gellan, in the polymeric mixture, undergoes coil-helix transition near physiological temperature, enabling instantaneous in situ physical gelation of the solution. The thiol cross-linkers that later oxidize to disulfides under physiological conditions, make the hydrogel non-absorbable, non-degradable, and reversible, for facile removal if needed. We used response surface methodology to investigate the structure-property relationships of eleven two-component hydrogels, and identified two hydrogel formulations that match the primary properties of the vitreous. We determined how each component of the hydrogel affects their optical, mechanical, sol-gel transition temperature, and osmotic swelling properties. All the hydrogels were transparent to visible light, with density and refractive indexes nearly equivalent to those of the natural vitreous. The shear storage moduli of the hydrogels, at 1Hz, ranged from 3 to 358 Pa, and the sol-gel transition temperatures, from 35.5 to 43 °C. In addition, as expected, all the hydrogels swelled in physiological solutions. Interestingly, we discovered that the relatively large swelling capacity of the semi-flexible ionic copolymer was significantly restricted by the minimally swellable fibrillary gellan network. The tightly swollen gel of two dissimilar networks produced Donnan osmotic swelling pressure in physiological solutions, which is also the driving force for re-attachment of the retina. Insights from the biomimetic nature of the gel, led us to propose that the natural vitreous also exhibits controlled swelling, where ionic HA’s swelling capacity is restricted by fibrillary collagen. The Donnan swelling pressure produced by the tightly swollen vitreous gel maintains the delicate internal structure of the eye, and perhaps plays a critical role during the ocular development. We evaluated the biocompatibility of the two optimized formulations of the hydrogels on different cell lines, and in rabbits. Both hydrogels were found to be biocompatible on primary porcine retinal pigment epithelial cells, human retinal pigment epithelial cells, and fibroblast (3T3/NIH) cells, by electric cell-substrate impedance sensing system. Furthermore, the hydrogels did not impair tight junction formation or affect proliferation of the cells. The hydrogels were also non-degradable in enzymatic solutions and in contact with ocular cell line for four weeks. Judged against silicone oil, a clinically-accepted vitreous replacement, both hydrogel formulations were biocompatible in rabbits for 30 days. Both hydrogels maintained optical clarity, physiological intra-ocular pressure, and intact retinal layers that displayed normal electroretinographs. In two cases of the iatrogenic retinal tear, the hydrogels reattached the retina by producing osmotic swelling pressure. The hydrogels also maintained the low oxygen environment, compared to silicone oil, in the rabbit’s vitreous cavity for 30 days post-surgery. In conclusion, the two hydrogels reattach the retina via a unique mechanism of osmotic swelling pressure. They overcome the limitations of silicone oil with comparable in-vivo biocompatibility, and merit further evaluations as an artificial vitreous. In addition, the ability to control the mechanical and swelling properties of the two-component hydrogels over a wide range suggests their utility as biomimetic replacements of other soft tissues, such as cornea, nucleus pulposus, and cartilage

Advancing treatment of retinal disease through in silico trials

Abstract Treating retinal diseases to prevent sight loss is an increasingly important challenge. Thanks to the configuration of the eye, the retina can be examined relatively easily in situ. Owing to recent technological development in scanning devices, much progress has been made in understanding the structure of the retina and characterising retinal biomarkers. However, treatment options remain limited and are often of low efficiency and efficacy.&amp;#xD;&amp;#xD;In recent years, the concept of in silico clinical trials has been adopted by many pharmaceutical companies to optimise and accelerate the development of therapeutics. In silico clinical trials rely on the use of mathematical models based on the physical and biochemical mechanisms underpinning a biological system. With appropriate simplifications and assumptions, one can generate computer simulations of various treatment regimens, new therapeutic molecules, delivery strategies and so forth, rapidly and at a fraction of the cost required for the equivalent experiments. Such simulations have the potential not only to hasten the development of therapies and strategies but also to&amp;#xD;optimise the use of existing therapeutics.&amp;#xD;&amp;#xD;In this paper, we review the state-of-the-art in in silico models of the retina for mathematicians, biomedical scientists and clinicians, highlighting the challenges to developing in silico clinical trials. Throughout this paper, we highlight key findings from in silico models about the physiology of the retina in health and disease. We describe the main building blocks of in silico clinical trials and identify challenges to developing in silico clinical trials of retinal diseases.</jats:p