Understanding the biological pathways that regulate ocular growth

Abstract

The visual disorder myopia (short-sightedness) is brought about by a mismatch between the optical power of the eye and its axial length. This is due most commonly to excessive elongation of the eye which causes objects viewed at a distance to appear blurred as the focal plane falls in front of the neural layer of the eye (the retina) instead of on it. Myopia is now recognised as the leading cause of visual impairment and low vision world-wide, with some estimates indicating that half of the world’s population may be myopic by 2050. Although our understanding of this visual disorder has dramatically improved over the past three decades, several critical questions remain. Central to this, and forming the basis of this thesis, is to understand the biological pathways controlling eye growth and how such pathways are modulated by external cues.
Animal models have demonstrated that a diverse array of treatments, broadly categorised as optical, pharmacological and environmental, can inhibit the excessive growth associated with experimental myopia. Although diverse in their mechanisms of action, each of these treatments leads to a common set of biological outcomes when inhibiting myopia development, that of choroidal expansion and reduced scleral growth. Therefore, this thesis tests the hypothesis that, irrespective of the means by which growth is inhibited, such common physiological changes are driven by a conserved set of retinal signals. One retinal molecule that is consistently reported to be associated with the modulation of growth across several species is the immediate early gene (IEG) early growth response-1 (Egr-1). Therefore, this thesis explored if Egr-1 represents a conserved component of the growth regulatory pathway emanating from the retina by investigating whether its expression is consistently altered across a wide variety of growth modulating paradigms (Aim 1). Expanding on this, using whole-transcriptome analysis, this thesis investigated what other molecules, in addition to Egr-1, show a conserved response across five well-characterised growth inhibitory paradigms also investigated in Aim 1 (Aim 2). Following this, and expanding on Aim 1, this thesis investigates if the unexpected differences seen in the expression of Egr-1 between the two forms of myopic defocus (growth suppression), that of positive lens-wear and recovery from experimental myopia, could suggest mechanistic differences (Aim 3). This thesis tests if such differences may be explained by the previous state of eye growth and/or the initial size of the eye at treatment onset. Finally, this thesis examines whether the two primary forms of experimental myopia, namely form-deprivation myopia (FDM) and lens-induced myopia (LIM), are affected in a similar manner by the external environmental cue of light, and tests whether this response is driven by the same underlying
mechanism, that of retinal dopamine release (Aim 4).
This thesis reports the following major findings:
Aim 1 - The retinal expression of Egr-1 showed a robust bi-directional response to a
wide range of growth modulating paradigms. Specifically, its expression was down- regulated in
response to enhanced growth, while being up-regulated in response to suppressed growth.
Aim 2 - In addition to Egr-1, at the transcriptome level, significant overlap was observed in the
expression profiles seen across the five diverse growth inhibitory treatments
investigated. This overlap was even greater at the pathway level, with a distinct
enrichment in processes associated with cell signalling and circadian entrainment. More broadly,
the molecular profile of the retina during the onset and prevention of FDM was indicative of IEG
activity, particularly the activity of Egr-1.
Aim 3 - The unexpected down-regulation of Egr-1 mRNA levels seen following positive
lens-wear remained even after adjusting for differences in the state of the eye before treatment
onset. Instead, it appears that the retinal expression of Egr-1, although a powerful biomarker of
changes in growth rates, may be insensitive to the direction of growth. Therefore, this aim will
test whether a down-regulation in Egr-1 levels is the default response to any initial stimulus that
alters ocular growth rates, irrespective of the direction of growth.
Aim 4 - Exposure to bright light inhibits the development of LIM in a similar intensity- dependent
manner as that seen for FDM, with this protective effect abolished by
blocking the dopaminergic system using a pharmacological antagonist.

Date of Award	2020
Original language	English
Awarding Institution	University of Canberra
Supervisor	Regan Ashby (Supervisor) & Sudha Rao (Supervisor)