3.2.2. Models of diabetic retinopathy
Rodent models are the most commonly used animal models to study diabetic
retinopathy. Traditionally, the mouse model of retinopathy of
prematurity (ROP) has been used as a surrogate model to study the
formation of new malformed and leaky vessels, a hallmark of diabetic
retinopathy. In this model, pups with their mothers are placed in an
oxygen chamber (75% O2 ± 5% and 2% CO2 for 22h/day) at postnatal day
7 until postnatal day 11, after which they are returned to room air
until day 18. The early exposure to elevated oxygen causes a central
obliteration of the vasculature, whilst the return to room air mediates
an hypoxia-driven increase in VEGF via HIF-1a resulting vascular
neogenesis. Using this model, we have shown that oxidative stress is a
key driver of neovascularisation[123], and that interventions like
ebselen to bolster specific antioxidants such glutathione
peroxidase-1[124] leads to significant attenuation of
vaso-obliteration and neovascularisation. In addition, by targeting the
master regulator of oxidative stress, namely the transcription factor
Nrf2 via the bardoxolone-methyl mimetic dh404, we additionally
demonstrated that a global approach to bolstering antioxidant defence
improved ROP in our mouse models[125].
To study diabetic retinopathy per se mouse or rat models can be
used after exposure to STZ. Rat models (a single tail vein injection of
STZ at 55mg/kg with daily insulin) enable robust analysis of vascular
leakage after 10-weeks of diabetes. In most experimental models,
Sprague-Dawley rats are used for diabetic retinopathy studies. Gene and
protein expression analysis of diabetic retinae enable the study of the
molecular pathways involved, with emphasis on VEGF since this growth
factor is the main causal factor in the vascular permeability that
accompanies diabetic retinopathy. Similarly, the use of STZ has enabled
the long-term study of diabetic retinopathy in mice. As mentioned
earlier, female mice are known to be resistant to STZ. Recently, a study
by Saadane et al have reported that the resistance of female C57BL/6J
mice to STZ-induced diabetes can be overcome by injecting higher doses
of STZ[26]. Female mice administered a dose of 75mg/kg of body
weight over 5 consecutive days, developed equivalent levels of
hyperglycaemia as male counterparts administered 55mg/kg of body weight
over 5 consecutive days, which was maintained for at least 8months,
enabling a robust evaluation of diabetic retinopathy in this model.
Importantly, both male and female diabetic mice developed similar
abnormalities in retinal oxidative stress as measured by the level of
superoxide in the retinae. This was accompanied by significantly
increased expression of pro-inflammatory proteins, leucocyte mediated
cytotoxicity and similarities in the number of degenerated capillaries
in both male and female mice[26]. Thus, varying STZ dosage is an
easy adjustment to enable the inclusion of both sexes in the exploration
of therapeutic agents for complications such as diabetic retinopathy.
Inflammatory processes play a key role in driving diabetic retinopathy.
To directly assess a role for inflammatory cytokines in the progression
of diabetic retinopathy, Mugisho et al have developed a novel animal
model that incorporates both hyperglycaemia and inflammation via single
intravitreal injections of the pro-inflammatory cytokines, IL-1ß and
TNF-α, in non-obese diabetic (NOD) mice[126]. The injection of
intravitreal cytokines evokes inflammation in the surrounding ocular
tissue and exhibits retinal oedema, increased astrogliosis and microglia
upregulation. In addition, pericyte loss observed by advanced vessel
dilation and vessel beading, endothelial cell death and blood-retina
barrier breakdown, are prominent[126]. The model may provide a new
opportunity to further study the pathology of diabetic retinopathy,
although its proneness to autoimmunity due to a genetic defect causing
cytokine dysregulation could be a limitation. However, the effectiveness
of this model was further highlighted in a follow-up study where
connexcin43, an inhibitor of inflammation and inflammasome activation,
was shown to significantly attenuate diabetic retinopathy[127].