Retinal Dysfunction in a Mouse Model of HCN1 Genetic Epilepsy.

Pathogenic variants in HCN1 are associated with a range of epilepsy syndromes including a developmental and epileptic encephalopathy. The recurrent de novo HCN1 pathogenic variant (M305L) results in a cation leak, allowing the flux of excitatory ions at potentials where the wild-type channels are closed. The Hcn1M294L mouse recapitulates patient seizure and behavioral phenotypes. As HCN1 channels are highly expressed in rod and cone photoreceptor inner segments, where they shape the light response, mutated channels are likely to impact visual function. Electroretinogram (ERG) recordings from male and female mice Hcn1M294L mice revealed a significant decrease in the photoreceptor sensitivity to light, as well as attenuated bipolar cell (P2) and retinal ganglion cell responses. Hcn1M294L mice also showed attenuated ERG responses to flickering lights. ERG abnormalities are consistent with the response recorded from a single female human subject. There was no impact of the variant on the structure or expression of the Hcn1 protein in the retina. In silico modeling of photoreceptors revealed that the mutated HCN1 channel dramatically reduced light-induced hyperpolarization, resulting in more Ca2+ flux during the response when compared with the wild-type situation. We propose that the light-induced change in glutamate release from photoreceptors during a stimulus will be diminished, significantly blunting the dynamic range of this response. Our data highlight the importance of HCN1 channels to retinal function and suggest that patients with HCN1 pathogenic variants are likely to have a dramatically reduced sensitivity to light and a limited ability to process temporal information.SIGNIFICANCE STATEMENT Pathogenic variants in HCN1 are emerging as an important cause of catastrophic epilepsy. HCN1 channels are ubiquitously expressed throughout the body, including the retina. Electroretinogram recordings from a mouse model of HCN1 genetic epilepsy showed a marked decrease in the photoreceptor sensitivity to light and a reduced ability to respond to high rates of light flicker. No morphologic deficits were noted. Simulation data suggest that the mutated HCN1 channel blunts light-induced hyperpolarization and consequently limits the dynamic range of this response. Our results provide insights into the role HCN1 channels play in retinal function as well as highlighting the need to consider retinal dysfunction in disease caused by HCN1 variants. The characteristic changes in the electroretinogram open the possibility of using this tool as a biomarker for this HCN1 epilepsy variant and to facilitate development of treatments.

Fractalkine-induced microglial vasoregulation occurs within the retina and is altered early in diabetic retinopathy.

Local blood flow control within the central nervous system (CNS) is critical to proper function and is dependent on coordination between neurons, glia, and blood vessels. Macroglia, such as astrocytes and Müller cells, contribute to this neurovascular unit within the brain and retina, respectively. This study explored the role of microglia, the innate immune cell of the CNS, in retinal vasoregulation, and highlights changes during early diabetes. Structurally, microglia were found to contact retinal capillaries and neuronal synapses. In the brain and retinal explants, the addition of fractalkine, the sole ligand for monocyte receptor Cx3cr1, resulted in capillary constriction at regions of microglial contact. This vascular regulation was dependent on microglial Cx3cr1 involvement, since genetic and pharmacological inhibition of Cx3cr1 abolished fractalkine-induced constriction. Analysis of the microglial transcriptome identified several vasoactive genes, including angiotensinogen, a constituent of the renin-angiotensin system (RAS). Subsequent functional analysis showed that RAS blockade via candesartan abolished microglial-induced capillary constriction. Microglial regulation was explored in a rat streptozotocin (STZ) model of diabetic retinopathy. Retinal blood flow was reduced after 4 wk due to reduced capillary diameter and this was coincident with increased microglial association. Functional assessment showed loss of microglial-capillary response in STZ-treated animals and transcriptome analysis showed evidence of RAS pathway dysregulation in microglia. While candesartan treatment reversed capillary constriction in STZ-treated animals, blood flow remained decreased likely due to dilation of larger vessels. This work shows microglia actively participate in the neurovascular unit, with aberrant microglial-vascular function possibly contributing to the early vascular compromise during diabetic retinopathy.

TAK1 blockade as a therapy for retinal neovascularization.

Retinal neovascularization, or pathological angiogenesis in the retina, is a leading cause of blindness in developed countries. Transforming growth factor-ß-activated kinase 1 (TAK1) is a mitogen-activated protein kinase kinase kinase (MAPKKK) activated by TGF-ß1 and other proinflammatory cytokines. TAK1 is also a key mediator of proinflammatory signals and plays an important role in maintaining vascular integrity upon proinflammatory cytokine stimulation such as TNFα. However, its role in pathological angiogenesis, particularly in retinal neovascularization, remains unclear. Here, we investigate the regulatory role of TAK1 in human endothelial cells responding to inflammatory stimuli and in a rat model of oxygen-induced retinopathy (OIR) featured retinal neovascularization. Using TAK1 knockout human endothelial cells that subjected to inflammatory stimuli, transcriptome analysis revealed that TAK1 is required for activation of NFκB signaling and mediates its downstream gene expression related to endothelial activation and angiogenesis. Moreover, pharmacological inhibition of TAK1 by 5Z-7-oxozeaenol attenuated angiogenic activities of endothelial cells. Transcriptome analysis also revealed enrichment of TAK1-mediated NFκB signaling pathway in the retina of OIR rats and retinal neovascular membrane from patients with proliferative diabetic retinopathy. Intravitreal injection of 5Z-7-oxozeaenol significantly reduced hypoxia-induced inflammation and microglial activation, thus attenuating aberrant retinal angiogenesis in OIR rats. Our data suggest that inhibition of TAK1 may have therapeutic potential for the treatment of retinal neovascular pathologies.

A drug-tunable Flt23k gene therapy for controlled intervention in retinal neovascularization.

Gene therapies that chronically suppress vascular endothelial growth factor (VEGF) represent a new approach for managing retinal vascular leakage and neovascularization. However, constitutive suppression of VEGF in the eye may have deleterious side effects. Here, we developed a novel strategy to introduce Flt23k, a decoy receptor that binds intracellular VEGF, fused to the destabilizing domain (DD) of Escherichia coli dihydrofolate reductase (DHFR) into the retina. The expressed DHFR(DD)-Flt23k fusion protein is degraded unless "switched on" by administering a stabilizer; in this case, the antibiotic trimethoprim (TMP). Cells transfected with the DHFR(DD)-Flt23k construct expressed the fusion protein at levels correlated with the TMP dose. Stabilization of the DHFR(DD)-Flt23k fusion protein by TMP was able to inhibit intracellular VEGF in hypoxic cells. Intravitreal injection of self-complementary adeno-associated viral vector (scAAV)-DHFR(DD)-Flt23k and subsequent administration of TMP resulted in tunable suppression of ischemia-induced retinal neovascularization in a rat model of oxygen-induced retinopathy (OIR). Hence, our study suggests a promising novel approach for the treatment of retinal neovascularization. Schematic diagram of the tunable system utilizing the DHFR(DD)-Flt23k approach to reduce VEGF secretion. a The schematic shows normal VEGF secretion. b Without the ligand TMP, the DHFR(DD)-Flt23k protein is destabilized and degraded by the proteasome. c In the presence of the ligand TMP, DHFR(DD)-Flt23k is stabilized and sequestered in the ER, thereby conditionally inhibiting VEGF. Green lines indicate the intracellular and extracellular distributions of VEGF. Blue lines indicate proteasomal degradation of the DHFR(DD)-Flt23k protein. Orange lines indicate the uptake of cell-permeable TMP. TMP, trimethoprim; VEGF, vascular endothelial growth factor; ER, endoplasmic reticulum.

Increased episcleral venous pressure in a mouse model of circumlimbal suture induced ocular hypertension.

PURPOSE: To investigate changes in aqueous humor dynamics during intraocular pressure (IOP) elevation induced by circumlimbal suture in mice. METHODS: Ocular hypertension (OHT) was induced by applying a circumlimbal suture behind the limbus in male adult C57BL6/J mice. In the OHT group, the suture was left in place for an average of 8 weeks (n = 10, OHT group). In the sham control group the suture was cut at 2 days (n = 9, sham group) and in the naïve control group (n = 5) no suture was implanted. IOP was measured at baseline across 3 days, 1 h post-suture implantation, and at the chronic endpoint. Anterior segments were assessed using optical coherence tomography (OCT). Episcleral venous pressure (EVP), total outflow facility (C), uveoscleral outflow (Fu) and aqueous humor flow rate (Fin) were determined using a constant-flow infusion model. RESULTS: All aqueous dynamic and chronic IOP outcome measures showed no difference between sham and naïve controls (p > 0.05) and thus these groups were combined into a single control group. IOP was elevated in OHT group compared with controls (p < 0.01). Chronic suture implantation did not change pupil size, anterior chamber depth or iridocorneal angles (p > 0.05). EVP was significantly higher in OHT eyes compared to control eyes (p < 0.01). There was no statistical difference in C, Fu and Fin between groups (p > 0.05). A significant linear correlation was found between IOP and EVP (R2 = 0.35, p = 0.001). CONCLUSIONS: Circumlimbal suture implantation in mouse eyes results in chronic IOP elevation without angle closure. Chronic IOP elevation is likely to reflect higher EVP.

Gene Therapy Intervention in Neovascular Eye Disease: A Recent Update.

Aberrant growth of blood vessels (neovascularization) is a key feature of severe eye diseases that can cause legal blindness, including neovascular age-related macular degeneration (nAMD) and diabetic retinopathy (DR). The development of anti-vascular endothelial growth factor (VEGF) agents has revolutionized the treatment of ocular neovascularization. Novel proangiogenic targets, such as angiopoietin and platelet-derived growth factor (PDGF), are under development for patients who respond poorly to anti-VEGF therapy and to reduce adverse effects from long-term VEGF inhibition. A rapidly advancing area is gene therapy, which may provide significant therapeutic benefits. Viral vector-mediated transgene delivery provides the potential for continuous production of antiangiogenic proteins, which would avoid the need for repeated anti-VEGF injections. Gene silencing with RNA interference to target ocular angiogenesis has been investigated in clinical trials. Proof-of-concept gene therapy studies using gene-editing tools such as CRISPR-Cas have already been shown to be effective in suppressing neovascularization in animal models, highlighting the therapeutic potential of the system for treatment of aberrant ocular angiogenesis. This review provides updates on the development of anti-VEGF agents and novel antiangiogenic targets. We also summarize current gene therapy strategies already in clinical trials and those with the latest approaches utilizing CRISPR-Cas gene editing against aberrant ocular neovascularization.

Therapeutic applications of chelating drugs in iron metabolic disorders of the brain and retina.

Iron is essential for normal cellular function, however, excessive accumulation of iron in neural tissue has been implicated in both cortical and retinal diseases. The exact role of iron in the pathogenesis of neurodegenerative disorders remains incompletely understood. However, iron-induced damage to the brain and retina is often attributed to the redox ability of iron to generate dangerous free radicals, which exacerbates local oxidative stress and neuronal damage. Iron chelators are compounds designed to scavenge labile iron, aiding to regulate iron bioavailability. Recently there has been growing interest in the application of chelating agents for treatment of diseases including neurodegenerative conditions, characterized by increased oxidative stress. This article reviews both clinical and preclinical evidence relating to the effectiveness of iron chelation therapy in conditions of iron dyshomeostasis linked to neurodegeneration in the brain and retina. The limitations as well as future opportunities iron chelation therapy are discussed.

Hypercapnia Impairs Vasoreactivity to Changes in Blood Pressure and Intraocular Pressure in Rat Retina.

SIGNIFICANCE: The balance between oxygen and carbon dioxide sets the resting tone (or diameter) of retinal blood vessels. Eyes that are hypercapnic use up their "vasodilatory reserve" and therefore fail to respond adequately to changes in intraocular or blood pressure. PURPOSE: Retinal vessels are regulated by both myogenic and metabolic mechanisms. We considered whether alteration of metabolic status would modify the vascular response to ocular perfusion pressure (OPP) lowering in rat retina. METHODS: In pentobarbital anesthetized adult Brown-Norway rats, normocapnia or hypercapnia was achieved by artificially ventilating animals with air or 5% carbon dioxide in ~30% oxygen, respectively. Ocular perfusion pressure was gradually reduced to ~20 mmHg by either lowering blood pressure (slowly drawing blood from a femoral artery/vein) or manometrically increasing intraocular pressure under normocapnic or hypercapnic conditions. In all four groups (n = 7 eyes for each), a confocal scanning laser ophthalmoscope was used to acquire image sequences centered on the optic nerve throughout pressure modification. The diameter of arterioles and venules at various OPP levels was measured and expressed as percentage relative to their own baseline. The response of arterioles and venules to OPP lowering was compared between normocapnic and hypercapnic groups. RESULTS: Average arterial carbon dioxide partial pressures were 36.9 ± 2.6 mmHg in normocapnic and 64.1 ± 5.9 mmHg in hypercapnic (P < .001) animals. In the normocapnic groups, blood pressure lowering and intraocular pressure elevation resulted in significant vasodilation of both arterioles and venules (P < .0001). In the hypercapnic groups, OPP lowering-induced vasodilation was significantly attenuated compared with the corresponding normocapnic groups (P < .0001 for both, two-way analysis of variance). CONCLUSION: Hypercapnia significantly modified myogenic vascular autoregulation in response to OPP reduction.

AAV-mediated gene delivery of the calreticulin anti-angiogenic domain inhibits ocular neovascularization.

Ocular neovascularization is a common pathological feature in diabetic retinopathy and neovascular age-related macular degeneration that can lead to severe vision loss. We evaluated the therapeutic efficacy of a novel endogenous inhibitor of angiogenesis, the calreticulin anti-angiogenic domain (CAD180), and its functional 112-residue fragment, CAD-like peptide 112 (CAD112), delivered using a self-complementary adeno-associated virus serotype 2 (scAAV2) in rodent models of oxygen-induced retinopathy and laser-induced choroidal neovascularization. The expression of CAD180 and CAD112 was elevated in human umbilical vein endothelial cells transduced with scAAV2-CAD180 or scAAV2-CAD112, respectively, and both inhibited angiogenic activity in vitro. Intravitreal gene delivery of scAAV2-CAD180 or scAAV2-CAD112 significantly inhibited ischemia-induced retinal neovascularization in rat eyes (CAD180: 52.7% reduction; CAD112: 49.2% reduction) compared to scAAV2-mCherry, as measured in retinal flatmounts stained with isolectin B4. Moreover, the retinal structure and function were unaffected by scAAV2-CAD180 or scAAV2-CAD112, as measured by optical coherence tomography and electroretinography. Moreover, subretinal delivery of scAAV2-CAD180 or scAAV2-CAD112 significantly attenuated laser-induced choroidal neovascularization in mouse eyes compared to scAAV2-mCherry, as measured by fundus fluorescein angiography (CAD180: 62.4% reduction; CAD112: 57.5% reduction) and choroidal flatmounts (CAD180: 40.21% reduction; CAD112: 43.03% reduction). Gene delivery using scAAV2-CAD180 or scAAV2-CAD112 has significant potential as a therapeutic option for the management of ocular neovascularization.

Optic nerve tissue displacement during mild intraocular pressure elevation: its relationship to central corneal thickness and corneal hysteresis.

PURPOSE: To determine the extent to which (1) optic nerve tissue is displaced following mild acute elevation of intraocular pressure, and (2) clinically accessible measures at the anterior eye can be used as a surrogate for such displacements. METHODS: We imaged the optic disc of 21 healthy subjects before and after intraocular pressure (IOP) elevation of ~10 mmHg delivered by ophthalmodynamometry. Steady-state tissue displacement during IOP elevation was assessed axially from OCT data, and laterally from SLO data. Recovery from IOP elevation was assessed by tracking a single vertical B-scan through the cup centre. Anatomical structures were demarcated by three masked clinicians to determine lateral shifts for temporal cup edge and central disc vessels, and axial shifts of disc surface and anterior lamina cribrosa. Spatial maps of deformation were constructed within the demarcated cup and disc to assess within-tissue displacement. Measured displacements were correlated with corneal hysteresis, corneal thickness, and IOP. RESULTS: The temporal cup edge moved more temporally with higher baseline IOP (R2  = 0.33, p = 0.006) and with lesser elevation of IOP (R2  = 0.43, p = 0.001); it moved more superiorly for thinner corneas (R2  = 0.35, p = 0.007). Thinner corneas also produced less within-cup deformation, relative to that of the disc (R2  = 0.39, p = 0.004). Axial displacement of the lamina and lateral displacement of vessels were often substantial (lamina 20 ± 15 µm, range 1-60 µm; vessels 37 ± 25 µm, range 2-102 µm) but did not correlate with measured parameters. Recovery from IOP elevation did not take more than 300-400 ms in any subject. CONCLUSIONS: Mild acute elevation of IOP produces large and rapidly reversible shifts in optic nerve tissue in young, healthy eyes. The resulting degree, direction and spatial distribution of cup movement are associated with IOP status and corneal thickness, but not corneal hysteresis.

Reversal of functional loss in a rat model of chronic intraocular pressure elevation.

PURPOSE: This pilot study considered whether intraocular pressure (IOP) lowering could reverse ganglion cell dysfunction in a rat model of chronic ocular hypertension. METHODS: A circumlimbal suture was applied in one eye to induce ocular hypertension (n = 7) in Long-Evans rats. The contralateral eye served as an untreated control. After 8 weeks of IOP elevation the suture was removed to lower IOP for the remaining 7 weeks. Electroretinogram (ERG) and optical coherence tomography (OCT) were measured at baseline, 2, 4, 8, 12 and 15 weeks. Retinae were collected for histology at week 15. RESULTS: In sutured eyes, IOP was elevated by 7-11 mmHg above control eyes (12 ± 0.2 mmHg [standard error of the mean]). Eight weeks of chronic IOP elevation resulted in a reduction of the ganglion cell mediated positive Scotopic Threshold Response (pSTR, -25 ± 7% of baseline), as well as smaller photoreceptor (-7 ± 4%) and bipolar cell mediated responses (-6 ± 5%). After suture removal, IOP recovered to normal. By 15 weeks the a-wave (0 ± 6%), b-wave (-2 ± 6%) and pSTR had recovered back to baseline (from -25 ± 7% to -4 ± 6%). The retinal nerve fiber layer was thinned by -9 ± 3% at week 8 and showed no further decline at week 15 (-10 ± 2%). Cell numbers in the ganglion cell layer were similar between suture removal and control eyes at week 15 (3543 ± 478 vs 4057 ± 476 cells mm-2 ). CONCLUSIONS: The circumlimbal suture model might be a useful platform to study the reversibility of neuronal dysfunction from chronic IOP challenge.

An acute intraocular pressure challenge to assess retinal ganglion cell injury and recovery in the mouse.

We describe a model of acute intraocular pressure (IOP) elevation in the mouse eye that induces reversible loss of inner retinal function associated with oxidative stress, glial cell activation and minimal loss of retinal ganglion cell (RGC) number. Young healthy mouse eyes recover inner retinal function within 7-days but more persistent functional loss is seen in older mice. Manipulation of diet and exercise further modify RGC recovery demonstrating the utility of this injury model for investigating lifestyle and therapeutic interventions. We believe that systematic investigation into the characteristics and determinants of RGC recovery following an IOP challenge will shed light on processes that govern RGC vulnerability in the early stages of glaucoma.

Provocative intraocular pressure challenge preferentially decreases venous oxygen saturation despite no reduction in blood flow.

PURPOSE: Ocular disease can both alter the retina's oxygen requirements, and decrease its ability to cope with changes in metabolic demand. We examined the influence of a moderate intraocular pressure (IOP) elevation on three outcome measures: arterial and venous oxygen saturation, blood flow, and the pattern electroretinogram (PERG). METHODS: We increased IOP to ˜30 mmHg in 23 healthy participants (22-39 years) using a mechanical probe applied to the eyelid, thereby lowering ocular perfusion pressure (OPP) by ~30%. The Oxymap retinal oximeter was used to measure oxygen saturation for arteries and veins. Blood flow, volume and velocity were measured using the Heidelberg retinal flowmeter and steady-state PERG waveforms (8.34 Hz) were recorded bilaterally (200 sweeps). For each outcome measure, data was obtained three times: at baseline, 1 min into sustained IOP elevation, and 1 min after the probe was removed. RESULTS: During IOP elevation, changes in oxygen saturation of retinal arteries failed to reach statistical significance [F(1,30) = 3.69, p = 0.05], whereas venous oxygen saturation was significantly reduced [F(1,21) = 27.43, p < 0.01]. Blood flow increased slightly [F(2,40) = 6.28, p < 0.0001], PERG amplitude significantly reduced [F(2,44) = 24.24, p < 0.0001] and PERG phase was significantly delayed [F(2,44) = 17.00, p < 0.0001]. Contralateral eyes were unchanged. OPP reduction correlated little with PERG amplitude, PERG phase or venous oxygen saturation. CONCLUSIONS: Mild, acute IOP elevation increases arterio-venous oxygen saturation differences primarily through lowering venous oxygen saturation, suggesting increased oxygen consumption by healthy neurons when physiologically stressed.

Chronic intraocular pressure elevation impairs autoregulatory capacity in streptozotocin-induced diabetic rat retina.

PURPOSE: To assess ocular blood flow responses to acute IOP stress following 4 weeks of chronic IOP elevation in streptozotocin (STZ)-induced diabetic and control rats. We hypothesise that chronic IOP elevation for 4 weeks will further impair blood flow regulation in STZ-induced diabetic rats eyes. METHODS: Two weeks following citrate buffer or STZ-injections chronic IOP elevation was induced in Long Evans rats via fortnightly intracameral injections of microspheres (15 µm) suspended in 5% polyethylene glycol. IOP was monitored daily. Electroretinography (ERG, -6.79-2.07 log cd s m(-2) ) was undertaken at Week 4 to compare photoreceptor (RmPIII ), ON-bipolar cell (Vmax ) and ganglion cell dominant ERG [scotopic threshold response (STR)] components. 4 weeks post-chronic IOP induction, ocular blood flow (laser Doppler flowmetry) was measured in response to acute IOP challenge (10-100 mmHg, in 5 mmHg steps, each 3 min). RESULTS: Four weeks of chronic IOP (mean ± S.E.M., citrate: 24.0 ± 0.3 to 30.7 ± 1.3 and STZ-diabetes: 24.2 ± 0.2 to 31.1 ± 1.2 mmHg) was associated with reduced photoreceptor amplitude in both groups (-25.3 ± 2.2% and -17.2 ± 3.0%, respectively). STZ-diabetic eyes showed reduced photoreceptor sensitivity (citrate: 0.5 ± 1.8%, STZ-diabetic: -8.1 ± 2.4%). Paradoxically ON-bipolar cell sensitivity was increased, particularly in citrate control eyes (citrate: 166.8 ± 25.9%, STZ-diabetic: 64.8 ± 18.7%). The ganglion cell dominant STR was not significantly reduced in STZ-diabetic rats. Using acute IOP elevation to probe autoregulation, we show that STZ-diabetes impaired autoregulation compared with citrate control animals. The combination of STZ-diabetes and chronic IOP elevation further impaired autoregulation. CONCLUSIONS: STZ-diabetes and chronic IOP elevation appear to be additive risk factors for impairment of ocular blood flow autoregulation.

Sildenafil alters retinal function in mouse carriers of retinitis pigmentosa.

Sildenafil, the active ingredient in Viagra, has been reported to cause transient visual disturbance from inhibition of phosphodiesterase 6 (PDE6), a key enzyme in the visual phototransduction pathway. This study investigated the effects of sildenafil on the rd1(+/-) mouse, a model for carriers of Retinitis Pigmentosa which exhibit normal vision but may have a lower threshold for cellular stress caused by sildenafil due to a heterozygous mutation in PDE6. Sildenafil caused a dose-dependent decrease in electroretinogram (ERG) responses of normal mice which mostly recovered two days post administration. In contrast, rd1(+/-) mice exhibited a significantly reduced photoreceptor and a supernormal bipolar cell response to sildenafil within 1 h of treatment. Carrier mice retinae took two weeks to return to baseline levels suggesting sildenafil has direct effects on both the inner and outer retina and these effects differ significantly between normal and carrier mice. Anatomically, an increase in expression of the early apoptotic marker, cytochrome C in rd1(+/-) mice indicated that the effects of sildenafil on visual function may lead to degeneration. The results of this study are significant considering approximately 1 in 50 people are likely to be carriers of recessive traits leading to retinal degeneration.

Test-retest reliability of retinal oxygen saturation measurement.

PURPOSE: To determine intrasession and intersession repeatability of retinal vessel oxygen saturation from the Oxymap Retinal Oximeter using a whole image-based analysis technique and so determine optimal analysis parameters to reduce variability. METHODS: Ten fundus oximetry images were acquired through dilated pupils from 18 healthy participants (aged 22 to 38) using the Oxymap Retinal Oximeter T1. A further 10 images were obtained 1 to 2 weeks later from each individual. Analysis was undertaken for subsets of images to determine the number of images needed to return a stable coefficient of variation (CoV). Intrasession and intersession variability were quantified by evaluating the CoV and establishing the 95% limits of agreement using Bland and Altman analysis. Retinal oxygenation was derived from the distribution of oxygenation values from all vessels of a given width in an image or set of images, as described by Paul et al. in 2013. RESULTS: Grouped in 10-µm-wide bins, oxygen saturation varied significantly for both arteries and veins (p < 0.01). Between 110 and 150 µm, arteries had the least variability between individuals, with average CoVs less than 5% whose confidence intervals did not overlap with the greater than 10% average CoVs for veins across the same range. Bland and Altman analysis showed that there was no bias within or between recording sessions and that the 95% limits of agreement were generally lower in arteries. CONCLUSIONS: Retinal vessel oxygen saturation measurements show variability within and between clinical sessions when the whole image is used, which we believe more accurately reflects the true variability in Oxymap images than previous studies on select image segments. Averaging data from vessels 100 to 150 µm in width may help to minimize such variability.

Understanding how ageing impacts ganglion cell susceptibility to injury in glaucoma.

Glaucoma is a leading cause of blindness worldwide, with a marked increase in prevalence with advancing age. Due to the multifactorial nature of glaucoma pathogenesis, dissecting how ageing impacts upon glaucoma risk requires analysis and synthesis of evidence from a vast literature. While there is a wealth of human clinical studies examining glaucoma pathogenesis and why older patients have increased risk, many aspects of the disease such as adaptations of retinal ganglion cells to stress, autophagy and the role of glial cells in glaucoma, require the use of animal models to study the complex cellular processes and interactions. Additionally, the accelerated nature of ageing in rodents facilitates the longitudinal study of changes that would not be feasible in human clinical studies. This review article examines evidence derived predominantly from rodent models on how the ageing process impacts upon various aspects of glaucoma pathology from the retinal ganglion cells themselves, to supporting cells and tissues such as glial cells, connective tissue and vasculature, in addition to oxidative stress and autophagy. An improved understanding of how ageing modifies these factors may lead to the development of different therapeutic strategies that target specific risk factors or processes involved in glaucoma.

Altered Outer Retinal Structure, Electrophysiology and Visual Perception in Parkinson's Disease.

BACKGROUND: Visual biomarkers of Parkinson's disease (PD) are attractive as the retina is an outpouching of the brain. Although inner retinal neurodegeneration in PD is well-established this has overlap with other neurodegenerative diseases and thus outer retinal (photoreceptor) measures warrant further investigation. OBJECTIVE: To examine in a cross-sectional study whether clinically implementable measures targeting outer retinal function and structure can differentiate PD from healthy ageing and whether these are sensitive to intraday levodopa (L-DOPA) dosing. METHODS: Centre-surround perceptual contrast suppression, macular visual field sensitivity, colour discrimination, light-adapted electroretinography and optical coherence tomography (OCT) were tested in PD participants (n = 16) and controls (n = 21). Electroretinography and OCT were conducted before and after midday L-DOPA in PD participants, or repeated after ∼2 hours in controls. RESULTS: PD participants had decreased center-surround contrast suppression (p < 0.01), reduced macular visual field sensitivity (p < 0.05), color vision impairment (p < 0.01) photoreceptor dysfunction (a-wave, p < 0.01) and photoreceptor neurodegeneration (outer nuclear layer thinning, p < 0.05), relative to controls. Effect size comparison between inner and outer retinal parameters showed that photoreceptor metrics were similarly robust in differentiating the PD group from age-matched controls as inner retinal changes. Electroretinography and OCT were unaffected by L-DOPA treatment or time. CONCLUSIONS: We show that outer retinal outcomes of photoreceptoral dysfunction (decreased cone function and impaired color vision) and degeneration (i.e., outer nuclear layer thinning) were equivalent to inner retinal metrics at differentiating PD from healthy age-matched adults. These findings suggest outer retinal metrics may serve as useful biomarkers for PD.

Levodopa Rescues Retinal Function in the Transgenic A53T Alpha-Synuclein Model of Parkinson's Disease.

BACKGROUND: Loss of substantia nigra dopaminergic cells and alpha-synuclein (α-syn)-rich intraneuronal deposits within the central nervous system are key hallmarks of Parkinson's disease (PD). Levodopa (L-DOPA) is the current gold-standard treatment for PD. This study aimed to evaluate in vivo retinal changes in a transgenic PD model of α-syn overexpression and the effect of acute levodopa (L-DOPA) treatment. METHODS: Anaesthetised 6-month-old mice expressing human A53T alpha-synuclein (HOM) and wildtype (WT) control littermates were intraperitoneally given 20 mg/kg L-DOPA (50 mg levodopa, 2.5 mg benserazide) or vehicle saline (n = 11-18 per group). In vivo retinal function (dark-adapted full-field ERG) and structure (optical coherence tomography, OCT) were recorded before and after drug treatment for 30 min. Ex vivo immunohistochemistry (IHC) on flat-mounted retina was conducted to assess tyrosine hydroxylase (TH) positive cell counts (n = 7-8 per group). RESULTS: We found that photoreceptor (a-wave) and bipolar cell (b-wave) ERG responses (p < 0.01) in A53T HOM mice treated with L-DOPA grew in amplitude more (47 ± 9%) than WT mice (16 ± 9%) treated with L-DOPA, which was similar to the vehicle group (A53T HOM 25 ± 9%; WT 19 ± 7%). While outer retinal thinning (outer nuclear layer, ONL, and outer plexiform layer, OPL) was confirmed in A53T HOM mice (p < 0.01), L-DOPA did not have an ameliorative effect on retinal layer thickness. These findings were observed in the absence of changes to the number of TH-positive amacrine cells across experiment groups. Acute L-DOPA treatment transiently improves visual dysfunction caused by abnormal alpha-synuclein accumulation. CONCLUSIONS: These findings deepen our understanding of dopamine and alpha-synuclein interactions in the retina and provide a high-throughput preclinical framework, primed for translation, through which novel therapeutic compounds can be objectively screened and assessed for fast-tracking PD drug discovery.

Electroretinography in streptozotocin diabetic rats following acute intraocular pressure elevation.

BACKGROUND: We consider whether pre-existing streptozotocin induced hyperglycemia in rats affects the ability of the eye to cope with a single episode of acute intraocular pressure (IOP) elevation. METHODS: Electroretinogram (ERG) responses were measured (-6.08 to 1.92 log cd.s.m(-2)) in anaesthetized (60:5 mg/kg ketamine:xylazine) dark-adapted (>12 h) adult Sprague-Dawley rats 1 week after a single acute IOP elevation to 70 mmHg for 60 min. This was undertaken in rats treated 11 weeks earlier with streptozotocin (STZ, n = 12, 50 mg/kg at 6 weeks of age) or citrate buffer (n = 12). ERG responses were analyzed to derive an index of photoreceptor (a-wave), ON-bipolar (b-wave), amacrine (oscillatory potentials) and inner retinal (positive scotopic threshold response, pSTR) function. RESULTS: One week following acute IOP elevation there was a significant reduction of the ganglion cell pSTR (-35 ± 11 %, P = 0.0161) in STZ-injected animals. In contrast the pSTR in citrate-injected animals was not significant changed (+16 ± 14 %). The negative component of the STR was unaffected by IOP elevation in either citrate or STZ-treated groups. Photoreceptoral (a-wave, citrate-control +4 ± 3 %, STZ +4 ± 5 %) and ON-bipolar cell (b-wave, control +4 ± 3 %, STZ +4 ± 5 %) mediated responses were not significantly affected by IOP elevation in either citrate- or STZ-injected rats. Finally, oscillatory potentials (citrate-control +8 ± 23 %, STZ +1 ± 17 %) were not reduced 1 week after IOP challenge. CONCLUSIONS: The ganglion cell dominated pSTR was reduced following a single episode of IOP elevation in STZ diabetic, but not control rats. These data indicate that hyperglycemia renders the inner retina more susceptible to IOP elevation.