Retinal hyperspectral imaging in mouse models of Parkinson's disease and healthy aging.

Retinal hyperspectral imaging (HSI) is a non-invasive in vivo approach that has shown promise in Alzheimer's disease. Parkinson's disease is another neurodegenerative disease where brain pathobiology such as alpha-synuclein and iron overaccumulation have been implicated in the retina. However, it remains unknown whether HSI is altered in in vivo models of Parkinson's disease, whether it differs from healthy aging, and the mechanisms which drive these changes. To address this, we conducted HSI in two mouse models of Parkinson's disease across different ages; an alpha-synuclein overaccumulation model (hA53T transgenic line M83, A53T) and an iron deposition model (Tau knock out, TauKO). In comparison to wild-type littermates the A53T and TauKO mice both demonstrated increased reflectivity at short wavelengths ~ 450 to 600 nm. In contrast, healthy aging in three background strains exhibited the opposite effect, a decreased reflectance in the short wavelength spectrum. We also demonstrate that the Parkinson's hyperspectral signature is similar to that from an Alzheimer's disease model, 5xFAD mice. Multivariate analyses of HSI were significant when plotted against age. Moreover, when alpha-synuclein, iron or retinal nerve fibre layer thickness were added as a cofactor this improved the R2 values of the correlations in certain groups. This study demonstrates an in vivo hyperspectral signature in Parkinson's disease that is consistent in two mouse models and is distinct from healthy aging. There is also a suggestion that factors including retinal deposition of alpha-synuclein and iron may play a role in driving the Parkinson's disease hyperspectral profile and retinal nerve fibre layer thickness in advanced aging. These findings suggest that HSI may be a promising translation tool in Parkinson's disease.

Retinal electrophysiology in central nervous system disorders. A review of human and mouse studies.

The retina and brain share similar neurochemistry and neurodevelopmental origins, with the retina, often viewed as a "window to the brain." With retinal measures of structure and function becoming easier to obtain in clinical populations there is a growing interest in using retinal findings as potential biomarkers for disorders affecting the central nervous system. Functional retinal biomarkers, such as the electroretinogram, show promise in neurological disorders, despite having limitations imposed by the existence of overlapping genetic markers, clinical traits or the effects of medications that may reduce their specificity in some conditions. This narrative review summarizes the principal functional retinal findings in central nervous system disorders and related mouse models and provides a background to the main excitatory and inhibitory retinal neurotransmitters that have been implicated to explain the visual electrophysiological findings. These changes in retinal neurochemistry may contribute to our understanding of these conditions based on the findings of retinal electrophysiological tests such as the flash, pattern, multifocal electroretinograms, and electro-oculogram. It is likely that future applications of signal analysis and machine learning algorithms will offer new insights into the pathophysiology, classification, and progression of these clinical disorders including autism, attention deficit/hyperactivity disorder, bipolar disorder, schizophrenia, depression, Parkinson's, and Alzheimer's disease. New clinical applications of visual electrophysiology to this field may lead to earlier, more accurate diagnoses and better targeted therapeutic interventions benefiting individual patients and clinicians managing these individuals and their families.

Effect of hydroxychloroquine or chloroquine and short wavelength light on in vivo retinal function and structure in mouse eyes.

CLINICAL RELEVANCE: The use of chloroquine or hydroxychloroquine can lead to both acute and chronic changes to both retinal structure and function. BACKGROUND: Chloroquine (CQ) and hydroxychloroquine (HCQ) have the potential for retina toxicity. The acute impact of short-term drug exposure (2-4 weeks) on in vivo retinal structure and function and assess whether short wavelength light exposure further exacerbates any structural and functional changes was assessed in a murine model. METHODS: Adult C57BL/6 J mice received intraperitoneal injection of vehicle or hydroxychloroquine (10 mg/kg) 3 times per week for 2 or 4 weeks, or chloroquine for 4 weeks (10 mg/kg). Over this period, animals were exposed to room light (8 hours) or short-wavelength light 4 hours per day (4 hours of normal room light) for 5 days each week. Retinal changes were assessed using electroretinography (ERG), in vivo optical coherence tomography (OCT) imaging. RESULTS: Short-term low-dose HCQ and CQ treatment led to RPE thickening and elongation of photoreceptors. These structural changes were associated with a no dysfunction in the case of HCQ treatments and widespread functional changes (photoreceptor sensitivity, bipolar cell amplitude and oscillatory potential amplitude) in the case of CQ treatment. Exposure to low intensity short-wavelength light does not appear to alter the effect of HCQ or CQ. CONCLUSIONS: HCQ and CQ treatment has acute effects on both retinal structure and function, effects that were not exacerbated by short wavelength light exposure. Whether chronic short wavelength light exposure exacerbates these changes require further study.

Optimizing Retinal Thermofusion in Retinal Detachment Repair: Achieving Instant Adhesion without Air Tamponade.

Purpose: Rhegmatogenous retinal detachment repair by intraoperative sealing of the tear without a tamponade agent should enable faster restoration of vision and resumption of normal activities. It avoids the need for further surgery in the case of silicone oil endotamponade. This study evaluated the retinal thermofusion (RTF) retinopexy method of subretinal space dehydration before photocoagulation to create an instantaneous intraoperative retina reattachment in a preclinical model. Design: Preclinical study. Participants: Twenty Dutch Belt, pigmented rabbits that underwent RTF repair after experimental retinal detachment. Methods: This ex vivo model quantified adhesion force between the retina and underlying retinal pigment epithelium and choroid after treatment of 1 retinal edge using postmortem porcine or human retina (6 × 12 mm). We compared (1) control, (2) laser photocoagulation alone, (3) dehydration alone, and (4) dehydration followed by photocoagulation (RTF). Optimized parameters for RTF were then applied in the in vivo rabbit model of retinal detachment. Animals were followed up for 14 days. Main Outcome Measures: For this ex vivo model, we measured adhesion force and related this to tissue temperature. For the in vivo study, we assessed retinal attachment using funduscopy and histologic analysis. Results: The ex vivo model showed that RTF repair produced significantly higher adhesion force than photocoagulation alone independent of dehydration method: warm (60° C) high airflow (50-70 ml/minute) or using laser wavelengths targeting water absorption peaks (1470 or 1940 nm) with coaxial low airflow (10-20 ml/minute). The latter approach produced a smaller footprint of dehydration. Application of RTF (1940-nm laser with coaxial airflow) in an in vivo retinal detachment model in rabbit eyes resulted in immediate retinal adhesion, achieving forces similar to those in the ex vivo experiments. Retinal thermofusion repair resulted in stable reattachment of the retina over the 2-week follow-up period. Conclusions: We showed that a short preliminary dehydrating laser treatment of a retinal tear margin before traditional laser photocoagulation creates an immediate intraoperative waterproof retinopexy adhesion independent of tamponade and a wound-healing response. This approach potentially will allow rapid postoperative recovery regardless of the tear location and improved vision.

Characterization of retinal function and structure in the MPTP murine model of Parkinson's disease.

In addition to well characterized motor symptoms, visual disturbances are increasingly recognized as an early manifestation in Parkinson's disease (PD). A better understanding of the mechanisms underlying these changes would facilitate the development of vision tests which can be used as preclinical biomarkers to support the development of novel therapeutics for PD. This study aims to characterize the retinal phenotype of a mouse model of dopaminergic dysfunction and to examine whether these changes are reversible with levodopa treatment. We use a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD to characterize the neurotoxic effects of MPTP on in vivo retinal function (electroretinography, ERG), retinal structure (optical coherence tomography, OCT) and retinal dopaminergic cell number (tyrosine hydroxylase immunohistochemistry, IHC) at two time points (21 and 45 days) post MPTP model induction. We also investigate the effect of levodopa (L-DOPA) as a proof-of-principle chronic intervention against MPTP-induced changes in the retina. We show that MPTP decreases dopaminergic amacrine cell number (9%, p < 0.05) and that a component of the ERG that involves these cells, in particular oscillatory potential (OP) peak timing, was significantly delayed at Day 45 (7-13%, p < 0.01). This functional deficit was paralleled by outer plexiform layer (OPL) thinning (p < 0.05). L-DOPA treatment ameliorated oscillatory potential deficits (7-13%, p < 0.001) in MPTP animals. Our data suggest that the MPTP toxin slows the timing of inner retinal feedback circuits related to retinal dopaminergic pathways which mirrors findings from humans with PD. It also indicates that the MPTP model causes structural thinning of the outer retinal layer on OCT imaging that is not ameliorated with L-DOPA treatment. Together, these non-invasive measures serve as effective biomarkers for PD diagnosis as well as for quantifying the effect of therapy.

Assessment of patient-reported outcome measures used in corneal transplantation: a systematic review.

The aim of this study was to review all the articles that have implemented patient-reported outcome measures (PROMs) to evaluate the quality of life (QoL) in corneal transplantation and discuss quality assessments of the PROMs. An extensive literature review was undertaken to identify all the studies that used PROMs to assess the QoL in corneal transplantation. Non-original or review articles, articles on other subject area and articles on cost-effectiveness/utility without PROM data/results were excluded. Each PROM was assessed against the following criteria: content development (item identification and item selection), psychometric properties, validity, reliability, and responsiveness. 425 articles were identified of which 35 articles were included in the final review. PROMs in corneal transplantation were used to (a) evaluate the QoL after surgery, (b) compare the QoL scores between different surgical techniques and (c) determine the relationship between QoL and objective measures such as visual acuity, visual field and stereoacuity. A total of 17 PROMs were used to assess QoL in corneal transplantation. Whilst this search did not produce any PROMs that were specifically designed to assess corneal transplantation, most studies were found to have employed the National Eye Institute Visual Function Questionnaire 25 (NEI VFQ 25). The Visual Function Index 14 (VF 14) performed better in the present quality assessment criteria compared to other PROMs, however, the NEI VFQ 25 and the VF 14 PROMs were not specifically developed for corneal transplantation and therefore the QoL assessment made using these PROMs may be incomplete. As improvements in various forms of lamellar transplantation surgery techniques such as UT-DSAEK and FT-DSAEK have resulted in better visual outcomes, improved graft survival and reduced complications, a corneal transplantation specific PROM will be useful in clinical settings to compare the outcomes of different surgical techniques from the patient perspective.

Progressive impairments in executive function in the APP/PS1 model of Alzheimer's disease as measured by translatable touchscreen testing.

Executive function deficits in Alzheimer's disease (AD) occur early in disease progression and may be predictive of cognitive decline. However, no preclinical studies have identified deficits in rewarded executive function in the commonly used APPSwe/PS1∆E9 (APP/PS1) mouse model. To address this, we assessed 12-26 month old APP/PS1 mice on rewarded reversal and/or extinction tasks. 16-month-old, but not 13- or 26-month-old, APP/PS1 mice showed an attenuated rate of extinction. Reversal deficits were seen in 22-month-old, but not 13-month-old APP/PS1 animals. We then confirmed that impairments in reversal were unrelated to previously reported visual impairments in both AD mouse models and humans. Age, but not genotype, had a significant effect on markers of retinal health, indicating the deficits seen in APP/PS1 mice were directly related to cognition. This is the first characterisation of rewarded executive function in APP/PS1 mice, and has great potential to facilitate translation from preclinical models to the clinic.

Retinal hyperspectral imaging in the 5xFAD mouse model of Alzheimer's disease.

Hyperspectral imaging of the retina has recently been posited as a potentially useful form of spectroscopy of amyloid-beta (Aß) protein in the eyes of those with Alzheimer's disease (AD). The concept of using the retina as a biomarker for AD is an attractive one, as current screening tools for AD are either expensive or inaccessible. Recent studies have investigated hyperspectral imaging in Aß models however these studies have been in younger mice. Here we characterised hyperspectral reflectance profile in 6 to 17 months old 5xFAD mice and compare this to Aß in isolated preparations. Hyperspectral imaging was conducted across two preparations of Aß using a custom built bench ophthalmoscope. In the in vitro condition, 1 mg of purified human Aß42 was solubilised and left to aggregate for 72 h. This soluble/insoluble Aß mixture was then imaged by suspending the solution at a pipette tip and compared against phosphate buffered saline (PBS) control (n = 10 ROIs / group). In the in vivo condition, a 5xFAD transgenic mouse model was used and retinae were imaged at the age of 6 (n = 9), 12 (n = 9) and 17 months (n = 8) with age matched wildtype littermates as control (n = 12, n = 13, n = 15 respectively). In the vitro condition, hyperspectral imaging of the solution showed greater reflectance compared with vehicle (p < 0.01), with the greatest differences occurring in the short visible spectrum (< 500 nm). In the in vivo preparation, 5xFAD showed greater hyperspectral reflectance at all ages (6, 12, 17 months, p < 0.01). These differences were noted most in the short wavelengths at younger ages, with an additional peak appearing at longer wavelengths (~ 550 nm) with advancing age. This study shows that the presence of Aß (soluble/insoluble mixture) can increase the hyperspectral reflectance profile in vitro as well as in vivo. Differences were evident in the short wavelength spectrum (< 500 nm) in vitro and were preserved when imaged through the ocular media in the in vivo conditions. With advancing age a second hump around ~ 550 nm became more apparent. Hyperspectral imaging of the retina does not require the use of contrast agents and is a potentially useful and non-invasive biomarker for AD.

Retinal changes in Alzheimer's disease- integrated prospects of imaging, functional and molecular advances.

Alzheimer's Disease (AD) is a devastating neurodegenerative disorder of the brain, clinically characterised by cognitive deficits that gradually worsen over time. There is, at present, no established cure, or disease-modifying treatments for AD. As life expectancy increases globally, the number of individuals suffering from the disease is projected to increase substantially. Cumulative evidence indicates that AD neuropathological process is initiated several years, if not decades, before clinical signs are evident in patients, and diagnosis made. While several imaging, cognitive, CSF and blood-based biomarkers have been proposed for the early detection of AD; their sensitivity and specificity in the symptomatic stages is highly variable and it is difficult to justify their use in even earlier, pre-clinical stages of the disease. Research has identified potentially measurable functional, structural, metabolic and vascular changes in the retina during early stages of AD. Retina offers a distinctively accessible insight into brain pathology and current and developing ophthalmic technologies have provided us with the possibility of detecting and characterising subtle, disease-related changes. Recent human and animal model studies have further provided mechanistic insights into the biochemical pathways that are altered in the retina in disease, including amyloid and tau deposition. This information coupled with advances in molecular imaging has allowed attempts to monitor biochemical changes and protein aggregation pathology in the retina in AD. This review summarises the existing knowledge that informs our understanding of the impact of AD on the retina and highlights some of the gaps that need to be addressed. Future research will integrate molecular imaging innovation with functional and structural changes to enhance our knowledge of the AD pathophysiological mechanisms and establish the utility of monitoring retinal changes as a potential biomarker for AD.

Morphometric Changes to Corneal Dendritic Cells in Individuals With Mild Cognitive Impairment.

PURPOSE: There has been increasing interest in identifying non-invasive, imaging biomarkers for neurodegenerative disorders of the central nervous system (CNS). The aim of this proof-of-concept study was to investigate whether corneal sensory nerve and dendritic cell (DC) parameters, captured using in vivo confocal microscopy (IVCM), are altered in individuals with mild cognitive impairment (MCI) and Alzheimer's disease (AD). METHODS: Fifteen participants were recruited from the Australian Imaging Biomarkers and Lifestyle (AIBL) study in Melbourne, VIC, Australia. The cohort consisted of cognitively normal (CN) individuals (n = 5), and those with MCI (n = 5) and AD (n = 5). Participants underwent a slit lamp examination of the anterior segment, followed by corneal imaging using laser-scanning in vivo confocal microscopy (IVCM) of the central and inferior whorl regions. Corneal DC density, field area, perimeter, circularity index, aspect ratio, and roundness were quantified using Image J. Quantitative data were derived for corneal nerve parameters, including nerve fiber length (CNFL), fiber density (CNFD), branch density (CNBD), and diameter. RESULTS: Corneal DC field area and perimeter were greater in individuals with MCI, relative to CN controls, in both the central and inferior whorl regions (p < 0.05 for all comparisons). In addition, corneal DCs in the whorl region of MCI eyes had lower circularity and roundness indices and a higher aspect ratio relative to CNs (p < 0.05 for all comparisons). DC density was similar across participant groups in both corneal regions. There was a trend toward lower quantitative parameters for corneal nerve architecture in the AD and MCI groups compared with CN participants, however, the inter-group differences did not reach statistical significance. Central corneal nerve diameters were similar between groups. CONCLUSION: This study is the first to report morphological differences in corneal DCs in humans with MCI. These differences were evident in both the central and mid-peripheral cornea, and in the absence of significant nerve abnormalities or a difference in DC density. These findings justify future large-scale studies to assess the utility of corneal IVCM and DC analysis for identifying early stage pathology in neurodegenerative disorders of the CNS.

Retinal Functional and Structural Changes in the 5xFAD Mouse Model of Alzheimer's Disease.

Alzheimer's disease is characterized by the aberrant deposition of protein in the brain and is the leading cause of dementia worldwide. Increasingly, there have been reports of the presence of these protein hallmarks in the retina. In this study, we assayed the retina of 5xFAD mice, a transgenic model of amyloid deposition known to exhibit dementia-like symptoms with age. Using OCT, we found that the retinal nerve fiber layer was thinner in 5xFAD at 6, 12, and 17 months of age compared with wild-type littermates, but the inner plexiform layer was thicker at 6 months old. Retinal function showed reduced ganglion cell responses to light in 5xFAD at 6, 12, and 17 months of age. This functional loss was observed in the outer retina at 17 months of age but not in younger mice. We showed using immunohistochemistry and ELISA that soluble and insoluble amyloid was present in the retina and brain at all ages. In conclusion, we report that amyloid is present in brain and retina of 5xFAD mice and that the pattern of neuronal dysfunction occurs in the inner retina at the early ages and progresses to encompass the outer retina with age. This implies that the inner retina is more sensitive to amyloid changes in early disease and that the outer retina is also affected with disease progression.

Age-Specific Retinal and Cerebral Immunodetection of Amyloid-ß Plaques and Oligomers in a Rodent Model of Alzheimer's Disease.

BACKGROUND: Amyloid-ß soluble oligomers (Aßo) are believed to be the cause of the pathophysiology underlying Alzheimer's disease (AD) and are normally detected some two decades before clinical onset of the disease. Retinal pathology associated with AD pathogenesis has previously been reported, including ganglion cell loss, accumulation of Aß deposits in the retina, and reduction of nerve fiber layer thickness as well as abnormalities of the microvasculature. OBJECTIVE: This study's aim is to better understand the relationship between brain and retinal Aßo deposition and in particular to quantify levels of the toxic Aßo as a function of age in the retina of a rodent model of AD. METHODS: Retinas and brain tissue from 5×FAD mice were stained with Congo red, Thioflavin-T (Th-T), and Aß plaque-specific and Aßo-specific antibodies. RESULTS: We show that retinas displayed an age-dependent increase of Th-T-specific amyloid fibrils. Staining with anti-Aß antibody confirmed the presence of the Aß plaques in all 5×FAD retinas tested. In contrast, staining with anti-Aßo antibody showed an age-dependent decrease of retinal Aßo. Of note, Aßo was observed mainly in the retinal nuclear layers. Finally, we confirmed the localization of Aßo to neurons, typically accumulating in late endosomes, indicating possible impairment of the endocytic pathway. CONCLUSION: Our results demonstrate the presence of intraneuronal Aßo in the retina and its accumulation inversely correlated with retinal Aß plaque deposition, indicating an age-related conversion in this animal model. These results support the development of an early AD diagnostic test targeting Aßo in the eye.

Response of the Trilaminar Retinal Vessel Network to Intraocular Pressure Elevation in Rat Eyes.

Purpose: The purpose of this study was to test the hypothesis that the superficial, intermediate, and deep retinal vascular plexus show different responses to intraocular pressure (IOP) elevation. Methods: Anesthetized adult Long Evans rats (n = 14) were imaged using optical coherence tomography angiography (OCTA; Spectralis) at baseline (IOP 10 mm Hg) and in follow-up mode to examine the vasculature during IOP elevation (10 to 110 mm Hg, 10 mm Hg steps, each step 3 minutes). A 20° × 10° field was imaged. Vessel density within a 2D projection image was determined (%) for the superficial vascular complex (SVC), intermediate capillary plexus (ICP), and deep capillary plexus (DCP). Comparisons were made between layers using 2-way repeated measures ANOVA (layer versus IOP) following normalization to baseline (% relative to 10 mm Hg). Results: The three vascular layers responded differently to IOP elevation. For IOPs between 40 and 60 mm Hg, DCP and ICP capillaries were significantly more resistant to IOP elevation than those in the SVC. When IOP was elevated above 70 mm Hg, all layers showed reduced vessel density. IOP induced change in SVC vessel density closely followed reductions in thickness of the inner retinal layers (nerve fiber, ganglion cell, and inner plexiform layer). This close relationship between reductions in tissue thickness and vessel density was less apparent for the ICP and DCP. Conclusions: These data show that the intermediate and deep vascular plexus in the rat retina have a greater capacity for autoregulation against mild IOP elevation but are more affected at high IOP.

Non-invasive in vivo hyperspectral imaging of the retina for potential biomarker use in Alzheimer's disease.

Studies of rodent models of Alzheimer's disease (AD) and of human tissues suggest that the retinal changes that occur in AD, including the accumulation of amyloid beta (Aß), may serve as surrogate markers of brain Aß levels. As Aß has a wavelength-dependent effect on light scatter, we investigate the potential for in vivo retinal hyperspectral imaging to serve as a biomarker of brain Aß. Significant differences in the retinal reflectance spectra are found between individuals with high Aß burden on brain PET imaging and mild cognitive impairment (n = 15), and age-matched PET-negative controls (n = 20). Retinal imaging scores are correlated with brain Aß loads. The findings are validated in an independent cohort, using a second hyperspectral camera. A similar spectral difference is found between control and 5xFAD transgenic mice that accumulate Aß in the brain and retina. These findings indicate that retinal hyperspectral imaging may predict brain Aß load.

A Model of Glaucoma Induced by Circumlimbal Suture in Rats and Mice.

The circumlimbal suture is a technique for inducing experimental glaucoma in rodents by chronically elevating intraocular pressure (IOP), a well-known risk factor for glaucoma. This protocol demonstrates a step-by-step guide on this technique in Long Evans rats and C57BL/6 mice. Under general anesthesia, a "purse-string" suture is applied on the conjunctiva, around the equator and behind the limbus of the eye. The fellow eye serves as an untreated control. Over the duration of our study, which was a period of 8 weeks for rats and 12 weeks for mice, IOP remained elevated, as measured regularly by rebound tonometry in conscious animals without topical anesthesia. In both species, the sutured eyes showed electroretinogram features consistent with preferential inner retinal dysfunction. Optical coherence tomography showed selective thinning of the retinal nerve fiber layer. Histology of the rat retina in cross-section found reduced cell density in the ganglion cell layer, but no change in other cellular layers. Staining of flat-mounted mouse retinae with a ganglion cell specific marker (RBPMS) confirmed ganglion cell loss. The circumlimbal suture is a simple, minimally invasive and cost-effective way to induce ocular hypertension that leads to ganglion cell injury in both rats and mice.

Use of an automated feedback application to improve communication skills.

This article was migrated. The article was marked as recommended. Effective communication skills are a professional competency, yet are often overlooked during training. Providing immediate and constructive feedback is imperative to assist students in developing better communication skills. We sought to evaluate the educational value of using a university-developed application, Rapid Feedback, to provide feedback following students' oral presentations over two years. An online survey comprising of eight 5-point Likert scale items and one open-ended question was conducted in 114 (response rate = 86.5%) students. Students either strongly agreed or agreed that the feedback delivered was timely (98%), relevant (96%), high quality (90%), and specific to enhance their learning (87%). The feedback obtained has helped to identify strengths and weaknesses (87%). Students commented that feedback received will improve their communication skills (90%). The report was also shown to supplement verbal feedback (95%). Overall, students expressed that the feedback report was valuable, allowing for critical self-reflection and future retention. Staff have also found the application easy to use and administer. In a time- and resource-constrained teaching environment, educators constantly explore technology to support student learning and teaching outcomes. We have implemented an application that is user-friendly to staff, efficient, and has provided effective feedback that is well-received and valued by students.

Retinal biomarkers provide "insight" into cortical pharmacology and disease.

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

Characterization of the Circumlimbal Suture Model of Chronic IOP Elevation in Mice and Assessment of Changes in Gene Expression of Stretch Sensitive Channels.

To consider whether a circumlimbal suture can be used to chronically elevate intraocular pressure (IOP) in mice and to assess its effect on retinal structure, function and gene expression of stretch sensitive channels. Anesthetized adult C57BL6/J mice had a circumlimbal suture (10/0) applied around the equator of one eye. In treated eyes (n = 23) the suture was left in place for 12 weeks whilst in sham control eyes the suture was removed at day two (n = 17). Contralateral eyes served as untreated controls. IOP was measured after surgery and once a week thereafter. After 12 weeks, electroretinography (ERG) was performed to assess photoreceptor, bipolar cell and retinal ganglion cell (RGC) function. Retinal structure was evaluated using optical coherence tomography. Retinae were processed for counts of ganglion cell density or for quantitative RT-PCR to quantify purinergic (P2x7, Adora3, Entpd1) or stretch sensitive channel (Panx1, Trpv4) gene expression. Immediately after suture application, IOP spiked to 33 ± 3 mmHg. After 1 day, IOP had recovered to 27 ± 3 mmHg. Between weeks 2 and 12, IOP remained elevated above baseline (control 14 ± 1 mmHg, ocular hypertensive 19 ± 1 mmHg). Suture removal at day 2 (Sham) restored IOP to baseline levels, where it remained through to week 12. ERG analysis showed that 12 weeks of IOP elevation reduced photoreceptor (-15 ± 4%), bipolar cell (-15 ± 4%) and ganglion cell responses (-19 ± 6%) compared to sham controls and respective contralateral eyes (untreated). The retinal nerve fiber layer was thinned in the presence of normal total retinal thickness. Ganglion cell density was reduced across all quadrants (superior -12 ± 5%; temporal, -7% ± 2%; inferior -9 ± 4%; nasal -8 ± 5%). Quantitative RT-PCR revealed a significant increase in Entpd1 gene expression (+11 ± 4%), whilst other genes were not significantly altered (P2x7, Adora3, Trpv4, Panx1). Our results show that circumlimbal ligation produces mild chronic ocular hypertension and retinal dysfunction in mice. Consistent with a sustained change to purinergic signaling we found an up-regulation of Entpd1.

Early Postnatal Hyperoxia in Mice Leads to Severe Persistent Vitreoretinopathy.

Purpose: To describe a mouse model of hyperoxia-induced vitreoretinopathy that replicated some of the clinical and pathologic features encountered in infants with severe retinopathy of prematurity and congenital ocular conditions such as persistent hyperplastic primary vitreous. Methods: Experimental mice (C57BL/6J) were exposed to 65% oxygen between postnatal days (P)0 to P7 and studied at P10, P14, and 3, 5, 8, 20, and 40 weeks. Controls were exposed to normoxic conditions. Fundus imaging and fluorescein angiography were performed at all time points, and spectral-domain optical coherence tomography (SD-OCT) and electroretinography were performed at 8- and 20-week time points. Eyes were processed for resin histology, frozen sections, and retinal whole mounts. Immunostaining was performed to visualize vasculature isolectin B4 (Ib4), collagen type IV, glial fibrillary acidic protein, and α-smooth muscle actin. Results: Early exposure to hyperoxia resulted in bilateral vitreous hemorrhages at 3 weeks. From 5 weeks onward there were extensive zones of retinal degeneration, scarring or gliosis, retinal folding, and detachments caused by traction of α-smooth muscle actin-positive vitreous membranes. Tortuous retinal vessels, together with hyperplastic and persistence of hyaloid vessels are evident into adulthood. In the early stages (P10-3 weeks), branches from the tunica vasculosa lentis (TVL) supplied the marginal retina until retinal vessels were established. The peripheral retina remained poorly vascularized into adulthood. Electroretinography revealed 50% to 60% diminution in retinal function in adult mice that strongly correlated with vitreal changes identified using SD-OCT. Conclusions: This animal model displays a mixture of vitreoretinal pathologic changes that persist into adulthood. The model may prove valuable in experimental investigations of therapeutic approaches to blinding conditions caused by vitreous and retinal abnormalities.

The Eye As a Biomarker for Alzheimer's Disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder resulting in dementia and eventual death. It is the leading cause of dementia and the number of cases are projected to rise in the next few decades. Pathological hallmarks of AD include the presence of hyperphosphorylated tau and amyloid protein deposition. Currently, these pathological biomarkers are detected either through cerebrospinal fluid analysis, brain imaging or post-mortem. Though effective, these methods are not widely available due to issues such as the difficulty in acquiring samples, lack of infrastructure or high cost. Given that the eye possesses clear optics and shares many neural and vascular similarities to the brain, it offers a direct window to cerebral pathology. These unique characteristics lend itself to being a relatively inexpensive biomarker for AD which carries the potential for wide implementation. The development of ocular biomarkers can have far implications in the discovery of treatments which can improve the quality of lives of patients. In this review, we consider the current evidence for ocular biomarkers in AD and explore potential future avenues of research in this area.