Advances in retinal ganglion cell imaging

In vivo scanning laser fundus and high-resolution OCT imaging of retinal ganglion cell injury in a non-human primate model with an activatable fluorescent-labeled TAT peptide probe

The optical imaging agent TcapQ488 has enabled imaging of retinal ganglion cell (RGC) injury in vivo in rodents and has potential as an effective diagnostic probe for early detection and intervention monitoring in glaucoma patients. In the present study, we investigated TcapQ488 in non-human primates (NHPs) to identify labeling efficacy and early signals of injured RGC, to determine species-dependent changes in RGC probe uptake and clearance, and to determine dose-limiting toxicities. Doses of 3, 6, and 12 nmol of TcapQ488 were delivered intravitreally to normal healthy NHP eyes and eyes that had undergone hemiretinal endodiathermy axotomy (HEA) in the inferior retina. Post-injection fundus fluorescence imaging using a Spectralis imaging platform (Heidelberg Engineering) documented TcapQ488 activation in RGC cell bodies. Optical coherence tomography (OCT), slit-lamp examinations, intraocular pressure measurements, and visual electrophysiology testing were performed to monitor probe tolerability. For comparison, a negative control, non-cleavable, non-quenched probe (dTcap488, 6 nmol), was delivered intravitreally to a normal healthy eye. In normal healthy eyes, intravitreal injection of 3 nmol of TcapQ488 was well-tolerated, while 12 nmol of TcapQ488 to the healthy eye caused extensive probe activation in the ganglion cell layer (GCL) and eventual retinal nerve fiber layer thinning. In HEA eyes, the HEA procedure followed by intravitreal TcapQ488 (3 nmol) injection resulted in probe activation within cell bodies in the GCL, confined to the HEA-treated inferior retina, indicating cell injury and slow axonal transport in the GCL. However, in contrast to rodents, a vitreal haze that lasted 2-12 weeks obscured rapid high-resolution imaging of the fundus. By contrast, intravitreal TcapQ488 injection prior to the HEA procedure led to minimal probe labeling in the GCL. The results of the dTcap488 control experiments indicated that fast axonal transport carried the probe out of the retina after cell body uptake. No evidence of pan-retinal toxicity or loss of retino-cortical function was detected in any of the three NHPs tested. Overall, these data provide evidence of TcapQ488 activation, without toxicity, in NHP HEA eyes that had been intravitreally injected with 3 nmol of the probe. Compared to rodents, unexpectedly rapid axonal transport in the NHPs reduced the capacity to visualize RGC cell bodies and axons through the backdrop of an intravitreal haze. Nonetheless, although intravitreal clearance rates did not scale to NHPs, HEA-induced reductions in axonal transport enhanced probe visualization in the cell body.

Retinal ganglion cell vulnerability to pathogenic tau in Alzheimer's disease

Accumulation of pathological tau isoforms, especially hyperphosphorylated tau at serine 396 (pS396-tau) and tau oligomers, has been demonstrated in the retinas of patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD). Previous studies have noted a decrease in retinal ganglion cells (RGCs) in AD patients, but the presence and impact of pathological tau isoforms in RGCs and RGC integrity, particularly in early AD stages, have not been explored. To investigate this, we examined retinal superior temporal cross-sections from 25 patients with MCI (due to AD) or AD dementia and 16 cognitively normal (CN) controls, matched for age and gender. We utilized the RGC marker ribonucleic acid binding protein with multiple splicing (RBPMS) and Nissl staining to assess neuronal density in the ganglion cell layer (GCL). Our study found that hypertrophic RGCs containing pS396-tau and T22-positive tau oligomers were more frequently observed in MCI and AD patients compared to CN subjects. Quantitative analyses indicated a decline in RGC integrity, with 46-55% and 55-56% reductions of RBPMS+ RGCs (P<0.01) and Nissl+ GCL neurons (P<0.01-0.001), respectively, in MCI and AD patients. This decrease in RGC count was accompanied by increases in necroptotic-like morphology and the cleaved caspase-3 apoptotic marker in RGCs of AD patients. Furthermore, there was a 2.1 to 3.1-fold increase (P<0.05-0.0001) in pS396-tau-laden RGCs in MCI and AD patients, with a greater abundance observed in individuals with higher Braak stages (V-VI), more severe clinical dementia ratings (CDR=3), and lower mini-mental state examination (MMSE) scores. Strong correlations were noted between the decline in RGCs and the total amount of retinal pS396-tau and pS396-tau+ RGCs, with pS396-tau+ RGC counts correlating significantly with brain neurofibrillary tangle scores (r= 0.71, P= 0.0001), Braak stage (r= 0.65, P= 0.0009), and MMSE scores (r= -0.76, P= 0.0004). These findings suggest that retinal tauopathy, characterized by pS396-tau and oligomeric tau in hypertrophic RGCs, is associated with and may contribute to RGC degeneration in AD. Future research should validate these findings in larger cohorts and explore noninvasive retinal imaging techniques that target tau pathology in RGCs to improve AD detection and monitor disease progression.

Characterizing Presumed Displaced Retinal Ganglion Cells in the Living Human Retina of Healthy and Glaucomatous Eyes

Purpose

The purpose of this study was to investigate the large somas presumed to be displaced retinal ganglion cells (dRGCs) located in the inner nuclear layer (INL) of the living human retina. Whereas dRGCs have previously been studied in mammals and human donor tissue, they have never been investigated in the living human retina.

Methods

Five young, healthy subjects and three subjects with varying types of glaucoma were imaged at multiple locations in the macula using adaptive optics optical coherence tomography. In the acquired volumes, bright large somas at the INL border with the inner plexiform layer were identified, and the morphometric biomarkers of soma density, en face diameter, and spatial distribution were measured at up to 13 degrees retinal eccentricity. Susceptibility to glaucoma was assessed.

Results

In the young, healthy individuals, mean density of the bright, large somas was greatest foveally (550 and 543 cells/mm2 at 2 degrees temporal and nasal, respectively) and decreased with increasing retinal eccentricity (38 cells/mm2 at 13 degrees temporal, the farthest we measured). Soma size distribution showed the opposite trend with diameters and size variation increasing with retinal eccentricity, from 12.7 ± 1.8 µm at 2 degrees to 15.7 ± 3.5 µm at 13 degrees temporal, and showed evidence of a bimodal distribution in more peripheral locations. Within and adjacent to the arcuate defects of the subjects with glaucoma, density of the bright large somas was significantly lower than found in the young, healthy individuals.

Conclusions

Our results suggest that the bright, large somas at the INL border are likely comprised of dRGCs but amacrine cells may contribute too. These somas appear highly susceptible to glaucomatous damage.

Comparison of Brn3a and RBPMS Labeling to Assess Retinal Ganglion Cell Loss During Aging and in a Model of Optic Neuropathy

Purpose

Retinal ganglion cell (RGC) loss provides the basis for diagnosis and stage determination of many optic neuropathies, and quantification of RGC survival is a critical outcome measure in models of optic neuropathy. This study examines the accuracy of manual RGC counting using two selective markers, Brn3a and RBPMS.

Methods

Retinal flat mounts from 1- to 18-month-old C57BL/6 mice, and from mice after microbead (MB)-induced intraocular pressure (IOP) elevation, are immunostained with Brn3a and/or RBPMS antibodies. Four individuals masked to the experimental conditions manually counted labeled RGCs in three copies of five images, and inter- and intra-person reliability was evaluated by the intraclass correlation coefficient (ICC).

Results

A larger population (approximately 10% higher) of RGCs are labeled with RBPMS than Brn3a antibody up to 6 months of age, but differences decrease to approximately 1% at older ages. Both RGC-labeled populations significantly decrease with age. MB-induced IOP elevation is associated with a significant decrease of both Brn3a- and RBPMS-positive RGCs. Notably, RGC labeling with Brn3a provides more consistent cell counts than RBPMS in interpersonal (ICC = 0.87 to 0.11, respectively) and intra-personal reliability (ICC = 0.97 to 0.66, respectively).

Conclusions

Brn3a and RBPMS markers are independently capable of detecting significant decreases of RGC number with age and in response to IOP elevation despite RPBMS detecting a larger number of RGCs up to 6 months of age. Brn3a labeling is less prone to manual cell counting variability than RBPMS labeling. Overall, either marker can be used as a single marker to detect significant changes in RGC survival, each offering distinct advantages.

The Role of Retinal Ganglion Cell Structure and Function in Glaucoma

Glaucoma, a leading cause of irreversible blindness globally, primarily affects retinal ganglion cells (RGCs). This review dives into the anatomy of RGC subtypes, covering the different underlying theoretical mechanisms that lead to RGC susceptibility in glaucoma, including mechanical, vascular, excitotoxicity, and neurotrophic factor deficiency, as well as oxidative stress and inflammation. Furthermore, we examined numerous imaging methods and functional assessments to gain insight into RGC health. Finally, we investigated the current possible neuroprotective targets for RGCs that could help with future glaucoma research and management.

Longitudinal in vivo evaluation of retinal ganglion cell complex layer and dendrites in mice with experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE), induced by the immunization of myelin oligodendrocyte glycoprotein (MOG), is related to human MOG antibody-associated disease (MOGAD). Neuroinflammation and demyelination of the optic nerve can lead to retinal ganglion cell (RGC) death and axonal damage in MOGAD. Here, we aimed to evaluate the structural changes in RGCs longitudinally by in vivo imaging in mice with RGCs expressing yellow fluorescent protein along the course of EAE. Successful induction of EAE was confirmed by the neurological function scores and histology analyses. The changes in the thickness of ganglion cell complex (GCC) layer and RGC survival and dendrites were monitored longitudinally along the course of EAE. Before the onset of EAE, there were no significant changes in the number and morphology of RGCs and the thickness of the GCC layer as compared to the mice without EAE induction. After the onset of EAE, the thickness of the GCC layer and the RGC number and dendritic network all gradually decreased along the course of EAE. Notably, dendritic shrinkage could be detected earlier than the thinning of the GCC layer. In summary, this study delineated the longitudinal profile of RGC structural changes in EAE mice, providing an assessment platform for monitoring outcomes of RGC treatments.

Graphene-based microfluidic perforated microelectrode arrays for retinal electrophysiological studies

Perforated microelectrode arrays (pMEAs) have become essential tools for ex vivo retinal electrophysiological studies. pMEAs increase the nutrient supply to the explant and alleviate the accentuated curvature of the retina, allowing for long-term culture and intimate contacts between the retina and electrodes for electrophysiological measurements. However, commercial pMEAs are not compatible with in situ high-resolution optical imaging and lack the capability of controlling the local microenvironment, which are highly desirable features for relating function to anatomy and probing physiological and pathological mechanisms in retina. Here we report on microfluidic pMEAs (μpMEAs) that combine transparent graphene electrodes and the capability of locally delivering chemical stimulation. We demonstrate the potential of μpMEAs by measuring the electrical response of ganglion cells to locally delivered high K+ stimulation under controlled microenvironments. Importantly, the capability for high-resolution confocal imaging of the retina tissue on top of the graphene electrodes allows for further analyses of the electrical signal source. The new capabilities provided by μpMEAs could allow for retinal electrophysiology assays to address key questions in retinal circuitry studies.

Automatic counting of retinal ganglion cells in the entire mouse retina based on improved YOLOv5

Glaucoma is characterized by the progressive loss of retinal ganglion cells (RGCs), although the pathogenic mechanism remains largely unknown. To study the mechanism and assess RGC degradation, mouse models are often used to simulate human glaucoma and specific markers are used to label and quantify RGCs. However, manually counting RGCs is time-consuming and prone to distortion due to subjective bias. Furthermore, semi-automated counting methods can produce significant differences due to different parameters, thereby failing objective evaluation. Here, to improve counting accuracy and efficiency, we developed an automated algorithm based on the improved YOLOv5 model, which uses five channels instead of one, with a squeeze-and-excitation block added. The complete number of RGCs in an intact mouse retina was obtained by dividing the retina into small overlapping areas and counting, and then merging the divided areas using a non-maximum suppression algorithm. The automated quantification results showed very strong correlation (mean Pearson correlation coefficient of 0.993) with manual counting. Importantly, the model achieved an average precision of 0.981. Furthermore, the graphics processing unit (GPU) calculation time for each retina was less than 1 min. The developed software has been uploaded online as a free and convenient tool for studies using mouse models of glaucoma, which should help elucidate disease pathogenesis and potential therapeutics.

Prediction of visual field defects from macular optical coherence tomography in glaucoma using cluster analysis

Purpose

To assess the accuracy of cluster analysis‐based models in predicting visual field (VF) defects from macular ganglion cell‐inner plexiform layer (GCIPL) measurements in glaucomatous and healthy cohorts.

Methods

GCIPL measurements were extracted from posterior pole optical coherence tomography (OCT), from locations corresponding to central VF test grids. Models incorporating cluster analysis methods and corrections for age and fovea to optic disc tilt were developed from 493 healthy participants, and 5th and 1st percentile limits of GCIPL thickness were derived. These limits were compared with pointwise 5th and 1st percentile limits by calculating sensitivities and specificities in an additional 40 normal and 37 glaucomatous participants, as well as applying receiver operating characteristic (ROC) curve analyses to assess the accuracy of predicting VF results from co‐localised GCIPL measurements.

Results

Clustered models demonstrated globally low sensitivity, but high specificity in the glaucoma cohort (0.28–0.53 and 0.77–0.91, respectively), and high specificity in the healthy cohort (0.91–0.98). Clustered models showed similar sensitivities and superior specificities compared with pointwise methods (0.41–0.65 and 0.71–0.98, respectively). There were significant differences in accuracy between clusters, with relatively poor accuracy at peripheral macular locations (p < 0.0001 for all comparisons).

Conclusions

Cluster analysis‐based models incorporating age correction and holistic consideration of fovea to optic disc tilt demonstrated superior performance in predicting VF results to pointwise methods in both glaucomatous and healthy eyes. However, relatively low sensitivity and poorer performance at the peripheral macula indicate that OCT in isolation may be insufficient to predict visual function across the macula accurately. With modifications to criteria for abnormality, the concepts suggested by the described normative models may guide prioritisation of VF assessment requirements, with the potential to limit excessive VF testing.

Tools and Biomarkers for the Study of Retinal Ganglion Cell Degeneration

The retina is part of the central nervous system, its analysis may provide an idea of the health and functionality, not only of the retina, but also of the entire central nervous system, as has been shown in Alzheimer’s or Parkinson’s diseases. Within the retina, the ganglion cells (RGC) are the neurons in charge of processing and sending light information to higher brain centers. Diverse insults and pathological states cause degeneration of RGC, leading to irreversible blindness or impaired vision. RGCs are the measurable endpoints in current research into experimental therapies and diagnosis in multiple ocular pathologies, like glaucoma. RGC subtype classifications are based on morphological, functional, genetical, and immunohistochemical aspects. Although great efforts are being made, there is still no classification accepted by consensus. Moreover, it has been observed that each RGC subtype has a different susceptibility to injury. Characterizing these subtypes together with cell death pathway identification will help to understand the degenerative process in the different injury and pathological models, and therefore prevent it. Here we review the known RGC subtypes, as well as the diagnostic techniques, probes, and biomarkers for programmed and unprogrammed cell death in RGC.

Quantification of Retinal Ganglion Cell Morphology in Human Glaucomatous Eyes

Purpose

To characterize retinal ganglion cell morphological changes in patients with primary open-angle glaucoma associated with hemifield defect (HD) using adaptive optics–optical coherence tomography (AO-OCT).

Methods

Six patients with early to moderate primary open-angle glaucoma with an average age of 58 years associated with HD and six age-matched healthy controls with an average age of 61 years were included. All participants underwent in vivo retinal ganglion cell (RGC) imaging at six primary locations across the macula with AO-OCT. Ganglion cell layer (GCL) somas were manually counted, and morphological parameters of GCL soma density, size, and symmetry were calculated. RGC cellular characteristics were correlated with functional visual field measurements.

Results

GCL soma density was 12,799 ± 7747 cells/mm², 9370 ± 5572 cells/mm², and 2134 ± 1494 cells/mm² at 3°, 6°, and 12°, respectively, in glaucoma patients compared with 25,058 ± 4649 cells/mm², 15,551 ± 2301 cells/mm², and 3891 ± 1105 cells/mm² (P < 0.05 for all locations) at the corresponding retinal locations in healthy participants. Mean soma diameter was significantly larger in glaucoma patients (14.20 ± 2.30 µm) compared with the health controls (12.32 ± 1.94 µm, P < 0.05 for all locations); symmetry was 0.36 ± 0.32 and 0.86 ± 0.13 in glaucoma and control cohorts, respectively.

Conclusions

Glaucoma patients had lower GCL soma density and symmetry, greater soma size, and increased variation of GCL soma reflectance compared with age-matched control subjects. The morphological changes corresponded with HD, and the cellular level structural loss correlated with visual function loss in glaucoma. AO-based morphological parameters could be potential sensitive biomarkers for glaucoma.

Evidence-based understanding of disc hemorrhage in glaucoma

Disc hemorrhage is a characteristic finding that is highly associated with glaucoma development or progression. Consequently, the literature commonly designates disc hemorrhage as a "risk factor" for glaucoma progression; however, the exact cause-and-effect relationship or mechanism remains unclear. In this review, we discuss the emerging evidence that disc hemorrhage is a secondary development that follows glaucomatous damage. As our understanding of disc hemorrhage has progressed in recent decades, we suggest the terminology be changed from "risk factor" to "indicator" of ongoing glaucomatous development or progression for a more accurate description, better indication of the clinical implications and, ultimately, a better guide for future research.

The eye, the kidney, and cardiovascular disease: old concepts, better tools, and new horizons

Chronic kidney disease (CKD) is common, with hypertension and diabetes mellitus acting as major risk factors for its development. Cardiovascular disease is the leading cause of death worldwide and the most frequent end point of CKD. There is an urgent need for more precise methods to identify patients at risk of CKD and cardiovascular disease. Alterations in microvascular structure and function contribute to the development of hypertension, diabetes, CKD, and their associated cardiovascular disease. Homology between the eye and the kidney suggests that noninvasive imaging of the retinal vessels can detect these microvascular alterations to improve targeting of at-risk patients. Retinal vessel-derived metrics predict incident hypertension, diabetes, CKD, and cardiovascular disease and add to the current renal and cardiovascular risk stratification tools. The advent of optical coherence tomography (OCT) has transformed retinal imaging by capturing the chorioretinal microcirculation and its dependent tissue with near-histological resolution. In hypertension, diabetes, and CKD, OCT has revealed vessel remodeling and chorioretinal thinning. Clinical and preclinical OCT has linked retinal microvascular pathology to circulating and histological markers of injury in the kidney. The advent of OCT angiography allows contrast-free visualization of intraretinal capillary networks to potentially detect early incipient microvascular disease. Combining OCT's deep imaging with the analytical power of deep learning represents the next frontier in defining what the eye can reveal about the kidney and broader cardiovascular health.

Piwnica-Worms D, Schellingerhout D. Axonal Transport as an In Vivo Biomarker for Retinal Neuropathy

We illuminate a possible explanatory pathophysiologic mechanism for retinal cellular neuropathy by means of a novel diagnostic method using ophthalmoscopic imaging and a molecular imaging agent targeted to fast axonal transport. The retinal neuropathies are a group of diseases with damage to retinal neural elements. Retinopathies lead to blindness but are typically diagnosed late, when substantial neuronal loss and vision loss have already occurred. We devised a fluorescent imaging agent based on the non-toxic C fragment of tetanus toxin (TTc), which is taken up and transported in neurons using the highly conserved fast axonal transport mechanism. TTc serves as an imaging biomarker for normal axonal transport and demonstrates impairment of axonal transport early in the course of an N-methyl-D-aspartic acid (NMDA)-induced excitotoxic retinopathy model in rats. Transport-related imaging findings were dramatically different between normal and retinopathic eyes prior to presumed neuronal cell death. This proof-of-concept study provides justification for future clinical translation.

The Hub-and-Spoke Management of Glaucoma

Glaucoma is an extremely significant public health issue, since it is the most common cause of irreversible blindness worldwide, nevertheless it is still widely undiagnosed because of its devious nature. Glaucoma diagnosis criteria are well-defined and have to be strictly observed and recognized: the earlier the disease is diagnosed, the earlier the patient can undergo the most suitable treatment, the better can be the prognosis. The three levels of prevention are essential in the approach to the disease and its pathophysiological features make it eligible for screening. This review provides an overview of the current state of the art in glaucoma management, starting from its prevention and coming to the hub-and-spoke organization. This model applied to glaucoma aims to direct patients toward professional and not professional figures who may guide them in integrated care pathway. This path should be designed in accordance with best practice to coordinate glaucoma prevention, diagnosis, treatment and follow up with the best cost-benefit ratio, protecting both the interests of the patient and of the society.

Suite of methods for assessing inner retinal temporal dynamics across spatial and temporal scales in the living human eye

Significance: There are no label-free imaging descriptors related to physiological activity of inner retinal cells in the living human eye. A major reason is that inner retinal neurons are highly transparent and reflect little light, making them extremely difficult to visualize and quantify. Aim: To measure physiologically-induced optical changes of inner retinal cells despite their challenging optical properties. Approach: We developed an imaging method based on adaptive optics and optical coherence tomography (AO-OCT) and a suite of postprocessing algorithms, most notably a new temporal correlation method. Results: We captured the temporal dynamics of entire inner retinal layers, of specific tissue types, and of individual cells across three different timescales from fast (seconds) to extremely slow (one year). Time correlation analysis revealed significant differences in time constant (up to 0.4 s) between the principal layers of the inner retina with the ganglion cell layer (GCL) being the most dynamic. At the cellular level, significant differences were found between individual GCL somas. The mean time constant of the GCL somas ( 0.69 ± 0.17    s ) was ∼ 30 % smaller than that of nerve fiber bundles and inner plexiform layer synapses and processes. Across longer durations, temporal speckle contrast and time-lapse imaging revealed motion of macrophage-like cells (over minutes) and GCL neuron loss and remodeling (over one year). Conclusions: Physiological activity of inner retinal cells is now measurable in the living human eye.

Midget retinal ganglion cell dendritic and mitochondrial degeneration is an early feature of human glaucoma

Glaucoma is characterized by the progressive dysfunction and loss of retinal ganglion cells. However, the earliest degenerative events that occur in human glaucoma are relatively unknown. Work in animal models has demonstrated that retinal ganglion cell dendrites remodel and atrophy prior to the loss of the cell soma. Whether this occurs in human glaucoma has yet to be elucidated. Serial block face scanning electron microscopy is well established as a method to determine neuronal connectivity at high resolution but so far has only been performed in normal retina from animal models. To assess the structure-function relationship of early human glaucomatous neurodegeneration, regions of inner retina assessed to have none-to-moderate loss of retinal ganglion cell number were processed using serial block face scanning electron microscopy (n = 4 normal retinas, n = 4 glaucoma retinas). This allowed detailed 3D reconstruction of retinal ganglion cells and their intracellular components at a nanometre scale. In our datasets, retinal ganglion cell dendrites degenerate early in human glaucoma, with remodelling and redistribution of the mitochondria. We assessed the relationship between visual sensitivity and retinal ganglion cell density and discovered that this only partially conformed to predicted models of structure-function relationships, which may be affected by these early neurodegenerative changes. In this study, human glaucomatous retinal ganglion cells demonstrate compartmentalized degenerative changes as observed in animal models. Importantly, in these models, many of these changes have been demonstrated to be reversible, increasing the likelihood of translation to viable therapies for human glaucoma.

Retinal α-synuclein deposits in Parkinson's disease patients and animal models

Despite decades of research, accurate diagnosis of Parkinson's disease remains a challenge, and disease-modifying treatments are still lacking. Research into the early (presymptomatic) stages of Parkinson's disease and the discovery of novel biomarkers is of utmost importance to reduce this burden and to come to a more accurate diagnosis at the very onset of the disease. Many have speculated that non-motor symptoms could provide a breakthrough in the quest for early biomarkers of Parkinson's disease, including the visual disturbances and retinal abnormalities that are seen in the majority of Parkinson's disease patients. An expanding number of clinical studies have investigated the use of in vivo assessments of retinal structure, electrophysiological function, and vision-driven tasks as novel means for identifying patients at risk that need further neurological examination and for longitudinal follow-up of disease progression in Parkinson's disease patients. Often, the results of these studies have been interpreted in relation to α-synuclein deposits and dopamine deficiency in the retina, mirroring the defining pathological features of Parkinson's disease in the brain. To better understand the visual defects seen in Parkinson's disease patients and to propel the use of retinal changes as biomarkers for Parkinson's disease, however, more conclusive neuropathological evidence for the presence of retinal α-synuclein aggregates, and its relation to the cerebral α-synuclein burden, is urgently needed. This review provides a comprehensive and critical overview of the research conducted to unveil α-synuclein aggregates in the retina of Parkinson's disease patients and animal models, and thereby aims to aid the ongoing discussion about the potential use of the retinal changes and/or visual symptoms as biomarkers for Parkinson's disease.

Rapidly purified ganglion cells from neonatal mouse retinas allow studies of mitochondrial morphology and autophagy

Retinal explants and mixed primary cultures are currently used to investigate retinal ganglion cells (RGCs) pathophysiology and pharmacology, but information on yield, quality and quantity of contaminant cells for the available RGCs enrichment techniques is lacking. Here we compare two methods of mouse primary RGCs purification and show that mitochondrial and autophagy parameters can be measured in rapidly purified RGCs. RGCs were purified from P0 mouse eyes using two methods based on the surface antigen Thy1. In a two-step immunopanning purification, a subtraction plate bound macrophage antiserum removed contaminant macrophages and endothelial cells; unbound RGCs were then affinity selected using a plate-bound antiThy1 antibody. In an immunopanning-magnetic separation, macrophage-antiserum bound cells were first subtracted and then RGCs were positively selected using an antiThy1 antibody bound to a magnetic column. The two-steps immunopanning yielded low amounts of 90% pure RGCs, whereas RGCs represented 30% of the 6-fold more cells collected by immunopanning-magnetic separation. RGCs purified with both methods could be microelectroporated to image expressed mitochondria and autophagosomes fluorescent probes and to show that expression of pathogenic Optic atrophy 1 mutants causes mitochondrial fragmentation. Thus, these two methods purify primary mouse RGCs amenable to studies of cell morphology, mitochondrial biology and autophagy.

Real-Time Imaging of Retinal Cell Apoptosis by Confocal Scanning Laser Ophthalmoscopy and Its Role in Glaucoma

Glaucoma is one of the leading causes of irreversible blindness in the world. It is characterized by the progressive loss of retinal ganglion cells (RGCs), mainly through the process of apoptosis. Glaucoma patients often come to clinical attention when irreversible loss of visual function has been already established; therefore, early recognition of RGC apoptosis is inordinately important in disease prevention. The novel technology called Detection of Apoptosing Retinal Cells (DARC) allows real-time in vivo quantification of apoptosing cells through the use of a fluorescent biomarker and a confocal scanning ophthalmoscope. A recent Phase I clinical trial has evaluated the safety of DARC and its ability to detect retinal apoptosis in glaucoma patients and healthy volunteers. Results suggest that DARC may have potential in the early detection of glaucoma, which could help alleviate the medical, social, and economic burden associated with this blinding condition.

FluoroGold-Labeled Organotypic Retinal Explant Culture for Neurotoxicity Screening Studies

Preclinical toxicity screening of the new retinal compounds is an absolute requirement in the pathway of further drug development. Since retinal neuron cultivation and in vivo studies are relatively expensive and time consuming, we aimed to create a fast and reproducible ex vivo system for retinal toxicity screening. For this purpose, we used rat retinal explant culture that was retrogradely labeled with the FluoroGold before the isolation. Explants were exposed to a toxic concentration of gentamicin and ciliary neurotrophic factor (CNTF), a known neuroprotective agent. The measured outcomes showed the cell density in retinal ganglion cell layer (GCL) and the activity of lactate dehydrogenase (LDH) in the culture medium. Gentamicin-induced oxidative stress resulted in retinal cell damage and rapid LDH release to the culture medium (p < 0.05). Additional CNTF supplementation minimized the cell damage, and the increase of LDH release was insignificant when compared to LDH levels before gentamicin insult (p > 0.05). As well as this, the LDH activity was directly correlated with the cell count in GCL (R = −0.84, p < 0.00001), making a sensitive marker of retinal neuron damage. The FLOREC protocol could be considered as a fast, reproducible, and sensitive method to detect neurotoxicity in the screening studies of the retinal drugs.

In vivo imaging of adeno-associated viral vector labelled retinal ganglion cells

A defining characteristic of optic neuropathies, such as glaucoma, is progressive loss of retinal ganglion cells (RGCs). Current clinical tests only provide weak surrogates of RGC loss, but the possibility of optically visualizing RGCs and quantifying their rate of loss could represent a radical advance in the management of optic neuropathies. In this study we injected two different adeno-associated viral (AAV) vector serotypes in the vitreous to enable green fluorescent protein (GFP) labelling of RGCs in wild-type mice for in vivo and non-invasive imaging. GFP-labelled cells were detected by confocal scanning laser ophthalmoscopy 1-week post-injection and plateaued in density at 4 weeks. Immunohistochemical analysis 5-weeks post-injection revealed labelling specificity to RGCs to be significantly higher with the AAV2-DCX-GFP vector compared to the AAV2-CAG-GFP vector. There were no adverse functional or structural effects of the labelling method as determined with electroretinography and optical coherence tomography, respectively. The RGC-specific positive and negative scotopic threshold responses had similar amplitudes between control and experimental eyes, while inner retinal thickness was also unchanged after injection. As a positive control experiment, optic nerve transection resulted in a progressive loss of labelled RGCs. AAV vectors provide strong and long-lasting GFP labelling of RGCs without detectable adverse effects.

iDrugs and iDevices Discovery Research: Preclinical Assays, Techniques, and Animal Model Studies for Ocular Hypotensives and Neuroprotectants

Discovery ophthalmic research is centered around delineating the molecular and cellular basis of ocular diseases and finding and exploiting molecular and genetic pathways associated with them. From such studies it is possible to determine suitable intervention points to address the disease process and hopefully to discover therapeutics to treat them. An investigational new drug (IND) filing for a new small-molecule drug, peptide, antibody, genetic treatment, or a device with global health authorities requires a number of preclinical studies to provide necessary safety and efficacy data. Specific regulatory elements needed for such IND-enabling studies are beyond the scope of this article. However, to enhance the overall data packages for such entities and permit high-quality foundation-building publications for medical affairs, additional research and development studies are always desirable. This review aims to provide examples of some target localization/verification, ocular drug discovery processes, and mechanistic and portfolio-enhancing exploratory investigations for candidate drugs and devices for the treatment of ocular hypertension and glaucomatous optic neuropathy (neurodegeneration of retinal ganglion cells and their axons). Examples of compound screening assays, use of various technologies and techniques, deployment of animal models, and data obtained from such studies are also presented.

Novel Therapeutics in Glaucoma Management

BACKGROUND

Glaucoma is a progressive optic neuropathy characterized by retinal ganglion cell death and alterations of visual field. Elevated intraocular pressure (IOP) is considered the main risk factor of glaucoma, even though other factors cannot be ruled out, such as epigenetic mechanisms.

OBJECTIVE

An overview of the ultimate promising experimental drugs to manage glaucoma has been provided.

RESULTS

In particular, we have focused on purinergic ligands, KATP channel activators, gases (nitric oxide, carbon monoxide and hydrogen sulfide), non-glucocorticoid steroidal compounds, neurotrophic factors, PI3K/Akt activators, citicoline, histone deacetylase inhibitors, cannabinoids, dopamine and serotonin receptors ligands, small interference RNA, and Rho kinase inhibitors.

CONCLUSIONS

The review has been also endowed of a brief chapter on last reports about potential neuroprotective benefits of anti-glaucoma drugs already present in the market.

Clinical translation of handheld optical coherence tomography: practical considerations and recent advancements

Since the inception of optical coherence tomography (OCT), advancements in imaging system design and handheld probes have allowed for numerous advancements in disease diagnostics and characterization of the structural and optical properties of tissue. OCT system developers continue to reduce form factor and cost, while improving imaging performance (speed, resolution, etc.) and flexibility for applicability in a broad range of fields, and nearly every clinical specialty. An extensive array of components to construct customized systems has also become available, with a range of commercial entities that produce high-quality products, from single components to full systems, for clinical and research use. Many advancements in the development of these miniaturized and portable systems can be linked back to a specific challenge in academic research, or a clinical need in medicine or surgery. Handheld OCT systems are discussed and explored for various applications. Handheld systems are discussed in terms of their relative level of portability and form factor, with mention of the supporting technologies and surrounding ecosystem that bolstered their development. Additional insight from our efforts to implement systems in several clinical environments is provided. The trend toward well-designed, efficient, and compact handheld systems paves the way for more widespread adoption of OCT into point-of-care or point-of-procedure applications in both clinical and commercial settings.

The Future of Imaging in Detecting Glaucoma Progression

Ocular imaging has been heavily incorporated into glaucoma management and provides important information that aids in the detection of disease progression. Longitudinal studies have shown that the circumpapillary retinal nerve fiber layer is an important parameter for glaucoma progression detection, whereas other studies have demonstrated that macular parameters, such as the ganglion cell inner plexiform layer and optic nerve head parameters, also are useful for progression detection. The introduction of novel technologies with faster scan speeds, wider scanning fields, higher resolution, and improved tissue penetration has enabled the precise quantification of additional key ocular structures, such as the individual retinal layers, optic nerve head, choroid, and lamina cribrosa. Furthermore, extracting functional information from scans such as blood flow rate and oxygen consumption provides new perspectives on the disease and its progression. These novel methods promise improved detection of glaucoma progression and better insight into the mechanisms of progression that will lead to better targeted treatment options to prevent visual damage and blindness.

Statistically strong label-free quantitative identification of native fluorophores in a biological sample

Bioimaging using endogenous cell fluorescence, without any external biomarkers makes it possible to explore cells and tissues in their original native state, also in vivo. In order to be informative, this label-free method requires careful multispectral or hyperspectral recording of autofluorescence images followed by unsupervised extraction (unmixing) of biochemical signatures. The unmixing is difficult due to the scarcity of biochemically pure regions in cells and also because autofluorescence is weak compared with signals from labelled cells, typically leading to low signal to noise ratio. Here, we solve the problem of unsupervised hyperspectral unmixing of cellular autofluorescence by introducing the Robust Dependent Component Analysis (RoDECA). This approach provides sophisticated and statistically robust quantitative biochemical analysis of cellular autofluorescence images. We validate our method on artificial images, where the addition of varying known level of noise has allowed us to quantify the accuracy of our RoDECA analysis in a way that can be applied to real biological datasets. The same unsupervised statistical minimisation is then applied to imaging of mouse retinal photoreceptor cells where we establish the identity of key endogenous fluorophores (free NADH, FAD and lipofuscin) and derive the corresponding molecular abundance maps. The pre-processing methodology of image datasets is also presented, which is essential for the spectral unmixing analysis, but mostly overlooked in the previous studies.

Imaging and quantifying ganglion cells and other transparent neurons in the living human retina

Ganglion cells (GCs) are fundamental to retinal neural circuitry, processing photoreceptor signals for transmission to the brain via their axons. However, much remains unknown about their role in vision and their vulnerability to disease leading to blindness. A major bottleneck has been our inability to observe GCs and their degeneration in the living human eye. Despite two decades of development of optical technologies to image cells in the living human retina, GCs remain elusive due to their high optical translucency. Failure of conventional imaging-using predominately singly scattered light-to reveal GCs has led to a focus on multiply-scattered, fluorescence, two-photon, and phase imaging techniques to enhance GC contrast. Here, we show that singly scattered light actually carries substantial information that reveals GC somas, axons, and other retinal neurons and permits their quantitative analysis. We perform morphometry on GC layer somas, including projection of GCs onto photoreceptors and identification of the primary GC subtypes, even beneath nerve fibers. We obtained singly scattered images by: (i) marrying adaptive optics to optical coherence tomography to avoid optical blurring of the eye; (ii) performing 3D subcellular image registration to avoid motion blur; and (iii) using organelle motility inside somas as an intrinsic contrast agent. Moreover, through-focus imaging offers the potential to spatially map individual GCs to underlying amacrine, bipolar, horizontal, photoreceptor, and retinal pigment epithelium cells, thus exposing the anatomical substrate for neural processing of visual information. This imaging modality is also a tool for improving clinical diagnosis and assessing treatment of retinal disease.

Making Basic Science Studies in Glaucoma More Clinically Relevant: The Need for a Consensus

Glaucoma is a chronic, progressive, and debilitating optic neuropathy that causes retinal damage and visual defects. The pathophysiologic mechanisms of glaucoma remain ill-defined, and there is an indisputable need for contributions from basic science researchers in defining pathways for translational research. However, glaucoma researchers today face significant challenges due to the lack of a map of integrated pathways from bench to bedside and the lack of consensus statements to guide in choosing the right research questions, techniques, and model systems. Here, we present the case for the development of such maps and consensus statements, which are critical for faster development of the most efficacious glaucoma therapy. We underscore that interrogating the preclinical path of both successful and unsuccessful clinical programs is essential to defining future research. One aspect of this is evaluation of available preclinical research tools. To begin this process, we highlight the utility of currently available animal models for glaucoma and emphasize that there is a particular need for models of glaucoma with normal intraocular pressure. In addition, we outline a series of discoveries from cell-based, animal, and translational research that begin to reveal a map of glaucoma from cell biology to physiology to disease pathology. Completion of these maps requires input and consensus from the global glaucoma research community. This article sets the stage by outlining various approaches to such a consensus. Together, these efforts will help accelerate basic science research, leading to discoveries with significant clinical impact for people with glaucoma.

Parameters of ocular fundus on spectral-domain optical coherence tomography for glaucoma diagnosis

In this review, we summarize the progression of several parameters assessed by spectral-domain optical coherence tomography (SD-OCT) in recent years for the detection of glaucoma. Monitoring the progression of defects in the retinal nerve fiber layer (RNFL) thickness is essential. Imaging and analysis of retinal ganglion cells (RGCs) and inner plexiform layer (IPL), respectively, have been of great importance. Optic nerve head (ONH) topography obtained from 3D SD-OCT images is another crucial step. Other important assessments involve locating the Bruch's membrane opening (BMO), estimating the optic disc size and rim area, and measuring the lamina cribrosa displacement. Still other parameters found in the past three years for glaucoma diagnosis comprise central retinal artery resistive index, optic disc perfusion in optical coherence tomography angiography (OCTA) study, peripapillary choroidal thickness, and choroidal area in SD-OCT. Recently, several more ocular fundus parameters have been found, and compared with the earlier parameters to judge the accuracy of diagnosis. While a few of these parameters have been widely used in clinical practice, a fair number are still in the experimental stage.

Normative database for separate inner retinal layers thickness using spectral domain optical coherence tomography in Caucasian population

PURPOSE

Develop the first normative database of the thickness of every inner retinal layer in the macular area in a healthy, Caucasian population between 18 to 87 years old, using Spectralis Optical Coherence Tomography (OCT).

METHODS

On this transversal, observational study, 300 patients between 18 to 87 years old and without an ophthalmological condition were recruited. Macular OCT scans were performed on all patients (Spectralis OCT, Heidelberg Engineering). An axial length measurement, and keratometry were performed using an optical biometer. The volume and thickness of the different macular sectors of the inner retinal layers (retinal nerve fiber layer (RNFL), ganglion cells layer (CGL) and inner plexiform layer (IPL)) were analyzed with the Spectralis OCT segmentation software. An eye was randomly selected for each patient.

RESULTS

297 patients (179 females and 118 males) were included in the study. The mean age was 56.07 years (range: 40.50-72). The multivariate analysis showed a positive correlation between the RNFL thickness and the axial length (p < 0.001). The mean central retinal thickness was 278.2 μm (range: 266-291), the mean central RNFL thickness was 12.61 μm (range: 11-14), the mean central CGL thickness was 17.63 μm (range: 14-21) and the mean central IPL thickness was 22.02 μm (range: 20-25). The multivariate analysis showed a negative correlation between age and CGL thickness and inner IPL thickness (p< 0.001).

CONCLUSION

This study provides a normative database of the volume of each of the inner retinal layers on a Caucasian population.

Imaging individual neurons in the retinal ganglion cell layer of the living eye

Although imaging of the living retina with adaptive optics scanning light ophthalmoscopy (AOSLO) provides microscopic access to individual cells, such as photoreceptors, retinal pigment epithelial cells, and blood cells in the retinal vasculature, other important cell classes, such as retinal ganglion cells, have proven much more challenging to image. The near transparency of inner retinal cells is advantageous for vision, as light must pass through them to reach the photoreceptors, but it has prevented them from being directly imaged in vivo. Here we show that the individual somas of neurons within the retinal ganglion cell (RGC) layer can be imaged with a modification of confocal AOSLO, in both monkeys and humans. Human images of RGC layer neurons did not match the quality of monkey images for several reasons, including safety concerns that limited the light levels permissible for human imaging. We also show that the same technique applied to the photoreceptor layer can resolve ambiguity about cone survival in age-related macular degeneration. The capability to noninvasively image RGC layer neurons in the living eye may one day allow for a better understanding of diseases, such as glaucoma, and accelerate the development of therapeutic strategies that aim to protect these cells. This method may also prove useful for imaging other structures, such as neurons in the brain.

A freely available semi-automated method for quantifying retinal ganglion cells in entire retinal flatmounts

Glaucomatous optic neuropathies are characterized by progressive loss of retinal ganglion cells (RGCs), the neurons that connect the eye to the brain. Quantification of these RGCs is a cornerstone in experimental optic neuropathy research and commonly performed via manually quantifying parts of the retina. However, this is a time-consuming process subject to inter- and intra-observer variability. Here we present a freely available ImageJ script to semi-automatically quantify RGCs in entire retinal flatmounts after immunostaining for the RGC-specific transcription factor Brn3a. The blob-like signal of Brn3a-immunopositive RGCs is enhanced via eigenvalues of the Hessian matrix and the resulting local maxima are counted as RGCs. After the user has outlined the retinal flatmount area, the total RGC number and retinal area are reported and an isodensity map, showing the RGC density distribution across the retina, is created. The semi-automated quantification shows a very strong correlation (Pearson's r ≥ 0.99) with manual counts for both widefield and confocal images, thereby validating the data generated via the developed script. Moreover, application of this method in established glaucomatous optic neuropathy models such as N-methyl-D-aspartate-induced excitotoxicity, optic nerve crush and laser-induced ocular hypertension revealed RGC loss conform with literature. Compared to manual counting, the described automated quantification method is faster and shows user-independent consistency. Furthermore, as the script detects the RGC number in entire retinal flatmounts, the method allows detection of regional differences in RGC density. As such, it can help advance research investigating the degenerative mechanisms of glaucomatous optic neuropathies and the effectiveness of new neuroprotective treatments. Because the script is flexible and easy to optimize due to a low number of critical parameters, it can potentially be applied in combination with other tissues or alternative labeling protocols.

Glaucoma Suspect: Diagnosis and Management

Glaucoma suspect is a diagnosis reserved for individuals who do not definitively have glaucoma at the present time but have characteristics suggesting that they are at high risk of developing the disease in the future based on a variety of factors. This review provides a practical approach to individuals classified as glaucoma suspects caused by one or more of the following risk factors or indicators of disease: ocular hypertension, optic nerve features suggestive of glaucoma, visual field abnormalities, and other characteristics placing them at greater risk than the average population. In addition to diagnostic considerations, this overview provides information on therapeutic approaches to the glaucoma suspect.