Ocular Manifestations of Alzheimer's and Other Neurodegenerative Diseases: The Prospect of the Eye as a Tool for the Early Diagnosis of Alzheimer's Disease

A Look into Ocular Diseases: The Pivotal Role of Omics Sciences in Ophthalmology Research

Precision medicine is a new medical approach which considers both population characteristics and individual variability to provide customized healthcare. The transition from traditional reactive medicine to personalized medicine is based on a biomarker-driven process and a deep knowledge of biological mechanisms according to which the development of diseases occurs. In this context, the advancements in high-throughput omics technologies represent a unique opportunity to discover novel biomarkers and to provide an unbiased picture of the biological system. One of the medical fields in which omics science has started to be recently applied is that of ophthalmology. Ocular diseases are very common, and some of them could be highly disabling, thus leading to vision loss and blindness. The pathogenic mechanism of most ocular diseases may be dependent on various genetic and environmental factors, whose effect has not been yet completely understood. In this context, large-scale omics approaches are fundamental to have a comprehensive evaluation of the whole system and represent an essential tool for the development of novel therapies. This Review summarizes the recent advancements in omics science applied to ophthalmology in the last ten years, in particular by focusing on proteomics, metabolomics and lipidomics applications from an analytical perspective. The role of high-efficiency separation techniques coupled to (high-resolution) mass spectrometry ((HR)MS) is also discussed, as well as the impact of sampling, sample preparation and data analysis as integrating parts of the analytical workflow.

Alzheimer's disease as a causal risk factor for diabetic retinopathy: a Mendelian randomization study

Objectives

This study aims to investigate the causal relationship between Alzheimer's Disease (AD) and Diabetic Retinopathy (DR).

Methods

Employing Mendelian Randomization (MR), Generalized Summary-data-based Mendelian Randomization (GSMR), and the MR-Steiger test, this study scrutinizes the genetic underpinnings of the hypothesized causal association between AD and DR, as well as its Proliferative DR (PDR) and Non-Proliferative DR (NPDR) subtypes. Comprehensive data from Genome-Wide Association Studies (GWAS) were analyzed, specifically AD data from the Psychiatric Genomics Consortium (71,880 cases/383,378 controls), and DR, PDR, and NPDR data from both the FinnGen consortium (FinnGen release R8, DR: 5,988 cases/314,042 controls; PDR: 8,383 cases/329,756 controls; NPDR: 3,446 cases/314,042 controls) and the IEU OpenGWAS (DR: 14,584 cases/176,010 controls; PDR: 8,681 cases/204,208 controls; NPDR: 2,026 cases/204,208 controls). The study also incorporated Functional Mapping and Annotation (FUMA) for an in-depth analysis of the GWAS results.

Results

The MR analyses revealed that genetic susceptibility to AD significantly increases the risk of DR, as evidenced by GWAS data from the FinnGen consortium (OR: 2.5090; 95% confidence interval (CI):1.2102-5.2018, false discovery rate P-value (PFDR)=0.0201; GSMR: bxy=0.8936, bxy_se=0.3759, P=0.0174), NPDR (OR: 2.7455; 95% CI: 1.3178-5.7197, PFDR=0.0166; GSMR: bxy=0.9682, bxy_se=0.3802, P=0.0126), and PDR (OR: 2.3098; 95% CI: 1.2411-4.2986, PFDR=0.0164; GSMR: bxy=0.7962, bxy_se=0.3205, P=0.0129) using DR GWAS from FinnGen consortium. These results were corroborated by DR GWAS datasets from IEU OpenGWAS. The MR-Steiger test confirmed a significant association of all identified instrumental variables (IVs) with AD. While a potential causal effect of DR and its subtypes on AD was identified, the robustness of these results was constrained by a low power value. FUMA analysis identified OARD1, NFYA, TREM1 as shared risk genes between DR and AD, suggesting a potential genetic overlap between these complex diseases.

Discussion

This study underscores the contribution of AD to an increased risk of DR, as well as NPDR and PDR subtypes, underscoring the necessity of a holistic approach in the management of patients affected by these conditions.

Categorization of Alzheimer's disease stages using deep learning approaches with McNemar's test

Early diagnosis is crucial in Alzheimer's disease both clinically and for preventing the rapid progression of the disease. Early diagnosis with awareness studies of the disease is of great importance in terms of controlling the disease at an early stage. Additionally, early detection can reduce treatment costs associated with the disease. A study has been carried out on this subject to have the great importance of detecting Alzheimer's disease at a mild stage and being able to grade the disease correctly. This study's dataset consisting of MRI images from the Alzheimer's Disease Neuroimaging Initiative (ADNI) was split into training and testing sets, and deep learning-based approaches were used to obtain results. The dataset consists of three classes: Alzheimer's disease (AD), Cognitive Normal (CN), and Mild Cognitive Impairment (MCI). The achieved results showed an accuracy of 98.94% for CN vs AD in the one vs one (1 vs 1) classification with the EfficientNetB0 model and 99.58% for AD vs CNMCI in the one vs All (1 vs All) classification with AlexNet model. In addition, in the study, an accuracy of 98.42% was obtained with the EfficientNet121 model in MCI vs CN classification. These results indicate the significant potential for mild stage Alzheimer's disease detection of Alzheimer's disease. Early detection of the disease in the mild stage is a critical factor in preventing the progression of Alzheimer's disease. In addition, a variant of the non-parametric statistical McNemar's Test was applied to determine the statistical significance of the results obtained in the study. Statistical significance of 1 vs 1 and 1 vs all classifications were obtained for EfficientNetB0, DenseNet, and AlexNet models.

Retina Oculomics in Neurodegenerative Disease

Ophthalmic biomarkers have long played a critical role in diagnosing and managing ocular diseases. Oculomics has emerged as a field that utilizes ocular imaging biomarkers to provide insights into systemic diseases. Advances in diagnostic and imaging technologies including electroretinography, optical coherence tomography (OCT), confocal scanning laser ophthalmoscopy, fluorescence lifetime imaging ophthalmoscopy, and OCT angiography have revolutionized the ability to understand systemic diseases and even detect them earlier than clinical manifestations for earlier intervention. With the advent of increasingly large ophthalmic imaging datasets, machine learning models can be integrated into these ocular imaging biomarkers to provide further insights and prognostic predictions of neurodegenerative disease. In this manuscript, we review the use of ophthalmic imaging to provide insights into neurodegenerative diseases including Alzheimer Disease, Parkinson Disease, Amyotrophic Lateral Sclerosis, and Huntington Disease. We discuss recent advances in ophthalmic technology including eye-tracking technology and integration of artificial intelligence techniques to further provide insights into these neurodegenerative diseases. Ultimately, oculomics opens the opportunity to detect and monitor systemic diseases at a higher acuity. Thus, earlier detection of systemic diseases may allow for timely intervention for improving the quality of life in patients with neurodegenerative disease.

Potential Ocular Biomarkers for Early Detection of Alzheimer's Disease and Their Roles in Artificial Intelligence Studies

Alzheimer's disease (AD) is the leading cause of dementia worldwide. Early detection is believed to be essential to disease management because it enables physicians to initiate treatment in patients with early-stage AD (early AD), with the possibility of stopping the disease or slowing disease progression, preserving function and ultimately reducing disease burden. The purpose of this study was to review prior research on the use of eye biomarkers and artificial intelligence (AI) for detecting AD and early AD. The PubMed database was searched to identify studies for review. Ocular biomarkers in AD research and AI research on AD were reviewed and summarized. According to numerous studies, there is a high likelihood that ocular biomarkers can be used to detect early AD: tears, corneal nerves, retina, visual function and, in particular, eye movement tracking have been identified as ocular biomarkers with the potential to detect early AD. However, there is currently no ocular biomarker that can be used to definitely detect early AD. A few studies that used AI with ocular biomarkers to detect AD reported promising results, demonstrating that using AI with ocular biomarkers through multimodal imaging could improve the accuracy of identifying AD patients. This strategy may become a screening tool for detecting early AD in older patients prior to the onset of AD symptoms.

Alzheimer's disease: a continuum with visual involvements

Introduction

Alzheimer's disease (AD) is the most common form of dementia affecting the central nervous system, and alteration of several visual structures has been reported. Structural retinal changes are usually accompanied by changes in visual function in this disease. The aim of this study was to analyse the differences in visual function at different stages of the pathology (family history group (FH+), mild cognitive impairment (MCI), mild AD and moderate AD) in comparison with a control group of subjects with no cognitive decline and no family history of AD.

Methods

We included 53 controls, 13 subjects with FH+, 23 patients with MCI, 25 patients with mild AD and, 21 patients with moderate AD. All were ophthalmologically healthy. Visual acuity (VA), contrast sensitivity (CS), colour perception, visual integration, and fundus examination were performed.

Results

The analysis showed a statistically significant decrease in VA, CS and visual integration score between the MCI, mild AD and moderate AD groups compared to the control group. In the CS higher frequencies and in the colour perception test (total errors number), statistically significant differences were also observed in the MCI, mild AD and moderate AD groups with respect to the FH+ group and also between the control and AD groups. The FH+ group showed no statistically significant difference in visual functions compared to the control group. All the test correlated with the Mini Mental State Examination score and showed good predictive value when memory decline was present, with better values when AD was at a more advanced stage.

Conclusion

Alterations in visual function appear in subjects with MCI and evolve when AD is established, being stable in the initial stages of the disease (mild AD and moderate AD). Therefore, visual psychophysical tests are a useful, simple and complementary tool to neuropsychological tests to facilitate diagnosis in the preclinical and early stages of AD.

Retinal vessels as a window on amyotrophic lateral sclerosis pathophysiology: A systematic review

Amyotrophic lateral sclerosis (ALS) is a rare fatal motor neuron disease. Although many potential mechanisms have been proposed, the pathophysiology of the disease remains unknown. Currently available treatments can only delay the progression of the disease and prolong life expectancy by a few months. There is still no definitive cure for ALS, and the development of new treatments is limited by a lack of understanding of the underlying biological processes that trigger and promote neurodegeneration. Several scientific results suggest a neurovascular impairment in ALS providing perspectives for the development of new biomarkers and treatments. In this article, we performed a systematic review using PRISMA guidelines including PubMed, EmBase, GoogleScholar, and Web of Science Core Collection to analyze the scientific literature published between 2000 and 2021 discussing the neurocardiovascular involvement and ophthalmologic abnormalities in ALS. In total, 122 articles were included to establish this systematic review. Indeed, microvascular pathology seems to be involved in ALS, affecting all the neurovascular unit components. Retinal changes have also been recently highlighted without significant alteration of the visual pathways. Despite the peripheral location of the retina, it is considered as an extension of the central nervous system (CNS) as it displays similarities to the brain, the inner blood-retinal barrier, and the blood-brain barrier. This suggests that the eye could be considered as a 'window' into the brain in many CNS disorders. Thus, studying ocular manifestations of brain pathologies seems very promising in understanding neurodegenerative disorders, mainly ALS. Optical coherence tomography angiography (OCT-A) could therefore be a powerful approach for exploration of retinal microvascularization allowing to obtain new diagnostic and prognostic biomarkers of ALS.

Vitreous Humor Biomarkers Reflect Pathological Changes in the Brain for Alzheimer's Disease and Chronic Traumatic Encephalopathy

BACKGROUND

Patients with eye disease have an increased risk for developing neurodegenerative disease. Neurodegenerative proteins can be measured in the eye; however, correlations between biomarker levels in eye fluid and neuropathological diagnoses have not been established.

OBJECTIVE

This exploratory, retrospective study examined vitreous humor from 41 postmortem eyes and brain tissue with neuropathological diagnoses of Alzheimer's disease (AD, n = 7), chronic traumatic encephalopathy (CTE, n = 15), both AD + CTE (n = 10), and without significant neuropathology (controls, n = 9).

METHODS

Protein biomarkers i.e., amyloid-β (Aβ 40,42), total tau (tTau), phosphorylated tau (pTau181,231), neurofilament light chain (NfL), and eotaxin-1 were quantitatively measured by immunoassay. Non-parametric tests were used to compare vitreous biomarker levels between groups. Spearman's rank correlation tests were used to correlate biomarker levels in vitreous and cortical tissue. The level of significance was set to α= 0.10.

RESULTS

In pairwise comparisons, tTau levels were significantly increased in AD and CTE groups versus controls (p = 0.08 for both) as well as AD versus AD+CTE group and CTE versus AD+CTE group (p = 0.049 for both). Vitreous NfL levels were significantly increased in low CTE (Stage I/II) versus no CTE (p = 0.096) and in low CTE versus high CTE stage (p = 0.03). Vitreous and cortical tissue levels of pTau 231 (p = 0.02, r = 0.38) and t-Tau (p = 0.04, r = -0.34) were significantly correlated.

CONCLUSION

The postmortem vitreous humor biomarker levels significantly correlate with AD and CTE pathology in corresponding brains, while vitreous NfL was correlated with the CTE staging. This exploratory study indicates that biomarkers in the vitreous humor may serve as a proxy for neuropathological disease.

Review: Emerging Eye-Based Diagnostic Technologies for Traumatic Brain Injury

The study of ocular manifestations of neurodegenerative disorders, Oculomics, is a growing field of investigation for early diagnostics, enabling structural and chemical biomarkers to be monitored overtime to predict prognosis. Traumatic brain injury (TBI) triggers a cascade of events harmful to the brain, which can lead to neurodegeneration. TBI, termed the "silent epidemic" is becoming a leading cause of death and disability worldwide. There is currently no effective diagnostic tool for TBI, and yet, early-intervention is known to considerably shorten hospital stays, improve outcomes, fasten neurological recovery and lower mortality rates, highlighting the unmet need for techniques capable of rapid and accurate point-of-care diagnostics, implemented in the earliest stages. This review focuses on the latest advances in the main neuropathophysiological responses and the achievements and shortfalls of TBI diagnostic methods. Validated and emerging TBI-indicative biomarkers are outlined and linked to ocular neuro-disorders. Methods detecting structural and chemical ocular responses to TBI are categorised along with prospective chemical and physical sensing techniques. Particular attention is drawn to the potential of Raman spectroscopy as a non-invasive sensing of neurological molecular signatures in the ocular projections of the brain, laying the platform for the first tangible path towards alternative point-of-care diagnostic technologies for TBI.

Association of cognitive function with Neurofilament light chain in the aqueous humor of human eye

OBJECTIVES

To evaluate the predictive clinical role of neurofilament light chain (NfL), amyloid-β (Aβ), glial fibrillary acidic protein (GFAP), and phosphorylated tau at threonine 181 (p-tau181) proteins in human aqueous humor (AH) and quantify the retinal macular microvascular parameters by optical coherence tomography angiography (OCTA) as early diagnostic markers of Alzheimer's disease (AD).

METHODS

This prospective, single-site, cross-sectional, cohort study enrolled 55 participants, including 38 patients with neovascular age-related macular degeneration (nAMD) and 17 individuals with senile cataracts. The single-molecule array platform was used to quantitatively measure the levels of AH NfL, Aβ40, Aβ42, GFAP, and p-tau181 proteins in AH. The mini-mental state examination (MMSE) score was used to assess the global cognitive function. OCTA scan with 6 × 6 mm macular area was used to quantify the retinal thickness and microvascular densities of superficial retinal capillary plexuses and deep retinal capillary plexuses.

RESULTS

NfL, Aβ40, Aβ42, GFAP, and p-tau181 were detected in all AH samples by Simoa platform. Individuals with cataract had higher concentrations of NfL and p-tau181 but lower Aβ40 and Aβ42 and similar GFAP compared to those with nAMD. Lower MMSE scores showed a negative correlation with NfL concentration of AH not only in the nAMD group (p = 0.043), but also in the cataract group (p = 0.032). However, the MMSE scores were not associated with the levels of Aβ40, Aβ42, GFAP, or p-Tau181. Further analysis found that the Aβ40 and Aβ42 concentrations showed a strong positive correlation (p < 0.0001). In addition, the NfL concentration showed a mild positive correlation with that of GFAP in the cataract group (p = 0.021). Although it has not reached statistical significance, there was a correlation between the levels of NfL and Aβ42 in the nAMD group (p = 0.051). Moreover, the macular superficial vessel density values had a negative correlation with the concentration of NfL (p = 0.004) but a positive correlation with MMSE scores (p = 0.045). The macular deep vessel density values were negatively correlated with the concentration of p-tau181 (p = 0.031) and positively correlated with MMSE scores (p = 0.020).

CONCLUSION

The examination of AD-related biomarkers in human AH and OCTA may improve the ocular-based AD detection methods and contribute to forestalling the progression of preclinical AD.

A systematic survey of advances in retinal imaging modalities for Alzheimer's disease diagnosis

Recent advances in retinal imaging pathophysiology have shown a new function for biomarkers in Alzheimer's disease diagnosis and prognosis. The significant improvements in Optical coherence tomography (OCT) retinal imaging have led to significant clinical translation, particularly in Alzheimer's disease detection. This systematic review will provide a comprehensive overview of retinal imaging in clinical applications, with a special focus on biomarker analysis for use in Alzheimer's disease detection. Articles on OCT retinal imaging in Alzheimer's disease diagnosis were identified in PubMed, Google Scholar, IEEE Xplore, and Research Gate databases until March 2021. Those studies using simultaneous retinal imaging acquisition were chosen, while those using sequential techniques were rejected. "Alzheimer's disease" and "Dementia" were searched alone and in combination with "OCT" and "retinal imaging". Approximately 1000 publications were searched, and after deleting duplicate articles, 145 relevant studies focused on the diagnosis of Alzheimer's disease utilizing retinal imaging were chosen for study. OCT has recently been demonstrated to be a valuable technique in clinical practice as according to this survey, 57% of the researchers employed optical coherence tomography, 19% used ocular fundus imaging, 13% used scanning laser ophthalmoscopy, and 11% have used multimodal imaging to diagnose Alzheimer disease. Retinal imaging has become an important diagnostic technique for Alzheimer's disease. Given the scarcity of available literature, it is clear that future prospective trials involving larger and more homogeneous groups are necessary, and the work can be expanded by evaluating its significance utilizing a machine-learning platform rather than simply using statistical methodologies.

Structural retinal changes in cerebral small vessel disease

Cerebral small vessel disease (CSVD) is an important contributor to cognitive impairment and stroke. Previous research has suggested associations with alterations in single retinal layers. We have assessed changes of all individual retinal layers in CSVD using high-resolution optical coherence tomography (OCT) for the first time. Subjects with recent magnetic resonance imaging (MRI) underwent macular and peripapillary retinal imaging using OCT for this case-control study. Number and volume ratio index (WMRI) of white matter lesions (WML) were determined on MRI. Data were analyzed using multiple linear regression models. 27 CSVD patients and 9 control participants were included. Ganglion cell layer (GCL) volume was significantly reduced in patients with CSVD compared to age-matched controls (p = 0.008). In patients with CSVD, larger foveal outer plexiform layer (OPL) volume and decreased temporal peripapillary retinal nerve fiber layer (RNFL) thickness were significantly associated with a higher WMRI in linear regression when controlling for age (p ≤ 0.033). Decreased foveal GCL volume and temporal-inferior RNFL thickness at Bruch's membrane opening (MRW), and increased temporal MRW were associated with a higher WML burden (p ≤ 0.037). Thus, we identified alterations in several OCT layers in individuals with CSVD (GCL, OPL, MRW and RNFL). Their potential diagnostic value merits further study.

Histological and Top-Down Proteomic Analyses of the Visual Pathway in the Cuprizone Demyelination Model

A change in visual perception is a frequent early symptom of multiple sclerosis (MS), the pathoaetiology of which remains unclear. Following a slow demyelination process caused by 12 weeks of low-dose (0.1%) cuprizone (CPZ) consumption, histology and proteomics were used to investigate components of the visual pathway in young adult mice. Histological investigation did not identify demyelination or gliosis in the optic tracts, pretectal nuclei, superior colliculi, lateral geniculate nuclei or visual cortices. However, top-down proteomic assessment of the optic nerve/tract revealed a significant change in the abundance of 34 spots in high-resolution two-dimensional (2D) gels. Subsequent liquid chromatography-tandem mass spectrometry (LC-TMS) analysis identified alterations in 75 proteoforms. Literature mining revealed the relevance of these proteoforms in terms of proteins previously implicated in animal models, eye diseases and human MS. Importantly, 24 proteoforms were not previously described in any animal models of MS, eye diseases or MS itself. Bioinformatic analysis indicated involvement of these proteoforms in cytoskeleton organization, metabolic dysregulation, protein aggregation and axonal support. Collectively, these results indicate that continuous CPZ-feeding, which evokes a slow demyelination, results in proteomic changes that precede any clear histological changes in the visual pathway and that these proteoforms may be potential early markers of degenerative demyelinating conditions.

Abnormalities of Retinal Structure and Microvasculature are Associated with Cerebral White Matter Hyperintensities

PURPOSE

While retinal microvasculature represents cerebral small vessels, the retinal nerve fiber layer is the extended white matter of the brain. We aimed to investigate the correlation between changes in retina and white matter hyperintensities (WMH).

METHODS

64 candidates with WMH received the optical coherence tomography angiography (OCTA) examination. WMH were divided into mild or moderate/severe groups according to the Fazekas score. After imaging superficial capillary plexus (SCP) and deep capillary plexus (DCP), we revealed the microvascular density parameters (vascular perfusion density (VPD), vascular length density (VLD), and fovea avascular zone area (FAZ-A)) and morphological parameters (vessel diameter index (VDI), fractal dimension (FD), and vessel tortuosity (VT)). The software algorithm measured the thickness of the peripapillary retina nerve fiber layer (PRNFL).

RESULTS

32 were classified as having mild WMH and 32 were moderate/severe. The Median (interquartile range) ages of the two groups were 58 (54-64) and 61 (57-67) years, respectively. A decrease of FD, VPD and VLD in either SCP or DCP appeared with an increased risk of moderate/severe WMH. Although changes of capillary plexus were not associated with PWMH, decreased FD, VPD, VLD and FAZ-A in either SCP or DCP was associated with an increased risk of moderate/severe DWMH. Participants with moderate/severe WMH demonstrated reduced PRNFL thickness, particularly in the DWMH, compared with mild WMH.

CONCLUSIONS

Abnormalities of retinal microvascular density, morphological parameters, and PRNFL thickness are correlated with the incidence of moderate/severe WMH, particularly the DWMH, suggesting that arteriosclerosis and hypoperfusion are the causes of DWMH.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Confirmatory factor analysis of the Dutch Screening Visual Complaints questionnaire in people with multiple sclerosis

Background

Visual complaints among people with multiple sclerosis (pwMS) are common, but often difficult to recognize. The Screening Visual Complaints questionnaire (SVCq) has been developed to screen for visual complaints in people with a neurodegenerative disease, including multiple sclerosis (MS). A previous study performed a factor analysis in a normal population which revealed an acceptable one-factor model, a three-factor model and a five-factor model within the SVCq. To increase the usability of the SVCq in people with MS, the purpose of the current study was to investigate the fit of the three models in a cohort of pwMS.

Results

The confirmatory factor analysis on the SVCq in 493 people with MS showed good fit for all the models. The three-factor model (diminished visual perception, altered visual perception and ocular discomfort) outperformed the one-factor model. The five-factor model outperformed both models, which showed that dividing the first factor (diminished visual perception) into three more factors (function-related, luminance-related and task-related) has merit.

Conclusions

All models may be useful in clinical care for pwMS. The one-factor model may give a quick overview of the presence and severity of visual complaints in general. The individual factors, of either the three- or the five factor models, may contribute to a better recognition of the nature of visual complaints in pwMS and may guide further steps in rehabilitation for pwMS with visual complaints.

Common pathways in dementia and diabetic retinopathy: understanding the mechanisms of diabetes-related cognitive decline

Type 2 diabetes (T2D) is associated with multiple comorbidities, including diabetic retinopathy (DR) and cognitive decline, and T2D patients have a significantly higher risk of developing Alzheimer's disease (AD). Both DR and AD are characterized by a number of pathological mechanisms that coalesce around the neurovascular unit, including neuroinflammation and degeneration, vascular degeneration, and glial activation. Chronic hyperglycemia and insulin resistance also play a significant role, leading to activation of pathological mechanisms such as increased oxidative stress and the accumulation of advanced glycation end-products (AGEs). Understanding these common pathways and the degree to which they occur simultaneously in the brain and retina during diabetes will provide avenues to identify T2D patients at risk of cognitive decline.

Recognizing visual complaints in people with multiple sclerosis: Prevalence, nature and associations with key characteristics of MS

BACKGROUND

Visual disturbances are common in multiple sclerosis (MS), but visual complaints may be underestimated. While these complaints decrease quality of life, they may not be discussed during clinic visits. People with MS (pwMS) may not be referred to appropriate care. We therefore investigated the prevalence, nature and associations of visual complaints of pwMS.

METHODS

We performed a cohort study with a comparison group. PwMS (n = 493) and healthy controls (n = 661) filled out the Screening Visual Complaints questionnaire (SVCq). Primary outcomes were the percentage of pwMS and controls that reported the 19 complaints, and total scores of the SVCq. We also compared the scores on the SVCq between different groups of pwMS.

RESULTS

In general, the complaints were reported more often by pwMS than by controls. PwMS especially reported experiencing complaints 'often/always', while controls reported the complaints primarily 'sometimes'. PwMS with and without a history of optic neuritis showed similar complaints. PwMS with a longer disease duration and those with SPMS reported more complaints. EDSS score and disease duration only showed a limited association with discomfort of visual complaints.

CONCLUSION

The prevalence of visual complaints among pwMS is high and any person with MS may experience a wide array of different visual complaints anywhere along the disease course, regardless of a history of optic neuritis. Using the SVCq may help detect pwMS' visual complaints and may facilitate referrals to appropriate care.

Ophthalmic manifestations of dementing disorders

PURPOSE OF REVIEW

Dementia is a term for loss of memory, language, problem-solving, and other thinking abilities, which significantly interferes with daily life. Certain dementing conditions may also affect visual function. The eye is an accessible window to the brain that can provide valuable information for the early diagnosis of people who suffer from Alzheimer's disease, Parkinson's disease, dementia with Lewy bodies as well as from more rare causes of dementias, such as Creutzfeldt-Jacob and Huntington's diseases. Herein, we present the ocular manifestations of neurocognitive disorders focusing on the neuro-ophthalmic ones and further discuss potential ocular biomarkers that could help in early detection of these disorders.

RECENT FINDINGS

Ophthalmic examination along with the recent developments in in-vivo testing have provided a strong foundation of useful knowledge about brain disorder in neurodegenerative diseases without the need for invasive studies. Currently, a number of visual measures, such as visual acuity, contrast sensitivity, pupil response, and saccades in addition to various ophthalmic tests, such as electroretinogram, visual evoked potential, optical coherence tomography (OCT), and OCT-angiography have been widely used and evaluated as potential biomarkers for different stages of dementia.

SUMMARY

Ophthalmologic and neuro-ophthalmic evaluation is evolving as an important part of the early diagnosis and management of people with dementia. A particular focus on ocular biomarkers in dementing illnesses has arisen over the past few years and there are several promising measures and imaging tools that have been proposed as potential biomarkers for these diseases.

Cognitive Dysfunctions in Glaucoma: An Overview of Morpho-Functional Mechanisms and the Impact on Higher-Order Visual Function

Background: Glaucoma is a chronic, vision-threatening disease, and a major cause of legal blindness. The current view is no longer limited to the progressive optic nerve injury, since growing evidence strongly support the interpretation of glaucoma as a complex neurodegenerative disease. However, the precise pathogenic mechanisms leading to the onset and progression of central nervous system (CNS) impairment, and the functional consequences of this damage, are still partially understood. The main aim of this review is to provide a complete and updated overview of the current knowledge regarding the CNS involvement in glaucoma, and the possible therapeutic perspectives. Methods: We made a careful survey of the current literature reporting all the relevant findings related to the cognitive dysfunctions occurring in glaucoma, with specific remarks dedicated on the higher-order visual function impairment and the possible employment of neuroprotective agents. Results: The current literature strongly support the interpretation of glaucoma as a multifaceted chronic neurodegenerative disease, widely affecting the CNS. The cognitive impairment may vary in terms of higher-order functions involvement and in the severity of the degeneration. Although several neuroprotective agents are currently available, the development of new molecules represents a major topic of investigation for future clinical trials. Conclusions: Glaucoma earned the right to be fully considered a neurodegenerative disease. Glaucomatous patients may experience a heterogeneous set of visual and cognitive symptoms, progressively deteriorating the quality of life. Neuroprotection is nowadays a necessary therapeutic goal and a future promising way to preserve visual and cognitive functions, thus improving patients' quality of life.

Ophthalmic Biomarkers for Alzheimer's Disease: A Review

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by neuronal loss, extracellular amyloid-β (Aβ) plaques, and intracellular neurofibrillary tau tangles. A diagnosis is currently made from the presenting symptoms, and the only definitive diagnosis can be done post-mortem. Over recent years, significant advances have been made in using ocular biomarkers to diagnose various neurodegenerative diseases, including AD. As the eye is an extension of the central nervous system (CNS), reviewing changes in the eye's biology could lead to developing a series of non-invasive, differential diagnostic tests for AD that could be further applied to other diseases. Significant changes have been identified in the retinal nerve fiber layer (RNFL), cornea, ocular vasculature, and retina. In the present paper, we review current research and assess some ocular biomarkers' accuracy and reliability that could potentially be used for diagnostic purposes. Additionally, we review the various imaging techniques used in the measurement of these biomarkers.

Tear Liquid for Predictive Diagnosis of Alzheimer’s Disease

The common approach of the diagnosis of Alzheimer’s Disease (AD) is made with an analysis of the cerebrospinal fluid or the study of retinal fundus and the plaques formation through optical corneal tomography (OCT), or more simply with a fundus camera. Tears analysis is widely discussed in literature as an essential method to describe molecular and biochemical alterations in different diseases. The aim of our study was the identification with immunocytochemistry of Amyloid Beta-42 in tears from patients with or without familiarity for Alzheimer Disease, in order to make the diagnosis earlier and more accessible compared to other invasive methods. Our study was performed on tears from three phenotypically healthy subjects: two of them were Caucasian with Alzheimer familiarity (48 and 55 years old) and the other one was Asian without Alzheimer familiarity (45 years old) and affected by an adenoviral keratoconjunctivitis at the moment of withdrawal. Tear samples were collected from eye fornix and were examinated by immunocytochemistry (ICC) assay using anti-Amyloid Beta X-42 antibody. Two out of three tears samples showed positive Amyloid Beta-42. Considering that our patients were phenotypically healthy, the identification of Amyloid Beta-42 by ICC could be a candidable method to make the diagnosis of the disease earlier and more accessible and available then other current and invasive methods and it could be a candidate for a screening method too.

Assessment of the outer retina and choroid in white matter lesions participants using swept-source optical coherence tomography

PURPOSE

To assess the three-dimensional outer retina thickness and choroid in eyes with white matter lesions (WMLs) using swept-source optical coherence tomography (SS-OCT).

METHODS

Participants without dementia and stroke with cerebral WMLs were enrolled in our study. Optical coherence tomography (OCT) and OCT angiography (OCTA) were used to image and evaluate the outer retinal layer, choroidal structure, and perfusion of the choriocapillaris, microvessels of the choroid, respectively. Measurement of the outer retinal thickness, choroidal thickness and perfusion of the choriocapillaris was done by the SS-OCT tool.

RESULTS

Thirty-one eyes from 16 WMLs and 40 eyes from 20 healthy controls were included in the data analyses. Outer retinal thickness was significantly reduced (P < .001) in WMLs participants when compared to healthy controls. Choroidal thickness was also significantly reduced (P < .001) in WMLs participants when compared to healthy controls. Choriocapillaris perfusion was significantly reduced (P = .002) in WMLs when compared to healthy controls. A significant correlation (Rho = .392, P = .032) was seen between the outer retinal thickness and choriocapillaris perfusion in WMLs participants.

CONCLUSIONS

Assessing retinal thickness and choroidal changes with the SS-OCTA as a proxy for WML could prove to be a potentially valuable tool for early detection of cognitive decline and other neurodegenerative diseases.

The Role of MicroRNAs in Mitochondria-Mediated Eye Diseases

The retina is among the most metabolically active tissues with high-energy demands. The peculiar distribution of mitochondria in cells of retinal layers is necessary to assure the appropriate energy supply for the transmission of the light signal. Photoreceptor cells (PRs), retinal pigment epithelium (RPE), and retinal ganglion cells (RGCs) present a great concentration of mitochondria, which makes them particularly sensitive to mitochondrial dysfunction. To date, visual loss has been extensively correlated to defective mitochondrial functions. Many mitochondrial diseases (MDs) show indeed neuro-ophthalmic manifestations, including retinal and optic nerve phenotypes. Moreover, abnormal mitochondrial functions are frequently found in the most common retinal pathologies, i.e., glaucoma, age-related macular degeneration (AMD), and diabetic retinopathy (DR), that share clinical similarities with the hereditary primary MDs. MicroRNAs (miRNAs) are established as key regulators of several developmental, physiological, and pathological processes. Dysregulated miRNA expression profiles in retinal degeneration models and in patients underline the potentiality of miRNA modulation as a possible gene/mutation-independent strategy in retinal diseases and highlight their promising role as disease predictive or prognostic biomarkers. In this review, we will summarize the current knowledge about the participation of miRNAs in both rare and common mitochondria-mediated eye diseases. Definitely, given the involvement of miRNAs in retina pathologies and therapy as well as their use as molecular biomarkers, they represent a determining target for clinical applications.

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

BACKGROUND

Amyloid-β soluble oligomers (Aβo) 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 Aβ 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 Aβo deposition and in particular to quantify levels of the toxic Aβo 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 Aβ plaque-specific and Aβo-specific antibodies.

RESULTS

We show that retinas displayed an age-dependent increase of Th-T-specific amyloid fibrils. Staining with anti-Aβ antibody confirmed the presence of the Aβ plaques in all 5×FAD retinas tested. In contrast, staining with anti-Aβo antibody showed an age-dependent decrease of retinal Aβo. Of note, Aβo was observed mainly in the retinal nuclear layers. Finally, we confirmed the localization of Aβo to neurons, typically accumulating in late endosomes, indicating possible impairment of the endocytic pathway.

CONCLUSION

Our results demonstrate the presence of intraneuronal Aβo in the retina and its accumulation inversely correlated with retinal Aβ plaque deposition, indicating an age-related conversion in this animal model. These results support the development of an early AD diagnostic test targeting Aβo in the eye.

Detection of retinal and blood Aβ oligomers with nanobodies

Introduction

Abnormal retinal changes are increasingly recognized as an early pathological change in Alzheimer's disease (AD). Although amyloid beta oligomers (Aβo) have been shown to accumulate in the blood and retina of AD patients and animals, it is not known whether the early Aβo deposition precedes their accumulation in brain.

Methods and results

Using nanobodies targeting Aβ_1‐40 and Aβ_1‐42 oligomers we were able to detect Aβ oligomers in the retina and blood but not in the brain of 3‐month‐old APP/PS1 mice. Furthermore, Aβ plaques were detected in the brain but not the retina of 3‐month‐old APP/PS1 mice.

Conclusion

These results suggest that retinal accumulation of Aβo originates from peripheral blood and precedes cognitive decline and Aβo deposition in the brain. This provides a very strong basis to develop and implement an “eye test” for early detection of AD using nanobodies targeting retinal Aβ.

Mesenchymal stem cell-conditioned medium attenuates the retinal pathology in amyloid-β-induced rat model of Alzheimer's disease: Underlying mechanisms

Amyloid‐beta (Aβ) oligomer is known to contribute to the pathophysiology of age‐related macular degeneration. Herein, we aimed to elucidate the in vivo and in vitro effects of Aβ_1‐42 application on retinal morphology in rats. Our in vivo studies revealed that intracerebroventricular administration of Aβ_1‐42 oligomer caused dysmorphological changes in both retinal ganglion cells and retinal pigment epithelium. In addition, in vitro studies revealed that ARPE‐19 cells following Aβ_1‐42 oligomer application had decreased viability along with apoptosis and decreased expression of the tight junction proteins, increased expression of both phosphor‐AKT and phosphor‐GSK3β and decreased expression of both SIRT1 and β‐catenin. Application of conditioned medium (CM) obtained from mesenchymal stem cells (MSC) protected against Aβ_1‐42 oligomer‐induced retinal pathology in both rats and ARPE‐19 cells. In order to explore the potential role of peptides secreted from the MSCs, we applied mass spectrometry to compare the peptidomics profiles of the MSC‐CM. Gene ontology enrichment analysis and String analysis were performed to explore the differentially expressed peptides by predicting the functions of their precursor proteins. Bioinformatics analysis showed that 3‐8 out of 155–163 proteins in the MSC‐CM maybe associated with SIRT1/pAKT/pGSK3β/β‐catenin, tight junction proteins, and apoptosis pathway. In particular, the secretomes information on the MSC‐CM may be helpful for the prevention and treatment of retinal pathology in age‐related macular degeneration.

Systemic delivery of a specific antibody targeting the pathological N-terminal truncated tau peptide reduces retinal degeneration in a mouse model of Alzheimer's Disease

Retina and optic nerve are sites of extra-cerebral manifestations of Alzheimer's Disease (AD). Amyloid-β (Aβ) plaques and neurofibrillary tangles of hyperphosphorylated tau protein are detected in eyes from AD patients and transgenic animals in correlation with inflammation, reduction of synapses, visual deficits, loss of retinal cells and nerve fiber. However, neither the pathological relevance of other post-translational tau modifications-such as truncation with generation of toxic fragments-nor the potential neuroprotective action induced by their in vivo clearance have been investigated in the context of AD retinal degeneration. We have recently developed a monoclonal tau antibody (12A12mAb) which selectively targets the neurotoxic 20-22 kDa NH2-derived peptide generated from pathological truncation at the N-terminal domain of tau without cross-reacting with its full-length normal protein. Previous studies have shown that 12A12mAb, when intravenously (i.v.)-injected into 6-month-old Tg2576 animals, markedly improves their AD-like, behavioural and neuropathological syndrome. By taking advantage of this well-established tau-directed immunization regimen, we found that 12A12mAb administration also exerts a beneficial action on biochemical, morphological and metabolic parameters (i.e. APP/Aβ processing, tau hyperphosphorylation, neuroinflammation, synaptic proteins, microtubule stability, mitochondria-based energy production, neuronal death) associated with ocular injury in the AD phenotype. These findings prospect translational implications in the AD field by: (1) showing for the first time that cleavage of tau takes part in several pathological changes occurring in vivo in affected retinas and vitreous bodies and that its deleterious effects are successfully antagonized by administration of the specific 12A12mAb; (2) shedding further insights on the tight connections between neurosensory retina and brain, in particular following tau-based immunotherapy. In our view, the parallel response we detected in this preclinical animal model, both in the eye and in the hippocampus, following i.v. 12A12mAb injection opens novel diagnostic and therapeutic avenues for the clinical management of cerebral and extracerebral AD signs in human beings.

Color and contrast vision in mouse models of aging and Alzheimer's disease using a novel visual-stimuli four-arm maze

We introduce a novel visual-stimuli four-arm maze (ViS4M) equipped with spectrally- and intensity-controlled LED emitters and dynamic grayscale objects that relies on innate exploratory behavior to assess color and contrast vision in mice. Its application to detect visual impairments during normal aging and over the course of Alzheimer’s disease (AD) is evaluated in wild-type (WT) and transgenic APP_SWE/PS1_∆E9 murine models of AD (AD⁺) across an array of irradiance, chromaticity, and contrast conditions. Substantial color and contrast-mode alternation deficits appear in AD⁺ mice at an age when hippocampal-based memory and learning is still intact. Profiling of timespan, entries and transition patterns between the different arms uncovers variable AD-associated impairments in contrast sensitivity and color discrimination, reminiscent of tritanomalous defects documented in AD patients. Transition deficits are found in aged WT mice in the absence of alternation decline. Overall, ViS4M is a versatile, controlled device to measure color and contrast-related vision in aged and diseased mice.

Macula Structure and Microvascular Changes in Recent Small Subcortical Infarct Patients

Purpose: This study aimed to assess the macula structure and capillaries in the macula and optic nerve head in recent small subcortical infarct (RSSI) patients. Methods: This observational cross-sectional study included 40 RSSI patients and 46 healthy controls. Optical coherence tomography angiography was used to image the capillaries in the macula and optic nerve head. An inbuilt algorithm was used to measure the densities in the microvasculature of the macula [superficial retinal capillary plexus (SRCP) and deep retinal capillary plexus (DRCP)] and optic nerve head [radial peripapillary capillary (RPC)] and thickness around the optic nerve head, peripapillary retinal nerve fiber layer (pRNFL). Results: Densities in RPC (P < 0.001), SRCP (P = 0.001), and DRCP (P = 0.003) were reduced in RSSI patients when compared with healthy controls. The pRNFL thickness was thinner (P < 0.001) in RSSI patients than healthy controls. In the RSSI group, the SRCP density significantly correlated with the DRCP density (rho = 0.381, P = 0.042). The pRNFL thickness displayed a significant relationship with the RPC density (rho = 0.482, P = 0.003) in the RSSI group. Conclusions: RSSI patients showed interrupted capillary plexuses leading to its significant impairment and neurodegeneration. Our report provides insight into the macula capillary microcirculation changes in RSSI.

[The relationship between cognitive impairment and changes in retinal neuroarchitectonics]

OBJECTIVE

To study the relationship between cognitive deficits and retinal neuroarchitectonics in Alzheimer's disease, vascular dementia, and glaucoma based on optical coherence tomography.

MATERIAL AND METHODS

A comprehensive examination of 90 patients with Alzheimer's disease, vascular dementia and glaucoma was conducted. The patients were divided into three groups of 30 people each. The groups were comparable by gender and age and initial socio-economic status. All patients underwent a comprehensive neurological and neuropsychological study as well as optical coherence tomography.

RESULTS AND CONCLUSION

The results of optical coherence tomography in Alzheimer's disease and glaucoma reveal retinal changes in the perifocal region in the upper and lower quadrants. In patients with vascular dementia, the process is observed in the foveal (central) region of the retina, which can be considered as a potential biomarker of the neurodegenerative damage. The severity of cognitive deficit in the Alzheimer's disease group correlates with the degree of degeneration in the layers of the peripapillary layer of the nerve fibers of the retina of the temporal region, the perifocal region of the lower quadrant of the retina, ganglion cells, and the inner plexiform layers of the retina. In the vascular dementia group, the severity of cognitive deficit positively correlates with the degree of cell degeneration in the foveal region of the inner plexiform retinal layer.

Alzheimer's Retinopathy: Seeing Disease in the Eyes

The neurosensory retina emerges as a prominent site of Alzheimer's disease (AD) pathology. As a CNS extension of the brain, the neuro retina is easily accessible for noninvasive, high-resolution imaging. Studies have shown that along with cognitive decline, patients with mild cognitive impairment (MCI) and AD often suffer from visual impairments, abnormal electroretinogram patterns, and circadian rhythm disturbances that can, at least in part, be attributed to retinal damage. Over a decade ago, our group identified the main pathological hallmark of AD, amyloid β-protein (Aβ) plaques, in the retina of patients including early-stage clinical cases. Subsequent histological, biochemical and in vivo retinal imaging studies in animal models and in humans corroborated these findings and further revealed other signs of AD neuropathology in the retina. Among these signs, hyperphosphorylated tau, neuronal degeneration, retinal thinning, vascular abnormalities and gliosis were documented. Further, linear correlations between the severity of retinal and brain Aβ concentrations and plaque pathology were described. More recently, extensive retinal pericyte loss along with vascular platelet-derived growth factor receptor-β deficiency were discovered in postmortem retinas of MCI and AD patients. This progressive loss was closely associated with increased retinal vascular amyloidosis and predicted cerebral amyloid angiopathy scores. These studies brought excitement to the field of retinal exploration in AD. Indeed, many questions still remain open, such as queries related to the temporal progression of AD-related pathology in the retina compared to the brain, the relations between retinal and cerebral changes and whether retinal signs can predict cognitive decline. The extent to which AD affects the retina, including the susceptibility of certain topographical regions and cell types, is currently under intense investigation. Advances in retinal amyloid imaging, hyperspectral imaging, optical coherence tomography, and OCT-angiography encourage the use of such modalities to achieve more accurate, patient- and user-friendly, noninvasive detection and monitoring of AD. In this review, we summarize the current status in the field while addressing the many unknowns regarding Alzheimer's retinopathy.

Smell, an Underrated Early Biomarker for Brain Aging

Olfaction is in addition to touch the most ancient of our senses, developing already in the womb it decays progressively from 65 years of age with a more pronounced impairment associated with dementia. Despite its clinical relevance and testing accessibility, smell remains an overlooked biomarker, which is rarely used by neurologists in the early screening phase. In this perspective article, we outline the reasons underlying the lack of awareness for this sense. In an attempt to put olfaction forward as an early biomarker for pathological brain aging, we draw a comparison with vision and hearing, regarded as more relevant for general health. This perspective article wants to encourage further studies aimed at understanding the mechanisms responsible for the early smell dysfunction in individuals a decade or more before the onset of cognitive symptoms.

OPA1 deficiency accelerates hippocampal synaptic remodelling and age-related deficits in learning and memory

A healthy mitochondrial network is essential for the maintenance of neuronal synaptic integrity. Mitochondrial and metabolic dysfunction contributes to the pathogenesis of many neurodegenerative diseases including dementia. OPA1 is the master regulator of mitochondrial fusion and fission and is likely to play an important role during neurodegenerative events. To explore this, we quantified hippocampal dendritic and synaptic integrity and the learning and memory performance of aged Opa1 haploinsufficient mice carrying the Opa1Q285X mutation (B6; C3-Opa1Q285STOP ; Opa1+/- ). We demonstrate that heterozygous loss of Opa1 results in premature age-related loss of spines in hippocampal pyramidal CA1 neurons and a reduction in synaptic density in the hippocampus. This loss is associated with subtle memory deficits in both spatial novelty and object recognition. We hypothesize that metabolic failure to maintain normal neuronal activity at the level of a single spine leads to premature age-related memory deficits. These results highlight the importance of mitochondrial homeostasis for maintenance of neuronal function during ageing.

3-Dimensional magnetic resonance imaging of the freely moving human eye

Eye motion is a major confound for magnetic resonance imaging (MRI) in neuroscience or ophthalmology. Currently, solutions toward eye stabilisation include participants fixating or administration of paralytics/anaesthetics. We developed a novel MRI protocol for acquiring 3-dimensional images while the eye freely moves. Eye motion serves as the basis for image reconstruction, rather than an impediment. We fully reconstruct videos of the moving eye and head. We quantitatively validate data quality with millimetre resolution in two ways for individual participants. First, eye position based on reconstructed images correlated with simultaneous eye-tracking. Second, the reconstructed images preserve anatomical properties; the eye's axial length measured from MRI images matched that obtained with ocular biometry. The technique operates on a standard clinical setup, without necessitating specialized hardware, facilitating wide deployment. In clinical practice, we anticipate that this may help reduce burdens on both patients and infrastructure, by integrating multiple varieties of assessments into a single comprehensive session. More generally, our protocol is a harbinger for removing the necessity of fixation, thereby opening new opportunities for ethologically-valid, naturalistic paradigms, the inclusion of populations typically unable to stably fixate, and increased translational research such as in awake animals whose eye movements constitute an accessible behavioural readout.

Structural and Microvascular Changes in the Macular Are Associated With Severity of White Matter Lesions

Purpose: This study aimed to characterize the microvascular and structural changes in the macular that occur in white matter hyperintensities (WMH) using optical coherence tomographic angiography. We also aimed to explore the association between macular microvascular and structural changes with focal markers of brain tissue on MRI in WMH using the Fazekas scale.

Methods: This study enrolled healthy participants who were stroke- and dementia-free. MRI was used to image the cerebral white matter lesions, and Fazekas scale was used to evaluate the severity of the white matter lesions. Optical coherence tomography angiography (OCT-A) was used to image the radial peripapillary capillaries (RPCs), macular capillary plexuses [superficial capillary plexus (SCP) and deep capillary plexus (DCP)] and thickness around the optic nerve head, peripapillary retinal nerve fiber layer (pRNFL).

Results: Seventy-four participants were enrolled and divided into two groups according to their Fazekas score (Fazekas scores ≤ 1 and ≥2). Participants with Fazekas score ≥2 showed significantly reduced RPC density (P = 0.02) and DCP density (P = 0.012) when compared with participants with Fazekas score ≤ 1. Participants with Fazekas score ≥2 showed reduced pRNFL (P = 0.004) when compared to participants with Fazekas score ≤ 1. Fazekas scores were significantly associated with the pRNFL thickness (Rho = −0.389, P = 0.001), RPC density (Rho = −0.248, P = 0.035), and DCP density (Rho = −0.283, P = 0.015), respectively.

Conclusions: Microvascular impairment and neuro-axonal damage are associated with the disease cascade in WMH. We have shown that RPC and DCP densities are significantly affected, and these impairments are associated with the severity of the disease and cognitive function. OCT-A could be a useful tool in quantifying the retinal capillary densities in WMH.

Ocular hypertension suppresses homeostatic gene expression in optic nerve head microglia of DBA/2 J mice

Glaucoma is the leading cause of irreversible vision loss. Ocular hypertension is a major risk factor for glaucoma and recent work has demonstrated critical early neuroinflammatory insults occur in the optic nerve head following ocular hypertension. Microglia and infiltrating monocytes are likely candidates to drive these neuroinflammatory insults. However, the exact molecular identity / transcriptomic profile of microglia following ocular hypertensive insults is unknown. To elucidate the molecular identity of microglia after long-term exposure to ocular hypertension, we used a mouse model of glaucoma (DBA/2 J). We performed RNA-sequencing of microglia mRNA from the optic nerve head at a time point following ocular hypertensive insults, but preceding detectable neurodegeneration (with microglia identified as being CD45^lo/CD11b⁺/CD11c⁻). Furthermore, RNA-sequencing was performed on optic nerve head microglia from mice treated with radiation therapy, a potent therapy preventing neuroinflammatory insults. Transcriptomic profiling of optic nerve head microglia mRNA identifies metabolic priming with marked changes in mitochondrial gene expression, and changes to phagocytosis, inflammatory, and sensome pathways. The data predict that many functions of microglia that help maintain tissue homeostasis are affected. Comparative analysis of these data with data from previously published whole optic nerve head tissue or monocyte-only samples from DBA/2 J mice demonstrate that many of the neuroinflammatory signatures in these data sets arise from infiltrating monocytes and not reactive microglia. Finally, our data demonstrate that prophylactic radiation therapy of DBA/2 J mice potently abolishes these microglia metabolic transcriptomic changes at the same time points. Together, our data provide a unique resource for the community to help drive further hypothesis generation and testing in glaucoma.

Novel alterations in corneal neuroimmune phenotypes in mice with central nervous system tauopathy

BACKGROUND

Tauopathy in the central nervous system (CNS) is a histopathological hallmark of frontotemporal dementia (FTD) and Alzheimer's disease (AD). Although AD is accompanied by various ocular changes, the effects of tauopathy on the integrity of the cornea, which is densely innervated by the peripheral nervous system and is populated by resident dendritic cells, is still unknown. The aim of this study was to investigate if neuroimmune interactions in the cornea are affected by CNS tauopathy.

METHODS

Corneas from wild type (WT) and transgenic rTg4510 mice that express the P301L tau mutation were examined at 2, 6, 8, and 11 months. Clinical assessment of the anterior segment of the eye was performed using spectral domain optical coherence tomography. The density of the corneal epithelial sensory nerves and the number and field area of resident epithelial dendritic cells were assessed using immunofluorescence. The immunological activation state of corneal and splenic dendritic cells was examined using flow cytometry and compared between the two genotypes at 9 months of age.

RESULTS

Compared to age-matched WT mice, rTg4510 mice had a significantly lower density of corneal nerve axons at both 8 and 11 months of age. Corneal nerves in rTg4510 mice also displayed a higher percentage of beaded nerve axons and a lower density of epithelial dendritic cells compared to WT mice. From 6 months of age, the size of the corneal dendritic cells was significantly smaller in rTg4510 compared to WT mice. Phenotypic characterization by flow cytometry demonstrated an activated state of dendritic cells (CD86⁺ and CD45⁺ CD11b⁺CD11c⁺) in the corneas of rTg4510 compared to WT mice, with no distinct changes in the spleen monocytes/dendritic cells. At 2 months of age, there were no significant differences in the neural or immune structures between the two genotypes.

CONCLUSIONS

Corneal sensory nerves and epithelial dendritic cells were altered in the rTg4510 mouse model of tauopathy, with temporal changes observed with aging. The activation of corneal dendritic cells prior to the gradual loss of neighboring sensory nerves suggests an early involvement of corneal immune cells in tau-associated pathology originating in the CNS.

Rhodopsin: A Potential Biomarker for Neurodegenerative Diseases

Retinal alterations have recently been associated with numerous neurodegenerative diseases. Rhodopsin is a G-protein coupled receptor found in the rod cells of the retina. As a biomarker associated with retinal thinning and degeneration, it bears potential in the early detection and monitoring of several neurodegenerative diseases. In this review article, we summarize the findings of correlations between rhodopsin and several neurodegenerative disorders as well as the potential of a novel technique, cSLO, in the quantification of rhodopsin.

Use of ocular biomarkers as a potential tool for early diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide which unfortunately has no known effective cure to date. Despite many clinical trials indicating the effectiveness of preclinical treatment, a sensitive tool for screening of AD is yet to be developed. Due to multiple similarities between ocular and the brain tissue, the eye is being explored by researchers for this purpose, with utmost attention focused on the retinal tissue. Besides visual functional impairment, neuronal degeneration and apoptosis, retinal nerve fiber degeneration, increase in the cup-to-disc ratio, and retinal vascular thinning and tortuosity are the changes observed in the retinal tissue which are related to AD. Studies have shown that targeting these changes in the retina is an effective way of reducing the degeneration of retinal neuronal tissue. Similar mechanisms of neurodegeneration have been demonstrated in the brain and the eyes of AD patients. Multiple studies are underway to investigate the potential of diagnosing AD and detection of amyloid-β (Aβ) levels in the retinal tissue. Since the tissues in the anterior segment of the eye are more accessible for in vivo imaging and examination, they have more potential as screening biomarkers. This article provides a concise review of available literature on the ocular biomarkers in anterior and posterior segments of the eye including the cornea, aqueous humour (AH), crystalline lens, and retina in AD. This review will also highlight the newer technological tools available for the detection of potential biomarkers in the eye for early diagnosis of AD.

Alterations of aqueous humor Aβ levels in Aβ-infused and transgenic mouse models of Alzheimer disease

Alzheimer's disease (AD) is an ageing-related neurodegenerative disease characterized and diagnosed by deposition of insoluble amyloid-β (Aβ) plaques in the brain. The plaque accumulation in the brain directly affects reduced levels of Aβ in cerebrospinal fluid (CSF) and blood, as Aβ can freely transport the blood-brain barrier, and clinical investigations have suggested these two biofluids as promising samples for in vitro diagnosis. Given that the human eye structurally resembles the brain and Aβ accumulation often observed in the ocular region of AD patients, in this study, we examined aqueous humor Aβ as another possible surrogate biomarker. First, using the acute Aβ-infused AD mouse model by injecting Aβ to the CSF in intracerebroventricular region of normal ICR mice, we investigated whether Aβ concentration in the aqueous humor in AD models is positively correlated with the concentration in the CSF. Then, we examined the correlation of aqueous humor Aβ levels with increased plaque deposition in the brain and reduced Aβ levels in both CSF and blood in adult and aged 5XFAD Alzheimer transgenic mice. Collectively, the synthetic Aβ injected into CSF immediately migrate to the aqueous humor, however, the age-dependently reducing pattern of Aβ levels in CSF and blood was not observed in the aqueous humor.

Pathogenic Tau Protein Species: Promising Therapeutic Targets for Ocular Neurodegenerative Diseases

Tau is a microtubule-associated protein, which is highly expressed in the central nervous system as well as ocular neurons and stabilizes microtubule structure. It is a phospho-protein being moderately phosphorylated under physiological conditions but its abnormal hyperphosphorylation or some post-phosphorylation modifications would result in a pathogenic condition, microtubule dissociation, and aggregation. The aggregates can induce neuroinflammation and trigger some pathogenic cascades, leading to neurodegeneration. Taking these together, targeting pathogenic tau employing tau immunotherapy may be a promising therapeutic strategy in fighting with cerebral and ocular neurodegenerative disorders.

A longitudinal assessment of retinal function and structure in the APP/PS1 transgenic mouse model of Alzheimer's disease

Background

A great body of evidence suggests that there are retinal functional and structural changes that occur in Alzheimer’s disease (AD). However, whether such changes are primary or secondary remains to be elucidated. We studied a range of retinal functional and structural parameters in association with AD- specific pathophysiological markers in the double transgenic APP/PS1 and control mice across age.

Methods

Electroretinogram (ERG) and optical coherence tomography (OCT) was performed in APP/PS1 and wild type (WT) control mice every 3 months from 3 to 12 months of age. For functional assessment, the a- and b-wave of the ERG, amplitude of oscillatory potentials (OP) and the positive scotopic threshold response (pSTR) were quantified at each time point. For structural assessment, the inner and outer retinal thickness was segmented and measured from OCT scans. Episodic memory was evaluated at 6, 9 and 12 months of age using the novel object recognition test. Amyloid beta (Aβ) distribution in the hippocampus and the retina were visualised at 3, 6 and 12 months of age. Inter- and intra- group analysis was performed to study rate of change for each parameter between the two groups.

Results

Inter-group analysis revealed a significant difference in b-wave and OPs of APP/PS1 compared to WT controls starting from 3 months (p < 0.001). There was also a significant difference in the amplitude of pSTR between the two groups starting from 6 months (p < 0.001). Furthermore, a significant difference in the inner retinal thickness, between the two groups, was observed starting from 9 months (p < 0.001).

Conclusions

We observed an age-related decline in retinal functional and structural parameters in both APP/PS1 and WT controls, however, inter-group analysis revealed that inner retinal functional and structural decline is exacerbated in APP/PS1 mice, and that retinal functional changes precede structural changes in this strain. Further studies are required to confirm whether such phenomenon occurs in humans and if studying retinal functional changes can aid-in early assessment of AD.

Electronic supplementary material

The online version of this article (10.1186/s40035-019-0170-z) contains supplementary material, which is available to authorized users.

Adult Goat Retinal Neuronal Culture: Applications in Modeling Hyperglycemia

Culture of adult neurons of the central nervous system (CNS) can provide a unique model system to explore neurodegenerative diseases. The CNS includes neurons and glia of the brain, spinal cord and retina. Neurons in the retina have the advantage of being the most accessible cells of the CNS, and can serve as a reliable mirror to the brain. Typically, primary cultures utilize fetal rodent neurons, but very rarely adult neurons from larger mammals. Here, we cultured primary retinal neurons isolated from adult goat up to 10 days, and established an in vitro model of hyperglycemia for performing morphological and molecular characterization studies. Immunofluorescence staining revealed that approximately 30–40% of cultured cells expressed neuronal markers. Next, we examined the relative expression of cell adhesion molecules (CAMs) in adult goat brain and retina. We also studied the effect of different glucose concentrations and media composition on the growth and expression of CAMs in cultured retinal neurons. Hyperglycemia significantly enhances neurite outgrowth in adult retinal neurons in culture. Expression of CAMs such as Caspr1, Contactin1 and Prion is downregulated in the presence of high glucose. Hyperglycemia downregulates the expression of the transcription factor CCAAT/enhancer binding protein (C/EBP α), predicted to bind CAM gene promoters. Collectively, our study demonstrates that metabolic environment markedly affects transcriptional regulation of CAMs in adult retinal neurons in culture. The effect of hyperglycemia on CAM interactions, as well as related changes in intracellular signaling pathways in adult retinal neurons warrants further investigation.

The clinical relevance of visualising the peripheral retina

Recent developments in imaging technologies now allow the documentation, qualitative and quantitative evaluation of peripheral retinal lesions. As wide field retinal imaging, capturing both the central and peripheral retina up to 200° eccentricity, is becoming readily available the question is: what is it that we gain by imaging the periphery? Based on accumulating evidence it is clear that findings in the periphery do not always associate to those observed in the posterior pole. However, the newly acquired information may provide useful clues to previously unrecognised disease features and may facilitate more accurate disease prognostication. In this review, we explore the anatomy and physiology of the peripheral retina, focusing on how it differs from the posterior pole, recount the history of peripheral retinal imaging, describe various peripheral retinal lesions and evaluate the overall relevance of peripheral retinal findings to different diseases.