Retinal macroglia changes in a triple transgenic mouse model of Alzheimer's disease

Raddeanin A Protects the BRB Through Inhibiting Inflammation and Apoptosis in the Retina of Alzheimer's Disease

Neuroinflammation and endothelial cell apoptosis are prominent features of blood-brain barrier (BBB) disruption, which have been described in Alzheimer's disease (AD) and can predict cognitive decline. Recent reports revealed vascular β-amyloid (Aβ) deposits, Muller cell degeneration and microglial dysfunction in the retina of AD patients. However, there has been no in-depth research on the roles of inflammation, retinal endothelial cell apoptosis, and blood-retinal barrier (BRB) damage in AD retinopathy. We found that Raddeanin A (RDA) could improve pathological and cognitive deficits in a mouse model of Alzheimer's disease by targeting β-amyloidosis, However, the effects of RDA on AD retinal function require further study. To clarify whether RDA inhibits inflammation and apoptosis and thus improves BRB function in AD-related retinopathy. In vitro we used Aβ-treated HRECs and MIO-M1 cells, and in vivo we used 3×Tg-AD mice to investigate the effect of RDA on BRB in AD-related retinopathy. We found that RDA could improve BRB function in AD-related retinopathy by inhibiting NLRP3-mediated inflammation and suppressing Wnt/β-catenin pathway-mediated apoptosis, which is expected to improve the pathological changes in AD-related retinopathy and the quality of life of AD patients.

Age-Related Retinal Layer Thickness Changes Measured by OCT in APPNL-F/NL-F Mice: Implications for Alzheimer's Disease

In Alzheimer's disease (AD), transgenic mouse models have established links between abnormalities in the retina and those in the brain. APPNL-F/NL-F is a murine, humanized AD model that replicates several pathological features observed in patients with AD. Research has focused on obtaining quantitative parameters from optical coherence tomography (OCT) in AD. The aim of this study was to analyze, in a transversal case-control study using manual retinal segmentation via SD-OCT, the changes occurring in the retinal layers of the APPNL/F-NF/L AD model in comparison to C57BL/6J mice (WT) at 6, 9, 12, 15, 17, and 20 months of age. The analysis focused on retinal thickness in RNFL-GCL, IPL, INL, OPL, and ONL based on the Early Treatment Diabetic Retinopathy Study (ETDRS) sectors. Both APPNL-F/NL-F-model and WT animals exhibited thickness changes at the time points studied. While WT showed significant changes in INL, OPL, and ONL, the AD model showed changes in all retinal layers analyzed. The APPNL-F/NL-F displayed significant thickness variations in the analyzed layers except for the IPL compared to related WT. These thickness changes closely resembled those found in humans during preclinical stages, as well as during mild and moderate AD stages, making this AD model behave more similarly to the disease in humans.

Alzheimer's disease pathophysiology in the Retina

The retina is an emerging CNS target for potential noninvasive diagnosis and tracking of Alzheimer's disease (AD). Studies have identified the pathological hallmarks of AD, including amyloid β-protein (Aβ) deposits and abnormal tau protein isoforms, in the retinas of AD patients and animal models. Moreover, structural and functional vascular abnormalities such as reduced blood flow, vascular Aβ deposition, and blood-retinal barrier damage, along with inflammation and neurodegeneration, have been described in retinas of patients with mild cognitive impairment and AD dementia. Histological, biochemical, and clinical studies have demonstrated that the nature and severity of AD pathologies in the retina and brain correspond. Proteomics analysis revealed a similar pattern of dysregulated proteins and biological pathways in the retina and brain of AD patients, with enhanced inflammatory and neurodegenerative processes, impaired oxidative-phosphorylation, and mitochondrial dysfunction. Notably, investigational imaging technologies can now detect AD-specific amyloid deposits, as well as vasculopathy and neurodegeneration in the retina of living AD patients, suggesting alterations at different disease stages and links to brain pathology. Current and exploratory ophthalmic imaging modalities, such as optical coherence tomography (OCT), OCT-angiography, confocal scanning laser ophthalmoscopy, and hyperspectral imaging, may offer promise in the clinical assessment of AD. However, further research is needed to deepen our understanding of AD's impact on the retina and its progression. To advance this field, future studies require replication in larger and diverse cohorts with confirmed AD biomarkers and standardized retinal imaging techniques. This will validate potential retinal biomarkers for AD, aiding in early screening and monitoring.

Quantifying Putative Retinal Gliosis in Preclinical Alzheimer's Disease

Purpose

Neuroinflammation plays a significant role in the pathology of Alzheimer's disease (AD). Mouse models of AD and postmortem biopsy of patients with AD reveal retinal glial activation comparable to central nervous system immunoreactivity. We hypothesized that the surface area of putative retinal gliosis observed in vivo using en face optical coherence tomography (OCT) imaging will be larger in patients with preclinical AD versus controls.

Methods

The Spectralis II instrument was used to acquire macular centered 20 × 20 and 30 × 25-degrees spectral domain OCT images of 76 participants (132 eyes). A cohort of 22 patients with preclinical AD (40 eyes, mean age = 69 years, range = 60-80 years) and 20 control participants (32 eyes, mean age = 66 years, range = 58-82 years, P = 0.11) were included for the assessment of difference in surface area of putative retinal gliosis and retinal nerve fiber layer (RNFL) thickness. The surface area of putative retinal gliosis and RNFL thickness for the nine sectors of the Early Treatment Diabetic Retinopathy Study (ETDRS) map were compared between groups using generalized linear mixed models.

Results

The surface area of putative retinal gliosis was significantly greater in the preclinical AD group (0.97 ± 0.55 mm2) compared to controls (0.68 ± 0.40 mm2); F(1,70) = 4.41, P = 0.039; Cohen's d = 0.61. There was no significant difference between groups for RNFL thickness in the 9 ETDRS sectors, P > 0.05.

Conclusions

Our analysis shows greater putative retinal gliosis in preclinical AD compared to controls. This demonstrates putative retinal gliosis as a potential biomarker for AD-related neuroinflammation.

Bridging Retinal and Cerebral Neurodegeneration: A Focus on Crosslinks between Alzheimer-Perusini's Disease and Retinal Dystrophies

In the early stages of Alzheimer-Perusini's disease (AD), individuals often experience vision-related issues such as color vision impairment, reduced contrast sensitivity, and visual acuity problems. As the disease progresses, there is a connection with glaucoma and age-related macular degeneration (AMD) leading to retinal cell death. The retina's involvement suggests a link with the hippocampus, where most AD forms start. A thinning of the retinal nerve fiber layer (RNFL) due to the loss of retinal ganglion cells (RGCs) is seen as a potential AD diagnostic marker using electroretinography (ERG) and optical coherence tomography (OCT). Amyloid beta fragments (Aβ), found in the eye's vitreous and aqueous humor, are also present in the cerebrospinal fluid (CSF) and accumulate in the retina. Aβ is known to cause tau hyperphosphorylation, leading to its buildup in various retinal layers. However, diseases like AD are now seen as mixed proteinopathies, with deposits of the prion protein (PrP) and α-synuclein found in affected brains and retinas. Glial cells, especially microglial cells, play a crucial role in these diseases, maintaining immunoproteostasis. Studies have shown similarities between retinal and brain microglia in terms of transcription factor expression and morphotypes. All these findings constitute a good start to achieving better comprehension of neurodegeneration in both the eye and the brain. New insights will be able to bring the scientific community closer to specific disease-modifying therapies.

Targeted Metabolomic Analysis of the Eye Tissue of Triple Transgenic Alzheimer's Disease Mice at an Early Pathological Stage

Alzheimer's disease (AD) is a severe neurodegenerative disease in older people. Despite some consensus on pathogenesis of AD established by previous researches, further elucidation is still required for better understanding. This study analyzed the eye tissues of 2- and 6-month-old triple transgenic AD (3 × Tg-AD) male mice and age-sex-matched wild-type (WT) mice using a targeted metabolomics approach. Compared with WT mice, 20 and 44 differential metabolites were identified in 2- and 6-month-old AD mice, respectively. They were associated with purine metabolism, pantothenate and CoA biosynthesis, pyruvate metabolism, lysine degradation, glycolysis/gluconeogenesis, and pyrimidine metabolism pathways. Among them, 8 metabolites presented differences in both the two groups, and 5 of them showed constant trend of change. The results indicated that the eye tissues of 3 × Tg-AD mice underwent changes in the early stages of the disease, with changes in metabolites observed at 2 months of age and more pronounced at 6 months of age, which is consistent with our previous studies on hippocampal targeted metabolomics in 3 × Tg-AD mice. Therefore, a joint analysis of data from this study and previous hippocampal study was performed, and the differential metabolites and their associated mechanisms were similar in eye and hippocampal tissues, but with tissue specificity.

Salvianolic acid B ameliorates retinal deficits in an early-stage Alzheimer's disease mouse model through downregulating BACE1 and Aβ generation

Alzheimer's disease (AD) is a neurodegenerative disease with subtle onset, early diagnosis remains challenging. Accumulating evidence suggests that the emergence of retinal damage in AD precedes cognitive impairment, and may serve as a critical indicator for early diagnosis and disease progression. Salvianolic acid B (Sal B), a bioactive compound isolated from the traditional Chinese medicinal herb Salvia miltiorrhiza, has been shown promise in treating neurodegenerative diseases, such as AD and Parkinson's disease. In this study we investigated the therapeutic effects of Sal B on retinopathy in early-stage AD. One-month-old transgenic mice carrying five familial AD mutations (5×FAD) were treated with Sal B (20 mg·kg-1·d-1, i.g.) for 3 months. At the end of treatment, retinal function and structure were assessed, cognitive function was evaluated in Morris water maze test. We showed that 4-month-old 5×FAD mice displayed distinct structural and functional deficits in the retinas, which were significantly ameliorated by Sal B treatment. In contrast, untreated, 4-month-old 5×FAD mice did not exhibit cognitive impairment compared to wild-type mice. In SH-SY5Y-APP751 cells, we demonstrated that Sal B (10 μM) significantly decreased BACE1 expression and sorting into the Golgi apparatus, thereby reducing Aβ generation by inhibiting the β-cleavage of APP. Moreover, we found that Sal B effectively attenuated microglial activation and the associated inflammatory cytokine release induced by Aβ plaque deposition in the retinas of 5×FAD mice. Taken together, our results demonstrate that functional impairments in the retina occur before cognitive decline, suggesting that the retina is a valuable reference for early diagnosis of AD. Sal B ameliorates retinal deficits by regulating APP processing and Aβ generation in early AD, which is a potential therapeutic intervention for early AD treatment.

Potential Retinal Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) represents a major diagnostic challenge, as early detection is crucial for effective intervention. This review examines the diagnostic challenges facing current AD evaluations and explores the emerging field of retinal alterations as early indicators. Recognizing the potential of the retina as a noninvasive window to the brain, we emphasize the importance of identifying retinal biomarkers in the early stages of AD. However, the examination of AD is not without its challenges, as the similarities shared with other retinal diseases introduce complexity in the search for AD-specific markers. In this review, we address the relevance of using the retina for the early diagnosis of AD and the complex challenges associated with the search for AD-specific retinal biomarkers. We provide a comprehensive overview of the current landscape and highlight avenues for progress in AD diagnosis by retinal examination.

Microglial Hemoxygenase-1 Deletion Reduces Inflammation in the Retina of Old Mice with Tauopathy

Tauopathies such as Alzheimer's disease are characterized by the accumulation of neurotoxic aggregates of tau protein. With aging and, especially, in Alzheimer's patients, the inducible enzyme heme oxygenase 1 (HO-1) progressively increases in microglia, causing iron accumulation, neuroinflammation, and neurodegeneration. The retina is an organ that can be readily accessed and can reflect changes that occur in the brain. In this context, we evaluated how the lack of microglial HO-1, using mice that do not express HO-1 in microglia (HMO-KO), impacts retinal macro and microgliosis of aged subjects (18 months old mice) subjected to tauopathy by intrahippocampal delivery of AAV-hTauP301L (TAU). Our results show that although tauopathy, measured as anti-TAUY9 and anti-AT8 positive immunostaining, was not observed in the retina of WT-TAU or HMO-KO+TAU mice, a morphometric study of retinal microglia and macroglia showed significant retinal changes in the TAU group compared to the WT group, such as: (i) increased number of activated microglia, (ii) retraction of microglial processes, (iii) increased number of CD68+ microglia, and (iv) increased retinal area occupied by GFAP (AROA) and C3 (AROC3). This retinal inflammatory profile was reduced in HMO-KO+TAU mice. Conclusion: Reduction of microglial HO-1 could be beneficial to prevent tauopathy-induced neuroinflammation.

Müller cell degeneration and microglial dysfunction in the Alzheimer's retina

Amyloid beta (Aβ) deposits in the retina of the Alzheimer's disease (AD) eye may provide a useful diagnostic biomarker for AD. This study focused on the relationship of Aβ with macroglia and microglia, as these glial cells are hypothesized to play important roles in homeostasis and clearance of Aβ in the AD retina. Significantly higher Aβ load was found in AD compared to controls, and specifically in the mid-peripheral region. AD retina showed significantly less immunoreactivity against glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) compared to control eyes. Immunoreactivity against ionized calcium binding adapter molecule-1 (IBA-1), a microglial marker, demonstrated a higher level of microgliosis in AD compared to control retina. Within AD retina, more IBA-1 immunoreactivity was present in the mid-peripheral retina, which contained more Aβ than the central AD retina. GFAP co-localized rarely with Aβ, while IBA-1 co-localized with Aβ in more layers of control than AD donor retina. These results suggest that dysfunction of the Müller and microglial cells may be key features of the AD retina.

Regulating microglial miR-155 transcriptional phenotype alleviates Alzheimer's-induced retinal vasculopathy by limiting Clec7a/Galectin-3+ neurodegenerative microglia

Single cell RNA sequencing studies identified novel neurodegeneration-associated microglial (MGnD/DAM) subtypes activated around cerebral amyloid plaques. Micro-RNA (miR)-155 of the TREM2-APOE pathway was shown to be a key transcriptional regulator of MGnD microglial phenotype. Despite growing interest in studying manifestations of Alzheimer's disease (AD) in the retina, a CNS organ accessible to noninvasive high-resolution imaging, to date MGnD microglia have not been studied in the AD retina. Here, we discovered the presence and increased populations of Clec7a+ and Galectin-3+ MGnD microglia in retinas of transgenic APPSWE/PS1L166P AD-model mice. Conditionally targeting MGnD microglia by miR-155 ablation via the tamoxifen-inducible CreERT2 system in APPSWE/PS1L166P mice diminished retinal Clec7a+ and Galectin-3+ microglial populations while increasing homeostatic P2ry12+ microglia. Retinal MGnD microglia were often adhering to microvessels; their depletion protected the inner blood-retina barrier and reduced vascular amyloidosis. Microglial miR-155 depletion further limits retinal inflammation. Mass spectrometry analysis revealed enhanced retinal PI3K-Akt signaling and predicted IL-8 and Spp1 decreases in mice with microglia-specific miR-155 knockout. Overall, this study identified MGnD microglia in APPSWE/PS1L166P mouse retina. Transcriptional regulation of these dysfunctional microglia mitigated retinal inflammation and vasculopathy. The protective effects of microglial miR-155 ablation should shed light on potential treatments for retinal inflammation and vascular damage during AD and other ocular diseases.

Alterations in Retinal Signaling Across Age and Sex in 3xTg Alzheimer’s Disease Mice

Background:

Visual disturbances often precede cognitive dysfunction in patients with Alzheimer’s disease (AD) and may coincide with early accumulation of amyloid-β (Aβ) protein in the retina. These findings have inspired critical research on in vivo ophthalmic Aβ imaging for disease biomarker detection but have not fully answered mechanistic questions on how retinal pathology affects visual signaling between the eye and brain.

Objective:

The goal of this study was to provide a functional and structural assessment of eye-brain communication between retinal ganglion cells (RGCs) and their primary projection target, the superior colliculus, in female and male 3xTg-AD mice across disease stages.

Methods:

Retinal electrophysiology, axonal transport, and immunofluorescence were used to determine RGC projection integrity, and retinal and collicular Aβ levels were assessed with advanced protein quantitation techniques.

Results:

3xTg mice exhibited nuanced deficits in RGC electrical signaling, axonal transport, and synaptic integrity that exceeded normal age-related decrements in RGC function in age- and sex-matched healthy control mice. These deficits presented in sex-specific patterns among 3xTg mice, differing in the timing and severity of changes.

Conclusion:

These data support the premise that retinal Aβ is not just a benign biomarker in the eye, but may contribute to subtle, nuanced visual processing deficits. Such disruptions might enhance the biomarker potential of ocular amyloid and differentiate patients with incipient AD from patients experiencing normal age-related decrements in visual function.

Concomitant Retinal Alterations in Neuronal Activity and TNFα Pathway Are Detectable during the Pre-Symptomatic Stage in a Mouse Model of Alzheimer's Disease

The pre-symptomatic stage of Alzheimer’s disease (AD) is associated with increased amyloid-β (Aβ) precursor protein (APP) processing and Aβ accumulation in the retina and hippocampus. Because neuronal dysfunctions are among the earliest AD-related alterations, we asked whether they are already detectable in the retina during the pre-symptomatic stage in a APPswePS1dE9 (APP/PS1) mouse model. The age chosen for the study (3–4 months) corresponds to the pre-symptomatic stage because no retinal Aβ was detected, in spite of the presence of βCTF (the first cleavage product of APP). We observed an increase in ERG amplitudes in APP/PS1 mice in comparison to the controls, which indicated an increased retinal neuron activity. These functional changes coincided with an increased expression of retinal TNFα and its receptors type-1 (TNFR1). Consistently, the IkB expression increased in APP/PS1 mice with a greater proportion of the phosphorylated protein (P-IkB) over total IkB, pointing to the putative involvement of the NFkB pathway. Because TNFα plays a crucial role in the control of neuronal excitability, it is likely that, as in the hippocampus, TNFα signaling via the TNFR1/NFkB pathway may be also involved in early, AD-associated, retinal neuron hyperexcitability. These results further demonstrate the interest of the retina for early disease detection with a potential to assess future therapeutic strategies.

The retina: A window in which to view the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most familiar type of dementia affecting elderly populations worldwide. Studies of AD patients and AD transgenic mice have revealed alterations in the retina similar to alterations which occur in the AD brain. Moreover, AD retinal pathology occurs even earlier than AD brain pathology. Importantly, non-invasive imaging techniques can be utilized for retinal observation due to the unique optical transparency of the eye, which acts as a convenient window in which preclinical pathology in the AD brain can be monitored. In this review, we overview the existing literature covering different forms of AD retinal pathology and propose a basis for the clinical application of using the retina as a window to view AD during preclinical and clinical stages.

Preservation of Retinal Function Through Synaptic Stabilization in Alzheimer's Disease Model Mouse Retina by Lycium Barbarum Extracts

In Alzheimer's disease (AD), amyloid β deposition-induced hippocampal synaptic dysfunction generally begins prior to neuronal degeneration and memory impairment. Lycium barbarum extracts (LBE) have been demonstrated to be neuroprotective in various animal models of neurodegeneration. In this study, we aimed to investigate the effects of LBE on the synapse loss in AD through the avenue of the retina in a triple transgenic mouse model of AD (3xTg-AD). We fed 3xTg-AD mice with low (200 mg/kg) or high (2 g/kg) dose hydrophilic LBE daily for 2 months from the starting age of 4- or 6-month-old. For those started at 6 month age, at 1 month (though not 2 months) after starting treatment, mice given high dose LBE showed a significant increase of a wave and b wave in scotopic ERG. After 2 months of treatment with high dose LBE, calpain-2, calpain-5, and the oxidative RNA marker 8-OHG were downregulated, and presynaptic densities in the inner plexiform layer but not the outer plexiform layer of the retina were significantly increased, suggesting the presynaptic structure of retina was preserved. Our results indicate that LBE feeding may preserve synapse stability in the retina of 3xTg-AD mice, probably by decreasing both oxidative stress and intracellular calcium influx. Thus, LBE might have potential as a neuroprotectant for AD through synapse preservation.

Retinal Changes in Transgenic Mouse Models of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder, the most common form of dementia. AD is characterised by amyloid-β (Aβ) plaques and neurofibrillary tangles (NFT) in the brain, in association with neuronal loss and synaptic failure, causing cognitive deficits. Accurate and early diagnosis is currently unavailable in lifespan, hampering early intervention of potential new treatments. Visual deficits have been well documented in AD patients, and the pathological changes identified in the brain are also believed to be found in the retina, an integral part of the central nervous system. Retinal changes can be detected by real-time non-invasive imaging, due to the transparent nature of the ocular media, potentially allowing an earlier diagnosis as well as monitoring disease progression and treatment outcome. Animal models are essential for AD research, and this review has a focus on retinal changes in various transgenic AD mouse models with retinal imaging and immunohistochemical analysis as well as therapeutic effects in those models. We also discuss the limitations of transgenic AD models in clinical translations.

Touchscreen cognitive testing: Cross-species translation and co-clinical trials in neurodegenerative and neuropsychiatric disease

Translating results from pre-clinical animal studies to successful human clinical trials in neurodegenerative and neuropsychiatric disease presents a significant challenge. While this issue is clearly multifaceted, the lack of reproducibility and poor translational validity of many paradigms used to assess cognition in animal models are central contributors to this challenge. Computer-automated cognitive test batteries have the potential to substantially improve translation between pre-clinical studies and clinical trials by increasing both reproducibility and translational validity. Given the structured nature of data output, computer-automated tests also lend themselves to increased data sharing and other open science good practices. Over the past two decades, computer automated, touchscreen-based cognitive testing methods have been developed for non-human primate and rodent models. These automated methods lend themselves to increased standardization, hence reproducibility, and have become increasingly important for the elucidation of the neurobiological basis of cognition in animal models. More recently, there have been increased efforts to use these methods to enhance translational validity by developing task batteries that are nearly identical across different species via forward (i.e., translating animal tasks to humans) and reverse (i.e., translating human tasks to animals) translation. An additional benefit of the touchscreen approach is that a cross-species cognitive test battery makes it possible to implement co-clinical trials-an approach developed initially in cancer research-for novel treatments for neurodegenerative disorders. Co-clinical trials bring together pre-clinical and early clinical studies, which facilitates testing of novel treatments in mouse models with underlying genetic or other changes, and can help to stratify patients on the basis of genetic, molecular, or cognitive criteria. This approach can help to determine which patients should be enrolled in specific clinical trials and can facilitate repositioning and/or repurposing of previously approved drugs. This has the potential to mitigate the resources required to study treatment responses in large numbers of human patients.

Retinal Ganglion Cells Functional Changes in a Mouse Model of Alzheimer's Disease Are Linked with Neurotransmitter Alterations

BACKGROUND

Alzheimer's disease (AD) is the most prevalent form of dementia worldwide. This neurodegenerative syndrome affects cognition, memory, behavior, and the visual system, particularly the retina.

OBJECTIVE

This work aims to determine whether the 5xFAD mouse, a transgenic model of AD, displays changes in the function of retinal ganglion cells (RGCs) and if those alterations are correlated with changes in the expression of glutamate and gamma-aminobutyric acid (GABA) neurotransmitters.

METHODS

In young (2-3-month-old) and adult (6-7-month-old) 5xFAD and WT mice, we have studied the physiological response, firing rate, and burst of RGCs to various types of visual stimuli using a multielectrode array system.

RESULTS

The firing rate and burst response in 5xFAD RGCs showed hyperactivity at the early stage of AD in young mice, whereas hypoactivity was seen at the later stage of AD in adults. The physiological alterations observed in 5xFAD correlate well with an increase in the expression of glutamate in the ganglion cell layer in young and adults. GABA staining increased in the inner nuclear and plexiform layer, which was more pronounced in the adult than the young 5xFAD retina, altering the excitation/inhibition balance, which could explain the observed early hyperactivity and later hypoactivity in RGC physiology.

CONCLUSION

These findings indicate functional changes may be caused by neurochemical alterations of the retina starting at an early stage of the AD disease.

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.

Retina and Brain Display Early and Differential Molecular and Cellular Changes in the 3xTg-AD Mouse Model of Alzheimer's Disease

The concept 'the retina as a window to the brain' has been increasingly explored in Alzheimer´s disease (AD) in recent years, since some patients present visual alterations before the first symptoms of dementia. The retina is an extension of the brain and can be assessed by noninvasive methods. However, assessing the retina for AD diagnosis is still a matter of debate. Using the triple transgenic mouse model of AD (3xTg-AD; males), this study was undertaken to investigate whether the retina and brain (hippocampus and cortex) undergo similar molecular and cellular changes during the early stages (4 and 8 months) of the pathology, and if the retina can anticipate the alterations occurring in the brain. We assessed amyloid-beta (Aβ) and hyperphosphorylated tau (p-tau) levels, barrier integrity, cell death, neurotransmitter levels, and glial changes. Overall, the retina, hippocampus, and cortex of 3xTg-AD are not significantly affected at these early stages. However, we detected a few differential changes in the retina and brain regions, and particularly a different profile in microglia branching in the retina and hippocampus, only at 4 months, where the number and length of the processes decreased in the retina and increased in the hippocampus. In summary, at the early stages of pathology, the retina, hippocampus, and cortex are not significantly affected but already present some molecular and cellular alterations. The retina did not mirror the changes detected in the brain, and these observations should be taking into account when using the retina as a potential diagnostic tool for AD.

Retinal Degeneration and Alzheimer's Disease: An Evolving Link

Age-related macular degeneration (AMD) and glaucoma are degenerative conditions of the retina and a significant cause of irreversible blindness in developed countries. Alzheimer’s disease (AD), the most common dementia of the elderly, is often associated with AMD and glaucoma. The cardinal features of AD include extracellular accumulation of amyloid β (Aβ) and intracellular deposits of hyper-phosphorylated tau (p-tau). Neuroinflammation and brain iron dyshomeostasis accompany Aβ and p-tau deposits and, together, lead to progressive neuronal death and dementia. The accumulation of Aβ and iron in drusen, the hallmark of AMD, and Aβ and p-tau in retinal ganglion cells (RGC), the main retinal cell type implicated in glaucoma, and accompanying inflammation suggest overlapping pathology. Visual abnormalities are prominent in AD and are believed to develop before cognitive decline. Some are caused by degeneration of the visual cortex, while others are due to RGC loss or AMD-associated retinal degeneration. Here, we review recent information on Aβ, p-tau, chronic inflammation, and iron dyshomeostasis as common pathogenic mechanisms linking the three degenerative conditions, and iron chelation as a common therapeutic option for these disorders. Additionally discussed is the role of prion protein, infamous for prion disorders, in Aβ-mediated toxicity and, paradoxically, in neuroprotection.

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.

Telomere length and oxidative stress variations in a murine model of Alzheimer's disease progression

Alzheimer's disease (AD) is the most common cause of dementia, and ageing is its major risk factor. Changes in telomere length have been associated with ageing and some degenerative diseases. Our aim was to explore some of the molecular changes caused by the progression of AD in a transgenic murine model (3xTg-AD; B6; 129-Psen1 <tm1Mpm> Tg (APPSwe, tauP301L) 1Lfa). Telomere length was assessed by qPCR in both brain tissue and peripheral blood cells and compared between three age groups: 5, 9 and 13 months. In addition, a possible effect of oxidative stress on telomere length and AD progression was explored. Shorter telomeres were found in blood cells of older transgenic mice compared to younger and wild-type mice but no changes in telomere length in the hippocampus. An increase in oxidative stress with age was found for all strains, but no correlation was found between oxidative stress and shorter telomere length for transgenic mice. Telomere length and oxidative stress are affected by AD progression in the 3xTg-AD murine model. Changes in blood cells are more noticeable than changes in brain tissue, suggesting that systemic changes can be detected early in the disease in this murine model.

Interactions between Amyloid-Β Proteins and Human Brain Pericytes: Implications for the Pathobiology of Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease that is the most common cause of dementia, especially among aging populations. Despite advances in AD research, the underlying cause and the discovery of disease-modifying treatments have remained elusive. Two key features of AD pathology are the aberrant deposition of amyloid beta (amyloid-β or Aβ) proteins in the brain parenchyma and Aβ toxicity in brain pericytes of the neurovascular unit/blood–brain barrier (NVU/BBB). This toxicity induces oxidative stress in pericytes and leads to capillary constriction. The interaction between pericytes and Aβ proteins results in the release of endothelin-1 in the pericytes. Endothelin-1 interacts with ET_A receptors to cause pericyte contraction. This pericyte-mediated constriction of brain capillaries can cause chronic hypoperfusion of the brain microvasculature, subsequently leading to the neurodegeneration and cognitive decline observed in AD patients. The interaction between Aβ proteins and brain pericytes is largely unknown and requires further investigation. This review provides an updated overview of the interaction between Aβ proteins with pericytes, one the most significant and often forgotten cellular components of the BBB and the inner blood–retinal barrier (IBRB). The IBRB has been shown to be a window into the central nervous system (CNS) that could allow the early diagnosis of AD pathology in the brain and the BBB using modern photonic imaging systems such as optical coherence tomography (OCT) and two-photon microscopy. In this review, I explore the regulation of Aβ proteins in the brain parenchyma, their role in AD pathobiology, and their association with pericyte function. This review discusses Aβ proteins and pericytes in the ocular compartment of AD patients as well as strategies to rescue or protect pericytes from the effects of Aβ proteins, or to replace them with healthy cells.

Reflections on the Utility of the Retina as a Biomarker for Alzheimer's Disease: A Literature Review

As a part of the central nervous system, the retina may reflect both physiologic processes and abnormalities related to diseases of the brain. Indeed, a concerted effort has been put forth to understand how Alzheimer's disease (AD) pathology may manifest in the retina as a means to assess the state of the AD brain. The development and refinement of ophthalmologic techniques for studying the retina in vivo have produced evidence of retinal degeneration in AD diagnosed patients. In this review, we will discuss retinal imaging techniques implemented to study the changes in AD retina as well as highlight the recent efforts made to correlate such findings to other clinical hallmarks of AD to assess the viability of the retina as a biomarker for AD.

Inhibition of AMD-Like Pathology With a Neurotrophic Compound in Aged Rats and 3xTg-AD Mice

Age-associated macular degeneration (AMD), which leads to loss of vision at its end stage, is one of the most common neurodegenerative diseases among the elderly. However, to date, no effective drug therapy is available for the prevention of AMD. Here, we report the occurrence of AMD pathology and its prevention by chronic treatment with the neurotrophic peptidergic compound P021, in aged rats and 3xTg-AD mice. We found photoreceptor degeneration, lipofuscin granules, vacuoles, and atrophy in retinal pigment epithelium (RPE) as well as Bruch’s membrane (BM) thickening; in aged rats, we even found rosette-like structure formation. Microgliosis and astrogliosis were observed in different retinal layers. In addition, we also found that total tau, phosphorylated tau, Aβ/APP, and VEGF were widely distributed in the sub-retina of aged rats and 3xTg mice. Importantly, chronic treatment with P021 for 3 months in rats and for 18 months in 3xTg mice ameliorated the pathological changes above. These findings indicate the therapeutic potential of P021 for prevention and treatment of AMD and retinal changes associated with aging and Alzheimer’s disease.

Vascular retinal biomarkers improves the detection of the likely cerebral amyloid status from hyperspectral retinal images

Introduction

This study investigates the relationship between retinal image features and β-amyloid (Aβ) burden in the brain with the aim of developing a noninvasive method to predict the deposition of Aβ in the brain of patients with Alzheimer's disease.

Methods

Retinal images from 20 cognitively impaired and 26 cognitively unimpaired cases were acquired (3 images per subject) using a hyperspectral retinal camera. The cerebral amyloid status was determined from binary reads by a panel of 3 expert raters on ¹⁸F-florbetaben positron-emission tomography (PET) studies. Image features from the hyperspectral retinal images were calculated, including vessels tortuosity and diameter and spatial-spectral texture measures in different retinal anatomical regions.

Results

Retinal venules of amyloid-positive subjects (Aβ+) showed a higher mean tortuosity compared with the amyloid-negative (Aβ−) subjects. Arteriolar diameter of Aβ+ subjects was found to be higher than the Aβ− subjects in a zone adjacent to the optical nerve head. Furthermore, a significant difference between texture measures built over retinal arterioles and their adjacent regions were observed in Aβ+ subjects when compared with the Aβ−. A classifier was trained to automatically discriminate subjects combining the extracted features. The classifier could discern Aβ+ subjects from Aβ− subjects with an accuracy of 85%.

Discussion

Significant differences in texture measures were observed in the spectral range 450 to 550 nm which is known as the spectral region known to be affected by scattering from amyloid aggregates in the retina. This study suggests that the inclusion of metrics related to the retinal vasculature and tissue-related textures extracted from vessels and surrounding regions could improve the discrimination performance of the cerebral amyloid status.

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

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

The use of PET for detection of Aβ in the brain in AD has limitations; studies also indicate that retinal changes, including Aβ deposition, occur in AD. Here the authors demonstrate the potential to use in vivo retinal hyperspectral imaging as a surrogate for brain accumulation of Aβ.

Characterization of Inner Retinal Hyperreflective Alterations in Early Cognitive Impairment on Adaptive Optics Scanning Laser Ophthalmoscopy

Purpose

To examine inner retinal hyperreflective features on adaptive optics scanning laser ophthalmoscopy (AOSLO) in individuals with early cognitive impairment.

Methods

In this prospective, cross-sectional study, we enrolled 12 participants with either amnestic mild cognitive impairment (aMCI, n = 10) or early dementia due to Alzheimer's disease (eAD, n = 2) and 12 age-, sex-, and race-matched cognitively normal controls. All participants completed AOSLO imaging of the inner retina. AOSLO montages of the peripapillary area were graded for hyperreflective features including granular membranes, mottled membranes, and nummular features. Regions of interest on AOSLO were compared qualitatively to corresponding optical coherence tomography (OCT) cross sections. OCT was also used to analyze peripapillary retinal nerve fiber layer (RNFL) thickness.

Results

Cognitively impaired individuals had a significantly higher number of granular membranes with a larger overall area compared to controls. The proportion of cognitively impaired individuals with two or more granular membranes was 41.7% compared to none in the control group. Granular membrane area was also inversely correlated with cognitive performance on the Montreal Cognitive Assessment. There was no difference between the two groups in terms of other membrane types or RNFL thickness.

Conclusions

Individuals with early cognitive impairment related to Alzheimer's show hyperreflective granular membranes on high-resolution imaging, which we hypothesize to be manifestations of inner retinal gliosis. The presence of these subtle hyperreflective membranes may obscure underlying RNFL thinning in these eyes on OCT imaging. The distinctive phenotype of granular membranes surrounding the optic nerve on AOSLO may represent a new potential biomarker of early Alzheimer's.

Retinal glial changes in Alzheimer's disease - A review

Alzheimer's disease (AD) is a neurodegenerative dementia characterized by the deposition of extracellular β-amyloid (Aβ) plaques and the presence of neurofibrillary tangles. Until now, the techniques used to analyze these deposits have been difficult to access, invasive, and expensive. This leads us to consider new access routes to the central nervous system (CNS), allowing us to diagnose the disease before the first symptoms appear. Recent studies have shown that microglial and macroglial cell activation could play a role in the development of this disease. Glial cells in the CNS can respond to various damages, such as neurodegenerative pathologies, with morphological and functional changes. These changes are a common feature in neurodegenerative diseases, including AD. The retina is considered an extension of the CNS and has a population of glial cells similar to that of the CNS. When glial cells are activated, various molecules are released and changes in glial cell expression occur, which can be indicators of neuronal damage. The objective of this review is to compile the most relevant findings in the last 10 years relating to alterations in the eye in AD, and the role that glial cells play in the degenerative process in the retina in the context of neurodegeneration.

Age-Related Changes in the Spatial Frequency Threshold of Male and Female 3xTg-AD Mice Using OptoMotry

Visual impairments and retinal abnormalities occur in patients with Alzheimer's disease (AD) and in mouse models of AD. It is important to know the visual ability of mouse models of AD to ensure that age-related cognitive deficits are not confounded by visual impairments. Using OptoMotry, the spatial frequency thresholds of male and female 3xTg-AD mice did not differ from their B6129SF2 wildtype controls between 1-18 months of age, but females had higher spatial frequency thresholds than males. However, the differences were quite small, and the visual ability of all mice was comparable to that of C57BL/6 mice.

The Retina as a Window or Mirror of the Brain Changes Detected in Alzheimer's Disease: Critical Aspects to Unravel

Alzheimer's disease is the most frequent cause of dementia worldwide, representing a global health challenge, with a massive impact on the quality of life of Alzheimer's disease patients and their relatives. The diagnosis of Alzheimer's disease constitutes a real challenge, because the symptoms manifest years after the first degenerative changes occurring in the brain and the diagnosis is based on invasive and/or expensive techniques. Therefore, there is an urgent need to identify new reliable biomarkers to detect Alzheimer's disease at an early stage. Taking into account the evidence for visual deficits in Alzheimer's disease patients, sometimes even before the appearance of the first disease symptoms, and that the retina is an extension of the brain, the concept of the retina as a window to look into the brain or a mirror of the brain has received increasing interest in recent years. However, only a few studies have assessed the changes occurring in the retina and the brain at the same time points. Unlike previous reviews on this subject, which are mainly focused on brain changes, we organized this review by comprehensively summarizing findings related with structural, functional, cellular, and molecular parameters in the retina reported in both Alzheimer's disease patients and animal models. Moreover, we separated the studies that assessed only the retina, and those that assessed both the retina and brain, which are few but allow establishing correlations between the retina and brain. This review also highlights some inconsistent results in the literature as well as relevant missing gaps in this field.

Visual Features in Alzheimer's Disease: From Basic Mechanisms to Clinical Overview

Alzheimer's disease (AD) is the leading cause of dementia worldwide. It compromises patients' daily activities owing to progressive cognitive deterioration, which has elevated direct and indirect costs. Although AD has several risk factors, aging is considered the most important. Unfortunately, clinical diagnosis is usually performed at an advanced disease stage when dementia is established, making implementation of successful therapeutic interventions difficult. Current biomarkers tend to be expensive, insufficient, or invasive, raising the need for novel, improved tools aimed at early disease detection. AD is characterized by brain atrophy due to neuronal and synaptic loss, extracellular amyloid plaques composed of amyloid-beta peptide (Aβ), and neurofibrillary tangles of hyperphosphorylated tau protein. The visual system and central nervous system share many functional components. Thus, it is plausible that damage induced by Aβ, tau, and neuroinflammation may be observed in visual components such as the retina, even at an early disease stage. This underscores the importance of implementing ophthalmological examinations, less invasive and expensive than other biomarkers, as useful measures to assess disease progression and severity in individuals with or at risk of AD. Here, we review functional and morphological changes of the retina and visual pathway in AD from pathophysiological and clinical perspectives.

Ocular amyloid imaging at the crossroad of Alzheimer's disease and age-related macular degeneration: implications for diagnosis and therapy

Alzheimer's disease (AD) and age-related macular degeneration (AMD) are important disorders of aging, but significant challenges remain in diagnosis and therapy. Amyloid-beta (Aβ), found in the brain and a defining feature of AD, has also been observed in the retina in both AD and AMD. While current diagnostic modalities for detecting Aβ in the brain are costly or invasive, Aβ in the retina can be noninvasively and conveniently imaged using modern photonic imaging systems such as optical coherence tomography (OCT). Moreover, since many of these retinal changes occur before degenerative changes can be detected in the brain, ocular amyloid biomarkers could be utilized to detect AD as well as AMD in their earliest stages when therapy may be most effective in halting disease progression. Novel technologies to quantify retinal biomarkers have the potential to facilitate early diagnosis and noninvasive monitoring of disease progression with important therapeutic implications.

Inflammation, neurodegeneration and protein aggregation in the retina as ocular biomarkers for Alzheimer's disease in the 3xTg-AD mouse model

Alzheimer's disease (AD) is the most common cause of dementia in the elderly. In the pathogenesis of AD a pivotal role is played by two neurotoxic proteins that aggregate and accumulate in the central nervous system: amyloid beta and hyper-phosphorylated tau. Accumulation of extracellular amyloid beta plaques and intracellular hyper-phosphorylated tau tangles, and consequent neuronal loss begins 10–15 years before any cognitive impairment. In addition to cognitive and behavioral deficits, sensorial abnormalities have been described in AD patients and in some AD transgenic mouse models. Retina can be considered a simple model of the brain, as some pathological changes and therapeutic strategies from the brain may be observed or applicable to the retina. Here we propose new retinal biomarkers that could anticipate the AD diagnosis and help the beginning and the follow-up of possible future treatments. We analyzed retinal tissue of triple-transgenic AD mouse model (3xTg-AD) for the presence of pathological hallmarks during disease progression. We found the presence of amyloid beta plaques, tau tangles, neurodegeneration, and astrogliosis in the retinal ganglion cell layer of 3xTg-AD mice, already at pre-symptomatic stage. Moreover, retinal microglia in pre-symptomatic mice showed a ramified, anti-inflammatory phenotype which, during disease progression, switches to a pro-inflammatory, less ramified one, becoming neurotoxic. We hypothesize retina as a window through which monitor AD-related neurodegeneration process.

Investigations into Retinal Pathology in the Early Stages of a Mouse Model of Alzheimer's Disease

There is increasing recognition that visual performance is impaired in early stages of Alzheimer’s disease (AD); however, no consensus exists as to the mechanisms underlying this visual dysfunction, in particular regarding the timing, nature, and extent of retinal versus cortical pathology. If retinal pathology presents sufficiently early, it offers great potential as a source of novel biomarkers for disease diagnosis. The current project utilized an array of immunochemical and molecular tools to perform a characterization of retinal pathology in the early stages of disease progression using a well-validated mouse model of AD (APP_SWE/PS1_ΔE9). Analytical endpoints included examination of aberrant amyloid and tau in the retina, quantification of any neuronal degeneration, delineation of cellular stress responses of neurons and particularly glial cells, and investigation of oxidative stress. Brain, eyes, and optic nerves were taken from transgenic and wild-type mice of 3 to 12 months of age and processed for immunohistochemistry, qPCR, or western immunoblotting. The results revealed robust expression of the human APP transgene in the retinas of transgenic mice, but a lack of identifiable retinal pathology during the period when amyloid deposits were dramatically escalating in the brain. We were unable to demonstrate the presence of amyloid plaques, dystrophic neurites, neuronal loss, macro- or micro-gliosis, aberrant cell cycle re-entry, oxidative stress, tau hyperphosphorylation, or upregulations of proinflammatory cytokines or stress signaling molecules in the retina. The overall results do not support the hypothesis that detectable retinal pathology occurs concurrently with escalating amyloid deposition in the brains of APP_SWE/PS1_ΔE9 mice.

Revolution of Alzheimer Precision Neurology. Passageway of Systems Biology and Neurophysiology

The Precision Neurology development process implements systems theory with system biology and neurophysiology in a parallel, bidirectional research path: a combined hypothesis-driven investigation of systems dysfunction within distinct molecular, cellular, and large-scale neural network systems in both animal models as well as through tests for the usefulness of these candidate dynamic systems biomarkers in different diseases and subgroups at different stages of pathophysiological progression. This translational research path is paralleled by an "omics"-based, hypothesis-free, exploratory research pathway, which will collect multimodal data from progressing asymptomatic, preclinical, and clinical neurodegenerative disease (ND) populations, within the wide continuous biological and clinical spectrum of ND, applying high-throughput and high-content technologies combined with powerful computational and statistical modeling tools, aimed at identifying novel dysfunctional systems and predictive marker signatures associated with ND. The goals are to identify common biological denominators or differentiating classifiers across the continuum of ND during detectable stages of pathophysiological progression, characterize systems-based intermediate endophenotypes, validate multi-modal novel diagnostic systems biomarkers, and advance clinical intervention trial designs by utilizing systems-based intermediate endophenotypes and candidate surrogate markers. Achieving these goals is key to the ultimate development of early and effective individualized treatment of ND, such as Alzheimer's disease. The Alzheimer Precision Medicine Initiative (APMI) and cohort program (APMI-CP), as well as the Paris based core of the Sorbonne University Clinical Research Group "Alzheimer Precision Medicine" (GRC-APM) were recently launched to facilitate the passageway from conventional clinical diagnostic and drug development toward breakthrough innovation based on the investigation of the comprehensive biological nature of aging individuals. The APMI movement is gaining momentum to systematically apply both systems neurophysiology and systems biology in exploratory translational neuroscience research on ND.

Retinal amyloid pathology and proof-of-concept imaging trial in Alzheimer's disease

BACKGROUND

Noninvasive detection of Alzheimer's disease (AD) with high specificity and sensitivity can greatly facilitate identification of at-risk populations for earlier, more effective intervention. AD patients exhibit a myriad of retinal pathologies, including hallmark amyloid β-protein (Aβ) deposits.

METHODS

Burden, distribution, cellular layer, and structure of retinal Aβ plaques were analyzed in flat mounts and cross sections of definite AD patients and controls (n = 37). In a proof-of-concept retinal imaging trial (n = 16), amyloid probe curcumin formulation was determined and protocol was established for retinal amyloid imaging in live patients.

RESULTS

Histological examination uncovered classical and neuritic-like Aβ deposits with increased retinal Aβ42 plaques (4.7-fold; P = 0.0063) and neuronal loss (P = 0.0023) in AD patients versus matched controls. Retinal Aβ plaque mirrored brain pathology, especially in the primary visual cortex (P = 0.0097 to P = 0.0018; Pearson's r = 0.84-0.91). Retinal deposits often associated with blood vessels and occurred in hot spot peripheral regions of the superior quadrant and innermost retinal layers. Transmission electron microscopy revealed retinal Aβ assembled into protofibrils and fibrils. Moreover, the ability to image retinal amyloid deposits with solid-lipid curcumin and a modified scanning laser ophthalmoscope was demonstrated in live patients. A fully automated calculation of the retinal amyloid index (RAI), a quantitative measure of increased curcumin fluorescence, was constructed. Analysis of RAI scores showed a 2.1-fold increase in AD patients versus controls (P = 0.0031).

CONCLUSION

The geometric distribution and increased burden of retinal amyloid pathology in AD, together with the feasibility to noninvasively detect discrete retinal amyloid deposits in living patients, may lead to a practical approach for large-scale AD diagnosis and monitoring.

FUNDING

National Institute on Aging award (AG044897) and The Saban and The Marciano Family Foundations.

Subretinal Glial Membranes in Eyes With Geographic Atrophy

Purpose

Müller cells create the external limiting membrane (ELM) by forming junctions with photoreceptor cells. This study evaluated the relationship between focal photoreceptors and RPE loss in geographic atrophy (GA) and Müller cell extension into the subretinal space.

Methods

Human donor eyes with no retinal disease or geographic atrophy (GA) were fixed and the eye cups imaged. The retinal posterior pole was stained for glial fibrillary acidic protein (GFAP; astrocytes and activated Müller cells) and vimentin (Müller cells) while the submacular choroids were labeled with Ulex Europaeus Agglutinin lectin (blood vessels). Choroids and retinas were imaged using a Zeiss 710 confocal microscope. Additional eyes were cryopreserved or processed for transmission electron microscopy (TEM) to better visualize the Müller cells.

Results

Vimentin staining of aged control retinas (n = 4) revealed a panretinal cobblestone-like ELM. While this pattern was also observed in the GA retinas (n = 7), each also had a distinct area in which vimentin⁺ and vimentin⁺/GFAP⁺ processes created a subretinal membrane. Subretinal glial membranes closely matched areas of RPE atrophy in the gross photos. Choroidal vascular loss was also evident in these atrophic areas. Smaller glial projections were noted, which correlated with drusen in gross photos. The presence of glia in the subretinal space was confirmed by TEM and cross cross-section immunohistochemistry.

Conclusions

In eyes with GA, subretinal Müller cell membranes present in areas of RPE atrophy may be a Müller cell attempt to replace the ELM. These membranes could interfere with treatments such as stem cell therapy.

Beta-amyloid sequelae in the eye: a critical review on its diagnostic significance and clinical relevance in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disorder. There is no test for its definitive diagnosis in routine clinical practice. Although phase III clinical trials have failed, only symptomatic treatment is currently available; a possible reason for these failed trials is that intervention commenced at an advanced stage of the disease. The hallmarks of an AD brain include plaques comprising of extracellular beta-amyloid (Aβ) protein aggregates and intracellular hyperphosphorylated neurofibrillary tangles of tau. Research into the preclinical diagnosis of AD has provided considerable evidence regarding early neuropathological changes using brain Aβ imaging and the cerebrospinal fluid biomarkers, Aβ and tau. Both these approaches have limitations that are expensive, invasive or time consuming and thus preclude them from screening at-risk population. Recent studies have demonstrated the presence of Aβ plaques in the eyes of AD subjects, which is positively associated with their brain Aβ burden. Thus ocular biomarkers point to a potential avenue for an earlier, relatively low-cost diagnosis in order for therapeutic interventions to be effective. Here we review the literature that spans the investigation for the presence of Aβ in aging eyes and the significance of its deposition in relation to AD pathology. We discuss clinical studies investigating in vivo imaging of Aβ in the eye and its association with brain Aβ burden and therapies that target ocular Aβ. Finally, we focus on the need to characterize AD-specific retinal Aβ to differentiate Aβ found in some eye diseases. Based on the current evidence, we conclude that integration of ocular biomarkers that can correctly predict brain Aβ burden would have an important role as a non-invasive, yet economical surrogate marker in the diagnostic process of AD.

Ocular indicators of Alzheimer's: exploring disease in the retina

Although historically perceived as a disorder confined to the brain, our understanding of Alzheimer's disease (AD) has expanded to include extra-cerebral manifestation, with mounting evidence of abnormalities in the eye. Among ocular tissues, the retina, a developmental outgrowth of the brain, is marked by an array of pathologies in patients suffering from AD, including nerve fiber layer thinning, degeneration of retinal ganglion cells, and changes to vascular parameters. While the hallmark pathological signs of AD, amyloid β-protein (Aβ) plaques and neurofibrillary tangles (NFT) comprising hyperphosphorylated tau (pTau) protein, have long been described in the brain, identification of these characteristic biomarkers in the retina has only recently been reported. In particular, Aβ deposits were discovered in post-mortem retinas of advanced and early stage cases of AD, in stark contrast to non-AD controls. Subsequent studies have reported elevated Aβ42/40 peptides, morphologically diverse Aβ plaques, and pTau in the retina. In line with the above findings, animal model studies have reported retinal Aβ deposits and tauopathy, often correlated with local inflammation, retinal ganglion cell degeneration, and functional deficits. This review highlights the converging evidence that AD manifests in the eye, especially in the retina, which can be imaged directly and non-invasively. Visual dysfunction in AD patients, traditionally attributed to well-documented cerebral pathology, can now be reexamined as a direct outcome of retinal abnormalities. As we continue to study the disease in the brain, the emerging field of ocular AD warrants further investigation of how the retina may faithfully reflect the neurological disease. Indeed, detection of retinal AD pathology, particularly the early presenting amyloid biomarkers, using advanced high-resolution imaging techniques may allow large-scale screening and monitoring of at-risk populations.

Immune response in the eye following epileptic seizures

BACKGROUND

Epileptic seizures are associated with an immune response in the brain. However, it is not known whether it can extend to remote areas of the brain, such as the eyes. Hence, we investigated whether epileptic seizures induce inflammation in the retina.

METHODS

Adult rats underwent electrically induced temporal status epilepticus, and the eyes were studied 6 h, 1, and 7 weeks later with biochemical and immunohistochemical analyses. An additional group of animals received CX3CR1 antibody intracerebroventricularly for 6 weeks after status epilepticus.

RESULTS

Biochemical analyses and immunohistochemistry revealed no increased cell death and unaltered expression of several immune-related cytokines and chemokines as well as no microglial activation, 6 h post-status epilepticus compared to non-stimulated controls. At 1 week, again, retinal cytoarchitecture appeared normal and there was no cell death or micro- or macroglial reaction, apart from a small decrease in interleukin-10. However, at 7 weeks, even if the cytoarchitecture remained normal and no ongoing cell death was detected, the numbers of microglia were increased ipsi- and contralateral to the epileptic focus. The microglia remained within the synaptic layers but often in clusters and with more processes extending into the outer nuclear layer. Morphological analyses revealed a decrease in surveying and an increase in activated microglia. In addition, increased levels of the chemokine KC/GRO and cytokine interleukin-1β were found. Furthermore, macroglial activation was noted in the inner retina. No alterations in numbers of phagocytic cells, infiltrating macrophages, or vascular pericytes were observed. Post-synaptic density-95 cluster intensity was reduced in the outer nuclear layer, reflecting seizure-induced synaptic changes without disrupted cytoarchitecture in areas with increased microglial activation. The retinal gliosis was decreased by a CX3CR1 immune modulation known to reduce gliosis within epileptic foci, suggesting a common immunological reaction.

CONCLUSIONS

Our results are the first evidence that epileptic seizures induce an immune response in the retina. It has a potential to become a novel non-invasive tool for detecting brain inflammation through the eyes.

Visual and Ocular Manifestations of Alzheimer's Disease and Their Use as Biomarkers for Diagnosis and Progression

Alzheimer's disease (AD) is the most common form of dementia affecting the growing aging population today, with prevalence expected to rise over the next 35 years. Clinically, patients exhibit a progressive decline in cognition, memory, and social functioning due to deposition of amyloid β (Aβ) protein and intracellular hyperphosphorylated tau protein. These pathological hallmarks of AD are measured either through neuroimaging, cerebrospinal fluid analysis, or diagnosed post-mortem. Importantly, neuropathological progression occurs in the eye as well as the brain, and multiple visual changes have been noted in both human and animal models of AD. The eye offers itself as a transparent medium to cerebral pathology and has thus potentiated the development of ocular biomarkers for AD. The use of non-invasive screening, such as retinal imaging and visual testing, may enable earlier diagnosis in the clinical setting, minimizing invasive and expensive investigations. It also potentially improves disease management and quality of life for AD patients, as an earlier diagnosis allows initiation of medication and treatment. In this review, we explore the evidence surrounding ocular changes in AD and consider the biomarkers currently in development for early diagnosis.

Increased CSF tau level is correlated with decreased lamina cribrosa thickness

Background

This study was to investigate whether the previously proposed link between Alzheimer’s disease (AD) and decreased retinal nerve fiber layer thickness could be explained by the relationship between abnormal CSF profiles and optic nerve head characteristics, focusing on the influence of CSF tau protein on the lamina cribrosa (LC) thickness (LCT).

Methods

A total of 44 eyes from 18 patients with AD and 26 healthy subjects were subjected to enhanced-depth-imaging volume scanning of the optic nerve using spectral-domain optical coherence tomography. The B-scan images were constructed three-dimensionally using maximum intensity projection (MIP), and the LCT was measured at three locations (superior midperipheral, midhorizontal, and inferior midperipheral) using the thin-slab MIP images. CSF levels of amyloid β 1-42 peptide, (Aβ_1–42), total tau (T-tau) and tau phosphorylated at threonine 181 (P-tau_181P) were measured from CSF samples of each subject. The relationship between the level of CSF proteins and the LCT was determined using linear regression and fractional polynomial analyses.

Results

Univariate regression analysis revealed that higher CSF levels of T-tau (P = 0.004) and P-tau_181P (P = 0.027), as well as a smaller central corneal thickness (P = 0.032), were significantly associated with a smaller LCT. Multivariate analysis indicated that only CSF T-tau (P = 0.041) was significantly associated with the LCT. The relationship was well explained by both linear regression (R² = 0.179, P = 0.004) and fractional polynomial analysis (R² = 0.275, P = 0.001). When we performed an assessment by linear regression with an indicator, the relationship was significant both in the healthy and AD groups, with a stronger correlation found in the healthy group (regression coefficients = -1.098 vs. -0.280, P = 0.018).

Conclusions

An increased CSF level of T-tau was significantly associated with a thinner LCT in both the healthy and AD groups. This result suggests that LCT could serve as a potential non-invasive indicator for increased CSF tau. The clinical meaning of the higher level of CSF T-tau in axonal degeneration of the optic nerve remains to be determined.

Glia-Mediated Retinal Neuroinflammation as a Biomarker in Alzheimer's Disease

Alzheimer's disease (AD) is the most common type of dementia worldwide; it is characterized by a progressive decline in cognitive functions and memory, resulting from synaptic and cell loss, and accompanied by a strong neuroinflammatory response. Besides the vast progress in the understanding of the pathophysiology of AD in the past decades, there is still no effective treatment. Moreover, the diagnosis occurs usually at an advanced stage of the disease, where the neurological damage has already occurred. The identification of biomarkers that would allow an early diagnosis of this disease is a major goal that would also help managing AD progression. Due to its cellular and physiological resemblances with the brain, the retina has long been regarded as a window to the brain. Several brain manifestations have been associated with retinal alterations. In AD patients, some structural and functional alterations in the retina can be associated with disease onset. However, only a few studies have focused on the alterations in retinal glial cells associated with AD. This review aims at giving an overview of the AD-associated retinal alterations, particularly in glial cells. The documented alterations in retinal glia will be discussed concerning their potential to predict the brain alterations occurring in AD.

Alzheimer's Disease-Related Protein Expression in the Retina of Octodon degus

New studies show that the retina also undergoes pathological changes during the development of Alzheimer's disease (AD). While transgenic mouse models used in these previous studies have offered insight into this phenomenon, they do not model human sporadic AD, which is the most common form. Recently, the Octodon degus has been established as a sporadic model of AD. Degus display age-related cognitive impairment associated with Aβ aggregates and phosphorylated tau in the brain. Our aim for this study was to examine the expression of AD-related proteins in young, adult and old degus retina using enzyme-linked or fluorescence immunohistochemistry and to quantify the expression using slot blot and western blot assays. Aβ4G8 and Aβ6E10 detected Aβ peptides in some of the young animals but the expression was higher in the adults. Aβ peptides were observed in the inner and outer segment of the photoreceptors, the nerve fiber layer (NFL) and ganglion cell layer (GCL). Expression was higher in the central retinal region than in the retinal periphery. Using an anti-oligomer antibody we detected Aβ oligomer expression in the young, adult and old retina. Immunohistochemical labeling showed small discrete labeling of oligomers in the GCL that did not resemble plaques. Congo red staining did not result in green birefringence in any of the animals analyzed except for one old (84 months) animal. We also investigated expression of tau and phosphorylated tau. Expression was seen at all ages studied and in adults it was more consistently observed in the NFL-GCL. Hyperphosphorylated tau detected with AT8 antibody was significantly higher in the adult retina and it was localized to the GCL. We confirm for the first time that Aβ peptides and phosphorylated tau are expressed in the retina of degus. This is consistent with the proposal that AD biomarkers are present in the eye.

Reduction of amyloid-beta levels in mouse eye tissues by intra-vitreally delivered neprilysin

Amyloid-beta (Aβ) is a group of aggregation-prone, 38- to 43-amino acid peptides generated in the eye and other organs. Numerous studies suggest that the excessive build-up of low-molecular-weight soluble oligomers of Aβ plays a role in the progression of Alzheimer's disease and other brain degenerative diseases. Recent studies raise the hypothesis that excessive Aβ levels may contribute also to certain retinal degenerative diseases. These findings, together with evidence that a major portion of Aβ is released as monomer into the extracellular space, raise the possibility that a technology enabling the enzymatic break-down of monomeric Aβ in the living eye under physiological conditions could prove useful for research on ocular Aβ physiology and, perhaps ultimately, for therapeutic applications. Neprilysin (NEP), an endopeptidase known to cleave Aβ monomer into inactive products, is a membrane-associated protein. However, sNEP, a recombinant form of the NEP catalytic domain, is soluble in aqueous medium. With the aim of determining the Aβ-cleaving activity of exogenous sNEP in the microenvironment of the intact eye, we analyzed the effect of intra-vitreally delivered sNEP on ocular Aβ levels in mice that exhibit readily measurable, aqueous buffer-extractable Aβ40 and Aβ42, two principal forms of Aβ. Anesthetized 10-month wild-type (C57BL/6J) and 2-3-month 5XFAD transgenic mice received intra-vitreal injections of sNEP (0.004-10 μg) in one eye and were sacrificed at defined post-treatment times (30 min - 12 weeks). Eye tissues (combined lens, vitreous, retina, RPE and choroid) were homogenized in phosphate-buffered saline, and analyzed for Aβ40 and Aβ42 (ELISA) and for total protein (Bradford assay). The fellow, untreated eye of each mouse served as control, and concentrations of Aβ (pmol/g protein) in the treated eye were normalized to that of the untreated control eye. In C57BL/6J mice, as measured at 2 h after sNEP treatment, increasing amounts of injected sNEP yielded progressively greater reductions of Aβ40, ranging from 12% ± 3% (mean ± SEM; n = 3) with 4 ng sNEP to 85% ± 13% (n = 5) with 10 μg sNEP. At 4 ng sNEP the average Aβ40 reduction reached >70% by 24 h following treatment and remained near this level for about 8 weeks. In 5XFAD mice, 10 μg sNEP produced an Aβ40 decrease of 99% ± 1% (n = 4) and a substantial although smaller decrease in Aβ42 (42% ± 36%; n = 4) within 24 h. Electroretinograms (ERGs) were recorded from eyes of C57BL/6J and 5XFAD mice at 9 days following treatment with 4 ng or 10 μg sNEP, conditions that on average led, respectively, to an 82% and 91% Aβ40 reduction in C57BL/6J eyes, an 87% and 92% Aβ40 reduction in 5XFAD eyes, and a 23% and 52% Aβ42 reduction in 5XFAD eyes. In all cases, sNEP-treated eyes exhibited robust ERG responses, consistent with a general tolerance of the posterior eye tissues to the investigated conditions of sNEP treatment. The sNEP-mediated decrease of ocular Aβ levels reported here represents a possible approach for determining effects of Aβ reduction in normally functioning eyes and in models of retinal degenerative disease.