Alzheimer’s-Related Amyloid Beta Peptide Aggregates in the Ageing Retina: Implications for Sight Loss and Dementia

Although visual problems are reported by patients with Alzheimer’s disease and dementia, studies into this particular aspect of neuropathology are scarce. The growing awareness of complex pathological processes in the ageing retina and brain, however, enables us to consider this from a new perspective. Here we discuss the latest findings on the wide-ranging visual defects experienced by those suffering from Alzheimer’s disease and dementia. We propose that events leading to chronic degeneration of the retina and the brain in fact share many striking similarities. In particular, we discuss the role of the Alzheimer’s-related amyloid beta (Aβ) group of peptides that has been shown to accumulate in senescent retinas, correlated with increased risk of retinal degeneration. The high photo-oxidative retinal environment creates ideal conditions for Aβ aggregation, evidenced by high Aβ loads reported in aged and donor eyes from patients with age-related macular degeneration. Consequently, longitudinal and non-invasive retinal assessments may provide invaluable information on incipient pathology and disease progression in the retina as well as the senescent brain. Such insights may not only lead to identifying new pathogenic mechanisms in the retina with implications for understanding Alzheimer’s disease but reveal the underlying causes of visual abnormalities reported in patients with dementia

Elevated amyloid beta disrupts the nanoscale organization and function of synaptic vesicle pools in hippocampal neurons

Alzheimer’s disease is linked to increased levels of amyloid beta (Aβ) in the brain, but the mechanisms underlying neuronal dysfunction and neurodegeneration remain enigmatic. Here, we investigate whether organizational characteristics of functional presynaptic vesicle pools, key determinants of information transmission in the central nervous system, are targets for elevated Aβ. Using an optical readout method in cultured hippocampal neurons, we show that acute Aβ42 treatment significantly enlarges the fraction of functional vesicles at individual terminals. We observe the same effect in a chronically elevated Aβ transgenic model (APPSw,Ind) using an ultrastructure-function approach that provides detailed information on nanoscale vesicle pool positioning. Strikingly, elevated Aβ is correlated with excessive accumulation of recycled vesicles near putative endocytic sites, which is consistent with deficits in vesicle retrieval pathways. Using the glutamate reporter, iGluSnFR, we show that there are parallel functional consequences, where ongoing information signaling capacity is constrained. Treatment with levetiracetam, an antiepileptic that dampens synaptic hyperactivity, partially rescues these transmission defects. Our findings implicate organizational and dynamic features of functional vesicle pools as targets in Aβ-driven synaptic impairment, suggesting that interventions to relieve the overloading of vesicle retrieval pathways might have promising therapeutic value

Proteinopathy in the retinal pigment epithelium (RPE): implications for sight-loss in old age

Damage to the retinal pigment epithelium (RPE), which maintains overlying photoreceptors in the retina, is linked with irreversible sight-loss including age-related macular degeneration (AMD), the most common cause of blindness in developed societies. RPE cells internalize and degrade photoreceptor outer segments (POS) as part of the daily photoreceptor renewal, which subjects the RPE to the highest proteolytic burden in the body. However, age and onset of retinopathy correlates with partially degraded POS, which accumulate as lipofuscin and related molecules, constituting a clinically well-documented pathway of RPE death in geographic atrophy AMD that has no effective treatment. To elucidate its molecular mechanisms, we exploited an in-vitro RPE model, which structurally and physiologically recapitulates the native RPE monolayer. Disease conditions linked with AMD including oxidative stress and impaired autophagy were recapitulated using 100m H2O2, oxidatively-modified POS (OxPOS) or 10nM bafilomycin A1, respectively. We also studied effects of the Alzheimer’s-related amyloid beta (A) proteins, which accumulate in aged/AMD retinas. Confocal-immunofluorescence studies combined with ultrastructural imaging revealed the fate of trafficked POS in the phagosome and autophagy-lysosomal pathways, which, depending on the insult, became sequestered in early compartments or were trafficked prematurely to lysosomes. A proportion of POS were also prematurely targeted to autophagy bodies. OxPOS accumulated in late compartments generating increased autofluorescence, which recapitulates aged/damaged RPE in patients. A rapidly accumulated in lysosomes, which, unlike POS cargos, RPE cells were unable to degrade effectively. Our findings revealed contrasting molecular mechanisms underpinning proteinopathy in the RPE that could be manipulated to develop future treatments

Serial block face scanning electron microscopy reveals novel organizational details of the retinal pigment epithelium

Advances in imaging have led to the development of several new types of microscopes such as serial block face scanning electron microscopy (SBF-SEM), lightsheet microscopy, as well as X-ray micro-computed tomography (micro-CT), which enables the study of samples in fundamentally different ways. Significantly, these are now commercially available, which facilitates their widespread use in research. With SBF-SEM, fixed and resin-embedded specimens can be serially sectioned and imaged to construct a 3D dataset of the ultrastructure of cells and tissues at high resolution. We used this technique on perfusion-fixed C57BL/6 mouse eyes to image the outer retina. Our findings revealed novel organizational details of the retinal pigment epithelium (RPE) (Keeling et al., 2020b); a specialized cell monolayer that maintains the overlying photoreceptors and also forms the outer blood-retinal-barrier. RPE cells were found to look after far more photoreceptors than was widely assumed. 3D-data enabled measurements of the RPE cytoplasmic and nuclear volumes, the length and angle of microvilli on the apical RPE surface, as well as sub-RPE spaces under the basolateral membrane. The study also compared between mono-nucleate vs. bi-nucleate RPE cells, whilst the use of computing microinstructions (macros) provided information on interactions between adjacent cells in the RPE monolayer. Analysis of SBF-SEM stacks showed several hundred mitochondria which were rendered in 3D, providing information on their volume and spatial distribution in healthy RPE. Mitochondria were found in varying shapes and sizes, and predominantly localized to the mid and basal-zones of cells. The capabilities of SBF-SEM alongside other imaging techniques are being increasingly harnessed by investigators to gain novel insights into the organization of cells and tissues in the eye. These findings also help improve the current understanding of pathology linked with common blinding conditions such as age-related macular degeneration (AMD), as well as rare forms of retinopathy which leads to irreversible sight-los

Impaired cargo clearance in the Retinal Pigment Epithelium (RPE) underlies irreversible blinding diseases

Chronic degeneration of the Retinal Pigment Epithelium (RPE) is a precursor to pathological changes in the outer retina. The RPE monolayer, which lies beneath the neuroretina, daily internalises and digests large volumes of spent photoreceptor outer segments. Impaired cargo handling and processing in the endocytic/phagosome and autophagy pathways lead to the accumulation of lipofuscin and pyridinium bis-retinoid A2E aggregates and chemically modified compounds such as malondialdehyde and 4-hydroxynonenal within RPE. These contribute to increased proteolytic and oxidative stress, resulting in irreversible damage to post-mitotic RPE cells and development of blinding conditions such as age-related macular degeneration, Stargardt disease and choroideremia. Here, we review how impaired cargo handling in the RPE results in their dysfunction, discuss new findings from our laboratory and consider how newly discovered roles for lysosomes and the autophagy pathway could provide insights into retinopathies. Studies of these dynamic, molecular events have also been spurred on by recent advances in optics and imaging technology. Mechanisms underpinning lysosomal impairment in other degenerative conditions including storage disorders, α-synuclein pathologies and Alzheimer’s disease are also discussed. Collectively, these findings help transcend conventional understanding of these intracellular compartments as simple waste disposal bags to bring about a paradigm shift in the way lysosomes are perceived

Challenges in studying geographic atrophy (GA) age-related macular degeneration: the potential of a new mouse model with GA-like features

Loss of central vision critical to everyday activities such as reading, face-recognition and driving due to damage in the central retina (the macula) is the leading cause of irreversible blindness amongst adults in the developed world. This condition, termed age-related macular degeneration (AMD), is a complex, chronic degenerative disease driven by a combination of genetic and lifestyle risk factors. Early signs of retinal changes in people as young as 30-40 years have been reported, although these individuals appear to be asymptomatic. However, by the age of 65 the disease is present in ~3% of individuals, which increases dramatically to affect 1/3 of individuals by the eighth decade of life. Early to intermediate AMD is estimated to affect ~150 million individuals globally, with another 10 million individuals suffering from end-stage, sight-threatening forms. These terminal stages are broadly grouped into dry (GA: geographic atrophy) or wet (CNV: choroidal neovascular) AMD (Sarks et al., 1988; Bird et al., 2014), with similar frequencies reported in patients. Recent advances in identifying genetic risk factors, including our discoveries in this field, indicate an initial shared pathology before progressing to aforementioned late-stage phenotypes. Currently, GA patients have no effective treatment, which may in part be due to the lack of good in-vivo models for GA studies. Here, we summarise our new findings that describe an altogether new mouse model with GA-like features which shows progressive outer retinal pathology (Ibbett et al., 2019) that can be used to gain novel insights into GA and potentially as a tool for drug development