Noninvasive monitoring of β-amyloid (Aβ) plaques, the neuropathological hallmarks of Alzheimer's disease (AD), is critical for AD diagnosis and prognosis. Current visualization of Aβ plaques in brains of live patients and animal models is limited in specificity and resolution. The retina as an extension of the brain portrays an appealing target for a live, noninvasive optical imaging of AD if disease pathology is manifested there. We identified retinal Aβ plaques in postmortem eyes from AD patients (n=8) and in suspected early stage cases (n=5), consistent with brain pathology and clinical reports; plaques were undetectable in age-matched non-AD individuals (n=5). In APPSWE/PS1ΔE9 transgenic mice (AD-Tg; n=18) and not in non-Tg wt mice (n=10), retinal Aβ plaques were detected following systemic administration of curcumin, a safe plaque-labeling fluorochrome. Moreover, retinal plaques were detectable earlier than in the brain and accumulated with disease progression. An immune-based therapy effective in reducing brain plaques, significantly reduced retinal Aβ plaque burden in immunized versus non-immunized AD mice (n=4 mice per group). In live AD-Tg mice (n=24), systemic administration of curcumin allowed noninvasive optical imaging of retinal Aβ plaques in vivo with high resolution and specificity; plaques were undetectable in non-Tg wt mice (n=11). Our discovery of Aβ specific plaques in retinas from AD patients, and the ability to noninvasively detect individual retinal plaques in live AD mice establish the basis for developing high resolution optical imaging for early AD diagnosis, prognosis assessment and response to therapies.

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 Aβ 42 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.

ObjectiveMelanopsin retinal ganglion cells (mRGCs) are photoreceptors driving circadian photoentrainment, and circadian dysfunction characterizes Alzheimer disease (AD). We investigated mRGCs in AD, hypothesizing that they contribute to circadian dysfunction.MethodsWe assessed retinal nerve fiber layer (RNFL) thickness by optical coherence tomography (OCT) in 21 mild‐moderate AD patients, and in a subgroup of 16 we evaluated rest–activity circadian rhythm by actigraphy. We studied postmortem mRGCs by immunohistochemistry in retinas, and axons in optic nerve cross‐sections of 14 neuropathologically confirmed AD patients. We coimmunostained for retinal amyloid β (Aβ) deposition and melanopsin to locate mRGCs. All AD cohorts were compared with age‐matched controls.ResultsWe demonstrated an age‐related optic neuropathy in AD by OCT, with a significant reduction of RNFL thickness (p = 0.038), more evident in the superior quadrant (p = 0.006). Axonal loss was confirmed in postmortem AD optic nerves. Abnormal circadian function characterized only a subgroup of AD patients. Sleep efficiency was significantly reduced in AD patients (p = 0.001). We also found a significant loss of mRGCs in postmortem AD retinal specimens (p = 0.003) across all ages and abnormal mRGC dendritic morphology and size (p = 0.003). In flat‐mounted AD retinas, Aβ accumulation was remarkably evident inside and around mRGCs.InterpretationWe show variable degrees of rest–activity circadian dysfunction in AD patients. We also demonstrate age‐related loss of optic nerve axons and specifically mRGC loss and pathology in postmortem AD retinal specimens, associated with Aβ deposition. These results all support the concept that mRGC degeneration is a contributor to circadian rhythm dysfunction in AD. ANN NEUROL 2016;79:90–109

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.

Pericyte loss and deficient vascular platelet-derived growth factor receptor-β (PDGFRβ) signaling are prominent features of the blood-brain barrier breakdown described in Alzheimer's disease (AD) that can predict cognitive decline yet have never been studied in the retina. Recent reports using noninvasive retinal amyloid imaging, optical coherence tomography angiography, and histological examinations support the existence of vascular-structural abnormalities and vascular amyloid β-protein (Aβ) deposits in retinas of AD patients. However, the cellular and molecular mechanisms of such retinal vascular pathology were not previously explored. Here, by modifying a method of enzymatically clearing non-vascular retinal tissue and fluorescent immunolabeling of the isolated blood vessel network, we identified substantial pericyte loss together with significant Aβ deposition in retinal microvasculature and pericytes in AD. Evaluation of postmortem retinas from a cohort of 56 human donors revealed an early and progressive decrease in vascular PDGFRβ in mild cognitive impairment (MCI) and AD compared to cognitively normal controls. Retinal PDGFRβ loss significantly associated with increased retinal vascular Aβ 40 and Aβ 42 burden. Decreased vascular LRP-1 and early apoptosis of pericytes in AD retina were also detected. Mapping of PDGFRβ and Aβ 40 levels in pre-defined retinal subregions indicated that certain geometrical and cellular layers are more susceptible to AD pathology. Further, correlations were identified between retinal vascular abnormalities and cerebral Aβ burden, cerebral amyloid angiopathy (CAA), and clinical status. Overall, the identification of pericyte and PDGFRβ loss accompanying increased vascular amyloidosis in Alzheimer's retina implies compromised blood-retinal barrier integrity and provides new targets for AD diagnosis and therapy.