Novel Early Retinal Imaging Biomarkers for Treating Later Spatial Memory Loss in Experimental Alzheimer's Disease

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Therapeutically delaying the progressive decline in cognition in patients with Alzheimer’s disease (AD) would transform AD into a manageable morbidity, a goal that has not been achieved using drugs targeted to β-amyloid (Aβ) plaque deposition. Accumulating results indicate that cognitive loss (linked to circuit / synaptic dysfunction) and β-amyloid (Aβ) plaque deposition can occur independent of each other, with both driven by a cross-linked soluble amyloid β-peptide oligomer - neuronal hyperactivity “AD cycle”. Remarkably, the prediction that cognitive dysfunction can be restored without altering plaque deposition has been confirmed in several AD models, for example, by drugs that prolong the opening time of the endoplasmic reticulum (ER) ryanodine receptor type 2 (RyR2) calcium channel and suppress neuronal hyperactivity. Conventional biomarkers are unable to interrogate either part of the “AD cycle” in patients at cellular resolution, an unmet goal for evaluating treatment efficacy at the prodromal stage. Here, we propose a novel solution to this problem based on the retina, a readily accessible part of the nervous system with damage similar to that found in the brain of patients with AD. The retina develops soluble amyloid β-peptide oligomers and plaque deposition before their appearance in the brain, as well as phosphorylated tau and neurofibrillary tangles. Before overt AD pathology and cognitive decline are evident, patients report impaired contrast sensitivity (CS), a major risk factor for falls as well as decreased survival. CS is driven by photoreceptors. Our first-in-kind preliminary results in an AD model when there is sparse plaque deposition in the retina show early impairment of CS, and rod hyperactivity measured using three OCT mitochondria-driven biomarkers developed in our laboratory. We have also discovered that CS impairment and rod hyperactivity biomarkers in 5xFAD male C57BL6/J (B6J) mice occur faster than in 5xFAD male C57BL/6Tac (B6NTac) mice. In WT male B6J mice, rods showed a lower OCT energy signature than in age-matched WT male B6NTac mice, indicating strain differences in baseline mitochondria activity. We propose to test two working hypotheses with three Specific Aims. First, that impaired CS, a hyperactive rod energy signature, and/or synaptic dysfunction occur earlier B6J 5xFAD mice than in B6NTac 5xFAD mice. Second that in 5xFAD mice, RyR2-targeted treatments that delay cognitive declines mitigate changes in early CS and energy biomarkers, declines in rod synaptic activity, and later spatial memory deficits but do not change the rate of plaque deposition.