Assessing synaptic integrity in Alzheimer’s brain with UCB‐J

Abstract

AbstractBackgroundSynaptic loss is an eminent feature of Alzheimer’s disease (AD). Synapses critical for neuronal transmission are regulated by synaptic vesicles (SV), which expresses an integral transmembrane protein called SV protein 2A (SV2A) throughout the brain. Notably, reduced SV2A expression has been implicated in AD and other proteinopathies. UCB‐J is a recently developed PET‐tracer, which target SV2A, to study synaptic integrity in different neurological disorders. Though several UCB‐J in vivo PET studies have been published in AD, there have been some discrepancies between the findings. Many of these studies indicated potential correlations between tau deposition, atrophy, cognitive‐impairment, and loss of UCB‐J binding. Based on these in vivo observations, we found it of utmost relevance to perform extensive pre‐clinical validation of UCB‐J in AD brains with postmortem brain imaging techniques.MethodWe performed radioligand binding assays (saturation and competition) alongside large brain section autoradiography in AD and control brains.Result 3H‐UCB‐J competition studies with unlabelled‐UCB‐J showed one‐binding site (IC50: ∼ 10 nM) in the frontal cortex (FC) of AD and control brains. Saturation studies in FC region of AD and control brains showed much higher specific binding in synaptosomal membrane P2‐fraction (Bmax: 5.8 ‐ 8.7 pmol/mg, Kd ∼ 5.0 nM) as compared to brain homogenates (Bmax: 0.09 ‐ 0.18 pmol/mg, Kd ∼ 4.0 nM). P2‐fraction saturation binding study clearly highlighted the loss of UCB‐J binding in FC of AD brain as compared to control brain. In contrast, large brain hemisphere section autoradiography incubated with 3H‐UCB‐J showed unexpectedly higher regional binding in AD brain sections as compared to control, motivating further mechanistic pre‐clinical validation with other synaptic markers in AD and other proteinopathies.ConclusionOur findings explicitly demonstrated high specificity of UCB‐J for SV2A in AD and control brains. UCB‐J specific binding was much higher in P2‐fraction than brain homogenate (P2‐fraction > brain homogenate) and highlighted the expected loss of UCB‐J binding in AD brain. Competition and saturation binding studies complemented each other and showed only one‐binding site with nM‐affinity in AD and control brains. Unanticipated high UCB‐J binding in AD brain in autoradiography studies indicate potential interaction with other brain tissue component, warrant further investigation.