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UNDERSTANDING SYNAPTIC INTEGRITY IN ALZHEIMER’S BRAIN WITH UCB-J
Abstract
Aims
UCB-J is a recently developed PET-tracer to study synaptic integrity in different neurological disorders. Though several UCB-J in vivo PET studies have been published in AD and other neuropsychological disorders, 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 in vitro characterization of UCB-J in postmortem AD brains.
Methods
We performed in vitro radioligand binding assays (saturation, competition, and regional distribution) alongside large brain section autoradiography in AD and control brains.
Results
3H-UCB-J saturation studies showed much higher specific binding (above 90%) in synaptosomal membrane P2-fraction than brain homogenates prepared from the same frontal cortex (FC) region of AD and control brains. The specific binding was in nanomolar-range (Kd ~ 5.0 nM) and clearly highlighted the loss of UCB-J binding in FC of AD brain compared to control (33% decrease in Bmax vs. control). This was very interesting as UCB-J showed almost 1.8-fold higher binding in FC of AD brain than control when brain homogenates were used. Competition studies with unlabelled-UCB-J showed one-binding site with nanomolar-affinity in FC of AD and control brains, complementing our saturation-data. In contrast to P2-fraction data, autoradiography with 3H-UCB-J showed unexpectedly higher binding in AD than control brain.
Conclusions
Our in vitro findings indicated off-target binding of UCB-J, if P2-fraction is not used. The ongoing regional distribution and other experiments will provide further understanding of UCB-J binding mechanism.