Understanding synaptic loss in PSP brains with UCB‐J

Abstract

AbstractBackgroundProgressive supranuclear palsy (PSP) is a 4R‐tauopathy causing problems with balance, movement, vision, speech, and swallowing. There is a lack of biomarkers for PSP, which often leads to misdiagnosis, prescription of incorrect pharmacological treatments and likely underdiagnosis. Synaptic vesicle protein 2A (SV2A) PET‐tracer UCB‐J has shown great promise in imaging synaptic loss in many neurodegenerative diseases in vivo including PSP. UCB‐J‐PET imaging holds great potential for the early and accurate diagnosis of PSP, however, there is a substantial gap in knowledge around the extent of synaptic loss in PSP. Hence, it is of utmost importance to investigate synaptic changes in PSP brains with UCB‐J and to establish its correlation with other synaptic and neuropathological markers to gain a deeper understanding of underlying mechanisms for future biomarker validation.MethodWe performed specialized postmortem brain imaging (small and large frozen brain autoradiography) and radioligand binding assays (saturation, competition, and regional distribution), alongside immunohistochemistry and biochemical analyses in PSP and control (CN) brains.ResultSaturation studies in brain homogenates (BH) from the frontal cortex (FC) showed lower specific binding of 3H‐UCB‐J in PSP (Bmax: 348‐409 fmol/mg, Kd∼2.7nM) as compared to CN brains (Bmax:0.4‐0.5 pmol/mg, Kd∼3.5nM), indicating a loss in SV2A. Importantly, PSP brains showed much higher 3H‐UCB‐J specific binding in nuclear membrane P1‐fractions (Bmax 18.7‐22.1 pmol/mg, Kd ∼3.0nM) and synaptosomal‐membrane P2‐fractions (Bmax: 10.3‐12.3 pmol/mg, Kd∼2.4nM) as compared to BH (see above). Small section autoradiography showed lower 3H‐UCB‐J specific binding in PSP FC, but unexpectedly higher average specific binding in globus pallidus as compared to CN, suggesting regional differences and potential interaction with other brain tissue components/proteins.ConclusionOur findings clearly demonstrate that there are region‐specific changes in UCB‐J binding in PSP brains compared to CN and further reflect the need for its in‐depth validation in postmortem brains and correlation with other synaptic and neuropathological markers. Moreover, P2 fraction is most optimal for reflecting synaptic loss with UCB‐J. The ongoing regional distribution, biochemical and immunostaining studies will help elucidate changes in synaptic integrity in PSP.