Measurement of protein synthesis rate and synaptic density by using [11C]leucine and [18F]UCB-H PET tracers respectively in a model for Alzheimer’s disease, rat TgF344-AD

Abstract

Introduction: Memory alterations are one of the main hallmarks of Alzheimer's disease (AD). Long-term memory storage requires the synthesis of new proteins and synaptic modifications. The aims of this PhD study were hence to (i) investigate the best approach to use [11C]leucine to measure the cerebral protein synthesis rate (PSR) in-vivo, (ii) evaluate the PSR in an animal model for AD, the TgF344-AD rats, and (iii) investigate synaptic density with [18F]UCB-H in the same model compared to wild type (WT). In parallel behavioural tests were performed in TG and WT. Methods: Wistar and 6, 12 and 18 months old TG and WT rats were scanned for 60 min with [11C]leucine with a Siemens Inveon PET-CT. Arterial blood activity was monitored online (Twilite) and in discrete blood samples (Wistar rats, n=11; WT, n=4; TG, n=5) by gamma-counting of whole blood and plasma and cold AA quantification in plasma. The sensitivity of [11C]leucine PET was assessed in Wistar rats by injection of PSR inhibitor anisomycin (60mg/kg) 10min prior to PET acquisition. A different cohort of 7 and 15 months old of age WT and TG rats were scanned with [18F]UCB-H (baseline scans). Non-specific binding was measured in additional rats by injecting cold UCB-J (1mg/kg) 10min prior to [18F]UCB-H injection (block scans). Data were expressed as normalised uptake values (NUVND) of the ratio between the 9-20 minutes SUV of the baseline over the blocked (non-displaceable SUVUCB-J) SUV for each ROI. For both [11C]leucine and [18F]UCB-H studies, PET images were quantified in BrainVisa and Anatomist software using 28 brain ROIs based on a MRI rat brain atlas. Open field (OF), Morris water maze (MWM) and reversal Morris water maze (RMWM) were all performed at 6, 12 and 18 months of age. Smell tests (ST) were performed only at 12 and 18 months of age. All behavioural tests were analysed using the ANYMAZE software. All data were analysed by using GraphPad Prism 8.4.1. Results: [11C]leucine study: the concentration of unlabelled leucine in plasma was statistically different between arterial and venous samples and between strains (Wistar vs. Fischer-344), with no significant difference found between WT and TG. Anisomycin administration significantly reduced the net uptake rate constant (Kcplx) of [11C]leucine and PSR (from -77% to -89%) proving the sensitivity of the method to this acute pharmacological inhibition of PSR. For the longitudinal study, averaged population-based whole-blood and plasma input function (IF) and cold leucine concentration were used to calculate Kcplx and PSR respectively. Kcplx, but not PSR, was significantly decreased in TG (genotype effect in a mixed-analysis model) in the hippocampus (p=0.0442), whole brain (p=0.0448), globus pallidus (p=0.0325) and the caudate putamen (p=0.0423). In the globus pallidus Kcplx and PSR were significantly reduced (-15%, p=0.0427 and p=0.0488 respectively, unpaired t-test) in TG vs. WT at 18 months of age. [18F]UCB-H study: pre-saturation with UCB-J blocked up to 70% of the specific binding signal and removed regional differences. Due to the significant variation of non-specific binding with age and genotype, a population-based SUVUCB-J was used to calculate the NUVND for each group. TG rats had a significantly lower NUVND than WT at 7m of age in the frontal (-17%, p=0.0028) and motor cortex (-16%, p=0.0066), hippocampus (-12%, p=0.0256), thalamus (-13%, p=0.0202), cerebellum ( 17%, p=0.0012), caudate putamen (-15%, p=0.0096) and globus pallidus (-13%, p=0.0177) but not in temporal cortex (p=0.0699). There was an age effect of in both WT and TG so that the 15m WT rats were significantly lower than at 7m and TG were no longer significantly different from WT at 15m. Behaviour: In the MWM, WT and TG were able to learn the platform position at all age. In the RMWM, all rats showed an inability to reverse their spatial memory (no statistical differences were found between age or genotype). In the ST, rats memorise an odour whe

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Hervé Boutin (Supervisor) & Rainer Hinz (Supervisor)