Animal PET/MR with the new radioligand [18F]AQ28A demonstrates the involvement of phosphodiesterase 10A in the regulation of energy homeostasis

OBJECTIVES: Cyclic nucleotide phosphodiesterases (PDEs) are enzymes that cleave the phosphodiester bond in the second messenger molecules cAMP and cGMP. PDE10A is dual-specific and mainly expressed in the striatum, a brain region which coordinates a great variety of cognitive functions, including motor and action planning, decision-making, motivation, reinforcement, and reward perception. Knock-out mice provided evidence for involvement of PDE10A in the regulation of energy balance as shown by their resistance to diet induced obesity (DIO). Therefore, we have used the novel selective radioligand [18F]AQ28A [1] to investigate the expression of PDE10A in the striatum and brown adipose tissue (BAT) of lean, diet-induced (DIO) and leptin-deficient genetically obese mice. As BAT activation could previously be visualized by using [18F]FDG [2], we also assessed whether inhibition of PDE10A modulates BAT activity.
METHODS: Isoflurane-anaesthetized female mice were injected with either [18F]AQ28A (11±3 MBq) or [18F]FDG (14±2 MBq) and subjected to 1 h PET/MR (Mediso nanoScan®). After co-registration of the dual modality image data volumes of interest (striatum, hypothalamus, interscapular BAT and muscle) were drawn manually with reference to the structural MRI data using ROVER software (ABX, Radeberg).The selectivity of [18F]AQ28A towards PDE10A was investigated by baseline (n=3) and blocking (n=3) experiments with the PDE10A inhibitor MP-10 (5 mg/kg). Another set of animals was provided over 16 weeks either with standard food (lean, n=5) or high-fat high-sugar diet (DIO, n=5) and received [18F]AQ28A PET/MR thereafter. A further group of lean mice (n=10), which was overnight fasted and housed under thermoneutral conditions, received either i.p. injection of MP-10 (n=5) or vehicle (n=5) followed 30 min later by 1 h [18F]FDG PET/MR. Thereafter the mice were sacrificed and striatum, hypothalamus and BAT were collected. Relative mRNA expressions of PDE10A, thermoregulatory genes and the indirect neuronal activity marker Fos were analyzed by real-time qPCR.
RESULTS: PDE10A selectivity of [18F]AQ28A was proven by blocking with MP-10 (SUV15min striatum baseline/blocking: 1.02±0.19/0.54±0.08; p<0.01). A 7-fold higher mRNA expression of PDE10A in striatum compared to hypothalamus was found (p<0.001). Acute pharmacological inhibition of PDE10A altered cAMP levels (p<0.01) and thermoregulatory gene and Fos expression in striatum (p<0.05), but not in hypothalamus. DIO resulted in ~60% and 80% higher PDE10A expression (p<0.05) in striatum and BAT, respectively, accompanied by an increased SUV of [18F]AQ28A in both targets (p<0.05). Acute administration of MP-10 to lean mice resulted in significantly higher FDG uptake by BAT (SUV55min: 0.40±0.01) compared to vehicle administration (SUV55min: 0.25±0.02; p<0.01).
CONCLUSION: Distinct alterations of gene expression together with significant changes of PDE10A availability and glucose metabolism in BAT after PDE10A inhibition reveal a novel thermoregulatory role for PDE10A. The data suggest that PDE10A inhibitors offer the potential to treat obesity by increasing thermogenesis and reducing hedonic feeding through recruiting striatal and BAT circuits.
[1]Wagner, S., et al. Eur J Med Chem 2016 107;97.
[2]Gnad, T.;…;Kranz, M.;…Pfeifer, A. Nature. 2014 18;516