Imaging of brain TSPO expression with [18F]DPA-714 PET/CT and immunofluorescence analysis in mouse model of ALS.

Introduction: 18kDa translocator protein TSPO is a hallmark of neuroinflammation. Under pathological conditions its expression increases markedly in brain activated glial cells [1]. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by inflammatory reaction in brain and spinal cord [2]. We have used microPET/CT and 18F-DPA-714, a selective radioligand for TSPO, and post-mortem immunofluorescence to study microglial activation in transgenic mouse model SOD1G93A. Methods: 9 symptomatic SOD1G93A mice (CS: 1-4) and 5 WT SOD1 control mice underwent 18F-DPA-714 PET/CT (GE Healthcare eXplore Vista). Images were acquired 20-50 minutes after radiotracer injection (SRA: 200-800 Gbq/μmol). PET frames were summed and used for data analysis. SUV ratios were calculated by normalizing cerebellum, brainstem, motor cortex and cervical spinal cord activity to that of frontal association cortex. After animals were transcardially perfused and brains and spinal cord were dissected out, post-fixed and cryoprotected. 40 μm thickness sections were incubated with anti-TSPO and anti-Iba1 primary antibodies, followed by fluorescent secondary antibodies. Fluorescence was examined using a confocal microscope (Zeiss LSM 700). Results: In SOD1G93A mice, cerebellum, brainstem and cervical spinal cord radiotracer uptake were increased as compared to WT SOD1, with a statistically significantly difference in brainstem (2.340±0.784 vs 1.576±0.287, p=0.014). Immunofluorescence studies showed that TSPO expression was increased in trigeminal, facial, ambiguus and hypoglossal brainstem nuclei, as well as in spinal cord, of SOD1G93A mice with CS of 4, with colocalization with Iba1 positive cells. Conclusion: Increased [18F]DPA-714 uptake can be detected with high resolution PET/CT in brainstem of transgenic SOD1G93A mice. Immunofluorescence showed an increased TSPO and Iba1 immunoreactivity in brainstem nuclei, suggesting that increased microglial activation might be the cellular counterpart of in vivo [18F]DPA-714 uptake. Overall 18F-DPA-714 PET/CT might be a suitable tool to evaluate microglial activation in the SOD1G93A mouse model.