Imaging neuroinflammation in an animal model of diet-induced obesity

Introduction: Obesity has been linked with an increased risk of cardiometabolic defects, cognitive decline and Alzheimer's disease. Mechanisms of obesity-induced neuropathologies are not fully understood but neuroinflammation is reported to play a central role in their progression. Neuroinflammation is characterized by the activation of certain inflammatory modulator biomarkers which have been successfully targeted by positron imaging tomography (PET) imaging agents. Sphingosine 1 phosphate receptor 1 (S1PR1) and the translocator protein (TSPO) have emerged as important biomarkers for targeting neuroinflammation. In this study, we investigate and validate the pathogenesis of neuroinflammation in an animal model of diet-induced obesity by targeting TSPO with [¹⁸F]DPA-714 [1] and S1PR1 with [¹⁸F]TZ4877 [2].

Methods: Two separate cohorts of six-week-old C57BL6/J male mice were placed on high fat based diet (HFD, N=6 per cohort) of 60 kcal % fat for 30 weeks along with the control group which were fed low fat diet (LFD, N=6 per cohort) of 10 kcal % fat. Preclinical PET imaging was performed at 24 weeks and 30 weeks using the Medico nanoScan PET/CT scanner. Whole body preclinical dynamic imaging was performed for 60 minutes for both tracers following 150 µCi administration of [¹⁸F]DPA-714 or [¹⁸F]TZ4877 on separate days. Immunofluorescence staining was performed against IBA1, GFAP, TSPO and S1PR1 to correlate with PET imaging data. Regional ROIs were drawn over the brain to extract time activity curves (TACs). To compensate for the differences in the weight between the two groups, standardized uptake values (SUV) were normalized to metabolic body weight based on Kleiber law. Kleiber's law is based on metabolic activity according to which proportion of weight actively involved in the uptake of metabolites and radiotracer is bodyweight^0.74. Student t-test and two-way ANOVA were performed to compare significant difference between different groups.

Results: As expected, HFD fed mice exhibited significant weight gain following 30 weeks of HFD relative to LFD fed group. SUV-normalized time activity curve exhibited higher uptake and stable retention in brain of mice on HFD relative to mice on LFD. We observed 59% higher retention of [¹⁸F]DPA-714 in the brain at 24 weeks which persisted at 30 weeks of HFD compared to LFD group. Consistent with higher retention of [¹⁸F]DPA-714 in the brain, immunofluorescence staining revealed increased TSPO and S1PR1 expression in HFD group. Similarly, we observed elevated levels of IBA1 and GFAP in the nucleus accumbens and hippocampus region of the brain in HFD group relative to LFD group.

Conclusions: [¹⁸F]DPA-714 PET exhibited increased retention in an animal model of obesity as a biomarker for neuroinflammation. PET findings correlated positively with the expression of neuroinflammatory biomarkers. Immunofluorescence staining for TSPO and S1PR1 confirmed the role of neuroinflammation in HFD-induced obesity. While the inflammatory signal of TSPO is well established in the brain, it's not fully understood in peripheral organs. Therefore, more studies are warranted to understand the interaction between peripheral inflammation and neuroinflammation.

[1]. James ML, Fulton RR, Vercoullie J, Henderson DJ, Garreau L, Chalon S, Dolle F, Costa B, Guilloteau D, Kassiou M. DPA-714, a new translocator protein-specific ligand: synthesis, radiofluorination, and pharmacologic characterization. J Nucl Med. 2008 May; 49(5):814-22.

[2]. Liu H, Luo Z, Gu J, Jiang H, Joshi S, Shoghi KI, Zhou Y, Gropler RJ, Benzinger TLS, Tu Z. In vivo Characterization of Four ¹⁸F-Labeled S1PR1 Tracers for Neuroinflammation. Mol Imaging Biol. 2020 Oct;22(5):1362-1369.

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