Mitochondrial Translocator Protein (TSPO) Expression in the Brain After Whole Body Gamma Irradiation.

Calina Betlazar,Guo-Jun Liu,Nicholas Howell,Justin Davies,Ryan J Middleton,Ben Storer,Richard Banati,Emma Davis

doi:10.3389/fcell.2021.715444

Abstract

The brain’s early response to low dose ionizing radiation, as may be encountered during diagnostic procedures and space exploration, is not yet fully characterized. In the brain parenchyma, the mitochondrial translocator protein (TSPO) is constitutively expressed at low levels by endothelial cells, and can therefore be used to assess the integrity of the brain’s vasculature. At the same time, the inducible expression of TSPO in activated microglia, the brain’s intrinsic immune cells, is a regularly observed early indicator of subtle or incipient brain pathology. Here, we explored the use of TSPO as a biomarker of brain tissue injury following whole body irradiation. Post-radiation responses were measured in C57BL/6 wild type (Tspo+/+) and TSPO knockout (Tspo–/–) mice 48 h after single whole body gamma irradiations with low doses 0, 0.01, and 0.1 Gy and a high dose of 2 Gy. Additionally, post-radiation responses of primary microglial cell cultures were measured at 1, 4, 24, and 48 h at an irradiation dose range of 0 Gy-2 Gy. TSPO mRNA and protein expression in the brain showed a decreased trend after 0.01 Gy relative to sham-irradiated controls, but remained unchanged after higher doses. Immunohistochemistry confirmed subtle decreases in TSPO expression after 0.01 Gy in vascular endothelial cells of the hippocampal region and in ependymal cells, with no detectable changes following higher doses. Cytokine concentrations in plasma after whole body irradiation showed differential changes in IL-6 and IL-10 with some variations between Tspo–/– and Tspo+/+ animals. The in vitro measurements of TSPO in primary microglial cell cultures showed a significant reduction 1 h after low dose irradiation (0.01 Gy). In summary, acute low and high doses of gamma irradiation up to 2 Gy reduced TSPO expression in the brain’s vascular compartment without de novo induction of TSPO expression in parenchymal microglia, while TSPO expression in directly irradiated, isolated, and thus highly activated microglia, too, was reduced after low dose irradiation. The potential link between TSPO, its role in mitochondrial energy metabolism and the selective radiation sensitivity, notably of cells with constitutive TSPO expression such as vascular endothelial cells, merits further exploration.

Highlights

Ionizing radiation is widely used for both diagnostic and therapeutic purposes in medicine
A downregulation of translocator protein (TSPO) expression was found after 0.01 Gy for both mRNA and protein compared with controls, though this did not reach significance
A smaller trend toward reduced expression was found for mRNA and protein expression of Iba1 (Supplementary Figure 1). These results suggest a subtle downregulation in TSPO expression and microglial reactivity after low dose ionizing radiation (LDIR) exposure

Summary

Introduction

Ionizing radiation is widely used for both diagnostic and therapeutic purposes in medicine. Microglial cells, the intrinsic tissue macrophages of the brain, are key to the understanding of the brain’s injury responses, including those caused by ionizing radiation (Acharya et al, 2016; Hladik and Tapio, 2016; Krukowski et al, 2018). Some of the earliest and most important responses to ionizing radiation are the generation of increased reactive oxygen species (ROS) and oxidative stress (Kam and Banati, 2013; Szumiel, 2015; Richardson and Harper, 2016; Farhood et al, 2019) produced by mitochondria, including in macrophages, as part of radiationinduced inflammatory tissue responses (Banati et al, 2004; Simpson and Oliver, 2020). Even higher doses have been shown to induce oxidative damage, as well as the expression of mitochondrial fission and fusion proteins concomitantly with microglial activation (Zhou et al, 2017)

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Discussion

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