Cellular Sources and Regional Variations in the Expression of the Neuroinflammatory Marker Translocator Protein (TSPO) in the Normal Brain

Abstract

The inducible expression of the mitochondrial translocator protein 18 kDa (TSPO) by activated microglia is a prominent, regular feature of acute and chronic-progressive brain pathology. This expression is also the rationale for the continual development of new TSPO binding molecules for the diagnosis of “neuroinflammation” by molecular imaging. However, there is in the normal brain an ill-defined, low-level constitutive expression of TSPO. Taking advantage of healthy TSPO knockout mouse brain tissue to validate TSPO antibody specificity, this study uses immunohistochemistry to determine the regional distribution and cellular sources of TSPO in the normal mouse brain. Fluorescence microscopy revealed punctate TSPO immunostaining in vascular endothelial cells throughout the brain. In the olfactory nerve layers and glomeruli of the olfactory bulb, choroid plexus and ependymal layers, we confirm constitutive TSPO expression levels similar to peripheral organs, while some low TSPO expression is present in regions of known neurogenesis, as well as cerebellar Purkinje cells. The distributed-sparse expression of TSPO in endothelial mitochondria throughout the normal brain can be expected to give rise to a low baseline signal in TSPO molecular imaging studies. Finally, our study emphasises the need for valid and methodologically robust verification of the selectivity of TSPO ligands through the use of TSPO knockout tissues.

Imaging of translocator protein upregulation is selective for pro‐inflammatory polarized astrocytes and microglia

“…A recent study by Betlazar, Harrison-Brown, Middleton, Banati, and Liu (2018) showed that endothelial cells contribute to TSPO expression in the normal brain. In order to determine the contribution of endothelial cells to the TSPO signal in our imaging studies, we used flow cytometry to evaluate endothelial expression.…”

Translocator protein mediates olfactory repulsion

“…43 However, there have been no reports about the relationship between TSPO and GPCR in modulating olfactory behaviors. 46 The strong tspo expression signal was also observed in olfactory nerve layers and glomeruli across the olfactory bulb from the normal mouse brain. tspo RNAi knockdown abrogated the inductive effects of TA on olfactory repulsion.…”

“…1 The inhibition of TSPO by Ro5-4864 has been shown to prevent the function of GPCRs in mesenteric postcapillary venules in vivo, and TSPO acts as a modulator of pathways of neutrophil adhesion and locomotion induced by GPCR signaling, thus connecting TSPO functions with the onset of an innate inflammatory response. [44][45][46][47] Earlier studies of tspo expression in the rodent brain using autoradiography with the prototypical tspo ligand PK 11195, indirectly revealed that TSPO-binding sites in the olfactory bulb, particularly in the lateral olfactory tract. 43 However, there have been no reports about the relationship between TSPO and GPCR in modulating olfactory behaviors.…”

“…In this study, tspo was found to be the crucial intermediate of TyR-mediated olfactory repulsion. 46 Moreover, Mirzaei et al (2016) identified that the TSPO-binding sites in the normal mouse brains are relatively high in the olfactory bulb and cerebellar cortex. Thus, TSPO may act as an effector protein to transduce the signals from the GPCR TyR.…”

“…The previous studies have found the high constitutive TSPO expression in the normal olfactory nerve layer and glomeruli of the olfactory bulb. [44][45][46][47] Earlier studies of tspo expression in the rodent brain using autoradiography with the prototypical tspo ligand PK 11195, indirectly revealed that TSPO-binding sites in the olfactory bulb, particularly in the lateral olfactory tract. 44,45 In olfactory nerve layers and glomeruli of the olfactory bulb, choroid plexus, and epdendymal layers, the constitutive tspo expression level is similar to that in peripheral organs.…”