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      Translocator protein (TSPO): the new story of the old protein in neuroinflammation

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          Abstract

          Translocator protein (TSPO), also known as peripheral benzodiazepine receptor, is a transmembrane protein located on the outer mitochondria membrane (OMM) and mainly expressed in glial cells in the brain. Because of the close correlation of its expression level with neuropathology and therapeutic efficacies of several TSPO binding ligands under many neurological conditions, TSPO has been regarded as both biomarker and therapeutic target, and the biological functions of TSPO have been a major research focus. However, recent genetic studies with animal and cellular models revealed unexpected results contrary to the anticipated biological importance of TSPO and cast doubt on the action modes of the TSPO-binding drugs. In this review, we summarize recent controversial findings on the discrepancy between pharmacological and genetic studies of TSPO and suggest some future direction to understand this old and mysterious protein.

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          Isolation of the mitochondrial benzodiazepine receptor: association with the voltage-dependent anion channel and the adenine nucleotide carrier.

          Rosario Trifiletti, S. Snyder, Adele Snowman … (1992)
          The mitochondrial benzodiazepine receptor (mBzR) has been solubilized with retention of reversible ligand binding, and the associated subunits were characterized. mBzR comprises immunologically distinct protein subunits of 18-, 30-, and 32-kDa. The 18-kDa protein is labeled by the isoquinoline carboxamide mBzR ligand [3H]PK14105, whereas the 30- and 32-kDa subunits are labeled by the benzodiazepine (Bz) ligands [3H]flunitrazepam and [3H]AHN-086. Selective antibodies and reagents identify the 32- and 30-kDa proteins as the voltage-dependent anion channel (VDAC) and the adenine nucleotide carrier (ADC), respectively. While isoquinoline carboxamide and Bz ligands target different subunits, they interact allosterically, as the binding of Bz and isoquinoline carboxamide ligands is mutually competitive at low nanomolar concentrations. Moreover, eosin-5-maleimide and mercuric chloride inhibit [3H]PK11195 binding to the intact receptor via sulfhydryl groups that are present in ADC. VDAC and ADC, outer and inner mitochondrial membrane channel proteins, respectively, together with the 18-kDa subunit, may comprise mBzR at functionally important transport sites at the junction of two mitochondrial membranes.
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            Macroglia-microglia interactions via TSPO signaling regulates microglial activation in the mouse retina.

            Xu Wang, Ellis W T Wong, Lian Zhao … (2014)
            Chronic retinal inflammation in the form of activated microglia and macrophages are implicated in the etiology of neurodegenerative diseases of the retina, including age-related macular degeneration, diabetic retinopathy, and glaucoma. However, molecular biomarkers and targeted therapies for immune cell activation in these disorders are currently lacking. To address this, we investigated the involvement and role of translocator protein (TSPO), a biomarker of microglial and astrocyte gliosis in brain degeneration, in the context of retinal inflammation. Here, we find that TSPO is acutely and specifically upregulated in retinal microglia in separate mouse models of retinal inflammation and injury. Concomitantly, its endogenous ligand, diazepam-binding inhibitor (DBI), is upregulated in the macroglia of the mouse retina such as astrocytes and Müller cells. In addition, we discover that TSPO-mediated signaling in microglia via DBI-derived ligands negatively regulates features of microglial activation, including reactive oxygen species production, TNF-α expression and secretion, and microglial proliferation. The inducibility and effects of DBI-TSPO signaling in the retina reveal a mechanism of coordinated macroglia-microglia interactions, the function of which is to limit the magnitude of inflammatory responses after their initiation, facilitating a return to baseline quiescence. Our results indicate that TSPO is a promising molecular marker for imaging inflammatory cell activation in the retina and highlight DBI-TSPO signaling as a potential target for immodulatory therapies.
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              Pro-inflammatory activation of primary microglia and macrophages increases 18 kDa translocator protein expression in rodents but not humans

              David R J Owen, Nehal Narayan, Lisa Wells … (2017)
              The 18kDa Translocator Protein (TSPO) is the most commonly used tissue-specific marker of inflammation in positron emission tomography (PET) studies. It is expressed in myeloid cells such as microglia and macrophages, and in rodent myeloid cells expression increases with cellular activation. We assessed the effect of myeloid cell activation on TSPO gene expression in both primary human and rodent microglia and macrophages in vitro, and also measured TSPO radioligand binding with 3H-PBR28 in primary human macrophages. As observed previously, we found that TSPO expression increases (∼9-fold) in rodent-derived macrophages and microglia upon pro-inflammatory stimulation. However, TSPO expression does not increase with classical pro-inflammatory activation in primary human microglia (fold change 0.85 [95% CI 0.58–1.12], p = 0.47). In contrast, pro-inflammatory activation of human monocyte-derived macrophages is associated with a reduction of both TSPO gene expression (fold change 0.60 [95% CI 0.45–0.74], p = 0.02) and TSPO binding site abundance (fold change 0.61 [95% CI 0.49–0.73], p < 0.0001). These findings have important implications for understanding the biology of TSPO in activated macrophages and microglia in humans. They are also clinically relevant for the interpretation of PET studies using TSPO targeting radioligands, as they suggest changes in TSPO expression may reflect microglial and macrophage density rather than activation phenotype.
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                Author and article information

                Journal
                Journal ID (nlm-ta): BMB Rep
                Journal ID (iso-abbrev): BMB Rep
                Title: BMB Reports
                Publisher: Korean Society for Biochemistry and Molecular Biology
                ISSN (Print): 1976-6696
                ISSN (Electronic): 1976-670X
                Publication date (Print): January 2020
                Publication date (Electronic): 31 January 2020
                Volume: 53
                Issue: 1
                Pages: 20-27
                Affiliations
                [1 ]Department of Brain and Cognitive Sciences, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
                [2 ]Neurometabolomics Research Center, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea
                Author notes
                [* ]Corresponding author: Tel: +82-53-785-6113; Fax: +82-53-785-1209; E-mail: yusw@ 123456dgist.ac.kr
                Article
                Publisher ID: bmb-53-020
                DOI: 10.5483/BMBRep.2020.53.1.273
                PMC ID: 6999824
                PubMed ID: 31818362
                SO-VID: 757010cd-76c2-4d25-ac13-f7226a7f0631
                Copyright © Copyright © 2020 by the The Korean Society for Biochemistry and Molecular Biology
                License:

                This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                Date received : 22 October 2019
                Categories
                Subject: Invited Mini Review

                Keywords: microglia,mitochondria,neuroinflammation,steroidogenesis,tspo
                Data availability:
                Keywords: microglia, mitochondria, neuroinflammation, steroidogenesis, tspo

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