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Review
, 20 (13)

VDAC1 and the TSPO: Expression, Interactions, and Associated Functions in Health and Disease States

Affiliations
Review

VDAC1 and the TSPO: Expression, Interactions, and Associated Functions in Health and Disease States

Varda Shoshan-Barmatz et al. Int J Mol Sci.

Abstract

The translocator protein (TSPO), located at the outer mitochondrial membrane (OMM), serves multiple functions and contributes to numerous processes, including cholesterol import, mitochondrial metabolism, apoptosis, cell proliferation, Ca2+ signaling, oxidative stress, and inflammation. TSPO forms a complex with the voltage-dependent anion channel (VDAC), a protein that mediates the flux of ions, including Ca2+, nucleotides, and metabolites across the OMM, controls metabolism and apoptosis and interacts with many proteins. This review focuses on the two OMM proteins TSPO and VDAC1, addressing their structural interaction and associated functions. TSPO appears to be involved in the generation of reactive oxygen species, proposed to represent the link between TSPO activation and VDAC, thus playing a role in apoptotic cell death. In addition, expression of the two proteins in healthy brains and diseased states is considered, as is the relationship between TSPO and VDAC1 expression. Both proteins are over-expressed in in brains from Alzheimer's disease patients. Finally, TSPO expression levels were proposed as a biomarker of some neuropathological settings, while TSPO-interacting ligands have been considered as a potential basis for drug development.

Keywords: TSPO; VDAC1; mitochondria.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Schematic representation of VDAC1 as a multi-functional protein involved in Ca2+ and metabolite transport, energy production and the structural and functional association of mitochondria with the ER. The various functions of VDAC1 in cell and mitochondria functions are presented. These include: (A) Transporting Ca2+ across the OMM, thereby modulating Ca2+ signaling. In the IMM, Ca2+ uptake into the matrix is mediated by a Ca2+-selective transporter, the mitochondrial Ca2+ uniporter (MCU), regulated by a calcium-sensing accessory subunit (MCU1). Ca2+ efflux is mediated by NCLX, a Na+/Ca2+ exchanger. Ca2+ controls energy production via activation of PDH, ICDH, and α KGDH by intra-mitochondrial Ca2+, leading to enhanced activity of the citric acid cycle; (B) Control of metabolic cross-talk between the mitochondria and the rest of the cell, by transporting metabolites; (C) Mediating cellular energy production by transporting ATP/ADP and NADH and acyl-CoA from the cytosol to the IMS, and regulating glycolysis via the association with HK; (D) Involvement in structural and functional association with the ER, mediating Ca2+ transport from the ER to mitochondria. Key proteins, such as the inositol 3 phosphate receptor type 3 (IP3R3), the sigma1 receptor (Sig1R), the chaperone HSP70, and glucose-regulated protein 75 (GRP75) are presented; (E) Participation in apoptosis via its oligomerization to form a protein-conducting channel, allowing Cyto c release and cell death; and (F) Mediation of the transfer of fatty acid acyl-CoAs across the OMM to the IMS, where they are converted into acylcarnitine by CPT1a for further processing by β-oxidation. VDAC1 is involved in cholesterol transport as a constituent of a multi-protein complex, the transduceosome, containing Star, TSPO and VDAC1. (G) The permeability transition pore (PTP), composed of VDAC at the OMM, ANT at the IMM and Cyp D in the matrix, allows release of apoptogenic proteins.
Figure 2
Figure 2
Structures of VDAC1, TSPO and their structural and functional complex (A) Crystal structure of dimeric Bacillus cercus TSPO (PDB ID: 4RYI) bound to PK11195. This structure is in good agreement with dimeric Rhodobacter sphaeroides TSPO (PDB ID:4UC3) and Homo sapiens TSPO structures solved with cholesterol analog PK11195 (PDB ID: 2MGY). Monomers of TSPO are in green and cyan, while PK11195 is in red [29]. (B) Crystal structure of human VDAC1 (PDB ID: 5XDN): (a) Monomeric VDAC1, (b) the dimeric form of human VDAC1. The N-terminal α-helix is in blue, cholesterol is in red, and E73 is in magenta. Cholesterol was manually docked for visual purposes [35]. (C) Proposed model for VDAC1-TSPO-associated functions: (a) In the OMM, VDAC1 and TSPO form dimers and associate with cholesterol. VDAC1 can be associated with hexokinase and anti-apoptotic proteins. TSPO is associated with anti-mitophagy partners to inhibit autophagy. Upon increased [Ca2+], acidification occurs, which in turn increases [ROS]. (b) Increased acidification, Ca2+ or ROS levels lead to VDAC1 oligomerization, concomitant with detachment of VDAC1 and TSPO-associated proteins. VDAC1 oligomers (likely hexamers) now create a large channel allowing the release of cytochrome c (Cyto c) from the IMS to the cytosol, activating apoptosis. TSPO is likely to stabilize the newly formed VDAC1 oligomer. (D) TSPO and VDAC1 sequences with the GXXXG motif labeled, and E73 in VDAC1 and the cholesterol-binding site in TSPO and the ATG8-binding motif (WYAGL, green) are also indicated.
Figure 3
Figure 3
TSPO and VDAC1 are over-expressed in the brains of transgenic mice. (A) Cross-sections of brains from wild-type (WT) and 5XFAD transgenic mice, immunofluorescently stained for TSPO or VDAC1. Formalin-fixed and paraffin-embedded 5 μm-thick brain sections were deparaffinized, rehydrated, and subjected to antigen retrieval in 0.01 M citrate buffer (pH 6.0). For confocal fluorescence microscopic imaging of immuno-stained brain sections from WT and 5XFAD transgenic mice, the tissues were stained with anti-TSPO or anti-VDAC1 antibodies. Nuclei were stained by DAPI. Immunofluorescent staining were performed using mouse anti-VDAC1 (1:1000) and rabbit anti-TSPO (1:500) antibodies, followed by incubation (2 h, 25 °C) with secondary ant-rabbit Alexa-flur-488 or anti-mouse Alexa-Flu 555 (1:1000) antibodies. The cells were then stained with DAPI and viewed with an Olympus IX81 confocal microscope. (B) For immunohistochemistry, endogenous peroxidase activity was blocked by incubating the sections in 3% H2O2 for 15 min, after which the slides were washed and incubated overnight at 4 °C with primary rabbit anti-TSPO antibodies (1:200) and then for 2 h with anti-rabbit (1:500) secondary antibodies conjugated to horseradish peroxidase (HRP). Sections were washed and incubated with the HRP substrate, DAB. Images were collected at 20× magnification using a microscope (Leica DM2500). Non-specific control experiments were conducted using the same protocols but omitting incubation with primary antibodies. Arrows points to β plaques enriched with TSPO-expressing microglia.
Figure 3
Figure 3
TSPO and VDAC1 are over-expressed in the brains of transgenic mice. (A) Cross-sections of brains from wild-type (WT) and 5XFAD transgenic mice, immunofluorescently stained for TSPO or VDAC1. Formalin-fixed and paraffin-embedded 5 μm-thick brain sections were deparaffinized, rehydrated, and subjected to antigen retrieval in 0.01 M citrate buffer (pH 6.0). For confocal fluorescence microscopic imaging of immuno-stained brain sections from WT and 5XFAD transgenic mice, the tissues were stained with anti-TSPO or anti-VDAC1 antibodies. Nuclei were stained by DAPI. Immunofluorescent staining were performed using mouse anti-VDAC1 (1:1000) and rabbit anti-TSPO (1:500) antibodies, followed by incubation (2 h, 25 °C) with secondary ant-rabbit Alexa-flur-488 or anti-mouse Alexa-Flu 555 (1:1000) antibodies. The cells were then stained with DAPI and viewed with an Olympus IX81 confocal microscope. (B) For immunohistochemistry, endogenous peroxidase activity was blocked by incubating the sections in 3% H2O2 for 15 min, after which the slides were washed and incubated overnight at 4 °C with primary rabbit anti-TSPO antibodies (1:200) and then for 2 h with anti-rabbit (1:500) secondary antibodies conjugated to horseradish peroxidase (HRP). Sections were washed and incubated with the HRP substrate, DAB. Images were collected at 20× magnification using a microscope (Leica DM2500). Non-specific control experiments were conducted using the same protocols but omitting incubation with primary antibodies. Arrows points to β plaques enriched with TSPO-expressing microglia.

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References

    1. Levin E., Premkumar A., Veenman L., Kugler W., Leschiner S., Spanier I., Weisinger G., Lakomek M., Weizman A., Snyder S.H., et al. The peripheral-type benzodiazepine receptor and tumorigenicity: Isoquinoline binding protein (ibp) antisense knockdown in the c6 glioma cell line. Biochemistry. 2005;44:9924–9935. doi: 10.1021/bi050150s. - DOI - PubMed
    1. Veenman L., Papadopoulos V., Gavish M. Channel-like functions of the 18-kda translocator protein (tspo): Regulation of apoptosis and steroidogenesis as part of the host-defense response. Curr. Pharm. Des. 2007;13:2385–2405. doi: 10.2174/138161207781368710. - DOI - PubMed
    1. Zisterer D.M., Williams D.C. Calmidazolium and other imidazole compounds affect steroidogenesis in y1 cells: Lack of involvement of the peripheral-type benzodiazepine receptor. J. Steroid Biochem. Mol. Biol. 1997;60:189–195. doi: 10.1016/S0960-0760(96)00189-6. - DOI - PubMed
    1. Jaremko Ł., Jaremko M., Giller K., Becker S., Zweckstetter M. Structure of the mitochondrial translocator protein in complex with a diagnostic ligand. Science. 2014;343:1363–1366. doi: 10.1126/science.1248725. - DOI - PMC - PubMed
    1. Jaremko M., Jaremko Ł., Jaipuria G., Becker S., Zweckstetter M. Structure of the mammalian tspo/pbr protein. Biochem. Soc. Trans. 2015;43:566–571. doi: 10.1042/BST20150029. - DOI - PubMed

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