Translocator protein

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Translocator protein (TSPO) is an 18 kDa protein mainly found on the outer mitochondrial membrane. It was first described as peripheral benzodiazepine receptor (PBR), a secondary binding site for diazepam, but subsequent research has found the receptor to be expressed throughout the body and brain.[1] In humans, the translocator protein is encoded by the TSPO gene.[2][3] It belongs to family of tryptophan-rich sensory proteins. Regarding intramitochondrial cholesterol transport, TSPO has been proposed to interact with StAR (steroidogenic acute regulatory protein) to transport cholesterol into mitochondria, though evidence is mixed.[4]

Function

In animals, TSPO (PBR) is a mitochondrial protein usually located in the outer mitochondrial membrane and characterised by its ability to bind a variety of benzodiazepine-like drugs, as well as to dicarboxylic tetrapyrrole intermediates of the haem biosynthetic pathway.

TSPO has many proposed functions depending on the tissue.[5] The most studied of these include roles in the immune response, steroid synthesis and apoptosis.

Cholesterol transport and bile acid biosynthesis

Mitochondrial cholesterol transport is a molecular function closely tied to TSPO in the scientific literature. TSPO binds with high affinity to the lipid cholesterol, and pharmacological ligands of TSPO facilitate cholesterol transport across the mitochondrial intermembrane space to stimulate steroid synthesis and bile acid synthesis in relevant tissues.[6] However, TSPO deletion in genetically engineered mouse models has yielded mixed results regarding the physiological necessity of TSPO's role in steroidogenesis. Deletion of TSPO in steroidogenic Leydig cells did not impair synthesis of the steroid testosterone.[7] Thus, though biochemical and pharmacological experimentation suggest an important role for TSPO in cellular cholesterol transport and steroid biosynthesis,[8] TSPO's necessity in this process remains controversial.

Regulation in the heart

TSPO (Translocator protein) acts to regulate heart rate and contractile force by its interaction with voltage-dependent calcium channels in cardiac myocytes.[9] The interaction between TSPO and calcium channels can alter cardiac action potential durations, thus contractility of the heart. In healthy individuals, TSPO has a cardio-protective role. When TSPO is up-regulated in the presence of infections, it can limit the inflammatory response, which can be cardio-damaging.[10]

Immunomodulation

PBRs (TSPOs) have many actions on immune cells including modulation of oxidative bursts by neutrophils and macrophages, inhibition of the proliferation of lymphoid cells and secretion of cytokines by macrophages.[11] Expression of TSPO is also linked to inflammatory responses that occur after ischemia-reperfusion injury, following brain injury, and in some neurodegenerative diseases.[citation needed]

Increased expression of TSPO is linked to the inflammatory responses in the heart that may cause myocarditis, which can lead to myocardial necrosis. TSPO is present in mast cells and macrophages, indicating its role in the immune system.[9] Oxidative stress is a strong contributing factor to cardiovascular disease, and often occurs because of inflammation caused by ischemia reperfusion injury.[12] Coxsackievirus B3 (CVB3) causes immune cells CD11b+ (present on macrophages) to stimulate inflammatory infiltration. Functionally, CD11b+ regulates leukocyte adhesion and migration to regulate the inflammatory response.[10] Following infection, CD11b+ is up-regulated, activating these immune responses, which then activate an increased expression of TSPO. These immune cells can cause myocarditis which can progress to dilated cardiomyopathy and heart failure.[10]

Apoptosis

Ligands of TSPO have been shown to induce apoptosis in human colorectal cancer cells.[citation needed] In lymphatic tissues, TSPO modulates apoptosis of thymocytes via reduction of mitochondrial transmembrane potential.[13]

Stress adaptation

TSPO in the basal land plant Physcomitrella patens, a moss, is essential for adaptation to salt stress.[14]

Tissue distribution

TSPO is found in many regions of the body including the human iris/ciliary-body.[15] Other tissues include the heart, liver, adrenal and testis, as well as hemopoietic and lymphatic cells.[16] "Peripheral" benzodiazepine receptors are also found in the brain, although only at around a quarter the expression levels of the "central" benzodiazepine receptors located at the plasma membrane.[17]

Therapeutic applications

TSPO has been shown to be involved in a number of processes such as inflammation,[18] and TSPO ligands may be useful anti-cancer drugs.[19][20]

Pharmacological activation of TSPO has been observed to be a potent stimulator of steroid biosynthesis [21][22] including neuroactive steroids such as allopregnanolone in the brain, which exert anxiolytic properties.[23] Thus, TSPO ligands such as emapunil (XBD-173) or alpidem have been proposed to be useful as potential anxiolytics which may have less addiction-based side effects than traditional benzodiazepine-type drugs.,[24][25][26][27] though toxicity side-effects remain a significant barrier in drug development.[28]

A 2013 study led by researchers from USC Davis School of Gerontology showed that TSPO ligands can prevent and at least partially correct abnormalities present in a mouse model of Alzheimer's disease.[29]

TSPO as a biomarker is a newly discovered non-invasive procedure, and has also been linked as a biomarker for other cardiovascular related diseases including: myocardial infarction (due to ischemic reperfusion), cardiac hypertrophy, atherosclerosis, arrhythmias, and large vessel vasculitis.[12] TSPO can be used as a biomarker to detect the presence and severity of inflammation in the heart and atherosclerotic plaques.[10]Inhibiting the over-production of TSPO can lead to a reduced incidence of arrhythmias which are most often caused by ischemia reperfusion injury.[12] TSPO ligands are used as a therapy after ischemia reperfusion injury to preserve the action potentials in cardiac tissue and restore normal electrical activity of the heart.[9] Higher levels of TSPO are present in those with heart disease, a change that is more common in men than women because testosterone worsens the inflammation causing permanent damage to the heart.[10]

The first high-resolution 3D solution structure of mammalian (mouse) translocator protein (TSPO) in a complex with its diagnostic PK11195 ligand was determined by means of NMR spectroscopy techniques by scientists from the Max-Planck Institute for Biophysical Chemistry in Goettingen in Germany in March 2014 (Jaremko et al., 2014) and has a PDB id: 2MGY. Obtained high-resolution clearly confirms a helical character of a protein and its complex with a diagnostic ligand in solution. The 3D structure of the mTSPO-PK11195 complex comprises five transmembrane α-helices (TM1 to TM5) that tightly pack together in the clockwise order TM1-TM2-TM5-TM4-TM3 (cytosol view). The mammalian TSPO in a complex with diagnostic ligand is nomomeric. The loop located in between TM1 and TM2 helices closes the entrance to the space between helices in which are bound with PK11195 molecule. Site-directed mutagenesis studies of mTSPO revealed that region important for PK11195 binding comprise amino acids from 41 to 51, because the deletion of this region resulted in the decrease in PK11195 binding (Fan et al., 2012).

The mammalian TSPO in a complex with the diagnostic ligand PK11195 is monomeric.[30][31]

Imaging

Ligands of the TSPO are very useful for imaging of inflammation. For example, the radioligand [3H]-PK-11195 has been used in receptor autoradiography to study neuroinflammation following brain injury. The affinity of [11C]-PBR28 depends on a single polymorphism (rs6971) in the TSPO gene.[32]

Measuring microglial activation in vivo is possible using PET imaging and radioligands binding to 18 kDa translocator protein (TSPO).[33] Activation can be meassured using the PET tracer (R)-[11C]PK11195 and others like PBR28 are under research.[34]

Selective ligands

Agonists

  • YL-IPA08
  • Ro5-4864 - original ligand with which TSPO receptor was characterised, now less used due to inter-species differences in binding affinity. Sedative yet also convulsant and anxiogenic in mice.[35]
Peptides
  • Anthralin - 16kDa polypeptide, binds to both TSPO receptor and dihydropyridine-sensitive calcium channels with high affinity.[36]
  • Diazepam binding inhibitor (DBI) - 11kDa neuropeptide, potent agonist for TSPO receptor and stimulates steroidogenesis in vivo,[37][38][39] also negative allosteric modulator of benzodiazepine-sensitive GABAA receptors.[40]
  • DBI 17-50 fragment - active processing product of DBI
Non-peptides

Antagonists

  • PK-11195 - potent and selective antagonist for both rat and human forms of TSPO.

See also

References

  1. ↑ Papadopoulos V, Baraldi M, Guilarte TR, Knudsen TB, Lacapère JJ, Lindemann P, Norenberg MD, Nutt D, Weizman A, Zhang MR, Gavish M (August 2006). "Translocator protein (18kDa): new nomenclature for the peripheral-type benzodiazepine receptor based on its structure and molecular function". Trends Pharmacol. Sci. 27 (8): 402–9. doi:10.1016/j.tips.2006.06.005. PMID 16822554.
  2. ↑ Chang YJ, McCabe RT, Rennert H, Budarf ML, Sayegh R, Emanuel BS, Skolnick P, Strauss JF (1992). "The human "peripheral-type" benzodiazepine receptor: regional mapping of the gene and characterization of the receptor expressed from cDNA". DNA Cell Biol. 11 (6): 471–80. doi:10.1089/dna.1992.11.471. PMID 1326278.
  3. ↑ Riond J, Mattei MG, Kaghad M, Dumont X, Guillemot JC, Le Fur G, Caput D, Ferrara P (January 1991). "Molecular cloning and chromosomal localization of a human peripheral-type benzodiazepine receptor". Eur. J. Biochem. 195 (2): 305–11. doi:10.1111/j.1432-1033.1991.tb15707.x. PMID 1847678.
  4. ↑ Bogan RL, Davis TL, Niswender GD (April 2007). "Peripheral-type benzodiazepine receptor (PBR) aggregation and absence of steroidogenic acute regulatory protein (StAR)/PBR association in the mitochondrial membrane as determined by bioluminescence resonance energy transfer (BRET)". J. Steroid Biochem. Mol. Biol. 104 (1–2): 61–7. doi:10.1016/j.jsbmb.2006.10.007. PMID 17197174.
  5. ↑ Casellas P, Galiegue S, Basile AS (2002). "Peripheral benzodiazepine receptors and mitochondrial function". Neurochem Int. 40 (6): 475–86. doi:10.1016/S0197-0186(01)00118-8. PMID 11850104.
  6. ↑ Lacapère JJ, Papadopoulos V (September 2003). "Peripheral-type benzodiazepine receptor: structure and function of a cholesterol-binding protein in steroid and bile acid biosynthesis". Steroids. 68 (7–8): 569–585. doi:10.1016/s0039-128x(03)00101-6. PMID 12957662.
  7. ↑ Morohaku K, Pelton SH, Daugherty DJ, Ronald Butler W, Deng W, Selvaraj V (2013). "Translocator Protein/Peripheral Benzodiazepine Receptor Is Not Required for Steroid Hormone Biosynthesis". Endocrinology. 155 (1): 89–97. doi:10.1210/en.2013-1556. PMC 3868810. PMID 24174323.
  8. ↑ Midzak A, Papadopoulos V (Sep 2014). "Binding domain-driven intracellular trafficking of sterols for synthesis of steroid hormones, bile acids and oxysterols". Traffic. 15 (9): 895–914. doi:10.1111/tra.12177. PMID 24890942.
  9. ↑ 9.0 9.1 9.2 Qi X, Xu J, Wang F, Xiao J (2012). "Translocator protein (18 kDa): a promising therapeutic target and diagnostic tool for cardiovascular diseases" (PDF). Oxid Med Cell Longev. 2012: 162934. doi:10.1155/2012/162934. PMC 3516045. PMID 23251719.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Fairweather D, Coronado MJ, Garton AE, Dziedzic JL, Bucek A, Cooper LT, Brandt JE, Alikhan FS, Wang H, Endres CJ, Choi J, Pomper MG, Guilarte TR (March 2014). "Sex differences in translocator protein 18 kDa (TSPO) in the heart: implications for imaging myocardial inflammation". J Cardiovasc Transl Res. 7 (2): 192–202. doi:10.1007/s12265-013-9538-0. PMC 3951973. PMID 24402571.
  11. ↑ Pawlikowski M (1993). "Immunomodulating effects of peripherally acting benzodiazepines". New York: In Peripheral Benzodiazepine Receptors. Academic press. pp. 125–135.
  12. ↑ 12.0 12.1 12.2 Batarseh A, Papadopoulos V (2010). "Regulation of translocator protein 18 kDa (TSPO) expression in health and disease states" (PDF). Mol. Cell. Endocrinol. 327: 1–12. doi:10.1016/j.mce.2010.06.013. PMC 2922062. PMID 20600583.
  13. ↑ Tanimoto Y, Onishi Y, Sato Y, Kizaki H (February 1999). "Benzodiazepine receptor agonists modulate thymocyte apoptosis through reduction of the mitochondrial transmembrane potential". Jpn. J. Pharmacol. 79 (2): 177–83. doi:10.1254/jjp.79.177. PMID 10202853.
  14. ↑ Frank W, Baar KM, Qudeimat E, Woriedh M, Alawady A, Ratnadewi D, Gremillon L, Grimm B, Reski R (September 2007). "A mitochondrial protein homologous to the mammalian peripheral-type benzodiazepine receptor is essential for stress adaptation in plants". Plant J. 51 (6): 1004–18. doi:10.1111/j.1365-313X.2007.03198.x. PMID 17651369.
  15. ↑ Valtier D, Malgouris C, Gilbert JC, Guicheney P, Uzan A, Gueremy C, Le Fur G, Saraux H, Meyer P (June 1987). "Binding sites for a peripheral type benzodiazepine antagonist ([3H]PK 11195) in human iris". Neuropharmacology. 26 (6): 549–52. doi:10.1016/0028-3908(87)90146-8. PMID 3037422.
  16. ↑ Woods MG, Williams DC (1996). Multiple forms and locations for the peripheral-type benzodiazepine receptor. Biochemical Pharmacology. 52. pp. 1805–1814. doi:10.1016/S0006-2952(96)00558-8. PMID 8951338.
  17. ↑ Marangos PJ, Patel J, Boulenger JP, Clark-Rosenberg R (July 1982). "Characterization of peripheral-type benzodiazepine binding sites in brain using [3H]Ro 5-4864". Molecular Pharmacology. 22 (1): 26–32. PMID 6289073.
  18. ↑ Chen MK, Guilarte TR (April 2008). "Translocator protein 18 kDa (TSPO): molecular sensor of brain injury and repair". Pharmacology & Therapeutics. 118 (1): 1–17. doi:10.1016/j.pharmthera.2007.12.004. PMC 2453598. PMID 18374421.
  19. ↑ Santidrián AF, Cosialls AM, Coll-Mulet L, Iglesias-Serret D, de Frias M, González-Gironès DM, Campàs C, Domingo A, Pons G, Gil J (December 2007). "The potential anticancer agent PK11195 induces apoptosis irrespective of p53 and ATM status in chronic lymphocytic leukemia cells". Haematologica. 92 (12): 1631–8. doi:10.3324/haematol.11194. PMID 18055986.
  20. ↑ Kugler W, Veenman L, Shandalov Y, Leschiner S, Spanier I, Lakomek M, Gavish M (2008). "Ligands of the mitochondrial 18 kDa translocator protein attenuate apoptosis of human glioblastoma cells exposed to erucylphosphohomocholine". Cellular Oncology. 30 (5): 435–50. PMID 18791274.
  21. ↑ Veenman L, Papadopoulos V, Gavish M (2007). "Channel-like functions of the 18-kDa translocator protein (TSPO): regulation of apoptosis and steroidogenesis as part of the host-defense response". Current Pharmaceutical Design. 13 (23): 2385–405. doi:10.2174/138161207781368710. PMID 17692008.
  22. ↑ Falchi AM, Battetta B, Sanna F, Piludu M, Sogos V, Serra M, Melis M, Putzolu M, Diaz G (August 2007). "Intracellular cholesterol changes induced by translocator protein (18 kDa) TSPO/PBR ligands". Neuropharmacology. 53 (2): 318–29. doi:10.1016/j.neuropharm.2007.05.016. PMID 17631921.
  23. ↑ Farb DH, Ratner MH (October 2014). "Targeting the modulation of neural circuitry for the treatment of anxiety disorders". Pharmacol Rev. 66 (4): 1002–1032. doi:10.1124/pr.114.009126. PMID 25237115.
  24. ↑ Mealy NE, Bayés M, Lupone B (2006). "Psychiatric Disorders". Drugs of the Future. 31 (3): 259.
  25. ↑ Da Settimo F, Simorini F, Taliani S, La Motta C, Marini AM, Salerno S, Bellandi M, Novellino E, Greco G, Cosimelli B, Da Pozzo E, Costa B, Simola N, Morelli M, Martini C (September 2008). "Anxiolytic-like effects of N,N-dialkyl-2-phenylindol-3-ylglyoxylamides by modulation of translocator protein promoting neurosteroid biosynthesis". Journal of Medicinal Chemistry. 51 (18): 5798–806. doi:10.1021/jm8003224. PMID 18729350.
  26. ↑ Taliani S, Da Settimo F, Da Pozzo E, Chelli B, Martini C (September 2009). "Translocator Protein Ligands as Promising Therapeutic Tools for Anxiety Disorders". Current Medicinal Chemistry. 16 (26): 3359–80. doi:10.2174/092986709789057653. PMID 19548867.
  27. ↑ Rupprecht R, Rammes G, Eser D, Baghai TC, Schüle C, Nothdurfter C, Troxler T, Gentsch C, Kalkman HO, Chaperon F, Uzunov V, McAllister KH, Bertaina-Anglade V, La Rochelle CD, Tuerck D, Floesser A, Kiese B, Schumacher M, Landgraf R, Holsboer F, Kucher K (June 2009). "Translocator Protein (18 kD) as Target for Anxiolytics Without Benzodiazepine-Like Side Effects". Science. 325 (5939): 490–3. doi:10.1126/science.1175055. PMID 19541954.
  28. ↑ Skolnick P (November 2012). "Anxioselective anxiolytics: on a quest for the Holy Grail". Trends Pharmacol Sci. 33 (11): 611–620. doi:10.1016/j.tips.2012.08.003. PMC 3482271. PMID 22981367.
  29. ↑ Barron, A. M.; Garcia-Segura, L. M.; Caruso, D.; Jayaraman, A.; Lee, J. -W.; Melcangi, R. C.; Pike, C. J. (2013). "Ligand for Translocator Protein Reverses Pathology in a Mouse Model of Alzheimer's Disease". The Journal of Neuroscience. 33 (20): 8891–8897. doi:10.1523/JNEUROSCI.1350-13.2013. PMC 3733563. PMID 23678130.
  30. ↑ Jaremko L, Jaremko M, Giller K, Becker S, Zweckstetter M (March 2014). "Structure of the mitochondrial translocator protein in complex with a diagnostic ligand". Science. 343 (6177): 1363–6. doi:10.1126/science.1248725. PMC 5650047. PMID 24653034.
  31. ↑ Fan J, Lindemann P, Feuilloley MG, Papadopoulos V (May 2012). "Structural and functional evolution of the translocator protein (18 kDa)". Curr. Mol. Med. 12 (4): 369–86. doi:10.2174/156652412800163415. PMID 22364126.
  32. ↑ Owen DR, Yeo AJ, Gunn RN, Song K, Wadsworth G, Lewis A, Rhodes C, Pulford DJ, Bennacef I, Parker CA, Stjean PL, Cardon LR, Mooser VE, Matthews PM, Rabiner EA, Rubio JP (October 2011). "An 18-kDa Translocator Protein (TSPO) polymorphism explains differences in binding affinity of the PET radioligand PBR28". J Cereb Blood Flow Metab. 32 (1): 1–5. doi:10.1038/jcbfm.2011.147. PMC 3323305. PMID 22008728.
  33. ↑ Airas L, Rissanen E, Tuisku J, Rinne J (April 2015). "Microglial Activation Correlates with Disease Progression in Multiple Sclerosis". Neurology. 86 (16 Supplement P4): 167.
  34. ↑ Mirzaei N, Tang SP, Ashworth S, Coello C, Plisson C, Passchier J, Selvaraj V, Tyacke RJ, Nutt DJ, Sastre M (2016). "In vivo imaging of microglial activation by positron emission tomography with [(11)C]PBR28 in the 5XFAD model of Alzheimer's disease". Glia. 64 (6): 993–1006. doi:10.1002/glia.22978. PMID 26959396.
  35. ↑ Pellow S, File SE (July 1984). "Behavioural actions of Ro 5-4864: a peripheral-type benzodiazepine?". Life Sciences. 35 (3): 229–40. doi:10.1016/0024-3205(84)90106-1. PMID 6087055.
  36. ↑ Gavish M, Bachman I, Shoukrun R, Katz Y, Veenman L, Weisinger G, Weizman A (December 1999). "Enigma of the peripheral benzodiazepine receptor". Pharmacological Reviews. 51 (4): 629–50. PMID 10581326.
  37. ↑ Papadopoulos V, Amri H, Boujrad N, Cascio C, Culty M, Garnier M, Hardwick M, Li H, Vidic B, Brown AS, Reversa JL, Bernassau JM, Drieu K (January 1997). "Peripheral benzodiazepine receptor in cholesterol transport and steroidogenesis". Steroids. 62 (1): 21–8. doi:10.1016/S0039-128X(96)00154-7. PMID 9029710.
  38. ↑ Costa E, Auta J, Guidotti A, Korneyev A, Romeo E (June 1994). "The pharmacology of neurosteroidogenesis". The Journal of Steroid Biochemistry and Molecular Biology. 49 (4–6): 385–9. doi:10.1016/0960-0760(94)90284-4. PMID 8043504.
  39. ↑ Garnier M, Boujrad N, Ogwuegbu SO, Hudson JR, Papadopoulos V (September 1994). "The polypeptide diazepam-binding inhibitor and a higher affinity mitochondrial peripheral-type benzodiazepine receptor sustain constitutive steroidogenesis in the R2C Leydig tumor cell line". The Journal of Biological Chemistry. 269 (35): 22105–12. PMID 8071335.
  40. ↑ Bormann J, Ferrero P, Guidotti A, Costa E (1985). "Neuropeptide modulation of GABA receptor C1- channels". Regulatory Peptides. Supplement. 4: 33–8. doi:10.1016/0167-0115(85)90215-0. PMID 2414820.

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