Translocator protein

Translocator protein (18 kDa) (TSPO), previously called peripheral benzodiazepine receptor (PBR), is a glial protein

It is an 18 kDa protein mainly found on the outer mitochondrial membrane. It interacts with StAR (steroidogenic acute regulatory protein) to transport cholesterol into mitochondria. 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.

In humans, the translocator protein is encoded by the TSPO gene.

It belongs to family of tryptophan-rich sensory proteins.

Translocator protein 18kDa (TSPO) is now an attractive drug target for controlling neuroinflammation. Studies applying TSPO ligands to neurodegenerative diseases, especially Alzheimer's disease (AD), were rare.

The 18-kDa mitochondrial translocator protein (TSPO) was reported to be upregulated in gliomas. 18F-GE-180 is a novel 3rd generation TSPO receptor ligand with improved target-to-background contrast compared to previous tracers. In this pilot study, we compared PET imaging with 18F-GE-180 and MRI of patients with untreated and recurrent pretreated glioblastoma. METHODS: Eleven patients with histologically confirmed IDH wildtype gliomas (10 glioblastomas, 1 anaplastic astrocytoma) underwent 18F-GE-180 PET at initial diagnosis or recurrence. The PET parameters mean background uptake (SUVBG), maximal tumour-to-background ratio (TBRmax) and PET volume using different thresholds (SUVBG × 1.6, 1.8 and 2.0) were evaluated in the 60-80 min p.i. summation images. The different PET volumes were compared to the contrast-enhancing tumour volume on MRI.

All gliomas were positive on 18F-GE-180 PET and were depicted with extraordinarily high tumour-to-background contrast (median SUVBG 0.47 (0.37-0.93), TBRmax 6.61 (3.88-9.07)). 18F-GE-180 uptake could be found even in areas without contrast enhancement on MRI, leading to significantly larger PET volumes than MRI-based volumes (median 90.5, 74.5, and 63.8 mL vs. 31.0 mL; p = 0.003, 0.004, 0.013). In percentage difference, the PET volumes were on average 179%, 135%, and 90% larger than the respective MRI volumes. The median spatial volumetric correlation (Sørensen-Dice coefficient) of PET volumes and MRI volumes prior to radiotherapy was 0.48, 0.54, and 0.58.

18F-GE-180 PET provides a remarkably high tumour-to-background contrast in untreated and pretreated glioblastoma and shows tracer uptake even beyond contrast enhancement on MRI. To what extent 18F-GE-180 uptake reflects the tumour extent of human gliomas and inflammatory cells remains to be evaluated in future prospective studies with guided stereotactic biopsies and correlation of histopathological results 1).

A study was aimed to evaluate the effect of PK11195, a specific TSPO ligand, in an animal model of neuroinflammation caused by systemic LPS administration. C57/BL6 mice were treated with lipopolysaccharide (LPS, 500μg/kg, i.p.) three days after PK11195 administration (3mg/kg, i.p.). The drugs were not discontinued until the mice were sacrificed. Cognitive function was assessed by Morris water maze (MWM) seven days after LPS injection. Chronic LPS-injection in mice was characterized by cognitive dysfunction, increased expression of cyclooxygenase (COX)-2 and TSPO, elevated Aβ content with increased expression of β-site APP cleaving enzyme-1 (BACE-1) and Insulin-degrading enzyme (IDE) as well as decreased brain progesterone and brain-derived neurophic factor (BDNF) level. PK11195 pretreatment protected cognitive function in LPS-injected animals and normalized the inflammatory proteins. Moreover, PK11195 pre-administration decreased elevated hippocampal Aβx-42 levels and increased brain levels of progesterone, allopregnanolone. However, LPS-induced BDNF decrease was not reversed by PK11195 administration. Our data demonstrated that PK11195 could protect cognitive deficits induced by chronic LPS administration. The underling mechanism may involve alleviated neuroinflammation, increased synthesis of neurosteroid and decreased Aβ accumulation accompanied by down-regulation of BACE-1 2).

Albert NL, Unterrainer M, Fleischmann DF, Lindner S, Vettermann F, Brunegraf A, Vomacka L, Brendel M, Wenter V, Wetzel C, Rupprecht R, Tonn JC, Belka C, Bartenstein P, Niyazi M. TSPO PET for glioma imaging using the novel ligand (18)F-GE-180: first results in patients with glioblastoma. Eur J Nucl Med Mol Imaging. 2017 Aug 19. doi: 10.1007/s00259-017-3799-9. [Epub ahead of print] PubMed PMID: 28821920.
Ma L, Zhang H, Liu N, Wang PQ, Guo WZ, Fu Q, Jiao LB, Ma YQ, Mi WD. TSPO ligand PK11195 alleviates neuroinflammation and beta-amyloid generation induced by systemic LPS administration. Brain Res Bull. 2016 Feb 2. pii: S0361-9230(16)30014-4. doi: 10.1016/j.brainresbull.2016.02.001. [Epub ahead of print] PubMed PMID: 26851069.
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