Translocator protein

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Translocator protein (TSPO) is a small protein, about 18 kilodaltons, located on the outer membrane of mitochondria. It was once called the peripheral benzodiazepine receptor (PBR) because it binds certain benzodiazepine-related drugs, but TSPO is found in many tissues throughout the body and brain. The human TSPO gene encodes this protein.

A central idea in TSPO research is its role in moving cholesterol into mitochondria, a key step in making steroid hormones. TSPO is thought to work with other cholesterol-handling proteins, but evidence from animal studies is mixed. In some experiments, removing TSPO from steroid-producing cells does not completely stop steroid synthesis, so TSPO’s essential role in this process remains uncertain.

In the heart, TSPO can interact with calcium channels and influence heart rate and the strength of contraction. When TSPO levels rise during infection or inflammation, it may help limit damaging inflammatory responses, which can protect heart tissue.

TSPO also plays a part in the immune system. It affects how immune cells produce reactive oxygen species, respond to inflammation, and undergo programmed cell death. Increased TSPO expression is seen in various inflammatory conditions, including brain injury, certain neurodegenerative diseases, and cardiovascular inflammation such as myocarditis.

Ligands that bind TSPO can influence cell survival and steroid production. Some TSPO ligands promote apoptosis in certain cancer cells and can stimulate the production of neuroactive steroids in the brain, which may have anxiolytic (anti-anxiety) effects. However, toxicity and other challenges limit their therapeutic use.

Because TSPO levels rise with inflammation, the protein serves as a biomarker for inflammation. Radiolabeled TSPO ligands are used to image inflammation in the brain and heart (for example, to assess microglial activation or inflammatory damage). A common genetic variation in TSPO (rs6971) affects how strongly some ligands bind, which can influence imaging results.

Structurally, TSPO comprises five transmembrane helices and functions as a monomer. The best-studied complex is TSPO bound to the diagnostic ligand PK11195, whose high-resolution structure helped confirm its helical architecture and how ligands fit into the binding pocket.

In summary, TSPO is a mitochondria-associated protein with roles in cholesterol transport, steroid production, inflammation, immune function, and cell death. Its activity is context-dependent and modulated by various ligands, making it a useful target for imaging inflammation and exploring potential therapies, even as scientists continue to clarify its essential physiological functions.