9.A.24 The Mitochondrial Cholesterol/Porphyrin Uptake Translocator Protein (TSPO) Family
The central channel Tom40 of the preprotein translocase of the outer membrane (TOM) complex is thought to be responsible for the import of virtually all preproteins synthesized outside the mitochondria. Otera et al. (2007) analyzed the topogenesis of the peripheral benzodiazepine receptor (PBR), which integrates into the mitochondrial outer membrane (MOM) through five hydrophobic transmembrane segments (TMSs) and functions in cholesterol and porphyrin import into the inner membrane. Analyses of in vitro and in vivo import into TOM component-depleted mitochondria reveal that PBR import (1) depends on the import receptor Tom70 but requires neither the Tom20 and Tom22 import receptors nor the import channel Tom40, (2) shares the post-Tom70 pathway with the C-tail-anchored proteins, and (3) requires factors in the mitochondrial intermembrane space. Furthermore, membrane integration of mitofusins and mitochondrial ubiquitin ligase, MOM proteins with two and four TMSs, respectively, proceeds through the same initial pathway. Thus, an unidentified pathway for the membrane integration of MOM proteins with multiple TMSs was identified (Otera et al., 2007).
Translocator protein (TSPO; PBR) has been reported to be an 18 kDa high affinity cholesterol, porphyrin uptake and drug-binding protein found primarily in the outer mitochondrial membrane. Although TSPO is found in many tissue types, it is expressed at the highest levels under normal conditions in tissues that synthesize steroids (Batarseh and Papadopoulos, 2010). TSPO, the 3-d structure of which is known (PDB# 2MGY) has been associated with cholesterol import into mitochondria, a key function in steroidogenesis, and directly or indirectly with multiple other cellular functions including apoptosis, cell proliferation, differentiation, anion transport, porphyrin transport, heme synthesis, and regulation of mitochondrial function (Jaremko et al. 2014). Aberrant expression of TSPO has been linked to multiple diseases, including cancer, brain injury, neurodegeneration, Parkinson''s and Alheimer''s diseases, and ischemia-reperfusion injury. It forms a large complex that includes VDAC-1, TSPO-associated protein-7 (PAP7; ACBD3), a protein kinase regulatory subunit, PKAR1A, and the StAR regluatory protein (Miller 2013). TSPO is conformationally flexible (Jaremko et al. 2015).
Neurosteroids are able to rapidly control the excitability of the central nervous system, acting as regulators of type A receptors for GABA. Neurosteroid level alterations occur in psychiatric disorders, including anxiety disorders. Investigators have manipulated neurosteroidogenesis in an effort to correct neuronal excitation and inhibition imbalances, which may lie at the root of neuropsychiatric conditions. A promising target for therapy of anxiety disorders is the Translocator Protein (TSPO). TSPO is expressed predominantly in steroid-synthesizing tissues and is localized to contact sites between the outer and inner mitochondrial membranes. It may mediate the rate-limiting step of neurosteroidogenesis. Brain concentrations of neurosteroids can be affected by selective TSPO activation. Indeed, TSPO drug ligands are able to stimulate primary neurosteroid formation that enhances GABAA receptor activity, pregnenolone and allopregnenalone, in both in vitro steroidogenic cells and in vivo animal models. A spectrum of TSPO ligands has been shown to exert anxiolytic actions when administered in rodents. The selective TSPO ligand, XBD173 (AC-5216, Emapunil), exerts anxiolytic effects not only in animal models, but also in humans. (Costa et al., 2012) reviewed the literature regarding the central nervous system biology of TSPO.
The TSPO (18 kDa translocator protein) is involved in cholesterol transport in organs that synthesize steroids and bile salts. Different natural and synthetic high-affinity TSPO ligands have been characterized through their ability to stimulate cholesterol transport, but they also stimulate other physiological processes including cell proliferation, apoptosis and calcium-dependent transepithelial ion secretion. TSPO is present in enterocyte mitochondria but not rat intestinal goblet cells (Ostuni et al. 2009). Enterocyte cytoplasm also contains the endogenous TSPO ligand, polypeptide DBI (diazepam-binding inhibitor). Whereas intestinal TSPO had high affinity for the synthetic ligand PK 11195, the pharmacological profile of TSPO in the duodenum was distinct from that in the jejunum and ileum. Specifically, benzodiazepine Ro5-4864 and protoporphyrin IX showed 5-13-fold lower affinity for duodenal TSPO. PK 11195 stimulated calcium-dependent chloride secretion in the duodenum and calcium-dependent chloride absorption in the ileum, but did not affect jejunum ion transport. Thus, the functional differences in subpopulations of TSPO in different regions of the intestine could be related to the structural organization of mitochondrial protein complexes that mediate the ability of TSPO to modulate either chloride secretion or absorption in the duodenum and ileum, respectively.
A 5 TMS bacterial homologue called the tryptophan-rich sensory protein of Rhodobacter spheroides binds retinoic acid, cucumin and an inhibitor of Bcl-2 actioin called gossypol (Li et al. 2013). It appears to function in porphyrin degredation in a light- and oxygen-dependent process (Ginter et al. 2013). The protein and its function(s) have been reviewed. TSPO is believed to be involved either directly or indirectly in numerous biological functions, including mitochondrial cholesterol transport and steroid hormone biosynthesis, porphyrin transport and heme synthesis, apoptosis, cell proliferation, and anion transport. Localized to the outer mitochondrial membrane of steroidogenic cells, TSPO has been shown to associate with cytosolic and mitochondrial proteins as part of a large multiprotein complex involved in mitochondrial cholesterol transport, the rate-limiting step in steroidogenesis. It has been concluded that TSPO is a unique mitochondrial pharmacological target for diseases that involve increased mitochondrial activity, including steroidogenesis, but the specific function is not clear (Papadopoulos et al. 2017).