Regulation of Mitochondrial, Cellular, and Organismal Functions by TSPO

In 1999, the enigma of the 18kDa mitochondrial translocator protein (TSPO), also known as the peripheral-type benzodiazepine receptor, was the seeming disparity of the many functions attributed to TSPO, ranging from the potential of TSPO acting as a housekeeping gene at molecular biological levels to adaptations to stress, and even involvement in higher emotional and cognitive functioning, such as anxiety and depression. In the years since then, knowledge regarding the many functions modulated by TSPO has expanded, and understanding has deepened. In addition, new functions could be firmly associated with TSPO, such as regulation of programmed cell death and modulation of gene expression. Interestingly, control by the mitochondrial TSPO over both of these life and death functions appears to include Ca⁺⁺ homeostasis, generation of reactive oxygen species (ROS), and ATP production. Other mitochondrial functions under TSPO control are considered to be steroidogenesis and tetrapyrrole metabolism. As TSPO effects on gene expression and on programmed cell death can be related to the wide range of functions that can be associated with TSPO, several of these five elements of Ca⁺⁺, ROS, ATP, steroids, and tetrapyrroles may indeed form the basis of TSPO's capability to operate as a multifunctional housekeeping gene to maintain homeostasis of the cell and of the whole multicellular organism.

Changes of TSPO-mediated mitophagy signaling pathway in learned helplessness mice

Low response rate was witnessed with the present monoaminergic based antidepressants, urging a need for new therapeutic target identification. Accumulated evidences strongly suggest that mitochondrial deficit is implicated in major depression and 18kDa translocator protein (TSPO) plays an important role in regulating mitochondrial function. However the changes of TSPO and TSPO mediated mitophagy pathway in the depressive brain is unclear. In present study, a well validated animal model of depression, learned helplessness (LH), was employed to investigate the relevant changes. Significant behavioral changes were observed in the LH mice. Results showed that TSPO and other mitophagy related proteins, such as VDAC1, Pink1 and Beclin1 were significantly decreased by LH challenge. Moreover, KIFC2, relevant to the mitochondrial transport and Snap25, relevant to neurotransmitter vesicle release, were also obviously down-regulated in the LH mice, which further rendered supportive evidence for the existing mitochondrial dysfunction in LH mice. Present results demonstrated that LH induced depressive symptoms and affected TSPO-mediated mitophagy pathway, indicating a potential target candidate for depression treatment.

Steroid hormone synthesis in mitochondria

Mitochondria are essential sites for steroid hormone biosynthesis. Mitochondria in the steroidogenic cells of the adrenal, gonad, placenta and brain contain the cholesterol side-chain cleavage enzyme, P450scc, and its two electron-transfer partners, ferredoxin reductase and ferredoxin. This enzyme system converts cholesterol to pregnenolone and determines net steroidogenic capacity, so that it serves as the chronic regulator of steroidogenesis. Several other steroidogenic enzymes, including 3β-hydroxysteroid dehydrogenase, 11β-hydroxylase and aldosterone synthase also reside in mitochondria. Similarly, the mitochondria of renal tubular cells contain two key enzymes participating in the activation and degradation of vitamin D. The access of cholesterol to the mitochondria is regulated by the steroidogenic acute regulatory protein, StAR, serving as the acute regulator of steroidogenesis. StAR action requires a complex multi-component molecular machine on the outer mitochondrial membrane (OMM). Components of this machine include the 18 kDa translocator protein (TSPO), the voltage-dependent anion chanel (VDAC-1), TSPO-associated protein 7 (PAP7, ACBD3), and protein kinase A regulatory subunit 1α (PKAR1A). The precise fashion in which these proteins interact and move cholesterol from the OMM to P450scc, and the means by which cholesterol is loaded into the OMM, remain unclear. Human deficiency diseases have been described for StAR and for all the mitochondrial steroidogenic enzymes, but not for the electron transfer proteins or for the components of the cholesterol import machine.

Repeated administration of AC-5216, a ligand for the 18 kDa translocator protein, improves behavioral deficits in a mouse model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a severely disabling anxiety disorder that may occur following exposure to a serious traumatic event. It is a psychiatric condition that can afflict anyone who has experienced a life-threatening or violent event. Previous studies have shown that changes in 18 kDa translocator protein (TSPO) expression (or function), a promising target for treating neurological disorders without benzodiazepine-like side effects, may correlate with PTSD. However, few studies have investigated the anti-PTSD effects of TSPO ligands. AC-5216, a ligand for TSPO, induces anxiolytic- and anti-depressant-like effects in animal models. The present study aimed to determine whether AC-5216 ameliorates PTSD behavior in mice. Following the training session consisting of exposure to inescapable electric foot shocks, animals were administered AC-5216 daily during the behavioral assessments, i.e., situational reminders (SRs), the open field (OF) test, the elevated plus-maze (EPM) test, and the staircase test (ST). The results indicated that exposure to foot shocks induced long-term behavioral deficiencies in the mice, including freezing and anxiety-like behavior, which were significantly ameliorated by repeated treatment with AC-5216 but without any effect on spontaneous locomotor activity or body weight. In summary, this study demonstrated the anti-PTSD effects of AC-5216 treatment, suggesting that TSPO may represent a therapeutic target for anti-PTSD drug discovery and that TSPO ligands may be a promising new class of drugs for the future treatment of PTSD.