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The translocator protein (18 kDa) (TSPO) is localized primarily in the outer mitochondrial membrane of steroid-synthesizing cells, including those in the central and peripheral nervous system. One of its main functions is the transport of the substrate cholesterol into mitochondria, a prerequisite for steroid synthesis. TSPO expression may constitute a biomarker of brain inflammation and reactive gliosis that could be monitored by using TSPO ligands as neuroimaging agents. Moreover, initial clinical trials have indicated that TSPO ligands might be valuable in the treatment of neurological and psychiatric disorders. This Review focuses on the biology and pathophysiology of TSPO and the potential of currently available TSPO ligands for the diagnosis and treatment of neurological and psychiatric disorders.

Leydig cells (LCs) are thought to differentiate from spindle-shaped precursor cells that exhibit some aspects of differentiated function, including 3␤-hydroxysteroid dehydrogenase (3␤HSD) activity. The precursor cells ultimately derive from undifferentiated stem LCs (SLCs), which are postulated to be present in testes before the onset of precursor cell differentiation. We searched for cells in the neonatal rat testis with the abilities to: (i) proliferate and expand indefinitely in vitro (self renew); (ii) differentiate (i.e., 3␤HSD and ultimately synthesize testosterone); and (iii) when transplanted into host rat testes, colonize the interstitium and subsequently differentiate in vivo. At 1 week postpartum, spindle-shaped cells were seen in the testicular interstitium that differed from the precursor cells in that they were 3␤HSD-negative, luteinizing hormone (LH) receptor (LHR)-negative, and platelet-derived growth factor receptor ␣ (PDGFR␣)-positive. These cells were purified from the testes of 1-week-old rats. The cells contained proteins known to be involved in LC development, including GATA4, c-kit receptor, and leukemia inhibitory factor receptor. The putative SLCs expanded over the course of 6 months while remaining undifferentiated. When treated in media that contained thyroid hormone, insulin-like growth factor I, and LH, 40% of the putative SLCs came to express 3␤HSD and to synthesize testosterone. When transplanted into host rat testes from which LCs had been eliminated, the putative SLCs colonized the interstitium and subsequently expressed 3␤HSD, demonstrating their ability to differentiate in vivo. We conclude that these cells are likely to be the sought-after SLCs.c-kit ͉ leukemia inhibitory factor ͉ platelet-derived growth factor receptor ␣ ͉ puberty ͉ steroidogenesis L eydig cells (LCs) are the primary source of testosterone in the male, and their differentiation in the testes during puberty is a signature event in the development of the male body plan. It is hypothesized, but far from proven, that LCs first arise from undifferentiated stem cells [stem LCs (SLCs)] (1-3). It has been suggested that, in rats, the putative SLCs are present in the testis at birth, and that by 11 days postpartum, at least some of their progeny express LC-specific genes and thus become committed to the LC lineage (4, 5).The committed cells subsequently undergo phased transitions through progenitor and immature stages and ultimately to terminally differentiated adult LC stage (6). In particular, progenitor LCs (PLCs) form during days 12-28 postpartum (presumably from SLCs). The PLCs proliferate and also exhibit some aspects of differentiated function, including 3␤-hydroxysteroid dehydrogenase (3␤HSD) activity (7). Luteinizing hormone (LH) receptors (LHRs) first appear as the PLCs differentiate, suggesting that SLCs are likely to be independent of LH control (8). The development of the steroidogenic capacity of PLCs requires stimulation by LH (9). The mitotic activity of PLCs gradually is reduced, and the cells enlarge...

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The transfer of cholesterol from the outer to the inner mitochondrial membrane is the rate-limiting step in hormone-induced steroid formation. To ensure that this step is achieved efficiently, free cholesterol must accumulate in excess at the outer mitochondrial membrane and then be transferred to the inner membrane. This is accomplished through a series of steps that involve various intracellular organelles, including lysosomes and lipid droplets, and proteins such as the translocator protein (18 kDa, TSPO) and steroidogenic acute regulatory (StAR) proteins. TSPO, previously known as the peripheral-type benzodiazepine receptor, is a high-affinity drug-and cholesterolbinding mitochondrial protein. StAR is a hormone-induced mitochondria-targeted protein that has been shown to initiate cholesterol transfer into mitochondria. Through the assistance of proteins such as the cAMP-dependent protein kinase regulatory subunit Iα (PKA-RIα) and the PKA-Rlα TSPOassociated acyl-coenzyme A binding domain containing 3 (ACBD3) protein, PAP7, cholesterol is transferred to and docked at the outer mitochondrial membrane. The TSPO-dependent import of StAR into mitochondria, and the association of TSPO with the outer/inner mitochondrial membrane contact sites, drives the intramitochondrial cholesterol transfer and subsequent steroid formation. The focus of this review is on (i) the intracellular pathways and protein-protein interactions involved in cholesterol transport and steroid biosynthesis and (ii) the roles and interactions of these proteins in endocrine pathologies and neurological diseases where steroid synthesis plays a critical role. KeywordsSteroid biosynthesis; translocator protein; steroidogenic acute regulatory protein; peripheral benzodiazepine receptor Cholesterol and Steroid SynthesisCholesterol is the sole precursor of steroids. Steroid synthesis is initiated at the inner mitochondrial membrane (IMM), where the cytochrome P450 cholesterol side chain cleavage