TSPO: kaleidoscopic 18-kDa amid biochemical pharmacology, control and targeting of mitochondria

The 18-kDa translocator protein (TSPO) localizes in the outer mitochondrial membrane (OMM) of cells and is readily up-regulated under various pathological conditions such as cancer, inflammation, mechanical lesions and neurological diseases. Able to bind with high affinity synthetic and endogenous ligands, its core biochemical function resides in the translocation of cholesterol into the mitochondria influencing the subsequent steps of (neuro-)steroid synthesis and systemic endocrine regulation. Over the years, however, TSPO has also been linked to core cellular processes such as apoptosis and autophagy. It interacts and forms complexes with other mitochondrial proteins such as the voltage-dependent anion channel (VDAC) via which signalling and regulatory transduction of these core cellular events may be influenced. Despite nearly 40 years of study, the precise functional role of TSPO beyond cholesterol trafficking remains elusive even though the recent breakthroughs on its high-resolution crystal structure and contribution to quality-control signalling of mitochondria. All this along with a captivating pharmacological profile provides novel opportunities to investigate and understand the significance of this highly conserved protein as well as contribute the development of specific therapeutics as presented and discussed in the present review

Developmental expression of translocator protein/peripheral benzodiazepine receptor in reproductive tissues.

Translocator protein (TSPO) present in the outer mitochondrial membrane has been suggested to be critical for cholesterol import, a rate-limiting step for steroid hormone biosynthesis. Despite the importance of steroidogenesis in regulating reproductive functions, the developmental profile of TSPO expression in the gonads and accessory sex organs has not been completely characterized. As a first step towards understanding the function of TSPO, we studied its expression in male and female murine reproductive organs. We examined testes and ovaries at embryonic days 14.5 and 18.5, and postnatal days 0, 7, 14, 21 and 56 of development. In the adult testis, TSPO was expressed in both Leydig cells and Sertoli cells. In the developing testes TSPO expression was seen in immature Sertoli cells, fetal Leydig cells and gonocytes. In the ovary, TSPO was expressed in the ovarian surface epithelium, interstitial cells granulosa cells and luteal cells. Corpora lutea of ovaries from pregnant mice showed strong expression of TSPO. In the developing ovary, TSPO expression was seen in the squamous pregranulosa cells associated with germ line cysts, together with progressively increasing expression in interstitial cells and the ovarian surface epithelium. In adult mice, the epithelia of other reproductive tissues like the epididymis, prostate, seminal vesicle, oviduct and uterus also showed distinct patterns of TSPO expression. In summary, TSPO expression in both male and female reproductive tissues was not only restricted to steroidogenic cells. Expression in Sertoli cells, ovarian surface epithelium, efferent ductal epithelium, prostatic epithelium, seminal vesiclular epithelium, uterine epithelium and oviductal epithelium suggest either previously unknown sites for de novo steroidogenesis or functions for TSPO distinct from its well-studied role in steroid hormone production

TSPO colocalizes with VDAC1 in Leydig and Sertoli cells.

<p>(A) Panel shows confocal images of TSPO and VDAC1 localization in the testis. TSPO labeling is not apparent in germ cells but specific fluorescence is visible in Sertoli cells (arrowhead) and Leydig cells. VDAC1 localization is seen in all cellular mitochondria including developing germ cells and the midpiece of spermatozoa. TSPO colocalization with VDAC1 is seen specifically in Leydig and Sertoli cells. (B) Negative control panel. (C) A 21-image Z-stack shows colocalization of TSPO and VDAC1 across optical sections in the X (green line) and Y (red line) axes. Specific overlap is seen at regions representing Leydig cells (L) and Sertoli cells (S). Scale bar 20 µm.</p

TSPO expression in the adult ovary is localized to the interstitial cells and granulosa cells.

<p>Immunohistochemical localization of TSPO in ovaries from an adult 8-week-old mouse. (A) The staining pattern for TSPO in a section dissecting the entire ovary. There was strong expression of TSPO observed in the interstitial cells. (B) TSPO expression was also strong in the ovarian surface epithelium in addition to the interstitial cells. Granulosa cells of a primary follicle (arrowhead) also showed TSPO expression. (C) Granulosa cells of a secondary follicle (arrowhead) expressed TSPO. Few theca cells around the follicle also showed weak expression of TSPO. (D) Squamous granulosa layer of a primordial follicle (arrowhead) showed TSPO expression. (E) Granulosa cells of most antral follicles show very weak to no expression of TSPO. (F) Regressing corpora lutea (arrowheads) also contained cells that show strong expression of TSPO. Scale bars 20 µm.</p

Developmental expression of TSPO in the ovary.

<p>Immunohistochemical localization of TSPO in embryonic (E14.5 and E18.5), and early postnatal stages (P0, P7, P14 and P21) of the developing ovary. At stages E14.5 and E18.5, TSPO was weak and appeared diffusely expressed in most cells forming the ovarian structure. Expression levels were higher at E18.5 compared to E14.5. Higher magnification images showed specific expression in the epithelial pregranulosa cells associated with the ovarian cysts (E14.5′ and E18.5′; a – progenitor in a germline cyst; b – pregranulosa cells). Compared to E18.5, TSPO expression at P0 was lower in the pregranulosa layer (P0′; c – pregranulosa cells) but there was strong expression in the germinal epithelium. Starting at P7, TSPO expression was observed in few clusters of interstitial cells that were very prominent in both numbers and expression at P14 and P21. TSPO expression pattern at P21 closely resembled that observed in the adult ovary. Scale bars 20 µm.</p

TSPO is expressed in the epithelium of the seminal vesicle and prostate.

<p>Immunohistochemical localization of TSPO in the seminal vesicle and prostate from an 8-week-old mouse. (A) Epithelium of the seminal vesicle showed weak expression of TSPO that was diffuse throughout the cytoplasm. (B) Epithelium of the ventral prostate showed strong expression of TSPO also distributed evenly in the cytoplasm. (C) Epithelium of the dorsolateral prostate showed a distinct apically polarized TSPO expression. Boxed regions in panels A, B and C are magnified in panels A’, B’ and C’. Scale bars 20 µm (Low magnification panel) and 10 µm (High magnification panel).</p

TSPO is expressed in the active corpus luteum.

<p>Immunohistochemical localization of TSPO in an ovary from a 14.5-day post coitus pregnant mouse. (A) The staining pattern for TSPO in a section dissecting the entire ovary. There was strong expression of TSPO in the two corpora lutea (arrowheads). (B) Higher magnification image of the boxed region in panel A showing variable levels of TSPO expression between different cells that form the corpus luteum. Both large and small luteal cells of the corpus luteum showed expression of TSPO. Scale bars 20 µm.</p