History of Tspo deletion and induction in vivo: Phenotypic outcomes under physiological and pathological situations

The mitochondrial translocator protein (TSPO) is an outer mitochondrial protein membrane with high affinity for cholesterol. It is expressed in most tissues but is more particularly enriched in steroidogenic tissues. TSPO is involved in various biological mechanisms and TSPO regulation has been related to several diseases. However, despite a considerable number of published studies interested in TSPO over the past forty years, the precise function of the protein remains obscure. Most of the functions attributed to TSPO have been identified using pharmacological ligands of this protein, leading to much debate about the accuracy of these findings. In addition, research on the physiological role of TSPO has been hampered by the lack of in vivo deletion models. Studies to perform genetic deletion of Tspo in animal models have long been unsuccessful, which led to the conclusions that the deletion was deleterious and the gene essential to life. During the last decades, thanks to the significant technical advances allowing genome modification, several models of animal genetically modified for TSPO have been developed. These models have modified our view regarding TSPO and profoundly improved our fundamental knowledge on this protein. However, to date, they did not allow to elucidate the precise molecular function of TSPO and numerous questions persist concerning the physiological role of TSPO and its future as a therapeutic target. This article chronologically reviews the development of deletion and induction models of TSPO.

Among the recombinant TSPOs, the BcTSPO

Overexpression of recombinant Bacillus cereus TSPO (BcTSPO) in E. coli bacteria leads to its recovery with a bound hemin both in bacterial membrane (MB) and inclusion bodies (IB). Unlike mouse TSPO, BcTSPO purified in SDS detergent from IB is well structured and can bind various ligands such as high-affinity PK 11195, protoporphyrin IX (PPIX) and δ-aminolevulinic acid (ALA). For each of the three ligands, 1H-15N HSQC titration NMR experiments suggest that different amino acids of BcTSPO binding cavity are involved in the interaction. PPIX, an intermediate of heme biosynthesis, binds to the cavity of BcTSPO and its fluorescence can be significantly reduced in the presence of light and oxygen. The light irradiation leads to two products that have been isolated and characterized as photoporphyrins. They result from the addition of singlet oxygen to the two vinyl groups hence leading to the formation of hydroxyaldehydes. The involvement of water molecules, recently observed along with the binding of heme in Rhodobacter sphaeroides (RsTSPO) is highly probable. Altogether, these results raise the question of the role of TSPO in heme biosynthesis regulation as a possible scavenger of reactive intermediates.

In vitro functional screening as a means to identify new plasticizers devoid of reproductive toxicity

Plasticizers are indispensable additives providing flexibility and malleability to plastics. Among them, several phthalates, including di (2-ethylhexyl) phthalate (DEHP), have emerged as endocrine disruptors, leading to their restriction in consumer products and creating a need for new, safer plasticizers. The goal of this project was to use in vitro functional screening tools to select novel non-toxic plasticizers suitable for further in vivo evaluation. A panel of novel compounds with satisfactory plasticizer properties and biodegradability were tested, along with several commercial plasticizers, such as diisononyl-cyclohexane-1,2-dicarboxylate (DINCH®). MEHP, the monoester metabolite of DEHP was also included as reference compound. Because phthalates target mainly testicular function, including androgen production and spermatogenesis, we used the mouse MA-10 Leydig and C18-4 spermatogonial cell lines as surrogates to examine cell survival, proliferation, steroidogenesis and mitochondrial integrity. The most promising compounds were further assessed on organ cultures of rat fetal and neonatal testes, corresponding to sensitive developmental windows. Dose-response studies revealed the toxicity of most maleates and fumarates, while identifying several dibenzoate and succinate plasticizers as innocuous on Leydig and germ cells. Interestingly, DINCH®, a plasticizer marketed as a safe alternative to phthalates, exerted a biphasic effect on steroid production in MA-10 and fetal Leydig cells. MEHP was the only plasticizer inducing the formation of multinucleated germ cells (MNG) in organ culture. Overall, organ cultures corroborated the cell line data, identifying one dibenzoate and one succinate as the most promising candidates. The adoption of such collaborative approaches for developing new chemicals should help prevent the development of compounds potentially harmful to human health.

ACBD2/ECI2-Mediated Peroxisome-Mitochondria Interactions in Leydig Cell Steroid Biosynthesis

Fatty acid metabolism and steroid biosynthesis are 2 major pathways shared by peroxisomes and mitochondria. Both organelles are in close apposition to the endoplasmic reticulum, with which they communicate via interorganelle membrane contact sites to promote cellular signaling and the exchange of ions and lipids. To date, no convincing evidence of the direct contact between peroxisomes and mitochondria was reported in mammalian cells. Hormone-induced, tightly controlled steroid hormone biosynthesis requires interorganelle interactions. Using immunofluorescent staining and live-cell imaging, we found that dibutyryl-cAMP treatment of MA-10 mouse tumor Leydig cells rapidly induces peroxisomes to approach mitochondria and form peroxisome-mitochondrial contact sites/fusion, revealed by the subcellular distribution of the endogenous acyl-coenzyme A-binding domain (ACBD)2/ECI2 isoform A generated by alternative splicing, and further validated using a proximity ligation assay. This event occurs likely via a peroxisome-like structure, which is mediated by peroxisomal and mitochondrial matrix protein import complexes: peroxisomal import receptor peroxisomal biogenesis factor 5 (PEX5), and the mitochondrial import receptor subunit translocase of outer mitochondrial membrane 20 homolog (yeast) protein. Similar results were obtained using the mLTC-1 mouse tumor Leydig cells. Ectopic expression of the ACBD2/ECI2 isoform A in MA-10 cells led to increased basal and hormone-stimulated steroid formation, indicating that ACBD2/ECI2-mediated peroxisomes-mitochondria interactions favor in the exchange of metabolites and/or macromolecules between these 2 organelles in support of steroid biosynthesis. Considering the widespread occurrence of the ACBD2/ECI2 protein, we propose that this protein might serve as a tool to assist in understanding the contact between peroxisomes and mitochondria.

Translocator Protein-Mediated Stabilization of Mitochondrial Architecture during Inflammation Stress in Colonic Cells

Chronic inflammation of the gastrointestinal tract increasing the risk of cancer has been described to be linked to the high expression of the mitochondrial translocator protein (18 kDa; TSPO). Accordingly, TSPO drug ligands have been shown to regulate cytokine production and to improve tissue reconstruction. We used HT-29 human colon carcinoma cells to evaluate the role of TSPO and its drug ligands in tumor necrosis factor (TNF)-induced inflammation. TNF-induced interleukin (IL)-8 expression, coupled to reactive oxygen species (ROS) production, was followed by TSPO overexpression. TNF also destabilized mitochondrial ultrastructure, inducing cell death by apoptosis. Treatment with the TSPO drug ligand PK 11195 maintained the mitochondrial ultrastructure, reducing IL-8 and ROS production and cell death. TSPO silencing and overexpression studies demonstrated that the presence of TSPO is essential to control IL-8 and ROS production, so as to maintain mitochondrial ultrastructure and to prevent cell death. Taken together, our data indicate that inflammation results in the disruption of mitochondrial complexes containing TSPO, leading to cell death and epithelia disruption.

SIGNIFICANCE

This work implicates TSPO in the maintenance of mitochondrial membrane integrity and in the control of mitochondrial ROS production, ultimately favoring tissue regeneration.

Mitochondria-associated membrane formation in hormone-stimulated Leydig cell steroidogenesis: role of ATAD3

Leydig cell steroidogenesis is a multistep process that takes place in the mitochondria and endoplasmic reticulum (ER). The physical association between these 2 organelles could facilitate both steroidogenesis substrate availability and mitochondrial product passage to steroidogenic enzymes in the ER, thus regulating the rate of steroid formation. Confocal microscopy, using antisera against organelle-specific antigens, and electron microscopy studies demonstrated that there is an increase in the number of mitochondria-ER contact sites in response to hormone treatment in MA-10 mouse tumor Leydig cells. Electron tomography and 3-dimensional reconstruction allowed for the visualization of mitochondria-associated membranes (MAMs). MAMs were isolated and found to contain the 67-kDa long isoform of the adenosine triphosphatase (ATPase) family, AAA domain-containing protein 3 (ATAD3). The 67-kDa ATAD3 is anchored in the inner mitochondrial membrane and is enriched in outer-inner mitochondrial membrane contact sites. ATAD3-depleted MA-10 cells showed reduced production of steroids in response to human choriogonadotropin but not to 22R-hydroxycholesterol treatment, indicating a role of ATAD3 in the delivery of the substrate cholesterol into the mitochondria. The N terminus of ATAD3 contains 50 amino acids that have been proposed to insert into the outer mitochondrial membrane and associated organelles such as the ER. Deletion of the ATAD3 N terminus resulted in the reduction of hormone-stimulated progesterone biosynthesis, suggesting a role of ATAD3 in mitochondria-ER contact site formation. Taken together, these results demonstrate that the hormone-induced, ATAD3-mediated, MAM formation participates in the optimal transfer of cholesterol from the ER into mitochondria for steroidogenesis.

Organelle plasticity and interactions in cholesterol transport and steroid biosynthesis

Steroid biosynthesis is a multi-step process controlled by pituitary hormones, which, via cAMP-dependent signaling pathways, drive tissue-specific steroid formation. Steroidogenesis begins with the transport of the substrate, cholesterol, from intracellular stores into the inner mitochondrial membrane, where the steroidogenic enzyme CYP11A1 converts cholesterol to pregnenolone. This process is accelerated by hormones and involves a number of proteins and protein-protein interactions. Indeed, cholesterol, stored in lipid droplets and membranes, is transferred through a hormone-induced complex of proteins derived from the cytosol, mitochondria, and other organelles termed the transduceosome to the outer mitochondrial membrane. From there, cholesterol reaches CYP11A1 through outer/inner membrane contact sites. Thus, cholesterol transfer is likely achieved through a hormone-dependent reorganization of organelles and protein distribution and interactions. The findings reviewed herein suggest the presence of a hormone-dependent organelle communication network mediated by protein-protein interactions and inter-organelle trafficking, resulting in the efficient and timely delivery of cholesterol into mitochondria for steroid synthesis.

Identification of a dynamic mitochondrial protein complex driving cholesterol import, trafficking, and metabolism to steroid hormones

Steroid hormones are critical for organismal development and health. The rate-limiting step in steroidogenesis is the transport of cholesterol from the outer mitochondrial membrane (OMM) to the cytochrome P450 enzyme CYP11A1 in the inner mitochondrial membrane (IMM). Cholesterol transfer occurs through a complex termed the "transduceosome," in which cytosolic steroidogenic acute regulatory protein interacts with OMM proteins translocator protein and voltage-dependent anion channel (VDAC) to assist with the transfer of cholesterol to OMM. It has been proposed that cholesterol transfer from OMM to IMM occurs at specialized contact sites bridging the two membranes composed of VDAC and IMM adenine nucleotide translocase (ANT). Blue native PAGE of Leydig cell mitochondria identified two protein complexes that were able to bind cholesterol at 66- and 800-kDa. Immunoblot and mass spectrometry analyses revealed that the 800-kDa complex contained the OMM translocator protein (18-kDa) and VDAC along with IMM CYP11A1, ATPase family AAA domain-containing protein 3A (ATAD3A), and optic atrophy type 1 proteins, but not ANT. Knockdown of ATAD3A, but not ANT or optic atrophy type 1, in Leydig cells resulted in a significant decrease in hormone-induced, but not 22R-hydroxycholesterol-supported, steroid production. Using a 22-phenoxazonoxy-5-cholene-3-beta-ol CYP11A1-specific probe, we further demonstrated that the 800-kDa complex offers the microenvironment needed for CYP11A1 activity. Addition of steroidogenic acute regulatory protein to the complex mobilized the cholesterol bound at the 800-kDa complex, leading to increased steroid formation. These results identify a bioactive, multimeric protein complex spanning the OMM and IMM unit that is responsible for the hormone-induced import, segregation, targeting, and metabolism of cholesterol.

Overexpression of translocator protein in inflammatory bowel disease: potential diagnostic and treatment value

BACKGROUND

Inflammatory bowel diseases (IBDs), such as ulcerative colitis and Crohn's disease, are chronic inflammatory disorders that increase the risk for colorectal cancer. The mitochondrial translocator protein (TSPO) is a high-affinity drug- and cholesterol-binding protein expressed in the colon and its expression is increased in colon cancers. The aim of this study was to investigate TSPO expression in IBD biopsies and to establish an animal model of IBD to examine the role of TSPO. In addition, we evaluated the potential use of TSPO drug ligands in diagnosing and treating IBD.

METHODS

TSPO expression in IBD biopsies was evaluated using immunohistochemistry. IBD was induced in a rat experimental model via treatment with dextran sodium sulfate (DSS). Colon morphology, TSPO expression, and proinflammatory cytokine production were evaluated in addition to the effect of TSPO drug ligands on disease pathology.

RESULTS

TSPO protein levels were elevated in the enterocytes of IBD biopsies. TSPO expression was localized to the enterocyte mitochondria. DSS treatment induced a time-dependent phenotype mimicking IBD with tissue injury and subsequent tissue regeneration. Coadministration of DSS and the TSPO drug ligands PK 11195 or Ro5-4864 increased both the rate of colon ulceration and regeneration, whereas administration of the TSPO drug ligand flunitrazepam partially prevented this pathology. These data correlated with changes in proinflammatory cytokine plasma levels, as well as increased cytokine production and secretion from the colon.

CONCLUSIONS

TSPO may serve as a marker of the IBD repair process, and TSPO drug ligands should be further evaluated for IBD treatment.

Electron microscope tomography of native membranes

Membrane proteins are often present in low amounts in cells. Their function can be modulated by interactions with other proteins. Moreover, these complexes can be transiently formed, thus making them difficult to be isolated and to be purified. One way to overcome these difficulties is to visualize these complexes in situ in the cells. For such purpose, electron microscopy coupled to tomography is a promising approach that has been developed over the last decades.Mitochondria are a good example of organelles where many membrane proteins form different functional complexes within the outer and the inner membranes. The latter is either close to the former or projects within the matrix to form cristae. Structure of these cristae involves different proteins and can vary from lamellar to tubular forms in normal mitochondria. In pathological conditions, other mitochondrial morphologies have been described, for instance, vesicular structures for inner boundary membrane have been observed.