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

Novel insights into the mechanisms of seasonal cyclicity of testicles by proteomics and transcriptomics analyses in goose breeder lines

Spermatogenesis is a crucial indicator of geese reproduction performance and production. The testis is the main organ responsible for sperm production, and the egg-laying cycle in geese is a complex physiological process that demands precise orchestration of hormonal cues and cellular events within the testes, however, the seasonal changes in the transcriptomic and proteomic profiles of goose testicles remain unclear. To explore various aspects of the mechanisms of the seasonal cyclicity of testicles in different goose breeds, in this study, we used an integrative transcriptomic and proteomic approach to screen the key genes and proteins in the testes of 2 goose males, the Hungarian white goose and the Wanxi white goose, at 3 different periods of the laying cycle: beginning of laying cycle (BLC), peak of laying cycle (PLC), and end of laying cycle (ELC). The results showed that a total of 9,273 differentially expressed genes and 4,543 differentially expressed proteins were identified in the geese testicles among the comparison groups. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis suggested that the DEGs, in the comparison groups, were mainly enrichment in metabolic pathways, neuroactive ligand-receptor interaction, cyctokine-cyctokine receptor interaction, calcium signaling pathway, apelin signaling pathway, ether lipid metabolism, cysteine, and methionine metabolism. While the DEPs, in the 3 comparison groups, were mainly involved in the ribosome, metabolic pathways, carbon metabolism, proteasome, endocytosis, lysosome, regulation of actin cytoskeleton, oxidative phosphorylation, nucleocytoplasmic transport, and tight junction. The protein-protein interaction network analysis (PPI) indicated that selected DEPs, such as CHD1L, RAB18, FANCM, TAF5, TSC1/2, PHLDB2, DNAJA2, NCOA5, DEPTOR, TJP1, and RAPGEF2, were highly associated with male reproductive regulation. Further, the expression trends of 4 identified DEGs were validated by qRT-PCR. In conclusion, this work offers a new perspective on comprehending the molecular mechanisms and pathways involved in the seasonal cyclicity of testicles in the Hungarian white goose and the Wanxi white goose, as well as contributing to improving goose reproductive performance.

Translocator protein expression and localization in human endometrium and endometriosis: A potential target for a noninvasive diagnosis?

The limitations of current imaging methods to detect small or superficial endometriotic lesions prompt the search for new molecular targets. TSPO is an 18 KDa protein located in the outer mitochondrial membrane, which can be traced by positron emission tomography (PET) using specific ligands. TSPO is located mostly in neurons and inflammatory sites outside the brain. We hypothesized that it might also be expressed in the human endometrium and endometrial-like tissue, being a target for molecular imaging of endometriosis. This prospective cross-sectional study included 28 women with endometriosis and 11 endometriosis-free controls. Endometriotic lesions (n = 49) and normal peritoneum (n = 13) from endometriosis patients were obtained during laparoscopy, while samples of eutopic endometrium from patients with endometriosis (n = 28) and from control women (n = 11) were collected in the operating room using a flexible device. TSPO mRNA expression was evaluated by quantitative reverse-transcription real-time PCR while protein expression was evaluated by immunohistochemistry with a monoclonal antibody antihuman TSPO. TSPO mRNA expression was detected in an invariable fashion in all tissue types evaluated; however, TSPO protein was found to be more abundant in the glandular epithelium than in the stroma, both in the endometrium and in the endometriotic lesions. Interestingly, hormone therapies did not alter the expression of TSPO, and its presence was mostly negative in tissues adjacent to endometriotic implants. As a proof of concept, the protein expression pattern of TSPO in endometriotic tissue and along the adjacent areas suggests that TSPO-based molecular imaging might be used for noninvasive endometriosis detection.

Intergenerational Hyperglycemia Impairs Mitochondrial Function and Follicular Development and Causes Oxidative Stress in Rat Ovaries Independent of the Consumption of a High-Fat Diet

We analyzed the influence of maternal hyperglycemia and the post-weaning consumption of a high-fat diet on the mitochondrial function and ovarian development of the adult pups of diabetic rats. Female rats received citrate buffer (Control-C) or Streptozotocin (for diabetes induction-D) on postnatal day 5. These adult rats were mated to obtain female pups (O) from control dams (OC) or from diabetic dams (OD), and they received a standard diet (SD) or high-fat diet (HFD) from weaning to adulthood and were distributed into OC/SD, OC/HFD, OD/SD, and OD/HFD. In adulthood, the OGTT and AUC were performed. These rats were anesthetized and euthanized for sample collection. A high percentage of diabetic rats were found to be in the OD/HFD group (OD/HFD 40% vs. OC/SD 0% p < 0.05). Progesterone concentrations were lower in the experimental groups (OC/HFD 0.40 ± 0.04; OD/SD 0.30 ± 0.03; OD/HFD 0.24 ± 0.04 vs. OC/SD 0.45 ± 0.03 p < 0.0001). There was a lower expression of MFF (OD/SD 0.34 ± 0.33; OD/HFD 0.29 ± 0.2 vs. OC/SD 1.0 ± 0.41 p = 0.0015) and MFN2 in the OD/SD and OD/HFD groups (OD/SD 0.41 ± 0.21; OD/HFD 0.77 ± 0.18 vs. OC/SD 1.0 ± 0.45 p = 0.0037). The number of follicles was lower in the OD/SD and OD/HFD groups. A lower staining intensity for SOD and Catalase and higher staining intensity for MDA were found in ovarian cells in the OC/HFD, OD/SD, and OD/HFD groups. Fetal programming was responsible for mitochondrial dysfunction, ovarian reserve loss, and oxidative stress; the association of maternal diabetes with an HFD was responsible for the higher occurrence of diabetes in female adult pups.

Functional metabolite reserves and lipid homeostasis revealed by the MA-10 Leydig cell metabolome

In Leydig cells, intrinsic factors that determine cellular steroidogenic efficiency is of functional interest to decipher and monitor pathophysiology in many contexts. Nevertheless, beyond basic regulation of cholesterol storage and mobilization, systems biology interpretation of the metabolite networks in steroidogenic function is deficient. To reconstruct and describe the different molecular systems regulating steroidogenesis, we profiled the metabolites in resting MA-10 Leydig cells. Our results identified 283-annotated components (82 neutral lipids, 154 membrane lipids, and 47 other metabolites). Neutral lipids were represented by an abundance of triacyglycerols (97.1%), and low levels of cholesterol esters (2.0%). Membrane lipids were represented by an abundance of glycerophospholipids (77.8%), followed by sphingolipids (22.2%). Acylcarnitines, nucleosides, amino acids and their derivatives were the other metabolite classes identified. Among nonlipid metabolites, we recognized substantial reserves of aspartic acid, choline, creatine, betaine, glutamine, homoserine, isoleucine, and pantothenic acid none of which have been previously considered as a requirement in steroidogenic function. Individually limiting use of betaine, choline, or pantothenic acid, during luteinizing hormone-induced steroidogenesis in MA-10 cells resulted in substantial decreases to acute steroidogenic capacity, explained by intermediary metabolite imbalances affecting homeostasis. As such, our dataset represents the current level of baseline characterization and unravels the functional resting state of steroidogenic MA-10 Leydig cells. In identifying metabolite stockpiles and causal mechanisms, these results serve to further comprehend the cellular setup and regulation of steroid biosynthesis.

Mitochondrial translocator protein deficiency exacerbates pathology in acute experimental ulcerative colitis

In human patients and animal models of ulcerative colitis (UC), upregulation of the mitochondrial translocator protein (TSPO) in the colon is consistent with inflammation. Although the molecular function for TSPO remains unclear, it has been investigated as a therapeutic target for ameliorating UC pathology. In this study, we examined the susceptibility of Tspo gene-deleted (Tspo ^-/- ) mice to insults as provided by the dextran sodium sulfate (DSS)-induced acute UC model. Our results show that UC clinical signs and pathology were severely exacerbated in Tspo ^-/- mice compared to control Tspo ^fl/fl cohorts. Histopathology showed extensive inflammation and epithelial loss in Tspo ^-/- mice that caused an aggravated disease. Colonic gene expression in UC uncovered an etiology linked to precipitous loss of epithelial integrity and disproportionate mast cell activation assessed by tryptase levels in Tspo ^-/- colons. Evaluation of baseline homeostatic shifts in Tspo ^-/- colons revealed gene expression changes noted in elevated epithelial Cdx2, mast cell Cd36 and Mcp6, with general indicators of lower proliferation capacity and elevated mitochondrial fatty acid oxidation. These findings demonstrate that intact physiological TSPO function serves to limit inflammation in acute UC, and provide a systemic basis for investigating TSPO-targeting mechanistic therapeutics.

Efficient induction and sustenance of pluripotent stem cells from bovine somatic cells

Although derivation of naïve bovine embryonic stem cells is unachieved, the possibility for generation of bovine induced pluripotent stem cells (biPSCs) has been generally reported. However, attempts to sustain biPSCs by promoting self-renewal have not been successful. Methods established for maintaining murine and human induced pluripotent stem cells (iPSCs) do not support self-renewal of iPSCs for any bovid species. In this study, we examined methods to enhance complete reprogramming and concurrently investigated signaling relevant to pluripotency of the bovine blastocyst inner cell mass (ICM). First, we identified that forced expression of SV40 large T antigen together with the reprogramming genes (OCT4, SOX2, KLF4 and MYC) substantially enhanced the reprogramming efficacy of bovine fibroblasts to biPSCs. Second, we uncovered that TGFβ signaling is actively perturbed in the ICM. Inhibition of ALK4/5/7 to block TGFβ/activin/nodal signaling together with GSK3β and MEK1/2 supported robust in vitro self-renewal of naïve biPSCs with unvarying colony morphology, steady expansion, expected pluripotency gene expression and committed differentiation plasticity. Core similarities between biPSCs and stem cells of the 16-cell-stage bovine embryo indicated a stable ground state of pluripotency; this allowed us to reliably gain predictive understanding of signaling in bovine pluripotency using systems biology approaches. Beyond defining a high-fidelity platform for advancing biPSC-based biotechnologies that have not been previously practicable, these findings also represent a significant step towards understanding corollaries and divergent aspects of bovine pluripotency.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: Pluripotency reprogramming by overcoming the stable epigenome of bovine cells, and uncovering precise early embryo self-renewal mechanisms enables sustenance and expansion of authentic induced pluripotent stem cells in vitro.

TSPO: an emerging role in appetite for a therapeutically promising biomarker

There is accumulating evidence that an obesogenic Western diet causes neuroinflammatory damage to the brain, which then promotes further appetitive behaviour. Neuroinflammation has been extensively studied by analysing the translocator protein of 18 kDa (TSPO), a protein that is upregulated in the inflamed brain following a damaging stimulus. As a result, there is a rich supply of TSPO-specific agonists, antagonists and positron emission tomography ligands. One TSPO ligand, etifoxine, is also currently used clinically for the treatment of anxiety with a minimal side-effect profile. Despite the neuroinflammatory pathogenesis of diet-induced obesity, and the translational potential of targeting TSPO, there is sparse literature characterizing the effect of TSPO on appetite. Therefore, in this review, the influence of TSPO on appetite is discussed. Three putative mechanisms for TSPO's appetite-modulatory effect are then characterized: the TSPO–allopregnanolone–GABA_AR signalling axis, glucosensing in tanycytes and association with the synaptic protein RIM-BP1. We highlight that, in addition to its plethora of functions, TSPO is a regulator of appetite. This review ultimately suggests that the appetite-modulating function of TSPO should be further explored due to its potential therapeutic promise.

Temporal kinetics of bovine mammary IgG secretion into colostrum and transition milk

Neonatal calf survival and health is predominantly dependent on sufficient consumption of immunoglobulin G (IgG) and the resulting transfer of passive immunity (TPI). In this study, we investigate the potential for continued IgG secretion and temporal kinetics of mammary IgG output in sequential milkings performed at 0, 4, 16, 28, 40, and 52 hr postcalving in Holstein dairy cows. For colostrum (0 hr), we also scrutinize the relationships between IgG concentration, volume, refractometer readings (˚Bx values, Brix) and concentration of sugars (lactose and glucose). Mammary transcripts postpartum (0 hr) indicated that active IgG secretion continues beyond the first milking (colostrum; n = 4 to 5). IgG measurements at the different timepoints indicated that colostrum represents only 25.1% of the total IgG produced across the 6 sequential milking timepoints, with a substantial 48.9% being secreted into transition milk over the next 3 timepoints (4-, 6-, and 28-hr) combined. The differences on the basis of IgG concentrations across 0-, 4-, and 16-hr milking timepoints were not statistically significant (P = 0.1522; n = 9). For colostrum, volume remained highly variable, even with induced let-down prior to milking (n = 27). Nonetheless, colostrum IgG secretion was significantly co-regulated with volume (R2 = 0.915; P < 0.001; n = 18), an association that was stronger than that measured for lactose (R2 = 0.803; P < 0.001; n = 18) and glucose (R2 = 0.467; P = 0.002; n = 17). Comparing colostrum ˚Bx values to absolute IgG concentrations showed no correlation (R2 = 0.127; P = 0.07; n = 27); biochemical separation of colostrum components indicated that both proteins and nonprotein solutes could affect ˚Bx values (P < 0.0001 for both; n = 5). This suggests that ˚Bx values do not reasonably indicate IgG concentration to serve as a measure of "colostrum quality." Additionally, our finding that early transition milk (4-, 6-, and 28-hr) can contribute substantially more IgG than colostrum forces a rethink of existing feeding paradigms and means to maximize TPI in calves. Collectively, our results reveal the remarkable value of early transition milk and caveats to colostrum assessments that could advance application in enhancing neonatal calf health.

Gonadotropin-inhibiting hormone promotes apoptosis of bovine ovary granulosa cells

Gonadotropin-inhibiting hormone (GnIH) inhibits the synthesis and release of gonadotropin by binding to its receptor. GnIH is involved in animal reproductive regulation, especially ovary function. It can regulate the proliferation, apoptosis and hormone secretion of follicular cells. However, the role and molecular mechanism of GnIH in bovine granulosa cell (bGC) apoptosis is unclear. Here, the effects of GnIH on proliferation, apoptosis, and mitochondrial function of bGCs were detected. A 10^-6 mol/mL concentration of GnIH inhibited bGC proliferation, promoted GC apoptosis, and damaged mitochondrial function. Additionally, GnIH significantly decreased the phosphorylation level of p38 (P < 0.01). To explore the role of the p38 signaling pathway in the process of GnIH-induced apoptosis in bGCs, an activator of p38 (U46619) was used to pretreat bGCs. U46619 pretreatment significantly alleviated GnIH damage to bGCs, including proliferation, apoptosis, and mitochondrial function. In conclusion, these results demonstrated that GnIH inhibited proliferation and promoted apoptosis of bGCs via the p38 signaling pathway.

The adiponectin agonist, AdipoRon, inhibits steroidogenesis and cell proliferation in human luteinized granulosa cells

During obesity, excess body weight is not only associated with an increased risk of type 2-diabetes, but also several other pathological processes, such as infertility. Adipose tissue is the largest endocrine organ of the body that produces adipokines, including adiponectin. Adiponectin has been reported to control fertility through the hypothalamic-pituitary-gonadal axis, and folliculogenesis in the ovaries. In this study, we focused on a recent adiponectin-like synthetic agonist called AdipoRon, and its action in human luteinized granulosa cells. We demonstrated that AdipoRon activated the adenosine monophosphate-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor alpha (PPAR) signalling pathways in human luteinized granulosa cells. A 25 μM AdipoRon stimulation reduced granulosa cell proliferation by inducing cell cycle arrest in G₁, associated with PTEN and p53 pathway activation. In addition, AdipoRon perturbed cell metabolism by decreasing mitochondrial activity and ATP production. In human luteinized granulosa cells, AdipoRon increased phosphodiesterase activity, leading to a drop in cyclic adenosine monophosphate (cAMP) production, aromatase expression and oestrogens secretion. In conclusion, AdipoRon impacted folliculogenesis by altering human luteinized granulosa cell function, via steroid production and cell proliferation. This agonist may have applications for improving ovarian function in metabolic disorders or granulosa cancers.

Lack of adrenal TSPO/PBR expression in hamsters reinforces correlation to triglyceride metabolism

Despite being a highly conserved protein, the precise role of the mitochondrial translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), remains elusive. The void created by studies that overturned a presumptive model that described TSPO/PBR as a mitochondrial cholesterol transporter for steroidogenesis has been filled with evidence that it can affect mitochondrial metabolic functions across different model systems. We previously reported that TSPO/PBR deficient steroidogenic cells upregulate mitochondrial fatty acid oxidation and presented a strong positive correlation between TSPO/PBR expression and tissues active in triglyceride metabolism or lipid storage. Nevertheless, the highlighting of inconsistencies in prior work has provoked reprisals that threaten to stifle progress. One frequent factoid presented as being supportive of a cholesterol import function is that there are no steroid-synthesizing cell types without high TSPO/PBR expression. In this study, we examine the hamster adrenal gland that is devoid of lipid droplets in the cortex and largely relies on de novo cholesterol biosynthesis and uptake for steroidogenesis. We find that Tspo expression in the hamster adrenal is imperceptible compared to the mouse. This observation is consistent with a substantially low expression of Cpt1a in the hamster adrenal, indicating minimal mitochondrial fatty acid oxidation capacity compared to the mouse. These findings provide further reinforcement that the much sought-after mechanism of TSPO/PBR function remains correlated with the extent of cellular triglyceride metabolism. Thus, TSPO/PBR could have a homeostatic function relevant only to steroidogenic systems that manage triglycerides associated with lipid droplets.

Mitochondrial respiratory chain complex abnormal expressions and fusion disorder are involved in fluoride-induced mitochondrial dysfunction in ovarian granulosa cells

Excessive fluoride intake has a strong female reproductive toxicity, which can result in follicular developmental dysplasia and decrease oocytes developmental potential. The underlying mechanisms of fluoride-induced mitochondrial dysfunction in ovarian granulosa cells remain largely unknown. In this study, the ultrastructure changes of mitochondria and DNA damage in ovarian granulosa cells were observed under transmission electron microscope and TUNEL staining. Then, the ATP content and ROS level in granulosa cells were measured. The expression of mitochondrial fusion proteins and mitochondrial respiratory chain complexes, including OPA1 and Mfn1, and NDUFV2, SDHA and CYC1, in the ovarian tissues were measured by immunohistochemistry, Western blot and Quantitative real-time PCR analyses. The expression of ATP5j and ATP5h in the ovarian tissues was also measured. Results show that fluoride treatment considerably damages mitochondrial ultrastructure and enhances the apoptosis of granulosa cells. The ATP content greatly decreased, whereas the ROS level increased after fluoride treatment. The expression level of Mfn1 in the ovarian tissue was up-regulated, whereas OPA1 expression had no significant change. The expression levels of NDUFV2, SDHA and CYC1 were considerably up-regulated, and the expression of ATP5j and ATP5h were down-regulated after fluoride treatment. In summary, the damage in the mitochondrial ultrastructure, ATP content decrease, ROS level increase and the abnormal expression of OPA1, Mfn1, NDUFV2, SDHA, CYC1, ATP5j and ATP5h in ovary tissue are closely associated with fluoride-induced mitochondrial dysfunction, which might be responsible for the follicular developmental dysplasia and the potential decrease in oocyte development induced by fluoride in female mice.

Translocator Protein (TSPO) Affects Mitochondrial Fatty Acid Oxidation in Steroidogenic Cells

Translocator protein (TSPO), also known as the peripheral benzodiazepine receptor, is a highly conserved outer mitochondrial membrane protein present in specific subpopulations of cells within different tissues. In recent studies, the presumptive model depicting mammalian TSPO as a critical cholesterol transporter for steroidogenesis has been refuted by studies examining effects of Tspo gene deletion in vivo and in vitro, biochemical testing of TSPO cholesterol transport function, and specificity of TSPO-mediated pharmacological responses. Nevertheless, high TSPO expression in steroid-producing cells seemed to indicate an alternate function for this protein in steroidogenic mitochondria. To seek an explanation, we used CRISPR/Cas9-mediated TSPO knockout steroidogenic MA-10 Leydig cell (MA-10:TspoΔ/Δ) clones to examine changes to core mitochondrial functions resulting from TSPO deficiency. We observed that 1) MA-10:TspoΔ/Δ cells had a shift in substrate utilization for energy production from glucose to fatty acids with significantly higher mitochondrial fatty acid oxidation (FAO), and increased reactive oxygen species production; and 2) oxygen consumption rate, mitochondrial membrane potential, and proton leak were not different between MA-10:TspoΔ/Δ and MA-10:Tspo+/+ control cells. Consistent with this finding, TSPO-deficient adrenal glands from global TSPO knockout (Tspo(-/-)) mice also showed up-regulation of genes involved in FAO compared with the TSPO floxed (Tspo(fl/fl)) controls. These results demonstrate the first experimental evidence that TSPO can affect mitochondrial energy homeostasis through modulation of FAO, a function that appears to be consistent with high levels of TSPO expression observed in cell types active in lipid storage/metabolism.

Mitochondrial Translocator Protein (TSPO) Function Is Not Essential for Heme Biosynthesis

Function of the mammalian translocator protein (TSPO; previously known as the peripheral benzodiazepine receptor) remains unclear because its presumed role in steroidogenesis and mitochondrial permeability transition established using pharmacological methods has been refuted in recent genetic studies. Protoporphyrin IX (PPIX) is considered a conserved endogenous ligand for TSPO. In bacteria, TSPO was identified to regulate tetrapyrrole metabolism and chemical catalysis of PPIX in the presence of light, and in vertebrates, TSPO function has been linked to porphyrin transport and heme biosynthesis. Positive correlation between high TSPO expression in cancer cells and susceptibility to photodynamic therapy based on their increased ability to convert the precursor 5-aminolevulinic acid (ALA) to PPIX appeared to reinforce this mechanism. In this study, we used TSPO knock-out (Tspo(-/-)) mice, primary cells, and different tumor cell lines to examine the role of TSPO in erythropoiesis, heme levels, PPIX biosynthesis, phototoxic cell death, and mitochondrial bioenergetic homeostasis. In contrast to expectations, our results demonstrate that TSPO deficiency does not adversely affect erythropoiesis, heme biosynthesis, bioconversion of ALA to PPIX, and porphyrin-mediated phototoxic cell death. TSPO expression levels in cancer cells do not correlate with their ability to convert ALA to PPIX. In fibroblasts, we observed that TSPO deficiency decreased the oxygen consumption rate and mitochondrial membrane potential (ΔΨm) indicative of a cellular metabolic shift, without a negative impact on porphyrin biosynthetic capability. Based on these findings, we conclude that mammalian TSPO does not have a critical physiological function related to PPIX and heme biosynthesis.

The changing landscape in translocator protein (TSPO) function

Translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is an outer mitochondrial membrane protein. TSPO has been shown to cooperate with steroidogenic acute regulatory protein (StAR) and function in the transport of cholesterol into mitochondria. TSPO has also been considered as a structural component of the mitochondrial permeability transition pore (MPTP). However, recent advances have changed these views of TSPO's functions and have prompted a re-evaluation of established concepts. This review summarizes the history of TSPO, key elements of the debate, and functional experiments that have changed our understanding. Moving forward, we examine how this fundamental change impacts our understanding of TSPO and affects the future of TSPO as a therapeutic and diagnostic target.

Transcriptional activation of LON Gene by a new form of mitochondrial stress: A role for the nuclear respiratory factor 2 in StAR overload response (SOR)

High output of steroid hormone synthesis in steroidogenic cells of the adrenal cortex and the gonads requires the expression of the steroidogenic acute regulatory protein (StAR) that facilitates cholesterol mobilization to the mitochondrial inner membrane where the CYP11A1/P450scc enzyme complex converts the sterol to the first steroid. Earlier studies have shown that StAR is active while pausing on the cytosolic face of the outer mitochondrial membrane while subsequent import of the protein into the matrix terminates the cholesterol mobilization activity. Consequently, during repeated activity cycles, high level of post-active StAR accumulates in the mitochondrial matrix. To prevent functional damage due to such protein overload effect, StAR is degraded by a sequence of three to four ATP-dependent proteases of the mitochondria protein quality control system, including LON and the m-AAA membranous proteases AFG3L2 and SPG7/paraplegin. Furthermore, StAR expression in both peri-ovulatory ovarian cells, or under ectopic expression in cell line models, results in up to 3-fold enrichment of the mitochondrial proteases and their transcripts. We named this novel form of mitochondrial stress as StAR overload response (SOR). To better understand the SOR mechanism at the transcriptional level we analyzed first the unexplored properties of the proximal promoter of the LON gene. Our findings suggest that the human nuclear respiratory factor 2 (NRF-2), also known as GA binding protein (GABP), is responsible for 88% of the proximal promoter activity, including the observed increase of transcription in the presence of StAR. Further studies are expected to reveal if common transcriptional determinants coordinate the SOR induced transcription of all the genes encoding the SOR proteases.

Minireview: translocator protein (TSPO) and steroidogenesis: a reappraisal

The 18-kDa translocator protein (TSPO), also known as the peripheral benzodiazepine receptor, is a transmembrane protein in the outer mitochondrial membrane. TSPO has long been described as being indispensable for mitochondrial cholesterol import that is essential for steroid hormone production. In contrast to this initial proposition, recent experiments reexamining TSPO function have demonstrated that it is not involved in steroidogenesis. This fundamental change has forced a reexamination of the functional interpretations made for TSPO that broadly impacts both basic and clinical research across multiple fields. In this minireview, we recapitulate the key studies from 25 years of TSPO research and concurrently examine their limitations that perhaps led towards the incorrect association of TSPO and steroid hormone production. Although this shift in understanding raises new questions regarding the molecular function of TSPO, these recent developments are poised to have a significant positive impact for research progress in steroid endocrinology.

PK11195 effect on steroidogenesis is not mediated through the translocator protein (TSPO)

Translocator protein (TSPO) is a mitochondrial outer membrane protein of unknown function with high physiological expression in steroidogenic cells. Using TSPO gene-deleted mice, we recently demonstrated that TSPO function is not essential for steroidogenesis. The first link between TSPO and steroidogenesis was established in studies showing modest increases in progesterone production by adrenocortical and Leydig tumor cell lines after treatment with PK11195. To reconcile discrepancies between physiological and pharmacological interpretations of TSPO function, we generated TSPO-knockout MA-10 mouse Leydig tumor cells (MA-10:TspoΔ/Δ) and examined their steroidogenic potential after exposure to either dibutyryl-cAMP or PK11195. Progesterone production in MA-10:TspoΔ/Δ after dibutyryl-cAMP was not different from control MA-10:Tspo+/+ cells, confirming that TSPO function is not essential for steroidogenesis. Interestingly, when treated with increasing concentrations of PK11195, both control MA-10:Tspo+/+ cells and MA-10:TspoΔ/Δ cells responded in a similar dose-dependent manner showing increases in progesterone production. These results show that the pharmacological effect of PK11195 on steroidogenesis is not mediated through TSPO.

Peripheral benzodiazepine receptor/translocator protein global knock-out mice are viable with no effects on steroid hormone biosynthesis

Translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is a mitochondrial outer membrane protein implicated as essential for cholesterol import to the inner mitochondrial membrane, the rate-limiting step in steroid hormone biosynthesis. Previous research on TSPO was based entirely on in vitro experiments, and its critical role was reinforced by an early report that claimed TSPO knock-out mice were embryonic lethal. In a previous publication, we examined Leydig cell-specific TSPO conditional knock-out mice that suggested TSPO was not required for testosterone production in vivo. This raised controversy and several questions regarding TSPO function. To examine the definitive role of TSPO in steroidogenesis and embryo development, we generated global TSPO null (Tspo(-/-)) mice. Contrary to the early report, Tspo(-/-) mice survived with no apparent phenotypic abnormalities and were fertile. Examination of adrenal and gonadal steroidogenesis showed no defects in Tspo(-/-) mice. Adrenal transcriptome comparison of gene expression profiles showed that genes involved in steroid hormone biosynthesis (Star, Cyp11a1, and Hsd3b1) were unchanged in Tspo(-/-) mice. Adrenocortical ultrastructure illustrated no morphological alterations in Tspo(-/-) mice. In an attempt to correlate our in vivo findings to previously used in vitro models, we also determined that siRNA knockdown or the absence of TSPO in different mouse and human steroidogenic cell lines had no effect on steroidogenesis. These findings directly refute the dogma that TSPO is indispensable for steroid hormone biosynthesis and viability. By amending the current model, this study advances our understanding of steroidogenesis with broad implications in biology and medicine.

Peripheral benzodiazepine receptor/translocator protein global knock-out mice are viable with no effects on steroid hormone biosynthesis

Translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is a mitochondrial outer membrane protein implicated as essential for cholesterol import to the inner mitochondrial membrane, the rate-limiting step in steroid hormone biosynthesis. Previous research on TSPO was based entirely on in vitro experiments, and its critical role was reinforced by an early report that claimed TSPO knock-out mice were embryonic lethal. In a previous publication, we examined Leydig cell-specific TSPO conditional knock-out mice that suggested TSPO was not required for testosterone production in vivo. This raised controversy and several questions regarding TSPO function. To examine the definitive role of TSPO in steroidogenesis and embryo development, we generated global TSPO null (Tspo(-/-)) mice. Contrary to the early report, Tspo(-/-) mice survived with no apparent phenotypic abnormalities and were fertile. Examination of adrenal and gonadal steroidogenesis showed no defects in Tspo(-/-) mice. Adrenal transcriptome comparison of gene expression profiles showed that genes involved in steroid hormone biosynthesis (Star, Cyp11a1, and Hsd3b1) were unchanged in Tspo(-/-) mice. Adrenocortical ultrastructure illustrated no morphological alterations in Tspo(-/-) mice. In an attempt to correlate our in vivo findings to previously used in vitro models, we also determined that siRNA knockdown or the absence of TSPO in different mouse and human steroidogenic cell lines had no effect on steroidogenesis. These findings directly refute the dogma that TSPO is indispensable for steroid hormone biosynthesis and viability. By amending the current model, this study advances our understanding of steroidogenesis with broad implications in biology and medicine.

Translocator protein/peripheral benzodiazepine receptor is not required for steroid hormone biosynthesis

Molecular events that regulate cellular biosynthesis of steroid hormones have been a topic of intense research for more than half a century. It has been established that transport of cholesterol into the mitochondria forms the rate-limiting step in steroid hormone production. In current models, both the steroidogenic acute regulatory protein (StAR) and the translocator protein (TSPO) have been implicated to have a concerted and indispensable effort in this cholesterol transport. Deletion of StAR in mice resulted in a critical failure of steroid hormone production, but deletion of TSPO in mice was found to be embryonic lethal. As a result, the role of TSPO in cholesterol transport has been established only using pharmacologic and genetic tools in vitro. To allow us to explore in more detail the function of TSPO in cell type-specific experimental manipulations in vivo, we generated mice carrying TSPO floxed alleles (TSPOfl/fl). In this study we made conditional knockout mice (TSPOcΔ/Δ) with TSPO deletion in testicular Leydig cells by crossing with an anti-Mullerian hormone receptor type II cre/+ mouse line. Genetic ablation of TSPO in steroidogenic Leydig cells in mice did not affect testosterone production, gametogenesis, and reproduction. Expression of StAR, cytochrome P450 side chain cleavage enzyme, 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase type I, and TSPO2 in TSPOcΔ/Δ testis was unaffected. These results challenge the prevailing dogma that claims an essential role for TSPO in steroid hormone biosynthesis and force reexamination of functional interpretations made for this protein. This is the first study examining conditional TSPO gene deletion in mice. The results show that TSPO function is not essential for steroid hormone biosynthesis.