Is there a mitochondrial signaling complex facilitating cholesterol import?

Structure-based drug design for TSPO: Challenges and opportunities

The translocator protein 18 kDa (TSPO) is an evolutionarily conserved mitochondrial transmembrane protein implicated in various neuropathologies and inflammatory conditions, making it a longstanding diagnostic and therapeutic target of interest. Despite the development of various classes of TSPO ligand chemotypes, and the elucidation of bacterial and non-human mammalian experimental structures, many unknowns exist surrounding its differential structural and functional features in health and disease. There are several limitations associated with currently used computational methodologies for modelling the native structure and ligand-binding behaviour of this enigmatic protein. In this perspective, we provide a critical analysis of the developments in the uses of these methods, outlining their uses, inherent limitations, and continuing challenges. We offer suggestions of unexplored opportunities that exist in the use of computational methodologies which offer promise for enhancing our understanding of the TSPO.

Neuroactive Steroids, Toll-like Receptors, and Neuroimmune Regulation: Insights into Their Impact on Neuropsychiatric Disorders

Pregnane neuroactive steroids, notably allopregnanolone and pregnenolone, exhibit efficacy in mitigating inflammatory signals triggered by toll-like receptor (TLR) activation, thus attenuating the production of inflammatory factors. Clinical studies highlight their therapeutic potential, particularly in conditions like postpartum depression (PPD), where the FDA-approved compound brexanolone, an intravenous formulation of allopregnanolone, effectively suppresses TLR-mediated inflammatory pathways, predicting symptom improvement. Additionally, pregnane neurosteroids exhibit trophic and anti-inflammatory properties, stimulating the production of vital trophic proteins and anti-inflammatory factors. Androstane neuroactive steroids, including estrogens and androgens, along with dehydroepiandrosterone (DHEA), display diverse effects on TLR expression and activation. Notably, androstenediol (ADIOL), an androstane neurosteroid, emerges as a potent anti-inflammatory agent, promising for therapeutic interventions. The dysregulation of immune responses via TLR signaling alongside reduced levels of endogenous neurosteroids significantly contributes to symptom severity across various neuropsychiatric disorders. Neuroactive steroids, such as allopregnanolone, demonstrate efficacy in alleviating symptoms of various neuropsychiatric disorders and modulating neuroimmune responses, offering potential intervention avenues. This review emphasizes the significant therapeutic potential of neuroactive steroids in modulating TLR signaling pathways, particularly in addressing inflammatory processes associated with neuropsychiatric disorders. It advances our understanding of the complex interplay between neuroactive steroids and immune responses, paving the way for personalized treatment strategies tailored to individual needs and providing insights for future research aimed at unraveling the intricacies of neuropsychiatric disorders.

Ameliorative effects of elderberry (Sambucus nigra L.) extract and extract-derived monosaccharide-amino acid on H2O2-induced decrease in testosterone-deficiency syndrome in a TM3 Leydig cell

With aging, men develop testosterone-deficiency syndrome (TDS). The development is closely associated with age-related mitochondrial dysfunction of Leydig cell and oxidative stress-induced reactive oxygen species (ROS). Testosterone-replacement therapy (TRT) is used to improve the symptoms of TDS. However, due to its various side effects, research on functional ingredients derived from natural products that do not have side effects is urgently needed. In this study, using the mitochondrial dysfunction TM3 (mouse Leydig) cells, in which testosterone biosynthesis is reduced by H2O2, we evaluated the effects of elderberry extract and monosaccharide-amino acid (fructose-leucine; FL) on mRNA and protein levels related to steroidogenesis-related enzymes steroidogenic acute regulatory protein (StAR), cytochrome P450 11A1(CYP11A1, cytochrome P450 17A1(CYP17A1), cytochrome P450 19A1(CYP19A1, aromatase), 3β-hydroxysteroid dehydrogenase (3β-HSD), and 17β-hydroxysteroid dehydrogenase(17β-HSD). We analyzed elderberry extract and extract-derived FL for changes in ROS scavenging activity and testosterone secretion. Elderberry extract and FL significantly reduced H2O2-induced intracellular ROS levels, improved testosterone secretion, and increased the mRNA and protein expression levels of steroidogenesis-related enzymes (StAR, 3b-HSD, 17b-HSD, CYP11A1, CYp17A1). However, the conversion of testosterone to estradiol was inhibited by elderberry extract and extract-derived FL, which reduced the mRNA and protein expression of CYP19A1. In conclusion, elderberry extract and FL are predicted to have value as novel functional ingredients that may contribute to the prevention of TDS by ameliorating reduced steroidogenesis.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

FGF1ΔHBS ameliorates retinal inflammation via suppressing TSPO signal in a type 2 diabetes mouse model

Translocator protein (18 kDa) (TSPO) plays an important role in retinal neuroinflammation in the early stage of diabetic retinopathy (DR). Studies have found that a FGF1 variant (FGF1ΔHBS) with reduced proliferative potency exerts excellent anti-inflammatory effects and potential therapeutic value for diabetic complications. In this study, intravitreal injection of FGF1ΔHBS was administrated every week for one month in db/db mice, which are genetically predisposed to develop type 2 diabetes mellitus and early retinopathy. Changes in retinal function and structure in the animal models were detected by electrophysiology (ERG) and optical tomography coherence (OCT). TSPO expression and retinal inflammation were analyzed by immunofluorescence, Western blot and real-time qPCR. In the retina of T2D (db/db) mice, FGF1 was significantly down-regulated while FGFR1 was up-regulated (both p < 0.05). TSPO and retinal inflammatory factors were all up-regulated. TSPO and FGFR1 were mainly co-stained in the inner retina. After FGF1ΔHBS treatment, ERG showed that the total amplitude of dark-adapted b-wave and oscillating potentials (Ops) was significantly improved, and OCT showed that the thickness of the retina around the optical nerve head was significantly preserved in T2D mice (all p < 0.05). The TSPO signal was significantly suppressed by FGF1ΔHBS. The activation of NF-κB p65 and the expression of inflammatory factors such as TNF-α, IL-1β, IL-6, COX-2, MIP-1α, and iNOS were all significantly down-regulated (all p < 0.05). Collectively, our current data demonstrated that intravitreal FGF1ΔHBS treatment can effectively inhibit retinal inflammation via suppressing TSPO signal and to preserve retinal function and structure in a T2D mouse model.

Cross-talk between neurosteroid and endocannabinoid systems in cannabis addiction

Steroids and endocannabinoids are part of two modulatory systems and some evidence has shown their interconnections in several functions. Homeostasis is a common steady-state described in the body, which is settled by regulatory systems to counterbalance deregulated or allostatic set points towards an equilibrium. This regulation is of primary significance in the central nervous system for maintaining neuronal plasticity and preventing brain-related disorders. In this context, the recent discovery of the shutdown of the endocannabinoid system (ECS) overload by the neurosteroid pregnenolone has highlighted new endogenous mechanisms of ECS regulation related to cannabis-induced intoxication. These mechanisms involve a regulatory loop mediated by overactivation of the central type-1 cannabinoid receptor (CB1R), which triggers the production of its own regulator, pregnenolone. Therefore, this highlights a new process of regulation of steroidogenesis in the brain. Pregnenolone, long considered an inactive precursor of neurosteroids, can then act as an endogenous negative allosteric modulator of CB1R. The present review aims to shed light on a new framework for the role of ECS in the addictive characteristics of cannabis with the novel endogenous mechanism of ECS involving the neurosteroid pregnenolone. In addition, this new endogenous regulatory loop could provide a relevant therapeutic model in the current context of increasing recreational and medical use of cannabis.

Translocator Protein 18 kDa (TSPO) as a Novel Therapeutic Target for Chronic Pain

Chronic pain is an enormous modern public health problem, with significant numbers of people debilitated by chronic pain from a variety of etiologies. Translocator protein 18 kDa (TSPO) was discovered in 1977 as a peripheral benzodiazepine receptor. It is a five transmembrane domain protein, mainly localized in the outer mitochondrial membrane. Recent and increasing studies have found changes in TSPO and its ligands in various chronic pain models. Reversing their expressions has been shown to alleviate chronic pain in these models, illustrating the effects of TSPO and its ligands. Herein, we review recent evidence and the mechanisms of TSPO in the development of chronic pain associated with peripheral nerve injury, spinal cord injury, cancer, and inflammatory responses. The cumulative evidence indicates that TSPO-based therapy may become an alternative strategy for treating chronic pain.

Translocator Protein Ligand Etifoxine Attenuates MPTP-Induced Neurotoxicity

Parkinson’s disease (PD) is a neurodegenerative disease, but the currently available treatments for this disease are symptomatic treatments. There is evidence that translocator protein (18 kDa) (TSPO) expression is upregulated in some neurodegenerative diseases, and TSPO ligands have obvious neuroprotective effects. However, the neuroprotective effects and other potential effects of the TSPO ligand etifoxine in PD remain unclear. Therefore, the present study was designed to explore the impacts of etifoxine on a mouse model of PD induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We found that etifoxine significantly reduced motor function deficits, decreased the loss of tyrosine hydroxylase-positive neurons in the substantia nigra, and attenuated the decrease in striatal dopamine levels in mice that received MPTP. Etifoxine diminished the production of inflammatory mediators and infiltration of leukocytes in the brain after MPTP exposure. In vitro studies suggested that microglia contribute to etifoxine’s neuroprotective effect. The results showed that etifoxine can alleviate MPTP-induced neurotoxicity and neuroinflammation, providing a new idea for the treatment of PD.

New perspectives on the role of the neurosteroid pregnenolone as an endogenous regulator of type-1 cannabinoid receptor (CB1R) activity and function

Pregnenolone is a steroid with specific characteristics, being the first steroid to be synthesised from cholesterol at all sites of steroidogenesis, including the brain. For many years, pregnenolone was defined as an inactive precursor of all steroids because no specific target had been discovered. However, over the last decade, it has become a steroid of interest because it has been recognised as being a biomarker for brain-related disorders through the development of metabolomic approaches and advanced analytical methods. In addition, physiological roles for pregnenolone emerged when specific targets were discovered. In this review, we highlight the discovery of the selective interaction of pregnenolone with the type-1 cannabinoid receptor (CB1R). After describing the specific characteristic of CB1Rs, we discuss the newly discovered mechanisms of their regulation by pregnenolone. In particular, we describe the action of pregnenolone as a negative allosteric modulator and a specific signalling inhibitor of the CB1R. These particular characteristics of pregnenolone provide a great strategic opportunity for therapeutic development in CB1-related disorders. Finally, we outline new perspectives using innovative genetic tools for the discovery of original regulatory mechanisms of pregnenolone on CB1-related functions.

Bioinformatic Analyses of Canonical Pathways of TSPOAP1 and its Roles in Human Diseases

TSPO-associated protein 1 (TSPOAP1) is a cytoplasmic protein and is closely associated with its mitochondrial transmembrane protein partner translocator protein (TSPO). To decipher the canonical signalling pathways of TSPOAP1, its role in human diseases and disorders, and relationship with TSPO; expression analyses of TSPOAP1- and TSPO-associated human genes were performed by Qiagen Ingenuity Pathway Analysis (IPA). In the expression analysis, necroptosis and sirtuin signalling pathways, mitochondrial dysfunction, and inflammasome were the top canonical pathways for both TSPOAP1 and TSPO, confirming the close relationship between these two proteins. A distribution analysis of common proteins in all the canonical pathways predicted for TSPOAP1 revealed that tumor necrosis factor receptor 1 (TNFR1), vascular cell adhesion molecule 1 (VCAM1), cyclic AMP response element-binding protein 1 (CREB1), T-cell receptor (TCR), nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain containing 3 (NLRP3), DNA-dependent protein kinase (DNA-PK or PRKDC), and mitochondrial permeability transition pore (mPTP) were the major interaction partners of TSPOAP1, highlighting the role of TSPOAP1 in inflammation, particularly neuroinflammation. An analysis of the overlap between TSPO and TSPOAP1 Homo sapiens genes and top-ranked canonical pathways indicated that TSPO and TSPOAP1 interact via voltage-dependent anion-selective channels (VDAC1/2/3). A heat map analysis indicated that TSPOAP1 has critical roles in inflammatory, neuroinflammatory, psychiatric, and metabolic diseases and disorders, and cancer. Taken together, this information improves our understanding of the mechanism of action and biological functions of TSPOAP1 as well as its relationship with TSPO; furthermore, these results could provide new directions for in-depth functional studies of TSPOAP1 aimed at unmasking its detailed functions.

The Ligands of Translocator Protein: Design and Biological Properties

In 2020, it is already 43 years since Braestrup and Squires discovered 18 kDa translocator protein (TSPO), known until 2006 as "peripheral benzodiazepine receptor". During this time, the functions of this receptor, which is located on the outer membrane of mitochondria, were studied in detail. One of the key functions of TSPO is the transfer of cholesterol from the outer to the inner mitochondrial membrane, which is the limiting stage in the synthesis of neurosteroids. TSPO is also involved in the transport of porphyrins, mitochondrial respiration, the opening of mitochondrial pores, apoptosis and cell proliferation. This review presents current information on the structure of TSPO, the mechanism of its participation in neurosteroidogenesis, as well as endogenous and synthetic TSPO ligands. Particular emphasis is placed on the analysis of approaches to the design of synthetic ligands and their neuropsychotropic activity in vitro and in vivo. The presented review demonstrates the promise of constructing new neuropsychotropic drugs in the series of TSPO ligands.

Dynamic Remodeling of Membranes and Their Lipids during Acute Hormone-Induced Steroidogenesis in MA-10 Mouse Leydig Tumor Cells

Lipids play essential roles in numerous cellular processes, including membrane remodeling, signal transduction, the modulation of hormone activity, and steroidogenesis. We chose steroidogenic MA-10 mouse tumor Leydig cells to investigate subcellular lipid localization during steroidogenesis. Electron microscopy showed that cAMP stimulation increased associations between the plasma membrane (PM) and the endoplasmic reticulum (ER) and between the ER and mitochondria. cAMP stimulation also increased the movement of cholesterol from the PM compared to untreated cells, which was partially inhibited when ATPase family AAA-domain containing protein 3 A (ATAD3A), which functions in ER and mitochondria interactions, was knocked down. Mitochondria, ER, cytoplasm, PM, PM-associated membranes (PAMs), and mitochondria-associated membranes (MAMs) were isolated from control and hormone-stimulated cells. Lipidomic analyses revealed that each isolated compartment had a unique lipid composition, and the induction of steroidogenesis caused the significant remodeling of its lipidome. cAMP-induced changes in lipid composition included an increase in phosphatidylserine and cardiolipin levels in PAM and PM compartments, respectively; an increase in phosphatidylinositol in the ER, mitochondria, and MAMs; and a reorganization of phosphatidic acid, cholesterol ester, ceramide, and phosphatidylethanolamine. Abundant lipids, such as phosphatidylcholine, were not affected by hormone treatment. Our data suggested that PM–ER–mitochondria tethering may be involved in lipid trafficking between organelles and indicated that hormone-induced acute steroid production involves extensive organelle remodeling.

Leydig cell aging: Molecular mechanisms and treatments

Late-onset hypogonadism, resulting from deficiency in serum testosterone (T), affects the health and quality of life of millions of aging men. T is synthesized by Leydig cells (LCs) in response to luteinizing hormone (LH). LH binds LC plasma membrane receptors, inducing the formation of a supramolecular complex of cytosolic and mitochondrial proteins, the Steroidogenic InteracTomE (SITE). SITE proteins are involved in targeting cholesterol to CYP11A1 in the mitochondria, the first enzyme of the steroidogenic cascade. Cholesterol translocation is the rate-determining step in T formation. With aging, LC defects occur that include changes in SITE, an increasingly oxidative intracellular environment, and reduced androgen formation and serum T levels. T replacement therapy (TRT) will restore T levels, but reported side effects make it desirable to develop additional strategies for increasing T. One approach is to target LC protein-protein interactions and thus increase T production by the hypofunctional Leydig cells themselves.

Enhancing glucose metabolism via gluconeogenesis is therapeutic in a zebrafish model of Dravet syndrome

Energy-producing pathways are novel therapeutic targets for the treatment of neurodevelopmental disorders. Here, we focussed on correcting metabolic defects in a catastrophic paediatric epilepsy, Dravet syndrome which is caused by mutations in sodium channel NaV1.1 gene, SCN1A. We utilized a translatable zebrafish model of Dravet syndrome (scn1lab) which exhibits key characteristics of patients with Dravet syndrome and shows metabolic deficits accompanied by down-regulation of gluconeogenesis genes, pck1 and pck2. Using a metabolism-based small library screen, we identified compounds that increased gluconeogenesis via up-regulation of pck1 gene expression in scn1lab larvae. Treatment with PK11195, a pck1 activator and a translocator protein ligand, normalized dys-regulated glucose levels, metabolic deficits, translocator protein expression and significantly decreased electrographic seizures in mutant larvae. Inhibition of pck1 in wild-type larvae mimicked metabolic and behaviour defects observed in scn1lab mutants. Together, this suggests that correcting dys-regulated metabolic pathways can be therapeutic in neurodevelopmental disorders such as Dravet syndrome arising from ion channel dysfunction.

Cholesterol homeostasis: Researching a dialogue between the brain and peripheral tissues

Cholesterol homeostasis is a highly regulated process in human body because of its several functions underlying the biology of cell membranes, the synthesis of all steroid hormones and bile acids and the need of trafficking lipids destined to cell metabolism. In particular, it has been recognized that peripheral and central nervous system cholesterol metabolism are separated by the blood brain barrier and are regulated independently; indeed, peripherally, it depends on the balance between dietary intake and hepatic synthesis on one hand and its degradation on the other, whereas in central nervous system it is synthetized de novo to ensure brain physiology. In view of this complex metabolism and its relevant functions in mammalian, impaired levels of cholesterol can induce severe cellular dysfunction leading to metabolic, cardiovascular and neurodegenerative diseases. The aim of this review is to clarify the role of cholesterol homeostasis in health and disease highlighting new intriguing aspects of the cross talk between its central and peripheral metabolism.

Translocator Protein 18 kDa (TSPO) Deficiency Inhibits Microglial Activation and Impairs Mitochondrial Function

TSPO is mainly expressed in the mitochondrial outer membrane of microglia in the central nervous system, and its expression is greatly increased when microglia are activated. However, the role and mechanism of this protein in microglial activation is not well characterized. In this study, we investigated the role of TSPO in microglial activation by isolating primary microglia from TSPO knockout mice and constructing TSPO-knockdown microglial cell line. We found that TSPO deficiency significantly inhibited microglial activation induced by LPS or IL-4. Mechanistically, TSPO deficiency greatly decreased the mitochondrial membrane potential and ATP production. Moreover, an analysis of cellular energy metabolism showed that TSPO deficiency suppressed mitochondrial oxidative phosphorylation (OXPHOS) and glycolysis, resulting in microglial overall metabolic deficits. Together, our results reveal a crucial role of TSPO in microglial activation through the regulation of mitochondrial metabolism, thus providing a potential therapeutic target for neuroinflammation-related diseases of the central nervous system.

ACBD3 is required for FAPP2 transferring glucosylceramide through maintaining the Golgi integrity

Glycosphingolipid (GSL) metabolism is involved in various physiological processes, including all major cell signaling pathways, and its dysregulation is linked to some diseases. The four-phosphate adaptor protein FAPP2-mediated glucosylceramide (GlcCer) transport for complex GSL synthesis has been studied extensively. However, the molecular machinery of FAPP2 as a GlcCer-transferring protein remains poorly defined. Here, we identify a Golgi-resident protein, acyl-coenzyme A binding domain containing 3 (ACBD3), as an interacting partner of FAPP2. We find that ACBD3 knockdown leads to dramatic Golgi fragmentation, which subsequently causes FAPP2 dispersal throughout the cytoplasm and a decreased localization at trans-Golgi network. The further quantitative lipidomic analysis indicates that ACBD3 knockdown triggers abnormal sphingolipid metabolism. Interestingly, the expression of siRNA-resistant full-length ACBD3 can rescue these defects caused by ACBD3 knockdown. These data reveal critical roles for ACBD3 in maintaining the integrity of Golgi morphology and cellular sphingolipid homeostasis and establish the importance of the integrated Golgi complex for the transfer of GlcCer and complex GSL synthesis.

Identification of Sec23ip, Part of 14-3-3γ Protein Network, as a Regulator of Acute Steroidogenesis in MA-10 Leydig Cells

Testosterone production occurs in the Leydig cells of the testes and is essential for virilization, development, reproduction, and quality of life. Although the steroidogenic proteins involved in cholesterol conversion to testosterone (T) are well characterized, the causes of reduced T during fetal, neonatal, and adult life remain uncertain. It is well established that normal cellular function is achieved through fine-tuning of multiple rather than single protein networks. Our objective was to use mass spectrometry (MS)-based proteomics to identify which cellular pathways, other than the steroidogenic machinery, influence testosterone production in MA-10 mouse tumor Leydig cells. The 14-3-3 family of scaffolds mediate protein-protein interactions facilitating the crosstalk between protein networks. We previously showed that in MA-10 cells, 14-3-3γ is a critical regulator of steroidogenesis. Therefore, identifying proteins that interact with 14-3-3γ during steroidogenesis could provide clues into the other networks involved. Using liquid chromatography (LC)-MS, we identified 688 proteins that interact with 14-3-3γ and thus potentially impact MA-10 cell steroidogenesis. The identified proteins belong to multiple protein networks, including endoplasmic reticulum-Golgi cargo sorting and vesicle biogenesis, micro ribonucleic acid-induced gene silencing, inflammation, and vesicle trafficking, to name a few. We found that silencing one of the candidates, Sec23ip, a protein known to be involved in vesicle trafficking, resulted in decreased steroidogenesis. We further showed that in Sec23ip-silenced MA-10 cells, cholesterol mobilization from the cytoplasmic membrane to mitochondria is impaired. Taken together these data suggest that Sec23ip is involved in cholesterol trafficking to supply cholesterol for acute steroidogenesis through its interactions with 14-3-3γ.

Insight into the Structural Features of TSPO: Implications for Drug Development

The translocator protein (TSPO), an 18-kDa transmembrane protein primarily found in the outer mitochondrial membrane, is evolutionarily conserved and widely distributed across species. In mammals, TSPO has been described as a key member of a multiprotein complex involved in many putative functions and, over the years, several classes of ligand have been developed to modulate these functions. In this review, we consider the currently available atomic structures of mouse and bacterial TSPO and propose a rationale for the development of new ligands for the protein. We provide a review of TSPO monomeric and oligomeric states and their conformational flexibility, together with ligand-binding site and interaction mechanisms. These data are expected to help considerably the development of high-affinity ligands for TSPO-based therapies or diagnostics.

Long-chain acyl-CoA synthetase 1 interacts with key proteins that activate and direct fatty acids into niche hepatic pathways

Fatty acid channeling into oxidation or storage modes depends on physiological conditions and hormonal signaling. However, the directionality of this channeling may also depend on the association of each of the five acyl-CoA synthetase isoforms with specific protein partners. Long-chain acyl-CoA synthetases (ACSLs) catalyze the conversion of long-chain fatty acids to fatty acyl-CoAs, which are then either oxidized or used in esterification reactions. In highly oxidative tissues, ACSL1 is located on the outer mitochondrial membrane (OMM) and directs fatty acids into mitochondria for β-oxidation. In the liver, however, about 50% of ACSL1 is located on the endoplasmic reticulum (ER) where its metabolic function is unclear. Because hepatic fatty acid partitioning is likely to require the interaction of ACSL1 with other specific proteins, we used an unbiased protein interaction technique, BioID, to discover ACSL1-binding partners in hepatocytes. We targeted ACSL1 either to the ER or to the OMM of Hepa 1-6 cells as a fusion protein with the Escherichia coli biotin ligase, BirA*. Proteomic analysis identified 98 proteins that specifically interacted with ACSL1 at the ER, 55 at the OMM, and 43 common to both subcellular locations. We found subsets of peroxisomal and lipid droplet proteins, tethering proteins, and vesicle proteins, uncovering a dynamic role for ACSL1 in organelle and lipid droplet interactions. Proteins involved in lipid metabolism were also identified, including acyl-CoA-binding proteins and ceramide synthase isoforms 2 and 5. Our results provide fundamental and detailed insights into protein interaction networks that control fatty acid metabolism.

Translocator protein agonist Ro5-4864 alleviates neuropathic pain and promotes remyelination in the sciatic nerve

Our previous study reported the translocator protein to play a critical role in neuropathic pain and the possible mechanisms in the spinal cord. However, its mechanism in the peripheral nervous system is poorly understood. This study was undertaken to explore the distribution of translocator protein in the dorsal root ganglion and the possible mechanisms in peripheral nervous system in a rat model of spared nerve injury. Our results showed that translocator protein was activated in dorsal root ganglion after spared nerve injury. The translocator protein signals were primarily colocalized with neurons in dorsal root ganglion. A single intrathecal (i.t.) injection of translocator protein agonist (7-chloro-5-4-chlorophenyl)-1,3-dihydro-1-methyl-2-H-1,4-benzodiaze-pine-2) (Ro5-4864) exerted remarkable analgesic effect compared with the spared nerve injury group ( P < 0.01). After i.t. administration of 2 µg Ro5-4864 on day 3, the expression of translocator protein in ipsilateral dorsal root ganglion was significantly increased on day 7( P < 0.01) but decreased on day 14 ( P < 0.05) compared with the same point in time in the control group. The duration of translocator protein activation in dorsal root ganglion was remarkably shortened. Ro5-4864 also inhibited the activation of phospho-extracellular signal-regulated kinase 1(p-ERK1) ( P < 0.01), p-ERK2 (D7: P < 0.01, D14: P < 0.05), and brain-derived neurotrophic factor ( P < 0.05) in dorsal root ganglion. Meanwhile, i.t. administration of 2 µg Ro5-4864 on day 3 further accelerated the expression of myelin protein zero(P0) and peripheral myelin protein 22 (PMP22). Our results suggested Ro5-4864 could alleviate neuropathic pain and attenuate p-ERK and brain-derived neurotrophic factor activation in dorsal root ganglion. Furthermore, Ro5-4864 stimulated the expression of myelin regeneration proteins which may also be an important factor against neuropathic pain development. Translocator protein may present a novel target for the treatment of neuropathic pain both in the central and peripheral nervous systems.

Steroidogenesis in Leydig cells: effects of aging and environmental factors

Serum testosterone (TS) levels decrease with aging in both humans and rodents. Using the rat as a model system, it was found that age-related reductions in serum TS were not due to loss of Leydig cells, but rather to the reduced ability of the Leydig cells to produce TS in response to luteinizing hormone (LH). Detailed analyses of the steroidogenic pathway have suggested that two defects along the pathway, LH-stimulated cAMP production and cholesterol transport to and into the mitochondria, are of particular importance in age-related reductions in TS production. Although the mechanisms involved in these defects are far from certain, increasing oxidative stress appears to play a particularly important role. Interestingly, increased oxidative stress also appears to be involved in the suppressive effects of endocrine disruptors on Leydig cell TS production.

Expression of StAR and Key Genes Regulating Cortisol Biosynthesis in Near Term Ovine Fetal Adrenocortical Cells: Effects of Long-Term Hypoxia

We previously demonstrated decreased expression of key genes regulating cortisol biosynthesis in long-term hypoxic (LTH) sheep fetal adrenals compared to controls. We also showed that inhibition of the extracellular signal-regulated kinases (ERKs) with the mitogen-activated protein kinase (MEK)/ERK inhibitor UO126 limited adrenocorticotropic (ACTH)-induced cortisol production in ovine fetal adrenocortical cells (FACs), suggesting a role for ERKs in cortisol synthesis. This study was designed to determine whether the previously observed decrease in LTH cytochrome P45011A1/cytochrome P450c17 (CYP11A1/CYP17) in adrenal glands was maintained in vitro, and whether ACTH alone with or without UO126 treatment had altered the expression of CYP11A1, CYP17, and steroidogenic acute regulatory protein (StAR) in control versus LTH FACs. Ewes were maintained at high altitude (3820 m) from ∼40 days of gestation (dG). At 138 to 141 dG, fetal adrenal glands were collected from LTH (n = 5) and age-matched normoxic controls (n = 6). Fetal adrenocortical cells were challenged with ACTH (10^-8 M) with or without UO126 (10 µM) for 18 hours. Media samples were collected for cortisol analysis and messenger RNA (mRNA) for CYP11A1, CYP17, and StAR was quantified by quantitative real-time polymerase chain reaction. Cortisol was higher in the LTH versus control ( P < .05). StAR mRNA was decreased in LTH versus control ( P < .05). U0126 alone had no effect on mRNA in either group. UO126 prevented the increase in CYP11A1 and CYP17 in control FACs. Basal CYP11A1 and CYP17 were not different in LTH versus control. ACTH increased CYP11A1 and CYP17 only in control FACs ( P < .05). U1026 attenuated the ACTH response indicative of a role for ERK in CYP11A1 and CYP17 expression. ACTH may require additional factors in FACs to fully regulate StAR expression.

Effect of ZBD-2 on chronic pain, depressive-like behaviors, and recovery of motor function following spinal cord injury in mice

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ZBD-2 significantly attenuated the symptoms of chronic SCI-pain and pain-induced depressive-like behaviors.

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ZBD-2 inhibited the decreases in the expression of synaptic plasticity-related signaling proteins.

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ZBD-2 reversed chronic, SCI-induced gliocyte activation at the lesion site.

In addition to debilitating sensory and motor deficits, patients with spinal cord injury (SCI) may experience chronic hyperpathic pain (SCI-pain). Recent studies have revealed that translocator protein (TSPO) is involved in repairing neural cells as well as reducing anxiety and depression. However, the role of TSPO in SCI-pain and pain-induced depression remains unknown. The present study aimed to determine the effects of a new TSPO ligand, ZBD-2, on SCI-pain and consequent pain-induced depressive-like behaviors in mice. Treatment with ZBD-2 at either dose significantly attenuated the symptoms of chronic SCI-pain and pain-induced depressive-like behaviors. ZBD-2 reversed SCI-induced elevation of serum corticosterone levels, an index of hyper-activation of the hypothalamic–pituitary–adrenal (HPA) axis. Additionally, administration of ZBD-2 inhibited decreases in the expression of synaptic plasticity-related signaling proteins, including brain-derived neurotrophic factor (BDNF) and cyclic AMP-responsive element binding protein (CREB). Moreover, ZBD-2 administration reversed chronic, SCI-induced gliocyte activation at the lesion site. Therefore, ZBD-2 may improve chronic SCI-pain and pain-induced depressive-like behaviors via suppression of gliocyte activation and restoration of the synaptic plasticity-related signaling systems.

Plasma Membrane Origin of the Steroidogenic Pool of Cholesterol Used in Hormone-induced Acute Steroid Formation in Leydig Cells

Hormone-sensitive acute steroid biosynthesis requires trafficking of cholesterol from intracellular sources to the inner mitochondrial membrane. The precise location of the intracellular cholesterol and its transport mechanism are uncertain. Perfringolysin O, produced by Clostridium perfringens, binds cholesterol. Its fourth domain (D4) retains cholesterol-binding properties but not cytotoxicity. We transfected steroidogenic MA-10 cells of mouse Leydig cell tumors with the mCherry-D4 plasmid. Tagged D4 with fluorescent proteins enabled us to track cholesterol. The staining was primarily localized to the inner leaflet of the plasma membrane and was partially released upon treatment with dibutyryl-cAMP (Bt₂cAMP), a cAMP analog. Inhibitors of cholesterol import into mitochondria blocked steroidogenesis and prevented release of D4 (and presumably cholesterol) from the plasma membrane. We conclude that the bulk of the steroidogenic pool of cholesterol, mobilized by Bt₂cAMP for acute steroidogenesis, originates from the plasma membrane. Treatment of the cells with steroid metabolites, 22(R)-hydroxycholesterol and pregnenolone, also reduced D4 release from the plasma membrane, perhaps evidence for a feedback effect of elevated steroid formation on cholesterol release. Interestingly, D4 staining was localized to endosomes during Bt₂cAMP stimulation suggesting that these organelles are on the route of cholesterol trafficking from the plasma membrane to mitochondria. Finally, D4 was expressed in primary rat Leydig cells with a lentivirus and was released from the plasma membrane following Bt₂cAMP treatment. We conclude that the plasma membrane is the source of cholesterol for steroidogenesis in these cells as well as in MA-10 cells.

TSPO-ligands prevent oxidative damage and inflammatory response in C6 glioma cells by neurosteroid synthesis

Translocator protein 18kDa (TSPO) is predominantly located in the mitochondrial outer membrane, playing an important role in steroidogenesis, inflammation, cell survival and proliferation. Its expression in central nervous system, mainly in glial cells, has been found to be upregulated in neuropathology, and brain injury. In this study, we investigated the anti-oxidative and anti-inflammatory effects of a group of TSPO ligands from the N,N-dialkyl-2-phenylindol-3-ylglyoxylamide class (PIGAs), highlighting the involvement of neurosteroids in their pharmacological effects. To this aim we used a well-known in vitro model of neurosteroidogenesis: the astrocytic C6 glioma cell line, where TSPO expression and localization, as well as cell response to TSPO ligand treatment, have been established. All PIGAs reduced l-buthionine-(S,R)-sulfoximine (BSO)-driven cell cytotoxicity and lipid peroxidation. Moreover, an anti-inflammatory effect was observed due to the reduction of inducible nitric oxide synthase and cyclooxygenase-2 induction in LPS/IFNγ challenged cells. Both effects were blunted by aminoglutethimide (AMG), an inhibitor of pregnenolone synthesis, suggesting neurosteroids' involvement in PIGA protective mechanism. Finally, pregnenolone evaluation in PIGA exposed cells revealed an increase in its synthesis, which was prevented by AMG pre-treatment. These findings indicate that these TSPO ligands reduce oxidative stress and pro-inflammatory enzymes in glial cells through the de novo synthesis of neurosteroids, suggesting that these compounds could be potential new therapeutic tools for the treatment of inflammatory-based neuropathologies with beneficial effects possibly comparable to steroids, but potentially avoiding the negative side effects of long-term therapies with steroid hormones.

Targeting translocator protein (18 kDa) (TSPO) dampens pro-inflammatory microglia reactivity in the retina and protects from degeneration

BACKGROUND

Reactive microglia are commonly seen in retinal degenerative diseases, and neurotoxic microglia responses can contribute to photoreceptor cell death. We and others have previously shown that translocator protein (18 kDa) (TSPO) is highly induced in retinal degenerations and that the selective TSPO ligand XBD173 (AC-5216, emapunil) exerts strong anti-inflammatory effects on microglia in vitro and ex vivo. Here, we investigated whether targeting TSPO with XBD173 has immuno-modulatory and neuroprotective functions in two mouse models of acute retinal degeneration using bright white light exposure.

METHODS

BALB/cJ and Cx3cr1(GFP/+) mice received intraperitoneal injections of 10 mg/kg XBD173 or vehicle for five consecutive days, starting 1 day prior to exposure to either 15,000 lux white light for 1 h or 50,000 lux focal light for 10 min, respectively. The effects of XBD173 treatment on microglia and Müller cell reactivity were analyzed by immuno-stainings of retinal sections and flat mounts, fluorescence-activated cell sorting (FACS) analysis, and mRNA expression of microglia markers using quantitative real-time PCR (qRT-PCR). Optical coherence tomography (OCT), terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) stainings, and morphometric analyses were used to quantify the extent of retinal degeneration and photoreceptor apoptosis.

RESULTS

Four days after the mice were challenged with bright white light, a large number of amoeboid-shaped alerted microglia appeared in the degenerating outer retina, which was nearly completely prevented by treatment with XBD173. This treatment also down-regulated the expression of TSPO protein in microglia but did not change the TSPO levels in the retinal pigment epithelium (RPE). RT-PCR analysis showed that the microglia/macrophage markers Cd68 and activated microglia/macrophage whey acidic protein (Amwap) as well as the pro-inflammatory genes Ccl2 and Il6 were reduced after XBD173 treatment. Light-induced degeneration of the outer retina was nearly fully blocked by XBD173 treatment. We further confirmed these findings in an independent mouse model of focal light damage. Retinas of animals receiving XBD173 therapy displayed significantly more ramified non-reactive microglia and more viable arrestin-positive cone photoreceptors than vehicle controls.

CONCLUSIONS

Targeting TSPO with XBD173 effectively counter-regulates microgliosis and ameliorates light-induced retinal damage, highlighting a new pharmacological concept for the treatment of retinal degenerations.

Mechanism of testosterone deficiency in the transgenic sickle cell mouse

Testosterone deficiency is associated with sickle cell disease (SCD), but its underlying mechanism is not known. We investigated the possible occurrence and mechanism of testosterone deficiency in a mouse model of human SCD. Transgenic sickle male mice (Sickle) exhibited decreased serum and intratesticular testosterone and increased luteinizing hormone (LH) levels compared with wild type (WT) mice, indicating primary hypogonadism in Sickle mice. LH-, dbcAMP-, and pregnenolone- (but not 22-hydroxycholesterol)- stimulated testosterone production by Leydig cells isolated from the Sickle mouse testis was decreased compared to that of WT mice, implying defective Leydig cell steroidogenesis. There also was reduced protein expression of steroidogenic acute regulatory protein (STAR), but not cholesterol side-chain cleavage enzyme (P450scc), in the Sickle mouse testis. These data suggest that the capacity of P450scc to support testosterone production may be limited by the supply of cholesterol to the mitochondria in Sickle mice. The sickle mouse testis exhibited upregulated NADPH oxidase subunit gp91phox and increased oxidative stress, measured as 4-hydroxy-2-nonenal, and unchanged protein expression of an antioxidant glutathione peroxidase-1. Mice heterozygous for the human sickle globin (Hemi) exhibited intermediate hypogonadal changes between those of WT and Sickle mice. These results demonstrate that testosterone deficiency occurs in Sickle mice, mimicking the human condition. The defects in the Leydig cell steroidogenic pathway in Sickle mice, mainly due to reduced availability of cholesterol for testosterone production, may be related to NADPH oxidase-derived oxidative stress. Our findings suggest that targeting testicular oxidative stress or steroidogenesis mechanisms in SCD offers a potential treatment for improving phenotypic changes associated with testosterone deficiency in this disease.

Pharmacological regulation of the cholesterol transport machinery in steroidogenic cells of the testis

Reduced serum testosterone (T), or hypogonadism, is estimated to affect about 5 million American men, including both aging and young men. Low serum T has been linked to mood changes, worsening cognition, fatigue, depression, decreased lean body mass and bone mineral density, increased visceral fat, metabolic syndrome, decreased libido, and sexual dysfunction. Administering exogenous T, known as T-replacement therapy (TRT), reverses many of the symptoms of low T levels. However, this treatment can result in luteinizing hormone suppression which, in turn, can lead to reduced sperm numbers and infertility, making TRT inappropriate for men who wish to father children. Additionally, TRT may result in supraphysiologic T levels, skin irritation, and T transfer to others upon contact; and there may be increased risk of prostate cancer and cardiovascular disease, particularly in aging men. Therefore, the development of alternate therapies for treating hypogonadism would be highly desirable. To do so requires greater understanding of the series of steps leading to T formation and how they are regulated, and the identification of key steps that are amenable to pharmacological modulation so as to induce T production. We review herein our current understanding of mechanisms underlying the pharmacological induction of T formation in hypogonadal testis.

Mitochondrial cholesterol: mechanisms of import and effects on mitochondrial function

Mitochondria require cholesterol for biogenesis and membrane maintenance, and for the synthesis of steroids, oxysterols and hepatic bile acids. Multiple pathways mediate the transport of cholesterol from different subcellular pools to mitochondria. In steroidogenic cells, the steroidogenic acute regulatory protein (StAR) interacts with a mitochondrial protein complex to mediate cholesterol delivery to the inner mitochondrial membrane for conversion to pregnenolone. In non-steroidogenic cells, several members of a protein family defined by the presence of a StAR-related lipid transfer (START) domain play key roles in the delivery of cholesterol to mitochondrial membranes. Subdomains of the endoplasmic reticulum (ER), termed mitochondria-associated ER membranes (MAM), form membrane contact sites with mitochondria and may contribute to the transport of ER cholesterol to mitochondria, either independently or in conjunction with lipid-transfer proteins. Model systems of mitochondria enriched with cholesterol in vitro and mitochondria isolated from cells with (patho)physiological mitochondrial cholesterol accumulation clearly demonstrate that mitochondrial cholesterol levels affect mitochondrial function. Increased mitochondrial cholesterol levels have been observed in several diseases, including cancer, ischemia, steatohepatitis and neurodegenerative diseases, and influence disease pathology. Hence, a deeper understanding of the mechanisms maintaining mitochondrial cholesterol homeostasis may reveal additional targets for therapeutic intervention. Here we give a brief overview of mitochondrial cholesterol import in steroidogenic cells, and then focus on cholesterol trafficking pathways that deliver cholesterol to mitochondrial membranes in non-steroidogenic cells. We also briefly discuss the consequences of increased mitochondrial cholesterol levels on mitochondrial function and their potential role in disease pathology.

Structure-to-function relationships of bacterial translocator protein (TSPO): a focus on Pseudomonas

The translocator protein (TSPO), which was previously designated as the peripheral-type benzodiazepine receptor, is a 3.5 billion year-old evolutionarily conserved protein expressed by most Eukarya, Archae and Bacteria, but its organization and functions differ remarkably. By taking advantage of the genomic data available on TSPO, we focused on bacterial TSPO and attempted to define functions of TSPO in Pseudomonas via in silico approaches. A tspo ortholog has been identified in several fluorescent Pseudomonas. This protein presents putative binding motifs for cholesterol and PK 11195, which is a specific drug ligand of mitochondrial TSPO. While it is a common surface distribution, the sense of insertion and membrane localization differ between α- and γ-proteobacteria. Experimental published data and STRING analysis of common TSPO partners in fluorescent Pseudomonas indicate a potential role of TSPO in the oxidative stress response, iron homeostasis and virulence expression. In these bacteria, TSPO could also take part in signal transduction and in the preservation of membrane integrity.

The mitochondrial voltage-dependent anion channel 1 in tumor cells

VDAC1 is found at the crossroads of metabolic and survival pathways. VDAC1 controls metabolic cross-talk between mitochondria and the rest of the cell by allowing the influx and efflux of metabolites, ions, nucleotides, Ca2+ and more. The location of VDAC1 at the outer mitochondrial membrane also enables its interaction with proteins that mediate and regulate the integration of mitochondrial functions with cellular activities. As a transporter of metabolites, VDAC1 contributes to the metabolic phenotype of cancer cells. Indeed, this protein is over-expressed in many cancer types, and silencing of VDAC1 expression induces an inhibition of tumor development. At the same time, along with regulating cellular energy production and metabolism, VDAC1 is involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. The engagement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space involves VDAC1 oligomerization that mediates the release of cytochrome c and AIF to the cytosol, subsequently leading to apoptotic cell death. Apoptosis can also be regulated by VDAC1, serving as an anchor point for mitochondria-interacting proteins, such as hexokinase (HK), Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. By binding to VDAC1, HK provides both a metabolic benefit and apoptosis-suppressive capacity that offer the cell a proliferative advantage and increase its resistance to chemotherapy. Thus, these and other functions point to VDAC1 as an excellent target for impairing the re-programed metabolism of cancer cells and their ability to evade apoptosis. Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to both cancer development and therapy. In addressing the recently solved 3D structures of VDAC1, this review will point to structure-function relationships of VDAC as critical for deciphering how this channel can perform such a variety of roles, all of which are important for cell life and death. Finally, this review will also provide insight into VDAC function in Ca2+ homeostasis, protection against oxidative stress, regulation of apoptosis and involvement in several diseases, as well as its role in the action of different drugs. We will discuss the use of VDAC1-based strategies to attack the altered metabolism and apoptosis of cancer cells. These strategies include specific siRNA able to impair energy and metabolic homeostasis, leading to arrested cancer cell growth and tumor development, as well VDAC1-based peptides that interact with anti-apoptotic proteins to induce apoptosis, thereby overcoming the resistance of cancer cell to chemotherapy. Finally, small molecules targeting VDAC1 can induce apoptosis. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Protein modifications regulate the role of 14-3-3γ adaptor protein in cAMP-induced steroidogenesis in MA-10 Leydig cells

The 14-3-3 protein family comprises adaptors and scaffolds that regulate intracellular signaling pathways. The 14-3-3γ isoform is a negative regulator of steroidogenesis that is hormonally induced and transiently functions at the initiation of steroidogenesis by delaying maximal steroidogenesis in MA-10 mouse tumor Leydig cells. Treatment of MA-10 cells with the cAMP analog 8-bromo-cAMP (8-Br-cAMP), which stimulates steroidogenesis, triggers the interaction of 14-3-3γ with the steroidogenic acute regulatory protein (STAR) in the cytosol, limiting STAR activity to basal levels. Over time, this interaction ceases, allowing for a 2-fold induction in STAR activity and maximal increase in the rate of steroid formation. The 14-3-3γ/STAR pattern of interaction was found to be opposite that of the 14-3-3γ homodimerization pattern. Phosphorylation and acetylation of 14-3-3γ showed similar patterns to homodimerization and STAR binding, respectively. 14-3-3γ Ser(58) phosphorylation and 14-3-3γ Lys(49) acetylation were blocked using trans-activator of HIV transcription factor 1 peptides coupled to 14-3-3γ sequences containing Ser(58) or Lys(49). Blocking either one of these modifications further induced 8-Br-cAMP-induced steroidogenesis while reducing lipid storage, suggesting that the stored cholesterol is used for steroid formation. Taken together, these results indicate that Ser(58) phosphorylation and Lys(49) acetylation of 14-3-3γ occur in a coordinated time-dependent manner to regulate 14-3-3γ homodimerization. 14-3-3γ Ser(58) phosphorylation is required for STAR interactions under control conditions, and 14-3-3γ Lys(49) acetylation is important for the cAMP-dependent induction of these interactions.

Carbenoxolone induces permeability transition pore opening in rat mitochondria via the translocator protein TSPO and connexin43

Ca(2+)-induced permeability transition pore (mPTP) opening in isolated rat brain mitochondria is promoted through targeting of connexin43. After a threshold Ca(2+) load, mitochondrial membrane potential drops and efflux of accumulated Ca(2+) from the mitochondrial matrix occurs, indicating the mPTP opening. Specific antibodies were used to assess the role of the translocator protein (18kDa; TSPO) and connexin43 in swelling of isolated rat liver and brain mitochondria induced by carbenoxolone and the endogenous TSPO ligand protoporphyrin IX. Mitochondrial membrane potential, Ca(2+) transport and oxygen consumption were determined using selective electrodes. All the parameters were detected simultaneously in a chamber with the selective electrodes. The phosphorylation state of mitochondrial protein targets was assessed. We report that Ca(2+)-induced mitochondrial swelling was strengthened in the presence of both carbenoxolone and protoporphyrin IX. The carbenoxolone- and protoporphyrin IX-accelerated mPTP induction in brain mitochondria was completely prevented by antibodies specific for the mitochondrial translocator protein (TSPO). The anti-TSPO antibodies were more effective than anti-сonnexin43 antibodies. Moreover, carbenoxolone-stimulated phosphorylation of mitochondrial proteins was inhibited by anti-TSPO antibodies. Taken together, the data suggests that, in addition to acting via connexion43, carbenoxolone may exert its effect on mPTP via mitochondrial outer membrane TSPO.

Induction of androgen formation in the male by a TAT-VDAC1 fusion peptide blocking 14-3-3ɛ protein adaptor and mitochondrial VDAC1 interactions

Low testosterone (T), a major cause of male hypogonadism and infertility, is linked to mood changes, fatigue, osteoporosis, reduced bone-mass index, and aging. The treatment of choice, T replacement therapy, has been linked with increased risk for prostate cancer and luteinizing hormone (LH) suppression, and shown to lead to infertility, cardiovascular diseases, and obesity. Alternate methods to induce T with lower side effects are desirable. In search of the mechanisms regulating T synthesis in the testes, we identified the 14-3-3ɛ protein adaptor as a negative regulator of steroidogenesis. Steroidogenesis begins in mitochondria. 14-3-3ɛ interacts with the outer mitochondrial membrane voltage-dependent anion channel (VDAC1) protein, forming a scaffold that limits the availability of cholesterol for steroidogenesis. We report the development of a tool able to induce endogenous T formation. Peptides able to penetrate testes conjugated to 14-3-3ɛ site of interaction with VDAC1 blocked 14-3-3ɛ-VDAC1 interactions while at the same time increased VDAC1-translocator protein (18 kDa) interactions that induced steroid formation in rat testes, leading to increased serum T levels. These peptides rescued intratesticular and serum T formation in adult male rats treated with gonadotropin-releasing hormone antagonist, which dampened LH and T production.

Translocator protein (18 kDa) (TSPO) is expressed in reactive retinal microglia and modulates microglial inflammation and phagocytosis

BACKGROUND

The translocator protein (18 kDa) (TSPO) is a mitochondrial protein expressed on reactive glial cells and a biomarker for gliosis in the brain. TSPO ligands have been shown to reduce neuroinflammation in several mouse models of neurodegeneration. Here, we analyzed TSPO expression in mouse and human retinal microglia and studied the effects of the TSPO ligand XBD173 on microglial functions.

METHODS

TSPO protein analyses were performed in retinoschisin-deficient mouse retinas and human retinas. Lipopolysaccharide (LPS)-challenged BV-2 microglial cells were treated with XBD173 and TSPO shRNAs in vitro and pro-inflammatory markers were determined by qRT-PCR. The migration potential of microglia was determined with wound healing assays and the proliferation was studied with Fluorescence Activated Cell Sorting (FACS) analysis. Microglial neurotoxicity was estimated by nitrite measurement and quantification of caspase 3/7 levels in 661 W photoreceptors cultured in the presence of microglia-conditioned medium. The effects of XBD173 on filopodia formation and phagocytosis were analyzed in BV-2 cells and human induced pluripotent stem (iPS) cell-derived microglia (iPSdM). The morphology of microglia was quantified in mouse retinal explants treated with XBD173.

RESULTS

TSPO was strongly up-regulated in microglial cells of the dystrophic mouse retina and also co-localized with microglia in human retinas. Constitutive TSPO expression was high in the early postnatal Day 3 mouse retina and declined to low levels in the adult tissue. TSPO mRNA and protein were also strongly induced in LPS-challenged BV-2 microglia while the TSPO ligand XBD173 efficiently suppressed transcription of the pro-inflammatory marker genes chemokine (C-C motif) ligand 2 (CCL2), interleukin 6 (IL6) and inducible nitric oxide (NO)-synthase (iNOS). Moreover, treatment with XBD173 significantly reduced the migratory capacity and proliferation of microglia, their level of NO secretion and their neurotoxic activity on 661 W photoreceptor cells. Furthermore, XBD173 treatment of murine and human microglial cells promoted the formation of filopodia and increased their phagocytic capacity to ingest latex beads or photoreceptor debris. Finally, treatment with XBD173 reversed the amoeboid alerted phenotype of microglial cells in explanted organotypic mouse retinal cultures after challenge with LPS.

CONCLUSIONS

These findings suggest that TSPO is highly expressed in reactive retinal microglia and a promising target to control microglial reactivity during retinal degeneration.

De novo synthesis of steroids and oxysterols in adipocytes

Local production and action of cholesterol metabolites such as steroids or oxysterols within endocrine tissues are currently recognized as an important principle in the cell type- and tissue-specific regulation of hormone effects. In adipocytes, one of the most abundant endocrine cells in the human body, the de novo production of steroids or oxysterols from cholesterol has not been examined. Here, we demonstrate that essential components of cholesterol transport and metabolism machinery in the initial steps of steroid and/or oxysterol biosynthesis pathways are present and active in adipocytes. The ability of adipocyte CYP11A1 in producing pregnenolone is demonstrated for the first time, rendering adipocyte a steroidogenic cell. The oxysterol 27-hydroxycholesterol (27HC), synthesized by the mitochondrial enzyme CYP27A1, was identified as one of the major de novo adipocyte products from cholesterol and its precursor mevalonate. Inhibition of CYP27A1 activity or knockdown and deletion of the Cyp27a1 gene induced adipocyte differentiation, suggesting a paracrine or autocrine biological significance for the adipocyte-derived 27HC. These findings suggest that the presence of the 27HC biosynthesis pathway in adipocytes may represent a defense mechanism to prevent the formation of new fat cells upon overfeeding with dietary cholesterol.

Mitochondria and mammalian reproduction

Mitochondria are cellular organelles with crucial roles in ATP synthesis, metabolic integration, reactive oxygen species (ROS) synthesis and management, the regulation of apoptosis (namely via the intrinsic pathway), among many others. Additionally, mitochondria in different organs or cell types may have distinct properties that can decisively influence functional analysis. In terms of the importance of mitochondria in mammalian reproduction, and although there are species-specific differences, these aspects involve both energetic considerations for gametogenesis and fertilization, control of apoptosis to ensure the proper production of viable gametes, and ROS signaling, as well as other emerging aspects. Crucially, mitochondria are the starting point for steroid hormone biosynthesis, given that the conversion of cholesterol to pregnenolone (a common precursor for all steroid hormones) takes place via the activity of the cytochrome P450 side-chain cleavage enzyme (P450scc) on the inner mitochondrial membrane. Furthermore, mitochondrial activity in reproduction has to be considered in accordance with the very distinct strategies for gamete production in the male and female. These include distinct gonad morpho-physiologies, different types of steroids that are more prevalent (testosterone, estrogens, progesterone), and, importantly, the very particular timings of gametogenesis. While spermatogenesis is complete and continuous since puberty, producing a seemingly inexhaustible pool of gametes in a fixed environment; oogenesis involves the episodic production of very few gametes in an environment that changes cyclically. These aspects have always to be taken into account when considering the roles of any common element in mammalian reproduction.

Evolutionary origin of the mitochondrial cholesterol transport machinery reveals a universal mechanism of steroid hormone biosynthesis in animals

Steroidogenesis begins with the transport of cholesterol from intracellular stores into mitochondria via a series of protein-protein interactions involving cytosolic and mitochondrial proteins located at both the outer and inner mitochondrial membranes. In adrenal glands and gonads, this process is accelerated by hormones, leading to the production of high levels of steroids that control tissue development and function. A hormone-induced multiprotein complex, the transduceosome, was recently identified, and is composed of cytosolic and outer mitochondrial membrane proteins that control the rate of cholesterol entry into the outer mitochondrial membrane. More recent studies unveiled the steroidogenic metabolon, a bioactive, multimeric protein complex that spans the outer-inner mitochondrial membranes and is responsible for hormone-induced import, segregation, targeting, and metabolism of cholesterol by cytochrome P450 family 11 subfamily A polypeptide 1 (CYP11A1) in the inner mitochondrial membrane. The availability of genome information allowed us to systematically explore the evolutionary origin of the proteins involved in the mitochondrial cholesterol transport machinery (transduceosome, steroidogenic metabolon, and signaling proteins), trace the original archetype, and predict their biological functions by molecular phylogenetic and functional divergence analyses, protein homology modeling and molecular docking. Although most members of these complexes have a history of gene duplication and functional divergence during evolution, phylogenomic analysis revealed that all vertebrates have the same functional complex members, suggesting a common mechanism in the first step of steroidogenesis. An archetype of the complex was found in invertebrates. The data presented herein suggest that the cholesterol transport machinery is responsible for steroidogenesis among all vertebrates and is evolutionarily conserved throughout the entire animal kingdom.

Genome-brain-behavior interdependencies as a framework to understand hormone effects on learned behavior

Hormones have profound effects on the maturation and function of the zebra finch song system. Hormones often signal through receptors that directly or indirectly regulate transcription. In this way, hormones and the genome are functionally connected. Genome-brain-behavior interdependencies are often studied on evolutionary timescales but we can now apply and test these relationships on short timescales, relevant to an individual. Here, we begin to place patterns of hormone-related gene expression into the timeframe of an individual's lifespan to consider how hormones contribute to organization of neural systems necessary for learned behavior, and how they might signal during experience in ways that affect future behavior. This framework illustrates both how much investigations into genome and hormone function are intertwined, and how much we still need to learn.

The spatial and temporal regulation of the hormonal signal. Role of mitochondria in the formation of a protein complex required for the activation of cholesterol transport and steroids synthesis

The mitochondria are critical for steroidogenesis since the ability of cholesterol to move into mitochondria to be available for cytochrome P450, CYP11A1, determines the efficacy of steroid production. Several proteins kinases, such as PKA, MEK and ERK which are essential to complete steroidogenesis, form a mitochondria-associated complex. The protein-protein interactions between kinases and key factors during the transport of cholesterol takes place in the contact sites between the two mitochondrial membranes; however, no mitochondrial targeting sequence has been described for these kinases. Here we discuss the possibility that mitochondrial reorganization may be mediating a compartmentalized cellular response. This reorganization could allow the physical interaction between the hormone-receptor complex and the enzymatic and lipidic machinery necessary for the complete steroid synthesis and release. The movement of organelles in specialized cells could impact on biological processes that include, but are not limited to, steroid synthesis.

Synthesis and characterization of an MRI Gd-based probe designed to target the translocator protein

DPA-713 is the lead compound of a recently reported pyrazolo[1,5-a]pyrimidineacetamide series, targeting the translocator protein (TSPO 18 kDa), and as such, this structure, as well as closely related derivatives, have been already successfully used as positron emission tomography radioligands. On the basis of the pharmacological core of this ligands series, a new magnetic resonance imaging probe, coded DPA-C(6)-(Gd)DOTAMA was designed and successfully synthesized in six steps and 13% overall yield from DPA-713. The Gd-DOTA monoamide cage (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) represents the magnetic resonance imaging reporter, which is spaced from the phenylpyrazolo[1,5-a]pyrimidineacetamide moiety (DPA-713 motif) by a six carbon-atom chain. DPA-C(6)-(Gd)DOTAMA relaxometric characterization showed the typical behavior of a small-sized molecule (relaxivity value: 6.02 mM(-1) s(-1) at 20 MHz). The good hydrophilicity of the metal chelate makes DPA-C(6)-(Gd)DOTAMA soluble in water, affecting thus its biodistribution with respect to the parent lipophilic DPA-713 molecule. For this reason, it was deemed of interest to load the probe to a large carrier in order to increase its residence lifetime in blood. Whereas DPA-C(6)-(Gd)DOTAMA binds to serum albumin with a low affinity constant, it can be entrapped into liposomes (both in the membrane and in the inner aqueous cavity). The stability of the supramolecular adduct formed by the Gd-complex and liposomes was assessed by a competition test with albumin.

Role of mitochondria in steroidogenesis

Adrenal gonadal, placental and brain mitochondria contain several steroidogenic enzymes, notably the cholesterol side chain cleavage enzyme, P450scc, which is the enzymatic rate-limiting step in steroidogenesis which determines cellular steroidogenic capacity. Even before this step, the access of cholesterol to this enzyme system is both rate-limiting and the site of acute regulation via the steroidogenic acute regulatory protein (StAR) which interacts with a complex multi-component 'transduceosome' on the outer mitochondrial membrane (OMM). The components of the transduceosome include the 18 kDa translocator protein (TSPO), the voltage-dependent anion channel (VDAC-1), TSPO-associated protein 7 (PAP7, ACBD3 for acyl-CoA-binding-domain 3), 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 P450scc. Mitochondria also contain several 'downstream' steroidogenic enzymes.