Classical and Novel TSPO Ligands for the Mitochondrial TSPO Can Modulate Nuclear Gene Expression: Implications for Mitochondrial Retrograde Signaling

TSPO deficiency promotes the progression of malignant peripheral sheath tumors by regulating the G2/M phase of the cell cycle via CDK1

Malignant peripheral nerve sheath tumors (MPNSTs) are highly aggressive Schwann cell-derived sarcomas that are sporadic or associated with Neurofibromatosis 1 (NF1) gene mutations. Traditional therapies are usually ineffective for treating MPNSTs, so new targets need to be identified for the treatment of MPNSTs. In the present study, the role of the mitochondrial translocator protein (TSPO) in the regulation of cell proliferation and the cell cycle in MPNSTs was investigated. TSPO expression was lower in MPNSTs than in NFs. Loss-of-function experiments revealed that TSPO deficiency promoted MPNST cell growth, migration, and invasion and influenced the cell cycle in vitro and in vivo. In addition, TSPO depletion suppressed cell apoptosis by downregulating the expression of caspase-3, caspase-8, HSP60, p27, p53, and BCL-2 and suppressed the cell cycle by upregulating CDK1, CDK2, CCNB1 and CCNA2. Furthermore, CDK1 was determined to be an upstream target of TSPO-mediated regulation via RNA-seq, qPCR, and Western blotting. Specifically, depletion of CDK1 weakened the effect of TSPO deficiency on cell proliferation and migration. More importantly, CDK1 knockdown induced significant cell cycle arrest in the G2/M phase. In summary, TSPO deficiency regulates the cell cycle in MPNSTs by targeting CDK1, which may be an effective molecular target for prognosis evaluation and treatment.

Dysregulation of lipid metabolism in the liver of Tspo knockout mice

The translocator protein, TSPO, has been implicated in a wide range of cellular processes exerted from its position in the outer mitochondrial membrane from where it influences lipid metabolism and mitochondrial oxidative activity. Understanding how this protein regulates a profusion of processes requires further elucidation and to that end we have examined lipid metabolism and used an RNAseq strategy to compare transcript abundance in wildtype and Tspo knockout (KO) mouse liver. The levels of cholesterol, triglyceride and phospholipid were significantly elevated in the KO mouse liver. The expression of cholesterol homeostasis genes was markedly downregulated. Determination of the differential expression revealed that many genes were either up- or downregulated in the KO animals. However, a striking observation within the results was a decrease of transcripts for protein degradation proteins in KO animals while protease inhibitors were enriched. When the entire abundance data-set was analysed with CEMiTool, and revealed a module of proteins that were under-represented in the KO animals. These could subsequently be formed into a network comprising three interlinked clusters at the centre of which were proteins of cytoplasmic ribosomes with gene ontology terms suggesting impairment to translation. The largest cluster was dominated by proteins of lipid metabolism but also contained disparate systems of iron metabolism and behaviour. The third cluster was dominated by proteins of the electron transport chain and oxidative phosphorylation. These findings suggest that TSPO contributes to lipid metabolism, detoxification of active oxygen species and oxidative phosphorylation, and regulates mitochondrial retrograde signalling.

Human oligodendrocyte-like cell differentiation is promoted by TSPO-mediated endogenous steroidogenesis

Mature oligodendrocytes (OLs) arise from oligodendrocyte precursor cells that, in case of demyelination, are recruited at the lesion site to remyelinate the axons and therefore restore the transmission of nerve impulses. It has been widely documented that exogenously administered steroid molecules are potent inducers of myelination. However, little is known about how neurosteroids produced de novo by OLs can impact this process. Here, we employed a human OL precursor cell line to investigate the role of de novo neurosteroidogenesis in the regulation of OLs differentiation, paying particular attention to the 18 kDa Translocator Protein (TSPO) which controls the rate-limiting step of the neurosteroidogenic process. Our results showed that, over the time of OL maturation, the availability of cholesterol, which is the neurosteroidogenesis initial substrate, and key members of the neurosteroidogenic machinery, including TSPO, were upregulated. In addition, OLs differentiation was impaired following neurosteroidogenesis inhibition and TSPO silencing. On the contrary, TSPO pharmacological stimulation promoted neurosteroidogenic function and positively impacted differentiation. Collectively, our results suggest that de novo neurosteroidogenesis is actively involved in the autocrine and paracrine regulation of human OL differentiation. Moreover, since TSPO was able to promote OL differentiation through a positive modulation of the neurosteroid biosynthetic process, it could be exploited as a promising target to tackle demyelinating diseases.

Enhanced angiogenesis in the thalamus induced by a novel TSPO ligand ameliorates cognitive deficits after focal cortical infarction

Neuronal loss in the ipsilateral thalamus after focal cortical infarction participates in post-stroke cognitive deficits, and enhanced angiogenesis in the thalamus is expected to reduce neuronal damage. We hypothesize that novel translocator protein (TSPO) ligand, 2-Cl-MGV-1, can promote angiogenesis, attenuate neuronal loss in the thalamus, and ameliorate post-stroke cognitive deficits. Cortical infarction was induced by distal middle cerebral artery occlusion (dMCAO) in stroke-prone renovascular hypertensive rats. 2-Cl-MGV-1 or dimethyl sulfoxide was administered 24 h after dMCAO and then for 6 or 13 days. Spatial learning and memory were assessed using the Morris water maze. Neuronal loss, TSPO expression, angiogenesis, and intrinsic pathway were determined by immunofluorescence and immunoblotting 7 and 14 days after dMCAO. Cortical infarction caused post-stroke cognitive deficits and secondary neuronal loss with gliosis in the ipsilateral thalamus within 14 days of dMCAO. Increased angiogenesis and elevated expression of vascular TSPO were detected in the ipsilateral thalamus, and treatment with 2-Cl-MGV-1 enhanced angiogenesis by stimulating the PI3K-AKT-mTOR pathway. The effects of 2-Cl-MGV-1 on angiogenesis coincided with reduced neuronal loss in the thalamus and contributed to improvements in post-stroke cognitive deficits. Our findings suggest that 2-Cl-MGV-1 stimulates angiogenesis, ameliorates neuronal loss in the thalamus, and improves post-stroke cognitive deficits.

TSPO exacerbates acute cerebral ischemia/reperfusion injury by inducing autophagy dysfunction

Autophagy is considered a double-edged sword, with a role in the regulation of the pathophysiological processes of the central nervous system (CNS) after cerebral ischemia-reperfusion injury (CIRI). The 18-kDa translocator protein (TSPO) is a highly conserved protein, with its expression level in the nervous system closely associated with the regulation of pathophysiological processes. In addition, the ligand of TSPO reduces neuroinflammation in brain diseases, but the potential role of TSPO in CIRI is largely undiscovered. On this basis, we investigated whether TSPO regulates neuroinflammatory response by affecting autophagy in microglia. In our study, increased expression of TSPO was detected in rat brain tissues with transient middle cerebral artery occlusion (tMCAO) and in BV2 microglial cells exposed to oxygen-glucose deprivation or reoxygenation (OGD/R) treatment, respectively. In addition, we confirmed that autophagy was over-activated during CIRI by increased expression of autophagy activation related proteins with Beclin-1 and LC3B, while the expression of p62 was decreased. The degradation process of autophagy was inhibited, while the expression levels of LAMP-1 and Cathepsin-D were significantly reduced. Results of confocal laser microscopy and transmission electron microscopy (TEM) indicated that autophagy flux was disordered. In contrast, inhibition of TSPO prevented autophagy over-activation both in vivo and in vitro. Interestingly, suppression of TSPO alleviated nerve cell damage by reducing reactive oxygen species (ROS) and pro-inflammatory factors, including TNF-α and IL-6 in microglia cells. In summary, these results indicated that TSPO might affect CIRI by mediating autophagy dysfunction and thus might serve as a potential target for ischemic stroke treatment.

Translocator protein in the rise and fall of central nervous system neurons

Translocator protein (TSPO), a 18 kDa protein found in the outer mitochondrial membrane, has historically been associated with the transport of cholesterol in highly steroidogenic tissues though it is found in all cells throughout the mammalian body. TSPO has also been associated with molecular transport, oxidative stress, apoptosis, and energy metabolism. TSPO levels are typically low in the central nervous system (CNS), but a significant upregulation is observed in activated microglia during neuroinflammation. However, there are also a few specific regions that have been reported to have higher TSPO levels than the rest of the brain under normal conditions. These include the dentate gyrus of the hippocampus, the olfactory bulb, the subventricular zone, the choroid plexus, and the cerebellum. These areas are also all associated with adult neurogenesis, yet there is no explanation of TSPO's function in these cells. Current studies have investigated the role of TSPO in microglia during neuron degeneration, but TSPO's role in the rest of the neuron lifecycle remains to be elucidated. This review aims to discuss the known functions of TSPO and its potential role in the lifecycle of neurons within the CNS.

TSPO acts as an immune resistance gene involved in the T cell mediated immune control of glioblastoma

Glioblastoma (GB) IDH-wildtype is the most malignant primary brain tumor. It is particularly resistant to current immunotherapies. Translocator protein 18 kDa (TSPO) is upregulated in GB and correlates with malignancy and poor prognosis, but also with increased immune infiltration. Here, we studied the role of TSPO in the regulation of immune resistance of human GB cells. The role of TSPO in tumor immune resistance was experimentally determined in primary brain tumor initiating cells (BTICs) and cell lines through genetic manipulation of TSPO expression and subsequent cocultures with antigen specific cytotoxic T cells and autologous tumor-infiltrating T cells. Death inducing intrinsic and extrinsic apoptotic pathways affected by TSPO were investigated. TSPO-regulated genes mediating apoptosis resistance in BTICs were identified through gene expression analysis and subsequent functional analyses. TSPO transcription in primary GB cells correlated with CD8⁺ T cell infiltration, cytotoxic activity of T cell infiltrate, expression of TNFR and IFNGR and with the activity of their downstream signalling pathways, as well as with the expression of TRAIL receptors. Coculture of BTICs with tumor reactive cytotoxic T cells or with T cell-derived factors induced TSPO up-regulation through T cell derived TNFα and IFNγ. Silencing of TSPO sensitized BTICs against T cell-mediated cytotoxicity. TSPO selectively protected BTICs against TRAIL-induced apoptosis by regulating apoptosis pathways. TSPO also regulated the expression of multiple genes associated with resistance against apoptosis. We conclude that TSPO expression in GB is induced through T cell-derived cytokines TNFα and IFNγ and that TSPO expression protects GB cells against cytotoxic T cell attack through TRAIL. Our data thereby provide an indication that therapeutic targeting of TSPO may be a suitable approach to sensitize GB to immune cell-mediated cytotoxicity by circumventing tumor intrinsic TRAIL resistance.

Energy metabolism disturbance in migraine: From a mitochondrial point of view

Migraine is a serious central nervous system disease with a high incidence rate. Its pathogenesis is very complex, which brings great difficulties for clinical treatment. Recently, many studies have revealed that mitochondrial dysfunction may play a key role in migraine, which affects the hyperosmotic of Ca²⁺, the excessive production of free radicals, the decrease of mitochondrial membrane potential, the imbalance of mPTP opening and closing, and the decrease of oxidative phosphorylation level, which leads to neuronal energy exhaustion and apoptosis, and finally lessens the pain threshold and migraine attack. This article mainly introduces cortical spreading depression, a pathogenesis of migraine, and then damages the related function of mitochondria, which leads to migraine. Oxidative phosphorylation and the tricarboxylic acid cycle are the main ways to provide energy for the body. 95 percent of the energy needed for cell survival is provided by the mitochondrial respiratory chain. At the same time, hypoxia can lead to cell death and migraine. The pathological opening of the mitochondrial permeability transition pore can promote the interaction between pro-apoptotic protein and mitochondrial, destroy the structure of mPTP, and further lead to cell death. The increase of mPTP permeability can promote the accumulation of reactive oxygen species, which leads to a series of changes in the expression of proteins related to energy metabolism. Both Nitric oxide and Calcitonin gene-related peptide are closely related to the attack of migraine. Recent studies have shown that changes in their contents can also affect the energy metabolism of the body, so this paper reviews the above mechanisms and discusses the mechanism of brain energy metabolism of migraine, to provide new strategies for the prevention and treatment of migraine and promote the development of individualized and accurate treatment of migraine.

Environmental concentrations of a delorazepam-based drug impact on embryonic development of non-target Xenopus laevis

Benzodiazepines, psychotropics drugs used for treating sleep disorders, anxiety and epilepsy, represent a major class of emerging water pollutants. As occurs for other pharmaceutical residues, they are not efficiently degraded during sewage treatment and persist in effluent waters. Bioaccumulation is already reported in fish and small crustaceans, but the impact and consequences on other "non-target" aquatic species are still unclear and nowadays of great interest. In this study, we investigated the effects of a pharmaceutical preparation containing the benzodiazepine delorazepam on the embryogenesis of Xenopus laevis, amphibian model species, taxa at high risk of exposure to water contaminants. Environmental (1 μg/L) and two higher (5 and 10 μg/L) concentrations were tested on tadpoles up to stage 45/46. Results demonstrate that delorazepam interferes with embryo development and that the effects are prevalently dose-dependent. Delorazepam reduces vitality by decreasing heart rate and motility, induces marked cephalic and abdominal edema, as well as intestinal and retinal defects. At the molecular level, delorazepam increases ROS production, modifies the expression of some master developmental genes and pro-inflammatory cytokines. The resulting stress condition significantly affects embryos' development and threatens their survival. Similar effects should be expected as well in embryos belonging to other aquatic species that have not been yet considered targets for these pharmaceutical residues.

The 18-kDa Translocator Protein PET Tracers as a Diagnostic Marker for Neuroinflammation: Development and Current Standing

Translocator protein (TSPO, 18 kDa) is an evolutionary, well-preserved, and tryptophan-rich 169-amino-acid protein which localizes on the contact sites between the outer and inner mitochondrial membranes of steroid-synthesizing cells. This mitochondrial protein is implicated in an extensive range of cellular activities, including steroid synthesis, cholesterol transport, apoptosis, mitochondrial respiration, and cell proliferation. The upregulation of TSPO is well documented in diverse disease conditions including neuroinflammation, cancer, brain injury, and inflammation in peripheral organs. On the basis of these outcomes, TSPO has been assumed to be a fascinating subcellular target for early stage imaging of the diseased state and for therapeutic purposes. The main outline of this Review is to give an update on dealing with the advances made in TSPO PET tracers for neuroinflammation, synchronously emphasizing the approaches applied for the design and advancement of new tracers with reference to their structure-activity relationship (SAR).

Imaging neuroinflammation with TSPO: A new perspective on the cellular sources and subcellular localization

Translocator Protein 18 kDa (TSPO), previously named Peripheral Benzodiazepine Receptor, is a well-validated and widely used biomarker of neuroinflammation to assess diverse central nervous system (CNS) pathologies in preclinical and clinical studies. Many studies have shown that in animal models of human neurological and neurodegenerative disease and in the human condition, TSPO levels increase in the brain neuropil, and this increase is driven by infiltration of peripheral inflammatory cells and activation of glial cells. Therefore, a clear understanding of the dynamics of the cellular sources of the TSPO response is critically important in the interpretation of Positron Emission Tomography (PET) studies and for understanding the pathophysiology of CNS diseases. Within the normal brain compartment, there are tissues and cells such as the choroid plexus, ependymal cells of the lining of the ventricles, and vascular endothelial cells that also express TSPO at even higher levels than in glial cells. However, there is a paucity of knowledge if these cell types respond and increase TSPO in the diseased brain. These cells do provide a background signal that needs to be accounted for in TSPO-PET imaging studies. More recently, there are reports that TSPO may be expressed in neurons of the adult brain and TSPO expression may be increased by neuronal activity. Therefore, it is essential to study this topic with a great deal of detail, methodological rigor, and rule out alternative interpretations and imaging artifacts. High levels of TSPO are present in the outer mitochondrial membrane. Recent studies have provided evidence of its localization in other cellular compartments including the plasma membrane and perinuclear regions which may define functions that are different from that in mitochondria. A greater understanding of the TSPO subcellular localization in glial cells and infiltrating peripheral immune cells and associated function(s) may provide an additional layer of information to the understanding of TSPO neurobiology. This review is an effort to outline recent advances in understanding the cellular sources and subcellular localization of TSPO in brain cells and to examine remaining questions that require rigorous investigation.

Neurogenic Potential of the 18-kDa Mitochondrial Translocator Protein (TSPO) in Pluripotent P19 Stem Cells

The 18-kDa translocator protein (TSPO) is a key mitochondrial target by which different TSPO ligands exert neuroprotective effects. We assayed the neurogenic potential of TSPO to induce the neuronal differentiation of pluripotent P19 stem cells in vitro. We studied changes in cell morphology, cell proliferation, cell death, the cell cycle, mitochondrial functionality, and the levels of pluripotency and neurogenesis of P19 stem cells treated with the TSPO ligand, PK 11195, in comparison to differentiation induced by retinoid acid (RA) and undifferentiated P19 stem cells. We observed that PK 11195 was able to activate the differentiation of P19 stem cells by promoting the development of embryoid bodies. PK 11195 also induced changes in the cell cycle, decreased cell proliferation, and activated cell death. Mitochondrial metabolism was also enhanced by PK 11195, thus increasing the levels of reactive oxygen species, Ca²⁺, and ATP as well as the mitochondrial membrane potential. Markers of pluripotency and neurogenesis were also altered during the cell differentiation process, as PK 11195 induced the differentiation of P19 stem cells with a high predisposition toward a neuronal linage, compared to cell differentiation induced by RA. Thus, we suggest a relevant neurogenic potential of TSPO along with broad therapeutic implications.

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.

Fucoidan: a promising agent for brain injury and neurodegenerative disease intervention

Brain injury and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis are urgent medical problems, which severely threaten the life quality of patients and their carers. However, there are currently no effective therapies. Fucoidan is a natural compound found in brown algae and some animals, which has multiple biological and pharmacological activities, such as antioxidant, anti-tumor, anti-coagulant, anti-thrombotic, immunoregulatory, anti-viral, and anti-inflammatory effects. A growing number of studies have shown that fucoidan also exerts a neuroprotective function. Particularly, recent findings have indicated that fucoidan could slow down the neurodegenerative processes and show protective effects against brain injury, which might be of therapeutic value for intervening in brain injury and neurodegenerative diseases. In this review, we have discussed the pharmacokinetics of fucoidan as well as the molecular mechanisms by which fucoidan exerts its neuroprotective effect on some neurological disorders. Along with this, we have also summarized the potential benefits of fucoidan in combination with other drugs in the treatment of neurodegenerative diseases and brain injury. Although the extraction process of fucoidan has been improved well, more efforts should be devoted to the translational research and clinical trials of fucoidan in the near future.

PK11195 Protects From Cell Death Only When Applied During Reperfusion: Succinate-Mediated Mechanism of Action

Aim: Reperfusion after myocardial ischemia causes cellular injury, in part due to changes in mitochondrial Ca²⁺ handling, oxidative stress, and myocyte energetics. We have previously shown that the 18-kDa translocator protein of the outer mitochondrial membrane (TSPO) can modulate Ca²⁺ handling. Here, we aim to evaluate the role of the TSPO in ischemia/reperfusion (I/R) injury.

Methods: Rabbit ventricular myocytes underwent simulated acute ischemia (20 min) and reperfusion (at 15 min, 1 h, and 3 h) in the absence and presence of 50 μM PK11195, a TSPO inhibitor. Cell death was measured by lactate dehydrogenase (LDH) assay, while changes in mitochondrial Ca²⁺, membrane potential (ΔΨ_m), and reactive oxygen species (ROS) generation were monitored using confocal microscopy in combination with fluorescent indicators. Substrate utilization was measured with Biolog mitochondrial plates.

Results: Cell death was increased by ~200% following I/R compared to control untreated ventricular myocytes. Incubation with 50 μM PK11195 during both ischemia and reperfusion did not reduce cell death but increased mitochondrial Ca²⁺ uptake and ROS generation. However, application of 50 μM PK11195 only at the onset and during reperfusion effectively protected against cell death. The large-scale oscillations in ΔΨ_m observed after ~1 h of reperfusion were significantly delayed by 1 μM cyclosporin A and almost completely prevented by 50 μM PK11195 applied during 3 h of reperfusion. After an initial increase, mitochondrial Ca²⁺, measured with Myticam, rapidly declined during 3 h of reperfusion after the initial transient increase. This decline was prevented by application of PK11195 at the onset and during reperfusion. PK11195 prevented a significant increase in succinate utilization following I/R and succinate-induced forward-mode ROS generation. Treatment with PK11195 was also associated with a significant increase in glutamate and a decrease in leucine utilization.

Conclusion: PK11195 administered specifically at the moment of reperfusion limited ROS-induced ROS release and cell death, likely in part, by a shift from succinate to glutamate utilization. These data demonstrate a unique mechanism to limit cardiac injury after I/R.

De novo Neurosteroidogenesis in Human Microglia: Involvement of the 18 kDa Translocator Protein

Neuroactive steroids are potent modulators of microglial functions and are capable of counteracting their excessive reactivity. This action has mainly been ascribed to neuroactive steroids released from other sources, as microglia have been defined unable to produce neurosteroids de novo. Unexpectedly, immortalized murine microglia recently exhibited this de novo biosynthesis; herein, de novo neurosteroidogenesis was characterized in immortalized human microglia. The results demonstrated that C20 and HMC3 microglial cells constitutively express members of the neurosteroidogenesis multiprotein machinery-in particular, the transduceosome members StAR and TSPO, and the enzyme CYP11A1. Moreover, both cell lines produce pregnenolone and transcriptionally express the enzymes involved in neurosteroidogenesis. The high TSPO expression levels observed in microglia prompted us to assess its role in de novo neurosteroidogenesis. TSPO siRNA and TSPO synthetic ligand treatments were used to reduce and prompt TSPO function, respectively. The TSPO expression downregulation compromised the de novo neurosteroidogenesis and led to an increase in StAR expression, probably as a compensatory mechanism. The pharmacological TSPO stimulation the de novo neurosteroidogenesis improved in turn the neurosteroid-mediated release of Brain-Derived Neurotrophic Factor. In conclusion, these results demonstrated that de novo neurosteroidogenesis occurs in human microglia, unravelling a new mechanism potentially useful for future therapeutic purposes.

The Interplay of Cholesterol and Ligand Binding in hTSPO from Classical Molecular Dynamics Simulations

The translocator protein (TSPO) is a 18kDa transmembrane protein, ubiquitously present in human mitochondria. It is overexpressed in tumor cells and at the sites of neuroinflammation, thus representing an important biomarker, as well as a promising drug target. In mammalian TSPO, there are cholesterol–binding motifs, as well as a binding cavity able to accommodate different chemical compounds. Given the lack of structural information for the human protein, we built a model of human (h) TSPO in the apo state and in complex with PK11195, a molecule routinely used in positron emission tomography (PET) for imaging of neuroinflammatory sites. To better understand the interactions of PK11195 and cholesterol with this pharmacologically relevant protein, we ran molecular dynamics simulations of the apo and holo proteins embedded in a model membrane. We found that: (i) PK11195 stabilizes hTSPO structural fold; (ii) PK11195 might enter in the binding site through transmembrane helices I and II of hTSPO; (iii) PK11195 reduces the frequency of cholesterol binding to the lower, N–terminal part of hTSPO in the inner membrane leaflet, while this impact is less pronounced for the upper, C–terminal part in the outer membrane leaflet, where the ligand binding site is located; (iv) very interestingly, cholesterol most frequently binds simultaneously to the so-called CRAC and CARC regions in TM V in the free form (residues L150–X–Y152–X(3)–R156 and R135–X(2)–Y138–X(2)–L141, respectively). However, when the protein is in complex with PK11195, cholesterol binds equally frequently to the CRAC–resembling motif that we observed in TM I (residues L17–X(2)–F20–X(3)–R24) and to CRAC in TM V. We expect that the CRAC–like motif in TM I will be of interest in future experimental investigations. Thus, our MD simulations provide insight into the structural features of hTSPO and the previously unknown interplay between PK11195 and cholesterol interactions with this pharmacologically relevant protein.

The TSPO-specific Ligand PK11195 Protects Against LPS-Induced Cognitive Dysfunction by Inhibiting Cellular Autophagy

Perioperative neurocognitive disorders (PND) is a common postoperative neurological complication. Neuroinflammation is a major cause that leads to PND. Autophagy, an intracellular process of lysosomal degradation, plays an important role in the development and maintenance of nervous system. PK11195 is a classic translocator protein (TSPO) ligand, which can improve the cognitive function of rats. In this study, we evaluate the protective effect of PK11195 on the learning and memory of rats. A rat model of lipopolysaccharide (LPS)-induced cognitive dysfunction was established by intraperitoneal injection of LPS. Morris Water Maze (MWM), Western blot, qRT-PCR, confocal microscopy and transmission electron microscopy (TEM) were used to study the role of TSPO-specific ligand PK11195 in LPS-activated mitochondrial autophagy in rat hippocampus. We found that PK11195 ameliorated LPS-induced learning and memory impairment, as indicated by decreased escape latencies, swimming distances and increased target quadrant platform crossing times and swimming times during MWM tests. TSPO, ATG7, ATG5, LC3B and p62 protein and mRNA expression increased in the hippocampus of PND model rats. The hippocampal microglia of PND model rats also have severe mitochondrial damage, and a large number of autophagosomes and phagocytic vesicles can be seen. PK11195 pretreatment significantly decreased the expression of TSPO, ATG7, ATG5, LC3B and p62 protein and mRNA, as well as mitochondrial damage. These findings suggested that PK11195 may alleviate the damage of LPS-induced cognitive dysfunction of rats by inhibiting microglia activation and autophagy.

Cigarette smoke induces apoptosis via 18 kDa translocator protein in human bronchial epithelial cells

AIMS

The 18 kDa translocator protein (TSPO) - also known as peripheral benzodiazepine receptor, is found to be expressed in lung epithelium and pneumocytes, which is closely associated with the mitochondrial permeability transition pore (mPTP) and apoptosis. Cigarette smoking, a key risk factor for the development of chronic obstructive pulmonary disease (COPD), is known to induce apoptosis. We aimed to investigate TSPO subcellular localization and to examine whether cigarette smoke medium (CSM) induce apoptosis via TSPO in airway epithelial cells.

MAIN METHODS

TSPO subcellular localization and expression were evaluated using immunofluorescent staining and Western blot analysis respectively. TSPO ligands either PK 11195 (a specific antagonist) or AC-5216 (a specific agonist) were pre-incubated in human bronchial epithelial cells before treating with 2% CSM for measurements of apoptotic cells, mitochondrial membrane potential (ΔΨm), cytoplasmic/mitochondrial reactive oxygen species (ROS) and inflammatory marker interleukin (IL)-8 respectively.

KEY FINDINGS

TSPO was localized around the nucleus and overlapped with mitochondria in BEAS-2B cells. CSM caused an increase in apoptotic cells along with elevation of TSPO protein expression. Pretreatment of PK 11195 suppressed while AC-5216 potentiated CSM-induced apoptosis, collapse of ΔΨm, elevation of cytoplasmic/mitochondrial ROS levels and IL-8 release. In support, knockdown of TSPO caused a significant suppression of CSM-induced IL-8 release in BEAS-2B cells.

SIGNIFICANCE

The findings suggest that TSPO may play a crucial role in the regulation of cigarette smoke-induced mitochondrial dysfunction via mPTP. Therefore, the development of specific TSPO antagonists like PK11195 may be beneficial to combat smoking-related diseases, such as COPD.

Mitochondrial TSPO Deficiency Triggers Retrograde Signaling in MA-10 Mouse Tumor Leydig Cells

The mitochondrial translocator protein (TSPO) has been shown to bind cholesterol with high affinity and is involved in mediating its availability for steroidogenesis. We recently reported that targeted Tspo gene deletion in MA-10 mouse tumor Leydig cells resulted in reduced cAMP-stimulated steroid formation and significant reduction in the mitochondrial membrane potential (ΔΨ_m) compared to control cells. We hypothesized that ΔΨ_m reduction in the absence of TSPO probably reflects the dysregulation and/or maintenance failure of some basic mitochondrial function(s). To explore the consequences of TSPO depletion via CRISPR-Cas9-mediated deletion (indel) mutation in MA-10 cells, we assessed the transcriptome changes in TSPO-mutant versus wild-type (Wt) cells using RNA-seq. Gene expression profiles were validated using real-time PCR. We report herein that there are significant changes in nuclear gene expression in Tspo mutant versus Wt cells. The identified transcriptome changes were mapped to several signaling pathways including the regulation of membrane potential, calcium signaling, extracellular matrix, and phagocytosis. This is a retrograde signaling pathway from the mitochondria to the nucleus and is probably the result of changes in expression of several transcription factors, including key members of the NF-κB pathway. In conclusion, TSPO regulates nuclear gene expression through intracellular signaling. This is the first evidence of a compensatory response to the loss of TSPO with transcriptome changes at the cellular level.

TSPO Ligands Boost Mitochondrial Function and Pregnenolone Synthesis

Translocator protein 18 kDa (TSPO) is located in the mitochondrial outer membrane and plays an important role in steroidogenesis and cell survival. In the central nervous system (CNS), its expression is upregulated in neuropathologies such as Alzheimer's disease (AD). Previously, we demonstrated that two new TSPO ligands based on an imidazoquinazolinone termed 2a and 2b, stimulated pregnenolone synthesis and ATP production in vitro. In the present study, we compared their effects to those of TSPO ligands described in the literature (XBD173, SSR-180,575, and Ro5-4864) by profiling the mitochondrial bioenergetic phenotype before and after treatment and investigating the protective effects of these ligands after oxidative injury in a cellular model of AD overexpressing amyloid-β (Aβ). Of note, ATP levels increased with rising pregnenolone levels suggesting that the energetic performance of mitochondria is linked to an increased production of this neurosteroid via TSPO modulation. Our results further demonstrate that the TSPO ligands 2a and 2b exerted neuroprotective effects by improving mitochondrial respiration, reducing reactive oxygen species and thereby decreasing oxidative stress-induced cell death as well as lowering Aβ levels. The compounds 2a and 2b show similar or even better functional effects than those obtained with the reference TSPO ligands XBD173 and SSR-180.575. These findings indicate that the new TSPO ligands modulate mitochondrial bioenergetic phenotype and protect against oxidative injury probably through the de novo synthesis of neurosteroids, suggesting that these compounds could be potential new therapeutic tools for the treatment of neurodegenerative disease.

Nanomedicines for Subcellular Targeting: The Mitochondrial Perspective

BACKGROUND

Over the past decade, there has been a surge in the number of mitochondrialactive therapeutics for conditions ranging from cancer to aging. Subcellular targeting interventions can modulate adverse intracellular processes unique to the compartments within the cell. However, there is a dearth of reviews focusing on mitochondrial nano-delivery, and this review seeks to fill this gap with regards to nanotherapeutics of the mitochondria.

METHODS

Besides its potential for a higher therapeutic index than targeting at the tissue and cell levels, subcellular targeting takes into account the limitations of systemic drug administration and significantly improves pharmacokinetics. Hence, an extensive literature review was undertaken and salient information was compiled in this review.

RESULTS

From literature, it was evident that nanoparticles with their tunable physicochemical properties have shown potential for efficient therapeutic delivery, with several nanomedicines already approved by the FDA and others in clinical trials. However, strategies for the development of nanomedicines for subcellular targeting are still emerging, with an increased understanding of dysfunctional molecular processes advancing the development of treatment modules. For optimal delivery, the design of an ideal carrier for subcellular delivery must consider the features of the diseased microenvironment. The functional and structural features of the mitochondria in the diseased state are highlighted and potential nano-delivery interventions for treatment and diagnosis are discussed.

CONCLUSION

This review provides an insight into recent advances in subcellular targeting, with a focus on en route barriers to subcellular targeting. The impact of mitochondrial dysfunction in the aetiology of certain diseases is highlighted, and potential therapeutic sites are identified.

The Role of Translocator Protein TSPO in Hallmarks of Glioblastoma

Simple Summary

The translocator protein (TSPO) has been under extensive investigation as a specific marker in positron emission tomography (PET) to visualize brain lesions following injury or disease. In recent years, TSPO is increasingly appreciated as a potential novel therapeutic target in cancer. In Glioblastoma (GBM), the most malignant primary brain tumor, TSPO expression levels are strongly elevated and scientific evidence accumulates, hinting at a pivotal role of TSPO in tumorigenesis and glioma progression. The aim of this review is to summarize the current literature on TSPO with respect to its role both in diagnostics and especially with regard to the critical hallmarks of cancer postulated by Hanahan and Weinberg. Overall, our review contributes to a better understanding of the functional significance of TSPO in Glioblastoma and draws attention to TSPO as a potential modulator of treatment response and thus an important factor that may influence the clinical outcome of GBM.

Abstract

Glioblastoma (GBM) is the most fatal primary brain cancer in adults. Despite extensive treatment, tumors inevitably recur, leading to an average survival time shorter than 1.5 years. The 18 kDa translocator protein (TSPO) is abundantly expressed throughout the body including the central nervous system. The expression of TSPO increases in states of inflammation and brain injury due to microglia activation. Not least due to its location in the outer mitochondrial membrane, TSPO has been implicated with a broad spectrum of functions. These include the regulation of proliferation, apoptosis, migration, as well as mitochondrial functions such as mitochondrial respiration and oxidative stress regulation. TSPO is frequently overexpressed in GBM. Its expression level has been positively correlated to WHO grade, glioma cell proliferation, and poor prognosis of patients. Several lines of evidence indicate that TSPO plays a functional part in glioma hallmark features such as resistance to apoptosis, invasiveness, and proliferation. This review provides a critical overview of how TSPO could regulate several aspects of tumorigenesis in GBM, particularly in the context of the hallmarks of cancer proposed by Hanahan and Weinberg in 2011.

Targeting Mitochondria in Tumor-Associated Macrophages using a Dendrimer-Conjugated TSPO Ligand that Stimulates Antitumor Signaling in Glioblastoma

Mitochondria mediate critical cellular processes, including proliferation, apoptosis, and immune responses; as such, their dysfunction is pathogenic in many neurodegenerative disorders and cancers. In glioblastoma, targeted delivery of mitochondria-focused anticancer therapies has failed to translate into clinical success due to the nonspecific cellular localization, heterogeneity of receptor expression across patients, poor transport across biological barriers to reach the brain, tumor, and mitochondria, and systemic side effects. Strategies that can overcome brain and solid tumor barriers and selectively target mitochondria within specific cell types may lead to improvements in glioblastoma treatment. Developments in dendrimer-mediated nanomedicines have shown promise targeting tumor-associated macrophages (TAMs) in glioblastoma, following systemic administration. Here, we present a novel dendrimer conjugated to the translocator protein (18 kDa) (TSPO) ligand 5,7-dimethylpyrazolo[1,5-α]pyrimidin-3-ylacetamide (DPA). We developed a clickable DPA for conjugation on the dendrimer surface and demonstrated in vitro that the dendrimer-DPA conjugate (D-DPA) significantly increases dendrimer colocalization with mitochondria. Compared to free TSPO ligand PK11195, D-DPA stimulates greater antitumor immune signaling. In vivo, we show that D-DPA targets mitochondria specifically within TAMs following systemic administration. Our results demonstrate that dendrimers can achieve TAM-specific targeting in glioblastoma and can be further modified to target specific intracellular compartments for organelle-specific drug delivery.

PET Imaging of the P2X7 Ion Channel with a Novel Tracer [18F]JNJ-64413739 in a Rat Model of Neuroinflammation

PURPOSE

The P2X7 receptor, an adenosine triphosphate (ATP)-gated purinoreceptor, has emerged as one of the key players in neuroinflammatory processes. Therefore, developing a positron emission tomography (PET) tracer for imaging of P2X7 receptors in vivo presents a promising approach to diagnose, monitor, and study neuroinflammation in a variety of brain disorders. To fulfill the goal of developing a P2X7 PET ligand as a biomarker of neuroinflammation, [¹⁸F]JNJ-64413739 has been recently disclosed.

PROCEDURES

We evaluated [¹⁸F]JNJ-64413739 in a rat model of neuroinflammation induced by an intracerebral injection of lipopolysaccharide (LPS). In vivo brain uptake was determined by PET imaging. Upregulation of neuroinflammatory biomarkers was determined by quantitative polymerase chain reaction (qPCR). Distribution of the tracer in the brain was determined by ex vivo autoradiography (ARG). The specificity of [¹⁸F]JNJ-64413739 was confirmed by performing blocking experiments with the P2X7 antagonist JNJ-54175446.

RESULTS

Brain regions of rats injected with LPS had a significantly increased uptake (34 % ± 3 % s.e.m., p = 0.036, t test, standardized uptake value measured over the entire scanning period) of [¹⁸F]JNJ-64413739 relative to the corresponding brain regions of control animals injected with phosphate-buffered saline (PBS). The uptake in the contralateral regions and cerebellum was not significantly different between the groups of animals. The increase in uptake of [¹⁸F]JNJ-64413739 at the LPS-injected site observed by PET imaging was concordant with ex vivo ARG, upregulation of neuroinflammatory biomarkers, and elevated P2X7 expression levels.

CONCLUSIONS

While further work is needed to study [¹⁸F]JNJ-64413739 in other types of neuroinflammation, the current results favorably characterize [¹⁸F]JNJ-64413739 as a potential PET tracer of central neuroinflammation.

Efficaciousness of Low Affinity Compared to High Affinity TSPO Ligands in the Inhibition of Hypoxic Mitochondrial Cellular Damage Induced by Cobalt Chloride in Human Lung H1299 Cells

The 18 kDa translocator protein (TSPO) plays an important role in apoptotic cell death, including apoptosis induced by the hypoxia mimicking agent cobalt chloride (CoCl₂). In this study, the protective effects of a high (CB86; Ki = 1.6 nM) and a low (CB204; Ki = 117.7 nM) affinity TSPO ligands were investigated in H1299 lung cancer cell line exposed to CoCl₂. The lung cell line H1299 was chosen in the present study since they express TSPO and able to undergo programmed cell death. The examined cell death markers included: ATP synthase reversal, reactive oxygen species (ROS) generation, mitochondrial membrane potential (Δψm) depolarization, cellular toxicity, and cellular viability. Pretreatment of the cells with the low affinity ligand CB204 at a concentration of 100 µM suppressed significantly (p < 0.05 for all) CoCl₂-induced cellular cytotoxicity (100%), ATP synthase reversal (67%), ROS generation (82%), Δψm depolarization (100%), reduction in cellular density (97%), and also increased cell viability (85%). Furthermore, the low affinity TSPO ligand CB204, was harmless when given by itself at 100 µM. In contrast, the high affinity ligand (CB86) was significantly effective only in the prevention of CoCl₂–induced ROS generation (39%, p < 0.001), and showed significant cytotoxic effects when given alone at 100 µM, as reflected in alterations in ADP/ATP ratio, oxidative stress, mitochondrial membrane potential depolarization and cell death. It appears that similar to previous studies on brain-derived cells, the relatively low affinity for the TSPO target enhances the potency of TSPO ligands in the protection from hypoxic cell death. Moreover, the high affinity TSPO ligand CB86, but not the low affinity ligand CB204, was lethal to the lung cells at high concentration (100 µM). The low affinity TSPO ligand CB204 may be a candidate for the treatment of pulmonary diseases related to hypoxia, such as pulmonary ischemia and chronic obstructive pulmonary disease COPD.

Diagnostic and Therapeutic Potential of TSPO Studies Regarding Neurodegenerative Diseases, Psychiatric Disorders, Alcohol Use Disorders, Traumatic Brain Injury, and Stroke: An Update

Neuroinflammation and cell death are among the common symptoms of many central nervous system diseases and injuries. Neuroinflammation and programmed cell death of the various cell types in the brain appear to be part of these disorders, and characteristic for each cell type, including neurons and glia cells. Concerning the effects of 18-kDa translocator protein (TSPO) on glial activation, as well as being associated with neuronal cell death, as a response mechanism to oxidative stress, the changes of its expression assayed with the aid of TSPO-specific positron emission tomography (PET) tracers' uptake could also offer evidence for following the pathogenesis of these disorders. This could potentially increase the number of diagnostic tests to accurately establish the stadium and development of the disease in question. Nonetheless, the differences in results regarding TSPO PET signals of first and second generations of tracers measured in patients with neurological disorders versus healthy controls indicate that we still have to understand more regarding TSPO characteristics. Expanding on investigations regarding the neuroprotective and healing effects of TSPO ligands could also contribute to a better understanding of the therapeutic potential of TSPO activity for brain damage due to brain injury and disease. Studies so far have directed attention to the effects on neurons and glia, and processes, such as death, inflammation, and regeneration. It is definitely worthwhile to drive such studies forward. From recent research it also appears that TSPO ligands, such as PK11195, Etifoxine, Emapunil, and 2-Cl-MGV-1, demonstrate the potential of targeting TSPO for treatments of brain diseases and disorders.

The Translocator Protein (TSPO) in Mitochondrial Bioenergetics and Immune Processes

The translocator protein (TSPO) is an outer mitochondrial membrane protein that is widely used as a biomarker of neuroinflammation, being markedly upregulated in activated microglia in a range of brain pathologies. Despite its extensive use as a target in molecular imaging studies, the exact cellular functions of this protein remain in question. The long-held view that TSPO plays a fundamental role in the translocation of cholesterol through the mitochondrial membranes, and thus, steroidogenesis, has been disputed by several groups with the advent of TSPO knockout mouse models. Instead, much evidence is emerging that TSPO plays a fundamental role in cellular bioenergetics and associated mitochondrial functions, also part of a greater role in the innate immune processes of microglia. In this review, we examine the more direct experimental literature surrounding the immunomodulatory effects of TSPO. We also review studies which highlight a more central role for TSPO in mitochondrial processes, from energy metabolism, to the propagation of inflammatory responses through reactive oxygen species (ROS) modulation. In this way, we highlight a paradigm shift in approaches to TSPO functioning.

Rewiring of Cancer Cell Metabolism by Mitochondrial VDAC1 Depletion Results in Time-Dependent Tumor Reprogramming: Glioblastoma as a Proof of Concept

Reprograming of the metabolism of cancer cells is an event recognized as a hallmark of the disease. The mitochondrial gatekeeper, voltage-dependent anion channel 1 (VDAC1), mediates transport of metabolites and ions in and out of mitochondria, and is involved in mitochondria-mediated apoptosis. Here, we compared the effects of reducing hVDAC1 expression in a glioblastoma xenograft using human-specific si-RNA (si-hVDAC1) for a short (19 days) and a long term (40 days). Tumors underwent reprograming, reflected in rewired metabolism, eradication of cancer stem cells (CSCs) and differentiation. Short- and long-term treatments of the tumors with si-hVDAC1 similarly reduced the expression of metabolism-related enzymes, and translocator protein (TSPO) and CSCs markers. In contrast, differentiation into cells expressing astrocyte or neuronal markers was noted only after a long period during which the tumor cells were hVDAC1-depleted. This suggests that tumor cell differentiation is a prolonged process that precedes metabolic reprograming and the “disappearance” of CSCs. Tumor proteomics analysis revealing global changes in the expression levels of proteins associated with signaling, synthesis and degradation of proteins, DNA structure and replication and epigenetic changes, all of which were highly altered after a long period of si-hVDAC1 tumor treatment. The depletion of hVDAC1 greatly reduced the levels of the multifunctional translocator protein TSPO, which is overexpressed in both the mitochondria and the nucleus of the tumor. The results thus show that VDAC1 depletion-mediated cancer cell metabolic reprograming involves a chain of events occurring in a sequential manner leading to a reversal of the unique properties of the tumor, indicative of the interplay between metabolism and oncogenic signaling networks.

Microglial Pro-Inflammatory and Anti-Inflammatory Phenotypes Are Modulated by Translocator Protein Activation

A key role of the mitochondrial Translocator Protein 18 KDa (TSPO) in neuroinflammation has been recently proposed. However, little is known about TSPO-activated pathways underlying the modulation of reactive microglia. In the present work, the TSPO activation was explored in an in vitro human primary microglia model (immortalized C20 cells) under inflammatory stimulus. Two different approaches were used with the aim to (i) pharmacologically amplify or (ii) silence, by the lentiviral short hairpin RNA, the TSPO physiological function. In the TSPO pharmacological stimulation model, the synthetic steroidogenic selective ligand XBD-173 attenuated the activation of microglia. Indeed, it reduces and increases the release of pro-inflammatory and anti-inflammatory cytokines, respectively. Such ligand-induced effects were abolished when C20 cells were treated with the steroidogenesis inhibitor aminoglutethimide. This suggests a role for neurosteroids in modulating the interleukin production. The highly steroidogenic ligand XBD-173 attenuated the neuroinflammatory response more effectively than the poorly steroidogenic ones, which suggests that the observed modulation on the cytokine release may be influenced by the levels of produced neurosteroids. In the TSPO silencing model, the reduction of TSPO caused a more inflamed phenotype with respect to scrambled cells. Similarly, during the inflammatory response, the TSPO silencing increased and reduced the release of pro-inflammatory and anti-inflammatory cytokines, respectively. In conclusion, the obtained results are in favor of a homeostatic role for TSPO in the context of dynamic balance between anti-inflammatory and pro-inflammatory mediators in the human microglia-mediated inflammatory response. Interestingly, our preliminary results propose that the TSPO expression could be stimulated by NF-κB during activation of the inflammatory response.

VDAC1 and the TSPO: Expression, Interactions, and Associated Functions in Health and Disease States

The translocator protein (TSPO), located at the outer mitochondrial membrane (OMM), serves multiple functions and contributes to numerous processes, including cholesterol import, mitochondrial metabolism, apoptosis, cell proliferation, Ca²⁺ signaling, oxidative stress, and inflammation. TSPO forms a complex with the voltage-dependent anion channel (VDAC), a protein that mediates the flux of ions, including Ca²⁺, nucleotides, and metabolites across the OMM, controls metabolism and apoptosis and interacts with many proteins. This review focuses on the two OMM proteins TSPO and VDAC1, addressing their structural interaction and associated functions. TSPO appears to be involved in the generation of reactive oxygen species, proposed to represent the link between TSPO activation and VDAC, thus playing a role in apoptotic cell death. In addition, expression of the two proteins in healthy brains and diseased states is considered, as is the relationship between TSPO and VDAC1 expression. Both proteins are over-expressed in in brains from Alzheimer’s disease patients. Finally, TSPO expression levels were proposed as a biomarker of some neuropathological settings, while TSPO-interacting ligands have been considered as a potential basis for drug development.

Inhibitory Effects of the Two Novel TSPO Ligands 2-Cl-MGV-1 and MGV-1 on LPS-induced Microglial Activation

The 18 kDa translocator protein (TSPO) ligands 2-Cl-MGV-1 and MGV-1 can attenuate cell death of astrocyte-like cells (U118MG) and induce differentiation of neuronal progenitor cells (PC-12). Lipopolysaccharide (LPS) is a bacterial membrane endotoxin that activates cellular inflammatory pathways by releasing pro-inflammatory molecules, including cytokines and chemokines. The aim of the present study was to assess the immuno-modulatory effect of TSPO ligands in activated microglial cells. We demonstrated that the TSPO ligands 2-Cl-MGV-1 and MGV-1 can prevent LPS-induced activation of microglia (BV-2 cell line). Co-treatment of LPS (100 ng/mL) with these TSPO ligands (final concentration- 25 µM) reduces significantly the LPS-induced release of interleukin-6 (IL-6) from 16.9-fold to 2.5-fold, IL-β from 8.3-fold to 1.6-fold, interferon-γ from 16.0-fold to 2.2-fold, and tumor necrosis factor-α from 16.4-fold to 1.8-fold. This anti-inflammatory activity seems to be achieved by inhibition of NF-κB p65 activation. Assessment of initiation of ROS generation and cell metabolism shows significant protective effects of these two novel TSPO ligands. The IL-10 and IL-13 levels were not affected by any of the TSPO ligands. Thus, it appears that the ligands suppress the LPS-induced activation of some inflammatory responses of microglia. Such immunomodulatory effects may be relevant to the pharmacotherapy of neuro-inflammatory diseases.

Translocator protein 18 kDa ligand alleviates neointimal hyperplasia in the diabetic rat artery injury model via activating PKG

AIMS

The proliferation of VSMCs is the pathologic basis for intimal hyperplasia after angioplasty in diabetic patients. Translocator protein (TSPO), located in the outer mitochondrial membrane, has been found to regulate redox intermediate components in cell dysfunction. We hypothesized that TSPO may regulate VSMC proliferation and migration, and be involved in the intimal hyperplasia after angioplasty in diabetes.

MATERIALS AND METHODS

Cell proliferation was measured by cell counting and MTT assays. Cell migration was measured by Transwell® and scratch-wound assays. TSPO expression in arteries of rats and high glucose-treated A10 cells were detected by immunoblotting and immunofluorescence staining. Neointimal formation of carotid artery was induced by balloon injury in type 2 diabetic rat.

KEY FINDINGS

TSPO expression was increased in the arterial samples from diabetic rats and A10 cells treated with high glucose. Down-regulation of TSPO expression by siRNA decreased the high-glucose-induced VSMC proliferation and migration in A10 cells. This phenomenon could be simulated by using TSPO ligands, PK 11195 and Ro5-4864. cGMP/PKG signals were involved in the TSPO ligand action, since in the presence of cGMP or PKG inhibitor ODQ or KT5823 respectively, the effect of PK 11195 on VSMC proliferation was blocked. Furthermore, PK 11195 significantly inhibited neointimal formation by the inhibition of VSMC proliferation.

SIGNIFICANCE

This study suggests that TSPO inhibition suppresses the proliferation and migration of VSMCs induced by hyperglycemia, consequently, preventing atherosclerosis and restenosis after angioplasty in diabetic conditions. TSPO may be a potential therapeutic target to reduce arterial remodeling induced by angioplasty in diabetes.

The TSPO Ligands 2-Cl-MGV-1, MGV-1, and PK11195 Differentially Suppress the Inflammatory Response of BV-2 Microglial Cell to LPS

The 18 kDa Translocator Protein (TSPO) is a marker for microglial activation as its expression is enhanced in activated microglia during neuroinflammation. TSPO ligands can attenuate neuroinflammation and neurotoxicity. In the present study, we examined the efficacy of new TSPO ligands designed by our laboratory, MGV-1 and 2-Cl-MGV-1, in mitigating an in vitro neuroinflammatory process compared to the classic TSPO ligand, PK 11195. We exposed BV-2 microglial cells to lipopolysaccharide (LPS) for 24 h to induce inflammatory response and added the three TSPO ligands: (1) one hour before LPS treatment (pretreatment), (2) simultaneously with LPS (cotreatment), and (3) one hour after LPS exposure (post-treatment). We evaluated the capability of TSPO ligands to reduce the levels of three glial inflammatory markers: cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and nitric oxide (NO). We compared the effects of the two novel ligands to PK 11195. Both 2-Cl-MGV-1 and MGV-1 reduced the levels of glial COX-2, iNOS, and NO in LPS-treated BV-2 cells more efficiently than PK 11195. Notably, even when added after exposure to LPS, all ligands were able to suppress the inflammatory response. Due to their pronounced anti-inflammatory activity, 2-Cl-MGV-1 and MGV-1 may serve as potential therapeutics in neuroinflammatory and neurodegenerative diseases.

Binding Affinity of Some Endogenous and Synthetic TSPO Ligands Regarding the rs6971 Polymorphism

An intriguing target involved in several pathophysiological processes is the 18 kDa translocator protein (TSPO), of which exact functions remained elusive until now. A single nucleotide polymorphism in the TSPO gene influences the binding affinity of endogenous and synthetic TSPO ligands by facilitating a lower-affinity conformation, which modifies a potential ligand binding site, ultimately leading to a binding profile classification according to each genotype. For instance, some clinical effects of the distinctive binding affinity profile of cholesterol toward the TSPO of individuals with different genotypes have been extensively discussed. Therefore, we conducted an investigation based on a radioligand binding assay, to determine the inhibition constants of some reported endogenous TSPO ligands (diazepam binding inhibitor and protoporphyrin IX), as well as synthetic ligands (disulfiram and derivatives). We observed no dependency of the polymorphism on the binding affinity of the evaluated endogenous ligands, whereas a high dependency on the binding affinity of the tested synthetic ligands was evident.

TSPO Ligands Promote Cholesterol Efflux and Suppress Oxidative Stress and Inflammation in Choroidal Endothelial Cells

Choroidal endothelial cells supply oxygen and nutrients to retinal pigment epithelial (RPE) cells and photoreceptors, recycle metabolites, and dispose of metabolic waste through the choroidal blood circulation. Death of the endothelial cells of the choroid may cause abnormal deposits including unesterified and esterified cholesterol beneath RPE cells and within Bruch’s membrane that contribute to the progression of age-related macular degeneration (AMD), the most prevalent cause of blindness in older people. Translocator protein (TSPO) is a cholesterol-binding protein that is involved in mitochondrial cholesterol transport and other cellular functions. We have investigated the role of TSPO in choroidal endothelial cells. Immunocytochemistry showed that TSPO was localized to the mitochondria of choroidal endothelial cells. Choroidal endothelial cells exposed to TSPO ligands (Etifoxine or XBD-173) had significantly increased cholesterol efflux, higher expression of cholesterol homeostasis genes (LXRα, CYP27A1, CYP46A1, ABCA1 and ABCG1), and reduced biosynthesis of cholesterol and phospholipids from [¹⁴C]acetate, when compared to untreated controls. Treatment with TSPO ligands also resulted in reduced production of reactive oxygen species (ROS), increased antioxidant capacity, and reduced release of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α and VEGF) induced by oxidized LDL. These data suggest TSPO ligands may offer promise for the treatment of AMD.

The 18 kDa Translocator Protein (TSPO) Overexpression in Hippocampal Dentate Gyrus Elicits Anxiolytic-Like Effects in a Mouse Model of Post-traumatic Stress Disorder

The translocator protein (18 kDa) (TSPO) recently attracted increasing attention in the pathogenesis of post-traumatic stress disorder (PTSD). This study is testing the hypothesis that the overexpression of TSPO in hippocampus dentate gyrus (DG) could alleviate the anxiogenic-like response in the mice model of PTSD induced by foot-shock. In this study, hippocampal DG overexpression of TSPO significantly reversed the increase of the contextual freezing response, the decrease of the percentage of both entries into and time spent in the open arms in elevated plus maze test and the decrease of the account of crossings from the dark to light compartments in light–dark transition test induced by electric foot-shocks procedure. It was further showed that the behavioral effects of TSPO overexpression were blocked by PK11195, a selective TSPO antagonist. In addition, the expression of TSPO and level of allopregnanolone (Allo) decreased in the mouse model of PTSD, which was blocked by overexpression of TSPO in hippocampal dentate gyrus. The difference of neurogenesis among groups was consistent with the changes of TSPO and Allo, as evidenced by bromodeoxyuridine (BrdU)- positive cells in the hippocampal dentate gyrus. These results firstly suggested that TSPO in hippocampal dentate gyrus could exert a great effect on the occurrence and recovery of PTSD in this animal model, and the anti-PTSD-like effect of hippocampal TSPO over-expression could be at least partially mediated by up-regulation of Allo and subsequent stimulation of the adult hippocampal neurogenesis.

The angiotensin II type 2 receptors protect renal tubule mitochondria in early stages of diabetes mellitus

Diabetic nephropathy correlates more closely to defective mitochondria and increased oxidative stress in the kidney than to hyperglycemia. A key driving factor of diabetic nephropathy is angiotensin II acting via the G-protein-coupled cell membrane type 1 receptor. The present study aimed to investigate the role of the angiotensin II type 2 receptor (AT2R) at the early stages of diabetic nephropathy. Using receptor binding studies and immunohistochemistry we found that the mitochondria in renal tubules contain high-affinity AT2Rs. Increased renal mitochondrial AT2R density by transgenic overexpression was associated with reduced superoxide production of isolated mitochondria from non-diabetic rats. Streptozotocin-induced diabetes (28 days) caused a drop in the ATP/oxygen ratio and an increase in the superoxide production of isolated renal mitochondria from wild-type diabetic rats. This correlated with changes in the renal expression profile and increased tubular epithelial cell proliferation. AT2R overexpression in tubular epithelial cells inhibited all diabetes-induced renal changes including a drop in mitochondrial bioenergetics efficiency, a rise in mitochondrial superoxide production, metabolic reprogramming, and increased proliferation. Thus, AT2Rs translocate to mitochondria and can contribute to reno-protective effects at early stages of diabetes. Hence, targeted AT2R overexpression in renal cells may open new avenues to develop novel types of drugs preventing diabetic nephropathy.

TSPO: An Evolutionarily Conserved Protein with Elusive Functions

TSPO (18 kDa translocator protein) was identified decades ago in a search for peripheral tissue binding sites for benzodiazepines, and was formerly called the peripheral benzodiazepine receptor. TSPO is a conserved protein throughout evolution and it is implicated in the regulation of many cellular processes, including inflammatory responses, oxidative stress, and mitochondrial homeostasis. TSPO, apart from its broad expression in peripheral tissues, is highly expressed in neuroinflammatory cells, such as activated microglia. In addition, emerging studies employing the ligands of TSPO suggest that TSPO plays an important role in neuropathological settings as a biomarker and therapeutic target. However, the precise molecular function of this protein in normal physiology and neuropathology remains enigmatic. This review provides an overview of recent advances in our understanding of this multifaceted molecule and identifies the knowledge gap in the field for future functional studies.

Mitochondrial Dysfunctions Regulated Radioresistance through Mitochondria-to-Nucleus Retrograde Signaling Pathway of NF-κB/PI3K/AKT2/mTOR

We investigated the relationship between significantly different genes of the mitochondria-to-nucleus retrograde signaling pathway (RTG) in H1299 ρ⁰ cells (mtDNA depleted cell) and compared their radiosensitivity to that of parental ρ⁺ cells, to determine the possible intervention targets of radiosensitization. ρ⁰ cells were depleted of mitochondrial DNA by chronic culturing in ethidium bromide at low concentration. Radiosensitivity was analyzed using clonogenic assay. Western blot was used to analyze the cell cycle-related proteins, serine/threonine kinase ataxia telangiectasia mutant (ATM), ataxia telangiectasia and Rad3-related protein (ATR) and cyclin B1 (CCNB1). The γ-H2AX foci were detected using confocal fluorescence microscopy. RNA samples were hybridized using the Agilent human genome expression microarray. The Kyoto Encyclopedia of Genes and Genomes (KEGG) database was used for Gene Ontology (GO) Consortium and pathway annotations of differentially expressed genes, respectively. The H1299 ρ⁰ cells were found to be more radioresistant than ρ⁺ cells. The ATP production of H1299 ρ⁰ cells was lower than that of the ρ⁺ cells before or after irradiation. Both H1299 ρ⁰ and ρ⁺ cells had higher ROS levels after irradiation, however, the radiation-induced ROS production in ρ⁰ cells was significantly lower than in ρ⁺ cells. In addition, the percentage of apoptosis in H1299 ρ⁰ cells was lower than in ρ⁺ cells after 6 Gy irradiation. As for the cell cycle and DNA damage response-related proteins ATM, ATR and CCNB1, the expression levels in ρ⁰ cells were significantly higher than in ρ⁺ cells, and there were less γ-H2AX foci in the ρ⁰ than ρ⁺ cells after irradiation. Furthermore, the results of the human genome expression microarray demonstrated that the phosphorylated protein levels of the NF-κB/PI3K/AKT2/mTOR signaling pathway were increased after 6 Gy irradiation and were decreased after treatment with the AKT2-specific inhibitor MK-2206 combined with radiation in H1299 ρ⁰ cells. MK-2206 treatment also led to an increase in pro-apoptotic proteins. In conclusion, these results demonstrate that mtDNA depletion might activate the mitochondria-to-nucleus retrograde signaling pathway of NF-κB/PI3K/AKT2/mTOR and induce radioresistance in H1299 ρ⁰ cells by evoking mitochondrial dysfunctions.

An Updated View of Translocator Protein (TSPO)

Decades of study on the role of mitochondria in living cells have evidenced the importance of the 18 kDa mitochondrial translocator protein (TSPO), first discovered in the 1977 as an alternative binding site for the benzodiazepine diazepam in the kidneys. This protein participates in a variety of cellular functions, including cholesterol transport, steroid hormone synthesis, mitochondrial respiration, permeability transition pore opening, apoptosis, and cell proliferation. Thus, TSPO has become an extremely attractive subcellular target for the early detection of disease states that involve the overexpression of this protein and the selective mitochondrial drug delivery. This special issue was programmed with the aim of summarizing the latest findings about the role of TSPO in eukaryotic cells and as a potential subcellular target of diagnostics or therapeutics. A total of 9 papers have been accepted for publication in this issue, in particular, 2 reviews and 7 primary data manuscripts, overall describing the main advances in this field.

Adrenal Oncocytic Neoplasm with Paradoxical Loss of Important Mitochondrial Steroidogenic Protein: The 18 kDA Translocator Protein

The adrenal glands produce a variety of hormones that play a key role in the regulation of blood pressure, electrolyte homeostasis, metabolism, immune system suppression, and the body's physiologic response to stress. Adrenal neoplasms can be asymptomatic or can overproduce certain hormones that lead to different clinical manifestations. Oncocytic adrenal neoplasms are infrequent tumors that arise from cells in the adrenal cortex and display a characteristic increase in the number of cytoplasmic mitochondria. Since the rate-limiting step in steroidogenesis includes the transport of cholesterol across the mitochondrial membranes, in part carried out by the 18-kDa translocator protein (TSPO), we assessed the expression of TSPO in a case of adrenal oncocytic neoplasm using residual adrenal gland of the patient as internal control. We observed a significant loss of TSPO immunofluorescence expression in the adrenal oncocytic tumor cells when compared to adjacent normal adrenal tissue. We further confirmed this finding by employing Western blot analysis to semiquantify TSPO expression in tumor and normal adrenal cells. Our findings could suggest a potential role of TSPO in the tumorigenesis of this case of adrenocortical oncocytic neoplasm.

Regulation of Mitochondrial, Cellular, and Organismal Functions by TSPO

In 1999, the enigma of the 18kDa mitochondrial translocator protein (TSPO), also known as the peripheral-type benzodiazepine receptor, was the seeming disparity of the many functions attributed to TSPO, ranging from the potential of TSPO acting as a housekeeping gene at molecular biological levels to adaptations to stress, and even involvement in higher emotional and cognitive functioning, such as anxiety and depression. In the years since then, knowledge regarding the many functions modulated by TSPO has expanded, and understanding has deepened. In addition, new functions could be firmly associated with TSPO, such as regulation of programmed cell death and modulation of gene expression. Interestingly, control by the mitochondrial TSPO over both of these life and death functions appears to include Ca⁺⁺ homeostasis, generation of reactive oxygen species (ROS), and ATP production. Other mitochondrial functions under TSPO control are considered to be steroidogenesis and tetrapyrrole metabolism. As TSPO effects on gene expression and on programmed cell death can be related to the wide range of functions that can be associated with TSPO, several of these five elements of Ca⁺⁺, ROS, ATP, steroids, and tetrapyrroles may indeed form the basis of TSPO's capability to operate as a multifunctional housekeeping gene to maintain homeostasis of the cell and of the whole multicellular organism.