Effects of translocator protein (18 kDa) ligands on microglial activation and neuronal death in the quinolinic-acid-injected rat striatum

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.

Targeting TSPO Reduces Inflammation and Apoptosis in an In Vitro Photoreceptor-Like Model of Retinal Degeneration

The 18 kDa translocator protein (TSPO) is predominantly located in the mitochondrial outer membrane, playing an important role in steroidogenesis, inflammation, survival, and cell proliferation. Its expression in the CNS, and mainly in glial cells, is upregulated in neuropathologies and brain injury. In this study, the potential of targeting TSPO for the therapeutic treatment of inflammatory-based retinal neurodegeneration was evaluated by means of an in vitro model of lipopolysaccharide (LPS)-induced degeneration in 661 W cells, a photoreceptor-like cell line. After the assessment of the expression of TSPO in 661W cells, which, to the best of our knowledge, was never investigated so far, the anti-inflammatory and cytoprotective effects of a number of known TSPO ligands, belonging to the class of N,N-dialkyl-2-arylindol-3-ylglyoxylamides (PIGAs), were evaluated, using the classic TSPO ligand PK11195 as the reference standard. All tested PIGAs showed the ability to modulate the inflammatory and apoptotic processes in 661 W photoreceptor-like cells and to reduce LPS-driven cellular cytotoxicity. The protective effect of PIGAs was, in all cases, reduced by cotreatment with the pregnenolone synthesis inhibitor SU-10603, suggesting the involvement of neurosteroids in the protective mechanism. As inflammatory processes play a crucial role in the retinal neurodegenerative disease progression toward photoreceptors' death and complete blindness, targeting TSPO might represent a successful strategy to slow down this degenerative process that may lead to the inexorable loss of vision.

The mitochondrial translocator protein (TSPO): a key multifunctional molecule in the nervous system

Translocator protein (TSPO, 18 kDa), formerly known as peripheral benzodiazepine receptor, is an evolutionary well-conserved protein located on the outer mitochondrial membrane. TSPO is involved in a variety of fundamental physiological functions and cellular processes. Its expression levels are regulated under many pathological conditions, therefore, TSPO has been proposed as a tool for diagnostic imaging and an attractive therapeutic drug target in the nervous system. Several synthetic TSPO ligands have thus been explored as agonists and antagonists for innovative treatments as neuroprotective and regenerative agents. In this review, we provide state-of-the-art knowledge of TSPO functions in the brain and peripheral nervous system. Particular emphasis is placed on its contribution to important physiological functions such as mitochondrial homeostasis, energy metabolism and steroidogenesis. We also report how it is involved in neuroinflammation, brain injury and diseases of the nervous system.

Activation of translocator protein alleviates mechanical allodynia and bladder dysfunction in cyclophosphamide-induced cystitis through repression of BDNF-mediated neuroinflammation

BACKGROUND

Bladder pain syndrome/interstitial cystitis (BPS/IC) is a refractory disease accompanied by bladder-related pain and hyperactivity. Studies have shown that the translocator protein (TSPO) modulates neuroinflammation and central sensitisation associated with pain. Moreover, we previously demonstrated that brain-derived neurotrophic factor (BDNF) regulates neuroinflammation and mechanical allodynia in cyclophosphamide (CYP)-induced cystitis through activation of glial cells. Here, we aimed to explore whether activation of TSPO attenuates mechanical allodynia and bladder dysfunction by regulating BDNF induced neuroinflammation in a CYP-induced cystitis model.

METHODS

Injection of CYP was performed to form a rat model of BPS/IC. The expression of TSPO was regulated by intrathecal injection of the TSPO agonist Ro5-4864. The von Frey filament test was applied to evaluate suprapubic allodynia. Bladder function was assessed using filling cystometry. Western blotting was used to detect the expression of TSPO, BDNF, GFAP, Iba-1, p-p38, p-JNK, TNF-α, and IL-1β, and double immunofluorescence was performed to localise TSPO in the L6-S1 spinal dorsal horn (SDH).

RESULTS

TSPO was activated in the SDH after CYP injection and was primarily colocalised with astrocytes. Ro5-4864 reversed mechanical allodynia and bladder dysfunction induced by CYP. Moreover, the upregulation of BDNF and activation of astrocytes and microglia was suppressed by Ro5-4864, resulting in downregulation of p-p38, p-JNK, TNF-α, and IL-1β.

CONCLUSIONS

Ro5-4864 alleviated mechanical allodynia and bladder dysfunction in the CYP model, possibly by inhibiting the elevation of BDNF and consequent activation of astrocytes and microglia induced neuroinflammation. TSPO may be a potential target for the treatment of BPS/IC.

The translocator protein ligands as mitochondrial functional modulators for the potential anti-Alzheimer agents

Small molecule modulators of mitochondrial function have been attracted much attention in recent years due to their potential therapeutic applications for neurodegenerative diseases. The mitochondrial translocator protein (TSPO) is a promising target for such compounds, given its involvement in the formation of the mitochondrial permeability transition pore in response to mitochondrial stress. In this study, we performed a ligand-based pharmacophore design and virtual screening, and identified a potent hit compound, 7 (VH34) as a TSPO ligand. After validating its biological activity against amyloid-β (Aβ) induced mitochondrial dysfunction and in acute and transgenic Alzheimer's disease (AD) model mice, we developed a library of analogs, and we found two most active compounds, 31 and 44, which restored the mitochondrial membrane potential, ATP production, and cell viability under Aβ-induced mitochondrial toxicity. These compounds recovered learning and memory function in acute AD model mice with improved pharmacokinetic properties.

TSPO imaging in animal models of brain diseases

Over the last 30 years, the 18-kDa TSPO protein has been considered as the PET imaging biomarker of reference to measure increased neuroinflammation. Generally assumed to image activated microglia, TSPO has also been detected in endothelial cells and activated astrocytes. Here, we provide an exhaustive overview of the recent literature on the TSPO-PET imaging (i) in the search and development of new TSPO tracers and (ii) in the understanding of acute and chronic neuroinflammation in animal models of neurological disorders. Generally, studies testing new TSPO radiotracers against the prototypic [11C]-R-PK11195 or more recent competitors use models of acute focal neuroinflammation (e.g. stroke or lipopolysaccharide injection). These studies have led to the development of over 60 new tracers during the last 15 years. These studies highlighted that interpretation of TSPO-PET is easier in acute models of focal lesions, whereas in chronic models with lower or diffuse microglial activation, such as models of Alzheimer's disease or Parkinson's disease, TSPO quantification for detection of neuroinflammation is more challenging, mirroring what is observed in clinic. Moreover, technical limitations of preclinical scanners provide a drawback when studying modest neuroinflammation in small brains (e.g. in mice). Overall, this review underlines the value of TSPO imaging to study the time course or response to treatment of neuroinflammation in acute or chronic models of diseases. As such, TSPO remains the gold standard biomarker reference for neuroinflammation, waiting for new radioligands for other, more specific targets for neuroinflammatory processes and/or immune cells to emerge.

Translocator Protein Regulate Polarization Phenotype Transformation of Microglia after Cerebral Ischemia-reperfusion Injury

Microglia cells are activated after cerebral ischemia-reperfusion injury (CIRI), playing a dual role in aggravating the injury or promoting tissue repair by polarization. Translocator protein (TSPO) is a biomarker of neuroinflammation or microglia activation. Its expression is significantly increased while brain injury and neuroinflammation occur. However, the relationship between TSPO and microglia polarization in CIRI is still not clear. In the present study, the middle cerebral artery occlusion (MCAO) methods in rats were used to simulate CIRI. We found that the expressions of M1 markers (CD86, IL-1β, and TNF-α) and M2 markers (CD206, IL-10, and TGF-β) were significantly increased. Moreover, the injection of TSPO ligand, PK11195, inhibited the increase of M1 polarization markers but promoted the expressions of M2 polarization markers, which significantly ameliorated the neurological damage after MCAO in rats. In vitro studies showed that shRNA-mediated TSPO knock-down promoted M1 polarization but inhibited M2 polarization, accompanied by a significant decrease in cell viability. On the contrary, overexpression of TSPO inhibited M1 polarization, promoted M2 polarization, and significantly improved cell viability. In summary, TSPO plays a neuroprotective role in CIRI by inhibiting M1 polarization and promoting M2 polarization, which suggests that TSPO may have the potential to serve as a therapeutic target for stroke.

TSPO deficiency accelerates amyloid pathology and neuroinflammation by impairing microglial phagocytosis

Increasing evidence has placed inflammation and immune dysfunction at the center of the pathogenesis of Alzheimer's disease (AD). The mitochondrial protein translocator protein (18 kDa) (TSPO) is highly upregulated in microglia and astrocytes in response to inflammatory stimulation. However, the biological action of TSPO in the pathogenesis of AD has not been determined to date. In this study, we showed that TSPO expression was upregulated in brain tissues from AD patients and AD model mice. APP/PS1 mice lacking TSPO generated significantly higher levels of Aβ_1-40 and Aβ_1-42 peptides and more Aβ plaques, as well as enhanced microglial activation, in the brain. TSPO-deficient microglia cultured in vitro showed a significant decrease in the ability to phagocytose Aβ peptides or latex beads and generated more proinflammatory cytokines (TNF-α and IL-1β) in response to Aβ peptides. Our findings suggest that TSPO has protective functions against neuroinflammation and Aβ pathogenesis in AD. TSPO may be a potential drug target for the development of drugs that have therapeutic or preventive effects in neuroinflammatory diseases.

The Protective Effect of PK-11195 on Cognitive Impairment in Rats Survived of Polymicrobial Sepsis

Sepsis is an organ dysfunction caused by a host's unregulated response to infection, causing long-term brain dysfunction with microglial activation, the release of inflammatory components, and mitochondrial changes. Neuroinflammation can increase the expression of the 18-kD translocator protein (TSPO) in the mitochondria, leading to the activation of the microglia and the release of inflammatory components. The antagonist PK-11195 can modulate TSPO and reduce microglial activation and cognitive damage presented in an animal model of sepsis. The aim of this was to evaluate the effects of PK-11195 on long-term brain inflammation and cognitive impairment in an animal model of sepsis. Wistar rats, 60 days old, were submitted to cecal ligation and puncture (CLP) surgery, divided into groups control/saline, control/PK-11195, sepsis/saline, and sepsis/PK-11195. Immediately after surgery, the antagonist PK-11195 was administered at a dose of 3 mg/kg. Ten days after CLP surgery, the animals were submitted to behavioral tests and determination of brain inflammatory parameters. The sepsis/saline group presented cognitive damage. However, there was damage prevention in animals that received PK-11195. Besides, the sepsis increased the levels of cytokines and M1 microglia markers and caused oxidative damage. However, PK-11195 had the potential to decrease inflammation. These events show that the modulation of neuroinflammation during sepsis by PK-11195, possibly related to changes in TSPO, improves mitochondrial function in the animals' brains. In conclusion, the antagonist PK-11195 attenuated brain inflammation and prevented cognitive impairment in animals subjected to sepsis.

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.

Adventures in Translocation: Studies of the Translocator Protein (TSPO) 18 kDa

The 18 kDa translocator protein (TSPO) is an evolutionarily conserved transmembrane protein found embedded in the outer mitochondrial membrane. A secondary target for the benzodiazepine diazepam, TSPO has been a protein of interest for researchers for decades, particularly owing to its well-established links to inflammatory conditions in the central and peripheral nervous systems. It has become a key biomarker for assessing microglial activation using positron emission tomography (PET) imaging in patients with diseases ranging from atherosclerosis to Alzheimer’s disease. This Account describes research published by our group over the past 15 years surrounding the development of TSPO ligands and their use in probing the function of this high-value target.

Translation Imaging in Parkinson's Disease: Focus on Neuroinflammation

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the appearance of α-synuclein insoluble aggregates known as Lewy bodies. Neurodegeneration is accompanied by neuroinflammation mediated by cytokines and chemokines produced by the activated microglia. Several studies demonstrated that such an inflammatory process is an early event, and contributes to oxidative stress and mitochondrial dysfunctions. α-synuclein fibrillization and aggregation activate microglia and contribute to disease onset and progression. Mutations in different genes exacerbate the inflammatory phenotype in the monogenic compared to sporadic forms of PD. Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) with selected radiopharmaceuticals allow in vivo imaging of molecular modifications in the brain of living subjects. Several publications showed a reduction of dopaminergic terminals and dopamine (DA) content in the basal ganglia, starting from the early stages of the disease. Moreover, non-dopaminergic neuronal pathways are also affected, as shown by in vivo studies with serotonergic and glutamatergic radiotracers. The role played by the immune system during illness progression could be investigated with PET ligands that target the microglia/macrophage Translocator protein (TSPO) receptor. These agents have been used in PD patients and rodent models, although often without attempting correlations with other molecular or functional parameters. For example, neurodegeneration and brain plasticity can be monitored using the metabolic marker 2-Deoxy-2-[¹⁸F]fluoroglucose ([¹⁸F]-FDG), while oxidative stress can be probed using the copper-labeled diacetyl-bis(N-methyl-thiosemicarbazone) ([Cu]-ATSM) radioligand, whose striatal-specific binding ratio in PD patients seems to correlate with a disease rating scale and motor scores. Also, structural and functional modifications during disease progression may be evaluated by Magnetic Resonance Imaging (MRI), using different parameters as iron content or cerebral volume. In this review article, we propose an overview of in vivo clinical and non-clinical imaging research on neuroinflammation as an emerging marker of early PD. We also discuss how multimodal-imaging approaches could provide more insights into the role of the inflammatory process and related events in PD development.

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.

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.

Synthesis and evaluation of novel potent TSPO PET ligands with 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamide

Translocator protein (TSPO) expression is closely related with neuroinflammation and neuronal damage which might cause several central nervous system diseases. Herein, a series of TSPO ligands (11a-c and 13a-d) with a 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamide structure were prepared and evaluated via an in vitro binding assay. Most of the novel ligands exhibited a nano-molar affinity for TSPO, which was better than that of DPA-714. Particularly, 11a exhibited a subnano-molar TSPO binding affinity with suitable lipophilicity for in vivo brain studies. After radiolabeling with fluorine-18, [¹⁸F]11a was used for a dynamic positron emission tomography (PET) study in a rat LPS-induced neuroinflammation model; the inflammatory lesion was clearly visualized with a superior target-to-background ratio compared to [¹⁸F]DPA-714. An immunohistochemical examination of the dissected brains confirmed that the uptake location of [¹⁸F]11a in the PET study was consistent with a positively activated microglia region. This study proved that [¹⁸F]11a could be employed as a potential PET tracer for detecting neuroinflammation and could give possibility for diagnosis of other diseases, such as cancers related with TSPO expression.

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.

Translocator protein ligand, YL-IPA08, attenuates lipopolysaccharide-induced depression-like behavior by promoting neural regeneration

Translocator protein has received attention for its involvement in the pathogenesis of depression. This study assessed the effects of the new translocator protein ligand, YL-IPA08, on alleviating inflammation-induced depression-like behavior in mice and investigated its mechanism of action. Mice were intracerebroventricularly injected with 1, 10, 100 or 1000 ng lipopolysaccharide. The tail-suspension test and the forced swimming test confirmed that 100 ng lipopolysaccharide induced depression-like behavior. A mouse model was then established by intraventricular injection of 100 ng lipopolysaccharide. On days 16-24 after model establishment, mice were intragastrically administered 3 mg/kg YL-IPA08 daily. Immunohistochemistry was used to determine BrdU and NeuN expression in the hippocampus. YL-IPA08 effectively reversed the depression-like behavior of lipopolysaccharide-treated mice, restored body mass, increased the number of BrdU-positive cells, and the number and proportion of BrdU and NeuN double-positive cells. These findings indicate that YL-IPA08 can attenuate lipopolysaccharide-induced depression-like behavior in mice by promoting the formation of hippocampal neurons.

Novel potential pyrazolopyrimidine based translocator protein ligands for the evaluation of neuroinflammation with PET

Translocator protein (TSPO) is an interesting biological target because TSPO overexpression is associated with microglial activation caused by neuronal damage or neuroinflammation, and these activated microglia are involved in several central nervous system diseases. Herein, novel fluorinated ligands (14a-c and 16a-c) based on a 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamide scaffold were synthesized, and in vitro characterization of each of the novel ligands was performed to elucidate structure activity relationships. All of the newly synthesized ligands displayed nano-molar affinity for TSPO. Particularly, an in vitro affinity study suggests that 2-(5,7-diethyl-2-(4-(3-fluoro-2-methylpropoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide (14a), which exhibited high nano-molar affinity for TSPO and proper lipophilicity, was suitable for in vivo brain studies. Thus, radiosynthesis from tosylate precursor 13a using fluorine-18 was performed, and [¹⁸F]14a was obtained in a 31% radiochemical yield (decay-corrected). Dynamic positron emission tomography (PET) imaging studies were performed in a lipopolysaccharide (LPS)-induced neuroinflammation rat model using [¹⁸F]14a to identify the location of inflammation in the brain with a high target-to-background signal ratio. In addition, we validated that the locations of inflammatory lesions found by PET imaging were consistent with the locations observed by histological examination of dissected brains using antibodies. These results suggest that [¹⁸F]14a is a novel promising PET imaging agent for diagnosing neuroinflammation, and it may also prove to be applicable for diagnosing other diseases, including cancers associated with altered TSPO expression, using PET techniques.

Positron Emission Tomography Studies of the Glial Cell Marker Translocator Protein in Patients With Psychosis: A Meta-analysis Using Individual Participant Data

BACKGROUND

Accumulating evidence suggests that the immune system may be an important target for new treatment approaches in schizophrenia. Positron emission tomography and radioligands binding to the translocator protein (TSPO), which is expressed in glial cells in the brain including immune cells, represents a potential method for patient stratification and treatment monitoring. This study examined whether patients with first-episode psychosis and schizophrenia had altered TSPO levels compared with healthy control subjects.

METHODS

PubMed was searched for studies comparing patients with psychosis with healthy control subjects using second-generation TSPO radioligands. The outcome measure was total distribution volume (VT), an index of TSPO levels, in frontal cortex, temporal cortex, and hippocampus. Bayes factors (BFs) were applied to examine the relative support for higher, lower, or no difference in patients' TSPO levels compared with healthy control subjects.

RESULTS

Five studies, with 75 participants with first-episode psychosis or schizophrenia and 77 healthy control subjects, were included. BFs showed strong support for lower VT in patients relative to no difference (all BFs > 32), or relative to higher VT (all BFs > 422), in all brain regions. From the posterior distributions, mean patient-control differences in standardized VT values were -0.48 for frontal cortex (95% credible interval [CredInt] = -0.88 to 0.09), -0.47 for temporal cortex (CredInt = -0.87 to -0.07), and -0.63 for hippocampus (CredInt = -1.00 to -0.25).

CONCLUSIONS

The lower levels of TSPO observed in patients may correspond to altered function or lower density of brain immune cells. Future studies should focus on investigating the underlying biological mechanisms and their relevance for treatment.

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.

Reconceptualization of translocator protein as a biomarker of neuroinflammation in psychiatry

A great deal of interest in psychiatric research is currently centered upon the pathogenic role of inflammatory processes. Positron emission tomography (PET) using radiolabeled ligands selective for the 18 kDa translocator protein (TSPO) has become the most widely used technique to assess putative neuroimmune abnormalities in vivo. Originally used to detect discrete neurotoxic damages, TSPO has generally turned into a biomarker of 'neuroinflammation' or 'microglial activation'. Psychiatric research has mostly accepted these denotations of TSPO, even if they may be inadequate and misleading under many pathological conditions. A reliable and neurobiologically meaningful diagnosis of 'neuroinflammation' or 'microglial activation' is unlikely to be achieved by the sole use of TSPO PET imaging. It is also very likely that the pathological meanings of altered TSPO binding or expression are disease-specific, and therefore, not easily generalizable across different neuropathologies or inflammatory conditions. This difficulty is intricately linked to the varying (and still ill-defined) physiological functions and cellular expression patterns of TSPO in health and disease. While altered TSPO binding or expression may indeed mirror ongoing neuroinflammatory processes in some cases, it may reflect other pathophysiological processes such as abnormalities in cell metabolism, energy production and oxidative stress in others. Hence, the increasing popularity of TSPO PET imaging has paradoxically introduced substantial uncertainty regarding the nature and meaning of neuroinflammatory processes and microglial activation in psychiatry, and likely in other neuropathological conditions as well. The ambiguity of conceiving TSPO simply as a biomarker of 'neuroinflammation' or 'microglial activation' calls for alternative interpretations and complimentary approaches. Without the latter, the ongoing scientific efforts and excitement surrounding the role of the neuroimmune system in psychiatry may not turn into therapeutic hope for affected individuals.

Thromboxane A2 receptor antagonist SQ29548 reduces ischemic stroke-induced microglia/macrophages activation and enrichment, and ameliorates brain injury

Thromboxane A2 receptor (TXA2R) activation is thought to be involved in thrombosis/hemostasis and inflammation responses. We have previously shown that TXA2R antagonist SQ29548 attenuates BV2 microglia activation by suppression of ERK pathway, but its effect is not tested in vivo. The present study aims to explore the role of TXA2R on microglia/macrophages activation after ischemia/reperfusion brain injury in mice. Adult male ICR mice underwent 90-min transient middle cerebral artery occlusion (tMCAO). Immediately and 24 h after reperfusion, SQ29548 was administered twice to the ipsilateral ventricle (10 μl, 2.6 μmol/ml, per dose). Cerebral infarction volume, inflammatory cytokines release and microglia/macrophages activation were measured using the cresyl violet method, quantitative polymerase chain reaction (qPCR), and immunofluorescence double staining, respectively. Expression of TXA2R was significantly increased in the ipsilateral brain tissue after ischemia/reperfusion, which was also found to co-localize with activated microglia/macrophages in the infarct area. Administration of SQ29548 inhibited microglia/macrophages activation and enrichment, including both M1 and M2 phenotypes, and attenuated ischemia-induced IL-1ß, IL-6, and TNF-α up-regulation and iNOS release. TXA2R antagonist SQ29548 inhibited ischemia-induced inflammatory response and furthermore reduced microglia/macrophages activation and ischemic/reperfusion brain injury.

Quinolinic acid neurotoxicity: Differential roles of astrocytes and microglia via FGF-2-mediated signaling in redox-linked cytoskeletal changes

QUIN is a glutamate agonist playing a role in the misregulation of the cytoskeleton, which is associated with neurodegeneration in rats. In this study, we focused on microglial activation, FGF2/Erk signaling, gap junctions (GJs), inflammatory parameters and redox imbalance acting on cytoskeletal dynamics of the in QUIN-treated neural cells of rat striatum. FGF-2/Erk signaling was not altered in QUIN-treated primary astrocytes or neurons, however cytoskeleton was disrupted. In co-cultured astrocytes and neurons, QUIN-activated FGF2/Erk signaling prevented the cytoskeleton from remodeling. In mixed cultures (astrocyte, neuron, microglia), QUIN-induced FGF-2 increased level failed to activate Erk and promoted cytoskeletal destabilization. The effects of QUIN in mixed cultures involved redox imbalance upstream of Erk activation. Decreased connexin 43 (Cx43) immunocontent and functional GJs, was also coincident with disruption of the cytoskeleton in primary astrocytes and mixed cultures. We postulate that in interacting astrocytes and neurons the cytoskeleton is preserved against the insult of QUIN by activation of FGF-2/Erk signaling and proper cell-cell interaction through GJs. In mixed cultures, the FGF-2/Erk signaling is blocked by the redox imbalance associated with microglial activation and disturbed cell communication, disrupting the cytoskeleton. Thus, QUIN signal activates differential mechanisms that could stabilize or destabilize the cytoskeleton of striatal astrocytes and neurons in culture, and glial cells play a pivotal role in these responses preserving or disrupting a combination of signaling pathways and cell-cell interactions. Taken together, our findings shed light into the complex role of the active interaction of astrocytes, neurons and microglia in the neurotoxicity of QUIN.

Neuroprotective effects of a novel translocator protein (18 kDa) ligand, ZBD-2, against focal cerebral ischemia and NMDA-induced neurotoxicity

Ligands of the translocator protein (18 kDa) (TSPO) have demonstrated rapid anxiolytic efficacy in stress responses and stress-related disorders. This protein is involved in the synthesis of endogenous neurosteroids including pregnenolone, dehydroepiandrosterone, and progesterone. These neurosteroids promote γ-aminobutyric acid-mediated neurotransmission in the central neural system (CNS). A TSPO ligand, N-benzyl-N-ethyl-2-(7,8-dihydro-7-benzyl-8-oxo-2-phenyl-9H-purin-9-yl) acetamide (ZBD-2) was recently synthesized. The purpose of the present study was to investigate the neuroprotective effects of ZBD-2 and. In cultured cortical neurons, treatment with ZBD-2 attenuated excitotoxicity induced by N-methyl-d-aspartate (NMDA) exposure. It significantly decreased the number of apoptotic cells by downregulating GluN2B-containing NMDA receptors (NMDARs), the ratio of Bax/Bcl-2, and levels of pro-caspase-3. Systemic treatment of ZBD-2 provided significant neuroprotection in mice subjected to middle cerebral artery occlusion. These findings provide direct evidence that neuroprotection by ZBD-2 is partially mediated by inhibiting GluN2B-containing NMDA receptor-mediated excitotoxicity.

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.

Cerebrovascular and microglial states are not altered by functional neuroinflammatory gene variant

The translocator protein, a microglial-expressed marker of neuroinflammation, has been implicated in Alzheimer's disease, which is characterized by alterations in vascular and inflammatory states. ATSPOvariant, rs6971, determines binding affinity of exogenous radioligandsin vivo; however, the effect of these altered binding characteristics on inflammatory and cerebrovascular biomarkers has not been assessed. In 2345 living subjects (Alzheimer's Disease Neuroimaging Initiative, n = 1330) and postmortem brain samples (Religious Orders Study and Memory and Aging Project, n = 1015), we analyzed effects of rs6971 on white matter hyperintensisites, cerebral infarcts, circulating inflammatory biomarkers, amyloid angiopathy, and microglial activation. We found that rs6971 does not alter translocator protein in a way that impacts cerebrovascular and inflammatory states known to be affected in dementia.

Quinazoline-based tricyclic compounds that regulate programmed cell death, induce neuronal differentiation, and are curative in animal models for excitotoxicity and hereditary brain disease

Expanding on a quinazoline scaffold, we developed tricyclic compounds with biological activity. These compounds bind to the 18 kDa translocator protein (TSPO) and protect U118MG (glioblastoma cell line of glial origin) cells from glutamate-induced cell death. Fascinating, they can induce neuronal differentiation of PC12 cells (cell line of pheochromocytoma origin with neuronal characteristics) known to display neuronal characteristics, including outgrowth of neurites, tubulin expression, and NeuN (antigen known as 'neuronal nuclei', also known as Rbfox3) expression. As part of the neurodifferentiation process, they can amplify cell death induced by glutamate. Interestingly, the compound 2-phenylquinazolin-4-yl dimethylcarbamate (MGV-1) can induce expansive neurite sprouting on its own and also in synergy with nerve growth factor and with glutamate. Glycine is not required, indicating that N-methyl-D-aspartate receptors are not involved in this activity. These diverse effects on cells of glial origin and on cells with neuronal characteristics induced in culture by this one compound, MGV-1, as reported in this article, mimic the diverse events that take place during embryonic development of the brain (maintenance of glial integrity, differentiation of progenitor cells to mature neurons, and weeding out of non-differentiating progenitor cells). Such mechanisms are also important for protective, curative, and restorative processes that occur during and after brain injury and brain disease. Indeed, we found in a rat model of systemic kainic acid injection that MGV-1 can prevent seizures, counteract the process of ongoing brain damage, including edema, and restore behavior defects to normal patterns. Furthermore, in the R6-2 (transgenic mouse model for Huntington disease; Strain name: B6CBA-Tg(HDexon1)62Gpb/3J) transgenic mouse model for Huntington disease, derivatives of MGV-1 can increase lifespan by >20% and reduce incidence of abnormal movements. Also in vitro, these derivatives were more effective than MGV-1.

Targeting the 18-kDa translocator protein: recent perspectives for neuroprotection

The translocator protein (TSPO, 18 kDa), mainly localized in the outer mitochondrial membrane of steroidogenic tissues, is involved in several cellular functions. TSPO level alterations have been reported in a number of human disorders, particularly in cancer, psychiatric and neurological diseases. In the central nervous system (CNS), TSPO is usually expressed in glial cells, but also in some neuronal cell types. Interestingly, the expression of TSPO on glial cells rises after brain injury and increased TSPO expression is often observed in neurological disorders, gliomas, encephalitis and traumatic injury. Since TSPO is up-regulated in brain diseases, several structurally different classes of ligands targeting TSPO have been described as potential diagnostic or therapeutic agents. Recent researches have reported that TSPO ligands might be valuable in the treatment of brain diseases. This review focuses on currently available TSPO ligands, as useful tools for the treatment of neurodegeneration, neuro-inflammation and neurotrauma.

[11C]PBR28 PET imaging is sensitive to neuroinflammation in the aged rat

Neuroinflammation in the aging rat brain was investigated using [(11)C]PBR28 microPET (positron emission tomography) imaging. Normal rats were studied alongside LRRK2 p.G2019S transgenic rats; this mutation increases the risk of Parkinson's disease in humans. Seventy [(11)C]PBR28 PET scans were acquired. Arterial blood sampling enabled tracer kinetic modeling and estimation of VT. In vitro autoradiography was also performed. PBR28 uptake increased with age, without differences between nontransgenic and transgenic rats. In 12 months of aging (4 to 16 months), standard uptake value (SUV) increased by 56% from 0.44 to 0.69 g/mL, whereas VT increased by 91% from 30 to 57 mL/cm(3). Standard uptake value and VT were strongly correlated (r = 0.52, 95% confidence interval (CI) = 0.31 to 0.69, n = 37). The plasma free fraction, fp, was 0.21 ± 0.03 (mean ± standard deviation, n = 53). In vitro binding increased by 19% in 16 months of aging (4 to 20 months). The SUV was less variable across rats than VT; coefficients of variation were 13% (n = 27) and 29% (n = 12). The intraclass correlation coefficient for SUV was 0.53, but was effectively zero for VT. These data show that [(11)C]PBR28 brain uptake increases with age, implying increased microglial activation in the aged brain.

Evaluation of [(11)C]CB184 for imaging and quantification of TSPO overexpression in a rat model of herpes encephalitis

Purpose

Evaluation of translocator protein (TSPO) overexpression is considered an attractive research tool for monitoring neuroinflammation in several neurological and psychiatric disorders. [¹¹C]PK11195 PET imaging has been widely used for this purpose. However, it has a low sensitivity and a poor signal-to-noise ratio. For these reasons, [¹¹C]CB184 was evaluated as a potentially more sensitive PET tracer.

Methods

A model of herpes simplex encephalitis (HSE) was induced in male Wistar rats. On day 6 or 7 after virus inoculation, [¹¹C]CB184 PET scans were acquired followed by ex vivo evaluation of biodistribution. In addition, [¹¹C]CB184 and [¹¹C]PK11195 PET scans with arterial blood sampling were acquired to generate input for pharmacokinetic modelling. Differences between the saline-treated control group and the virus-treated HSE group were explored using volumes of interest and voxel-based analysis.

Results

The biodistribution study showed significantly higher [¹¹C]CB184 uptake in the amygdala, olfactory bulb, medulla, pons and striatum (p < 0.05) in HSE rats than in control rats, and the voxel-based analysis showed higher bilateral uptake in the pons and medulla (p < 0.05, corrected at the cluster level). A high correlation was found between tracer uptake in the biodistribution study and on the PET scans (p < 0.001, r² = 0.71). Pretreatment with 5 mg/kg of unlabelled PK11195 effectively reduced (p < 0.001) [¹¹C]CB184 uptake in the whole brain. Both, [¹¹C]CB184 and [¹¹C]PK11195, showed similar amounts of metabolites in plasma, and the binding potential (BP_ND) was not significantly different between the HSE rats and the control rats. In HSE rats BP_ND for [¹¹C]CB184 was significantly higher (p < 0.05) in the amygdala, hypothalamus, medulla, pons and septum than in control rats, whereas higher uptake of [¹¹C]PK11195 was only detected in the medulla.

Conclusion

[¹¹C]CB184 showed nonspecific binding to healthy tissue comparable to that observed for [¹¹C]PK11195, but it displayed significantly higher specific binding in those brain regions affected by the HSE. Our results suggest that [¹¹C]CB184 PET is a good alternative for imaging of neuroinflammatory processes.

Computational discovery and experimental verification of tyrosine kinase inhibitor pazopanib for the reversal of memory and cognitive deficits in rat model neurodegeneration

Pazopanib, a tyrosine kinase inhibitor marketed for cancer treatment, abrogates the course of neurodegeneration.

Cognition and memory impairment are hallmarks of the pathological cascade of various neurodegenerative disorders. Herein, we developed a novel computational strategy with two-dimensional virtual screening for not only affinity but also specificity. We integrated the two-dimensional virtual screening with ligand screening for 3D shape, electrostatic similarity and local binding site similarity to find existing drugs that may reduce the signs of cognitive deficits. For the first time, we found that pazopanib, a tyrosine kinase inhibitor marketed for cancer treatment, inhibits acetylcholinesterase (AchE) activities at sub-micromolar concentration. We evaluated and compared the effects of intragastrically-administered pazopanib with donepezil, a marketed AchE inhibitor, in cognitive and behavioral assays including the novel object recognition test, Y maze and Morris water maze test. Surprisingly, we found that pazopanib can restore memory loss and cognitive dysfunction to a similar extent as donepezil in a dosage of 15 mg kg^–1, only one fifth of the equivalent clinical dosage for cancer treatment. Furthermore, we demonstrated that pazopanib dramatically enhances the hippocampal Ach levels and increases the expression of the synaptic marker SYP. These findings suggest that pazopanib may become a viable treatment option for memory and cognitive deficits with a good safety profile in humans.

The translocator protein as a drug target in Alzheimer's disease

The translocator protein (TSPO) recently emerged as a potential drug target in Alzheimer's disease (AD). This has been fuelled mainly by positron emission topography studies that show the upregulation of TSPO in AD, especially in relation to microgliosis and astrogliosis in amyloid-β and tau pathology. Although data as to the exact role of TSPO in AD is still inconclusive, TSPO appears to be involved in neuroinflammatory processes and AD has been shown to involve substantial inflammation. Therefore, further development and investigation of the pharmacological effect of TSPO ligands in AD pathology are warranted.

Allopregnanolone and neuroinflammation: a focus on multiple sclerosis

The progesterone derivative allopregnanolone (ALLO) is one of the most widely studied compounds among neurosteroids. Through interactions with GABA-A receptors expressed by neurons and glial cells, ALLO has been shown to affect diverse aspects of neural cell physiology, including cell proliferation and survival, migration, and gene expression. Recent data point to important roles for ALLO in different neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis (MS). Dysregulation in ALLO biosynthesis pathways has been reported in brain tissue from MS patients as well as in the central nervous system (CNS) tissue derived from MS animal models. Administration of ALLO has been shown to ameliorate neurobehavioral deficits together with neuropathology and inflammation in the CNS of animals with autoimmune demyelination. These findings are in line with previous reports indicating growth- and differentiation-promoting actions of ALLO on neurons and glial cells as well as its neuroprotective effects in the context of other CNS diseases. Nonetheless, these findings have also raised the possibility that ALLO might influence leukocyte biology and associated neuroinflammatory mechanisms independent of its neuroregenerative properties. Herein, we review the current knowledge regarding the role of ALLO in the pathogenesis of MS, and discuss the potential cellular and molecular pathways that might be influenced by ALLO in the context of disease.

The role of inflammation in brain cancer

Malignant brain tumors are among the most lethal of human tumors, with limited treatment options currently available. A complex array of recurrent genetic and epigenetic changes has been observed in gliomas that collectively result in derangements of common cell signaling pathways controlling cell survival, proliferation, and invasion. One important determinant of gene expression is DNA methylation status, and emerging studies have revealed the importance of a recently identified demethylation pathway involving 5-hydroxymethylcytosine (5hmC). Diminished levels of the modified base 5hmC is a uniform finding in glioma cell lines and patient samples, suggesting a common defect in epigenetic reprogramming. Within the tumor microenvironment, infiltrating immune cells increase oxidative DNA damage, likely promoting both genetic and epigenetic changes that occur during glioma evolution. In this environment, glioma cells are selected that utilize multiple metabolic changes, including changes in the metabolism of the amino acids glutamate, tryptophan, and arginine. Whereas altered metabolism can promote the destruction of normal tissues, glioma cells exploit these changes to promote tumor cell survival and to suppress adaptive immune responses. Further understanding of these metabolic changes could reveal new strategies that would selectively disadvantage tumor cells and redirect host antitumor responses toward eradication of these lethal tumors.