The 18 kDa translocator protein, microglia and neuroinflammation

Thalamic inflammation after brain trauma is associated with thalamo-cortical white matter damage

Background

Traumatic brain injury can trigger chronic neuroinflammation, which may predispose to neurodegeneration. Animal models and human pathological studies demonstrate persistent inflammation in the thalamus associated with axonal injury, but this relationship has never been shown in vivo.

Findings

Using [¹¹C]-PK11195 positron emission tomography, a marker of microglial activation, we previously demonstrated thalamic inflammation up to 17 years after traumatic brain injury. Here, we use diffusion MRI to estimate axonal injury and show that thalamic inflammation is correlated with thalamo-cortical tract damage.

Conclusions

These findings support a link between axonal damage and persistent inflammation after brain injury.

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography

Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Therapeutic actions of translocator protein (18 kDa) ligands in experimental models of psychiatric disorders and neurodegenerative diseases

Translocator protein (TSPO) is an 18kDa protein located at contact sites between the outer and the inner mitochondrial membrane. Numerous studies have associated TSPO with the translocation of cholesterol across the aqueous mitochondrial intermembrane space and the regulation of steroidogenesis, as well as with the control of some other mitochondrial functions, such as mitochondrial respiration, mitochondrial permeability transition pore opening, apoptosis and cell proliferation. In the brain, changes in TSPO expression occur in several neuropathological conditions including neurodegenerative diseases and psychiatric disorders. Furthermore, TSPO ligands have been shown to promote neuroprotection in animal models of brain pathology. At least in some cases, the mechanisms of neuroprotection are associated with modifications in brain steroidogenesis. In addition, regulation of neuroinflammation seems to be a common mechanism in the neuroprotective actions of TSPO ligands in different animal models of brain pathology.

Imaging robust microglial activation after lipopolysaccharide administration in humans with PET

Neuroinflammation is associated with a broad spectrum of neurodegenerative and psychiatric diseases. The core process in neuroinflammation is activation of microglia, the innate immune cells of the brain. We measured the neuroinflammatory response produced by a systemic administration of the Escherichia coli lipopolysaccharide (LPS; also called endotoxin) in humans with the positron emission tomography (PET) radiotracer [11C]PBR28, which binds to translocator protein, a molecular marker that is up-regulated by microglial activation. In addition, inflammatory cytokines in serum and sickness behavior profiles were measured before and after LPS administration to relate brain microglial activation with systemic inflammation and behavior. Eight healthy male subjects each had two 120-min [11C]PBR28 PET scans in 1 d, before and after an LPS challenge. LPS (1.0 ng/kg, i.v.) was administered 180 min before the second [11C]PBR28 scan. LPS administration significantly increased [11C]PBR28 binding 30-60%, demonstrating microglial activation throughout the brain. This increase was accompanied by an increase in blood levels of inflammatory cytokines, vital sign changes, and sickness symptoms, well-established consequences of LPS administration. To our knowledge, this is the first demonstration in humans that a systemic LPS challenge induces robust increases in microglial activation in the brain. This imaging paradigm to measure brain microglial activation with [11C]PBR28 PET provides an approach to test new medications in humans for their putative antiinflammatory effects.

Assessing brain immune activation in psychiatric disorders: clinical and preclinical PET imaging studies of the 18-kDa translocator protein

Accumulating evidence from different lines of research suggests an involvement of the immune system in the pathophysiology of several psychiatric disorders. During recent years, a series of positron emission tomography (PET) studies have been published using radioligands for the translocator protein (TSPO) to study microglia activation in schizophrenia, bipolar I disorder, major depression, autism spectrum disorder, and drug abuse. The results have been somewhat conflicting, which could be due to differences both in patient sample characteristics and in PET methods. In particular, further work is needed to address both methodological and biological sources of variability in TSPO levels, a process in which the use of animal models and small animal PET systems can be a valuable tool. Given this development, PET studies of immune activation have the potential to further increase our understanding of disease mechanisms in psychiatric disorders, which is a requisite in the search for new treatment approaches. Furthermore, molecular imaging could become an important clinical tool for identifying specific subgroups of patients or disease stages that would benefit from treatment targeting the immune system.

Single and Compound Knock-outs of MicroRNA (miRNA)-155 and Its Angiogenic Gene Target CCN1 in Mice Alter Vascular and Neovascular Growth in the Retina via Resident Microglia

The response of the retina to ischemic insult typically leads to aberrant retinal neovascularization, a major cause of blindness. The epigenetic regulation of angiogenic gene expression by miRNAs provides new prospects for their therapeutic utility in retinal neovascularization. Here, we focus on miR-155, a microRNA functionally important in inflammation, which is of paramount importance in the pathogenesis of retinal neovascularization. Whereas constitutive miR-155-deficiency in mice results in mild vascular defects, forced expression of miR-155 causes endothelial hyperplasia and increases microglia count and activation. The mouse model of oxygen-induced retinopathy, which recapitulates ischemia-induced aberrant neovessel growth, is characterized by increased expression of miR-155 and localized areas of microglia activation. Interestingly, miR-155 deficiency in mice reduces microglial activation, curtails abnormal vessel growth, and allows for rapid normalization of the retinal vasculature following ischemic insult. miR-155 binds to the 3'-UTR and represses the expression of the CCN1 gene, which encodes an extracellular matrix-associated integrin-binding protein that both promotes physiological angiogenesis and harnesses growth factor-induced abnormal angiogenic responses. Single CCN1 deficiency or double CCN1 and miR-155 knock-out in mice causes retinal vascular malformations typical of faulty maturation, mimicking the vascular alterations of miR-155 gain of function. During development, the miR-155/CCN1 regulatory axis balances the proangiogenic and proinflammatory activities of microglia to allow for their function as guideposts for sprout fusion and anastomosis. Under ischemic conditions, dysregulated miR-155 and CCN1 expression increases the inflammatory load and microglial activation, prompting aberrant angiogenic responses. Thus, miR-155 functions in tandem with CCN1 to modulate inflammation-induced vascular homeostasis and repair.

TSPO as a target for glioblastoma therapeutics

The translocator protein (TSPO) is an 18-kDa five-transmembrane protein, which is primarily found in the outer mitochondrial membrane. Levels of this protein are up-regulated in the most aggressive and common glioma, glioblastoma multiforme (GM). Levels of TSPO also correlate with GM clinical outcome, suggesting that TSPO may be a novel GM diagnostic imaging agent. Therapeutically, targeting the TSPO may provide a mechanism to abrogate the apoptotic-resistant, invasive and aggressive nature of GM and may also provide a way of targeting other anti-cancer treatments to GM sites. This review highlights recent progress in research on TSPO-based diagnostic imaging and therapeutics for GM.

The 18-kDa mitochondrial translocator protein in gliomas: from the bench to bedside

The 18-kDa mitochondrial translocator protein (TSPO) is known to be highly expressed in several types of cancer, including gliomas, whereas expression in normal brain is low. TSPO functions in glioma are still incompletely understood. The TSPO can be quantified pre-operatively with molecular imaging making it an ideal candidate for personalized treatment of patient with glioma. Studies have proposed to exploit the TSPO as a transporter of chemotherapics to selectively target tumour cells in the brain. Our studies proved that positron emission tomography (PET)-imaging can contribute to predict progression of patients with glioma and that molecular imaging with TSPO-specific ligands is suitable to stratify patients in view of TSPO-targeted treatment. Finally, we proved that TSPO in gliomas is predominantly expressed by tumour cells.

Structure of the mammalian TSPO/PBR protein

The 3D structure of the 18-kDa transmembrane (TM) protein TSPO (translocator protein)/PBR (peripheral benzodiazepine receptor), which contains a binding site for benzodiazepines, is important to better understand its function and regulation by endogenous and synthetic ligands. We have recently determined the structure of mammalian TSPO/PBR in complex with the diagnostic ligand PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide; Jaremko et al. (2014) Science 343: , 1363-1366], providing for the first time atomic-level insight into the conformation of this protein, which is up-regulated in various pathological conditions including Alzheimer's disease and Parkinson's disease. Here, we review the studies which have probed the structural properties of mammalian TSPO/PBR as well as the homologues bacterial tryptophan-rich sensory proteins (TspOs) over the years and provide detailed insight into the 3D structure of mouse TSPO (mTSPO)/PBR in complex with PK11195.

Guwiyang Wurra--'Fire Mouse': a global gene knockout model for TSPO/PBR drug development, loss-of-function and mechanisms of compensation studies

The highly conserved 18-kDa translocator protein (TSPO) or peripheral benzodiazepine receptor (PBR), is being investigated as a diagnostic and therapeutic target for disease conditions ranging from inflammation to neurodegeneration and behavioural illnesses. Many functions have been attributed to TSPO/PBR including a role in the mitochondrial permeability transition pore (MPTP), steroidogenesis and energy metabolism. In this review, we detail the recent developments in determining the physiological role of TSPO/PBR, specifically based on data obtained from the recently generated Tspo knockout mouse models. In addition to defining the role of TSPO/PBR, we also describe the value of Tspo knockout mice in determining the selectivity, specificity and presence of any off-target effects of TSPO/PBR ligands.

Targeting mitochondrial energy metabolism with TSPO ligands

The translocator protein (18 kDa) (TSPO) resides on the outer mitochondrial membrane where it is believed to participate in cholesterol transport and steroid hormone synthesis. Although it is almost ubiquitously expressed, what TSPO does in non-steroidogenic tissues is largely unexplored. Recent studies report changes in glucose homoeostasis and cellular energy production when TSPO function is modulated by selective ligands or by genetic loss-of-function. This review summarizes findings that connect TSPO function with the regulation of mitochondrial energy metabolism. The juxtaposition of TSPO at the cytosolic/mitochondrial interface and the existence of endogenous ligands that are regulated by metabolism suggest that TSPO functions to adapt mitochondrial to cellular metabolism. From a pharmacological perspective the specific up-regulation of TSPO in neuro-inflammatory and injury-induced conditions make TSPO an interesting, druggable target of mitochondrial metabolism.

The changing landscape in translocator protein (TSPO) function

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

Lost in translocation: the functions of the 18-kD translocator protein

Research spanning nearly four decades has assigned to the translocator protein (18 kDa) (TSPO) a critical role, among others, in the mitochondrial import of cholesterol, the subsequent steps of (neuro)steroid production, and systemic endocrine regulation, with implications for the pathophysiology of immune, inflammatory, neurodegenerative, and psychiatric as well as neoplastic diseases. Recent knockout studies in mice unexpectedly report normal or latent phenotypes, raising doubts about the protein's role in steroidogenesis and other previously postulated functions and challenging the validity of earlier data on the selectivity of TSPO-binding drugs. Here we provide a synthesis of the current debate from a structural and molecular biology perspective, discuss the limits of inference in loss-of-function (gene knockout) studies, and suggest new functions of TSPO.

Structural Integrity of the A147T Polymorph of Mammalian TSPO

Ligands of the transmembrane protein TSPO are used for imaging of brain inflammation, but a common polymorphism in TSPO complicates their application to humans. Here we determined the three-dimensional structure and side-chain dynamics of the A147T polymorph of mammalian TSPO in complex with the first-generation ligand PK11195. We show that A147T TSPO is able to retain the same structural and dynamic profile as the wild-type protein and thus binds PK11195 with comparable affinity. Our study is important for the design of more potent diagnostic and therapeutic ligands of TSPO.

Astroglia-Microglia Cross Talk during Neurodegeneration in the Rat Hippocampus

Brain injury triggers a progressive inflammatory response supported by a dynamic astroglia-microglia interplay. We investigated the progressive chronic features of the astroglia-microglia cross talk in the perspective of neuronal effects in a rat model of hippocampal excitotoxic injury. N-Methyl-D-aspartate (NMDA) injection triggered a process characterized within 38 days by atrophy, neuronal loss, and fast astroglia-mediated S100B increase. Microglia reaction varied with the lesion progression. It presented a peak of tumor necrosis factor-α (TNF-α) secretion at one day after the lesion, and a transient YM1 secretion within the first three days. Microglial glucocorticoid receptor expression increased up to day 5, before returning progressively to sham values. To further investigate the astroglia role in the microglia reaction, we performed concomitant transient astroglia ablation with L-α-aminoadipate and NMDA-induced lesion. We observed a striking maintenance of neuronal death associated with enhanced microglial reaction and proliferation, increased YM1 concentration, and decreased TNF-α secretion and glucocorticoid receptor expression. S100B reactivity only increased after astroglia recovery. Our results argue for an initial neuroprotective microglial reaction, with a direct astroglial control of the microglial cytotoxic response. We propose the recovery of the astroglia-microglia cross talk as a tissue priority conducted to ensure a proper cellular coordination that retails brain damage.

Possible immunosuppressive effects of drug exposure and environmental and nutritional effects on infection and vaccination

A variety of drugs which are not primarily considered to be immunosuppressive agents have been described to modulate the humoral and cellular immune response in humans or animals. Thereby they may have an influence on the effectiveness and possible side effects of vaccines. This mini review lists some of the different substance classes and also some of endogeneous, infectious, nutritional, and environmental influences with suspected capability to interfere with immunizations. Studies in most cases focused on substances with known immunosuppressive functions, but there is growing evidence for immunomodulatory effects also of commonly used drugs with wide distribution. In particular combinations of those antiproliferative and antiphlogistic side effects of different substance classes have not been studied in detail but may substantially interfere with the development of a functional humoral and cellular immune response. The drugs of importance include antipyretics, anticoagulants, tranquilizers, and substances influencing lipid metabolism but also commonly used drugs of abuse like alcohol or cannabinoids. Additional substances of environmental, nutritional, or microbiological origin may also play a role but their combinatory/synergistic effects have been disregarded so far due to the lack of systematic data and the complex study designs necessary to elucidate those complex epidemiologic questions.

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

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

[18F]FPBMP: – a potential new positron emission tomography radioligand for imaging of translocator protein (18 kDa) in peripheral organs of rats

The five transmembrane translocator protein (18 kDa, TSPO) is abundantly expressed in the mitochondria of activated microglia (brain) and peripheral tissues, including those of the heart, lung and kidney.

Positron emission tomography and functional characterization of a complete PBR/TSPO knockout

The evolutionarily conserved peripheral benzodiazepine receptor (PBR), or 18-kDa translocator protein (TSPO), is thought to be essential for cholesterol transport and steroidogenesis, and thus life. TSPO has been proposed as a biomarker of neuroinflammation and a new drug target in neurological diseases ranging from Alzheimer's disease to anxiety. Here we show that global C57BL/6-Tspo(tm1GuWu(GuwiyangWurra))-knockout mice are viable with normal growth, lifespan, cholesterol transport, blood pregnenolone concentration, protoporphyrin IX metabolism, fertility and behaviour. However, while the activation of microglia after neuronal injury appears to be unimpaired, microglia from (GuwiyangWurra)TSPO knockouts produce significantly less ATP, suggesting reduced metabolic activity. Using the isoquinoline PK11195, the ligand originally used for the pharmacological and structural characterization of the PBR/TSPO, and the imidazopyridines CLINDE and PBR111, we demonstrate the utility of (GuwiyangWurra)TSPO knockouts to provide robust data on drug specificity and selectivity, both in vitro and in vivo, as well as the mechanism of action of putative TSPO-targeting drugs.