Brain inflammation co-localizes highly with tau in mild cognitive impairment due to early-onset Alzheimer's disease.

Brain inflammation, with an increased density of microglia and macrophages, is an important component of Alzheimer's disease (AD) and a potential therapeutic target. However, it is incompletely characterized, particularly in patients whose disease begins before the age of 65 years and, thus, have few co-pathologies. Inflammation has been usefully imaged with translocator protein (TSPO) positron emission tomography (PET), but most inflammation PET tracers cannot image subjects with a low-binder TSPO rs6971 genotype. In an important development, participants with any TSPO genotype can be imaged with a novel tracer, [11C]ER176, that has a high binding potential and a more favorable metabolite profile than other TSPO tracers currently available. We applied [11C]ER176 to detect brain inflammation in mild cognitive impairment (MCI) caused by early-onset AD. Furthermore, we sought to correlate the brain localization of inflammation, volume loss, elevated Aß and tau. We studied brain inflammation in 25 patients with early-onset amnestic MCI (average age 59 ± 4.5 years, 10 women) and 23 healthy controls (average age 65 ± 6.0 years, 12 women), both groups with a similar proportion of all three TSPO-binding affinities. [11C]ER176 total distribution volume (VT), obtained with an arterial input function, was compared across patients and controls using voxel-wise and region-wise analyses. In addition to inflammation PET, most MCI patients had Aß (n=23), and tau PET (n=21). For Aß and tau tracers, standard uptake value ratios (SUVRs) were calculated using cerebellar grey matter as region of reference. Regional correlations among the three tracers were determined. Data were corrected for partial volume effect. Cognitive performance was studied with standard neuropsychological tools. In MCI caused by early-onset AD, there was inflammation in the default network, reaching statistical significance in precuneus and lateral temporal and parietal association cortex bilaterally, and in the right amygdala. Topographically, inflammation co-localized most strongly with tau (r= 0.63 ± 0.24). This correlation was higher than the co-localization of Aß with tau (r= 0.55±0.25) and of inflammation with Aß (0.43±0.22). Inflammation co-localized least with atrophy (-0.29±0.26). These regional correlations could be detected in participants with any of the three rs6971 TSPO polymorphisms. Inflammation in AD-related regions correlated with impaired cognitive scores. Our data highlight the importance of inflammation, a potential therapeutic target, in the AD process. Furthermore, they support the notion that, as shown in experimental tissue and animal models, the propagation of tau in humans is associated with brain inflammation.

Neuroimmune activation is associated with neurological outcome in anoxic and traumatic coma.

The pathophysiological underpinnings of critically disrupted brain connectomes resulting in coma are poorly understood. Inflammation is potentially an important but still undervalued factor. Here, we present a first-in-human prospective study using the 18-kDa translocator protein (TSPO) radioligand 18F-DPA714 for PET imaging to allow in vivo neuroimmune activation quantification in patients with coma (n = 17) following either anoxia or traumatic brain injuries in comparison with age- and sex-matched controls. Our findings yielded novel evidence of an early inflammatory component predominantly located within key cortical and subcortical brain structures that are putatively implicated in consciousness emergence and maintenance after severe brain injury (i.e. mesocircuit and frontoparietal networks). We observed that traumatic and anoxic patients with coma have distinct neuroimmune activation profiles, both in terms of intensity and spatial distribution. Finally, we demonstrated that both the total amount and specific distribution of PET-measurable neuroinflammation within the brain mesocircuit were associated with the patient's recovery potential. We suggest that our results can be developed for use both as a new neuroprognostication tool and as a promising biometric to guide future clinical trials targeting glial activity very early after severe brain injury.

Regulation of astrocyte metabolism by mitochondrial translocator protein 18 kDa.

The mitochondrial translocator protein 18 kDa (TSPO) has been linked to functions from steroidogenesis to regulation of cellular metabolism and is an attractive therapeutic target for chronic CNS inflammation. Studies in Leydig cells and microglia indicate that TSPO function may vary between cells depending on their specialized roles. Astrocytes are critical for providing trophic and metabolic support in the brain. Recent work has highlighted that TSPO expression increases in astrocytes under inflamed conditions and may drive astrocyte reactivity. Relatively little is known about the role TSPO plays in regulating astrocyte metabolism and whether this protein is involved in immunometabolic processes in these cells. Using TSPO-deficient (TSPO-/-) mouse primary astrocytes in vitro (MPAs) and a human astrocytoma cell line (U373 cells), we performed extracellular metabolic flux analyses. We found that TSPO deficiency reduced basal cellular respiration and attenuated the bioenergetic response to glucopenia. Fatty acid oxidation was increased, and lactate production was reduced in TSPO-/- MPAs and U373 cells. Co-immunoprecipitation studies revealed that TSPO forms a complex with carnitine palmitoyltransferase 1a in U373 and MPAs, presenting a mechanism wherein TSPO may regulate FAO in these cells. Compared to TSPO+/+ cells, in TSPO-/- MPAs we observed attenuated tumor necrosis factor release following 3 h lipopolysaccharide (LPS) stimulation, which was enhanced at 24 h post-LPS stimulation. Together these data suggest that while TSPO acts as a regulator of metabolic flexibility, TSPO deficiency does not appear to modulate the metabolic response of MPAs to inflammation, at least in response to the model used in this study.

Associations between sex, body mass index and the individual microglial response in Alzheimer's disease.

BACKGROUND AND OBJECTIVES: 18-kDa translocator protein position-emission-tomography (TSPO-PET) imaging emerged for in vivo assessment of neuroinflammation in Alzheimer's disease (AD) research. Sex and obesity effects on TSPO-PET binding have been reported for cognitively normal humans (CN), but such effects have not yet been systematically evaluated in patients with AD. Thus, we aimed to investigate the impact of sex and obesity on the relationship between ß-amyloid-accumulation and microglial activation in AD. METHODS: 49 patients with AD (29 females, all Aß-positive) and 15 Aß-negative CN (8 female) underwent TSPO-PET ([18F]GE-180) and ß-amyloid-PET ([18F]flutemetamol) imaging. In 24 patients with AD (14 females), tau-PET ([18F]PI-2620) was additionally available. The brain was parcellated into 218 cortical regions and standardized-uptake-value-ratios (SUVr, cerebellar reference) were calculated. Per region and tracer, the regional increase of PET SUVr (z-score) was calculated for AD against CN. The regression derived linear effect of regional Aß-PET on TSPO-PET was used to determine the Aß-plaque-dependent microglial response (slope) and the Aß-plaque-independent microglial response (intercept) at the individual patient level. All read-outs were compared between sexes and tested for a moderation effect of sex on associations with body mass index (BMI). RESULTS: In AD, females showed higher mean cortical TSPO-PET z-scores (0.91 ± 0.49; males 0.30 ± 0.75; p = 0.002), while Aß-PET z-scores were similar. The Aß-plaque-independent microglial response was stronger in females with AD (+ 0.37 ± 0.38; males with AD - 0.33 ± 0.87; p = 0.006), pronounced at the prodromal stage. On the contrary, the Aß-plaque-dependent microglial response was not different between sexes. The Aß-plaque-independent microglial response was significantly associated with tau-PET in females (Braak-II regions: r = 0.757, p = 0.003), but not in males. BMI and the Aß-plaque-independent microglial response were significantly associated in females (r = 0.44, p = 0.018) but not in males (BMI*sex interaction: F(3,52) = 3.077, p = 0.005). CONCLUSION: While microglia response to fibrillar Aß is similar between sexes, women with AD show a stronger Aß-plaque-independent microglia response. This sex difference in Aß-independent microglial activation may be associated with tau accumulation. BMI is positively associated with the Aß-plaque-independent microglia response in females with AD but not in males, indicating that sex and obesity need to be considered when studying neuroinflammation in AD.

Arabidopsis transcriptomic analysis reveals cesium inhibition of root growth involves abscisic acid signaling.

MAIN CONCLUSION: This is the first report on the involvement of abscisic acid signaling in regulating post-germination growth under Cs stress, not related to potassium deficiency. Cesium (Cs) is known to exert toxicity in plants by competition and interference with the transport of potassium (K). However, the precise mechanism of how Cs mediates its damaging effect is still unclear. This fact is mainly attributed to the large effects of lower K uptake in the presence of Cs that shadow other crucial effects by Cs that were not related to K. RNA-seq was conducted on Arabidopsis roots grown to identify putative genes that are functionally involved to investigate the difference between Cs stress and low K stress. Our transcriptome data demonstrated Cs-regulated genes only partially overlap to low K-regulated genes. In addition, the divergent expression trend of High-affinity K+ Transporter (HAK5) from D4 to D7 growth stage suggested participation of other molecular events besides low K uptake under Cs stress. Potassium deficiency triggers expression level change of the extracellular matrix, transfer/carrier, cell adhesion, calcium-binding, and DNA metabolism genes. Under Cs stress, genes encoding translational proteins, chromatin regulatory proteins, membrane trafficking proteins and defense immunity proteins were found to be primarily regulated. Pathway enrichment and protein network analyses of transcriptome data exhibit that Cs availability are associated with alteration of abscisic acid (ABA) signaling, photosynthesis activities and nitrogen metabolism. The phenotype response of ABA signaling mutants supported the observation and revealed Cs inhibition of root growth involved in ABA signaling pathway. The rather contrary response of loss-of-function mutant of Late Embryogenesis Abundant 7 (LEA7) and Translocator Protein (TSPO) further suggested low K stress and Cs stress may activate different salt tolerance responses. Further investigation on the crosstalk between K transport, signaling, and salt stress-responsive signal transduction will provide a deeper understanding of the mechanisms and molecular regulation underlying Cs toxicity.

Differences in neuroinflammation in people who started antiretroviral treatment during primary versus chronic HIV infection: an 18kDa Translocator protein (TSPO) positron emission tomography (PET) study.

Persistent inflammation is described in people with HIV (PWH) on antiretroviral treatment (ART). Early ART initiation is associated with reduced inflammation. We aimed to evaluate neuroinflammation, using translocator protein (TSPO) [11C]PBR28 PET neuroimaging in PWH who initiated ART during acute HIV (aPWH) versus chronic HIV infection (cPWH) versus a control population. This was a cross-sectional, observational study. All participants underwent [11C]PBR28 PET-CT neuroimaging. Using a two-tissue compartment model, total volume of distribution (VT) and distribution volume ratios (DVR) using cortical grey matter as a pseudo-reference region at 20 regions of interest (ROIs) were calculated. Differences in VT and DVR were compared between groups using the Kruskall-Wallis test. Seventeen neuro-asymptomatic male PWH on ART (9 aPWH, 8 cPWH) and 8 male control participants (CPs) were included. Median (interquartile range, IQR) age was 40 (30, 46), 44 (41, 47) and 21 (20, 25) years in aPWH, cPWH and CPs, respectively. Median (IQR) CD4 (cells/µL) and CD4:CD8 were 687 (652, 1014) and 1.37 (1.24, 1.42), and 700 (500, 720) and 0.67 (0.64, 0.82) in aPWH and cPWH, respectively. Overall, no significant difference in VT and DVR were observed between the three groups at any ROIs. cPWH demonstrated a trend towards higher mean VT compared with aPWH and CPs at most ROIs. No significant differences in neuroinflammation, using [11C]PBR28 binding as a proxy, were identified between cPWH, aPWH and CPs. A trend towards lower absolute [11C]PBR28 binding was seen amongst aPWH and CPs, suggesting early ART may mitigate neuroinflammation.

Efficacy of a combined anti-seizure treatment against cholinergic established status epilepticus following a sarin nerve agent insult in rats.

The development of refractory status epilepticus (SE) following sarin intoxication presents a therapeutic challenge. Here, we evaluated the efficacy of delayed combined double or triple treatment in reducing abnormal epileptiform seizure activity (ESA) and the ensuing long-term neuronal insult. SE was induced in rats by exposure to 1.2 LD50 sarin followed by treatment with atropine and TMB4 (TA) 1 min later. Double treatment with ketamine and midazolam or triple treatment with ketamine, midazolam and levetiracetam was administered 30 min post-exposure, and the results were compared to those of single treatment with midazolam alone or triple treatment with ketamine, midazolam, and valproate, which was previously shown to ameliorate this neurological insult. Toxicity and electrocorticogram activity were monitored during the first week, and behavioral evaluations were performed 2 weeks post-exposure, followed by biochemical and immunohistopathological analyses. Both double and triple treatment reduced mortality and enhanced weight recovery compared to TA-only treatment. Triple treatment and, to a lesser extent, double treatment significantly ameliorated the ESA duration. Compared to the TA-only or the TA+ midazolam treatment, both double and triple treatment reduced the sarin-induced increase in the neuroinflammatory marker PGE2 and the brain damage marker TSPO and decreased gliosis, astrocytosis and neuronal damage. Finally, both double and triple treatment prevented a change in behavior, as measured in the open field test. No significant difference was observed between the efficacies of the two triple treatments, and both triple combinations completely prevented brain injury (no differences from the naïve rats). Delayed double and, to a greater extent, triple treatment may serve as an efficacious delayed therapy, preventing brain insult propagation following sarin-induced refractory SE.

Integrating TSPO PET imaging and transcriptomics to unveil the role of neuroinflammation and amyloid-ß deposition in Alzheimer's disease.

PURPOSE: Despite the revealed role of immunological dysfunctions in the development and progression of Alzheimer's disease (AD) through animal and postmortem investigations, direct evidence regarding the impact of genetic factors on microglia response and amyloid-ß (Aß) deposition in AD individuals is lacking. This study aims to elucidate this mechanism by integrating transcriptomics and TSPO, Aß PET imaging in clinical AD cohort. METHODS: We analyzed 85 patients with PET/MR imaging for microglial activation (TSPO, [18F]DPA-714) and Aß ([18F]AV-45) within the prospective Alzheimer's Disease Immunization and Microbiota Initiative Study Cohort (ADIMIC). Immune-related differentially expressed genes (IREDGs), identified based on AlzData, were screened and verified using blood samples from ADIMIC. Correlation and mediation analyses were applied to investigate the relationships between immune-related genes expression, TSPO and Aß PET imaging. RESULTS: TSPO uptake increased significantly both in aMCI (P < 0.05) and AD participants (P < 0.01) and showed a positive correlation with Aß deposition (r = 0.42, P < 0.001). Decreased expression of TGFBR3, FABP3, CXCR4 and CD200 was observed in AD group. CD200 expression was significantly negatively associated with TSPO PET uptake (r =-0.33, P = 0.013). Mediation analysis indicated that CD200 acted as a significant mediator between TSPO uptake and Aß deposition (total effect B = 1.92, P = 0.004) and MMSE score (total effect B =-54.01, P = 0.003). CONCLUSION: By integrating transcriptomics and TSPO PET imaging in the same clinical AD cohort, this study revealed CD200 played an important role in regulating neuroinflammation, Aß deposition and cognitive dysfunction.

Enhancing predictability of IDH mutation status in glioma patients at initial diagnosis: a comparative analysis of radiomics from MRI, [18F]FET PET, and TSPO PET.

PURPOSE: According to the World Health Organization classification for tumors of the central nervous system, mutation status of the isocitrate dehydrogenase (IDH) genes has become a major diagnostic discriminator for gliomas. Therefore, imaging-based prediction of IDH mutation status is of high interest for individual patient management. We compared and evaluated the diagnostic value of radiomics derived from dual positron emission tomography (PET) and magnetic resonance imaging (MRI) data to predict the IDH mutation status non-invasively. METHODS: Eighty-seven glioma patients at initial diagnosis who underwent PET targeting the translocator protein (TSPO) using [18F]GE-180, dynamic amino acid PET using [18F]FET, and T1-/T2-weighted MRI scans were examined. In addition to calculating tumor-to-background ratio (TBR) images for all modalities, parametric images quantifying dynamic [18F]FET PET information were generated. Radiomic features were extracted from TBR and parametric images. The area under the receiver operating characteristic curve (AUC) was employed to assess the performance of logistic regression (LR) classifiers. To report robust estimates, nested cross-validation with five folds and 50 repeats was applied. RESULTS: TBRGE-180 features extracted from TSPO-positive volumes had the highest predictive power among TBR images (AUC 0.88, with age as co-factor 0.94). Dynamic [18F]FET PET reached a similarly high performance (0.94, with age 0.96). The highest LR coefficients in multimodal analyses included TBRGE-180 features, parameters from kinetic and early static [18F]FET PET images, age, and the features from TBRT2 images such as the kurtosis (0.97). CONCLUSION: The findings suggest that incorporating TBRGE-180 features along with kinetic information from dynamic [18F]FET PET, kurtosis from TBRT2, and age can yield very high predictability of IDH mutation status, thus potentially improving early patient management.

PET imaging for the early evaluation of ocular inflammation in diabetic rats by using [18F]-DPA-714.

Ocular complications of diabetes mellitus (DM) are the leading cause of vision loss. Ocular inflammation often occurs in the early stage of DM; however, there are no proven quantitative methods to evaluate the inflammatory status of eyes in DM. The 18 kDa translocator protein (TSPO) is an evolutionarily conserved cholesterol binding protein localized in the outer mitochondrial membrane. It is a biomarker of activated microglia/macrophages; however, its role in ocular inflammation is unclear. In this study, fluorine-18-DPA-714 ([18F]-DPA-714) was evaluated as a specific TSPO probe by cell uptake, cell binding assays and micro positron emission tomography (microPET) imaging in both in vitro and in vivo models. Primary microglia/macrophages (PMs) extracted from the cornea, retina, choroid or sclera of neonatal rats with or without high glucose (50 mM) treatment were used as the in vitro model. Sprague-Dawley (SD) rats that received an intraperitoneal administration of streptozotocin (STZ, 60 mg/kg once) were used as the in vivo model. Increased cell uptake and high binding affinity of [18F]-DPA-714 were observed in primary PMs under hyperglycemic stress. These findings were consistent with cellular morphological changes, cell activation, and TSPO up-regulation. [18F]-DPA-714 PET imaging and biodistribution in the eyes of DM rats revealed that inflammation initiates in microglia/macrophages in the early stages (3 weeks and 6 weeks), corresponding with up-regulated TSPO levels. Thus, [18F]-DPA-714 microPET imaging may be an effective approach for the early evaluation of ocular inflammation in DM.

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

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

Neurosteroids and translocator protein 18 kDa (TSPO) ligands as novel treatment options in depression.

Recently, the gamma-aminobutyric acid (GABA) system has come into focus for the treatment of anxiety, postpartum depression, and major depressive disorder. Endogenous 3α-reduced steroids such as allopregnanolone are potent positive allosteric modulators of GABAA receptors and have been known for decades. Current industry developments and first approvals by the U.S. food and drug administration (FDA) for the treatment of postpartum depression with exogenous analogues of these steroids represent a major step forward in the field. 3α-reduced steroids target both synaptic and extrasynaptic GABAA receptors, unlike benzodiazepines, which bind to synaptic receptors. The first FDA-approved 3α-reduced steroid for postpartum depression is brexanolone, an intravenous formulation of allopregnanolone. It has been shown to provide rapid relief of depressive symptoms. An orally available 3α-reduced steroid is zuranolone, which also received FDA approval in 2023 for the treatment of postpartum depression. Although a number of studies have been conducted, the efficacy data were not sufficient to achieve approval of zuranolone in major depressive disorder by the FDA in 2023. The most prominent side effects of these 3α-reduced steroids are somnolence, dizziness and headache. In addition to the issue of efficacy, it should be noted that current data limit the use of these compounds to two weeks. An alternative to exogenous 3α-reduced steroids may be the use of substances that induce endogenous neurosteroidogenesis, such as the translocator protein 18 kDa (TSPO) ligand etifoxine. TSPO has been extensively studied for its role in steroidogenesis, in addition to other functions such as anti-inflammatory and neuroregenerative properties. Currently, etifoxine is the only clinically available TSPO ligand in France for the treatment of anxiety disorders. Studies are underway to evaluate its antidepressant potential. Hopefully, neurosteroid research will lead to the development of fast-acting antidepressants.

Translocator protein (18 kDa) positron emission tomography imaging as a biomarker of neuroinflammation in epilepsy.

Increasing evidence indicate that neuroinflammation triggered by glial cells plays a significant role in epileptogenesis. To this effect, the overexpression of translocator protein 18 kDa (TSPO) in activated microglia and astrocytes has been identified as an inflammatory biomarker in epilepsy. It is now possible to quantify neuroinflammation using non-invasive positron emission tomography (PET) imaging of TSPO. With the advancement of radiotracers, TSPO PET has become an innovative tool in elucidating the "neuroinflammatory machinery" of drug-resistant epilepsy. Furthermore, TSPO PET has demonstrated potential in detecting MRI-negative epileptogenic zones (EZ) and provided an innovative perspective in epileptic medical treatment. This manuscript presents a comprehensive exploration of the neuroinflammatory mechanisms of epilepsy, alongside a thorough review of TSPO PET studies conducted in clinical and preclinical settings. The primary objective is to deepen our understanding of epilepsy progression and to establish TSPO PET as an effective monitoring tool for treatment efficacy.

An Improved HPLC Separation Method for TSPO Radioligand [11C]ER176 Clinical Production.

Mitochondrial membrane translocator protein 18 kDa (TSPO) expression is increased in activated microglia, established as a plausible target of neuroinflammation imaging. [11C]ER176, specifically binding to TSPO, has been developed as the third generation of radioligand for PET imaging of TSPO, which showed the potential in better quantifying neuroinflammation than its predecessors. In the current study, we developed an automated radiosynthesis with an improved HPLC purification method for [11C]ER176 clinical production. The improved HPLC separation was integrated into the automated production of [11C]ER176 using a reverse phase semi-preparative HPLC column with an isocratic pump and the mixture of methanol and 50 mM ammonium acetate as the mobile phase. The fraction corresponding to [11C]ER176 was collected around 8.5-9.0 min without the risk of getting contaminations from nearby impurities. The automated production process took about 30 min after end of bombardment (EOB) and the quality of the final product [11C]ER176 met all specifications for clinical use based on current US Pharmacopeia and FDA CGMP requirements.

Microglia measured by TSPO PET are associated with Alzheimer's disease pathology and mediate key steps in a disease progression model.

INTRODUCTION: Evidence suggests microglial activation precedes regional tau and neurodegeneration in Alzheimer's disease (AD). We characterized microglia with translocator protein (TSPO) positron emission tomography (PET) within an AD progression model where global amyloid beta (Aß) precedes local tau and neurodegeneration, resulting in cognitive impairment. METHODS: Florbetaben, PBR28, and MK-6240 PET, T1 magnetic resonance imaging, and cognitive measures were performed in 19 cognitively unimpaired older adults and 22 patients with mild cognitive impairment or mild AD to examine associations among microglia activation, Aß, tau, and cognition, adjusting for neurodegeneration. Mediation analyses evaluated the possible role of microglial activation along the AD progression model. RESULTS: Higher PBR28 uptake was associated with higher Aß, higher tau, and lower MMSE score, independent of neurodegeneration. PBR28 mediated associations between tau in early and middle Braak stages, between tau and neurodegeneration, and between neurodegeneration and cognition. DISCUSSION: Microglia are associated with AD pathology and cognition and may mediate relationships between subsequent steps in AD progression.

Application of TSPO-Specific Positron Emission Tomography Radiotracer as an Early Indicator of Acute Liver Failure Induced by Propacetamol, a Prodrug of Paracetamol.

Acetaminophen overdose is a leading cause of acute liver failure (ALF), and effective treatment depends on early prediction of disease progression. ALF diagnosis currently requires blood collection 24-72 h after APAP ingestion, necessitating repeated tests and hospitalization. Here, we assessed earlier ALF diagnosis using positron emission tomography (PET) imaging of translocator proteins (TSPOs), which are involved in molecular transport, oxidative stress, apoptosis, and energy metabolism, with the radiotracer [18F]GE180. We intraperitoneally administered propacetamol hydrochloride to male C57BL/6 mice to induce ALF. We performed in vivo PET/CT imaging 3 h later using the TSPO-specific radiotracer [18F]GE180 and quantitatively analyzed the PET images by determining the averaged standardized uptake value (SUVav) in the liver parenchyma. We assessed liver TSPO expression levels via real-time polymerase chain reaction, Western blotting, and immunohistochemistry. [18F]GE180 PET imaging 3 h after propacetamol administration (1500 mg/kg) significantly increased liver SUVav compared to controls (p = 0.001). Analyses showed a 10-fold and 4-fold increase in TSPO gene and protein expression, respectively, in the liver, 3 h after propacetamol induction compared to controls. [18F]GE180 PET visualized and quantified propacetamol-induced ALF through TSPO overexpression. These findings highlight TSPO PET's potential as a non-invasive imaging biomarker for early-stage ALF.

GRT-X Stimulates Dorsal Root Ganglia Axonal Growth in Culture via TSPO and Kv7.2/3 Potassium Channel Activation.

GRT-X, which targets both the mitochondrial translocator protein (TSPO) and the Kv7.2/3 (KCNQ2/3) potassium channels, has been shown to efficiently promote recovery from cervical spine injury. In the present work, we investigate the role of GRT-X and its two targets in the axonal growth of dorsal root ganglion (DRG) neurons. Neurite outgrowth was quantified in DRG explant cultures prepared from wild-type C57BL6/J and TSPO-KO mice. TSPO was pharmacologically targeted with the agonist XBD173 and the Kv7 channels with the activator ICA-27243 and the inhibitor XE991. GRT-X efficiently stimulated DRG axonal growth at 4 and 8 days after its single administration. XBD173 also promoted axonal elongation, but only after 8 days and its repeated administration. In contrast, both ICA27243 and XE991 tended to decrease axonal elongation. In dissociated DRG neuron/Schwann cell co-cultures, GRT-X upregulated the expression of genes associated with axonal growth and myelination. In the TSPO-KO DRG cultures, the stimulatory effect of GRT-X on axonal growth was completely lost. However, GRT-X and XBD173 activated neuronal and Schwann cell gene expression after TSPO knockout, indicating the presence of additional targets warranting further investigation. These findings uncover a key role of the dual mode of action of GRT-X in the axonal elongation of DRG neurons.

Multimodal comparisons of QSM and PET in neurodegeneration and aging.

Quantitative susceptibility mapping (QSM) has been used to study susceptibility changes that may occur based on tissue composition and mineral deposition. Iron is a primary contributor to changes in magnetic susceptibility and of particular interest in applications of QSM to neurodegeneration and aging. Iron can contribute to neurodegeneration through inflammatory processes and via interaction with aggregation of disease-related proteins. To better understand the local susceptibility changes observed on QSM, its signal has been studied in association with other imaging metrics such as positron emission tomography (PET). The associations of QSM and PET may provide insight into the pathophysiology of disease processes, such as the role of iron in aging and neurodegeneration, and help to determine the diagnostic utility of QSM as an indirect indicator of disease processes typically evaluated with PET. In this review we discuss the proposed mechanisms and summarize prior studies of the associations of QSM and amyloid PET, tau PET, TSPO PET, FDG-PET, 15O-PET, and F-DOPA PET in evaluation of neurologic diseases with a focus on aging and neurodegeneration.

Pharmacological modulation of TSPO in microglia/macrophages and neurons in a chronic neurodegenerative model of prion disease.

Neuroinflammation is an important component of many neurodegenerative diseases, whether as a primary cause or a secondary outcome. For that reason, either as diagnostic tools or to monitor progression and/or pharmacological interventions, there is a need for robust biomarkers of neuroinflammation in the brain. Mitochondrial TSPO (18 kDa Translocator protein) is one of few available biomarkers of neuroinflammation for which there are clinically available PET imaging agents. In this study, we further characterised neuroinflammation in a mouse model of prion-induced chronic neurodegeneration (ME7) including a pharmacological intervention via a CSF1R inhibitor. This was achieved by autoradiographic binding of the second-generation TSPO tracer, [3H]PBR28, along with a more comprehensive examination of the cellular contributors to the TSPO signal changes by immunohistochemistry. We observed regional increases of TSPO in the ME7 mouse brains, particularly in the hippocampus, cortex and thalamus. This increased TSPO signal was detected in the cells of microglia/macrophage lineage as well as in astrocytes, endothelial cells and neurons. Importantly, we show that the selective CSF1R inhibitor, JNJ-40346527 (JNJ527), attenuated the disease-dependent increase in TSPO signal, particularly in the dentate gyrus of the hippocampus, where JNJ527 attenuated the number of Iba1+ microglia and neurons, but not GFAP+ astrocytes or endothelial cells. These findings suggest that [3H]PBR28 quantitative autoradiography in combination with immunohistochemistry are important translational tools for detecting and quantifying neuroinflammation, and its treatments, in neurodegenerative disease. Furthermore, we demonstrate that although TSPO overexpression in the ME7 brains was driven by various cell types, the therapeutic effect of the CSF1R inhibitor was primarily to modulate TSPO expression in microglia and neurons, which identifies an important route of biological action of this particular CSF1R inhibitor and provides an example of a cell-specific effect of this type of therapeutic agent on the neuroinflammatory process.

Microglial activation, tau and amyloid deposition in TREM2 p.R47H carriers and mild cognitive impairment patients: a multi-modal/multi-tracer PET/MRI imaging study with influenza vaccine immune challenge.

BACKGROUND: Microglia are increasingly understood to play an important role in the pathogenesis of Alzheimer's disease. The rs75932628 (p.R47H) TREM2 variant is a well-established risk factor for Alzheimer's disease. TREM2 is a microglial cell surface receptor. In this multi-modal/multi-tracer PET/MRI study we investigated the effect of TREM2 p.R47H carrier status on microglial activation, tau and amyloid deposition, brain structure and cognitive profile. METHODS: We compared TREM2 p.R47H carriers (n = 8; median age = 62.3) and participants with mild cognitive impairment (n = 8; median age = 70.7). Participants underwent two [18F]DPA-714 PET/MRI scans to assess TSPO signal, indicative of microglial activation, before and after receiving the seasonal influenza vaccination, which was used as an immune stimulant. Participants also underwent [18F]florbetapir and [18F]AV1451 PET scans to assess amyloid and tau burden, respectively. Regional tau and TSPO signal were calculated for regions of interest linked to Braak stage. An additional comparison imaging healthy control group (n = 8; median age = 45.5) had a single [18F]DPA-714 PET/MRI. An expanded group of participants underwent neuropsychological testing, to determine if TREM2 status influenced clinical phenotype. RESULTS: Compared to participants with mild cognitive impairment, TREM2 carriers had lower TSPO signal in Braak II (P = 0.04) and Braak III (P = 0.046) regions, despite having a similar burden of tau and amyloid. There were trends to suggest reduced microglial activation following influenza vaccine in TREM2 carriers. Tau deposition in the Braak VI region was higher in TREM2 carriers (P = 0.04). Furthermore, compared to healthy controls TREM2 carriers had smaller caudate (P = 0.02), total brain (P = 0.049) and white matter volumes (P = 0.02); and neuropsychological assessment revealed worse ADAS-Cog13 (P = 0.03) and Delayed Matching to Sample (P = 0.007) scores. CONCLUSIONS: TREM2 p.R47H carriers had reduced levels of microglial activation in brain regions affected early in the Alzheimer's disease course and differences in brain structure and cognition. Changes in microglial response may underlie the increased Alzheimer's disease risk in TREM2 p.R47H carriers. Future therapeutic agents in Alzheimer's disease should aim to enhance protective microglial actions.