Translocator protein (TSPO) inhibits Nosema bombycis proliferation in silkworm, Bombyx mori.

Pebrine disease, caused by Nosema bombycis (Nb) infection in silkworms, is a severe and long-standing disease that threatens sericulture. As parasitic pathogens, a complex relationship exists between microsporidia and their hosts at the mitochondrial level. Previous studies have found that the translocator protein (TSPO) is involved in various biological functions, such as membrane potential regulation, mitochondrial autophagy, immune responses, calcium ion channel regulation, and cell apoptosis. In the present study, we found that TSPO expression in silkworms (BmTSPO) was upregulated following Nb infection, leading to an increase in cytoplasmic calcium, adenosine triphosphate, and reactive oxygen species levels. Knockdown and overexpression of BmTSPO resulted in the promotion and inhibition of Nb proliferation, respectively. We also demonstrated that the overexpression of BmTSPO promotes host cell apoptosis and significantly increases the expression of genes involved in the immune deficiency and Janus kinase-signal transducer and the activator of the transcription pathways. These findings suggest that BmTSPO activates the innate immune signalling pathway in silkworms to regulate Nb proliferation. Targeting TSPO represents a promising approach for the development of new treatments for microsporidian infections.

A Multimodal, In Vivo Approach for Assessing Structurally and Phenotypically Related Neuroactive Molecules.

A recently reported behavioral screen in larval zebrafish for phenocopiers of known anesthetics and associated drugs yielded an isoflavone. Related isoflavones have also been reported as GABAA potentiators. From this, we synthesized a small library of isoflavones and incorporated an in vivo phenotypic approach to perform structure-behavior relationship studies of the screening hit and related analogs via behavioral profiling, patch-clamp experiments, and whole brain imaging. This revealed that analogs effect a range of behavioral responses, including sedation with and without enhancing the acoustic startle response. Interestingly, a subset of compounds effect sedation and enhancement of motor responses to both acoustic and light stimuli. Patch clamp recordings of cells with a human GABAA receptor confirmed that behavior-modulating isoflavones modify the GABA signaling. To better understand these molecules' nuanced effects on behavior, we performed whole brain imaging to reveal that analogs differentially effect neuronal activity. These studies demonstrate a multimodal approach to assessing activities of neuroactives.

18 kDa Translocator protein (TSPO) is upregulated in rat brain after peripheral nerve injury and downregulated by diroximel fumarate.

Neuroimmune signaling is a key process underlying neuropathic pain. Clinical studies have demonstrated that 18 kDa translocator protein (TSPO), a putative marker of neuroinflammation, is upregulated in discrete brain regions of patients with chronic pain. However, no preclinical studies have investigated TSPO dynamics in the brain in the context of neuropathic pain and in response to analgesic treatments. We used positron emission tomography-computed tomography (PET-CT) and [18F]-PBR06 radioligand to measure TSPO levels in the brain across time after chronic constriction injury (CCI) of the sciatic nerve in both male and female rats. Up to 10 weeks post-CCI, TSPO expression was increased in discrete brain regions, including medial prefrontal cortex, somatosensory cortex, insular cortex, anterior cingulate cortex, motor cortex, ventral tegmental area, amygdala, midbrain, pons, medulla, and nucleus accumbens. TSPO was broadly upregulated across these regions at 4 weeks post CCI in males, and 10 weeks in females, though there were regional differences between the sexes. Using immunohistochemistry, we confirmed TSPO expression in these regions. We further demonstrated that TSPO was upregulated principally in microglia in the nucleus accumbens core, and astrocytes and endothelial cells in the nucleus accumbens shell. Finally, we tested whether TSPO upregulation was sensitive to diroximel fumarate, a drug that induces endogenous antioxidants via nuclear factor E2-related factor 2 (Nrf2). Diroximel fumarate alleviated neuropathic pain and reduced TSPO upregulation. Our findings indicate that TSPO is upregulated over the course of neuropathic pain development and is resolved by an antinociceptive intervention in animals with peripheral nerve injury.

Regional desynchronization of microglial activity is associated with cognitive decline in Alzheimer's disease.

BACKGROUND: Microglial activation is one hallmark of Alzheimer disease (AD) neuropathology but the impact of the regional interplay of microglia cells in the brain is poorly understood. We hypothesized that microglial activation is regionally synchronized in the healthy brain but experiences regional desynchronization with ongoing neurodegenerative disease. We addressed the existence of a microglia connectome and investigated microglial desynchronization as an AD biomarker. METHODS: To validate the concept, we performed microglia depletion in mice to test whether interregional correlation coefficients (ICCs) of 18 kDa translocator protein (TSPO)-PET change when microglia are cleared. Next, we evaluated the influence of dysfunctional microglia and AD pathophysiology on TSPO-PET ICCs in the mouse brain, followed by translation to a human AD-continuum dataset. We correlated a personalized microglia desynchronization index with cognitive performance. Finally, we performed single-cell radiotracing (scRadiotracing) in mice to ensure the microglial source of the measured desynchronization. RESULTS: Microglia-depleted mice showed a strong ICC reduction in all brain compartments, indicating microglia-specific desynchronization. AD mouse models demonstrated significant reductions of microglial synchronicity, associated with increasing variability of cellular radiotracer uptake in pathologically altered brain regions. Humans within the AD-continuum indicated a stage-depended reduction of microglia synchronicity associated with cognitive decline. scRadiotracing in mice showed that the increased TSPO signal was attributed to microglia. CONCLUSION: Using TSPO-PET imaging of mice with depleted microglia and scRadiotracing in an amyloid model, we provide first evidence that a microglia connectome can be assessed in the mouse brain. Microglia synchronicity is closely associated with cognitive decline in AD and could serve as an independent personalized biomarker for disease progression.

Rediscovering a Forgotten Link: TSPO and RIM-BP1 in Appetite Regulation.

The translocator protein of 18 kDa (TSPO) and RIM binding protein 1 (RIM-BP1) are both heavily expressed in neurons at the olfactory bulb. These proteins have overlapping functional profiles and are both implicated in the development of obesity. Over 20 years ago, a yeast 2-hybrid experiment discovered that RIM-BP1 interacts with a peptide constructed from a fraction of the TSPO sequence. Considering these data, the authors predict that the interaction between RIM-BP1 and TSPO could alter the olfactory system's mediation of appetite. Despite the therapeutic potential of this interaction, it has never been confirmed if the full TSPO protein and RIM-BP1 interact. The interaction is instead often cited as physiologically irrelevant. This commentary revisits the forgotten interaction between TSPO and RIM-BP1, reviewing all relevant literature discussing their relationship. Contrary to common discourse that the RIM-BP1 and TSPO are potential binding partners, while the interaction may regulate many neurological functions, existing evidence suggests that the interaction would have a specific role in odor-guided appetite. Further research into the nutritional neuroscientific consequences of TSPO/RIM-BP1 interactions should therefore be conducted.

A blood-free modeling approach for the quantification of the blood-to-brain tracer exchange in TSPO PET imaging.

Introduction: Recent evidence suggests the blood-to-brain influx rate (K1 ) in TSPO PET imaging as a promising biomarker of blood-brain barrier (BBB) permeability alterations commonly associated with peripheral inflammation and heightened immune activity in the brain. However, standard compartmental modeling quantification is limited by the requirement of invasive and laborious procedures for extracting an arterial blood input function. In this study, we validate a simplified blood-free methodologic framework for K1 estimation by fitting the early phase tracer dynamics using a single irreversible compartment model and an image-derived input function (1T1K-IDIF). Methods: The method is tested on a multi-site dataset containing 177 PET studies from two TSPO tracers ([11C]PBR28 and [18F]DPA714). Firstly, 1T1K-IDIF K1 estimates were compared in terms of both bias and correlation with standard kinetic methodology. Then, the method was tested on an independent sample of [11C]PBR28 scans before and after inflammatory interferon-α challenge, and on test-retest dataset of [18F]DPA714 scans. Results: Comparison with standard kinetic methodology showed good-to-excellent intra-subject correlation for regional 1T1K-IDIF-K1 (ρintra = 0.93 ± 0.08), although the bias was variable depending on IDIF ability to approximate blood input functions (0.03-0.39 mL/cm3/min). 1T1K-IDIF-K1 unveiled a significant reduction of BBB permeability after inflammatory interferon-α challenge, replicating results from standard quantification. High intra-subject correlation (ρ = 0.97 ± 0.01) was reported between K1 estimates of test and retest scans. Discussion: This evidence supports 1T1K-IDIF as blood-free alternative to assess TSPO tracers' unidirectional blood brain clearance. K1 investigation could complement more traditional measures in TSPO studies, and even allow further mechanistic insight in the interpretation of TSPO signal.

Microglia-mediated neuroimmune suppression in PTSD is associated with anhedonia.

Dynamic brain immune function in individuals with posttraumatic stress disorder is rarely studied, despite evidence of peripheral immune dysfunction. Positron emission tomography brain imaging using the radiotracer [11C]PBR28 was used to measure the 18-kDa translocator protein (TSPO), a microglial marker, at baseline and 3 h after administration of lipopolysaccharide (LPS), a potent immune activator. Data were acquired in 15 individuals with PTSD and 15 age-matched controls. The PTSD group exhibited a significantly lower magnitude LPS-induced increase in TSPO availability in an a priori prefrontal-limbic circuit compared to controls. Greater anhedonic symptoms in the PTSD group were associated with a more suppressed neuroimmune response. In addition, while a reduced granulocyte-macrophage colony-stimulating factor response to LPS was observed in the PTSD group, other measured cytokine responses and self-reported sickness symptoms did not differ between groups; these findings highlight group differences in central-peripheral immune system relationships. The results of this study provide evidence of a suppressed microglia-mediated neuroimmune response to a direct immune system insult in individuals with PTSD that is associated with the severity of symptoms. They also provide further support to an emerging literature challenging traditional concepts of microglial and immune function in psychiatric disease.

Physically informed deep neural networks for metabolite-corrected plasma input function estimation in dynamic PET imaging.

INTRODUCTION: We propose a novel approach for the non-invasive quantification of dynamic PET imaging data, focusing on the arterial input function (AIF) without the need for invasive arterial cannulation. METHODS: Our method utilizes a combination of three-dimensional depth-wise separable convolutional layers and a physically informed deep neural network to incorporatea priori knowledge about the AIF's functional form and shape, enabling precise predictions of the concentrations of [11C]PBR28 in whole blood and the free tracer in metabolite-corrected plasma. RESULTS: We found a robust linear correlation between our model's predicted AIF curves and those obtained through traditional, invasive measurements. We achieved an average cross-validated Pearson correlation of 0.86 for whole blood and 0.89 for parent plasma curves. Moreover, our method's ability to estimate the volumes of distribution across several key brain regions - without significant differences between the use of predicted versus actual AIFs in a two-tissue compartmental model - successfully captures the intrinsic variability related to sex, the binding affinity of the translocator protein (18 kDa), and age. CONCLUSIONS: These results not only validate our method's accuracy and reliability but also establish a foundation for a streamlined, non-invasive approach to dynamic PET data quantification. By offering a precise and less invasive alternative to traditional quantification methods, our technique holds significant promise for expanding the applicability of PET imaging across a wider range of tracers, thereby enhancing its utility in both clinical research and diagnostic settings.

Microglial Positron Emission Tomography Imaging In Vivo : Positron Emission Tomography Radioligands: Utility in Research and Clinical Practice.

Microglia, the resident immune cells of the central nervous system (CNS) play a key role in regulating and maintaining homeostasis in the brain. However, the CNS is also vulnerable to infections and inflammatory processes. In response to CNS perturbations, microglia become reactive, notably with expression of the translocator protein (TSPO), primarily on their outer mitochondrial membrane. Despite TSPO being commonly used as a marker for microglia, it is also present in other cell types such as astrocytes. Positron emission tomography (PET) ligands that target the TSPO enable the noninvasive detection and quantification of glial reactivity. While some limitations were raised, TSPO PET remains an attractive biomarker of CNS infection and inflammation. This book chapter delves into the development and application of microglial PET imaging with a focus on the TSPO PET. First, we provide an overview of the evolution of TSPO PET radioligands from first-generation to second-generation ligands and their applications in studying neuroinflammation (or CNS inflammation). Subsequently, we discuss the limitations and challenges associated with TSPO PET. Then we go on to explore non-TSPO targets for microglial PET imaging. Finally, we conclude with future directions for research and clinical practice in this field.

Preclinical characterization of [18F]D2-LW223: an improved metabolically stable PET tracer for imaging the translocator protein 18 kDa (TSPO) in neuroinflammatory rodent models and non-human primates.

Positron emission tomography (PET) targeting translocator protein 18 kDa (TSPO) can be used for the noninvasive detection of neuroinflammation. Improved in vivo stability of a TSPO tracer is beneficial for minimizing the potential confounding effects of radiometabolites. Deuteration represents an important strategy for improving the pharmacokinetics and stability of existing drug molecules in the plasma. This study developed a novel tracer via the deuteration of [18F]LW223 and evaluated its in vivo stability and specific binding in neuroinflammatory rodent models and nonhuman primate (NHP) brains. Compared with LW223, D2-LW223 exhibited improved binding affinity to TSPO. Compared with [18F]LW223, [18F]D2-LW223 has superior physicochemical properties and favorable brain kinetics, with enhanced metabolic stability and reduced defluorination. Preclinical investigations in rodent models of LPS-induced neuroinflammation and cerebral ischemia revealed specific [18F]D2-LW223 binding to TSPO in regions affected by neuroinflammation. Two-tissue compartment model analyses provided excellent model fits and allowed the quantitative mapping of TSPO across the NHP brain. These results indicate that [18F]D2-LW223 holds significant promise for the precise quantification of TSPO expression in neuroinflammatory pathologies of the brain.

Evidence for inflammation in normal-appearing brain regions in patients with growing sporadic vestibular schwannoma: A PET study.

Background: Nonauditory symptoms can be a prominent feature in patients with sporadic vestibular schwannoma (VS), but the cause of these symptoms is unknown. Inflammation is hypothesized to play a key role in the growth and symptomatic presentation of sporadic VS, and in this study, we investigated through translocator protein (TSPO) positron emission tomography (PET) whether inflammation occurred within the "normal appearing" brain of such patients and its association with tumor growth. Methods: Dynamic PET datasets from 15 patients with sporadic VS (8 static and 7 growing) who had been previously imaged using the TSPO tracer [11C](R)-PK11195 were included. Parametric images of [11C](R)-PK11195 binding potential (BPND) and the distribution volume ratio (DVR) were derived and compared across VS growth groups within both contralateral and ipsilateral gray (GM) and white matter (WM) regions. Voxel-wise cluster analysis was additionally performed to identify anatomical regions of increased [11C](R)-PK11195 binding. Results: Compared with static tumors, growing VS demonstrated significantly higher cortical (GM, 1.070 vs. 1.031, P = .03) and whole brain (GM & WM, 1.045 vs. 1.006, P = .03) [11C](R)-PK11195 DVR values. The voxel-wise analysis supported the region-based analysis and revealed clusters of high TSPO binding within the precentral, postcentral, and prefrontal cortex in patients with growing VS. Conclusions: We present the first in vivo evidence of increased TSPO expression and inflammation within the brains of patients with growing sporadic VS. These results provide a potential mechanistic insight into the development of nonauditory symptoms in these patients and highlight the need for further studies interrogating the role of neuroinflammation in driving VS symptomatology.

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.

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.

A p- adic approach to the TSPO gene.

TSPO protein is known to be involved in various cellular functions and dysregulations of TSPO expression has been found to be associated with pathologies of different human diseases, including cardiovascular disease, cancer, neuroinflammatory, neurodegenerative, neoplastic disorders. However, there are limited studies in the literature on the effects of sequence variations in the TSPO gene on the function of the protein and their relationship with human diseases. Evaluating the pathogenicity of genetic variants is crucial in terms of prioritizing the functional importance and clinical use. Therefore, various in-silico prediction tools have been developed that combine different algorithms to predict the effects of sequence variations on protein functions or gene regulation. In this study, the p-adic distance approach in modeling the genetic code, proposed and developed by Dragovich and Dragovich, was discussed in order to obtain an alternative to the existing in-silico prediction tools. Dragovichs' approach is expressed as follows: A 5-adic space of codons is constructed and 5-adic and 2-adic distances between codons are taken into account. As a result, two codons with the smallest value of 5-adic and 2-adic distances are obtained, encoded for the same amino acid and stop signal. This model describes well the degeneration of the genetic code. This study combined the data obtained from in-silico prediction tools and used a bioinformatics approach to determine the functional relevance of coding SNPs in the TSPO. Overall, we evaluate the potential utility of Dragovichs' approach by comparing it with other existing prediction tools for variant classification and prioritization.

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.

Translocator protein (TSPO) genotype does not change cerebrospinal fluid levels of glial activation, axonal and synaptic damage markers in early Alzheimer's disease.

BACKGROUND: PET imaging of the translocator protein (TSPO) is used to assess in vivo brain inflammation. One of the main methodological issues with this method is the allelic dependence of the radiotracer affinity. In Alzheimer's disease (AD), previous studies have shown similar clinical and patho-biological profiles between TSPO genetic subgroups. However, there is no evidence regarding the effect of the TSPO genotype on cerebrospinal-fluid biomarkers of glial activation, and synaptic and axonal damage. METHOD: We performed a trans-sectional study in early AD to compare cerebrospinal-fluid levels of GFAP, YKL-40, sTREM2, IL-6, IL-10, NfL and neurogranin between TSPO genetic subgroups. RESULTS: We recruited 33 patients with early AD including 16 (48%) high affinity binders, 13 (39%) mixed affinity binders, and 4/33 (12%) low affinity binders. No difference was observed in terms of demographics, and cerebrospinal fluid levels of each biomarker for the different subgroups. CONCLUSION: TSPO genotype is not associated with a change in glial activation, synaptic and axonal damage in early AD. Further studies with larger numbers of participants will be needed to confirm that the inclusion of specific TSPO genetic subgroups does not introduce selection bias in studies and trials of AD that combine TSPO imaging with cerebrospinal fluid biomarkers.

TSPO in pancreatic beta cells and its possible involvement in type 2 diabetes.

Amyloidosis forms a large family of pathologies associated with amyloid deposit generated by the formation of amyloid fibrils or plaques. The amyloidogenic proteins and peptides involved in these processes are targeted against almost all organs. In brain they are associated with neurodegenerative disease, and the Translocator Protein (TSPO), overexpressed in these inflammatory conditions, is one of the target for the diagnostic. Moreover, TSPO ligands have been described as promising therapeutic drugs for neurodegenerative diseases. Type 2 diabetes, another amyloidosis, is due to a beta cell mass decrease that has been linked to hIAPP (human islet amyloid polypeptide) fibril formation, leading to the reduction of insulin production. In the present study, in a first approach, we link overexpression of TSPO and inflammation in potentially prediabetic patients. In a second approach, we observed that TSPO deficient rats have higher level of insulin secretion in basal conditions and more IAPP fibrils formation compared with wild type animals. In a third approach, we show that diabetogenic conditions also increase TSPO overexpression and IAPP fibril formation in rat beta pancreatic cell line (INS-1E). These data open the way for further studies in the field of type 2 diabetes treatment or prevention.

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.

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.

[123I]CLINDE SPECT as a neuroinflammation imaging approach in a rat model of stroke.

Poststroke neuroinflammation exacerbates disease progression. [11C]PK11195-positron emission tomography (PET) imaging has been used to visualize neuroinflammation; however, its short half-life of 20 min limits its clinical use. [123I]CLINDE has a longer half-life (13h); therefore, [123I]CLINDE-single-photon emission computed tomography (SPECT) imaging is potentially more practical than [11C]PK11195-PET imaging in clinical settings. The objectives of this study were to 1) validate neuroinflammation imaging using [123I]CLINDE and 2) investigate the mechanisms underlying stroke in association with neuroinflammation using multimodal techniques, including magnetic resonance imaging (MRI), gas-PET, and histological analysis, in a rat model of ischemic stroke, that is, permanent middle cerebral artery occlusion (pMCAo). At 6 days post-pMCAo, [123I]CLINDE-SPECT considerably corresponded to the immunohistochemical images stained with the CD68 antibody (a marker for microglia/microphages), comparable to the level observed in [11C]PK11195-PET images. In addition, the [123I]CLINDE-SPECT images corresponded well with autoradiography images. Rats with severe infarcts, as defined by MRI, exhibited marked neuroinflammation in the peri-infarct area and less neuroinflammation in the ischemic core, accompanied by a substantial reduction in the cerebral metabolic rate of oxygen (CMRO2) in 15O-gas-PET. Rats with moderate-to-mild infarcts exhibited neuroinflammation in the ischemic core, where CMRO2 levels were mildly reduced. This study demonstrates that [123I]CLINDE-SPECT imaging is suitable for neuroinflammation imaging and that the distribution of neuroinflammation varies depending on the severity of infarction.