TSPO imaging in animal models of brain diseases

Imaging quantitative changes in blood-brain barrier permeability using [18F]2-fluoro-2-deoxy-sorbitol ([18F]FDS) PET in relation to glial cell recruitment in a mouse model of endotoxemia

The quantitative relationship between the disruption of the blood-brain barrier (BBB) and the recruitment of glial cells was explored in a mouse model of endotoxemia. [18F]2-Fluoro-2-deoxy-sorbitol ([18F]FDS) PET imaging was used as a paracellular marker for quantitative monitoring of BBB permeability after i.v injection of increasing doses of lipopolysaccharide (LPS) or vehicle (saline, n = 5). The brain distribution of [18F]FDS (VT, mL.cm-3) was estimated using kinetic modeling. LPS dose-dependently increased the brain VT of [18F]FDS after injection of LPS 4 mg/kg (5.2 ± 2.4-fold, n = 4, p < 0.01) or 5 mg/kg (9.0 ± 9.1-fold, n = 4, p < 0.01) but not 3 mg/kg (p > 0.05, n = 7). In 12 individuals belonging to the different groups, changes in BBB permeability were compared with expression of markers of astrocyte (GFAP) and microglial cell (CD11b) using ex vivo immunohistochemistry. Increased expression of CD11b and GFAP expression was observed in mice injected with 3 mg/kg of LPS, which did not increase with higher LPS doses. Quantitative [18F]FDS PET imaging can capture different levels of BBB permeability in vivo. A biphasic effect was observed with the lowest dose of LPS that triggered neuroinflammation without disruptive changes in BBB permeability, and higher LPS doses that increased BBB permeability without additional recruitment of glial cells.

A longitudinal MRI and TSPO PET-based investigation of brain region-specific neuroprotection by diazepam versus midazolam following organophosphate-induced seizures

Acute poisoning with organophosphorus cholinesterase inhibitors (OPs), such as OP nerve agents and pesticides, can cause life threatening cholinergic crisis and status epilepticus (SE). Survivors often experience significant morbidity, including brain injury, acquired epilepsy, and cognitive deficits. Current medical countermeasures for acute OP poisoning include a benzodiazepine to mitigate seizures. Diazepam was long the benzodiazepine included in autoinjectors used to treat OP-induced seizures, but it is now being replaced in many guidelines by midazolam, which terminates seizures more quickly, particularly when administered intramuscularly. While a direct correlation between seizure duration and the extent of brain injury has been widely reported, there are limited data comparing the neuroprotective efficacy of diazepam versus midazolam following acute OP intoxication. To address this data gap, we used non-invasive imaging techniques to longitudinally quantify neuropathology in a rat model of acute intoxication with the OP diisopropylfluorophosphate (DFP) with and without post-exposure intervention with diazepam or midazolam. Magnetic resonance imaging (MRI) was used to monitor neuropathology and brain atrophy, while positron emission tomography (PET) with a radiotracer targeting translocator protein (TSPO) was utilized to assess neuroinflammation. Animals were scanned at 3, 7, 28, 65, 91, and 168 days post-DFP and imaging metrics were quantitated for the hippocampus, amygdala, piriform cortex, thalamus, cerebral cortex and lateral ventricles. In the DFP-intoxicated rat, neuroinflammation persisted for the duration of the study coincident with progressive atrophy and ongoing tissue remodeling. Benzodiazepines attenuated neuropathology in a region-dependent manner, but neither benzodiazepine was effective in attenuating long-term neuroinflammation as detected by TSPO PET. Diffusion MRI and TSPO PET metrics were highly correlated with seizure severity, and early MRI and PET metrics were positively correlated with long-term brain atrophy. Collectively, these results suggest that anti-seizure therapy alone is insufficient to prevent long-lasting neuroinflammation and tissue remodeling.

TOM40 mediates the effect of TSPO on postpartum depression partially through regulating calcium homeostasis in microglia

AIMS

To assess the effect of the translocator protein 18 kDa (TSPO) on postpartum depression and explore its mechanism.

METHODS

Postpartum depression (PPD) mouse model was established, and flow cytometry, immunofluorescence, Western blot analysis, real-time quantitative PCR, adeno-associated virus (AAV), co-immunoprecipitation-mass spectrometry and immunofluorescence co-staining were used to detect the effect of TSPO ligand ZBD-2 on PPD mice.

RESULTS

ZBD-2 inhibits the overactivation of microglia in the hippocampus and amygdala of PPD model mice. ZBD-2 not only inhibited the inflammation but also repressed the burst of reactive oxygen species (ROS) and mitochondrial ROS (mtROS). Meanwhile, ZBD-2 protects mitochondria from LPS-induced damages through inhibiting the influx of calcium. ZBD-2 modulated the calcium influx by increasing the level of translocase of the outer mitochondrial membrane 40 (TOM40) and reducing the interaction of TSPO and TOM40. In addition, the effect of ZBD-2 was partially dependent on anti-oxidative process. Knockdown of TOM40 by adeno-associated virus (AAV) in the hippocampus or amygdala dramatically reduced the effect of ZBD-2 on PPD, indicating that TOM40 mediates the effect of ZBD-2 on PPD.

CONCLUSIONS

TOM40 is required for the effect of ZBD-2 on treating anxiety and depression in PPD mice. This study reveals the role of microglia TSPO in PPD development and provides the new therapeutic strategy for PPD.

The Traumatic Inoculation Process Affects TSPO Radioligand Uptake in Experimental Orthotopic Glioblastoma

BACKGROUND

The translocator protein (TSPO) has been proven to have great potential as a target for the positron emission tomography (PET) imaging of glioblastoma. However, there is an ongoing debate about the potential various sources of the TSPO PET signal. This work investigates the impact of the inoculation-driven immune response on the PET signal in experimental orthotopic glioblastoma.

METHODS

Serial [18F]GE-180 and O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) PET scans were performed at day 7/8 and day 14/15 after the inoculation of GL261 mouse glioblastoma cells (n = 24) or saline (sham, n = 6) into the right striatum of immunocompetent C57BL/6 mice. An additional n = 25 sham mice underwent [18F]GE-180 PET and/or autoradiography (ARG) at days 7, 14, 21, 28, 35, 50 and 90 in order to monitor potential reactive processes that were solely related to the inoculation procedure. In vivo imaging results were directly compared to tissue-based analyses including ARG and immunohistochemistry.

RESULTS

We found that the inoculation process represents an immunogenic event, which significantly contributes to TSPO radioligand uptake. [18F]GE-180 uptake in GL261-bearing mice surpassed [18F]FET uptake both in the extent and the intensity, e.g., mean target-to-background ratio (TBRmean) in PET at day 7/8: 1.22 for [18F]GE-180 vs. 1.04 for [18F]FET, p < 0.001. Sham mice showed increased [18F]GE-180 uptake at the inoculation channel, which, however, continuously decreased over time (e.g., TBRmean in PET: 1.20 at day 7 vs. 1.09 at day 35, p = 0.04). At the inoculation channel, the percentage of TSPO/IBA1 co-staining decreased, whereas TSPO/GFAP (glial fibrillary acidic protein) co-staining increased over time (p < 0.001).

CONCLUSION

We identify the inoculation-driven immune response to be a relevant contributor to the PET signal and add a new aspect to consider for planning PET imaging studies in orthotopic glioblastoma models.

Sex Differences in Stress Response: Classical Mechanisms and Beyond

Neuropsychiatric disorders, which are associated with stress hormone dysregulation, occur at different rates in men and women. Moreover, nowadays, preclinical and clinical evidence demonstrates that sex and gender can lead to differences in stress responses that predispose males and females to different expressions of similar pathologies. In this curated review, we focus on what is known about sex differences in classic mechanisms of stress response, such as glucocorticoid hormones and corticotrophin-releasing factor (CRF), which are components of the hypothalamicpituitary- adrenal (HPA) axis. Then, we present sex differences in neurotransmitter levels, such as serotonin, dopamine, glutamate and GABA, as well as indices of neurodegeneration, such as amyloid β and Tau. Gonadal hormone effects, such as estrogens and testosterone, are also discussed throughout the review. We also review in detail preclinical data investigating sex differences caused by recentlyrecognized regulators of stress and disease, such as the immune system, genetic and epigenetic mechanisms, as well neurosteroids. Finally, we discuss how understanding sex differences in stress responses, as well as in pharmacology, can be leveraged into novel, more efficacious therapeutics for all. Based on the supporting evidence, it is obvious that incorporating sex as a biological variable into preclinical research is imperative for the understanding and treatment of stress-related neuropsychiatric disorders, such as depression, anxiety and Alzheimer's disease.

Systemic treatment with a selective TNFR2 agonist alters the central and peripheral immune responses and transiently improves functional outcome after experimental ischemic stroke

Ischemic stroke often leaves survivors with permanent disabilities and therapies aimed at limiting detrimental inflammation and improving functional outcome are still needed. Tumor necrosis factor (TNF) levels increase rapidly after ischemic stroke, and while signaling through TNF receptor 1 (TNFR1) is primarily detrimental, TNFR2 signaling mainly has protective functions. We therefore investigated how systemic stimulation of TNFR2 with the TNFR2 agonist NewSTAR2 affects ischemic stroke in mice. We found that NewSTAR2 treatment induced changes in peripheral immune cell numbers and transiently affected microglial numbers and neuroinflammation. However, this was not sufficient to improve long-term functional outcome after stroke in mice.

New Perspectives in Radiological and Radiopharmaceutical Hybrid Imaging in Progressive Supranuclear Palsy: A Systematic Review

Progressive supranuclear palsy (PSP) is a neurodegenerative disease characterized by four-repeat tau deposition in various cell types and anatomical regions, and can manifest as several clinical phenotypes, including the most common phenotype, Richardson's syndrome. The limited availability of biomarkers for PSP relates to the overlap of clinical features with other neurodegenerative disorders, but identification of a growing number of biomarkers from imaging is underway. One way to increase the reliability of imaging biomarkers is to combine different modalities for multimodal imaging. This review aimed to provide an overview of the current state of PSP hybrid imaging by combinations of positron emission tomography (PET) and magnetic resonance imaging (MRI). Specifically, combined PET and MRI studies in PSP highlight the potential of [18F]AV-1451 to detect tau, but also the challenge in differentiating PSP from other neurodegenerative diseases. Studies over the last years showed a reduced synaptic density in [11C]UCB-J PET, linked [11C]PK11195 and [18F]AV-1451 markers to disease progression, and suggested the potential role of [18F]RO948 PET for identifying tau pathology in subcortical regions. The integration of quantitative global and regional gray matter analysis by MRI may further guide the assessment of reduced cortical thickness or volume alterations, and diffusion MRI could provide insight into microstructural changes and structural connectivity in PSP. Challenges in radiopharmaceutical biomarkers and hybrid imaging require further research targeting markers for comprehensive PSP diagnosis.

Prenatal infection and adolescent social adversity affect microglia, synaptic density, and behavior in male rats

Maternal infection during pregnancy and childhood social trauma have been associated with neurodevelopmental and affective disorders, such as schizophrenia, autism spectrum disorders, bipolar disorder and depression. These disorders are characterized by changes in microglial cells, which play a notable role in synaptic pruning, and synaptic deficits. Here, we investigated the effect of prenatal infection and social adversity during adolescence - either alone or in combination - on behavior, microglia, and synaptic density. Male offspring of pregnant rats injected with poly I:C, mimicking prenatal infection, were exposed to repeated social defeat during adolescence. We found that maternal infection during pregnancy prevented the reduction in social behavior and increase in anxiety induced by social adversity during adolescence. Furthermore, maternal infection and social adversity, alone or in combination, induced hyperlocomotion in adulthood. Longitudinal in vivo imaging with [11C]PBR28 positron emission tomography revealed that prenatal infection alone and social adversity during adolescence alone induced a transient increase in translocator protein TSPO density, an indicator of glial reactivity, whereas their combination induced a long-lasting increase that remained until adulthood. Furthermore, only the combination of prenatal infection and social adversity during adolescence induced an increase in microglial cell density in the frontal cortex. Prenatal infection increased proinflammatory cytokine IL-1β protein levels in hippocampus and social adversity reduced anti-inflammatory cytokine IL-10 protein levels in hippocampus during adulthood. This reduction in IL-10 was prevented if rats were previously exposed to prenatal infection. Adult offspring exposed to prenatal infection or adolescent social adversity had a higher synaptic density in the frontal cortex, but not hippocampus, as evaluated by synaptophysin density. Interestingly, such an increase in synaptic density was not observed in rats exposed to the combination of prenatal infection and social adversity, perhaps due to the long-lasting increase in microglial density, which may lead to an increase in microglial synaptic pruning. These findings suggest that changes in microglia activity and cytokine release induced by prenatal infection and social adversity during adolescence may be related to a reduced synaptic pruning, resulting in a higher synaptic density and behavioral changes in adulthood.

Neuroinflammation in the Evolution of Motor Function in Stroke and Trauma Patients: Treatment and Potential Biomarkers

Neuroinflammation has a significant impact on different pathologies, such as stroke or spinal cord injury, intervening in their pathophysiology: expansion, progression, and resolution. Neuroinflammation involves oxidative stress, damage, and cell death, playing an important role in neuroplasticity and motor dysfunction by affecting the neuronal connection responsible for motor control. The diagnosis of this pathology is performed using neuroimaging techniques and molecular diagnostics based on identifying and measuring signaling molecules or specific markers. In parallel, new therapeutic targets are being investigated via the use of bionanomaterials and electrostimulation to modulate the neuroinflammatory response. These novel diagnostic and therapeutic strategies have the potential to facilitate the development of anticipatory patterns and deliver the most beneficial treatment to improve patients' quality of life and directly impact their motor skills. However, important challenges remain to be solved. Hence, the goal of this study was to review the implication of neuroinflammation in the evolution of motor function in stroke and trauma patients, with a particular focus on novel methods and potential biomarkers to aid clinicians in diagnosis, treatment, and therapy. A specific analysis of the strengths, weaknesses, threats, and opportunities was conducted, highlighting the key challenges to be faced in the coming years.

The systemic immune response in Parkinson's disease: focus on the peripheral immune component

During Parkinson's disease (PD), both the central nervous system (CNS) and peripheral nervous system (PNS) are affected. In parallel, innate immune cells respond early to neuronal changes and alpha-synuclein (α-syn) pathology. Moreover, some of the affected neuronal groups innervate organs with a relevant role in immunity. Consequently, not only microglia, but also peripheral immune cells are altered, resulting in a systemic immune response. Innate and adaptive immune cells may participate in the neurodegenerative process by acting peripherally, infiltrating the brain, or releasing mediators that can protect or harm neurons. However, the sequence of the changes and the significance of each immune compartment in the disease remain to be clarified. In this review, we describe current understanding of the peripheral immune response in PD and discuss the road ahead.

Evidence of neuroinflammation in fibromyalgia syndrome: a [ 18 F]DPA-714 positron emission tomography study

ABSTRACT

This observational study aimed to determine whether individuals with fibromyalgia (FM) exhibit higher levels of neuroinflammation than healthy controls (HCs), as measured with positron emission tomography using [ 18 F]DPA-714, a second-generation radioligand for the translocator protein (TSPO). Fifteen women with FM and 10 HCs underwent neuroimaging. Distribution volume (V T ) was calculated for in 28 regions of interest (ROIs) using Logan graphical analysis and compared between groups using multiple linear regressions. Group (FM vs HC) was the main predictor of interest and TSPO binding status (high- vs mixed-affinity) was added as a covariate. The FM group had higher V T in the right postcentral gyrus ( b = 0.477, P = 0.033), right occipital gray matter (GM; b = 0.438, P = 0.039), and the right temporal GM ( b = 0.466, P = 0.042). The FM group also had lower V T than HCs in the left isthmus of the cingulate gyrus ( b = -0.553, P = 0.014). In the subgroup of high-affinity binders, the FM group had higher V T in the bilateral precuneus, postcentral gyrus, parietal GM, occipital GM, and supramarginal gyrus. Group differences in the right parietal GM were associated with decreased quality of life, higher pain severity and interference, and cognitive problems. In support of our hypothesis, we found increased radioligand binding (V T ) in the FM group compared with HCs in several brain regions regardless of participants' TSPO binding status. The ROIs overlapped with prior reports of increased TSPO binding in FM. Overall, increasing evidence supports the hypothesis that FM involves microglia-mediated neuroinflammation in the brain.

Mapping acute neuroinflammation in vivo with diffusion-MRI in rats given a systemic lipopolysaccharide challenge

It is becoming increasingly apparent that neuroinflammation plays a critical role in an array of neurological and psychiatric disorders. Recent studies have demonstrated the potential of diffusion MRI (dMRI) to characterize changes in microglial density and morphology associated with neuroinflammation, but these were conducted mostly ex vivo and/or in extreme, non-physiological animal models. Here, we build upon these studies by investigating the utility of well-established dMRI methods to detect neuroinflammation in vivo in a more clinically relevant animal model of sickness behavior. We show that diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI) indicate widespread increases in diffusivity in the brains of rats given a systemic lipopolysaccharide challenge (n = 20) vs. vehicle-treated controls (n = 12). These diffusivity changes correlated with histologically measured changes in microglial morphology, confirming the sensitivity of dMRI to neuroinflammatory processes. This study marks a further step towards establishing a noninvasive indicator of neuroinflammation, which would greatly facilitate early diagnosis and treatment monitoring in various neurological and psychiatric diseases.

Synthesis and biological profile of benzoxazolone derivatives

The benzoxazolone nucleus is an ideal scaffold for drug design, owing to its discrete physicochemical profile, bioisosteric preference over pharmacokinetically weaker moieties, weakly acidic behavior, presence of both lipophilic and hydrophilic fragments on a single framework, and a wider choice of chemical modification on the benzene and oxazolone rings. These properties apparently influence the interactions of benzoxazolone-based derivatives with their respective biological targets. Hence, the benzoxazolone ring is implicated in the synthesis and development of pharmaceuticals with a diverse biological profile ranging from anticancer, analgesics, insecticides, anti-inflammatory, and neuroprotective agents. This has further led to the commercialization of several benzoxazolone-based molecules and a few others under clinical trials. Nevertheless, the SAR exploration of benzoxazolone derivatives for the identification of potential "hits" followed by the screening of "leads" provides a plethora of opportunities for further exploration of the pharmacological profile of the benzoxazolone nucleus. In this review, we aim to present the biological profile of different derivatives based on the benzoxazolone framework.

Advances in the application of neuroinflammatory molecular imaging in brain malignancies

The prevalence of brain cancer has been increasing in recent decades, posing significant healthcare challenges. The introduction of immunotherapies has brought forth notable diagnostic imaging challenges for brain tumors. The tumor microenvironment undergoes substantial changes in induced immunosuppression and immune responses following the development of primary brain tumor and brain metastasis, affecting the progression and metastasis of brain tumors. Consequently, effective and accurate neuroimaging techniques are necessary for clinical practice and monitoring. However, patients with brain tumors might experience radiation-induced necrosis or other neuroinflammation. Currently, positron emission tomography and various magnetic resonance imaging techniques play a crucial role in diagnosing and evaluating brain tumors. Nevertheless, differentiating between brain tumors and necrotic lesions or inflamed tissues remains a significant challenge in the clinical diagnosis of the advancements in immunotherapeutics and precision oncology have underscored the importance of clinically applicable imaging measures for diagnosing and monitoring neuroinflammation. This review summarizes recent advances in neuroimaging methods aimed at enhancing the specificity of brain tumor diagnosis and evaluating inflamed lesions.

Sex-specific microglia state in the Neuroligin-4 knock-out mouse model of autism spectrum disorder

Neuroligin-4 (NLGN4) loss-of-function mutations are associated with monogenic heritable autism spectrum disorder (ASD) and cause alterations in both synaptic and behavioral phenotypes. Microglia, the resident CNS macrophages, are implicated in ASD development and progression. Here we studied the impact of NLGN4 loss in a mouse model, focusing on microglia phenotype and function in both male and female mice. NLGN4 depletion caused lower microglia density, less ramified morphology, reduced response to injury and purinergic signaling specifically in the hippocampal CA3 region predominantly in male mice. Proteomic analysis revealed disrupted energy metabolism in male microglia and provided further evidence for sexual dimorphism in the ASD associated microglial phenotype. In addition, we observed impaired gamma oscillations in a sex-dependent manner. Lastly, estradiol application in male NLGN4-/- mice restored the altered microglial phenotype and function. Together, these results indicate that loss of NLGN4 affects not only neuronal network activity, but also changes the microglia state in a sex-dependent manner that could be targeted by estradiol treatment.

The historical progression of positron emission tomography research in neuroendocrinology

The rapid and continual development of a number of radiopharmaceuticals targeting different receptor, enzyme and small molecule systems has fostered Positron Emission Tomography (PET) imaging of endocrine system actions in vivo in the human brain for several decades. PET radioligands have been developed to measure changes that are regulated by hormone action (e.g., glucose metabolism, cerebral blood flow, dopamine receptors) and actions within endocrine organs or glands such as steroids (e.g., glucocorticoids receptors), hormones (e.g., estrogen, insulin), and enzymes (e.g., aromatase). This systematic review is targeted to the neuroendocrinology community that may be interested in learning about positron emission tomography (PET) imaging for use in their research. Covering neuroendocrine PET research over the past half century, researchers and clinicians will be able to answer the question of where future research may benefit from the strengths of PET imaging.

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

Activated Microglia in the Early Stage of a Rat Model of Parkinson's Disease: Revealed by PET-MRI Imaging by [18F]DPA-714 Targeting TSPO

In the past decades, translocator protein (TSPO) has been considered as an in vivo biomarker to measure the presence of neuroinflammatory reactions. In this study, expression of TSPO was quantified via [¹⁸F]DPA-714 positron emission tomography-magnetic resonance imaging (PET-MRI) imaging to investigate the effects of microglial activation associated with motor behavioral impairments in the 6-hydroxydopamine (6-OHDA)-treated rodent model of Parkinson's disease (PD). [¹⁸F]FDG PET-MRI (for non-specific inflammation), [¹⁸F]D₆-FP-(+)-DTBZ PET-MRI (for damaged dopaminergic neurons), post-PET immunofluorescence, and Pearson's correlation analyses were also performed. The time course of the striatal [¹⁸F]DPA-714 binding ratio elevated in 6-OHDA-treated rats during 1-3 weeks post-treatment, with the peak TSPO binding in the 1st week. No differences between bilateral striatum in [¹⁸F]FDG PET imaging were found. Moreover, an obvious correlation between [¹⁸F]DPA-714 SUV_R^R/L and rotation numbers was found (r = 0.434, *p = 0.049). No correlation between [¹⁸F]FDG SUV_R^R/L and rotation behavior was found. [¹⁸F]DPA-714 appeared to be a potential PET tracer for imaging the microglia-mediated neuroinflammation in the early stage of PD.

Activated Microglia in the Early Stage of a Rat Model of Parkinson's Disease: Revealed by PET-MRI Imaging by [18F]DPA-714 Targeting TSPO

In the past decades, translocator protein (TSPO) has been considered as an in vivo biomarker to measure the presence of neuroinflammatory reactions. In this study, expression of TSPO was quantified via [¹⁸F]DPA-714 positron emission tomography-magnetic resonance imaging (PET-MRI) to investigate the effects of microglial activation associated with motor behavioral impairments in the 6-hydroxydopamine (6-OHDA)-treated rodent model of Parkinson's disease (PD). [¹⁸F]FDG PET-MRI (for non-specific inflammation), [¹⁸F]D₆-FP-(+)-DTBZ PET-MRI (for damaged dopaminergic (DA) neurons), post-PET immunofluorescence, and Pearson's correlation analyses were also performed. The time course of striatal [¹⁸F]DPA-714 binding ratio was elevated in 6-OHDA-treated rats during 1-3 weeks post-treatments, with peak TSPO binding in the 1st week. No difference between the bilateral striatum in [¹⁸F]FDG PET imaging were found. Moreover, an obvious correlation between [¹⁸F]DPA-714 SUV_R^R/L and rotation numbers was found (r = 0.434, *p = 0.049). No correlation between [¹⁸F]FDG SUV_R^R/L and rotation behavior was found. [¹⁸F]DPA-714 appeared to be a potential PET tracer for imaging the microglia-mediated neuroinflammation in the early stage of PD.

A comparison of machine learning approaches for the quantification of microglial cells in the brain of mice, rats and non-human primates

Microglial cells are brain-specific macrophages that swiftly react to disruptive events in the brain. Microglial activation leads to specific modifications, including proliferation, morphological changes, migration to the site of insult, and changes in gene expression profiles. A change in inflammatory status has been linked to many neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. For this reason, the investigation and quantification of microglial cells is essential for better understanding their role in disease progression as well as for evaluating the cytocompatibility of novel therapeutic approaches for such conditions. In the following study we implemented a machine learning-based approach for the fast and automatized quantification of microglial cells; this tool was compared with manual quantification (ground truth), and with alternative free-ware such as the threshold-based ImageJ and the machine learning-based Ilastik. We first trained the algorithms on brain tissue obtained from rats and non-human primate immunohistochemically labelled for microglia. Subsequently we validated the accuracy of the trained algorithms in a preclinical rodent model of Parkinson's disease and demonstrated the robustness of the algorithms on tissue obtained from mice, as well as from images provided by three collaborating laboratories. Our results indicate that machine learning algorithms can detect and quantify microglial cells in all the three mammalian species in a precise manner, equipotent to the one observed following manual counting. Using this tool, we were able to detect and quantify small changes between the hemispheres, suggesting the power and reliability of the algorithm. Such a tool will be very useful for investigation of microglial response in disease development, as well as in the investigation of compatible novel therapeutics targeting the brain. As all network weights and labelled training data are made available, together with our step-by-step user guide, we anticipate that many laboratories will implement machine learning-based quantification of microglial cells in their research.

[18F]GE-180-PET and Post Mortem Marker Characteristics of Long-Term High-Fat-Diet-Induced Chronic Neuroinflammation in Mice

Obesity is characterized by immoderate fat accumulation leading to an elevated risk of neurodegenerative disorders, along with a host of metabolic disturbances. Chronic neuroinflammation is a main factor linking obesity and the propensity for neurodegenerative disorders. To determine the cerebrometabolic effects of diet-induced obesity (DIO) in female mice fed a long-term (24 weeks) high-fat diet (HFD, 60% fat) compared to a group on a control diet (CD, 20% fat), we used in vivo PET imaging with the radiotracer [¹⁸F]FDG as a marker for brain glucose metabolism. In addition, we determined the effects of DIO on cerebral neuroinflammation using translocator protein 18 kDa (TSPO)-sensitive PET imaging with [¹⁸F]GE-180. Finally, we performed complementary post mortem histological and biochemical analyses of TSPO and further microglial (Iba1, TMEM119) and astroglial (GFAP) markers as well as cerebral expression analyses of cytokines (e.g., Interleukin (IL)-1β). We showed the development of a peripheral DIO phenotype, characterized by increased body weight, visceral fat, free triglycerides and leptin in plasma, as well as increased fasted blood glucose levels. Furthermore, we found obesity-associated hypermetabolic changes in brain glucose metabolism in the HFD group. Our main findings with respect to neuroinflammation were that neither [¹⁸F]GE-180 PET nor histological analyses of brain samples seem fit to detect the predicted cerebral inflammation response, despite clear evidence of perturbed brain metabolism along with elevated IL-1β expression. These results could be interpreted as a metabolically activated state in brain-resident immune cells due to a long-term HFD.

Development of a minimally invasive simultaneous estimation method for quantifying translocator protein binding with [18F]FEPPA positron emission tomography

BACKGROUND

The purpose of this study was to assess the feasibility of using a minimally invasive simultaneous estimation method (SIME) to quantify the binding of the 18-kDa translocator protein tracer [¹⁸F]FEPPA. Arterial sampling was avoided by extracting an image-derived input function (IDIF) that was metabolite-corrected using venous blood samples. The possibility of reducing scan duration to 90 min from the recommended 2-3 h was investigated by assuming a uniform non-displaceable distribution volume (V_ND) to simplify the SIME fitting.

RESULTS

SIME was applied to retrospective data from healthy volunteers and was comprised of both high-affinity binders (HABs) and mixed-affinity binders (MABs). Estimates of global V_ND and regional total distribution volume (V_T) from SIME were not significantly different from values obtained using a two-tissue compartment model (2CTM). Regional V_T estimates were greater for HABs compared to MABs for both the 2TCM and SIME, while the SIME estimates had lower inter-subject variability (41 ± 17% reduction). Binding potential (BP_ND) values calculated from regional V_T and brain-wide V_ND estimates were also greater for HABs, and reducing the scan time from 120 to 90 min had no significant effect on BP_ND. The feasibility of using venous metabolite correction was evaluated in a large animal model involving a simultaneous collection of arterial and venous samples. Strong linear correlations were found between venous and arterial measurements of the blood-to-plasma ratio and the remaining [¹⁸F]FEPPA fraction. Lastly, estimates of BP_ND and the specific distribution volume (i.e., V_S = V_T - V_ND) from a separate group of healthy volunteers (90 min scan time, venous-scaled IDIFs) agreed with estimates from the retrospective data for both genotypes.

CONCLUSIONS

The results of this study demonstrate that accurate estimates of regional V_T, BP_ND and V_S can be obtained by applying SIME to [¹⁸F]FEPPA data. Furthermore, the application of SIME enabled the scan time to be reduced to 90 min, and the approach worked well with IDIFs that were scaled and metabolite-corrected using venous blood samples.

TSPO expression in a Zika virus murine infection model as an imaging target for acute infection-induced neuroinflammation

INTRODUCTION

Zika virus (ZIKV) is a neurotropic human pathogen that causes neuroinflammation, whose hallmark is elevated translocator protein (TSPO) expression in the brain. This study investigates ZIKV-associated changes in adult brain TSPO expression, evaluates the effectiveness of TSPO radioligands in detecting TSPO expression, and identifies cells that drive brain TSPO expression in a mouse infection model.

METHODS

The interferon-deficient AG129 mouse infected with ZIKV was used as neuroinflammation model. TSPO expression was evaluated by tissue immunostaining. TSPO radioligands, [³H]PK11195 and [¹⁸F]FEPPA, were used for in vitro and ex vivo detection of TSPO in infected brains. [¹⁸F]FEPPA-PET was used for in vivo detection of TSPO expression. Cell subsets that contribute to TSPO expression were identified by flow cytometry.

RESULTS

Brain TSPO expression increased with ZIKV disease severity. This increase was contributed by TSPO-positive microglia and infiltrating monocytes; and by influx of TSPO-expressing immune cells into the brain. [³H]PK11195 and [¹⁸F]FEPPA distinguish ZIKV-infected brains from normal controls in vitro and ex vivo. [¹⁸F]FEPPA brain uptake by PET imaging correlated with disease severity and neuroinflammation. However, TSPO expression by immune cells contributed to significant blood pool [¹⁸F]FEPPA activity which could confound [¹⁸F]FEPPA-PET imaging results.

CONCLUSIONS

TSPO is a biologically relevant imaging target for ZIKV neuroinflammation. Brain [¹⁸F]FEPPA uptake can be a surrogate marker for ZIKV disease and may be a potential PET imaging marker for ZIKV-induced neuroinflammation. Future TSPO-PET/SPECT studies on viral neuroinflammation and related encephalitis should assess the contribution of immune cells on TSPO expression and employ appropriate image correction methods to subtract blood pool activity.

Longitudinal evaluation of neuroinflammation and oxidative stress in a mouse model of Alzheimer disease using positron emission tomography

Background

Validation of new biomarkers of Alzheimer disease (AD) is crucial for the successful development and implementation of treatment strategies. Additional to traditional AT(N) biomarkers, neuroinflammation biomarkers, such as translocator protein (TSPO) and cystine/glutamine antiporter system (x_c^-), could be considered when assessing AD progression. Herein, we report the longitudinal investigation of [¹⁸F]DPA-714 and [¹⁸F]FSPG for their ability to detect TSPO and x_c^- biomarkers, respectively, in the 5xFAD mouse model for AD.

Methods

Expression of TSPO and x_c^- system was assessed longitudinally (2–12 months of age) on 5xFAD mice and their respective controls by positron emission tomography (PET) imaging using radioligands [¹⁸F]DPA-714 and [¹⁸F]FSPG. In parallel, in the same mice, amyloid-β plaque deposition was assessed with the amyloid PET radiotracer [¹⁸F]florbetaben. In vivo findings were correlated to ex vivo immunofluorescence staining of TSPO and x_c^- in microglia/macrophages and astrocytes on brain slices. Physiological changes of the brain tissue were assessed by magnetic resonance imaging (MRI) in 12-month-old mice.

Results

PET studies showed a significant increase in the uptake of [¹⁸F]DPA-714 and [¹⁸F]FSPG in the cortex, hippocampus, and thalamus in 5xFAD but not in WT mice over time. The results correlate with Aβ plaque deposition. Ex vivo staining confirmed higher TSPO overexpression in both, microglia/macrophages and astrocytes, and overexpression of x_c^- in non-glial cells of 5xFAD mice. Additionally, the results show that Aβ plaques were surrounded by microglia/macrophages overexpressing TSPO. MRI studies showed significant tissue shrinkage and microstructural alterations in 5xFAD mice compared to controls.

Conclusions

TSPO and x_c^- overexpression can be assessed by [¹⁸F]DPA-714 and [¹⁸F]FSPG, respectively, and correlate with the level of Aβ plaque deposition obtained with a PET amyloid tracer. These results position the two tracers as promising imaging tools for the evaluation of disease progression.

Graphical abstract

Longitudinal in vivo study in the 5xFAD mouse model shows that TSPO and oxidative stress assessment through [¹⁸F]DPA-714 and [¹⁸F]FSPG-PET imaging, respectively, could serve as a potential tool for the evaluation of Alzheimer disease progression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-01016-5.

In vivo molecular imaging in preclinical research

In vivo molecular imaging is a research field in which molecular biology and advanced imaging techniques are combined for imaging molecular-level biochemical and physiological changes that occur in a living body. For biomolecular imaging, the knowledge of molecular biology, cell biology, biochemistry, and physiology must be applied. Imaging techniques such as fluorescence, luminescence, single-photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT), and magnetic resonance imaging (MRI) are used for biomolecular imaging. These imaging techniques are used in various fields, i.e., diagnosis of various diseases, development of new drugs, development of treatments, and evaluation of effects. Moreover, as biomolecular imaging can repeatedly acquire images without damaging biological tissues or sacrificing the integrity of objects, changes over time can be evaluated.

Phenotypes or diseases in a living body are caused by the accumulation of various biological phenomena. Genetic differences cause biochemical and physiological differences, which accumulate and cause anatomical or structural changes. Biomolecular imaging techniques are suitable for each step. In evaluating anatomical or structural changes, MRI, CT, and ultrasound have advantages in obtaining high-resolution images. SPECT and MRI are advantageous for the evaluation of various physiological phenomena. PET and magnetic resonance spectroscopy can be used to image biochemical phenomena in vivo. Although various biomolecular imaging techniques can be used to evaluate various biological phenomena, it is important to use imaging techniques suitable for each purpose.

Peroxisome Injury in Multiple Sclerosis: Protective Effects of 4-Phenylbutyrate in CNS-Associated Macrophages

Multiple sclerosis (MS) is a progressive and inflammatory demyelinating disease of the CNS. Peroxisomes perform critical functions that contribute to CNS homeostasis. We investigated peroxisome injury and mitigating effects of peroxisome-restorative therapy on inflammatory demyelination in models of MS. Human autopsied CNS tissues (male and female), human cell cultures, and cuprizone-mediated demyelination mice (female) were examined by RT-PCR, Western blotting, and immunolabeling. The therapeutic peroxisome proliferator, 4-phenylbutyrate (4-PBA) was investigated in vitro and in vivo White matter from MS patients showed reduced peroxisomal transcript and protein levels, including PMP70, compared with non-MS controls. Cultured human neural cells revealed that human microglia contained abundant peroxisomal proteins. TNF-α-exposed microglia displayed reduced immunolabeling of peroxisomal proteins, PMP70 and PEX11β, which was prevented with 4-PBA. In human myeloid cells exposed to TNF-α or nigericin, suppression of PEX11β and catalase protein levels were observed to be dependent on NLRP3 expression. Hindbrains from cuprizone-exposed mice showed reduced Abcd1, Cat, and Pex5l transcript levels, with concurrent increased Nlrp3 and Il1b transcript levels, which was abrogated by 4-PBA. In the central corpus callosum, Iba-1 in CNS-associated macrophages and peroxisomal thiolase immunostaining after cuprizone exposure was increased by 4-PBA. 4-PBA prevented decreased myelin basic protein and neurofilament heavy chain immunoreactivity caused by cuprizone exposure. Cuprizone-induced neurobehavioral deficits were improved by 4-PBA treatment. Peroxisome injury in CNS-associated macrophages contributed to neuroinflammation and demyelination that was prevented by 4-PBA treatment. A peroxisome-targeted therapy might be valuable for treating inflammatory demyelination and neurodegeneration in MS.SIGNIFICANCE STATEMENT Multiple sclerosis (MS) is a common and disabling disorder of the CNS with no curative therapies for its progressive form. The present studies implicate peroxisome impairment in CNS-associated macrophages (CAMs), which include resident microglia and blood-derived macrophages, as an important contributor to inflammatory demyelination and neuroaxonal injury in MS. We also show that the inflammasome molecule NLRP3 is associated with peroxisome injury in vitro and in vivo, especially in CAMs. Treatment with the peroxisome proliferator 4-phenylbutyrate exerted protective effects with improved molecular, morphologic, and neurobehavioral outcomes that were associated with a neuroprotective CAM phenotype. These findings offer novel insights into the contribution of peroxisome injury in MS together with preclinical testing of a rational therapy for MS.

Dogs as a Natural Animal Model of Epilepsy

Epilepsy is a common neurological disease in both humans and domestic dogs, making dogs an ideal translational model of epilepsy. In both species, epilepsy is a complex brain disease characterized by an enduring predisposition to generate spontaneous recurrent epileptic seizures. Furthermore, as in humans, status epilepticus is one of the more common neurological emergencies in dogs with epilepsy. In both species, epilepsy is not a single disease but a group of disorders characterized by a broad array of clinical signs, age of onset, and underlying causes. Brain imaging suggests that the limbic system, including the hippocampus and cingulate gyrus, is often affected in canine epilepsy, which could explain the high incidence of comorbid behavioral problems such as anxiety and cognitive alterations. Resistance to antiseizure medications is a significant problem in both canine and human epilepsy, so dogs can be used to study mechanisms of drug resistance and develop novel therapeutic strategies to benefit both species. Importantly, dogs are large enough to accommodate intracranial EEG and responsive neurostimulation devices designed for humans. Studies in epileptic dogs with such devices have reported ictal and interictal events that are remarkably similar to those occurring in human epilepsy. Continuous (24/7) EEG recordings in a select group of epileptic dogs for >1 year have provided a rich dataset of unprecedented length for studying seizure periodicities and developing new methods for seizure forecasting. The data presented in this review substantiate that canine epilepsy is an excellent translational model for several facets of epilepsy research. Furthermore, several techniques of inducing seizures in laboratory dogs are discussed as related to therapeutic advances. Importantly, the development of vagus nerve stimulation as a novel therapy for drug-resistant epilepsy in people was based on a series of studies in dogs with induced seizures. Dogs with naturally occurring or induced seizures provide excellent large-animal models to bridge the translational gap between rodents and humans in the development of novel therapies. Furthermore, because the dog is not only a preclinical species for human medicine but also a potential patient and pet, research on this species serves both veterinary and human medicine.

[18F]BIBD-239: 18F-Labeled ER176, a Positron Emission Tomography Tracer Specific for the Translocator Protein

[¹¹C]ER176 has adequate sensitivity to image the human brain translocator protein (TSPO) in all three genotypes by positron emission tomography (PET). However, its clinical application is limited by the short half-life of ¹¹C (20.38 min). To overcome the deficiency of [¹¹C]ER176 and keep the pharmacophore features of ER176 to the maximum extent, we designed four fluorine-labeled ER176 derivatives using the deuterium method. In vitro competition binding confirmed that the designed compounds had high affinity for TSPO. Biodistribution experiments showed that tissues with high expression of TSPO had high uptake of these compounds, as well as that the compound showed high brain penetration and mild defluorination in vivo. Therefore, [¹⁸F]BIBD-239 with simple synthesis conditions was selected for further biological evaluation. Theoretical simulations showed that BIBD-239 and ER176 have similar binding modes and sites to Ala147-TSPO and Thr147-TSPO, which indicated that the tracers may have consistent sensitivity to the three affinity genotypes. In vitro autoradiography and in vivo PET studies of the ischemic rat brain showed dramatically higher uptake of [¹⁸F]BIBD-239 on the lesion site compared to the contralateral side with good brain kinetics. Additionally, [¹⁸F]BIBD-239 provided clear tumor PET images in a GL261 glioma model. Importantly, PET imaging and liquid chromatography-high-resolution mass spectrometry (LC-HRMS) results showed that in vivo defluorination and other metabolites of [¹⁸F]BIBD-239 did not interfere with brain imaging. Conclusively, [¹⁸F]BIBD-239, similar to ER176 with low polymorphism sensitivity, has simple labeling conditions, high labeling yield, high affinity, and high specificity for TSPO, and it is planned for further evaluation in higher species.

Neuroinflammation, Stroke, Blood-Brain Barrier Dysfunction, and Imaging Modalities

Maintaining blood-brain barrier (BBB) integrity is crucial for the homeostasis of the central nervous system. Structurally comprising the BBB, brain endothelial cells interact with pericytes, astrocytes, neurons, microglia, and perivascular macrophages in the neurovascular unit. Brain ischemia unleashes a profound neuroinflammatory response to remove the damaged tissue and prepare the brain for repair. However, the intense neuroinflammation occurring during the acute phase of stroke is associated with BBB breakdown, neuronal injury, and worse neurological outcomes. Here, we critically discuss the role of neuroinflammation in ischemic stroke pathology, focusing on the BBB and the interactions between central nervous system and peripheral immune responses. We highlight inflammation-driven injury mechanisms in stroke, including oxidative stress, increased MMP (matrix metalloproteinase) production, microglial activation, and infiltration of peripheral immune cells into the ischemic tissue. We provide an updated overview of imaging techniques for in vivo detection of BBB permeability, leukocyte infiltration, microglial activation, and upregulation of cell adhesion molecules following ischemic brain injury. We discuss the possibility of clinical implementation of imaging modalities to assess stroke-associated neuroinflammation with the potential to provide image-guided diagnosis and treatment. We summarize the results from several clinical studies evaluating the efficacy of anti-inflammatory interventions in stroke. Although convincing preclinical evidence suggests that neuroinflammation is a promising target for ischemic stroke, thus far, translating these results into the clinical setting has proved difficult. Due to the dual role of inflammation in the progression of ischemic damage, more research is needed to mechanistically understand when the neuroinflammatory response begins the transition from injury to repair. This could have important implications for ischemic stroke treatment by informing time- and context-specific therapeutic interventions.

Longitudinal [18F]GE-180 PET Imaging Facilitates In Vivo Monitoring of TSPO Expression in the GL261 Glioblastoma Mouse Model

The 18 kDa translocator protein (TSPO) is increasingly recognized as an interesting target for the imaging of glioblastoma (GBM). Here, we investigated TSPO PET imaging and autoradiography in the frequently used GL261 glioblastoma mouse model and aimed to generate insights into the temporal evolution of TSPO radioligand uptake in glioblastoma in a preclinical setting. We performed a longitudinal [18F]GE-180 PET imaging study from day 4 to 14 post inoculation in the orthotopic syngeneic GL261 GBM mouse model (n = 21 GBM mice, n = 3 sham mice). Contrast-enhanced computed tomography (CT) was performed at the day of the final PET scan (±1 day). [18F]GE-180 autoradiography was performed on day 7, 11 and 14 (ex vivo: n = 13 GBM mice, n = 1 sham mouse; in vitro: n = 21 GBM mice; n = 2 sham mice). Brain sections were also used for hematoxylin and eosin (H&E) staining and TSPO immunohistochemistry. [18F]GE-180 uptake in PET was elevated at the site of inoculation in GBM mice as compared to sham mice at day 11 and later (at day 14, TBRmax +27% compared to sham mice, p = 0.001). In GBM mice, [18F]GE-180 uptake continuously increased over time, e.g., at day 11, mean TBRmax +16% compared to day 4, p = 0.011. [18F]GE-180 uptake as depicted by PET was in all mice co-localized with contrast-enhancement in CT and tissue-based findings. [18F]GE-180 ex vivo and in vitro autoradiography showed highly congruent tracer distribution (r = 0.99, n = 13, p < 0.001). In conclusion, [18F]GE-180 PET imaging facilitates non-invasive in vivo monitoring of TSPO expression in the GL261 GBM mouse model. [18F]GE-180 in vitro autoradiography is a convenient surrogate for ex vivo autoradiography, allowing for straightforward identification of suitable models and scan time-points on previously generated tissue sections.

Positron Emission Tomography in Animal Models of Tauopathies

The microtubule-associated protein tau (MAPT) plays an important role in Alzheimer's disease and primary tauopathy diseases. The abnormal accumulation of tau contributes to the development of neurotoxicity, inflammation, neurodegeneration, and cognitive deficits in tauopathy diseases. Tau synergically interacts with amyloid-beta in Alzheimer's disease leading to detrimental consequence. Thus, tau has been an important target for therapeutics development for Alzheimer's disease and primary tauopathy diseases. Tauopathy animal models recapitulating the tauopathy such as transgenic, knock-in mouse and rat models have been developed and greatly facilitated the understanding of disease mechanisms. The advance in PET and imaging tracers have enabled non-invasive detection of the accumulation and spread of tau, the associated microglia activation, metabolic, and neurotransmitter receptor alterations in disease animal models. In vivo microPET studies on mouse or rat models of tauopathy have provided significant insights into the phenotypes and time course of pathophysiology of these models and allowed the monitoring of treatment targeting at tau. In this study, we discuss the utilities of PET and recently developed tracers for evaluating the pathophysiology in tauopathy animal models. We point out the outstanding challenges and propose future outlook in visualizing tau-related pathophysiological changes in brain of tauopathy disease animal models.

Inflammatory Markers in Substance Use and Mood Disorders: A Neuroimaging Perspective

Chronic exposure to addictive drugs in substance use disorders and stressors in mood disorders render the brain more vulnerable to inflammation. Inflammation in the brain, or neuroinflammation, is characterized by gliosis, microglial activation, and sustained release of cytokines, chemokines, and pro-inflammatory factors compromising the permeability of the blood-brain barrier. There is increased curiosity in understanding how substance misuse and/or repeated stress exposure affect inflammation and contribute to abnormal neuronal activity, altered neuroplasticity, and impaired cognitive control, which eventually promote compulsive drug-use behaviors and worsen mood disorders. This review will emphasize human imaging studies to explore the link between brain function and peripheral markers of inflammation in substance use disorders and mood disorders.

Positron Emission Tomography in Animal Models of Alzheimer's Disease Amyloidosis: Translational Implications

Animal models of Alzheimer's disease amyloidosis that recapitulate cerebral amyloid-beta pathology have been widely used in preclinical research and have greatly enabled the mechanistic understanding of Alzheimer's disease and the development of therapeutics. Comprehensive deep phenotyping of the pathophysiological and biochemical features in these animal models is essential. Recent advances in positron emission tomography have allowed the non-invasive visualization of the alterations in the brain of animal models and in patients with Alzheimer's disease. These tools have facilitated our understanding of disease mechanisms and provided longitudinal monitoring of treatment effects in animal models of Alzheimer's disease amyloidosis. In this review, we focus on recent positron emission tomography studies of cerebral amyloid-beta accumulation, hypoglucose metabolism, synaptic and neurotransmitter receptor deficits (cholinergic and glutamatergic system), blood-brain barrier impairment, and neuroinflammation (microgliosis and astrocytosis) in animal models of Alzheimer's disease amyloidosis. We further propose the emerging targets and tracers for reflecting the pathophysiological changes and discuss outstanding challenges in disease animal models and future outlook in the on-chip characterization of imaging biomarkers towards clinical translation.