PK11195 binding to the peripheral benzodiazepine receptor as a marker of microglia activation in multiple sclerosis and experimental autoimmune encephalomyelitis

Remimazolam attenuates inflammation and kidney fibrosis following folic acid injury

The transition of acute kidney injury (AKI) to chronic kidney disease (CKD) is characterized by intense inflammation and progressive fibrosis. Remimazolam is widely used for procedural sedation in intensive care units, such as AKI patients. Remimazolam has been shown to possess anti-inflammatory and organ-protective properties. However, the role of remimazolam in inflammation and renal fibrosis following AKI remains unclear. Here, we explored the effects of remimazolam on the inflammatory response and kidney fibrogenesis of mice subjected to folic acid (FA) injury. Our results showed that remimazolam treatment alleviated kidney damage and dysfunction. Mice treated with remimazolam presented less collagen deposition in FA-injured kidneys compared with FA controls, which was accompanied by a reduction of extracellular matrix proteins accumulation and fibroblasts activation. Furthermore, remimazolam treatment reduced inflammatory cells infiltration into the kidneys of mice with FA injury and inhibited proinflammatory or profibrotic molecules expression. Finally, remimazolam treatment impaired the activation of bone marrow-derived fibroblasts and blunted the transformation of macrophages to myofibroblasts in FA nephropathy. Additionally, the benzodiazepine receptor antagonist PK-11195 partially reversed the protective effect of remimazolam on the FA-injured kidneys. Overall, remimazolam attenuates the inflammatory response and renal fibrosis development following FA-induced AKI, which may be related to the peripheral benzodiazepine receptor pathway.

Microglia in neurodegenerative diseases: mechanism and potential therapeutic targets

Microglia activation is observed in various neurodegenerative diseases. Recent advances in single-cell technologies have revealed that these reactive microglia were with high spatial and temporal heterogeneity. Some identified microglia in specific states correlate with pathological hallmarks and are associated with specific functions. Microglia both exert protective function by phagocytosing and clearing pathological protein aggregates and play detrimental roles due to excessive uptake of protein aggregates, which would lead to microglial phagocytic ability impairment, neuroinflammation, and eventually neurodegeneration. In addition, peripheral immune cells infiltration shapes microglia into a pro-inflammatory phenotype and accelerates disease progression. Microglia also act as a mobile vehicle to propagate protein aggregates. Extracellular vesicles released from microglia and autophagy impairment in microglia all contribute to pathological progression and neurodegeneration. Thus, enhancing microglial phagocytosis, reducing microglial-mediated neuroinflammation, inhibiting microglial exosome synthesis and secretion, and promoting microglial conversion into a protective phenotype are considered to be promising strategies for the therapy of neurodegenerative diseases. Here we comprehensively review the biology of microglia and the roles of microglia in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, multiple system atrophy, amyotrophic lateral sclerosis, frontotemporal dementia, progressive supranuclear palsy, corticobasal degeneration, dementia with Lewy bodies and Huntington's disease. We also summarize the possible microglia-targeted interventions and treatments against neurodegenerative diseases with preclinical and clinical evidence in cell experiments, animal studies, and clinical trials.

Measuring Pathology in Patients with Multiple Sclerosis Using Positron Emission Tomography

PURPOSE OF REVIEW

Multiple sclerosis is characterized by a diverse and complex pathology. Clinical relapses, the hallmark of the disease, are accompanied by focal white matter lesions with intense inflammatory and demyelinating activity. Prevention of these relapses has been the major focus of pharmaceutical development, and it is now possible to dramatically reduce this inflammatory activity. Unfortunately, disability accumulation persists for many people living with multiple sclerosis owing to ongoing damage within existing lesions, pathology outside of discrete lesions, and other yet unknown factors. Understanding this complex pathological cascade will be critical to stopping progressive multiple sclerosis. Positron emission tomography uses biochemically specific radioligands to quantitatively measure pathological processes with molecular specificity. This review examines recent advances in the understanding of multiple sclerosis facilitated by positron emission tomography and identifies future avenues to expand understanding and treatment options.

RECENT FINDINGS

An increasing number of radiotracers allow for the quantitative measurement of inflammatory abnormalities, de- and re-myelination, and metabolic disruption associated with multiple sclerosis. The studies have identified contributions of ongoing, smoldering inflammation to accumulating tissue injury and clinical worsening. Myelin studies have quantified the dynamics of myelin loss and recovery. Lastly, metabolic changes have been found to contribute to symptom worsening. The molecular specificity facilitated by positron emission tomography in people living with multiple sclerosis will critically inform efforts to modulate the pathology leading to progressive disability accumulation. Existing studies show the power of this approach applied to multiple sclerosis. This armamentarium of radioligands allows for new understanding of how the brain and spinal cord of people is impacted by multiple sclerosis.

Study protocol for a phase II, double-blind, randomised controlled trial of cannabidiol (CBD) compared with placebo for reduction of brain neuroinflammation in adults with chronic low back pain

INTRODUCTION

Chronic pain is a debilitating medical problem that is difficult to treat. Neuroinflammatory pathways have emerged as a potential therapeutic target, as preclinical studies have demonstrated that glial cells and neuroglial interactions play a role in the establishment and maintenance of pain. Recently, we used positron emission tomography (PET) to demonstrate increased levels of 18 kDa translocator protein (TSPO) binding, a marker of glial activation, in patients with chronic low back pain (cLBP). Cannabidiol (CBD) is a glial inhibitor in animal models, but studies have not assessed whether CBD reduces neuroinflammation in humans. The principal aim of this trial is to evaluate whether CBD, compared with placebo, affects neuroinflammation, as measured by TSPO levels.

METHODS AND ANALYSIS

This is a double-blind, randomised, placebo-controlled, phase II clinical trial. Eighty adults (aged 18-75) with cLBP for >6 months will be randomised to either an FDA-approved CBD medication (Epidiolex) or matching placebo for 4 weeks using a dose-escalation design. All participants will undergo integrated PET/MRI at baseline and after 4 weeks of treatment to evaluate neuroinflammation using [¹¹C]PBR28, a second-generation radioligand for TSPO. Our primary hypothesis is that participants randomised to CBD will demonstrate larger reductions in thalamic [¹¹C]PBR28 signal compared with those receiving placebo. We will also assess the effect of CBD on (1) [¹¹C]PBR28 signal from limbic regions, which our prior work has linked to depressive symptoms and (2) striatal activation in response to a reward task. Additionally, we will evaluate self-report measures of cLBP intensity and bothersomeness, depression and quality of life at baseline and 4 weeks.

ETHICS AND DISSEMINATION

This protocol is approved by the Massachusetts General Brigham Human Research Committee (protocol number: 2021P002617) and FDA (IND number: 143861) and registered with ClinicalTrials.gov. Results will be published in peer-reviewed journals and presented at conferences.

TRIAL REGISTRATION NUMBER

NCT05066308; ClinicalTrials.gov.

Microglia in multiple sclerosis - pathogenesis and imaging

PURPOSE OF REVIEW

Microglia normally protects the central nervous system (CNS) against insults. However, their persistent activation in multiple sclerosis (MS) contributes to injury. Here, we review microglia activation in MS and their detection using positron emission tomography (PET).

RECENT FINDINGS

During lesion evolution and the progression of MS, microglia activity may contribute to neurotoxicity through the release of pro-inflammatory cytokines, reactive oxidative species, proteases and glutamate. A means to detect and monitor microglia activation in individuals living with MS is provided by positron emission tomography (PET) imaging using the mitochondrial 18-kDa translocator protein (TSPO) ligand. TSPO PET imaging shows increased microglial activation within the normal appearing white matter that precedes radiological signs of neurodegeneration measured by T2 lesion enlargement. PET-detected microglia activation increases with progression of MS. These findings demand the use of CNS penetrant inhibitors that affect microglia. Such therapies may include hydroxychloroquine that is recently reported in a small study to reduce the expected progression in primary progressive MS, and Bruton's tyrosine kinase inhibitors for which there are now eleven Phase 3 registered trials in MS.

SUMMARY

Microglial activation drives injury in MS. PET imaging with microglia-specific ligands offer new insights into progression of MS and as a monitor for treatment responses.

Opportunities for Molecular Imaging in Multiple Sclerosis Management: Linking Probe to Treatment

Imaging has been a critical component of multiple sclerosis (MS) management for nearly 40 years. The visual information derived from structural MRI, that is, signs of blood-brain barrier disruption, inflammation and demyelination, and brain and spinal cord atrophy, are the primary metrics used to evaluate therapeutic efficacy in MS. The development of targeted imaging probes has expanded our ability to evaluate and monitor MS and its therapies at the molecular level. Most molecular imaging probes evaluated for MS applications are small molecules initially developed for PET, nearly half of which are derived from U.S. Food and Drug Administration-approved drugs and those currently undergoing clinical trials. Superparamagnetic and fluorinated particles have been used for tracking circulating immune cells (in situ labeling) and immunosuppressive or remyelinating therapeutic stem cells (ex vivo labeling) clinically using proton (hydrogen 1 [¹H]) and preclinically using fluorine 19 MRI. Translocator protein PET and ¹H MR spectroscopy have been demonstrated to complement imaging metrics from structural (gadolinium-enhanced) MRI in nine and six trials for MS disease-modifying therapies, respectively. Still, despite multiple demonstrations of the utility of molecular imaging probes to evaluate the target location and to elucidate the mechanisms of disease-modifying therapies for MS applications, their use has been sparse in both preclinical and clinical settings.

The long-term interaction of diet and dopamine D2 gene expression on brain microglial activation

Dopamine D2 receptors are expressed on microglial in the central nervous system and promote anti-inflammatory responses. Little work has been done on the interaction between the dopamine D2 receptors and diet on activated microglial expression in the brain. To assess this, the current study uses in vitro autoradiography to look at microglial activation in the brain as a marker for neuroinflammation. Mice with different levels of expression of the DA D2 gene were given a chronic diet of either normal diet chow or high fat diet chow for 30 weeks. Mice were then euthanized and their brains were processed for [³H]PK11195 autoradiography. Mice with reductions or lack of the D2 gene showed higher [³H]PK11195 binding in a diet-specific manner within somatosensory and striatal regions, as well as the piriform, frontal, insular, and entorhinal regions compared to mice with normal D2 gene levels. These brain regions are important for sensory processing, habit formation, as well as cognitive function tasks related to learning, motivation, and memory. These results suggest that decreased D2R levels may increase vulnerability to specific inflammatory markers. Future studies will need to examine the implications of these inflammatory changes on brain function and behavior.

Remimazolam Protects Against LPS-Induced Endotoxicity Improving Survival of Endotoxemia Mice

Remimazolam is a new benzodiazepine of sedative drugs with an ultra-short-acting anesthetic effect, commonly used for critically ill patients (especially septic patients) in intensive care units (ICUs). Although some anesthetics have been reported to show certain anti-inflammatory effects, the role of remimazolam in inflammation is still remained unknown. Here, we studied the effects of remimazolam on macrophage in response to LPS both in vivo and in vitro. Interestingly, compared with LPS treatment group, remimazolam remarkably improved survival rate of endotoxemia mice and decreased the release of LPS-induced inflammatory mediators (such as TNF-α, IL-6, and IL-1β). We further found that remimazolam not only inhibited the activation of MAPK signal pathway at 15 min after LPS treatment but also disturbed Rab5a related TLR4 expression at cell surface in response to LPS at a later time. Such evidence suggests that remimazolam might be beneficial to septic patients who are suffering from uncontrolled inflammatory responses.

TSPO imaging in animal models of brain diseases

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

Have (R)-[11C]PK11195 challengers fulfilled the promise? A scoping review of clinical TSPO PET studies

PURPOSE

The prototypical TSPO radiotracer (R)-[11C]PK11195 has been used in humans for more than thirty years to visualize neuroinflammation in several pathologies. Alternative radiotracers have been developed to improve signal-to-noise ratio and started to be tested clinically in 2008. Here we examined the scientific value of these "(R)-[11C]PK11195 challengers" in clinical research to determine if they could supersede (R)-[11C]PK11195.

METHODS

A systematic MEDLINE (PubMed) search was performed (up to end of year 2020) to extract publications reporting TSPO PET in patients with identified pathologies, excluding studies in healthy subjects and methodological studies.

RESULTS

Of the 288 publications selected, 152 used 13 challengers, and 142 used (R)-[11C]PK11195. Over the last 20 years, the number of (R)-[11C]PK11195 studies remained stable (6 ± 3 per year), but was surpassed by the total number of challenger studies for the last 6 years. In total, 3914 patients underwent a TSPO PET scan, and 47% (1851 patients) received (R)-[11C]PK11195. The 2 main challengers were [11C]PBR28 (24%-938 patients) and [18F]FEPPA (11%-429 patients). Only one-in-ten patients (11%-447) underwent 2 TSPO scans, among whom 40 (1%) were scanned with 2 different TSPO radiotracers.

CONCLUSIONS

Generally, challengers confirmed disease-specific initial (R)-[11C]PK11195 findings. However, while their better signal-to-noise ratio seems particularly useful in diseases with moderate and widespread neuroinflammation, most challengers present an allelic-dependent (Ala147Thr polymorphism) TSPO binding and genetic stratification is hindering their clinical implementation. As new challengers, insensitive to TSPO human polymorphism, are about to enter clinical evaluation, we propose this systematic review to be regularly updated (living review).

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

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

Thalamic neurometabolite alterations in patients with knee osteoarthritis before and after total knee replacement

ABSTRACT

The weak association between disability levels and "peripheral" (ie, knee) findings suggests that central nervous system alterations may contribute to the pathophysiology of knee osteoarthritis (KOA). Here, we evaluated brain metabolite alterations in patients with KOA, before and after total knee arthroplasty (TKA), using 1H-magnetic resonance spectroscopy (MRS). Thirty-four presurgical patients with KOA and 13 healthy controls were scanned using a PRESS sequence (TE = 30 ms, TR = 1.7 seconds, voxel size = 15 × 15 × 15 mm). In addition, 13 patients were rescanned 4.1 ± 1.6 (mean ± SD) weeks post-TKA. When using creatine (Cr)-normalized levels, presurgical KOA patients demonstrated lower N-acetylaspartate (NAA) (P < 0.001), higher myoinositol (mIns) (P < 0.001), and lower Choline (Cho) (P < 0.05) than healthy controls. The mIns levels were positively correlated with pain severity scores (r = 0.37, P < 0.05). These effects reached statistical significance also using water-referenced concentrations, except for the Cho group differences (P ≥ 0.067). Post-TKA patients demonstrated an increase in NAA (P < 0.01), which returned to the levels of healthy controls (P > 0.05), irrespective of metric. In addition, patients demonstrated postsurgical increases in Cr-normalized (P < 0.001), but not water-referenced mIns, which were proportional to the NAA/Cr increases (r = 0.61, P < 0.05). Because mIns is commonly regarded as a glial marker, our results are suggestive of a possible dual role for neuroinflammation in KOA pain and post-TKA recovery. Moreover, the apparent postsurgical normalization of NAA, a putative marker of neuronal integrity, might implicate mitochondrial dysfunction, rather than neurodegenerative processes, as a plausible pathophysiological mechanism in KOA. More broadly, our results add to a growing body of literature suggesting that some pain-related brain alterations can be reversed after peripheral surgical treatment.

Thalamic neuroinflammation as a reproducible and discriminating signature for chronic low back pain

ABSTRACT

Using positron emission tomography, we recently demonstrated elevated brain levels of the 18 kDa translocator protein (TSPO), a glial activation marker, in chronic low back pain (cLBP) patients, compared to healthy controls (HCs). Here, we first sought to replicate the original findings in an independent cohort (15 cLBP, 37.8 ± 12.5 y/o; 18 HC, 48.2 ± 12.8 y/o). We then trained random forest machine learning algorithms based on TSPO imaging features combining discovery and replication cohorts (totaling 25 cLBP, 42.4 ± 13.2 y/o; 27 HC, 48.9 ± 12.6 y/o), to explore whether image features other than the mean contain meaningful information that might contribute to the discrimination of cLBP patients and HC. Feature importance was ranked using SHapley Additive exPlanations values, and the classification performance (in terms of area under the curve values) of classifiers containing only the mean, other features, or all features was compared using the DeLong test. Both region-of-interest and voxelwise analyses replicated the original observation of thalamic TSPO signal elevations in cLBP patients compared to HC (P < 0.05). The random forest-based analyses revealed that although the mean is a discriminating feature, other features demonstrate similar level of importance, including the maximum, kurtosis, and entropy. Our observations suggest that thalamic neuroinflammatory signal is a reproducible and discriminating feature for cLBP, further supporting a role for glial activation in human cLBP, and the exploration of neuroinflammation as a therapeutic target for chronic pain. This work further shows that TSPO signal contains a richness of information that the simple mean might fail to capture completely.

Microglia Activation in Basal Ganglia Is a Late Event in Huntington Disease Pathophysiology

OBJECTIVE

To define the role played by microglia in different stages of Huntington disease (HD), we used the TSPO radioligand [11C]-ER176 and PET to evaluate microglial activation in relation to neurodegeneration and in relation to the clinical features seen at premanifest and manifest stages of the disease.

METHODS

This is a cross-sectional study in which 18 subjects (6 controls, 6 premanifest, and 6 manifest HD gene carriers) underwent a [11C]-ER176 PET scan and an MRI for anatomic localization. Segmentation of regions of interest (ROIs) was performed, and group differences in [11C]-ER176 binding (used to evaluate the extent of microglial activation) were assessed by the standardized uptake value ratio (SUVR). Microglial activation was correlated with ROIs volumes, disease burden, and the scores obtained in the clinical scales. As an exploratory aim, we evaluated the dynamic functions of microglia in vitro, by using induced microglia-like (iMG) cells from peripheral blood monocytes.

RESULTS

Individuals with manifest HD present higher [11C]-ER176 SUVR in both globi pallidi and putamina in comparison with controls. No differences were observed when we compared premanifest HD with controls or with manifest HD. We also found a significant correlation between increased microglial activation and cumulative disease burden, and with reduced volumes. iMG from controls, premanifest HD, and manifest HD patients showed similar phagocytic capacity.

CONCLUSIONS

Altogether, our data demonstrate that microglial activation is involved in HD pathophysiology and is associated with disease progression.

Dynamics of Cyclooxygenase-1 Positive Microglia/Macrophage in the Retina of Pathological Model Mice as a Biomarker of the Retinal Inflammatory Diseases

In an intraocular inflammatory state, microglia residing in the retina become active and migrate inside the retina. In this study, we investigated whether cyclooxygenase-1 (COX-1) expressed by retinal microglia/macrophage can be a biomarker for the diagnosis of retinal diseases. COX-1 was immunopositive in microglia/macrophage and neutrophils, while COX-2 was immunopositive in astrocytes and neurons in the inner layer of normal retina. The number of COX-1 positive cells per section of the retinal tissue was 14 ± 2.8 (mean ± standard deviation) in normal mice, which showed significant increase in the lipopolysaccharide (LPS)-administrated model (62 ± 5.0, p = 8.7 × 10⁻⁹). In addition to microglia, we found neutrophils that were positive for COX-1. In the early stage of inflammation in the experimental autoimmune uveoretinitis (EAU), COX-1 positive cells, infiltrating from the ciliary body into the retinal outer nuclear layer, were observed. The number of infiltrating COX-1 positive cells correlated with the severity of EAU. Taken together, the increased number of COX-1 positive microglia/macrophage with morphological changes were observed in the retinas of retinal inflammatory disease models. This suggests that COX-1 can be a marker of disease-related activities of microglia/macrophage, which should be useful for the diagnosis of retinal diseases.

Neuroimaging Correlates of Cognitive Dysfunction in Adults with Multiple Sclerosis

Cognitive impairment is a frequent and meaningful symptom in multiple sclerosis (MS), caused by the accrual of brain structural damage only partially counteracted by effective functional reorganization. As both these aspects can be successfully investigated through the application of advanced neuroimaging, here, we offer an up-to-date overview of the latest findings on structural, functional and metabolic correlates of cognitive impairment in adults with MS, focusing on the mechanisms sustaining damage accrual and on the identification of useful imaging markers of cognitive decline.

PET Agents in Dementia: An Overview

This article presents an overview of imaging agents for PET that have been applied for research and diagnostic purposes in patients affected by dementia. Classified by the target which the agents visualize, seven groups of tracers can be distinguished, namely radiopharmaceuticals for: (1) Misfolded proteins (ß-amyloid, tau, α-synuclein), (2) Neuroinflammation (overexpression of translocator protein), (3) Elements of the cholinergic system, (4) Elements of monoamine neurotransmitter systems, (5) Synaptic density, (6) Cerebral energy metabolism (glucose transport/ hexokinase), and (7) Various other proteins. This last category contains proteins involved in mechanisms underlying neuroinflammation or cognitive impairment, which may also be potential therapeutic targets. Many receptors belong to this category: AMPA, cannabinoid, colony stimulating factor 1, metabotropic glutamate receptor 1 and 5 (mGluR1, mGluR5), opioid (kappa, mu), purinergic (P2X7, P2Y12), sigma-1, sigma-2, receptor for advanced glycation endproducts, and triggering receptor expressed on myeloid cells-1, besides several enzymes: cyclooxygenase-1 and 2 (COX-1, COX-2), phosphodiesterase-5 and 10 (PDE5, PDE10), and tropomyosin receptor kinase. Significant advances in neuroimaging have been made in the last 15 years. The use of 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG) for quantification of regional cerebral glucose metabolism is well-established. Three tracers for ß-amyloid plaques have been approved by the Food and Drug Administration and European Medicines Agency. Several tracers for tau neurofibrillary tangles are already applied in clinical research. Since many novel agents are in the preclinical or experimental stage of development, further advances in nuclear medicine imaging can be expected in the near future. PET studies with established tracers and tracers for novel targets may result in early diagnosis and better classification of neurodegenerative disorders and in accurate monitoring of therapy trials which involve these targets. PET data have prognostic value and may be used to assess the response of the human brain to interventions, or to select the appropriate treatment strategy for an individual patient.

Investigation of the impact of preconditioning with lipopolysaccharide on inflammation-induced gene expression in the brain and depression-like behavior in male mice

Although several recent studies have suggested that neuroinflammation plays a role in depression, both medication and neuroinflammatory preventive strategies have been poorly investigated. Recent studies have indicated that preconditioning with lipopolysaccharide (LPS) reduces the damage that occurs following ischemic stroke and brain trauma. However, to date, the effects of LPS preconditioning on psychiatric symptoms have not been reported. Thus, we assessed gene expression and behavioral changes affected by preconditioning with low-dose (LD) LPS in male mice with systemic inflammation induced by administration of high-dose (HD) LPS. mRNA expression analyses of cytokine-, glial-, and oxidative stress-associated genes revealed that majority of these genes responded to HD LPS. Differential gene expression in the presence and absence of LD LPS preconditioning, identified a subset of genes that may contribute to the mechanism of LPS preconditioning in the brain. Notably, LPS preconditioning attenuated an increase in expression of the astrocyte marker Gfap caused by systemic inflammation, suggesting that astrocytes have a key role in endotoxin tolerance in the brain induced by LPS preconditioning. As increased astrocyte in the brain of patients with depression is suggested to contribute to the pathophysiology of major depression, LPS preconditioning might be applicable to the prevention and treatment of depression. Unfortunately, in this study, LPS preconditioning did not show a reversal effect on behavior decline due to high-dose LPS-induced systemic inflammation. Alternative aspects of behavioral changes should be assessed to identify behavioral components that are affected by LPS preconditioning. Nonetheless, the findings in the present study indicate the possibility of the mechanism of endotoxin tolerance induction in the brain via astrocyte regulation by LPS preconditioning. Since there has been reported pharmacological significance of astrocytes in psychiatric disorders, regulation of endotoxin tolerance might be a key method to control psychiatric symptoms.

High-Resolution Transcriptomic and Proteomic Profiling of Heterogeneity of Brain-Derived Microglia in Multiple Sclerosis

Microglia are important for central nervous system (CNS) homeostasis and first to respond to tissue damage and perturbations. Microglia are heterogeneous cells; in case of pathology, microglia adopt a range of phenotypes with altered functions. However, how these different microglia subtypes are implicated in CNS disease is largely unresolved. Multiple sclerosis (MS) is a chronic demyelinating disease of the CNS, characterized by inflammation and axonal degeneration, ultimately leading to neurological decline. One way microglia are implicated in MS is through stimulation of remyelination. They facilitate efficient remyelination by phagocytosis of myelin debris. In addition, microglia recruit oligodendrocyte precursor cells (OPCs) to demyelinated areas and stimulate remyelination. The development of high-resolution technologies to profile individual cells has greatly contributed to our understanding of microglia heterogeneity and function under normal and pathological conditions. Gene expression profiling technologies have evolved from whole tissue RNA sequencing toward single-cell or nucleus sequencing. Single microglia proteomic profiles are also increasingly generated, offering another layer of high-resolution data. Here, we will review recent studies that have employed these technologies in the context of MS and their respective advantages and disadvantages. Moreover, recent developments that allow for (single) cell profiling while retaining spatial information and tissue context will be discussed.

Brain Renin-Angiotensin System at the Intersect of Physical and Cognitive Frailty

The renin–angiotensin system (RAS) was initially considered to be part of the endocrine system regulating water and electrolyte balance, systemic vascular resistance, blood pressure, and cardiovascular homeostasis. It was later discovered that intracrine and local forms of RAS exist in the brain apart from the endocrine RAS. This brain-specific RAS plays essential roles in brain homeostasis by acting mainly through four angiotensin receptor subtypes; AT₁R, AT₂R, MasR, and AT₄R. These receptors have opposing effects; AT₁R promotes vasoconstriction, proliferation, inflammation, and oxidative stress while AT₂R and MasR counteract the effects of AT₁R. AT₄R is critical for dopamine and acetylcholine release and mediates learning and memory consolidation. Consequently, aging-associated dysregulation of the angiotensin receptor subtypes may lead to adverse clinical outcomes such as Alzheimer’s disease and frailty via excessive oxidative stress, neuroinflammation, endothelial dysfunction, microglial polarization, and alterations in neurotransmitter secretion. In this article, we review the brain RAS from this standpoint. After discussing the functions of individual brain RAS components and their intracellular and intracranial locations, we focus on the relationships among brain RAS, aging, frailty, and specific neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and vascular cognitive impairment, through oxidative stress, neuroinflammation, and vascular dysfunction. Finally, we discuss the effects of RAS-modulating drugs on the brain RAS and their use in novel treatment approaches.

Tracing the History of the Human Translocator Protein to Recent Neurodegenerative and Psychiatric Imaging

The human 18 kDa translocator protein (TSPO) has been widely used as a measure of glial activation in health and disease. With the continuous progress of radiotracers with increased affinity and selectivity, associations between TSPO expression, disease severity, and progression have been examined, particularly in neurodegenerative disorders such as multiple sclerosis (MS), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). However, findings in psychiatric disorders have prompted reassessment of the interpretation of regional TSPO expression differences in the brain, specifically with respect to potential neuroinflammatory components. This "mini" Review aims to guide readers through the complexity of TSPO imaging research by identifying the successes, challenges, and promising new directions of the field. We will provide a brief history of how TSPO imaging has evolved over the last three decades and present lessons learned in the context of neurodegenerative and psychiatric disorders.

Microglia, inflammation and gut microbiota responses in a progressive monkey model of Parkinson's disease: A case series

Inflammation has been linked to the development of nonmotor symptoms in Parkinson's disease (PD), which greatly impact patients' quality of life and can often precede motor symptoms. Suitable animal models are critical for our understanding of the mechanisms underlying disease and the associated prodromal disturbances. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkey model is commonly seen as a "gold standard" model that closely mimics the clinical motor symptoms and the nigrostriatal dopaminergic loss of PD, however MPTP toxicity extends to other nondopaminergic regions. Yet, there are limited reports monitoring the MPTP-induced progressive central and peripheral inflammation as well as other nonmotor symptoms such as gastrointestinal function and microbiota. We report 5 cases of progressive parkinsonism in non-human primates to gain a broader understanding of MPTP-induced central and peripheral inflammatory dysfunction to understand the potential role of inflammation in prodromal/pre-motor features of PD-like degeneration. We measured inflammatory proteins in plasma and CSF and performed [¹⁸F]FEPPA PET scans to evaluate translocator proteins (TSPO) or microglial activation. Monkeys were also evaluated for working memory and executive function using various behavior tasks and for gastrointestinal hyperpermeability and microbiota composition. Additionally, monkeys were treated with a novel TNF inhibitor XPro1595 (10 mg/kg, n = 3) or vehicle (n = 2) every three days starting 11 weeks after the initiation of MPTP to determine whether XPro1595 would alter inflammation and microglial behavior in a progressive model of PD. The case studies revealed that earlier and robust [¹⁸F]FEPPA PET signals resulted in earlier and more severe parkinsonism, which was seen in male cases compared to female cases. Potential other sex differences were observed in circulating inflammation, microbiota diversity and their metabolites. Additional studies with larger group sizes of both sexes would enable confirmation and extension of these findings. If these findings reflect potential differences in humans, these sex differences have significant implications for therapeutic development of inflammatory targets in the clinic.

A quantitative neuropathological assessment of translocator protein expression in multiple sclerosis

The 18 kDa translocator protein (TSPO) is increasingly used to study brain and spinal cord inflammation in degenerative diseases of the CNS such as multiple sclerosis. The enhanced TSPO PET signal that arises during disease is widely considered to reflect activated pathogenic microglia, although quantitative neuropathological data to support this interpretation have not been available. With the increasing interest in the role of chronic microglial activation in multiple sclerosis, characterising the cellular neuropathology associated with TSPO expression is of clear importance for understanding the cellular and pathological processes on which TSPO PET imaging is reporting. Here we have studied the cellular expression of TSPO and specific binding of two TSPO targeting radioligands (3H-PK11195 and 3H-PBR28) in tissue sections from 42 multiple sclerosis cases and 12 age-matched controls. Markers of homeostatic and reactive microglia, astrocytes, and lymphocytes were used to investigate the phenotypes of cells expressing TSPO. There was an approximate 20-fold increase in cells double positive for TSPO and HLA-DR in active lesions and in the rim of chronic active lesion, relative to normal appearing white matter. TSPO was uniformly expressed across myeloid cells irrespective of their phenotype, rather than being preferentially associated with pro-inflammatory microglia or macrophages. TSPO+ astrocytes were increased up to 7-fold compared to normal-appearing white matter across all lesion subtypes and accounted for 25% of the TSPO+ cells in these lesions. To relate TSPO protein expression to ligand binding, specific binding of the TSPO ligands 3H-PK11195 and 3H-PBR28 was determined in the same lesions. TSPO radioligand binding was increased up to seven times for 3H-PBR28 and up to two times for 3H-PK11195 in active lesions and the centre of chronic active lesions and a strong correlation was found between the radioligand binding signal for both tracers and the number of TSPO+ cells across all of the tissues examined. In summary, in multiple sclerosis, TSPO expression arises from microglia of different phenotypes, rather than being restricted to microglia which express classical pro-inflammatory markers. While the majority of cells expressing TSPO in active lesions or chronic active rims are microglia/macrophages, our findings also emphasize the significant contribution of activated astrocytes, as well as smaller contributions from endothelial cells. These observations establish a quantitative framework for interpretation of TSPO in multiple sclerosis and highlight the need for neuropathological characterization of TSPO expression for the interpretation of TSPO PET in other neurodegenerative disorders.

The P2X7 receptor tracer [11C]SMW139 as an in vivo marker of neuroinflammation in multiple sclerosis: a first-in man study

Purpose

The novel PET tracer [¹¹C]SMW139 binds with high affinity to the P2X₇ receptor, which is expressed on pro-inflammatory microglia. The purposes of this first in-man study were to characterise pharmacokinetics of [¹¹C]SMW139 in patients with active relapsing remitting multiple sclerosis (RRMS) and healthy controls (HC) and to evaluate its potential to identify in vivo neuroinflammation in RRMS.

Methods

Five RRMS patients and 5 age-matched HC underwent 90-min dynamic [¹¹C]SMW139 PET scans, with online continuous and manual arterial sampling to generate a metabolite-corrected arterial plasma input function. Tissue time activity curves were fitted to single- and two-tissue compartment models, and the model that provided the best fits was determined using the Akaike information criterion.

Results

The optimal model for describing [¹¹C]SMW139 kinetics in both RRMS and HC was a reversible two-tissue compartment model with blood volume parameter and with the dissociation rate k₄ fixed to the whole-brain value. Exploratory group level comparisons demonstrated an increased volume of distribution (V_T) and binding potential (BP_ND) in RRMS compared with HC in normal appearing brain regions. BP_ND in MS lesions was decreased compared with non-lesional white matter, and a further decrease was observed in gadolinium-enhancing lesions. In contrast, increased V_T was observed in enhancing lesions, possibly resulting from disruption of the blood-brain barrier in active MS lesions. In addition, there was a high correlation between parameters obtained from 60- to 90-min datasets, although analyses using 60-min data led to a slight underestimation in regional V_T and BP_ND values.

Conclusions

This first in-man study demonstrated that uptake of [¹¹C]SMW139 can be quantified with PET using BP_ND as a measure for specific binding in healthy controls and RRMS patients. Additional studies are warranted for further clinical evaluation of this novel neuroinflammation tracer.

Electronic supplementary material

The online version of this article (10.1007/s00259-019-04550-x) contains supplementary material, which is available to authorized users.

Cien Años de Microglía: Milestones in a Century of Microglial Research

The year 2019 marks the 100-year anniversary of the discovery of microglia by Pío del Río-Hortega. We will recount the state of neuroscience research at the beginning of the 20th century and the heated scientific dispute regarding microglial identity. We will then walk through some of the milestones of microglial research in the decades since then. In the last 20 years, the field has grown exponentially. Researchers have shown that microglia are unlike any other resident macrophages: they have a unique origin and distinguishing features. Microglia are extraordinarily motile cells and constantly survey their environment, interacting with neurons, astrocytes, oligodendrocytes, neural stem cells, and infiltrating immune cells. We finally highlight some open questions for future research regarding microglia's identity, population dynamics, and dual (beneficial and detrimental) role in pathology.

Chronic inflammation in multiple sclerosis - seeing what was always there

Activation of innate immune cells and other compartmentalized inflammatory cells in the brains and spinal cords of people with relapsing-remitting multiple sclerosis (MS) and progressive MS has been well described histopathologically. However, conventional clinical MRI is largely insensitive to this inflammatory activity. The past two decades have seen the introduction of quantitative dynamic MRI scanning with contrast agents that are sensitive to the reduction in blood-brain barrier integrity associated with inflammation and to the trafficking of inflammatory myeloid cells. New MRI imaging sequences provide improved contrast for better detection of grey matter lesions. Quantitative lesion volume measures and magnetic resonance susceptibility imaging are sensitive to the activity of macrophages in the rims of white matter lesions. PET and magnetic resonance spectroscopy methods can also be used to detect contributions from innate immune activation in the brain and spinal cord. Some of these advanced research imaging methods for visualization of chronic inflammation are practical for relatively routine clinical applications. Observations made with the use of these techniques suggest ways of stratifying patients with MS to improve their care. The imaging methods also provide new tools to support the development of therapies for chronic inflammation in MS.

Recent Developments in TSPO PET Imaging as A Biomarker of Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation is an inflammatory response in the brain and spinal cord, which can involve the activation of microglia and astrocytes. It is a common feature of many central nervous system disorders, including a range of neurodegenerative disorders. An overlap between activated microglia, pro-inflammatory cytokines and translocator protein (TSPO) ligand binding was shown in early animal studies of neurodegeneration. These findings have been translated in clinical studies, where increases in TSPO positron emission tomography (PET) signal occur in disease-relevant areas across a broad spectrum of neurodegenerative diseases. While this supports the use of TSPO PET as a biomarker to monitor response in clinical trials of novel neurodegenerative therapeutics, the clinical utility of current TSPO PET radioligands has been hampered by the lack of high affinity binding to a prevalent form of polymorphic TSPO (A147T) compared to wild type TSPO. This review details recent developments in exploration of ligand-sensitivity to A147T TSPO that have yielded ligands with improved clinical utility. In addition to developing a non-discriminating TSPO ligand, the final frontier of TSPO biomarker research requires developing an understanding of the cellular and functional interpretation of the TSPO PET signal. Recent insights resulting from single cell analysis of microglial phenotypes are reviewed.

PET Evaluation of Microglial Activation in Non-neurodegenerative Brain Diseases

PURPOSE OF THE REVIEW

Microglial cell activation is an important component of neuroinflammation, and it is generally well accepted that chronic microglial activation is indicative of accumulating tissue damage in neurodegenerative conditions, particularly in the earlier stages of disease. Until recently, there has been less focus on the role of neuroinflammation in other forms of neurological and neuropsychiatric conditions. Through this review, we hope to demonstrate the important role TSPO PET imaging has played in illuminating the pivotal role of neuroinflammation and microglial activation underpinning these conditions.

RECENT FINDINGS

TSPO is an 18 kDa protein found on the outer membrane of mitochondria and can act as a marker of microglial activation using nuclear imaging. Through the development of radiopharmaceuticals targeting TSPO, researchers have been able to better characterise the spatial-temporal evolution of chronic neurological conditions, ranging from the focal autoimmune reactions seen in multiple sclerosis to the Wallerian degeneration at remote parts of the brain months following acute cerebral infarction. Development of novel techniques to investigate neuroinflammation within the central nervous system, for the purposes of diagnosis and therapeutics, has flourished over the past few decades. TSPO has proven itself a robust and sensitive biomarker of microglial activation and neuroimaging affords a minimally invasive technique to characterise neuroinflammatory processes in vivo.

Imaging outcome measures of neuroprotection and repair in MS: A consensus statement from NAIMS

OBJECTIVE

To summarize current and emerging imaging techniques that can be used to assess neuroprotection and repair in multiple sclerosis (MS), and to provide a consensus opinion on the potential utility of each technique in clinical trial settings.

METHODS

Clinicians and scientists with expertise in the use of MRI in MS convened in Toronto, Canada, in November 2016 at a North American Imaging in Multiple Sclerosis (NAIMS) Cooperative workshop meeting. The discussion was compiled into a manuscript and circulated to all NAIMS members in attendance. Edits and feedback were incorporated until all authors were in agreement.

RESULTS

A wide spectrum of imaging techniques and analysis methods in the context of specific study designs were discussed, with a focus on the utility and limitations of applying each technique to assess neuroprotection and repair. Techniques were discussed under specific themes, and included conventional imaging, magnetization transfer ratio, diffusion tensor imaging, susceptibility-weighted imaging, imaging cortical lesions, magnetic resonance spectroscopy, PET, advanced diffusion imaging, sodium imaging, multimodal techniques, imaging of special regions, statistical considerations, and study design.

CONCLUSIONS

Imaging biomarkers of neuroprotection and repair are an unmet need in MS. There are a number of promising techniques with different strengths and limitations, and selection of a specific technique will depend on a number of factors, notably the question the trial seeks to answer. Ongoing collaborative efforts will enable further refinement and improved methods to image the effect of novel therapeutic agents that exert benefit in MS predominately through neuroprotective and reparative mechanisms.

Antibody-Based Tracers for PET/SPECT Imaging of Chronic Inflammatory Diseases

Chronic inflammatory diseases are often progressive, resulting not only in physical damage to patients but also social and economic burdens, making early diagnosis of them critical. Nuclear medicine techniques can enhance the detection of inflammation by providing functional as well as anatomical information when combined with other modalities such as magnetic resonance imaging, computed tomography or ultrasonography. Although small molecules and peptides were mainly used for the treatment and imaging of chronic inflammatory diseases in the past, antibodies and their fragments have also been emerging for chronic inflammatory diseases as they show high specificity to their targets and can have various biological half-lives depending on how they are engineered. In addition, imaging with antibodies or their fragments can visualize the in vivo biodistribution of the probes or help monitor therapeutic responses, thereby providing physicians with a greater understanding of drug behavior in vivo and another means of monitoring their patients. In this review, we introduce various targets and radiolabeled antibody-based probes for the molecular imaging of chronic inflammatory diseases in preclinical and clinical studies. Targets can be classified into three different categories: 1) cell-adhesion molecules, 2) surface markers on immune cells, and 3) cytokines or enzymes. The limitations and future directions of using radiolabeled antibodies for imaging inflammatory diseases are also discussed.

The TSPO Ligands 2-Cl-MGV-1, MGV-1, and PK11195 Differentially Suppress the Inflammatory Response of BV-2 Microglial Cell to LPS

The 18 kDa Translocator Protein (TSPO) is a marker for microglial activation as its expression is enhanced in activated microglia during neuroinflammation. TSPO ligands can attenuate neuroinflammation and neurotoxicity. In the present study, we examined the efficacy of new TSPO ligands designed by our laboratory, MGV-1 and 2-Cl-MGV-1, in mitigating an in vitro neuroinflammatory process compared to the classic TSPO ligand, PK 11195. We exposed BV-2 microglial cells to lipopolysaccharide (LPS) for 24 h to induce inflammatory response and added the three TSPO ligands: (1) one hour before LPS treatment (pretreatment), (2) simultaneously with LPS (cotreatment), and (3) one hour after LPS exposure (post-treatment). We evaluated the capability of TSPO ligands to reduce the levels of three glial inflammatory markers: cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and nitric oxide (NO). We compared the effects of the two novel ligands to PK 11195. Both 2-Cl-MGV-1 and MGV-1 reduced the levels of glial COX-2, iNOS, and NO in LPS-treated BV-2 cells more efficiently than PK 11195. Notably, even when added after exposure to LPS, all ligands were able to suppress the inflammatory response. Due to their pronounced anti-inflammatory activity, 2-Cl-MGV-1 and MGV-1 may serve as potential therapeutics in neuroinflammatory and neurodegenerative diseases.

Expression of Translocator Protein and [18F]-GE180 Ligand Uptake in Multiple Sclerosis Animal Models

Positron emission tomography (PET) ligands targeting the translocator protein (TSPO) represent promising tools to visualize neuroinflammation in multiple sclerosis (MS). Although it is known that TSPO is expressed in the outer mitochondria membrane, its cellular localization in the central nervous system under physiological and pathological conditions is not entirely clear. The purpose of this study was to assess the feasibility of utilizing PET imaging with the TSPO tracer, [18F]-GE180, to detect histopathological changes during experimental demyelination, and to determine which cell types express TSPO. C57BL/6 mice were fed with cuprizone for up to 5 weeks to induce demyelination. Groups of mice were investigated by [18F]-GE180 PET imaging at week 5. Recruitment of peripheral immune cells was triggered by combining cuprizone intoxication with MOG_35–55 immunization (i.e., Cup/EAE). Immunofluorescence double-labelling and transgene mice were used to determine which cell types express TSPO. [18F]-GE180-PET reliably detected the cuprizone-induced pathology in various white and grey matter regions, including the corpus callosum, cortex, hippocampus, thalamus and caudoputamen. Cuprizone-induced demyelination was paralleled by an increase in TSPO expression, glia activation and axonal injury. Most of the microglia and around one-third of the astrocytes expressed TSPO. TSPO expression induction was more severe in the white matter corpus callosum compared to the grey matter cortex. Although mitochondria accumulate at sites of focal axonal injury, these mitochondria do not express TSPO. In Cup/EAE mice, both microglia and recruited monocytes contribute to the TSPO expressing cell populations. These findings support the notion that TSPO is a valuable marker for the in vivo visualization and quantification of neuropathological changes in the MS brain. The pathological substrate of an increase in TSPO-ligand binding might be diverse including microglia activation, peripheral monocyte recruitment, or astrocytosis, but not axonal injury.

In vivo assessment of neuroinflammation in progressive multiple sclerosis: a proof of concept study with [18F]DPA714 PET

Background

Over the past decades, positron emission tomography (PET) imaging has become an increasingly useful research modality in the field of multiple sclerosis (MS) research, as PET can visualise molecular processes, such as neuroinflammation, in vivo. The second generation PET radioligand [¹⁸F]DPA714 binds with high affinity to the 18-kDa translocator-protein (TSPO), which is mainly expressed on activated microglia. The aim of this proof of concept study was to evaluate this in vivo marker of neuroinflammation in primary and secondary progressive MS.

Methods

All subjects were genotyped for the rs6971 polymorphism within the TSPO gene, and low-affinity binders were excluded from participation in this study. Eight patients with progressive MS and seven age and genetic binding status matched healthy controls underwent a 60 min dynamic PET scan using [¹⁸F]DPA714, including both continuous on-line and manual arterial blood sampling to obtain metabolite-corrected arterial plasma input functions.

Results

The optimal model for quantification of [¹⁸F]DPA714 kinetics was a reversible two-tissue compartment model with additional blood volume parameter. For genetic high-affinity binders, a clear increase in binding potential was observed in patients with MS compared with age-matched controls. For both high and medium affinity binders, a further increase in binding potential was observed in T2 white matter lesions compared with non-lesional white matter. Volume of distribution, however, did not differentiate patients from healthy controls, as the large non-displaceable compartment of [¹⁸F]DPA714 masks its relatively small specific signal.

Conclusion

The TSPO radioligand [¹⁸F]DPA714 can reliably identify increased focal and diffuse neuroinflammation in progressive MS when using plasma input-derived binding potential, but observed differences were predominantly visible in high-affinity binders.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1352-9) contains supplementary material, which is available to authorized users.

Chronic oral methylphenidate treatment increases microglial activation in rats

Methylphenidate (MP) is a widely prescribed psychostimulant used to treat attention deficit hyperactivity disorder. Previously, we established a drinking paradigm to deliver MP to rats at doses that result in pharmacokinetic profiles similar to treated patients. In the present study, adolescent male rats were assigned to one of three groups: control (water), low-dose MP (LD; 4/10 mg/kg), and high dose MP (HD; 30/60 mg/kg). Following 3 months of treatment, half of the rats in each group were euthanized, and the remaining rats received only water throughout a 1-month-long abstinence phase. In vitro autoradiography using [³H] PK 11195 was performed to measure microglial activation. HD MP rats showed increased [³H] PK 11195 binding compared to control rats in several cerebral cortical areas: primary somatosensory cortex including jaw (68.6%), upper lip (80.1%), barrel field (88.9%), and trunk (78%) regions, forelimb sensorimotor area (87.3%), secondary somatosensory cortex (72.5%), motor cortices 1 (73.2%) and 2 (69.3%), insular cortex (59.9%); as well as subcortical regions including the thalamus (62.9%), globus pallidus (79.4%) and substantia nigra (22.7%). Additionally, HD MP rats showed greater binding compared to LD MP rats in the hippocampus (60.6%), thalamus (59.6%), substantia nigra (38.5%), and motor 2 cortex (55.3%). Following abstinence, HD MP rats showed no significant differences compared to water controls; however, LD MP rats showed increased binding in pre-limbic cortex (78.1%) and ventromedial caudate putamen (113.8%). These findings indicate that chronic MP results in widespread microglial activation immediately after treatment and following the cessation of treatment in some brain regions.

TSPO in diverse CNS pathologies and psychiatric disease: A critical review and a way forward

The use of Translocator Protein 18 kDa (TSPO) as a clinical neuroimaging biomarker of brain injury and neuroinflammation has increased exponentially in the last decade. There has been a furious pace in the development of new radiotracers for TSPO positron emission tomography (PET) imaging and its use has now been extensively described in many neurological and mental disorders. This fast pace of research and the ever-increasing number of new laboratories entering the field often times lack an appreciation of the historical perspective of the field and introduce dogmatic, but unproven facts, related to the underlying neurobiology of the TSPO response to brain injury and neuroinflammation. Paradoxically, while in neurodegenerative disorders and in all types of CNS pathologies brain TSPO levels increase, a new observation in psychiatric disorders such as schizophrenia is decreased brain levels of TSPO measured by PET. The neurobiological bases for this new finding is currently not known, but rigorous experimental design using multiple experimental approaches and careful interpretation of results is critically important to provide the methodological and/or biological underpinnings to this new observation. This review provides a perspective of the early history of validating TSPO as a biomarker of brain injury and neuroinflammation and a critical analysis of controversial topics in the literature related to the cellular sources of the TSPO response. The latter is important in order to provide the correct interpretation of PET studies in neurodegenerative and psychiatric disorders. Furthermore, this review proposes some yet to be explored explanations to new findings in psychiatric disorders and new approaches to quantitatively assess the glial sources of the TSPO response in order to move the field forward.

Exploring the heterogeneity of MS lesions using positron emission tomography: a reappraisal of their contribution to disability

The biological mechanisms driving disability worsening in multiple sclerosis (MS) are only partly understood. Monitoring changes in lesion load on MRI has a limited predictive value on the progression of clinical disability, and there is an essential need for novel imaging markers specific for the main candidate mechanisms underlying neurodegeneration which include failing myelin repair, innate immune cell activation and gray matter neuronal damage. Positron Emission Tomography (PET) is an imaging technology based on the injection of radiotracers directed against specific molecular targets, which has recently allowed the selective quantification in-vivo of the key biological mechanisms relevant to MS pathophysiology. Pilot PET studies performed in patients with all forms of MS allowed to revisit the contribution of MS lesions to disability worsening and showed that the evolution of lesions toward chronic activation, together with their remyelination profile were relevant predictors of disability worsening. PET offers the opportunity to bridge a critical gap between neuropathology and in-vivo imaging. This technique provides an original approach to disentangle some of the most relevant pathological components driving MS progression, to follow-up their temporal evolution, to investigate their clinical relevance and to evaluate novel therapeutics aimed to prevent disease progression.

(Micro)Glia as Effectors of Cortical Volume Loss in Schizophrenia

Contrary to the notion that neurology but not psychiatry is the domain of disorders evincing structural brain alterations, it is now clear that there are subtle but consistent neuropathological changes in schizophrenia. These range from increases in ventricular size to dystrophic changes in dendritic spines. A decrease in dendritic spine density in the prefrontal cortex (PFC) is among the most replicated of postmortem structural findings in schizophrenia. Examination of the mechanisms that account for the loss of dendritic spines has in large part focused on genes and molecules that regulate neuronal structure. But the simple question of what is the effector of spine loss, ie, where do the lost spines go, is unanswered. Recent data on glial cells suggest that microglia (MG), and perhaps astrocytes, play an important physiological role in synaptic remodeling of neurons during development. Synapses are added to the dendrites of pyramidal cells during the maturation of these neurons; excess synapses are subsequently phagocytosed by MG. In the PFC, this occurs during adolescence, when certain symptoms of schizophrenia emerge. This brief review discusses recent advances in our understanding of MG function and how these non-neuronal cells lead to structural changes in neurons in schizophrenia.

Evaluation of Microglial Activation in Multiple Sclerosis Patients Using Positron Emission Tomography

Understanding the mechanisms underlying progression in multiple sclerosis (MS) is one of the key elements contributing to the identification of appropriate therapeutic targets for this under-managed condition. In addition to plaque-related focal inflammatory pathology typical for relapsing remitting MS there are, in progressive MS, widespread diffuse alterations in brain areas outside the focal lesions. This diffuse pathology is tightly related to microglial activation and is co-localized with signs of neurodegeneration. Microglia are brain-resident cells of the innate immune system and overactivation of microglia is associated with several neurodegenerative diseases. Understanding the role of microglial activation in relation to developing neurodegeneration and disease progression may provide a key to developing therapies to target progressive MS. 18-kDa translocator protein (TSPO) is a mitochondrial molecule upregulated in microglia upon their activation. Positron emission tomography (PET) imaging using TSPO-binding radioligands provides a method to assess microglial activation in patients in vivo. In this mini-review, we summarize the current status of TSPO imaging in the field of MS. In addition, the review discusses new insights into the potential use of this method in treatment trials and in clinical assessment of progressive MS.

Development of a PET radioligand for potassium channels to image CNS demyelination

Central nervous system (CNS) demyelination represents the pathological hallmark of multiple sclerosis (MS) and contributes to other neurological conditions. Quantitative and specific imaging of demyelination would thus provide critical clinical insight. Here, we investigated the possibility of targeting axonal potassium channels to image demyelination by positron emission tomography (PET). These channels, which normally reside beneath the myelin sheath, become exposed upon demyelination and are the target of the MS drug, 4-aminopyridine (4-AP). We demonstrate using autoradiography that 4-AP has higher binding in non-myelinated and demyelinated versus well-myelinated CNS regions, and describe a fluorine-containing derivative, 3-F-4-AP, that has similar pharmacological properties and can be labeled with ¹⁸F for PET imaging. Additionally, we demonstrate that [¹⁸F]3-F-4-AP can be used to detect demyelination in rodents by PET. Further evaluation in Rhesus macaques shows higher binding in non-myelinated versus myelinated areas and excellent properties for brain imaging. Together, these data indicate that [¹⁸F]3-F-4-AP may be a valuable PET tracer for detecting CNS demyelination noninvasively.

Novel MRI and PET markers of neuroinflammation in multiple sclerosis

PURPOSE OF REVIEW

Gadolinium-enhancement depicts blood-brain barrier disruption associated with new inflammatory MRI lesions in multiple sclerosis (MS) and is widely used for diagnosis and therapeutic monitoring. However, earlier and more specific markers of inflammation are urgently needed.

RECENT FINDINGS

Susceptibility-weighted images demonstrate the importance of the central vein in the formation of MS lesions. Perfusion weighted imaging techniques can show focal and diffuse low-grade inflammatory changes not visible on conventional MRI. Leptomeningeal enhancement could be part of the aetiology of subpial cortical MS lesions. Ultrasmall superparamagnetic particles of iron oxide can identify neuroinflammatory changes in addition to gadolinium enhancement and as such identify different types and phases of MS lesions. 18kD-translocator protein PET tracers identify activated microglia and an increase in TSPO uptake in both MS lesions and normal appearing brain tissue is related to disease severity and progression. A range of novel tracers for microglia activation is under development as well as radioligands that can label therapeutic drugs.

SUMMARY

Novel MRI and PET techniques improve in-vivo visualization and quantification of the pleomorphic aspects of neuroinflammation, providing us with a unique insight in its pathogenesis, clinical relevance, and therapy responsiveness in MS.

Alzheimer's Disease: The Role of Microglia in Brain Homeostasis and Proteopathy

Brain aging is central to late-onset Alzheimer's disease (LOAD), although the mechanisms by which it occurs at protein or cellular levels are not fully understood. Alzheimer's disease is the most common proteopathy and is characterized by two unique pathologies: senile plaques and neurofibrillary tangles, the former accumulating earlier than the latter. Aging alters the proteostasis of amyloid-β peptides and microtubule-associated protein tau, which are regulated in both autonomous and non-autonomous manners. Microglia, the resident phagocytes of the central nervous system, play a major role in the non-autonomous clearance of protein aggregates. Their function is significantly altered by aging and neurodegeneration. This is genetically supported by the association of microglia-specific genes, TREM2 and CD33, and late onset Alzheimer's disease. Here, we propose that the functional characterization of microglia, and their contribution to proteopathy, will lead to a new therapeutic direction in Alzheimer's disease research.

TSPO imaging using the novel PET ligand [18F]GE-180: quantification approaches in patients with multiple sclerosis

Background

PET ligands targeting the translocator protein (TSPO) represent promising tools to visualise neuroinflammation. Here, we analysed parameters obtained in dynamic and static PET images using the novel TSPO ligand [¹⁸F]GE-180 in patients with relapsing remitting multiple sclerosis (RRMS) and an approach for semi-quantitative assessment of this disease in clinical routine.

Seventeen dynamic [¹⁸F]GE-180 PET scans of RRMS patients were evaluated (90 min). A pseudo-reference region (PRR) was defined after identification of the least disease-affected brain area by voxel-based comparison with six healthy controls (HC) and upon exclusion of voxels suspected of being affected in static 60–90 min p.i. images. Standardised uptake value ratios (SUVR) obtained from static images normalised to PRR were correlated to the distribution volume ratios (DVR) derived from dynamic data with Logan reference tissue model.

Results

Group comparison with HC revealed white matter and thalamus as most affected regions. Fewest differences were found in grey matter, and normalisation to frontal cortex (FC) yielded the greatest reduction in variability of healthy grey and white matter. Hence, FC corrected for affected voxels was chosen as PRR, leading to time-activity curves of FC which were congruent to HC data (SUV_60–90 0.37, U test P = 0.42). SUVR showed a very strong correlation with DVR (Pearson ρ > 0.9). Focal MS lesions exhibited a high SUVR (range, 1.3–3.2).

Conclusions

This comparison with parameters from dynamic data suggests that SUVR normalised to corrected frontal cortex as PRR is suitable for the quantification of [¹⁸F]GE-180 uptake in lesions and different brain regions of RRMS patients. This efficient diagnostic protocol based on static [¹⁸F]GE-180 PET scans acquired 60–90 min p.i. allows the semi-quantitative assessment of neuroinflammation in RRMS patients in clinical routine.

The macrophage marker translocator protein (TSPO) is down-regulated on pro-inflammatory 'M1' human macrophages

The translocator protein (TSPO) is a mitochondrial membrane protein, of as yet uncertain function. Its purported high expression on activated macrophages, has lent utility to TSPO targeted molecular imaging in the form of positron emission tomography (PET), as a means to detect and quantify inflammation in vivo. However, existing literature regarding TSPO expression on human activated macrophages is lacking, mostly deriving from brain tissue studies, including studies of brain malignancy, and inflammatory diseases such as multiple sclerosis. Here, we utilized three human sources of monocyte derived macrophages (MDM), from THP-1 monocytes, healthy peripheral blood monocytes and synovial fluid monocytes from patients with rheumatoid arthritis, to undertake a detailed investigation of TSPO expression in activated macrophages. In this work, we demonstrate a consistent down-regulation of TSPO mRNA and protein in macrophages activated to a pro-inflammatory, or ‘M1’ phenotype. Conversely, stimulation of macrophages to an M2 phenotype with IL-4, dexamethasone or TGF-β1 did not alter TSPO expression, regardless of MDM source. The reasons for this are uncertain, but our study findings add some supporting evidence for recent investigations concluding that TSPO may be involved in negative regulation of inflammatory responses in macrophages.

The Role of Advanced Magnetic Resonance Imaging Techniques in Multiple Sclerosis Clinical Trials

Magnetic resonance imaging has been crucial in the development of anti-inflammatory disease-modifying treatments. The current landscape of multiple sclerosis clinical trials is currently expanding to include testing not only of anti-inflammatory agents, but also neuroprotective, remyelinating, neuromodulating, and restorative therapies. This is especially true of therapies targeting progressive forms of the disease where neurodegeneration is a prominent feature. Imaging techniques of the brain and spinal cord have rapidly evolved in the last decade to permit in vivo characterization of tissue microstructural changes, connectivity, metabolic changes, neuronal loss, glial activity, and demyelination. Advanced magnetic resonance imaging techniques hold significant promise for accelerating the development of different treatment modalities targeting a variety of pathways in MS.

The translocator protein ligand XBD173 improves clinical symptoms and neuropathological markers in the SJL/J mouse model of multiple sclerosis

Multiple sclerosis (MS) is a severe autoimmune disease characterized by inflammatory, demyelinating and neurodegenerative components causing motor, sensory, visual and/or cognitive symptoms. The relapsing-remitting MS affecting 85% of patients is reliably mimicked by the proteolipid-protein (PLP)-induced experimental autoimmune encephalomyelitis (EAE) SJL/J-mouse model. Significant progress was made for MS treatment but the development of effective therapies devoid of severe side-effects remains a great challenge. Here, we combine clinical, behavioral, histopathological, biochemical and molecular approaches to demonstrate that low and well tolerated doses (10-20mg/kg) of TSPO ligand XBD173 (Emapunil) efficiently ameliorate clinical signs and neuropathology of PLP-EAE mice. In addition to the conventional clinical scoring of symptoms, we applied the robust behavioral Catwalk-method to confirm that XBD173 (10mg/kg) increases the maximum contact area parameter at EAE-disease peak, indicating an improvement/recovery of motor functions. Consistently, histopathological studies coupled with microscope-cellSens quantification and RT-qPCR analyzes showed that XBD173 prevented demyelination by restoring normal protein and mRNA levels of myelin basic protein that was significantly repressed in PLP-EAE mice spinal cord and brain. Interestingly, ELISA-based measurement revealed that XBD173 increased allopregnanolone concentrations in PLP-EAE mice spinal and brain tissues. Furthermore, flow cytometry assessment demonstrated that XBD173 therapy decreased serum level of pro-inflammatory cytokines, including interleukin-17A, Interleukin-6 and tumor-necrosis-factor alpha in PLP-EAE mice. As the optimal XBD173 dosing exerting the maximal beneficial action in EAE mice is the lower 10mg/kg dose, the paper opens interesting perspectives for the development of efficient and safe therapies against MS with slight or no side-effects.

Bridging Pharmaceutical Chemistry with Drug and Nanoparticle Targeting to Investigate the Role of the 18-kDa Translocator Protein TSPO

An interesting mitochondrial biomarker is the 18-kDa mitochondrial translocator protein (TSPO). Decades of study have shown that this protein plays an important role in a wide range of cellular functions, including opening of the mitochondrial permeability transition pore as well as programmed cell death and proliferation. Variations in TSPO expression have been correlated to different diseases, from tumors to endocrine and neurological disorders. TSPO has therefore become an appealing target for both early diagnosis and selective mitochondrial drug delivery. The number of structurally different TSPO ligands examined has increased over time, highlighting the scientific community's growing understanding of the roles of TSPO in normal and pathological conditions. However, only few TSPO ligands are characterized by the presence of groups that are potentially derivatizable; therefore only few such ligands are well suited for the preparation of targeted prodrugs or nanocarriers able to deliver therapeutics and/or diagnostic agents to mitochondria. This review provides an overview of the very few examples of drug delivery systems characterized by moieties that target TSPO.

Increased White Matter Inflammation in Aging- and Alzheimer's Disease Brain

Chronic neuroinflammation, which is primarily mediated by microglia, plays an essential role in aging and neurodegeneration. It is still unclear whether this microglia-induced neuroinflammation occurs globally or is confined to distinct brain regions. In this study, we investigated microglia activity in various brain regions upon healthy aging and Alzheimer's disease (AD)-related pathology in both human and mouse samples. In purified microglia isolated from aging mouse brains, we found a profound gene expression pattern related to pro-inflammatory processes, phagocytosis, and lipid homeostasis. Particularly in white matter microglia of 24-month-old mice, abundant expression of phagocytic markers including Mac-2, Axl, CD16/32, Dectin1, CD11c, and CD36 was detected. Interestingly, in white matter of human brain tissue the first signs of inflammatory activity were already detected during middle age. Thus quantification of microglial proteins, such as CD68 (commonly associated with phagocytosis) and HLA-DR (associated with antigen presentation), in postmortem human white matter brain tissue showed an age-dependent increase in immunoreactivity already in middle-aged people (53.2 ± 2.0 years). This early inflammation was also detectable by non-invasive positron emission tomography imaging using [¹¹C]-(R)-PK11195, a ligand that binds to activated microglia. Increased microglia activity was also prominently present in the white matter of human postmortem early-onset AD (EOAD) brain tissue. Interestingly, microglia activity in the white matter of late-onset AD (LOAD) CNS was similar to that of the aged clinically silent AD cases. These data indicate that microglia-induced neuroinflammation is predominant in the white matter of aging mice and humans as well as in EOAD brains. This white matter inflammation may contribute to the progression of neurodegeneration, and have prognostic value for detecting the onset and progression of aging and neurodegeneration.