In vivo (R)-[(11)C]PK11195 PET imaging of 18kDa translocator protein in recent onset psychosis

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

Comprehensive review of multidimensional biomarkers in the ShangHai At Risk for Psychosis (SHARP) program for early psychosis identification

Psychosis is recognized as one of the largest contributors to nonfatal health loss, and early identification can largely improve routine clinical activity by predicting the psychotic course and guiding treatment. Clinicians have used the clinical high-risk for psychosis (CHR) paradigm to better understand the risk factors that contribute to the onset of psychotic disorders. Clinical factors have been widely applied to calculate the individualized risks for conversion to psychosis 1-2 years later. However, there is still a dearth of valid biomarkers to predict psychosis. Biomarkers, in the context of this paper, refer to measurable biological indicators that can provide valuable information about the early identification of individuals at risk for psychosis. The aim of this paper is to critically review studies assessing CHR and suggest possible biomarkers for application of prediction. We summarized the studies on biomarkers derived from the findings of the ShangHai at Risk for Psychosis (SHARP) program, including those that are considered to have the most potential. This comprehensive review was conducted based on expert opinions within the SHARP research team, and the selection of studies and results presented in this paper reflects the collective expertise of the team in the field of early psychosis identification. The three dimensions with potential candidates include neuroimaging dimension of brain structure and function, electrophysiological dimension of event-related potentials (ERPs), and immune dimension of inflammatory cytokines and complement proteins, which proved to be useful in supporting the prediction of psychosis from the CHR state. We suggest that these three dimensions could be useful as risk biomarkers for treatment optimization. In the future, when available for the integration of multiple dimensions, clinicians may be able to obtain a comprehensive report with detailed information of psychosis risk and specific indications about preferred prevention.

Microglia and Other Cellular Mediators of Immunological Dysfunction in Schizophrenia: A Narrative Synthesis of Clinical Findings

Schizophrenia is a complex psychiatric condition that may involve immune system dysregulation. Since most putative disease mechanisms in schizophrenia have been derived from genetic association studies and fluid-based molecular analyses, this review aims to summarize the emerging evidence on clinical correlates to immune system dysfunction in this psychiatric disorder. We conclude this review by attempting to develop a unifying hypothesis regarding the relative contributions of microglia and various immune cell populations to the development of schizophrenia. This may provide important translational insights that can become useful for addressing the multifaceted clinical presentation of schizophrenia.

A consideration of the increased risk of schizophrenia due to prenatal maternal stress, and the possible role of microglia

Schizophrenia is caused by interaction of a combination of genetic and environmental factors. Of the latter, prenatal exposure to maternal stress is reportedly associated with elevated disease risk. The main orchestrators of inflammatory processes within the brain are microglia, and aberrant microglial activation/function has been proposed to contribute to the aetiology of schizophrenia. Here, we evaluate the epidemiological and preclinical evidence connecting prenatal stress to schizophrenia risk, and consider the possible mediating role of microglia in the prenatal stress-schizophrenia relationship. Epidemiological findings are rather consistent in supporting the association, albeit they are mitigated by effects of sex and gestational timing, while the evidence for microglial activation is more variable. Rodent models of prenatal stress generally report lasting effects on offspring neurobiology. However, many uncertainties remain as to the mechanisms underlying the influence of maternal stress on the developing foetal brain. Future studies should aim to characterise the exact processes mediating this aspect of schizophrenia risk, as well as focussing on how prenatal stress may interact with other risk factors.

New insight in the cross-talk between microglia and schizophrenia: From the perspective of neurodevelopment

Characterized by psychotic symptoms, negative symptoms and cognitive deficits, schizophrenia had a catastrophic effect on patients and their families. Multifaceted reliable evidence indicated that schizophrenia is a neurodevelopmental disorder. Microglia, the immune cells in central nervous system, related to many neurodevelopmental diseases. Microglia could affect neuronal survival, neuronal death and synaptic plasticity during neurodevelopment. Anomalous microglia during neurodevelopment may be associated with schizophrenia. Therefore, a hypothesis proposes that the abnormal function of microglia leads to the occurrence of schizophrenia. Nowadays, accumulating experiments between microglia and schizophrenia could afford unparalleled probability to assess this hypothesis. Herein, this review summarizes the latest supporting evidence in order to shed light on the mystery of microglia in schizophrenia.

Microglia sequelae: brain signature of innate immunity in schizophrenia

Schizophrenia is a psychiatric disorder with significant impact on individuals and society. The current pharmacologic treatment, which principally alleviates psychosis, is focused on neurotransmitters modulation, relying on drugs with severe side effects and ineffectiveness in a significant percentage of cases. Therefore, and due to difficulties inherent to diagnosis and treatment, it is vital to reassess alternative cellular and molecular drug targets. Distinct risk factors - genetic, developmental, epigenetic, and environmental - have been associated with disease onset and progression, giving rise to the proposal of different pathophysiological mechanisms and putative pharmacological targets. Immunity is involved and, particularly microglia - innate immune cells of the central nervous system, critically involved in brain development - have captured attention as cellular players. Microglia undergo marked morphologic and functional alterations in the human disease, as well as in animal models of schizophrenia, as reported in several original papers. We cluster the main findings of clinical studies by groups of patients: (1) at ultra-high risk of psychosis, (2) with a first episode of psychosis or recent-onset schizophrenia, and (3) with chronic schizophrenia; in translational studies, we highlight the time window of appearance of particular microglia alterations in the most well studied animal model in the field (maternal immune activation). The organization of clinical and translational findings based on schizophrenia-associated microglia changes in different phases of the disease course may help defining a temporal pattern of microglia changes and may drive the design of novel therapeutic strategies.

Essential Principles and Recent Progress in the Development of TSPO PET Ligands for Neuroinflammation Imaging

The translocator protein 18kDa (TSPO) is expressed in the outer mitochondrial membrane and is implicated in several functions, including cholesterol transport and steroidogenesis. Under normal physiological conditions, TSPO is present in very low concentrations in the human brain but is markedly upregulated in response to brain injury and inflammation. This upregulation is strongly associated with activated microglia. Therefore, TSPO is particularly suited for assessing active gliosis associated with brain lesions following injury or disease. For over three decades, TSPO has been studied as a biomarker. Numerous radioligands for positron emission tomography (PET) that target TSPO have been developed for imaging inflammatory progression in the brain. Although [11C]PK11195, the prototypical first-generation PET radioligand, is still widely used for in vivo studies, mainly now as its single more potent R-enantiomer, it has severe limitations, including low sensitivity and poor amenability to quantification. Second-generation radioligands are characterized by higher TSPO specific signals but suffer from other drawbacks, such as sensitivity to the TSPO single nucleotide polymorphism (SNP) rs6971. Therefore, their applications in human studies have the burden of needing to genotype subjects. Consequently, recent efforts are focused on developing improved radioligands that combine the optimal features of the second generation with the ability to overcome the differences in binding affinities across the population. This review presents essential principles in the design and development of TSPO PET ligands and discusses prominent examples among the main chemotypes.

New concepts in the development of schizophrenia, autism spectrum disorders, and degenerative brain diseases based on chronic inflammation: A working hypothesis from continued advances in neuroscience research

This paper was written prompted by a poignant film about adolescent girl with schizophrenia who babysits for a younger girl in an isolated cabin. Schizophrenia is an illness that both authors are fascinated with and that they continue to study and investigate. There is now compelling evidence that schizophrenia is a very complex syndrome that involves numerous neural pathways in the brain, far more than just dopaminergic and serotonergic systems. One of the more popular theories in recent literature is that it represents a hypo glutaminergic deficiency of certain pathways, including thalamic ones. After much review of research and study in this area, we have concluded that most such theories contain a number of shortcomings. Most are based on clinical responses to certain drugs, particularly antipsychotic drugs affecting the dopaminergic neurotransmitters; thus, assuming dopamine release was the central cause of the psychotic symptoms of schizophrenia. The theory was limited in that dopamine excess could only explain the positive symptoms of the disorder. Antipsychotic medications have minimal effectiveness for the negative and cognitive symptoms associated with schizophrenia. It has been estimated that 20–30% of patients show either a partial or no response to antipsychotic medications. In addition, the dopamine hypothesis does not explain the neuroanatomic findings in schizophrenia.

Applicability, potential and limitations of TSPO PET imaging as a clinical immunopsychiatry biomarker

PURPOSE

TSPO PET imaging may hold promise as a single-step diagnostic work-up for clinical immunopsychiatry. This review paper on the clinical applicability of TSPO PET for primary psychiatric disorders discusses if and why TSPO PET imaging might become the first clinical immunopsychiatry biomarker and the investment prerequisites and scientific advancements needed to accommodate this transition from bench to bedside.

METHODS

We conducted a systematic search of the literature to identify clinical studies of TSPO PET imaging in patients with primary psychiatric disorders. We included both original case-control studies as well as longitudinal cohort studies of patients with a primary psychiatric diagnosis.

RESULTS

Thirty-one original studies met our inclusion criteria. In the field of immunopsychiatry, TSPO PET has until now mostly been studied in schizophrenia and related psychotic disorders, and to a lesser extent in mood disorders and neurodevelopmental disorders. Quantitative TSPO PET appears most promising as a predictive biomarker for the transdiagnostic identification of subgroups or disease stages that could benefit from immunological treatments, or as a prognostic biomarker forecasting patients' illness course. Current scanning protocols are still too unreliable, impractical and invasive for clinical use in symptomatic psychiatric patients.

CONCLUSION

TSPO PET imaging in its present form does not yet offer a sufficiently attractive cost-benefit ratio to become a clinical immunopsychiatry biomarker. Its translation to psychiatric clinical practice will depend on the prioritising of longitudinal research and the establishment of a uniform protocol rendering clinically meaningful TSPO uptake quantification at the shortest possible scan duration without arterial cannulation.

Supervised clustering for TSPO PET imaging

PURPOSE

This technical note seeks to act as a practical guide for implementing a supervised clustering algorithm (SVCA) reference region approach and to explain the main strengths and limitations of the technique in the context of 18-kilodalton translocator protein (TSPO) positron emission tomography (PET) studies in experimental medicine.

BACKGROUND

TSPO PET is the most widely used imaging technique for studying neuroinflammation in vivo in humans. Quantifying neuroinflammation with PET can be a challenging and invasive procedure, especially in frail patients, because it often requires blood sampling from an arterial catheter. A widely used alternative to arterial sampling is SVCA, which identifies the voxels with minimal specific binding in the PET images, thus extracting a pseudo-reference region for non-invasive quantification. Unlike other reference region approaches, SVCA does not require specification of an anatomical reference region a priori, which alleviates the limitation of TSPO contamination in anatomically-defined reference regions in individuals with underlying inflammatory processes. Furthermore, SVCA can be applied to any TSPO PET tracer across different neurological and neuropsychiatric conditions, providing noninvasivequantification of TSPO expression.

METHODS

We provide an overview of the development of SVCA as well as step-by-step instructions for implementing SVCA with suggestions for specific settings. We review the literature on SVCAapplications using first- and second- generation TSPO PET tracers and discuss potential clinically relevant limitations and applications.

CONCLUSIONS

The correct implementation of SVCA can provide robust and reproducible estimates of brain TSPO expression. This review encourages the standardisation of SVCA methodology in TSPO PET analysis, ultimately aiming to improve replicability and comparability across study sites.

Specific and non-specific binding of a tracer for the translocator-specific protein in schizophrenia: an [11C]-PBR28 blocking study

OBJECTIVE

The mitochondrial 18-kDa translocator protein (TSPO) is expressed by activated microglia and positron emission tomography enables the measurement of TSPO levels in the brain. Findings in schizophrenia have shown to vary depending on the outcome measure used and this discrepancy in TSPO results could be explained by lower non-displaceable binding (V_ND) in schizophrenia, which could obscure increases in specific binding. In this study, we have used the TSPO ligand XBD173 to block the TSPO radioligand [¹¹C]-PBR28 and used an occupancy plot to quantify V_ND in patients with schizophrenia.

METHODS

A total of 7 patients with a diagnosis of schizophrenia were recruited for this study. Each patient received two separate PET scans with [¹¹C]PBR28, one at baseline and one after the administration of the TSPO ligand XBD173. All patients were high-affinity binders (HABs) for the TSPO gene. We used an occupancy plot to quantify the non-displaceable component (V_ND) using 2TCM kinetic estimates with and without vascular correction. Finally we computed the V_ND at a single subject level using the SIME method.

RESULTS

All patients showed a global and generalized reduction in [¹¹C]PBR28 uptake after the administration of XBD173. Constraining the V_ND to be equal for all patients, the population V_ND was estimated to be 1.99 mL/cm³ (95% CI 1.90 to 2.08). When we used vascular correction, the fractional TSPO occupancy remained similar.

CONCLUSIONS

In schizophrenia patients, a substantial component of the [¹¹C]PBR28 signal represents specific binding to TSPO. Furthermore, the V_ND in patients with schizophrenia is similar to that previously reported in healthy controls. These results suggest that changes in non-specific binding between schizophrenia patients and healthy controls do not account for discrepant PET findings in this disorder.

Anti-inflammatory Combination Therapy for the Treatment of Schizophrenia

BACKGROUND

Despite adequate antipsychotic treatment, most people with schizophrenia continue to exhibit persistent positive and negative symptoms and cognitive impairments. The current study was designed to examine the efficacy and safety of adjunctive anti-inflammatory combination therapy for these illness manifestations.

METHODS

Thirty-nine people with either Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision, schizophrenia or schizoaffective disorder were entered into a 12-week double-blind, 2-arm, triple-dummy, placebo-controlled, randomized clinical trial: 19 were randomized to anti-inflammatory combination therapy and 20 were randomized to placebo. The Brief Psychiatric Rating Scale positive symptom item total score was used to assess positive symptom change, the Scale for the Assessment of Negative Symptoms total score was used to assess negative symptom change, the Calgary Depression Scale total score was used to assess depressive symptom change, and the MATRICS Consensus Cognitive Battery was used to assess neuropsychological test performance.

RESULTS

There was a significant time effect for Brief Psychiatric Rating Scale positive symptom item score (t226 = -2.66, P = 0.008), but the treatment (t54=1.52, P = 0.13) and treatment × time (t223 = 0.47, P = 0.64) effects were not significant. There were no significant time (t144 = 0.53, P = 0.72), treatment (t58=0.48, P = 0.63), or treatment × time (t143 = -0.20, P = 0.84) effects for the Scale for the Assessment of Negative Symptoms total score; or for any of the other symptom measures. There were no significant group differences in the change in the MATRICS Consensus Cognitive Battery composite score over the course of the study (F1,26=2.20, P = 0.15).

CONCLUSIONS

The study results suggest that there is no significant benefit of combined anti-inflammatory treatment for persistent positive symptoms or negative symptoms or cognitive impairments (clinicaltrials.gov trial number: NCT01514682).

Differential gene regulation in the anterior cingulate cortex and superior temporal cortex in schizophrenia: A molecular network approach

The closely connected anterior cingulate cortex (ACC) and superior temporal cortex (STC) are important for higher cognitive functions. Both brain regions are disturbed in schizophrenia, i.e., functional and structural alterations have been reported. This postmortem investigation in brains from patients with schizophrenia and controls compared gene expression in the left ACC and left STC. Most differentially expressed genes were unique to each brain region, but some clusters of genes were equally dysregulated in both, giving rise to a more general disease-specific pattern of gene regulation. The data was used to construct a molecular network of the genes identically expressed in both regions as primary nodes and the metabolically connected genes as secondary nodes. The network analysis identified downregulated clusters of immune-associated gene products and upregulated clusters belonging to the ubiquitin-proteasome system. These findings could help to identify new potential therapeutic targets for future approaches.

Therapeutic Drug Monitoring of Second- and Third-Generation Antipsychotic Drugs-Influence of Smoking Behavior and Inflammation on Pharmacokinetics

Both inflammation and smoking can influence a drug’s pharmacokinetic properties, i.e., its liberation, absorption, distribution, metabolism, and elimination. Depending on, e.g., pharmacogenetics, these changes may alter treatment response or cause serious adverse drug reactions and are thus of clinical relevance. Antipsychotic drugs, used in the treatment of psychosis and schizophrenia, should be closely monitored due to multiple factors (e.g., the narrow therapeutic window of certain psychotropic drugs, the chronicity of most mental illnesses, and the common occurrence of polypharmacotherapy in psychiatry). Therapeutic drug monitoring (TDM) aids with drug titration by enabling the quantification of patients’ drug levels. Recommendations on the use of TDM during treatment with psychotropic drugs are presented in the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology; however, data on antipsychotic drug levels during inflammation or after changes in smoking behavior—both clinically relevant in psychiatry—that can aid clinical decision making are sparse. The following narrative review provides an overview of relevant literature regarding TDM in psychiatry, particularly in the context of second- and third-generation antipsychotic drugs, inflammation, and smoking behavior. It aims to spread awareness regarding TDM (most pronouncedly of clozapine and olanzapine) as a tool to optimize drug safety and provide patient-tailored treatment.

Therapeutic Drug Monitoring of Second- and Third-Generation Antipsychotic Drugs—Influence of Smoking Behavior and Inflammation on Pharmacokinetics

Both inflammation and smoking can influence a drug’s pharmacokinetic properties, i.e., its liberation, absorption, distribution, metabolism, and elimination. Depending on, e.g., pharmacogenetics, these changes may alter treatment response or cause serious adverse drug reactions and are thus of clinical relevance. Antipsychotic drugs, used in the treatment of psychosis and schizophrenia, should be closely monitored due to multiple factors (e.g., the narrow therapeutic window of certain psychotropic drugs, the chronicity of most mental illnesses, and the common occurrence of polypharmacotherapy in psychiatry). Therapeutic drug monitoring (TDM) aids with drug titration by enabling the quantification of patients’ drug levels. Recommendations on the use of TDM during treatment with psychotropic drugs are presented in the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology; however, data on antipsychotic drug levels during inflammation or after changes in smoking behavior—both clinically relevant in psychiatry—that can aid clinical decision making are sparse. The following narrative review provides an overview of relevant literature regarding TDM in psychiatry, particularly in the context of second- and third-generation antipsychotic drugs, inflammation, and smoking behavior. It aims to spread awareness regarding TDM (most pronouncedly of clozapine and olanzapine) as a tool to optimize drug safety and provide patient-tailored treatment.

Therapeutic Drug Monitoring of Second- and Third-Generation Antipsychotic Drugs-Influence of Smoking Behavior and Inflammation on Pharmacokinetics

Both inflammation and smoking can influence a drug's pharmacokinetic properties, i.e., its liberation, absorption, distribution, metabolism, and elimination. Depending on, e.g., pharmacogenetics, these changes may alter treatment response or cause serious adverse drug reactions and are thus of clinical relevance. Antipsychotic drugs, used in the treatment of psychosis and schizophrenia, should be closely monitored due to multiple factors (e.g., the narrow therapeutic window of certain psychotropic drugs, the chronicity of most mental illnesses, and the common occurrence of polypharmacotherapy in psychiatry). Therapeutic drug monitoring (TDM) aids with drug titration by enabling the quantification of patients' drug levels. Recommendations on the use of TDM during treatment with psychotropic drugs are presented in the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology; however, data on antipsychotic drug levels during inflammation or after changes in smoking behavior-both clinically relevant in psychiatry-that can aid clinical decision making are sparse. The following narrative review provides an overview of relevant literature regarding TDM in psychiatry, particularly in the context of second- and third-generation antipsychotic drugs, inflammation, and smoking behavior. It aims to spread awareness regarding TDM (most pronouncedly of clozapine and olanzapine) as a tool to optimize drug safety and provide patient-tailored treatment.

Extracellular free water and glutathione in first-episode psychosis-a multimodal investigation of an inflammatory model for psychosis

Evidence has been accumulating for an immune-based component to the etiology of psychotic disorders. Advancements in diffusion magnetic resonance imaging (MRI) have enabled estimation of extracellular free water (FW), a putative biomarker of neuroinflammation. Furthermore, inflammatory processes may be associated with altered brain levels of metabolites, such as glutathione (GSH). Consequently, we sought to test the hypotheses that FW is increased and associated with decreased GSH in patients with first-episode schizophrenia (SZ) compared with healthy controls (HC). SZ (n = 36) and HC (n = 40) subjects underwent a multi-shell diffusion MRI scan on a Siemens 3T scanner. ¹H-MR spectroscopy data were acquired using a GSH-optimized MEGA-PRESS editing sequence and GSH/creatine ratios were calculated for DLPFC (SZ: n = 33, HC: n = 37) and visual cortex (SZ: n = 29, HC: n = 35) voxels. Symptoms and functioning were measured using the SANS, SAPS, BPRS, and GSF/GRF. SZ demonstrated significantly elevated FW in whole-brain gray (p = .001) but not white matter (p = .060). There was no significant difference between groups in GSH in either voxel. However, there was a significant negative correlation between DLPFC GSH and both whole-brain and DLPFC-specific gray matter FW in SZ (r = -.48 and -.47, respectively; both p < .05), while this relationship was nonsignificant in HC and in both groups in the visual cortex. These data illustrate an important relationship between a metabolite known to be important for immune function-GSH-and the diffusion extracellular FW measure, which provides additional support for these measures as neuroinflammatory biomarkers that could potentially provide tractable treatment targets to guide pharmacological intervention.

Dopamine and Neuroinflammation in Schizophrenia - Interpreting the Findings from Translocator Protein (18kDa) PET Imaging

Schizophrenia is a complex disease whose pathophysiology is not yet fully understood. In addition to the long prevailing dopaminergic hypothesis, the evidence suggests that neuroinflammation plays a role in the pathophysiology of the disease. Recent studies using positron emission tomography (PET) that target a 18kDa translocator protein (TSPO) in activated microglial cells in an attempt to measure neuroinflammation in patients have shown a decrease or a lack of an increase in TSPO binding. Many biological and methodological considerations have been formulated to explain these findings. Although dopamine has been described as an immunomodulatory molecule, its potential role in neuroinflammation has not been explored in the aforementioned studies. In this review, we discuss the interactions between dopamine and neuroinflammation in psychotic states. Dopamine may inhibit neuroinflammation in activated microglia. Proinflammatory molecules released from microglia may decrease dopaminergic transmission. This could potentially explain why the levels of neuroinflammation in the brain of patients with schizophrenia seem to be unchanged or decreased compared to those in healthy subjects. However, most data are indirect and are derived from animal studies or from studies performed outside the field of schizophrenia. Further studies are needed to combine TSPO and dopamine imaging to study the association between microglial activation and dopamine system function.

Neuroinflammation as measured by positron emission tomography in patients with recent onset and established schizophrenia: implications for immune pathogenesis

Positron emission tomography (PET) imaging of the 18 kDa translocator protein (TSPO), which is upregulated in activated microglia, is a method for investigating whether immune activation is evident in the brain of adults with schizophrenia. This study aimed to measure TSPO availability in the largest patient group to date, and to compare it between patients with recent onset (ROS) and established (ES) schizophrenia. In total, 20 ROS patients (14 male), 21 ES (13 male), and 21 healthy controls completed the study. Patients were predominantly antipsychotic-medicated. Participants underwent a PET scan using the TSPO-specific radioligand [¹¹C](R)-PK11195. The primary outcome was binding potential (BP_ND) in the anterior cingulate cortex (ACC). Secondary outcomes were BP_ND in six other regions. Correlations were investigated between TSPO availability and symptom severity. Data showed that mean BP_ND was higher in older (ES and controls) compared with younger (ROS and controls) individuals, but did not significantly differ between ROS or ES and their respective age-matched controls (ACC; ANOVA main effect of diagnosis: F_1,58 = 0.407, p = 0.526). Compared with controls, BP_ND was lower in antipsychotic-free (n = 6), but not in medicated, ROS patients. BP_ND in the ES group was negatively correlated with positive symptoms, and positively correlated with negative symptom score. Our data suggest ageing is associated with higher TSPO but a diagnosis of schizophrenia is not. Rather, subnormal TSPO levels in drug-free recent-onset patients may imply impaired microglial development and/or function, which is counteracted by antipsychotic treatment. The development of novel radioligands for specific immune-mechanisms is needed for further clarification.

Personality traits in psychosis and psychosis risk linked to TSPO expression: a neuroimmune marker

Personality has been correlated with differences in cytokine expression, an indicator of peripheral inflammation; however, the associations between personality and central markers of inflammation have never been investigated in vivo in humans. Microglia are the resident macrophages of the central nervous system, and the first responders to tissue damage and brain insult. Microglial activation is associated with elevated expression of translocator protein 18kDa (TSPO), which can be imaged with positron emission tomography (PET) to quantify immune activation in the human brain. This study aimed to investigate the association between personality and TSPO expression across the psychosis spectrum. A total of 61 high-resolution [¹⁸F]FEPPA PET scans were conducted in 28 individuals at clinical high risk (CHR) for psychosis, 19 First-Episode Psychosis (FEP), and 14 healthy volunteers (HVs), and analyzed using a two-tissue compartment model and plasma input function to obtain a total volume of distribution (V_T) as an index of brain TSPO expression (controlling for the rs6971 TSPO polymorphism). Personality was assessed using the Revised NEO Personality Inventory (NEO-PI-R). We found TSPO expression to be specifically associated with neuroticism. A positive association between TSPO expression and neuroticism was found in HVs, in contrast to a nonsignificant, negative association in CHR and significant negative association in FEP. The TSPO-associated neuroticism trait indicates an unexplored connection between neuroimmune activation and personality that varies across the psychosis spectrum.

Effects of Antipsychotic Drugs: Cross Talk Between the Nervous and Innate Immune System

Converging lines of evidence suggest that activation of microglia (innate immune cells in the central nervous system [CNS]) is present in a subset of patients with schizophrenia. The extent to which antipsychotic drug treatment contributes to or combats this effect remains unclear. To address this question, we reviewed the literature for evidence that antipsychotic exposure influences brain microglia as indexed by in vivo neuroimaging and post-mortem studies in patients with schizophrenia and experimental animal models. We found no clear evidence from clinical studies for an effect of antipsychotics on either translocator protein (TSPO) radioligand binding (an in vivo neuroimaging measure of putative gliosis) or markers of brain microglia in post-mortem studies. In experimental animals, where drug and illness effects may be differentiated, we also found no clear evidence for consistent effects of antipsychotic drugs on TSPO radioligand binding. By contrast, we found evidence that chronic antipsychotic exposure may influence central microglia density and morphology. However, these effects were dependent on the dose and duration of drug exposure and whether an immune stimulus was present or not. In the latter case, antipsychotics were generally reported to suppress expression of inflammatory cytokines and inducible inflammatory enzymes such as cyclooxygenase and microglia activation. No clear conclusions could be drawn with regard to any effect of antipsychotics on brain microglia from current clinical data. There is evidence to suggest that antipsychotic drugs influence brain microglia in experimental animals, including possible anti-inflammatory actions. However, we lack detailed information on how these drugs influence brain microglia function at the molecular level. The clinical relevance of the animal data with regard to beneficial treatment effects and detrimental side effects of antipsychotic drugs also remains unknown, and further studies are warranted.

In Vivo TSPO Signal and Neuroinflammation in Alzheimer's Disease

In the last decade, positron emission tomography (PET) and single-photon emission computed tomography (SPECT) in in vivo imaging has attempted to demonstrate the presence of neuroinflammatory reactions by measuring the 18 kDa translocator protein (TSPO) expression in many diseases of the central nervous system. We focus on two pathological conditions for which neuropathological studies have shown the presence of neuroinflammation, which translates in opposite in vivo expression of TSPO. Alzheimer's disease has been the most widely assessed with more than forty preclinical and clinical studies, showing overall that TSPO is upregulated in this condition, despite differences in the topography of this increase, its time-course and the associated cell types. In the case of schizophrenia, a reduction of TSPO has instead been observed, though the evidence remains scarce and contradictory. This review focuses on the key characteristics of TSPO as a biomarker of neuroinflammation in vivo, namely, on the cellular origin of the variations in its expression, on its possible biological/pathological role and on its variations across disease phases.

PET Imaging of the P2X7 Ion Channel with a Novel Tracer [18F]JNJ-64413739 in a Rat Model of Neuroinflammation

PURPOSE

The P2X7 receptor, an adenosine triphosphate (ATP)-gated purinoreceptor, has emerged as one of the key players in neuroinflammatory processes. Therefore, developing a positron emission tomography (PET) tracer for imaging of P2X7 receptors in vivo presents a promising approach to diagnose, monitor, and study neuroinflammation in a variety of brain disorders. To fulfill the goal of developing a P2X7 PET ligand as a biomarker of neuroinflammation, [¹⁸F]JNJ-64413739 has been recently disclosed.

PROCEDURES

We evaluated [¹⁸F]JNJ-64413739 in a rat model of neuroinflammation induced by an intracerebral injection of lipopolysaccharide (LPS). In vivo brain uptake was determined by PET imaging. Upregulation of neuroinflammatory biomarkers was determined by quantitative polymerase chain reaction (qPCR). Distribution of the tracer in the brain was determined by ex vivo autoradiography (ARG). The specificity of [¹⁸F]JNJ-64413739 was confirmed by performing blocking experiments with the P2X7 antagonist JNJ-54175446.

RESULTS

Brain regions of rats injected with LPS had a significantly increased uptake (34 % ± 3 % s.e.m., p = 0.036, t test, standardized uptake value measured over the entire scanning period) of [¹⁸F]JNJ-64413739 relative to the corresponding brain regions of control animals injected with phosphate-buffered saline (PBS). The uptake in the contralateral regions and cerebellum was not significantly different between the groups of animals. The increase in uptake of [¹⁸F]JNJ-64413739 at the LPS-injected site observed by PET imaging was concordant with ex vivo ARG, upregulation of neuroinflammatory biomarkers, and elevated P2X7 expression levels.

CONCLUSIONS

While further work is needed to study [¹⁸F]JNJ-64413739 in other types of neuroinflammation, the current results favorably characterize [¹⁸F]JNJ-64413739 as a potential PET tracer of central neuroinflammation.

Microglial Pruning of Synapses in the Prefrontal Cortex During Adolescence

Exaggerated synaptic elimination in the prefrontal cortex (PFC) during adolescence has been suggested to contribute to the neuropathological changes of schizophrenia. Recent data indicate that microglia (MG) sculpt synapses during early postnatal development. However, it is not known if MG contribute to the structural maturation of the PFC, which has a protracted postnatal development. We determined if MG are involved in developmentally specific synapse elimination in the PFC, focusing on adolescence. Layer 5 PFC pyramidal cells (PCs) were intracellularly filled with Lucifer Yellow for dendritic spine measurements in postnatal day (P) 24, P30, P35, P39, and P50 rats. In the contralateral PFC we evaluated if MG engulfed presynaptic (glutamatergic) and postsynaptic (dendritic spines) elements. Dendritic spine density increased from P24 to P35, when spine density peaked. There was a significant increase in MG engulfment of spines at P39 relative to earlier ages; this subsided by P50. MG also phagocytosed presynaptic glutamatergic terminals. These data indicate that MG transiently prune synapses of PFC PCs during adolescence, when the symptoms of schizophrenia typically first appear. An increase in MG-mediated synaptic remodeling of PFC PCs may contribute to the structural changes observed in schizophrenia.

A Genetics Perspective on the Role of the (Neuro)Immune System in Schizophrenia

The immune system has long been hypothesized to play a role in schizophrenia pathogenesis based on data from diverse disciplines. Recent reports of the identification of schizophrenia-associated genetic variants and their initial biological characterization have renewed investigation of the role of the immune system in schizophrenia. In the current review, the plausibility of a role of the immune system in schizophrenia pathogenesis is examined, by revisiting epidemiology, neuroimaging, pharmacology, and developmental biology from a genetics perspective, as well as by synthesizing diverse findings from the emerging and dynamic schizophrenia genomics field. Genetic correlations between schizophrenia and immunological disorders are inconsistent and often contradictory, as are neuroimaging studies of microglia markers. Small therapeutic trials of anti-inflammatory agents targeting immune function have been consistently negative. Some gene expression analyses of post-mortem brains of patients with schizophrenia have reported an upregulation of genes of immune function though others report downregulation, and overall transcriptome profiling to date does not support an upregulation of immune pathways associated with schizophrenia genetic risk. The currently reviewed genetic data do not converge to reveal consistent evidence of the neuroimmune system in schizophrenia pathogenesis, and indeed, a substantive role for the neuroimmune system in schizophrenia has yet to be established.

Overlapping evidence of innate immune dysfunction in psychotic and affective disorders

Disturbances of the immune system and immune responses after activation are a common finding in neuropsychiatric disorders. Psychotic and affective disorders such as major depressive disorder (MDD), schizophrenia (SCZ) and bipolar disorder (BD) also share high rates of comorbidity with inflammatory and metabolic disorders. Evidence of elevated circulating inflammatory cytokines, altered numbers and function of immune cells, and evidence of neuroinflammation including activation of microglia in the brain have been found in patients with SCZ, BD and MDD. Often these findings correlate to psychological state at the time of measurement. However, significant variation exists across these studies in many aspects, creating challenges in identifying a specific signature of immune dysfunction in these disorders. Innate immune dysfunction, and alterations in monocytes, the critical sentinel cells of the innate immune system, have been seen repeatedly in all three of these disorders, with frequent overlap in findings. In this review, dysfunction specific to the innate arm of the immune system is compared for overlapping evidence across three major psychotic and affective disorders.

Perspective: Solving the Heterogeneity Conundrum of TSPO PET Imaging in Psychosis

Positron emission tomography using ligands targeting translocator protein 18 kDa (TSPO PET) is an innovative method to visualize and quantify glial inflammatory responses in the central nervous system in vivo. Compared to some other neuropsychiatric disorders, findings of TSPO PET in schizophrenia and related psychotic disorders have been considerably more heterogeneous. Two conflicting meta-analyses have been published on the topic within the last year: one asserting evidence for decreased TSPO uptake, while the other observed increased TSPO uptake in a selection of studies. In this paper, we review and discuss five hypotheses which may explain the observed variability of TSPO PET findings in psychotic illness, namely that (1) an inflammatory phenotype is only present in a subgroup of psychosis patients; (2) heterogeneity is caused by interference of antipsychotic medication; (3) interference of other clinical confounders in the study populations (such as age, sex, BMI, smoking, and substance use); or (4) methodological variability between studies (such as choice of tracer and kinetic model, genotyping, study power, and diurnal effects); and (5) the glial responses underlying changes in TSPO expression are themselves heterogeneous and dynamic. Finally, we propose four key recommendations for future research proposals to mitigate these different causes of heterogeneity.

Preliminary data indicating a connection between stress-induced prefrontal dopamine release and hippocampal TSPO expression in the psychosis spectrum

Prolonged stress can cause neuronal loss in the hippocampus resulting in disinhibition of glutamatergic neurons proposed to enhance dopaminergic firing in subcortical regions including striatal areas. Supporting this, imaging studies show increased striatal dopamine release in response to psychosocial stress in healthy individuals with low childhood maternal care, individuals at clinical high risk for psychosis (CHR) and patients with schizophrenia. The prefrontal cortex (PFC) is connected to the hippocampus and a key region to control neurochemical responses to stressful stimuli. We recently reported a disrupted PFC dopamine-stress regulation in schizophrenia, which was intact in CHR. Given the available evidence on the link between psychosocial stress, PFC dopamine release and hippocampal immune activation in psychosis, we explored, for the first time in vivo, whether stress-induced PFC dopamine release is associated with hippocampal TSPO expression (a neuroimmune marker) in the psychosis spectrum. We used an overlapping sample of antipsychotic-naïve subjects with CHR (n = 6) and antipsychotic-free schizophrenia patients (n = 9) from our previously published studies, measuring PFC dopamine release induced by a psychosocial stress task with [¹¹C]FLB457 positron emission tomography (PET) and TSPO expression with [¹⁸F]FEPPA PET. We observed that participants on the psychosis spectrum with lower stress-induced dopamine release in PFC had significantly higher TSPO expression in hippocampus (β = -2.39, SE = 0.96, F(1,11) = 6.17, p = 0.030). Additionally, we report a positive association between stress-induced PFC dopamine release, controlled for hippocampal TSPO expression, and Global Assessment of Functioning. This is the first exploration of the relationship between PFC dopamine release and hippocampal TSPO expression in vivo in humans.

Neuroinflammation in schizophrenia: meta-analysis of in vivo microglial imaging studies

BACKGROUND

Converging lines of evidence implicate an important role for the immune system in schizophrenia. Microglia are the resident immune cells of the central nervous system and have many functions including neuroinflammation, axonal guidance and neurotrophic support. We aimed to provide a quantitative review of in vivo PET imaging studies of microglia activation in patients with schizophrenia compared with healthy controls.

METHODS

Demographic, clinical and imaging measures were extracted from each study and meta-analysis was conducted using a random-effects model (Hedge's g). The difference in 18-kDa translocator protein (TSPO) binding between patients with schizophrenia and healthy controls, as quantified by either binding potential (BP) or volume of distribution (VT), was used as the main outcome. Sub-analysis and sensitivity analysis were carried out to investigate the effects of genotype, ligand and illness stage.

RESULTS

In total, 12 studies comprising 190 patients with schizophrenia and 200 healthy controls met inclusion criteria. There was a significant elevation in tracer binding in schizophrenia patients relative to controls when BP was used as an outcome measure, (Hedge's g = 0.31; p = 0.03) but no significant differences when VT was used (Hedge's g = -0.22; p = 0.29).

CONCLUSIONS

In conclusion, there is evidence for moderate elevations in TSPO tracer binding in grey matter relative to other brain tissue in schizophrenia when using BP as an outcome measure, but no difference when VT is the outcome measure. We discuss the relevance of these findings as well as the methodological issues that may underlie the contrasting difference between these outcomes.

Neuroinflammation in the dorsolateral prefrontal cortex in elderly chronic schizophrenia

Cognitive deterioration and symptom progression occur in schizophrenia over the course of the disorder. A dysfunction of the immune system/neuroinflammatory pathways has been linked to schizophrenia (SZ). These altered processes in the dorsolateral prefrontal cortex (DLPFC) could contribute to the worsening of the deficits. However, limited studies are available in this brain region in elderly population with long-term treatments. In this study, we explore the possible deregulation of 21 key genes involved in immune homeostasis, including pro- and anti-inflammatory cytokines, cytokine modulators (toll-like receptors, colony-stimulating factors, and members of the complement system) and microglial and astroglial markers in the DLPFC in elderly chronic schizophrenia. We used quantitative real-time reverse transcriptase polymerase chain reaction (RT-PCR) on extracts from postmortem DLPFC of elderly subjects with chronic SZ (n = 14) compared to healthy control individuals (n = 14). We report that CSF1R, TLR4, IL6, TNFα, TNFRSF1A, IL10, IL10RA, IL10RB, and CD68 were down-regulated in elderly SZ subjects. Moreover, we found that the expression levels of all the altered inflammatory genes in SZ correlated with the microglial marker CD68. However, no associations were found with the astroglial marker GFAP. This study reveals a decrease in the gene expression of cytokines and immune response/inflammation mediators in the DLPFC of elderly subjects with chronic schizophrenia, supporting the idea of a dysfunction of these processes in aged patients and its possible relationship with active microglia abundance. These findings include elements that might contribute to the cognitive decline and symptom progression linked to DLPFC functioning at advanced stages of the disease.

Automatic Extraction of a Reference Region for the Noninvasive Quantification of Translocator Protein in Brain Using 11C-PBR28

Brain inflammation is associated with various types of neurodegenerative diseases, including Alzheimer disease (AD). Quantifying inflammation with PET is a challenging and invasive procedure, especially in frail patients, because it requires blood sampling from an arterial catheter. A widely used alternative to arterial sampling is a supervised clustering algorithm (SVCA), which identifies the voxels with minimal specific binding in the PET images, thus extracting a reference region for noninvasive kinetic modeling. Methods: We tested this algorithm on a large population of subjects injected with the translocator protein radioligand ¹¹C-PBR28 and compared the kinetic modeling results obtained with the gold standard of arterial input function (V _T/f _p) with those obtained by SVCA (distribution volume ratio [DVR] with Logan plot). The study comprised 57 participants (21 healthy controls, 11 mild cognitive impairment patients, and 25 AD patients). Results: We found that V _T/f _p was greater in AD patients than in controls in the inferior parietal, combined middle and inferior temporal, and entorhinal cortices. SVCA-DVR identified increased binding in the same regions and in an additional one, the parahippocampal region. We noticed however that the average amplitude of the reference curve obtained from subjects with genetic high-affinity binding for ¹¹C-PBR28 was significantly larger than that from subjects with moderate affinity. This suggests that the reference curve extracted by SVCA was contaminated by specific binding. Conclusion: SVCA allows the noninvasive quantification of inflammatory biomarker translocator protein measured with ¹¹C-PBR28 but without the need of arterial sampling. Although the reference curves were contaminated with specific binding, the decreased variance of the outcome measure, SVCA DVR, allowed for an apparent greater sensitivity to detect regional abnormalities in brains of patients with AD.

Ultrastructural pathology of oligodendrocytes adjacent to microglia in prefrontal white matter in schizophrenia

Microglial activation has been proposed to be involved in the pathophysiology of schizophrenia (SCZ). We hypothesized that dystrophic alterations of oligodendrocytes previously reported in the prefrontal white matter in SCZ might be associated with microglial activation in the acute state of SCZ. White matter of the prefrontal cortex (BA10) was studied in post-mortem brain tissue from 21 SCZ cases and 20 normal controls. The SCZ group included 12 subjects with predominantly positive symptoms and 9 subjects with predominantly negative symptoms. Electron microscopy was applied to estimate cell density, size, volume fraction (Vv) and the number (N) of organelles in oligodendrocytes adjacent to microglia and in oligodendrocytes adjacent to myelin, neurons and capillaries and not adjacent to microglia. Cell density of oligodendrocytes was not changed in the SCZ group as compared to controls. Vv and N of mitochondria were significantly decreased, while Vv of vacuoles of endoplasmic reticulum and lipofuscin granules were significantly increased in oligodendrocytes adjacent to either microglia or myelin in the SCZ group and in patients displaying predominantly positive symptoms as compared to the control group. There were no significant differences between oligodendrocytes adjacent to microglia and to myelin. Vv and N of lipofuscin were also increased in peri-capillary oligodendrocytes. There was no effect of clinical subgroups on the parameters of peri-capillary and peri-neuronal oligodendrocytes. Though many ameboid and dystrophic microglia adjacent to oligodendrocytes were found in the SCZ samples, we provide no quantitative evidence that oligodendrocyte dystrophy is associated with microglial activation in white matter in SCZ.

Test-retest reliability and convergent validity of (R)-[11C]PK11195 outcome measures without arterial input function

Purpose

The PET radioligand (R)-[¹¹C]PK11195 is used to quantify the 18-kDa translocator protein (TSPO), a marker for glial activation. Since there is no brain region devoid of TSPO, an arterial input function (AIF) is ideally required for quantification of binding. However, obtaining an AIF is experimentally demanding, is sometimes uncomfortable for participants, and can introduce additional measurement error during quantification. The objective of this study was to perform an evaluation of the test-retest reliability and convergent validity of techniques used for quantifying (R)-[¹¹C]PK11195 binding without an AIF in clinical studies.

Methods

Data from six healthy individuals who participated in two PET examinations, 6 weeks apart, were analyzed. Regional non-displaceable binding potential (BP_ND) values were calculated using the simplified reference tissue model, with either cerebellum as reference region or a reference input derived using supervised cluster analysis (SVCA). Standardized uptake values (SUVs) were estimated for the time interval of 40–60 min.

Results

Test-retest reliability for BP_ND estimates were poor (80% of ICCs < 0.5). BP_ND estimates derived without an AIF were not correlated with BP_ND, total or specific distribution volume from the 2TCM using an AIF (all R² < 12%). SUVs showed moderate reliability but no correlation to any other outcome measure.

Conclusions

Caution is warranted when interpreting patient-control comparisons employing (R)-[¹¹C]PK11195 outcome measures obtained without an AIF.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0455-8) contains supplementary material, which is available to authorized users.

TSPO expression and brain structure in the psychosis spectrum

Psychosis is associated with abnormal structural changes in the brain including decreased regional brain volumes and abnormal brain morphology. However, the underlying causes of these structural abnormalities are less understood. The immune system, including microglial activation, has been implicated in the pathophysiology of psychosis. Although previous studies have suggested a connection between peripheral proinflammatory cytokines and structural brain abnormalities in schizophrenia, no in-vivo studies have investigated whether microglial activation is also linked to brain structure alterations previously observed in schizophrenia and its putative prodrome. In this study, we investigated the link between mitochondrial 18 kDa translocator protein (TSPO) and structural brain characteristics (i.e. regional brain volume, cortical thickness, and hippocampal shape) in key brain regions such as dorsolateral prefrontal cortex and hippocampus of a large group of participants (N = 90) including individuals at clinical high risk (CHR) for psychosis, first-episode psychosis (mostly antipsychotic-naïve) patients, and healthy volunteers. The participants underwent structural brain MRI scan and [¹⁸F]FEPPA positron emission tomography (PET) targeting TSPO. A significant [¹⁸F]FEPPA binding-by-group interaction was observed in morphological measures across the left hippocampus. In first-episode psychosis, we observed associations between [¹⁸F]FEPPA V_T (total volume of distribution) and outward and inward morphological alterations, respectively, in the dorsal and ventro-medial portions of the left hippocampus. These associations were not significant in CHR or healthy volunteers. There was no association between [¹⁸F]FEPPA V_T and other structural brain characteristics. Our findings suggest a link between TSPO expression and alterations in hippocampal morphology in first-episode psychosis.

The effects of haloperidol on microglial morphology and translocator protein levels: An in vivo study in rats using an automated cell evaluation pipeline

BACKGROUND

Altered microglial markers and morphology have been demonstrated in patients with schizophrenia in post-mortem and in vivo studies. However, it is unclear if changes are due to antipsychotic treatment.

AIMS

Here we aimed to determine whether antipsychotic medication affects microglia in vivo.

METHODS

To investigate this we administered two clinically relevant doses (0.05 mg n=12 and 2.5 mg n=7 slow-release pellets, placebo n=20) of haloperidol, over 2 weeks, to male Sprague Dawley rats to determine the effect on microglial cell density and morphology (area occupied by processes and microglial cell area). We developed an analysis pipeline for the automated assessment of microglial cells and used lipopolysaccharide (LPS) treatment ( n=13) as a positive control for analysis. We also investigated the effects of haloperidol ( n=9) or placebo ( n=10) on the expression of the translocator protein 18 kDa (TSPO) using autoradiography with [³H]PBR28, a TSPO ligand used in human positron emission tomography (PET) studies.

RESULTS

Here we demonstrated that haloperidol at either dose does not alter microglial measures compared with placebo control animals ( p > 0.05). Similarly there was no difference in [³H]PBR28 binding between placebo and haloperidol tissue ( p > 0.05). In contrast, LPS was associated with greater cell density ( p = 0.04) and larger cell size ( p = 0.01).

CONCLUSION

These findings suggest that haloperidol does not affect microglial cell density, morphology or TSPO expression, indicating that clinical study alterations are likely not the consequence of antipsychotic treatment. The automated cell evaluation pipeline was able to detect changes in microglial morphology induced by LPS and is made freely available for future use.

Hippocampal glutamate metabolites and glial activation in clinical high risk and first episode psychosis

Alterations in glutamate neurotransmission have been implicated in the pathophysiology of schizophrenia, as well as in symptom severity and cognitive deficits. The hippocampus, in particular, is a site of key functional and structural abnormalities in schizophrenia. Yet few studies have investigated hippocampal glutamate in antipsychotic-naïve first episode psychosis patients or in individuals at clinical high risk (CHR) of developing psychosis. Using proton magnetic resonance spectroscopy (1H-MRS), we investigated glutamate metabolite levels in the left hippocampus of 25 CHR (19 antipsychotic-naïve), 16 patients with first-episode psychosis (13 antipsychotic-naïve) and 31 healthy volunteers. We also explored associations between hippocampal glutamate metabolites and glial activation, as indexed by [18F]FEPPA positron emission tomography (PET); symptom severity; and cognitive function. Groups differed significantly in glutamate plus glutamine (Glx) levels (F(2, 69) = 6.39, p = 0.003). Post-hoc analysis revealed that CHR had significantly lower Glx levels than both healthy volunteers (p = 0.003) and first-episode psychosis patients (p = 0.050). No associations were found between glutamate metabolites and glial activation. Our findings suggest that glutamate metabolites are altered in CHR.

Positron Emission Tomography Studies of the Glial Cell Marker Translocator Protein in Patients With Psychosis: A Meta-analysis Using Individual Participant Data

BACKGROUND

Accumulating evidence suggests that the immune system may be an important target for new treatment approaches in schizophrenia. Positron emission tomography and radioligands binding to the translocator protein (TSPO), which is expressed in glial cells in the brain including immune cells, represents a potential method for patient stratification and treatment monitoring. This study examined whether patients with first-episode psychosis and schizophrenia had altered TSPO levels compared with healthy control subjects.

METHODS

PubMed was searched for studies comparing patients with psychosis with healthy control subjects using second-generation TSPO radioligands. The outcome measure was total distribution volume (VT), an index of TSPO levels, in frontal cortex, temporal cortex, and hippocampus. Bayes factors (BFs) were applied to examine the relative support for higher, lower, or no difference in patients' TSPO levels compared with healthy control subjects.

RESULTS

Five studies, with 75 participants with first-episode psychosis or schizophrenia and 77 healthy control subjects, were included. BFs showed strong support for lower VT in patients relative to no difference (all BFs > 32), or relative to higher VT (all BFs > 422), in all brain regions. From the posterior distributions, mean patient-control differences in standardized VT values were -0.48 for frontal cortex (95% credible interval [CredInt] = -0.88 to 0.09), -0.47 for temporal cortex (CredInt = -0.87 to -0.07), and -0.63 for hippocampus (CredInt = -1.00 to -0.25).

CONCLUSIONS

The lower levels of TSPO observed in patients may correspond to altered function or lower density of brain immune cells. Future studies should focus on investigating the underlying biological mechanisms and their relevance for treatment.

(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.

Translocator protein (TSPO) and stress cascades in mouse models of psychosis with inflammatory disturbances

Changes in inflammatory cascades have been implicated in the underlying pathophysiology of psychosis. Translocator protein 18 kDa (TSPO) has been used to assess neuroinflammatory processes in psychotic disorders. Nonetheless, it is unclear whether TSPO, a mitochondrial protein, can be interpreted as a general marker for inflammation in diseases involving psychosis. To address this question, we investigated TSPO signaling in representative mouse models for psychosis with inflammatory disturbances. The maternal immune activation and cuprizone short-term exposure models show different TSPO signaling. Furthermore, we observed similarities and differences in their respective stress pathways including stress hormone signaling and oxidative stress that are functionally interconnected with the inflammatory responses. We propose that more careful studies of TSPO distribution in neuroinflammation and other stress cascades associated with psychotic symptoms will allow us to understand the biological mechanisms underlying psychosis-related behaviors.

Brain TSPO imaging and gray matter volume in schizophrenia patients and in people at ultra high risk of psychosis: An [11C]PBR28 study

Patients with schizophrenia show whole brain and cortical gray matter (GM) volume reductions which are progressive early in their illness. Microglia, the resident immune cells in the CNS, phagocytose neurons and synapses. Some post mortem and in vivo studies in schizophrenia show evidence for elevated microglial activation compared to matched controls. However, it is currently unclear how these results relate to changes in cortical structure.

METHODS

Fourteen patients with schizophrenia and 14 ultra high risk for psychosis (UHR) subjects alongside two groups of age and genotype matched healthy controls received [11C]PBR28 PET scans to index TSPO expression, a marker of microglial activation and a 3T MRI scan. We investigated the relationship between the volume changes of cortical regions and microglial activation in cortical GM (as indexed by [11C]PBR28 distribution volume ratio (DVR).

RESULTS

The total cortical GM volume was significantly lower in SCZ than the controls [mean (SD)/cm3: SCZ=448.83 (39.2) and controls=499.6 (59.2) (p=0.02) but not in UHR (mean (SD)=503.06 (57.9) and controls=524.46 (45.3) p=0.3). Regression model fitted the total cortical GM DVR values with the cortical regional volumes in SCZ (r=0.81; p<0.001) and in UHR (r=0.63; p=0.02). We found a significant negative correlation between the TSPO signal and total cortical GM volume in SCZ with the highest absolute correlation coefficient in the right superior-parietal cortex (r=-0.72; p=0.006).

CONCLUSIONS

These findings suggest that microglial activity is related to the altered cortical volume seen in schizophrenia. Longitudinal investigations are required to determine whether microglial activation leads to cortical gray matter loss.

18F-PBR111 PET Imaging in Healthy Controls and Schizophrenia: Test-Retest Reproducibility and Quantification of Neuroinflammation

Activated microglia express the translocator protein (TSPO) on the outer mitochondrial membrane. ¹⁸F-PBR111 is a second-generation PET ligand that specifically binds the TSPO, allowing in vivo visualization and quantification of neuroinflammation. The aim of this study was to evaluate whether the test-retest variability of ¹⁸F-PBR111 in healthy controls is acceptable to detect a psychosis-associated neuroinflammatory signal in schizophrenia. Methods: Dynamic 90-min ¹⁸F-PBR111 scans were obtained in 17 healthy male controls (HCs) and 11 male schizophrenia patients (SPs) during a psychotic episode. Prior genotyping for the rs6917 polymorphism distinguished high-affinity binders (HABs) and mixed-affinity binders (MABs). Total volume of distribution (V_T) was determined from 2-tissue-compartment modeling with vascular trapping and a metabolite-corrected plasma input function. A subgroup of HCs (n = 12; 4 HABs and 8 MABs) was scanned twice to assess absolute test-retest variability and intraclass correlation coefficients of the regional V_T values. Differences in TSPO binding between HC and SP were assessed using mixed model analysis adjusting for age, genotype, and age*cohort. The effect of using different scan durations (V_{T-60 min} versus V_{T-90 min}) was determined based on Pearson r. Data were mean ± SD. Results: Mean absolute variability in V_T ranged from 16% ± 14% (19% ± 20% HAB; 15% ± 11% MAB) in the cortical gray matter to 22% ± 15% (23% ± 15% HAB; 22% ± 16% MAB) in the hippocampus. Intraclass correlation coefficients were consistently between 0.64 and 0.82 for all tested regions. TSPO binding in SP compared with HC depended on age (cohort*age: P < 0.05) and was increased by +14% ± 4% over the regions. There was a significant effect of genotype on TSPO binding, and V_T of HABs was 31% ± 8% (HC: 17% ± 5%, SP: 61% ± 14%) higher than MABs. Across all clinical groups, V_{T-60 min} and V_{T-90 min} were strongly correlated (r > 0.7, P < 0.0001). Conclusion:¹⁸F-PBR111 can be used for monitoring of TSPO binding, as shown by medium test-retest variability and reliability of V_T in HCs. Microglial activation is present in SPs depending on age and needs to be adjusted for genotype.

Using molecular imaging to understand early schizophrenia-related psychosis neurochemistry: a review of human studies

Schizophrenia is a chronic psychiatric disorder generally preceded by a so-called prodromal phase, which is characterized by attenuated psychotic symptoms. Advances in clinical research have enabled prospective identification of those individuals who are at clinical high risk (CHR) for psychosis, with the power to predict psychosis onset within the near future. Changes in several brain neurochemical systems and molecular mechanisms are implicated in the pathophysiology of schizophrenia and the psychosis spectrum, including the dopaminergic, γ-aminobutyric acid (GABA)-ergic, glutamatergic, endocannabinoid, and immunologic (i.e. glial activation) system and other promising future directions such as synaptic density, which are possible to quantify in vivo using positron emission tomography (PET). This paper aims to review in vivo PET studies in the mentioned systems in the early course of psychosis (i.e. CHR and first-episode psychosis (FEP)). The results of reviewed studies are promising; however, the current understanding of the underlying pathology of psychosis is still limited. Importantly, promising efforts involve the development of novel PET radiotracers targeting systems with growing interest in schizophrenia, like the nociceptive system and synaptic density.

PET imaging of putative microglial activation in individuals at ultra-high risk for psychosis, recently diagnosed and chronically ill with schizophrenia

We examined putative microglial activation as a function of illness course in schizophrenia. Microglial activity was quantified using [11C](R)-(1-[2-chrorophynyl]-N-methyl-N-[1-methylpropyl]-3 isoquinoline carboxamide (11C-(R)-PK11195) positron emission tomography (PET) in: (i) 10 individuals at ultra-high risk (UHR) of psychosis; (ii) 18 patients recently diagnosed with schizophrenia; (iii) 15 patients chronically ill with schizophrenia; and, (iv) 27 age-matched healthy controls. Regional-binding potential (BPND) was calculated using the simplified reference-tissue model with four alternative reference inputs. The UHR, recent-onset and chronic patient groups were compared to age-matched healthy control groups to examine between-group BPND differences in 6 regions: dorsal frontal, orbital frontal, anterior cingulate, medial temporal, thalamus and insula. Correlation analysis tested for BPND associations with gray matter volume, peripheral cytokines and clinical variables. The null hypothesis of equality in BPND between patients (UHR, recent-onset and chronic) and respective healthy control groups (younger and older) was not rejected for any group comparison or region. Across all subjects, BPND was positively correlated to age in the thalamus (r=0.43, P=0.008, false discovery rate). No correlations with regional gray matter, peripheral cytokine levels or clinical symptoms were detected. We therefore found no evidence of microglial activation in groups of individuals at high risk, recently diagnosed or chronically ill with schizophrenia. While the possibility of 11C-(R)-PK11195-binding differences in certain patient subgroups remains, the patient cohorts in our study, who also displayed normal peripheral cytokine profiles, do not substantiate the assumption of microglial activation in schizophrenia as a regular and defining feature, as measured by 11C-(R)-PK11195 BPND.

Brain microglia in psychiatric disorders

The role of immune activation in psychiatric disorders has attracted considerable attention over the past two decades, contributing to the rise of a new era for psychiatry. Microglia, the macrophages of the brain, are progressively becoming the main focus of the research in this field. In this Review, we assess the literature on microglia activation across different psychiatric disorders, including post-mortem and in-vivo studies in humans and experimental studies in animals. Although microglia activation has been noted in all types of psychiatric disorder, no association was seen with specific diagnostic categories. Furthermore, the findings from these studies highlight that not all psychiatric patients have microglial activation. Therefore, the cause of the neuroinflammation in these cohorts and its implications are unclear. We discuss psychosocial stress as one of the main factors determining microglial activation in patients with psychiatric disorders, and explore the relevance of these findings for future treatment strategies.

Neuroinflammation and Oxidative Stress in Psychosis and Psychosis Risk

Although our understanding of psychotic disorders has advanced substantially in the past few decades, very little has changed in the standard of care for these illnesses since the development of atypical anti-psychotics in the 1990s. Here, we integrate new insights into the pathophysiology with the increasing interest in early detection and prevention. First, we explore the role of N-methyl-d-aspartate receptors in a subpopulation of cortical parvalbumin-containing interneurons (PVIs). Postmortem and preclinical data has implicated these neurons in the positive and negative symptoms, as well as the cognitive dysfunction present in schizophrenia. These neurons also appear to be sensitive to inflammation and oxidative stress during the perinatal and peripubertal periods, which may be mediated in large part by aberrant synaptic pruning. After exploring some of the molecular mechanisms through which neuroinflammation and oxidative stress are thought to exert their effects, we highlight the progress that has been made in identifying psychosis prior to onset through the identification of individuals at clinical high risk for psychosis (CHR). By combining our understanding of psychosis pathogenesis with the increasing characterization of endophenotypes that precede frank psychosis, it may be possible to identify patients before they present with psychosis and intervene to reduce the burden of the disease to both patients and families.

Immune involvement in the pathogenesis of schizophrenia: a meta-analysis on postmortem brain studies

Although the precise pathogenesis of schizophrenia is unknown, genetic, biomarker and imaging studies suggest involvement of the immune system. In this study, we performed a systematic review and meta-analysis of studies investigating factors related to the immune system in postmortem brains of schizophrenia patients and healthy controls. Forty-one studies were included, reporting on 783 patients and 762 controls. We divided these studies into those investigating histological alterations of cellular composition and those assessing molecular parameters; meta-analyses were performed on both categories. Our pooled estimate on cellular level showed a significant increase in the density of microglia (P=0.0028) in the brains of schizophrenia patients compared with controls, albeit with substantial heterogeneity between studies. Meta-regression on brain regions demonstrated this increase was most consistently observed in the temporal cortex. Densities of macroglia (astrocytes and oligodendrocytes) did not differ significantly between schizophrenia patients and healthy controls. The results of postmortem histology are paralleled on the molecular level, where we observed an overall increase in expression of proinflammatory genes on transcript and protein level (P=0.0052) in patients, while anti-inflammatory gene expression levels were not different between schizophrenia and controls. The results of this meta-analysis strengthen the hypothesis that components of the immune system are involved in the pathogenesis of schizophrenia.

Inflammation and the neural diathesis-stress hypothesis of schizophrenia: a reconceptualization

An interaction between external stressors and intrinsic vulnerability is one of the longest standing pathoaetiological explanations for schizophrenia. However, novel lines of evidence from genetics, preclinical studies, epidemiology and imaging have shed new light on the mechanisms that may underlie this, implicating microglia as a key potential mediator. Microglia are the primary immune cells of the central nervous system. They have a central role in the inflammatory response, and are also involved in synaptic pruning and neuronal remodeling. In addition to immune and traumatic stimuli, microglial activation occurs in response to psychosocial stress. Activation of microglia perinatally may make them vulnerable to subsequent overactivation by stressors experienced in later life. Recent advances in genetics have shown that variations in the complement system are associated with schizophrenia, and this system has been shown to regulate microglial synaptic pruning. This suggests a mechanism via which genetic and environmental influences may act synergistically and lead to pathological microglial activation. Microglial overactivation may lead to excessive synaptic pruning and loss of cortical gray matter. Microglial mediated damage to stress-sensitive regions such as the prefrontal cortex and hippocampus may lead directly to cognitive and negative symptoms, and account for a number of the structural brain changes associated with the disorder. Loss of cortical control may also lead to disinhibition of subcortical dopamine-thereby leading to positive psychotic symptoms. We review the preclinical and in vivo evidence for this model and consider the implications this has for treatment, and future directions.