Moving Toward Multicenter Therapeutic Trials in Amyotrophic Lateral Sclerosis: Feasibility of Data Pooling Using Different Translocator Protein PET Radioligands

Potential of neuroimaging as a biomarker in amyotrophic lateral sclerosis: from structure to metabolism

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron degeneration. The development of ALS involves metabolite alterations leading to tissue lesions in the nervous system. Recent advances in neuroimaging have significantly improved our understanding of the underlying pathophysiology of ALS, with findings supporting the corticoefferent axonal disease progression theory. Current studies on neuroimaging in ALS have demonstrated inconsistencies, which may be due to small sample sizes, insufficient statistical power, overinterpretation of findings, and the inherent heterogeneity of ALS. Deriving meaningful conclusions solely from individual imaging metrics in ALS studies remains challenging, and integrating multimodal imaging techniques shows promise for detecting valuable ALS biomarkers. In addition to giving an overview of the principles and techniques of different neuroimaging modalities, this review describes the potential of neuroimaging biomarkers in the diagnosis and prognostication of ALS. We provide an insight into the underlying pathology, highlighting the need for standardized protocols and multicenter collaborations to advance ALS research.

PET and SPECT Imaging of ALS: An Educational Review

Amyotrophic lateral sclerosis (ALS) is a disease leading to progressive motor degeneration and ultimately death. It is a complex disease that can take a significantly long time to be diagnosed, as other similar pathological conditions must be ruled out for a definite diagnosis of ALS. Noninvasive imaging of ALS has shed light on disease pathology and altered biochemistry in the ALS brain. Other than magnetic resonance imaging (MRI), two types of functional imaging, positron emission tomography (PET) and single photon emission computed tomography (SPECT), have provided valuable data about what happens in the brain of ALS patients compared to healthy controls. PET imaging has revealed a specific pattern of brain metabolism through [18F]FDG, while other radiotracers have uncovered neuroinflammation, changes in neuronal density, and protein aggregation. SPECT imaging has shown a general decrease in regional cerebral blood flow (rCBF) in ALS patients. This educational review summarizes the current state of ALS imaging with various PET and SPECT radiopharmaceuticals to better understand the pathophysiology of ALS.

Characterization of neuroinflammation pattern in anti-LGI1 encephalitis based on TSPO PET and symptom clustering analysis

PURPOSE

TSPO PET with radioligand [18F]DPA-714 is an emerging molecular imaging technique that reflects cerebral inflammation and microglial activation, and it has been recently used in central nervous system diseases. In this study, we aimed to investigate the neuroinflammation pattern of anti-leucine-rich glioma-inactivated 1 (LGI1) protein autoimmune encephalitis (AIE) and to evaluate its possible correlation with clinical phenotypes.

METHODS

Twenty patients with anti-LGI1 encephalitis from the autoimmune encephalitis cohort in Huashan Hospital and ten with other AIE and non-inflammatory diseases that underwent TSPO PET imaging were included in the current study. Increased regional [18F]DPA-714 retention in anti-LGI1 encephalitis was detected on a voxel basis using statistic parametric mapping analysis. Multiple correspondence analysis and hierarchical clustering were conducted for discriminate subgroups in anti-LGI1 encephalitis. Standardized uptake value ratios normalized to the cerebellum (SUVRc) were calculated for semiquantitative analysis of TSPO PET features between different LGI1-AIE subgroups.

RESULTS

Increased regional retention of [18F]DPA-714 was identified in the bilateral hippocampus, caudate nucleus, and frontal cortex in LGI1-AIE patients. Two subgroups of LGI1-AIE patients were distinguished based on the top seven common symptoms. Patients in cluster 1 had a high frequency of facio-brachial dystonic seizures than those in cluster 2 (p = 0.004), whereas patients in cluster 2 had a higher frequency of general tonic-clonic (GTC) seizures than those in cluster 1 (p < 0.001). Supplementary motor area and superior frontal gyrus showed higher [18F]DPA-714 retention in cluster 2 patients compared with those in cluster 1 (p = 0.024; p = 0.04, respectively).

CONCLUSIONS

Anti-LGI1 encephalitis had a distinctive molecular imaging pattern presented by TSPO PET scan. LGI1-AIE patients with higher retention of [18F]DPA-714 in the frontal cortex were more prone to present with GTC seizures. Further studies are required for verifying its value in clinical application.

Molecular Imaging of Innate Immunity and Immunotherapy

The innate immune system plays a key role as the first line of defense in various human diseases including cancer, cardiovascular and inflammatory diseases. In contrast to tissue biopsies and blood biopsies, in vivo imaging of the innate immune system can provide whole body measurements of immune cell location and function and changes in response to disease progression and therapy. Rationally developed molecular imaging strategies can be used in evaluating the status and spatio-temporal distributions of the innate immune cells in near real-time, mapping the biodistribution of novel innate immunotherapies, monitoring their efficacy and potential toxicities, and eventually for stratifying patients that are likely to benefit from these immunotherapies. In this review, we will highlight the current state-of-the-art in noninvasive imaging techniques for preclinical imaging of the innate immune system particularly focusing on cell trafficking, biodistribution, as well as pharmacokinetics and dynamics of promising immunotherapies in cancer and other diseases; discuss the unmet needs and current challenges in integrating imaging modalities and immunology and suggest potential solutions to overcome these barriers.

Differences in Cerebral Glucose Metabolism in ALS Patients with and without C9orf72 and SOD1 Mutations

Amyotrophic lateral sclerosis (ALS) is characterized by progressive loss of upper and lower motor neurons. In 10% of patients, the disorder runs in the family. Our aim was to study the impact of ALS-causing gene mutations on cerebral glucose metabolism. Between October 2010 and October 2022, 538 patients underwent genetic testing for mutations with strong evidence of causality for ALS and 18F-2-fluoro-2-deoxy-D-glucose-PET (FDG PET), at University Hospitals Leuven. We identified 48 C9orf72-ALS and 22 SOD1-ALS patients. After propensity score matching, two cohorts of 48 and 21 matched sporadic ALS patients, as well as 20 healthy controls were included. FDG PET images were assessed using a voxel-based and volume-of-interest approach. We observed widespread frontotemporal involvement in all ALS groups, in comparison to healthy controls. The degree of relative glucose metabolism in SOD1-ALS in motor and extra-motor regions did not differ significantly from matched sporadic ALS patients. In C9orf72-ALS, we found more pronounced hypometabolism in the peri-rolandic region and thalamus, and hypermetabolism in the medulla extending to the pons, in comparison to matched sporadic ALS patients. Our study revealed C9orf72-dependent differences in glucose metabolism in the peri-rolandic region, thalamus, and brainstem (i.e., medulla, extending to the pons) in relation to matched sporadic ALS patients.

Positron Emission Tomography of Neuroimmune Responses in Humans: Insights and Intricacies

The brain's immune system plays a critical role in responding to immune challenges and maintaining homeostasis. However, dysregulated neuroimmune function contributes to neurodegenerative disease and neuropsychiatric conditions. In vivo positron emission tomography (PET) imaging of the neuroimmune system has facilitated a greater understanding of its physiology and the pathology of some neuropsychiatric conditions. This review presents an in-depth look at PET findings from human neuroimmune function studies, highlighting their importance in current neuropsychiatric research. Although the majority of human PET studies feature radiotracers targeting the translocator protein 18 kDa (TSPO), this review also considers studies with other neuroimmune targets, including monoamine oxidase B, cyclooxygenase-1 and cyclooxygenase-2, nitric oxide synthase, and the purinergic P2X7 receptor. Promising new targets, such as colony-stimulating factor 1, Sphingosine-1-phosphate receptor 1, and the purinergic P2Y12 receptor, are also discussed. The significance of validating neuroimmune targets and understanding their function and expression is emphasized in this review to better identify and interpret PET results.

Advances in the neuroimaging of motor disorders

Neuroimaging is a valuable adjunct to the history and examination in the evaluation of motor system disorders. Conventional imaging with computed tomography or magnetic resonance imaging depicts important anatomic information and helps to identify imaging patterns which may support diagnosis of a specific motor disorder. Advanced imaging techniques can provide further detail regarding volume, functional, or metabolic changes occurring in nervous system pathology. This chapter is an overview of the advances in neuroimaging with particular emphasis on both standard and less well-known advanced imaging techniques and findings, such as diffusion tensor imaging or volumetric studies, and their application to specific motor disorders. In addition, it provides reference to emerging imaging biomarkers in motor system disorders such as Parkinson disease, amyotrophic lateral sclerosis, and Huntington disease, and briefly reviews the neuroimaging findings in different causes of myelopathy and peripheral nerve disorders.

18F-Radiolabeled Translocator Protein (TSPO) PET Tracers: Recent Development of TSPO Radioligands and Their Application to PET Study

Translocator protein 18 kDa (TSPO) is a transmembrane protein in the mitochondrial membrane, which has been identified as a peripheral benzodiazepine receptor. TSPO is generally present at high concentrations in steroid-producing cells and plays an important role in steroid synthesis, apoptosis, and cell proliferation. In the central nervous system, TSPO expression is relatively modest under normal physiological circumstances. However, some pathological disorders can lead to changes in TSPO expression. Overexpression of TSPO is associated with several diseases, such as neurodegenerative diseases, neuroinflammation, brain injury, and cancers. TSPO has therefore become an effective biomarker of related diseases. Positron emission tomography (PET), a non-invasive molecular imaging technique used for the clinical diagnosis of numerous diseases, can detect diseases related to TSPO expression. Several radiolabeled TSPO ligands have been developed for PET. In this review, we describe recent advances in the development of TSPO ligands, and ¹⁸F-radiolabeled TSPO in particular, as PET tracers. This review covers pharmacokinetic studies, preclinical and clinical trials of ¹⁸F-labeled TSPO PET ligands, and the synthesis of TSPO ligands.

Imaging Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation plays a major role in the etiopathology of neurodegenerative diseases, including Alzheimer and Parkinson diseases, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. In vivo monitoring of neuroinflammation using PET is critical to understand this process, and data are accumulating in this regard, thus a review is useful. From PubMed, we retrieved publications using any of the available PET tracers to image neuroinflammation in humans as well as selected articles dealing with experimental animal models or the chemistry of currently used or potential radiotracers. We reviewed 280 articles. The most common PET neuroinflammation target, translocator protein (TSPO), has limitations, lacking cellular specificity and the ability to separate neuroprotective from neurotoxic inflammation. However, TSPO PET is useful to define the amount and location of inflammation in the brain of people with neurodegenerative disorders. We describe the characteristics of TSPO and other potential PET neuroinflammation targets and PET tracers available or in development. Despite target and tracer limitations, in recent years there has been a sharp increase in the number of reports of neuroinflammation PET in humans. The most studied has been Alzheimer disease, in which neuroinflammation seems initially neuroprotective and neurotoxic later in the progression of the disease. We describe the findings in all the major neurodegenerative disorders. Neuroinflammation PET is an indispensable tool to understand the process of neurodegeneration, particularly in humans, as well as to validate target engagement in therapeutic clinical trials.

Functional Imaging for Neurodegenerative Diseases

Diagnosis and monitoring of neurodegenerative diseases has changed profoundly over the past twenty years. Biomarkers are now included in most diagnostic procedures as well as in clinical trials. Neuroimaging biomarkers provide access to brain structure and function over the course of neurodegenerative diseases. They have brought new insights into a wide range of neurodegenerative diseases and have made it possible to describe some of the imaging challenges in clinical populations. MRI mainly explores brain structure while molecular imaging, functional MRI and electro- and magnetoencephalography examine brain function. In this paper, we describe and analyse the current and potential contribution of MRI and molecular imaging in the field of neurodegenerative diseases.

Neuroinflammation as potential precursor of leukoencephalopathy in early-stage breast cancer patients: A cross-sectional PET-MRI study

Background

Although chemotherapy-induced leukoencephalopathy has been described in case and cohort studies, literature remains inconclusive about its prevalence and mechanisms. Therefore, we investigated the presence of leukoencephalopathy after multiagent chemotherapy in women treated for breast cancer and potential underlying neuroinflammatory processes.

Methods

In this exploratory study, 15 chemotherapy-treated and 15 age-matched chemotherapy-naïve patients with early-stage breast cancer, as well as 15 healthy controls underwent simultaneous PET-MR neuroimaging, including T1-weighted MPRAGE, T2-weighted FLAIR and dynamic PET with the 18-kDA translocator protein (TSPO) radioligand [¹⁸F]DPA-714. Total and regional (juxtacortical, periventricular, deep white matter and infratentorial) lesion burden were compared between the groups with one-way ANOVA. With paired t-tests, [¹⁸F]DPA-714 volume of distribution [V_T, including partial volume correction (PVC)] in lesioned and normal appearing white matter (NAWM) were compared within subjects, to investigate inflammation. Finally, two general linear models were used to examine the predictive values of neurofilament light-chain (NfL) serum levels on (1) total lesion burden or (2) PVC [¹⁸F]DPA-714 V_T of lesions showing elevated inflammation.

Results

No significant differences were found in total or localized lesion burden. However, significantly higher (20–45%) TSPO uptake was observed in juxtacortical lesions (p ≤ 0.008, t ≥ 3.90) compared to NAWM in both cancer groups, but only persisted for chemotherapy-treated patients after PVC (p = 0.005, t = 4.30). NfL serum levels were not associated with total lesion volume or tracer uptake in juxtacortical lesions.

Conclusion

This multimodal neuroimaging study suggests that neuroinflammatory processes could be involved in the development of juxtacortical, but not periventricular or deep white matter, leukoencephalopathy shortly after chemotherapy for early-stage breast cancer.

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No increased white matter lesion load in breast cancer patients.

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No differences in TSPO uptake in periventricular or deep white matter lesions.

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Higher TSPO uptake in juxtacortical lesions in chemotherapy-treated breast cancer patients.

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TSPO uptake in inflammatory lesions and NfL levels not significantly associated, despite a trend.

The role of inflammation in neurodegeneration: novel insights into the role of the immune system in C9orf72 HRE-mediated ALS/FTD

Neuroinflammation is an important hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). An inflammatory reaction to neuronal injury is deemed vital for neuronal health and homeostasis. However, a continued activation of the inflammatory response can be detrimental to remaining neurons and aggravate the disease process. Apart from a disease modifying role, some evidence suggests that neuroinflammation may also contribute to the upstream cause of the disease. In this review, we will first focus on the role of neuroinflammation in the pathogenesis of chromosome 9 open reading frame 72 gene (C9orf72) hexanucleotide repeat expansions (HRE)-mediated ALS/FTD (C9-ALS/FTD). Additionally, we will discuss evidence from ex vivo and in vivo studies and finally, we briefly summarize the trials and progress of anti-inflammatory therapies.

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).

A pilot [11C]PBR28 PET/MRI study of neuroinflammation and neurodegeneration in chronic stroke patients

Neuroinflammation occurs in response to acute ischemic stroke, and has been speculated to underlie secondary poststroke pathologies, such as depression, that often develop over time poststroke. However, no study has examined whether neuroinflammation is present in chronic stroke patients (e.g., ​≥ ​1 year poststroke). This study tested whether neuroinflammation is present in chronic stroke patients, and is associated with neurodegeneration, using [¹¹C]PBR28 PET and diffusion MRI.

Eight patients with middle cerebral artery (MCA) ischemic stroke incurred 1–3 years prior and 16 healthy controls underwent [¹¹C]PBR28 PET to measure glial activation and diffusion MRI to measure microstructural integrity by mean diffusivity (MD) and fractional anisotropy (FA) using an integrated PET/MRI scanner. Group differences in [¹¹C]PBR28 binding, MD and FA were analyzed voxelwise across the whole brain excluding the infarct zone defined as voxels containing the infarct in any patient.

Compared to controls, patients showed elevations in [¹¹C]PBR28 binding in several brain regions outside the infarct zone, including regions with presumed direct neuroanatomical connections to the infarct (e.g., ipsilesional internal capsule and thalamus) and those without known direct connections (e.g., contralesional thalamus and cingulate gyrus). Patients also showed widespread elevations in MD, with a subset of these regions having reduced FA. In patients, MD was more elevated in regions with co-localized elevations in [¹¹C]PBR28 binding than in contralateral regions without elevations in [¹¹C]PBR28 binding.

This pilot study supports the presence of extensive glial activation along with widespread loss in microstructural integrity in non-infarcted tissue in a cohort of patients with chronic MCA stroke. The loss in microstructural integrity was greater in regions with co-localized glial activation. It is possible that stroke risk factors (e.g., hypertension) contributed to these tissue changes in patients.

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Chronic neuroinflammation speculated to underlie secondary poststroke pathologies such as depression.

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Measured neuroinflammation in chronic stroke patients using [¹¹C]PBR28 PET.

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First study showing extensive neuroinflammation in non-infarcted tissue in chronic stroke patients.

Novel Tracers and Radionuclides in PET Imaging

The use of PET imaging agents in oncology, cardiovascular disease, and neurodegenerative disease shows the power of this technique in evaluating the molecular and biological characteristics of numerous diseases. These agents provide crucial information for designing therapeutic strategies for individual patients. Novel PET tracers are in continual development and many have potential use in clinical and research settings. This article discusses the potential applications of tracers in diagnostics, the biological characteristics of diseases, the ability to provide prognostic indicators, and using this information to guide treatment strategies including monitoring treatment efficacy in real time to improve outcomes and survival.

Interplay between immunity and amyotrophic lateral sclerosis: Clinical impact

Amyotrophic lateral sclerosis (ALS) is a debilitating and rapidly fatal neurodegenerative disease. Despite decades of research and many new insights into disease biology over the 150 years since the disease was first described, causative pathogenic mechanisms in ALS remain poorly understood, especially in sporadic cases. Our understanding of the role of the immune system in ALS pathophysiology, however, is rapidly expanding. The aim of this manuscript is to summarize the recent advances regarding the immune system involvement in ALS, with particular attention to clinical translation. We focus on the potential pathophysiologic mechanism of the immune system in ALS, discussing local and systemic factors (blood, cerebrospinal fluid, and microbiota) that influence ALS onset and progression in animal models and people. We also explore the potential of Positron Emission Tomography to detect neuroinflammation in vivo, and discuss ongoing clinical trials of therapies targeting the immune system. With validation in human patients, new evidence in this emerging field will serve to identify novel therapeutic targets and provide realistic hope for personalized treatment strategies.

Biomarkers in amyotrophic lateral sclerosis: a review of new developments

PURPOSE OF REVIEW

This review draws together the most recent findings in ALS biomarker research from biochemical, imaging and neurophysiology techniques.

RECENT FINDINGS

The potential of circulating RNA is highlighted, including new retrieval techniques. With ongoing genetic clinical trials, the need for pharmacodynamic biomarkers is essential. There is a strong case for neurofilament proteins being validated in ALS; their biomarker profile is discussed. Oxidative stress and neuroinflammation studies offer insight into disease mechanisms and offer good biomarker potential. Recent metabolic studies include investigation of lipid profiles, creatinine and ferritin. The potential of chitinase proteins as pharmacodynamic and prognostic biomarkers is highlighted. The role of tau and amyloidβ is debated, as evidenced by the articles presented here. Proteomic approaches provide unbiased discoveries of novel biomarkers, together with confirmation of previous findings. The use of imaging techniques is outlined to demonstrate selective atrophy, volume loss, muscle and tract involvement. In-vivo imaging is discussed with reference to histone deacetylase, oxidative stress, neuroinflammation and metabolic changes. New applications of electrophysiology demonstrate objective muscle biomarkers and brain network perturbations.

SUMMARY

The biomarker research field continues to provide insight into the disease. Multicentre collaborations are needed to validate these promising recent findings.

The Use, Standardization, and Interpretation of Brain Imaging Data in Clinical Trials of Neurodegenerative Disorders

Imaging biomarkers play a wide-ranging role in clinical trials for neurological disorders. This includes selecting the appropriate trial participants, establishing target engagement and mechanism-related pharmacodynamic effect, monitoring safety, and providing evidence of disease modification. In the early stages of clinical drug development, evidence of target engagement and/or downstream pharmacodynamic effect-especially with a clear relationship to dose-can provide confidence that the therapeutic candidate should be advanced to larger and more expensive trials, and can inform the selection of the dose(s) to be further tested, i.e., to "de-risk" the drug development program. In these later-phase trials, evidence that the therapeutic candidate is altering disease-related biomarkers can provide important evidence that the clinical benefit of the compound (if observed) is grounded in meaningful biological changes. The interpretation of disease-related imaging markers, and comparability across different trials and imaging tools, is greatly improved when standardized outcome measures are defined. This standardization should not impinge on scientific advances in the imaging tools per se but provides a common language in which the results generated by these tools are expressed. PET markers of pathological protein aggregates and structural imaging of brain atrophy are common disease-related elements across many neurological disorders. However, PET tracers for pathologies beyond amyloid β and tau are needed, and the interpretability of structural imaging can be enhanced by some simple considerations to guard against the possible confound of pseudo-atrophy. Learnings from much-studied conditions such as Alzheimer's disease and multiple sclerosis will be beneficial as the field embraces rarer diseases.

Moving Toward Patient-Tailored Treatment in ALS and FTD: The Potential of Genomic Assessment as a Tool for Biological Discovery and Trial Recruitment

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two devastating and intertwined neurodegenerative diseases. Historically, ALS and FTD were considered distinct disorders given differences in presenting clinical symptoms, disease duration, and predicted risk of developing each disease. However, research over recent years has highlighted the considerable clinical, pathological, and genetic overlap of ALS and FTD, and these two syndromes are now thought to represent different manifestations of the same neuropathological disease spectrum. In this review, we discuss the need to shift our focus from studying ALS and FTD in isolation to identifying the biological mechanisms that drive these diseases-both common and distinct-to improve treatment discovery and therapeutic development success. We also emphasize the importance of genomic data to facilitate a "precision medicine" approach for treating ALS and FTD.

Heterogeneity of Neuroinflammatory Responses in Amyotrophic Lateral Sclerosis: A Challenge or an Opportunity?

Amyotrophic Lateral Sclerosis (ALS) is a complex pathology: (i) the neurodegeneration is chronic and progressive; it starts focally in specific central nervous system (CNS) areas and spreads to different districts; (ii) multiple cell types further than motor neurons (i.e., glial/immune system cells) are actively involved in the disease; (iii) both neurosupportive and neurotoxic neuroinflammatory responses were identified. Microglia cells (a key player of neuroinflammation in the CNS) attracted great interest as potential target cell population that could be modulated to counteract disease progression, at least in preclinical ALS models. However, the heterogeneous/multifaceted microglia cell responses occurring in different CNS districts during the disease represent a hurdle for clinical translation of single-drug therapies. To address this issue, over the past ten years, several studies attempted to dissect the complexity of microglia responses in ALS. In this review, we shall summarize these results highlighting how the heterogeneous signature displayed by ALS microglia reflects not only the extent of neuronal demise in different regions of the CNS, but also variable engagement in the attempts to cope with the neuronal damage. We shall discuss novel avenues opened by the advent of single-cell and spatial transcriptomics technologies, underlining the potential for discovery of novel therapeutic targets, as well as more specific diagnostic/prognostic not-invasive markers of neuroinflammation.

PET Imaging for Oxidative Stress in Neurodegenerative Disorders Associated with Mitochondrial Dysfunction

Oxidative stress based on mitochondrial dysfunction is assumed to be the principal molecular mechanism for the pathogenesis of many neurodegenerative disorders. However, the effects of oxidative stress on the neurodegeneration process in living patients remain to be elucidated. Molecular imaging with positron emission tomography (PET) can directly evaluate subtle biological changes, including the redox status. The present review focuses on recent advances in PET imaging for oxidative stress, in particular the use of the Cu-ATSM radioligand, in neurodegenerative disorders associated with mitochondrial dysfunction. Since reactive oxygen species are mostly generated by leakage of excess electrons from an over-reductive state due to mitochondrial respiratory chain impairment, PET with ⁶²Cu-ATSM, the accumulation of which depends on an over-reductive state, is able to image oxidative stress. ⁶²Cu-ATSM PET studies demonstrated enhanced oxidative stress in the disease-related brain regions of patients with mitochondrial disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, the magnitude of oxidative stress increased with disease severity, indicating that oxidative stress based on mitochondrial dysfunction contributes to promoting neurodegeneration in these diseases. Oxidative stress imaging has improved our insights into the pathological mechanisms of neurodegenerative disorders, and is a promising tool for monitoring further antioxidant therapies.

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.