Neuroinflammation and Tau Colocalize in vivo in Progressive Supranuclear Palsy

Neuroinflammation Parallels 18F-PI-2620 Positron Emission Tomography Patterns in Primary 4-Repeat Tauopathies

BACKGROUND

Preclinical, postmortem, and positron emission tomography (PET) imaging studies have pointed to neuroinflammation as a key pathophysiological hallmark in primary 4-repeat (4R) tauopathies and its role in accelerating disease progression.

OBJECTIVE

We tested whether microglial activation (1) progresses in similar spatial patterns as the primary pathology tau spreads across interconnected brain regions, and (2) whether the degree of microglial activation parallels tau pathology spreading.

METHODS

We examined in vivo associations between tau aggregation and microglial activation in 31 patients with clinically diagnosed 4R tauopathies, using 18F-PI-2620 PET and 18F-GE180 (translocator protein [TSPO]) PET. We determined tau epicenters, defined as subcortical brain regions with highest tau PET signal, and assessed the connectivity of tau epicenters to cortical regions of interest using a 3-T resting-state functional magnetic resonance imaging template derived from age-matched healthy elderly controls.

RESULTS

In 4R tauopathy patients, we found that higher regional tau PET covaries with elevated TSPO-PET across brain regions that are functionally connected to each other (β = 0.414, P < 0.001). Microglial activation follows similar distribution patterns as tau and distributes primarily across brain regions strongly connected to patient-specific tau epicenters (β = -0.594, P < 0.001). In these regions, microglial activation spatially parallels tau distribution detectable with 18F-PI-2620 PET.

CONCLUSIONS

Our findings indicate that the spatial expansion of microglial activation parallels tau distribution across brain regions that are functionally connected to each other, suggesting that tau and inflammation are closely interrelated in patients with 4R tauopathies. The combination of in vivo tau and inflammatory biomarkers could therefore support the development of immunomodulatory strategies for disease-modifying treatments in these conditions. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Association between hippocampal microglia, AD and LATE-NC, and cognitive decline in older adults

INTRODUCTION

This study investigates the relationship between microglia inflammation in the hippocampus, brain pathologies, and cognitive decline.

METHODS

Participants underwent annual clinical evaluations and agreed to brain donation. Neuropathologic evaluations quantified microglial burden in the hippocampus, amyloid beta (Aβ), tau tangles, and limbic age-related transactive response DNA-binding protein 43 (TDP-43) encephalopathy neuropathologic changes (LATE-NC), and other common brain pathologies. Mixed-effect and linear regression models examined the association of microglia with a decline in global and domain-specific cognitive measures, and separately with brain pathologies. Path analyses estimated direct and indirect effects of microglia on global cognition.

RESULT

Hippocampal microglia were associated with a faster decline in global cognition, specifically in episodic memory, semantic memory, and perceptual speed. Tau tangles and LATE-NC were independently associated with microglia. Other pathologies, including Aβ, were not related. Regional hippocampal burden of tau tangles and TDP-43 accounted for half of the association of microglia with cognitive decline.

DISCUSSION

Microglia inflammation in the hippocampus contributes to cognitive decline. Tau tangles and LATE-NC partially mediate this association.

Colocalization of Increased Midbrain Signals in Neuroinflammation and Tau PET Imaging Suggests the Diagnosis of Progressive Supranuclear Palsy

ABSTRACT

Clinical overlap with multiple other neurological diseases makes the diagnosis of autoimmune encephalitis challenging; consequently, a broad range of neurological diseases are misdiagnosed as autoimmune encephalitis. A 58-year-old man presented with abnormal behavior, irritability for 3 years, oculomotor disturbance, unsteady walking, and dysphagia and was suspected as having anti-dipeptidyl-peptidase-like protein 6 (DPPX) encephalitis as the anti-DPPX antibody was positive in the serum. However, the therapeutic effect of immunotherapy was unsatisfactory. Subsequently, colocalization of increased midbrain signals was observed in neuroinflammation PET using [ 18 F]DPA-714 and in tau PET using [ 18 F]florzolotau, suggesting the diagnosis of progressive supranuclear palsy.

Mechanisms linking cerebrovascular dysfunction and tauopathy: Adding a layer of epiregulatory complexity

Intracellular accumulation of hyperphosphorylated misfolded tau proteins are found in many neurodegenerative tauopathies, including Alzheimer's disease (AD). Tau pathology can impact cerebrovascular physiology and function through multiple mechanisms. In vitro and in vivo studies have shown that alterations in the blood-brain barrier (BBB) integrity and function can result in synaptic abnormalities and neuronal damage. In the present review, we will summarize how tau proteostasis dysregulation contributes to vascular dysfunction and, conversely, we will examine the factors and pathways leading to tau pathological alterations triggered by cerebrovascular dysfunction. Finally, we will highlight the role epigenetic and epitranscriptomic factors play in regulating the integrity of the cerebrovascular system and the progression of tauopathy including a few observartions on potential therapeutic interventions. LINKED ARTICLES: This article is part of a themed issue From Alzheimer's Disease to Vascular Dementia: Different Roads Leading to Cognitive Decline. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.6/issuetoc.

Neuroinflammation is linked to dementia risk in Parkinson's disease

The development of dementia is a devastating aspect of Parkinson's disease (PD), affecting nearly half of patients within 10 years post-diagnosis. For effective therapies to prevent and slow progression to PD dementia (PDD), the key mechanisms that determine why some people with PD develop early dementia, while others remain cognitively unaffected, need to be understood. Neuroinflammation and tau protein accumulation have been demonstrated in post-mortem PD brains, and in many other neurodegenerative disorders leading to dementia. However, whether these processes mediate dementia risk early on in the PD disease course is not established. To this end, we used PET neuroimaging with 11C-PK11195 to index neuroinflammation and 18F-AV-1451 for misfolded tau in early PD patients, stratified according to dementia risk in our 'Neuroinflammation and Tau Accumulation in Parkinson's Disease Dementia' (NET-PDD) study. The NET-PDD study longitudinally assesses newly-diagnosed PD patients in two subgroups at low and high dementia risk (stratified based on pentagon copying, semantic fluency, MAPT genotype), with comparison to age- and sex-matched controls. Non-displaceable binding potential (BPND) in 43 brain regions (Hammers' parcellation) was compared between groups (pairwise t-tests), and associations between BPND of the tracers tested (linear-mixed-effect models). We hypothesized that people with higher dementia risk have greater inflammation and/or tau accumulation in advance of significant cognitive decline. We found significantly elevated neuroinflammation (11C-PK11195 BPND) in multiple subcortical and restricted cortical regions in the high dementia risk group compared with controls, while in the low-risk group this was limited to two cortical areas. The high dementia risk group also showed significantly greater neuroinflammation than the low-risk group concentrated on subcortical and basal ganglia regions. Neuroinflammation in most of these regions was associated with worse cognitive performance (Addenbrooke's Cognitive Examination-III score). Overall neuroinflammation burden also correlated with serum levels of pro-inflammatory cytokines. In contrast, increases in 18F-AV-1451 (tau) BPND in PD versus controls were restricted to subcortical regions where off-target binding is typically seen, with no relationship to cognition found. Whole-brain 18F-AV-1451 burden correlated with serum phosphorylated tau181 levels. Although there was minimal regional tau accumulation in PD, regional neuroinflammation and tau burden correlated in PD participants, with the strongest association in the high dementia risk group, suggesting possible co-localization of these pathologies. In conclusion, our findings suggest that significant regional neuroinflammation in early PD might underpin higher risk for PDD development, indicating neuroinflammation as a putative early modifiable aetiopathological disease factor to prevent or slow dementia development using immunomodulatory strategies.

Preclinical translational platform of neuroinflammatory disease biology relevant to neurodegenerative disease

Neuroinflammation is a key driver of neurodegenerative disease, however the tools available to model this disease biology at the systems level are lacking. We describe a translational drug discovery platform based on organotypic culture of murine cortical brain slices that recapitulate disease-relevant neuroinflammatory biology. After an acute injury response, the brain slices assume a chronic neuroinflammatory state marked by transcriptomic profiles indicative of activation of microglia and astrocytes and loss of neuronal function. Microglia are necessary for manifestation of this neuroinflammation, as depletion of microglia prior to isolation of the brain slices prevents both activation of astrocytes and robust loss of synaptic function genes. The transcriptomic pattern of neuroinflammation in the mouse platform is present in published datasets derived from patients with amyotrophic lateral sclerosis, Huntington's disease, and frontotemporal dementia. Pharmacological utility of the platform was validated by demonstrating reversal of microglial activation and the overall transcriptomic signature with transforming growth factor-β. Additional anti-inflammatory targets were screened and inhibitors of glucocorticoid receptors, COX-2, dihydrofolate reductase, and NLRP3 inflammasome all failed to reverse the neuroinflammatory signature. Bioinformatics analysis of the neuroinflammatory signature identified protein tyrosine phosphatase non-receptor type 11 (PTPN11/SHP2) as a potential target. Three structurally distinct inhibitors of PTPN11 (RMC-4550, TN0155, IACS-13909) reversed the neuroinflammatory disease signature. Collectively, these results highlight the utility of this novel neuroinflammatory platform for facilitating identification and validation of targets for neuroinflammatory neurodegenerative disease drug discovery.

PET Imaging to Measure Neuroinflammation In Vivo

This paper provides an overview of the role of neuroinflammation in Alzheimer's disease and other neurodegenerative diseases, highlighting the potential of anti-inflammatory treatments to slow or prevent decline. This research focuses on the use of positron emission tomography (PET) imaging to visualize and quantify molecular brain changes in patients, specifically microglial activation and reactive astrogliosis. We discuss the development and application of several PET radioligands, including first-generation ligands like PK11195 and Ro5-4864, as well as second- and third-generation ligands such as [11C]PBR28, [18F]DPA-714, [18F]GE-180, and [11C]ER176. These ligands target the 18-kDa translocator protein (TSPO), which is overexpressed in activated microglia and upregulated in astrocytes. We also address the limitations of these ligands, such as low brain uptake, poor penetration of the blood-brain barrier, short half-life, and variable kinetic behavior. Furthermore, we demonstrate the impact of genetic polymorphisms on ligand binding.

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

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

A review of the flortaucipir literature for positron emission tomography imaging of tau neurofibrillary tangles

Alzheimer's disease is defined by the presence of β-amyloid plaques and neurofibrillary tau tangles potentially preceding clinical symptoms by many years. Previously only detectable post-mortem, these pathological hallmarks are now identifiable using biomarkers, permitting an in vivo definitive diagnosis of Alzheimer's disease. 18F-flortaucipir (previously known as 18F-T807; 18F-AV-1451) was the first tau positron emission tomography tracer to be introduced and is the only Food and Drug Administration-approved tau positron emission tomography tracer (Tauvid™). It has been widely adopted and validated in a number of independent research and clinical settings. In this review, we present an overview of the published literature on flortaucipir for positron emission tomography imaging of neurofibrillary tau tangles. We considered all accessible peer-reviewed literature pertaining to flortaucipir through 30 April 2022. We found 474 relevant peer-reviewed publications, which were organized into the following categories based on their primary focus: typical Alzheimer's disease, mild cognitive impairment and pre-symptomatic populations; atypical Alzheimer's disease; non-Alzheimer's disease neurodegenerative conditions; head-to-head comparisons with other Tau positron emission tomography tracers; and technical considerations. The available flortaucipir literature provides substantial evidence for the use of this positron emission tomography tracer in assessing neurofibrillary tau tangles in Alzheimer's disease and limited support for its use in other neurodegenerative disorders. Visual interpretation and quantitation approaches, although heterogeneous, mostly converge and demonstrate the high diagnostic and prognostic value of flortaucipir in Alzheimer's disease.

Microglia in neurodegenerative diseases: mechanism and potential therapeutic targets

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

Longitudinal Synaptic Loss in Primary Tauopathies: An In Vivo [11 C]UCB-J Positron Emission Tomography Study

BACKGROUND

Synaptic loss is characteristic of many neurodegenerative diseases; it occurs early and is strongly related to functional deficits.

OBJECTIVE

In this longitudinal observational study, we determine the rate at which synaptic density is reduced in the primary tauopathies of progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD), and we test the relationship with disease progression.

METHODS

Our cross-sectional cohort included 32 participants with probable PSP and 16 with probable CBD (all amyloid-negative corticobasal syndrome), recruited from tertiary care centers in the United Kingdom, and 33 sex- and age-matched healthy control subjects. Synaptic density was estimated by positron emission tomography imaging with the radioligand [11 C]UCB-J that binds synaptic vesicle 2A. Clinical severity and cognition were assessed by the PSP Rating Scale and the Addenbrooke's cognitive examination. Regional [11 C]UCB-J nondisplaceable binding potential was estimated in Hammersmith Atlas regions of interest. Twenty-two participants with PSP/CBD had a follow-up [11 C]UCB-J positron emission tomography scan after 1 year. We calculated the annualized change in [11 C]UCB-J nondisplaceable binding potential and correlated this with the change in clinical severity.

RESULTS

We found significant annual synaptic loss within the frontal lobe (-3.5%, P = 0.03) and the right caudate (-3.9%, P = 0.046). The degree of longitudinal synaptic loss within the frontal lobe correlated with the rate of change in the PSP Rating Scale (R = 0.47, P = 0.03) and cognition (Addenbrooke's Cognitive Examination-Revised, R = -0.62, P = 0.003).

CONCLUSIONS

We provide in vivo evidence for rapid progressive synaptic loss, correlating with clinical progression in primary tauopathies. Synaptic loss may be an important therapeutic target and outcome variable for early-phase clinical trials of disease-modifying treatments. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Inflammation in multiple system atrophy

Misfolding protein aggregation inside or outside cells is the major pathological hallmark of several neurodegenerative diseases. Among proteinopathies are neurodegenerative diseases with atypical Parkinsonism and an accumulation of insoluble fibrillary alpha-synuclein (synucleinopathies) or hyperphosphorylated tau protein fragments (tauopathies). As there are no therapies available to slow or halt the progression of these disea ses, targeting the inflammatory process is a promising approach. The inflammatory biomarkers could also help in the differential diagnosis of Parkinsonian syndromes. Here, we review inflammation's role in multiple systems atrophy pathogenesis, diagnosis, and treatment.

Imaging of Tauopathies with PET Ligands: State of the Art and Future Outlook

(1) Background: Tauopathies are a group of diseases characterized by the deposition of abnormal tau protein. They are distinguished into 3R, 4R, and 3R/4R tauopathies and also include Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). Positron emission tomography (PET) imaging represents a pivotal instrument to guide clinicians. This systematic review aims to summarize the current and novel PET tracers. (2) Methods: Literature research was conducted on Pubmed, Scopus, Medline, Central, and the Web of Science using the query "pet ligands" and "tauopathies". Articles published from January 2018 to 9 February, 2023, were searched. Only studies on the development of novel PET radiotracers for imaging in tauopathies or comparative studies between existing PET tracers were included. (3) Results: A total of 126 articles were found, as follows: 96 were identified from PubMed, 27 from Scopus, one on Central, two on Medline, and zero on the Web of Science. Twenty-four duplicated works were excluded, and 63 articles did not satisfy the inclusion criteria. The remaining 40 articles were included for quality assessment. (4) Conclusions: PET imaging represents a valid instrument capable of helping clinicians in diagnosis, but it is not always perfect in differential diagnosis, even if further investigations on humans for novel promising ligands are needed.

Pathomechanisms of depression in progressive supranuclear palsy

Depression is one of the most frequent neuropsychiatric symptoms in progressive supranuclear palsy (PSP), a four-repeat tauopathy and most common atypical parkinsonian disorder, but its pathophysiology and pathogenesis are poorly understood. Pubmed/Medline was systematically analyzed until January 2023, with focus on the prevalence, major clinical features, neuroimaging findings and treatment options of depression in PSP. The average prevalence of depression in PSP is around 50%; it does usually not correlate with most other clinical parameters. Depression is associated with multi-regional patterns of morphometric gray matter variations, e.g., reduced thickness of temporo-parieto-occipital cortices, and altered functional orbitofrontal and medial frontal circuits with disturbances of mood-related brain networks. Unfortunately, no specific neuropathological data about depression in PSP are available. Antidepressive and electroconvulsive therapies are effective in improving symptoms; the efficacy of transcranial stimulation needs further confirmation. Depression in PSP is a common symptom, related to multi-regional patterns of cerebral disturbances and complex pathogenic mechanisms that deserve further elucidation as a basis for adequate treatment to improve the quality of life in this fatal disease.

Pathomechanisms of cognitive impairment in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative disorder characterized by early postural instability and falls, oculomotor dysfunction (vertical supranuclear gaze palsy), parkinsonism with poor response to levodopa, pseudobulbar palsy, and cognitive impairment. This four-repeat tauopathy is morphologically featured by accumulation of tau protein in neurons and glia causing neuronal loss and gliosis in the extrapyramidal system associated with cortical atrophy and white matter lesions. Cognitive impairment being frequent in PSP and more severe than in multiple system atrophy and Parkinson disease, is dominated by executive dysfunction, with milder difficulties in memory, and visuo-spatial and naming dysfunctions. Showing longitudinal decline, it has been related to a variety of pathogenic mechanisms associated with the underlying neurodegenerative process, such as involvement of cholinergic and muscarinergic dysfunctions, and striking tau pathology in frontal and temporal cortical regions associated with reduced synaptic density. Altered striatofrontal, fronto-cerebellar, parahippocampal, and multiple subcortical structures, as well as widespread white matter lesions causing extensive connectivity disruptions in cortico-subcortical and cortico-brainstem connections, support the concept that PSP is a brain network disruption disorder. The pathophysiology and pathogenesis of cognitive impairment in PSP, as in other degenerative movement disorders, are complex and deserve further elucidation as a basis for adequate treatment to improve the quality of life of patients with this fatal disease.

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.

Immunology and microbiome: Implications for motor systems

Immune-inflammatory mechanisms seem to play a relevant role in neurodegenerative disorders affecting motor systems, particularly Parkinson's disease, where activity changes in inflammatory cells and evidence of neuroinflammation in experimental models and patients is available. Amyotrophic lateral sclerosis is also characterized by neuroinflammatory changes that involve primarily glial cells, both microglia and astrocytes, as well as systemic immune dysregulation associated with more rapid progression. Similarly, the exploration of gut dysbiosis in these two prototypical neurodegenerative motor disorders is advancing rapidly. Altered composition of gut microbial constituents and related metabolic and putative functional pathways is supporting a pathophysiological link that is currently explored in preclinical, germ-free animal models. Less compelling, but still intriguing, evidence suggests that motor neurodevelopmental disorders, e.g., Tourette syndrome, are associated with abnormal trajectories of maturation that include also immune system development. Microglia has a key role also in these disorders, and new therapeutic avenues aiming at its modulation are exciting prospects. Preclinical and clinical research on the role of gut dysbiosis in Tourette syndrome and related behavioral disorders is still in its infancy, but early findings support the rationale to delve deeper into its contribution to neural and immune maturation abnormalities in its spectrum.

Synaptic Loss in Frontotemporal Dementia Revealed by [11 C]UCB-J Positron Emission Tomography

OBJECTIVE

Synaptic loss is an early feature of neurodegenerative disease models, and is severe in post mortem clinical studies, including frontotemporal dementia. Positron emission tomography (PET) with radiotracers that bind to synaptic vesicle glycoprotein 2A enables quantification of synaptic density in vivo. This study used [11 C]UCB-J PET in participants with behavioral variant frontotemporal dementia (bvFTD), testing the hypothesis that synaptic loss is severe and related to clinical severity.

METHODS

Eleven participants with clinically probable bvFTD and 25 age- and sex-matched healthy controls were included. Participants underwent dynamic [11 C]UCB-J PET, structural magnetic resonance imaging, and a neuropsychological battery, including the revised Addenbrooke Cognitive Examination, and INECO frontal screening. General linear models compared [11 C]UCB-J binding potential maps and gray matter volume between groups, and assessed associations between synaptic density and clinical severity in patients. Analyses were also performed using partial volume corrected [11 C]UCB-J binding potential from regions of interest (ROIs).

RESULTS

Patients with bvFTD showed severe synaptic loss compared to controls. [11 C]UCB-J binding was reduced bilaterally in medial and dorsolateral frontal regions, inferior frontal gyri, anterior and posterior cingulate gyrus, insular cortex, and medial temporal lobe. Synaptic loss in the frontal and cingulate regions correlated significantly with cognitive impairments. Synaptic loss was more severe than atrophy. Results from ROI-based analyses mirrored the voxelwise results.

INTERPRETATION

In accordance with preclinical models, and human postmortem evidence, there is widespread frontotemporal loss of synapses in symptomatic bvFTD, in proportion to severity. [11 C]UCB-J PET could support translational studies and experimental medicine strategies for new disease-modifying treatments for neurodegeneration. ANN NEUROL 2023;93:142-154.

18 F-Florzolotau Tau Positron Emission Tomography Imaging in Patients with Multiple System Atrophy-Parkinsonian Subtype

BACKGROUND

Anecdotal evidence suggests that patients diagnosed with the parkinsonian subtype of multiple system atrophy (MSA-P) may show uptake of the second-generation tau positron emission tomography (PET) tracer ¹⁸ F-Florzolotau (previously known as ¹⁸ F-APN-1607) in the putamen.

OBJECTIVES

This study systematically investigated the localization and magnitude of ¹⁸ F-Florzolotau uptake in a relatively large cohort of patients with MSA-P.

METHODS

¹⁸ F-Florzolotau PET imaging was performed in 31 patients with MSA-P, 24 patients with Parkinson's disease (PD), and 20 age-matched healthy controls. ¹⁸ F-Florzolotau signal in the striatum was analyzed by visual inspection and classified as either positive or negative. Regional ¹⁸ F-Florzolotau binding was also expressed as standardized uptake value ratio (SUVR) to assess whether it was associated with core symptoms of MSA-P after adjustment for potential confounders.

RESULTS

By visual inspection and semiquantitative SUVR comparisons, patients with MSA-P showed elevated ¹⁸ F-Florzolotau uptake in the putamen, globus pallidus, and dentate-a finding that was not observed in PD. This increased signal was significantly associated with the core symptoms of MSA-P. In addition, patients with MSA-P with cerebellar ataxia showed an elevated ¹⁸ F-Florzolotau uptake in the cerebellar dentate.

CONCLUSIONS

¹⁸ F-Florzolotau tau PET imaging findings may reflect the clinical severity of MSA-P and can potentially discriminate between this condition and PD. © 2022 International Parkinson and Movement Disorder Society.

Type-I Interferons in Alzheimer's Disease and Other Tauopathies

The detection of pathogen-associated molecular patterns can elicit the production of type-I interferons (IFNs), soluble cytokines that induce a transcriptional state inhibitory to viral replication. Signatures of type-I IFN-driven gene expression, and type-I IFNs themselves, are observed in the central nervous system during neurodegenerative diseases including Alzheimer's disease and other tauopathies, the umbrella term for diseases that feature aggregation of the cytosolic protein tau. The contribution of the type-I IFN response to pathological progression of these diseases, however, is not well-understood. The wholesale transcriptional changes that ensue from type-I IFN production can both promote protective effects and lead to damage dependent on the context and duration of the response. The type-I IFN system therefore represents a signaling pathway with a potential disease-modifying role in the progression of neurodegenerative disease. In this review we summarize the evidence for a type-I IFN signature in AD and other tauopathies and examine the role of aggregated proteins as inflammatory stimuli. We explore both the protective role of IFN against protein pathologies as well as their downstream toxic consequences, which include the exacerbation of protein pathology as a potentially destructive feed-forward loop. Given the involvement of type-I IFNs in other neurogenerative diseases, we draw comparisons with other categories of homotypic protein aggregation. Understanding how type-I IFN influences progression of AD and other tauopathies may yield important insight to neurodegeneration and identify new targets in an area currently lacking disease-modifying therapies.

In Vivo 18F-Flortaucipir PET Does Not Accurately Support the Staging of Progressive Supranuclear Palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative disorder characterized by neuroglial tau pathology. A new staging system for PSP pathology postmortem has been described and validated. We used a data-driven approach to test whether postmortem pathologic staging in PSP can be reproduced in vivo with 18F-flortaucipir PET. Methods: Forty-two patients with probable PSP and 39 controls underwent 18F-flortaucipir PET. Conditional inference tree analyses on regional binding potential values identified absent/present pathology thresholds to define in vivo staging. Following the postmortem staging approach for PSP pathology, we evaluated the combinations of absent/present pathology (or abnormal/normal PET signal) across all regions to assign each participant to in vivo stages. ANOVA was applied to analyze differences among means of disease severity between stages. In vivo staging was compared with postmortem staging in 9 patients who also had postmortem confirmation of the diagnosis and stage. Results: Stage assignment was estimable in 41 patients: 10, 26, and 5 patients were classified in stage I/II, stage III/IV, and stage V/VI, respectively, whereas 1 patient was not classifiable. Explorative substaging identified 2 patients in stage I, 8 in stage II, 9 in stage III, 17 in stage IV, and 5 in stage V. However, the nominal 18F-flortaucipir--derived stage was not associated with clinical severity and was not indicative of pathology staging postmortem. Conclusion:18F-flortaucipir PET in vivo does not correspond to neuropathologic staging in PSP. This analytic approach, seeking to mirror in vivo neuropathology staging with PET-to-autopsy correlational analyses, might enable in vivo staging with next-generation tau PET tracers; however, further evidence and comparisons with postmortem data are needed.

Importin-Mediated Pathological Tau Nuclear Translocation Causes Disruption of the Nuclear Lamina, TDP-43 Mislocalization and Cell Death

Tau is a cytosolic protein that has also been observed in the nucleus, where it has multiple proposed functions that are regulated by phosphorylation. However, the mechanism underlying the nuclear import of tau is unclear, as is the contribution of nuclear tau to the pathology of tauopathies. We have previously generated a pathological form of tau, PH-tau (pseudophosphorylation mutants S199E, T212E, T231E, and S262E) that mimics AD pathological behavior in cells, Drosophila, and a mouse model. Here, we demonstrated that PH-tau translocates into the nucleus of transiently transfected HEK-293 cells, but wildtype tau does not. We identified a putative importin binding site in the tau sequence, and showed that disruption of this site prevents tau from entering the nucleus. We further showed that this nuclear translocation is prevented by inhibitors of both importin-α and importin-β. In addition, expression of PH-tau resulted in an enlarged population of dying cells, which is prevented by blocking its entry into the nucleus. PH-tau-expressing cells also exhibited disruption of the nuclear lamina and mislocalization of TDP-43 to the cytoplasm. We found that PH-tau does not bundle microtubules, and this effect is independent of nuclear translocation. These results demonstrate that tau translocates into the nucleus through the importin-α/β pathway, and that PH-tau exhibits toxicity after its nuclear translocation. We propose a model where hyperphosphorylated tau not only disrupts the microtubule network, but also translocates into the nucleus and interferes with cellular functions, such as nucleocytoplasmic transport, inducing mislocalization of proteins like TDP-43 and, ultimately, cell death.

Tauopathies: new perspectives and challenges

BACKGROUND

Tauopathies are a class of neurodegenerative disorders characterized by neuronal and/or glial tau-positive inclusions.

MAIN BODY

Clinically, tauopathies can present with a range of phenotypes that include cognitive/behavioral-disorders, movement disorders, language disorders and non-specific amnestic symptoms in advanced age. Pathologically, tauopathies can be classified based on the predominant tau isoforms that are present in the inclusion bodies (i.e., 3R, 4R or equal 3R:4R ratio). Imaging, cerebrospinal fluid (CSF) and blood-based tau biomarkers have the potential to be used as a routine diagnostic strategy and in the evaluation of patients with tauopathies. As tauopathies are strongly linked neuropathologically and genetically to tau protein abnormalities, there is a growing interest in pursuing of tau-directed therapeutics for the disorders. Here we synthesize emerging lessons on tauopathies from clinical, pathological, genetic, and experimental studies toward a unified concept of these disorders that may accelerate the therapeutics.

CONCLUSIONS

Since tauopathies are still untreatable diseases, efforts have been made to depict clinical and pathological characteristics, identify biomarkers, elucidate underlying pathogenesis to achieve early diagnosis and develop disease-modifying therapies.

Gender differences in progressive supranuclear palsy

The gender differences in progressive supranuclear palsy (PSP) are not extensively studied. The objective of this study was to determine the gender differences in the phenotypic expression and progression in PSP. We did a retrospective review of medical records of patients diagnosed with PSP over a 21-year period. The interval between disease onset and attainment of the five clinical disability milestones namely wheel chair dependency, unintelligible speech, severe dysphagia, severe cognitive impairment and urinary catheterization was used to determine the progression. Data was analysed from the case records of 334 patients with PSP. 209 patients (62.2%) were male and 125 (37.4%) among the patients were women (male:female ratio = 1.6:1). Males had older age at onset with longer duration of illness at time of presentation. Tremors were more common, PSP-P phenotype was more frequent and time to attain wheelchair dependency was earlier in males. Falls within 1 year of disease onset, apathy and executive dysfunction were more frequent and time to attain unintelligible speech, severe dysphagia and cognitive impairment were earlier in females. This study in a large cohort of clinically diagnosed cases of PSP has showed that gender differences exist in PSP in terms of clinical characteristics, progression of the disease.

Positron Emission Tomography in Animal Models of Tauopathies

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

Imaging pathological tau in atypical parkinsonisms: A review

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[18F]AV-1451 displays mixed results for specificity to 4R CBD- and PSP-tau.

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[18F]PI-2620 and [18F]PM-PBB3 are the most promising second-generation tau PET tracers.

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Research using second-generation tau PET tracers in CBD and PSP is still limited.

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Finding an imaging diagnostic biomarker requires further work with larger samples.

Atypical parkinsonisms (APs) are a group of diseases linked to tau pathology. These include progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). In the initial stages, these APs may have similar clinical manifestations to Parkinson’s disease (PD) and other parkinsonisms: bradykinesia, postural instability, tremor, and cognitive decline. Because of this, one major hurdle is the accurate early diagnosis of APs. Recent advances in positron emission tomography (PET) radiotracer development have allowed for targeting pathological tau in Alzheimer’s disease (AD). Currently, work is still in progress for identifying a first-in-class radiotracer for imaging tau in APs. In this review, we evaluate the literature on in vitro and in vivo testing of current tau PET radiotracers in APs. The tau PET tracers assessed include both first-generation tracers ([18F]AV-1451, [18F]FDDNP, [18F]THK derivatives, and [11C]PBB3) and second-generation tracers ([18F]PM-PBB3, [18F]PI-2620, [18F]RO-948, [18F]JNJ-067, [18F]MK-6240, and [18F]CBD-2115). Concerns regarding off-target binding to cerebral white matter and the basal ganglia are still prominent with first-generation tracers, but this seems to have been mediated in a handful of second-generation tracers, including [18F]PI-2620 and [18F]PM-PBB3. Additionally, these two tracers and [18F]MK-6240 show promising results for imaging PSP- and CBD-tau. Overall, [18F]AV-1451 is the most widely studied tracer but the mixed results regarding its efficacy for use in imaging AP-tau is a cause for concern moving forward. Instead, future work may benefit from focusing on the second-generation radiotracers which seem to have a higher specificity for AP-tau than those originally developed for imaging AD-tau.

OUP accepted manuscript

The progression of cognitive decline is heterogeneous in the three most common idiopathic parkinsonian diseases: Parkinson disease, multiple system atrophy and progressive supranuclear palsy. The causes for this heterogeneity are not fully understood, and there are no validated biomarkers that can accurately identify patients who will develop dementia and when. In this population-based, prospective study, comprehensive neuropsychological testing was performed repeatedly in new-onset, idiopathic parkinsonism. Dementia was diagnosed until 10 years and participants (N = 210) were deeply phenotyped by multimodal clinical, biochemical, genetic and brain imaging measures. At baseline, before the start of dopaminergic treatment, mild cognitive impairment was prevalent in 43.4% of the patients with Parkinson disease, 23.1% of the patients with multiple system atrophy and 77.8% of the patients with progressive supranuclear palsy. Longitudinally, all three diseases had a higher incidence of cognitive decline compared with healthy controls, but the types and severity of cognitive dysfunctions differed. In Parkinson disease, psychomotor speed and attention showed signs of improvement after dopaminergic treatment, while no such improvement was seen in other diseases. The 10-year cumulative probability of dementia was 54% in Parkinson disease and 71% in progressive supranuclear palsy, while there were no cases of dementia in multiple system atrophy. An easy-to-use, multivariable model that predicts the risk of dementia in Parkinson disease within 10 years with high accuracy (area under the curve: 0.86, P < 0.001) was developed. The optimized model adds CSF biomarkers to four easily measurable clinical features at baseline (mild cognitive impairment, olfactory function, motor disease severity and age). The model demonstrates a highly variable but predictable risk of dementia in Parkinson disease, e.g. a 9% risk within 10 years in a patient with normal cognition and CSF amyloid-β₄₂ in the highest tertile, compared with an 85% risk in a patient with mild cognitive impairment and CSF amyloid-β₄₂ in the lowest tertile. Only small or no associations with cognitive decline were found for factors that could be easily modifiable (such as thyroid dysfunction). Risk factors for cognitive decline in multiple system atrophy and progressive supranuclear palsy included signs of systemic inflammation and eye movement abnormalities. The predictive model has high accuracy in Parkinson disease and might be used for the selection of patients into clinical trials or as an aid to improve the prevention of dementia.

Neuroimaging correlates of gait abnormalities in progressive supranuclear palsy

Progressive supranuclear palsy is a neurodegenerative disorder characterized primarily by tau inclusions and neurodegeneration in the midbrain, basal ganglia, thalamus, premotor and frontal cortex. Neurodegenerative change in progressive supranuclear palsy has been assessed using MRI. Degeneration of white matter tracts is evident with diffusion tensor imaging and PET methods have been used to assess brain metabolism or presence of tau protein deposits. Patients with progressive supranuclear palsy present with a variety of clinical syndromes; however early onset of gait impairments and postural instability are common features. In this study we assessed the relationship between multimodal imaging biomarkers (i.e., MRI atrophy, white matter tracts degeneration, flortaucipir-PET uptake) and laboratory-based measures of gait and balance abnormalities in a cohort of nineteen patients with progressive supranuclear palsy, using univariate and multivariate statistical analyses. The PSP rating scale and its gait midline sub-score were strongly correlated to gait abnormalities but not to postural imbalance. Principal component analysis on gait variables identified velocity, stride length, gait stability ratio, length of gait phases and dynamic stability as the main contributors to the first component, which was associated with diffusion tensor imaging measures in the posterior thalamic radiation, external capsule, superior cerebellar peduncle, superior fronto-occipital fasciculus, body and splenium of the corpus callosum and sagittal stratum, with MRI volumes in frontal and precentral regions and with flortaucipir-PET uptake in the precentral gyrus. The main contributor to the second principal component was cadence, which was higher in patients presenting more abnormalities on mean diffusivity: this unexpected finding might be related to compensatory gait strategies adopted in progressive supranuclear palsy. Postural imbalance was the main contributor to the third principal component, which was related to flortaucipir-PET uptake in the left paracentral lobule and supplementary motor area and white matter disruption in the superior cerebellar peduncle, putamen, pontine crossing tract and corticospinal tract. A partial least square model identified flortaucipir-PET uptake in midbrain, basal ganglia and thalamus as the main correlate of speed and dynamic component of gait in progressive supranuclear palsy. Although causality cannot be established in this analysis, our study sheds light on neurodegeneration of brain regions and white matter tracts that underlies gait and balance impairment in progressive supranuclear palsy.

Molecular pathology and synaptic loss in primary tauopathies: an 18F-AV-1451 and 11C-UCB-J PET study

The relationship between in vivo synaptic density and molecular pathology in primary tauopathies is key to understanding the impact of tauopathy on functional decline and in informing new early therapeutic strategies. In this cross-sectional observational study, we determine the in vivo relationship between synaptic density and molecular pathology in the primary tauopathies of progressive supranuclear palsy and corticobasal degeneration as a function of disease severity.

Twenty-three patients with progressive supranuclear palsy and 12 patients with corticobasal syndrome were recruited from a tertiary referral centre. Nineteen education-, sex- and gender-matched control participants were recruited from the National Institute for Health Research ‘Join Dementia Research’ platform. Cerebral synaptic density and molecular pathology, in all participants, were estimated using PET imaging with the radioligands ¹¹C-UCB-J and ¹⁸F-AV-1451, respectively. Patients with corticobasal syndrome also underwent amyloid PET imaging with ¹¹C-PiB to exclude those with likely Alzheimer’s pathology—we refer to the amyloid-negative cohort as having corticobasal degeneration, although we acknowledge other underlying pathologies exist. Disease severity was assessed with the progressive supranuclear palsy rating scale; regional non-displaceable binding potentials of ¹¹C-UCB-J and ¹⁸F-AV-1451 were estimated in regions of interest from the Hammersmith Atlas, excluding those with known off-target binding for ¹⁸F-AV-1451. As an exploratory analysis, we also investigated the relationship between molecular pathology in cortical brain regions and synaptic density in subcortical areas.

Across brain regions, there was a positive correlation between ¹¹C-UCB-J and ¹⁸F-AV-1451 non-displaceable binding potentials (β = 0.4, t = 3.6, P = 0.001), independent of age or time between PET scans. However, this correlation became less positive as a function of disease severity in patients (β = −0.02, t = −2.9, P = 0.007, R = −0.41). Between regions, cortical ¹⁸F-AV-1451 binding was negatively correlated with synaptic density in subcortical areas (caudate nucleus, putamen). Brain regions with higher synaptic density are associated with a higher ¹⁸F-AV-1451 binding in progressive supranuclear palsy/corticobasal degeneration, but this association diminishes with disease severity. Moreover, higher cortical ¹⁸F-AV-1451 binding correlates with lower subcortical synaptic density. Longitudinal imaging is required to confirm the mediation of synaptic loss by molecular pathology. However, the effect of disease severity suggests a biphasic relationship between synaptic density and molecular pathology with synapse-rich regions vulnerable to accrual of pathological aggregates, followed by a loss of synapses in response to the molecular pathology.

Given the importance of synaptic function for cognition and action, our study elucidates the pathophysiology of primary tauopathies and may inform the design of future clinical trials.

Neuroinflammation predicts disease progression in progressive supranuclear palsy

INTRODUCTION

In addition to tau pathology and neuronal loss, neuroinflammation occurs in progressive supranuclear palsy (PSP). However, the prognostic value of the in vivo imaging markers for these processes in PSP remains unclear. We test the primary hypothesis that baseline in vivo imaging assessment of neuroinflammation in subcortical regions predicts clinical progression in patients with PSP.

METHODS

Seventeen patients with PSP-Richardson's syndrome underwent a baseline multimodal imaging assessment, including [11C]PK11195 positron emission tomography (PET) to index microglial activation, [18F]AV-1451 PET for tau pathology and structural MRI. Disease severity was measured at baseline and serially up to 4 years with the Progressive Supranuclear Palsy Rating Scale (PSPRS) (average interval of 5 months). Regional grey-matter volumes and PET ligand binding potentials were summarised by three principal component analyses (PCAs). A linear mixed-effects model was applied to the longitudinal PSPRS scores. Single-modality imaging predictors were regressed against the individuals' estimated rate of progression to identify the prognostic value of baseline imaging markers.

RESULTS

PCA components reflecting neuroinflammation and tau burden in the brainstem and cerebellum correlated with the subsequent annual rate of change in the PSPRS. PCA-derived PET markers of neuroinflammation and tau pathology correlated with regional brain volume in the same regions. However, MRI volumes alone did not predict the rate of clinical progression.

CONCLUSIONS

Molecular imaging with PET for microglial activation and tau pathology can predict clinical progression in PSP. These data encourage the evaluation of immunomodulatory approaches to disease-modifying therapies in PSP and the potential for PET to stratify patients in early phase clinical trials.

Update on neuroimaging for categorization of Parkinson's disease and atypical parkinsonism

PURPOSE OF REVIEW

Differential diagnosis of Parkinsonism may be difficult. The objective of this review is to present the work of the last three years in the field of imaging for diagnostic categorization of parkinsonian syndromes focusing on progressive supranuclear palsy (PSP) and multiple system atrophy (MSA).

RECENT FINDINGS

Two main complementary approaches are being pursued. The first seeks to develop and validate manual qualitative or semi-quantitative imaging markers that can be easily used in clinical practice. The second is based on quantitative measurements of magnetic resonance imaging abnormalities integrated in a multimodal approach and in automatic categorization machine learning tools.

SUMMARY

These two complementary approaches obtained high diagnostic around 90% and above in the classical Richardson form of PSP and probable MSA. Future work will determine if these techniques can improve diagnosis in other PSP variants and early forms of the diseases when all clinical criteria are not fully met.

Mechanisms of Neurodegeneration in Various Forms of Parkinsonism-Similarities and Differences

Parkinson’s disease (PD), dementia with Lewy body (DLB), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and multiple system atrophy (MSA) belong to a group of neurodegenerative diseases called parkinsonian syndromes. They share several clinical, neuropathological and genetic features. Neurodegenerative diseases are characterized by the progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra- and intracellular accumulation of misfolded proteins. The parkinsonian diseases affect distinct areas of the brain. PD and MSA belong to a group of synucleinopathies that are characterized by the presence of fibrillary aggregates of α-synuclein protein in the cytoplasm of selected populations of neurons and glial cells. PSP is a tauopathy associated with the pathological aggregation of the microtubule associated tau protein. Although PD is common in the world’s aging population and has been extensively studied, the exact mechanisms of the neurodegeneration are still not fully understood. Growing evidence indicates that parkinsonian disorders to some extent share a genetic background, with two key components identified so far: the microtubule associated tau protein gene (MAPT) and the α-synuclein gene (SNCA). The main pathways of parkinsonian neurodegeneration described in the literature are the protein and mitochondrial pathways. The factors that lead to neurodegeneration are primarily environmental toxins, inflammatory factors, oxidative stress and traumatic brain injury.