Corticobasal degeneration with TDP-43 pathology presenting with progressive supranuclear palsy syndrome: a distinct clinicopathologic subtype

Current concepts and molecular pathology of neurodegenerative diseases

Neurodegenerative diseases are a pathologically, clinically and genetically diverse group of diseases characterised by selective dysfunction, loss of synaptic connectivity and neurodegeneration, ​and are associated with the deposition of misfolded proteins in neurons and/or glia. Molecular studies have highlighted the role of conformationally altered proteins in the pathogenesis of neurodegenerative diseases and have paved the way for developing disease-specific biomarkers that capture and differentiate the main type/s of protein abnormality responsible for neurodegenerative diseases, some of which are currently used in clinical practice. These proteins follow sequential patterns of anatomical involvement and disease spread in the brain and may also be detected in peripheral organs. Recent studies suggest that glia are likely to have an important role in pathological spread throughout the brain and even follow distinct progression patterns from neurons. In addition to morphological and molecular approaches to the classification of these disorders, a further new stratification level incorporates the structure of protein filaments detected by cryogenic electron microscopy. Rather than occurring in isolation, combined deposition of tau, amyloid-β, α-synuclein and TDP-43 are frequently observed in neurodegenerative diseases and in the ageing brain. These can be overlooked, and their clinicopathological relevance is difficult to interpret. This review provides an overview of disease pathogenesis and diagnostic implications, recent molecular and ultrastructural classification of neurodegenerative diseases, how to approach ageing-related and mixed pathologies, ​and the importance of the protein-based classification system for practising neuropathologists and clinicians. This review also informs general pathologists about the relevance of ongoing full body autopsy studies to understand the spectrum and pathogenesis of neurodegenerative diseases.

Upper motor neuron-predominant motor neuron disease presenting as atypical parkinsonism: A clinicopathological study

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by upper and lower motor neuron signs. There are, however, cases where upper motor neurons (UMNs) are predominantly affected, leading to clinical presentations of UMN-dominant ALS or primary lateral sclerosis. Furthermore, cases exhibiting an UMN-predominant pattern of motor neuron disease (MND) presenting with corticobasal syndrome (CBS) have been sparsely reported. This study aims to clarify the clinicopathological features of patients with UMN-predominant MND. We reviewed 24 patients with UMN-predominant MND with TDP-43 pathology in the presence or absence of frontotemporal lobar degeneration. Additionally, we reviewed the medical records of patients with pathologically-confirmed corticobasal degeneration (CBD) who received a final clinical diagnosis of CBS (n = 10) and patients with pathologically-confirmed progressive supranuclear palsy (PSP) who received a final clinical diagnosis of PSP syndrome (n = 10). Of 24 UMN-predominant MND patients, 20 had a clinical diagnosis of an atypical parkinsonian disorder, including CBS (n = 11) and PSP syndrome (n = 8). Only two patients had antemortem diagnoses of motor neuron disease. UMN-predominant MND patients with CBS less frequently exhibited apraxia than those with CBD, and they were less likely to meet clinical criteria for possible or probable CBS. Similarly, UMN-predominant MND patients with PSP syndrome less often met clinical criteria for probable PSP than PSP patients with PSP syndrome. Our findings suggest that UMN-predominant MND can mimic atypical parkinsonism, and should be considered in the differential diagnosis of CBS and PSP syndrome, in particular when criteria are not met.

Decoding TDP-43: the molecular chameleon of neurodegenerative diseases

TAR DNA-binding protein 43 (TDP-43) has emerged as a critical player in neurodegenerative disorders, with its dysfunction implicated in a wide spectrum of diseases including amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and Alzheimer's disease (AD). This comprehensive review explores the multifaceted roles of TDP-43 in both physiological and pathological contexts. We delve into TDP-43's crucial functions in RNA metabolism, including splicing regulation, mRNA stability, and miRNA biogenesis. Particular emphasis is placed on recent discoveries regarding TDP-43's involvement in DNA interactions and chromatin dynamics, highlighting its broader impact on gene expression and genome stability. The review also examines the complex pathogenesis of TDP-43-related disorders, discussing the protein's propensity for aggregation, its effects on mitochondrial function, and its non-cell autonomous impacts on glial cells. We provide an in-depth analysis of TDP-43 pathology across various neurodegenerative conditions, from well-established associations in ALS and FTLD to emerging roles in diseases such as Huntington's disease and Niemann-Pick C disease. The potential of TDP-43 as a therapeutic target is explored, with a focus on recent developments in targeting cryptic exon inclusion and other TDP-43-mediated processes. This review synthesizes current knowledge on TDP-43 biology and pathology, offering insights into the protein's central role in neurodegeneration and highlighting promising avenues for future research and therapeutic interventions.

Co-Aggregation of TDP-43 with Other Pathogenic Proteins and Their Co-Pathologies in Neurodegenerative Diseases

Pathological aggregation of a specific protein into insoluble aggregates is a common hallmark of various neurodegenerative diseases (NDDs). In the earlier literature, each NDD is characterized by the aggregation of one or two pathogenic proteins, which can serve as disease-specific biomarkers. The aggregation of these specific proteins is thought to be a major cause of or deleterious result in most NDDs. However, accumulating evidence shows that a pathogenic protein can interact and co-aggregate with other pathogenic proteins in different NDDs, thereby contributing to disease onset and progression synergistically. During the past years, more than one type of NDD has been found to co-exist in some individuals, which may increase the complexity and pathogenicity of these diseases. This article reviews and discusses the biochemical characteristics and molecular mechanisms underlying the co-aggregation and co-pathologies associated with TDP-43 pathology. The TDP-43 aggregates, as a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), can often be detected in other NDDs, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and spinocerebellar ataxia type 2 (SCA2). In many cases, TDP-43 is shown to interact and co-aggregate with multiple pathogenic proteins in vitro and in vivo. Furthermore, the co-occurrence and co-aggregation of TDP-43 with other pathogenic proteins have important consequences that may aggravate the diseases. Thus, the current viewpoint that the co-aggregation of TDP-43 with other pathogenic proteins in NDDs and their relevance to disease progression may gain insights into the patho-mechanisms and therapeutic potential of various NDDs.

The Imbalance of Homocysteine, Vitamin B12 and Folic Acid in Parkinson Plus Syndromes: A Review beyond Parkinson Disease

While there is a link between homocysteine (Hcy), B12 and folic acid and neurodegeneration, especially in disorders like Parkinson's and Alzheimer's diseases, its role in Parkinson plus syndromes (PPS) has only been partially investigated. It appears that elevated Hcy, along with an imbalance of its essential vitamin cofactors, are both implicated in the development and progression of parkinsonian syndromes, which represent different disease pathologies, namely alpha-synucleinopathies and tauopathies. Attributing a potential pathogenetic role in hyperhomocysteinemia would be crucial in terms of improving the diagnostic and prognostic accuracy of these syndromes and also for providing a new target for possible therapeutic intervention. The scope of this review is to focus on vitamin imbalance in PPS, with a special emphasis on the role of Hcy, B12 and folic acid in the neurodegenerative process and their implication in the therapeutic approach of these disorders.

Depletion of TDP-43 exacerbates tauopathy-dependent brain atrophy by sensitizing vulnerable neurons to caspase 3-mediated endoproteolysis of tau in a mouse model of Multiple Etiology Dementia

TDP-43 proteinopathy, initially disclosed in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), coexists with tauopathy in a variety of neurodegenerative disorders, termed multiple etiology dementias (MEDs), including Alzheimer's Disease (AD). While such co-pathology of TDP-43 is strongly associated with worsened neurodegeneration and steeper cognitive decline, the pathogenic mechanism underlying the exacerbated neuron loss remains elusive. The loss of TDP-43 splicing repression that occurs in presymptomatic ALS-FTD individuals suggests that such early loss could facilitate the pathological conversion of tau to accelerate neuron loss. Here, we report that the loss of TDP-43 repression of cryptic exons in forebrain neurons (CaMKII-CreER;Tardbp f/f mice) is necessary to exacerbate tauopathy-dependent brain atrophy by sensitizing vulnerable neurons to caspase 3-dependent cleavage of endogenous tau to promote tauopathy. Corroborating this finding within the human context, we demonstrate that loss of TDP-43 function in iPSC-derived cortical neurons promotes early cryptic exon inclusion and subsequent caspase 3-mediated endoproteolysis of tau. Using a genetic approach to seed tauopathy in CaMKII-CreER;Tardbp f/f mice by expressing a four-repeat microtubule binding domain of human tau, we show that the amount of tau seed positively correlates with levels of caspase 3-cleaved tau. Importantly, we found that the vulnerability of hippocampal neurons to TDP-43 depletion is dependent on the amount of caspase 3-cleaved tau: from most vulnerable neurons in the CA2/3, followed by those in the dentate gyrus, to the least in CA1. Taken together, our findings strongly support the view that TDP-43 loss-of-function exacerbates tauopathy-dependent brain atrophy by increasing the sensitivity of vulnerable neurons to caspase 3-mediated endoproteolysis of tau, resulting in a greater degree of neurodegeneration in human disorders with co-pathologies of tau and TDP-43. Our work thus discloses novel mechanistic insights and therapeutic targets for human tauopathies harboring co-pathology of TDP-43 and provides a new MED model for testing therapeutic strategies.

Gliovascular transcriptional perturbations in Alzheimer's disease reveal molecular mechanisms of blood brain barrier dysfunction

To uncover molecular changes underlying blood-brain-barrier dysfunction in Alzheimer's disease, we performed single nucleus RNA sequencing in 24 Alzheimer's disease and control brains and focused on vascular and astrocyte clusters as main cell types of blood-brain-barrier gliovascular-unit. The majority of the vascular transcriptional changes were in pericytes. Of the vascular molecular targets predicted to interact with astrocytic ligands, SMAD3, upregulated in Alzheimer's disease pericytes, has the highest number of ligands including VEGFA, downregulated in Alzheimer's disease astrocytes. We validated these findings with external datasets comprising 4,730 pericyte and 150,664 astrocyte nuclei. Blood SMAD3 levels are associated with Alzheimer's disease-related neuroimaging outcomes. We determined inverse relationships between pericytic SMAD3 and astrocytic VEGFA in human iPSC and zebrafish models. Here, we detect vast transcriptome changes in Alzheimer's disease at the gliovascular-unit, prioritize perturbed pericytic SMAD3-astrocytic VEGFA interactions, and validate these in cross-species models to provide a molecular mechanism of blood-brain-barrier disintegrity in Alzheimer's disease.

Senile plaque-associated transactive response DNA-binding protein 43 in Alzheimer's disease: A case report spanning 16 years of memory loss

Transactive response DNA-binding protein 43 (TDP-43) pathological inclusions are found in frontotemporal lobar degeneration (FTLD-TDP) and Alzheimer's disease (AD-TDP). While clinically different, TDP-43 inclusions in FTLD-TDP and AD can have similar morphological characteristics. However, TDP-43 colocalizing with tau and forming "apple-bite" or "flame-shaped" neuronal cytoplasmic inclusions (NCI) are only found in AD-TDP. Here, we describe a case with AD and neuritic plaque-associated TDP-43. The patient was a 96-year-old right-handed Caucasian woman who had developed a slowly progressive amnestic syndrome compatible with typical AD at age 80. Genetic testing revealed APOE ε3/ε4, GRN r5848 CT, and MAPT H1/H2 genotype. Consistent with the old age at onset and long disease duration, limbic-predominant AD was found at autopsy, with high hippocampal yet low cortical neurofibrillary tangle (NFT) counts. Hippocampal and amygdala sclerosis were present. Immunohistochemistry for phospho-TDP-43 showed NCIs, dystrophic neurites, and rare neuronal intranuclear inclusions consistent with FTLD-TDP type A, as well as tau NFT-associated TDP-43 inclusions. These were frequent in the amygdala, entorhinal cortex, hippocampus, occipitotemporal gyrus, and inferior temporal gyrus but sparse in the mid-frontal cortex. Additionally, there were TDP-43-immunoreactive inclusions forming plaque-like structures in the molecular layer of the dentate fascia of the hippocampus. The presence of neuritic plaques in the same region was confirmed using thioflavin-S fluorescent microscopy and immunohistochemistry for phospho-tau. Double labeling immunofluorescence showed colocalization of TDP-43 and tau within neuritic plaques. Other pathologies included mild Lewy body pathology predominantly affecting the amygdala and olfactory bulb, aging-related tau astrogliopathy, and mixed small vessel disease (arteriolosclerosis and amyloid angiopathy) with several cortical microinfarcts. In conclusion, we have identified TDP-43 colocalizing with tau in neuritic plaques in AD, which expands the association of TDP-43 and tau in AD beyond NFTs. The clinical correlate of this plaque-associated TDP-43 appears to be a slowly progressive amnestic syndrome.

Clinicopathologic features of a novel star-shaped transactive response DNA-binding protein 43 (TDP-43) pathology in the oldest old

Transactive response DNA-binding protein 43 (TDP-43) pathology is categorized as type A-E in frontotemporal lobar degeneration and as type α-β in Alzheimer disease (AD) based on inclusion type. We screened amygdala slides of 131 cases with varying ages at death, clinical/neuroimaging findings, and AD neuropathologic changes for TDP-43 pathology using anti-phospho-TDP-43 antibodies. Seven cases (5%) only showed atypical TDP-43 inclusions that could not be typed. Immunohistochemistry and immunofluorescence assessed the atypical star-shaped TDP-43 pathology including its distribution, species, cellular localization, and colocalization with tau. All 7 had died at an extremely old age (median: 100 years [IQR: 94-101]) from nonneurological causes and none had dementia (4 cognitively unimpaired, 3 with amnestic mild cognitive impairment). Neuroimaging showed mild medial temporal involvement. Pathologically, the star-shaped TDP-43-positive inclusions were found in medial (subpial) amygdala and, occasionally, in basolateral regions. Hippocampus only showed TDP-43-positive neurites in the fimbria and subiculum while the frontal lobe was free of TDP-43 inclusions. The star-shaped inclusions were better detected with antibodies against N-terminal than C-terminal TDP-43. Double-labeling studies confirmed deposition of TDP-43 within astrocytes and colocalization with tau. We have identified a novel TDP-43 pathology with star-shaped morphology associated with superaging, with a homogeneous clinicopathologic picture, possibly representing a novel, true aging-related TDP-43 pathology.

Pathomechanisms of cognitive and behavioral impairment in corticobasal degeneration

Corticobasal degeneration (CBD) is a rare, sporadic, late-onset progressive neurodegenerative disorder of unknown etiology, clinically characterized by an akinetic-rigid syndrome, behavior and personality disorders, language problems (aphasias), apraxia, executive and cognitive abnormalities and limb dystonia. The syndrome is not specific, as clinical features of pathologically proven CBD include several phenotypes. This 4-repeat (4R) tauopathy is morphologically featured by often asymmetric frontoparietal atrophy, ballooned/achromatic neurons containing filamentous 4R-tau aggregates in cortex and striatum, thread-like processes that are more widespread than in progressive supranuclear palsy (PSP), pathognomonic "astroglial plaques", and numerous inclusions in both astrocytes and oligodendroglia ("coiled bodies") in the white matter. Cognitive deficits in CBD are frequent initial presentations before onset of motor symptoms, depending on the phenotypic variant. They predominantly include executive and visuospatial dysfunction, sleep disorders and language deficits with usually preserved memory domains. Neuroimaging studies showed heterogenous locations of brain atrophy, particularly contralateral to the dominant symptoms, with disruption of striatal connections to prefrontal cortex and basal ganglia circuitry. Asymmetric hypometabolism, mainly involving frontal and parietal regions, is associated with brain cholinergic deficits, and dopaminergic nigrostriatal degeneration. Widespread alteration of cortical and subcortical structures causing heterogenous changes in various brain functional networks support the concept that CBD, similar to PSP, is a brain network disruption disorder. Putative pathogenic factors are hyperphosphorylated tau-pathology, neuroinflammation and oxidative injury, but the basic mechanisms of cognitive impairment in CBD, as in other degenerative movement disorders, are complex and deserve further elucidation as a basis for early diagnosis and adequate treatment of this fatal disorder.

Clinical course in corticobasal syndrome and corticobasal degeneration: implications for diagnosis and management

This scientific commentary relates to 'Clinical course of pathologically confirmed corticobasal degeneration and corticobasal syndrome', by Aiba et al. (https://doi.org/10.1093/braincomms/fcad296).

Diffuse argyrophilic grain disease with TDP-43 proteinopathy and neuronal intermediate filament inclusion disease: FTLD with mixed tau, TDP-43 and FUS pathologies

Frontotemporal lobar degeneration (FTLD) is a group of disorders characterized by degeneration of the frontal and temporal lobes, leading to progressive decline in language, behavior, and motor function. FTLD can be further subdivided into three main subtypes, FTLD-tau, FTLD-TDP and FTLD-FUS based which of the three major proteins - tau, TDP-43 or FUS - forms pathological inclusions in neurons and glia. In this report, we describe an 87-year-old woman with a 7-year history of cognitive decline, hand tremor and gait problems, who was thought to have Alzheimer's disease. At autopsy, histopathological analysis revealed severe neuronal loss, gliosis and spongiosis in the medial temporal lobe, orbitofrontal cortex, cingulate gyrus, amygdala, basal forebrain, nucleus accumbens, caudate nucleus and anteromedial thalamus. Tau immunohistochemistry showed numerous argyrophilic grains, pretangles, thorn-shaped astrocytes, and ballooned neurons in the amygdala, hippocampus, parahippocampal gyrus, anteromedial thalamus, insular cortex, superior temporal gyrus and cingulate gyrus, consistent with diffuse argyrophilic grain disease (AGD). TDP-43 pathology in the form of small, dense, rounded neuronal cytoplasmic inclusion with few short dystrophic neurites was observed in the limbic regions, superior temporal gyrus, striatum and midbrain. No neuronal intranuclear inclusion was observed. Additionally, FUS-positive inclusions were observed in the dentate gyrus. Compact, eosinophilic intranuclear inclusions, so-called "cherry spots," that were visible on histologic stains were immunopositive for α-internexin. Taken together, the patient had a mixed neurodegenerative disease with features of diffuse AGD, TDP-43 proteinopathy and neuronal intermediate filament inclusion disease. She met criteria for three subtypes of FTLD: FTLD-tau, FTLD-TDP and FTLD-FUS. Her amnestic symptoms that were suggestive of Alzheimer's type dementia are best explained by diffuse AGD and medial temporal TDP-43 proteinopathy, and her motor symptoms were likely explained by neuronal loss and gliosis due to tau pathology in the substantia nigra. This case underscores the importance of considering multiple proteinopathies in the diagnosis of neurodegenerative diseases.

Charting the Next Road Map for CSF Biomarkers in Alzheimer's Disease and Related Dementias

Clinical prediction of underlying pathologic substrates in people with Alzheimer's disease (AD) dementia or related dementia syndromes (ADRD) has limited accuracy. Etiologic biomarkers - including cerebrospinal fluid (CSF) levels of AD proteins and cerebral amyloid PET imaging - have greatly modernized disease-modifying clinical trials in AD, but their integration into medical practice has been slow. Beyond core CSF AD biomarkers (including beta-amyloid 1-42, total tau, and tau phosphorylated at threonine 181), novel biomarkers have been interrogated in single- and multi-centered studies with uneven rigor. Here, we review early expectations for ideal AD/ADRD biomarkers, assess these goals' future applicability, and propose study designs and performance thresholds for meeting these ideals with a focus on CSF biomarkers. We further propose three new characteristics: equity (oversampling of diverse populations in the design and testing of biomarkers), access (reasonable availability to 80% of people at risk for disease, along with pre- and post-biomarker processes), and reliability (thorough evaluation of pre-analytical and analytical factors influencing measurements and performance). Finally, we urge biomarker scientists to balance the desire and evidence for a biomarker to reflect its namesake function, indulge data- as well as theory-driven associations, re-visit the subset of rigorously measured CSF biomarkers in large datasets (such as Alzheimer's disease neuroimaging initiative), and resist the temptation to favor ease over fail-safe in the development phase. This shift from discovery to application, and from suspended disbelief to cogent ingenuity, should allow the AD/ADRD biomarker field to live up to its billing during the next phase of neurodegenerative disease research.

APOE2 Exacerbates TDP-43 Related Toxicity in the Absence of Alzheimer Pathology

OBJECTIVE

Recent evidence supports a link between increased TDP-43 burden and the presence of an APOE4 gene allele in Alzheimer's disease (AD); however, it is difficult to conclude the direct effect of APOE on TDP-43 pathology due to the presence of mixed AD pathologies. The goal of this study is to address how APOE isoforms impact TDP-43 pathology and related neurodegeneration in the absence of typical AD pathologies.

METHODS

We overexpressed human TDP-43 via viral transduction in humanized APOE2, APOE3, APOE4 mice, and murine Apoe-knockout (Apoe-KO) mice. Behavior tests were performed across ages. Animals were harvested at 11 months of age and TDP-43 overexpression-related neurodegeneration and gliosis were assessed. To further address the human relevance, we analyzed the association of APOE with TDP-43 pathology in 160 postmortem brains from autopsy-confirmed amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with motor neuron disease (FTLD-MND) in the Mayo Clinic Brain Bank.

RESULTS

We found that TDP-43 overexpression induced motor function deficits, neuronal loss, and gliosis in the motor cortex, especially in APOE2 mice, with much milder or absent effects in APOE3, APOE4, or Apoe-KO mice. In the motor cortex of the ALS and FTLD-MND postmortem human brains, we found that the APOE2 allele was associated with more severe TDP-43-positive dystrophic neurites.

INTERPRETATION

Our data suggest a genotype-specific effect of APOE on TDP-43 proteinopathy and neurodegeneration in the absence of AD pathology, with the strongest association seen with APOE2. ANN NEUROL 2023;93:830-843.

Molecular fragment characteristics and distribution of tangle associated TDP-43 (TATs) and other TDP-43 lesions in Alzheimer's disease

TAR DNA binding protein 43 (TDP-43) pathology is a defining feature of frontotemporal lobar degeneration (FTLD). In FTLD-TDP there is a moderate-to-high burden of morphologically distinctive TDP-43 immunoreactive inclusions distributed throughout the brain. In Alzheimer's disease (AD), similar TDP-43 immunoreactive inclusions are observed. In AD, however, there is a unique phenomenon of neurofibrillary tangle-associated TDP-43 (TATs) whereby TDP-43 intermingles with neurofibrillary tangles. Little is known about the characteristics and distribution of TATs, or how burden and distribution of TATs compares to burden and distribution of other FTLD-TDP-like lesions observed in AD. Here we characterize molecular fragment characteristics, burden and distribution of TATs and assess how these features compare to features of other TDP-43 lesions. We performed TDP-43 immunohistochemistry with anti-phosphorylated, C- and N-terminal TDP-43 antibodies in 20 high-probability AD cases and semi-quantitative burden of seven inclusion types within five brain regions (entorhinal cortex, subiculum, CA1 and dentate gyrus of hippocampus, occipitotemporal cortex). Hierarchical cluster analysis was used to analyze the dataset that consisted of 75 different combinations of neuropathological features. TATs were nonspherical with heterogeneous staining patterns and present in all regions except hippocampal dentate. All three antibodies detected TATs although N-terminal antibody sensitivity was low. Three clusters were identified: Cluster-1 had mild-moderate TATs, moderate-frequent neuronal cytoplasmic inclusions, dystrophic neurites, neuronal intranuclear inclusions and fine neurites, and perivascular and granular inclusions identified only with the N-terminal antibody throughout the brain; Cluster-2 had scant TATs in limbic regions and Cluster-3 mild-moderate TATs and mild-moderate neuronal cytoplasmic inclusions and dystrophic neurites throughout the brain and moderate fine neurites. Only 17% of cluster 1 cases had the TMEM106b GG (protective) haplotype and 83% had hippocampal sclerosis. Both features differed across clusters (p=0.03 & p=0.01). TATs have molecular characteristics, distribution and burden, and genetic and pathologic associations like FTLD-TDP lesions.

TDP-43 Proteinopathy and Tauopathy: Do They Have Pathomechanistic Links?

Transactivation response DNA binding protein 43 kDa (TDP-43) and tau are major pathological proteins of neurodegenerative disorders, of which neuronal and glial aggregates are pathological hallmarks. Interestingly, accumulating evidence from neuropathological studies has shown that comorbid TDP-43 pathology is observed in a subset of patients with tauopathies, and vice versa. The concomitant pathology often spreads in a disease-specific manner and has morphological characteristics in each primary disorder. The findings from translational studies have suggested that comorbid TDP-43 or tau pathology has clinical impacts and that the comorbid pathology is not a bystander, but a part of the disease process. Shared genetic risk factors or molecular abnormalities between TDP-43 proteinopathies and tauopathies, and direct interactions between TDP-43 and tau aggregates, have been reported. Further investigations to clarify the pathogenetic factors that are shared by a broad spectrum of neurodegenerative disorders will establish key therapeutic targets.

Galactosylceramidase deficiency and pathological abnormalities in cerebral white matter of Krabbe disease

Krabbe Disease (KD) is an autosomal recessive disorder that results from loss-of-function mutations in the GALC gene, which encodes lysosomal enzyme galactosylceramidase (GALC). Functional deficiency of GALC is toxic to myelin-producing cells, which leads to progressive demyelination in both the central and peripheral nervous systems. It is hypothesized that accumulation of psychosine, which can only be degraded by GALC, is a primary initiator of pathologic cascades. Despite the central role of GALC in KD pathomechanism, investigations of GALC deficiency at a protein level are largely absent, due in part, to the lack of sensitive antibodies in the field. Leveraging two custom antibodies that can detect GALC at endogenous levels, we demonstrated that GALC protein is predominantly localized to oligodendrocytes in cerebral white matter of an infant brain, consistent with its functional role in myelination. Mature GALC could also be quantitatively detected as a 26 kDa band by western blotting and correlated to enzyme activity in brain tissues. The p.Ile562Thr polymorphic variant, which is over-represented in the KD population, was associated with reduced mature GALC protein and activity. In three infantile KD cases, homozygous null mutations in GALC lead to deficiency in total GALC protein and activity. Interestingly, although GALC activity was absent, normal levels of total GALC protein were detected by a sandwich ELISA using our custom antibodies in a later-onset KD brain, which suggests that the assay has the potential to differentiate infantile- and later-onset KD cases. Among the infantile KD cases, we quantified a 5-fold increase in psychosine levels, and observed increased levels of acid ceramidase, a key enzyme for psychosine production, and hyperglycosylated lysosomal-associated membrane protein 1, a marker for lysosomal activation, in periventricular white matter, a major pathological brain region, when compared with age-matched normal controls. While near complete demyelination was observed in these cases, we quantified that an early-infantile case (age of death at 10 months) had about 3-fold increases in both globoid cells, a pathological hallmark for KD, and CD8-positive T lymphocytes, a pathological marker for multiple sclerosis, in the white matter when compared with a slower progressing infantile case (age of death at 21 months), which suggests a positive correlation between clinical severity and neuropathology. Taken together, our findings have advanced the understanding of GALC protein biology in the context of normal and KD brain white matter. We also revealed new neuropathological changes that may provide insights to understand KD pathogenesis.

New developments of biofluid-based biomarkers for routine diagnosis and disease trajectories in frontotemporal dementia

Frontotemporal dementia (FTD) covers a spectrum of neurodegenerative disorders with different phenotypes, genetic backgrounds, and pathological states. Its clinicopathological diversity challenges the diagnostic process and the execution of clinical trials, calling for specific diagnostic biomarkers of pathologic FTD types. There is also a need for biomarkers that facilitate disease staging, quantification of severity, monitoring in clinics and observational studies, and for evaluation of target engagement and treatment response in clinical trials. This review discusses current FTD biofluid-based biomarker knowledge taking into account the differing applications. The limitations, knowledge gaps, and challenges for the development and implementation of such markers are also examined. Strategies to overcome these hurdles are proposed, including the technologies available, patient cohorts, and collaborative research initiatives. Access to robust and reliable biomarkers that define the exact underlying pathophysiological FTD process will meet the needs for specific diagnosis, disease quantitation, clinical monitoring, and treatment development.

Old age amyotrophic lateral sclerosis and limbic TDP-43 pathology

This study aimed to assess and compare the burden of transactive response DNA-binding protein of 43 kDa (TDP-43) pathology and clinical features of amyotrophic lateral sclerosis (ALS) in three age groups. All cases were from the Mayo Clinic brain bank for neurodegenerative disorders and most were followed longitudinally in the ALS Clinic. Cases with moderate-to-severe Alzheimer's disease neuropathological change were excluded. The 55 cases included in the study were divided into three groups by age at death: 75 years or older (old-ALS, n = 8), 64-74 years (middle-ALS, n = 23), and 63 years or younger (young-ALS, n = 24). Clinical features, including disease duration, initial symptoms, and ALS Cognitive Behavior Score (ALS-CBS), were summarized. Sections of paraffin-embedded tissue from the motor cortex, basal forebrain, medial temporal lobe, and middle frontal gyrus were processed for phospho-TDP-43 immunohistochemistry. The burden of TDP-43 pathology was analyzed using digital image analysis. The TDP-43 burden in the limbic system (i.e., amygdala, dentate gyrus and CA1 sector of the hippocampus, subiculum, and entorhinal cortex) was greater in old-ALS than in young-ALS and middle-ALS. TDP-43 burden in the middle frontal gyrus was sparse and did not differ between the three groups. The average of ALS-CBS was not different between the three groups. The present study shows that the amygdala and hippocampus are vulnerable to TDP-43 pathology in older patients with ALS. We discuss the evidence for and against this pathology being related to concurrent limbic-predominant, age-related TDP-43 encephalopathy neuropathologic change.

Frontotemporal lobar degeneration with TAR DNA-binding protein 43 (TDP-43): its journey of more than 100 years

Frontotemporal lobar degeneration (FTLD) with TDP-43-immunoreactive inclusions (FTLD-TDP) is a neurodegenerative disease associated with clinical, genetic, and neuropathological heterogeneity. An association between TDP-43, FTLD and amyotrophic lateral sclerosis (ALS) was first described in 2006. However, a century before immunohistochemistry existed, atypical dementias displaying behavioral, language and/or pyramidal symptoms and showing non-specific FTLD with superficial cortical neuronal loss, gliosis and spongiosis were often confused with Alzheimer's or Pick's disease. Initially this pathology was termed dementia lacking distinctive histopathology (DLDH), but this was later renamed when ubiquitinated inclusions originally found in ALS were also discovered in (DLDH), thus warranting a recategorization as FTLD-U (ubiquitin). Finally, the ubiquitinated protein was identified as TDP-43, which aggregates in cortical, subcortical, limbic and brainstem neurons and glial cells. The topography and morphology of TDP-43 inclusions associate with specific clinical syndromes and genetic mutations which implies different pathomechanisms that are yet to be discovered; hence, the TDP-43 journey has actually just begun. In this review, we describe how FTLD-TDP was established and defined clinically and neuropathologically throughout the past century.

Magnetic Resonance Planimetry in the Differential Diagnosis between Parkinson’s Disease and Progressive Supranuclear Palsy

The clinical differential diagnosis between Parkinson’s disease (PD) and progressive supranuclear palsy (PSP) is often challenging. The description of milder PSP phenotypes strongly resembling PD, such as PSP-Parkinsonism, further increased the diagnostic challenge and the need for reliable neuroimaging biomarkers to enhance the diagnostic certainty. This review aims to summarize the contribution of a relatively simple and widely available imaging technique such as MR planimetry in the differential diagnosis between PD and PSP, focusing on the recent advancements in this field. The development of accurate MR planimetric biomarkers, together with the implementation of automated algorithms, led to robust and objective measures for the differential diagnosis of PSP and PD at the individual level. Evidence from longitudinal studies also suggests a role of MR planimetry in predicting the development of the PSP clinical signs, allowing to identify PSP patients before they meet diagnostic criteria when their clinical phenotype can be indistinguishable from PD. Finally, promising evidence exists on the possible association between MR planimetric measures and the underlying pathology, with important implications for trials with new disease-modifying target therapies.

Parallel Appearance of Polyglutamine and Transactivation-Responsive DNA-Binding Protein 43 and Their Complementary Subcellular Localization in Brains of Patients With Spinocerebellar Ataxia Type 2

Spinocerebellar ataxia type 2 (SCA2) is caused by mutations in the ATXN2 gene in which toxic effects are triggered by expanded polyglutamine repeats within ataxin-2. SCA2 is accompanied by motor neuron degeneration as occurs in amyotrophic lateral sclerosis (ALS). We investigated the distribution patterns of ataxin-2 and transactivation-responsive DNA-binding protein 43 (TDP-43), a major disease-related protein in ALS, in the CNS of 3 SCA2 patients. Phosphorylated TDP-43 (pTDP-43)-positive lesions were widely distributed throughout the CNS and generally overlapped with 1C2 (expanded polyglutamine)-immunoreactive lesions. This distribution pattern is different from the pattern in limbic-predominant age-related TDP-43 encephalopathy. In SCA2, double immunostaining of TDP-43 and 1C2 in motor neurons revealed 3 staining patterns: cytoplasmic 1C2 and nuclear TDP-43, nucleocytoplasmic 1C2 and nuclear TDP-43, and nuclear 1C2 and cytoplasmic TDP-43, which reflect the early, active, and final stages of pathological change, respectively. The translocation of TDP-43 from the nucleus to the cytoplasm along with the translocation of 1C2 in the opposite direction indicates that nuclear accumulation of the disease-specific protein ataxin-2 affects the intracellular dynamics of TDP-43. Such a close interrelationship between mutant ataxin-2 and TDP-43 in the cell might account for the similarity of their distribution in the CNS of patients with SCA2.

Neuropathology and emerging biomarkers in corticobasal syndrome

Corticobasal syndrome (CBS) is a clinical syndrome characterised by progressive asymmetric limb rigidity and apraxia with dystonia, myoclonus, cortical sensory loss and alien limb phenomenon. Corticobasal degeneration (CBD) is one of the most common underlying pathologies of CBS, but other disorders, such as progressive supranuclear palsy (PSP), Alzheimer's disease (AD) and frontotemporal lobar degeneration with TDP-43 inclusions, are also associated with this syndrome.In this review, we describe common and rare neuropathological findings in CBS, including tauopathies, synucleinopathies, TDP-43 proteinopathies, fused in sarcoma proteinopathy, prion disease (Creutzfeldt-Jakob disease) and cerebrovascular disease, based on a narrative review of the literature and clinicopathological studies from two brain banks. Genetic mutations associated with CBS, including GRN and MAPT, are also reviewed. Clinicopathological studies on neurodegenerative disorders associated with CBS have shown that regardless of the underlying pathology, frontoparietal, as well as motor and premotor pathology is associated with CBS. Clinical features that can predict the underlying pathology of CBS remain unclear. Using AD-related biomarkers (ie, amyloid and tau positron emission tomography (PET) and fluid biomarkers), CBS caused by AD often can be differentiated from other causes of CBS. Tau PET may help distinguish AD from other tauopathies and non-tauopathies, but it remains challenging to differentiate non-AD tauopathies, especially PSP and CBD. Although the current clinical diagnostic criteria for CBS have suboptimal sensitivity and specificity, emerging biomarkers hold promise for future improvements in the diagnosis of underlying pathology in patients with CBS.

Current concepts of mixed pathologies in neurodegenerative diseases

Neurodegenerative diseases are a pathologically, clinically and genetically diverse group of disorders without effective disease-modifying therapies. Pathologically, these disorders are characterised by disease-specific protein aggregates in neurons and/or glia and referred to as proteinopathies. Many neurodegenerative diseases show pathological overlap with the same abnormally deposited protein occurring in anatomically distinct regions, which give rise to specific patterns of cognitive and motor clinical phenotypes. Sequential distribution patterns of protein inclusions throughout the brain have been described. Rather than occurring in isolation, it is increasingly recognised that combinations of one or more proteinopathies with or without cerebrovascular disease frequently occur in individuals with neurodegenerative diseases. In addition, complex constellations of ageing-related and incidental pathologies associated with tau, TDP-43, Aβ, α-synuclein deposition have been commonly reported in longitudinal ageing studies. This review provides an overview of current classification of neurodegenerative and age-related pathologies and presents the spectrum and complexity of mixed pathologies in community-based, longitudinal ageing studies, in major proteinopathies, and genetic conditions. Mixed pathologies are commonly reported in individuals >65 years with and without cognitive impairment; however, they are increasingly recognised in younger individuals (<65 years). Mixed pathologies are thought to lower the threshold for developing cognitive impairment and dementia. Hereditary neurodegenerative diseases also show a diverse range of mixed pathologies beyond the proteinopathy primarily linked to the genetic abnormality. Cases with mixed pathologies might show a different clinical course, which has prognostic relevance and obvious implications for biomarker and therapy development, and stratifying patients for clinical trials.

Motor neuron TDP-43 proteinopathy in progressive supranuclear palsy and corticobasal degeneration

Transactive response DNA-binding protein 43 kDa (TDP-43) is mislocalized from the nucleus and aggregates within the cytoplasm of affected neurons in amyotrophic lateral sclerosis (ALS) cases. TDP-43 pathology has also been found in brain tissues under non-ALS conditions, suggesting mechanistic links between TDP-43-related ALS (ALS-TDP) and various neurological disorders. This study aimed to assess TDP-43 pathology in the spinal cord motor neurons of tauopathies. We examined 106 spinal cords from consecutively autopsied cases with progressive supranuclear palsy (PSP, n = 26), corticobasal degeneration (CBD, n = 12), globular glial tauopathy (GGT, n = 5), Alzheimer's disease (AD, n = 21), or Pick disease (PiD, n = 6) and neurologically healthy controls (n = 36). Ten of the PSP cases (38%) and seven of the CBD cases (58%) showed mislocalization and cytoplasmic aggregation of TDP-43 in spinal cord motor neurons, which was prominent in the cervical cord. TDP-43-aggregates were found to be skein-like, round-shaped, granular, or dot-like and contained insoluble C-terminal fragments showing blotting pattern of ALS or frontotemporal lobar degeneration (FTLD). The lower motor neurons also showed cystatin-C aggregates, although Bunina bodies were absent in hematoxylin-eosin staining. The spinal cord TDP-43 pathology was often associated with TDP-43 pathology of the primary motor cortex. Positive correlations were shown between the severities of TDP-43 and 4-repeat (4R)-tau aggregates in the cervical cord. TDP-43 and 4R-tau aggregates burdens positively correlated with microglial burden in anterior horn. TDP-43 pathology of spinal cord motor neuron did not develop in an age-dependent manner and was not found in the AD, PiD, GGT, and control groups. Next, we assessed splicing factor proline/glutamine rich (SFPQ) expression in spinal cord motor neurons; SFPQ is a recently-identified regulator of ALS/FTLD pathogenesis, and it is also reported that interaction between SFPQ and fused-in-sarcoma (FUS) regulates splicing of microtubule-associated protein tau exon 10. Immunofluorescent and proximity-ligation assays revealed altered SFPQ/FUS-interactions in the neuronal nuclei of PSP, CBD, and ALS-TDP cases but not in AD, PiD, and GGT cases. Moreover, SFPQ expression was depleted in neurons containing TDP-43 or 4R-tau aggregates of PSP and CBD cases. Our results indicate that PSP and CBD may have properties of systematic motor neuron TDP-43 proteinopathy, suggesting mechanistic links with ALS-TDP. SFPQ dysfunction, arising from altered interaction with FUS, may be a candidate of the common pathway.

Pathophysiological Underpinnings of Extra-Motor Neurodegeneration in Amyotrophic Lateral Sclerosis: New Insights From Biomarker Studies

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) lie at opposing ends of a clinical, genetic, and neuropathological continuum. In the last decade, it has become clear that cognitive and behavioral changes in patients with ALS are more frequent than previously recognized. Significantly, these non-motor features can impact the diagnosis, prognosis, and management of ALS. Partially overlapping neuropathological staging systems have been proposed to describe the distribution of TAR DNA-binding protein 43 (TDP-43) aggregates outside the corticospinal tract. However, the relationship between TDP-43 inclusions and neurodegeneration is not absolute and other pathophysiological processes, such as neuroinflammation (with a prominent role of microglia), cortical hyperexcitability, and synaptic dysfunction also play a central role in ALS pathophysiology. In the last decade, imaging and biofluid biomarker studies have revealed important insights into the pathophysiological underpinnings of extra-motor neurodegeneration in the ALS-FTLD continuum. In this review, we first summarize the clinical and pathophysiological correlates of extra-motor neurodegeneration in ALS. Next, we discuss the diagnostic and prognostic value of biomarkers in ALS and their potential to characterize extra-motor neurodegeneration. Finally, we debate about how biomarkers could improve the diagnosis and classification of ALS. Emerging imaging biomarkers of extra-motor neurodegeneration that enable the monitoring of disease progression are particularly promising. In addition, a growing arsenal of biofluid biomarkers linked to neurodegeneration and neuroinflammation are improving the diagnostic accuracy and identification of patients with a faster progression rate. The development and validation of biomarkers that detect the pathological aggregates of TDP-43 in vivo are notably expected to further elucidate the pathophysiological underpinnings of extra-motor neurodegeneration in ALS. Novel biomarkers tracking the different aspects of ALS pathophysiology are paving the way to precision medicine approaches in the ALS-FTLD continuum. These are essential steps to improve the diagnosis and staging of ALS and the design of clinical trials testing novel disease-modifying treatments.

TDP-43 Pathology in Alzheimer's Disease

Transactive response DNA binding protein of 43 kDa (TDP-43) is an intranuclear protein encoded by the TARDBP gene that is involved in RNA splicing, trafficking, stabilization, and thus, the regulation of gene expression. Cytoplasmic inclusion bodies containing phosphorylated and truncated forms of TDP-43 are hallmarks of amyotrophic lateral sclerosis (ALS) and a subset of frontotemporal lobar degeneration (FTLD). Additionally, TDP-43 inclusions have been found in up to 57% of Alzheimer's disease (AD) cases, most often in a limbic distribution, with or without hippocampal sclerosis. In some cases, TDP-43 deposits are also found in neurons with neurofibrillary tangles. AD patients with TDP-43 pathology have increased severity of cognitive impairment compared to those without TDP-43 pathology. Furthermore, the most common genetic risk factor for AD, apolipoprotein E4 (APOE4), is associated with increased frequency of TDP-43 pathology. These findings provide strong evidence that TDP-43 pathology is an integral part of multiple neurodegenerative conditions, including AD. Here, we review the biology and pathobiology of TDP-43 with a focus on its role in AD. We emphasize the need for studies on the mechanisms that lead to TDP-43 pathology, especially in the setting of age-related disorders such as AD.

Brain TDP-43 pathology in corticobasal degeneration: topographical correlation with neuronal loss

AIMS

Neuronal and glial inclusions comprising transactive response DNA-binding protein of 43 kDa (TDP-43) have been identified in the brains of patients with corticobasal degeneration (CBD), and a possible correlation between the presence of these inclusions and clinical phenotypes has been speculated. However, the significance of TDP-43 pathology in the pathomechanism of CBD has remained unclear. Here we investigated the topographical relationship between TDP-43 inclusions and neuronal loss in CBD.

METHODS

We estimated semi-quantitatively neuronal loss and TDP-43 pathology in the form of neuronal cytoplasmic inclusions (NCIs), astrocytic inclusions (AIs), oligodendroglial cytoplasmic inclusions (GCIs), and dystrophic neurites in 22 CNS regions in 10 patients with CBD. Then, the degree of correlation between the severity of neuronal loss and the quantity of each type of TDP-43 inclusion was assessed. We also investigated tau pathology in a similar manner.

RESULTS

TDP-43 pathology was evident in 9 patients. The putamen and globus pallidus were the regions most frequently affected (80%). NCIs were the most prominent form, and their quantity was significantly correlated with the severity of neuronal loss in more than half of the regions examined. The quantities of TDP-43 NCIs and tau NCIs were correlated in only a few regions. The number of regions where the quantities of TDP-43 AIs and GCIs were correlated with the severity of neuronal loss was apparently small in comparison with that of NCIs.

CONCLUSIONS

TDP-43 alterations in neurons, not closely associated with tau pathology, may be involved in the pathomechanism underlying neuronal loss in CBD.

Concurrent tau pathologies in frontotemporal lobar degeneration with TDP-43 pathology

Aims

Accumulating evidence suggests that patients with frontotemporal lobar degeneration (FTLD) can have pathologic accumulation of multiple proteins, including tau and TDP‐43. This study aimed to determine the frequency and characteristics of concurrent tau pathology in FTLD with TDP‐43 pathology (FTLD‐TDP).

Methods

The study included 146 autopsy‐confirmed cases of FTLD‐TDP and 55 cases of FTLD‐TDP with motor neuron disease (FTLD‐MND). Sections from the basal forebrain were screened for tau pathology with phosphorylated‐tau immunohistochemistry. For cases with tau pathology on the screening section, additional brain sections were studied to establish a diagnosis. Genetic analysis of C9orf72, GRN and MAPT was performed on select cases.

Results

We found 72 cases (36%) with primary age‐related tauopathy (PART), 85 (42%) with ageing‐related tau astrogliopathy (ARTAG), 45 (22%) with argyrophilic grain disease (AGD) and 2 cases (1%) with corticobasal degeneration (CBD). Patients with ARTAG or AGD were significantly older than those without these comorbidities. One of the patients with FTLD‐TDP and CBD had C9orf72 mutation and relatively mild tau pathology, consistent with incidental CBD.

Conclusion

The coexistence of TDP‐43 and tau pathologies was relatively common, particularly PART and ARTAG. Although rare, patients with FTLD can have multiple neurodegenerative proteinopathies. The absence of TDP‐43‐positive astrocytic plaques may suggest that CBD and FTLD‐TDP were independent disease processes in the two patients with both tau and TDP‐43 pathologies. It remains to be determined if mixed cases represent a unique disease process or two concurrent disease processes in an individual.

Deep learning-based model for diagnosing Alzheimer's disease and tauopathies

AIMS

This study aimed to develop a deep learning-based model for differentiating tauopathies, including Alzheimer's disease (AD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and Pick's disease (PiD), based on tau-immunostained digital slide images.

METHODS

We trained the YOLOv3 object detection algorithm to detect five tau lesion types: neuronal inclusions, neuritic plaques, tufted astrocytes, astrocytic plaques and coiled bodies. We used 2522 digital slide images of CP13-immunostained slides of the motor cortex from 10 cases each of AD, PSP and CBD for training. Data augmentation was performed to increase the size of the training dataset. We next constructed random forest classifiers using the quantitative burdens of each tau lesion from motor cortex, caudate nucleus and superior frontal gyrus, ascertained from the object detection model. We split 120 cases (32 AD, 36 PSP, 31 CBD and 21 PiD) into training (90 cases) and test (30 cases) sets to train random forest classifiers.

RESULTS

The resultant random forest classifier achieved an average test score of 0.97, indicating that 29 out of 30 cases were correctly diagnosed. A validation study using hold-out datasets of CP13- and AT8-stained slides from 50 cases (10 AD, 17 PSP, 13 CBD and 10 PiD) showed >92% (without data augmentation) and >95% (with data augmentation) diagnostic accuracy in both CP13- and AT8-stained slides.

CONCLUSION

Our diagnostic model trained with CP13 also works for AT8; therefore, our diagnostic tool can be potentially used by other investigators and may assist medical decision-making in neuropathological diagnoses of tauopathies.

Pathway from TDP-43-Related Pathology to Neuronal Dysfunction in Amyotrophic Lateral Sclerosis and Frontotemporal Lobar Degeneration

Transactivation response DNA binding protein 43 kDa (TDP-43) is known to be a pathologic protein in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). TDP-43 is normally a nuclear protein, but affected neurons of ALS or FTLD patients exhibit mislocalization of nuclear TDP-43 and cytoplasmic inclusions. Basic studies have suggested gain-of-neurotoxicity of aggregated TDP-43 or loss-of-function of intrinsic, nuclear TDP-43. It has also been hypothesized that the aggregated TDP-43 functions as a propagation seed of TDP-43 pathology. However, a mechanistic discrepancy between the TDP-43 pathology and neuronal dysfunctions remains. This article aims to review the observations of TDP-43 pathology in autopsied ALS and FTLD patients and address pathways of neuronal dysfunction related to the neuropathological findings, focusing on impaired clearance of TDP-43 and synaptic alterations in TDP-43-related ALS and FTLD. The former may be relevant to intraneuronal aggregation of TDP-43 and exocytosis of propagation seeds, whereas the latter may be related to neuronal dysfunction induced by TDP-43 pathology. Successful strategies of disease-modifying therapy might arise from further investigation of these subcellular alterations.

Machine learning-based decision tree classifier for the diagnosis of progressive supranuclear palsy and corticobasal degeneration

Aims

This study aimed to clarify the different topographical distribution of tau pathology between progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) and establish a machine learning‐based decision tree classifier.

Methods

Paraffin‐embedded sections of the temporal cortex, motor cortex, caudate nucleus, globus pallidus, subthalamic nucleus, substantia nigra, red nucleus, and midbrain tectum from 1020 PSP and 199 CBD cases were assessed by phospho‐tau immunohistochemistry. The severity of tau lesions (i.e., neurofibrillary tangle, coiled body, tufted astrocyte or astrocytic plaque, and tau threads) was semi‐quantitatively scored in each region. Hierarchical cluster analysis was performed using tau pathology scores. A decision tree classifier was made with tau pathology scores using 914 cases. Cross‐validation was done using 305 cases. An additional ten cases were used for a validation study.

Results

Cluster analysis displayed two distinct clusters; the first cluster included only CBD, and the other cluster included all PSP and six CBD cases. We built a decision tree, which used only seven decision nodes. The scores of tau threads in the caudate nucleus were the most decisive factor for predicting CBD. In a cross‐validation, 302 out of 305 cases were correctly diagnosed. In the pilot validation study, three investigators made a correct diagnosis in all cases using the decision tree.

Conclusion

Regardless of the morphology of astrocytic tau lesions, semi‐quantitative tau pathology scores in select brain regions are sufficient to distinguish PSP and CBD. The decision tree simplifies neuropathologic differential diagnosis of PSP and CBD.

A novel temporal-predominant neuro-astroglial tauopathy associated with TMEM106B gene polymorphism in FTLD/ALS-TDP

Polymorphisms in TMEM106B, a gene on chromosome 7p21.3 involved in lysosomal trafficking, correlates to worse neuropathological, and clinical outcomes in frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) with TDP-43 inclusions. In a small cohort of C9orf72 expansion carriers, we previously found an atypical, neuroglial tauopathy in cases harboring a TMEM106B rs1990622 A/A genotype. To test whether TMEM106B genotype affects the risk of developing atypical tauopathy under a recessive genotype model (presence versus absence of two major alleles: A/A vs. A/G and G/G). We characterized the atypical tauopathy neuropathologically and determined its frequency by TMEM106B rs1990622 genotypes in 90 postmortem cases with a primary diagnosis of FTLD/ALS-TDP [mean age at death 65.5 years (±8.1), 40% female]. We investigated the effect of this new atypical tauopathy on demographics and clinical and neuropsychological metrics. We also genotyped TMEM106B in an independent series with phenotypically similar cases. Sixteen cases (16/90, 17.7 %) showed the temporal-predominant neuro-astroglial tauopathy, and 93.7% of them carried an A/A genotype (vs. ~35% in a population cohort). The odds ratio of FTLD/ALS-TDP individuals with the A/A genotype showing neuro-astroglial tauopathy was 13.9. Individuals with this tauopathy were older at onset (p = 0.01). The validation cohort had a similarly high proportion of rs1990622 A/A genotype. TDP-43 and tau changes co-occur in a subset of neurons. Our data add to the growing body of evidence that TMEM106B polymorphisms may modulate neurodegeneration. A distinctive medial temporal predominant, 4-repeat, neuro-astroglial tauopathy strongly correlates to TMEM106B A/A genotype in FTLD/ALS-TDP cases.

Post-Mortem 7.0-Tesla Magnetic Resonance Imaging of the Hippocampus in Progressive Supranuclear Palsy with and without Cerebral Amyloid Angiopathy

Introduction and Purpose: Cerebral amyloid angiopathy (CAA) can be observed in patients with progressive supranuclear palsy (PSP), though to a lesser degree than in Alzheimer’s disease. The present post-mortem 7.0-tesla magnetic resonance imaging (MRI) evaluates whether CAA has an influence on the degree of hippocampal atrophy (HA) and on the incidence of associated micro-infarcts (HMIs) and cortical micro-bleeds (HMBs). Material and Methods: Eight brains with PSP-CAA were compared to 20 PSP brains without CAA. In addition to the neuropathological examination, the hippocampus was evaluated on the most representative coronal section with T2 and T2*-weighted MRI sequences. The average degree of HA was determined in both groups. The incidence of HMIs and HMBs was also compared as well as the frequency of cortical micro-infarcts (CoMIs) and cortical micro-bleeds (CoMBs) in the hemispheric neocortex. Results: The neuropathological examination showed a higher incidence of lacunar infarcts in the PSP-CAA brains compared to the PSP ones. With magnetic resonance imaging (MRI), the severity of HA and the incidence of HMIs and HMBs was similar between both groups. Additionally, the frequency of CoMIs and CoMBs in the neocortex was comparable. Conclusions: The association of CAA in PSP brains has no influence on the degree of HA and on the incidence of the small cerebrovascular lesions in the hippocampus as well as in the neocortex.

Truncated stathmin-2 is a marker of TDP-43 pathology in frontotemporal dementia

No treatment for frontotemporal dementia (FTD), the second most common type of early-onset dementia, is available, but therapeutics are being investigated to target the 2 main proteins associated with FTD pathological subtypes: TDP-43 (FTLD-TDP) and tau (FTLD-tau). Testing potential therapies in clinical trials is hampered by our inability to distinguish between patients with FTLD-TDP and FTLD-tau. Therefore, we evaluated truncated stathmin-2 (STMN2) as a proxy of TDP-43 pathology, given the reports that TDP-43 dysfunction causes truncated STMN2 accumulation. Truncated STMN2 accumulated in human induced pluripotent stem cell-derived neurons depleted of TDP-43, but not in those with pathogenic TARDBP mutations in the absence of TDP-43 aggregation or loss of nuclear protein. In RNA-Seq analyses of human brain samples from the NYGC ALS cohort, truncated STMN2 RNA was confined to tissues and disease subtypes marked by TDP-43 inclusions. Last, we validated that truncated STMN2 RNA was elevated in the frontal cortex of a cohort of patients with FTLD-TDP but not in controls or patients with progressive supranuclear palsy, a type of FTLD-tau. Further, in patients with FTLD-TDP, we observed significant associations of truncated STMN2 RNA with phosphorylated TDP-43 levels and an earlier age of disease onset. Overall, our data uncovered truncated STMN2 as a marker for TDP-43 dysfunction in FTD.

Microglial depletion prevents extracellular matrix changes and striatal volume reduction in a model of Huntington's disease

Huntington's disease is associated with a reactive microglial response and consequent inflammation. To address the role of these cells in disease pathogenesis, we depleted microglia from R6/2 mice, a rapidly progressing model of Huntington's disease marked by behavioural impairment, mutant huntingtin (mHTT) accumulation, and early death, through colony-stimulating factor 1 receptor inhibition (CSF1Ri) with pexidartinib (PLX3397) for the duration of disease. Although we observed an interferon gene signature in addition to downregulated neuritogenic and synaptic gene pathways with disease, overt inflammation was not evident by microglial morphology or cytokine transcript levels in R6/2 mice. Nonetheless, CSF1Ri-induced microglial elimination reduced or prevented disease-related grip strength and object recognition deficits, mHTT accumulation, astrogliosis, and striatal volume loss, the latter of which was not associated with reductions in cell number but with the extracellular accumulation of chondroitin sulphate proteoglycans (CSPGs)-a primary component of glial scars. A concurrent loss of proteoglycan-containing perineuronal nets was also evident in R6/2 mice, and microglial elimination not only prevented this but also strikingly increased perineuronal nets in the brains of naïve littermates, suggesting a new role for microglia as homeostatic regulators of perineuronal net formation and integrity.

Nuclear Imaging in the Diagnosis of Clinically Uncertain Parkinsonian Syndromes

BACKGROUND

Parkinsonian syndromes are classified by etiology mainly on clinical grounds, that is, on the basis of the clinical manifestations and with the aid of conventional ancillary studies. In most cases, the clinical diagnosis is clear. In up to 30% of cases, however, the etiological classification remains uncertain after completion of the basic clinical diagnostic evaluation, and additional investigation with nuclear imaging may be indicated. In particular, cerebral single-photon emission computed tomography (SPECT) with dopamine transporter (DAT) ligands may be helpful. DAT-SPECT can be used to demonstrate or rule out nigrostriatal degeneration and thereby differentiate neurodegenerative parkinsonian syndromes from symptomatic parkinsonian syndromes and other differential diagnoses. Positron emission tomography (PET) with the glucose analogue [18F]fluorodeoxyglucose (FDG) can be used to identify disease-specific patterns of neuronal dysfunction/degeneration in order to differentiate the various neurodegenerative parkinsonian syndromes from one another.

METHODS

In this review, we summarize the current state of the evidence on DAT-SPECT and FDG-PET for the indications mentioned above on the basis of a selective review of the literature.

RESULTS

DAT-SPECT has been adequately validated as an in vivo marker for nigrostriatal degeneration. Studies using the clinical diagnosis of a movement disorders specialist over the course of the disease as a reference have shown that DAT- SPECT is 78-100% sensitive (median, 93%) and 70-100% specific (median, 89%) for the differentiation of neurodegenerative parkinsonian syndromes from symptomatic parkinsonism and other differential diagnoses in clinically unclear cases. DAT- SPECT scanning led to a change of diagnosis in 27-56% of patients (median, 43%) and to a change of treatment in 33-72% (median, 43%). FDG-PET enables the differentiation of atypical neurodegenerative parkinsonian syndromes from the idiopathic parkinsonian syndrome (i.e., Parkinson's disease proper) with high sensitivity and specificity (both approximately 90%), when the clinical diagnosis by a movement disorders specialist over the course of the disease is used as a reference.

CONCLUSION

DAT-SPECT has been well documented to be highly diagnostically accurate and to have a relevant influence on the diagnosis and treatment of patients with clinically uncertain parkinsonian or tremor syndrome. It has not yet been shown to improve patient-relevant endpoints such as mortality, morbidity, and health-related quality of life; proof of this will probably have to await the introduction of neuroprotective treatments. The current evidence for the high differential diagnostic accuracy of FDG-PET in neurodegenerative parkinsonian syndromes needs to be reinforced by prospective studies with neuropathological verification of the diagnosis.

Clinical and pathologic features of cognitive-predominant corticobasal degeneration

OBJECTIVE

To describe clinical and pathologic characteristics of corticobasal degeneration (CBD) with cognitive predominant problems during the disease course.

METHODS

In a series of autopsy-confirmed cases of CBD, we identified patients with cognitive rather than motor predominant features (CBD-Cog), including 5 patients thought to have Alzheimer disease (AD) and 10 patients thought to have behavioral variant frontotemporal dementia (FTD). We compared clinical and pathologic features of CBD-Cog with those from a series of 31 patients with corticobasal syndrome (CBD-CBS). For pathologic comparisons between CBD-Cog and CBD-CBS, we used semiquantitative scoring of neuronal and glial lesion types in multiple brain regions and quantitative assessments of tau burden from image analysis.

RESULTS

Five of 15 patients with CBD-Cog never had significant motor problems during their disease course. The most common cognitive abnormalities in CBD-Cog were executive and visuospatial dysfunction. The frequency of language problems did not differ between CBD-Cog and CBD-CBS. Argyrophilic grain disease, which is a medial temporal tauopathy associated with mild cognitive impairment, was more frequent in CBD-Cog. Apathy was also more frequent in CBD-Cog. Tau pathology in CBD-Cog was greater in the temporal and less in perirolandic cortices than in CBD-CBS.

CONCLUSION

A subset of patients with CBD has a cognitive predominant syndrome than can be mistaken for AD or FTD. Our findings suggest that distribution of tau cortical pathology (greater in temporal and less in perirolandic cortices) may be the basis of this uncommon clinical variant of CBD.

TDP-43 Is Efficiently Transferred Between Neuron-Like Cells in a Manner Enhanced by Preservation of Its N-Terminus but Independent of Extracellular Vesicles

The misfolding of transactive response DNA-binding protein (TDP-43) is a major contributor to the pathogenesis of TDP-43 proteinopathies, including amyotrophic lateral sclerosis and frontotemporal lobar degeneration with TDP-43 inclusions, but also plays a role in other neurodegenerative diseases including Alzheimer disease. It is thought that different truncations at the N- and C-termini of TDP-43 contribute to its misfolding and aggregation in the brain, and that these aberrant TDP-43 fragments contribute to disease. Despite this, little is known about whether different truncation events influence the protein’s transmissibility between cells and how this cell-to-cell transfer occurs. In this study, we use a well-established cellular model to study the efficiency by which full-length and truncated TDP-43 fragments are transferred between neuron-like cells. We demonstrate that preservation of the N-terminus of TDP-43 enhances its transmissibility between cells and that this protein transmission occurs in a manner exclusive of extracellular vesicles, instead requiring cellular proximity for efficient propagation. These data indicate that the N-terminus of TDP-43 might be a useful target in the generation of therapeutics to limit the spread of TDP-43 pathology.

Fulminant corticobasal degeneration: a distinct variant with predominant neuronal tau aggregates

Corticobasal degeneration typically progresses gradually over 5–7 years from onset till death. Fulminant corticobasal degeneration cases with a rapidly progressive course were rarely reported (RP-CBD). This study aimed to investigate their neuropathological characteristics. Of the 124 autopsy-confirmed corticobasal degeneration cases collected from 14 centres, we identified 6 RP-CBD cases (4.8%) who died of advanced disease within 3 years of onset. These RP-CBD cases had different clinical phenotypes including rapid global cognitive decline (N = 2), corticobasal syndrome (N = 2) and Richardson’s syndrome (N = 2). We also studied four corticobasal degeneration cases with an average disease duration of 3 years or less, who died of another unrelated illness (Intermediate-CBD). Finally, we selected 12 age-matched corticobasal degeneration cases out of a cohort of 110, who had a typical gradually progressive course and reached advanced clinical stage (End-stage-CBD). Quantitative analysis showed high overall tau burden (p = 0.2) and severe nigral cell loss (p = 0.47) in both the RP-CBD and End-stage-CBD groups consistent with advanced pathological changes, while the Intermediate-CBD group (mean disease duration = 3 years) had milder changes than End-stage-CBD (p < 0.05). These findings indicated that RP-CBD cases had already developed advanced pathological changes as those observed in End-stage-CBD cases (mean disease duration = 6.7 years), but within a significantly shorter duration (2.5 years; p < 0.001). Subgroup analysis was performed to investigate the cellular patterns of tau aggregates in the anterior frontal cortex and caudate by comparing neuronal-to-astrocytic plaque ratios between six RP-CBD cases, four Intermediate-CBD and 12 age-matched End-stage-CBD. Neuronal-to-astrocytic plaque ratios of Intermediate-CBD and End-stage-CBD, but not RP-CBD, positively correlated with disease duration in both the anterior frontal cortex and caudate (p = 0.02). In contrast to the predominance of astrocytic plaques we previously reported in preclinical asymptomatic corticobasal degeneration cases, neuronal tau aggregates predominated in RP-CBD exceeding those in Intermediate-CBD (anterior frontal cortex: p < 0.001, caudate: p = 0.001) and End-stage-CBD (anterior frontal cortex: p = 0.03, caudate: p = 0.01) as demonstrated by its higher neuronal-to-astrocytic plaque ratios in both anterior frontal cortex and caudate. We did not identify any difference in age at onset, any pathogenic tau mutation or concomitant pathologies that could have contributed to the rapid progression of these RP-CBD cases. Mild TDP-43 pathology was observed in three RP-CBD cases. All RP-CBD cases were men. The MAPT H2 haplotype, known to be protective, was identified in one RP-CBD case (17%) and 8 of the matched End-stage-CBD cases (67%). We conclude that RP-CBD is a distinct aggressive variant of corticobasal degeneration with characteristic neuropathological substrates resulting in a fulminant disease process as evident both clinically and pathologically. Biological factors such as genetic modifiers likely play a pivotal role in the RP-CBD variant and should be the subject of future research.

Primary Tau Pathology, Not Copathology, Correlates With Clinical Symptoms in PSP and CBD

Distinct neuronal and glial tau pathologies define corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP). Additional Alzheimer disease, TDP-43, and Lewy body copathologies are also common. The interplay of these pathologies with clinical symptoms remains unclear as individuals can present with corticobasal syndrome, frontotemporal dementia, PSP, or atypical Parkinsonism and may have additional secondary impairments. We report clinical, pathological, and genetic interactions in a cohort of CBD and PSP cases. Neurofibrillary tangles and plaques were common. Apolipoprotein E (APOE)ε4 carriers had more plaques while PSP APOEε2 carriers had fewer plaques. TDP-43 copathology was present and age-associated in 14% of PSP, and age-independent in 33% of CBD. Lewy body copathology varied from 9% to 15% and was not age-associated. The primary FTD-Tau burden-a sum of the neuronal, astrocytic and oligodendrocytic tau-was not age-, APOE-, or MAPT-related. In PSP, FTD-Tau, independent of copathology, associated with executive, language, motor, and visuospatial impairments, while PSP with Parkinsonism had a lower FTD-Tau burden, but this was not the case in CBD. Taken together, our results indicate that the primary tauopathy burden is the strongest correlate of clinical PSP, while copathologies are principally determined by age and genetic risk factors.

Are comorbidities compatible with a molecular pathological classification of neurodegenerative diseases?

PURPOSE OF REVIEW

The purpose of this review is to provide an update on comorbidities in neurodegenerative conditions. The term comorbidity is used here to distinguish cases with overlapping pathogenic mechanisms, which includes combinations of neurodegenerative proteinopathies from cases with multimorbidity, which is defined as concomitant brain and systemic disorders with different pathogenic mechanisms.

RECENT FINDINGS

Comorbid proteinopathies are more frequent in both sporadic and hereditary neurodegenerative diseases than previously assumed. The most frequent additional proteinopathies are related to Alzheimer's disease, Lewy body disorder, and limbic predominant transactive response DNA-binding protein 43 proteinopathy, however, different forms of tau pathologies are also increasingly recognized. In addition to ageing, synergistic interaction of proteins, common disease pathways, and the influence of genetic variations are discussed as possible pathogenic players.

SUMMARY

Comorbid proteinopathies might influence the clinical course and have implications for biomarker and therapeutic development. As pure forms of proteinopathies are still observed, the notion of current molecular classification is justified. This corroborates elucidation of various pathogenic pathways leading to neurodegeneration. Assuming that single proteins and associated pathways are targeted in therapy trials, efforts are needed to better stratify patients and to select pure proteinopathy forms lacking unfavorable genetic constellations. Otherwise combined therapeutic strategies might be necessary for comorbid proteinopathies.

Multiple system atrophy - a clinicopathological update

Multiple system atrophy (MSA) is a fatal, adult-onset neurodegenerative disorder of uncertain etiology, clinically characterized by various combinations of Levo-dopa-unresponsive parkinsonism, and cerebellar, motor, and autonomic dysfunctions. MSA is an α-synucleinopathy with specific glioneuronal degeneration involving striatonigral, olivopontocerebellar, autonomic and peripheral nervous systems. The pathologic hallmark of this unique proteinopathy is the deposition of aberrant α-synuclein (αSyn) in both glia (mainly oligodendroglia) and neurons forming pathological inclusions that cause cell dysfunction and demise. The major variants are striatonigral degeneration (MSA with predominant parkinsonism / MSA-P) and olivopontocerebellar atrophy (MSA with prominent cerebellar ataxia / MSA-C). However, the clinical and pathological features of MSA are broader than previously considered. Studies in various mouse models and human patients have helped to better understand the molecular mechanisms that underlie the progression of the disease. The pathogenesis of MSA is characterized by propagation of disease-specific strains of αSyn from neurons to oligodendroglia and cell-to-cell spreading in a "prion-like" manner, oxidative stress, proteasomal and mitochondrial dysfunctions, myelin dysregulation, neuroinflammation, decreased neurotrophic factors, and energy failure. The combination of these mechanisms results in neurodegeneration with widespread demyelination and a multisystem involvement that is specific for MSA. Clinical diagnostic accuracy and differential diagnosis of MSA have improved by using combined biomarkers. Cognitive impairment, which has been a non-supporting feature of MSA, is not uncommon, while severe dementia is rare. Despite several pharmacological approaches in MSA models, no effective disease-modifying therapeutic strategies are currently available, although many clinical trials targeting disease modification, including immunotherapy and combined approaches, are under way. Multidisciplinary research to elucidate the genetic and molecular background of the noxious processes as the basis for development of an effective treatment of the hitherto incurable disorder are urgently needed.

Corticobasal degeneration

Corticobasal degeneration (CBD) is a rare neurodegenerative disease characterized by the predominance of pathological 4 repeat tau deposition in various cell types and anatomical regions. Corticobasal syndrome (CBS) is one of the clinical phenotypes associated with CBD pathology, manifesting as a progressive asymmetric akinetic-rigid, poorly levodopa-responsive parkinsonism, with cerebral cortical dysfunction. CBD can manifest as several clinical phenotypes, and similarly, CBS can also have a pathologic diagnosis other than CBD. This chapter discusses the clinical manifestations of pathologically confirmed CBD cases, the current diagnostic criteria, as well as the pathologic and neuroimaging findings of CBD/CBS. At present, therapeutic options for CBD remain symptomatic. Further research is needed to improve the clinical diagnosis of CBD, as well as studies on disease-modifying therapies for this relentlessly progressive neurodegenerative disorder.