Classification of FTLD-TDP cases into pathological subtypes using antibodies against phosphorylated and non-phosphorylated TDP43

Neuropeptide FF (NPFF)-positive nerve cells of the human cerebral cortex and white matter in controls, selected neurodegenerative diseases, and schizophrenia

We quantified and determined for the first time the distribution pattern of the neuropeptide NPFF in the human cerebral cortex and subjacent white matter. To do so, we studied n = 9 cases without neurological disorders and n = 22 cases with neurodegenerative diseases, including sporadic amyotrophic lateral sclerosis (ALS, n = 8), Alzheimer's disease (AD, n = 8), Pick's disease (PiD, n = 3), and schizophrenia (n = 3). NPFF-immunopositive cells were located chiefly, but not exclusively, in the superficial white matter and constituted there a subpopulation of white matter interstitial cells (WMIC): Pyramidal-like and multipolar somata predominated in the gyral crowns, whereas bipolar and ovoid somata predominated in the cortex surrounding the sulci. Their sparsely ramified axons were unmyelinated and exhibited NPFF-positive bead-like varicosities. We found significantly fewer NPFF-immunopositive cells in the gray matter of the frontal, cingulate, and superior temporal gyri of both sporadic ALS and late-stage AD patients than in controls, and significantly fewer NPFF-positive cells in the subjacent as well as deep white matter of the frontal gyrus of these patients compared to controls. Notably, the number of NPFF-positive cells was also significantly lower in the hippocampal formation in AD compared to controls. In PiD, NPFF-positive cells were present in significantly lower numbers in the gray and white matter of the cingulate and frontal gyrii in comparison to controls. In schizophrenic patients, lower wNPFF cell counts in the neocortex were significant and global (cingulate, frontal, superior temporal gyrus, medial, and inferior gyri). The precise functions of NPFF-positive cells and their relationship to the superficial corticocortical white matter U-fibers are currently unknown. Here, NPFF immunohistochemistry and expression characterize a previously unrecognized population of cells in the human brain, thereby providing a new entry-point for investigating their physiological and pathophysiological roles.

Amplifying the Heat Shock Response Ameliorates ALS and FTD Pathology in Mouse and Human Models

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are now known as parts of a disease spectrum with common pathological features and genetic causes. However, as both conditions are clinically heterogeneous, patient groups may be phenotypically similar but pathogenically and genetically variable. Despite numerous clinical trials, there remains no effective therapy for these conditions, which, in part, may be due to challenges of therapy development in a heterogeneous patient population. Disruption to protein homeostasis is a key feature of different forms of ALS and FTD. Targeting the endogenous protein chaperone system, the heat shock response (HSR) may, therefore, be a potential therapeutic approach. We conducted a preclinical study of a known pharmacological amplifier of the HSR, called arimoclomol, in mice with a mutation in valosin-containing protein (VCP) which causes both ALS and FTD in patients. We demonstrate that amplification of the HSR ameliorates the ALS/FTD-like phenotype in the spinal cord and brain of mutant VCP mice and prevents neuronal loss, replicating our earlier findings in the SOD1 mouse model of ALS. Moreover, in human cell models, we demonstrate improvements in pathology upon arimoclomol treatment in mutant VCP patient fibroblasts and iPSC-derived motor neurons. Our findings suggest that targeting of the HSR may have therapeutic potential, not only in non-SOD1 ALS, but also for the treatment of FTD.

Interaction of Proteins Involved in Neuronal Proteinopathies

Proteinopathy is characterized by the accumulation of aggregates of a specific protein in a target organ, tissue, or cell. The aggregation of the same protein can cause different pathologies as single protein can adopt various amyloidogenic, disease-specific conformations. The conformation governs the interaction of amyloid aggregates with other proteins that are prone to misfolding and, thus, determines disease-specific spectrum of concomitant pathologies. In this regard, a detailed description of amyloid protein conformation as well as spectrum of its interaction with other proteins become a key point for drafting of precise description of the disease. The majority of clinical cases of neuronal proteinopathies is caused by the aggregation of rather limited range of amyloidogenic proteins. Here, we provided the characterization of pathologies, related to the aggregation of amyloid β peptide, tau protein, α-synuclein, TDP-43, and amylin, giving a short description of pathologies themselves, recent advances in elucidation of misfolded protein conformation, with emphasis on those protein aggregates extracted from biological samples, what is known about the interaction of this proteins, and the influence of this interaction on the progression of underlying disease and comorbidities.

Elevated GRO-α and IL-18 in serum and brain implicate the NLRP3 inflammasome in frontotemporal dementia

Neuroinflammation is a hallmark of frontotemporal dementia (FTD), a heterogeneous group of proteinopathies characterized by the progressive degeneration of the frontal and temporal lobes. It is marked by microglial activation and subsequent cytokine release. Although cytokine levels in FTD brain and CSF have been examined, the number of cytokines measured in each study is limited and knowledge on cytokine concentrations in FTD serum is scarce. Here, we assessed 48 cytokines in FTD serum and brain. The aim was to determine common cytokine dysregulation pathways in serum and brain in FTD. Blood samples and brain tissue samples from the superior frontal cortex (SFC) were collected from individuals diagnosed with behavioral variant FTD (bvFTD) and healthy controls, and 48 cytokines were measured using a multiplex immunological assay. The data were evaluated by principal component factor analysis to determine the contribution from different components of the variance in the cohort. Levels of a number of cytokines were altered in serum and SFC in bvFTD compared to controls, with increases in GRO-α and IL-18 in both serum and SFC. These changes could be associated with NLRP3 inflammasome activation or the NFκB pathway, which activates NLRP3. The results suggest the possible importance of the NLRP3 inflammasome in FTD. An improved understanding of the role of inflammasomes in FTD could provide valuable insights into the pathogenesis, diagnosis and treatment of FTD.

TDP-43 and tau concurrence in the entorhinal subfields in primary age-related tauopathy and preclinical Alzheimer's disease

Phosphorylated tau (p-tau) pathology correlates strongly with cognitive decline and is a pathological hallmark of Alzheimer's Disease (AD). In recent years, phosphorylated transactive response DNA-binding protein (pTDP-43) has emerged as a common comorbidity, found in up to 70% of all AD cases (Josephs et al., Acta Neuropathol, 131(4), 571-585; Josephs, Whitwell, et al., Acta Neuropathol, 127(6), 811-824). Current staging schemes for pTDP-43 in AD and primary age-related tauopathy (PART) track its progression throughout the brain, but the distribution of pTDP-43 within the entorhinal cortex (EC) at the earliest stages has not been studied. Moreover, the exact nature of p-tau and pTDP-43 co-localization is debated. We investigated the selective vulnerability of the entorhinal subfields to phosphorylated pTDP-43 pathology in preclinical AD and PART postmortem tissue. Within the EC, posterior-lateral subfields showed the highest semi-quantitative pTDP-43 density scores, while the anterior-medial subfields had the lowest. On the rostrocaudal axis, pTDP-43 scores were higher posteriorly than anteriorly (p < 0.010), peaking at the posterior-most level (p < 0.050). Further, we showed the relationship between pTDP-43 and p-tau in these regions at pathology-positive but clinically silent stages. P-tau and pTDP-43 presented a similar pattern of affected subregions (p < 0.0001) but differed in density magnitude (p < 0.0001). P-tau burden was consistently higher than pTDP-43 at every anterior-posterior level and in most EC subfields. These findings highlight pTDP-43 burden heterogeneity within the EC and the posterior-lateral subfields as the most vulnerable regions within stage II of the current pTDP-43 staging schemes for AD and PART. The EC is a point of convergence for p-tau and pTDP-43 and identifying its most vulnerable neuronal populations will prove key for early diagnosis and disease intervention.

Pathology vs pathogenesis: Rationale and pitfalls in the clinicopathology model of neurodegeneration

In neurodegenerative disorders, the term pathology is often implicitly referred to as pathogenesis. Pathology has been conceived as a window into the pathogenesis of neurodegenerative disorders. This clinicopathologic framework posits that what can be identified and quantified in postmortem brain tissue can explain both premortem clinical manifestations and the cause of death, a forensic approach to understanding neurodegeneration. As the century-old clinicopathology framework has yielded little correlation between pathology and clinical features or neuronal loss, the relationship between proteins and degeneration is ripe for revisitation. There are indeed two synchronous consequences of protein aggregation in neurodegeneration: the loss of the soluble/normal proteins on one; the accrual of the insoluble/abnormal fraction of these proteins on the other. The omission of the first part in the protein aggregation process is an artifact of the early autopsy studies: soluble, normal proteins have disappeared, with only the remaining insoluble fraction amenable to quantification. We here review the collective evidence from human data suggesting that protein aggregates, known collectively as pathology, are the consequence of many biological, toxic, and infectious exposures, but may not explain alone the cause or pathogenesis of neurodegenerative disorders.

TDP-43 Proteinopathy Specific Biomarker Development

TDP-43 is the primary or secondary pathological hallmark of neurodegenerative diseases, such as amyotrophic lateral sclerosis, half of frontotemporal dementia cases, and limbic age-related TDP-43 encephalopathy, which clinically resembles Alzheimer's dementia. In such diseases, a biomarker that can detect TDP-43 proteinopathy in life would help to stratify patients according to their definite diagnosis of pathology, rather than in clinical subgroups of uncertain pathology. For therapies developed to target pathological proteins that cause the disease a biomarker to detect and track the underlying pathology would greatly enhance such undertakings. This article reviews the latest developments and outlooks of deriving TDP-43-specific biomarkers from the pathophysiological processes involved in the development of TDP-43 proteinopathy and studies using biosamples from clinical entities associated with TDP-43 pathology to investigate biomarker candidates.

Molecular Investigations of Protein Aggregation in the Pathogenesis of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating progressive neurodegenerative disorder characterized by selective loss of lower and upper motor neurons (MNs) in the brain and spinal cord, resulting in paralysis and eventually death due to respiratory insufficiency. Although the fundamental physiological mechanisms underlying ALS are not completely understood, the key neuropathological hallmarks of ALS pathology are the aggregation and accumulation of ubiquitinated protein inclusions within the cytoplasm of degenerating MNs. Herein, we discuss recent insights into the molecular mechanisms that lead to the accumulation of protein aggregates in ALS. This will contribute to a better understanding of the pathophysiology of the disease and may open novel avenues for the development of therapeutic strategies.

ATP-binding cassette transporter expression is widely dysregulated in frontotemporal dementia with TDP-43 inclusions

The human brain is highly enriched in lipids and increasing evidence indicates that dysregulation of lipids in the brain is associated with neurodegeneration. ATP-binding cassette subfamily A (ABCA) transporters control the movement of lipids across cellular membranes and are implicated in a number of neurodegenerative diseases. However, very little is known about the role of ABCA transporters in frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP), which is a common form of younger-onset dementia. We therefore undertook a comprehensive analysis of the expression of ABCA transporters (ABCA1-13) in five key brain regions (amygdala, inferior temporal cortex, superior frontal cortex, cerebellum and parietal cortex) in FTLD-TDP and controls. We found that the expression of ABCA2, ABCA3, ABCA4, ABCA7, ABCA9, ABCA10 and ABCA13 was significantly altered in FTLD-TDP in a region-specific manner. In addition, the expression of ABCA transporters correlated specifically to different neural markers and TARDBP. These results suggest substantial dysregulation of ABCA transporters and lipid metabolism in FTLD-TDP and these changes are associated with neuroinflammation.

Utility of the Addenbrooke’s Cognitive Examination III online calculator to differentiate the primary progressive aphasia variants

The Addenbrooke’s Cognitive Examination III is a brief cognitive screening tool that is widely used for the detection and monitoring of dementia. Recent findings suggest that the three variants of primary progressive aphasia can be distinguished based on their distinct profiles on the five subdomain scores of this test. Here, we investigated the utility of the Addenbrooke’s Cognitive Examination III to differentiate the primary progressive aphasia variants based on their item-by-item performance profiles on this test. From these results, we created an interactive primary progressive aphasia Addenbrooke’s Cognitive Examination III calculator which predicts the variant based on a patient’s unique item-by-item profile. Twenty-eight logopenic variant, 25 non-fluent variant and 37 semantic variant primary progressive aphasia patients and 104 healthy controls completed the Addenbrooke’s Cognitive Examination III at first clinical presentation. Multinomial regression analyses were conducted to establish performance profiles among groups, and R Shiny from RStudio was used to create the interactive Addenbrooke’s Cognitive Examination III diagnostic calculator. To verify its accuracy, probability values of the regression model were derived based on a 5-fold cross-validation of cases. The calculator’s accuracy was then verified in an independent sample of 17 logopenic, 19 non-fluent and 13 semantic variant primary progressive aphasia patients and 68 Alzheimer’s disease patients who had completed the Addenbrooke’s Cognitive Examination III (or an older version of this test: Revised) and had in vivo amyloid-PET imaging and/or brain autopsy pathological confirmation. Cross-validation of cases in the calculator model revealed different rates of sensitivity in classifying variants: semantic = 100%, non-fluent = 80.6% and logopenic = 79.9%; healthy controls were distinguished from primary progressive aphasia patients with 100% sensitivity. Verification of in vivo amyloid and/or autopsy-confirmed patients showed that the calculator correctly classified 10/13 (77%) semantic variant, 3/19 (16%) non-fluent variant and 4/17 (24%) logopenic variant patients. Importantly, for patients who were not classified, diagnostic probability values mostly pointed toward the correct clinical diagnosis. Furthermore, misclassified diagnoses of the primary progressive aphasia cohort were rare (1/49; 2%). Although 22 of the 68 Alzheimer’s disease patients (32%) were misclassified with primary progressive aphasia, 19/22 were misclassified with the logopenic variant (i.e. falling within the same neuropathological entity). The Addenbrooke’s Cognitive Examination III primary progressive aphasia diagnostic calculator demonstrates sound accuracy in differentiating the variants based on an item-by-item Addenbrooke’s Cognitive Examination III profile. This calculator represents a new frontier in using data-driven approaches to differentiate the primary progressive aphasia variants.

A C-Terminally Truncated TDP-43 Splice Isoform Exhibits Neuronal Specific Cytoplasmic Aggregation and Contributes to TDP-43 Pathology in ALS

Neuronal cytoplasmic aggregation and ubiquitination of TDP-43 is the most common disease pathology linking Amyotrophic Lateral Sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). TDP-43 pathology is characterized by the presence of low molecular weight TDP-43 species generated through proteolytic cleavage and/or abnormal RNA processing events. In addition to N-terminally truncated TDP-43 species, it has become evident that C-terminally truncated variants generated through alternative splicing in exon 6 also contribute to the pathophysiology of ALS/FTLD. Three such variants are listed in UCSD genome browser each sharing the same C-terminal unique sequence of 18 amino acids which has been shown to contain a putative nuclear export sequence. Here we have identified an additional C-terminally truncated variant of TDP-43 in human spinal cord tissue. This variant, called TDP43C-spl, is generated through use of non-canonical splice sites in exon 6, skipping 1,020 bp and encoding a 272 aa protein lacking the C-terminus with the first 256 aa identical to full-length TDP-43 and the same 18 amino acid C-terminal unique sequence. Ectopic expression studies in cells revealed that TDP43C-spl was localized to the nucleus in astrocytic and microglial cell lines but formed cytoplasmic ubiquitinated aggregates in neuronal cell lines. An antibody raised to the unique 18 amino acid sequence showed elevated levels of C-terminally truncated variants in ALS spinal cord tissues, and co-labeled TDP-43 pathology in disease affected spinal motor neurons. The retention of this 18 amino acid sequence among several C-terminally truncated TDP-43 variants suggests important functional relevance. Our studies of TDP43C-spl suggest this may be related to the selective vulnerability of neurons to TDP-43 pathology and cell-subtype differences in nuclear export.

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.

FTLD-TDP assemblies seed neoaggregates with subtype-specific features via a prion-like cascade

Morphologically distinct TDP-43 aggregates occur in clinically different FTLD-TDP subtypes, yet the mechanism of their emergence and contribution to clinical heterogeneity are poorly understood. Several lines of evidence suggest that pathological TDP-43 follows a prion-like cascade, but the molecular determinants of this process remain unknown. We use advanced microscopy techniques to compare the seeding properties of pathological FTLD-TDP-A and FTLD-TDP-C aggregates. Upon inoculation of patient-derived aggregates in cells, FTLD-TDP-A seeds amplify in a template-dependent fashion, triggering neoaggregation more efficiently than those extracted from FTLD-TDP-C patients, correlating with the respective disease progression rates. Neoaggregates are sequentially phosphorylated with N-to-C directionality and with subtype-specific timelines. The resulting FTLD-TDP-A neoaggregates are large and contain densely packed fibrils, reminiscent of the pure compacted fibrils present within cytoplasmic inclusions in postmortem brains. In contrast, FTLD-TDP-C dystrophic neurites show less dense fibrils mixed with cellular components, and their respective neoaggregates are small, amorphous protein accumulations. These cellular seeding models replicate aspects of the patient pathological diversity and will be a useful tool in the quest for subtype-specific therapeutics.

Anatomic survey of seeding in Alzheimer's disease brains reveals unexpected patterns

Tauopathies are heterogeneous neurodegenerative diseases defined by progressive brain accumulation of tau aggregates. The most common tauopathy, sporadic Alzheimer's disease (AD), involves progressive tau deposition that can be divided into specific stages of neurofibrillary tangle pathology. This classification is consistent with experimental data which suggests that network-based propagation is mediated by cell-cell transfer of tau "seeds", or assemblies, that serve as templates for their own replication. Until now, seeding assays of AD brain have largely been limited to areas previously defined by NFT pathology. We now expand this work to additional regions. We selected 20 individuals with AD pathology of NFT stages I, III, and V. We stained and classified 25 brain regions in each using the anti-phospho-tau monoclonal antibody AT8. We measured tau seeding in each of the 500 samples using a cell-based tau "biosensor" assay in which induction of intracellular tau aggregation is mediated by exogenous tau assemblies. We observed a progressive increase in tau seeding according to NFT stage. Seeding frequently preceded NFT pathology, e.g., in the basolateral subnucleus of the amygdala and the substantia nigra, pars compacta. We observed seeding in brain regions not previously known to develop tau pathology, e.g., the globus pallidus and internal capsule, where AT8 staining revealed mainly axonal accumulation of tau. AT8 staining in brain regions identified because of tau seeding also revealed pathology in a previously undescribed cell type: Bergmann glia of the cerebellar cortex. We also detected tau seeding in brain regions not previously examined, e.g., the intermediate reticular zone, dorsal raphe nucleus, amygdala, basal nucleus of Meynert, and olfactory bulb. In conclusion, tau histopathology and seeding are complementary analytical tools. Tau seeding assays reveal pathology in the absence of AT8 signal in some instances, and previously unrecognized sites of tau deposition. The variation in sites of seeding between individuals could underlie differences in the clinical presentation and course of AD.

Neuroimaging in Frontotemporal Dementia: Heterogeneity and Relationships with Underlying Neuropathology

Frontotemporal dementia encompasses a group of clinical syndromes defined pathologically by degeneration of the frontal and temporal lobes. Historically, these syndromes have been challenging to diagnose, with an average of about three years between the time of symptom onset and the initial evaluation and diagnosis. Research in the field of neuroimaging has revealed numerous biomarkers of the various frontotemporal dementia syndromes, which has provided clinicians with a method of narrowing the differential diagnosis and improving diagnostic accuracy. As such, neuroimaging is considered a core investigative tool in the evaluation of neurodegenerative disorders. Furthermore, patterns of neurodegeneration correlate with the underlying neuropathological substrates of the frontotemporal dementia syndromes, which can aid clinicians in determining the underlying etiology and improve prognostication. This review explores the advancements in neuroimaging and discusses the phenotypic and pathologic features of behavioral variant frontotemporal dementia, semantic variant primary progressive aphasia, and nonfluent variant primary progressive aphasia, as seen on structural magnetic resonance imaging and positron emission tomography.

The Role of White Matter Dysfunction and Leukoencephalopathy/Leukodystrophy Genes in the Aetiology of Frontotemporal Dementias: Implications for Novel Approaches to Therapeutics

Frontotemporal dementia (FTD) is a common cause of presenile dementia and is characterized by behavioural and/or language changes and progressive cognitive deficits. Genetics is an important component in the aetiology of FTD, with positive family history of dementia reported for 40% of cases. This review synthesizes current knowledge of the known major FTD genes, including C9orf72 (chromosome 9 open reading frame 72), MAPT (microtubule-associated protein tau) and GRN (granulin), and their impact on neuronal and glial pathology. Further, evidence for white matter dysfunction in the aetiology of FTD and the clinical, neuroimaging and genetic overlap between FTD and leukodystrophy/leukoencephalopathy are discussed. The review highlights the role of common variants and mutations in genes such as CSF1R (colony-stimulating factor 1 receptor), CYP27A1 (cytochrome P450 family 27 subfamily A member 1), TREM2 (triggering receptor expressed on myeloid cells 2) and TMEM106B (transmembrane protein 106B) that play an integral role in microglia and oligodendrocyte function. Finally, pharmacological and non-pharmacological approaches for enhancing remyelination are discussed in terms of future treatments of FTD.

Motor cortical excitability predicts cognitive phenotypes in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are well-recognised as an extended disease spectrum. This study hypothesised that cortical hyperexcitability, an early pathophysiological abnormality in ALS, would distinguish cognitive phenotypes, as a surrogate marker of pathological disease burden. 61 patients with ALS, matched for disease duration (pure motor ALS, n = 39; ALS with coexistent FTD, ALS-FTD, n = 12; ALS with cognitive/behavioural abnormalities not meeting FTD criteria, ALS-Cog, n = 10) and 30 age-matched healthy controls. Cognitive function on the Addenbrooke's cognitive examination (ACE) scale, behavioural function on the motor neuron disease behavior scale (MiND-B) and cortical excitability using transcranial magnetic stimulation (TMS) were documented. Cortical resting motor threshold (RMT), lower threshold indicating hyperexcitability, was lower in ALS-FTD (50.2 ± 6.9) compared to controls (64.3 ± 12.6, p < 0.005), while ALS-Cog (63.3 ± 12.7) and ALS (60.8 ± 13.9, not significant) were similar to controls. Short interval intracortical inhibition (SICI) was reduced across all ALS groups compared to controls, indicating hyperexcitability. On receiver operating characteristic curve analysis, RMT differentiated ALS-FTD from ALS (area under the curve AUC = 0.745, p = 0.011). The present study has identified a distinct pattern of cortical excitability across cognitive phenotypes in ALS. As such, assessment of cortical physiology may provide more precise clinical prognostication in ALS.

TDP-43 Vasculopathy in the Spinal Cord in Sporadic Amyotrophic Lateral Sclerosis (sALS) and Frontal Cortex in sALS/FTLD-TDP

Sporadic amyotrophic lateral sclerosis (sALS) and FTLD-TDP are neurodegenerative diseases within the spectrum of TDP-43 proteinopathies. Since abnormal blood vessels and altered blood-brain barrier have been described in sALS, we wanted to know whether TDP-43 pathology also occurs in blood vessels in sALS/FTLD-TDP. TDP-43 deposits were identified in association with small blood vessels of the spinal cord in 7 of 14 cases of sALS and in small blood vessels of frontal cortex area 8 in 6 of 11 FTLD-TDP and sALS cases, one of them carrying a GRN mutation. This was achieved using single and double-labeling immunohistochemistry, and double-labeling immunofluorescence and confocal microscopy. In the sALS spinal cord, P-TDP43 Ser403-404 deposits were elongated and parallel to the lumen, whereas others were granular, seldom forming clusters. In the frontal cortex, the inclusions were granular, or elongated and parallel to the lumen, or forming small globules within or in the external surface of the blood vessel wall. Other deposits were localized in the perivascular space. The present findings are in line with previous observations of TDP-43 vasculopathy in a subset of FTLD-TDP cases and identify this pathology in the spinal cord and frontal cortex in a subset of cases within the sALS/FTLD-TDP spectrum.

Frontotemporal Lobar Degeneration TDP-43-Immunoreactive Pathological Subtypes: Clinical and Mechanistic Significance

Frontotemporal lobar degeneration with TPD-43-immunoreactive pathology (FTLD-TDP) is subclassified based on the type and cortical laminar distribution of neuronal inclusions. The relevance of these pathological subtypes is supported by the presence of relatively specific clinical and genetic correlations. Recent evidence suggests that the different patterns of pathology are a reflection of biochemical differences in the pathological TDP-43 species, each of which is influenced by differing genetic factors. As a result, patient FTLD-TDP subtype may be an important factor to consider when developing biomarkers and targeted therapies for frontotemporal dementia. In this chapter, we first describe the pathological features, clinical and genetic correlations of the currently recognized FTLD-TDP subtypes. We then discuss a number of novel patterns of TDP-43 pathology. Finally, we provide an overview of what is currently known about the biochemical basis of the different FTLD-TDP subtypes and how this may explain the observed phenotypic and pathological heterogeneity.

Fabry Disease With Concomitant Lewy Body Disease

Although Gaucher disease can be accompanied by Lewy pathology (LP) and extrapyramidal symptoms, it is unknown if LP exists in Fabry disease (FD), another progressive multisystem lysosomal storage disorder. We aimed to elucidate the distribution patterns of FD-related inclusions and LP in the brain of a 58-year-old cognitively unimpaired male FD patient suffering from predominant hypokinesia. Immunohistochemistry (CD77, α-synuclein, collagen IV) and neuropathological staging were performed on 100-µm sections. Tissue from the enteric or peripheral nervous system was unavailable. As controls, a second cognitively unimpaired 50-year-old male FD patient without LP or motor symptoms and 3 age-matched individuals were examined. Inclusion body pathology was semiquantitatively evaluated. Although Lewy neurites/bodies were not present in the 50-year-old individual or in controls, severe neuronal loss in the substantia nigra pars compacta and LP corresponding to neuropathological stage 4 of Parkinson disease was seen in the 58-year-old FD patient. Major cerebrovascular lesions and/or additional pathologies were absent in this individual. We conclude that Lewy body disease with parkinsonism can occur within the context of FD. Further studies determining the frequencies of both inclusion pathologies in large autopsy-controlled FD cohorts could help clarify the implications of both lesions for disease pathogenesis, potential spreading mechanisms, and therapeutic interventions.

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.

Antibody against TDP-43 phosphorylated at serine 375 suggests conformational differences of TDP-43 aggregates among FTLD-TDP subtypes

Aggregation of hyperphosphorylated TDP-43 is the hallmark pathological feature of the most common molecular form of frontotemporal lobar degeneration (FTLD-TDP) and in the vast majority of cases with amyotrophic lateral sclerosis (ALS-TDP). However, most of the specific phosphorylation sites remain to be determined, and their relevance regarding pathogenicity and clinical and pathological phenotypic diversity in FTLD-TDP and ALS-TDP remains to be identified. Here, we generated a novel antibody raised against TDP-43 phosphorylated at serine 375 (pTDP-43^S375) located in the low-complexity domain, and used it to investigate the presence of S375 phosphorylation in a series (n = 44) of FTLD-TDP and ALS-TDP cases. Immunoblot analysis demonstrated phosphorylation of S375 to be a consistent feature of pathological TDP-43 species, including full-length and C-terminal fragments, in all FTLD-TDP subtypes examined (A-C) and in ALS-TDP. Of particular interest, however, detailed immunohistochemical analysis showed striking differences in the immunoreactivity profile of inclusions with the pTDP-43^S375 antiserum among pathological subtypes. TDP-43 pathology of ALS-TDP, FTLD-TDP type B (including cases with the C9orf72 mutation), and FTLD-TDP type C all showed strong pTDP-43^S375 immunoreactivity that was similar in amount and morphology to that seen with an antibody against TDP-43 phosphorylated at S409/410 used as the gold standard. In stark contrast, TDP-43 pathology in sporadic and genetic forms of FTLD-TDP type A (including cases with GRN and C9orf72 mutations) was found to be almost completely negative by pTDP-43^S375 immunohistochemistry. These data suggest a subtype-specific, conformation-dependent binding of pTDP-43^S375 antiserum to TDP-43 aggregates, consistent with the idea of distinct structural TDP-43 conformers (i.e., TDP-43 strains) as the molecular basis for the phenotypic diversity in TDP-43 proteinopathies.

Review: Neuropathology of non-tau frontotemporal lobar degeneration

Frontotemporal dementia (FTD) is a heterogeneous clinical syndrome associated with frontotemporal lobar degeneration (FTLD) as a relatively consistent neuropathological hallmark feature. However, the discoveries in the past decade of many of the relevant pathological proteins aggregating in human FTD brains in addition to several new FTD causing gene mutations underlined that FTD is a diverse condition on the neuropathological and genetic basis. This resulted in a novel molecular classification of these conditions based on the predominant protein abnormality and allows most cases of FTD to be placed now into one of three broad molecular subgroups; FTLD with tau, TAR DNA-binding protein 43 or FET protein accumulation (FTLD-tau, FTLD-TDP and FTLD-FET respectively). This review will provide an overview of the molecular neuropathology of non-tau FTLD, insights into disease mechanisms gained from the study of human post mortem tissue as well as discussion of current controversies in the field.

Subcortical TDP-43 pathology patterns validate cortical FTLD-TDP subtypes and demonstrate unique aspects of C9orf72 mutation cases

Frontotemporal lobar degeneration with TDP-43 immunoreactive (TDP-ir) inclusions (FTLD-TDP) is sub-classified based on the pattern of neocortical pathology, with each subtype showing clinical and genetic correlations. Recent studies indicate that accurate subtyping of cases may be important to help identify genetic risk factors and develop biomarkers. Although most FTLD-TDP cases are easily classified, some do not match well to one of the existing subtypes. In particular, cases with the C9orf72 repeat expansion (C9+) have been reported to show FTLD-TDP type A, type B or a combination of A and B pathology (A + B). In our series of FTLD-TDP cases, we found that those lacking the C9orf72 mutation (non-C9) were all readily classified as type A (n = 29), B (n = 16) or C (n = 18), using current criteria and standard observational methods. This classification was validated using non-biased hierarchical cluster analysis (HCA) of neocortical pathology data. In contrast, only 14/28 (50%) of the C9+ cases were classified as either pure type A or pure type B, with the remainder showing A + B features. HCA confirmed separation of the C9+ cases into three groups. We then investigated whether patterns of subcortical TDP-ir pathology helped to classify the difficult cases. For the non-C9 cases, each subtype showed a consistent pattern of subcortical involvement with significant differences among the groups. The most distinguishing features included white matter threads, neuronal intranuclear inclusions in hippocampus and striatum, and delicate threads in CA1 in type A; glial cytoplasmic inclusions in white matter and neuronal cytoplasmic inclusions (NCI) in lower motor neurons in type B; compact NCI in striatum in type C. HCA of the C9+ cases based on subcortical features increased the number that clustered with the non-C9 type A (46%) or non-C9 type B (36%); however, there remained a C9+ group with A + B features (18%). These findings suggest that most FTLD-TDP cases can be classified using existing criteria and that each group also shows characteristic subcortical TDP-ir pathology. However, C9+ cases may be unique in the degree to which their pathology overlaps between FTLD-TDP types A and B.

C-terminal and full length TDP-43 specie differ according to FTLD-TDP lesion type but not genetic mutation

The transactive response DNA binding protein of 43 kDa (TDP-43) is an intranuclear protein involved in RNA splicing. Abnormally deposited TDP-43 is found in the brains of patients with frontotemporal lobar degeneration (FTLD). Different morphological characteristics of TDP-43 immunoreactive inclusions define the different variants of FTLD-TDP. Little is known about the relationships between TDP-43 specie (phosphorylated TDP-43, C-terminal fragments and full length TDP-43) and lesion types. Using novel antibodies that recognize phosphorylated TDP-43 (pTDP-43), a neoepitope in the C-terminal fragment of TDP-43 (cTDP-43) and the N-terminal, i.e. full length (nTDP-43) we assess the relative burden of pTDP-43, cTDP-43 and nTDP-43 in 8 different lesion types across FTLD-TDP type A-C. These include neuronal cytoplasmic inclusions, dystrophic neurites, neuronal intranuclear inclusions, fine neurites of the hippocampus, peri-vascular inclusions, Pick body-like inclusions, long thick dystrophic neurites and granular pre-inclusions. We also assess for associations with progranulin (GRN) and C9ORF72 genetic mutations. For all eight lesion types, the highest burden was observed for pTDP-43. In six of the eight lesions studied, cTDP-43 burden was greater than nTDP-43 burden. However, we observed a higher burden of nTDP-43 to cTDP-43 for pre-inclusions. We also noted an equal-to-slightly higher burden of nTDP-43 to cTDP-43 for peri-vascular inclusions. There was not strong evidence for associations to be driven by mutation status although for neuronal cytoplasmic inclusions and dystrophic neurites GRN+ cases had higher burden of pTDP-43, cTDP-43 and nTDP-43 compared to GRN- cases, with nTDP-43 inclusions only observed in GRN+ cases. In addition, for pre-inclusions, cTDP-43 and nTDP-43 burden tended to be higher in C9ORF72- cases compared to C9ORF72+ cases, although this was not the case for pTDP-43. There is clear evidence that phosphorylation and C terminal fragments play an important role in lesion formation in FTLD-TDP. However, for some inclusions, particularly pre-inclusions, full-length TDP-43 appears to be playing a role.

Hot-spot KIF5A mutations cause familial ALS

Heterozygous missense mutations in the N-terminal motor or coiled-coil domains of the kinesin family member 5A (KIF5A) gene cause monogenic spastic paraplegia (HSP10) and Charcot-Marie-Tooth disease type 2 (CMT2). Moreover, heterozygous de novo frame-shift mutations in the C-terminal domain of KIF5A are associated with neonatal intractable myoclonus, a neurodevelopmental syndrome. These findings, together with the observation that many of the disease genes associated with amyotrophic lateral sclerosis disrupt cytoskeletal function and intracellular transport, led us to hypothesize that mutations in KIF5A are also a cause of amyotrophic lateral sclerosis. Using whole exome sequencing followed by rare variant analysis of 426 patients with familial amyotrophic lateral sclerosis and 6137 control subjects, we detected an enrichment of KIF5A splice-site mutations in amyotrophic lateral sclerosis (2/426 compared to 0/6137 in controls; P = 4.2 × 10⁻³), both located in a hot-spot in the C-terminus of the protein and predicted to affect splicing exon 27. We additionally show co-segregation with amyotrophic lateral sclerosis of two canonical splice-site mutations in two families. Investigation of lymphoblast cell lines from patients with KIF5A splice-site mutations revealed the loss of mutant RNA expression and suggested haploinsufficiency as the most probable underlying molecular mechanism. Furthermore, mRNA sequencing of a rare non-synonymous missense mutation (predicting p.Arg1007Gly) located in the C-terminus of the protein shortly upstream of the splice donor of exon 27 revealed defective KIF5A pre-mRNA splicing in respective patient-derived cell lines owing to abrogation of the donor site. Finally, the non-synonymous single nucleotide variant rs113247976 (minor allele frequency = 1.00% in controls, n = 6137), also located in the C-terminal region [p.(Pro986Leu) in exon 26], was significantly enriched in familial amyotrophic lateral sclerosis patients (minor allele frequency = 3.40%; P = 1.28 × 10⁻⁷). Our study demonstrates that mutations located specifically in a C-terminal hotspot of KIF5A can cause a classical amyotrophic lateral sclerosis phenotype, and underline the involvement of intracellular transport processes in amyotrophic lateral sclerosis pathogenesis.

The Pathobiology of TDP-43 C-Terminal Fragments in ALS and FTLD

During neurodegenerative disease, the multifunctional RNA-binding protein TDP-43 undergoes a vast array of post-translational modifications, including phosphorylation, acetylation, and cleavage. Many of these alterations may directly contribute to the pathogenesis of TDP-43 proteinopathies, which include most forms of amyotrophic lateral sclerosis (ALS) and approximately half of all frontotemporal dementia, pathologically identified as frontotemporal lobar degeneration (FTLD) with TDP-43 pathology. However, the relative contributions of the various TDP-43 post-translational modifications to disease remain unclear, and indeed some may be secondary epiphenomena rather than disease-causative. It is therefore critical to determine the involvement of each modification in disease processes to allow the design of targeted treatments. In particular, TDP-43 C-terminal fragments (CTFs) accumulate in the brains of people with ALS and FTLD and are therefore described as a neuropathological signature of these diseases. Remarkably, these TDP-43 CTFs are rarely observed in the spinal cord, even in ALS which involves dramatic degeneration of spinal motor neurons. Therefore, TDP-43 CTFs are not produced non-specifically in the course of all forms of TDP-43-related neurodegeneration, but rather variably arise due to additional factors influenced by regional heterogeneity in the central nervous system. In this review, we summarize how TDP-43 CTFs are generated and degraded by cells, and critique evidence from studies of TDP-43 CTF pathology in human disease tissues, as well as cell and animal models, to analyze the pathophysiological relevance of TDP-43 CTFs to ALS and FTLD. Numerous studies now indicate that, although TDP-43 CTFs are prevalent in ALS and FTLD brains, disease-related pathology is only variably reproduced in TDP-43 CTF cell culture models. Furthermore, TDP-43 CTF expression in both transgenic and viral-mediated in vivo models largely fails to induce motor or behavioral dysfunction reminiscent of human disease. We therefore conclude that although TDP-43 CTFs are a hallmark of TDP-43-related neurodegeneration in the brain, they are not a primary cause of ALS or FTLD.

Psychiatric Symptoms in Amyotrophic Lateral Sclerosis: Beyond a Motor Neuron Disorder

The historical view that Amyotrophic Lateral Sclerosis (ALS) as a pure motor disorder has been increasingly challenged by the discovery of cognitive and behavioral changes in the spectrum of Frontotemporal Dementia (FTD). Less recognized and still significant comorbidities that ALS patients may present are prior or concomitant psychiatric illness, such as psychosis and schizophrenia, or mood disorders. These non-motor symptoms disturbances have a close time relationship with disease onset, may constitute part of a larger framework of network disruption in motor neuron disorders, and may impact ALS patients and families, with regards to ethical choices and end-of-life decisions. This review aims at identifying the most common psychiatric alterations related to ALS and its prognosis, looking at a common genetic background and shared structural brain pathology.

Primary progressive aphasia and the FTD-MND spectrum disorders: clinical, pathological, and neuroimaging correlates

Objective: Behavioral variant frontotemporal dementia (bvFTD), is commonly considered the cognitive presentation of the frontotemporal dementia-motor neuron disease (FTD-MND) spectrum disorder. We evaluated the prevalence of primary progressive aphasia in a series of pathologically confirmed cases of FTD-MND spectrum. Methods: Pathologically confirmed cases of frontotemporal lobar degeneration-motor neuron disease (FTLD-MND) were obtained from the UCSF brain bank. Cases were analyzed for presence of language impairment via retrospective chart review of research visits that include neurologic exam, in-depth cognitive testing and magnetic resonance imaging (MRI) imaging. Forty one cases were included. Thirty two were diagnosed with FTD-MND, while nine cases were diagnosed as MND-only from clinical evaluation. Results: Ten FTLD-MND cases (31%) presented with prominent or isolated language involvement consistent with a diagnosis of primary progressive aphasia (PPA), which we called progressive aphasia with motor neuron disease (PA-MND). Of these, three cases that mirrored the non-fluent variant of PPA (nfvPPA) were named nfvPA-MND. The imaging pattern of these nfvPA-MND showed atrophy strictly confined to the frontal and anterior temporal language cortical areas. Another group of seven cases that resembled patients with the semantic variant PPA (svPPA) were named svPA-MND. The group of svPPA-MND on imaging analysis showed selective atrophy of the temporal lobe and orbitofrontal cortex. Conclusions: Language impairment was a frequent phenotype of FTD-MND associated with focal atrophy patterns within the language networks. This data suggest patients with FTD-MND can present quite often with language phenotype of nfvPPA and svPPA, as opposed to exclusive bvFTD symptoms.

Apolipoprotein D Upregulation in Alzheimer's Disease but Not Frontotemporal Dementia

Frontotemporal dementia (FTD) and Alzheimer’s disease (AD) are the two common forms of dementia. FTD syndromes are characterized by lobar atrophy (frontotemporal lobar degeneration or FTLD) and the presence of either cellular TDP43 (FTLD-TDP), tau (FTLD-tau), or FUS aggregates, while extracellular β-amyloid plaques and hyperphosphorylated tau tangles develop in AD. Oxidative stress can induce these pathological modifications in disease models, and is thought to play a role in these syndromes. Apolipoprotein D (apoD) is a glial-expressed lipocalin known to protect against oxidative stress, with increased levels in AD, supporting a protective role. The expression of apoD has not been studied in FTLD. This study assesses apoD expression in FTLD-TDP and FTLD-tau in comparison to AD and controls. It also analyzes the effect of apoD on TARDBP (TDP43 gene) and β-amyloid precursor protein (APP). The expression of apoD was analyzed by Western blotting in FTLD-TDP, FTLD-tau, AD, and control post-mortem brain tissue. An apoD-overexpressing cell model was used to study the impact of increased apoD on APP and TARDBP expression. We confirm that apoD expression was increased in AD but surprisingly it was not affected in either of the two main pathological forms of FTLD. Under oxidative stress conditions, apoD had no effect on TDP43 expression but it did decrease APP expression. This suggests that apoD does not act as a neuroprotective factor in FTLD in the same way as in AD. This could contribute to the more rapid degeneration observed in FTLD.

TARDBP mutation associated with semantic variant primary progressive aphasia, case report and review of the literature

Semantic variant primary progressive aphasia (svPPA) is a clinical syndrome included in the frontotemporal dementia (FTD) spectrum. Unlike other forms of FTD, it is sporadic in the majority of cases and not commonly associated with motor neuron disease (MND). We describe a case of svPPA associated with MND in the same family, due to a mutation of the transactive response DNA binding protein (TARDBP) gene, and review the literature.

Spreading of Tau Pathology in Sporadic Alzheimer's Disease Along Cortico-cortical Top-Down Connections

By using AT8-immunocytochemistry that visualizes hyperphosphorylated tau protein, we examined neurofibrillary changes related to sporadic Alzheimer's disease (AD) in N = 40 individuals at neurofibrillary tangle (NFT) stages I-IV. We report the presence of abnormal tau changes within solitary pyramidal neurons in layers III and V of the neocortex. These pyramidal cells showed pathology in different cell compartments (dendritic, somatic, axonal) that appeared to occur sequentially: Tau pathology was seen in distal segments of the basal dendrites, then in proximal dendrites, the soma, and, finally, in the axon of affected neurons. These findings are remarkable in that they point to the existence of neurofibrillary changes in regions routinely associated with later NFT stages. In addition, they lend support to the idea that it may be the axons of cortico-cortical top-down neurons in neocortical fields involved in AD that carry and spread abnormal tau seeds in a focused manner (transsynaptically) into the distal dendritic segments of nerve cells following directly in the neuronal chain, thereby sustaining further tau-seeded templating.

Prevalence of transactive response DNA-binding protein 43 (TDP-43) proteinopathy in cognitively normal older adults: systematic review and meta-analysis

OBJECTIVE

To perform a systematic review and meta-analysis on the prevalence of transactive response DNA-binding protein 43 (TDP-43) proteinopathy in cognitively normal older adults.

METHODS

We systematically reviewed and performed a meta-analysis on the prevalence of TDP-43 proteinopathy in older adults with normal cognition, evaluated by the Mini-Mental State Examination or the Clinical Dementia Rating. We estimated the overall prevalence of TDP-43 using random-effect models, and stratified by age, sex, sample size, study quality, antibody used to assess TDP-43 aggregates, analysed brain regions, Braak stage, Consortium to Establish a Registry for Alzheimer's Disease score, hippocampal sclerosis and geographic location.

RESULTS

A total of 505 articles were identified in the systematic review, and 7 were included in the meta-analysis with 1196 cognitively normal older adults. We found an overall prevalence of TDP-43 proteinopathy of 24%. Prevalence of TDP-43 proteinopathy varied widely across geographic location (North America: 37%, Asia: 29%, Europe: 14%, and Latin America: 11%). Estimated prevalence of TDP-43 proteinopathy also varied according to study quality (quality score >7: 22% vs. quality score <7: 42%), antibody used to assess TDP-43 proteinopathy (native: 18% vs. hyperphosphorylated: 24%) and presence of hippocampal sclerosis (without 24% vs. with hippocampal sclerosis: 48%). Other stratified analyses by age, sex, analysed brain regions, sample size and severity of AD neuropathology showed similar pooled TDP-43 prevalence.

CONCLUSIONS

Different methodology to access TDP-43, and also differences in lifestyle and genetic factors across different populations could explain our results. Standardization of TDP-43 measurement, and future studies about the impact of genetic and lifestyle characteristics on the development of neurodegenerative diseases are needed.

Dissecting the pathobiology of altered MRI signal in amyotrophic lateral sclerosis: A post mortem whole brain sampling strategy for the integration of ultra-high-field MRI and quantitative neuropathology

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a clinically and histopathologically heterogeneous neurodegenerative disorder, in which therapy is hindered by the rapid progression of disease and lack of biomarkers. Magnetic resonance imaging (MRI) has demonstrated its potential for detecting the pathological signature and tracking disease progression in ALS. However, the microstructural and molecular pathological substrate is poorly understood and generally defined histologically. One route to understanding and validating the pathophysiological correlates of MRI signal changes in ALS is to directly compare MRI to histology in post mortem human brains.

RESULTS

The article delineates a universal whole brain sampling strategy of pathologically relevant grey matter (cortical and subcortical) and white matter tracts of interest suitable for histological evaluation and direct correlation with MRI. A standardised systematic sampling strategy that was compatible with co-registration of images across modalities was established for regions representing phosphorylated 43-kDa TAR DNA-binding protein (pTDP-43) patterns that were topographically recognisable with defined neuroanatomical landmarks. Moreover, tractography-guided sampling facilitated accurate delineation of white matter tracts of interest. A digital photography pipeline at various stages of sampling and histological processing was established to account for structural deformations that might impact alignment and registration of histological images to MRI volumes. Combined with quantitative digital histology image analysis, the proposed sampling strategy is suitable for routine implementation in a high-throughput manner for acquisition of large-scale histology datasets. Proof of concept was determined in the spinal cord of an ALS patient where multiple MRI modalities (T1, T2, FA and MD) demonstrated sensitivity to axonal degeneration and associated heightened inflammatory changes in the lateral corticospinal tract. Furthermore, qualitative comparison of R2* and susceptibility maps in the motor cortex of 2 ALS patients demonstrated varying degrees of hyperintense signal changes compared to a control. Upon histological evaluation of the same region, intensity of signal changes in both modalities appeared to correspond primarily to the degree of microglial activation.

CONCLUSION

The proposed post mortem whole brain sampling methodology enables the accurate intraindividual study of pathological propagation and comparison with quantitative MRI data, to more fully understand the relationship of imaging signal changes with underlying pathophysiology in ALS.

TDP-43 post-translational modifications in health and disease

INTRODUCTION

Nuclear factor TDP-43 is a ubiquitously expressed RNA binding protein that plays a key causative role in several neurodegenerative diseases, especially in the ALS/FTD spectrum. In addition, its aberrant aggregation and expression has been recently observed in other type of diseases, such as myopathies and Niemann-Pick C, a lysosomal storage disease. Areas covered: This review aims to specifically cover the post-translational modifications (PTMs) that can affect TDP-43 function and cellular status both in health and disease. To this date, these include phosphorylation, formation of C-terminal fragments, disulfide bridge formation, ubiquitination, acetylation, and sumoylation. Recently published articles on these subjects have been reviewed in this manuscript. Expert opinion: Targeting aberrant TDP-43 expression in neurodegenerative diseases is a very challenging task due to the fact that both its overexpression and downregulation are considerably toxic to cells. This characteristic makes it difficult to therapeutically target this protein in a generalized manner. An alternative approach could be the identification of specific aberrant PTMs that promote its aggregation or toxicity, and developing novel therapeutic approaches toward their selective modification.

Reappraisal of TDP-43 pathology in FTLD-U subtypes

Frontotemporal lobar degeneration with tau-negative, ubiquitin-immunoreactive (-ir) pathology (FTLD-U) is subclassified based on the type and cortical laminar distribution of neuronal inclusions. Following the discovery of the transactive response DNA-binding protein Mr 43 kD (TDP-43) as the ubiquitinated protein in most FTLD-U, the same pathological criteria have been used to classify FTLD cases based on TDP-43-ir changes. However, the fact that immunohistochemistry (IHC) for ubiquitin and TDP-43 each recognizes slightly different pathological changes in these cases means that the original FTLD-U subtype criteria may not be directly applicable for use with TDP-43 IHC. We formally re-evaluated the TDP-43-ir pathological features that characterize the different FTLD-U subtypes to see if the current classification could be refined. In our series of 78 cases, 81% were classified as one of the common FTLD-U subtypes (29% A, 35% B, 17% C). With TDP-43 IHC, each subtype demonstrated consistent intra-group pathological features and clear inter-group differences. The TDP-43-ir changes that characterized type A and C cases were similar to those seen with ubiquitin IHC; specifically, compact neuronal cytoplasmic inclusions (NCI), short thick dystrophic neurites (DN), and lentiform neuronal intranuclear inclusions concentrated in cortical layer II in type A cases, and a predominance of long thick DN in type C. However, type B cases showed significant differences with TDP-43 compared with ubiquitin IHC; with many diffuse granular NCI and wispy thread and dots-like profiles in all cortical layers. The remaining 15 cases (12 with C9orf72 mutations) showed changes that were consistent with combined type A and type B pathology. These findings suggest that the pathological criteria for subtyping FTLD cases based on TDP-43 IHC might benefit from some refinement that recognizes differences in the morphologies of NCI and neurites. Furthermore, there is a significant subset of cases (most with the C9orf72 mutation) with the pathological features of multiple FTLD-TDP subtypes for which appropriate classification is difficult.

Pathological TDP-43 changes in Betz cells differ from those in bulbar and spinal α-motoneurons in sporadic amyotrophic lateral sclerosis

Two nerve cells types, Betz cells in layer Vb of the primary motor neocortex and α-motoneurons of the lower brainstem and spinal cord, become involved at the beginning of the pathological cascade underlying sporadic amyotrophic lateral sclerosis (sALS). In both neuronal types, the cell nuclei forfeit their normal (non-phosphorylated) expression of the 43-kDa transactive response DNA-binding protein (TDP-43). Here, we present initial evidence that in α-motoneurons the loss of normal nuclear TDP-43 expression is followed by the formation of phosphorylated TDP-43 aggregates (pTDP-43) within the cytoplasm, whereas in Betz cells, by contrast, the loss of normal nuclear TDP-43 expression remains mostly unaccompanied by the development of cytoplasmic aggregations. We discuss some implications of this phenomenon of nuclear clearing in the absence of cytoplasmic inclusions, namely, abnormal but soluble (and, thus, probably toxic) cytoplasmic TDP-43 could enter the axoplasm of Betz cells, and following its transmission to the corresponding α-motoneurons in the lower brainstem and spinal cord, possibly contribute in recipient neurons to the dysregulation of the normal nuclear protein. Because the cellular mechanisms that possibly inhibit the aggregation of TDP-43 in the cytoplasm of involved Betz cells are unknown, insight into such mechanisms could disclose a pathway by which the development of aggregates in this cell population could be accelerated, thereby opening an avenue for a causally based therapy.

An update on semantic dementia: genetics, imaging, and pathology

Progressive and relatively circumscribed loss of semantic knowledge, referred to as semantic dementia (SD) which falls under the broader umbrella of frontotemporal dementia, was officially identified as a clinical syndrome less than 50 years ago. Here, we review recent neuroimaging, pathological, and genetic research in SD. From a neuroimaging perspective, SD is characterised by hallmark asymmetrical atrophy of the anterior temporal pole and anterior fusiform gyrus, which is usually left lateralised. Functional magnetic resonance imaging (fMRI) studies have revealed widespread changes in connectivity, implicating the anterior temporal regions in semantic deficits in SD. Task-related fMRI have also demonstrated the relative preservation of frontal and parietal regions alongside preserved memory performance. In addition, recent longitudinal studies have demonstrated that, with disease progression, atrophy encroaches into the contralateral temporal pole and medial prefrontal cortices, which reflects emerging changes in behaviour and social cognition. Notably, unlike other frontotemporal dementia subtypes, recent research has demonstrated strong clinicopathological concordance in SD, with TDP43 type C as the most common pathological subtype. Moreover, an underlying genetic cause appears to be relatively rare in SD, with the majority of cases having a sporadic form of the disease. The relatively clear diagnosis, clinical course, and pathological homogeneity of SD make this syndrome a promising target for novel disease-modifying interventions. The development of neuroimaging markers of disease progression at the individual level is an important area of research for future studies to address, in order to assist with this endeavour.

A Clinical Guide to Frontotemporal Dementias

The term frontotemporal dementia (FTD) describes a diverse group of clinical syndromes, including behavioral-variant FTD (bvFTD), nonfluent/agrammatic-variant primary progressive aphasia (nfvPPA), semantic-variant primary progressive aphasia (svPPA), FTD motor neuron disease (FTD-MND), progressive supranuclear palsy syndrome (PSP-S), and corticobasal syndrome (CBS). Although each of these syndromes may be distinguished by their respective disturbances in behavior, language, or motor function and characteristic imaging findings, they may present a diagnostic dilemma when encountered clinically. In this article, we review the clinical features, diagnostic criteria, pathology, genetics, and therapeutic interventions for FTD spectrum disorders.

The frontotemporal dementia-motor neuron disease continuum

Early reports of cognitive and behavioural deficits in motor neuron disease might have been overlooked initially, but the concept of a frontotemporal dementia-motor neuron disease continuum has emerged during the past decade. Frontotemporal dementia-motor neuron disease is now recognised as an important dementia syndrome, which presents substantial challenges for diagnosis and management. Frontotemporal dementia, motor neuron disease, and frontotemporal dementia-motor neuron disease are characterised by overlapping patterns of TAR DNA binding protein (TDP-43) pathology, while the chromosome 9 open reading frame 72 (C9orf72) repeat expansion is common across the disease spectrum. Indeed, the C9orf72 repeat expansion provides important clues to disease pathogenesis and suggests potential therapeutic targets. Variable diagnostic criteria identify motor, cognitive, and behavioural deficits, but further refinement is needed to define the clinical syndromes encountered in frontotemporal dementia-motor neuron disease.

Semi-Automated Digital Image Analysis of Pick's Disease and TDP-43 Proteinopathy

Digital image analysis of histology sections provides reliable, high-throughput methods for neuropathological studies but data is scant in frontotemporal lobar degeneration (FTLD), which has an added challenge of study due to morphologically diverse pathologies. Here, we describe a novel method of semi-automated digital image analysis in FTLD subtypes including: Pick's disease (PiD, n=11) with tau-positive intracellular inclusions and neuropil threads, and TDP-43 pathology type C (FTLD-TDPC, n=10), defined by TDP-43-positive aggregates predominantly in large dystrophic neurites. To do this, we examined three FTLD-associated cortical regions: mid-frontal gyrus (MFG), superior temporal gyrus (STG) and anterior cingulate gyrus (ACG) by immunohistochemistry. We used a color deconvolution process to isolate signal from the chromogen and applied both object detection and intensity thresholding algorithms to quantify pathological burden. We found object-detection algorithms had good agreement with gold-standard manual quantification of tau- and TDP-43-positive inclusions. Our sampling method was reliable across three separate investigators and we obtained similar results in a pilot analysis using open-source software. Regional comparisons using these algorithms finds differences in regional anatomic disease burden between PiD and FTLD-TDP not detected using traditional ordinal scale data, suggesting digital image analysis is a powerful tool for clinicopathological studies in morphologically diverse FTLD syndromes.

TDP-43 proteinopathies: pathological identification of brain regions differentiating clinical phenotypes

The pathological sequestration of TAR DNA-binding protein 43 (TDP-43, encoded by TARDBP) into cytoplasmic pathological inclusions characterizes the distinct clinical syndromes of amyotrophic lateral sclerosis and behavioural variant frontotemporal dementia, while also co-occurring in a proportion of patients with Alzheimer's disease, suggesting that the regional concentration of TDP-43 pathology has most relevance to specific clinical phenotypes. This has been reflected in the three different pathological staging schemes for TDP-43 pathology in these different clinical syndromes, with none of these staging schemes including a preclinical phase similar to that which has proven beneficial in other neurodegenerative diseases. To apply each of these three staging schemes for TDP-43 pathology, the clinical phenotype must be known undermining the potential predictive value of the pathological examination. The present study set out to test whether a more unified approach could accurately predict clinical phenotypes based solely on the regional presence and severity of TDP-43 pathology. The selection of brain regions of interest was based on key regions routinely sampled for neuropathological assessment under current consensus criteria that have also been used in the three TDP-43 staging schemes. The severity of TDP-43 pathology in these regions of interest was assessed in four clinicopathological phenotypes: amyotrophic lateral sclerosis (n = 27, 47-78 years, 15 males), behavioural variant frontotemporal dementia (n = 15, 49-82 years, seven males), Alzheimer's disease (n = 26, 51-90 years, 11 males) and cognitively normal elderly individuals (n = 17, 80-103 years, nine males). Our results demonstrate that the presence of TDP-43 in the hypoglossal nucleus discriminates patients with amyotrophic lateral sclerosis with an accuracy of 98%. The severity of TDP-43 deposited in the anterior cingulate cortex identifies patients with behavioural variant frontotemporal dementia with an accuracy of 99%. This identification of regional pathology associated with distinct clinical phenotypes suggests key regions on which probabilistic pathological criteria, similar to those currently available for Alzheimer's disease and dementia with Lewy bodies, can be developed for TDP-43 proteinopathies. We propose and validate a simplified probabilistic statement that involves grading the presence of TDP-43 in the hypoglossal nucleus and the severity of TDP-43 in the anterior cingulate for the pathological identification of TDP-43 proteinopathy cases with clinical amyotrophic lateral sclerosis and behavioural variant frontotemporal dementia.

Neuropathological assessments of the pathology in frontotemporal lobar degeneration with TDP43-positive inclusions: an inter-laboratory study by the BrainNet Europe consortium

The BrainNet Europe consortium assessed the reproducibility in the assignment of the type of frontotemporal lobar degeneration (FTLD) with TAR DNA-binding protein (TDP) 43 following current recommendations. The agreement rates were influenced by the immunohistochemical (IHC) method and by the classification strategy followed. p62-IHC staining yielded good uniform quality of stains, but the most reliable results were obtained implementing specific Abs directed against the hallmark protein TDP43. Both assessment of the type and the extent of lesions were influenced by the Abs and by the quality of stain. Assessment of the extent of the lesions yielded poor results repeatedly; thus, the extent of pathology should not be used in diagnostic consensus criteria. Whilst 31 neuropathologists typed 30 FTLD-TDP cases, inter-rater agreement ranged from 19 to 100 per cent, being highest when applying phosphorylated TDP43/IHC. The agreement was highest when designating Type C or Type A/B. In contrast, there was a poor agreement when attempting to separate Type A or Type B FTLD-TDP. In conclusion, we can expect that neuropathologist, independent of his/her familiarity with FTLD-TDP pathology, can identify a TDP43-positive FTLD case. The goal should be to state a Type (A, B, C, D) or a mixture of Types (A/B, A/C or B/C). Neuropathologists, other clinicians and researchers should be aware of the pitfalls whilst doing so. Agreement can be reached in an inter-laboratory setting regarding Type C cases with thick and long neurites, whereas the differentiation between Types A and B may be more troublesome.

New criteria for frontotemporal dementia syndromes: clinical and pathological diagnostic implications

OBJECTIVE

To assess the impact of new clinical diagnostic criteria for frontotemporal dementia (FTD) syndromes, including primary progressive aphasias (PPA), on prior clinical diagnosis and to explore clinicopathological correlations.

METHODS

178 consecutive neuropathologically ascertained cases initially diagnosed with a FTD syndrome were collected through specialist programmes: the Cambridge Brain Bank, UK, and Sydney Brain Bank, Australia. 135 cases were reclassified using the revised diagnostic criteria into behavioural variant (bvFTD), semantic variant PPA (sv-PPA), non-fluent/agrammatic variant PPA (nfv-PPA) and logopenic variant PPA (lv-PPA). Pathological diagnoses included FTLD-tau, FTLD-TDP, FTLD-FUS, FTLD-UPS, FLTD-ni and Alzheimer's disease (AD). Statistical analyses included χ(2) tests, analyses of variance and discriminant statistics.

RESULTS

Comparison of the original and revised diagnosis revealed no change in 90% of bvFTD and sv-PPA cases. By contrast, 51% of nfv-PPA cases were reclassified as lv-PPA, with apraxia of speech and sentence repetition assisting in differentiation. Previous patterns of pathology were confirmed, although more AD cases occurred in FTD syndromes (10% bvFTD, ∼15% sv-PPA and ∼30% nfv-PPA) than expected. AD was the dominant pathology (77%) of lv-PPA. Discriminant analyses revealed that object agnosia, phonological errors and neuropsychiatric features differentiated AD from FTLD.

CONCLUSIONS

This study provides pathological validation that the new criteria assist with separating PPA cases with AD pathology into the new lv-PPA syndrome and found that a number of diagnostic clinical features (disinhibition, food preferences and naming) did not assist in discriminating the different FTD syndromes.