An autopsy case of incipient Pick's disease: immunohistochemical profile of early-stage Pick body formation

G272V and P301L Mutations Induce Isoform Specific Tau Mislocalization to Dendritic Spines and Synaptic Dysfunctions in Cellular Models of 3R and 4R Tau Frontotemporal Dementia

Tau pathologies are detected in the brains of some of the most common neurodegenerative diseases including Alzheimer's disease (AD), Lewy body dementia (LBD), chronic traumatic encephalopathy (CTE), and frontotemporal dementia (FTD). Tau proteins are expressed in six isoforms with either three or four microtubule-binding repeats (3R tau or 4R tau) due to alternative RNA splicing. AD, LBD, and CTE brains contain pathological deposits of both 3R and 4R tau. FTD patients can exhibit either 4R tau pathologies in most cases or 3R tau pathologies less commonly in Pick's disease, which is a subfamily of FTD. Here, we report the isoform-specific roles of tau in FTD. The P301L mutation, linked to familial 4R tau FTD, induces mislocalization of 4R tau to dendritic spines in primary hippocampal cultures that were prepared from neonatal rat pups of both sexes. Contrastingly, the G272V mutation, linked to familial Pick's disease, induces phosphorylation-dependent mislocalization of 3R tau but not 4R tau proteins to dendritic spines. The overexpression of G272V 3R tau but not 4R tau proteins leads to the reduction of dendritic spine density and suppression of mEPSCs in 5-week-old primary rat hippocampal cultures. The decrease in mEPSC amplitude caused by G272V 3R tau is dynamin-dependent whereas that caused by P301L 4R tau is dynamin-independent, indicating that the two tau isoforms activate different signaling pathways responsible for excitatory synaptic dysfunction. Our 3R and 4R tau studies here will shed new light on diverse mechanisms underlying FTD, AD, LBD, and CTE.

Frontotemporal lobar degeneration proteinopathies have disparate microscopic patterns of white and grey matter pathology

Frontotemporal lobar degeneration proteinopathies with tau inclusions (FTLD-Tau) or TDP-43 inclusions (FTLD-TDP) are associated with clinically similar phenotypes. However, these disparate proteinopathies likely differ in cellular severity and regional distribution of inclusions in white matter (WM) and adjacent grey matter (GM), which have been understudied. We performed a neuropathological study of subcortical WM and adjacent GM in a large autopsy cohort (n = 92; FTLD-Tau = 37, FTLD-TDP = 55) using a validated digital image approach. The antemortem clinical phenotype was behavioral-variant frontotemporal dementia (bvFTD) in 23 patients with FTLD-Tau and 42 with FTLD-TDP, and primary progressive aphasia (PPA) in 14 patients with FTLD-Tau and 13 with FTLD-TDP. We used linear mixed-effects models to: (1) compare WM pathology burden between proteinopathies; (2) investigate the relationship between WM pathology burden and WM degeneration using luxol fast blue (LFB) myelin staining; (3) study regional patterns of pathology burden in clinico-pathological groups. WM pathology burden was greater in FTLD-Tau compared to FTLD-TDP across regions (beta = 4.21, SE = 0.34, p < 0.001), and correlated with the degree of WM degeneration in both FTLD-Tau (beta = 0.32, SE = 0.10, p = 0.002) and FTLD-TDP (beta = 0.40, SE = 0.08, p < 0.001). WM degeneration was greater in FTLD-Tau than FTLD-TDP particularly in middle-frontal and anterior cingulate regions (p < 0.05). Distinct regional patterns of WM and GM inclusions characterized FTLD-Tau and FTLD-TDP proteinopathies, and associated in part with clinical phenotype. In FTLD-Tau, WM pathology was particularly severe in the dorsolateral frontal cortex in nonfluent-variant PPA, and GM pathology in dorsolateral and paralimbic frontal regions with some variation across tauopathies. Differently, FTLD-TDP had little WM regional variability, but showed severe GM pathology burden in ventromedial prefrontal regions in both bvFTD and PPA. To conclude, FTLD-Tau and FTLD-TDP proteinopathies have distinct severity and regional distribution of WM and GM pathology, which may impact their clinical presentation, with overall greater severity of WM pathology as a distinguishing feature of tauopathies.

Evidence of corticofugal tau spreading in patients with frontotemporal dementia

Common neurodegenerative diseases feature progressive accumulation of disease-specific protein aggregates in selectively vulnerable brain regions. Increasing experimental evidence suggests that misfolded disease proteins exhibit prion-like properties, including the ability to seed corruptive templating and self-propagation along axons. Direct evidence for transneuronal spread in patients, however, remains limited. To test predictions made by the transneuronal spread hypothesis in human tissues, we asked whether tau deposition within axons of the corticospinal and corticopontine pathways can be predicted based on clinical syndromes and cortical atrophy patterns seen in frontotemporal lobar degeneration (FTLD). Sixteen patients with Pick's disease, 21 with corticobasal degeneration, and 3 with FTLD-MAPT were included, spanning a range of clinical syndromes across the frontotemporal dementia (FTD) spectrum. Cortical involvement was measured using a neurodegeneration score, a tau score, and a composite score based on semiquantitative ratings and complemented by an MRI-based cortical atrophy W-map based on antemortem imaging. Midbrain cerebral peduncle and pontine base descending fibers were divided into three subregions, representing prefrontopontine, corticospinal, and parieto-temporo-occipital fiber pathways. Tau area fraction was calculated in each subregion and related to clinical syndrome and cortical measures. Within each clinical syndrome, there were predicted relationships between cortical atrophy patterns and axonal tau deposition in midbrain cerebral peduncle and pontine base. Between syndromes, contrasting and predictable patterns of brainstem axonal tau deposition emerged, with, for example, greater tau in prefrontopontine fibers in behavioral variant FTD and in corticospinal fibers in corticobasal syndrome. Finally, semiquantitative and quantitative cortical degeneration scores predicted brainstem axonal tau deposition based on anatomical principles. Taken together, these findings provide important human evidence in support of axonal tau spreading in patients with specific forms of tau-related neurodegeneration.

The role of lysosomes and autophagosomes in frontotemporal lobar degeneration

INTRODUCTION

Cell biological and genetic evidence implicate failures in degrading aggregating proteins, such as tau and TDP-43, through the autophagy or lysosomal pathways in the pathogenesis of frontotemporal lobar degeneration (FTLD).

METHODS

We investigated changes in the degradative pathways in 60 patients with different pathological or genetic forms of FTLD employing immunohistochemistry for marker proteins such as lysosomal-associated membrane proteins 1 (LAMP-1) and 2 (LAMP-2), cathepsin D (CTSD) and microtubule-associated protein 1 light chain 3 alpha (LC3A). Immunostained sections were qualitatively and semi-quantitatively assessed for the appearance, distribution and intensity of staining in neurones of the dentate gyrus (DG) and CA4 region of the hippocampus, and the temporal cortex (Tcx).

RESULTS

Lower levels of neuronal LAMP-1 immunostaining were present in the DG and Tcx in FTLD-tau compared to FTLD-TDP. There was less LAMP-1 immunostaining in FTLD-tau with MAPT mutations, and FTLD-tau with Pick bodies, compared to FTLD-TDP types A and B, and less LAMP-1 immunostaining in FTLD-TDP type C than in FTLD-TDP types A and B. There was greater LAMP-1 immunostaining in GRN mutation which may reflect the underlying type A histology rather than mutation. There were no differences in neuronal LAMP-2, CTSD, EEA-1 or LC3A immunostaining between any of the five FTLD histological or four genetic groups, nor between FTLD-TDP and FTLD-tau.

CONCLUSIONS

The underlying pathological mechanism in FTLD-tau may lie with a relative deficiency of lysosomes, or defective vesicular transport, whereas the failure to clear TDP-43 aggregates may lie with lysosomal dysfunction rather than a lack of available lysosomes or degradative enzymes.

Structures of filaments from Pick's disease reveal a novel tau protein fold

The ordered assembly of tau protein into abnormal filamentous inclusions underlies many human neurodegenerative diseases1. Tau assemblies seem to spread through specific neural networks in each disease2, with short filaments having the greatest seeding activity3. The abundance of tau inclusions strongly correlates with disease symptoms4. Six tau isoforms are expressed in the normal adult human brain-three isoforms with four microtubule-binding repeats each (4R tau) and three isoforms that lack the second repeat (3R tau)1. In various diseases, tau filaments can be composed of either 3R or 4R tau, or of both. Tau filaments have distinct cellular and neuroanatomical distributions5, with morphological and biochemical differences suggesting that they may be able to adopt disease-specific molecular conformations6,7. Such conformers may give rise to different neuropathological phenotypes8,9, reminiscent of prion strains10. However, the underlying structures are not known. Using electron cryo-microscopy, we recently reported the structures of tau filaments from patients with Alzheimer's disease, which contain both 3R and 4R tau11. Here we determine the structures of tau filaments from patients with Pick's disease, a neurodegenerative disorder characterized by frontotemporal dementia. The filaments consist of residues Lys254-Phe378 of 3R tau, which are folded differently from the tau filaments in Alzheimer's disease, establishing the existence of conformers of assembled tau. The observed tau fold in the filaments of patients with Pick's disease explains the selective incorporation of 3R tau in Pick bodies, and the differences in phosphorylation relative to the tau filaments of Alzheimer's disease. Our findings show how tau can adopt distinct folds in the human brain in different diseases, an essential step for understanding the formation and propagation of molecular conformers.

Mapping Neurodegenerative Disease Onset and Progression

Brain networks have been of long-standing interest to neurodegeneration researchers, including but not limited to investigators focusing on conventional prion diseases, which are known to propagate along neural pathways. Tools for human network mapping, however, remained inadequate, limiting our understanding of human brain network architecture and preventing clinical research applications. Until recently, neuropathological studies were the only viable approach to mapping disease onset and progression in humans but required large autopsy cohorts and laborious methods for whole-brain sectioning and staining. Despite important advantages, postmortem studies cannot address in vivo, physiological, or longitudinal questions and have limited potential to explore early-stage disease except for the most common disorders. Emerging in vivo network-based neuroimaging strategies have begun to address these issues, providing data that complement the neuropathological tradition. Overall, findings to date highlight several fundamental principles of neurodegenerative disease anatomy and pathogenesis, as well as some enduring mysteries. These principles and mysteries provide a road map for future research.

Timing and significance of pathological features in C9orf72 expansion-associated frontotemporal dementia

SEE SCABER AND TALBOT DOI101093/AWW264 FOR A SCIENTIFIC COMMENTARY ON THIS ARTICLE: A GGGGCC repeat expansion in C9orf72 leads to frontotemporal dementia and/or amyotrophic lateral sclerosis. Diverse pathological features have been identified, and their disease relevance remains much debated. Here, we describe two illuminating patients with frontotemporal dementia due to the C9orf72 repeat expansion. Case 1 was a 65-year-old female with behavioural variant frontotemporal dementia accompanied by focal degeneration in subgenual anterior cingulate cortex, amygdala, and medial pulvinar thalamus. At autopsy, widespread RNA foci and dipeptide repeat protein inclusions were observed, but TDP-43 pathology was nearly absent, even in degenerating brain regions. Case 2 was a 74-year-old female with atypical frontotemporal dementia-motor neuron disease who underwent temporal lobe resection for epilepsy 5 years prior to her first frontotemporal dementia symptoms. Archival surgical resection tissue contained RNA foci, dipeptide repeat protein inclusions, and loss of nuclear TDP-43 but no TDP-43 inclusions despite florid TDP-43 inclusions at autopsy 8 years after first symptoms. These findings suggest that C9orf72-specific phenomena may impact brain structure and function and emerge before first symptoms and TDP-43 aggregation.

Deep clinical and neuropathological phenotyping of Pick disease

OBJECTIVE

To characterize sequential patterns of regional neuropathology and clinical symptoms in a well-characterized cohort of 21 patients with autopsy-confirmed Pick disease.

METHODS

Detailed neuropathological examination using 70μm and traditional 6μm sections was performed using thioflavin-S staining and immunohistochemistry for phosphorylated tau, 3R and 4R tau isoforms, ubiquitin, and C-terminally truncated tau. Patterns of regional tau deposition were correlated with clinical data. In a subset of cases (n = 5), converging evidence was obtained using antemortem neuroimaging measures of gray and white matter integrity.

RESULTS

Four sequential patterns of pathological tau deposition were identified starting in frontotemporal limbic/paralimbic and neocortical regions (phase I). Sequential involvement was seen in subcortical structures, including basal ganglia, locus coeruleus, and raphe nuclei (phase II), followed by primary motor cortex and precerebellar nuclei (phase III) and finally visual cortex in the most severe (phase IV) cases. Behavioral variant frontotemporal dementia was the predominant clinical phenotype (18 of 21), but all patients eventually developed a social comportment disorder. Pathological tau phases reflected the evolution of clinical symptoms and degeneration on serial antemortem neuroimaging, directly correlated with disease duration and inversely correlated with brain weight at autopsy. The majority of neuronal and glial tau inclusions were 3R tau-positive and 4R tau-negative in sporadic cases. There was a relative abundance of mature tau pathology markers in frontotemporal limbic/paralimbic regions compared to neocortical regions.

INTERPRETATION

Pick disease tau neuropathology may originate in limbic/paralimbic cortices. The patterns of tau pathology observed here provide novel insights into the natural history and biology of tau-mediated neurodegeneration.