Pathologic Involvement of Glutamatergic Striatal Inputs From the Cortices in TAR DNA-Binding Protein 43 kDa-Related Frontotemporal Lobar Degeneration and Amyotrophic Lateral Sclerosis

C9orf72 gene networks in the human brain correlate with cortical thickness in C9-FTD and implicate vulnerable cell types

Introduction

A hexanucleotide repeat expansion (HRE) intronic to chromosome 9 open reading frame 72 (C9orf72) is recognized as the most common genetic cause of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and ALS-FTD. Identifying genes that show similar regional co-expression patterns to C9orf72 may help identify novel gene targets and biological mechanisms that mediate selective vulnerability to ALS and FTD pathogenesis.

Methods

We leveraged mRNA expression data in healthy brain from the Allen Human Brain Atlas to evaluate C9orf72 co-expression patterns. To do this, we correlated average C9orf72 expression values in 51 regions across different anatomical divisions (cortex, subcortex, and cerebellum) with average gene expression values for 15,633 protein-coding genes, including 54 genes known to be associated with ALS, FTD, or ALS-FTD. We then performed imaging transcriptomic analyses to evaluate whether the identified C9orf72 co-expressed genes correlated with patterns of cortical thickness in symptomatic C9orf72 pathogenic HRE carriers (n = 19) compared to controls (n = 23). Lastly, we explored whether genes with significant C9orf72 imaging transcriptomic correlations (i.e., "C9orf72 imaging transcriptomic network") were enriched in specific cell populations in the brain and enriched for specific biological and molecular pathways.

Results

A total of 2,120 genes showed an anatomical distribution of gene expression in the brain similar to C9orf72 and significantly correlated with patterns of cortical thickness in C9orf72 HRE carriers. This C9orf72 imaging transcriptomic network was differentially expressed in cell populations previously implicated in ALS and FTD, including layer 5b cells, cholinergic neurons in the spinal cord and brainstem and medium spiny neurons of the striatum, and was enriched for biological and molecular pathways associated with protein ubiquitination, autophagy, cellular response to DNA damage, endoplasmic reticulum to Golgi vesicle-mediated transport, among others.

Conclusion

Considered together, we identified a network of C9orf72 associated genes that may influence selective regional and cell-type-specific vulnerabilities in ALS/FTD.

C9orf72 gene networks in the human brain correlate with cortical thickness in C9-FTD and implicate vulnerable cell types

Introduction

A hexanucleotide repeat expansion (HRE) intronic to chromosome 9 open reading frame 72 (C9orf72) is recognized as the most common genetic cause of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and ALS-FTD. Identifying genes that show similar regional co-expression patterns to C9orf72 may help identify novel gene targets and biological mechanisms that mediate selective vulnerability to ALS and FTD pathogenesis.

Methods

We leveraged mRNA expression data in healthy brain from the Allen Human Brain Atlas to evaluate C9orf72 co-expression patterns. To do this, we correlated average C9orf72 expression values in 51 regions across different anatomical divisions (cortex, subcortex, cerebellum) with average gene expression values for 15,633 protein-coding genes, including 50 genes known to be associated with ALS, FTD, or ALS-FTD. We then evaluated whether the identified C9orf72 co-expressed genes correlated with patterns of cortical thickness in symptomatic C9orf72 pathogenic HRE carriers (n=19). Lastly, we explored whether genes with significant C9orf72 radiogenomic correlations (i.e., 'C9orf72 gene network') were enriched in specific cell populations in the brain and enriched for specific biological and molecular pathways.

Results

A total of 1,748 genes showed an anatomical distribution of gene expression in the brain similar to C9orf72 and significantly correlated with patterns of cortical thickness in C9orf72 HRE carriers. This C9orf72 gene network was differentially expressed in cell populations previously implicated in ALS and FTD, including layer 5b cells, cholinergic motor neurons in the spinal cord, and medium spiny neurons of the striatum, and was enriched for biological and molecular pathways associated with multiple neurotransmitter systems, protein ubiquitination, autophagy, and MAPK signaling, among others.

Conclusions

Considered together, we identified a network of C9orf72-associated genes that may influence selective regional and cell-type-specific vulnerabilities in ALS/FTD.

Brain imaging signatures in amyotrophic lateral sclerosis: Correlation with peripheral motor degeneration

OBJECTIVE

This study aimed to explore the clinical significance of brain imaging signatures in the context of clinical neurological deficits in association with upper and lower motor neuron degeneration in amyotrophic lateral sclerosis (ALS).

METHODS

We performed brain MRI examinations to quantitatively evaluate (1) gray matter volume and (2) white matter tract fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD), and mean diffusivity (MD). Image-derived indices were correlated with (1) global neurological deficits of MRC muscle strength sum score, revised amyotrophic lateral sclerosis functional rating scale (ALSFRS-R), and forced vital capacity (FVC), and (2) focal scores of University of Pennsylvania Upper motor neuron score (Penn score) and the summation of compound muscle action potential Z scores (CMAP Z sum score).

RESULTS

There were 39 ALS patients and 32 control subjects matched for age and gender. Compared to controls, ALS patients had a lower gray matter volume in the precentral gyrus of the primary motor cortex, which was correlated with FA of corticofugal tracts. The gray matter volume of the precentral gyrus was correlated with FVC, MRC sum score, and CMAP Z sum score, while the FA of the corticospinal tract was linearly associated with CMAP Z sum score and Penn score on multivariate linear regression model.

INTERPRETATION

This study indicated that clinical assessment of muscle strength and routine measurements on nerve conduction studies provided surrogate markers of brain structural changes for ALS. Furthermore, these findings suggested parallel involvement of both upper and lower motor neurons in ALS.

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.

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.

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.

The neural network basis of altered decision-making in patients with amyotrophic lateral sclerosis

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is a multisystem disorder associated with motor impairment and behavioral/cognitive involvement. We examined decision-making features and changes in the neural hub network in patients with ALS using a probabilistic reversal learning task and resting-state network analysis, respectively.

METHODS

Ninety ALS patients and 127 cognitively normal participants performed this task. Data from 62 ALS patients and 63 control participants were fitted to a Q-learning model.

RESULTS

ALS patients had anomalous decision-making features with little shift in choice until they thought the value of the two alternatives had become equal. The quantified parameters (Pαβ) calculated by logistic regression analysis with learning rate and inverse temperature well represented the unique choice pattern of ALS patients. Resting-state network analysis demonstrated a strong correlation between Pαβ and decreased degree centrality in the anterior cingulate gyrus and frontal pole.

INTERPRETATION

Altered decision-making in ALS patients may be related to the decreased hub function of medial prefrontal areas.

Reappraisal of the anatomical spreading and propagation hypothesis about TDP-43 aggregation in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Neuronal inclusion of transactivation response DNA-binding protein 43 kDa (TDP-43) is known to be a pathologic hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). TDP-43, which is physiologically a nuclear protein, is mislocalized from the nucleus and aggregated within the cytoplasm of affected neurons in ALS and FTLD patients. Neuropathologic or experimental studies have addressed mechanisms underlying spreading of TDP-43 inclusions in the central nervous system of ALS and FTLD patients. On the basis of postmortem observations, it is hypothesized that TDP-43 inclusions spread along the neural projections. A centrifugal gradient of TDP-43 pathology in certain anatomical systems and axonal or synaptic aggregation of TDP-43 may support the hypothesis. Experimental studies have revealed cell-to-cell propagation of aggregated or truncated TDP-43, which indicates a direct transmission of TDP-43 inclusions to contiguous cells. However, discrepancies remain between the cell-to-cell propagation suggested in the experimental models and the anatomical spreading of TDP-43 aggregations based on postmortem observations. Trans-synaptic transmission, rather than the direct cell-to-cell transmission, may be consistent with the anatomical spreading of TDP-43 aggregations, but cellular mechanisms of trans-synaptic transmission of aggregated proteins remain to be elucidated. Moreover, the spreading of TDP-43 inclusions varies among patients and genetic backgrounds, which indicates host-dependent factors for spreading of TDP-43 aggregations. Perturbation of cellular TDP-43 clearance may be a possible factor modifying the aggregation and spreading. This review discusses postmortem and experimental evidence that address mechanisms of spreading of TDP-43 pathology in the central nervous system of ALS and FTLD patients.

Increased prevalence of granulovacuolar degeneration in C9orf72 mutation

Granulovacuolar degeneration (GVD) is usually found in Alzheimer's disease (AD) cases or in elderly individuals. Its severity correlates positively with the density of neurofibrillary tangles (NFTs). Mechanisms underlying GVD formation are unknown. We assessed the prevalence and distribution of GVD in cases with TDP-43-related frontotemporal lobar degeneration (FTLD-TDP) and amyotrophic lateral sclerosis (ALS-TDP). Consecutively autopsied cases with FTLD/ALS-TDP and C9orf72 mutations (FTLD/ALS-C9; N = 29), cases with FTLD/ALS-TDP without C9orf72 mutations (FTLD/ALS-nonC9; N = 46), and age-matched healthy controls (N = 40) were studied. The prevalence of GVD was significantly higher in the FTLD/ALS-C9 cases (26/29 cases) than in the FTLD/ALS-nonC9 cases (15/46 cases; Fisher exact test; p < 2×10^-6) or in the control group (12/40 individuals; p < 1×10^-6). Average Braak stages and ages of death were not significantly different among the groups. The CA2 sector was most frequently affected in the FTLD/ALS-C9 group, whereas the CA1/subiculum was the most vulnerable area in the other groups. Extension of GVD correlated with the clinical duration of the disease in the FTLD/ALS-C9 cases but not in the FTLD/ALS-nonC9 cases. The GVD-containing neurons frequently had dipeptide repeat (DPR) protein inclusions. GVD granules labeled with antibodies directed against charged multivesicular body protein 2B or casein kinase 1δ were attached to DPR inclusions within GVD. Our results suggest that development of GVD and DPR inclusions is related to common pathogenic mechanisms and that GVD is not only associated with NFTs seen in AD cases or aging individuals.

Age-related impairment in Addenbrooke's cognitive examination revised scores in patients with amyotrophic lateral sclerosis

OBJECTIVE

Older age is thought to be a risk factor for cognitive impairment in amyotrophic lateral sclerosis (ALS). However, very few clinical studies have investigated this relationship using sufficient numbers of healthy controls that correspond to each generation. The purpose of this study was to determine the age-related changes of Addenbrooke's Cognitive Examination-Revised (ACE-R) score in ALS patients by comparing healthy controls of various ages.

METHODS

131 ALS patients (86 males, 45 females; mean age: 64.8 ± 10.2; mean education: 12.5 ± 2.7) and 151 age-, gender-, and education-matched healthy controls were enrolled. We applied ACE-R, which could evaluate not only global cognition but five cognitive subdomains that included orientation/attention, memory, verbal fluency, language, and visuospatial ability.

RESULTS

ALS patients had significantly lower total and subdomain scores of ACE-R than healthy controls. Multiple regression analysis suggested that age at examination and age at onset had significant influence on ACE-R scores. When we divided ALS patients and healthy controls into 4 groups according to age at examination for ALS, total and each subdomain scores were significantly lower with age, particularly in the older-middle and the oldest group (66.31 years or more) of ALS compared with healthy controls. Locally weighted scatterplot smoothing analysis supported that these reductions of ACE-R total and subdomain scores in ALS patients were more accelerated by approximately 60 years as compared with healthy controls.

CONCLUSION

ALS patients showed accelerated age-related ACE-R score reduction beyond normal ageing processes.

Pathogenesis of Frontotemporal Lobar Degeneration: Insights From Loss of Function Theory and Early Involvement of the Caudate Nucleus

Frontotemporal lobar degeneration (FTLD) is a group of clinically, pathologically and genetically heterogeneous neurodegenerative disorders that involve the frontal and temporal lobes. Behavioral variant frontotemporal dementia (bvFTD), semantic dementia (SD), and progressive non-fluent aphasia (PNFA) are three major clinical syndromes. TDP-43, FUS, and tau are three major pathogenetic proteins. In this review, we first discuss the loss-of-function mechanism of FTLD. We focus on FUS-associated pathogenesis in which FUS is linked to tau by regulating its alternative splicing machinery. Moreover, FUS is associated with abnormalities in post-synaptic formation, which can be an early disease marker of FTLD. Second, we discuss clinical and pathological aspects of FTLD. Recently, FTLD and amyotrophic lateral sclerosis (ALS) have been recognized as the same disease entity; indeed, nearly all sporadic ALS cases show TDP-43 pathology irrespective of FTD phenotype. Thus, investigating early structural and network changes in the FTLD/ALS continuum can be useful for developing early diagnostic markers of FTLD. MRI studies have revealed the involvement of the caudate nucleus and its anatomical networks in association with the early phase of behavioral/cognitive decline in FTLD/ALS. In particular, even ALS patients with normal cognition have shown a significant decrease in structural connectivity between the caudate head networks. In pathological studies, FTLD/ALS has shown striatal involvement of both efferent system components and glutamatergic inputs from the cerebral cortices even in ALS patients. Thus, the caudate nucleus may be primarily associated with behavioral abnormality and cognitive involvement in FTLD/ALS. Although several clinical trials have been conducted, there is still no therapy that can change the disease course in patients with FTLD. Therefore, there is an urgent need to establish a strategy for predominant sporadic FTLD cases.

Synapse loss in the prefrontal cortex is associated with cognitive decline in amyotrophic lateral sclerosis

In addition to motor neurone degeneration, up to 50% of amyotrophic lateral sclerosis (ALS) patients present with cognitive decline. Understanding the neurobiological changes underlying these cognitive deficits is critical, as cognitively impaired patients exhibit a shorter survival time from symptom onset. Given the pathogenic role of synapse loss in other neurodegenerative diseases in which cognitive decline is apparent, such as Alzheimer’s disease, we aimed to assess synaptic integrity in the ALS brain. Here, we have applied a unique combination of high-resolution imaging of post-mortem tissue with neuropathology, genetic screening and cognitive profiling of ALS cases. Analyses of more than 1 million synapses using two complimentary high-resolution techniques (electron microscopy and array tomography) revealed a loss of synapses from the prefrontal cortex of ALS patients. Importantly, synapse loss was significantly greater in cognitively impaired cases and was not due to cortical atrophy, nor associated with dementia-associated neuropathology. Interestingly, we found a trend between pTDP-43 pathology and synapse loss in the frontal cortex and discovered pTDP-43 puncta at a subset of synapses in the ALS brains. From these data, we postulate that synapse loss in the prefrontal cortex represents an underlying neurobiological substrate of cognitive decline in ALS.