The neuropathology associated with repeat expansions in the C9ORF72 gene

Axonopathy Underlying Amyotrophic Lateral Sclerosis: Unraveling Complex Pathways and Therapeutic Insights

Amyotrophic Lateral Sclerosis (ALS) is a complex neurodegenerative disorder characterized by progressive axonopathy, jointly leading to the dying back of the motor neuron, disrupting both nerve signaling and motor control. In this review, we highlight the roles of axonopathy in ALS progression, driven by the interplay of multiple factors including defective trafficking machinery, protein aggregation, and mitochondrial dysfunction. Dysfunctional intracellular transport, caused by disruptions in microtubules, molecular motors, and adaptors, has been identified as a key contributor to disease progression. Aberrant protein aggregation involving TDP-43, FUS, SOD1, and dipeptide repeat proteins further amplifies neuronal toxicity. Mitochondrial defects lead to ATP depletion, oxidative stress, and Ca2+ imbalance, which are regarded as key factors underlying the loss of neuromuscular junctions and axonopathy. Mitigating these defects through interventions including neurotrophic treatments offers therapeutic potential. Collaborative research efforts aim to unravel ALS complexities, opening avenues for holistic interventions that target diverse pathological mechanisms.

Pure argyrophilic grain disease revisited: independent effects on limbic, neocortical, and striato-pallido-nigral degeneration and the development of dementia in a series with a low to moderate Braak stage

Agyrophilic grains (AGs) are age-related limbic-predominant lesions in which four-repeat tau is selectively accumulated. Because previous methodologically heterogeneous studies have demonstrated inconsistent findings on the relationship between AGs and dementia, whether AGs affect cognitive function remains unclear. To address this question, we first comprehensively evaluated the distribution and quantity of Gallyas-positive AGs and the severity of neuronal loss in the limbic, neocortical, and subcortical regions in 30 cases of pure argyrophilic grain disease (pAGD) in Braak stages I-IV and without other degenerative diseases, and 34 control cases that had only neurofibrillary tangles with Braak stages I-IV and no or minimal Aβ deposits. Then, we examined whether AGs have independent effects on neuronal loss and dementia by employing multivariate ordered logistic regression and binomial logistic regression. Of 30 pAGD cases, three were classified in diffuse form pAGD, which had evident neuronal loss not only in the limbic region but also in the neocortex and subcortical nuclei. In all 30 pAGD cases, neuronal loss developed first in the amygdala, followed by temporo-frontal cortex, hippocampal CA1, substantia nigra, and finally, the striatum and globus pallidus with the progression of Saito AG stage. In multivariate analyses of 30 pAGD and 34 control cases, the Saito AG stage affected neuronal loss in the amygdala, hippocampal CA1, temporo-frontal cortex, striatum, globus pallidus, and substantia nigra independent of the age, Braak stage, and limbic-predominant age-related TDP-43 encephalopathy (LATE-NC) stage. In multivariate analyses of 23 pAGD and 28 control cases that lacked two or more lacunae and/or one or more large infarctions, 100 or more AGs per × 400 visual field in the amygdala (OR 10.02, 95% CI 1.12-89.43) and hippocampal CA1 (OR 12.22, 95% CI 1.70-87.81), and the presence of AGs in the inferior temporal cortex (OR 8.18, 95% CI 1.03-65.13) affected dementia independent of age, moderate Braak stages (III-IV), and LATE-NC. Given these findings, the high density of limbic AGs and the increase of AGs in the inferior temporal gyrus may contribute to the occurrence of dementia through neuronal loss, at least in cases in a low to moderate Braak stage.

Progressive Cerebrocerebellar Uncoupling in Sporadic and Genetic Forms of Amyotrophic Lateral Sclerosis

BACKGROUND AND OBJECTIVES

Amyotrophic lateral sclerosis (ALS) is predominantly associated with motor cortex, corticospinal tract (CST), brainstem, and spinal cord degeneration, and cerebellar involvement is much less well characterized. However, some of the cardinal clinical features of ALS, such as dysarthria, dysphagia, gait impairment, falls, and impaired dexterity, are believed to be exacerbated by coexisting cerebellar pathology. Cerebellar pathology may also contribute to cognitive, behavioral, and pseudobulbar manifestations. Our objective was to systematically assess both intracerebellar pathology and cerebrocerebellar connectivity alterations in a genetically stratified cohort of ALS.

METHODS

A prospective, multimodal neuroimaging study was conducted to evaluate the longitudinal evolution of intracerebellar pathology and cerebrocerebellar connectivity, using structural and functional measures.

RESULTS

A total of 113 healthy controls and 212 genetically stratified individuals with ALS were included: (1) C9orf72 hexanucleotide carriers ("C9POS"), (2) sporadic patients who tested negative for ALS-associated genetic variants, and (3) intermediate-length CAG trinucleotide carriers in ATXN2 ("ATXN2"). Flocculonodular lobule (padj = 0.014, 95% CI -5.06e-5 to -3.98e-6) and crura (padj = 0.031, 95% CI -1.63e-3 to -5.55e-5) volume reductions were detected at baseline in sporadic patients. Cerebellofrontal and cerebelloparietal structural connectivity impairment was observed in both C9POS and sporadic patients at baseline, and both projections deteriorated further over time in sporadic patients (padj = 0.003, t(249) = 3.04 and padj = 0.05, t(249) = 1.93). Functional cerebelloparietal uncoupling was evident in sporadic patients at baseline (padj = 0.004, 95% CI -0.19 to -0.03). ATXN2 patients exhibited decreased cerebello-occipital functional connectivity at baseline (padj = 0.004, 95% CI -0.63 to -0.06), progressive cerebellotemporal functional disconnection (padj = 0.025, t(199) = -2.26), and progressive flocculonodular lobule degeneration (padj = 0.017, t(249) = -2.24). C9POS patients showed progressive ventral dentate atrophy (padj = 0.007, t(249) = -2.75). The CSTs (padj < 0.001, 95% CI 4.89e-5 to 1.14e-4) and transcallosal interhemispheric fibers (padj < 0.001, 95% CI 5.21e-5 to 1.31e-4) were affected at baseline in C9POS and exhibited rapid degeneration over the 4 time points. The rate of decline in CST and corpus callosum integrity was faster than the rate of cerebrocerebellar disconnection (padj = 0.001, t(190) = 6.93).

DISCUSSION

ALS is associated with accruing intracerebellar disease burden as well as progressive corticocerebellar uncoupling. Contrary to previous suggestions, we have not detected evidence of compensatory structural or functional changes in response to supratentorial degeneration. The contribution of cerebellar disease burden to dysarthria, dysphagia, gait impairment, pseudobulbar affect, and cognitive deficits should be carefully considered in clinical assessments, monitoring, and multidisciplinary interventions.

Limbic Network and Papez Circuit Involvement in ALS: Imaging and Clinical Profiles in GGGGCC Hexanucleotide Carriers in C9orf72 and C9orf72-Negative Patients

Background: While frontotemporal involvement is increasingly recognized in Amyotrophic lateral sclerosis (ALS), the degeneration of limbic networks remains poorly characterized, despite growing evidence of amnestic deficits, impaired emotional processing and deficits in social cognition. Methods: A prospective neuroimaging study was conducted with 204 individuals with ALS and 111 healthy controls. Patients were stratified for hexanucleotide expansion status in C9orf72. A deep-learning-based segmentation approach was implemented to segment the nucleus accumbens, hypothalamus, fornix, mammillary body, basal forebrain and septal nuclei. The cortical, subcortical and white matter components of the Papez circuit were also systematically evaluated. Results: Hexanucleotide repeat expansion carriers exhibited bilateral amygdala, hypothalamus and nucleus accumbens atrophy, and C9orf72 negative patients showed bilateral basal forebrain volume reductions compared to controls. Both patient groups showed left rostral anterior cingulate atrophy, left entorhinal cortex thinning and cingulum and fornix alterations, irrespective of the genotype. Fornix, cingulum, posterior cingulate, nucleus accumbens, amygdala and hypothalamus degeneration was more marked in C9orf72-positive ALS patients. Conclusions: Our results highlighted that mesial temporal and parasagittal subcortical degeneration is not unique to C9orf72 carriers. Our radiological findings were consistent with neuropsychological observations and highlighted the importance of comprehensive neuropsychological testing in ALS, irrespective of the underlying genotype.

Clinicopathological correlates in the frontotemporal lobar degeneration-motor neuron disease spectrum

Amyotrophic lateral sclerosis (ALS) is a devastating motor neuron disease (MND) that shares a common clinical, genetic and pathologic spectrum with frontotemporal dementia (FTD). It is highly heterogeneous in its presentation and features. Up to 50% of patients with MND develop cognitive-behavioural symptoms during the course of the disease, meeting criteria for FTD in 10%-15% of cases. In the absence of a precise biomarker, neuropathology is still a valuable tool to understand disease nosology, reach a definite diagnostic confirmation and help define specific subgroups of patients with common phenotypic, genetic and biomarker profiles. However, few neuropathological series have been published, and the frequency of frontotemporal lobar degeneration (FTLD) in MND is difficult to estimate. In this work we describe a large clinicopathological series of MND patients, analysing the frequency of concurrent FTLD changes and trying to define specific subgroups of patients based on their clinical, genetic and pathological characteristics. We performed an observational, retrospective, multicentre case study. We included all cases meeting neuropathological criteria for MND from the Neurological Tissue Bank of the FRCB-IDIBAPS-Hospital Clínic Barcelona Biobank between 1994 and 2022, regardless of their last clinical diagnosis. While brain donation is encouraged in all patients, it is performed in very few, and representativeness of the cohort might not be precise for all patients with MND. We retrospectively reviewed clinical and neuropathological data and describe the main clinical, genetic and pathogenic features, comparing neuropathologic groups between MND with and without FTLD changes and aiming to define specific subgroups. We included brain samples from 124 patients, 44 of whom (35.5%) had FTLD neuropathologic features (i.e. FTLD-MND). Pathologic TDP-43 aggregates were present in 93.6% of the cohort and were more extensive (higher Brettschneider stage) in those with concurrent FTLD (P < 0.001). Motor symptom onset was more frequent in the bulbar region in FTLD-MND cases than in those with isolated MND (P = 0.023), with no differences in survival. We observed a better clinicopathological correlation in the MND group than in the FTLD-MND group (93.8% versus 61.4%; P < 0.001). Pathogenic genetic variants were more common in the FTLD-MND group, especially C9orf72. We describe a frequency of FTLD of 35.5% in our series of neuropathologically confirmed cases of MND. The FTLD-MND spectrum is highly heterogeneous in all aspects, especially in patients with FTLD, in whom it is particularly difficult to define specific subgroups. In the absence of definite biomarkers, neuropathology remains a valuable tool for a definite diagnosis, increasing our knowledge in disease nosology.

Detection of TDP-43 seeding activity in the olfactory mucosa from patients with frontotemporal dementia

INTRODUCTION

We assessed TAR DNA-binding protein 43 (TDP-43) seeding activity and aggregates detection in olfactory mucosa of patients with frontotemporal lobar degeneration with TDP-43-immunoreactive pathology (FTLD-TDP) by TDP-43 seeding amplification assay (TDP43-SAA) and immunocytochemical analysis.

METHODS

The TDP43-SAA was optimized using frontal cortex samples from 16 post mortem cases with FTLD-TDP, FTLD with tau inclusions, and controls. Subsequently, olfactory mucosa samples were collected from 17 patients with FTLD-TDP, 15 healthy controls, and three patients carrying MAPT variants.

RESULTS

TDP43-SAA discriminated with 100% accuracy post mortem cases presenting or lacking TDP-43 neuropathology. TDP-43 seeding activity was detectable in the olfactory mucosa, and 82.4% of patients with FTLD-TDP tested positive, whereas 86.7% of controls tested negative (P < 0.001). Two out of three patients with MAPT mutations tested negative. In TDP43-SAA positive samples, cytoplasmatic deposits of phosphorylated TDP-43 in the olfactory neural cells were detected.

DISCUSSION

TDP-43 aggregates can be detectable in olfactory mucosa, suggesting that TDP43-SAA might be useful for identifying and monitoring FTLD-TDP in living patients.

G2C4 targeting antisense oligonucleotides potently mitigate TDP-43 dysfunction in human C9orf72 ALS/FTD induced pluripotent stem cell derived neurons

The G4C2 repeat expansion in the C9orf72 gene is the most common genetic cause of Amyotrophic Lateral Sclerosis and Frontotemporal Dementia. Many studies suggest that dipeptide repeat proteins produced from this repeat are toxic, yet, the contribution of repeat RNA toxicity is under investigated and even less is known regarding the pathogenicity of antisense repeat RNA. Recently, two clinical trials targeting G4C2 (sense) repeat RNA via antisense oligonucleotide failed despite a robust decrease in sense-encoded dipeptide repeat proteins demonstrating target engagement. Here, in this brief report, we show that G2C4 antisense, but not G4C2 sense, repeat RNA is sufficient to induce TDP-43 dysfunction in induced pluripotent stem cell (iPSC) derived neurons (iPSNs). Unexpectedly, only G2C4, but not G4C2 sense strand targeting, ASOs mitigate deficits in TDP-43 function in authentic C9orf72 ALS/FTD patient iPSNs. Collectively, our data suggest that the G2C4 antisense repeat RNA may be an important therapeutic target and provide insights into a possible explanation for the recent G4C2 ASO clinical trial failure.

Glycine-Alanine Dipeptide Repeat Protein from C9-ALS Interacts with Sulfide Quinone Oxidoreductase (SQOR) to Induce the Activity of the NLRP3 Inflammasome in HMC3 Microglia: Irisflorentin Reverses This Interaction

Amyotrophic lateral sclerosis (ALS) is a fatal rare disease of progressive degeneration of motor neurons. The most common genetic mutation in ALS is the hexanucleotide repeat expansion (HRE) located in the first intron of the C9orf72 gene (C9-ALS). HRE can produce dipeptide repeat proteins (DPRs) such as poly glycine-alanine (GA) in a repeat-associated non-ATG (RAN) translation. GA-DPR has been shown to be toxic to motor neurons in various biological models. However, its effects on microglia involved in C9-ALS have not been reported. Here, we show that GA-DPR (GA50) activates the NLR family pyrin domain containing 3 (NLRP3) inflammasome in a human HMC3 microglia model. MCC950 (specific inhibitor of the NLRP3) treatment can abrogate this activity. Next, using yeast two-hybrid screening, we identified sulfide quinone oxidoreductase (SQOR) as a GA50 interacting protein. SQOR knockdown in HMC3 cells can significantly induce the activity of the NLRP3 inflammasome by upregulating the level of intracellular reactive oxygen species and the cytoplasmic escape of mitochondrial DNA. Furthermore, we obtained irisflorentin as an effective blocker of the interaction between SQOR and GA50, thus inhibiting NLRP3 inflammasome activity in GA50-expressing HMC3 cells. These results imply the association of GA-DPR, SQOR, and NLRP3 inflammasomes in microglia and establish a treatment strategy for C9-ALS with irisflorentin.

C9orf72-catalyzed GTP loading of Rab39A enables HOPS-mediated membrane tethering and fusion in mammalian autophagy

The multi-subunit homotypic fusion and vacuole protein sorting (HOPS) membrane-tethering complex is required for autophagosome-lysosome fusion in mammals, yet reconstituting the mammalian HOPS complex remains a challenge. Here we propose a "hook-up" model for mammalian HOPS complex assembly, which requires two HOPS sub-complexes docking on membranes via membrane-associated Rabs. We identify Rab39A as a key small GTPase that recruits HOPS onto autophagic vesicles. Proper pairing with Rab2 and Rab39A enables HOPS complex assembly between proteoliposomes for its tethering function, facilitating efficient membrane fusion. GTP loading of Rab39A is important for the recruitment of HOPS to autophagic membranes. Activation of Rab39A is catalyzed by C9orf72, a guanine exchange factor associated with amyotrophic lateral sclerosis and familial frontotemporal dementia. Constitutive activation of Rab39A can rescue autophagy defects caused by C9orf72 depletion. These results therefore reveal a crucial role for the C9orf72-Rab39A-HOPS axis in autophagosome-lysosome fusion.

Nuclear pore complex and nucleocytoplasmic transport disruption in neurodegeneration

Nuclear pore complexes (NPCs) play a critical role in maintaining the equilibrium between the nucleus and cytoplasm, enabling bidirectional transport across the nuclear envelope, and are essential for proper nuclear organization and gene regulation. Perturbations in the regulatory mechanisms governing NPCs and nuclear envelope homeostasis have been implicated in the pathogenesis of several neurodegenerative diseases. The ESCRT-III pathway emerges as a critical player in the surveillance and preservation of well-assembled, functional NPCs, as well as nuclear envelope sealing. Recent studies have provided insights into the involvement of nuclear ESCRT-III in the selective reduction of specific nucleoporins associated with neurodegenerative pathologies. Thus, maintaining quality control of the nuclear envelope and NPCs represents a pivotal element in the pathological cascade leading to neurodegenerative diseases. This review describes the constituents of the nuclear-cytoplasmic transport machinery, encompassing the nuclear envelope, NPC, and ESCRT proteins, and how their structural and functional alterations contribute to the development of neurodegenerative diseases.

C9ORF72 Gene GGGGCC Hexanucleotide Expansion: A High Clinical Variability from Amyotrophic Lateral Sclerosis to Frontotemporal Dementia

The expanded GGGGCC hexanucleotide repeat (HRE) in the non-coding region of the C9ORF72 gene (C9ORF72-HRE) is the most common genetic cause of familial forms of amyotrophic lateral sclerosis (ALS), FTD, and concurrent ALS and FTD (ALS-FTD), in addition to contributing to the sporadic forms of these diseases. Both syndromes overlap not only genetically, but also sharing similar clinical and neuropathological findings, being considered as a spectrum. In this paper we describe the clinical-genetic findings in a Basque family with different manifestations within the spectrum, our difficulties in reaching the diagnosis, and a narrative review, carried out as a consequence, of the main features associated with C9ORF72-HRE. Family members underwent a detailed clinical assessment, neurological examination, and genetic analysis by repeat-primed PCR. We studied 10 relatives of a symptomatic carrier of the C9ORF72-HRE expansion. Two of them presented the expansion in the pathological range, one of them was symptomatic whereas the other one remained asymptomatic at 72 years. Given the great intrafamilial clinical variability of C9ORF72-HRE, the characterization of patients and family members with particular clinical and genetic subgroups within ALS and FTD becomes a bottleneck for medication development, in particular for genetically focused medicines for ALS and FTD.

A toxic gain-of-function mechanism in C9orf72 ALS impairs the autophagy-lysosome pathway in neurons

BACKGROUND

Motor neurons (MNs), which are primarily affected in amyotrophic lateral sclerosis (ALS), are a specialized type of neurons that are long and non-dividing. Given their unique structure, these cells heavily rely on transport of organelles along their axons and the process of autophagy to maintain their cellular homeostasis. It has been shown that disruption of the autophagy pathway is sufficient to cause progressive neurodegeneration and defects in autophagy have been associated with various subtypes of ALS, including those caused by hexanucleotide repeat expansions in the C9orf72 gene. A more comprehensive understanding of the dysfunctional cellular mechanisms will help rationalize the design of potent and selective therapies for C9orf72-ALS.

METHODS

In this study, we used induced pluripotent stem cell (iPSC)-derived MNs from C9orf72-ALS patients and isogenic control lines to identify the underlying mechanisms causing dysregulations of the autophagy-lysosome pathway. Additionally, to ascertain the potential impact of C9orf72 loss-of-function on autophagic defects, we characterized the observed phenotypes in a C9orf72 knockout iPSC line (C9-KO).

RESULTS

Despite the evident presence of dysfunctions in several aspects of the autophagy-lysosome pathway, such as disrupted lysosomal homeostasis, abnormal lysosome morphology, inhibition of autophagic flux, and accumulation of p62 in C9orf72-ALS MNs, we were surprised to find that C9orf72 loss-of-function had minimal influence on these phenotypes. Instead, we primarily observed impairment in endosome maturation as a result of C9orf72 loss-of-function. Additionally, our study shed light on the pathological mechanisms underlying C9orf72-ALS, as we detected an increased TBK1 phosphorylation at S172 in MNs derived from C9orf72 ALS patients.

CONCLUSIONS

Our data provides further insight into the involvement of defects in the autophagy-lysosome pathway in C9orf72-ALS and strongly indicate that those defects are mainly due to the toxic gain-of-function mechanisms underlying C9orf72-ALS.

Cryptic exon detection and transcriptomic changes revealed in single-nuclei RNA sequencing of C9ORF72 patients spanning the ALS-FTD spectrum

The C9ORF72-linked diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are characterized by the nuclear depletion and cytoplasmic accumulation of TAR DNA-binding protein 43 (TDP-43). Recent studies have shown that the loss of TDP-43 function leads to the inclusion of cryptic exons (CE) in several RNA transcript targets of TDP-43. Here, we show for the first time the detection of CEs in a single-nuclei RNA sequencing (snRNA-seq) dataset obtained from frontal and occipital cortices of C9ORF72 patients that phenotypically span the ALS-FTD disease spectrum. We assessed each cellular cluster for detection of recently described TDP-43-induced CEs. Transcripts containing CEs in the genes STMN2 and KALRN were detected in the frontal cortex of all C9ORF72 disease groups with the highest frequency in excitatory neurons in the C9ORF72-FTD group. Within the excitatory neurons, the cluster with the highest proportion of cells containing a CE had transcriptomic similarities to von Economo neurons, which are known to be vulnerable to TDP-43 pathology and selectively lost in C9ORF72-FTD. Differential gene expression and pathway analysis of CE-containing neurons revealed multiple dysregulated metabolic processes. Our findings reveal novel insights into the transcriptomic changes of neurons vulnerable to TDP-43 pathology.

Reversing lysosome-ribosome circuit dysregulation mitigates C9FTD/ALS neurodegeneration and behaviors

G4C2 repeat expansion in C9orf72 causes the most common familial frontotemporal dementia and amyotrophic lateral sclerosis (C9FTD/ALS). The pathogenesis includes haploinsufficiency of C9orf72, which forms a protein complex with Smcr8, as well as G4C2 repeat-induced gain of function including toxic dipeptide repeats (DPRs). The key in vivo disease-driving mechanisms and how loss- and gain-of-function interplay remain poorly understood. Here, we identified dysregulation of a lysosome-ribosome biogenesis circuit as an early and key disease mechanism using a physiologically relevant mouse model with combined loss- and gain-of-function across the aging process. C9orf72 deficiency exacerbates FTD/ALS-like pathologies and behaviors in C9ORF72 bacterial artificial chromosome (C9-BAC) mice with G4C2 repeats under endogenous regulatory elements from patients. Single nucleus RNA sequencing (snRNA-seq) and bulk RNA-seq revealed that C9orf72 depletion disrupts lysosomes in neurons and leads to transcriptional dysregulation of ribosomal protein genes, which are likely due to the proteotoxic stress response and resemble ribosomopathy defects. Importantly, ectopic expression of C9orf72 or its partner Smcr8 in C9FTD/ALS mutant mice promotes lysosomal functions and restores ribosome biogenesis gene transcription, resulting in the mitigation of DPR accumulation, neurodegeneration as well as FTD/ALS-like motor and cognitive behaviors. Therefore, we conclude that loss- and gain-of-function crosstalk in C9FTD/ALS converges on neuronal dysregulation of a lysosome-ribosome biogenesis circuit leading to proteotoxicity, neurodegeneration and behavioral defects.

LATE-NC staging in routine neuropathologic diagnosis: an update

An international consensus report in 2019 recommended a classification system for limbic-predominant age-related TDP-43 encephalopathy neuropathologic changes (LATE-NC). The suggested neuropathologic staging system and nomenclature have proven useful for autopsy practice and dementia research. However, some issues remain unresolved, such as cases with unusual features that do not fit with current diagnostic categories. The goal of this report is to update the neuropathologic criteria for the diagnosis and staging of LATE-NC, based primarily on published data. We provide practical suggestions about how to integrate available genetic information and comorbid pathologies [e.g., Alzheimer's disease neuropathologic changes (ADNC) and Lewy body disease]. We also describe recent research findings that have enabled more precise guidance on how to differentiate LATE-NC from other subtypes of TDP-43 pathology [e.g., frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS)], and how to render diagnoses in unusual situations in which TDP-43 pathology does not follow the staging scheme proposed in 2019. Specific recommendations are also made on when not to apply this diagnostic term based on current knowledge. Neuroanatomical regions of interest in LATE-NC are described in detail and the implications for TDP-43 immunohistochemical results are specified more precisely. We also highlight questions that remain unresolved and areas needing additional study. In summary, the current work lays out a number of recommendations to improve the precision of LATE-NC staging based on published reports and diagnostic experience.

C9ORF72 knockdown triggers FTD-like symptoms and cell pathology in mice

The GGGGCC intronic repeat expansion within C9ORF72 is the most common genetic cause of ALS and FTD. This mutation results in toxic gain of function through accumulation of expanded RNA foci and aggregation of abnormally translated dipeptide repeat proteins, as well as loss of function due to impaired transcription of C9ORF72. A number of in vivo and in vitro models of gain and loss of function effects have suggested that both mechanisms synergize to cause the disease. However, the contribution of the loss of function mechanism remains poorly understood. We have generated C9ORF72 knockdown mice to mimic C9-FTD/ALS patients haploinsufficiency and investigate the role of this loss of function in the pathogenesis. We found that decreasing C9ORF72 leads to anomalies of the autophagy/lysosomal pathway, cytoplasmic accumulation of TDP-43 and decreased synaptic density in the cortex. Knockdown mice also developed FTD-like behavioral deficits and mild motor phenotypes at a later stage. These findings show that C9ORF72 partial loss of function contributes to the damaging events leading to C9-FTD/ALS.

ANXA11 mutations are associated with amyotrophic lateral sclerosis–frontotemporal dementia

Background

The Annexin A11 (ANXA11) gene has been newly identified as a causative gene of amyotrophic lateral sclerosis (ALS) with or without frontotemporal dementia (FTD). The current study aimed to investigate the ANXA11 mutations in a Chinese ALS–FTD or FTD cohort.

Methods

We included ten probands/patients with suspected ALS–FTD or FTD. Mutational analysis of ANXA11 was performed through Next Generation Sequencing (NGS) and Sanger sequencing. We collected and reviewed clinical presentation, neuropsychology test results, brain-imaging findings, and electrophysiological examination findings.

Results

In total, six probands presented with ALS–FTD, and four with behavior variant FTD (bv-FTD). We identified a non-synonymous heterozygous mutation (c.119A>G, p.D40G) of ANXA11 in proband 1, which is associated with ALS. However, this is the first report of the mutation causing ALS–FTD. Proband 1 started with abnormal behavior and progressed to classic upper motor nervous disease. Magnetic resonance imaging (MRI) showed significant bilateral temporal lobe atrophy and bilateral hyperintensities along the corticospinal tracts.18F-AV45-PET imaging showed negative amyloid deposits.

Conclusion

ANXA11-related diseases have high clinical and genetic heterogeneity. Our study confirmed the contribution of ANXA11 mutations to ALS–FTD. The ANXA11 mutations established a complex genotype–phenotype correlation in ALS–FTD. Our research further elucidated the genetic mechanism of ALS–FTD and contributed to setting the foundation of future targeted therapy.

Bioenergetic and Autophagic Characterization of Skin Fibroblasts from C9orf72 Patients

The objective of this study is to describe the alterations occurring during the neurodegenerative process in skin fibroblast cultures from C9orf72 patients. We characterized the oxidative stress, autophagy flux, small ubiquitin-related protein SUMO2/3 levels as well as the mitochondrial function in skin fibroblast cultures from C9orf72 patients. All metabolic and bioenergetic findings were further correlated with gene expression data obtained from RNA sequencing analysis. Fibroblasts from C9orf72 patients showed a 30% reduced expression of C9orf72, ~3-fold increased levels of oxidative stress and impaired mitochondrial function obtained by measuring the enzymatic activities of mitochondrial respiratory chain complexes, specifically of complex III activity. Furthermore, the results also reveal that C9orf72 patients showed an accumulation of p62 protein levels, suggesting the alteration of the autophagy process, and significantly higher protein levels of SUMO2/3 (p = 0.03). Our results provide new data reinforcing that C9orf72 cells suffer from elevated oxidative damage to biomolecules and organelles and from increased protein loads, leading to insufficient autophagy and an increase in SUMOylation processes.

C9orf72 Hexanucleotide Repeat Expansion-Related Neuropathology Is Attenuated by Nasal Rifampicin in Mice

The non-coding GGGGCC hexanucleotide repeat expansion (HRE) in C9orf72 gene is a dominant cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). This intronic mutation elicits the formation of nuclear and cytoplasmic inclusions containing RNA, RNA-binding proteins, and HRE-derived dipeptide repeat proteins (DPRs), leading to neurodegeneration via the gain-of-toxic function or loss-of-function of relevant proteins. Using C9-500 mice harboring ~500 repeats of the GGGGCC sequence in human C9orf72 gene, we investigated the effects of rifampicin against HRE-related pathological phenotypes. Rifampicin was administered intranasally to 4.5- to 5-month-old mice for 1 month, and their cognitive function and neuropathology were assessed by the Morris water maze test and immunohistochemical staining. Rifampicin treatment reduced the formation of RNA foci and cytoplasmic inclusions containing DPRs or phosphorylated TDP-43, and furthermore, the levels of phosphorylated double-strand RNA-dependent protein kinase (PKR) that regulates repeat-associated non-ATG (RAN) translation. Synapse loss in the hippocampus and neuronal loss and microglial activation in the prefrontal and motor cortices were also attenuated, and mouse memory was significantly improved. Our findings suggest a therapeutic potential of nasal rifampicin in the prevention of C9orf72-linked neurodegenerative disorders.

How can we define the presymptomatic C9orf72 disease in 2022? An overview on the current definitions of preclinical and prodromal phases

Repeat expansions in C9orf72 gene are the main genetic cause of frontotemporal dementia, amyotrophic lateral sclerosis and related phenotypes. With the advent of disease-modifying treatments, the presymptomatic disease phase is getting increasing interest as an ideal time window in which innovant therapeutic approaches could be administered. Recommendations issued from international study groups distinguish between a preclinical disease stage, during which lesions accumulate in absence of any symptoms or signs, and a prodromal stage, marked by the appearance the first subtle cognitive, behavioral, psychiatric and motor signs, before the full-blown disease. This paper summarizes the current definitions and criteria for these stages, in particular focusing on how fluid-based, neuroimaging and cognitive biomarkers can be useful to monitor disease trajectory across the presymptomatic phase, as well as to detect the earliest signs of clinical conversion. Continuous advances in the knowledge of C9orf72 pathophysiology, and the integration of biomarkers in the clinical evaluation of mutation carriers will allow a better diagnostic definition of C9orf72 disease spectrum from the earliest stages, with relevant impact on the possibility of disease prevention.

HDAC6 Interacts With Poly (GA) and Modulates its Accumulation in c9FTD/ALS

The aberrant translation of a repeat expansion in chromosome 9 open reading frame 72 (C9orf72), the most common cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), results in the accumulation of toxic dipeptide repeat (DPR) proteins in the central nervous system We have found that, among the sense DPR proteins, HDAC6 specifically interacts with the poly (GA) and co-localizes with inclusions in both patient tissue and a mouse model of this disease (c9FTD/ALS). Overexpression of HDAC6 increased poly (GA) levels in cultured cells independently of HDAC6 deacetylase activity, suggesting that HDAC6 can modulate poly (GA) pathology through a mechanism that depends upon their physical interaction. Moreover, decreasing HDAC6 expression by stereotaxic injection of antisense oligonucleotides significantly reduced the number of poly (GA) inclusions in c9FTD/ALS mice. These findings suggest that pharmacologically reducing HDAC6 levels could be of therapeutic value in c9FTD/ALS.

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

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

Reduced mtDNA Copy Number in the Prefrontal Cortex of C9ORF72 Patients

Hexanucleotide repeat expansion in C9ORF72 gene is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9ALS/FTD). Loss of C9ORF72 protein function and a toxic gain-of-function directly by the RNA or RAN translation have been proposed as triggering pathological mechanisms, along with the accumulation of TDP-43 protein. In addition, mitochondrial defects have been described to be a major driver of disease initiation. Mitochondrial DNA copy number has been proposed as a useful biomarker of mitochondrial dysfunction. The aim of our study was to determine the presence of mtDNA copy number alterations in C9ALS/FTD patients. Therefore, we assessed mtDNA copy number in postmortem prefrontal cortex from 18 C9ORF72 brain donors and 9 controls using digital droplet PCR. A statistically significant decrease of 50% was obtained when comparing C9ORF72 samples and controls. This decrease was independent of age and sex. The reduction of mtDNA copy number was found to be higher in patients' samples presenting abundant TDP-43 protein inclusions. A growing number of studies demonstrated the influence of mtDNA copy number reduction on neurodegeneration. Our results provide new insights into the role of mitochondrial dysfunction in the pathogenesis of C9ALS/FTD.

Biomarkers for C9orf7-ALS in Symptomatic and Pre-symptomatic Patients: State-of-the-art in the New Era of Clinical Trials

The development of new possible treatments for C9orf72-related ALS and the possibility of early identification of subjects genetically at risk of developing the disease is creating a critical need for biomarkers to track neurodegeneration that could be used as outcome measures in clinical trials. Current candidate biomarkers in C9orf72-ALS include neuropsychology tests, imaging, electrophysiology as well as different circulating biomarkers. Neuropsychology tests show early executive and verbal function involvement both in symptomatic and asymptomatic mutation carriers. At brain MRI, C9orf72-ALS patients present diffuse white and grey matter degeneration, which are already identified up to 20 years before symptom onset and that seem to be slowly progressive over time, while regions of altered connectivity at fMRI and of hypometabolism at [¹⁸F]FDG PET have been described as well. At the same time, spinal cord MRI has also shown progressive decrease of FA in the cortico-spinal tract over time. On the side of wet biomarkers, neurofilament proteins are increased both in the CSF and serum just before symptom onset and tend to slowly increase over time, while poly(GP) protein can be detected in the CSF and probably used as target engagement marker in clinical trials.

Ribonuclease recruitment using a small molecule reduced c9ALS/FTD r(G4C2) repeat expansion in vitro and in vivo ALS models

[Figure: see text].

Analysis of Heritability Across the Clinical Phenotypes of Frontotemporal Dementia and the Frequency of the C9ORF72 in a Colombian Population

Frontotemporal dementia (FTD) is a highly heritable condition. Up to 40% of FTD is familial and an estimated 15% to 40% is due to single-gene mutations. It has been estimated that the G4C2 hexanucleotide repeat expansions in the C9ORF72 gene can explain up to 37.5% of the familial cases of FTD, especially in populations of Caucasian origin. The purpose of this paper is to evaluate hereditary risk across the clinical phenotypes of FTD and the frequency of the G4C2 expansion in a Colombian cohort diagnosed with FTD. Methods: A total of 132 FTD patients were diagnosed according to established criteria in the behavioral variant FTD, logopenic variant PPA, non-fluent agrammatic PPA, and semantic variant PPA. Hereditary risk across the clinical phenotypes was established in four categories that indicate the pathogenic relationship of the mutation: high, medium, low, and apparently sporadic, based on those proposed by Wood and collaborators. All subjects were also examined for C9ORF72 hexanucleotide expansion (defined as >30 repetitions). Results: There were no significant differences in the demographic characteristics of the patients between the clinical phenotypes of FTD. The higher rate phenotype was bvFTD (62.12%). In accordance with the risk classification, we found that 72 (54.4%) complied with the criteria for the sporadic cases; for the familial cases, 23 (17.4%) fulfilled the high-risk criteria, 23 (17.4%) fulfilled the low risk criteria, and 14 (10.6%) fulfilled the criteria to be classified as subject to medium risk. C9ORF72 expansion frequency was 0.76% (1/132). Conclusion: The FTD heritability presented in this research was very similar to the results reported in the literature. The C9ORF72 expansion frequency was low. Colombia is a triethnic country, with a high frequency of genetic Amerindian markers; this shows consistency with the present results of a low repetition frequency. This study provides an initial report of the frequency for the hexanucleotide repeat expansions in C9ORF72 in patients with FTD in a Colombian population and paves the way for further study of the possible genetic causes of FTD in Colombia.

Emerging hiPSC Models for Drug Discovery in Neurodegenerative Diseases

Neurodegenerative diseases affect millions of people worldwide and are characterized by the chronic and progressive deterioration of neural function. Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), represent a huge social and economic burden due to increasing prevalence in our aging society, severity of symptoms, and lack of effective disease-modifying therapies. This lack of effective treatments is partly due to a lack of reliable models. Modeling neurodegenerative diseases is difficult because of poor access to human samples (restricted in general to postmortem tissue) and limited knowledge of disease mechanisms in a human context. Animal models play an instrumental role in understanding these diseases but fail to comprehensively represent the full extent of disease due to critical differences between humans and other mammals. The advent of human-induced pluripotent stem cell (hiPSC) technology presents an advantageous system that complements animal models of neurodegenerative diseases. Coupled with advances in gene-editing technologies, hiPSC-derived neural cells from patients and healthy donors now allow disease modeling using human samples that can be used for drug discovery.

Molecular Pathology of ALS: What We Currently Know and What Important Information Is Still Missing

Despite an early understanding of amyotrophic lateral sclerosis (ALS) as a disease affecting the motor system, including motoneurons in the motor cortex, brainstem, and spinal cord, today, many cases involving dementia and behavioral disorders are reported. Therefore, we currently divide ALS not only based on genetic predisposition into the most common sporadic variant (90% of cases) and the familial variant (10%), but also based on cognitive and/or behavioral symptoms, with five specific subgroups of clinical manifestation—ALS with cognitive impairment, ALS with behavioral impairment, ALS with combined cognitive and behavioral impairment, the fully developed behavioral variant of frontotemporal dementia in combination with ALS, and comorbid ALS and Alzheimer’s disease (AD). Generally, these cases are referred to as amyotrophic lateral sclerosis-frontotemporal spectrum disorder (ALS-FTSD). Clinical behaviors and the presence of the same pathognomonic deposits suggest that FTLD and ALS could be a continuum of one entity. This review was designed primarily to compare neuropathological findings in different types of ALS relative to their characteristic locations as well as the immunoreactivity of the inclusions, and thus, foster a better understanding of the immunoreactivity, distribution, and morphology of the pathological deposits in relation to genetic mutations, which can be useful in specifying the final diagnosis.

Autophagy and ALS: mechanistic insights and therapeutic implications

Mechanisms of protein homeostasis are crucial for overseeing the clearance of misfolded and toxic proteins over the lifetime of an organism, thereby ensuring the health of neurons and other cells of the central nervous system. The highly conserved pathway of autophagy is particularly necessary for preventing and counteracting pathogenic insults that may lead to neurodegeneration. In line with this, mutations in genes that encode essential autophagy factors result in impaired autophagy and lead to neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS). However, the mechanistic details underlying the neuroprotective role of autophagy, neuronal resistance to autophagy induction, and the neuron-specific effects of autophagy-impairing mutations remain incompletely defined. Further, the manner and extent to which non-cell autonomous effects of autophagy dysfunction contribute to ALS pathogenesis are not fully understood. Here, we review the current understanding of the interplay between autophagy and ALS pathogenesis by providing an overview of critical steps in the autophagy pathway, with special focus on pivotal factors impaired by ALS-causing mutations, their physiologic effects on autophagy in disease models, and the cell type-specific mechanisms regulating autophagy in non-neuronal cells which, when impaired, can contribute to neurodegeneration. This review thereby provides a framework not only to guide further investigations of neuronal autophagy but also to refine therapeutic strategies for ALS and related neurodegenerative diseases.Abbreviations: ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CHMP2B: charged multivesicular body protein 2B; DPR: dipeptide repeat; FTD: frontotemporal dementia; iPSC: induced pluripotent stem cell; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; PINK1: PTEN induced kinase 1; RNP: ribonuclear protein; sALS: sporadic ALS; SPHK1: sphingosine kinase 1; TARDBP/TDP-43: TAR DNA binding protein; TBK1: TANK-binding kinase 1; TFEB: transcription factor EB; ULK: unc-51 like autophagy activating kinase; UPR: unfolded protein response; UPS: ubiquitin-proteasome system; VCP: valosin containing protein.

C9ORF72: What It Is, What It Does, and Why It Matters

When the non-coding repeat expansion in the C9ORF72 gene was discovered to be the most frequent cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) in 2011, this gene and its derived protein, C9ORF72, were completely unknown. The mutation appeared to produce both haploinsufficiency and gain-of-function effects in the form of aggregating expanded RNAs and dipeptide repeat proteins (DPRs). An unprecedented effort was then unleashed to decipher the pathogenic mechanisms and the functions of C9ORF72 in order to design therapies. A decade later, while the toxicity of accumulating gain-of-function products has been established and therapeutic strategies are being developed to target it, the contribution of the loss of function starts to appear more clearly. This article reviews the current knowledge about the C9ORF72 protein, how it is affected by the repeat expansion in models and patients, and what could be the contribution of its haploinsufficiency to the disease in light of the most recent findings. We suggest that these elements should be taken into consideration to refine future therapeutic strategies, compensating for the decrease of C9ORF72 or at least preventing a further reduction.

Plasma microRNA signature in presymptomatic and symptomatic subjects with C9orf72-associated frontotemporal dementia and amyotrophic lateral sclerosis

OBJECTIVE

To identify potential biomarkers of preclinical and clinical progression in chromosome 9 open reading frame 72 gene (C9orf72)-associated disease by assessing the expression levels of plasma microRNAs (miRNAs) in C9orf72 patients and presymptomatic carriers.

METHODS

The PREV-DEMALS study is a prospective study including 22 C9orf72 patients, 45 presymptomatic C9orf72 mutation carriers and 43 controls. We assessed the expression levels of 2576 miRNAs, among which 589 were above noise level, in plasma samples of all participants using RNA sequencing. The expression levels of the differentially expressed miRNAs between patients, presymptomatic carriers and controls were further used to build logistic regression classifiers.

RESULTS

Four miRNAs were differentially expressed between patients and controls: miR-34a-5p and miR-345-5p were overexpressed, while miR-200c-3p and miR-10a-3p were underexpressed in patients. MiR-34a-5p was also overexpressed in presymptomatic carriers compared with healthy controls, suggesting that miR-34a-5p expression is deregulated in cases with C9orf72 mutation. Moreover, miR-345-5p was also overexpressed in patients compared with presymptomatic carriers, which supports the correlation of miR-345-5p expression with the progression of C9orf72-associated disease. Together, miR-200c-3p and miR-10a-3p underexpression might be associated with full-blown disease. Four presymptomatic subjects in transitional/prodromal stage, close to the disease conversion, exhibited a stronger similarity with the expression levels of patients.

CONCLUSIONS

We identified a signature of four miRNAs differentially expressed in plasma between clinical conditions that have potential to represent progression biomarkers for C9orf72-associated frontotemporal dementia and amyotrophic lateral sclerosis. This study suggests that dysregulation of miRNAs is dynamically altered throughout neurodegenerative diseases progression, and can be detectable even long before clinical onset.

TRIAL REGISTRATION NUMBER

NCT02590276.

Altered Phase Separation and Cellular Impact in C9orf72-Linked ALS/FTD

Since the discovery of the C9orf72 repeat expansion mutation as causative for chromosome 9-linked amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in 2011, a multitude of cellular pathways have been implicated. However, evidence has also been accumulating for a key mechanism of cellular compartmentalization—phase separation. Liquid-liquid phase separation (LLPS) is fundamental for the formation of membraneless organelles including stress granules, the nucleolus, Cajal bodies, nuclear speckles and the central channel of the nuclear pore. Evidence has now accumulated showing that the formation and function of these membraneless organelles is impaired by both the toxic arginine rich dipeptide repeat proteins (DPRs), translated from the C9orf72 repeat RNA transcript, and the repeat RNA itself. Both the arginine rich DPRs and repeat RNA themselves undergo phase separation and disrupt the physiological phase separation of proteins involved in the formation of these liquid-like organelles. Hence abnormal phase separation may explain a number of pathological cellular phenomena associated with C9orf72-ALS/FTD. In this review article, we will discuss the principles of phase separation, phase separation of the DPRs and repeat RNA themselves and how they perturb LLPS associated with membraneless organelles and the functional consequences of this. We will then discuss how phase separation may impact the major pathological feature of C9orf72-ALS/FTD, TDP-43 proteinopathy, and how LLPS may be targeted therapeutically in disease.

Ultra-high field (7T) functional magnetic resonance imaging in amyotrophic lateral sclerosis: a pilot study

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Participants with ALS exhibited impaired function between the cortex and cerebellum.

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The cerebellum is associated with complex motor and cognitive processing tasks.

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These findings add to the growing number of ALS reports implicating the cerebellum.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of the central nervous system that results in a progressive loss of motor function and ultimately death. It is critical, yet also challenging, to develop non-invasive biomarkers to identify, localize, measure and/or track biological mechanisms implicated in ALS. Such biomarkers may also provide clues to identify potential molecular targets for future therapeutic trials. Herein we report on a pilot study involving twelve participants with ALS and nine age-matched healthy controls who underwent high-resolution resting state functional magnetic resonance imaging at an ultra-high field of 7 Tesla. A group-level whole-brain analysis revealed a disruption in long-range functional connectivity between the superior sensorimotor cortex (in the precentral gyrus) and bilateral cerebellar lobule VI. Post hoc analyses using atlas-derived left and right cerebellar lobule VI revealed decreased functional connectivity in ALS participants that predominantly mapped to bilateral postcentral and precentral gyri. Cerebellar lobule VI is a transition zone between anterior motor networks and posterior non-motor networks in the cerebellum, and is associated with a wide range of key functions including complex motor and cognitive processing tasks. Our observation of the involvement of cerebellar lobule VI adds to the growing number of studies implicating the cerebellum in ALS. Future avenues of scientific investigation should consider how high-resolution imaging at 7T may be leveraged to visualize differences in functional connectivity disturbances in various genotypes and phenotypes of ALS along the ALS-frontotemporal dementia spectrum.

Limbic-predominant age-related TDP-43 encephalopathy differs from frontotemporal lobar degeneration

TAR-DNA binding protein-43 (TDP-43) proteinopathy is seen in multiple brain diseases. A standardized terminology was recommended recently for common age-related TDP-43 proteinopathy: limbic-predominant, age-related TDP-43 encephalopathy (LATE) and the underlying neuropathological changes, LATE-NC. LATE-NC may be co-morbid with Alzheimer's disease neuropathological changes (ADNC). However, there currently are ill-defined diagnostic classification issues among LATE-NC, ADNC, and frontotemporal lobar degeneration with TDP-43 (FTLD-TDP). A practical challenge is that different autopsy cohorts are composed of disparate groups of research volunteers: hospital- and clinic-based cohorts are enriched for FTLD-TDP cases, whereas community-based cohorts have more LATE-NC cases. Neuropathological methods also differ across laboratories. Here, we combined both cases and neuropathologists' diagnoses from two research centres-University of Pennsylvania and University of Kentucky. The study was designed to compare neuropathological findings between FTLD-TDP and pathologically severe LATE-NC. First, cases were selected from the University of Pennsylvania with pathological diagnoses of either FTLD-TDP (n = 33) or severe LATE-NC (mostly stage 3) with co-morbid ADNC (n = 30). Sections from these University of Pennsylvania cases were cut from amygdala, anterior cingulate, superior/mid-temporal, and middle frontal gyrus. These sections were stained for phospho-TDP-43 immunohistochemically and evaluated independently by two University of Kentucky neuropathologists blinded to case data. A simple set of criteria hypothesized to differentiate FTLD-TDP from LATE-NC was generated based on density of TDP-43 immunoreactive neuronal cytoplasmic inclusions in the neocortical regions. Criteria-based sensitivity and specificity of differentiating severe LATE-NC from FTLD-TDP cases with blind evaluation was ∼90%. Another proposed neuropathological feature related to TDP-43 proteinopathy in aged individuals is 'Alpha' versus 'Beta' in amygdala. Alpha and Beta status was diagnosed by neuropathologists from both universities (n = 5 raters). There was poor inter-rater reliability of Alpha/Beta classification (mean κ = 0.31). We next tested a separate cohort of cases from University of Kentucky with either FTLD-TDP (n = 8) or with relatively 'pure' severe LATE-NC (lacking intermediate or severe ADNC; n = 14). The simple criteria were applied by neuropathologists blinded to the prior diagnoses at University of Pennsylvania. Again, the criteria for differentiating LATE-NC from FTLD-TDP was effective, with sensitivity and specificity ∼90%. If more representative cases from each cohort (including less severe TDP-43 proteinopathy) had been included, the overall accuracy for identifying LATE-NC was estimated at >98% for both cohorts. Also across both cohorts, cases with FTLD-TDP died younger than those with LATE-NC (P < 0.0001). We conclude that in most cases, severe LATE-NC and FTLD-TDP can be differentiated by applying simple neuropathological criteria.

Emotional disorders and the cerebellum: Neurobiological substrates, neuropsychiatry, and therapeutic implications

The notion that the cerebellum is devoted exclusively to motor control has been replaced by a more sophisticated understanding of its role in neurological function, one that includes cognition and emotion. Early clinical reports, as well as physiological and behavioral studies in animal models, raised the possibility of a nonmotor role for the cerebellum. Anatomical studies demonstrate cerebellar connectivity with the distributed neural circuits linked with autonomic, sensorimotor, vestibular, associative, and limbic/paralimbic brain areas. Identification of the cerebellar cognitive affective syndrome in adults and children underscored the clinical relevance of the role of the cerebellum in cognition and emotion. It opened new avenues of investigation into higher-order deficits that accompany the ataxias and other cerebellar diseases, as well as the contribution of cerebellar dysfunction to neuropsychiatric and neurocognitive disorders. Brain imaging studies have demonstrated the complexity of cerebellar functional topography, revealing a double representation of the sensorimotor cerebellum in the anterior lobe and lobule VIII and a triple cognitive representation in the cerebellar posterior lobe, as well as representation in the cerebellum of the intrinsic connectivity networks identified in the cerebral hemispheres. This paradigm shift in thinking about the cerebellum has been advanced by the theories of dysmetria of thought and the universal cerebellar transform, harmonizing the dual anatomic realities of homogeneously repeating cerebellar cortical microcircuitry set against the heterogeneous and topographically arranged cerebellar connections with extracerebellar structures. This new appreciation of cerebellar incorporation into circuits that subserve cognition and emotion mandates a deeper understanding of the cerebellum by practitioners in behavioral neurology and neuropsychiatry because it impacts the understanding and diagnosis of disorders of emotion and intellect and has potential for novel cerebellar-based approaches to therapy.

Human ALS/FTD brain organoid slice cultures display distinct early astrocyte and targetable neuronal pathology

Amyotrophic lateral sclerosis overlapping with frontotemporal dementia (ALS/FTD) is a fatal and currently untreatable disease characterized by rapid cognitive decline and paralysis. Elucidating initial cellular pathologies is central to therapeutic target development, but obtaining samples from presymptomatic patients is not feasible. Here, we report the development of a cerebral organoid slice model derived from human induced pluripotent stem cells (iPSCs) that recapitulates mature cortical architecture and displays early molecular pathology of C9ORF72 ALS/FTD. Using a combination of single-cell RNA sequencing and biological assays, we reveal distinct transcriptional, proteostasis and DNA repair disturbances in astroglia and neurons. We show that astroglia display increased levels of the autophagy signaling protein P62 and that deep layer neurons accumulate dipeptide repeat protein poly(GA), DNA damage and undergo nuclear pyknosis that could be pharmacologically rescued by GSK2606414. Thus, patient-specific iPSC-derived cortical organoid slice cultures are a reproducible translational platform to investigate preclinical ALS/FTD mechanisms as well as novel therapeutic approaches.

Clinical Update on C9orf72: Frontotemporal Dementia, Amyotrophic Lateral Sclerosis, and Beyond

The identification of C9orf72 gene has led to important scientific progresses and has considerably changed our clinical practice. However, a decade after C9orf72 discovery, some important clinical questions remain unsolved. The reliable cutoff for the pathogenic repeat number and the implication of intermediate alleles in frontotemporal dementia, amyotrophic lateral sclerosis, or in other diseases are still uncertain. The occurrence of an anticipation phenomenon - at the clinical and molecular levels - in C9orf72 kindreds is still debated as well, and the factors driving age at onset and phenotype variability are largely unknown. All these questions have a significant impact not only in clinical practice for diagnosis and genetic counseling but also in a research context for the initiation of therapeutic trials. In this chapter, we will address all those issues and summarize the recent updates about clinical aspects of C9orf72 disease, focusing on both the common and the less typical phenotypes.

TFEB/Mitf links impaired nuclear import to autophagolysosomal dysfunction in C9-ALS

Disrupted nucleocytoplasmic transport (NCT) has been implicated in neurodegenerative disease pathogenesis; however, the mechanisms by which disrupted NCT causes neurodegeneration remain unclear. In a Drosophila screen, we identified ref(2)P/p62, a key regulator of autophagy, as a potent suppressor of neurodegeneration caused by the GGGGCC hexanucleotide repeat expansion (G4C2 HRE) in C9orf72 that causes amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). We found that p62 is increased and forms ubiquitinated aggregates due to decreased autophagic cargo degradation. Immunofluorescence and electron microscopy of Drosophila tissues demonstrate an accumulation of lysosome-like organelles that precedes neurodegeneration. These phenotypes are partially caused by cytoplasmic mislocalization of Mitf/TFEB, a key transcriptional regulator of autophagolysosomal function. Additionally, TFEB is mislocalized and downregulated in human cells expressing GGGGCC repeats and in C9-ALS patient motor cortex. Our data suggest that the C9orf72-HRE impairs Mitf/TFEB nuclear import, thereby disrupting autophagy and exacerbating proteostasis defects in C9-ALS/FTD.

Co-incidental C9orf72 expansion mutation-related frontotemporal lobar degeneration pathology and sporadic Creutzfeldt-Jakob disease

Background

The C9orf72 hexanucleotide expansion mutation is the most common cause of genetic frontotemporal dementia (FTD), amyotrophic lateral sclerosis (ALS) and combined FTD‐ALS. Its underlying neuropathology combines TDP‐43 pathology and dipeptide repeat protein (DPR) deposits and may also associate with other neurodegeneration‐associated protein aggregates. Herein we present a unique combination of C9orf72 mutation with sporadic Creutzfeldt−Jakob disease (CJD) in a 74‐year‐old patient with rapidly progressive dementia.

Methods

Detailed neuropathological examination including immunohistochemistry for several proteinopathies. Genetic analysis was conducted by repeat primed polymerase chain reaction (PCR). Furthermore, we analyzed additional C9orf72 mutation carriers for prion−protein (PrP) deposits in brain tissue and screened the cerebellar cortex of other CJD cases for p62/DPR neuronal inclusions to assess the frequency of combined pathologies.

Results

Postmortem brain examination of a patient with a rapidly progressive neurological deterioration of 8 months’ duration confirmed the diagnosis of CJD. She harbored valine homozygosity at PRNP codon 129. In addition, a frontotemporal lobar degeneration (FTLD)‐pattern with TDP‐43 protein aggregates and p62+/C9RANT+ positive inclusions along with a high degree of Alzheimer‐related pathology (A3B3C3) were identified. The suspected C9orf72 expansion mutation was confirmed by repeat‐primed PCR. Screening of 13 C9orf72 cases showed no pathological PrP aggregates and screening of 100 CJD cases revealed no other C9orf72 expansion mutation carriers.

Conclusion

A combination of a C9orf72 expansion mutation‐related FTLD with sporadic CJD in the same patient is rare. While the rarity of both diseases makes this concurrence most likely to be coincidental, questions regarding a potential link between these two neurodegenerative pathologies deserve further studies.

Insights into disease mechanisms and potential therapeutics for C9orf72-related amyotrophic lateral sclerosis/frontotemporal dementia

In 2011, a hexanucleotide repeat expansion (HRE) in the noncoding region of C9orf72 was associated with the most frequent genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). The main pathogenic mechanisms in C9-ALS/FTD are haploinsufficiency of the C9orf72 protein and gain of function toxicity from bidirectionally-transcribed repeat-containing RNAs and dipeptide repeat proteins (DPRs) resulting from non-canonical RNA translation. Additionally, abnormalities in different downstream cellular mechanisms, such as nucleocytoplasmic transport and autophagy, play a role in pathogenesis. Substantial research efforts using in vitro and in vivo models have provided valuable insights into the contribution of each mechanism in disease pathogenesis. However, conflicting evidence exists, and a unifying theory still lacks. Here, we provide an overview of the recently published literature on clinical, neuropathological and molecular features of C9-ALS/FTD. We highlight the supposed neuronal role of C9orf72 and the HRE pathogenic cascade, mainly focusing on the contribution of RNA foci and DPRs to neurodegeneration and discussing the several downstream mechanisms. We summarize the emerging biochemical and neuroimaging biomarkers, as well as the potential therapeutic approaches. Despite promising results, a specific disease-modifying treatment is still not available to date and greater insights into disease mechanisms may help in this direction.

Generation and Characterization of Novel Monoclonal Antibodies Targeting p62/sequestosome-1 Across Human Neurodegenerative Diseases

Human neurodegenerative diseases can be characterized as disorders of protein aggregation. As a key player in cellular autophagy and the ubiquitin proteasome system, p62 may represent an effective immunohistochemical target, as well as mechanistic operator, across neurodegenerative proteinopathies. In this study, 2 novel mouse-derived monoclonal antibodies 5G3 and 2A5 raised against residues 360-380 of human p62/sequestosome-1 were characterized via immunohistochemical application upon human tissues derived from cases of C9orf72-expansion spectrum diseases, Alzheimer disease, progressive supranuclear palsy, Lewy body disease, and multiple system atrophy. 5G3 and 2A5 reliably highlighted neuronal dipeptide repeat, tau, and α-synuclein inclusions in a distribution similar to a polyclonal antibody to p62, phospho-tau antibodies 7F2 and AT8, and phospho-α-synuclein antibody 81A. However, antibodies 5G3 and 2A5 consistently stained less neuropil structures, such as tau neuropil threads and Lewy neurites, while 2A5 marked fewer glial inclusions in progressive supranuclear palsy. Both 5G3 and 2A5 revealed incidental astrocytic tau immunoreactivity in cases of Alzheimer disease and Lewy body disease with resolution superior to 7F2. Through their unique ability to highlight specific types of pathological deposits in neurodegenerative brain tissue, these novel monoclonal p62 antibodies may provide utility in both research and diagnostic efforts.

Genetic screening of ANXA11 revealed novel mutations linked to amyotrophic lateral sclerosis

ANXA11 mutations have previously been discovered in amyotrophic lateral sclerosis (ALS) motor neuron disease. To confirm the contribution of ANXA11 mutations to ALS, a large exome data set obtained from 330 French patients, including 150 familial ALS index cases and 180 sporadic ALS cases, was analyzed, leading to the identification of 3 rare ANXA11 variants in 5 patients. The novel p.L254V variant was associated with early onset sporadic ALS. The novel p.D40Y mutation and the p.G38R variant concerned patients with predominant pyramidal tract involvement and cognitive decline. Neuropathologic findings in a p.G38R carrier associated the presence of ALS typical inclusions within the spinal cord, massive degeneration of the lateral tracts, and type A frontotemporal lobar degeneration. This mutant form of annexin A11 accumulated in various brain regions and in spinal cord motor neurons, although its stability was decreased in patients' lymphoblasts. Because most ANXA11 inclusions were not colocalized with transactive response DNA-binding protein 43 or p62 deposits, ANXA11 aggregation does not seem mandatory to trigger neurodegeneration with additional participants/partner proteins that could intervene.

Modelling frontotemporal dementia using patient-derived induced pluripotent stem cells

Frontotemporal dementia (FTD) describes a group of clinically heterogeneous conditions that frequently affect people under the age of 65 (Le Ber et al., 2013). There are multiple genetic causes of FTD, including coding or splice-site mutations in MAPT, GRN mutations that lead to haploinsufficiency of progranulin protein, and a hexanucleotide GGGGCC repeat expansion in C9ORF72. Pathologically, FTD is characterised by abnormal protein accumulations in neurons and glia. These aggregates can be composed of the microtubule-associated protein tau (observed in FTD with MAPT mutations), the DNA/RNA-binding protein TDP-43 (seen in FTD with mutations in GRN or C9ORF72 repeat expansions) or dipeptide proteins generated by repeat associated non-ATG translation of the C9ORF72 repeat expansion. There are currently no disease-modifying therapies for FTD and the availability of in vitro models that recapitulate pathologies in a disease-relevant cell type would accelerate the development of novel therapeutics. It is now possible to generate patient-specific stem cells through the reprogramming of somatic cells from a patient with a genotype/phenotype of interest into induced pluripotent stem cells (iPSCs). iPSCs can subsequently be differentiated into a plethora of cell types including neurons, astrocytes and microglia. Using this approach has allowed researchers to generate in vitro models of genetic FTD in human cell types that are largely inaccessible during life. In this review we explore the recent progress in the use of iPSCs to model FTD, and consider the merits, limitations and future prospects of this approach.

Modeling ALS using iPSCs: is it possible to reproduce the phenotypic variations observed in patients in vitro?

Amyotrophic lateral sclerosis (ALS) is a fatal disease that leads to progressive degeneration of motoneurons. Mutations in the C9ORF72, SOD1, TARDBP and FUS genes, among others, have been associated with ALS. Although motoneuron degeneration is the common outcome of ALS, different pathological mechanisms seem to be involved in this process, depending on the genotypic background of the patient. The advent of induced pluripotent stem cell (iPSC) technology enabled the development of patient-specific cell lines, from which it is possible to generate different cell types and search for phenotypic alterations. In this review, we summarize the pathophysiological markers detected in cells differentiated from iPSCs of ALS patients. In a translational perspective, iPSCs from ALS patients could be useful for drug screening, through stratifying patients according to their genetic background.

Divergence, Convergence, and Therapeutic Implications: A Cell Biology Perspective of C9ORF72-ALS/FTD

Ever since a GGGGCC hexanucleotide repeat expansion mutation in C9ORF72 was identified as the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), three competing but nonexclusive hypotheses to explain how this mutation causes diseases have been proposed and are still under debate. Recent studies in the field have tried to understand how the repeat expansion disrupts cellular physiology, which has suggested interesting convergence of these hypotheses on downstream, functional defects in cells, such as nucleocytoplasmic transport disruption, membrane-less organelle defects, and DNA damage. These studies have not only provided an integrated view of the disease mechanism but also revealed novel cell biology implicated in neurodegeneration. Furthermore, some of the discoveries have given rise to new ideas for therapeutic development. Here, we review the research progress on cellular pathophysiology of C9ORF72-mediated ALS and FTD and its therapeutic implication. We suggest that the repeat expansion drives pathogenesis through a combination of downstream defects, of which some can be therapeutic targets.

Development of neurodegeneration in amyotrophic lateral sclerosis: from up or down?

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disease associated with neurodegeneration and intracellular pathological 43-kDa transactive response sequence DNA-binding protein (TDP-43) positive inclusions. The various clinical symptoms, such as motor disorders and cognitive impairment, reflect the degeneration of certain areas of the nervous system. Since the discovery of the significance of pathological TDP-43 for human disease including ALS, there has been an increasing number of studies reporting on the distribution and severity of neurodegeneration. These have rekindled the old debate about whether the first or second motor neuron is the primary site of degeneration in ALS. To shed light on this question, the following is a review of the relevant neuropathological studies.

Smcr8 deficiency disrupts axonal transport-dependent lysosomal function and promotes axonal swellings and gain of toxicity in C9ALS/FTD mouse models

G4C2 repeat expansions in an intron of C9ORF72 cause the most common familial amyotrophic lateral sclerosis and frontotemporal dementia (collectively, C9ALS/FTD). Mechanisms and mediators of C9ALS/FTD pathogenesis remain poorly understood. C9orf72 and Smcr8 form a protein complex. Here, we show that expression of Smcr8, like C9orf72, is reduced in C9ALS/FTD mouse models and patient tissues. Since Smcr8 is highly conserved between human and mouse, we evaluated the effects of Smcr8 downregulation in mice. Smcr8 knockout (KO) mice exhibited motor behavior deficits, which resemble those of C9ALS/FTD mouse models, and displayed axonal swellings in their spinal cords and neuromuscular junctions. These deficits are caused by impaired autophagy-lysosomal functions due to disrupted axonal transport in mutant motor neurons. Consistent with its interaction with C9orf72 and their downregulation in patient tissues, Smcr8 deficiency exacerbated autophagy-lysosomal impairment in C9orf72 KO mice. The disease relevance of Smcr8 downregulation was reflected by exacerbated axonal swellings and gain of toxicity pathology arising from Smcr8 haploinsufficiency in a mouse model of C9ALS/FTD. Thus, our in vivo studies suggested that Smcr8 deficiency impairs axonal transport dependent autophagy-lysosomal function and exacerbates axonal degeneration and gain of toxicity in C9ALS/FTD mouse models.

Genetic forms of frontotemporal lobar degeneration: Current diagnostic approach and new directions in therapeutic strategies

Recent advances in the genetics of neurodegenerative diseases have substantially improved our knowledge about the genetic causes of frontotemporal lobar degeneration (FTLD). Three major genes, namely progranulin (GRN), C9orf72 and MAPT, as well as several less common genes, are responsible for the majority of familial cases and for a significant proportion of sporadic forms, including FTLD with or without associated amyotrophic lateral sclerosis and some rarer clinical presentations. Plasma progranulin dosage and next-generation sequencing are currently available tools which allow the detection of a genetic cause in a more rapid and efficient way. This has important consequences for clinical practice and genetic counseling for patients and families. The ongoing investigations on some therapeutic candidates targeting different biological pathways involved in the most frequent genetic forms of FTLD, as well as a better understanding of the early pathophysiological modifications occurring during the presymptomatic phase of the disease could hopefully contribute to develop effective disease-modifying therapies. The identification of a causal mutation in a family is of outmost importance indeed to propose to presymptomatic carriers their inclusion in clinical trials with the aim to prevent or delay the onset of disease.

C9orf72 arginine-rich dipeptide repeat proteins disrupt karyopherin-mediated nuclear import

Disruption of nucleocytoplasmic transport is increasingly implicated in the pathogenesis of neurodegenerative diseases, including ALS caused by a C9orf72 hexanucleotide repeat expansion. However, the mechanism(s) remain unclear. Karyopherins, including importin β and its cargo adaptors, have been shown to co-precipitate with the C9orf72 arginine-containing dipeptide repeat proteins (R-DPRs), poly-glycine arginine (GR) and poly-proline arginine (PR), and are protective in genetic modifier screens. Here, we show that R-DPRs interact with importin β, disrupt its cargo loading, and inhibit nuclear import of importin β, importin α/β, and transportin cargoes in permeabilized mouse neurons and HeLa cells, in a manner that can be rescued by RNA. Although R-DPRs induce widespread protein aggregation in this in vitro system, transport disruption is not due to nucleocytoplasmic transport protein sequestration, nor blockade of the phenylalanine-glycine (FG)-rich nuclear pore complex. Our results support a model in which R-DPRs interfere with cargo loading on karyopherins.