Ubiquitinated, p62 immunopositive cerebellar cortical neuronal inclusions are evident across the spectrum of TDP-43 proteinopathies but are only rarely additionally immunopositive for phosphorylation-dependent TDP-43

Multiple Neuropathologies Underly Hippocampal Subfield Atrophy in a Case With a Slowly Progressive Amnestic Syndrome: Challenging the Notion of Pure LATE-NC

Alzheimer's disease (AD) is the leading cause of dementia in the elderly, marked by abnormal protein buildup (beta-amyloid and tau) resulting in neuronal loss, especially in the medial temporal lobe and other limbic regions. The presence of transactive response DNA binding protein 43 (TDP-43) immunoreactive inclusions in medial temporal lobe regions has also been associated with neuroimaging changes in limbic regions. It has been proposed that hypometabolism in limbic regions on [18F] fluorodeoxyglucose positron emission tomography (FDG-PET) in a patient with a slowly evolving amnestic syndrome may be a signature of the presence of TDP-43. In this context, we observed an 86-year-old Caucasian female with dementia characterized by a slowly evolving amnestic syndrome, along with focal medial temporal atrophy evident on MRI and hypometabolism in limbic regions on FDG-PET. The patient subsequently died and underwent an autopsy. We performed detailed neuroimaging and digital neuropathological analyses of the hippocampal subfields to better understand the relationship between clinico-imaging findings and histopathology. In addition to TDP-43, we identified three other pathological processes in the medial temporal lobe: sequestosome-1/p62, argyrophilic grain disease (AGD), and primary age-related tauopathy (PART). Hippocampal subfield volumes and rates of atrophy were no different from those of matched healthy controls, except for the atrophy rate in cornu ammonis 1 (CA1). Digital histopathology revealed the relative highest burden of pathology for p62, followed by TDP-43, AGD, and PART in CA1. Multiple pathological processes appear to have contributed to the hippocampal atrophy and hypometabolism in our patient with a slowly progressive amnestic syndrome.

A robust evaluation of TDP-43, poly GP, cellular pathology and behavior in an AAV-C9ORF72 (G4C2)66 mouse model

The G4C2 hexanucleotide repeat expansion in C9ORF72 is the major genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9-ALS/FTD). Despite considerable efforts, the development of mouse models of C9-ALS/FTD useful for therapeutic development has proven challenging due to the intricate interplay of genetic and molecular factors underlying this neurodegenerative disorder, in addition to species differences. This study presents a robust investigation of the cellular pathophysiology and behavioral outcomes in a previously described AAV mouse model of C9-ALS expressing 66 G4C2 hexanucleotide repeats. The model displays key molecular ALS pathological markers including RNA foci, dipeptide repeat (DPR) protein aggregation, p62 positive stress granule formation as well as mild gliosis. However, the AAV-(G4C2)66 mouse model in this study has marginal neurodegeneration with negligible neuronal loss, or clinical deficits. Human C9orf72 is typically associated with altered TAR DNA-binding protein (TDP-43) function, yet studies of this rodent model revealed no significant evidence of TDP-43 dysfunction. While our findings indicate and support that this is a highly valuable robust and pharmacologically tractable model for investigating the molecular mechanisms and cellular consequences of (G4C2) repeat driven DPR pathology, it is not suitable for investigating the development of disease- associated TDP-43 dysfunction or clinical impairment. Our findings underscore the complexity of ALS pathogenesis involving genetic mutations and protein dysregulation and highlight the need for more comprehensive model systems that reliably replicate the multifaceted cellular and behavioral aspects of C9-ALS.

A robust evaluation of TDP-43, poly GP, cellular pathology and behavior in a AAV- C9ORF72 (G 4 C 2) 66 mouse model

The G4C2 hexanucleotide repeat expansion in C9ORF72is the major genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9-ALS/FTD). Despite considerable efforts, the development of mouse models of C9-ALS/FTD useful for therapeutic development has proven challenging due to the intricate interplay of genetic and molecular factors underlying this neurodegenerative disorder, in addition to species differences. This study presents a robust investigation of the cellular pathophysiology and behavioral outcomes in a previously described AAV mouse model of C9-ALS expressing 66 G4C2 hexanucleotide repeats. The model displays key molecular ALS pathological markers including RNA foci, dipeptide repeat (DPR) protein aggregation, p62 positive stress granule formation as well as mild gliosis. However, the AAV-(G4C2)66 mouse model in this study has marginal neurodegeneration with negligible neuronal loss, or clinical deficits. Human C9orf72 is typically associated with altered TAR DNA-binding protein (TDP-43) function, yet studies of this rodent model revealed no significant evidence of TDP-43 dysfunction. While our findings indicate and support that this is a highly valuable robust and pharmacologically tractable model for investigating the molecular mechanisms and cellular consequences of (G4C2) repeat driven DPR pathology, it is not suitable for investigating the development of disease- associated TDP-43 dysfunction or clinical impairment. Our findings underscore the complexity of ALS pathogenesis involving genetic mutations and protein dysregulation and highlight the need for more comprehensive model systems that reliably replicate the multifaceted cellular and behavioral aspects of C9-ALS.

A robust evaluation of TDP-43, poly GP, cellular pathology and behavior in a AAV-C9ORF72 (G4C2)66 mouse model

The G4C2 hexanucleotide repeat expansion in C9ORF72 is the major genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9-ALS/FTD). Despite considerable efforts, the development of mouse models of C9-ALS/FTD useful for therapeutic development has proven challenging due to the intricate interplay of genetic and molecular factors underlying this neurodegenerative disorder, in addition to species differences. This study presents a robust investigation of the cellular pathophysiology and behavioral outcomes in a previously described AAV mouse model of C9-ALS expressing 66 G4C2 hexanucleotide repeats. Despite displaying key molecular ALS pathological markers including RNA foci, dipeptide repeat (DPR) protein aggregation, p62 positive stress granule formation as well as mild gliosis, the AAV-(G4C2)66 mouse model in this study exhibits negligible neuronal loss, no motor deficits, and functionally unimpaired TAR DNA-binding protein-43 (TDP-43). While our findings indicate and support that this is a robust and pharmacologically tractable model for investigating the molecular mechanisms and cellular consequences of (G4C2) repeat driven DPR pathology, it is not suitable for investigating the development of disease associated neurodegeneration, TDP-43 dysfunction, gliosis, and motor performance. Our findings underscore the complexity of ALS pathogenesis involving genetic mutations and protein dysregulation and highlight the need for more comprehensive model systems that reliably replicate the multifaceted cellular and behavioral aspects of C9-ALS.

Cerebellum in Alzheimer's disease and other neurodegenerative diseases: an emerging research frontier

The cerebellum is crucial for both motor and nonmotor functions. Alzheimer's disease (AD), alongside other dementias such as vascular dementia (VaD), Lewy body dementia (DLB), and frontotemporal dementia (FTD), as well as other neurodegenerative diseases (NDs) like Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and spinocerebellar ataxias (SCA), are characterized by specific and non-specific neurodegenerations in central nervous system. Previously, the cerebellum's significance in these conditions was underestimated. However, advancing research has elevated its profile as a critical node in disease pathology. We comprehensively review the existing evidence to elucidate the relationship between cerebellum and the aforementioned diseases. Our findings reveal a growing body of research unequivocally establishing a link between the cerebellum and AD, other forms of dementia, and other NDs, supported by clinical evidence, pathological and biochemical profiles, structural and functional neuroimaging data, and electrophysiological findings. By contrasting cerebellar observations with those from the cerebral cortex and hippocampus, we highlight the cerebellum's distinct role in the disease processes. Furthermore, we also explore the emerging therapeutic potential of targeting cerebellum for the treatment of these diseases. This review underscores the importance of the cerebellum in these diseases, offering new insights into the disease mechanisms and novel therapeutic strategies.

Preclinical investigation in FAAH inhibition as a neuroprotective therapy for frontotemporal dementia using TDP-43 transgenic male mice

BACKGROUND

Frontotemporal dementia (FTD) is a heterogeneous group of early onset and progressive neurodegenerative disorders, characterized by degeneration in the frontal and temporal lobes, which causes deterioration in cognition, personality, social behavior and language. Around 45% of the cases are characterized by the presence of aggregates of the RNA-binding protein TDP-43.

METHODS

In this study, we have used a murine model of FTD that overexpresses this protein exclusively in the forebrain (under the control of the CaMKIIα promoter) for several biochemical, histological and pharmacological studies focused on the endocannabinoid system.

RESULTS

These mice exhibited at postnatal day 90 (PND90) important cognitive deficits, signs of emotional impairment and disinhibited social behaviour, which were, in most of cases, maintained during the first year of life of these animals. Motor activity was apparently normal, but FTD mice exhibited higher mortality. Their MRI imaging analysis and their ex-vivo histopathological evaluation proved changes compatible with atrophy (loss of specific groups of pyramidal neurons: Ctip2- and NeuN-positive cells) and inflammatory events (astroglial and microglial reactivities) in both cortical (medial prefrontal cortex) and subcortical (hippocampus) structures at PND90 and also at PND365. The analysis of the endocannabinoid system in these mice proved a decrease in the hydrolysing enzyme FAAH in the prefrontal cortex and the hippocampus, with an increase in the synthesizing enzyme NAPE-PLD only in the hippocampus, responses that were accompanied by modest elevations in anandamide and related N-acylethanolamines. The potentiation of these elevated levels of anandamide after the pharmacological inactivation of FAAH with URB597 resulted in a general improvement in behaviour, in particular in cognitive deterioration, associated with the preservation of pyramidal neurons of the medial prefrontal cortex and the CA1 layer of the hippocampus, and with the reduction of gliosis in both structures.

CONCLUSIONS

Our data confirmed the potential of elevating the endocannabinoid tone as a therapy against TDP-43-induced neuropathology in FTD, limiting glial reactivity, preserving neuronal integrity and improving cognitive, emotional and social deficits.

Random forest modelling demonstrates microglial and protein misfolding features to be key phenotypic markers in C9orf72-ALS

Clinical heterogeneity observed across patients with amyotrophic lateral sclerosis (ALS) is a known complicating factor in identifying potential therapeutics, even within cohorts with the same mutation, such as C9orf72 hexanucleotide repeat expansions (HREs). Thus, further understanding of pathways underlying this heterogeneity is essential for appropriate ALS trial stratification and the meaningful assessment of clinical outcomes. It has been shown that both inflammation and protein misfolding can influence ALS pathogenesis, such as the manifestation or severity of motor or cognitive symptoms. However, there has yet to be a systematic and quantitative assessment of immunohistochemical markers to interrogate the potential relevance of these pathways in an unbiased manner. To investigate this, we extensively characterised features of commonly used glial activation and protein misfolding stains in thousands of images of post-mortem tissue from a heterogeneous cohort of deeply clinically profiled patients with a C9orf72 HRE. Using a random forest model, we show that microglial staining features are the most accurate classifiers of disease status in our panel and that clinicopathological relationships exist between microglial activation status, TDP-43 pathology, and language dysfunction. Furthermore, we detected spatially resolved changes in fused in sarcoma (FUS) staining, suggesting that liquid-liquid phase shift of this aggregation-prone RNA-binding protein may be important in ALS caused by a C9orf72 HRE. Interestingly, no one feature alone significantly impacted the predictiveness of the model, indicating that the collective examination of all features, or a combination of several features, is what allows the model to be predictive. Our findings provide further support to the hypothesis of dysfunctional immune regulation and proteostasis in the pathogenesis of C9-ALS and provide a framework for digital analysis of commonly used neuropathological stains as a tool to enrich our understanding of clinicopathological relationships within and between cohorts. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Molecular Dissection of TDP-43 as a Leading Cause of ALS/FTLD

TAR DNA binding protein 43 (TDP-43) is a DNA/RNA binding protein involved in pivotal cellular functions, especially in RNA metabolism. Hyperphosphorylated and ubiquitinated TDP-43-positive neuronal cytoplasmic inclusions are identified in the brain and spinal cord in most cases of amyotrophic lateral sclerosis (ALS) and a substantial proportion of frontotemporal lobar degeneration (FTLD) cases. TDP-43 dysfunctions and cytoplasmic aggregation seem to be the central pathogenicity in ALS and FTLD. Therefore, unraveling both the physiological and pathological mechanisms of TDP-43 may enable the exploration of novel therapeutic strategies. This review highlights the current understanding of TDP-43 biology and pathology, describing the cellular processes involved in the pathogeneses of ALS and FTLD, such as post-translational modifications, RNA metabolism, liquid–liquid phase separation, proteolysis, and the potential prion-like propagation propensity of the TDP-43 inclusions.

Elucidating the Role of Cerebellar Synaptic Dysfunction in C9orf72-ALS/FTD - a Systematic Review and Meta-Analysis

A hexanucleotide repeat expansion in the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) with synaptic dysfunction identified as an early pathological hallmark. Although TDP-43 pathology and overt neurodegeneration are largely absent from the cerebellum, the pathological hallmarks of RNA foci and dipeptide repeat protein (DPR) inclusions are most abundant. Here, we present a systematic literature search in the databases of PubMed, Scopus, Embase, Web of Science and Science Direct up until March 5, 2021, which yielded 19,515 publications. Following the exclusion criteria, 72 articles were included having referred to C9orf72, synapses and the cerebellum. Meta-analyses were conducted on studies which reported experimental and control groups with means and standard deviations extracted from figures using the online tool PlotDigitizer. This revealed dendritic defects (P = 0.03), reduced C9orf72 in human patients (P = 0.005) and DPR-related neuronal loss (P = 0.0006) but no neuromuscular junction abnormalities (P = 0.29) or cerebellar neuronal loss (P = 0.23). Our results suggest that dendritic arborisation defects, synaptic gene dysregulation and altered synaptic neurotransmission may drive cerebellar synaptic dysfunction in C9-ALS/FTD. In this review, we discuss how the chronological appearance of the different pathological hallmarks alters synaptic integrity which may have profound implications for disease progression. We conclude that a reduction in C9orf72 protein levels combined with the accumulation of RNA foci and DPRs act synergistically to drive C9 synaptopathy in the cerebellum of C9-ALS/FTD patients.

Modelling amyotrophic lateral sclerosis in rodents

The efficient study of human disease requires the proper tools, one of the most crucial of which is an accurate animal model that faithfully recapitulates the human condition. The study of amyotrophic lateral sclerosis (ALS) is no exception. Although the majority of ALS cases are considered sporadic, most animal models of this disease rely on genetic mutations identified in familial cases. Over the past decade, the number of genes associated with ALS has risen dramatically and, with each new genetic variant, there is a drive to develop associated animal models. Rodent models are of particular importance as they allow for the study of ALS in the context of a living mammal with a comparable CNS. Such models not only help to verify the pathogenicity of novel mutations but also provide critical insight into disease mechanisms and are crucial for the testing of new therapeutics. In this Review, we aim to summarize the full spectrum of ALS rodent models developed to date.

The converging roles of sequestosome-1/p62 in the molecular pathways of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)

Investigations into the pathogenetic mechanisms underlying amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have provided significant insight into the disease. At the cellular level, ALS and FTD are classified as proteinopathies, which is motor neuron degeneration and death characterized by pathological protein aggregates or dysregulated proteostasis. At both the clinical and molecular level there are common signaling pathways dysregulated across the ALS and FTD spectrum (ALS/FTD). Sequestosome-1/p62 is a multifunctional scaffold protein with roles in several signaling pathways including proteostasis, protein degradation via the ubiquitin proteasome system and autophagy, the antioxidant response, inflammatory response, and apoptosis. Notably these pathways are dysregulated in ALS and FTD. Mutations in the functional domains of p62 provide links to the pathogenetic mechanisms of p62 and dyshomeostasis of p62 levels is noted in several types of ALS and FTD. We present here that the dysregulated ALS and FTD signaling pathways are linked, with p62 converging the molecular mechanisms. This review summarizes the current literature on the complex role of p62 in the pathogenesis across the ALS/FTD spectrum. The focus is on the underlying convergent molecular mechanisms of ALS and FTD-associated proteins and pathways that dysregulate p62 levels or are dysregulated by p62, with emphasis on how p62 is implicated across the ALS/FTD spectrum.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Effects of TDP-43 overexpression on neuron proteome and morphology in vitro

TDP-43 is pathologically and genetically with associated amyotrophic lateral sclerosis and frontotemporal lobar degeneration. These diseases are characterized by significant neurite defects, including cytoskeletal pathology. The involvement of TDP-43 in the degeneration of neurons in these diseases are not yet well understood, however accumulating evidence shows involvement in neurite outgrowth, remodelling and in regulation of many components of the neuronal cytoskeleton. In order to investigate how alterations to TDP-43 expression levels may exert effects on the neuronal cytoskeleton, primary cortical neurons from transgenic mice overexpressing one or two copies of human wildtype TDP-43 under the prion promoter were examined. Label-free quantitative proteomic analysis, followed by functional annotation clustering to identify protein families that clustered together within up- or down-regulated protein groups, revealed that actin-binding proteins were significantly more abundant in neurons overexpressing TDP-43 compared to wildtype neurons. Morphological analysis demonstrated that during early development neurons expressing one copy of human TDP-43 had an increased number of neurite branches and alterations to growth cone morphology, while no changes were observed in neurons expressing two copies of TDP-43. These developmental processes require specific expression and organization of the cytoskeleton. The results from these studies provide further insight into the normal function of TDP-43 and how alterations in TDP-43 expression may impact the cytoskeleton.

Old age genetically confirmed frontotemporal lobar degeneration with TDP-43 has limbic predominant TDP-43 deposition

AIMS

To assess the burden of transactive response DNA-binding protein of 43 kDa (TDP-43) inclusions in a unique cohort of old-age patients with genetic frontotemporal lobar degeneration (gFTLD-TDP) and compare these patients with sporadic old-age individuals with TDP-43, either in the presence of Alzheimer's disease (AD-TDP) or in isolation (pure-TDP).

METHODS

The brain bank at Mayo Clinic-Jacksonville was searched for cases ≥75 years old at death with TDP-43 extending into middle frontal cortex. Cases were split into the following groups: (1) gFTLD-TDP (n = 15) with progranulin (GRN)/C9ORF72 mutations; (2) AD-TDP (n = 10)-cases with median Braak neurofibrillary tangle (NFT) stage VI, Thal phase V; (3) pure-TDP (n = 10)-cases with median Braak NFT stage I, Thal phase I. Clinical data were abstracted; TDP-43 burden was calculated using digital pathology.

RESULTS

Amnestic Alzheimer's dementia was the clinical diagnosis in ≥50% patients in each group. The distribution of TDP-43 burden in gFTLD-TDP and AD-TDP, but not pure-TDP, was limbic-predominant targeting CA1 and subiculum. Patients with gFTLD-TDP had higher burden in entorhinal cortex compared to AD-TDP. TDP-43 burden in middle frontal cortex did not differ between the three groups.

CONCLUSIONS

In old age it is challenging to clinically and pathologically differentiate gFTLD-TDP from AD-TDP and pure-TDP-43 based on burden. Like AD-TDP, old age gFTLD-TDP have a limbic predominant TDP-43 distribution. The finding that amnestic Alzheimer's dementia was the most common clinical diagnosis regardless of group suggests that TDP-43 directly and indirectly targets limbic regions.

Chronic traumatic encephalopathy and the nucleus basalis of Meynert

Due to the growing number of chronic traumatic encephalopathy (CTE) cases in the military and contact sports, defining the cellular and molecular substrate of this disorder is crucial. Most classic neuropathological investigations describe cortical tau and, to a lesser extent, amyloid lesions, which may underlie the clinical sequela associated with CTE. The application of modern molecular biologic technology to postmortem human brain tissue has made it possible to evaluate the genetic signature of specific neuronal phenotypes at different stages of CTE pathology. Most recently, molecular pathobiology has been used in the field of CTE, with an emphasis on the cholinergic neurons located within the nucleus basalis of Meynert, which develop tau pathology and are associated with cognitive dysfunction similar to that found in Alzheimer's disease (AD). Quantitative findings derived from single-cell transcript investigations provide clues to our understanding of the selective vulnerability of neurons containing AD-like tau pathology at different stages of CTE. Since human tissue-based studies provide a gold standard for the field of CTE, continued molecular pathological studies are needed to reveal novel drug targets for the treatment of this disorder.

Maintaining the balance of TDP-43, mitochondria, and autophagy: a promising therapeutic strategy for neurodegenerative diseases

Mitochondria are the energy center of cell operations and are involved in physiological functions and maintenance of metabolic balance and homeostasis in the body. Alterations of mitochondrial function are associated with a variety of degenerative and acute diseases. As mitochondria age in cells, they gradually become inefficient and potentially toxic. Acute injury can trigger the permeability of mitochondrial membranes, which can lead to apoptosis or necrosis. Transactive response DNA-binding protein 43 kDa (TDP-43) is a protein widely present in cells. It can bind to RNA, regulate a variety of RNA processes, and play a role in the formation of multi-protein/RNA complexes. Thus, the normal physiological functions of TDP-43 are particularly important for cell survival. Normal TDP-43 is located in various subcellular structures including mitochondria, mitochondrial-associated membrane, RNA particles and stress granules to regulate the endoplasmic reticulum–mitochondrial binding, mitochondrial protein translation, and mRNA transport and translation. Importantly, TDP-43 is associated with a variety of neurodegenerative diseases, including amyotrophic lateral sclerosis, frontotemporal dementia and Alzheimer's disease, which are characterized by abnormal phosphorylation, ubiquitination, lysis or nuclear depletion of TDP-43 in neurons and glial cells. Although the pathogenesis of TDP-43 proteinopathy remains unknown, the presence of pathological TDP-43 inside or outside of mitochondria and the functional involvement of TDP-43 in the regulation of mitochondrial morphology, transport, and function suggest that mitochondria are associated with TDP-43-related diseases. Autophagy is a basic physiological process that maintains the homeostasis of cells, including targeted clearance of abnormally aggregated proteins and damaged organelles in the cytoplasm; therefore, it is considered protective against neurodegenerative diseases. However, the combination of abnormal TDP-43 aggregation, mitochondrial dysfunction, and insufficient autophagy can lead to a variety of aging-related pathologies. In this review, we describe the current knowledge on the associations of mitochondria with TDP-43 and the role of autophagy in the clearance of abnormally aggregated TDP-43 and dysfunctional mitochondria. Finally, we discuss a novel approach for neurodegenerative treatment based on the knowledge.

Selective neuronal degeneration in MATR3 S85C knock-in mouse model of early-stage ALS

A missense mutation, S85C, in the MATR3 gene is a genetic cause for amyotrophic lateral sclerosis (ALS). It is unclear how the S85C mutation affects MATR3 function and contributes to disease. Here, we develop a mouse model that harbors the S85C mutation in the endogenous Matr3 locus using the CRISPR/Cas9 system. MATR3 S85C knock-in mice recapitulate behavioral and neuropathological features of early-stage ALS including motor impairment, muscle atrophy, neuromuscular junction defects, Purkinje cell degeneration and neuroinflammation in the cerebellum and spinal cord. Our neuropathology data reveals a loss of MATR3 S85C protein in the cell bodies of Purkinje cells and motor neurons, suggesting that a decrease in functional MATR3 levels or loss of MATR3 function contributes to neuronal defects. Our findings demonstrate that the MATR3 S85C mouse model mimics aspects of early-stage ALS and would be a promising tool for future basic and preclinical research.

Immiscible inclusion bodies formed by polyglutamine and poly(glycine-alanine) are enriched with distinct proteomes but converge in proteins that are risk factors for disease and involved in protein degradation

Poly(glycine-alanine) (polyGA) is one of the polydipeptides expressed in Frontotemporal Dementia and/or Amyotrophic Lateral Sclerosis 1 caused by C9ORF72 mutations and accumulates as inclusion bodies in the brain of patients. Superficially these inclusions are similar to those formed by polyglutamine (polyQ)-expanded Huntingtin exon 1 (Httex1) in Huntington’s disease. Both have been reported to form an amyloid-like structure suggesting they might aggregate via similar mechanisms and therefore recruit the same repertoire of endogenous proteins. When co-expressed in the same cell, polyGA₁₀₁ and Httex1(Q₉₇) inclusions adopted immiscible phases suggesting different endogenous proteins would be enriched. Proteomic analyses identified 822 proteins in the inclusions. Only 7 were specific to polyGA and 4 specific to Httex1(Q₉₇). Quantitation demonstrated distinct enrichment patterns for the proteins not specific to each inclusion type (up to ~8-fold normalized to total mass). The proteasome, microtubules, TriC chaperones, and translational machinery were enriched in polyGA aggregates, whereas Dnaj chaperones, nuclear envelope and RNA splicing proteins were enriched in Httex1(Q₉₇) aggregates. Both structures revealed a collection of folding and degradation machinery including proteins in the Httex1(Q₉₇) aggregates that are risk factors for other neurodegenerative diseases involving protein aggregation when mutated, which suggests a convergence point in the pathomechanisms of these diseases.

Molecular Mechanisms of TDP-43 Misfolding and Pathology in Amyotrophic Lateral Sclerosis

TAR DNA binding protein 43 (TDP-43) is a versatile RNA/DNA binding protein involved in RNA-related metabolism. Hyper-phosphorylated and ubiquitinated TDP-43 deposits act as inclusion bodies in the brain and spinal cord of patients with the motor neuron diseases: amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). While the majority of ALS cases (90-95%) are sporadic (sALS), among familial ALS cases 5-10% involve the inheritance of mutations in the TARDBP gene and the remaining (90-95%) are due to mutations in other genes such as: C9ORF72, SOD1, FUS, and NEK1 etc. Strikingly however, the majority of sporadic ALS patients (up to 97%) also contain the TDP-43 protein deposited in the neuronal inclusions, which suggests of its pivotal role in the ALS pathology. Thus, unraveling the molecular mechanisms of the TDP-43 pathology seems central to the ALS therapeutics, hence, we comprehensively review the current understanding of the TDP-43's pathology in ALS. We discuss the roles of TDP-43's mutations, its cytoplasmic mis-localization and aberrant post-translational modifications in ALS. Also, we evaluate TDP-43's amyloid-like in vitro aggregation, its physiological vs. pathological oligomerization in vivo, liquid-liquid phase separation (LLPS), and potential prion-like propagation propensity of the TDP-43 inclusions. Finally, we describe the various evolving TDP-43-induced toxicity mechanisms, such as the impairment of endocytosis and mitotoxicity etc. and also discuss the emerging strategies toward TDP-43 disaggregation and ALS therapeutics.

Mechanisms of selective autophagy and mitophagy: Implications for neurodegenerative diseases

Over the past 20 years, the concept of mammalian autophagy as a nonselective degradation system has been repudiated, due in part to important discoveries in neurodegenerative diseases, which opened the field of selective autophagy. Protein aggregates and damaged mitochondria represent key pathological hallmarks shared by most neurodegenerative diseases. The landmark discovery in 2007 of p62/SQSTM1 as the first mammalian selective autophagy receptor defined a new family of autophagy-related proteins that serve to target protein aggregates, mitochondria, intracellular pathogens and other cargoes to the core autophagy machinery via an LC3-interacting region (LIR)-motif. Notably, mutations in the LIR-motif proteins p62 (SQSTM1) and optineurin (OPTN) contribute to familial forms of frontotemporal dementia and amyotrophic lateral sclerosis. Moreover, a subset of LIR-motif proteins is involved in selective mitochondrial degradation initiated by two recessive familial Parkinson's disease genes. PTEN-induced kinase 1 (PINK1) activates the E3 ubiquitin ligase Parkin (PARK2) to mark depolarized mitochondria for degradation. An extensive body of literature delineates key mechanisms in this pathway, based mostly on work in transformed cell lines. However, the potential role of PINK1-triggered mitophagy in neurodegeneration remains a conundrum, particularly in light of recent in vivo mitophagy studies. There are at least three major mechanisms by which mitochondria are targeted for mitophagy: transmembrane receptor-mediated, ubiquitin-mediated and cardiolipin-mediated. This review summarizes key features of the major cargo recognition pathways for selective autophagy and mitophagy, highlighting their potential impact in the pathogenesis or amelioration of neurodegenerative diseases.

The Genetics of C9orf72 Expansions

Repeat expansions in the promoter region of C9orf72 are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and related disorders of the ALS/frontotemporal lobar degeneration (FTLD) spectrum. Remarkable clinical heterogeneity among patients with a repeat expansion has been observed, and genetic anticipation over different generations has been suggested. Genetic factors modifying the clinical phenotype have been proposed, including genetic variation in other known disease genes, the genomic context of the C9orf72 repeat, and expanded repeat size, which has been estimated between 45 and several thousand units. The role of variability in normal and expanded repeat sizes for disease risk and clinical phenotype is under debate. Different pathogenic mechanisms have been proposed, including loss of function, RNA toxicity, and dipeptide repeat (DPR) protein toxicity resulting from abnormal translation of the expanded repeat, but the major mechanism is yet unclear.

Sense-encoded poly-GR dipeptide repeat proteins correlate to neurodegeneration and uniquely co-localize with TDP-43 in dendrites of repeat-expanded C9orf72 amyotrophic lateral sclerosis

Hexanucleotide repeat expansions in C9orf72 are the most common genetic cause of amyotrophic lateral sclerosis (C9 ALS). The main hypothesized pathogenic mechanisms are C9orf72 haploinsufficiency and/or toxicity from one or more of bi-directionally transcribed repeat RNAs and their dipeptide repeat proteins (DPRs) poly-GP, poly-GA, poly-GR, poly-PR and poly-PA. Recently, nuclear import and/or export defects especially caused by arginine-containing poly-GR or poly-PR have been proposed as significant contributors to pathogenesis based on disease models. We quantitatively studied and compared DPRs, nuclear pore proteins and C9orf72 protein in clinically related and clinically unrelated regions of the central nervous system, and compared them to phosphorylated TDP-43 (pTDP-43), the hallmark protein of ALS. Of the five DPRs, only poly-GR was significantly abundant in clinically related areas compared to unrelated areas (p < 0.001), and formed dendritic-like aggregates in the motor cortex that co-localized with pTDP-43 (p < 0.0001). While most poly-GR dendritic inclusions were pTDP-43 positive, only 4% of pTDP-43 dendritic inclusions were poly-GR positive. Staining for arginine-containing poly-GR and poly-PR in nuclei of neurons produced signals that were not specific to C9 ALS. We could not detect significant differences of nuclear markers RanGap, Lamin B1, and Importin β1 in C9 ALS, although we observed subtle nuclear changes in ALS, both C9 and non-C9, compared to control. The C9orf72 protein itself was diffusely expressed in cytoplasm of large neurons and glia, and nearly 50% reduced, in both clinically related frontal cortex and unrelated occipital cortex, but not in cerebellum. In summary, sense-encoded poly-GR DPR was unique, and localized to dendrites and pTDP43 in motor regions of C9 ALS CNS. This is consistent with new emerging ideas about TDP-43 functions in dendrites.

TDP-43 post-translational modifications in health and disease

INTRODUCTION

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

Mechanistic View of hnRNPA2 Low-Complexity Domain Structure, Interactions, and Phase Separation Altered by Mutation and Arginine Methylation

hnRNPA2, a component of RNA-processing membraneless organelles, forms inclusions when mutated in a syndrome characterized by the degeneration of neurons (bearing features of amyotrophic lateral sclerosis [ALS] and frontotemporal dementia), muscle, and bone. Here we provide a unified structural view of hnRNPA2 self-assembly, aggregation, and interaction and the distinct effects of small chemical changes-disease mutations and arginine methylation-on these assemblies. The hnRNPA2 low-complexity (LC) domain is compact and intrinsically disordered as a monomer, retaining predominant disorder in a liquid-liquid phase-separated form. Disease mutations D290V and P298L induce aggregation by enhancing and extending, respectively, the aggregation-prone region. Co-aggregating in disease inclusions, hnRNPA2 LC directly interacts with and induces phase separation of TDP-43. Conversely, arginine methylation reduces hnRNPA2 phase separation, disrupting arginine-mediated contacts. These results highlight the mechanistic role of specific LC domain interactions and modifications conserved across many hnRNP family members but altered by aggregation-causing pathological mutations.

Autophagy and Its Impact on Neurodegenerative Diseases: New Roles for TDP-43 and C9orf72

Autophagy is a catabolic mechanism where intracellular material is degraded by vesicular structures called autophagolysosomes. Autophagy is necessary to maintain the normal function of the central nervous system (CNS), avoiding the accumulation of misfolded and aggregated proteins. Consistently, impaired autophagy has been associated with the pathogenesis of various neurodegenerative diseases. The proteins TAR DNA-binding protein-43 (TDP-43), which regulates RNA processing at different levels, and chromosome 9 open reading frame 72 (C9orf72), probably involved in membrane trafficking, are crucial in the development of neurodegenerative diseases such as Amyotrophic lateral sclerosis (ALS) and Frontotemporal Lobar Degeneration (FTLD). Additionally, recent studies have identified a role for these proteins in the control of autophagy. In this manuscript, we review what is known regarding the autophagic mechanism and discuss the involvement of TDP-43 and C9orf72 in autophagy and their impact on neurodegenerative diseases.

Protein Homeostasis in Amyotrophic Lateral Sclerosis: Therapeutic Opportunities?

Protein homeostasis (proteostasis), the correct balance between production and degradation of proteins, is essential for the health and survival of cells. Proteostasis requires an intricate network of protein quality control pathways (the proteostasis network) that work to prevent protein aggregation and maintain proteome health throughout the lifespan of the cell. Collapse of proteostasis has been implicated in the etiology of a number of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), the most common adult onset motor neuron disorder. Here, we review the evidence linking dysfunctional proteostasis to the etiology of ALS and discuss how ALS-associated insults affect the proteostasis network. Finally, we discuss the potential therapeutic benefit of proteostasis network modulation in ALS.

Impaired protein degradation in FTLD and related disorders

Impaired protein degradation has been discussed as a cause or consequence of various neurodegenerative diseases, such as Alzheimer's, Parkinson's and Huntington's disease. More recently, evidence accumulated that dysfunctional protein degradation may play a role in frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Since in almost all neurodegenerative diseases, protein aggregates are disease-defining hallmarks, it is most likely that impaired protein degradation contributes to disease onset and progression. In the majority of FTD cases, the pathological protein aggregates contain either microtubuleassociated protein tau or TAR DNA-binding protein (TDP)-43. Aggregates are also positive for ubiquitin and p62/sequestosome 1 (SQSTM1) indicating that these aggregates are targeted for degradation. FTD-linked mutations in genes encoding three autophagy adaptor proteins, p62/SQSTM1, ubiquilin 2 and optineurin, indicate that impaired autophagy might cause FTD. Furthermore, the strongest evidence for lysosomal impairment in FTD is provided by the progranulin (GRN) gene, which is linked to FTD and neuronal ceroid lipofuscinosis. In this review, we summarize the observations that have been made during the last years linking the accumulation of disease-associated proteins in FTD to impaired protein degradation pathways. In addition, we take resent findings for nucleocytoplasmic transport defects of TDP-43, as discussed for hexanucleotide repeat expansions in C9orf72 into account and provide a hypothesis how the interplay of altered nuclear transport and protein degradation leads to the accumulation of protein deposits.

Structural explanation of poor prognosis of amyotrophic lateral sclerosis in the non-demented state

BACKGROUND AND PURPOSE

Amyotrophic lateral sclerosis (ALS), a motor neuron disease, is associated with various cortical symptoms including mild cognitive decline with behavior changes, suggesting the involvement of extra-motor areas in ALS. Our aim was to investigate the specific patterns of brain atrophy in sporadic, impaired ALS patients without commonly known genetic mutations using voxel-based morphometry.

MATERIALS AND METHODS

Forty-seven patients with sporadic ALS and 28 age-matched healthy controls were recruited. ALS participants were divided into three groups according to comprehensive neuropsychological testing: pure (ALS-pure), cognitive impairment (ALSci) and behavioral impairment (ALSbi). Quantitative comparison of brain atrophy patterns was performed amongst these three groups using voxel-based analysis. All analyses were adjusted for total intracranial volume, age, sex, disease duration and functional disability score.

RESULTS

The ALSci group exhibited decreased volume in the left cerebellum, fusiform gyrus, optic radiations and corticospinal tracts compared to healthy controls. ALSci patient imaging showed decreased brain volume in the bilateral cerebellum, right putamen gray matter and bilateral superior longitudinal fasciculi white matter compared to pure ALS patients (P < 0.001 uncorrected, corrected for the entire volume). Compared to healthy controls, ALS-pure and ALSbi groups did not show any significant volume changes in gray and white matter.

CONCLUSIONS

These findings also support the hypothesis that ALS pathogenesis has a dual focality of onset (cortex and anterior horn) with contiguous spread outwards. Additionally, neuropsychological features may be an important predictor of progression and survival rates in ALS.

Motor neuron degeneration in spastic paraplegia 11 mimics amyotrophic lateral sclerosis lesions

The most common form of autosomal recessive hereditary spastic paraplegia is caused by mutations in the SPG11/KIAA1840 gene on chromosome 15q. The nature of the vast majority of SPG11 mutations found to date suggests a loss-of-function mechanism of the encoded protein, spatacsin. The SPG11 phenotype is, in most cases, characterized by a progressive spasticity with neuropathy, cognitive impairment and a thin corpus callosum on brain MRI. Full neuropathological characterization has not been reported to date despite the description of >100 SPG11 mutations. We describe here the clinical and pathological features observed in two unrelated females, members of genetically ascertained SPG11 families originating from Belgium and Italy, respectively. We confirm the presence of lesions of motor tracts in medulla oblongata and spinal cord associated with other lesions of the central nervous system. Interestingly, we report for the first time pathological hallmarks of SPG11 in neurons that include intracytoplasmic granular lysosome-like structures mainly in supratentorial areas, and others in subtentorial areas that are partially reminiscent of those observed in amyotrophic lateral sclerosis, such as ubiquitin and p62 aggregates, except that they are never labelled with anti-TDP-43 or anti-cystatin C. The neuropathological overlap with amyotrophic lateral sclerosis, associated with some shared clinical manifestations, opens up new fields of investigation in the physiopathological continuum of motor neuron degeneration.

Progression of tau pathology within cholinergic nucleus basalis neurons in chronic traumatic encephalopathy: A chronic effects of neurotrauma consortium study

OBJECTIVE

To test the hypothesis that the nucleus basalis of Meynert (nbM), a cholinergic basal forebrain (CBF) cortical projection system, develops neurofibrillary tangles (NFTs) during the progressive pathological stages of chronic traumatic encephalopathy (CTE) in the brain of athletes.

METHOD

To characterize NFT pathology, tau-antibodies marking early, intermediate and late stages of NFT development in CBF tissue obtained at autopsy from eighteen former athletes and veterans with a history of repetitive mild traumatic brain injury (TBI) were used.

RESULTS

Analysis revealed that cholinergic nbM neurons develop intracellular tau-immunoreactive changes progressively across the pathological stages of CTE. In particular, there was an increase in pre-tangle (phosphorylated pS422) and oligomeric (TOC1 and TNT1) forms of tau in stage IV compared to stage II CTE cases. The nbM neurons also displayed pathologic TDP-43 inclusions and diffuse extracellular and vascular amyloid-β (Aβ) deposits in CTE. A higher percentage of pS422/p75NTR, pS422 and TNT1 labelled neurons were significantly correlated with age at symptom onset, interval between symptom onset and death and age at death.

CONCLUSION

The development of NFTs within the cholinergic nbM neurons could contribute to an axonal disconnection in CTE. Further studies are needed to determine the mechanism driving NFT formation in the nbM neurons and its relation to chronic cognitive dysfunction in CTE.

What does imaging reveal about the pathology of amyotrophic lateral sclerosis?

Amyotrophic lateral sclerosis (ALS) is now recognised to be a heterogeneous neurodegenerative syndrome of the motor system and its frontotemporal cortical connections. The development and application of structural and functional imaging over the last three decades, in particular magnetic resonance imaging (MRI), has allowed traditional post mortem histopathological and emerging molecular findings in ALS to be placed in a clinical context. Cerebral grey and white matter structural MRI changes are increasingly being understood in terms of brain connectivity, providing insights into the advancing degenerative process and producing candidate biomarkers. Such markers may refine the prognostic stratification of patients and the diagnostic pathway, as well as providing an objective assessment of changes in disease activity in response to future therapeutic agents. Studies are being extended to the spinal cord, and the application of neuroimaging to unaffected carriers of highly penetrant genetic mutations linked to the development of ALS offers a unique window to the pre-symptomatic landscape.

Drosha inclusions are new components of dipeptide-repeat protein aggregates in FTLD-TDP and ALS C9orf72 expansion cases

Supplemental digital content is available in the text.

Frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) are 2 neurodegenerative disorders that share clinical, genetic, and neuropathologic features. The presence of abnormal expansions of GGGGCC repeats (G4C2 repeats) in a noncoding region of the Chromosome 9 open reading frame 72 (C9orf72) gene is the major genetic cause of both FTLD and ALS. Transcribed G4C2 repeats can form nuclear RNA foci and recruit RNA-binding proteins, thereby inhibiting their normal function. Moreover, through a repeat-associated non-ATG translation mechanism, G4C2 repeats translation leads to dipeptide-repeat protein aggregation in the cytoplasm of neurons. Here, we identify Drosha protein as a new component of these dipeptide-repeat aggregates. In C9orf72 mutation cases of FTLD-TDP (c9FTLD-TDP) and ALS (c9ALS), but not in FTLD or ALS cases without C9orf72 mutation, Drosha is mislocalized to form neuronal cytoplasmic inclusions in the hippocampus, frontal cortex, and cerebellum. Further characterization of Drosha-positive neuronal cytoplasmic inclusions in the hippocampus, frontal cortex, and cerebellum revealed colocalization with p62 and ubiquilin-2, 2 pathognomonic signatures of c9FTLD-TDP and c9ALS cases; however, Drosha inclusions rarely colocalized with TDP-43 pathology. We conclude that Drosha may play a unique pathogenic role in the onset or progression of FTLD-TDP/ALS in patients with the C9orf72 mutation.

Patterns of microglial cell activation in frontotemporal lobar degeneration

AIMS

Pathological heterogeneity within patients with frontotemporal lobar degeneration (FTLD) in general precludes the accurate assignment of diagnostic subtype in life. The aim of this study was to assess the extent of microglial cell activation in FTLD in order to determine whether it might be possible to employ this as a diagnostic marker in vivo using PET ligand [11C](R)-PK11195 in order to differentiate cases of FTLD according to histological subtype.

METHODS

The distribution and extent of microglial cell activation was assessed semi-quantitatively in cortical grey and subcortical white matter of CD68 immunostained sections of frontal and temporal cortex from 78 pathologically confirmed cases of FTLD, 13 of Alzheimer's disease (AD) and 13 controls.

RESULTS

Significantly higher levels of microglial cell activation than controls occurred in all four regions in FTLD, and in three of the four regions in AD. Microglial activation was greater in frontal subcortical white matter in FTLD than AD, whereas it was higher in temporal cortical grey matter in AD than FTLD. Microglial cell activation was significantly higher in temporal subcortical white matter in FTLD-MAPT than in other genetic (GRN, C9ORF72) or non-genetic forms of FTLD.

CONCLUSIONS

The present study suggests that high levels of microglial cell involvement in temporal lobe (subcortical white matter) might serve as a marker of inherited FTLD associated with intronic mutations in MAPT, with a relatively intense signal in this region in PET studies using [11C](R)-PK11195 as microglial cell marker could indicate the presence of MAPT mutation in vivo.

Analysis of β-N-methylamino-L-alanine (L-BMAA) neurotoxicity in rat cerebellum

Due to its structural similarity to glutamate, L-BMAA could be a trigger for neurodegenerative disorders caused by changes in the intracellular medium, such as increased oxidative stress, mitochondrial dysfunction, impaired synthesis and protein degradation and the imbalance of some enzymes. It is also important to note that according to some published studies, L-BMAA will be incorporated into proteins, causing the alteration of protein homeostasis. Neuronal cells are particularly prone to suffer damage in protein folding and protein accumulation because they have not performed cellular division. In this work, we will analyse the cerebellum impairment triggered by L-BMAA in treated rats. The cerebellum is one of the most important subcortical motor centres and ensures that movements are performed with spatial and temporal precision. Cerebellum damage caused by L-BMAA can contribute to motor impairment. To characterize this neurodegenerative pathology, we first carried out ultrastructure analysis in Purkinje cells showing altered mitochondria, endoplasmic reticulum (ER), and Golgi apparatus (GA). We then performed biochemical assays of GSK3 and TDP-43 in cerebellum, obtaining an increase of both biomarkers with L-BMAA treatment and, finally, performed autophagy studies that revealed a higher level of these processes after treatment. This work provides evidence of cerebellar damage in rats after treatment with L-BMAA. Three months after treatment, affected rats cannot restore the normal functions of the cerebellum regarding motor coordination and postural control.

The Spectrum of C9orf72-mediated Neurodegeneration and Amyotrophic Lateral Sclerosis

The discovery that a hexanucleotide repeat expansion in C9orf72 is the most numerous genetic variant of both amyotrophic lateral sclerosis and frontotemporal dementia has opened a rapidly growing field, which may provide long hoped for advances in the understanding and treatment of these devastating diseases. In this review we describe the various phenotypes, clinical and pathological, associated with expansion of C9orf72, which go beyond amyotrophic lateral sclerosis and frontotemporal dementia to include neurodegeneration more broadly. Next we take a step back and summarize the current understanding of the C9orf72 expansion and its protein products at a molecular level. Three mechanisms are prominent: toxicity mediated directly by RNA transcribed from the repeat; toxicity mediated by dipeptide repeat proteins translated from the repeat sequence; and haploinsufficiency resulting from reduced transcription of the C9orf72 exonic sequence. A series of exciting advances have recently described how dipeptide repeat proteins might interfere with the normal role of the nucleolus in maturation of RNA binding proteins and in production of ribosomes. Importantly, these mechanisms are unlikely to be mutually exclusive. We draw attention to the fact that clinical and pathological similarities to other genetic variants without a repeat expansion must not be overlooked in ascribing a pathogenic mechanism to C9orf72-disease. Finally, with a view to impact on patient care, we discuss current practice with respect to genetic screening in patients with and without a family history of disease, and the most promising developments towards therapy that have been reported to date.

Brain distribution of dipeptide repeat proteins in frontotemporal lobar degeneration and motor neurone disease associated with expansions in C9ORF72

A hexanucleotide (GGGGCC) expansion in C9ORF72 gene is the most common genetic change seen in familial Frontotemporal Lobar Degeneration (FTLD) and familial Motor Neurone Disease (MND). Pathologically, expansion bearers show characteristic p62 positive, TDP-43 negative inclusion bodies within cerebellar and hippocampal neurons which also contain dipeptide repeat proteins (DPR) formed from sense and antisense RAN (repeat associated non ATG-initiated) translation of the expanded repeat region itself. 'Inappropriate' formation, and aggregation, of DPR might therefore confer neurotoxicity and influence clinical phenotype. Consequently, we compared the topographic brain distribution of DPR in 8 patients with Frontotemporal dementia (FTD), 6 with FTD + MND and 7 with MND alone (all 21 patients bearing expansions in C9ORF72) using a polyclonal antibody to poly-GA, and related this to the extent of TDP-43 pathology in key regions of cerebral cortex and hippocampus. There were no significant differences in either the pattern or severity of brain distribution of DPR between FTD, FTD + MND and MND groups, nor was there any relationship between the distribution of DPR and TDP-43 pathologies in expansion bearers. Likewise, there were no significant differences in the extent of TDP-43 pathology between FTLD patients bearing an expansion in C9ORF72 and non-bearers of the expansion. There were no association between the extent of DPR pathology and TMEM106B or APOE genotypes. However, there was a negative correlation between the extent of DPR pathology and age at onset. Present findings therefore suggest that although the presence and topographic distribution of DPR may be of diagnostic relevance in patients bearing expansion in C9ORF72 this has no bearing on the determination of clinical phenotype. Because TDP-43 pathologies are similar in bearers and non-bearers of the expansion, the expansion may act as a major genetic risk factor for FTLD and MND by rendering the brain highly vulnerable to those very same factors which generate FTLD and MND in sporadic disease.

Pathological mechanisms underlying TDP-43 driven neurodegeneration in FTLD-ALS spectrum disorders

Aggregation of misfolded TAR DNA-binding protein 43 (TDP-43) is a striking hallmark of neurodegenerative processes that are observed in several neurological disorders, and in particular in most patients diagnosed with frontotemporal lobar degeneration (FTLD) or amyotrophic lateral sclerosis (ALS). A direct causal link with TDP-43 brain proteinopathy was provided by the identification of pathogenic mutations in TARDBP, the gene encoding TDP-43, in ALS families. However, TDP-43 proteinopathy has also been observed in carriers of mutations in several other genes associated with both ALS and FTLD demonstrating a key role for TDP-43 in neurodegeneration. To date, and despite substantial research into the biology of TDP-43, its functioning in normal brain and in neurodegeneration processes remains largely elusive. Nonetheless, breakthroughs using cellular and animal models have provided valuable insights into ALS and FTLD pathogenesis. Accumulating evidence has redirected the research focus towards a major role for impaired RNA metabolism and protein homeostasis. At the same time, the concept that toxic TDP-43 protein aggregates promote neurodegeneration is losing its credibility. This review aims at highlighting and discussing the current knowledge on TDP-43 driven pathomechanisms leading to neurodegeneration as observed in TDP-43 proteinopathies. Based on the complexity of the associated neurological diseases, a clear understanding of the essential pathological modifications will be crucial for further therapeutic interventions.

Dipeptide repeat proteins are present in the p62 positive inclusions in patients with frontotemporal lobar degeneration and motor neurone disease associated with expansions in C9ORF72

BACKGROUND

Cases of Frontotemporal Lobar Degeneration (FTLD) and Motor Neurone Disease (MND) associated with expansions in C9ORF72 gene are characterised pathologically by the presence of TDP-43 negative, but p62 positive, inclusions in granule cells of the cerebellum and in cells of dentate gyrus and area CA4 of the hippocampus.

RESULTS

We screened 84 cases of pathologically confirmed FTLD and 23 cases of MND for the presence of p62 positive inclusions in these three brain regions, and identified 13 positive cases of FTLD and 3 of MND. All cases demonstrated expansions in C9ORF72 by Southern blotting where frozen tissues were available. The p62 positive inclusions in both cerebellum and hippocampus were immunostained by antibodies to dipeptide repeat proteins (DPR), poly Gly-Ala (poly-GA), poly Gly-Pro (poly-GP) and poly Gly-Arg (poly-GR), these arising from a putative non-ATG initiated (RAN) sense translation of the GGGGCC expansion. There was also some slight, but variable, immunostaining with poly-AP antibody implying some antisense translation might also occur, though the relative paucity of immunostaining could reflect poor antigen avidity on the part of the antisense antibodies. Of the FTLD cases with DPR, 6 showed TDP-43 type A and 6 had TDP-43 type B histology; one had FTLD-tau with the pathology of corticobasal degeneration. There were no qualitative or quantitative differences in the pattern of immunostaining with antibodies to DPR, or p62, proteins between TDP-43 type A and type B cases. Ratings for frequency of inclusions immunostained by these poly-GA, poly-GP and poly-GR antibodies broadly correlated with those for immunolabelled by p62 antibody in all three regions.

CONCLUSION

We conclude that DPR are a major component of p62 positive inclusions in FTLD and MND.

Sumoylation of critical proteins in amyotrophic lateral sclerosis: emerging pathways of pathogenesis

Emerging lines of evidence suggest a relationship between amyotrophic lateral sclerosis (ALS) and protein sumoylation. Multiple studies have demonstrated that several of the proteins involved in the pathogenesis of ALS, including superoxide dismutase 1, fused in liposarcoma, and TAR DNA-binding protein 43 (TDP-43), are substrates for sumoylation. Additionally, recent studies in cellular and animal models of ALS revealed that sumoylation of these proteins impact their localization, longevity, and how they functionally perform in disease, providing novel areas for mechanistic investigations and therapeutics. In this article, we summarize the current literature examining the impact of sumoylation of critical proteins involved in ALS and discuss the potential impact for the pathogenesis of the disease. In addition, we report and discuss the implications of new evidence demonstrating that sumoylation of a fragment derived from the proteolytic cleavage of the astroglial glutamate transporter, EAAT2, plays a direct role in downregulating the expression levels of full-length EAAT2 by binding to a regulatory region of its promoter.

Pathological assessments for the presence of hexanucleotide repeat expansions in C9ORF72 in Alzheimer's disease

Background

We have sought histological evidence, using TDP-43 and p62 immunohistochemistry, for the presence of expansions in C9ORF72 among 200 patients with pathologically confirmed AD.

Results

We noted TDP-43 pathological changes in hippocampus and temporal cortex in 45 (22.5%) of these patients, but did not detect any cases where p62 positive changes in hippocampus and cerebellum, considered pathognomic for C9ORF72 expansions, were present.

Conclusion

We conclude that expansions in C9ORF72 associated with AD are a rare occurrence, and in those instances in the literature where these have been reported, the presence of AD may in fact be coincidental and unrelated to the expansion.

p62/SQSTM1 differentially removes the toxic mutant androgen receptor via autophagy and inclusion formation in a spinal and bulbar muscular atrophy mouse model

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders that are caused by the expansion of trinucleotide CAG repeats in the causative genes. Spinal and bulbar muscular atrophy (SBMA) is an inherited motor neuron disease that is caused by the expansion of a polyQ tract within the androgen receptor (AR). p62 is a ubiquitin- and light-chain 3-binding protein that is known to regulate the degradation of targeted proteins via autophagy and inclusion formation. In this study, we examined the effects of p62 depletion and overexpression on cultured cells and in a transgenic mouse model that overexpressed the mutant AR. Here, we demonstrate that depletion of p62 significantly exacerbated motor phenotypes and the neuropathological outcome, whereas overexpression of p62 protected against mutant AR toxicity in SBMA mice. Depletion of p62 significantly increased the levels of monomeric mutant AR and mutant AR protein complexes in an SBMA mouse model via the impairment of autophagic degradation. In addition, p62 overexpression improved SBMA mouse phenotypes by inducing cytoprotective inclusion formation. Our results demonstrate that p62 provides two different therapeutic targets in SBMA pathogenesis: (1) autophagy-dependent degradation and (2) benevolent inclusion formation of the mutant AR.

Mixed tau, TDP-43 and p62 pathology in FTLD associated with a C9ORF72 repeat expansion and p.Ala239Thr MAPT (tau) variant

A massive intronic GGGGCC hexanucleotide repeat expansion in C9ORF72 has recently been identified as the most common cause of familial and sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). We have previously demonstrated that C9ORF72 mutant cases have a specific pathological profile with abundant p62-positive, TDP-43-negative cytoplasmic and intranuclear inclusions within cerebellar granular cells of the cerebellum and pyramidal cells of the hippocampus in addition to classical TDP-43 pathology. Here, we report mixed tau and TDP-43 pathology in a woman with behavioural variant FTLD who had the C9ORF72 mutation, and the p.Ala239Thr variant in MAPT (microtubule associated protein tau) gene not previously associated with tau pathology. Two of her brothers, who carried the C9ORF72 mutation, but not the MAPT variant, developed classical ALS without symptomatic cognitive changes. The dominant neuropathology in this woman with FTLD was a tauopathy with Pick's disease-like features. TDP-43 labelling was mainly confined to Pick bodies, but p62-positive, TDP-43-negative inclusions, characteristic of C9ORF72 mutations, were present in the cerebellum and hippocampus. Mixed pathology to this degree is unusual. One might speculate that the presence of the C9ORF72 mutation might influence tau deposition in what was previously thought to be a "benign" variant in MAPT in addition to the aggregation of TDP-43 and other as yet unidentified proteins decorated with ubiquitin and p62.

The genetics and neuropathology of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of motor neurons leading to death from respiratory failure within about 3 years of symptom onset. A family history of ALS is obtained in about 5 % but the distinction between familial and apparently sporadic ALS is artificial and genetic factors play a role in all types. For several years, only one gene was known to have a role in ALS pathogenesis, SOD1. In the last few years there has been a rapid advance in our genetic knowledge of the causes of ALS, and the relationship of the genetic subtypes with pathological subtypes and clinical phenotype. Mutations in the gene for TDP-43 protein, TARDBP, highlight this, with pathology mimicking closely that found in other types of ALS, and a phenotypic spectrum that includes frontotemporal dementia. Mutations in the FUS gene, closely related to TDP-43, lead to a similar clinical phenotype but distinct pathology, so that the three pathological groups represented by SOD1, TARDBP, and FUS are distinct. In this review, we explore the genetic architecture of ALS, highlight some of the genes implicated in pathogenesis, and describe their phenotypic range and overlap with other diseases.

Oligoclonal bands in the cerebrospinal fluid of amyotrophic lateral sclerosis patients with disease-associated mutations

In amyotrophic lateral sclerosis (ALS) cerebrospinal fluid (CSF) analysis is usually performed to exclude inflammatory processes of the central nervous system. Although in a small subset of patients an intrathecal synthesis of IgG is detectable, usually there is no clear explanation for this evidence. This study investigates the occurrence of oligoclonal bands (OCBs) in the CSF of a large series of ALS patients, attempting a correlation with genotype data. CSF was collected from 259 ALS patients. CSF parameters were measured according to standard procedures, and detection of OCBs performed by isoelectric focusing. The patients were screened for mutations in SOD1, FUS, TARDBP, ANG, OPTN, and C9ORF72. We observed the presence of OCBs in the CSF of 9/259 ALS patients (3.5 %), and of disease-associated mutations in 12 cases. OCBs were significantly more frequent in mutation carriers compared to the remaining cohort (3/12 vs 6/247; p < 0.01). Among patients with OCBs, two patients had the TARDBP p.A382T mutation (one of which in homozygous state), and one the ANG p.P-4S variant. Both patients carrying the p.A382T mutation had an atypical phenotype, one of them manifesting signs suggestive of a cerebellar involvement, and the other presenting neuroradiological findings suggestive of an inflammatory disorder of the central nervous system. Our results suggest that ALS patients with OCBs may harbor mutations in disease-causing genes. We speculate that mutations in both TARDBP and ANG genes may disrupt the blood-brain barrier (BBB), promoting local immune responses and neuroinflammation. The role of mutant TARDBP and ANG genes on BBB integrity of ALS patients warrants further investigation.