Distinct TDP-43 pathology in ALS patients with ataxin 2 intermediate-length polyQ expansions

Parallel Appearance of Polyglutamine and Transactivation-Responsive DNA-Binding Protein 43 and Their Complementary Subcellular Localization in Brains of Patients With Spinocerebellar Ataxia Type 2

Spinocerebellar ataxia type 2 (SCA2) is caused by mutations in the ATXN2 gene in which toxic effects are triggered by expanded polyglutamine repeats within ataxin-2. SCA2 is accompanied by motor neuron degeneration as occurs in amyotrophic lateral sclerosis (ALS). We investigated the distribution patterns of ataxin-2 and transactivation-responsive DNA-binding protein 43 (TDP-43), a major disease-related protein in ALS, in the CNS of 3 SCA2 patients. Phosphorylated TDP-43 (pTDP-43)-positive lesions were widely distributed throughout the CNS and generally overlapped with 1C2 (expanded polyglutamine)-immunoreactive lesions. This distribution pattern is different from the pattern in limbic-predominant age-related TDP-43 encephalopathy. In SCA2, double immunostaining of TDP-43 and 1C2 in motor neurons revealed 3 staining patterns: cytoplasmic 1C2 and nuclear TDP-43, nucleocytoplasmic 1C2 and nuclear TDP-43, and nuclear 1C2 and cytoplasmic TDP-43, which reflect the early, active, and final stages of pathological change, respectively. The translocation of TDP-43 from the nucleus to the cytoplasm along with the translocation of 1C2 in the opposite direction indicates that nuclear accumulation of the disease-specific protein ataxin-2 affects the intracellular dynamics of TDP-43. Such a close interrelationship between mutant ataxin-2 and TDP-43 in the cell might account for the similarity of their distribution in the CNS of patients with SCA2.

A C-terminal ataxin-2 disordered region promotes Huntingtin protein aggregation and neurodegeneration in Drosophila models of Huntington’s disease

The Ataxin-2 (Atx2) protein contributes to the progression of neurodegenerative phenotypes in animal models of amyotrophic lateral sclerosis (ALS), type 2 spinocerebellar ataxia (SCA-2), Parkinson’s disease, and Huntington’s disease (HD). However, because the Atx2 protein contains multiple separable activities, deeper understanding requires experiments to address the exact mechanisms by which Atx2 modulates neurodegeneration (ND) progression. Recent work on two ALS models, C9ORF72 and FUS, in Drosophila has shown that a C-terminal intrinsically disordered region (cIDR) of Atx2 protein, required for assembly of ribonucleoprotein (RNP) granules, is essential for the progression of neurodegenerative phenotypes as well as for accumulation of protein inclusions associated with these ALS models. Here, we show that the Atx2-cIDR also similarly contributes to the progression of degenerative phenotypes and accumulation of Huntingtin protein aggregates in Drosophila models of HD. Because Huntingtin is not an established component of RNP granules, these observations support a recently hypothesized, unexpected protein-handling function for RNP granules, which could contribute to the progression of Huntington’s disease and, potentially, other proteinopathies.

Toxicity of pathogenic ataxin-2 in Drosophila shows dependence on a pure CAG repeat sequence

Spinocerebellar ataxia type 2 is a polyglutamine (polyQ) disease associated with an expanded polyQ domain within the protein product of the ATXN2 gene. Interestingly, polyQ repeat expansions in ATXN2 are also associated with amyotrophic lateral sclerosis (ALS) and parkinsonism depending upon the length of the polyQ repeat expansion. The sequence encoding the polyQ repeat also varies with disease presentation: a pure CAG repeat is associated with SCA2, whereas the CAG repeat in ALS and parkinsonism is typically interrupted with the glutamine encoding CAA codon. Here, we asked if the purity of the CAG sequence encoding the polyQ repeat in ATXN2 could impact the toxicity of the ataxin-2 protein in vivo in Drosophila. We found that ataxin-2 encoded by a pure CAG repeat conferred toxicity in the retina and nervous system, whereas ataxin-2 encoded by a CAA-interrupted repeat or CAA-only repeat failed to confer toxicity, despite expression of the protein at similar levels. Furthermore, the CAG-encoded ataxin-2 protein aggregated in the fly eye, while ataxin-2 encoded by either a CAA/G or CAA repeat remained diffuse. The toxicity of the CAG-encoded ataxin-2 protein was also sensitive to the translation factor eIF4H, a known modifier of the toxic GGGGCC repeat in flies. These data indicate that ataxin-2 encoded by a pure CAG versus interrupted CAA/G polyQ repeat domain is associated with differential toxicity, indicating that mechanisms associated with the purity of the sequence of the polyQ domain contribute to disease.

Phenotypic variability in amyotrophic lateral sclerosis

Clinically, ALS phenotypes depend on the areas of the body that are affected, the different degrees of involvement of upper and lower motor neurons, the degrees of involvement of other systems, particularly cognition and behavior, and rates of progression. Phenotypic variability of ALS is characteristic and can be declined on the distribution of motor manifestations but also on the presence of extra-motor signs present in a variable manner in ALS patients. Neuropathologically, ALS is defined by the loss of UMN and LMN and the presence of two representative motor neuronal cytoplasmic inclusions, Bunina bodies and 43kDa Transactivation Response DNA Binding Protein (TDP-43) - positive cytoplasmic inclusions. The distribution of cytopathology and neuronal loss in patients is variable and this variability is directly related to phenotypic variability. Key regulators of phenotypic variability in ALS have not been determined. The functional decrement of TDP-43, and region-specific neuronal susceptibility to ALS, may be involved. Due to the selective vulnerability among different neuronal systems, lesions are multicentric, region-oriented, and progress at different rates. They may vary from patient to patient, which may be linked to the clinicopathological variability across patients.

Simultaneous ALS and SCA2 associated with an intermediate-length ATXN2 CAG-repeat expansion

Spinocerebellar ataxia type 2 (SCA2) and amyotrophic lateral sclerosis (ALS) share a common molecular basis: both are associated with CAG-repeat expansion of ATXN2 and TDP-43-positive neuronal cytoplasmic inclusions. To date, the two disorders are viewed as clinically distinct with ALS resulting from 30-33 CAG-repeats and SCA2 from >34 CAG-repeats. We describe a 67-year old with a 32 CAG-repeat expansion of ATXN2 who presented with simultaneous symptoms of ALS and SCA2. Our case demonstrates that the clinical dichotomy between SCA2 and ATXN2-ALS is false. We suggest instead that CAG-repeat expansion length determines the timing of SCA2 clinical symptoms relative to onset of ALS; consistent with this age of onset of SCA2 but not ATXN2-ALS, is dependent upon expansion length. Review of the literature and our local cohort provides evidence for occurrence of ALS in late stage SCA2, which may be under-recognised by clinicians who think of the two diseases as distinct.

Mouse Ataxin-2 Expansion Downregulates CamKII and Other Calcium Signaling Factors, Impairing Granule-Purkinje Neuron Synaptic Strength

Spinocerebellar ataxia type 2 (SCA2) is caused by polyglutamine expansion in Ataxin-2 (ATXN2). This factor binds RNA/proteins to modify metabolism after stress, and to control calcium (Ca2+) homeostasis after stimuli. Cerebellar ataxias and corticospinal motor neuron degeneration are determined by gain/loss in ATXN2 function, so we aimed to identify key molecules in this atrophic process, as potential disease progression markers. Our Atxn2-CAG100-Knock-In mouse faithfully models features observed in patients at pre-onset, early and terminal stages. Here, its cerebellar global RNA profiling revealed downregulation of signaling cascades to precede motor deficits. Validation work at mRNA/protein level defined alterations that were independent of constant physiological ATXN2 functions, but specific for RNA/aggregation toxicity, and progressive across the short lifespan. The earliest changes were detected at three months among Ca2+ channels/transporters (Itpr1, Ryr3, Atp2a2, Atp2a3, Trpc3), IP3 metabolism (Plcg1, Inpp5a, Itpka), and Ca2+-Calmodulin dependent kinases (Camk2a, Camk4). CaMKIV-Sam68 control over alternative splicing of Nrxn1, an adhesion component of glutamatergic synapses between granule and Purkinje neurons, was found to be affected. Systematic screening of pre/post-synapse components, with dendrite morphology assessment, suggested early impairment of CamKIIα abundance together with the weakening of parallel fiber connectivity. These data reveal molecular changes due to ATXN2 pathology, primarily impacting excitability and communication.

Diagnosis of idiopathic amyotrophic lateral sclerosis using Fourier-transform infrared spectroscopic analysis of patient-derived skin

One of the great challenges in identifying effective therapy in many neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), is the lack of reliable biomarkers. In this study, we applied infrared imaging microspectroscopy, a valuable technique to investigate biomolecule fingerprints and secondary structure of proteins within biological tissue. We hypothesized that, since skin and CNS have the same embryonic origin, spectral differences associated with ALS-specific pathological events will be readily detectable through skin testing using this technique. Cells from healthy individuals and ALS patients were isolated from skin biopsies in order to generate tissue-engineered in vitro skin (TES). Infrared spectra of the generated TES were recorded using a focal-plane-array Fourier transform infrared (FPA-FTIR) spectrometer, and hierarchical cluster analysis of the spectral data was performed in order to establish clear differences between the tested TES specimens. Interestingly, our analyses showed that it was readily possible to discriminate ALS- and control-TES solely based on differences in associated FTIR spectra, mainly located between 1149 and 1473 cm^-1, attributed to disruption of phospholipid cell membranes, extracellular matrix remodeling or cholesterol accumulation. Spectral differences within the TES samples may therefore be associated with disease state, paving the way for the identification of biomarkers in ALS.

ATXN1 repeat expansions confer risk for amyotrophic lateral sclerosis and contribute to TDP-43 mislocalization

Increasingly, repeat expansions are being identified as part of the complex genetic architecture of amyotrophic lateral sclerosis. To date, several repeat expansions have been genetically associated with the disease: intronic repeat expansions in C9orf72, polyglutamine expansions in ATXN2 and polyalanine expansions in NIPA1. Together with previously published data, the identification of an amyotrophic lateral sclerosis patient with a family history of spinocerebellar ataxia type 1, caused by polyglutamine expansions in ATXN1, suggested a similar disease association for the repeat expansion in ATXN1. We, therefore, performed a large-scale international study in 11 700 individuals, in which we showed a significant association between intermediate ATXN1 repeat expansions and amyotrophic lateral sclerosis (P = 3.33 × 10^-7). Subsequent functional experiments have shown that ATXN1 reduces the nucleocytoplasmic ratio of TDP-43 and enhances amyotrophic lateral sclerosis phenotypes in Drosophila, further emphasizing the role of polyglutamine repeat expansions in the pathophysiology of amyotrophic lateral sclerosis.

Phenotypic variability and its pathological basis in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by its inherent clinicopathological variability. The concurrence of upper and lower motor neuron signs is a common feature in the majority of patients with ALS. However, some patients manifest an atypical clinical course, with only upper or lower motor neuron signs, or various extra-motor symptoms including cognitive dysfunction, parkinsonism, autonomic dysfunction, or ophthalmoparesis. This variability indicates different manifestations of ALS and is reflected by ALS pathology spreading into the central nervous system. The presence of cytoplasmic inclusions positive for transactivation response DNA-binding protein 43 kDa (TDP-43) is a key feature in ALS. Loss of TDP-43 from the nucleus and its subsequent aggregation in the cytoplasm may occur in susceptible regions and may be associated with neuronal loss. However, in some regions, there is no apparent neuronal loss while TDP-43 accumulation is evident; in contrast, in other regions, neuronal loss is apparent without any evidence of TDP-43 accumulation. Therefore, in addition to TDP-43 dysfunction, underlying region-specific cellular vulnerability may exist in the upper and lower motor neurons and frontotemporal system in patients with ALS. The microscopic discrepancy and selective vulnerability may be linked to the macroscopic propensities of the sites of onset, and may also determine the direction and rate of progression of the lesions. Thus, there may be multicentric sites of onset, region-oriented disease development, and different speeds of disease progression across patients with ALS. ALS lesions occur in motor-related areas but may spread to neighboring areas. However, since lesions may spread in a discontinuous manner, and the dynamics of disease propagation have not been able to be identified, it remains controversial whether the stepwise appearance of TDP-43-positive inclusions is based on direct cell-to-cell protein propagation. Further understanding of the phenotypic variability of ALS and its pathological basis may serve as a guide for investigating the underlying pathogenesis of ALS.

TDP-43 levels in the brain tissue of ALS cases with and without C9ORF72 or ATXN2 gene expansions

OBJECTIVE

To assess the amount of phosphorylated and nonphosphorylated TAR DNA-binding protein 43 (TDP-43) in the motor brain regions of cases of amyotrophic lateral sclerosis (ALS) with and without repeat expansions in the ATXN2 or C9ORF72 genes.

METHODS

The 45-kDa phosphorylated form of TDP-43 and 43-kDa nonphosphorylated form of TDP-43 were quantified by immunoblot in postmortem brain tissue from the motor cortex, spinal cord, and cerebellar vermis of 23 cases with ALS with repeat expansions in the ATXN2 or C9ORF72 genes and sporadic disease and 10 controls.

RESULTS

Significantly greater levels of phosphorylated TDP-43 were identified in the motor cortex of cases with ALS with C9ORF72 expansions, and significantly greater amounts of phosphorylated TDP-43 were found in the spinal cord of cases with ALS with intermediate ATXN2 expansions. In contrast, however, similar levels of nonphosphorylated TDP-43 were found in all 3 regions between ALS groups.

CONCLUSION

Despite its central role in the pathogenesis of ALS and the emergence of potential targets to modify its aggregation, TDP-43 levels have not been quantified in pathologically confirmed cases with ALS. The present results demonstrating significant differences in phosphorylated but not nonphosphorylated TDP-43 levels suggest that different posttranslational modifications are involved in the generation of greater pathologic TDP-43 levels identified here in our cohort of cases with genetic expansions. These findings are consistent with emerging studies implicating distinct pathomechanisms in the generation of pathologic TDP-43 in cases with ALS with C9ORF72 or ATXN2 expansions and are of relevance to therapeutic research aimed at reducing pathologic TDP-43 in all or a subset of patients with ALS.

Misfolded SOD1 pathology in sporadic Amyotrophic Lateral Sclerosis

Aggregation of mutant superoxide dismutase 1 (SOD1) is a pathological hallmark of a subset of familial ALS patients. However, the possible role of misfolded wild type SOD1 in human ALS is highly debated. To ascertain whether or not misfolded SOD1 is a common pathological feature in non-SOD1 ALS, we performed a blinded histological and biochemical analysis of post mortem brain and spinal cord tissues from 19 sporadic ALS, compared with a SOD1 A4V patient as well as Alzheimer’s disease (AD) and non-neurological controls. Multiple conformation- or misfolded-specific antibodies for human SOD1 were compared. These were generated independently by different research groups and were compared using standardized conditions. Five different misSOD1 staining patterns were found consistently in tissue sections from SALS cases and the SOD1 A4V patient, but were essentially absent in AD and non-neurological controls. We have established clear experimental protocols and provide specific guidelines for working, with conformational/misfolded SOD1-specific antibodies. Adherence to these guidelines will aid in the comparison of the results of future studies and better interpretation of staining patterns. This blinded, standardized and unbiased approach provides further support for a possible pathological role of misSOD1 in SALS.

RNP-Granule Assembly via Ataxin-2 Disordered Domains Is Required for Long-Term Memory and Neurodegeneration

Human Ataxin-2 is implicated in the cause and progression of amyotrophic lateral sclerosis (ALS) and type 2 spinocerebellar ataxia (SCA-2). In Drosophila, a conserved atx2 gene is essential for animal survival as well as for normal RNP-granule assembly, translational control, and long-term habituation. Like its human homolog, Drosophila Ataxin-2 (Atx2) contains polyQ repeats and additional intrinsically disordered regions (IDRs). We demonstrate that Atx2 IDRs, which are capable of mediating liquid-liquid phase transitions in vitro, are essential for efficient formation of neuronal mRNP assemblies in vivo. Remarkably, ΔIDR mutants that lack neuronal RNP granules show normal animal development, survival, and fertility. However, they show defects in long-term memory formation/consolidation as well as in C9ORF72 dipeptide repeat or FUS-induced neurodegeneration. Together, our findings demonstrate (1) that higher-order mRNP assemblies contribute to long-term neuronal plasticity and memory, and (2) that a targeted reduction in RNP-granule formation efficiency can alleviate specific forms of neurodegeneration.

The roles of intrinsic disorder-based liquid-liquid phase transitions in the "Dr. Jekyll-Mr. Hyde" behavior of proteins involved in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Pathological developments leading to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are associated with misbehavior of several key proteins, such as SOD1 (superoxide dismutase 1), TARDBP/TDP-43, FUS, C9orf72, and dipeptide repeat proteins generated as a result of the translation of the intronic hexanucleotide expansions in the C9orf72 gene, PFN1 (profilin 1), GLE1 (GLE1, RNA export mediator), PURA (purine rich element binding protein A), FLCN (folliculin), RBM45 (RNA binding motif protein 45), SS18L1/CREST, HNRNPA1 (heterogeneous nuclear ribonucleoprotein A1), HNRNPA2B1 (heterogeneous nuclear ribonucleoprotein A2/B1), ATXN2 (ataxin 2), MAPT (microtubule associated protein tau), and TIA1 (TIA1 cytotoxic granule associated RNA binding protein). Although these proteins are structurally and functionally different and have rather different pathological functions, they all possess some levels of intrinsic disorder and are either directly engaged in or are at least related to the physiological liquid-liquid phase transitions (LLPTs) leading to the formation of various proteinaceous membrane-less organelles (PMLOs), both normal and pathological. This review describes the normal and pathological functions of these ALS- and FTLD-related proteins, describes their major structural properties, glances at their intrinsic disorder status, and analyzes the involvement of these proteins in the formation of normal and pathological PMLOs, with the ultimate goal of better understanding the roles of LLPTs and intrinsic disorder in the "Dr. Jekyll-Mr. Hyde" behavior of those proteins.

Biology and Pathobiology of TDP-43 and Emergent Therapeutic Strategies

Cytoplasmic TDP-43 mislocalization and aggregation is a pathological hallmark of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. TDP-43 is an RNA-binding protein (RBP) with a prion-like domain (PrLD) that promotes TDP-43 misfolding. PrLDs possess compositional similarity to canonical prion domains of various yeast proteins, including Sup35. Strikingly, disease-causing TDP-43 mutations reside almost exclusively in the PrLD and can enhance TDP-43 misfolding and toxicity. Another ∼70 human RBPs harbor PrLDs, including FUS, TAF15, EWSR1, hnRNPA1, and hnRNPA2, which have surfaced in the etiology of neurodegenerative diseases. Importantly, PrLDs enable RBP function and mediate phase transitions that partition functional ribonucleoprotein compartments. This PrLD activity, however, renders RBPs prone to populating deleterious oligomers or self-templating fibrils that might spread disease, and disease-linked PrLD mutations can exacerbate this risk. Several strategies have emerged to counter TDP-43 proteinopathies, including engineering enhanced protein disaggregases based on Hsp104.

Engineering enhanced protein disaggregases for neurodegenerative disease

Protein misfolding and aggregation underpin several fatal neurodegenerative diseases, including Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and frontotemporal dementia (FTD). There are no treatments that directly antagonize the protein-misfolding events that cause these disorders. Agents that reverse protein misfolding and restore proteins to native form and function could simultaneously eliminate any deleterious loss-of-function or toxic gain-of-function caused by misfolded conformers. Moreover, a disruptive technology of this nature would eliminate self-templating conformers that spread pathology and catalyze formation of toxic, soluble oligomers. Here, we highlight our efforts to engineer Hsp104, a protein disaggregase from yeast, to more effectively disaggregate misfolded proteins connected with PD, ALS, and FTD. Remarkably subtle modifications of Hsp104 primary sequence yielded large gains in protective activity against deleterious α-synuclein, TDP-43, FUS, and TAF15 misfolding. Unusually, in many cases loss of amino acid identity at select positions in Hsp104 rather than specific mutation conferred a robust therapeutic gain-of-function. Nevertheless, the misfolding and toxicity of EWSR1, an RNA-binding protein with a prion-like domain linked to ALS and FTD, could not be buffered by potentiated Hsp104 variants, indicating that further amelioration of disaggregase activity or sharpening of substrate specificity is warranted. We suggest that neuroprotection is achievable for diverse neurodegenerative conditions via surprisingly subtle structural modifications of existing chaperones.

Frontotemporal lobar degeneration: defining phenotypic diversity through personalized medicine

Frontotemporal lobar degeneration (FTLD) comprises two main classes of neurodegenerative diseases characterized by neuronal/glial proteinaceous inclusions (i.e., proteinopathies) including tauopathies (i.e., FTLD-Tau) and TDP-43 proteinopathies (i.e., FTLD-TDP) while other very rare forms of FTLD are known such as FTLD with FUS pathology (FTLD-FUS). This review focuses mainly on FTLD-Tau and FLTD-TDP, which may present as several clinical syndromes: a behavioral/dysexecutive syndrome (behavioral variant frontotemporal dementia); language disorders (primary progressive aphasia variants); and motor disorders (amyotrophic lateral sclerosis, corticobasal syndrome, progressive supranuclear palsy syndrome). There is considerable heterogeneity in clinical presentations of underlying neuropathology and current clinical criteria do not reliably predict underlying proteinopathies ante-mortem. In contrast, molecular etiologies of hereditary FTLD are consistently associated with specific proteinopathies. These include MAPT mutations with FTLD-Tau and GRN, C9orf72, VCP and TARDBP with FTLD-TDP. The last decade has seen a rapid expansion in our knowledge of the molecular pathologies associated with this clinically and neuropathologically heterogeneous group of FTLD diseases. Moreover, in view of current limitations to reliably diagnose specific FTLD neuropathologies prior to autopsy, we summarize the current state of the science in FTLD biomarker research including neuroimaging, biofluid and genetic analyses. We propose that combining several of these biomarker modalities will improve diagnostic specificity in FTLD through a personalized medicine approach. The goals of these efforts are to enhance power for clinical trials focused on slowing or preventing progression of spread of tau, TDP-43 and other FTLD-associated pathologies and work toward the goal of defining clinical endophenotypes of FTD.

Dynamic droplets: the role of cytoplasmic inclusions in stress, function, and disease

Neurodegenerative diseases and other proteinopathies constitute a class of several dozen illnesses etiologically linked to pathological protein misfolding and aggregation. Because of this strong association with disease pathology, cell death, and aging, accumulation of proteins in aggregates or aggregation-associated structures (inclusions) has come to be regarded by many as a deleterious process, to be avoided if possible. Recent work has led us to see inclusion structures and disordered aggregate-like protein mixtures (which we call dynamic droplets) in a new light: not necessarily as a result of a pathological breakdown of cellular order, but as an elaborate cellular architecture regulating function and stress response. In this review, we discuss what is currently known about the role of inclusion structures in cellular homeostasis, stress response, toxicity, and disease. We will focus on possible mechanisms of aggregate toxicity, in contrast to the homeostatic function of several inclusion structures.

Early detection of structural abnormalities and cytoplasmic accumulation of TDP-43 in tissue-engineered skins derived from ALS patients

Amyotrophic lateral sclerosis (ALS) is an adult-onset disease characterized by the selective degeneration of motor neurons in the brain and spinal cord progressively leading to paralysis and death. Current diagnosis of ALS is based on clinical assessment of related symptoms. The clinical manifestations observed in ALS appear relatively late in the disease course after degeneration of a significant number of motor neurons. As a result, the identification and development of disease-modifying therapies is difficult. Therefore, novel strategies for early diagnosis of neurodegeneration, to monitor disease progression and to assess response to existing and future treatments are urgently needed. Factually, many neurological disorders, including ALS, are accompanied by skin changes that often precede the onset of neurological symptoms. Aiming to generate an innovative human-based model to facilitate the identification of predictive biomarkers associated with the disease, we developed a unique ALS tissue-engineered skin model (ALS-TES) derived from patient’s own cells. The ALS-TES presents a number of striking features including altered epidermal differentiation, abnormal dermo-epidermal junction, delamination, keratinocyte infiltration, collagen disorganization and cytoplasmic TDP-43 inclusions. Remarkably, these abnormal skin defects, uniquely seen in the ALS-derived skins, were detected in pre-symtomatic C9orf72-linked ALS patients carrying the GGGGCC DNA repeat expansion. Consequently, our ALS skin model could represent a renewable source of human tissue, quickly and easily accessible to better understand the physiopathological mechanisms underlying this disease, to facilitate the identification of disease-specific biomarkers, and to develop innovative tools for early diagnosis and disease monitoring.

Poly-A binding protein-1 localization to a subset of TDP-43 inclusions in amyotrophic lateral sclerosis occurs more frequently in patients harboring an expansion in C9orf72

Amyotrophic lateral sclerosis (ALS) is an adult-onset motor neuron disease in which the loss of spinal cord motor neurons leads to paralysis and death within a few years of clinical disease onset. In almost all cases of ALS, transactive response DNA binding protein of 43 kDa (TDP-43) forms cytoplasmic neuronal inclusions. A second causative gene for a subset of ALS is fused in sarcoma, an RNA binding protein that also forms cytoplasmic inclusions in spinal cord motor neurons. Poly-A binding protein-1 (PABP-1) is a marker of stress granules (i.e. accumulations of proteins and RNA indicative of translational arrest in cells under stress). We report on the colocalization of PABP-1 to both TDP-43 and fused-in-sarcoma inclusions in 4 patient cohorts: ALS without a mutation, ALS with an intermediate polyglutamine repeat expansion in ATXN2, ALS with a GGGGCC hexanucleotide repeat expansion in C9orf72, and ALS with basophilic inclusion body disease. Notably, PABP-1 colocalization to TDP-43 was twice as frequent in ALS with C9orf72 expansions compared to ALS with no mutation. This study highlights PABP-1 as a protein that is important to the pathology of ALS and indicates that the proteomic profile of TDP-43 inclusions in ALS may differ depending on the causative genetic mutation.

FTLD-ALS of TDP-43 type and SCA2 in a family with a full ataxin-2 polyglutamine expansion

Polyglutamine expansions in the ataxin-2 gene (ATXN2) cause autosomal dominant spinocerebellar ataxia type 2 (SCA2), but have recently also been associated with amyotrophic lateral sclerosis (ALS). We present clinical and pathological features of a family in which a pathological ATXN2 expansion led to frontotemporal lobar degeneration with ALS (FTLD-ALS) in the index case, but typical SCA2 in a son, and compare the neuropathology with a case of typical SCA2. The index case shares the molecular signature of SCA2 with prominent polyglutamine and p62-positive intranuclear neuronal inclusions mainly in the pontine nuclei, while harbouring more pronounced neocortical and spinal TDP-43 pathology. We conclude that ATXN2 mutations can cause not only ALS, but also a neuropathological overlap syndrome of SCA2 and FTLD presenting clinically as pure FTLD-ALS without ataxia. The cause of the phenotypic heterogeneity remains unexplained, but the presence of a CAA-interrupted CAG repeat in the FTLD case in this family suggests that one potential mechanism may be variation in repeat tract composition between members of the same family.

Intermediate CAG repeat expansion in the ATXN2 gene is a unique genetic risk factor for ALS--a systematic review and meta-analysis of observational studies

Amyotrophic lateral sclerosis (ALS) is a rare degenerative condition of the motor neurons. Over 10% of ALS cases are linked to monogenic mutations, with the remainder thought to be due to other risk factors, including environmental factors, genetic polymorphisms, and possibly gene-environmental interactions. We examined the association between ALS and an intermediate CAG repeat expansion in the ATXN2 gene using a meta-analytic approach. Observational studies were searched with relevant disease and gene terms from MEDLINE, EMBASE, and PsycINFO from January 2010 through to January 2014. All identified articles were screened using disease terms, gene terms, population information, and CAG repeat information according to PRISMA guidelines. The final list of 17 articles was further evaluated based on the study location, time period, and authors to exclude multiple usage of the same study populations: 13 relevant articles were retained for this study. The range 30-33 CAG repeats in the ATXN2 gene was most strongly associated with ALS. The meta-analysis revealed that the presence of an intermediate CAG repeat (30-33) in the ATXN2 gene was associated with an increased risk of ALS [odds ratio (OR) = 4.44, 95%CI: 2.91-6.76)] in Caucasian ALS patients. There was no significant difference in the association of this CAG intermediate repeat expansion in the ATXN2 gene between familial ALS cases (OR = 3.59, 1.58-8.17) and sporadic ALS cases (OR = 3.16, 1.88-5.32). These results indicate that the presence of intermediate CAG repeat expansion in the ATXN2 gene is a specific genetic risk factor for ALS, unlike monogenic mutations with an autosomal dominant transmission mode, which cause a more severe phenotype of ALS, with a higher prevalence in familial ALS.

Amyotrophic lateral sclerosis and spinocerebellar ataxia type 2 in a family with full CAG repeat expansions of ATXN2

IMPORTANCE

A family with coexistence of spinocerebellar ataxia type 2 and amyotrophic lateral sclerosis (ALS) is described.

OBSERVATIONS

Intermediate or full CAG repeat expansions of ATXN2 are associated with ALS. However, no coexistence of spinocerebellar ataxia type 2 and ALS in a family has been reported in the literature.We describe a 47-year-old woman with an 11-year history of ataxia and her paternal uncle with ALS who were evaluated at Columbia University Medical Center since July 2006. Both our patient with ataxia and her uncle with ALS have full pathological CAG repeat expansions of ATXN2.

CONCLUSIONS AND RELEVANCE

The diverse clinical phenotypes of ATXN2 CAG expansions and their coexistence in a single family are highlighted. A clinician should consider the diagnosis of spinocerebellar ataxia type 2 when encountering a patient with ataxia and a family history of ALS.

De novo mutations in ataxin-2 gene and ALS risk

Pathogenic CAG repeat expansion in the ataxin-2 gene (ATXN2) is the genetic cause of spinocerebellar ataxia type 2 (SCA2). Recently, it has been associated with Parkinsonism and increased genetic risk for amyotrophic lateral sclerosis (ALS). Here we report the association of de novo mutations in ATXN2 with autosomal dominant ALS. These findings support our previous conjectures based on population studies on the role of large normal ATXN2 alleles as the source for new mutations being involved in neurodegenerative pathologies associated with CAG expansions. The de novo mutations expanded from ALS/SCA2 non-risk alleles as proven by meta-analysis method. The ALS risk was associated with SCA2 alleles as well as with intermediate CAG lengths in the ATXN2. Higher risk for ALS was associated with pathogenic CAG repeat as revealed by meta-analysis.

TDP-43 pathology in polyglutamine diseases: with reference to amyotrphic lateral sclerosis

A nuclear protein, transactivation response (TAR) DNA binding protein 43 kDa (TDP-43), is the major component of neuronal cytoplasmic inclusions (NCIs) in frontotemporal lobar degeneration with ubiquitin inclusions (FTLD-U) and sporadic amyotrophic lateral sclerosis (SALS). While initially thought to be relatively specific to FTLD-U and ALS, TDP-43 pathology has now been detected in a number of other neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. In such tauopathies and α-synucleinopathies, occurrence of TDP-43-positive neuronal cytoplasmic inclusions may be associated with other distinct molecular pathologic processes primarily involving their own pathological proteins, tau and α-synuclein, respectively (secondary TDP-43 proteinopathies). On the other hand, in several polyglutamine (polyQ) diseases, TDP-43 appears to play an important pathomechanistic role. Interestingly, intermediate-length polyQ expansions (27-33 Qs) in ataxin 2, the causative gene of spinocerebellar ataxia type 2, have recently been reported to be a genetic risk factor for SALS. Here, with a review of the literature, we discuss the relationship between ALS and polyQ diseases from the viewpoint of TDP-43 neuropathology.

Amyotrophic lateral sclerosis (ALS)-associated VAPB-P56S inclusions represent an ER quality control compartment

BACKGROUND

Protein aggregation and the formation of intracellular inclusions are a central feature of many neurodegenerative disorders, but precise knowledge about their pathogenic role is lacking in most instances. Here we have characterized inclusions formed in transgenic mice carrying the P56S mutant form of VAPB that causes various motor neuron syndromes including ALS8.

RESULTS

Inclusions in motor neurons of VAPB-P56S transgenic mice are characterized by the presence of smooth ER-like tubular profiles, and are immunoreactive for factors that operate in the ER associated degradation (ERAD) pathway, including p97/VCP, Derlin-1, and the ER membrane chaperone BAP31. The presence of these inclusions does not correlate with signs of axonal and neuronal degeneration, and axotomy leads to their gradual disappearance, indicating that they represent reversible structures. Inhibition of the proteasome and knockdown of the ER membrane chaperone BAP31 increased the size of mutant VAPB inclusions in primary neuron cultures, while knockdown of TEB4, an ERAD ubiquitin-protein ligase, reduced their size. Mutant VAPB did not codistribute with mutant forms of seipin that are associated with an autosomal dominant motor neuron disease, and accumulate in a protective ER derived compartment termed ERPO (ER protective organelle) in neurons.

CONCLUSIONS

The data indicate that the VAPB-P56S inclusions represent a novel reversible ER quality control compartment that is formed when the amount of mutant VAPB exceeds the capacity of the ERAD pathway and that isolates misfolded and aggregated VAPB from the rest of the ER. The presence of this quality control compartment reveals an additional level of flexibility of neurons to cope with misfolded protein stress in the ER.

Mutations in SQSTM1 encoding p62 in amyotrophic lateral sclerosis: genetics and neuropathology

Mutations in SQSTM1 encoding the sequestosome 1/p62 protein have recently been identified in familial and sporadic cases of amyotrophic lateral sclerosis (ALS). p62 is a component of the ubiquitin inclusions detected in degenerating neurons in ALS patients. We sequenced SQSTM1 in 90 French patients with familial ALS (FALS) and 74 autopsied ALS cases with sporadic ALS (SALS). We identified, at the heterozygote state, one missense c.1175C>T, p.Pro392Leu (exon 8) in one of our FALS and one substitution in intron 7 (the c.1165+1G>A, previously called IVS7+1 G-A, A390X) affecting the exon 7 splicing site in one SALS. These mutations that are located in the ubiquitin-associated domain (UBA domain) of the p62 protein have already been described in Paget's disease and ALS patients carrying these mutations had both concomitant Paget's disease. However, we also identified two novel missense mutations in two SALS: the c.259A>G, p.Met87Val in exon 2 and the c.304A>G, p.Lys102Glu in exon 3. These mutations that were not detected in 360 control subjects are possibly pathogenic. Neuropathology analysis of three patients carrying SQSTM1 variants revealed the presence of large round p62 inclusions in motor neurons, and immunoblot analysis showed an increased p62 and TDP-43 protein levels in the spinal cord. Our results confirm that SQSTM1 gene mutations could be the cause or genetic susceptibility factor of ALS in some patients.

Amyotrophic lateral sclerosis

PURPOSE OF REVIEW

The field of amyotrophic lateral sclerosis (ALS) has seen a number of remarkable advances during recent years that will be summarized in this review.

RECENT FINDINGS

In particular, the progress in the molecular neuropathology with the discovery of pathogenic mutations in TAR DNA binding protein (TARDBP), fused in sarcoma (FUS), ubiquilin2 (UBQLN2) and most recently C9ORF72 (abbreviation for the open reading frame 72 on chromosome 9) has further substantiated the - clinically temporarily forgotten - relation of classic ALS to frontotemporal degeneration (FTD). Also, major progress has been made by the discovery of genes relevant for the disease, and pathogenetic concepts have been suggested which imply that not one, but multiple genetic and cell biological hits are involved in the causation of the disease. Progress in interventional therapies has remained poor; important recent examples are the failure of the interventional lithium and pioglitazone trials. However, a study of a third interventional compound - dexpramipexol - raises substantial hopes that the class of chemicals originally represented by riluzole - benzothiazoles - may provide additional therapeutic progress for ALS patients.

SUMMARY

Tremendous progress has been made in the field of ALS based on recent neuropathological and genetic discoveries. Moreover, the role of metabolism and nutrition in the pathogenesis of the disease is debated and may potentially serve as a future therapeutic target. For the facilitation and cost reduction of clinical trials, the development and international standardization of disease-specific 'wet' and 'dry' biomarkers is essential.

ALS-associated ataxin 2 polyQ expansions enhance stress-induced caspase 3 activation and increase TDP-43 pathological modifications

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease caused by the loss of motor neurons. The degenerating motor neurons of ALS patients are characterized by the accumulation of cytoplasmic inclusions containing phosphorylated and truncated forms of the RNA-binding protein TDP-43. Ataxin 2 intermediate-length polyglutamine (polyQ) expansions were recently identified as a risk factor for ALS; however, the mechanism by which they contribute to disease is unknown. Here, we show that intermediate-length ataxin 2 polyQ expansions enhance stress-induced TDP-43 C-terminal cleavage and phosphorylation in human cells. We also connect intermediate-length ataxin 2 polyQ expansions to the stress-dependent activation of multiple caspases, including caspase 3. Caspase activation is upstream of TDP-43 cleavage and phosphorylation since caspase inhibitors block these pathological modifications. Analysis of the accumulation of activated caspase 3 in motor neurons revealed a striking association with ALS cases harboring ataxin 2 polyQ expansions. These findings indicate that activated caspase 3 defines a new pathological feature of ALS with intermediate-length ataxin 2 polyQ expansions. These results provide mechanistic insight into how ataxin 2 intermediate-length polyQ expansions could contribute to ALS--by enhancing stress-induced TDP-43 pathological modifications via caspase activation. Because longer ataxin 2 polyQ expansions are associated with a different disease, spinocerebellar ataxia 2, these findings help explain how different polyQ expansions in the same protein can have distinct cellular consequences, ultimately resulting in different clinical features. Finally, since caspase inhibitors are effective at reducing TDP-43 pathological modifications, this pathway could be pursued as a therapeutic target in ALS.