Juvenile ALS with basophilic inclusions is a FUS proteinopathy with FUS mutations

ALS-associated mutations in FUS disrupt the axonal distribution and function of SMN

Mutations in the RNA binding protein fused in sarcoma/translated in liposarcoma (FUS/TLS) cause amyotrophic lateral sclerosis (ALS). Although ALS-linked mutations in FUS often lead to a cytosolic mislocalization of the protein, the pathogenic mechanisms underlying these mutations remain poorly understood. To gain insight into these mechanisms, we examined the biochemical, cell biological and functional properties of mutant FUS in neurons. Expression of different FUS mutants (R521C, R521H, P525L) in neurons caused axonal defects. A protein interaction screen performed to explain these phenotypes identified numerous FUS interactors including the spinal muscular atrophy (SMA) causing protein survival motor neuron (SMN). Biochemical experiments showed that FUS and SMN interact directly and endogenously, and that this interaction can be regulated by FUS mutations. Immunostaining revealed co-localization of mutant FUS aggregates and SMN in primary neurons. This redistribution of SMN to cytosolic FUS accumulations led to a decrease in axonal SMN. Finally, cell biological experiments showed that overexpression of SMN rescued the axonal defects induced by mutant FUS, suggesting that FUS mutations cause axonal defects through SMN. This study shows that neuronal aggregates formed by mutant FUS protein may aberrantly sequester SMN and concomitantly cause a reduction of SMN levels in the axon, leading to axonal defects. These data provide a functional link between ALS-linked FUS mutations, SMN and neuronal connectivity and support the idea that different motor neuron disorders such as SMA and ALS may be caused, in part, by defects in shared molecular pathways.

Topography of FUS pathology distinguishes late-onset BIBD from aFTLD-U

Background

Multiple neurodegenerative diseases are characterized by the abnormal accumulation of FUS protein including various subtypes of frontotemporal lobar degeneration with FUS inclusions (FTLD-FUS). These subtypes include atypical frontotemporal lobar degeneration with ubiquitin-positive inclusions (aFTLD-U), basophilic inclusion body disease (BIBD) and neuronal intermediate filament inclusion disease (NIFID). Despite considerable overlap, certain pathologic features including differences in inclusion morphology, the subcellular localization of inclusions, and the relative paucity of subcortical FUS pathology in aFTLD-U indicate that these three entities represent related but distinct diseases. In this study, we report the clinical and pathologic features of three cases of aFTLD-U and two cases of late-onset BIBD with an emphasis on the anatomic distribution of FUS inclusions.

Results

The aFTLD-U cases demonstrated FUS inclusions in cerebral cortex, subcortical grey matter and brainstem with a predilection for anterior forebrain and rostral brainstem. In contrast, the distribution of FUS pathology in late-onset BIBD cases demonstrated a predilection for pyramidal and extrapyramidal motor regions with relative sparing of cerebral cortex and limbic regions.

Conclusions

The topography of FUS pathology in these cases demonstrate the diversity of sporadic FUS inclusion body diseases and raises the possibility that late-onset motor neuron disease with BIBD neuropathology may exhibit unique clinical and pathologic features.

How do the RNA-binding proteins TDP-43 and FUS relate to amyotrophic lateral sclerosis and frontotemporal degeneration, and to each other?

PURPOSE OF REVIEW

This review examines the recent research developments aimed at defining the role of RNA-binding proteins (TDP-43 and FUS) in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD).

RECENT FINDINGS

TAR DNA-binding protein 43 kDa (TDP-43) and fused in sarcoma (FUS) are RNA-binding proteins that form aggregates in ALS and FTLD, and when mutated can drive the pathogenesis of these disorders. However, fundamental questions remain as to the relationship between TDP-43 and FUS aggregation and disease, their normal and pathologic function, and where they converge on the same cellular pathways. Autopsy series point to distinct molecular actions as TDP-43 and FUS neuronal inclusions do not overlap, with FUS inclusions being present in only a small subgroup of patients. By contrast, modeling experiments in lower organisms support a genetic interaction between TDP-43 and FUS, although it is likely indirect. Regardless, the recent finding that additional RNA-binding proteins may also cause ALS, and the observation that TDP-43 aggregation remains a core feature in all of the recently identified genetic forms of ALS (C9ORF72, VCP, UBQLN2, and PFN1), underscores the central role of TDP-43 and RNA metabolism in ALS and FTLD.

SUMMARY

Recent discoveries point to an unprecedented convergence of molecular pathways in ALS and FTLD involving RNA metabolism. Defining the exact points of convergence will likely be key to advancing therapeutics development in the coming years.

Mimics and chameleons in motor neurone disease

The progression of motor neurone disease (MND) is currently irreversible, and the grave implications of diagnosis naturally fuels concern among neurologists over missing a potential mimic disorder. There is no diagnostic test for MND but in reality there are few plausible mimics in routine clinical practice. In the presence of a progressive pure motor disorder, signs such as florid fasciculations, bilateral tongue wasting, the ‘split hand’, head drop, emotionality, and cognitive or behavioural impairment carry high positive predictive value. MND is clinically heterogeneous, however, with some important chameleon-like presentations and considerable variation in clinical course. Lack of confidence about the scope of such variation, or an approach to diagnosis emphasising investigations over clinical common sense, has the potential to exacerbate diagnostic delay in MND and impede timely planning of the care which is essential to maximising quality of life.

Controversies and priorities in amyotrophic lateral sclerosis

Two decades after the discovery that 20% of familial amyotrophic lateral sclerosis (ALS) cases were linked to mutations in the superoxide dismutase-1 (SOD1) gene, a substantial proportion of the remainder of cases of familial ALS have now been traced to an expansion of the intronic hexanucleotide repeat sequence in C9orf72. This breakthrough provides an opportunity to re-evaluate longstanding concepts regarding the cause and natural history of ALS, coming soon after the pathological unification of ALS with frontotemporal dementia through a shared pathological signature of cytoplasmic inclusions of the ubiquitinated protein TDP-43. However, with profound clinical, prognostic, neuropathological, and now genetic heterogeneity, the concept of ALS as one disease appears increasingly untenable. This background calls for the development of a more sophisticated taxonomy, and an appreciation of ALS as the breakdown of a wider network rather than a discrete vulnerable population of specialised motor neurons. Identification of C9orf72 repeat expansions in patients without a family history of ALS challenges the traditional division between familial and sporadic disease. By contrast, the 90% of apparently sporadic cases and incomplete penetrance of several genes linked to familial cases suggest that at least some forms of ALS arise from the interplay of multiple genes, poorly understood developmental, environmental, and age-related factors, as well as stochastic events.

Functional diversity of protein fibrillar aggregates from physiology to RNA granules to neurodegenerative diseases

Many proteins exhibit propensities to form fibrillar aggregates called amyloids that are rich in β-sheet structures. Abnormal accumulation of amyloids in the brain and spinal cords is well known as a major pathological change in neurodegenerative diseases; therefore, amyloids have long been considered as disease culprits formed via protein misfolding and should be avoided in healthy cells. Recently, however, increasing numbers of proteins have been identified that require formation of fibrillar states for exertion of their physiological functions, and the critical roles of such functional amyloids include a molecular switch for environmental adaptation, a structural template for catalysis, and a regulator of intracellular signaling. Protein amyloids will, therefore, be more prevailed in our physiologies than we have expected so far. Here, we have reviewed recent studies on such regulatory roles of protein fibrillar aggregates in various physiologies and further discussed possible relations of functional to pathological amyloids.

Fused in sarcoma (FUS): an oncogene goes awry in neurodegeneration

Fused in sarcoma (FUS) is a nuclear DNA/RNA binding protein that regulates different steps of gene expression, including transcription, splicing and mRNA transport. FUS has been implicated in neurodegeneration, since mutations in FUS cause familial amyotrophic lateral sclerosis (ALS-FUS) and lead to the cytosolic deposition of FUS in the brain and spinal cord of ALS-FUS patients. Moreover, FUS and two related proteins of the same protein family (FET family) are co-deposited in cytoplasmic inclusions in a subset of patients with frontotemporal lobar degeneration (FTLD-FUS). Cytosolic deposition of these otherwise nuclear proteins most likely causes the loss of a yet unknown essential nuclear function and/or the gain of a toxic function in the cytosol. Here we summarize what is known about the physiological functions of the FET proteins in the nucleus and cytoplasm and review the distinctive pathomechanisms that lead to the deposition of only FUS in ALS-FUS, but all three FET proteins in FTLD-FUS. We suggest that ALS-FUS is caused by a selective dysfunction of FUS, while FTLD-FUS may be caused by a dysfunction of the entire FET family. This article is part of a Special Issue entitled 'RNA and splicing regulation in neurodegeneration'.

The changing scene of amyotrophic lateral sclerosis

Several recent breakthroughs have provided notable insights into the pathogenesis of amyotrophic lateral sclerosis (ALS), with some even shifting our thinking about this neurodegenerative disease and raising the question as to whether this disorder is a proteinopathy, a ribonucleopathy or both. In addition, these breakthroughs have revealed mechanistic links between ALS and frontotemporal dementia, as well as between ALS and other neurodegenerative diseases, such as the cerebellar atrophies, myotonic dystrophy and inclusion body myositis. Here, we summarize the new findings in ALS research, discuss what they have taught us about this disease and examine issues that are still outstanding.

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.

Truncating mutations in FUS/TLS give rise to a more aggressive ALS-phenotype than missense mutations: a clinico-genetic study in Germany

BACKGROUND AND PURPOSE

Mutations in the FUS/TLS have been associated with amyotrophic lateral sclerosis (ALS) in a few percent of patients.

METHODS

We screened 184 familial (FALS) and 200 sporadic German patients with ALS for FUS/TLS mutations by sequence analysis of exons 5, 6 and 13-15. We compared the phenotypes of patients with different FUS/TLS mutations.

RESULTS

We identified three missense mutations p.K510R, p.R514G, p.R521H, and the two truncating mutations p.R495X and p.G478LfsX23 in samples from eight pedigrees. Both truncating mutations were associated with young onset and very aggressive disease courses, whereas the p.R521H, p.R514G and in particular the p.K510R mutation showed a milder phenotype with disease durations ranging from 3 years to more than 26 years, the longest reported for a patient with a FUS/TLS mutation. Also, in a pair of monozygous twins with the p.K510R mutation, a remarkable similar disease course was observed.

CONCLUSIONS

Mutations in FUS/TLS account for 8.7% (16 of 184) of FALS in Germany. This is a higher prevalence than reported from other countries. Truncating FUS/TLS mutations result in a more severe phenotype than most missense mutations. The wide phenotypic differences have implications for genetic counselling.

The molecular basis of the frontotemporal lobar degeneration-amyotrophic lateral sclerosis spectrum

There is increasing evidence that frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) represent a continuum of neurodegenerative diseases. FTLD is complicated by ALS in a significant proportion of patients, and neuropsychological studies have demonstrated frontotemporal dysfunction in up to 50% of ALS patients. More recently, advances in neuropathology and molecular genetics have started to disclose the biological basis for the observed clinical concurrence. TDP-43 and FUS have been discovered as key pathological proteins in both FTLD and ALS. The most recent discovery of a pathological hexanucleotide repeat expansion in the gene C9orf72 as a frequent cause of both FTLD and ALS has eventually confirmed the association of these two at first sight distinct neurodegenerative diseases. Mutations in the TARDBP, FUS, and VCP genes had previously been associated with different phenotypes of the FTLD-ALS spectrum, although in these cases one end of the spectrum predominates. Whilst on the one hand providing evidence for overlap, these discoveries have also highlighted that FTLD and ALS are etiologically diverse. In this review, we review the recent advances that support the existence of an FTLD-ALS spectrum, with particular emphasis on the molecular genetic aspect.

Endogenous TDP-43, but not FUS, contributes to stress granule assembly via G3BP

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the selective loss of upper and lower motor neurons, a cell type that is intrinsically more vulnerable than other cell types to exogenous stress. The interplay between genetic susceptibility and environmental exposures to toxins has long been thought to be relevant to ALS. One cellular mechanism to overcome stress is the formation of small dense cytoplasmic domains called stress granules (SG) which contain translationally arrested mRNAs. TDP-43 (encoded by TARDBP) is an ALS-causative gene that we have previously implicated in the regulation of the core stress granule proteins G3BP and TIA-1. TIA-1 and G3BP localize to SG under nearly all stress conditions and are considered essential to SG formation. Here, we report that TDP-43 is required for proper SG dynamics, especially SG assembly as marked by the secondary aggregation of TIA-1. We also show that SG assembly, but not initiation, requires G3BP. Furthermore, G3BP can rescue defective SG assembly in cells depleted of endogenous TDP-43. We also demonstrate that endogenous TDP-43 and FUS do not have overlapping functions in this cellular process as SG initiation and assembly occur normally in the absence of FUS. Lastly, we observe that SG assembly is a contributing factor in the survival of neuronal-like cells responding to acute oxidative stress. These data raise the possibility that disruptions of normal stress granule dynamics by loss of nuclear TDP-43 function may contribute to neuronal vulnerability in ALS.

FUS-NLS/Transportin 1 complex structure provides insights into the nuclear targeting mechanism of FUS and the implications in ALS

The C-terminal nuclear localization sequence of FUsed in Sarcoma (FUS-NLS) is critical for its nuclear import mediated by transportin (Trn1). Familial amyotrophic lateral sclerosis (ALS) related mutations are clustered in FUS-NLS. We report here the structural, biochemical and cell biological characterization of the FUS-NLS and its clinical implications. The crystal structure of the FUS-NLS/Trn1 complex shows extensive contacts between the two proteins and a unique α-helical structure in the FUS-NLS. The binding affinity between Trn1 and FUS-NLS (wide-type and 12 ALS-associated mutants) was determined. As compared to the wide-type FUS-NLS (K(D) = 1.7 nM), each ALS-associated mutation caused a decreased affinity and the range of this reduction varied widely from 1.4-fold over 700-fold. The affinity of the mutants correlated with the extent of impaired nuclear localization, and more importantly, with the duration of disease progression in ALS patients. This study provides a comprehensive understanding of the nuclear targeting mechanism of FUS and illustrates the significance of FUS-NLS in ALS.

De novo FUS gene mutations are associated with juvenile-onset sporadic amyotrophic lateral sclerosis in China

Juvenile amyotrophic lateral sclerosis (ALS) is a rare form of motor neuron disease and occurs before 25 years of age. Only very few sporadic cases of juvenile-onset ALS have been reported. Rare SOD1 mutations and several FUS mutations have been identified in juvenile-onset ALS patients. To define the genetics of juvenile-onset sporadic ALS (SALS) of Chinese origin, we sequenced all 5 exons of SOD1, exons 3-6 and 12-15 of FUS in 11 juvenile-onset SALS patients, 105 adult-onset ALS patients (including 6 familial ALS [FALS] pedigrees), and 245 healthy controls. For the 11 juvenile-onset SALS and 6 FALS cases, the other 7 exons of FUS were also screened. A heterozygous de novo missense mutation c.1574C>T (p.P525L), a heterozygous de novo 2-base pair deletion c.1509_1510delAG (p.G504Wfs*12), and a nonsense mutation c.1483C>T (p.R495X) was each identified in 1 juvenile SALS patient. A heterozygous missense mutation c.1561C>G (p.R521G) was identified in a FALS proband. In the Chinese population, the frequency of FUS mutation in FALS is 11.4% (95% confidence interval [CI], 0.9%-22.0%), higher than the Japanese (10%; 95% CI, 0.7%-19.3%), and Caucasians (4.9%; 95% CI, 3.9%-6.0%). The frequency of FUS mutation in SALS patients is 1.5% (95% CI, 0.2%-2.9%), which is similar to Koreans (1.6%; 95% CI, 0%-3.2%), but higher than in Caucasians (0.6%; 95% CI, 0.4%-0.8%). Our findings suggest that de novo FUS mutations are associated with juvenile-onset SALS of Chinese origin and that this gene should be screened in ALS patients with a young age of onset, aggressive progression, and sporadic occurrence.

Familial ALS with FUS P525L mutation: two Japanese sisters with multiple systems involvement

We evaluated the clinicopathological features of familial amyotrophic lateral sclerosis (ALS) with the fused in sarcoma (FUS) P525L mutation. Two sisters and their mother had a similar clinical course, which was characterized by the development of limb weakness at a young age with rapid disease progression. An elder sister, patient 1, progressed into a totally locked-in state requiring mechanical ventilation and died 26 years after the onset of the disease. In contrast, the younger sister, patient 2, died in the early stages of the disease. The patients had neuropathological findings that indicated a very active degeneration of motor neurons and multiple system degeneration, which led to marked brain and spinal cord atrophy in the long term clinical outcome. The multiple system degeneration included the frontal lobe, the basal ganglia and substantia nigra, cerebellum and related area. Compared with previously reported ALS cases, the severe degeneration of the frontal lobe and the striatum were the characteristic features in the patient 1 in this case study. The degeneration spread over multiple systems might be caused not only by the appearance of the FUS immunoreactive neuronal cytoplasmic inclusions but also by the degeneration of neuronal connections from the primary motor cortex and related areas.

Arginine methylation next to the PY-NLS modulates Transportin binding and nuclear import of FUS

Fused in sarcoma (FUS) is a nuclear protein that carries a proline-tyrosine nuclear localization signal (PY-NLS) and is imported into the nucleus via Transportin (TRN). Defects in nuclear import of FUS have been implicated in neurodegeneration, since mutations in the PY-NLS of FUS cause amyotrophic lateral sclerosis (ALS). Moreover, FUS is deposited in the cytosol in a subset of frontotemporal lobar degeneration (FTLD) patients. Here, we show that arginine methylation modulates nuclear import of FUS via a novel TRN-binding epitope. Chemical or genetic inhibition of arginine methylation restores TRN-mediated nuclear import of ALS-associated FUS mutants. The unmethylated arginine-glycine-glycine domain preceding the PY-NLS interacts with TRN and arginine methylation in this domain reduces TRN binding. Inclusions in ALS-FUS patients contain methylated FUS, while inclusions in FTLD-FUS patients are not methylated. Together with recent findings that FUS co-aggregates with two related proteins of the FET family and TRN in FTLD-FUS but not in ALS-FUS, our study provides evidence that these two diseases may be initiated by distinct pathomechanisms and implicates alterations in arginine methylation in pathogenesis.

Metal imaging in neurodegenerative diseases

Metal ions are known to play an important role in many neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and prion diseases. In these diseases, aberrant metal binding or improper regulation of redox active metal ions can induce oxidative stress by producing cytotoxic reactive oxygen species (ROS). Altered metal homeostasis is also frequently seen in the diseased state. As a result, the imaging of metals in intact biological cells and tissues has been very important for understanding the role of metals in neurodegenerative diseases. A wide range of imaging techniques have been utilized, including X-ray fluorescence microscopy (XFM), particle induced X-ray emission (PIXE), energy dispersive X-ray spectroscopy (EDS), laser ablation inductively coupled mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS), all of which allow for the imaging of metals in biological specimens with high spatial resolution and detection sensitivity. These techniques represent unique tools for advancing the understanding of the disease mechanisms and for identifying possible targets for developing treatments. In this review, we will highlight the advances in neurodegenerative disease research facilitated by metal imaging techniques.

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.

Olfaction in Parkinson's disease and related disorders

Olfactory dysfunction is an early 'pre-clinical' sign of Parkinson's disease (PD). The present review is a comprehensive and up-to-date assessment of such dysfunction in PD and related disorders. The olfactory bulb is implicated in the dysfunction, since only those syndromes with olfactory bulb pathology exhibit significant smell loss. The role of dopamine in the production of olfactory system pathology is enigmatic, as overexpression of dopaminergic cells within the bulb's glomerular layer is a common feature of PD and most animal models of PD. Damage to cholinergic, serotonergic, and noradrenergic systems is likely involved, since such damage is most marked in those diseases with the most smell loss. When compromised, these systems, which regulate microglial activity, can influence the induction of localized brain inflammation, oxidative damage, and cytosolic disruption of cellular processes. In monogenetic forms of PD, olfactory dysfunction is rarely observed in asymptomatic gene carriers, but is present in many of those that exhibit the motor phenotype. This suggests that such gene-related influences on olfaction, when present, take time to develop and depend upon additional factors, such as those from aging, other genes, formation of α-synuclein- and tau-related pathology, or lowered thresholds to oxidative stress from toxic insults. The limited data available suggest that the physiological determinants of the early changes in PD-related olfactory function are likely multifactorial and may include the same determinants as those responsible for a number of other non-motor symptoms of PD, such as dysautonomia and sleep disturbances.

Young-onset amyotrophic lateral sclerosis: historical and other observations

There is a wide range of age at initial symptom onset in amyotrophic lateral sclerosis despite a mean age of 65 years in population-based studies. 'Young-onset' amyotrophic lateral sclerosis typically refers to patients younger than ∼45 years and accounts for about 10% of cases in contemporary series. A review of published cases of amyotrophic lateral sclerosis from 1850 to 1950 revealed a far higher proportion of cases with young onset (>50%), with a steady decline to the contemporary figure. It is possible that this is not solely explained by increases in life expectancy. While there is still a rich variation in phenotypes among cases of young-onset amyotrophic lateral sclerosis, bulbar onset was found to be significantly under-represented in analysis of a large patient database, with implications for age-related vulnerabilities pertaining to focality of symptom onset. The timing of initiating pathological processes in relation to the emergence of symptoms is discussed, including the potential role of very early development and the interaction of epigenetic and environmental factors.

Requirements for stress granule recruitment of fused in sarcoma (FUS) and TAR DNA-binding protein of 43 kDa (TDP-43)

Cytoplasmic inclusions containing TAR DNA-binding protein of 43 kDa (TDP-43) or Fused in sarcoma (FUS) are a hallmark of amyotrophic lateral sclerosis (ALS) and several subtypes of frontotemporal lobar degeneration (FTLD). FUS-positive inclusions in FTLD and ALS patients are consistently co-labeled with stress granule (SG) marker proteins. Whether TDP-43 inclusions contain SG markers is currently still debated. We determined the requirements for SG recruitment of FUS and TDP-43 and found that cytoplasmic mislocalization is a common prerequisite for SG recruitment of FUS and TDP-43. For FUS, the arginine-glycine-glycine zinc finger domain, which is the protein's main RNA binding domain, is most important for SG recruitment, whereas the glycine-rich domain and RNA recognition motif (RRM) domain have a minor contribution and the glutamine-rich domain is dispensable. For TDP-43, both the RRM1 and the C-terminal glycine-rich domain are required for SG localization. ALS-associated point mutations located in the glycine-rich domain of TDP-43 do not affect SG recruitment. Interestingly, a 25-kDa C-terminal fragment of TDP-43, which is enriched in FTLD/ALS cortical inclusions but not spinal cord inclusions, fails to be recruited into SG. Consistently, inclusions in the cortex of FTLD patients, which are enriched for C-terminal fragments, are not co-labeled with the SG marker poly(A)-binding protein 1 (PABP-1), whereas inclusions in spinal cord, which contain full-length TDP-43, are frequently positive for this marker protein.

Mutant FUS induces endoplasmic reticulum stress in amyotrophic lateral sclerosis and interacts with protein disulfide-isomerase

Mutations in the gene encoding fused in sarcoma (FUS) are linked to amyotrophic lateral sclerosis (ALS), but the mechanisms by which these mutants trigger neurodegeneration remain unknown. Endoplasmic reticulum (ER) stress is increasingly recognized as an important and early pathway to motor neuron death in ALS. FUS is normally located in the nucleus but in ALS, FUS redistributes to the cytoplasm and forms inclusions. In this study, we investigated whether FUS induces ER stress in a motor neuron like cell line (NSC-34). We demonstrate that ER stress is triggered in cells expressing mutant FUS, and this is closely associated with redistribution of mutant FUS to the cytoplasm. Mutant FUS also colocalized with protein disulfide-isomerase (PDI), an important ER chaperone, in NSC-34 cells and PDI was colocalized with FUS inclusions in human ALS lumbar spinal cords, in both sporadic ALS and mutant FUS-linked familial ALS tissues. These findings implicate ER stress in the pathophysiology of FUS, and provide evidence for common pathogenic pathways in ALS linked to the ER.

Sporadic amyotrophic lateral sclerosis: new hypothesis regarding its etiology and pathogenesis suggests that astrocytes might be the primary target hosting a still unknown external agent

This article briefly describes the already known clinical features and pathogenic mechanisms underlying sporadic amyotrophic lateral sclerosis, namely excitoxicity, oxidative stress, protein damage, inflammation, genetic abnormalities and neuronal death. Thereafter, it puts forward the hypothesis that astrocytes may be the cells which serve as targets for the harmful action of a still unknown environmental agent, while neuronal death may be a secondary event following the initial insult to glial cells. The article also suggests that an emergent virus or a misfolded infectious protein might be potential candidates to accomplish this task.

FUS-related proteinopathies: lessons from animal models

The recent identification of ALS-linked mutations in FUS and TDP-43 has led to a major shift in our thinking in regard to the potential molecular mechanisms of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). RNA-mediated proteinopathy is increasingly being recognized as a potential cause of neurodegenerative disorders. FUS and TDP-43 are structurally and functionally similar proteins. FUS is a DNA/RNA binding protein that may regulate aspects of RNA metabolism, including splicing, mRNA processing, and micro RNA biogenesis. It is unclear how ALS-linked mutations perturb the functions of FUS. This review highlights recent advances in understanding the functions of FUS and discusses findings from FUS animal models that provide several key insights into understanding the molecular mechanisms that might contribute to ALS pathogenesis.

The tip of the iceberg: RNA-binding proteins with prion-like domains in neurodegenerative disease

Prions are self-templating protein conformers that are naturally transmitted between individuals and promote phenotypic change. In yeast, prion-encoded phenotypes can be beneficial, neutral or deleterious depending upon genetic background and environmental conditions. A distinctive and portable 'prion domain' enriched in asparagine, glutamine, tyrosine and glycine residues unifies the majority of yeast prion proteins. Deletion of this domain precludes prionogenesis and appending this domain to reporter proteins can confer prionogenicity. An algorithm designed to detect prion domains has successfully identified 19 domains that can confer prion behavior. Scouring the human genome with this algorithm enriches a select group of RNA-binding proteins harboring a canonical RNA recognition motif (RRM) and a putative prion domain. Indeed, of 210 human RRM-bearing proteins, 29 have a putative prion domain, and 12 of these are in the top 60 prion candidates in the entire genome. Startlingly, these RNA-binding prion candidates are inexorably emerging, one by one, in the pathology and genetics of devastating neurodegenerative disorders, including: amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U), Alzheimer's disease and Huntington's disease. For example, FUS and TDP-43, which rank 1st and 10th among RRM-bearing prion candidates, form cytoplasmic inclusions in the degenerating motor neurons of ALS patients and mutations in TDP-43 and FUS cause familial ALS. Recently, perturbed RNA-binding proteostasis of TAF15, which is the 2nd ranked RRM-bearing prion candidate, has been connected with ALS and FTLD-U. We strongly suspect that we have now merely reached the tip of the iceberg. We predict that additional RNA-binding prion candidates identified by our algorithm will soon surface as genetic modifiers or causes of diverse neurodegenerative conditions. Indeed, simple prion-like transfer mechanisms involving the prion domains of RNA-binding proteins could underlie the classical non-cell-autonomous emanation of neurodegenerative pathology from originating epicenters to neighboring portions of the nervous system. This article is part of a Special Issue entitled RNA-Binding Proteins.

Sporadic juvenile amyotrophic lateral sclerosis caused by mutant FUS/TLS: possible association of mental retardation with this mutation

We present two cases of patients with juvenile amyotrophic lateral sclerosis (ALS), who had no history of familial ALS. The symptoms of both patients started as weakness of the unilateral upper limb and neck, and extended to bulbar and respiratory weakness in a relatively short period. Of note, the first patient was mentally retarded before the onset of weakness. Fused in sarcoma/translocated in liposarcoma (FUS/TLS) gene analyses revealed mutations of p. G492EfsX527 (c. 1475delG), which is a novel deletion/frameshift mutation, in the first patient and p. R514S mutation (c. 1542G > T) in the second patient. Molecular analysis revealed that the mutant FUS/TLS, especially the deletion/frameshift mutation, showed significant cytoplasmic localization in transfected motor neuron-like cells. Our findings suggest the association of mental retardation with the FUS/TLS mutation. Further investigation, including the effect of FUS/TLS on cognitive function, would aid better understanding of FUS/TLS proteinopathies.

Modeling ALS and FTLD proteinopathies in yeast: an efficient approach for studying protein aggregation and toxicity

In recent years there have been several reports of human neurodegenerative diseases that involve protein misfolding being modeled in the yeast Saccharomyces cerevisiae. This review summarizes recent advances in understanding the specific mechanisms underlying intracellular neuronal pathology during Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Lobar Degeneration (FTLD), including SOD1, TDP-43 and FUS protein inclusions and the potential of these proteins to be involved in pathogenic prion-like mechanisms. More specifically, we focus on findings from yeast systems that offer tremendous possibilities for screening for genetic and chemical modifiers of disease-related proteotoxicity.

P525L FUS mutation is consistently associated with a severe form of juvenile amyotrophic lateral sclerosis

Some FUS mutations have been observed in patients with the juvenile form of Amyotrophic Lateral Sclerosis starting before 25 years. We report an 11-year-old girl affected by sporadic juvenile ALS with a rapid course resulting in tracheostomy after 14 months from the onset. Sequencing FUS gene revealed a de novo P525L mutation. Our findings, together with literature data, indicate that this mutation is consistently associated with a specific phenotype characterized by juvenile onset, severe course and high proportion of de novo mutations in sporadic cases.

Making connections: pathology and genetics link amyotrophic lateral sclerosis with frontotemporal lobe dementia

Over the last couple of decades, there has been a growing body of clinical, genetic, and histopathological evidence that similar pathological processes underlie amyotrophic lateral sclerosis (ALS) and some types of frontotemporal lobe dementia (FTD). Even though there is great diversity in the genetic causes of these disorders, there is a high degree of overlap in their histopathology. Genes linked to rare cases of familial ALS and/or FTD, like FUS, TARDBP, OPTN, and UBQLN2 may converge onto a unifying pathogenic pathway and thereby provide novel therapeutic targets common to a spectrum of etiologically diverse forms of ALS and ALS-FTD. Additionally, there are major loci for ALS-FTD on chromosomes 9p and 15q. Identification of causative genetic alterations at those loci will be an important step in understanding the pathogenesis of juvenile- and adult-onset ALS and ALS-FTD. Interactions between TDP-43, FUS, optineurin, and ubiquilin 2 need to be studied to understand their common molecular pathways. Future efforts should also be directed towards generation and characterization of in vivo models to dissect the pathogenic mechanisms of these diseases. Such efforts will rapidly accelerate the discovery of new drugs that regulate accumulation of pathogenic proteins and their downstream consequences.

On the development of markers for pathological TDP-43 in amyotrophic lateral sclerosis with and without dementia

Pathological 43-kDa transactive response sequence DNA-binding protein (TDP-43) has been recognized as the major disease protein in amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration with ubiquitin positive, tau and α-synuclein negative inclusions (FTLD-U) and the transitional forms between these multisystem conditions. In order to develop TDP-43 into a successful ALS biomarker, the natural history of TDP-43 pathology needs to be characterized and the underlying pathophysiology established. Here we propose a spatial and temporal "two-axes" model of central nervous system vulnerability for TDP-43 linked degeneration and review recent studies on potential biomarkers related to pathological TDP-43 in the cerebrospinal fluid (CSF), blood, and skeletal muscle. The model includes the following two arms: Firstly, a "motor neuron disease" or "spinal cord/brainstem to motor cortex" axis (with degeneration possibly ascending from the lower motor neurons to the upper motor neurons); and secondly, a "dementia" or "corticoid/allocortex to neocortex" axis (with a probable spread of TDP-43 linked degeneration from the mediotemporal lobe to wider mesocortical and neocortical brain areas). At the cellular level, there is a gradual disappearance of normal TDP-43 in the nucleus in combination with the formation of pathological aggregates in the cell body and cellular processes, which can also be used to identify the stage of the disease process. Moreover, TDP-43 lesions in subpial/subependymal or perivascular localizations have been noted, and this might account for increased CSF and blood TDP-43 levels through mechanisms that remain to be elucidated.

Emerging drugs for amyotrophic lateral sclerosis

INTRODUCTION

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disease that results in increasing disability and that is uniformly fatal. Since its approval in the 1990s, riluzole remains the sole treatment for ALS offering modest survival benefit. While significant advances have been made in the symptomatic management of the disease, more effective drug therapy targeting disease progression is sorely needed.

AREAS COVERED

Advances in the understanding of pathogenic mechanisms involved in disease development and progression have provided multiple avenues for developing effective treatment strategies. This review highlights recent discoveries relating to these diverse mechanisms and their implications for the development of drug therapy. Previous human clinical trials that have targeted these pathways are mentioned and ongoing drug trials are discussed.

EXPERT OPINION

The search for effective drug therapy faces important challenges in the areas of basic science and animal research, translation of these results into human clinical trials, inherent bias in human studies and issues related to delays in clinical diagnosis. How these issues may be addressed and why ALS research constitutes fertile grounds for drug development not only for this devastating disease, but also for other more prevalent neurodegenerative diseases, is discussed in this review.

Expression of human FUS protein in Drosophila leads to progressive neurodegeneration

Mutations in the Fused in sarcoma/Translated in liposarcoma gene (FUS/TLS, FUS) have been identified among patients with amyotrophic lateral sclerosis (ALS). FUS protein aggregation is a major pathological hallmark of FUS proteinopathy, a group of neurodegenerative diseases characterized by FUS-immunoreactive inclusion bodies. We prepared transgenic Drosophila expressing either the wild type (Wt) or ALS-mutant human FUS protein (hFUS) using the UAS-Gal4 system. When expressing Wt, R524S or P525L mutant FUS in photoreceptors, mushroom bodies (MBs) or motor neurons (MNs), transgenic flies show age-dependent progressive neural damages, including axonal loss in MB neurons, morphological changes and functional impairment in MNs. The transgenic flies expressing the hFUS gene recapitulate key features of FUS proteinopathy, representing the first stable animal model for this group of devastating diseases.

RNA-binding proteins with prion-like domains in ALS and FTLD-U

Amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease) is a debilitating, and universally fatal, neurodegenerative disease that devastates upper and lower motor neurons. The causes of ALS are poorly understood. A central role for RNA-binding proteins and RNA metabolism in ALS has recently emerged. The RNA-binding proteins, TDP-43 and FUS, are principal components of cytoplasmic inclusions found in motor neurons of ALS patients and mutations in TDP-43 and FUS are linked to familial and sporadic ALS. Pathology and genetics also connect TDP-43 and FUS with frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). It was unknown whether mechanisms of FUS aggregation and toxicity were similar or different to those of TDP-43. To address this issue, we have employed yeast models and pure protein biochemistry to define mechanisms underlying TDP-43 and FUS aggregation and toxicity, and to identify genetic modifiers relevant for human disease. We have identified prion-like domains in FUS and TDP-43 and provide evidence that these domains are required for aggregation. Our studies have defined key similarities as well as important differences between the two proteins. Collectively, however, our findings lead us to suggest that FUS and TDP-43, though similar RNA-binding proteins, likely aggregate and confer disease phenotypes via distinct mechanisms.

Pathological heterogeneity in amyotrophic lateral sclerosis with FUS mutations: two distinct patterns correlating with disease severity and mutation

Mutations in the gene encoding the fused in sarcoma (FUS) protein are responsible for ~3% of familial amyotrophic lateral sclerosis (ALS) and <1% of sporadic ALS (ALS-FUS). Descriptions of the associated neuropathology are few and largely restricted to individual case reports. To better define the neuropathology associated with FUS mutations, we have undertaken a detailed comparative analysis of six cases of ALS-FUS that include sporadic and familial cases, with both juvenile and adult onset, and with four different FUS mutations. We found significant pathological heterogeneity among our cases, with two distinct patterns that correlated with the disease severity and the specific mutation. Frequent basophilic inclusions and round FUS-immunoreactive (FUS-ir) neuronal cytoplasmic inclusions (NCI) were a consistent feature of our early-onset cases, including two with the p.P525L mutation. In contrast, our late-onset cases that included two with the p.R521C mutation had tangle-like NCI and numerous FUS-ir glial cytoplasmic inclusions. Double-labeling experiments demonstrated that many of the glial inclusions were in oligodendrocytes. Comparison with the neuropathology of cases of frontotemporal lobar degeneration with FUS-ir pathology showed significant differences and suggests that FUS mutations are associated with a distinct pathobiology.

Novel types of frontotemporal lobar degeneration: beyond tau and TDP-43

Most cases of frontotemporal lobar degeneration (FTLD) are characterized by the abnormal accumulation of either the microtubule-associated protein tau or the transactive response DNA-binding protein with M(r) 43 kDa, TDP-43 (FTLD-tau and FTLD-TDP, respectively). However, there remain ∼10% of cases, composed of a heterogenous collection of uncommon disorders, for which the molecular basis remains uncertain. In this review, we describe the characteristic genetic, clinical, and pathological features of the major tau/TDP-negative FTLD subtypes, with focus on recent advances in our understanding of their molecular basis. This includes the discovery that the pathological changes in atypical FTLD with ubiquitinated inclusions, neuronal intermediate filament inclusion disease, and basophilic inclusion body disease are immunoreactive for the fused in sarcoma (FUS) protein, resulting in the creation of a new molecular subgroup (FTLD-FUS), and studies clarifying the functional consequences of pathogenic CHMP2B mutations.

Molecular determinants and genetic modifiers of aggregation and toxicity for the ALS disease protein FUS/TLS

TDP-43 and FUS are RNA-binding proteins that form cytoplasmic inclusions in some forms of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Moreover, mutations in TDP-43 and FUS are linked to ALS and FTLD. However, it is unknown whether TDP-43 and FUS aggregate and cause toxicity by similar mechanisms. Here, we exploit a yeast model and purified FUS to elucidate mechanisms of FUS aggregation and toxicity. Like TDP-43, FUS must aggregate in the cytoplasm and bind RNA to confer toxicity in yeast. These cytoplasmic FUS aggregates partition to stress granule compartments just as they do in ALS patients. Importantly, in isolation, FUS spontaneously forms pore-like oligomers and filamentous structures reminiscent of FUS inclusions in ALS patients. FUS aggregation and toxicity requires a prion-like domain, but unlike TDP-43, additional determinants within a RGG domain are critical for FUS aggregation and toxicity. In further distinction to TDP-43, ALS-linked FUS mutations do not promote aggregation. Finally, genome-wide screens uncovered stress granule assembly and RNA metabolism genes that modify FUS toxicity but not TDP-43 toxicity. Our findings suggest that TDP-43 and FUS, though similar RNA-binding proteins, aggregate and confer disease phenotypes via distinct mechanisms. These differences will likely have important therapeutic implications.

Research advances in amyotrophic lateral sclerosis, 2009 to 2010

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of upper and lower motor neurons that causes progressive weakness and death. The breadth of research in ALS continues to grow with exciting new discoveries in disease pathogenesis and potential future therapeutics. There is a growing list of identified mutations in familial ALS, including those in genes encoding TDP-43 and FUS/TLS, which are expanding our understanding of the role of RNA modulation in ALS pathogenesis. There is a greater appreciation for the role of glial cells in motor neuron disease. Mitochondrial dysfunction is also being shown to be critical for motor neuron degeneration. In addition to pharmacotherapy, there are promising early developments with therapeutic implications in the areas of RNA interference, stem cell therapies, viral vector-mediated gene therapy, and immunotherapy. With greater understanding of ALS pathogenesis and exciting new therapeutic technologies, there is hope for future progress in treating this disease.

Mutation in the senataxin gene found in a patient affected by familial ALS with juvenile onset and slow progression

We report an Italian male with juvenile onset familial disease characterized by progressive weakness and wasting of four limbs and prolonged survival. Diagnostic work-up revealed the diffuse involvement of central and peripheral motor neurons. Genetic analysis revealed a L389S mutation in the senataxin (SETX) gene.

Distinct pathological subtypes of FTLD-FUS

Most cases of frontotemporal lobar degeneration (FTLD) are characterized by abnormal intracellular accumulation of either tau or TDP-43 protein. However, in ~10% of cases, composed of a heterogenous collection of uncommon disorders, the molecular basis remains to be uncertain. We recently discovered that the pathological changes in several tau/TDP-43-negative FTLD subtypes are immunoreactive (ir) for the fused in sarcoma (FUS) protein. In this study, we directly compared the pattern of FUS-ir pathology in cases of atypical FTLD-U (aFTLD-U, N = 10), neuronal intermediate filament inclusion disease (NIFID, N = 5) and basophilic inclusion body disease (BIBD, N = 8), to determine whether these are discrete entities or represent a pathological continuum. All cases had FUS-ir pathology in the cerebral neocortex, hippocampus and a similar wide range of subcortical regions. Although there was significant overlap, each group showed specific differences that distinguished them from the others. Cases of aFTLD-U consistently had less pathology in subcortical regions. In addition, the neuronal inclusions in aFTLD-U usually had a uniform, round shape, whereas NIFID and BIBD were characterized by a variety of inclusion morphologies. In all cases of aFTLD-U and NIFID, vermiform neuronal intranuclear inclusions (NII) were readily identified in the hippocampus and neocortex. In contrast, only two cases of BIBD had very rare NII in a single subcortical region. These findings support aFTLD-U, NIFID and BIBD as representing closely related, but distinct entities that share a common molecular pathogenesis. Although cases with overlapping pathology may exist, we recommend retaining the terms aFTLD-U, NIFID and BIBD for specific FTLD-FUS subtypes.

Regulation of TDP-43 aggregation by phosphorylation and p62/SQSTM1

TAR DNA-binding protein-43 (TDP-43) proteinopathy has been linked to several neurodegenerative diseases, such as frontotemporal lobar degeneration with ubiquitin-positive inclusions and amyotrophic lateral sclerosis. Phosphorylated and ubiquitinated TDP-43 C-terminal fragments have been found in cytoplasmic inclusions in frontotemporal lobar degeneration with ubiquitin-positive inclusions and amyotrophic lateral sclerosis patients. However, the factors and pathways that regulate TDP-43 aggregation are still not clear. We found that the C-terminal 15 kDa fragment of TDP-43 is sufficient to induce aggregation but the aggregation phenotype is modified by additional sequences. Aggregation is accompanied by phosphorylation at serine residues 409/410. Mutation of 409/410 to phosphomimetic aspartic acid residues significantly reduces aggregation. Inhibition of either proteasome or autophagy dramatically increases TDP-43 aggregation. Furthermore, TDP-43 aggregates colocalize with markers of autophagy and the adaptor protein p62/SQSTM1. Over-expression of p62/SQSTM1 reduces TDP-43 aggregation in an autophagy and proteasome-dependent manner. These studies suggest that aggregation of TDP-43 C-terminal fragments is regulated by phosphorylation events and both the autophagy and proteasome-mediated degradation pathways.

Modeling ALS and FTLD proteinopathies in yeast: an efficient approach for studying protein aggregation and toxicity

In recent years there have been several reports of human neurodegenerative diseases that involve protein misfolding being modeled in the yeast Saccharomyces cerevisiae. This review summarizes recent advances in understanding the specific mechanisms underlying intracellular neuronal pathology during Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Lobar Degeneration (FTLD), including SOD1, TDP-43 and FUS protein inclusions and the potential of these proteins to be involved in pathogenic prion-like mechanisms. More specifically, we focus on findings from yeast systems that offer tremendous possibilities for screening for genetic and chemical modifiers of disease-related proteotoxicity.

Lockhart Clarke's contribution to the description of amyotrophic lateral sclerosis

The definition of the clinicopathological entity of amyotrophic lateral sclerosis evolved over half a century. Although the definitive term amyotrophic lateral sclerosis that acknowledged both upper and lower motor neuron involvement was attributed to Jean-Martin Charcot in 1874, his initial case was published nearly a decade earlier; and it is accepted that, from at least the 1830s, several others (including Charles Bell, François-Amilcar Aran and Jean Cruveilhier) had already recognized a progressive lower motor neuron-only syndrome within a broader, clinically-defined group of disorders, termed progressive muscular atrophy. Although William Gowers first grouped the three phenotypes of amyotrophic lateral sclerosis, progressive muscular atrophy and progressive bulbar palsy together as part of the same syndrome, the term motor neuron disease, as an over-arching label, was not suggested until nearly a century later by W. Russell Brain. Augustus Jacob Lockhart Clarke (1817-80) is best known for his descriptions of spinal cord anatomy. However, in two detailed case reports from the 1860s, he carried out rigorous post-mortem neuropathological studies of what appear to be classical cases of amyotrophic lateral sclerosis. Furthermore, he recognized the additional involvement of the corticospinal tracts that distinguished this from progressive muscular atrophy. Several aspects of the exquisite clinical histories documented as part of both studies, one by Charles Bland Radcliffe, resonate with contemporary debates concerning the evolution of disease in amyotrophic lateral sclerosis. These 'past masters' still have much to teach us.