TDP-43 is directed to stress granules by sorbitol, a novel physiological osmotic and oxidative stressor

The Physiological and Pathological Implications of the Formation of Hydrogels, with a Specific Focus on Amyloid Polypeptides

Hydrogels are water-swollen and viscoelastic three-dimensional cross-linked polymeric network originating from monomer polymerisation. Hydrogel-forming polypeptides are widely found in nature and, at a cellular and organismal level, they provide a wide range of functions for the organism making them. Amyloid structures, arising from polypeptide aggregation, can be damaging or beneficial to different types of organisms. Although the best-known amyloids are those associated with human pathologies, this underlying structure is commonly used by higher eukaryotes to maintain normal cellular activities, and also by microbial communities to promote their survival and growth. Amyloidogenesis occurs by nucleation-dependent polymerisation, which includes several species (monomers, nuclei, oligomers, and fibrils). Oligomers of pathological amyloids are considered the toxic species through cellular membrane perturbation, with the fibrils thought to represent a protective sink for toxic species. However, both functional and disease-associated amyloids use fibril cross-linking to form hydrogels. The properties of amyloid hydrogels can be exploited by organisms to fulfil specific physiological functions. Non-physiological hydrogelation by pathological amyloids may provide additional toxic mechanism(s), outside of membrane toxicity by oligomers, such as physical changes to the intracellular and extracellular environments, with wide-spread consequences for many structural and dynamic processes, and overall effects on cell survival.

Reversible induction of TDP-43 granules in cortical neurons after traumatic injury

Traumatic brain injury (TBI) has been proposed as a risk factor for neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). To determine whether TBI might trigger or exacerbate ALS-relevant pathology, we delivered a mild stab-wound injury to the motor cortex of three different ALS mouse models expressing mutations in SOD1, TDP-43 or FUS and scrutinized the effects on the formation of phospho-TDP-43 (pTDP-43) cytoplasmic granules. Stab-injury induced the formation of cytoplasmic TDP-43 granules in wt animals, peaking at 3dpi; a much larger response was seen in mutant TDP-43 mice, whose response peaked at 7dpi. The pTDP-43 granules did not colocalize with the stress markers TIAR-1 and FUS but colocalized with FMRP (35%) and with p62 (65%), suggesting their involvement in transport granules and their clearance by autophagy. A similar, albeit smaller effect, was seen in mutant FUS mice. In the SOD1^G93A mouse model, neither increase in pTDP-43 granules nor in SOD1 aggregates were detected. In all cases, pTDP-43 granules were cleared and the number of pTDP-43-positive neurons returned to baseline by 40dpi. Neither injury-related neuronal loss nor motor performance or survival was significantly different in transgenic mice receiving injury vs sham mice. Thus, trauma can trigger ALS-related TDP-43 pathology, the extent of which is modulated by ALS-related mutations. However, the pathological findings prove reversible and do not affect disease progression and neuronal vulnerability.

Dysregulated molecular pathways in amyotrophic lateral sclerosis-frontotemporal dementia spectrum disorder

Frontotemporal dementia (FTD), the second most common form of dementia in people under 65 years of age, is characterized by progressive atrophy of the frontal and/or temporal lobes. FTD overlaps extensively with the motor neuron disease amyotrophic lateral sclerosis (ALS), especially at the genetic level. Both FTD and ALS can be caused by many mutations in the same set of genes; the most prevalent of these mutations is a GGGGCC repeat expansion in the first intron of C9ORF72 As shown by recent intensive studies, some key cellular pathways are dysregulated in the ALS-FTD spectrum disorder, including autophagy, nucleocytoplasmic transport, DNA damage repair, pre-mRNA splicing, stress granule dynamics, and others. These exciting advances reveal the complexity of the pathogenic mechanisms of FTD and ALS and suggest promising molecular targets for future therapeutic interventions in these devastating disorders.

Failure to Deliver and Translate-New Insights into RNA Dysregulation in ALS

Amyotrophic Lateral Sclerosis (ALS) is a progressive and fatal neurodegenerative disease affecting both upper and lower motor neurons. The molecular mechanisms underlying disease pathogenesis remain largely unknown. Multiple genetic loci including genes involved in proteostasis and ribostasis have been linked to ALS providing key insights into the molecular mechanisms underlying disease. In particular, the identification of the RNA binding proteins TDP-43 and fused in sarcoma (FUS) as causative factors of ALS resulted in a paradigm shift centered on the study of RNA dysregulation as a major mechanism of disease. With wild-type TDP-43 pathology being found in ~97% of ALS cases and the identification of disease causing mutations within its sequence, TDP-43 has emerged as a prominent player in ALS. More recently, studies of the newly discovered C9orf72 repeat expansion are lending further support to the notion of defects in RNA metabolism as a key factor underlying ALS. RNA binding proteins are involved in all aspects of RNA metabolism ranging from splicing, transcription, transport, storage into RNA/protein granules, and translation. How these processes are affected by disease-associated mutations is just beginning to be understood. Considerable work has gone into the identification of splicing and transcription defects resulting from mutations in RNA binding proteins associated with disease. More recently, defects in RNA transport and translation have been shown to be involved in the pathomechanism of ALS. A central hypothesis in the field is that disease causing mutations lead to the persistence of RNA/protein complexes known as stress granules. Under times of prolonged cellular stress these granules sequester specific mRNAs preventing them from translation, and are thought to evolve into pathological aggregates. Here we will review recent efforts directed at understanding how altered RNA metabolism contributes to ALS pathogenesis.

Intrinsic Disorder, Protein-Protein Interactions, and Disease

It is recognized now that biologically active proteins without stable tertiary structure (known as intrinsically disordered proteins, IDPs) and hybrid proteins containing ordered domains and intrinsically disordered protein regions (IDPRs) are important players found in any given proteome. These IDPs/IDPRs possess functions that complement functional repertoire of their ordered counterparts, being commonly related to recognition, as well as control and regulation of various signaling pathways. They are interaction masters, being able to utilize a wide spectrum of interaction mechanisms, ranging from induced folding to formation of fuzzy complexes where significant levels of disorder are preserved, to polyvalent stochastic interactions playing crucial roles in the liquid-liquid phase transitions leading to the formation of proteinaceous membrane-less organelles. IDPs/IDPRs are tightly controlled themselves via various means, including alternative splicing, precisely controlled expression and degradation, binding to specific partners, and posttranslational modifications. Distortions in the regulation and control of IDPs/IDPRs, as well as their aberrant interactivity are commonly associated with various human diseases. This review presents some aspects of the intrinsic disorder-based functionality and dysfunctionality, paying special attention to the normal and pathological protein-protein interactions.

Functional and dynamic polymerization of the ALS-linked protein TDP-43 antagonizes its pathologic aggregation

TDP-43 is a primarily nuclear RNA-binding protein, whose abnormal phosphorylation and cytoplasmic aggregation characterizes affected neurons in patients with amyotrophic lateral sclerosis and frontotemporal dementia. Here, we report that physiological nuclear TDP-43 in mouse and human brain forms homo-oligomers that are resistant to cellular stress. Physiological TDP-43 oligomerization is mediated by its N-terminal domain, which can adopt dynamic, solenoid-like structures, as revealed by a 2.1 Å crystal structure in combination with nuclear magnetic resonance spectroscopy and electron microscopy. These head-to-tail TDP-43 oligomers are unique among known RNA-binding proteins and represent the functional form of the protein in vivo, since their destabilization results in loss of alternative splicing regulation of known neuronal RNA targets. Our findings indicate that N-terminal domain-driven oligomerization spatially separates the adjoining highly aggregation-prone, C-terminal low-complexity domains of consecutive TDP-43 monomers, thereby preventing low-complexity domain inter-molecular interactions and antagonizing the formation of pathologic aggregates.TDP-43 aggregation is observed in amyotrophic lateral sclerosis. Here the authors combine X-ray crystallography, nuclear magnetic resonance and electron microscopy studies and show that physiological oligomerization of TDP-43 is mediated through its N-terminal domain, which forms functional and dynamic oligomers antagonizing pathologic aggregation.

Prion-like propagation as a pathogenic principle in frontotemporal dementia

Frontotemporal dementia is a devastating neurodegenerative disease causing stark alterations in personality and language. Characterized by severe atrophy of the frontal and temporal brain lobes, frontotemporal dementia (FTD) shows extreme heterogeneity in clinical presentation, genetic causes, and pathological findings. Like most neurodegenerative diseases, the initial symptoms of FTD are subtle, but increase in severity over time, as the disease progresses. Clinical progression is paralleled by exacerbation of pathological findings and the involvement of broader brain regions, which currently lack mechanistic explanation. Yet, a flurry of studies indicate that protein aggregates accumulating in neurodegenerative diseases can act as propagating entities, amplifying their pathogenic conformation, in a way similar to infectious prions. In this prion‐centric view, FTD can be divided into three subtypes, TDP‐43 or FUS proteinopathy and tauopathy. Here, we review the current evidence that FTD‐linked pathology propagates in a prion‐like manner and discuss the implications of these findings for disease progression and heterogeneity.

Frontotemporal dementia (FTD) is a progressive neurodegenerative disease causing severe personality dysfunctions, characterized by profound heterogeneity. Accumulation of tau, TDP‐43 or FUS cytoplasmic aggregates characterize molecularly distinct and non‐overlapping FTD subtypes. Here, we discuss the current evidence suggesting that prion‐like propagation and cell‐to‐cell spread of each of these cytoplasmic aggregates may underlie disease progression and heterogeneity.

This article is part of the Frontotemporal Dementia special issue.

Truncation of the TAR DNA-binding protein 43 is not a prerequisite for cytoplasmic relocalization, and is suppressed by caspase inhibition and by introduction of the A90V sequence variant

The RNA-binding and -processing protein TAR DNA-binding protein 43 (TDP-43) is heavily linked to the underlying causes and pathology of neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal lobar degeneration. In these diseases, TDP-43 is mislocalized, hyperphosphorylated, ubiquitinated, aggregated and cleaved. The importance of TDP-43 cleavage in the disease pathogenesis is still poorly understood. Here we detail the use of D-sorbitol as an exogenous stressor that causes TDP-43 cleavage in HeLa cells, resulting in a 35 kDa truncated product that accumulates in the cytoplasm within one hour of treatment. We confirm that the formation of this 35 kDa cleavage product is mediated by the activation of caspases. Inhibition of caspases blocks the cleavage of TDP-43, but does not prevent the accumulation of full-length protein in the cytoplasm. Using D-sorbitol as a stressor and caspase activator, we also demonstrate that the A90V variant of TDP-43, which lies adjacent to the caspase cleavage site within the nuclear localization sequence of TDP-43, confers partial resistance against caspase-mediated generation of the 35 kDa cleavage product.

Mutant KRAS Enhances Tumor Cell Fitness by Upregulating Stress Granules

There is growing evidence that stress-coping mechanisms represent tumor cell vulnerabilities that may function as therapeutically beneficial targets. Recent work has delineated an integrated stress adaptation mechanism that is characterized by the formation of cytoplasmic mRNA and protein foci, termed stress granules (SGs). Here, we demonstrate that SGs are markedly elevated in mutant KRAS cells following exposure to stress-inducing stimuli. The upregulation of SGs by mutant KRAS is dependent on the production of the signaling lipid molecule 15-deoxy-delta 12,14 prostaglandin J2 (15-d-PGJ2) and confers cytoprotection against stress stimuli and chemotherapeutic agents. The secretion of 15-d-PGJ2 by mutant KRAS cells is sufficient to enhance SG formation and stress resistance in cancer cells that are wild-type for KRAS. Our findings identify a mutant KRAS-dependent cell non-autonomous mechanism that may afford the establishment of a stress-resistant niche that encompasses different tumor subclones. These results should inform the design of strategies to eradicate tumor cell communities.

Energy Homeostasis and Abnormal RNA Metabolism in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease that is clinically characterized by progressive muscle weakness and impaired voluntary movement due to the loss of motor neurons in the brain, brain stem and spinal cord. To date, no effective treatment is available. Ample evidence suggests that impaired RNA homeostasis and abnormal energy status are two major pathogenesis pathways in ALS. In the present review article, we focus on recent studies that report molecular insights of both pathways, and discuss the possibility that energy dysfunction might negatively regulate RNA homeostasis via the impairment of cytoplasmic-nuclear shuttling in motor neurons and subsequently contribute to the development of ALS.

VEGF alleviates ALS-CSF induced cytoplasmic accumulations of TDP-43 and FUS/TLS in NSC-34 cells

Cytoplasmic mislocalisation and aggregation of TDP-43 and FUS/TLS proteins in spinal motor neurons contribute to the pathogenesis of the highly fatal disorder amyotrophic lateral sclerosis (ALS). We investigated the neuroprotective effect of VEGF on expression of these proteins in the motor neuronal cell line NSC-34 modelled to reminisce sporadic form of ALS. We studied the expression of TDP-43 and FUS/TLS proteins after exposure to ALS-CSF and following VEGF supplementation by quantitative confocal microscopy and electron microscopy. ALS-CSF caused cytoplasmic overexpression of both the proteins and stress-granule formation in the cells. These alterations were alleviated by VEGF supplementation. The related ultrastructural changes like nuclear membrane dysmorphism and p-bodies associated changes were also reversed. However the protein expression did not completely translocate to the nucleus, as some cells continued to show to cytoplasmic mislocalisation. Thus, the present findings indicate that VEGF alleviates TDP43 and FUS pathology by complimenting its role in controlling apoptosis and reversing choline acetyl transferase expression. Hence, VEGF appears to target multiple pathogenic processes in the neurodegenerative cascade of ALS.

Phase to Phase with TDP-43

TDP-43 is a dimeric nuclear protein that plays a central role in RNA metabolism. In recent years, this protein has become a focal point of research in the amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) disease spectrum, as pathognomonic inclusions within affected neurons contain post-translationally modified TDP-43. A key question in TDP-43 research involves determining the mechanisms and triggers that cause TDP-43 to form pathological aggregates. This review gives a brief overview of the physiological and pathological roles of TDP-43 and focuses on the structural features of its protein domains and how they may contribute to normal protein function and to disease. A special emphasis is placed on the C-terminal prion-like region thought to be implicated in pathology, as it is where nearly all ALS/FTD-associated mutations reside. Recent structural studies of this domain revealed its crucial role in the formation of phase-separated liquid droplets through a partially populated α-helix. This new discovery provides further support for the theory that liquid droplets such as stress granules may be precursors to pathological aggregates, linking environmental effects such as stress to the potential etiology of the disease. The transition of TDP-43 among soluble, droplet, and aggregate phases and the implications of these transitions for pathological aggregation are summarized and discussed.

Cytoplasmic stress granules: Dynamic modulators of cell signaling and disease

Stress granule (SG) assembly is a conserved cellular strategy to minimize stress-related damage and promote cell survival. Beyond their fundamental role in the stress response, SGs have emerged as key players for human health. As such, SG assembly is associated with cancer, neurodegenerative disorders, ischemia, and virus infections. SGs and granule-related signaling circuits are therefore promising targets to improve therapeutic intervention for several diseases. This is clinically relevant, because pharmacological drugs can affect treatment outcome by modulating SG formation. As membraneless and highly dynamic compartments, SGs regulate translation, ribostasis and proteostasis. Moreover, they serve as signaling hubs that determine cell viability and stress recovery. Various compounds can modulate SG formation and dynamics. Rewiring cell signaling through SG manipulation thus represents a new strategy to control cell fate under various physiological and pathological conditions.

Motoneuron Disease: Basic Science

ALS is a relentless neurodegenerative disease in which motor neurons are the susceptible neuronal population. Their death results in progressive paresis of voluntary and respiratory muscles. The unprecedented rate of discoveries over the last two decades have broadened our knowledge of genetic causes and helped delineate molecular pathways. Here we critically review ALS epidemiology, genetics, pathogenic mechanisms, available animal models, and iPS cell technologies with a focus on their translational therapeutic potential. Despite limited clinical success in treatments to date, the new discoveries detailed here offer new models for uncovering disease mechanisms as well as novel strategies for intervention.

ALS mutant SOD1 interacts with G3BP1 and affects stress granule dynamics

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease. Mutations in Cu/Zn superoxide dismutase (SOD1) are responsible for approximately 20 % of the familial ALS cases. ALS-causing SOD1 mutants display a gain-of-toxicity phenotype, but the nature of this toxicity is still not fully understood. The Ras GTPase-activating protein-binding protein G3BP1 plays a critical role in stress granule dynamics. Alterations in the dynamics of stress granules have been reported in several other forms of ALS unrelated to SOD1. To our surprise, the mutant G93A SOD1 transgenic mice exhibited pathological cytoplasmic inclusions that co-localized with G3BP1-positive granules in spinal cord motor neurons. The co-localization was also observed in fibroblast cells derived from familial ALS patient carrying SOD1 mutation L144F. Mutant SOD1, unlike wild-type SOD1, interacted with G3BP1 in an RNA-independent manner. Moreover, the interaction is specific for G3BP1 since mutant SOD1 showed little interaction with four other RNA-binding proteins implicated in ALS. The RNA-binding RRM domain of G3BP1 and two particular phenylalanine residues (F380 and F382) are critical for this interaction. Mutant SOD1 delayed the formation of G3BP1- and TIA1-positive stress granules in response to hyperosmolar shock and arsenite treatment in N2A cells. In summary, the aberrant mutant SOD1-G3BP1 interaction affects stress granule dynamics, suggesting a potential link between pathogenic SOD1 mutations and RNA metabolism alterations in ALS.

TDP-43/FUS in motor neuron disease: Complexity and challenges

Amyotrophic lateral sclerosis (ALS), a common motor neuron disease affecting two per 100,000 people worldwide, encompasses at least five distinct pathological subtypes, including, ALS-SOD1, ALS-C9orf72, ALS-TDP-43, ALS-FUS and Guam-ALS. The etiology of a major subset of ALS involves toxicity of the TAR DNA-binding protein-43 (TDP-43). A second RNA/DNA binding protein, fused in sarcoma/translocated in liposarcoma (FUS/TLS) has been subsequently associated with about 1% of ALS patients. While mutations in TDP-43 and FUS have been linked to ALS, the key contributing molecular mechanism(s) leading to cell death are still unclear. One unique feature of TDP-43 and FUS pathogenesis in ALS is their nuclear clearance and simultaneous cytoplasmic aggregation in affected motor neurons. Since the discoveries in the last decade implicating TDP-43 and FUS toxicity in ALS, a majority of studies have focused on their cytoplasmic aggregation and disruption of their RNA-binding functions. However, TDP-43 and FUS also bind to DNA, although the significance of their DNA binding in disease-affected neurons has been less investigated. A recent observation of accumulated genomic damage in TDP-43 and FUS-linked ALS and association of FUS with neuronal DNA damage repair pathways indicate a possible role of deregulated DNA binding function of TDP-43 and FUS in ALS. In this review, we discuss the different ALS disease subtypes, crosstalk of etiopathologies in disease progression, available animal models and their limitations, and recent advances in understanding the specific involvement of RNA/DNA binding proteins, TDP-43 and FUS, in motor neuron diseases.

Neurodegeneration and RNA-binding proteins

In the eukaryotic nucleus, RNA-binding proteins (RBPs) play a very important role in the life cycle of both coding and noncoding RNAs. As soon as they are transcribed, in fact, all RNA molecules within a cell are bound by distinct sets of RBPs that have the task of regulating its correct processing, transport, stability, and function/translation up to its final degradation. These tasks are particularly important in cells that have a complex RNA metabolism, such as neurons. Not surprisingly, therefore, recent findings have shown that the misregulation of genes involved in RNA metabolism or the autophagy/proteasome pathway plays an important role in the onset and progression of several neurodegenerative diseases. In this article, we aim to review the recent advances that link neurodegenerative processes and RBP proteins. WIREs RNA 2017, 8:e1394. doi: 10.1002/wrna.1394 For further resources related to this article, please visit the WIREs website.

In Vivo Formation of Vacuolated Multi-phase Compartments Lacking Membranes

Eukaryotic cells contain membrane-less organelles, including nucleoli and stress granules, that behave like liquid droplets. Such endogenous condensates often have internal substructure, but how this is established in the absence of membrane encapsulation remains unclear. We find that the N- and C-terminal domains of TDP43, a heterogeneous nuclear ribonucleoprotein implicated in neurodegenerative diseases, are capable of driving the formation of sub-structured liquid droplets in vivo. These droplets contain dynamic internal "bubbles" of nucleoplasm, reminiscent of membrane-based multi-vesicular endosomes. A conserved sequence embedded within the intrinsically disordered region (IDR) of TDP43 promotes the formation of these multi-phase assemblies. Disease-causing point mutations in the IDR can change the propensity to form bubbles, protein dynamics within the phase, or phase-environment exchange rates. Our results show that a single IDR-containing protein can nucleate the assembly of compartmentalized liquid droplets approximating the morphological complexity of membrane-bound organelles.

Altered mRNP granule dynamics in FTLD pathogenesis

In neurons, RNA-binding proteins (RBPs) play a key role in post-transcriptional gene regulation, for example alternative splicing, mRNA localization in neurites and local translation upon synaptic stimulation. There is increasing evidence that defective or mislocalized RBPs - and consequently altered mRNA processing - lead to neuronal dysfunction and cause neurodegeneration, including frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Cytosolic RBP aggregates containing TAR DNA-binding protein of 43 kDa (TDP-43) or fused in sarcoma (FUS) are a common hallmark of both disorders. There is mounting evidence that translationally silent mRNP granules, such as stress granules or transport granules, play an important role in the formation of these RBP aggregates. These granules are thought to be 'catalytic convertors' of RBP aggregation by providing a high local concentration of RBPs. As recently shown in vitro, RBPs that contain a so-called low-complexity domain start to 'solidify' and eventually aggregate at high protein concentrations. The same may happen in mRNP granules in vivo, leading to 'solidified' granules that lose their dynamic properties and ability to fulfill their physiological functions. This may result in a disturbed stress response, altered mRNA transport and local translation, and formation of pathological TDP-43 or FUS aggregates, all of which may contribute to neuronal dysfunction and neurodegeneration. Here, we discuss the general functional properties of these mRNP granules, how their dynamics may be disrupted in frontotemporal lobar degeneration/amyotrophic lateral sclerosis, for example by loss or gain of function of TDP-43 and FUS, and how this may contribute to the development of RBP aggregates and neurotoxicity. In this review, we discuss how dynamic mRNP granules, such as stress granules or neuronal transport granules, may be converted into pathological aggregates containing misfolded RNA-binding proteins (RBPs), such as TDP-43 and FUS. Abnormal interactions between low-complexity domains in RBPs may cause dynamic mRNP granules to solidify and become dysfunctional. This may result in a disturbed stress response, altered mRNA transport and local translation, as well as RBP aggregation, all of which may contribute to neuronal dysfunction and neurodegeneration.

Stress granules at the intersection of autophagy and ALS

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal disease caused by loss of upper and lower motor neurons. The majority of ALS cases are classified as sporadic (80-90%), with the remaining considered familial based on patient history. The last decade has seen a surge in the identification of ALS-causing genes - including TARDBP (TDP-43), FUS, MATR3 (Matrin-3), C9ORF72 and several others - providing important insights into the molecular pathways involved in pathogenesis. Most of the protein products of ALS-linked genes fall into two functional categories: RNA-binding/homeostasis and protein-quality control (i.e. autophagy and proteasome). The RNA-binding proteins tend to be aggregation-prone with low-complexity domains similar to the prion-forming domains of yeast. Many also incorporate into stress granules (SGs), which are cytoplasmic ribonucleoprotein complexes that form in response to cellular stress. Mutant forms of TDP-43 and FUS perturb SG dynamics, lengthening their cytoplasmic persistence. Recent evidence suggests that SGs are regulated by the autophagy pathway, suggesting a unifying connection between many of the ALS-linked genes. Persistent SGs may give rise to intractable aggregates that disrupt neuronal homeostasis, thus failure to clear SGs by autophagic processes may promote ALS pathogenesis. This article is part of a Special Issue entitled SI:Autophagy.

Sequestration of cellular interacting partners by protein aggregates: implication in a loss-of-function pathology

Protein misfolding and aggregation are a hallmark of several neurodegenerative diseases (NDs). However, how protein aggregation leads to cytotoxicity and neurodegeneration is still controversial. Emerging evidence demonstrates that sequestration of cellular-interacting partners by protein aggregates contributes to the pathogenesis of these diseases. Here, we review current research on sequestration of cellular proteins by protein aggregates and its relation to proteinopathies. Based on different interaction modes, we classify these protein sequestrations into four types: protein coaggregation, domain/motif-mediated sequestration, RNA-assisted sequestration, and sequestration of molecular chaperones. Thus, the cellular essential proteins and/or RNA hijacked by protein aggregates may lose their biological functions, consequently resulting in cytotoxicity and neurodegeneration. We have proposed a hijacking model recapitulating the sequestration process and the loss-of-function pathology of ND.

The heat shock response plays an important role in TDP-43 clearance: evidence for dysfunction in amyotrophic lateral sclerosis

Detergent-resistant, ubiquitinated and hyperphosphorylated Tar DNA binding protein 43 (TDP-43, encoded by TARDBP) neuronal cytoplasmic inclusions are the pathological hallmark in ∼95% of amyotrophic lateral sclerosis and ∼60% of frontotemporal lobar degeneration cases. We sought to explore the role for the heat shock response in the clearance of insoluble TDP-43 in a cellular model of disease and to validate our findings in transgenic mice and human amyotrophic lateral sclerosis tissues. The heat shock response is a stress-responsive protective mechanism regulated by the transcription factor heat shock factor 1 (HSF1), which increases the expression of chaperones that refold damaged misfolded proteins or facilitate their degradation. Here we show that manipulation of the heat shock response by expression of dominant active HSF1 results in a dramatic reduction of insoluble and hyperphosphorylated TDP-43 that enhances cell survival, whereas expression of dominant negative HSF1 leads to enhanced TDP-43 aggregation and hyperphosphorylation. To determine which chaperones were mediating TDP-43 clearance we over-expressed a range of heat shock proteins (HSPs) and identified DNAJB2a (encoded by DNAJB2, and also known as HSJ1a) as a potent anti-aggregation chaperone for TDP-43. DNAJB2a has a J domain, allowing it to interact with HSP70, and ubiquitin interacting motifs, which enable it to engage the degradation of its client proteins. Using functionally deleted DNAJB2a constructs we demonstrated that TDP-43 clearance was J domain-dependent and was not affected by ubiquitin interacting motif deletion or proteasome inhibition. This indicates that TDP-43 is maintained in a soluble state by DNAJB2a, leaving the total levels of TDP-43 unchanged. Additionally, we have demonstrated that the levels of HSF1 and heat shock proteins are significantly reduced in affected neuronal tissues from a TDP-43 transgenic mouse model of amyotrophic lateral sclerosis and patients with sporadic amyotrophic lateral sclerosis. This implies that the HSF1-mediated DNAJB2a/HSP70 heat shock response pathway is compromised in amyotrophic lateral sclerosis. Defective refolding of TDP-43 is predicted to aggravate the TDP-43 proteinopathy. The finding that the pathological accumulation of insoluble TDP-43 can be reduced by the activation of HSF1/HSP pathways presents an exciting opportunity for the development of novel therapeutics.

The Myoblast C2C12 Transfected with Mutant Valosin-Containing Protein Exhibits Delayed Stress Granule Resolution on Oxidative Stress

Valosin-containing protein (VCP) mutations cause inclusion body myopathy with Paget disease and frontotemporal dementia. However, the mechanisms by which mutant VCP triggers degeneration remain unknown. Here, we investigated the role of VCP in cellular stress and found that the oxidative stressor arsenite and heat shock-activated stress responses evident by T-intracellular antigen-1-positive granules in C2C12 myoblasts. Granules also contained phosphorylated transactive response DNA-binding protein 43, ubiquitin, microtubule-associated protein 1A/1B light chains 3, and lysosome-associated membrane protein 2. Mutant VCP produced more T-intracellular antigen-1-positive granules than wild-type in the postarsenite exposure period. Similar results were observed for other granule components, indicating that mutant VCP delayed clearance of stress granules. Furthermore, stress granule resolution was impaired on differentiated C2C12 cells expressing mutant VCP. To address whether mutant VCP triggers dysregulation of the stress granule pathway in vivo, we analyzed skeletal muscle of aged VCPR155H-knockin mice. We found significant increments in oxidated proteins but observed the stress granule markers RasGAP SH3-binding protein and phosphorylated eukaryotic translation initiation factor 2α unchanged. The mixed results indicate that mutant VCP together with aging lead to higher oxidative stress in skeletal muscle but were insufficient to disrupt the stress granule pathway. Our findings support that deficiencies in recovery from stressors may result in attenuated tolerance to stress that could trigger muscle degeneration.

TDP-43 is intercellularly transmitted across axon terminals

Transactive response DNA-binding protein 43 kD (TDP-43) is an aggregation-prone prion-like domain-containing protein and component of pathological intracellular aggregates found in most amyotrophic lateral sclerosis (ALS) patients. TDP-43 oligomers have been postulated to be released and subsequently nucleate TDP-43 oligomerization in recipient cells, which might be the molecular correlate of the systematic symptom spreading observed during ALS progression. We developed a novel protein complementation assay allowing quantification of TDP-43 oligomers in living cells. We demonstrate the exchange of TDP-43 between cell somata and the presence of TDP-43 oligomers in microvesicles/exosomes and show that microvesicular TDP-43 is preferentially taken up by recipient cells where it exerts higher toxicity than free TDP-43. Moreover, studies using microfluidic neuronal cultures suggest both anterograde and retrograde trans-synaptic spreading of TDP-43. Finally, we demonstrate TDP-43 oligomer seeding by TDP-43-containing material derived from both cultured cells and ALS patient brain lysate. Thus, using an innovative detection technique, we provide evidence for preferentially microvesicular uptake as well as both soma-to-soma "horizontal" and bidirectional "vertical" synaptic intercellular transmission and prion-like seeding of TDP-43.

PERK-opathies: An Endoplasmic Reticulum Stress Mechanism Underlying Neurodegeneration

The unfolded protein response (UPR) plays a vital role in maintaining cell homeostasis as a consequence of endoplasmic reticulum (ER) stress. However, prolonged UPR activity leads to cell death. This time-dependent dual functionality of the UPR represents the adaptive and cytotoxic pathways that result from ER stress. Chronic UPR activation in systemic and neurodegenerative diseases has been identified as an early sign of cellular dyshomeostasis. The Protein Kinase R-like ER Kinase (PERK) pathway is one of three major branches in the UPR, and it is the only one to modulate protein synthesis as an adaptive response. The specific identification of prolonged PERK activity has been correlated with the progression of disorders such as diabetes, Alzheimer's disease, and cancer, suggesting that PERK plays a role in the pathology of these disorders. For the first time, the term "PERK-opathies" is used to group these diseases in which PERK mediates detriment to the cell culminating in chronic disorders. This article reviews the literature documenting links between systemic disorders with the UPR, but with a specific emphasis on the PERK pathway. Then, articles reporting links between the UPR, and more specifically PERK, and neurodegenerative disorders are presented. Finally, a therapeutic perspective is discussed, where PERK interventions could be potential remedies for cellular dysfunction in chronic neurodegenerative disorders.

RNA Granules and Diseases: A Case Study of Stress Granules in ALS and FTLD

RNA granules are microscopically visible cellular structures that aggregate by protein-protein and protein-RNA interactions. Using stress granules as an example, we discuss the principles of RNA granule formation, which rely on the multivalency of RNA and multi-domain proteins as well as low-affinity interactions between proteins with prion-like/low-complexity domains (e.g. FUS and TDP-43). We then explore how dysregulation of RNA granule formation is linked to neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), and discuss possible strategies for therapeutic intervention.

Folliculin, a tumor suppressor associated with Birt-Hogg-Dubé (BHD) syndrome, is a novel modifier of TDP-43 cytoplasmic translocation and aggregation

TDP-43 was identified as the major component of ubiquitin and autophagosome-positive cytoplasmic inclusions in neurons in the large majority of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar dementia (FTLD) patients. It has been shown that a loss of nuclear TDP-43 in combination with enhanced cytoplasmic mislocalization of TDP-43, which is associated with accumulation of TDP-43 aggregates in the cytosol, is an early and key event in TDP-43-mediated neurodegeneration. However, the mechanism underlying TDP-43 nucleocytoplasmic shuttling is still not clear. Here, we show that the tumor suppressor folliculin (FLCN) is a novel positive regulator of TDP-43 cytoplasmic translocation. FLCN directly interacts with TDP-43. The amino acids 202-299 of FLCN and RNA-recognition motif domains of TDP-43 are necessary for their interaction. In addition, both exogenous and endogenous FLCNs are required for TDP-43 cytoplasmic accumulation, protein aggregation and stress granule formation. Overall, our study suggests that FLCN may play an important role in the regulation of TDP-43 nucleocytoplasmic shuttling and TDP-43-mediated proteinopathy.

Alterations in stress granule dynamics driven by TDP-43 and FUS: a link to pathological inclusions in ALS?

Stress granules (SGs) are RNA-containing cytoplasmic foci formed in response to stress exposure. Since their discovery in 1999, over 120 proteins have been described to be localized to these structures (in 154 publications). Most of these components are RNA binding proteins (RBPs) or are involved in RNA metabolism and translation. SGs have been linked to several pathologies including inflammatory diseases, cancer, viral infection, and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). In ALS and FTD, the majority of cases have no known etiology and exposure to external stress is frequently proposed as a contributor to either disease initiation or the rate of disease progression. Of note, both ALS and FTD are characterized by pathological inclusions, where some well-known SG markers localize with the ALS related proteins TDP-43 and FUS. We propose that TDP-43 and FUS serve as an interface between genetic susceptibility and environmental stress exposure in disease pathogenesis. Here, we will discuss the role of TDP-43 and FUS in SG dynamics and how disease-linked mutations affect this process.

Fragile X protein mitigates TDP-43 toxicity by remodeling RNA granules and restoring translation

RNA dysregulation is a newly recognized disease mechanism in amyotrophic lateral sclerosis (ALS). Here we identify Drosophila fragile X mental retardation protein (dFMRP) as a robust genetic modifier of TDP-43-dependent toxicity in a Drosophila model of ALS. We find that dFMRP overexpression (dFMRP OE) mitigates TDP-43 dependent locomotor defects and reduced lifespan in Drosophila. TDP-43 and FMRP form a complex in flies and human cells. In motor neurons, TDP-43 expression increases the association of dFMRP with stress granules and colocalizes with polyA binding protein in a variant-dependent manner. Furthermore, dFMRP dosage modulates TDP-43 solubility and molecular mobility with overexpression of dFMRP resulting in a significant reduction of TDP-43 in the aggregate fraction. Polysome fractionation experiments indicate that dFMRP OE also relieves the translation inhibition of futsch mRNA, a TDP-43 target mRNA, which regulates neuromuscular synapse architecture. Restoration of futsch translation by dFMRP OE mitigates Futsch-dependent morphological phenotypes at the neuromuscular junction including synaptic size and presence of satellite boutons. Our data suggest a model whereby dFMRP is neuroprotective by remodeling TDP-43 containing RNA granules, reducing aggregation and restoring the translation of specific mRNAs in motor neurons.

Dysregulated miRNA biogenesis downstream of cellular stress and ALS-causing mutations: a new mechanism for ALS

Interest in RNA dysfunction in amyotrophic lateral sclerosis (ALS) recently aroused upon discovering causative mutations in RNA-binding protein genes. Here, we show that extensive down-regulation of miRNA levels is a common molecular denominator for multiple forms of human ALS. We further demonstrate that pathogenic ALS-causing mutations are sufficient to inhibit miRNA biogenesis at the Dicing step. Abnormalities of the stress response are involved in the pathogenesis of neurodegeneration, including ALS. Accordingly, we describe a novel mechanism for modulating microRNA biogenesis under stress, involving stress granule formation and re-organization of DICER and AGO2 protein interactions with their partners. In line with this observation, enhancing DICER activity by a small molecule, enoxacin, is beneficial for neuromuscular function in two independent ALS mouse models. Characterizing miRNA biogenesis downstream of the stress response ties seemingly disparate pathways in neurodegeneration and further suggests that DICER and miRNAs affect neuronal integrity and are possible therapeutic targets.

Aberrant RNA homeostasis in amyotrophic lateral sclerosis: potential for new therapeutic targets?

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive motor neuron degeneration. The disease pathogenesis is multifaceted in that multiple cellular and molecular pathways have been identified as contributors to the disease progression. Consequently, numerous therapeutic targets have been pursued for clinical development, unfortunately with little success. The recent discovery of mutations in RNA modulating genes such as TARDBP/TDP-43, FUS/TLS or C9ORF72 changed our understanding of neurodegenerative mechanisms in ALS and introduced the role of dysfunctional RNA processing as a significant contributor to disease pathogenesis. This article discusses the latest findings on such RNA toxicity pathways in ALS and potential novel therapeutic approaches.

The function of RNA-binding proteins at the synapse: implications for neurodegeneration

The loss of synapses is a central event in neurodegenerative diseases. Synaptic proteins are often associated with disease neuropathology, but their role in synaptic loss is not fully understood. Of the many processes involved in sustaining the integrity of synapses, local protein translation can directly impact synaptic formation, communication, and maintenance. RNA-binding proteins and their association with RNA granules serve to regulate mRNA transportation and translation at synapses and in turn regulate the synapse. Genetic mutations in RNA-binding proteins FUS and TDP-43 have been linked with causing neurodegenerative diseases: amyotrophic lateral sclerosis and frontotemporal dementia. The observation that mutations in FUS and TDP-43 coincide with changes in RNA granules provides evidence that dysfunction of RNA metabolism may underlie the mechanism of synaptic loss in these diseases. However, we do not know how mutations in RNA-binding proteins would affect RNA granule dynamics and local translation, or if these alterations would cause neurodegeneration. Further investigation into this area will lead to important insights into how disruption of RNA metabolism and local translation at synapses can cause neurodegenerative diseases.

Amyotrophic lateral sclerosis: mechanisms and therapeutics in the epigenomic era

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease of the motor neurons, which results in weakness and atrophy of voluntary skeletal muscles. Treatments do not modify the disease trajectory effectively, and only modestly improve survival. A complex interaction between genes, environmental exposure and impaired molecular pathways contributes to pathology in patients with ALS. Epigenetic mechanisms control the hereditary and reversible regulation of gene expression without altering the basic genetic code. Aberrant epigenetic patterns-including abnormal microRNA (miRNA) biogenesis and function, DNA modifications, histone remodeling, and RNA editing-are acquired throughout life and are influenced by environmental factors. Thus, understanding the molecular processes that lead to epigenetic dysregulation in patients with ALS might facilitate the discovery of novel therapeutic targets and biomarkers that could reduce diagnostic delay. These achievements could prove crucial for successful disease modification in patients with ALS. We review the latest findings regarding the role of miRNA modifications and other epigenetic mechanisms in ALS, and discuss their potential as therapeutic targets.

G3BP1 promotes stress-induced RNA granule interactions to preserve polyadenylated mRNA

The TDP-43 target G3BP1 is essential for a functional interaction between stress granules and processing bodies.

G3BP1, a target of TDP-43, is required for normal stress granule (SG) assembly, but the functional consequences of failed SG assembly remain unknown. Here, using both transformed cell lines and primary neurons, we investigated the functional impact of this disruption in SG dynamics. While stress-induced translational repression and recruitment of key SG proteins was undisturbed, depletion of G3BP1 or its upstream regulator TDP-43 disturbed normal interactions between SGs and processing bodies (PBs). This was concomitant with decreased SG size, reduced SG–PB docking, and impaired preservation of polyadenylated mRNA. Reintroduction of G3BP1 alone was sufficient to rescue all of these phenotypes, indicating that G3BP1 is essential for normal SG–PB interactions and SG function.

TDP-43 Proteinopathy and ALS: Insights into Disease Mechanisms and Therapeutic Targets

Therapeutic options for patients with amyotrophic lateral sclerosis (ALS) are currently limited. However, recent studies show that almost all cases of ALS, as well as tau-negative frontotemporal dementia (FTD), share a common neuropathology characterized by the deposition of TAR-DNA binding protein (TDP)-43-positive protein inclusions, offering an attractive target for the design and testing of novel therapeutics. Here we demonstrate how diverse environmental stressors linked to stress granule formation, as well as mutations in genes encoding RNA processing proteins and protein degradation adaptors, initiate ALS pathogenesis via TDP-43. We review the progressive development of TDP-43 proteinopathy from cytoplasmic mislocalization and misfolding through to macroaggregation and the addition of phosphate and ubiquitin moieties. Drawing from cellular and animal studies, we explore the feasibility of therapeutics that act at each point in pathogenesis, from mitigating genetic risk using antisense oligonucleotides to modulating TDP-43 proteinopathy itself using small molecule activators of autophagy, the ubiquitin-proteasome system, or the chaperone network. We present the case that preventing the misfolding of TDP-43 and/or enhancing its clearance represents the most important target for effectively treating ALS and frontotemporal dementia.

Linking RNA Dysfunction and Neurodegeneration in Amyotrophic Lateral Sclerosis

The degeneration of motor neurons in amyotrophic lateral sclerosis (ALS) inevitably causes paralysis and death within a matter of years. Mounting genetic and functional evidence suggest that abnormalities in RNA processing and metabolism underlie motor neuron loss in sporadic and familial ALS. Abnormal localization and aggregation of essential RNA-binding proteins are fundamental pathological features of sporadic ALS, and mutations in genes encoding RNA processing enzymes cause familial disease. Also, expansion mutations occurring in the noncoding region of C9orf72-the most common cause of inherited ALS-result in nuclear RNA foci, underscoring the link between abnormal RNA metabolism and neurodegeneration in ALS. This review summarizes the current understanding of RNA dysfunction in ALS, and builds upon this knowledge base to identify converging mechanisms of neurodegeneration in ALS. Potential targets for therapy development are highlighted, with particular emphasis on early and conserved pathways that lead to motor neuron loss in ALS.

Cytoplasmic hGle1A regulates stress granules by modulation of translation

When eukaryotic cells respond to stress, gene expression pathways change to selectively export and translate subsets of mRNAs. Translationally repressed mRNAs accumulate in cytoplasmic foci known as stress granules (SGs). SGs are in dynamic equilibrium with the translational machinery, but mechanisms controlling this are unclear. Gle1 is required for DEAD-box protein function during mRNA export and translation. We document that human Gle1 (hGle1) is a critical regulator of translation during stress. hGle1 is recruited to SGs, and hGLE1 small interfering RNA-mediated knockdown perturbs SG assembly, resulting in increased numbers of smaller SGs. The rate of SG disassembly is also delayed. Furthermore, SG hGle1-depletion defects correlate with translation perturbations, and the hGle1 role in SGs is independent of mRNA export. Interestingly, we observe isoform-specific roles for hGle1 in which SG function requires hGle1A, whereas mRNA export requires hGle1B. We find that the SG defects in hGle1-depleted cells are rescued by puromycin or DDX3 expression. Together with recent links of hGLE1 mutations in amyotrophic lateral sclerosis patients, these results uncover a paradigm for hGle1A modulating the balance between translation and SGs during stress and disease.

From nucleation to widespread propagation: A prion-like concept for ALS

Propagation of pathological protein assemblies via a prion-like mechanism has been suggested to drive neurodegenerative diseases, such as Parkinson's and Alzheimer's. Recently, amyotrophic lateral sclerosis (ALS)-linked proteins, such as SOD1, TDP-43 and FUS were shown to follow self-perpetuating seeded aggregation, thereby adding ALS to the group of prion-like disorders. The cell-to-cell spread of these pathological protein assemblies and their pathogenic mechanism is poorly understood. However, as ALS is a non-cell autonomous disease and pathology in glial cells was shown to contribute to motor neuron damage, spreading mechanisms are likely to underlie disease progression via the interplay between affected neurons and their neighboring glial cells.

Futsch/MAP1B mRNA is a translational target of TDP-43 and is neuroprotective in a Drosophila model of amyotrophic lateral sclerosis

TDP-43 is an RNA-binding protein linked to amyotrophic lateral sclerosis (ALS) that is known to regulate the splicing, transport, and storage of specific mRNAs into stress granules. Although TDP-43 has been shown to interact with translation factors, its role in protein synthesis remains unclear, and no in vivo translation targets have been reported to date. Here we provide evidence that TDP-43 associates with futsch mRNA in a complex and regulates its expression at the neuromuscular junction (NMJ) in Drosophila. In the context of TDP-43-induced proteinopathy, there is a significant reduction of futsch mRNA at the NMJ compared with motor neuron cell bodies where we find higher levels of transcript compared with controls. TDP-43 also leads to a significant reduction in Futsch protein expression at the NMJ. Polysome fractionations coupled with quantitative PCR experiments indicate that TDP-43 leads to a futsch mRNA shift from actively translating polysomes to nontranslating ribonuclear protein particles, suggesting that in addition to its effect on localization, TDP-43 also regulates the translation of futsch mRNA. We also show that futsch overexpression is neuroprotective by extending life span, reducing TDP-43 aggregation, and suppressing ALS-like locomotor dysfunction as well as NMJ abnormalities linked to microtubule and synaptic stabilization. Furthermore, the localization of MAP1B, the mammalian homolog of Futsch, is altered in ALS spinal cords in a manner similar to our observations in Drosophila motor neurons. Together, our results suggest a microtubule-dependent mechanism in motor neuron disease caused by TDP-43-dependent alterations in futsch mRNA localization and translation in vivo.

Staurosporine induced apoptosis rapidly downregulates TDP- 43 in glioma cells

TDP-43 is a ubiquitously expressed DNA/RNA binding protein that has recently attracted attention for its involvement in neurodegenerative diseases. While TDP-43 has been found to participate in various important cellular activities including stress and apoptosis, little is known about its role in cancer cells. Here we report that staurosporine (STS) induced apoptosis in U87 glioma cells is associated with rapid downregulation of TDP-43 at both mRNA and protein levels. The latter is dependent on activation of caspase 3. More importantly, we have shown that knockdown of TDP-43 by specific siRNA dramatically enhanced cytotoxicity of STS. These results suggest that normal level of TDP-43 may be protective for cancer cells under apoptotic insult.

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.

RNA-processing protein TDP-43 regulates FOXO-dependent protein quality control in stress response

Protein homeostasis is critical for cell survival and functions during stress and is regulated at both RNA and protein levels. However, how the cell integrates RNA-processing programs with post-translational protein quality control systems is unknown. Transactive response DNA-binding protein (TARDBP/TDP-43) is an RNA-processing protein that is involved in the pathogenesis of major neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we report a conserved role for TDP-43, from C. elegans to mammals, in the regulation of protein clearance via activation of FOXO transcription factors. In response to proteotoxic insults, TDP-43 redistributes from the nucleus to the cytoplasm, promoting nuclear translocation of FOXOs and relieving an inhibition of FOXO activity in the nucleus. The interaction between TDP-43 and the FOXO pathway in mammalian cells is mediated by their competitive binding to 14-3-3 proteins. Consistent with FOXO-dependent protein quality control, TDP-43 regulates the levels of misfolded proteins. Therefore, TDP-43 mediates stress responses and couples the regulation of RNA metabolism and protein quality control in a FOXO-dependent manner. The results suggest that compromising the function of TDP-43 in regulating protein homeostasis may contribute to the pathogenesis of related neurodegenerative diseases.

TDP-43 is linked to pathogenesis of major neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). How TDP-43 contributes to the development of these degenerative diseases remains unsolved, and the full range of TDP-43 functions has yet to be established. In the present study, we explored a conversed function of TDP-43 in regulating protein homeostasis from C. elegans to mammals. Under conditions of stress, TDP-43 translocates from the nucleus to the cytoplasm, competes with FOXO transcription factors for binding to 14-3-3 proteins, and releases FOXO for nuclear translocation and activation. These data are consistent with the ability of TDP-43 to regulate protein aggregation. Together the results provide important insight into the role of TDP-43 in stress responses and disease mechanisms. Since chronic stress is associated with neurodegenerative diseases, the TDP-43 switch could be kept in overdrive mode in these disorders, with its capacity to buffer further stress and maintain protein homeostasis being compromised. This mechanism also suggests that other RNA-processing proteins that exhibit similar stress-induced behavior may be coupled to other cellular pathways to provide coordinated reprogramming in stress responses.

Activity-dependent FUS dysregulation disrupts synaptic homeostasis

The RNA-binding protein fused-in-sarcoma (FUS) has been associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), two neurodegenerative disorders that share similar clinical and pathological features. Both missense mutations and overexpression of wild-type FUS protein can be pathogenic in human patients. To study the molecular and cellular basis by which FUS mutations and overexpression cause disease, we generated novel transgenic mice globally expressing low levels of human wild-type protein (FUS(WT)) and a pathological mutation (FUS(R521G)). FUS(WT) and FUS(R521G) mice that develop severe motor deficits also show neuroinflammation, denervated neuromuscular junctions, and premature death, phenocopying the human diseases. A portion of FUS(R521G) mice escape early lethality; these escapers have modest motor impairments and altered sociability, which correspond with a reduction of dendritic arbors and mature spines. Remarkably, only FUS(R521G) mice show dendritic defects; FUS(WT) mice do not. Activation of metabotropic glutamate receptors 1/5 in neocortical slices and isolated synaptoneurosomes increases endogenous mouse FUS and FUS(WT) protein levels but decreases the FUS(R521G) protein, providing a potential biochemical basis for the dendritic spine differences between FUS(WT) and FUS(R521G) mice.

The histone deacetylase inhibitor, Vorinostat, represses hypoxia inducible factor 1 alpha expression through translational inhibition

Hypoxia inducible factor 1α (HIF-1α) is a master regulator of tumor angiogenesis being one of the major targets for cancer therapy. Previous studies have shown that Histone Deacetylase Inhibitors (HDACi) block tumor angiogenesis through the inhibition of HIF-1α expression. As such, Vorinostat (Suberoylanilide Hydroxamic Acid/SAHA) and Romidepsin, two HDACis, were recently approved by the Food and Drug Administration (FDA) for the treatment of cutaneous T cell lymphoma. Although HDACis have been shown to affect HIF-1α expression by modulating its interactions with the Hsp70/Hsp90 chaperone axis or its acetylation status, the molecular mechanisms by which HDACis inhibit HIF-1α expression need to be further characterized. Here, we report that the FDA-approved HDACi Vorinostat/SAHA inhibits HIF-1α expression in liver cancer-derived cell lines, by a new mechanism independent of p53, prolyl-hydroxylases, autophagy and proteasome degradation. We found that SAHA or silencing of HDAC9 mechanism of action is due to inhibition of HIF-1α translation, which in turn, is mediated by the eukaryotic translation initiation factor - eIF3G. We also highlighted that HIF-1α translation is dramatically inhibited when SAHA is combined with eIF3H silencing. Taken together, we show that HDAC activity regulates HIF-1α translation, with HDACis such as SAHA representing a potential novel approach for the treatment of hepatocellular carcinoma.

Pathological stress granules in Alzheimer's disease

A feature of neurodegenerative disease is the accumulation of insoluble protein aggregates in the brain. In some conditions, including Amyotrophic Lateral Sclerosis and Frontotemporal lobar degeneration, the primary aggregating entities are RNA binding proteins. Through regulated prion-like assembly, RNA binding proteins serve many functions in RNA metabolism that are essential for the healthy maintenance of cells of the central nervous system. Those RNA binding proteins that are the core nucleating factors of stress granules (SGs), including TIA-1, TIAR, TTP and G3BP1, are also found in the pathological lesions of other neurological conditions, such as Alzheimer's disease, where the hallmark aggregating protein is not an RNA binding protein. This discovery suggests that the regulated cellular pathway, which utilizes assembly of RNA binding proteins to package and silence mRNAs during stress, may be integral in the aberrant pathological protein aggregation that occurs in numerous neurodegenerative conditions.

Autophagy induction enhances TDP43 turnover and survival in neuronal ALS models

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have distinct clinical features but a common pathology--cytoplasmic inclusions rich in transactive response element DNA-binding protein of 43 kDa (TDP43). Rare TDP43 mutations cause ALS or FTD, but abnormal TDP43 levels and localization may cause disease even if TDP43 lacks a mutation. Here we show that individual neurons vary in their ability to clear TDP43 and are exquisitely sensitive to TDP43 levels. To measure TDP43 clearance, we developed and validated a single-cell optical method that overcomes the confounding effects of aggregation and toxicity and discovered that pathogenic mutations shorten TDP43 half-life. New compounds that stimulate autophagy improved TDP43 clearance and localization and enhanced survival in primary murine neurons and in human stem cell-derived neurons and astrocytes harboring mutant TDP43. These findings indicate that the levels and localization of TDP43 critically determine neurotoxicity and show that autophagy induction mitigates neurodegeneration by acting directly on TDP43 clearance.