Targeted depletion of TDP-43 expression in the spinal cord motor neurons leads to the development of amyotrophic lateral sclerosis-like phenotypes in mice

Casein kinase II induced polymerization of soluble TDP-43 into filaments is inhibited by heat shock proteins

Background

Trans-activation Response DNA-binding Protein-43 (TDP-43) lesions are observed in Amyotrophic Lateral Sclerosis (ALS), Frontotemporal Lobar Degeneration with ubiquitin inclusions (FTLD-TDP) and 25–50% of Alzheimer's Disease (AD) cases. These abnormal protein inclusions are composed of either amorphous TDP-43 aggregates or highly ordered filaments. The filamentous TDP-43 accumulations typically contain clean 10–12 nm filaments though wider 18–20 nm coated filaments may be observed. The TDP-43 present within these lesions is phosphorylated, truncated and ubiquitinated, and these modifications appear to be abnormal as they are linked to both a cellular heat shock response and microglial activation. The mechanisms associated with this abnormal TDP-43 accumulation are believed to result in a loss of TDP-43 function, perhaps due to the post-translational modifications or resulting from physical sequestration of the TDP-43. The formation of TDP-43 inclusions involves cellular translocation and conversion of TDP-43 into fibrillogenic forms, but the ability of these accumulations to sequester normal TDP-43 and propagate this behavior between neurons pathologically is mostly inferred. The lack of methodology to produce soluble full length TDP-43 and recapitulate this polymerization into filaments as observed in disease has limited our understanding of these pathogenic cascades.

Results

The protocols described here generate soluble, full-length and untagged TDP-43 allowing for a direct assessment of the impact of various posttranslational modifications on TDP-43 function. We demonstrate that Casein Kinase II (CKII) promotes the polymerization of this soluble TDP-43 into 10 nm diameter filaments that resemble the most common TDP-43 structures observed in disease. Furthermore, these filaments are recognized as abnormal by Heat Shock Proteins (HSPs) which can inhibit TDP-43 polymerization or directly promote TDP-43 filament depolymerization.

Conclusion

These findings demonstrate CKII induces polymerization of soluble TDP-43 into filaments and Hsp90 promotes TDP-43 filament depolymerization. These findings provide rational for potential therapeutic intervention at these points in TDP-43 proteinopathies.

Divergent phenotypes in mutant TDP-43 transgenic mice highlight potential confounds in TDP-43 transgenic modeling

The majority of cases of frontotemporal lobar degeneration and amyotrophic lateral sclerosis are pathologically defined by the cleavage, cytoplasmic redistribution and aggregation of TAR DNA binding protein of 43 kDa (TDP-43). To examine the contribution of these potentially toxic mechanisms in vivo, we generated transgenic mice expressing human TDP-43 containing the familial amyotrophic lateral sclerosis-linked M337V mutation and identified two lines that developed neurological phenotypes of differing severity and progression. The first developed a rapid cortical neurodegenerative phenotype in the early postnatal period, characterized by fragmentation of TDP-43 and loss of endogenous murine Tdp-43, but entirely lacking aggregates of ubiquitin or TDP-43. A second, low expressing line was aged to 25 months without a severe neurodegenerative phenotype, despite a 30% loss of mouse Tdp-43 and accumulation of lower molecular weight TDP-43 species. Furthermore, TDP-43 fragments generated during neurodegeneration were not C-terminal, but rather were derived from a central portion of human TDP-43. Thus we find that aggregation is not required for cell loss, loss of murine Tdp-43 is not necessarily sufficient in order to develop a severe neurodegenerative phenotype and lower molecular weight TDP-43 positive species in mouse models should not be inherently assumed to be representative of human disease. Our findings are significant for the interpretation of other transgenic studies of TDP-43 proteinopathy.

A nonsense mutation in mouse Tardbp affects TDP43 alternative splicing activity and causes limb-clasping and body tone defects

Mutations in TARDBP, encoding Tar DNA binding protein-43 (TDP43), cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Attempts to model TDP43 dysfunction in mice have used knockouts or transgenic overexpressors, which have revealed the difficulties of manipulating TDP43, whose level is tightly controlled by auto-regulation. In a complementary approach, to create useful mouse models for the dissection of TDP43 function and pathology, we have identified a nonsense mutation in the endogenous mouse Tardbp gene through screening an N-ethyl-N-nitrosourea (ENU) mutant mouse archive. The mutation is predicted to cause a Q101X truncation in TDP43. We have characterised Tardbp(Q101X) mice to investigate this mutation in perturbing TDP43 biology at endogenous expression levels. We found the Tardbp(Q101X) mutation is homozygous embryonic lethal, highlighting the importance of TDP43 in early development. Heterozygotes (Tardbp(+/Q101X) ) have abnormal levels of mutant transcript, but we find no evidence of the truncated protein and mice have similar full-length TDP43 protein levels as wildtype littermates. Nevertheless, Tardbp(+/Q101X) mice have abnormal alternative splicing of downstream gene targets, and limb-clasp and body tone phenotypes. Thus the nonsense mutation in Tardbp causes a mild loss-of-function phenotype and behavioural assessment suggests underlying neurological abnormalities. Due to the role of TDP43 in ALS, we investigated potential interactions with another known causative gene, mutant superoxide dismutase 1 (SOD1). Tardbp(+/Q101X) mice were crossed with the SOD1(G93Adl) transgenic mouse model of ALS. Behavioural and physiological assessment did not reveal modifying effects on the progression of ALS-like symptoms in the double mutant progeny from this cross. In summary, the Tardbp(Q101X) mutant mice are a useful tool for the dissection of TDP43 protein regulation, effects on splicing, embryonic development and neuromuscular phenotypes. These mice are freely available to the community.

Physiological protein aggregation run amuck: stress granules and the genesis of neurodegenerative disease

Recent advances in neurodegenerative diseases point to novel mechanisms of protein aggregation. RNA binding proteins are abundant in the nucleus, where they carry out processes such as RNA splicing. Neurons also express RNA binding proteins in the cytoplasm and processes to enable functions such as mRNA transport and local protein synthesis. The biology of RNA binding proteins turns out to have important features that appear to promote the pathophysiology of amyotrophic lateral sclerosis and might contribute to other neurodegenerative disease. RNA binding proteins consolidate transcripts to form complexes, termed RNA granules, through a process of physiological aggregation mediated by glycine rich domains that exhibit low protein complexity and in some cases share homology to similar domains in known prion proteins. Under conditions of cell stress these RNA granules expand, leading to form stress granules, which function in part to sequester specialized transcript and promote translation of protective proteins. Studies in humans show that pathological aggregates occurring in ALS, Alzheimer's disease, and other dementias co-localize with stress granules. One increasingly appealing hypothesis is that mutations in RNA binding proteins or prolonged periods of stress cause formation of very stable, pathological stress granules. The consolidation of RNA binding proteins away from the nucleus and neuronal arbors into pathological stress granules might impair the normal physiological activities of these RNA binding proteins causing the neurodegeneration associated with these diseases. Conversely, therapeutic strategies focusing on reducing formation of pathological stress granules might be neuroprotective.

The ALS/FTLD-related RNA-binding proteins TDP-43 and FUS have common downstream RNA targets in cortical neurons

TDP-43 and FUS are linked to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), and loss of function of either protein contributes to these neurodegenerative conditions. To elucidate the TDP-43- and FUS-regulated pathophysiological RNA metabolism cascades, we assessed the differential gene expression and alternative splicing profiles related to regulation by either TDP-43 or FUS in primary cortical neurons. These profiles overlapped by >25% with respect to gene expression and >9% with respect to alternative splicing. The shared downstream RNA targets of TDP-43 and FUS may form a common pathway in the neurodegenerative processes of ALS/FTLD.

Converging mechanisms in ALS and FTD: disrupted RNA and protein homeostasis

Breakthrough discoveries identifying common genetic causes for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have transformed our view of these disorders. They share unexpectedly similar signatures, including dysregulation in common molecular players including TDP-43, FUS/TLS, ubiquilin-2, VCP, and expanded hexanucleotide repeats within the C9ORF72 gene. Dysfunction in RNA processing and protein homeostasis is an emerging theme. We present the case here that these two processes are intimately linked, with disease-initiated perturbation of either leading to further deviation of both protein and RNA homeostasis through a feedforward loop including cell-to-cell prion-like spread that may represent the mechanism for relentless disease progression.

Amyotrophic lateral sclerosis: an update on recent genetic insights

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease affecting both upper and lower motor neurons. The prognosis for ALS is extremely poor, but there is a limited course of treatment with only one approved medication. A most striking recent discovery is that TDP-43 is identified as a key molecule that is associated with both sporadic and familial forms of ALS. TDP-43 is not only a pathological hallmark, but also a genetic cause for ALS. Subsequently, a number of ALS-causative genes have been found. Above all, the RNA-binding protein, such as FUS, TAF15, EWSR1 and hnRNPA1, have structural and functional similarities to TDP-43, and physiological functions of some molecules, including VCP, UBQLN2, OPTN, FIG4 and SQSTM1, are involved in a protein degradation system. These discoveries provide valuable insight into the pathogenesis of ALS, and open doors for developing an effective disease-modifying therapy.

Downregulation of microRNA-9 in iPSC-derived neurons of FTD/ALS patients with TDP-43 mutations

Transactive response DNA-binding protein 43 (TDP-43) is a major pathological protein in frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). There are many disease-associated mutations in TDP-43, and several cellular and animal models with ectopic overexpression of mutant TDP-43 have been established. Here we sought to study altered molecular events in FTD and ALS by using induced pluripotent stem cell (iPSC) derived patient neurons. We generated multiple iPSC lines from an FTD/ALS patient with the TARDBP A90V mutation and from an unaffected family member who lacked the mutation. After extensive characterization, two to three iPSC lines from each subject were selected, differentiated into postmitotic neurons, and screened for relevant cell-autonomous phenotypes. Patient-derived neurons were more sensitive than control neurons to 100 nM straurosporine but not to other inducers of cellular stress. Three disease-relevant cellular phenotypes were revealed under staurosporine-induced stress. First, TDP-43 was localized in the cytoplasm of a higher percentage of patient neurons than control neurons. Second, the total TDP-43 level was lower in patient neurons with the A90V mutation. Third, the levels of microRNA-9 (miR-9) and its precursor pri-miR-9-2 decreased in patient neurons but not in control neurons. The latter is likely because of reduced TDP-43, as shRNA-mediated TDP-43 knockdown in rodent primary neurons also decreased the pri-miR-9-2 level. The reduction in miR-9 expression was confirmed in human neurons derived from iPSC lines containing the more pathogenic TARDBP M337V mutation, suggesting miR-9 downregulation might be a common pathogenic event in FTD/ALS. These results show that iPSC models of FTD/ALS are useful for revealing stress-dependent cellular defects of human patient neurons containing rare TDP-43 mutations in their native genetic contexts.

TDP-43 regulates the microprocessor complex activity during in vitro neuronal differentiation

TDP-43 (TAR DNA-binding protein 43) is an RNA-binding protein implicated in RNA metabolism at several levels. Even if ubiquitously expressed, it is considered as a neuronal activity-responsive factor and a major signature for neurological pathologies, making the comprehension of its activity in the nervous system a very challenging issue. TDP-43 has also been described as an accessory component of the Drosha-DGCR8 (DiGeorge syndrome critical region gene 8) microprocessor complex, which is crucially involved in basal and tissue-specific RNA processing events. In the present study, we exploited in vitro neuronal differentiation systems to investigate the TDP-43 demand for the microprocessor function, focusing on both its canonical microRNA biosynthetic activity and its alternative role as a post-transcriptional regulator of gene expression. Our findings reveal a novel role for TDP-43 as an essential factor that controls the stability of Drosha protein during neuronal differentiation, thus globally affecting the production of microRNAs. We also demonstrate that TDP-43 is required for the Drosha-mediated regulation of Neurogenin 2, a master gene orchestrating neurogenesis, whereas post-transcriptional control of Dgcr8, another Drosha target, resulted to be TDP-43-independent. These results implicate a previously uncovered contribution of TDP-43 in regulating the abundance and the substrate specificity of the microprocessor complex and provide new insights into TDP-43 as a key player in neuronal differentiation.

Disease animal models of TDP-43 proteinopathy and their pre-clinical applications

Frontotemperal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) are two common neurodegenerative diseases. TDP-43 is considered to be a major disease protein in FTLD/ALS, but it’s exact role in the pathogenesis and the effective treatments remains unknown. To address this question and to determine a potential treatment for FTLD/ALS, the disease animal models of TDP-43 proteinopathy have been established. TDP-43 proteinopathy is the histologic feature of FTLD/ALS and is associated with disease progression. Studies on the disease animal models with TDP-43 proteinopathy and their pre-clinical applications are reviewed and summarized. Through these disease animal models, parts of TDP-43 functions in physiological and pathological conditions will be better understood and possible treatments for FTLD/ALS with TDP-43 proteinopathy may be identified for possible clinical applications in the future.

Rodent models of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterised by the degeneration of upper and lower motor neurons. Recent advances in our understanding of some of the genetic causes of ALS, such as mutations in SOD1, TARDBP, FUS and VCP have led to the generation of rodent models of the disease, as a strategy to help our understanding of the pathophysiology of ALS and to assist in the development of therapeutic strategies. This review provides detailed descriptions of TDP-43, FUS and VCP models of ALS, and summarises potential therapeutics which have been recently trialled in rodent models of the disease. This article is part of a Special Issue entitled: Animal Models of Disease.

The pathological phenotypes of human TDP-43 transgenic mouse models are independent of downregulation of mouse Tdp-43

Tar DNA binding protein 43 (TDP-43) is the major component of pathological deposits in frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP) and in amyotrophic lateral sclerosis (ALS). It has been reported that TDP-43 transgenic mouse models expressing human TDP-43 wild-type or ALS-associated mutations recapitulate certain ALS and FTLD pathological phenotypes. Of note, expression of human TDP-43 (hTDP-43) reduces the levels of mouse Tdp-43 (mTdp-43). However, it remained unclear whether the mechanisms through which TDP-43 induces ALS or FTLD-like pathologies resulted from a reduction in mTdp-43, an increase in hTDP-43, or a combination of both. In elucidating the role of mTdp-43 and hTDP-43 in hTDP-43 transgenic mice, we observed that reduction of mTdp-43 in non-transgenic mice by intraventricular brain injection of AAV1-shTardbp leads to a dramatic increase in the levels of splicing variants of mouse sortilin 1 and translin. However, the levels of these two abnormal splicing variants are not increased in hTDP-43 transgenic mice despite significant downregulation of mTdp-43 in these mice. Moreover, further downregulation of mTdp-43 in hTDP-43 hemizygous mice, which are asymptomatic, to the levels equivalent to that of mTdp-43 in hTDP-43 homozygous mice does not induce the pathological phenotypes observed in the homozygous mice. Lastly, the number of dendritic spines and the RNA levels of TDP-43 RNA targets critical for synapse formation and function are significantly decreased in symptomatic homozygous mice. Together, our findings indicate that mTdp-43 downregulation does not lead to a loss of function mechanism or account for the pathological phenotypes observed in hTDP-43 homozygous mice because hTDP-43 compensates for the reduction, and associated functions of mTdp-43. Rather, expression of hTDP-43 beyond a certain threshold leads to abnormal metabolism of TDP-43 RNA targets critical for neuronal structure and function, which might be responsible for the ALS or FTLD-like pathologies observed in homozygous hTDP-43 transgenic mice.

RNA binding mediates neurotoxicity in the transgenic Drosophila model of TDP-43 proteinopathy

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive and selective loss of motor neurons. The discovery of mutations in the gene encoding an RNA-binding protein, TAR DNA-binding protein of 43 kD (TDP-43), in familial ALS, strongly implicated abnormalities in RNA processing in the pathogenesis of ALS, although the mechanisms whereby TDP-43 leads to neurodegeneration remain elusive. To clarify the mechanism of degeneration caused by TDP-43, we generated transgenic Drosophila melanogaster expressing a series of systematically modified human TDP-43 genes in the retinal photoreceptor neurons. Overexpression of wild-type TDP-43 resulted in vacuolar degeneration of the photoreceptor neurons associated with thinning of the retina, which was significantly exacerbated by mutations of TDP-43 linked to familial ALS or disrupting its nuclear localization signal (NLS). Remarkably, these degenerative phenotypes were completely normalized by addition of a mutation or deletion of the RNA recognition motif that abolishes the RNA binding ability of TDP-43. Altogether, our results suggest that RNA binding is key to the neurodegeneration caused by overexpression of TDP-43, and that abnormalities in RNA processing may be crucial to the pathogenesis of TDP-43 proteinopathy.

Similar dose-dependence of motor neuron cell death caused by wild type human TDP-43 and mutants with ALS-associated amino acid substitutions

Background

TDP-43, a multi-functional DNA/ RNA-binding protein encoded by the TARDBP gene, has emerged as a major patho-signature factor of the ubiquitinated intracellular inclusions (UBIs) in the diseased cells of a range of neurodegenerative diseases. Mutations in at least 9 different genes including TARDBP have been identified in ALS with TDP-43 (+)-UBIs. Thus far, the pathogenic role(s) of the more than 30 ALS-associated mutations in the TARDBP gene has not been well defined.

Results

By transient DNA transfection studies, we show that exogenously expressed human TDP-43 (hTDP-43), either wild type (WT) or 2 different ALS mutant (MT) forms, could cause significantly higher apoptotic death rate of a mouse spinal motor neuron-like cell line (NSC34) than other types of cells, e.g. mouse neuronal Neuro2a and human fibroblast HEK293T cells. Furthermore, at the same plasmid DNA dose(s) used for transfection, the percentages of NSC34 cell death caused by the 2 exogenously expressed hTDP-43 mutants are all higher than that caused by the WT hTDP-43. Significantly, the above observations are correlated with higher steady-state levels of the mutant hTDP-43 proteins as well as their stabilities than the WT.

Conclusions

Based on these data and previous transgenic TDP-43 studies in animals or cell cultures, we suggest that one major common consequence of the different ALS-associated TDP-43 mutations is the stabilization of the hTDP-43 polypeptide. The resulting elevation of the steady state level of hTDP-43 in combination with the relatively low tolerance of the spinal motor neurons to the increased amount of hTDP-43 lead to the neurodegeneration and pathogenesis of ALS, and of diseases with TDP-43 proteinopathies in general.

Expression of ALS-linked TDP-43 mutant in astrocytes causes non-cell-autonomous motor neuron death in rats

Mutation of Tar DNA-binding protein 43 (TDP-43) is linked to amyotrophic lateral sclerosis. Although astrocytes have important roles in neuron function and survival, their potential contribution to TDP-43 pathogenesis is unclear. Here, we created novel lines of transgenic rats that express a mutant form of human TDP-43 (M337V substitution) restricted to astrocytes. Selective expression of mutant TDP-43 in astrocytes caused a progressive loss of motor neurons and the denervation atrophy of skeletal muscles, resulting in progressive paralysis. The spinal cord of transgenic rats also exhibited a progressive depletion of the astroglial glutamate transporters GLT-1 and GLAST. Astrocytic expression of mutant TDP-43 led to activation of astrocytes and microglia, with an induction of the neurotoxic factor Lcn2 in reactive astrocytes that was independent of TDP-43 expression. These results indicate that mutant TDP-43 in astrocytes is sufficient to cause non-cell-autonomous death of motor neurons. This motor neuron death likely involves deficiency in neuroprotective genes and induction of neurotoxic genes in astrocytes.

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.

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.

Loss of ALS-associated TDP-43 in zebrafish causes muscle degeneration, vascular dysfunction, and reduced motor neuron axon outgrowth

Mutations in the Tar DNA binding protein of 43 kDa (TDP-43; TARDBP) are associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with TDP-43(+) inclusions (FTLD-TDP). To determine the physiological function of TDP-43, we knocked out zebrafish Tardbp and its paralogue Tardbp (TAR DNA binding protein-like), which lacks the glycine-rich domain where ALS- and FTLD-TDP-associated mutations cluster. tardbp mutants show no phenotype, a result of compensation by a unique splice variant of tardbpl that additionally contains a C-terminal elongation highly homologous to the glycine-rich domain of tardbp. Double-homozygous mutants of tardbp and tardbpl show muscle degeneration, strongly reduced blood circulation, mispatterning of vessels, impaired spinal motor neuron axon outgrowth, and early death. In double mutants the muscle-specific actin binding protein Filamin Ca is up-regulated. Strikingly, Filamin C is similarly increased in the frontal cortex of FTLD-TDP patients, suggesting aberrant expression in smooth muscle cells and TDP-43 loss-of-function as one underlying disease mechanism.

Mouse models of frontotemporal dementia

The pace of discovery in frontotemporal dementia (FTD) has accelerated dramatically with the discovery of new genetic causes and pathological substrates of the disease. MAPT/tau, GRN/progranulin, and C9ORF72 have emerged as common FTD genes, and TARDBP/TDP-43, VCP, FUS, and CHMP2B have been identified as less common genetic causes. TDP-43 and FUS have joined tau as common neuropathological substrates of the disease. Mouse models provide an important tool for understanding the role of these molecules in FTD pathogenesis. Here, we review recent progress with mouse models based on tau, TDP-43, progranulin, VCP, and CHMP2B. We also consider future prospects for FTD models, including developing new models to address unanswered questions. There are also opportunities for capitalizing on conservation of the salience network, which is selectively vulnerable in FTD, and the availability of FTD-related behavioral paradigms to analyze mouse models of the disease.

ALS-linked TDP-43 mutations produce aberrant RNA splicing and adult-onset motor neuron disease without aggregation or loss of nuclear TDP-43

Transactivating response region DNA binding protein (TDP-43) is the major protein component of ubiquitinated inclusions found in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) with ubiquitinated inclusions. Two ALS-causing mutants (TDP-43(Q331K) and TDP-43(M337V)), but not wild-type human TDP-43, are shown here to provoke age-dependent, mutant-dependent, progressive motor axon degeneration and motor neuron death when expressed in mice at levels and in a cell type-selective pattern similar to endogenous TDP-43. Mutant TDP-43-dependent degeneration of lower motor neurons occurs without: (i) loss of TDP-43 from the corresponding nuclei, (ii) accumulation of TDP-43 aggregates, and (iii) accumulation of insoluble TDP-43. Computational analysis using splicing-sensitive microarrays demonstrates alterations of endogenous TDP-43-dependent alternative splicing events conferred by both human wild-type and mutant TDP-43(Q331K), but with high levels of mutant TDP-43 preferentially enhancing exon exclusion of some target pre-mRNAs affecting genes involved in neurological transmission and function. Comparison with splicing alterations following TDP-43 depletion demonstrates that TDP-43(Q331K) enhances normal TDP-43 splicing function for some RNA targets but loss-of-function for others. Thus, adult-onset motor neuron disease does not require aggregation or loss of nuclear TDP-43, with ALS-linked mutants producing loss and gain of splicing function of selected RNA targets at an early disease stage.

TDP-43 loss-of-function causes neuronal loss due to defective steroid receptor-mediated gene program switching in Drosophila

TDP-43 proteinopathy is strongly implicated in the pathogenesis of amyotrophic lateral sclerosis and related neurodegenerative disorders. Whether TDP-43 neurotoxicity is caused by a novel toxic gain-of-function mechanism of the aggregates or by a loss of its normal function is unknown. We increased and decreased expression of TDP-43 (dTDP-43) in Drosophila. Although upregulation of dTDP-43 induced neuronal ubiquitin and dTDP-43-positive inclusions, both up- and downregulated dTDP-43 resulted in selective apoptosis of bursicon neurons and highly similar transcriptome alterations at the pupal-adult transition. Gene network analysis and genetic validation showed that both up- and downregulated dTDP-43 directly and dramatically increased the expression of the neuronal microtubule-associated protein Map205, resulting in cytoplasmic accumulations of the ecdysteroid receptor (EcR) and a failure to switch EcR-dependent gene programs from a pupal to adult pattern. We propose that dTDP-43 neurotoxicity is caused by a loss of its normal function.

Fast motor axon loss in SMARD1 does not correspond to morphological and functional alterations of the NMJ

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a childhood motoneuron disease caused by mutations in the gene encoding for IGHMBP2, an ATPase/Helicase. Paralysis of the diaphragm is an early and prominent clinical sign resulting both from denervation and myopathy. In skeletal muscles, muscle atrophy mainly results from loss of motoneuron cell bodies and axonal degeneration. Although it is well known that loss of motoneurons at the lumbar spinal cord is an early event in the pathogenesis of the disease, it is not clear whether the corresponding proximal axons and NMJs are also early affected. In order to address this question, we have investigated the time course of the disease progression at the level of the motoneuron cell body, proximal axon (ventral root), distal axon (sciatic nerve), NMJ, and muscle fiber in Nmd(2J) mice, a mouse model for SMARD1. Our results show an early and apparently parallel loss of motoneurons, proximal axons, and NMJs. In affected muscles, however, denervated fibers coexist with NMJs with normal morphology and unaltered neurotransmission. Furthermore, unaffected axons are able to sprout and reinnervate muscle fibers, suggesting selective vulnerability of neurons to Ighmbp2 deficiency. The preservation of the NMJ morphology and neurotransmission in the Nmd(2J) mouse until motor axon loss takes place, differs from that observed in SMA mouse models in which NMJ impairment is an early and more general phenomenon in affected muscles.

RNA-mediated toxicity in neurodegenerative disease

Cellular viability depends upon the well-orchestrated functions carried out by numerous protein-coding and non-coding RNAs, as well as RNA-binding proteins. During the last decade, it has become increasingly evident that abnormalities in RNA processing represent a common feature among many neurodegenerative diseases. In "RNAopathies", which include diseases caused by non-coding repeat expansions, RNAs exert toxicity via diverse mechanisms: RNA foci formation, bidirectional transcription, and the production of toxic RNAs and proteins by repeat associated non-ATG translation. The mechanisms of toxicity in "RNA-binding proteinopathies", diseases in which RNA-binding proteins like TDP-43 and FUS play a prominent role, have yet to be fully elucidated. Nonetheless, both loss of function of the RNA binding protein, and a toxic gain of function resulting from its aggregation, are thought to be involved in disease pathogenesis. As part of the special issue on RNA and Splicing Regulation in Neurodegeneration, this review intends to explore the diverse RNA-related mechanisms contributing to neurodegeneration, with a special emphasis on findings emerging from animal models.

XBP1 depletion precedes ubiquitin aggregation and Golgi fragmentation in TDP-43 transgenic rats

Protein inclusion is a prominent feature of neurodegenerative diseases including frontotemporal lobar degeneration (FTLD) that is characterized by the presence of ubiquitinated TDP-43 inclusion. Presence of protein inclusions indicates an interruption to protein degradation machinery or the overload of misfolded proteins. In response to the increase in misfolded proteins, cells usually initiate a mechanism called unfolded protein response (UPR) to reduce misfolded proteins in the lumen of endoplasmic reticules. Here, we examined the effects of mutant TDP-43 on the UPR in transgenic rats that express mutant human TDP-43 restrictedly in the neurons of the forebrain. Over-expression of mutant TDP-43 in rats caused prominent aggregation of ubiquitin and remarkable fragmentation of Golgi complexes prior to neuronal loss. While ubiquitin aggregates and Golgi fragments were accumulating, neurons expressing mutant TDP-43 failed to up-regulate chaperones residing in the endoplasmic reticules and failed to initiate the UPR. Prior to ubiquitin aggregation and Golgi fragmentation, neurons were depleted of X-box-binding protein 1 (XBP1), a key player of UPR machinery. Although it remains to determine how mutation of TDP-43 leads to the failure of the UPR, our data demonstrate that failure of the UPR is implicated in TDP-43 pathogenesis.