Kensuke Ikenaka

Loss of TDP-43 causes age-dependent progressive motor neuron degeneration.

Amyotrophic lateral sclerosis is a devastating, progressive neurodegenerative disease that affects upper and lower motor neurons. Although several genes are identified as the cause of familial cases, the pathogeneses of sporadic forms, which account for 90% of amyotrophic lateral sclerosis, have not been elucidated. Transactive response DNA-binding protein 43 a nuclear protein regulating RNA processing, redistributes to the cytoplasm and forms aggregates, which are the histopathological hallmark of sporadic amyotrophic lateral sclerosis, in affected motor neurons, suggesting that loss-of-function of transactive response DNA-binding protein 43 is one of the causes of the neurodegeneration. To test this hypothesis, we assessed the effects of knockout of transactive response DNA-binding protein 43 in mouse postnatal motor neurons using Cre/loxp system. These mice developed progressive weight loss and motor impairment around the age of 60 weeks, and exhibited degeneration of large motor axon, grouped atrophy of the skeletal muscle, and denervation in the neuromuscular junction. The spinal motor neurons lacking transactive response DNA-binding protein 43 were not affected for 1 year, but exhibited atrophy at the age of 100 weeks; whereas, extraocular motor neurons, that are essentially resistant in amyotrophic lateral sclerosis, remained preserved even at the age of 100 weeks. Additionally, ultra structural analysis revealed autolysosomes and autophagosomes in the cell bodies and axons of motor neurons of the 100-week-old knockout mice. In summary, the mice in which transactive response DNA-binding protein 43 was knocked-out specifically in postnatal motor neurons exhibited an age-dependent progressive motor dysfunction accompanied by neuropathological alterations, which are common to sporadic amyotrophic lateral sclerosis. These findings suggest that transactive response DNA-binding protein 43 plays an essential role in the long term maintenance of motor neurons and that loss-of-function of this protein seems to contribute to the pathogenesis of amyotrophic lateral sclerosis.

dnc-1/dynactin 1 Knockdown Disrupts Transport of Autophagosomes and Induces Motor Neuron Degeneration

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive loss of motor neurons. We previously showed that the expression of dynactin 1, an axon motor protein regulating retrograde transport, is markedly reduced in spinal motor neurons of sporadic ALS patients, although the mechanisms by which decreased dynactin 1 levels cause neurodegeneration have yet to be elucidated. The accumulation of autophagosomes in degenerated motor neurons is another key pathological feature of sporadic ALS. Since autophagosomes are cargo of dynein/dynactin complexes and play a crucial role in the turnover of several organelles and proteins, we hypothesized that the quantitative loss of dynactin 1 disrupts the transport of autophagosomes and induces the degeneration of motor neuron. In the present study, we generated a Caenorhabditis elegans model in which the expression of DNC-1, the homolog of dynactin 1, is specifically knocked down in motor neurons. This model exhibited severe motor defects together with axonal and neuronal degeneration. We also observed impaired movement and increased number of autophagosomes in the degenerated neurons. Furthermore, the combination of rapamycin, an activator of autophagy, and trichostatin which facilitates axonal transport dramatically ameliorated the motor phenotype and axonal degeneration of this model. Thus, our results suggest that decreased expression of dynactin 1 induces motor neuron degeneration and that the transport of autophagosomes is a novel and substantial therapeutic target for motor neuron degeneration.

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.

Disruption of axonal transport in motor neuron diseases.

Motor neurons typically have very long axons, and fine-tuning axonal transport is crucial for their survival. The obstruction of axonal transport is gaining attention as a cause of neuronal dysfunction in a variety of neurodegenerative motor neuron diseases. Depletions in dynein and dynactin-1, motor molecules regulating axonal trafficking, disrupt axonal transport in flies, and mutations in their genes cause motor neuron degeneration in humans and rodents. Axonal transport defects are among the early molecular events leading to neurodegeneration in mouse models of amyotrophic lateral sclerosis (ALS). Gene expression profiles indicate that dynactin-1 mRNA is downregulated in degenerating spinal motor neurons of autopsied patients with sporadic ALS. Dynactin-1 mRNA is also reduced in the affected neurons of a mouse model of spinal and bulbar muscular atrophy, a motor neuron disease caused by triplet CAG repeat expansion in the gene encoding the androgen receptor. Pathogenic androgen receptor proteins also inhibit kinesin-1 microtubule-binding activity and disrupt anterograde axonal transport by activating c-Jun N-terminal kinase. Disruption of axonal transport also underlies the pathogenesis of spinal muscular atrophy and hereditary spastic paraplegias. These observations suggest that the impairment of axonal transport is a key event in the pathological processes of motor neuron degeneration and an important target of therapy development for motor neuron diseases.

Altered Tau Isoform Ratio Caused by Loss of FUS and SFPQ Function Leads to FTLD-like Phenotypes

Fused in sarcoma (FUS) and splicing factor, proline- and glutamine-rich (SFPQ) are RNA binding proteins that regulate RNA metabolism. We found that alternative splicing of the Mapt gene at exon 10, which generates 4-repeat tau (4R-T) and 3-repeat tau (3R-T), is regulated by interactions between FUS and SFPQ in the nuclei of neurons. Hippocampus-specific FUS- or SFPQ-knockdown mice exhibit frontotemporal lobar degeneration (FTLD)-like behaviors, reduced adult neurogenesis, accumulation of phosphorylated tau, and hippocampal atrophy with neuronal loss through an increased 4R-T/3R-T ratio. Normalization of this increased ratio by 4R-T-specific silencing results in recovery of the normal phenotype. These findings suggest a biological link among FUS/SFPQ, tau isoform alteration, and phenotypic expression, which may function in the early pathomechanism of FTLD.

Pioglitazone suppresses neuronal and muscular degeneration caused by polyglutamine-expanded androgen receptors

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by the expansion of a CAG repeat in the androgen receptor (AR) gene. Mutant AR has been postulated to alter the expression of genes important for mitochondrial function and induce mitochondrial dysfunction. Here, we show that the expression levels of peroxisome proliferator-activated receptor-γ (PPARγ), a key regulator of mitochondrial biogenesis, were decreased in mouse and cellular models of SBMA. Treatment with pioglitazone (PG), an activator of PPARγ, improved the viability of the cellular model of SBMA. The oral administration of PG also improved the behavioral and histopathological phenotypes of the transgenic mice. Furthermore, immunohistochemical and biochemical analyses demonstrated that the administration of PG suppressed oxidative stress, nuclear factor-κB (NFκB) signal activation and inflammation both in the spinal cords and skeletal muscles of the SBMA mice. These findings suggest that PG is a promising candidate for the treatment of SBMA.

Neuropathology and omics in motor neuron diseases

Motor neuron diseases, including amyotrophic lateral sclerosis (ALS), are devastating disorders and effective therapies have not yet been established. One of the reasons for this lack of therapeutics, especially in sporadic ALS (SALS), is attributed to the absence of excellent disease models reflecting its pathology. For this purpose, identifying important key molecules for ALS pathomechanisms and developing disease models is crucial, and omics approaches, including genomics, transcriptomics and proteomics, have been employed. In particular, transcriptome analysis using cDNA microarray is the most popular omics approach and we have previously identified dynactin‐1 as an important molecule downregulated in the motor neurons of SALS patients from the early stage of the disease. Dynactin‐1 is also known as a causative gene in familial ALS (FALS). Dynactin‐1 is a major component of the dynein/dynactin motor protein complex functioning in retrograde axonal transport. In motor neuron diseases as well as other neurodegenerative diseases, the role of axonal transport dysfunction in their pathogenesis always draws attention, but its precise mechanisms remain to be fully elucidated. In this article, we review our previous omics approach to SALS and the role of dynactin‐1 in the pathogenesis of ALS. Finally, we emphasize the need for creating novel SALS disease models based on the results of omics analysis, especially based on the observation that dynactin‐1 gene expression was downregulated in SALS motor neurons.

RNP2 of RNA Recognition Motif 1 Plays a Central Role in the Aberrant Modification of TDP-43

Phosphorylated and truncated TAR DNA-binding protein-43 (TDP-43) is a major component of ubiquitinated cytoplasmic inclusions in neuronal and glial cells of two TDP-43 proteinopathies, amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Modifications of TDP-43 are thus considered to play an important role in the pathogenesis of TDP-43 proteinopathies. However, both the initial cause of these abnormal modifications and the TDP-43 region responsible for its aggregation remain uncertain. Here we report that the 32 kDa C-terminal fragment of TDP-43, which lacks the RNP2 motif of RNA binding motif 1 (RRM1), formed aggregates in cultured cells, and that similar phenotypes were obtained when the RNP2 motif was either deleted from or mutated in full-length TDP-43. These aggregations were ubiquitinated, phosphorylated and truncated, and sequestered the 25 kDa C-terminal TDP-43 fragment seen in the neurons of TDP-43 proteinopathy patients. In addition, incubation with RNase decreased the solubility of TDP-43 in cell lysates. These findings suggest that the RNP2 motif of RRM1 plays a substantial role in pathological TDP-43 modifications and that it is possible that disruption of RNA binding may underlie the process of TDP-43 aggregation.

Cutaneous arteritis associated with peripheral neuropathy: two case reports.

Dear Editor, Primary vasculitis confined to a single organ without systemic manifestation is referred to as single organ vasculitis (SOV). SOV that affects the skins or peripheral nerves has been named cutaneous arteritis or non-systemic vasculitic neuropathy (NSVN), respectively. These isolated vasculitides predominantly affect middleor small-sized vessels within the lower dermis or epineurium and show relatively benign prognosis obtained by immunosuppressive therapies. We describe two cases of necrotizing vasculitis confined to the skin and peripheral nerves and discuss pathogenetic relationship between these isolated vasculitides. Case 1 was a 24-year-old woman who presented with sensory disturbance, progressive weakness, and pain in her hands and feet, which had started 3 months prior. Chemical exposure, daily alcohol consumption and a history of diabetes mellitus were absent. On admission, the skin of the lower extremities demonstrated multiple livedo reticularis, purpura and tenderness (Fig. 1a–b). The weakness and sensory disturbance showed a pattern of mononeuritis multiplex predominantly in her left foot, which was supported by electrophysiological studies. Other constitutional symptoms were absent. Routine examinations of blood chemistry, urine and cerebrospinal fluid were normal. Tests for eosinophil count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), thiamine, anti-immunoglobulin (Ig)G cardiolipin, anti-IgG prothrombin, anti-SS-A/B, anti-DNA, anti-Sm, antineutrophil cytoplasmic antibodies, lupus anticoagulants, cryoglobulins,