Kumi Kaneko

beta Subunits of voltage-gated sodium channels are novel substrates of beta-site amyloid precursor protein-cleaving enzyme (BACE1) and gamma-secretase

Sequential processing of amyloid precursor protein (APP) by membrane-bound proteases, BACE1 and γ-secretase, plays a crucial role in the pathogenesis of Alzheimer disease. Much has been discovered on the properties of these proteases; however, regulatory mechanisms of enzyme-substrate interaction in neurons and their involvement in pathological changes are still not fully understood. It is mainly because of the membrane-associated cleavage of these proteases and the lack of information on new substrates processed in a similar way to APP. Here, using RNA interference-mediated BACE1 knockdown, mouse embryonic fibroblasts that are deficient in either BACE1 or presenilins, and BACE1-deficient mouse brain, we show clear evidence that β subunits of voltage-gated sodium channels are sequentially processed by BACE1 and γ-secretase. These results may provide new insights into the underlying pathology of Alzheimer disease.

Complete loss of post-translational modifications triggers fibrillar aggregation of SOD1 in the familial form of amyotrophic lateral sclerosis

Dominant mutations in Cu,Zn-superoxide dismutase (SOD1) cause a familial form of amyotrophic lateral sclerosis (fALS), and aggregation of mutant SOD1 has been proposed to play a role in neurodegeneration. A growing body of evidence suggests that fALS-causing mutations destabilize the native structure of SOD1, leading to aberrant protein interactions for aggregation. SOD1 becomes stabilized and enzymatically active after copper and zinc binding and intramolecular disulfide formation, but it remains unknown which step(s) in the SOD1 maturation process is important in the pathological aggregation. In this study we have shown that apoSOD1 without disulfide is the most facile state for formation of amyloid-like fibrillar aggregates. fALS mutations impair either zinc binding, disulfide formation, or both, leading to accumulation of the aggregation-prone, apo, and disulfide-reduced SOD1. Moreover, we have found that the copper chaperone for SOD1 (CCS) facilitates maturation of SOD1 and that CCS overexpression ameliorates intracellular aggregation of mutant SOD1 in vivo. Based on our in vivo and in vitro results, we propose that facilitation of post-translational modifications is a promising strategy to reduce SOD1 aggregation in the cell.

A Seeding Reaction Recapitulates Intracellular Formation of Sarkosyl-insoluble Transactivation Response Element (TAR) DNA-binding Protein-43 Inclusions

The transactivation response element (TAR) DNA-binding protein-43 (TDP-43) is a nuclear protein that normally regulates transcription and splicing. Abnormal accumulation of insoluble inclusions containing TDP-43 has been recently reported in the affected tissues of amyotrophic lateral sclerosis (ALS) patients. Here, we show that intracellular aggregation of TDP-43 can be triggered by transduction of fibrillar aggregates prepared from in vitro functional TDP-43. Sarkosyl is found to be incapable of solubilizing those intracellularly seeded aggregates of TDP-43, which is consistent with the observation that TDP-43 inclusions in ALS patients are sarkosyl-insoluble. In addition, intracellular seeding in our cell models reproduces ubiquitination of TDP-43 aggregates, which is another prominent feature of TDP-43 inclusions in ALS patients. Although it has been so far difficult to initiate disease-associated changes of TDP-43 using cultured cell models, we propose that a seeding reaction is a key to construct a model to monitor TDP-43 pathologies.

Crystal structure of an active form of BACE1, an enzyme responsible for amyloid beta protein production

ABSTRACT BACE1 (β-secretase) is a transmembrane aspartic protease that cleaves the β-amyloid precursor protein and generates the amyloid β peptide (Aβ). BACE1 cycles between the cell surface and the endosomal system many times and becomes activated interconvertibly during its cellular trafficking, leading to the production of Aβ. Here we report the crystal structure of the catalytically active form of BACE1. The active form has novel structural features involving the conformation of the flap and subsites that promote substrate binding. The functionally essential residues and water molecules are well defined and play a key role in the iterative activation of BACE1. We further describe the crystal structure of the dehydrated form of BACE1, showing that BACE1 activity is dependent on the dynamics of a catalytically required Asp-bound water molecule, which directly affects its catalytic properties. These findings provide insight into a novel regulation of BACE1 activity and elucidate how BACE1 modulates its activity during cellular trafficking.

Sodium channel β4 subunit : down-regulation and possible involvement in neuritic degeneration in Huntington's disease transgenic mice

Sodium channel β4 is a very recently identified auxiliary subunit of the voltage‐gated sodium channels. To find the primarily affected gene in Huntingtons disease (HD) pathogenesis, we profiled HD transgenic mice using a high‐density oligonucleotide array and identified β4 as an expressed sequence tag (EST) that was significantly down‐regulated in the striatum of HD model mice and patients. Reduction in β4 started at a presymptomatic stage in HD mice, whereas other voltage‐gated ion channel subunits were decreased later. In contrast, spinal cord neurons, which generate only negligible levels of expanded polyglutamine aggregates, maintained normal levels of β4 expression even at the symptomatic stage. Overexpression of β4 induced neurite outgrowth in Neuro2a cells, and caused a thickening of dendrites and increased density of dendritic spines in hippocampal primary neurons, indicating that β4 modulates neurite outgrowth activities. These results suggest that down‐regulation of β4 may lead to abnormalities of sodium channel and neurite degeneration in the striatum of HD transgenic mice and patients with HD.

Cross-Seeding Fibrillation of Q/N-Rich Proteins Offers New Pathomechanism of Polyglutamine Diseases

A pathological hallmark of the Huntingtons disease (HD) is intracellular inclusions containing a huntingtin (Htt) protein with an elongated polyglutamine tract. Aggregation of mutant Htt causes abnormal protein–protein interactions, and the functional dysregulation of aggregate-interacting proteins (AIPs) has been proposed as a pathomechanism of HD. Despite this, a molecular mechanism remains unknown how Htt aggregates sequester AIPs. We note an RNA-binding protein, TIA-1, as a model of AIPs containing a Q/N-rich sequence and suggest that in vitro and in vivo Htt fibrillar aggregates function as a structural template for inducing insoluble fibrillation of TIA-1. It is also plausible that such a cross-seeding activity of Htt aggregates represses the physiological function of TIA-1. We thus propose that Htt aggregates act as an intracellular hub for the cross-seeded fibrillation of Q/N-rich AIPs and that a cross-seeding reaction is a molecular origin to cause diverse pathologies in a polyglutamine disease.

Mutation-dependent polymorphism of Cu,Zn-superoxide dismutase aggregates in the familial form of amyotrophic lateral sclerosis

More than 100 different mutations in Cu,Zn-superoxide dismutase (SOD1) are linked to a familial form of amyotrophic lateral sclerosis (fALS). Pathogenic mutations facilitate fibrillar aggregation of SOD1, upon which significant structural changes of SOD1 have been assumed; in general, however, a structure of protein aggregate remains obscure. Here, we have identified a protease-resistant core in wild-type as well as fALS-causing mutant SOD1 aggregates. Three different regions within an SOD1 sequence are found as building blocks for the formation of an aggregate core, and fALS-causing mutations modulate interactions among these three regions to form a distinct core, namely SOD1 aggregates exhibit mutation-dependent structural polymorphism, which further regulates biochemical properties of aggregates such as solubility. Based upon these results, we propose a new pathomechanism of fALS in which mutation-dependent structural polymorphism of SOD1 aggregates can affect disease phenotypes.

Membrane microdomain switching: a regulatory mechanism of amyloid precursor protein processing

Neuronal activity has an impact on β cleavage of amyloid precursor protein (APP) by BACE1 to generate amyloid-β peptide (Aβ). However, the molecular mechanisms underlying this effect remain to be elucidated. Cholesterol dependency of β cleavage prompted us to analyze immunoisolated APP-containing detergent-resistant membranes from rodent brains. We found syntaxin 1 as a key molecule for activity-dependent regulation of APP processing in cholesterol-dependent microdomains. In living cells, APP associates with syntaxin 1–containing microdomains through X11–Munc18, which inhibits the APP–BACE1 interaction and β cleavage via microdomain segregation. Phosphorylation of Munc18 by cdk5 causes a shift of APP to BACE1-containing microdomains. Neuronal hyperactivity, implicated in Aβ overproduction, promotes the switching of APP microdomain association as well as β cleavage in a partially cdk5-dependent manner. We propose that microdomain switching is a mechanism of cholesterol- and activity-dependent regulation of APP processing in neurons.

Accelerated Disease Onset with Stabilized Familial Amyotrophic Lateral Sclerosis (ALS)-linked Mutant TDP-43 Proteins

Background: Dominant mutations in TDP-43 cause familial ALS. Results: Longer half-lives of mutant TDP-43 proteins correlated with earlier disease onset, and stabilized TDP-43 provoked protein insolubility, cleavage, RNA dysregulation, and cytotoxicity. Conclusion: Increased stability of TDP-43 causes toxicity through abnormal proteostasis and RNA dysregulation. Significance: This is the first TDP-43 cell model based on genotype-phenotype correlation of ALS patients. Abnormal protein accumulation is a pathological hallmark of neurodegenerative diseases, including accumulation of TAR DNA-binding protein 43 (TDP-43) in amyotrophic lateral sclerosis (ALS). Dominant mutations in the TDP-43 gene are causative for familial ALS; however, the relationship between mutant protein biochemical phenotypes and disease course and their significance to disease pathomechanism are not known. Here, we found that longer half-lives of mutant proteins correlated with accelerated disease onset. Based on our findings, we established a cell model in which chronic stabilization of wild-type TDP-43 protein provoked cytotoxicity and recapitulated pathogenic protein cleavage and insolubility to the detergent Sarkosyl, TDP-43 properties that have been observed in sporadic ALS lesions. Furthermore, these cells showed proteasomal impairment and dysregulation of their own mRNA levels. These results suggest that chronically increased stability of mutant or wild-type TDP-43 proteins results in a gain of toxicity through abnormal proteostasis.

Intranuclear Aggregation of Mutant FUS/TLS as a Molecular Pathomechanism of Amyotrophic Lateral Sclerosis

Background: Abnormal accumulation of mutant FUS/TLS is a pathological change in patients with amyotrophic lateral sclerosis (ALS). Results: A pathogenic mutation, G156E, increases propensities of FUS/TLS for aggregation in vitro and in vivo. Conclusion: Intranuclear aggregation of mutant FUS/TLS is a molecular pathomechanism of ALS. Significance: A loss of functional TLS/FUS in the nucleus will lead to neurodegeneration. Dominant mutations in FUS/TLS cause a familial form of amyotrophic lateral sclerosis (fALS), where abnormal accumulation of mutant FUS proteins in cytoplasm has been observed as a major pathological change. Many of pathogenic mutations have been shown to deteriorate the nuclear localization signal in FUS and thereby facilitate cytoplasmic mislocalization of mutant proteins. Several other mutations, however, exhibit no effects on the nuclear localization of FUS in cultured cells, and their roles in the pathomechanism of fALS remain obscure. Here, we show that a pathogenic mutation, G156E, significantly increases the propensities for aggregation of FUS in vitro and in vivo. Spontaneous in vitro formation of amyloid-like fibrillar aggregates was observed in mutant but not wild-type FUS, and notably, those fibrils functioned as efficient seeds to trigger the aggregation of wild-type protein. In addition, the G156E mutation did not disturb the nuclear localization of FUS but facilitated the formation of intranuclear inclusions in rat hippocampal neurons with significant cytotoxicity. We thus propose that intranuclear aggregation of FUS triggered by a subset of pathogenic mutations is an alternative pathomechanism of FUS-related fALS diseases.