Tomohiro Kabuta

Degradation of amyotrophic lateral sclerosis-linked mutant Cu,Zn-superoxide dismutase proteins by macroautophagy and the proteasome

Mutations in the Cu,Zn-superoxide dismutase (SOD1) gene cause ∼20% of familial cases of amyotrophic lateral sclerosis (fALS). Accumulating evidence indicates that a gain of toxic function of mutant SOD1 proteins is the cause of the disease. It has also been shown that the ubiquitin-proteasome pathway plays a role in the clearance and toxicity of mutant SOD1. In this study, we investigated the degradation pathways of wild-type and mutant SOD1 in neuronal and nonneuronal cells. We provide here the first evidence that wild-type and mutant SOD1 are degraded by macroautophagy as well as by the proteasome. Based on experiments with inhibitors of these degradation pathways, the contribution of macroautophagy to mutant SOD1 clearance is comparable with that of the proteasome pathway. Using assays that measure cell viability and cell death, we observed that under conditions where expression of mutant SOD1 alone does not induce toxicity, macroautophagy inhibition induced mutant SOD1-mediated cell death, indicating that macroautophagy reduces the toxicity of mutant SOD1 proteins. We therefore propose that both macroautophagy and the proteasome are important for the reduction of mutant SOD1-mediated neurotoxicity in fALS. Inhibition of macroautophagy also increased SOD1 levels in detergent-soluble and -insoluble fractions, suggesting that both detergent-soluble and -insoluble SOD1 are degraded by macroautophagy. These findings may provide further insights into the mechanisms of pathogenesis of fALS.

Aberrant interaction between Parkinson disease-associated mutant UCH-L1 and the lysosomal receptor for chaperone-mediated autophagy.

Parkinson disease (PD) is the most common neurodegenerative movement disorder. An increase in the amount of α-synuclein protein could constitute a cause of PD. α-Synuclein is degraded at least partly by chaperone-mediated autophagy (CMA). The I93M mutation in ubiquitin C-terminal hydrolase L1 (UCH-L1) is associated with familial PD. However, the relationship between α-synuclein and UCH-L1 in the pathogenesis of PD has remained largely unclear. In this study, we found that UCH-L1 physically interacts with LAMP-2A, the lysosomal receptor for CMA, and Hsc70 and Hsp90, which can function as components of the CMA pathway. These interactions were abnormally enhanced by the I93M mutation and were independent of the monoubiquitin binding of UCH-L1. In a cell-free system, UCH-L1 directly interacted with the cytosolic region of LAMP-2A. Expression of I93M UCH-L1 in cells induced the CMA inhibition-associated increase in the amount of α-synuclein. Our findings may provide novel insights into the molecular links betweenα-synuclein and UCH-L1 and suggest that aberrant interaction of mutant UCH-L1 with CMA machinery, at least partly, underlies the pathogenesis of PD associated with I93M UCH-L1.

Dopaminergic neuronal loss in transgenic mice expressing the Parkinson's disease-associated UCH-L1 I93M mutant

The I93M mutation in ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) was reported in one German family with autosomal dominant Parkinsons disease (PD). The causative role of the mutation has, however, been questioned. We generated transgenic (Tg) mice carrying human UCHL1 under control of the PDGF-B promoter; two independent lines were generated with the I93M mutation (a high- and low-expressing line) and one line with wild-type human UCH-L1. We found a significant reduction in the dopaminergic neurons in the substantia nigra and the dopamine content in the striatum in the high-expressing I93M Tg mice as compared with non-Tg mice at 20 weeks of age. Although these changes were absent in the low-expressing I93M Tg mice, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment profoundly reduced dopaminergic neurons in this line as compared with wild-type Tg or non-Tg mice. Abnormal neuropathologies were also observed, such as silver staining-positive argyrophilic grains in the perikarya of degenerating dopaminergic neurons, in I93M Tg mice. The midbrains of I93M Tg mice contained increased amounts of insoluble UCH-L1 as compared with those of non-Tg mice, perhaps resulting in a toxic gain of function. Collectively, our data represent in vivo evidence that expression of UCHL1(I93M) leads to the degeneration of dopaminergic neurons.

Aberrant molecular properties shared by familial Parkinson’s disease-associated mutant UCH-L1 and carbonyl-modified UCH-L1

Parkinsons disease (PD) is a neurodegenerative disorder characterized by loss of dopaminergic neurons. The I93M mutation in ubiquitin C-terminal hydrolase L1 (UCH-L1) is associated with familial PD, and we have previously shown that the I93M UCH-L1-transgenic mice exhibit dopaminergic cell loss. Over 90% of neurodegenerative diseases, including PD, occur sporadically. However, the molecular mechanisms underlying sporadic PD as well as PD associated with I93M UCH-L1 are largely unknown. UCH-L1 is abundant (1-5% of total soluble protein) in the brain and is a major target of oxidative/carbonyl damage associated with sporadic PD. As well, abnormal microtubule dynamics and tubulin polymerization are associated with several neurodegenerative diseases including frontotemporal dementia and parkinsonism linked to chromosome 17. Here we show that familial PD-associated mutant UCH-L1 and carbonyl-modified UCH-L1 display shared aberrant properties: compared with wild-type UCH-L1, they exhibit increased insolubility and elevated interactions with multiple proteins, which are characteristics of several neurodegenerative diseases-linked mutants. Circular dichroism analyses suggest similar structural changes in both UCH-L1 variants. We further report that one of the proteins interacting with UCH-L1 is tubulin, and that aberrant interaction of mutant or carbonyl-modified UCH-L1 with tubulin modulates tubulin polymerization. These findings may underlie the toxic gain of function by mutant UCH-L1 in familial PD. Our results also suggest that the carbonyl modification of UCH-L1 and subsequent abnormal interactions of carbonyl-modified UCH-L1 with multiple proteins, including tubulin, constitute one of the causes of sporadic PD.

Discovery of a novel type of autophagy targeting RNA

Regulated degradation of cellular components by lysosomes is essential to maintain biological homeostasis. In mammals, three forms of autophagy, macroautophagy, microautophagy and chaperone-mediated autophagy (CMA), have been identified. Here, we showed a novel type of autophagy, in which RNA is taken up directly into lysosomes for degradation. This pathway, which we term “RNautophagy,” is ATP-dependent, and unlike CMA, is independent of HSPA8/Hsc70. LAMP2C, a lysosomal membrane protein, serves as a receptor for this pathway. The cytosolic tail of LAMP2C specifically binds to almost all total RNA derived from mouse brain. The cytosolic sequence of LAMP2C and its affinity for RNA are evolutionarily conserved from nematodes to humans. Our findings shed light on the mechanisms underlying RNA homeostasis in higher eukaryotes.

A novel 4EHP-GIGYF2 translational repressor complex is essential for mammalian development

ABSTRACT The binding of the eukaryotic initiation factor 4E (eIF4E) to the mRNA 5′ cap structure is a rate-limiting step in mRNA translation initiation. eIF4E promotes ribosome recruitment to the mRNA. In Drosophila, the eIF4E homologous protein (d4EHP) forms a complex with binding partners to suppress the translation of distinct mRNAs by competing with eIF4E for binding the 5′ cap structure. This repression mechanism is essential for the asymmetric distribution of proteins and normal embryonic development in Drosophila. In contrast, the physiological role of the mammalian 4EHP (m4EHP) was not known. In this study, we have identified the Grb10-interacting GYF protein 2 (GIGYF2) and the zinc finger protein 598 (ZNF598) as components of the m4EHP complex. GIGYF2 directly interacts with m4EHP, and this interaction is required for stabilization of both proteins. Disruption of the m4EHP-GIGYF2 complex leads to increased translation and perinatal lethality in mice. We propose a model by which the m4EHP-GIGYF2 complex represses translation of a subset of mRNAs during embryonic development, as was previously reported for d4EHP.

Reduction in memory in passive avoidance learning, exploratory behaviour and synaptic plasticity in mice with a spontaneous deletion in the ubiquitin C-terminal hydrolase L1 gene

Overexpression of ubiquitin C‐terminal hydrolase L1 (UCH‐L1) in mice rescues amyloid β‐protein‐induced decreases in synaptic plasticity and memory. However, the physiological role of UCH‐L1 in the brain is not fully understood. In the present study, we investigated the role of UCH‐L1 in the brain by utilizing gracile axonal dystrophy (gad) mice with a spontaneous deletion in the gene Uch‐l1 as a loss‐of‐function model. Although gad mice exhibit motor paresis beginning at ∼ 12 weeks of age, it is possible to analyse their brain phenotypes at a younger age when no motor paresis is evident. Maintenance of memory in a passive avoidance test and exploratory behaviour in an open field test were reduced in 6‐week‐old gad mice. The maintenance of theta‐burst stimulation‐induced long‐term potentiation (LTP) of field synaptic responses from Schaffer collaterals to CA1 pyramidal cells in hippocampal slices was also impaired in gad mice. The LTP in gad mice was insensitive to actinomycin D, suggesting that a transcription‐dependent component of the LTP is impaired. Phosphorylation of cyclic AMP response element binding protein (CREB) in the CA1 region of hippocampal slices from gad mice occurred earlier than in the slices from wild‐type mice and was transient, suggesting that CREB phosphorylation is altered in gad mice. These results suggest that memory in passive avoidance learning, exploratory behaviour and hippocampal CA1 LTP are reduced in gad mice. We propose that UCH‐L1‐mediated maintenance of the temporal integrity and persistence of CREB phosphorylation underlies these impairments.

Direct uptake and degradation of DNA by lysosomes

Lysosomes contain various hydrolases that can degrade proteins, lipids, nucleic acids and carbohydrates. We recently discovered “RNautophagy,” an autophagic pathway in which RNA is directly taken up by lysosomes and degraded. A lysosomal membrane protein, LAMP2C, a splice variant of LAMP2, binds to RNA and acts as a receptor for this pathway. In the present study, we show that DNA is also directly taken up by lysosomes and degraded. Like RNautophagy, this autophagic pathway, which we term “DNautophagy,” is dependent on ATP. The cytosolic sequence of LAMP2C also directly interacts with DNA, and LAMP2C functions as a receptor for DNautophagy, in addition to RNautophagy. Similarly to RNA, DNA binds to the cytosolic sequences of fly and nematode LAMP orthologs. Together with the findings of our previous study, our present findings suggest that RNautophagy and DNautophagy are evolutionarily conserved systems in Metazoa.

Motility Response to Insulin-like Growth Factor-I (IGF-I) in MCF-7 Cells is Associated with IRS-2 Activation and Integrin Expression

In MCF-7L cells, insulin-like growth factor-I (IGF-I) stimulates activation of insulin receptor substrate-1 (IRS-1) and enhances cell proliferation. While others have shown that IGF-I enhances cell motility in MCF-7 cells, we have not been able to demonstrate this. To determine if the source of MCF-7 cells account for these reported differences, we examined the MCF-7 cells available from the American Type Culture Collection (MCF-7/ATCC) and compared them to the MCF-7L cells maintained in our laboratory. Both MCF-7L and MCF-7/ATCC grew in response to 5 nM IGF-I and 1 nM estradiol. However, only MCF-7/ATCC demonstrated IGF-I stimulated motility. Immunoprecipitation of IRS substrates followed by anti-phosphotyrosine blotting demonstrated that both IRS-1 and IRS-2 were activated by IGF-I in these cells. However, MCF-7/ATCC cells had greater phosphorylation of IRS-2 compared to MCF-7L. Immunoblots showed that levels of IRS-1 and IRS-2 were comparable between cell lines. We have previously shown that fibronectin-binding integrins are necessary for IGF-stimulated motility. Similar levels of β1 integrin were detected in both strains of MCF-7. However, low levels of α5 and α3 were detected in MCF-7L cells whereas high levels of α3 and α5 integrin were expressed in MCF-7/ATCC cells. Inhibition of integrin function by a blocking antibody or inhibitory peptide diminished IGF-mediated motility in MCF-7/ATCC. In MCF-7/ATCC cells, IGF-I stimulation was associated with a movement of IRS-2 to the leading edge of filopodia. Thus, patterns of integrin expression among breast cancer cell lines may partially explain the different motility behavior of cells in response to IGF-I. IRS-2 activation and integrin occupancy are both required for IGF-stimulated motility.

Effects of UCH-L1 on α-synuclein over-expression mouse model of Parkinson’s disease

The rare inherited form of Parkinson’s disease (PD), PARK5, is caused by a missense mutation in ubiquitin carboxy‐terminal hydrolase‐L1 (UCH‐L1) gene, resulting in Ile93Met substitution in its gene product (UCH‐L1Ile93Met). PARK5 is inherited in an autosomal‐dominant mode, but whether the Ile93Met mutation gives rise to a gain‐of‐toxic‐function or loss‐of‐function of UCH‐L1 protein remains controversial. Here, we investigated the selective vulnerabilities of dopaminergic (DA) neurons in UCH‐L1‐transgenic (Tg) and spontaneous UCH‐L1‐null gracile axonal dystrophy mice to an important PD‐causing insult, abnormal accumulation of α‐synuclein (αSyn). Immunohistochemistry of midbrain sections of a patient with sporadic PD showed αSyn‐ and UCH‐L1‐double‐positive Lewy bodies in nigral DA neurons, suggesting physical and/or functional interaction between the two proteins in human PD brain. Recombinant adeno‐associated viral vector‐mediated over‐expression of αSyn for 4 weeks significantly enhanced the loss of nigral DA cell bodies in UCH‐L1Ile93Met‐Tg mice, but had weak effects in age‐matched UCH‐L1wild‐type‐Tg mice and non‐Tg littermates. In contrast, the extent of αSyn‐induced DA cell loss in gracile axonal dystrophy mice was not significantly different from wild‐type littermates at 13‐weeks post‐injection. Our results support the hypothesis that PARK5 is caused by a gain‐of‐toxic‐function of UCH‐L1Ile93Met mutant, and suggest that regulation of UCH‐L1 in nigral DA cells could be a future target for treatment of PD.