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Impaired turnover of the autophagy substrate p62 leads to liver injury. p62 inhibits the ubiquitin ligase Keap1, leading to stabilization of the transcription factor Nrf2. High levels of p62 in autophagy deficient animals leads to unusually high expression of Nrf2 targets genes and results in liver injury. Impaired selective turnover of p62 by autophagy causes severe liver injury accompanied by the formation of p62-positive inclusions and upregulation of detoxifying enzymes. These phenotypes correspond closely to the pathological conditions seen in human liver diseases, including alcoholic hepatitis and hepatocellular carcinoma. However, the molecular mechanisms and pathophysiological processes in these events are still unknown. Here we report the identification of a novel regulatory mechanism by p62 of the transcription factor Nrf2, whose target genes include antioxidant proteins and detoxification enzymes. p62 interacts with the Nrf2-binding site on Keap1, a component of Cullin-3-type ubiquitin ligase for Nrf2. Thus, an overproduction of p62 or a deficiency in autophagy competes with the interaction between Nrf2 and Keap1, resulting in stabilization of Nrf2 and transcriptional activation of Nrf2 target genes. Our findings indicate that the pathological process associated with p62 accumulation results in hyperactivation of Nrf2 and delineates unexpected roles of selective autophagy in controlling the transcription of cellular defence enzyme genes.

A tidy cell is a healthy cell Two papers this week suggest that the process of protein degradation and clearance of cellular components may be more important in maintaining the health of the nervous system than was thought. Both groups show that inhibition of autophagy in mouse brain cells results in neurodegeneration and early death. Autophagy, the protein degradation and recycling of cellular components, is important for the normal growth and development of a cell. The finding that the continual clearance of cellular components is essential for maintaining neuronal health should open up new avenues of research into the nature of neurodegenerative diseases. One of two papers showing that loss of autophagy in the central nervous system of mice causes the accumulation of protein aggregates in inclusion bodies, neurodegeneration and premature death of the mice. This demonstrates that continuous clearance of cellular components is essential for proper housekeeping and vital to keep the neurons in tiptop shape. Protein quality-control, especially the removal of proteins with aberrant structures, has an important role in maintaining the homeostasis of non-dividing neural cells 1 . In addition to the ubiquitin–proteasome system, emerging evidence points to the importance of autophagy—the bulk protein degradation pathway involved in starvation-induced and constitutive protein turnover—in the protein quality-control process 2 , 3 . However, little is known about the precise roles of autophagy in neurons. Here we report that loss of Atg7 (autophagy-related 7), a gene essential for autophagy, leads to neurodegeneration. We found that mice lacking Atg7 specifically in the central nervous system showed behavioural defects, including abnormal limb-clasping reflexes and a reduction in coordinated movement, and died within 28 weeks of birth. Atg7 deficiency caused massive neuronal loss in the cerebral and cerebellar cortices. Notably, polyubiquitinated proteins accumulated in autophagy-deficient neurons as inclusion bodies, which increased in size and number with ageing. There was, however, no obvious alteration in proteasome function. Our results indicate that autophagy is essential for the survival of neural cells, and that impairment of autophagy is implicated in the pathogenesis of neurodegenerative disorders involving ubiquitin-containing inclusion bodies.

Background Autophagy has been reported to play a pivotal role on the replication of various RNA viruses. In this study, we investigated the role of autophagy on hepatitis C virus (HCV) RNA replication and demonstrated anti-HCV effects of an autophagic proteolysis inhibitor, chloroquine. Methods Induction of autophagy was evaluated following the transfection of HCV replicon to Huh-7 cells. Next, we investigated the replication of HCV subgenomic replicon in response to treatment with lysosomal protease inhibitors or pharmacological autophagy inhibitor. The effect on HCV replication was analyzed after transfection with siRNA of ATG5, ATG7 and light-chain (LC)-3 to replicon cells. The antiviral effect of chloroquine and/or interferon-α (IFNα) was evaluated. Results The transfection of HCV replicon increased the number of autophagosomes to about twofold over untransfected cells. Pharmacological inhibition of autophagic proteolysis significantly suppressed expression level of HCV replicon. Silencing of autophagy-related genes by siRNA transfection significantly blunted the replication of HCV replicon. Treatment of replicon cells with chloroquine suppressed the replication of the HCV replicon in a dose-dependent manner. Furthermore, combination treatment of chloroquine to IFNα enhanced the antiviral effect of IFNα and prevented re-propagation of HCV replicon. Protein kinase R was activated in cells treated with IFNα but not with chloroquine. Incubation with chloroquine decreased degradation of long-lived protein leucine. Conclusion The results of this study suggest that the replication of HCV replicon utilizes machinery involving cellular autophagic proteolysis. The therapy targeted to autophagic proteolysis by using chloroquine may provide a new therapeutic option against chronic hepatitis C.

Autophagy is a regulated lysosomal degradation process that involves autophagosome formation and transport. Although recent evidence indicates that basal levels of autophagy protect against neurodegeneration, the exact mechanism whereby this occurs is not known. By using conditional knockout mutant mice, we report that neuronal autophagy is particularly important for the maintenance of local homeostasis of axon terminals and protection against axonal degeneration. We show that specific ablation of an essential autophagy gene, Atg7, in Purkinje cells initially causes cell-autonomous, progressive dystrophy (manifested by axonal swellings) and degeneration of the axon terminals. Consistent with suppression of autophagy, no autophagosomes are observed in these dystrophic swellings, which is in contrast to accumulation of autophagosomes in the axonal dystrophic swellings under pathological conditions. Axonal dystrophy of mutant Purkinje cells proceeds with little sign of dendritic or spine atrophy, indicating that axon terminals are much more vulnerable to autophagy impairment than dendrites. This early pathological event in the axons is followed by cell-autonomous Purkinje cell death and mouse behavioral deficits. Furthermore, ultrastructural analyses of mutant Purkinje cells reveal an accumulation of aberrant membrane structures in the axonal dystrophic swellings. Finally, we observe double-membrane vacuole-like structures in wild-type Purkinje cell axons, whereas these structures are abolished in mutant Purkinje cell axons. Thus, we conclude that the autophagy protein Atg7 is required for membrane trafficking and turnover in the axons. Our study implicates impairment of axonal autophagy as a possible mechanism for axonopathy associated with neurodegeneration.

Several studies have addressed the importance of various ub iquitin‐ l ike (UBL) post‐translational modifiers. These UBLs are covalently linked to most, if not all, target protein(s) through an enzymatic cascade analogous to ubiquitylation, consisting of E1 (activating), E2 (conjugating), and E3 (ligating) enzymes. In this report, we describe the identification of a novel u biquitin‐ f old m odifier 1 (Ufm1) with a molecular mass of 9.1 kDa, displaying apparently similar tertiary structure, although lacking obvious sequence identity, to ubiquitin. Ufm1 is first cleaved at the C‐terminus to expose its conserved Gly residue. This Gly residue is essential for its subsequent conjugating reactions. The C‐terminally processed Ufm1 is activated by a novel E1‐like enzyme, Uba5, by forming a high‐energy thioester bond. Activated Ufm1 is then transferred to its cognate E2‐like enzyme, Ufc1, in a similar thioester linkage. Ufm1 forms several complexes in HEK293 cells and mouse tissues, revealing that it conjugates to the target proteins. Ufm1, Uba5, and Ufc1 are all conserved in metazoa and plants but not in yeast, suggesting its potential roles in various multicellular organisms.

Autophagy is a dynamic membrane phenomenon for bulk protein degradation in the lysosome/vacuole 1 , 2 . Apg8/Aut7 is an essential factor for autophagy in yeast 3 , 4 , 5 . We previously found that the carboxy-terminal arginine of nascent Apg8 is removed by Apg4/Aut2 protease, leaving a glycine residue at the C terminus 6 . Apg8 is then converted to a form (Apg8-X) that is tightly bound to the membrane 6 . Here we report a new mode of protein lipidation. Apg8 is covalently conjugated to phosphatidylethanolamine through an amide bond between the C-terminal glycine and the amino group of phosphatidylethanolamine. This lipidation is mediated by a ubiquitination-like system. Apg8 is a ubiquitin-like protein that is activated by an E1 protein, Apg7 (refs 7 , 8 ), and is transferred subsequently to the E2 enzymes Apg3/Aut1 (ref. 9 ). Apg7 activates two different ubiquitin-like proteins, Apg12 (ref. 10 ) and Apg8, and assigns them to specific E2 enzymes, Apg10 (ref. 11 ) and Apg3, respectively. These reactions are necessary for the formation of Apg8-phosphatidylethanolamine. This lipidation has an essential role in membrane dynamics during autophagy 6 .

GABAA receptor-associated protein (GABARAP) was initially identified as a protein that interacts with GABAA receptor. Although LC3 (microtubule-associated protein 1 light chain 3), a GABARAP homolog, has been localized in the dendrites and cell bodies of neurons under normal conditions, the subcellular distribution of GABARAP in neurons remains unclear. Subcellular fractionation indicated that endogenous GABARAP was localized to the microsome-enriched and synaptic vesicle-enriched fractions of mouse brain as GABARAP-I, an unlipidated form. To investigate the distribution of GABARAP in neurons, we generated GFP-GABARAP transgenic mice. Immunohistochemistry in these transgenic mice showed that positive signals for GFP-GABARAP were widely distributed in neurons in various brain regions, including the hippocampus and cerebellum. Interestingly, intense diffuse and/or fibrillary expression of GFP-GABARAP was detected along the axonal initial segments (AIS) of hippocampal pyramidal neurons and cerebellar Purkinje cells, in addition to the cell bodies and dendrites of these neurons. In contrast, only slight amounts of LC3 were detected along the AIS of these neurons, while diffuse and/or fibrillary staining for LC3 was mainly detected in their cell bodies and dendrites. These results indicated that, compared with LC3, GABARAP is enriched in the AIS, in addition to the cell bodies and dendrites, of these hippocampal pyramidal neurons and cerebellar Purkinje cells.

Although the gap junction or connexin (Cx) is considered to be a tumor-suppressor, it is also required for tumor promotion. Therefore, we examined hepatic gap junctions in hepatocarcinogen-resistant (DRH) rats. Specifically, we investigated gap junction structure and Cx32 expression during normal conditions and in response to a hepatocarcinogen, 3′-methyl-4-dimethylaminoazobenzene (3′-MeDAB). On a basal diet without 3′-MeDAB, hepatic gap junctions and Cx32 protein expression were greater in DRH rats than in control Donryu rats, as evidenced by morphometry, immunohistochemistry and immunoblotting. On a diet containing 3′-MeDAB, gap junctions and expressed Cx32 were increased significantly in Donryu rats, but not in DRH rats. In this condition, Donryu rats lost weight but DRH rats increased relative liver weight. After 3′-MeDAB treatment, cathepsin D expression in hepatocytes was significantly increased only in Donryu rats, indicating that DRH rats were less susceptible to 3′-MeDAB. The abundance of mitogen-activated protein kinase, some constituent of which might be associated with the degree of Cx protein phosphorylation, was reduced to a greater extent in Donryu than in DRH rats after 3′-MeDAB treatment. The resistance of DRH rats to carcinogenesis may be due partially to their stabilized gap junctions, which could coordinate metabolic coupling to evade 3′-MeDAB toxicity.

Previously, we reported \"calpain-induced leakage of lysosomal enzyme cathepsin\" as a mechanism of ischemic neuronal death specific for primates. Cathepsin inhibitors such as CA-074 and E-64c were demonstrated to significantly inhibit hippocampal neuronal death. Pyramidal neurons of the hippocampus, Purkinje cells in the cerebellum, and neurons in the caudate nucleus, outer putamen and cortical III, V layers, are known to be vulnerable to ischemia. However, regional differences of the vulnerability and response to neuroprotectants, have not been studied in detail. Here, the monkey brains undergoing transient ischemia were studied to clarify such regional differences by the microscopic counting of surviving neurons. The dead neurons were characterized by eosinophilic coagulation necrosis without apoptotic bodies. The control postischemic brain without treatment showed surviving neurons in caudate nucleus (55.8%), outer putamen (44.4%), cortical III layer (37.8%), CA4 (35.3%), cortical V layer (34.1%), cerebellum (28.2%), CA3 (24.3%), CA2 (16.2%), and CA1 (2.0%). Only the CA1 showed an almost total neuronal loss. In contrast, a single postictal injection of CA-074 or E-64c led to significant inhibition of postischemic neuronal death in all brain regions studied. Overall, more surviving neurons were seen after E-64c treatment than with CA-074: cerebellum, 91.6% vs 85.6%; CA4, 88.6% vs 77.3%; caudate nucleus, 86.1% vs 89.8%; CA2, 83.6% vs 53.0%; outer putamen, 81.3% vs 87.7%; CA1, 80.1% vs 47.4%; CA3, 79.6% vs 60.3%; cortical layer III, 75.5% vs 67.7%; and cortical layer V, 75.0% vs 65.9%, for E-64c and CA-074, respectively. Cathepsin plays a critical role in ischemic neuronal death, and its inhibitors may protect neurons throughout the brain.

β-Amyloid formation requires multiple abnormal proteolytic cleavages of amyloid precursor protein (APP), including one within its intramembrane domain. Lysosomes, which contain a wide variety of proteases (cathepsins) and other acid hydrolases, are major sites for the turnover of membrane proteins and other cell constituents. Using immunocytochemistry, immunoelectron microscopy, and enzyme histochemistry, we studied the expression and cellular distributions of 10 lysosomal hydrolases, including 4 cathepsins, in neocortex from patients with Alzheimer disease and control (non-Alzheimer-disease) individuals. In control brains, acid hydrolases were localized exclusively to intracellular lysosomerelated compartments, and 8 of the 10 enzymes predominated in neurons. In Alzheimer disease brains, strongly immunoreactive lysosomes and lipofuscin granules accumulated markedly in the perikarya and proximal dendrites of many cortical neurons, some of which were undergoing degeneration. More strikingly, these same hydrolases were present in equally high or higher levels in senile plaques in Alzheimer disease, but they were not found extracellularly in control brains, including those from Parkinson or Huntington disease patients. At the ultrastructural level, hydrolase immunoreactivity in senile plaques was localized to extracellular lipofuscin granules similar in morphology to those within degenerating neurons. Two cathepsins that were undetectable in neurons were absent from senile plaques. These results show that lysosome function is altered in cortical neurons in Alzheimer disease. The presence of a broad spectrum of acid hydrolases in senile plaques indicates that lysosomes and their contents may be liberated from cells, principally neurons and their processes, as they degenerate. Because cathepsins can cleave polypeptide sites on APP relevant for β-amyloid formation, their abnormal extracellular localization and dysregulation in Alzheimer disease can account for the multiple hydrolytic events in β-amyloid formation. The actions of membrane-degrading acid hydrolases could also explain how the intramembrane portion of APP containing the C terminus of β-amyloid becomes accessible to proteases.