Only d-allose, among various rare monosaccharides tested, induced resistance to Xanthomonas oryzae pv. oryzae in susceptible rice leaves with defence responses: reactive oxygen species, lesion mimic formation, and PR-protein gene expression. These responses were suppressed by ascorbic acid or diphenylene iodonium. Transgenic rice plants overexpressing OsrbohC, encoding NADPH oxidase, were enhanced in sensitivity to d-allose. d-Allose-mediated defence responses were suppressed by the presence of a hexokinase inhibitor. 6-Deoxy-d-allose, a structural derivative of d-allose unable to be phosphorylated, did not confer resistance. Transgenic rice plants expressing Escherichia coli AlsK encoding d-allose kinase to increase d-allose 6-phosphate synthesis were more sensitive to d-allose, but E. coli AlsI encoding d-allose 6-phosphate isomerase expression to decrease d-allose 6-phosphate reduced sensitivity. A d-glucose 6-phosphate dehydrogenase-defective mutant was also less sensitive, and OsG6PDH1 complementation restored full sensitivity. These results reveal that a monosaccharide, d-allose, induces rice resistance to X. oryzae pv. oryzae by activating NADPH oxidase through the activity of d-glucose 6-phosphate dehydrogenase, initiated by hexokinase-mediated conversion of d-allose to d-allose 6-phosphate, and treatment with d-allose might prove to be useful for reducing disease development in rice.
d-Allose; d-glucose 6-phosphate dehydrogenase; hexokinase; NADPH oxidase; Oryza sativa L; rare sugar.
Haemolytic anaemia due to a stenosed graft is a rare complication after surgery for aortic dissection. We present the case of a patient with haemolytic anaemia and heart failure, who had undergone emergent ascending aorta replacement for type A acute aortic dissection 5 years earlier. Chest computed tomography revealed severe graft stenosis of the proximal anastomosis and transthoracic echocardiography showed severe aortic regurgitation. Surgical treatment was necessary because of heart failure and myocardial ischaemia due to haemolytic anaemia and aortic regurgitation. During the operation, we found an inner graft surrounded by an outer graft and a dilated lumen between the double-reinforced grafts compressing the inner graft. We successfully reconstructed the aortic root with a total arch replacement. To the best of our knowledge, there are no cases in which haemolytic anaemia and AR developed in a patient with acute aortic dissection surgically treated by such a mechanism.
Haemolytic anaemia; Surgical repaired aortic dissection and aortic regurgitation
Disseminated intravascular coagulation (DIC) is the most frequent coagulation disorder in patients with prostate cancer. However, renal involvement in DIC associated with prostate cancer has rarely been documented. Herein, we present a case of metastatic prostate cancer presenting with acute renal failure (RF) triggered by DIC. An 80 year old man with metastatic prostate cancer was treated with antihormone therapy at an outpatient clinic. He was admitted to our hospital because of severe dyspnea and progressive RF. A hemorrhagic tendency was not clinically evident. Laboratory tests exhibited a significant coagulation disorder, suggestive of DIC. Despite treatment, his RF and dyspnea worsened, and he eventually passed away. An autopsy study revealed hypertensive nephrosclerosis superimposed by fibrin rich thrombi formation involving glomerular capillaries and arterioles characteristic of DIC. Additionally, focal segmental glomerulosclerosis was identified, which was presumably secondary to the glomerular endothelial and/or podocyte injury augmented by DIC. Those findings showed that glomerular injury, which was induced and subsequently exacerbated by DIC associated with prostate cancer, highly contributed to the progression of RF in our case. A differential diagnosis of DIC should be considered when a patient with prostate cancer presents with renal dysfunction.
disseminated intravascular coagulation; prostate cancer; renal pathology
The efficacy of sivelestat in the treatment of acute lung injury/acute respiratory distress syndrome (ALI/ARDS) has not been established. In part, this is due to the wide variety of factors involved in the etiology of ALI/ARDS. In this study, we examined the efficacy of sivelestat in patients with ALI/ARDS associated with abdominal sepsis.
The subjects were 49 patients with ALI/ARDS after surgery for abdominal sepsis. The efficacy of sivelestat was retrospectively assessed in two treatment groups, ie, a sivelestat group (n = 34) and a non-sivelestat group (n = 15).
The sivelestat group showed significant improvements in oxygenation, thrombocytopenia, and multiple organ dysfunction score. The number of ventilator days (6.6 ± 6.1 versus 11.1 ± 8.4 days; P = 0.034) and length of stay in the intensive care unit (8.5 ± 6.2 versus 13.3 ± 9.5 days; P = 0.036) were significantly lower in the sivelestat group. The hospital mortality rate decreased by half in the sivelestat group, but was not significantly different between the two groups.
Administration of sivelestat to patients with ALI/ARDS following surgery for abdominal sepsis resulted in early improvements of oxygenation and multiple organ dysfunction score, early ventilator weaning, and early discharge from the intensive care unit.
sivelestat; acute lung injury; acute respiratory distress syndrome; abdominal sepsis
Cells in the animal early visual cortex are sensitive to contour orientations and form repeated structures known as orientation columns. At the behavioral level, there exist 2 well-known global biases in orientation perception (oblique effect and radial bias) in both animals and humans. However, their neural bases are still under debate. To unveil how these behavioral biases are achieved in the early visual cortex, we conducted high-resolution functional magnetic resonance imaging experiments with a novel continuous and periodic stimulation paradigm. By inserting resting recovery periods between successive stimulation periods and introducing a pair of orthogonal stimulation conditions that differed by 90° continuously, we focused on analyzing a blood oxygenation level-dependent response modulated by the change in stimulus orientation and reliably extracted orientation preferences of single voxels. We found that there are more voxels preferring horizontal and vertical orientations, a physiological substrate underlying the oblique effect, and that these over-representations of horizontal and vertical orientations are prevalent in the cortical regions near the horizontal- and vertical-meridian representations, a phenomenon related to the radial bias. Behaviorally, we also confirmed that there exists perceptual superiority for horizontal and vertical orientations around horizontal and vertical meridians, respectively. Our results, thus, refined the neural mechanisms of these 2 global biases in orientation perception.
fMRI; oblique effect; orientation; primary visual cortex; radial bias
Acute hypoxemic respiratory failure (AHRF) is one of the most serious complications after cardiovascular surgery. It remains unclear whether noninvasive ventilation (NIV) has potential as an effective therapy for AHRF after cardiovascular surgery, although many reports have described the use of NIV for AHRF after extubation. The aim of this study was to investigate the effectiveness of NIV in the early stage of mild AHRF after cardiovascular surgery.
We retrospectively analyzed all patients admitted to the intensive care unit after cardiovascular surgery, whose oxygenation transfer (PaO2/FIO2) deteriorated mildly after extubation, and in whom NIV was initiated. A two-way analysis of variance and the Bonferroni multiple comparisons procedure, the Mann–Whitney test, Fisher’s exact test or the χ2test was performed.
A total of 94 patients with AHRF received NIV, of whom 89 patients (94%) successfully avoided endotracheal intubation (successful group) and five patients required reintubation (reintubation group). All patients, including the reintubated patients, were successfully weaned from mechanical ventilation and discharged from the intensive care unit. In the successful group, PaO2/FIO2 improved and the respiratory rate decreased significantly within 1 h after the start of NIV, and the improvement in PaO2/FIO2 remained during the whole NIV period.
We conclude that NIV is beneficial for mild AHRF after cardiovascular surgery when it is started within 3 h after mild deterioration of PaO2/FIO2. We also think that it is important not to hesitate before performing reintubation when NIV is judged to be ineffective.
Acute hypoxemic respiratory failure; Cardiovascular surgery; Post-extubation; Noninvasive ventilation; PaO2/FIO2
Fever is frequently observed in critically ill patients. An independent association of fever with increased mortality has been observed in non-neurological critically ill patients with mixed febrile etiology. The association of fever and antipyretics with mortality, however, may be different between infective and non-infective illness.
We designed a prospective observational study to investigate the independent association of fever and the use of antipyretic treatments with mortality in critically ill patients with and without sepsis. We included 1,425 consecutive adult critically ill patients (without neurological injury) requiring > 48 hours intensive care admitted in 25 ICUs. We recorded four-hourly body temperature and all antipyretic treatments until ICU discharge or 28 days after ICU admission, whichever occurred first. For septic and non-septic patients, we separately assessed the association of maximum body temperature during ICU stay (MAXICU) and the use of antipyretic treatments with 28-day mortality.
We recorded body temperature 63,441 times. Antipyretic treatment was given 4,863 times to 737 patients (51.7%). We found that treatment with non-steroidal anti-inflammatory drugs (NSAIDs) or acetaminophen independently increased 28-day mortality for septic patients (adjusted odds ratio: NSAIDs: 2.61, P = 0.028, acetaminophen: 2.05, P = 0.01), but not for non-septic patients (adjusted odds ratio: NSAIDs: 0.22, P = 0.15, acetaminophen: 0.58, P = 0.63). Application of physical cooling did not associate with mortality in either group. Relative to the reference range (MAXICU 36.5°C to 37.4°C), MAXICU ≥ 39.5°C increased risk of 28-day mortality in septic patients (adjusted odds ratio 8.14, P = 0.01), but not in non-septic patients (adjusted odds ratio 0.47, P = 0.11).
In non-septic patients, high fever (≥ 39.5°C) independently associated with mortality, without association of administration of NSAIDs or acetaminophen with mortality. In contrast, in septic patients, administration of NSAIDs or acetaminophen independently associated with 28-day mortality, without association of fever with mortality. These findings suggest that fever and antipyretics may have different biological or clinical or both implications for patients with and without sepsis.
body temperature; antipyretic; fever; critical illness; mortality
The crystallization and preliminary X-ray diffraction studies of the Skp1–Fbg3 complex are reported. Crystallization by repeated microseeding using selected crystals as a source of microseeds is described.
F-box proteins are the substrate-recognition components of Skp1–Cullin1–F-box protein–Rbx1 (SCF) ubiquitin ligase complexes. Fbs1, an F-box protein, binds specifically to proteins modified with high-mannose oligosaccharides. Fbg3, another F-box protein, has 51% sequence identity to Fbs1. Although the residues that are necessary for binding to oligosaccharides are conserved between Fbs1 and Fbg3, Fbg3 does not bind glycoproteins. Skp1 and Fbg3 were co-expressed in Escherichia coli and their complex was purified to homogeneity and crystallized. Microseeding combined with the sandwiched hanging-drop technique improved the quality of the resulting crystals. The plate-shaped crystals belonged to space group P212121, with unit-cell parameters a = 34.1, b = 76.6, c = 193.9 Å and one molecule per asymmetric unit.
F-box proteins; Fbg3; Skp1–Fbg3 complex; ubiquitin ligases
Impaired autophagy stabilizes p62 and promotes tumorigenesis through activation of the Nrf2 transcription factor.
Suppression of autophagy is always accompanied by marked accumulation of p62, a selective autophagy substrate. Because p62 interacts with the Nrf2-binding site on Keap1, which is a Cullin 3–based ubiquitin ligase adapter protein, autophagy deficiency causes competitive inhibition of the Nrf2–Keap1 interaction, resulting in stabilization of Nrf2 followed by transcriptional activation of Nrf2 target genes. Herein, we show that liver-specific autophagy-deficient mice harbor adenomas linked to both the formation of p62- and Keap1-positive cellular aggregates and induction of Nrf2 targets. Importantly, similar aggregates were identified in more than 25% of human hepatocellular carcinomas (HCC), and induction of Nrf2 target genes was recognized in most of these tumors. Gene targeting of p62 in an HCC cell line markedly abrogates the anchorage-independent growth, whereas forced expression of p62, but not a Keap1 interaction-defective mutant, resulted in recovery of the growth defect. These results indicate the involvement of persistent activation of Nrf2 through the accumulation of p62 in hepatoma development.
Proteasomes are multisubunit proteases that play a critical role in maintaining cellular function through the selective degradation of ubiquitinated proteins. When 3 additional β subunits, expression of which is induced by IFN-γ, are substituted for their constitutively expressed counterparts, the structure is converted to an immunoproteasome. However, the underlying roles of immunoproteasomes in human diseases are poorly understood. Using exome analysis, we found a homozygous missense mutation (G197V) in immunoproteasome subunit, β type 8 (PSMB8), which encodes one of the β subunits induced by IFN-γ in patients from 2 consanguineous families. Patients bearing this mutation suffered from autoinflammatory responses that included recurrent fever and nodular erythema together with lipodystrophy. This mutation increased assembly intermediates of immunoproteasomes, resulting in decreased proteasome function and ubiquitin-coupled protein accumulation in the patient’s tissues. In the patient’s skin and B cells, IL-6 was highly expressed, and there was reduced expression of PSMB8. Downregulation of PSMB8 inhibited the differentiation of murine and human adipocytes in vitro, and injection of siRNA against Psmb8 in mouse skin reduced adipocyte tissue volume. These findings identify PSMB8 as an essential component and regulator not only of inflammation, but also of adipocyte differentiation, and indicate that immunoproteasomes have pleiotropic functions in maintaining the homeostasis of a variety of cell types.
Epithelial cells of the thymus cortex express a unique proteasome particle involved in positive T cell selection. This thymoproteasome contains the recently discovered β5t subunit that has an uncharted activity, if any. We synthesized fluorescent epoxomicin probes that were used in a chemical proteomics approach, entailing activity-based profiling, affinity purification, and LC-MS identification, to demonstrate that the β5t subunit is catalytically active in the murine thymus. A panel of established protea-some inhibitors showed that the broad-spectrum inhibitor epoxomicin blocks the β5t activity and that the subunit-specific antagonists bortezomib and NC005 do not inhibit β5t. We show that β5t has a substrate preference distinct from β5/β5i that might explain how the thymoproteasome generates the MHC class I peptide repertoire needed for positive T cell selection.
Axon degeneration is a hallmark of neurodegenerative diseases, including Alzheimer’s and Parkinson’s disease. Such degeneration is not a passive event, but rather is an active process, mediated by mechanisms that are distinct from the canonical pathways of programmed cell death that mediate destruction of the cell soma. Little is known of the diverse mechanisms involved, particularly those of retrograde axon degeneration. We have previously observed in living animal models of degeneration in the nigro-striatal projection that a constitutively active form of the kinase Akt (Myr-Akt) demonstrates an ability to suppress programmed cell death and preserve the soma of dopamine neurons. Here we show in both neurotoxin and physical injury (axotomy) models that Myr-Akt is also able to preserve their axons due to suppression of acute retrograde axon degeneration. This cellular phenotype is associated with increased mTor activity, and can be recapitulated by a constitutively active form of the small GTPase Rheb, an upstream activator of mTor. Axon degeneration in these models is accompanied by the occurrence of macroautophagy, which is suppressed by Myr-Akt. Conditional deletion of the essential autophagy mediator Atg7 in adult mice also achieves striking axon protection in these acute models of retrograde degeneration. The protection afforded by both Myr-Akt and Atg7 deletion is robust and lasting, because it is still observed as protection of both axons and dopaminergic striatal innervation weeks after injury. We conclude that acute retrograde axon degeneration is regulated by Akt/Rheb/mTor signaling pathways.
degeneration; Parkinson’s disease; apoptosis; axon; autophagy; substantia nigra
Inferior temporal (IT) object representations have been intensively studied in monkeys and humans, but representations of the same particular objects have never been compared between the species. Moreover, IT’s role in categorization is not well understood. Here, we presented monkeys and humans with the same images of real-world objects and measured the IT response pattern elicited by each image. In order to relate the representations between the species and to computational models, we compare response-pattern dissimilarity matrices. IT response patterns form category clusters, which match between man and monkey. The clusters correspond to animate and inanimate objects; within the animate objects, faces and bodies form subclusters. Within each category, IT distinguishes individual exemplars, and the within-category exemplar similarities also match between the species. Our findings suggest that primate IT across species may host a common code, which combines a categorical and a continuous representation of objects.
Both anabolism and catabolism of the amino acids released by starvation-induced autophagy are essential for cell survival, but their actual metabolic contributions in adult animals are poorly understood. Herein, we report that, in mice, liver autophagy makes a significant contribution to the maintenance of blood glucose by converting amino acids to glucose via gluconeogenesis. Under a synchronous fasting-initiation regimen, autophagy was induced concomitantly with a fall in plasma insulin in the presence of stable glucagon levels, resulting in a robust amino acid release. In liver-specific autophagy (Atg7)-deficient mice, no amino acid release occurred and blood glucose levels continued to decrease in contrast to those of wild-type mice. Administration of serine (30 mg/animal) exerted a comparable effect, raising the blood glucose levels in both control wild-type and mutant mice under starvation. Thus, the absence of the amino acids that were released by autophagic proteolysis is a major reason for a decrease in blood glucose. Autophagic amino acid release in control wild-type livers was significantly suppressed by the prior administration of glucose, which elicited a prompt increase in plasma insulin levels. This indicates that insulin plays a dominant role over glucagon in controlling liver autophagy. These results are the first to show that liver-specific autophagy plays a role in blood glucose regulation.
amino acid; autophagy; liver; gluconeogenesis; insulin; phosphoenolpyruvate carboxykinase
Mutations in the parkin gene, which encodes a ubiquitin ligase (E3), are a major cause of autosomal recessive parkinsonism. Although parkin-mediated ubiquitination was initially linked to protein degradation, accumulating evidence suggests that the enzyme is capable of catalyzing multiple forms of ubiquitin modifications including monoubiquitination, K48- and K63-linked polyubiquitination. In this study, we sought to understand how a single enzyme could exhibit such multifunctional catalytic properties.
Methods and Findings
By means of in vitro ubiquitination assays coupled with mass spectrometry analysis, we were surprised to find that parkin is apparently capable of mediating E2-independent protein ubiquitination in vitro, an unprecedented activity exhibited by an E3 member. Interestingly, whereas full length parkin catalyzes solely monoubiquitination regardless of the presence or absence of E2, a truncated parkin mutant containing only the catalytic moiety supports both E2-independent and E2-dependent assembly of ubiquitin chains.
Our results here suggest a complex regulation of parkin's activity and may help to explain how a single enzyme like parkin could mediate diverse forms of ubiquitination.
Inactivation of the essential autophagy gene Atg5 results in selective accumulation of aggregation-prone proteins independently of substrate ubiquitination.
Genetic ablation of autophagy in mice leads to liver and brain degeneration accompanied by the appearance of ubiquitin (Ub) inclusions, which has been considered to support the hypothesis that ubiquitination serves as a cis-acting signal for selective autophagy. We show that tissue-specific disruption of the essential autophagy genes Atg5 and Atg7 leads to the accumulation of all detectable Ub–Ub topologies, arguing against the hypothesis that any particular Ub linkage serves as a specific autophagy signal. The increase in Ub conjugates in Atg7−/− liver and brain is completely suppressed by simultaneous knockout of either p62 or Nrf2. We exploit a novel assay for selective autophagy in cell culture, which shows that inactivation of Atg5 leads to the selective accumulation of aggregation-prone proteins, and this does not correlate with an increase in substrate ubiquitination. We propose that protein oligomerization drives autophagic substrate selection and that the accumulation of poly-Ub chains in autophagy-deficient circumstances is an indirect consequence of activation of Nrf2-dependent stress response pathways.
Little is known about how synaptic activity is modulated in the central nervous system. We have identified SCRAPPER, a synapse-localized E3 ubiquitin ligase, which regulates neural transmission. SCRAPPER directly binds and ubiquitinates RIM1, a modulator of presynaptic plasticity. In neurons from Scrapper-knockout (SCR-KO) mice, RIM1 had a longer half-life with significant reduction in ubiquitination, indicating that SCRAPPER is the predominant ubiquitin ligase that mediates RIM1 degradation. As anticipated in a RIM1 degradation defect mutant, SCR-KO mice displayed altered electrophysiological synaptic activity, i.e., increased frequency of miniature excitatory postsynaptic currents. This phenotype of SCR-KO mice was phenocopied by RIM1 overexpression and could be rescued by re-expression of SCRAPPER or knockdown of RIM1. The acute effects of proteasome inhibitors, such as upregulation of RIM1 and the release probability, were blocked by the impairment of SCRAPPER. Thus, SCRAPPER has an essential function in regulating proteasome-mediated degradation of RIM1 required for synaptic tuning.
The 26S proteasome, a central enzyme for ubiquitin-dependent proteolysis, is a highly complex structure comprising 33 distinct subunits. Recent studies have revealed multiple dedicated chaperones involved in proteasome assembly both in yeast and in mammals. However, none of these chaperones is essential for yeast viability. PAC1 is a mammalian proteasome assembly chaperone that plays a role in the initial assembly of the 20S proteasome, the catalytic core of the 26S proteasome, but does not cause a complete loss of the 20S proteasome when knocked down. Thus, both chaperone-dependent and -independent assembly pathways exist in cells, but the contribution of the chaperone-dependent pathway remains unclear. To elucidate its biological significance in mammals, we generated PAC1 conditional knockout mice. PAC1-null mice exhibited early embryonic lethality, demonstrating that PAC1 is essential for mammalian development, especially for explosive cell proliferation. In quiescent adult hepatocytes, PAC1 is responsible for producing the majority of the 20S proteasome. PAC1-deficient hepatocytes contained normal amounts of the 26S proteasome, but they completely lost the free latent 20S proteasome. They also accumulated ubiquitinated proteins and exhibited premature senescence. Our results demonstrate the importance of the PAC1-dependent assembly pathway and of the latent 20S proteasomes for maintaining cellular integrity.
Autophagy degrades pathogens in vitro. The autophagy gene Atg5 has been reported to be required for IFN-γ-dependent host protection in vivo. However, these protective effects occur independently of autophagosome formation. Thus, the in vivo role of classic autophagy in protection conferred by adaptive immunity and how adaptive immunity triggers autophagy are incompletely understood. Employing biochemical, genetic and morphological studies, we found that CD40 upregulates the autophagy molecule Beclin 1 in microglia and triggers killing of Toxoplasma gondii dependent on the autophagy machinery. Infected CD40−/− mice failed to upregulate Beclin 1 in microglia/macrophages in vivo. Autophagy-deficient Beclin 1+/− mice, mice with deficiency of the autophagy protein Atg7 targeted to microglia/macrophages as well as CD40−/− mice exhibited impaired killing of T. gondii and were susceptible to cerebral and ocular toxoplasmosis. Susceptibility to toxoplasmosis occurred despite upregulation of IFN-γ, TNF-α and NOS2, preservation of IFN-γ-induced microglia/macrophage anti-T. gondii activity and the generation of anti-T. gondii T cell immunity. CD40 upregulated Beclin 1 and triggered killing of T. gondii by decreasing protein levels of p21, a molecule that degrades Beclin 1. These studies identified CD40-p21-Beclin 1 as a pathway by which adaptive immunity stimulates autophagy. In addition, they support that autophagy is a mechanism through which CD40-dependent immunity mediates in vivo protection and that the CD40-autophagic machinery is needed for host resistance despite IFN-γ.
Defective mitochondrial quality control is shown to be a mechanism for neurodegeneration in some forms of Parkinson's disease.
Parkinson's disease (PD) is a prevalent neurodegenerative disorder. Recent identification of genes linked to familial forms of PD such as Parkin and PINK1 (PTEN-induced putative kinase 1) has revealed that ubiquitylation and mitochondrial integrity are key factors in disease pathogenesis. However, the exact mechanism underlying the functional interplay between Parkin-catalyzed ubiquitylation and PINK1-regulated mitochondrial quality control remains an enigma. In this study, we show that PINK1 is rapidly and constitutively degraded under steady-state conditions in a mitochondrial membrane potential–dependent manner and that a loss in mitochondrial membrane potential stabilizes PINK1 mitochondrial accumulation. Furthermore, PINK1 recruits Parkin from the cytoplasm to mitochondria with low membrane potential to initiate the autophagic degradation of damaged mitochondria. Interestingly, the ubiquitin ligase activity of Parkin is repressed in the cytoplasm under steady-state conditions; however, PINK1-dependent mitochondrial localization liberates the latent enzymatic activity of Parkin. Some pathogenic mutations of PINK1 and Parkin interfere with the aforementioned events, suggesting an etiological importance. These results provide crucial insight into the pathogenic mechanisms of PD.
Parkinson disease (PD) is the second most prevalent neurodegenerative disorder, and thus elucidation of the pathogenic mechanism and establishment of a fundamental cure is essential in terms of public welfare. Fortunately, our understanding of the pathogenesis of two types of recessive familial PDs—early-onset familial PD caused by dysfunction of the PTEN-induced putative kinase 1 (PINK1) gene and autosomal recessive juvenile Parkinsonism (ARJP) caused by a mutation in the Parkin gene—has evolved and continues to expand.
PINK1; parkin; ubiquitin; mitochondria; autophagy; mitophagy; membrane potential; quality control
Autophagy is a tightly regulated cell self-eating process. It has been shown to be associated with various neuropathological conditions and therefore, traditionally known as a stress-induced process. Recent studies, however, reveal that autophagy is constitutively active in healthy neurons. Neurons are highly specialized, post-mitotic cells that are typically composed of a soma (cell body), a dendritic tree, and an axon. Despite the vast growth of our current knowledge of autophagy, the detailed process in such a highly differentiated cell type remains elusive. Current evidence strongly suggests that autophagy is uniquely regulated in neurons and is also highly adapted to local physiology in the axons. In addition, the molecular mechanism for basal autophagy in neurons may be significantly divergent from “classical” induced autophagy. A considerable number of studies has increasingly shown an important role for autophagy in neurodegenerative diseases and has explored autophagy as a potential drug target. Thus, understanding the neuronal autophagy process will ultimately aid in drug target identification and rational design of drug screening to combat neurodegenerative diseases.
The intracellular storage and utilization of lipids are critical to maintain cellular energy homeostasis. During nutrient deprivation, cellular lipids stored as triglycerides in lipid droplets are hydrolysed into fatty acids for energy. A second cellular response to starvation is the induction of autophagy, which delivers intracellular proteins and organelles sequestered in double-membrane vesicles (autophagosomes) to lysosomes for degradation and use as an energy source. Lipolysis and autophagy share similarities in regulation and function but are not known to be interrelated. Here we show a previously unknown function for autophagy in regulating intracellular lipid stores (macrolipophagy). Lipid droplets and autophagic components associated during nutrient deprivation, and inhibition of autophagy in cultured hepatocytes and mouse liver increased triglyceride storage in lipid droplets. This study identifies a critical function for autophagy in lipid metabolism that could have important implications for human diseases with lipid over-accumulation such as those that comprise the metabolic syndrome.
Skeletal muscle atrophy caused by unloading is characterized by both decreased responsiveness to myogenic growth factors (e.g., insulin-like growth factor 1 [IGF-1] and insulin) and increased proteolysis. Here, we show that unloading stress resulted in skeletal muscle atrophy through the induction and activation of the ubiquitin ligase Cbl-b. Upon induction, Cbl-b interacted with and degraded the IGF-1 signaling intermediate IRS-1. In turn, the loss of IRS-1 activated the FOXO3-dependent induction of atrogin-1/MAFbx, a dominant mediator of proteolysis in atrophic muscle. Cbl-b-deficient mice were resistant to unloading-induced atrophy and the loss of muscle function. Furthermore, a pentapeptide mimetic of tyrosine608-phosphorylated IRS-1 inhibited Cbl-b-mediated IRS-1 ubiquitination and strongly decreased the Cbl-b-mediated induction of atrogin-1/MAFbx. Our results indicate that the Cbl-b-dependent destruction of IRS-1 is a critical dual mediator of both increased protein degradation and reduced protein synthesis observed in unloading-induced muscle atrophy. The inhibition of Cbl-b-mediated ubiquitination may be a new therapeutic strategy for unloading-mediated muscle atrophy.