The ubiquitin-proteasome and autophagy-lysosomal pathways are the two main routes of protein and organelle clearance in eukaryotic cells. The proteasome system is responsible for unfolded, ...short-lived proteins, which precludes the clearance of oligomeric and aggregated proteins, whereas macroautophagy, a process generally referred to as autophagy, mediates mainly the bulk degradation of long-lived cytoplasmic proteins, large protein complexes or organelles.
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Recently, the autophagy-lysosomal pathway has been implicated in neurodegenerative disorders as an important pathway for the clearance of abnormally accumulated intracellular proteins, such as huntingtin, tau, and mutant and modified α-synuclein.
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Our recent study illustrated the induction of adaptive autophagy in response to mutant glial fibrillary acidic protein (GFAP) accumulation in astrocytes, in the brains of patients with Alexander disease (AxD), and in mutant GFAP knock-in mouse brains.
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This autophagic response is negatively regulated by mammalian target of rapamycin (mTOR). The activation of p38 MAPK by GFAP accumulation is responsible for mTOR inactivation and the induction of autophagy. We also found that the accumulation of GFAP impairs proteasome activity.
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In this commentary we discuss the potential compensatory relationship between an impaired proteasome and activated autophagy, and propose that the MLK-MAPK (mixed lineage kinase-mitogen-activated protein kinase) cascade is a regulator of this crosstalk.
Addendum to: Tang G, Yue Z, Talloczy, Z, Hagemann T, Cho W, Sulzer D, Messing A, Goldman JE. Alexander disease-mutant GFAP accumulation stimulates autophagy through p38 MAPK and mTOR signaling pathways. Hum Mol Genetics 2008; In press.
Autophagy is postulated to play a role in antiviral innate immunity. However, it is unknown whether viral evasion of autophagy is important in disease pathogenesis. Here we show that the herpes ...simplex virus type 1 (HSV-1)-encoded neurovirulence protein ICP34.5 binds to the mammalian autophagy protein Beclin 1 and inhibits its autophagy function. A mutant HSV-1 virus lacking the Beclin 1-binding domain of ICP34.5 fails to inhibit autophagy in neurons and demonstrates impaired ability to cause lethal encephalitis in mice. The neurovirulence of this Beclin 1-binding mutant virus is restored in pkr(-/-) mice. Thus, ICP34.5-mediated antagonism of the autophagy function of Beclin 1 is essential for viral neurovirulence, and the antiviral signaling molecule PKR lies genetically upstream of Beclin 1 in host defense against HSV-1. Our findings suggest that autophagy inhibition is a novel molecular mechanism by which viruses evade innate immunity and cause fatal disease.
The lysosomal pathway of autophagy is the major catabolic mechanism for degrading long-lived cellular proteins and cytoplasmic organelles. Recent studies have also shown that autophagy (xenophagy) ...may be used to degrade bacterial pathogens that invade intracellularly. However, it is not yet known whether xenophagy is a mechanism for degrading viruses. Previously, we showed that autophagy induction requires the antiviral eIF2alpha kinase signaling pathway (including PKR and eIF2alpha) and that this function ofeIF2alpha kinase signaling is antagonized by the herpes simplex virus (HSV-1) neurovirulence gene product, ICP34.5. Here, we show quantitative morphologic evidence of PKR-dependent xenophagic degradation of herpes simplex virions and biochemical evidence of PKR and eIF2alpha-dependent degradation of HSV-1 proteins, both of which are blocked by ICP34.5. Together, these findings indicate that xenophagy degrades HSV-1 and that this cellular function is antagonized by the HSV-1 neurovirulence gene product, ICP34.5. Thus, autophagy-related pathways are involved in degrading not only cellular constituents and intracellular bacteria, but also viruses.
The elF2α kinases are a family of evolutionarily conserved serine/threonine kinases that regulate stress-induced translational arrest. Here, we demonstrate that the yeast elF2α kinase, GCN2, the ...target phosphorylation site of Gcn2p, Ser-51 of elF2α, and the elF2α-regulated transcriptional transactivator, GCN4, are essential for another fundamental stress response, starvation-induced autophagy. The mammalian IFN-inducible elF2α kinase, PKR, rescues starvation-induced autophagy in GCN2-disrupted yeast, and pkr null and Ser-51 nonphosphorylatable mutant elF2α murine embryonic fibroblasts are defective in autophagy triggered by herpes simplex virus infection. Furthermore, PKR and elF2α Ser-51-dependent autophagy is antagonized by the herpes simplex virus neurovirulence protein, ICP34.5. Thus, autophagy is a novel evolutionarily conserved function of the elF2α kinase pathway that is targeted by viral virulence gene products.
Altered degradation of alpha-synuclein (alpha-syn) has been implicated in the pathogenesis of Parkinson disease (PD). We have shown that alpha-syn can be degraded via chaperone-mediated autophagy ...(CMA), a selective lysosomal mechanism for degradation of cytosolic proteins. Pathogenic mutants of alpha-syn block lysosomal translocation, impairing their own degradation along with that of other CMA substrates. While pathogenic alpha-syn mutations are rare, alpha-syn undergoes posttranslational modifications, which may underlie its accumulation in cytosolic aggregates in most forms of PD. Using mouse ventral medial neuron cultures, SH-SY5Y cells in culture, and isolated mouse lysosomes, we have found that most of these posttranslational modifications of alpha-syn impair degradation of this protein by CMA but do not affect degradation of other substrates. Dopamine-modified alpha-syn, however, is not only poorly degraded by CMA but also blocks degradation of other substrates by this pathway. As blockage of CMA increases cellular vulnerability to stressors, we propose that dopamine-induced autophagic inhibition could explain the selective degeneration of PD dopaminergic neurons.
KIL-d is a cytoplasmically inherited genetic trait that causes killer virus-infected cells of Saccharomyces cerevisiae to express the normal killer phenotypes in a/alpha cells, but to show variegated ...defective killer phenotypes in a or alpha type cells. Mating of KIL-d haploids results in "healing" of their phenotypic defects, while meiosis of the resulting diploids results in "resetting" of the variegated, but mitotically stable, defects. We show that KIL-d does not reside on the double-stranded RNA genome of killer virus. Thus, the KIL-d effect on viral gene expression is epigenetic in nature. Resetting requires nuclear events of meiosis, since KIL-d can be cytoplasmically transmitted during cytoduction without causing defects in killer virus expression. Subsequently, mating of these cytoductants followed by meiosis generates spore clones expressing variegated defective phenotypes. Cytoduction of wild-type cytoplasm into a phenotypically defective KIL-d haploid fails to heal, nor does simultaneous or sequential expression of both MAT alleles cause healing. Thus, healing is not triggered by the appearance of heterozygosity at the MAT locus, but rather requires the nuclear fusion events which occur during mating. Therefore, KIL-d appears to interact with the nucleus in order to exert its effects on gene expression by the killer virus RNA genome.
The cytoplasmically inherited KIL-d element epigenetically regulates killer virus gene expression in Saccharomyces cerevisiae. KIL-d results in variegated defects in expression of the M ...double-stranded RNA viral segment in haploid cells that are "healed" in diploids. We report that the KIL-d element is spontaneously lost with a frequency of 10(-4)-10(-5) and reappears with variegated phenotypic expression with a frequency of > or =10(-3). This high rate of loss and higher rate of reappearance is unlike any known nucleic acid replicon but resembles the behavior of yeast prions. However, KIL-d is distinct from the known yeast prions in its relative guanidinium hydrochloride incurability and independence of Hsp104 protein for its maintenance. Despite its transmissibility by successive cytoplasmic transfers, multiple cytoplasmic nucleic acids have been proven not to carry the KIL-d trait. KIL-d epigenetically regulates the expression of the M double-stranded RNA satellite virus genome, but fails to alter the expression of M cDNA. This specificity remained even after a cycle of mating and meiosis. Due to its unique genetic properties and viral RNA specificity, KIL-d represents a new type of genetic element that interacts with a viral RNA genome.
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![Dopamine-modified [alpha]-s...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0021-9738/small "Dopamine-modified [alpha]-synuclein blocks chaperone-mediated autophagy")
![The [KIL-d] cytoplasmic gen...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0016-6731_1943-2631/small "The [KIL-d] cytoplasmic genetic element of yeast results in epigenetic regulation of viral M double-stranded RNA gene expression")