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K63-linked ubiquitin chains attached to plasma membrane proteins serve as tags for endocytosis and endosome-to-lysosome sorting. USP8 is an essential deubiquitinase for the maintenance of endosomal functions. Prolonged depletion of USP8 leads to cell death, but the major effects on cellular signaling pathways are poorly understood. Here, we show that USP8 depletion causes aberrant accumulation of K63-linked ubiquitin chains on endosomes and induces immune and stress responses. Upon USP8 depletion, two different decoders for K63-linked ubiquitin chains, TAB2/3 and p62, were recruited to endosomes and activated the TAK1-NF-κB and Keap1-Nrf2 pathways, respectively. Oxidative stress, an environmental stimulus that potentially suppresses USP8 activity, induced accumulation of K63-linked ubiquitin chains on endosomes, recruitment of TAB2, and expression of the inflammatory cytokine. The results demonstrate that USP8 is a gatekeeper of misdirected ubiquitin signals and inhibits immune and stress response pathways by removing K63-linked ubiquitin chains from endosomes. [ABSTRACT FROM AUTHOR]

Su(var)3-9, Enhancer-of-zeste, and Trithorax (SET) domain-containing protein 8 (SET8) is the sole enzyme that monomethylates Lys-20 of histone H4 (H4K20). SET8 has been implicated in the regulation of multiple biological processes, such as gene transcription, the cell cycle, and senescence. SET8 quickly undergoes ubiquitination and degradation by several E3 ubiquitin ligases; however, the enzyme that deubiquitinates SET8 has not yet been identified. Here we demonstrated that ubiquitin-specific peptidase 17-like family member (USP17) deubiquitinates and therefore stabilizes the SET8 protein. We observed that USP17 interacts with SET8 and removes polyubiquitin chains from SET8. USP17 knockdown not only decreased SET8 protein levels and H4K20 monomethylation but also increased the levels of the cyclin-dependent kinase inhibitor p21. As a consequence, USP17 knockdown suppressed cell proliferation. We noted that USP17 was down-regulated in replicative senescence and that USP17 inhibition alone was sufficient to trigger cellular senescence. These results reveal a regulatory mechanism whereby USP17 prevents cellular senescence by removing ubiquitin marks from and stabilizing SET8 and transcriptionally repressing p21. [ABSTRACT FROM AUTHOR]

Disorganization of nodes of Ranvier is associated with motor and sensory dysfunctions. Mechanisms that allow nodal recovery during pathological processes remain poorly understood. A highly enriched nodal cytoskeletal protein βIV spectrin anchors and stabilizes the nodal complex to actin cytoskeleton. Loss of murine βIV spectrin allows the initial nodal organization, but causes gradual nodal destabilization. Mutations in human βIV spectrin cause auditory neuropathy and impairment in motor coordination. Similar phenotypes are caused by nodal disruption due to demyelination. Here we report on the precise timelines of nodal disorganization and reorganization by following disassembly and reassembly of key nodal proteins in βIV spectrin mice of both sexes before and after βIV spectrin re-expression at specifically chosen developmental time points. We show that the timeline of nodal restoration has different outcomes in the PNS and CNS with respect to nodal reassembly and functional restoration. In the PNS, restoration of nodes occurs within 1 month regardless of the time of βIV spectrin re-expression. In contrast, the CNS nodal reorganization and functional restoration occurs within a critical time window; after that, nodal reorganization diminishes, leading to less efficient motor recovery. We demonstrate that timely restoration of nodes can improve both the functional properties and the ultrastructure of myelinated fibers affected by long-term nodal disorganization. Our studies, which indicate a critical timeline for nodal restoration together with overall motor performance and prolonged life span, further support the idea that nodal restoration is more beneficial if initiated before any axonal damage, which is critically relevant to demyelinating disorders. [ABSTRACT FROM AUTHOR]

Stress granules are transient cytoplasmic foci induced by various stresses that contain translation-stalled mRNAs and RNA-binding proteins. They are proposed to modulate mRNA translation and stress responses. Here, we show that the deubiquitylases USP5 and USP13 are recruited to heat-induced stress granules. Heat-induced stress granules also contained K48- and K63-linked ubiquitin chains. Depletion of USP5 or USP13 resulted in elevated ubiquitin chain levels and accelerated assembly of heat-induced stress granules, suggesting that these enzymes regulate the stability of the stress granules through their ubiquitin isopeptidase activity. Moreover, disassembly of heat-induced stress granules after returning the cells to normal temperatures was markedly repressed by individual depletion of USP5 or USP13. Finally, overexpression of a ubiquitin mutant lacking the C-terminal diglycine motif caused the accumulation of unanchored ubiquitin chains and the repression of the disassembly of heat-induced stress granules. As unanchored ubiquitin chains are preferred substrates for USP5, we suggest that USP5 regulates the assembly and disassembly of heat-induced stress granules by mediating the hydrolysis of unanchored ubiquitin chains while USP13 regulates stress granules through deubiquitylating protein-conjugated ubiquitin chains. [ABSTRACT FROM AUTHOR]

Caveolin 1 (Cav-1) is an oligomeric protein that forms flaskshaped, lipid-rich pits, termed caveolae, on the plasma membrane. Cav-1 is targeted for lysosomal degradation in ubiquitination- and valosin-containing protein (VCP)-dependent manners. VCP, an ATPase associated with diverse cellular activities that remodels or segregates ubiquitinated protein complexes, has been proposed to disassemble Cav-1 oligomers on the endosomal membrane, facilitating the trafficking of Cav-1 to the lysosome. Genetic mutations in VCP compromise the lysosomal trafficking of Cav-1, leading to a disease called inclusion body myopathy with Paget disease of bone and/or frontotemporal dementia (IBMPFD). Here we identified the Ankrd13 family of ubiquitin-interacting motif (UIM)-containing proteins as novel VCP-interacting molecules on the endosome. Ankrd13 proteins formed a ternary complex with VCP and Cav-1 and exhibited high binding affinity for ubiquitinated Cav-1 oligomers in an UIM-dependent manner. Mass spectrometric analyses revealed that Cav-1 undergoes Lys-63-linked polyubiquitination, which serves as a lysosomal trafficking signal, and that the UIMs of Ankrd13 proteins bind preferentially to this ubiquitin chain type. The overexpression of Ankrd13 caused enlarged hollow lateendosomes, which was reminiscent of the phenotype of the VCP mutations in IBMPFD. Overexpression of Ankrd13 proteins also stabilized ubiquitinated Cav-1 oligomers on the limiting membrane of enlarged endosomes. The interaction with Ankrd13 was abrogated in IMBPFD-associatedVCPmutants. Collectively, our results suggest that Ankrd13 proteins cooperate with VCP to regulate the lysosomal trafficking of ubiquitinated Cav-1. [ABSTRACT FROM AUTHOR]

Context: Somatic mutations in the ubiquitin-specific peptidase USP8 gene were recently detected in one- to two-third(s) of corticotroph adenomas of Cushing's disease (CD). These mutations may lead to the deubiquitination of EGFR, thereby increasing EGFR signaling, which has been implicated in ACTH hypersecretion. Objective: Our objective was to determine the impact of USP8 mutations on the clinicopathological features of CD. Subjects and methods: USP8 mutations as well as clinicopathological characteristics were examined in 60 corticotroph adenomas including 15 Crooke's cell adenomas (CCAs), a rare histological variant presenting with generally aggressive behavior, using qRT-PCR and/or immunohistochemistry. Results: USP8 mutations were exclusively detected in women, except for one case, with a prevalence of 42.2% in non-CCA and 13.3% in CCA (overall 35%). Clinically well-behaved presentations including microadenoma and curative resection were more common in mutated cases. The expression of EGFR was not associated with the mutation status. In contrast, mutated tumors expressed significantly higher levels of POMC, SSTR5, and MGMT. Conclusions: Microadenomas that strongly express POMC were common among mutated tumors, which may lead to the mechanisms by which very small adenomas secrete excess ACTH to present overt CD. While USP8 mutations were less likely to enhance tumorous ACTH hypersecretion via EGFR-mediated activation, the presence of USP8 mutations may predict favorable responses to the somatostatin analog pasireotide, which exhibits high affinity for SSTR5. In contrast, non-mutated aggressive tumors such as CCA may respond better to the alkylating agent temozolomide because of their significantly weak expression of MGMT. [ABSTRACT FROM AUTHOR]

Ubiquitin-specific proteases (USPs) consist of a family of deubiquitinating enzymes with more than 50 members in humans. Three of them, including USP37, contain ubiquitin-interacting motifs (UIMs), an ~20-amino acid α-helical stretch that binds to ubiquitin. However, the roles of the UIMs in these USP enzymes remain unknown. USP37 has three UIMs, designated here as UIMs 1, 2, and 3 from the N-terminal side, between the Cys and His boxes comprising the catalytic core. Here, we examined the role of the UIMs in USP37 using its mutants that harbor mutations in the UIMs. The nuclear localization of USP37 was not affected by the UIM mutations. However, mutations in UIM2 or UIM3, but not UIM1, resulted in a significant decrease in USP37 binding to ubiquitinated proteins in the cell. In vitro, a region of USP37 harboring the three UIMs also bound to both Lys48-linked and Lys63-linked ubiquitin chains in a UIM2- and UIM3-dependent manner. The level of USP37 ubiquitination was also reduced by mutations in UIM2 or UIM3, suggesting their role in ubiquitination of USP37 itself. Finally, mutants lacking functional UIM2 or UIM3 exhibited a reduced isopeptidase activity toward ubiquitinated proteins in the cell and both Lys48-linked and Lys63-linked ubiquitin chains. These results suggested that the UIMs in USP37 contribute to the full enzymatic activity, but not ubiquitin chain substrate specificity, of USP37 possibly by holding the ubiquitin chain substrate in the proximity of the catalytic core. [ABSTRACT FROM AUTHOR]

Clinical relevance is implicated between the genetic polymorphisms of the ABC (ATP-binding cassette) transporter ABCG2 (ABC subfamily G, member 2) and the individual differences in drug response. We expressed a total of seven non-synonymous SNP (single nucleotide polymorphism) variants in Flp-In-293 cells by using the Flp (flippase) recombinase system. Of these, ABCG2 F208S and S441N variants were found to be expressed at markedly low levels, whereas their mRNA levels were equal to those of the other SNP variants and ABCG2 WT (wild-type). Interestingly, protein expression levels of the ABCG2 F208S and S441N variants increased 6- to 12-fold when Flp-In-293 cells were treated with MG132, a proteasome inhibitor. Immunoprecipitation followed by immunoblot analysis showed that the ABCG2 F208S and S441N variant proteins were endogenously ubiquitinated in Flp-In-293 cells, and treatment with MG132 significantly enhanced the level of these ubiquitinated variants. Immunofluorescence microscopy demonstrated that MG132 greatly affected the ABCG2 F208S and S441N variants in terms of both protein levels and intracellular distribution. Immunoblot analysis revealed that those variants were N-glycosylated; however, their oligosaccharides were immature compared with those present on ABCG2 WT. The ABCG2 F208S and S441N variant proteins do not appear to be processed in the Golgi apparatus, but undergo ubiquitin-mediated protein degradation in proteasomes, whereas ABCG2 WT is sorted to the plasma membrane and then degraded via the lysosomal pathway. The present study provides the first evidence that certain genetic polymorphisms can affect the protein stability of ABCG2. Control of proteasomal degradation of ABCG2 would provide a novel approach in cancer chemotherapy to circumvent multidrug resistance of human cancers. [ABSTRACT FROM AUTHOR]