The Ubiquitin-Specific Protease 18 Promotes Hepatitis C Virus Production by Increasing Viral Infectivity

Roles of ubiquitin-specific proteases in inflammatory diseases

Ubiquitin-specific proteases (USPs), as one of the deubiquitinating enzymes (DUBs) families, regulate the fate of proteins and signaling pathway transduction by removing ubiquitin chains from the target proteins. USPs are essential for the modulation of a variety of physiological processes, such as DNA repair, cell metabolism and differentiation, epigenetic modulations as well as protein stability. Recently, extensive research has demonstrated that USPs exert a significant impact on innate and adaptive immune reactions, metabolic syndromes, inflammatory disorders, and infection via post-translational modification processes. This review summarizes the important roles of the USPs in the onset and progression of inflammatory diseases, including periodontitis, pneumonia, atherosclerosis, inflammatory bowel disease, sepsis, hepatitis, diabetes, and obesity. Moreover, we highlight a comprehensive overview of the pathogenesis of USPs in these inflammatory diseases as well as post-translational modifications in the inflammatory responses and pave the way for future prospect of targeted therapies in these inflammatory diseases.

Structural insights into the catalytic mechanism and ubiquitin recognition of USP34

Ubiquitination, an important posttranslational modification, participates in virtually all aspects of cellular functions and is reversed by deubiquitinating enzymes (DUBs). Ubiquitin-specific protease 34 (USP34) plays an essential role in cancer, neurodegenerative diseases, and osteogenesis. Despite its functional importance, how USP34 recognizes ubiquitin and catalyzes deubiquitination remains structurally uncharacterized. Here, we report the crystal structures of the USP34 catalytic domain in free state and after binding with ubiquitin. In the free state, USP34 adopts an inactive conformation, which contains a misaligned catalytic histidine in the triad. Comparison of USP34 structures before and after ubiquitin binding reveals a structural basis for ubiquitin recognition and elucidates a mechanism by which the catalytic triad is realigned. Transition from an open inactive state to a relatively closed active state is coupled to a process by which the "fingertips" of USP34 intimately grip ubiquitin, and this has not been reported before. Our structural and biochemical analyses provide important insights into the catalytic mechanism and ubiquitin recognition of USP34.

Discovery of Genes that Modulate Flavivirus Replication in an Interferon-Dependent Manner

Establishment of the interferon (IFN)-mediated antiviral state provides a crucial initial line of defense against viral infection. Numerous genes that contribute to this antiviral state remain to be identified. Using a loss-of-function strategy, we screened an original library of 1156 siRNAs targeting 386 individual curated human genes in stimulated microglial cells infected with Zika virus (ZIKV), an emerging RNA virus that belongs to the flavivirus genus. The screen recovered twenty-one potential host proteins that modulate ZIKV replication in an IFN-dependent manner, including the previously known IFITM3 and LY6E. Further characterization contributed to delineate the spectrum of action of these genes towards other pathogenic RNA viruses, including Hepatitis C virus and SARS-CoV-2. Our data revealed that APOL3 acts as a proviral factor for ZIKV and several other related and unrelated RNA viruses. In addition, we showed that MTA2, a chromatin remodeling factor, possesses potent flavivirus-specific antiviral functions induced by IFN. Our work identified previously unrecognized genes that modulate the replication of RNA viruses in an IFN-dependent manner, opening new perspectives to target weakness points in the life cycle of these viruses.

Intrinsic Type 1 Interferon (IFN1) Profile of Uncultured Human Bone Marrow CD45lowCD271+ Multipotential Stromal Cells (BM-MSCs): The Impact of Donor Age, Culture Expansion and IFNα and IFNβ Stimulation

Skeletal aging is associated with reduced proliferative potential of bone marrow (BM) multipotential stromal cells (MSCs). Recent data suggest the involvement of type 1 interferon (IFN1) signalling in hematopoietic stem cell (HSC) senescence. Considering that BM-HSCs and BM-MSCs share the same BM niche, we investigated IFN1 expression profile in human BM-MSCs in relation to donor age, culture-expansion and IFN1 (α and β) stimulation. Fluorescence-activated cell sorting was used to purify uncultured BM-MSCs from younger (19-41, n = 6) and older (59-89, n = 6) donors based on the CD45^lowCD271⁺ phenotype, and hematopoietic-lineage cells (BM-HLCs, CD45⁺CD271^-) were used as controls. Gene expression was analysed using integrated circuits arrays in sorted fractions as well as cultured/stimulated BM-MSCs and Y201/Y202 immortalised cell lines. IFN1 stimulation led to BM-MSC growth arrest and upregulation of many IFN1-stimulated genes (ISGs), with IFNβ demonstrating stronger effects. Uncultured MSCs were characterised by a moderate-level ISG expression similar to Y201 cells. Age-related changes in ISG expression were negligible in BM-MSCs compared to BM-HLCs, and intracellular reactive oxygen species (ROS) levels in BM-MSCs did not significantly correlate with donor age. Antiaging genes Klotho and SIRT6 correlated with more ISGs in BM-MSCs than in BM-HLCs. In patients with osteoarthritis (OA), BM-MSCs expressed considerably lower levels of several ISGs, indicating that their IFN1 signature is affected in a pathological condition. In summary, BM-MSCs possess homeostatic IFN1 gene expression signature in health, which is sensitive to in vitro culture and external IFN1 stimulation. IFN signalling may facilitate in vivo BM-MSC responses to DNA damage and combating senescence and aberrant immune activation.

USP18 deubiquitinates and stabilizes Twist1 to promote epithelial-mesenchymal transition in glioblastoma cells

Aberrant activation of epithelial-mesenchymal transition (EMT) pathway drives the invasion and migration of multiple cancers including glioblastoma (GBM). Clinical interventions focused on inhibiting EMT are of increasing interest in the treatment of GBM. In the present study, we discovered that glioma tissues and cells, especially GBMs show significantly up-modulated ubiquitin-specific protease 18 (USP18) expression. Functionally, decreased USP18 expression attenuated GBM cell invasion and migration through repressing EMT. Moreover, a critical EMT-inducing transcription factor Twist1 that activates EMT, was identified as a downstream target of USP18. Mechanistically, USP18 interacts with Twist1, removes its ubiquitination off, and subsequently stabilizes it. Short hairpin RNA-mediated downregulation of USP18 accelerates Twist1 degradation, resulting in the inhibition of GBM cell invasion and migration in vitro and in a nude mouse model. Importantly, reconstituted expression of Twist1 almost completely rescues the inhibitory effect of USP18 depletion on GBM cell invasion, migration and tumor formation. Clinically, the expression levels of USP18 and Twist1 are positively relevant in GBM specimens, and high expression of USP18 correlates with patient's poor outcome. Finally, our findings unveil the crucial role of USP18 on GBM malignancy. Targeting USP18-Twist1 regulatory axis may open a novel avenue for GBM treatment.