Overview of the membrane-associated RING-CH (MARCH) E3 ligase family

MARCH5 promotes hepatocellular carcinoma progression by inducing p53 ubiquitination degradation

BACKGROUND

Human MARCH5 is a mitochondria-localized E3 ubiquitin-protein ligase that is essential for the regulation of mitochondrial dynamics. A large body of evidence suggests that imbalances in mitochondrial dynamics are strongly associated with cancer. However, the expression, biological function and prognostic significance of MARCH5 in hepatocellular carcinoma (HCC) have not been determined.

MATERIALS AND METHODS

The mRNA and protein expression of MARCH5 in HCC cell lines and tumor tissues was assessed by real-time quantitative PCR, Western blot analysis and immunohistochemistry. The clinical prognostic significance of MARCH5 was evaluated in 135 HCC patients. Knockdown or overexpression of MARCH5 in HCC cells was determined by in vitro cell proliferation, migration and invasion assays, and in vivo tumor growth and metastasis assays. In addition, the intrinsic mechanisms by which MARCH5 regulates HCC cell growth and metastasis were explored.

RESULTS

MARCH5 was significantly overexpressed in HCC cells and was closely associated with patients' poor postoperative prognosis. In vivo and in vitro experiments revealed that MARCH5 significantly promoted the increase and invasive and migratory ability of hepatocellular carcinoma cells, which was mainly due to the promotion of autophagy by MARCH5. Mechanistic studies revealed that MARCH5 promoted autophagy through ubiquitination degradation of p53 leading to malignant progression of hepatocellular carcinoma.

CONCLUSION

Our findings suggest that MARCH5 plays a critical oncogenic role in HCC cells, which provides experimental evidence for the use of MARCH5 as a potential target for HCC therapy.

MARCH2, a Novel Oncogene-regulated SNAIL E3 Ligase, Suppresses Triple-negative Breast Cancer Metastases

Epithelial-mesenchymal transition (EMT) in cancer promotes metastasis and chemotherapy resistance. A subset of triple-negative breast cancer (TNBC) exhibits a mesenchymal gene signature that is associated with poor patient outcomes. We previously identified PTK6 tyrosine kinase as an oncogenic driver of EMT in a subset of TNBC. PTK6 induces EMT by stabilizing SNAIL, a key EMT-initiating transcriptional factor. Inhibition of PTK6 activity reverses mesenchymal features of TNBC cells and suppresses their metastases by promoting SNAIL degradation via a novel mechanism. In the current study, we identify membrane-associated RING-CH2 (MARCH2) as a novel PTK6-regulated E3 ligase that promotes the ubiquitination and degradation of SNAIL protein. The MARCH2 RING domain is critical for SNAIL ubiquitination and subsequent degradation. PTK6 inhibition promotes the interaction of MARCH2 with SNAIL. Overexpression of MARCH2 exhibits tumor suppressive properties and phenocopies the effects of SNAIL downregulation and PTK6 inhibition in TNBC cells, such as inhibition of migration, anoikis resistance, and metastasis. Consistent with this, higher levels of MARCH2 expression in breast and other cancers are associated with better prognosis. We have identified MARCH2 as a novel SNAIL E3 ligase that regulates EMT and metastases of mesenchymal TNBC.

SIGNIFICANCE

EMT is a process directly linked to drug resistance and metastasis of cancer cells. We identified MARCH2 as a novel regulator of SNAIL, a key EMT driver, that promotes SNAIL ubiquitination and degradation in TNBC cells. MARCH2 is oncogene regulated and inhibits growth and metastasis of TNBC. These insights could contribute to novel strategies to therapeutically target TNBC.

TAC-tics for Leveraging Proximity Biology in Drug Discovery

Chemically induced proximity (CIP) refers to co-opting naturally occurring biological pathways using synthetic molecules to recruit neosubstrates that are not normally encountered or to enhance the affinity of naturally occurring interactions. Leveraging proximity biology through CIPs has become a rapidly evolving field and has garnered considerable interest in basic research and drug discovery. PROteolysis TArgeting Chimera (PROTAC) is a well-established CIP modality that induces the proximity between a target protein and an E3 ubiquitin ligase, causing target protein degradation via the ubiquitin-proteasome system. Inspired by PROTACs, several other induced proximity modalities have emerged to modulate both proteins and RNA over recent years. In this review, we summarize the critical advances and opportunities in the field, focusing on protein degraders, RNA degraders and non-degrader modalities such as post-translational modification (PTM) and protein-protein interaction (PPI) modulators. We envision that these emerging proximity-based drug modalities will be valuable resources for both biological research and therapeutic discovery in the future.

Advances in research on potential inhibitors of multiple myeloma

Multiple myeloma (MM) is a common hematological malignancy. Although recent clinical applications of immunomodulatory drugs, proteasome inhibitors and CD38-targeting antibodies have significantly improved the outcome of MM patient with increased survival, the incidence of drug resistance and severe treatment-related complications is gradually on the rise. This review article summarizes the characteristics and clinical investigations of several MM drugs in clinical trials, including their structures, mechanisms of action, structure-activity relationships, and clinical study progress. Furthermore, the application potentials of the drugs that have not yet entered clinical trials are also reviewed. The review also outlines the future directions of MM drug development.

MARCH5 promotes STING pathway activation by suppressing polymer formation of oxidized STING

Stimulator of interferon genes (STING) is a core DNA sensing adaptor in innate immune signaling. STING activity is regulated by a variety of post-translational modifications (PTMs), including phosphorylation, ubiquitination, sumoylation, palmitoylation, and oxidation, as well as the balance between active and inactive polymer formation. It remains unclear, though, how different PTMs and higher order structures cooperate to regulate STING activity. Here, we report that the mitochondrial ubiquitin ligase MARCH5 (Membrane Associated Ring-CH-type Finger 5, also known as MITOL) ubiquitinates STING and enhances its activation. A long-term MARCH5 deficiency, in contrast, leads to the production of reactive oxygen species, which then facilitate the formation of inactive STING polymers by oxidizing mouse STING cysteine 205. We show that MARCH5-mediated ubiquitination of STING prevents the oxidation-induced STING polymer formation. Our findings highlight that MARCH5 balances STING ubiquitination and polymer formation and its control of STING activation is contingent on oxidative conditions.

Transcriptome screening identifies TIPARP as an antiviral host factor against the Getah virus

IMPORTANCE

Alphaviruses threaten public health continuously, and Getah virus (GETV) is a re-emerging alphavirus that can potentially infect humans. Approved antiviral drugs and vaccines against alphaviruses are few available, but several host antiviral factors have been reported. Here, we used GETV as a model of alphaviruses to screen for additional host factors. Tetrachlorodibenzo-p-dioxin-inducible poly(ADP ribose) polymerase was identified to inhibit GETV replication by inducing ubiquitination of the glycoprotein E2, causing its degradation by recruiting the E3 ubiquitin ligase membrane-associated RING-CH8 (MARCH8). Using GETV as a model virus, focusing on the relationship between viral structural proteins and host factors to screen antiviral host factors provides new insights for antiviral studies on alphaviruses.

MARCH5-dependent NLRP3 ubiquitination is required for mitochondrial NLRP3-NEK7 complex formation and NLRP3 inflammasome activation

The NLRP3 inflammasome plays a key role in responding to pathogens, and endogenous damage and mitochondria are intensively involved in inflammasome activation. The NLRP3 inflammasome forms multiprotein complexes and its sequential assembly is important for its activation. Here, we show that NLRP3 is ubiquitinated by the mitochondria-associated E3 ligase, MARCH5. Myeloid cell-specific March5 conditional knockout (March5 cKO) mice failed to secrete IL-1β and IL-18 and exhibited an attenuated mortality rate upon LPS or Pseudomonas aeruginosa challenge. Macrophages derived from March5 cKO mice also did not produce IL-1β and IL-18 after microbial infection. Mechanistically, MARCH5 interacts with the NACHT domain of NLRP3 and promotes K27-linked polyubiquitination on K324 and K430 residues of NLRP3. Ubiquitination-defective NLRP3 mutants on K324 and K430 residues are not able to bind to NEK7, nor form NLRP3 oligomers leading to abortive ASC speck formation and diminished IL-1β production. Thus, MARCH5-dependent NLRP3 ubiquitination on the mitochondria is required for NLRP3-NEK7 complex formation and NLRP3 oligomerization. We propose that the E3 ligase MARCH5 is a regulator of NLRP3 inflammasome activation on the mitochondria.

Targeting E3 ubiquitin ligases and their adaptors as a therapeutic strategy for metabolic diseases

Posttranslational modification of proteins via ubiquitination determines their activation, translocation, dysregulation, or degradation. This process targets a large number of cellular proteins, affecting all biological pathways involved in the cell cycle, development, growth, and differentiation. Thus, aberrant regulation of ubiquitination is likely associated with several diseases, including various types of metabolic diseases. Among the ubiquitin enzymes, E3 ubiquitin ligases are regarded as the most influential ubiquitin enzymes due to their ability to selectively bind and recruit target substrates for ubiquitination. Continued research on the regulatory mechanisms of E3 ligases and their adaptors in metabolic diseases will further stimulate the discovery of new targets and accelerate the development of therapeutic options for metabolic diseases. In this review, based on recent discoveries, we summarize new insights into the roles of E3 ubiquitin ligases and their adaptors in the pathogenesis of metabolic diseases by highlighting recent evidence obtained in both human and animal model studies.

The emerging roles of MARCH8 in viral infections: A double-edged Sword

The host cell membrane-associated RING-CH 8 protein (MARCH8), a member of the E3 ubiquitin ligase family, regulates intracellular turnover of many transmembrane proteins and shows potent antiviral activities. Generally, 2 antiviral modes are performed by MARCH8. On the one hand, MARCH8 catalyzes viral envelope glycoproteins (VEGs) ubiquitination and thus leads to their intracellular degradation, which is the cytoplasmic tail (CT)-dependent (CTD) mode. On the other hand, MARCH8 traps VEGs at some intracellular compartments (such as the trans-Golgi network, TGN) but without inducing their degradation, which is the cytoplasmic tail-independent (CTI) mode, by which MARCH8 hijacks furin, a cellular proprotein convertase, to block VEGs cleavage. In addition, the MARCH8 C-terminal tyrosine-based motif (TBM) 222YxxL225 also plays a key role in its CTI antiviral effects. In contrast to its antiviral potency, MARCH8 is occasionally hijacked by some viruses and bacteria to enhance their invasion, indicating a duplex role of MARCH8 in host pathogenic infections. This review summarizes MARCH8's antiviral roles and how viruses evade its restriction, shedding light on novel antiviral therapeutic avenues.

Regulation of inflammation and immunity in sepsis by E3 ligases

Sepsis is a life-threatening organ dysfunction caused by an abnormal infection-induced immune response. Despite significant advances in supportive care, sepsis remains a considerable therapeutic challenge and is the leading cause of death in the intensive care unit (ICU). Sepsis is characterized by initial hyper-inflammation and late immunosuppression. Therefore, immune-modulatory therapies have great potential for novel sepsis therapies. Ubiquitination is an essential post-translational protein modification, which has been known to be intimately involved in innate and adaptive immune responses. Several E3 ubiquitin ligases have been implicated in innate immune signaling and T-cell activation and differentiation. In this article, we review the current literature and discuss the role of E3 ligases in the regulation of immune response and their effects on the course of sepsis to provide insights into the prevention and therapy for sepsis.

Influenza virus reduces ubiquitin E3 ligase MARCH10 expression to decrease ciliary beat frequency

Respiratory viruses, such as influenza, decrease airway cilia function and expression, which leads to reduced mucociliary clearance and inhibited overall immune defense. Ubiquitination is a posttranslational modification using E3 ligases, which plays a role in the assembly and disassembly of cilia. We examined the role of membrane-associated RING-CH (MARCH) family of E3 ligases during influenza infection and determined that MARCH10, specifically expressed in ciliated epithelial cells, is significantly decreased during influenza infection in mice, human lung epithelial cells, and human lung tissue. Cellular depletion of MARCH10 in differentiated human bronchial epithelial cells (HBECs) using CRISPR/Cas9 showed a decrease in ciliary beat frequency. Furthermore, MARCH10 cellular knockdown in combination with influenza infection selectively decreased immunoreactive levels of the ciliary component, dynein axonemal intermediate chain 1. Cellular overexpression of MARCH10 significantly decreased influenza hemagglutinin protein levels in the differentiated HBECs and knockdown of MARCH10 increased IL-1β cytokine expression, whereas overexpression had the reciprocal effect. These findings suggest that MARCH10 may have a protective role in airway pulmonary host defense and innate immunity during influenza infection.

Zebrafish MARCH7 negatively regulates IFN antiviral response by degrading TBK1

Membrane-associated RING-CH-type finger (MARCH) proteins have been reported to regulate type I IFN production during host antiviral innate immunity. The present study reported the zebrafish MARCH family member, MARCH7, as a negative regulator in virus-triggered type I IFN induction via targeting TANK-binding kinase 1 (TBK1) for degradation. As an IFN-stimulated gene (ISG), we discovered that MARCH7 was significantly induced by spring viremia of carp virus (SVCV) or poly(I:C) stimulation. Ectopic expression of MARCH7 reduced the activity of IFN promoter and dampened the cellular antiviral responses triggered by SVCV and grass carp reovirus (GCRV), which concomitantly accelerated the viral replication. Accordingly, the knockdown of MARCH7 by siRNA transfection significantly promoted the transcription of ISG genes and inhibited SVCV replication. Mechanistically, we found that MARCH7 interacted with TBK1 and degraded it via K48-linked ubiquitination. Further characterization of truncated mutants of MARCH7 and TBK1 confirmed that the C-terminal RING of MARCH7 is essential in the MARCH7-mediated degradation of TBK1 and the negative regulation of IFN antiviral response. This study reveals a molecular mechanism by which zebrafish MARCH7 negatively regulates the IFN response by targeting TBK1 for protein degradation, providing new insights into the essential role of MARCH7 in antiviral innate immunity.

HPV upregulates MARCHF8 ubiquitin ligase and inhibits apoptosis by degrading the death receptors in head and neck cancer

The membrane-associated RING-CH-type finger ubiquitin ligase MARCHF8 is a human homolog of the viral ubiquitin ligases Kaposi's sarcoma herpesvirus K3 and K5 that promote host immune evasion. Previous studies have shown that MARCHF8 ubiquitinates several immune receptors, such as the major histocompatibility complex II and CD86. While human papillomavirus (HPV) does not encode any ubiquitin ligase, the viral oncoproteins E6 and E7 are known to regulate host ubiquitin ligases. Here, we report that MARCHF8 expression is upregulated in HPV-positive head and neck cancer (HNC) patients but not in HPV-negative HNC patients compared to normal individuals. The MARCHF8 promoter is highly activated by HPV oncoprotein E6-induced MYC/MAX transcriptional activation. The knockdown of MARCHF8 expression in human HPV-positive HNC cells restores cell surface expression of the tumor necrosis factor receptor superfamily (TNFRSF) death receptors, FAS, TRAIL-R1, and TRAIL-R2, and enhances apoptosis. MARCHF8 protein directly interacts with and ubiquitinates the TNFRSF death receptors. Further, MARCHF8 knockout in mouse oral cancer cells expressing HPV16 E6 and E7 augments cancer cell apoptosis and suppresses tumor growth in vivo. Our findings suggest that HPV inhibits host cell apoptosis by upregulating MARCHF8 and degrading TNFRSF death receptors in HPV-positive HNC cells.

MARCH9 Mediates NOX2 Ubiquitination to Alleviate NLRP3 Inflammasome-Dependent Pancreatic Cell Pyroptosis in Acute Pancreatitis

OBJECTIVE

The pathogenesis of acute pancreatitis mainly involves NLRP3 inflammasome-mediated pancreatic cell injury, although regulators of this inflammasome machinery are still not fully identified. Membrane-associated RING-CH 9 (MARCH9) is a member of MARCH-type finger proteins, which regulates innate immunity through catalyzing polyubiquitination of critical immune factors. The aim of present research is to examine the function of MARCH9 in acute pancreatitis.

METHODS

Cerulein-induced acute pancreatitis was established on pancreatic cell line AR42J and rat model. Reactive oxygen species (ROS) accumulation and NLRP3 inflammasome-dependent cell pyroptosis in pancreas were examined by flow cytometry.

RESULTS

MARCH9 was downregulated by cerulein, but overexpressing MARCH9 could inhibit NLRP3 inflammasome activation and ROS accumulation, thus suppressing pancreatic cell pyroptosis and mitigating pancreatic injury. We further uncovered that the mechanism underlying such an effect of MARCH9 is through mediating the ubiquitination of NADPH oxidase-2, whose deficiency reduces cellular ROS accumulation and inflammasome formation.

CONCLUSIONS

Our results suggested that MARCH9 suppresses NLRP3 inflammasome-mediated pancreatic cell injury through mediating the ubiquitination and degradation of NADPH oxidase-2, which compromises ROS generation and NLRP3 inflammasomal activation.

Membrane compartmentalisation of the ubiquitin system

We now have a comprehensive inventory of ubiquitin system components. Understanding of any system also needs an appreciation of how components are organised together. Quantitative proteomics has provided us with a census of their relative populations in several model cell types. Here, by examining large scale unbiased data sets, we seek to identify and map those components, which principally reside on the major organelles of the endomembrane system. We present the consensus distribution of > 50 ubiquitin modifying enzymes, E2s, E3s and DUBs, that possess transmembrane domains. This analysis reveals that the ER and endosomal compartments have a diverse cast of resident E3s, whilst the Golgi and mitochondria operate with a more restricted palette. We describe key functions of ubiquitylation that are specific to each compartment and relate this to their signature complement of ubiquitin modifying components.

Isoforms of Human MARCH1 Differ in Ability to Restrict Influenza A Viruses Due to Differences in Their N Terminal Cytoplasmic Domain

MARCH1 and MARCH8 are closely related E3 ubiquitin ligases that ubiquitinate an overlapping spectrum of host proteins and restrict replication of certain viruses. While the antiviral activity of MARCH8 has been intensively studied, less is known regarding virus inhibition by MARCH1. Isoforms 1 and 2 of MARCH1 are very similar in overall structure but show major differences in their N-terminal cytoplasmic domain (N-CT). Herein, we used a doxycycline-inducible overexpression system to demonstrate that MARCH1.1 reduces titres of influenza A virus (IAV) released from infected cells whereas MARCH1.2 does not. The deletion of the entire N-CT of MARCH1.2 restored its ability to restrict IAV infectivity and sequential deletions mapped the restoration of IAV inhibition to delete the 16 N-terminal residues within the N-CT of MARCH1.2. While only MARCH1.1 mediated anti-IAV activity, qPCR demonstrated the preferential expression of MARCH1.2 over MARCH1.1 mRNA in unstimulated human peripheral blood mononuclear cells and also in monocyte-derived macrophages. Together, these studies describe the differential ability of MARCH1 isoforms to restrict IAV infectivity for the first time. Moreover, as published immunological, virological and biochemical studies examining the ability of MARCH1 to target particular ligands generally use only one of the two isoforms, these findings have broader implications for our understanding of how MARCH1 isoforms might differ in their ability to modulate particular host and/or viral proteins.

Membrane-associated RING-CH protein (MARCH8) is a novel glycolysis repressor targeted by miR-32 in colorectal cancer

BACKGROUND

Colorectal cancer (CRC) is the third most common cancer and leading cause of cancer-related deaths worldwide. Aberrant cellular metabolism is a hallmark of cancer cells, and disturbed metabolism showed clinical significance in CRC. The membrane-associated RING-CH 8 (MARCH8) protein, the first MARCH E3 ligase, plays an oncogenic role and serves as a prognostic marker in multiple cancers, however, the role of MARCH8 in CRC is unclear. In the present study, we aimed to investigate the biomarkers and their underlying mechanism for CRC.

METHOD

In this study, we first examined the function of MARCH8 in CRC by analysing public database. Besides, we performing gene silencing studies and generating cellular overexpression and xenograft models. Then its protein substrate was identified and validated. In addition, the expression of MARCH8 was investigated in tissue samples from CRC patients, and the molecular basis for decreased expression was analysed.

RESULTS

Systematic analysis reveals that MARCH8 is a beneficial prognostic marker in CRC. In CRC, MARCH8 exhibited tumor-suppressive activity both in vivo and in vitro. Furthermore, we found that MARCH8 is negatively correlated with hexokinase 2 (HK2) protein in CRC patients. MARCH8 regulates glycolysis and promotes ubiquitination-mediated proteasome degradation to reduces HK2 protein levels. Then HK2 inhibitor partially rescues the effect of MARCH8 knockdown in CRC. Poised chromatin and elevated miR-32 repressed MARCH8 expression.

CONCLUSION

In summary, we propose that in CRC, poised chromatin and miR-32 decrease the expression of MARCH8, further bind and add ubiquitin, induce HK2 degradation, and finally repress glycolysis to promote tumor suppressors in CRC.

The RING finger protein family in health and disease

Ubiquitination is a highly conserved and fundamental posttranslational modification (PTM) in all eukaryotes regulating thousands of proteins. The RING (really interesting new gene) finger (RNF) protein, containing the RING domain, exerts E3 ubiquitin ligase that mediates the covalent attachment of ubiquitin (Ub) to target proteins. Multiple reviews have summarized the critical roles of the tripartite-motif (TRIM) protein family, a subgroup of RNF proteins, in various diseases, including cancer, inflammatory, infectious, and neuropsychiatric disorders. Except for TRIMs, since numerous studies over the past decades have delineated that other RNF proteins also exert widespread involvement in several diseases, their importance should not be underestimated. This review summarizes the potential contribution of dysregulated RNF proteins, except for TRIMs, to the pathogenesis of some diseases, including cancer, autoimmune diseases, and neurodegenerative disorder. Since viral infection is broadly involved in the induction and development of those diseases, this manuscript also highlights the regulatory roles of RNF proteins, excluding TRIMs, in the antiviral immune responses. In addition, we further discuss the potential intervention strategies targeting other RNF proteins for the prevention and therapeutics of those human diseases.

MARCH1 silencing suppresses growth of oral squamous cell carcinoma through regulation of PHLPP2

PURPOSE

Oral squamous cell carcinoma (OSCC) is the most frequent type of oral cancer and is associated with high mortality. Membrane-associated ring-CH type finger 1 (MARCH1) is an E3 ubiquitin ligase with roles in immune regulation and cancer development. Whether MARCH1 has a specific role in OSCC, and if so through what mechanism, has not been explored.

METHODS

Immunohistochemistry was performed to examine MARCH1 expression in OSCC clinical samples and adjacent paracancerous tissues. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and Western blot were conducted to determine mRNA expression and protein levels, respectively. Knockdown and overexpression experiments were carried out to evaluate the effects of MARCH1 on proliferation and apoptosis. To test protein-protein interaction, co-immunoprecipitation assay was performed. Finally, tumor cell grafting was utilized to test the function of MARCH in vivo.

RESULTS

High MARCH1 expression in OSCC clinical samples correlated with poor patient prognosis. Functionally, MARCH1 knockdown in OSCC cells suppressed proliferation and promoted apoptosis, while MARCH1 overexpression displayed the opposite effects. We identified PH Domain And Leucine Rich Repeat Protein Phosphatase (PHLPP) 2 as an important target of MARCH1. Mechanistically, MARCH1 interacted with PHLPP2 and promoted PHLPP2 ubiquitination. Lastly, MARCH1 knockdown suppressed OSCC tumorigenicity in vivo and increased PHLPP2 protein level.

CONCLUSION

Our study uncovered a function of MARCH1 in OSCC and identified PHLPP2 as an important target of MARCH1 to modulate OSCC cell proliferation and apoptosis.

The Role of Membrane-Associated E3 Ubiquitin Ligases in Cancer

The cell membrane system comprises the plasma membrane, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondria, and nuclear membrane, which are essential for maintaining normal physiological functions of cells. The proteins associated with these membrane-organelles are frequently modified to regulate their functions, the most common of which is ubiquitin modification. So far, many ubiquitin E3 ligases anchored in the membrane system have been identified as critical players facilitating intracellular biofunctions whose dysfunction is highly related to cancer. In this review, we summarized membrane-associated E3 ligases and revealed their relationship with cancer, which is of great significance for discovering novel drug targets of cancer and may open up new avenues for inducing ubiquitination-mediated degradation of cancer-associated membrane proteins via small chemicals such as PROTAC and molecular glue.

Phospholipids alter activity and stability of mitochondrial membrane-bound ubiquitin ligase MARCH5

This study shows that lipids can act as regulators for the ubiquitination process and can control the stability and activity of a membrane-embedded E3 ubiquitin ligase.

Mitochondrial homeostasis is tightly controlled by ubiquitination. The mitochondrial integral membrane ubiquitin ligase MARCH5 is a crucial regulator of mitochondrial membrane fission, fusion, and disposal through mitophagy. In addition, the lipid composition of mitochondrial membranes can determine mitochondrial dynamics and organelle turnover. However, how lipids influence the ubiquitination processes that control mitochondrial homeostasis remains unknown. Here, we show that lipids common to the mitochondrial membranes interact with MARCH5 and affect its activity and stability depending on the lipid composition in vitro. As the only one of the tested lipids, cardiolipin binding to purified MARCH5 induces a significant decrease in thermal stability, whereas stabilisation increases the strongest in the presence of phosphatidic acid. Furthermore, we observe that the addition of lipids to purified MARCH5 alters the ubiquitination pattern. Specifically, cardiolipin enhances auto-ubiquitination of MARCH5. Our work shows that lipids can directly affect the activity of ubiquitin ligases and suggests that the lipid composition in mitochondrial membranes could control ubiquitination-dependent mechanisms that regulate the dynamics and turnover of mitochondria.

Receptor control by membrane-tethered ubiquitin ligases in development and tissue homeostasis

Paracrine cell-cell communication is central to all developmental processes, ranging from cell diversification to patterning and morphogenesis. Precise calibration of signaling strength is essential for the fidelity of tissue formation during embryogenesis and tissue maintenance in adults. Membrane-tethered ubiquitin ligases can control the sensitivity of target cells to secreted ligands by regulating the abundance of signaling receptors at the cell surface. We discuss two examples of this emerging concept in signaling: (1) the transmembrane ubiquitin ligases ZNRF3 and RNF43 that regulate WNT and bone morphogenetic protein receptor abundance in response to R-spondin ligands and (2) the membrane-recruited ubiquitin ligase MGRN1 that controls Hedgehog and melanocortin receptor abundance. We focus on the mechanistic logic of these systems, illustrated by structural and protein interaction models enabled by AlphaFold. We suggest that membrane-tethered ubiquitin ligases play a widespread role in remodeling the cell surface proteome to control responses to extracellular ligands in diverse biological processes.

Potential of E3 Ubiquitin Ligases in Cancer Immunity: Opportunities and Challenges

Cancer immunotherapies, including immune checkpoint inhibitors and immune pathway–targeted therapies, are promising clinical strategies for treating cancer. However, drug resistance and adverse reactions remain the main challenges for immunotherapy management. The future direction of immunotherapy is mainly to reduce side effects and improve the treatment response rate by finding new targets and new methods of combination therapy. Ubiquitination plays a crucial role in regulating the degradation of immune checkpoints and the activation of immune-related pathways. Some drugs that target E3 ubiquitin ligases have exhibited beneficial effects in preclinical and clinical antitumor treatments. In this review, we discuss mechanisms through which E3 ligases regulate tumor immune checkpoints and immune-related pathways as well as the opportunities and challenges for integrating E3 ligases targeting drugs into cancer immunotherapy.

MARCH8 Restricts Influenza A Virus Infectivity but Does Not Downregulate Viral Glycoprotein Expression at the Surface of Infected Cells

Membrane-associated RING-CH8 (MARCH8) impairs the cell surface expression of envelope glycoproteins from different viruses, reducing their incorporation into virions. Using stable cell lines with inducible MARCH8 expression, we show that MARCH8 did not alter susceptibility to influenza A virus (IAV) infection, but virions released from infected cells were markedly less infectious. Knockdown of endogenous MARCH8 confirmed its effect on IAV infectivity. The expression of MARCH8 impaired the infectivity of both H3N2 and H1N1 strains and was dependent on its E3 ligase activity. Although virions released in the presence of MARCH8 expressed smaller amounts of viral hemagglutinin (HA) and neuraminidase (NA) proteins, there was no impact on levels of the viral HA, NA, or matrix 2 (M2) proteins detected on the surface of infected cells. Moreover, mutation of lysine residues in the cytoplasmic tails of HA, NA, and/or M2, or in the viral M1 protein, did not abrogate MARCH8-mediated restriction. While MARCH1 and -8 target similar immunological ligands and both restrict HIV-1, only MARCH8 inhibited IAV infectivity. Deletion of the N-terminal cytoplasmic (N-CT) domain of MARCH8 confirmed it to be a critical determinant of IAV inhibition. Of interest, deletion of the MARCH1 N-CT or its replacement with the MARCH8 N-CT resulted in acquisition of IAV restriction. Together, these data demonstrate that MARCH8 restricts a late stage in IAV replication by a mechanism distinct to its reported activity against other viruses. Moreover, we show that the N-CT of MARCH8 is essential for anti-IAV activity, whereas the MARCH1 N-CT inhibits its ability to restrict IAV.

HSC70 Inhibits Spring Viremia of Carp Virus Replication by Inducing MARCH8-Mediated Lysosomal Degradation of G Protein

As a fierce pathogen, spring viremia of carp virus (SVCV) can cause high mortality in the common carp, and its glycoprotein (G protein) is a component of the viral structure on the surface of virion, which is crucial in viral life cycle. This report adopted tandem affinity purification (TAP), mass spectrometry analysis (LC-MS/MS), immunoprecipitation, and confocal microscopy assays to identify Heat shock cognate protein 70 (HSC70) as an interaction partner of SVCV G protein. It was found that HSC70 overexpression dramatically inhibited SVCV replication, whereas its loss of functions elicited opposing effects on SVCV replication. Mechanistic studies indicate that HSC70 induces lysosomal degradation of ubiquitinated-SVCV G protein. This study further demonstrates that Membrane-associated RING-CH 8 (MARCH8), an E3 ubiquitin ligase, is critical for SVCV G protein ubiquitylation and leads to its lysosomal degradation. Furthermore, the MARCH8 mediated ubiquitylation of SVCV G protein required the participation of HSC70 through forming a multicomponent complex. Taken together, these results demonstrate that HSC70 serves as a scaffold for MARCH8 and SVCV G, which leads to the ubiquitylation and degradation of SVCV G protein and thus inhibits viral replication. These findings have established a novel host defense mechanism against SVCV.

RNA sequencing analysis reveals the competing endogenous RNAs interplay in resected hepatocellular carcinoma patients who received interferon-alpha therapy

BACKGROUND

Interferon-alpha (IFN-α) is a general therapeutic regimen to be utilized in hepatocellular carcinoma (HCC). However, regulatory mechanisms of IFN-α on competing endogenous RNAs (ceRNAs) level in anti-HCC relapse are rarely understood.

METHODS

HCC patients with and without IFN-α treatment were calculated to analyze the expression profile of mRNA, long non-coding RNA (lncRNA), microRNA (miRNA), and circular RNA (circRNA) by RNA sequence, and significant differential expression (DE) of these types of RNAs were selected for further analysis. A ceRNA regulatory network was constructed to explore the potential mechanisms of IFN-α intervention on anti-HCC relapse. Finally, the potential prognostic associated genes among these DE RNAs were identified.

RESULTS

Totally, 556 mRNAs, 120 circRNAs, 87 lncRNAs, and 96 miRNAs were differentially expressed in patients who received IFN-α treatment. A ceRNA regulatory network including a circRNA-miRNA-mRNA network which composed of 4 up- and 10 down-regulated circRNAs, 8 up- and 5 down-regulated miRNAs, 28 up- and 9 down-regulated mRNAs, and a lncRNA-miRNA-mRNA network which composed of 10 up- and 3 down-regulated lncRNAs, 11 up- and 5 down-regulated miRNAs, 28 up- and 10 down-regulated mRNAs was constructed. Gene enrichment and pathway analysis revealed that the ceRNA network was associated with immune-related pathway and corresponding molecular function in patients who accepted IFN-α treatment. Next, we identified 3 most relevant to IFN-α treatment to HCC among these DE RNAs, namely FAM20A, IGFBP4 and MARCH3, as the prognostic associated genes for HCC. Furthermore, MARCH3 expression correlated with infiltrating levels of tumor infiltrating immune cells (TICCs) in HCC. MARCH3 expression also showed strong correlations with the gene markers of diverse immune cells in HCC.

CONCLUSION

Our data discovered a novel ceRNA network in HCC patients receiving IFN-α therapy, which might lay the foundation for better understand the regulatory mechanism of IFN-α treatment.

Membrane-Associated RING-CH 8 Functions as a Novel PD-L1 E3 Ligase to Mediate PD-L1 Degradation Induced by EGFR Inhibitors

Expression of programmed death-ligand 1 (PD-L1) on cancer cells is a critical mechanism contributing to immunosuppression and immune escape. PD-L1 expression may also affect therapeutic outcomes of epidermal growth factor receptor (EGFR)-targeted therapy (e.g., with osimertinib/AZD9291) against EGFR-mutant non-small cell lung cancers (NSCLC) and can even be altered during the treatment albeit with largely undefined mechanisms. This study primarily focuses on elucidating the mechanism by which osimertinib induces PD-L1 degradation in addition to validating osimertinib's effect on decreasing PD-L1 expression in EGFR-mutant NSCLC cells and tumors. Osimertinib and other EGFR inhibitors effectively decreased PD-L1 levels primarily in EGFR-mutant NSCLCs and xenografted tumors. Osimertinib not only decreased PD-L1 mRNA expression, but also prompted proteasomal degradation of PD-L1 protein, indicating both transcriptional and posttranslational mechanisms accounting for osimertinib-induced reduction of PD-L1. Knockdown of β-TrCP or inhibition of GSK3 failed to prevent PD-L1 reduction induced by osimertinib. Rather, knockdown of membrane-associated RING-CH 8 (MARCH8) that encodes a membrane-bound E3 ubiquitin ligase rescued osimertinib-induced PD-L1 reduction. Furthermore, manipulation of MARCH8 expression accordingly altered PD-L1 degradation rate. Critically, MARCH8 interacted with PD-L1 through its N-terminal region and also ubiquitinated PD-L1 in cells. Collectively, these results strongly suggest that MARCH8 is a previously undiscovered E3 ubiquitin ligase responsible for PD-L1 degradation including osimertinib-induced PD-L1 degradation, establishing a novel connection between MARCH8 and PD-L1 regulation. IMPLICATIONS: This study has demonstrated a previously undiscovered function of MARCH8 in mediating PD-L1 degradation induced by EGFR inhibitors in EGFR-mutant NSCLC cells, establishing a novel connection between MARCH8 and PD-L1 regulation.

Proteomic analyses reveal that immune integrins are major targets for regulation by Membrane-Associated Ring-CH (MARCH) proteins MARCH2, 3, 4 and 9

MARCH proteins are membrane-associated Ring-CH E3 ubiquitin ligases that dampen immune responses by downregulating cell surface expression of major histocompatibility complexes I and II as well as immune co-stimulatory receptors. We recently showed that MARCH2,3,4 and 9 also downregulate cell surface expression of the inflammatory cytokine receptor for interleukin-6 (IL6Rα). Here we use over-expression of these MARCH proteins in the M1 myeloid leukaemia cell line and cell surface proteomic analyses to globally analyse other potential targets of these proteins. A large range of cell surface proteins regulated by more than one MARCH protein in addition to several MARCH protein-specific cell surface targets were identified most of which were downregulated by MARCH expression. Prominent among these were several integrin complexes associated with immune cell homing, adhesion and migration. Integrin α4β1 (VLA4 or VCAM-1 receptor) was downregulated only by MARCH2 and we showed that in MARCH2 knockout mice, Integrin α4 was upregulated specifically in mature B-lymphocytes and this was accompanied by decreased numbers of B-cells in the spleen.

The Engineered MARCH8-Resistant Vesicular Stomatitis Virus Glycoprotein Enhances Lentiviral Vector Transduction

Lentiviral vectors are one of the most commonly used viral delivery systems for gene therapy. Vesicular stomatitis virus-G envelope glycoprotein (VSV G)-pseudotyped lentiviral vectors have been widely used in clinical studies for treatment of virus infections and genetic deficient diseases. However, the efficiency of lentiviral vector transduction has been long recognized as a limiting factor in clinical gene therapy application, especially in transducing hematopoietic stem cells. MARCH8 (membrane-associated RING-CH 8), an E3 ubiquitin ligase, has been reported to target and downregulate VSV G. Results in this study show that MARCH8 induces ubiquitination and lysosome degradation of VSV G, and knockout of MARCH8 in virus-producing cells increases lentiviral vector transduction by elevating the level of VSV G protein. We then engineered VSV G mutant that has the lysine residues in the cytoplasmic domain substituted for arginine, and showed that this G mutant resists degradation by MARCH8, and allows the enhancement of transduction efficiency of lentiviral vector particles than the parental VSV G protein. This engineered VSV G mutant thus further advances the lentiviral vector system as a powerful tool in gene therapy.

Mechanism of Viral Glycoprotein Targeting by Membrane-Associated RING-CH Proteins

Viral envelope glycoproteins are an important structural component on the surfaces of enveloped viruses that direct virus binding and entry and also serve as targets for the host adaptive immune response. In this study, we investigate the mechanism of action of the MARCH family of cellular proteins that disrupt the trafficking and virion incorporation of viral glycoproteins across several virus families.

An emerging class of cellular inhibitory proteins has been identified that targets viral glycoproteins. These include the membrane-associated RING-CH (MARCH) family of E3 ubiquitin ligases that, among other functions, downregulate cell surface proteins involved in adaptive immunity. The RING-CH domain of MARCH proteins is thought to function by catalyzing the ubiquitination of the cytoplasmic tails (CTs) of target proteins, leading to their degradation. MARCH proteins have recently been reported to target retroviral envelope glycoproteins (Env) and vesicular stomatitis virus G glycoprotein (VSV-G). However, the mechanism of antiviral activity remains poorly defined. Here we show that MARCH8 antagonizes the full-length forms of HIV-1 Env, VSV-G, Ebola virus glycoprotein (EboV-GP), and the spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), thereby impairing the infectivity of virions pseudotyped with these viral glycoproteins. This MARCH8-mediated targeting of viral glycoproteins requires the E3 ubiquitin ligase activity of the RING-CH domain. We observe that MARCH8 protein antagonism of VSV-G is CT dependent. In contrast, MARCH8-mediated targeting of HIV-1 Env, EboV-GP, and SARS-CoV-2 S protein by MARCH8 does not require the CT, suggesting a novel mechanism of MARCH-mediated antagonism of these viral glycoproteins. Confocal microscopy data demonstrate that MARCH8 traps the viral glycoproteins in an intracellular compartment. We observe that the endogenous expression of MARCH8 in several relevant human cell types is rapidly inducible by type I interferon. These results help to inform the mechanism by which MARCH proteins exert their antiviral activity and provide insights into the role of cellular inhibitory factors in antagonizing the biogenesis, trafficking, and virion incorporation of viral glycoproteins.

When MARCH family proteins meet viral infections

Membrane-associated RING-CH (MARCH) ubiquitin ligases belong to a RING finger domain E3 ligases family. Recent studies have demonstrated that MARCH proteins play critical roles during various viral infections. MARCH proteins can directly antagonize different steps of the viral life cycle and promote individual viral infection. This mini-review will focus on the latest advances of MARCH family proteins' emerging roles during viral infections.

Elucidating the Antiviral Mechanism of Different MARCH Factors

The membrane-associated RING-CH (MARCH) proteins belong to a family of E3 ubiquitin ligases, whose main function is to remove transmembrane proteins from the plasma membrane. Recent work has shown that the human MARCH1, 2, and 8 are antiretroviral factors that target the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins by reducing their incorporation in the budding virions. Nevertheless, the dearth of information regarding the antiviral mechanism of this family of proteins necessitates further examination. In this study, using both the human MARCH proteins and their mouse homologues, we provide a comprehensive analysis of the antiretroviral mechanism of this family of proteins. Moreover, we show that human MARCH proteins restrict to various degrees the envelope glycoproteins of a diverse number of viruses. This report sheds light on the important antiviral function of MARCH proteins and their significance in cell intrinsic immunity.IMPORTANCE This study examines the mechanism utilized by different MARCH proteins to restrict retrovirus infection. MARCH proteins block the incorporation of envelope glycoproteins to the budding virions. In this report, by comparing the human and mouse MARCH genes and using murine leukemia virus (MLV) and HIV-1, we identify differences in the mechanism of restriction among MARCH proteins. Furthermore, we perform a comprehensive analysis on a number of envelope glycoproteins and show that MARCH proteins have broad antiviral functions.

Multiple assay systems to analyze the dynamics of mitochondrial nucleoids in living mammalian cells

BACKGROUND

Mitochondria, which play a critical role in energy production by oxidative respiration, are highly dynamic organelles and their double membranes undergo frequent events of fusion and fission. Mitochondria are believed to be derived from the endosymbiosis of proteobacteria, and thus mitochondria still contain their own DNA, mitochondrial DNA (mtDNA). Several copies of mtDNA form mitochondrial nucleoid with DNA-binding proteins. Recently, the morphology and distribution of the mitochondrial membrane and nucleoid were reported to be cooperatively regulated during their dynamic movement. However, the molecular mechanism is unclear, because the involved molecules are poorly understood, and suitable techniques to analyze nucleoid have not been fully developed.

RESULTS

To solve these issues, we examined the molecular mechanism of nucleoid dynamics by two approaches. First, we constructed a new probe to perform live imaging of nucleoid dynamics using the DNA-binding domain of mitochondrial transcriptional factor A (TFAM) and the photo-convertible fluorescent protein Kikume Green-Red (KikGR). Nucleoids were visualized stably for a long period using the new probe. Second, we searched for nucleoid regulatory factors by small interfering RNA screening using HeLa cells and identified a subset of MARCH family ubiquitin ligases that affect nucleoid morphology.

CONCLUSION

The factors and probe, reported in this study, would be useful to reveal novel mechanisms of mitochondrial regulation.

GENERAL SIGNIFICANCE

The mtDNA dynamics should be concerned in the regulation of mitochondrial activity and its quality control, associated with mitochondrial membrane dynamics.

Mechanism of Viral Glycoprotein Targeting by Membrane-associated-RING-CH Proteins

An emerging class of cellular inhibitory proteins has been identified that targets viral glycoproteins. These include the membrane-associated RING-CH (MARCH) family of E3 ubiquitin ligases that, among other functions, downregulate cell-surface proteins involved in adaptive immunity. The RING-CH domain of MARCH proteins is thought to function by catalyzing the ubiquitination of the cytoplasmic tails (CTs) of target proteins, leading to their degradation. MARCH proteins have recently been reported to target retroviral envelope glycoproteins (Env) and vesicular stomatitis virus G glycoprotein (VSV-G). However, the mechanism of antiviral activity remains poorly defined. Here we show that MARCH8 antagonizes the full-length forms of HIV-1 Env, VSV-G, Ebola virus glycoprotein (EboV-GP), and the spike (S) protein of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) thereby impairing the infectivity of virions pseudotyped with these viral glycoproteins. This MARCH8-mediated targeting of viral glycoproteins requires the E3 ubiquitin ligase activity of the RING-CH domain. We observe that MARCH8 protein antagonism of VSV-G is CT dependent. In contrast, MARCH8-mediated targeting of HIV-1 Env, EboV-GP and SARS-CoV-2 S protein by MARCH8 does not require the CT, suggesting a novel mechanism of MARCH-mediated antagonism of these viral glycoproteins. Confocal microscopy data demonstrate that MARCH8 traps the viral glycoproteins in an intracellular compartment. We observe that the endogenous expression of MARCH8 in several relevant human cell types is rapidly inducible by type I interferon. These results help to inform the mechanism by which MARCH proteins exert their antiviral activity and provide insights into the role of cellular inhibitory factors in antagonizing the biogenesis, trafficking, and virion incorporation of viral glycoproteins.

5-FU inhibits migration and invasion of CRC cells through PI3K/AKT pathway regulated by MARCH1

Colorectal cancer is a major health problem with a significant impact on the patients' quality of life. 5‐Fluorouracil is the most common chemotherapy drug used for this type of cancer. While its molecular mechanism is the inhibition of DNA synthesis via the inhibition of thymine nucleotide synthetase, its complete anticancer mechanism is not clear. Membrane‐associated RING‐CH‐1 (MARCH1) is an E3 ubiquitin ligase that plays an important role in antigen presentation. However, MARCH1 has not been studied in the context of colorectal cancer. In this study, we demonstrated that MARCH1 is highly expressed in colorectal cancer tissues and cell lines. Furthermore, migration and invasion of colorectal tumor cells were inhibited via transfection with small interfering RNAs to suppress the expression of MARCH1. The western blot analysis showed that MARCH1 regulates epithelial–mesenchymal transition and the PI3K/AKT pathway. Moreover, 5‐fluorouracil inhibited the proliferation, migration, and invasion of tumor cells, via the targeting of MARCH1 and the consequent downregulation of the PI3K/AKT pathway, impacting the progression of epithelial–mesenchymal transition. In conclusion, our study shows that MARCH1 may play a role as an oncogene in colorectal cancer and may represent a new target molecule of 5‐fluorouracil.

The E3 ubiquitin ligase MARCHF6 as a metabolic integrator in cholesterol synthesis and beyond

MARCHF6 is a large multi-pass E3 ubiquitin ligase embedded in the membranes of the endoplasmic reticulum. It participates in endoplasmic reticulum associated degradation, including autoubiquitination, and many of its identified substrates are involved in sterol and lipid metabolism. Post-translationally, MARCHF6 expression is attuned to cholesterol status, with high cholesterol preventing its degradation and hence boosting MARCHF6 levels. By modulating MARCHF6 activity, cholesterol may regulate other aspects of cell metabolism beyond the known repertoire. Whilst we have learnt much about MARCHF6 in the past decade, there are still many more mysteries to be unravelled to fully understand its regulation, substrates, and role in human health and disease.

MARCH8 Inhibits Ebola Virus Glycoprotein, Human Immunodeficiency Virus Type 1 Envelope Glycoprotein, and Avian Influenza Virus H5N1 Hemagglutinin Maturation

Enveloped viruses express three classes of fusion proteins that are required for their entry into host cells via mediating virus and cell membrane fusion. Class I fusion proteins are produced from influenza viruses, retroviruses, Ebola viruses, and coronaviruses. They are first synthesized as a type I transmembrane polypeptide precursor that is subsequently glycosylated and oligomerized. Most of these precursors are cleaved en route to the plasma membrane by a cellular protease furin in the late secretory pathway, generating the trimeric N-terminal receptor-binding and C-terminal fusion subunits. Here, we show that a cellular protein, MARCH8, specifically inhibits the furin-mediated cleavage of EBOV GP, HIV-1 Env, and H5N1 HA. Further analyses uncovered that MARCH8 blocked the EBOV GP glycosylation in the Golgi and inhibited its transport from the Golgi to the plasma membrane. Thus, MARCH8 has a very broad antiviral activity by specifically inactivating different viral fusion proteins.

Membrane-associated RING-CH-type 8 (MARCH8) strongly blocks human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) incorporation into virions by downregulating its cell surface expression, but the mechanism is still unclear. We now report that MARCH8 also blocks the Ebola virus (EBOV) glycoprotein (GP) incorporation via surface downregulation. To understand how these viral fusion proteins are downregulated, we investigated the effects of MARCH8 on EBOV GP maturation and externalization via the conventional secretion pathway. MARCH8 interacted with EBOV GP and furin when detected by immunoprecipitation and retained the GP/furin complex in the Golgi when their location was tracked by a bimolecular fluorescence complementation (BiFC) assay. MARCH8 did not reduce the GP expression or affect the GP modification by high-mannose N-glycans in the endoplasmic reticulum (ER), but it inhibited the formation of complex N-glycans on the GP in the Golgi. Additionally, the GP O-glycosylation and furin-mediated proteolytic cleavage were also inhibited. Moreover, we identified a novel furin cleavage site on EBOV GP and found that only those fully glycosylated GPs were processed by furin and incorporated into virions. Furthermore, the GP shedding and secretion were all blocked by MARCH8. MARCH8 also blocked the furin-mediated cleavage of HIV-1 Env (gp160) and the highly pathogenic avian influenza virus H5N1 hemagglutinin (HA). We conclude that MARCH8 has a very broad antiviral activity by prohibiting different viral fusion proteins from glycosylation and proteolytic cleavage in the Golgi, which inhibits their transport from the Golgi to the plasma membrane and incorporation into virions.

Orchestration of signaling by structural disorder in class 1 cytokine receptors

BACKGROUND

Class 1 cytokine receptors (C1CRs) are single-pass transmembrane proteins responsible for transmitting signals between the outside and the inside of cells. Remarkably, they orchestrate key biological processes such as proliferation, differentiation, immunity and growth through long disordered intracellular domains (ICDs), but without having intrinsic kinase activity. Despite these key roles, their characteristics remain rudimentarily understood.

METHODS

The current paper asks the question of why disorder has evolved to govern signaling of C1CRs by reviewing the literature in combination with new sequence and biophysical analyses of chain properties across the family.

RESULTS

We uncover that the C1CR-ICDs are fully disordered and brimming with SLiMs. Many of these short linear motifs (SLiMs) are overlapping, jointly signifying a complex regulation of interactions, including network rewiring by isoforms. The C1CR-ICDs have unique properties that distinguish them from most IDPs and we forward the perception that the C1CR-ICDs are far from simple strings with constitutively bound kinases. Rather, they carry both organizational and operational features left uncovered within their disorder, including mechanisms and complexities of regulatory functions.

CONCLUSIONS

Critically, the understanding of the fascinating ability of these long, completely disordered chains to orchestrate complex cellular signaling pathways is still in its infancy, and we urge a perceptional shift away from the current simplistic view towards uncovering their full functionalities and potential. Video abstract.

The CD28 Transmembrane Domain Contains an Essential Dimerization Motif

CD28 plays a critical role in regulating immune responses both by enhancing effector T cell activation and differentiation and controlling the development and function of regulatory T cells. CD28 is expressed at the cell surface as a disulfide linked homodimer that is thought to bind ligand monovalently. How ligand binding triggers CD28 to induce intracellular signaling as well as the proximal signaling pathways that are induced are not well-understood. In addition, recent data suggest inside-out signaling initiated by the T cell antigen receptor can enhance CD28 ligand binding, possibly by inducing a rearrangement of the CD28 dimer interface to allow for bivalent binding. To understand how possible conformational changes during ligand-induced receptor triggering and inside-out signaling are mediated, we examined the CD28 transmembrane domain. We identified an evolutionarily conserved YxxxxT motif that is shared with CTLA-4 and resembles the transmembrane dimerization motif within CD3ζ. We show that the CD28 transmembrane domain can drive protein dimerization in a bacterial expression system at levels equivalent to the well-known glycophorin A transmembrane dimerization motif. In addition, ectopic expression of the CD28 transmembrane domain into monomeric human CD25 can drive dimerization in murine T cells as detected by an increase in FRET by flow cytometry. Mutation of the polar YxxxxT motif to hydrophobic leucine residues (Y145L/T150L) attenuated CD28 transmembrane mediated dimerization in both the bacterial and mammalian assays. Introduction of the Y145L/T150L mutation of the CD28 transmembrane dimerization motif into the endogenous CD28 locus by CRISPR resulted in a dramatic loss in CD28 cell surface expression. These data suggest that under physiological conditions the YxxxxT dimerization motif within the CD28 transmembrane domain plays a critical role in the assembly and/or expression of stable CD28 dimers at the cell surface.

Regulation of Cancer Immune Checkpoint: Mono- and Poly-Ubiquitination: Tags for Fate

The antagonism, stalemate and compromise between the immune system and tumor cells is closely associated with tumor development and progression. In recent years, tumor immunotherapy has made continuous breakthroughs. It has become an important approach for cancer treatment, improving the survival and prognosis of more and more tumor patients. Further investigating the mechanism of tumor immune regulation, and exploring tumor immunotherapy targets with high specificity and wide applicability will provide researchers and clinicians with favorable weapons towards cancer. Ubiquitination affects protein fate through influencing the activity, stability and location of target protein. The regulation of substrate protein fate by ubiquitination is involved in cell cycle, apoptosis, transcriptional regulation, DNA repair, immune response, protein degradation and quality control. E3 ubiquitin ligase selectively recruits specific protein substrates through specific protein-protein interactions to determine the specificity of the overall ubiquitin modification reaction. Immune-checkpoint inhibitory pathway is an important mechanism for tumor cells to evade immune killing, which can inhibit T cell activity. Blocking the immune checkpoints and activating T cells through targeting the negative regulatory factors of T cell activation and removing the "brake" of T lymphocytes can enhance T cells immune response against tumors. Therefore, blocking the immune checkpoint is one of the methods to enhance the activity of T cells, and it is also a hot target for the development of anti-tumor drugs in recent years, whose inhibitors have shown good effect in specific tumor treatment. Ubiquitination, as one of the most important posttranslational modification of proteins, also modulates the expression, intracellular trafficking, subcellular and membranous location of immune checkpoints, regulating the immune surveillance of T cells to tumors.

Mitochondria ubiquitin ligase, MARCH5 resolves hepatitis B virus X protein aggregates in the liver pathogenesis

Infection of hepatitis B virus (HBV) increase the incidence of chronic liver disease and hepatocellular carcinoma (HCC). The hepatitis B viral x (HBx) protein encoded by the HBV genome contributes to the pathogenesis of HCC and thus, negative regulation of HBx is beneficial for the alleviation of the disease pathogenesis. MARCH5 is a mitochondrial E3 ubiquitin ligase and here, we show that high MARCH5 expression levels are correlated with improved survival in HCC patients. MARCH5 interacts with HBx protein mainly accumulated in mitochondria and targets it for degradation. The N-terminal RING domain of MARCH5 was required for the interaction with HBx, and MARCH5^H43W lacking E3 ligase activity failed to reduce HBx protein levels. High expression of HBx results in the formation of protein aggregates in semi-denaturing detergent agarose gels and MARCH5 mediates the elimination of protein aggregates through the proteasome pathway. HBx-induced ROS production, mitophagy, and cyclooxygenase-2 gene expression were suppressed in the presence of high MARCH5 expression. These results suggest MARCH5 as a target for alleviating HBV-mediated liver disease.

The Identification of Potential Therapeutic Targets for Cutaneous Squamous Cell Carcinoma

We performed a small interfering RNA screen to identify targets for cutaneous squamous cell carcinoma (cSCC) therapy in the ubiquitin/ubiquitin-like system. We provide evidence for selective anti-cSCC activity of knockdown of the E3 ubiquitin ligase MARCH4, the ATPase p97/VCP, the deubiquitinating enzyme USP8, the cullin-RING ligase (CRL) 4 substrate receptor CDT2/DTL, and components of the anaphase-promoting complex/cyclosome (APC/C). Specifically attenuating CRL4^CDT2 by CDT2 knockdown can be more potent in killing cSCC cells than targeting CRLs or CRL4s in general by RBX1 or DDB1 depletion. Suppression of the APC/C or forced APC/C activation by targeting its repressor EMI1 are both potential therapeutic approaches. We observed that cSCC cells can be selectively killed by small-molecule inhibitors of USP8 (DUBs-IN-3/compound 22c) and the NEDD8 E1 activating enzyme/CRLs (MLN4924/pevonedistat). A substantial proportion of cSCC cell lines are very highly MLN4924-sensitive. Pathways that respond to defects in proteostasis are involved in the anti-cSCC activity of p97 suppression. Targeting USP8 can reduce the expression of growth factor receptors that participate in cSCC development. EMI1 and CDT2 depletion can selectively cause DNA re-replication and DNA damage in cSCC cells.

Membrane-associated RING-CH (MARCH) proteins down-regulate cell surface expression of the interleukin-6 receptor alpha chain (IL6Rα)

Interleukin 6 (IL6) is a cytokine that regulates a number of important immune and inflammatory pathways. We used the ability of IL6 to inhibit the clonal proliferation of the mouse M1 myeloid leukemia cell line in agar to positively screen a cDNA expression library for proteins that inhibited IL6 activity. We found three clones completely resistant to IL6 that contained the cDNA for the Membrane-Associated RING-CH E3 ubiquitin ligase MARCH2. MARCH2 is a member of a family of membrane-bound E3 ubiquitin ligases that target cell surface receptors for degradation. MARCH2 overexpressing M1 clones retained responsiveness to the related cytokines leukemia inhibitory factor and oncostatin M and we showed that its inhibitory effect was a result of selective down-regulation of the IL6 receptor alpha chain and not the shared receptor subunit, gp130 or other signalling molecules. This activity of MARCH2 was also shared with related proteins MARCH4, MARCH9 and an isoform of MARCH3. The transmembrane domains and C-terminal domains, as well as a functional RING domain, of MARCH proteins were all required for substrate recognition and down-regulation. Genetic deletion of individual MARCH proteins in mice had no or little effect on IL6Rα levels but combined deletions of MARCH2,3 and 4 displayed elevated steady-state levels of IL6Rα in selected haemopoietic cell subsets including CD8+ and CD4+ T cells. These studies extend the potential immunosuppressive roles of MARCH proteins to include down-regulation of IL6 inflammatory responses.

The E3 Ubiquitin Ligase MARCH8 Regulates TNF-α-Induced Apoptosis in Hippocampal Neurons by Targeting Myosin Light Chain 2 for Degradation

Tumor necrosis factor-α (TNF-α) is an important inflammatory cytokine that plays a key role in neuronal damage. Elevated expression of TNF-α is associated with numerous neurodegenerative diseases including Alzheimer's Disease and Parkinson's Disease. However, the specific mechanism of the signaling events that trigger TNF-α-mediated neurotoxicity remain unknown. In this study, we report that intracerebroventricular injection of TNF-α in rat hippocampal neurons down-regulates MLC2 and up-regulates MARCH8, an essential light chain and regulatory myosin light chain of NM Myosin II, respectively. MARCH8 overexpression attenuates the degradation of MLC2 by promoting its ubiquitination and degradation. Inhibition of MARCH8 by siRNA blocks caspase-3 activation and apoptosis signaling, suggesting that TNF-α-induced apoptosis of neurons is partially dependent on the accumulation of MARCH8 and the ubiquitination of MLC2. Taken together, our data not only clarify the function of MARCH8 in TNF-α-induced neurotoxicity, but also demonstrates that TNF-α promotes the MARCH8-MLC2 mediated apoptosis of hippocampal neurons. Anat Rec, 302:2271-2278, 2019. © 2019 American Association for Anatomy.

The Membrane-Associated MARCH E3 Ligase Family: Emerging Roles in Immune Regulation

The membrane-associated RING-CH-type finger (MARCH) proteins of E3 ubiquitin ligases have emerged as critical regulators of immune responses. MARCH proteins target immune receptors, viral proteins as well as components in innate immune response for polyubiquitination and degradations via distinct routes. This review summarizes the current progress about MARCH proteins and their regulation on immune responses.

MARCH ligases in immunity

Membrane associated RING-CH (MARCH) ubiquitin ligases control the stability, trafficking and function of important immunoreceptors, including MHC molecules and costimulatory molecule CD86. Regulation of the critical antigen presenting molecule MHC II by MARCH1 and the control of MARCH1 expression by inflammatory stimuli is a key step in the function of antigen presenting cells. MHC II ubiquitination by MARCH8 and CD83 plays a critical role in T cell thymic selection. Recent studies reveal new immune functions of MARCH ligases in innate immunity, regulation of FcγR expression and Treg development. In addition, we review the importance of MARCH in immunomodulation at the host-pathogen interface. Both bacterial and viral pathogens manipulate MARCH function, while MARCH ligases act as an important host anti-viral defence mechanism. Here, we review the role of membrane-bound MARCH ligases in immune function and provide an update on new substrates and concepts. Understanding the increasingly complex roles of MARCH E3 ligases will be vital to develop therapeutic strategies for their regulation.

A serine in the first transmembrane domain of the human E3 ubiquitin ligase MARCH9 is critical for down-regulation of its protein substrates

The membrane-associated RING-CH (MARCH) family of membrane-bound E3 ubiquitin ligases regulates the levels of cell-surface membrane proteins, many of which are involved in immune responses. Although their role in ubiquitin-dependent endocytosis and degradation of cell-surface proteins is extensively documented, the features of MARCH proteins and their substrates that drive the molecular recognition events leading to ubiquitin transfer remain poorly defined. In this study, we sought to determine the features of human MARCH9 that are required for regulating the surface levels of its substrate proteins. Consistent with previous studies of other MARCH proteins, we found that susceptibility to MARCH9 activity is encoded in the transmembrane (TM) domains of its substrates. Accordingly, substitutions at specific residues and motifs within MARCH9's TM domains resulted in varying degrees of functional impairment. Most notably, a single serine-to-alanine substitution in the first of its two TM domains rendered MARCH9 completely unable to alter the surface levels of two different substrates: the major histocompatibility class I molecule HLA-A2 and the T-cell co-receptor CD4. Solution NMR analysis of a MARCH9 fragment encompassing the two TM domains and extracellular connecting loop revealed that the residues contributing most to MARCH9 activity are located in the α-helical portions of TM1 and TM2 that are closest to the extracellular face of the lipid bilayer. This observation defines a key region required for substrate regulation. In summary, our biochemical and structural findings demonstrate that specific sequences in the α-helical MARCH9 TM domains make crucial contributions to its ability to down-regulate its protein substrates.

Under control: The innate immunity of fish from the inhibitors' perspective

The innate immune response involves a concerted network of induced gene products, preformed immune effectors, biochemical signalling cascades and specialised cells. However, the multifaceted activation of these defensive measures can derail or overshoot and, if left unchecked, overwhelm the host. A plenty of regulatory devices therefore mediate the fragile equilibrium between pathogen defence and pathophysiological manifestations. Over the past decade in particular, an almost complete set of teleostean sequences orthologous to mammalian immunoregulatory factors has been identified in various fish species, which prove the remarkable conservation of innate immune-control concepts among vertebrates. This review will present the current knowledge on more than 50 teleostean regulatory factors (plus additional fish-specific paralogs) that are of paramount importance for controlling the clotting cascade, the complement system, pattern-recognition pathways and cytokine-signalling networks. A special focus lies on those immunoregulatory features that have emerged as potential biomarker genes in transcriptome-wide research studies. Moreover, we report on the latest progress in elucidating control elements that act directly with immune-gene-encoding nucleic acids, such as transcription factors, hormone receptors and micro- and long noncoding RNAs. Investigations into the function of teleostean inhibitory factors are still mainly based on gene-expression profiling or overexpression studies. However, in support of structural and in-vitro analyses, evidence from in-vivo trials is also available and revealed many biochemical details on piscine immune regulation. The presence of multiple gene copies in fish adds a degree of complexity, as it is so far hardly understood if they might play distinct roles during inflammation. The present review addresses this and other open questions that should be tackled by fish immunologists in future.