TRIM Proteins and Their Roles in Antiviral Host Defenses

The translational values of TRIM family in pan-cancers: From functions and mechanisms to clinics

Cancer is the second leading cause of human death across the world. Tripartite motif (TRIM) family, with E3 ubiquitin ligase activities in majority of its members, is reported to be involved in multiple cellular processes and signaling pathways. TRIM proteins have critical effects in the regulation of biological behaviors of cancer cells. Here, we discussed the current understanding of the molecular mechanism of TRIM proteins regulation of cancer cells. We also comprehensively reviewed published studies on TRIM family members as oncogenes or tumor suppressors in the oncogenesis, development, and progression of a variety of types of human cancers. Finally, we highlighted that certain TRIM family members are potential molecular biomarkers for cancer diagnosis and prognosis, and potential therapeutic targets.

TRIM28 functions as a negative regulator of aggresome formation

Selective recognition and elimination of misfolded polypeptides are crucial for protein homeostasis. When the ubiquitin-proteasome system is impaired, misfolded polypeptides tend to form small cytosolic aggregates and are transported to the aggresome and eventually eliminated by the autophagy pathway. Despite the importance of this process, the regulation of aggresome formation remains poorly understood. Here, we identify TRIM28/TIF1β/KAP1 (tripartite motif containing 28) as a negative regulator of aggresome formation. Direct interaction between TRIM28 and CTIF (cap binding complex dependent translation initiation factor) leads to inefficient aggresomal targeting of misfolded polypeptides. We also find that either treatment of cells with poly I:C or infection of the cells by influenza A viruses triggers the phosphorylation of TRIM28 at S473 in a way that depends on double-stranded RNA-activated protein kinase. The phosphorylation promotes association of TRIM28 with CTIF, inhibits aggresome formation, and consequently suppresses viral proliferation. Collectively, our data provide compelling evidence that TRIM28 is a negative regulator of aggresome formation.AbbreviationsBAG3: BCL2-associated athanogene 3; CTIF: CBC-dependent translation initiation factor; CED: CTIF-EEF1A1-DCTN1; DCTN1: dynactin subunit 1; EEF1A1: eukaryotic translation elongation factor 1 alpha 1; EIF2AK2: eukaryotic translation initiation factor 2 alpha kinase 2; HDAC6: histone deacetylase 6; IAV: influenza A virus; IP: immunoprecipitation; PLA: proximity ligation assay; polypeptidyl-puro: polypeptidyl-puromycin; qRT-PCR: quantitative reverse-transcription PCR; siRNA: small interfering RNA.

Knockdown of TRIM52 alleviates LPS-induced inflammatory injury in human periodontal ligament cells through the TLR4/NF-κB pathway

Tripartite motif-containing (TRIM) 52 (TRIM52) is a vital regulator of inflammation. However, the function and mechanisms of TRIM52 in lipopolysaccharide (LPS)-induced inflammatory injury of human periodontal ligament cells (HPDLCs) in periodontitis remain undefined. In the present research, gene expression was determined using a quantitative polymerase chain reaction and Western blot. The effect of TRIM52 on LPS-induced inflammatory injury was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, flow cytometry, and enyzme-linked immunosorbent assay (ELISA). We found that TRIM52 expression was up-regulated in LPS-treated HPDLCs. Knockdown of TRIM52 alleviated LPS-induced proliferative inhibition and apoptosis promotion in HPDLCs, as evidenced by a decrease in cleaved caspase-3 expression and caspase-3 activity. Silencing TRIM52 suppressed LPS-induced inflammatory response of HPDLCs, as indicated by the decrease in interleukin (IL)-6, IL-8, tumor necrosis factor-α (TNF-α) levels, and increase in IL-10 levels. TRIM52 knockdown inhibited LPS-induced activation of TLR4/nuclear factor-κ B (NF-κB) signaling pathway. Taken together, knockdown of TRIM52 mitigated LPS-induced inflammatory injury via the TLR4/NF-κB signaling pathway, providing an effective therapeutic target for periodontitis.

TRIM26 Induces Ferroptosis to Inhibit Hepatic Stellate Cell Activation and Mitigate Liver Fibrosis Through Mediating SLC7A11 Ubiquitination

Hepatic stellate cells (HSCs) are activated by inflammatory mediators to secrete extracellular matrix for collagen deposition, leading to liver fibrosis. Ferroptosis is iron- and lipid hydroperoxide-dependent programmed cell death, which has recently been targeted for inhibiting liver fibrogenic processes. Tripartite motif-containing protein 26 (TRIM26) is an E3 ubiquitin ligase that functions as a tumor suppressor in hepatocellular carcinoma, while little is known about its function in liver fibrosis. In the present study, the differential expression of TRIM26 in normal and fibrotic liver tissues was examined based on both online databases and specimens collected from patient cohort. The effects of TRIM26 on HSCs ferroptosis were examined in vitro through evaluating cell proliferation, lipid peroxidation, and expression of key ferroptosis-related factors. In vivo function of TRIM26 in liver fibrosis was examined based on CCl₄-induced mice model. We found that TRIM26 was downregulated in fibrotic liver tissues. The overexpression of TRIM26 inhibited HSCs proliferation, promoted lipid peroxidation, manipulated ferroptosis-related factor expressions, and counteracted the effect of iron inhibitor deferoxamine. Moreover, TRIM26 physically interacted with solute carrier family-7 member-11 (SLC7A11), a critical protein for lipid reactive oxygen species (ROS) scavenging, and mediated its ubiquitination. In addition, TRIM26 overexpression induced HSCs ferroptosis and mitigated CCl₄-induced liver fibrosis in mice. In conclusion, TRIM26 promotes HSCs ferroptosis to suppress liver fibrosis through mediating the ubiquitination of SLC7A11. The TRIM26-targeted SLC7A11 suppression can be a novel therapeutic strategy for liver fibrosis.

TRIM29 Reverses Oxaliplatin Resistance of P53 Mutant Colon Cancer Cell

Background

Oxaliplatin is the first-choice chemotherapy method for patients with advanced colon cancer. However, its resistance leads to treatment failure for many patients. In our experiments, we aim to elucidate the associations among TRIM29 protein, mutant P53, and the resistance of colon cancer cells to oxaliplatin.

Methods

HCT116 and HT-29 cells were cultured and transfected with plasmids pIRES2-ZsGreen1-TRIM29-flag. Western blot and real-time qRT-PCR were utilized to examine the protein and mRNA expressions of TRIM29 and other related markers, respectively. MTT assay was utilized to determine the cell growth rate and generate the inhibition curve. Continuous culture in low-concentration oxaliplatin was conducted to construct oxaliplatin-resistant cell lines. The coimmunoprecipitation method and immunofluorescence detection were used to examine the interaction between TRIM29 and mutant P53 protein in HT29 cells.

Results

We successfully transfected pIRES2-ZsGreen1-TRIM29-flag into HCT116 and HT29 cells, which were utilized in the whole experiments. TRIM29 significantly increased the sensitivity of P53 mutant colon cancer cell HT29 to oxaliplatin. The oxaliplatin-resistant model of P53 mutant colon cancer cell HT29 was successfully constructed. TRIM29 physically bound with mutant P53 and retained it in the cytoplasm from the nucleus, which inhibited its transcription function of downstream genes such as MDR1. In addition, TRIM29 successfully reversed the resistance of HT29-OX resistant cell model to oxaliplatin.

Conclusion

In mutant P53 colon cancer cell HT29, TRIM29 greatly increased the sensitivity of HT29 to oxaliplatin and reverse oxaliplatin resistance. The underlying mechanism is TRIM29 may increase the sensitivity of HT29 to oxaliplatin by blocking the transcriptional function of mutant P53, which inhibits the transcription function of its downstream gene such as MDR1.

The Roles of TRIMs in Antiviral Innate Immune Signaling

The Tripartite motif (TRIM) protein family, which contains over 80 members in human sapiens, is the largest subfamily of the RING-type E3 ubiquitin ligase family. It is implicated in regulating various cellular functions, including cell cycle process, autophagy, and immune response. The dysfunction of TRIMs may lead to numerous diseases, such as systemic lupus erythematosus (SLE). Lots of studies in recent years have demonstrated that many TRIM proteins exert antiviral roles. TRIM proteins could affect viral replication by regulating the signaling pathways of antiviral innate immune responses. Besides, TRIM proteins can directly target viral components, which can lead to the degradation or functional inhibition of viral protein through degradative or non-degradative mechanisms and consequently interrupt the viral lifecycle. However, new evidence suggests that some viruses may manipulate TRIM proteins for their replication. Here, we summarize the latest discoveries on the interactions between TRIM protein and virus, especially TRIM proteins’ role in the signaling pathway of antiviral innate immune response and the direct “game” between them.

TRIM41 is required to innate antiviral response by polyubiquitinating BCL10 and recruiting NEMO

Sensing of pathogenic nucleic acids by pattern recognition receptors (PRR) not only initiates anti-microbe defense but causes inflammatory and autoimmune diseases. E3 ubiquitin ligase(s) critical in innate response need to be further identified. Here we report that the tripartite motif-containing E3 ubiquitin ligase TRIM41 is required to innate antiviral response through facilitating pathogenic nucleic acids-triggered signaling pathway. TRIM41 deficiency impairs the production of inflammatory cytokines and type I interferons in macrophages after transfection with nucleic acid-mimics and infection with both DNA and RNA viruses. In vivo, TRIM41 deficiency leads to impaired innate response against viruses. Mechanistically, TRIM41 directly interacts with BCL10 (B cell lymphoma 10), a core component of CARD proteins−BCL10 − MALT1 (CBM) complex, and modifies the Lys63-linked polyubiquitylation of BCL10, which, in turn, hubs NEMO for activation of NF-κB and TANK-binding kinase 1 (TBK1) − interferon regulatory factor 3 (IRF3) pathways. Our study suggests that TRIM41 is the potential universal E3 ubiquitin ligase responsible for Lys63 linkage of BCL10 during innate antiviral response, adding new insight into the molecular mechanism for the control of innate antiviral response.

RNA polymerase III and antiviral innate immune response

The innate immune system has numerous signal transduction pathways that lead to the production of type I interferons in response to exposure of cells to external stimuli. One of these pathways comprises RNA polymerase (Pol) III that senses common DNA viruses, such as cytomegalovirus, vaccinia, herpes simplex virus-1 and varicella zoster virus. This polymerase detects and transcribes viral genomic regions to generate AU-rich transcripts that bring to the induction of type I interferons. Remarkably, Pol III is also stimulated by foreign non-viral DNAs and expression of one of its subunits is induced by an RNA virus, the Sindbis virus. Moreover, a protein subunit of RNase P, which is known to associate with Pol III in initiation complexes, is induced by viral infection. Accordingly, alliance of the two tRNA enzymes in innate immunity merits a consideration.

TRIM24 facilitates antiviral immunity through mediating K63-linked TRAF3 ubiquitination

Ubiquitination is an essential mechanism in the control of antiviral immunity upon virus infection. Here, we identify a series of ubiquitination-modulating enzymes that are modulated by vesicular stomatitis virus (VSV). Notably, TRIM24 is down-regulated through direct transcriptional suppression induced by VSV-activated IRF3. Reducing or ablating TRIM24 compromises type I IFN (IFN-I) induction upon RNA virus infection and thus renders mice more sensitive to VSV infection. Mechanistically, VSV infection induces abundant TRIM24 translocation to mitochondria, where TRIM24 binds with TRAF3 and directly mediates K63-linked TRAF3 ubiquitination at K429/K436. This modification of TRAF3 enables its association with MAVS and TBK1, which consequently activates downstream antiviral signaling. Together, these findings establish TRIM24 as a critical positive regulator in controlling the activation of antiviral signaling and describe a previously unknown mechanism of TRIM24 function.

HIV-1 Infection Transcriptomics: Meta-Analysis of CD4+ T Cells Gene Expression Profiles

HIV-1 infection elicits a complex dynamic of the expression various host genes. High throughput sequencing added an expressive amount of information regarding HIV-1 infections and pathogenesis. RNA sequencing (RNA-Seq) is currently the tool of choice to investigate gene expression in a several range of experimental setting. This study aims at performing a meta-analysis of RNA-Seq expression profiles in samples of HIV-1 infected CD4+ T cells compared to uninfected cells to assess consistently differentially expressed genes in the context of HIV-1 infection. We selected two studies (22 samples: 15 experimentally infected and 7 mock-infected). We found 208 differentially expressed genes in infected cells when compared to uninfected/mock-infected cells. This result had moderate overlap when compared to previous studies of HIV-1 infection transcriptomics, but we identified 64 genes already known to interact with HIV-1 according to the HIV-1 Human Interaction Database. A gene ontology (GO) analysis revealed enrichment of several pathways involved in immune response, cell adhesion, cell migration, inflammation, apoptosis, Wnt, Notch and ERK/MAPK signaling.

TSLP-Driven Chromatin Remodeling and Trained Systemic Immunity after Neonatal Respiratory Viral Infection

Our studies have previously shown a role for persistent TSLP production in the lungs of mice after early-life respiratory syncytial virus (RSV) infection that leads to an altered immune phenotype, including accumulation of "inflammatory" dendritic cells (DC). This study investigates the role of TSLP driving systemic trained immunity in DC in early-life RSV-infected mice. Bone marrow-derived DCs (BMDC) from early-life RSV-infected mice at 4 wk postinfection showed enhanced expression of costimulatory molecules and cytokines, including Tslp, that regulate immune cell function. The adoptive transfer of BMDC grown from early-life RSV-infected mice was sufficient to exacerbate allergic disease development. The addition of recombinant TSLP during differentiation of BMDC from naive mice induced a similar altered phenotype as BMDC grown from early-life RSV-infected mice, suggesting a role for TSLP in the phenotypic changes. To assess the role of TSLP in these changes, global transcriptomic characterization of TSLPR^-/- BMDC infected with RSV was performed and showed a higher upregulation of type 1 IFN genes and concomitant downregulation of inflammatory genes. Assay for transposase-accessible chromatin using sequencing analysis demonstrated that TSLPR^-/- BMDC had a parallel gain in physical chromatin accessibility near type 1 genes and loss in accessibility near genes related to RSV pathology, with IFN regulatory factor 4 (IRF4) and STAT3 predicted as top transcription factors binding within differentially accessible regions in wild-type. Importantly, these studies show that in the absence of TSLP signaling, BMDC are able to mount an appropriate type 1 IFN-associated antiviral response to RSV. In summary, RSV-induced TSLP alters chromatin structure in DC to drive trained innate immunity and activates pathogenic gene programs in mice.

The role of TRIM family proteins in autophagy, pyroptosis, and diabetes mellitus

The ubiquitin-proteasome system, which is one of the systems for cell protein homeostasis and degradation, happens through the ordered and coordinated action of three types of enzymes, E1 ubiquitin-activating enzyme, E2 ubiquitin-carrier enzyme, E3 ubiquitin-protein ligase. Tripartite motif-containing (TRIM) family proteins are the richest subfamily of really interesting new gene E3 ubiquitin ligases, which play a critical role not only in many biological processes, including proliferation, apoptosis, pyroptosis, innate immunity, and autophagy, but also many diseases like cancer, diabetes mellitus, and neurodegenerative disease. Increasing evidence suggests that TRIM family proteins play a vital role in modulating autophagy, pyroptosis, and diabetes mellitus. The aim of this review is to discuss the role of TRIM proteins in the regulation of autophagy, pyroptosis, diabetes mellitus, and diabetic complications.

MG53, A Tissue Repair Protein with Broad Applications in Regenerative Medicine

Under natural conditions, injured cells can be repaired rapidly through inherent biological processes. However, in the case of diabetes, cardiovascular disease, muscular dystrophy, and other degenerative conditions, the natural repair process is impaired. Repair of injury to the cell membrane is an important aspect of physiology. Inadequate membrane repair function is implicated in the pathophysiology of many human disorders. Recent studies show that Mitsugumin 53 (MG53), a TRIM family protein, plays a key role in repairing cell membrane damage and facilitating tissue regeneration. Clarifying the role of MG53 and its molecular mechanism are important for the application of MG53 in regenerative medicine. In this review, we analyze current research dissecting MG53′s function in cell membrane repair and tissue regeneration, and highlight the development of recombinant human MG53 protein as a potential therapeutic agent to repair multiple-organ injuries.

Knockdown of TRIM5α or TRIM11 increases lentiviral vector transduction efficiency of human Muller cells

The goal of this study was to determine the expression and distribution of the host restriction factors (RFs) TRIM5α and TRIM11 in non-human primate (NHP) neural retina tissue and the human Muller cell line MIO-M1. In addition, experiments were performed to determine the effect of TRIM5α and TRIM11 knockdown on FIVGFP transduction of MIO-M1 cells with the goal of devising strategies to increase the efficiency of lentiviral (LV) gene delivery. Immunofluorescence (IF) studies indicated that TRIM5α and TRIM11 were localized predominantly in nuclei within the outer nuclear layer (ONL) and inner nuclear layer (INL) of NHP retina tissue. Double label IF indicated that TRIM5α and TRIM11 were localized to some of the retinal Muller cell nuclei. MIO-M1 cells expressed TRIM5α predominantly in the nucleus and TRIM11 primarily in the cytosol. FIVGFP transduction efficiency was significantly increased, at 4 and 7 days post transduction, in TRIM5α and TRIM11 knockdown clones (KD) compared to WT MIO-M1 cells. In addition, pretreatment with the proteasome inhibitor MG132 increased the transduction efficiency of FIVGFP in WT MIO-M1 cells. The nuclear translocation of NF-κB (p65), at 72 h post FIVGFP transduction, was enhanced in TRIM5α and TRIM11 KD clones. The expression of TRIM5α and TRIM11 in macaque neural retina tissue and MIO-M1 cells indicate the presence of these RFs in NHP retina and human Muller cells. Our data indicate that even partial knockdown of TRIM5α or TRIM11, or a short proteasome inhibitor pretreatment, can increase the transduction efficiency of a LV vector.

TRIM26 Facilitates HSV-2 Infection by Downregulating Antiviral Responses through the IRF3 Pathway

Herpes simplex virus type 2 (HSV-2) is the primary cause of genital herpes which results in significant morbidity and mortality, especially in women, worldwide. HSV-2 is transmitted primarily through infection of epithelial cells at skin and mucosal surfaces. Our earlier work to examine interactions between HSV-2 and vaginal epithelial cells demonstrated that infection of the human vaginal epithelial cell line (VK2) with HSV-2 resulted in increased expression of TRIM26, a negative regulator of the Type I interferon pathway. Given that upregulation of TRIM26 could negatively affect anti-viral pathways, we decided to further study the role of TRIM26 in HSV-2 infection and replication. To do this, we designed and generated two cell lines derived from VK2s with TRIM26 overexpressed (OE) and knocked out (KO). Both, along with wildtype (WT) VK2, were infected with HSV-2 and viral titres were measured in supernatants 24 h later. Our results showed significantly enhanced virus production by TRIM26 OE cells, but very little replication in TRIM26 KO cells. We next examined interferon-β production and expression of two distinct interferon stimulated genes (ISGs), MX1 and ISG15, in all three cell lines, prior to and following HSV-2 infection. The absence of TRIM26 (KO) significantly upregulated interferon-β production at baseline and even further after HSV-2 infection. TRIM26 KO cells also showed significant increase in the expression of MX1 and ISG15 before and after HSV-2 infection. Immunofluorescent staining indicated that overexpression of TRIM26 substantially decreased the nuclear localization of IRF3, the primary mediator of ISG activation, before and after HSV-2 infection. Taken together, our data indicate that HSV-2 utilizes host factor TRIM26 to evade anti-viral response and thereby increase its replication in vaginal epithelial cells.

Crystal structure and mutational analysis of the human TRIM7 B30.2 domain provide insights into the molecular basis of its binding to glycogenin-1

Tripartite motif (TRIM)7 is an E3 ubiquitin ligase that was first identified through its interaction with glycogenin-1 (GN1), the autoglucosyltransferase that initiates glycogen biosynthesis. A growing body of evidence indicates that TRIM7 plays an important role in cancer development, viral pathogenesis, and atherosclerosis and, thus, represents a potential therapeutic target. TRIM family proteins share a multidomain architecture with a conserved N-terminal TRIM and a variable C-terminal domain. Human TRIM7 contains the canonical TRIM motif and a B30.2 domain at the C terminus. To contribute to the understanding of the mechanism of action of TRIM7, we solved the X-ray crystal structure of its B30.2 domain (TRIM7B30.2) in two crystal forms at resolutions of 1.6 Å and 1.8 Å. TRIM7B30.2 exhibits the typical B30.2 domain fold, consisting of two antiparallel β-sheets of seven and six strands, arranged as a distorted β-sandwich. Furthermore, two long loops partially cover the concave face of the β-sandwich defined by the β-sheet of six strands, thus forming a positively charged cavity. We used sequence conservation and mutational analyses to provide evidence of a putative binding interface for GN1. These studies showed that Leu423, Ser499, and Cys501 of TRIM7B30.2 and the C-terminal 33 amino acids of GN1 are critical for this binding interaction. Molecular dynamics simulations also revealed that hydrogen bond and hydrophobic interactions play a major role in the stability of a modeled TRIM7B30.2-GN1 C-terminal peptide complex. These data provide useful information that could be used to target this interaction for the development of potential therapeutic agents.

STING, a promising target for small molecular immune modulator: A review

Stimulator of interferon genes (STING) plays a crucial role in human innate immune system, which is gradually concerned following the emerging immunotherapy. Activated STING induces the production of type I interferons (IFNs) and proinflammatory cytokines through STING-TBK1-IRF3/NF-κB pathway, which could be applied into the treatment of infection, inflammation, and tumorigenesis. Here, we provided a detailed summary of STING from its structure, function and regulation. Especially, we illustrated the canonical or noncanonical cyclic dinucleotides (CDNs) and synthetic small molecules for STING activation or inhibition and their efficacy in related diseases. Importantly, we particularly emphasized the discovery, development and modification of STING agonist or antagonist, attempting to enlighten reader's mind for enriching small molecular modulator of STING. In addition, we summarized biological evaluation methods for the assessment of small molecules activity.

Retroviral Restriction Factors and Their Viral Targets: Restriction Strategies and Evolutionary Adaptations

The evolutionary conflict between retroviruses and their vertebrate hosts over millions of years has led to the emergence of cellular innate immune proteins termed restriction factors as well as their viral antagonists. Evidence accumulated in the last two decades has substantially increased our understanding of the elaborate mechanisms utilized by these restriction factors to inhibit retroviral replication, mechanisms that either directly block viral proteins or interfere with the cellular pathways hijacked by the viruses. Analyses of these complex interactions describe patterns of accelerated evolution for these restriction factors as well as the acquisition and evolution of their virus-encoded antagonists. Evidence is also mounting that many restriction factors identified for their inhibition of specific retroviruses have broader antiviral activity against additional retroviruses as well as against other viruses, and that exposure to these multiple virus challenges has shaped their adaptive evolution. In this review, we provide an overview of the restriction factors that interfere with different steps of the retroviral life cycle, describing their mechanisms of action, adaptive evolution, viral targets and the viral antagonists that evolved to counter these factors.

Nasopharyngeal carcinoma MHC region deep sequencing identifies HLA and novel non-HLA TRIM31 and TRIM39 loci

Despite pronounced associations of major histocompatibility complex (MHC) regions with nasopharyngeal carcinoma (NPC), causal variants underlying NPC pathogenesis remain elusive. Our large-scale comprehensive MHC region deep sequencing study of 5689 Hong Kong Chinese identifies eight independent NPC-associated signals and provides mechanistic insight for disrupted transcription factor binding, altering target gene transcription. Two novel protective variants, rs2517664 (T_rs2517664 = 4.6%, P = 6.38 × 10⁻²¹) and rs117495548 (G_rs117495548 = 3.0%, P = 4.53 × 10⁻¹³), map near TRIM31 and TRIM39/TRIM39-RPP21; multiple independent protective signals map near HLA-B including a previously unreported variant, rs2523589 (P = 1.77 × 10⁻³⁶). The rare HLA-B*07:05 allele (OR < 0.015, P = 5.83 × 10⁻²¹) is absent in NPC, but present in controls. The most prevalent haplotype lacks seven independent protective alleles (OR = 1.56) and the one with additional Asian-specific susceptibility rs9391681 allele (OR = 2.66) significantly increased NPC risk. Importantly, this study provides new evidence implicating two non-human leukocyte antigen (HLA) genes, E3 ubiquitin ligases, TRIM31 and TRIM39, impacting innate immune responses, with NPC risk reduction, independent of classical HLA class I/II alleles.

Here the authors report a major histocompatibility complex (MHC) association analysis for nasopharyngeal carcinoma in Chinese individuals from Hong Kong, finding 8 independent associated loci associated with lower risk for developing nasopharyngeal carcinoma. Two non-human leukocyte antigen (HLA) genes are E3 ubiquitin ligases, TRIM31 and TRIM39, having a role in the innate immune response and implicating the importance of host Epstein-Barr virus interactions in this cancer.

All About the RNA: Interferon-Stimulated Genes That Interfere With Viral RNA Processes

Interferon (IFN) signaling induces the expression of a wide array of genes, collectively referred to as IFN-stimulated genes (ISGs) that generally function to inhibit viral replication. RNA viruses are frequently targeted by ISGs through recognition of viral replicative intermediates and molecular features associated with viral genomes, or the lack of molecular features associated with host mRNAs. The ISGs reviewed here primarily inhibit viral replication in an RNA-centric manner, working to sense, degrade, or repress expression of viral RNA. This review focuses on dissecting how these ISGs exhibit multiple antiviral mechanisms, often through use of varied co-factors, highlighting the complexity of the type I IFN response. Specifically, these ISGs can mediate antiviral effects through viral RNA degradation, viral translation inhibition, or both. While the OAS/RNase L pathway globally degrades RNA and arrests translation, ISG20 and ZAP employ targeted RNA degradation and translation inhibition to block viral replication. Meanwhile, SHFL targets translation by inhibiting -1 ribosomal frameshifting, which is required by many RNA viruses. Finally, a number of E3 ligases inhibit viral transcription, an attractive antiviral target during the lifecycle of negative-sense RNA viruses which must transcribe their genome prior to translation. Through this review, we aim to provide an updated perspective on how these ISGs work together to form a complex network of antiviral arsenals targeting viral RNA processes.

Tripartite motif‑containing 14 regulates cell proliferation and apoptosis in cervical cancer via the Akt signaling pathway

Tripartite motif‑containing (TRIM) 14 is a protein of the TRIM family. Studies have indicated that TRIM14 may be used as an oncogene in tumor cells, such as osteosarcoma, non‑small cell lung cancer and breast cancer through different pathways. However, the functions of TRIM14 in cervical cancer cells remain unclear. Therefore, this study aimed to investigate the functions of TRIM14 in cervical cancer cells and its underlying mechanism. Caski cells stably expressing TRIM14 and SiHa, and HeLa cells stably expressing TRIM14 short hairpin RNA were constructed by lentivirus‑mediated overexpression or knockdown systems. The effects of TRIM14 on proliferation and apoptosis of cervical cancer cells were detected by Cell Counting Kit‑8 (CCK‑8) assay and flow cytometry, respectively. In addition, reverse transcription‑quantitative (RT‑q) PCR and western blotting were used to investigate the expression levels of TRIM14 and of signaling pathway marker protein including P21, caspase‑3, cleaved caspase‑3, Akt and phosphorylated Akt. The results of RT‑qPCR and western blotting revealed that TRIM14 was highly expressed in human cervical cancer tissues and cell lines compared with adjacent normal tissues and normal cervical epithelial cells. TRIM14 also regulated cell proliferation and apoptosis of human SiHa, HeLa and Caski cervical cancer cell lines through the Akt signaling pathway. Additionally, TRIM14 protein levels were related to the clinical and pathological features of cervical cancer. CCK‑8 assay and flow cytometry demonstrated that TRIM14 expression could promote cervical cancer cell proliferation and autophagy suppression. Taken together, TRIM14‑induced cell proliferation and apoptosis inhibition may by evoked by the activation of the Akt pathway. This study demonstrated the role of TRIM14 in cervical cancer, and reveals its mechanism of action as a potential therapeutic target for cervical cancer.

Expression profile of Tripartite motif family (TRIM) in human fibroblasts (NHDF) infected with porcine endogenous retrovirus (PERV)

BACKGROUND

Understanding the interactions between the microRNA (miRNA) and mRNA of genes encoding restriction factors (RFs) can lead to the development of new antiretroviral strategies aimed at providing the resistance and reducing susceptibility of human cells to potential PERV infection. Among RFs TRIM family play an important role in shaping the immune response during various stages of infection. The aim of the study was to evaluate in vitro the transcriptional profile of TRIM family genes and identify complementary miRNAs in NHDF cells infected with PERVs and induced by gram negative lipopolysacharide (LPS).

METHODS

Human dermal fibroblasts cells were cultured in four separate conditions- 2 monocultures: control (N), treated with LPS (NL) and 2 co-cultures with porcine PK15 cells: without LPS (NP) and treated with LPS (NLP). Bacterial LPS was used in this study as an inducer of inflammation in NHDF cells. After extraction of DNA and RNA from cells PERV infection was confirmed in co-cultures by qPCR and RTqPCR. RNA extracts served as a matrix for HGU 133A 2.0 and miRNA 2.0 microarrays to evaluate the expression profile of the selected TRIM family genes and miRNAs adequately. TRIM 2, 14, 22, and 28 were selected for the validation of HGU 133A 2.0 results. Statistical analyses were performed with the use of REST^© 2009 and Genespring GX 13.0. Transcriptome Analysis Console 4.0 program (Affymetrix) was used to identify miRNAs that differentiate the studied genes in all conditions.

RESULTS

Porcine endogenous retrovirus infection at DNA and RNA level was confirmed in NHDF cells in each of the tested groups (NP and NLP). Contamination was excluded in N and NL groups. Based on the analysis of HGU 133A 2.0 results 93 mRNA IDs of TRIM family genes differentiating analyzed conditions were selected P < .05. HGU 133A 2.0 mRNA fluorescence profile was confirmed with RTqPCR of TRIM2, TRIM14, TRIM22 and TRIM28 P < .05. TRIM14 down regulation was specific only in PERV monoinfection (group NP). In miRNA 2.0 microarray 346 miRNAs were identified as differentiating NHDF cells in all analyzed conditions, P < .05. According to the analysis with mirTAR platform and Microrna.org datatbase none of the selected miRNAs had the potential to regulate the selected genes of the TRIM family.

CONCLUSION

Porcine endogenous retrovirus infection of NHDF cells is accompanied by TRIM14 down regulation suggesting TRIM14 as a possible marker of retroviral infection. None of the selected miRNAs had statistically significant potential to regulate the expression of the selected TRIMs in any of the analyzed conditions.

Post-Translation Regulation of Influenza Virus Replication

Influenza virus exploits cellular factors to complete each step of viral replication. Yet, multiple host proteins actively block replication. Consequently, infection success depends on the relative speed and efficacy at which both the virus and host use their respective effectors. Post-translational modifications (PTMs) afford both the virus and the host means to readily adapt protein function without the need for new protein production. Here we use influenza virus to address concepts common to all viruses, reviewing how PTMs facilitate and thwart each step of the replication cycle. We also discuss advancements in proteomic methods that better characterize PTMs. Although some effectors and PTMs have clear pro- or antiviral functions, PTMs generally play regulatory roles to tune protein functions, levels, and localization. Synthesis of our current understanding reveals complex regulatory schemes where the effects of PTMs are time and context dependent as the virus and host battle to control infection.

TRIM Proteins and Their Roles in the Influenza Virus Life Cycle

The ubiquitin-proteasome system (UPS) has been recognized for regulating fundamental cellular processes, followed by induction of proteasomal degradation of target proteins, and triggers multiple signaling pathways that are crucial for numerous aspects of cellular physiology. Especially tripartite motif (TRIM) proteins, well-known E3 ubiquitin ligases, emerge as having critical roles in several antiviral signaling pathways against varying viral infections. Here we highlight recent advances in the study of antiviral roles of TRIM proteins toward influenza virus infection in terms of the modulation of pathogen recognition receptor (PRR)-mediated innate immune sensing, direct obstruction of influenza viral propagation, and participation in virus-induced autophagy.

"Repair Me if You Can": Membrane Damage, Response, and Control from the Viral Perspective

Cells are constantly challenged by pathogens (bacteria, virus, and fungi), and protein aggregates or chemicals, which can provoke membrane damage at the plasma membrane or within the endo-lysosomal compartments. Detection of endo-lysosomal rupture depends on a family of sugar-binding lectins, known as galectins, which sense the abnormal exposure of glycans to the cytoplasm upon membrane damage. Galectins in conjunction with other factors orchestrate specific membrane damage responses such as the recruitment of the endosomal sorting complex required for transport (ESCRT) machinery to either repair damaged membranes or the activation of autophagy to remove membrane remnants. If not controlled, membrane damage causes the release of harmful components including protons, reactive oxygen species, or cathepsins that will elicit inflammation. In this review, we provide an overview of current knowledge on membrane damage and cellular responses. In particular, we focus on the endo-lysosomal damage triggered by non-enveloped viruses (such as adenovirus) and discuss viral strategies to control the cellular membrane damage response. Finally, we debate the link between autophagy and inflammation in this context and discuss the possibility that virus induced autophagy upon entry limits inflammation.

RIG-I-like receptors: their regulation and roles in RNA sensing

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are key sensors of virus infection, mediating the transcriptional induction of type I interferons and other genes that collectively establish an antiviral host response. Recent studies have revealed that both viral and host-derived RNAs can trigger RLR activation; this can lead to an effective antiviral response but also immunopathology if RLR activities are uncontrolled. In this Review, we discuss recent advances in our understanding of the types of RNA sensed by RLRs in the contexts of viral infection, malignancies and autoimmune diseases. We further describe how the activity of RLRs is controlled by host regulatory mechanisms, including RLR-interacting proteins, post-translational modifications and non-coding RNAs. Finally, we discuss key outstanding questions in the RLR field, including how our knowledge of RLR biology could be translated into new therapeutics.

The antiviral activities of TRIM proteins

Tripartite motif (TRIM) proteins are a highly versatile family of host-cell factors that play an integral role in the mammalian defense against pathogens. TRIM proteins regulate either transcription-dependent antiviral responses such as pro-inflammatory cytokine induction, or they modulate other important cell-intrinsic defense pathways like autophagy. Additionally, TRIM proteins exert direct antiviral activity whereby they antagonize specific viral components through diverse mechanisms. Here, we summarize the latest discoveries on the molecular mechanisms of antiviral TRIM proteins and also discuss current and future trends in this fast-evolving field.

The Expression of Tripartite Motif Protein 36 and β-Catenin Correlates with the Prognosis of Esophageal Cancer

Aims

Tripartite motif protein 36 (TRIM36) plays a tumor-suppressive role in prostate cancer. However, there is little information on the clinical relevance of TRIM36 expression in esophageal cancer (ESCA).

Methods

TRIM36 expression was analyzed by using The Cancer Genome Atlas (TCGA) ESCA dataset as well as by quantitative real-time polymerase chain reaction (PCR) and immunohistochemical (IHC) staining on samples from our hospital.

Results

In the current study, the analysis of TCGA ESCA dataset suggested the decreased expression of TRIM36 in ESCA tissues. Further analyses on samples from our hospital demonstrated that TRIM36 was significantly downregulated in ESCA tissues than in the noncancerous controls at both the mRNA and protein levels. Moreover, gene set enrichment analysis on TCGA ESCA dataset suggested that TRIM36 expression was inversely correlated with the β-catenin pathway. IHC staining data showed that 66.25% (53/80) and 51.25% (41/80) of ESCA cases had a low expression of TRIM36 and a high expression of β-catenin, respectively. By Fisher's exact test, we found that TRIM36 protein expression was significantly correlated with tumor size (P = 0.0104), tumor stage (P = 0.0169), lymph node metastasis (P = 0.0021), vital status (P = 0.0443), and β-catenin expression (P = 0.0329). These findings suggest the potential clinical significance of TRIM36 in ESCA. Kaplan–Meier and log-rank test demonstrated that a low expression of TRIM6 and a high expression of β-catenin were associated with poor overall survival of ESCA patients.

Conclusions

Our study provides evidence for the prognostic value of TRIM36 in ESCA.

TRIM Proteins in Host Defense and Viral Pathogenesis

Purpose of Review

Tripartite motif (TRIM) proteins are a large group of E3 ubiquitin ligases involved in different cellular functions. Of special interest are their roles in innate immunity, inflammation, and virus replication. We discuss novel roles of TRIM proteins during virus infections that lead to increased pathogenicity.

Recent Findings

TRIM proteins regulate different antiviral and inflammatory signaling pathways, mostly by promoting ubiquitination of important factors including pattern recognition receptors, adaptor proteins, kinases, and transcription factors that are involved in type I interferon and NF-κB pathways. Therefore, viruses have developed mechanisms to target TRIMs for immune evasion. New evidence is emerging indicating that viruses have the ability to directly use TRIMs and the ubiquitination process to enhance the viral replication cycle and cause increased pathogenesis. A new report on TRIM7 also highlights the potential pro-viral role of TRIMs via ubiquitination of viral proteins and suggests a novel mechanism by which ubiquitination of virus envelope protein may provide determinants of tissue and species tropism.

Summary

TRIM proteins have important functions in promoting host defense against virus infection; however, viruses have adapted to evade TRIM-mediated immune responses and can hijack TRIMs to ultimately increase virus pathogenesis. Only by understanding specific TRIM-virus interactions and by using more in vivo approaches can we learn how to harness TRIM function to develop therapeutic approaches to reduce virus pathogenesis.

FTR67, a member of the fish-specific finTRIM family, triggers IFN pathway and against spring viremia of carp virus

Tripartite motif (TRIM) proteins have attracted particular research interest because of their multiple functions in the antiviral innate immune response. TRIM proteins perform different functions during virus infection, some play a role in inhibiting while others play a role in promoting. In this study, we described a species-specific TRIM gene named ftr67. Analysis of tissue distribution showed that ftr67 was mainly expressed in the gill and liver in five examined tissues of zebrafish. The phylogenic analysis showed that ftr67 was closest to the grass carp TRIM67. Overexpression of ftr67 resulted in a significantly decreased SVCV entry and impaired SVCV replication in FHM cells. Furthermore, overexpression of ftr67 could significantly induce the upregulation of molecular sensor RIG-I, IRF3/7, IFN and ISGs. In addition, RING domain of ftr67 was a required part essential for the antiviral effect. In summary, our results demonstrated that the important role of ftr67 in regulating SVCV infection, which offers a potential target for development of anti-SVCV therapies.

Autoimmunity gene IRGM suppresses cGAS-STING and RIG-I-MAVS signaling to control interferon response

Activation of the type 1 interferon response is extensively connected to the pathogenesis of autoimmune diseases. Loss of function of Immunity Related GTPase M (IRGM) has also been associated to several autoimmune diseases, but its mechanism of action is unknown. Here, we found that IRGM is a master negative regulator of the interferon response. Several nucleic acid‐sensing pathways leading to interferon‐stimulated gene expression are highly activated in IRGM knockout mice and human cells. Mechanistically, we show that IRGM interacts with nucleic acid sensor proteins, including cGAS and RIG‐I, and mediates their p62‐dependent autophagic degradation to restrain interferon signaling. Further, IRGM deficiency results in defective mitophagy leading to the accumulation of defunct leaky mitochondria that release cytosolic DAMPs and mtROS. Hence, IRGM deficiency increases not only the levels of the sensors, but also those of the stimuli that trigger the activation of the cGAS‐STING and RIG‐I‐MAVS signaling axes, leading to robust induction of IFN responses. Taken together, this study defines the molecular mechanisms by which IRGM maintains interferon homeostasis and protects from autoimmune diseases.

Tumor suppressor p53 cross-talks with TRIM family proteins

p53 is a key tumor suppressor. As a transcription factor, p53 accumulates in cells in response to various stress signals and selectively transcribes its target genes to regulate a wide variety of cellular stress responses to exert its function in tumor suppression. In addition to tumor suppression, p53 is also involved in many other physiological and pathological processes, e.g. anti-infection, immune response, development, reproduction, neurodegeneration and aging. To maintain its proper function, p53 is under tight and delicate regulation through different mechanisms, particularly the posttranslational modifications. The tripartite motif (TRIM) family proteins are a large group of proteins characterized by the RING, B-Box and coiled-coil (RBCC) domains at the N-terminus. TRIM proteins play important roles in regulation of many fundamental biological processes, including cell proliferation and death, DNA repair, transcription, and immune response. Alterations of TRIM proteins have been linked to many diseases including cancer, infectious diseases, developmental disorders, and neurodegeneration. Interestingly, recent studies have revealed that many TRIM proteins are involved in the regulation of p53, and at the same time, many TRIM proteins are also regulated by p53. Here, we review the cross-talk between p53 and TRIM proteins, and its impact upon cellular biological processes as well as cancer and other diseases.

The MID1 gene product in physiology and disease

MID1 is an E3 ubiquitin ligase of the Tripartite Motif (TRIM) subfamily of RING-containing proteins, hence also known as TRIM18. MID1 is a microtubule-binding protein found in complex with the catalytic subunit of PP2A (PP2Ac) and its regulatory subunit alpha 4 (α4). To date, several substrates and interactors of MID1 have been described, providing evidence for the involvement of MID1 in a plethora of essential biological processes, especially during embryonic development. Mutations in the MID1 gene are responsible of the X-linked form of Opitz syndrome (XLOS), a multiple congenital disease characterised by defects in the development of midline structures during embryogenesis. Here, we review MID1-related physiological mechanisms as well as the pathological implication of the MID1 gene in XLOS and in other clinical conditions.

Distinct and Orchestrated Functions of RNA Sensors in Innate Immunity

Faithful maintenance of immune homeostasis relies on the capacity of the cellular immune surveillance machinery to recognize "nonself", such as the presence of pathogenic RNA. Several families of pattern-recognition receptors exist that detect immunostimulatory RNA and then induce cytokine-mediated antiviral and proinflammatory responses. Here, we review the distinct features of bona fide RNA sensors, Toll-like receptors and retinoic-acid inducible gene-I (RIG-I)-like receptors in particular, with a focus on their functional specificity imposed by cell-type-dependent expression, subcellular localization, and ligand preference. Furthermore, we highlight recent advances on the roles of nucleotide-binding oligomerization domain (NOD)-like receptors and DEAD-box or DEAH-box RNA helicases in an orchestrated RNA-sensing network and also discuss the relevance of RNA sensor polymorphisms in human disease.

TRIM35 mediates protection against influenza infection by activating TRAF3 and degrading viral PB2

Tripartite motif (TRIM) family proteins are important effectors of innate immunity against viral infections. Here we identified TRIM35 as a regulator of TRAF3 activation. Deficiency in or inhibition of TRIM35 suppressed the production of type I interferon (IFN) in response to viral infection. Trim35-deficient mice were more susceptible to influenza A virus (IAV) infection than were wild-type mice. TRIM35 promoted the RIG-I-mediated signaling by catalyzing Lys63-linked polyubiquitination of TRAF3 and the subsequent formation of a signaling complex with VISA and TBK1. IAV PB2 polymerase countered the innate antiviral immune response by impeding the Lys63-linked polyubiquitination and activation of TRAF3. TRIM35 mediated Lys48-linked polyubiquitination and proteasomal degradation of IAV PB2, thereby antagonizing its suppression of TRAF3 activation. Our in vitro and in vivo findings thus reveal novel roles of TRIM35, through catalyzing Lys63- or Lys48-linked polyubiquitination, in RIG-I antiviral immunity and mechanism of defense against IAV infection.

To TRIM or not to TRIM: the balance of host-virus interactions mediated by the ubiquitin system

The innate immune system responds rapidly to protect against viral infections, but an overactive response can cause harmful damage. To avoid this, the response is tightly regulated by post-translational modifications (PTMs). The ubiquitin system represents a powerful PTM machinery that allows for the reversible linkage of ubiquitin to activate and deactivate a target's function. A precise enzymatic cascade of ubiquitin-activating, conjugating and ligating enzymes facilitates ubiquitination. Viruses have evolved to take advantage of the ubiquitin pathway either by targeting factors to dampen the antiviral response or by hijacking the system to enhance their replication. The tripartite motif (TRIM) family of E3 ubiquitin ligases has garnered attention as a major contributor to innate immunity. Many TRIM family members limit viruses either indirectly as components in innate immune signalling, or directly by targeting viral proteins for degradation. In spite of this, TRIMs and other ubiquitin ligases can be appropriated by viruses and repurposed as valuable tools in viral replication. This duality of function suggests a new frontier of research for TRIMs and raises new challenges for discerning the subtleties of these pro-viral mechanisms. Here, we review current findings regarding the involvement of TRIMs in host-virus interactions. We examine ongoing developments in the field, including novel roles for unanchored ubiquitin in innate immunity, the direct involvement of ubiquitin ligases in promoting viral replication, recent controversies on the role of ubiquitin and TRIM25 in activation of the pattern recognition receptor RIG-I, and we discuss the implications these studies have on future research directions.

Specificity in Ubiquitination Triggered by Virus Infection

Ubiquitination is a prominent posttranslational modification, in which the ubiquitin moiety is covalently attached to a target protein to influence protein stability, interaction partner and biological function. All seven lysine residues of ubiquitin, along with the N-terminal methionine, can each serve as a substrate for further ubiquitination, which effectuates a diverse combination of mono- or poly-ubiquitinated proteins with linear or branched ubiquitin chains. The intricately composed ubiquitin codes are then recognized by a large variety of ubiquitin binding domain (UBD)-containing proteins to participate in the regulation of various pathways to modulate the cell behavior. Viruses, as obligate parasites, involve many aspects of the cell pathways to overcome host defenses and subjugate cellular machineries. In the virus-host interactions, both the virus and the host tap into the rich source of versatile ubiquitination code in order to compete, combat, and co-evolve. Here, we review the recent literature to discuss the role of ubiquitin system as the infection progresses in virus life cycle and the importance of ubiquitin specificity in the regulation of virus-host relation.

Vaccinia Virus as a Master of Host Shutoff Induction: Targeting Processes of the Central Dogma and Beyond

The synthesis of host cell proteins is adversely inhibited in many virus infections, whereas viral proteins are efficiently synthesized. This phenomenon leads to the accumulation of viral proteins concurrently with a profound decline in global host protein synthesis, a phenomenon often termed “host shutoff”. To induce host shutoff, a virus may target various steps of gene expression, as well as pre- and post-gene expression processes. During infection, vaccinia virus (VACV), the prototype poxvirus, targets all major processes of the central dogma of genetics, as well as pre-transcription and post-translation steps to hinder host cell protein production. In this article, we review the strategies used by VACV to induce host shutoff in the context of strategies employed by other viruses. We elaborate on how VACV induces host shutoff by targeting host cell DNA synthesis, RNA production and processing, mRNA translation, and protein degradation. We emphasize the topics on VACV’s approaches toward modulating mRNA processing, stability, and translation during infection. Finally, we propose avenues for future investigations, which will facilitate our understanding of poxvirus biology, as well as fundamental cellular gene expression and regulation mechanisms.

TRIM14 Is a Key Regulator of the Type I IFN Response during Mycobacterium tuberculosis Infection

Tripartite motif-containing proteins (TRIMs) play a variety of recently described roles in innate immunity. Although many TRIMs regulate type I IFN expression following cytosolic nucleic acid sensing of viruses, their contribution to innate immune signaling and gene expression during bacterial infection remains largely unknown. Because Mycobacterium tuberculosis is an activator of cGAS-dependent cytosolic DNA sensing, we set out to investigate a role for TRIM proteins in regulating macrophage responses to M. tuberculosis In this study, we demonstrate that TRIM14, a noncanonical TRIM that lacks an E3 ubiquitin ligase RING domain, is a critical negative regulator of the type I IFN response in Mus musculus macrophages. We show that TRIM14 interacts with both cGAS and TBK1 and that macrophages lacking TRIM14 dramatically hyperinduce IFN stimulated gene (ISG) expression following M. tuberculosis infection, cytosolic nucleic acid transfection, and IFN-β treatment. Consistent with a defect in resolution of the type I IFN response, Trim14 knockout macrophages have more phospho-Ser754 STAT3 relative to phospho-Ser727 and fail to upregulate the STAT3 target Socs3, which is required to turn off IFNAR signaling. These data support a model whereby TRIM14 acts as a scaffold between TBK1 and STAT3 to promote phosphorylation of STAT3 at Ser727 and resolve ISG expression. Remarkably, Trim14 knockout macrophages hyperinduce expression of antimicrobial genes like Nos2 and are significantly better than control cells at limiting M. tuberculosis replication. Collectively, these data reveal an unappreciated role for TRIM14 in resolving type I IFN responses and controlling M. tuberculosis infection.

Pattern Recognition Receptor Signaling and Innate Responses to Influenza A Viruses in the Mallard Duck, Compared to Humans and Chickens

Mallard ducks are a natural host and reservoir of avian Influenza A viruses. While most influenza strains can replicate in mallards, the virus typically does not cause substantial disease in this host. Mallards are often resistant to disease caused by highly pathogenic avian influenza viruses, while the same strains can cause severe infection in humans, chickens, and even other species of ducks, resulting in systemic spread of the virus and even death. The differences in influenza detection and antiviral effectors responsible for limiting damage in the mallards are largely unknown. Domestic mallards have an early and robust innate response to infection that seems to limit replication and clear highly pathogenic strains. The regulation and timing of the response to influenza also seems to circumvent damage done by a prolonged or dysregulated immune response. Rapid initiation of innate immune responses depends on viral recognition by pattern recognition receptors (PRRs) expressed in tissues where the virus replicates. RIG-like receptors (RLRs), Toll-like receptors (TLRs), and Nod-like receptors (NLRs) are all important influenza sensors in mammals during infection. Ducks utilize many of the same PRRs to detect influenza, namely RIG-I, TLR7, and TLR3 and their downstream adaptors. Ducks also express many of the same signal transduction proteins including TBK1, TRIF, and TRAF3. Some antiviral effectors expressed downstream of these signaling pathways inhibit influenza replication in ducks. In this review, we summarize the recent advances in our understanding of influenza recognition and response through duck PRRs and their adaptors. We compare basal tissue expression and regulation of these signaling components in birds, to better understand what contributes to influenza resistance in the duck.

Emerging RNA-binding roles in the TRIM family of ubiquitin ligases

TRIM proteins constitute a large, diverse and ancient protein family which play a key role in processes including cellular differentiation, autophagy, apoptosis, DNA repair, and tumour suppression. Mostly known and studied through the lens of their ubiquitination activity as E3 ligases, it has recently emerged that many of these proteins are involved in direct RNA binding through their NHL or PRY/SPRY domains. We summarise the current knowledge concerning the mechanism of RNA binding by TRIM proteins and its biological role. We discuss how RNA-binding relates to their previously described functions such as E3 ubiquitin ligase activity, and we will consider the potential role of enrichment in membrane-less organelles.

A recessive Trim2 mutation causes an axonal neuropathy in mice

We analyzed Trim2^A/A mice, generated by CRISPR-Cas9, which have a recessive, null mutation of Trim2. Trim2^A/A mice develop ataxia that is associated with a severe loss of cerebellar Purkinje cells and a peripheral neuropathy. Myelinated axons in the CNS, including those in the deep cerebellar nuclei, have focal enlargements that contain mitochondria and neurofilaments. In the PNS, there is a loss of myelinated axons, particularly in the most distal nerves. The pathologically affected neuronal populations - primary sensory and motor neurons as well as cerebellar Purkinje cells - express TRIM2, suggesting that loss of TRIM2 in these neurons results in cell autonomous effects on their axons. In contrast, these pathological findings were not found in a second strain of Trim2 mutant mice (Trim2^C/C), which has a partial deletion in the RING domain that is needed for ubiquitin ligase activity. Both the Trim2^Aand the Trim2^C alleles encode mutant TRIM2 proteins with reduced ubiquitination activity. In sum, Trim2^A/A mice are a genetically authentic animal model of a recessive axonal neuropathy of humans, apparently for a function that does not depend on the ubiquitin ligase activity.

TRIM5α self-assembly and compartmentalization of the HIV-1 viral capsid

The tripartite-motif protein, TRIM5α, is an innate immune sensor that potently restricts retrovirus infection by binding to human immunodeficiency virus capsids. Higher-ordered oligomerization of this protein forms hexagonally patterned structures that wrap around the viral capsid, despite an anomalously low affinity for the capsid protein (CA). Several studies suggest TRIM5α oligomerizes into a lattice with a symmetry and spacing that matches the underlying capsid, to compensate for the weak affinity, yet little is known about how these lattices form. Using a combination of computational simulations and electron cryo-tomography imaging, we reveal the dynamical mechanisms by which these lattices self-assemble. Constrained diffusion allows the lattice to reorganize, whereas defects form on highly curved capsid surfaces to alleviate strain and lattice symmetry mismatches. Statistical analysis localizes the TRIM5α binding interface at or near the CypA binding loop of CA. These simulations elucidate the molecular-scale mechanisms of viral capsid cellular compartmentalization by TRIM5α.

Tripartite-motif containing (TRIM) proteins modulate cellular responses to viral infection. Here the authors use molecular dynamics simulations to demonstrate that TRIM5α uses a two-dimensional lattice hopping mechanism to aggregate on the HIV capsid surface and initiate lattice growth.

The Restrictome of Flaviviruses

Flaviviruses are a genus of mostly arthropod-borne RNA viruses that cause a range of pathologies in humans. Basic knowledge on flaviviruses is rapidly expanding, partly due to their status as frequent emerging or re-emerging pathogens. Flaviviruses include the dengue, Zika, West Nile, tick-borne encephalitis and yellow fever viruses (DENV, ZIKV, WNV, TBEV and YFV, respectively). As is the case with other families of viruses, the success of productive infection of human cells by flaviviruses depends in part on the antiviral activity of a heterogeneous group of cellular antiviral proteins called restriction factors. Restriction factors are the effector proteins of the cell-autonomous innate response against viruses, an immune pathway that also includes virus sensors as well as intracellular and extracellular signal mediators such as type I interferons (IFN-I). In this review, I summarize recent progress toward the identification and characterization of flavivirus restriction factors. In particular, I focus on IFI6, Schlafen 11, FMRP, OAS-RNase L, RyDEN, members of the TRIM family of proteins (TRIM5α, TRIM19, TRIM56, TRIM69 and TRIM79α) and a new mechanism of action proposed for viperin. Recent and future studies on this topic will lead to a more complete picture of the flavivirus restrictome, defined as the ensemble of cellular factors with demonstrated anti-flaviviral activity.

TRIM proteins in autophagy: selective sensors in cell damage and innate immune responses

Autophagy, a main intracellular catabolic process, is induced in response to a variety of cellular stresses to promptly degrade harmful agents and to coordinate the activity of prosurvival and prodeath processes in order to determine the fate of the injured cells. While the main components of the autophagy machinery are well characterized, the molecular mechanisms that confer selectivity to this process both in terms of stress detection and cargo engulfment have only been partly elucidated. Here, we discuss the emerging role played by the E3 ubiquitin ligases of the TRIM family in regulating autophagy in physiological and pathological conditions, such as inflammation, infection, tumorigenesis, and muscle atrophy. TRIM proteins employ different strategies to regulate the activity of the core autophagy machinery, acting either as scaffold proteins or via ubiquitin-mediated mechanisms. Moreover, they confer high selectivity to the autophagy-mediated degradation as described for the innate immune response, where TRIM proteins mediate both the engulfment of pathogens within autophagosomes and modulate the immune response by controlling the stability of signaling regulators. Importantly, the elucidation of the molecular mechanisms underlying the regulation of autophagy by TRIMs is providing important insights into how selective types of autophagy are altered under pathological conditions, as recently shown in cancer and muscular dystrophy.

Vesicular Stomatitis Virus Transcription Is Inhibited by TRIM69 in the Interferon-Induced Antiviral State

Interferons (IFNs) induce the expression of interferon-stimulated genes (ISGs), many of which are responsible for the cellular antiviral state in which the replication of numerous viruses is blocked. How the majority of individual ISGs inhibit the replication of particular viruses is unknown. We conducted a loss-of-function screen to identify genes required for the activity of alpha interferon (IFN-α) against vesicular stomatitis virus, Indiana serotype (VSVIND), a prototype negative-strand RNA virus. Our screen revealed that TRIM69, a member of the tripartite motif (TRIM) family of proteins, is a VSVIND inhibitor. TRIM69 potently inhibited VSVIND replication through a previously undescribed transcriptional inhibition mechanism. Specifically, TRIM69 physically associates with the VSVIND phosphoprotein (P), requiring a specific peptide target sequence encoded therein. P is a cofactor for the viral polymerase and is required for viral RNA synthesis, as well as the assembly of replication compartments. By targeting P, TRIM69 inhibits pioneer transcription of the incoming virion-associated minus-strand RNA, thereby preventing the synthesis of viral mRNAs, and consequently impedes all downstream events in the VSVIND replication cycle. Unlike some TRIM proteins, TRIM69 does not inhibit viral replication by inducing degradation of target viral proteins. Rather, higher-order TRIM69 multimerization is required for its antiviral activity, suggesting that TRIM69 functions by sequestration or anatomical disruption of the viral machinery required for VSVIND RNA synthesis.IMPORTANCE Interferons are important antiviral cytokines that work by inducing hundreds of host genes whose products inhibit the replication of many viruses. While the antiviral activity of interferon has long been known, the identities and mechanisms of action of most interferon-induced antiviral proteins remain to be discovered. We identified gene products that are important for the antiviral activity of interferon against vesicular stomatitis virus (VSV), a model virus that whose genome consists of a single RNA molecule with negative-sense polarity. We found that a particular antiviral protein, TRIM69, functions by a previously undescribed molecular mechanism. Specifically, TRIM69 interacts with and inhibits the function of a particular phosphoprotein (P) component of the viral transcription machinery, preventing the synthesis of viral messenger RNAs.

Many Faces of TRIM Proteins on the Road from Pluripotency to Neurogenesis

Tripartite motif (TRIM) proteins participate in numerous biological processes. They are the key players in immune system and are involved in the oncogenesis. Moreover, TRIMs are the highly conserved regulators of developmental pathways in both vertebrates and invertebrates. In particular, numerous data point to the participation of TRIMs in the determination of stem cell fate, as well as in the neurogenesis. TRIMs apply various mechanisms to perform their functions. Their common feature is the ability to ubiquitinate proteins mediated by the Really Interesting New Gene (RING) domain. Different C-terminal domains of TRIMs are involved in DNA and RNA binding, protein/protein interactions, and chromatin-mediated transcriptional regulation. Mutations and alterations of TRIM expression cause significant disturbances in the stem cells' self-renewal and neurogenesis, which result in the various pathologies of the nervous system (neurodegeneration, neuroinflammation, and malignant transformation). This review discusses the diverse molecular mechanisms of participation of TRIMs in stem cell maintenance and self-renewal as well as in neural differentiation processes and neuropathology.

TRIM32, but not its muscular dystrophy-associated mutant, positively regulates and is targeted to autophagic degradation by p62/SQSTM1

The tripartite motif (TRIM) proteins constitute a family of ubiquitin E3 ligases involved in a multitude of cellular processes, including protein homeostasis and autophagy. TRIM32 is characterized by six protein-protein interaction domains termed NHL, various point mutations in which are associated with limb-girdle-muscular dystrophy 2H (LGMD2H). Here, we show that TRIM32 is an autophagy substrate. Lysosomal degradation of TRIM32 was dependent on ATG7 and blocked by knockout of the five autophagy receptors p62 (also known as SQSTM1), NBR1, NDP52 (also known as CALCOCO2), TAX1BP1 and OPTN, pointing towards degradation by selective autophagy. p62 directed TRIM32 to lysosomal degradation, while TRIM32 mono-ubiquitylated p62 on lysine residues involved in regulation of p62 activity. Loss of TRIM32 impaired p62 sequestration, while reintroduction of TRIM32 facilitated p62 dot formation and its autophagic degradation. A TRIM32^LGMD2H disease mutant was unable to undergo autophagic degradation and to mono-ubiquitylate p62, and its reintroduction into the TRIM32-knockout cells did not affect p62 dot formation. In light of the important roles of autophagy and p62 in muscle cell proteostasis, our results point towards impaired TRIM32-mediated regulation of p62 activity as a pathological mechanisms in LGMD2H.

Autophagy and Viral Infection

Autophagy is an intracellular recycling process that maintains cellular homeostasis by orchestrating immunity upon viral infection. Following viral infection, autophagy is often initiated to curtail infection by delivering viral particles for lysosomal degradation and further integrating with innate pattern recognition receptor signaling to induce interferon (IFN)-mediated viral clearance. However, some viruses have evolved anti-autophagy strategies to escape host immunity and to promote viral replication. In this chapter, we illustrate how autophagy prevents viral infection to generate an optimal anti-viral milieu, and then concentrate on how viruses subvert and hijack the autophagic process to evade immunosurveillance, thereby facilitating viral replication and pathogenesis. Understanding the interplays between autophagy and viral infection is anticipated to guide the development of effective anti-viral therapeutics to fight against infectious diseases.