NLRC5/MHC class I transactivator: A key target for immune escape by SARS-CoV-2.

Antigen presentation to CD8+ T cells by MHC class I molecules is essential for host defense against viral infections. Various mechanisms have evolved in multiple viruses to escape immune surveillance and defense to support viral proliferation in host cells. Through in vitro SARS-CoV-2 infection studies and analysis of COVID-19 patient samples, we found that SARS-CoV-2 suppresses the induction of the MHC class I pathway by inhibiting the expression and function of NLRC5, a major transcriptional regulator of MHC class I genes. In this review, we discuss the molecular mechanisms for suppression of the MHC class I pathway and clinical implications for COVID-19.

FBXO11 constitutes a major negative regulator of MHC class II through ubiquitin-dependent proteasomal degradation of CIITA.

Major histocompatibility complex (MHC) class I and II molecules play critical roles in the activation and regulation of adaptive immunity through antigen presentation to CD8+ and CD4+ T cells, respectively. Strict regulation of MHC expression is critical for proper immune responses. CIITA (MHC class II transactivator), an NLR (nucleotide-binding domain, leucine-rich-repeat containing) protein, is a master regulator of MHC class II (MHC-II) gene transcription. Although it has been known that CIITA activity is regulated at the transcriptional and protein levels, the mechanism to determine CIITA protein level has not been elucidated. Here, we show that FBXO11 is a bona fide E3 ligase of CIITA and regulates CIITA protein level through ubiquitination-mediated degradation. A nonbiased proteomic approach for CIITA-binding protein identified FBXO11, a member of the Skp1-Cullin-1-F-box E3 ligase complex, as a binding partner of CIITA but not MHC class I transactivator, NLRC5. The cycloheximide chase assay showed that the half-life of CIITA is mainly regulated by FBXO11 via the ubiquitin-proteasome system. The expression of FBXO11 led to the reduced MHC-II at the promoter activity level, transcriptional level, and surface expression level through downregulation of CIITA. Moreover, human and mouse FBXO11-deficient cells display increased levels of MHC-II and related genes. In normal and cancer tissues, FBXO11 expression level is negatively correlated with MHC-II. Interestingly, the expression of FBXO11, along with CIITA, is associated with prognosis of cancer patients. Therefore, FBXO11 is a critical regulator to determine the level of MHC-II, and its expression may serve as a biomarker for cancer.

Mitochondrial LETM1 drives ionic and molecular clock rhythms in circadian pacemaker neurons.

The mechanisms that generate robust ionic oscillation in circadian pacemaker neurons are under investigation. Here, we demonstrate critical functions of the mitochondrial cation antiporter leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1), which exchanges K+/H+ in Drosophila and Ca2+/H+ in mammals, in circadian pacemaker neurons. Letm1 knockdown in Drosophila pacemaker neurons reduced circadian cytosolic H+ rhythms and prolonged nuclear PERIOD/TIMELESS expression rhythms and locomotor activity rhythms. In rat pacemaker neurons in the hypothalamic suprachiasmatic nucleus (SCN), circadian rhythms in cytosolic Ca2+ and Bmal1 transcription were dampened by Letm1 knockdown. Mitochondrial Ca2+ uptake peaks late during the day were also observed in rat SCN neurons following photolytic elevation of cytosolic Ca2+. Since cation transport by LETM1 is coupled to mitochondrial energy synthesis, we propose that LETM1 integrates metabolic, ionic, and molecular clock rhythms in the central clock system in both invertebrates and vertebrates.

SARS-CoV-2 inhibits induction of the MHC class I pathway by targeting the STAT1-IRF1-NLRC5 axis.

The MHC class I-mediated antigen presentation pathway plays a critical role in antiviral immunity. Here we show that the MHC class I pathway is targeted by SARS-CoV-2. Analysis of the gene expression profile from COVID-19 patients as well as SARS-CoV-2 infected epithelial cell lines reveals that the induction of the MHC class I pathway is inhibited by SARS-CoV-2 infection. We show that NLRC5, an MHC class I transactivator, is suppressed both transcriptionally and functionally by the SARS-CoV-2 ORF6 protein, providing a mechanistic link. SARS-CoV-2 ORF6 hampers type II interferon-mediated STAT1 signaling, resulting in diminished upregulation of NLRC5 and IRF1 gene expression. Moreover, SARS-CoV-2 ORF6 inhibits NLRC5 function via blocking karyopherin complex-dependent nuclear import of NLRC5. Collectively, our study uncovers an immune evasion mechanism of SARS-CoV-2 that targets the function of key MHC class I transcriptional regulators, STAT1-IRF1-NLRC5.

Innate immune sensing of coronavirus and viral evasion strategies.

The innate immune system is the first line of the host defense program against pathogens and harmful substances. Antiviral innate immune responses can be triggered by multiple cellular receptors sensing viral components. The activated innate immune system produces interferons (IFNs) and cytokines that perform antiviral functions to eliminate invading viruses. Coronaviruses are single-stranded, positive-sense RNA viruses that have a broad range of animal hosts. Coronaviruses have evolved multiple means to evade host antiviral immune responses. Successful immune evasion by coronaviruses may enable the viruses to adapt to multiple species of host organisms. Coronavirus transmission from zoonotic hosts to humans has caused serious illnesses, such as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and coronavirus disease-2019 (COVID-19), resulting in global health and economic crises. In this review, we summarize the current knowledge of the mechanisms underlying host sensing of and innate immune responses against coronavirus invasion, as well as host immune evasion strategies of coronaviruses.

A substrate-trapping strategy to find E3 ubiquitin ligase substrates identifies Parkin and TRIM28 targets.

The identification of true substrates of an E3 ligase is biologically important but biochemically difficult. In recent years, several techniques for identifying substrates have been developed, but these approaches cannot exclude indirect ubiquitination or have other limitations. Here we develop an E3 ligase substrate-trapping strategy by fusing a tandem ubiquitin-binding entity (TUBE) with an anti-ubiquitin remnant antibody to effectively identify ubiquitinated substrates. We apply this method to one of the RBR-type ligases, Parkin, and to one of the RING-type ligases, TRIM28, and identify previously unknown substrates for TRIM28 including cyclin A2 and TFIIB. Furthermore, we find that TRIM28 promotes cyclin A2 ubiquitination and degradation at the G1/S phase and suppresses premature entry into S phase. Taken together, the results indicate that this method is a powerful tool for comprehensively identifying substrates of E3 ligases.

Characterization of a long noncoding RNA Pcdh17it as a novel marker for immature premyelinating oligodendrocytes.

Oligodendrocyte precursors (OPs) proliferate and differentiate into oligodendrocytes (OLs) during postnatal development and into adulthood in the central nervous system (CNS). Following the initiation of differentiation, OPs give rise to immature, premyelinating OLs, which undergo further differentiation and mature into myelin-forming OLs. We identified an immature OL-specific long noncoding RNA, named Pcdh17it. Through co-localization analysis and morphological characterization of OLs, we found that Pcdh17it is a specific marker for newly born immature OLs in the developing and adult forebrain of mice, and we used this new marker to analyze OL generation over the lifespan of mice. Pcdh17it is an effective tool for monitoring newly born OLs in adult brain, allowing detailed study of the dynamics of OP differentiation into OLs in the normal and pathological CNS.

[Carcinoma of the small intestine diagnosed by double balloon endoscopy].

There are no specific symptoms of patients with carcinoma of the small intestine. Therefore, it is difficult to diagnose in the early stage by imaging modalities such as radiological enteroclysis, computed tomography, and classical endoscopy. However, double balloon endoscopy makes it possible to diagnose the carcinoma of the entire small bowel by taking tissue samples for pathological assessment. The characteristic of endoscopic findings is irregular ulcerated tumor with malignant stricture. It is still difficult to find carcinoma of small intestine in patients without symptoms and most cases are advanced when diagnosis is achieved. We should try to diagnose in early stage by combining images modalities, capsule endoscopy and double balloon endoscopy safely and efficiently, resulting in improving the patients' prognosis.