Critical roles of the miR-17∼92 family in thymocyte development, leukemogenesis, and autoimmunity.

Thymocyte development requires precise control of PI3K-Akt signaling to promote proliferation and prevent leukemia and autoimmune disorders. Here, we show that ablating individual clusters of the miR-17∼92 family has a negligible effect on thymocyte development, while deleting the entire family severely impairs thymocyte proliferation and reduces thymic cellularity, phenocopying genetic deletion of Dicer. Mechanistically, miR-17∼92 expression is induced by Myc-mediated pre-T cell receptor (TCR) signaling, and miR-17∼92 promotes thymocyte proliferation by suppressing the translation of Pten. Retroviral expression of miR-17∼92 restores the proliferation and differentiation of Myc-deficient thymocytes. Conversely, partial deletion of the miR-17∼92 family significantly delays Myc-driven leukemogenesis. Intriguingly, thymocyte-specific transgenic miR-17∼92 expression does not cause leukemia or lymphoma but instead aggravates skin inflammation, while ablation of the miR-17∼92 family ameliorates skin inflammation. This study reveals intricate roles of the miR-17∼92 family in balancing thymocyte development, leukemogenesis, and autoimmunity and identifies those microRNAs (miRNAs) as potential therapeutic targets for leukemia and autoimmune diseases.

Dhx33 promotes B-cell growth and proliferation by controlling activation-induced rRNA upregulation.

Upon recognition of foreign antigens, naïve B cells undergo rapid activation, growth, and proliferation. How B-cell growth and proliferation are coupled with activation remains poorly understood. Combining CRISPR/Cas9-mediated functional analysis and mouse genetics approaches, we found that Dhx33, an activation-induced RNA helicase, plays a critical role in coupling B-cell activation with growth and proliferation. Mutant mice with B-cell-specific deletion of Dhx33 exhibited impaired B-cell development, germinal center reactions, plasma cell differentiation, and antibody production. Dhx33-deficient B cells appeared normal in the steady state and early stage of activation but were retarded in growth and proliferation. Mechanistically, Dhx33 played an indispensable role in activation-induced upregulation of ribosomal DNA (rDNA) transcription. In the absence of Dhx33, activated B cells were compromised in their ability to ramp up 47S ribosomal RNA (rRNA) production and ribosome biogenesis, resulting in nucleolar stress, p53 accumulation, and cellular death. Our findings demonstrate an essential role for Dhx33 in coupling B-cell activation with growth and proliferation and suggest that Dhx33 inhibition is a potential therapy for lymphoma and antibody-mediated autoimmune diseases.

Glycogen synthase kinase 3 drives thymocyte egress by suppressing ß-catenin activation of Akt.

Molecular pathways controlling emigration of mature thymocytes from thymus to the periphery remain incompletely understood. Here, we show that T cell­specific ablation of glycogen synthase kinase 3 (GSK3) led to severely impaired thymic egress. In the absence of GSK3, ß-catenin accumulated in the cytoplasm, where it associated with and activated Akt, leading to phosphorylation and degradation of Foxo1 and downregulation of Klf2 and S1P1 expression, thereby preventing emigration of thymocytes. A cytoplasmic membrane-localized ß-catenin excluded from the nucleus promoted Akt activation, suggesting a new function of ß-catenin independent of its role as a transcriptional activator. Furthermore, genetic ablation of ß-catenin, retroviral expression of a dominant negative Akt mutant, and transgenic expression of a constitutively active Foxo1 restored emigration of GSK3-deficient thymocytes. Our findings establish an essential role for GSK3 in thymocyte egress and reveal a previously unidentified signaling function of ß-catenin in the cytoplasm.

Ascorbic Acid Promotes Plasma Cell Differentiation through Enhancing TET2/3-Mediated DNA Demethylation.

Plasma cells provide high-affinity antibodies against invading pathogens. Although transcriptional and epigenetic mechanisms have been extensively studied for plasma cell differentiation, how these mechanisms respond to environmental cues remains largely unexplored. In this study, we show that ascorbic acid (vitamin C), an essential nutrient, is able to promote plasma cell differentiation and humoral immune response by enhancing TET2/3-mediated DNA demethylation. Ascorbic acid treatment during B cell activation has persistent effects on later plasma cell differentiation by predisposing germinal center B cells toward the plasma cell lineage. Conversely, ascorbic acid deficiency in vivo blocks plasma cell differentiation and attenuates the humoral immune response following antigen immunization. We further demonstrate that such effects of ascorbic acid on plasma cell differentiation require DNA methylcytosine oxidases TET2 and TET3. Our study thus reveals a previously uncharacterized link between essential nutrients and epigenetic regulation of plasma cell differentiation and humoral immune response.

Differential Sensitivity of Target Genes to Translational Repression by miR-17~92.

MicroRNAs (miRNAs) are thought to exert their functions by modulating the expression of hundreds of target genes and each to a small degree, but it remains unclear how small changes in hundreds of target genes are translated into the specific function of a miRNA. Here, we conducted an integrated analysis of transcriptome and translatome of primary B cells from mutant mice expressing miR-17~92 at three different levels to address this issue. We found that target genes exhibit differential sensitivity to miRNA suppression and that only a small fraction of target genes are actually suppressed by a given concentration of miRNA under physiological conditions. Transgenic expression and deletion of the same miRNA gene regulate largely distinct sets of target genes. miR-17~92 controls target gene expression mainly through translational repression and 5'UTR plays an important role in regulating target gene sensitivity to miRNA suppression. These findings provide molecular insights into a model in which miRNAs exert their specific functions through a small number of key target genes.

MicroRNA-17~92 inhibits colorectal cancer progression by targeting angiogenesis.

The miR-17~92 microRNA (miRNA) cluster host gene is upregulated in a broad spectrum of human cancers including colorectal cancer (CRC). Previous studies have shown that miR-17~92 promotes tumorigenesis and cancer angiogenesis in some tumor models. However, its role in the initiation and progression of CRC remains unknown. In this study, we found that transgenic mice overexpressing miR-17~92 specifically in epithelial cells of the small and large intestines exhibited decreased tumor size and tumor angiogenesis in azoxymethane and dextran sulfate sodium salt (AOM-DSS)-induced CRC model as compared to their littermates control. Further study showed that miR-17~92 inhibited the progression of CRC via suppressing tumor angiogenesis through targeting multiple tumor angiogenesis-inducing genes, TGFBR2, HIF1α, and VEGFA in vivo and in vitro. Collectively, we demonstrated that miR-17~92 suppressed tumor progression by inhibiting tumor angiogenesis in a genetically engineered mouse model, indicating the presence of cellular context-dependent pro- and anti-cancer effects of miR-17~92.

Gasdermin D is an executor of pyroptosis and required for interleukin-1ß secretion.

Inflammasome is an intracellular signaling complex of the innate immune system. Activation of inflammasomes promotes the secretion of interleukin 1ß (IL-1ß) and IL-18 and triggers pyroptosis. Caspase-1 and -11 (or -4/5 in human) in the canonical and non-canonical inflammasome pathways, respectively, are crucial for inflammasome-mediated inflammatory responses. Here we report that gasdermin D (GSDMD) is another crucial component of inflammasomes. We discovered the presence of GSDMD protein in nigericin-induced NLRP3 inflammasomes by a quantitative mass spectrometry-based analysis. Gene deletion of GSDMD demonstrated that GSDMD is required for pyroptosis and for the secretion but not proteolytic maturation of IL-1ß in both canonical and non-canonical inflammasome responses. It was known that GSDMD is a substrate of caspase-1 and we showed its cleavage at the predicted site during inflammasome activation and that this cleavage was required for pyroptosis and IL-1ß secretion. Expression of the N-terminal proteolytic fragment of GSDMD can trigger cell death and N-terminal modification such as tagging with Flag sequence disrupted the function of GSDMD. We also found that pro-caspase-1 is capable of processing GSDMD and ASC is not essential for GSDMD to function. Further analyses of LPS plus nigericin- or Salmonella typhimurium-treated macrophage cell lines and primary cells showed that apoptosis became apparent in Gsdmd(-/-) cells, indicating a suppression of apoptosis by pyroptosis. The induction of apoptosis required NLRP3 or other inflammasome receptors and ASC, and caspase-1 may partially contribute to the activation of apoptotic caspases in Gsdmd(-/-) cells. These data provide new insights into the molecular mechanisms of pyroptosis and reveal an unexpected interplay between apoptosis and pyroptosis.

Translocation of mixed lineage kinase domain-like protein to plasma membrane leads to necrotic cell death.

Mixed lineage kinase domain-like protein (MLKL) was identified to function downstream of receptor interacting protein 3 (RIP3) in tumor necrosis factor-α (TNF)-induced necrosis (also called necroptosis). However, how MLKL functions to mediate necroptosis is unknown. By reconstitution of MLKL function in MLKL-knockout cells, we showed that the N-terminus of MLKL is required for its function in necroptosis. The oligomerization of MLKL in TNF-treated cells is essential for necroptosis, as artificially forcing MLKL together by using the hormone-binding domain (HBD*) triggers necroptosis. Notably, forcing together the N-terminal domain (ND) but not the C-terminal kinase domain of MLKL causes necroptosis. Further deletion analysis showed that the four-α-helix bundle of MLKL (1-130 amino acids) is sufficient to trigger necroptosis. Both the HBD*-mediated and TNF-induced complexes of MLKL(ND) or MLKL are tetramers, and translocation of these complexes to lipid rafts of the plasma membrane precedes cell death. The homo-oligomerization is required for MLKL translocation and the signal sequence for plasma membrane location is located in the junction of the first and second α-helices of MLKL. The plasma membrane translocation of MLKL or MLKL(ND) leads to sodium influx, and depletion of sodium from the cell culture medium inhibits necroptosis. All of the above phenomena were not seen in apoptosis. Thus, the MLKL oligomerization leads to translocation of MLKL to lipid rafts of plasma membrane, and the plasma membrane MLKL complex acts either by itself or via other proteins to increase the sodium influx, which increases osmotic pressure, eventually leading to membrane rupture.