Intestinal Epithelial Sirtuin 1 Regulates Intestinal Inflammation During Aging in Mice by Altering the Intestinal Microbiota.

BACKGROUND & AIMS: Intestinal epithelial homeostasis is maintained by complex interactions among epithelial cells, commensal gut microorganisms, and immune cells. Disruption of this homeostasis is associated with disorders such as inflammatory bowel disease (IBD), but the mechanisms of this process are not clear. We investigated how Sirtuin 1 (SIRT1), a conserved mammalian NAD+-dependent protein deacetylase, senses environmental stress to alter intestinal integrity. METHODS: We performed studies of mice with disruption of Sirt1 specifically in the intestinal epithelium (SIRT1 iKO, villin-Cre+, Sirt1flox/flox mice) and control mice (villin-Cre-, Sirt1flox/flox) on a C57BL/6 background. Acute colitis was induced in some mice by addition of 2.5% dextran sodium sulfate to drinking water for 5-9 consecutive days. Some mice were given antibiotics via their drinking water for 4 weeks to deplete their microbiota. Some mice were fed with a cholestyramine-containing diet for 7 days to sequester their bile acids. Feces were collected and proportions of microbiota were analyzed by 16S rRNA amplicon sequencing and quantitative PCR. Intestines were collected from mice and gene expression profiles were compared by microarray and quantitative PCR analyses. We compared levels of specific mRNAs between colon tissues from age-matched patients with ulcerative colitis (n=10) vs without IBD (n=8, controls). RESULTS: Mice with intestinal deletion of SIRT1 (SIRT1 iKO) had abnormal activation of Paneth cells starting at the age of 5-8 months, with increased activation of NF-κB, stress pathways, and spontaneous inflammation at 22-24 months of age, compared with control mice. SIRT1 iKO mice also had altered fecal microbiota starting at 4-6 months of age compared with control mice, in part because of altered bile acid metabolism. Moreover, SIRT1 iKO mice with defective gut microbiota developed more severe colitis than control mice. Intestinal tissues from patients with ulcerative colitis expressed significantly lower levels of SIRT1 mRNA than controls. Intestinal tissues from SIRT1 iKO mice given antibiotics, however, did not have signs of inflammation at 22-24 months of age, and did not develop more severe colitis than control mice at 4-6 months. CONCLUSIONS: In analyses of intestinal tissues, colitis induction, and gut microbiota in mice with intestinal epithelial disruption of SIRT1, we found this protein to prevent intestinal inflammation by regulating the gut microbiota. SIRT1 might therefore be an important mediator of host-microbiome interactions. Agents designed to activate SIRT1 might be developed as treatments for IBDs.

HIF-2α-dependent induction of miR-29a restrains TH1 activity during T cell dependent colitis.

Metabolic imbalance leading to inflammatory hypoxia and stabilization of hypoxia-inducible transcription factors (HIFs) is a hallmark of inflammatory bowel diseases. We hypothesize that HIF could be stabilized in CD4+ T cells during intestinal inflammation and alter the functional responses of T cells via regulation of microRNAs. Our assays reveal markedly increased T cell-intrinsic hypoxia and stabilization of HIF protein during experimental colitis. microRNA screen in primary CD4+ T cells points us towards miR-29a and our subsequent studies identify a selective role for HIF-2α in CD4-cell-intrinsic induction of miR-29a during hypoxia. Mice with T cell-intrinsic HIF-2α deletion display elevated T-bet (target of miR-29a) levels and exacerbated intestinal inflammation. Mice with miR-29a deficiency in T cells show enhanced intestinal inflammation. T cell-intrinsic overexpression of HIF-2α or delivery of miR-29a mimetic dampen TH1-driven colitis. In this work, we show a previously unrecognized function for hypoxia-dependent induction of miR-29a in attenuating TH1-mediated inflammation.

CD73 is critical for the resolution of murine colonic inflammation.

CD73 is a glycosyl-phosphatidylinositol-(GPI-) linked membrane protein that catalyzes the extracellular dephosphorylation of adenosine monophosphate (AMP) to adenosine. Adenosine is a negative regulator of inflammation and prevents excessive cellular damage. We investigated the role of extracellular adenosine in the intestinal mucosa during the development of Dextran-Sulfate-Sodium-(DSS-)salt-induced colitis in mice that lack CD73 (CD73(-/-)) and are unable to synthesize extracellular adenosine. We have found that, compared to wild-type (WT) mice, CD73(-/-) mice are highly susceptible to DSS-induced colitis. CD73(-/-) mice exhibit pronounced weight loss, slower weight recovery, an increase in gut permeability, a decrease in expression of tight junctional adhesion molecules, as well as unresolved inflammation following the removal of DSS. Moreover, colonic epithelia in CD73(-/-) mice exhibited increased TLR9 expression, high levels of IL-1ß and TNF-α, and constitutive activation of NF-κB. We conclude that CD73 expression in the colon is critical for regulating the magnitude and the resolution of colonic immune responses.

SARS-CoV-2 Infection: Host Response, Immunity, and Therapeutic Targets.

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has resulted in a global pandemic with severe socioeconomic effects. Immunopathogenesis of COVID-19 leads to acute respiratory distress syndrome (ARDS) and organ failure. Binding of SARS-CoV-2 spike protein to human angiotensin-converting enzyme 2 (hACE2) on bronchiolar and alveolar epithelial cells triggers host inflammatory pathways that lead to pathophysiological changes. Proinflammatory cytokines and type I interferon (IFN) signaling in alveolar epithelial cells counter barrier disruption, modulate host innate immune response to induce chemotaxis, and initiate the resolution of inflammation. Here, we discuss experimental models to study SARS-CoV-2 infection, molecular pathways involved in SARS-CoV-2-induced inflammation, and viral hijacking of anti-inflammatory pathways, such as delayed type-I IFN response. Mechanisms of alveolar adaptation to hypoxia, adenosinergic signaling, and regulatory microRNAs are discussed as potential therapeutic targets for COVID-19.

A high G418-resistant neo(R) transgenic mouse and mouse embryonic fibroblast (MEF) feeder layers for cytotoxicity and gene targeting in vivo and in vitro.

Aminoglycoside antibiotics have been in use since 1944 with the discovery of streptomycin. The aim of this study was to derive a new, highly resistant multicopy neo(R) transgenic mouse strain, named TgN3Ems, by random insertion of the plasmid, pPGKneobpA, and compare the level of drug resistance of wild-type and transgenic mice in vivo and corresponding primary mouse embryonic fibroblasts (MEFs) in vitro to a model neomycin analog, G418. The expression neoR in transgenic animals caused a 5-fold increase in the approximate lethal dose of G418, compared to wild type. No adverse pathological changes were found for the transgenic mice treated with G418, as they all died within minutes after injection. In contrast, the G418 treatment of wild-type mice resulted in a marked liver and kidney toxicity detected microscopically and via increases of serum biomarkers for liver and kidney damage. In addition, there was a mild bone marrow and lymphoid depletion. In in vitro studies, the transgenic MEFs survived 20-fold higher G418 levels, compared to the wild-type MEF cells. Therefore, TgN3Ems transgenic mice could be used as a source of G418-resistant feeder cells for gene targeting. Since the expression of drug-resistance genes in transgenic animals confers resistance to toxicity, the TgN3Ems mice might serve as a tool applicable in drug design.

Myeloid cell sirtuin-1 expression does not alter host immune responses to Gram-negative endotoxemia or Gram-positive bacterial infection.

The role of sirtuin-1 (SIRT1) in innate immunity, and in particular the influence of SIRT1 on antimicrobial defense against infection, has yet to be reported but is important to define since SIRT1 inhibitors are being investigated as therapeutic agents in the treatment of cancer, Huntington's disease, and autoimmune diseases. Given the therapeutic potential of SIRT1 suppression, we sought to characterize the role of SIRT1 in host defense. Utilizing both pharmacologic methods and a genetic knockout, we demonstrate that SIRT1 expression has little influence on macrophage and neutrophil antimicrobial functions. Myeloid SIRT1 expression does not change mortality in gram-negative toxin-induced shock or gram-positive bacteremia, suggesting that therapeutic suppression of SIRT1 may be done safely without suppression of myeloid cell-specific immune responses to severe bacterial infections.

SIRT1 transgenic mice show phenotypes resembling calorie restriction.

We generated mice that overexpress the sirtuin, SIRT1. Transgenic mice have been generated by knocking in SIRT1 cDNA into the beta-actin locus. Mice that are hemizygous for this transgene express normal levels of beta-actin and higher levels of SIRT1 protein in several tissues. Transgenic mice display some phenotypes similar to mice on a calorie-restricted diet: they are leaner than littermate controls; are more metabolically active; display reductions in blood cholesterol, adipokines, insulin and fasted glucose; and are more glucose tolerant. Furthermore, transgenic mice perform better on a rotarod challenge and also show a delay in reproduction. Our findings suggest that increased expression of SIRT1 in mice elicits beneficial phenotypes that may be relevant to human health and longevity.

Semaphorin 7A is a negative regulator of T cell responses.

Semaphorins play an essential role in axonal guidance, and emerging evidence points to diverse functions of several Semaphorin family members in the immune system. Semaphorin 7A (Sema7A) promotes axonal growth in the central nervous system. Here, we show that Sema7A also plays a critical role in negative regulation of T cell activation and function. T cells deficient in Sema7A exhibit enhanced homeostatic and antigen-induced proliferative response. Moreover, autoreactive Sema7A-deficient T cells mediate aggressive autoimmune disease. The deficiency in Sema7A leads to defective TCR downmodulation and T cell hyperresponsiveness. These results demonstrate an important role of Sema7A in limiting autoimmune responses and add to growing evidence of shared signaling pathways used by the immune and nervous systems.