Poly(I:C) Priming Exacerbates Cecal Ligation and Puncture-Induced Polymicrobial Sepsis in Mice

Sepsis continues to be a major healthcare issue with one of the highest mortality rates in intensive care units. Toll-like receptors are pattern recognition receptors that are intricately involved in the pathogenesis of sepsis. TLR3 is a major receptor for double-stranded RNA and is largely associated with immunity to viral infection. In this study, we examined the role of TLR3 priming in the immunopathology of sepsis using cecal-ligation and puncture (CLP) model of sepsis in mice. Mice injected with vehicle or poly(I:C) were subjected to sham or CLP surgery and various parameters of sepsis, including mortality, inflammation, and bacterial clearance were assessed. Poly(I:C) pre-treatment significantly enhanced mortality in mice subjected to CLP. Consistent with this, inflammatory cytokines including TNFα, IL-12p40, IFNγ, and MCP-1 were enhanced both systemically and locally in the poly(I:C)-treated group compared to the vehicle control. In addition, bacterial load was significantly higher in the poly(I:C)-treated septic mice. These changes were associated with reduced macrophage activation (but not neutrophils) in the peritoneal cavity of poly(I:C) pre-treated mice compared to vehicle pre-treatment. Together our results demonstrate that poly(I:C) priming in sepsis is likely to be detrimental to the host due to effects on systemic inflammatory cytokines and bacterial clearance.

Epithelial Barrier Function in Gut-Bone Signaling

The intestinal epithelial barrier plays an essential role in maintaining host homeostasis. The barrier regulates nutrient absorption as well as prevents the invasion of pathogenic bacteria in the host. It is composed of epithelial cells, tight junctions, and a mucus layer. Several factors, such as cytokines, diet, and diseases, can affect this barrier. These factors have been shown to increase intestinal permeability, inflammation, and translocation of pathogenic bacteria. In addition, dysregulation of the epithelial barrier can result in inflammatory diseases such as inflammatory bowel disease. Our lab and others have also shown that barrier disruption can have systemic effects including bone loss. In this chapter, we will discuss the current literature to understand the link between intestinal barrier and bone. We will discuss how inflammation, aging, dysbiosis, and metabolic diseases can affect intestinal barrier-bone link. In addition, we will highlight the current suggested mechanism between intestinal barrier and bone.

G protein-coupled receptor kinase-2-deficient mice are protected from dextran sodium sulfate-induced acute colitis

G protein-coupled receptor kinase 2 (GRK2) is a serine/threonine kinase and plays a key role in different disease processes. Previously, we showed that GRK2 knockdown enhances wound healing in colonic epithelial cells. Therefore, we hypothesized that ablation of GRK2 would protect mice from dextran sodium sulfate (DSS)-induced acute colitis. To test this, we administered DSS to wild-type (GRK2^+/+) and GRK2 heterozygous (GRK^+/-) mice in their drinking water for 7 days. As predicted, GRK2^+/- mice were protected from colitis as demonstrated by decreased weight loss (20% loss in GRK2^+/+ vs. 11% loss in GRK2^+/-). lower disease activity index (GRK2^+/+ 9.1 vs GRK2^+/- 4.1), and increased colon lengths (GRK2^+/+ 4.7 cm vs GRK2^+/- 5.3 cm). To examine the mechanisms by which GRK2^+/- mice are protected from colitis, we investigated expression of inflammatory genes in the colon as well as immune cell profiles in colonic lamina propria, mesenteric lymph node, and in bone marrow. Our results did not reveal differences in immune cell profiles between the two genotypes. However, expression of inflammatory genes was significantly decreased in DSS-treated GRK2^+/- mice compared with GRK2^+/+. To understand the mechanisms, we generated myeloid-specific GRK2 knockout mice and subjected them to DSS-induced colitis. Similar to whole body GRK2 heterozygous knockout mice, myeloid-specific knockout of GRK2 was sufficient for the protection from DSS-induced colitis. Together our results indicate that deficiency of GRK2 protects mice from DSS-induced colitis and further suggests that the mechanism of this effect is likely via GRK2 regulation of inflammatory genes in the myeloid cells.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

Immunology of Gut-Bone Signaling

In recent years a link between the gastrointestinal tract and bone health has started to gain significant attention. Dysbiosis of the intestinal microbiota has been linked to the pathology of a number of diseases which are associated with bone loss. In addition modulation of the intestinal microbiota with probiotic bacteria has revealed to have both beneficial local and systemic effects. In the present chapter, we discuss the intestinal and bone immune systems, explore how intestinal disease affects the immune system, and examine how these pathologic changes could adversely impact bone health.

Paroxetine differentially modulates LPS-induced TNFα and IL-6 production in mouse macrophages

Paroxetine is a selective serotonin reuptake inhibitor (SSRI) that is clinically used for the treatment of depression in human patients. Because of recent reports on the role of serotonin in modulating inflammation and the link between inflammation and depression, we sought to test the effect of paroxetine directly on macrophage response to an inflammatory stimulus. Lipopolysaccharide (LPS) treatment of mouse macrophages significantly enhanced TNFα and IL-6 production. Paroxetine treatment of macrophages, however, significantly inhibited LPS-induced IL-6 production. In contrast, paroxetine enhanced LPS-induced TNFα production in macrophages. These effects of paroxetine were mimicked by fluoxetine, another SSRI. To determine if the effects of paroxetine are mediated via modulation of the 5-HT system, we treated macrophages with 5-HT or 5-HT receptor antagonist (LY215840) in the presence of LPS and/or paroxetine. 5-HT treatment by itself did not affect LPS-induced cytokine production. LY215840, however, reversed paroxetine's effect on LPS-induced TNFα production but not IL-6. To understand the signaling mechanisms, we examined paroxetine's effect on MAPK and NFκB pathways. While paroxetine inhibited LPS-induced IκBα phosphorylation, MAPK pathways were mostly unaffected. Together these data demonstrate that paroxetine has critical but differential effects on IL-6 and TNFα production in macrophages and that it likely regulates these cytokines via distinct mechanisms.