Innate Immune Nod1/RIP2 Signaling Is Essential for Cardiac Hypertrophy but Requires Mitochondrial Antiviral Signaling Protein for Signal Transductions and Energy Balance

BACKGROUND

Cardiac hypertrophy is a key biological response to injurious stresses such as pressure overload and, when excessive, can lead to heart failure. Innate immune activation by danger signals, through intracellular pattern recognition receptors such as nucleotide-binding oligomerization domain 1 (Nod1) and its adaptor receptor-interacting protein 2 (RIP2), might play a major role in cardiac remodeling and progression to heart failure. We hypothesize that Nod1/RIP2 are major contributors to cardiac hypertrophy, but may not be sufficient to fully express the phenotype alone.

METHODS

To elucidate the contribution of Nod1/RIP2 signaling to cardiac hypertrophy, we randomized Nod1^-/-, RIP2^-/-, or wild-type mice to transverse aortic constriction or sham operations. Cardiac hypertrophy, fibrosis, and cardiac function were examined in these mice.

RESULTS

Nod1 and RIP2 proteins were upregulated in the heart after transverse aortic constriction, and this was paralleled by increased expression of mitochondrial proteins, including mitochondrial antiviral signaling protein (MAVS). Nod1^-/- and RIP2^-/- mice subjected to transverse aortic constriction exhibited better survival, improved cardiac function, and decreased cardiac hypertrophy. Downstream signal transduction pathways that regulate inflammation and fibrosis, including NF (nuclear factor) κB and MAPK (mitogen-activated protein kinase)-GATA4/p300, were reduced in both Nod1^-/- and RIP2^-/- mice after transverse aortic constriction compared with wild-type mice. Coimmunoprecipitation of extracted cardiac proteins and confocal immunofluorescence microscopy showed that Nod1/RIP2 interaction was robust and that this complex also included MAVS as an essential component. Suppression of MAVS expression attenuated the complex formation, NF κB signaling, and myocyte hypertrophy. Interrogation of mitochondrial function compared in the presence or ablation of MAVS revealed that MAVS serves to suppress mitochondrial energy output and mediate fission/fusion related dynamic changes. The latter is possibly linked to mitophagy during cardiomyocytes stress, which may provide an intriguing link between innate immune activation and mitochondrial energy balance under stress or injury conditions.

CONCLUSIONS

We have identified that innate immune Nod1/RIP2 signaling is a major contributor to cardiac remodeling after stress. This process is critically joined by and regulated through the mitochondrial danger signal adapter MAVS. This novel complex coordinates remodeling, inflammatory response, and mitochondrial energy metabolism in stressed cardiomyocytes. Thus, Nod1/RIP2/MAVS signaling complex may represent an attractive new therapeutic approach toward heart failure.

Characterization of a murine model with arteritis induced by Nod1 ligand, FK565: A comparative study with a CAWS-induced model

OBJECTIVE

Kawasaki disease (KD) occurs via activation of the innate immune system. Nucleotide oligomerization domain-1 (NOD1) is a pattern recognition receptor regulating the innate immunity. We characterized histopathology of arteritis induced by FK565, a ligand for NOD1, in mice, compared with Candida albicans water-soluble fraction (CAWS)-induced model.

METHODS

Vasculitis was induced by injection of FK565 or CAWS into C57BL6/J mice (n = 9 and n = 11, respectively). At 4 weeks, they were sacrificed, and plasma cytokines and chemokines were measured.

RESULTS

FK565 injection induced vasculitis mainly involving bilateral coronary arteries whereas the aortic root was diffusely affected in CAWS mice. In FK565 animals, the abdominal aorta and its branching arteries also exhibited inflammation with atherosclerosis. IL-1α, IL-1β, IL-5 and RANTES were increased in FK565 group whereas IL-6, IL-13, G-CSF, IFN-γ, and TNF-α were higher in CAWS animals (p < .05 for all variables). The total area of inflammation in FK565 mice appeared to correlate with IL-1β levels (r = 0.71, p = .05).

CONCLUSIONS

Histopathology of FK565-induced model demonstrated 'site-specific' coronary arteritis mimicking KD. This histopathological difference from CAWS model may be due to different cytokine expression profiles.

A high-sensitivity bi-directional reporter to monitor NF-κB activity in cell culture and zebrafish in real time

Nuclear factor (NF)-κB transcription factors play major roles in numerous biological processes including development and immunity. Here, we engineered a novel bi-directional NF-κB-responsive reporter, pSGNluc, in which a high-affinity NF-κB promoter fragment simultaneously drives expression of luciferase and GFP. Treatment with TNFα (also known as TNF) induced a strong, dose-dependent luciferase signal in cell culture. The degree of induction over background was comparable to that of other NF-κB-driven luciferase reporters, but the absolute level of expression was at least 20-fold higher. This extends the sensitivity range of otherwise difficult assays mediated exclusively by endogenously expressed receptors, as we show for Nod1 signaling in HEK293 cells. To measure NF-κB activity in the living organism, we established a transgenic zebrafish line carrying the pSGNluc construct. Live in toto imaging of transgenic embryos revealed the activation patterns of NF-κB signaling during embryonic development and as responses to inflammatory stimuli. Taken together, by integrating qualitative and quantitative NF-κB reporter activity, pSGNluc is a valuable tool for studying NF-κB signaling at high spatiotemporal resolution in cultured cells and living animals that goes beyond the possibilities provided by currently available reporters.

Nucleotide Binding Oligomerization Domain 1 Is an Essential Signal Transducer in Human Epithelial Cells Infected with Helicobacter pylori That Induces the Transepithelial Migration of Neutrophils

Background/Aims

The cytosolic host protein nucleotide binding oligomerization domain 1 (Nod1) has emerged as a key pathogen recognition molecule for innate immune responses in epithelial cells. The purpose of the study was to elucidate the mechanism by which Helicobacter pylori infection leads to transepithelial neutrophil migration in a Nod1-mediated manner.

Methods

Human epithelial cell lines AGS and Caco-2 were grown and infected with H. pylori. Interleukin (IL)-8 mRNA expression and IL-8 secretion were assessed, and nuclear factor κB (NF-κB) activation was determined. Stable transfections of AGS and Caco-2 cells with dominant negative Nod1 were generated. Neutrophil migration across the monolayer was quantified.

Results

Cytotoxin-associated gene pathogenicity island (cagPAI)(+) H. pylori infection upregulated IL-8 mRNA expression and IL-8 secretion in AGS and Caco-2 cells compared with controls. NF-κB activation, IL-8 mRNA expression and IL-8 secretion by cagPAI knockdown strains were reduced compared with those infected with the wild-type strain. NF-κB activation, IL-8 mRNA expression and IL-8 secretion in dominant-negative (DN)-Nod1 stably transfected cells were reduced compared with the controls. The transepithelial migration of neutrophils in DN-Nod1 stably transfected cells was reduced compared with that in controls.

Conclusions

Signaling through Nod1 plays an essential role in neutrophil migration induced by the upregulated NF-κB activation and IL-8 expression in H. pylori-infected human epithelial cells.

Expeditious synthesis of enantiopure, orthogonally protected bis-α-amino acids (OPBAAs) and their use in a study of Nod1 stimulation

A convenient approach towards the synthesis of orthogonally protected chiral bis-α-amino acids (OPBAAs) is described. The key transformations include: (1) a highly stereoselective conjugation (alkylation) of the Schöllkopf bis-lactim ethers and oxazolidinyl alkyl halides to build a backbone skeleton; and (2) our orthogonal protection strategy. A series of enantiopure OPBAAs bearing a variety of alkyl chain as a spacer; two stereogenic centers; and three protecting groups were prepared as examples. These versatile molecules were applied to the synthesis of biologically interesting di- or tri-peptide analogues, including chiral iE-meso-DAP and A-iE-meso-DAP, for the study of Nod1 activation in the innate immune response.

Synthesis of conformationally constrained γ-D-glutamyl-meso-diaminopimelic acid derivatives as ligands of nucleotide-binding oligomerization domain protein 1 (Nod1)

Nod1, an important member of the pattern recognition receptor family, remains a virtually unexploited target. Harnessing its innate immune stimulatory properties still remains an unfulfilled goal of medicinal chemistry. Nucleotide-binding oligomerization domain protein 1 (Nod1) agonists have been shown to boost the inflammatory responses against pathogenic microbes and could thus constitute a new class of broad spectrum antimicrobial agents. To gain additional insight into the structure/activity relationships of Nod1 agonistic compounds, a series of novel, conformationally constrained γ-D-glutamyl-meso-diaminopimelic acid (iE-DAP) analogs have been designed and synthesized. Ramos-Blue cells expressing Nod1 were used to screen and validate our compounds for their Nod1-agonist activity. Their immunomodulatory properties were subsequently determined in vitro, by evaluating their capacity to induce pro-inflammatory cytokine and chemokine production from human peripheral blood mononuclear cells (PBMC), by themselves and in synergy with lipopolysaccharide (LPS), a Toll-like receptor 4 (TLR4) ligand. The synthesized iE-DAP analogs were shown to possess immuno-enhancing properties as a result of their potent and specific Nod1-agonistic effect. The activity of the compound exhibiting the greatest capacity to induce pro-inflammatory cytokine release from PBMC surpassed that of lauroyl-γ-D-glutamyl-meso-diaminopimelic acid (C12-iE-DAP).