Activation of an innate immune receptor, Nod1, accelerates atherogenesis in Apoe-/- mice

Sterile inflammation and the NLRP3 inflammasome in cardiometabolic disease

Inflammation plays an important role in the pathophysiology of cardiometabolic diseases. Sterile inflammation, a non-infectious and damage-associated molecular pattern (DAMP)-induced innate response, is now well-established to be closely associated with development and progression of cardiometabolic diseases. The NOD-like receptor (NLR) family pyrin domain-containing 3 (NLRP3) inflammasome is well-established as a major player in sterile inflammatory responses. It is a multimeric cytosolic protein complex which regulates the activation of caspase-1 and subsequently promotes cleavage and release of interleukin (IL)-1 family cytokines, which have a deleterious impact on the development of cardiometabolic diseases. Therefore, targeting NLRP3 itself or the downstream consequences of NLRP3 activation represent excellent potential therapeutic targets in inflammatory cardiometabolic diseases. Here, we review our current understanding of the role which NLRP3 inflammasome regulation plays in cardiometabolic diseases such as obesity, diabetes, non-alcoholic steatohepatitis (NASH), atherosclerosis, ischemic heart disease and cardiomyopathy. Finally, we highlight the potential of targeting NLPR3 or related signaling molecules as a therapeutic approach.

Innate Immunity System in Patients With Cardiovascular and Kidney Disease

With a global burden of 844 million, chronic kidney disease (CKD) is now considered a public health priority. Cardiovascular risk is pervasive in this population, and low-grade systemic inflammation is an established driver of adverse cardiovascular outcomes in these patients. Accelerated cellular senescence, gut microbiota-dependent immune activation, posttranslational lipoprotein modifications, neuroimmune interactions, osmotic and nonosmotic sodium accumulation, acute kidney injury, and precipitation of crystals in the kidney and the vascular system all concur in determining the unique severity of inflammation in CKD. Cohort studies documented a strong link between various biomarkers of inflammation and the risk of progression to kidney failure and cardiovascular events in patients with CKD. Interventions targeting diverse steps of the innate immune response may reduce the risk of cardiovascular and kidney disease. Among these, inhibition of IL-1β (interleukin-1 beta) signaling by canakinumab reduced the risk for cardiovascular events in patients with coronary heart disease, and this protection was equally strong in patients with and without CKD. Several old (colchicine) and new drugs targeting the innate immune system, like the IL-6 (interleukin 6) antagonist ziltivekimab, are being tested in large randomized clinical trials to thoroughly test the hypothesis that mitigating inflammation may translate into better cardiovascular and kidney outcomes in patients with CKD.

DBDPE upregulates NOD-like receptor signaling to induce NLRP3 inflammasome-mediated HAECs pyroptosis

Decabromodiphenyl ethane (DBDPE), a typical new brominated flame retardant (BFR), is a widespread new pollutant in the environment. Several studies and our previous studies have found that DBDPE can cause aortic endothelial injury and aortic endothelial cell pyroptosis, whereas the molecular mechanism involved has not been elucidated. In this study, we exposed human aortic endothelial cells (HAECs) to 25 μmol/L of DBDPE and analyzed the gene expression profiles by Affymetrix PrimeView™ Human Gene Expression Chip. The results showed that 886 genes were differentially expressed in the DBDPE exposure group. Enrichment analyses revealed that differentially expressed genes were mainly enriched in the inflammatory response and NOD-like receptor signal pathway. Gene-gene functional interaction analyses and crossover genes and pathways analyses found that the NOD-like receptor signal pathway may be involved in regulating NLRP3 and IL-18. We found that NOD2 cannot interact with NLRP3 directly through an immunoprecipitation experiment. Thus, we construct the RIPK2 knockdown HAECs cell line to repress the NOD-like receptor signaling and further study the mechanism of DBDPE-activated NLRP3 inflammasome to induce HAECs pyroptosis. The results showed that RIPK2 knockdown could repress DBDPE-induced NOD-like receptor signaling pathway upregulation, inhibit NLRP3 inflammasome activation, and decrease HAECs pyroptosis. In addition, RIPK2 knockdown decreased the ROS generation in HAECs induced by DBDPE. And NAC pretreated HAECs inhibited DBDPE-induced NLRP3 inflammasome activation and HAECs pyroptosis. These results demonstrated that DBDPE upregulated NOD-like receptor signaling to induce ROS generation and, in turn, activated NLRP3 inflammasome, leading to HAECs pyroptosis.

High-fat diet activates splenic NOD1 and enhances neutrophil recruitment and neutrophil extracellular traps release in the spleen of ApoE-deficient mice

In the course of atherogenesis, the spleen plays an important role in the regulation of extramedullary hematopoiesis, and in the control of circulating immune cells, which contributes to plaque progression. Here, we have investigated the role of splenic nucleotide-binding oligomerization domain 1 (NOD1) in the recruitment of circulating immune cells, as well as the involvement of this immune organ in extramedullary hematopoiesis in mice fed on a high-fat high-cholesterol diet (HFD). Under HFD conditions, the absence of NOD1 enhances the mobilization of immune cells, mainly neutrophils, from the bone marrow to the blood. To determine the effect of NOD1-dependent mobilization of immune cells under pro-atherogenic conditions, Apoe^−/− and Apoe^−/−Nod1^−/− mice fed on HFD for 4 weeks were used. Splenic NOD1 from Apoe^−/− mice was activated after feeding HFD as inferred by the phosphorylation of the NOD1 downstream targets RIPK2 and TAK1. Moreover, this activation was accompanied by the release of neutrophil extracellular traps (NETs), as determined by the increase in the expression of peptidyl arginine deiminase 4, and the identification of citrullinated histone H3 in this organ. This formation of NETs was significantly reduced in Apoe^−/−Nod1^−/− mice. Indeed, the presence of Ly6G⁺ cells and the lipidic content in the spleen of mice deficient in Apoe and Nod1 was reduced when compared to the Apoe^−/− counterparts, which suggests that the mobilization and activation of circulating immune cells are altered in the absence of NOD1. Furthermore, confirming previous studies, Apoe^−/−Nod1^−/− mice showed a reduced atherogenic disease, and diminished recruitment of neutrophils in the spleen, compared to Apoe^−/− mice. However, splenic artery ligation reduced the atherogenic burden in Apoe^−/− mice an effect that, unexpectedly was lost in Apoe^−/−Nod1^−/− mice. Together, these results suggest that neutrophil accumulation and activity in the spleen are driven in part by NOD1 activation in mice fed on HFD, contributing in this way to regulating atherogenic progression.

NOD1 splenic activation confers ferroptosis protection and reduces macrophage recruitment under pro-atherogenic conditions

The bioavailability and regulation of iron is essential for central biological functions in mammals. The role of this element in ferroptosis and the dysregulation of its metabolism contribute to diseases, ranging from anemia to infections, alterations in the immune system, inflammation and atherosclerosis. In this sense, monocytes and macrophages modulate iron metabolism and splenic function, while at the same time they can worsen the atherosclerotic process in pathological conditions. Since the nucleotide-binding oligomerization domain 1 (NOD1) has been linked to numerous disorders, including inflammatory and cardiovascular diseases, we investigated its role in iron homeostasis. The iron content was measured in various tissues of Apoe^-/- and Apoe^-/-Nod1^-/- mice fed a high-fat diet (HFD) for 4 weeks, under normal or reduced splenic function after ligation of the splenic artery. In the absence of NOD1 the iron levels decreased in spleen, heart and liver regardless the splenic function. This iron decrease was accompanied by an increase in the recruitment of F4/80⁺-macrophages in the spleen through a CXCR2-dependent signaling, as deduced by the reduced recruitment after administration of a CXCR2 inhibitor. CXCR2 mediates monocyte/macrophage chemotaxis to areas of inflammation and accumulation of leukocytes in the atherosclerotic plaque. Moreover, in the absence of NOD1, inhibition of CXCR2 enhanced atheroma progression. NOD1 activation increased the levels of GPX4 and other iron and ferroptosis regulatory proteins in macrophages. Our findings highlight the preeminent role of NOD1 in iron homeostasis and ferroptosis. These results suggest promising avenues of investigation for the diagnosis and treatment of iron-related diseases directed by NOD1.

The Intriguing Role of TLR Accessory Molecules in Cardiovascular Health and Disease

Activation of Toll like receptors (TLR) plays an important role in cardiovascular disease development, progression and outcomes. Complex TLR mediated signaling affects vascular and cardiac function including tissue remodeling and repair. Being central components of both innate and adaptive arms of the immune system, TLRs interact as pattern recognition receptors with a series of exogenous ligands and endogenous molecules or so-called danger associated molecular patterns (DAMPs) that are released upon tissue injury and cellular stress. Besides immune cells, a number of structural cells within the cardiovascular system, including endothelial cells, smooth muscle cells, fibroblasts and cardiac myocytes express TLRs and are able to release or sense DAMPs. Local activation of TLR-mediated signaling cascade induces cardiovascular tissue repair but in a presence of constant stimuli can overshoot and cause chronic inflammation and tissue damage. TLR accessory molecules are essential in guiding and dampening these responses toward an adequate reaction. Furthermore, accessory molecules assure specific and exclusive TLR-mediated signal transduction for distinct cells and pathways involved in the pathogenesis of cardiovascular diseases. Although much has been learned about TLRs activation in cardiovascular remodeling, the exact role of TLR accessory molecules is not entirely understood. Deeper understanding of the role of TLR accessory molecules in cardiovascular system may open therapeutic avenues aiming at manipulation of inflammatory response in cardiovascular disease. The present review outlines accessory molecules for membrane TLRs that are involved in cardiovascular disease progression. We first summarize the up-to-date knowledge on TLR signaling focusing on membrane TLRs and their ligands that play a key role in cardiovascular system. We then survey the current evidence of the contribution of TLRs accessory molecules in vascular and cardiac remodeling including myocardial infarction, heart failure, stroke, atherosclerosis, vein graft disease and arterio-venous fistula failure.

Ca2+ mishandling in heart failure: Potential targets

Ca²⁺ mishandling is a common feature in several cardiovascular diseases such as heart failure (HF). In many cases, impairment of key players in intracellular Ca²⁺ homeostasis has been identified as the underlying mechanism of cardiac dysfunction and cardiac arrhythmias associated with HF. In this review, we summarize primary novel findings related to Ca²⁺ mishandling in HF progression. HF research has increasingly focused on the identification of new targets and the contribution of their role in Ca²⁺ handling to the progression of the disease. Recent research studies have identified potential targets in three major emerging areas implicated in regulation of Ca²⁺ handling: the innate immune system, bone metabolism factors and post-translational modification of key proteins involved in regulation of Ca²⁺ handling. Here, we describe their possible contributions to the progression of HF.

NOD1-Targeted Immunonutrition Approaches: On the Way from Disease to Health

Immunonutrition appears as a field with great potential in modern medicine. Since the immune system can trigger serious pathophysiological disorders, it is essential to study and implement a type of nutrition aimed at improving immune system functioning and reinforcing it individually for each patient. In this sense, the nucleotide-binding oligomerization domain-1 (NOD1), one of the members of the pattern recognition receptors (PRRs) family of innate immunity, has been related to numerous pathologies, such as cancer, diabetes, or cardiovascular diseases. NOD1, which is activated by bacterial-derived peptidoglycans, is known to be present in immune cells and to contribute to inflammation and other important pathways, such as fibrosis, upon recognition of its ligands. Since immunonutrition is a significant developing research area with much to discover, we propose NOD1 as a possible target to consider in this field. It is relevant to understand the cellular and molecular mechanisms that modulate the immune system and involve the activation of NOD1 in the context of immunonutrition and associated pathological conditions. Surgical or pharmacological treatments could clearly benefit from the synergy with specific and personalized nutrition that even considers the health status of each subject.

Network analysis of the effects of long non-coding RNAs in artemisinin treatment of atherosclerosis in APOE-/- mice

Introduction

Atherosclerosis has become a worldwide medical burden. Our previous studies have shown that artemisinin (ART) had the capability to reduce atherosclerosis. Emerging evidence indicates that long non-coding RNAs (lncRNAs) are involved in the development of atherosclerosis. However, whether lncRNAs might participate in the mechanism through which artemisinin mitigates atherosclerosis has not been reported.

Material and methods

Eight-week-old apolipoprotein E deficient (APOE-/-) mice were divided into two groups, one of which was treated with artemisinin. Red oil O staining was used to measure the sizes of the atherosclerotic lesions. We conducted deep sequencing to investigate lncRNA profiles in the aorta tissue in high-fat diet fed APOE knockdown mice with and without artemisinin treatment. CeRNA network, Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analyses were performed through bioinformatics analysis. RT-PCR was used to validate the differentially expressed lncRNAs.

Results

A total of 102 lncRNAs and 4,630 mRNAs were differentially expressed (p < 0.05) between the artemisinin treatment group and atherosclerosis model group. KEGG and GO analyses indicated that the categories metabolic process, specific amino acid degradation and PI3K-Akt signaling pathway are involved in the effects of artemisinin treatment in atherosclerosis (q < 0.05). LncRNA ENSMUST00000099676.4, ENSMUST00000143673.1, ENSMUST00000070085.5 and ENSMUST00000224554 might be engaged in the treatment mechanism through which artemisinin alleviates atherosclerosis.

Conclusions

These findings indicated the possible mechanism and therapeutic role of lncRNAs in artemisinin treatment of atherosclerosis and provided a theoretical basis for the future application of artemisinin in patients with atherosclerosis.

[Changes in the intestinal microbiota as a risk factor for dyslipidemia, atherosclerosis and the role of probiotics in their prevention]

The review presents an analysis of studies on the role of the intestinal microbiota and microbiome in lipid metabolism and the development of dyslipidemia, atherosclerosis and cardiovascular diseases. The role of the intestine as a metabolic organ with a multifactorial strain evolution, involved in lipid metabolism, cholesterol homeostasis and enterohepatic circulation is shown. The influence of microbial imbalance on the development of dyslipidemia and atherosclerosis is considered. Special attention is paid to preventive therapy with hypolipidemic probiotics. It is shown that the use of probiotics with hypolipidemic properties and consisting of a mixture of such strains asLactobacillus plantarumCECT7527, CET7528 and CECT7529, mixtures ofLactobacillus acidophilusLa-5,Bifidobacterium lactisBB-12,Bifidobacterium animalis lactisBB-12 contribute to reducing the level of LDL-C, CCS, TG, are safe and well tolerated, can be used as an adjuvant non-drug therapy in combination with hypolipidemic drugs for dyslipidemia, multifocal atherosclerosis.

Deletion or Inhibition of NOD1 Favors Plaque Stability and Attenuates Atherothrombosis in Advanced Atherogenesis †

Atherothrombosis, the main cause of acute coronary syndromes (ACS), is characterized by the rupture of the atherosclerotic plaque followed by the formation of thrombi. Fatal plaque rupture sites show large necrotic cores combined with high levels of inflammation and thin layers of collagen. Plaque necrosis due to the death of macrophages and smooth muscle cells (SMCs) remains critical in the process. To determine the contribution of the innate immunity receptor NOD1 to the stability of atherosclerotic plaque, Apoe^-/- and Apoe^-/- Nod1^-/- atherosclerosis prone mice were placed on a high-fat diet for 16 weeks to assess post-mortem advanced atherosclerosis in the aortic sinus. The proliferation and apoptosis activity were analyzed, as well as the foam cell formation capacity in these lesions and in primary cultures of macrophages and vascular SMCs obtained from both groups of mice. Our results reinforce the preeminent role for NOD1 in human atherosclerosis. Advanced plaque analysis in the Apoe^-/- atherosclerosis model suggests that NOD1 deficiency may decrease the risk of atherothrombosis by decreasing leukocyte infiltration and reducing macrophage apoptosis. Furthermore, Nod1^-/- SMCs exhibit higher proliferation rates and decreased apoptotic activity, contributing to thicker fibrous caps with reduced content of pro-thrombotic collagen. These findings demonstrate a direct link between NOD1 and plaque vulnerability through effects on both macrophages and SMCs, suggesting promising insights for early detection of biomarkers for treating patients before ACS occurs.

Innate Immune Receptors, Key Actors in Cardiovascular Diseases

Cardiovascular diseases (CVDs) are the leading cause of death in the industrialized world. Most CVDs are associated with increased inflammation that arises mainly from innate immune system activation related to cardiac damage. Sustained activation of the innate immune system frequently results in maladaptive inflammatory responses that promote cardiovascular dysfunction and remodeling. Much research has focused on determining whether some mediators of the innate immune system are potential targets for CVD therapy. The innate immune system has specific receptors-termed pattern recognition receptors (PRRs)-that not only recognize pathogen-associated molecular patterns, but also sense danger-associated molecular signals. Activation of PRRs triggers the inflammatory response in different physiological systems, including the cardiovascular system. The classic PRRs, toll-like receptors (TLRs), and the more recently discovered nucleotide-binding oligomerization domain-like receptors (NLRs), have been recently proposed as key partners in the progression of several CVDs (e.g., atherosclerosis and heart failure). The present review discusses the key findings related to the involvement of TLRs and NLRs in the progression of several vascular and cardiac diseases, with a focus on whether some NLR subtypes (nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain-containing receptor 3 and nucleotide-binding oligomerization domain-containing protein 1) can be candidates for the development of new therapeutic strategies for several CVDs.

Deficiency of Nucleotide-binding oligomerization domain-containing proteins (NOD) 1 and 2 reduces atherosclerosis

Atherosclerosis is crucially fueled by inflammatory pathways including pattern recognition receptor (PRR)-related signaling of the innate immune system. Currently, the impact of the cytoplasmic PRRs nucleotide-binding oligomerization domain-containing protein (NOD) 1 and 2 is incompletely characterized. We, therefore, generated Nod1/Nod2 double knockout mice on a low-density lipoprotein receptor (Ldlr)-deficient background (= Ldlr^−/−Nod1/2^−/−) which were subsequently analyzed regarding experimental atherosclerosis, lipid metabolism, insulin resistance and gut microbiota composition. Compared to Ldlr^−/− mice, Ldlr^−/−Nod1/2^−/− mice showed reduced plasma lipids and increased hepatic expression of the scavenger receptor LDL receptor-related protein 1 after feeding a high-fat diet for 12 weeks. Furthermore, intestinal cholesterol and its bacterial degradation product coprostanol were elevated in Ldlr^−/−Nod1/2^−/− mice, correlating with the increased abundance of Eubacterium coprostanoligenes as assessed by 3rd generation sequencing of the gut microbiota. Atherosclerotic plaques of Ldlr^−/−Nod1/2^−/− mice exhibited less lipid deposition and macrophage accumulation. Moreover, macrophages from Ldlr^−/−Nod1/2^−/− mice showed higher expression of the cholesterol efflux transporters Abca1 and Abcg1 and accordingly reduced foam cell formation. Deficiency of Nod1 and Nod2 led to reduced plaque lipid deposition and inflammatory cell infiltration in atherosclerotic plaques. This might be explained by diminished plasma lipid levels and foam cell formation due to altered expression of key regulators of the hepatic cholesterol pathway as well as differential intestinal cholesterol metabolism and microbiota composition.

Renoprotective Effects of a Novel Receptor-Interacting Protein Kinase 2 Inhibitor, AS3334034, in Uninephrectomized Adriamycin-Induced Chronic Kidney Disease Rats

Renal inflammation is a final common pathway of chronic kidney disease (CKD), and its progression can be used to effectively gauge the degree of renal dysfunction. Inflammatory mechanisms contribute to glomerulosclerosis and tubulointerstitial fibrosis, which are hallmarks of CKD leading to end-stage renal disease. Receptor-interacting protein kinase 2 (RIP2) is largely committed to nucleotide-binding oligomerization domain signaling as a direct effector and transmits nuclear factor-κB (NF-κB)-mediated proinflammatory cytokine production. In the present study, we hypothesized that if inflammation via RIP2 and NF-κB signaling plays an important role in renal failure, then the anti-inflammatory effect of RIP2 inhibitors should be effective in improving CKD. To determine its pharmacologic potency, we investigated the renoprotective properties of the novel RIP2 inhibitor AS3334034 [7-methoxy-6-(2-methylpropane-2-sulfonyl)-N-(4-methyl-1H-pyrazol-3-yl)quinolin-4-amine] in uninephrectomized adriamycin-induced CKD rats. Six weeks' repeated administration of AS3334034 (10 mg/kg, once daily) significantly reduced urinary protein excretion and prevented the development of glomerulosclerosis and tubulointerstitial fibrosis. In addition, AS3334034 showed beneficial effects on renal function, as demonstrated by a decrease in levels of plasma creatinine and blood urea nitrogen and attenuation of a decline in creatinine clearance. Furthermore, AS3334034 significantly attenuated inflammation, renal apoptosis, and glomerular podocyte loss. These results suggest that the RIP2 inhibitor AS3334034 suppresses the progression of chronic renal failure via an anti-inflammatory effect and is therefore potentially useful in treating patients with CKD. SIGNIFICANCE STATEMENT: The receptor-interacting protein kinase 2 (RIP2) inhibitor AS3334034 suppresses the progression of chronic renal failure via an anti-inflammatory effect, suggesting that the nucleotide-binding oligomerization domain-RIP2 axis might play a crucial role in the pathogenesis of inflammatory kidney diseases. AS3334034 is expected to be potentially useful in the treatment of patients with chronic kidney disease.

Relationship between non-alcoholic fatty liver disease and cardiovascular disease

With the in-depth study of non-alcoholic fatty liver disease (NAFLD), it has been found in recent years that NAFLD is closely related to cardiovascular disease (CVD). It has been proved that NAFLD is not only an important risk factor for CVD, but it is also an important mechanism of atherosclerosis, coronary heart disease, and hypertension in young people. This article reviews the recent progress in the understanding of the relationship between NAFLD and CVD, with an aim to improve the knowledge of CVD physicians on liver disease and provide reference for prevention and treatment of these conditions.

Pattern Recognition Receptor-Mediated Chronic Inflammation in the Development and Progression of Obesity-Related Metabolic Diseases

Obesity-induced chronic inflammation is known to promote the development of many metabolic diseases, especially insulin resistance, type 2 diabetes mellitus, nonalcoholic fatty liver disease, and atherosclerosis. Pattern recognition receptor-mediated inflammation is an important determinant for the initiation and progression of these metabolic diseases. Here, we review the major features of the current understanding with respect to obesity-related chronic inflammation in metabolic tissues, focus on Toll-like receptors and nucleotide-binding oligomerization domain-like receptors with an emphasis on how these receptors determine metabolic disease progression, and provide a summary on the development and progress of PRR antagonists for therapeutic intervention.

Endothelial NOD1 directs myeloid cell recruitment in atherosclerosis through VCAM-1

Atherosclerosis is a chronic disease characterized by vascular lipid retention and inflammation, and pattern recognition receptors (PRRs) are important contributors in early stages of the disease. Given the implication of the intracellular PRR nucleotide-binding oligomerization domain 1 (NOD1) in cardiovascular diseases, we investigated its contribution to early atherosclerosis. We evidenced NOD1 induction in atherosclerotic human and mouse tissues, predominantly in vascular endothelial cells. Accordingly, NOD1 genetic inactivation in Apoe^-/- mice reduced not only atherosclerosis burden, but also monocyte and neutrophil accumulation in atheromata. Of note, in the presence of either peptidoglycan or oxidized LDLs, endothelial NOD1 triggered VCAM-1 up-regulation through the RIP2-NF-κB axis in an autocrine manner, enhancing firm adhesion of both sets of myeloid cells to the inflamed micro- and macrovasculature in vivo. Our data define a major proatherogenic role for endothelial NOD1 in early leukocyte recruitment to the athero-prone vasculature, thus introducing NOD1 as an innovative therapeutic target and potential prognostic molecule.-González-Ramos, S., Paz-García, M., Rius, C., del Monte-Monge, A., Rodríguez, C., Fernández-García, V., Andrés, V., Martínez-González, J., Lasunción, M. A., Martín-Sanz, P., Soehnlein, O., Boscá, L. Endothelial NOD1 directs myeloid cell recruitment in atherosclerosis through VCAM-1.

Innate Immune Signaling and Its Role in Metabolic and Cardiovascular Diseases

The innate immune system is an evolutionarily conserved system that senses and defends against infection and irritation. Innate immune signaling is a complex cascade that quickly recognizes infectious threats through multiple germline-encoded cell surface or cytoplasmic receptors and transmits signals for the deployment of proper countermeasures through adaptors, kinases, and transcription factors, resulting in the production of cytokines. As the first response of the innate immune system to pathogenic signals, inflammatory responses must be rapid and specific to establish a physical barrier against the spread of infection and must subsequently be terminated once the pathogens have been cleared. Long-lasting and low-grade chronic inflammation is a distinguishing feature of type 2 diabetes and cardiovascular diseases, which are currently major public health problems. Cardiometabolic stress-induced inflammatory responses activate innate immune signaling, which directly contributes to the development of cardiometabolic diseases. Additionally, although the innate immune elements are highly conserved in higher-order jawed vertebrates, lower-grade jawless vertebrates lack several transcription factors and inflammatory cytokine genes downstream of the Toll-like receptors (TLRs) and retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) pathways, suggesting that innate immune signaling components may additionally function in an immune-independent way. Notably, recent studies from our group and others have revealed that innate immune signaling can function as a vital regulator of cardiometabolic homeostasis independent of its immune function. Therefore, further investigation of innate immune signaling in cardiometabolic systems may facilitate the discovery of new strategies to manage the initiation and progression of cardiometabolic disorders, leading to better treatments for these diseases. In this review, we summarize the current progress in innate immune signaling studies and the regulatory function of innate immunity in cardiometabolic diseases. Notably, we highlight the immune-independent effects of innate immune signaling components on the development of cardiometabolic disorders.

The Role of Gut Microbiota in Atherosclerosis and Hypertension

In recent years, accumulating evidence has indicated the importance of gut microbiota in maintaining human health. Gut dysbiosis is associated with the pathogenesis of a number of metabolic diseases including obesity, type 2 diabetes mellitus (T2DM), non-alcoholic fatty liver disease (NAFLD), and cardiovascular diseases (CVDs). Indeed, CVD has become the leading cause of death worldwide, especially in developed countries. In this review, we mainly discuss the gut microbiota-involved mechanisms of CVD focusing on atherosclerosis and hypertension, two major risk factors for serious CVD. Then, we briefly discuss the prospects of gut microbiota-targeted therapeutic strategies for the treatment of CVD in the future.

Innate Immune Signaling in Nonalcoholic Fatty Liver Disease and Cardiovascular Diseases

The physiological significance of innate immune signaling lies primarily in its role in host defense against invading pathogens. It is becoming increasingly clear that innate immune signaling also modulates the development of metabolic diseases, especially nonalcoholic fatty liver disease and cardiovascular diseases, which are characterized by chronic, low-grade inflammation due to a disarrangement of innate immune signaling. Notably, recent studies indicate that in addition to regulating canonical innate immune-mediated inflammatory responses (or immune-dependent signaling-induced responses), molecules of the innate immune system regulate pathophysiological responses in multiple organs during metabolic disturbances (termed immune-independent signaling-induced responses), including the disruption of metabolic homeostasis, tissue repair, and cell survival. In addition, emerging evidence from the study of immunometabolism indicates that the systemic metabolic status may have profound effects on cellular immune function and phenotypes through the alteration of cell-intrinsic metabolism. We summarize how the innate immune system interacts with metabolic disturbances to trigger immune-dependent and immune-independent pathogenesis in the context of nonalcoholic fatty liver disease, as representative of metabolic diseases, and cardiovascular diseases.

Linkage of Infection to Adverse Systemic Complications: Periodontal Disease, Toll-Like Receptors, and Other Pattern Recognition Systems

Toll-like receptors (TLRs) are a group of pattern recognition receptors (PRRs) that provide innate immune sensing of conserved pathogen-associated molecular patterns (PAMPs) to engage early immune recognition of bacteria, viruses, and protozoa. Furthermore, TLRs provide a conduit for initiation of non-infectious inflammation following the sensing of danger-associated molecular patterns (DAMPs) generated as a consequence of cellular injury. Due to their essential role as DAMP and PAMP sensors, TLR signaling also contributes importantly to several systemic diseases including cardiovascular disease, diabetes, and others. The overlapping participation of TLRs in the control of infection, and pathogenesis of systemic diseases, has served as a starting point for research delving into the poorly defined area of infection leading to increased risk of various systemic diseases. Although conflicting studies exist, cardiovascular disease, diabetes, cancer, rheumatoid arthritis, and obesity/metabolic dysfunction have been associated with differing degrees of strength to infectious diseases. Here we will discuss elements of these connections focusing on the contributions of TLR signaling as a consequence of bacterial exposure in the context of the oral infections leading to periodontal disease, and associations with metabolic diseases including atherosclerosis and type 2 diabetes.

Microbial modulation of cardiovascular disease

Although diet has long been known to contribute to the pathogenesis of cardiovascular disease (CVD), research over the past decade has revealed an unexpected interplay between nutrient intake, gut microbial metabolism and the host to modify the risk of developing CVD. Microbial-associated molecular patterns are sensed by host pattern recognition receptors and have been suggested to drive CVD pathogenesis. In addition, the host microbiota produces various metabolites, such as trimethylamine-N-oxide, short-chain fatty acids and secondary bile acids, that affect CVD pathogenesis. These recent advances support the notion that targeting the interactions between the host and microorganisms may hold promise for the prevention or treatment of CVD. In this Review, we summarize our current knowledge of the gut microbial mechanisms that drive CVD, with special emphasis on therapeutic interventions, and we highlight the need to establish causal links between microbial pathways and CVD pathogenesis.

Deficiency of NOD1 Improves the β-Adrenergic Modulation of Ca2+ Handling in a Mouse Model of Heart Failure

Heart failure (HF) is a complex syndrome characterized by cardiac dysfunction, Ca²⁺ mishandling, and chronic activation of the innate immune system. Reduced cardiac output in HF leads to compensatory mechanisms via activation of the adrenergic nervous system. In turn, chronic adrenergic overstimulation induces pro-arrhythmic events, increasing the rate of sudden death in failing patients. Nucleotide-binding oligomerization domain-containing protein 1 (NOD1) is an innate immune modulator that plays a key role in HF progression. NOD1 deficiency in mice prevents Ca²⁺ mishandling in HF under basal conditions, but its role during β-adrenergic stimulation remains unknown. Here, we evaluated whether NOD1 regulates the β-adrenergic modulation of Ca²⁺ signaling in HF. Ca²⁺ dynamics were examined before and after isoproterenol perfusion in cardiomyocytes isolated from healthy and from post-myocardial infarction (PMI) wild-type (WT) and Nod1^-/- mice. Isoproterenol administration induced similar effects on intracellular [Ca²⁺]_i transients, cell contraction, and sarcoplasmic reticulum (SR)-Ca²⁺ load in healthy WT and Nod1^-/- cells. However, compared with WT-PMI cells, isoproterenol exposure induced a significant increase in the [Ca²⁺]_i transients and cell contraction parameters in Nod1^-/--PMI cells, which mainly due to an increase in SR-Ca²⁺ load. NOD1 deficiency also prevented the increase in diastolic Ca²⁺ leak (Ca²⁺ waves) induced by isoproterenol in PMI cells. mRNA levels of β1 and β2 adrenergic receptors were significantly higher in Nod1^-/--PMI hearts vs WT-PMI hearts. Healthy cardiomyocytes pre-treated with the selective agonist of NOD1, iE-DAP, and perfused with isoproterenol showed diminished [Ca²⁺]_i transients amplitude, cell contraction, and SR-Ca²⁺ load compared with vehicle-treated cells. iE-DAP-treated cells also presented increased diastolic Ca²⁺ leak under β-adrenergic stimulation. The selectivity of iE-DAP on Ca²⁺ handling was validated by pre-treatment with the inactive analog of NOD1, iE-Lys. Overall, our data establish that NOD1 deficiency improves the β-adrenergic modulation of Ca²⁺ handling in failing hearts.

NOD1 and NOD2: Molecular targets in prevention and treatment of infectious diseases

Nucleotide-binding oligomerization domain (NOD) 1 and NOD2 are pattern-recognition receptors responsible for sensing fragments of bacterial peptidoglycan known as muropeptides. Stimulation of innate immunity by systemic or local administration of NOD1 and NOD2 agonists is an attractive means to prevent and treat infectious diseases. In this review, we discuss novel data concerning structural features of selective and non-selective (dual) NOD1 and NOD2 agonists, main signaling pathways and biological effects induced by NOD1 and NOD2 stimulation, including induction of pro-inflammatory cytokines, type I interferons and antimicrobial peptides, induction of autophagy, alterations of metabolism. We also discuss interactions between NOD1/NOD2 and Toll-like receptor agonists in terms of synergy and cross-tolerance. Finally, we review available animal data on the role of NOD1 and NOD2 in protection against infections, and discuss how these data could be applied in human infectious diseases.

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.

Role of NOD1 in Heart Failure Progression via Regulation of Ca2+ Handling

BACKGROUND

Heart failure (HF) is a complex syndrome associated with a maladaptive innate immune system response that leads to deleterious cardiac remodeling. However, the underlying mechanisms of this syndrome are poorly understood. Nucleotide-binding oligomerization domain-containing protein 1 (NOD1) is a newly recognized innate immune sensor involved in cardiovascular diseases.

OBJECTIVES

This study evaluated the role of NOD1 in HF progression.

METHODS

NOD1 was examined in human failing myocardium and in a post-myocardial infarction (PMI) HF model evaluated in wild-type (wt-PMI) and Nod1^-/- mice (Nod1^-/--PMI).

RESULTS

The NOD1 pathway was up-regulated in human and murine failing myocardia. Compared with wt-PMI, hearts from Nod1^-/--PMI mice had better cardiac function and attenuated structural remodeling. Ameliorated cardiac function in Nod1^-/--PMI mice was associated with prevention of Ca²⁺ dynamic impairment linked to HF, including smaller and longer intracellular Ca²⁺ concentration transients and a lesser sarcoplasmic reticulum Ca²⁺ load due to a down-regulation of the sarcoplasmic reticulum Ca²⁺-adenosine triphosphatase pump and by augmented levels of the Na⁺/Ca²⁺ exchanger. Increased diastolic Ca²⁺ release in wt-PMI cardiomyocytes was related to hyperphosphorylation of ryanodine receptors, which was blunted in Nod1^-/--PMI cardiomyocytes. Pharmacological blockade of NOD1 also prevented Ca²⁺ mishandling in wt-PMI mice. Nod1^-/--PMI mice showed significantly fewer ventricular arrhythmias and lower mortality after isoproterenol administration. These effects were associated with lower aberrant systolic Ca²⁺ release and with a prevention of the hyperphosphorylation of ryanodine receptors under isoproterenol administration in Nod1^-/--PMI mice.

CONCLUSIONS

NOD1 modulated intracellular Ca²⁺ mishandling in HF, emerging as a new target for HF therapy.

Are the innate and adaptive immune systems setting hypertension on fire?

Hypertension is the most common chronic cardiovascular disease and is associated with several pathological states, being an important cause of morbidity and mortality around the world. Low-grade inflammation plays a key role in hypertension and the innate and adaptive immune systems seem to contribute to hypertension development and maintenance. Hypertension is associated with vascular inflammation, increased vascular cytokines levels and infiltration of immune cells in the vasculature, kidneys and heart. However, the mechanisms that trigger inflammation and immune system activation in hypertension are completely unknown. Cells from the innate immune system express pattern recognition receptors (PRR), which detect conserved pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) that induce innate effector mechanisms to produce endogenous signals, such as inflammatory cytokines and chemokines, to alert the host about danger. Additionally, antigen-presenting cells (APC) act as sentinels that are activated by PAMPs and DAMPs to sense the presence of the antigen/neoantigen, which ensues the adaptive immune system activation. In this context, different lymphocyte types are activated and contribute to inflammation and end-organ damage in hypertension. This review will focus on experimental and clinical evidence demonstrating the contribution of the innate and adaptive immune systems to the development of hypertension.

Rare Variants in NOD1 Associated with Carotid Bifurcation Intima-Media Thickness in Dominican Republic Families

Cardiovascular disorders including ischemic stroke (IS) and myocardial infarction (MI) are heritable; however, few replicated loci have been identified. One strategy to identify loci influencing these complex disorders is to study subclinical phenotypes, such as carotid bifurcation intima-media thickness (bIMT). We have previously shown bIMT to be heritable and found evidence for linkage and association with common variants on chromosome 7p for bIMT. In this study, we aimed to characterize contributions of rare variants (RVs) in 7p to bIMT. To achieve this aim, we sequenced the 1 LOD unit down region on 7p in nine extended families from the Dominican Republic (DR) with strong evidence for linkage to bIMT. We then performed the family-based sequence kernel association test (famSKAT) on genes within the 7p region. Analyses were restricted to single nucleotide variants (SNVs) with population based minor allele frequency (MAF) <5%. We first analyzed all exonic RVs and then the subset of only non-synonymous RVs. There were 68 genes in our analyses. Nucleotide-binding oligomerization domain (NOD1) was the most significantly associated gene when analyzing exonic RVs (famSKAT p = 9.2x10^-4; number of SNVs = 14). We achieved suggestive replication of NOD1 in an independent sample of twelve extended families from the DR (p = 0.055). Our study provides suggestive statistical evidence for a role of rare variants in NOD1 in bIMT. Studies in mice have shown Nod1 to play a role in heart function and atherosclerosis, providing biologic plausibility for a role in bIMT thus making NOD1 an excellent bIMT candidate.

Understanding the regulation of pattern recognition receptors in inflammatory diseases - a 'Nod' in the right direction

Nucleotide-binding oligomerization domain (NOD) -like receptors (NLRs) are a family of 23 receptors known as pattern recognition receptors; they are expressed in many cell types and play a key role in the innate immune response. The NLRs are activated by pathogen-associated molecular patterns, which include structurally conserved molecules present on the surfaces of bacteria. The activation of these NLRs by pathogens results in the downstream activation of signalling kinases and transcription factors, culminating in the transcription of genes coding for pro-inflammatory factors. Expression of NLR is altered in many cellular, physiological and disease states. There is a lack of understanding of the mechanisms by which NLR expression is regulated, particularly in chronic inflammatory states. Genetic polymorphisms and protein interactions are included in such mechanisms. This review seeks to examine the current knowledge regarding the regulation of this family of receptors and their signalling pathways as well as how their expression changes in disease states with particular focus on NOD1 and NOD2 in inflammatory bowel diseases among others.

Kawasaki disease: a matter of innate immunity

Kawasaki disease (KD) is an acute systemic vasculitis of childhood that does not have a known cause or aetiology. The epidemiological features (existence of epidemics, community outbreaks and seasonality), unique age distribution and clinical symptoms and signs of KD suggest that the disease is caused by one or more infectious environmental triggers. However, KD is not transmitted person-to-person and does not occur in clusters within households, schools or nurseries. KD is a self-limited illness that is not associated with the production of autoantibodies or the deposition of immune complexes, and it rarely recurs. Regarding the underlying pathophysiology of KD, innate immune activity (the inflammasome) is believed to play a role in the development of KD vasculitis, based on the results of studies with animal models and the clinical and laboratory findings of KD patients. Animal studies have demonstrated that innate immune pathogen-associated molecular patterns (PAMPs) can cause vasculitis independently of acquired immunity and have provided valuable insights regarding the underlying mechanisms of this phenomenon. To validate this concept, we recently searched for KD-specific PAMPs and identified such molecules with high specificity and sensitivity. These molecules have structures similar to those of microbe-associated molecular patterns (MAMPs), as shown by liquid chromatography-tandem mass spectrometry. We propose herein that KD is an innate immune disorder resulting from the exposure of a genetically predisposed individual to microbe-derived innate immune stimulants and that it is not a typical infectious disease.

Activation of Nod1 Signaling Induces Fetal Growth Restriction and Death through Fetal and Maternal Vasculopathy

Intrauterine fetal growth restriction (IUGR) and death (IUFD) are both serious problems in the perinatal medicine. Fetal vasculopathy is currently considered to account for a pathogenic mechanism of IUGR and IUFD. We previously demonstrated that an innate immune receptor, the nucleotide-binding oligomerization domain-1 (Nod1), contributed to the development of vascular inflammations in mice at postnatal stages. However, little is known about the deleterious effects of activated Nod1 signaling on embryonic growth and development. We report that administration of FK565, one of the Nod1 ligands, to pregnant C57BL/6 mice induced IUGR and IUFD. Mass spectrometry analysis revealed that maternally injected FK565 was distributed to the fetal tissues across placenta. In addition, maternal injection of FK565 induced robust increases in the amounts of CCL2, IL-6, and TNF proteins as well as NO in maternal, placental and fetal tissues. Nod1 was highly expressed in fetal vascular tissues, where significantly higher levels of CCL2 and IL-6 mRNAs were induced with maternal injection of FK565 than those in other tissues. Using Nod1-knockout mice, we verified that both maternal and fetal tissues were involved in the development of IUGR and IUFD. Furthermore, FK565 induced upregulation of genes associated with immune response, inflammation, and apoptosis in fetal vascular tissues. Our data thus provided new evidence for the pathogenic role of Nod1 in the development of IUGR and IUFD at the maternal-fetal interface.

Role of Nucleotide-binding and Oligomerization Domain 2 Protein (NOD2) in the Development of Atherosclerosis

NOD2 (nucleotide-binding and oligomerization domain 2) was initially reported as a susceptibility gene for Crohn's disease, with several studies focused on elucidating its molecular mechanism in the progression of Crohn's disease. We now know that NOD2 is an intracellular bacterial sensing receptor, and that MDP-mediated NOD2 activation drives inflammatory signaling. Various mutations in NOD2 have been reported, with NOD2 loss of function being associated with the development of Crohn's disease and other autoimmune diseases. These results suggest that NOD2 not only has an immune stimulatory function, but also an immune regulatory function. Atherosclerosis is a chronic inflammatory disease of the arterial wall; its pathologic progression is highly dependent on the immune balance. This immune balance is regulated by infiltrating monocytes and macrophages, both of which express NOD2. These findings indicate a potential role of NOD2 in atherosclerosis. The purpose of this review is to outline the known roles of NOD2 signaling in the pathogenesis of atherosclerosis.

Identification of Pathogenic Cardiac CD11c+ Macrophages in Nod1-Mediated Acute Coronary Arteritis

OBJECTIVE

Nod1 is an intracellular pattern recognition receptor for bacterial peptidoglycan fragments. We previously reported that a synthetic Nod1 ligand, FK565, induced acute coronary arteritis in mice similar to that of Kawasaki disease. However, the molecular mechanisms underlying this characteristic inflammation have remained elusive.

APPROACH AND RESULTS

We found that CD11c(+)MHC class II(+) cells accumulated in the heart of FK565-treated mice before arteritis development. Morphological features and gene expression signatures of the cardiac CD11c(+)MHC class II(+) cells suggested that this population is closely related to macrophages, and thus, we designated them cardiac CD11c(+) macrophages. Nod1 in nonhematopoietic cells, rather than hematopoietic cells, was required for the increase of cardiac CD11c(+) macrophages and arteritis development. Among nonhematopoietic cells, cardiac endothelial cells produced a large amount of chemokines in response to FK565. Endothelial cell-specific blockade of Nod1 signaling suppressed FK565-induced expression of these chemokines, accumulation of cardiac CD11c(+) macrophages, and subsequent coronary arteritis development. We also found that CCR2(+)Ly6C(hi) inflammatory monocytes in peripheral blood supplied precursors of cardiac CD11c(+) macrophages. CCR2-deficient mice or pertussis toxin-treated mice exhibited decreased numbers of cardiac CD11c(+) macrophages and reduced arteritis.

CONCLUSIONS

These results suggest that Ly6C(hi) monocytes are recruited to FK565-activated endothelial cells to generate cardiac CD11c(+) macrophages, which play a pivotal role in the pathogenesis of acute coronary arteritis.