NOD1 Activation Induces Cardiac Dysfunction and Modulates Cardiac Fibrosis and Cardiomyocyte Apoptosis

The innate immune system is responsible for the initial response of an organism to potentially harmful stressors, pathogens or tissue injury, and accordingly plays an essential role in the pathogenesis of many inflammatory processes, including some cardiovascular diseases. Toll like receptors (TLR) and nucleotide-binding oligomerization domain-like receptors (NLRs) are pattern recognition receptors that play an important role in the induction of innate immune and inflammatory responses. There is a line of evidence supporting that activation of TLRs contributes to the development and progression of cardiovascular diseases but less is known regarding the role of NLRs. Here we demonstrate the presence of the NLR member NOD1 (nucleotide-binding oligomerization domain containing 1) in the murine heart. Activation of NOD1 with the specific agonist C12-iEDAP, but not with the inactive analogue iE-Lys, induces a time- and dose-dependent cardiac dysfunction that occurs concomitantly with cardiac fibrosis and apoptosis. The administration of iEDAP promotes the activation of the NF-κB and TGF-β pathways and induces apoptosis in whole hearts. At the cellular level, both native cardiomyocytes and cardiac fibroblasts expressed NOD1. The NLR activation in cardiomyocytes was associated with NF-κB activation and induction of apoptosis. NOD1 stimulation in fibroblasts was linked to NF-κB activation and to increased expression of pro-fibrotic mediators. The down-regulation of NOD1 by specific siRNAs blunted the effect of iEDAP on the pro-fibrotic TGF-β pathway and cell apoptosis. In conclusion, our report uncovers a new pro-inflammatory target that is expressed in the heart, NOD1. The specific activation of this NLR induces cardiac dysfunction and modulates cardiac fibrosis and cardiomyocyte apoptosis, pathological processes involved in several cardiac diseases such as heart failure.

固有免疫系统（innate immune system）是机体针对潜在有害应激源、病原体或组织损伤产生初始应答的核心系统，因此在包括部分心血管疾病在内的多种炎症过程的发病机制中发挥关键作用。Toll样受体（Toll-like receptors, TLR）与核苷酸结合寡聚化结构域样受体（nucleotide-binding oligomerization domain-like receptors, NLRs）均为模式识别受体（pattern recognition receptors），在固有免疫与炎症应答的诱导过程中发挥重要作用。已有多项研究证据表明，TLR的活化参与心血管疾病的发生与进展，但目前对于NLRs在该过程中的作用仍知之甚少。本研究证实，NLR家族成员NOD1（核苷酸结合寡聚化结构域包含1，nucleotide-binding oligomerization domain containing 1）存在于小鼠心脏组织中。使用特异性激动剂C12-iEDAP活化NOD1，而非无活性类似物iE-Lys，可引发呈时间与剂量依赖性的心脏功能障碍，该过程伴随心脏纤维化与细胞凋亡的发生。给予iEDAP可促进NF-κB与转化生长因子-β（transforming growth factor-β, TGF-β）通路的活化，并诱导整体心脏组织发生细胞凋亡。在细胞层面，原代心肌细胞与心脏成纤维细胞均表达NOD1。心肌细胞中的NLR活化与NF-κB活化及细胞凋亡诱导相关；而成纤维细胞中的NOD1刺激则与NF-κB活化及促纤维化介质表达上调相关。通过特异性小干扰RNA（small interfering RNA, siRNA）下调NOD1的表达，可削弱iEDAP对促纤维化TGF-β通路及细胞凋亡的影响。综上，本研究揭示了心脏中表达的新型促炎性靶点NOD1。该NLR的特异性活化可诱导心脏功能障碍，并调控心脏纤维化与心肌细胞凋亡——这些病理过程参与包括心力衰竭在内的多种心脏疾病的发生发展。