MyD88 mediated inflammatory signaling leads to CaMKII oxidation, cardiac hypertrophy and death after myocardial infarction

Abstract 

The toll-like receptors (TLR) and myocardial infarction (MI) promote NF-κB-dependent inflammatory transcription and oxidative injury in myocardium. The multifunctional Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is activated by oxidation and contributes to NF-κB-dependent transcription, myocardial hypertrophy and post-MI death. The myeloid differentiation protein 88 (MyD88) is an adapter protein critical for many TLR functions, but downstream targets for TLR/MyD88 signaling in MI are not well understood. We asked if CaMKII and TLR/MyD88 pathways are interconnected and if TLR/MyD88 contributes to adverse outcomes after MI. Here we show that TLR-4 activation by lipopolysaccharide (LPS) induces CaMKII oxidation (ox-CaMKII) in cardiomyocytes. MI enhances ox-CaMKII in wild type (WT) hearts but not in MyD88^−/− hearts that are defective in MyD88-dependent TLR signaling. In post-MI WT hearts expression of pro-inflammatory genes TNF-α (Tnfa), complement factor B (Cfb), myocyte death and fibrosis were significantly increased, but increases were significantly less in MyD88^−/− hearts after MI. MyD88^−/− cardiomyocytes were defective in NF-κB activation by LPS but not by the MyD88-independent TLR agonist poly(I:C). In contrast, TNF-α induced Cfb gene expression was not deficient in MyD88^−/− cardiomyocytes. Several hypertrophy marker genes were upregulated in both WT and MyD88^−/− hearts after MI, but Acta1 was significantly attenuated in MyD88^−/− hearts, suggesting that MyD88 selectively affects expression of hypertrophic genes. Post-MI cardiac hypertrophy, inflammation, apoptosis, ox-CaMKII expression and mortality were significantly reduced in MyD88^−/− compared to WT littermates. These data suggest that MyD88 contributes to CaMKII oxidation and is important for adverse hypertrophic and inflammatory responses to LPS and MI.

Keywords: Myocardiial infarction, Hypertrophy, Inflammation, Oxidant stress, CaMKII, Innate immunity