Innate Immune Nod1/RIP2 Signaling is Essential for Cardiac Hypertrophy, but Requires Mitochondrial Antiviral Signaling Protein (MAVS) 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-containing protein 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 (WT) mice to transverse aortic constriction (TAC) or sham operations. Cardiac hypertrophy, fibrosis, and cardiac function were examined in these mice. Results: Nod1 and RIP2 proteins were up-regulated in the heart after TAC, and this was paralleled by increased expression of mitochondrial proteins, including mitochondrial antiviral signaling protein (MAVS). Nod1-/- and RIP2-/- mice subjected to TAC exhibited better survival, improved cardiac function and decreased cardiac hypertrophy. Downstream signal transduction pathways that regulate inflammation and fibrosis including NF-κB and MAPK-GATA4/p300, were reduced in both Nod1-/- and RIP2-/- mice after TAC compared with WT mice. Co-immunoprecipitation 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 signalling 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 following 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.