TLR9-HIF-1α-VEGF Axis Is Essential for HMGB1-Mediate Post-Myocardial Infarction Tissue Repair

Background: Toll-like receptor 9 (TLR9) as an essential part of the innate immune system plays a vital role in regulating cardiomyocyte survival and wound healing. During hypoxia, High Mobility Group Box 1 (HMGB1) is rapidly released extracellularly and translocates from the nucleus to bind with cytoplasmic TLR9. However, the mechanism by which TLR9 interacts with HMGB1 and regulates myocardial damage remains unclear. Our current study investigated the effects of TLR9 and HMGB1 on experimental MI.   Methods: Experimental MI was induced by left anterior descending (LAD) ligation in wild type (WT) or TLR9 knockout (TLR9KO)mice, to delineate the functional effects of TLR9 on cardiac healing, apoptosis, angiogenesis. Left ventricular injection of recombination human HMGB1(rhHMGB1)was performed to explore the mechanism of TLR9 and HMGB1 in MI.   Findings: The survival rate of TLR9KO mice with a higher rate of cardiac rupture was significantly lower than that in WT mice after 28-days postoperation. The effect of TLR9 knockout on insufficient wound healing in experimental MI is caused by a diminished number of myofibroblasts and defective matrix synthetic capability. Moreover, the increased myocardial apoptotic cells and decreased angiogenic capacity were found in TLR9 knockout mice after MI. The results showed contrary in rhHMGB1 (Recombinant Human High Mobility Group Box 1) treated WT mice and similarity after applying rhHMGB1 in TLR9KO mice.   Interpretation: Our study demonstrates that TLR9 is essential for the repair of infarcted myocardium and interaction of HMGB1 and TLR9 is involved in the survival and wound healing of myocardial cells after myocardial infarction.    Funding Statement: This work was supported by grants from the National Natural Science Foundation of China (No: 81470402, 81800216, 81700218, 81770399 and 81700254), the Key Project of the National Natural Science Foundation (No. 81530012) and the Fundamental Research Funds for the Central Universities (No. 2042017kf0085, 2042018kf1032 and 2042015kf0073). Declaration of Interests: The authors declare: None. Ethics Approval Statement: All human heart samples are dedicated for research purposes only, approved by the Renmin Hospital of Wuhan University Review Board, according to the Helsinki Declaration. All animal care and experimental procedures were approved by the Animal Care and Use Committee of Renmin Hospital of Wuhan University (approval no. 2016005) in accordance with the guidelines for the Care and Use of Laboratory Animals published by the United States National Institutes of Health (NIH).