Heart surgery for both coronary artery bypass graft (CABG) and transplantation often involves cardiac ischemia/reperfusion (I/R), which leads to a switch of the myocardial energy source from fatty acid ?- oxidation to anaerobic glycolysis. As an adaptation, Glut-4, a major isoform of the glucose transporters in the heart, is recruited to the cardiomyocyte surface (also called sarcolemma) to take up glucose and stimulate cardiac ATP production. Nonetheless, such compensatory Glut4 translocation is not sufficient to meet cardiac glucose demands for ATP generation in I/R hearts and thereby, results in an energy crisis. Notably, recent multiple large clinical trials involving infusion of glucose-insulin-potassium (GIK) solution into patients undergoing cardiac surgery have not shown any positive results (or even worse). Therefore, exploring how to augment Glut-4 translocation and glucose utilization, independent of insulin, is desperately needed to counter I/R-triggered cardiac energy loss. We recently made the novel findings that the tumor susceptibility gene 101 (Tsg101), a central component of the ESCRT (endosomal sorting complexes required for transport) machinery, is able to regulate the endosomal recycling of membrane receptors in animal hearts. Our newest data further showed that: 1) Tsg101 binds directly to Glut-4 in adult mouse hearts; 2) forced expression of Tsg101 in cultured myocytes resulted in higher levels of sarcolemma Glut-4 and improved cell survival when challenged with hypoxia/reoxygenation; and 3) Tsg101 up-regulates the expression of Rab11a and FIP3 (Rab11-family interacting protein 3), two key factors involved in endosomal recycling. Most importantly, our pilot data also showed that a group of naturally-occurring nano-vesicles, exosomes, released by bone- marrow stem cells, can effectively deliver Tsg101 into cardiac myocytes. Based on these initial findings, we hypothesize that Tsg101 can reduce or prevent cardiac I/R-induced energy crisis/injury by promoting Rab11a/ FIP3-mediated endosomal recycling of Glut-4. Treatment of mouse hearts with Tsg101-containing exosomes before ischemia or during early reperfusion can elevate myocardial Tsg101, thereby limiting I/R-triggered cardiac damage. The work proposed here will address three specific aims: 1) Define the role of Tsg101 in glucose-dependent energy generation and cardio-protection from I/R injury, using both heart-specific Tsg101-overexpressing and inducible knockdown mouse models; 2) Identify whether Tsg101-induced cardio-protection is dependent on Rab11a/FIP3-mediated endosomal recycling of Glut-4; and 3) Investigate the therapeutic potential of Tsg101 to prevent/reduce I/R-induced cardiac energy stress and injury, using Tsg101-loaded exosomes. The proposed studies are expected to identify Tsg101 as a novel regulator of Glut-4 translocation and as a major cardio-protector against I/R-induced energy stress. If verified, the findings from this proposal should provide new and safe strategies to increase energy generation in I/R hearts and hopefully, minimize surgically induced cardiac I/R injury.
Cardiac surgery-caused ischemia-reperfusion injury remains a major source of post-operative morbidity and mortality. The proposed research is relevant to public health because the elucidation of Tsg101 and the incorporated exosomes in cardiac energy generation is ultimately expected to open new avenues and develop novel delivery tools for transferring those beneficial proteins into the heart, thereby limiting surgically induced cardiac injury. Thus, the proposed project is relevant to the part of NIH?s mission that pertains to advancing fundamental knowledge and translational study that will help to reduce the burdens of human disability.
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