Enhancing endothelial cell engraftment via transplantation of exogenous mitochondria

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PROJECT SUMMARY/ABSTRACT Ischemic diseases, including critical limb ischemia and myocardial infarction, afflict millions of people in the United States. Currently, these diseases are predominately treated by surgical interventions. However, the inability to regenerate microvascular beds in ischemic tissues remains a challenge. Alternatively, the development of therapies based on transplanting endothelial cells (ECs) continues to be a priority in vascular medicine. Unfortunately, engrafting ECs is not trivial. Studies have repeatedly shown that in order to achieve significant engraftment resulting in functional new blood vessels, ECs require co-transplantation with supporting cells such as mesenchymal stromal cells (MSCs). However, this paradigm is problematic because it increases the complexity of clinical trials exponentially. Previously, we have published extensively on all aspects of human EC+MSC engraftment. However, the underlying mechanisms by which MSCs facilitate EC engraftment remain incompletely understood. Recently, we found that upon implantation, MSCs transfer mitochondria to ECs via tunneling nanotubes and that when this transfer was genetically abrogated, EC engraftment was drastically impaired. Based on this insight, we propose a new concept: artificially transplanting mitochondria into human ECs as a means to preemptively enhance their ability to engraft without a secondary cell type. Indeed, our preliminary data show that transplanting exogenous mitochondria into ECs renders the cells (termed mitoT-ECs) capable of forming functional vessels in vivo in ischemic tissues, without the support of MSCs. We also found that transplanted mitochondria co-localized with LC3B-marked autophagosomes and that genetic ablation of PINK1 and Parkin (both central players in mitophagy) eliminated the enhanced engraftment ability of mitoT-ECs. Together, our overarching hypothesis is that transplanting exogenous mitochondria into ECs renders transient cytoprotection via mitophagy; this, in turn, enhances the engraftment ability of the cells. To test this hypothesis and to determine the efficacy of mitoT-ECs to treat ischemic diseases, we propose two specific aims. In Aim-1, we will determine conditions (e.g., concentration and timing) for optimal EC engraftment in immunodeficient mice. We will dissect the role of mitophagy and will examine the fate and persistence of the transplanted mitochondria. We will also determine if selective drugs with mitophagy-enhancing properties could also enhance EC engraftment. In Aim-2, we will determine the efficacy of mitochondrial transplantation-enabled EC therapy in two well-established models of ischemic diseases: critical hind limb ischemia (in mice) and myocardial ischemia/reperfusion injury (in rats). In summary, we propose studies to develop a novel approach to engraft ECs more successfully. We envision this research could become the basis for a new strategy in vascular cell therapies.