No abstract

Phagocytosis of apoptotic cells is fundamentally important throughout life, because non-cleared cells become secondarily necrotic and release intracellular contents, thus instigating inflammatory and autoimmune responses. Secreted "find-me" and exposed "eat-me" signals displayed by the dying cell in concert with the phagocyte receptors comprise the phagocytic synapse of apoptotic cell clearance. In this scenario, lysophospholipids (lysoPLs) are assumed to act as find-me signals for the attraction of phagocytes. However, both the identity of the lysoPLs released from apoptotic cells and the nature of the phagocyte receptor are largely unknown. By a detailed analysis of the structural requirements we show here that lysophosphatidylcholine (lysoPC), but none of the lysoPC metabolites or other lysoPLs, represents the essential apoptotic attraction signal able to trigger a phagocyte chemotactic response. Furthermore, using RNA interference and expression studies, we demonstrate that the G-protein-coupled receptor G2A, unlike its relative GPR4, is involved in the chemotaxis of monocytic cells. Thus, our study identifies lysoPC and G2A as the crucial receptor/ligand system for the attraction of phagocytes to apoptotic cells and the prevention of autoimmunity.

Background-Mesenchymal stem cell (MSC)-based regenerative strategies were investigated to treat acute myocardial infarction and improve left ventricular function. Methods and Results-Murine AMI was induced by coronary ligation with subsequent injection of MSCs, hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), or MSCs ϩHGF/VEGF into the border zone. Left ventricular ejection fraction was calculated using micro-computed tomography imaging after 6 months. HGF and VEGF protein injection (with or without concomitant MSC injection) significantly and similarly improved the left ventricular ejection fraction and reduced scar size compared with the MSC group, suggesting that myocardial recovery was due to the cytokines rather than myocardial regeneration. To provide sustained paracrine effects, HGF or VEGF overexpressing MSCs were generated (MSC-HGF, MSC-VEGF). MSC-HGF and MSC-VEGF showed significantly increased in vitro proliferation and increased in vivo proliferation within the border zone. Cytokine production correlated with MSC survival. MSC-HGF-and MSC-VEGF-treated animals showed smaller scar sizes, increased peri-infarct vessel densities, and better preserved left ventricular function when compared with MSCs transfected with empty vector. Murine cardiomyocytes were exposed to hypoxic in vitro conditions. The LDH release was reduced, fewer cardiomyocytes were apoptotic, and Akt activity was increased if cardiomyocytes were maintained in conditioned medium obtained from MSC-HGF or MSC-VEGF cultures. Conclusions-This study showed that (1) elevating the tissue levels of HGF and VEGF after acute myocardial infarction seems to be a promising reparative therapeutic approach, (2) HGF and VEGF are cardioprotective by increasing the tolerance of cardiomyocytes to ischemia, reducing cardiomyocyte apoptosis and increasing prosurvival Akt activation, and (3) MSC-HGF and MSC-VEGF are a valuable source for increased cytokine production and maximize the beneficial effect of MSC-based repair strategies. However, the optimal MSC source (bone marrow versus adipose tissue), the ideal application method (intracoronary versus intramyocardial), and the underlying mechanisms are unclear. Although initially aimed at regeneration, MSC therapy may limit maladaptive remodeling and improve heart function mainly through paracrine mechanisms in the absence of regeneration. Due to the stressful procedure of cell harvest causing morphological changes with loss of spindle-shaped fibroblast-like shape and intercellular connections and shear stress during needle aspiration and cell injection, combined with a harsh milieu in the ischemic heart of the host, substantial cell losses are common early after cell transplantation. 3 Despite these significant losses, beneficial effects on cardiac remodeling have been consistently observed, andFrom the Departments of Cardiothoracic Surgery (T.D., W.S., R.C.R., S.S.) and Cardiovascular Medicine (C.P., J.C.W.), and Pediatrics (T.D.), Stanford University, Stanford, Calif; Libin Cardiova...