Background-Promoting survival of transplanted cells or endogenous precursors is an important goal. We hypothesized that a novel approach to promote vascularization would be to create injectable microenvironments within the myocardium that recruit endothelial cells and promote their survival and organization. Methods and Results-In this study we demonstrate that self-assembling peptides can be injected and that the resulting nanofiber microenvironments are readily detectable within the myocardium. Furthermore, the self-assembling peptide nanofiber microenvironments recruit progenitor cells that express endothelial markers, as determined by staining with isolectin and for the endothelial-specific protein platelet-endothelial cell adhesion molecule-1. Vascular smooth muscle cells are recruited to the microenvironment and appear to form functional vascular structures. After the endothelial cell population, cells that express ␣-sarcomeric actin and the transcription factor Nkx2.5 infiltrate the peptide microenvironment. When exogenous donor green fluorescent protein-positive neonatal cardiomyocytes were injected with the self-assembling peptides, transplanted cardiomyocytes in the peptide microenvironment survived and also augmented endogenous cell recruitment. Conclusions-These experiments demonstrate that self-assembling peptides can create nanofiber microenvironments in the myocardium and that these microenvironments promote vascular cell recruitment. Because these peptide nanofibers may be modified in a variety of ways, this approach may enable injectable tissue regeneration strategies.

There is a great need for delivery strategies capable of efficiently localizing drugs to the damaged myocardium that do not require direct intramyocardial injection of therapeutic molecules. In the work discussed here, we exploited the myocardium-specific upregulation of matrix metalloproteinases (MMPs) that occurs during myocardium remodeling by designing a micellar vehicle containing an MMP-targeting peptide (MMP-TP). The binding of MMP-TP to MMP was evaluated with purified MMP-2 protein and U-937 cells induced to overexpress MMP. Inhibition of MMP-2 activity was not observed in the presence of unmodified micelles but was pronounced at a 5 mol % MMP-TP ligand density. In a FACS analysis, MMP-TP micelles containing 5 mol % of the MMP-targeting peptide showed ∼10-fold higher binding to activated U937 cells than plain micelles and micelles containing a control peptide with two amino acid replacements. MMP-TP-micelles and plain micelles were injected intravenously into C57BL/6 mice 1, 3, and 7 days after the induction of a myocardial infarction (MI). Immunohistochemistry performed on heart tissue sections revealed that MMP-TP-micelles colocalize with both MMP and infiltrating macrophages. MMP-TP micelles showed significantly enhanced accumulation to the necrotic area of the heart after MI on days 3 and 7 when compared to plain micelles and negative control peptide micelles. This is coincident with the measured temporal profile of MMP gene expression in the heart after MI. These results suggest that MMP-TP micelles are candidates for the development of targeted regenerative heart therapeutics because of their ability to target the infarcted myocardium in a MMP dependent manner.

License to fuse! A phosphorylated fusion peptide can mediate membrane fusion when the phosphates (green triangles, see scheme) are removed by phosphatases (blue spheres), delivering the contents of the liposome into the cytosol. This phosphatase‐triggered approach may be useful to create target‐specific lipid nanocarriers.