Thymosin beta4 induces adult epicardial progenitor mobilization and neovascularization
Smart N, Risebro CA, Melville AAD, Moses K, Schwartz RJ, Chien KR, Riley PR
Nature (2007)
Thymosin β4 stimulated outgrowth from quiescent adult epicardial explants, restoring pluripotency and triggering differentiation into fibroblasts, smooth muscle cells, and endothelial cells — the second foundational Nature paper that drove the cardiac-repair clinical-development arc.
This 2007 Nature paper from Paul Riley's Oxford group sits beside Bock-Marquette 2004 as one of the two foundational mechanistic papers behind the entire thymosin β4 cardiac-repair clinical program. Where Bock-Marquette identified the integrin-linked-kinase / Akt pathway driving cardiomyocyte survival, Smart and colleagues showed that thymosin β4 acts on a different cellular target — the adult epicardium — and converts quiescent epicardial cells back into multipotent vascular progenitors.
The investigators first established that thymosin β4 is essential for all aspects of coronary vessel development in mice during embryogenesis. They then asked whether the same molecule could reawaken vascular progenitor capacity in adult hearts. In quiescent adult epicardial explants, thymosin β4 stimulated significant outgrowth, restored pluripotency, and triggered differentiation along three vascular lineages: fibroblasts, smooth muscle cells, and endothelial cells. Thymosin β4 knockdown in the heart produced a significant reduction in the pro-angiogenic cleavage product N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP). When AcSDKP alone was injected, it could not rescue the full thymosin β4 mutant phenotype, but it did significantly enhance endothelial cell differentiation from adult epicardially derived precursor cells — separating the molecule's two mechanistic arms.
The paper's enduring claim is that thymosin β4 and its AcSDKP cleavage product together act as potent stimulators of coronary vasculogenesis and angiogenesis, and that thymosin β4-induced adult epicardial cells represent a viable source of vascular progenitors for renewal of regressed vessels at low basal level or sustained neovascularization following cardiac injury.
This is a mouse mechanistic study — the translational claim that adult human epicardium can be similarly mobilized is an inference. The most ambitious framing in the early press coverage — that thymosin β4 could trigger epicardial-to-cardiomyocyte conversion in injured adult hearts — has been substantially contested in the subsequent decade. Zhou et al. (Circulation Research 2013) failed to demonstrate epicardial-to-cardiomyocyte conversion after MI in a more stringent lineage-tracing model, and the broader regeneration claim has been narrowed in follow-up work to the vascular-progenitor mobilization documented here, rather than full cardiomyocyte reprogramming.
The numerical effect sizes are not provided in the abstract; this entry should be supplemented with the full paper before any specific magnitude claim is downstream-cited. The clinical-development arc that followed (RegeneRx's RGN-352 cardiac program) did not progress to a successful Phase 3 readout in MI patients — the Phase 2 Ruff 2010 healthy-volunteer safety study was followed by a small MI Phase 2 trial that produced inconclusive efficacy signals.
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