Claudin-dependent clonal expansion couples epicardial activation to regenerative fate acquisition

Epicardial activation is a conserved response to injury across vertebrates, yet only in regenerative species do epicardium-derived cells (EPDCs) acquire the capacity to invade the injury site and contribute to cardiac regeneration. How regenerative competence is established in epicardial cells downstream of activation remains poorly understood. Using multicolour lineage tracing we find that epicardial cells of the salamander species Pleurodeles waltl undergo a transient phase of clonal expansion that precedes EPDC accumulation at the injury site. Depletion of cCpE-sensitive Claudins from otherwise intact tight junctions alters the organisation and the mechanical state of the epicardium, impairs clonal expansion and reduces the capacity of epicardial cells to delaminate. Consequently, EPDCs fail to activate a regenerative transcriptional programme and complete their transition into injury-induced migratory EPDCs that are required to sculpt a Tenascin-C-rich matrix supporting cardiomyocyte protrusion at the injury site. Together, our findings demonstrate that epicardial activation alone is insufficient for regeneration and identify Claudin-dependent clonal expansion as a mechanism that couples epicardial activation to the acquisition of a regenerative EPDC state.