PROJECT SUMMARY Cardiac fibrosis is the pathologic development of scar tissue around the muscle cells of the heart. It occurs with nearly every form of heart disease and rapidly progresses the heart to failure. A fundamental cellular determinant of fibrosis is the transition of fibroblasts into myofibroblasts, which are the primary producers of fibrotic matrix. Notably, myofibroblast revert to quiescence as organs including the heart recover from some injuries and fibrosis resolves. New evidence from liver and skin suggests these deactivated myofibroblasts are primed to reactivate with enhanced fibrotic responsiveness and wound healing capabilities as a protective adaptation to future injury. This has never been examined in the heart, but our preliminary data suggests cardiac fibrosis more than doubles when recovered hearts experience a second bout of pathologic stress. Given heart disease develops from cumulative rounds of stress and recovery, cardiac myofibroblasts likely undergo multiple cycles of deactivation and reactivation providing impetus for delineating these processes and determining how they are regulated. Our previous work suggests that p38 MAPK regulates the stability of the myofibroblast phenotype and the heart's fibrotic response. Moreover, recent findings suggest p38 inhibition appears to deactivate myofibroblasts reverting them to quiescence. This finding suggests that we can use experimental p38 perturbations of the myofibroblast state to study its impact on the heart's fibrotic response. Hence two comprehensive specific aims will be used to examine the overarching hypothesis that p38 regulation of myofibroblast state stability and the kinetics and fate trajectory of deactivation underlies long-term fibrotic responsiveness of the heart. Here single cell RNA sequencing in combination with myofibroblast lineage reporter mice engineered with targeted gain or loss of p38 function will be used to determine: (Aim 1) the role of p38 in regulating myofibroblast deactivation and resolving cardiac fibrosis, and (Aim 2) the effects of p38 activity on deactivated cardiac myofibroblast reactivation, fibrotic responsiveness, and fibrotic memory. Together these aims will attempt (A) to define the process and regulation of cardiac myofibroblast deactivation and reactivation, (B) to identify compensatory responses to perturbations in myofibroblast activity, and (C) to identify the function of deactivated myofibroblasts, their epigenetic memories of injury, and their role in enhancing the heart's fibrotic responsiveness.