PROJECT SUMMARY This project provides the applicant with Ph.D. training intersecting cell, developmental and regenerative biology. Zebrafish regenerate resected fins, including their bony ray skeletons, back to the original size irrespective of injury extent. Yet, how fins “know” when to stop growing as the correct size is attained is poorly understood. The applicant’s laboratory recently proposed a new model for robust fin size restoration based on initial amount of a skeletal geometry-defined “niche” population and its progressive depletion by a cell state transition. Further, the group found dramatic fin overgrowth in the classic zebrafish mutant longfint2 is caused by cis-ectopic expression of the kcnh2a K+ channel. Kcnh2a in longfint2 results in niche cell perdurance and therefore excessive outgrowth during regeneration. The applicant’s transplant experiments show ectopic kcnh2a in the mesenchyme lineage that generates niche cells is sufficient for overgrowth. Yet, how kcnh2a disrupts orderly niche depletion is unknown. Published studies show inhibiting the Ca2+-dependent phosphatase calcineurin also causes striking fin overgrowth. The applicant’s new epistasis experiments suggest Kcnh2a functions upstream of calcineurin. Their new transgenic Ca2+ reporter identifies dynamic Ca2+ levels in niche/mesenchyme cells that preliminary transcriptomics indicate depend on specifically expressed Ca2+ channels. This leads to the central hypothesis that Ca2+ channel activity and dynamic Ca2+ levels activate calcineurin signaling to promote niche-to-mesenchymal cell state transitions and end fin regrowth. The applicant will pursue three specific aims: 1) Determine how Kcnh2a impacts intracellular Ca2+ dynamics during fin regeneration, 2) Identify Ca2+ channels influencing regenerated fin size, and 3) Determine if calcineurin promotes niche-to-mesenchymal cell state transitions to slow fin outgrowth. In parallel with thesis research, the applicant will pursue extensive scientific communication training and activities, mentoring and teaching experiences, professional development workshops and networking events, and a diversity-supporting leadership role in the Women in Graduate Science organization.