CCAM Seminar Series

Speaker: Dr. Timothy Duerr Ph.D., Assistant Professor, Department of Orthopaedic Surgery, UConn Health

Title: “Cellular and molecular mechanisms regulating proximodistal positional identity during axolotl limb regeneration”

Abstract: Axolotl salamanders can regenerate their limbs following amputation anywhere along the proximodistal (PD) axis. During this process, resident connective tissue cells, including dermal fibroblasts and periskeletal cells, dedifferentiate and accumulate at the limb stump to form a blastema. Blastema cells retain their genetically encoded PD positional identity, enabling them to redifferentiate and regenerate the appropriate PD limb structures. Retinoic acid (RA) signaling is essential for establishing PD positional identity, as elevated levels of RA instruct blastema cells to adopt a proximal limb identity. We found that RA concentration along the PD axis is regulated by Cyp26b1 expression, which degrades excess RA to generate distal identity during limb regeneration. These differences in RA concentration lead to differential expression of homeobox genes that provide blastema cells with the correct PD positional identity. One such gene identified was Shox, which we show is required for activation of the PTHrP/IHH feedback loop in regenerating chondrocytes of proximally amputated limbs. Once blastema cells genetically adopt positional identity via RA, they express PD-specific cell surface proteins that modify cellular adhesivity. This enables self-sorting with cells of similar positional identities. We have identified several cell surface proteins, including LPHN2, TENM4, and FLRT3 that follow the same spatiotemporal expression patterns as 5’ Hox genes and are both RA responsive and differentially expressed between proximally and distally amputated limbs. Collectively, our results support a model whereby RA directs blastema cells to establish PD identity through transcription factors that regulate the expression of positional identity-specific cell surface proteins.