Contributions of Fli1a and Hox13 During Zebrafish Pectoral Fin Development and Implications for Ewing Sarcoma

Abstract

The zebrafish embryonic pectoral fin is comprised of the endoskeletal disc (ED) and the pectoral fin fold (PFF). Throughout larval development, the ED will give rise to the endoskeleton, consisting of proximal and distal radials, while the PFF will contribute to the formation of exoskeletal fin rays. The development of the pectoral fin is dependent on the coordinated activity of various transcription factors including Hox13 which are essential for patterning of the pectoral fin. Another transcription factor, Fli1a, is well-characterized in blood vessels, however, its role in the ED remains elusive. Using transgenic reporter lines, our laboratory has demonstrated a complementary pattern between fli1a and hox13 domains of expression, suggesting a regulatory relationship between these genes during pectoral fin development. Using time-course analysis, we discover that hox13-expressing cells in the pectoral fin emerge from fli1a-negative chondrocytes of the ED. Furthermore, we show that a brighter fli1a-expressing cell population gives rise to distal radials, while hox13-expressing cells localize with presumptive fin rays, concomitant with blood vessel remodeling in the PFF. Surprisingly, our analyses reveal that fli1a and hox13 domains of activity remain complementary throughout larval development. This pattern was also present during fin regeneration and recapitulated in the mouse autopod. In support of the regulatory relationship between fli1a and hox13, studies in Ewing sarcoma, an aggressive bone and soft tissue cancer, describe that FLI1 dysregulation is associated with an increase in HOXA13 and HOXD13 expression. To examine this regulation in zebrafish, we have induced ectopic fli1a expression in the hox13 domain. The mis-expression of fli1a was associated with an increase in hox13 expression during larval development and long-lasting aberrations in the morphology and migration of hox13- expressing cells, leading to defects in the adult pectoral fin. The ectopic fli1a anomalies also recapitulated some Ewing sarcoma phenotypes. Taken together, our results confirm the coordinated participation of fli1a and hox13 during pectoral fin development and provide important insights into fin fold cell migration, distal radial formation, fin ray morphogenesis, and blood vessel remodeling. Understanding the regulation between fli1a and hox13 could not only elucidate the development of the paired appendages but also shed light on the roles of FLI1 and HOX13 in the pathology of Ewing sarcoma.