p16.1 and p16.2 , new HSPC markers, play redundant roles in zebrafish T-cell lymphopoiesis

In the recent years, the zebrafish model has become a first-choice animal model in the field of hematopoiesis, due to extra-uterine development and optical transparency that allow an easy observation of early stages of hematopoietic stem and progenitor cells (HSPCs) development. Here, we characterized the function of two undescribed genes, si:ch211-214p16.1 and si:ch211-214p16.2 (shortened p16.1 and p16.2 ) that are expressed by HSPCs, as early as their emergence. By combining different strategies, we have demonstrated that p16.1 and p16.2 play redundant roles in embryonic thymopoiesis. Knocking-down both genes at the same time – or one gene in the mutant background for the other gene – impaired HSPCs commitment towards lymphoid fate and homing to the thymus. However double mutant embryos did not show any phenotype. As these double-mutant animals were viable, their thymocytes were compared to their wild-type counterparts by RNA sequencing, revealing that double mutant thymocytes compensated the lack of p16.1 and p16.2 by upregulating genes involved in chemotaxis and cell migration. One of these transcripts, ccr9b , was indeed validated as an important regulator of thymic homing in double mutant embryos, only. In summary, we found two new markers of embryonic HSPCs, functionally linked to lymphoid fate. The description of such markers will be important for our understanding of heterogeneity among emerging HSPCs in the embryo.