Multilevel Regulation Of The Microtubule Plus-End Tracking Protein Clasp2 During Mouse And Zebrafish Hematopoietic Stem Cell Development.

During embryonic development, the first definitive hematopoietic stem cells (HSCs) are generated from hemogenic endothelial cells through an endothelial to hematopoietic transition (EHT) occurring in the main arteries of vertebrate embryos. HSCs then mature and expand during development prior to colonizing the adult hematopoietic niche. The complex and highly regulated process of HSC production is not fully understood yet. The journey of HSCs through their subsequent niches requires cell shape remodelling, attachment, division, migration and homing, all steps involving extensive cytoskeletal rearrangements. CLASP2 is a microtubule-associated protein that regulates microtubule dynamics by stabilizing the plus-end of microtubules. We have previously shown that Clasp2 knockout mice (Clasp2-/-) have an HSC defect in the bone marrow, resulting in severe pancytopenia. Our goal is now to investigate the role of CLASP2 in hematopoietic stem and progenitor cells (HSPCs) during embryonic development. We studied the role of CLASP2 on hematopoiesis by performing loss-of-function experiments in mouse and zebrafish embryos. Here, we show that CLASP2 ensures the proper dynamics of EHT and is involved in the maturation of HSC precursors into fully functional HSCs in the aorta. At later time points during embryogenesis, CLASP2 prevents the exhaustion of the blood cell pool by ensuring self-renewal of HSCs and by precluding their premature differentiation. Our data indicate that CLASP2 is involved in both, cellular fidelity during mitosis and protein formation at the post-translational level in the Golgi and/or their trafficking to the cell membrane, as exemplified for KIT. In conclusion, our data emphasize an earlier role of CLASP2 as an important and conserved regulator of HSC generation and expansion at multiple steps of hematopoiesis during mouse and zebrafish development.