Hapln1b organizes the ECM to modulate kit signaling and control developmental hematopoiesis in zebrafish

During early vertebrate development, hematopoietic stem cells (HSCs) are produced from a hemogenic endothelium located in the dorsal aorta, before they migrate to a transient niche where they expand, the fetal liver and the caudal hematopoietic tissue (CHT), in mammals and zebrafish, respectively. In zebrafish, previous studies have shown that the extracellular matrix (ECM) around the aorta needs to be degraded to allow HSCs to leave the aortic floor and reach blood circulation. However, the role of the ECM components in HSC specification has never been addressed. We show here that hapln1b, a key component of the ECM is specifically expressed in hematopoietic sites in the zebrafish embryo. Gain- and loss-of-function experiments all resulted in the absence of HSCs in the early embryo, showing that hapln1b is required, at the correct level, to specify HSCs in the hemogenic endothelium. Furthermore, we show that the expression of hapln1b is necessary to maintain the integrity of the ECM through its link domain. In addition, by combining functional analyses and computer modelling, we show that kitlgb interacts with the ECM, to specify HSCs. Overall, we have demonstrated that the ECM is an integral component of the microenvironment as it mediates specific cytokine signaling that is required for normal HSC specification.