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Objective— As they travel through the blood stream, plasma lipoproteins interact continuously with endothelial cells (ECs). Although the focus of research has mostly been guided by the importance of lipoproteins as risk factors for atherosclerosis, thrombosis, and other cardiovascular diseases, little is known about the mechanisms linking lipoproteins and angiogenesis under physiological conditions, and particularly, during embryonic development. In this work, we performed global mRNA expression profiling of endothelial cells from hypo-, and hyperlipidemic zebrafish embryos with the goal of uncovering novel mediators of lipoprotein signaling in the endothelium. Approach and Results— Microarray analysis was conducted on fluorescence-activated cell sorting–isolated fli1:EGFP + ECs from normal, hypo-, and hyperlipidemic zebrafish embryos. We found that opposed levels of apoprotein B lipoproteins result in differential expression of the secreted enzyme autotaxin in ECs, which in turn affects EC sprouting and angiogenesis. We further demonstrate that the effects of autotaxin in vivo are mediated by lysophosphatidic acid (LPA)—a well-known autotaxin activity product—and that LPA and LPA receptors participate as well in the response of ECs to lipoprotein levels. Conclusions— Our findings provide the first in vivo gene expression profiling of ECs facing different levels of plasma apoprotein B lipoproteins and uncover a novel lipoprotein–autotaxin–LPA axis as regulator of EC behavior. These results highlight new roles for lipoproteins as signaling molecules, which are independent of their canonical function as cholesterol transporters.

Formation and remodeling of vascular beds are complex processes orchestrated by multiple signaling pathways. While it is well accepted that vessels of a particular organ display specific features that enable them to fulfill distinct functions, the embryonic origins of tissue-specific vessels, as well as the molecular mechanisms regulating their formation, are poorly understood. The subintestinal plexus of the zebrafish embryo comprises vessels that vascularize the gut, liver and pancreas, and as such represents an ideal model to investigate the early steps of organ-specific vessel formation. Here we show that both arterial and venous components of the subintestinal plexus originate from a pool of specialized angioblasts residing in the floor of the Posterior Cardinal Vein (PCV). Using live imaging of zebrafish embryos, in combination with photoconvertable transgenic reporters, we demonstrate that these angioblasts undergo two phases of migration and differentiation. Initially, a subintestinal vein (SIV) forms and expands ventrally through a bone morphogenetic protein (BMP)-dependent step of collective migration. Concomitantly, a VEGF-dependent shift in the directionality of migration, coupled to the upregulation of arterial markers is observed, which culminates with the generation of the supraintestinal artery (SIA). Altogether our results establish the zebrafish subintestinal plexus as an advantageous model for the study of organ-specific vessel development, and provide new insights into the molecular mechanisms controlling its formation. More broadly, our findings suggest that PCV-specialized angioblasts contribute not only to the formation of the early trunk vasculature, but also to the establishment of late forming-, tissue specific vascular beds.

How cells acquire their fate is a fundamental question in developmental and regenerative biology. Multipotent progenitors undergo cell-fate restriction in response to cues from the microenvironment, the nature of which is poorly understood. In the case of the lymphatic system, venous cells from the cardinal vein are thought to generate lymphatic vessels through trans-differentiation. Here we show that in zebrafish, lymphatic progenitors arise from a previously uncharacterized niche of specialized angioblasts within the cardinal vein, which also generates arterial and venous fates. We further identify Wnt5b as a novel lymphatic inductive signal and show that it also promotes the ‘angioblast-to-lymphatic’ transition in human embryonic stem cells, suggesting that this process is evolutionarily conserved. Our results uncover a novel mechanism of lymphatic specification, and provide the first characterization of the lymphatic inductive niche. More broadly, our findings highlight the cardinal vein as a heterogeneous structure, analogous to the haematopoietic niche in the aortic floor.