The cardiovascular system is essential for all aspects of tissue growth and physiology. Haematopoietic stem cells (HSCs) are derived from the embryonic artery during early development and give rise to all adult blood cell lineages throughout life. The shared origin of arterial endothelial cells (ECs) and HSCs means these cells share common mechanisms which regulate their formation. Our understanding of these mechanisms remain incomplete. In mammals, the transcription factors foxc1 and foxc2 are required for both cardiovascular and haematopoietic development, however, how these genes interact with endothelial signalling pathways to regulate these processes remains unclear.
We have generated novel zebrafish mutants in orthologues of mammalian foxc1 (foxc1a and foxc1b) to determine the function of these genes during blood vessel and HSC formation. foxc1a mutants display aberrant cranial blood vessel formation and defective blood-brain barrier formation with almost total loss of central arteries (CtAs). flt4 expression is drastically reduced within the primordial hindbrain channel (PHBC) of foxc1a mutants, indicating foxc1a may regulate the ability of ECs within the PHBC to receive VEGF. Arteriovenous differentiation is disrupted within the trunk of foxc1a mutants. While the dorsal aorta and posterior cardinal vein are present in foxc1a mutants, arterial gene expression is substantially reduced and venous expression is increased, indicating foxc1a is required for correct specification of arteries and veins. Segmental arteries (SeA) are correctly specified and sprout normally in foxc1a mutants, however foxc1a; foxc1b double mutants display ectopic secondary SeA branching indicating foxc1a and foxc1b genetically interact during angiogenesis. In keeping with observed reductions in arterial gene expression, foxc1a mutants exhibit reduced HSC numbers and HSC progeny. Interestingly, foxc1a mutants also display defective somite patterning as has been reported previously. Furthermore, somitic Wnt16 and Dlc/Dld signalling are known to instruct HSC formation non-cell-autonomously and wnt16 and dlc expression are reduced in foxc1a mutants. Collectively, this indicates foxc1a/b play compensatory and pleiotropic roles in co-ordinating both blood vessel and HSC formation and may influence these processes via distinct pathways.