Alk1 Signaling in Vascular Development

Heterozygous loss of the endothelial-specific transforming growth factor-beta (TGF-β) Type 1 receptor, activin receptor-like kinase 1 (ALK1), results in the autosomal dominant disorder, hereditary hemorrhagic telangiectasia type 2 (HHT2), which is characterized by mucocutaneous telangiectasias as well as arteriovenous malformations (AVMs) in the brain, lungs, liver, gastrointestinal tract, and spinal cord. As a result, patients suffer from a range of clinical symptoms including epistaxis, hemorrhage, and stroke. Using zebrafish, our laboratory has demonstrated that AVMs form via a two-step mechanism involving an initial increase in endothelial cell number caused by lack of alk1, and then an adaptive response to increased blood flow in downstream vessels. This adaptive response involves increased arterial caliber and maintenance of normally transient connections between arteries and veins, thereby forming high-flow AVMs. Furthermore, we have demonstrated that alk1 expression is dependent on blood flow, and that lack of flow mimics loss of alk1, suggesting that Alk1 might act downstream of blood flow to stabilize arterial caliber. To date, the in vivo ligand and intracellular mediators required for flow-dependent, Alk1-mediated endothelial quiescence and AVM prevention remain unknown. In this work, I demonstrate that bone morphogenetic protein 10 (Bmp10) is the physiologically relevant Alk1 ligand during zebrafish embryonic development. Bmp10 paralogs are expressed exclusively in the heart, and loss of blood flow affects arterial pSmad1/5/9, cxcr4a, and edn1 expression similarly to loss of alk1, even when alk1 expression is restored via a flow-independent transgene. Together, these data suggest that flow is required not only for alk1 expression but also to deliver cardiac-derived Bmp10 ligand to arterial endothelial cell Alk1 to promote endothelial cell quiescence. Downstream of Bmp10/Alk1, Alk1 kinase activity is required to prevent AVMs. However characterization of a pSmad1/5-responsive transgenic reporter, Tg(BRE:EGFP), suggests that although phosphorylation of Smad1/5/9 in arterial endothelium is clearly dependent on Alk1, pSmad1/5/9 may not activate transcription via a canonical mechanism within these cells. In sum, the work presented in this thesis constructs a novel blood flow-responsive signaling pathway, suggests novel mechanisms by which Alk1 may control gene expression, and finally, describes a new tool for studying Alk1 and BMP signaling in vivo