Abstract
Activin receptor-like kinase 1 (ALK1, encoded by the gene ACVRL1) is a type I BMP/TGF-β receptor that mediates signalling in endothelial cells via phosphorylation of SMAD1/5/8. During angiogenesis, sprouting endothelial cells specialise into tip cells and stalk cells. ALK1 synergises with Notch in stalk cells to induce expression of the Notch targets HEY1 and HEY2 and thereby represses tip cell formation and angiogenic sprouting. The ALK1-Fc soluble protein fusion has entered clinic trials as a therapeutic strategy to sequester the high-affinity extracellular ligand BMP9. Here, we determined the crystal structure of the ALK1 intracellular kinase domain and explored the effects of a small molecule kinase inhibitor K02288 on angiogenesis. K02288 inhibited BMP9-induced phosphorylation of SMAD1/5/8 in human umbilical vein endothelial cells to reduce both the SMAD and the Notch-dependent transcriptional responses. In endothelial sprouting assays, K02288 treatment induced a hypersprouting phenotype reminiscent of Notch inhibition. Furthermore, K02288 caused dysfunctional vessel formation in a chick chorioallantoic membrane assay of angiogenesis. Such activity may be advantageous for small molecule inhibitors currently in preclinical development for specific BMP gain of function conditions, including diffuse intrinsic pontine glioma and fibrodysplasia ossificans progressiva, as well as more generally for other applications in tumour biology.
Highlights
The Notch and ALK1 signalling pathways play critical roles in the vasculature as evidenced by their respective linkage to the diseases Alagille syndrome [1] and hereditary haemorrhagic telangiectasia (HHT) [2]. Angiogenic stimuli such as VEGF induce quiescent endothelial cells to specialise into tip cells that migrate towards the stimulus and into attached stalk cells that proliferate behind for lumen formation
Dll4 expression on tip cells allows for the binding and activation of Notch receptors on adjacent stalk cells triggering the release of the Notch intracellular domain (NICD) for transcriptional activation
We show that a small molecule inhibitor targeting intracellular BMP receptor domains can interfere with angiogenesis in a similar fashion to ALK1-Fc and Notch pathway inhibitors consistent with the known functional synergy between these two signalling pathways (Fig. 4b)
Summary
The Notch and ALK1 signalling pathways play critical roles in the vasculature as evidenced by their respective linkage to the diseases Alagille syndrome [1] and hereditary haemorrhagic telangiectasia (HHT) [2]. Angiogenic stimuli such as VEGF induce quiescent endothelial cells to specialise into tip cells that migrate towards the stimulus and into attached stalk cells that proliferate behind for lumen formation. ALK1 signals via phosphorylation of the transcription factors SMAD1/5/8, which act synergistically with NICD to induce the expression of Notch target genes, including HEY1 and HEY2
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
