Bone morphogenetic proteins go endothelial

Mouse knock-out studies, joined later by studies in zebrafish, have shown that the transforming growth factor type β (TGFβ) receptors are essential for angiogenesis, while mutations in the endothelial cell–specific and signaling receptor for TGFβ, ALK1, are associated with the genetic vascular disease hereditary hemorrhagic telangiectasia (HHT) type 2. Now, bone morphogenetic proteins (BMPs) join this stage.

Studies in cell culture have suggested that angiogenesis is regulated by a switch between 2 TGFβ signaling pathways, one emanating from (the ubiquitously expressed) ALK5, the other one from ALK1.^1,2  This switch is regulated by the coreceptor endoglin present on angiogenic vessels, and endoglin mutations cause hereditary hemorrhagic telangiectasia (HHT) type 1. ALK5 activates in cultured endothelial cells the downstream Smad proteins Smad2/3, whereas ALK1 acts through Smad1/5/8. Smad1/5/8 are bone morphogenetic protein (BMP) Smads because they are activated by the BMP receptors ALK2, ALK3, and ALK6 in tissues ranging from cartilage and bone tissue to many soft tissues in the embryo and the adult animal. Candidate target genes for each of the 2 TGFβ pathways in mainly human umbilical vascular endothelial cells (HUVECs) but also human microvascular endothelial cells (HMECs) have been identified.^3–5  For this, either stimulation of the cells with TGFβ was used or overexpression of a constitutively active form of ALK1 and ALK5.

In this issue of Blood, David and colleagues report important new results on the signaling activity of ALK1 in HMECs from dermis (HMVEC-d's). The authors were intrigued by recent results on the crystal structure of BMP9 (also named GDF2) demonstrating that BMP9 can bind to receptor complexes consisting of ALK1 and BMP type II receptor (BMPRII) and that complex and mature forms of BMP9 exhibit similar biologic activity.⁶  Some groups have reported, mainly at meetings, preliminary data on BMP binding (in particular BMP2, BMP4, and BMP7 have been tried as these were initially the only ligands widely available) to ALK1. For the many BMP ligands with possible therapeutic potential, the affinities of the individual ligands for the multiple combinations of BMP receptors are well-kept secrets. To date, no convincing data have emerged showing that a BMP is a physiologic ligand for ALK1 in the vessel wall. David and colleagues show that BMP9 (and BMP10) induces in HMEC-d's sustained Smad1/5/8 phosphorylation and activates transcription of a BMP-Smad reporter gene and the BMP-Smad target gene Id1. Using siRNA approaches and others, the authors show that this stimulation is dependent on ALK1 (and BMPRII), while cotransfected endoglin increases the response to BMP9. David et al also demonstrate that BMP9 (and BMP10) induces a number of genes that have been identified in each of the microarray studies done so far and that searched for ALK1 target genes.^3–5 

The results of the paper by David and colleagues add endothelial cells as targets for certain ligands of the BMP subgroup of the TGFβ family. This work will attract the attention of many groups studying the in vivo function of BMP signaling in various processes in embryogenesis and disease. Future challenges are to include the documentation of Smad but also non-Smad signaling emanating from ALK1-TβRII and ALK1-BMPRII complexes when activated by TGFβ and BMP9, respectively, and where in vivo the BMP9→ALK1 pathway is operational and relevant. Also, the role of the coreceptor endoglin will have to be revisited and perhaps other coreceptors should be searched for. Alternative approaches would be those that involve the identification of proteins that interact with the intracytoplasmic tail of TβRII and/or BMPRII. Such proteins may lead to a better understanding of how the signaling and perhaps the endocytic routing of these liganded receptors, and thereby the cellular response, is regulated, or whether in the case of BMP ligands preformed versus BMP-induced receptor complexes are assembled and kept separated,⁷  including in endothelial cells.