Bone Morphogenetic Protein-9 Is a Circulating Vascular Quiescence Factor

Abstract

Abstract-Angiogenesis is a complex process, requiring a finely tuned balance between numerous stimulatory and inhibitory signals. ALK1 (activin receptor like-kinase 1) is an endothelial-specific type 1 receptor of the transforming growth factor-␤ receptor family. Heterozygotes with mutations in the ALK1 gene develop hereditary hemorrhagic telangiectasia type 2 (HHT2). Recently, we reported that bone morphogenetic protein (BMP)9 and BMP10 are specific ligands for ALK1 that potently inhibit microvascular endothelial cell migration and growth. These data lead us to suggest that these factors may play a role in the control of vascular quiescence. To test this hypothesis, we checked their presence in human serum. We found that human serum induced Smad1/5 phosphorylation. To identify the active factor, we tested neutralizing antibodies against BMP members and found that only the anti-BMP9 inhibited serum-induced Smad1/5 phosphorylation. The concentration of circulating BMP9 was found to vary between 2 and 12 ng/mL in sera and plasma from healthy humans, a value well above its EC 50 (50 pg/mL). These data indicated that BMP9 is circulating at a biologically active concentration. We then tested the effects of BMP9 in 2 in vivo angiogenic assays. We found that BMP9 strongly inhibited sprouting angiogenesis in the mouse sponge angiogenesis assay and that BMP9 could inhibit blood circulation in the chicken chorioallantoic membrane assay. Taken together, our results demonstrate that BMP9, circulating under a biologically active form, is a potent antiangiogenic factor that is likely to play a physiological role in the control of adult blood vessel quiescence. Key Words: BMP9 Ⅲ ALK1 Ⅲ HHT Ⅲ angiogenesis B one morphogenetic proteins (BMPs), which belong to the transforming growth factor (TGF)␤ superfamily, were originally identified as inducers of ectopic bone growth and cartilage formation. Since then, there has been substantial progress in our knowledge of the multiple functions of these growth factors. 1 BMPs regulate cell growth, differentiation, and apoptosis of various cell types, and they are critically important in the morphogenesis and differentiation of tissues and organs. BMP9, also known as growth differentiation factor-2, is expressed in the adult liver by nonparenchymal cells (ie, endothelial, stellate, and Kupffer cells) 2 and in the septum and spinal cord of mouse embryos. 3 BMP9 has been described as a hematopoietic, hepatogenic, osteogenic, and chondrogenic factor. It has also been identified as a regulator of glucose metabolism, capable of reducing glycemia in diabetic mice and as a differentiation factor for cholinergic neurons in the central nervous system. 3 More recently, it was shown to induce the expression of hepcidin, a hormone that plays a key role in iron homeostasis. 4 ALK1 (activin receptor like-kinase 1) is an endothelialspecific type I receptor of the TGF␤ receptor family that is implicated in the pathogenesis of hereditary hemorrhagic telangiectasia type 2 (HHT2), also known as the Rendu-Osler...

TGF-β signaling in vascular biology and dysfunction

“…Both the endoglin extracellular and intracellular domains interact with TβRII and ALK5 [7]. Endoglin is able to bind directly to BMPs [13][14][15].…”

Maladie de Rendu-Osler

“…L'expression de BMP10 semble restreinte au stade embryonnaire et jouerait un rôle dans le développement du coeur. BMP9, en revanche, est produite chez l'adulte dans le foie et circule dans le plasma et le sérum (5 ng/ml) [26]. L'ensemble de ces données suggère que BMP9 serait le ligand d'ALK1 impliqué dans la maladie de Rendu-Osler.…”

“…La liaison de BMP9 à l'endogline, et l'augmentation par l'endogline de la réponse Smad dépendante de BMP9 en aval d'ALK1 ont aussi été démontrées [22]. Aucune compensation du taux circulant de BMP9 n'a été mis en évidence chez les patients atteints de la maladie de Rendu-Osler [26]. Implication d'ALK1 et de l'endogline dans la phase de maturation de l'angiogenèse L'angiogenèse est la formation de nouveaux vaisseaux sanguins à partir d'un réseau vasculaire existant.…”

“…Le rôle du récepteur ALK1, en l'absence de ligand spécifique, a longtemps été débattu dans la littérature [27,28]. L'identification de son ligand BMP9 a permis de confirmer clairement son rôle dans l'inhibition de la prolifération, de la migration et du bourgeonnement des cellules endothéliales in vitro ainsi que de la néo-angiogenèse in vivo [24][25][26]. Ces données indiquent que BMP9 serait un facteur-clé de la phase de maturation de l'angiogenèse ( Figure 1) et sa présence dans le sang suggère son rôle dans le maintien de la quiescence vasculaire chez l'adulte [26].…”

“…L'identification de son ligand BMP9 a permis de confirmer clairement son rôle dans l'inhibition de la prolifération, de la migration et du bourgeonnement des cellules endothéliales in vitro ainsi que de la néo-angiogenèse in vivo [24][25][26]. Ces données indiquent que BMP9 serait un facteur-clé de la phase de maturation de l'angiogenèse ( Figure 1) et sa présence dans le sang suggère son rôle dans le maintien de la quiescence vasculaire chez l'adulte [26]. Ces résultats nous permettent de proposer le modèle suivant : lorsque la voie BMP9/ALK1/endogline est perturbée, la quiescence est diminuée, ce qui entraîne un dérèglement de la balance angiogénique et donc une néoactivation de l'angiogenèse (Figure 2A et B).…”