The Role of BMP Signaling in Pulmonary Arterial Hypertension

PAH is characterized by progressive narrowing of small pulmonary arteries, frequently culminating in right heart failure and death. The pathology of PAH involves abnormal function, proliferation, and apoptosis of vascular endothelial (EC) and smooth muscle (VSMC) lineages, and the presence of plexiform lesions suggesting disordered angiogenesis. Recently, an important role for circulating bone marrow-derived vascular progenitor cells, including endothelial progenitor cells, in lesion formation has been suggested. Despite the advent of several new therapies, survival without lung-transplantation at 5 years remains less than 50%. About 10% of idiopathic PAH patients have a family history of the disease. The identification of mutations in the bone morphogenetic protein type 2 receptor (BMPR2) gene in the majority of patients with heritable PAH (hPAH) and around 20% of PAH patients without a family history represented an important advance. More rarely, mutations in another transforming growth factor-b family receptor, ALK-1, can give rise to PAH. These and other findings suggest that altered BMP signaling contributes to the pathogenesis of PAH due to a variety of etiologies. However, mechanistic links between abnormal BMP signaling and PAH pathogenesis remain incompletely defined.

The award of the Fondation Leducq Transatlantic Network of Excellence has allowed us to assemble a collaborative team to tackle the question of how BMPR2 mutations cause PAH and how this can be exploited for the development of novel therapies.  The role of BMPs and the impact of BMPR2 mutations on vascular cell function and cell-cell interactions will be studied in a range of informative in vivo models including transgenic zebrafish and novel mouse models of BMPR2 dysfunction. We will also employ circulating late-outgrowth endothelial progenitor cells from patents with heritable and idiopathic PAH and unaffected family members to study endothelial biology in these groups. Induced pluripotent stem cells will be generated from EPCs and fibroblasts derived from PAH patients and embryonic stem cells and iPS cells from mice carrying Bmpr2 mutations to study cardiovascular differentiation. These models will provide greater insight into the downstream and specific targets of BMP signaling in vascular cells, including microRNAs.

The role of BMPR2 in endothelial barrier function will be investigated, since this may be an early trigger for the development of PAH. The contribution of SMAD-dependent and -independent pathways to the abnormal EC phenotype will be determined. Current hypotheses regarding the initiation of PAH in patients carrying BMPR2 mutations suggest the need for second hits or triggers necessary for disease manifestation, including the induction of EC apoptosis. The availability of new genetic mouse models, including conditional knockin of a dominant-negative mutant BMPR2 known to cause human disease, conditional Bmpr2 knockout mice, as well as mice carrying a mutant gene specifying a BMPR-II protein lacking its long carboxyl terminus, will greatly assist this research.

Emphasis will be placed on characterizing how presence of mutant BMPR-II receptors modulates BMP type I receptor utilization (by specific BMP ligands) and alters receptor interactions with downstream signaling molecules. Pulmonary vascular tone will be evaluated in wild mice and mice with BMPR2 mutations using novel non-invasive echocardiographic methods and invasive hemodynamic techniques. Pulmonary vascular structure will be characterized using micro-CT angiography and histology. We will determine whether inhibition of BMP type I receptor function, using small molecule inhibitors can modify the development of pulmonary hypertension in mice with BMPR2 mutations. Similar studies will be performed using miRNA mimics and anti-miRNAs. These studies will enable us to determine whether pulmonary hypertension associated with Bmpr2 mutation in mice is attributable to gain or loss of BMP signaling.

Specific collaborative projects

The Role of Tmem100 in Pulmonary Arterial Hypertension and Development

Drs. Smith and Morrell are investigating the functional roles of Tmem100 in pulmonary vascular remodeling and early zebrafish development. Tmem100 is activated by BMP9 and BMP10 through the ALK1 receptor, and mutations in the gene may be a direct cause of PAH or may exacerbate the disease. We are investigating the mode of action of this poorly-understood gene using experimental embryology and developmental genetics in the zebrafish.

The Role of ALK2 in Pulmonary Arterial Hypertension and Vasculogenesis

Drs. Smith and Yu are analysing the function of the selective ALK2 inhibitor LDN212854 and exploring its effects on early zebrafish development, particularly in vasculogenesis and haematopoiesis. LDN212854 is a more selective inhibitor of ALK2 signaling than previously described BMP inhibitors, yet retains in vivo potency in several species. We will test the ability of this novel probe compound to discern the role of ALK2 in a temporally restricted manner during early embryongenesis.

The Role of microRNAs in Human Pulmonary Arterial Hypertension

Drs. Hata and Morrell are profiling microRNAs in pulmonary vascular cells (smooth muscle and endothelial cells) from healthy individuals and PAH patients and elucidating the role of microRNAs in the pathogenesis of PAH.