658

BMPR1B

ALK-6|ALK6|CDw293;bone morphogenetic protein receptor, type IB;BMPR1B;bone morphogenetic protein receptor, type IB

BMP type-1B receptor|BMPR-1B|activin receptor-like kinase 6|bone morphogenetic protein receptor type-1B|serine/threonine receptor kinase

4q22-q24

protein-coding

Count: BMPR1B; 658

The functional involvement of bone morphogenetic protein (BMP) system in primary pulmonary hypertension (PPH) remains unclear. Here we demonstrate a crucial role of the BMP type IB receptor, activin receptor-like kinase (ALK)-6 for pulmonary arterial smooth muscle cell (pphPASMC) mitosis isolated from a sporadic PPH patient bearing no mutations in BMPR2 gene. A striking increase in the levels of ALK-6 mRNA was revealed in pphPASMC compared with control PASMCs, in which ALK-6 transcripts were hardly detectable. BMP-2 and -7 stimulated the mitosis of pphPASMCs, which was opposite to their suppressive effects on the mitosis of the control PASMCs. BMP-4 and -6 and activin inhibited pphPASMC mitosis, whereas these did not affect control PASMCs. The presence of BMP signaling machinery in pphPASMCs was elucidated based on the analysis on Id-1 transcription and Smad-reporter genes. Overexpression of a dominant-negative ALK-6 construct revealed that ALK-6 plays a key role in the mitosis as well as intracellular BMP signaling of pphPASMCs. Gene silencing of ALK-6 using small interfering RNA also reduced DNA synthesis as well as Id-1 transcription in pphPASMCs regardless of BMP-2 stimulation. Although Id-1 response was not stimulated by BMP-2 in control PASMCs, the gene delivery of wild-type ALK-6 caused significant increase in the Id-1 transcripts in response to BMP-2. Additionally, inhibitors of ERK and p38 MAPK pathways suppressed pphPASMC mitosis induced by BMP-2, implying that the mitotic action is in part MAPK dependent. Thus, the BMP system is strongly involved in pphPASMC mitosis through ALK-6, which possibly leads to activation of Smad and MAPK, resulting in the progression of vascular remodeling of pulmonary arteries in PPH.

BACKGROUND: mutations in the bone morphogenetic protein receptor type 2 (BMPR2) gene, the activin receptor-like kinase 1 (ALK1) gene, and SMAD8 gene have been reported in heritable pulmonary arterial hypertension (HPAH) and in idiopathic pulmonary arterial hypertension (IPAH). However, almost 30% of HPAH cases and 60-90% of IPAH cases have no mutations in those genes. This suggests that there remain unidentified genes associated with HPAH and IPAH. METHODS AND RESULTS: This study screened for mutations in endoglin, SMAD1, SMAD2, SMAD3, SMAD4, SMAD5, SMAD6, SMAD7, bone morphogenetic protein receptor type 1A (BMPR1A) and bone morphogenetic protein receptor type 1B (BMPR1B) genes in 43 IPAH patients who had no mutations in BMPR2, ALK1 and SMAD8. Two missense mutations (c.479 G>A S160N, c.1176 C>A F392L) in BMPR1B were each identified in 2 IPAH patients. Immunoblot analysis revealed that the BMPR1B F392L protein promoted SMAD8 phosphorylation. The response to BMP was analyzed using promoter-reporter activities. The transcriptional activation of the BMPR1B F392L protein with SMAD8 increased above that of wild-type BMPR1B with SMAD8, and those of BMPR1B S160N and F392L with SMAD8 and SMAD4 were each increased above those of the wild-type BMPR1B with SMAD8 and SMAD4. CONCLUSIONS: We identified 2 novel mutations in BMPR1B in 2 patients with IPAH. Our study suggests that BMPR1B mutations are associated with the pathogenesis of IPAH.

BACKGROUND: mutations in the bmpr2 gene, encoding the type II bone morphogenetic protein (BMP) receptor, have been identified in patients with pulmonary arterial hypertension (PAH), implicating BMP signaling in PAH. The aim of this study was to assess BMP signaling and its physiological effects in a monocrotaline (MCT) model of PAH. METHODS AND RESULTS: expression of BMP receptors Ib and II, and Smads 4, 5, 6, and 8, was downregulated in lungs but not kidneys of MCT-treated rats. Smad1 phosphorylation and expression of BMP/Smad target genes id1 and id3 was also reduced, although ERK1/2 and p38(MAPK) phosphorylation remained unaffected. BMP receptor and Smad expression, Smad1 phosphorylation, and induction of the BMP/Smad-responsive element of the id1 promoter were reduced in pulmonary artery smooth muscle cells (PASMCs) from MCT-treated rats. As a consequence of impaired BMP/Smad signaling, PASMCs from MCT-treated rats were resistant to apoptosis induced by BMP-4 and BMP-7, and were also resistant to BMP-4 antagonism of proliferation induced by platelet-derived growth factor. CONCLUSION: BMP signaling and BMP-regulated physiological phenomena are perturbed in MCT-treated rats, lending solid support to the proposed roles for BMP signaling in the pathogenesis of human PAH.

mutations in the gene encoding bone morphogenetic protein (BMP) receptor type 2 (BMPR-2) have been reported in pulmonary arterial hypertension (PAH), but their functional relevance remains incompletely understood. BMP receptor expression was evaluated in human lungs and in cultured pulmonary artery smooth muscle cells (PASMCs) isolated from 19 idiopathic PAH patients and nine heritable PAH patients with demonstrated BMPR-2 mutations. BMP4-treated PASMCs were assessed for Smad and p38 mitogen-activated protein kinase (MAPK) signalling associated with mitosis and apoptosis. Lung tissue and PASMCs from heritable PAH patients presented with decreased BMPR-2 expression and variable increases in BMPR-1A and BMPR-1B expression, while a less important decreased BMPR-2 expression was observed in PASMCs from idiopathic PAH patients. Heritable PAH PASMCs showed no increased phosphorylation of Smad1/5/8 in the presence of BMP4, which actually activated the p38MAPK pathway. Individual responses varied from one mutation to another. PASMCs from PAH patients presented with an in vitro proliferative pattern, which could be inhibited by BMP4 in idiopathic PAH but not in heritable PAH. PASMCs from idiopathic PAH and more so from heritable PAH presented an inhibition of BMP4-induced apoptosis. Most heterogeneous BMPR-2 mutations are associated with defective Smad signalling compensated for by an activation of p38MAPK signalling, accounting for PASMC proliferation and deficient apoptosis.