Bone Morphogenetic Proteins Signal Via SMAD and Mitogen-activated Protein (MAP) Kinase Pathways at Distinct Times during Osteoclastogenesis

To investigate the role of bone morphogenetic protein (BMP) signaling in osteoclastogenesis in vivo, we eliminated BMPRII in osteoclasts by creating a BMPRII^fl/fl;lysM-Cre mouse strain. Conditional knock-out (cKO) mice are osteopetrotic when compared with WT controls due to a decrease in osteoclast activity. Bone marrow macrophages (BMMs) isolated from cKO mice are severely inhibited in their capacity to differentiate into mature osteoclasts in the presence of M-CSF and receptor activator of NF-κB (RANK) ligand. We also show that BMP noncanonical (MAPK) and canonical (SMAD) pathways are utilized at different stages of osteoclast differentiation. BMP2 induces p38 phosphorylation in pre-fusion osteoclasts and increases SMAD phosphorylation around osteoclast precursor fusion. Phosphorylation of MAPKs was decreased in differentiated BMMs from cKO animals. Treating BMMs with the SMAD inhibitor dorsomorphin confirms the requirement for the canonical pathway around the time of fusion. These results demonstrate the requirement for BMP signaling in osteoclasts for proper bone homeostasis and also explore the complex signaling mechanisms employed by BMP signaling during osteoclast differentiation.     

Bone morphogenetic protein receptor II is a novel mediator of endothelial nitric-oxide synthase activation

Activation of bone morphogenetic protein (BMP) receptor II (BMPRII) promotes pulmonary artery endothelial cell (PAEC) survival, proliferation, and migration. Mutations to BMPRII are associated with the development of pulmonary arterial hypertension (PAH). Endothelial dysfunction, including decreased endothelial nitric-oxide synthase (eNOS) activity and loss of bioactive nitric oxide (NO), plays a prominent role in the development of PAH. We hypothesized that stimulation of BMPRII promotes normal PAEC function by activating eNOS. We report that BMPRII ligands, BMP2 and BMP4, (i) stimulate eNOS phosphorylation at a critical regulatory site, (ii) increase eNOS activity, and (iii) result in canonical changes in eNOS protein-protein interactions. The stimulation of eNOS activity by BMPRII ligands was largely dependent on protein kinase A (PKA) activation, as demonstrated using the PKA inhibitors H89 and myristoylated PKI(6-22) amide. PAEC migration stimulated by BMP2 and BMP4 was inhibited by the NOS inhibitor l-nitroarginine methyl ester, providing functional evidence of eNOS activation. Furthermore, BMP2 and BMP4 failed to stimulate eNOS phosphorylation when BMPRII was knocked down by siRNA. Most important to the pathophysiology of the disease, BMP2 and BMP4 failed to stimulate eNOS phosphorylation in PAECs isolated from patients with mutations in the BMPR2 gene. These data demonstrate a new action of BMPs/BMPRII in the pulmonary endothelium and provide novel mechanistic insight into the pathogenesis of PAH.     

Caveolin-1 regulates BMPRII localization and signaling in vascular smooth muscle cells

Recent studies demonstrate the interaction of BMPRII and caveolin-1 in various cell types. In this study we test the hypothesis that caveolin-1 interacts with and regulates BMPRII-dependent signaling in vascular smooth muscle cells. We demonstrate that BMPRII localizes to caveolae and directly interacts with caveolin-1 in mouse aortic smooth muscle cells. We demonstrate that this interaction is mediated by the caveolin-1 scaffolding domain and is regulated by caveolin-1 phosphorylation. Downregulation of caveolin-1 via siRNA resulted in a loss of BMP-dependent SMAD phosphorylation and gene regulation. Further studies revealed that loss of caveolin-1 results in decreased BMPRII membrane localization and decreased association of BMPRII with the type I BMP receptor BMPRIa. Dominant negative caveolin-1 decreased BMPRII membrane localization suggesting a role for caveolin-1 in BMPRII trafficking. Taken together, our findings establish caveolin-1 as an important regulator of downstream signaling and membrane targeting of BMPRII in vascular smooth muscle cells.     

Cellular interplay in pulmonary arterial hypertension: Implications for new therapies

Pulmonary arterial hypertension (PAH) is a complex and multifactorial disease characterized by vascular remodeling, vasoconstriction, inflammation and thrombosis. Although the available therapies have resulted in improvements in morbidity and survival, PAH remains a severe and devastating disease with a poor prognosis and a high mortality, justifying the need of novel therapeutic targets. An increasing number of studies have demonstrated that endothelial cells (ECs), smooth muscle cells (SMCs) and fibroblasts of the pulmonary vessel wall, as well as platelets and inflammatory cells have a role in PAH pathogenesis. This review aims to integrate the interplay among different types of cells, during PAH development and progression, and the impact of current therapies in cellular modulation. The interplay among endothelial cells, smooth muscle cells and fibroblasts present in pulmonary vessels wall, platelets and inflammatory cells is regulated by several mediators produced by these cells, contributing to the pathophysiologic features of PAH. Current therapies are mainly focused in the pulmonary vascular tone and in the endothelial dysfunction. However, once they have not been effective, novel therapies targeting other PAH features, such as inflammation and platelet dysfunction are emerging. Further understanding of the interplay among different vascular cell types involved in PAH development and progression can contribute to find novel therapeutic targets, decreasing PAH mortality and morbidity in the future.     

Epigenetic repression of bone morphogenetic protein receptor II expression in scleroderma

Germline mutations in the bone morphogenetic protein type II receptor (BMPRII) gene play an essential role in the pathogenesis of familial pulmonary arterial hypertension (FPAH). In view of the histological similarities between scleroderma (SSc) and FPAH arterial lesion, we examined the expression levels of BMPRII in SSc microvascular endothelial cells (MVEC). Oxidative stress and serum starvation were used to examine apoptotic responses of MVECs. BMPRII expression levels were determined by RT‐PCR and by Western blot. Epigenetic regulation of BMPRII expression was examined by the addition of epigenetic inhibitors to MVECs cultures, by methylation‐specific PCR, and by sequence analysis of DNA methylation pattern of the BMPRII promotor region. SSc‐MVECs were more sensitive to apoptotic signals than were normal‐MVECs. A significant decrease in BMPRII expression levels in SSc‐MVECs was noted, whereas no significant differences in the expression levels of BMPRIA and BMPRIB were observed. Similar reduction in expression levels was noted in SSc skin biopsies. The expression level of BMPRII in SSc‐MVECs was normalized by the addition of 2‐deoxy‐5‐azacytidine and trichostatin A to cell cultures. Extensive CpG sites methylation in the BMPRII promoter region was noted in SSc‐MVECs with no detectable site methylation in control‐MVECs. SSc‐MVECs are more sensitive to apoptotic triggers than are control‐MVECs. The enhanced apoptosis may be related to epigenetic repression of BMPRII expression as apoptosis of control‐MVECs can be augmented by knocking down BMPRII expression. The role of BMPRII underexpression in the pathogenesis of SSc vasculopathy is suggested and should be investigated further.