The BMP Pathway and Its Inhibitors in the Skeleton.

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor-ß family of ligands. BMPs exhibit widespread utility and pleiotropic, context-dependent effects, and the strength and duration of BMP pathway signaling is tightly regulated at numerous levels via mechanisms operating both inside and outside the cell. Defects in the BMP pathway or its regulation underlie multiple human diseases of different organ systems. Yet much remains to be discovered about the BMP pathway in its original context, i.e., the skeleton. In this review, we provide a comprehensive overview of the intricacies of the BMP pathway and its inhibitors in bone development, homeostasis, and disease. We frame the content of the review around major unanswered questions for which incomplete evidence is available. First, we consider the gene regulatory network downstream of BMP signaling in osteoblastogenesis. Next, we examine why some BMP ligands are more osteogenic than others and what factors limit BMP signaling during osteoblastogenesis. Then we consider whether specific BMP pathway components are required for normal skeletal development, and if the pathway exerts endogenous effects in the aging skeleton. Finally, we propose two major areas of need of future study by the field: greater resolution of the gene regulatory network downstream of BMP signaling in the skeleton, and an expanded repertoire of reagents to reliably and specifically inhibit individual BMP pathway components.

Muscle-derived factors influencing bone metabolism.

A significant amount of attention has been brought to the endocrine-like function of skeletal muscle on various tissues, particularly with bone. Several lines of investigation indicate that the physiology of both bone and muscle systems may be regulated by a given stimulus, such as exercise, aging, and inactivity. Moreover, emerging evidence indicates that bone is heavily influenced by soluble factors derived from skeletal muscle (i.e., muscle-to-bone communication). The purpose of this review is to discuss the regulation of bone remodeling (formation and/or resorption) through skeletal muscle-derived cytokines (hereafter myokines) including the anti-inflammatory cytokine METRNL and pro-inflammatory cytokines (e.g., TNF-α, IL-6, FGF-2 and others). Our goal is to highlight possible therapeutic opportunities to improve muscle and bone health in aging.

PTHrP intracrine actions divergently influence breast cancer growth through p27 and LIFR.

The role of parathyroid hormone (PTH)-related protein (PTHrP) in breast cancer remains controversial, with reports of PTHrP inhibiting or promoting primary tumor growth in preclinical studies. Here, we provide insight into these conflicting findings by assessing the role of specific biological domains of PTHrP in tumor progression through stable expression of PTHrP (-36-139aa) or truncated forms with deletion of the nuclear localization sequence (NLS) alone or in combination with the C-terminus. Although the full-length PTHrP molecule (-36-139aa) did not alter tumorigenesis, PTHrP lacking the NLS alone accelerated primary tumor growth by downregulating p27, while PTHrP lacking the NLS and C-terminus repressed tumor growth through p27 induction driven by the tumor suppressor leukemia inhibitory factor receptor (LIFR). Induction of p27 by PTHrP lacking the NLS and C-terminus persisted in bone disseminated cells, but did not prevent metastatic outgrowth, in contrast to the primary tumor site. These data suggest that the PTHrP NLS functions as a tumor suppressor, while the PTHrP C-terminus may act as an oncogenic switch to promote tumor progression through differential regulation of p27 signaling.

Soft Tissue Manipulation Alters RANTES/CCL5 and IL-4 Cytokine Levels in a Rat Model of Chronic Low Back Pain.

Low back pain (LBP) is a common musculoskeletal complaint that can impede physical function and mobility. Current management often involves pain medication, but there is a need for non-pharmacological and non-invasive interventions. Soft tissue manipulation (STM), such as massage, has been shown to be effective in human subjects, but the molecular mechanisms underlying these findings are not well understood. In this paper, we evaluated potential changes in the soft tissue levels of more than thirty pro- or anti-inflammatory cytokines following instrument-assisted STM (IASTM) in rats with chronic, induced LBP using Complete Freund's Adjuvant. Our results indicate that IASTM is associated with reduced soft tissue levels of Regulated on Activation, Normal T cell Expressed and Secreted (RANTES)/Chemokine (C-C motif) ligand 5 (CCL5) and increased soft tissue levels of Interleukin (IL)-4, which are pro-inflammatory and anti-inflammatory factors, respectively, by 120 min post-treatment. IASTM was not associated with tissue-level changes in C-X-C Motif Chemokine Ligand (CXCL)-5/Lipopolysaccharide-Induced CXC Chemokine (LIX)-which is the murine homologue of IL-8, CXCL-7, Granulocyte-Macrophage-Colony Simulating Factor (GM-CSF), Intercellular Adhesion Molecule (ICAM)-1, IL1-Receptor Antagonist (IL-1ra), IL-6, Interferon-Inducible Protein (IP)-10/CXCL-10, L-selectin, Tumor Necrosis Factor (TNF)-α, or Vascular Endothelial Growth Factor (VEGF) at either 30 or 120 min post-treatment. Combined, our findings raise the possibility that IASTM may exert tissue-level effects associated with improved clinical outcomes and potentially beneficial changes in pro-/anti-inflammatory cytokines in circulation and at the tissue level.

Neuromedin U (NMU) regulates osteoblast differentiation and activity.

Osteoporosis is a disease of low bone mass that places individuals at enhanced risk for fracture, disability, and death. Osteoporosis rates are expected to rise significantly in the coming decades yet there are limited pharmacological treatment options, particularly for long-term management of this chronic condition. The drug development pipeline is relatively bereft of new strategies, causing an urgent and unmet need for developing new strategies and targets for treating osteoporosis. Here, we examine a lesser-studied bone remodeling pathway, Neuromedin U (NMU), which is expressed in the bone microenvironment along with its cognate receptors NMU receptor 1 (NMUR1) and 2 (NMUR2). We independently corroborate a prior report that global loss of NMU expression leads to high bone mass and test the hypothesis that NMU negatively regulates osteoblast differentiation. Consistent with this, in vitro studies reveal NMU represses osteoblastic differentiation of osteogenic precursors but, in contrast, promotes osteoblastic marker expression, proliferation and activity of osteoblast-like cells. Phospho-profiling arrays were used to detail differential signaling outcomes that may underlie the opposite responses of these cell types. Collectively, our findings indicate that NMU exerts cell-type-specific responses to regulate osteoblast differentiation and activity.

The transcriptional co-repressor TLE3 regulates myogenic differentiation by repressing the activity of the MyoD transcription factor.

Satellite cells are skeletal muscle stem cells that provide myonuclei for postnatal muscle growth, maintenance, and repair/regeneration in adults. Normally, satellite cells are mitotically quiescent, but they are activated in response to muscle injury, in which case they proliferate extensively and exhibit up-regulated expression of the transcription factor MyoD, a master regulator of myogenesis. MyoD forms a heterodimer with E proteins through their basic helix-loop-helix domain, binds to E boxes in the genome and thereby activates transcription at muscle-specific promoters. The central role of MyoD in muscle differentiation has increased interest in finding potential MyoD regulators. Here we identified transducin-like enhancer of split (TLE3), one of the Groucho/TLE family members, as a regulator of MyoD function during myogenesis. TLE3 was expressed in activated and proliferative satellite cells in which increased TLE3 levels suppressed myogenic differentiation, and, conversely, reduced TLE3 levels promoted myogenesis with a concomitant increase in proliferation. We found that, via its glutamine- and serine/proline-rich domains, TLE3 interferes with MyoD function by disrupting the association between the basic helix-loop-helix domain of MyoD and E proteins. Our findings indicate that TLE3 participates in skeletal muscle homeostasis by dampening satellite cell differentiation via repression of MyoD transcriptional activity.

Loss of BMPR2 leads to high bone mass due to increased osteoblast activity.

Imbalances in the ratio of bone morphogenetic protein (BMP) versus activin and TGFß signaling are increasingly associated with human diseases yet the mechanisms mediating this relationship remain unclear. The type 2 receptors ACVR2A and ACVR2B bind BMPs and activins but the type 2 receptor BMPR2 only binds BMPs, suggesting that type 2 receptor utilization might play a role in mediating the interaction of these pathways. We tested this hypothesis in the mouse skeleton, where bone mass is reciprocally regulated by BMP signaling and activin and TGFß signaling. We found that deleting Bmpr2 in mouse skeletal progenitor cells (Bmpr2-cKO mice) selectively impaired activin signaling but had no effect on BMP signaling, resulting in an increased bone formation rate and high bone mass. Additionally, activin sequestration had no effect on bone mass in Bmpr2-cKO mice but increased bone mass in wild-type mice. Our findings suggest a novel model whereby BMPR2 availability alleviates receptor-level competition between BMPs and activins and where utilization of ACVR2A and ACVR2B by BMPs comes at the expense of activins. As BMP and activin pathway modulation are of current therapeutic interest, our findings provide important mechanistic insight into the relationship between these pathways in human health.

On the Emerging Role of the Taste Receptor Type 1 (T1R) Family of Nutrient-Sensors in the Musculoskeletal System.

The special sense of taste guides and guards food intake and is essential for body maintenance. Salty and sour tastes are sensed via ion channels or gated ion channels while G protein-coupled receptors (GPCRs) of the taste receptor type 1 (T1R) family sense sweet and umami tastes and GPCRs of the taste receptor type 2 (T2R) family sense bitter tastes. T1R and T2R receptors share similar downstream signaling pathways that result in the stimulation of phospholipase-C-ß2. The T1R family includes three members that form heterodimeric complexes to recognize either amino acids or sweet molecules such as glucose. Although these functions were originally described in gustatory tissue, T1R family members are expressed in numerous non-gustatory tissues and are now viewed as nutrient sensors that play important roles in monitoring global glucose and amino acid status. Here, we highlight emerging evidence detailing the function of T1R family members in the musculoskeletal system and review these findings in the context of the musculoskeletal diseases sarcopenia and osteoporosis, which are major public health problems among the elderly that affect locomotion, activities of daily living, and quality of life. These studies raise the possibility that T1R family member function may be modulated for therapeutic benefit.

Muscle regulatory factors regulate T1R3 taste receptor expression.

T1R3 is a T1R class of G protein-coupled receptors, composing subunit of the umami taste receptor when complexed with T1R1. T1R3 was originally discovered in gustatory tissue but is now known to be expressed in a wide variety of tissues and cell types such the intestine, pancreatic ß-cells, skeletal muscle, and heart. In addition to taste recognition, the T1R1/T1R3 complex functions as an amino acid sensor and has been proposed to be a control mechanism for the secretion of hormones, such as cholecystokinin, insulin, and duodenal HCO3(-) and activates the mammalian rapamycin complex 1 (MTORC1) to inhibit autophagy. T1R3 knockout mice have increased rate of autophagy in the heart, skeletal muscle and liver. Thus, T1R3 has multiple physiological functions and is widely expressed in vivo. However, the exact mechanisms regulating T1R3 expression are largely unknown. Here, we used comparative genomics and functional analyses to characterize the genomic region upstream of the annotated transcriptional start of human T1R3. This revealed that the T1R3 promoter in human and mouse resides in an evolutionary conserved region (ECR). We also identified a repressive element located upstream of the human T1R3 promoter that has relatively high degree of conservation with rhesus macaque. Additionally, the muscle regulatory factors MyoD and Myogenin regulate T1R3 expression and T1R3 expression increases with skeletal muscle differentiation of murine myoblast C2C12 cells. Taken together, our study raises the possibility that MyoD and Myogenin might control skeletal muscle metabolism and homeostasis through the regulation of T1R3 promoter activity.

N-linked glycosylation of the bone morphogenetic protein receptor type 2 (BMPR2) enhances ligand binding.

The bone morphogenetic protein (BMP) signaling pathway is essential for normal development and tissue homeostasis. BMP signal transduction occurs when ligands interact with a complex of type 1 and type 2 receptors to activate downstream transcription factors. It is well established that a single BMP receptor may bind multiple BMP ligands with varying affinity, and this has been largely attributed to conformation at the amino acid level. However, all three type 2 BMP receptors (BMPR2, ACVR2A/B) contain consensus N-glycosylation sites in their extracellular domains (ECDs), which could play a role in modulating interaction with ligand. Here, we show a differential pattern of N-glycosylation between BMPR2 and ACVR2A/B. Site-directed mutagenesis reveals that BMPR2 is uniquely glycosylated near its ligand binding domain and at a position that is mutated in patients with heritable pulmonary arterial hypertension. We further demonstrate using a cell-free pulldown assay that N-glycosylation of the BMPR2-ECD enhances its ability to bind BMP2 ligand but has no impact on binding by the closely-related ACVR2B. Our results illuminate a novel aspect of BMP signaling pathway mechanics and demonstrate a functional difference resulting from post-translational modification of type 2 BMP receptors. Additionally, since BMPR2 is required for several aspects of normal development and defects in its function are strongly implicated in human disease, our findings are likely to be relevant in several biological contexts in normal and abnormal human physiology.

Examining the Role of Hypothalamus-Derived Neuromedin-U (NMU) in Bone Remodeling of Rats.

Global loss of the neuropeptide Neuromedin-U (NMU) is associated with increased bone formation and high bone mass in male and female mice by twelve weeks of age, suggesting that NMU suppresses osteoblast differentiation and/or activity in vivo. NMU is highly expressed in numerous anatomical locations including the skeleton and the hypothalamus. This raises the possibility that NMU exerts indirect effects on bone remodeling from an extra-skeletal location such as the brain. Thus, in the present study we used microinjection to deliver viruses carrying short-hairpin RNA designed to knockdown Nmu expression in the hypothalamus of 8-week-old male rats and evaluated the effects on bone mass in the peripheral skeleton. Quantitative RT-PCR confirmed approximately 92% knockdown of Nmu in the hypothalamus. However, after six weeks, micro computed tomography on tibiae from Nmu-knockdown rats demonstrated no significant change in trabecular or cortical bone mass as compared to controls. These findings are corroborated by histomorphometric analyses which indicate no differences in osteoblast or osteoclast parameters between controls and Nmu-knockdown samples. Collectively, these data suggest that hypothalamus-derived NMU does not regulate bone remodeling in the postnatal skeleton. Future studies are necessary to delineate the direct versus indirect effects of NMU on bone remodeling.

The role of BMP2 signaling in the skeleton.

While new roles for the adult skeleton as an endocrine organ continue to emerge, our understanding of how bone homeostasis is maintained is also changing. Here we focus on BMP2, a molecule identified by its ability to induce bone formation at extraskeletal sites. We detail specific roles for BMP2 in the adult skeleton, where it acts to regulate the differentiation of periosteal skeletal progenitors during fracture healing and also mediates osteoblast formation in the bone marrow microenvironment. We highlight two areas of BMP2 biology that deserve further study: the specific signaling pathways used by BMP2 to affect bone formation, and the factors that regulate BMP2 production in the adult skeleton. These activities serve to distinguish BMP2 from other members of the TGF-b/BMP/Activin gene superfamily.

The Metabolic Bone Disease X-linked Hypophosphatemia: Case Presentation, Pathophysiology and Pharmacology.

The authors present a stereotypical case presentation of X-linked hypophosphatemia (XLH) and provide a review of the pathophysiology and related pharmacology of this condition, primarily focusing on the FDA-approved medication burosumab. XLH is a renal phosphate wasting disorder caused by loss of function mutations in the PHEX gene (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). Typical biochemical findings include elevated serum levels of bioactive/intact fibroblast growth factor 23 (FGF23) which lead to (i) low serum phosphate levels, (ii) increased fractional excretion of phosphate, and (iii) inappropriately low or normal 1,25-dihydroxyvitamin D (1,25-vitD). XLH is the most common form of heritable rickets and short stature in patients with XLH is due to chronic hypophosphatemia. Additionally, patients with XLH experience joint pain and osteoarthritis from skeletal deformities, fractures, enthesopathy, spinal stenosis, and hearing loss. Historically, treatment for XLH was limited to oral phosphate supplementation, active vitamin D supplementation, and surgical intervention for cases of severe bowed legs. In 2018, the United States Food and Drug Administration (FDA) approved burosumab for the treatment of XLH and this medication has demonstrated substantial benefit compared with conventional therapy. Burosumab binds circulating intact FGF23 and blocks its biological effects in target tissues, resulting in increased serum inorganic phosphate (Pi) concentrations and increased conversion of inactive vitamin D to active 1,25-vitD.

NMUR1 in the NMU-Mediated Regulation of Bone Remodeling.

Neuromedin-U (NMU) is an evolutionarily conserved peptide that regulates varying physiologic effects including blood pressure, stress and allergic responses, metabolic and feeding behavior, pain perception, and neuroendocrine functions. Recently, several lines of investigation implicate NMU in regulating bone remodeling. For instance, global loss of NMU expression in male and female mice leads to high bone mass due to elevated bone formation rate with no alteration in bone resorption rate or observable defect in skeletal patterning. Additionally, NMU treatment regulates the activity of osteoblasts in vitro. The downstream pathway utilized by NMU to carry out these effects is unknown as NMU signals via two G-protein-coupled receptors (GPCRs), NMU receptor 1 (NMUR1), and NMU receptor 2 (NMUR2), and both are expressed in the postnatal skeleton. Here, we sought to address this open question and build a better understanding of the downstream pathway utilized by NMU. Our approach involved the knockdown of Nmur1 in MC3T3-E1 cells in vitro and a global knockout of Nmur1 in vivo. We detail specific cell signaling events (e.g., mTOR phosphorylation) that are deficient in the absence of NMUR1 expression yet trabecular bone volume in femora and tibiae of 12-week-old male Nmur1 knockout mice are unchanged, compared to controls. These results suggest that NMUR1 is required for NMU-dependent signaling in MC3T3-E1 cells, but it is not required for the NMU-mediated effects on bone remodeling in vivo. Future studies examining the role of NMUR2 are required to determine the downstream pathway utilized by NMU to regulate bone remodeling in vivo.

ID family protein expression and regulation in hypoxic pulmonary hypertension.

Bone morphogenetic protein (BMP) signaling has been linked to the development of pulmonary hypertension (PH). Inhibitors of differentiation (ID) proteins (ID1-4) are a family of basic helix-loop-helix transcription factors that are downstream targets of the BMP signaling pathway, but the role that ID proteins play in the development of PH is unknown. To address this, we evaluated pulmonary expression of ID proteins in a mouse model of hypoxia-induced PH. There is selective induction of ID1 and ID3 expression in hypoxic pulmonary vascular smooth muscle cells (VSMCs) in vivo, and ID1 and ID3 expression are increased by hypoxia in cultured pulmonary VSMCs in a BMP-dependent fashion. ID4 protein is barely detectable in the mouse lung, and while ID2 is induced in hypoxic peripheral VSMCs in vivo, it is not increased by hypoxia or BMP signaling in cultured pulmonary VSMCs. In addition, the PH response to chronic hypoxia is indistinguishable between wild type and Id1 null mice. This is associated with a compensatory increase in ID3 but not ID2 expression in pulmonary VSMCs of Id1 null mice. These findings indicate that ID1 is dispensable for mounting a normal pulmonary vascular response to hypoxia, but suggest that ID3 may compensate for loss of ID1 expression in pulmonary VSMCs. Taken together, these findings indicate that ID1 and ID3 expression are regulated in a BMP-dependent fashion in hypoxic pulmonary VSMCs, and that ID1 and ID3 may play a cooperative role in regulating BMP-dependent VSMC responses to chronic hypoxia.

Bmp2 and Bmp4 exert opposing effects in hypoxic pulmonary hypertension.

The bone morphogenetic protein (BMP) type 2 receptor ligand, Bmp2, is upregulated in the peripheral pulmonary vasculature during hypoxia-induced pulmonary hypertension (PH). This contrasts with the expression of Bmp4, which is expressed in respiratory epithelia throughout the lung. Unlike heterozygous null Bmp4 mice (Bmp4(LacZ/+)), which are protected from the development of hypoxic PH, mice that are heterozygous null for Bmp2 (Bmp2(+/-)) develop more severe hypoxic PH than their wild-type littermates. This is associated with reduced endothelial nitric oxide synthase (eNOS) expression and activity in the pulmonary vasculature of hypoxic Bmp2(+/-) but not Bmp4(LacZ/+) mutant mice. Furthermore, exogenous BMP2 upregulates eNOS expression and activity in intrapulmonary artery and pulmonary endothelial cell preparations, indicating that eNOS is a target of Bmp2 signaling in the pulmonary vasculature. Together, these data demonstrate that Bmp2 and Bmp4 exert opposing roles in hypoxic PH and suggest that the protective effects of Bmp2 are mediated by increasing eNOS expression and activity in the hypoxic pulmonary vasculature.

Mechanical stimulation of human dermal fibroblasts regulates pro-inflammatory cytokines: potential insight into soft tissue manual therapies.

OBJECTIVE: Soft tissue manual therapies are commonly utilized by osteopathic physicians, chiropractors, physical therapists and massage therapists. These techniques are predicated on subjecting tissues to biophysical mechanical stimulation but the cellular and molecular mechanism(s) mediating these effects are poorly understood. Previous studies established an in vitro model system for examining mechanical stimulation of dermal fibroblasts and established that cyclical strain, intended to mimic overuse injury, induces secretion of numerous pro-inflammatory cytokines. Moreover, mechanical strain intended to mimic soft tissue manual therapy reduces strain-induced secretion of pro-inflammatory cytokines. Here, we sought to partially confirm and extend these reports and provide independent corroboration of prior results. RESULTS: Using cultures of primary human dermal fibroblasts, we confirm cyclical mechanical strain increases levels of IL-6 and adding long-duration stretch, intended to mimic therapeutic soft tissue stimulation, after cyclical strain results in lower IL-6 levels. We also extend the prior work, reporting that long-duration stretch results in lower levels of IL-8. Although there are important limitations to this experimental model, these findings provide supportive evidence that therapeutic soft tissue stimulation may reduce levels of pro-inflammatory cytokines. Future work is required to address these open questions and advance the mechanistic understanding of therapeutic soft tissue stimulation.

Assessment of the Research Interests and Perceptions of First-Year Medical Students at 4 Colleges of Osteopathic Medicine.

CONTEXT: There are limited data regarding the experiences of and attitudes toward research participation among osteopathic medical students despite rapidly increasing enrollment and expansion of the number of osteopathic medical schools. OBJECTIVE: To assess first-year osteopathic medical students' experience with research, their interest in it, their perceptions of its value, and barriers to participation. METHODS: An anonymous, online survey was sent to 868 medical students in the class of 2021 at 4 colleges of osteopathic medicine. The survey consisted of 14 multiple-choice items (7 of which offered the option of a written response) and 1 open-ended item that asked them to report their age. The survey remained open for 2 weeks, with 1 reminder email sent on the last day of the survey. Incomplete responses were excluded from the analysis. RESULTS: A total of 328 participants were included, for a response rate of 38%. A majority of respondents reported previous research experience (261 [79.6%]), consistent with a strong perception that research participation is important (315 [96.0%]). Fewer students (177 [54.0%]) were either currently participating in research or affirmed interest in performing research during medical school, with the highest level of interest in clinical research (259 [79.0%]) followed by basic science (166 [50.6%]). Regarding incentives that might encourage participation in research, students preferred monetary compensation (213 [64.9%]) or extra credit in courses (195 [59.5%]). A commonly reported barrier to performing research during medical school was the possibility of a negative impact on performance in coursework (289 [88.1%]). CONCLUSION: First-year osteopathic medical students are interested in research, view research experience as valuable, and consider research experience as beneficial to future career development. This study's findings highlight opportunities for increasing student participation in research through incentives or removal of perceived barriers.

Identification of a bone morphogenetic protein type 2 receptor neutralizing antibody.

OBJECTIVE: The bone morphogenetic protein (BMP) signaling pathway comprises the largest subdivision of the transforming growth factor (TGFß) superfamily. BMP signaling plays essential roles in both embryonic development and postnatal tissue homeostasis. Dysregulated BMP signaling underlies human pathologies ranging from pulmonary arterial hypertension to heterotopic ossification. Thus, understanding the basic mechanisms and regulation of BMP signaling may yield translational opportunities. Unfortunately, limited tools are available to evaluate this pathway, and genetic approaches are frequently confounded by developmental requirements or ability of pathway components to compensate for one another. Specific inhibitors for type 2 receptors are poorly represented. Thus, we sought to identify and validate an antibody that neutralizes the ligand-binding function of BMP receptor type 2 (BMPR2) extracellular domain (ECD). RESULTS: Using a modified, cell-free immunoprecipitation assay, we examined the neutralizing ability of the mouse monoclonal antibody 3F6 and found a dose-dependent inhibition of BMPR2-ECD ligand-binding. Consistent with this, 3F6 blocks endogenous BMPR2 function in the BMP-responsive cell line HEK293T. The specificity of 3F6 action was confirmed by demonstrating that this antibody has no effect on BMP-responsiveness in HEK293T cells in which BMPR2 expression is knocked-down. Our results provide important proof-of-concept data for future studies interrogating BMPR2 function.