iMSC-mediated delivery of ACVR2B-Fc fusion protein reduces heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva.

BACKGROUND: Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease caused by a gain-of-function mutation in ACVR1, which is a bone morphogenetic protein (BMP) type I receptor. Moreover, it causes progressive heterotopic ossification (HO) in connective tissues. Using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) and mouse models, we elucidated the underlying mechanisms of FOP pathogenesis and identified a candidate drug for FOP. METHODS: In the current study, healthy mesenchymal stem/stromal cells derived from iPSCs (iMSCs) expressing ACVR2B-Fc (iMSCACVR2B-Fc), which is a neutralizing receptobody, were constructed. Furthermore, patient-derived iMSCs and FOP mouse model (ACVR1R206H, female) were used to confirm the inhibitory function of ACVR2B-Fc fusion protein secreted by iMSCACVR2B-Fc on BMP signaling pathways and HO development, respectively. RESULTS: We found that secreted ACVR2B-Fc attenuated BMP signaling initiated by Activin-A and BMP-9 in both iMSCs and FOP-iMSCs in vitro. Transplantation of ACVR2B-Fc-expressing iMSCs reduced primary HO in a transgenic mouse model of FOP. Notably, a local injection of ACVR2B-Fc-expressing iMSCs and not an intraperitoneal injection improved the treadmill performance, suggesting compound effects of ACVR2B-Fc and iMSCs. CONCLUSIONS: These results offer a new perspective for treating FOP through stem cell therapy.

Impact of heterozygous ALK1 mutations on the transcriptomic response to BMP9 and BMP10 in endothelial cells from hereditary hemorrhagic telangiectasia and pulmonary arterial hypertension donors.

Heterozygous activin receptor-like kinase 1 (ALK1) mutations are associated with two vascular diseases: hereditary hemorrhagic telangiectasia (HHT) and more rarely pulmonary arterial hypertension (PAH). Here, we aimed to understand the impact of ALK1 mutations on BMP9 and BMP10 transcriptomic responses in endothelial cells. Endothelial colony-forming cells (ECFCs) and microvascular endothelial cells (HMVECs) carrying loss of function ALK1 mutations were isolated from newborn HHT and adult PAH donors, respectively. RNA-sequencing was performed on each type of cells compared to controls following an 18 h stimulation with BMP9 or BMP10. In control ECFCs, BMP9 and BMP10 stimulations induced similar transcriptomic responses with around 800 differentially expressed genes (DEGs). ALK1-mutated ECFCs unexpectedly revealed highly similar transcriptomic profiles to controls, both at the baseline and upon stimulation, and normal activation of Smad1/5 that could not be explained by a compensation in cell-surface ALK1 level. Conversely, PAH HMVECs revealed strong transcriptional dysregulations compared to controls with > 1200 DEGs at the baseline. Consequently, because our study involved two variables, ALK1 genotype and BMP stimulation, we performed two-factor differential expression analysis and identified 44 BMP9-dysregulated genes in mutated HMVECs, but none in ECFCs. Yet, the impaired regulation of at least one hit, namely lunatic fringe (LFNG), was validated by RT-qPCR in three different ALK1-mutated endothelial models. In conclusion, ALK1 heterozygosity only modified the BMP9/BMP10 regulation of few genes, including LFNG involved in NOTCH signaling. Future studies will uncover whether dysregulations in such hits are enough to promote HHT/PAH pathogenesis, making them potential therapeutic targets, or if second hits are necessary.

Antibody blockade of activin type II receptors preserves skeletal muscle mass and enhances fat loss during GLP-1 receptor agonism.

OBJECTIVE: Glucagon-like peptide 1 (GLP-1) receptor agonists reduce food intake, producing remarkable weight loss in overweight and obese individuals. While much of this weight loss is fat mass, there is also a loss of lean mass, similar to other approaches that induce calorie deficit. Targeting signaling pathways that regulate skeletal muscle hypertrophy is a promising avenue to preserve lean mass and modulate body composition. Myostatin and Activin A are TGFß-like ligands that signal via the activin type II receptors (ActRII) to antagonize muscle growth. Pre-clinical and clinical studies demonstrate that ActRII blockade induces skeletal muscle hypertrophy and reduces fat mass. In this manuscript, we test the hypothesis that combined ActRII blockade and GLP-1 receptor agonism will preserve muscle mass, leading to improvements in skeletomuscular and metabolic function and enhanced fat loss. METHODS: In this study, we explore the therapeutic potential of bimagrumab, a monoclonal antibody against ActRII, to modify body composition alone and during weight loss induced by GLP-1 receptor agonist semaglutide in diet-induced obese mice. Mechanistically, we define the specific role of the anabolic kinase Akt in mediating the hypertrophic muscle effects of ActRII inhibition in vivo. RESULTS: Treatment of obese mice with bimagrumab induced a ∼10 % increase in lean mass while simultaneously decreasing fat mass. Daily treatment of obese mice with semaglutide potently decreased body weight; this included a significant decrease in both muscle and fat mass. Combination treatment with bimagrumab and semaglutide led to superior fat mass loss while simultaneously preserving lean mass despite reduced food intake. Treatment with both drugs was associated with improved metabolic outcomes, and increased lean mass was associated with improved exercise performance. Deletion of both Akt isoforms in skeletal muscle modestly reduced, but did not prevent, muscle hypertrophy driven by ActRII inhibition. CONCLUSIONS: Collectively, these data demonstrate that blockade of ActRII signaling improves body composition and metabolic parameters during calorie deficit driven by GLP-1 receptor agonism and demonstrate the existence of Akt-independent pathways supporting muscle hypertrophy in the absence of ActRII signaling.

ACVRL1 drives resistance to multitarget tyrosine kinase inhibitors in colorectal cancer by promoting USP15-mediated GPX2 stabilization.

BACKGROUND: Multitarget tyrosine kinase inhibitors (mTKIs) such as Regorafenib and Sorafenib have already been approved for the treatment of many solid tumours. However, the efficacy of mTKIs in colorectal cancer (CRC) is limited; the underlined mechanism remains largely elusive. Our study was aimed to find out the resistance mechanism of mTKIs in CRC. METHODS: RNA sequencing was used to identify the expression of Activin A receptor-like type 1 (ACVRL1) under the treatment of mTKIs. Gain/loss-of-function experiments were performed to assess the biological function of ACVRL1 in resistance to mTKIs. The underlying mechanisms of ACVRL1-mediated mTKI resistance were investigated by using liquid chromatography-mass spectrometry assays (LC-MS), co-immunoprecipitation assays (Co-IP), chromatin immunoprecipitation assays, ubiquitination assays, dual luciferase reporter assays, etc. RESULTS: RNA sequencing identified the activation of ACVRL1 under the treatment of mTKIs in CRC cells. ACVRL1 knockdown and overexpression significantly affects the sensitivity of CRC cells to mTKIs both in vitro and vivo. Mechanistically, we found the ß-catenin/TCF-1-KCNQ1OT1/miR-7-5p axis mediated the activation of ACVRL1. Furthermore, LC-MS assays indicated the interaction between ACVRL1 and glutathione peroxidase 2(GPX2) protein. IP assay defined ACVRL1 truncation (282-503aa) could be responsible for interacting with GPX2, and rescue experiments with ACVRL1 truncations confirmed the importance of this interaction in driving mTKI resistance. Co-IP assays confirmed that ACVRL1 associates with ubiquitin-specific peptidase 15(USP15) which directly deubiquinates GPX2 at the K187(K, lysine) site, leading to the accumulation of GPX2 protein. Rescue experiments performed with the lysine mutants in GPX2 CRISPR knockout cell model confirmed the importance of GPX2 K187 mutant. As a result, the increased ROS clearance and decreased cell apoptosis eventually lead to mTKI resistance in CRC. CONCLUSIONS: Our results demonstrate that the Wnt/ß-catenin/KCNQ1OT1/miR-7-5p/ACVRL1/GPX2 biological axis plays a vital role in CRC, targeting which may be an effective approach for overcoming mTKI resistance.

Understanding the pathogenesis of brain arteriovenous malformation: genetic variations, epigenetics, signaling pathways, and immune inflammation.

Brain arteriovenous malformation (BAVM) is a rare but serious cerebrovascular disease whose pathogenesis has not been fully elucidated. Studies have found that epigenetic regulation, genetic variation and their signaling pathways, immune inflammation, may be the cause of BAVM the main reason. This review comprehensively analyzes the key pathways and inflammatory factors related to BAVMs, and explores their interplay with epigenetic regulation and genetics. Studies have found that epigenetic regulation such as DNA methylation, non-coding RNAs and m6A RNA modification can regulate endothelial cell proliferation, apoptosis, migration and damage repair of vascular malformations through different target gene pathways. Gene defects such as KRAS, ACVRL1 and EPHB4 lead to a disordered vascular environment, which may promote abnormal proliferation of blood vessels through ERK, NOTCH, mTOR, Wnt and other pathways. PDGF-B and PDGFR-ß were responsible for the recruitment of vascular adventitial cells and smooth muscle cells in the extracellular matrix environment of blood vessels, and played an important role in the pathological process of BAVM. Recent single-cell sequencing data revealed the diversity of various cell types within BAVM, as well as the heterogeneous expression of vascular-associated antigens, while neutrophils, macrophages and cytokines such as IL-6, IL-1, TNF-α, and IL-17A in BAVM tissue were significantly increased. Currently, there are no specific drugs targeting BAVMs, and biomarkers for BAVM formation, bleeding, and recurrence are lacking clinically. Therefore, further studies on molecular biological mechanisms will help to gain insight into the pathogenesis of BAVM and develop potential therapeutic strategies.

Nonfunctional TGF-ß/ALK1/ENG signaling pathway supports neutrophil proangiogenic activity in hereditary hemorrhagic telangiectasia.

The transforming growth factor ß (TGF-ß)/ALK1/ENG signaling pathway maintains quiescent state of endothelial cells, but at the same time, it regulates neutrophil functions. Importantly, mutations of this pathway lead to a rare autosomal disorder called hereditary hemorrhagic telangiectasia (HHT), characterized with abnormal blood vessel formation (angiogenesis). As neutrophils are potent regulators of angiogenesis, we investigated how disturbed TGF-ß/ALK1/ENG signaling influences angiogenic properties of these cells in HHT. We could show for the first time that not only endothelial cells, but also neutrophils isolated from such patients are ENG/ALK1 deficient. This deficiency obviously stimulates proangiogenic switch of such neutrophils. Elevated proangiogenic activity of HHT neutrophils is mediated by the increased spontaneous degranulation of gelatinase granules, resulting in high release of matrix-degrading matrix metalloproteinase 9 (MMP9). In agreement, therapeutic disturbance of this process using Src tyrosine kinase inhibitors impaired proangiogenic capacity of such neutrophils. Similarly, inhibition of MMP9 activity resulted in significant impairment of neutrophil-mediated angiogenesis. All in all, deficiency in TGF-ß/ALK1/ENG signaling in HHT neutrophils results in their proangiogenic activation and disease progression. Therapeutic strategies targeting neutrophil degranulation and MMP9 release and activity may serve as a potential therapeutic option for HHT.

High concentrations of soluble endoglin can inhibit BMP9 signaling in non-endothelial cells.

Endoglin (ENG) is a single-pass transmembrane protein highly expressed on vascular endothelial cells, although low expression levels can be detected in many other cell types. Its extracellular domain can be found in circulation known as soluble endoglin (sENG). Levels of sENG are elevated in many pathological conditions, in particular preeclampsia. We have shown that while loss of cell surface ENG decreases BMP9 signaling in endothelial cells, knocking down ENG in blood cancer cells enhances BMP9 signaling. Despite sENG binding to BMP9 with high affinity and blocking the type II receptor binding site on BMP9, sENG did not inhibit BMP9 signaling in vascular endothelial cells, but the dimeric form of sENG inhibited BMP9 signaling in blood cancer cells. Here we report that in non-endothelial cells such as human multiple myeloma cell lines and the mouse myoblast cell line C2C12, both monomeric and dimeric forms of sENG inhibit BMP9 signaling when present at high concentrations. Such inhibition can be alleviated by the overexpression of ENG and ACVRL1 (encoding ALK1) in the non-endothelial cells. Our findings suggest that the effects of sENG on BMP9 signaling is cell-type specific. This is an important consideration when developing therapies targeting the ENG and ALK1 pathway.

Blood endothelial ALK1-BMP4 signaling axis regulates adult hair follicle stem cell activation.

Blood vessels can play dual roles in tissue growth by transporting gases and nutrients and by regulating tissue stem cell activity via signaling. Correlative evidence implicates skin endothelial cells (ECs) as signaling niches of hair follicle stem cells (HFSCs), but functional demonstration from gene depletion of signaling molecules in ECs is missing to date. Here, we show that depletion of the vasculature-factor Alk1 increases BMP4 secretion from ECs, which delays HFSC activation. Furthermore, while previous evidence suggests a lymphatic vessel role in adult HFSC activation possibly through tissue drainage, a blood vessel role has not yet been addressed. Genetic perturbation of the ALK1-BMP4 axis in all ECs or the lymphatic ECs specifically unveils inhibition of HFSC activation by blood vessels. Our work suggests a broader relevance of blood vessels, adding adult HFSCs to the EC functional repertoire as signaling niches for the adult stem cells.

Associated genetic variants and potential pathogenic mechanisms of brain arteriovenous malformation.

BACKGROUND: The pathogenic mechanism of brain arteriovenous malformation (bAVM) is poorly understood. A growing body of evidence indicates that genetic factors play crucial roles in bAVM. This study examined genetic variants associated with bAVM through quantitative synthesis and qualitative description of literature. METHODS: Five databases were searched to gather potentially relevant articles published up to January 2022. STATA 14.0 software was used for statistical analyses. Pooled odds ratios and 95% confidence intervals were calculated with random effect models, and heterogeneity was assessed using the Cochran Q test and quantified with the I 2 test. Sensitivity and publication bias were analyzed to test the robustness of the associations. Variants identified in only one study or with great heterogeneity were not suitable for pooling association analysis, and therefore a qualitative systematic review was performed. RESULTS: In total, 30 papers were included in a systematic review involving 4709 cases and 7832 controls, where 17 papers were in a meta-analysis. A suggested association of bAVM was observed with ACVRL1 rs2071219 in the additive model and CDKN2B-AS1 rs1333040 in the recessive and additive models. Other variants of genes that could not be analyzed were summarized by qualitative description. These genes were mostly involved in bone morphogenic protein/transforming growth factor beta (BMP/TGF-ß), vascular endothelial growth factor/vascular endothelial growth factor receptor (VEGF/VEGFR), and RAS-mitogen activated protein kinase (MAPK) signaling and inflammation. CONCLUSIONS: According to our meta-analysis, ACVRL1 rs2071219 and CDKN2B-AS1 rs1333040 were potentially associated with bAVM. Multiple pathological signaling pathways could affect disease development. Future studies should aim to determine the interaction of candidate genes with environmental risk factors and to elucidate detailed mechanisms of action of variants and genes.1.

BMP10 functions independently from BMP9 for the development of a proper arteriovenous network.

Hereditary hemorrhagic telangiectasia (HHT) is a genetic vascular disorder characterized by the presence of arteriovenous malformation (AVM) in multiple organs. HHT is caused by mutations in genes encoding major constituents for transforming growth factor-ß (TGF-ß) family signaling: endoglin (ENG), activin receptor-like kinase 1 (ALK1), and SMAD4. The identity of physiological ligands for this ENG-ALK1 signaling pertinent to AVM formation has yet to be clearly determined. To investigate whether bone morphogenetic protein 9 (BMP9), BMP10, or both are physiological ligands of ENG-ALK1 signaling involved in arteriovenous network formation, we generated a novel Bmp10 conditional knockout mouse strain. We examined whether global Bmp10-inducible knockout (iKO) mice develop AVMs at neonatal and adult stages in comparison with control, Bmp9-KO, and Bmp9/10-double KO (dKO) mice. Bmp10-iKO and Bmp9/10-dKO mice showed AVMs in developing retina, postnatal brain, and adult wounded skin, while Bmp9-KO did not display any noticeable vascular defects. Bmp10 deficiency resulted in increased proliferation and size of endothelial cells in AVM vessels. The impaired neurovascular integrity in the brain and retina of Bmp10-iKO and Bmp9/10-dKO mice was detected. Bmp9/10-dKO mice exhibited the lethality and vascular malformation similar to Bmp10-iKO mice, but their phenotypes were more pronounced. Administration of BMP10 protein, but not BMP9 protein, prevented retinal AVM in Bmp9/10-dKO and endothelial-specific Eng-iKO mice. These data indicate that BMP10 is indispensable for the development of a proper arteriovenous network, whereas BMP9 has limited compensatory functions for the loss of BMP10. We suggest that BMP10 is the most relevant physiological ligand of the ENG-ALK1 signaling pathway pertinent to HHT pathogenesis.

The Influence of Adiponectin on Transport of Low-Density Lipoproteins through Human Endothelial Cell Monolayer In Vitro.

The influence of adiponectin, a protein secreted by adipocytes, on the activation of transendothelial LDL transport, the initial event of atherogenesis, was studied. The addition of adiponectin to the cultured endothelial hybridoma EA.hy926 cells did not affect both basal and TNF-stimulated transendothelial transport of LDL. In addition, adiponectin affects neither expression levels of CAV1, SCARB1, and ACVRL1 genes encoding proteins involved in transendothelial LDL transport, nor the MMP secretion by the EA.hy926cells. At the same time, adiponectin suppressed the TNF-stimulated IL-8 production and expression of the adhesion molecule gene ICAM1 in these cells. Thus, adiponectin reduces proinflammatory activation of EA.hy926 cells, which is not accompanied by changes in the transendothelial LDL transport. We speculate that anti-inflammatory action of adiponectin is the base for the influence of this adipokine on atherogenesis.

Detection of activin receptor type IIA and IIB-Fc fusion proteins by automated capillary immunoassay.

Activin receptor type IIA and type IIB fusion protein have been designed to sequester circulating molecules of the transforming growth factor-ß (TGF-ß) superfamily and inactivate their actions. Members of this superfamily have been reported as essential regulators of erythropoiesis by triggering the formation of activated ternary complexes containing different combinations of type I and type II receptors, which can limit RBC production by accelerating erythroid differentiation and inhibiting erythroid progenitor expansion. The recent approval of Luspatercept for the treatment of anemia associated to transfusion-dependent MDS and Beta-thalassemia in afflicted patients means that it can now pose a real threat of being abused in sport for its ability to stimulate erythropoiesis. Several methods for the detection of these molecules in blood have been proposed for the purpose of sport antidoping control. Here we propose the detection of the ActRIIA-Fc and ActRIIB-Fc fusion proteins by automated capillary Western immunoassay (Simple Western). The use of these immunoassays for the detection of protein targets has become widespread in the recent years. The work presented here demonstrates that this methodology enables a versatile, rapid, and sensitive detection of activin ligand traps in blood samples: plasma, serum, or dried blood spots (DBS). Preliminary results indicate that detection in urine samples is also possible. The option to use different antibodies allows the possibility to use this method as an initial testing procedure as well as a confirmation procedure. Finally, results coming from an administration study confirm that the method is suitable for routine analysis.

Atheroprone fluid shear stress-regulated ALK1-Endoglin-SMAD signaling originates from early endosomes.

BACKGROUND: Fluid shear stress enhances endothelial SMAD1/5 signaling via the BMP9-bound ALK1 receptor complex supported by the co-receptor Endoglin. While moderate SMAD1/5 activation is required to maintain endothelial quiescence, excessive SMAD1/5 signaling promotes endothelial dysfunction. Increased BMP signaling participates in endothelial-to-mesenchymal transition and inflammation culminating in vascular diseases such as atherosclerosis. While the function of Endoglin has so far been described under picomolar concentrations of BMP9 and short-term shear application, we investigated Endoglin under physiological BMP9 and long-term pathophysiological shear conditions. RESULTS: We report here that knock-down of Endoglin leads to exacerbated SMAD1/5 phosphorylation and atheroprone gene expression profile in HUVECs sheared for 24 h. Making use of the ligand-trap ALK1-Fc, we furthermore show that this increase is dependent on BMP9/10. Mechanistically, we reveal that long-term exposure of ECs to low laminar shear stress leads to enhanced Endoglin expression and endocytosis of Endoglin in Caveolin-1-positive early endosomes. In these endosomes, we could localize the ALK1-Endoglin complex, labeled BMP9 as well as SMAD1, highlighting Caveolin-1 vesicles as a SMAD signaling compartment in cells exposed to low atheroprone laminar shear stress. CONCLUSIONS: We identified Endoglin to be essential in preventing excessive activation of SMAD1/5 under physiological flow conditions and Caveolin-1-positive early endosomes as a new flow-regulated signaling compartment for BMP9-ALK1-Endoglin signaling axis in atheroprone flow conditions.

The activin receptor ligand trap ActRIIB:ALK4-Fc ameliorates cardiomyopathy induced by neuromuscular disease and diabetes.

Cardiomyopathies are ascribed to a variety of etiologies, present with diverse clinical phenotypes, and lack disease-modifying treatments. Mounting evidence implicates dysregulated activin receptor signaling in heart disease and highlights inhibition of this pathway as a potential therapeutic target. Here, we explored the effects of activin ligand inhibition using ActRIIB:ALK4-Fc, a heterodimeric receptor fusion protein, in two mechanistically distinct murine models of cardiomyopathy. Treatment with ActRIIB:ALK4-Fc significantly improved systolic or diastolic function in cardiomyopathy induced by neuromuscular disease or diabetes mellitus. Moreover, ActRIIB:ALK4-Fc corrected Ca2+ handling protein expression in diseased heart tissues, suggesting that activin signaling inhibition could alleviate cardiomyopathies in part by rebalancing aberrant intracellular Ca2+ homeostasis-a common underlying pathomechanism in diverse heart diseases.

Hsa_circ_0129047 regulates the miR-375/ACVRL1 axis to attenuate the progression of lung adenocarcinoma.

BACKGROUND: Circular RNAs (circRNAs) are attractive candidates to be used as biomarkers of human cancers, including lung adenocarcinoma (LUAD). Our study aimed to investigate the functions and regulatory mechanisms of hsa_circ_0129047 in the tumorigenesis of LUAD. METHODS: Reverse transcription-quantitative polymerase chain reaction was performed to determine the circRNA, microRNA (miRNA), and mRNA expression levels in LUAD cell lines and tissues. Tumor xenografts were established in nude mice to evaluate whether hsa_circ_0129047 affected LUAD tumor development in vivo. Cell counting kit-8 and transwell assays were performed to assess the mechanisms by which hsa_circ_0129047 influenced the viability and migration of LUAD cells, respectively. Apoptosis was evaluated via determination of the levels of the apoptotic markers, B-cell lymphoma-2, and Bcl-2-associated X, via Western blotting. Dual-luciferase reporter assay, RNA immunoprecipitation assay, and Pearson's correlation analysis were performed to determine the relationships among miR-375 and hsa_circ_0129047 and activin A receptor-like type 1 (ACVRL1). RESULTS: Downregulation of hsa_circ_0129047 levels was observed in LUAD cell lines and tissues. Meanwhile, the upregulation of hsa_circ_0129047 levels repressed the proliferative, migratory, and survival capacities of LUAD cells in vitro. Hsa_circ_0129047 exerted antitumor effects during in vivo tumor development. Finally, we demonstrated that hsa_circ_0129047 sponged miR-375. This interaction facilitated the expression of the downstream target of miR-375, ACVRL1, whose upregulation inhibited the development and malignancy of LUAD. CONCLUSION: These findings demonstrate that hsa_circ_0129047 functions as a tumor inhibitor in LUAD by modulating the miR-375/ACVRL1 axis. Hence, hsa_circ_0129047 may be a promising biomarker and gene target for LUAD treatment.

EPDR1 is a noncanonical effector of insulin-mediated angiogenesis regulated by an endothelial-specific TGF-ß receptor complex.

Insulin signaling in blood vessels primarily functions to stimulate angiogenesis and maintain vascular homeostasis through the canonical PI3K and MAPK signaling pathways. However, angiogenesis is a complex process coordinated by multiple other signaling events. Here, we report a distinct crosstalk between the insulin receptor and endoglin/activin receptor-like kinase 1 (ALK1), an endothelial cell-specific TGF-ß receptor complex essential for angiogenesis. While the endoglin-ALK1 complex normally binds to TGF-ß or bone morphogenetic protein 9 (BMP9) to promote gene regulation via transcription factors Smad1/5, we show that insulin drives insulin receptor oligomerization with endoglin-ALK1 at the cell surface to trigger rapid Smad1/5 activation. Through quantitative proteomic analysis, we identify ependymin-related protein 1 (EPDR1) as a major Smad1/5 gene target induced by insulin but not by TGF-ß or BMP9. We found endothelial EPDR1 expression is minimal at the basal state but is markedly enhanced upon prolonged insulin treatment to promote cell migration and formation of capillary tubules. Conversely, we demonstrate EPDR1 depletion strongly abrogates these angiogenic effects, indicating that EPDR1 is a crucial mediator of insulin-induced angiogenesis. Taken together, these results suggest important therapeutic implications for EPDR1 and the TGF-ß pathways in pathologic angiogenesis during hyperinsulinemia and insulin resistance.

Hypoxia-induced inhibin promotes tumor growth and vascular permeability in ovarian cancers.

Hypoxia, a driver of tumor growth and metastasis, regulates angiogenic pathways that are targets for vessel normalization and ovarian cancer management. However, toxicities and resistance to anti-angiogenics can limit their use making identification of new targets vital. Inhibin, a heteromeric TGFß ligand, is a contextual regulator of tumor progression acting as an early tumor suppressor, yet also an established biomarker for ovarian cancers. Here, we find that hypoxia increases inhibin levels in ovarian cancer cell lines, xenograft tumors, and patients. Inhibin is regulated primarily through HIF-1, shifting the balance under hypoxia from activins to inhibins. Hypoxia regulated inhibin promotes tumor growth, endothelial cell invasion and permeability. Targeting inhibin in vivo through knockdown and anti-inhibin strategies robustly reduces permeability in vivo and alters the balance of pro and anti-angiogenic mechanisms resulting in vascular normalization. Mechanistically, inhibin regulates permeability by increasing VE-cadherin internalization via ACVRL1 and CD105, a receptor complex that we find to be stabilized directly by inhibin. Our findings demonstrate direct roles for inhibins in vascular normalization via TGF-ß receptors providing new insights into the therapeutic significance of inhibins as a strategy to normalize the tumor vasculature in ovarian cancer.

Type II BMP and activin receptors BMPR2 and ACVR2A share a conserved mode of growth factor recognition.

BMPR2 is a type II Transforming Growth Factor (TGF)-ß family receptor that is fundamentally associated with pulmonary arterial hypertension (PAH) in humans. BMPR2 shares functional similarities with the type II activin receptors ACVR2A and ACVR2B, as it interacts with an overlapping group of TGF-ß family growth factors (GFs). However, how BMPR2 recognizes GFs remains poorly understood. Here, we solved crystal structures of BMPR2 in complex with the GF activin B and of ACVR2A in complex with the related GF activin A. We show that both BMPR2 and ACVR2A bind GFs with nearly identical geometry using a conserved hydrophobic hot spot, while differences in contacting residues are predominantly found in loop areas. Upon further exploration of the GF-binding spectrum of the two receptors, we found that although many GFs bind both receptors, the high-affinity BMPR2 GFs comprise BMP15, BMP10, and Nodal, whereas those of ACVR2A are activin A, activin B, and GDF11. Lastly, we evaluated GF-binding domain BMPR2 variants found in human PAH patients. We demonstrate that mutations within the GF-binding interface resulted in loss of GF binding, while mutations in loop areas allowed BMPR2 to retain the ability to bind cognate GFs with high affinity. In conclusion, the in vitro activities of BMPR2 variants and the crystal structures reported here indicate biochemically relevant complexes that explain how some GF-binding domain variants can lead to PAH.

Endothelial progenitor cells stimulate neonatal lung angiogenesis through FOXF1-mediated activation of BMP9/ACVRL1 signaling.

Pulmonary endothelial progenitor cells (EPCs) are critical for neonatal lung angiogenesis and represent a subset of general capillary cells (gCAPs). Molecular mechanisms through which EPCs stimulate lung angiogenesis are unknown. Herein, we used single-cell RNA sequencing to identify the BMP9/ACVRL1/SMAD1 pathway signature in pulmonary EPCs. BMP9 receptor, ACVRL1, and its downstream target genes were inhibited in EPCs from Foxf1WT/S52F mutant mice, a model of alveolar capillary dysplasia with misalignment of pulmonary veins (ACDMPV). Expression of ACVRL1 and its targets were reduced in lungs of ACDMPV subjects. Inhibition of FOXF1 transcription factor reduced BMP9/ACVRL1 signaling and decreased angiogenesis in vitro. FOXF1 synergized with ETS transcription factor FLI1 to activate ACVRL1 promoter. Nanoparticle-mediated silencing of ACVRL1 in newborn mice decreased neonatal lung angiogenesis and alveolarization. Treatment with BMP9 restored lung angiogenesis and alveolarization in ACVRL1-deficient and Foxf1WT/S52F mice. Altogether, EPCs promote neonatal lung angiogenesis and alveolarization through FOXF1-mediated activation of BMP9/ACVRL1 signaling.

The m6A methyltransferase METTL3 regulates muscle maintenance and growth in mice.

Skeletal muscle serves fundamental roles in organismal health. Gene expression fluctuations are critical for muscle homeostasis and the response to environmental insults. Yet, little is known about post-transcriptional mechanisms regulating such fluctuations while impacting muscle proteome. Here we report genome-wide analysis of mRNA methyladenosine (m6A) dynamics of skeletal muscle hypertrophic growth following overload-induced stress. We show that increases in METTL3 (the m6A enzyme), and concomitantly m6A, control skeletal muscle size during hypertrophy; exogenous delivery of METTL3 induces skeletal muscle growth, even without external triggers. We also show that METTL3 represses activin type 2 A receptors (ACVR2A) synthesis, blunting activation of anti-hypertrophic signaling. Notably, myofiber-specific conditional genetic deletion of METTL3 caused spontaneous muscle wasting over time and abrogated overload-induced hypertrophy; a phenotype reverted by co-administration of a myostatin inhibitor. These studies identify a previously unrecognized post-transcriptional mechanism promoting the hypertrophic response of skeletal muscle via control of myostatin signaling.