Characterization of the different oligomeric states of the DAN family antagonists SOSTDC1 and SOST

miR4352b a cross-species modulator of SOSTDC1, targets dual pathway to regulate bone health and fracture healing

Mutations in SOST can lead to various monogenic bone diseases. Its paralog, SOSTDC1, shares 55 % protein sequence homology and belongs to the BMP antagonist class. Sostdc1-/- mice exhibit distinct effects on cortical and trabecular bone. Genetic polymorphisms in SOSTDC1 impacting peak bone mass makes SOSTDC1 gene, a candidate for influencing BMD variation in humans. SOSTDC1 is upregulated in bone loss conditions, altering BMP-responsive genes and signaling modulators, suggesting its dual BMP/Wnt antagonist role may enhance both pathways. Overexpression of SOSTDC1 confirmed its role as an osteogenic antagonist. Glycine max (Soy)-derived miR4352b, identified for cross-kingdom applications, precisely targets SOSTDC1, a key regulator of bone. SOSTDC1 competitively binds to BMP2 receptor, BMPR1A. Gma-miR4352b suppresses SOSTDC1 expression, enhancing osteogenesis and countering SOSTDC1's inhibition of osteogenic potential. Modeling estrogen deficiency to mimic elevated SOSTDC1 levels, we observed an inverse correlation with SOSTDC1 expression, while serum BMP2 and PINP levels increased following gma-miR4352b supplementation. In fracture healing, SOSTDC1's crucial role becomes evident in conditions of delayed fracture healing. As healing progresses, SOSTDC1 expression decreases. Gma-miR4352b, compared to scrambled miRNA, remarkably promotes callus formation, achieving 68 % healing by day 10, surpassing the scrambled group at 44 %. By the day 13, the treatment group exhibits advanced healing, challenging to find the callus, while the scrambled group maintains a healing rate similar to day10. The accelerated healing in the treatment group underscores the importance of SOSTDC1 in influencing early fracture healing, potentially through the activation of both BMP2 and Wnt signaling pathways.

Structural Basis of Non-Latent Signaling by the Anti-Müllerian Hormone Procomplex

Most TGFβ family ligands exist as procomplexes consisting of a prodomain noncovalently bound to a growth factor (GF); Whereas some prodomains confer latency, the Anti-Müllerian Hormone (AMH) prodomain maintains a remarkably high affinity for the GF yet remains active. Using single particle EM methods, we show the AMH prodomain consists of two subdomains: a vestigial TGFβ prodomain-like fold and a novel, helical bundle GF-binding domain, the result of an exon insertion 450 million years ago, that engages both receptor epitopes. When associated with the prodomain, the AMH GF is distorted into a strained, open conformation whose closure upon bivalent binding of AMHR2 displaces the prodomain through a conformational shift mechanism to allow for signaling.

Characterization of erythroferrone oligomerization and its impact on BMP antagonism

Hepcidin, a peptide hormone that negatively regulates iron metabolism, is expressed by bone morphogenetic protein (BMP) signaling. Erythroferrone (ERFE) is an extracellular protein that binds and inhibits BMP ligands, thus positively regulating iron import by indirectly suppressing hepcidin. This allows for rapid erythrocyte regeneration after blood loss. ERFE belongs to the C1Q/TNF-related protein family and is suggested to adopt multiple oligomeric forms: a trimer, a hexamer, and a high molecular weight species. The molecular basis for how ERFE binds BMP ligands and how the different oligomeric states impact BMP inhibition are poorly understood. In this study, we demonstrated that ERFE activity is dependent on the presence of stable dimeric or trimeric ERFE and that larger species are dispensable for BMP inhibition. Additionally, we used an in silico approach to identify a helix, termed the ligand-binding domain, that was predicted to bind BMPs and occlude the type I receptor pocket. We provide evidence that the ligand-binding domain is crucial for activity through luciferase assays and surface plasmon resonance analysis. Our findings provide new insight into how ERFE oligomerization impacts BMP inhibition, while identifying critical molecular features of ERFE essential for binding BMP ligands.

NBL1 Reduces Corneal Fibrosis and Scar Formation after Wounding

Corneal scarring is a leading cause of blindness. Currently, there is no treatment to prevent and/or reduce corneal scar formation under pathological conditions. Our previous data showed that the NBL1 protein, also termed the DAN Family BMP (Bone morphogenetic protein) Antagonist, was highly expressed in corneal stromal cells upon wounding. Here, we examined the function of NBL1 in corneal wound healing. Mouse corneas were mechanically wounded, followed by a 2-week treatment using NBL1. Wounded corneas treated with vehicle or an Fc tag served as controls. Compared with the controls, NBL1 treatment facilitated wound re-epithelialization, partially restored the stromal thickness, and significantly reduced corneal scar formation. NBL1 treatment did not decrease immune cell infiltration, indicating that the anti-scarring effect was not dependent on immune suppression. We further examined the anti-fibrotic effect of NBL1 on human corneas. Pairs of human corneas were induced to form myofibroblasts (a key player in fibrosis and scarring) upon wounding and incubation in a medium containing TGF-β1. The OS corneas were treated with Fc as a control, and the OD corneas were treated with NBL1. Compared with the control, human corneas treated with NBL1 had significantly fewer myofibroblasts, which was consistent with these mouse data. A further study revealed that NBL1 treatment inhibited BMP canonical (phospho-Smad1/5) and no-canonical (phospho-p38) pathways in human corneas. Data show that NBL1 reduced corneal fibrosis and scar formation in mice and cultured human corneas. The underlying molecular mechanism is not certain because both anti-fibrotic Smad1/5 and pro-fibrotic p38 pathways were inhibited upon NBL1 treatment. Whether the p38 pathway dominates the Smad1/5 pathway during corneal fibrosis, leading to the anti-fibrotic effect of NBL1, needs further investigation.

Characterization of erythroferrone oligomerization and its impact on BMP antagonism

Hepcidin, a peptide hormone that negatively regulates iron metabolism, is expressed by bone morphogenetic protein (BMP) signaling. Erythroferrone (ERFE) is an extracellular protein that binds and inhibits BMP ligands, thus positively regulating iron import by indirectly suppressing hepcidin. This allows for rapid erythrocyte regeneration after blood loss. ERFE belongs to the C1Q/TNF related protein (CTRP) family and is suggested to adopt multiple oligomeric forms: a trimer, a hexamer, and a high molecular weight species. The molecular basis for how ERFE binds BMP ligands and how the different oligomeric states impact BMP inhibition are poorly understood. In this study, we demonstrated that ERFE activity is dependent on the presence of stable dimeric or trimeric ERFE, and that larger species are dispensable for BMP inhibition. Additionally, we used an in-silico approach to identify a helix, termed the ligand binding domain (LBD), that was predicted to bind BMPs and occlude the type I receptor pocket. We provide evidence that the LBD is crucial for activity through luciferase assays and surface plasmon resonance (SPR) analysis. Our findings provide new insight into how ERFE oligomerization impacts BMP inhibition, while identifying critical molecular features of ERFE essential for binding BMP ligands.

Sostdc1 Suppression in the Absence of Sclerostin Potentiates Anabolic Action of Cortical Bone in Mice

The development of Wnt-based osteoanabolic agents has progressed rapidly in recent years, given the potent effects of Wnt modulation on bone homeostasis. Simultaneous pharmacologic inhibition of the Wnt antagonists sclerostin and Dkk1 can be optimized to create potentiated effects in the cancellous bone compartment. We looked for other candidates that might be co-inhibited along with sclerostin to potentiate the effects in the cortical compartment. Sostdc1 (Wise), like sclerostin and Dkk1, also binds and inhibits Lrp5/6 coreceptors to impair canonical Wnt signaling, but Sostdc1 has greater effects in the cortical bone. To test this concept, we deleted Sostdc1 and Sost from mice and measured the skeletal effects in cortical and cancellous compartments individually. Sost deletion alone produced high bone mass in all compartments, whereas Sostdc1 deletion alone had no measurable effects on either envelope. Mice with codeletion of Sostdc1 and Sost had high bone mass and increased cortical properties (bone mass, formation rates, mechanical properties), but only among males. Combined administration of sclerostin antibody and Sostdc1 antibody in wild-type female mice produced potentiation of cortical bone gain despite no effect of Sostdc1 antibody alone. In conclusion, Sostdc1 inhibition/deletion can work in concert with sclerostin deficiency to improve cortical bone properties. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Follistatin Forms a Stable Complex With Inhibin A That Does Not Interfere With Activin A Antagonism

Inhibins are transforming growth factor-β family heterodimers that suppress follicle-stimulating hormone (FSH) secretion by antagonizing activin class ligands. Inhibins share a common β chain with activin ligands. Follistatin is another activin antagonist, known to bind the common β chain of both activins and inhibins. In this study, we characterized the antagonist-antagonist complex of inhibin A and follistatin to determine if their interaction impacted activin A antagonism. We isolated the inhibin A:follistatin 288 complex, showing that it forms in a 1:1 stoichiometric ratio, different from previously reported homodimeric ligand:follistatin complexes, which bind in a 1:2 ratio. Small angle X-ray scattering coupled with modeling provided a low-resolution structure of inhibin A in complex with follistatin 288. Inhibin binds follistatin via the shared activin β chain, leaving the α chain free and flexible. The inhibin A:follistatin 288 complex was also shown to bind heparin with lower affinity than follistatin 288 alone or in complex with activin A. Characterizing the inhibin A:follistatin 288 complex in an activin-responsive luciferase assay and by surface plasmon resonance indicated that the inhibitor complex readily dissociated upon binding type II receptor activin receptor type IIb, allowing both antagonists to inhibit activin signaling. Additionally, injection of the complex in ovariectomized female mice did not alter inhibin A suppression of FSH. Taken together, this study shows that while follistatin binds to inhibin A with a substochiometric ratio relative to the activin homodimer, the complex can dissociate readily, allowing both proteins to effectively antagonize activin signaling.

Role of Sostdc1 in skeletal biology and cancer

Sclerostin domain-containing protein-1 (Sostdc1) is a member of the sclerostin family and encodes a secreted 28–32 kDa protein with a cystine knot-like domain and two N-linked glycosylation sites. Sostdc1 functions as an antagonist to bone morphogenetic protein (BMP), mediating BMP signaling. It also interacts with LRP6, mediating LRP6 and Wnt signaling, thus regulating cellular proliferation, differentiation, and programmed cell death. Sostdc1 plays various roles in the skin, intestines, brain, lungs, kidneys, and vasculature. Deletion of Sostdc1 gene in mice resulted in supernumerary teeth and improved the loss of renal function in Alport syndrome. In the skeletal system, Sostdc1 is essential for bone metabolism, bone density maintenance, and fracture healing. Recently, Sostdc1 has been found to be closely related to the development and progression of multiple cancer types, including breast, renal, gastric, and thyroid cancers. This article summarises the role of Sostdc1 in skeletal biology and related cancers to provide a theoretical basis for the treatment of related diseases.