Heterodimer-heterotetramer formation mediates enhanced sensor activity in a biophysical model for BMP signaling

Elevated BMPR2 expression amplifies osteoblast differentiation in ankylosing spondylitis

Objective

Bone morphogenetic protein receptor type 2 (BMPR2) has been associated with radiographic changes in ankylosing spondylitis (AS), but further characterization of the cellular signaling pathway in osteoprogenitor (OP) is not clearly understood. The aim of this study was to investigate the expression of BMPR2 and bone morphogenetic protein 2 (BMP2)-mediated responsibility in AS.

Methods

We collected 10 healthy control (HC) and 14 AS-OPs derived from facet joints. Subsequently, we then conducted RNA sequencing with two samples per group and selected BMP-related genes. Facet joint tissues and derived primary OPs were evaluated by validation of selected RNA sequencing data, immunohistochemistry, and comparison of osteogenic differentiation potential.

Results

Based on RNA-sequencing analysis, we found that BMPR2 expression is higher in AS-OPs compared to in HC-OPs. We also validated the increased BMPR2 expression in facet joint tissues with AS and its derived OPs in messenger RNA and protein levels. Additionally, primary AS-OPs showed much greater response to osteogenic differentiation induced by BMP2 and a higher capacity for smad1/5/8-induced RUNX2 expression compared to HCs.

Conclusion

The expression of BMPR2 was found to be significantly increased in facet joint tissues of patients with AS. These findings suggest that BMPR2 may play a role in the BMP2-mediated progression of AS.

Moderate-Affinity Affibodies Modulate the Delivery and Bioactivity of Bone Morphogenetic Protein-2

Uncontrolled bone morphogenetic protein-2 (BMP-2) release can lead to off-target bone growth and other adverse events. To tackle this challenge, yeast surface display is used to identify unique BMP-2-specific protein binders known as affibodies that bind to BMP-2 with different affinities. Biolayer interferometry reveals an equilibrium dissociation constant of 10.7 nm for the interaction between BMP-2 and high-affinity affibody and 34.8 nm for the interaction between BMP-2 and the low-affinity affibody. The low-affinity affibody-BMP-2 interaction also exhibits an off-rate constant that is an order of magnitude higher. Computational modeling of affibody-BMP-2 binding predicts that the high- and low-affinity affibodies bind to two distinct sites on BMP-2 that function as different cell-receptor binding sites. BMP-2 binding to affibodies reduces expression of the osteogenic marker alkaline phosphatase (ALP) in C2C12 myoblasts. Affibody-conjugated polyethylene glycol-maleimide hydrogels increase uptake of BMP-2 compared to affibody-free hydrogels, and high-affinity hydrogels exhibit lower BMP-2 release into serum compared to low-affinity hydrogels and affibody-free hydrogels over four weeks. Loading BMP-2 into affibody-conjugated hydrogels prolongs ALP activity of C2C12 myoblasts compared to soluble BMP-2. This work demonstrates that affibodies with different affinities can modulate BMP-2 delivery and activity, creating a promising approach for controlling BMP-2 delivery in clinical applications.

Early radial positional information in the cochlea is optimized by a precise linear BMP gradient and enhanced by SOX2

Positional information encoded in signaling molecules is essential for early patterning in the prosensory domain of the developing cochlea. The sensory epithelium, the organ of Corti, contains an exquisite repeating pattern of hair cells and supporting cells. This requires precision in the morphogen signals that set the initial radial compartment boundaries, but this has not been investigated. To measure gradient formation and morphogenetic precision in developing cochlea, we developed a quantitative image analysis procedure measuring SOX2 and pSMAD1/5/9 profiles in mouse embryos at embryonic day (E)12.5, E13.5, and E14.5. Intriguingly, we found that the pSMAD1/5/9 profile forms a linear gradient up to the medial ~ 75% of the PSD from the pSMAD1/5/9 peak in the lateral edge during E12.5 and E13.5. This is a surprising activity readout for a diffusive BMP4 ligand secreted from a tightly constrained lateral region since morphogens typically form exponential or power-law gradient shapes. This is meaningful for gradient interpretation because while linear profiles offer the theoretically highest information content and distributed precision for patterning, a linear morphogen gradient has not yet been observed. Furthermore, this is unique to the cochlear epithelium as the pSMAD1/5/9 gradient is exponential in the surrounding mesenchyme. In addition to the information-optimized linear profile, we found that while pSMAD1/5/9 is stable during this timeframe, an accompanying gradient of SOX2 shifts dynamically. Last, through joint decoding maps of pSMAD1/5/9 and SOX2, we see that there is a high-fidelity mapping between signaling activity and position in the regions that will become Kölliker's organ and the organ of Corti. Mapping is ambiguous in the prosensory domain precursory to the outer sulcus. Altogether, this research provides new insights into the precision of early morphogenetic patterning cues in the radial cochlea prosensory domain.

Heterodimerization-dependent secretion of bone morphogenetic proteins in Drosophila

Combinatorial signaling is key to instruct context-dependent cell behaviors. During embryonic development, adult homeostasis, and disease, bone morphogenetic proteins (BMPs) act as dimers to instruct specific cellular responses. BMP ligands can form both homodimers or heterodimers; however, obtaining direct evidence of the endogenous localization and function of each form has proven challenging. Here, we make use of precise genome editing and direct protein manipulation via protein binders to dissect the existence and functional relevance of BMP homodimers and heterodimers in the Drosophila wing imaginal disc. This approach identified in situ the existence of Dpp (BMP2/4)/Gbb (BMP5/6/7/8) heterodimers. We found that Gbb is secreted in a Dpp-dependent manner in the wing imaginal disc. Dpp and Gbb form a gradient of heterodimers, whereas neither Dpp nor Gbb homodimers are evident under endogenous physiological conditions. We find that the formation of heterodimers is critical for obtaining optimal signaling and long-range BMP distribution.

Computational modeling of TGF-β2:TβRI:TβRII receptor complex assembly as mediated by the TGF-β coreceptor betaglycan

Transforming growth factor-β1, -β2, and -β3 (TGF-β1, -β2, and -β3) are secreted signaling ligands that play essential roles in tissue development, tissue maintenance, immune response, and wound healing. TGF-β ligands form homodimers and signal by assembling a heterotetrameric receptor complex comprised of two type I receptor (TβRI):type II receptor (TβRII) pairs. TGF-β1 and TGF-β3 ligands signal with high potency due to their high affinity for TβRII, which engenders high-affinity binding of TβRI through a composite TGF-β:TβRII binding interface. However, TGF-β2 binds TβRII 200-500 more weakly than TGF-β1 and TGF-β3 and signals with lower potency compared with these ligands. Remarkably, the presence of an additional membrane-bound coreceptor, known as betaglycan, increases TGF-β2 signaling potency to levels similar to TGF-β1 and -β3. The mediating effect of betaglycan occurs even though it is displaced from and not present in the heterotetrameric receptor complex through which TGF-β2 signals. Published biophysics studies have experimentally established the kinetic rates of the individual ligand-receptor and receptor-receptor interactions that initiate heterotetrameric receptor complex assembly and signaling in the TGF-β system; however, current experimental approaches are not able to directly measure kinetic rates for the intermediate and latter steps of assembly. To characterize these steps in the TGF-β system and determine the mechanism of betaglycan in the potentiation of TGF-β2 signaling, we developed deterministic computational models with different modes of betaglycan binding and varying cooperativity between receptor subtypes. The models identified conditions for selective enhancement of TGF-β2 signaling. The models provide support for additional receptor binding cooperativity that has been hypothesized but not evaluated in the literature. The models further showed that betaglycan binding to the TGF-β2 ligand through two domains provides an effective mechanism for transfer to the signaling receptors that has been tuned to efficiently promote assembly of the TGF-β2(TβRII)2(TβRI)2 signaling complex.

Progress of Wnt Signaling Pathway in Osteoporosis

Osteoporosis, one of the serious health diseases, involves bone mass loss, bone density diminishing, and degeneration of bone microstructure, which is accompanied by a tendency toward bone fragility and a predisposition to fracture. More than 200 million people worldwide suffer from osteoporosis, and the cost of treating osteoporotic fractures is expected to reach at least $25 billion by 2025. The generation and development of osteoporosis are regulated by genetic factors and regulatory factors such as TGF-β, BMP, and FGF through multiple pathways, including the Wnt signaling pathway, the Notch signaling pathway, and the MAPK signaling pathway. Among them, the Wnt signaling pathway is one of the most important pathways. It is not only involved in bone development and metabolism but also in the differentiation and proliferation of chondrocytes, mesenchymal stem cells, osteoclasts, and osteoblasts. Dkk-1 and SOST are Wnt inhibitory proteins that can inhibit the activation of the canonical Wnt signaling pathway and block the proliferation and differentiation of osteoblasts. Therefore, they may serve as potential targets for the treatment of osteoporosis. In this review, we analyzed the mechanisms of Wnt proteins, β-catenin, and signaling molecules in the process of signal transduction and summarized the relationship between the Wnt signaling pathway and bone-related cells. We hope to attract attention to the role of the Wnt signaling pathway in osteoporosis and offer new perspectives and approaches to making a diagnosis and giving treatment for osteoporosis.

Ligand-receptor promiscuity enables cellular addressing

In multicellular organisms, secreted ligands selectively activate, or "address," specific target cell populations to control cell fate decision-making and other processes. Key cell-cell communication pathways use multiple promiscuously interacting ligands and receptors, provoking the question of how addressing specificity can emerge from molecular promiscuity. To investigate this issue, we developed a general mathematical modeling framework based on the bone morphogenetic protein (BMP) pathway architecture. We find that promiscuously interacting ligand-receptor systems allow a small number of ligands, acting in combinations, to address a larger number of individual cell types, defined by their receptor expression profiles. Promiscuous systems outperform seemingly more specific one-to-one signaling architectures in addressing capability. Combinatorial addressing extends to groups of cell types, is robust to receptor expression noise, grows more powerful with increases in the number of receptor variants, and is maximized by specific biochemical parameter relationships. Together, these results identify design principles governing cellular addressing by ligand combinations.