Tuning cellular responses to BMP-2 with material surfaces

Recent Developments in Layer-by-Layer Assembly for Drug Delivery and Tissue Engineering Applications

Surfaces with biological functionalities are of great interest for biomaterials, tissue engineering, biophysics, and for controlling biological processes. The layer-by-layer (LbL) assembly is a highly versatile methodology introduced 30 years ago, which consists of assembling complementary polyelectrolytes or biomolecules in a stepwise manner to form thin self-assembled films. In view of its simplicity, compatibility with biological molecules, and adaptability to any kind of supporting material carrier, this technology has undergone major developments over the past decades. Specific applications have emerged in different biomedical fields owing to the possibility to load or immobilize biomolecules with preserved bioactivity, to use an extremely broad range of biomolecules and supporting carriers, and to modify the film's mechanical properties via crosslinking. In this review, the focus is on the recent developments regarding LbL films formed as 2D or 3D objects for applications in drug delivery and tissue engineering. Possible applications in the fields of vaccinology, 3D biomimetic tissue models, as well as bone and cardiovascular tissue engineering are highlighted. In addition, the most recent technological developments in the field of film construction, such as high-content liquid handling or machine learning, which are expected to open new perspectives in the future developments of LbL, are presented.

Use of biosilica to improve loading and delivery of bone morphogenic protein 2

The clinical utility of bone morphogenetic protein 2 (BMP2) is limited because of the poor attraction between BMP2 and carriers, resulting in low loading efficiency and initial burst release. Here, the high binding affinity of BMP2 to the biosilica surface was utilized to overcome this limitation. Atomic force microscopy revealed that BMP2 bound nearly 8- and 2-fold more strongly to biosilica-coated hydroxyapatite than to uncoated and plain silica-coated hydroxyapatite, respectively. To achieve controlled release, collagen was introduced between the silica layers on hydroxyapatite, which was optimized by adjusting the collagen concentration and number of layers. The optimal biosilica/collagen formulation induced sustained BMP2 release without compromising loading efficiency. BMP2 combined with the mentioned formulation led to an increase in osteogenesis, as compared to the combination of BMP2 with either biosilica-coated or non-coated hydroxyapatite in vitro. In rat calvarial defect models, the biosilica/collagen-coated hydroxyapatite with 1 μg BMP2 showed 26 % more bone regeneration than the same dose of BMP2-loaded hydroxyapatite and 10.6 % more than hydroxyapatite with 2.5-fold dose of BMP2. Using BMP2 affinity carriers coated with biosilica/collagen allows for more efficacious in situ loading and delivery of BMP2, making them suitable for the clinical application of growth factors through a soaking method.

3D-Printed Osteoinductive Polymeric Scaffolds with Optimized Architecture to Repair a Sheep Metatarsal Critical-Size Bone Defect

The reconstruction of critical-size bone defects in long bones remains a challenge for clinicians. A new osteoinductive medical device is developed here for long bone repair by combining a 3D-printed architectured cylindrical scaffold made of clinical-grade polylactic acid (PLA) with a polyelectrolyte film coating delivering the osteogenic bone morphogenetic protein 2 (BMP-2). This film-coated scaffold is used to repair a sheep metatarsal 25-mm long critical-size bone defect. In vitro and in vivo biocompatibility of the film-coated PLA material is proved according to ISO standards. Scaffold geometry is found to influence BMP-2 incorporation. Bone regeneration is followed using X-ray scans, µCT scans, and histology. It is shown that scaffold internal geometry, notably pore shape, influenced bone regeneration, which is homogenous longitudinally. Scaffolds with cubic pores of ≈870 µm and a low BMP-2 dose of ≈120 µg cm-3 induce the best bone regeneration without any adverse effects. The visual score given by clinicians during animal follow-up is found to be an easy way to predict bone regeneration. This work opens perspectives for a clinical application in personalized bone regeneration.

Magnetic microspheres mimicking certain functions of macrophages: Towards precise antibacterial potency for bone defect healing

A variety of novel biomaterials have recently been developed to promote bone regeneration. However, the current biomaterials cannot accurately and effectively resist bacterial invasion. In this study, we constructed microspheres that mimic certain functions of macrophages as additives to bone repair materials, which can be manipulated as demanded to resist bacteria effectively and protect bone defect healing. Firstly, we prepared gelatin microspheres (GMSs) by an emulsion-crosslinking method, which were subsequently coated with polydopamine (PDA). Then, amino antibacterial nanoparticles obtained by a nanoprecipitation-self-assembly method and commercial amino magnetic nanoparticles were modified onto these PDA-coated GMSs to construct the functionalized microspheres (FMSs). The results showed that the FMSs possessed a rough topography and could be manipulated by a 100-400 ​mT static magnetic field to migrate directionally in unsolidified hydrogels. Moreover, in vitro experiments with near-infrared (NIR) showed that the FMSs had a sensitive and recyclable photothermal performance and could capture and kill Porphyromonas gingivalis by releasing reactive oxygen species. Finally, the FMSs were mixed with osteogenic hydrogel precursor, injected into the Sprague-Dawley rat periodontal bone defect of maxillary first molar (M1), and subsequently driven by magnetism to the cervical surface of M1 and the outer surface of the gel system for targeted sterilization under NIR, thus protecting the bone defect healing. In conclusion, the FMSs had excellent manipulation and antimicrobial performances. This provided us with a promising strategy to construct light-magnetism-responsive antibacterial materials to build a beneficial environment for bone defect healing.

Dopamine/DOPAC-assisted immobilization of bone morphogenetic protein-2 loaded Heparin/PEI nanogels onto three-dimentional printed calcium phosphate ceramics for enhanced osteoinductivity and osteogenicity

Nowadays, the three-dimensional (3D) printed calcium phosphate (CaP) ceramics have well-designed geometric structure, but suffer from relative weak osteoinductivity. Surface modification by incorporating bone morphogenetic protein-2 (BMP2) onto scaffolds is considered as an efficient approach to improve their bioactivity. However, high dose and uncontrolled burst release of BMP2 may cause undesired side effect. In the present study, porous BCP ceramics with inverse face-centred cube structure prepared by digital light processing (DLP)-based 3D printing technique were used as the substrates. BMP2 proteins were loaded in the self-assembled Heparin/PEI nanogels (NP/BMP2), and then immobilized onto BCP substrates through the intermediate mussel-derived bioactive dopamine and dihydroxyphenylacetic acid (DA/DOPAC) coating layers to construct functional BCP/layer/NP/BMP2 scaffolds. Our results showed that Heparin/PEI nanogel was a potent delivery system for BMP2, and BCP/layer/NP/BMP2 scaffolds exhibited the high loading capacity, controlled release rate, and sustained local delivery of BMP2. In vitro cell experiments with bone marrow stromal cells (BMSCs) found that BCP/layer/NP/BMP2 could promote cell proliferation, facilitate cell spreading, accelerate cell migration, up-regulate expression of osteogenic genes, and improve synthesis of osteoblast-related proteins. Moreover, the murine intramuscular implantation model suggested that BCP/layer/NP/BMP2 had a superior osteoinductive capacity, and the rat femoral condyle defect repair model showed that BCP/layer/NP/BMP2 could enhance in situ bone repair and regeneration. These findings demonstrate that the incorporation of BMP2 loaded Heparin/PEI nanogels to 3D printed scaffolds holds great promise in fabricating bone graft with a superior biological performance for orthopedic application.

The Effect of the Topmost Layer and the Type of Bone Morphogenetic Protein-2 Immobilization on the Mesenchymal Stem Cell Response

Recombinant human bone morphogenetic protein-2 (rhBMP-2) plays a key role in the stem cell response, not only via its influence on osteogenesis, but also on cellular adhesion, migration, and proliferation. However, when applied clinically, its supra-physiological levels cause many adverse effects. Therefore, there is a need to concomitantly retain the biological activity of BMP-2 and reduce its doses. Currently, the most promising strategies involve site-specific and site-directed immobilization of rhBMP-2. This work investigated the covalent and electrostatic binding of rhBMP-2 to ultrathin-multilayers with chondroitin sulfate (CS) or diazoresin (DR) as the topmost layer. Angle-resolved X-ray photoelectron spectroscopy was used to study the exposed chemical groups. The rhBMP-2 binding efficiency and protein state were studied with time-of-flight secondary ion mass spectrometry. Quartz crystal microbalance, atomic force microscopy, and enzyme-linked immunosorbent assay were used to analyze protein–substrate interactions. The effect of the topmost layer was tested on initial cell adhesion and short-term osteogenesis marker expression. The results show the highest expression of selected osteomarkers in cells cultured on the DR-ended layer, while the cellular flattening was rather poor compared to the CS-ended system. rhBMP-2 adhesion was observed only on negatively charged layers. Cell flattening became more prominent in the presence of the protein, even though the osteogenic gene expression decreased.

Statistic Copolymers Working as Growth Factor-Binding Mimics of Fibronectin

Growth factors (GFs) play important roles in biological system and are widely used in tissue regeneration. However, their application is greatly hindered by short in vivo lifetime of GFs. GFs are bound to fibronectin dynamically in the extracellular matrix, which inspired the authors to mimic the GF binding domain of fibronectin and design GF-binding amphiphilic copolymers bearing positive charges. The optimal amino acid polymer can bind to a variety of representative GFs, such as bone morphogenetic protein-2 (BMP-2) and TGF-β1 from the transforming growth factor-β superfamily, PDGF-AA and PDGF-BB from the platelet-derived growth factor family, FGF-10 and FGF-21 from the fibroblast growth factor family, epidermal growth factor from the EGF family and hepatocyte growth factor from the plasminogen-related growth factor family, with binding affinities up to the nanomolar level. 3D scaffolds immobilized with the optimal copolymer enable sustained release of loaded BMP-2 without burst release and significantly enhances the in vivo function of BMP-2 for bone formation. This strategy opens new avenues in designing GF-binding copolymers as synthetic mimics of fibronectin for diverse applications.

Decellularized Human Adipose Tissue as an Alternative Graft Material for Bone Regeneration

BACKGROUND

Tissue engineering approaches to treat damaged bone include various tissue transplants such as autologous, allogeneic, and xenografts. Artificial materials have been widely introduced to meet the demand for graft materials, but insufficiency in supply is still not resolved. In this study, human adipose tissue, easily obtained from the human body, was harvested, and the tissue was decellularized to fabricate a decellularized human adipose tissue matrix (DM) as an alternative graft material.

METHODS

Human adipose tissue was obtained via liposuction. The obtained fresh adipose tissue sample was cut into pieces then put into decellularization solution (1% antibiotic-antimycotic solution and 1% phenylmethanesulphonyl fluoride). Lipids were further removed via treatment in isopropanol. The sample was then subjected to another enzymatic digestion and lipid removal processes. The obtained decellularized adipose tissue matrix was lyophilized to form a graft material in disc shape.

RESULTS

Decellularization was confirmed by nuclear staining methods and detection of RNA and DNA via PCR. Bone morphogenetic protein 2 (BMP2)-loaded DM showed the ability to form new bone tissue when implanted in subcutaneous tissue. In recovery of a mouse calvarial defect model, BMP2-loaded DM exhibited similar levels of bone tissue regeneration efficiency compared with a well-defined commercial product, BMP2-loaded CollaCote®.

CONCLUSION

The DM developed in this study is expected to address the problem of insufficient supply of graft materials and contribute to the treatment of bone defects of critical size as an alternative bone graft material with preserved extracellular matrix components.

Site-Directed Immobilization of an Engineered Bone Morphogenetic Protein 2 (BMP2) Variant to Collagen-Based Microspheres Induces Bone Formation In Vivo

For the treatment of large bone defects, the commonly used technique of autologous bone grafting presents several drawbacks and limitations. With the discovery of the bone-inducing capabilities of bone morphogenetic protein 2 (BMP2), several delivery techniques were developed and translated to clinical applications. Implantation of scaffolds containing adsorbed BMP2 showed promising results. However, off-label use of this protein-scaffold combination caused severe complications due to an uncontrolled release of the growth factor, which has to be applied in supraphysiological doses in order to induce bone formation. Here, we propose an alternative strategy that focuses on the covalent immobilization of an engineered BMP2 variant to biocompatible scaffolds. The new BMP2 variant harbors an artificial amino acid with a specific functional group, allowing a site-directed covalent scaffold functionalization. The introduced artificial amino acid does not alter BMP2′s bioactivity in vitro. When applied in vivo, the covalently coupled BMP2 variant induces the formation of bone tissue characterized by a structurally different morphology compared to that induced by the same scaffold containing ab-/adsorbed wild-type BMP2. Our results clearly show that this innovative technique comprises translational potential for the development of novel osteoinductive materials, improving safety for patients and reducing costs.

Engineering of a Microscale Niche for Pancreatic Tumor Cells Using Bioactive Film Coatings Combined with 3D-Architectured Scaffolds

Two-photon polymerization has recently emerged as a promising technique to fabricate scaffolds for three-dimensional (3D) cell culture and tissue engineering. Here, we combined 3D-printed microscale scaffolds fabricated using two-photon polymerization with a bioactive layer-by-layer film coating. This bioactive coating consists of hyaluronic acid and poly(l-lysine) of controlled stiffness, loaded with fibronectin and bone morphogenic proteins 2 and 4 (BMP2 and BMP4) as matrix-bound proteins. Planar films were prepared using a liquid handling robot directly in 96-well plates to perform high-content studies of cellular processes, especially cell adhesion, proliferation, and BMP-induced signaling. The behaviors of two human pancreatic cell lines PANC1 (immortalized) and PAN092 (patient-derived cell line) were systematically compared and revealed important context-specific cell responses, notably in response to film stiffness and matrix-bound BMPs (bBMPs). Fibronectin significantly increased cell adhesion, spreading, and proliferation for both cell types on soft and stiff films; BMP2 increased cell adhesion and inhibited proliferation of PANC1 cells and PAN092 on soft films. BMP4 enhanced cell adhesion and proliferation of PANC1 and showed a bipolar effect on PAN092. Importantly, PANC1 exhibited a strong dose-dependent BMP response, notably for bBMP2, while PAN092 was insensitive to BMPs. Finally, we proved that it is possible to combine a microscale 3D Ormocomp scaffold fabricated using the two-photon polymerization technique with the bioactive film coating to form a microscale tumor tissue and mimic the early stages of metastatic cancer.

Materials-driven fibronectin assembly on nanoscale topography enhances mesenchymal stem cell adhesion, protecting cells from bacterial virulence factors and preventing biofilm formation

Post-operative infection is a major complication in patients recovering from orthopaedic surgery. As such, there is a clinical need to develop biomaterials for use in regenerative surgery that can promote mesenchymal stem cell (MSC) osteospecific differentiation and that can prevent infection caused by biofilm-forming pathogens. Nanotopographical approaches to pathogen control are being identified, including in orthopaedic materials such as titanium and its alloys. These topographies use high aspect ratio nanospikes or nanowires to prevent bacterial adhesion but these features also significantly reduce MSC adhesion and activity. Here, we use a poly (ethyl acrylate) (PEA) polymer coating on titanium nanowires to spontaneously organise fibronectin (FN) and to deliver bone morphogenetic protein 2 (BMP2) to enhance MSC adhesion and osteospecific signalling. Using a novel MSC-Pseudomonas aeruginosa co-culture, we show that the coated nanotopographies protect MSCs from cytotoxic quorum sensing and signalling molecules, enhance MSC adhesion and osteoblast differentiation and reduce biofilm formation. We conclude that the PEA polymer-coated nanotopography can both support MSCs and prevent pathogens from adhering to a biomaterial surface, thus protecting from biofilm formation and bacterial infection, and supporting osteogenic repair.

Single Cell Center of Mass for the Analysis of BMP Receptor Heterodimers Distributions

At the plasma membrane, transmembrane receptors are at the interface between cells and their environment. They allow sensing and transduction of chemical and mechanical extracellular signals. The spatial distribution of receptors and the specific recruitment of receptor subunits to the cell membrane is crucial for the regulation of signaling and cell behavior. However, it is challenging to define what regulates such spatial patterns for receptor localization, as cell shapes are extremely diverse when cells are maintained in standard culture conditions. Bone morphogenetic protein receptors (BMPRs) are serine-threonine kinases, which build heteromeric complexes of BMPRI and II. These are especially interesting targets for receptor distribution studies, since the signaling pathways triggered by BMPR-complexes depends on their dimerization mode. They might exist as preformed complexes, or assemble upon binding of BMP, triggering cell signaling which leads to differentiation or migration. In this work we analyzed BMPR receptor distributions in single cells grown on micropatterns, which allow not only to control cell shape, but also the distribution of intracellular organelles and protein assemblies. We developed a script called ComRed (Center Of Mass Receptor Distribution), which uses center of mass calculations to analyze the shift and spread of receptor distributions according to the different cell shapes. ComRed was tested by simulating changes in experimental data showing that shift and spread of distributions can be reliably detected. Our ComRed-based analysis of BMPR-complexes indicates that receptor distribution depends on cell polarization. The absence of a coordinated internalization after addition of BMP suggests that a rapid and continual recycling of BMPRs might occur. Receptor complexes formation and localization in cells induced by BMP might yield insights into the local regulation of different signaling pathways.

Enhanced bone regeneration via spatiotemporal and controlled delivery of a genetically engineered BMP-2 in a composite Hydrogel

Scaffolds functionalized with bone morphogenetic protein-2 (BMP-2) have shown great potential for bone regeneration. However, structural instability and the necessity for supra-physiological dose have thus far limited practical applications for BMP-2. Protein modification and site-specific covalent immobilization of BMP-2 to carrier materials might be optimal strategies to overcome these problems. Here, we report a broadly applicable strategy where the polyhistidine tag-T4 Lysozyme (His6-T4L) was genetically fused at the N-terminus of BMP-2 and used as a protein spacer, which on one hand enhanced protein solubility and stability, and on the other hand mediated site-specific covalent anchoring of BMP-2 upon binding to nickel-chelated nitrilotriacetic acid (Ni-NTA) microparticles (denoted as MPs-His6-T4L-BMP2) to further maximize its rescued activity. We also constructed a novel gelatin-based hydrogel that was crosslinked by transglutaminase (TG) and tannic acid (TA). This hydrogel, when incorporated with MPs-His6-T4L-BMP2, displayed excellent in-situ injectability, thermosensitivity, adhesiveness and improved mechanical properties. The effective loading mode led to a controlled and long-term sustained release of His6-T4L-BMP2, thereby resulting in enhancement of bone regeneration in a critical-sized bone defect. We believe that the protein modification strategy proposed here opens up new route not only for BMP-2 applications, but can be used to inform novel uses for other macromolecules.

Experimental Study on the Effect of miR-200b Regulation of Bone Morphogenetic Protein (BMP)-2 Expression on the Proliferation and Invasion of Nasopharyngeal Carcinoma Cells

BMP is an important member of the skeletal junction proteins associated with cell invasion, metastasis, and migration. MiR-200b is associated with the progression of several tumors. This study investigated whether miR-200b plays a role in regulating BMP-2 expression and affecting the proliferation and invasion of nasopharyngeal carcinoma cells. The nasopharyngeal carcinoma tissues with different TNM stages were collected and the proliferative nasal tissues alone were used as controls to detect the expression of miR-200b and BMP by RT-PCR. The nasopharyngeal carcinoma cell line CNE1 cells were divided into miR-NC group, miR-200b mimic group, siRNA-NC group, and siRNA-BMP-2 group, to detect BMP-2 level, cell invasion and proliferation ability by transwell. The BMP-2 mRNA expressed in nasopharyngeal carcinoma tissues was significantly elevated compared to controls and correlated with TNM stage. BMP-2 was higher in tumor tissues than in controls, however, the expression profile of miR-200b was opposite to BMP-2. Transfection with miR-200b mimic or siRNA-BMP-2 significantly down-regulated BMP-2 in CNE1 cells and attenuated cell invasive and proliferative capacity. Reduced expression of miR-200b is associated with elevated BMP-2 expression and increased invasive capacity of nasopharyngeal cancer cells. Overexpression of miR-200b reduces the invasive and proliferative capacity of nasopharyngeal cancer cells by targeting and inhibiting BMP-2 expression.

3D-printed scaffold combined to 2D osteoinductive coatings to repair a critical-size mandibular bone defect

The reconstruction of large bone defects (12 cm³) remains a challenge for clinicians. We developed a new critical-size mandibular bone defect model on a minipig, close to human clinical issues. We analyzed the bone reconstruction obtained by a 3D-printed scaffold made of clinical-grade polylactic acid (PLA), coated with a polyelectrolyte film delivering an osteogenic bioactive molecule (BMP-2). We compared the results (computed tomography scans, microcomputed tomography scans, histology) to the gold standard solution, bone autograft. We demonstrated that the dose of BMP-2 delivered from the scaffold significantly influenced the amount of regenerated bone and the repair kinetics, with a clear BMP-2 dose-dependence. Bone was homogeneously formed inside the scaffold without ectopic bone formation. The bone repair was as good as for the bone autograft. The BMP-2 doses applied in our study were reduced 20- to 75-fold compared to the commercial collagen sponges used in the current clinical applications, without any adverse effects. Three-dimensional printed PLA scaffolds loaded with reduced doses of BMP-2 may be a safe and simple solution for large bone defects faced in the clinic.

Surface modifications to enhance osseointegration-Resulting material properties and biological responses

As life expectancy and the age of the general population increases so does the need for improved implants. A major contributor to the failure of implants is poor osseointegration, which is typically described as the direct connection between bone and implant. This leads to unnecessary complications and an increased burden on the patient population. Modification of the implant surfaces through novel techniques, such as varying topography and/or applying coatings, has become a popular method to enhance the osseointegration capability of implants. Recent research has shown that particular surface features influence how bone cells interact with a material; however, it is unknown which exact features achieve optimal bone integration. In this review, current methods of modifying surfaces will be highlighted, and the resulting surface characteristics and biological responses are discussed. Review of the current strategies of surface modifications found that many coating types are more advantageous when used in combination; however, finding a surface modification that utilizes the mutual beneficial effects of important surface characteristics while still maintaining commercial viability is where future challenges exist.

Fabricating a novel HLC-hBMP2 fusion protein for the treatment of bone defects

Treating serious bone trauma with an osteo-inductive agent such as bone morphogenetic proteins (BMPs) has been considered as an optimized option when delivered via a collagen sponge (CS). Previous works have shown that the BMP concentration and release rate from approved CS carriers is difficult to control with precision. Here we presented the fabrication of a recombinant fusion protein from recombinant human-like collagen (HLC) and human BMP-2 (hBMP2). The fusion protein preserved the characteristic of HLC allowing the recombinant protein to be expressed in Yeast (such as Pichia pastoris GS115) and purified rapidly and easily with mass production after methanol induction. It also kept the stable properties of HLC and hBMP2 in the body fluid environment with good biocompatibility and no cytotoxicity. Moreover, the recombinant fusion protein fabricated a vertical through-hole structure with improved mechanical properties, and thus facilitated migration of bone marrow mesenchymal stem cells (MSCs) into the fusion materials. Furthermore, the fusion protein degraded and released hBMP-2 in vivo allowing osteoinductive activity and the enhancement of utilization rate and the precise control of the hBMP2 release. This fusion protein when applied to cranial defects in rats was osteoinductively active and improved bone repairing enhancing the repairing rate 3.5- fold and 4.2- fold when compared to the HLC alone and the control, respectively. There were no visible inflammatory reactions, infections or extrusions around the implantation sites observed. Our data strongly suggests that this novel recombinant fusion protein could be more beneficial in the treatment of bone defects than the simple superposition of the hBMP2/collagen sponge.

Learning from BMPs and their biophysical extracellular matrix microenvironment for biomaterial design

It is nowadays well-accepted that the extracellular matrix (ECM) is not a simple reservoir for growth factors but is an organization center of their biological activity. In this review, we focus on the ability of the ECM to regulate the biological activity of BMPs. In particular, we survey the role of the ECM components, notably the glycosaminoglycans and fibrillary ECM proteins, which can be promoters or repressors of the biological activities mediated by the BMPs. We examine how a process called mechano-transduction induced by the ECM can affect BMP signaling, including BMP internalization by the cells. We also focus on the spatio-temporal regulation of the BMPs, including their release from the ECM, which enables to modulate their spatial localization as well as their local concentration. We highlight how biomaterials can recapitulate some aspects of the BMPs/ECM interactions and help to answer fundamental questions to reveal previously unknown molecular mechanisms. Finally, the design of new biomaterials inspired by the ECM to better present BMPs is discussed, and their use for a more efficient bone regeneration in vivo is also highlighted.

Enhanced Osteogenic Differentiation of Human Bone Marrow Mesenchymal Stem Cells in Ossification of the Posterior Longitudinal Ligament Through Activation of the BMP2-Smad1/5/8 Pathway

Ossification of the posterior longitudinal ligament (OPLL) is characterized by ectopic OPLL. To date, the specific molecular pathogenesis of OPLL has not been clearly elucidated. In this study, bone marrow-derived mesenchymal stem cells obtained from healthy donors (HD-MSCs) and patients with OPLL (OPLL-MSCs) were cultured in osteogenic differentiation medium for 21 days. The osteogenic differentiation capacity was determined by alizarin red S (ARS) and alkaline phosphatase (ALP) assays. Gene expression levels of osteoblastic markers were measured by quantitative reverse transcription-polymerase chain reaction. Protein levels of related genes and the activation of related signaling pathways were measured by western blotting. LDN193189 was used to inhibit the Smad1/5/8 pathway, and small interfering RNA was used to regulate BMP2 expression. Our results showed that the OPLL-MSCs had stronger ARS staining and ALP activity and higher expression of RUNX2, Osterix, and OCN than the HD-MSCs. During osteogenic differentiation, the Smad1/5/8 pathway was overactivated in the OPLL-MSCs, and LDN193189 inhibition reversed the enhanced osteogenic ability of these cells. Besides, BMP2 was upregulated in the OPLL-MSCs. After BMP2 knockdown, the abnormal osteogenic differentiation of OPLL-MSCs was rescued. Thus, abnormal activation of the BMP2-Smad1/5/8 pathway induces enhanced osteogenic differentiation of OPLL-MSCs compared with HD-MSCs. These findings reveal a mechanism of pathological osteogenesis in OPLL and provide a new perspective on inhibiting pathological osteogenesis by regulating BMP2.

Surface Co-presentation of BMP-2 and integrin selective ligands at the nanoscale favors α5β1 integrin-mediated adhesion

Here we present the use of surface nanopatterning of covalently immobilized BMP-2 and integrin selective ligands to determine the specificity of their interactions in regulating cell adhesion and focal adhesion assembly. Gold nanoparticle arrays carrying single BMP-2 dimers are prepared by block-copolymer micellar nanolithography and azide-functionalized integrin ligands (cyclic-RGD peptides or α₅β₁ integrin peptidomimetics) are immobilized on the surrounding polyethylene glycol alkyne by click chemistry. Compared to BMP-2 added to the media, surface immobilized BMP-2 (iBMP-2) favors the spatial segregation of adhesion clusters and enhances focal adhesion (FA) size in cells adhering to α₅β₁ integrin selective ligands. Moreover, iBMP-2 copresented with α₅β₁ integrin ligands induces the recruitment of α_vβ₃ integrins in FAs. When copresented with RGD, iBMP-2 induces the assembly of a higher number of FAs, which are not affected by α₅β₁ integrin blocking. Our dual-functionalized platforms offer the possibility to study the crosstalk between integrins and BMP receptors, and more in general they could be used to address the spatial regulation of growth factors and adhesion receptors crosstalk on biomimetic surfaces.

Heparan sulfate co-immobilized with cRGD ligands and BMP2 on biomimetic platforms promotes BMP2-mediated osteogenic differentiation

The chemical and physical properties of the extracellular matrix (ECM) are known to be fundamental for regulating growth factor bioactivity. The role of heparan sulfate (HS), a glycosaminoglycan, and of cell adhesion proteins (containing the cyclic RGD (cRGD) ligands) on bone morphogenetic protein 2 (BMP2)-mediated osteogenic differentiation has not been fully explored. In particular, it is not known whether and how their effects can be potentiated when they are presented in controlled close proximity, as in the ECM. Here, we developed streptavidin platforms to mimic selective aspects of the in vivo presentation of cRGD, HS and BMP2, with a nanoscale-control of their surface density and orientation to study cell adhesion and osteogenic differentiation. We showed that whereas a controlled increase in cRGD surface concentration upregulated BMP2 signaling due to β₃ integrin recruitment, silencing either β₁ or β₃ integrins negatively affected BMP2-mediated phosphorylation of SMAD1/5/9 and alkaline phosphatase expression. Furthermore, the presence of adsorbed BMP2 promoted cellular adhesion at very low cRGD concentrations. Finally, we proved that HS co-immobilized with cRGD both sustained BMP2 signaling and enhanced osteogenic differentiation compared to BMP2 directly immobilized on streptavidin, even with a low cRGD surface concentration. Altogether, our results show that HS facilitated and sustained the synergy between BMP2 and integrin pathways and that the co-immobilization of HS and cRGD peptides optimised BMP2-mediated osteogenic differentiation. Statement of significance The growth factor BMP2 is used to treat large bone defects. Previous studies have shown that the presentation of BMP2 via extracellular matrix molecules, such as heparan sulfate (HS), can upregulate BMP2 signaling. The potential advantages of dose reduction and local specificity have stimulated interest in further investigations into biomimetic approaches. We designed a streptavidin model surface eligible for immobilizing tunable amounts of molecules from the extracellular space, such as HS, adhesion motifs (cyclic RGD) and BMP2. By studying cellular adhesion, BMP2 bioactivity and its osteogenic potential we reveal the combined effect of integrins, HS and BMP2, which contribute in answering fundamental questions regarding cell-matrix interaction.

Bone Morphogenetic Protein-2 in Development and Bone Homeostasis

Bone morphogenetic proteins (BMPs) are multi-functional growth factors belonging to the Transforming Growth Factor-Beta (TGF-β) superfamily. These proteins are essential to many developmental processes, including cardiogenesis, neurogenesis, and osteogenesis. Specifically, within the BMP family, Bone Morphogenetic Protein-2 (BMP-2) was the first BMP to be characterized and has been well-studied. BMP-2 has important roles during embryonic development, as well as bone remodeling and homeostasis in adulthood. Some of its specific functions include digit formation and activating osteogenic genes, such as Runt-Related Transcription Factor 2 (RUNX2). Because of its diverse functions and osteogenic potential, the Food and Drug Administration (FDA) approved usage of recombinant human BMP-2 (rhBMP-2) during spinal fusion surgery, tibial shaft repair, and maxillary sinus reconstructive surgery. However, shortly after initial injections of rhBMP-2, several adverse complications were reported, and alternative therapeutics have been developed to limit these side-effects. As the clinical application of BMP-2 is largely implicated in bone, we focus primarily on its role in bone. However, we also describe briefly the role of BMP-2 in development. We then focus on the structure of BMP-2, its activation and regulation signaling pathways, BMP-2 clinical applications, and limitations of using BMP-2 as a therapeutic. Further, this review explores other potential treatments that may be useful in treating bone disorders.

Microfluidic fabrication of microcarriers with sequential delivery of VEGF and BMP-2 for bone regeneration

Wound instability and poor functional vascularization in bone tissue engineering lead to lack of tissue integration and ultimate failure of engineered grafts. In order to harness the regenerative potential of growth factors and stimulate bone healing, present study aims to design multifunctional cell therapy microcarriers with the capability of sequential delivery of essential growth factors, bone morphogenetic protein 2 (BMP-2) and vascular endothelial growth factor (VEGF). An on-chip double emulsion method was implemented to generate monodisperse VEGF encapsulated microcarriers. Bio-inspired poly(3,4-dihydroxyphenethylamine) (PDA) was then functionalized to the microcarriers surface for BMP-2 conjugation. The microcarriers were seeded with mesenchymal stem cells (MSCs) using a dynamic culture technique for cells expansion. Finally, the microcarriers were incorporated into an injectable alginate-RGD hydrogel laden with endothelial cells (ECs) for further analysis. The DNA and calcium content, as well as ALP activity of the construct were analyzed. The confocal fluorescent microscopy was employed to monitor the MSCs and tunneling structure of ECs. Eventually, the capability of developed microcarriers for bone tissue formation was examined in vivo. Microfluidic platform generated monodisperse VEGF-loaded PLGA microcarriers with size-dependent release patterns. Microcarriers generated with the on-chip technique showed more sustained VEGF release profiles compared to the conventional bulk mixing method. The PDA functionalization of microcarriers surface not only provided immobilization of BMP-2 with prolonged bioavailability, but also enhanced the attachment and proliferation of MSCs. Dynamic culturing of microcarriers showcased their great potential to boost MSCs population required for stem cell therapy of bone defects. ALP activity and calcium content analysis of MSCs-laden microcarriers loaded into injectable hydrogels revealed their capability of tunneling formation, vascular cell growth and osteogenic differentiation. The in vivo histology and real-time polymerase chain reaction analysis revealed that transplantation of MSC-laden microcarriers supports ectopic bone formation in the rat model. The presented approach to design bioactive microcarriers offer sustained sequential delivery of bone ECM chemical cues and offer an ideal stabilized 3D microenvironment for patient-specific cell therapy applications. The proposed methodology is readily expandable to integrate other cells and cytokines in a tuned spatiotemporal manner for personalized regenerative medicine.

Anagliptin stimulates osteoblastic cell differentiation and mineralization

Osteoporosis is a common debilitating bone disease characterized by loss of bone mass and degradation of the bone architecture, which is primarily driven by dysregulated differentiation of mesenchymal stem cells into bone-producing osteoblasts. Osteoblasts contribute to bone formation by secreting various proteins that guide the deposition of bone extracellular matrix, such as alkaline phosphatase (ALP), osteocalcin (OCN), and osteopontin (OPN). The Wnt/β-catenin pathway is widely recognized as a regulator of bone mass and is required to maintain bone homeostasis. Hormones have long been recognized as playing a key role in bone metabolism, and in recent years, growing evidence has shown that diabetes is a risk factor for osteoporosis. In the present study, we investigated the effects of the antidiabetic drug anagliptin on the differentiation and mineralization of osteoblasts induced by osteogenic medium. Anagliptin promotes insulin production via inhibition of dipeptidyl peptidase IV (DPP-4), an enzyme that targets the incretin hormone glucagon-like peptide 1 (GLP-1) for degradation. Our findings show that anagliptin significantly increases the differentiation of MSCs into osteoblasts via activation of RUNX2. Anagliptin significantly increased matrix deposition and mineralization by osteoblasts, as evidenced by elevated levels of ALP, OCN, OPN, and BMP-2. We further demonstrate that anagliptin activates the canonical and noncannonical Wnt signaling pathways and that silencing of Wnt/β-catenin signaling completely abolished the effects of anagliptin. Thus, anagliptin might be a safe, effective therapy for type II diabetes that might show promise as a therapy against osteoporosis.

A BMP/activin A chimera is superior to native BMPs and induces bone repair in nonhuman primates when delivered in a composite matrix

Bone morphogenetic protein (BMP)/carriers approved for orthopedic procedures achieve efficacy superior or equivalent to autograft bone. However, required supraphysiological BMP concentrations have been associated with potential local and systemic adverse events. Suboptimal BMP/receptor binding and rapid BMP release from approved carriers may contribute to these outcomes. To address these issues and improve efficacy, we engineered chimeras with increased receptor binding by substituting BMP-6 and activin A receptor binding domains into BMP-2 and optimized a carrier for chimera retention and tissue ingrowth. BV-265, a BMP-2/BMP-6/activin A chimera, demonstrated increased binding affinity to BMP receptors, including activin-like kinase-2 (ALK2) critical for bone formation in people. BV-265 increased BMP intracellular signaling, osteogenic activity, and expression of bone-related genes in murine and human cells to a greater extent than BMP-2 and was not inhibited by BMP antagonist noggin or gremlin. BV-265 induced larger ectopic bone nodules in rats compared to BMP-2 and was superior to BMP-2, BMP-2/6, and other chimeras in nonhuman primate bone repair models. A composite matrix (CM) containing calcium-deficient hydroxyapatite granules suspended in a macroporous, fenestrated, polymer mesh-reinforced recombinant human type I collagen matrix demonstrated improved BV-265 retention, minimal inflammation, and enhanced handling. BV-265/CM was efficacious in nonhuman primate bone repair models at concentrations ranging from ¹/₁₀ to ¹/₃₀ of the BMP-2/absorbable collagen sponge (ACS) concentration approved for clinical use. Initial toxicology studies were negative. These results support evaluations of BV-265/CM as an alternative to BMP-2/ACS in clinical trials for orthopedic conditions requiring augmented healing.

Tag-Free Site-Specific BMP-2 Immobilization with Long-Acting Bioactivities via a Simple Sugar-Lectin Interaction

The construction of a biomaterial matrix with biological properties is of great importance to developing functional materials for clinical use. However, the site-specific immobilization of growth factors to endow materials with bioactivities has been a challenge to date. Considering the wide existence of glycosylation in mammalian proteins or recombinant proteins, we establish a bioaffinity-based protein immobilization strategy (bioanchoring method) utilizing the native sugar-lectin interaction between concanavalin A (Con A) and the oligosaccharide chain on glycosylated bone morphogenetic protein-2 (GBMP-2). The interaction realizes the site-specific immobilization of GBMP-2 to a substrate modified with Con A while preserving its bioactivity in a sustained and highly efficient way, as evidenced by its enhanced ability to induce osteodifferentiation compared with that of the soluble GBMP-2. Moreover, the surface with Con A-bioanchored GBMP-2 can be reused to stimulate multiple batches of C2C12 cells to differentiate almost to the same degree. Even after 4 month storage at 4 °C in phosphate-buffered saline (PBS), the Con A-bioanchored GBMP-2 still maintains the bioactivity to stimulate the differentiation of C2C12 cells. Furthermore, the ectopic ossification test proves the in vivo bioactivity of bioanchored GBMP-2. Overall, our results demonstrate that the tag-free and site (i.e., sugar chain)-specific protein immobilization strategy represents a simple and generic alternative, which is promising to apply for other glycoprotein immobilization and application. It should be noted that although the lectin we utilized can only bind to d-mannose/d-glucose, the diversity of the lectin family assures that a specific lectin could be offered for other sugar types, thus expanding the applicable scope further.

Bone marrow niche crosses paths with BMPs: a road to protection and persistence in CML

Chronic myeloid leukaemia (CML) is a paradigm of precision medicine, being one of the first cancers to be treated with targeted therapy. This has revolutionised CML therapy and patient outcome, with high survival rates. However, this now means an ever-increasing number of patients are living with the disease on life-long tyrosine kinase inhibitor (TKI) therapy, with most patients anticipated to have near normal life expectancy. Unfortunately, in a significant number of patients, TKIs are not curative. This low-level disease persistence suggests that despite a molecularly targeted therapeutic approach, there are BCR-ABL1-independent mechanisms exploited to sustain the survival of a small cell population of leukaemic stem cells (LSCs). In CML, LSCs display many features akin to haemopoietic stem cells, namely quiescence, self-renewal and the ability to produce mature progeny, this all occurs through intrinsic and extrinsic signals within the specialised microenvironment of the bone marrow (BM) niche. One important avenue of investigation in CML is how the disease highjacks the BM, thereby remodelling this microenvironment to create a niche, which enables LSC persistence and resistance to TKI treatment. In this review, we explore how changes in growth factor levels, in particular, the bone morphogenetic proteins (BMPs) and pro-inflammatory cytokines, impact on cell behaviour, extracellular matrix deposition and bone remodelling in CML. We also discuss the challenges in targeting LSCs and the potential of dual targeting using combination therapies against BMP receptors and BCR-ABL1.

Copresentation of BMP-6 and RGD Ligands Enhances Cell Adhesion and BMP-Mediated Signaling

We report on the covalent immobilization of bone morphogenetic protein 6 (BMP-6) and its co-presentation with integrin ligands on a nanopatterned platform to study cell adhesion and signaling responses which regulate the transdifferentiation of myoblasts into osteogenic cells. To immobilize BMP-6, the heterobifunctional linker MU-NHS is coupled to amine residues of the growth factor; this prevents its internalization while ensuring that its biological activity is maintained. Additionally, to allow cells to adhere to such platform and study signaling events arising from the contact to the surface, we used click-chemistry to immobilize cyclic-RGD carrying an azido group reacting with PEG-alkyne spacers via copper-catalyzed 1,3-dipolar cycloaddition. We show that the copresentation of BMP-6 and RGD favors focal adhesion formation and promotes Smad 1/5/8 phosphorylation. When presented in low amounts, BMP-6 added to culture media of cells adhering to the RGD ligands is less effective than BMP-6 immobilized on the surfaces in inducing Smad complex activation and in inhibiting myotube formation. Our results suggest that a local control of ligand density and cell signaling is crucial for modulating cell response.

Release kinetics of the model protein FITC-BSA from different polymer-coated bovine bone substitutes

Background

Controlled release of proteins bound to conventional bone substitutes is still insufficient. Therefore, this study evaluates in-vitro release kinetics of the model protein FITC-BSA (fluorescein conjugated bovine serum albumine) from insoluble bovine collagenous bone matrices (ICBM) with different polymer coatings. Analyzes aim at comparing FITC-BSA release from uncoated versus coated ICBM over time to find bone substitute coatings with consistent release profiles.

Methods

Release kinetics of FITC-BSA from uncoated as well as coated ICBM with five different polymers (RESOMER R 203 H, RG 503 H, RG 504 H, RG 505, L 206 S) were measured over a period of 11 days (d). Measurements were conducted after 6 h (h), 12 h, 24 h, 3 d, 5 d, 7 d, 9 d and 11 d with six samples for each coated ICBM. Two groups were formed (1) with and (2) without medium change at times of measurement. For each group ANOVA with post-hoc Bonferroni testing was used. Scanning electron microscopy assessed morphologic differences between ICBM coating.

Results

In group 1 approx. 70% of FITC-BSA release from uncoated ICBM occurred after 6 h compared to approx. 50% in group 2. Only polymers with medium inherent viscosity, i.e. RESOMER RG 503 H, constantly showed significantly more FITC-BSA release throughout 11 d than uncoated ICBM (p = 0.007). The same was found for group 2 (p = 0.005). No significant differences between PLA and PLGA polymers were found. Scanning electron microscopy results indicate a weak adhesion of polymer coatings to ICBM explaining its rather weak retentive effect on overall FITC-BSA release.

Conclusions

Medium molecular size polymers reduce the overall released FITC-BSA from ICBM over time. In clinical practice these polymers may prove ideal for bone substitute materials.

Enhanced selective cellular proliferation by multi-biofunctionalization of medical implant surfaces with heterodimeric BMP-2/6, fibronectin, and FGF-2

Increasing cell adhesion on implant surfaces is an issue of high biomedical importance. Early colonization with endogenous cells reduces the risk of bacterial contamination and enhances the integration of an implant into the diverse cellular tissues surrounding it. In vivo integration of implants is controlled by a complex spatial and temporal interplay of cytokines and adhesive molecules. The concept of a multi-biofunctionalized TiO₂ surface for stimulating bone and soft tissue growth is presented here. All supramolecular architectures were built with a biotin-streptavidin coupling system. Biofunctionalization of TiO₂ with immobilized FGF-2 and heparin could be shown to selectively increase the proliferation of fibroblasts while immobilized BMP-2 only stimulated the growth of osteoblasts. Furthermore, TiO₂ surfaces biofunctionalized with either the BMP-2 or BMP-2/6 growth factor and the cell adhesion-enhancing protein fibronectin showed higher osteoblast adhesion than a TiO₂ surface functionalized with only one of these proteins. In conclusion, the presented immobilization strategy is applicable in vivo for a selective surface coating of implants in both hard and connective tissue. The combined immobilization of different extracellular proteins on implants has the potential to further influence cell-specific reactions. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2910-2922, 2018.

Hyaluronan hydrogels delivering BMP-6 for local targeting of malignant plasma cells and osteogenic differentiation of mesenchymal stromal cells

Multiple myeloma is a malignant disease characterized by accumulation of clonal plasma cells in the bone marrow. Uncoupling of bone formation and resorption by myeloma cells leads to osteolytic lesions. These are prone to fracture and represent a possible survival space for myeloma cells under treatment causing disease relapse. Here we report on a novel approach suitable for local treatment of multiple myeloma based on hyaluronic acid (HA) hydrogels mimicking the physical properties of the bone marrow. The HA hydrogels are complexed with heparin to achieve sustained presentation and controlled release of bone morphogenetic protein 6 (BMP-6). Others and we have shown that BMP-6 induces myeloma cell apoptosis and bone formation. Using quartz crystal microbalance and enzyme-linked immunosorbent assay, we measured an initial surface density of 400 ng BMP6/cm², corresponding to two BMP-6 per heparin molecule, with 50% release within two weeks. HA-hydrogels presenting BMP-6 enhanced the phosphorylation of Smad 1/5 while reducing the activity of BMP-6 antagonist sclerostin. These materials induced osteogenic differentiation of mesenchymal stromal cells and decreased the viability of myeloma cell lines and primary myeloma cells. BMP-6 functionalized HA-hydrogels represent a promising material for local treatment of myeloma-induced bone disease and residual myeloma cells within lesions to minimize disease relapse or fractures. STATEMENT OF SIGNIFICANCE: Multiple myeloma is a hematological cancer characterized by the accumulation of clonal plasma cells in the bone marrow and local suppression of bone formation, resulting in osteolytic lesions and fractures. Despite recent advances in systemic treatment of multiple myeloma, it is rare to achieve a targeted suppression of myeloma cells and healing of bone lesions. Here we present hydrogels which mimic the physico-chemical properties of the bone marrow, consisting of hyaluronic acid with crosslinked heparin for the controlled presentation of bioactive BMP-6. The hydrogels decrease the viability of myeloma cell lines and primary myeloma cells and induces osteogenic differentiation of mesenchymal stromal cells. The presentation of BMP-6 in the hyaluronan hydrogels enhances the phosphorylation of Smad1/5 while reducing the activity of the BMP-6 antagonist sclerostin. As such, BMP-6 functionalized hyaluronan hydrogels represent a promising material for the localized eradication of myeloma cells.

Enhancing cell seeding and osteogenesis of MSCs on 3D printed scaffolds through injectable BMP2 immobilized ECM-Mimetic gel

OBJECTIVE

Design of bioactive scaffolds with osteogenic capacity is a central challenge in cell-based patient-specific bone tissue engineering. Efficient and spatially uniform seeding of (stem) cells onto such constructs is vital to attain functional tissues. Herein we developed heparin functionalized collagen gels supported by 3D printed bioceramic scaffolds, as bone extracellular matrix (ECM)-mimetic matrices. These matrices were designed to enhance cell seeding efficiency of mesenchymal stem cells (MSCs) as well as improve their osteogenic differentiation through immobilized bone morphogenic protein 2 (BMP2) to be used for personalized bone regeneration.

METHODS

A 3D gel based on heparin-conjugated collagen matrix capable of immobilizing recombinant human bone morphogenic protein 2 (BMP2) was synthesized. Isolated dental pulp Mesenchymal stem cells (MSCs) were then encapsulated into the bone ECM microenvironment to efficiently and uniformly seed a bioactive ceramic-based scaffold fabricated using additive manufacturing technique. The designed 3D cell-laden constructs were comprehensively investigated trough in vitro assays and in vivo study.

RESULTS

In-depth rheological characterizations of heparin-conjugated collagen gel revealed that elasticity of the matrix is significantly improved compared with freely incorporated heparin. Investigation of the MSCs laden collagen-heparin hydrogels revealed their capability to provide spatiotemporal bioavailability of BMP2 while suppressing the matrix contraction over time. The in vivo histology and real-time polymerase chain reaction (qPCR) analysis showed that the designed construct supported the osteogenic differentiation of MSCs and induced the ectopic bone formation in rat model.

SIGNIFICANCE

The presented hybrid constructs combine bone ECM chemical cues with mechanical function providing an ideal 3D microenvironment for patient-specific bone tissue engineering and cell therapy applications. The implemented methodology in design of ECM-mimetic 3D matrix capable of immobilizing BMP2 to improve seeding efficiency of customized scaffolds can be exploited for other bioactive molecules.

Orientation effects on the nanoscale adsorption behavior of bone morphogenetic protein-2 on hydrophilic silicon dioxide

Bone Morphogenetic Protein-2 (BMP-2) is a growth factor associated with different developmental functions in regenerative medicine and tissue engineering. Because of its favorable properties for the development of bone and cartilage tissue, BMP-2 promotes the biocompatibility of medical implants. In this research, molecular dynamics simulations were implemented to simulate the interaction of BMP-2 with a flat hydrophilic silicon dioxide substrate, an important biomaterial for medical applications. We considered the influence of four orthogonal protein orientations on the adsorption behavior. Results showed that arginine and lysine were the main residues to interact with the silicon dioxide substrate, directly adsorbing onto the surface and overcoming water layers. However, between these charged residues, we observed a preference for arginine to adsorb. Orientations with the α-helix loop closer to the surface at the beginning of the simulations had greater loss of secondary structure as compared to the other configurations. Among all the orientations, the end-on B configuration had favorable adsorption characteristics with a binding energy of 14 000 kJ mol⁻¹ and retention of 21.7% β-sheets as confirmed by the Ramachandran plots. This research provides new insights into the nanoscale interaction of BMP-2 and silicon dioxide substrate with applications in orthopedic implants and regenerative medicine.

Preferential binding of charged residues with hydrophilic silicon dioxide substrate during adsorption of BMP-2 in end-on B configuration.

Practical guide to characterize biomolecule adsorption on solid surfaces (Review)

The control over the adsorption or grafting of biomolecules from a liquid to a solid interface is of fundamental importance in different fields, such as drug delivery, pharmaceutics, diagnostics, and tissue engineering. It is thus important to understand and characterize how biomolecules interact with surfaces and to quantitatively measure parameters such as adsorbed amount, kinetics of adsorption and desorption, conformation of the adsorbed biomolecules, orientation, and aggregation state. A better understanding of these interfacial phenomena will help optimize the engineering of biofunctional surfaces, preserving the activity of biomolecules and avoiding unwanted side effects. The characterization of molecular adsorption on a solid surface requires the use of analytical techniques, which are able to detect very low quantities of material in a liquid environment without modifying the adsorption process during acquisition. In general, the combination of different techniques will give a more complete characterization of the layers adsorbed onto a substrate. In this review, the authors will introduce the context, then the different factors influencing the adsorption of biomolecules, as well as relevant parameters that characterize their adsorption. They review surface-sensitive techniques which are able to describe different properties of proteins and polymeric films on solid two-dimensional materials and compare these techniques in terms of sensitivity, penetration depth, ease of use, and ability to perform "parallel measurements."

Tuning the bioactivity of bone morphogenetic protein-2 with surface immobilization strategies

Bone morphogenetic protein-2 (BMP-2) involved therapy is of great potential for bone regeneration. However, its clinical application is restricted due to the undesirable bioactivity and relevant complications in vivo. Immobilization of recombinant BMP-2 (rhBMP-2) is an efficient strategy to mimic natural microenvironment and retain its bioactivity. Herein, we present evidences indicating that osteoinductive capacity of rhBMP-2 can be regulated via variant immobilizing approaches. Three representative superficial immobilizing models were employed to fabricate rhBMP-2-immobilized surfaces including physical adsorption (Au/rhBMP-2), covalent grafting (rhBMP-2-SAM-Au) and heparin binding (Hep-SAM-Au/rhBMP-2) (SAM: self-assembled monolayer). Loading capacity, releasing behavior, osteogenic differentiation and signaling pathways involved, as well as the cellular recognition of rhBMP-2 under various immobilization modes were systematically investigated. As a result, disparate immobilizing approaches not only have effects on loading capacity, but also lead to disparity of osteoinduction at the same dosage. Notably, heparin could reinforce the recognition between rhBMP-2 and its receptors (BMPRs) whereas weaken its binding to its antagonist Noggin. Owing to this "selective" binding feature, the favorable osteoinduction and maximum ectopic bone formation can be achieved with the heparin-binding approach. In particular, manipulation of orientation-mediated BMP-2-cell recognition efficiency may be a potential target to design more therapeutic efficient rhBMP-2 delivery system. STATEMENT OF SIGNIFICANCE: Bone morphogenetic protein-2 (BMP-2) is crucial in bone regeneration. However, its clinical application is challenged due to its shorten half-life and supra-physiological dose associated complications. In this study, three representative superficial immobilizing patterns were fabricated through physical adsorption, covalent grafting and electrostatic interaction with heparin respectively. We provided evidences indicating an dose-dependent osteoinductive capacity of immobilized BMP-2. Further, a possible mechanism of rhBMP-2-cell recognition at the interface was presented, highlighting the superior effect of heparin on rhBMP-2 bioactivity. Finally, We proposed a dual mechanism of tuning the bioactivity of immobilized rhBMP-2 through surface immobilization approaches: regulation of the saturated loading capacity and orientation-mediated rhBMP-2-cell recognition. These results provide novel insights into designing criterion of efficient delivery vehicle for rhBMP-2.

Molecular dynamics simulations of adsorption and desorption of bone morphogenetic protein-2 on textured hydroxyapatite surfaces

Interactions between bone morphogenetic protein-2 (BMP-2) and biomaterial surfaces are of great significance in the fields of regenerative medicine and bone tissue engineering. In this work, the adsorption and desorption behaviors of BMP-2 on a series of nano-textured hydroxyapatite (HAP) surfaces were systematically investigated by combined molecular dynamic (MD) simulations and steered molecular dynamic (SMD) simulations. The textured HAP surfaces exhibited nanostructured topographies and played a critical role in the mediation of dynamic behaviors of BMP-2. Compared to the HAP-flat model, the HAP-1:1 group (means ridge vs groove = 1:1) showed the excellent ability to capture BMP-2, less conformation change of BMP-2 molecule, and high cysteine-knot stability during the adsorption and desorption processes. These findings suggest that nano-textured HAP surfaces are more capable of loading BMP-2 molecules, and most importantly, they can help maintain a higher biological activity of BMP-2 cargos. In the present study, for the first time, we have deeply clarified the adsorption and desorption dynamics of BMP-2 on various nano-textured HAP surfaces at the atomic level, which can provide significant guidelines for the future design of BMP-2-based tissue engineering implants/scaffolds. STATEMENT OF SIGNIFICANCE: By using combined molecular dynamic (MD) simulations and steered molecular dynamic (SMD) simulations, the adsorption and desorption dynamics of bone morphogenetic protein-2 (BMP-2) dimer on a series of nano-textured hydroxyapatite (HAP) surfaces at the atomic level were presented in details for the first time. We have proved that the HAP-1:1 model (means ridge vs groove = 1:1) possessed excellent ability to capture BMP-2, less conformation change, and high cysteine-knot stability. As a result, the nano-textured topography of HAP-1:1 could maintain a relatively high biological activity of BMP-2 cargos. This work could provide theoretical guidelines for the design of BMP-2-based implants/scaffolds for bone tissue engineering.

Hydrolytically Labile Linkers Regulate Release and Activity of Human Bone Morphogenetic Protein-6

Release of growth factors while simultaneously maintaining their full biological activity over a period of days to weeks is an important issue in controlled drug delivery and in tissue engineering. In addition, the selected strategy to immobilize growth factors largely determines their biological activity. Silica surfaces derivatized with glycidyloxy propyl trimethoxysilane and poly(glycidyl methacrylate) brushes yielded epoxide-functionalized surfaces onto which human bone morphogenetic protein-6 (hBMP-6) was immobilized giving stable secondary amine bonds. The biological activity of hBMP-6 was unleashed by hydrolysis of the surface siloxane and ester bonds. We demonstrate that this type of labile bonding strategy can be applied to biomaterial surfaces with relatively simple and biocompatible chemistry, such as siloxane, ester, and imine bonds. Our data indicates that the use of differential hydrolytically labile linkers is a versatile method for functionalization of biomaterials with a variety of growth factors providing control over their biological activity.

Localization and promotion of recombinant human bone morphogenetic protein-2 bioactivity on extracellular matrix mimetic chondroitin sulfate-functionalized calcium phosphate cement scaffolds

Localization of recombinant human bone morphogenetic protein-2 (rhBMP-2) with continuous and effective osteogenic stimulation is still a great challenge in the field of bone regeneration. To achieve this aim, rhBMP-2 was tethered on chondroitin sulfate (CS)-functionalized calcium phosphate cement (CPC) scaffolds through specific noncovalent interactions. CS, one of the core glycosaminoglycans, was covalently conjugated onto CPC scaffolds with the assistance of polydopamine (PDA) and further immobilized rhBMP-2 in a biomimetic form. The CPC-PDA-CS scaffolds not only controlled the release kinetics and presentation state of rhBMP-2 but also effectively increased the expression levels of bone morphogenetic protein receptors (BMPRs) and enhanced the recognitions of the remaining rhBMP-2 to BMPRs. Strikingly, the rhBMP-2-loaded CPC-PDA-CS significantly promoted the cellular surface translocation of BMPRs (especially BMPR-IA). In vivo studies demonstrated that, compared with the rhBMP-2 upon CPC and CPC-PDA, the rhBMP-2 upon CPC-PDA-CS exhibited sustained release and induced high quality and more ectopic bone formation. Collectively, these results suggest that rhBMP-2 can be localized within CS-functionalized CPC scaffolds and exert continuous, long-term, and effective osteogenic stimulation. Thus, this work could provide new avenues in mimicking bone extracellular matrix microenvironment and localizing growth factor activity for enhanced bone regeneration.

STATEMENT OF SIGNIFICANCE

A bioinspired chondroitin sulfate (CS)-functionalized calcium phosphate cement (CPC) platform was developed to tether recombinant human bone morphogenetic protein-2 (rhBMP-2), which could exhibit continuous, long-term, and effective osteogenic stimulation in bone tissue engineering. Compared with rhBMP-2-loaded CPC, the rhBMP-2-loaded CPC-polydopamine-CS scaffolds induced higher expression of bone morphogenetic protein receptors (BMPRs), greater cellular surface translocation of bone morphogenetic protein receptor-IA, higher binding affinity of BMPRs/rhBMP-2, and thus higher activation of the drosophila gene mothers against decapentaplegic protein-1/5/8 (Smad1/5/8) and extracellular-regulated protein kinases-1/2 (ERK1/2) signaling. This work can provide new guidelines for the design of BMP-2-based bioactive materials for bone regeneration.

TGF-β family co-receptor function and signaling

Transforming growth factor-β (TGF-β) family members, which include TGF-βs, activins and bone morphogenetic proteins, are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues. These secreted protein ligands signal via single-transmembrane Type I and Type II serine/threonine kinase receptors and intracellular SMAD transcription factors. Deregulation in signaling has been implicated in a broad array of diseases, and implicate the need for intricate fine tuning in cellular signaling responses. One important emerging mechanism by which TGF-β family receptor signaling intensity, duration, specificity and diversity are regulated and/or mediated is through cell surface co-receptors. Here, we provide an overview of the co-receptors that have been identified for TGF-β family members. While some appear to be specific to TGF-β family members, others are shared with other pathways and provide possible ways for signal integration. This review focuses on novel functions of TGF-β family co-receptors, which continue to be discovered.

Covalent Binding of Bone Morphogenetic Protein-2 and Transforming Growth Factor-β3 to 3D Plotted Scaffolds for Osteochondral Tissue Regeneration

Engineering the osteochondral tissue presents some challenges mainly relying in its function of transition from the subchondral bone to articular cartilage and the gradual variation in several biological, mechanical, and structural features. A possible solution for osteochondral regeneration might be the design and fabrication of scaffolds presenting a gradient able to mimic this transition. Covalent binding of biological factors proved to enhance cell adhesion and differentiation in two-dimensional culture substrates. Here, we used polymer brushes as selective linkers of bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β3 (TGF-β3) on the surface of 3D scaffolds fabricated via additive manufacturing (AM) and subsequent controlled radical polymerization. These growth factors (GFs) are known to stimulate the differentiation of human mesenchymal stromal cells (hMSCs) toward the osteogenic and chondrogenic lineages, respectively. BMP-2 and TGF-β3 were covalently bound both homogeneously within a poly(ethylene glycol) (PEG)-based brush-functionalized scaffolds, and following a gradient composition by varying their concentration along the axial section of the 3D constructs. Following an approach previously developed by our group and proved to be successful to generate fibronectin gradients, opposite brush-supported gradients of BMP-2 and TGF-β3 were finally generated and subsequently tested to differentiate cells in a gradient fashion. The brush-supported GFs significantly influenced hMSCs osteochondral differentiation when the scaffolds were homogenously modified, yet no effect was observed in the gradient scaffolds. Therefore, this technique seems promising to maintain the biological activity of growth factors covalently linked to 3D scaffolds, but needs to be further optimized in case biological gradients are desired.

Interplay of Geometric Cues and RGD/BMP-2 Crosstalk in Directing Stem Cell Fate

Within the native microenvironment, extracellular matrix (ECM) components are thought to display a complex and heterogeneous distribution, spanning several length scales. Herein, the objective is to mimic, in vitro, the hierarchical organization of proteins and growth factors as well as their crosstalk. Photolithography technique was used to adjacently pattern geometrically defined regions of RGD and BMP-2 mimetic peptides onto glass substrates. These ECM-derived ligands are known to jointly regulate mesenchymal stem cells (MSCs) osteogenic differentiation. By manipulating the spatial distribution of dually grafted peptides, the extent of human MSCs osteogenic differentiation was significantly affected, depending on the shape of peptide micropatterns. Our data highlight the existence of a strong interplay between geometric cues and biochemical signals. Such in vitro systems provide a valuable tool to investigate mechanisms by which multiple ECM cues overlap to regulate stem cell fate, thereby contributing to the design of bioinspired biomaterials for bone tissue engineering applications.

Physicochemical regulation of TGF and VEGF delivery from mesoporous calcium phosphate bone substitutes

Aim: Determination of the physicochemical parameters governing growth factors (GFs) adsorption and release from mesoporous calcium phosphate ceramics. Materials & methods: Six mesoporous calcium phosphate ceramics prepared by soft and hard templating were loaded with two different physiological concentrations of TGF-β1 or VEGF165 and their in vitro kinetics of adsorption/release were studied. Results: This low GF loading promotes adsorption on the highest binding sites. The usually encountered detrimental burst release is thus considerably reduced for samples prepared by hard-templating method. Conclusion: Our findings highlight that the strong affinity of GFs with the ceramic surfaces, demonstrated by a slow GFs release, is enhanced by the large surface area, confinement into mesopores of ceramics and high difference of surface charge between ceramic surfaces and GFs.

Strontium doping promotes bioactivity of rhBMP-2 upon calcium phosphate cement via elevated recognition and expression of BMPR-IA

Preserving and improving osteogenic activity of bone morphogenetic protein-2 (BMP-2) upon implants remains one of the key limitations in bone regeneration. With calcium phosphate cement (CPC) as model, we have developed a series of strontium (Sr)-doped CPC (SCPC) to address this issue. The effects of fixed Sr on the bioactivity of recombinant human BMP-2 (rhBMP-2) as well as the underlying mechanism were investigated. The results suggested that the rhBMP-2-induced osteogenic activity was significantly promoted upon SCPCs, especially with a low amount of fixed Sr (SrCO₃ content <10wt%). Further studies demonstrated that the Sr-induced enhancement of bioactivity of rhBMP-2 was related to an elevated recognition of bone morphogenetic protein receptor-IA (BMPR-IA) to rhBMP-2 and an increased expression of BMPR-IA in C2C12 model cells. As a result, the activations of BMP-induced signaling pathways were different in C2C12 cells incubated upon CPC/rhBMP-2 and SCPCs/rhBMP-2. These findings explicitly decipher the mechanism of SCPCs promoting osteogenic bioactivity of rhBMP-2 and signify the promising application of the SCPCs/rhBMP-2 matrix in bone regeneration implants.