Dual effects of heparin on BMP-2-induced osteogenic activity in MC3T3-E1 cells

Regulating the bioactivity of non-glycosylated recombinant human bone morphogenetic protein-2 to enhance bone regeneration

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is the predominant growth factor that effectively induces osteogenic differentiation in orthopedic procedures. However, the bioactivity and stability of rhBMP-2 are intrinsically associated with its sequence, structure, and storage conditions. In this study, we successfully determined the amino acid sequence and protein secondary structure model of non-glycosylated rhBMP-2 expressed by an E. coli expression system through X-ray crystal structure analysis. Furthermore, we observed that acidic storage conditions enhanced the proliferative and osteoinductive activity of rhBMP-2. Although the osteogenic activity of non-glycosylated rhBMP-2 is relatively weaker compared to glycosylated rhBMP-2; however, this discrepancy can be mitigated by incorporating exogenous chaperone molecules. Overall, such information is crucial for rationalizing the design of stabilization methods and enhancing the bioactivity of rhBMP-2, which may also be applicable to other growth factors.

Human iPSCs as Model Systems for BMP-Related Rare Diseases

Disturbances in bone morphogenetic protein (BMP) signalling contribute to onset and development of a number of rare genetic diseases, including Fibrodysplasia ossificans progressiva (FOP), Pulmonary arterial hypertension (PAH), and Hereditary haemorrhagic telangiectasia (HHT). After decades of animal research to build a solid foundation in understanding the underlying molecular mechanisms, the progressive implementation of iPSC-based patient-derived models will improve drug development by addressing drug efficacy, specificity, and toxicity in a complex humanized environment. We will review the current state of literature on iPSC-derived model systems in this field, with special emphasis on the access to patient source material and the complications that may come with it. Given the essential role of BMPs during embryonic development and stem cell differentiation, gain- or loss-of-function mutations in the BMP signalling pathway may compromise iPSC generation, maintenance, and differentiation procedures. This review highlights the need for careful optimization of the protocols used. Finally, we will discuss recent developments towards complex in vitro culture models aiming to resemble specific tissue microenvironments with multi-faceted cellular inputs, such as cell mechanics and ECM together with organoids, organ-on-chip, and microfluidic technologies.

High Molecular Weight Poly(glutamic acid) to Improve BMP2-Induced Osteogenic Differentiation

FDA-approved bone morphogenetic protein 2 (BMP2) has serious side effects due to the super high dose requirement. Heparin is one of the most well-studied sulfated polymers to stabilize BMP2 and improve its functionality. However, the clinical use of heparin is questionable because of its undesired anticoagulant activity. Recent studies suggest that poly(glutamic acid) (pGlu) has the potential to improve BMP2 bioactivity with less safety concerns; however, the knowledge on pGlu's contribution remains largely unknown. Therefore, we aimed to study the role of pGlu in BMP2-induced osteogenesis and its potential application in bone tissue engineering. Our data, for the first time, indicated that both low (L-pGlu) and high molecular weight pGlu (H-pGlu) were able to significantly improve the BMP2-induced early osteoblastic differentiation marker (ALP) in MC3T3-E1 preosteoblasts. Importantly, the matrix mineralization was more rapidly enhanced by H-pGlu compared to L-pGlu. Additionally, our data indicated that only α-H-pGlu could significantly improve BMP2's activity, whereas γ-H-pGlu failed to do so. Moreover, both gene expression and mineralization data demonstrated that α-H-pGlu enabled a single dose of BMP2 to induce a high level of osteoblastic differentiation without multiple doses of BMP2. To study the potential application of pGlu in tissue engineering, we incorporated the H-pGlu+BMP2 nanocomplexes into the collagen hydrogel with significantly elevated osteoblastic differentiation. Furthermore, H-pGlu-coated 3D porous gelatin and chitosan scaffolds significantly enhanced osteogenic differentiation through enabling sustained release of BMP2. Thus, our findings suggest that H-pGlu is a promising new alternative with great potential for bone tissue engineering applications.

A 3D-printed bioactive polycaprolactone scaffold assembled with core/shell microspheres as a sustained BMP2-releasing system for bone repair

Integration of biological factors and hierarchical rigid scaffolds is of great interest in bone tissue engineering for fabrication of biomimetic constructs with high physical and biological performance for enhanced bone repair. Core/shell microspheres (CSMs) delivering bone morphogenetic protein-2 (BMP-2) and a strategy to integrate CSMs with 3D-printed scaffolds were developed herein to form a hybrid 3D system for bone repair. The scaffold was printed with polycaprolactone (PCL) and then coated with polydopamine. Shells of CSMs were electrosprayed with alginate. Cores were heparin-coated polylactic acid (PLA) microparticles fabricated via simple emulsion and heparin coating strategy. Assembly of microspheres and scaffolds was realized via a self-locking method with the assistance of controlled expansion of CSMs. The hybrid system was evaluated in the rat critical-sized bone defect model. CSMs released BMP-2 in a tunable manner and boosted osteogenic performance in vitro. CSMs were then successfully integrated inside the scaffolds. The assembled system effectively promoted osteogenesis in vitro and in vivo. These observations show the importance of how BMP-2 is delivered, and the core/shell microspheres represent effective BMP-2 carriers that could be integrated into scaffolds, together forming a hybrid system as a promising candidate for enhanced bone regeneration.

Cyclodextrin-Based Supramolecular Complexes of Osteoinductive Agents for Dental Tissue Regeneration

Oral tissue regeneration has received growing attention for improving the quality of life of patients. Regeneration of oral tissues such as alveolar bone and widely defected bone has been extensively investigated, including regenerative treatment of oral tissues using therapeutic cells and growth factors. Additionally, small-molecule drugs that promote bone formation have been identified and tested as new regenerative treatment. However, treatments need to progress to realize successful regeneration of oral functions. In this review, we describe recent progress in development of regenerative treatment of oral tissues. In particular, we focus on cyclodextrin (CD)-based pharmaceutics and polyelectrolyte complexation of growth factors to enhance their solubility, stability, and bioactivity. CDs can encapsulate hydrophobic small-molecule drugs into their cavities, resulting in inclusion complexes. The inclusion complexation of osteoinductive small-molecule drugs improves solubility of the drugs in aqueous solutions and increases in vitro osteogenic differentiation efficiency. Additionally, various anionic polymers such as heparin and its mimetic polymers have been developed to improve stability and bioactivity of growth factors. These polymers protect growth factors from deactivation and degradation by complex formation through electrostatic interaction, leading to potentiation of bone formation ability. These approaches using an inclusion complex and polyelectrolyte complexes have great potential in the regeneration of oral tissues.

Structural perspective of BMP ligands and signaling

The Bone Morphogenetic Proteins (BMPs) are the largest class signaling molecules within the greater Transforming Growth Factor Beta (TGFβ) family, and are responsible for a wide array of biological functions, including dorsal-ventral patterning, skeletal development and maintenance, as well as cell homeostasis. As such, dysregulation of BMPs results in a number of diseases, including fibrodysplasia ossificans progressiva (FOP) and pulmonary arterial hypertension (PAH). Therefore, understanding BMP signaling and regulation at the molecular level is essential for targeted therapeutic intervention. This review discusses the recent advances in the structural and biochemical characterization of BMPs, from canonical ligand-receptor interactions to co-receptors and antagonists. This work aims to highlight how BMPs differ from other members of the TGFβ family, and how that information can be used to further advance the field. Lastly, this review discusses several gaps in the current understanding of BMP structures, with the aim that discussion of these gaps will lead to advancements in the field.

The differentiation effect of bone morphogenetic protein (BMP) on human amniotic epithelial stem cells to express ectodermal lineage markers

Stem cells are a promising tool for treatment of a variety of degenerative diseases. Human amniotic epithelial stem cells (hAECs) have desirable and unique characteristics that make them a proper candidate for cell therapy. In this study, we have investigated the effects of BMP-4 (bone morphogenetic protein-4) and its inhibition on differentiation of AECs into ectodermal lineages. Analysis of AEC-derived ectodermal lineages (neurons and keratinocytes) was performed by using flow cytometry technique for Map2 and β-tubulin (as neuron markers), Olig2 and MBP (as oligodendrocyte markers), and K14 and K10 (as keratinocyte markers). The results of this study illustrated that noggin (as BMP antagonist), BMP4, and both BMP4 and heparin (together or separately) increased neural and keratinocyte marker expression, respectively. The expression of markers MAP2, olig2, and K14 in hAECs has been significantly decreased 21 days after exposure to differentiation medium (without growth factors) compared with isolation day, which supports the hypothesis that AECs can be dedifferentiated into pluripotent cells. Moreover, activation and inhibition of BMP signaling have no effects on viability of hAECs. The results of this study showed that BMP signaling and its inhibition are the key factors for ectodermal lineage differentiation of amnion-derived stem cells.

Fibrin Glue/Fibronectin/Heparin-Based Delivery System of BMP2 Induces Osteogenesis in MC3T3-E1 Cells and Bone Formation in Rat Calvarial Critical-Sized Defects

Bone morphogenetic proteins (BMPs) have been used to promote bone formation in many clinical scenarios. However, the BMPs are inherently unstable in vivo and therefore need to be combined with carriers for controlled delivery. In this study, an innovative and efficient fibrin glue/fibronectin/heparin (FG/Fn/Hep)-based delivery system was developed for controlled release of BMP2. The incorporation of heparin can significantly slow the release of BMP2 without substantially affecting the structure and stiffness of the FG/Fn. The BMP2 release from the FG/Fn/Hep-BMP2 hydrogel is largely dominated by hydrogel degradation rather than simple diffusion. In vitro release experiments and MC3T3-E1 cell induction experiments showed that BMP2 can be released steadily and can induce MC3T3-E1 cells to differentiate into osteoblasts efficiently. This process is characterized by the significantly increased expression of calcium deposits, alkaline phosphatase, runt-related transcription factor-2, osteopontin, osteocalcin, and collagen I in comparison with the negative control. In vivo assessments revealed that the FG/Fn/Hep-BMP2 hydrogel significantly promotes bone regeneration in a rat calvarial critical-sized defect model. Our investigation indicates that FG/Fn/Hep-BMP2 hydrogel holds promise to be used as an alternative biomaterial for the repair of bone defects.

Polyelectrolyte Complexes between Polycarboxylates and BMP-2 for Enhancing Osteogenic Differentiation: Effect of Chemical Structure of Polycarboxylates

Bone morphogenetic protein 2 (BMP-2) has received considerable attention because of its osteoinductivity, but its use is limited owing to its instability and adverse effects. To reduce the dose of BMP-2, complexation with heparin is a promising approach, because heparin enhances the osteoinductivity of BMP-2. However, the clinical use of heparin is restricted because of its anticoagulant activity. Herein, to explore alternative polymers that show heparin-like activity, four polycarboxylates, poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), poly(aspartic acid) (PAsp), and poly(glutamic acid) (PGlu), were selected and their capability to modulate the osteoinductivity of BMP-2 was evaluated. Dynamic light scattering indicated that these polycarboxylates formed polyelectrolyte complexes with BMP-2. The osteogenic differentiation efficiency of MC3T3-E1 cells treated with the polycarboxylate/BMP-2 complexes was investigated in comparison to that of the heparin/BMP-2 complex. As a result, PGlu/BMP-2 complex showed the highest activity of alkaline phosphatase, which is an early-stage marker of osteogenic differentiation, and rapid mineralization. Based on these observations, PGlu could serve as an alternative to heparin in the regenerative therapy of bone using BMP-2.

A silencing-mediated enhancement of osteogenic differentiation by supramolecular ternary siRNA polyplexes comprising biocleavable cationic polyrotaxanes and anionic fusogenic peptides

Gene silencing of noggin by small interfering RNA (siRNA) is a promising approach for the treatment of bone defects, because noggin deactivates bone morphogenetic protein-2 (BMP-2) and suppresses osteogenic differentiation. Here, we demonstrated the silencing of the noggin gene by siRNA polyplexes composed of noggin-targeted siRNA and biocleavable cationic polyrotaxanes (DMAE-SS-PRX). To improve the endosomal escape efficiencies of the DMAE-SS-PRX/siRNA polyplexes, anionic and fusogenic GALA peptides were integrated onto the DMAE-SS-PRX/siRNA polyplexes via simple electrostatic interactions. The formation of ternary complexes was confirmed by gel electrophoresis, dynamic light scattering, and zeta-potential measurements. Although the association of GALA peptides with the DMAE-SS-PRX/siRNA polyplexes did not remarkably affect the cellular uptake efficiency of siRNA, the endosomal escape efficiency was remarkably increased for GALA/DMAE-SS-PRX/siRNA ternary polyplexes because of the endosomal and lysosomal membrane destabilization by GALA peptides. Consequently, GALA/DMAE-SS-PRX/siRNA ternary polyplexes showed significantly higher gene silencing efficiency against noggin and enhanced the BMP-2-mediated osteogenic differentiation efficiency. Therefore, we concluded that GALA/DMAE-SS-PRX/siRNA ternary polyplexes can be effective siRNA carriers for suppressing the expression of specific endogenous genes. Consequently, we believe that a more practical approach in vivo will be the combined use of BMP-2 and GALA/DMAE-SS-PRX/siRNA ternary polyplexes, because it will improve the efficacy of bone regeneration therapy.

Heparin-dopamine functionalized graphene foam for sustained release of bone morphogenetic protein-2

The recently developed three-dimensional (3D) graphene foam (GrF) is intriguing for potential bone tissue engineering applications because it provides stem cells with a 3D porous substrate for osteogenic differentiation. However, the nature of graphene's structure lacks functional groups, thus making it difficult for further modification such as immobilization or conjugation of growth factors, which are normally required to promote tissue regeneration. To explore the potential of GrF functionalization and sustained release of therapeutic proteins, we fabricated a modified 3D GrF scaffold with bio-inspired heparin-dopamine (Hepa-Dopa) molecules using a highly scalable chemical vapour deposition method. Our data indicated that Hepa-Dopa modification resulted in significantly higher bone morphogenetic protein-2 (BMP2) binding ability and longer release capacity compared with the untreated scaffolds. Importantly, the heparin-functionalized 3D GrF significantly improved the exogenous BMP2-induced osteogenic differentiation. Therefore, our study, for the first time, indicated that the 3D GrF can be biomimetically functionalized with Hepa-Dopa and be used for sustained release of BMP2, thereby inducing osteogenic differentiation and suggesting promising potential as a new multifunctional carrier for therapeutic proteins and stem cells in bone tissue engineering.

Bilayered BMP2 Eluting Coatings on Graphene Foam by Electrophoretic Deposition: Electroresponsive BMP2 Release and Enhancement of Osteogenic Differentiation

Recent development of three-dimensional graphene foam (GF) with conductive and interconnected macroporous structure is attracting particular attention as platforms for tissue engineering. However, widespread application of GF as bone scaffolds is restricted due to its poor mechanical property and inert surface character. To overcome these drawbacks, in this study, a bilayered biopolymer coating was designed and successfully deposited covering the entire surface area of GF skeleton. A poly(lactic-co-glycolic acid) layer was first dip-coated to strengthen the GF substrate, followed by the electrophoretic codeposition of a hybrid layer, consisting of chitosan and BMP2, to functionalize GF with the ability to recruit and induce osteogenic differentiation of hMSC. Our data indicated that the mechanical property of GF was significantly increased without compromising the macroporous structure. Importantly, the immobilized BMP2 exhibited sustained and electroresponsive release profiles with rapid response to the electric field exerted on GF, which is beneficial to balancing BMP2 dose in a physiological environment. Moreover, the osteogenic differentiation of hMSC was significantly improved on the functionalized GF. Taking advantage of the unique macrostructure from GF as well as the superior mechanical properties and BMP2 release profile supported by the deposited coatings, it is therefore expected that the developed GF could be a promising alternative as innovative bone-forming favorable scaffolds.

Sprouty2 is involved in the control of osteoblast proliferation and differentiation through the FGF and BMP signaling pathways

Fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) play essential roles in bone formation and osteoblast activity through the extracellular signal-regulated kinase 1/2 (ERK1/2) and Smad pathways. Sprouty family members are intracellular inhibitors of the FGF signaling pathway, and four orthologs of Sprouty have been identified in mammals. In vivo analyses have revealed that Sprouty2 is associated with bone formation. However, the mechanism by which the Sprouty family controls bone formation has not been clarified. In this study, we investigated the involvement of Sprouty2 in osteoblast proliferation and differentiation. We examined Sprouty2 expression in MC3T3-E1 cells, and found that high levels of Sprouty2 expression were induced by basic FGF stimulation. Overexpression of Sprouty2 in MC3T3-E1 cells resulted in suppressed proliferation compared with control cells. Sprouty2 negatively regulated the phosphorylation of ERK1/2 after basic FGF stimulation, and of Smad1/5/8 after BMP stimulation. Furthermore, Sprouty2 suppressed the expression of osterix, alkaline phosphatase, and osteocalcin mRNA, which are markers of osteoblast differentiation. Additionally, Sprouty2 inhibited osteoblast matrix mineralization. These results suggest that Sprouty2 is involved in the control of osteoblast proliferation and differentiation by downregulating the FGF-ERK1/2 and BMP-Smad pathways, and suppresses the induction of markers of osteoblast differentiation.

Heparin, Heparan Sulphate and the TGF-β Cytokine Superfamily

Of the circa 40 cytokines of the TGF-β superfamily, around a third are currently known to bind to heparin and heparan sulphate. This includes TGF-β1, TGF-β2, certain bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), as well as GDNF and two of its close homologues. Experimental studies of their heparin/HS binding sites reveal a diversity of locations around the shared cystine-knot protein fold. The activities of the TGF-β cytokines in controlling proliferation, differentiation and survival in a range of cell types are in part regulated by a number of specific, secreted BMP antagonist proteins. These vary in structure but seven belong to the CAN or DAN family, which shares the TGF-β type cystine-knot domain. Other antagonists are more distant members of the TGF-β superfamily. It is emerging that the majority, but not all, of the antagonists are also heparin binding proteins. Any future exploitation of the TGF-β cytokines in the therapy of chronic diseases will need to fully consider their interactions with glycosaminoglycans and the implications of this in terms of their bioavailability and biological activity.

Differential circRNA expression profiles during the BMP2-induced osteogenic differentiation of MC3T3-E1 cells

OBJECTIVE

Recent studies have indicated that circular RNAs (circRNAs) might play important roles in various diseases. However, little is known about the functions of circRNAs in the skeletal system, and the role of circRNAs in the mechanism by which bone morphogenetic protein 2 (BMP2) promotes bone differentiation remains unknown. Here, we performed RNA-seq to analyze differential expression of circRNA during different osteoblast differentiation stages and investigated the relevant mechanisms.

MATERIALS AND METHODS

Alkaline phosphatase (ALP) staining and activity were performed to assess osteogenic differentiation in MC3T3-E1 cells. The expression of osteogenic markers in MC3T3-E1 cells and the differential expression levels of circRNAs were measured and validated by qRT-PCR. Osteogenic marker proteins were measured by western blot. RNA-seq was performed to detect differential expression of circRNAs during the osteogenic differentiation of MC3T3-E1 cells induced by BMP2. Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and PANTHER pathway analyses were performed to predict the functions of differentially expressed circRNAs and potentially co-expressed target genes. The microRNA (miRNA) targets of the circRNAs and circRNA-miRNA interactions were predicted by miRanda. The circRNA-miRNA co-expression network was constructed based on the correlation analysis between the differentially expressed circRNAs and miRNAs. A graph of the circRNA-miRNA network was created using Cytoscape 3.01.

RESULTS

The Cell Counting Kit 8 (CCK-8) assay showed that BMP2 promoted the proliferation of osteoblasts in vitro. Both the intracellular ALP content and activity were increased in BMP2-treated MC3T3-E1 cells. In addition, the mRNA and protein levels of the osteoblastic markers ALP, Sp7 transcription factor (SP7) and runt-related transcription factor 2 (RUNX2) were substantially up-regulated. In the present study, 158 circRNAs were differentially expressed by a fold-change ≥2.0, P<0.05 and false discovery rate <0.05. Among these, 74 circRNAs were up-regulated, while 84 circRNAs were down-regulated. In addition, the expression levels of circRNA.5846, circRNA.19142 and circRNA.10042 were significantly increased in the BMP2 group. Furthermore, by analyzing the target mRNAs of miR-7067-5p using GO and PANTHER pathway analyses, circ19142 and circ5846 were found to be not only strongly associated with the biological process of the positive regulation of developmental processes but also related to the fibroblast growth factor, epidermal growth factor, platelet-derived growth factor and Wnt signaling pathways, which are involved in cell growth and differentiation.

CONCLUSION

The present study identified circ19142 and circ5846 as being associated with osteoblast differentiation and BMP2 may induce osteogenic differentiation through a circ19142/circ5846-targeted miRNA-mRNA axis.

WFIKKN1 and WFIKKN2: "Companion" proteins regulating TGFB activity

The WFIKKN (WAP, Follistatin/kazal, Immunoglobulin, Kunitz and Netrin domain-containing) protein family is composed of two multidomain proteins: WFIKKN1 and WFIKKN2. They were formed by domain shuffling and are likely present in deuterostoms. The WFIKKN (also called GASP) proteins are well known for their function in muscle and skeletal tissues, namely, inhibition of certain members of the transforming growth factor beta (TGFB) superfamily such as myostatin (MSTN) and growth and differentiation factor 11 (GDF11). However, the role of the WFIKKN proteins in other tissues is still poorly understood in spite of evidence suggesting possible action in the inner ear, brain and reproduction. Further, several recent studies based on next generation technologies revealed differential expression of WFIKKN1 and WFIKKN2 in various tissues suggesting that their function is not limited to MSTN and GDF11 inhibition in musculoskeletal tissue. In this review, we summarize current knowledge about the WFIKKN proteins and propose that they are "companion" proteins for various growth factors by providing localized and sustained presentation of TGFB proteins to their respective receptors, thus regulating the balance between the activation of Smad and non-Smad pathways by TGFB.

Biomaterial strategies for controlling stem cell fate via morphogen sequestration

This review explores the role of protein sequestration in the stem cell niche and how it has inspired the design of biomaterials that exploit natural protein sequestration to influence stem cell fate.

Heparin affects human bone marrow stromal cell fate: Promoting osteogenic and reducing adipogenic differentiation and conversion

Heparins are broadly used for the prevention and treatment of thrombosis and embolism. Yet, osteoporosis is considered to be a severe side effect in up to one third of all patients on long-term treatment. However, the mechanisms underlying this clinical problem are only partially understood. To investigate if heparin affects differentiation of skeletal precursors, we examined the effects of heparin on the osteogenic and adipogenic lineage commitment and differentiation of primary human bone marrow stromal cells (hBMSCs). Due to the known inverse relationship between adipogenesis and osteogenesis and the capacity of pre-differentiated cells to convert into the respective other lineage, we also determined heparin effects on osteogenic conversion and adipogenic differentiation/conversion. Interestingly, heparin did not only significantly increase mRNA expression and enzyme activity of the osteogenic marker alkaline phosphatase (ALP), but it also promoted mineralization during osteogenic differentiation and conversion. Furthermore, the mRNA expression of the osteogenic marker bone morphogenic protein 4 (BMP4) was enhanced. In addition, heparin administration partly prevented adipogenic differentiation and conversion demonstrated by reduced lipid droplet formation along with a decreased expression of adipogenic markers. Moreover, luciferase reporter assays, inhibitor experiments and gene expression analyses revealed that heparin had putative permissive effects on osteogenic signaling via the BMP pathway and reduced the mRNA expression of the Wnt pathway inhibitors dickkopf 1 (DKK1) and sclerostin (SOST). Taken together, our data show a rather supportive than inhibitory effect of heparin on osteogenic hBMSC differentiation and conversion in vitro. Further studies will have to investigate the net effects of heparin administration on bone formation versus bone resorption in vivo to unravel the molecular mechanisms of heparin-associated osteoporosis and reconcile conflicting experimental data with clinical observations.

Heparin microparticle effects on presentation and bioactivity of bone morphogenetic protein-2

Biomaterials capable of providing localized and sustained presentation of bioactive proteins are critical for effective therapeutic growth factor delivery. However, current biomaterial delivery vehicles commonly suffer from limitations that can result in low retention of growth factors at the site of interest or adversely affect growth factor bioactivity. Heparin, a highly sulfated glycosaminoglycan, is an attractive growth factor delivery vehicle due to its ability to reversibly bind positively charged proteins, provide sustained delivery, and maintain protein bioactivity. This study describes the fabrication and characterization of heparin methacrylamide (HMAm) microparticles for recombinant growth factor delivery. HMAm microparticles were shown to efficiently bind several heparin-binding growth factors (e.g. bone morphogenetic protein-2 (BMP-2), vascular endothelial growth factor (VEGF), and basic fibroblast growth factor (FGF-2)), including a wide range of BMP-2 concentrations that exceeds the maximum binding capacity of other common growth factor delivery vehicles, such as gelatin. BMP-2 bioactivity was assessed on the basis of alkaline phosphatase (ALP) activity induced in skeletal myoblasts (C2C12). Microparticles loaded with BMP-2 stimulated comparable C2C12 ALP activity to soluble BMP-2 treatment, indicating that BMP-2-loaded microparticles retain bioactivity and potently elicit a functional cell response. In summary, our results suggest that heparin microparticles stably retain large amounts of bioactive BMP-2 for prolonged periods of time, and that presentation of BMP-2 via heparin microparticles can elicit cell responses comparable to soluble BMP-2 treatment. Consequently, heparin microparticles present an effective method of delivering and spatially retaining growth factors that could be used in a variety of systems to enable directed induction of cell fates and tissue regeneration.

Novel mineralized heparin-gelatin nanoparticles for potential application in tissue engineering of bone

Nanoparticles (NPs) were prepared from succinylated gelatin (s-GL) cross-linked with aldehyde heparin (a-HEP) and used subsequently as a nano-template for the mineralization of hydroxyapatite (HAP). Gelatin was functionalized with succinyl groups that made it soluble at room temperature. Heparin was oxidized to generate aldehyde groups and then used as a cross-linker that can react with s-GL to form NPs via Schiff's base linkage. The polymer concentrations, feed molar ratios and pH conditions were varied to fabricate NPs suspension. NPs were obtained with a spheroid shape of an average size of 196 nm at pH 2.5 and 202 nm at pH 7.4. These NPs had a positive zeta potential of 7.3 ± 3.0 mV and a narrow distribution with PDI 0.123 at pH 2.5, while they had a negative zeta potential of -2.6 ± 0.3 mV and formed aggregates (PDI 0.257) at pH 7.4. The NPs prepared at pH 2.5 with a mean particle size of 196 nm were further used for mineralization studies. The mineralization process was mediated by solution without calcination at 37 °C. The HAP formed on NPs was analyzed by Fourier transform infrared spectroscopy and X-ray diffraction. HAP coated s-GL/a-HEP NPs developed in this study may be used in future as osteoinductive fillers enhancing the mechanical properties of injectable hydrogel or use as potential multifunctional device for nanotherapeutic approaches.

Single bout short duration fluid shear stress induces osteogenic differentiation of MC3T3-E1 cells via integrin β1 and BMP2 signaling cross-talk

Fluid shear stress plays an important role in bone osteogenic differentiation. It is traditionally believed that pulsed and continuous stress load is more favorable for fracture recovery and bone homeostasis. However, according to our clinical practice, we notice that one single stress load is also sufficient to trigger osteogenic differentiation. In the present study, we subject osteoblast MC3T3-E1 cells to single bout short duration fluid shear stress by using a parallel plate flow system. The results show that 1 hour of fluid shear stress at 12 dyn/cm² promotes terminal osteogenic differentiation, including rearrangement of F-actin stress fiber, up-regulation of osteogenic genes expression, elevation of alkaline phosphatase activity, secretion of type I collagen and osteoid nodule formation. Moreover, collaboration of BMP2 and integrin β1 pathways plays a significant role in such differentiation processes. Our findings provide further experimental evidence to support the notion that single bout short duration fluid shear stress can promote osteogenic differentiation.

The factor VII-activating protease (FSAP) enhances the activity of bone morphogenetic protein-2 (BMP-2)

Factor VII-activating protease (FSAP) is a circulating protease involved in the pathogenesis of atherosclerosis, calcification, and fibrotic processes. To understand how FSAP controls the balance of local growth factors, we have investigated its effect on the regulation of bone morphogenetic proteins (BMPs). BMP-2 is produced as a large pro-form and secreted as a mature heparin-binding growth factor after intracellular processing by pro-protein convertases (PCs). In this study, we discovered that FSAP enhances the biological activity of mature BMP-2 as well as its pro-form, as shown by osteogenic differentiation of C2C12 myoblasts. These findings were complemented by knockdown of FSAP in hepatocytes, which revealed BMP-2 processing by endogenous FSAP. N-terminal sequencing indicated that pro-BMP-2 was cleaved by FSAP at the canonical PC cleavage site, giving rise to mature BMP-2 (Arg(282)↓Gln(283)), as well as in the N-terminal heparin binding region of mature BMP-2, generating a truncated mature BMP-2 peptide (Arg(289)↓Lys(290)). Similarly, mature BMP-2 was also cleaved to a truncated peptide within its N-terminal region (Arg(289)↓Lys(290)). Plasmin exhibited a similar activity, but it was weaker compared with FSAP. Thrombin, Factor VIIa, Factor Xa, and activated protein C were not effective. These results were further supported by the observation that the mutation of the heparin binding region of BMP-2 inhibited the processing by FSAP but not by PC. Thus, the proteolysis and activation of pro-BMP-2 and mature BMP-2 by FSAP can regulate cell differentiation and calcification in vasculature and may explain why polymorphisms in the gene encoding for FSAP are related to vascular diseases.