Bone Morphogenetic Protein-2 Promotes Human Mesenchymal Stem Cell Survival and Resultant Bone Formation When Entrapped in Photocrosslinked Alginate Hydrogels

Structurally and mechanically tuned macroporous hydrogels for scalable mesenchymal stem cell-extracellular matrix spheroid production

Mesenchymal stem cells (MSCs) are essential in regenerative medicine. However, conventional expansion and harvesting methods often fail to maintain the essential extracellular matrix (ECM) components, which are crucial for their functionality and efficacy in therapeutic applications. Here, we introduce a bone marrow-inspired macroporous hydrogel designed for the large-scale production of MSC-ECM spheroids. Through a soft-templating approach leveraging liquid-liquid phase separation, we engineer macroporous hydrogels with customizable features, including pore size, stiffness, bioactive ligand distribution, and enzyme-responsive degradability. These tailored environments are conducive to optimal MSC proliferation and ease of harvesting. We find that soft hydrogels enhance mechanotransduction in MSCs, establishing a standard for hydrogel-based 3D cell culture. Within these hydrogels, MSCs exist as both cohesive spheroids, preserving their innate vitality, and as migrating entities that actively secrete functional ECM proteins. Additionally, we also introduce a gentle, enzymatic harvesting method that breaks down the hydrogels, allowing MSCs and secreted ECM to naturally form MSC-ECM spheroids. These spheroids display heightened stemness and differentiation capacity, mirroring the benefits of a native ECM milieu. Our research underscores the significance of sophisticated materials design in nurturing distinct MSC subpopulations, facilitating the generation of MSC-ECM spheroids with enhanced therapeutic potential.

Advancements in drug-loaded hydrogel systems for bone defect repair

Bone defects are primarily the result of high-energy trauma, pathological fractures, bone tumor resection, or infection debridement. The treatment of bone defects remains a huge clinical challenge. The current treatment options for bone defects include bone traction, autologous/allogeneic bone transplantation, gene therapy, and bone tissue engineering amongst others. With recent developments in the field, composite scaffolds prepared using tissue engineering techniques to repair bone defects are used more often. Among the various composite scaffolds, hydrogel exhibits the advantages of good biocompatibility, high water content, and degradability. Its three-dimensional structure is similar to that of the extracellular matrix, and as such it is possible to load stem cells, growth factors, metal ions, and small molecule drugs upon these scaffolds. Therefore, the hydrogel-loaded drug system has great potential in bone defect repair. This review summarizes the various natural and synthetic materials used in the preparation of hydrogels, in addition to the latest research status of hydrogel-loaded drug systems.

Degradation-Kinetics-Controllable and Tissue-Regeneration-Matchable Photocross-linked Alginate Hydrogels for Bone Repair

Photocross-linked alginate hydrogels, due to their biodegradability, biocompatibility, strong control for gelling kinetics in space and time, and admirable adaptability for in situ polymerization with a minimally invasive approach in surgical procedures, have created great expectations in bone regeneration. However, hydrogels with suitable degradation kinetics that can match the tissue regeneration process have not been designed, which limits their further application in bone tissue engineering. Herein, we finely developed an oxidation strategy for alginate to obtain hydrogels with more suitable degradation rates and comprehensively explored their physical and biological performances in vitro and in vivo to further advance the clinical application for the hydrogels in bone repair. The physical properties of the gels can be tuned via tailoring the degree of alginate oxidation. In particular, in vivo degradation studies showed that the degradation rates of the gels were significantly increased by oxidizing alginate. The activity, proliferation, initial adhesion, and osteogenic differentiation of rat and rabbit bone marrow stromal cells (BMSCs) cultured with/in the hydrogels were explored, and the results demonstrated that the gels possessed excellent biocompatibility and that the encapsulated BMSCs were capable of osteogenic differentiation. Furthermore, in vivo implantation of rabbit BMSC-loaded gels into tibial plateau defects of rabbits demonstrated the feasibility of hydrogels with appropriate degradation rates for bone repair. This study indicated that hydrogels with increasingly controllable and matchable degradation kinetics and satisfactory bioproperties demonstrate great clinical potential in bone tissue engineering and regenerative medicine and could also provide references for drug/growth-factor delivery therapeutic strategies for diseases requiring specific drug/growth-factor durations of action.

Engineered Cell-Secreted Extracellular Matrix Modulates Cell Spheroid Mechanosensing and Amplifies Their Response to Inductive Cues for the Formation of Mineralized Tissues

The clinical translation of mesenchymal stromal cell (MSC)-based therapies remains challenging due to rapid cell death and poor control over cell behavior. Compared to monodisperse cells, the aggregation of MSCs into spheroids increases their tissue-forming potential by promoting cell-cell interactions. However, MSCs initially lack engagement with an endogenous extracellular matrix (ECM) when formed into spheroids. Previously the instructive nature of an engineered, cell-secreted ECM is demonstrated to promote survival and differentiation of adherent MSCs. Herein, it is hypothesized that the incorporation of this cell-secreted ECM during spheroid aggregation would enhance MSC osteogenic potential by promoting cell-matrix and cell-cell interactions. ECM-loaded spheroids contained higher collagen and glycosaminoglycan content, and MSCs exhibited increased mechanosensitivity to ECM through Yes-associated protein (YAP) activation via integrin α2β1 binding. ECM-loaded spheroids sustained greater MSC viability and proliferation and are more responsive to soluble cues for lineage-specific differentiation than spheroids without ECM or loaded with collagen. The encapsulation of ECM-loaded spheroids in instructive alginate gels resulted in spheroid fusion and enhanced osteogenic differentiation. These results highlight the clinical potential of ECM-loaded spheroids as building blocks for the repair of musculoskeletal tissues.

Two Hits for Bone Regeneration in Aged Patients: Vertebral Bone Marrow Clot as a Biological Scaffold and Powerful Source of Mesenchymal Stem Cells

Recently, the use of a new formulation of bone marrow aspirate (BMA), the BMA clot, has been described. This product entails a naturally formed clot from the harvested bone marrow, which retains all the BMA components preserved in a matrix biologically molded by the clot. Even though its beneficial effects were demonstrated by some studies, the impact of aging and aging-associated processes on biological properties and the effect of BMA cell-based therapy are currently unknown. The purpose of our study was to compare selected parameters and properties of clotted BMA and BMA-derived mesenchymal stem cells (MSCs) from younger (<45 years) and older (>65 years) female donors. Clotted BMA growth factors (GFs) expression, MSCs morphology and viability, doubling time, surface marker expression, clonogenic potential, three-lineage differentiation, senescence-associated factors, and Klotho synthesis from younger and older donors were analyzed. Results indicated that donor age does not affect tissue-specific BMA clot regenerative properties such as GFs expression and MSCs morphology, viability, doubling time, surface antigens expression, colony-forming units, osteogenic and adipogenic differentiation, and Klotho and senescence-associated gene expression. Only few differences, i.e., increased platelet-derived growth factor-AB (PDGF-AB) synthesis and MSCs Aggrecan (ACAN) expression, were detected in younger donors in comparison with older ones. However, these differences do not interfere with all the other BMA clot biological properties. These results demonstrated that BMA clot can be applied easily, without any sample processing and avoiding potential contamination risks as well as losing cell viability, proliferation, and differentiation ability, for autologous transplantation in aged patients. The vertebral BMA clot showed two successful hits since it works as a biological scaffold and as a powerful source of mesenchymal stem cells, thus representing a novel and advanced therapeutic alternative for the treatment of orthopedic injuries.

Stromal cell-derived factor-1 promotes osteoblastic differentiation of human bone marrow mesenchymal stem cells via the lncRNA-H19/miR-214-5p/BMP2 axis

BACKGROUND

Stromal cell-derived factor-1 (SDF-1) plays an important role in the osteoblastic differentiation of human bone marrow mesenchymal stem cells (hBMMSCs), but the specific mechanism remains unclear. Our study aimed to clarify the role of the lncRNA-H19/miR-214-5p/BMP2 axis in the osteoblastic differentiation of hBMMSCs induced by SDF-1.

METHODS

We used reverse-transcriptase polymerase chain reaction, western blotting, alkaline phosphatase activity test, and Alizarin red staining to evaluate the osteoblastic differentiation of primary hBMMSCs and the luciferase reporter assay to determine if lncRNA-H19 binds with miR-214-5p.

RESULTS

Our results indicated that SDF-1 (50 ng/mL) promotes the osteoblastic differentiation of hBMMSCs, significantly upregulates osteoblastogenic genes (OCN, OSX, RUNX2, and ALP), and increases Alizarin red staining, alkaline phosphatase activity, and lncRNA-H19 expression. Luciferase reporter assay verified that lncRNA-H19 binds with and represses miR-214-5p, thereby upregulating BMP2 expression. Use of miR-214-5p inhibitor or overexpression of lncRNA-H19 can promote the osteoblastic differentiation of hBMMSCs, but miR-214-5p or shH19 inhibits the osteoblastic differentiation of hBMMSCs. Treatment with an miR-214-5p inhibitor could rescue the inhibitory effect of shH19 on the osteoblastic differentiation of hBMMSCs.

CONCLUSIONS

Taken together, SDF-1 promotes the osteoblastic differentiation of hBMMSCs through the lncRNA-H19/miR-214-5p/BMP2 axis. Increased osteoblastic differentiation by an miR-214-5p inhibitor reveals a new possible strategy for the treatment of bone defect and osteoporosis.

Progress in the mechanical modulation of cell functions in tissue engineering

In mammals, mechanics at multiple stages-nucleus to cell to ECM-underlie multiple physiological and pathological functions from its development to reproduction to death. Under this inspiration, substantial research has established the role of multiple aspects of mechanics in regulating fundamental cellular processes, including spreading, migration, growth, proliferation, and differentiation. However, our understanding of how these mechanical mechanisms are orchestrated or tuned at different stages to maintain or restore the healthy environment at the tissue or organ level remains largely a mystery. Over the past few decades, research in the mechanical manipulation of the surrounding environment-known as substrate or matrix or scaffold on which, or within which, cells are seeded-has been exceptionally enriched in the field of tissue engineering and regenerative medicine. To do so, traditional tissue engineering aims at recapitulating key mechanical milestones of native ECM into a substrate for guiding the cell fate and functions towards specific tissue regeneration. Despite tremendous progress, a big puzzle that remains is how the cells compute a host of mechanical cues, such as stiffness (elasticity), viscoelasticity, plasticity, non-linear elasticity, anisotropy, mechanical forces, and mechanical memory, into many biological functions in a cooperative, controlled, and safe manner. High throughput understanding of key cellular decisions as well as associated mechanosensitive downstream signaling pathway(s) for executing these decisions in response to mechanical cues, solo or combined, is essential to address this issue. While many reports have been made towards the progress and understanding of mechanical cues-particularly, substrate bulk stiffness and viscoelasticity-in regulating the cellular responses, a complete picture of mechanical cues is lacking. This review highlights a comprehensive view on the mechanical cues that are linked to modulate many cellular functions and consequent tissue functionality. For a very basic understanding, a brief discussion of the key mechanical players of ECM and the principle of mechanotransduction process is outlined. In addition, this review gathers together the most important data on the stiffness of various cells and ECM components as well as various tissues/organs and proposes an associated link from the mechanical perspective that is not yet reported. Finally, beyond addressing the challenges involved in tuning the interplaying mechanical cues in an independent manner, emerging advances in designing biomaterials for tissue engineering are also explored.

3D‑printed Ti6Al4V scaffolds combined with pulse electromagnetic fields enhance osseointegration in osteoporosis

The loosening and displacement of prostheses after dental implantation and arthroplasty is a substantial medical burden due to the complex correction surgery. Three-dimensional (3D)-printed porous titanium (pTi) alloy scaffolds are characterized by low stiffness, are beneficial to bone ingrowth, and may be used in orthopedic applications. However, for the bio-inert nature between host bone and implants, titanium alloy remains poorly compatible with osseointegration, especially in disease conditions, such as osteoporosis. In the present study, 3D-printed pTi scaffolds with ideal pore size and porosity matching the bone tissue, were combined with pulse electromagnetic fields (PEMF), an exogenous osteogenic induction stimulation, to evaluate osseointegration in osteoporosis. In vitro, external PEMF significantly improved osteoporosis-derived bone marrow mesenchymal stem cell proliferation and osteogenic differentiation on the surface of pTi scaffolds by enhancing the expression of alkaline phosphatase, runt-related transcription factor-2, osteocalcin, and bone morphogenetic protein-2. In vivo, Microcomputed tomography analysis and histological evaluation indicated the external PEMF markedly enhanced bone regeneration and osseointegration. This novel therapeutic strategy has potential to promote osseointegration of dental implants or artificial prostheses for patients with osteoporosis.

Biomaterial Properties Modulating Bone Regeneration

Biomaterial scaffolds have been gaining momentum in the past several decades for their potential applications in the area of tissue engineering. They function as three-dimensional porous constructs to temporarily support the attachment of cells, subsequently influencing cell behaviors such as proliferation and differentiation to repair or regenerate defective tissues. In addition, scaffolds can also serve as delivery vehicles to achieve sustained release of encapsulated growth factors or therapeutic agents to further modulate the regeneration process. Given the limitations of current bone grafts used clinically in bone repair, alternatives such as biomaterial scaffolds have emerged as potential bone graft substitutes. This review summarizes how physicochemical properties of biomaterial scaffolds can influence cell behavior and its downstream effect, particularly in its application to bone regeneration.

Evaluation of Demineralized Bone Matrix Particles Delivered by Alginate Hydrogel for a Bone Graft Substitute: An Animal Experimental Study

BACKGROUND Our objective was to explore a synthetic alginate hydrogel delivery system for the delivery of demineralized bone matrix (DBM) particles for bone graft substitutes. MATERIAL AND METHODS The physiochemical properties of surface morphology, porosity measurements, in vitro degradation, equilibrium swelling, and mechanical testing of combined DBM powder and alginate in amounts of 0 mg/1 mL, 25 mg/1 mL, 50 mg/1 mL, and 100 mg/1 mL were detected. In vitro cell culture and in vivo studies using Sprague-Dawley rats were performed to evaluate the biocompatibility and osteoinductivity of DBM-alginate (ADBM) composites. RESULTS DBM particles were uniformly scattered in all composites, and macro-scale pores were omnipresent. All composites showed a similar low degradation rate, with approximately 85% of weight remaining after 15 days. As the concentration of DBM particles in composites increased, degradation in collagenase and elastic modulus increased and the pore area and swelling ratio significantly decreased. No cytotoxicity of ADBM or alginate on mesenchymal stem cells (MSCs) was observed. Cell cultivation with ADBM showed greater osteogenic potential, evidenced by the upregulation of alkaline phosphatase and alizarin red staining activity and the mRNA expression level of marker genes RUNX2, OCN, OPN, and collagen I compared with the cells grown in alginate. Evaluation of ectopic bone formation revealed the osteoinductivity of the ADBM composites was significantly greater than that of DBM particles. Osteoinduction of the composites was demonstrated by a cranial defect model study. CONCLUSIONS The delivery of DBM particles using a synthetic alginate hydrogel carrier may be a promising approach in bone tissue engineering for bone defects.

A novel hydrogel scaffold contained bioactive glass nanowhisker (BGnW) for osteogenic differentiation of human mesenchymal stem cells (hMSCs) in vitro

Employing hydrogels as an alternative strategy for repairing bone defects has received great attention in bone tissue engineering. In this study, hydrogel scaffold based on collagen, gelatin, and glutaraldehyde was combined with bioactive glass nanowhiskers (BGnW) to differentiate human mesenchymal stem cells (hMSCs) into the osteogenic lineage and inducing biomineralization. Pure Gel-Glu-Col and bioactive glass nanowhiskers were used as control throughout the paper. Chemical, physical and morphological characteristics of the nanocomposite scaffold were assessed meticulously using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), porosity measurement, water uptake ability, tensile test, and scanning electron microscopy (SEM). To determine the cytotoxicity and cell viability of the hydrogel, MTT assay and Acridine orange (AO) staining were performed. hMSCs seeded on Gel-Glu-Col/BGnW were then incubated with osteogenic differentiation media for 14 days. Biomineralization assays (alkaline phosphatase (ALP) activity, calcium content assay, von Kossa, and Alizarin red staining) were carried out, and osteogenic genes and protein markers were examined using real time-PCR and immunocytochemistry. Results showed that the components of the hydrogel were properly integrated. The mechanical property of hydrogel was enhanced following the addition of BGnW. Cell viability assays confirmed the biocompatibility of the scaffold and increasing the proliferation after incorporating BGnW into pure Ge1-Glu-Col. Our nanocomposite maintained an enhanced ability of biomineralization as compared to its pure counterparts. Molecular investigations revealed an elevated level of osteogenic markers as compared to Ge1-Glu-Col and BGnW. All in all, Gel-Glu-Col/BGnW seems to be a potential candidate for the regeneration of bone tissue.

Ad-Dressing Stem Cells: Hydrogels for Encapsulation

Regenerative medicine is a novel scientific field that employs the use of stem cells as cell-based therapy for the regeneration and functional restoration of damaged tissues and organs. Stem cells bear characteristics such as the capacity for self-renewal and differentiation towards specific lineages and, therefore, serve as a backup reservoir in case of tissue injuries. Therapeutically, they can be autologously or allogeneically transplanted for tissue regeneration; however, allogeneic stem cell transplantation can provoke host immune responses leading to a host-versus-transplant reaction. A probable solution to this problem is stem cell encapsulation, a technique that utilizes various biomaterials for the creation of a semi-permeable membrane that encases the stem cells. Stem cell encapsulation can be accomplished by employing a great variety of natural and/or synthetic hydrogels and offers many benefits in regenerative medicine, including protection from the host’s immune system and mechanical stress, improved cell viability, proliferation and differentiation, cryopreservation and controlled and continuous delivery of the stem-cell-secreted therapeutic agents. Here, in this review, we report and discuss almost all natural and synthetic hydrogels used in stem cell encapsulation, along with the benefits that these materials, alone or in combination, could offer to cell therapy through functional cell encapsulation.

A New Polycaprolactone-Based Biomembrane Functionalized with BMP-2 and Stem Cells Improves Maxillary Bone Regeneration

Oral diseases have an impact on the general condition and quality of life of patients. After a dento-alveolar trauma, a tooth extraction, or, in the case of some genetic skeletal diseases, a maxillary bone defect, can be observed, leading to the impossibility of placing a dental implant for the restoration of masticatory function. Recently, bone neoformation was demonstrated after in vivo implantation of polycaprolactone (PCL) biomembranes functionalized with bone morphogenic protein 2 (BMP-2) and ibuprofen in a mouse maxillary bone lesion. In the present study, human bone marrow derived mesenchymal stem cells (hBM-MSCs) were added on BMP-2 functionalized PCL biomembranes and implanted in a maxillary bone lesion. Viability of hBM-MSCs on the biomembranes has been observed using the "LIVE/DEAD" viability test and scanning electron microscopy (SEM). Maxillary bone regeneration was observed for periods ranging from 90 to 150 days after implantation. Various imaging methods (histology, micro-CT) have demonstrated bone remodeling and filling of the lesion by neoformed bone tissue. The presence of mesenchymal stem cells and BMP-2 allows the acceleration of the bone remodeling process. These results are encouraging for the effectiveness and the clinical use of this new technology combining growth factors and mesenchymal stem cells derived from bone marrow in a bioresorbable membrane.

Photo-crosslinked alginate nano-hydroxyapatite paste for bone tissue engineering

In this study, methacrylation of alginate was carried out by reacting sodium alginate with methacrylic anhydride in the presence of sodium hydroxide. Separately synthesized nano-hydroxyapatite (nano-HAp) powder was surface functionalized using mercaptopropionic acid and ethylene glycol methacrylate phosphate (EGMP) in the presence of azobisisobutyronitrile benzene as a free radical initiator in a nitrogen atmosphere. Methacrylated alginate solution was mixed with the required amount of surface-functionalized HAp nanoparticles in the presence of 0.05% Irgacure 2959 as a photoinitiator and was placed at the centre of a 8 kW UV light source (265 nm) to prepare photo-crosslinked bone paste. X-ray diffraction analysis indicated that surface functionalization did not alter phase purity of HAp nanopowder in the prepared paste. The graft polymerization of EGMP on the surface of HAp was confirmed by the presence of the 1732 cm^-1 band, which belongs to C=-O stretching of EGMP, in addition to the characteristic peaks of nano-HAp and alginate in the composite paste. The storage and loss moduli of all the prepared pastes increased non-linearly with time up to 100 s, demonstrating their pseudo plastic behaviour. The rate of release of bone morphogenetic protein 2 (BMP-2) was significantly faster in the first few days, and the release curve gradually levelled off prior to slowing down up to 22 d. Mesenchymal stem cell adhesion studies revealed that cells could attach to the paste material and stretch over the surface of the material after 14 d of incubation. MTT assay showed that prepared paste materials were conducive to attachment and proliferation of mesenchymal stem cells. Immunocytochemical analysis revealed that the addition of surface-functionalized nano-HAp and BMP-2 to alginate hydrogel enhanced the osteogenic potential of the prepared paste. The results indicate that the newly developed photo-crosslinked paste may be physically and biologically suitable for application as a bone filler.

Optimizing an Injectable Composite Oxygen-Generating System for Relieving Tissue Hypoxia

Oxygen deficiency resulting from bone fracture-induced vascular disruption leads to massive cell death and delayed osteoblast differentiation, ultimately impairing new bone formation and fracture healing. Enhancing local tissue oxygenation can help promote bone regeneration. In this work, an injectable composite oxygen-generating system consisting of calcium peroxide (CaO₂)/manganese dioxide (MnO₂)-encapsulated poly lactic-co-glycolic acid (PLGA) microparticles (CaO₂ + MnO₂@PLGA MPs) is proposed for the local delivery of oxygen. By utilizing a series of methodologies, the impacts of each component used for MP fabrication on the oxygen release behavior and cytotoxicity of the CaO₂ + MnO₂@ PLGA MPs are thoroughly investigated. Our analytical data obtained from in vitro studies indicate that the optimized CaO₂ + MnO₂@PLGA MPs developed in this study can effectively relieve the hypoxia of preosteoblast MC3T3-E1 cells that are grown under low oxygen tension and promote their osteogenic differentiation, thus holding great promise in enhancing fractural healing by increasing tissue oxygenation.

The Delivery of RNA-Interference Therapies Based on Engineered Hydrogels for Bone Tissue Regeneration

RNA interference (RNAi) is an efficient post-transcriptional gene modulation strategy mediated by small interfering RNAs (siRNAs) and microRNAs (miRNAs). Since its discovery, RNAi has been utilized extensively to diagnose and treat diseases at both the cellular and molecular levels. However, the application of RNAi therapies in bone regeneration has not progressed to clinical trials. One of the major challenges for RNAi therapies is the lack of efficient and safe delivery vehicles that can actualize sustained release of RNA molecules at the target bone defect site and in surrounding cells. One promising approach to achieve these requirements is encapsulating RNAi molecules into hydrogels for delivery, which enables the nucleic acids to be delivered as RNA conjugates or within nanoparticles. Herein, we reviewed recent investigations into RNAi therapies for bone regeneration where RNA delivery was performed by hydrogels.

Alginate-hydrogel versus alginate-solid system. Efficacy in bone regeneration in osteoporosis

In the present study, two different PLGA-Alginate scaffolds, a hydrogel (HY) and a solid sponge (SS), were developed for β-estradiol and BMP-2 sustained delivery for bone regeneration in osteoporosis. β-Estradiol and BMP-2 were encapsulated in PLGA and PLGA-Alginate microspheres respectively. Scaffolds were characterized in vitro in terms of porosity, water uptake, release rate and HY rheological properties. BMP-2 release profiles were also analysed in vivo. The bone regeneration induced by both HY and SS was evaluated using a critical-sized bone defect in an osteoporotic (OP) rat model. Compared to HY, SS presented 30% higher porosity, more than double water absorption capacity and almost negligible mass loss compared to the 40% of HY. Both systems were flexible and fit well the defect shape, however, HY has the advantage of being injectable. Despite both delivery systems had similar composition and release profile, bone repair was around 30% higher with SS than with HY, possibly due to its longer residence time at the defect site. The incorporation of mesenchymal stem cells obtained from OP rats did not result in any improvement or synergistic effect on bone repair.

Enhanced osseointegration of three-dimensional supramolecular bioactive interface through osteoporotic microenvironment regulation

Purpose: Osteoporosis is more likely to cause serious complications after joint replacement, mainly due to physiological defects of endogenous osteogenic cells and the pathological osteoclast activity. It is a feasible solution to design a prosthetic surface interface that specifically addresses this troublesome situation. Methods: A novel "three-dimensional (3D) inorganic-organic supramolecular bioactive interface" was constructed consisting of stiff 3D printing porous metal scaffold and soft multifunctional, self-healable, injectable, and biodegradable supramolecular polysaccharide hydrogel. Apart from mimicking the bone extracellular matrix, the bioactive interface could also encapsulate bioactive substances, namely bone marrow mesenchymal stem cells (BMSCs) and bone morphogenetic protein-2 (BMP-2). A series of in vitro characterizations, such as topography and mechanical characterization, in vitro release of BMP-2, biocompatibility analysis, and osteogenic induction of BMSCs were carried out. After that, the in vivo osseointegration effect of the bioactive interface was investigated in detail using an osteoporotic model. Results: The administration of injectable supramolecular hydrogel into the inner pores of 3D printing porous metal scaffold could obviously change the morphology of BMSCs and facilitate its cell proliferation. Meanwhile, BMP-2 was capable of being sustained released from supramolecular hydrogel, and subsequently induced osteogenic differentiation of BMSCs and promoted the integration of the metal microspores-bone interface in vitro and in vivo. Moreover, the osteoporosis condition of bone around the bioactive interface was significantly ameliorated. Conclusion: This study demonstrates that the 3D inorganic-organic supramolecular bioactive interface can serve as a novel artificial prosthesis interface for various osteogenesis-deficient patients, such as osteoporosis and rheumatoid arthritis.

Effects of Physical, Chemical, and Biological Stimulus on h-MSC Expansion and Their Functional Characteristics

Human adult mesenchymal stem or stromal cells (h-MSC) therapy has gained considerable attention due to the potential to treat or cure diseases given their immunosuppressive properties and tissue regeneration capabilities. Researchers have explored diverse strategies to promote high h-MSC production without losing functional characteristics or properties. Physical stimulus including stiffness, geometry, and topography, chemical stimulus, like varying the surface chemistry, and biochemical stimuli such as cytokines, hormones, small molecules, and herbal extracts have been studied but have yet to be translated to industrial manufacturing practice. In this review, we describe the role of those stimuli on h-MSC manufacturing, and how these stimuli positively promote h-MSC properties, impacting the cell manufacturing field for cell-based therapies. In addition, we discuss other process considerations such as bioreactor design, good manufacturing practice, and the importance of the cell donor and ethics factors for manufacturing potent h-MSC.

The combination of multi-functional ingredients-loaded hydrogels and three-dimensional printed porous titanium alloys for infective bone defect treatment

Biomaterial with the dual-functions of bone regeneration and antibacterial is a novel therapy for infective bone defects. Three-dimensional (3D)-printed porous titanium (pTi) benefits bone ingrowth, but its microporous structure conducive to bacteria reproduction. Herein, a multifunctional hydrogel was prepared from dynamic supramolecular assembly of sodium tetraborate (Na2B4O7), polyvinyl alcohol (PVA), silver nanoparticles (AgNPs) and tetraethyl orthosilicate (TEOS), and composited with pTi as an implant system. The pTi scaffolds have ideal pore size and porosity matching with bone, while the supramolecular hydrogel endows pTi scaffolds with antibacterial and biological activity. In vitro assessments indicated the 3D composite implant was biocompatible, promoted bone marrow mesenchymal stem cells (BMSCs) proliferation and osteogenic differentiation, and inhibited bacteria, simultaneously. In vivo experiments further demonstrated that the implant showed effective antibacterial ability while promoting bone regeneration. Besides distal femur defect, the innovative scaffolds may also serve as an ideal biomaterial (e.g. dental implants) for other contaminated defects.

Potential of electrospun cationic BSA fibers to guide osteogenic MSC differentiation via surface charge and fibrous topography

Large or complex bone fractures often need clinical treatments for sufficient bone repair. New treatment strategies have pursued the idea of using mesenchymal stromal cells (MSCs) in combination with osteoinductive materials to guide differentiation of MSCs into bone cells ensuring complete bone regeneration. To overcome the challenge of developing such materials, fundamental studies are needed to analyze and understand the MSC behavior on modified surfaces of applicable materials for bone healing. For this purpose, we developed a fibrous scaffold resembling the bone/bone marrow extracellular matrix (ECM) based on protein without addition of synthetic polymers. With this biomimetic in vitro model we identified the fibrous structure as well as the charge of the material to be responsible for its effects on MSC differentiation. Positive charge was introduced via cationization that additionally supported the stability of the scaffold in cell culture, and acted as nucleation point for mineralization during osteogenesis. Furthermore, we revealed enhanced focal adhesion formation and osteogenic differentiation of MSCs cultured on positively charged protein fibers. This pure protein-based and chemically modifiable, fibrous ECM model allows the investigation of MSC behavior on biomimetic materials to unfold new vistas how to direct cells' differentiation for the development of new bone regenerating strategies.

Cell sheets of co-cultured BMP-2-modified bone marrow stromal cells and endothelial progenitor cells accelerate bone regeneration in vitro

Bone tissue engineering provides a substitute for bone transplantation to address various bone defects. However, bone regeneration involves a large number of cellular events. In addition, obtaining sufficient source material for autogenous bone or alloplastic bone substitutes remains an unsolved issue. In previous studies, it was confirmed that bone marrow stromal cells (BMSCs) and endothelial progenitor cells (EPCs) had the capacity to promote bone regeneration. Additionally, bone morphogenetic protein-2 (BMP-2) has been demonstrated to be an active inducer of osteoblast differentiation. Therefore, the aim of the present study was to produce an effective integration system, including a scaffold, reparative cells and growth factors, that may enhance bone regeneration. Firstly, bone marrow-derived BMSCs and EPCs were isolated and identified by flow cytometry. Cell proliferation ability, secreted BMP-2 levels and alkaline phosphatase (ALP) activity were highest in the cell sheets containing BMP-2-modified BMSCs and EPCs. In addition, the expression levels of osteogenesis-associated genes, including runt related transcription factor 2 (Runx2), distal-less homeobox 5 (Dlx5), ALP and integrin-binding sialoprotein (Ibsp), and osteogenesis-associated proteins, including Runx2, Dlx, ALP, Ibsp, vascular endothelial growth factor, osteonectin, osteopontin and type I collagen, gradually increased during the co-culture of ad-BMP-2-BMSCs/EPCs. The levels of these genes and proteins were increased compared with those observed in the BMSC, EPC and BMP-2-modified BMSC groups. Finally, scanning electron microscopy observation also demonstrated that the BMP2-modified BMSCs were able to combine well with EPCs to construct a cell sheet for bone formation. Collectively, these results describe an adenovirus (ad)-BMP2-BMSCs/EPCs co-culture system that may significantly accelerate bone regeneration compared with a BMSCs/EPCs co-culture system or ad-BMP2-BMSCs alone.

In Situ Gelling Hydrogel with Anti-Bacterial Activity and Bone Healing Property for Treatment of Osteomyelitis

Background

Despite the development of progressive surgical techniques and antibiotics, osteomyelitis is a big challenge for orthopedic surgeons. The main aim of this study is to fabricate an in situ gelling hydrogel that permits sustained release of antibiotic (for control of infection) and growth factor (for induction of new bone formation) for effective treatment of osteomyelitis.

Methods

An in situ gelling alginate (ALG)/hyaluronic acid (HA) hydrogel containing vancomycin (antibiotic) and bone morphogenetic protein-2 (BMP-2; growth factor) was prepared by simple mixing of ALG/HA/Na₂HPO₄ solution and CaSO₄/vancomycin/BMP-2 solution. The release behaviors of vancomycin and BMP-2, anti-bacterial effect (in vitro); and therapeutic efficiency for osteomyelitis and bone regeneration (in vivo, osteomyelitis rat model) of the vancomycin and BMP-2-incorporated ALG/HA hydrogel were investigated.

Results

The gelation time of the ALG/HA hydrogel was controlled into approximately 4 min, which is sufficient time for handling and injection into osteomyelitis lesion. Both vancomycin and BMP-2 were continuously released from the hydrogel for 6 weeks. From the in vitro studies, the ALG/HA hydrogel showed an effective anti-bacterial activity without significant cytotoxicity for 6 weeks. From an in vivo animal study using Sprague-Dawley rats with osteomyelitis in femur as a model animal, it was demonstrated that the ALG/HA hydrogel was effective for suppressing bacteria (Staphylococcus aureus) proliferation at the osteomyelitis lesion and enhancing bone regeneration without additional bone grafts.

Conclusions

From the results, we suggest that the in situ gelling ALG/HA hydrogel containing vancomycin and BMP-2 can be a feasible therapeutic tool to treat osteomyelitis.

Conditioning of myoblast secretome using mesenchymal stem/stromal cell spheroids improves bone repair

Local muscle loss associated with open fractures remains an obstacle to functional recovery and bone healing. Muscle cells secrete bioactive myokines that elicit autocrine and paracrine effects and initiate signaling pathways for regenerating damaged muscle and bone. Mesenchymal stem/stromal cells (MSCs) are under investigation for the regeneration of both muscle and bone through their potent secretome. Compared to monodisperse cells, MSC spheroids exhibit a more complex secretome with heightened therapeutic potential. We hypothesized that the osteogenic potential of myokines would be enhanced when myoblasts were exposed to the MSC spheroid secretome. Conditioned media from MSC spheroids increased osteogenic response of MC3T3 pre-osteoblasts compared to myokines from L6 myoblasts alone. This effect was synergistically enhanced when conditioned media of MSC spheroids was serially delivered to myoblasts and then osteoprogenitor cells in vitro. We then delivered myoblast-stimulated conditioned media in the presence or absence of syngeneic rat bone marrow stromal cells (rBMSCs) from alginate hydrogels to a rat critical-sized segmental defect. We observed increased bone formation in defects treated with conditioned media compared to rBMSCs alone, while bone formation was greatest in defects treated with both conditioned media and rBMSCs over 12 weeks. This foundational study demonstrates a novel approach for capitalizing on the paracrine signaling of muscle cells to promote bone repair and provides additional evidence of the synergistic interaction between muscle and bone.

Is Tissue Engineering Helping Orthopaedic Care in Trauma?

Tissue engineering in orthopaedic trauma is needed. Progress has been made in all areas including regenerating bone, cartilage, soft tissue, and making up for bone defects with scaffolds. Bone regeneration and managing bone defects with scaffolds continue to be successful in the basic science realm with promising results, but currently, these successes are mostly limited to small animal models. Cartilage defects have more clinically available treatment options, but the benefits of "off-the-shelf" allograft options, and scaffolds, have little clinical evidence in the acute fracture setting. Most of the true chondrocyte replacement therapies such as matrix-induced autologous chondrocyte implantation and osteochondral allografts require delayed treatment while cell growth or graft matching occurs. Soft-tissue defects can be managed with tissue engineering for the skin with success, but muscle and nerve defects are still limited to the basic science arena. Although significant gains have been made in all areas for tissue engineering in basic science, and is very promising, this success currently comes with limited translation into clinical availability for the orthopaedic trauma patient.

Dynamic expression of SMAD3 is critical in osteoblast differentiation of PDMCs

Human pluripotent stem cells have the potential assist in the identification of genes involved in mammalian development. The human placenta is considered a repository of stem cells, termed placenta‑derived multipotent cells (PDMCs), which are able to differentiate into cells with an osteoblastic phenotype. This plasticity of PDMCs maybe applied clinically to the understanding of osteogenesis and osteoporosis. In the presentstudy, osteoblasts were generated by culturing PDMCs in osteogenic medium. Reverse transcription quantitative polymerase chain reactionand the degree of osteoblast calcification were used to evaluate the efficacy of osteogenesis. The results suggestedthat the expression of mothers against decapentaplegic homolog 3 (SMAD3) increased in the initial stages of osteogenic differentiation but decreased in the later stages. However, osteogenesis was inhibitedwhen the PDMCs overexpressed SMAD3 throughout the differentiation period. In addition, the rate of osteogenic differentiation was decreased when SMAD3 signaling was impaired. In conclusion, SMAD3 serves an important role in osteoblast differentiation and bone formation in a time‑dependent manner. The data from the present study indicate that arapid increase in SMAD3 expression is crucial for osteogenesis and suggest a role for PDMCs in the treatment of patients with osteoporosis.

Functional Hydrogels With Tunable Structures and Properties for Tissue Engineering Applications

Tissue engineering (TE) has been used as an attractive and efficient process to restore the original tissue structures and functions through the combination of biodegradable scaffolds, seeded cells, and biological factors. As a unique type of scaffolds, hydrogels have been frequently used for TE because of their similar 3D structures to the native extracellular matrix (ECM), as well as their tunable biochemical and biophysical properties to control cell functions such as cell adhesion, migration, proliferation, and differentiation. Various types of hydrogels have been prepared from naturally derived biomaterials, synthetic polymers, or their combination, showing their promise in TE. This review summarizes the very recent progress of hydrogels used for TE applications. The strategies for tuning biophysical and biochemical properties, and structures of hydrogels are first introduced. Their influences on cell functions and promotive effects on tissue regeneration are then highlighted.

Enzymatically degradable alginate hydrogel systems to deliver endothelial progenitor cells for potential revasculature applications

The objective of this study was to design an injectable biomaterial system that becomes porous in situ to deliver and control vascular progenitor cell release. Alginate hydrogels were loaded with outgrowth endothelial cells (OECs) and alginate lyase, an enzyme which cleaves alginate polymer chains. We postulated and confirmed that higher alginate lyase concentrations mediated loss of hydrogel mechanical properties. Hydrogels incorporating 5 and 50 mU/mL of alginate lyase experienced approximately 28% and 57% loss of mass as well as 81% and 91% reduction in storage modulus respectively after a week. Additionally, computational methods and mechanical analysis revealed that hydrogels with alginate lyase significantly increased in mesh size over time. Furthermore, alginate lyase was not found to inhibit OEC proliferation, viability or sprouting potential. Finally, alginate hydrogels incorporating OECs and alginate lyase promoted up to nearly a 10 fold increase in OEC migration in vitro than nondegradable hydrogels over the course of a week and increased functional vasculature in vivo via a chick chorioallantoic membrane (CAM) assay. Overall, these findings demonstrate that alginate lyase incorporated hydrogels can provide a simple and robust system to promote controlled outward cell migration into native tissue for potential therapeutic revascularization applications.

Identification of core genes and prediction of miRNAs associated with osteoporosis using a bioinformatics approach

Osteoporosis (OP) is an age-related disease, and osteoporotic fracture is one of the major causes of disability and mortality in elderly patients (>70 years old). As the pathogenesis and molecular mechanism of OP remain unclear, the identification of disease biomarkers is important for guiding research and providing therapeutic targets. In the present study, core genes and microRNAs (miRNAs) associated with OP were identified. Differentially expressed genes (DEGs) between human mesenchymal stem cell specimens from normal osseous tissues and OP tissues were detected using the GEO2R tool of the Gene Expression Omnibus database and Morpheus. Network topological parameters were determined using NetworkAnalyzer. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed using the Database for Annotation, Visualization and Integrated Discovery, and ClueGO. Cytoscape with the Search Tool for the Retrieval of Interacting Genes and Molecular Complex Detection plug-in was used to visualize protein-protein interactions (PPIs). Additionally, miRNA-gene regulatory modules were predicted using CyTargetLinker in order to guide future research. In total, 915 DEGs were identified, including 774 upregulated and 141 downregulated genes. Enriched GO terms and pathways were determined, including 'nervous system development', 'regulation of molecular function', 'glutamatergic synapse pathway' and 'pathways in cancer'. The node degrees of DEGs followed power-law distributions. A PPI network with 541 nodes and 1,431 edges was obtained. Overall, 3 important modules were identified from the PPI network. The following 10 genes were identified as core genes based on high degrees of connectivity: Albumin, PH domain leucine-rich repeat-containing protein phosphatase 2 (PHLPP2), DNA topoisomerase 2-α, kininogen 1 (KNG1), interleukin 2 (IL2), leucine-rich repeats and guanylate kinase domain containing, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit γ (PIK3CG), leptin, transferrin and RNA polymerase II subunit A (POLR2A). Additionally, 15 miRNA-target interactions were obtained using CyTargetLinker. Overall, 7 miRNAs co-regulated IL2, 3 regulated PHLPP2, 3 regulated KNG1, 1 regulated PIK3CG and 1 modulated POLR2A. These results indicate potential biomarkers in the pathogenesis of OP and therapeutic targets.

Hypoxic Preconditioning of Mesenchymal Stem Cells with Subsequent Spheroid Formation Accelerates Repair of Segmental Bone Defects

Cell-based approaches for musculoskeletal tissue repair are limited by poor cell survival and engraftment. Short-term hypoxic preconditioning of mesenchymal stem cells (MSCs) can prolong cell viability in vivo, while the aggregation of MSCs into spheroids increases cell survival, trophic factor secretion, and tissue formation in vivo. We hypothesized that preconditioning MSCs in hypoxic culture before spheroid formation would increase cell viability, proangiogenic potential, and resultant bone repair compared with that of individual MSCs. Human MSCs were preconditioned in 1% O2 in monolayer culture for 3 days (PC3) or kept in ambient air (PC0), formed into spheroids of increasing cell density, and then entrapped in alginate hydrogels. Hypoxia-preconditioned MSC spheroids were more resistant to apoptosis than ambient air controls and this response correlated with duration of hypoxia exposure. Spheroids of the highest cell density exhibited the greatest osteogenic potential in vitro and vascular endothelial growth factor (VEGF) secretion was greatest in PC3 spheroids. PC3 spheroids were then transplanted into rat critical-sized femoral segmental defects to evaluate their potential for bone healing. Spheroid-containing gels induced significantly more bone healing compared with gels containing preconditioned individual MSCs or acellular gels. These data demonstrate that hypoxic preconditioning represents a simple approach for enhancing the therapeutic potential of MSC spheroids when used for bone healing. Stem Cells 2018;36:1393-1403.

Materials-Directed Differentiation of Mesenchymal Stem Cells for Tissue Engineering and Regeneration

Cell-based therapies are a promising alternative to grafts and organ transplantation for treating tissue loss or damage due to trauma, malfunction, or disease. Over the past two decades, mesenchymal stem cells (MSCs) have attracted much attention as a potential cell population for use in regenerative medicine. While the proliferative capacity and multilineage potential of MSCs provide an opportunity to generate clinically relevant numbers of transplantable cells, their use in tissue regenerative applications has met with relatively limited success to date apart from secreting paracrine-acting factors to modulate the defect microenvironment. Presently, there is significant effort to engineer the biophysical properties of biomaterials to direct MSC differentiation and further expand on the potential of MSCs in tissue engineering, regeneration, and repair. Biomaterials can dictate MSC differentiation by modulating features of the substrate including composition, mechanical properties, porosity, and topography. The purpose of this review is to highlight recent approaches for guiding MSC fate using biomaterials and provide a description of the underlying characteristics that promote differentiation toward a desired phenotype.

Dual non-viral gene delivery from microparticles within 3D high-density stem cell constructs for enhanced bone tissue engineering

High-density mesenchymal stem cell (MSC) aggregates can be guided to form bone-like tissue via endochondral ossification in vitro when culture media is supplemented with proteins, such as growth factors (GFs), to first guide the formation of a cartilage template, followed by culture with hypertrophic factors. Recent reports have recapitulated these results through the controlled spatiotemporal delivery of chondrogenic transforming growth factor-β1 (TGF-β1) and chondrogenic and osteogenic bone morphogenetic protein-2 (BMP-2) from microparticles embedded within human MSC aggregates to avoid diffusion limitations and the lengthy, costly in vitro culture necessary with repeat exogenous supplementation. However, since GFs have limited stability, localized gene delivery is a promising alternative to the use of proteins. Here, mineral-coated hydroxyapatite microparticles (MCM) capable of localized delivery of Lipofectamine-plasmid DNA (pDNA) nanocomplexes encoding for TGF-β1 (pTGF-β1) and BMP-2 (pBMP-2) were incorporated, alone or in combination, within MSC aggregates from three healthy porcine donors to induce sustained production of these transgenes. Three donor populations were investigated in this work due to the noted MSC donor-to-donor variability in differentiation capacity documented in the literature. Delivery of pBMP-2 within Donor 1 aggregates promoted chondrogenesis at week 2, followed by an enhanced osteogenic phenotype at week 4. Donor 2 and 3 aggregates did not promote robust glycosaminoglycan (GAG) production at week 2, but by week 4, Donor 2 aggregates with pTGF-β1/pBMP-2 and Donor 3 aggregates with both unloaded MCM and pBMP-2 enhanced osteogenesis compared to controls. These results demonstrate the ability to promote osteogenesis in stem cell aggregates through controlled, non-viral gene delivery within the cell masses. These findings also indicate the need to screen donor MSC regenerative potential in response to gene transfer prior to clinical application. Taken together, this work demonstrates a promising gene therapy approach to control stem cell fate in biomimetic 3D condensations for treatment of bone defects.

BMP2-modified injectable hydrogel for osteogenic differentiation of human periodontal ligament stem cells

This is the first report on the development of a covalently bone morphogenetic protein-2 (BMP2)-immobilized hydrogel that is suitable for osteogenic differentiation of human periodontal ligament stem cells (hPLSCs). O-propargyl-tyrosine (OpgY) was site-specifically incorporated into BMP2 to prepare BMP2-OpgY with an alkyne group. The engineered BMP2-OpgY exhibited osteogenic characteristics after in vitro osteogenic differentiation of hPLSCs, indicating the osteogenic ability of BMP2-OpgY. A methoxy polyethylene glycol-(polycaprolactone-(N₃)) block copolymer (MC-N₃) was prepared as an injectable in situ-forming hydrogel. BMP2 covalently immobilized on an MC hydrogel (MC-BMP2) was prepared quantitatively by a simple biorthogonal reaction between alkyne groups on BMP2-OpgY and azide groups on MC-N₃ via a Cu(I)-catalyzed click reaction. The hPLSCs-loaded MC-BMP2 formed a hydrogel almost immediately upon injection into animals. In vivo osteogenic differentiation of hPLSCs in the MC-BMP2 formulation was confirmed by histological staining and gene expression analyses. Histological staining of hPLSC-loaded MC-BMP2 implants showed evidence of mineralized calcium deposits, whereas hPLSC-loaded MC-Cl or BMP2-OpgY mixed with MC-Cl, implants showed no mineral deposits. Additionally, MC-BMP2 induced higher levels of osteogenic gene expression in hPLSCs than in other groups. In conclusion, BMP2-OpgY covalently immobilized on MC-BMP2 induced osteogenic differentiation of hPLSCs as a noninvasive method for bone tissue engineering.

Cellularizing hydrogel-based scaffolds to repair bone tissue: How to create a physiologically relevant micro-environment?

Tissue engineering is a promising alternative to autografts or allografts for the regeneration of large bone defects. Cell-free biomaterials with different degrees of sophistication can be used for several therapeutic indications, to stimulate bone repair by the host tissue. However, when osteoprogenitors are not available in the damaged tissue, exogenous cells with an osteoblast differentiation potential must be provided. These cells should have the capacity to colonize the defect and to participate in the building of new bone tissue. To achieve this goal, cells must survive, remain in the defect site, eventually proliferate, and differentiate into mature osteoblasts. A critical issue for these engrafted cells is to be fed by oxygen and nutrients: the transient absence of a vascular network upon implantation is a major challenge for cells to survive in the site of implantation, and different strategies can be followed to promote cell survival under poor oxygen and nutrient supply and to promote rapid vascularization of the defect area. These strategies involve the use of scaffolds designed to create the appropriate micro-environment for cells to survive, proliferate, and differentiate in vitro and in vivo. Hydrogels are an eclectic class of materials that can be easily cellularized and provide effective, minimally invasive approaches to fill bone defects and favor bone tissue regeneration. Furthermore, by playing on their composition and processing, it is possible to obtain biocompatible systems with adequate chemical, biological, and mechanical properties. However, only a good combination of scaffold and cells, possibly with the aid of incorporated growth factors, can lead to successful results in bone regeneration. This review presents the strategies used to design cellularized hydrogel-based systems for bone regeneration, identifying the key parameters of the many different micro-environments created within hydrogels.