Enzymatic Crosslinking and Degradation of Gelatin as a Switch for Bone Morphogenetic Protein-2 Activity

Precisely Localized Bone Regeneration Mediated by Marine-Derived Microdroplets with Superior BMP-2 Binding Affinity

Prompt and robust bone regeneration has been clinically achieved using supraphysiological doses of bone morphogenetic protein-2 (BMP-2) to overcome the short half-life and rapid clearance. However, uncontrolled burst release of exogenous BMP-2 causes severe complications such as heterotopic ossification and soft tissue inflammation. Therefore, numerous researches have focused on developing a new BMP-2 delivery system for a sustained release profile by immobilizing BMP-2 in various polymeric vehicles. Herein, to avoid denaturation of BMP-2 and enhance therapeutic action via localized delivery, a complex coacervate consisting of fucoidan, a marine-derived glycosaminoglycan, and poly-l-lysine (PLL) is fabricated. Superior BMP-2 binding ability and electrostatic interaction-driven engulfment enable facile and highly efficient microencapsulation of BMP-2. The microencapsulation ability of the coacervate significantly improves BMP-2 bioactivity and provides protection against antagonist and proteolysis, while allowing prolonged release. Moreover, BMP-2 containing coacervate is coated on conventional collagen sponges. The bioactivity and localized bone regenerating ability are confirmed through in vitro (human-derived stem cells), and in vivo (calvarial bone defect model) evaluations.

Implantable Biomaterials for Peripheral Nerve Regeneration–Technology Trends and Translational Tribulations

The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon’s perspective and give an outlook toward future challenges in the field.

Influence of Cellular Microenvironment on Human Articular Chondrocyte Cell Signaling

OBJECTIVE

Alteration of the cellular microenvironment may influence the intra- and intercellular communication and contribute to cartilage injury and repair. The purpose of this study was to investigate how matrix elasticity/stiffness affects chondrogenic activities, including cell survival, phenotypic expression, and the release of both pro- and anti-inflammatory cytokines.

DESIGN

Human articular chondrocytes (HACs) cultured on traditional 2-dimensional (2D) plastic surfaces were compared with those cultured within 3D hydrogel matrices of varying stiffness. Chondrogenic proliferation, differentiation, and the expression of pro- and anti-inflammatory cytokines were evaluated. Both interleukin-1-beta (IL-1β) and human synovial fluid-derived cells (hSFCs) were introduced to study the effects of matrix stiffness on chondrocyte response.

RESULTS

Cells demonstrated the most robust chondrogenic differentiation and secreted the least pro-inflammatory cytokines when the matrix stiffness was close to their native microenvironment. The IL-1β effects were attenuated when HACs were co-cultured with hSFCs.

CONCLUSION

Modifying the matrix stiffness to mimic the native cartilage microenvironment not only optimized chondrogenic expression but also was essential for the regulation of physiological homeostasis. This study proposed a new toolkit to study cell-molecule, cell-cell, and cell-matrix influence on cartilage physiology.

High ligand density drives extensive spreading and motility on soft GelMA gels

In comparison to synthetic hydrogels where ligand density and stiffness can be independently tuned, cell responses are expected to deviate on native biopolymer networks where ligand density and stiffness are coupled. Here we probe the tensional homeostasis of fibroblasts on methacrylated gelatin (GelMA) gels, which are widely used in tissue engineering applications. On 5%-15% GelMA gels which are very soft (10-100's of Pa's in stiffness), fibroblasts were found to spread extensively and assemble prominent stress fibers and focal adhesions. Probing of contractile mechanics using trypsin-induced detachment revealed adhesive drag, but not contractility, was sensitive to GelMA concentration. Contractility-altering drugs blebbistatin and nocodazole, which exhibited opposite effects on focal adhesion size, both led to reduction in adhesive drag and cell rounding. However, cell motility was impacted only in nocodazole-treated cells. Collectively, our experiments suggest that on soft GelMA gels, contractility-independent adhesion clustering mediated by high ligand density can drive cell spreading and motility.

Robust phenotypic maintenance of limb cells during heterogeneous culture in a physiologically relevant polymeric-based constructed graft system

A major challenge during the simultaneous regeneration of multiple tissues is the ability to maintain the phenotypic characteristics of distinct cell populations on one construct, especially in the presence of different exogenous soluble cues such as growth factors. Therefore, in this study, we questioned whether phenotypic maintenance over a distinct population of cells can be achieved by providing biomimetic structural cues relevant to each cell phenotype into the construct's design and controlling the presentation of growth factors in a region-specific manner. To address this question, we developed a polymeric-based constructed graft system (CGS) as a physiologically relevant model that consists of three combined regions with distinct microstructures and growth factor types. Regions A and B of the CGS exhibited similar microstructures to the skin and soft tissues and contained rhPDGF-BB and rhIGF-I, while region C exhibited a similar microstructure to the bone tissue and contained rhBMP-2. Primary rat skin fibroblasts, soft tissue fibroblasts, and osteoblasts were then cultured on regions A, B, and C of the CGS, respectively and their phenotypic characteristics were evaluated in this heterogenous environment. In the absence of growth factors, we found that the structural cues presented in every region played a key role in maintaining the region-specific cell functions and heterogeneity during a heterogeneous culture. In the presence of growth factors, we found that spatially localizing the growth factors at their respective regions resulted in enhanced region-specific cell functions and maintained region-specific cell heterogeneity compared to supplementation, which resulted in a significant reduction of cell growth and loss of phenotype. Our data suggest that providing biomimetic structural cues relevant to each cell phenotype and controlling the presentation of growth factors play a crucial role in ensuring heterogeneity maintenance of distinct cell populations during a heterogeneous culture. The presented CGS herein provides a reliable platform for investigating different cells responses to heterogeneous culture in a physiologically relevant microenvironment. In addition, the model provides a unique platform for evaluating the feasibility and efficacy of different approaches for simultaneously delivering multiple growth factors or molecules from a single construct to achieve enhanced cell response while maintaining cellular heterogeneity during a heterogenous culture.

Repair of Rotator Cuff Tendon Defects in Aged Rats Using a Growth Factor Injectable Gel Scaffold

PURPOSE

To determine if the tendon-specific crosslinking gelatin (Col-Tgel) impregnated with growth factors promotes tendon healing at the bone interface and in a tendon window model.

METHODS

Two different Col-Tgel formulations were first tested in vitro by evaluating cell morphology and tendogenic differentiation. After the optimum formulation was determined, the gel was mixed with either transforming growth factor-β3 (TGF-β3) or growth differentiation factor-7 (GDF-7) growth factor and prepared for injections. Window defects were induced in 12 animals, which were randomized into the following treatments: (1) sham, (2) empty Col-Tgel, (3) Col-Tgel containing TGF-β3, or (4) Col-Tgel containing GDF-7. Based on these results, the sham, empty Col-Tgel, and Col-Tgel containing TGF-β3 were applied to the supraspinatus repair interface. Tendons were analyzed biomechanically and histologically using hematoxylin and eosin and Masson's trichrome staining.

RESULTS

In the window defect model, histologic scores were the best in rats treated with TGF-β3 containing Col-Tgel, followed by the empty Col-Tgel scaffold, and finally the sham control. The GDF-7 Col-Tgel was not further tested because occasional ectopic cartilage and bone formation was found in the prior window defect model. In the supraspinatus repair model, there was no statistical difference (P > .05) in the biomechanical strength among the 3 treatment groups, but load-to-failure ratio improved when TGF-β3 was added to the scaffold, suggesting improved tendon healing.

CONCLUSIONS

This pilot study evaluated the performance of an injectable gel tendon graft in a population of retired breeder rats. The results suggest that Col-Tgel containing TGF-β3 may be a useful adjunctive treatment for surgical repair of full-thickness rotator cuff tears. Histologic and biomechanical scores suggest that Col-Tgel containing TGF-β3 promotes tendon healing.

CLINICAL RELEVANCE

The results of this study suggest that shoulders injected with Col-Tgel may be a useful adjunctive treatment for repair of rotator cuff tears.

Building capacity for macrophage modulation and stem cell recruitment in high-stiffness hydrogels for complex periodontal regeneration: Experimental studies in vitro and in rats

Recently, we found that although high-stiffness matrices stimulated osteogenic differentiation of bone marrow-derived stromal cells (BMSCs), the macrophages (Mφs) in high-stiffness transglutaminase crosslinked gelatins (TG-gels) tended to undergo M1 polarization and hence compromised cell osteogenesis. In this study, we hypothesized that the copresentation of interleukin (IL)-4 and stromal cell-derived factor (SDF)-1α in high-stiffness TG-gels may enhance periodontal regeneration by modulating Mφ polarization and promoting endogenous stem cell recruitment. We found that Mφs were more likely to polarize toward an immunomodulatory M2 state in the presence of IL-4 and hence positively influence the osteogenic differentiation of BMSCs when these cells coexisted in either indirect or direct co-culture systems. In cell migration assays, BMSCs exhibited an enhanced capability to move toward gels containing SDF-1α, and more cells could be recruited into the three-dimensional matrix of TG-gels. When TG-gels containing IL-4 and/or SDF-1α were used to repair periodontal defects, more new bone (MicroCT) was formed in animals that received the dual cytokine-loaded transplants at 4 weeks postsurgery. Mφs were recruited to all the transplanted gels, and after one week, more M1-phenotype cells were found in the groups without IL-4, while the presence of IL-4 was more likely to result in M2 polarization (immunofluorescence staining). When the tissue biopsies were histologically examined, the TG-gels containing both IL-4 and SDF-1α led to a generally satisfactory regeneration with respect to attachment recovery (epithelial and connective tissue) and hybrid tissue regeneration (bone, periodontal ligament and cementum). Our data suggest that the incorporation of IL-4 into high-stiffness TG-gels may promote the M2 polarization of Mφs and that SDF-1α can be applied to guide endogenous cell homing. Overall, building capacity for Mφ modulation and cell recruitment in high-stiffness hydrogels represents a simple and effective strategy that can support high levels of periodontal tissue regeneration. STATEMENT OF SIGNIFICANCE: The development of hydrogel-based regenerative therapies centered on the mobilization and stimulation of native cells for therapeutics opens a window toward realizing periodontal endogenous regeneration. In the present study, the parallel use of immunomodulatory and homing factors in high-stiffness hydrogel materials is shown to induce stem cell homing, modulate cell differentiation and indeed induce regrowth of the periodontium. We found that incorporation of interleukin (IL)-4 in high-stiffness TG-gels coaxed macrophages to polarize into M2 phenotypes, and stromal cell-derived factor (SDF)-1α could be applied to direct endogenous cell homing. Hence, we present for the first time a clinically relevant strategy based on macrophage modulation and host cell recruitment that can support high levels of periodontal tissue regeneration.

Macrophage involvement affects matrix stiffness-related influences on cell osteogenesis under three-dimensional culture conditions

Accumulating evidence indicates that the physicochemical properties of biomaterials exert profound influences on stem cell fate decisions. However, matrix-based regulation selected through in vitro analyses based on a given cell population do not genuinely reflect the in vivo conditions, in which multiple cell types are involved and interact dynamically. This study constitutes the first investigation of how macrophages (Mφs) in stiffness-tunable transglutaminase cross-linked gelatin (TG-gel) affect the osteogenesis of bone marrow-derived mesenchymal stem cells (BMMSCs). When a single cell type was cultured, low-stiffness TG-gels promoted BMMSC proliferation, whereas high-stiffness TG-gels supported cell osteogenic differentiation. However, Mφs in high-stiffness TG-gels were more likely to polarize toward the pro-inflammatory M1 phenotype. Using either conditioned medium (CM)-based incubation or Transwell-based co-culture, we found that Mφs encapsulated in the low-stiffness matrix exerted a positive effect on the osteogenesis of co-cultured BMMSCs. Conversely, Mφs in high-stiffness TG-gels negatively affected cell osteogenic differentiation. When both cell types were cultured in the same TG-gel type and placed into the Transwell system, the stiffness-related influences of Mφs on BMMSCs were significantly altered; both the low- and high-stiffness matrix induced similar levels of BMMSC osteogenesis. Although the best material parameter for synergistically affecting Mφs and BMMSCs remains unknown, our data suggest that Mφ involvement in the co-culture system alters previously identified material-related influences on BMMSCs, such as matrix stiffness-related effects, which were identified based on a culture system involving a single cell type. Such Mφ-stem cell interactions should be considered when establishing proper matrix parameter-associated cell regulation in the development of biomimetic biomaterials for regenerative applications.

STATEMENT OF SIGNIFICANCE

The substrate stiffness of a scaffold plays critical roles in modulating both reparative cells, such as mesenchymal stem cells (MSCs), and immune cells, such as macrophages (Mφs). Although the influences of material stiffness on either Mφs or MSCs, have been extensively described, how the two cell types respond to matrix cues to dynamically affect each other in a three-dimensional (3D) biosystem remains largely unknown. Here, we report our findings that, in a platform wherein Mφs and bone marrow-derived MSCs coexist, matrix stiffness can influence stem cell fate through both direct matrix-associated regulation and indirect Mφ-based modulation. Our data support future studies of the MSC-Mφ-matrix interplay in the 3D context to optimize matrix parameters for the development of the next biomaterial.

A self-organising biomimetic collagen/nano-hydroxyapatite-glycosaminoglycan scaffold for spinal fusion

The use of spinal fusion surgery as a treatment for degenerative spinal conditions and chronic back pain is increasing. However, this technique requires use of a bone grafting material to fuse the vertebrae, traditionally autologous bone, which consists of an optimal combination of osteogenic cell precursors, extracellular matrix proteins and mineral components. To date, this remains the 'gold standard' material but its supply is limited and is associated with a number of clinical and ethical difficulties; consequently, various combinations of cells with biological scaffold materials have been tested but have failed to achieve fusion rates even comparable to autologous bone. We successfully fabricated a novel collagen-based scaffold using self-organising atelocollagen combined with nano-hydroxyapatite and chondroitin sulphate, cross-linked by microbial transglutaminase. The scaffold was characterised using a range of imaging, chemical composition and thermal analysis techniques. It was found to exhibit appropriate stiffness and suitable pore size for the adhesion, growth and differentiation of MSCs. The low toxicity makes it suitable for clinical application, and its slow degradation profile would enable the scaffold to promote bone growth over an extended period. This material therefore shows promise for clinical use in spinal fusion and other procedures requiring the use of bone grafts.

Muscle tissue engineering and regeneration through epigenetic reprogramming and scaffold manipulation

Efficiency of cell-based tissue engineering and regenerative medicine has been limited by inadequate cellular responses to injury because of aging and poor controllability of cellular interactions. Since cell progression is under a tight epigenetic regulation, epigenetic modulators such as 5-azacytidine (5-Aza-CR) have been utilized to facilitate reprogramming and development of somatic cells in 2-dimensional (2-D) settings. Nonetheless, progression of a specific tissue lineage toward the terminal phenotype is dependent not only on the genomic potential, but also on the microenvironment cues that are beyond the capability of 2-D approaches. In this study, we investigated the combined effects of matrices of variable rigidities and the treatment with the epigenetic modulator 5-Aza-CR on reprogramming adipose-derived stromal cells (ADSCs) into myoblast-like cells by utilizing tunable transglutaminase cross-linked gelatin (Col-Tgel) in vitro and in vivo. Our experiments demonstrated that cellular plasticity and trans-differentiation were significantly enhanced when ADSCs were treated with an effective dose of 5-Aza-CR (1.25 to 12.5 ng) in the optimal myogenic matrix (15 ± 5 kPa Col-Tgel). Our findings suggest that both physical signals and chemical milieu are critical for the regulation of cellular responses.

Tumor bioengineering using a transglutaminase crosslinked hydrogel

Development of a physiologically relevant 3D model system for cancer research and drug development is a current challenge. We have adopted a 3D culture system based on a transglutaminase-crosslinked gelatin gel (Col-Tgel) to mimic the tumor 3D microenvironment. The system has several unique advantages over other alternatives including presenting cell-matrix interaction sites from collagen-derived peptides, geometry-initiated multicellular tumor spheroids, and metabolic gradients in the tumor microenvironment. Also it provides a controllable wide spectrum of gel stiffness for mechanical signals, and technical compatibility with imaging based screening due to its transparent properties. In addition, the Col-Tgel provides a cure-in-situ delivery vehicle for tumor xenograft formation in animals enhancing tumor cell uptake rate. Overall, this distinctive 3D system could offer a platform to more accurately mimic in vivo situations to study tumor formation and progression both in vitro and in vivo.

Sustained delivery of biomolecules from gelatin carriers for applications in bone regeneration

Local delivery of therapeutic biomolecules to stimulate bone regeneration has matured considerably during the past decades, but control over the release of these biomolecules still remains a major challenge. To this end, suitable carriers that allow for tunable spatial and temporal delivery of biomolecules need to be developed. Gelatin is one of the most widely used natural polymers for the controlled and sustained delivery of biomolecules because of its biodegradability, biocompatibility, biosafety and cost–effectiveness. The current study reviews the applications of gelatin as carriers in form of bulk hydrogels, microspheres, nanospheres, colloidal gels and composites for the programmed delivery of commonly used biomolecules for applications in bone regeneration with a specific focus on the relationship between carrier properties and delivery characteristics.

The synergetic effect of hydrogel stiffness and growth factor on osteogenic differentiation

Cells respond to various chemical signals as well as environmental aspects of the extracellular matrix (ECM) that may alter cellular structures and functions. Hence, better understanding of the mechanical stimuli of the matrix is essential for creating an adjuvant material that mimics the physiological environment to support cell growth and differentiation, and control the release of the growth factor. In this study, we utilized the property of transglutaminase cross-linked gelatin (TG-Gel), where modification of the mechanical properties of TG-Gel can be easily achieved by tuning the concentration of gelatin. Modifying one or more of the material parameters will result in changes of the cellular responses, including different phenotype-specific gene expressions and functional differentiations. In this study, stiffer TG-Gels itself facilitated focal contact formation and osteogenic differentiation while soft TG-Gel promoted cell proliferation. We also evaluated the interactions between a stimulating factor (i.e. BMP-2) and matrix rigidity on osteogenesis both in vitro and in vivo. The results presented in this study suggest that the interactions of chemical and physical factors in ECM scaffolds may work synergistically to enhance bone regeneration.

Injectable gel graft for bone defect repair

Aim: To examine the performance of an injectable gel graft made of transglutaminase (Tg)-crosslinked gelatin gel with BMP-2 (BMP-2–Tg–Gel) for bone defect repair in animal models. Materials & methods: BMP-2 mixed with gelatin gel was crosslinked using Tg. The release of tethered BMP-2 through autocrine and paracrine pathways was demonstrated by using C2C12 and NIH 3T3 cells, respectively. BMP-2–Tg–Gel was injected into the induced cranial defect site. After 14 days, the sample was removed for x-ray imaging and histological evaluation. Results: Our in vivo results demonstrated that the injectable Tg–Gel with its osteoconductivity and controllable BMP-2 activity induced bone formation in our rat models when tethered with BMP-2. Conclusion: Tg–Gel as an injectable functional bone graft may enable the use of minimally invasive surgical procedures to treat irregular-shaped bone defects. Furthermore, this novel approach is capable of incorporating and controlling the release of therapeutic agents that may advance the science of tissue regeneration.

Influence of solvent composition on the performance of carbodiimide cross-linked gelatin carriers for retinal sheet delivery

Gelatin is a protein molecule that displays bioaffinity and provides a template to guide retinal pigment epithelial (RPE) cell organization and growth. We have recently demonstrated that the carbodiimide cross-linked gelatin membranes can be used as retinal sheet carriers. The purpose of this work was to further determine the role of solvent composition in the tissue delivery performance of chemically modified biopolymer matrices. The gelatin molecules were treated with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) in the presence of binary ethanol/water mixtures with varying ethanol concentrations (70-95 vol%) to obtain the carriers with different cross-linking efficiencies and mechanical properties. Results of melting point measurements and in vitro degradation tests showed that when the cross-linking index reached a high level of around 45 %, the EDC cross-linked gelatin materials have sufficient thermal stability and resistance to enzymatic degradation, indicating their suitability for the development of carriers for retinal sheet delivery. Irrespective of the solvent composition, the chemically modified gelatin samples are compatible toward human RPE cells without causing toxicity and inflammation. In particular, the membrane carriers prepared by the cross-linking in the presence of solvent mixtures containing 80-90 vol% of ethanol have no impact on the proliferative capacity of ARPE-19 cultures and possess good efficiency in transferring and encapsulating the retinal tissues. It is concluded that, except for cell viability and pro-inflammatory cytokine expression, the retinal sheet delivery performance strongly depends on the solvent composition for EDC cross-linking of gelatin molecules.

Enhanced osteogenesis of adipose derived stem cells with Noggin suppression and delivery of BMP-2

Bone morphogenetic proteins (BMPs) are believed to be the most potent osteoinductive factors. However, BMPs are highly pleiotropic molecules and their supra-physiological high dose requirement leads to adverse side effects and inefficient bone formation. Thus, there is a need to develop alternative osteoinductive growth factor strategies that can effectively complement BMP activity. In this study, we intrinsically stimulated BMP signaling in adipose derived stem cells (ASCs) by downregulating noggin, a potent BMP antagonist, using an RNAi strategy. ASCs transduced with noggin shRNA significantly enhanced osteogenic differentiation of cells. The potency of endogenous BMPs was subsequently enhanced by stimulating ASCs with exogenous BMPs at a significantly reduced dose. The level of mineralization in noggin shRNA treated ASCs when treated with BMP-2 was comparable to that of control shRNA treated cell treated with 10-fold more BMP-2. The complementary strategy of noggin suppression + BMP-2 to enhance osteogenesis was further confirmed in 3D in vitro environments using scaffolds consisting of chitosan (CH), chondroitin sulfate (CS), and apatite layer on their surfaces designed to slowly release BMP-2. This finding supports the novel therapeutic potential of this complementary strategy in bone regeneration.