The efficiency of genetically modified mesenchymal stromal cells combined with a functionally graded scaffold for bone regeneration in corticosteroid-induced osteonecrosis of the femoral head in rabbits

Core decompression (CD) with mesenchymal stromal cells (MSCs) is an effective therapy for early-stage osteonecrosis of the femoral head (ONFH). Preconditioning of MSCs, using inflammatory mediators, is widely used in immunology and various cell therapies. We developed a three-dimensional printed functionally graded scaffold (FGS), made of β-TCP and PCL, for cell delivery at a specific location. The present study examined the efficacy of CD treatments with genetically modified (GM) MSCs over-expressing PDGF-BB (PDGF-MSCs) or GM MSCs co-over-expressing IL-4 and PDGF-BB and preconditioned for three days of exposure to lipopolysaccharide and tumor necrosis factor-alpha (IL-4-PDGF-pMSCs) using the FGS for treating steroid-induced ONFH in rabbits. We compared CD without cell-therapy, with IL-4-PDGF-pMSCs alone, and with FGS loaded with PDGF-MSCs or IL-4-PDGF-pMSCs. For the area inside the CD, the bone volume in the CD alone was higher than in both FGS groups. The IL-4-PDGF-pMSCs alone and FGS + PDGF-MSCs reduced the occurrence of empty lacunae and improved osteoclastogenesis. There was no significant difference in angiogenesis among the four groups. The combined effect of GM MSCs or pMSCs and the FGS was not superior to the effect of each alone. To establish an important adjunctive therapy for CD for early ONFH in the future, it is necessary and essential to develop an FGS that delivers biologics appropriately and provides structural and mechanical support.

An osteoinductive and biodegradable intramedullary implant accelerates bone healing and mitigates complications of bone transport in male rats

Bone transport is a surgery-driven procedure for the treatment of large bone defects. However, challenging complications include prolonged consolidation, docking site nonunion and pin tract infection. Here, we develop an osteoinductive and biodegradable intramedullary implant by a hybrid tissue engineering construct technique to enable sustained delivery of bone morphogenetic protein-2 as an adjunctive therapy. In a male rat bone transport model, the eluting bone morphogenetic protein-2 from the implants accelerates bone formation and remodeling, leading to early bony fusion as shown by imaging, mechanical testing, histological analysis, and microarray assays. Moreover, no pin tract infection but tight osseointegration are observed. In contrast, conventional treatments show higher proportion of docking site nonunion and pin tract infection. The findings of this study demonstrate that the novel intramedullary implant holds great promise for advancing bone transport techniques by promoting bone regeneration and reducing complications in the treatment of bone defects.

Development and systematic characterization of GelMA/alginate/PEGDMA/xanthan gum hydrogel bioink system for extrusion bioprinting

Gelatin methacryloyl (GelMA)/alginate-based hydrogels have shown great promise in bioprinting, but their printability is limited at room temperature. In this paper, we present our development of a room temperature printable hydrogel bioink by introducing polyethylene glycol dimethacrylate (PEGDMA) and xanthan gum into the GelMA/alginate system. The inclusion of PEGDMA facilitates tuning of the hydrogel's mechanical property, while xanthan gum improves the viscosity of the hydrogel system and allows easy extrusion at room temperature. To fine-tune the mechanical and degradation properties, methacrylated xanthan gum was synthesized and chemically crosslinked to the system. We systematically characterized this hydrogel with attention to printability, strut size, mechanical property, degradation and cytocompatibility, and achieved a broad range of compression modulus (∼10-100 kPa) and degradation profile (100% degradation by 24 h-40% by 2 weeks). Moreover, xanthan gum demonstrated solubility in ionic solutions such as cell culture medium, which is essential for biocompatibility. Live/dead staining showed that cell viability in the printed hydrogels was over 90% for 7 days. Metabolic activity analysis demonstrated excellent cell proliferation and survival within 4 weeks of incubation. In summary, the newly developed hydrogel system has demonstrated distinct features including extrusion printability, widely tunable mechanical property and degradation, ionic solubility, and cytocompatibility. It offers great flexibility in bioprinting and tissue engineering.

A bioactive compliant vascular graft modulates macrophage polarization and maintains patency with robust vascular remodeling

Conventional synthetic vascular grafts are associated with significant failure rates due to their mismatched mechanical properties with the native vessel and poor regenerative potential. Though different tissue engineering approaches have been used to improve the biocompatibility of synthetic vascular grafts, it is still crucial to develop a new generation of synthetic grafts that can match the dynamics of native vessel and direct the host response to achieve robust vascular regeneration. The size of pores within implanted biomaterials has shown significant effects on macrophage polarization, which has been further confirmed as necessary for efficient vascular formation and remodeling. Here, we developed biodegradable, autoclavable synthetic vascular grafts from a new polyurethane elastomer and tailored the grafts' interconnected pore sizes to promote macrophage populations with a pro-regenerative phenotype and improve vascular regeneration and patency rate. The synthetic vascular grafts showed similar mechanical properties to native blood vessels, encouraged macrophage populations with varying M2 to M1 phenotypic expression, and maintained patency and vascular regeneration in a one-month rat carotid interposition model and in a four-month rat aortic interposition model. This innovative bioactive synthetic vascular graft holds promise to treat clinical vascular diseases.

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Small diameter vascular grafts were fabricated from a new elastomeric polyurethane designed for vascular tissue engineering.

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The grafts combined excellent elasticity, strength, porosity, hemocompatibility, degradability, and biocompatibility.

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In vivo, grafts maintained patency for four months and supported tissue regeneration resembling the native arterial wall.

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Pore size was found to influence graft characteristics and efficacy.

Dual Delivery of BMP2 and IGF1 Through Injectable Hydrogel Promotes Cranial Bone Defect Healing

Critical-sized cranial bone defect remains a great clinical challenge. With advantages in regenerative medicine, injectable hydrogels incorporated with bioactive molecules show great potential in promoting cranial bone repair. Recently, we developed a dual delivery system by sequential release of bone morphogenetic protein 2 (BMP2) followed by insulin-like growth factor 1 (IGF1) in microparticles (MPs), and an injectable alginate/collagen (alg/col)-based hydrogel. In this study, we aim to evaluate the effect of dual delivery of BMP2 and IGF1 in MPs through the injectable hydrogel in critical-sized cranial bone defect healing. The gelatin MPs loaded with BMP2 and poly(lactic-co-glycolic acid)-poly(ethylene glycol)-carboxyl (PLGA-PEG-COOH) MPs loaded with IGF1 were prepared, respectively. The encapsulation efficiency and release profile of growth factors in MPs were measured. A cranial defect model was applied to evaluate the efficacy of the dual delivery system in bone regeneration. Adult Sprague Dawley rats were subjected to osteotomy to make an ⌀8-mm cranial defect. The injectable hydrogel containing MPs loaded with BMP2 (2 μg), IGF1 (2 μg), or a combination of BMP2 (1 μg) and IGF1 (1 μg) were injected to the defect site. New bone formation was evaluated by microcomputed tomography, histological analysis, and immunohistochemistry after 4 or 8 weeks. Data showed that dual delivery of the low-dose BMP2 and IGF1 in MPs through alg/col-based hydrogel successfully restored cranial bone as early as 4 weeks after implantation, whose effect was comparable to the single delivery of high-dose BMP2 in MPs. In conclusion, this study suggests that dual delivery of BMP2 and IGF1 in MPs in alg/col-based hydrogel achieves early bone regeneration in critical-sized bone defect, with advantage in reducing the dose of BMP2. Impact Statement Sequential release of bone morphogenetic protein 2 (BMP2) followed by insulin-like growth factor 1 (IGF1) in two different microparticles promotes critical-sized bone defect healing. This dual delivery system reduces the dose of BMP2 by supplementing IGF1, which may diminish the potential side effects of BMP2.

Applying deep learning to quantify empty lacunae in histologic sections of osteonecrosis of the femoral head

Osteonecrosis of the femoral head (ONFH) is a disease in which inadequate blood supply to the subchondral bone causes the death of cells in the bone marrow. Decalcified histology and assessment of the percentage of empty lacunae are used to quantify the severity of ONFH. However, the current clinical practice of manually counting cells is a tedious and inefficient process. We utilized the power of artificial intelligence by training an established deep convolutional neural network framework, Faster-RCNN, to automatically classify and quantify osteocytes (healthy and pyknotic) and empty lacunae in 135 histology images. The adjusted correlation coefficient between the trained cell classifier and the ground truth was R = 0.98. The methods detailed in this study significantly reduced the manual effort of cell counting in ONFH histological samples and can be translated to other fields of image quantification.

Hybprinting for musculoskeletal tissue engineering

This review presents bioprinting methods, biomaterials, and printing strategies that may be used for composite tissue constructs for musculoskeletal applications. The printing methods discussed include those that are suitable for acellular and cellular components, and the biomaterials include soft and rigid components that are suitable for soft and/or hard tissues. We also present strategies that focus on the integration of cell-laden soft and acellular rigid components under a single printing platform. Given the structural and functional complexity of native musculoskeletal tissue, we envision that hybrid bioprinting, referred to as hybprinting, could provide unprecedented potential by combining different materials and bioprinting techniques to engineer and assemble modular tissues.

The effect of genetically modified platelet-derived growth factor-BB over-expressing mesenchymal stromal cells during core decompression for steroid-associated osteonecrosis of the femoral head in rabbits

BACKGROUND

Approximately one third of patients undergoing core decompression (CD) for early-stage osteonecrosis of the femoral head (ONFH) experience progression of the disease, and subsequently require total hip arthroplasty (THA). Thus, identifying adjunctive treatments to optimize bone regeneration during CD is an unmet clinical need. Platelet-derived growth factor (PDGF)-BB plays a central role in cell growth and differentiation. The aim of this study was to characterize mesenchymal stromal cells (MSCs) that were genetically modified to overexpress PDGF-BB (PDGF-BB-MSCs) in vitro and evaluate their therapeutic effect when injected into the bone tunnel at the time of CD in an in vivo rabbit model of steroid-associated ONFH.

METHODS

In vitro studies: Rabbit MSCs were transduced with a lentivirus vector carrying the human PDGF-BB gene under the control of either the cytomegalovirus (CMV) or phosphoglycerate (PGK) promoter. The proliferative rate, PDGF-BB expression level, and osteogenic differentiation capacity of unmodified MSCs, CMV-PDGF-BB-MSCs, and PGK-PDGF-BB-MSCs were assessed. In vivo studies: Twenty-four male New Zealand white rabbits received an intramuscular (IM) injection of methylprednisolone 20 mg/kg. Four weeks later, the rabbits were divided into four groups: the CD group, the hydrogel [HG, (a collagen-alginate mixture)] group, the MSC group, and the PGK-PDGF-BB-MSC group. Eight weeks later, the rabbits were sacrificed, their femurs were harvested, and microCT, mechanical testing, and histological analyses were performed.

RESULTS

In vitro studies: PGK-PDGF-BB-MSCs proliferated more rapidly than unmodified MSCs (P < 0.001) and CMV-PDGF-BB-MSCs (P < 0.05) at days 3 and 7. CMV-PDGF-BB-MSCs demonstrated greater PDGF-BB expression than PGK-PDGF-BB-MSCs (P < 0.01). However, PGK-PDGF-BB-MSCs exhibited greater alkaline phosphatase staining at 14 days (P < 0.01), and osteogenic differentiation at 28 days (P = 0.07) than CMV-PDGF-BB-MSCs. In vivo: The PGK-PDGF-BB-MSC group had a trend towards greater bone mineral density (BMD) than the CD group (P = 0.074). The PGK-PDGF-BB-MSC group demonstrated significantly lower numbers of empty lacunae (P < 0.001), greater osteoclast density (P < 0.01), and greater angiogenesis (P < 0.01) than the other treatment groups.

CONCLUSION

The use of PGK-PDGF-BB-MSCs as an adjunctive treatment with CD may reduce progression of osteonecrosis and enhance bone regeneration and angiogenesis in the treatment of early-stage ONFH.

The efficacy of lapine preconditioned or genetically modified IL4 over-expressing bone marrow-derived mesenchymal stromal cells in corticosteroid-associated osteonecrosis of the femoral head in rabbits

Cell-based therapy for augmentation of core decompression (CD) using mesenchymal stromal cells (MSCs) is a promising treatment for early stage osteonecrosis of the femoral head (ONFH). Recently, the therapeutic potential for immunomodulation of osteogenesis using preconditioned (with pro-inflammatory cytokines) MSCs (pMSCs), or by the timely resolution of inflammation using MSCs that over-express anti-inflammatory cytokines has been described. Here, pMSCs exposed to tumor necrosis factor-alpha and lipopolysaccharide for 3 days accelerated osteogenic differentiation in vitro. Furthermore, injection of pMSCs encapsulated with injectable hydrogels into the bone tunnel facilitated angiogenesis and osteogenesis in the femoral head in vivo, using rabbit bone marrow-derived MSCs and a model of corticosteroid-associated ONFH in rabbits. In contrast, in vitro and in vivo studies demonstrated that genetically-modified MSCs that over-express IL4 (IL4-MSCs), established by using a lentiviral vector carrying the rabbit IL4 gene under the cytomegalovirus promoter, accelerated proliferation of MSCs and decreased the percentage of empty lacunae in the femoral head. Therefore, adjunctive cell-based therapy of CD using pMSCs and IL4-MSCs may hold promise to heal osteonecrotic lesions in the early stage ONFH. These interventions must be applied in a temporally sensitive fashion, without interfering with the mandatory acute inflammatory phase of bone healing.

Investigation of a Prevascularized Bone Graft for Large Defects in the Ovine Tibia

In vivo bioreactors are a promising approach for engineering vascularized autologous bone grafts to repair large bone defects. In this pilot parametric study, we first developed a three-dimensional (3D) printed scaffold uniquely designed to accommodate inclusion of a vascular bundle and facilitate growth factor delivery for accelerated vascular invasion and ectopic bone formation. Second, we established a new sheep deep circumflex iliac artery (DCIA) model as an in vivo bioreactor for engineering a vascularized bone graft and evaluated the effect of implantation duration on ectopic bone formation. Third, after 8 weeks of implantation around the DCIA, we transplanted the prevascularized bone graft to a 5 cm segmental bone defect in the sheep tibia, using the custom 3D printed bone morphogenic protein 2 (BMP-2) loaded scaffold without prior in vivo bioreactor maturation as a control. Analysis by micro-computed tomography and histomorphometry found ectopic bone formation in BMP-2 loaded scaffolds implanted for 8 and 12 weeks in the iliac pouch, with greater bone formation occurring after 12 weeks. Grafts transplanted to the tibial defect supported bone growth, mainly on the periphery of the graft, but greater bone growth and less soft tissue invasion was observed in the avascular BMP-2 loaded scaffold implanted directly into the tibia without prior in vivo maturation. Histopathological evaluation noted considerably greater vascularity in the bone grafts that underwent in vivo maturation with an inserted vascular bundle compared with the avascular BMP-2 loaded graft. Our findings indicate that the use of an initial DCIA in vivo bioreactor maturation step is a promising approach to developing vascularized autologous bone grafts, although scaffolds with greater osteoinductivity should be further studied. Impact statement This translational pilot study aims at combining a tissue engineering scaffold strategy, in vivo prevascularization, and a modified transplantation technique to accelerate large segmental bone defect repair. First, we three-dimensional (3D) printed a 5 cm scaffold with a unique design to facilitate vascular bundle inclusion and osteoinductive growth factor delivery. Second, we established a new sheep deep circumflex iliac artery model as an in vivo bioreactor for prevascularizing the novel 3D printed osteoinductive scaffold. Subsequently, we transplanted the prevascularized bone graft to a clinically relevant 5 cm segmental bone defect in the sheep tibia for bone regeneration.

Osteoinductive 3D printed scaffold healed 5 cm segmental bone defects in the ovine metatarsus

Autologous bone grafts are considered the gold standard grafting material for the treatment of nonunion, but in very large bone defects, traditional autograft alone is insufficient to induce repair. Recombinant human bone morphogenetic protein 2 (rhBMP-2) can stimulate bone regeneration and enhance the healing efficacy of bone grafts. The delivery of rhBMP-2 may even enable engineered synthetic scaffolds to be used in place of autologous bone grafts for the treatment of critical size defects, eliminating risks associated with autologous tissue harvest. We here demonstrate that an osteoinductive scaffold, fabricated by combining a 3D printed rigid polymer/ceramic composite scaffold with an rhBMP-2-eluting collagen sponge can treat extremely large-scale segmental defects in a pilot feasibility study using a new sheep metatarsus fracture model stabilized with an intramedullary nail. Bone regeneration after 24 weeks was evaluated by micro-computed tomography, mechanical testing, and histological characterization. Load-bearing cortical bridging was achieved in all animals, with increased bone volume observed in sheep that received osteoinductive scaffolds compared to sheep that received an rhBMP-2-eluting collagen sponge alone.

Effect of porosity of a functionally-graded scaffold for the treatment of corticosteroid-associated osteonecrosis of the femoral head in rabbits

Background/Objective: Core decompression (CD) with scaffold and cell-based therapies is a promising strategy for providing both mechanical support and regeneration of the osteonecrotic area for early stage osteonecrosis of the femoral head (ONFH). We designed a new 3D printed porous functionally-graded scaffold (FGS) with a central channel to facilitate delivery of transplanted cells in a hydrogel to the osteonecrotic area. However, the optimal porous structural design for the FGS for the engineering of bone in ONFH has not been elucidated. The aim of this study was to fabricate and evaluate two different porous structures (30% or 60% porosity) of the FGSs in corticosteroid-associated ONFH in rabbits.

Methods

Two different FGSs with 30% or 60% porosity containing a 1-mm central channel were 3D printed using polycaprolactone and β-tricalcium phosphate. The FGS was 3-mm diameter and 32-mm length and was composed of three segments: 1-mm in length for the non-porous proximal segment, 22-mm in length for the porous (30% versus 60%) middle segment, and 9-mm in length for the 15% porous distal segment. Eighteen male New Zealand White rabbits were given a single dose of 20 ​mg/kg methylprednisolone acetate intramuscularly. Four weeks later, rabbits were divided into three groups: the CD group, the 30% porosity FGS group, and the 60% porosity FGS group. In the CD group, a 3-mm diameter drill hole was created into the left femoral head. In the FGS groups, a 30% or 60% porosity implant was inserted into the bone tunnel. Eight weeks postoperatively, femurs were harvested and microCT, mechanical, and histological analyses were performed.

Results

The actual porosity and pore size of the middle segments were 26.4% ​± ​2.3% and 699 ​± ​56 ​μm in the 30% porosity FGS, and 56.0% ​± ​4.5% and 999 ​± ​71 ​μm in the 60% porosity FGS, respectively using microCT analysis. Bone ingrowth ratio in the 30% porosity FGS group was 73.9% ​± ​15.8%, which was significantly higher than 39.5% ​± ​13.0% in the CD group on microCT (p ​< ​0.05). Bone ingrowth ratio in the 60% porosity FGS group (61.3% ​± ​30.1%) showed no significant differences compared to the other two groups. The stiffness at the bone tunnel site in the 30% porosity FGS group was 582.4 ​± ​192.3 ​N/mm³, which was significantly higher than 338.7 ​± ​164.6 ​N/mm³ in the 60% porosity FGS group during push-out testing (p ​< ​0.05). Hematoxylin and eosin staining exhibited thick and mature trabecular bone around the porous FGS in the 30% porosity FGS group, whereas thinner, more immature trabecular bone was seen around the porous FGS in the 60% porosity FGS group.

Conclusion

These findings indicate that the 30% porosity FGS may enhance bone regeneration and have superior biomechanical properties in the bone tunnel after CD in ONFH, compared to the 60% porosity FGS.

Translation potential statement

The translational potential of this article: This FGS implant holds promise for improving outcomes of CD for early stage ONFH.

In-situ stable injectable collagen-based hydrogels for cell and growth factor delivery

Here we report development of in-situ stable injectable hydrogels for delivery of cells and growth factors based on two precursors, alginate, and collagen/calcium sulfate (CaSO4). The alg/col hydrogels were shear-thinning, injectable through commercially available needles and stable right after injection. Rheological measurements revealed that pre-crosslinked alg/col hydrogels fully crosslinked at 37°C and that the storage modulus of alg/col hydrogels increased with increasing the collagen content or the concentration of CaSO4. The viscoelastic characteristics and injectability of the alg/col hydrogels were not significantly impacted by the storage of precursor solutions for 28 days. An osteoinductive bone morphogenic protein-2 (BMP-2) loaded into alg/col hydrogels was released in 14 days. Human mesenchymal stem cells (hMSCs) encapsulated in alg/col hydrogels had over 90% viability over 7 days after injection. The DNA content of hMSC-laden alg/col hydrogels increased by 6-37 folds for 28 days, depending on the initial cell density. In addition, hMSCs encapsulated in alg/col hydrogels and incubated in osteogenic medium were osteogenically differentiated and formed a mineralized matrix. Finally, a BMP-2 loaded alg/col hydrogel was used to heal a critical size calvarial bone defect in rats after 8 weeks of injection. The alg/col hydrogel holds great promise in tissue engineering and bioprinting applications.

Development of PLGA-PEG-COOH and gelatin-based microparticles dual delivery system and E-beam sterilization effects for controlled release of BMP-2 and IGF-1

The purpose of this study was to develop a PLGA-PEG-COOH- and gelatin-based microparticles (MPs) dual delivery system for release of BMP-2 and IGF-1. We made and characterized the delivery system based on its morphology, loading capacity, Encapsulation efficiency and release kinetics. Second, we examined the effects of electron beam (EB) sterilization on BMP-2 and IGF-1 loaded MPs and their biological effects. Third, we evaluated the synergistic effect of a controlled dual release of BMP-2 and IGF-1 on osteogenesis of MSCs. Encapsulation efficiency of growth factors into gelatin and PLGA-PEG-COOH MPs are in the range of 64.78% to 76.11%. E-beam sterilized growth factor delivery systems were effective in significantly promoting osteogenesis of MSCs, although E-beam sterilization decreased the bioactivity of growth factors in MPs by approximately 22%. BMP-2 release behavior from gelatin MPs/PEG hydrogel shows a faster release (52.7%) than that of IGF-1 from the PLGA-PEG-COOH MPs/PEG hydrogel (27.3%). The results demonstrate that the gelatin and PLGA-PEG-COOH MPs based delivery system could realize temporal release of therapeutic biomolecules by incorporating different growth factors into distinct microparticles. EB sterilization was an accessible method for sterilizing growth factors loaded carriers, which could pave the way for implementing growth factor delivery in clinical applications.

Regenerative Scar-Free Skin Wound Healing

IMPACT STATEMENT

Millions of people every year develop scars in response to skin injuries after surgery, trauma, or burns with significant undesired physical and psychological effects. This review provides an update on engineering strategies for scar-free wound healing and discusses the role of different cell types, growth factors, cytokines, and extracellular components in regenerative wound healing. The use of pro-regenerative matrices combined with engineered cells with less intrinsic potential for fibrogenesis is a promising strategy for achieving scar-free skin tissue regeneration.

Material and regenerative properties of an osteon-mimetic cortical bone-like scaffold

The objective of this work was to fabricate a rigid, resorbable and osteoconductive scaffold by mimicking the hierarchical structure of the cortical bone. Aligned peptide-functionalize nanofiber microsheets were generated with calcium phosphate (CaP) content similar to that of the natural cortical bone. Next, the CaP-rich fibrous microsheets were wrapped around a microneedle to form a laminated microtube mimicking the structure of an osteon. Then, a set of the osteon-mimetic microtubes were assembled around a solid rod and the assembly was annealed to fuse the microtubes and form a shell. Next, an array of circular microholes were drilled on the outer surface of the shell to generate a cortical bone-like scaffold with an interconnected network of Haversian- and Volkmann-like microcanals. The CaP content, porosity and density of the bone-mimetic microsheets were 240 wt%, 8% and 1.9 g/ml, respectively, which were close to that of natural cortical bone. The interconnected network of microcanals in the fused microtubes increased permeability of a model protein in the scaffold. The cortical scaffold induced osteogenesis and vasculogenesis in the absence of bone morphogenetic proteins upon seeding with human mesenchymal stem cells and endothelial colony-forming cells. The localized and timed-release of morphogenetic factors significantly increased the extent of osteogenic and vasculogenic differentiation of human mesenchymal stem cells and endothelial colony-forming cells in the cortical scaffold. The cortical bone-mimetic nature of the cellular construct provided balanced rigidity, resorption rate, osteoconductivity and nutrient diffusivity to support vascularization and osteogenesis.

Sequential Zonal Chondrogenic Differentiation of Mesenchymal Stem Cells in Cartilage Matrices

IMPACT STATEMENT

The higher regenerative capacity of fetal articular cartilage compared with the adult is rooted in differences in cell density and matrix composition. We hypothesized that the zonal organization of articular cartilage can be engineered by encapsulation of mesenchymal stem cells in a single superficial zone-like matrix followed by sequential addition of zone-specific growth factors within the matrix, similar to the process of fetal cartilage development. The results demonstrate that the zonal organization of articular cartilage can potentially be regenerated using an injectable, monolayer cell-laden hydrogel with sequential release of growth factors.

Effect of Electron Beam Sterilization on Three-Dimensional-Printed Polycaprolactone/Beta-Tricalcium Phosphate Scaffolds for Bone Tissue Engineering

IMPACT STATEMENT

Providing customized geometries and improved control in physical and biological properties, 3D-printed polycaprolactone/beta-tricalcium phosphate (PCL/β-TCP) composite constructs are of high interest for bone tissue engineering applications. A critical step toward the translation and clinical applications of these types of scaffolds is terminal sterilization, and E-beam irradiation might be the most relevant method because of PCL properties. Through in vitro experimental testing of both physical and biological properties, it is proven in this article that E-beam irradiation is relevant for sterilization of 3D-printed PCL/β-TCP scaffolds for bone tissue engineering applications.

Devitalized Stem Cell Microsheets for Sustainable Release of Osteogenic and Vasculogenic Growth Factors and Regulation of Anti-Inflammatory Immune Response

The objective of this work was to investigate the effect of devitalized human mesenchymal stem cells (hMSCs) and endothelial colony-forming cells (ECFCs) seeded on mineralized nanofiber microsheets on protein release, osteogenesis, vasculogenesis, and macrophage polarization. Calcium phosphate nanocrystals were grown on the surface of aligned, functionalized nanofiber microsheets. The microsheets were seeded with hMSCs, ECFCs, or a mixture of hMSCs+ECFCs, cultured for cell attachment, differentiated to the osteogenic or vasculogenic lineage, and devitalized by lyophilization. The release kinetic of total protein, bone morphogenetic protein-2 (BMP2), and vascular endothelial growth factor (VEGF) from the devitalized microsheets was measured. Next, hMSCs and/or ECFCs were seeded on the devitalized cell microsheets and cultured in the absence of osteo-/vasculo-inductive factors to determine the effect of devitalized cell microsheets on hMSC/ECFC differentiation. Human macrophages were seeded on the microsheets to determine the effect of devitalized cells on macrophage polarization. Based on the results, devitalized undifferentiated hMSC and vasculogenic-differentiated ECFC microsheets had highest sustained release of BMP2 and VEGF, respectively. The devitalized hMSC microsheets did not affect M2 macrophage polarization while vascular-differentiated, devitalized ECFC microsheets did not affect M1 polarization. Both groups stimulated higher M2 macrophage polarization compared to M1.

Comparative effect of physicomechanical and biomolecular cues on zone-specific chondrogenic differentiation of mesenchymal stem cells

Current tissue engineering approaches to regeneration of articular cartilage rarely restore the tissue to its normal state because the generated tissue lacks the intricate zonal organization of the native cartilage. Zonal regeneration of articular cartilage is hampered by the lack of knowledge for the relation between physical, mechanical, and biomolecular cues and zone-specific chondrogenic differentiation of progenitor cells. This work investigated in 3D the effect of TGF-β1, zone-specific growth factors, optimum matrix stiffness, and adding nanofibers on the expression of chondrogenic markers specific to the superficial, middle, and calcified zones of articular cartilage by the differentiating human mesenchymal stem cells (hMSCs). Growth factors included BMP-7, IGF-1, and hydroxyapatite (HA) for the superficial, middle, and calcified zones, respectively; optimum matrix stiffness was 80 kPa, 2.1 MPa, and 320 MPa; and nanofibers were aligned horizontal, random, and perpendicular to the gel surface. hMSCs with zone-specific cell densities were encapsulated in engineered hydrogels and cultured with or without TGF-β1, zone-specific growth factor, optimum matrix modulus, and fiber addition and cultured in basic chondrogenic medium. The expression of encapsulated cells was measured by mRNA, protein, and biochemical analysis. Results indicated that zone-specific matrix stiffness had a dominating effect on chondrogenic differentiation of hMSCs to the superficial and calcified zone phenotypes. Addition of aligned nanofibers parallel to the direction of gel surface significantly enhanced expression of Col II in the superficial zone chondrogenic differentiation of hMSCs. Conversely, biomolecular factor IGF-1 in combination with TGF-β1 had a dominating effect on the middle zone chondrogenic differentiation of hMSCs. Results of this work could potentially lead to the development of multilayer grafts mimicking the zonal organization of articular cartilage.

Spatiotemporal release of BMP-2 and VEGF enhances osteogenic and vasculogenic differentiation of human mesenchymal stem cells and endothelial colony-forming cells co-encapsulated in a patterned hydrogel

Reconstruction of large bone defects is limited by insufficient vascularization and slow bone regeneration. The objective of this work was to investigate the effect of spatial and temporal release of recombinant human bone morphogenetic protein-2 (BMP2) and vascular endothelial growth factor (VEGF) on the extent of osteogenic and vasculogenic differentiation of human mesenchymal stem cells (hMSCs) and endothelial colony-forming cells (ECFCs) encapsulated in a patterned hydrogel. Nanogels (NGs) based on polyethylene glycol (PEG) macromers chain-extended with short lactide (L) and glycolide (G) segments were used for grafting and timed-release of BMP2 and VEGF. NGs with 12kDa PEG molecular weight (MW), 24 LG segment length, and 60/40L/G ratio (P12-II, NG(10)) released the grafted VEGF in 10days. NGs with 8kDa PEG MW, 26 LG segment length, and 60/40L/G ratio (P8-I, NG(21)) released the grafted BMP2 in 21days. hMSCs and NG-BMP2 were encapsulated in a patterned matrix based on acrylate-functionalized lactide-chain-extended star polyethylene glycol (SPELA) hydrogel and microchannel patterns filled with a suspension of hMSCs+ECFCs and NG-VEGF in a crosslinked gelatin methacryloyl (GelMA) hydrogel. Groups included patterned constructs without BMP2/VEGF (None), with directly added BMP2/VEGF, and NG-BMP2/NG-VEGF. Based on the results, timed-release of VEGF in the microchannels in 10days from NG(10) and BMP2 in the matrix in 21days from NG(21) resulted in highest extent of osteogenic and vasculogenic differentiation of the encapsulated hMSCs and ECFCs compared to direct addition of VEGF and BMP2. Further, timed-release of VEGF from NG(10) in hMSC+ECFC encapsulating microchannels and BMP2 from NG(21) in hMSC encapsulating matrix sharply increased bFGF expression in the patterned constructs. The results suggest that mineralization and vascularization are coupled by localized secretion of paracrine signaling factors by the differentiating hMSCs and ECFCs.

Gelation characteristics, physico-mechanical properties and degradation kinetics of micellar hydrogels

Due to their high water content and diffusivity of nutrients and biomolecules, hydrogels are very attractive as a matrix for growth factor immobilization and in situ delivery of cells to the site of regeneration in tissue engineering. The formation of micellar structures at the nanoscale in hydrogels alters the spatial distribution of the reactive groups and affects the rate and extent of crosslinking and mechanical properties of the hydrogel. Further, the degradation rate of a hydrogel is strongly affected by the proximity of water molecules to the hydrolytically degradable segments at the nanoscale. The objective of this review is to summarize the unique properties of micellar hydrogels with a focus on our previous work on star polyethylene glycol (PEG) macromonomers chain extended with short aliphatic hydroxy acid (HA) segments (SPEXA hydrogels). Micellar SPEXA hydrogels have faster gelation rates and higher compressive moduli compared to their non-micellar counterpart. Owing to their micellar structure, SPEXA hydrogels have a wide range of degradation rates from a few days to many months as opposed to non-degradable PEG gels while both gels possess similar water contents. Furthermore, the viability and differentiation of mesenchymal stem cells (MSCs) is enhanced when the cells are encapsulated in degradable micellar SPEXA gels compared with those cells encapsulated in non-micellar PEG gels.

A developmentally inspired combined mechanical and biochemical signaling approach on zonal lineage commitment of mesenchymal stem cells in articular cartilage regeneration

Articular cartilage is organized into multiple zones including superficial, middle and calcified zones with distinct cellular and extracellular components to impart lubrication, compressive strength, and rigidity for load transmission to bone, respectively. During native cartilage tissue development, changes in biochemical, mechanical, and cellular factors direct the formation of stratified structure of articular cartilage. The objective of this work was to investigate the effect of combined gradients in cell density, matrix stiffness, and zone-specific growth factors on the zonal organization of articular cartilage. Human mesenchymal stem cells (hMSCs) were encapsulated in acrylate-functionalized lactide-chain-extended polyethylene glycol (SPELA) gels simulating cell density and stiffness of the superficial, middle and calcified zones. The cell-encapsulated gels were cultivated in a medium supplemented with growth factors specific to each zone and the expression of zone-specific markers was measured with incubation time. Encapsulation of 60 × 10(6) cells per mL hMSCs in a soft gel (80 kPa modulus) and cultivation with a combination of TGF-β1 (3 ng mL(-1)) and BMP-7 (100 ng mL(-1)) led to the expression of markers for the superficial zone. Conversely, encapsulation of 15 × 10(6) cells per mL hMSCs in a stiff gel (320 MPa modulus) and cultivation with a combination of TGF-β1 (30 ng mL(-1)) and hydroxyapatite (3%) led to the expression of markers for the calcified zone. Further, encapsulation of 20 × 10(6) cells per mL hMSCs in a gel with 2.1 MPa modulus and cultivation with a combination of TGF-β1 (30 ng mL(-1)) and IGF-1 (100 ng mL(-1)) led to up-regulation of the middle zone markers. Results demonstrate that a developmental approach with gradients in cell density, matrix stiffness, and zone-specific growth factors can potentially regenerate zonal structure of the articular cartilage.

Optimum 3D Matrix Stiffness for Maintenance of Cancer Stem Cells Is Dependent on Tissue Origin of Cancer Cells

INTRODUCTION

The growth and expression of cancer stem cells (CSCs) depend on many factors in the tumor microenvironment. The objective of this work was to investigate the effect of cancer cells' tissue origin on the optimum matrix stiffness for CSC growth and marker expression in a model polyethylene glycol diacrylate (PEGDA) hydrogel without the interference of other factors in the microenvironment.

METHODS

Human MCF7 and MDA-MB-231 breast carcinoma, HCT116 colorectal and AGS gastric carcinoma, and U2OS osteosarcoma cells were used. The cells were encapsulated in PEGDA gels with compressive moduli in the 2-70 kPa range and optimized cell seeding density of 0.6x106 cells/mL. Micropatterning was used to optimize the growth of encapsulated cells with respect to average tumorsphere size. The CSC sub-population of the encapsulated cells was characterized by cell number, tumorsphere size and number density, and mRNA expression of CSC markers.

RESULTS

The optimum matrix stiffness for growth and marker expression of CSC sub-population of cancer cells was 5 kPa for breast MCF7 and MDA231, 25 kPa for colorectal HCT116 and gastric AGS, and 50 kPa for bone U2OS cells. Conjugation of a CD44 binding peptide to the gel stopped tumorsphere formation by cancer cells from different tissue origin. The expression of YAP/TAZ transcription factors by the encapsulated cancer cells was highest at the optimum stiffness indicating a link between the Hippo transducers and CSC growth. The optimum average tumorsphere size for CSC growth and marker expression was 50 μm.

CONCLUSION

The marker expression results suggest that the CSC sub-population of cancer cells resides within a niche with optimum stiffness which depends on the cancer cells' tissue origin.

Effect of organic acids on calcium phosphate nucleation and osteogenic differentiation of human mesenchymal stem cells on peptide functionalized nanofibers

Carboxylate-rich organic acids play an important role in controlling the growth of apatite crystals and the extent of mineralization in the natural bone. The objective of this work was to investigate the effect of organic acids on calcium phosphate (CaP) nucleation on nanofiber microsheets functionalized with a glutamic acid peptide and osteogenic differentiation of human mesenchymal stem cells (hMSCs) seeded on the CaP-nucleated microsheets. High molecular weight poly(dl-lactide) (DL-PLA) was mixed with low molecular weight L-PLA conjugated with Glu-Glu-Gly-Gly-Cys peptide, and the mixture was electrospun to generate aligned nanofiber microsheets. The nanofiber microsheets were incubated in a modified simulated body fluid (mSBF) supplemented with different organic acids for nucleation and growth of CaP crystals on the nanofibers. Organic acids included citric acid (CA), hydroxycitric acid (HCA), tartaric acid (TART), malic acid (MA), ascorbic acid (AsA), and salicylic acid (SalA). HCA microsheets had the highest CaP content at 240 ± 10% followed by TART and CA with 225 ± 8% and 225 ± 10%, respectively. The Ca/P ratio and percent crystallinity of the nucleated CaP in TART microsheets was closest to that of stoichiometric hydroxyapatite. The extent of CaP nucleation and growth on the nanofiber microsheets depended on the acidic strength and number of hydrogen-bonding hydroxyl groups of the organic acids. Compressive modulus and degradation of the CaP nucleated microsheets were related to percent crystallinity and CaP content. Osteogenic differentiation of hMSCs seeded on the microsheets and cultured in osteogenic medium increased only for those microsheets nucleated with CaP by incubation in CA or AsA-supplemented mSBF. Further, only CA microsheets stimulated bone nodule formation by the seeded hMSCs.

Morphogenic Peptides in Regeneration of Load Bearing Tissues

Morphogenic proteins due to their short half-life require high doses of growth factors in regeneration of load bearing tissues which leads to undesirable side effects. These side effects include bone overgrowth, tumor formation and immune reaction. An alternative approach to reduce undesirable side effects of proteins in regenerative medicine is to use morphogenic peptides derived from the active domains of morphogenic proteins or soluble and insoluble components of the extracellular matrix of mineralized load bearing tissues to induce differentiation of progenitor cells, mineralization, maturation and bone formation. In that regard, many peptides with osteogenic activity have been discovered. These include peptides derived from bone morphogenic proteins (BMPs), those based on interaction with integrin and heparin-binding receptors, collagen derived peptides, peptides derived from other soluble ECM proteins such as bone sialoprotein and enamel matrix proteins, and those peptides derived from vasculoinductive and neuro-inductive proteins. Although these peptides show significant osteogenic activity in vitro and increase mineralization and bone formation in animal models, they are not widely used in clinical orthopedic applications as an alternative to morphogenic proteins. This is partly due to the limited availability of data on structure and function of morphogenic peptides in physiological medium, particularly in tissue engineered scaffolds. Due to their amphiphilic nature, peptides spontaneously self-assemble and aggregate into micellar structures in physiological medium. Aggregation alters the sequence of amino acids in morphogenic peptides that interact with cell surface receptors thus affecting osteogenic activity of the peptide. Aggregation and micelle formation can dramatically reduce the active concentration of morphogenic peptides with many-fold increase in peptide concentration in physiological medium. Other factors that affect bioactivity are the non-specific interaction of morphogenic peptides with lipid bilayer of the cell membrane, interaction of the peptide with cell surface receptors that do not specifically induce osteogenesis leading to less-than-optimal osteogenic activity of the peptide, and less-than-optimal interaction of the peptide with osteogenic receptors on the cell surface. Covalent attachment or physical interaction with the tissue engineered matrix can also alter the bioactivity of morphogenic peptides and lead to a lower extent of osteogenesis and bone formation. This chapter reviews advances in discovery of morphogenic peptide, their structural characterization, and challenges in using morphogenic peptides in clinical applications as growth factors in tissue engineered devices for regeneration of load bearing tissues.

Experimental and computational investigation of the effect of hydrophobicity on aggregation and osteoinductive potential of BMP-2-derived peptide in a hydrogel matrix

An attractive approach to reduce the undesired side effects of bone morphogenetic proteins (BMPs) in regenerative medicine is to use osteoinductive peptide sequences derived from BMPs. Although the structure and function of BMPs have been studied extensively, there is limited data on structure and activity of BMP-derived peptides immobilized in hydrogels. The objective of this work was to investigate the effect of concentration and hydrophobicity of the BMP-2 peptide, corresponding to residues 73-92 of the knuckle epitope of BMP-2 protein, on peptide aggregation and osteogenic differentiation of human mesenchymal stem cells encapsulated in a polyethylene glycol (PEG) hydrogel. The peptide hydrophobicity was varied by capping PEG chain ends with short lactide segments. The BMP-2 peptide with a positive index of hydrophobicity had a critical micelle concentration (CMC) and formed aggregates in aqueous solution. Based on simulation results, there was a slight increase in the concentration of free peptide in solution with 1000-fold increase in peptide concentration. The dose-osteogenic response curve of the BMP-2 peptide was in the 0.0005-0.005 mM range, and osteoinductive potential of the BMP-2 peptide was significantly less than that of BMP-2 protein even at 1000-fold higher concentrations, which was attributed to peptide aggregation. Further, the peptide or PEG-peptide aggregates had significantly higher interaction energy with the cell membrane compared with the free peptide, which led to a higher nonspecific interaction with the cell membrane and loss of osteoinductive potential. Conjugation of the BMP-2 peptide to PEG increased CMC and osteoinductive potential of the peptide whereas conjugation to lactide-capped PEG reduced CMC and osteoinductive potential of the peptide. Experimental and simulation results revealed that osteoinductive potential of the BMP-2 peptide is correlated with its CMC and the free peptide concentration in aqueous medium and not the total concentration.

Time Dependence of Material Properties of Polyethylene Glycol Hydrogels Chain Extended with Short Hydroxy Acid Segments

The objective of this work was to investigate the effect of chemical composition and segment number (n) on gelation, stiffness, and degradation of hydroxy acid-chain-extended star polyethylene glycol acrylate (SPEXA) gels. The hydroxy acids included glycolide (G,), L-lactide (L), p-dioxanone (D) and -caprolactone (C). Chain-extension generated water soluble macromers with faster gelation rates, lower sol fractions, higher compressive moduli, and a wide-ranging degradation times when crosslinked into a hydrogel. SPEGA gels with the highest fraction of inter-molecular crosslinks had the most increase in compressive modulus with n whereas SPELA and SPECA had the lowest increase in modulus. SPEXA gels exhibited a wide range of degradation times from a few days for SPEGA to a few weeks for SPELA, a few months for SPEDA, and many months for SPECA. Marrow stromal cells and endothelial progenitor cells had the highest expression of vasculogenic markers when co-encapsulated in the faster degrading SPELA gel.

Nanostructure formation and transition from surface to bulk degradation in polyethylene glycol gels chain-extended with short hydroxy acid segments

Degradable, in situ gelling, inert hydrogels with tunable properties are very attractive as a matrix for cell encapsulation and delivery to the site of regeneration. Cell delivery is generally limited by the toxicity of gelation and degradation reactions. The objective of this work was to investigate by simulation and experimental measurement gelation kinetics and degradation rate of star acrylated polyethylene glycol (PEG) macromonomers chain-extended with short hydroxy acid (HA) segments (SPEXA) as a function of HA monomer type and number of HA repeat units. HA monomers included least hydrophobic glycolide (G), lactide (L), p-dioxanone (D), and most hydrophobic ε-caprolactone (C). Chain extension of PEG with short HA segments resulted in micelle formation for all HA types. There was a significant decrease in gelation time of SPEXA precursor solutions with HA chain-extension for all HA types due to micelle formation, consistent with the simulated increase in acrylate-acrylate (Ac-Ac) and Ac-initiator integration numbers. The hydrolysis rate of SPEXA hydrogels was strongly dependent on HA type and number of HA repeat units. SPEXA gels chain-extended with the least hydrophobic glycolide completely degraded within days, lactide within weeks, and p-dioxanone and ε-caprolactone degraded within months. The wide range of degradation rates observed for SPEXA gels can be explained by large differences in equilibrium water content of the micelles for different HA monomer types. A biphasic relationship between HA segment length and gel degradation rate was observed for all HA monomers, which was related to the transition from surface (controlled by HA segment length) to bulk (controlled by micelle equilibrium water content) hydrolysis within the micelle phase. To our knowledge, this is the first report on transition from surface to bulk degradation at the nanoscale in hydrogels.

Effect of surface modification of nanofibres with glutamic acid peptide on calcium phosphate nucleation and osteogenic differentiation of marrow stromal cells

Biomineralization is mediated by extracellular matrix (ECM) proteins with amino acid sequences rich in glutamic acid. The objective of this study was to investigate the effect of calcium phosphate deposition on aligned nanofibres surface-modified with a glutamic acid peptide on osteogenic differentiation of rat marrow stromal cells. Blend of EEGGC peptide (GLU) conjugated low molecular weight polylactide (PLA) and high molecular weight poly(lactide-co-glycolide) (PLGA) was electrospun to form aligned nanofibres (GLU-NF). The GLU-NF microsheets were incubated in a modified simulated body fluid for nucleation of calcium phosphate crystals on the fibre surface. To achieve a high calcium phosphate to fibre ratio, a layer-by-layer approach was used to improve diffusion of calcium and phosphate ions inside the microsheets. Based on dissipative particle dynamics simulation of PLGA/PLA-GLU fibres, > 80% of GLU peptide was localized to the fibre surface. Calcium phosphate to fibre ratios as high as 200%, between those of cancellous (160%) and cortical (310%) bone, was obtained with the layer-by-layer approach. The extent of osteogenic differentiation and mineralization of marrow stromal cells seeded on GLU-NF microsheets was directly related to the amount of calcium phosphate deposition on the fibres prior to cell seeding. Expression of osteogenic markers osteopontin, alkaline phosphatase (ALP), osteocalcin and type 1 collagen increased gradually with calcium phosphate deposition on GLU-NF microsheets. Results demonstrate that surface modification of aligned synthetic nanofibres with EEGGC peptide dramatically affects nucleation and growth of calcium phosphate crystals on the fibres leading to increased osteogenic differentiation of marrow stromal cells and mineralization.

Effect of CD44 binding peptide conjugated to an engineered inert matrix on maintenance of breast cancer stem cells and tumorsphere formation

INTRODUCTION

As cancer cells are affected by many factors in their microenvironment, a major challenge is to isolate the effect of a specific factor on cancer stem cells (CSCs) while keeping other factors unchanged. We have developed a synthetic inert 3D polyethylene glycol diacrylate (PEGDA) gel culture system as a unique tool to study the effect of microenvironmental factors on CSCs response. We have reported that CSCs formed in the inert PEGDA gel by encapsulation of breast cancer cells maintain their stemness within a certain range of gel stiffness. The objective was to investigate the effect of CD44 binding peptide (CD44BP) conjugated to the gel on the maintenance of breast CSCs.

METHODS

4T1 or MCF7 breast cancer cells were encapsulated in PEGDA gel with CD44BP conjugation. Control groups included dissolved CD44BP and the gel with mutant CD44BP conjugation. Tumorsphere size and density, and expression of CSC markers were determined after 9 days. For in vivo, cell encapsulated gels were inoculated in syngeneic Balb/C mice and tumor formation was determined after 4 weeks. Effect of CD44BP conjugation on breast CSC maintenance was compared with integrin binding RGD peptide (IBP) and fibronectin-derived heparin binding peptide (FHBP).

RESULTS

Conjugation of CD44BP to the gel inhibited breast tumorsphere formation in vitro and in vivo. The ability of the encapsulated cells to form tumorspheres in the peptide-conjugated gels correlated with the expression of CSC markers. Tumorsphere formation in vitro was enhanced by FHBP while it was abolished by IBP.

CONCLUSION

CD44BP and IBP conjugated to the gel abolished tumorsphere formation by encapsulated 4T1 cells while FHBP enhanced tumorsphere formation compared to cells in the gel without peptide. The PEGDA hydrogel culture system provides a novel tool to investigate the individual effect of factors in the microenvironment on CSC maintenance without interference of other factors.

Drug release kinetics, cell uptake, and tumor toxicity of hybrid VVVVVVKK peptide-assembled polylactide nanoparticles

An exciting approach to tumor delivery is encapsulation of the drug in self-assembled polymer-peptide nanoparticles. The objective of this work was to synthesize a conjugate of low molecular weight polylactide (LMW PLA) and V6K2 peptide and investigate self-assembly, drug release kinetics, cell uptake and toxicity, drug pharmacokinetics, and tumor cell invasion with Doxorubicin (DOX) or paclitaxel (PTX). The results for PLA-V6K2 self-assembled NPs were compared with those of polyethylene glycol stabilized PLA (PLA-EG) NPs. The size of PLA-V6K2 and PLA-EG NPs was 100 ± 20 and 130 ± 50 nm, respectively, with polydispersity index of 1.04 and 1.14. The encapsulation efficiency of DOX in PLA-V6K2 and PLA-EG NPs was 44 ± 9% and 55 ± 5%, respectively, and that of PTX was >90 for both NP types. The release of DOX and PTX from PLA-V6K2 was slower than that of PLA-EG, and the release rate was relatively constant with time. Based on molecular dynamic simulation, the less hydrophobic DOX was distributed in the lactide core as well as the peptide shell, while the hydrophobic PTX was localized mainly to the lactide core. PLA-V6K2 NPs had significantly higher cell uptake by 4T1 mouse breast carcinoma cells compared to PLA-EG NPs, which was attributed to the electrostatic interactions between the peptide and negatively charged moieties on the cell membrane. PLA-V6K2 NPs showed no toxicity to marrow stromal cells. DOX-loaded PLA-V6K2 NPs showed higher toxicity to 4T1 cells and the DNA damage response, and apoptosis was delayed compared to the free DOX. DOX or PTX encapsulated in PLA-V6K2 NPs significantly reduced invasion of 4T1 cells compared to those cells treated with the drug in PLA-EG NPs. Invasion of 4T1 cells treated with DOX in PLA-V6K2 and PLA-EG NPs was 5 ± 1% and 30 ± 5%, respectively, and that of PTX was 11 ± 2% and 40 ± 7%. The AUC of DOX in PLA-V6K2 NPs was 67% and 21% higher than those of free DOX and PLA-EG NPs, respectively. DOX-loaded PLA-V6K2 NPs injected in C3HeB/FeJ mice inoculated with MTCL syngeneic breast cancer cells displayed higher tumor toxicity than PLA-EG NPs and lower host toxicity than the free DOX. Cationic PLA-V6K2 NPs with higher tumor toxicity than the PLA-EG NPs are potentially useful in chemotherapy.

Three-dimensional-engineered matrix to study cancer stem cells and tumorsphere formation: effect of matrix modulus

Maintenance of cancer stem cells (CSCs) is regulated by the tumor microenvironment. Synthetic hydrogels provide the flexibility to design three-dimensional (3D) matrices to isolate and study individual factors in the tumor microenvironment. The objective of this work was to investigate the effect of matrix modulus on tumorsphere formation by breast cancer cells and maintenance of CSCs in an inert microenvironment without the interference of other factors. In that regard, 4T1 mouse breast cancer cells were encapsulated in inert polyethylene glycol diacrylate hydrogels and the effect of matrix modulus on tumorsphere formation and expression of CSC markers was investigated. The gel modulus had a strong effect on tumorsphere formation and the effect was bimodal. Tumorsphere formation and expression of CSC markers peaked after 8 days of culture. At day 8, as the matrix modulus was increased from 2.5 kPa to 5.3, 26.1, and 47.1 kPa, the average tumorsphere size changed from 37±6 μm to 57±6, 20±4, and 12±2 μm, respectively; cell number density in the gel changed from 0.8±0.1×10⁵ cells/mL to 1.7±0.2×10⁵, 0.4±0.1×10⁵, and 0.2±0.1×10⁵ cells/mL after initial encapsulation of 0.14×10⁵ cells/mL; and the expression of CD44 breast CSC marker changed from 17±4-fold to 38±9-, 3±1-, and 2±1-fold increase compared with the initial level. Similar results were obtained with MCF7 human breast carcinoma cells. Mouse 4T1 and human MCF7 cells encapsulated in the gel with 5.3 kPa modulus formed the largest tumorspheres and highest density of tumorspheres, and had highest expression of breast CSC markers CD44 and ABCG2. The inert polyethylene glycol hydrogel can be used as a model-engineered 3D matrix to study the role of individual factors in the tumor microenvironment on tumorigenesis and maintenance of CSCs without the interference of other factors.

Gelation characteristics and osteogenic differentiation of stromal cells in inert hydrolytically degradable micellar polyethylene glycol hydrogels

The use of poly(ethylene glycol) (PEG) hydrogels in tissue engineering is limited by their persistence in the site of regeneration. In an attempt to produce inert hydrolytically degradable PEG-based hydrogels, star (SPELA) poly(ethylene glycol-co-lactide) acrylate macromonomers with short lactide segments (<15 lactides per macromonomer) were synthesized. The SPELA hydrogel was characterized with respect to gelation time, modulus, water content, sol fraction, degradation, and osteogenic differentiation of encapsulated marrow stromal cells (MSCs). The properties of SPELA hydrogel were compared with those of the linear poly(ethylene glycol-co-lactide) acrylate (LPELA). The SPELA hydrogel had higher modulus, lower water content, and lower sol fraction than the LPELA. The shear modulus of SPELA hydrogel was 2.2 times higher than LPELA, whereas the sol fraction of SPELA hydrogel was 5 times lower than LPELA. The degradation of SPELA hydrogel depended strongly on the number of lactide monomers per macromonomer (nL) and showed a biphasic behavior. For example, as nL increased from 0 to 3.4, 6.4, 11.6, and 14.8, mass loss increased from 7 to 37, 80, 100% and then deceased to 87%, respectively, after 6 weeks of incubation. The addition of 3.4 lactides per macromonomer (<10 wt % dry macromonomer or <2 wt % swollen hydrogel) increased mass loss to 50% after 6 weeks. Molecular dynamic simulations demonstrated that the biphasic degradation behavior was related to aggregation and micelle formation of lactide monomers in the macromonomer in aqueous solution. MSCs encapsulated in SPELA hydrogel expressed osteogenic markers Dlx5, Runx2, osteopontin, and osteocalcin and formed a mineralized matrix. The expression of osteogenic markers and extent of mineralization was significantly higher when MSCs were encapsulated in SPELA hydrogel with the addition of bone morphogenetic protein-2 (BMP2). Results demonstrate that hydrolytically degradable PEG-based hydrogels are potentially useful as a delivery matrix for stem cells in regenerative medicine.