Engineered Fibrin Gels for Parallel Stimulation of Mesenchymal Stem Cell Proangiogenic and Osteogenic Potential

Using biomaterials to improve mesenchymal stem cell therapies for chronic, nonhealing wounds

Historically, treatment of chronic, nonhealing wounds has focused on managing symptoms using biomaterial-based wound dressings, which do not adequately address the underlying clinical issue. Mesenchymal stem cells (MSCs) are a promising cell-based therapy for the treatment of chronic, nonhealing wounds, yet inherent cellular heterogeneity and susceptibility to death during injection limit their clinical use. Recently, researchers have begun to explore the synergistic effects of combined MSC-biomaterial therapies, where the biomaterial serves as a scaffold to protect the MSCs and provides physiologically relevant physicochemical cues that can direct MSC immunomodulatory behavior. In this review, we highlight recent progress in this field with a focus on the most commonly used biomaterials, classified based on their source, including natural biomaterials, synthetic biomaterials, and the combination of natural and synthetic biomaterials. We also discuss current challenges regarding the clinical translation of these therapies, as well as a perspective on the future outlook of the field.

Supporting wound healing by mesenchymal stem cells (MSCs) therapy in combination with scaffold, hydrogel, and matrix; State of the art

Non-healing wounds impose a huge annual cost on the survival of different countries and large populations in the world. Wound healing is a complex and multi-step process, the speed and quality of which can be changed by various factors. To promote wound healing, compounds such as platelet-rich plasma, growth factors, platelet lysate, scaffolds, matrix, hydrogel, and cell therapy, in particular, with mesenchymal stem cells (MSCs) are suggested. Nowadays, the use of MSCs has attracted a lot of attention. These cells can induce their effect by direct effect and secretion of exosomes. On the other hand, scaffolds, matrix, and hydrogels provide suitable conditions for wound healing and the growth, proliferation, differentiation, and secretion of cells. In addition to generating suitable conditions for wound healing, the combination of biomaterials and MSCs increases the function of these cells at the site of injury by favoring their survival, proliferation, differentiation, and paracrine activity. In addition, other compounds such as glycol, sodium alginate/collagen hydrogel, chitosan, peptide, timolol, and poly(vinyl) alcohol can be used along with these treatments to increase the effectiveness of treatments in wound healing. In this review article, we take a glimpse into the merging scaffolds, hydrogels, and matrix application with MSCs therapy to favor wound healing.

A bioactive material with dual integrin-targeting ligands regulates specific endogenous cell adhesion and promotes vascularized bone regeneration in adult and fetal bone defects

Significant progress has been made in designing bone materials capable of directing endogenous cells to promote vascularized bone regeneration. However, current strategies lack regulation of the specific endogenous cell populations for vascularized bone regeneration, thus leading to adverse tissue formation and decreased regenerative efficiency. Here, we engineered a biomaterial to regulate endogenous cell adhesion and promote vascularized bone regeneration. The biomaterial works by presenting two synthetic ligands, LLP2A and LXW7, explicitly targeting integrins α4β1 and αvβ3, respectively, expressed on the surfaces of the cells related to bone formation and vascularization, such as mesenchymal stem cells (MSCs), osteoblasts, endothelial progenitor cells (EPCs), and endothelial cells (ECs). In vitro, the LLP2A/LXW7 modified biomaterial improved the adhesion of MSCs, osteoblasts, EPCs, and ECs via integrin α4β1 and αvβ3, respectively. In an adult rat calvarial bone defect model, the LLP2A/LXW7 modified biomaterial enhanced bone formation and vascularization by synergistically regulating endogenous cells with osteogenic and angiogenic potentials, such as DLX5+ cells, osteocalcin+ cells, CD34+/CD45- cells and CD31+ cells. In a fetal sheep spinal bone defect model, the LLP2A/LXW7 modified biomaterial augmented bone formation and vascularization without any adverse effects. This innovative biomaterial offers an off-the-shelf, easy-to-use, and biologically safe product suitable for vascularized bone regeneration in both fetal and adult disease environments.

Ectopic Bone Tissue Engineering in Mice Using Human Gingiva or Bone Marrow-Derived Stromal/Progenitor Cells in Scaffold-Hydrogel Constructs

Three-dimensional (3D) spheroid culture can promote the osteogenic differentiation and bone regeneration capacity of mesenchymal stromal cells (MSC). Gingiva-derived progenitor cells (GPC) represent a less invasive alternative to bone marrow MSC (BMSC) for clinical applications. The aim of this study was to test the in vivo bone forming potential of human GPC and BMSC cultured as 3D spheroids or dissociated cells (2D). 2D and 3D cells encapsulated in constructs of human platelet lysate hydrogels (HPLG) and 3D-printed poly (L-lactide-co-trimethylene carbonate) scaffolds (HPLG-PLATMC) were implanted subcutaneously in nude mice; cell-free HPLG-PLATMC constructs served as a control. Mineralization was assessed using micro-computed tomography (µCT), histology, scanning electron microscopy (SEM) and in situ hybridization (ISH). After 4–8 weeks, µCT revealed greater mineralization in 3D-BMSC vs. 2D-BMSC and 3D-GPC (p < 0.05), and a similar trend in 2D-GPC vs. 2D-BMSC (p > 0.05). After 8 weeks, greater mineralization was observed in cell-free constructs vs. all 2D- and 3D-cell groups (p < 0.05). Histology and SEM revealed an irregular but similar mineralization pattern in all groups. ISH revealed similar numbers of 2D and 3D BMSC/GPC within and/or surrounding the mineralized areas. In summary, spheroid culture promoted ectopic mineralization in constructs of BMSC, while constructs of dissociated GPC and BMSC performed similarly. The combination of HPLG and PLATMC represents a promising scaffold for bone tissue engineering applications.

Bone regeneration in rat calvarial defects using dissociated or spheroid mesenchymal stromal cells in scaffold-hydrogel constructs

Background

Three-dimensional (3D) spheroid culture can promote the osteogenic differentiation of bone marrow mesenchymal stromal cells (BMSC). 3D printing offers the possibility to produce customized scaffolds for complex bone defects. The aim of this study was to compare the potential of human BMSC cultured as 2D monolayers or 3D spheroids encapsulated in constructs of 3D-printed poly-L-lactide-co-trimethylene carbonate scaffolds and modified human platelet lysate hydrogels (PLATMC-HPLG) for bone regeneration.

Methods

PLATMC-HPLG constructs with 2D or 3D BMSC were assessed for osteogenic differentiation based on gene expression and in vitro mineralization. Subsequently, PLATMC-HPLG constructs with 2D or 3D BMSC were implanted in rat calvarial defects for 12 weeks; cell-free constructs served as controls. Bone regeneration was assessed via in vivo computed tomography (CT), ex vivo micro-CT and histology.

Results

Osteogenic gene expression was significantly enhanced in 3D versus 2D BMSC prior to, but not after, encapsulation in PLATMC-HPLG constructs. A trend for greater in vitro mineralization was observed in constructs with 3D versus 2D BMSC (p > 0.05). In vivo CT revealed comparable bone formation after 4, 8 and 12 weeks in all groups. After 12 weeks, micro-CT revealed substantial regeneration in 2D BMSC (62.47 ± 19.46%), 3D BMSC (51.01 ± 24.43%) and cell-free PLATMC-HPLG constructs (43.20 ± 30.09%) (p > 0.05). A similar trend was observed in the histological analysis.

Conclusion

Despite a trend for superior in vitro mineralization, constructs with 3D and 2D BMSC performed similarly in vivo. Regardless of monolayer or spheroid cell culture, PLATMC-HPLG constructs represent promising scaffolds for bone tissue engineering applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02642-w.

Spheroid Coculture of Human Gingiva-Derived Progenitor Cells With Endothelial Cells in Modified Platelet Lysate Hydrogels

Cell coculture strategies can promote angiogenesis within tissue engineering constructs. This study aimed to test the angiogenic potential of human umbilical vein endothelial cells (HUVEC) cocultured with gingiva-derived progenitor cells (GPC) as spheroids in a xeno-free environment. Human platelet lysate (HPL) was used as a cell culture supplement and as a hydrogel matrix (HPLG) for spheroid encapsulation. HUVEC and HUVEC + GPC (1:1 or 5:1) spheroids were encapsulated in various HPLG formulations. Angiogenesis was assessed via in vitro sprouting and in vivo chick chorioallantoic membrane (CAM) assays. HUVEC revealed characteristic in vitro sprouting in HPL/HPLG and this was significantly enhanced in cocultures with GPC (p < 0.05). A trend for greater sprouting was observed in 5:1 vs 1:1 HUVEC + GPC spheroids and in certain HPLG formulations (p > 0.05). Both HUVEC and HUVEC + GPC spheroids in HPLG revealed abundant and comparable neoangiogenesis in the CAM assay (p > 0.05). Spheroid coculture of HUVEC + GPC in HPLG represents a promising strategy to promote angiogenesis.

Multi-peptide presentation and hydrogel mechanics jointly enhance therapeutic duo-potential of entrapped stromal cells

The native extracellular matrix (ECM) contains a host of matricellular proteins and bioactive factors that regulate cell behavior, and many ECM components have been leveraged to guide cell fate. However, the large size and chemical characteristics of these constituents complicate their incorporation into biomaterials without interfering with material properties, motivating the need for alternative approaches to regulate cellular responses. Mesenchymal stromal cells (MSCs) can promote osseous regeneration in vivo directly or indirectly through multiple means including (1) secretion of proangiogenic and mitogenic factors to initiate formation of a vascular template and recruit host cells into the tissue site or (2) direct differentiation into osteoblasts. As MSC behavior is influenced by the properties of engineered hydrogels, we hypothesized that the biochemical and biophysical properties of alginate could be manipulated to promote the dual contributions of encapsulated MSCs toward bone formation. We functionalized alginate with QK peptide to enhance proangiogenic factor secretion and RGD to promote adhesion, while biomechanical-mediated osteogenic cues were controlled by modulating viscoelastic properties of the alginate substrate. A 1:1 ratio of QK:RGD resulted in the highest levels of both proangiogenic factor secretion and mineralization in vitro. Viscoelastic alginate outperformed purely elastic gels in both categories, and this effect was enhanced by stiffness up to 20 kPa. Furthermore, viscoelastic constructs promoted vessel infiltration and bone regeneration in a rat calvarial defect over 12 weeks. These data suggest that modulating viscoelastic properties of biomaterials, in conjunction with dual peptide functionalization, can simultaneously enhance multiple aspects of MSC regenerative potential and improve neovascularization of engineered tissues.

Osteogenic preconditioning in perfusion bioreactors improves vascularization and bone formation by human bone marrow aspirates

Cell-derived extracellular matrix (ECM) provides a niche to promote osteogenic differentiation, cell adhesion, survival, and trophic factor secretion. To determine whether osteogenic preconditioning would improve the bone-forming potential of unfractionated bone marrow aspirate (BMA), we perfused cells on ECM-coated scaffolds to generate naïve and preconditioned constructs, respectively. The composition of cells selected from BMA was distinct on each scaffold. Naïve constructs exhibited robust proangiogenic potential in vitro, while preconditioned scaffolds contained more mesenchymal stem/stromal cells (MSCs) and endothelial cells (ECs) and exhibited an osteogenic phenotype. Upon implantation into an orthotopic calvarial defect, BMA-derived ECs were present in vessels in preconditioned implants, resulting in robust perfusion and greater vessel density over the first 14 days compared to naïve implants. After 10 weeks, human ECs and differentiated MSCs were detected in de novo tissues derived from naïve and preconditioned scaffolds. These results demonstrate that bioreactor-based preconditioning augments the bone-forming potential of BMA.

Assessment of stem cell viability in the initial healing period in rabbits with a cranial bone defect according to the type and form of scaffold

PURPOSE

Increased bone regeneration has been achieved through the use of stem cells in combination with graft material. However, the survival of transplanted stem cells remains a major concern. The purpose of this study was to evaluate the viability of transplanted mesenchymal stem cells (MSCs) at an early time point (24 hours) based on the type and form of the scaffold used, including type I collagen membrane and synthetic bone.

METHODS

The stem cells were obtained from the periosteum of the otherwise healthy dental patients. Four symmetrical circular defects measuring 6 mm in diameter were made in New Zealand white rabbits using a trephine drill. The defects were grafted with 1) synthetic bone (β-tricalcium phosphate/hydroxyapatite [β-TCP/HA]) and 1×105 MSCs, 2) collagen membrane and 1×105 MSCs, 3) β-TCP/HA+collagen membrane and 1×105 MSCs, or 4) β-TCP/HA, a chipped collagen membrane and 1×105 MSCs. Cellular viability and the cell migration rate were analyzed.

RESULTS

Cells were easily separated from the collagen membrane, but not from synthetic bone. The number of stem cells attached to synthetic bone in groups 1, 3, and 4 seemed to be similar. Cellular viability in group 2 was significantly higher than in the other groups (P<0.05). The cell migration rate was highest in group 2, but this difference was not statistically significant (P>0.05).

CONCLUSIONS

This study showed that stem cells can be applied when a membrane is used as a scaffold under no or minimal pressure. When space maintenance is needed, stem cells can be loaded onto synthetic bone with a chipped membrane to enhance the survival rate.

Dual functional approaches for osteogenesis coupled angiogenesis in bone tissue engineering

Bone fracture healing is a multistep and overlapping process of inflammation, angiogenesis and osteogenesis. It is initiated by inflammation, causing the release of various cytokines and growth factors. It leads to the recruitment of stem cells and formation of vasculature resulting in the functional bone formation. This combined phenomenon is used by bone tissue engineers from past few years to address the problem of vasculature and osteogenic differentiation during bone regeneration. In this review, we have discussed all major studies reporting the dual functioning approach to promote osteogenesis coupled angiogenesis using various scaffolds. These scaffolds are broadly classified into four types based on the nature of their structural and functional components. The functionality of the scaffold is either due to the structural components or the loaded cargo which conducts or induces the coupled functionality. Dual delivery system for osteoinductive and angioinductive factors ensures the co-delivery of two different types of molecules to induce osteogenesis and angiogenesis. Single delivery scaffold for angioinductive and osteoinductive molecule releases single type of molecules which could induce both angiogenesis and osteogenesis. Osteoconductive scaffold consisted of bone constituents releases angioinductive factors. Osteoconductive and angioconductive scaffold composed of components which provide the native substrate features for osteogenesis and angiogenesis. This review article also discusses the studies highlighting the synergism of physico-chemical stimuli as dual functioning feature to enhance angiogenesis and osteogenesis simultaneously. In addition, this article covers one of the least discussed area of the bone regeneration i.e. 'cartilage formation as a median between angiogenesis and osteogenesis'.

Injectable mineralized microsphere-loaded composite hydrogels for bone repair in a sheep bone defect model

The efficacy of cell-based therapies as an alternative to autologous bone grafts requires biomaterials to localize cells at the defect and drive osteogenic differentiation. Hydrogels are ideal cell delivery vehicles that can provide instructional cues via their composition or mechanical properties but commonly lack osteoconductive components that nucleate mineral. To address this challenge, we entrapped mesenchymal stromal cells (MSCs) in a composite hydrogel based on two naturally-derived polymers (alginate and hyaluronate) containing biomineralized polymeric microspheres. Mechanical properties of the hydrogels were dependent upon composition. The presentation of the adhesive tripeptide Arginine-Glycine-Aspartic Acid (RGD) from both polymers induced greater osteogenic differentiation of ovine MSCs in vitro compared to gels formed of RGD-alginate or RGD-alginate/hyaluronate alone. We then evaluated the capacity of this construct to stimulate bone healing when transplanting autologous, culture-expanded MSCs into a surgical induced, critical-sized ovine iliac crest bone defect. At 12 weeks post-implantation, defects treated with MSCs transplanted in composite gels exhibited significant increases in blood vessel density, osteoid formation, and bone formation compared to acellular gels or untreated defects. These findings demonstrate the capacity of osteoconductive hydrogels to promote bone formation with autologous MSCs in a large animal bone defect model and provide a promising vehicle for cell-based therapies of bone healing.

Engineering fibrin hydrogels to promote the wound healing potential of mesenchymal stem cell spheroids

Mesenchymal stem cells (MSCs) secrete endogenous factors such as vascular endothelial growth factor (VEGF) and prostaglandin E2 (PGE2) that promote angiogenesis, modulate the inflammatory microenvironment, and stimulate wound repair, and MSC spheroids secrete more trophic factors than dissociated, individual MSCs. Compared to injection of cells alone, transplantation of MSCs in a biomaterial can enhance their wound healing potential by localizing cells at the defect site and upregulating trophic factor secretion. To capitalize on the therapeutic potential of spheroids, we engineered a fibrin gel delivery vehicle to simultaneously enhance the proangiogenic and anti-inflammatory potential of entrapped human MSC spheroids. We used multifactorial statistical analysis to determine the interaction between four input variables derived from fibrin gel synthesis on four output variables (gel stiffness, gel contraction, and secretion of VEGF and PGE2). Manipulation of the four input variables tuned fibrin gel biophysical properties to promote the simultaneous secretion of VEGF and PGE2 by entrapped MSC spheroids while maintaining overall gel integrity. MSC spheroids in stiffer gels secreted the most VEGF, while PGE2 secretion was highest in more compliant gels. Simultaneous VEGF and PGE2 secretion was greatest using hydrogels with intermediate mechanical properties, as small increases in stiffness increased VEGF secretion while maintaining PGE2 secretion by entrapped spheroids. The fibrin gel formulation predicted to simultaneously increase VEGF and PGE2 secretion stimulated endothelial cell proliferation, enhanced macrophage polarization, and promoted angiogenesis when used to treat a wounded three-dimensional human skin equivalent. These data demonstrate that a statistical approach is an effective strategy to formulate fibrin gel formulations that enhance the wound healing potential of human MSCs.

STATEMENT OF SIGNIFICANCE

Mesenchymal stem cells (MSCs) are under investigation for wound healing applications due to their secretion of bioactive factors that enhance granulation tissue formation, blood vessel ingrowth, and reduce inflammation. However, the effectiveness of cell-based therapies is reduced due to poor engraftment and high rates of cell death when transplanted into harsh environments characteristic of large wounds. Compared to dissociated cells, MSCs exhibit increased overall function when aggregated into three-dimensional spheroids, and transplantation of cells using biomaterials is one strategy for guiding cell function in the defect site. The present study demonstrates that the biophysical properties of fibrin hydrogels, designed for use as a cell carrier, can be engineered to dictate the secretion of bioactive factors by entrapped MSC spheroids. This strategy enables MSCs to contribute to wound healing by synergistically promoting neovascularization and modulating the inflammatory milieu.

A review of fibrin and fibrin composites for bone tissue engineering

Tissue engineering has emerged as a new treatment approach for bone repair and regeneration seeking to address limitations associated with current therapies, such as autologous bone grafting. While many bone tissue engineering approaches have traditionally focused on synthetic materials (such as polymers or hydrogels), there has been a lot of excitement surrounding the use of natural materials due to their biologically inspired properties. Fibrin is a natural scaffold formed following tissue injury that initiates hemostasis and provides the initial matrix useful for cell adhesion, migration, proliferation, and differentiation. Fibrin has captured the interest of bone tissue engineers due to its excellent biocompatibility, controllable biodegradability, and ability to deliver cells and biomolecules. Fibrin is particularly appealing because its precursors, fibrinogen, and thrombin, which can be derived from the patient's own blood, enable the fabrication of completely autologous scaffolds. In this article, we highlight the unique properties of fibrin as a scaffolding material to treat bone defects. Moreover, we emphasize its role in bone tissue engineering nanocomposites where approaches further emulate the natural nanostructured features of bone when using fibrin and other nanomaterials. We also review the preparation methods of fibrin glue and then discuss a wide range of fibrin applications in bone tissue engineering. These include the delivery of cells and/or biomolecules to a defect site, distributing cells, and/or growth factors throughout other pre-formed scaffolds and enhancing the physical as well as biological properties of other biomaterials. Thoughts on the future direction of fibrin research for bone tissue engineering are also presented. In the future, the development of fibrin precursors as recombinant proteins will solve problems associated with using multiple or single-donor fibrin glue, and the combination of nanomaterials that allow for the incorporation of biomolecules with fibrin will significantly improve the efficacy of fibrin for numerous bone tissue engineering applications.

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

There is a substantial need to prolong cell persistence and enhance functionality in situ to enhance cell-based tissue repair. Bone morphogenetic protein-2 (BMP-2) is often used at high concentrations for osteogenic differentiation of mesenchymal stem cells (MSCs) but can induce apoptosis. Biomaterials facilitate the delivery of lower doses of BMP-2, reducing side effects and localizing materials at target sites. Photocrosslinked alginate hydrogels (PAHs) can deliver osteogenic materials to irregular-sized bone defects, providing improved control over material degradation compared to ionically cross-linked hydrogels. It is hypothesized that the delivery of MSCs and BMP-2 from a PAH increases cell persistence by reducing apoptosis, while promoting osteogenic differentiation and enhancing bone formation compared to MSCs in PAHs without BMP-2. BMP-2 significantly decreases apoptosis and enhances survival of photoencapsulated MSCs, while simultaneously promoting osteogenic differentiation in vitro. Bioluminescence imaging reveals increased MSC survival when implanted in BMP-2 PAHs. Bone defects treated with MSCs in BMP-2 PAHs demonstrate 100% union as early as 8 weeks and significantly higher bone volumes at 12 weeks, while defects with MSC-entrapped PAHs alone do not fully bridge. This study demonstrates that transplantation of MSCs with BMP-2 in PAHs achieves robust bone healing, providing a promising platform for bone repair.

Mesenchymal Stem Cell Spheroids Retain Osteogenic Phenotype Through α2β1 Signaling

The induction of mesenchymal stem cells (MSCs) toward the osteoblastic lineage using osteogenic supplements prior to implantation is one approach under examination to enhance their bone-forming potential. Spheroids formed from induced cells exhibited improved retention of osteogenic markers as a function of integrin binding to cell-secreted extracellular matrix (ECM). These results demonstrate the capacity of spheroidal culture to sustain the mineral-producing phenotype of MSCs, thus enhancing their contribution toward bone formation and repair.

The induction of mesenchymal stem cells (MSCs) toward the osteoblastic lineage using osteogenic supplements prior to implantation is one approach under examination to enhance their bone-forming potential. MSCs rapidly lose their induced phenotype upon removal of the soluble stimuli; however, their bone-forming potential can be sustained when provided with continued instruction via extracellular matrix (ECM) cues. In comparison with dissociated cells, MSC spheroids exhibit improved survival and secretion of trophic factors while maintaining their osteogenic potential. We hypothesized that entrapment of MSC spheroids formed from osteogenically induced cells would exhibit better preservation of their bone-forming potential than would dissociated cells from monolayer culture. Spheroids exhibited comparable osteogenic potential and increased proangiogenic potential with or without osteogenic preconditioning versus monolayer-cultured MSCs. Spheroids were then entrapped in collagen hydrogels, and the osteogenic stimulus was removed. In comparison with entrapped dissociated MSCs, spheroids exhibited significantly increased markers of osteogenic differentiation. The capacity of MSC spheroids to retain their osteogenic phenotype upon withdrawal of inductive cues was mediated by α₂β₁ integrin binding to cell-secreted ECM. These results demonstrate the capacity of spheroidal culture to sustain the mineral-producing phenotype of MSCs, thus enhancing their contribution toward bone formation and repair.

Significance

Despite the promise of mesenchymal stem cells (MSCs) for cell-based therapies for tissue repair and regeneration, there is little evidence that transplanted MSCs directly contribute to new bone formation, suggesting that induced cells rapidly lose their osteogenic phenotype or undergo apoptosis. In comparison with dissociated cells, MSC spheroids exhibit increased trophic factor secretion and improved cell survival. The loss of phenotype represents a significant clinical challenge for cell therapies, yet there is no evidence for whether MSC spheroids retain their osteogenic phenotype upon entrapment in a clinically relevant biomaterial. These findings demonstrate that MSC spheroids retain their osteogenic phenotype better than do dissociated MSCs, and this is due to integrin engagement with the cell-secreted extracellular matrix. These data provide evidence for a novel approach for potentiating the use of MSCs in bone repair.

Neurogenic potential of engineered mesenchymal stem cells overexpressing VEGF

Numerous signaling molecules are altered following nerve injury, serving as a blueprint for drug delivery approaches that promote nerve repair. However, challenges with achieving the appropriate temporal duration of recombinant protein delivery have limited the therapeutic success of this approach. Genetic engineering of mesenchymal stem cells (MSCs) to enhance the secretion of proangiogenic molecules such as vascular endothelial growth factor (VEGF) may provide an alternative. We hypothesized that the administration of VEGF-expressing human MSCs would stimulate neurite outgrowth and proliferation of cell-types involved in neural repair. When cultured with dorsal root ganglion (DRG) explants in vitro, control and VEGF-expressing MSCs (VEGF-MSCs) increased neurite extension and proliferation of Schwann cells (SCs) and endothelial cells, while VEGF-MSCs stimulated significantly greater proliferation of endothelial cells. When embedded within a 3D fibrin matrix, VEGF-MSCs maintained overexpression and expressed detectable levels over 21 days. After transplantation into a murine sciatic nerve injury model, VEGF-MSCs maintained high VEGF levels for 2 weeks. This study provides new insight into the role of VEGF on peripheral nerve injury and the viability of transplanted genetically engineered MSCs. The study aims to provide a framework for future studies with the ultimate goal of developing an improved therapy for nerve repair.

Hydrogel biophysical properties instruct coculture-mediated osteogenic potential

Cell-based approaches for bone formation require instructional cues from the surrounding environment. As an alternative to pharmacological strategies or transplanting single cell populations, one approach is to coimplant populations that can establish a new vasculature and differentiate to bone-forming osteoblasts. Mesenchymal stem/stromal cells (MSCs) possess osteogenic potential and produce numerous angiogenic growth factors. Endothelial colony-forming cells (ECFCs) are a subpopulation of endothelial progenitor cells capable of vasculogenesis in vivo and may provide endogenous cues to support MSC function. We investigated the contribution of the carrier biophysical properties to instruct entrapped human MSCs and ECFCs to simultaneously promote their osteogenic and proangiogenic potential. Compared with gels containing MSCs alone, fibrin gels engineered with increased compressive stiffness simultaneously increased the osteogenic and proangiogenic potential of entrapped cocultured cells. ECFCs produced bone morphogenetic protein-2 (BMP-2), a potent osteoinductive molecule, and increases in BMP-2 secretion correlated with gel stiffness. Coculture of MSCs with ECFCs transduced to knockdown BMP-2 production abrogated the osteogenic response to levels observed with MSCs alone. These results demonstrate that physical properties of engineered hydrogels modulate the function of cocultured cells in the absence of inductive cues, thus increasing the translational potential of coimplantation to speed bone formation and repair.