Mineralized Synthetic Matrices as an Instructive Microenvironment for Osteogenic Differentiation of Human Mesenchymal Stem Cells

Influence of extracellular cues of hydrogel biomaterials on stem cell fate

Tissue engineering is a multidisciplinary field that focuses on creating functional tissue through the combination of biomimetic scaffolds, a cell source, and biochemical/physiochemical cues. Stem cells are often used as the cell source due to their multipotent properties and autologous sourcing; however, the combination of physical and chemical cues that regulate their behavior creates challenges in reproducibly directing them to a specific fate. Hydrogel biomaterials are widely explored as tissue scaffolds due to their innate biomimetic properties and tailorability. For these constructs to be successful, properties such as surface chemistry and spatial configuration, stiffness, and degradability of the biomaterial used for the scaffold framework should be analogous to the natural environment of the tissue they are repairing/replacing. This is imperative, as cues from the surrounding extracellular matrix (ECM) influence stem cell behavior and direct cell differentiation to a specific lineage. Hydrogels offer great promise as tools to control stem cell fate, as researchers can modulate the degradation rates, mechanical properties, swelling behavior, and chemical properties of the biomaterial scaffold to mimic the instructive cues of the native ECM. Discussion of the advantages and challenges of utilizing hydrogel biomaterials as the basis of tissue scaffolds is reviewed herein, as well as specific examples of hydrogels in tissue engineering and advances in hydrogel research to achieve desired cell phenotypes.

Silk Biomaterials for Bone Tissue Engineering

Silk is a natural fibrous polymer with application potential in regenerative medicine. Increasing interest remains for silk materials in bone tissue engineering due to their characteristics in biocompatibility, biodegradability and mechanical properties. Plenty of the in vitro and in vivo studies confirmed the advantages of silk in accelerating bone regeneration. Silk is processed into scaffolds, hydrogels, and films to facilitate different bone regenerative applications. Bioactive factors such as growth factors and drugs, and stem cells are introduced to silk-based matrices to create friendly and osteogenic microenvironments, directing cell behaviors and bone regeneration. The recent progress in silk-based bone biomaterials is discussed and focused on different fabrication and functionalization methods related to osteogenesis. The challenges and potential targets of silk bone materials are highlighted to evaluate the future development of silk-based bone materials.

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

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

Biomaterial-assisted local and systemic delivery of bioactive agents for bone repair

Although bone tissues possess an intrinsic capacity for repair, there are cases where bone healing is either impaired or insufficient, such as fracture non-union, osteoporosis, osteomyelitis, and cancers. In these cases, treatments like surgical interventions are used, either alone or in combination with bioactive agents, to promote tissue repair and manage associated clinical complications. Improving the efficacy of bioactive agents often requires carriers, with biomaterials being a pivotal player. In this review, we discuss the role of biomaterials in realizing the local and systemic delivery of biomolecules to the bone tissue. The versatility of biomaterials enables design of carriers with the desired loading efficiency, release profile, and on-demand delivery. Besides local administration, systemic administration of drugs is necessary to combat diseases like osteoporosis, warranting bone-targeting drug delivery systems. Thus, chemical moieties with the affinity towards bone extracellular matrix components like apatite minerals have been widely utilized to create bone-targeting carriers with better biodistribution, which cannot be achieved by the drugs alone. Bone-targeting carriers combined with the desired drugs or bioactive agents have been extensively investigated to enhance bone healing while minimizing off-target effects. Herein, these advancements in the field have been systematically reviewed. STATEMENT OF SIGNIFICANCE: Drug delivery is imperative when surgical interventions are not sufficient to address various bone diseases/defects. Biomaterial-assisted delivery systems have been designed to provide drugs with the desired loading efficiency, sustained release, and on-demand delivery to enhance bone healing. By surveying recent advances in the field, this review outlines the design of biomaterials as carriers for the local and systemic delivery of bioactive agents to the bone tissue. Particularly, biomaterials that bear chemical moieties with affinity to bone are attractive, as they can present the desired bioactive agents to the bone tissue efficiently and thus enhance the drug efficacy for bone repair.

Tissue engineered bone mimetics to study bone disorders ex vivo: Role of bioinspired materials

Recent advances in materials development and tissue engineering has resulted in a substantial number of bioinspired materials that recapitulate cardinal features of bone extracellular matrix (ECM) such as dynamic inorganic and organic environment(s), hierarchical organization, and topographical features. Bone mimicking materials, as defined by its self-explanatory term, are developed based on the current understandings of the natural bone ECM during development, remodeling, and fracture repair. Compared to conventional plastic cultures, biomaterials that resemble some aspects of the native environment could elicit a more natural molecular and cellular response relevant to the bone tissue. Although current bioinspired materials are mainly developed to assist tissue repair or engineer bone tissues, such materials could nevertheless be applied to model various skeletal diseases in vitro. This review summarizes the use of bioinspired materials for bone tissue engineering, and their potential to model diseases of bone development and remodeling ex vivo. We largely focus on biomaterials, designed to re-create different aspects of the chemical and physical cues of native bone ECM. Employing these bone-inspired materials and tissue engineered bone surrogates to study bone diseases has tremendous potential and will provide a closer portrayal of disease progression and maintenance, both at the cellular and tissue level. We also briefly touch upon the application of patient-derived stem cells and introduce emerging technologies such as organ-on-chip in disease modeling. Faithful recapitulation of disease pathologies will not only offer novel insights into diseases, but also lead to enabling technologies for drug discovery and new approaches for cell-based therapies.

Stimuli-Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

Supramolecular hydrogels are a class of self-assembled network structures formed via non-covalent interactions of the hydrogelators. These hydrogels capable of responding to external stimuli are considered to be smart materials due to their ability to undergo sol-gel and/or gel-sol transition upon subtle changes in their surroundings. Such stimuli-responsive hydrogels are intriguing biomaterials with applications in tissue engineering, delivery of cells and drugs, modulating tissue environment to promote innate tissue repair, and imaging for medical diagnostics among others. This review summarizes the recent developments in stimuli-responsive supramolecular hydrogels and their potential applications in regenerative medicine. Specifically, various structural aspects of supramolecular hydrogelators involved in self-assembly, the role of external stimuli in tuning/controlling their phase transitions, and how these functions could be harnessed to advance applications in regenerative medicine are focused on. Finally, the key challenges and future prospects for these versatile materials are briefly described.

Effect of age on biomaterial-mediated in situ bone tissue regeneration

Emerging studies show the potential application of synthetic biomaterials that are intrinsically osteoconductive and osteoinductive as bone grafts to treat critical bone defects. Here, the biomaterial not only assists recruitment of endogenous cells, but also supports cellular activities relevant to bone tissue formation and function. While such biomaterial-mediated in situ tissue engineering is highly attractive, success of such an approach relies largely on the regenerative potential of the recruited cells, which is anticipated to vary with age. In this study, we investigated the effect of the age of the host on mineralized biomaterial-mediated bone tissue repair using critical-sized cranial defects as a model system. Mice of varying ages, 1-month-old (juvenile), 2-month-old (young-adult), 6-month-old (middle-aged), and 14-month-old (elderly), were used as recipients. Our results show that the bio-mineralized scaffolds support bone tissue formation by recruiting endogenous cells for all groups albeit with differences in an age-related manner. Analyses of bone tissue formation after 2 and 8 weeks post-treatment show low mineral deposition and reduced number of osteocalcin and tartrate-resistant acid phosphatase (TRAP)-expressing cells in elderly mice.

STATEMENT OF SIGNIFICANCE

Tissue engineering strategies that promote tissue repair through recruitment of endogenous cells will have a significant impact in regenerative medicine. Previous studies from our group have shown that biomineralized materials containing calcium phosphate minerals can contribute to neo-bone tissue through recruitment and activation of endogenous cells. In this study, we investigated the effect of age of the recipient on biomaterial-mediated bone tissue repair. Our results show that the age of the recipient mouse had a significant impact on the quality and quantity of the engineered neo-bone tissues, in which delayed/compromised bone tissue formation was observed in older mice. These findings are in agreement with the clinical observations that age of patients is a key factor in bone repair.

Functionally graded multilayer scaffolds for in vivo osteochondral tissue engineering

Osteochondral tissue repair remains a significant challenge in orthopedic surgery. Tissue engineering of osteochondral tissue has transpired as a potential therapeutic solution as it can effectively regenerate bone, cartilage, and the bone-cartilage interface. While advancements in scaffold fabrication and stem cell engineering have made significant progress towards the engineering of composite tissues, such as osteochondral tissue, new approaches are required to improve the outcome of such strategies. Herein, we discuss the use of a single-unit trilayer scaffold with depth-varying pore architecture and mineral environment to engineer osteochondral tissues in vivo. The trilayer scaffold includes a biomineralized bottom layer mimicking the calcium phosphate (CaP)-rich bone microenvironment, a cryogel middle layer with anisotropic pore architecture, and a hydrogel top layer. The mineralized bottom layer was designed to support bone formation, while the macroporous middle layer and hydrogel top layer were designed to support cartilage tissue formation. The bottom layer was kept acellular and the top two layers were loaded with cells prior to implantation. When implanted in vivo, these trilayer scaffolds resulted in the formation of osteochondral tissue with a lubricin-rich cartilage surface. The osteochondral tissue formation was a result of continuous differentiation of the transplanted cells to form cartilage tissue and recruitment of endogenous cells through the mineralized bottom layer to form bone tissue. Our results suggest that integrating exogenous cell-based cartilage tissue engineering along with scaffold-driven in situ bone tissue engineering could be a powerful approach to engineer analogs of osteochondral tissue. In addition to offering new therapeutic opportunities, such approaches and systems could also advance our fundamental understanding of osteochondral tissue regeneration and repair.

STATEMENT OF SIGNIFICANCE

In this work, we describe the use of a single-unit trilayer scaffold with depth-varying pore architecture and mineral environment to engineer osteochondral tissues in vivo. The trilayer scaffold was designed to support continued differentiation of the donor cells to form cartilage tissue while supporting bone formation through recruitment of endogenous cells. When implanted in vivo, these trilayer scaffolds partially loaded with cells resulted in the formation of osteochondral tissue with a lubricin-rich cartilage surface. Approaches such as the one presented here that integrates ex vivo tissue engineering along with endogenous cell-mediated tissue engineering can have a significant impact in tissue engineering composite tissues with diverse cell populations and functionality.

Fabrication of in vitro 3D mineralized tissue by fusion of composite spheroids incorporating biomineral-coated nanofibers and human adipose-derived stem cells

Development of a bone-like 3D microenvironment with stem cells has always been intriguing in bone tissue engineering. In this study, we fabricated composite spheroids by combining functionalized fibers and human adipose-derived stem cells (hADSCs), which were fused to form a 3D mineralized tissue construct. We prepared fragmented poly (ι-lactic acid) (PLLA) fibers approximately 100 μm long by partial aminolysis of electrospun fibrous mesh. PLLA fibers were then biomineralized with various concentrations of NaHCO₃ (0.005, 0.01, and 0.04 M) to form mineralized fragmented fibers (mFF1, mFF2, and mFF3, respectively). SEM analysis showed that the minerals in mFF2 and mFF3 completely covered the fiber surface, and surface chemistry analysis confirmed the presence of hydroxyapatite peaks. Additionally, mFFs formed composite spheroids with hADSCs, demonstrating that the cells were strongly attached to mFFs and homogeneously distributed throughout the spheroid. In vitro culture of spheroids in the media without osteogenic supplements showed significantly enhanced expression of osteogenic genes including Runx2 (20.83 ± 2.83 and 22.36 ± 2.18 fold increase), OPN (14.24 ± 1.71 and 15.076 ± 1.38 fold increase), and OCN (4.36 ± 0.41 and 5.63 ± 0.51 fold increase) in mFF2 and mFF3, respectively, compared to the no mineral fiber group. In addition, mineral contents were significantly increased at day 7. Blocking the biomineral-mediated signaling by PSB 603 significantly down regulated the expression of these genes in mFF3 at day 7. Finally, we fused composite spheroids to form a mineralized 3D tissue construct, which maintained the viability of cells and showed pervasively distributed minerals within the structure. Our composite spheroids could be used as an alternative platform for the development of in vitro bone models, in vivo cell carriers, and as building blocks for bioprinting 3D bone tissue.

STATEMENT OF SIGNIFICANCE

This manuscript described our recent work for the preparation of biomimeral-coated fibers that can be assembled with mesenchymal stem cells and provide bone-like environment for directed control over osteogenic differentiation. Biomineral coating onto synthetic, biodegradable single fibers was successfully carried out using multiple steps, combination of template protein coating inspired from mussel adhesion and charge-charge interactions between template proteins and mineral ions. The biomineral-coated single micro-scale fibers (1-2.5 μm in diameter) were then assembled with human adipose tissue derived stem cells (hADSCs). The assembled structure exhibited spheroidal architecture with few hundred micrometers. hADSCs within the spheroids were differentiated into osteogenic lineage in vitro and mineralized in the growth media. These spheroids were fused to form in vitro 3D mineralized tissue with larger size.

Mineralized alginate hydrogels using marine carbonates for bone tissue engineering applications

The search for an ideal bone tissue replacement has led to the development of new composite materials designed to simulate the complex inorganic/organic structure of bone. The present work is focused on the development of mineralized calcium alginate hydrogels by the addition of marine derived calcium carbonate biomineral particles. Following a novel approach, we were able to obtain calcium carbonate particles of high purity and complex micro and nanostructure dependent on the source material. Three different types of alginates were selected to develop inorganic/organic scaffolds in order to correlate alginate composition with scaffold properties and cell behavior. The incorporation of calcium carbonates into alginate networks was able to promote extracellular matrix mineralization and osteoblastic differentiation of mesenchymal stem cells when added at 7 mg/ml. We demonstrated that the selection of the alginate type and calcium carbonate origin is crucial to obtain adequate systems for bone tissue engineering as they modulate the mechanical properties and cell differentiation.

Mineralized Biomaterials Mediated Repair of Bone Defects Through Endogenous Cells

Synthetic biomaterials that create a dynamic calcium (Ca²⁺)-, phosphate (PO₄^3-) ion-, and calcium phosphate (CaP)-rich microenvironment, similar to that found in native bone tissue, have been shown to promote osteogenic commitment of stem cells in vitro and in vivo. The intrinsic osteoconductivity and osteoinductivity of such biomaterials make them promising bone grafts for the treatment of bone defects. We thus aimed to evaluate the potential of mineralized biomaterials to induce bone repair of a critical-sized cranial defect in the absence of exogenous cells and growth factors. Our results demonstrate that the mineralized biomaterial alone can support complete bone formation within critical-sized bone defects through recruitment of endogenous cells and neo-bone tissue formation in mice. The newly formed bone tissue recapitulated many key characteristics of native bone such as formation of bone minerals reaching similar bone mineral density, presence of bone-forming osteoblasts and tartrate-resistant acid phosphatase-expressing osteoclasts, as well as vascular networks. Biomaterials that recruit endogenous cells and provide a tissue-specific microenvironment to modulate cellular behavior and support generation of functional tissues are a key step forward in moving bench-side tissue engineering approaches to the bedside. Such tissue engineering strategies could eventually pave the path toward readily available therapies that significantly reduce patient cost of care and improve overall clinical outcomes.

Poly(N-acryloyl glycinamide): a fascinating polymer that exhibits a range of properties from UCST to high-strength hydrogels

This feature article introduces the diverse intriguing properties of poly(N-acryloyl glycinamide) aqueous systems spanning from low to high concentrations.

iPS cell technologies and their prospect for bone regeneration and disease modeling: A mini review

Bone disorders are a group of varied acute and chronic traumatic, degenerative, malignant or congenital conditions affecting the musculoskeletal system. They are prevalent in society and, with an ageing population, the incidence and impact on the population’s health is growing. Severe persisting pain and limited mobility are the major symptoms of the disorder that impair the quality of life in affected patients. Current therapies only partially treat the disorders, offering management of symptoms, or temporary replacement with inert materials. However, during the last few years, the options for the treatment of bone disorders have greatly expanded, thanks to the advent of regenerative medicine. Skeletal cell-based regeneration medicine offers promising reparative therapies for patients. Mesenchymal stem (stromal) cells from different tissues have been gradually translated into clinical practice; however, there are a number of limitations. The introduction of reprogramming methods and the subsequent production of induced pluripotent stem cells provides a possibility to create human-specific models of bone disorders. Furthermore, human-induced pluripotent stem cell-based autologous transplantation is considered to be future breakthrough in the field of regenerative medicine. The main goal of the present paper is to review recent applications of induced pluripotent stem cells in bone disease modeling and to discuss possible future therapy options. The present article contributes to the dissemination of scientific and pre-clinical results between physicians, mainly orthopedist and thus supports the translation to clinical practice.

Repair of volumetric bone defects with a high strength BMP-loaded-mineralized hydrogel tubular scaffold

A high strength and BMP-2-loaded tubular scaffold was engineered by in situ mineralization of a supramolecular hydrogel. This tubular scaffold could lead to an efficient volumetric bone repair.

In vivo comparison of biomineralized scaffold-directed osteogenic differentiation of human embryonic and mesenchymal stem cells

Human pluripotent stem cells such as embryonic stem cells (hESCs) and multipotent stem cells like mesenchymal stem cells (hMSCs) hold great promise as potential cell sources for bone tissue regeneration. Comparing the in vivo osteogenesis of hESCs and hMSCs by biomaterial-based cues provides insight into the differentiation kinetics of these cells as well as their potential to contribute to bone tissue repair in vivo. Here, we compared in vivo osteogenic differentiation of hESCs and hMSCs within osteoinductive calcium phosphate (CaP)-bearing biomineralized scaffolds that recapitulate a bone-specific mineral microenvironment. Both hESCs and hMSCs underwent osteogenic differentiation responding to the biomaterial-based instructive cues. Furthermore, hMSCs underwent earlier in vivo osteogenesis compared to hESCs, but both stem cell types acquired a similar osteogenic maturation by 8 weeks of implantation.

Hydrogels as Extracellular Matrix Analogs

The extracellular matrix (ECM) is the non-cellular component of tissue that provides physical scaffolding to cells. Emerging studies have shown that beyond structural support, the ECM provides tissue-specific biochemical and biophysical cues that are required for tissue morphogenesis and homeostasis. Hydrogel-based platforms have played a key role in advancing our knowledge of the role of ECM in regulating various cellular functions. Synthetic hydrogels allow for tunable biofunctionality, as their material properties can be tailored to mimic those of native tissues. This review discusses current advances in the design of hydrogels with defined physical and chemical properties. We also highlight research findings that demonstrate the impact of matrix properties on directing stem cell fate, such as self-renewal and differentiation. Recent and future efforts towards understanding cell-material interactions will not only advance our basic understanding, but will also help design tissue-specific matrices and delivery systems to transplant stem cells and control their response in vivo.

Small molecule-driven direct conversion of human pluripotent stem cells into functional osteoblasts

The abilities of human pluripotent stem cells (hPSCs) to proliferate without phenotypic alteration and to differentiate into tissue-specific progeny make them a promising cell source for regenerative medicine and development of physiologically relevant in vitro platforms. Despite this potential, efficient conversion of hPSCs into tissue-specific cells still remains a challenge. Herein, we report direct conversion of hPSCs into functional osteoblasts through the use of adenosine, a naturally occurring nucleoside in the human body. The hPSCs treated with adenosine not only expressed the molecular signatures of osteoblasts but also produced calcified bone matrix. Our findings show that the adenosine-mediated osteogenesis of hPSCs involved the adenosine A2bR. When implanted in vivo, using macroporous synthetic matrices, the human induced pluripotent stem cell (hiPSC)-derived donor cells participated in the repair of critical-sized bone defects through the formation of neobone tissue without teratoma formation. The newly formed bone tissues exhibited various attributes of the native tissue, including vascularization and bone resorption. To our knowledge, this is the first demonstration of adenosine-induced differentiation of hPSCs into functional osteoblasts and their subsequent use to regenerate bone tissues in vivo. This approach that uses a physiologically relevant single small molecule to generate hPSC-derived progenitor cells is highly appealing because of its simplicity, cost-effectiveness, scalability, and impact in cell manufacturing, all of which are decisive factors for successful translational applications of hPSCs.

Adenosine Signaling Mediates Osteogenic Differentiation of Human Embryonic Stem Cells on Mineralized Matrices

Human embryonic stem cells (hESCs) are attractive cell sources for tissue engineering and regenerative medicine due to their self-renewal and differentiation ability. Design of biomaterials with an intrinsic ability that promotes hESC differentiation to the targeted cell type boasts significant advantages for tissue regeneration. We have previously developed biomineralized calcium phosphate (CaP) matrices that inherently direct osteogenic differentiation of hESCs without the need of osteogenic-inducing chemicals or growth factors. Here, we show that CaP matrix-driven osteogenic differentiation of hESCs occurs through A2b adenosine receptor (A2bR). The inhibition of the receptor with an A2bR-specific antagonist attenuated mineralized matrix-mediated osteogenic differentiation of hESCs. In addition, when cultured on matrices in an environment deficient of CaP minerals, exogenous adenosine promoted osteogenic differentiation of hESCs, but was attenuated by the inhibition of A2bR. Such synthetic matrices that intrinsically support osteogenic commitment of hESCs are not only beneficial for bone tissue engineering but can also be used as a platform to study the effect of the physical and chemical cues to the extracellular milieu on stem cell commitment. Insights into the cell signaling during matrix-induced differentiation of stem cells will also help define the key processes and enable discovery of new targets that promote differentiation of pluripotent stem cells for bone tissue engineering.

Improvement of Distribution and Osteogenic Differentiation of Human Mesenchymal Stem Cells by Hyaluronic Acid and β-Tricalcium Phosphate-Coated Polymeric Scaffold In Vitro

Bone tissue engineering requires a well-designed scaffold that can be biodegradable, biocompatible, and support the stem cells to osteogenic differentiation. Porous polycaprolactone (PCL) scaffold prepared by fused deposition modeling is an attractive biomaterial that has been used in clinic. However, PCL scaffolds lack biological function and osteoinductivity. In this study, we functionalized the PCL scaffolds by embedding them with a matrix of hyaluronic acid/β-tricalcium phosphate (HA/TCP). Human mesenchymal stem cells (MSCs) were cultured on scaffolds with and without coating to investigate proliferation and osteogenic differentiation. The DNA amount was significantly higher in the HA/TCP-coated scaffold on day 21. At the gene expression level, HA/TCP coating significantly increased the expression of ALP and COLI on day 4. These data correlated with the ALP activity peaking on day 7 in the HA/TCP-coated scaffold. Scanning electron microscope and histological analysis revealed that the cell matrix and calcium deposition were distributed more uniformly in the coated scaffolds compared to scaffolds without coating. In conclusion, the HA/TCP coating improved cellular proliferation, osteogenic differentiation, and uniform distribution of the cellular matrix in vitro. The HA/TCP-PCL scaffold holds great promise to accommodate human bone marrow-derived MSCs for bone reconstruction purposes, which warrants future in vivo studies.

Synthetic bone mimetic matrix-mediated in situ bone tissue formation through host cell recruitment

Advances in tissue engineering have offered new opportunities to restore anatomically and functionally compromised tissues. Although traditional tissue engineering approaches that utilize biomaterials and cells to create tissue constructs for implantation or biomaterials as a scaffold to deliver cells are promising, strategies that can activate endogenous cells to promote tissue repair are more clinically attractive. Here, we demonstrate that an engineered injectable matrix mimicking a calcium phosphate (CaP)-rich bone-specific microenvironment can recruit endogenous cells to form bone tissues in vivo. Comparison of matrix alone with that of bone marrow-soaked or bFGF-soaked matrix demonstrates similar extent of neo-bone formation and bridging of decorticated transverse processes in a posterolateral lumbar fusion rat model. Synthetic biomaterials that stimulate endogenous cells without the need for biologics to assist tissue repair could circumvent limitations associated with conventional tissue engineering approaches, including ex vivo cell processing and laborious efforts, thereby accelerating the translational aspects of regenerative medicine.

Biomineralized matrices dominate soluble cues to direct osteogenic differentiation of human mesenchymal stem cells through adenosine signaling

Stem cell differentiation is determined by a repertoire of signals from its microenvironment, which includes the extracellular matrix (ECM) and soluble cues. The ability of mesenchymal stem cells (MSCs), a common precursor for the skeletal system, to differentiate into osteoblasts and adipocytes in response to their local cues plays an important role in skeletal tissue regeneration and homeostasis. In this study, we investigated whether a bone-specific calcium phosphate (CaP) mineral environment could induce osteogenic differentiation of human MSCs, while inhibiting their adipogenic differentiation, in the presence of adipogenic-inducing medium. We also examined the mechanism through which the mineralized matrix suppresses adipogenesis of hMSCs to promote their osteogenic differentiation. Our results show that hMSCs cultured on mineralized matrices underwent osteogenic differentiation despite being cultured in the presence of adipogenic medium, which indicates the dominance of matrix-based cues of the mineralized matrix in directing osteogenic commitment of stem cells. Furthermore, the mineralized matrix-driven attenuation of adipogenesis was reversed with the inhibition of A2b adenosine receptor (A2bR), implicating a role of adenosine signaling in mineralized environment-mediated inhibition of adipogenesis. Such synthetic matrices with an intrinsic ability to direct differentiation of multipotent adult stem cells toward a targeted phenotype while inhibiting their differentiation into other lineages not only will be a powerful tool in delineating the role of complex microenvironmental cues on stem cell commitment but also will contribute to functional tissue engineering and their translational applications.

Mineralized gelatin methacrylate-based matrices induce osteogenic differentiation of human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSC) are a promising cell source with pluripotency and self-renewal properties. Design of simple and robust biomaterials with an innate ability to induce lineage-specificity of hiPSC is desirable to realize their application in regenerative medicine. In this study, the potential of biomaterials containing calcium phosphate minerals to induce osteogenic differentiation of hiPSC was investigated. hiPSC cultured using mineralized gelatin methacrylate-based matrices underwent osteogenic differentiation ex vivo, in both two-dimensional and three-dimensional cultures, in growth medium devoid of any osteogenic-inducing chemical components or growth factors. The findings that osteogenic differentiation of hiPSC can be achieved through biomaterial-based cues alone present new avenues for personalized regenerative medicine. Such biomaterials that could not only act as structural scaffolds, but could also provide tissue-specific functions such as directing stem cell differentiation commitment, have great potential in bone tissue engineering.

Biomimetic Material-Assisted Delivery of Human Embryonic Stem Cell Derivatives for Enhanced In Vivo Survival and Engraftment

The ability of human embryonic stem cells (hESCs) and their derivatives to differentiate and contribute to tissue repair has enormous potential to treat various debilitating diseases. However, improving the in vivo viability and function of the transplanted cells, a key determinant of translating cell-based therapies to the clinic, remains a daunting task. Here, we develop a hybrid biomaterial consisting of hyaluronic acid (HA) grafted with 6-aminocaproic acid moieties (HA-6ACA) to improve cell delivery and their subsequent in vivo function using skeletal muscle as a model system. Our findings show that the biomimetic material-assisted delivery of hESC-derived myogenic progenitor cells into cardiotoxin-injured skeletal muscles of NOD/SCID mice significantly promotes survival and engraftment of transplanted cells in a dose-dependent manner. The donor cells were found to contribute to the regeneration of damaged muscle fibers and to the satellite cell (muscle specific stem cells) compartment. Such biomimetic cell delivery vehicles that are cost-effective and easy-to-synthesize could play a key role in improving the outcomes of other stem cell-based therapies.

Biomineralized matrix-assisted osteogenic differentiation of human embryonic stem cells

The physical and chemical properties of a matrix play an important role in determining various cellular behaviors, including lineage specificity. We demonstrate that the differentiation commitment of human embryonic stem cells (hESCs), both in vitro and in vivo, can be solely achieved through synthetic biomaterials. hESCs were cultured using mineralized synthetic matrices mimicking a calcium phosphate (CaP)-rich bone environment differentiated into osteoblasts in the absence of any osteogenic inducing supplements. When implanted in vivo, these hESC-laden mineralized matrices contributed to ectopic bone tissue formation. In contrast, cells within the corresponding non-mineralized matrices underwent either osteogenic or adipogenic fate depending upon the local cues present in the microenvironment. To our knowledge, this is the first demonstration where synthetic matrices are shown to induce terminal cell fate specification of hESCs exclusively by biomaterial-based cues both in vitro and in vivo. Technologies that utilize tissue specific cell-matrix interactions to control stem cell fate could be a powerful tool in regenerative medicine. Such approaches can be used as a tool to advance our basic understanding and assess the translational potential of stem cells.

Biomimetic polymer scaffolds to promote stem cell-mediated osteogenesis

Bone tissue engineering using stem cells with osteogenic potential is a promising avenue of research for bone defect reconstruction. Organic, inorganic, and composite scaffolds have all been engineered to provide biomimetic microenvironments for stem cells. These scaffolds are designed to promote stem cell osteogenesis. Here, we review current technologies for developing biomimetic, osteoinductive scaffolds for stem cell applications. We summarize the reported in vitro and in vivo osteogenic effects of these scaffolds on stem cells.

Materials as stem cell regulators

The stem cell/material interface is a complex, dynamic microenvironment in which the cell and the material cooperatively dictate one another's fate: the cell by remodelling its surroundings, and the material through its inherent properties (such as adhesivity, stiffness, nanostructure or degradability). Stem cells in contact with materials are able to sense their properties, integrate cues via signal propagation and ultimately translate parallel signalling information into cell fate decisions. However, discovering the mechanisms by which stem cells respond to inherent material characteristics is challenging because of the highly complex, multicomponent signalling milieu present in the stem cell environment. In this Review, we discuss recent evidence that shows that inherent material properties may be engineered to dictate stem cell fate decisions, and overview a subset of the operative signal transduction mechanisms that have begun to emerge. Further developments in stem cell engineering and mechanotransduction are poised to have substantial implications for stem cell biology and regenerative medicine.

A combinatorial cell-laden gel microarray for inducing osteogenic differentiation of human mesenchymal stem cells

Development of three dimensional (3D) microenvironments that direct stem cell differentiation into functional cell types remains a major challenge in the field of regenerative medicine. Here, we describe a new platform to address this challenge by utilizing a robotic microarray spotter for testing stem cell fates inside various miniaturized cell-laden gels in a systematic manner. To demonstrate the feasibility of our platform, we evaluated the osteogenic differentiation of human mesenchymal stem cells (hMSCs) within combinatorial 3D niches. We were able to identify specific combinations, that enhanced the expression of osteogenic markers. Notably, these 'hit' combinations directed hMSCs to form mineralized tissue when conditions were translated to 3D macroscale hydrogels, indicating that the miniaturization of the experimental system did not alter stem cell fate. Overall, our findings confirmed that the 3D cell-laden gel microarray can be used for screening of different conditions in a rapid, cost-effective, and multiplexed manner for a broad range of tissue engineering applications.

Calcium phosphate-bearing matrices induce osteogenic differentiation of stem cells through adenosine signaling

Synthetic matrices emulating the physicochemical properties of tissue-specific ECMs are being developed at a rapid pace to regulate stem cell fate. Biomaterials containing calcium phosphate (CaP) moieties have been shown to support osteogenic differentiation of stem and progenitor cells and bone tissue formation. By using a mineralized synthetic matrix mimicking a CaP-rich bone microenvironment, we examine a molecular mechanism through which CaP minerals induce osteogenesis of human mesenchymal stem cells with an emphasis on phosphate metabolism. Our studies show that extracellular phosphate uptake through solute carrier family 20 (phosphate transporter), member 1 (SLC20a1) supports osteogenic differentiation of human mesenchymal stem cells via adenosine, an ATP metabolite, which acts as an autocrine/paracrine signaling molecule through A2b adenosine receptor. Perturbation of SLC20a1 abrogates osteogenic differentiation by decreasing intramitochondrial phosphate and ATP synthesis. Collectively, this study offers the demonstration of a previously unknown mechanism for the beneficial role of CaP biomaterials in bone repair and the role of phosphate ions in bone physiology and regeneration. These findings also begin to shed light on the role of ATP metabolism in bone homeostasis, which may be exploited to treat bone metabolic diseases.