Nanostructured Tendon-Derived Scaffolds for Enhanced Bone Regeneration by Human Adipose-Derived Stem Cells

Decellularized extracellular matrix biomaterials for regenerative therapies: Advances, challenges and clinical prospects

Tissue engineering and regenerative medicine have shown potential in the repair and regeneration of tissues and organs via the use of engineered biomaterials and scaffolds. However, current constructs face limitations in replicating the intricate native microenvironment and achieving optimal regenerative capacity and functional recovery. To address these challenges, the utilization of decellularized tissues and cell-derived extracellular matrix (ECM) has emerged as a promising approach. These biocompatible and bioactive biomaterials can be engineered into porous scaffolds and grafts that mimic the structural and compositional aspects of the native tissue or organ microenvironment, both in vitro and in vivo. Bioactive dECM materials provide a unique tissue-specific microenvironment that can regulate and guide cellular processes, thereby enhancing regenerative therapies. In this review, we explore the emerging frontiers of decellularized tissue-derived and cell-derived biomaterials and bio-inks in the field of tissue engineering and regenerative medicine. We discuss the need for further improvements in decellularization methods and techniques to retain structural, biological, and physicochemical characteristics of the dECM products in a way to mimic native tissues and organs. This article underscores the potential of dECM biomaterials to stimulate in situ tissue repair through chemotactic effects for the development of growth factor and cell-free tissue engineering strategies. The article also identifies the challenges and opportunities in developing sterilization and preservation methods applicable for decellularized biomaterials and grafts and their translation into clinical products.

Decellularized extracellular matrix-based composite scaffolds for tissue engineering and regenerative medicine

Despite the considerable advancements in fabricating polymeric-based scaffolds for tissue engineering, the clinical transformation of these scaffolds remained a big challenge because of the difficulty of simulating native organs/tissues' microenvironment. As a kind of natural tissue-derived biomaterials, decellularized extracellular matrix (dECM)-based scaffolds have gained attention due to their unique biomimetic properties, providing a specific microenvironment suitable for promoting cell proliferation, migration, attachment and regulating differentiation. The medical applications of dECM-based scaffolds have addressed critical challenges, including poor mechanical strength and insufficient stability. For promoting the reconstruction of damaged tissues or organs, different types of dECM-based composite platforms have been designed to mimic tissue microenvironment, including by integrating with natural polymer or/and syntenic polymer or adding bioactive factors. In this review, we summarized the research progress of dECM-based composite scaffolds in regenerative medicine, highlighting the critical challenges and future perspectives related to the medical application of these composite materials.

An engineered lamellar bone mimicking full-scale hierarchical architecture for bone regeneration

Lamellar bone, compactly and ingeniously organized in the hierarchical pattern with 6 ordered scales, is the structural motif of mature bone. Each hierarchical scale exerts an essential role in determining physiological behavior and osteogenic bioactivity of bone. Engineering lamellar bone with full-scale hierarchy remains a longstanding challenge. Herein, using bioskiving and mineralization, we attempt to engineer compact constructs resembling full-scale hierarchy of lamellar bone. Through systematically investigating the effect of mineralization on physicochemical properties and bioactivities of multi-sheeted collagen matrix fabricated by bioskiving, the hierarchical mimicry and hierarchy-property relationship are elucidated. With prolongation of mineralization, hierarchical mimicry and osteogenic bioactivity of constructs are performed in a bidirectional manner, i.e. first rising and then descending, which is supposed to be related with transformation of mineralization mechanism from nonclassical to classical crystallization. Construct mineralized 9 days can accurately mimic each hierarchical scale and efficiently promote osteogenesis. Bioinformatic analysis further reveals that this construct potently activates integrin α5-PI3K/AKT signaling pathway through mechanical and biophysical cues, and thereby repairing critical-sized bone defect. The present study provides a bioinspired strategy for completely resembling complex hierarchy of compact mineralized tissue, and offers a critical research model for in-depth studying the structure-function relationship of bone.

Functionalized Decellularized Bone Matrix Promotes Bone Regeneration by Releasing Osteogenic Peptides

The decellularized bone matrix (DCB) provides a promising bone substitute for the treatment of bone defects because of its similar biochemical, biophysical, and mechanical properties to normal bone tissue. However, the decellularized procedure also greatly reduced its osteogenic induction activity. In this study, peptides derived from the knuckle epitope of bone morphogenetic protein-2 were incorporated into the thermo-sensitive hydrogel poloxamer 407, and the peptide-loaded hydrogel was then filled into the pores of DCB to construct a functionalized scaffold with enhanced osteogenesis. In vitro studies have shown that the functionalized DCB scaffold possessed appropriate mechanical properties and biocompatibility and exhibited a sustained release profile of osteogenic peptide. These performances critically facilitated cell proliferation and cell spreading of bone marrow mesenchymal stem cells and upregulated the expression of osteogenic-related genes by activating the Smad/Runx2 signaling pathway, thereby promoting osteogenic differentiation and extracellular matrix mineralization. Further in vivo studies demonstrated that the functionalized DCB scaffold accelerated the repair of critical radial defects in rabbits without inducing excessive graft-related inflammatory responses. These results suggest a clinically meaningful strategy for the treatment of large segmental bone defects, and the prepared osteogenic peptide modified composite DCB scaffold has great application potential for bone regeneration.

How Mechanical and Physicochemical Material Characteristics Influence Adipose-Derived Stem Cell Fate

Adipose-derived stem cells (ASCs) are a subpopulation of mesenchymal stem cells. Compared to bone marrow-derived stem cells, they can be harvested with minimal invasiveness. ASCs can be easily expanded and were shown to be able to differentiate into several clinically relevant cell types. Therefore, this cell type represents a promising component in various tissue engineering and medical approaches (e.g., cell therapy). In vivo cells are surrounded by the extracellular matrix (ECM) that provides a wide range of tissue-specific physical and chemical cues, such as stiffness, topography, and chemical composition. Cells can sense the characteristics of their ECM and respond to them in a specific cellular behavior (e.g., proliferation or differentiation). Thus, in vitro biomaterial properties represent an important tool to control ASCs behavior. In this review, we give an overview of the current research in the mechanosensing of ASCs and current studies investigating the impact of material stiffens, topography, and chemical modification on ASC behavior. Additionally, we outline the use of natural ECM as a biomaterial and its interaction with ASCs regarding cellular behavior.

Natural bone-mimicking nanopore-incorporated hydroxyapatite scaffolds for enhanced bone tissue regeneration

Background

A considerable number of studies has been carried out to develop alloplastic bone graft materials such as hydroxyapatite (HAP) that mimic the hierarchical structure of natural bones with multiple levels of pores: macro-, micro-, and nanopores. Although nanopores are known to play many essential roles in natural bones, only a few studies have focused on HAPs containing them; none of those studies investigated the functions of nanopores in biological systems.

Method

We developed a simple yet powerful method to introduce nanopores into alloplastic HAP bone graft materials in large quantities by simply pressing HAP nanoparticles and sintering them at a low temperature.

Results

The size of nanopores in HAP scaffolds can be controlled between 16.5 and 30.2 nm by changing the sintering temperature. When nanopores with a size of ~ 30.2 nm, similar to that of nanopores in natural bones, are introduced into HAP scaffolds, the mechanical strength and cell proliferation and differentiation rates are significantly increased. The developed HAP scaffolds containing nanopores (SNPs) are biocompatible, with negligible erythema and inflammatory reactions. In addition, they enhance the bone regeneration when are implanted into a rabbit model. Furthermore, the bone regeneration efficiency of the HAP-based SNP is better than that of a commercially available bone graft material.

Conclusion

Nanopores of HAP scaffolds are very important for improving the bone regeneration efficiency and may be one of the key factors to consider in designing highly efficient next-generation alloplastic bone graft materials.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40824-022-00253-x.

The role of tendon derived stem/progenitor cells and extracellular matrix components in the bone tendon junction repair

Fibrocartilage enthesis is the junction between bone and tendon with a typical characteristics of fibrocartilage transition zones. The regeneration of this transition zone is the bottleneck for functional restoration of bone tendon junction (BTJ). Biomimetic approaches, especially decellularized extracellular matrix (ECM) materials, are strategies which aim to mimic the components of tissues to the utmost extent, and are becoming popular in BTJ healing because of their ability not only to provide scaffolds to allow cells to attach and migrate, but also to provide a microenvironment to guide stem/progenitor cells lineage-specific differentiation. However, the cellular and molecular mechanisms of those approaches, especially the ECM proteins, remain unclear. For BTJ reconstruction, fibrocartilage regeneration is the key for good integrity of bone and tendon as well as its mechanical recovery, so the components which can guide stem cells to a chondrogenic commitment in biomimetic approaches might well be the key for fibrocartilage regeneration and eventually for the better BTJ healing. In this review, we firstly discuss the importance of cartilage-like formation in the healing process of BTJ. Next, we explore the possibility of tendon-derived stem/progenitor cells as cell sources for BTJ regeneration due to their multi-differentiation potential. Finally, we summarize the role of extracellular matrix components of BTJ in guiding stem cell fate to a chondrogenic commitment, so as to provide cues for understanding the mechanisms of lineage-specific potential of biomimetic approaches as well as to inspire researchers to incorporate unique ECM components that facilitate BTJ repair into design.

Current evidence on potential of adipose derived stem cells to enhance bone regeneration and future projection

Injuries to the postnatal skeleton are naturally repaired through successive steps involving specific cell types in a process collectively termed “bone regeneration”. Although complex, bone regeneration occurs through a series of well-orchestrated stages wherein endogenous bone stem cells play a central role. In most situations, bone regeneration is successful; however, there are instances when it fails and creates non-healing injuries or fracture nonunion requiring surgical or therapeutic interventions. Transplantation of adult or mesenchymal stem cells (MSCs) defined by the International Society for Cell and Gene Therapy (ISCT) as CD105+CD90+CD73+CD45-CD34-CD14orCD11b-CD79αorCD19-HLA-DR- is being investigated as an attractive therapy for bone regeneration throughout the world. MSCs isolated from adipose tissue, adipose-derived stem cells (ADSCs), are gaining increasing attention since this is the most abundant source of adult stem cells and the isolation process for ADSCs is straightforward. Currently, there is not a single Food and Drug Administration (FDA) approved ADSCs product for bone regeneration. Although the safety of ADSCs is established from their usage in numerous clinical trials, the bone-forming potential of ADSCs and MSCs, in general, is highly controversial. Growing evidence suggests that the ISCT defined phenotype may not represent bona fide osteoprogenitors. Transplantation of both ADSCs and the CD105^- sub-population of ADSCs has been reported to induce bone regeneration. Most notably, cells expressing other markers such as CD146, AlphaV, CD200, PDPN, CD164, CXCR4, and PDGFRα have been shown to represent osteogenic sub-population within ADSCs. Amongst other strategies to improve the bone-forming ability of ADSCs, modulation of VEGF, TGF-β1 and BMP signaling pathways of ADSCs has shown promising results. The U.S. FDA reveals that 73% of Investigational New Drug applications for stem cell-based products rely on CD105 expression as the “positive” marker for adult stem cells. A concerted effort involving the scientific community, clinicians, industries, and regulatory bodies to redefine ADSCs using powerful selection markers and strategies to modulate signaling pathways of ADSCs will speed up the therapeutic use of ADSCs for bone regeneration.

Human adipose-derived stem cell spheroids incorporating platelet-derived growth factor (PDGF) and bio-minerals for vascularized bone tissue engineering

Stem cells with mineralized materials have been used for bone regeneration; however, engineering the complex vascularized structure of the natural bone remains a challenge. Here, we developed platelet-derived growth factor (PDGF) and bio-mineral coated fibers which were then assembled with human adipose-derived stem cells (hADSCs) to form spheroids as building blocks for vascularized bone regeneration. The PDGF incorporated within the spheroid increased the proliferation of hADSCs, which was characterized by Ki-67 staining and DNA contents. Furthermore, the PDGF enhanced not only osteogenic differentiation, but also endothelial differentiation of hADSCs; the cells within the spheroids showed significantly greater gene expression by 2.46 ± 0.14 fold for osteocalcin (OCN) and by 12.85 ± 3.36 fold for von Willebrand factor (vWF) than those without PDGF. Finally, at two months following transplantation of PDGF-incorporated spheroids onto in vivo mouse calvarial defect, the regenerated bone area (42.48 ± 10.84%) was significantly enhanced and the greatest number of capillaries and arterioles with indication of transplanted hADSCs were observed. Moreover, millimeter-scale in vitro tissue prepared by fused assembly of the spheroids exhibited greater mRNA expression-associated to endothelial lineage. Taken together, these findings indicate that stem cell spheroids incorporating PDGF and bio-minerals could be used as a module for successful vascularized bone regeneration.

The Construction of Biomimetic Cementum Through a Combination of Bioskiving and Fluorine-Containing Biomineralization

Despite tremendous attention is given to the construction of biomimetic cementum for regeneration of tooth cementum, the lack of recapitulating the composition and hierarchical structure of cementum often leads to the poor performance of constructed materials. How to highly mimic the sophisticated composition and hierarchy of cementum remains a longstanding challenge in constructing the biomimetic cementum. Inspired by cementum formation process, a novel construction approach via a combination of bioskiving and fluorine-containing biomineralization is developed in this study. The alternative collagen lamellae (ACL) that can highly mimic the rotated plywood structure of cementum collagen matrix is fabricated via bioskiving. Followed by biomineralization in the amorphous calcium phosphate (ACP) solution with different concentration of fluorine, a series of biomimetic cementum is constructed. Screened by physicochemical characterization, the biomimetic cementum with the composition and hierarchical structure highly similar to human cementum is selected. Through in vitro biological assay, this biomimetic cementum is proven to significantly promote the adhesion, proliferation, and cementogenic differentiation of periodontal ligament cells (PDLCs). Furthermore, in vivo study demonstrates that biomimetic cementum could induce cementogenesis. This biomimetic cementum constructed via combinatory application of bioskiving and fluorine-containing biomineralization stands as a promising candidate for achieving cementum regeneration.

Thiolated bone and tendon tissue particles covalently bound in hydrogels for in vivo calvarial bone regeneration

Bone regeneration of large cranial defects, potentially including traumatic brain injury (TBI) treatment, presents a major problem with non-crosslinking, clinically available products due to material migration outside the defect. Commercial products such as bone cements are permanent and thus not conducive to bone regeneration, and typical commercial bioactive materials for bone regeneration do not crosslink. Our previous work demonstrated that non-crosslinking materials may be prone to material migration following surgical placement, and the current study attempted to address these problems by introducing a new hydrogel system where tissue particles are themselves the crosslinker. Specifically, a pentenoate-modified hyaluronic acid (PHA) polymer was covalently linked to thiolated tissue particles of demineralized bone matrix (TDBM) or devitalized tendon (TDVT), thereby forming an interconnected hydrogel matrix for calvarial bone regeneration. All hydrogel precursor solutions exhibited sufficient yield stress for surgical placement and an adequate compressive modulus post-crosslinking. Critical-size calvarial defects were filled with a 4% PHA hydrogel containing 10 or 20% TDBM or TDVT, with the clinical product DBX^Ⓡ being employed as the standard of care control for the in vivo study. At 12 weeks, micro-computed tomography analysis demonstrated similar bone regeneration among the experimental groups, TDBM and TDVT, and the standard of care control DBX^Ⓡ. The group with 10% TDBM was therefore identified as an attractive material for potential calvarial defect repair, as it additionally exhibited a sufficient initial recovery after shearing (i.e., > 80% recovery). Future studies will focus on applying a hydrogel in a rat model for treatment of TBI. STATEMENT OF SIGNIFICANCE: Non-crosslinking materials may be prone to material migration from a calvarial bone defect following surgical placement, which is problematic for materials intended for bone regeneration. Unfortunately, typical crosslinking materials such as bone cements are permanent and thus not conducive to bone regeneration, and typical bioactive materials for bone regeneration such as tissue matrix are not crosslinked in commercial products. The current study addressed these problems by introducing a new biomaterial where tissue particles are themselves the crosslinker in a hydrogel system. The current study successfully demonstrated a new material based on pentenoate-modified hyaluronic acid with thiolated demineralized bone matrix that is capable of rapid crosslinking, with desirable paste-like rheology of the precursor material for surgical placement, and with bone regeneration comparable to a commercially available standard-of-care product. Such a material may hold promise for a single-surgery treatment of severe traumatic brain injury (TBI) following hemicraniectomy.

Adipose-Derived Stem Cells in Bone Tissue Engineering: Useful Tools with New Applications

Adipose stem cells (ASCs) are a crucial element in bone tissue engineering (BTE). They are easy to harvest and isolate, and they are available in significative quantities, thus offering a feasible and valid alternative to other sources of mesenchymal stem cells (MSCs), like bone marrow. Together with an advantageous proliferative and differentiative profile, they also offer a high paracrine activity through the secretion of several bioactive molecules (such as growth factors and miRNAs) via a sustained exosomal release which can exert efficient conditioning on the surrounding microenvironment. BTE relies on three key elements: (1) scaffold, (2) osteoprogenitor cells, and (3) bioactive factors. These elements have been thoroughly investigated over the years. The use of ASCs has offered significative new advancements in the efficacy of each of these elements. Notably, the phenotypic study of ASCs allowed discovering cell subpopulations, which have enhanced osteogenic and vasculogenic capacity. ASCs favored a better vascularization and integration of the scaffolds, while improvements in scaffolds' materials and design tried to exploit the osteogenic features of ASCs, thus reducing the need for external bioactive factors. At the same time, ASCs proved to be an incredible source of bioactive, proosteogenic factors that are released through their abundant exosome secretion. ASC exosomes can exert significant paracrine effects in the surroundings, even in the absence of the primary cells. These paracrine signals recruit progenitor cells from the host tissues and enhance regeneration. In this review, we will focus on the recent discoveries which have involved the use of ASCs in BTE. In particular, we are going to analyze the different ASCs' subpopulations, the interaction between ASCs and scaffolds, and the bioactive factors which are secreted by ASCs or can induce their osteogenic commitment. All these advancements are ultimately intended for a faster translational and clinical application of BTE.

[Research progress of structured repair of tendon-bone interface]

In sports system, the tendon-bone interface has the effect of tensile and bearing load, so the effect of healing plays a crucial role in restoring joint function. The process of repair is the formation of scar tissue, so it is difficult to achieve the ideal effect for morphology and biomechanical strength. The tissue engineering method can promote the tendon-bone interface healing from the seed cells, growth factors, and scaffolds, and is a new direction in the field of development of the tendon-bone interface healing.

Biomimetic Polymer-Based Engineered Scaffolds for Improved Stem Cell Function

Scaffolds are considered promising materials for tissue engineering applications due to their unique physiochemical properties. The high porosity and adequate mechanical properties of the scaffolds facilitate greater cell adhesion, proliferation, and differentiation. Stem cells are frequently applied in tissue engineering applications due to their excellent potential. It has been noted that cell functions are profoundly affected by the nature of the extracellular matrix (ECM). Naturally derived ECM contains the bioactive motif that also influences the immune response of the organism. The properties of polymer scaffolds mean they can resemble the native ECM and can regulate cellular responses. Various techniques such as electrospinning and 3D printing, among others, are frequently used to fabricate polymer scaffolds, and their cellular responses are different with each technique. Furthermore, enhanced cell viability, as well as the differentiation ability of stem cells on the surface of scaffolds, opens a fascinating approach to the formation of ECM-like environments for tissue engineering applications.

Recent Advances in Scaffold Design and Material for Vascularized Tissue-Engineered Bone Regeneration

Bone tissue is a highly vascularized tissue and concomitant development of the vascular system and mineralized matrix requires a synergistic interaction between osteogenesis and angioblasts. Several strategies have been applied to achieve vascularized tissue-engineered bone, including the addition of cytokines as well as pre-vascularization strategies and co-culture systems. However, the scaffold is another extremely important component to consider, and development of vascularized bone scaffolds remains one of the greatest challenges for engineering clinically relevant bone substitutes. Here, this review highlights the biomaterial selection, preparation of pre-vascularized scaffolds, composition modification of the scaffold, structural design, and the comprehensive use of the above synergistic modifications of scaffold materials for vascular scaffolds in bone tissue engineering. Moreover, a strategy is proposed for the design of future scaffold structures, in which promoting the regeneration of vascularized bone by regulating the microenvironment should be the main focus. This overview can help illuminate progress in this field and identify the most recently developed scaffolds that show the greatest potential for achieving clinically vascularized bone.

Elevated Expression of Dkk-1 by Glucocorticoid Treatment Impairs Bone Regenerative Capacity of Adipose Tissue-Derived Mesenchymal Stem Cells

Glucocorticoids are steroid hormones used as anti-inflammatory treatments. However, this strong immunomodulation causes undesirable side effects that impair bones, such as osteoporosis. Glucocorticoid therapy is a major risk factor for developing steroid-induced osteonecrosis of the femur head (ONFH). Since ONFH is incurable, therapy with mesenchymal stem cells (MSCs) that can differentiate into osteoblasts are a first-line choice. Bone marrow-derived MSCs (BM-MSCs) are often used as a source of stem cell therapy for ONFH, but their proliferative activity is impaired after steroid treatment. Adipose tissue-derived MSCs (AT-MSCs) may be an attractive alternative source; however, it is unknown whether AT-MSCs from steroid-induced ONFH (sAT-MSCs) have the same differentiation ability as BM-MSCs or normal AT-MSCs (nAT-MSCs). In this study, we demonstrate that nAT-MSCs chronically exposed to glucocorticoids show lower alkaline phosphatase activity leading to reduced osteogenic differentiation ability. This impaired osteogenesis is mediated by high expression of Dickkopf1 (Dkk-1) that inhibits wnt/β-catenin signaling. Increased Dkk-1 also causes impaired osteogenesis along with reductions in bone regenerative capacity in sAT-MSCs. Of note, plasma Dkk-1 levels are elevated in steroid-induced ONFH patients. Collectively, our findings suggest that glucocorticoid-induced expression of Dkk-1 could be a key factor in modulating the differentiation ability of MSCs used for ONFH and other stem cell therapies.

Pioneering Effects and Enhanced Neurite Complexity of Primary Hippocampal Neurons on Hierarchical Neurotemplated Scaffolds

In this work, the use of scaffolds is reported, templated from live neurons as an advanced culture platform for primary neurons. Hippocampal neurons cultured on neurotemplated scaffolds exhibit an affinity for templated somas, revealing a preference for micrometric structures amidst nanotopographical features. It is also reported, for the first time, that neurite complexity can be topographically controlled by increasing the density of nanometric features on neurotemplated scaffolds. Neurotemplated scaffolds are versatile, hierarchical topographies that feature biologically relevant structures, in both form and scale, and capture the true complexity of an in vivo environment. The introduction and implementation of neurotemplated scaffolds is sure to advance research in the fields of neurodevelopment, network development, and neuroregeneration.

Superior calvarial bone regeneration using pentenoate-functionalized hyaluronic acid hydrogels with devitalized tendon particles

Traumatic brain injury (TBI) is a life-threatening condition defined by internal brain herniation. Severe TBI is commonly treated by a two-stage surgical intervention, where decompressive craniectomy is first conducted to remove a large portion of calvarial bone and allow unimpeded brain swelling. In the second surgery, spaced weeks to months after the first, cranioplasty is performed to restore the cranial bone. Hydrogels with paste-like precursor solutions for surgical placement may potentially revolutionize TBI treatment by permitting a single-stage surgical intervention, capable of being implanted with the initial surgery, remaining pliable during brain swelling, and tuned to regenerate calvarial bone after brain swelling has subsided. The current study evaluated the use of photocrosslinkable pentenoate-functionalized hyaluronic acid (PHA) and non-crosslinking hyaluronic acid (HA) hydrogels encapsulating naturally derived tissue particles of demineralized bone matrix (DBM), devitalized cartilage (DVC), devitalized meniscus (DVM), or devitalized tendon (DVT) for bone regeneration in critical-size rat calvarial defects. All hydrogel precursors exhibited a yield stress for placement and addition of particles increased the average material compressive modulus. The HA-DBM (4-30%), PHA (4%), and PHA-DVT (4-30%) groups had 5 (p < 0.0001), 3.1, and 3.2 (p < 0.05) times greater regenerated bone volume compared to the sham (untreated defect) group, respectively. In vitro cell studies suggested that the PHA-DVT (4-10%) group would have the most desirable performance. Overall, hydrogels containing DVT particles outperformed other materials in terms of bone regeneration in vivo and calcium deposition in vitro. Hydrogels containing DVT will be further evaluated in future rat TBI studies.

STATEMENT OF SIGNIFICANCE

Traumatic brain injury (TBI) is a life-threatening condition characterized by severe brain swelling and is currently treated by a two-stage surgical procedure. Complications associated with the two-stage surgical intervention include the occurrence of the condition termed syndrome of the trephined; however, the condition is completely reversible once the secondary surgery is performed. A desirable TBI treatment would include a single surgical intervention to avoid syndrome of the trephined altogether. The first hurdle in reaching the overall goal is to develop a pliable hydrogel material that can regenerate the patient's bone. The development of a pliable hydrogel technology would greatly impact the field of bone regeneration for TBI application and other areas of bone regeneration.

Electrospun Silk Fibroin Nanofibrous Scaffolds with Two-Stage Hydroxyapatite Functionalization for Enhancing the Osteogenic Differentiation of Human Adipose-Derived Mesenchymal Stem Cells

The development of functional scaffolds with improved osteogenic potential is important for successful bone formation and mineralization in bone tissue engineering. In this study, we developed a functional electrospun silk fibroin (SF) nanofibrous scaffold functionalized with two-stage hydroxyapatite (HAp) particles, using mussel adhesive-inspired polydopamine (PDA) chemistry. HAp particles were first incorporated into SF scaffolds during the electrospinning process, and then immobilized onto the electrospun SF nanofibrous scaffolds containing HAp via PDA-mediated adhesive chemistry. We obtained two-stage HAp-functionalized SF nanofibrous scaffolds with improved mechanical properties and capable of providing a bone-specific physiological microenvironment. The developed scaffolds were tested for their ability to enhance the osteogenic differentiation of human adipose-derived mesenchymal stem cells (hADMSCs) in vitro and repair bone defect in vivo. To boost their ability for bone repair, we genetically modified hADMSCs with the transcriptional coactivator with PDZ-binding motif (TAZ) via polymer nanoparticle-mediated gene delivery. TAZ is a well-known transcriptional modulator that activates the osteogenic differentiation of mesenchymal stem cells (MSCs). Two-stage HAp-functionalized SF scaffolds significantly promoted the osteogenic differentiation of TAZ-transfected hADMSCs in vitro and enhanced mineralized bone formation in a critical-sized calvarial bone defect model. Our study shows the potential utility of SF scaffolds with nanofibrous structures and enriched inorganic components in bone tissue engineering.

The potential of using semitendinosus tendon as autograft in rabbit meniscus reconstruction

Since transplantation of meniscal allograft or artificial menisci is limited by graft sources and a series of adverse events, substitution for meniscus reconstruction still needs to be explored. Natural biomaterials, which can provide a unique 3-D microenvironment, remain a promising alternative for tissue engineering. Among them, autograft is a preferred option for its safety and excellent biocompatibility. In this study, we utilized semitendinosus tendon autograft in meniscus reconstruction to investigate its fibrochondrogenic metaplasticity potential and chondroprotective effect. Tendon-derived stem cells (TDSCs) and synovial-derived mesenchymal stem cells (SMSCs), two most important stem cell sources in our strategy, exhibited excellent viability, distribution, proliferation and fibrochondrogenic differentiation ability in decellularized semitendinosus tendon (DST) scaffolds in vitro. Histologic evaluation of the tendon grafts in vivo suggested endogenous stem cells differentiated into fibrochondrocytes, synthesized proteoglycan, type II collagen and radial type I collagen at 12 weeks and 24 weeks post-surgery. As for elastic modulus and hardness of the grafts, there were no significant differences between native meniscus and regenerated meniscus at 24 weeks. The protection of condylar cartilage from degeneration was significantly better in the reconstruction group comparing to control group. Overall, semitendinosus tendon autograft seems to be a promising substitution in meniscus reconstruction.

Adipose-derived stem cell therapies for bone regeneration

INTRODUCTION

Cell-based therapies exploit the heterogeneous and self-sufficient biological environment of stem cells to restore, maintain and improve tissue functions. Adipose-derived stem cells (ASCs) are, to this aim, promising cell types thanks to advantageous isolation procedures, growth kinetics, plasticity and trophic properties. Specifically, bone regeneration represents a suitable, though often challenging, target setting to test and apply ASC-based therapeutic strategies. Areas covered: ASCs are extremely plastic and secrete bioactive peptides that mediate paracrine functions, mediating their trophic actions in vivo. Numerous preclinical studies demonstrated that ASCs improve bone healing. Clinical trials are ongoing to validate the clinical feasibility of these approaches. This review is intended to define the state-of-the-art on ASCs, encompassing the biological features that make them suitable for bone regenerative strategies, and to provide an update on existing preclinical and clinical applications. Expert opinion: ASCs offer numerous advantages over other stem cells in terms of feasibility of clinical translation. Data obtained from in vivo experimentation are encouraging, and clinical trials are ongoing. More robust validations are thus expected to be achieved during the next few years, and will likely pave the way to optimized patient-tailored treatments for bone regeneration.

Effect of tetrahedral DNA nanostructures on osteogenic differentiation of mesenchymal stem cells via activation of the Wnt/β-catenin signaling pathway

Adipose-derived stem cells (ADSCs) are considered to be ideal stem cell sources for bone regeneration owing to their ability to differentiate into osteo-like cells. Therefore, they have attracted increasing attention in recent years. Tetrahedral DNA nanostructures (TDNs), a new type of DNA-based biomaterials, have shown great potential for biomedical applications. In the present work, we aimed to investigate the role played by TDNs in osteogenic differentiation and proliferation of ADSCs and tried to explore if the canonical Wnt signal pathway could be the vital biological mechanism driving these cellular responses. Upon exposure to TDNs, ADSCs proliferation and osteogenic differentiation were significantly enhanced, accompanied by the up-regulation of genes correlated with the Wnt/β-catenin pathway. In conclusion, our results indicate that TDNs are crucial regulators of the increase in osteogenic potential and ADSCs proliferation, and this noteworthy discovery could provide a promising novel approach toward ADSCs-based bone defect regeneration.