Osteogenic differentiation of human mesenchymal stem cells directed by extracellular matrix-mimicking ligands in a biomimetic self-assembled peptide amphiphile nanomatrix

Maneuvering the mineralization of self-assembled peptide nanofibers for designing mechanically-stiffened self-healable composites toward bone-mimetic ECM

Extracellular matrix (ECM) elasticity remains a crucial parameter to determine cell-material interactions (viz. adhesion, growth, and differentiation), cellular communication, and migration that are essential to tissue repair and regeneration. Supramolecular peptide hydrogels with their 3-dimensional porous network and tuneable mechanical properties have emerged as an excellent class of ECM-mimetic biomaterials with relevant dynamic attributes and bioactivity. Here, we demonstrate the design of minimalist amyloid-inspired peptide amphiphiles, CnPA (n = 6, 8, 10, 12) with tuneable peptide nanostructures that are efficiently biomineralized and cross-linked using bioactive silicates. Such hydrogel composites, CnBG exhibit excellent mechanical attributes and possess excellent self-healing abilities and collagen-like strain-stiffening ability as desired for bone ECM mimetic scaffold. The composites exhibited the formation of a hydroxyapatite mineral phase upon incubation in a simulated body fluid that rendered mechanical stiffness akin to the hydroxyapatite-bridged collagen fibers to match the bone tissue elasticity eventually. In a nutshell, peptide nanostructure-guided temporal effects and mechanical attributes demonstrate C8BG to be an optimal composite. Finally, such constructs feature the potential for adhesion, proliferation of U2OS cells, high alkaline phosphatase activity, and osteoconductivity.

Supramolecular Peptide Nanofiber Hydrogels for Bone Tissue Engineering: From Multihierarchical Fabrications to Comprehensive Applications

Bone tissue engineering is becoming an ideal strategy to replace autologous bone grafts for surgical bone repair, but the multihierarchical complexity of natural bone is still difficult to emulate due to the lack of suitable biomaterials. Supramolecular peptide nanofiber hydrogels (SPNHs) are emerging biomaterials because of their inherent biocompatibility, satisfied biodegradability, high purity, facile functionalization, and tunable mechanical properties. This review initially focuses on the multihierarchical fabrications by SPNHs to emulate natural bony extracellular matrix. Structurally, supramolecular peptides based on distinctive building blocks can assemble into nanofiber hydrogels, which can be used as nanomorphology-mimetic scaffolds for tissue engineering. Biochemically, bioactive motifs and bioactive factors can be covalently tethered or physically absorbed to SPNHs to endow various functions depending on physiological and pharmacological requirements. Mechanically, four strategies are summarized to optimize the biophysical microenvironment of SPNHs for bone regeneration. Furthermore, comprehensive applications about SPNHs for bone tissue engineering are reviewed. The biomaterials can be directly used in the form of injectable hydrogels or composite nanoscaffolds, or they can be used to construct engineered bone grafts by bioprinting or bioreactors. Finally, continuing challenges and outlook are discussed.

Uni-Directionally Oriented Fibro-Porous PLLA/Fibrin Bio-Hybrid Scaffold: Mechano-Morphological and Cell Studies

In this study, we report a biohybrid oriented fibrous scaffold based on nanofibers of poly(l-lactic acid) (PLLA)/fibrin produced by electrospinning and subsequent post-treatment. Induced hydrolytic degradation of the fibers in 0.25 M NaOH solution for various time periods followed by the immobilization of fibrin on the hydrolyzed fiber surfaces was shown to significantly affect the mechanical properties, with the tensile strength (40.6 MPa ± 1.3) and strain at failure (38% ± 4.5) attaining a value within the range of human ligaments and ligament-replacement grafts. Unidirectional electrospinning with a mandrel rotational velocity of 26.4 m/s produced highly aligned fibers with an average diameter of 760 ± 96 nm. After a 20-min hydrolysis treatment in NaOH solution, this was further reduced to an average of 457 ± 89 nm, which is within the range of collagen bundles found in ligament tissue. Based on the results presented herein, the authors hypothesize that a combination of fiber orientation/alignment and immobilization of fibrin can result in the mechanical and morphological modification of PLLA tissue scaffolds for ligament-replacement grafts. Further, it was found that treatment with NaOH enhanced the osteogenic differentiation of hMSCs and the additional inclusion of fibrin further enhanced osteogenic differentiation, as demonstrated by decreased proliferative rates and increased ALP activity.

Osteogenic Peptides and Attachment Methods Determine Tissue Regeneration in Modified Bone Graft Substitutes

The inclusion of biofunctional molecules with synthetic bone graft substitutes has the potential to enhance tissue regeneration during treatment of traumatic bone injuries. The clinical use of growth factors has though been associated with complications, some serious. The use of smaller, active peptides has the potential to overcome these problems and provide a cost-effective, safe route for the manufacture of enhanced bone graft substitutes. This review considers the design of peptide-enhanced bone graft substitutes, and how peptide selection and attachment method determine clinical efficacy. It was determined that covalent attachment may reduce the known risks associated with growth factor-loaded bone graft substitutes, providing a predictable tissue response and greater clinical efficacy. Peptide choice was found to be critical, but even within recognised families of biologically active peptides, the configurations that appeared to most closely mimic the biological molecules involved in natural bone healing processes were most potent. It was concluded that rational, evidence-based design of peptide-enhanced bone graft substitutes offers a pathway to clinical maturity in this highly promising field.

Understanding and Regulating Cell-Matrix Interactions Using Hydrogels of Designable Mechanical Properties

Similar to natural tissues, hydrogels contain abundant water, so they are considered as promising biomaterials for studying the influence of the mechanical properties of extracellular matrices (ECM) on various cell functions. In recent years, the growing research on cellular mechanical response has revealed that many cell functions, including cell spreading, migration, tumorigenesis and differentiation, are related to the mechanical properties of ECM. Therefore, how cells sense and respond to the extracellular mechanical environment has gained considerable attention. In these studies, hydrogels are widely used as the in vitro model system. Hydrogels of tunable stiffness, viscoelasticity, degradability, plasticity, and dynamical properties have been engineered to reveal how cells respond to specific mechanical features. In this review, we summarize recent process in this research direction and specifically focus on the influence of the mechanical properties of the ECM on cell functions, how cells sense and respond to the extracellular mechanical environment, and approaches to adjusting the stiffness of hydrogels.

Self-Assembling Peptide-Based Hydrogels in Angiogenesis

Ischemic diseases, especially in the heart and the brain, have become a serious threat to human health. Growth factor and cell therapy are emerging as promising therapeutic strategies; however, their retention and sustainable functions in the injured tissue are limited. Self-assembling peptide (SAP)-based hydrogels, mimicking the extracellular matrix, are therefore introduced to encapsulate and controllably release cells, cell-derived exosomes or growth factors, thus promoting angiogenesis and tissue recovery after ischemia. We will summarize the classification, composition and structure of SAPs, and the influencing factors for SAP gelation. Moreover, we will describe the functionalized SAPs, and the combinatorial therapy of cells, exosomes or growth factors with functionalized SAPs for angiogenic process as well as its advantage in immunogenicity and injectability. Finally, an outlook on future directions and challenges is provided.

Review of Integrin-Targeting Biomaterials in Tissue Engineering

The ability to direct cell behavior has been central to the success of numerous therapeutics to regenerate tissue or facilitate device integration. Biomaterial scientists are challenged to understand and modulate the interactions of biomaterials with biological systems in order to achieve effective tissue repair. One key area of research investigates the use of extracellular matrix-derived ligands to target specific integrin interactions and induce cellular responses, such as increased cell migration, proliferation, and differentiation of mesenchymal stem cells. These integrin-targeting proteins and peptides have been implemented in a variety of different polymeric scaffolds and devices to enhance tissue regeneration and integration. This review first presents an overview of integrin-mediated cellular processes that have been identified in angiogenesis, wound healing, and bone regeneration. Then, research utilizing biomaterials are highlighted with integrin-targeting motifs as a means to direct these cellular processes to enhance tissue regeneration. In addition to providing improved materials for tissue repair and device integration, these innovative biomaterials provide new tools to probe the complex processes of tissue remodeling in order to enhance the rational design of biomaterial scaffolds and guide tissue regeneration strategies.

Ocular co-Delivery of Timolol and Brimonidine from a Self-Assembling Peptide Hydrogel for the Treatment of Glaucoma: In Vitro and Ex Vivo Evaluation

Effective pharmacotherapy during glaucoma treatment depends on interventions that reduce intraocular pressure (IOP) and retain the IOP lowering effect for sufficient time so as to reduce dosing frequency and enhance patient adherence. Combination anti-glaucoma therapy and dosage forms that increase precorneal residence time could therefore constitute a promising therapeutic intervention. The in-situ gel forming self-assembling peptide ac-(RADA)4-CONH2 was evaluated as carrier for the ocular co-delivery of timolol maleate (TM) and brimonidine tartrate (BR). The hydrogel's microstructure and mechanical properties were assessed with atomic force microscopy and rheology, respectively. Drug diffusion from the hydrogel was evaluated in vitro in simulated tear fluid and ex vivo across porcine corneas and its effect on the treated corneas was assessed through physicochemical characterization and histological analysis. Results indicated that TM and BR co-delivery affected hydrogel's microstructure resulting in shorter nanofibers and a less rigid hydrogel matrix. Rapid and complete release of both drugs was achieved within 8 h, while a 2.8-fold and 5.4-fold higher corneal permeability was achieved for TM and BR, respectively. No significant alterations were induced in the structural integrity of the corneas treated with the hydrogel formulation, suggesting that self-assembling peptide hydrogels might serve as promising systems for combination anti-glaucoma therapy.

Hybrid Nanofiber Scaffold-Based Direct Conversion of Neural Precursor Cells/Dopamine Neurons

The concept of cellular reprogramming was developed to generate induced neural precursor cells (iNPCs)/dopaminergic (iDA) neurons using diverse approaches. Here, we investigated the effects of various nanoscale scaffolds (fiber, dot, and line) on iNPC/iDA differentiation by direct reprogramming. The generation and maturation of iDA neurons (microtubule-associated protein 2-positive and tyrosine hydroxylase-positive) and iNPCs (NESTIN-positive and SOX2-positive) increased on fiber and dot scaffolds as compared to that of the flat (control) scaffold. This study demonstrates that nanotopographical environments are suitable for direct differentiation methods and may improve the differentiation efficiency.

Peptide Self-Assembly into Hydrogels for Biomedical Applications Related to Hydroxyapatite

Amphiphilic peptides can be self-assembled by establishing physical cross-links involving hydrogen bonds and electrostatic interactions with divalent ions. The derived hydrogels have promising properties due to their biocompatibility, reversibility, trigger capability, and tunability. Peptide hydrogels can mimic the extracellular matrix and favor the growth of hydroxyapatite (HAp) as well as its encapsulation. Newly designed materials offer great perspectives for applications in the regeneration of hard tissues such as bones, teeth, and cartilage. Furthermore, development of drug delivery systems based on HAp and peptide self-assembly is attracting attention.

Angiogenic and Osteogenic Synergy of Human Mesenchymal Stem Cells and Human Umbilical Vein Endothelial Cells Cocultured on a Nanomatrix

To date, bone tissue regeneration strategies lack an approach that effectively provides an osteogenic and angiogenic environment conducive to bone growth. In the current study, we evaluated the osteogenic and angiogenic response of human mesenchymal stem cells (hMSCs) and green fluorescent protein-expressing human umbilical vein endothelial cells (GFP-HUVECs) cocultured on a self-assembled, peptide amphiphile nanomatrix functionalized with the cell adhesive ligand RGDS (PA-RGDS). Analysis of alkaline phosphatase activity, von Kossa staining, Alizarin Red quantification, and osteogenic gene expression, indicates a significant synergistic effect between the PA-RGDS nanomatrix and coculture that promoted hMSC osteogenesis. In addition, coculturing on PA-RGDS resulted in enhanced HUVEC network formation and upregulated vascular endothelial growth factor gene and protein expression. Though PA-RGDS and coculturing hMSCs with HUVECs were each previously reported to individually enhance hMSC osteogenesis, this study is the first to demonstrate a synergistic promotion of HUVEC angiogenesis and hMSC osteogenesis by integrating coculturing with the PA-RGDS nanomatrix. We believe that using the combination of hMSC/HUVEC coculture and PA-RGDS substrate is an efficient method for promoting osteogenesis and angiogenesis, which has immense potential as an efficacious, engineered platform for bone tissue regeneration.

Peptide Amphiphile Micelle Vaccine Size and Charge Influence the Host Antibody Response

Vaccines are one of the best health care advances ever developed, having led to the eradication of smallpox and near eradication of polio and diphtheria. While tremendously successful, traditional vaccines (i.e., whole-killed or live-attenuated) have been associated with some undesirable side effects, including everything from mild injection site inflammation to the autoimmune disease Guillain-Barré syndrome. This has led recent research to focus on developing subunit vaccines (i.e., protein, peptide, or DNA vaccines) since they are inherently safer because they deliver only the bioactive components necessary (i.e., antigens) to produce a protective immune response against the pathogen of interest. However, a major challenge in developing subunit vaccines is overcoming numerous biological barriers to effectively deliver the antigen to the secondary lymphoid organs where adaptive immune responses are orchestrated. Peptide amphiphile micelles are a class of biomaterials that have been shown to possess potent self-adjuvanting vaccine properties, but their optimization capacity and underlying immunostimulatory mechanism are not well understood. The present work investigated the influence of micelle size and charge on the materials' bioactivity, including lymph node accumulation, cell uptake ability, and immunogenicity. The results generated provide considerable insight into how micelles exert their biological effects, yielding a micellar toolbox that can be exploited to either enhance or diminish host immune responses. This exciting development makes peptide amphiphile micelles an attractive candidate for both immune activation and suppression applications.

Biofunctionalized peptide nanofiber-based composite scaffolds for bone regeneration

Bone tissue had moderate self-healing capabilities, but biomaterial scaffolds were required for the repair of some defects such as large bone defects. Peptide nanofiber scaffolds demonstrated important potential in regenerative medicine. Functional modification and controlled release of signal molecules were two significant approaches to increase the bioactivity of biofunctionalized peptide nanofiber scaffolds, but peptide scaffolds were limited by insufficient mechanical strength. Thus, it was necessary to combine peptide scaffolds with other materials including polymers, hydroxyapatite, demineralized bone matrix (DBM) and metal materials based on the requirement of different bone defects. As the development of peptide-based composite scaffolds continued to evolve, ultimate translation to the clinical environment may allow for improved therapeutic outcomes for bone repair.

Driving mesenchymal stem cell differentiation from self-assembled monolayers

The utilization of self-assembled monolayer (SAM) systems to direct Mesenchymal Stem Cell (MSC) differentiation has been covered in the literature for years, but finding a general consensus pertaining to its exact role over the differentiation of stem cells had been rather challenging. Although there are numerous reports on surface functional moieties activating and inducing differentiation, the results are often different between reports due to the varying surface conditions, such as topography or surface tension. Herein, in view of the complexity of the subject matter, we have sought to catalogue the recent developments around some of the more common functional groups on predominantly hard surfaces and how these chemical groups may influence the overall outcome of the mesenchymal stem cells (MSC) differentiation so as to better establish a clearer underlying relationship between stem cells and their base substratum interactions.

Graphical illustration showing the functional groups that drive MSC differentiation without soluble bioactive cues within the first 14 days.

Encapsulation of Human Islets Using a Biomimetic Self-Assembled Nanomatrix Gel for Protection against Cellular Inflammatory Responses

The major concern of pancreatic islet transplantation is that the implanted islets are exposed to the immune system of the recipient. To overcome this challenge, the peptide amphiphile (PA) nanomatrix gel was used for immunoisolation of islets through microencapsulation. The PA can self-assemble to form a nanomatrix gel with an extracellular matrix-mimicking, islet nurturing microenvironment and a semipermeable immune barrier. In this study, the islet protective effect of the PA nanomatrix gel was evaluated by coculture of PA-encapsulated human islets with differentiated U937 cells (human monocyte cell-line) for 3 and 7 days. The coculture of the bare islets with the differentiated U937 cells stimulated proinflammatory cytokine (IL-1β and TNF-α) secretion and caused islet death after 7 days, which simulated an early inflammatory response environment after islet transplantation. The PA-encapsulated islets, however, did not stimulate proinflammatory cytokine secretion and maintained islet viability up to 7 days. More insulin-producing β cells were observed when islets were PA-encapsulated than control islets with the differentiated U937 cells for 7 days compared to the bare islets. This result was also confirmed by dithizone staining analysis. Further evaluation of islet functionality was assessed by a glucose-stimulated insulin secretion test. The PA-encapsulated islets showed greater insulin secretion response to glucose stimulation than the bare islets with the differentiated U937 cells after 3 and 7 days. These results demonstrated that islet encapsulation with the PA nanomatrix gel was able to improve islet survival and function in the presence of inflammatory responses, which will increase the success rate of islet engraftment and the efficacy of islet transplantation.

Osteogenic differentiation of 3D cultured mesenchymal stem cells induced by bioactive peptides

OBJECTIVES

Bioactive peptides derived from receptor binding motifs of native proteins are a potent source of bioactive molecules that can induce signalling pathways. These peptides could substitute for osteogenesis promoting supplements. The work presented here compares three kinds of bioactive peptides derived from collagen III, bone morphogenetic protein 7 (BMP-7) and BMP-2 with their potential osteogenic activity on the model of porcine mesenchymal stem cells (pMSCs).

MATERIALS AND METHODS

pMSCs were cultured on electrospun polycaprolactone nanofibrous scaffolds with different concentrations of the bioactive peptides without addition of any osteogenic supplement. Analysis of pMSCs cultures included measurement of the metabolic activity and proliferation, immunofluorescence staining and also qPCR.

RESULTS

Results showed no detrimental effect of the bioactive peptides to cultured pMSCs. Based on qPCR analysis, the bioactive peptides are specific for osteogenic differentiation with no detectable expression of collagen II. Our results further indicate that peptide derived from BMP-2 protein promoted the expression of mRNA for osteocalcin (OCN) and collagen I significantly compared to control groups and also supported deposition of OCN as observed by immunostaining method.

CONCLUSION

The data suggest that bioactive peptide with an amino acid sequence of KIPKASSVPTELSAISTLYL derived from BMP-2 protein was the most potent for triggering osteogenic differentiation of pMSCs.

Presentation of functional groups on self-assembled supramolecular peptide nanofibers mimicking glycosaminoglycans for directed mesenchymal stem cell differentiation

Organizational complexity and functional diversity of the extracellular matrix regulate cellular behaviors.

Osteoinduction of stem cells by collagen peptide-immobilized hydrolyzed poly(butylene succinate)/β-tricalcium phosphate scaffold for bone tissue engineering

Bone substitute is a therapeutic approach to treat bone abnormalities. A scaffold serves mainly as osteoconductive elements. To facilitate a better biological performance, short collagen peptide was immobilized onto hydrolyzed poly(butylene succinate)/β-tricalcium phosphate (HPBSu/TCP) scaffolds. PBSu/TCP (80:20) scaffolds were fabricated by a supercritical CO2 technique, hydrolyzed with 0.6 M NaOH and conjugated with short collagen peptide tagged with or without red fluorescence. The surface morphology and porous structure of scaffolds were characterized by scanning electron microscopy and micro-computed tomography. Human mesenchymal stem cells were cultured onto the scaffolds and examined for osteogenic differentiation and biomineralization in vitro by means of alkaline phosphatase activity, alizarin red staining, and reverse transcription-polymerase chain reaction. The PBSu/TCP and HPBSu/TCP scaffolds were successfully prepared. Scanning electron microscopy and micro-computed tomography results showed that the porosity was distributed throughout the scaffolds with the pore sizes in the range of 250–900 µm. Fluorescence microscopy demonstrated retention of tagged short collagen peptide on the scaffold. Mesenchymal stem cells adhered and grew well on the material. Under osteogenic induction, cells cultured on the short collagen peptide -immobilized scaffold significantly produced a greater amount of alkaline phosphatase activity and positive mineralization than those cultured on the control scaffold. The present results have shown that the short collagen peptide-immobilized HPBSu/TCP scaffold enhanced osteoinduction and biomineralization of stem cell-derived osteoblasts, possibly via stimulation of alkaline phosphatase activity. This suggests the potential use of osteogenic peptide-immobilized material in bone tissue engineering for correcting bone defects.

Adhesive Peptide Sequences Regulate Valve Interstitial Cell Adhesion, Phenotype and Extracellular Matrix Deposition

Knowledge of how extracellular matrix (ECM) binding impacts valve interstitial cells (VICs) is critical not only to better understanding the etiology of valvular diseases but also to constructing living valve substitutes that can grow and remodel. Use of ECM-mimicking adhesive peptides with specific affinity to different receptors provides insights into adhesion-mediated cell signaling and downstream outcomes. Expression of adhesion receptors by VICs was assessed by flow cytometry and used to guide the choice of peptides studied. The peptide RGDS with affinity to multiple integrin receptors, and specific receptor-targeting peptides DGEA (integrin α2β1), YIGSR (67kDa laminin/elastin receptor; 67LR), and VAPG (67LR) were incorporated into hydrogels to investigate their effects on VICs. DGEA, YIGSR, and VAPG alone were insufficient to induce stable VIC adhesion. As a result, these peptides were studied in combination with 1 mM RGDS. For VICs cultured on two-dimensional hydrogel surfaces, YIGSR and VAPG down-regulated the expression of smooth muscle α-actin (myofibroblast activation marker); DGEA promoted VIC adhesion and VIC-mediated ECM deposition and inhibited the activity of alkaline phosphatase (osteogenic differentiation marker). Further, YIGSR and DGEA in combination promoted ECM deposition while inhibiting both myofibroblastic and osteogenic differentiation. However, VICs behaved differently to adhesive ligands when cultured within three-dimensional hydrogels, with most VICs assuming a healthy, quiescent phenotype under all peptide conditions tested. DGEA promoted ECM deposition by VICs within hydrogels. Overall, we demonstrate that the presentation of defined peptides targeting specific adhesion receptors can be used to regulate VIC adhesion, phenotype and ECM synthesis.

Modular Peptide Amphiphile Micelles Improving an Antibody-Mediated Immune Response to Group A Streptococcus

Inducing a strong and specific immune response is the hallmark of a successful vaccine. Nanoparticles have emerged as promising vaccine delivery devices to discover and elicit immune responses. Fine-tuning a nanoparticle vaccine to create an immune response with specific antibody and other cellular responses is influenced by many factors such as shape, size, and composition. Peptide amphiphile micelles are a unique biomaterials platform that can function as a modular vaccine delivery system, enabling control over many of these important factors and delivering payloads more efficiently to draining lymph nodes. In this study, the modular properties of peptide amphiphile micelles are utilized to improve an immune response against a Group A Streptococcus B cell antigen (J8). The hydrophobic/hydrophilic interface of peptide amphiphile micelles enabled the precise entrapment of amphiphilic adjuvants which were found to not alter micelle formation or shape. These heterogeneous micelles significantly enhanced murine antibody responses when compared to animals vaccinated with nonadjuvanted micelles or soluble J8 peptide supplemented with a classical adjuvant. The heterogeneous micelle induced antibodies also showed cross-reactivity with wild-type Group A Streptococcus providing evidence that micelle-induced immune responses are capable of identifying their intended pathogenic targets.

Biomimetic microenvironments for regenerative endodontics

Regenerative endodontics has been proposed to replace damaged and underdeveloped tooth structures with normal pulp-dentin tissue by providing a natural extracellular matrix (ECM) mimicking environment; stem cells, signaling molecules, and scaffolds. In addition, clinical success of the regenerative endodontic treatments can be evidenced by absence of signs and symptoms; no bony pathology, a disinfected pulp, and the maturation of root dentin in length and thickness. In spite of the various approaches of regenerative endodontics, there are several major challenges that remain to be improved: a) the endodontic root canal is a strong harbor of the endodontic bacterial biofilm and the fundamental etiologic factors of recurrent endodontic diseases, (b) tooth discolorations are caused by antibiotics and filling materials, (c) cervical root fractures are caused by endodontic medicaments, (d) pulp tissue is not vascularized nor innervated, and (e) the dentin matrix is not developed with adequate root thickness and length. Generally, current clinical protocols and recent studies have shown a limited success of the pulp-dentin tissue regeneration. Throughout the various approaches, the construction of biomimetic microenvironments of pulp-dentin tissue is a key concept of the tissue engineering based regenerative endodontics. The biomimetic microenvironments are composed of a synthetic nano-scaled polymeric fiber structure that mimics native pulp ECM and functions as a scaffold of the pulp-dentin tissue complex. They will provide a framework of the pulp ECM, can deliver selective bioactive molecules, and may recruit pluripotent stem cells from the vicinity of the pulp apex. The polymeric nanofibers are produced by methods of self-assembly, electrospinning, and phase separation. In order to be applied to biomedical use, the polymeric nanofibers require biocompatibility, stability, and biodegradability. Therefore, this review focuses on the development and application of the biomimetic microenvironments of pulp-dentin tissue among the current regenerative endodontics.

Enhancement of the Regenerative Potential of Anorganic Bovine Bone Graft Utilizing a Polyglutamate-Modified BMP2 Peptide with Improved Binding to Calcium-Containing Materials

Autogenous bone is the gold standard material for bone grafting in craniofacial and orthopedic regenerative medicine. However, due to complications associated with harvesting donor bone, clinicians often use commercial graft materials that may lose their osteoinductivity due to processing. This study was aimed to functionalize one of these materials, anorganic bovine bone (ABB), with osteoinductive peptides to enhance regenerative capacity. Two peptides known to induce osteoblastic differentiation of mesenchymal stem cells were evaluated: (1) DGEA, an amino acid motif within collagen I and (2) a biomimetic peptide derived from bone morphogenic protein 2 (BMP2pep). To achieve directed coupling of the peptides to the graft surface, the peptides were engineered with a heptaglutamate domain (E7), which confers specific binding to calcium moieties within bone mineral. Peptides with the E7 domain exhibited greater anchoring to ABB than unmodified peptides, and E7 peptides were retained on ABB for at least 8 weeks in vivo. To assess the osteoinductive potential of the peptide-conjugated ABB, ectopic bone formation was evaluated utilizing a rat subcutaneous pouch model. ABB conjugated with full-length recombinant BMP2 (rBMP2) was also implanted as a model for current clinical treatments utilizing rBMP2 passively adsorbed to carriers. These studies showed that E7BMP2pep/ABB samples induced more new bone formation than all other peptides, and an equivalent amount of new bone as compared with rBMP2/ABB. A mandibular defect model was also used to examine intrabony healing of peptide-conjugated ABB. Bone healing was monitored at varying time points by positron emission tomography imaging with (18)F-NaF, and it was found that the E7BMP2pep/ABB group had greater bone metabolic activity than all other groups, including rBMP2/ABB. Importantly, animals implanted with rBMP2/ABB exhibited complications, including inflammation and formation of cataract-like lesions in the eye, whereas no side effects were observed with E7BMP2pep/ABB. Furthermore, histological analysis of the tissues revealed that grafts with rBMP2, but not E7BMP2pep, induced formation of adipose tissue in the defect area. Collectively, these results suggest that E7-modified BMP2-mimetic peptides may enhance the regenerative potential of commercial graft materials without the deleterious effects or high costs associated with rBMP2 treatments.

Bone Tissue Engineering: Past-Present-Future

Bone is one of the few tissues to display a true potential for regeneration. Fracture healing is an obvious example where regeneration occurs through tightly regulated sequences of molecular and cellular events which recapitulate tissue formation seen during embryogenesis. Still in some instances, bone regeneration does not occur properly (i.e. critical size lesions) and an appropriate therapeutic intervention is necessary. Successful replacement of bone by tissue engineering will likely depend on the recapitulation of this flow of events. In fact, bone regeneration requires cross-talk between microenvironmental factors and cells; for example, resident mesenchymal progenitors are recruited and properly guided by soluble and insoluble signaling molecules. Tissue engineering attempts to reproduce and to mimic this natural milieu by delivering cells capable of differentiating into osteoblasts, inducing growth factors and biomaterials to support cellular attachment, proliferation, migration, and matrix deposition. In the last two decades, a significant effort has been made by the scientific community in the development of methods and protocols to repair and regenerate tissues such as bone, cartilage, tendons, and ligaments. In this same period, great advancements have been achieved in the biology of stem cells and on the mechanisms governing "stemness". Unfortunately, after two decades, effective clinical translation does not exist, besides a few limited examples. Many years have passed since cell-based regenerative therapies were first described as "promising approaches", but this definition still engulfs the present literature. Failure to envisage translational cell therapy applications in routine medical practice evidences the existence of unresolved scientific and technical struggles, some of which still puzzle researchers in the field and are presented in this chapter.

Comparing variable-length polyglutamate domains to anchor an osteoinductive collagen-mimetic peptide to diverse bone grafting materials

PURPOSE

Allografts, xenografts, and alloplasts are commonly used in craniofacial medicine as alternatives to autogenous bone grafts; however, these materials lack important bone-inducing proteins. A method for enhancing the osteoinductive potential of these commercially available materials would provide a major clinical advance. In this study, a calcium-binding domain, polyglutamate, was added to an osteoinductive peptide derived from collagen type I, Asp-Gly-Glu-Ala (DGEA), to anchor the peptide onto four different materials: freeze-dried bone allograft (FDBA); anorganic bovine bone (ABB); β-tricalcium phosphate (β-TCP); and a calcium sulfate bone cement (CaSO4). The authors also examined whether peptide binding and retention could be tuned by altering the number of glutamate residues within the polyglutamate domain.

MATERIALS AND METHODS

DGEA or DGEA modified with diglutamate (E2DGEA), tetraglutamate (E4DGEA), or heptaglutamate (E7DGEA) were evaluated for binding and release to the grafting materials. Peptides were conjugated with a fluorescein isothiocyanate (FITC) tag to allow monitoring by fluorescent microscopy or through measurements of solution fluorescence. In vivo retention was evaluated by implanting graft materials coated with FITC-peptides into rat subcutaneous pouches.

RESULTS

Significantly more peptide was loaded onto the four graft materials as the number of glutamates increased, with E7DGEA exhibiting the greatest binding. There was also significantly greater retention of peptides with longer glutamate domains following a 3-day incubation with agitation. Importantly, E7DGEA peptides remained on the grafts after a 2-month implantation into skin pouches, a sufficient interval to influence bony healing.

CONCLUSION

Variable-length polyglutamate domains can be added to osteoinductive peptides to control the amount of peptide bound and rate of peptide released. The lack of methods for tunable coupling of biologics to commercial graft sources has been a major barrier toward developing materials that approach the clinical efficacy of autogenous bone. Modification of osteoinductive factors with polyglutamate domains constitutes a technically straightforward and cost-effective strategy for enhancing osteoinductivity of diverse graft products.

Physical Stimuli-Induced Chondrogenic Differentiation of Mesenchymal Stem Cells Using Magnetic Nanoparticles

Chondrogenic commitments of mesenchymal stem cells (MSCs) require 3D cellular organization. Furthermore, recent progresses in bioreactor technology have contributed to the development of various biophysical stimulation platforms for efficient cartilage tissue formation. Here, an approach is reported to drive 3D cellular organization and enhance chondrogenic commitment of bone-marrow-derived human mesenchymal stem cells (BM-hMSCs) via magnetic nanoparticle (MNP)-mediated physical stimuli. MNPs isolated from Magnetospirillum sp. AMB-1 are endocytosed by the BM-hMSCs in a highly efficient manner. MNPs-incorporated BM-hMSCs are pelleted and then subjected to static magnetic field and/or magnet-derived shear stress. Magnetic-based stimuli enhance level of sulfated glycosaminoglycan (sGAG) and collagen synthesis, and facilitate the chondrogenic differentiation of BM-hMSCs. In addition, both static magnetic field and magnet-derived shear stress applied for the chondrogenic differentiation of BM-hMSCs do not show increament of hypertrophic differentiation. This MNP-mediated physical stimulation platform demonstrates a promising strategy for efficient cartilage tissue engineering.

Three-Dimensional Cell Entrapment as a Function of the Weight Percent of Peptide-Amphiphile Hydrogels

The design of scaffolds which mimic the stiffness, nanofiber structure, and biochemistry of the native extracellular matrix (ECM) has been a major objective for the tissue engineering field. Furthermore, mimicking the innate three-dimensional (3D) environment of the ECM has been shown to significantly altered cellular response compared to that of traditional two-dimensional (2D) culture. We report the development of a self-assembling, fibronectin-mimetic, peptide-amphiphile nanofiber scaffold for 3D cell culture. To form such a scaffold, 5 mol % of a bioactive PR_g fibronectin-mimetic peptide-amphiphile was mixed with 95 mol % of a diluent peptide-amphiphile (E2) whose purpose was to neutralize electrostatic interactions, increase the gelation kinetics, and promote cell survival. Atomic force microscopy verified the fibrilar structure of the gels, and the mechanical properties were characterized for various weight percent (wt %) formulations of the 5 mol % PR_g-95 mol % E2 peptide-amphiphile mixture. The 0.5 wt % formulations had an elastic modulus of 429.0 ± 21.3 Pa whereas the 1.0 wt % peptide-amphiphile hydrogels had an elastic modulus of 808.6 ± 38.1 Pa. The presence of entrapped cells in the gels decreased the elastic modulus, and the decrease was a function of cell loading. Although both formulations supported cell proliferation, the 0.5 wt % gels supported significantly greater NIH3T3/GFP fibroblast cell proliferation throughout the gels than the 1.0 wt % gels. However, compared to the 0.5 wt % formulations, the 1.0 wt % hydrogels promoted greater increases in mRNA expression and the production of fibronectin and type IV collagen ECM proteins. This study suggests that this fibronectin-mimetic scaffold holds great promise in the advancement of 3D culture applications and cell therapies.

Towards the design of 3D multiscale instructive tissue engineering constructs: Current approaches and trends

The design of 3D constructs with adequate properties to instruct and guide cells both in vitro and in vivo is one of the major focuses of tissue engineering. Successful tissue regeneration depends on the favorable crosstalk between the supporting structure, the cells and the host tissue so that a balanced matrix production and degradation are achieved. Herein, the major occurring events and players in normal and regenerative tissue are overviewed. These have been inspiring the selection or synthesis of instructive cues to include into the 3D constructs. We further highlight the importance of a multiscale perception of the range of features that can be included on the biomimetic structures. Lastly, we focus on the current and developing tissue-engineering approaches for the preparation of such 3D constructs: top-down, bottom-up and integrative. Bottom-up and integrative approaches present a higher potential for the design of tissue engineering devices with multiscale features and higher biochemical control than top-down strategies, and are the main focus of this review.

Biomimetic approaches in bone tissue engineering: Integrating biological and physicomechanical strategies

The development of responsive biomaterials capable of demonstrating modulated function in response to dynamic physiological and mechanical changes in vivo remains an important challenge in bone tissue engineering. To achieve long-term repair and good clinical outcomes, biologically responsive approaches that focus on repair and reconstitution of tissue structure and function through drug release, receptor recognition, environmental responsiveness and tuned biodegradability are required. Traditional orthopedic materials lack biomimicry, and mismatches in tissue morphology, or chemical and mechanical properties ultimately accelerate device failure. Multiple stimuli have been proposed as principal contributors or mediators of cell activity and bone tissue formation, including physical (substrate topography, stiffness, shear stress and electrical forces) and biochemical factors (growth factors, genes or proteins). However, optimal solutions to bone regeneration remain elusive. This review will focus on biological and physicomechanical considerations currently being explored in bone tissue engineering.

Physical cues of cell culture materials lead the direction of differentiation lineages of pluripotent stem cells

Differentiation methods of hPSCs into specific cell lineages. Differentiation of hPSCsviaEB formation (types AB, A–D) or without EB formation (types E–H).

Biocompatible succinic acid-based polyesters for potential biomedical applications: fungal biofilm inhibition and mesenchymal stem cell growth

Poly(alkene succinates) are promising materials for specialized medical devices and tissue engineering, presenting intrinsic properties, such as; fungal biofilm inhibition, biocompatibility and stem cells controlled growth promotion.

Peptide amphiphile micelles self-adjuvant group A streptococcal vaccination

Delivery system design and adjuvant development are crucially important areas of research for improving vaccines. Peptide amphiphile micelles are a class of biomaterials that have the unique potential to function as both vaccine delivery vehicles and self-adjuvants. In this study, peptide amphiphiles comprised of a group A streptococcus B cell antigen (J8) and a dialkyl hydrophobic moiety (diC16) were synthesized and organized into self-assembled micelles, driven by hydrophobic interactions among the alkyl tails. J8-diC16 formed cylindrical micelles with highly α-helical peptide presented on their surfaces. Both the micelle length and secondary structure were shown to be enhanced by annealing. When injected into mice, J8-diC16 micelles induced a strong IgG1 antibody response that was comparable to soluble J8 peptide supplemented with two classical adjuvants. It was discovered that micelle adjuvanticity requires the antigen be a part of the micelle since separation of J8 and the micelle was insufficient to induce an immune response. Additionally, the diC16 tail appears to be non-immunogenic since it does not stimulate a pathogen recognition receptor whose agonist (Pam3Cys) possesses a very similar chemical structure. The research presented in this paper demonstrates the promise peptide amphiphile micelles have in improving the field of vaccine engineering.

A comprehensive review on the role of various materials in the osteogenic differentiation of mesenchymal stem cells with a special focus on the association of heat shock proteins and nanoparticles

Mesenchymal stem cells (MSCs) have important roles in the area of regenerative medicine and clinical applications due to their pluripotent nature. Osteogenic differentiation of MSCs has been studied extensively using various stimulants to develop models of bone repair. There are several factors that enhance the differentiation of MSCs into bone tissues. This review focuses on the effects of various inducers on the osteoblast differentiation of MSCs at different stages of cellular development. We discuss the various growth factors, hormones, vitamins, cytokines, chemical stimulants, and mechanical forces applied in bioreactors that play an essential role in the proliferation, differentiation, and matrix mineralization of stem cells during osteogenesis. Various nanoparticles have also been used recently for the same purpose and the results are promising. Moreover, we review the role of various stresses, including thermal stress, and the subsequent involvement of heat shock proteins as inducers of the proliferation and differentiation of osteoblasts. We also report how various proteasome inhibitors have been shown to induce proliferation and osteogenic differentiation of MSCs in a number of cases. In this communication, the role of peptide-based scaffolds in osteoblast proliferation and differentiation is also reviewed. Based on the reviewed information, this article proposes novel possibilities for the enhancement of proliferation, differentiation, and migration of osteoblasts from MSCs. © 2014 S. Karger AG, Basel.

Cell therapy with embryonic stem cell-derived cardiomyocytes encapsulated in injectable nanomatrix gel enhances cell engraftment and promotes cardiac repair

A significant barrier to the therapeutic use of stem cells is poor cell retention in vivo. Here, we evaluate the therapeutic potential and long-term engraftment of cardiomyocytes (CMs) derived from mouse embryonic stem cells (mESCs) encapsulated in an injectable nanomatrix gel consisting of peptide amphiphiles incorporating cell adhesive ligand Arg-Gly-Asp-Ser (PA-RGDS) in experimental myocardial infarction (MI). We cultured rat neonatal CMs in PA-RGDS for 7 days and found that more than 90% of the CMs survived. Next, we intramyocardially injected mouse CM cell line HL-1 CMs with or without PA-RGDS into uninjured hearts. Histologic examination and flow cytometry analysis of digested heart tissues showed approximately 3-fold higher engraftment in the mice that received CMs with PA-RGDS compared to those without PA-RGDS. We further investigated the therapeutic effects and long-term engraftment of mESC-CMs with PA-RGDS on MI in comparison with PBS control, CM-only, and PA-RGDS only. Echocardiography demonstrated that the CM-only and CM+PA-RGDS groups showed higher cardiac function at week 2 compared to other groups. However, from 3 weeks, higher cardiac function was maintained only in the CM+PA-RGDS group; this was sustained for 12 weeks. Confocal microscopic examination of the cardiac tissues harvested at 14 weeks demonstrated sustained engraftment and integration of mESC-CMs into host myocardium in the CM+PA-RGDS group only. This study for the first time demonstrated that PA-RGDS encapsulation can enhance survival of mESC-derived CMs and improve cardiac function post-MI. This nanomatrix gel-mediated stem cell therapy can be a promising option for treating MI.

Bioactive supramolecular peptide nanofibers for regenerative medicine

Recent advances in understanding of cell-matrix interactions and the role of the extracellular matrix (ECM) in regulation of cellular behavior have created new perspectives for regenerative medicine. Supramolecular peptide nanofiber systems have been used as synthetic scaffolds in regenerative medicine applications due to their tailorable properties and ability to mimic ECM proteins. Through designed bioactive epitopes, peptide nanofiber systems provide biomolecular recognition sites that can trigger specific interactions with cell surface receptors. The present Review covers structural and biochemical properties of the self-assembled peptide nanofibers for tissue regeneration, and highlights studies that investigate the ability of ECM mimetic peptides to alter cellular behavior including cell adhesion, proliferation, and/or differentiation.

Enhanced MIN-6 beta cell survival and function on a nitric oxide-releasing peptide amphiphile nanomatrix

Innovative biomaterial strategies are required to improve islet cell retention, viability, and functionality, and thereby obtain clinically successful outcomes from pancreatic islet cell transplantation. To address this need, we have developed a peptide amphiphile-based nanomatrix that incorporates multifunctional bioactive cues and sustained release of nitric oxide. The goal of this study was to evaluate the effect of this peptide amphiphile nanomatrix on the viability and functionality of MIN-6 islet cells. Additionally, this study provides insight into the role of nitric oxide in islet cell biology, given that conventional nitric oxide donors are unable to release nitric oxide in a controlled, sustained manner, leading to ambiguous results. It was hypothesized that controlled nitric oxide release in synergy with multifunctional bioactive cues would promote islet cell viability and functionality. Nitric oxide-releasing peptide amphiphile nanomatrices within the range of 16.25 μmol to 130 μmol were used to analyze MIN-6 cell behavior. Both 32.5 μmol and 65 μmol peptide amphiphiles showed improved MIN-6 functionality in response to glucose over a 7-day time period, and the elevated functionality was correlated with both PDX-1 and insulin gene expression. Our results demonstrate that nitric oxide has a beneficial effect on MIN-6 cells in a concentration-dependent manner.

Lysine-appended polydiacetylene scaffolds for human mesenchymal stem cells

We report on the self-assembly based fabrication of fibrous polymers for tissue engineering applications. Directed self-assembly followed by polymerization of lysine-appended diacetylenes generated a variety of polymers (P1-P5) with distinct chemical properties. The self-assembly along with the conjugated double and triple bonds and rigid geometry of diacetylene backbone imposed a nanofibrous morphology on the resulting polymers. Chemical properties including wettability of the polymers were tuned by using lysine (Lys) with orthogonal protecting groups (Boc and Fmoc). These Lys-appended polydiacetylene scaffolds were compared in terms of their efficiency toward human mesenchymal stem cells adhesion and spreading. Interestingly, polymer P4 containing Lys N(α)-NH2 and Lys N(ε)-Boc with balanced wettability supported cell adhesion better than the more hydrophobic polymer P2 with N(ε)-Boc and N(α)-Fmoc or more hydrophilic polymer P5 containing free N(ε) and N(α) amino groups. The molecular level control in the fabrication of nanofibrous polymers compared with other existing methods for the generation of fibrous polymers is the hallmark of this work.

Improved MIN6 β-cell function on self-assembled peptide amphiphile nanomatrix inscribed with extracellular matrix-derived cell adhesive ligands

Understanding the role of the pancreatic extracellular matrix (ECM) in supporting islet survival and function drives the pursuit to create biomaterials that imitate and restore the pancreatic ECM microenvironment. To create an ECM mimic holding bioinductive cues for β-cells, self-assembled peptide amphiphiles (PAs) inscribed with four selected ECM-derived cell adhesive ligands are synthesized. After 7 days, compared to control groups cultured on biologically inert substrates, MIN6 β-cells cultured on PAs functionalized with YIGSR and RGDS cell adhesive ligands exhibit elevated insulin secretion in responses to glucose and also form β-cell clusters. These findings suggest that the self-assembled PA nanomatrix may be utilized to improve pancreatic islet transplantation for treating type 1 diabetes.

Nanostructured surfaces of dental implants

The structural and functional fusion of the surface of the dental implant with the surrounding bone (osseointegration) is crucial for the short and long term outcome of the device. In recent years, the enhancement of bone formation at the bone-implant interface has been achieved through the modulation of osteoblasts adhesion and spreading, induced by structural modifications of the implant surface, particularly at the nanoscale level. In this context, traditional chemical and physical processes find new applications to achieve the best dental implant technology. This review provides an overview of the most common manufacture techniques and the related cells-surface interactions and modulation. A Medline and a hand search were conducted to identify studies concerning nanostructuration of implant surface and their related biological interaction. In this paper, we stressed the importance of the modifications on dental implant surfaces at the nanometric level. Nowadays, there is still little evidence of the long-term benefits of nanofeatures, as the promising results achieved in vitro and in animals have still to be confirmed in humans. However, the increasing interest in nanotechnology is undoubted and more research is going to be published in the coming years.

Peptide nanofiber scaffolds for multipotent stromal cell culturing

Self-assembled peptide nanofibers are versatile materials providing suitable platforms for regenerative medicine applications. This chapter describes the use of peptide nanofibers as extracellular matrix mimetic scaffolds for two-dimensional (2D) and three-dimensional (3D) multipotent stromal cell culture systems and procedures for in vitro experiments using these scaffolds. Preparation of 2D and 3D peptide nanofiber scaffolds and cell culturing procedures are presented as part of in vitro experiments including cell adhesion, viability, and spreading analysis. Analysis of cellular differentiation on peptide nanofiber scaffolds is described through immunocytochemistry, qRT-PCR, and other biochemical experiments towards osteogenic and chondrogenic lineage.

Extracellular matrix mimetic peptide scaffolds for neural stem cell culture and differentiation

Self-assembled peptide nanofibers form three-dimensional networks that are quite similar to fibrous extracellular matrix (ECM) in their physical structure. By incorporating short peptide sequences derived from ECM proteins, these nanofibers provide bioactive platforms for cell culture studies. This protocol provides information about preparation and characterization of self-assembled peptide nanofiber scaffolds, culturing of neural stem cells (NSCs) on these scaffolds, and analysis of cell behavior. As cell behavior analyses, viability and proliferation of NSCs as well as investigation of differentiation by immunocytochemistry, qRT-PCR, western blot, and morphological analysis on ECM mimetic peptide nanofiber scaffolds are described.

Polyglutamate directed coupling of bioactive peptides for the delivery of osteoinductive signals on allograft bone

Allograft bone is commonly used as an alternative to autograft, however allograft lacks many osteoinductive factors present in autologous bone due to processing. In this study, we investigated a method to reconstitute allograft with osteoregenerative factors. Specifically, an osteoinductive peptide from collagen I, DGEA, was engineered to express a heptaglutamate (E7) domain, which binds the hydroxyapatite within bone mineral. Addition of E7 to DGEA resulted in 9× greater peptide loading on allograft, and significantly greater retention after a 5-day interval with extensive washing. When factoring together greater initial loading and retention, the E7 domain directed a 45-fold enhancement of peptide density on the allograft surface. Peptide-coated allograft was also implanted subcutaneously into rats and it was found that E7DGEA was retained in vivo for at least 3 months. Interestingly, E7DGEA peptides injected intravenously accumulated within bone tissue, implicating a potential role for E7 domains in drug delivery to bone. Finally, we determined that, as with DGEA, the E7 modification enhanced coupling of a bioactive BMP2-derived peptide on allograft. These results suggest that E7 domains are useful for coupling many types of bone-regenerative molecules to the surface of allograft to reintroduce osteoinductive signals and potentially advance allograft treatments.

Hydroxyapatite nanoparticle reinforced peptide amphiphile nanomatrix enhances the osteogenic differentiation of mesenchymal stem cells by compositional ratios

In the field of bone tissue engineering, there is a need for materials that mimic the native bone extracellular matrix (ECM). This need is met through the creation of biphasic composites intended to mimic both the organic and inorganic facets of the native bone ECM. However, few studies have created composites with organic ECM analogous components capable of directing cellular behaviors and many are not fabricated in the nanoscale. Furthermore, few attempts have been made at investigating how variations of organic and inorganic components affect the osteogenic differentiation of human mesenchymal stem cells (hMSCs). To address these issues, biphasic nanomatrix composites consisting of hydroxyapatite nanoparticles (HANPs) embedded within peptide amphiphile (PA) nanofibers tailored with the RGDS cellular adhesion motif (PA-RGDS) were created at various HANP to PA-RGDS ratios. Fabrication of these biphasic nanomatrix composites was confirmed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The long-term cellularity and osteogenic differentiation of hMSCs in response to the different compositional ratios were then assessed by quantifying the timed expression of genes indicative of osteogenic differentiation, alkaline phosphatase activity, and DNA content over time. Decreased cellularity and the expression of genes over time correlated with increasing compositional ratios between HANP and PA-RGDS. The highest HANP to PA-RGDS ratio (66% HANP) exhibited the greatest improvement to the osteogenic differentiation of hMSCs. Overall, these results demonstrate that the compositional ratio of biphasic nanomatrix composites plays an important role in influencing the osteogenic differentiation of hMSCs. Based on the observations presented within this study, these biphasic nanomatrix composites show promise for future usage in bone tissue engineering applications.