Substrate mineralization stimulates focal adhesion contact redistribution and cell motility of bone marrow stromal cells

Review on material parameters to enhance bone cell function in vitro and in vivo

Bone plays critical roles in support, protection, movement, and metabolism. Although bone has an innate capacity for regeneration, this capacity is limited, and many bone injuries and diseases require intervention. Biomaterials are a critical component of many treatments to restore bone function and include non-resorbable implants to augment bone and resorbable materials to guide regeneration. Biomaterials can vary considerably in their biocompatibility and bioactivity, which are functions of specific material parameters. The success of biomaterials in bone augmentation and regeneration is based on their effects on the function of bone cells. Such functions include adhesion, migration, inflammation, proliferation, communication, differentiation, resorption, and vascularization. This review will focus on how different material parameters can enhance bone cell function both in vitro and in vivo.

Cell and Material-Specific Phage Display Peptides Increase iPS-MSC Mediated Bone and Vasculature Formation In Vivo

Biomimetically designed materials matching the chemical and mechanical properties of tissue support higher mesenchymal stem cell (MSC) adhesion. However, directing cell-specific attachment and ensuring uniform cell distribution within the interior of 3D biomaterials remain key challenges in healing critical sized defects. Previously, a phage display derived MSC-specific peptide (DPIYALSWSGMA, DPI) was combined with a mineral binding sequence (VTKHLNQISQSY, VTK) to increase the magnitude and specificity of MSC attachment to calcium-phosphate biomaterials in 2D. This study investigates how DPI-VTK influences quantity and uniformity of iPS-MSC mediated bone and vasculature formation in vivo. There is greater bone formation in vivo when iPS-MSCs are transplanted on bone-like mineral (BLM) constructs coated with DPI-VTK compared to VTK (p < 0.002), uncoated BLM (p < 0.037), acellular BLM/DPI-VTK (p < 0.003), and acellular BLM controls (p < 0.01). This study demonstrates, for the first time, the ability of non-native phage-display designed peptides to spatially control uniform cell distribution on 3D scaffolds and increase the magnitude and uniformity of bone and vasculature formation in vivo. Taken together, the study validates phage display as a novel technology platform to engineer non-native peptides with the ability to drive cell specific attachment on biomaterials, direct bone regeneration, and engineer uniform vasculature in vivo.

Cell adhesion and mechanical stimulation in the regulation of mesenchymal stem cell differentiation

Stem cells have been shown to have the potential to provide a source of cells for applications to tissue engineering and organ repair. The mechanisms that regulate stem cell fate, however, mostly remain unclear. Mesenchymal stem cells (MSCs) are multipotent progenitor cells that are isolated from bone marrow and other adult tissues, and can be differentiated into multiple cell lineages, such as bone, cartilage, fat, muscles and neurons. Although previous studies have focused intensively on the effects of chemical signals that regulate MSC commitment, the effects of physical/mechanical cues of the microenvironment on MSC fate determination have long been neglected. However, several studies provided evidence that mechanical signals, both direct and indirect, played important roles in regulating a stem cell fate. In this review, we summarize a number of recent studies on how cell adhesion and mechanical cues influence the differentiation of MSCs into specific lineages. Understanding how chemical and mechanical cues in the microenvironment orchestrate stem cell differentiation may provide new insights into ways to improve our techniques in cell therapy and organ repair.

Using polymeric materials to control stem cell behavior for tissue regeneration

Patients with organ failure often suffer from increased morbidity and decreased quality of life. Current strategies of treating organ failure have limitations, including shortage of donor organs, low efficiency of grafts, and immunological problems. Tissue engineering emerged about two decades ago as a strategy to restore organ function with a living, functional engineered substitute. However, the ability to engineer a functional organ is limited by a limited understanding of the interactions between materials and cells that are required to yield functional tissue equivalents. Polymeric materials are one of the most promising classes of materials for use in tissue engineering, due to their biodegradability, flexibility in processing and property design, and the potential to use polymer properties to control cell function. Stem cells offer potential in tissue engineering because of their unique capacity to self-renew and differentiate into neurogenic, osteogenic, chondrogenic, and myogenic lineages under appropriate stimuli from extracellular components. This review examines recent advances in stem cell-polymer interactions for tissue regeneration, specifically highlighting control of polymer properties to direct adhesion, proliferation, and differentiation of stem cells, and how biomaterials can be designed to provide some of the stimuli to cells that the natural extracellular matrix does.

The development of non-viral gene-activated matrices for bone regeneration using polyethyleneimine (PEI) and collagen-based scaffolds

The healing potential of scaffolds for tissue engineering can be enhanced by combining them with genes to produce gene-activated matrices (GAMs) for tissue regeneration. We examined the potential of using polyethyleneimine (PEI) as a vector for transfection of mesenchymal stem cells (MSCs) in monolayer culture and in 3D collagen-based GAMs. PEI-pDNA polyplexes were fabricated at a range of N/P ratios and their optimal transfection parameters (N/P 7 ratio, 2μg dose) and transfection efficiencies (30±8%) determined in monolayer culture. The polyplexes were then loaded onto collagen, collagen-glycosaminoglycan and collagen-nanohydroxyapatite scaffolds where gene expression was observed up to 21 days with a polyplex dose as low as 2μg. Transient expression profiles indicated that the GAMs act as a polyplex depot system whereby infiltrating cells become transfected over time as they migrate throughout the scaffold. The collagen-nHa GAM exhibited the most prolonged and elevated levels of transgene expression. This research has thus demonstrated that PEI is a highly efficient pDNA transfection agent for both MSC monolayer cultures and in the 3D GAM environment. By combining therapeutic gene therapy with highly engineered scaffolds, it is proposed that these GAMs might have immense capability to promote tissue regeneration.

Biomineralized composite substrates increase gene expression with nonviral delivery

Current strategies to enhance gene transfer have focused on the development of vectors to increase the efficiency of DNA delivery. However, the extracellular matrix and microenvironment have a profound impact on numerous cellular activities including spreading and proliferation; two processes that have been associated with gene transfer efficiency. This study was designed to test the hypothesis that the presence of a biomineralized coating on biodegradable substrates would affect transgene expression following nonviral gene delivery. Thin films were prepared from polymeric microspheres, while biomineralized films were fabricated from microspheres previously soaked in modified simulated body fluid. Mineralized films were significantly more rigid and had widespread mineral coverage compared with nonmineralized substrates. Human mesenchymal stem cells (MSCs) were cultured on biomineralized or nonmineralized films and transfected with plasmid DNA condensed with linear polyethyleneimine (PEI). Compared with cells transfected on nonmineralized films, increases in gene expression were detected in the presence of biomineral at all charge ratios examined. We observed increased uptake of both PEI and DNA by cells on mineralized films. The results of these studies offer an approach to modulate gene delivery and improve the potential benefit of nonviral gene delivery approaches.

Apatite content of collagen materials dose-dependently increases pre-osteoblastic cell deposition of a cement line-like matrix

Bone matrix, mainly composed of type I collagen and apatite, is constantly modified during the bone remodeling process, which exposes bone cells to various proportions of mineralized collagen within bone structural units. Collagen-mineralized substrates have been shown to increase osteoblast activities. We hypothesized that such effects may be explained by a rapid secretion of specific growth factors and/or deposition of specific matrix proteins. Using MC3T3-E1 seeded for 32h on collagen substrates complexed with various apatite contents, we found that pre-osteoblasts in contact with mineralized collagen gave rise to a dose-dependent deposit of Vascular Endothelial Growth Factor-A (VEGF-A) and RGD-containing proteins such as osteopontin (OPN) and fibronectin (FN). This RGD-matrix deposition reinforced the cell adhesion to collagen-mineralized substrates. It was also observed that, on these substrates, this matrix was elaborated concomitantly to an increased cell migration, allowing a homogeneous coverage of the sample. This particular surface activation was probably done firstly to reinforce cell survival (VEGF-A) and adhesion (OPN, FN) and secondly to recruit and prepare surfaces for subsequent bone cell activity.

Porous polymer/hydroxyapatite scaffolds: characterization and biocompatibility investigations

Poly-lactic-glycolic acid (PLGA) has been widely used as a scaffold material for bone tissue engineering applications. 3D sponge-like porous scaffolds have previously been generated through a solvent casting and salt leaching technique. In this study, polymer-ceramic composite scaffolds were created by immersing PLGA scaffolds in simulated body fluid, leading to the formation of a hydroxyapatite (HAP) coating. The presence of a HAP layer was confirmed using scanning electron microscopy, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy in attenuated total reflection mode. HAP-coated PLGA scaffolds were tested for their biocompatibility in vitro using human osteoblast cell cultures. Biocompatibility was assessed by standard tests for cell proliferation (MTT, WST), as well as fluorescence microscopy after standard cell vitality staining procedures. It was shown that PLGA-HAP composites support osteoblast growth and vitality, paving the way for applications as bone tissue engineering scaffolds.

Connexin 43 as a signaling platform for increasing the volume and spatial distribution of regenerated tissue

Gap junction intercellular communication (GJIC) is ubiquitous in the majority of vertebrate cells and is required for the proper development of most tissues. The loss of gap junction-mediated cell-to-cell communication leads to compromised development in many tissues and organs. Because cells constantly interact through gap junctions to coordinate tissue functions and homeostasis, we hypothesized that increasing cell-to-cell communication, via genetically engineering cells to overexpress gap junction proteins, could enhance cell differentiation in the interior regions of 3D tissue equivalents, thereby increasing the ability to regenerate larger and more uniform volumes of tissue. To test this hypothesis, we used bone as a model tissue because of the difficulty in achieving spatially uniform bone regeneration in 3D. In bone marrow stromal cells (BMSC), GJIC and osteogenic differentiation were compromised in 3D cultures relative to 2D monolayers and in the core of 3D cultures relative to the surface. Overexpression of connexin 43 (Cx43) via transduction of BMSCs with a lentivirus overcame this problem, enhancing both the magnitude and spatial distribution of GJIC and osteogenic differentiation markers throughout 3D constructs. Transplantation of cells overexpressing Cx43 resulted in an increased volume fraction and spatial uniformity of bone in vivo, relative to nontransduced BMSCs. Increased GJIC also enhanced the effect of a potent osteoinductive agent (BMP-7), suggesting a synergism between the soluble factor and GJIC. These findings present a platform to improve cell-to-cell communication in 3D and to achieve uniformly distributed tissue regeneration in 3D.

Tissue engineering: state of the art in oral rehabilitation

More than 85% of the global population requires repair or replacement of a craniofacial structure. These defects range from simple tooth decay to radical oncologic craniofacial resection. Regeneration of oral and craniofacial tissues presents a formidable challenge that requires synthesis of basic science, clinical science and engineering technology. Identification of appropriate scaffolds, cell sources and spatial and temporal signals (the tissue engineering triad) is necessary to optimize development of a single tissue, hybrid organ or interface. Furthermore, combining the understanding of the interactions between molecules of the extracellular matrix and attached cells with an understanding of the gene expression needed to induce differentiation and tissue growth will provide the design basis for translating basic science into rationally developed components of this tissue engineering triad. Dental tissue engineers are interested in regeneration of teeth, oral mucosa, salivary glands, bone and periodontium. Many of these oral structures are hybrid tissues. For example, engineering the periodontium requires growth of alveolar bone, cementum and the periodontal ligament. Recapitulation of biological development of hybrid tissues and interfaces presents a challenge that exceeds that of engineering just a single tissue. Advances made in dental interface engineering will allow these tissues to serve as model systems for engineering other tissues or organs of the body. This review will begin by covering basic tissue engineering principles and strategic design of functional biomaterials. We will then explore the impact of biomaterials design on the status of craniofacial tissue engineering and current challenges and opportunities in dental tissue engineering.

Effect of ionic activity products on the structure and composition of mineral self assembled on three-dimensional poly(lactide-co-glycolide) scaffolds

A biomimetic approach involving the self-assembly of mineral within the pores of three-dimensional porous polymer scaffolds is a promising strategy to integrate advantages of inorganic and organic phases into a single material for hard tissue engineering. Such a material enhances the ability of progenitor cells to differentiate down an osteoblast lineage in vitro and in vivo, compared with polymer scaffolds. The mechanisms regulating mineral formation in this one-step process, however, are poorly understood, especially the effects of ionic activity products (IP) of the mineralizing solution and incubation time. The aims of this study were to define the structure and composition of mineral formed within the pores of biodegradable polymer scaffolds as a function of IP and time. Three-dimensional poly(lactide-co-glycolide) scaffolds were fabricated by solvent casting/particulate leaching and incubated for 4-16 days in six variants of simulated body fluid whose IPs were varied by adjusting ionic concentrations. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy demonstrated the formation of carbonated apatite with sub-micrometer sized crystals that grew into spherical globules extending out of the scaffold pore surfaces. As IP increased, more mineral grew on the scaffold pore surfaces, but the apatite became less crystalline and the Ca/P molar ratio decreased from 1.63 +/- 0.005 to 1.51 +/- 0.002. Since morphology, composition, and structure of mineral are factors that affect cell function, this study demonstrates that the IP of the mineralizing solution is an important modulator of material properties, potentially leading to enhanced control of cell function.