Stem cell plasticity, osteogenic differentiation and the third dimension

Development of an integrin αv-based universal marker, capable of both prediction and direction of stem cell fate

Integrin-mediated focal adhesion (FA) and subsequent cytoskeletal reorganization influence cell morphology, migration, and ultimately cell fate. Previous studies have used various patterned surfaces with defined macroscopic cell shapes or nanoscopic FA distributions to explore how different substrates affect the fate of human bone marrow mesenchymal stem cells (BMSCs). However, there is currently no straightforward relationship between BMSC cell fates induced by patterned surfaces and FA distribution substrates. In this study, we conducted single-cell image analysis of integrin αv-mediated FA and cell morphological features of BMSCs during biochemically induced differentiation. This enabled the identification of distinct FA features that can discriminate between osteogenic and adipogenic differentiation, demonstrating that integrin αv-mediated focal adhesion (FA) can be used as a non-invasive biomarker for real time observation. Based on these results, we developed an organized microscale fibronectin (FN) patterned surface where the fate of BMSC could be precisely manipulated by these FA features. Notably, even in the absence of any biochemical inducers, such as those contained in the differentiation medium, BMSCs cultured on these FN patterned surfaces exhibited upregulation of differentiation markers comparable to BMSCs cultured using conventional differentiation methods. Therefore, the present study reveals the application of these FA features as universal markers not only for predicting differentiation status, but also for regulating cell fate by precisely controlling the FA features with a new cell culture platform. STATEMENT OF SIGNIFICANCE: Although the effects of material physiochemical properties on cell morphology and subsequent cell fate decisions have been extensively studied, a simple yet intuitive correlation between cellular features and differentiation remains unavailable. We present a single cell image-based strategy for predicting and directing stem cell fate. By using a specific integrin isoform, integrin αv, we identified distinct geometric features that can be used as a marker for discriminating between osteogenic and adipogenic differentiation in real-time. From these data, new cell culture platforms capable of regulating cell fate by precisely controlling FA features and cell area can be developed.

A quantitative approach for determining the role of geometrical constraints when shaping mesenchymal condensations

In embryogenesis, mesenchymal condensation is a critical event during the formation of many organ systems, including cartilage and bone. During organ formation, mesenchymal cells aggregate and undergo compaction while activating developmental programmes. The final three-dimensional form of the organ, as well as cell fates, can be influenced by the size and shape of the forming condensation. This process is hypothesized to result from multiscale cell interactions within mesenchymal microenvironments; however, these are complex to investigate in vivo. Three-dimensional in vitro models that recapitulate key phenotypes can contribute to our understanding of the microenvironment interactions regulating this fundamental developmental process. Here we devise such models by using image analysis to guide the design of polydimethylsiloxane 3D microstructures as cell culture substrates. These microstructures establish geometrically constrained micromass cultures of mouse embryonic skeletal progenitor cells which influence the development of condensations. We first identify key phenotypes differentiating face and limb bud micromass cultures by linear discriminant analysis of the shape descriptors for condensation morphology, which are used to guide the rational design of a micropatterned polydimethylsiloxane substrate. High-content imaging analysis highlights that the geometry of the microenvironment affects the establishment and growth of condensations. Further, cells commit to establish condensations within the first 5 h; condensations reach their full size within 17 h; following which they increase cell density while maintaining size for at least 7 days. These findings elucidate the value of our model in dissecting key aspects of mesenchymal condensation development.

Three-Dimensional Stem Cell Bioprinting

Stem cells have become a revived biotechnology that is beginning to expand the field of regenerative medicine. Although stem cells are capable of regenerating tissues, current research trends tend to side on developing fully functional organs and other clinical uses including in situ stem cell repair through three-dimensional printing methods. Through several tests and techniques, it can be shown that most stem cell printing methods are possible and that most tests come out with high cell viability. Furthermore, the importance of bioprinting is to benefit the field of regenerative medicine, which looks into artificial organ transplants for the thousands of patients without donors. Although the field is not brand new, understanding the integration and use of additive manufacturing with biomaterials is essential in developing fully functional organs. There is a heavy emphasis on the biomaterials themselves since they have a crucial role in creating an organ that is mechanically robust and adaptable in vivo. Covered in this review article are many featured tests, which also touch on the importance of including a biomaterial that is capable of maintaining a viable microenvironment. These include biomaterials such as hydrogels, biopolymers, and synthetic extra cellular matrices (ECM) built for stem cells to proliferate, differentiate, and give freedom to cell communication after printing.

Engineering of synthetic cellular microenvironments: implications for immunity

In this article, we discuss novel synthetic approaches for studying the interactions of cells with their microenvironment. Notably, critical cellular processes such as growth, differentiation, migration, and fate determination, are tightly regulated by interactions with neighboring cells, and the surrounding extracellular matrix. Given the huge complexity of natural cellular environments, and their rich molecular and physical diversity, the mission of understanding "environmental signaling" at a molecular-mechanistic level appears to be extremely challenging. To meet these challenges, attempts have been made in recent years to design synthetic matrices with defined chemical and physical properties, which, artificial though they may be, could reveal basic "design principles" underlying the physiological processes. Here, we summarize recent developments in the characterization of the chemical and physical properties of cell sensing and adhesion, as well as the design and use of engineered, micro- to nanoscale patterned and confined environments, for systematic, comprehensive modulation of the cells' environment. The power of these biomimetic surfaces to highlight environmental signaling events in cells, and in immune cells in particular, will be discussed.

Photopolymerizable chitosan-collagen hydrogels for bone tissue engineering

Photopolymerizable hydrogels derived from naturally occurring polymers have attracted significant interest in tissue-engineering applications due to their excellent biocompatibility, hydrophilic nature favourable for cell ingrowth and ability to be cured in situ through a minimally invasive procedure. In this study, we developed a composite hydrogel consisting of photocrosslinkable methacrylated glycol chitosan (MeGC) and semi-interpenetrating collagen (Col) with a riboflavin photoinitiator under blue light. The incorporation of Col in MeGC hydrogels enhanced the compressive modulus and slowed the degradation rate of the hydrogels. MeGC-Col composite hydrogels significantly enhanced cellular attachment, spreading, proliferation and osteogenic differentiation of mouse bone marrow stromal cells (BMSCs) seeded on the hydrogels compared with pure MeGC hydrogels, as observed by upregulated alkaline phosphatase (ALP) activity as well as increased mineralization. Similarly, when cells were encapsulated within hydrogels, BMSCs exhibited greater proliferation, ALP activity and mineral deposits in the presence of Col. These findings demonstrate that MeGC-Col composite hydrogels may be useful in promoting bone regeneration. Copyright © 2014 John Wiley & Sons, Ltd.

Topographical control of multiple cell adhesion molecules for traction force microscopy

Cellular traction forces are important quantitative measures in cell biology as they have provided much insight into cell behavior in contexts such as cellular migration, differentiation, and disease progression. However, the complex environment in vivo permits application of cell traction forces through multiple types of cell adhesion molecules. Currently available approaches to differentiate traction forces among multiple cell adhesion molecules are limited to specialized approaches to decouple cell-cell from cell-extracellular matrix (ECM) tractions. Here, we present a technique which uses indirect micropatterning onto a polyacrylamide gel to pattern multiple, spatially distinct fluorescently labeled ECM proteins, specifically gelatin and fibronectin (Fn), and confine the area to which cells can adhere. We found that cells interacting with both gelatin and Fn altered their traction forces significantly in comparison to cells on Fn-only substrates. This crosstalk interaction resulted in a decrease in overall traction forces on dual-patterned substrates as compared to cells on Fn-only substrates. This illustrates the unique need to study such interactions and demonstrates great potential in future studies in multi-ligand environments. Current micropatterning techniques on glass can easily be adapted to present other protein classes, such as cadherins, while maintaining control of adhesion spacing, cell spread area, and stiffness, each of which are important regulators of cell mechanobiology.

Epidermal growth factor can optimize a serum-free culture system for bone marrow stem cell proliferation in a miniature pig model

Bone marrow-derived mesenchymal stem cells have become an attractive cell source for periodontal ligament regeneration treatment because of their potential to engraft to several tissue types after injury. Most researchers have focused on the transplantation process, but few have paid attention to cell safety concerns and rapid proliferation before transplantation. Using serum-free medium to culture stem cells may be an effective method to avoid problems associated with exogenous serum and the addition of growth factors to promote cell proliferation. Here, we randomly divided our serum-free cultures and treated them with different levels of epidermal growth factor (EGF). We then evaluated changes in rates of cell adhesion, proliferation, apoptosis, and cell cycle ratio as well as their differentiation potential. The data showed that all of these parameters were significantly different when comparing serum-free cultures with and without 10 nM/L EGF (p < 0.05/0.01); however, cells with 10 nM/L EGF did not respond differently than cells grown in standard serum-containing media without EGF (p > 0.05). In summary, our results demonstrate that 10 nM/L EGF was the optimal dose for serum-free culture, which can replace traditional standard serum medium for in vitro expansion of miniature pig bone marrow-derived mesenchymal stem cells.

Monitoring the dynamics of primary T cell activation and differentiation using long term live cell imaging in microwell arrays

Methods that allow monitoring of individual cells over time, using live cell imaging, are essential for studying dynamical cellular processes in heterogeneous cell populations such as primary T lymphocytes. However, applying single cell time-lapse microscopy to study activation and differentiation of these cells was limited due to a number of reasons. First, primary naïve T cells are non-adherent and become highly motile upon activation through their antigen receptor. Second, CD4(+) T cell differentiation is a relatively slow process which takes 3-4 days. As a result, long-term dynamic monitoring of individual cells during the course of activation and differentiation is challenging as cells rapidly escape out of the microscope field of view. Here we present and characterize a platform which enables capture and growth of primary T lymphocytes with minimal perturbation, allowing for long-term monitoring of cell activation and differentiation. We use standard cell culture plates combined with PDMS based arrays containing thousands of deep microwells in which primary CD4(+) T cells are trapped and activated by antigen coated microbeads. We demonstrate that this system allows for live cell imaging of individual T cells for up to 72 h, providing quantitative data on cell proliferation and death times. In addition, we continuously monitor dynamics of gene expression in those cells, of either intracellular proteins using cells from transgenic mice expressing fluorescent reporter proteins, or cell surface proteins using fluorescently labeled antibodies. Finally, we show how intercellular interactions between different cell types can be investigated using our device. This system provides a new platform in which dynamical processes and intercellular interactions within heterogeneous populations of primary T cells can be studied at the single cell level.

Microengineered synthetic cellular microenvironment for stem cells

Stem cells possess the ability of self-renewal and differentiation into specific cell types. Therefore, stem cells have great potentials in fundamental biology studies and clinical applications. The most urgent desire for stem cell research is to generate appropriate artificial stem cell culture system, which can mimic the dynamic complexity and precise regulation of the in vivo biochemical and biomechanical signals, to regulate and direct stem cell behaviors. Precise control and regulation of the biochemical and biomechanical stimuli to stem cells have been successfully achieved using emerging micro/nanoengineering techniques. This review provides insights into how these micro/nanoengineering approaches, particularly microcontact printing and elastomeric micropost array, are applied to create dynamic and complex environment for stem cells culture.