Angelo S. Mao

Harnessing traction-mediated manipulation of the cell/matrix interface to control stem-cell fate

Stem cells sense and respond to the mechanical properties of the extracellular matrix. However, both the extent to which extracellular matrix mechanics affect stem cell fate in 3D micro-environments and the underlying biophysical mechanisms are unclear. We demonstrate that the commitment of mesenchymal stem cell (MSC) populations changes in response to the rigidity of 3D micro-environments, with osteogenesis occurring predominantly at 11–30 kPa. In contrast to previous 2D work, however, cell fate was not correlated with morphology. Instead, matrix stiffness regulated integrin binding as well as reorganization of adhesion ligands on the nanoscale, both of which were traction-dependent and correlated with osteogenic commitment of MSC populations. These findings suggest that cells interpret changes in the physical properties of adhesion substrates as changes in adhesion ligand presentation, and that cells themselves can be harnessed as tools to mechanically process materials into structures that feedback to manipulate their fate.

Regenerative medicine: Current therapies and future directions

Organ and tissue loss through disease and injury motivate the development of therapies that can regenerate tissues and decrease reliance on transplantations. Regenerative medicine, an interdisciplinary field that applies engineering and life science principles to promote regeneration, can potentially restore diseased and injured tissues and whole organs. Since the inception of the field several decades ago, a number of regenerative medicine therapies, including those designed for wound healing and orthopedics applications, have received Food and Drug Administration (FDA) approval and are now commercially available. These therapies and other regenerative medicine approaches currently being studied in preclinical and clinical settings will be covered in this review. Specifically, developments in fabricating sophisticated grafts and tissue mimics and technologies for integrating grafts with host vasculature will be discussed. Enhancing the intrinsic regenerative capacity of the host by altering its environment, whether with cell injections or immune modulation, will be addressed, as well as methods for exploiting recently developed cell sources. Finally, we propose directions for current and future regenerative medicine therapies.

Microfluidic Generation of Monodisperse, Structurally Homogeneous Alginate Microgels for Cell Encapsulation and 3D Cell Culture.

Monodisperse alginate microgels (10-50 μm) are created via droplet-based microfluidics by a novel crosslinking procedure. Ionic crosslinking of alginate is induced by release of chelated calcium ions. The process separates droplet formation and gelation reaction enabling excellent control over size and homogeneity under mild reaction conditions. Living mesenchymal stem cells are encapsulated and cultured in the generated 3D microenvironments.

Deterministic encapsulation of single cells in thin tunable microgels for niche modelling and therapeutic delivery

Existing techniques to encapsulate cells into microscale hydrogels generally yield high polymer-to-cell ratios and lack control over the hydrogel’s mechanical properties1. Here, we report a microfluidic-based method for encapsulating single cells in a ~6 micron layer of alginate that increases the proportion of cell-containing microgels by 10-fold, with encapsulation efficiencies over 90%. We show that in vitro cell viability was maintained over a three-day period, that the microgels are mechanically tractable, and that for microscale cell assemblages of encapsulated marrow stromal cells cultured in microwells, osteogenic differentiation of encapsulated cells depends on gel stiffness and cell density. We also show that intravenous injection of singly-encapsulated marrow stromal cells into mice delays clearance kinetics and sustains donor-derived soluble factors in vivo. The encapsulation of single cells in tunable hydrogels should find use in a variety of tissue engineering and regenerative medicine applications.

Effects of substrate stiffness and cell-cell contact on mesenchymal stem cell differentiation

The mechanical properties of the microenvironment and direct contact-mediated cell-cell interactions are two variables known to be important in the determination of stem cell differentiation fate, but little is known about the interplay of these cues. Here, we use a micropatterning approach on polyacrylamide gels of tunable stiffnesses to study how homotypic cell-cell contacts and mechanical stiffness affect different stages of osteogenesis of mesenchymal stem cells (MSCs). Nuclear localization of transcription factors associated with osteogenesis depended on substrate stiffness and was independent of the degree of cell-cell contact. However, expression of alkaline phosphatase, an early protein marker for osteogenesis, increased only in cells with both direct contact with neighboring cells and adhesion to stiffer substrates. Finally, mature osteogenesis, as assessed by calcium deposition, was low in micropatterned cells, even on stiff substrates and in multicellular clusters. These results indicate that substrate stiffness and the presence of neighboring cells regulate osteogenesis in MSCs.

Linear patterning of mesenchymal condensations is modulated by geometric constraints

The development of the vertebral column starts with the formation of a linear array of mesenchymal condensations, forming the blueprint for the eventual alternating pattern of bone and cartilage. Despite growing insight into the molecular mechanisms of morphogenesis, the impact of the physical aspects of the environment is not well understood. We hypothesized that geometric boundary conditions may play a pivotal role in the linear patterning of condensations, as neighbouring tissues provide physical constraints to the cell population. To study the process of condensation and the patterning thereof under tightly controlled geometric constraints, we developed a novel in vitro model that combines micropatterning with the established micromass assay. The spacing and alignment of condensations changed with the width of the cell adhesive patterns, a phenomenon that could not be explained by cell availability alone. Moreover, the extent of chondrogenic commitment was increased on substrates with tighter geometric constraints. When the in vivo pattern of condensations was investigated in the developing vertebral column of chicken embryos, the measurements closely fit into the quantitative relation between geometric constraints and inter-condensation distance found in vitro. Together, these findings suggest a potential role of geometric constraints in skeletal patterning in a cellular process of self-organization.

Microfluidic Templated Multicompartment Microgels for 3D Encapsulation and Pairing of Single Cells

Controlled encapsulation and pairing of single cells within a confined 3D matrix can enable the replication of the highly ordered cellular structure of human tissues. Microgels with independently controlled compartments that can encapsulate cells within separately confined hydrogel matrices would provide precise control over the route of pairing single cells. Here, a one-step microfluidic method is presented to generate monodisperse multicompartment microgels that can be used as a 3D matrix to pair single cells in a highly biocompatible manner. A method is presented to induce microgels formation on chip, followed by direct extraction of the microgels from oil phase, thereby avoiding prolonged exposure of the microgels to the oil. It is further demonstrated that by entrapping stem cells with niche cells within separate but adjacent compartments of the microgels, it can create complex stem cell niche microenvironments in a controlled manner, which can serve as a useful tool for the study of cell-cell interactions. This microfluidic technique represents a significant step toward high-throughput single cells encapsulation and pairing for the study of intercellular communications at single cell level, which is of significant importance for cell biology, stem cell therapy, and tissue engineering.