Extracellular-matrix tethering regulates stem-cell fate

Force-dependent cell signaling in stem cell differentiation

Stem cells interact with biochemical and biophysical signals in their extracellular environment. The biophysical signals are transduced to the stem cells either through the underlying extracellular matrix or externally applied forces. Increasing evidence has shown that these biophysical cues such as substrate stiffness and topography can direct stem cell differentiation and determine the cell fate. The mechanism of the biophysically induced differentiation is not understood; however, several key signaling components have been demonstrated to be involved in the force-mediated differentiation. This review will focus on focal adhesions, cytoskeletal contractility, Rho GTPase signaling and nuclear regulation in connection with biophysically induced differentiation. We will briefly introduce the important components of the mechanotransduction machinery, and the recent developments in the study of force-dependent stem cell differentiation.

Fibroblast cluster formation on 3D collagen matrices requires cell contraction dependent fibronectin matrix organization

Fibroblasts incubated on 3D collagen matrices in serum or lysophosphatidic acid (LPA)-containing medium self-organize into clusters through a mechanism that requires cell contraction. However, in platelet-derived growth factor (PDGF)-containing medium, cells migrate as individuals and do not form clusters even though they constantly encounter each other. Here, we present evidence that a required function of cell contraction in clustering is formation of fibronectin (FN) fibrillar matrix. We found that in serum or LPA but not in PDGF or basal medium, cells organized FN (both serum and cellular) into a fibrillar, detergent-insoluble matrix. Cell clusters developed concomitant with FN matrix formation. FN fibrils accumulated beneath cells and along the borders of cell clusters in regions of cell-matrix tension. Blocking Rho kinase or myosin II activity prevented FN matrix assembly and cell clustering. Using siRNA silencing and function-blocking antibodies and peptides, we found that cell clustering and FN matrix assembly required α5β1 integrins and fibronectin. Cells were still able to exert contractile force and compact the collagen matrix under the latter conditions, which showed that contraction was not sufficient for cell clustering to occur. Our findings provide new insights into how procontractile (serum/LPA) and promigratory (PDGF) growth factor environments can differentially regulate FN matrix assembly by fibroblasts interacting with collagen matrices and thereby influence mesenchymal cell morphogenetic behavior under physiologic circumstances such as wound repair, morphogenesis and malignancy.

Scalable production of controllable dermal papilla spheroids on PVA surfaces and the effects of spheroid size on hair follicle regeneration

Organ size and numbers are vital issues in bioengineering for hair follicle (HF) regeneration. Murine HF dermal papilla (DP) cells are able to induce HF neogenesis when transplanted as aggregates. However, how the preparation of murine and human DP aggregates affects HF inductivity and the size of regenerated HF is yet to be determined. Here we report a scalable method for production of controllable human and rat DP spheroids in general labs for reproducible experiments. Compared with more hydrophobic polyethylene and poly(ethylene-co-vinyl alcohol), DP cells are poorly adhesive to hydrophilic polyvinyl alcohol (PVA). Seeded in PVA-coated 96-welled commercial PCR tube arrays, DP cells quickly aggregate into single spheroids with progressive compaction. Varying seeded cell numbers and culture periods enables us to control the size and cell number of the spheroids. The spheroids obtained have high viability and preserve DP characters. A proof of principle experiment was conducted to examine the size effect on the efficiency and efficacy of HF regeneration. We found that both human and rat DP spheroids are able to induce HF neogenesis and larger DP spheroids exhibit higher HF inductivity. However, the average diameter of regenerated hair fiber did not significantly change with the increasing size of transplanted DP spheroids. The result suggests that an appropriate size of DP spheroid is essential for HF inductivity, but its size cannot be directly translated to a thicker regenerated hair. Our results also have implications on the efficiency and efficacy in the regeneration of other epithelial organs.

Microarrayed Materials for Stem Cells

Stem cells hold remarkable promise for applications in disease modeling, cancer therapy and regenerative medicine. Despite the significant progress made during the last decade, designing materials to control stem cell fate remains challenging. As an alternative, materials microarray technology has received great attention because it allows for high throughput materials synthesis and screening at a reasonable cost. Here, we discuss recent developments in materials microarray technology and their applications in stem cell engineering. Future opportunities in the field will also be reviewed.

Matrix compliance regulates Rac1b localization, NADPH oxidase assembly, and epithelial-mesenchymal transition

Substratum stiffness controls the subcellular localization of Rac1b, a highly activated splice variant of the small GTPase Rac1. On stiff substrata, Rac1b localizes to the plasma membrane, forming a complex with NADPH oxidase and generating ROS, thus inducing the expression of the transcription factor Snail and downstream signaling to EMT.

Epithelial–mesenchymal transition (EMT) is a form of epithelial plasticity implicated in fibrosis and tumor metastasis. Here we show that the mechanical rigidity of the microenvironment plays a pivotal role in the promotion of EMT by controlling the subcellular localization and downstream signaling of Rac GTPases. Soft substrata, with compliances comparable to that of normal mammary tissue, are protective against EMT, whereas stiffer substrata, with compliances characteristic of breast tumors, promote EMT. Rac1b, a highly activated splice variant of Rac1 found in tumors, localizes to the plasma membrane in cells cultured on stiff substrata or in collagen-rich regions of human breast tumors. At the membrane, Rac1b forms a complex with NADPH oxidase and promotes the production of reactive oxygen species, expression of Snail, and activation of the EMT program. In contrast, soft microenvironments inhibit the membrane localization of Rac1b and subsequent redox changes. These results reveal a novel mechanotransduction pathway in the regulation of epithelial plasticity via EMT.

Transduction of mechanical and cytoskeletal cues by YAP and TAZ

The physical and mechanical properties of the cellular microenvironment regulate cell shape and can strongly influence cell fate. How mechanical cues are sensed and transduced to regulate gene expression has long remained elusive. Recently, cues from the extracellular matrix, cell adhesion sites, cell shape and the actomyosin cytoskeleton were found to converge on the regulation of the downstream effectors of the Hippo pathway YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ-binding motif) in vertebrates and Yorkie in flies. This convergence may explain how mechanical signals can direct normal and pathological cell behaviour.

Polymeric biomaterials for stem cell bioengineering

This review covers the application of polymeric materials in stem cell bioengineering. Main emphasis is directed towards current material design concepts that mimic distinct exogenous signals of the stem cell microenvironment. Progress within the field of stem cell-specific biomaterials will be discussed, focusing on pluripotent, hematopoietic, mesenchymal and neural stem cells. The future role of biomaterials will be outlined with possible applications for cell reprogramming and engineering cancer cell microenvironments.

Controlling matrix stiffness and topography for the study of tumor cell migration

Cellular studies have long been performed on the bench top, within Petri dishes and flasks that expose cells to surroundings that differ greatly from their native environment. The complexity of a human tissue is such that to truly replicate a cell's physiologic microenvironment in vitro is currently impossible. It is nevertheless important to determine how various factors of the microenvironment interact to drive cell behavior, particularly with regard to disease states, such as cancer. Here we focus on two key elements of the cellular microenvironment, matrix stiffness and architecture, in the context of tumor cell behavior. We discuss recent work focusing on the effects of these individual properties on cancer cell migration and describe one technique developed by our lab that could be applied to dissect the effects of specific structural and mechanical cues, and which may lead to useful insights into the potentially synergistic effects of these properties on tumor cell behavior.