Inhibition of human embryonic stem cell differentiation by mechanical strain

Mechanical Force Induces Osteogenic Differentiation of Murine Induced Pluripotent Stem Cells via TGF-β Signalling

OBJECTIVES

Mechanical forces are pivotal in regulating various cellular responses, particularly in periodontal ligament and bone. However, the effects of mechanical force are contingent upon several factors, including force types, duration, magnitude and the differentiation stage of the cells. The present study investigated the impact of intermittent compressive force (ICF) on osteogenic differentiation in murine gingiva-derived induced pluripotent stem cells (miPS).

MATERIALS AND METHODS

Adherent retinoic acid-treated miPS were subjected to ICF in a serum-free medium for 24 h. Real-time polymerase chain reaction, western blot analysis and immunofluorescence staining were employed to evaluate mRNA and protein expression patterns. In vitro mineralisation was assessed using alizarin red S staining.

RESULTS

Our findings revealed that ICF treatment induced the expression of osteogenic markers, including Runx2, Col1a1, Opn and Dlx5. Furthermore, ICF promoted the release of extracellular adenosine triphosphate (ATP) at 24 h. Pretreatment with ICF increased in vitro mineralisation, while ATP priming did not enhance mineralisation in adherent retinoic acid-treated miPS. A TGF-β inhibitor attenuated the ICF-upregulated Runx2, Col1a1, Opn and Dlx5 as well as the ICF-induced in vitro mineralisation.

CONCLUSION

Collectively, our results suggest that ICF can induce osteogenic differentiation of adherent retinoic acid-treated miPS, mediated through TGF-β signalling. Eventually, such information could be of value in controlling iPS responses during regenerative treatment applications.

Human Induced Pluripotent Spheroids' Growth Is Driven by Viscoelastic Properties and Macrostructure of 3D Hydrogel Environment

Stem cells, particularly human iPSCs, constitute a powerful tool for tissue engineering, notably through spheroid and organoid models. While the sensitivity of stem cells to the viscoelastic properties of their direct microenvironment is well-described, stem cell differentiation still relies on biochemical factors. Our aim is to investigate the role of the viscoelastic properties of hiPSC spheroids' direct environment on their fate. To ensure that cell growth is driven only by mechanical interaction, bioprintable alginate-gelatin hydrogels with significantly different viscoelastic properties were utilized in differentiation factor-free culture medium. Alginate-gelatin hydrogels of varying concentrations were developed to provide 3D environments of significantly different mechanical properties, ranging from 1 to 100 kPa, while allowing printability. hiPSC spheroids from two different cell lines were prepared by aggregation (⌀ = 100 µm, n > 1 × 104), included and cultured in the different hydrogels for 14 days. While spheroids within dense hydrogels exhibited limited growth, irrespective of formulation, porous hydrogels prepared with a liquid-liquid emulsion method displayed significant variations of spheroid morphology and growth as a function of hydrogel mechanical properties. Transversal culture (adjacent spheroids-laden alginate-gelatin hydrogels) clearly confirmed the separate effect of each hydrogel environment on hiPSC spheroid behavior. This study is the first to demonstrate that a mechanically modulated microenvironment induces diverse hiPSC spheroid behavior without the influence of other factors. It allows one to envision the combination of multiple formulations to create a complex object, where the fate of hiPSCs will be independently controlled by their direct microenvironment.

Insight into muscle stem cell regeneration and mechanobiology

Stem cells possess the unique ability to differentiate into specialized cell types. These specialized cell types can be used for regenerative medicine purposes such as cell therapy. Myosatellite cells, also known as skeletal muscle stem cells (MuSCs), play important roles in the growth, repair, and regeneration of skeletal muscle tissues. However, despite its therapeutic potential, the successful differentiation, proliferation, and expansion processes of MuSCs remain a significant challenge due to a variety of factors. For example, the growth and differentiation of MuSCs can be greatly influenced by actively replicating the MuSCs microenvironment (known as the niche) using mechanical forces. However, the molecular role of mechanobiology in MuSC growth, proliferation, and differentiation for regenerative medicine is still poorly understood. In this present review, we comprehensively summarize, compare, and critically analyze how different mechanical cues shape stem cell growth, proliferation, differentiation, and their potential role in disease development (Fig. 1). The insights developed from the mechanobiology of stem cells will also contribute to how these applications can be used for regenerative purposes using MuSCs.

Dentin Particulate for Bone Regeneration: An In Vitro Study

The aim of this in vitro study was to investigate the commitment and behavior of dental pulp stem cells (DPSCs) seeded onto two different grafting materials, human dentin particulate (DP) and deproteinized bovine bone matrix (BG), with those cultured in the absence of supplements. Gene expression analyses along with epigenetic and morphological tests were carried out to examine odontogenic and osteogenic differentiation and cell proliferation. Compressive testing of the grafting materials seeded with DPSCs was performed as well. DPSC differentiation into odontoblast-like cells was identified from the upregulation of odontogenic markers (DSPP and MSX) and osteogenic markers (RUNX2, alkaline phosphatase, osteonectin, osteocalcin, collagen type I, bmp2, smad5/8). Epigenetic tests confirmed the presence of miRNAs involved in odontogenic or osteogenic commitment of DPSCs cultured for up to 21 days on DP. Compressive strength values obtained from extracellular matrix (ECM) synthesized by DPSCs showed a trend of being higher when seeded onto DP than onto BG. High expression of VEGF factor, which is related to angiogenesis, and of dentin sialoprotein was observed only in the presence of DP. Morphological analyses confirmed the typical phenotype of adult odontoblasts. In conclusion, the odontogenic and osteogenic commitment of DPSCs and their respective functions can be achieved on DP, which enables exceptional dentin and bone regeneration.

Mechano-Biological Computer Model of Scaffold-Supported Bone Regeneration: Effect of Bone Graft and Scaffold Structure on Large Bone Defect Tissue Patterning

Large segmental bone defects represent a clinical challenge for which current treatment procedures have many drawbacks. 3D-printed scaffolds may help to support healing, but their design process relies mainly on trial and error due to a lack of understanding of which scaffold features support bone regeneration. The aim of this study was to investigate whether existing mechano-biological rules of bone regeneration can also explain scaffold-supported bone defect healing. In addition, we examined the distinct roles of bone grafting and scaffold structure on the regeneration process. To that end, scaffold-surface guided migration and tissue deposition as well as bone graft stimulatory effects were included in an in silico model and predictions were compared to in vivo data. We found graft osteoconductive properties and scaffold-surface guided extracellular matrix deposition to be essential features driving bone defect filling in a 3D-printed honeycomb titanium structure. This knowledge paves the way for the design of more effective 3D scaffold structures and their pre-clinical optimization, prior to their application in scaffold-based bone defect regeneration.

Developmentally-Inspired Biomimetic Culture Models to Produce Functional Islet-Like Cells From Pluripotent Precursors

Insulin-producing beta cells sourced from pluripotent stem cells hold great potential as a virtually unlimited cell source to treat diabetes. Directed pancreatic differentiation protocols aim to mimic various stimuli present during embryonic development through sequential changes of in vitro culture conditions. This is commonly accomplished by the timed addition of soluble signaling factors, in conjunction with cell-handling steps such as the formation of 3D cell aggregates. Interestingly, when stem cells at the pancreatic progenitor stage are transplanted, they form functional insulin-producing cells, suggesting that in vivo microenvironmental cues promote beta cell specification. Among these cues, biophysical stimuli have only recently emerged in the context of optimizing pancreatic differentiation protocols. This review focuses on studies of cell–microenvironment interactions and their impact on differentiating pancreatic cells when considering cell signaling, cell–cell and cell–ECM interactions. We highlight the development of in vitro cell culture models that allow systematic studies of pancreatic cell mechanobiology in response to extracellular matrix proteins, biomechanical effects, soluble factor modulation of biomechanics, substrate stiffness, fluid flow and topography. Finally, we explore how these new mechanical insights could lead to novel pancreatic differentiation protocols that improve efficiency, maturity, and throughput.

A high throughput screening system for studying the effects of applied mechanical forces on reprogramming factor expression

Mechanical forces are important in the regulation of physiological homeostasis and the development of disease. The application of mechanical forces to cultured cells is often performed using specialized systems that lack the flexibility and throughput of other biological techniques. In this study, we developed a high throughput platform for applying complex dynamic mechanical forces to cultured cells. We validated the system for its ability to accurately apply parallel mechanical stretch in a 96 well plate format in 576 well simultaneously. Using this system, we screened for optimized conditions to stimulate increases in Oct-4 and other transcription factor expression in mouse fibroblasts. Using high throughput mechanobiological screening assays, we identified small molecules that can synergistically enhance the increase in reprograming-related gene expression in mouse fibroblasts when combined with mechanical loading. Taken together, our findings demonstrate a new powerful tool for investigating the mechanobiological mechanisms of disease and performing drug screening in the presence of applied mechanical load.

BEaTS-α an open access 3D printed device for in vitro electromechanical stimulation of human induced pluripotent stem cells

3D printing was used to develop an open access device capable of simultaneous electrical and mechanical stimulation of human induced pluripotent stem cells in 6-well plates. The device was designed using Computer-Aided Design (CAD) and 3D printed with autoclavable, FDA-approved materials. The compact design of the device and materials selection allows for its use inside cell incubators working at high humidity without the risk of overheating or corrosion. Mechanical stimulation of cells was carried out through the cyclic deflection of flexible, translucent silicone membranes by means of a vacuum-controlled, open-access device. A rhythmic stimulation cycle was programmed to create a more physiologically relevant in vitro model. This mechanical stimulation was coupled and synchronized with in situ electrical stimuli. We assessed the capabilities of our device to support cardiac myocytes derived from human induced pluripotent stem cells, confirming that cells cultured under electromechanical stimulation presented a defined/mature cardiomyocyte phenotype. This 3D printed device provides a unique high-throughput in vitro system that combines both mechanical and electrical stimulation, and as such, we foresee it finding applications in the study of any electrically responsive tissue such as muscles and nerves.

Cyclic Strain Promotes H19 Expression and Vascular Tube Formation in iPSC-Derived Endothelial Cells

Introduction

Induced pluripotent stem cell (iPSC)-derived endothelial cells (ECs) have the potential for therapeutic application in several cardiovascular diseases. Mechanical strain is known to regulate EC behavior and stem cell differentiation and may play a role in directing EC differentiation of iPSCs. H19, a long non-coding RNA (lncRNA), is known to affect ECs in several mechanically relevant pathologies and may play a role in this process as well. Therefore, we investigated expression changes of H19 resulting from mechanical stimulation during EC differentiation, as well as functional effects on EC tube formation.

Methods

iPSCs were subjected to 5% cyclic mechanical strain during EC differentiation. RT-PCR and flow cytometry were used to assess changes in mesoderm differentiation and gene expression in the final ECs as a result of strain. Functional outcomes of mechanically differentiated ECs were assessed with a tube formation assay and changes in H19. H19 was also overexpressed in human umbilical vein endothelial cells (HUVECs) to assess its role in non-H19-expressing ECs.

Results

Mechanical strain promoted mesoderm differentiation, marked by increased expression of brachyury 24 h after initiation of differentiation. Strain also increased expression of H19, CD31, VE-cadherin, and VEGFR2 in differentiated ECs. Strain-differentiated ECs formed tube networks with higher junction and endpoint density than statically-differentiated ECs. Overexpression of H19 in HUVECs resulted in similar patterns of tube formation.

Conclusions

H19 expression is increased by mechanical strain and promotes tube branching in iPSC-derived ECs.

Advances in Pluripotent Stem Cells: History, Mechanisms, Technologies, and Applications

Over the past 20 years, and particularly in the last decade, significant developmental milestones have driven basic, translational, and clinical advances in the field of stem cell and regenerative medicine. In this article, we provide a systemic overview of the major recent discoveries in this exciting and rapidly developing field. We begin by discussing experimental advances in the generation and differentiation of pluripotent stem cells (PSCs), next moving to the maintenance of stem cells in different culture types, and finishing with a discussion of three-dimensional (3D) cell technology and future stem cell applications. Specifically, we highlight the following crucial domains: 1) sources of pluripotent cells; 2) next-generation in vivo direct reprogramming technology; 3) cell types derived from PSCs and the influence of genetic memory; 4) induction of pluripotency with genomic modifications; 5) construction of vectors with reprogramming factor combinations; 6) enhancing pluripotency with small molecules and genetic signaling pathways; 7) induction of cell reprogramming by RNA signaling; 8) induction and enhancement of pluripotency with chemicals; 9) maintenance of pluripotency and genomic stability in induced pluripotent stem cells (iPSCs); 10) feeder-free and xenon-free culture environments; 11) biomaterial applications in stem cell biology; 12) three-dimensional (3D) cell technology; 13) 3D bioprinting; 14) downstream stem cell applications; and 15) current ethical issues in stem cell and regenerative medicine. This review, encompassing the fundamental concepts of regenerative medicine, is intended to provide a comprehensive portrait of important progress in stem cell research and development. Innovative technologies and real-world applications are emphasized for readers interested in the exciting, promising, and challenging field of stem cells and those seeking guidance in planning future research direction.

Low intensity pulsed ultrasound (LIPUS) maintains osteogenic potency by the increased expression and stability of Nanog through spleen tyrosine kinase (Syk) activation

Mesenchymal stem cells (MSCs) are a powerful tool for cell-based, clinical therapies like bone regeneration. Therapeutic use of cell transplantation requires many cells, however, the expansion process needed to produce large quantities of cells reduces the differentiation potential of MSCs. Here, we examined the protective effects of low intensity pulsed ultrasound (LIPUS) on the maintenance of osteogenic potency. Primary osteoblastic cells were serially passaged between 2 and 12 times with daily LIPUS treatment. We found that LIPUS stimulation maintains osteogenic differentiation capacity in serially passaged cells, as characterized by improved matrix mineralization and Osteocalcin mRNA expression. Decreased expression of Nanog, Sox2, and Msx2, and increased expression of Pparg2 from serial passaging was recovered in LIPUS-stimulated cells. We found that LIPUS stimulation not only increased but also sustained expression of Nanog in primary osteoblasts and ST2 cells, a mouse mesenchymal stromal cell line. Nanog overexpression in serially passaged cells mimicked the recuperative effects of LIPUS on osteogenic potency, highlighting the important role of Nanog in LIPUS stimulation. Additionally, we found that spleen tyrosine kinase (Syk) is an important signaling molecule to induce Nanog expression in LIPUS-stimulated cells. Syk activation was regulated by both Rho-associated kinase 1 (ROCK1) and extracellular ATP in a paracrine manner. Interestingly, the LIPUS-induced increase in Nanog mRNA expression was regulated by ATP-P2X4-Syk Y323 activation, while the improvement of Nanog protein stability was controlled by the ROCK1-Syk Y525/526 pathway. Taken together, these results indicate that LIPUS stimulation recovers and maintains the osteogenic potency of serially passaged cells through a Syk-Nanog axis.

Agili-C implant promotes the regenerative capacity of articular cartilage defects in an ex vivo model

PURPOSE

Osteochondral implants are currently adopted for the treatment of symptomatic full-thickness chondral and osteochondral defects. Agili-C™ is a cell-free aragonite-based scaffold which aims to reproduce the original structure and function of the articular joint while directing the growth and regeneration of both cartilage and its underlying subchondral bone. The goal of the present study was to investigate the ex vivo mechanisms of action (MOA) of the Agili-C™ implant in the repair of full-thickness cartilage defects. In particular, we tested whether Agili-C™ implant has the potential to stimulate cartilage ingrowth through chondrocytes migration into the 3D interconnected porous structure of the scaffold, along with maintaining their viability and phenotype and the deposition of hyaline cartilage matrix.

METHODS

Articular cartilage samples were collected through the Gift of Hope Organ and Tissue Donor Network (Itasca, IL) within 24 h from death. For this study, cartilage from a total of 14 donors was used. To model a chondral defect, donut-shaped cartilage explants were prepared from each tissue specimen. The chondral phase of the Agili-C™ implant was placed inside the tissue in full contact and press fit manner. Cartilage explants with the Agili-C™ implant inside were cultured for 60 days. As a control, the same donut-shaped cartilage explants were cultured without Agili-C™, under the same culture conditions.

RESULTS

Using fresh human cadaveric articular cartilage tissue in a 60-day culture, it was demonstrated that chondrocytes were able to migrate into the Agili-C™ scaffold and contribute to the deposition of the extracellular matrix (ECM) rich in collagen type II and aggrecan, and lacking collagen type I. Additionally, we were able to show the formation of a layer populated by progenitor-like cells on the articular surface of the implant.

CONCLUSIONS

The analysis of samples taken from knee and ankle joints of human donors with a wide age range and both genders supports the potential of Agili-C™ scaffold to stimulate cartilage regeneration and repair. Based on these results, the present scaffold can be used in the clinical practice as a one-step procedure to treat full-thickness chondral defects.

Visco-Node-Pore Sensing: A Microfluidic Rheology Platform to Characterize Viscoelastic Properties of Epithelial Cells

Viscoelastic properties of cells provide valuable information regarding biological or clinically relevant cellular characteristics. Here, we introduce a new, electronic-based, microfluidic platform-visco-node-pore sensing (visco-NPS)-which quantifies cellular viscoelastic properties under periodic deformation. We measure the storage (G') and loss (G″) moduli (i.e., elasticity and viscosity, respectively) of cells. By applying a wide range of deformation frequencies, our platform quantifies the frequency dependence of viscoelastic properties. G' and G″ measurements show that the viscoelastic properties of malignant breast epithelial cells (MCF-7) are distinctly different from those of non-malignant breast epithelial cells (MCF-10A). With its sensitivity, visco-NPS can dissect the individual contributions of different cytoskeletal components to whole-cell mechanical properties. Moreover, visco-NPS can quantify the mechanical transitions of cells as they traverse the cell cycle or are initiated into an epithelial-mesenchymal transition. Visco-NPS identifies viscoelastic characteristics of cell populations, providing a biophysical understanding of cellular behavior and a potential for clinical applications.

Periostin plays role in force-induced stem cell potential by periodontal ligament stem cells

Mechanical stimuli have been shown to play an important role in directing stem cell fate and maintenance of tissue homeostasis. One of the functions of the mechanoresponsive tissue periodontal ligament (PDL) is to withstand the functional forces within the oral cavity. Periodontal ligament stem cells (PDLSCs) derived from periodontal tissue have been demonstrated to be able to respond directly to mechanical forces. However, the mechanisms of action of mechanical force on PDLSCs are not totally understood. The aim of this study was to investigate the mechanisms by which compressive force affects PDLSCs, especially their stemness properties. PDLSCs were established from extracted human third molars; their stem cell characteristics were validated by detecting the expression of stem cell markers and confirming their ability to differentiate into osteogenic and adipogenic lineages. PDLSCs were subjected to various magnitudes of static compressive force (0 [control], 0.5, 1.0, 1.5, or 2 g/cm² ). Application of 1.0 g/cm² compressive force significantly upregulated a panel of stem cell marker genes, including NANOG and OCT4. Conversely, higher force magnitudes downregulated these genes. Mechanical loading also upregulated periostin, a matrix protein that plays important roles in tissue morphogenesis. Interestingly, knockdown of periostin using siRNA abolished force-induced stem cell marker expression in PDLSCs. This study suggests a proper magnitude of compressive force could be one important factor involved in the modulation of the pluripotency of PDLSCs through the action of periostin. The precise mechanism by which periostin regulates stemness requires further detailed investigation.

Rapid translocation of pluripotency-related transcription factors by external uniaxial forces

Human embryonic stem cells subjected to a one-time uniaxial stretch for as short as 30-min on a flexible substrate coated with Matrigel experienced rapid and irreversible nuclear-to-cytoplasmic translocation of NANOG and OCT4, but not Sox2. Translocations were directed by intracellular transmission of biophysical signals from cell surface integrins to nuclear CRM1 and were independent of exogenous soluble factors. On E-CADHERIN-coated substrates, presumably with minimal integrin engagement, mechanical strain-induced rapid nuclear-to-cytoplasmic translocation of the three transcription factors. These findings might provide fundamental insights into early developmental processes and may facilitate mechanotransduction-mediated bioengineering approaches to influencing stem cell fate determination.

Acoustic Tweezing Cytometry Induces Rapid Initiation of Human Embryonic Stem Cell Differentiation

Mechanical forces play critical roles in influencing human embryonic stem cell (hESC) fate. However, it remains largely uncharacterized how local mechanical forces influence hESC behavior in vitro. Here, we used an ultrasound (US) technique, acoustic tweezing cytometry (ATC), to apply targeted cyclic subcellular forces to hESCs via integrin-bound microbubbles (MBs). We found that ATC-mediated cyclic forces applied for 30 min to hESCs near the edge of a colony induced immediate global responses throughout the colony, suggesting the importance of cell-cell connection in the mechanoresponsiveness of hESCs to ATC-applied forces. ATC application generated increased contractile force, enhanced calcium activity, as well as decreased expression of pluripotency transcription factors Oct4 and Nanog, leading to rapid initiation of hESC differentiation and characteristic epithelial-mesenchymal transition (EMT) events that depend on focal adhesion kinase (FAK) activation and cytoskeleton (CSK) tension. These results reveal a unique, rapid mechanoresponsiveness and community behavior of hESCs to integrin-targeted cyclic forces.

Controlling Differentiation of Stem Cells for Developing Personalized Organ-on-Chip Platforms

Organ-on-chip (OOC) platforms have attracted attentions of pharmaceutical companies as powerful tools for screening of existing drugs and development of new drug candidates. OOCs have primarily used human cell lines or primary cells to develop biomimetic tissue models. However, the ability of human stem cells in unlimited self-renewal and differentiation into multiple lineages has made them attractive for OOCs. The microfluidic technology has enabled precise control of stem cell differentiation using soluble factors, biophysical cues, and electromagnetic signals. This study discusses different tissue- and organ-on-chip platforms (i.e., skin, brain, blood-brain barrier, bone marrow, heart, liver, lung, tumor, and vascular), with an emphasis on the critical role of stem cells in the synthesis of complex tissues. This study further recaps the design, fabrication, high-throughput performance, and improved functionality of stem-cell-based OOCs, technical challenges, obstacles against implementing their potential applications, and future perspectives related to different experimental platforms.

Mechanical forces direct stem cell behaviour in development and regeneration

Stem cells and their local microenvironment, or niche, communicate through mechanical cues to regulate cell fate and cell behaviour and to guide developmental processes. During embryonic development, mechanical forces are involved in patterning and organogenesis. The physical environment of pluripotent stem cells regulates their self-renewal and differentiation. Mechanical and physical cues are also important in adult tissues, where adult stem cells require physical interactions with the extracellular matrix to maintain their potency. In vitro, synthetic models of the stem cell niche can be used to precisely control and manipulate the biophysical and biochemical properties of the stem cell microenvironment and to examine how the mode and magnitude of mechanical cues, such as matrix stiffness or applied forces, direct stem cell differentiation and function. Fundamental insights into the mechanobiology of stem cells also inform the design of artificial niches to support stem cells for regenerative therapies.

Responses of human adipose-derived stem cells to interstitial level of extremely low shear flows regarding differentiation, morphology, and proliferation

Human cells encounter a range of shear stress levels in situ and this natural variability in shear stress implies that realistic investigations of cell type characteristics may depend on nontrivial shear stress models. Human adipose-derived stem cells (hASCs) differentiate near the blood capillary vessels where interstitial flows predominate. However, the effects of interstitial levels of shear on hASCs are not fully understood. In this study, we propose a microfluidic shear generation system, in which a gradient distribution of the interstitial level of shear flow is created to investigate the effects of interstitial-level shear flow on hASCs. To generate such a gradient profile of interstitial-level shear stress, we fabricated a semicircle-shaped microfluidic channel, and generated an extremely low flow using an osmosis-driven pump. Changes to hASC morphology, proliferation, and differentiation were observed under shear stresses of 1.8 × 10^-3-2.4 × 10^-3 Pa. At higher shear stresses, we found higher proliferation rates, stronger actin structures, and lower differentiation. We also conducted computational simulations of a monolayer culture, which showed that the shear stress level even on a single cell varies owing to the change of the cell thickness between the pseudopodia and the nucleus. We found that hASCs detectably respond to extremely low levels of shear flow, above a threshold of ∼2.0 × 10^-3 Pa. Our microplatform may be useful for quantitating biological responses and function changes of other stem cells and cancer cells to interstitial-level shear flows.

The extracellular microscape governs mesenchymal stem cell fate

Each cell forever interacts with its extracellular matrix (ECM); a stem cell relies on this interaction to guide differentiation. The stiffness, nanotopography, protein composition, stress and strain inherent to any given ECM influences stem cell lineage commitment. This interaction is dynamic, multidimensional and reciprocally evolving through time, and from this concerted exchange the macroscopic tissues that comprise living organisms are formed. Mesenchymal stem cells can give rise to bone, cartilage, tendon and muscle; thus attempts to manipulate their differentiation must heed the physical properties of incredibly complex native microenvironments to realize regenerative goals.

Mechanical Forces Reshape Differentiation Cues That Guide Cardiomyogenesis

Soluble morphogen gradients have long been studied in the context of heart specification and patterning. However, recent data have begun to challenge the notion that long-standing in vivo observations are driven solely by these gradients alone. Evidence from multiple biological models, from stem cells to ex vivo biophysical assays, now supports a role for mechanical forces in not only modulating cell behavior but also inducing it de novo in a process termed mechanotransduction. Structural proteins that connect the cell to its niche, for example, integrins and cadherins, and that couple to other growth factor receptors, either directly or indirectly, seem to mediate these changes, although specific mechanistic details are still being elucidated. In this review, we summarize how the wingless (Wnt), transforming growth factor-β, and bone morphogenetic protein signaling pathways affect cardiomyogenesis and then highlight the interplay between each pathway and mechanical forces. In addition, we will outline the role of integrins and cadherins during cardiac development. For each, we will describe how the interplay could change multiple processes during cardiomyogenesis, including the specification of undifferentiated cells, the establishment of heart patterns to accomplish tube and chamber formation, or the maturation of myocytes in the fully formed heart.

Engineering Embryonic Stem Cell Microenvironments for Tailored Cellular Differentiation

The rapid progress of embryonic stem cell (ESCs) research offers great promise for drug discovery, tissue engineering, and regenerative medicine. However, a major limitation in translation of ESCs technology to pharmaceutical and clinical applications is how to induce their differentiation into tailored lineage commitment with satisfactory efficiency. Many studies indicate that this lineage commitment is precisely controlled by the ESC microenvironment in vivo. Engineering and biomaterial-based approaches to recreate a biomimetic cellular microenvironment provide valuable strategies for directing ESCs differentiation to specific lineages in vitro. In this review, we summarize and examine the recent advances in application of engineering and biomaterial-based approaches to control ESC differentiation. We focus on physical strategies (e.g., geometrical constraint, mechanical stimulation, extracellular matrix (ECM) stiffness, and topography) and biochemical approaches (e.g., genetic engineering, soluble bioactive factors, coculture, and synthetic small molecules), and highlight the three-dimensional (3D) hydrogel-based microenvironment for directed ESC differentiation. Finally, future perspectives in ESCs engineering are provided for the subsequent advancement of this promising research direction.

Capsule stiffness regulates the efficiency of pancreatic differentiation of human embryonic stem cells

Encapsulation of donor islets using a hydrogel material is a well-studied strategy for islet transplantation, which protects donor islets from the host immune response. Replacement of donor islets by human embryonic stem cell (hESC) derived islets will also require a means of immune-isolating hESCs by encapsulation. However, a critical consideration of hESC differentiation is the effect of surrounding biophysical environment, in this case capsule biophysical properties, on differentiation. The objective of this study, thus, was to evaluate the effect of capsule properties on growth, viability, and differentiation of encapsulated hESCs throughout pancreatic induction. It was observed that even in the presence of soluble chemical cues for pancreatic induction, substrate properties can significantly modulate pancreatic differentiation, hence necessitating careful tuning of capsule properties. Capsules in the range of 4-7kPa supported cell growth and viability, whereas capsules of higher stiffness suppressed cell growth. While an increase in capsule stiffness enhanced differentiation at the intermediate definitive endoderm (DE) stage, increased stiffness strongly suppressed pancreatic progenitor (PP) induction. Signaling pathway analysis indicated an increase in pSMAD/pAKT levels with substrate stiffness likely the cause of enhancement of DE differentiation. In contrast, sonic hedgehog inhibition was more efficient under softer gel conditions, which is necessary for successful PP differentiation.

STATEMENT OF SIGNIFICANCE

Cell replacement therapy for type 1 diabetes (T1D), affecting millions of people worldwide, requires the immunoisolation of insulin-producing islets by encapsulation with a semi-impermeable material. Due to the shortage of donor islets, human pluripotent stem cell (hPSC) derived islets are an attractive alternative. However, properties of the encapsulating substrate are known to influence hPSC cell fate. In this work, we determine the effect of substrate stiffness on growth and pancreatic fate of encapsulated hPSCs. We precisely identify the range of substrate properties conducive for pancreatic cell fate, and also the mechanism by which substrate properties modify the cell signaling pathways and hence cell fate. Such information will be critical in driving regenerative cell therapy for long term treatment of T1D.

Features of Microsystems for Cultivation and Characterization of Stem Cells with the Aim of Regenerative Therapy

Stem cells have infinite potential for regenerative therapy thanks to their advantageous ability which is differentiable to requisite cell types for recovery and self-renewal. The microsystem has been proved to be more helpful to stem cell studies compared to the traditional methods, relying on its advantageous feature of mimicking in vivo cellular environments as well as other profitable features such as minimum sample consumption for analysis and multiprocedures. A wide variety of microsystems were developed for stem cell studies; however, regenerative therapy-targeted applications of microtechnology should be more emphasized and gain more attractions since the regenerative therapy is one of ultimate goals of biologists and bioengineers. In this review, we introduce stem cell researches harnessing well-known microtechniques (microwell, micropattern, and microfluidic channel) in view point of physical principles and how these systems and principles have been implemented appropriately for characterizing stem cells and finding possible regenerative therapies. Biologists may gain information on the principles of microsystems to apply them to find solutions for their current challenges, and engineers may understand limitations of the conventional microsystems and find new chances for further developing practical microsystems. Through the well combination of engineers and biologists, the regenerative therapy-targeted stem cell researches harnessing microtechnology will find better suitable treatments for human disorders.

Edges of human embryonic stem cell colonies display distinct mechanical properties and differentiation potential

In order to understand the mechanisms that guide cell fate decisions during early human development, we closely examined the differentiation process in adherent colonies of human embryonic stem cells (hESCs). Live imaging of the differentiation process reveals that cells on the outer edge of the undifferentiated colony begin to differentiate first and remain on the perimeter of the colony to eventually form a band of differentiation. Strikingly, this band is of constant width in all colonies, independent of their size. Cells at the edge of undifferentiated colonies show distinct actin organization, greater myosin activity and stronger traction forces compared to cells in the interior of the colony. Increasing the number of cells at the edge of colonies by plating small colonies can increase differentiation efficiency. Our results suggest that human developmental decisions are influenced by cellular environments and can be dictated by colony geometry of hESCs.

Heart-on-a-chip based on stem cell biology

Heart diseases are one of the main causes of death around the world. The great challenge for scientists is to develop new therapeutic methods for these types of ailments. Stem cells (SCs) therapy could be one of a promising technique used for renewal of cardiac cells and treatment of heart diseases. Conventional in vitro techniques utilized for investigation of heart regeneration do not mimic natural cardiac physiology. Lab-on-a-chip systems may be the solution which could allow the creation of a heart muscle model, enabling the growth of cardiac cells in conditions similar to in vivo conditions. Microsystems can be also used for differentiation of stem cells into heart cells, successfully. It will help better understand of proliferation and regeneration ability of these cells. In this review, we present Heart-on-a-chip systems based on cardiac cell culture and stem cell biology. This review begins with the description of the physiological environment and the functions of the heart. Next, we shortly described conventional techniques of stem cells differentiation into the cardiac cells. This review is mostly focused on describing Lab-on-a-chip systems for cardiac tissue engineering. Therefore, in the next part of this article, the microsystems for both cardiac cell culture and SCs differentiation into cardiac cells are described. The section about SCs differentiation into the heart cells is divided in sections describing biochemical, physical and mechanical stimulations. Finally, we outline present challenges and future research concerning Heart-on-a-chip based on stem cell biology.

Analysis of stochastic stem cell models with control

Understanding the dynamics of stem cell lineages is of central importance both for healthy and cancerous tissues. We study stochastic population dynamics of stem cells and differentiated cells, where cell decisions, such as proliferation vs. differentiation decisions, or division and death decisions, are under regulation from surrounding cells. The goal is to understand how different types of control mechanisms affect the means and variances of cell numbers. We use the assumption of weak dependencies of the regulatory functions (the controls) on the cell populations near the equilibrium to formulate moment equations. We then study three different methods of closure, showing that they all lead to the same results for the highest order terms in the expressions for the moments. We derive simple explicit expressions for the means and the variances of stem cell and differentiated cell numbers. It turns out that the variance is expressed as an algebraic function of partial derivatives of the controls with respect to the population sizes at the equilibrium. We demonstrate that these findings are consistent with the results previously obtained in the context of particular systems, and also present two novel examples with negative and positive control of division and differentiation decisions. This methodology is formulated without any specific assumptions on the functional form of the controls, and thus can be used for any biological system.

Application of biomaterials to advance induced pluripotent stem cell research and therapy

Derived from any somatic cell type and possessing unlimited self-renewal and differentiation potential, induced pluripotent stem cells (iPSCs) are poised to revolutionize stem cell biology and regenerative medicine research, bringing unprecedented opportunities for treating debilitating human diseases. To overcome the limitations associated with safety, efficiency, and scalability of traditional iPSC derivation, expansion, and differentiation protocols, biomaterials have recently been considered. Beyond addressing these limitations, the integration of biomaterials with existing iPSC culture platforms could offer additional opportunities to better probe the biology and control the behavior of iPSCs or their progeny in vitro and in vivo. Herein, we discuss the impact of biomaterials on the iPSC field, from derivation to tissue regeneration and modeling. Although still exploratory, we envision the emerging combination of biomaterials and iPSCs will be critical in the successful application of iPSCs and their progeny for research and clinical translation.

Engineering myocardial tissue patches with hierarchical structure-function

Complex hierarchical organization is a hallmark of tissues and their subsequent integration into organs. A major challenge in tissue engineering is to generate arrays of cells with defined structural organization that display appropriate functional properties. Given what is known about cellular responses to physiochemical cues from the surrounding environment, we can build tissue structures that mimic these microenvironments and validate these platforms using both experimental and computational approaches. Tissue generation encompasses many methods and tissue types, but here we review layering cell sheets to create scaffold-less myocardial patches. We discuss surgical criteria that can drive the design of myocardial cell sheets and the methods used to fabricate, mechanically condition, and functionally test them. We also focus on how computational and experimental approaches could be integrated to optimize tissue mechanical properties by using measurements of biomechanical properties and tissue anisotropy to create predictive computational models. Tissue anisotropy and dynamic mechanical stimuli affect cell phenotype in terms of protein expression and secretion, which in turn, leads to compositional and structural changes that ultimately impact tissue function. Therefore, a combinatorial approach of design, fabrication, testing, and modeling can be carried out iteratively to optimize engineered tissue function.

Paramagnetic beads and magnetically mediated strain enhance cardiomyogenesis in mouse embryoid bodies

Mechanical forces play an important role in proper embryologic development, and similarly such forces can directly impact pluripotency and differentiation of mouse embryonic stem cells (mESC) in vitro. In addition, manipulation of the embryoid body (EB) microenvironment, such as by incorporation of microspheres or microparticles, can similarly influence fate determination. In this study, we developed a mechanical stimulation regimen using permanent neodymium magnets to magnetically attract cells within an EB. Arginine-Glycine-Aspartic Acid (RGD)-conjugated paramagnetic beads were incorporated into the interior of the EBs during aggregation, allowing us to exert force on individual cells using short-term magnetization. EBs were stimulated for one hour at different magnetic field strengths, subsequently exerting a range of force intensity on the cells at different stages of early EB development. Our results demonstrated that following exposure to a 0.2 Tesla magnetic field, ESCs respond to magnetically mediated strain by activating Protein Kinase A (PKA) and increasing phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) expression. The timing of stimulation can also be tailored to guide ESC differentiation: the combination of bone morphogenetic protein 4 (BMP4) supplementation with one hour of magnetic attraction on Day 3 enhances cardiomyogenesis by increasing contractile activity and the percentage of sarcomeric α-actin-expressing cells compared to control samples with BMP4 alone. Interestingly, we also observed that the beads alone had some impact on differentiation by increasingly slightly, albeit not significantly, the percentage of cardiomyocytes. Together these results suggest that magnetically mediated strain can be used to enhance the percentage of mouse ESC-derived cardiomyocytes over current differentiation protocols.

Effect of substrate mechanics on cardiomyocyte maturation and growth

Cardiac tissue engineering constructs are a promising therapeutic treatment for myocardial infarction, which is one of the leading causes of death. In order to further advance the development and regeneration of engineered cardiac tissues using biomaterial platforms, it is important to have a complete overview of the effects that substrates have on cardiomyocyte (CM) morphology and function. This article summarizes recent studies that investigate the effect of mechanical cues on the CM differentiation, maturation, and growth. In these studies, CMs derived from embryos, neonates, and mesenchymal stem cells were seeded on different substrates of various elastic modulus. Measuring the contractile function by force production, work output, and calcium handling, it was seen that cell behavior on substrates was optimized when the substrate stiffness mimicked that of the native tissue. The contractile function reflected changes in the sarcomeric protein confirmation and organization that promoted the contractile ability. The analysis of the literature also revealed that, in addition to matrix stiffness, mechanical stimulation, such as stretching the substrate during cell seeding, also played an important role during cell maturation and tissue development.

Conjoint propagation and differentiation of human embryonic stem cells to cardiomyocytes in a defined microcarrier spinner culture

Introduction

Myocardial infarction is accompanied by a significant loss of cardiomyocytes (CMs). Functional CMs, differentiated from human embryonic stem cells (hESCs), offer a potentially unlimited cell source for cardiac disease therapies and regenerative cardiovascular medicine. However, conventional production methods on monolayer culture surfaces cannot adequately supply the large numbers of cells required for such treatments. To this end, an integrated microcarrier (MC) bioprocessing system for hESC propagation and subsequent CM differentiation was developed.

Methods

Production of hESC-derived CMs was initially established in monolayer cultures. This control condition was compared against hESC expansion on laminin-coated MC with cationic surface charge, in a stirred serum-free defined culture. Following expansion, the hESC/MC aggregates were placed in a CM differentiation medium, using Wnt signalling modulators in four different culture conditions. This process eliminated the need for manual colony cutting. The final optimized protocol was tested in stirred spinner flasks, combining expansion and differentiation on the same MC, with only media changes during the culture process.

Results

In the propagation phase, a 15-fold expansion of viable pluripotent HES-3 was achieved, with homogeneous sized aggregates of 316 ± 11 μm. Of the four differentiation conditions, stirred spinner flask cultures (MC-Sp) provided the best controlled aggregate sizes and yielded 1.9 × 10⁶ CM/ml, as compared to 0.5 × 10⁶ CM/ml using the monolayer cultures method: a four-fold increase in CM/ml. Similar results (1.3 × 10⁶ CM/ml) were obtained with an alternative hESC H7 line. The hESC/MC-derived CM expressed cardiac-specific transcription factors, structural, ion channel genes, and exhibited cross-striations of sarcomeric proteins, thus confirming their cardiac ontogeny. Moreover, E-4031 (0.3 μM) prolonged the QT-interval duration by 40% and verapamil (3 μM) reduced it by 45%, illustrating the suitability of these CM for pharmacological assays.

Conclusions

We have demonstrated a robust and scalable microcarrier system for generating hESC-derived CM. This platform is enabled by defined microcarrier matrices and it integrates cell propagation and differentiation within a continuous process, in serum-free culture media. It can generate significant numbers of CM, which are potentially suitable for future clinical therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/scrt498) contains supplementary material, which is available to authorized users.

Tensile forces applied on a cell-embedded three-dimensional scaffold can direct early differentiation of embryonic stem cells toward the mesoderm germ layer

Mechanical forces play an important role in the initial stages of embryo development; yet, the influence of forces, particularly of tensile forces, on embryonic stem cell differentiation is still unknown. The effects of tensile forces on mouse embryonic stem cell (mESC) differentiation within a three-dimensional (3D) environment were examined using an advanced bioreactor system. Uniaxial static or dynamic stretch was applied on cell-embedded collagen constructs. Six-day-long cyclic stretching of the seeded constructs led to a fourfold increase in Brachyury (BRACH-T) expression, associated with the primitive streak phase in gastrulation, confirmed also by immunofluorescence staining. Further examination of gene expression characteristic of mESC differentiation and pluripotency, under the same conditions, revealed changes mostly related to mesodermal processes. Additionally, downregulation of genes related to pluripotency and stemness was observed. Cyclic stretching of the 3D constructs resulted in actin fiber alignment parallel to the stretching direction. BRACH-T expression decreased under cyclic stretching with addition of myosin II inhibitor. No significant changes in gene expression were observed when mESCs were first differentiated in the form of embryoid bodies and then exposed to cyclic stretching, suggesting that forces primarily influence nondifferentiated cells. Understanding the effects of forces on stem cell differentiation provides a means of controlling their differentiation for later use in regenerative medicine applications and sheds light on their involvement in embryogenesis.

Nanoengineered Platforms to Guide Pluripotent Stem Cell Fate

Tissue engineering utilizes cells, signaling molecules, and scaffolds towards creating functional tissue to repair damaged organs. Pluripotent stem cells (PSCs) are a promising cell source due to their ability to self-renewal indefinitely and their potential to differentiate into almost any cell type. Great strides have been taken to parse the physiological mechanisms by which PSCs respond to their microenvironment and commit to a specific lineage. The combination of physical cues and chemical factors is thought to have the most profound influence on stem cell behavior, therefore a major focus of tissue engineering strategies is scaffold design to incorporate these signals. One overlooked component of the in vivo microenvironment researchers attempt to recapitulate with three dimensional (3D) substrates is the nanoarchitecture formed by the fibrillar network of extracellular matrix (ECM) proteins. These nanoscale features have the ability to impact cell adhesion, migration, proliferation, and lineage commitment. Significant advances have been made in deciphering how these nanoscale cues interact with stem cells to determine phenotype, but much is still unknown as to how the interplay between physical and chemical signals regulate in vitro and in vivo cellular fate. This review dives deeper to investigate nanoscale platforms for engineering tissue, as well use the use of these nanotechnologies to drive pluripotent stem cell lineage determination.

A nanoscale ridge/groove pattern arrayed surface enhances adipogenic differentiation of human supernumerary tooth-derived dental pulp stem cells in vitro

OBJECTIVE

The aim of this study was to establish human dental pulp stem cells (hDPSCs) from supernumerary teeth and determine the effects of a 350-nm nano-patterned surface on adipogenic and osteogenic differentiation of hDPSCs.

DESIGN

Several surface markers were analysed by FACS to confirm the isolated cells as hDPSCs. To demonstrate the effects of a nano-patterned surface on the differentiation of hDPSCs, the cells were cultured on a nano-patterned surface with or without adipogenic or osteogenic induction factors. Cells were then stained with Oil red O or Alizarin red, and the lineage specific genes LPL and Runx-2 were analysed by real-time PCR at 3, 6 and 9 days after culture.

RESULTS

The hDPSCs on a nano-patterned surface showed a linear arrangement compared to irregular cells on a conventional surface. During adipogenic differentiation, more Oil red O stained cells were found in the nano-patterned group than in the conventional group. On the other hand, there was no significant difference in Alizarin red staining between the nano-pattern and conventional surface groups after induction of osteogenic differentiation. Gene expression analyses revealed significantly higher expression of LPL in the nano-patterned group than in the conventional group, whereas Runx-2 expression was higher in the conventional group than in the nano-patterned group.

CONCLUSION

This study showed that a nano-patterned surface may be able to enhance adipogenic differentiation of hDPSCs by altering their morphology and gene expression patterns, whereas the same surface may inhibit or suppress osteogenic differentiation of the cells.

A global assessment of stem cell engineering

Over the last 2 years a global assessment of stem cell engineering (SCE) was conducted with the sponsorship of the National Science Foundation, the National Cancer Institute at the National Institutes of Health, and the National Institute of Standards and Technology. The purpose was to gather information on the worldwide status and trends in SCE, that is, the involvement of engineers and engineering approaches in the stem cell field, both in basic research and in the translation of research into clinical applications and commercial products. The study was facilitated and managed by the World Technology Evaluation Center. The process involved site visits in both Asia and Europe, and it also included several different workshops. From this assessment, the panel concluded that there needs to be an increased role for engineers and the engineering approach. This will provide a foundation for the generation of new markets and future economic growth. To do this will require an increased investment in engineering, applied research, and commercialization as it relates to stem cell research and technology. It also will require programs that support interdisciplinary teams, new innovative mechanisms for academic-industry partnerships, and unique translational models. In addition, the global community would benefit from forming strategic partnerships between countries that can leverage existing and emerging strengths in different institutions. To implement such partnerships will require multinational grant programs with appropriate review mechanisms.

Biomaterial strategies for stem cell maintenance during in vitro expansion

Stem cells, having the potential for self-renewal and multilineage differentiation, are the building blocks for tissue/organ regeneration. Stem cells can be isolated from various sources but are, in general, available in too small numbers to be used directly for clinical purpose without intermediate expansion procedures in vitro. Although this in vitro expansion of undifferentiated stem cells is necessary, stem cells typically diminish their ability to self-renew and proliferate during passaging. Consequently, maintaining the stemness of stem cells has been recognized as a major challenge in stem cell-based research. This review focuses on the latest developments in maintaining the self-renewal ability of stem cells during in vitro expansion by biomaterial strategies. Further, this review highlights what should be the focus for future studies using stem cells for regenerative applications.

Engineering the human pluripotent stem cell microenvironment to direct cell fate

Human pluripotent stem cells (hPSCs), including both embryonic stem cells and induced pluripotent stem cells, offer a potential cell source for research, drug screening, and regenerative medicine applications due to their unique ability to self-renew or differentiate to any somatic cell type. Before the full potential of hPSCs can be realized, robust protocols must be developed to direct their fate. Cell fate decisions are based on components of the surrounding microenvironment, including soluble factors, substrate or extracellular matrix, cell-cell interactions, mechanical forces, and 2D or 3D architecture. Depending on their spatio-temporal context, these components can signal hPSCs to either self-renew or differentiate to cell types of the ectoderm, mesoderm, or endoderm. Researchers working at the interface of engineering and biology have identified various factors which can affect hPSC fate, often based on lessons from embryonic development, and they have utilized this information to design in vitro niches which can reproducibly direct hPSC fate. This review highlights culture systems that have been engineered to promote self-renewal or differentiation of hPSCs, with a focus on studies that have elucidated the contributions of specific microenvironmental cues in the context of those culture systems. We propose the use of microsystem technologies for high-throughput screening of spatial-temporal presentation of cues, as this has been demonstrated to be a powerful approach for differentiating hPSCs to desired cell types.

Symmetric vs. asymmetric stem cell divisions: an adaptation against cancer?

Traditionally, it has been held that a central characteristic of stem cells is their ability to divide asymmetrically. Recent advances in inducible genetic labeling provided ample evidence that symmetric stem cell divisions play an important role in adult mammalian homeostasis. It is well understood that the two types of cell divisions differ in terms of the stem cells' flexibility to expand when needed. On the contrary, the implications of symmetric and asymmetric divisions for mutation accumulation are still poorly understood. In this paper we study a stochastic model of a renewing tissue, and address the optimization problem of tissue architecture in the context of mutant production. Specifically, we study the process of tumor suppressor gene inactivation which usually takes place as a consequence of two “hits”, and which is one of the most common patterns in carcinogenesis. We compare and contrast symmetric and asymmetric (and mixed) stem cell divisions, and focus on the rate at which double-hit mutants are generated. It turns out that symmetrically-dividing cells generate such mutants at a rate which is significantly lower than that of asymmetrically-dividing cells. This result holds whether single-hit (intermediate) mutants are disadvantageous, neutral, or advantageous. It is also independent on whether the carcinogenic double-hit mutants are produced only among the stem cells or also among more specialized cells. We argue that symmetric stem cell divisions in mammals could be an adaptation which helps delay the onset of cancers. We further investigate the question of the optimal fraction of stem cells in the tissue, and quantify the contribution of non-stem cells in mutant production. Our work provides a hypothesis to explain the observation that in mammalian cells, symmetric patterns of stem cell division seem to be very common.

Engineered micromechanical cues affecting human pluripotent stem cell regulations and fate

The survival, growth, self-renewal, and differentiation of human pluripotent stem cells (hPSCs) are influenced by their microenvironment, or so-called "niche," consisting of particular chemical and physical cues. Previous studies on mesenchymal stem cells and other stem cells have collectively uncovered the importance of physical cues and have begun to shed light on how stem cells sense and process such cues. In an attempt to support similar progress in mechanobiology of hPSCs, we review mechanosensory machinery, which plays an important role in cell-extracellular matrix interactions, cell-cell interactions, and subsequent intracellular responses. In addition, we review recent studies on the mechanobiology of hPSCs, in which engineered micromechanical environments were used to investigate effects of specific physical cues. Identifying key physical cues and understanding their mechanism will ultimately help in harnessing the full potential of hPSCs for clinical applications.

Principles of regulation of self-renewing cell lineages

Identifying the exact regulatory circuits that can stably maintain tissue homeostasis is critical for our basic understanding of multicellular organisms, and equally critical for identifying how tumors circumvent this regulation, thus providing targets for treatment. Despite great strides in the understanding of the molecular components of stem-cell regulation, the overall mechanisms orchestrating tissue homeostasis are still far from being understood. Typically, tissue contains the stem cells, transit amplifying cells, and terminally differentiated cells. Each of these cell types can potentially secrete regulatory factors and/or respond to factors secreted by other types. The feedback can be positive or negative in nature. This gives rise to a bewildering array of possible mechanisms that drive tissue regulation. In this paper, we propose a novel method of studying stem cell lineage regulation, and identify possible numbers, types, and directions of control loops that are compatible with stability, keep the variance low, and possess a certain degree of robustness. For example, there are exactly two minimal (two-loop) control networks that can regulate two-compartment (stem and differentiated cell) tissues, and 20 such networks in three-compartment tissues. If division and differentiation decisions are coupled, then there must be a negative control loop regulating divisions of stem cells (e.g. by means of contact inhibition). While this mechanism is associated with the highest robustness, there could be systems that maintain stability by means of positive divisions control, coupled with specific types of differentiation control. Some of the control mechanisms that we find have been proposed before, but most of them are new, and we describe evidence for their existence in data that have been previously published. By specifying the types of feedback interactions that can maintain homeostasis, our mathematical analysis can be used as a guide to experimentally zero in on the exact molecular mechanisms in specific tissues.

Microfluidic platforms for mechanobiology

Mechanotransduction has been a topic of considerable interest since early studies demonstrated a link between mechanical force and biological response. Until recently, studies of fundamental phenomena were based either on in vivo experiments with limited control or direct access, or on large-scale in vitro studies lacking many of the potentially important physiological factors. With the advent of microfluidics, many of the previous limitations of in vitro testing were eliminated or reduced through greater control or combined functionalities. At the same time, imaging capabilities were tremendously enhanced. In this review, we discuss how microfluidics has transformed the study of mechanotransduction. This is done in the context of the various cell types that exhibit force-induced responses and the new biological insights that have been elucidated. We also discuss new microfluidic studies that could produce even more realistic models of in vivo conditions by combining multiple stimuli or creating a more realistic microenvironment.

Mechanobiology of human pluripotent stem cells

Human pluripotent stem cells (hPSCs) are self-renewing and have the potential to differentiate into any cell type in the body, making them attractive cell sources for applications in tissue engineering and regenerative medicine. However, in order for hPSCs to find use in the clinic, the mechanisms underlying their self-renewal and lineage commitment must be better understood. Many technologies that have been developed for the maintenance and directed differentiation of hPSCs involve the use of soluble growth factors, but recent studies suggest that other elements of the hPSC microenvironment also influence the growth and differentiation of hPSCs. This includes the influences of cell-cell interactions, substrate mechanics, cellular interactions with extracellular matrix, as well as the nanotopography of the substrate and physical forces such as shear stress, cyclic mechanical strain, and compression. In this review, we highlight the recent progress of this area of research and discuss ways in which the mechanical cues may be incorporated into hPSC culture regimes to improve methods for expanding and differentiating hPSCs.