Human Pluripotent Stem Cell Mechanobiology: Manipulating the Biophysical Microenvironment for Regenerative Medicine and Tissue Engineering Applications

3D Bioprinting of Hyaline Articular Cartilage: Biopolymers, Hydrogels, and Bioinks

The musculoskeletal system, consisting of bones and cartilage of various types, muscles, ligaments, and tendons, is the basis of the human body. However, many pathological conditions caused by aging, lifestyle, disease, or trauma can damage its elements and lead to severe disfunction and significant worsening in the quality of life. Due to its structure and function, articular (hyaline) cartilage is the most susceptible to damage. Articular cartilage is a non-vascular tissue with constrained self-regeneration capabilities. Additionally, treatment methods, which have proven efficacy in stopping its degradation and promoting regeneration, still do not exist. Conservative treatment and physical therapy only relieve the symptoms associated with cartilage destruction, and traditional surgical interventions to repair defects or endoprosthetics are not without serious drawbacks. Thus, articular cartilage damage remains an urgent and actual problem requiring the development of new treatment approaches. The emergence of biofabrication technologies, including three-dimensional (3D) bioprinting, at the end of the 20th century, allowed reconstructive interventions to get a second wind. Three-dimensional bioprinting creates volume constraints that mimic the structure and function of natural tissue due to the combinations of biomaterials, living cells, and signal molecules to create. In our case-hyaline cartilage. Several approaches to articular cartilage biofabrication have been developed to date, including the promising technology of 3D bioprinting. This review represents the main achievements of such research direction and describes the technological processes and the necessary biomaterials, cell cultures, and signal molecules. Special attention is given to the basic materials for 3D bioprinting-hydrogels and bioinks, as well as the biopolymers underlying the indicated products.

Microenvironments Matter: Advances in Brain-on-Chip

To highlight the particular needs with respect to modeling the unique and complex organization of the human brain structure, we reviewed the state-of-the-art in devising brain models with engineered instructive microenvironments. To acquire a better perspective on the brain's working mechanisms, we first summarize the importance of regional stiffness gradients in brain tissue, varying per layer and the cellular diversities of the layers. Through this, one can acquire an understanding of the essential parameters in emulating the brain in vitro. In addition to the brain's organizational architecture, we addressed also how the mechanical properties have an impact on neuronal cell responses. In this respect, advanced in vitro platforms emerged and profoundly changed the methods of brain modeling efforts from the past, mainly focusing on animal or cell line research. The main challenges in imitating features of the brain in a dish are with regard to composition and functionality. In neurobiological research, there are now methods that aim to cope with such challenges by the self-assembly of human-derived pluripotent stem cells (hPSCs), i.e., brainoids. Alternatively, these brainoids can be used stand-alone or in conjunction with Brain-on-Chip (BoC) platform technology, 3D-printed gels, and other types of engineered guidance features. Currently, advanced in vitro methods have made a giant leap forward regarding cost-effectiveness, ease-of-use, and availability. We bring these recent developments together into one review. We believe our conclusions will give a novel perspective towards advancing instructive microenvironments for BoCs and the understanding of the brain's cellular functions either in modeling healthy or diseased states of the brain.

Assessing the combined effect of surface topography and substrate rigidity in human bone marrow stem cell cultures

The combined effect of surface topography and substrate rigidity in stem cell cultures is still under-investigated, especially when biodegradable polymers are used. Herein, we assessed human bone marrow stem cell response on aliphatic polyester substrates as a function of anisotropic grooved topography and rigidity (7 and 12 kPa). Planar tissue culture plastic (TCP, 3 GPa) and aliphatic polyester substrates were used as controls. Cell morphology analysis revealed that grooved substrates caused nuclei orientation/alignment in the direction of the grooves. After 21 days in osteogenic and chondrogenic media, the 3 GPa TCP and the grooved 12 kPa substrate induced significantly higher calcium deposition and alkaline phosphatase (ALP) activity and glycosaminoglycan (GAG) deposition, respectively, than the other groups. After 14 days in tenogenic media, the 3 GPa TCP upregulated four and downregulated four genes; the planar 7 kPa substrate upregulated seven genes and downregulated one gene; and the grooved 12 kPa substrate upregulated seven genes and downregulated one gene. After 21 days in adipogenic media, the softest (7 kPa) substrates induced significantly higher oil droplet deposition than the other substrates and the grooved substrate induced significantly higher droplet deposition than the planar. Our data pave the way for more rational design of bioinspired constructs.

Harnessing conserved signaling and metabolic pathways to enhance the maturation of functional engineered tissues

The development of induced-pluripotent stem cell (iPSC)-derived cell types offers promise for basic science, drug testing, disease modeling, personalized medicine, and translatable cell therapies across many tissue types. However, in practice many iPSC-derived cells have presented as immature in physiological function, and despite efforts to recapitulate adult maturity, most have yet to meet the necessary benchmarks for the intended tissues. Here, we summarize the available state of knowledge surrounding the physiological mechanisms underlying cell maturation in several key tissues. Common signaling consolidators, as well as potential synergies between critical signaling pathways are explored. Finally, current practices in physiologically relevant tissue engineering and experimental design are critically examined, with the goal of integrating greater decision paradigms and frameworks towards achieving efficient maturation strategies, which in turn may produce higher-valued iPSC-derived tissues.

Soft substrate maintains stemness and pluripotent stem cell-like phenotype of human embryonic stem cells under defined culture conditions

Human embryonic stem cells (hESCs) are derived from the inner cell mass (ICM) of the pre-implantation blastocyst. Prior to embryo implantation, the ICM cells are surrounded by trophoblasts which have mechanical stiffness ranging from Pascal (Pa) to kilopascal (kPa). However, under in vitro conditions these cells are cultured on stiff tissue culture treated plastic plates (TCP) which have stiffness of approximately 1 gigapascal (GPa). This obvious dichotomy motivated us to investigate the fate of hESCs cultured on softer substrate, and to probe if the hESCs undergo differentiation or they retain pluripotency on soft substrates. We investigated the expression of pluripotency markers, and lineage-specific markers; we particularly looked at the expression of transcriptional coactivator YAP (Yes-associated protein), an important mediator of extracellular matrix (ECM) mechanical cues and a known downstream transducer of Hippo pathway. Downregulation of YAP has been correlated to the loss of multipotency of human mesenchymal stem cells (hMSCs) and pluripotency in mouse ESCs (mESCs); but we report that hESCs maintain their stemness on soft substrate of varying stiffness. Our findings revealed that on soft substrate hESCs express pluripotency markers and does not undergo substrate-mediated differentiation. Interestingly we show that hESCs maintained basal level of YAP expression for cell survival and proliferation, but YAP expression does not correlate directly with pluripotency in hESCs. To summarize, our results show that hESCs retain their stemness on soft substrate despite downregulation of YAP.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-022-00537-z.

Micro/nanoengineered technologies for human pluripotent stem cells maintenance and differentiation

Human pluripotent stem cells (hPSCs) are a promising source of cells for cell replacement-based therapies as well as modeling human development and diseases in vitro. However, achieving fate control of hPSC with a high yield and specificity remains challenging. The fate specification of hPSCs is regulated by biochemical and biomechanical cues in their environment. Driven by this knowledge, recent exciting advances in micro/nanoengineering have been leveraged to develop a broad range of tools for the generation of extracellular biomechanical and biochemical signals that determine the behavior of hPSCs. In this review, we summarize such micro/nanoengineered technologies for controlling hPSC fate and highlight the role of biochemical and biomechanical cues such as substrate rigidity, surface topography, and cellular confinement in the hPSC-based technologies that are on the horizon.

Performance of the Polydopamine-Graphene Oxide Composite Substrate in the Osteogenic Differentiation of Mouse Embryonic Stem Cells

Graphene oxide (GO) is a biocompatible material considered a favorable stem cell culture substrate. In this study, GO was modified with polydopamine (PDA) to facilitate depositing GO onto a tissue culture polystyrene (PT) surface, and the osteogenic performance of the PDA/GO composite in pluripotent embryonic stem cells (ESCs) was investigated. The surface chemistry of the PDA/GO-coated PT surface was analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). A high cell viability of ESCs cultured on the PDA/GO composite-coated surface was initially ensured. Then, the osteogenic differentiation of the ESCs in response to the PDA/GO substrate was assessed by alkaline phosphatase (ALP) activity, intracellular calcium levels, matrix mineralization assay, and evaluation of the mRNA and protein levels of osteogenic factors. The culture of ESCs on the PDA/GO substrate presented higher osteogenic potency than that on the uncoated control surface. ESCs cultured on the PDA/GO substrate expressed significantly higher levels of integrin α5 and β1, as well as bone morphogenetic protein receptor (BMPR) types I and II, compared with the control groups. The phosphorylation of extracellular signal-regulated kinase (ERK)1/2, p38, and c-Jun-N-terminal kinase (JNK) mitogen-activated protein kinases (MAPKs) was observed in ESCs culture on the PDA/GO substrate. Moreover, BMP signal transduction by SMAD1/5/8 phosphorylation was increased more in cells on PDA/GO than in the control. The nuclear translocation of SMAD1/5/8 in cells was also processed in response to the PDA/GO substrate. Blocking activation of the integrin α5/β1, MAPK, or SMAD signaling pathways downregulated the PDA/GO-induced osteogenic differentiation of ESCs. These results suggest that the PDA/GO composite stimulates the osteogenic differentiation of ESCs via the integrin α5/β1, MAPK, and BMPR/SMAD signaling pathways.

Induced Pluripotent Stem Cells on a Chip: A Self-Contained, Accessible, Pipette-less iPSC Culturing and Differentiation Kit

Over the past decade, induced pluripotent stem cells (iPSCs) have become a major focus of stem cell and developmental biology research, offering researchers a clinically relevant source of cells that are amenable to genetic engineering approaches. Though stem cells are promising for both research and commercial endeavors, iPSC-based assays require tedious protocols that include complex treatments, expensive reagents, and specialized equipment that limit their integration into academic curricula and cell biology research groups. Expanding on existing Kit-On-A-Lid-Assay (KOALA) technologies, we have developed a self-contained, injection molded, pipette-less iPSC culture and differentiation platform that significantly reduces associated costs and labor of stem cell maintenance and differentiation. The KOALA kit offers users the full range of iPSC culture necessities, including cell cryopreservation, media exchanges, differentiation, endpoint analysis, and a new capability, cell passaging. Using the KOALA kit, we were able to culture ~20,000 iPSCs per microchannel for at least 7 days, while maintaining stable expression of stemness markers (SSEA4 and Oct4) and normal iPSC phenotype. We also adapted protocols for differentiating iPSCs into neuroepithelial cells, cardiomyocytes, and definitive endodermal cells, a cell type from each germ layer of human development.

Radial extracorporeal shockwave promotes subchondral bone stem/progenitor cell self-renewal by activating YAP/TAZ and facilitates cartilage repair in vivo

BACKGROUND

Radial extracorporeal shockwave (r-ESW), an innovative and noninvasive technique, is gaining increasing attention in regenerative medicine due to its mechanobiological effects. Subchondral bone stem/progenitor cells (SCB-SPCs), originating from the pivotal zone of the osteochondral unit, have been shown to have multipotency and self-renewal properties. However, thus far, little information is available regarding the influences of r-ESW on the biological properties of SCB-SPCs and their therapeutic effects in tissue regeneration.

METHODS

SCB-SPCs were isolated from human knee plateau osteochondral specimens and treated with gradient doses of r-ESW in a suspension stimulation system. The optimized parameters for SCB-SPC self-renewal were screened out by colony-forming unit fibroblast assay (CFU-F). Then, the effects of r-ESW on the proliferation, apoptosis, and multipotency of SCB-SPCs were evaluated. Moreover, the repair efficiency of radial shockwave-preconditioned SCB-SPCs was evaluated in vivo via an osteochondral defect model. Potential mechanisms were explored by western blotting, confocal laser scanning, and high-throughput sequencing.

RESULTS

The CFU-F data indicate that r-ESW could augment the self-renewal of SCB-SPCs in a dose-dependent manner. The CCK-8 and flow cytometry results showed that the optimized shockwave markedly promoted SCB-SPC proliferation but had no significant influence on cell apoptosis. Radial shockwave exerted no significant influence on osteogenic capacity but strongly suppressed adipogenic ability in the current study. For chondrogenic potentiality, the treated SCB-SPCs were mildly enhanced, while the change was not significant. Importantly, the macroscopic scores and further histological analysis strongly demonstrated that the in vivo therapeutic effects of SCB-SPCs were markedly improved post r-ESW treatment. Further analysis showed that the cartilage-related markers collagen II and proteoglycan were expressed at higher levels compared to their counterpart group. Mechanistic studies suggested that r-ESW treatment strongly increased the expression of YAP and promoted YAP nuclear translocation in SCB-SPCs. More importantly, self-renewal was partially blocked by the YAP-specific inhibitor verteporfin. Moreover, the high-throughput sequencing data indicated that other self-renewal-associated pathways may also be involved in this process.

CONCLUSION

We found that r-ESW is capable of promoting the self-renewal of SCB-SPCs in vitro by targeting YAP activity and strengthening its repair efficiency in vivo, indicating promising application prospects.

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.

Building an Artificial Cardiac Microenvironment: A Focus on the Extracellular Matrix

The increased knowledge in cell signals and stem cell differentiation, together with the development of new technologies, such as 3D bioprinting, has made the generation of artificial tissues more feasible for in vitro studies and in vivo applications. In the human body, cell fate, function, and survival are determined by the microenvironment, a rich and complex network composed of extracellular matrix (ECM), different cell types, and soluble factors. They all interconnect and communicate, receiving and sending signals, modulating and responding to cues. In the cardiovascular field, the culture of stem cells in vitro and their differentiation into cardiac phenotypes is well established, although differentiated cardiomyocytes often lack the functional maturation and structural organization typical of the adult myocardium. The recreation of an artificial microenvironment as similar as possible to the native tissue, though, has been shown to partly overcome these limitations, and can be obtained through the proper combination of ECM molecules, different cell types, bioavailability of growth factors (GFs), as well as appropriate mechanical and geometrical stimuli. This review will focus on the role of the ECM in the regulation of cardiac differentiation, will provide new insights on the role of supporting cells in the generation of 3D artificial tissues, and will also present a selection of the latest approaches to recreate a cardiac microenvironment in vitro through 3D bioprinting approaches.

Use of a PTFE Micro-Bioreactor to Promote 3D Cell Rearrangement and Maintain High Plasticity in Epigenetically Erased Fibroblasts

Phenotype definition is driven by epigenetic mechanisms as well as directly influenced by the cell microenvironment and by biophysical signals deriving from the extracellular matrix. The possibility to interact with the epigenetic signature of an adult mature cell, reversing its differentiated state and inducing a short transient high plasticity window, was previously demonstrated. In parallel, in vitro studies have shown that 3D culture systems, mimicking cell native tissue, exert significant effects on cell behavior and functions. Here we report the production of "PTFE micro-bioreactors" for long-term culture of epigenetically derived high plasticity cells. The system promotes 3D cell rearrangement, global DNA demethylation and elevated transcription of pluripotency markers, that is dependent on WW domain containing transcription regulator 1 (TAZ) nuclear accumulation and SMAD family member 2 (SMAD2) co-shuttling. Our findings demonstrate that the use of 3D culture strategies greatly improves the induction and maintenance of a high plasticity state.

Functional nanoarrays for investigating stem cell fate and function

Stem cells show excellent potential in the field of tissue engineering and regenerative medicine based on their excellent capability to not only self-renew but also differentiate into a specialized cell type of interest. However, the lack of a non-destructive monitoring system makes it challenging to identify and characterize differentiated cells before their transplantation without compromising cell viability. Thus, the development of a non-destructive monitoring method for analyzing cell function is highly desired and can significantly benefit stem cell-based therapies. Recently, nanomaterial-based scaffolds (e.g., nanoarrays) have made possible considerable advances in controlling the differentiation of stem cells and characterization of the differentiation status sensitively in real time. This review provides a selective overview of the recent progress in the synthesis methods of nanoarrays and their applications in controlling stem cell fate and monitoring live cell functions electrochemically. We believe that the topics discussed in this review can provide brief and concise guidelines for the development of novel nanoarrays and promote the interest in live cell study applications. A method which can not only control but also monitor stem cell fate and function will be a promising technology that can accelerate stem cell therapies.

Combinatorial extracellular matrix microarray identifies novel bioengineered substrates for xeno-free culture of human pluripotent stem cells

Stem cells in their microenvironment are exposed to a plethora of biochemical signals and biophysical forces. Interrogating the role of each factor in the cell microenvironment, however, remains difficult due to the inability to study microenvironmental cues and tease apart their interactions in high throughput. To address this need, we developed an extracellular matrix (ECM) microarray screening platform capable of tightly controlling substrate stiffness and ECM protein composition to screen the effects of these cues and their interactions on cell fate. We combined this platform with a design of experiments screening strategy to identify optimal conditions that can maintain human pluripotent stem cell (hPSC) pluripotency in chemically defined, xeno-free conditions. Combinations of ECM proteins (fibronectin, vitronectin, laminin-521, and collagen IV) were deposited on polydimethylsiloxane substrates with elastic moduli ranging from ~1 to 60 kPa using a high throughput protein plotter. Through our screening approach, we identified several non-intuitive protein-protein and protein-stiffness interactions and developed three novel culture substrates. hPSCs grown on these novel culture substrates displayed higher proliferation rates and pluripotency marker expression than current gold-standard culture substrates Geltrex- and vitronectin-coated plastic. This ECM microarray and screening approach is not limited to the factors studied here and can be broadly applied to other cell types to systematically screen microenvironmental conditions to optimally guide cell phenotype.

Engineering Biomaterials with Micro/Nanotechnologies for Cell Reprogramming

Cell reprogramming is a revolutionized biotechnology that offers a powerful tool to engineer cell fate and function for regenerative medicine, disease modeling, drug discovery, and beyond. Leveraging advances in biomaterials and micro/nanotechnologies can enhance the reprogramming performance in vitro and in vivo through the development of delivery strategies and the control of biophysical and biochemical cues. In this review, we present an overview of the state-of-the-art technologies for cell reprogramming and highlight the recent breakthroughs in engineering biomaterials with micro/nanotechnologies to improve reprogramming efficiency and quality. Finally, we discuss future directions and challenges for reprogramming technologies and clinical translation.

Emerging Methods for Enhancing Pluripotent Stem Cell Expansion

Pluripotent stem cells (PSCs) have great potential to revolutionize the fields of tissue engineering and regenerative medicine as well as stem cell therapeutics. However, the end goal of using PSCs for therapeutic use remains distant due to limitations in current PSC production. Conventional methods for PSC expansion have limited potential to be scaled up to produce the number of cells required for the end-goal of therapeutic use due to xenogenic components, high cost or low efficiency. In this mini review, we explore novel methods and emerging technologies of improving PSC expansion: the use of the two-dimensional mechanobiological strategies of topography and stiffness and the use of three-dimensional (3D) expansion methods including encapsulation, microcarrier-based culture, and suspension culture. Additionally, we discuss the limitations of conventional PSC expansion methods as well as the challenges in implementing non-conventional methods.

Mimicking megakaryopoiesis in vitro using biomaterials: Recent advances and future opportunities

Presently donor-derived platelets used in the clinic are associated with concerns about adequate availability, expense, risk of bacterial contamination and complications due to immunological reaction. To prevail over our dependence on transfusion of donor-derived platelets, efforts are being made to generate them in vitro. Development of biomaterials that support or mimic bone marrow niche micro-environmental cues could improve the in vitro production of platelets from megakaryocytes (MKs) derived from various stem cell sources. In spite of significant advances in the production of MKs from various stem cell sources using 2D as well as 3D culture approaches in vitro and the development of biomaterials-based platelet systems, yield and quality of these platelets remains unsuitable for clinical use. Thus, in vitro production of clinically useful platelets on a large scale remains an unmet target to date. This review summarizes the most frequently used 2D and 3D approaches to generate MKs and platelets in vitro, emphasizing the importance of mimicking in vivo micro-environment. Further, this review proposes the use of interpenetrating network (IPN) biomaterial-based approach as a promising strategy for improving the generation of MK and platelets in sufficient numbers in vitro. STATEMENT OF SIGNIFICANCE: Thrombocytopenia is one of the major global health and socio-economic problems. Transfusion with donor-derived platelets (PLTs) is the only effective treatment for this condition. However, this approach is limited by factors like short shelf-life of PLTs, PLT activation, alloimmunization, risk of bacterial contamination, infection etc. In vitro generated MKs and PLTs derived from non-donor-dependent sources may help to overcome the platelet transfusion concerns. Here we have reviewed various 2D and 3D strategies used for in vitro generation of MKs and PLTs, with special emphasis on various biomaterial platforms and different physico/chemical cues being used for the purpose. We have also proposed a biomaterial-based approach of using interpenetrating network (IPN) for generating clinically relevant numbers of MKs and PLTs.

Heterogeneity in Adipose Stem Cells

Adipose stem cells (ASCs) are the basis of procedures intended for tissue regeneration. These cells are heterogeneous, owing to various factors, including the donor age, sex, body mass index, and clinical condition; the isolation procedure (liposuction or fat excision); the place from where the cells were sampled (body site and depth of each adipose depot); culture surface; type of medium (whether supplemented with fetal bovine serum or xeno-free), that affect the principal phenotypic features of ASCs. The features related to ASCs heterogeneity are relevant for the success of therapeutic procedures; these features include proliferation capacity, differentiation potential, immunophenotype, and the secretome. These are important characteristics for the success of regenerative tissue engineering, not only because of their effects upon the reconstruction and healing exerted by ASCs themselves, but also because of the paracrine signaling of ASCs and its impact on recipient tissues. Knowledge of sources of heterogeneity will be helpful in the standardization of ASCs-based procedures. New avenues of research could include evaluation of the effects of the use of more homo1geneous ASCs for specific purposes, the study of ASCs-recipient interactions in heterologous cell transplantation, and the characterization of epigenetic changes in ASCs, as well as investigations of the effect of the metabolome upon ASCs behavior in culture.

Microphysiological systems meet hiPSC technology - New tools for disease modeling of liver infections in basic research and drug development

Complex cell culture models such as microphysiological models (MPS) mimicking human liver functionality in vitro are in the spotlight as alternative to conventional cell culture and animal models. Promising techniques like microfluidic cell culture or micropatterning by 3D bioprinting are gaining increasing importance for the development of MPS to address the needs for more predictivity and cost efficiency. In this context, human induced pluripotent stem cells (hiPSCs) offer new perspectives for the development of advanced liver-on-chip systems by recreating an in vivo like microenvironment that supports the reliable differentiation of hiPSCs to hepatocyte-like cells (HLC). In this review we will summarize current protocols of HLC generation and highlight recently established MPS suitable to resemble physiological hepatocyte function in vitro. In addition, we are discussing potential applications of liver MPS for disease modeling related to systemic or direct liver infections and the use of MPS in testing of new drug candidates.

Microfluidics for mechanobiology of model organisms

Mechanical stimuli play a critical role in organ development, tissue homeostasis, and disease. Understanding how mechanical signals are processed in multicellular model systems is critical for connecting cellular processes to tissue- and organism-level responses. However, progress in the field that studies these phenomena, mechanobiology, has been limited by lack of appropriate experimental techniques for applying repeatable mechanical stimuli to intact organs and model organisms. Microfluidic platforms, a subgroup of microsystems that use liquid flow for manipulation of objects, are a promising tool for studying mechanobiology of small model organisms due to their size scale and ease of customization. In this work, we describe design considerations involved in developing a microfluidic device for studying mechanobiology. Then, focusing on worms, fruit flies, and zebrafish, we review current microfluidic platforms for mechanobiology of multicellular model organisms and their tissues and highlight research opportunities in this developing field.

New substrates for stem cell control

The capacity to culture stem cells in a controllable, robust and scalable manner is necessary in order to develop successful strategies for the generation of cellular and tissue platforms for drug screening, toxicity testing, tissue engineering and regenerative medicine. Creating substrates that support the expansion, maintenance or directional differentiation of stem cells would greatly aid these efforts. Optimally, the substrates used should be chemically defined and synthetically scalable, allowing growth under defined, serum-free culture conditions. To achieve this, the chemical and physical attributes of the substrates should mimic the natural tissue environment and allow control of their biological properties. Herein, recent advances in the development of materials to study/manipulate stem cells, both in vitro and in vivo, are described with a focus on the novelty of the substrates' properties, and on application of substrates to direct stem cells.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Bacterial Cellulose Shifts Transcriptome and Proteome of Cultured Endothelial Cells Towards Native Differentiation

Preserving the native phenotype of primary cells in vitro is a complex challenge. Recently, hydrogel-based cellular matrices have evolved as alternatives to conventional cell culture techniques. We developed a bacterial cellulose-based aqueous gel-like biomaterial, dubbed Xellulin, which mimics a cellular microenvironment and seems to maintain the native phenotype of cultured and primary cells. When applied to human umbilical vein endothelial cells (HUVEC), it allowed the continuous cultivation of cell monolayers for more than one year without degradation or dedifferentiation. To investigate the impact of Xellulin on the endothelial cell phenotype in detail, we applied quantitative transcriptomics and proteomics and compared the molecular makeup of native HUVEC, HUVEC on collagen-coated Xellulin and collagen-coated cell culture plastic (polystyrene).Statistical analysis of 12,475 transcripts and 7831 proteins unveiled massive quantitative differences of the compared transcriptomes and proteomes. K-means clustering followed by network analysis showed that HUVEC on plastic upregulate transcripts and proteins controlling proliferation, cell cycle and protein biosynthesis. In contrast, HUVEC on Xellulin maintained, by and large, the expression levels of genes supporting their native biological functions and signaling networks such as integrin, receptor tyrosine kinase MAP/ERK and PI3K signaling pathways, while decreasing the expression of proliferation associated proteins. Moreover, CD34-an endothelial cell differentiation marker usually lost early during cell culture - was re-expressed within 2 weeks on Xellulin but not on plastic. And HUVEC on Xellulin showed a significantly stronger functional responsiveness to a prototypic pro-inflammatory stimulus than HUVEC on plastic.Taken together, this is one of the most comprehensive transcriptomic and proteomic studies of native and propagated HUVEC, which underscores the importance of the morphology of the cellular microenvironment to regulate cellular differentiation, and demonstrates, for the first time, the potential of Xellulin as versatile tool promoting an in vivo-like phenotype in primary and propagated cell culture.

Effects of clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) protein 9 system-Based Deletion of miR-451 in Mouse Embryonic Stem Cells on Their Self-Renewal and Hematopoietic Differentiation

Pluripotent stem cells (PSCs) are a useful source of cells for exploring the role of genes related with early developmental processes and specific diseases due to their ability to differentiate into all somatic cell types. Recently, the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) protein 9 system has proven to be a robust tool for targeted genetic modification. Here, we generated miR-451-deficient PSCs using the CRISPR/Cas9 system with PCR-based homologous recombination donor and investigated the impact of its deletion on self-renewal and hematopoietic development. CRISPR/Cas9-mediated miR-451 knockout did not alter the gene expressions of pluripotency, cellular morphology, and cell cycle, but led to impaired erythrocyte development. These findings propose that a combination of PSCs and CRISPR/Cas9 system could be useful to promote biomedical applications of PSCs by elucidating the function and manipulating of specific miRNAs during lineage specification and commitment.

Graphene Oxide promotes embryonic stem cell differentiation to haematopoietic lineage

Pluripotent stem cells represent a promising source of differentiated tissue-specific stem and multipotent progenitor cells for regenerative medicine and drug testing. The realisation of this potential relies on the establishment of robust and reproducible protocols of differentiation. Several reports have highlighted the importance of biomaterials in assisting directed differentiation. Graphene oxide (GO) is a novel material that has attracted increasing interest in the field of biomedicine. In this study, we demonstrate that GO coated substrates significantly enhance the differentiation of mouse embryonic stem (ES) cells to both primitive and definitive haematopoietic cells. GO does not affect cell proliferation or survival of differentiated cells but rather enhances the transition of haemangioblasts to haemogenic endothelial cells, a key step during haematopoietic specification. Importantly, GO also improves, in addition to murine, human ES cell differentiation to blood cells. Taken together, our study reveals a positive role for GO in haematopoietic differentiation and suggests that further functionalization of GO could represent a valid strategy for the generation of large numbers of functional blood cells. Producing these cells would accelerate haematopoietic drug toxicity testing and treatment of patients with blood disorders or malignancies.

A microfabricated platform with hydrogel arrays for 3D mechanical stimulation of cells

Cellular microenvironments present cells with multiple stimuli, including not only soluble biochemical and insoluble matrix cues but also mechanical factors. Biomaterial array platforms have been used to combinatorially and efficiently probe and define two-dimensional (2D) and 3D microenvironmental cues to guide cell functions for tissue engineering applications. However, there are few examples of array platforms that include dynamic mechanical forces, particularly to enable stretching of 3D cell-seeded biomaterials, which is relevant to engineering connective and cardiovascular tissues. Here we present a deformable membrane platform that enables 3D dynamic mechanical stretch of arrayed biomaterial constructs. Cell-seeded polyethylene glycol norbornene (PEG-NB) hydrogels were bound to miniaturized deformable membranes via a thiol-ene reaction with off-stoichiometry thiol-ene based polydimethylsiloxane (OSTE-PDMS) as the membrane material. Bonding to OSTE-PDMS enabled the 3D hydrogel microconstructs to be cyclically deformed and stretched by the membrane. As a first demonstration, human mesenchymal stromal cells (MSCs) embedded in PEG-NB were stretched for several days. They were found to be viable, spread in the 3D hydrogels, and exhibited a contractile myofibroblast phenotype when exposed to dynamic 3D mechanical deformation. This platform, which is readily scalable to larger arrays, enables systematic interrogation of the relationships between combinations of 3D mechanobiological cues and cellular responses, and thus has the potential to identify strategies to predictably control the construction of functional engineered tissues.

STATEMENT OF SIGNIFICANCE

Current high-throughput biomaterial screening approaches fail to consider the effects of dynamic mechanical stimulation, despite its importance in a wide variety of regenerative medicine applications. To meet this need, we developed a deformable membrane platform that enables 3D dynamic stretch of arrayed biomaterial constructs. Our approach combines microtechnologies fabricated with off-stoichiometry thiol-ene based polydimethylsiloxane membranes that can covalently bond cell-seeded polyethylene glycol norbornene 3D hydrogels, a model biomaterial with tunable adhesive, elastic and degradation characteristics. As a first demonstration, we show that human mesenchymal stromal cells embedded in hydrogels and subjected to dynamic mechanical stimulation undergo myofibroblast differentiation. This system is readily scaled up to larger arrays, and will enable systematic and efficient screening of combinations of 3D mechanobiological and biomaterial cues on cell fate and function.