Shear stress during early embryonic stem cell differentiation promotes hematopoietic and endothelial phenotypes

Recent progress in stem cell differentiation directed by material and mechanical cues

Stem cells play essential roles in tissue regeneration in vivo via specific lineage differentiation induced by environmental factors. In the past, biochemical signals were the focus of induced stem cell differentiation. As reported by Engler et al (2006 Cell 126 677-89), biophysical signal mediated stem cell differentiation could also serve as an important inducer. With the advancement of material science, it becomes a possible strategy to generate active biophysical signals for directing stem cell fate through specially designed material microstructures. In the past five years, significant progress has been made in this field, and these designed biophysical signals include material elasticity/rigidity, micropatterned structure, extracellular matrix (ECM) coated materials, material transmitted extracellular mechanical force etc. A large number of investigations involved material directed differentiation of mesenchymal stem cells, neural stem/progenitor cells, adipose derived stem cells, hematopoietic stem/progenitor cells, embryonic stem cells and other cells. Hydrogel based materials were commonly used to create varied mechanical properties via modifying the ratio of different components, crosslinking levels, matrix concentration and conjugation with other components. Among them, polyacrylamide (PAM) and polydimethylsiloxane (PDMS) hydrogels remained the major types of material. Specially designed micropatterning was not only able to create a unique topographical surface to control cell shape, alignment, cell-cell and cell-matrix contact for basic stem cell biology study, but also could be integrated with 3D bioprinting to generate micropattered 3D structure and thus to induce stem cell based tissue regeneration. ECM coating on a specific topographical structure was capable of inducing even more specific and potent stem cell differentiation along with soluble factors and mechanical force. The article overviews the progress of the past five years in this particular field.

Cytoskeletal Expression and Remodeling in Pluripotent Stem Cells

Many emerging cell-based therapies are based on pluripotent stem cells, though complete understanding of the properties of these cells is lacking. In these cells, much is still unknown about the cytoskeletal network, which governs the mechanoresponse. The objective of this study was to determine the cytoskeletal state in undifferentiated pluripotent stem cells and remodeling with differentiation. Mouse embryonic stem cells (ESCs) and reprogrammed induced pluripotent stem cells (iPSCs), as well as the original un-reprogrammed embryonic fibroblasts (MEFs), were evaluated for expression of cytoskeletal markers. We found that pluripotent stem cells overall have a less developed cytoskeleton compared to fibroblasts. Gene and protein expression of smooth muscle cell actin, vimentin, lamin A, and nestin were markedly lower for ESCs than MEFs. Whereas, iPSC samples were heterogeneous with most cells expressing patterns of cytoskeletal proteins similar to ESCs with a small subpopulation similar to MEFs. This indicates that dedifferentiation during reprogramming is associated with cytoskeletal remodeling to a less developed state. In differentiation studies, it was found that shear stress-mediated differentiation resulted in an increase in expression of cytoskeletal intermediate filaments in ESCs, but not in iPSC samples. In the embryoid body model of spontaneous differentiation of pluripotent stem cells, however, both ESCs and iPSCs had similar gene expression for cytoskeletal proteins during early differentiation. With further differentiation, however, gene levels were significantly higher for iPSCs compared to ESCs. These results indicate that reprogrammed iPSCs more readily reacquire cytoskeletal proteins compared to the ESCs that need to form the network de novo. The strategic selection of the parental phenotype is thus critical not only in the context of reprogramming but also the ultimate functionality of the iPSC-differentiated cell population. Overall, this increased characterization of the cytoskeleton in pluripotent stem cells will allow for the better understanding and design of stem cell-based therapies.

Applying Shear Stress to Pluripotent Stem Cells

Thorough understanding of the effects of shear stress on stem cells is critical for the rationale design of large-scale production of cell-based therapies. This is of growing importance as emerging tissue engineering and regenerative medicine applications drive the need for clinically relevant numbers of both pluripotent stem cells (PSCs) and cells derived from PSCs. Here, we describe the use of a custom parallel plate bioreactor system to impose fluid shear stress on a layer of PSCs adhered to protein-coated glass slides. This system can be useful both for basic science studies in mechanotransduction and as a surrogate model for bioreactors used in large-scale production.

Production of human pluripotent stem cell therapeutics under defined xeno-free conditions: progress and challenges

Recent advances on human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs) have brought us closer to the realization of their clinical potential. Nonetheless, tissue engineering and regenerative medicine applications will require the generation of hPSC products well beyond the laboratory scale. This also mandates the production of hPSC therapeutics in fully-defined, xeno-free systems and in a reproducible manner. Toward this goal, we summarize current developments in defined media free of animal-derived components for hPSC culture. Bioinspired and synthetic extracellular matrices for the attachment, growth and differentiation of hPSCs are also reviewed. Given that most progress in xeno-free medium and substrate development has been demonstrated in two-dimensional rather than three dimensional culture systems, translation from the former to the latter poses unique difficulties. These challenges are discussed in the context of cultivation platforms of hPSCs as aggregates, on microcarriers or after encapsulation in biocompatible scaffolds.

Combined effects of flow-induced shear stress and electromagnetic field on neural differentiation of mesenchymal stem cells

Differentiation of bone marrow-derived mesenchymal stem cells (MSCs) into neural phenotype has been induced by either flow-induced shear stress (FSS) or electromagnetic fields (EMF). However, procedures are still expensive and time consuming. In the present work, induction for 1 h with the combination of both forces showed the presence of the neural precursor nestin as early as 9 h in culture after treatment and this result lasted for the following 6 d. In conclusion, the use of a combination of FSS and EMF for a short-time renders in neurite-like cells, although further investigation is required to analyze cell functionality.

Cell shape-dependent shear stress on adherent cells in a micro-physiologic system as revealed by FEM

Flow-induced shear stress on adherent cells leads to biochemical signaling and mechanical responses of the cells. To determine the flow-induced shear stress on adherent cells cultured in a micro-scaled reaction chamber, we developed a suitable finite element method model. The influence of the most important parameters-cell shape, cell density, shear modulus and fluid velocity-was investigated. Notably, the cell shape strongly influences the resulting shear stress. Long and smooth cells undergo lower shear stress than more rounded cells. Changes in the curvature of the cells lead to stress peaks and single cells experience higher shear stress values than cells of a confluent monolayer. The computational results of the fluid flow simulation were validated experimentally. We also analyzed the influence of flow-induced shear stress on the metabolic activity and shape of L929, a mouse fibroblast cell line, experimentally. The results indicate that threshold stress values for continuous flow conditions cannot be transferred to quasi static flow conditions interrupted by short fluid exchange events.

Advances in cell culture: anchorage dependence

Anchorage-dependent cells are of great interest for various biotechnological applications. (i) They represent a formidable production means of viruses for vaccination purposes at very large scales (in 1000-6000 l reactors) using microcarriers, and in the last decade many more novel viral vaccines have been developed using this production technology. (ii) With the advent of stem cells and their use/potential use in clinics for cell therapy and regenerative medicine purposes, the development of novel culture devices and technologies for adherent cells has accelerated greatly with a view to the large-scale expansion of these cells. Presently, the really scalable systems--microcarrier/microcarrier-clump cultures using stirred-tank reactors--for the expansion of stem cells are still in their infancy. Only laboratory scale reactors of maximally 2.5 l working volume have been evaluated because thorough knowledge and basic understanding of critical issues with respect to cell expansion while retaining pluripotency and differentiation potential, and the impact of the culture environment on stem cell fate, etc., are still lacking and require further studies. This article gives an overview on critical issues common to all cell culture systems for adherent cells as well as specifics for different types of stem cells in view of small- and large-scale cell expansion and production processes.

Human pluripotent stem cell-derived products: advances towards robust, scalable and cost-effective manufacturing strategies

The ability to develop cost-effective, scalable and robust bioprocesses for human pluripotent stem cells (hPSCs) will be key to their commercial success as cell therapies and tools for use in drug screening and disease modelling studies. This review outlines key process economic drivers for hPSCs and progress made on improving the economic and operational feasibility of hPSC bioprocesses. Factors influencing key cost metrics, namely capital investment and cost of goods, for hPSCs are discussed. Step efficiencies particularly for differentiation, media requirements and technology choice are amongst the key process economic drivers identified for hPSCs. Progress made to address these cost drivers in hPSC bioprocessing strategies is discussed. These include improving expansion and differentiation yields in planar and bioreactor technologies, the development of xeno-free media and microcarrier coatings, identification of optimal bioprocess operating conditions to control cell fate and the development of directed differentiation protocols that reduce reliance on expensive morphogens such as growth factors and small molecules. These approaches offer methods to further optimise hPSC bioprocessing in terms of its commercial feasibility.

Efficient differentiation of human pluripotent stem cells to endothelial progenitors via small-molecule activation of WNT signaling

Human pluripotent stem cell (hPSC)-derived endothelial cells and their progenitors may provide the means for vascularization of tissue-engineered constructs and can serve as models to study vascular development and disease. Here, we report a method to efficiently produce endothelial cells from hPSCs via GSK3 inhibition and culture in defined media to direct hPSC differentiation to CD34⁺CD31⁺ endothelial progenitors. Exogenous vascular endothelial growth factor (VEGF) treatment was dispensable, and endothelial progenitor differentiation was β-catenin dependent. Furthermore, by clonal analysis, we showed that CD34⁺CD31⁺CD117⁺TIE-2⁺ endothelial progenitors were multipotent, capable of differentiating into calponin-expressing smooth muscle cells and CD31⁺CD144⁺vWF⁺I-CAM1⁺ endothelial cells. These endothelial cells were capable of 20 population doublings, formed tube-like structures, imported acetylated low-density lipoprotein, and maintained a dynamic barrier function. This study provides a rapid and efficient method for production of hPSC-derived endothelial progenitors and endothelial cells and identifies WNT/β-catenin signaling as a primary regulator for generating vascular cells from hPSCs.

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WNT pathway activation directs hPSC differentiation to endothelial progenitors

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hPSC-derived endothelial progenitors can differentiate to endothelial cells

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Purified hPSC-derived endothelial cells are capable of 20 population doublings

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WNT pathway activation permits defined production of endothelial cells from hPSCs

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.

Alginate encapsulation parameters influence the differentiation of microencapsulated embryonic stem cell aggregates

Pluripotent embryonic stem cells (ESCs) have tremendous potential as tools for regenerative medicine and drug discovery, yet the lack of processes to manufacture viable and homogenous cell populations of sufficient numbers limits the clinical translation of current and future cell therapies. Microencapsulation of ESCs within microbeads can shield cells from hydrodynamic shear forces found in bioreactor environments while allowing for sufficient diffusion of nutrients and oxygen through the encapsulation material. Despite initial studies examining alginate microbeads as a platform for stem cell expansion and directed differentiation, the impact of alginate encapsulation parameters on stem cell phenotype has not been thoroughly investigated. Therefore, the objective of this study was to systematically examine the effects of varying alginate compositions on microencapsulated ESC expansion and phenotype. Pre-formed aggregates of murine ESCs were encapsulated in alginate microbeads composed of a high or low ratio of guluronic to mannuronic acid residues (High G and High M, respectively), with and without a poly-L-lysine (PLL) coating, thereby providing four distinct alginate bead compositions for analysis. Encapsulation in all alginate compositions was found to delay differentiation, with encapsulation within High G alginate yielding the least differentiated cell population. The addition of a PLL coating to the High G alginate prevented cell escape from beads for up to 14 days. Furthermore, encapsulation within High M alginate promoted differentiation toward a primitive endoderm phenotype. Taken together, the findings of this study suggest that distinct ESC expansion capacities and differentiation trajectories emerge depending on the alginate composition employed, indicating that encapsulation material physical properties can be used to control stem cell fate.

Hypertensive rat lungs retain hallmarks of vascular disease upon decellularization but support the growth of mesenchymal stem cells

There are an insufficient number of donor organs available to meet the demand for lung transplantation. This issue could be addressed by regenerating functional tissue from diseased or damaged lungs that would otherwise be deemed unsuitable for transplant. Detergent-mediated whole-lung decellularization produces a three-dimensional natural scaffold that can be repopulated with various cell types. In this study, we investigated the decellularization and initial recellularization of diseased lungs using a rat model of monocrotaline-induced pulmonary hypertension (MCT-PHT). Decellularization of control and MCT-PHT Sprague-Dawley rat lungs was accomplished by treating the lungs with a combination of Triton X-100, sodium deoxycholate, NaCl, and DNase. The resulting acellular matrices were characterized by DNA quantification, Western blotting, immunohistochemistry, and proteomic analyses revealing that decellularization was able to remove cells while leaving the extracellular matrix (ECM) components and lung ultrastructure intact. Decellularization significantly reduced DNA content (∼30-fold in MCT-PHT lungs and ∼50-fold in the control lungs) and enriched ECM components (>60-fold in both the control and MCT-PHT lungs) while depleting cellular proteins. MicroCT visualization of MCT-PHT rat lungs indicated that the vasculature was narrowed as a result of MCT treatment, and this characteristic was unchanged by decellularization. Mean arterial vessel diameter of representative decellularized MCT-PHT and control scaffolds was estimated to be 0.152±0.134 mm and 0.247±0.160 mm, respectively. Decellularized MCT-PHT lung scaffolds supported attachment and survival of rat adipose-derived stem cells (rASCs), seeded into the airspace or the vasculature, for at least 2 weeks. The cells seeded in MCT-PHT lung scaffolds proliferated and underwent apoptosis similar to control scaffolds; however, the initial percentage of apoptotic cells was slightly higher in MCT-PHT lungs (2.79±2.03% vs. 1.05±1.02% of airway-seeded rASCs, and 4.47±1.21% vs. 2.66±0.10% of vascular seeded rASCs). The ECM of cell-seeded scaffolds showed no signs of degradation by the cells after 14 days in culture. These data suggest that diseased hypertensive lungs can be efficiently decellularized similar to control lungs and have the potential to be recellularized with mesenchymal stem cells with the ultimate goal of generating healthy, functional pulmonary tissue.

Engineering physical microenvironment for stem cell based regenerative medicine

Regenerative medicine has rapidly evolved over the past decade owing to its potential applications to improve human health. Targeted differentiations of stem cells promise to regenerate a variety of tissues and/or organs despite significant challenges. Recent studies have demonstrated the vital role of the physical microenvironment in regulating stem cell fate and improving differentiation efficiency. In this review, we summarize the main physical cues that are crucial for controlling stem cell differentiation. Recent advances in the technologies for the construction of physical microenvironment and their implications in controlling stem cell fate are also highlighted.

Stem cell bioprocess engineering towards cGMP production and clinical applications

Stem cells, including mesenchymal stem cells and pluripotent stem cells, are becoming an indispensable tool for various biomedical applications including drug discovery, disease modeling, and tissue engineering. Bioprocess engineering, targeting large scale production, provides a platform to generate a controlled microenvironment that could potentially recreate the stem cell niche to promote stem cell proliferation or lineage-specific differentiation. This survey aims at defining the characteristics of stem cell populations currently in use and the present-day limits in their applications for therapeutic purposes. Furthermore, a bioprocess engineering strategy based on bioreactors and 3-D cultures is discussed in order to achieve the improved stem cell yield, function, and safety required for production under current good manufacturing practices.

Dendritic cells derived from pluripotent stem cells: Potential of large scale production

Human pluripotent stem cells (hPSCs), including human embryonic stem cells and human induced pluripotent stem cells, are promising sources for hematopoietic cells due to their unlimited growth capacity and the pluripotency. Dendritic cells (DCs), the unique immune cells in the hematopoietic system, can be loaded with tumor specific antigen and used as vaccine for cancer immunotherapy. While autologous DCs from peripheral blood are limited in cell number, hPSC-derived DCs provide a novel alternative cell source which has the potential for large scale production. This review summarizes recent advances in differentiating hPSCs to DCs through the intermediate stage of hematopoietic stem cells. Step-wise growth factor induction has been used to derive DCs from hPSCs either in suspension culture of embryoid bodies (EBs) or in co-culture with stromal cells. To fulfill the clinical potential of the DCs derived from hPSCs, the bioprocess needs to be scaled up to produce a large number of cells economically under tight quality control. This requires the development of novel bioreactor systems combining guided EB-based differentiation with engineered culture environment. Hence, recent progress in using bioreactors for hPSC lineage-specific differentiation is reviewed. In particular, the potential scale up strategies for the multistage DC differentiation and the effect of shear stress on hPSC differentiation in bioreactors are discussed in detail.

Fluid shear stress pre-conditioning promotes endothelial morphogenesis of embryonic stem cells within embryoid bodies

Pluripotent embryonic stem cells (ESCs) are capable of differentiating into all mesoderm-derived cell lineages, including endothelial, hematopoietic, and cardiac cell types. Common strategies to direct mesoderm differentiation of ESCs rely on exposing the cells to a series of biochemical and biophysical cues at different stages of differentiation to promote maturation toward specific cell phenotypes. Shear forces that mimic cardiovascular physiological forces can evoke a myriad of responses in somatic and stem cell populations, and have, thus, been studied as a means to direct stem cell differentiation. However, elucidating the effects of shear pre-conditioning on the subsequent vascular differentiation and morphogenesis of ESCs has yet to be examined. In this study, ESC monolayers were subjected to physiological shear (5 dyn/cm(2)) or static conditions for 2 days on collagen IV-coated substrates before initiating embryoid body (EB) differentiation. Immediately after the pre-conditioning period, shear pre-conditioned and statically cultured ESCs exhibited similar morphologies and largely retained a pluripotent phenotype; however, ESCs exposed to fluid shear expressed increased levels of endothelial marker genes Flk-1 (∼3-fold), VE-cadherin (∼3-fold), and PECAM (∼2-fold), compared with statically cultured ESCs. After 7 days of EB culture, ∼70% of EBs formed from shear pre-conditioned ESCs expressed significantly higher levels of endothelial marker genes compared with EBs formed from statically cultured ESCs. Interestingly, unlike EBs formed from statically cultured ESCs, EBs formed from fluid shear stress pre-conditioned ESCs exhibited a centrally localized region of VE-cadherin(+) cells that persisted for at least 10 days of differentiation. These results demonstrate that fluid shear stress pre-conditioning not only promotes ESC endothelial gene expression but also subsequently impacts the organization of endothelial cells within EBs. Together, these studies highlight a novel approach to promote in vitro morphogenesis of developmental vasculogenic models and potentially promote pre-vascularization of tissue-engineered constructs derived from pluripotent stem cells.

Application of fluid mechanical force to embryonic sources of hemogenic endothelium and hematopoietic stem cells

During embryonic development, hemodynamic forces caused by blood flow support vascular remodeling, arterialization of luminal endothelium, and hematopoietic stem cell (HSC) emergence. Previously, we reported that fluid shear stress plays a key role in stimulating nitric oxide (NO) signaling in the aorta-gonad-mesonephros (AGM) and is essential for definitive hematopoiesis. We employed a Dynamic Flow System modified from a cone-and-plate assembly to precisely regulate in vitro exposure of AGM cells to a defined pattern of laminar shear stress. Here, we present the design of a microfluidic platform accessible to any research group that requires small cell numbers and allows for recirculation of paracrine signaling factors with minimal damage to nonadherent hematopoietic progenitors and stem cells. We detail the assembly of the microfluidic platform using commercially available components and provide specific guidance in the use of an emerging standard in the measurement of embryonic HSC potential, intravenous neonatal transplantation.

Biomechanical force in blood development: extrinsic physical cues drive pro-hematopoietic signaling

The hematopoietic system is dynamic during development and in adulthood, undergoing countless spatial and temporal transitions during the course of one's life. Microenvironmental cues in the many unique hematopoietic niches differ, characterized by distinct soluble molecules, membrane-bound factors, and biophysical features that meet the changing needs of the blood system. Research from the last decade has revealed the importance of substrate elasticity and biomechanical force in determination of stem cell fate. Our understanding of the role of these factors in hematopoiesis is still relatively poor; however, the developmental origin of blood cells from the endothelium provides a model for comparison. Many endothelial mechanical sensors and second messenger systems may also determine hematopoietic stem cell fate, self renewal, and homing behaviors. Further, the intimate contact of hematopoietic cells with mechanosensitive cell types, including osteoblasts, endothelial cells, mesenchymal stem cells, and pericytes, places them in close proximity to paracrine signaling downstream of mechanical signals. The objective of this review is to present an overview of the sensors and intracellular signaling pathways activated by mechanical cues and highlight the role of mechanotransductive pathways in hematopoiesis.

Restitutio ad integrum: a dream or a real possibility?

The subject of organ regeneration has attracted substantial investigative attention and has been extensively reviewed. Therefore, I shall focus on several only recently emerged issues and on those aspects of stem cell-mediated regeneration which, although are important in my opinion, have nevertheless evaded the radar of scientific pursuit. Specifically, I shall describe the recent work on the prominence of local lineage-restricted stem cells, as opposed to the bone marrow-derived or circulating ones, in regeneration. This will be followed by an attempt to re-interpret a bulk of published data on the beneficial effects of cell therapy with the focus on the secretome of stem cells. Multiple factors that conspire to cause insufficient or failed regeneration in adult mammals will be screened with emphasis placed on the mechanical forces, senescence and exhaustion, each leading to phenotypical switch and/or stem cell incompetence. Finally, I shall enumerate several potential pathways to induce or restore stem cell competence. Although a significant amount of work has been performed in the non-renal field, I would hope that some of the mechanisms and concepts discussed herein will eventually trickle into kidney regeneration.