Farlan S. Veraitch

Mechanical Dynamics of Single Cells During Early Apoptosis

Dynamic mechanical properties of cells are becoming recognized as indicators and regulators of physiological processes such as differentiation, malignant phenotypes and mitosis. A key process in development and homeostasis is apoptosis and whilst the molecular control over this pathway is well studied, little is known about the mechanical consequences of cell death. Here, we study the caspase-dependent mechanical kinetics of single cells during early apoptosis initiated with the general protein-kinase inhibitor staurosporine. This results in internal remodelling of the cytoskeleton and nucleus which is reflected in dynamic changes in the mechanical properties of the cell. Utilizing simultaneous confocal and atomic force microscopy (AFM), we measured distinct mechanical dynamics in the instantaneous cellular Youngs Modulus and longer timescale viscous deformation. This allowed us to visualize time-dependent nuclear and cytoskeletal control of force dissipation with fluorescent fusion proteins throughout the cell. This work reveals that the cell death program not only orchestrates biochemical dynamics but also controls the mechanical breakdown of the cell. Importantly, the consequences of mechanical disregulation during apoptosis may be a contributing factor to several human pathologies through the poorly timed release of dead cells and cell debris.

Hypoxia Enhances the Generation of Retinal Progenitor Cells from Human Induced Pluripotent and Embryonic Stem Cells

The efficient differentiation of retinal cells from human pluripotent stem cells remains a major challenge for the development of successful and cost-effective cellular therapies for various forms of blindness. Current differentiation strategies rely on exposing pluripotent stem cells to soluble growth factors that play key roles during early development (such as DKK-1, Noggin, and IGF-1) at 20% oxygen (O(2)). This O(2) tension is, however, considerably higher than O(2) levels during organogenesis and may impair the differentiation process. In this study, we examined the effect of mimicking the physiological O(2) tension (2%) on the generation of retinal progenitor cells (RPCs) from human induced pluripotent stem cells (iPSCs) and human embryonic stem cells (hESCs). Both cell types were induced to differentiate into RPCs at 20% and 2% O(2). After 3 days in suspension culture as embryoid bodies (EBs), 2% O(2) caused the activation of hypoxia inducible factor responsive genes VEGF and LDHA and was accompanied by elevated expression levels of the early eye field genes Six3 and Lhx2. Twenty-one days after plating EBs in an adherent culture, we observed more RPCs co-expressing Pax6 and Chx10 at 2% O(2). Quantitative polymerase chain reaction analysis confirmed that lowering O(2) tension had caused a rise in the expression of both genes compared with 20% O(2). Our results indicate that mimicking physiological O(2) is a favorable condition for the efficient generation of RPCs from both hiPSCs and hESCs.

Automated method for the rapid and precise estimation of adherent cell culture characteristics from phase contrast microscopy images

The quantitative determination of key adherent cell culture characteristics such as confluency, morphology, and cell density is necessary for the evaluation of experimental outcomes and to provide a suitable basis for the establishment of robust cell culture protocols. Automated processing of images acquired using phase contrast microscopy (PCM), an imaging modality widely used for the visual inspection of adherent cell cultures, could enable the non‐invasive determination of these characteristics. We present an image‐processing approach that accurately detects cellular objects in PCM images through a combination of local contrast thresholding and post hoc correction of halo artifacts. The method was thoroughly validated using a variety of cell lines, microscope models and imaging conditions, demonstrating consistently high segmentation performance in all cases and very short processing times (<1 s per 1,208 × 960 pixels image). Based on the high segmentation performance, it was possible to precisely determine culture confluency, cell density, and the morphology of cellular objects, demonstrating the wide applicability of our algorithm for typical microscopy image processing pipelines. Furthermore, PCM image segmentation was used to facilitate the interpretation and analysis of fluorescence microscopy data, enabling the determination of temporal and spatial expression patterns of a fluorescent reporter. We created a software toolbox (PHANTAST) that bundles all the algorithms and provides an easy to use graphical user interface. Source‐code for MATLAB and ImageJ is freely available under a permissive open‐source license. Biotechnol. Bioeng. 2014;111: 504–517.

Lowering oxygen tension enhances the differentiation of mouse embryonic stem cells into neuronal cells

Embryonic stem cells (ESC) are capable of proliferating indefinitely in vitro whilst retaining their ability to differentiate into cells of every adult lineage. Efficient, high yield processes, which direct differentiation of ESC to specific lineages, will underpin the development of cost‐effective drug screening and cell therapy products. The aim of this study was to investigate whether laboratory oxygen tension currently used for the neuronal differentiation of ESC was suboptimal resulting in inefficient process yields. An adherent monolayer protocol for the neuronal differentiation of mouse ESC (mESC) was performed in oxygen controlled chambers using a chemically defined media over an 8 day period of culture. When exposed to oxygen tensions more appropriate to in vivo neuronal development (2% O2), there was a 34‐fold increase in the yield of viable cells from the differentiation process. Low oxygen tension inhibited cell death during an early phase (48 to 96 h) and toward the end (120 to 192 h) of the process. The percentage of cells expressing neuronal markers was determined by flow cytometry, revealing a small rise in the βIII tubulin and a threefold increase in the MAP2 populations at 2% O2. The total increase in the yield of viable cells expressing neuronal markers was shown to be 55‐fold for βIII tubulin and 114‐fold for MAP2. In conclusion, this study revealed that low oxygen tension can be used to enhance the yield of neuronal cells derived from ESCs and has implications for the development of efficient, cost‐effective production processes.

Microfabricated modular scale-down device for regenerative medicine process development.

The capacity of milli and micro litre bioreactors to accelerate process development has been successfully demonstrated in traditional biotechnology. However, for regenerative medicine present smaller scale culture methods cannot cope with the wide range of processing variables that need to be evaluated. Existing microfabricated culture devices, which could test different culture variables with a minimum amount of resources (e.g. expensive culture medium), are typically not designed with process development in mind. We present a novel, autoclavable, and microfabricated scale-down device designed for regenerative medicine process development. The microfabricated device contains a re-sealable culture chamber that facilitates use of standard culture protocols, creating a link with traditional small-scale culture devices for validation and scale-up studies. Further, the modular design can easily accommodate investigation of different culture substrate/extra-cellular matrix combinations. Inactivated mouse embryonic fibroblasts (iMEF) and human embryonic stem cell (hESC) colonies were successfully seeded on gelatine-coated tissue culture polystyrene (TC-PS) using standard static seeding protocols. The microfluidic chip included in the device offers precise and accurate control over the culture medium flow rate and resulting shear stresses in the device. Cells were cultured for two days with media perfused at 300 µl.h−1 resulting in a modelled shear stress of 1.1×10−4 Pa. Following perfusion, hESC colonies stained positively for different pluripotency markers and retained an undifferentiated morphology. An image processing algorithm was developed which permits quantification of co-cultured colony-forming cells from phase contrast microscope images. hESC colony sizes were quantified against the background of the feeder cells (iMEF) in less than 45 seconds for high-resolution images, which will permit real-time monitoring of culture progress in future experiments. The presented device is a first step to harness the advantages of microfluidics for regenerative medicine process development.

Development of a multiplexed microfluidic platform for the automated cultivation of embryonic stem cells.

We present a multiplexed platform for a microfabricated stem cell culture device. The modular platform contains all the components to control stem cell culture conditions in an automated fashion. It does not require an incubator during perfusion culture and can be mounted on the stage of an inverted fluorescence microscope for high-frequency imaging of stem cell cultures. A pressure-driven pump provides control over the medium flow rate and offers switching of the flow rates. Flow rates of the pump are characterized for different pressure settings, and a linear correlation between the applied pressure and the flow rate in the cell culture devices is shown. In addition, the pump operates with two culture medium reservoirs, thus enabling the switching of the culture medium on-the-fly during a cell culture experiment. Also, with our platform, the culture medium reservoirs are cooled to prevent medium degradation during long-term experiments. Media temperature is then adjusted to a higher controlled temperature before entering the microfabricated cell culture device. Furthermore, the temperature is regulated in the microfabricated culture devices themselves. Preliminary culture experiments are demonstrated using mouse embryonic stem cells.

Precisely delivered nanomechanical forces induce blebbing in undifferentiated mouse embryonic stem cells

The aim of this study was to probe the morphological response of single mouse embryonic stem cells (mESC) to precisely delivered nanomechanical forces. Plating mESC as single cells gave rise to either round compact or flattened fibroblastic morphologies. The expression of OCT4 and Nanog was reduced in flattened cells, indicating that this population had begun to differentiate. Upon application of.> nN of force, using atomic force microscopy and simultaneous laser scanning confocal microscopy, round cells, but not flattened cells, were capable of forming mechanically induced blebs (miBlebs). Flattened cells appeared to have a more highly developed cytoskeleton than undifferentiated stem cells as characterized by the distribution of phospho-ezrin-radixin-moesin (pERM). Higher levels of pERM and an inability to form miBlebs in flattened cells imply that the earliest stages of embryonic stem cell differentiation are associated with the development of stronger mechanical links between the plasma membrane and the cytoskeleton

The effect of Young's modulus on the neuronal differentiation of mouse embryonic stem cells.

There is substantial evidence that cells produce a diverse response to changes in ECM stiffness depending on their identity. Our aim was to understand how stiffness impacts neuronal differentiation of embryonic stem cells (ESCs), and how this varies at three specific stages of the differentiation process. In this investigation, three effects of stiffness on cells were considered; attachment, expansion and phenotypic changes during differentiation. Stiffness was varied from 2 kPa to 18 kPa to finally 35 kPa. Attachment was found to decrease with increasing stiffness for both ESCs (with a 95% decrease on 35 kPa compared to 2 kPa) and neural precursors (with a 83% decrease on 35 kPa). The attachment of immature neurons was unaffected by stiffness. Expansion was independent of stiffness for all cell types, implying that the proliferation of cells during this differentiation process was independent of Youngs modulus. Stiffness had no effect upon phenotypic changes during differentiation for mESCs and neural precursors. 2 kPa increased the proportion of cells that differentiated from immature into mature neurons. Taken together our findings imply that the impact of Youngs modulus on attachment diminishes as neuronal cells become more mature. Conversely, the impact of Youngs modulus on changes in phenotype increased as cells became more mature.

The Full Spectrum of Physiological Oxygen Tensions and Step-Changes in Oxygen Tension Affects the Neural Differentiation of Mouse Embryonic Stem Cells

The beneficial impact of lowering oxygen tension to physiological levels has been demonstrated in a number of stem cell differentiation protocols. The majority of these studies compare normal laboratory oxygen tension with one physiological condition (typically 2–5% O2). In this article, we investigated whether the full spectrum of physiological oxygen tensions (0–20% O2) and step‐changes in oxygen tension could enhance the production of neural populations from of embryonic stem cells (ESCs). We used a model system for the conversion of mouse ESCs into cells expressing one neuroectoderm stem cell marker (nestin) and two neural markers (βIII tubulin and microtubule‐associated protein (MAP2)). 4–10% O2 was associated with large increases in the total production of viable cells and the highest number of cells expressing Nestin, βIII tubulin, and MAP2. However, 4–10% O2 also caused a reduction in the percentage of cells expressing all three markers. Step changes in oxygen tension at the mid‐point of the differentiation process affected the total production of viable cells and the percentage of cells expressing all three markers. We found that the initial oxygen tension and the magnitude of the step‐change were critical variables. A step increase from 0 to 2% O2 mid‐way through the protocol resulted in the highest percentage of cells expressing βIII tubulin (86.5%). In conclusion, we have demonstrated that the full spectrum of physiological oxygen tensions and step changes in oxygen tension represent a powerful tool for the optimisation of neural differentiation processes.

Real-time monitoring of specific oxygen uptake rates of embryonic stem cells in a microfluidic cell culture device

Abstract Oxygen plays a key role in stem cell biology as a signaling molecule and as an indicator of cell energy metabolism. Quantification of cellular oxygen kinetics, i.e. the determination of specific oxygen uptake rates (sOURs), is routinely used to understand metabolic shifts. However current methods to determine sOUR in adherent cell cultures rely on cell sampling, which impacts on cellular phenotype. We present real‐time monitoring of cell growth from phase contrast microscopy images, and of respiration using optical sensors for dissolved oxygen. Time‐course data for bulk and peri‐cellular oxygen concentrations obtained for Chinese hamster ovary (CHO) and mouse embryonic stem cell (mESCs) cultures successfully demonstrated this non‐invasive and label‐free approach. Additionally, we confirmed non‐invasive detection of cellular responses to rapidly changing culture conditions by exposing the cells to mitochondrial inhibiting and uncoupling agents. For the CHO and mESCs, sOUR values between 8 and 60 amol cell−1 s−1, and 5 and 35 amol cell−1 s−1 were obtained, respectively. These values compare favorably with literature data. The capability to monitor oxygen tensions, cell growth, and sOUR, of adherent stem cell cultures, non‐invasively and in real time, will be of significant benefit for future studies in stem cell biology and stem cell‐based therapies.