Search

Noninvasive biomarkers hold important potential for the characterization and purification of stem cells because the addition of exogenous labels, probes, or reporters, as well as the disruption of cell-cell and cell-extracellular matrix interactions, can unintentionally but dramatically alter stem cell state. We recently showed that intensity of the intrinsically fluorescent metabolite, nicotinamide adenine dinucleotide (NADH), fluctuates predictably with changes in stem cell viability and differentiation state. Here, we use multiphoton flow cytometry developed in our laboratory to rapidly and noninvasively characterize and purify populations of intact stem cell aggregates based on NADH intensity and assessed the differentiation capacity of sorted populations. We found removal of aggregates with NADH intensity indicative of cell death resulted in a remaining population of aggregates significantly more likely to produce beating cardiomyocytes (26% vs. 8%, P < 0.05). Similarly, we found isolation of stem cell aggregates with NADH intensity indicative of future cardiac differentiation gave rise to more aggregates with beating cardiomyocytes at later time points (50% vs. 28%, P < 0.05). Further, coupling NADH intensity with gating based on size, enhances the enrichment for EBs capable of giving rise to cardiomyocytes (59% vs. 27%, P < 0.05). Thus, we demonstrate that endogenous properties of cell aggregates, such as NADH and size, can serve as gating parameters for large particle sorting devices to purify populations of stem cells or their progeny in a noninvasive manner, leading the way for improved therapeutic applications., (© 2014 International Society for Advancement of Cytometry.)

Embryoid bodies (EBs) are large (>100 μm) 3D microtissues composed of stem cells, differentiating cells and extracellular matrix (ECM) proteins that roughly recapitulate early embryonic development. EBs are widely used as in vitro model systems to study stem cell differentiation and the complex physical and chemical interactions contributing to tissue development. Though much has been learned about differentiation from EBs, the practical and technical difficulties of effectively probing and properly analyzing these 3D microtissues has limited their utility and further application. We describe advancement of a technology platform developed in our laboratory, multiphoton flow cytometry (MPFC), to detect and sort large numbers of intact EBs based on size and fluorescent reporters. Real-time and simultaneous measurement of size and fluorescence intensity are now possible, through the implementation of image processing algorithms in the MPFC software. We applied this platform to purify populations of EBs generated from murine induced pluripotent stem (miPS) cells exhibiting enhanced potential for cardiomyocyte differentiation either as a consequence of size or expression of NKX2-5, a homeodomain protein indicative of precardiac cells. Large EBs (330-400 μm, diameter) purified soon after EB formation showed significantly higher potential to form cardiomyocytes at later time points than medium or small EBs. In addition, EBs expressing NKX2-5 soon after EB formation were more likely to form beating areas, indicative of cardiomyocyte differentiation, at later time points. Collectively, these studies highlight the ability of the MPFC to purify EBs and similar microtissues based on preferred features exhibited at the time of sorting or on features indicative of future characteristics or functional capacity.

Background Information: Continued advances in stem cell biology and stem cell transplantation rely on non-invasive biomarkers to characterise cells and stem cell aggregates. The non-invasive quality of such biomarkers is essential because exogenous labels, probes or reporters can unintentionally and dramatically alter stem cell state as can disruption of cell-cell and cell-matrix interactions. Here, we investigate the utility of the autofluorescent metabolite, nicotinamide adenine dinucleotide (NADH), as a non-invasive, intrinsic biomarker of cell death when detected with multi-photon optical-based approaches. To test this possibility, cell death was induced in murine embryoid bodies (EBs) at an early stage (day 3) of differentiation using staurosporine, an ATP-competitive kinase inhibitor of electron transport. Several hours after staurosporine treatment, EBs were stained with a single-colour, live/dead probe. A single-cross-sectional plane of each EB was imaged to detect the fluorescence intensity of the live/dead probe (extrinsic fluorescence) as well as the fluorescence intensity of NADH (intrinsic fluorescence). EBs were assessed at subsequent time points (days 6-12) for the formation of beating areas as an indicator of functional differentiation., Results: Statistical comparison indicated a strong positive correlation between extrinsic fluorescence intensity of the live/dead stain and intrinsic fluorescence of NADH, suggesting that the intensity of NADH fluorescence could be used to reliably and non-invasively assess death of cells of EBs. Furthermore, EBs that had high levels of cell death soon after aggregate formation had limited ability to give rise to functional cardiomyocytes at later time points., Conclusions: We demonstrate the utility of NADH fluorescence intensity as a non-invasive indicator of cell death in stem cell aggregates when measured using multi-photon excitation. In addition, we show that the degree of stem cell death at early stages of differentiation is predictive for the formation of functional cardiomyocytes., (Copyright © 2012 Soçiété Francaise des Microscopies and Société de Biologie Cellulaire de France.)

Increasingly, invitro culture of adherent cell types utilizes three-dimensional (3D) scaffolds or aggregate culture strategies to mimic tissue-like, microenvironmental conditions. In parallel, new flow cytometry-based technologies are emerging to accurately analyze the composition and function of these microtissues (i.e., large particles) in a non-invasive and high-throughput way. Lacking, however, is an accessible platform that can be used to effectively sort or purify large particles based on analysis parameters. Here we describe a microfluidic-based, electromechanical approach to sort large particles. Specifically, sheath-less asymmetric curving channels were employed to separate and hydrodynamically focus particles to be analyzed and subsequently sorted. This design was developed and characterized based on wall shear stress, tortuosity of the flow path, vorticity of the fluid in the channel, sorting efficiency and enrichment ratio. The large particle sorting device was capable of purifying fluorescently labelled embryoid bodies (EBs) from unlabelled EBs with an efficiency of 87.3% ± 13.5%, and enrichment ratio of 12.2 ± 8.4 (n = 8), while preserving cell viability, differentiation potential, and long-term function.

Detection and tracking of stem cell state are difficult due to insufficient means for rapidly screening cell state in a noninvasive manner. This challenge is compounded when stem cells are cultured in aggregates or three-dimensional (3D) constructs because living cells in this form are difficult to analyze without disrupting cellular contacts. Multiphoton laser scanning microscopy is uniquely suited to analyze 3D structures due to the broad tunability of excitation sources, deep sectioning capacity, and minimal phototoxicity but is throughput limited. A novel multiphoton fluorescence excitation flow cytometry (MPFC) instrument could be used to accurately probe cells in the interior of multicell aggregates or tissue constructs in an enhanced-throughput manner and measure corresponding fluorescent properties. By exciting endogenous fluorophores as intrinsic biomarkers or exciting extrinsic reporter molecules, the properties of cells in aggregates can be understood while the viable cellular aggregates are maintained. Here we introduce a first generation MPFC system and show appropriate speed and accuracy of image capture and measured fluorescence intensity, including intrinsic fluorescence intensity. Thus, this novel instrument enables rapid characterization of stem cells and corresponding aggregates in a noninvasive manner and could dramatically transform how stem cells are studied in the laboratory and utilized in the clinic.