Dual-laser flow cytometry of single mammalian cells.

An improved dual-laser flow cytometric system for quantitative analysis and sorting of mammalian cells has been developed using a low-power argon and high-power krypton laser as illumination sources, thus permitting the excitation of fluorescent dyes having absorption regions ranging from the ultraviolet to infrared. Cells stained in liquid suspension with fluorescent dyes enter a flow chamber where they intersect two spatially separated laser beams. Separate pairs of quartz beam-shaping optics focus each beam onto the cell stream. Electro-optical sensors measure fluorescence and light scatter signals from cells that are processed electronically and displayed as frequency distribution histograms. Cells also can be electronically separated and microscopically identified. The ease and versatility of operation designed into this system represent a marked technological improvement for dual-laser excited flow systems. Details of this instrument are described along with illustrative examples of cells stained with mithramycin and rhodamine and analyzed for DNA content, total protein, and nuclear and cytoplasmic diameter.     

Safe sorting of GFP-transduced live cells for subsequent culture using a modified FACS vantage.

BACKGROUND: A stream-in-air cell sorter enables rapid sorting to a high purity, but it is not well suited for sorting of infectious material due to the risk of airborne spread to the surroundings. METHODS: A FACS Vantage cell sorter was modified for safe use with potentially HIV infected cells. Safety tests with bacteriophages were performed to evaluate the potential spread of biologically active material during cell sorting. Cells transduced with a retroviral vector carrying the gene for GFP were sorted on the basis of their GFP fluorescence, and GFP expression was followed during subsequent culture. RESULTS: The bacteriophage sorting showed that the biologically active material was confined to the sorting chamber. A failure mode simulating a nozzle blockage resulted in detectable droplets inside the sorting chamber, but no droplets could be detected when an additional air suction from the sorting chamber had been put on. The GFP transduced cells were sorted to 99% purity. Cells not expressing GFP at the time of sorting did not turn on the gene during subsequent culture. Un-sorted cells and cells sorted to be positive for GFP showed a decrease in the fraction of GFP positive cells during culture. CONCLUSIONS: Sorting of live infected cells can be performed safely and with no deleterious effects on vector expression using the modified FACS Vantage instrument.     

Single-cell entrapment and microcolony development within uniform microspheres amenable to flow cytometry

A method is presented for encapsulating single microbial cells in small spheres suitable for analysis and sorting by flow cytometry. The entrapped cells are able to multiply and form colonies contained within their respective microspheres. The system is based on ejecting the cells suspended in a gellable liquid through an orifice vibrating at ultrasonic frequencies, thus shearing the cell-containing jet into uniform droplets. When low-melting-temperature agarose was used, the droplets could be gelled into solid spheres during flight by appropriately directed colling air streams. This gelling was accompanied by significant dehydration, resulting in a twofold decrease in bead diameter and a corresponding increase in agarose concentration. Nevertheless, the microbeads obtained were highly uniform and had diameters which could be precisely controlled in the range of 10 to 40 microns. A variety of bacterial and yeast species were entrapped in agarose beads by using this system. In all cases the cells were able to develop into microcolonies containing as many as several hundred cells. This system enables one to apply the powerful method of flow cytometry to the analysis and sorting of whole microbial colonies. Potential applications of this technology in various areas of microbiology are considered.     

Single-cell entrapment and microcolony development within uniform microspheres amenable to flow cytometry.

A method is presented for encapsulating single microbial cells in small spheres suitable for analysis and sorting by flow cytometry. The entrapped cells are able to multiply and form colonies contained within their respective microspheres. The system is based on ejecting the cells suspended in a gellable liquid through an orifice vibrating at ultrasonic frequencies, thus shearing the cell-containing jet into uniform droplets. When low-melting-temperature agarose was used, the droplets could be gelled into solid spheres during flight by appropriately directed colling air streams. This gelling was accompanied by significant dehydration, resulting in a twofold decrease in bead diameter and a corresponding increase in agarose concentration. Nevertheless, the microbeads obtained were highly uniform and had diameters which could be precisely controlled in the range of 10 to 40 microns. A variety of bacterial and yeast species were entrapped in agarose beads by using this system. In all cases the cells were able to develop into microcolonies containing as many as several hundred cells. This system enables one to apply the powerful method of flow cytometry to the analysis and sorting of whole microbial colonies. Potential applications of this technology in various areas of microbiology are considered.     

Cell sorting within the prestalk zone of Dictyostelium discoideum

Abstract. The prestalk zone of slugs of Dictyostelium discoideum has been shown to contain three subregions in which the extracellular matrix genes ecmA and ecmB are differentially expressed; it is generally thought that these regions are defined by extracellular signals. Using β-galactosidase as a cell marker, we have shown that cells can sort specifically to all three regions. Cells from the posterior-prestalk zone ("prestalk 0 zone") which are injected into the slug tip move within 60 min back to their position of origin. When cells from the anterior prestalk zone (presumably containing a mixture of ecmA and ecmB expressers) are transplanted to the posterior prestalk zone, they move to the tip ("prestalk A zone") within 1 h and about 30 min subsequently are often found in a cone-shaped region within the tip ("prestalk B zone"). Cells transplanted to their own positions do not move significantly within this period. Since the sub-regions of the prestalk zone can be defined by sorting, it is possible that they are normally formed in this way rather than by position-dependent signals. Cells transplanted without a change in anterior-posterior position and cells which have sorted back to their positions of origin eventually spread out throughout the prestalk zone. This suggests that sorting preferences of cells are respecified. When posterior prestalk cells are transplanted to the prespore zone, respecification of sorting preference is suspended until the cells return to the prestalk zone and anterior-prestalk cells acquire posterior-prestalk sorting preferences.     

Flow microfluorometric and light-scatter measurement of nuclear and cytoplasmic size in mammalian cells.

A technique for rapid measurement of nuclear and cytoplasmic size relationships in mammalian cell populations has been developed. Based on fluorescence staining of either the nucleus alone or in combination with the cytoplasm using two-color fluorescence methods, this technique permits the simultaneous determination of nuclear and cytoplasmic diameters from fluorescence and light-scatter measurements. Cells stained in liquid suspension pass through a flow chamber at a constant velocity, intersecting a laser beam which excites cell fluorescence and causes light scatter. Depending upon which analysis procedure is used, optical sensors measure nuclear fluorescence and light scatter (whole cell size) or two-color nuclear and cytoplasmic fluorescence from individual cells crossing the laser beam. The time durations of signals generated by the nucleus and cytoplasm are converted electronically into signals proportional to the respective diameters and are displayed as frequency distribution hitograms. Illustrative examples of measurements on uniform microspheres, cultured mammalian cells and human exfoliated gynecologic cells are presented.     

Flow cytometry of human gynecologic specimens using log chromomycin A3 fluorescence and log 90 degrees light scatter.

Flow cytometry and electronic cell sorting are being investigated to screen gynecologic specimens for cervical neoplasia. Cellular DNA content is quantitated by Chromomycin A3 fluorescence and cell size is quantitated by 90 degrees light scatter; the logarithms of the measured intensities are used to produce a two parameter histogram. To determine the cell types responsible for signals in various histogram regions, systematic electronic cell sorting is performed. The sorted fractions are sedimented into microscope slides and stained by the Papanicolaou technique. The cells in each fraction are identified by conventional cytomorphologic criteria. Morphologic analysis of sorted cells reveals histogram regions corresponding to specific cell types. One very important region contains the highest concentration of signals from abnormal cells and is therefore the best region to analyze for specimen abnormality. However, because a significant number of signals in this region are from normal cells, specimens cannot be diagnosed by their analysis. Another important histogram region is composed primarily of signals from endocervical columnar and metaplastic cells. The presence of such cells is a good criterion for specimen adequacy, therefore analysis of signals in this region is essential to assess specimen adequacy for automatic screening.     

Label‐free quantitative chemical imaging and classification analysis of adipogenesis using mouse embryonic stem cells

Stem cells have received much attention recently for their potential utility in regenerative medicine. The identification of their differentiated progeny often requires complex staining procedures, and is challenging for intermediary stages which are a priori unknown. In this work, the ability of label‐free quantitative coherent anti‐Stokes Raman scattering (CARS) micro‐spectroscopy to identify populations of intermediate cell states during the differentiation of murine embryonic stem cells into adipocytes is assessed. Cells were imaged at different days of differentiation by hyperspectral CARS, and images were analysed with an unsupervised factorization algorithm providing Raman‐like spectra and spatially resolved maps of chemical components. Chemical decomposition combined with a statistical analysis of their spatial distributions provided a set of parameters that were used for classification analysis. The first 2 principal components of these parameters indicated 3 main groups, attributed to undifferentiated cells, cells differentiated into committed white pre‐adipocytes, and differentiating cells exhibiting a distinct protein globular structure with adjacent lipid droplets. An unsupervised classification methodology was developed, separating undifferentiated cell from cells in other stages, using a novel method to estimate the optimal number of clusters. The proposed unsupervised classification pipeline of hyperspectral CARS data offers a promising new tool for automated cell sorting in lineage analysis.      

Global cell sorting is mediated by local cell-cell interactions in the C. elegans embryo

The Caenorhabditis elegans embryo achieves pattern formation by sorting cells into coherent regions before morphogenesis is initiated. The sorting of cells is coupled to their fate. Cells move extensively relative to each other to reach their correct position in the body plan. Analyzing the mechanism of cell sorting in in vitro culture experiments using 4D microscopy, we show that all AB-derived cells sort only according to their local neighbors, and that all cells are able to communicate with each other. The directions of cell movement do not depend on a cellular polarity but only on local cell-cell interactions; in experimental situations, this allows even the reversal of the polarity of whole regions of the embryo. The work defines a new mechanism of pattern formation we call "cell focusing".     

The developmental fate of Dictyostelium discoideum cells depends greatly on the cell-cycle position at the onset of starvation

The relationship between the development of Dictyostelium discoideum Ax-2 and the cell cycle at the onset of starvation was analysed with special reference to sorting behaviors during the formation of polarized cell masses (slugs), using a method for inducing good synchrony. Cells starved at different cell-cycle positions showed different developmental features during further culture. For example, cells just before mitosis and dividing cells were sorted out into the anterior prestalk zone of migrating slugs, while cells starved during most of the G2-phase, into the posterior prespore zone. Time courses of cell aggregation and tip formation were also found to vary greatly in a cell-cycle-related manner, and cells starved during the late G2-phase showed the most rapid development. Differential chemotaxis and cohesiveness are generally considered to be important for cell sorting in Dictyostelium development. In fact, remarkable differences in the chemotactic ability to a chemoattractant, cAMP, were detected among cells starved at any particular phase of the cell cycle. EDTA-resistant cohesiveness was also acquired differently depending on the cell cycle, and it was stronger in the cells showing more rapid aggregation. These findings indicate a close relation of the cell cycle to the cell sorting and pattern formation. The possible significance of the cell-cycle-related events presented here is discussed, with special emphasis on the process of cell aggregation.     

N-聚糖和O-聚糖在极性化细胞蛋白质分拣中的作用

极性化上皮细胞的质膜因其所含蛋白质、脂质等组分不同,可以分为细胞膜顶端和细胞膜基底侧端两个区域,而新合成的蛋白质向这两个区域的有效分拣是上皮细胞维持其自身极性及正常功能所必需的。细胞膜基底侧端蛋白质的分拣主要由位于该蛋白质胞质区的信号肽所介导,关于这方面的研究是比较深入的;而细胞膜顶端蛋白质的分拣机制目前尚未阐明,因而显得比较复杂。近年来,糖类分子作为生物体内细胞识别和调控过程的信息分子日益受到关注,人们通过干扰聚糖合成、基因突变以及构建糖基化缺陷细胞株等实验方法,逐渐地认识到糖类分子在极性化上皮细胞的蛋白质分拣调节中起重要作用。由于糖分子本身结构非常复杂,而且目前缺乏研究糖类分子的有效手段,使得糖生物学的研究远远落后于蛋白质和核酸的研究。从而导致探讨糖类分子在蛋白质分拣过程的具体机制相对来说比较困难。本综述拟简要概括糖类分子中N-聚糖和O-聚糖在极性化上皮细胞的蛋白质分拣过程中的作用,以及两种聚糖在此过程中行使分拣信号功能的可能机制。     

Integration of Droplet Microfluidic Tools for Single-cell Functional Metagenomics:An Engineering Head Start

Droplet microfluidic techniques have shown promising outcome to study single cells at high throughput. However, their adoption in laboratories studying"-omics"sciences is still irrelevant due to the complex and multi-disciplinary nature of the field. To facilitate their use, here we provide engineering details and organized protocols for integrating three droplet-based microfluidic technologies into the metagenomic pipeline to enable functional screening of bioproducts at high throughput. First, a device encapsulating single cells in droplets at a rate of～ 250 Hz is described considering droplet size and cell growth. Then, we expand on previously reported fluorescence-activated droplet sorting systems to integrate the use of 4 independent fluorescence-exciting lasers (i.e., 405, 488, 561, and 637 nm) in a single platform to make it compatible with different fluorescence-emitting biosensors. For this sorter, both hardware and software are provided and optimized for effortlessly sorting droplets at 60 Hz. Then, a passive droplet merger is also integrated into our pipeline to enable adding new reagents to already-made droplets at a rate of 200 Hz. Finally, we provide an optimized recipe for manufacturing these chips using silicon dry-etching tools. Because of the overall integration and the technical details presented here, our approach allows biologists to quickly use microfluidic technologies and achieve both single-cell resolution and high-throughput capability (>50,000 cells/day) for mining and bioprospecting metagenomic data.     

Rapid assay for cell age response to radiation by electronic volume flow cell sorting

A new technique is described for measuring cell survival as a function of cell cycle position using flow cytometric cell sorting on the basis of electronic volume signals. The sorting of cells into different cell age compartments is demonstrated for three different cell lines commonly used in radiobiological research. Using flow cytometric DNA content analysis and [3H]thymidine autoradiography of the sorted cell populations, we demonstrate that the resolution of the age compartment separation is as good as or better than that reported for other cell synchronizing techniques. The variation in cell survival as a function of position in the cell cycle after a single dose of radiation as measured by volume cell sorting is similar to that determined by other cell synchrony techniques. This new method has several advantages, including: no treatment of the cells is required, thus, this method is noncytotoxic; no cell cycle progression is needed to obtain different cell age compartments; the cell population can be held in complete growth medium at any desired temperature during sorting; and a complete radiation age-response assay can be plated in 2 h. The application of this method to problems in radiobiology and chemotherapy is discussed, along with some of the technical limitations.     

Hierarchy of sorting signals in chimeras of intestinal lactase-phlorizin hydrolase and the influenza virus hemagglutinin

Lactase-phlorizin hydrolase (LPH) is an apical protein in intestinal cells. The location of sorting signals in LPH was investigated by preparing a series of mutants that lacked the LPH cytoplasmic domain or had the cytoplasmic domain of LPH replaced by sequences that comprised basolateral targeting signals and overlapping internalization signals of various potency. These signals are mutants of the cytoplasmic domain of the influenza hemagglutinin (HA), which have been shown to be dominant in targeting HA to the basolateral membrane. The LPH-HA chimeras were expressed in Madin-Darby canine kidney (MDCK) and colon carcinoma (Caco-2) cells, and their transport to the cell surface was analyzed. All of the LPH mutants were targeted correctly to the apical membrane. Furthermore, the LPH-HA chimeras were internalized, indicating that the HA tails were available to interact with the cytoplasmic components of clathrin-coated pits. The introduction of a strong basolateral sorting signal into LPH was not sufficient to override the strong apical signals of the LPH external domain or transmembrane domains. These results show that basolateral sorting signals are not always dominant over apical sorting signals in proteins that contain each and suggest that sorting of basolateral from apical proteins occurs within a common compartment where competition for sorting signals can occur.     

High-speed photodamage cell selection using bromodeoxyuridine/Hoechst 33342 photosensitized cell killing

One of the major drawbacks of droplet sorting in a flow cytometer is the relatively low sorting speed. Thus, we have developed an alternative, faster sorting technique: photodamage cell sorting. In a photodamage cell sorter all unwanted cells, as detected with the first, measuring laser, are killed with the second, damaging laser. Thus, the cells need to be photosensitive to the second laser. In addition, a mechanism is needed to switch this laser on and off based on the sorting criteria. In our photodamage cell sorter, the ZAPPER, we use an acousto-optic crystal to switch the laser beam. Cells are made photosensitive by vital staining with photosensitizers. With cells grown in the presence of 5-bromo-2'-deoxyuridine (BrdUrd) and stained with Hoechst 33342 (H42) at least a 5-decade cell reduction is accomplished after irradiation with 400 mW UV light. With this system, sorting rates have been achieved of 30,000 cells per second. Due to the selection based on photodynamic killing, this sorting technique is restricted to the selection of viable cells. Photodamage cell sorting seems well suited for isolating viable cells occurring in low percentages or for the sorting of large numbers of cells. Another application can be the sorting of large or fragile cells.     

Formation of junctions and cell sorting in aggregates of chick and mouse cells

The frequency of desmosome formation was examined in aggregates of old cells, which form many junctions, combined with young cells, which form few. Cells of chick corneal epithelium and mouse epidermis, which can be distinguished morphologically, were combined. Desmosomes between these cell types are stable. Further, young cells make more desmosomes than they otherwise would on those surfaces adjoining old cells. Desmosomes increase in number in aggregates while cell sorting is occurring. Cells consistently sort, with those which form most desmosomes lying internally. Gap junctions and intermediate junctions are also present, but are uncommon. A carbohydrate cell-surface coat has regenerated by the time desmosome formation starts. The possible relation of desmosome formation to cell sorting is discussed.     

Sorting minor subpopulations of cells: use of fluorescence as the triggering signal

Flow cytometric cell sorting is commonly used to obtain purified subpopulations of cells for use in in vitro and in vivo assays. This can be time-consuming if the subpopulations of interest represent very low percentages of the cell suspension under study. Often the desired subpopulations are identified by two-color immunofluorescence staining. Generally, cell sorting is performed with a flow cytometer configured to trigger on light scatter signals, then sort windows are set based upon the signals from both fluorescent markers. We demonstrate that triggering the cytometer with the fluorescence signal from antibody staining common to both of the desired subpopulations, then sorting the subpopulations based upon staining of a second marker, substantially increases the speed of cell sorting vis-à-vis traditional methods. This is because undesired events are not analysed, allowing an increase in the throughput rate. While desired subpopulations of cells can be obtained by this method, undesired (i.e., nonstaining) cell "contaminants" increase and may require a second sort. The combined time for the initial enrichment sort and a second sort can be less than sorting once using standard methodology. Alternatively, the degree of contamination may be controlled by adjusting the concentration of the cell suspension and by the sample flow rate.     

Simultaneous use of rhodamine 123, phycoerythrin, Texas red, and allophycocyanin for the isolation of human hematopoietic progenitor cells

The supravital, mitochondrial specific dye Rhodamine 123 (R123) was used in conjunction with three monoclonal antibodies to isolate a population of human bone marrow (BM) cells enriched for hematopoietic progenitor cells. BM cells stained with phycoerythrin-HLA-DR, Texas red-CD34, allophycocyanin-CD15, and R123 were fractionated using four-color immunofluorescence cell sorting. Cells expressing CD34 but not HLA-DR and CD15 (CD34+ HLA-DR- CD15-) were subdivided according to their reactivity with R123 into quiescent, R123 dull (R+) or cycling, R123 bright (R++) subpopulations. Morphological analysis and hematopoietic progenitor cell assays indicated that CD34+ HLA-DR- CD15- R+ cells contained larger numbers of blast cells and colony forming units than CD34+ HLA-DR- CD15- R++ cells. The flow cytometer settings used to accommodate the detection of the R123 fluorescence in combination with that of three other fluorochromes are described.     

Mechanisms of apical protein sorting in polarized thyroid epithelial cells.

The process leading to thyroid hormone synthesis is vectorial and depends upon the polarized organization of the thyrocytes into the follicular unit. Thyrocyte membrane proteins are delivered to two distinct domains of the plasma membrane using apical (AP) and basolateral (BL) sorting signals. A recent hypothesis for AP sorting proposes that apically destined proteins cluster with glycosphingolipids (GSLs) and cholesterol, into microdomains (or rafts) of the Golgi membrane from which AP vesicles originate. In MDCK cells the human neurotrophin receptor, p75hNTR, is delivered to the AP surface through a sorting signal, rich in O-glycosylated sugars, identified in its ectodomain. We have investigated whether this signal is functional in the thyroid-derived FRT cell line and whether p75hNTR clusters into lipid rafts to be sorted to the AP membrane. We found that p75hNTR is apically delivered via a direct pathway and does not associate with rafts during its transport to the surface of FRT cells. Therefore, although the same signal could be recognized by different cell types thyroid cells may possess a tissue-specific sorting machinery.     

PCR-Activated Cell Sorting for Cultivation-Free Enrichment and Sequencing of Rare Microbes

Microbial systems often exhibit staggering diversity, making the study of rare, interesting species challenging. For example, metagenomic analyses of mixed-cell populations are often dominated by the sequences of the most abundant organisms, while those of rare microbes are detected only at low levels, if at all. To overcome this, selective cultivation or fluorescence-activated cell sorting (FACS) can be used to enrich for the target species prior to sequence analysis; however, since most microbes cannot be grown in the lab, cultivation strategies often fail, while cell sorting requires techniques to uniquely label the cell type of interest, which is often not possible with uncultivable microbes. Here, we introduce a culture-independent strategy for sorting microbial cells based on genomic content, which we term PCR-activated cell sorting (PACS). This technology, which utilizes the power of droplet-based microfluidics, is similar to FACS in that it uses a fluorescent signal to uniquely identify and sort target species. However, PACS differs importantly from FACS in that the signal is generated by performing PCR assays on the cells in microfluidic droplets, allowing target cells to be identified with high specificity with suitable design of PCR primers and TaqMan probes. The PACS assay is general, requires minimal optimization and, unlike antibody methods, can be developed without access to microbial antigens. Compared to non-specific methods in which cells are sorted based on size, granularity, or the ability to take up dye, PACS enables genetic sequence-specific sorting and recovery of the cell genomes. In addition to sorting microbes, PACS can be applied to eukaryotic cells, viruses, and naked nucleic acids.