Volume 52 (11): 1447-1457, 2004 Copyright ©The Histochemical Society, Inc. Spatial Distribution and Initial Changes of SSEA-1 and Other Cell Adhesion-related Molecules on Mouse Embryonic Stem Cells Before and During Differentiation
Institute of Organ Transplants, Reconstructive Medicine and Tissue Engineering, Shinshu University Graduate School of Medicine (LC,FY,KS), and Department of Anatomy and Organ Technology, Shinshu University School of Medicine (KJ,NO,YO,KA,KS), Matsumoto, Japan Correspondence to: Li Cui, Inst. of Organ Transplants, Reconstructive Medicine and Tissue Engineering, Shinshu University Graduate School of Medicine, 3-1-1 Asahi, Matsumoto 390-8621, Japan. E-mail: cui{at}sch.md.shinshu-u.ac.jp
We examined the distribution of cell adhesion-related molecules (CAMs) among mouse embryonic stem (ES) cells and the spatial distribution on cell surfaces before and during differentiation. The cellcell heterogeneity of SSEA-1, PECAM-1, and ICAM-1 among the undifferentiated cells in the ES cell colonies was evident by immunohistochemistry and immuno-SEM, supporting the flow cytometry findings. In contrast, most undifferentiated ES cells strongly expressed CD9. SSEA-1 was located preferentially on the edge of low protuberances and microvilli and formed clusters or linear arrays of 320 particles. PECAM-1 and ICAM-1 were randomly localized on the free cell surfaces, whereas CD9 was preferentially localized on the microvilli or protuberances, especially in the cell periphery. Both the SSEA-1+ fraction and the SSEA-1 fraction of magnetic cell sorting (MACS) formed undifferentiated colonies after plating. Flow cytometry showed that these populations reverted separately again to a culture with a mixed phenotype. Differentiation induced by retinoic acid downregulated the expression of all CAMs. Immuno-SEM showed decreases of SSEA-1 in the differentiated ES cells, although some clustering still remained. Our findings help to elucidate the significance of these molecules in ES cell maintenance and differentiation and suggest that cell surface antigens may be useful for defining the phenotype of undifferentiated and differentiated ES cells. (J Histochem Cytochem 52:14471457, 2004)
Key Words: embryonic stem cells SSEA-1 cell adhesion-related molecules immuno-SEM and -TEM flow cytometry retinoic acid
EMBRYONIC STEM (ES) cells are derived from the inner cell mass of blastocyst stage early embryos and have both pluripotency and capacity of self-renewal. Therefore, ES cells can serve as experimental models for the study of early embryonic development and differentiation, and potentially may serve as sources for cell therapy of various tissues and organs. Mouse ES cells can be maintained in an undifferentiated state for long periods in medium containing the leukemia inhibitory factor (LIF) (Smith et al. 1988
Embryonic cell surface molecules have been viewed generally as lineage markers and regulators of cellcell interactions. Cell surface carbohydrates are implicated in a number of membrane-modulated phenomena, such as cell aggregation and adhesion. They play a role in the cellular interactions of the immune system (Springer 1990
Expression patterns of cell adhesion-related molecules, such as SSEA-1, ICAM-1, PECAM-1, and CD9, occur in undifferentiated and differentiated ES cells (Tian et al. 1997 In this study we examined the surface ultrastructure of mouse ES cells and the spatial distribution of SSEA-1, ICAM-1, PECAM-1, and CD9 on the cells. In addition, we investigated the changes in the morphology and the expression of these CAMs on initiation of ES cell differentiation. We report for the first time the spatial distribution and expression levels of the above molecules on mouse ES cells.
Murine ES Cell Lines and Cell Culture Five karyotypically normal ES cell lines were used in the study. ES cell lines developed from 129/sv and DBA1 strain mice, passage 9 and 12, respectively, were purchased from Cell and Molecular Technologies (Phillipsburg, NJ). ES cell lines AB1 and AB2.2 at passage 12 were gifts from Dr. Allan Bradley. ES cell line ES-D3, developed by Doetschman, was obtained from American Type Culture Collection (Manassas, VA). Frozen stocks of cells were thawed and seeded onto feeder layers. They were cultured as described previously (Johkura et al. 2003
Differentiation was induced by adding 106 M trans-retinoic acid (RA) (Draper et al. 2002 As a control culture, ES cells were dissociated with 0.1% trypsinEDTA and resuspended in the differentiating medium without LIF for 48 hr, in which ES cells aggregated to form embryoid bodies. Aggregated embryoid bodies were collected and cultured on gelatin-coated coverslips in the same medium for 24 hr. ES cells were also cultured in the differentiating medium without LIF for 48 hr as a control culture for PCR.
Alkaline Phosphatase Staining and Immunofluorescence Labeling
Double-labeling Immunoelectron Microscopy
Preparation for Scanning Electron Microscopy
Preparation for Transmission Electron Microscopy
RT-PCR Analysis
Magnetic Cell Sorting (MACS) Separation SSEA-1-positive and -negative cells were separated using the MACS system (Miltenyi Biotec; Bergisch Gladbach, Germany) according to the instructions of the manufacturer. Briefly, cells were resuspended in 0.5% BSA/PBS and incubated with anti-SSEA-1 antibody (10 µg/ml) for 20 min at 8C, followed by a magnetically labeled rat anti-mouse IgM (Miltenyi Biotec) for 20 min at 8C. The cells were then washed three times with 0.5% BSA/PBS. For sorting and selection of SSEA-1-positive cells, labeled cells were loaded onto a sterile LS column installed in a magnetic field. The column was rinsed with 0.5% BSA/PBS and the negative unlabeled cells passed through. Trapped cells were eluted after the removal of the column from the magnetic field and were collected by centrifugation. For sorting and selection of SSEA-1-negative cells, an LD column was used and the negative unlabeled cells were collected in a centrifugation tube. The SSEA-1+ fraction and SSEA-1 fraction were cultured separately at a density of 2 x 104 cells/cm2 on feeder layers in ES culture medium with LIF.
Flow Cytometry
Characterization of Undifferentiated Mouse ES Cells The undifferentiated state of mouse ES cells was ascertained on the basis of morphological features, alkaline phosphatase (ALP) enzyme cytochemistry, and SSEA-1 immunostaining. Hoffman modulation contrast microscopy, which reveals surface features of cells in culture, showed high, dome-shaped ES colonies (Figure 1a) . Within the colonies, the undifferentiated ES cells had indistinct margins and were positive for ALP and SSEA-1 (Figures1b and 1c). The undifferentiated ES cells typically had high nucleus/cytoplasm ratios, prominent nucleoli, and were firmly and closely packed together in the colony (Figures 1d and 1e). Dividing cells were present 12 hr after plating (Figure 2a) , and many also displayed filopodia or lamellipodia. On day 2 of culture, typical three-dimensional, tightly packed undifferentiated colonies were present (Figure 2b). The free surfaces were rather rough, with many protuberances several nanometers high. In some cases, long and short microvilli were present.
Cell Adhesion-related Molecules on ES Cells Cell adhesion-related molecules that may be involved in the maintenance of undifferentiated ES cells were assessed by flow cytometry (Figure 3) . Nearly all undifferentiated ES cells, 98.5 ± 0.6% (mean ± SD), expressed high levels of CD9 antigen. The fluorescence intensity for SSEA-1, PECAM-1, and ICAM-1 varied from low to high. PECAM-1 and ICAM-1 patterns of distribution were similar to one another, i.e., 74.6 ± 1.4% and 80.7 ± 2.0% of the undifferentiated ES cells expressed the respective molecules. In contrast, only 51.5 ± 1.6% of the undifferentiated ES cells were positive for SSEA-1 (Figure 3). Nearly all SSEA-1-positive cells expressed PECAM-1, ICAM-1, and CD9. RT-PCR also confirmed the synthesis in undifferentiated ES cells of mRNA for PECAM-1, ICAM-1, and CD9 (Figure 4) . To further characterize SSEA-1-positive and -negative cells, we purified them from the undifferentiated ES colonies using MACS. By flow cytometry analysis, SSEA-1-positive cells were enriched from 51% to 98% after MACS (Figure 5a)
. SSEA-1 negative cells were purified from 49% to 78.2% (Figure 5c). In contrast, the percentage of CD9-positive cells before and after sorting was the same (not shown). Three days after plating, the morphology of colonies obtained from both SSEA-1+ and SSEA-1 fractions was the same as that of standard undifferentiated culture (Figures 5b and 5d insets). An SSEA-1- negative cell population emerged again in undifferentiated ES colonies derived from the SSEA-1+ fraction (Figure 5b). In undifferentiated ES colonies derived from the SSEA-1 fraction, the percentage of SSEA-1-positive cells increased to the level of that before sorting (Figure 5d).
Spatial Localization of Cell Adhesion-related Molecules on the Undifferentiated ES Cells When viewed by CLSM, antibodies to SSEA-1, CD9, ICAM-1, and PECAM-1 were localized at the contact regions of undifferentiated ES cells and on the free surfaces (Figure 6) , although the fluorescence intensity varied greatly from cell to cell. Cells stained for both CD9 and SSEA-1 showed that most were CD9-positive, but some of the same cells were SSEA-1-positive and others were SSEA-1-negative (Figures 6a and 6b), consistent with the results of flow cytometry. Double staining for ICAM-1 and SSEA-1 revealed that some cells were positive for both (Figure 6c), whereas others were positive for ICAM-1 alone or were negative for both (Figure 6d). Cellcell junctions exhibited stronger fluorescence for PECAM-1 than did free surfaces (Figure 6e). Some cells were positive for PECAM-1 alone (Figure 6f). SSEA-1 had a dot-like appearance on the surface of ES cells (Figure 6g). In general, SSEA-1-positive cells were positive for other CAMs, although occasionally one did not express detectable levels of any of the other CAMs.
Immuno-SEM and -TEM showed that CD9 was preferentially localized at cell boundaries, where the gold particles were mainly restricted to the microvilli or low protuberances and often formed clusters (Figures 7a and 7b) . In contrast, the gold particles for ICAM-1 and PECAM-1 were randomly distributed on the cell free surface (Figures 7c7e). In the undifferentiated ES cells, the distribution of SSEA-1 varied greatly from cell to cell (Figure 7e). On SSEA-1-positive cells, the 10-nm gold particles were located preferentially on the edge of low protuberances and microvilli, forming clusters ranging from 3 to 20 particles or in linear arrays (Figures 7c7e). These findings corresponded to the dot-like appearance of SSEA-1 seen by CLSM (Figure 6g). As a control, embryoid body outgrowths derived from suspension cultures were stained for SSEA-1 (Figure 8b) . Some undifferentiated ES cells were positive, whereas completely differentiated epithelium-like cells were SSEA-1- negative, showing the specificity of the antibody.
Cell Adhesion-related Molecules During ES Cell Differentiation by Retinoic Acid Treatment with retinoic acid caused the colonies to become flattened, and the character of the treated cells was clearly different from that of undifferentiated ES cells (Figure 8a, inset). After differentiation for 2 days, flow cytometry showed that the expression level of SSEA-1 and cell adhesion-related molecules decreased or disappeared (Figure 3). Fluorescence intensity was also generally reduced when viewed by CLSM (not shown). Immuno-SEM showed the general decrease of SSEA-1 in the differentiated ES cells, in which a few dot-like clusters of SSEA-1 still remained (Figure 8a). CD9, PECAM-1, and ICAM-1 labeling were clearly decreased. RT-PCR also confirmed the rapid decline in the expression levels of CD9, ICAM-1, and PECAM-1 mRNAs during initial cell differentiation with regard to ß-actin expression (Figure 4). ß1-Integrin was also highly expressed in the undifferentiated ES cells and did not significantly change throughout the time course of initial differentiation.
In this study we have demonstrated the intercellular heterogeneity of SSEA-1, ICAM-1, and PECAM-1 distribution and ubiquitous expression of CD9 in the undifferentiated mouse ES cells. The range of SSEA-1, ICAM-1, and PECAM-1 expression varied from low to high levels. SSEA-1 was not present in all undifferentiated ES cells, even though the cell morphology showed little variation. This pattern is consistent with a previous report showing that there are both SSEA-1-positive and -negative cells in preimplantation mouse embryos (Solter and Knowles 1978
SSEA-1 (Lex) is a homophilic adhesion molecule capable of interacting with itself and was localized preferentially on cell surface projections. Carbohydratecarbohydrate interactions are important in specific recognition between cells (Eggens et al. 1989
Immunocytochemistry and flow cytometry showed that ICAM-1 and PECAM-1 were also heterogeneously distributed on the undifferentiated ES cells. ICAM-1 was uniformly and randomly distributed on the ES cell surfaces and in the cellcell contact sites. Despite the discernible expression of ICAM-1, ligands for this molecule, i.e., LFA-1 and Mac-1, were not present in the ES cells (Tian et al. 1997
As shown by CLSM, PECAM-1 is the predominant CAM at the cellcell boundaries of ES cells. This is consistent with previous findings showing that staining for PECAM-1 is specific to cellcell borders in the inner cell mass of the mouse blastocyst (Robson et al. 2001
Because most undifferentiated ES cells are positive for CD9 and it quickly disappeared after initial differentiation, CD9 may be a more suitable marker for undifferentiated ES cells than SSEA-1. CD9 is a cell adhesion-related molecule and may play a role in cellextracellular matrix or cellcell interactions as a co-factor of integrin (Rubinstein et al. 1997 The association of cell surfaces containing microdomains of adhesion molecules plays an important role in the three-dimensional cellcell interactions that affect differentiation of ES cells. The data presented here enable us to further understand the roles of these cell adhesion-related molecules in cellcell interactions and in self-renewal of ES cells. In addition, the present study indicates that these antigens may be used as markers of cell status to test the phenotypic stability of long-term ES cell cultures. Simultaneous use of immunoreactivity for multiple surface antigens will assist in the identification of positive or negative selection of target cells derived from ES cells.
Supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (no.13558107), a Grant-in-Aid for 21st COE Program by the Ministry of Education, Culture, Sports, Science, and Technology, and a Grant from The Japan-China Medical Association. Mouse ES cell lines AB1 and AB2.2 were a kind gift from Dr Allan Bradley (Baylor College of Medicine). We thank Dr Kiyokazu Kametani and Ms Kayo Suzuki (Research Center for Instrumental Analysis of Shinshu University) and Mr Mitsuo Ueno (Fine Materials Engineering, Faculty of Textile Science and Technology of Shinshu University) for excellent technical assistance. We thank Dr Kei-ichi Uemura (Department of Aging Biochemistry, Neuro-aging Research Division, Research Center on Aging and Adaptation of Shinshu University School of Medicine) for advice concerning flow cytometry analysis. We also thank the Microscope Division of Carl Zeiss (Tokyo, Japan) for their support with respect to the CLSM experiment.
Received for publication December 21, 2003; accepted June 6, 2004
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