Membrane Glycolipids in Stem Cells

Abstract

Stem cells, such as embryonic stem cells, hematopoietic stem cells, neural stem cells, mesenchymal stem cells, and very small embryonic-like stem cells, are undifferentiated cells that are endowed with a high potential for proliferation and the capacity for self-renewal with retention of pluri/multipotency to differentiate into their progenies. Recently, studies regarding the biological functions of glycolipids and cell surface microdomains (caveolae, lipid rafts, or glycolipid-enriched microdomains) in stem cells are emerging. In this review, we introduce the expression patterns of glycolipids and the functional roles of cell surface microdomains in stem cells.

1. Introduction

Glycolipids are lipid molecules linked with one or more carbohydrate units. Based on their core lipid moiety, glycolipids are classified into glycosphingolipids (containing ceramide) and glycoglycerolipids (containing glycerol). Glycosphingolipids (GSLs) containing sialic acid residue(s) are referred to as gangliosides. Glycolipids are found in virtually all vertebrate cells and body fluids and are localized primarily, but not exclusively, on the plasma membrane. One of notable characteristics of glycolipids is the structural diversity; so far, 172 neutral GSLs, 24 sulfated GSLs, and 188 gangliosides with variations in the carbohydrate chain have been reported in vertebrate tissues and organs [1]. Their complexity may increase manifold when variations in their lipophilic components are taken into consideration. The expression patterns of glycolipids are known to change drastically during development or cellular differentiation. Therefore, glycolipids have been frequently used as important developmental marker molecules.

Glycolipids have also been suggested to play important biological functions. Mouse embryos deficient in glucosylceramide synthase, which catalyzes the initial step of GSL biosynthesis, are able to differentiate into endoderm, mesoderm, and ectoderm but are unable to form more differentiated tissues and die during midgastrulation by apoptosis in the ectoderm [2]. Although neural cell-specific disruption of glucosylceramide synthase does not impair late embryonic development, all glucosylceramide synthase-deficient mice die within 3 weeks after birth by dysfunction of cerebellum and peripheral nerves, associated with structural defects. This strongly indicates that GSLs are required for brain maturation after birth [3]. Mice deficient in ganglioside synthases are basically viable but exhibit multiple defects such as a lethal sound-induced seizure in GD3 synthase-/GM2 synthase-double knockout mice and a hearing loss in GM3 synthase-knockout mice [4, 5] (for further details and discussions of the mice deficient in ganglioside synthases, see [6]). Also in humans, a nonsense mutation of GM3 synthase causes autosomal recessive infantile-onset symptomatic epilepsy syndrome [7]. These studies revealed that mutation of GSL-synthesis enzymes leading to absence of specific GSL structures and accumulation of certain precursor GSLs. The consequences are manifested with developmental abnormalities, leading to certain clinical consequences and death. They further reinforce the concept that plasma membrane glycolipids are crucial in developmental events by playing important biological functions, such as modulation of cell signaling and cell-cell interaction. On the plasma membrane, glycolipids are believed to cluster with other membrane lipids, such as cholesterol and sphingomyelin, to form specialized microdomains termed caveolae, lipid rafts, or glycolipid-enriched microdomains [8–11]. In these microdomains, signaling and cell-adhesion molecules are localized, implicating that these domains may form platforms for signal transduction and cell adhesion.

Although they are still few, studies regarding the biological functions of glycolipids and cell surface microdomains in stem cells are emerging. A stem cell is defined as an undifferentiated cell endowed with a high potential for proliferation and the capacity for self-renewal with retention of pluripotency or multipotency to differentiate into their progenies. Stem cells represent cellular reservoirs for formation of tissues and organs during development and for replacement of cells lost during normal cellular turnover in adulthood. Stem cells are roughly classified into two types: embryonic stem cells and somatic stem cells. An embryonic stem cell is derived from epiblasts in the inner cell mass of blastocysts and has a pluripotency to generate all cells in three germ layers, endoderm, mesoderm and ectoderm. Somatic stem cells are multipotent cells which can restrictively differentiate into a related group of cells. As the representative somatic stem cells, hematopoietic stem cells, neural stem cells and mesenchymal stem cells are well known. In addition to these stem cells, induced pluripotent stem cells (iPS cells), pluripotent stem cells artificially generated from mouse somatic cells such as fibroblasts by introducing Oct3/4, Sox2, c-Myc, and Klf4 have recently been established [12]. These stem cells have attracted a great interest lately, owing to their potential in unlocking specific cellular events leading to differentiation, proliferation, and fate determination. Furthermore, it has been hypothesized that there is a sub-population of particular cancer cells having stem cell-like characteristics such as self-renewal and differentiation capacity and continually sustaining tumorigenesis in primary tumors. These stem cell-like cancer cells are referred to as cancer stem cells. Recently, cancer stem cells have been isolated from tumors. Cancer stem cells, namely tumor-initiating cells or tumor-propagating cells, can be important targets for cancer treatment.

In the stem cells, it has been expected that glycolipids and cell surface microdomains play important roles as markers and functional molecules. In this review, we will introduce the expression patterns of glycolipids and the functional roles of the cell surface microdomains in stem cells. The structures of the glycolipids and the glycolipid expressions in stem cells referred to in this paper are presented in Table 1 and 2.

Table 1.

Structures and nomenclature of glycosphingolipids referred to in this paper. The nomenclature for gangliosides is based on that of Svennerholm [58].

Abbreviation	Series	Structure
Asialo-GM1	asialo-series	Galβ1,3GalNAcβ1,4Galβ1,4Glcβ1,1Cer
Fucosyl-GM1		Fucα1,2Galβ1,3GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GM3	a-series	Neu5Acα2,3Galβ1,4Glcβ1,1Cer
GM2	a-series	GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GM1	a-series	Galβ1,3GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GD1a	a-series	Neu5Acα2,3Galβ1,3GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GalNAc-GD1a	a-series	GalNAcβ1,4(Neu5Acα2,3)Galβ1,3GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GT1a	a-series	Neu5Acα2,8Neu5Acα2,3Galβ1,3GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GT1aα	a-series	Neu5Acα2,3Galβ1,3(Neu5Acα2,6)GalNAcβ1,4(Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GD3	b-series	Neu5Acα2,8Neu5Acα2,3Galβ1,4Glcβ1,1Cer
9-O-acetyl-GD3	b-series	Neu5,9Ac2α2,8Neu5Acα2,3Galβ1,4Glcβ1,1Cer
GD2	b-series	GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GD1b	b-series	Galβ1,3GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GT1b	b-series	Neu5Acα2,3Galβ1,3GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GQ1b	b-series	Neu5Acα2,8Neu5Acα2,3Galβ1,3GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GQ1bα	b-series	Neu5Acα2,3Galβ1,3(NeuAcα2,6)GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GT3	c-series	Neu5Acα2,8Neu5Acα2,8Neu5Acα2,3Galβ1,4Glcβ1,1Cer
GT2	c-series	GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GT1c	c-series	Galβ1,3GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GQ1c	c-series	Neu5Acα2,3Galβ1,3GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
GP1c	c-series	Neu5Acα2,8Neu5Acα2,3Galβ1,3GalNAcβ1,4(Neu5Acα2,8Neu5Acα2,8Neu5Acα2,3)Galβ1,4Glcβ1,1Cer
SSEA-1		Galβ1,4(Fucα1,3)GlcNAcβ1,3Galβ1,4Glcβ1,1Cer
SSEA-3		Galβ1,3GalNAcβ1,3Galα1,4Galβ1,4Glcβ1,1Cer
SSEA-4		Neu5Acα2,3Galβ1,3GalNAcβ1,3Galα1,4Galβ1,4Glcβ1,1Cer

Table 2.

Glycolipids expressed in stem cells

Stem cells	Species	Glycolipid	Reference
Embryonic stem cell	Mouse	GM3	[13]
		GM1	[13, 14]
		GD1a	[13]
		Lactosylceramide	[13]
		Ceramide trihexoside	[13]
		Globoside	[13]
		Forssman antigen	[13]
		SSEA-1	[17, 18]
	Human	SSEA-3	[15, 21]
		SSEA-4	[15, 21]
Induced pluripotent stem cell	Mouse	SSEA-1 (glycoprotein/proteoglycan?)	[12]
Neural stem cell	Mouse, human	GD2	[30]
	Mouse	GD3	[31]
		GQ1b	[31]
		GT1b	[31]
		Chol-1α (GT1aα and GQ1bα)	[32]
	Mouse	SSEA-1 (glycoprotein/proteoglycan?)	[30, 34]
	Human	SSEA-1 (glycoprotein/proteoglycan?)	[30]
Brain cancer stem cell	Human	A2B5 antigens (c-series gangliosides)	[35, 36]
	Mouse	SSEA-1 (glycoprotein/proteoglycan?)	[37, 38]
	Human	SSEA-1 (glycoprotein/proteoglycan?)	[39]
Hematopoietic stem cell	Mouse	GM1?	[42–44]
Mesenchymal stem cell	Human	GD2	[46]
		GM1	[14]
		GM3	[48]
		GM2	[48]
		GD1a	[48]
	Mouse, human	SSEA-4	[45]
Very small embryonic-like stem cell	Mouse	SSEA-1 (glycoprotein/proteoglycan?)	[49]
	Human	SSEA-4	[50]

2. Glycolipids expressed in stem cells

2.1 Glycolipids expressed in embryonic stem cells

In undifferentiated E14 mouse embryonic stem cells, it has been biochemically elucidated that a-series gangliosides, GM3, GM1 and GD1a, are expressed [13]. On the other hand, it has also been reported that GM1 is exclusively expressed in J1 mouse embryonic stem cells [14]. During differentiation, the concentration of gangliosides is increased and the expression pattern of gangliosides becomes more complex, with GD3 predominating initially and a- and b-series emerging at the expense of GD3 [13]. One of the unusual differentiation-related gangliosides in E14 ES cells is GalNAc-GD1a, which was identified by fast-atom bombardment mass spectrometry [13]. With respect to neutral GSLs expressed in undifferentiated mouse embryonic stem cells, lactosylceramide, ceramide trihexoside, globoside and forssman antigen have been identified [13]. In H7 human embryonic stem cells, a flow cytometric study revealed that globo-series GSLs, stage specific embryonic antigen-3 (SSEA-3) and SSEA-4, are also expressed [15]. These antigens are down-regulated following induction of differentiation with concomitant increase in expression of the b-series gangliosides, GD3, 9-O-acetyl-GD3 and GD2, and c-series gangliosides.

SSEA-1 is a trisaccharide antigen recognized by a monoclonal antibody against embryonal carcinoma cells [16]. It is well recognized that mouse embryonic stem cells express the SSEA-1 antigen [17, 18]. It has also been reported that iPS cells are positive for SSEA-1 [12]. The SSEA-1 carbohydrate epitope is carried by glycoproteins, proteoglycans and glycolipids [19]. However, in the teratocarcinoma cell lines, glycoproteins rather than glycolipids are the major carrier molecules of SSEA-1 [20]. Interestingly, SSEA-1 is not expressed in human embryonic stem cells; SSEA-3 and SSEA-4 are expressed in these cells [21]. In contrast, SSEA-3 and SSEA-4 are not expressed in mouse embryonic stem cells.

Glycolipids expressed in embryonal carcinoma cells, stem cells in teratocarcinoma, have also been reported. In mouse P19 embryonal carcinoma cells, trace amounts of GM3 and GD3 are expressed [22, 23]. However, after neuronal differentiation induced by retinoic acid, the total ganglioside amount was elevated, and expression of GM2, GM1, GD1a, GD1b, GT1b and GQ1b was upregulated in these cells. In 2102Ep human teratocarcinoma cells, globo-series GSLs such as globotriaosylceramide, globotetraosylceramide (globoside), globopentaosylceramide (SSEA-3), fucosyl-globopentaosylceramide (SSEA-3) and sialyl-globopentaosylceramide (SSEA-3 and SSEA-4) have been characterized [24]. In addition, fucosyl-globoside and sialyl-globoside have been found in HT-E (833K) human teratocarcinoma cells [25]. It is interesting to note that globo-series GSLs are related to cellular differentiation [26].

2.2 Glycolipids expressed in neural stem cells

Neural stem cells are multipotent cells generating brain-forming cells such as neurons and glial cells; the latter include astrocytes and oligodendrocytes [27]. Neural stem cells appear in the neuroepithelium during development and are located in the subventricular zone of the lateral ventricles and the subgranular layer of the dentate gyrus in the hippocampus in adulthood [28, 29]. There are studies reporting the expression of glycolipids in these cells [27].

In mouse and human neural stem cells, the expression of a b-series ganglioside, GD2, was detected by flow cytometric analysis [30]. By a biochemical study of the mouse neuroepithelial cells that are known to be rich in neural stem cells, it was determined that another b-series ganglioside, GD3, is the predominant gangliosides [31]. In addition, GQ1b, GT1b and Chol-1α gangliosides (GT1aα and GQ1bα) are also expressed in the neuroepithelial cells in much lesser amount [31–33]. Interestingly, a-series gangliosides such as GM1 and GD1a are not detectable in the neuroepithelial cells, but are up-regulated after neuronal differentiation [32]. In mouse and human neural stem cells, a fucosylated carbohydrate antigen, SSEA-1 is also expressed [30, 34]. It has been found that the SSEA-1 antigen in neuroepithelial cells is carried not only by glycoproteins, but also by glycolipids [33].

2.3 Glycolipids expressed in cancer stem cells

Recently, glycolipids characterizing cancer stem cells in brain tumors have been reported. In human glioma such as glioblastoma, astrocytoma, oligodendroglioma and ependymoma, the cancer stem cells are positive for A2B5 antigens, c-series gangliosides such as GT3, GT2, GQ1c and GP1c [35, 36]. It has been confirmed by xenograft experiments that A2B5⁺ cells isolated from human glioblastoma by fluorescence-activated cell sorting (FACS) or magnetic cell sorting can form tumors in brains of nude rats or mice. These studies clearly indicate the usefulness of c-series gangliosides as marker molecules to identify brain cancer stem cells. In addition to c-series gangliosides, SSEA-1 has also been found in cancer stem cells. Tumors from Patched mutant mice, a mouse model of medulloblastoma, has been reported to highly express SSEA-1 [37, 38]. SSEA-1⁺ cells collected from the tumors by FACS gave rise to tumors in the brains of all to be positive for SSEA-1. Also, in human glioblastoma multiforme cells, SSEA-1-positive populations have been reported to be the cancer stem cells [39]. In this study, it was found that SSEA-1⁺ cells sorted from human glioblastoma multiforme by magnetic cell sorting and injected into the brains of NOD-SCID mice generated tumors more efficiently than SSEA-1⁻ cells. These findings suggest that SSEA-1 is a general marker antigen of cancer stem cells in brain tumors. The carrier molecules of SSEA-1 in these tumor stem cells have not yet been characterized, but there might be a possibility that they are glycolipids.

2.4 Glycolipids expressed in hematopoietic stem cells

Hematopoietic stem cells are multipotent cells which can give rise to cells forming blood and immune systems, such as myeloids (erythrocytes, basophils, eosinophils, neutrophils, monocytes, macrophages, megakaryocytes, and platelets) and lymphoids (T-cells, B-cells, and natural killer cells). Hematopoietic stem cells originally appear in the aorta-gonad-mesonephros in embryos and then are located in the bone marrow in adults.

So far there has not been biochemical analysis of glycolipids expressed in hematopoietic stem cells, probably because they are in very low abundance in bone marrow (mouse bone marrow hematopoietic stem cells account for 0.004% of bone marrow mononuclear cells) [40]. However, in rat lymphocyte progenitor cells prepared from bone marrow by Ficoll-Paque density centrifugation, the expression of gangliosides has been quantified by biochemical analysis [41]. The contents of each ganglioside fraction are as follows: monosialogangliosides, 22.8 ± 1.8 %; disialogangliosides-1, 17.7 ± 1.6 %; disialogangliosides-2, 15.8 ± 1.3 %; trisialogangliosides, 19.3 ± 2.0 %; and tetrasialogangliosides, 24.4 ± 1.9 %. The molecular species of these gangliosides has not yet been identified. Furthermore, it has been found that cholera toxin B subunit, a molecular probe recognizing GM1, are reactive with mouse hematopoietic stem cells [42–44], which suggests the presence of GM1 in these cells. However, it is known that the cholera toxin B-subunit is reactive with not only GM1, but also to a much lesser extent with other glycolipids, such as GD1a, asialo-GM1, fucosyl-GM1, GM2, GD1b, GT1b, and GM3. The possibility that hematopoietic stem cells express these gangliosides cannot be ruled out. To identify bona fide gangliosides expressed in hematopoietic stem cells, systematic biochemical studies are required.

2.5 Glycolipids expressed in mesenchymal stem cells

Mesenchymal stem cells are multipotent stromal cells which can differentiate into cells of the mesodermal lineage such as myocytes, osteocytes, adipocytes and chondrocytes. These cells can be isolated from all mesenchymal tissues, especially bone marrow. It has been reported that the SSEA-4 carbohydrate epitope is expressed in adult mouse and human mesenchymal stem cells and is useful for isolating these cells from bone marrow [45]. It was found in this study that SSEA-4⁺ cells isolated from mouse and human bone marrow cells by FACS have a high proliferative ability and a multipotency to differentiate into adipocytes and osteoblasts. Furthermore, an immunocytochemical and flow cytometric analysis revealed that GD2 is specifically expressed in human mesenchymal stem cells isolated from bone marrow and adipose tissue, but not in unfractionated bone marrow cells, CD45⁺ leukocytes, CD34⁺ hematopoietic progenitor cells, CD33⁺ myeloid cells, CD3⁺ T-lymphocytes, and CD19⁺ B-lymphocytes [46]. It has been confirmed that GD2⁺ cells isolated from bone marrow using immunomagnetic beads have the characteristics of mesenchymal stem cells such as the morphology, plastic adherence ability, and multipotency to differentiate into osteoblasts, adipocytes, and chondroblasts. In addition, mesenchymal stem cells isolated from human umbilical cord blood have been reported to be positive for GD2 [47]. These findings clearly indicate the expression of GD2 in human mesenchymal stem cells and its usefulness as a marker molecule. However, it has also been reported that GM1 is expressed in human mesenchymal stem cells prepared from periodontal bone marrow [14]. Recently, it also has been reported that GM3, GM2, and GD1a are expressed in human dental pulp-derived stem cells which share molecular and cellular characteristics with bone marrow-derived mesenchymal stem cells; during neuronal differentiation, GD3 is up-regulated [48]. It still remains to be undetermined whether the predominant ganglioside expressed in human mesenchymal stem cells is GD2 or other gangliosides.

2.6 Glycolipids expressed in very small embryonic-like stem cells

Very small embryonic-like stem cells (VSELs) are adult versatile cells, which were originally found in mouse bone marrow and human umbilical cord blood [49–51]. VSELs express embryonic stem cell markers such as Oct3/4 and Nanog and have pluripotency to differentiate into cells of all three germ layers (i.e., cardiomyocytes, neurons, astrocytes, oligodendrocytes, and pancreatic cells) in vitro like embryonic stem cells [49]. The most interesting characteristic of these cells is the size; the diameter of the cell body is just 2–4 μm in mice and 3–5 μm in human. VSELs were identified not only in bone marrow, but also in organs such as brains, kidneys, pancreas and skeletal muscles in mice [51].

VSELs are found in a population of CXCR4⁺ Sca-1⁺ lineage marker⁻ CD45⁻ cells in adult mouse bone marrow, and a population of CXCR4⁺ CD133⁺ CD34⁺ lineage marker⁻ CD45⁻ cells in human umbilical cord blood [49, 50]. In addition to these markers, SSEA-1 and SSEA-4 are expressed in mouse and human VSELs, respectively. It is interesting that the expression patterns of these antigens are similar to those in mouse and human embryonic stem cells. It is not yet clear whether the carrier molecule of SSEA-1 in the mouse VSELs is a glycolipid, but the molecular characteristics are expected to be similar with those in mouse embryonic stem cells. The immunoreactivity of SSEA-1 in mouse VSELs after permeabilization with Triton X-100 which can easily wash out glycolipid antigens on the cell surface [49, 50] suggests that the SSEA-1 carrier molecules in these cells are mainly glycoproteins.

3. Cell surface microdomains in stem cells

3.1 Cell surface microdomains in hematopoietic stem cells

Long-term hematopoietic stem cells in the bone marrow niche are in a noncycling state. In this hibernation or dormancy state of hematopoietic stem cells, cell surface microdomains play an important role [42]. In CD34⁻ c-Kit⁺ Sca-1⁺ lineage marker⁻ hematopoietic stem cells freshly isolated from the bone marrow niche, cell surface microdomains positive for GM1 are diffusely distributed [44]. Stimulation by cytokines, stem cell factor and thrombopoietin, induces clustering of cell surface microdomains and re-entry into the cell cycle, probably via segregation and concentration of the activated cytokine receptors. The factor from the niche which inhibits cell surface microdomain clustering and induces the hibernation of hematopoietic stem cells has been identified as transforming growth factor-β [52]. These studies clearly indicate the importance of cell surface microdomain as platforms to integrate the signals for the cells to maintain in a hibernation state.

Rac GTPases are known to be essential for filamentous-actin assembly and chemotaxis of hematopoietic stem/progenitor cells. It has been reported that disruption of cell surface microdomains with methyl-β-cyclodextrin suppresses the translocation of Rac1 to cell surface microdomains and the assembly of filamentous-actin in c-Kit⁺ hematopoietic progenitor cells derived from mouse low-density bone marrow [43]. This study suggests that cell surface microdomains are also involved in chemotaxis of hematopoietic stem/progenitor cells.

It has been suggested that the number of stem cells in physiological condition is regulated by a balance between symmetric and asymmetric cell division. Cell surface microdomains have been implicated also in polarization of human CD34⁺ hematopoietic stem/progenitor cells [53].

3.2 Cell surface microdomains in neural stem cells

In mouse neuroepithelial cells, cell surface microdomains have been reported to be involved in signal transduction and cell adhesion [31]. In these cells, disruption of cell surface microdomains with methyl-β-cyclodextrin suppressed basic fibroblast growth factor-induced activation of the Ras-mitogen-activated protein kinase pathway that is essential for cellular proliferation. In addition, disruption of cell surface microdomains inhibited the cell adhesion via β1 integrin which is localized in these microdomains. It has also been suggested that cell surface microdomains in mouse neurospheres act as integrators of the signal crosstalk by integrin, growth factors, and Notch, which are important for the cell fate regulation [54].

In mid-embryonic to early-postnatal stages of developing mouse cortex, astroglial lineage cell surface microdomains are characterized by enrichment of phosphatidylglucoside (PtdGlc), a phosphoglycerolipid containing glucose linked to phosphatidic acid [55, 56]. It has been recently suggested that the PtdGlc-rich cell surface microdomains are involved in astrocytogenesis during development [57]. Treatment of mouse neural stem cells with anti-PtdGlc antibody causes recruitment of epidermal growth factor receptor into cell surface microdomain compartments with subsequent activation of the downstream signaling pathway. Therefore, PtdGlc-rich cell surface microdomains are physiologically coupled and regulate the activation of epidermal growth factor receptor on neural stem cell during development.

4. Closing remark

In recent years, major progresses have been made in stem cell biology, including generation of iPS cells from somatic cells. Although many unresolved issues remain, such as low success rate and safety concerns, application of cell-based transplantation using stem cells is a very attractive and promising strategy for regenerative and restorative medicine. The identification of stem cell marker, especially those located on the plasma membrane, is becoming increasingly important for the isolation and classification of specific populations of stem cells. As we described above, specific glycolipids detected on each stem cell type are used for this purpose. Additionally, understanding the functional role of glycolipids during cellular differentiation and proliferation remains to be an important area of research. Such information will undoubtedly stimulate the progresses in developing stem cell-based therapeutic strategies for a variety of degenerative diseases or tissue damage.