Expression of an L-selectin ligand on hematopoietic progenitor cells

STAT3β is a tumor suppressor in acute myeloid leukemia

Signal transducer and activator of transcription 3 (STAT3) exists in 2 alternatively spliced isoforms, STAT3α and STAT3β. Although truncated STAT3β was originally postulated to act as a dominant-negative form of STAT3α, it has been shown to have various STAT3α-independent regulatory functions. Recently, STAT3β gained attention as a powerful antitumorigenic molecule in cancer. Deregulated STAT3 signaling is often found in acute myeloid leukemia (AML); however, the role of STAT3β in AML remains elusive. Therefore, we analyzed the STAT3β/α messenger RNA (mRNA) expression ratio in AML patients, where we observed that a higher STAT3β/α mRNA ratio correlated with a favorable prognosis and increased overall survival. To gain better understanding of the function of STAT3β in AML, we engineered a transgenic mouse allowing for balanced Stat3β expression. Transgenic Stat3β expression resulted in decelerated disease progression and extended survival in PTEN- and MLL-AF9-dependent AML mouse models. Our findings further suggest that the antitumorigenic function of STAT3β depends on the tumor-intrinsic regulation of a small set of significantly up- and downregulated genes, identified via RNA sequencing. In conclusion, we demonstrate that STAT3β plays an essential tumor-suppressive role in AML.

Fulfilling Koch's postulates in glycoscience: HCELL, GPS and translational glycobiology

Glycoscience-based research that is performed expressly to address medical necessity and improve patient outcomes is called "translational glycobiology". In the 19th century, Robert Koch proposed a set of postulates to rigorously establish causality in microbial pathogenesis, and these postulates can be reshaped to guide knowledge into how naturally-expressed glycoconjugates direct molecular processes critical to human well-being. Studies in the 1990s indicated that E-selectin, an endothelial lectin that binds sialofucosylated carbohydrate determinants, is constitutively expressed on marrow microvessels, and investigations in my laboratory indicated that human hematopoietic stem cells (HSCs) uniquely express high levels of a specialized glycoform of CD44 called "hematopoietic cell E-/L-selectin ligand" (HCELL) that functions as a highly potent E-selectin ligand. To assess the role of HCELL in directing HSC migration to marrow, a method called "glycosyltransferase-programmed stereosubstitution" (GPS) was developed to custom-modify CD44 glycans to enforce HCELL expression on viable cell surfaces. Human mesenchymal stem cells (MSCs) are devoid of E-selectin ligands, but GPS-based glycoengineering of CD44 on MSCs licenses homing of these cells to marrow in vivo, providing direct evidence that HCELL serves as a "bone marrow homing receptor". This review will discuss the molecular basis of cell migration in historical context, will describe the discovery of HCELL and its function as the bone marrow homing receptor, and will inform on how glycoengineering of CD44 serves as a model for adapting Koch's postulates to elucidate the key roles that glycoconjugates play in human biology and for realizing the immense impact of translational glycobiology in clinical medicine.

Glycoengineering of HCELL, the human bone marrow homing receptor: sweetly programming cell migration

The successful clinical implementation of adoptive cell therapeutics, including bone marrow transplantation and other stem cell-based treatments, depends critically on the ability to deliver cells to sites where they are needed. E-selectin, an endothelial C-type lectin, binds sialofucosylated carbohydrate determinants on its pertinent ligands. This molecule is expressed in a constitutive manner on bone marrow and dermal microvascular endothelium, and inducibly on post-capillary venules at all sites of tissue injury. Engagement of E-selectin with relevant ligand(s) expressed on circulating cells mediates initial "tethering/rolling" endothelial adhesive interactions prerequisite for extravasation of blood-borne cells at any target tissue. Most mammalian cells express high levels of a transmembrane glycoprotein known as CD44. A specialized glycoform of CD44 called "Hematopoietic Cell E-/L-selectin Ligand" (HCELL) is a potent E-selectin ligand expressed on human cells. Under native conditions, HCELL expression is restricted to human hematopoietic stem/progenitor cells. We have developed a technology called "Glycosyltransferase-Programmed Stereosubstitution" (GPS) for custom-modifying CD44 glycans to create HCELL on the surface of living cells. GPS-based glycoengineering of HCELL endows cell migration to endothelial beds expressing E-selectin. Enforced HCELL expression targets human mesenchymal stem cell homing to marrow, licensing transendothelial migration without chemokine signaling via a VLA-4/VCAM-1-dependent "Step 2-bypass pathway." This review presents an historical framework of the homing receptor concept, and will describe the discovery of HCELL, its function as the bone marrow homing receptor, and how enforced expression of this molecule via chemical engineering of CD44 glycans could enable stem cell-based regenerative medicine and other adoptive cell therapeutics.

Glycosyltransferase-programmed stereosubstitution (GPS) to create HCELL: engineering a roadmap for cell migration

During evolution of the vertebrate cardiovascular system, the vast endothelial surface area associated with branching vascular networks mandated the development of molecular processes to efficiently and specifically recruit circulating sentinel host defense cells and tissue repair cells at localized sites of inflammation/tissue injury. The forces engendered by high-velocity blood flow commensurately required the evolution of specialized cell surface molecules capable of mediating shear-resistant endothelial adhesive interactions, thus literally capturing relevant cells from the blood stream onto the target endothelial surface and permitting subsequent extravasation. The principal effectors of these shear-resistant binding interactions comprise a family of C-type lectins known as 'selectins' that bind discrete sialofucosylated glycans on their respective ligands. This review explains the 'intelligent design' of requisite reagents to convert native CD44 into the sialofucosylated glycoform known as hematopoietic cell E-/L-selectin ligand (HCELL), the most potent E-selectin counter-receptor expressed on human cells, and will describe how ex vivo glycan engineering of HCELL expression may open the 'avenues' for the efficient vascular delivery of cells for a variety of cell therapies.

The haemopoietic stem cell: between apoptosis and self renewal

Self renewal and apoptosis of haemopoietic stem cells (HSC) represent major factors that determine the size of the haemopoietic cell mass. Changes in self renewal above or below the steady state value of 0.5 will result in either bone marrow expansion or aplasia, respectively. Despite the growing body of research that describes the potential role of HSC, there is still very little information on the mechanisms that govern HSC self renewal and apoptosis. Considerable insight into the role of HSC in many diseases has been gained in recent years. In light of their crucial importance, this article reviews recent developments in the understanding of the molecular, biological, and physiological characteristics of haemopoietic stem cells.

The bone marrow is akin to skin: HCELL and the biology of hematopoietic stem cell homing

The recent findings that adult stem cells are capable of generating new blood vessels and parenchymal cells within tissues they have colonized has raised immense optimism that these cells may provide functional recovery of damaged organs. The use of adult stem cells for regenerative therapy poses the challenging task of getting these cells into the requisite sites with minimum morbidity and maximum efficiency. Ideally, tissue-specific colonization could be achieved by introducing the stem cells intravascularly and exploiting the native physiologic processes governing cell trafficking. Critical to the success of this approach is the use of stem cells bearing appropriate membrane molecules that mediate homing from vascular to tissue compartments. Hematopoietic stem cells (HSC) express a novel glycoform of CD44 known as hematopoietic cell E-/L-selectin ligand (HCELL). This molecule is the most potent E-selectin ligand natively expressed on any human cell. This article reviews our current understanding of the molecular basis of HSC homing and will describe the fundamental "roll" of HCELL in opening the avenues for efficient HSC trafficking to the bone marrow, the skin and other extramedullary sites.

N-linked sialyated sugar receptors support haematopoietic cell-osteoblast adhesions

Haematopoietic progenitor cells proliferate and develop predominantly when they adhere to bone marrow stromal cells such as osteoblasts. Therefore, changes in adhesion may be a common mechanism by which stem cells survive, mature and properly traffic between the bone marrow and the circulation. To characterize these adhesion molecules, we reduced the bone marrow cavity to a simple adhesion assay between KG1a (a CD34+ haematopoietic cell line) and osteosarcoma monolayers (MG-63 or SaOS-2). The data demonstrated that adhesion was mediated by cell-to-cell rather than cell-to-matrix contact, was sensitive to trypsin, calcium chelators and glycosylation inhibitors. Selective pretreatment attributed the constitutive binding to N-linked glycans on KG1a. When carboprocessing was inhibited later at the high mannose intermediate (via deoxymannojirimycin), adhesion was retained. Surprisingly, binding of KG1a to SaOS-2 increased past constitutive levels as doses of tunicamycin or deoxymannojirimycin dropped. Selective pretreatment suggested that this 'inducible' binding resides with molecule(s) on SaOS-2. If the terminal sialic acid was digested (via neuraminidase), this induced response was duplicated. These data, verified in primary cells, suggest that the initial tethering between blood and bone cells in this model is probably due to heavily glycosylated, rapidly processed protein(s) on both cell types.

Low l -selectin (CD62L) expression in acute myeloid leukemia correlates with a bad cytogenetic risk

OBJECTIVES

Interactions between hemopoietic cells and the stromal microenvironment or immunoreactive cells are mediated by specific cell surface receptors. The expression of those molecules may alter the adhesive qualities (mobility and homing) as well as immune response behavior of leukemic blasts. L-Selectin (CD62L) is suggested to play a role in the redistribution and homing of hemopoietic progenitor cells to the bone marrow (BM). Down-regulation of L-selectin is responsible for mobilization of blasts from the BM into the circulation and ligation of L-selectin stimulates proliferation of progenitor cells. This could have an influence on the process of leukemia.

METHOD

We have studied the expression of L-selectin on mononuclear BM cells of 36 acute myeloid leukemia (AML) patients at first diagnosis by FACS analysis using a directly fluorescein isothiocyanate conjugated antibody (clone DRE G56).

RESULTS

On average the patients presented with 88% blasts in the BM. The expression tended to be higher in primary (p) AML compared with secondary (s) AML. L-Selectin was very heterogenously expressed in all FAB groups. Highest expression was found in cases with AML-M4 with four of nine cases presenting with an inv(16) karyotype. Separating our patient cohort in cytogenetic risk groups we could detect a significantly higher expression of L-selectin in cases with a 'good risk' karyotype and a very low expression in cases with a 'bad risk' karyotype (P = 0.037). Comparing patients who achieved remission after double induction therapy (responders) with patients who showed persisting disease (non-responders) we found a higher percentage of L-selectin+ cases or cells in the responder group than in the non-responder group, although the differences were not significant because of only five cases in the 'non-responder' group. Evaluating cut-off points greatest differences in relapse-free survival probabilities were found in patients who presented with > or = 30% L-selectin+ BM cells compared with cases with < 30%: 86% of cases with > or = 30% L-selectin+ cells were still in remission after a mean follow up time of only 8 months compared with only 46% in the group with < 30% L-selectin+ cells.

CONCLUSIONS

We can conclude that (i) expression of L-selectin on AML blasts is variable. This reveals the great diversitiy of immunophenotypes in AML and might contribute to identify individual blast phenotypes in order to detect minimal residual disease in remission. (ii) Low L-selectin expression correlates with a bad cytogenetic risk, with a lower probability to achieve remission and with a shorter relapse-free survival time. This might reflect a decreased homing of the blasts to the BM as well as an impaired cytotoxic T-cell reaction against leukemic cells. The expression of L-selectin on leukemic blasts might be influenced by different cytokine therapies (e.g. with interferon alpha) and this might result in an altered hematologic reconstitution after cytotoxic therapies as well as in an altered immunologic recognition of blasts.

Identification of a putative pathway for the muscle homing of stem cells in a muscular dystrophy model

Attempts to repair muscle damage in Duchenne muscular dystrophy (DMD) by transplanting skeletal myoblasts directly into muscles are faced with the problem of the limited migration of these cells in the muscles. The delivery of myogenic stem cells to the sites of muscle lesions via the systemic circulation is a potential alternative approach to treat this disease. Muscle-derived stem cells (MDSCs) were obtained by a MACS(R) multisort method. Clones of MDSCs, which were Sca-1+/CD34-/L-selectin+, were found to adhere firmly to the endothelium of mdx dystrophic muscles after i.v. or i.m. injections. The subpopulation of Sca-1+/CD34- MDSCs expressing L-selectin was called homing MDSCs (HMDSCs). Treatment of HMDSCs with antibodies against L-selectin prevented adhesion to the muscle endothelium. Importantly, we found that vascular endothelium from striate muscle of young mdx mice expresses mucosal addressin cell adhesion molecule-1 (MAdCAM-1), a ligand for L-selectin. Our results showed for the first time that the expression of the adhesion molecule L-selectin is important for muscle homing of MDSCs. This discovery will aid in the improvement of a potential therapy for muscular dystrophy based on the systemic delivery of MDSCs.

Identification of cultured progenitor cells from human marrow stroma

The marrow stromal cells (MSC) are essential for regulation of bone remodeling and hematopoiesis. It is of prime importance to isolate MSC and to expand the proliferating cells ex vivo. In this study, we analyzed cultured MSC for various cellular parameters, including cell morphology, cell cycle, and expression of cell surface antigens by flow cytometry. MSC were divided based on cell size to small (S-cells) and large (L-cells) and were visualized by light and electron microscope. The S-cells were proliferating cells correlated with G0/G1 phase of cell cycle, and expressed cFOS. The expression of surface markers CD-34, -44, -51, -61, -62E, -62P, -62L was quantified using flow cytometry. CD-44 was ubiquitously expressed by S and L cells, CD-51 and -61 were expressed by 30%-38% of S-cells. CD-34 and -62 expressed 20% positive of the analyzed cells that were of the proliferating progenitors (S-cells). This study enables the identification of subpopulations from MSC with special attention paid to the proliferating cells from ex vivo cultures of marrow stroma.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.

Vascular cell adhesion molecule-1 expression and hematopoietic supportive capacity of immortalized murine stromal cell lines derived from fetal liver and adult bone marrow

Ontogeny-specific differences in hematopoietic behavior may be influenced by unique adhesive interactions between hematopoietic cells and the microenvironment, such as that mediated by vascular cell adhesion molecule-1 (VCAM-1, CD 106). Although VCAM-1 is variably expressed during vertebrate development, we hypothesized that VCAM-1 expression might be linked to the enhanced capacity of the fetal liver microenvironment to support hematopoiesis. To test this we used immortalized murine stromal cell lines derived from midgestation fetal liver and adult bone marrow to compare the functional expression of VCAM-1. Molecular analysis of VCAM-1 expression was performed on stromal cell lines using Northern blot analysis, immunoprecipitation studies, and solid-phase enzyme-linked immunosorbent assay. Hematopoietic studies were performed by coculturing fetal liver cells with stromal cell lines, and the functional readout was determined by high-proliferative potential colony-forming cell (HPP-CFC) adherence assays. In contrast to our initial hypothesis, we observed greater expression of VCAM-1 messenger ribonucleic acid and protein on an adult marrow stromal cell line. In functional studies, anti-VCAM-1 antibody inhibited the binding of nearly half of the HPP-CFCs to adult marrow stroma but had a minimal effect on their binding to fetal liver stroma, despite the greater adherence of HPP-CFCs to fetal stroma. We conclude that VCAM-1 influences the hematopoietic supportive capacity of immortalized murine stroma derived from adult bone marrow. Our studies suggest that cellular interactions other than those mediated by VCAM-1 are involved in the increased adhesive capacity of immortalized murine stroma derived from fetal liver.

Homing and clonogenic outgrowth of CD34(+) peripheral blood stem cells: a role for L-selectin?

OBJECTIVE

After transplantation of hematopoietic stem cells, adhesion molecules play a major role in the multistep process of engraftment in which L-selectin is suggested to be of relevance. A positive correlation previously was found between the number of reinfused L-selectin(+) stem cells and platelet recovery. In the present study, we determined the role of L-selectin in different engraftment steps, i.e., adhesion to endothelial cells, migration, and clonogenic outgrowth by in vitro assays that closely mimic the in vivo situation.

MATERIALS AND METHODS

Flow adhesion and migration experiments were performed using the human bone marrow endothelial cell line 4LHBMEC and isolated peripheral CD34(+) cells with or without blocking of L-selectin-ligand interaction. Various clonogenic assays, including serum-free colony-forming unit-megakaryocytes (CFU-MK) and burst-forming unit-megakaryocytes (BFU-MK), were performed with sorted L-selectin(+)L-selectin(-) cells or in the presence of antibodies.

RESULTS

Blocking of L-selectin on CD34(+) cells did not significantly affect rolling over and firm adhesion to 4LHBMEC. In addition, no role for L-selectin was found in transendothelial migration experiments. Finally, in clonogenic outgrowth of sorted or anti-L-selectin monoclonal antibody-incubated CD34(+) cells, no key role for L-selectin expression could be defined in BFU-MK and CFU-MK assays.

CONCLUSION

Using in vitro assays for CD34(+) stem cell adhesion, migration, and clonogenic capacity, we were not able to define a major role for L-selectin.

A distinct glycoform of CD44 is an L-selectin ligand on human hematopoietic cells

We previously have obtained operational evidence of a hematopoietic cell L-selectin ligand expressed on normal human hematopoietic cells and on leukemic blasts. Using a technique developed in our laboratory for analyzing and identifying adhesion molecules, we show here that hematopoietic cell L-selectin ligand is a specialized glycoform of CD44. This L-selectin ligand activity of CD44 requires sialofucosylated N-linked glycans and is sulfation-independent. These data provide important insights on the structural biology of CD44 and reveal a role for this protein as an L-selectin ligand on human hematopoietic cells.

A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan–dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PSGL-1, a ligand for all selectins. Using a shear-based adhesion assay, a hematopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoietic cell line KG1a and on normal human hematopoietic progenitors was previously identified. To characterize the structural biology of HCLL and to define its relationship to PSGL-1, the effects of chemical and enzymatic treatments on HCLL activity of KG1a cells and membrane preparations were analyzed. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein. Glycosidase digestions of membrane protein preparations and metabolic treatments of KG1a cells with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex-type N-glycans. Adhesion assays and biochemical studies showed that this glycoprotein is also expressed on circulating blasts in native acute leukemias. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 expression had equivalent HCLL activity; (2) anti–PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias with low expression of PSGL-1 and CD34 display high HCLL activity; and (4) despite high level expression of PSGL-1, HCLL activity was absent on HL60 cells. These data provide first evidence of a naturally expressed membrane L-selectin ligand expressing binding determinant(s) on an N-linked glycoconjugate. This novel ligand may help mediate L-selectin–dependent cell-cell adhesive interactions within the cytoarchitecture of the bone marrow microenvironment.

A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan–dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PSGL-1, a ligand for all selectins. Using a shear-based adhesion assay, a hematopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoietic cell line KG1a and on normal human hematopoietic progenitors was previously identified. To characterize the structural biology of HCLL and to define its relationship to PSGL-1, the effects of chemical and enzymatic treatments on HCLL activity of KG1a cells and membrane preparations were analyzed. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein. Glycosidase digestions of membrane protein preparations and metabolic treatments of KG1a cells with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex-type N-glycans. Adhesion assays and biochemical studies showed that this glycoprotein is also expressed on circulating blasts in native acute leukemias. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 expression had equivalent HCLL activity; (2) anti–PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias with low expression of PSGL-1 and CD34 display high HCLL activity; and (4) despite high level expression of PSGL-1, HCLL activity was absent on HL60 cells. These data provide first evidence of a naturally expressed membrane L-selectin ligand expressing binding determinant(s) on an N-linked glycoconjugate. This novel ligand may help mediate L-selectin–dependent cell-cell adhesive interactions within the cytoarchitecture of the bone marrow microenvironment.

Sulfated glycans induce rapid hematopoietic progenitor cell mobilization: evidence for selectin-dependent and independent mechanisms

The adhesive mechanisms leading to the mobilization of hematopoietic progenitor cells (HPCs) from the bone marrow into the blood are poorly understood. We report on a role for selectins and fucoidan in progenitor mobilization. Baseline levels of circulating HPCs are increased in endothelial selectin-deficient (P/E−/−) mice. Similar levels are observed when E-selectin null (E−/−) mice are treated with anti-P-selectin antibody or with fucoidan (which inhibits P- and L-selectin function). In particular, administration of 2 doses of fucoidan (25 mg/kg) over 6 hours produces profound mobilization of progenitors in wild-type mice and the response is greatly enhanced in E−/− and P/E−/− mice. Competitive reconstitution experiments reveal that fucoidan also elicits long-term (more than 6 months) repopulating stem cells. Mobilization assays using chimeric mice harboring L-selectin–deficient progenitors and wild-type progenitors expressing the green fluorescence protein suggest that L-selectin expression is not required but confers an advantage for fucoidan-induced mobilization. Sulfation is critical as desulfated fucoidan is ineffective. In addition, sulphogalactosylceramide (sulfatide) but not heparin can induce HPC mobilization. Our results indicate that administration of sulfated glycans, especially with concurrent inhibition of E-selectin function, represents a powerful novel method for rapid mobilization of long-term–repopulating stem cells. These findings may help elucidate the mechanisms of HPC trafficking during development and adult life.

Sulfated glycans induce rapid hematopoietic progenitor cell mobilization: evidence for selectin-dependent and independent mechanisms

The adhesive mechanisms leading to the mobilization of hematopoietic progenitor cells (HPCs) from the bone marrow into the blood are poorly understood. We report on a role for selectins and fucoidan in progenitor mobilization. Baseline levels of circulating HPCs are increased in endothelial selectin-deficient (P/E−/−) mice. Similar levels are observed when E-selectin null (E−/−) mice are treated with anti-P-selectin antibody or with fucoidan (which inhibits P- and L-selectin function). In particular, administration of 2 doses of fucoidan (25 mg/kg) over 6 hours produces profound mobilization of progenitors in wild-type mice and the response is greatly enhanced in E−/− and P/E−/− mice. Competitive reconstitution experiments reveal that fucoidan also elicits long-term (more than 6 months) repopulating stem cells. Mobilization assays using chimeric mice harboring L-selectin–deficient progenitors and wild-type progenitors expressing the green fluorescence protein suggest that L-selectin expression is not required but confers an advantage for fucoidan-induced mobilization. Sulfation is critical as desulfated fucoidan is ineffective. In addition, sulphogalactosylceramide (sulfatide) but not heparin can induce HPC mobilization. Our results indicate that administration of sulfated glycans, especially with concurrent inhibition of E-selectin function, represents a powerful novel method for rapid mobilization of long-term–repopulating stem cells. These findings may help elucidate the mechanisms of HPC trafficking during development and adult life.

Intercellular adhesion can be visualized using fluorescently labeled fibrosarcoma HT1080 cells cocultured with hematopoietic cell lines or CD34(+) enriched human mobilized peripheral blood cells

BACKGROUND

Intercellular contacts between adjacent cells migrating over each other are important in many cellular processes. However, it has been difficult to visualize and identify dynamic intercellular adhesions between migrating cells in situ.

METHODS

Two fluorescent membrane dyes, PKH2 and PKH26 for staining HT1080 and hematopoietic cells and cell lines, and an automated fluorescence microscopy system were used to monitor intercellular adhesion.

RESULTS

Cellular extensions connecting two or more adjacent cells were visualized, showing the intercellular adhesion between migrating cells for minutes and up to hours. After cells adhered to each other, followed by cell migration in different directions, cellular extensions were dragged from the pivotal contact points in different focal planes. CD34(+)-enriched mobilized peripheral blood cells and six hematopoietic cell lines showed intercellular connections in cocultures with HT1080. However, the frequency of intercellular connections was variable in different cocultures. A cell density of about 3.1 x 10(4) cells/cm(2) for both cell lines in cocultures provided an adequate number of cells in each field of view, showing up to four intercellular connections per 100 total cells plated.

DISCUSSION

The tools derived from this study will open new areas of investigation for understanding the mechanism of the intercellular adhesion process.

Mechanisms that regulate the function of the selectins and their ligands

Selectins are a family of three cell adhesion molecules (L-, E-, and P-selectin) specialized in capturing leukocytes from the bloodstream to the blood vessel wall. This initial cell contact is followed by the selectin-mediated rolling of leukocytes on the endothelial cell surface. This represents the first step in a cascade of molecular interactions that lead to leukocyte extravasation, enabling the processes of lymphocyte recirculation and leukocyte migration into inflamed tissue. The central importance of the selectins in these processes has been well documented in vivo by the use of adhesion-blocking antibodies as well as by studies on selectin gene-deficient mice. This review focuses on the molecular mechanisms that regulate expression and function(s) of the selectins and their ligands. Cell-surface expression of the selectins is regulated by a variety of different mechanisms. The selectins bind to carbohydrate structures on glycoproteins, glycolipids, and proteoglycans. Glycoproteins are the most likely candidates for physiologically relevant ligands. Only a few glycoproteins are appropriately glycosylated to allow strong binding to the selectins. Recently, more knowledge about the structure and the regulated expression of some of the carbohydrates on these ligands necessary for selectin binding has been accumulated. For at least one of these ligands, the physiological function is now well established. A novel and exciting aspect is the signaling function of the selectins and their ligands. Especially in the last two years, convincing data have been published supporting the idea that selectins and glycoprotein ligands of the selectins participate in the activation of leukocyte integrins.