CD43 Interacts With Moesin and Ezrin and Regulates Its Redistribution to the Uropods of T Lymphocytes at the Cell-Cell Contacts

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

O-glycans direct selectin ligands to lipid rafts on leukocytes

Palmitoylated cysteines typically target transmembrane proteins to domains enriched in cholesterol and sphingolipids (lipid rafts). P-selectin glycoprotein ligand-1 (PSGL-1), CD43, and CD44 are O-glycosylated proteins on leukocytes that associate with lipid rafts. During inflammation, they transduce signals by engaging selectins as leukocytes roll in venules, and they move to the raft-enriched uropods of polarized cells upon chemokine stimulation. It is not known how these glycoproteins associate with lipid rafts or whether this association is required for signaling or for translocation to uropods. Here, we found that loss of core 1-derived O-glycans in murine C1galt1(-/-) neutrophils blocked raft targeting of PSGL-1, CD43, and CD44, but not of other glycosylated proteins, as measured by resistance to solubilization in nonionic detergent and by copatching with a raft-resident sphingolipid on intact cells. Neuraminidase removal of sialic acids from wild-type neutrophils also blocked raft targeting. C1galt1(-/-) neutrophils or neuraminidase-treated neutrophils failed to activate tyrosine kinases when plated on immobilized anti-PSGL-1 or anti-CD44 F(ab')2. Furthermore, C1galt1(-/-) neutrophils incubated with anti-PSGL-1 F(ab')2 did not generate microparticles. In marked contrast, PSGL-1, CD43, and CD44 moved normally to the uropods of chemokine-stimulated C1galt1(-/-) neutrophils. These data define a role for core 1-derived O-glycans and terminal sialic acids in targeting glycoprotein ligands for selectins to lipid rafts of leukocytes. Preassociation of these glycoproteins with rafts is required for signaling but not for movement to uropods.

Podocalyxin increases the aggressive phenotype of breast and prostate cancer cells in vitro through its interaction with ezrin

Podocalyxin is an anti-adhesive transmembrane sialomucin that has been implicated in the development of more aggressive forms of breast and prostate cancer. The mechanism through which podocalyxin increases cancer aggressiveness remains poorly understood but may involve the interaction of podocalyxin with ezrin, an established mediator of metastasis. Here, we show that overexpression of podocalyxin in MCF7 breast cancer and PC3 prostate cancer cell lines increased their in vitro invasive and migratory potential and led to increased expression of matrix metalloproteases 1 and 9 (MMP1 and MMP9). Podocalyxin expression also led to an increase in mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) activity. To determine the role of ezrin in these podocalyxin-dependent phenotypic events, we first confirmed that podocalyxin formed a complex with ezrin in MCF7 and PC3 cells. Furthermore, expression of podocalyxin was associated with a changed ezrin subcellular localization and increased ezrin phosphorylation. Transient knockdown of ezrin protein abrogated MAPK and PI3K signaling as well as MMP expression and invasiveness in cancer cells overexpressing podocalyxin. These findings suggest that podocalyxin leads to increased in vitro migration and invasion, increased MMP expression, and increased activation of MAPK and PI3K activity in MCF7 and PC3 cells through its ability to form a complex with ezrin.

Syndecan-1 is targeted to the uropods of polarized myeloma cells where it promotes adhesion and sequesters heparin-binding proteins

Syndecan-1 (CD138) is a heparan sulfate-bearing proteoglycan present on the surface of myeloma cells where it mediates myeloma cell-cell and cell-extracellular matrix adhesion. In this study, we examined myeloma cell lines for cell membrane localization of syndecan-1. On some cells we note a striking localization of syndecan-1 to a single small membrane protrusion, with the remainder of the cell surface being mostly negative for syndecan-1. Examination of cell morphology reveals that a proportion of cells from myeloma cell lines, as well as primary myeloma cells, are polarized, with a uropod on one end and lamellipodia on the other end. On these polarized cells, syndecan-1 is specifically targeted to the uropod, but in contrast, on nonpolarized cells syndecan-1 is evenly distributed over the entire cell surface. In addition to syndecan-1, several other cell surface molecules localize specifically to the uropod, including CD44 and CD54. Functional assays reveal that myeloma cell lines with a high proportion of polarized cells have a much higher migratory potential than cell lines with few polarized cells. Moreover, the uropod is the cell pole preferentially involved in aggregation of myeloma cells and in adhesion of myeloma cells to osteoblast-like cells. When polarized myeloma cells are incubated with heparin-binding proteins, like hepatocyte growth factor or osteoprotegerin, they concentrate in the uropod. These data indicate that syndecan-1 is targeted to the uropod of polarized myeloma cells and that this targeting plays a role in promoting cell-cell adhesion and may also regulate the biological activity of heparin-binding cytokines.

Neutrophil polarity and locomotion are associated with surface redistribution of leukosialin (CD43), an antiadhesive membrane molecule

This study analyzed the behavior of an antiadhesive membrane molecule, CD43, in neutrophil polarization and locomotion. CD43 cross-linking by antibodies induced neutrophil locomotion, with CD43 molecules clustered at the uropod of polarized neutrophils. In contrast, CD11b/CD18 cross-linking by antibodies did not affect either cell polarization or locomotion. Stimulation of suspended or adherent neutrophils with chemotactic peptide results in cell polarization and locomotion and a concomitant redistribution of CD43 to the uropod. This process is entirely reversible. The study also investigated which actin-binding protein could be involved in CD43 lateral redistribution. -Actinin and moesin are preferentially adsorbed on Sepharose beads bearing a recombinant CD43 intracellular domain. Analysis by immunofluorescence confocal microscopy shows a codistribution of moesin during CD43 lateral redistribution. By contrast, -actinin is located at the leading edge, an area devoid of CD43. These results shed new light on the role of CD43 membrane redistribution, which appears to be directly related to neutrophil polarity and locomotion.

Polarization and interaction of adhesion molecules P-selectin glycoprotein ligand 1 and intercellular adhesion molecule 3 with moesin and ezrin in myeloid cells

In response to the chemoattractants interleukin 8, C5a,N-formyl-methionyl-leucyl-phenylalanine, and interleukin 15, adhesion molecules P-selectin glycoprotein ligand 1 (PSGL-1), intercellular adhesion molecule 3 (ICAM-3), CD43, and CD44 are redistributed to a newly formed uropod in human neutrophils. The adhesion molecules PSGL-1 and ICAM-3 were found to colocalize with the cytoskeletal protein moesin in the uropod of stimulated neutrophils. Interaction of PSGL-1 with moesin was shown in HL-60 cell lysates by isolating a complex with glutathione S-transferase fusions of the cytoplasmic domain of PSGL-1. Bands of 78- and 81-kd were identified as moesin and ezrin by Western blot analysis. ICAM-3 and moesin also coeluted from neutrophil lysates with an anti-ICAM-3 immunoaffinity assay. Direct interaction of the cytoplasmic domains of ICAM-3 and PSGL-1 with the amino-terminal domain of recombinant moesin was demonstrated by protein-protein binding assays. These results suggest that the redistribution of PSGL-1 and its association with intracellular molecules, including the ezrin-radixin-moesin actin-binding proteins, regulate functions mediated by PSGL-1 in leukocytes stimulated by chemoattractants.

A Common Signaling Pathway Via Syk and Lyn Tyrosine Kinases Generated From Capping of the Sialomucins CD34 and CD43 in Immature Hematopoietic Cells

The sialomucin CD34 is a useful marker for hematopoietic stem/progenitor cells. However, the role of CD34 remains poorly understood. Here we investigate the functions of CD34 and another sialomucin CD43 coexpressed on hematopoietic stem/progenitor cells. Stimulation of undifferentiated hematopoietic KG1a cells with anti-CD34 or anti-CD43 induced homotypic cytoadhesion, accompanied by formation of a long-lived cap of CD34 and CD43 respectively, which colocalized with F-actin. Stimulation with either antibody specifically increased tyrosine phosphorylation of the identical set of proteins of Lyn, Syk, pp60, pp69, and pp77 at the capping site. These events were similar to those observed in monocytic U937 cells ectopically expressing CD34. After stimulation of KG1a cells, coimmunoprecipitation of Lyn with pp69 and pp77 and of Syk with pp37 was detected in the membrane fraction. Blockade of antibody-induced cap formation by treatment with cytochalasin D leads to inhibition of tyrosine phosphorylation of Syk and pp77 and homotypic cytoadhesion. Moreover, normal human CD34+ bone marrow cells showed cap formation of CD34 or CD43 after stimulation. These results suggest that crosslinking of either CD34 or CD43 activates the same signaling pathway for cytoadhesion through Lyn, Syk, and the novel tyrosine-phosphorylated proteins within hematopoiesis.

Structural Requirements for CD43 Function

The regulation of T cell activation and adhesion by CD43 (leukosialin, sialophorin) has been thought to be mainly a function of the large size and negative charge of the extracellular domain of the protein. In this work, we demonstrate that the cytoplasmic tail is both necessary and sufficient for the negative regulatory effect of CD43 on cell-cell adhesion. Expression of mutant CD43 proteins in primary T cells from CD43-deficient mice demonstrated that the antiproliferative effect of CD43 is also dependent upon the cytoplasmic tail. In contrast, Ab-mediated costimulation through CD43 does not require the intracellular domain of CD43. These data demonstrate that CD43 primarily serves as a negative regulator of T cell activation and adhesion, and that this is mediated not exclusively by passive effects of the extracellular domain, but requires participation of the cytoplasmic tail, perhaps through interactions with the cytoskeleton, or alternatively, active regulation of intracellular signaling pathways.