Cells release membrane enclosed nano-sized vesicles termed extracellular vesicles (EVs) that function as mediators of intercellular communication by transferring biological information between cells. Tumor-derived EVs have emerged as important mediators in cancer development and progression, mainly through transfer of their bioactive content which can include oncoproteins, oncogenes, chemokine receptors, as well as soluble factors, transcripts of proteins and miRNAs involved in angiogenesis or inflammation. This transfer has been shown to influence the metastatic behavior of primary tumors. Moreover, tumor-derived EVs have been shown to influence distant cellular niches, establishing favorable microenvironments that support growth of disseminated cancer cells upon their arrival at these pre-metastatic niches. It is generally accepted that cells release a number of major EV populations with distinct biophysical properties and biological functions. Exosomes, microvesicles, and apoptotic bodies are EV populations most widely studied and characterized. They are discriminated based primarily on their intracellular origin. However, increasing evidence suggests that even within these EV populations various subpopulations may exist. This heterogeneity introduces an extra level of complexity in the study of EV biology and function. For example, EV subpopulations could have unique roles in the intricate biological processes underlying cancer biology. Here, we discuss current knowledge regarding the role of subpopulations of EVs in cancer development and progression and highlight the relevance of EV heterogeneity. The position of tetraspanins and integrins therein will be highlighted. Since addressing EV heterogeneity has become essential for the EV field, current and novel techniques for isolating EV subpopulations will also be discussed. Further dissection of EV heterogeneity will advance our understanding of the critical roles of EVs in health and disease.

Abstract. The integrin lymphocyte function-associated antigen-1 (LFA-1) expressed on T cells serves as a useful model for analysis of leukocyte integrin functional activity. We have assessed the role of divalent cations Mg", Call, and Mn2+ in LFA-1 binding to ligand intercellular adhesion molecule-1 (ICAM-1) and induction of the divalent cation-dependent epitope recognized by mAb 24 . Manganese strongly promoted both expression of the 24 epitope and T cell binding to ICAM-1 via LFA1, suggesting that Mn2+ is able to directly alter the conformation of LFA1 in a manner that favors ligand binding. Since Mn2+ also promotes functional activity of other integrins, parallels in mechanism of ligand binding may span the integrin family. In contrast, induction of 24 epitope expression by Mg 21 required removal of Ca 2+ from T cell LFA1 with

In multiple sclerosis, brain-reactive T cells invade the central nervous system (CNS) and induce a self-destructive inflammatory process. T-cell infiltrates are not only found within the parenchyma and the meninges, but also in the cerebrospinal fluid (CSF) that bathes the entire CNS tissue. How the T cells reach the CSF, their functionality, and whether they traffic between the CSF and other CNS compartments remains hypothetical. Here we show that effector T cells enter the CSF from the leptomeninges during Lewis rat experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. While moving through the three-dimensional leptomeningeal network of collagen fibres in a random Brownian walk, T cells were flushed from the surface by the flow of the CSF. The detached cells displayed significantly lower activation levels compared to T cells from the leptomeninges and CNS parenchyma. However, they did not represent a specialized non-pathogenic cellular sub-fraction, as their gene expression profile strongly resembled that of tissue-derived T cells and they fully retained their encephalitogenic potential. T-cell detachment from the leptomeninges was counteracted by integrins VLA-4 and LFA-1 binding to their respective ligands produced by resident macrophages. Chemokine signalling via CCR5/CXCR3 and antigenic stimulation of T cells in contact with the leptomeningeal macrophages enforced their adhesiveness. T cells floating in the CSF were able to reattach to the leptomeninges through steps reminiscent of vascular adhesion in CNS blood vessels, and invade the parenchyma. The molecular/cellular conditions for T-cell reattachment were the same as the requirements for detachment from the leptomeningeal milieu. Our data indicate that the leptomeninges represent a checkpoint at which activated T cells are licensed to enter the CNS parenchyma and non-activated T cells are preferentially released into the CSF, from where they can reach areas of antigen availability and tissue damage.

The very late activation antigens (VLA) or beta 1 integrins mediate cell attachment to different extracellular matrix proteins and intercellular adhesions. The ligand binding activity of these adhesion receptors is not constitutive and can be regulated by temperature, presence of extracellular divalent cations, stimulatory monoclonal antibodies (mAbs), and cellular activation. We have generated three novel mAbs, HUTS-4, HUTS-7, and HUTS-21, recognizing specific epitopes on the common beta 1 subunit (CD29) of VLA integrins whose expression correlates with the ligand binding activity of these heterodimeric glycoproteins. This correlation has been demonstrated for several integrin heterodimers in different cell systems using a variety of extracellular and intracellular stimuli for integrin activation. Thus, the presence of micromolar concentrations of extracellular Mn2+, preincubation with the activating anti-beta 1 mAb TS2/16, and cell treatment with phorbol esters or calcium ionophores, induced the expression of the HUTS beta 1 epitopes on T lymphoblasts. Using a panel of human-mouse beta 1 chimeric molecules, we have mapped these epitopes to the 355-425 sequence of the beta 1 polypeptide. This segment represents therefore a novel regulatory region of beta 1 that is exposed upon integrin activation. Interestingly, binding of HUTS mAbs to partially activated VLA integrins results in maximal activation of these adhesion receptors and enhancement of cell adhesion to beta 1 integrin ligands collagen, laminin, and fibronectin.

We show that the three conformational states of integrin α5β1 have discrete free energies and define activation by measuring intrinsic affinities for ligand of each state and the equilibria linking them. The 5,000‐fold higher affinity of the extended‐open state than the bent‐closed and extended‐closed states demonstrates profound regulation of affinity. Free energy requirements for activation are defined with protein fragments and intact α5β1. On the surface of K562 cells, α5β1 is 99.8% bent‐closed. Stabilization of the bent conformation by integrin transmembrane and cytoplasmic domains must be overcome by cellular energy input to stabilize extension. Following extension, headpiece opening is energetically favored. N‐glycans and leg domains in each subunit that connect the ligand‐binding head to the membrane repel or crowd one another and regulate conformational equilibria in favor of headpiece opening. The results suggest new principles for regulating signaling in the large class of receptors built from extracellular domains in tandem with single‐span transmembrane domains.

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IntroductionPlasma membrane contains small organized microdomains (lipid rafts) in which restricted repertoires of proteins are arranged together. 1,2 In resting cells, lipid rafts are estimated to be around 100 nm in diameter, including a few dozen proteins, and are distributed randomly on the cell surface, covering up to 50% of the plasma membrane. Upon cell activation, raft domains coalesce, recruiting and excluding different receptors, and allowing the proper organization of signaling complexes for efficient signal transduction. 1,2 Tetraspanins comprise a large number of small palmitoylated polypeptides that span the plasma membrane 4 times, [3][4][5][6] and form microdomains that contain a restricted repertoire of proteins. Biochemically, they share some properties with lipid rafts, but tetraspanin microdomains are based on protein-protein interactions. [7][8][9][10] Tetraspanins have a highly conserved structure with a short and a large extracellular loop (LEL) where 2 or 3 disulfide bonds can be formed. 11 This large loop interacts noncovalently with other tetraspanins and transmembrane proteins, including integrins and adhesion receptors of the immunoglobulin (Ig) superfamily. Although all mammalian cells express different tetraspanins, genetic approaches have been elusive and their function has not yet been fully elucidated. However, their role in antigen presentation and sperm-egg binding has been recently underscored. [12][13][14][15][16][17][18][19][20] The association of certain plasma membrane proteins to the cortical actin cytoskeleton is critical for their proper localization and function. Thus, the concentration of selectins and their ligands on the tip of microvilli 21,22 both at the leukocyte and the apical surface of endothelial cells favors their interaction during the rolling phase of leukocyte extravasation. Likewise, vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), which are relevant in the subsequent leukocyte firm adhesion step, are also displayed anchored to actin through ezrin-radixin-moesin proteins (ERMs) 23,24 at the apical surface on endothelial cells. Upon leukocyte firm adhesion, the engagement of VCAM-1 and ICAM-1 triggers the reorganization of the endothelial cortical actin cytoskeleton, building up a 3-dimensional docking structure that prevents the detachment of leukocytes by shear stress. 22,23 Here, we show that ICAM-1 and VCAM-1 are included in tetraspanin microdomains that regulate their membrane expression and the efficient adhesive function necessary for proper leukocyte transendothelial migration under flow conditions. Materials and methods Cells and cell culturesHuman umbilical vein endothelial cells (HUVECs) were obtained and cultured as previously described. 25 Cells were used up to the third passage in all assays. To activate HUVECs, tumor necrosis factor-␣ (TNF-␣; 20 ng/mL)(R&D Systems, Minneapolis, MN) was added to the culture media 20 hours before the assays were performed. T lymphoblasts were derived The online vers...

Endoglin is an endothelial homodimeric membrane antigen containing the tripeptide arginine-glycine-aspartic acid (RGD), which is a recognition motif for adhesion receptors of the integrin family. We have investigated the expression of endoglin by monocyte/macrophage cells from different tissue compartments and at different stages of cell differentiation. Although endoglin is absent from peripheral blood monocytes, it is expressed by in vitro differentiated monocytes as determined by flow cytometry using the endoglin-specific monoclonal antibody 44G4 and 8E11. Furthermore, Northern blot analyses revealed a correlation between the presence of endoglin mRNA and the surface expression of the antigen by in vitro differentiated monocytes. Immunostaining of frozen tissue sections with the 8E11 monoclonal antibody demonstrated the presence of endoglin not only in the endothelium of all the tissues studied, but also on the interstitial macrophages present in the red pulp of the spleen. Using as a model of macrophage differentiation monocytic cell lines treated with phorbol esters, we found that the reactivity of the 8E11 monoclonal antibody is greatly increased on U-937 and HL-60 cells during their PMA-induced differentiation. These findings clearly demonstrate for the first time the regulated expression of the putative adhesion molecule endoglin by macrophages.