Endoglycan, a member of the CD34 family, functions as an L-selectin ligand through modification with tyrosine sulfation and sialyl Lewis x

Total Synthesis of a PSGL-1 Glycopeptide Analogue for Targeted Inhibition of P-Selectin

The high affinity interaction between P-selectin glycoprotein ligand-1 (PSGL-1) and P-selectin is mediated by a multimotif glycosulfopeptide (GSP) recognition domain consisting of clustered tyrosine sulfates and a Core 2 O-glycan terminated with sialyl LewisX (C2-O-sLeX). These distinct GSP motifs are much more common than previously appreciated within a wide variety of functionally important domains involved in protein-protein interactions. However, despite the potential of GSPs to serve as tools for fundamental studies and prospects for drug discovery, their utility has been limited by the absence of chemical schemes for synthesis on scale. Herein, we report the total synthesis of GSnP-6, an analogue of the N-terminal domain of PSGL-1, and potent inhibitor of P-selectin. An efficient, scalable, hydrogenolysis-free synthesis of C2-O-sLeX-Thr-COOH was identified by both convergent and orthogonal one-pot assembly, which afforded this crucial building block, ready for direct use in solid phase peptide synthesis (SPPS). C2-O-sLeX-Thr-COOH was synthesized in 10 steps with an overall yield of 23% from the 4-O,5-N oxazolidinone thiosialoside donor. This synthesis represents an 80-fold improvement in reaction yield as compared to prior reports, achieving the first gram scale synthesis of SPPS ready C2-O-sLeX-Thr-COOH and enabling the scalable synthesis of GSnP-6 for preclinical evaluation. Significantly, we established that GSnP-6 displays dose-dependent inhibition of venous thrombosis in vivo and inhibits vaso-occlusive events in a human sickle cell disease equivalent microvasculature-on-a-chip system. The insights gained in formulating this design strategy can be broadly applied to the synthesis of a wide variety of biologically important oligosaccharides and O-glycan bearing glycopeptides.

CD34-Structure, Functions and Relationship with Cancer Stem Cells

The CD34 protein was identified almost four decades ago as a biomarker for hematopoietic stem cell progenitors. CD34 expression of these stem cells has been exploited for therapeutic purposes in various hematological disorders. In the last few decades, studies have revealed the presence of CD34 expression on other types of cells with non-hematopoietic origins, such as interstitial cells, endothelial cells, fibrocytes, and muscle satellite cells. Furthermore, CD34 expression may also be found on a variety of cancer stem cells. Nowadays, the molecular functions of this protein have been involved in a variety of cellular functions, such as enhancing proliferation and blocking cell differentiation, enhanced lymphocyte adhesion, and cell morphogenesis. Although a complete understanding of this transmembrane protein, including its developmental origins, its stem cell connections, and other functions, is yet to be achieved. In this paper, we aimed to carry out a systematic analysis of the structure, functions, and relationship with cancer stem cells of CD34 based on the literature overview.

Site-specific glycosylation of proteoglycans: a revisited frontier in proteoglycan research

Proteoglycans (PGs), a class of carbohydrate-modified proteins, are present in essentially all metazoan organisms investigated to date. PGs are composed of glycosaminoglycan (GAG) chains attached to various core proteins and are important for embryogenesis and normal homeostasis. PGs exert many of their functions via their GAG chains and understanding the details of GAG-ligand interactions has been an essential part of PG research. Although PGs are also involved in many diseases, the number of GAG-related drugs used in the clinic is yet very limited, indicating a lack of detailed structure-function understanding. Structural analysis of PGs has traditionally been obtained by first separating the GAG chains from the core proteins, after which the two components are analyzed separately. While this strategy greatly facilitates the analysis, it precludes site-specific information and introduces either a "GAG" or a "core protein" perspective on the data interpretation. Mass-spectrometric (MS) glycoproteomic approaches have recently been introduced, providing site-specific information on PGs. Such methods have revealed a previously unknown structural complexity of the GAG linkage regions and resulted in identification of several novel CSPGs and HSPGs in humans and in model organisms, thereby expanding our view on PG complexity. In light of these findings, we discuss here if the use of such MS-based techniques, in combination with various functional assays, can also be used to expand our functional understanding of PGs. We have also summarized the site-specific information of all human PGs known to date, providing a theoretical framework for future studies on site-specific functional analysis of PGs in human pathophysiology.

Endoglycan Regulates Purkinje Cell Migration by Balancing Cell-Cell Adhesion

The importance of cell adhesion molecules for the development of the nervous system has been recognized many decades ago. Functional in vitro and in vivo studies demonstrated a role of cell adhesion molecules in cell migration, axon growth and guidance, as well as synaptogenesis. Clearly, cell adhesion molecules have to be more than static glue making cells stick together. During axon guidance, cell adhesion molecules have been shown to act as pathway selectors but also as a means to prevent axons going astray by bundling or fasciculating axons. We identified Endoglycan as a negative regulator of cell-cell adhesion during commissural axon guidance across the midline. The presence of Endoglycan allowed commissural growth cones to smoothly navigate the floor-plate area. In the absence of Endoglycan, axons failed to exit the floor plate and turn rostrally. These observations are in line with the idea of Endoglycan acting as a lubricant, as its presence was important, but it did not matter whether Endoglycan was provided by the growth cone or the floor-plate cells. Here, we expand on these observations by demonstrating a role of Endoglycan during cell migration. In the developing cerebellum, Endoglycan was expressed by Purkinje cells during their migration from the ventricular zone to the periphery. In the absence of Endoglycan, Purkinje cells failed to migrate and, as a consequence, cerebellar morphology was strongly affected. Cerebellar folds failed to form and grow, consistent with earlier observations on a role of Purkinje cells as Shh deliverers to trigger granule cell proliferation.

Endoglycan (PODXL2) is proteolytically processed by ADAM10 (a disintegrin and metalloprotease 10) and controls neurite branching in primary neurons

Cell adhesion is tightly controlled in multicellular organisms, for example, through proteolytic ectodomain shedding of the adhesion-mediating cell surface transmembrane proteins. In the brain, shedding of cell adhesion proteins is required for nervous system development and function, but the shedding of only a few adhesion proteins has been studied in detail in the mammalian brain. One such adhesion protein is the transmembrane protein endoglycan (PODXL2), which belongs to the CD34-family of highly glycosylated sialomucins. Here, we demonstrate that endoglycan is broadly expressed in the developing mouse brains and is proteolytically shed in vitro in mouse neurons and in vivo in mouse brains. Endoglycan shedding in primary neurons was mediated by the transmembrane protease a disintegrin and metalloprotease 10 (ADAM10), but not by its homolog ADAM17. Functionally, endoglycan deficiency reduced the branching of neurites extending from primary neurons in vitro, whereas deletion of ADAM10 had the opposite effect and increased neurite branching. Taken together, our study discovers a function for endoglycan in neurite branching, establishes endoglycan as an ADAM10 substrate and suggests that ADAM10 cleavage of endoglycan may contribute to neurite branching.

ITGB6-Knockout Suppresses Cholangiocarcinoma Cell Migration and Invasion with Declining PODXL2 Expression

Intrahepatic cholangiocarcinoma (iCCA) is a heterogeneous bile duct cancer with a poor prognosis. Integrin αvβ6 (β6) has been shown to be upregulated in iCCA and is associated with its subclassification and clinicopathological features. In the present study, two ITGB6-knockout HuCCT1 CCA cell lines (ITGB6-ko cells) were established using the clustered regulatory interspaced short palindromic repeats (CRISPR), an associated nuclease 9 (Cas9) system, and single-cell cloning. RNA sequencing analysis, real-time polymerase chain reaction (PCR), and immunofluorescent methods were applied to explore possible downstream factors. ITGB6-ko cells showed significantly decreased expression of integrin β6 on flow cytometric analysis. Both cell lines exhibited significant inhibition of cell migration and invasion, decreased wound-healing capability, decreased colony formation ability, and cell cycle dysregulation. RNA sequencing and real-time PCR analysis revealed a remarkable decrease in podocalyxin-like protein 2 (PODXL2) expression in ITGB6-ko cells. Colocalization of PODXL2 and integrin β6 was also observed. S100 calcium-binding protein P and mucin 1, which are associated with CCA subclassification, were downregulated in ITGB6-ko cells. These results describe the successful generation of ITGB6-ko CCA cell clones with decreased migration and invasion and downregulation of PODXL2, suggesting the utility of integrin β6 as a possible therapeutic target or diagnostic marker candidate.

A sticky wicket: Defining molecular functions for CD34 in hematopoietic cells

The CD34 cell surface antigen is widely expressed in tissues on cells with progenitor-like properties and on mature vascular endothelia. In adult human bone marrow, CD34 marks hematopoietic stem and progenitor cells (HSPCs) starting from the bulk of hematopoietic stem cells with long-term repopulating potential (LT-HSCs) throughout expansion and differentiation of oligopotent and unipotent progenitors. CD34 protein surface expression is typically lost as cells mature into terminal effectors. Because of this expression pattern of HSPCs, CD34 has had a central role in the evaluation or selection of donor graft tissue in HSC transplant (HSCT). Given its clinical importance, it is surprising that the biological functions of CD34 are still poorly understood. This enigma is due, in part, to CD34's context-specific role as both a pro-adhesive and anti-adhesive molecule and its potential functional redundancy with other sialomucins. Moreover, there are also critical differences in the regulation of CD34 expression on HSPCs in humans and experimental mice. In this review, we highlight some of the more well-defined functions of CD34 in HSPCs with a focus on proposed functions most relevant to HSCT biology.

PODXL2 maintains cellular stemness and promotes breast cancer development through the Rac1/Akt pathway

The cluster of differentiation 34 (CD34) family, which includes CD34, podocalyxin-like protein 1 (PODXL), and PODXL2, are type-I transmembrane sialomucins and markers of hematopoietic stem cells (HSCs) and vascular-associated tissues. CD34 family proteins are expressed by endothelial cells and hematopoietic precursors. PODXL is well known to be associated with invadopodia formation and to promote the epithelial-mesenchymal transition, tumor migration and invasion. PODXL expression was correlated with poor survival of cancer patients. However, the role of PODXL2 in cancer has been less fully explored. To reveal the novel role of PODXL2 in breast cancer, the present study evaluated PODXL2 levels in relation to clinical outcomes of cancer patients by performing a bioinformatics analysis using the Oncomine database, Kaplan-Meier plots, and the CCLE database. Empirical validation of bioinformatics predictions was conducted utilizing the short hairpin (sh)-RNA silencing method for PODXL2 in the BT474 invasive ductal breast carcinoma cell line. The bioinformatics analysis revealed that PODXL2 overexpression was correlated with poor survival of breast cancer patients, suggesting an oncogenic role of PODXL2 in breast carcinoma. In a validation experiment, knockdown of PODXL2 in BT474 cells slightly influenced cell proliferation, suppressed migration, and inhibited expressions of downstream molecules, including Ras-related C3 botulinum toxin substrate 1 (Rac1), phosphorylated (p)-Akt (S473), and p-paxillin (Y31) proteins. In addition, knockdown of PODXL2 reduced expression levels of cancer stem cell (CSC) markers, including Oct-4 and Nanog, and the breast CSC marker aldehyde dehydrogenase 1 (ALDH1). Collectively, our present study demonstrated that PODXL2 plays a crucial role in cancer development and could serve as a potential prognostic biomarker in breast cancer patients.

A plasma proteogenomic signature for fibromuscular dysplasia

AIMS

Fibromuscular dysplasia (FMD) is a poorly understood disease that predominantly affects women during middle-life, with features that include stenosis, aneurysm, and dissection of medium-large arteries. Recently, plasma proteomics has emerged as an important means to understand cardiovascular diseases. Our objectives were: (i) to characterize plasma proteins and determine if any exhibit differential abundance in FMD subjects vs. matched healthy controls and (ii) to leverage these protein data to conduct systems analyses to provide biologic insights on FMD, and explore if this could be developed into a blood-based FMD test.

METHODS AND RESULTS

Females with 'multifocal' FMD and matched healthy controls underwent clinical phenotyping, dermal biopsy, and blood draw. Using dual-capture proximity extension assay and nuclear magnetic resonance-spectroscopy, we evaluated plasma levels of 981 proteins and 31 lipid sub-classes, respectively. In a discovery cohort (Ncases = 90, Ncontrols = 100), we identified 105 proteins and 16 lipid sub-classes (predominantly triglycerides and fatty acids) with differential plasma abundance in FMD cases vs. controls. In an independent cohort (Ncases = 23, Ncontrols = 28), we successfully validated 37 plasma proteins and 10 lipid sub-classes with differential abundance. Among these, 5/37 proteins exhibited genetic control and Bayesian analyses identified 3 of these as potential upstream drivers of FMD. In a 3rd cohort (Ncases = 506, Ncontrols = 876) the genetic locus of one of these upstream disease drivers, CD2-associated protein (CD2AP), was independently validated as being associated with risk of having FMD (odds ratios  = 1.36; P = 0.0003). Immune-fluorescence staining identified that CD2AP is expressed by the endothelium of medium-large arteries. Finally, machine learning trained on the discovery cohort was used to develop a test for FMD. When independently applied to the validation cohort, the test showed a c-statistic of 0.73 and sensitivity of 78.3%.

CONCLUSION

FMD exhibits a plasma proteogenomic and lipid signature that includes potential causative disease drivers, and which holds promise for developing a blood-based test for this disease.

CD34+ Stem Cells: Promising Roles in Cardiac Repair and Regeneration

Cell therapy has received significant attention as a novel therapeutic approach to restore cardiac function after injury. CD34-positive (CD34⁺) stem cells have been investigated for their ability to promote angiogenesis and contribute to the prevention of remodelling after infarct. However, there are significant differences between murine and human CD34⁺ cells; understanding these differences might benefit the therapeutic use of these cells. Herein we discuss the function of the CD34 cell and highlight the similarities and differences between murine and human CD34 cell function, which might explain some of the differences between the animal and human evolutions. We also summarize the studies that report the application of murine and human CD34⁺ cells in preclinical studies and clinical trials and current limitations with the application of cell therapy for cardiac repair. Finally, to overcome these limitations we discuss the application of novel humanized rodent models that can bridge the gap between preclinical and clinical studies as well as rejuvenation strategies for improving the quality of old CD34⁺ cells for future clinical trials of autologous cell transplantation.

Glycopeptide analogues of PSGL-1 inhibit P-selectin in vitro and in vivo

Blockade of P-selectin/PSGL-1 interactions holds significant potential for treatment of disorders of innate immunity, thrombosis, and cancer. Current inhibitors remain limited due to low binding affinity or by the recognized disadvantages inherent to chronic administration of antibody therapeutics. Here we report an efficient approach for generating glycosulfopeptide mimics of N-terminal PSGL-1 through development of a stereoselective route for multi-gram scale synthesis of the C2 O-glycan building block and replacement of hydrolytically labile tyrosine sulfates with isosteric sulfonate analogs. Library screening afforded a compound of exceptional stability, GSnP-6, that binds to human P-selectin with nanomolar affinity (K_d ~ 22 nM). Molecular dynamics simulation defines the origin of this affinity in terms of a number of critical structural contributions. GSnP-6 potently blocks P-selectin/PSGL-1 interactions in vitro and in vivo and represents a promising candidate for the treatment of diseases driven by acute and chronic inflammation.

Podocalyxin-like 1 promotes invadopodia formation and metastasis through activation of Rac1/Cdc42/cortactin signaling in breast cancer cells

Metastatic disease is the leading cause of cancer mortality. Identifying biomarkers and regulatory mechanisms is important toward developing diagnostic and therapeutic tools against metastatic cancer. In this study, we demonstrated that podocalyxin-like 1 (PODXL) is overexpressed in breast tumor cells and increased in lymph node metastatic cancer. Mechanistically, we found that the expression of PODXL was associated with cell motility and invasiveness. Suppression of PODXL in MDA-MB-231 cells reduced lamellipodia formation and focal adhesion kinase (FAK) and paxillin phosphorylation. PODXL knockdown reduced the formation of invadopodia, such as inhibiting the colocalization of F-actin with cortactin and suppressing phosphorylation of cortactin and neural Wiskott-Aldrich syndrome protein. Conversely, overexpression of PODXL in MCF7 cells induced F-actin/cortactin colocalization and enhanced invadopodia formation and activation. Invadopodia activity and tumor invasion in PODXL-knockdown cells are similar to that in cortactin-knockdown cells. We further found that the DTHL motif in PODXL is crucial for regulating cortactin phosphorylation and Rac1/Cdc42 activation. Inhibition of Rac1/Cdc42 impeded PODXL-mediated cortactin activation and FAK and paxillin phosphorylation. Moreover, inhibition of PODXL in MDA-MB-231 cells significantly suppressed tumor colonization in the lungs and distant metastases, similar to those in cortactin-knockdown cells. These findings show that overexpression of PODXL enhanced invadopodia formation and tumor metastasis by inducing Rac1/Cdc42/cortactin signaling network.

Glycophenotypic alterations induced by Pteridium aquilinum in mice gastric mucosa: synergistic effect with Helicobacter pylori infection

The bracken fern Pteridium aquilinum is a plant known to be carcinogenic to animals. Epidemiological studies have shown an association between bracken fern exposure and gastric cancer development in humans. The biological effects of exposure to this plant within the gastric carcinogenesis process are not fully understood. In the present work, effects in the gastric mucosa of mice treated with Pteridium aquilinum were evaluated, as well as molecular mechanisms underlying the synergistic role with Helicobacter pylori infection. Our results showed that exposure to Pteridium aquilinum induces histomorphological modifications including increased expression of acidic glycoconjugates in the gastric mucosa. The transcriptome analysis of gastric mucosa showed that upon exposure to Pteridium aquilinum several glycosyltransferase genes were differently expressed, including Galntl4, C1galt1 and St3gal2, that are mainly involved in the biosynthesis of simple mucin-type carbohydrate antigens. Concomitant treatment with Pteridium aquilinum and infection with Helicobacter pylori also resulted in differently expressed glycosyltransferase genes underlying the biosynthesis of terminal sialylated Lewis antigens, including Sialyl-Lewis^x. These results disclose the molecular basis for the altered pattern of glycan structures observed in the mice gastric mucosa. The gene transcription alterations and the induced glycophenotypic changes observed in the gastric mucosa contribute for the understanding of the molecular mechanisms underlying the role of Pteridium aquilinum in the gastric carcinogenesis process.

CD44 and HCELL: preventing hematogenous metastasis at step 1

Despite great strides in our knowledge of the genetic and epigenetic changes underlying malignancy, we have limited information on the molecular basis of metastasis. Over 90% of cancer deaths are caused by spread of tumor cells from a primary site to distant organs and tissues, highlighting the pressing need to define the molecular effectors of cancer metastasis. Mounting evidence suggests that circulating tumor cells (CTCs) home to specific tissues by hijacking the normal leukocyte trafficking mechanisms. Cancer cells characteristically express CD44, and there is increasing evidence that hematopoietic cell E-/L-selectin ligand (HCELL), a sialofucosylated glycoform of CD44, serves as the major selectin ligand on cancer cells, allowing interaction of tumor cells with endothelium, leukocytes, and platelets. Here, we review the structural biology of CD44 and of HCELL, and present current data on the function of these molecules in mediating organ-specific homing/metastasis of CTCs.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

Molecular stiffness of selectins

During inflammation, selectin-ligand interactions provide forces for circulating leukocytes to adhere to vascular surfaces, which stretch the interacting molecules, suggesting that mechanical properties may be pertinent to their biological function. From mechanical measurements with atomic force microscopy, we analyzed the molecular characteristics of selectins complexed with ligands and antibodies. Respective stiffness of L-, E-, and P-selectins (4.2, 1.4, and 0.85 piconewton/nm) correlated inversely with the number (2, 6, and 9) of consensus repeats in the selectin structures that acted as springs in series to dominate their compliance. After reconstitution into a lipid bilayer, purified membrane P-selectin remained a dimer, capable of forming dimeric bonds with P-selectin glycoprotein ligand (PSGL)-1, endoglycan-Ig, and a dimeric form of a glycosulfopeptide modeled after the N terminus of PSGL-1. By comparison, purified membrane L- and E-selectin formed only monomeric bonds under identical conditions. Ligands and antibodies were much less stretchable than selectins. The length of endoglycan-Ig was found to be 51 ± 12 nm. These results provide a comprehensive characterization of the molecular stiffness of selectins and illustrate how mechanical measurements can be utilized for molecular analysis, e.g. evaluating the multimericity of selectins and determining the molecular length of endoglycan.

Human L-selectin preferentially binds synthetic glycosulfopeptides modeled after endoglycan and containing tyrosine sulfate residues and sialyl Lewis x in core 2 O-glycans

Endoglycan is a mucin-like glycoprotein expressed by endothelial cells and some leukocytes and is recognized by L-selectin, a C-type lectin important in leukocyte trafficking and extravasation during inflammation. Here, we show that recombinant L-selectin and human T lymphocytes expressing L-selectin bind to synthetic glycosulfopeptides (GSPs). These synthetic glycosulfopeptides contain 37 amino acid residues modeled after the N-terminus of human endoglycan and contain one or two tyrosine sulfates (TyrSO(3)) along with a nearby core-2-based Thr-linked O-glycan with sialyl Lewis x (C2-SLe(x)). TyrSO(3) at position Y118 was more critical for binding than at Y97. C2-SLe(x) at T124 was required for L-selectin recognition. Interestingly, under similar conditions, neither L-selectin nor T lymphocytes showed appreciable binding to the sulfated carbohydrate epitope 6-sulfo-SLe(x). P-selectin also bound to endoglycan-based GSPs but with lower affinity than toward GSPs modeled after PSGL-1, the physiological ligand for P- and L-selectin that is expressed on leukocytes. These results demonstrate that TyrSO(3) residues in association with a C2-SLe(x) moiety within endoglycan and PSGL-1 are preferentially recognized by L-selectin.

Cancer cells in transit: the vascular interactions of tumor cells

Metastasis is a highly regulated, multistep process in which cancerous cells shed from the primary tumor and enter the circulatory system, where they interact extensively with host cells before they lodge and colonize the target organ. The adhesive interactions of circulating tumor cells with platelets, leukocytes, and endothelial cells facilitate their survival and extravasation from the vasculature, thus representing critical kick-off events for the colonization of distant organs. This review presents our current mechanistic knowledge on vascular interactions of tumor cells, and it discusses biochemical and cell and molecular biology techniques used for the identification of novel receptor-ligand pairs mediating these interactions. This review brings together diverse observations about the contributions of key molecular constituents, including selectins, fibrin(ogen), and CD44, in one mechanistic interpretation. Understanding the molecular underpinnings of adhesive interactions between tumor cells and host cells may provide guidelines for developing promising antimetastatic therapies when initiated early in the course of disease progression.

Novel functions of the CD34 family

For almost 30 years, the cell-surface protein CD34 has been widely used as a marker to assist in the identification and Summary isolation of hematopoietic stem cells (HSCs) and progenitors in preparation for bone-marrow transplantation. In addition, it has increasingly been used as a marker to help identify other tissue-specific stem cells, including muscle satellite cells and epidermal precursors. Despite its utility as a stem-cell marker, however, the function of CD34 has remained remarkably elusive. This is probably because: (1) it is subject to a range of tissue-specific post-transcriptional and post-translational modifications that are expected to alter its function dramatically; (2) the simple interpretation of CD34 gain- and loss-of-function experiments has been confounded by the overlapping expression of the two recently discovered CD34-related proteins podocalyxin and endoglycan; and (3) there has been a glaring lack of robust in vitro and in vivo functional assays that permit the structural and functional analysis of CD34 and its relatives. Here, we provide a brief review of the domain structure, genomic organization, and tissue distribution of the CD34 family. We also describe recent insights from gain- and loss-of-function experiments and improved assays, which are elucidating a fascinating role for these molecules in cell morphogenesis and migration.

Agent-based model of therapeutic adipose-derived stromal cell trafficking during ischemia predicts ability to roll on P-selectin

Intravenous delivery of human adipose-derived stromal cells (hASCs) is a promising option for the treatment of ischemia. After delivery, hASCs that reside and persist in the injured extravascular space have been shown to aid recovery of tissue perfusion and function, although low rates of incorporation currently limit the safety and efficacy of these therapies. We submit that a better understanding of the trafficking of therapeutic hASCs through the microcirculation is needed to address this and that selective control over their homing (organ- and injury-specific) may be possible by targeting bottlenecks in the homing process. This process, however, is incredibly complex, which merited the use of computational techniques to speed the rate of discovery. We developed a multicell agent-based model (ABM) of hASC trafficking during acute skeletal muscle ischemia, based on over 150 literature-based rules instituted in Netlogo and MatLab software programs. In silico, trafficking phenomena within cell populations emerged as a result of the dynamic interactions between adhesion molecule expression, chemokine secretion, integrin affinity states, hemodynamics and microvascular network architectures. As verification, the model reasonably reproduced key aspects of ischemia and trafficking behavior including increases in wall shear stress, upregulation of key cellular adhesion molecules expressed on injured endothelium, increased secretion of inflammatory chemokines and cytokines, quantified levels of monocyte extravasation in selectin knockouts, and circulating monocyte rolling distances. Successful ABM verification prompted us to conduct a series of systematic knockouts in silico aimed at identifying the most critical parameters mediating hASC trafficking. Simulations predicted the necessity of an unknown selectin-binding molecule to achieve hASC extravasation, in addition to any rolling behavior mediated by hASC surface expression of CD15s, CD34, CD62e, CD62p, or CD65. In vitro experiments confirmed this prediction; a subpopulation of hASCs slowly rolled on immobilized P-selectin at speeds as low as 2 µm/s. Thus, our work led to a fundamentally new understanding of hASC biology, which may have important therapeutic implications.

Ischemic pathologies, such as acute myocardial infarction and peripheral vascular disease, continue to be associated with high morbidities and mortalities. Recently, therapies wherein adult stem cells are injected into the circulation have been shown to increase blood flow and help to restore tissue function following injury. Pre-clinical animal models and human trials have shown successes utilizing this approach, but variable trafficking efficiencies and low incorporation of cells into the injured tissue severely limit effectiveness and may preclude clinical adoption. To address this, we sought to study the complex process of how injected stem cells traffic through the microcirculation and home to sites of injury, in an effort to identify bottlenecks in this process that could be manipulated for therapeutic gain. We developed an agent-based computer model to speed the rate of discovery, and we identified a key cell–cell adhesion interaction that could be targeted to enhance stem cell homing efficiencies during injectable stem cell therapies.

A common African polymorphism abolishes tyrosine sulfation of human anionic trypsinogen (PRSS2)

Human pancreatic trypsinogens undergo post-translational sulfation on Tyr(154), catalysed by the Golgi-resident enzyme tyrosylprotein sulfotransferase 2. Sequence alignments suggest that the sulfation of Tyr(154) is facilitated by a unique sequence context which is characteristically found in primate trypsinogens. In the search for genetic variants that might alter this sulfation motif, we identified a single nucleotide polymorphism (c.457G>C) in the PRSS2 (serine protease 2, human anionic trypsinogen) gene, which changed Asp(153) to a histidine residue (p.D153H). The p.D153H variant is common in subjects of African origin, with a minor allele frequency of 9.2%, whereas it is absent in subjects of European descent. We demonstrate that Asp(153) is the main determinant of tyrosine sulfation in anionic trypsinogen, as both the natural p.D153H variation and the p.D153N mutation result in a complete loss of trypsinogen sulfation. In contrast, mutation of Asp(156) and Glu(157) only slightly decrease tyrosine sulfation, whereas mutation of Gly(151) and Pro(155) has no effect. With respect to the biological relevance of the p.D153H variant, we found that tyrosine sulfation had no significant effect on the activation of anionic trypsinogen or the catalytic activity and inhibitor sensitivity of anionic trypsin. Taken together with previous studies, the observations of the present study suggest that the primary role of trypsinogen sulfation in humans is to stimulate autoactivation of PRSS1 (serine protease 1, human cationic trypsinogen), whereas the sulfation of anionic trypsinogen is unimportant for normal digestive physiology. As a result, the p.D153H polymorphism which eliminates this modification could become widespread in a healthy population.

Podocalyxin selectively marks erythroid-committed progenitors during anemic stress but is dispensable for efficient recovery

OBJECTIVE

Podocalyxin expression on Ter119(+) erythroblasts is induced following administration of erythropoietin (Epo) or phenylhydrazine treatment, but is notably absent on committed erythroid progenitors during homeostatic red cell turnover. Following high-dose Epo administration in vivo, podocalyxin surface expression is upregulated, in part, via a signal transducers and activators of transcription 5-dependent pathway and this expression has been postulated to play a role in the release of reticulocytes from hematopoietic organs into the periphery under conditions of increased erythropoietic rate. Here we have thoroughly addressed this hypothesis and further examined the expression profile of podocalyxin during Epo-induced erythroblast expansion and stress erythropoiesis.

MATERIALS AND METHODS

Following Epo induction, progenitor cells were sorted to characterize podocalyxin expression during stress. In addition, as podocalyxin-deficient mice die perinatally, we used chimeric mice reconstituted with wild-type or podocalyxin-deficient hematopoietic cells to analyze differences in response to high dose Epo administration and chemically induced anemia.

RESULTS

Podocalyxin surface expression is rapidly upregulated in response to stress and marks early erythroid progenitors and erythroblasts. Despite loss of podocalyxin, chimeras exhibit normal basal erythropoiesis and no differences in erythroid progenitor proportions in the spleen and marrow in response to Epo. Further, podocalyxin is dispensable for efficient recovery from models of anemia.

CONCLUSIONS

We demonstrate that podocalyxin is a highly specific marker of stress-induced blast-forming unit erythroid and colony-forming unit erythroid progenitors in mouse bone marrow and spleen. In addition, our findings suggest that podocalyxin is not necessary for efficient erythroblast expansion, erythroid differentiation, or reticulocyte release in response to Epo stimulation in vivo.

Endoglycan, a member of the CD34 family of sialomucins, is a ligand for the vascular selectins

The interactions of the selectin family of adhesion molecules with their ligands are essential for the initial rolling stage of leukocyte trafficking. Under inflammatory conditions, the vascular selectins, E- and P-selectin, are expressed on activated vessels and interact with carbohydrate-based ligands on the leukocyte surface. While several ligands have been characterized on human T cells, monocytes and neutrophils, there is limited information concerning ligands on B cells. Endoglycan (EG) together with CD34 and podocalyxin comprise the CD34 family of sialomucins. We found that EG, previously implicated as an L-selectin ligand on endothelial cells, was present on human B cells, T cells and peripheral blood monocytes. Upon activation of B cells, EG increased with a concurrent decrease in PSGL-1. Expression of EG on T cells remained constant under the same conditions. We further found that native EG from several sources (a B cell line, a monocyte line and human tonsils) was reactive with HECA-452, a mAb that recognizes sialyl Lewis X and related structures. Moreover, immunopurified EG from these sources was able to bind to P-selectin and where tested E-selectin. This interaction was divalent cation-dependent and required sialylation of EG. Finally, an EG construct supported slow rolling of E- and P-selectin bearing cells in a sialic acid and fucose dependent manner, and the introduction of intact EG into a B cell line facilitated rolling interactions on a P-selectin substratum. These in vitro findings indicate that EG can function as a ligand for the vascular selectins.

No detectable endothelial- or leukocyte-derived L-selectin ligand activity on the endothelium in inflamed cremaster muscle venules

L-selectin is important in mediating leukocyte recruitment in inflammation. The role of L-selectin was for long believed to be influenced by an inducible endothelial ligand; however, L-selectin ligand activity was recently shown to be mediated by leukocytic P-selectin glycoprotein ligand 1 (PSGL-1). Still, it is unknown whether PSGL-1 is deposited on the endothelium or whether leukocyte fragments or leukocytic uropods are presented on the venular surface. Moreover, it is unclear whether ligands for L-selectin other than PSGL-1 are present in inflammation. Overall, this has complicated understanding of the mechanisms that guide recruitment of inflammatory cells. Here, I used intravital microscopy on mouse cremaster muscle venules to show that L-selectin influences leukocyte rolling in inflammation exclusively by mediating L-selectin/PSGL-1-dependent, secondary capture to rolling and adherent leukocytes. I show that leukocyte primary capture in inflammation is mediated almost entirely by P-selectin, whereas the capacity of E-selectin to mediate capture appears to be minimal. In parallel, primary capture remaining after function inhibition of P-selectin is not decreased by blockage or absence of L-selectin. Rolling along the endothelium in venules following a number of inflammatory treatments was abolished by simultaneous blockage of P-selectin, E-selectin, and VCAM-1, indicating that there is no additional adhesive pathway involving L-selectin or any other molecule that can mediate leukocyte rolling in inflamed cremaster muscle venules in response to the used stimuli. Moreover, in vivo staining failed to detect any L-selectin ligand activity on the endothelium. These data demonstrate that expression of L-selectin on leukocytes is insufficient for mediating rolling and efficient recruitment of leukocytes in inflammation.

CD34 facilitates the development of allergic asthma

Asthma is a pulmonary inflammatory disease dependent on eosinophil and mast cell infiltration into the lung. CD34 is a sialomucin expressed by both of these cell types, and we have used CD34(-/-) mice and a standard mouse model of asthma to evaluate the importance of CD34 expression on disease development. In comparison with wild-type (wt) mice, CD34(-/-) mice exhibited a dramatic reduction in all hallmarks of allergic asthma, including lowered airway inflammatory cell infiltration, airway hyperresponsiveness, and mast-cell recruitment. Bone marrow transplantation experiments confirmed that these defects are due to CD34 expression by bone marrow-derived cells. This was not, however, due to an inability to respond to antigen as, on a per cell basis, wt and CD34(-/-) inflammatory cells exhibit identical responses in cytokine production. We found a striking reduction in mobility of CD34(-/-) eosinophils in vitro, the major component of inflammatory infiltrates, which was consistent with proposed models for CD34 as an inhibitor of cell-cell adhesion. In summary, our data suggest that CD34 enhances mast-cell and eosinophil invasiveness and that its expression by these cells is a prerequisite for development of allergic asthma.

Critical functions of N-glycans in L-selectin-mediated lymphocyte homing and recruitment

Lymphocyte homing is mediated by specific interaction between L-selectin on lymphocytes and the carbohydrate ligand 6-sulfo sialyl Lewis X on high endothelial venules. Here we generated mice lacking both core 1 extension and core 2 branching enzymes to assess the functions of O-glycan-borne L-selectin ligands in vivo. Mutant mice maintained robust lymphocyte homing, yet they lacked O-glycan L-selectin ligands. Biochemical analyses identified a class of N-glycans bearing the 6-sulfo sialyl Lewis X L-selectin ligand in high endothelial venules. These N-glycans supported the binding of L-selectin to high endothelial venules in vitro and contributed in vivo to O-glycan-independent lymphocyte homing in wild-type and mutant mice. Our results demonstrate the critical function of N-glycan-linked 6-sulfo sialyl Lewis X in L-selectin-dependent lymphocyte homing and recruitment.

Sulfated L-selectin ligands as a therapeutic target in chronic inflammation

The homing of lymphocytes to peripheral lymph nodes is initiated by an adhesive interaction between L-selectin on lymphocytes and peripheral node addressin (PNAd), a set of sialomucins displayed on high endothelial venules (HEVs) of lymph nodes. The monoclonal antibody MECA-79 reacts with the PNAd sialomucins by recognizing an N-acetylglucosamine (GlcNAc)-6-sulfated oligosaccharide, which overlaps with sialyl 6-sulfo Lewis X, the L-selectin recognition determinant. Two HEV-expressed sulfotransferases, GlcNAc6ST-1 and GlcNAc6ST-2, are essential for the expression of the MECA-79 epitope and L-selectin ligand activity on lymph-node HEVs. PNAd, as defined by MECA-79 staining, is also expressed on activated blood vessels at several sites of chronic inflammation. Recent evidence indicates that the same two sulfotransferases underlie the formation of functional PNAd at these sites. Experiments in a sheep model of asthma demonstrate that a chronic inflammatory disease can be ameliorated by targeting PNAd.

Beyond mere markers: functions for CD34 family of sialomucins in hematopoiesis

CD34, podocalyxin, and endoglycan are members of a family of single-pass transmembrane proteins that show distinct expression on early hematopoietic precursors and vascular-associated tissue. In spite of the fact that the expression of CD34 on these early progenitors has been known for over 20 yr and used clinically in hematopoietic stem cell transplantation for more than 15 yr, little is known about its exact role or function. More recently, CD34 expression has been shown to distinguish activated early progenitors from quiescent cells. With the subsequent identification of podocalyxin and endoglycan as related family members also expressed on early progenitor cells, attention is slowly shifting toward understanding how these molecules might contribute to progenitor function and behavior. In this review we examine the existing evidence and propose testable models to reveal the importance of these molecules for stem and progenitor cell function.

Beyond mere markers: functions for CD34 family of sialomucins in hematopoiesis

CD34, podocalyxin, and endoglycan are members of a family of single-pass transmembrane proteins that show distinct expression on early hematopoietic precursors and vascular-associated tissue. In spite of the fact that the expression of CD34 on these early progenitors has been known for over 20 yr and used clinically in hematopoietic stem cell transplantation for more than 15 yr, little is known about its exact role or function. More recently, CD34 expression has been shown to distinguish activated early progenitors from quiescent cells. With the subsequent identification of podocalyxin and endoglycan as related family members also expressed on early progenitor cells, attention is slowly shifting toward understanding how these molecules might contribute to progenitor function and behavior. In this review we examine the existing evidence and propose testable models to reveal the importance of these molecules for stem and progenitor cell function.

Binding of human serum amyloid P componentto L-selectin

Serum concentrations of soluble L-selectin by far exceed those of other soluble adhesion molecules, and serum soluble L-selectin concentrations are remarkably stable upon prolonged storage. We present evidence for Ca(2+)-dependent binding interactions between human serum amyloid P (SAP), a proteolysis-resistant pentraxin glycoprotein, and L-selectin, as shown by surface plasmon resonance measurements, protein band shift assays in a native PAGE system, and after SDS-PAGE and membrane transfer. Monoclonal antibodies to L-selectin strongly reduced binding of biotinylated SAP to L-selectin-IgG chimeras immobilized on microtiter plates. As binding was reduced by prior glycopeptidase F treatment of L-selectin but not of SAP, it appears to be based on SAP lectin domain interactions with N-linked L-selectin carbohydrates. In freshly prepared human lymphocytes, SAP incubation induced expression of a beta2 integrin neoepitope associated with high-affinity binding. This was partially blocked by pre-incubation with Fab fragments of two anti-L-selectin antibodies. In flow chamber experiments, SAP inhibited the adherence of human neutrophils to activated endothelium under shear stress. Thus, SAP binds to human L-selectin and affects L-selectin-dependent leukocyte-endothelial interactions.

N-acetylglucosamine-6-O-sulfotransferases 1 and 2 cooperatively control lymphocyte homing through L-selectin ligand biosynthesis in high endothelial venules

Lymphocyte homing is mediated by specific interactions between L-selectin on lymphocytes and sulfated carbohydrates restricted to high endothelial venules in lymph nodes. Here we generated mice deficient in both N-acetylglucosamine-6-O-sulfotransferase 1 (GlcNAc6ST-1) and GlcNAc6ST-2 and found that mutant mice had approximately 75% less homing of lymphocytes to the peripheral lymph nodes than did wild-type mice. Consequently, these mice had lower contact hypersensitivity responses than those of wild-type mice. Carbohydrate structural analysis showed that 6-sulfo sialyl Lewis X, a dominant ligand for L-selectin, was almost completely absent from the high endothelial venules of these mutant mice, whereas the amount of unsulfated sialyl Lewis X was much greater. These results demonstrate the essential function of GlcNAc6ST-1 and GlcNAc6ST-2 in L-selectin ligand biosynthesis in high endothelial venules and their importance in immune surveillance.

Identifying new candidate genes for hereditary facial paresis on chromosome 3q21–q22 by RNA in situ hybridization in mouse

Hereditary congenital facial paresis (HCFP) belongs to the family of congenital cranial dysinnervation disorders and is characterized by an isolated dysfunction of the facial nerve (nVII). While genetic defects have been identified for several members of this disease family, genes underlying congenital facial paresis and Möbius syndrome remain to be discovered. Here we focus on HCFP linked to chromosome 3q21-q22 and identify new candidate genes using expression analysis by means of RNA in situ hybridization during mouse embryogenesis. We selected 28 positional candidates and identified 17 genes with undetectable expression levels during mouse development, ubiquitous expression, or expression in tissues not affected in HCFP. Additionally, 7 genes were excluded by direct sequence or reverse transcription-PCR analysis. The remaining 4 genes (Klf15, Flj40083, Kiaa0779, and Podxl2) were found to be expressed at spatial and temporal positions during mouse development that correlate with HCFP regions in humans, defining these genes as primary candidates in HCFP.

Epitope recognition of antibodies that define the sialomucin, endolyn (CD164), a negative regulator of haematopoiesis

Endolyn (CD164) is a sialomucin that functions as an adhesion molecule and a negative regulator of CD34+ CD38- human haematopoietic precursor cell proliferation. The 105A5 and 103B2/9E10 CD164 monoclonal antibodies (mAbs), which act as surrogate ligands, recognize distinct glycosylation-dependent classes I and II epitopes located on domain I of the native and recombinant CD164 proteins. Here, we document five new CD164 mAbs, the 96 series, that rely on conformational integrity, but not glycosylation, of exons 2- and 3-encoded CD164 domains, thereby resembling the class III mAbs, N6B6 and 67D2. Although all the 96 series class III mAbs labelled both the 105A5+ and 103B2/9E10+ cells, cross-competition and immunoblotting studies allow them to be categorized into two distinct class III subgroups, i.e. the N6B6-like subgroup that only recognizes 80-100 kDa proteins and the 67D2-like subgroup that also recognizes a higher molecular weight (>220 kDa) form. To more closely define the reactivity patterns of mAbs to the classes I and II epitopes, the global glycosylation patterns of the soluble human (h) CD164 proteins were determined using lectin binding, high-performance liquid chromatography (HPLC) and mass spectrometry. hCD164 recombinant proteins bound to the lectins, Galanthus nivalis agglutinin, Datura stramonium agglutinin, Sambucus nigra agglutinin, Maackia amurensis agglutinin and peanut agglutinin, indicating the presence of high mannose and complex N-glycans, in addition to core 1 O-glycans (the Tn antigen) and alpha2-3 and alpha2-6 sialic acid moieties. Our HPLC and mass spectrometry results revealed both high mannose and complex N-glycosylation with various numbers of branches increasing the complexity of the glycosylation pattern. Most O-glycans were small, core 1 or 2 based. High levels of sialylation in alpha2-3 and alpha2-6 linkages, without sialyl-Lewis X, indicate that the majority of these hCD164 recombinant proteins are unable to bind to selectins in our assay system, but may interact with Siglec molecules.

Spatial and temporal sequence of events in cell adhesion: from molecular recognition to focal adhesion assembly

A new concept that attributes a pivotal role to the pericellular coat in the regulation of the early stages of cell adhesion is presented. Quick, adaptable, and transient adhesion through multiple cooperative weak interactions provides the cell with an additional level of modulation in the decision-making process that precedes the commitment to adhesion at a particular site. Hyaluronan emerges as a modulator of cell adhesion in certain cells, mediating binding or repulsion through its polyelectrolyte character, in addition to its chirality and molecular-recognition properties. The biophysical properties of hyaluronan as well as its ultrastructural organization are analyzed in relation to this proposed function.

A small-molecule switch for Golgi sulfotransferases

The study of glycan function is a major frontier in biology that could benefit from small molecules capable of perturbing carbohydrate structures on cells. The widespread role of sulfotransferases in modulating glycan function makes them prime targets for small-molecule modulators. Here, we report a system for conditional activation of Golgi-resident sulfotransferases using a chemical inducer of dimerization. Our approach capitalizes on two features shared by these enzymes: their requirement of Golgi localization for activity on cellular substrates and the modularity of their catalytic and localization domains. Fusion of these domains to the proteins FRB and FKBP enabled their induced assembly by the natural product rapamycin. We applied this strategy to the GlcNAc-6-sulfotransferases GlcNAc6ST-1 and GlcNAc6ST-2, which collaborate in the sulfation of L-selectin ligands. Both the activity and specificity of the inducible enzymes were indistinguishable from their WT counterparts. We further generated rapamycin-inducible chimeric enzymes comprising the localization domain of a sulfotransferase and the catalytic domain of a glycosyltransferase, demonstrating the generality of the system among other Golgi enzymes. The approach provides a means for studying sulfate-dependent processes in cellular systems and, potentially, in vivo.

Ligands for L-selectin: homing, inflammation, and beyond

Understanding the molecular basis of lymphocyte homing to lymphoid organs was originally a problem of concern only to immunologists. With the discovery of L-selectin and its ligands, interested scientists have expanded to include glycobiologists, immunopathologists, cancer biologists, and developmental biologists. Going beyond its first discovered role in homing to lymph nodes, the L-selectin system is implicated in such diverse processes as inflammatory leukocyte trafficking in both acute and chronic settings, hematogenous metastasis of carcinoma cells, effector mechanisms for inflammatory demyelination of axons, and implantation of the early mammalian embryo. This review focuses on the ligands for L-selectin that are found on vascular endothelium, leukocytes, carcinoma cells, and at various extravascular sites. The discovery of selectins and their ligands has validated the long-predicted hypothesis that carbohydrate-directed cell adhesion is relevant in eukaryotic systems. Emphasis will be given to the carbohydrate and sulfation modifications of the ligands, which enable recognition by L-selectin. The rapid "homing" of labeled cells into the lymph nodes presumably had its basis in the special affinity of small lymphocytes for the endothelium of the postcapillary venules.

Glycolipid-mediated cell-cell recognition in inflammation and nerve regeneration

Cell surface complex carbohydrates have emerged as key recognition molecules, mediating physiological interactions between cells. Typically, glycans on one cell surface are engaged by complementary carbohydrate binding proteins (lectins) on an apposing cell, initiating appropriate cellular responses. Although many cell surface lectins have been identified in vertebrates, only a few of their endogenous carbohydrate ligands have been established. Each major class of cell surface glycans-glycoproteins, glycolipids, and proteoglycans-has been implicated as physiologically relevant lectin ligands. The current minireview focuses on findings that implicate glycosphingolipids as especially important molecules in cell-cell recognition in two different systems: the recognition of human leukocytes by E-selectin on the vascular endothelium during inflammation and the recognition of nerve cell axons by myelin-associated glycoprotein in myelin-axon stabilization and the regulation of axon regeneration.

Endomucin, a sialomucin expressed in high endothelial venules, supports L-selectin-mediated rolling

Lymphocyte homing to lymph nodes is regulated by transient but specific interactions between lymphocytes and high endothelial venules (HEVs), the initial phase of which is mainly governed by the leukocyte adhesion molecule L-selectin, which recognizes sulfated and sialylated O-linked oligosaccharides displayed on sialomucin core proteins. One of the sialomucin proteins, endomucin, is predominantly expressed in vascular endothelial cells of a variety of tissues including the HEVs of lymph nodes; however, whether it functions as a ligand for L-selectin remains to be formally proven. Here we show that the endomucin splice isoform a is predominantly expressed in PNAd+ HEVs and MAdCAM-1+ HEVs, as seen in non-HEV-type vascular endothelial cells. Using affinity purification with soluble L-selectin, we found that HEV endomucin is specifically modified with L-selectin-reactive oligosaccharides and can bind L-selectin as well as an HEV-specific mAb, MECA-79. Our results also indicated that a 90-100 kDa endomucin species is preferentially decorated with L-selectin-reactive sugar chains, whereas an 80 kDa species represents conventional forms expressed in non-HEV-type vascular endothelial cells in lymph nodes. Furthermore, a CHO cell line expressing endomucin together with a specific combination of carbohydrate-modifying enzymes [core-2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT), alpha-1,3-fucosyltransferase VII (FucTVII) and L-selectin ligand sulfotransferase (LSST)] showed L-selectin-dependent rolling under flow conditions in vitro. These results suggest that when endomucin is appropriately modified by a specific set of glycosyltransferases and a sulfotransferase, it can function as a ligand for L-selectin, and that the endomucin expressed in HEVs may represent another sialomucin ligand for L-selectin.

Molecular Determinants Controlling Homeostatic Recirculation and Tissue-Specific Trafficking of Lymphocytes

The homeostasis of the immune system is maintained by the recirculation of naïve lymphocytes through the secondary lymphoid tissues, such as the lymph nodes, Peyer's patches, and spleen. Upon insult by pathogens or antigens, lymphocytes become activated, and the regulated trafficking of these cells results in the integration of systemic and regional immune responses. The exquisite specificity of such lymphocyte trafficking is determined by tissue-specific guidance signals expressed by the endothelial cells of postcapillary venules, combined with counterreceptors expressed by the circulating lymphocytes. The high endothelial venules can selectively guide naïve lymphocytes into the lymph nodes and Peyer's patches by expressing a unique combination of vascular addressins, lymphocyte-specific chemokines, and chemokine-binding molecules. The inflamed postcapillary venules in extralymphoid tissues, such as the skin and intestinal lamina propria, also use a distinct array of endothelial adhesion molecules and tissue- selective chemokines, and support the recruitment of effector and memory lymphocytes that express the appropriate receptors for tissue-specific trafficking. In this review, we summarize the present understanding of the homeostatic recirculation of naïve lymphocytes through the secondary lymphoid tissues and the specific targeting of antigen-experienced lymphocytes into the effector sites. We also revisit some previous studies that reported apparently conflicting observations.

Mechanisms of glucocorticoid-induced down-regulation of neutrophil L-selectin in cattle: evidence for effects at the gene-expression level and primarily on blood neutrophils

One anti-inflammatory action of glucocorticoids is down-regulation of surface L-selectin on circulating neutrophils. However, it is unclear if this is a result of release of affected bone marrow neutrophils or if the steroid has direct effects on L-selectin expression in existing blood neutrophils. We recently demonstrated that circulating neutrophils from cattle with high blood concentrations of endogenous glucocorticoid had reduced L-selectin mRNA, suggesting that the steroid interrupted L-selectin gene expression. In the current study, dexamethasone (DEX) was administered to cattle in vivo, and blood and bone marrow neutrophils were studied simultaneously within the animal to determine which pool of cells responds to glucocorticoids with inhibited L-selectin expression. Purified blood neutrophils were also treated with DEX +/- RU486 in vitro, and glucocorticoid effects on L-selectin expression were determined. Our results indicate that glucocorticoid-induced suppression of L-selectin, which accompanies neutrophilia, is likely mediated by direct effects of glucocorticoid receptor activation on intracellular reservoirs of L-selectin mRNA and protein in cattle, predominantly in blood neutrophils.

Core 2 Branching β1,6-N-Acetylglucosaminyltransferase and High Endothelial Venule-restricted Sulfotransferase Collaboratively Control Lymphocyte Homing

L-selectin mediates lymphocyte homing by facilitating lymphocyte adhesion to carbohydrate ligands expressed on high endothelial venules (HEV) of the secondary lymphoid organs. Previous studies demonstrated that L-selectin ligand sulfotransferase (LSST) forms 6-sulfo sialyl Lewis x (sLe(x)) on both core 2 branch and MECA-79-positive extended core 1 O-glycans, but the chemical nature and roles of HEV ligands elaborated by LSST and core 2 beta1,6-N-acetylglucosaminyltransferase-1 (Core2GlcNAcT) have been undefined. In the present study, we have generated mutant mice with deficient LSST and show that inactivation of LSST gene alone leads to only partial impairment of lymphocyte homing to peripheral lymph nodes and moderate reduction in lymphocyte counts in the peripheral lymph nodes, despite the fact that L-selectin ligands that contain 6-sulfo sLe(x) are reduced at HEV. By contrast, LSST/Core2GlcNAcT double null mice exhibited a markedly reduced lymphocyte homing and reduced lymphocyte counts as a result of significantly decreased 6-sulfo sLe(x) on HEV L-selectin counterreceptors, relative to LSST- or Core2GlcNAcT-single null mice. Moreover, induction of LSST and Core2GlcNAcT transcripts was observed in HEV-like structure formed in the salivary gland of the non-obese diabetic mouse, which displays chronic inflammation. These results indicate that LSST and Core2GlcNAcT cooperatively synthesize HEV-specific L-selectin ligands required for lymphocyte homing and suggest that LSST and Core2GlcNAcT play a critical role in lymphocyte trafficking during chronic inflammation.

Inhibition of P-selectin-mediated cell adhesion by a sulfated derivative of sialic acid

P-selectin, a carbohydrate-binding cell adhesion molecule expressed on activated endothelial cells and platelets, plays a key role in the recruitment of leukocytes to inflammatory and hemorrhagic sites. It simultaneously recognizes a sialic acid-containing carbohydrate chain and the sulfated tyrosine residues of a specific counter-receptor expressed on the leukocyte surface. We examined the inhibitory effects of a synthetic sulfated derivative of sialic acid (NMSO3) on P-selectin-mediated cell adhesion and found the following: (1) P-selectin/IgG chimera bound to immobilized NMSO3. (2) The binding of P-selectin/IgG chimera to purified P-selectin glycoprotein ligand-1 was inhibited by soluble NMSO3. (3) The adhesion of HL60 cells to P-selectin-expressing CHO cells was inhibited by NMSO3. (4) NMSO3 inhibited P-selectin-induced tumor necrosis factor-alpha production in monocytes and activated platelet-induced generation of reactive oxygen species in neutrophils. In conclusion, NMSO3 acts as a specific inhibitor for P-selectin-mediated cell adhesion and for adhesion-dependent leukocyte activation.

Low force decelerates L-selectin dissociation from P-selectin glycoprotein ligand-1 and endoglycan

Selectin-ligand interactions mediate the tethering and rolling of circulating leukocytes on vascular surfaces during inflammation and immune surveillance. To support rolling, these interactions are thought to have rapid off-rates that increase slowly as wall shear stress increases. However, the increase of off-rate with force, an intuitive characteristic named slip bonds, is at odds with a shear threshold requirement for selectin-mediated cell rolling. As shear drops below the threshold, fewer cells roll and those that do roll less stably and with higher velocity. We recently demonstrated a low force regime where the off-rate of P-selectin interacting with P-selectin glycoprotein ligand-1 (PSGL-1) decreased with increasing force. This counter-intuitive characteristic, named catch bonds, might partially explain the shear threshold phenomenon. Because L-selectin-mediated cell rolling exhibits a much more pronounced shear threshold, we used atomic force microscopy and flow chamber experiments to determine off-rates of L-selectin interacting with their physiological ligands and with an antibody. Catch bonds were observed at low forces for L-selectin-PSGL-1 interactions coinciding with the shear threshold range, whereas slip bonds were observed at higher forces. These catch-slip transitional bonds were also observed for L-selectin interacting with endoglycan, a newly identified PSGL-1-like ligand. By contrast, only slip bonds were observed for L-selectin-antibody interactions. These findings suggest that catch bonds contribute to the shear threshold for rolling and are a common characteristic of selectin-ligand interactions.