P-selectin glycoprotein ligand-1 decameric repeats regulate selectin-dependent rolling under flow conditions

Chemokine binding to PSGL-1 is controlled by O-glycosylation and tyrosine sulfation

Protein glycosylation influences cellular recognition and regulates protein interactions, but how glycosylation functions alongside other common posttranslational modifications (PTMs), like tyrosine sulfation (sTyr), is unclear. We produced a library of 53 chemoenzymatically synthesized glycosulfopeptides representing N-terminal domains of human and murine P-selectin glycoprotein ligand-1 (PSGL-1), varying in sTyr and O-glycosylation (structure and site). Using these, we identified key roles of PSGL-1 O-glycosylation and sTyr in controlling interactions with specific chemokines. Results demonstrate that sTyr positively affects CCL19 and CCL21 binding to PSGL-1 N terminus, whereas O-glycan branching and sialylation reduced binding. For murine PSGL-1, interference between PTMs is greater, attributed to proximity between the two PTMs. Using fluorescence polarization, we found sTyr is a positive determinant for some chemokines. We showed that synthetic sulfopeptides are potent in decreasing chemotaxis of human dendritic cells toward CCL19 and CCL21. Our results provide new research avenues into the interplay of PTMs regulating leukocyte/chemokine interactions.

Probing expression of E-selectin using CRISPR-Cas9-mediated tagging with HiBiT in human endothelial cells

E-selectin is expressed on endothelial cells in response to inflammatory cytokines and mediates leukocyte rolling and extravasation. However, studies have been hampered by lack of experimental approaches to monitor expression in real time in living cells. Here, NanoLuc Binary Technology (NanoBiT) in conjunction with CRISPR-Cas9 genome editing was used to tag endogenous E-selectin in human umbilical vein endothelial cells (HUVECs) with the 11 amino acid nanoluciferase fragment HiBiT. Addition of the membrane-impermeable complementary fragment LgBiT allowed detection of cell surface expression. This allowed the effect of inflammatory mediators on E-selectin expression to be monitored in real time in living endothelial cells. NanoBiT combined with CRISPR-Cas9 gene editing allows sensitive monitoring of real-time changes in cell surface expression of E-selectin and offers a powerful tool for future drug discovery efforts aimed at this important inflammatory protein.

Identification of the SHREK Family of Proteins as Broad-Spectrum Host Antiviral Factors

Mucins and mucin-like molecules are highly glycosylated, high-molecular-weight cell surface proteins that possess a semi-rigid and highly extended extracellular domain. P-selectin glycoprotein ligand-1 (PSGL-1), a mucin-like glycoprotein, has recently been found to restrict HIV-1 infectivity through virion incorporation that sterically hinders virus particle attachment to target cells. Here, we report the identification of a family of antiviral cellular proteins, named the Surface-Hinged, Rigidly-Extended Killer (SHREK) family of virion inactivators (PSGL-1, CD43, TIM-1, CD34, PODXL1, PODXL2, CD164, MUC1, MUC4, and TMEM123) that share similar structural characteristics with PSGL-1. We demonstrate that SHREK proteins block HIV-1 infectivity by inhibiting virus particle attachment to target cells. In addition, we demonstrate that SHREK proteins are broad-spectrum host antiviral factors that block the infection of diverse viruses such as influenza A. Furthermore, we demonstrate that a subset of SHREKs also blocks the infectivity of a hybrid alphavirus-SARS-CoV-2 (Ha-CoV-2) pseudovirus. These results suggest that SHREK proteins may be a part of host innate immunity against enveloped viruses.

Identification of the SHREK family of proteins as broad-spectrum host antiviral factors

Mucins and mucin-like molecules are highly glycosylated, high-molecular-weight cell surface proteins that possess a semi-rigid and highly extended extracellular domain. P-selectin glycoprotein ligand-1 (PSGL-1), a mucin-like glycoprotein, has recently been found to restrict HIV-1 infectivity through virion incorporation that sterically hinders virus particle attachment to target cells. Here, we report the identification of a family of antiviral cellular proteins, named the Surface-Hinged, Rigidly-Extended Killer (SHREK) family of virion inactivators (PSGL-1, CD43, TIM-1, CD34, PODXL1, PODXL2, CD164, MUC1, MUC4, and TMEM123), that share similar structural characteristics with PSGL-1. We demonstrate that SHREK proteins block HIV-1 infectivity by inhibiting virus particle attachment to target cells. In addition, we demonstrate that SHREK proteins are broad-spectrum host antiviral factors that block the infection of diverse viruses such as influenza A. Furthermore, we demonstrate that a subset of SHREKs also blocks the infectivity of a hybrid alphavirus-SARS-CoV-2 virus-like particle. These results suggest that SHREK proteins may be a part of host innate immunity against enveloped viruses.

PSGL-1 restricts HIV-1 infectivity by blocking virus particle attachment to target cells

PSGL-1 and CD43 are surface glycoproteins expressed on blood CD4 T cells to bind to selectins for T cell tethering, rolling, and migration into inflamed tissues. The PSGL-1 level is greatly up-regulated during inflammation. Here we found that PSGL-1 and CD43 expression inhibits HIV spreading infection. Mechanistically, PSGL-1 blocks the binding of virus particles to target cells. PSGL-1–mediated suppression of virus infectivity extends to another retrovirus—murine leukemia virus—and to influenza A virus. These results further our understanding of virus–host interactions and help elucidate mechanisms by which cellular host factors regulate viral infection and pathogenesis.

P-selectin glycoprotein ligand-1 (PSGL-1) is a dimeric, mucin-like, 120-kDa glycoprotein that binds to P-, E-, and L-selectins. PSGL-1 is expressed primarily on the surface of lymphoid and myeloid cells and is up-regulated during inflammation to mediate leukocyte tethering and rolling on the surface of endothelium for migration into inflamed tissues. Although it has been reported that PSGL-1 expression inhibits HIV-1 replication, the mechanism of PSGL-1–mediated anti-HIV activity remains to be elucidated. Here we report that PSGL-1 in virions blocks the infectivity of HIV-1 particles by preventing the binding of particles to target cells. This inhibitory activity is independent of the viral glycoprotein present on the virus particle; the binding of particles bearing the HIV-1 envelope glycoprotein or vesicular stomatitis virus G glycoprotein or even lacking a viral glycoprotein is impaired by PSGL-1. Mapping studies show that the extracellular N-terminal domain of PSGL-1 is necessary for its anti–HIV-1 activity, and that the PSGL-1 cytoplasmic tail contributes to inhibition. In addition, we demonstrate that the PSGL-1–related monomeric E-selectin–binding glycoprotein CD43 also effectively blocks HIV-1 infectivity. HIV-1 infection, or expression of either Vpu or Nef, down-regulates PSGL-1 from the cell surface; expression of Vpu appears to be primarily responsible for enabling the virus to partially escape PSGL-1–mediated restriction. Finally, we show that PSGL-1 inhibits the infectivity of other viruses, such as murine leukemia virus and influenza A virus. These findings demonstrate that PSGL-1 is a broad-spectrum antiviral host factor with a unique mechanism of action.

Acute Myeloid and Lymphoblastic Leukemia Cell Interactions with Endothelial Selectins: Critical Role of PSGL-1, CD44 and CD43

Acute myeloid and lymphoblastic leukemia are poor prognosis hematologic malignancies, which disseminate from the bone marrow into the blood. Blast interactions with selectins expressed by vascular endothelium promote the development of drug resistance and leukostasis. While the role of selectins in initiating leukemia blast adhesion is established, our knowledge of the involved selectin ligands is incomplete. Using various primary acute leukemia cells and U937 monoblasts, we identified here functional selectin ligands expressed by myeloblasts and lymphoblasts by performing biochemical studies, expression inhibition by RNA interference and flow adhesion assays on recombinant selectins or selectin ligands immunoadsorbed from primary blast cells. Results demonstrate that P-selectin glycoprotein ligand-1 (PSGL-1) is the major P-selectin ligand on myeloblasts, while it is much less frequently expressed and used by lymphoblasts to interact with endothelial selectins. To roll on E-selectin, myeloblasts use PSGL-1, CD44, and CD43 to various extents and the contribution of these ligands varies strongly among patients. In contrast, the interactions of PSGL-1-deficient lymphoblasts with E-selectin are mainly supported by CD43 and/or CD44. By identifying key selectin ligands expressed by acute leukemia blasts, this study offers novel insight into their involvement in mediating acute leukemia cell adhesion with vascular endothelium and may identify novel therapeutic targets.

The function of P-selectin glycoprotein ligand-1 is conserved from ancestral fishes to mammals

PSGL-1 is a mucin-like glycoprotein that supports, in mammals, leukocyte rolling on selectins. However, we have limited knowledge whether its function is conserved in non-mammals and how its structure adapted during evolution. To identify conserved amino acid sequences required for selectin binding, we performed multiple alignments of PSGL-1 sequences from 18 mammals, 4 birds, 3 reptiles, 1 amphibian, and 15 fishes. The amino-terminal T[D/E]PP[D/E] motif, which identifies in mammals a core-2 O-glycosylated threonine required for selectin-binding, is partially conserved in some fishes (e.g., T. rubripes) and birds (e.g., G. gallus), however, most non-mammals do not display it. The sulfated tyrosine residues of human PSGL-1, which bind L- and P-selectin, are not observed in non-mammals, suggesting that they are dispensable for selectin-binding or that other amino acids play their role. A mucin-like domain is present in all species. Interestingly, the alignment of cytoplasmic sequences of non-mammals reveals the conservation of ezrin/radixin/moesin binding site and two new motifs (M1 and M2). To examine the conservation of PSGL-1 function, we cloned PSGL-1 cDNA sequences of zebrafish and fugu, and established their cross-reactivity with human selectins under flow conditions. Importantly, deleting the well-conserved M1 motif strongly decreased PSGL-1 expression at leukocyte surface and induced retention of the precursor molecule in the endoplasmic reticulum, indicating that M1 motif provides a signal required to export PSGL-1 precursors to the Golgi complex. These data show for the first time the conservation of PSGL-1 function from fishes to mammals and reveal the function of a new motif.

Metabolomic fingerprint in patients at high risk of cardiovascular disease by cocoa intervention

SCOPE

Metabolomics approach is focused on identifying all metabolites present in a biological sample (metabolome). Consumption of cocoa products has been related to health benefits including positive effect on cardiovascular health.

METHODS AND RESULTS

Twenty volunteers were included in this randomized, crossover, and controlled clinical trial. After a 2-wk washout period, subjects received 40 g/day of cocoa powder with 500 mL skimmed milk (cocoa with skimmed milk intervention) or 500 mL/day of skimmed milk (skimmed milk intervention) for 4-wk. Urine (24 h) samples were collected at baseline and after each intervention and were analyzed by HPLC-hybrid quadrupole TOF in negative and positive ionization modes followed by multivariate analysis. This analysis revealed a marked separation between the cocoa with skimmed milk intervention and skimmed milk intervention and baseline periods. Thirty-nine compounds linked with cocoa intake, including alkaloid metabolites, polyphenol host and gut microbial metabolites (hydroxyphenylvalerolactones and hydroxyphenylvaleric acids), diketopiperazines and N-phenylpropenoyl-l-amino acids were identified. In the case of endogenous metabolites, putative identifications suggested that metabolites linked with carnitine metabolism and sulfation of tyrosine were decreased by the consumption of cocoa.

CONCLUSION

LC-MS metabolomics strategy allows the defining of a complex metabolic profile derived from cocoa phytochemicals. Likewise, the identification of endogenous markers could lead to new hypotheses to unravel the relationship between cocoa intake and cardiovascular diseases.

Ezrin-radixin-moesin-binding sequence of PSGL-1 glycoprotein regulates leukocyte rolling on selectins and activation of extracellular signal-regulated kinases

P-selectin glycoprotein ligand-1 (PSGL-1) mediates the capture (tethering) of free-flowing leukocytes and subsequent rolling on selectins. PSGL-1 interactions with endothelial selectins activate Src kinases and spleen tyrosine kinase (Syk), leading to α(L)β(2) integrin-dependent leukocyte slow rolling, which promotes leukocyte recruitment into tissues. In addition, but through a distinct pathway, PSGL-1 engagement activates ERK. Because ezrin, radixin and moesin proteins (ERMs) link PSGL-1 to actin cytoskeleton and because they serve as adaptor molecules between PSGL-1 and Syk, we examined the role of PSGL-1 ERM-binding sequence (EBS) on cell capture, rolling, and signaling through Syk and MAPK pathways. We carried out mutational analysis and observed that deletion of EBS severely reduced 32D leukocyte tethering and rolling on L-, P-, and E-selectin and slightly increased rolling velocity. Alanine substitution of Arg-337 and Lys-338 showed that these residues play a key role in supporting leukocyte tethering and rolling on selectins. Importantly, EBS deletion or Arg-337 and Lys-338 mutations abrogated PSGL-1-induced ERK activation, whereas they did not prevent Syk phosphorylation or E-selectin-induced leukocyte slow rolling. These studies demonstrate that PSGL-1 EBS plays a critical role in recruiting leukocytes on selectins and in activating the MAPK pathway, whereas it is dispensable to phosphorylate Syk and to lead to α(L)β(2)-dependent leukocyte slow rolling.

Mobilization and homing of hematopoietic stem cells

Hematopoietic stem cells (HSC) are a population of precursor cells that posses the capacity for self-renewal and multilineage differentiation. In the bone marrow (BM), HSCs warrant blood cell homeostasis, but at the same time a stable pool of functional cells must be constantly maintained. For this, HSCs constitute a model in which subpopulations of quiescent and active adult stem cells co-exist in the same tissue, in specific microenvironment called stem-cell "niches." These microenvironments keep the stem cells at quiescent (osteoblastic niche) for its self-renewal and activate the stem cells (vascular niche) for proliferation and/or injury repair, maintaining a dynamic balance between self-renewal and differentiation. HSC reside in the bone marrow but can be forces into the blood, a process termed mobilization used clinically to harvest large number of cells for transplantation. At the same time, homing to the BM is necessary to optimize cell engraftment. Here, we summarize current understanding of HSC niche characteristics, and the physiological and pathological mechanisms that guide HSC mobilization both within the BM and to distant niches in the periphery. Mobilization and Homing are mirror process depending on an interplay between chemokines, chemokine receptors, intracellular signaling, adhesion moleculas and proteases. The interaction between SDF-1/CXCL12 and its receptor CXCR4 is critical to retain HSCs within the bone marrow. Current mobilization strategies used in clinic, mainly G-CSF cytokine, are well tolerated but often produce suboptimal number of collected HSCs. Novel agents (AMD3100, stem cell factor, GROßT.) are being developed to enhance the mobilization to modify the signaling into the niche and boost the stem cell harvest, increasing the number of HSCs available for the transplant.

Biomechanics of leukocyte rolling

Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin-ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent 'catch-slip' bond kinetics are explained using the 'two pathway model' for bond dissociation. Both the 'sliding-rebinding' and the 'allosteric' mechanisms attribute 'catch-slip' bond behavior to the force-induced conformational changes in the lectin-EGF domain hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This 'shear-threshold' phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers.

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.

PSGL-1 function in immunity and steady state homeostasis

The substantial importance of P-selectin glycoprotein ligand 1 (PSGL-1) in leukocyte trafficking has continued to emerge beyond its initial identification as a selectin ligand. PSGL-1 seemed to be a relatively simple molecule with an extracellular mucin domain extended as a flexible rod, teleologically consistent with its primary role in tethering leukocytes to endothelial selectins. The rolling interaction between leukocyte and endothelium mediated by this selectin-PSGL-1 interaction requires branched O-glycan extensions on specific PSGL-1 amino acid residues. In some cells, such as neutrophils, the glycosyltransferases involved in formation of the O-glycans are constitutively expressed, while in other cells, such as T cells, they are expressed only after appropriate activation. Thus, PSGL-1 supports leukocyte recruitment in both innate and adaptive arms of the immune response. A complex array of amino acids within the selectins engage multiple sugar residues of the branched O-glycans on PSGL-1 and provide the molecular interactions responsible for the velcro-like catch bonds that support leukocyte rolling. Such binding of PSGL-1 can also induce signaling events that influence cell phenotype and function. Scrutiny of PSGL-1 has revealed a better understanding of how it performs as a selectin ligand and yielded unexpected insights that extend its scope from supporting leukocyte rolling in inflammatory settings to homeostasis including stem cell homing to the thymus and mature T-cell homing to secondary lymphoid organs. PSGL-1 has been found to bind homeostatic chemokines CCL19 and CCL21 and to support the chemotactic response to these chemokines. Surprisingly, the O-glycan modifications of PSGL-1 that support rolling mediated by selectins in inflammatory conditions interfere with PSGL-1 binding to homeostatic chemokines and thereby limit responsiveness to the chemotactic cues used in steady state T-cell traffic. The multi-level influence of PSGL-1 on cell traffic in both inflammatory and steady state settings is therefore substantially determined by the orchestrated addition of O-glycans. However, central as specific O-glycosylation is to PSGL-1 function, in vivo regulation of PSGL-1 glycosylation in T cells remains poorly understood. It is our purpose herein to review what is known, and not known, of PSGL-1 glycosylation and to update understanding of PSGL-1 functional scope.

New insights into leukocyte recruitment by intravital microscopy

Leukocyte recruitment to sites of inflammation requires adhesion to and transmigration through the blood vessel wall. Recent progress in optical equipment and new genetic and molecular tools have revealed additional steps in the leukocyte adhesion cascade beyond rolling, adhesion, and transmigration. In vivo studies using intravital microscopy (IVM) were essential for the discovery of slow rolling, postadhesion strengthening, intraluminal crawling, and different routes of transmigration. IVM revealed unique features of leukocyte recruitment in different organs. This review focuses on insights into the leukocyte adhesion cascade gained by IVM.