Residues throughout the cytoplasmic domain affect the internalization efficiency of P-selectin

Endothelial signaling by neutrophil-released oncostatin M enhances P-selectin-dependent inflammation and thrombosis

In the earliest phase of inflammation, histamine and other agonists rapidly mobilize P-selectin to the apical membranes of endothelial cells, where it initiates rolling adhesion of flowing neutrophils. Clustering of P-selectin in clathrin-coated pits facilitates rolling. Inflammatory cytokines typically signal by regulating gene transcription over a period of hours. We found that neutrophils rolling on P-selectin secreted the cytokine oncostatin M (OSM). The released OSM triggered signals through glycoprotein 130 (gp130)-containing receptors on endothelial cells that, within minutes, further clustered P-selectin and markedly enhanced its adhesive function. Antibodies to OSM or gp130, deletion of the gene encoding OSM in hematopoietic cells, or conditional deletion of the gene encoding gp130 in endothelial cells inhibited neutrophil rolling on P-selectin in trauma-stimulated venules of the mouse cremaster muscle. In a mouse model of P-selectin-dependent deep vein thrombosis, deletion of OSM in hematopoietic cells or of gp130 in endothelial cells markedly inhibited adhesion of neutrophils and monocytes and the rate and extent of thrombus formation. Our results reveal a paracrine-signaling mechanism by which neutrophil-released OSM rapidly influences endothelial cell function during physiological and pathological inflammation.

The Interaction of Selectins and PSGL-1 as a Key Component in Thrombus Formation and Cancer Progression

Cellular interaction is inevitable in the pathomechanism of human disease. Formation of heterotypic cellular aggregates, between distinct cells of hematopoietic and nonhematopoietic origin, may be involved in events leading to inflammation and the complex process of cancer progression. Among adhesion receptors, the family of selectins with their ligands have been considered as one of the major contributors to cell-cell interactions. Consequently, the inhibition of the interplay between selectins and their ligands may have potential therapeutic benefits. In this review, we focus on the current evidence on the selectins as crucial modulators of inflammatory, thrombotic, and malignant disorders. Knowing that there is promiscuity in selectin binding, we outline the importance of a key protein that serves as a ligand for all selectins. This dimeric mucin, the P-selectin glycoprotein ligand 1 (PSGL-1), has emerged as a major player in inflammation, thrombus, and cancer development. We discuss the interaction of PSGL-1 with various selectins in physiological and pathological processes with particular emphasis on mechanisms that lead to severe disease.

Circulating soluble P-selectin must dimerize to promote inflammation and coagulation in mice

Leukocyte adhesion to P-selectin on activated platelets and endothelial cells induces shedding of the P-selectin ectodomain into the circulation. Plasma soluble P-selectin (sP-selectin) is elevated threefold to fourfold in patients with cardiovascular disease. Circulating sP-selectin is thought to trigger signaling in leukocytes that directly contributes to inflammation and thrombosis. However, sP-selectin likely circulates as a monomer, and in vitro studies suggest that sP-selectin must dimerize to induce signaling in leukocytes. To address this discrepancy, we expressed the entire ectodomain of mouse P-selectin as a monomer (sP-selectin) or as a disulfide-linked dimer fused to the Fc portion of mouse immunoglobulin G (sP-selectin-Fc). Dimeric sP-selectin-Fc, but not monomeric sP-selectin, triggered integrin-dependent adhesion of mouse leukocytes in vitro. Antibody-induced oligomerization of sP-selectin or sP-selectin-Fc was required to trigger formation of neutrophil extracellular traps. Injecting sP-selectin-Fc, but not sP-selectin, into mice augmented integrin-dependent adhesion of neutrophils in venules, generated tissue factor-bearing microparticles, shortened plasma-clotting times, and increased thrombus frequency in the inferior vena cava. Furthermore, transgenic mice that overexpressed monomeric sP-selectin did not exhibit increased inflammation or thrombosis. We conclude that elevated plasma sP-selectin is a consequence rather than a cause of cardiovascular disease.

Expression of E- and P-selectin in Tumor Necrosis Factor—induced Dermatitis in Dogs

Adhesion molecules on endothelial cells play an important role in leukocyte recruitment in several inflammatory processes. Vascular selectins mediate the initial adhesion of leukocytes to the blood vessel wall during their extravasation into inflamed tissues, and in vitro studies in dogs have shown that selectin expression can be induced by cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1 (IL-1). The objective of this study was to determine whether vascular selectins are induced by cytokines in vivo in a cutaneous model of inflammation in dogs. Skin biopsies were collected from nine dogs at various time points after an intradermal injection of TNF-α (10 ng/site) or phosphate-buffered saline containing 0.1% bovine serum albumin, and immunohistochemistry was performed using anti-P-selectin (MD3) and anti-E-selectin (CL37) monoclonal antibodies. In all animals, TNF-α induced an inflammatory reaction that was maximal at 12 hours and then decreased by 24 and 48 hours. Control skin displayed no expression of E- and P-selectin, whereas TNF-α induced the expression of P-selectin and E-selectin on dermal vessels that was highest at 12 hours and 3 hours, respectively ( P < 0.05). Numerous platelet aggregates recognized by the anti-P-selectin antibody were present in the lumina of vessels and in perivascular tissues. These results demonstrate that TNF-α can induce the expression of P- and E-selectin in vivo in dog skin and suggest that these selectins are involved in leukocyte recruitment in canine dermatitis.

Differential regulation of human and murine P-selectin expression and function in vivo

Basal and inducible expression of human P-selectin in transgenic mice differs from that of murine P-selectin, resulting in distinct functions.

Leukocytes roll on P-selectin after its mobilization from secretory granules to the surfaces of platelets and endothelial cells. Tumor necrosis factor (TNF), IL-1β, and lipopolysaccharide increase synthesis of P-selectin in murine but not in human endothelial cells. To explore the physiological significance of this difference in gene regulation, we made transgenic mice bearing the human Selp gene and crossed them with mice lacking murine P-selectin (Selp^−/−). The transgenic mice constitutively expressed human P-selectin in platelets, endothelial cells, and macrophages. P-selectin mediated comparable neutrophil migration into the inflamed peritoneum of transgenic and wild-type (WT) mice. Leukocytes rolled similarly on human or murine P-selectin on activated murine platelets and in venules of the cremaster muscle subjected to trauma. However, TNF increased murine P-selectin in venules, slowing rolling and increasing adhesion, whereas it decreased human P-selectin, accelerating rolling and decreasing adhesion. Both P- and E-selectin mediated basal rolling in the skin of WT mice, but E-selectin dominated rolling in transgenic mice. During contact hypersensitivity, murine P-selectin messenger (m) RNA was up-regulated and P-selectin was essential for leukocyte recruitment. However, human P-selectin mRNA was down-regulated and P-selectin contributed much less to leukocyte recruitment. These findings reveal functionally significant differences in basal and inducible expression of human and murine P-selectin in vivo.

Rolling cell adhesion

Rolling adhesion on vascular surfaces is the first step in recruiting circulating leukocytes, hematopoietic progenitors, or platelets to specific organs or to sites of infection or injury. Rolling requires the rapid yet balanced formation and dissociation of adhesive bonds in the challenging environment of blood flow. This review explores how structurally distinct adhesion receptors interact through mechanically regulated kinetics with their ligands to meet these challenges. Remarkably, increasing force applied to adhesive bonds first prolongs their lifetimes (catch bonds) and then shortens their lifetimes (slip bonds). Catch bonds mediate the counterintuitive phenomenon of flow-enhanced rolling adhesion. Force-regulated disruptions of receptor interdomain or intradomain interactions remote from the ligand-binding surface generate catch bonds. Adhesion receptor dimerization, clustering in membrane domains, and interactions with the cytoskeleton modulate the forces applied to bonds. Both inside-out and outside-in cell signals regulate these processes.

The essential role of clathrin-mediated endocytosis in the infectious entry of human enterovirus 71

Little is currently known about the infectious entry process of human enterovirus 71 (HEV71) into host cells, which may represent potential anti-viral targeting sites. In this study a targeted small-interfering RNA (siRNA) screening platform assay was established and validated to identify and profile key cellular genes involved in processes of endocytosis, cytoskeletal dynamics, and endosomal trafficking essential for HEV71 infection. Screen evaluation was conducted via the expression of well characterized dominant-negative mutants, bioimaging studies (double-labeled immunofluorescence assays, transmission electron microscopy analysis), secondary siRNA-based dosage dependence studies, and drug inhibition assays. The infectious entry of HEV71 into rhabdomyosarcoma cells was shown to be significantly inhibited by siRNAs targeting genes associated with clathrin-mediated endocytosis (CME) that include AP2A1, ARRB1, CLTC, CLTCL1, SYNJ1, ARPC5, PAK1, ROCK1, and WASF1. The functional role of CME was verified by the observation of strong co-localization between HEV71 particles and clathrin as well as dose-dependent inhibition of HEV71 infection upon siRNA knockdown of CME-associated genes. HEV71 entry by CME was further confirmed via inhibition by dominant-negative EPS15 mutants and treatment of CME drug inhibitors, with more than 80% inhibition observed at 20 μm chlorpromazine. Furthermore, HEV71 infection was shown to be sensitive to the disruption of human genes in regulating early to late endosomal trafficking as well as endosomal acidic pH. The identification of clathrin-mediated endocytosis as the entry pathway for HEV71 infection of susceptible host cells contributes to a better understanding of HEV71 pathogenesis and enables future development of anti-viral strategies against HEV71 infection.

Numb3 is an endocytosis adaptor for the inflammatory marker P-selectin

The endocytic protein Numb3 was found to bind to the cytosolic tail of the leukocyte adhesion receptor P-selectin. The N-terminal phosphotyrosine-binding (PTB) domain of Numb3 is responsible for this activity. An alanine scan revealed the FTNAAFD sequence as recognition region in P-selectin. Structural modeling of the interaction between the Numb PTB domain and the P-selectin tail suggests that both phenylalanines within the recognition sequence fit into hydrophobic cavities of the PTB surface. Their exchange for alanine gave Numb-negative mutants detaining the inhibition of P-selectin endocytosis by Numb PTB overexpression. Cells stable expressing P-selectins internalized the negative mutants markedly slower than the wild type. Consistent with other reports on the phosphorylation of Numb, we found that only the dephospho-Numb is able to bind P-selectin. Our observations demonstrate that Numb3 is an endocytic receptor for P-selectin and may be responsible for the rapid internalization of P-selectin when endothelial activation ends.

Soluble but not platelet P-selectin correlates with spontaneous platelet aggregation: a pilot study

BACKGROUND

P-selectin (PS) is a marker of platelet activation measured on the platelet surface as platelet PS (pPS) or in serum as soluble PS (sPS). Controversy remains over the exact relationship between sPS, pPS, and other markers such as spontaneous platelet aggregation (SPA).

OBJECTIVE

To investigate correlations between pPS, sPS, and SPA in patients with peripheral arterial disease.

METHODS

SPA, pPS, and sPS levels were measured in venous blood sampled from patients following intermittent claudication (n = 18) or an acute stroke (n = 18).

RESULTS

SPA and sPS correlated significantly in the claudicants (Pearson correlation coefficient, r = 0.661; P = .0020) and stroke patients (r = 0.514; P = .020). No significant correlation was identified between pPS and SPA, or sPS and pPS.

CONCLUSIONS

The 2 methods of assessing PS are not comparable. Although pPS is accepted as a platelet activation marker, sPS may be a better indicator of aggregation represented by SPA.

Clustering endothelial E-selectin in clathrin-coated pits and lipid rafts enhances leukocyte adhesion under flow

During inflammation, E-selectin expressed on cytokine-activated endothelial cells mediates leukocyte rolling under flow. E-selectin undergoes endocytosis and may associate with lipid rafts. We asked whether distribution of E-selectin in membrane domains affects its functions. E-selectin was internalized in transfected CHO cells or cytokine-activated human umbilical vein endothelial cells (HUVECs). Confocal microscopy demonstrated colocalization of E-selectin with alpha-adaptin, a clathrin-associated protein. Deleting the cytoplasmic domain of E-selectin or disrupting clathrin-coated pits with hypertonic medium blocked internalization of E-selectin, reduced colocalization of E-selectin with alpha-adaptin, and inhibited E-selectin-mediated neutrophil rolling under flow. Unlike CHO cells, HUVECs expressed a small percentage of E-selectin in lipid rafts. Even fewer neutrophils rolled on E-selectin in HUVECs treated with hypertonic medium and with methyl-beta-cyclodextrin, which disrupts lipid rafts. These data demonstrate that E-selectin clusters in both clathrin-coated pits and lipid rafts of endothelial cells but is internalized in clathrin-coated pits. Distribution in both domains markedly enhances E-selectin's ability to mediate leukocyte rolling under flow.

In silico determination of intracellular glycosylation and phosphorylation sites in human selectins: implications for biological function

Post-translational modifications provide the proteins with the possibility to perform functions in addition to those determined by their primary sequence. However, analysis of multifunctional protein structures in the environment of cells and body fluids is made especially difficult by the presence of other interacting proteins. Bioinformatics tools are therefore helpful to predict protein multifunctionality through the identification of serine and threonine residues wherein the hydroxyl group is likely to become modified by phosphorylation or glycosylation. Moreover, serines and threonines where both modifications are likely to occur can also be predicted (YinYang sites), to suggest further functional versatility. Structural modifications of hydroxyl groups of P-, E-, and L-selectins have been predicted and possible functions resulting from such modifications are proposed. Functional changes of the three selectins are based on the assumption that transitory and reversible protein modifications by phosphate and O-GlcNAc cause specific conformational changes and generate binding sites for other proteins. The computer-assisted prediction of glycosylation and phosphorylation sites in selectins should be helpful to assess the contribution of dynamic protein modifications in selectin-mediated inflammatory responses and cell-cell adhesion processes that are difficult to determine experimentally.

Functions of sorting nexin 17 domains and recognition motif for P-selectin trafficking

SNX17 is a member of the sorting nexin family (SNX), a group of hydrophilic proteins whose common characteristic property is a phox homology (PX) domain. The PX domain directs SNXs to phosphatidylinositides containing membranes of the endosomal compartment, where the SNXs are involved in the sorting of transmembrane proteins. SNX17 is known to interact with P-selectin and the LDL receptor family. Here, we report that the PX domain of SNX17 specifically binds to phosphatidylinositol 3-phosphate-containing membranes. The functional part of SNX17 that binds P-selectin or Patched (PTCH) consists of a truncated FERM domain and a unique C terminus together (FC-unit). In a yeast two-hybrid analysis a putative recognition motif for the FC-unit was revealed within P-selectin as FxNaa(F/Y). When HepG2 cells overexpress P-selectin together with SNX17, SNX17 changes its distribution from early endosomes to lysobisphosphatidic acid-containing late endosomes. Furthermore, overexpressed SNX17 restrains P-selectin in the outer membrane of the late endosomal compartment, thus preventing the normal lysosomal accumulation of P-selectin. These results suggest that the PX domain is necessary for the intracellular localisation, while the FC-unit is required for cargo recognition. We hypothesise that the expression level of SNX17 may regulate the lysosomal degradation, at least for P-selectin, by suppressing its entry into the inner vesicles of the multi-vesicular bodies (MVBs).

Molecular Characterization of Equine P-Selectin (CD62P) and Its Regulation in Ovarian Follicles During the Ovulatory Process1

Ovulation is accompanied by a marked infiltration of leukocytes into thecal layers after the gonadotropin surge. P-selectin is known to play a critical role in the initial steps of leukocyte recruitment from the bloodstream during inflammation. Thus, the objective was to investigate the potential regulation of P-selectin by gonadotropins in equine preovulatory follicles. The full-length equine P-selectin cDNA was cloned by a combination of reverse transcription-polymerase chain reaction (RT-PCR) and 5'- and 3'-rapid amplification of cDNA ends. Results showed that equine P-selectin cDNA encodes an 829-amino acid protein that is highly conserved when compared to the human protein (80% identity). Semiquantitative RT-PCR/Southern blot analyses were performed to study the regulation of P-selectin transcript in preovulatory follicles isolated during estrus at 0, 12, 24, 30, 33, 36, and 39 h after an ovulatory dose of hCG (ovulation occurs between 39 and 42 h post-hCG in this model). Results showed that levels of P-selectin mRNA remained very low or undetectable throughout the ovulatory process in extracts prepared from the granulosa cell layer. In contrast, a significant increase in P-selectin transcript was observed between 30 and 39 h post-hCG in extracts obtained from thecal layers (P < 0.05). Likewise, immunohistochemistry revealed an increase of immunoreactive P-selectin protein in the vascular endothelium present in thecal layers of follicles isolated 36 and 39 h post-hCG. Thus, the present study describes, to our knowledge for the first time, the primary structure of equine P-selectin and the regulation of P-selectin transcript and protein in follicular thecal endothelial cells before ovulation.

Sorting nexin 17 accelerates internalization yet retards degradation of P-selectin

The transient appearance of P-selectin on the surface of endothelial cells helps recruit leukocytes into sites of inflammation. The tight control of cell surface P-selectin on these cells depends on regulated exocytosis of Weibel-Palade bodies where the protein is stored and on its rapid endocytosis. After endocytosis, P-selectin is either sorted via endosomes and the Golgi apparatus for storage in Weibel-Palade bodies or targeted to lysosomes for degradation. A potential player in this complex endocytic itinerary is SNX17, a member of the sorting nexin family, which has been shown in a yeast two-hybrid assay to bind P-selectin. Here, we show that overexpression of SNX17 in mammalian cells can influence two key steps in the endocytic trafficking of P-selectin. First, it promotes the endocytosis of P-selectin from the plasma membrane. Second, it inhibits the movement of P-selectin into lysosomes, thereby reducing its degradation.

Peritoneal macrophages express both P-selectin and PSGL-1

Macrophages, phagocytic cells involved in an early phase of host defense, are known to express the P-selectin ligand, PSGL-1. Heretofore, P-selectin has only been found on platelets and endothelial cells. Here, we demonstrate that peritoneal macrophages isolated by peritoneal lavage of unchallenged mice express P-selectin on the plasma membrane. The peritoneal macrophages synthesize P-selectin, as indicated by metabolic labeling experiments. P-Selectin is constitutively expressed on the extracellular surface of macrophages but is only partially colocalized with PSGL-1. P-Selectin is rapidly translocated from the macrophage plasma membrane to intracellular vesicles and to lysosomes. Peritoneal macrophages assemble into cell strings under flow conditions based upon macrophage–macrophage interactions mediated by P-selectin and PSGL-1. This is the first description of a leukocyte shown to express both P-selectin and PSGL-1.

Signal-dependent distribution of cell surface P-selectin in clathrin-coated pits affects leukocyte rolling under flow

Flowing leukocytes roll on P-selectin that is mobilized from secretory granules to the surfaces of endothelial cells after stimulation with histamine or thrombin. Before it is internalized, P-selectin clusters in clathrin-coated pits, which enhances its ability to support leukocyte rolling. We found that thrombin and histamine induced comparable exocytosis of P-selectin on endothelial cells. However, compared with histamine, thrombin decreased the recruitment of P-selectin into clathrin-coated pits, slowed the internalization of P-selectin, and reduced the number and stability of neutrophils rolling on P-selectin. Significantly more RhoA was activated in thrombin- than in histamine-stimulated endothelial cells. Inhibitors of RhoA or its effector, Rho kinase, reversed thrombin's ability to inhibit the internalization and adhesive function of P-selectin in endothelial cells. Experiments with transfected cells confirmed that the inhibitory actions of thrombin and Rho kinase on P-selectin required its cytoplasmic domain. Thus, a signaling event affects both the function and clearance of a protein that enters the constitutive clathrin-mediated endocytic pathway.

Endocytosis of synaptotagmin 1 is mediated by a novel, tryptophan-containing motif

The rate at which a membrane protein is internalized from the plasma membrane can be regulated by revealing a latent internalization signal in response to an appropriate stimulus. Internalization of the synaptic vesicle membrane protein, synaptotagmin 1, is controlled by two distinct regions of its intracytoplasmic C2B domain, an internalization signal present in the 29 carboxyterminal (CT) amino acids and a separate regulatory region. We have now characterized the internalization motif by mutagenesis and found that it involves an essential tryptophan in the last beta strand of the C2B domain, a region that is distinct from the AP2-binding site previously described. Internalization through the tryptophan-based motif is sensitive to eps15 and dynamin mutants and is therefore likely to be clathrin mediated. A tryptophan-to-phenylalanine mutation had no effect on internalization of the CT domain alone, but completely inhibited endocytosis of the folded C2B domain. This result suggests that recognition of sorting motifs can be influenced by their structural context. We conclude that endocytosis of synaptotagmin 1 requires a novel type of internalization signal that is subject to regulation by the rest of the C2B domain.

The P-selectin cytoplasmic domain directs the cellular storage of a recombinant chimeric factor IX

Hemophilia B was recognized as a good candidate for gene therapy. Several strategies have been attempted and gave promising results in hemophilic animals but failed to achieve corrective levels in humans. To overcome this inconvenience we aimed to generate intracellular pools of factor (F)IX in cells that are implicated in the hemostatic response, e.g. endothelial cells and platelets. Upon stimulation, these cells release their granule content, which in this case would result in an increase in local FIX concentration, and could locally produce an effective hemostasis. In an attempt to produce an intracellular pool of releasable coagulation FIX, the cytoplasmic domain of the P-selectin (pselCT) molecule was fused to the carboxy-terminal extremity of the human FIX protein. The properties of this chimeric molecule (FIX-pselCT) were studied in AtT20, a cell line which possesses storage granules. As previously shown for transmembrane molecules but not for a soluble protein such as FIX, the pselCT fragment induces the storage of FIX-pselCT. The coagulant activity of FIX-pselCT was not affected by the addition of the pselCT tail. The treatment of AtT20 cells with different inhibitors revealed that FIX-pselCT was not submitted to intracellular degradation and that the half-life of the chimeric molecule was at least two times longer than that of FIX-WT. An immunoelectron microscopic analysis demonstrated a specific localization of FIX-pselCT within the ACTH-containing granules. Cell stimulation using Phorbol Myristrate Acetate (PMA), ionophore A-23187 or 8-Br-cAMP induced efficient release of an active FIX-pselCT. These data demonstrate that the addition of the cytoplasmic domain of P-selectin to FIX modifies the cellular fate of the FIX molecule by directing the recombinant protein toward regulated-secretory granules without altering its coagulant activity.

Biogenesis of Weibel–Palade bodies

Weibel-Palade bodies (WPBs) are the lysosome-related secretory organelles of endothelial cells. Their content protein von Willebrand factor, plays a key role in haemostasis, whilst P-selectin in the membranes is critical in the initiation of inflammation. Biogenesis of these rod-shaped structures is driven by von Willebrand factor, since its heterologous expression leads to formation of organelles morphologically indistinguishable from bona fide WPBs. The two main membrane proteins of WPBs, CD63 and P-selectin, have complex itineraries controlled largely by cytoplasmic targeting signals. We are only just beginning to understand the way in which these three proteins come together to form mature WPBs.

The terminal six amino-acids of the carboxy cytoplasmic tail of CD36 contain a functional domain implicated in the binding and capture of oxidized low-density lipoprotein

CD36, a major adhesion molecule expressed by monocytes/macrophages, plays a key role in the binding and internalization of oxidized low-density lipoprotein (OxLDL). This adhesion molecule, a member of an important scavenger receptor family, contains a very short C-terminal cytoplasmic tail that is known to induce intracellular signalling events. However, the domains on the cytoplasmic tail involved in such signal transduction are unknown. In this study, we have investigated the functional components of the cytoplasmic tail by site-directed mutagenesis coupled with functional OxLDL and monoclonal antibody (mAb) binding studies. Seven truncated or punctual CD36 constructs, localized in the cytoplasmic tail, were produced by site-directed mutagenesis. Each construct was stably expressed in HEK293 cells. We used a quantitative and a qualitative method, labelling OxLDL with either iodine or rhodamine, to determine the functional importance of the cytoplasmic domains in OxLDL internalization. Results indicate that: (1) a deletion of the last amino-acid (construct K472STOP) significantly reduces, compared with wild-type, the binding, internalization and degradation of OxLDL; (2) truncation of the last six amino-acids (construct R467STOP) significantly reduces OxLDL binding; (3) the above two constructs (K472STOP and R467STOP) showed a reduced rate of OxLDL internalization compared with wild-type; (4) the binding and rate of internalization of an anti-CD36 monoclonal antibody (10/5) was not affected by the above mentioned mutants (K472STOP and R467STOP), compared with wild-type. This study shows, for the first time, a specific site on the CD36 cytoplasmic tail that is critical for the binding, endocytosis and targeting of OxLDL.

P-selectin targeting to secretory lysosomes of Rbl-2H3 cells

The biogenesis of secretory lysosomes, which combine characteristics of both lysosomes and secretory granules, is currently of high interest. In particular, it is not clear whether delivery of membrane proteins to the secretory lysosome requires lysosomal, secretory granule, or some novel targeting determinants. Heterologous expression of P-selectin has established that this membrane protein contains targeting signals for both secretory granules and lysosomes. P-selectin is therefore an ideal probe with which to determine the signals required for targeting to secretory lysosomes. We have exploited subcellular fractionation and immunofluorescence microscopy to monitor targeting of transiently expressed wild-type and mutant horseradish peroxidase (HRP)-P-selectin chimeras to secretory lysosomes of Rbl-2H3 cells. The exposure of the HRP chimeras to intracellular proteolysis was also determined as a third monitor of secretory lysosome targeting. Our data show that HRP-P-selectin accumulates in secretory lysosomes of Rbl-2H3 cells using those cytoplasmic sequences previously found to be sufficient for targeting to conventional lysosomes. This work highlights the similar sorting signals used for targeting of membrane proteins to conventional lysosomes and secretory lysosomes.

Endosomal sorting of amyloid precursor protein-P-selectin chimeras influences secretase processing

Amyloid beta protein, the major component of the senile plaques in Alzheimer's disease, is generated by secretory and endocytic processing of amyloid precursor protein. Internalized amyloid precursor protein either recycles to the plasma membrane, where alpha-secretase resides, or moves to acidic compartment(s) for beta-secretase exposure. While the trans-Golgi network contains beta-secretase activity, recent examination of the subcellular distribution of this proteinase, called BACE, has led to the suggestion that beta-secretase activity might also reside at the plasma membrane and in endosomes. To examine the role of endocytic compartments in beta-secretase processing of amyloid precursor protein, the wild-type and endosomal sorting mutant P-selectin cytoplasmic domains were used to control movement of amyloid precursor protein through endosomes. Amyloid precursor protein/P-selectin, which is sorted from early to late endosomes, undergoes significantly less alpha-secretase cleavage, and more beta-secretase cleavage, than amyloid precursor protein/P-selectin768A, a mutant that recycles more efficiently to the cell surface. Our results demonstrate that endosomal sorting influences relative exposure of the amyloid precursor protein/P-selectin chimeras to alpha- and beta-secretase activities, and suggest that, because delivery to late endocytic compartments favors beta-secretase processing of amyloid precursor protein, there is likely limited beta-secretase activity in early endosomes or at the cell surface. We propose that the trans-Golgi network may be involved in both secretory and endocytic generation of amyloid beta protein.

AP-3 adaptor functions in targeting P-selectin to secretory granules in endothelial cells

P-selectin, a cell adhesion protein participating in the early stages of inflammation, contains multiple sorting signals that regulate its cell surface expression. Targeting to secretory granules regulates delivery of P-selectin to the cell surface. Internalization followed by sorting from early to late endosomes mediates rapid removal of P-selectin from the surface. We show here that the P-selectin cytoplasmic domain bound AP-2 and AP-3 adaptor complexes in vitro. The amino acid substitution L768A, which abolishes endosomal sorting and impairs granule targeting of P-selectin, reduced binding of AP-3 adaptors but not AP-2 adaptors. Turnover of P-selectin was 2.4-fold faster than turnover of transferrin receptor in AP-3-deficient mocha fibroblasts, similar to turnover of these two proteins in AP-3-competent cells, demonstrating that AP-3 function is not required for endosomal sorting. However, sorting P-selectin to secretory granules was defective in endothelial cells from AP-3-deficient pearl mice, demonstrating a role for AP-3 adaptors in granule assembly in endothelial cells. P-selectin sorting to platelet alpha-granules was normal in pearl mice, consistent with earlier evidence that granule targeting of P-selectin is mechanistically distinct in endothelial cells and platelets. These observations establish that AP-3 adaptor functions in assembly of conventional secretory granules, in addition to lysosomes and the 'lysosome-like' secretory granules of platelets and melanocytes.

A single internalization signal from the di-leucine family is critical for constitutive endocytosis of the type II TGF-(β) receptor

Endocytosis has an important contribution to the regulation of the surface expression levels of many receptors. In spite of the central role of the transforming growth factor (β) (TGF-(β)) receptors in numerous cellular and physiological processes, their endocytosis is largely unexplored. Current information on TGF-(β) receptor endocytosis relies exclusively on studies with chimeric constructs containing the extracellular domain of the GM-CSF receptors, following the internalization of the GM-CSF ligand; the conformation and interactions of the chimeric receptors (and therefore their endocytosis) may differ considerably from those of the native TGF-(β) receptors. Furthermore, there are no data on the potential endocytosis motif(s) of the TGF-(β) receptors or other receptor Ser/Thr kinases. Here, we report the use of type II TGF-(β) receptors, myc-tagged at their extracellular terminus, to investigate their endocytosis. Employing fluorescent antibody fragments to label exclusively the cell surface myc-tagged receptors exposed to the external milieu, made it possible to follow the internalization of the receptors, without the complications that render labeling with TGF-(β) (which binds to many cellular proteins) unsuitable for such studies. The results demonstrate that the full-length type II TGF-(β) receptor undergoes constitutive endocytosis via clathrin-coated pits. Using a series of truncation and deletion mutants of this receptor, we identified a short peptide sequence (I(218)I(219)L(220)), which conforms to the consensus of internalization motifs from the di-leucine family, as the major endocytosis signal of the receptor. The functional importance of this sequence in the full-length receptor was validated by the near complete loss of internalization upon mutation of these three amino acids to alanine.

A new member of the sorting nexin family interacts with the C-terminus of P-selectin

P-selectin is a cell adhesion molecule found in platelets and endothelial cells mediating binding of leukocytes. It is stored in secretory granules and expressed at the plasma membrane after cell activation. After rapid internalisation P-selectin recycles or is degraded. The 35 amino acid cytoplasmic domain of P-selectin contains signals for sorting into secretory granules, for endocytosis and for delivery to lysosomes. To investigate protein-protein interactions, we performed two-hybrid screening using the cytoplasmic domain of P-selectin as bait. KIAA0064 was identified as a putative intracellular P-selectin binding protein. Because the protein contains a phox homology (PX) domain in the N-terminus which is a characteristic feature of the sorting nexin (SNX) family, it was named SNX17. The PX domain is not required for binding of P-selectin in the two-hybrid system. Expression of a fusion protein between SNX17 and green fluorescent protein demonstrated localisation of SNX17 in the cytosol and to membranes.

A third specificity-determining site in mu 2 adaptin for sequences upstream of Yxx phi sorting motifs

Internalization signals of the Yxx phi type (phi = bulky hydrophobic side chain) interact with the mu 2 chain of AP-2 adaptors. Internalization activity is intolerant of non-conservative substitution of either the tyrosine or the phi side chains, which bind to hydrophobic pockets in mu 2 adaptin in a conformation described as 'a two pinned plug into a socket'. P-selectin, a type I transmembrane protein, contains the Yxx phi-like sequence YGVF in its cytoplasmic domain, but substitution of either the tyrosine or phenylalanine with alanine in the full-length protein causes only small changes in the rate of endocytosis. It is shown here that the sequence YGVF contained within a peptide corresponding to the 17 COOH-terminal amino acids of P-selectin binds to mu 2 adaptin in the same fashion previously seen for other Yxx phi motifs. In addition, the P-selectin peptide binds to a third hydrophobic pocket in mu 2 adaptin through a leucine at position Y-3 in the peptide. This structure suggests that some sequences can function as a 'three pinned plug', in which internalization activity is not critically dependent on any one of the three interacting side chains.

Rapid transport of internalized P-selectin to late endosomes and the TGN: roles in regulating cell surface expression and recycling to secretory granules

Prior studies on receptor recycling through late endosomes and the TGN have suggested that such traffic may be largely limited to specialized proteins that reside in these organelles. We present evidence that efficient recycling along this pathway is functionally important for nonresident proteins. P-selectin, a transmembrane cell adhesion protein involved in inflammation, is sorted from recycling cell surface receptors (e.g., low density lipoprotein [LDL] receptor) in endosomes, and is transported from the cell surface to the TGN with a half-time of 20-25 min, six to seven times faster than LDL receptor. Native P-selectin colocalizes with LDL, which is efficiently transported to lysosomes, for 20 min after internalization, but a deletion mutant deficient in endosomal sorting activity rapidly separates from the LDL pathway. Thus, P-selectin is sorted from LDL receptor in early endosomes, driving P-selectin rapidly into late endosomes. P-selectin then recycles to the TGN as efficiently as other receptors. Thus, the primary effect of early endosomal sorting of P-selectin is its rapid delivery to the TGN, with rapid turnover in lysosomes a secondary effect of frequent passage through late endosomes. This endosomal sorting event provides a mechanism for efficiently recycling secretory granule membrane proteins and, more generally, for downregulating cell surface receptors.

Interleukin-13 induces PSGL-1/P–selectin–dependent adhesion of eosinophils, but not neutrophils, to human umbilical vein endothelial cells under flow

Selective eosinophil accumulation is a hallmark of diseases such as asthma. In a model of chronic eosinophilic inflammation, we have previously shown that the tethering step in eosinophil adhesion is mediated by PSGL-1 binding to P-selectin. The Th2-associated cytokine IL-13 is of potential importance in allergic disease. We have therefore investigated whether IL-13 can mediate eosinophil binding to human umbilical vein endothelial cells (HUVEC) through P-selectin. IL-13 caused dose- and time-dependent increases of P-selectin expression, as assessed by flow and laser scanning cytometry. A similar degree of expression was observed with IL-4. There was no effect on E-selectin or ICAM-1 expression. Tumor necrosis factor- induced the expression of VCAM-1, E-selectin, and ICAM-1 but had no effect on P-selectin expression. IL-13 increased the production of mRNA for surface and soluble variants of P-selectin. Under flow conditions, eosinophils, but not neutrophils, showed enhanced binding to IL-13 and to IL-4–stimulated HUVEC compared to medium-cultured cells. Eosinophil adhesion was completely inhibited by a blocking monoclonal antibody against PSGL-1 and P-selectin. Anti–VLA-4 and anti–VCAM-1 antibodies inhibited binding to a lesser extent. Thus, at physiologic levels of expression induced by Th2 cytokines, P-selectin/PSGL-1 supported eosinophil but not neutrophil adhesion. This mechanism is likely to be a key event leading to the selective accumulation of eosinophils in allergic inflammation.

Interleukin-13 induces PSGL-1/P–selectin–dependent adhesion of eosinophils, but not neutrophils, to human umbilical vein endothelial cells under flow

Selective eosinophil accumulation is a hallmark of diseases such as asthma. In a model of chronic eosinophilic inflammation, we have previously shown that the tethering step in eosinophil adhesion is mediated by PSGL-1 binding to P-selectin. The Th2-associated cytokine IL-13 is of potential importance in allergic disease. We have therefore investigated whether IL-13 can mediate eosinophil binding to human umbilical vein endothelial cells (HUVEC) through P-selectin. IL-13 caused dose- and time-dependent increases of P-selectin expression, as assessed by flow and laser scanning cytometry. A similar degree of expression was observed with IL-4. There was no effect on E-selectin or ICAM-1 expression. Tumor necrosis factor- induced the expression of VCAM-1, E-selectin, and ICAM-1 but had no effect on P-selectin expression. IL-13 increased the production of mRNA for surface and soluble variants of P-selectin. Under flow conditions, eosinophils, but not neutrophils, showed enhanced binding to IL-13 and to IL-4–stimulated HUVEC compared to medium-cultured cells. Eosinophil adhesion was completely inhibited by a blocking monoclonal antibody against PSGL-1 and P-selectin. Anti–VLA-4 and anti–VCAM-1 antibodies inhibited binding to a lesser extent. Thus, at physiologic levels of expression induced by Th2 cytokines, P-selectin/PSGL-1 supported eosinophil but not neutrophil adhesion. This mechanism is likely to be a key event leading to the selective accumulation of eosinophils in allergic inflammation.

A screen of random sequences for those that alter the trafficking of the influenza virus hemagglutinin in vivo

In order to determine if the sequence patterns known to specify internalization represent the majority of possible internalization signals, we identified random sequences capable of causing a reporter protein to be internalized at least several-fold faster than the rate of non-selective internalization of membrane by clathrin-coated pits. A library of influenza hemagglutinin (HA) proteins, bearing short random sequences in place of the wild-type cytoplasmic domain, was prepared in recombinant SV40 virus. The library was expressed and screened for HAs that could internalize anti-HA antibody from the medium. The cytoplasmic sequences of the selected proteins were determined. From a small sample of sequences we detected several that did not resemble those previously identified. The known internalization signals must represent only a subset of the sequences that can serve as internalization signals.

Divergent Inducible Expression of P-Selectin and E-Selectin in Mice and Primates

We used in vitro and in vivo approaches to examine whether tumor necrosis factor- (TNF-) and oncostatin M (OSM), cytokines that bind to distinct classes of receptors, differentially regulate expression of P- and E-selectin in murine and primate endothelial cells. In human umbilical vein endothelial cells, TNF- rapidly increased mRNA for E-selectin but not P-selectin. OSM elicited little or no change in mRNA for E-selectin, but induced a delayed and prolonged increase in P-selectin mRNA. TNF- and OSM did not cooperate to further enhance P- or E-selectin mRNA. Intravenous infusion of Escherichia coli, which markedly elevates plasma lipopolysaccharide and TNF-, increased mRNA for E-selectin but not P-selectin in baboons. In murine bEnd.3 endothelioma cells, TNF- and OSM individually and cooperatively increased mRNA and protein for both P- and E-selectin. Intravenous injection of these cytokines also individually and cooperatively increased mRNA for P- and E-selectin in mice. We conclude that the murine P- and E-selectin genes respond to both TNF- and OSM, whereas the primate P- and E-selectin genes have much more specialized responses. Such differences should be considered when extrapolating the functions of P- and E-selectin in murine models of inflammation to humans.

Divergent Inducible Expression of P-Selectin and E-Selectin in Mice and Primates

We used in vitro and in vivo approaches to examine whether tumor necrosis factor- (TNF-) and oncostatin M (OSM), cytokines that bind to distinct classes of receptors, differentially regulate expression of P- and E-selectin in murine and primate endothelial cells. In human umbilical vein endothelial cells, TNF- rapidly increased mRNA for E-selectin but not P-selectin. OSM elicited little or no change in mRNA for E-selectin, but induced a delayed and prolonged increase in P-selectin mRNA. TNF- and OSM did not cooperate to further enhance P- or E-selectin mRNA. Intravenous infusion of Escherichia coli, which markedly elevates plasma lipopolysaccharide and TNF-, increased mRNA for E-selectin but not P-selectin in baboons. In murine bEnd.3 endothelioma cells, TNF- and OSM individually and cooperatively increased mRNA and protein for both P- and E-selectin. Intravenous injection of these cytokines also individually and cooperatively increased mRNA for P- and E-selectin in mice. We conclude that the murine P- and E-selectin genes respond to both TNF- and OSM, whereas the primate P- and E-selectin genes have much more specialized responses. Such differences should be considered when extrapolating the functions of P- and E-selectin in murine models of inflammation to humans.

The signal for clathrin-mediated endocytosis of the paramyxovirus SV5 HN protein resides at the transmembrane domain-ectodomain boundary region

The hemagglutinin-neuraminidase (HN) glycoprotein of the paramyxovirus SV5 is internalized from the cell surface via clathrin-coated pits. However, the cytoplasmic domain of SV5 HN does not contain a previously characterized internalization motif. A cell-surface-expressed chimeric protein (APK), consisting of the cytoplasmic tail, transmembrane (TM) domain, and 12 residues of the ectodomain of HN joined to the cytoplasmic protein pyruvate kinase is internalized, indicating that the N-terminal region of HN contains an internalization signal. Although SV5 HN is internalized at a rate similar to that of influenza virus hemagglutinin (HA) mutant Y543, which contains a degenerate tyrosine-based signal in its cytoplasmic tail, the elimination of the majority of the HN cytoplasmic tail, or substitution of the HN TM domain with leucine residues, did not affect the rate of HN internalization. The HN protein of the closely related virus, Newcastle disease virus (NDV), is not internalized from the cell surface. Working under the usual convention that the TM domain consists of the hydrophobic residues bounded by two charged residues, analysis of internalization of mutant and chimeric NDV HN molecules indicates that the first seven SV5 HN ectodomain residues are critical for internalization of HN. A glutamic acid residue (E37) that abuts this presumptive HN TM domain/ectodomain boundary is important for SV5 HN internalization.

A complex web of signal-dependent trafficking underlies the triorganellar distribution of P-selectin in neuroendocrine PC12 cells

By analyzing the trafficking of HRP-P-selectin chimeras in which the lumenal domain of P-selectin was replaced with horseradish peroxidase, we determined the sequences needed for targeting to synaptic-like microvesicles (SLMV), dense core granules (DCG), and lysosomes in neuroendocrine PC12 cells. Within the cytoplasmic domain of P-selectin, Tyr777 is needed for the appearance of P-selectin in immature and mature DCG, as well as for targeting to SLMV. The latter destination also requires additional sequences (Leu768 and 786DPSP789) which are responsible for movement through endosomes en route to the SLMV. Leu768 also mediates transfer from early transferrin (Trn)-positive endosomes to the lysosomes; i.e., operates as a lysosomal targeting signal. Furthermore, SLMV targeting of HRP-P-selectin chimeras, but not the endogenous SLMV protein synaptophysin/p38, previously shown to be delivered to SLMV directly from the plasma membrane, is a Brefeldin A-sensitive process. Together, these data are consistent with a model of SLMV biogenesis which involves an endosomal intermediate in PC12 cells. In addition, we have discovered that impairment of SLMV or DCG targeting results in a concomitant increase in lysosomal delivery, illustrating the entwined relationships between routes leading to regulated secretory organelles (RSO) and to lysosomes.

Constitutive lysosomal targeting and degradation of bovine endothelin-converting enzyme-1a mediated by novel signals in its alternatively spliced cytoplasmic tail

Endothelin-converting enzyme-1 (ECE-1) is a type II membrane protein that catalyzes the proteolytic activation of big endothelin-1 to endothelin-1 (ET-1). The subcellular distribution of ECE-1, and hence the exact site of physiological activation of big ET-1, remains controversial. Here, we demonstrate with several complementary methods that the two alternatively spliced bovine ECE-1 isoforms, ECE-1a and ECE-1b, differing only in the first 30 amino acids of their N-terminal cytoplasmic tails, exhibit strikingly distinct intracellular sorting patterns. Bovine ECE-1a, which is responsible for the intracellular cleavage of big ET-1 in endothelial cells, is constitutively recruited into the lysosome, where it is rapidly degraded. In contrast, bovine ECE-1b, the isoform found in cultured smooth muscle cells, is transported to the plasma membrane by a default pathway and functions as an ectoenzyme. Mutational analyses reveal that the N-terminal tip of the cytoplasmic domain of bovine ECE-1a contains novel proline-containing signals that mediate constitutive lysosomal targeting. Analyses of chimeric ECE-1/transferrin receptors demonstrate that the cytoplasmic tail of bovine ECE-1a is sufficient for the lysosomal delivery and rapid degradation. Our results suggest that the distinct intracellular targeting of bovine ECE-1 isoforms may provide new insights into functional aspect of the endothelin system and that the cell permeability of ECE inhibitor compounds should be carefully considered during their pharmacological development.

Mechanisms that regulate the function of the selectins and their ligands

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

Role of P-selectin cytoplasmic domain in granular targeting in vivo and in early inflammatory responses

P-selectin is an adhesion receptor for leukocytes expressed on activated platelets and endothelial cells. The cytoplasmic domain of P-selectin was shown in vitro to contain signals required for both the sorting of this protein into storage granules and its internalization from the plasma membrane. To evaluate in vivo the role of the regulated secretion of P-selectin, we have generated a mouse that expresses P-selectin lacking the cytoplasmic domain (DeltaCT mice). The deletion did not affect the sorting of P-selectin into alpha-granules of platelets but severely compromised the storage of P-selectin in endothelial cells. Unstored P-selectin was proteolytically shed from the plasma membrane, resulting in increased levels of soluble P-selectin in the plasma. The DeltaCT-P-selectin appeared capable of mediating cell adhesion as it supported leukocyte rolling in the mutant mice. However, a secretagogue failed to upregulate leukocyte rolling in the DeltaCT mice, indicating an absence of a releasable storage pool of P-selectin in the endothelium. Furthermore, the neutrophil influx into the inflamed peritoneum was only 30% of the wild-type level 2 h after stimulation. Our results suggest that different sorting mechanisms for P-selectin are used in platelets and endothelial cells and that the storage pool of P-selectin in endothelial cells is functionally important during early stages of inflammation.

Determinants in the cytoplasmic domain of P-selectin required for sorting to secretory granules

P-selectin is a granule membrane protein of platelets and endothelial cells that is expressed at the plasma membrane after cell activation. To determine which residues in its cytoplasmic tail are important for sorting to storage granules during biosynthesis, we expressed P-selectin mutants in AtT-20, a murine cell line with secretory granules that contain the hormone corticotropin ('ACTH'). Immunofluorescence microscopy of permeabilized cells revealed that wild-type P-selectin and mutants with alanine substitutions at 14 different positions in the cytoplasmic tail were concentrated in the tips of the cellular processes, which contain the majority of corticotropin granules. However, targeting to the cell tips was greatly decreased for Tyr777-->Ala, Tyr777-->Phe, Gly778-->Ala, Phe780-->Ala and Leu768/Asn769-->Ala/Ala mutants. The reduced presence of these mutants in corticotropin granules was confirmed by immunoelectron microscopy. Stimulation of AtT-20 transfectants with 8-Br-cAMP resulted in a significant increase in membrane expression of wild-type P-selectin, but in only a marginal increase in the surface expression of the five mutants. Antibody binding studies with intact and permeabilized cells demonstrated that the percentage of P-selectin that is expressed on the surface of the cells was considerably higher for these mutants than for wild-type P-selectin (6%), ranging from approximately 20% for the Gly778 and Phe780 mutants to 63% for the Leu768/Asn769 mutant. Taken together, these results indicate that Tyr777, Gly778 and Phe780 form part of an atypical tyrosine-based motif, which also requires the presence Leu768 and/or Asn769 to mediate sorting of P-selectin to secretory granules.

A balance of opposing signals within the cytoplasmic tail controls the lysosomal targeting of P-selectin

The 35-amino acid cytoplasmic tail of the adhesion receptor P-selectin is subdivided into stop transfer, C1 and C2 domains. It contains structural signals needed for targeting this protein to specialized secretory organelles and to lysosomes. Recently, using site-directed mutagenesis of horseradish peroxidase-P-selectin chimeras, we have uncovered a novel sequence within the C1 domain, KCPL, that mediates sorting from early, transferrin-positive endosomes to lysosomes and therefore operates as a positive lysosomal targeting signal (Blagoveshchenskaya, A. D., Norcott, J. P. , and Cutler, D. F. (1998) J. Biol. Chem. 273, 2729-2737). In the current study, we examined lysosomal targeting by both subcellular fractionation and an intracellular proteolysis assay and found that a balance of positive and negative signals is required for proper lysosomal sorting of P-selectin. First, we have found that within the sequence KCPL, Cys-766 plays a major role along with Pro-767, whereas Lys-765 and Leu-768 make no contribution to promoting lysosomal targeting. In addition, horseradish peroxidase-P-selectin chimeras were capable of acylation in vivo with [3H]palmitic acid at Cys-766, since no labeling of a chimera in which Cys-766 was replaced with Ala was detected. Second, analysis of mutations within the C2 domain revealed that substitution of two sequences, YGVF and DPSP, causes an increase in both lysosomal targeting and intracellular proteolysis suggesting the presence of lysosomal avoidance signals. The inhibition or promotion of lysosomal targeting resulted from alterations in endosomal sorting since internalization was not changed in parallel with lysosomal delivery. Analysis of the double mutants KCPL/YGVF or KCPL/DPSP revealed that although the positive lysosomal targeting signal operates in the early/sorting transferrin-positive endosomes, the negative lysosomal targeting (lysosomal avoidance) signals act at later stages of the endocytic pathway, most likely in late endosomal compartments.

Intracellular trafficking of the vacuolar H+-ATPase accessory subunit Ac45

Ac45 is a type I transmembrane protein associated with vacuolar H+-ATPase, a proton pump mediating the acidification of multiple intracellular organelles. In this study, we examined the intracellular routing of Ac45 in transfected CV-1 fibroblasts. Steady state immunolabeling showed that Ac45 is located on the plasma membrane and in a vacuolar compartment in the juxtanuclear region. Antibody internalization experiments revealed that Ac45 is rapidly retrieved from the cell surface and is targeted to the vacuolar structures. The 26-residue cytoplasmic tail of Ac45 was intrinsically capable of mediating endocytosis of the cell surface protein Tac, indicating that the tail contains an autonomous internalization signal. Immunolocalization studies on cells expressing carboxy-terminally truncated Ac45 mutants showed the presence of essential routing information in the membrane-distal region of the cytoplasmic tail. Further mutational analysis of this region, which lacks the recognized tyrosine- or di-leucine-based sorting motifs, suggested that multiple sites rather than a short linear sequence are responsible for the internalization. Collectively, our results indicate that the cytoplasmic tail of Ac45 contains autonomous targeting information distinct from previously described routing determinants.

Endocytosis of the common cytokine receptor gammac chain. Identification of sequences involved in internalization and degradation

The common cytokine receptor gammac, shared by interleukin 2, 4, 7, 9, and 15 receptors, has a major role in lymphocyte proliferation and differentiation, leading, when mutated, to a genetic disease, X-linked severe combined immunodeficiency. In this study, we report that gammac is internalized and degraded in lymphoid cells. To identify gammac regions involved in sorting along the endocytic pathway, we have studied a chimeric protein composed of the extracellular part of interleukin 2-receptor alpha and transmembrane and intracellular part of gammac, alpha gamma gammawt. When transfected in Jurkat T cells, alpha gamma gammawt is as efficiently internalized and degraded as gammac, demonstrating that the transmembrane and cytosolic tail of gammac carry sequences involved in this process. To identify these motifs, we have analyzed the trafficking of chimeric proteins with serial truncations in their cytosolic tail. Internalization studies showed that the cytosolic tail of gammac contains three regions located between cytosolic amino acids 1-35, 35-40, and 40-65 involved in gammac endocytosis. Successive deletions of these motifs result in reduced endocytosis. One region containing the 5 cytosolic amino acids 36-40 is essential to direct gammac to the degradation pathway. These sorting sequences, by participating in the fine tuning of cell surface gammac expression, might somewhat regulate the cell responsiveness to interleukins whose receptors share this component.

Interactions of the cytoplasmic domain of P-selectin with clathrin-coated pits enhance leukocyte adhesion under flow

Flowing leukocytes tether to and roll on P-selectin, a receptor on endothelial cells that is rapidly internalized in clathrin-coated pits. We asked whether the association of P-selectin with clathrin-coated pits contributes to its adhesive function. Under flow, rolling neutrophils accumulated efficiently on CHO cells expressing wild-type P-selectin or a P-selectin construct with a substitution in the cytoplasmic domain that caused even faster internalization than that of the wild-type protein. By contrast, far fewer rolling neutrophils accumulated on CHO cells expressing P-selectin constructs with a deletion or a substitution in the cytoplasmic domain that impaired internalization. Neutrophils rolled on the internalization-competent constructs with greater adhesive strength, slower velocity, and more uniform motion. Flowing neutrophils tethered equivalently to internalization-competent or internalization-defective P-selectin, but after tethering, they rolled further on internalization-competent P-selectin. Confocal microscopy demonstrated colocalization of alpha-adaptin, a component of clathrin-coated pits, with wild-type P-selectin, but not with P-selectin lacking the cytoplasmic domain. Treatment of CHO cells or endothelial cells with hypertonic medium reversibly impaired the clathrin-mediated internalization of P-selectin and its ability to support neutrophil rolling. Interactions of the cytoplasmic domain of P-selectin with clathrin-coated pits provide a novel mechanism to enhance leukocyte adhesion under flow.

An atypical sorting determinant in the cytoplasmic domain of P-selectin mediates endosomal sorting

We previously identified the 11 amino acid C1 region of the cytoplasmic domain of P-selectin as essential for an endosomal sorting event that confers rapid turnover on P-selectin. The amino acid sequence of this region has no obvious similarity to other known sorting motifs. We have analyzed the sequence requirements for endosomal sorting by measuring the effects of site-specific mutations on the turnover of P-selectin and of the chimeric protein LLP, containing the lumenal and transmembrane domains of the low density lipoprotein receptor and the cytoplasmic domain of P-selectin. Endosomal sorting activity was remarkably tolerant of alanine substitutions within the C1 region. The activity was eliminated by alanine substitution of only one amino acid residue, leucine 768, where substitution with several other large side chains, hydrophobic and polar, maintained the sorting activity. The results indicate that the endosomal sorting determinant is not structurally related to previously reported sorting determinants. Rather, the results suggest that the structure of the sorting determinant is dependent on the tertiary structure of the cytoplasmic domain.

VCAM-1 is internalized by a clathrin-related pathway in human endothelial cells but its alpha 4 beta 1 integrin counter-receptor remains associated with the plasma membrane in human T lymphocytes

Lymphocyte extravasation involves a step(s) of de-adhesion to allow trans- and subendothelial migration in response to inflammatory signals. We show here that ligated VCAM-1 was rapidly internalized (t1/2 14.5 min) in ECV 304 endothelial cells and in TNF-alpha-primed human umbilical vein-derived endothelial cells (t1/2 11.2 min). The process required energy (ATP), intracellular Ca2+, an intact cytoskeletal network and active protein kinases. The internalization of VCAM-1 involved a clathrin-dependent pathway based on the observations that 1) it was inhibited in cells treated with lysosomotropic agents or with a hypertonic concentration of sucrose, and 2) internalized VCAM-1 colocalized with clathrin. In contrast, the cross-linked alpha 4 beta 1 integrin counter-receptor of VCAM-1 remained associated with the plasma membrane of purified peripheral T and Jurkat cells. Our results suggest a model where VCAM-1 would initially participate in the retention of T cells to the endothelium by binding alpha 4 beta 1 integrin. Lymphocyte de-adhesion would be facilitated as a result of the internalization of VCAM-1. The persistent cell surface expression of alpha 4 beta 1 integrin would allow the migrating T cells to interact with and receive signal(s) from its fibronectin ligand of the extracellular matrix.

The transmembrane domain enhances granular targeting of P-selectin

P-selectin is an integral membrane glycoprotein that is stored in granules of endothelial cells and platelets. The cytoplasmic domain of P-selectin is known to contain at least part of the signal that directs the protein to storage granules. In order to more fully understand how P-selectin is targeted to the regulated secretory pathway, we have expressed chimeric constructs between P- and E-selectin, a protein which is expressed on the cell surface, in a rat insulinoma cell line. Immunofluorescence studies indicated that replacing the cytoplasmic domain of E-selectin with that of P-selectin resulted in low-level granular expression. In contrast, when both the transmembrane and cytoplasmic domains of E-selectin were replaced with the analogous domains of P-selectin, the granular localization appeared greatly increased. This was confirmed by immunoelectron microscopy which demonstrated a three- to fourfold improvement in granular targeting, i.e. similar to wild-type P-selectin. The transmembrane domain had to be in the context of the P-selectin cytoplasmic domain as this membrane-spanning region could not induce granular targeting on its own. These results describe a novel function for the transmembrane domain of P-selectin in enhancing the efficiency of granular targeting and further implicate protein transmembrane domains in intracellular trafficking.

Lysosomal targeting of P-selectin is mediated by a novel sequence within its cytoplasmic tail

Signals controlling the intracellular targeting of many membrane proteins are present as short sequences within their cytoplasmic domains. P-selectin is a type I membrane protein receptor for leukocytes, acting during the inflammation response. Heterologous expression experiments have demonstrated that its 35-residue cytoplasmic tail contains signals for targeting to synaptic-like microvesicles, dense-cored granules, and lysosomes. We have examined the lysosomal targeting information present within the cytoplasmic tail by site-directed mutagenesis of horseradish peroxidase-P-selectin chimeras followed by transient transfection in H.Ep.2 cells. Assaying lysosomal targeting by subcellular fractionation as well as intracellular proteolysis, we have discovered a novel lysosomal targeting signal, KCPL, located within the C1 domain of the cytoplasmic tail. Alanine substitution of this tetrapeptide reduced lysosomal targeting to the level of a tailless horseradish peroxidase-P-selectin chimera, which was previously found to be deficient in both internalization and delivery to lysosomes. A proline residue within this lysosomal targeting signal makes a major contribution to the efficiency of lysosomal targeting. A diaminobenzidine density shift procedure established that chimeras with an inactivated KCPL sequence are present within transferrin-positive compartments. Such a mutant also displays an increased level of expression at the plasma membrane. Our results indicate that the sequence KCPL within the cytoplasmic tail of P-selectin is a structural element that mediates sorting from endosomes to lysosomes.

Cytoplasmic domain of E-selectin contains a non-tyrosine endocytosis signal

E-selectin is an activation-dependent, endothelial cell-restricted adhesion molecule that is internalized and degraded rapidly once expressed on the cell surface. Tyrosine-containing structural motifs play an important role in the internalization of a number of integral proteins, and the membrane-proximal E-selectin cytoplasmic tyrosine residue (Tyr582) conforms to the endocytosis motif proposed previously. To determine the endocytosis motif in E-selectin, we selectively introduced truncation, substitution, and deletion mutations to the cytoplasmic tail of E-selectin. We analyzed the internalization kinetics of surface-expressed wild-type and mutant E-selectin constructs in transiently transfected Chinese hamster ovary cells using 125I-labeled E-selectin monoclonal antibody (125I-P6E2) in an acid elution assay. Interestingly, truncation immediately membrane proximal to Tyr582 (DeltaDGS construct) did not alter internalization kinetics significantly (DeltaDGS versus wild-type, mean surface half-life = 42 versus 45 min, respectively). Thus, it appears that the tyrosine residues are not required for internalization of E-selectin. Additional analyses indicated that Ser581 was necessary but alone was insufficient for surface E-selectin endocytosis. Thus, we conclude that there exists a novel non-tyrosine-containing endocytosis signal in the cytoplasmic tail which involves Ser581 and residues membrane-proximal to it.

Identification of three internalization sequences in the cytoplasmic tail of the 46 kDa mannose 6-phosphate receptor

The cytoplasmic tail of the human 46 kDa mannose 6-phosphate receptor (MPR 46) is necessary for rapid internalization of the receptor and sufficient to mediate internalization of a resident plasma membrane protein. To localize the internalization sequences within the 67 amino acids of the cytoplasmic tail, the tail was progressively shortened from its C-terminus, internal deletions of between four and eight amino acids were introduced into the tail, and individual residues were substituted by alanine, glycine or serine. Three sequences were identified that contribute to the internalization of MPR 46. The first is located within the 23 juxtamembrane cytoplasmic residues of the tail. It contains four essential residues within a heptapeptide and does not resemble known internalization signals. The second sequence contains as a critical residue Tyr-45. The third region is located within the C-terminal seven residues and contains a di-leucine pair as essential residues. The first and third sequences were shown to function as autonomous internalization sequences. Substitution of critically important residues within a single internalization sequence was tolerated, with no or only a moderate decrease in the internalization rate. When essential residues from two or all three internalization sequences were substituted, however, the internalization rate was decreased by more than 60% and 90% respectively. This indicates that the autonomous internalization signals in the cytoplasmic tail of MPR 46 function in an additive manner, but are partly redundant.

Selectin-carbohydrate interactions during inflammation and metastasis

L-, E-, and P-selectin are membrane-anchored, C-type lectins that initiate tethering and rolling of flowing leukocytes on endothelial cells, platelets, or other leukocytes during inflammation. The selectins bind to sialylated, fucosylated, or, in some cases, sulfated glycans on glycoproteins, glycolipids, or proteoglycans. However, they bind with relatively high affinity or avidity to only a few, appropriately modified glycoproteins on leukocytes or endothelial cells. One leukocyte mucin, PSGL-1, tethers flowing leukocytes to P-selectin on activated platelets or endothelial cells, and also helps tether leukocytes to L-selectin on other leukocytes. The physiologic expression of the selectins is tightly controlled to limit the inflammatory response. But dysregulated expression of the selectins may contribute to inflammatory and thrombotic disorders, and perhaps to tumor metastases.