Expression of the neutrophil-activating CXC chemokine ENA-78/CXCL5 by human eosinophils

BACKGROUND

Eosinophils are seen at sites of inflammation in diseases such as helminthic infestation, asthma, ulcerative colitis and some neoplastic diseases. They are also associated with connective tissue remodelling, for example in longstanding asthma. In the present study, we investigated whether eosinophils express the CXC chemokine epithelial cell-derived neutrophil activating peptide (ENA-78/CXCL5), a chemokine that can activate neutrophils and in addition possesses angiogenic properties. Immunocytochemistry detected CXCL5 in eosinophils and the peptide was localized in the specific granules by immunoelectron microscopy.

METHODS AND RESULTS

In eosinophil lysates, 12 +/- 2 pg (mean +/- SEM) of CXCL5 was detected per 106 cells by enzyme-linked immunosorbent assay (ELISA). Weak constitutive expression of CXCL5, as well as the related CXC chemokine IL-8/CXCL8, could be detected in freshly isolated eosinophils by RT-PCR. However, during prolonged incubation of eosinophils, a strong increase in both CXCL5 and IL-8/CXCL8 expression was seen, as detected by RT-PCR, and increasing amounts of CXCL5 peptide with time were detected in the incubation medium by ELISA. Addition of TNF-alpha neutralizing antibodies during prolonged incubation significantly inhibited CXCL5 production, demonstrating involvement of auto- and paracrine effects from TNF-alpha produced by eosinophils themselves. Addition of IFN-gamma showed a strong inhibitory effect on CXCL5 synthesis.

CONCLUSION

These findings suggest that, through expression of CXCL5, eosinophils can recruit and activate CXC receptor 2 (CXCR2)-bearing cells such as neutrophils at sites of inflammation. Eosinophils may also promote connective tissue remodelling through release of this peptide.

Sorting for storage in myeloid cells of nonmyeloid proteins and chimeras with the propeptide of myeloperoxidase precursor

During formation of polymorphonuclear neutrophils, proteins are synthesized for storage in granules. Whereas sorting of proteins into distinct subtypes of cytoplasmic granules may reflect the coordinated expression of the proteins contained in them, still the mechanism(s) for the retrieval of proteins from the constitutive secretion is unknown. To investigate the mechanisms of retrieval, nonmyeloid secretory proteins were expressed in myeloid cell lines, and their subcellular fate was assessed. The contribution of the propeptide (MPOpro) of the myeloperoxidase (MPO) precursor was investigated by determining the fate of chimeras containing MPOpro. The nonmyeloid protein alpha(1)-microglobulin (alpha(1)-m) was targeted to storage organelles in 32D cells and colocalized with the lysosomal marker LAMP-1, whereas soluble TNF receptor 1 (sTNFR1) was secreted without granule targeting. Fusion of MPOpro to alpha(1)-m delayed exit from endoplasmic reticulum (ER), but subsequent targeting to dense organelles was indistinguishable from that of alpha(1)-m alone. Fusion proteins between MPOpro and sTNFR1 or green fluorescent protein expressed in myeloid 32D, K562, or PLB-985 cells did not associate stably with calreticulin or calnexin, molecular chaperones that normally interact transiently with the MPO precursor, but were still efficiently retained in the ER followed by degradation. We conclude that normally secreted, nonmyeloid proteins can be targeted efficiently to storage organelles in myeloid cells, that myeloid cells selectively target some proteins for storage but not others, and that MPOpro may contribute to the prolonged ER retention of the MPO precursor independent of the ER-molecular chaperones calreticulin and calnexin.

The Rab7 effector protein RILP controls lysosomal transport by inducing the recruitment of dynein-dynactin motors

Many intracellular compartments, including MHC class II-containing lysosomes, melanosomes, and phagosomes, move along microtubules in a bidirectional manner and in a stop-and-go fashion due to the alternating activities of a plus-end directed kinesin motor and a minus-end directed dynein-dynactin motor. It is largely unclear how motor proteins are targeted specifically to different compartments. Rab GTPases recruit and/or activate several proteins involved in membrane fusion and vesicular transport. They associate with specific compartments after activation, which makes Rab GTPases ideal candidates for controlling motor protein binding to specific membranes. We and others [7] have identified a protein, called RILP (for Rab7-interacting lysosomal protein), that interacts with active Rab7 on late endosomes and lysosomes. Here we show that RILP prevents further cycling of Rab7. RILP expression induces the recruitment of functional dynein-dynactin motor complexes to Rab7-containing late endosomes and lysosomes. Consequently, these compartments are transported by these motors toward the minus end of microtubules, effectively inhibiting their transport toward the cell periphery. This signaling cascade may be responsible for timed and selective dynein motor recruitment onto late endosomes and lysosomes.

Gap junction protein connexin-43 interacts directly with microtubules

Gap junctions are specialized cell-cell junctions that mediate intercellular communication. They are composed of connexin proteins, which form transmembrane channels for small molecules [1, 2]. The C-terminal tail of connexin-43 (Cx43), the most widely expressed connexin member, has been implicated in the regulation of Cx43 channel gating by growth factors [3-5]. The Cx43 tail contains various protein interaction sites, but little is known about binding partners. To identify Cx43-interacting proteins, we performed pull-down experiments using the C-terminal tail of Cx43 fused to glutathione-S-transferase. We find that the Cx43 tail binds directly to tubulin and, like full-length Cx43, sediments with microtubules. Tubulin binding to Cx43 is specific in that it is not observed with three other connexins. We established that a 35-amino acid juxtamembrane region in the Cx43 tail, which contains a presumptive tubulin binding motif, is necessary and sufficient for microtubule binding. Immunofluorescence and immunoelectron microscopy studies reveal that microtubules extend to Cx43-based gap junctions in contacted cells. However, intact microtubules are dispensable for the regulation of Cx43 gap-junctional communication. Our findings suggest that, in addition to its well-established role as a channel-forming protein, Cx43 can anchor microtubule distal ends to gap junctions and thereby might influence the properties of microtubules in contacted cells.

Regulation of MHC class II antigen presentation by sorting of recycling HLA-DM/DO and class II within the multivesicular body

MHC class II molecules bind antigenic peptides in the late endosomal/lysosomal MHC class II compartments (MIIC) before cell surface presentation. The class II modulatory molecules HLA-DM and HLA-DO mainly localize to the MIICs. Here we show that DM/DO complexes continuously recycle between the plasma membrane and the lysosomal MIICs. Like DMbeta and the class II-associated invariant chain, the DObeta cytoplasmic tail contains potential lysosomal targeting signals. The DObeta signals, however, are not essential for internalization of the DM/DO complex from the plasma membrane or targeting to the MIICs. Instead, the DObeta tail determines the distribution of both DM/DO and class II within the multivesicular MIIC by preferentially localizing them to the limiting membrane and, in lesser amounts, to the internal membranes. This distribution augments the efficiency of class II antigenic peptide loading by affecting the efficacy of lateral interaction between DM/DO and class II molecules. Sorting of DM/DO and class II molecules to specific localizations within the MIIC represents a novel way of regulating MHC class II Ag presentation.

Human cathelicidin, hCAP-18, is processed to the antimicrobial peptide LL-37 by extracellular cleavage with proteinase 3

Cathelicidins are a family of antimicrobial proteins found in the peroxidase-negative granules of neutrophils. The known biologic functions reside in the C-terminus, which must be cleaved from the holoprotein to become active. Bovine and porcine cathelicidins are cleaved by elastase from the azurophil granules to yield the active antimicrobial peptides. The aim of this study was to identify the physiological setting for cleavage of the only human cathelicidin, hCAP-18, to liberate the antibacterial and cytotoxic peptide LL-37 and to identify the protease responsible for this cleavage. Immunoelectron microscopy demonstrated that both hCAP-18 and azurophil granule proteins were present in the phagolysosome. Immunoblotting revealed no detectable cleavage of hCAP-18 in cells after phagocytosis. In contrast, hCAP-18 was cleaved to generate LL-37 in exocytosed material. Of the 3 known serine proteases from azurophil granules, proteinase 3 was solely responsible for cleavage of hCAP-18 after exocytosis. This is the first detailed study describing the generation of a human antimicrobial peptide from a promicrobicidal protein, and it demonstrates that the generation of active antimicrobial peptides from common proproteins occurs differently in related species. (Blood. 2001;97:3951-3959)

c-Cbl ubiquitinates the EGF receptor at the plasma membrane and remains receptor associated throughout the endocytic route

Cbl family members have an evolutionarily conserved role in attenuating receptor tyrosine kinase function. Their negative regulatory capacity depends on a Ring finger domain that interacts with ubiquitin conjugating enzymes. Cbl molecules constitute a novel type of E3 or ubiquitin ligase family that is recruited to phosphotyrosine motifs. Ubiquitination of the receptor system is coupled to its downregulation, but it is unclear at which point in the endocytic pathway Cbl molecules come into play. Using low temperature and a dynamin mutant, we find that c-Cbl associates with and ubiquitinates the activated epidermal growth factor (EGF) receptor at the plasma membrane in the absence of endocytosis. With the aid of confocal microscopy and immunogold electron microscopy, we could demonstrate that c-Cbl associates with the EGF receptor at the plasma membrane prior to receptor recruitment into clathrin-coated pits and remains associated throughout the clathrin-mediated endocytic pathway. c-Cbl and the EGF receptor also colocalize in internal vesicles of multivesicular endosomes. Our data are consistent with a role for c-Cbl in clathrin-mediated endocytosis of tyrosine kinase receptors, as well as their intracellular sorting.

Mass, charge, and subcellular localization of a unique secretory product identified by the basophil-specific antibody BB1

BACKGROUND

BB1 is a basophil-specific mAb (Lab Invest 1999;79:27-38). The identity of the corresponding antigen has not been determined, but it gives a granular appearance on staining and is secreted on activation of basophils.

OBJECTIVE

We sought to further characterize the basophilspecific antigen identified by BB1.

METHODS

Intracellular localization was determined by flow cytometry and by immunogold labeling and electron microscopy. Physical chemical properties were investigated by gel filtration chromatography and preparative isoelectric focusing.

RESULTS

In flow cytometry, permeabilization of cells increased immunofluorescence 100-fold, confirming the predominantly intracellular localization of the antigen. It was further localized to the secretory granules by immunoelectron microscopy. Double labeling with a CD63-specific antibody demonstrated selective binding of BB1 to the granule matrix. Gel filtration chromatography indicated that the antigen is secreted as a complex of approximately 5 x 10(6) d, which was well resolved from the 210-kd supramolecular complex containing tryptase. The antigen was degraded by pronase. Isoelectric focusing indicated a highly basic protein with an isoelectric point of 9.6.

CONCLUSION

With its granule localization, release on cell activation, and unique properties, the antigen identified by BB1 could be a novel mediator of allergic disease. We propose the name basogranulin for this novel basophil-specific protein.

Sequential migration of neutrophils across monolayers of endothelial and epithelial cells

In the course of granulocyte-dominated lung inflammation, granulocytes migrate across the endothelium and epithelium of the lung and cause severe tissue damage. To study this process in more detail, we developed a bilayer transmigration model composed of primary human endothelial and lung epithelial cells, simultaneously cultured on opposite sides of Transwell filters. Electron microscopical analysis showed that the morphology of the cells and the expression of junctional proteins remained unaltered and that matrix components were deposited onto the filter. Intriguingly, neutrophil migration was more efficient across the bilayers than across single epithelial monolayers and did not differ from migration across single endothelial monolayers. Coculture experiments showed that endothelial cells stimulated epithelial cells to release IL-6 and that epithelial cells enhanced release of IL-8 from endothelial cells. Together these data reveal bidirectional signaling and enhanced neutrophil migration in a transmigration model of primary human epithelial and endothelial cells.

Immortalisation of human bone marrow endothelial cells: characterisation of new cell lines

BACKGROUND

Adhesion of haematopoietic progenitor cells (HPC) to human bone marrow endothelial cells (HBMEC) plays a key role in homing of HPC to bone marrow. Here we describe four new HBMEC cell lines that can be used to study the (specific) adhesion of HPC to HBMEC.

DESIGN

HBMEC were immortalised with a retroviral construct containing the human papilloma virus 16 E6/E7 genes. Four cell lines were characterised.

RESULTS

The cell lines showed their endothelial nature by the expression of von Willebrand Factor and VE-cadherin (CD144). Electron microscopic analysis revealed normal endothelial-cell characteristics, including the presence of Weibel-Palade bodies and intercellular junction structures. An extensive phenotypic analysis of the cell-lines was performed, they were found to resemble primary HBMEC. The only difference found was the absence of expression of E-selectin (CD62e) and VCAM-1 (CD106) on resting HBMEC cell lines. Upon stimulation with IL-1beta the expression of E-selectin, VCAM-1 and ICAM-1 (CD54) was upregulated. All resting cell lines bound CD34+ HPC. Adhesion was increased by addition of the phorbol ester PMA. Two cell lines showed increased binding upon IL-1beta prestimulation. Highest adhesion was observed after the combination of IL-1beta prestimulation of the endothelial cells and addition of PMA. Binding of CD34+ HPC to HBMEC was compared with the binding to human umbilical vein endothelial cell lines and to a human dermal microvascular endothelial cell line (HMEC-1). So far, we have only found relatively less binding of HPC to IL-1beta prestimulated HMEC-1 cells, which could be explained by a reduced induction of E-selectin and VCAM-1 upon IL-1beta stimulation of these cells.

CONCLUSION

The immortalised HBMEC cell lines have maintained their normal phenotype for the majority of characteristics examined. The expression of E-selectin and VCAM-1, which are not constitutively expressed on the cell lines, can be induced by stimulation of the endothelial cells with IL-1beta. The cell lines have furthermore maintained their capability to bind HPC. They will therefore be useful to investigate the interactions between HPC and HBMEC involved in homing of HPC.

The bactericidal/permeability-increasing protein (BPI) is membrane-associated in azurophil granules of human neutrophils, and relocation occurs upon cellular activation

Neutrophilic granulocytes contain the 55 kDa bactericidal/permeability-increasing protein (BPI). BPI binds to lipopolysaccharides (LPS), and exerts bacteriostatic and bactericidal effects against a wide variety of Gram-negative bacterial species. We have investigated the subcellular location of BPI in immature and mature neutrophils using cryotechnique for immunoelectron microscopy. BPI was found to colocate with myeloperoxidase (MPO), a marker for azurophil granules, and it also showed the same pattern of distribution as CD63, a transmembrane-anchored protein. This suggests that BPI is membrane-associated in the azurophil granules in neutrophils. Its presence in azurophil granules was further confirmed by the finding of BPI in the azurophil granules of neutrophil promyelocytes of the bone marrow. Induction of selective release of azurophilic granules by the Na-ionophore monensin resulted in fusion of endosomes with azurophil granules, leading to the formation of large vacuoles containing MPO, CD63, and BPI. After phagocytosis of serum-treated zymosan (STZ), BPI was detected in phagosomes, both in association with membranes as well as in the lumen, suggesting the release of BPI into activated compartments. The results show that BPI is present in azurophil granules, is probably primarily membrane-associated, and is relocated after activation, following the same route as MPO and CD63.

A Rab11-containing rapidly recycling compartment in macrophages that promotes phagocytosis

Macrophages are specialized cells of the immune system that exhibit a prodigious capacity for phagocytosis. The ability of macrophages to internalize a substantial proportion of their plasma membrane during phagocytosis indicates that they possess a mechanism for the rapid renewal of plasma membrane. We examined the role of endocytic membrane recycling in promoting phagocytosis. In contrast to many other cell types, macrophages lack a morphologically distinct peri-centriolar recycling compartment but instead demonstrate an extensive network of transferrin receptor-positive tubules and vesicles that participated in recycling. The rate of transferrin recycling in thioglycollate-elicited murine peritoneal macrophages (thio-macrophages) was exceedingly rapid, with exocytic rate constants that were 2- to 3-fold higher than those of most other cells. Because the GTPase Rab11 has been implicated in transferrin recycling in other cells, we determined its role in transferrin recycling and phagocytosis in macrophages. Macrophages expressing epitope-tagged Rab11 demonstrated the presence of Rab11 in several intracellular membrane compartments, including endosomes and nascent phagosomes. Expression of Rab11 25N, a GTP binding-deficient allele of Rab11, led to a decreased rate of transferrin efflux and impaired Fc(gamma)R-mediated phagocytosis, where Fc(gamma)R is the receptor for the Fc portion of IgG. In contrast, expression of Rab11 70L, a GTPase-deficient allele of Rab11, led to an increased rate of transferrin efflux and enhanced phagocytosis. We conclude that macrophages have adapted a rapidly mobilizable, endocytic compartment to enhance phagocytosis. Rab11 participates in the recruitment of this compartment to the macrophage cell surface.

ARF6 is required for growth factor- and rac-mediated membrane ruffling in macrophages at a stage distal to rac membrane targeting

Activation of Rac1, a member of the Rho family of GTPases, is associated with multiple cellular responses, including membrane ruffling and focal complex formation. The mechanisms by which Rac1 is coupled to these functional responses are not well understood. It was recently shown that ARF6, a GTPase implicated in cytoskeletal alterations and a membrane recycling pathway, is required for Rac1-dependent phagocytosis in macrophages (Q. Zhang et al., J. Biol. Chem. 273:19977-19981, 1998). To determine whether ARF6 is required for Rac1-dependent cytoskeletal responses in macrophages, we expressed wild-type (WT) or guanine nucleotide binding-deficient alleles (T27N) of ARF6 in macrophages coexpressing activated alleles of Rac1 (Q61L) or Cdc42 (Q61L) or stimulated with colony-stimulating factor 1 (CSF-1). Expression of ARF6 T27N but not ARF6 WT inhibited ruffles mediated by Rac1 Q61L or CSF-1. In contrast, expression of ARF6 T27N did not inhibit Rac1 Q61L-mediated focal complex formation and did not impair Cdc42 Q61L-mediated filopodial formation. Cryoimmunogold electron microscopy demonstrated the presence of ARF6 in membrane ruffles induced by either CSF-1 or Rac1 Q61L. Addition of CSF-1 to macrophages led to the redistribution of ARF6 from the interior of the cell to the plasma membrane, suggesting that this growth factor triggers ARF6 activation. Direct targeting of Rac1 to the plasma membrane did not bypass the blockade in ruffling induced by ARF6 T27N, indicating that ARF6 regulates a pathway leading to membrane ruffling that occurs after the activation and membrane association of Rac. These data demonstrate that intact ARF6 function is required for coupling activated Rac to one of several effector pathways and suggest that a principal function of ARF6 is to coordinate Rac activation with plasma membrane-based protrusive events.

Accurate intracellular localization of HLA-DM requires correct spacing of a cytoplasmic YTPL targeting motif relative to the transmembrane domain

HLA-DM is an MHC class II-related heterodimer that is targeted to lysosomal compartments by a tyrosine-based signal YTPL, present in the cytoplasmic tail of the beta chain. Similar signals in other proteins control transport to different intracellular locations and can be recognized at several sorting sites within the cell including the trans-Golgi network, the plasma membrane and the early or sorting endosome. Therefore, in addition to recognizing the basic tyrosine motif, the sorting machinery must be sensitive to additional features associated with these elements. Here we show that efficient trafficking of HLA-DM to lysosomal compartments is dependent upon the proximity of its tyrosine motif to the transmembrane domain. Constructs in which the spacing is altered are rapidly internalized but are expressed at the cell surface. We conclude that the spacing of the HLA-DMB-encoded tyrosine motif relative to the transmembrane domain is an important feature controlling DM sorting in endosomes.

Partial rescue of epithelial phenotype in integrin beta4 null mice by a keratin-5 promoter driven human integrin beta4 transgene

Integrin beta4 null mice exhibit extensive epidermal detachment, reminiscent of the human skin blistering disease junctional epidermolysis bullosa associated with pyloric atresia. Hemidesmosomes, the stable adhesion structures of squamous epithelia, are not formed in the absence of alpha6beta4. Null mutant mice die shortly after birth, but apart from their striking epithelial phenotype, no obvious developmental defects have been observed. To elucidate the cause of death in these mice, we generated transgenic mice with a heterologous construct consisting of the squamous epithelial-specific keratin-5 promoter and a human integrin beta4 subunit cDNA. The transgene was not expressed in the presence of endogenous beta4, probably as a result of competition for a limited pool of alpha6 subunits. In a beta4 null background, however, the transgene was expressed, and its expression pattern followed that of squamous epithelial-specific keratins. These rescued pups appeared healthy and ultrastructural analysis revealed that the interspecies heterodimer alpha6(mouse)/beta4(human) was sufficient to trigger the assembly of hemidesmosomes. After a variable period of up to 48 hours after birth these animals began to exhibit haemorrhages at the plantar and palmar areas. We observed the formation of small blisters and found that the transgene was not detectably expressed in this region, which is devoid of hair follicles. The rescued neonates became increasingly cyanotic and died soon after the onset of this phenomenon. We performed a developmental study of the expression of beta4 in the complete respiratory tract, but we found no correlation between the spatiotemporal distribution of beta4 and the onset of the respiratory insufficiency. It became clear, however, that there was a gradual detachment of squamous epithelia in the oral and nasal cavities which led to obstruction of the respiratory tract, suggesting that in beta4 null and rescued mice, neonatal death was a direct consequence of decreased adhesion properties of hairless squamous epithelia, rather than a developmental defect of the lungs.

Recycling MHC class I molecules and endosomal peptide loading

MHC class I molecules usually present peptides derived from endogenous antigens that are bound in the endoplasmic reticulum. Loading of exogenous antigens on class I molecules, e.g., in cross-priming, sometimes occurs, but the intracellular location where interaction between the antigenic fragment and class I takes place is unclear. Here we show that measles virus F protein can be presented by class I in transporters associated with antigen processing-independent, NH(4)Cl-sensitive manner, suggesting that class I molecules are able to interact and bind antigen in acidic compartments, like class II molecules. Studies on intracellular transport of green fluorescent protein-tagged class I molecules in living cells confirmed that a small fraction of class I molecules indeed enters classical MHC class II compartments (MIICs) and is transported in MIICs back to the plasma membrane. Fractionation studies show that class I complexes in MIICs contain peptides. The pH in MIIC (around 5.0) is such that efficient peptide exchange can occur. We thus present evidence for a pathway for class I loading that is shared with class II molecules.

A combination of megakaryocyte growth and development factor and interleukin-1 is sufficient to culture large numbers of megakaryocytic progenitors and megakaryocytes for transfusion purposes

Chemotherapy-induced thrombocytopenia is a major risk factor in cancer treatment. The transfusion of autologous ex vivo expanded megakaryocytes could be a new therapy to shorten the period of thrombocytopenia. Therefore we investigated, in a liquid culture system, the effect of various cytokine combinations composed of pegylated megakaryocyte growth and development factor (PEG-rHuMGDF), interleukin-1 (IL-1), IL-3, IL-6, IL-11 and stem cell factor (SCF) on the proliferation and differentiation of CD34+ cells, in order to define the most optimal and minimum levels of cytokine combinations for megakaryocyte expansion. Besides PEG-rHuMGDF, IL-1 was found to be important for optimal megakaryocyte expansion. Depletion of either SCF, IL-6 or IL-11 did not exert a large effect, but the absence of IL-1 strongly diminished the number of megakaryocytic cells. Addition of IL-3 to the combination PEG-rHuMGDF, IL-1, IL-6, IL-11 and SCF significantly reduced the number of megakaryocyte progenitors (CD34+CD41+ cells) and the number of CFU-Meg. Furthermore, we found a strong correlation between the number of CD34+CD41+ cells and the number of CFU-Meg obtained after 8 d culture. Our study shows that optimal ex vivo expansion of megakaryocytes is achieved by the combination of PEG-rHuMGDF and IL-1. The numbers of megakaryocytes and megakaryocyte progenitors (CD34+CD41+) obtained in our liquid culture system with the growth factor combination PEG-rHuMGDF and IL-1 are suitable for transfusion purposes.

Multivesicular body morphogenesis requires phosphatidyl-inositol 3-kinase activity

Multivesicular bodies are endocytic compartments containing multiple small vesicles that originate from the invagination and 'pinching off' of the limiting membrane into the luminal space [1] [2] [3]. The molecular mechanisms responsible for the formation of these compartments are unknown. In the human melanoma cell line Mel JuSo, newly synthesised major histocompatibility complex (MHC) class II molecules accumulate in multivesicular early lysosomes [4]. The phosphatidylinositol (PI) 3-kinase inhibitor wortmannin induced the transient vacuolation of early MHC class II compartments, but also of early and late endosomes. We demonstrate that endocytic membrane influx is required for the wortmannin-induced swelling of vesicles. The wortmannin-induced vacuoles contained a reduced number of intraluminal vesicles that were linked to the limiting membrane by membraneous connections. These data suggest that wortmannin inhibits the invagination and/or pinching off of intraluminal vesicles and provide evidence of a role for PI 3-kinase in multivesicular body morphogenesis. We propose that the wortmannin-induced vacuolation occurs as a result of the inability of multivesicular bodies to store endocytosed membranes as intraluminal vesicles thereby causing the formation of large 'empty' vacuoles.

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.

Fusion of azurophil granules with phagosomes and activation of the tyrosine kinase Hck are specifically inhibited during phagocytosis of mycobacteria by human neutrophils

Pathogenic mycobacteria parasitize macrophages and reside within phagosomes, which do not fuse with lysosomal granules. Mycobacteria are also internalized by neutrophils, which possess at least two types of granules, specific and azurophil granules, the latter being specialized lysosomes. Here, we investigated the ability of mycobacteria to inhibit the fusion of these granules with their phagosomes in human neutrophils. It was found that when pathogenic (Mycobacterium kansasii and Mycobacterium avium) or nonpathogenic (Mycobacterium smegmatis and Mycobacterium phlei) mycobacteria were internalized by neutrophils, they induced the inhibition of azurophil granule fusion with phagosomes even when they were serum opsonized. In contrast, secretion of specific granule content and production of O2-, both of which contribute to the neutrophil bactericidal response, were triggered. Hck is a Src family tyrosine kinase associated with azurophil granules. During internalization of zymosan, azurophil granules fused with phagosomes and Hck was activated and translocated to the phagosomal membrane, whereas in neutrophils engulfing mycobacteria, Hck did not translocate and remained unactivated. The activation of the tyrosine kinase Fgr was not affected. These results indicate that 1) pathogenic and nonpathogenic mycobacteria trigger similar bactericidal responses in neutrophils, 2) phagocytosis and fusion of azurophil granules can be uncoupled by mycobacteria, and 3) Hck could be one of the key elements of the azurophil secretory pathway that are altered during phagocytosis of mycobacteria.

Cre-loxP-mediated inactivation of the alpha6A integrin splice variant in vivo: evidence for a specific functional role of alpha6A in lymphocyte migration but not in heart development

Two splice variants of the alpha6 integrin subunit, alpha6A and alpha6B, with different cytoplasmic domains, have previously been described. While alpha6B is expressed throughout the development of the mouse, the expression of alpha6A begins at 8.5 days post coitum and is initially restricted to the myocardium. Later in ontogeny, alpha6A is found in various epithelia and in certain cells of the immune system. In this study, we have investigated the function of alpha6A in vivo by generating knockout mice deficient for this splice variant. The Cre- loxP system of the bacteriophage P1 was used to specifically remove the exon encoding the cytoplasmic domain of alpha6A in embryonic stem cells, and the deletion resulted in the expression of alpha6B in all tissues that normally express alpha6A. We show that alpha6A-/- mice develop normally and are fertile. The substitution of alpha6A by alpha6B does not impair the development and function of the heart, hemidesmosome formation in the epidermis, or keratinocyte migration. Furthermore, T cells differentiated normally in alpha6A-/- mice. However, the substitution of alpha6A by alpha6B leads to a decrease in the migration of lymphocytes through laminin-coated Transwell filters and to a reduction of the number of T cells isolated from the peripheral and mesenteric lymph nodes. Lymphocyte homing to the lymph nodes, which involves various types of integrin-ligand interactions, was not affected in the alpha6A knockout mice, indicating that the reduced number of lymph node cells could not be directly attributed to defects in lymphocyte trafficking. Nevertheless, the expression of alpha6A might be necessary for optimal lymphocyte migration on laminin in certain pathological conditions.

Small GTP-binding proteins in human endothelial cells

Small GTP-binding proteins of the Ras superfamily control an extensive number of intracellular events by alternating between GDP- and GTP-bound conformation. The presence of members of this protein family was examined in human umbilical vein endothelial cells employing RT-PCR. Sequence analysis of 215 cDNA clones revealed the presence of a total of 28 different partial cDNAs encoding small GTP-binding proteins. Two sequences corresponded to novel isoforms of Rab2 and Rab9. In addition, human analogues of Rab4b, Rab7, Rab9, Rab14 and Rab15 were identified. Besides Rab proteins, members of other subfamilies were detected as well. As a first step towards elucidation of the function of the different small GTP-binding proteins identified we have isolated full length cDNA corresponding to Rab30 from a human endothelial cell cDNA library. In order to assess the subcellular localization of Rab30, we expressed epitope-tagged Rab30 cDNA in monkey kidney COS-1 cells. Immunoelectron-microscopy of transfected COS-1 cells indicated that Rab30 is associated with Golgi stacks.

Removal and degradation of the free MHC class II beta chain in the endoplasmic reticulum requires proteasomes and is accelerated by BFA

We have studied the degradation of the free major histocompatibility complex (MHC) class II beta subunit in the ER. Domain swapping experiments demonstrate that both the intra- and extracellular domain determine the rate of degradation. Recently, it has been shown that some ER-retained proteins are exported from the ER by the translocon followed by deglycosylation and degradation in the cytosol by proteasomes. Degradation of the beta chain follows a different route. The proteasome is involved but inhibition of the proteasome by lactacystin does not result in deglycosylation and export to the cytosol. Instead, the beta chain is retained in the ER implying that extraction of the beta chain from the ER membrane requires proteasome activity. Surprisingly, brefeldin A accelerates the degradation of the beta chain by the proteasome. This suggests that various processes outside the ER are involved in ER-degradation. The ER is the site from where misfolded class II beta chains enter a proteasome-dependent degradation pathway.