A tyrosine-based sorting signal in the beta2 integrin cytoplasmic domain mediates its recycling to the plasma membrane and is required for ligand-supported migration

Unique CD18 mutations involving a deletion in the extracellular stalk region and a major truncation of the cytoplasmic domain in a patient with leukocyte adhesion deficiency type 1

Two novel CD18 mutations were identified in a patient who was a compound heterozygote with type 1 leukocyte adhesion deficiency and whose phenotype was typical except that he exhibited hypertrophic scarring. A deletion of 36 nucleotides in exon 12 (1622del36) predicted the net loss of 12 amino acid (aa) residues in the third cysteine-rich repeat of the extracellular stalk region (mut-1). A nonsense mutation in exon 15 (2200G>T), predicted a 36-aa truncation of the cytoplasmic domain (mut-2). Lymphocyte function-associated antigen 1 (LFA-1) and macrophage antigen-1 (Mac-1) containing the mut-1 beta(2) subunit were expressed at very low levels compared with wild-type (wt) beta(2). Mac-1 and LFA-1 expression with the mut-2 beta(2) subunit were equivalent to results with wt beta(2). Binding function of Mac-1 with mut-2 beta(2) was equivalent to that with wt beta(2). However, binding function of LFA-1 with the mut-2 beta(2) subunit was reduced by 50% versus wt beta(2). It was concluded that (1) the portion of the CD18 stalk region deleted in mut-1 is critical for beta(2) integrin heterodimer expression but the portion of the cytoplasmic domain truncated in mut-2 is not; and (2) the mut-2 cytoplasmic domain truncation impairs binding function of LFA-1 but not of Mac-1. Studies with the patient's neutrophils (PMNs) were consistent with functional impairment of LFA-1 but not of Mac-1.

Constitutive GABAA receptor endocytosis is dynamin-mediated and dependent on a dileucine AP2 adaptin-binding motif within the beta 2 subunit of the receptor

Receptor endocytosis is an important mechanism for regulating the synaptic efficacy of neurotransmitters. There is strong evidence that GABA(A) receptor endocytosis is clathrin-dependent; however, this process is not well understood. Here we demonstrate that in HEK 293 cells, endocytosis of GABA(A) receptors composed of either alpha1beta2gamma2Lor alpha1beta2 subunits is blocked by the dominant negative dynamin construct K44A. Furthermore, we identify a dileucine AP2 adaptin-binding motif within the receptor beta2 subunit that is critical for endocytosis. Internalization of GABAA receptors lacking this motif is dramatically inhibited, and the receptors appear to accumulate on the cell surface. Patch clamp analysis of receptors lacking the dileucine motif show that there is an increase in the peak amplitude of GABA-gated chloride currents compared with wild-type receptors. Additionally, GABA-gated chloride currents in HEK 293 cells expressing wild-type receptors are increased by introduction of a peptide corresponding to the dileucine motif region of the receptor beta2 subunit but not by a control peptide containing alanine substitutions for the dileucine motif. In mouse brain cerebral cortical neurons, the dileucine motif peptide increases GABA-gated chloride currents of native GABA(A) receptors. This is the first report to our knowledge that an AP2 adaptin dileucine recognition motif is critical for the endocytosis of ligand-gated ion channels belonging to this superfamily.

The critical cytoplasmic regions of the alphaL/beta2 integrin in Rap1-induced adhesion and migration

Rap1 is a potent inside-out signal that increases LFA-1 adhesive activity. In this study, we have defined the cytoplasmic region of the alphaL and beta2 integrin that are required for Rap1-stimulated adhesion and subsequent migration on ICAM-1. Human LFA-1 bearing truncated and point-mutated alphaL and beta2 cytoplasmic regions were reconstituted in mouse IL-3-dependent proB cells, BAF/3. Truncation of the alphaL, but not beta2 subunit cytoplasmic region, abolished Rap1V12-dependent adhesion to ICAM-1. The alanine substitution of two lysine residues (K1097/K1099) in the alphaL subunit was found to be critical in adhesion induced by Rap1V12, but not PMA. This mutation suppressed Rap1V12-induced LFA-1 conformation changes and ligand-binding affinity. The K1097/K1099 mutation also impaired binding to ICAM-1 induced by TCR cross-linking or SDF-1. In contrast, the alanine substitution for tyrosine in the beta2 subunit endocytosis motif inhibited internalization of LFA-1, and severely impaired detachment at the cell rear, which resulted in long-elongated cell shapes. This result demonstrates that internalization of LFA-1 is a critical step in the deadhesion process. Our study revealed novel requirements of amino acid residues of the LFA-1 cytoplasmic region in the response to the inside-out signaling and the subsequent deadhesion process.

GIP, a G-protein-coupled receptor interacting protein

A novel protein was cloned while screening for partners interacting with the second intracellular loop of the V2 vasopressin receptor (V2R). The protein was named GIP as in G-protein-coupled receptor Interacting Protein; the corresponding gene was located on the 17th chromosome where three exons encode for a 379-amino-acid protein.GIP subcellular localization was studied by immunocytochemistry and also using a biotinylating agent. The protein was found to be localized, at least in part, on the plasma membrane, probably in the form of a trimer. The results indicated that GIP is a transmembrane protein and the most part of the molecule is intracellular. Sequence homology inferred that GIP cytosolic domain is folded as a collagen-like helix followed by a globular domain. The interaction of the globular domain with the V2R was confirmed by pull-down experiments indicating that this structural motif can also interact with cytosolic proteins.

Palmitoylation of tetraspanin proteins: modulation of CD151 lateral interactions, subcellular distribution, and integrin-dependent cell morphology

Here we demonstrate that multiple tetraspanin (transmembrane 4 superfamily) proteins are palmitoylated, in either the Golgi or a post-Golgi compartment. Using CD151 as a model tetraspanin, we identified and mutated intracellular N-terminal and C-terminal cysteine palmitoylation sites. Simultaneous mutations of C11, C15, C242, and C243 (each to serine) eliminated >90% of CD151 palmitoylation. Notably, palmitoylation had minimal influence on the density of tetraspanin protein complexes, did not promote tetraspanin localization into detergent-resistant microdomains, and was not required for CD151-alpha 3 beta 1 integrin association. However, the CD151 tetra mutant showed markedly diminished associations with other cell surface proteins, including other transmembrane 4 superfamily proteins (CD9, CD63). Thus, palmitoylation may be critical for assembly of the large network of cell surface tetraspanin-protein interactions, sometimes called the "tetraspanin web." Also, compared with wild-type CD151, the tetra mutant was much more diffusely distributed and showed markedly diminished stability during biosynthesis. Finally, expression of the tetra-CD151 mutant profoundly altered alpha 3 integrin-deficient kidney epithelial cells, such that they converted from a dispersed, elongated morphology to an epithelium-like cobblestone clustering. These results point to novel biochemical and biological functions for tetraspanin palmitoylation.

The tyrosine motifs of Lamp 1 and LAP determine their direct and indirect targetting to lysosomes

Lamp 1 and lysosomal acid phosphatase (LAP) are lysosomal membrane proteins that harbour a tyrosine-based sorting motif within their short cytoplasmic tails. Lamp 1 is delivered from the trans-Golgi network (TGN) via endosomes directly to lysosomes bypassing the plasma membrane, whereas LAP is indirectly transported to lysosomes and recycles between endosomes and the plasma membrane before being delivered to lysosomes. By analysing truncated forms of LAP and chimeras in which the cytoplasmic tail or part of the cytoplasmic tails of LAP and Lamp 1 were exchanged, we were able to show that the YRHV tyrosine motif of LAP is necessary and sufficient to mediate recycling between endosomes and the plasma membrane. When peptides corresponding to the cytoplasmic tails of LAP and Lamp 1 and chimeric or mutant forms of these tails were assayed for in vitro binding of AP1 and AP2, we found that AP2 bound to LAP- and Lamp-1-derived peptides, whereas AP1 bound only to peptides containing the YQTI tyrosine motif of Lamp 1. Residues +2 and +3 of the tyrosine motif were critical for the differential binding of adaptors. LAP in which these residues (-HV) were substituted for those of Lamp 1 (-TI) was transported directly to lysosomes, whereas a chimera carrying the Lamp 1 tail in which residues +2 and +3 were substituted for those of LAP (-HV) gained the ability to recycle. In conclusion, the residues +2 and +3 of the tyrosine motifs determine the sorting of Lamp 1 and LAP in endosomes, mediating either the direct or the indirect pathway to lysosomes.

Calcium signaling through the beta 2-cytoplasmic domain of LFA-1 requires intracellular elements of the T cell receptor complex

The beta(2) integrin LFA-1 is an important cell-cell adhesion receptor of the immune system. Evidence suggests that the molecule also participates in signaling and co-stimulatory function. We show here that clustering of the intracellular domain of the beta(2) chain but not of the alpha(L)- or beta(1)-cytoplasmic domains, respectively, triggers intracellular Ca(2+) mobilization in Jurkat cells. A beta(2)-specific NPXF motif, located in the C-terminal portion of the beta(2) tail, is required for Ca(2+) signaling, and we show that this motif is important for the induction of allo-specific target cell lysis by cytotoxic T cells in vitro. Significantly, the Ca(2+)-signaling capacity of the beta(2) integrin is abrogated in T cells that do not express the T cell receptor but may be reconstituted by co-expression of the T cell receptor-zeta chain. Our data suggest a specific function of the cytoplasmic domain of the beta(2) integrin chain in T cell signaling.

Structural and functional characterization of protein 4.1R-phosphatidylserine interaction: potential role in 4.1R sorting within cells

Erythrocyte protein 4.1R is a multifunctional protein that binds to various membrane proteins and to phosphatidylserine. In the present study, we report two important observations concerning 4.1R-phosphatidylserine interaction. Biochemically, a major finding of the present study is that 4.1R binding to phosphatidylserine appears to be a two-step process in which 4.1R first interacts with serine head group of phosphatidylserine through the positively charged amino acids YKRS and subsequently forms a tight hydrophobic interaction with fatty acid moieties. 4.1R failed to dissociate from phosphatidylserine liposomes under high ionic strength but could be released specifically by phospholipase A(2) but not by phospholipase C or D. Biochemical analyses showed that acyl chains were associated with 4.1R released by phospholipase A(2). Importantly, the association of acyl chains with 4.1R impaired its ability to interact with calmodulin, band 3, and glycophorin C. Removal of acyl chains restored 4.1R binding. These data indicate that acyl chains of phosphatidylserine play an important role in its interaction with 4.1R and on 4.1R function. In terms of biological significance, we have obtained evidence that 4.1R-phosphatidylserine interaction may play an important role in cellular sorting of 4.1R.

PDGF-regulated rab4-dependent recycling of alphavbeta3 integrin from early endosomes is necessary for cell adhesion and spreading

BACKGROUND

It has been postulated that the regulation of integrin vesicular traffic facilitates cell migration by internalizing integrins at the rear of the cell and transporting them forward within vesicles for exocytosis at the leading edge to form new contacts with the extracellular matrix. The rab family of GTPases control key targeting events in the endo/exocytic pathway; therefore, these GTPases may be involved in the regulation of cell-matrix contact assembly.

RESULTS

The endo/exocytic cycle of alphavbeta3 and alpha5beta1 integrins was studied using mouse 3T3 fibroblast cell lines. In serum-starved cells, internalized integrins were transported through rab4-positive, early endosomes and arrived at the rab11-positive, perinuclear recycling compartment approximately 30 min after endocytosis. From the recycling compartment, integrins were recycled to the plasma membrane in a rab11-dependent fashion. Following treatment with PDGF, alphavbeta3 integrin, but not alpha5beta1, was rapidly recycled directly back to the plasma membrane from the early endosomes via a rab4-dependent mechanism without the involvement of rab11. This rapid recycling pathway directed alphavbeta3 to numerous small puncta distributed evenly across the dorsal surface of the cell, and the integrin only became localized into focal complexes at later times following PDGF addition. Interestingly, inhibition of PDGF-stimulated alphavbeta3 recycling using dominant-negative rab4 mutants compromised cell adhesion and spreading on vitronectin (a ligand for alphavbeta3), but adhesion to fibronectin (a ligand for alpha5beta1 and alphavbeta3) was unchanged.

CONCLUSIONS

We propose that growth factor-regulated, rab4-dependent recycling of alphavbeta3 integrin from early endosomes to the plasma membrane is a critical upstream event in the assembly of cell-matrix contacts.

Rat neuronal leucine-rich repeat protein-3: cloning and regulation of the gene expression

Rat neuronal leucine-rich repeat protein-3 (rNLRR-3) gene was isolated and cloned from fibrosarcoma cells overexpressing c-Ha-ras. Stable expression of constitutively active forms of Ras (H-Ras(V12) or v-H-Ras) led to a two- to fourfold increase in rNLRR-3 mRNA in rat normal fibroblasts (3Y1). When cells expressing H-Ras(V12) were treated with mitogen activated protein kinase (MAPK) kinase inhibitors (U0126, PD98059), suppression of rNLRR-3 mRNA correlated well with a reduction in MAPK activity. Epidermal growth factor (EGF) led to elevation of rNLRR-3 gene expression about 4 h after stimulation of normal fibroblasts. U0126 completely suppressed the induction by EGF of rNLRR-3 mRNA with abrogation of MAPK phosphorylation. U0126 inhibited the basal transcription of rNLRR-3. LY294002, a PI3 kinase inhibitor, showed a lesser effect on expression of the gene. These results indicate that rNLRR-3 gene expression is regulated mainly through the Ras-MAPK signaling pathway in fibroblasts.

Green fluorescent protein (GFP) tagged to the cytoplasmic tail of alphaIIb or beta3 allows the expression of a fully functional integrin alphaIIb(beta3): effect of beta3GFP on alphaIIb(beta3) ligand binding

Using green fluorescent protein (GFP) as an autofluorescent tag, we report the first successful visualization of a beta3 integrin in a living cell. GFP fused in frame to the cytoplasmic tail of either alphaIIb or beta3 allowed normal expression, heterodimerization, processing and surface exposure of alphaIIbGFPbeta3 and alphaIIb(beta3)GFP receptors in Chinese hamster ovary (CHO) cells. Direct microscopic observation of the autofluorescent cells in suspension following antibody-induced alphaIIb(beta3) capping revealed an intense autofluorescent cap corresponding to unlabelled immunoclustered GFP-tagged alphaIIb(beta3). GFP-tagged alphaIIbbeta3 receptors mediated fibrinogen-dependent cell adhesion, were readily detectable in focal adhesions of unstained living cells and triggered p125(FAK) tyrosine phosphorylation similar to wild-type alphaIIb(beta3) (where FAK corresponds to focal adhesion kinase). However, GFP tagged to beta3, but not to alphaIIb, induced spontaneous CHO cell aggregation in the presence of soluble fibrinogen, as well as binding of the fibrinogen mimetic monoclonal antibody PAC1 in the absence of alphaIIb(beta3) receptor activation. Time-lapse imaging of living transfectants revealed a characteristic redistribution of GFP-tagged alphaIIb(beta3) during the early stages of cell attachment and spreading, starting with alphaIIb(beta3) clustering at the rim of the cell contact area, that gradually overlapped with the boundary of the attached cell, and, with the onset of cell spreading, to a reorganization of alphaIIb(beta3) in focal adhesions. Taken together, our results demonstrate that (1) fusion of GFP to the cytoplasmic tail of either alphaIIb or beta3 integrin subunits allows normal cell surface expression of a functional receptor, and (2) structural modification of the beta3 integrin cytoplasmic tail, rather than the alphaIIb subunit, plays a major role in alphaIIb(beta3) affinity modulation. With the successful direct visualization of functional alphaIIb(beta3) receptors in living cells, the generation of autofluorescent integrins in transgenic animals will become possible, allowing new approaches to study the dynamics of integrin functions.

Dual function of rhoD in vesicular movement and cell motility

The trafficking of intracellular membranes requires the coordination of membrane-cytoskeletal interactions. Rab proteins are key players in the regulation of vesicular transport, while Rho family members control actin-dependent cell functions. We have previously identified a rho protein, rhoD, which is localized to the plasma membrane and early endosomes. When overexpressed, rhoD alters the actin cytoskeleton and plays an important role in endosome organization. We found that a rhoD mutant exerts its effect on early endosome dynamics through an inhibition in organelle motility. In these studies, the effect of rhoD on endosome dynamics was evaluated in the presence of a constitutively active, GTPase-deficient mutant of rab5, rab5Q79L. As rab5Q79L itself stimulates endosome motility, rhoD might counteract this stimulation, without itself exerting any effect in the absence of rab5 activation. We have now addressed this issue by investigating the effect of rhoD in the absence of co-expressed rab5. We find that rhoDG26V alone alters vesicular dynamics. Vesicular movement, in particular the endocytic/recycling circuit, is altered during processes such as cell motility. Due to the participation of vesicular motility and cytoskeletal rearrangements in cell movement and the involvement of rhoD in both, we have addressed the role of rhoD in this process and have found that rhoDG26V inhibits endothelial cell motility.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

IgCAMs: bidirectional signals underlying neurite growth

Understanding how immunoglobulin superfamily cell adhesion molecules (IgCAMs) regulate nervous system development has lagged behind studies on integrins and cadherins. The recent characterization of IgCAM structures combined with cell biological studies on protein-protein interactions and membrane targeting/trafficking demonstrate that IgCAMs interact in exceedingly complex ways to regulate axonal growth and pathfinding.

Ceramide-induced TCR up-regulation

The TCR is a constitutively recycling receptor meaning that a constant fraction of TCR from the plasma membrane is transported inside the cell at the same time as a constant fraction of TCR from the intracellular pool is transported to the plasma membrane. TCR recycling is affected by protein kinase C activity. Thus, an increase in protein kinase C activity affects TCR recycling kinetics leading to a new TCR equilibrium with a reduced level of TCR expressed at the T cell surface. Down-regulation of TCR expression compromises T cell activation. Conversely, TCR up-regulation is expected to increase T cell responsiveness. The purpose of this study was to identify and characterize potential pathways for TCR up-regulation. We found that ceramide affected TCR recycling dynamics and induced TCR up-regulation in a concentration- and time-dependent manner. Experiments applying phosphatase inhibitors indicated that ceramide-induced TCR up-regulation was most probably mediated by serine/threonine protein phosphatase 2A. Analyses of T cell variants demonstrated that TCR up-regulation was dependent on the presence of an intact CD3gamma L-based motif and thus acted on TCR engaged in the recycling pathway. Finally, we showed that TCR up-regulation probably plays a physiological role by increasing T cell responsiveness. Thus, by affecting the TCR recycling kinetics, T cells have the potential both to up- and down-regulate TCR expression and thereby adjust T cell responsiveness.

Effect of integrin beta 2 subunit truncations on LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) assembly, surface expression, and function

LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) are members of the beta2 integrins involved in leukocyte function during immune and inflammatory responses. We aimed to determine a minimized beta2 subunit that forms functional LFA-1 and Mac-1. Using a series of truncated beta2 variants, we showed that the subregion Q23-D300 of the beta2 subunit is sufficient to combine with the alphaL and alphaM subunits intracellularly. However, only the beta2 variants terminating after Q444 promote cell surface expression of LFA-1 and Mac-1. Thus, the major cysteine-rich region and the three highly conserved cysteine residues at positions 445, 447, and 449 of the beta2 subunit are not required for LFA-1 and Mac-1 surface expression. The surface-expressed LFA-1 variants are constitutively active with respect to ICAM-1 adhesion and these variants express the activation reporter epitope of the mAb 24. In contrast, surface-expressed Mac-1, both the wild type and variants, require 0. 5 mM MnCl2 for adhesion to denatured BSA. These results suggest that the role of the beta2 subunit in LFA-1- and Mac-1-mediated adhesion may be different.

Integrin LFA-1 interacts with the transcriptional co-activator JAB1 to modulate AP-1 activity

Integrin adhesion receptors transduce signals that control complex cell functions which require the regulation of gene expression, such as proliferation, differentiation and survival. Their intracellular domain has no catalytic function, indicating that interaction with other transducing molecules is crucial for integrin-mediated signalling. Here we have identified a protein that interacts with the cytoplasmic domain of the beta2 subunit of the alphaL/beta2 integrin LFA-1. This protein is JAB1 (Jun activation domain-binding protein 1), a coactivator of the c-Jun transcription factor. We found that JAB1 is present both in the nucleus and in the cytoplasm of cells and that a fraction of JAB1 colocalizes with LFA-1 at the cell membrane. LFA-1 engagement is followed by an increase of the nuclear pool of JAB1, paralleled by enhanced binding of c-Jun-containing AP-1 complexes to their DNA consensus site and increased transactivation of an AP-1-dependent promoter. We suggest that signalling through the LFA-1 integrin may affect c-Jun-driven transcription by regulating JAB1 nuclear localization. This represents a new pathway for integrin-dependent modulation of gene expression.