A T-lymphoma transmembrane glycoprotein (gp180) is linked to the cytoskeletal protein, fodrin

A Review on Adducin from Functional to Pathological Mechanisms: Future Direction in Cancer

Adducin (ADD) is a family of membrane skeleton proteins including ADD1, ADD2, and ADD3 that are encoded by distinct genes on different chromosomes. Adducin is primarily responsible for the assembly of spectrin-actin network that provides physical support to the plasma membrane and mediates signal transduction in various cellular physiological processes upon regulation by protein kinase C-dependent and calcium/calmodulin-dependent pathways. Abnormal phosphorylation, genetic variations, and alternative splicing of adducin may contribute to alterations in cellular functions involved in pathogenic processes. These alterations are associated with a wide range of diseases including cancer. This paper begins with a discussion on how adducin partakes in the structural formation of membrane skeleton, its regulation, and related functional characteristics, followed by a review on the pathogenesis of hypertension, biliary atresia, and cancer with respect to increased disease susceptibility mediated by adducin polymorphism and/or dysregulation. Given the functional diversity of adducin in different cellular compartments, we aim to provide a knowledge base whereby its pathophysiological roles can be better understood. More importantly, we aim to provide novel insights that may be of significance in turning the adducin model to clinical application.

Role of DHA in aging-related changes in mouse brain synaptic plasma membrane proteome

Aging has been related to diminished cognitive function, which could be a result of ineffective synaptic function. We have previously shown that synaptic plasma membrane proteins supporting synaptic integrity and neurotransmission were downregulated in docosahexaenoic acid (DHA)-deprived brains, suggesting an important role of DHA in synaptic function. In this study, we demonstrate aging-induced synaptic proteome changes and DHA-dependent mitigation of such changes using mass spectrometry-based protein quantitation combined with western blot or messenger RNA analysis. We found significant reduction of 15 synaptic plasma membrane proteins in aging brains including fodrin-α, synaptopodin, postsynaptic density protein 95, synaptic vesicle glycoprotein 2B, synaptosomal-associated protein 25, synaptosomal-associated protein-α, N-methyl-D-aspartate receptor subunit epsilon-2 precursor, AMPA2, AP2, VGluT1, munc18-1, dynamin-1, vesicle-associated membrane protein 2, rab3A, and EAAT1, most of which are involved in synaptic transmission. Notably, the first 9 proteins were further reduced when brain DHA was depleted by diet, indicating that DHA plays an important role in sustaining these synaptic proteins downregulated during aging. Reduction of 2 of these proteins was reversed by raising the brain DHA level by supplementing aged animals with an omega-3 fatty acid sufficient diet for 2 months. The recognition memory compromised in DHA-depleted animals was also improved. Our results suggest a potential role of DHA in alleviating aging-associated cognitive decline by offsetting the loss of neurotransmission-regulating synaptic proteins involved in synaptic function.

Reorganization of the human neutrophil plasma membrane is associated with functional priming: implications for neutrophil preparations

Changes in the functional and plasma membrane organizational states of human neutrophils were examined using two isolation procedures, which may simulate altered physiological states in vivo. A gelatin-based method of blood-neutrophil isolation was used to model in vivo priming, and neutrophils isolated by this method were compared with control populations prepared by a pyrogen-free, dextran-based method. Gelatin-prepared neutrophils were functionally primed for adherence and agonist-stimulated superoxide generation relative to unprimed, control neutrophils. The organizational state of the membrane cortex was examined by mapping the subcellular distribution of select cortical and transmembrane proteins by several methods, including subcellular fractionation, indirect immunofluorescence, and compositional analysis of Triton X-100-insoluble membrane skeleton preparations. Filamentous actin, fodrin, and the fodrin anchor, CD45, were largely cytoplasmic in unprimed neutrophils but translocated to plasma membranes upon priming, whereas CD43 and ezrin were exclusively surface-associated in both populations. Isopycnic sucrose density gradient analysis of N(2)-cavitated neutrophils revealed a major shift in the distribution of surface-associated transmembrane and membrane cortical components relative to the plasma membrane marker alkaline phosphatase in primed but not unprimed neutrophils. Similar results were obtained after neutrophil stimulation with known priming agents, LPS, TNF-alpha, or GM-CSF. Together, these results may suggest that priming of suspended, circulating neutrophils is associated with a large-scale reorganization of the plasma membrane and associated membrane cortex in a process that is independent of cellular adhesion and gross morphologic polarization.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

CD45 autoantibodies mediate neutralization of activated T cells from lupus patients through anergy or apoptosis

The objectives were to provide estimates of the prevalence of autoantibody (Ab) directed to CD45 in lupus patients, identify the target autoantigen(s), and determine the ability of such reactivity to mediate neutralization of T lymphocytes. Sera from 64 patients were studied using 2 assays: Western blot and an ELISA with CD45 eluted from 3 cell lines as antigen (U937, Jurkat and Daudi). The role of carbohydrate specificity was investigated using enzyme digestion of blotted glycans, competition with sugars, and inhibition with lectins. Apoptosis was studied through annexin V binding, and cell cycle analysis using the propidium iodide method. AutoAb to CD45 were detected in 16/64 sera (25%) by Western blot, and 21/32 sera (66%) found positive in the ELISA. CD45 purified from Daudi cells was identified in the ELISA, but not in the blot. AutoAb were of the IgM and the IgG isotypes, but not specific for a particular cell type or CD45 isoform: 2 dominant specificities were recognized, one against p180, and another against high MW isoforms. Neuraminidase-induced enhancement of reactivity, together with the inhibitory effect of N-acetyl galactosamine and Dolichos diflorus lectin suggest that the epitopes are carbohydrates. AutoAb which were specific for activated CD4+T cells triggered the annexin V binding, and, in 2 of 4 cases, lymphocytes underwent apoptosis. In conclusion, carbohydrate conformational epitopes may be important as target antigens, and some CD45 autoAb have the capacity to neutralize activated T cells through anergy or apoptosis.

Local accumulation of alpha-spectrin-related protein under plasma membrane during capping and phagocytosis in Acanthamoeba

During capping and phagocytosis the interaction between cluster cell surface receptors and the submembraneous actin-based skeleton may be mediated by spectrin-like proteins. To test this possibility we examined the localization of an alpha-spectrin immunoanalogue, that had been previously identified in whole extracts of Acanthamoeba, during capping of Con A receptors and during phagocytosis of Con A-coated yeast. During capping alpha-spectrin and filamentous actin co-migrated with the Con A receptors and accumulated in the region of cap formation, as demonstrated by double immunofluorescence studies. Immunoelectron microscopy revealed submembraneous location of alpha-spectrin in cells exposed to Con A, both at the time of initial cross-linking and during accumulation of alpha-spectrin in the region of the cap. Phagocytosis studies showed that alpha-spectrin and actin filaments were concentrated around phagocytic cups that enclosed ConA-coated yeast upon internalization. The proteins also surrounded nascent phagosomes present in the vicinity of the plasma membrane but were absent at the later time point of phagosome maturation. These data demonstrate a correlation between clustering of cell surface receptors and submembraneous localization of alpha-spectrin, suggesting an involvement of spectrin-like proteins in mediating the interaction of receptor clusters with the actin cytoskeleton.

Association of L-arginine transporters with fodrin: implications for hypoxic inhibition of arginine uptake

In this study, we investigated the possible interaction between the cationic amino acid transporter (CAT)-1 arginine transporter and ankyrin or fodrin. Because ankyrin and fodrin are substrates for calpain and because hypoxia increases calpain expression and activity in pulmonary artery endothelial cells (PAEC), we also studied the effect of hypoxia on ankyrin, fodrin, and CAT-1 contents in PAEC. Exposure to long-term hypoxia (24 h) inhibited L-arginine uptake by PAEC, and this inhibition was prevented by calpain inhibitor 1. The effects of hypoxia and calpain inhibitor 1 were not associated with changes in CAT-1 transporter content in PAEC plasma membranes. However, hypoxia stimulated the hydrolysis of ankyrin and fodrin in PAEC, and this could be prevented by calpain inhibitor 1. Incubation of solubilized plasma membrane proteins with anti-fodrin antibodies resulted in a 70% depletion of CAT-1 immunoreactivity and in a 60% decrease in L-arginine transport activity in reconstituted proteoliposomes (3,291 +/- 117 vs. 8,101 +/- 481 pmol. mg protein(-1). 3 min(-1) in control). Incubation with anti-ankyrin antibodies had no effect on CAT-1 content or L-arginine transport in reconstituted proteoliposomes. These results demonstrate that CAT-1 arginine transporters in PAEC are associated with fodrin, but not with ankyrin, and that long-term hypoxia decreases L-arginine transport by a calpain-mediated mechanism that may involve fodrin proteolysis.

Mechanical anchoring strength of L-selectin, beta2 integrins, and CD45 to neutrophil cytoskeleton and membrane

The strength of anchoring of transmembrane receptors to cytoskeleton and membrane is important in cell adhesion and cell migration. With micropipette suction, we applied pulling forces to human neutrophils adhering to latex beads that were coated with antibodies to CD62L (L-selectin), CD18 (beta2 integrins), or CD45. In each case, the adhesion frequency between the neutrophil and bead was low, and our Monte Carlo simulation indicates that only a single bond was probably involved in every adhesion event. When the adhesion between the neutrophil and bead was ruptured, it was very likely that receptors were extracted from neutrophil surfaces. We found that it took 1-2 s to extract an L-selectin at a force range of 25-45 pN, 1-4 s to extract a beta2 integrin at a force range of 60-130 pN, and 1-11 s to extract a CD45 at a force range of 35-85 pN. Our results strongly support the conclusion that, during neutrophil rolling, L-selectin is unbound from its ligand when the adhesion between neutrophils and endothelium is ruptured.

Adenosine diphosphate (ADP)-ribosylation of the guanosine triphosphatase (GTPase) rho in resting peripheral blood human T lymphocytes results in pseudopodial extension and the inhibition of T cell activation

Scrape loading Clostridium botulinum C3 exoenzyme into primary peripheral blood human T lymphocytes (PB T cells) efficiently adenosine diphosphate (ADP)-ribosylates and thus inactivates the guanosine triphosphatase (GTPase) Rho. Basal adhesion of PB T cells to the beta1 integrin substrate fibronectin (Fn) was not inhibited by inactivation of Rho, nor was upregulation of adhesion using phorbol myristate acetate (PMA; 10 ng/ml) or Mn++ (1 mM) affected. Whereas untreated PB T cells adherent to Fn remain spherical, C3-treated PB T cells extend F-actin-containing pseudopodia. Inactivation of Rho delayed the kinetics of PMA-dependent PB T cell homotypic aggregation, a process involving integrin alphaLbeta2. Although C3 treatment of PB T cells did not prevent adhesion to the beta1 integrin substrate Fn, it did inhibit beta1 integrin/CD3-mediated costimulation of proliferation. Analysis of intracellular cytokine production at the single cell level demonstrated that ADP-ribosylation of Rho inhibited beta1 integrin/ CD3 and CD28/CD3 costimulation of IL-2 production within 6 h of activation. Strikingly, IL-2 production induced by PMA and ionomycin was unaffected by C3 treatment. Thus, the GTPase Rho is a novel regulator of T lymphocyte cytoarchitecture, and functional Rho is required for very early events regulating costimulation of IL-2 production in PB T cells.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Involvement of CD44 and the cytoskeletal linker protein ankyrin in human neutrophil bacterial phagocytosis

The leukocyte CD44 and CD45 cell surface receptors are associated via the linker proteins ankyrin and fodrin with the cytoskeleton, which itself is important in immune cell functions such as adherence, chemotaxis, and phagocytosis. The effects of rat antihuman CD44 and CD45 monoclonal antibodies on phagocytosis of fluoresceinated heat-killed Staphylococcus aureus 502A by normal human neutrophils (PMNs) during 2 hr incubation in RPMI-1640 was studied via flow cytometry and confocal microscopy. Flow cytometry was performed using an excitation wavelength of 488 nm, fluorescence being measured at 515-560 nm on 50,000 PMNs per sample. Confocal microscopy was performed on samples after further incubation with rhodamine-conjugated antiankyrin. Anti-CD44 resulted in an increase of 27-31% compared to control (P = 0.004) in the proportion of PMNs fluorescing, an increase of 17-24% (P = 0.001) in mean intracellular fluorescence per PMN, and an increase in total PMN fluorescence of 50-58% compared to control (P < 0.001). In contrast, anti-CD45 had little effect on phagocytosis. Colchicine (a microtubule-disrupting agent) enhanced, whereas cytochalasin-D (a microfilament inhibitor) inhibited bacterial phagocytosis; cytochalasin-D completely abrogated the effect of anti-CD44 on this PMN function. Hyaluronic acid augmented phagocytosis by an increment similar to that observed with anti-CD44. Two-color flow cytometry and confocal microscopy demonstrated that ankyrin always colocalized with ingested fluorescein isothiocyanate (FITC)-labeled bacteria. These data strongly suggest that CD44 is involved in bacterial phagocytosis, provide further evidence of CD44 receptor linkage to cytoskeletal elements in human leukocytes, and suggest that ankyrin has a significant role in the transport of phagosomes.

Supplemental L-arginine HCl augments bacterial phagocytosis in human polymorphonuclear leukocytes

That L-arginine (L-Arg) augments the host response to acute bacterial sepsis suggests that this amino acid intervenes early in the immune response, perhaps via the nitric oxide synthetase (NOS) pathway. The effect of L-Arg supplementation on in vitro phagocytosis of fluorescein-labeled, heat-killed Staphylococcus aureus by peripheral blood neutrophils (PMNs) from 12 normal human volunteers was studied. Separated PMNs were incubated for 2 h with labeled bacteria, with and without supplemental L-Arg, D-arginine, glycine, and/or the NOS inhibitors L-canavanine, aminoguanidine, or L-NG-nitroarginine methyl ester. PMNs were fixed and extracellular fluorescence quenched with crystal violet. By flow cytometry and confocal microscopy, L-Arg supplementation was shown to result in a highly significant increase in PMN bacterial phagocytosis, the maximal effect being seen with L-Arg 380 microM and falling off with higher concentrations. This augmentation was completely abrogated by NOS inhibitors in molar excess, but inhibitors alone did not suppress phagocytosis below that of unsupplemented controls. Neither D-arginine nor glycine affected phagocytosis; the L-Arg effect was stereospecific and not related to utilization of L-Arg as an energy source. L-Arg supplementation significantly enhances bacterial phagocytosis in human neutrophils, perhaps by effects on cytoskeletal phenomena, and this appears to be mediated through NOS activity. Phagocytosis by nonspecific immune cells which intervene early in the response to sepsis is critically important, and beneficial effects of L-Arg on the clinical course of sepsis may be due at least in part to augmentation of phagocyte function.

The extracellular domain of CD45 controls association with the CD4-T cell receptor complex and the response to antigen-specific stimulation

The CD45 tyrosine phosphatase plays an important role in regulating T lymphocyte activation, but the function of the different isoforms of CD45 is not known. T cell transfectants have been prepared that express individual CD45 isoforms in cells with a well-defined T cell receptor (TCR) from the D10 T helper 2 clone. We find that cells bearing low molecular weight CD45 isoforms are far more efficient in responding to stimulation with peptide and antigen-presenting cells compared with cells bearing high molecular weight CD45 isoforms. One hypothesis for the preferential activation of cells that express low molecular weight CD45 isoforms is that they interact with other cell surface antigens important in TCR signaling, altering their phosphorylation status and affecting the character of the signal transduction pathway. In this report, using cells expressing single isoforms, we demonstrate that low molecular weight isoforms of CD45 preferentially associate with CD4 and the TCR complex compared with high molecular weight isoforms. The molecular basis for this interaction was further examined using a glycosyl phosphatidyl inositol (GPI)-linked form of CD45Null (lacking tyrosine phosphatase domains), which preferentially associated with CD4 compared with GPI-linked CD45ABC, and cytoplasmic tail mutants of CD4, which retained the ability to coassociate. Using this panel of transfectants, it is clear that the interaction between CD4 and CD45 does not require the cytoplasmic domains of CD45, but is dependent on the specific external domain of the various isoforms: low molecular weight species were more likely to associate with the CD4-TCR complex than the higher molecular weight isoforms, and their ability to coassociate correlated with the magnitude of the response to specific antigen.

Genomic organization of the human alpha-adducin gene and its alternately spliced isoforms

The cDNA for the human alpha-adducin gene has been cloned, and different alternately spliced forms have been identified. We report the complete genomic organization of the human alpha-adducin gene and these alternately spliced forms. The human alpha-adducin gene, spanning approximately 85 kb, consists of 16 exons ranging in size from 34 to 1892 bp. One of the spliced forms of the human alpha-adducin gene results from alternate use of the 5' splice donor site for exon 10, while another results in a truncated protein following insertion of 34 bp comprising exon 15, followed by a premature stop codon. This alternate spliced form of alpha-adducin is predicted to result in an altered carboxyl terminus that would eliminate a protein kinase and calmodulin binding site. Seven nucleotide substitutions and 4 insertion/deletions were also identified. The 5' region of the human alpha-adducin gene contains one Sp1 site, two AP2 sites, and two CAAT boxes. No TATA box was apparent, consistent with features of a housekeeping gene. We have mapped another cDNA within the first intron of the human alpha-adducin gene, suggesting overlapping genes in this 4p16.3 genomic region.

A specifically apical sub-membrane intermediate filament cytoskeleton in non-brush-border epithelial cells

Although many pieces of evidence support the notion of a role for the cytoskeleton in epithelial polarization, no cytoskeletal component has been found to be specifically apical, except for some actin-binding proteins. Here we report the apical distribution of a 53 kDa cytokeratin. Furthermore, this cytokeratin co-purified with biotinylated apical plasma membrane proteins in high density complexes. Differential biotinylation of the basolateral domain showed that the 53 kDa protein is mainly attached to the apical membrane, although a companion 58 kDa protein attaches to both apical and basolateral membrane proteins. Immunoprecipitation experiments indicated that a number of apical components are directly or indirectly linked to the 53 kDa protein. These results indicate the existence of a terminal web-like structure in non-brush-border cells, which attaches to the apical domain and may play a role in apical polarization, especially during the acquisition of polarity from non-polarized cellular stages.

Ankyrin-binding domain of CD44(GP85) is required for the expression of hyaluronic acid-mediated adhesion function

GP85 is one of the most common hemopoietic isoforms of the cell adhesion molecule, CD44. CD44(GP85) is known to contain at least one ankyrin-binding site within its 70 aa cytoplasmic domain and to bind hyaluronic acid (HA) with its extracellular domain. In this study we have mapped the ankyrin-binding domain of CD44(GP85) by deleting various portions of the cytoplasmic region followed by expression of these truncated cDNAs in COS cells. The results of these experiments indicate that the ankyrin-binding domain resides between amino acids 305 and 355. Biochemical analyses, using competition binding assays and a synthetic peptide (NGGNGT-VEDRKPSEL) containing 15 aa between aa 305 and aa 320, support the conclusion that this region is required for ankryin binding. Furthermore, we have constructed a fusion protein in which this 15 aa sequence of CD44(GP85) is transplanted onto another transmembrane protein which does not bind ankyrin. Our results show that this fusion protein acquires the ability to bind ankyrin confirming that the sequence (306NGGNGTVEDRKPSE320L) is a critical part of the ankryin-binding domain of CD44(GP85). In addition, we have demonstrated that deletion of this 15 aa ankyrin-binding sequence from CD44(GP85) results in a drastic reduction (> or = 90%) of HA-binding and HA-mediated cell adhesion. These findings strongly suggest that ankyrin binding to the cytoplasmic domain of CD44(GP85) plays a pivotal role in regulating hyaluronic acid-mediated cell-cell and cell-extracellular matrix interactions.

Dynamic properties of ankyrin in T lymphocytes: colocalization with spectrin and protein kinase C beta

Ankyrin is a well characterized membrane skeletal protein which has been implicated in the anchorage of specific integral membrane proteins to the spectrin-based membrane skeleton in a number of systems. In this study, the organization of ankyrin was examined in lymphocytes in relation to T cell function. Light and electron microscope immunolocalization studies revealed extensive heterogeneity in the subcellular distribution of ankyrin in murine tissue-derived lymphocytes. While ankyrin can be localized at the lymphocyte plasma membrane, it can also be accumulated at some distance from the cell periphery, in small patches or in a single discrete, nonmembrane-bound structure. Double immunofluorescence studies demonstrated that ankyrin colocalizes with spectrin and with the signal transducing molecule protein kinase C beta (PKC beta) in tissue-derived lymphocytes, suggesting a functional association between these molecules in the lymphocyte cytoplasm. In addition, T lymphocyte activation-related signals and phorbol ester treatment, both of which lead to PKC activation, cause a rapid translocation of ankyrin, together with spectrin and PKC beta, to a single Triton X-100-insoluble aggregate in the cytoplasm. This finding suggests a mechanism for the reported appearance of PKC in the particulate fraction of cells after activation: activated lymphocyte PKC beta may interact with insoluble cytoskeletal elements like ankyrin and spectrin. Further evidence for a link between the subcellular organization of these proteins and PKC activity is provided by the observation that inhibitors of PKC activity cause their concomitant redistribution to the cell periphery. The dynamic nature of lymphocyte ankyrin and its ability to accumulate at sites distant from the plasma membrane are properties which may be unique to the lymphocyte form of the molecule. Its colocalization with PKC beta in the lymphocyte cytoplasm, together with its redistribution in response to physiological signals, suggests that structural protein(s) may play a role in signal transduction pathways in this cell type. Our data support the conclusion that ankyrin is not solely involved in anchorage of proteins at the plasma membrane in lymphoid cells.

Distinct involvement of CD45 in antigen receptor signalling in CD4+ and CD8+ primary T cells

We demonstrate that pretreatment of primary CD4+, but not CD8+ T cells with anti-CD45 inhibits activation signals induced through the T cell receptor for antigen (TCR alpha beta). Specifically, anti-TCR alpha beta-mediated tyrosine phosphorylation of phospholipase C-gamma 1 is inhibited, and this in turn correlates with the inhibition of subsequent Ca2+ mobilization and DNA synthesis. In marked contrast, none of these activation parameters are affected by anti-CD45 in CD8+ T cells. Perturbation of TCR alpha beta signalling in CD4+ cells is observed in conditions which do not detectably affect the level of CD45 expression, or its membrane distribution. Further, changes in the intrinsic phosphatase activity of CD45 are not detectable. While anti-CD45 ablates TCR alpha beta signalling, anti-CD3 epsilon-mediated activation is unaffected. This suggests that elements of the antigen receptor complex can be functionally uncoupled, and indicates that the requirements for CD45 in signalling through these two elements are different. The results demonstrate that the involvement of CD45 in coupling TCR alpha beta to second messenger-generating pathways is under distinct physical and/or functional constraints in primary CD4+ and CD8+ T cells.

Differential regulation of actin polymerization following activation of resting T lymphocytes from young and aged mice

Actin polymerization accompanies receptor-mediated responses and is correlated with motility-related events. In T lymphocytes, there is a lateral redistribution of surface receptors into caps and aggregation of actin-myosin in cytoplasmic subcaps, and these are impaired in T cells from aged individuals. This study documents marked changes in age-related cytoskeletal actin filament function which may account for the reduced motility. Basal levels of filamentous actin (F-actin) are significantly higher in purified G(o) T cells from aged C57BL/6 mice, due to a preferential increase in the CD8+ subpopulation. Following activation of the resting T cells with Concanavalin A (Con A), F-actin depolymerized in cells from young mice for 2 min, followed by rapid polymerization, reaching a plateau 200% above resting levels. In cells from 15-17-month-old mice, an attenuated depolymerization phase was seen for 45 sec, followed by little polymerization. No depolymerization or polymerization phases occurred in cells from aged mice. Phorbol 12 myristate 13-acetate (PMA), which activates protein kinase C (PKC), bypassing receptor mediated signals, induced actin polymerization to 57% of the levels of that after Con A stimulation in cells from both young and old animals and partially eliminated the differences in actin filament assembly due to age. Perturbation of the cytoskeleton with cytochalasin E (CE) potentiated proliferation of Con A-stimulated T cells from aged mice but did not completely restore the deficit attributed to immunosenescence. The results show an age-related impairment of cytoskeletal functions and suggest that differences in early signal transduction events contribute to the decrements in surface receptor motility and subsequent proliferation of T lymphocytes from older individuals.

Divalency of the monoclonal antibody 5-1-6 is required for induction of proteinuria in rats

A single i.v. injection of 3 mg of the F(ab')2 fragment of MoAb 5-1-6 into rats induced immediate proteinuria (128.1 +/- 80.7 mg/24 h on day 1) which lasted 1-2 days. In contrast, rats administered 10 mg of the corresponding Fab fragment did not develop abnormal proteinuria even though an equivalent dose of the intact MoAb 5-1-6 far exceeded the nephritogenic dose. The total kidney binding of 125I-Fab fragment was 209.5 +/- 34.3 micrograms/2 kidneys. This exceeded that obtained by injection of 3 mg MoAb 5-1-6 IgG1 (58.9 +/- 12.5 micrograms/2 kidneys at 1 h) and was similar to that obtained following injection of 3 mg F(ab')2 fragment (235.3 +/- 16.9 micrograms/2 kidneys). Immunofluorescence (IF) showed a linear pattern along the glomerular capillary wall at 1 h after the administration of MoAb 5-1-6 IgG1, F(ab')2 or Fab fragment. On day 5, fine to coarse granules were observed scattered in F(ab')2-injected rat glomeruli, whereas granules were densely localized in Fab-injected rat glomeruli. Complement-depleted rats injected with 3 mg of MoAb 5-1-6 IgG1 developed proteinuria with the same time course as non-depleted rats. This observation, together with the ability of F(ab')2 to induce proteinuria, indicates that proteinuria induced by MoAb 5-1-6 is complement-independent. This study suggests that MoAb 5-1-6-induced proteinuria is initiated by cross-linking of the epitopes by divalent MoAb 5-1-6 and is independent of complement activity.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

Adhesive interactions in thymic development: does selective expression of CD45 isoforms promote stage-specific microclustering in the assembly of functional adhesive complexes on differentiating T lineage lymphocytes?

CD45 isoform transitions appear to play essential roles in both life and death, and correlate with the stages of thymocyte development during which there is a change in physical location from medullary and/or outer cortical areas to the inner cortex. This work speculates that CD45 isoforms, through a focal role in the assembly of adhesive complexes mediated by the external domains, participate in the maintenance and/or modulation of migratory behaviour by differentiating thymocytes, or alternatively in the anchoring of thymocytes in a generative micro-environment. The objective of the sections that follow is to formulate the hypothesis that CD45 isoforms, through their differential interactions with adhesion molecules expressed by T cells, profoundly influence cell motility and consequent micro-environmental localization. An adhesive assembly of CD45 and adhesion molecules on the outside, and of the adhesive complex with the cytoskeleton on the inside, would promote CD45-mediated regulation of adhesion/de-adhesion through lateral external interactions mediated by external domains of CD45 isoforms, through enzymatic modulation of the cytoplasmic domains of adhesion molecules by the CD45 tyrosine phosphatase activity, and through phosphatase control of cytoskeletal assembly and disassembly.

A CD44-like endothelial cell transmembrane glycoprotein (GP116) interacts with extracellular matrix and ankyrin

We used complementary biochemical and immunological techniques to establish that an endothelial cell transmembrane glycoprotein, GP116, is a CD44-like molecule and binds directly both to extracellular matrix components (e.g., hyaluronic acid) and to ankyrin. The specific characteristics of GP116 are as follows: (i) GP116 can be surface labeled with Na 125I and contains a wheat germ agglutinin-binding site(s), indicating that it has an extracellular domain; (ii) GP116 displays immunological cross-reactivity with a panel of CD44 antibodies, shares some peptide similarity with CD44, and has a similar 52-kDa precursor molecule, indicating that it is a CD44-like molecule; (iii) GP116 displays specific hyaluronic acid-binding properties, indicating that it is a hyaluronic acid receptor; (iv) GP116 can be phosphorylated by endogenous protein kinase C activated by 12-O-tetradecanoylphorbol-13-acetate and by exogenously added protein kinase C; and (v) GP116 and a 20-kDa tryptic polypeptide fragment of GP116 from the intracellular domain are capable of binding the membrane-cytoskeleton linker molecule, ankyrin. Furthermore, phosphorylation of GP116 by protein kinase C significantly enhances GP116 binding to ankyrin. Together, these findings strongly suggest that phosphorylation of the transmembrane glycoprotein GP116 (a CD44-like molecule) by protein kinase C is required for effective GP116-ankyrin interaction during endothelial cell adhesion events.

A CD44-like endothelial cell transmembrane glycoprotein (GP116) interacts with extracellular matrix and ankyrin

We used complementary biochemical and immunological techniques to establish that an endothelial cell transmembrane glycoprotein, GP116, is a CD44-like molecule and binds directly both to extracellular matrix components (e.g., hyaluronic acid) and to ankyrin. The specific characteristics of GP116 are as follows: (i) GP116 can be surface labeled with Na 125I and contains a wheat germ agglutinin-binding site(s), indicating that it has an extracellular domain; (ii) GP116 displays immunological cross-reactivity with a panel of CD44 antibodies, shares some peptide similarity with CD44, and has a similar 52-kDa precursor molecule, indicating that it is a CD44-like molecule; (iii) GP116 displays specific hyaluronic acid-binding properties, indicating that it is a hyaluronic acid receptor; (iv) GP116 can be phosphorylated by endogenous protein kinase C activated by 12-O-tetradecanoylphorbol-13-acetate and by exogenously added protein kinase C; and (v) GP116 and a 20-kDa tryptic polypeptide fragment of GP116 from the intracellular domain are capable of binding the membrane-cytoskeleton linker molecule, ankyrin. Furthermore, phosphorylation of GP116 by protein kinase C significantly enhances GP116 binding to ankyrin. Together, these findings strongly suggest that phosphorylation of the transmembrane glycoprotein GP116 (a CD44-like molecule) by protein kinase C is required for effective GP116-ankyrin interaction during endothelial cell adhesion events.

Interaction of aggregated native and mutant IgE receptors with the cellular skeleton

When aggregated, cell surface proteins become resistant to solubilization by detergents, presumably because of aggregation-induced or -stabilized interactions between the membrane protein and the cytoskeleton or plasma membrane skeleton. We genetically engineered variants of the tetrameric high-affinity receptor for IgE (Fc epsilon RI) to identify a site on its alpha, beta, or gamma chains that mediates such putative interactions. Using flow cytofluorometry, we studied rat basophilic leukemia cells, transiently transfected COS cells, and stably transfected P815 cells bearing wild-type and mutated receptors. We observed that (i) solubilization was markedly dependent on the degree of aggregation, the extent varying somewhat with the cell type and, particularly at lower levels of aggregation, with the time after addition of detergent; (ii) truncation of no single cytoplasmic domain of the alpha, beta, or gamma chains ablated the insolubilization effect; and (iii) incomplete receptors were also efficiently insolubilized by aggregation. Thus receptors consisting only of alpha and gamma chains, a "receptor" consisting of only the ectodomain of the alpha chain attached to the plasma membrane by a glycosyl-phosphatidyl inositol anchor, and "receptors" consisting only of minimally modified gamma chains were resistant to solubilization after aggregation. We conclude that no unique subunit or domain of Fc epsilon RI mediates the insolubilization phenomenon. Our results support a model in which the bridging of membrane proteins leads to their becoming nonspecifically enmeshed in a network of membrane skeletal proteins on either the outside and/or the inside of the membrane so that dissolution of the lipid bilayer becomes irrelevant.

The cortical actomyosin system of cytochalasin D-treated lymphoblasts

Global cytoskeleton dynamics is likely to exist in animal cells and some experimental evidence for this has recently been obtained in cells from the human lymphoblastic cell line KE37. We have further investigated the dramatic and reversible microtubule-dependent cell elongation which occurs upon treatment of KE37 cells with cytochalasin D. This phenomenon results in a non-locomotory cell with definite polarity. It involves a sustained equatorial myosin II-dependent contraction of cortical, most of the myosin II being accumulated on segments of the main cellular extension. We report here that such a cell lengthening is energy-dependent and can be inhibited, or suppressed, by surface ligands such as wheat germ agglutinin but not by concanavalin A. Suppression of the cytochalasin D effect by wheat germ agglutinin is rapid and appears to be collapse of the cell extension and relocalization of the contracted actomyosin as a whole. It suggests that the binding of the wheat germ agglutinin to the cell surface results in the transient disassembly of microtubules, a possibility also raised by the potent antagonist effect of taxol on wheat germ agglutinin action. Taken together, the data are consistent with a specific role of microtubules in the control of the activity of the cortical actomyosin system.

Actin-binding proteins involved in the capping of epidermal growth factor receptors in A431 cells

A capping process of epidermal growth factor receptors (EGF-Rs) was used for the study of the relation between the receptors and the actin-binding proteins (spectrin, vinculin, annexin I) that may be involved in EGF-R-cytoskeleton interaction. In intact, adherent A431 cells, EGF-Rs were diffusively distributed on the cell surface. Spectrin, vinculin, and annexin I were located beneath the plasma membrane. An abundance of EGF-Rs as well as submembrane proteins was observed in regions of membrane ruffles and cell-cell contacts. Annexin I was localized also in cytoplasm being attached to filamentous structures surrounding the nucleus and extending to the cell periphery. Under polyvalent ligand treatment, EGF-Rs of adherent cells were aggregated on one side of the cell. Spectrin, vinculin, and annexin I dislocated together with EGF-Rs and were concentrated under plasma membrane at regions where cap formation took place. In suspended A431 cells only spectrin was located under the plasma membrane whereas annexin I and vinculin were diffusively distributed through the cells. During cap formation only spectrin was colocalized with EGF-Rs. The results confirmed the major role of spectrin as a receptor-microfilament linking protein.

Biochemical identification of alpha-fodrin and protein 4.1 in human keratinocytes

The mature erythrocyte has a cytoskeleton of less complexity than that of nucleated cells and has been elucidated in greater detail. Two of its major components are the heterodimeric protein spectrin and protein 4.1. We report here our isolation from human keratinocytes of immunoreactive forms of both protein 4.1 and of alpha-fodrin, the extra-erythrocytic form of alpha-spectrin. These keratinocyte proteins are approximately 125 kD and 240 kD in size, respectively. We also have isolated clones containing alpha-fodrin and protein 4.1 sequences from a human keratinocyte cDNA library. These sequences confirm the active transcription in keratinocytes of the alpha-fodrin and protein 4.1 genes. Both alpha-fodrin and protein 4.1 mRNA are detectable by Northern blot analysis in human keratinocytes, where their abundance appears not to be regulated by calcium concentration in the medium.

Protein tyrosine phosphatases: a diverse family of intracellular and transmembrane enzymes

Protein tyrosine phosphatases (PTPs) represent a diverse family of enzymes that exist as integral membrane and nonreceptor forms. The PTPs, with specific activities in vitro 10 to 1000 times greater than those of the protein tyrosine kinases would be expected to effectively control the amount of phosphotyrosine in the cell. They dephosphorylate tyrosyl residues in vivo and take part in signal transduction and cell cycle regulation. Most of the transmembrane forms, such as the leukocyte common antigen (CD45), contain two conserved intracellular catalytic domains; but their external segments are highly variable. The structural features of the transmembrane forms suggest that these receptor-linked PTPs are capable of transducing external signals; however, the ligands remain unidentified. A hypothesis is proposed explaining how phosphatases might act synergistically with the kinases to elicit a full physiological response, without regard to the state of phosphorylation of the target proteins.

Isolation and preliminary characterization of the major membrane boundaries of the endocytic pathway in lymphocytes

Plasma membrane, coated pits, endosomes, and lysosomes were isolated from a mouse T lymphoma cell line using a density shift protocol in which these compartments were selectively loaded with gold conjugates. The plasma membrane was prepared after selective labeling for 1 h at 2 degrees C with gold-ricin and gave a yield of 40% according to enzymatic and antigenic markers. Endosomes were obtained by loading the cells for 2 h at 22 degrees C with gold complexed to an antimouse transferrin receptor mAb. Coated pits were isolated using a similar procedure, but after an incubation at 10 degrees C, which allowed deep invagination of the pits but prevented internalization. The yield (calculated using the recovery of [125I]transferrin) was 32% for endosomes and 10% for coated pits. Finally lysosomes were prepared by loading the cells for 18 h at 37 degrees C with gold low density lipoproteins (LDLs) followed by a 3-h chase at 37 degrees C with LDL alone. The final lysosome yield (based on the recovery of lysosomal enzymes) was 16%. Studies of the protein composition of these cellular compartments on two-dimensional gels showed that while some major proteins are present throughout the pathway, specific proteins can be identified in each of the isolated fractions. The greatest change in the pattern of protein constituents seen along the pathway was between endosomal and lysosomal preparations.

Isolation of high-affinity murine interleukin 2 receptors as detergent-resistant membrane complexes

Murine T cells and T cell lines bearing high- and low-affinity receptors for interleukin (IL) 2 were chemically cross-linked to radiolabeled IL 2 and subjected to differential detergent extractions to evaluate the extent of IL 2 receptor association with the nonionic detergent-resistant framework of the plasma membrane. Low-affinity receptors were readily solubilized by nonionic detergent extraction of whole cross-linked cells, while solubilization of high-affinity receptors required a stronger ionic detergent suggesting their association with a membrane structure that is resistant to nonionic detergents. To achieve physical separation of low- and high-affinity receptors, cells cross-linked to 125I-labeled IL 2 were centrifuged through a sucrose barrier containing Triton X-100. Alternatively, Triton X-114 extracts of plasma membrane fractions were partitioned into aqueous and detergent phases. By either approach, high-affinity receptors differed from low-affinity ones by their increased density and consisted of detergent-resistant complexes containing p55-p75 heterodimers. The low-affinity receptors, on the contrary, were of low density and consisted exclusively of detergent-soluble p55 subunits. High density and resistance to nonionic detergent extraction of high-affinity IL 2 receptors suggest their integration into lateral microdomains of the detergent-resistant framework of the plasma membrane.

A 39-kD plasma membrane protein (IP39) is an anchor for the unusual membrane skeleton of Euglena gracilis

The major integral plasma membrane protein (IP39) of Euglena gracilis was radiolabeled, peptide mapped, and dissected with proteases to identify cytoplasmic domains that bind and anchor proteins of the cell surface. When plasma membranes were radioiodinated and extracted with octyl glucoside, 98% of the extracted label was found in IP39 or the 68- and 110-kD oligomers of IP39. The octyl glucoside extracts were incubated with unlabeled cell surface proteins immobilized on nitrocellulose (overlays). Radiolabel from the membrane extract bound one (80 kD) of the two (80 and 86 kD) major membrane skeletal protein bands. Resolubilization of the bound label yielded a radiolabeled polypeptide identical in Mr to IP39. Intact plasma membranes were also digested with papain before or after radioiodination, thereby producing a cytoplasmically truncated IP39. The octyl glucoside extract of truncated IP39 no longer bound to the 80-kD membrane skeletal protein in the nitrocellulose overlays. EM of intact or trypsin digested plasma membranes incubated with membrane skeletal proteins under stringent conditions similar to those used in the nitrocellulose overlays revealed a partially reformed membrane skeletal layer. Little evidence of a membrane skeletal layer was found, however, when plasma membranes were predigested with papain before reassociation. A candidate 80-kD binding domain of IP39 has been tentatively identified as a peptide fragment that was present after trypsin digestion of plasma membranes, but was absent after papain digestion in two-dimensional peptide maps of IP39. Together, these data suggest that the unique peripheral membrane skeleton of Euglena binds to the plasma membrane through noncovalent interactions between the major 80-kD membrane skeletal protein and a small, papain sensitive cytoplasmic domain of IP39. Other (62, 51, and 25 kD) quantitatively minor peripheral proteins also interact with IP39 on the nitrocellulose overlays, and the possible significance of this binding is discussed.

Expression of high molecular weight isoforms of CD45 by mouse thymic progenitor cells

We have studied the expression of isoforms of CD45 (leukocyte common antigen, LCA) among T cell precursors using the organ culture system of Jenkinson et al. (Eur. J. Immunol. 1982. 12: 583). These experiments show that cells capable of recolonizing alymphoid embryonic thymic lobes in vitro can be detected in the thymus of fetal and adult mice and are enriched when thymocytes are depleted of cells bearing CD4 or CD8. These data are consistent with results from in vivo experiments of Fowlkes et al. (J. Exp. Med. 1985. 162: 802) indicating that T cell precursors lie within the double-negative (CD4-CD8-) compartment. No precursors were detected among the reciprocal populations of cells bearing CD4 and/or CD8 (single and double positives). Double-negative cell fractions were then divided on the basis of reactivity with monoclonal antibodies RA3-2C2 and RA3-3A1. These antibodies recognize the high molecular weight species of the LCA or, more accurately, a product defined by exon A of the CD45 gene. Recolonizing cells were found predominantly in the CD45RA+ (RA3-2C2 and RA3-3A1 reactive) fraction of double-negative thymocytes; CD45RA- enriched populations had increased efficiency of recolonization and CD45RA- depleted populations had decreased ability to recolonize as compared with the whole CD4-CD8- fraction. To clarify whether progenitors enriched in the CD45RA+ fraction were capable of giving rise to mature CD4+, CD8+ and CD4+ CD8+ cells, we analyzed the progeny of lobes seeded with CD4-CD8-CD45RA+ fractions. After 7-9 days in organ culture the proportion of CD4+, CD8+ or CD4+ CD8+ cells had increased to 35.2%, 18.6% and 23.7%, respectively (mean of five experiments), indicating that progenitors among the CD45RA+ population were indeed multipotent. These results suggest that the majority of T stem cells in the thymus are among thymocytes that express the CD45RA molecule, an hypothesis supported by our finding that removal of CD45RA-expressing cells (using complement and antibody) eliminated recolonizing capacity of thymic cell fractions.

Anti-leucocyte common (CD45) antibodies inhibit dendritic cell stimulation of CD4 and CD8 T-lymphocyte proliferation

Dendritic cells (DC) act as primary antigen-presenting cells and probably initiate T-cell responses via the formation of DC-T-cell clusters. Both DC and T cells express members of the leucocyte common (CD45) family of molecules as major cell membrane proteins. The possibility that the leucocyte common antigens (LCA) play a functional role in the cellular events of a primary T-cell response was tested, by examining the effect of CD45 antibodies on the T-cell proliferative response induced by stimulating with allogeneic DC in the mixed leucocyte reaction (MLR). The presence of individual CD45 monoclonal antibodies during the MLR inhibited T-cell proliferation only weakly, but a mixture of monoclonal antibodies against different epitopes of the LCA had a more profound inhibitory effect. The effect of the CD45 antibodies was maximal during the first 24 hr and preincubation experiments indicated that the major blocking effect occurred at the T-cell level, although some inhibition after preincubation with DC was noted. Purified allogeneic CD8 T-cell responses were more susceptible to inhibition than purified CD4 cell responses, but no difference in the inhibition of the CD45RA (CMRF-11) or CD45RO (UCHL1) LCA-positive T-cell subsets was observed. The presence of CD45 antibodies did not inhibit DC-T-cell clustering but reduced cluster stability, suggesting that certain epitopes of the LCA may be involved in the process of cell adhesion.

Protein tyrosine dephosphorylation and signal transduction

The protein tyrosine phosphatases comprise a family of enzymes that specifically dephosphorylate tyrosyl residues. Determination of the amino acid sequence of a major low molecular mass form isolated from human placenta (PTPase 1B) provided the basis for the first identification of transmembrane proteins that bear intracellular phosphatase domains. The existence of such molecules, bearing the hallmarks of receptors, raises the exciting possibility of a novel mechanism of signal transduction in which the early events involve the ligand-induced dephosphorylation of tyrosyl residues in proteins.

Molecular characterization of GP50: a neuron-specific, synaptic-enriched glycoprotein

The molecular properties of the neuron-specific, synaptic-enriched glycoprotein GP50 have been investigated with the aid of the monoclonal antibody MabSM-GP50. GP50 immunoreactivity was detected in the brains of the frog, trout, pigeon, snake, rabbit, mouse, cow, and human, although variation in quantity and electrophoretic mobility of the immunoreactive protein between species was apparent. Deglycosylation of synaptic membranes (SMs) with endoglycosidase H, peptide:N-glycosidase F, trifluoromethane-sulfonic acid, and alkaline sodium borohydride indicated that GP50 is associated primarily, if not exclusively, with high-mannose and/or hybrid-type oligosaccharides and lacks complex N-linked and O-linked sugar chains. GP50 remained associated with the membrane fraction following extraction of SMs with alkaline sodium carbonate, was partially (55%) present in the detergent phase following the phase partitioning of SMs in the presence of Triton X-114, and was resistant to proteolytic digestion with trypsin when present as a component of intact membranes. Taken together, these results indicate that GP50 is an integral component of the SM. Sucrose gradient centrifugation of Triton X-100 extracts of SMs or of forebrain and cerebellar homogenates resolved GP50 into two fractions with sedimentation coefficients of 3.6S and 7.3S, which accounted for 45 and 55% of the total, respectively. The 7.3S form occurred exclusively in the aqueous phase following partitioning with Triton X-114, whereas the 3.6S species was found in both the aqueous and detergent phases.(ABSTRACT TRUNCATED AT 250 WORDS)