Further characterization of a fodrin-containing transmembrane complex from mouse T-lymphoma cells

Tyrosine phosphatases and their possible interplay with tyrosine kinases

Protein tyrosine phosphatases represent a new family of intracellular and receptor-linked enzymes. They are totally specific toward tyrosyl residues in proteins, and, with specific activities 10-1000-fold greater than those of the protein tyrosine kinases, they can be expected to tightly control the level of phosphotyrosine within the cell. Most transmembrane forms contain two conserved intracellular catalytic domains, as displayed by the leukocyte common antigen CD45, but highly variable external segments. Some are related to the neuronal cell adhesion molecules (NCAMs) or fasciclin II and others contain fibronectin III repeats; this suggests that these enzymes might be involved in cell-cell interaction. The intercellular enzymes appear to contain a highly conserved catalytic core linked to a regulatory segment. Deletion of the regulatory domain alters both substrate specificity and cellular localization. Likewise, overexpression of the full-length and truncated enzymes affects cell cycle progression and actin filament stability, respectively. The interplay between tyrosine kinases and phosphatases is considered. A hypothesis is presented suggesting that in some systems phosphatases might act synergistically with the kinases and elicit a physiological response, irrespective of the state of phosphorylation of the target protein.

Human Langerhans cells express a novel form of the leukocyte common antigen (CD45)

CD45 is a family of transmembrane glycoproteins that function as protein tyrosine phosphatases. All isoforms exhibit common CD45 epitopes, whereas the restricted CD45 epitopes (RA, RB, and RO) are each limited to one or more isoforms. In prior studies, we showed that human Langerhans cells in normal epidermis express a novel CD45 phenotype. They express common CD45 epitopes but are characteristically RA- RB- RO-. This suggests that Langerhans cells can express a novel form of CD45. In order to clarify this issue further, mRNA extracted from enriched Langerhans cell preparations was reverse transcribed into cDNA. The 5' portion of CD45 cDNA was then amplified using polymerase chain reaction primers complementary to exon 2 and exons 9-10, which flank the CD45 variable exon region (exons 4-6). Cloning and sequencing of the dominant 441 bp polymerase chain reaction product revealed the following exon configuration for the 5' translated region of Langerhans cells CD45: exon 3/7/8/9/10. This is the same exon configuration associated with the 180 kd CD45 isoform expressed by memory T cells and monocytes/macrophages; however, these cell types are RO+ whereas normal Langerhans cells are RO-. The RO epitope is known to be an oligosaccharide with a terminal sialic acid moiety. Therefore, we determined the expression of a related epitope, OPD4, by Langerhans cells. This is another terminal sialic acid moiety expressed by the 180 kd CD45 isoform of memory T cells but not by monocytes/macrophages. Langerhans cells were OPD4-. Our data suggest that memory T cells, monocytes/macrophages, and Langerhans cells all express a common CD45 transcript lacking exons 4-6; however, this transcript appears to undergo lineage-specific, post-translational glycosylation to create three distinct CD45 glycoproteins: RO+ OPD4+, RO+ OPD4-, and RO- OPD4-, which are expressed typically by memory T cells, monocytes/macrophages, and Langerhans cells, respectively. Because these epitopes are located extracellularly, they are postulated to allow differential responses to extracellular stimuli by creating differential ligand specificity.

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.

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.

Cytogenetic abnormalities in childhood acute lymphoblastic leukemia correlates with clinical features and treatment outcome

Virtually all cases of childhood acute lymphoblastic leukemia have chromosomal abnormalities. Non-random chromosomal abnormalities have been correlated with leukemic cell lineage, the degree of cell differentiation and certain clinical and biologic features. Cytogenetic findings have prognostic significance, but the adverse influence of many rearrangements, including most chromosomal translocations, may be offset by the greater cytoreductive effects of intensified therapy. Cytogenetic abnormalities have also provided focus for molecular studies of leukemogenesis. Such studies have recently identified key genes and their protein products which play important roles in malignant transformation and proliferation.

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.

Concanavalin A induces a cytoskeletal association of T200 molecules in T lymphocytes

A recent report indicated that T200 molecules interact with elements of the cytoskeleton in BW5147 T lymphoma cells. We have confirmed the cytoskeletal association of T200 by examining nonionic detergent-soluble and detergent-insoluble fractions of murine T cell tumor cell lines, cloned cytotoxic T lymphocyte lines, and thymocytes. Concanavalin A (Con A)-treated and untreated cells were extracted with 0.5% Triton X-100 and the remaining insoluble material was extracted under conditions allowing actin depolymerization. In the absence of Con A treatment, little T200 could be recovered from the depolymerized insoluble fraction. However, in T cells treated with capping concentrations of Con A, a considerable amount of T200 was rendered insoluble in nonionic detergent, and T200 could be recovered from the insoluble fraction by a buffer which dissociates actin polymers. A lesser, but still significant, amount of T200 associated with the detergent-insoluble fraction of thymocytes treated with concentrations of Con A and succinyl Con A, which are mitogenic for T cells. We also found that in T cells treated with mitogenic concentrations of succinyl Con A, more T200 associated with cytoskeleton than did H-2 or LFA-1 molecules. Because T200 is such a predominant molecule on the surface of T cells, such translocations of the molecule may have a major impact on the physiology of the cell, especially if T200 functions as a protein tyrosine phosphatase as recent evidence by others suggests.

A new member of the immunoglobulin superfamily that has a cytoplasmic region homologous to the leukocyte common antigen

A human gene (LAR) that hybridizes to mouse leukocyte common antigen cDNA under relaxed hybridization conditions was isolated. The LAR gene is expressed in a broad range of cells, including T lymphocytes, kidney, and prostate cells. The structure of the protein encoded by the LAR gene was deduced by determining the nucleotide sequences of a 7.7-kb LAR cDNA. The putative LAR protein is composed of a 1,234 amino acid extracellular region, a 24 amino acid transmembrane segment, and a 623 amino acid cytoplasmic region. The cytoplasmic region contains two homologous domains that have extensive sequence similarity to the cytoplasmic region of the leukocyte common antigens. The NH2-terminal region of the extracellular segment of the LAR protein contains three tandem Ig-like domains and nine non-Ig-like domains. Among the known Ig-like proteins, the LAR protein has the highest degree of similarity to neural-cell adhesion molecule. The non-Ig-like domains of the LAR protein are also similar to the non-Ig-like domains of neural-cell adhesion molecule.

Membrane-cytoskeleton interactions: inhibition of odontoblast differentiation by a monoclonal antibody directed against a membrane protein

It is known that high-molecular-weight (HMW) membrane proteins mediate interactions with constituents of the extracellular matrix and/or with cytoskeletal elements. To study participation of HMW membrane proteins in odontoblast or ameloblast differentiation, an immunological approach has been adopted. Antibodies directed against membrane proteins (Mr, 110-190) from mouse embryos have been produced by the hybridoma technique. Supernatants of hybridoma cultures were screened for their ability to stain dental tissues and also tested for their biological activities on dental cells in primary culture or on developing tooth germs in organ culture. An IgM monoclonal antibody, MC16A16, directed against a 165-kDa antigen present in plasma membrane preparations, reacted strongly with the dental epithelium and weakly with the mesenchyme. MC16A16 also reacted with the cell surface of nonpermeabilized cultured dental cells and could detach epithelial cells cultured on glass, but not mesenchymal cells which maintained vinculin-containing focal contacts. This antibody, which affected the organization of dental-cell microfilaments in primary culture, also inhibited the polarization of odontoblasts, but not that of ameloblasts.

Isolation of Thy-1 caps and analysis of their phospholipid composition in mouse T-lymphoma cells

In this study we have used a density perturbation method to isolate anti-Thy-1 antibody-induced Thy-1 caps from mouse T-lymphoma cells in the absence of detergents, and then compared the phospholipid composition of these capped membranes with that of uncapped membranes. Initial phospholipid analysis by two-dimensional thin layer chromatography (2-D TLC) reveals a significant increase in the amount of 32P-labeled phosphatidylcholine in the Thy-1 capped membrane. In contrast, no significant changes are observed in the labeling of phosphatidylserine, phosphatidylethanolamine, or the sphingomyelins. Therefore, it is suggested that phosphatidylcholine may be involved in the organization and/or regulation of Thy-1 antigen redistribution. The composition of phosphoinositide in uncapped and capped membranes was analysed separately using one-dimensional thin layer chromatography (1-D TLC) to resolve phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PlP), and phosphatidylinositol 4, 5-bisphosphate (PIP2) from all other phospholipids. This analysis reveals a significant reduction in levels of PIP and PIP2, but not PI, in Thy-1 caps. Through the use of ion exchange column chromatography, we have found an increased production of all three species of inositol phosphates during anti-Thy-1 antibody-induced capping. Inositol 1, 4, 5-triphosphate (IP3) shows the most significant increase, compared to the much smaller increases in inositol 4, 5-bisphosphate (IP2) and inositol monophosphate (IP). These results suggest that the binding of anti-Thy-1 antibody to Thy-1 antigen activates phospholipase C which, in turn, initiates polyphosphoinositide turnover and IP3 production. It is proposed that these observed effects are the result of early signal transducing events which are prerequisite steps in Thy-1 receptor cap formation.

Phorbol diesters increase the phosphorylation of the leukocyte common antigen CD45 in human T cells

We have studied the phosphorylation of the human leukocyte surface glycoprotein CD45, formerly known as leukocyte common antigen or T200, following activation of peripheral T cells with tumor-promoting phorbol diesters. The cells were incubated with [32P]orthophosphate, treated with phorbol diesters, and the CD45 was immunoprecipitated using the monoclonal antibody T29/33. CD45 was weakly phosphorylated in nonactivated T cells, but a treatment of the cells with 10, 60 or 200 nM phorbol 12,13-dibutyrate (PDBu) markedly increased the phosphorylation of the glycoprotein. This effect of PDBu was relatively selective for CD45 since the activation of the cells did not affect the phosphorylation of all membrane phosphoglycoproteins. The effect of another tumor promoter, phorbol 12-myristate 13-acetate (PMA), was comparable to that of PDBu, but a nontumor-promoting phorbol derivative, 4 alpha-phorbol 12,13-didecanoate, did not change the level of CD45 phosphorylation. The phosphorylation of CD45 was found to clearly increase after a treatment of T cells with 60 nM PDBu for only 30 s, and the phosphorylation reached its maximum in about 5 min.