Multisubunit receptors in the immune system and their association with the cytoskeleton: in search of functional significance

Inhibition of actin polymerization enhances commitment to and execution of apoptosis induced by withdrawal of trophic support

We have previously shown, using jasplakinolide, that stabilization of the actin cytoskeleton enhanced apoptosis induced upon cytokine withdrawal (Posey and Bierer [1999] J. Biol. Chem. 274:4259-4265). It remained possible, however, that a disruption in the regulation of actin dynamics, and not simply F-actin stabilization, was required to affect the transduction of an apoptotic signal. We have now tested the effects of cytochalasin D, a well-characterized agent that promoted actin depolymerization. Actin depolymerization did not affect CD95 (Fas)-induced death of Jurkat T cells in the time course studied but did enhance the commitment to cytokine withdrawal-induced apoptosis of factor-dependent cell lines. The induction of cell death was not the result of direct cytoskeletal collapse, since treatment of the cells with cytochalasin D in the presence of IL-2 did not promote death. As with jasplakinolide, the enhancement of commitment to apoptosis could be delayed by overexpression of the anti-apoptotic protein Bcl-x(L), but, unlike jasplakinolide, cytochalasin D modestly affected the "execution" stage of apoptosis as well. Taken together, these results suggest that changes in actin dynamics, i.e., the rate of actin polymerization and depolymerization, modulate the transduction of the apoptotic signal committing lymphocytes, withdrawn from required growth factors, to the death pathway.

Colocalization of the B cell receptor and CD20 followed by activation-dependent dissociation in distinct lipid rafts

The B cell Ag receptor (BCR) and CD20, a putative calcium channel, inducibly associate with cholesterol-dependent membrane microdomains known as lipid rafts. A functional association between the BCR and CD20 is suggested by the effects of CD20-specific mAbs, which can modulate cell cycle transitions elicited by BCR signaling. Using immunofluorescence microscopy we show here that the BCR and CD20 colocalize after receptor ligation and then rapidly dissociate at the cell surface before endocytosis of the BCR. After separation, surface BCR and CD20 were detected in distinct lipid rafts isolated as low density, detergent-resistant membrane fragments. Pretreatment with methyl-beta-cyclodextrin, which we have previously shown to enhance receptor-mediated calcium mobilization, did not prevent colocalization of the BCR and CD20, but slowed their dissociation. The data demonstrate rapid dynamics of the BCR in relation to CD20 at the cell surface. Activation-dependent dissociation of the BCR from CD20 occurs before receptor endocytosis and appears to require in part the integrity of lipid rafts.

Ordered just so: lipid rafts and lymphocyte function

Immunologists have long been occupied with the description of cellular activation signaling events that originate with the stimulation of multichain immunoreceptors at the cell surface. These signals are transmitted by a protein-partner-signaling cascade through the cytoplasm to the nucleus, where they culminate in changes in gene expression, metabolic state, and entry into cell cycle. For T cells and B cells, these signaling cascades start with the ligation of the T cell receptor (TCR) and B cell receptor (BCR), respectively, and result in the recruitment and activation of related families of signaling molecules at the cell surface. Until recently, this gathering of signaling proteins was thought to occur within the featureless plasma membrane, a cellular organ that was envisioned as a boundary between the inner and outer components of the cell, but which contributed little to the signaling process. However, the past few years have seen the gradual realization that activation of signaling in lymphocytes takes place in and around specialized membrane subdomains called lipid rafts (also known as DIGs and GEMs). Here, we provide a brief overview of the analogous structures and compositions of lipid raft-associated signaling complexes in T cells and B cells, and the ways in which lymphocytes--and their pathogen adversaries--use lipid rafts to their benefit.

The cytoskeleton-associated TCR zeta chain is constitutively phosphorylated in the absence of an active p56(lck) form

The TCR recognizes peptide-MHC complexes and transmits activation signals leading to cellular responses. We have previously characterized two TCR populations expressed on the T cell surface; one is linked to the cytoskeleton via a detergent-insoluble cytoskeleton-associated zeta (cska-zeta) chain, while the other is detergent soluble and not linked to the cytoskeleton. The cska-zeta form displays unique properties: it is constitutively phosphorylated, does not undergo hyperphosphorylation upon TCR stimulation as opposed to its non-cytoskeleton-associated counterpart (non-cska-zeta) and it maintains a molecular mass of 16 kDa. It is well established that p56(lck) and possibly p59(fyn) are responsible for the generation of the 21/23-kDa phosphorylated detergent-soluble zeta form. We now demonstrate that the phosphorylation of cska-zeta does not require the activity of p56(lck). We also show that although Lck does not phosphorylate cska-zeta in vivo, it retains the capacity to phosphorylate cska-zeta in vitro. Moreover, differences in zeta-associated kinase activity were detected for non-cska-zeta and cska-zeta. Our results indicating that different kinases phosphorylate the two zeta forms are consistent with a growing consensus that each TCR form may regulate distinct cellular functions.

Transient translocation of the B cell receptor and Src homology 2 domain-containing inositol phosphatase to lipid rafts: evidence toward a role in calcium regulation

Membrane microdomains (lipid rafts) are enriched in selected signaling molecules and may compartmentalize receptor-mediated signals. Here, we report that in primary human B lymphocytes and in Ramos B cells B cell receptor (BCR) stimulation induces rapid and transient redistribution of a subset of engaged BCRs to lipid rafts and phosphorylation of raft-associated tyrosine kinase substrates. Cholesterol sequestration disrupted the lipid rafts, preventing BCR redistribution, but did not inhibit tyrosine kinase activation or phosphorylation of mitogen-activated protein kinase/extracellular regulated kinase. However, raft disruption enhanced the release of calcium from intracellular stores, suggesting that rafts may sequester early signaling events that down-regulate calcium flux. Consistent with this, BCR stimulation induced rapid and transient translocation of the Src homology 2 domain-containing inositol phosphatase, SHIP, into lipid rafts.

Searching for significance in TCR-cytoskeleton interactions

Two T-cell receptor (TCR) populations are expressed on T cells; one is linked to the cytoskeleton via its zeta chain. These cytoskeleton-linked receptors (30-40% of the total number of TCRs) might be important in TCR-mediated signaling and/or concurrent events. Here, differences between the two populations are summarized, and new data are examined to speculate on the functional significance of cytoskeleton-linked TCRs.

CD40 associates with the MHC class II molecules on human B cells

The MHC class II and CD40 molecules are two major components of the immune system that are involved in cell-cell interactions and signal transduction. Data obtained in the course of the present investigation show that these two molecules are physically associated on the surface of various human B cell lines and on normal tonsilar B cells. The CD40 / MHC class II complexes were not detected on the germinal center B cell line Ramos. However, stimulation of these cells via CD40 or MHC class II triggered their association, suggesting that the formation of the complex is related to the activation status of the cells. The formation of these complexes did not alter the interaction of MHC class II molecules with one of their natural ligands, the staphylococcal enterotoxin A (SEA), as evidenced by the ability of SEA to bind MHC class II / CD40 complexes. Cross-linking of MHC class II or CD40 molecules leads to the association as well as the co-association of both molecules to the NP-49-insoluble cellular matrix. Such association allowed us to demonstrate that only a fraction of these molecules can be physically associated on the cell surface. Based on previous observations and those presented here, it is highly possible that the CD40 / MHC class II complexes may have an important role in signal(s) induced via both molecules and during T / B cells interactions.

The p185(neu)-containing glycoprotein complex of a microfilament-associated signal transduction particle. Purification, reconstitution, and molecular associations with p58(gag) and actin

Microfilaments associate with the microvillar membrane of 13762 ascites mammary adenocarcinoma cells via a large transmembrane complex (TMC) comprising the major glycoproteins TMC-gp120, -110, -80, -65, and -55, the receptor kinase p185(neu), and the cytoplasmic proteins actin and p58(gag), linking the receptor with microfilaments in a signal transduction particle. Immunoblot screening with polyclonal antisera to TMC glycoproteins showed selective epithelial expression in normal rat tissues and epithelially derived tumor cells. The TMC glycoproteins were isolated by solubilization of microfilament core preparations in SDS, dilution, and separation on a concanavalin A-agarose affinity column. The large p185(neu)-containing complex was reconstituted from the column eluate after displacement of SDS with nonionic detergent, demonstrated by gel filtration and co-immunoprecipitation of the glycoproteins with anti-gp55 or anti-p185(neu). Exhaustive biotinylation of the glycoproteins gave a stoichiometry of gp120:gp110:gp80:gp65:gp55 of approximately 1:1:1:0.5:1. Overlay blots with biotinylated actin and in vitro translated, [(35)S]methionine-labeled p58(gag), respectively, showed specific interactions of actin with gp55 and gp120 and of p58(gag) with gp65 and gp55. These results provide evidence for a specific complex of microfilament-associated glycoproteins containing p185(neu) and p58(gag) and suggest a role for the complex in signal transduction scaffolding.

Expression of the T cell antigen receptor zeta chain following activation is controlled at distinct checkpoints. Implications for cell surface receptor down-modulation and re-expression

The multisubunit T cell antigen receptor (TCR) is involved in antigen recognition and signal transduction, leading to T cell activation and rapid down-modulation of the cell surface expressed TCRs. Although the levels of TCR cell surface expression are pivotal to the efficiency and duration of the immune response, the molecular mechanisms controlling TCR down-modulation and re-expression upon activation, remain obscure. Here, we provide a biochemical characterization of the regulatory mechanisms governing TCR expression following long-term T cell activation. We focused primarily on the TCR zeta chain, as this is considered the limiting factor in TCR complex formation and transport to the cell surface. We found that following TCR-mediated activation zeta mRNA is up-regulated by a transcription-dependent mechanism. Concomitantly, zeta protein levels are modified according to a biphasic pattern: rapid degradation coinciding with TCR cell surface down-regulation, followed by a rebound to normal levels 24 h subsequent to T cell activation. Even though there are adequate levels of all the TCR subunits within the cell following 24 h of activation, TCR cell surface expression remained very low, provided the activating antibody is continuously present. Correlative with the latter, we detected a previously undescribed monomeric form of the zeta chain. This form could be indicative of adverse endoplasmic reticulum conditions affecting correct protein folding, dimerization, and TCR assembly, all critical for optimal receptor surface re-expression. Cumulatively, our results indicate that the levels of TCR expression following activation, are tightly controlled at several checkpoints.

GPI-microdomains: a role in signalling via immunoreceptors

Glycosylphosphatidylinositol (GPI)-anchored proteins and glycosphingolipids are assembled on the leukocyte surface within membrane microdomains, which also accommodate a set of cytoplasmic signalling molecules (Src family kinases, G-proteins, linker proteins). Recent results suggest that these membrane specializations mediate not only signal transduction via GPI-proteins and glycolipids but also play important roles in initiation of signalling via immunoreceptors.

Globular domains of agrin are functional units that collaborate to induce acetylcholine receptor clustering

Agrin, an extracellular matrix protein involved in neuromuscular junction formation, directs clustering of postsynaptic molecules, including acetylcholine receptors (AChRs). This activity resides entirely in the C-terminal portion of the protein, which consists of three laminin-like globular domains (G-domains: G1, G2 and G3) and four EGF-like repeats. Additionally, alternate mRNA splicing yields G-domain variants G2(0,4) with 0- or 4-amino-acid inserts, and G3(0, 8,11,19) with 0-, 8-, 11- or 19-amino-acid inserts. In order to better understand the contributions of individual domains and alternate splicing to agrin activity, single G-domains and covalently linked pairs of G-domains were expressed as soluble proteins and their AChR clustering activity measured on cultured C2 myotubes. These analyses reveal the following: (1) While only G3(8) exhibits detectable activity by itself, all G-domains studied (G1, G2(0), G2(4), G3(0) and G3(8)) enhance G3(8) activity when physically linked to G3(8). This effect is most pronounced when G2(4) is linked to G3(8) and is independent of the order of the G-domains. (2) The deletion of EGF-like repeats enhances activity. (3) Increasing the physical separation between linked G1 and G3(8) domains produces a significant increase in activity; similar alterations to linked G2 and G3(8) domains are without effect. (4) Clusters induced by two concatenated G3(8) domains are significantly smaller than all other agrin forms studied. These data suggest that agrin G-domains are the functional units which interact independently of their specific organization to yield AChR clustering. G-domain synergism resulting in biological output could be due to physical interactions between G-domains or, alternatively, independent interactions of G-domains with cell surface receptors which require spatially localized coactivation for optimal signal transduction.

Characterization of the detergent insolubility of the T cell receptor for antigen

Aggregation of cell surface receptors plays an important role in signal transduction in many receptor systems. In the T cell receptor (TCR), as in many other cell surface receptors, this aggregation results in insolubility in certain nonionic detergents. We have characterized this insolubility for TCR, and we show it is not preexisting in HPB-ALL cells but increases with increasing TCR aggregation. It is not likely to be due to a direct interaction with cellular cytoskeletal elements, as it is not affected by inhibitors of actin or tubulin polymerization. It may be due to interaction with detergent-resistant membrane domains that have been found in various cell types and contain tyrosine kinases, the earliest known participants in TCR signal transduction. This aggregation-dependent insolubility occurs as rapidly as the anti-TCR antibody binds, so the kinetics are consistent with an involvement in signal transduction. It is not, however, dependent on signal transduction, as inhibitors of tyrosine kinases do not inhibit the insolubility. Insolubility is also enhanced by preaggregation of CD4, an important T cell surface molecule which also associates with the tyrosine kinase p56lck.

Fluorescence depolarization as an early measure of T lymphocyte stimulation

We have used the Cellscan, an apparatus capable of measuring optical properties of individual cells, to study changes in fluorescence polarization associated with T cell stimulation. We show that the fluorescence polarization of human peripheral blood lymphocytes (PBL) labeled with fluorescein diacetate (FDA) is markedly reduced upon exposure to the mitogenic lectins phytohemagglutinin (PHA), concanavalin A (ConA), or to phorbol esters. Methyl alpha-D-mannopyranoside (alphaMM) is able to reverse the depolarizing effect induced by ConA as long as the cells are not committed to proliferate. H7 and staurosporin, both inhibitors of protein kinase C (PKC), inhibit the depolarization induced by PHA. The mitogen-induced depolarization is dependent on metabolic energy. The results support the use of fluorescence depolarization of FDA-labeled PBL, monitored by the Cellscan, as a sensitive means of measuring early lymphocyte stimulation.

Normal T cells express two T cell antigen receptor populations, one of which is linked to the cytoskeleton via zeta chain and displays a unique activation-dependent phosphorylation pattern

The TCR couples antigen recognition and the transmission of activation signals. We report the expression of two TCR populations on the surface of T lymphocytes, one of which is linked to the cytoskeleton via the zeta chain. We also demonstrate that assembly of the CD3 subunits with cytoskeleton-associated zeta is necessary for their maximal localization to the cytoskeleton. The potential significance of these two receptor forms is underscored by differences observed in non-activated T cells; while detergent-soluble phosphorylated zeta appears as a 21-kDa protein, phosphorylated cytoskeleton-associated zeta appears as a 16-kDa form. This dichotomous phosphorylation pattern is rigidly maintained following activation, although each of the receptor populations undergoes different activation-dependent modifications: 1) levels of soluble phosphorylated 21-kDa zeta are enhanced, while phosphorylated 16-kDa cytoskeleton-associated zeta exhibits little change; 2) soluble non-phosphorylated 16-kDa zeta translocates to the cytoskeleton; 3) activation-dependent ubiquitinated zeta forms localize to both fractions, albeit with different kinetics. We also show that the protein tyrosine kinase Lck undergoes activation-dependent modifications and translocates to the cytoskeleton. The phosphorylation profiles of the dichotomous TCR populations in both non-activated and activated lymphocytes suggest that each population could regulate distinct cellular functions, possibly by select intermolecular associations.