Membrane insertion of lymphocyte surface molecules

Sustained-release naltrexone: novel treatment for opioid dependence

The devastating costs of opioid abuse and dependence underscore the need for effective treatments for these disorders. At present, several different maintenance medications exist for treating opioid dependence, including methadone, buprenorphine and naltrexone. Of these, naltrexone is the only one that possesses no opioid agonist effects. Instead, naltrexone occupies opioid receptors and prevents or reverses the effects produced by opioid agonists. Despite its clear pharmacologic effectiveness, its clinical effectiveness in treating opioid dependence has been disappointing, primarily due to non-compliance with taking the medication. However, the recent availability of sustained-release formulations of naltrexone has renewed interest in this medication. The present paper describes the development of sustained-release naltrexone formulations and discusses the clinical issues associated with their use in treating opioid dependence.

5-iodonaphthyl-1-azide labeling of plasma membrane proteins adjacent to specific sites via energy transfer

We have examined conditions optimal for 5-iodonaphthyl-1-azide (INA4) labeling of membrane proteins proximal to known membrane sites. Membrane-bound INA can be indirectly activated by energy transfer from visible chromophores. We demonstrate that the efficiency of this sensitized activation is enhanced by use of triplet-forming chromophores such as eosin and by deoxygenation. Variation of sensitized activation efficiency with INA concentration indicates that the critical distance for eosin-INA energy transfer in solution is 8-14 A. We suggest that photosensitization occurs through triplet exchange and present an improved labeling protocol based on these findings. This protocol was used to examine whether different accessory proteins are associated with isolated and crosslinked Type I Fc epsilon receptors on 2H3 rat basophilic leukemia cells. 2H3 cells were incubated with eosin-conjugated IgE and irradiated at 514 nm yielding [125I]INA derivatized peptides at 53, 38, 34, and 29 kDa. Crosslinking IgE with mouse anti-rat IgE prior to irradiation labeled three additional proteins at 60, 54, and 43 kDa. These results demonstrate the utility of sensitized INA labeling in characterizing protein-protein interactions in membranes of intact cells and indicate the importance of considering photophysical factors when selecting sensitizers and reaction conditions. We discuss estimation of the size of the membrane region surrounding a sensitizing chromophore within which INA labeling of membrane proteins occurs.

Developmentally regulated association of a 56-kD member of the surface immunoglobulin M receptor complex

Immature and mature B cells differ in the signals generated and transduced through their antigen receptor, surface immunoglobulin M (sIgM). Whereas signals generated through sIgM on mature B cells initiate a program leading to the positive activation of these cells, signaling through this receptor at the immature stage of development leads to a state of induced unresponsiveness or tolerance. Our previous studies have described developmental differences in sIgM transmembrane signaling that are independent of ligand-receptor affinity. In an attempt to understand the molecular basis for signaling differences between immature and mature B cells, we have analyzed the sIgM receptor complex in neonatal and adult mouse splenic B cells. While previously described components of this complex do not exhibit marked developmentally regulated differences in their association with sIgM, we have identified a 56-kD protein that associates with sIgM in mature (antigen-responsive), but not immature (tolerance-sensitive) B cells. This protein (p56) associates with sIgM as a homodimer, is constitutively phosphorylated on tyrosine, and is coimmunoprecipitated with IgM but not IgD. The observed inability to iodinate p56 suggests it is an intracellular component of the receptor complex. Based upon its migration in one- and two-dimensional gel electrophoresis we show, however, that p56 is distinct from the blk, lyn, or fyn src family kinases that have been shown to be associated with sIgM in mature B cells. The developmentally regulated participation of p56 in the B cell antigen receptor complex suggests a role in the differential signaling mediated via sIgM on immature and mature B cells.

Cell surface display of rat invariant gamma chain: detection by monoclonal antibodies directed against a C-terminal gamma chain segment

A series of 14 monoclonal antibodies (mAb) directed against the C-terminal part of the rat invariant gamma chain (amino acid 142-216) was generated using distinct fusion proteins that contain this gamma segment for immunization and hybridoma screening. Additional fusion protein were prepared carrying discrete regions of the gamma chain. Employing these reagents confirmed that the obtained mAb do indeed recognize the C-terminal portion of the invariant chain, as demonstrated by Western blot analysis. All mAb established recognize epitopes present on the native gamma chain, as revealed by immunoprecipitation analysis using nonionic detergent extracts of metabolically labeled Lewis rat splenocytes combined with two-dimensional gel electrophoresis. However, while the majority of the gamma chain-specific mAb precipitated gamma chain-containing polypeptide chain complexes in which immature, sialic acid-deficient and mature, terminally sialylated forms of the gamma chain were predominantly represented, a fraction of the antibodies preferentially precipitated the immature gamma forms. Cell surface binding of these two groups of mAb correlated with the immunoprecipitation data in that the former group of antibodies did bind to intact Lewis rat spleen cells, while essentially no binding was observed with the antibodies of the latter group. Double-fluorescence staining with the class II-specific fluorescein isothiocyanate-conjugated mAb OX3 and OX6, respectively, as well as a representative gamma chain-specific mAb visualized with phycoerythrin-coupled secondary antibody shows coexpression of class II determinants and the invariant chain at the cell surface.

Hydrophobic labelling of membrane-embedded proteins with lipophilic reagents. Incorporation of [125I]INA and [125I]TID into B lymphocyte membrane immunoglobulins

Hydrophobic labelling is frequently used in the study of membrane-inserted domains of intrinsic proteins. However, the published procedures, fail to incorporate sufficient radioactivity into membrane immunoglobulins of B lymphocytes to permit investigation of their subunit structures and associations with other proteins. In order to increase the specific radioactivity of [125I]iodonaphthylazide ([125I]INA), an improved method for the synthesis of the reagent was developed. In addition, the optimal conditions for labelling B lymphocytes with [125I]INA and the commercially available reagent 3-(trifluoromethyl)-3-(3'-[125I]iodophenyl)diazirine ([125I]TID) were elaborated. Under these optimized conditions, Ig molecules labelled with [125I]INA and [125I]TID were isolated and analysed in detail by SDS-PAGE. The usefulness of the two reagents for the investigation of lipid-embedded domains of membrane proteins is discussed.

Selective labeling of proteins in biological systems by photosensitization of 5-iodonaphthalene-1-azide

The apolar azide of 5-iodonaphthalene-1-azide (Ina) partitions into the lipid bilayer of biological membranes. Upon photolysis at 314 nm, it is rapidly converted into the reactive nitrene, which efficiently attaches covalently to lipid-embedded domains of proteins and, to a lesser extent, to membrane phospholipids. Above 370 nm, Ina absorption is negligible and photolysis at these wavelengths does not occur. However, on addition of the photosensitizing molecule 3-aminopyrene, trifluoperazine, or 8-anilinonaphthalene-1-sulfonate, followed by irradiation at 380 nm, efficient conversion of Ina to reactive species was observed, as measured by [125I]Ina-labeling of membrane proteins and inactivation of the hormonal response of adenylate cyclase. Irradiation at 480 nm in the presence of a fluorescein derivative of n-undecylamine also resulted in a pattern of [125I]Ina-labeled membrane proteins and hormone uncoupling indistinguishable from that obtained following direct photolysis at 314 nm. Photosensitization of the azide molecules is confined to the vicinity of the photosensitizer chromophore. This allowed selective labeling of chromophore-bearing proteins in solution or in membranes. Bovine serum albumin-fluorescein conjugate, in the presence of nonderivatized soluble proteins, was exclusively labeled by [125I]Ina when irradiated at 480 nm, but random labeling occurred on photolysis at 314 nm. Likewise, rhodopsin in rod outer segment membranes from frog retina was exclusively labeled by [125I]Ina upon photosensitization at 380 nm. Random labeling again occurred on direct irradiation at 314 nm. The results suggest that selective labeling in complex biological systems may be achieved by photosensitized activation of azides.

Class II antigens in Hashimoto thyroiditis. I. Synthesis and expression of HLA-DR and HLA-DQ by thyroid epithelial cells

Aberrant expression of HLA-DR antigens on epithelial cells is seen in various organ-specific autoimmune disorders including Hashimoto thyroiditis (HT). Expression of HLA-DQ has so far not been demonstrated on these cells. We report here that thyroid epithelial cells (TEC) in HT, in addition to the known aberrant expression of HLA-DR, coexpress HLA-DQ antigens. Furthermore we provide evidence that class II antigens are synthesized by TEC themselves by demonstration of intracellular HLA-DR gamma-chain. These findings support the theory that TEC may be able to present (auto)antigens in vivo thus perhaps contributing to the perpetuation of thyroid destruction. As expression of class II antigens on TEC was never observed in non- or weakly infiltrated areas, we propose that infiltration by T cells is necessary to induce this aberrant expression of class II antigens.

A 30-kDa protein is disulfide linked to IgM on normal and neoplastic murine B lymphocytes

Four cloned continuously cultured mouse B lymphoma cells of the lines WEHI-279, BCL1 and 38C-13 were used to study the surface presentation of IgM. Cells of all lines expose, in addition to mu 2L2, various, disulfide-linked subunits as already shown for normal B lymphocytes (Koch, N. and Haustein, D., Mol. Immunol. 1982. 19:477). In contrast to normal B cells, B lymphoma cells also carry IgM structures of higher molecular weight than mu 2L2 on their surface. Furthermore, B lymphoma cells of all investigated lines expose a mu 2L2 structure to which a 30-kDa protein is disulfide-linked. This 30-kDa protein could also be identified on normal mouse B lymphocytes when more drastic conditions were employed. mu 2L2 disulfide linked to this 30-kDa protein is the main IgM structure on normal and neoplastic B cells whereas only small amounts of mu 2L2 (unlinked to this protein) can be detected.

Selective photoinduced uncoupling of the response of adenylate cyclase to gonadotropins by 5-iodonaphthyl 1-azide

5-Iodonaphthyl 1-azide (INA) has been previously shown to selectively label, on photolysis, only those proteins in contact with the membrane lipids. Low concentrations (less than 10 microM) of INA added to rat ovarian plasma membranes induced, on photoactivation, a selective and complete loss of the response of the adenylate cyclase to stimulation by human chorionic gonadotropin (hCG) or luteinizing hormone (LH). In contrast, this treatment affected neither hCG binding to the receptor nor the stimulation of the enzyme by NaF. That the uncoupling of the receptor from the enzyme by INA occurred within the lipid bilayer can be derived from the finding that the prior presence neither of saturating concentrations of hCG nor of the aqueous nitrene-scavenger glutathione (GSH) prevented this effect. Photolysis at higher concentrations of INA (0.1-1 mM) led to the inhibition of the adenylate cyclase stimulated by fluoride. This effect was totally prevented by glutathione. A similar behavior was obtained with a water-soluble analogue of INA, namely, 5-diazonionapthyl 1-azide (DAN). On photoactivation with 30 microM DAN, the NaF-stimulated adenylate cyclase was inhibited, but this effect was completely prevented by added GSH. At low concentrations where its effects are restricted to the lipid core, INA may represent a useful tool to define receptor coupling with the adenylate cyclase. The capacity of INA at low concentrations to uncouple the hormone receptor from the adenylate cyclase is not restricted to the LH/hCG receptor. Other hormone receptors tested behaved similarly. Therefore, the reported findings appear to represent a general phenomenon.

A tumor-associated antigen specific for human kappa myeloma cells

A monoclonal antibody (K-1-21) raised against a kappa Bence Jones protein exhibits unique binding properties to malignant plasma cells. K-1-21 is an IgG1 kappa antibody that reacts with human kappa light chains in free form, but shows no reactivity with heavy chain-associated kappa light chains. By immunofluorescence, K-1-21 binds to the surface of LICR LON/HMy2 (HMy2) kappa myeloma cells and to plasma cells from a majority (8/11) of patients with various types of kappa myeloma; it did not bind to the surface of normal cells, nor to malignant cells of non-kappa myeloma origin. Flow cytometry analysis of K-1-21 binding to HMy2 cells indicated that the surface reactivity of K-1-21 could be completely inhibited by preincubation of the antibody with purified kappa light chains, whereas no inhibition occurred after preincubation with lambda chains or intact human IgG. Thus, the epitope recognized by K-1-21 on the cell surface may be similar, if not identical, to the determinant recognized on soluble free kappa light chains, and constitutes a tumor-associated antigen with selectivity for kappa myeloma cells. K-1-21 may therefore have clinical potential in patients with kappa myeloma.

Biochemical characterization of mIgM- variants of the murine B-cell lymphoma, WEHI 279.1

The membrane immunoglobulin M (mIgM) of a B lymphocyte serves as a receptor for its cognate antigen. Our aim is to elucidate the structure and function of this membrane-bound receptor. The first step is to determine the requirements for proper membrane placement of IgM. We have used mIgM-positive B lymphocyte tumors from which we isolated mIgM negative variants by immunoselection. We report here the initial characterization of mIgM- variants isolated by repeated cycles of selection of the murine B lymphoma, WEHI 279.1, with goat anti-mouse immunoglobulin (G alpha MIg) and complement. These particular variants were chosen from a pool of more than 150 variants originally isolated because they resulted from several selection schemes and clearly had different origins. By analysis of their proteins, we have found three major phenotypes that do not produce mIgM: reduced microns, microseconds and L chain levels within cells, loss of microns and microseconds but retention of L chain synthesis, and loss of microns but retention of reduced amounts of microseconds and L chain. The defects underlying these phenotypes produce complex changes in the synthesis, turnover, and secretion of the mu or L chains involved. We performed experiments comparing the effects of the glycosylation inhibitor tunicamycin on variants with reduced mu and L levels with its effects on variants with L but no mu chains. These experiments suggested that mu and L chain synthesis are controlled coordinately at the level of protein synthesis. We have not yet isolated any variants lacking L chain synthesis or any appearing to have gross structural defects in the micron protein. This analysis is the first phase of the detailed characterization of the requirements for proper synthesis, processing, tetramer formation, and membrane display of mIgM on B lymphoma tumors in mice.

Further biochemical studies of the human B-cell differentiation antigens B1 and B2

Two human B lymphocyte-associated antigens, B1 and B2, were studied by immunoprecipitation using monoclonal antibodies. B1 was found to be a nonglycosylated protein of MW 35 kD, phosphorylated at serine and threonine. B2, a 140 kD MW antigen, was characterized as a glycoprotein without phosphorylated sites. Both proteins were found to contain portions which could be labeled with 125I-iodonaphthylazide. The B1 protein displayed hydrophobic properties whereas B2 had a more amphiphilic character.

mRNA for surface immunoglobulin gamma chains encodes a highly conserved transmembrane sequence and a 28-residue intracellular domain

To probe the structure of the gamma heavy chain of membrane IgG and the mRNA and gene segments that encode it, we have analyzed cDNA clones derived from a gamma 1 membrane RNA of B lymphoma 2PK-3. The nucleotide sequence of the clones indicated that membrane gamma 1 chains bear a COOH-terminal 71-residue segment that is absent from secretory gamma 1 chains. This terminus includes a 26-residue hydrophobic transmembrane region homologous to that of membrane mu chains and, significantly, a 28-residue intracellular domain found only on gamma chains. The extra domain suggests that receptor IgG, on memory B cells, may generate a different signal on binding antigen than does receptor IgM, on virgin B cells. The gamma 1 membrane terminus is encoded by two gene segments 1.5 and 2.4 kilobase pairs downstream from the C gamma 1 gene, and homologous segments occur 3' to the C gamma 2a and C gamma 3 genes. Small amounts of membrane gamma mRNAs persist in plasmacytomas secreting IgG1, IgG2a, or IgG2b, suggesting that competition between alternative RNA processing pathways governs the synthesis of membrane and secretory gamma chain mRNAs.