Distinct gamma2 subunit domains mediate clustering and synaptic function of postsynaptic GABAA receptors and gephyrin

Modulation of the concentration of postsynaptic GABA(A) receptors contributes to functional plasticity of inhibitory synapses. The gamma2 subunit of GABA(A) receptor is specifically required for clustering of these receptors, for recruitment of the submembrane scaffold protein gephyrin to postsynaptic sites, and for postsynaptic function of GABAergic inhibitory synapses. To elucidate this mechanism, we here have mapped the gamma2 subunit domains required for restoration of postsynaptic clustering and function of GABA(A) receptors in gamma2 subunit mutant neurons. Transfection of gamma2-/- neurons with the gamma2 subunit but not the alpha2 subunit rescues postsynaptic clustering of GABA(A) receptors, results in recruitment of gephyrin to postsynaptic sites, and restores the amplitude and frequency of miniature inhibitory postsynaptic currents to wild-type levels. Analogous analyses of chimeric gamma2/alpha2 subunit constructs indicate, unexpectedly, that the fourth transmembrane domain of the gamma2 subunit is required and sufficient for postsynaptic clustering of GABA(A) receptors, whereas cytoplasmic gamma2 subunit domains are dispensable. In contrast, both the major cytoplasmic loop and the fourth transmembrane domain of the gamma2 subunit contribute to efficient recruitment of gephyrin to postsynaptic receptor clusters and are essential for restoration of miniature IPSCs. Our study points to a novel mechanism involved in targeting of GABA(A) receptors and gephyrin to inhibitory synapses.

The GDP-GTP exchange factor collybistin: an essential determinant of neuronal gephyrin clustering

Glycine receptors (GlyRs) and specific subtypes of GABA(A) receptors are clustered at synapses by the multidomain protein gephyrin, which in turn is translocated to the cell membrane by the GDP-GTP exchange factor collybistin. We report the characterization of several new variants of collybistin, which are created by alternative splicing of exons encoding an N-terminal src homology 3 (SH3) domain and three alternate C termini (CB1, CB2, and CB3). The presence of the SH3 domain negatively regulates the ability of collybistin to translocate gephyrin to submembrane microaggregates in transfected mammalian cells. Because the majority of native collybistin isoforms appear to harbor the SH3 domain, this suggests that collybistin activity may be regulated by protein-protein interactions at the SH3 domain. We localized the binding sites for collybistin and the GlyR beta subunit to the C-terminal MoeA homology domain of gephyrin and show that multimerization of this domain is required for collybistin-gephyrin and GlyR-gephyrin interactions. We also demonstrate that gephyrin clustering in recombinant systems and cultured neurons requires both collybistin-gephyrin interactions and an intact collybistin pleckstrin homology domain. The vital importance of collybistin for inhibitory synaptogenesis is underlined by the discovery of a mutation (G55A) in exon 2 of the human collybistin gene (ARHGEF9) in a patient with clinical symptoms of both hyperekplexia and epilepsy. The clinical manifestation of this collybistin missense mutation may result, at least in part, from mislocalization of gephyrin and a major GABA(A) receptor subtype.

Isolation of the human gamma-carboxylase and a gamma-carboxylase-associated protein from factor IX-expressing mammalian cells

A model system for the analysis of intracellular events governing the modification of individual vitamin K-dependent (VKD) proteins by the carboxylase has been developed using recombinant VKD protein-transfected cell lines. When untransfected 293 cells were analyzed by in vitro carboxylation followed by SDS-PAGE, endogenous VKD proteins were not detected. With 293 cells stably-transfected with recombinant native factor IX, most (> 95%) of the carboxylase was in complex with the factor IX, as assayed by adsorption of carboxylase activity to immobilized anti-factor IX antibody. In contrast, with 293 cells stably-transfected with recombinant factor IX deleted in the propeptide sequence (amino acids -18 to -4, delta pro factor IX), no association of factor IX with the carboxylase was observed. This observation was used to specifically isolate and identify the human carboxylase, and carboxylase-associated protein. When the carboxylase was purified from solubilized microsomes from either native factor IX, or delta pro factor IX, stably-transfected 293 cells, a single 98 kDa band was specifically obtained from native factor IX microsomes, but not from delta pro factor IX microsomes. This band was subsequently shown by Western and microsequencing analysis to comprise both the carboxylase and carboxylase-associated protein. This isolation, which represents the first isolation to near homogeneity of both the human carboxylase and the carboxylase from cell lines, will be valuable in isolating enzymatically active recombinant carboxylase, which has been refractile to other purification attempts. This system was also used to show that the human carboxylase in 293 cells is capable of binding and modifying two different liver-derived proteins. Protein C-producing 293 cells were generated from the same 293 progenitor cell line used to created the factor IX-expressing cells. With both factor IX- and protein C-transfected 293 cells, the secreted proteins were almost completely carboxylated, and in microsomes from each cell line the carboxylase was found in near quantitative complex with the two different VKD proteins. Thus the carboxylase modifies both VKD proteins. The approach described here for the analysis of the carboxylase from recombinant VKD protein-transfected cell lines should provide an important new system for studying protein carboxylation and VKD protein-carboxylase interaction.

Secretion of interleukin-1 beta by a leukemia cell line in response to lipopolysaccharide and mezerein

The acute monocytic leukemia cell line, THP-1, is frequently used as a model to study the mechanism of cell-mediated immune response. The model is justified, in part, because THP-1 cells can be induced to express features of activated peripheral blood monocytes or tissue macrophages. The current investigation, however, demonstrates that THP-1 cells differ from normal monocytes in their secretion of interleukin-1 beta (IL-1 beta) following exposure to reagents that induce synthesis. For example, LPS treatment alone did not result in IL-1 beta secretion and very low concentrations were observed when lipopolysaccharide (LPS)-treated cells were simultaneously incubated with silica or silica in combination with hydroxyurea. Silica-enhanced release of IL-1 was related to changes in cell membrane permeability. Recombinant interferon-gamma (rIFN gamma) alone did not induce IL-1 beta secretion and did not significantly increase secretion by LPS- and silica-stimulated cells. In contrast, mezerein stimulation led to higher extracellular concentrations of IL-1 beta and rIFN gamma augmented secretion by mezerein-treated cells. Isoelectrofocusing of conditioned medium and titration of pooled fractions showed a direct correlation between extracellular levels of biologically active and immunoreactive IL-1. The role of IFN gamma in stimulating IL-1 beta secretion was not related to an enhancement of viability or an increase in the proportion of mezerein-treated cells synthesizing DNA. It was concluded that mezerein's regulation of secretion by THP-1 cells depended on the expression of monocyte features, including cell adherence and responsiveness to IFN gamma.

Demonstration of IL-1 alpha, and IL-1 beta secretion by the monocytic leukemia cell line, THP-1

This study examined the secretion of IL-1 alpha and IL-1 beta by THP-1 leukemia cells following activation with mezerein and promotion of synthesis by interferon (IFN-gamma). Interleukin-1 (IL-1) was not detected by co-mitogenic thymocyte assays of crude supernates. Isoelectrofocusing of concentrated medium showed that all biologically active IL-1 migrated at a pH of 6.8-7.2, indicating that the major secreted form was IL-1 beta. Double antibody ELISA confirmed the presence of IL-1 beta, but failed to detect IL-1 alpha in isofocused fractions. Although it appeared that THP-1 cells do not secrete IL-1 alpha; an inhibitor of thymocyte response to IL-1 was present in conditioned medium, migrated in an acidic pH range and masked the expression of biologically active rIL-1 alpha and rIL-1 beta. In contrast, IL-1 alpha was detected using a cell blotting assay. This technique permitted visualization of subpicogram levels of IL-1 when secreted by cells attached to an immunoblotting paper. Cell blotting showed that a greater proportion of attached cells incubated for 24 h in medium containing mezerein and IFN-gamma secreted IL-1 than cells in control medium. In conclusion, the amount of immunoreactive or biologically active IL-1 alpha secreted by stimulated THP-1 cells appeared to be much lower than that reported for human peripheral blood monocytes.

Correlation between human cell growth response to interleukin 1 and receptor binding

Human recombinant interleukin 1 alpha (IL-1 alpha) and IL-1 beta inhibited the replication of the mammary tumor cell line, MDA-MB-415; stimulated division in the colon carcinoma, SW-48; and had no effect on the growth of the milk mammary line, HBL-100. Inhibition of growth was reflected in a significant decrease in DNA synthesis accompanying a transient increase in RNA synthesis. Specific binding of 125I-labeled recombinant IL-1 beta by MDA-MB-415 and SW-48 reached a maximum by 2 h of incubation, and an equivalent amount was bound by each cell type. Binding was inhibited in a dose-dependent manner by unlabeled IL-1 alpha or IL-1 beta. Scatchard plot analysis revealed that MDA-MB-415 cells expressed approximately 700 binding sites with an apparent dissociation constant of 8.8 x 10(-10) M. Reversibility of growth inhibition was independent of dose or time of incubation, but DNA synthesis did not return to control values. Flow cytometric analysis of DNA content showed that growth inhibition was cell cycle phase nonspecific with a slight reduction in the proportion of cells in S phase. The major conclusion from these studies was that inhibition or stimulation of malignant cell growth by IL-1 was related to the presence of receptor sites.

Regulation by interferon gamma of function in the acute monocytic leukemia cell line, THP-1

THP-1 is an acute monocytic leukemia cell line which acquires phenotypic and functional monocytoid-like features following incubation with mezerein. The current study concerned the modulation of these features by rIFN gamma. rIFN gamma induces the time-dependent enhancement of HLA-DR expression in the presence or absence of mezerein but has no effect on the expression of Leu-M1, Leu-M2, or Leu-M3 antigens. CSF-1 production following mezerein activation was reduced by incubation in the presence of 10(3) and 10(4) units/ml rIFN gamma. This was confirmed through both biological assays with mouse bone marrow cells and an indirect ELISA. In contrast, the concentration of growth inhibitory activity in conditioned medium was increased by rIFN gamma. A small but significant increase in IL-1 beta concentration in conditioned medium was detected using a sensitive double-antibody ELISA and a radioimmunoassay. The results infer that the functional characteristics of this leukemia cell line are modulated by rIFN gamma in a manner qualitatively similar to that reported for IFN gamma treated normal monocytes.

Quantitation of human interleukin-1 beta with polyclonal antibodies

Polyclonal antisera to human recombinant interleukin-1 beta have been developed in rabbits and mice. When measured by indirect enzyme-linked immunoassays, the antisera bound rIL-1 beta in a dose-dependent manner but did not bind to rIL-1 alpha or murine rIL-1. Both antisera specifically neutralized thymocyte activation induced by rIL-1 beta or normal monocyte IL-1 and growth inhibition induced by rIL-1 beta. The antiserum specificities led us to examine a double antibody ELISA for quantitating IL-1 beta. The linear portion of the standard curve from an ELISA against rIL-1 beta ranged from 0.5 to 10.0 half-maximal units of [3H]thymidine incorporation in mouse thymocyte cultures. A similar correlation was observed using monocyte derived IL-1 from two sources. The major result of this work was the observation that polyclonal antibodies made against a recombinant molecule also recognized natural IL-1. We suggest that these antisera recognize epitopes unique to IL-1 beta which reside at or near regions of the molecule responsible for biological activity.

Characterization of a colony-stimulating factor produced by the human monocytic leukemia cell line, THP-1

The human monocytic leukemia cell line, THP-1, acquires macrophage-like characteristics following exposure to mezerein. Serum-free medium conditioned by mezerein-activated cells was observed to contain colony-stimulating factor (CSF) activity in assays with murine bone marrow cultures. Isoelectrofocusing revealed that CSF activity displayed charge heterogeneity and migrated in a pl range of 4.4-5.3. Treatment with neuraminidase did not affect biological activity but did reduce charge heterogeneity. Reisofocusing of neuraminidase-treated CSF revealed a peak of activity at pl 4.9. The active component was shown to be an acidic sialoglycoprotein, resistant to proteolytic cleavage but completely inactivated by 2-mercaptoethanol. This CSF has been purified from THP-1-conditioned serum-free medium by preparative isoelectrofocusing, gel filtration through Sephacryl S-200, ion exchange chromatography on DEAE-Sephacel, neuraminidase treatment, and tris-glycinate polyacrylamide gel electrophoresis (PAGE). Elution from SDS-PAGE revealed a single peak of activity corresponding to an apparent molecular weight of 70,000 daltons. Preliminary characterization of the bone marrow cells in colonies showed that THP-1 cells produced macrophage-specific CSF.

Biological response modifiers released by a mezerein-treated human monocytic leukemia cell line, THP-1

After mezerein treatment of suspension cultures of the acute monocytic human leukemia cell line THP-1, cells became adherent to plastic culture surfaces, lost division potential, acquired Fc receptors, displayed phagocytic activity, and expressed increased nonspecific esterase staining. Serum-free RPMI 1640 medium conditioned by adherent THP-1 cells was examined for the presence of biological response modifiers. Preparative isoelectrofocusing of concentrated medium in the presence of various pH gradients of Ampholine ampholytes resulted in the separation of the following activities: fibroblast growth-stimulating activities in pH ranges of 4.10-4.55 and 5.30-5.45; colony-stimulating factor (CSF) for mouse bone marrow cells at pH 4.10-4.55; and a malignant cell growth inhibitor comigrating with a lymphocyte-activating factor (interleukin-1) at pH 6.70-6.95. Both CSF and the fibroblast growth stimulator isofocused at pH 4.10-4.55 coeluted following molecular-sieve chromatography through P-100. CSF-induced colonies were composed of nongranulocytic mononuclear cells. Chromatography through an ACA-54 column separated interleukin-1 from most of the growth-inhibitory activity.

Stimulation of diploid fibroblast growth with serum-free medium conditioned by mezerein-treated monocytic leukemia cells

Medium conditioned by mezerein-treated human acute monocytic leukemia cells (THP-1) stimulated human fibroblast replication. Maximum mitogenic activity was elaborated by THP-1 cells with a 24-hr incubation in 10(-7) M mezerein (activator phase) followed by a 36-hr incubation in insulin-supplemented serum-free Roswell Park Memorial Institute (RPMI)-1640 medium (effector phase). Growth stimulation was not due to the presence of residual mezerein. We previously reported that leukemia cells also produced a growth inhibitor. Fibroblast stimulation was resolved by isoelectrofocusing into several active fractions separate from the growth inhibitory activity for malignant mammary cells. Conditioned medium was mitogenic for fibroblasts in the presence of high concentrations of fetal bovine and human whole blood sera. Growth stimulation was observed in plasma-derived serum only when supplemented with exogenous platelet-derived growth factor. Thus, this THP-1 cell product does not fulfill the role of a competence factor.

Production of growth-inhibitory activity in serum-free medium by human monocytic leukemia cells

Serum-free medium conditioned by activated cells of the acute monocytic leukemia line, THP-1, was examined for growth-inhibitory activity with several established human cell lines. Free-floating clusters of THP-1 cells were activated into adherent nonproliferating cells by a 24-hr exposure to 10(-7) M mezerein in Roswell Park Memorial Institute 1640 medium containing 1% fetal bovine serum. Adherent cells were incubated for an additional 24 hr in serum-free medium containing insulin (5 micrograms/ml). Dose-response studies revealed that a cervical carcinoma (HeLa), a melanoma (A375Ag5), and several mammary carcinoma cell lines (MCF-7, BT474, MDA-MB415, and T47D) were growth inhibited by this conditioned medium. We concluded, from the results of thymidine release assays and from experiments on reversibility, that inhibition was a cytostatic and not a cytolytic response. In contrast, THP-1 conditioned medium stimulated the growth of two mammary lines (ZR75-1 and HBL-100), a lung type II carcinoma (549), and a colon adenocarcinoma (SW48). Preliminary characterization showed that the inhibitory activity was stable to acid and urea treatment but was destroyed by trypsin and sodium dodecyl sulfate. Molecular sieve chromatography of acetic acid-extracted material separated the inhibitory and stimulatory components.