Oligomerization and ring closure of immunoglobulin E class antibodies by divalent haptens

Cross-linking of antibodies constitutes a widespread initiation signal for their respective effector functions. Cross-linking IgE-class antibodies provide the triggering signal to mast cells for their degranulation process. To obtain a quantitative insight into these cross-linking processes, the interactions between a DNP-specific monoclonal antibody of the IgE class and a series of divalent DNP haptens with spacers of different length and flexibility have been studied by fluorescence titration experiments. These were analyzed by employing the theoretical model developed by Dembo and Goldstein [Dembo, M., & Goldstein, B. (1978) J. Immunol. 121, 345-353] in a fitting procedure. Equilibrium constants that describe the aggregation and ring-closure processes caused by divalent hapten binding have been used as free parameters. The intrinsic binding constants were determined by fluorescence titrations with corresponding monovalent haptens. The main results are the following: (1) The divalent haptens with a short and flexible spacer [i.e., N alpha, N epsilon-di-(DNP)-L-lysine,meso-bis[(DNP-beta-Ala)amino]succinate, and bis[(DNP-tri-D-Ala)amino]heptane, having a maximal DNP-DNP distance of gamma = 14, 21, and 45 A, respectively] effect aggregation of the antibodies mainly into closed dimers. (2) The divalent hapten family with long and rigid oligoproline spacers di(DNP)-Ahx-Asp-(Pro)n-Lys with n = 24, 27, and 33 (i.e., gamma = 100, 110, and 130 A) causes aggregation of the antibodies predominantly into closed dimers and trimers. The corresponding equilibrium constants of the respective ring-closure processes decrease significantly with longer spacer length. (3) Evidence was found that intramolecularly monomeric ring closure of the IgE antibodies is caused by haptens containing oligoproline spacers with n = 37 or 42 (gamma = 130-150 A). The equilibrium constant of the ring-closure process increases with spacer length. This increase in stability indicates a difference in the imposed strain. Furthermore, the latter results imply that the distance between the two binding sites of the IgE molecule lies in the range dictated by the rigid oligoproline part of the respective hapten's spacer, i.e., 115-130 A. (4) Nearly all oligomeric ring-closure processes proceed relatively slowly with an approximate lower limit of a half-life of 5-10 s. This slowing down of the aggregation and ring-closure processes most probably reflects steric factors.     

Direct binding of a myasthenia gravis related epitope to MHC class II molecules on living murine antigen-presenting cells.

MHC gene products present antigenic epitopes to the antigen receptor on T cells. Nevertheless, direct binding of such epitopes to MHC class II proteins on normal living antigen-presenting cells (APCs) has not yet been demonstrated. We have previously shown a significant difference in the ability of T cells of myasthenia gravis (MG) patients to proliferate in response to the synthetic peptide p195-212 of the human acetylcholine receptor (AChR) alpha-subunit in comparison to healthy controls. The observed proliferative responses correlated significantly with HLA-DR5. Moreover, lymph node cells of various mouse strains that were primed with the T cell epitope, p195-212, were found to proliferate to different extents. To investigate these observations further, we designed an assay for direct binding of p195-212 to MHC class II proteins on the surface of freshly prepared splenic adherent cells. Binding of a biotinylated p195-212 was monitored using phycoerythrin-avidin by flow cytometry. Fifteen to sixty per cent of the cells were labeled following incubation with the biotinylated peptide. Binding was observed only to splenic adherent cells derived from mouse strains of which T cells were capable of proliferating in response to p195-212. The binding specificity, in terms of epitope structure and its site of interaction on the cells, was shown by its inhibition with an excess of the unlabeled peptide or with the relevant monoclonal anti-I-A antibodies. These results constitute the first direct evidence for the specific binding of a T cell epitope to live APC.     

Immunocytochemical and ultrastructural study of the frog (Rana pipiens) pars distalis with special reference to folliculo-stellate cell function during in vitro superfusion

Summary The colloidal gold immunocytochemical technique was used to determine the ultrastructural features of the glandular cells in the pituitaries of male frogs, Rana pipiens, both in vivo and after superfusion in vitro. Specific reactions to antisera against bullfrog gonadotropins, human prolactin, and synthetic 1–39 corticotropin allowed identification of the 3 corresponding types of glandular cells. No immunoreaction was obtained with antisera against human or ovine-growth hormone, human -thyrotropin hormone, and bovine S-100 protein. General morphological features of these immunocytochemically identified glandular cells were similar to those of equivalent cells previously described in other amphibian species. Non-glandular folliculo-stellate cells were distinctive. In freshly removed pituitaries, these folliculo-stellate cells contained lysosome-like structures, but did not show phagocytic vacuoles in the cytoplasm; they contained many mitochondria, and the Golgi complex and endoplasmic reticulum were relatively undeveloped. After 4 or 18 h of superfusion, some immunoreactive gonadotropic, prolactin, and corticotropic cells showed degeneration and destruction. In the same gland, folliculo-stellate cells retained a viable appearance, but showed phagocytic vacuoles containing secretory granule-like structures which were immunoreactive to gonadotropic, prolactin, and corticotropic antibodies. Some folliculo-stellate cells showed phagocytic vacuoles containing complete glandular cells. These results suggest that superfusion causes a destruction of some of the glandular cells, and that folliculo-stellate cells act as phagocytes when cellular debris or moribund cells are present in the intercellular space in the pituitary parenchyma.Supported by grant DCB 8710462 from the National Science Foundation, grant 2148-83 from the CAICYT (Spain) and the Junta de Andalucia (Spain)     

Effects of gonadectomy and steroids on pituitary gonadotropin secretion in a frog, Rana pipiens

Secretory dynamics of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured at various times following gonadectomy in adult male grass frogs, Rana pipiens. Plasma levels and in vitro initial secretory rates of both LH and FSH increased significantly within 1 wk and remained elevated for 3-4 wk of castration. Pituitary FSH and LH content were unchanged. However, dissociation between the two gonadotropins (Gth) occurred thereafter: Secretion of FSH remained elevated for 70 days, but those of LH declined to control levels after 30 days. In vitro secretion of Gth from gonadectomized (gonadx) frogs declined progressively over time reaching control levels after 24 h incubation. The results indicate that elevated pituitary secretion contributes to the observed circulating LH and FSH levels in gonadx frogs, and that FSH and LH may be controlled independently. Replacement therapy with 17 beta-estradiol (E2) suppressed post-gonadectomy increases in plasma Gth and in vitro responses to GnRH, whereas 5 alpha-dihydrotestosterone (DHT) had little effect in vivo and augmented GnRH responses in long-term castrates. In vitro, E2 also inhibited, while 48 h of DHT treatment had no effect on GnRH responsiveness of pituitaries from gonadx frogs. The actions of these steroids were opposite to those typically observed in mammals (and birds), and support the hypothesis that E2 may contribute to seasonal testicular regression in ranid frogs.     

Stereological study of gonadotropes in the frog,Rana pipiens,after GnRH stimulation in vitro

Summary Previous physiological results have indicated the existence of two releasable pools of gonadotropins in amphibian pituitaries: an acute releasable pool that appears independent of protein synthesis, and a storage pool involved in chronic release that depends on protein synthesis. To elucidate the ultrastructural localization of these pools and the morphological changes induced in gonadotrope cells after treatment with gonadotropin-releasing hormone, we carried out a morphometric study of immuno-identified gonadotrope cells using an in vitro superfusion system. Treatment with gonadotropin-releasing hormone induced a degranulation of small (110–255 nm) and medium (236–360 nm) secretory granules as well as hypertrophy of the endoplasmic reticulum and Golgi complex. Simultaneous incubation with gonadotropin-releasing hormone and cycloheximide inhibited the release of secretory granules although the endoplasmic reticulum and Golgi complex were hypertrophied. These morphological results strongly suggest: (1) that gonadotropin-releasing hormone induces degranulation and hypertrophy of the biosynthetic machinery in gonadotrope cells; and (2) that the activation of the endoplasmic reticulum and Golgi complex by stimulation with gonadotropin-releasing hormone is independent of protein synthesis, while the release of secretory granules is protein synthesis-dependent. In addition, the second or storage pool of gonadotropin is associated mainly with the small and medium secretory granules.     

Plasma steroid concentrations change in response to sexual behavior in Bufo marinus

Steroid hormone concentrations change in response to social or environmental stimuli in many vertebrates. To test this phenomenon in an amphibian, we examined plasma androgen (A) and corticosterone (B) concentrations in male marine toads (Bufo marinus), a tropical species exhibiting intermale competition, amplectic clasping of females, and bouts of breeding behavior following rains. When males clasped females for 0, 1, 2, or 3 hr, plasma A concentrations were significantly and positively correlated with hours spent in amplexus. In field-sampled males, plasma A concentrations were higher in amplexing males than in single males. Among single males those found closer to breeding ponds had higher A concentrations than those more distant. These data support the hypothesis that sexual stimuli enhance plasma A concentrations in this amphibian. In 3-hr experimental tests and field-sampled males, B concentrations were higher in amplexing than in single males. Unlike some amphibians, short-term elevations of B apparently are not associated with decreased reproductive function. However, as in other amphibians in which high B concentrations are associated with stress-induced inhibition of reproduction, after 48-72 hr in captivity male toads showed high B concentrations and low plasma androgen concentrations. The bursts of sexual activities exhibited by B. marinus following heavy rains were associated with no changes in A concentration and with increased B concentration.     

Translocation of cytosolic acetylcholine into synaptic vesicles and demonstration of vesicular release

The rate of translocation of newly synthesized acetylcholine (ACh) from the presynaptic cytosol of Torpedo electric organ nerve terminals into synaptic vesicles and the extent to which ACh release from these neurons is mediated by a vesicular mechanism were investigated. For this purpose the compound 2(4-phenylpiperidino)cyclohexanol (AH5183), which inhibits the active transport of ACh into isolated cholinergic synaptic vesicles, was employed. Preincubation of purified Torpedo nerve terminals (synaptosomes) with AH5183 does not affect the intraterminal synthesis of [3H]ACh but results in a marked inhibition (85%) of its Ca2+-dependent K+-evoked release. By contrast, the evoked release of the endogenous nonlabeled ACh is not affected by this compound. When AH5183 is added during radiolabeling, it causes a progressively smaller inhibition of [3H]ACh release which is completely abolished if the drug is added after the preparation has been labeled. These findings suggest that most of the newly synthesized synaptosomal [3H]ACh (85%) is released by a vesicular mechanism and that some [3H]ACh (15%) may be released by a different process. The translocation of cytosolic [3H]ACh into the synaptic vesicles was monitored by determining the time course of the loss of susceptibility of [3H]ACh release to AH5183. It was found not to be coupled kinetically to [3H]ACh synthesis and to lag behind it. The nature of the intraterminal processes underlying this lag is discussed.     

A selenomethionine-containing azurin from an auxotroph of Pseudomonas aeruginosa

The production and spectroscopic properties of an L-selenomethionine-containing homolog of Pseudomonas aeruginosa azurin are described. The amino acid substitution was carried out by developing an L-methionine-dependent bacterial strain from a fully functional ATCC culture. Uptake studies monitored using L-[75Se]methionine indicated that L-selenomethionine was incorporated into the protein synthetic pathway of Pseudomonas bacteria in a manner analogous to L-methionine. Several batches of bacteria were grown, and one sample of isolated and purified selenoazurin (azurin in which methionine was substituted by selenomethionine) was found (by neutron activation analysis) to contain 5.2 +/- 0.8 seleniums/copper. Correspondingly, a residual 0.35 methionines, relative to 6.0 in the native protein, were found by amino acid analysis in this azurin sample. The redox potential and extinction coefficient of this selenoazurin were found to be 333 +/- 1 mV (pH 7.0, I = 0.22) and 5855 +/- 160 M-1 cm-1 at 626 +/- 1 nm, respectively. Visible electronic, CD, and EPR spectra are reported and Gaussian curve fitting to the former spectrum allowed assignment of the selenomethionine Se----Cu(II) transition to a band found at 18034 cm-1, based upon an observed 450 cm-1 shift to the red from the analogous band position in the native protein. The data are consistent with a relatively more covalent copper site stabilizing the reduced, Cu(I), form in the selenoprotein. A role for the methionine as a modulator of the blue copper site redox potential by metal----ligand back bonding from Cu(I) is discussed in terms of a ligand sphere which limits the valence change at copper to much less than 1 during a redox cycle.     

Circannual and seasonal variations in plasma luteinizing hormone levels of ovariectomized ground squirrels (Spermophilus lateralis)

Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and ovariectomized squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. LH was undetectable (less than 0.9 ng/ml) in plasma of intact females at all times of year. Females ovariectomized (OVX) at 9.5 months of age in March showed substantial increases in plasma LH in May and June but LH was undetectable between July and November. Females ovariectomized at 13 months of age in July first manifested detectable LH levels the following January and February (6-7 months post-ovariectomy). Very few adult females trapped in May and ovariectomized in August had detectable LH levels within 2 months of ovariectomy; however, females ovariectomized the following February had detectable LH titers 1 month later. Long-term studies of individual OVX squirrels indicated peak LH levels between March and June, 1980, relatively low or undetectable titers between August and December and elevated LH levels between January and March, 1981. The results are suggestive of a circannual rhythm of LH secretion which appears restricted to one season of the year and occurs independently of steroid feedback from the ovaries; ovarian steroids only modulate the levels of plasma LH during the brief annual period of hypothalamo-hypophysial activity. We suggest that onset and termination of LH release are mediated by central nervous system circannual clocks.