Accelerated calcium ion uptake in murine thymocytes induced by concanavalin A.

The mechanism of enhancement of Ca2+ uptake by the T cell mitogen concanavalin A (Con A) was studied in murine thymocytes. Native Con A enhanced the rate of Ca2+ uptake as much as 9-fold, an increase being observed within five minutes after Con A addition. The effect of Con A was reversed completely by alpha-methyl mannopyranoside (alpha-MM). Increased Ca2+ uptake was observed with increasing concentrations of Con A, between 2 and 400 microgram/ml, indicating that the stimulation of Ca2+ uptake is not restricted to mitogenic lectin concentrations (0.5-2 microgram/ml). Succinyl Con A exhibits only a slight effect in the same concentration ranges as native Con A. Ca2+ uptake, both in the absence and presence of Con A, is strongly dependent on energy metabolism and is carrier mediated. The augmentation of Ca2+ uptake by native Con A is due to an enhanced Vmax. Uptake of the anion, CrO42-, by thymocytes, found to be a non-saturable process, was also enhanced by Con A. The effect of Con A on CrO42- permeability appears to be independent of its effect on Ca2+ uptake.     

Transport of 5-hydroxytryptamine by dense granules from porcine platelets

A method is described for the isolation of a homogeneous preparation of dense granules from procine platelets. The purified dense granule fraction contained approximately 400 nmol of 5-hydroxytryptamine/mg of protein and appeared to be homogeneous when examined by electron microscopy. Isolated dense granules transport exogenously added 5-hydroxytryptamine via two mechanisms: 1) a carrier-mediated process predominating at low substrate concentrations and 2) a diffusion-controlled process predominating at high substrate concentrations. Temperature studies revealed an apparent energy of activation of 14.9 kcal/mol for the carrier-mediated transport. Kinetic data yielded a Km of 3.3 micron and a Vmax of 0.79 nmol/min/mg of protein for the mediated transport process. Steady state uptake was sensitive to changes in medium osmotic pressure and a decline in uptake below 300 mosM was correlated with release of endogenous 5-hydroxytryptamine. The transport was inhibited by a number of structural analogs of 5-hydroxytryptamine. These results demonstrate the existence of a carrier-mediated transport system for 5-hydroxytryptamine in the membranes of the platelet dense granules.     

Evidence for two conformational forms of monhistone protein BA which differ in their affinity for DNA

Nonhistone protein BA_free was purified from the 0.075 M NaCl/0.025 M $\text{EDTA}\text{pH}$ 8 extract of whole rat liver nuclei while protein BA_bound was isolated from the 0.05 M Na₂HPO₄/8 M urea/1% β-mercaptoethanol/pH 7.6 extract of dehistonized rat liver chromatin. Chromatin associated protein BA_bound was able to bind 60% of the [³H] DNA in a nitrocellulose filter binding assay while nucleoplasmic protein BA_free showed essentially no DNA binding activity. Circular dichroism analysis of the two forms of protein BA revealed substantial differences in their conformations. Protein BA_free was found to have an α-helix content of 41% while protein BA_bound displayed a spectrum more typical of unordered or β-turn structures.     

Platelet-derived growth factor-mediated Ca2+ entry is blocked by antibodies to phosphatidylinositol 4,5-bisphosphate but does not involve heparin-sensitive inositol 1,4,5-trisphosphate receptors.

Elevation of intracellular Ca2+ by platelet-derived growth factor (PDGF) and other growth factors involves both release of Ca2+ from intracellular Ca2+ stores and Ca2+ entry from the extracellular medium. Release from intracellular stores is believed to be mediated by inositol 1,4,5-trisphosphate (IP3) and the heparin-sensitive IP3 receptor. We studied the mechanism by which entry of extracellular Ca2+ is induced by PDGF. Intracellular free Ca2+ (Ca2+i) was measured in single cultured rat vascular smooth muscle cells using fura 2 microspectrofluorometry. In nominally Ca2(+)-free medium, PDGF (recombinant BB, 10 ng/ml) raised intracellular Ca2+ transiently (less than 5 min); addition of 2 mM Ca2+ to the bathing medium after 5 min caused a second, prolonged increase in intracellular Ca2+. Repeated changes in extracellular Ca2+ from 0 to 2 mM over 90 min caused rapid, parallel changes in Ca2+i of approximately 200 nM. This change in Ca2+i in response to changes in extracellular Ca2+ was virtually undetectable in control or thrombin-treated cells. The intracellular response to changes in medium Ca2+ after PDGF was completely blocked by 10 mM CoCl2, but not by 10(-7) M nicardipine. Microinjection of monoclonal antibodies to phosphatidylinositol 4,5-bisphosphate (PIP2) (kt 10, 2 mg/ml) totally abolished both mobilization of intracellular Ca2+ stores and entry of extracellular Ca2+. Consistent with this finding, maintenance of Ca2+ entry required ongoing receptor occupancy, since displacement of PDGF from its receptor with suramin (1 mM) eradicated extracellular Ca2+ entry in less than 5 min. To determine whether extracellular Ca2+ entry involves the heparin-sensitive IP3 receptor, cells were microinjected with heparin (4 mg/ml) prior to addition of PDGF. Heparin, but not chondroitin sulfate, prevented mobilization of intracellular Ca2+ stores but did not affect extracellular Ca2+ entry. We PDGF requires ongoing receptor occupancy and involves PIP2 or PIP2 metabolism. However, the signal which mediates PDGF-induced Ca2+ entry does not require the heparin-sensitive IP3 receptor.     

Fusion of sphingomyelin vesicles induced by proteins from Taiwan cobra (Naja naja atra) venom. Interactions of zwitterionic phospholipids with cardiotoxin analogues

Egg sphingomyelin vesicles were used to assay aggregation/fusion activities of proteins from Taiwan (Naja naja atra) venom to avoid the problem of phospholipase A2 contamination during protein purification. It led to the identification of a new cardiotoxin (CTX) analogue protein (CTX V) with major aggregation/fusion, but few hemolysis, activities. On the contrary, cardiotoxin (CTX III) induced significant hemolysis of human red blood cells but exhibited few aggregation/fusion activities. To study the structure/activity relationship of these CTX-induced processes, the amino acid sequence of CTX V was determined and its aggregation/fusion activity was compared with that of CTX III by transmission electron microscopy, quasielastic laser light scattering, differential scanning calorimetry, and fluorescence spectroscopy. The results show that the CTX-induced fusion process at temperatures slightly above that of the gel to liquid-crystalline phase transition of sphingomyelin vesicles can ultimately convert small sonicated vesicles into large fused vesicles with sizes of 1-2 microns. The abilities of CTX V to induce the leakage of sphingomyelin vesicles content and to cause the fusion of vesicles are approximately 10-fold higher than those of CTX III. Based on the CTX structures determined in the present and other studies, it is suggested that the amino acid residue X within the well conserved sequence of -Cys-Pro-X-Gly-Lys-Gln-Leu-Cys- plays a role in the interaction of CTX with lipid molecules. The lipid phase transition could further enhance the protein-lipid interaction in the process leading to the fusion of vesicles.     

Activation of Fc gamma RII induces tyrosine phosphorylation of multiple proteins including Fc gamma RII.

Platelets provide a useful system for studying Fc gamma receptor-mediated signaling events because these cells express only a single class of Fc gamma receptors and because platelet aggregation and secretion can be activated through Fc gamma receptor stimulation. We report here that stimulation of platelets by cross-linking antibodies to Fc gamma RII or by treatment with an anti-CD9 monoclonal antibody, which acts through Fc gamma RII, causes an induction of tyrosine phosphorylation of multiple platelet proteins. Although the profile of tyrosine-phosphorylated proteins induced by stimulation of this Fc receptor was similar to that induced by thrombin, an additional 40-kDa phosphorylated protein was also detected. This protein co-migrated with Fc gamma RII and was immunoprecipitated with a monoclonal antibody to Fc gamma RII. In addition, after the cross-linking of Fc gamma RII in HEL cells or in COS-1 cells transfected with Fc gamma RII cDNA, the 40-kDa protein immunoprecipitated with anti-Fc gamma RII was also phosphorylated on tyrosine. These data strongly suggest that Fc gamma RII itself is a substrate for a tyrosine kinase(s) activated when Fc gamma RII is stimulated. Fc gamma RII was phosphorylated by the Src protein in vitro, suggesting that this kinase may be responsible for phosphorylation of Fc gamma RII in vivo. These studies establish that activation of platelets and human erythroleukemia cells through Fc gamma RII and CD9 involves an induction of tyrosine phosphorylation of multiple proteins including Fc gamma RII itself and suggest that these phosphorylation events may be involved in Fc gamma RII-mediated cell signaling.     

Central peptidergic mechanisms controlling reproductive hormone secretion: novel methodology reveals a role for the natriuretic peptides.

A variety of neural factors can influence reproductive hormone secretion by neuromodulatory actions within the hypothalamus or neuroendocrine actions within the anterior pituitary gland. Passive immunoneutralization and antagonist administration protocols have suggested physiological roles for a number of these factors; however, both experimental approaches have severe technical limitations. We have developed novel methodology utilizing cytotoxin cell targeting with neuropeptides linked to the toxic A chain of the plant cytotoxin ricin. With this methodology we can target and destroy in vivo or in vitro cells bearing receptors for that peptide. Ricin A chain conjugated to atrial natriuretic peptide (ANP), a neuropeptide known to pharmacologically inhibit luteinizing hormone-releasing hormone (LHRH) release, was injected into the cerebroventricular system of intact, cycling rats and ovariectomized rats. Cytotoxin conjugate treatment significantly lengthened the estrous cycle. In ovariectomized rats the luteinizing hormone surge induced by steroid priming was completely inhibited. LHRH content of the median eminences of these rats was not significantly altered. These data suggest that ANP binding to clearance receptors in the hypothalamus displaces the C-type natriuretic peptide (CNP) from the shared clearance receptor, making more CNP available to inhibit LHRH release. In the absence of cells bearing the clearance receptor all available CNP binds to the ANPR-B receptor and exerts its effect via an inhibitory interneuron, since LHRH fibers are spared by this treatment.