Phosphatidylinositol transfer proteins and protein kinase C make separate but non-interacting contributions to the phosphorylation state necessary for secretory competence in rat mast cells.

Mast cells permeabilized by streptolysin O undergo exocytosis when stimulated with Ca(2+) and guanosine 5'-[gamma-thio]triphosphate but become progressively refractory to this stimulus if it is delayed. This run-down of responsiveness occurs over a period of 20-30 min, during which the cells leak soluble and tethered proteins. We show here that withdrawal of ATP during the process of run-down is strongly inhibitory but that as little as 25 microM ATP can extend responsiveness significantly; this effect is maximal at 50 microM. When phosphatidylinositol transfer proteins (PITPs) are provided to cells at the time of permeabilization, run-down is retarded. We conclude that in the presence of ATP they convey substrates for phosphorylation that are essential for exocytosis and thus interact with the regulatory machinery. Furthermore, we show that PITPalpha and PITPbeta have additive effects in this mechanism, suggesting that they are not functionally redundant. Alternatively, secretion from run-down cells can be inhibited by the aminoglycoside antibiotic neomycin, which is understood to bind to phosphoinositide headgroups, and by a PH (pleckstrin homology) domain polypeptide that binds phosphoinositides. The apparent displacement of neomycin by exogenous PITPs suggests that these proteins screen essential lipids. Secretion from run-down cells is also inhibited by 1-O-hexadecyl-2-O-methyl-rac-glycerol (AMG-C(16)), an inhibitor of protein kinase C. The lack of synergy between neomycin and AMG-C(16) suggests that protein kinase C independently provides a second essential component through protein phosphorylation and that there are two independent phosphorylation pathways necessary for secretion competence.

Thirty years of stimulus-secretion coupling: from Ca(2+) toGTP in the regulation of exocytosis.

Calcium, initially considered as the universal link between receptor stimulation and the onset of exocytosis in secretory cells, is now recognised as only one of a number of intracellular activators. In cells of haematopoietic origin (including mast cells), the key activator is one or more GTPases. Cells of this class, stimulated with GTPgammaS can undergo exocytosis in the effective absence of Ca(2+). A number of GTP-binding proteins that mediate exocytosis (G(E)) have been proposed but the best evidence supports roles for members of the Rho family of monomeric GTPases and for betagamma-subunits derived from G(i3). While preactivated Rac and Cdc42 can induce secretion from permeabilised mast cells in the absence of a guanine nucleotide betagamma-subunits only act to enhance the secretion induced by other GTP-binding proteins (likely to be members of the Rho family of monomeric GTPases). Further work is required to identify downstream effectors activated by these GTP-binding proteins and to show how they interact with the SNAP and SNARE isoforms known to be present in these cells.

Regulation of exocytosis from rat peritoneal mast cells by G protein beta gamma-subunits.

We applied G protein-derived beta gamma-subunits to permeabilized mast cells to test their ability to regulate exocytotic secretion. Mast cells permeabilized with streptolysin-O leak soluble (cytosol) proteins over a period of 5 min and become refractory to stimulation by Ca2+ and GTPgammaS over approximately 20-30 min. beta gamma-Subunits applied to the permeabilized cells retard this loss of sensitivity to stimulation (run-down) and it can be inferred that they interact with the regulatory mechanism for secretion. While alpha-subunits are without effect, beta gamma-subunits at concentrations >10(-8 )M enhance the secretion due to Ca2+ and GTPgammaS. Unlike the small GTPases Rac and Cdc42, beta gamma-subunits cannot induce secretion in the absence of an activating guanine nucleotide, and thus further GTP-binding proteins (likely to be Rho-related GTPases) must be involved. The enhancement due to beta gamma-subunits is mediated largely through interaction with pleckstrin homology (PH) domains. It remains manifest in the face of maximum activation by PMA and inhibition of PKC with the pseudosubstrate inhibitory peptide. Soluble peptides mimicking PH domains inhibit the secretion due to GTPgammaS and block the enhancement due to beta gamma-subunits. Our data suggest that beta gamma-subunits are components of the pathway of activation of secretion due to receptor-mimetic ligands such as mastoparan and compound 48/80.

Ontogeny of epinephrine, norepinephrine, dopamine-beta-hydroxylase, and chromogranin A in the adrenal gland of pigs.

OBJECTIVE: To obtain data on the ontogeny of catecholamines and other chromaffin vesicle components, which could serve as a basis for the study of their role during fetal life in normal and pathologic conditions. DESIGN: Epinephrine, norepinephrine, dopamine-beta-hydroxylase, and chromogranin A contents were measured in the porcine adrenal gland during various stages of gestation. ANIMALS: 934 porcine fetuses representing 22 gestational ages between 43 and 108 days. PROCEDURE: Total homogenates of adrenal glands were extracted and contents of different neurochemical markers were measured, using high-performance liquid chromatography, immunoassays, and western blotting. Immunohistochemical studies also were performed. RESULTS: Epinephrine and norepinephrine contents as a function of gestational age can be represented by a sigmoidal curve. Norepinephrine content rises early in gestation, whereas epinephrine content increases later. Maximal increase was significantly higher for epinephrine content. A progressive appearance of separate epinephrine- and norepinephrine-storing cells was documented. Dopamine-beta-hydroxylase content as a function of gestational age can be adequately represented by a parabolic curve. No quantitative changes in chromogranin A concentration were observed, but western blotting revealed qualitative changes with progressing gestational age. CONCLUSIONS: Important changes occur in catecholamine formation around day 60 of gestation. The sharp increase in epinephrine/norepinephrine contents and the appearance of separate epinephrine- and norepinephrine-storing cells may be related to the progressive splanchnic innervation of the adrenal gland. The presence of chromogranin A early in gestation may indicate its necessity for catecholamine storage.