Contractile proteins in chromaffin cells

Actin filament disassembly is a sufficient final trigger for exocytosis in nonexcitable cells

Although the actin cytoskeleton has been implicated in vesicle trafficking, docking and fusion, its site of action and relation to the Ca(2+)-mediated activation of the docking and fusion machinery have not been elucidated. In this study, we examined the role of actin filaments in regulated exocytosis by introducing highly specific actin monomer-binding proteins, the beta-thymosins or a gelsolin fragment, into streptolysin O-permeabilized pancreatic acinar cells. These proteins had stimulatory and inhibitory effects. Low concentrations elicited rapid and robust exocytosis with a profile comparable to the initial phase of regulated exocytosis, but without raising [Ca2+], and even when [Ca2+] was clamped at low levels by EGTA. No additional cofactors were required. Direct visualization and quantitation of actin filaments showed that beta-thymosin, like agonists, induced actin depolymerization at the apical membrane where exocytosis occurs. Blocking actin depolymerization by phalloidin or neutralizing beta-thymosin by complexing with exogenous actin prevented exocytosis. These findings show that the cortical actin network acts as a dominant negative clamp which blocks constitutive exocytosis. In addition, actin filaments also have a positive role. High concentrations of the actin depolymerizing proteins inhibited all phases of exocytosis. The inhibition overrides stimulation by agonists and all downstream effectors tested, suggesting that exocytosis cannot occur without a minimal actin cytoskeletal structure.

Actin involvement in exocytosis from PC12 cells: studies on the influence of botulinum C2 toxin on stimulated noradrenaline release

Botulinum C2 toxin is known to ADP-ribosylate actin. The toxin effect was studied on [3H]noradrenaline secretion of PC12 cells. [3H]Noradrenaline release was stimulated five- to 15-fold by carbachol (100 microM) or K+ (50 mM) and 10-30-fold by the ionophore A23187 (5 microM). Pretreatment of PC12 cells with botulinum C2 toxin for 4-8 h at 20 degrees C, increased carbachol-, K+-, and A23187-induced, but not basal, [3H]noradrenaline release maximally 1.5-to three-fold, whereas approximately 75% of the cellular actin pool was ADP-ribosylated. Treatment of PC12 cells with botulinum C2 toxin for up to 1 h at 37 degrees C also increased stimulated [3H]noradrenaline secretion, whereas toxin treatment for greater than 1 h decreased the enhanced [3H]noradrenaline release stimulated by carbachol and K+ but not by A23187. Concomitantly with toxin-induced stimulation of secretion, 20-50% of the cellular actin was ADP-ribosylated, whereas greater than 60% of actin was modified when exocytosis was attenuated. The data indicate that ADP-ribosylation of actin by botulinum C2 toxin largely modulates stimulation of [3H]noradrenaline release. Moreover, the biphasic toxin effects suggest that distinct mechanisms are involved in the role of actin in secretion.

Effect of cytochalasin D on acinar cell structure and secretion in rat parotid salivary glands in vitro

Cytochalasin D, a microfilament disrupting agent, considerably inhibited isoproterenol-stimulated amylase release from enzymatically dispersed parotid acini. Histologically cytochalasin D caused a loss of microvilli lining acinar lumina and luminal enlargement. Nearly empty vacuoles appeared near the luminal and lateral surface, and the membrane bordering on the vacuoles was often continuous with the plasma membrane. Therefore, the vacuolization probably resulted from an elongation of the membrane lining the lumen. Fluorescence staining with rhodamine-phalloidin showed that cytochalasin D caused disruption of microfilaments. When stimulating the cytochalasin D-treated cells with isoproterenol, the number of secretory granules in the cytoplasm diminished markedly and secretory material was observed in the vacuoles, indicating that inhibition of amylase release by cytochalasin D is not due to blocking of exocytosis but to the retention of amylase discharged into vacuoles. These findings suggest that microfilaments are essential in maintaining the parotid acinar structure but do not play a direct part in the movement of secretory granules and their fusion with the luminal membrane.

Phosphorylation and dephosphorylation of chromaffin cell proteins in response to stimulation

Stimulation of bovine chromaffin cell in culture changed (increased or decreased) the phosphorylation state of several proteins as examined by 32P incorporation. Enhanced phosphorylation of 22 protein bands as well as increased dephosphorylation of a 20.4 kilodaltons protein band was observed when extracts of cultured chromaffin cells stimulated by either acetylcholine or high K+ were subjected to mono-dimensional gel electrophoresis. For several protein bands, the degree of phosphorylation was larger in cells stimulated by acetylcholine than in those challenged by a depolarizing concentration of K+. The most affected phosphoproteins have apparent molecular weights of 14,800, 29,000, 33,000, 57,000 (tubulin subunit), 63,000 (tyrosine hydroxylase subunit) and 94,000. The presence of a low extracellular calcium concentration (0.5 mM Ca2+ plus 15 mM Mg2+) in the incubation medium inhibited (38-100%) the acetylcholine-evoked increases in protein phosphorylation observed previously for 18 protein bands. Trifluoperazine at the concentration required for 50% inhibition of acetylcholine-induced catecholamine release decreases (33-100%) the stimulation-induced phosphorylation in all polypeptides, with the exception of the 14.8 kilodaltons and the dephosphorylated 20.4 kilodaltons components which were not affected. Two-dimensional gel electrophoresis analysis revealed that exposure of chromaffin cells to acetylcholine produced two types of effect on protein phosphorylation: activation of protein kinase activities affecting about 30 polypeptides; activation of protein phosphatase activities resulting in the dephosphorylation of about 40 polypeptides, most of them appearing as minor phosphoproteins, with the exception of the alpha-subunit of pyruvate dehydrogenase and the 20.4 kilodaltons polypeptide. On the basis of their molecular properties (molecular weight and pI) and their abundance in chromaffin cells, the 80 kilodaltons phosphoprotein which focused at pI 4.8 and the 117.5 kilodaltons phosphoprotein which focused at pI 5.0 were identified as chromogranins A and B, respectively. The relationship between acetylcholine-induced protein phosphorylation (or dephosphorylation) and catecholamine secretion was also investigated. The time course of protein phosphorylation (or dephosphorylation) paralleled or preceded [3H]noradrenaline release for 16 phosphoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of taxol on secretory cells: functional, morphological, and electrophysiological correlates

The effect of 0.5-1.0 microM taxol, a potent promoter of microtubule polymerization in vitro, was studied on the secretory activity of chromaffin cells of the adrenal medulla. Taxol was found to have a dual effect: the long-term effect (after a 1-h incubation) of taxol was to induce almost complete inhibition of catecholamine release, whereas after a short incubation (10 min) a massive, nicotine-independent release of catecholamine was produced. From results obtained using the patch-clamp technique to study the Ca++-dependent K+ channels (Ic channels), it was possible to conclude that taxol probably provokes an augmentation of free [Ca++]i in the cytoplasm, values increasing from 10(-8) M at rest to several 10(-7) M. The increased spontaneous release of stored neurohormones and the increased frequency of opening of Ic channels occur simultaneously and could both originate from a rise of [Ca++]i upon taxol addition. Immunofluorescence and ultrastructural studies showed that 13-h taxol treatment of chromaffin cells led to a different distribution of secretory organelles, and also to microtubule reorganization. In treated cells, microtubules were found to form bundles beneath the cell membrane and, at the ultrastructural level, to be packed along the cell axis. It is concluded that in addition to its action on microtubules, the antitumor drug taxol has side effects on the cell secretory activity, one of them being to modify free [Ca++]i.