Accumulation of synaptosomal-associated protein of 25 kDa (SNAP-25) and other proteins associated with the secretory pathway in GH4C1 cells upon treatment with estradiol, insulin, and epidermal growth factor

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

Synapsin associates with cyclophilin B in an ATP- and cyclosporin A-dependent manner

Immunophilins are ubiquitous enzymes responsible for proline isomerisation during protein synthesis and for the chaperoning of several membrane proteins. These activities can be blocked by the immunosuppressants cyclosporin A, FK506 and rapamycin. It has been shown that all three immunosuppressants have neurotrophic activity and can modulate neurotransmitter release, but the molecular basis of these effects is currently unknown. Here, we show that synapsin I, a synaptic vesicle-associated protein, can be purified from Torpedo cholinergic synaptosomes through its affinity to cyclophilin B, an immunophilin that is particularly abundant in brain. The interaction is direct and conserved in mammals, and shows a dissociation constant of about 0.5 microM in vitro. The binding between the two proteins can be disrupted by cyclosporin A and inhibited by physiological concentrations of ATP. Furthermore, cyclophilin B co-localizes with synapsin I in rat synaptic vesicle fractions and its levels in synaptic vesicle-containing fractions are decreased in synapsin knockout mice. These results suggest that immunophilins are involved in the complex protein networks operating at the presynaptic level and implicate the interaction between cyclophilin B and synapsins in presynaptic function.

Distinct protein kinases regulate SNAP-25 expression in chromaffin cells

The contribution of distinct Ca(2+)-sensitive protein kinases to the regulation of the expression of the synaptosomal-associated protein SNAP-25 was examined in bovine chromaffin cells. Prolonged incubation with high K(+) (38 mM) or 1,1-dimethyl-4-phenyl-piperazinium (DMPP), a nicotinic receptor agonist, significantly increased SNAP-25 protein and mRNA expression, as assessed by immunoblotting and semi-quantitative RT-PCR analysis. Both stimuli preferentially enhanced mRNA coding for the SNAP-25a isoform. Increase of SNAP-25 expression induced by K(+) or DMPP was inhibited over 70% by KN-62 and KN-93, two Ca(2+)/calmodulin-dependent protein kinase (CaMK) inhibitors, whereas the inactive analogue KN-92 only reduced the expression by 34%. The three compounds also inhibited the high K(+)-elicited [Ca(2+)](i) signal by 40%, suggesting that the effect of KN-62 and KN-93 was a combination of CaMK/ Ca(2+) influx inhibitory actions. Incubation of the cells with mitogen-activated protein kinase (MAPK) inhibitors PD98059 and U0126 reduced protein expression elicited by high K(+) by 50%, but did not modify the response to DMPP. Interestingly, although protein kinase A (PKA) inhibition by H-89 did not affect the high K(+) or DMPP-induced SNAP-25 expression, basal protein levels were significantly modified upon activation or inhibition of this pathway. Basal expression of SNAP-25 was also modified by the protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate, but not by Gö6976, a PKC-alpha inhibitor, suggesting that the Ca(2+)-insensitive PKC-epsilon isoform control basal expression of SNAP-25 in these cells. Taken together, these results provide the first evidence that diverse protein kinases might converge in the induction of SNAP-25 expression in chromaffin cells. The preferential contribution of one or another kinase would depend on the physiological or experimental conditions.

Effect of hypothyroidism on synaptosomal-associated protein of 25 kDa and syntaxin-1 expression in adenohypophyses of rat

The aim of this study was to analyze the expression of SNAP-25 and syntaxin-1 in the adenohypophyses of hypothyroid rats. Rats were divided into: 1) controls; 2) thyroidectomized 40 days; and 3) thyroidectomized 40 days with replacement of T4 20 days after surgery. Adenohypophyses were studied by immunohistochemistry and immunoblot analysis using antibodies against SNAP-25, syntaxin-1 and TSH. By immunostaining, SNAP-25 and syntaxin-1 were conspicuous and were localized around cytoplasmic vacuoles of thyroidectomy cells. Immunoblot analysis shows that thyroidectomy increases adenohypophysial SNAP-25 expression and decreases syntaxin-1 levels. T4 administration for 20 days produces a recovery similar to control values. In conclusion, thyroidectomy produces changes in both expression and immunoreactivity of SNAP-25 and syntaxin-1 in adenohypophyses of rats and these effects can be reversed by T4 administration.

Acquisition of Lubrol insolubility, a common step for growth hormone and prolactin in the secretory pathway of neuroendocrine cells

Rat prolactin in the dense cores of secretory granules of the pituitary gland is a Lubrol-insoluble aggregate. In GH(4)C(1) cells, newly synthesized rat prolactin and growth hormone were soluble, but after 30 min about 40% converted to a Lubrol-insoluble form. Transport from the endoplasmic reticulum is necessary for conversion to Lubrol insolubility, since incubating cells with brefeldin A or at 15 degrees C reduced formation of insoluble rat (35)S-prolactin. Formation of Lubrol-insoluble aggregates has protein and cell specificity; newly synthesized human growth hormone expressed in AtT20 cells underwent a 40% conversion to Lubrol insolubility with time, but albumin did not, and human growth hormone expressed in COS cells underwent less than 10% conversion to Lubrol insolubility. del32-46 growth hormone, a naturally occurring form of growth hormone, and P89L growth hormone underwent conversion, although they were secreted more slowly, indicating that there is some tolerance in structural requirements for aggregation. An intracellular compartment with an acidic pH is not necessary for conversion to Lubrol insolubility, because incubation with chloroquine or bafilomycin slowed, but did not prevent, the conversion. GH(4)C(1) cells treated with estradiol, insulin, and epidermal growth factor accumulate more secretory granules and store more prolactin, but not more growth hormone, than untreated cells; Lubrol-insoluble aggregates of prolactin and growth hormone formed to the same extent in hormone-treated or untreated GH(4)C(1) cells, but prolactin was retained longer in hormone-treated cells. These findings indicate that aggregation alone is not sufficient to cause retention of secretory granule proteins, and there is an additional selective process.