Regulated secretion

A report from the minisymposium on regulated secretion at the 39th Annual Meeting of the American Society for Cell Bilogy, Washington DC, December 11-15, 1999     

GRASPing unconventional secretion

GRASP proteins associate with the Golgi apparatus and have been implicated in the stacking of Golgi cisternae, vesicle tethering, and mitotic progression, but their specific functions are unclear. In this issue, Kinseth et al. (2007) show unexpectedly that a GRASP homolog is required for an unconventional secretory pathway that bypasses the usual route for Golgi-dependent membrane traffic.     

Dynamics of secretion

In this short review, kinetic aspects of exocytosis are discussed. A special emphasis is put on recent data that highlight dynamic differences between neurotransmission and other forms of secretion.     

Connexins and secretion

Connexin channels clustered at gap junctions are obligatory attributes of all macroscopic endocrine and exocrine glands investigated so far and also connect most types of cells which produce secretory products in other tissues. Increasing evidence indicates that connexins, and the cell-to-cell communications that these proteins permit, contribute to control the growth of secretory cells, their expression of specific genes and their differentiated function, including their characteristic ability to biosynthetize and release secretory products in a regulated manner. Since the previous reviews which have been published on this topic, several lines of evidence have been added in support of multiple regulatory roles of gland connexins. Here, we review this novel evidence, point to the many questions which are still open and discuss some interesting perspectives of the field.     

Bacterial secretion chaperones

Many Gram-negative bacteria are able to invade hosts by translocation of effectors directly into target cells in processes usually mediated by two very complex secretion systems (SSs), named type III (T3) and type IV (T4) SSs. These syringe-needle injection devices work with intervention of specialized secretion chaperones that, unlike traditional molecular chaperones, do not assist in protein folding and are not energized by ATP. Controversy still surrounds secretion chaperones primary role, but we can say that these chaperones act as: (i) bodyguards to prevent premature aggregation, or as (ii) pilots to direct substrate secretion through the correct secretion system. This family of chaperones does not share primary structure similarity but amazingly equal 3D folds. This mini review has the intent to present updated structural and functional data for several important secretion chaperones, either alone or in complex with their cognate substrates, as well to report on the common features and roles of T3, T4 and flagellar chaperones.     

Adrenalectomy increases beta-lipotropin secretion over beta-endorphin secretion from anterior pituitary corticotrophs

Previous studies have indicated that acute stress in vivo or ovine corticotropin releasing hormone (oCRH) in vitro, releases both beta-lipotropin (beta-LPH) and beta-endorphin (beta-END) from the anterior lobe, with beta-END predominating over beta-LPH by 2:1. However, repeated stress shifts this ratio to proportionately more beta-LPH released with re-stress or oCRH in vitro. Alternative hypotheses were that the glucocorticoids released during stress altered the processing of proopiomelanocortin (POMC) or that the increased biosynthetic drive resulted in an inability of the processing enzymes to keep pace with biosynthesis. To distinguish between these alternatives, adrenalectomy studies were performed. Following removal of glucocorticoid negative feedback there is greatly increased secretion of beta-END-IR from anterior lobe corticotrophs with a subsequent increase in biosynthetic drive. Under these conditions of increased biosynthetic drive in the absence of steroids, the corticotroph secretes primarily beta-LPH, suggesting that increased biosynthetic drive alters the posttranslational processing rate of POMC.