Chromogranins A and B as regulators of vesicle cargo and exocytosis

Abstract

Chromogranins (Cgs) are acidic proteins that have been implicated in several physiological processes such as vesicle sorting, the production of bioactive peptides and the accumulation of soluble species inside large dense core vesicles (LDCV). They constitute the main protein component in the vesicular matrix of LDCV. This latter characteristic of Cgs accounts for the ability of vesicles to concentrate catecholamines and Ca(2+). It is likely that Cgs are behind the delay in the neurotransmitter exit towards the extracellular milieu after vesicle fusion, due to their low affinity and high capacity to bind solutes present inside LDCV. The recent availability of mouse strains lacking Cgs, combined with the arrival of several techniques for the direct monitoring of exocytosis, have helped to expand our knowledge about the mechanisms used by granins to concentrate catecholamines and Ca(2+) in LDCV, and how they affect the kinetics of exocytosis. We will discuss the roles of Cgs A and B in maintaining the intravesicular environment of secretory vesicles and in exocytosis, bringing together the most recent findings from adrenal chromaffin cells.

Fig. 1

The lack of CgA promotes the overexpression of CgB and vice versa. a Western-blots of the adrenal medulla confirming the lack in CgA and the overexpression of CgB. b Quantification of CgB in the WT and CgA-KO as the average from 3 wells containing 4 different medullas (means ± SEM). c As in a but showing the overexpression of CgA in CgB-KO mouse. d Quantification of CgA in the WT and CgB-KO as the average from 3 wells containing 4 different medullas (means ± SEM). The gels’ densities are expressed in arbitrary units (a.u.), α-tubulin was used as an internal control. * P < 0.05; ** P < 0.01 Mann–Whitney test. Modified from Montesinos et al. (2008) and Diaz-Vera et al. (2010)

Fig. 2

Secretory characteristics of the Cgs-KO chromaffin cells. Data from amperometry experiments carried out in CgA-KO- (a–c), and from CgB-KO-cells (d–e). Secretory spikes were counted in the first 2 min after BaCl₂ stimulation (a, d). Total secretion was measured by integration of these spikes (b, e). The charges of individual spikes from CgA-KO and CgB-KO cells compared with their own controls (WT) are shown in panels (c) and (f), values expressed in pCoulombs. Means ± SEM, the number of cells analyzed are shown in brackets * P < 0.05; ** P < 0.01 Mann–Whitney test. Modified from Montesinos et al. (2008) and Diaz-Vera et al. (2010)

Fig. 3

Kinetic profiles of amperometric spikes from CgA- and CgB-KO chromaffin cells. Traces illustrate the kinetic changes observed in exocytosis from the cells lacking CgA or CgB. The spikes were constructed by averaging spikes from WT, CgA-KO and CgB-KO and normalized to the I _max (100%) of their own control cells. Discontinuous lines show the ascending slopes obtained by the linear fit of the 25–75% segment of the ascending portion of the spikes. The table shows the changes of amperometric parameters normalized to their own control cells from WT animals. * P < 0.05; ** P < 0.01 Mann–Whitney Modified from Diaz-Vera et al. (2010)