Mediators released from the granules of antigen-activated mast cells contribute to allergies, inflammation and diseases such as asthma. One of the major models used to study mucosal mast cells is the RBL-2H3 mast cell line. While there has been considerable research on the initial signalling events following IgE receptor (Fc??RI) cross-linking, the movement of granules to sites of exocytosis is poorly understood. Understanding the mechanisms that control granule movement to and fusion with the plasma membrane could provide novel targets for improved asthma and allergy therapeutics. To this end, an isolated intact population of granules from the RBL-2H3 mast cell provides a powerful research tool and as such the primary aim of this work was to isolate intact granules from the RBL-2H3 mast cell. Using iso-osmotic Percoll gradients we have isolated an intact granule population from RBL-2H3 mast cells. This granule population contained three granule markers: ??- hexosaminidase, serotonin and chymase. Triton X-100 pre-lysis resulted in loss of granule markers from this main peak, indicating that the isolated granules are in fact intact. Further analysis of the granule population showed that it is free from bulk contamination with other organelles and plasma membrane. The granules were estimated to have a density of 1.055 ??? 1.092g/mL, significantly less dense than that of rat peritoneal mast cell granules (1.2g/mL; [1]). Using an intact versus lysed approach, granule-associated proteins and phosphoproteins, from unactivated RBL-2H3 cells, were determined. Nine unknown granule-associated proteins were found using silver staining of gradient fractions separated on a SDSPAGE gel. In addition, four unknown serine or threonine granule-associated phosphoproteins were found. Molecular weight comparison suggested overlap in some of the unknown proteins and phosphoproteins. Probing for protein kinase C (PKC) isoforms confirmed previous results suggesting that a small population of PKC?? localised to the granules [2], and extended these results to include a population of PKC??I. The serine/threonine phosphatase PP1 does not appear to be granule associated. However, there was a small loss of PP2A from the granules (upon lysis), suggesting that perhaps a subpopulation of PP2A is granule-associated. The main granule peak represents a secretion competent population as Fc??RI-mediated activation of the cells resulted in a significant loss of granule markers from this peak. At the peak rate of antigen-induced secretion a number of changes occur in the phosphorylation of granule-associated phosphoproteins. In addition to an increase in the phosphorylation of three of the phosphoproteins seen in resting mast cell granules, eight new proteins were seen. Whether these proteins are granule-associated is currently unknown. PKC?? was found to translocate away from the granules at the peak rate of secretion, perhaps representing an important control mechanism in granule exocytosis. None of the tested PKC isoforms were found to translocate to the granules, providing little clue as to the identity of the kinase that may be involved in these phosphorylation events. However, as PKC??I is granule-associated and does not translocate off the granules, it would suggest that this kinase might be important for some of the observed phosphorylations. Overall the studies in this thesis show for the first time a rapid gradient-based method for the isolation of intact granules from unactivated and activated RBL-2H3 mast cells. These granules were used to determine granule-associated proteins and phosphoproteins, as well as to investigate changes that occur during the secretory process. In addition, the results show that a number of proteins have increased serine/threonine phosphorylation at the peak rate of antigen-stimulated secretion. This implies that phosphorylation is likely to play a role in the control of granule exocytosis. The identity of these proteins deserves further investigation. Thus, isolated intact RBL- 2H3 mast cell granules provide a powerful research tool to further investigate the mechanism and control of granule exocytosis.
| Identifer | oai:union.ndltd.org:ADTP/232642 |
| Date | January 2005 |
| Creators | Kranenburg, Tanya Ann, School of Medicine, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Medicine |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Tanya Ann Kranenburg, http://unsworks.unsw.edu.au/copyright |
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