Price, Leo Sebastian; (1995) Secretion and the actin cytoskeleton in permeabilised mast cells. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The secretory responses of streptolysin-O-permeabilised peritoneal rat mast cells have been studied extensively. Here, a novel flow cytometry technique was developed which enabled the secretory responses of large numbers of cells to be analysed. This method was used to identify differentially responding cell subpopulations and to study the factors which influence the transition between the various secretory states. Rac and Rho, have previously been shown to control various aspects of cytoskeletal organisation in different cell types including mast cells. The effect of constitutively active mutant forms of these proteins on the secretory response was investigated. When added to permeabilised cells, both proteins enhanced secretion and this effect was achieved by increasing the proportion of secreting cells. Furthermore, the presence of the endogenous Rac and Rho proteins in mast cells and their role in the secretory response was also demonstrated. It has been proposed that the cortical actin cytoskeleton acts as a barrier, which must be overcome to allow fusion of secretory granules with the plasma membrane. Upon cell stimulation, the F-actin cortex disassembled. Disassembly occurred in both secreting and non-secreting cells, indicating that this was not sufficient for exocytosis to occur. In addition to its previously described cytoskeletal effects, Rho was shown here to promote F-actin disassembly describing a novel cytoskeletal effect of this protein. It has been shown that F-actin filaments that disassemble from the cortex upon stimulation with GTP-γ-S relocate to the cell interior, a process which is dependent on the small GTP-binding protein Rac. Here, F-actin relocalisation was shown to occur preferentially in secreting cells, demonstrating a correlation between regulation of the cytoskeleton and secretion. Taken together, the results presented here indicate that the regulation of the cytoskeleton and secretion is closely correlated. However, a degree of independence suggests that the control of these two processes subsequently diverges.

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