Cellular effects of phosphoinositide derivatives on the actin cytoskeleton.

Filippi, Beatrice Maria (2006). Cellular effects of phosphoinositide derivatives on the actin cytoskeleton. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f666

Abstract

The glycerophosphoinositols are water-soluble, biologically active phospholipase A2 metabolites that are derived from phosphoinositides. The cellular concentrations of the glycerophosphoinositols have been shown to increase in hormone- and Ras-activated cells. Glycerophosphoinositol 4-phosphate (GroPIns4P) is the most active and well studied of the glycerophosphoinositols. When added exogenously, GroPIns4P can enter cells and affect various cellular functions, such as adenylyl cyclase (AC) activity (inhibition) and actin cytoskeleton organization (stimulation of ruffle and stress fibre formation). GroPIns4P acts at the level of the Racl and RhoA GTPases of the Rho family to form ruffle and stress fibres, and in addition, it increases the cytosolic fraction of GTP-bound Racl and favours its translocation to the plasma membrane.
In the present study, the mechanisms of action of the glycerophosphoinositols, with focus on GroPIns4P, have been characterized, with two potential mechanisms of action being revealed. On the one hand, GroPIns4P initiates a signal cascade that start from Src activation and leads to a phospholipase Cy-dependent increase in intracellular Ca²⁺ and the consequent activation of Ca²⁺/calmodulin kinase II (CaMKII); this kinase can, in turn, activate TIAM1, a specific exchange factor of Racl that is involved in Racl-dependent ruffle formation. On the other hand, GroPIns4P increases the binding between GDP-bound Racl and TIAM1 in pull-down assays and interacts with a protein complex that co-immunoprecipitates with TLAM1, and which also contains CaMKII. These data suggest that besides the signalling pathway initiate by Src, GroPIns4P can favour the formation of a large protein complex that could provide the machinery important for ruffle formation.
The other activities of GroPIns4P, such as AC inhibition and stress fibre formation, are not modulated by the same pathway as ruffle formation, thus suggesting that GroPIns4P is a multifunctional compound that is able to modulate different cellular functions by acting through multiple targets.