Signaling actin polymerization responses in hematopoietic cells.

The polymerization and depolymerization of the cellular protein actin are thought to provide the basis for the morphological changes that are necessary for cell motility. The actin polymerization response in stimulated hematopoietic cells can be divided into discrete phases: (1) The initial actin polymerization event and (2) The gradual depolymerization of F-actin back to basal levels. An additional intermediate phase has been characterized in neutrophils stimulated with leukotriene B$\sb4$ or platelet activating factor. This stimulation results in the production of a rapidly oscillating actin polymerization/depolymerization response immediately subsequent to the initial actin polymerization event. Protein kinase C was implicated in mediating these F-actin oscillations by the observation that inhibition of this enzyme resulted in a masking of this response. The motor actin binding protein myosin was subsequently implicated as a downstream mediator of the F-actin oscillations by the observation that disruption of intracellular actin-myosin resulted in a masking of this response. Additionally, stimulation of neutrophils with leukotriene B$\sb4$ resulted in the myosin light chain being serine phosphorylated in a protein kinase C-dependent manner. This phosphorylation was shown to occur in a manner that was kinetically distinct from the myosin phosphorylation induced by n-formyl-methionyl-leucyl-phenylalanine, a potent activator of actin polymerization that alone did not induce F-actin oscillations. These data suggest that protein kinase C and downstream phosphorylation of myosin by a protein kinase C-dependent pathway may play a role in mediating the production of neutrophil F-actin oscillations. Wiskott-Aldrich Syndrome Protein (WASP), the protein identified as being defective in the disease Wiskott-Aldrich Syndrome (WAS), has been hypothesized to play a role in signaling the initial actin polymerization event. In contrast, we found that stimulated actin polymerization is kinetically normal in the hematopoietic lineages affected in WAS. We also found that the actin cytoskeleton in WAS platelets is capable of producing the hallmark cytoarchitectural features associated with activation. Further analysis revealed increased caspase-3 activity in WAS lymphocytes. This increased activity resulted in accelerated apoptosis of these cells. These data indicate that WASP does not play a universal role in signaling actin polymerization, but plays a role in delaying cell death.Ph.D.BiochemistryBiological SciencesCellular biologyHealth and Environmental SciencesImmunologyPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131519/2/9909975.pd