NOVEL MECHANISMS IN THE REGULATION OF PERINUCLEAR ACTIN ASSEMBLY

Abstract

Proper organization and dynamics of actin cytoskeleton are critical for cells? functions and survival. Perinuclear actin contributes to maintaining of nuclear shape and cellular mechanical homeostasis, and integrating cell nucleus into the actin cytoskeleton architecture. Actin dynamics is regulated by a number of different factors in concert. It is widely accepted that extracellular physical signals can exert effects on actin dynamics and organization. Inside the cell, the formin protein family constitutes an important group of actin regulators. However, how these external and internal regulators control actin dynamics in the perinuclear area has not been sufficiently studied. Work in this thesis concentrates on understanding the regulation of perinuclear actin. First, we report that external mechanical force induced an immediate and transient perinuclear actin assembly. This involved actin polymerization, and disassembly. Intracellular Ca2+ bursts were found to be essential for the perinuclear actin response. Further, we showed that a potent actin polymerization factor, inverted formin 2 (INF2), was involved in the perinuclear actin remodeling. Specifically, INF2 co-localized with a transient actin structure in the perinuclear region. A reduction in the level of INF2 resulted in the attenuation of this actin remodeling process. These results reveal a novel mechanotransduction cell response: external mechanical stimulation induces a rapid transient perinuclear actin polymerization mediated by Ca2+ and formin INF2. Second, we showed that another formin, mDia2, localized to the cytoplasmic side of nuclear membrane. Further, quantitative measurement using fluorescence correlation spectroscopy revealed reduced motility of mDia2 in close proximity to the nuclear envelope compared to that in the bulk of cytoplasm. This means that mDia2 is trapped in perinuclear area by interactions with some associated proteins. By super-resolution imaging and immunoprecipitation assay, mDia2 was shown to co-localize and interact with the transport receptor importin b at the nuclear rim. These data suggest that mDia2 can be an additional factor participating in the assembly of perinuclear actin network. This thesis work has provided new findings and hypotheses on the regulation of perinuclear actin. It shows that the dynamics of perinuclear actin can be controlled by external mechanical factors and the molecular regulators from the formin protein family.