Abstract
The actin cytoskeleton is a classic biomechanical mediator of cell migration. While it is known that actin also shuttles in and out of the nucleus, its functions within this compartment remain poorly understood. In this study, we investigated how nuclear actin regulates keratinocyte gene expression and cell behavior. Gene expression profiling of normal HaCaT keratinocytes compared to HaCaTs over-expressing wild-type β-actin or β-actin tagged with a nuclear localization sequence (NLS-actin), identified multiple adhesive and cytoskeletal genes, such as MYL9, ITGB1, and VCL, which were significantly down-regulated in keratinocytes with high levels of nuclear actin. In addition, genes associated with transcriptional regulation and apoptosis were up-regulated in cells over expressing NLS-actin. Functionally, accumulation of actin in the nucleus altered cytoskeletal and focal adhesion organization and inhibited cell motility. Exclusion of endogenous actin from the nucleus by knocking down Importin 9 reversed this phenotype and enhanced cell migration. Based on these findings, we conclude that the level of actin in the nucleus is a transcriptional regulator for tuning keratinocyte migration.
Highlights
The actin cytoskeleton is a classic biomechanical mediator of cell migration
Nuclear actin mediates several key transcriptional processes. It physically interacts with all three RNA polymerases and helps stabilize the pre-transcriptional complex[12,13]. It interacts with chromatin remodeling complexes, such INO80 and BAF14,15, and nuclear G-actin negatively regulates the transcriptional activity of serum response factor (SRF) by binding and inhibiting the co-factor megakaryoblastic leukemia 1 (MKL1)[16,17]
To gain insight into how nuclear actin regulates cell function, we generated HaCaT keratinocyte lines stably expressing wild type β-actin-GFP (WT-actin) or β-actin-GFP tagged with a nuclear localization sequence (NLS-actin), which forces actin to accumulate within the nucleus (Fig. 1a)
Summary
The actin cytoskeleton is a classic biomechanical mediator of cell migration. While it is known that actin shuttles in and out of the nucleus, its functions within this compartment remain poorly understood. We investigated how nuclear actin regulates keratinocyte gene expression and cell behavior. Accumulation of actin in the nucleus altered cytoskeletal and focal adhesion organization and inhibited cell motility. Nuclear actin mediates several key transcriptional processes It physically interacts with all three RNA polymerases and helps stabilize the pre-transcriptional complex[12,13]. Through mRNA expression profiling, we demonstrate that nuclear actin negatively regulates a core set of adhesive and cytoskeletal genes and inhibits cell motility. Together, these results provide new insights into the broad range of functions of nuclear actin and identify a novel transcriptional mechanism by which actin regulates cell migration and motility
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