Impediments that that slow the rate of DNA replication are collectively referred to as “replication stress”. Replication stress is the main driver of genome instability in early cancer development and is recognized as a hallmark of cancer. Actin is a cytoskeletal protein that forms filaments to provide cells with mechanical support and driving force for movement. While actin is traditionally considered a cytoplasmic protein, recent evidence indicates actin polymerization can also occur inside the nucleus. However, the role for nuclear actin fibres, the mechanism(s) triggering their polymerization, and the impact of nuclear actin on the genome remains unclear.
Using live-cell and super-resolution imaging, chromatin fibre analysis, biochemistry, cell and molecular biology, we discovered that actin polymerization plays a prominent role in the replication stress response. Consistent with induced DNA replication stress, pharmacological inhibition of actin polymerization in human cells resulted in S-phase elongation, reduced DNA replication rate, shortened distance between replication origins, and increased occurrence of micronuclei and anaphase abnormalities. Pharmacological replication stress induced ATR and mTOR-dependent nuclear actin polymerization, which altered the nuclear architecture through the expansion of nuclear volume and redistribution of replication forks to the nuclear periphery. Inhibiting ATR, mTOR or actin polymerization, suppressed replication stress-dependent nuclear alteration and prevented the restart of stalled replication forks. Preliminary data indicate co-localization of stalled replication forks with certain repair factors is compartmentalized to the nuclear periphery, suggesting that actin polymerization facilitates movement of stalled forks to the nuclear periphery for repair.
Cumulatively, these data reveal a novel pathway where actin dependent forces shape the nucleus in response to replication stress to maintain genome health. This findings suggest nuclear actin polymerization may have additional nuclear roles that that impact genome function