Nuclear Filamentous Actin functions in the replication stress response

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 polymerizes to filamentous form (F-actin) to provide cells with mechanical support and transport pathways. While F-actin is traditionally considered a cytoplasmic structure, recently it was identified also inside the nucleus.

Here we used advanced microscopy, bespoke image analysis tools, and molecular biology to identify that nuclear F-actin plays a prominent role in the replication stress response. Consistent with DNA replication stress, we identified that inhibition of actin polymerization resulted in S-phase elongation, reduced DNA replication rates, shortened distance between replication origins, and increased occurrence of anaphase abnormalities. We also found that inducing replication stress resulted in ATR, IPMK and mTOR-dependent nuclear F-actin formation. The resulting nuclear F-actin altered nuclear architecture by promoting nuclear sphericity and expanding the nuclear volume to counteract nuclear membrane deformation induced by replication stress. Additionally, nuclear F-actin promoted the mobility of stalled replication foci including the directed movement of stalled foci along actin fibres to the nuclear periphery. Inhibiting ATR, mTOR or actin polymerization, suppressed the F-actin dependent alteration of the nuclear architecture in response to replication stress and prevented the restart of stalled replication forks. Additionally, inhibiting actin polymerization slowed resolution of ATR signalling in response to replication stress.

Our data reveal a novel pathway regulated by ATR and mTOR, where F-actin dependent forces shape nuclear architecture in response to replication stress to maintain genome stability.