Nuclear actin and DNA replication stress regulate the recruitment of human telomerase to telomeres

The recruitment of telomerase to telomeres is a tightly regulated process which is stimulated by replication stress and mediated by the DNA damage response regulatory kinase ATR. Here, we demonstrate that nuclear filamentous actin is important for telomerase recruitment under endogenous and replication stress conditions in immortal human cells. Inhibition of nuclear actin polymerization decreases the presence of telomerase at telomeres. This process is regulated by both ATR and mTOR kinases, and employs other regulators of actin structure and function, such as WASP, ARP2/3 and myosin. Nuclear filamentous actin serves as a site for telomerase recruitment, which is mediated by telomere tethering on actin fibres in response to replication stress, allowing telomerase to localize to telomeres containing stalled replication forks. Overall, these data demonstrate that, in human cells which express telomerase, telomeric replication stress triggers the recruitment of telomerase to telomeres via a nuclear actin network, enabling telomere length maintenance.