Inositol pyrophosphate synthesis by inositol hexakisphosphate kinase 1 is required for homologous recombination repair.

Inositol pyrophosphates, such as diphosphoinositol pentakisphosphate (IP(7)), are water-soluble inositol phosphates that contain high energy diphosphate moieties on the inositol ring. Inositol hexakisphosphate kinase 1 (IP6K1) participates in inositol pyrophosphate synthesis, converting inositol hexakisphosphate (IP(6)) to IP(7). In the present study, we show that mouse embryonic fibroblasts (MEFs) lacking IP6K1 exhibit impaired DNA damage repair via homologous recombination (HR). IP6K1 knock-out MEFs show decreased viability and reduced recovery after induction of DNA damage by the replication stress inducer, hydroxyurea, or the radiomimetic antibiotic, neocarzinostatin. Cells lacking IP6K1 arrest after genotoxic stress, and markers associated with DNA repair are recruited to DNA damage sites, indicating that HR repair is initiated in these cells. However, repair does not proceed to completion because these markers persist as nuclear foci long after drug removal. A fraction of IP6K1-deficient MEFs continues to proliferate despite the persistence of DNA damage, rendering the cells more susceptible to chromosomal aberrations. Expression of catalytically active but not inactive IP6K1 can restore the repair process in knock-out MEFs, implying that inositol pyrophosphates are required for HR-mediated repair. Our study therefore highlights inositol pyrophosphates as novel small molecule regulators of HR signaling in mammals.

Mechanosensing through talin 1 contributes to tissue mechanical homeostasis.

It is widely believed that tissue mechanical properties, determined mainly by the extracellular matrix (ECM), are actively maintained. However, despite its broad importance to biology and medicine, tissue mechanical homeostasis is poorly understood. To explore this hypothesis, we developed mutations in the mechanosensitive protein talin1 that alter cellular sensing of ECM stiffness. Mutation of a novel mechanosensitive site between talin1 rod domain helix bundles 1 and 2 (R1 and R2) shifted cellular stiffness sensing curves, enabling cells to spread and exert tension on compliant substrates. Opening of the R1-R2 interface promotes binding of the ARP2/3 complex subunit ARPC5L, which mediates the altered stiffness sensing. Ascending aortas from mice bearing these mutations show increased compliance, less fibrillar collagen, and rupture at lower pressure. Together, these results demonstrate that cellular stiffness sensing regulates ECM mechanical properties. These data thus directly support the mechanical homeostasis hypothesis and identify a novel mechanosensitive interaction within talin that contributes to this mechanism.