Hyperosmotic stress induces <scp>PARP1</scp> ‐mediated <scp>HPF1</scp> ‐dependent mono( <scp>ADP</scp> ‐ribosyl)ation

While the downstream effectors of the hyperosmotic stress response are relatively well characterized, the primary molecular sensors responsible for initial stress detection remain poorly defined. In this study, we demonstrate that hyperosmotic stress triggers a rapid and transient mono(ADP-ribosyl)ation (MARylation). Beside MARylation, signs of acute genotoxicity are missing and CHK1 activation is observed only upon recovery from osmotic stress. Our data indicate that PARP1 catalyzes its own MARylation in an HPF1 co-factor dependent manner. Biochemical assays further demonstrate that the mono-ADP-ribose moiety is resistant to hydroxylamine treatment, which is a feature of HPF1-directed O-glycosidic bonds. Together, these findings support a model in which PARP1 acts as a sensor of chromatin structure changes induced by hyperosmotic stress leading to its autoMARylation.