Reversible internalization of cytoplasm into the nucleus in senescent cells

Cellular senescence is a heterogeneous phenotype, driven by diverse effector programs, which result in irreversible growth arrest, DNA damage, production of complex secretome and metabolic reprogramming1,2. Here we show that in liver-specific Setd8-KO mice, after mitogen treatment, a significant number of hepatocytes become senescent, have enlarged nuclei and increased nuclear envelope plasticity. Despite cell cycle arrest, senescent cells in Setd8-deficient mice retain inherent capability to replicate DNA, resulting in endoreduplication-driven chromosomal hyperploidy. Remarkably, the enlarged nuclei had multiple nuclear engulfments progressing to the generation of large intranuclear vesicles surrounded by nuclear lamina. These vesicles had externally-positioned membrane-associated chromatin, contained glycogen, various cytoplasmic proteins, even whole organelles like ER or mitochondria. We term this process “endonucleosis”. Intranuclear vesicles were absent in hepatocytes of Setd8/Atg5 double knockout mice, suggesting that the process requires the canonical autophagy machinery. Endonucleosis and hyperploidization are temporary, early features of senescence. Larger vesicles were split into microvesicles over time and were eventually eliminated, while large nuclei were segregated into smaller ones without mitosis. The results reveal a unique senescence phenotype with several unusual features, such as hyperploidy, endonucleosis and nuclear segregation, all of which are anticipated to function as parts of survival mechanisms to escape necrotic death. RNA-Seq, CUT&Tag, ATAC-Seq, ChIP-Seq