Shaping of Nuclear Structure During Mitosis Impacts Stem Cell Survival

Nuclei of metazoan cells are diverse in shape and structure. Although most are spherical, others have an irregular shape that reflects their function. Nuclear structure depends upon proteins in the nuclear lamina (NL), a protein network comprised of lamins and hundreds of lamin associated proteins that line the inner nuclear membrane. NL proteins build contacts with the genome to regulate transcription, replication and DNA repair. In addition, these proteins transmit regulatory information between cellular compartments, by connecting the nucleoskeleton to the cytoskeleton. Prominent changes in nuclear morphology occur during physiological aging and in disease. Indeed, mutation of genes encoding NL proteins cause a remarkable spectrum of age-enhanced human diseases, known as laminopathies that include muscular dystrophies, lipodystrophies, bone disorders, and accelerated aging disorders. These diseases are linked to failures in maintenance of adult stem cell population, resulting in loss of tissue homeostasis. We use Drosophila melanogaster as a model to understand NL contributions to adult stem cell maintenance. Our studies capitalize on our finding that the NL LEM-domain (LEM-D) protein D-emerin/Otefin is required for survival of germline stem cells (GSCs), adult stem cells that reside in a well characterized natural stem cell niche. We have found that loss of D-emerin/Otefin or its binding partner Barrier-to-autointegration factor (BAF) results in activation of a novel checkpoint that promotes GSC loss. Activation of this NL checkpoint depends upon ATM- and Rad3-related (ATR) kinase and Checkpoint kinase 2 (Chk2), two kinases well known for their roles in the DNA damage response pathway. However, activation of the NL checkpoint is independent of canonical triggers of DNA damage and replication stress, instead correlating with alteration in nuclear structure. Based on a shared role for D-emerin/Otein and BAF in chromatin attachment to the NL during post-mitotic nuclear assembly, we have begun investigations of NL events during GSC mitosis. To begin these analyses, we examined the sequential events during wild type GSC mitosis. Surprisingly, we find that the NL and the nuclear envelope remain intact during all stages of mitosis. We find that the mitotic NL surrounds the mitotic spindle and embeds the centrosomes, suggesting that GSCs undergo a closed mitosis similar to yeast. Notably, d-emerin/otefin mutants are defective in GSC mitosis, evidenced by the disrupted spindle organization and centrosome structure. Indeed, in non-dividing d-emerin/otefin mutant GSCs, the centrosome is positioned at the site of extensive NL deformation found in interphase cells, providing a molecular basis for the observed altered NL structural. Our studies represent the first report of a “yeast-like” mitosis in a stem population. Further, our data suggest that defects in this atypical mitosis activate the NL checkpoint and lead to stem cell loss.