Causes and Consequences of Nuclear Envelope Rupture and Repair

The nuclear envelope is frequently depicted as a static membrane barrier that separates the chromatin from the cytosol. However, recent work has shown that the nucleus is highly tolerant of membrane integrity loss, and that it may even be beneficial in some situations. Currently, changes in nuclear membrane stability have been identified in cells and organisms harboring mutations in nuclear lamin proteins that cause a group of human diseases called laminopathies, and in cancer cells. Our lab currently studies transient nuclear envelope rupture in cultured cancer cells and has developed tools to study this process using live-cell imaging and other types of light microscopy.

Fluorescent time-lapse video of nucleoplasm and chromatin during transient nuclear membrane rupture Emily M Hatch and Martin W Hetzer “Nuclear envelope rupture is induced by actin-based nucleus confinement.” JCB, 2016 Oct 10; 215 (1): 27.
Fluorescent time-lapse video of micronucleus formation and disruption.

Our lab also studies nuclear envelope rupture in micronuclei. Micronuclei form when chromosomes missegregate during mitosis and recruit an independent nuclear envelope. Micronuclei occur spontaneously at a low rate in healthy tissue and are more frequent in cancer cells. In contrast to the main nucleus, the majority of micronuclei undergo nuclear membrane rupture during interphase, and they almost always fail to repair the membrane, leaving the chromatin in the cytosol for the rest of interphase. This process, called nuclear envelope collapse, has been associated with DNA damage, chromothripsis (massive chromosome rearrangements frequently found in cancer), and activation of innate immune pathways that can stimulate inflammation and metastasis.

Our current model is that nuclear membrane rupture occurs when lesions form in the nuclear lamina that lead to areas of weak membrane that are prone to both chromatin herniation and membrane rupture when force is applied to the nucleus. However, the molecular mechanisms driving these events, and why the membrane repairs in the main nucleus but not micronuclei, are unclear. In addition, how nuclear membrane rupture causes DNA damage and innate immune pathway activation, as well as the extent that nuclear membrane repair protects from these consequences, is not well understood. Our lab is working on a number of projects to address these questions.

1) What controls nuclear membrane rupture and repair in cancer cells?

The major questions from our model are a) where do large lesions in the nuclear lamina come from, and b) what determines when the membrane ruptures. In addition, there are still many unknowns about how the nuclear envelope repairs after membrane rupture. Thus, a major project in the lab is to identify the factors that determine nuclear lamina structure and nuclear membrane stability.

2) Why are micronuclei so unstable?

Our previous work showed that defects in the nuclear lamina are apparent in micronuclei soon after mitosis, but that the forces that drive membrane disruption in primary nuclei are not acting on micronuclei. Thus, another aim of the lab is to understand what causes altered nuclear lamina assembly or maintenance in micronuclei and the reason for membrane disruption. Our goal is to both improve our understanding of nuclear envelope assembly in general and to identify new ways to prevent nuclear membrane rupture and lamina defects in cancer cells.

3) What are the consequences of losing nucleus compartmentalization at the cellular level and how does this contribute to human disease?

Although several consequences of persistent nuclear membrane rupture have been identified in micronuclei, it is likely that we have not yet identified all the problems caused by this process in either micronuclei or the main nucleus. Thus, we are developing new tools to identify changes in chromatin structure and function, and cellular pathways after nuclear membrane instability. The overall goal of this work is to define causal links between nuclear membrane rupture, misregulation of cellular functions, and disease phenotypes.

Nuclear membrane rupture causes mislocalization of cytosolic and nuclear contents
Nuclear membrane rupture causes mislocalization of cytosolic and nuclear contents. When the membrane repairs, this can sometimes leave objects as large as organelles in the wrong compartment. Membrane rupture occurs at lamina lesions in both the primary nucleus and micronuclei, which are often the site of chromatin herniation in primary nuclei. High-res version