DNA density is a better indicator of a nuclear bleb than lamin B loss

Lamin B loss in nuclear blebs is rupture dependent while increased DNA damage is rupture independent

The nucleus houses genetic information and functions separate from the rest of the cell. Loss of nuclear shape results in nuclear ruptures. Nuclear blebs are deformations identified by decreased DNA density, while lamin B levels vary drastically. To determine if decreased lamin B levels are due to nuclear rupture, we used immunofluorescence to measure levels of lamin B and emerin, a nuclear envelope protein that enriches to sites of nuclear rupture. We observed that cell types that exhibit decreased levels of lamin B also show an enrichment of emerin in nuclear blebs. Oppositely, in other cell types, nuclear blebs display maintained levels of lamin B1 and showed no emerin enrichment. To determine how nuclear rupture affects DNA damage, we time lapse imaged nuclear rupture dynamics then fixed the same cells to conduct immunofluorescence of γH2AX and emerin. We find that DNA damage levels are higher in blebbed nuclei independent of nuclear rupture. Thus, we confirm that lamin B1 loss in nuclear blebs is due to nuclear rupture and blebbed nuclei have increased DNA damage that is independent of rupture.

Nuclear blebs are associated with destabilized chromatin packing domains

Disrupted nuclear shape is associated with multiple pathological processes including premature aging disorders, cancer-relevant chromosomal rearrangements, and DNA damage. Nuclear blebs (i.e., herniations of the nuclear envelope) have been induced by (1) nuclear compression, (2) nuclear migration (e.g., cancer metastasis), (3) actin contraction, (4) lamin mutation or depletion, and (5) heterochromatin enzyme inhibition. Recent work has shown that chromatin transformation is a hallmark of bleb formation, but the transformation of higher-order structures in blebs is not well understood. As higher-order chromatin has been shown to assemble into nanoscopic packing domains, we investigated if (1) packing domain organization is altered within nuclear blebs and (2) if alteration in packing domain structure contributed to bleb formation. Using Dual-Partial Wave Spectroscopic microscopy, we show that chromatin packing domains within blebs are transformed both by B-type lamin depletion and the inhibition of heterochromatin enzymes compared to the nuclear body. Pairing these results with single-molecule localization microscopy of constitutive heterochromatin, we show fragmentation of nanoscopic heterochromatin domains within bleb domains. Overall, these findings indicate that translocation into blebs results in a fragmented higher-order chromatin structure.

SUMMARY STATEMENT

Nuclear blebs are linked to various pathologies, including cancer and premature aging disorders. We investigate alterations in higher-order chromatin structure within blebs, revealing fragmentation of nanoscopic heterochromatin domains.