Bypassing Border Control: Nuclear Envelope Rupture in Disease.

Recent observations in laminopathy patient cells and cancer cells have revealed that the nuclear envelope (NE) can transiently rupture during interphase. NE rupture leads to an uncoordinated exchange of nuclear and cytoplasmic material, thereby deregulating cellular homeostasis. Moreover, concurrently inflicted DNA damage could prime rupture-prone cells for genome instability. Thus, NE rupture may represent a novel pathogenic mechanism that has far-reaching consequences for cell and organism physiology.

Statins and Histone Deacetylase Inhibitors Affect Lamin A/C - Histone Deacetylase 2 Interaction in Human Cells.

We recently identified lamin A/C as a docking molecule for human histone deacetylase 2 (HDAC2) and showed involvement of HDAC2-lamin A/C complexes in the DNA damage response. We further showed that lamin A/C-HDAC2 interaction is altered in Hutchinson-Gilford Progeria syndrome and other progeroid laminopathies. Here, we show that both inhibitors of lamin A maturation and small molecules inhibiting HDAC activity affect lamin A/C interaction with HDAC2. While statins, which inhibit prelamin A processing, reduce protein interaction, HDAC inhibitors strengthen protein binding. Moreover, treatment with HDAC inhibitors restored the enfeebled lamin A/C-HDAC2 interaction observed in HGPS cells. Based on these results, we propose that prelamin A levels as well as HDAC2 activation status might influence the extent of HDAC2 recruitment to the lamin A/C-containing platform and contribute to modulate HDAC2 activity. Our study links prelamin A processing to HDAC2 regulation and provides new insights into the effect of statins and histone deacetylase inhibitors on lamin A/C functionality in normal and progeroid cells.

Loss of Nuclear Envelope Integrity in Aging and Disease.

The nuclear envelope (NE) serves as a central organizing unit for the eukaryotic cell. By virtue of its highly selective, semipermeable barrier function, the NE shields the enclosed genetic material, while at the same time ensuring its regulated transcription, replication, and repair. The NE has long been considered to only dismantle during mitosis. However, in recent years it has become clear that in a variety of pathologies, NE integrity becomes compromised during interphase as well. Loss of NE integrity, or briefly NE stress, is manifested in various ways, ranging from a gradual reduction in nucleocytoplasmic transport function, to selective loss and degradation of NE components, and finally to catastrophic rupture events that provoke abhorrent molecular fluxes between the nucleus and cytoplasm. Although cells manage to cope with such forms of NE stress, the different insults to nuclear compartmentalization alter gene regulation and jeopardize genome stability. Hence, loss of NE integrity is emerging as a broad-spectrum pathogenic mechanism. In this review, we discuss the relevance of nuclear compartmentalization and the loss thereof in aging and disease development.

Targeted Perturbation of Nuclear Envelope Integrity with Vapor Nanobubble-Mediated Photoporation.

The nuclear envelope (NE) has long been considered to dismantle only during mitosis. However, recent observations in cancer cells and laminopathy patient cells have revealed that the NE can also transiently rupture during interphase, thereby perturbing cellular homeostasis. Although NE ruptures are promoted by mechanical force and the loss of lamins, their stochastic nature and variable frequency preclude the study of their direct downstream consequences. We have developed a method based on vapor nanobubble-mediated photoporation that allows for deliberately inducing NE ruptures in a spatiotemporally controlled manner. Our method relies on wide-field laser illumination of perinuclear gold nanoparticles, resulting in the formation of short-lived vapor nanobubbles that inflict minute mechanical damage to the NE, thus creating small pores. We demonstrate that perinuclear localization of gold nanoparticles can be achieved after endocytic uptake or electroporation-facilitated delivery and that both strategies result in NE rupture upon laser irradiation. Furthermore, we prove that photoporation-induced nuclear ruptures are transient and recapitulate hallmarks of spontaneous NE ruptures that occur in A-type lamin-depleted cells. Finally, we show that the same approach can be used to promote influx of macromolecules that are too large to passively migrate through the NE. Thus, by providing unprecedented control over nuclear compartmentalization, nuclear photoporation offers a powerful tool for both fundamental cell biology research and drug delivery applications.

Accurate Detection of Dysmorphic Nuclei Using Dynamic Programming and Supervised Classification.

A vast array of pathologies is typified by the presence of nuclei with an abnormal morphology. Dysmorphic nuclear phenotypes feature dramatic size changes or foldings, but also entail much subtler deviations such as nuclear protrusions called blebs. Due to their unpredictable size, shape and intensity, dysmorphic nuclei are often not accurately detected in standard image analysis routines. To enable accurate detection of dysmorphic nuclei in confocal and widefield fluorescence microscopy images, we have developed an automated segmentation algorithm, called Blebbed Nuclei Detector (BleND), which relies on two-pass thresholding for initial nuclear contour detection, and an optimal path finding algorithm, based on dynamic programming, for refining these contours. Using a robust error metric, we show that our method matches manual segmentation in terms of precision and outperforms state-of-the-art nuclear segmentation methods. Its high performance allowed for building and integrating a robust classifier that recognizes dysmorphic nuclei with an accuracy above 95%. The combined segmentation-classification routine is bound to facilitate nucleus-based diagnostics and enable real-time recognition of dysmorphic nuclei in intelligent microscopy workflows.

Cellular Redox Profiling Using High-content Microscopy.

Reactive oxygen species (ROS) regulate essential cellular processes including gene expression, migration, differentiation and proliferation. However, excessive ROS levels induce a state of oxidative stress, which is accompanied by irreversible oxidative damage to DNA, lipids and proteins. Thus, quantification of ROS provides a direct proxy for cellular health condition. Since mitochondria are among the major cellular sources and targets of ROS, joint analysis of mitochondrial function and ROS production in the same cells is crucial for better understanding the interconnection in pathophysiological conditions. Therefore, a high-content microscopy-based strategy was developed for simultaneous quantification of intracellular ROS levels, mitochondrial membrane potential (ΔΨm) and mitochondrial morphology. It is based on automated widefield fluorescence microscopy and image analysis of living adherent cells, grown in multi-well plates, and stained with the cell-permeable fluorescent reporter molecules CM-H2DCFDA (ROS) and TMRM (ΔΨm and mitochondrial morphology). In contrast with fluorimetry or flow-cytometry, this strategy allows quantification of subcellular parameters at the level of the individual cell with high spatiotemporal resolution, both before and after experimental stimulation. Importantly, the image-based nature of the method allows extracting morphological parameters in addition to signal intensities. The combined feature set is used for explorative and statistical multivariate data analysis to detect differences between subpopulations, cell types and/or treatments. Here, a detailed description of the assay is provided, along with an example experiment that proves its potential for unambiguous discrimination between cellular states after chemical perturbation.

Sustained accumulation of prelamin A and depletion of lamin A/C both cause oxidative stress and mitochondrial dysfunction but induce different cell fates.

The cell nucleus is structurally and functionally organized by lamins, intermediate filament proteins that form the nuclear lamina. Point mutations in genes that encode a specific subset of lamins, the A-type lamins, cause a spectrum of diseases termed laminopathies. Recent evidence points to a role for A-type lamins in intracellular redox homeostasis. To determine whether lamin A/C depletion and prelamin A accumulation differentially induce oxidative stress, we have performed a quantitative microscopy-based analysis of reactive oxygen species (ROS) levels and mitochondrial membrane potential (Δψm) in human fibroblasts subjected to sustained siRNA-mediated knockdown of LMNA and ZMPSTE24, respectively. We measured a highly significant increase in basal ROS levels and an even more prominent rise of induced ROS levels in lamin A/C depleted cells, eventually resulting in Δψm hyperpolarization and apoptosis. Depletion of ZMPSTE24 on the other hand, triggered a senescence pathway that was associated with moderately increased ROS levels and a transient Δψm depolarization. Both knockdowns were accompanied by an upregulation of several ROS detoxifying enzymes. Taken together, our data suggest that both persistent prelamin A accumulation and lamin A/C depletion elevate ROS levels, but to a different extent and with different effects on cell fate. This may contribute to the variety of disease phenotypes witnessed in laminopathies.