Chromatin condensation regulates endothelial cell adaptation to shear stress

Vascular endothelial cells (ECs) have been shown to be mechanoresponsive to the forces of blood flow, including fluid shear stress (FSS), the frictional force of blood on the vessel wall. Recent reports have shown that FSS induces epigenetic changes in chromatin. Epigenetic changes, such as methylation and acetylation of histones, not only affect gene expression but also affect chromatin condensation, which can alter nuclear stiffness. Thus, we hypothesized that changes in chromatin condensation may be an important component for how ECs adapt to FSS. Using both in vitro and in vivo models of EC adaptation to FSS, we observed an increase in histone acetylation and a decrease in histone methylation in ECs adapted to flow as compared with static. Using small molecule drugs, as well as vascular endothelial growth factor, to change chromatin condensation, we show that decreasing chromatin condensation enables cells to more quickly align to FSS, whereas increasing chromatin condensation inhibited alignment. Additionally, we show data that changes in chromatin condensation can also prevent or increase DNA damage, as measured by phosphorylation of γH2AX. Taken together, these results indicate that chromatin condensation, and potentially by extension nuclear stiffness, is an important aspect of EC adaptation to FSS.

SIRT6 facilitates directional telomere movement upon oxidative damage

Oxidative damage to telomeres leads to telomere attrition and genomic instability, resulting in poor cell viability. Telomere dynamics contribute to the maintenance of telomere integrity; however, whether oxidative damage induces telomere movement and how telomere mobility is regulated remain poorly understood. Here, we show that oxidative damage at telomeres triggers directional telomere movement. The presence of the human Sir2 homolog, Sirtuin 6 (SIRT6) is required for oxidative damage-induced telomeric movement. SIRT6 knock out (KO) cells show neither damage-induced telomere movement nor chromatin decondensation at damaged telomeres; both are observed in wild type (WT) cells. A deacetylation mutant of SIRT6 increases damage-induced telomeric movement in SIRT6 KO cells as well as WT SIRT6. SIRT6 recruits the chromatin-remodeling protein SNF2H to damaged telomeres, which appears to promote chromatin decondensation independent of its deacetylase activity. Together, our results suggest that SIRT6 plays a role in the regulation of telomere movement upon oxidative damage, shedding new light onto the function of SIRT6 in telomere maintenance.

SSRP1 Cooperates with PARP and XRCC1 to Facilitate Single-Strand DNA Break Repair by Chromatin Priming

DNA single-strand breaks (SSB) are the most common form of DNA damage, requiring repair processes that to initiate must overcome chromatin barriers. The FACT complex comprised of the SSRP1 and SPT16 proteins is important for maintaining chromatin integrity, with SSRP1 acting as an histone H2A/H2B chaperone in chromatin disassembly during DNA transcription, replication, and repair. In this study, we show that SSRP1, but not SPT16, is critical for cell survival after ionizing radiation or methyl methanesulfonate-induced single-strand DNA damage. SSRP1 is recruited to SSB in a PARP-dependent manner and retained at DNA damage sites by N-terminal interactions with the DNA repair protein XRCC1. Mutational analyses showed how SSRP1 function is essential for chromatin decondensation and histone H2B exchange at sites of DNA strand breaks, which are both critical to prime chromatin for efficient SSB repair and cell survival. By establishing how SSRP1 facilitates SSB repair, our findings provide a mechanistic rationale to target SSRP1 as a general approach to selectively attack cancer cells. Cancer Res; 77(10); 2674-85. ©2017 AACR.

Nuclear mechanical resilience but not stiffness is modulated by αII-spectrin

Spectrins are multi-domain, elastic proteins that provide elasticity to the plasma membrane of erythrocytes and select nucleated cells. Spectrins have also been found in the nucleus of non-erythrocytes, but their function remains to be uncovered. It has been hypothesized that a spring-like spectrin network exists within the lamina nucleoskeleton, however, experiments testing a spectrin network׳s mechanical impact on the nucleus are lacking. Here, we knock-down levels of nuclear αII-spectrin with the goal of disrupting this nucleoskeletal spectrin network. We mechanically test live cells with intranuclear particle tracking and compression assays to probe changes in nuclear mechanics with decreases in αII-spectrin. We show no changes in chromatin mechanics or in the stiffness of nuclei under compression. However, we do observe a reduction in the ability of nuclei with decreased αII-spectrin to recover after compression. These results establish spectrin as a nucleoskeletal component that specifically contributes to elastic recovery after compression.

Mechanobiology of Chromatin and the Nuclear Interior

The view of the cell nucleus has evolved from an isolated, static organelle to a dynamic structure integrated with other mechanical elements of the cell. Both dynamics and integration appear to contribute to a mechanical regulation of genome expression. Here, we review physical structures inside the nucleus at different length scales and the dynamic reorganization modulated by cellular forces. First, we discuss nuclear organization focusing on self-assembly and disassembly of DNA structures and various nuclear bodies. We then discuss the importance of connections from the chromatin fiber through the nuclear envelope to the rest of the cell as they relate to mechanobiology. Finally, we discuss how cell stimulation, both chemical and physical, can alter nuclear structures and ultimately cellular function in healthy cells and in some model diseases. The view of chromatin and nuclear bodies as mechanical entities integrated with force generation from the cytoskeleton combines polymer physics with cell biology and medicine.

Spatially Resolved Quantification of Chromatin Condensation through Differential Local Rheology in Cell Nuclei Fluorescence Lifetime Imaging

The linear sequence of DNA encodes access to the complete set of proteins that carry out cellular functions. Yet, much of the functionality appropriate for each cell is nested within layers of dynamic regulation and organization, including a hierarchy of chromatin structural states and spatial arrangement within the nucleus. There remain limitations in our understanding of gene expression within the context of nuclear organization from an inability to characterize hierarchical chromatin organization in situ. Here we demonstrate the use of fluorescence lifetime imaging microscopy (FLIM) to quantify and spatially resolve chromatin condensation state using cell-permeable, DNA-binding dyes (Hoechst 33342 and PicoGreen). Through in vitro and in situ experiments we demonstrate the sensitivity of fluorescence lifetime to condensation state through the mechanical effects that accompany the structural changes and are reflected through altered viscosity. The establishment of FLIM for resolving and quantifying chromatin condensation state opens the door for single-measurement mechanical studies of the nucleus and for characterizing the role of genome structure and organization in nuclear processes that accompany physiological and pathological changes.

Early Passage Dependence of Mesenchymal Stem Cell Mechanics Influences Cellular Invasion and Migration

The cellular structures and mechanical properties of human mesenchymal stem cells (hMSCs) vary significantly during culture and with differentiation. Previously, studies to measure mechanics have provided divergent results using different quantitative parameters and mechanical models of deformation. Here, we examine hMSCs prepared for clinical use and subject them to mechanical testing conducive to the relevant deformability associated with clinical injection procedures. Micropipette aspiration of hMSCs shows deformation as a viscoelastic fluid, with little variation from cell to cell within a population. After two passages, hMSCs deform as viscoelastic solids. Further, for clinical applicability during stem cell migration in vivo, we investigated the ability of hMSCs to invade into micropillar arrays of increasing confinement from 12 to 8 μm spacing between adjacent micropillars. We find that hMSC samples with reduced deformability and cells that are more solid-like with passage are more easily able to enter the micropillar arrays. Increased cell fluidity is an advantage for injection procedures and optimization of cell selection based on mechanical properties may enhance efficacy of injected hMSC populations. However, the ability to invade and migrate within tight interstitial spaces appears to be increased with a more solidified cytoskeleton, likely from increased force generation and contractility. Thus, there may be a balance between optimal injection survival and in situ tissue invasion.

Nuclear stiffening and chromatin softening with progerin expression leads to an attenuated nuclear response to force

Progerin is a mutant form of the nucleoskeletal protein lamin A, and its expression results in the rare premature aging disorder Hutchinson-Gilford progeria syndrome (HGPS). Patients with HGPS demonstrate several characteristic signs of aging including cardiovascular and skeletal dysfunction. Cells from HGPS patients show several nuclear abnormalities including aberrant morphology, nuclear stiffening and loss of epigenetic modifications including heterochromatin territories. However, it is unclear why these changes disproportionately impact mechanically-responsive tissues. Using micropipette aspiration, we show that nuclei in progerin-expressing cells are stiffer than control cells. Conversely, our particle tracking reveals the nuclear interior becomes more compliant in cells from HGPS patients or with progerin expression, as consistent with decreased chromatin condensation as shown previously. Additionally, we find the nuclear interior is less responsive to external mechanical force from shear or compression likely resulting from damped force propagation due to nucleoskeletal stiffening. Collectively our findings suggest that force is similarly transduced into the nuclear interior in normal cells. In HGPS cells a combination of a stiffened nucleoskeleton and softened nuclear interior leads to mechanical irregularities and dysfunction of mechanoresponsive tissues in HGPS patients.

The tail domain of lamin B1 is more strongly modulated by divalent cations than lamin A

The nucleoskeleton contains mainly nuclear intermediate filaments made of lamin proteins. Lamins provide nuclear structure and also play a role in various nuclear processes including signal transduction, transcription regulation and chromatin organization. The disparate functions of lamins may be related to the intrinsic disorder of the tail domains, which allows for altered and promiscuous binding. Here, we show modulation of lamin tail domain structures in the presence of divalent cations. We utilize changes in fluorescence of tryptophan residues within the Ig-fold flanked by disordered regions to experimentally measure protein thermodynamics. Using spectroscopy experiments and molecular dynamics simulations, we show that the tail domain of lamin B1 shows enhanced association with both Ca(2+) and Mg(2+) compared to the tail domain of lamin A. Binding curves show a similar KD between protein and ion (250-300 μM) for both proteins with both ions. However, we observe a maximum binding of ions to lamin B1 tail domain which is 2-3 times greater than that for lamin A tail domain by both experiment and simulation. Using simulations, we show that divalent ion association alters the Ig-fold by pinning flanking regions. With cells in culture, we observe altered lamin B1 organization in the presence of excess Mg(2+) more so than for lamin A. We suggest that the differential sensitivity to divalent cations contributes to the vastly different functionalities and binding of the 2 proteins.

Active cytoskeletal force and chromatin condensation independently modulate intranuclear network fluctuations

Chromatin remodeling, including the movement of genes and regulatory factors, precedes or accompanies stimulated changes in gene expression. Here we quantify chromatin fluctuations in primary human cells using particle-tracking microrheology and determine the physical mechanisms which influence chromatin reorganization. We find that intranuclear movements are enhanced beyond thermal motion by active force generation from cytoskeletal motor activity propagated through the LINC complex; intranuclear movements are also dependent on the viscoelasticity of the DNA-protein polymer network. Chromatin movements were dramatically altered by modulation of chromatin condensation state, which we independently verified using fluorescence lifetime imaging microscopy (FLIM). These findings suggest that chromatin condensation and cytoskeletal force generation play distinct functional roles in regulating intranuclear movements, and these effects are decoupled as measured by particle tracking. We further utilize this approach in identifying the nuclear responsiveness of primary human endothelial cells to vascular endothelial growth factor (VEGF): early in the response chromatin movements increase and are dominated by cytoskeletal force, which transitions at later times to a chromatin decondensation event. Given the hierarchical genome organization in primary cells, our work generally suggests an important role for force generation and chromatin mechanics in altered gene expression kinetics.