Hutchinson-Gilford progeria syndrome alters nuclear shape and reduces cell motility in three dimensional model substrates.

Cell migration through tight interstitial spaces in three dimensional (3D) environments impacts development, wound healing and cancer metastasis and is altered by the aging process. The stiffness of the extracellular matrix (ECM) increases with aging and affects the cells and cytoskeletal processes involved in cell migration. However, the nucleus, which is the largest and densest organelle, has not been widely studied during cell migration through the ECM. Additionally, the nucleus is stiffened during the aging process through the accumulation of a mutant nucleoskeleton protein lamin A, progerin. By using microfabricated substrates to mimic the confined environment of surrounding tissues, we characterized nuclear movements and deformation during cell migration into micropillars where interspacing can be tuned to vary nuclear confinement. Cell motility decreased with decreased micropillar (µP) spacing and correlated with increased dysmorphic shapes of nuclei. We examined the effects of increased nuclear stiffness which correlates with cellular aging by studying Hutchinson-Gilford progeria syndrome cells which are known to accumulate progerin. With the expression of progerin, cells showed a threshold response to decreased µP spacing. Cells became trapped in the close spacing, possibly from visible micro-defects in the nucleoskeleton induced by cell crawling through the µP and from reduced force generation, measured independently. We suggest that ECM changes during aging could be compounded by the increasing stiffness of the nucleus and thus changes in cell migration through 3D tissues.

Force-induced changes in subnuclear movement and rheology.

Extracellular mechanical forces result in changes in gene expression, but it is unclear how cells are able to permanently adapt to new mechanical environments because chemical signaling pathways are short-lived. We visualize force-induced changes in nuclear rheology to examine short- and long-time genome organization and movements. Punctate labels in the nuclear interior of HeLa, human umbilical vein endothelial, and osteosarcoma (Saos-2) cells allow tracking of nuclear movements in cells under varying levels of shear and compressive force. Under adequate shear stress two distinct regimes develop in cells under mechanical stimulation: an initial event of increased intranuclear movement followed by a regime of intranuclear movements that reflect the dose of applied force. At early times there is a nondirectionally oriented response with a small increase in nuclear translocations. After 30 min, there is a significant increase in nuclear movements, which scales with the amount of shear or compressive stress. The similarities in the nuclear response to shear and compressive stress suggest that the nucleus is a mechanosensitive element within the cell. Thus, applied extracellular forces stimulate intranuclear movements, resulting in repositioning of nuclear bodies and the associated chromatin within the nucleus.

In the middle of it all: mutual mechanical regulation between the nucleus and the cytoskeleton.

The nucleus is typically treated as the large phase-dense or easy-to-label structure at the center of the cell which is manipulated by the governing mechanical machinery inside the cytoplasm. However, recent evidence has suggested that the mechanical properties of the nucleus are important to cell fate. We will discuss many aspects of the structural and functional interconnections between nuclear mechanics and cellular mechanics in this review. There are numerous implications for the progression of many disease states associated with both nuclear structural proteins and cancers. The nucleus itself is a large organelle taking up significant volume within the cell, and most studies agree that nuclei are significantly stiffer than the surrounding cytoplasm. Thus when a cell is exposed to force, the nucleus is exposed to and helps resist that force. The nucleus and nucleoskeleton are interconnected with the cellular cytoskeleton, and these connections may aid in helping disperse forces within tissues and/or with mechanotransduction. During translocation and transmigration the nucleus can act as a resistive element. Understanding the role of mechanical regulation of the nucleus may aid in understanding cellular motility and crawling through confined geometries. Thus the nucleus plays a role in developing mechanical territories and niches, affecting rates of wound healing and allowing cells to transmigrate through tissues for developmental, repair or pathological means.