Search

Fibroblasts exhibit heterogeneous cell geometries in tissues and integrate both mechanical and biochemical signals in their local microenvironment to regulate genomic programs via chromatin remodelling. While in connective tissues fibroblasts experience tensile and compressive forces (CFs), the role of compressive forces in regulating cell behavior and, in particular, the impact of cell geometry in modulating transcriptional response to such extrinsic mechanical forces is unclear. Here we show that CF on geometrically well-defined mouse fibroblast cells reduces actomyosin contractility and shuttles histone deacetylase 3 (HDAC3) into the nucleus. HDAC3 then triggers an increase in the heterochromatin content by initiating removal of acetylation marks on the histone tails. This suggests that, in response to CF, fibroblasts condense their chromatin and enter into a transcriptionally less active and quiescent states as also revealed by transcriptome analysis. On removal of CF, the alteration in chromatin condensation was reversed. We also present a quantitative model linking CF-dependent changes in actomyosin contractility leading to chromatin condensation. Further, transcriptome analysis also revealed that the transcriptional response of cells to CF was geometry dependent. Collectively, our results suggest that CFs induce chromatin condensation and geometry-dependent differential transcriptional response in fibroblasts that allows maintenance of tissue homeostasis.

Physical boundary conditions of the cell define its cytoskeletal organization and thus regulate the force balance on the nucleus. The boundary can be very precisely engineered by plating cells on micropatterned substrate of various geometries. Such modulations of cell shape and size impinge on nuclear morphology and dynamics. Understanding the regulation of nuclear dynamics is crucial for both physiological processes such as migration, wound healing, stem cell differentiation, and gene expression program and for disease conditions like cancer progression and laminopathies. In this chapter, we discuss the different types of nuclear dynamics, translation, rotation, and fluctuations of the nuclear envelope, forces contributing to such dynamics, and their implications in cell physiology.

The collective activity of several molecular motors and other active processes generate large forces for directional motion within the cell, which is vital for a multitude of cellular functions such as migration, division, contraction, transport, and positioning of various organelles. These processes also generate a background of fluctuating forces, which influence intracellular dynamics and thereby create unique biophysical signatures, which are altered in many diseases. In this study, we have used the nucleus as a probe particle to understand the microrheological properties of altered intracellular environments by using micropatterning to confine cells in two structurally and functionally extreme geometries. We find that nuclear positional dynamics is sensitive to the cytoskeletal organization by studying the effect of actin polymerization and nuclear rigidity on the diffusive behavior of the nucleus. Taken together, our results suggest that mapping nuclear positional dynamics provides important insights into biophysical properties of the active cytoplasmic medium. These biophysical signatures have the potential to be used as an ultrasensitive single-cell assay for early disease diagnostics.

Electrostatic self-assembly of colloidal and nanoparticles has attracted a lot of attention in recent years, since it offers the possibility of producing novel crystalline structures that have the potential to be used as advanced materials for photonic and other applications. The stoichiometry of these crystals is not constrained by charge neutrality of the two types of particles due to the presence of counterions, and hence a variety of three-dimensional structures have been observed depending on the relative sizes of the particles and their charge. Here we report structural polymorphism of two-dimensional crystals of oppositely charged linear macroions, namely DNA and self-assembled cylindrical micelles of cationic amphiphiles. Our system differs from those studied earlier in terms of the presence of a strongly binding counterion that competes with DNA to bind to the micelle. The presence of these counterions leads to novel structures of these crystals, such as a square lattice and a superlattice of an underlying hexagonal lattice, determined from a detailed analysis of the small-angle diffraction data. These lower-dimensional equilibrium systems can play an important role in developing a deeper theoretical understanding of the stability of crystals of oppositely charged particles. Further, it should be possible to use the same design principles to fabricate structures on a longer length-scale by an appropriate choice of the two macroions.