Contextualizing the Contributions of HP1α-Mediated Chromatin Cross-Linking to Total and Internal Nuclear Mechanics

Abstract

The physical properties of the nucleus dictate global cell responses to mechanical challenges, and their disruption is implicated in human disease. For instance, nuclear softening facilitates the rate-limiting deformation step in cancer cell invasion. Recent work suggests that chromatin-chromatin cross-linking is an important (but under-explored) determinant of nuclear mechanical properties. Here, I examine whether depletion of HP1α, a chromatin cross-linker whose expression is lost in metastatic cells, changes total and internal nuclear mechanics in ways that facilitate confined cell migration. Additionally, I aim to contextualize the scope of HP1α’s mechanical contributions by comparing them to those of a second, better-characterized determinant of nuclear stiffness (Lamin A). Using a set of imaging-based readouts, I demonstrate that HP1α contributes to nuclear morphology and nucleoplasmic diffusion dynamics, but that its effects are comparatively subtle. However, I also show that HP1α is a major contributor to intra-nucleolar diffusion dynamics, meaning that its impacts on internal nuclear mechanics exhibit an intriguing spatial heterogeneity. Using atomic force microscopy, a custom nuclear deformation assay, and an in vitro assessment of confined migration frequency, I then demonstrate that loss of HP1α-mediated cross-linking yields total nuclear softening and facilitates migration through narrow pores. Collectively, these results suggest an interesting comparative model of Lamin A and HP1α’s contributions to nuclear mechanics and demonstrate that spatially heterogenous changes in internal mechanical properties can result in total nuclear softening. In addition, the presented findings indicate that a mechanical model of HP1α’s contributions to cancer cell migration – and, by extension, to disease progression – is worth exploring further in future work.