Specificity of Nuclear Size Scaling in Frog Erythrocytes

Physical forces modulate interphase nuclear size

The eukaryotic nucleus exhibits remarkable plasticity in size, adjusting dynamically to changes in cellular conditions such as during development and differentiation, and across species. Traditionally, the supply of structural constituents to the nuclear envelope has been proposed as the principal determinant of nuclear size. However, recent experimental and theoretical analyses have provided an alternative perspective, which emphasizes the crucial role of physical forces such as osmotic pressure and chromatin repulsion forces in regulating nuclear size. These forces can be modulated by the molecular profiles that traverse the nuclear envelope and assemble in the macromolecular complex. This leads to a new paradigm wherein multiple nuclear macromolecules that are not limited to only the structural constituents of the nuclear envelope, are involved in the control of nuclear size and related functions.

Differential contribution of nuclear size scaling mechanisms between Xenopus species

Size of the nucleus, a membrane-bound organelle for DNA replication and transcription in eukaryotic cells, varies to adapt nuclear functions to the surrounding environment. Nuclear size strongly correlates with cytoplasmic size and genomic content. Previous studies using Xenopus laevis have unraveled two modes, cytoplasmic and chromatin-based mechanisms, for controlling nuclear size. However, owing to limited comparative analyses of the mechanisms among eukaryotic species, the contribution of each mechanism in controlling nuclear size has not been comprehensively elucidated. Here, we compared the relative contribution utilizing a cell-free reconstruction system from the cytoplasmic extract of unfertilized eggs of Xenopus tropicalis to that of the sister species X. laevis. In this system, interphase nuclei were reconstructed in vitro from sperm chromatin and increased in size throughout the incubation period. Using extracts from X. tropicalis, growth rate of the reconstructed nuclei was decreased by obstructing the effective cytoplasmic space, decreasing DNA quantity, or inhibiting molecules involved in various cytoplasmic mechanisms. Although these features are qualitatively identical to that shown by the extract of X. laevis, the sensitivities of experimental manipulation for each cellular parameter were different between the extracts from two Xenopus species. These quantitative differences implied that the contribution of each mode to expansion of the nuclear envelope is coordinated in a species-specific manner, which sets the species-specific nuclear size for in vivo physiological function.

Nuclear size rectification: A potential new therapeutic approach to reduce metastasis in cancer

Research on metastasis has recently regained considerable interest with the hope that single cell technologies might reveal the most critical changes that support tumor spread. However, it is possible that part of the answer has been visible through the microscope for close to 200 years. Changes in nuclear size characteristically occur in many cancer types when the cells metastasize. This was initially discarded as contributing to the metastatic spread because, depending on tumor types, both increases and decreases in nuclear size could correlate with increased metastasis. However, recent work on nuclear mechanics and the connectivity between chromatin, the nucleoskeleton, and the cytoskeleton indicate that changes in this connectivity can have profound impacts on cell mobility and invasiveness. Critically, a recent study found that reversing tumor type-dependent nuclear size changes correlated with reduced cell migration and invasion. Accordingly, it seems appropriate to now revisit possible contributory roles of nuclear size changes to metastasis.