Boudreau, V. | bioRxiv Science

bioRxiv2018

Centrosome-nuclear envelope tethering and microtubule motor-based pulling forces collaborate in centrosome positioning during mitotic entry

Centrosome positioning relative to the nucleus and cell shape is highly regulated across cell types, during cell migration and during spindle formation in cell division. Across most sexually reproducing animals, centrosomes are provided to the oocyte through fertilization and must be positioned properly to establish the zygotic mitotic spindle. How centrosomes are positioned in space and time through the concerted action of key mitotic entry biochemical regulators including Protein Phosphatase 2A (PP2A-B55/SUR-6), biophysical regulators including Dynein and the nuclear lamina is unclear. Here, we uncover a role for PP2A-B55/SUR-6 in regulating centrosome positioning. Mechanistically, PP2A-B55/SUR-6 regulates nuclear size prior to mitotic entry, in turn affecting nuclear envelope-based Dynein density and motor capacity. Using computational simulations, PP2A-B55/ SUR-6 regulation of nuclear size and nuclear envelope Dynein density were both predicted to be required for proper centrosome positioning. Conversely, compromising nuclear lamina integrity led to centrosome detachment from the nuclear envelope and migration defects. Removal of PP2A-B55/SUR-6 and the nuclear lamina simultaneously further disrupted centrosome positioning, leading to unseparated centrosome pairs dissociated from the nuclear envelope. Taken together, we propose a model in which centrosomes migrate and are positioned through the concerted action of nuclear envelope-based Dynein pulling forces and cen-trosome-nuclear envelope tethering.

cell biology↗

bioRxiv2018

Nucleo-cytoplasmic trafficking regulates nuclear surface area during nuclear organogenesis

Throughout development, nuclei must be assembled following every cell division to establish a functional organelle from compact, mitotic chromatin. During nuclear organogenesis, chromatin expands to establish a nucleus of a given size seperate from the cytoplasm. Determining how nuclear organogenesis is regulated is particularly significant in the context of certain cancers in which scaling relationships between cell and nuclear sizes are not maintained. Controlling cell size in vitro using a microfluidics approach, we determined that neither nuclear volume nor surface area scale directly with cell size. Looking to explain differential nuclear scaling relationships, we developed a simple mechano-chemical mathematical model. In simulating biological perturbations in silico, our model predicted crucial roles for nucleo-cytoplasmic trafficking in regulating nuclear expansion and in restricting the recruitment of a potential nuclear surface area factor. In mammalian tissue culture, inhibiting nuclear export increased nuclear expansion rates and reduced the amount of nuclear lamin, a candidate surface area factor, being recruited to assembling nuclei, supporting our models predictions. Targeting the principal nuclear export component in the Drosophila syncytial embryo, Embargoed, we show that nuclear expansion rates are also increased in this developmental context, consistent with our model. Using the MS2-reporter system in fly embryos, we demonstrate a role for nuclear export in regulating transcription activation timing and dynamics, suggesting that regulating nuclear assembly is crucial for downstream nuclear function. Taken together, we propose a simple model through which nuclear organogenesis is achieved and demonstrate a role for nuclear export in regulating nuclear assembly.

cell biology↗