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Periodic Report Summary 1 - MECHANICUS (The mechanics of nuclear division and positioning)

During mitosis the genetic material is separated by an assembly of polymers, molecular motors and adapter proteins – the mitotic spindle. This super-molecular machine moves the chromosome halves in opposite directions over impressive cellular scales (tens of μm) within a relatively short time (minutes). Positioning of the chromosomes is especially challenging and fascinating in large egg cells where the center-to-cell cortex distance is in the order of millimeters. Understanding the mechanical properties responsible for pulling chromosomes and positioning nuclei is at the heart of spindle function, which is critical to the life cycle of a cell. This knowledge is pivotal in developing biomedical applications. This project investigates the mechanical basis leading to directed movement within the cell during mitosis in large cells such as in embryos. Our model system is the fruit fly in which embryos undergo several nuclear divisions and nuclear distribution without cell division.
In the course of installing an independent lab at the host institute, the fellow’s group has started their quest by designing and developing a robust, quantitative experimental assay that allows them to test the response of the spindle machinery to spatial perturbations. The assay is based on embryo explant generation – a cell-free assay in which the content of the embryo is extracted and deposited onto functional surfaces or 3D compartments. Because the cell membrane is now absent we can perform mechanical measurements and perturbations. This is done with simultaneous live imaging on a confocal fluorescence microscope, giving the researchers visual dynamic information of the spindle machinery and chromatin. The fruit fly, Drosophila melanogaster, is an excellent model with an extensive genome editing toolbox, and the fellow’s group has meanwhile generated several transgenic constructs that allow visualization and gene knock-down. The first results from experimentally testing the mechanics of nuclear separation suggest a coordinated and robust machinery governing nuclear positioning independent of spatial constraints or obstacles. As part of the mitotic spindle the microtubule cytoskeleton plays a crucial role in defining and maintaining the distance between nuclei after mitosis, and this is presumably mediated by microtubule cross-linking. Nuclear positioning is intimately linked to the mitotic cycle. With the ongoing development of nano-fabricated mechanical probes the fellow expects to finally characterize the biomechanics of chromosome and nuclear movement and linking it to the key molecular activities.
This project lies at the interdisciplinary interface between mechanical engineering, microscopy, cell biology and fly genetics.

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Life Sciences