Mechanisms of Nuclear Migration

University Of California At Davis, Davis CA

Investigators

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Abstract

Summary Proper nuclear positioning is central to many cellular and developmental events. We focus on the roles of linker of nucleoskeleton and cytoskeleton (LINC) complexes and nuclear lamins in mediating nuclear positioning. LINC complexes are formed by the interaction between KASH proteins in the outer nuclear membrane and SUN proteins in the inner nuclear membrane. The nucleoplasmic domains of SUN proteins interact with nuclear lamins and chromatin. Our preliminary data show that LINC complexes regulate cytoplasmic macromolecular crowding, which is important for many biochemical reactions and phase separations. Our goal is to address knowledge gaps in two research themes. In the first theme, we address how LINC complexes and parallel pathways mediate nuclear positioning. We do not understand how nuclei determine which direction to move on microtubules, a basic question in the intracellular trafficking of many cargos. We hypothesize that specific isoforms of the KASH protein UNC-83 direct nuclear migration at different developmental stages; a short UNC-83 isoform activates kinesin-1, while a longer isoform favors dynein motors. We also do not understand how nuclei are deformed to squeeze though small constrictions, a hallmark of the immune response and metastasis. In our working model, four parallel pathways function to deform and protect nuclei, including LINC complexes and dynein, FLN-2 through unknown mechanisms, heterochromatin anchored to the nuclear periphery, and a Cdc42/actin pathway. We developed an in vivo model, where C. elegans larval P-cell nuclei migrate through narrow constrictions as part of normal development. We combine forward genetics, live imaging, and in vitro microtubule motor assays to address these questions. In research theme 2, we address how LINC complexes regulate the biophysical properties of the cytoplasm during development. Our studies are based on our unexpected findings that the giant KASH protein ANC-1 functions to anchor the endoplasmic reticulum (ER), lipid droplets, mitochondria, and nuclei by preserving the mechanical properties of the cytoplasm. In our model, ANC-1 maintains ER morphology through its spectrin repeats acting as a scaffold for the ER. Furthermore, ANC-1 works through LINC complexes, lamins, and ribosomes to regulate cytoplasmic macromolecular crowding. To test our model, we combine developmental and biophysical approaches to perform passive rheology experiments in C. elegans tissues by expressing genetically encoded macromolecular nanoparticles (GEMs) and measuring their diffusion rates in the cytoplasm. Disrupting LINC complexes, ribosomes, or lamins leads to significantly decreased cytoplasmic macromolecular crowding, which could affect reaction rates, cellular mechanics, and phase separations. The significance of our research lies in its potential to shed light on the molecular basis of a wide range of human diseases associated with LINC complex dysfunction, including cancer, infertility, muscular dystrophy, and neurological disorders. The innovative combination of developmental and biophysical approaches positions our studies to provide novel insights into cellular architecture, developmental processes, and disease pathogenesis.