Regulation of Directional Cell Migration from within the Nuclear Envelope

Abstract

Directional cell migration is fundamental to development, wound healing, and is mis-regulated in many cancers. A critical aspect of directional cell migration is the positioning of the nucleus and centrosome. In many migratory cell types, the centrosome is positioned in the center of the cell, while the nucleus is found in the rear of the cell, as in fibroblasts, or in the front of the cell, as in amoebas. Nuclear positioning is achieved through the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex. The LINC complex is a conserved molecular bridge that spans the nuclear envelope (NE) and is comprised of lamin anchored SUN proteins localized to the inner nuclear membrane and nesprin proteins localized to the outer nuclear membrane, which interact with the cytoskeleton. SUN2 trimerization is required for binding of the nesprins within the lumen of the NE. A candidate regulator of the LINC complex is the NE lumen-localized AAA+ protein torsinA. AAA+ proteins hydrolyze ATP to structurally remodel substrates, and torsinA has been shown to interact with LINC complex components. Underscoring the importance of these proteins, mutations in SUNs and nesprins are associated with cancers, cerebellar ataxia, and Emery-Dreifuss Muscular Dystrophy, while mutations in torsinA are associated with primary dystonia. Interrogation of the oligomeric state of the two most widely expressed SUN proteins, SUN1 and SUN2, within the NE of living cells using Z-scan Fluorescence Fluctuation Spectroscopy (FFS) reveals that SUN2 forms trimers while SUN1 forms higher order oligomers. During rearward nuclear movement in fibroblasts orienting for migration, torsinA and its inner nuclear membrane localized ATPase stimulator Lamina Associated Polypeptide1 (LAP1) are required for the formation and stability of linear arrays of SUN2 and nesprin2-Giant LINC complexes as well as perinuclear retrograde actin flow. Lastly, phylogenetic analysis shows that torsin homologs are more ancient than reported, suggesting they arose with the advent of the nucleus. The social amoeba Dictyostelium was developed as a model system to study the conserved role of torsins during cellular migration. Deletion of the torsin homolog, tsin, in Dictyostelium altered their directional migration and decreased their cell-substratum interaction during directional cell migration.