The GFP-nesprin2G TAN lines in nesprin2G-depleted (by siRNA) NIH3T3 cells. TAN lines at higher intensity in yellow on a less intensely stained nucleus (green). The cell leading edge is toward the upper left and the nucleus would move with the TAN lines toward the bottom right. Image courtesy of Dr. Gregg G. Gundersen, Department of Pathology & Cell Biology, Columbia University, New York, USA.

During migration, nuclei move to specific locations in the cell to mediate cell polarity. Although most nuclear movements are controlled by microtubule-dependent mechanisms, there have been some hints that actin might also be at play here. Gregg Gundersen and colleagues now report in Science an actin-dependent mechanism for nuclei positioning in migrating fibroblasts, whereby structures called transmembrane actin-associated nuclear (TAN) lines — consisting of cytoplasmic actin cables, and the nuclear envelope proteins Nesprin-2 and Sun2 — directly link actin filaments to the nucleus.

The authors first tested the effects of displacing nesprins from the nuclear envelope in wound-edge fibroblast cells by expressing dominant-negative constructs of the LINC (linker of nucleoskeleton and cytoskeleton) complex that consists of outer nuclear membrane proteins Nesprin-1 and Nesprin-2 and inner nuclear membrane proteins Sun1 and Sun2. This inhibited centrosome orientation and blocked rearward nuclear positioning. Furthermore, knock down of Nesprin-2 with short-interfering (si)RNA blocked centrosome orientation and rearward nuclear movement. These defects were rescued by expression of a 'mini' Nesprin-2 protein, indicating that Nesprin-2 is involved in nuclear positioning.

They go on to describe linear structures called TAN (transmembrane actin-associated nuclear) lines, which colocalize with dorsal actin cables in Nesprin-2-depleted and mini-Nesprin-2 rescued cells. Using live fluorescence imaging, they further elucidated that TAN lines are molecular assemblies of Nesprin-2 and Sun2, which couple actin filaments to the nuclear envelope, controlling rearward nuclear movement. Consistently, Sun2 depletion inhibited nuclear positioning and centrosome orientation, thereby disrupting cell polarity; this was restored by expression of Sun2. Interestingly, Nesprin-2 expression could not rescue nuclear movement in Sun2-depleted cells, showing that both Nesprin-2 and Sun2 are required for rearward nuclear movement in polarizing fibroblasts. Further investigation revealed that depletion of the LINC complex, Nesprin-2 or Sun2 slows down the migration of cells. This indicates that both LINC complex and Nesprin-2- and Sun2-mediated nuclear movement are essential for proper cell migration.

This study demonstrates that the LINC complex components Nesprin-2 and Sun2 assemble into TAN lines, directly linking the nucleus to the cytoplasmic actin filaments, thereby allowing retrograde actin flow to be used for nuclear movement. Further research will help elucidate how TAN lines regulate migration during development and cancer metastasis.

Author: Iley Ozerlat - Copyright © 2010 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission