The Endoplasmic Spreading Mechanism of Fibroblasts

Cell motility is an essential process that depends on a coherent, cross-linked cytoskeleton that physically coordinates the actions of numerous structural and signaling molecules. In culture, a common feature of cells is the coherent movement of the endoplasmic reticulum and membranous organelles toward the periphery during substrate adhesion and spreading. The actin cross-linking protein, filamin (Fln), has been implicated in the support of three-dimensional cortical actin networks capable of both maintaining cellular integrity and withstanding large forces. Although numerous studies have examined cells lacking one of the multiple Fln isoforms, compensatory mechanisms can mask novel phenotypes only observable by further Fln depletion. Indeed, shRNA-mediated knockdown of FlnA in FlnB-/- mouse embryonic fibroblasts (MEFs) causes a novel endoplasmic spreading deficiency as detected by endoplasmic reticulum markers. Microtubule (MT) extension rates are also decreased but not by peripheral actin flow, because this is also decreased in the Fln-depleted system. Additionally, Fln-depleted MEFs exhibit decreased adhesion stability that leads to increased ruffling of the cell edge, reduced