Received date:: January 30, 2015; Accepted date:: June 23, 2015; Published date:: June 29, 2015
Citation: Kim Y, Yeo MG (2015) Role of Focal Adhesions in Lamellipodia Dynamics. J Biotechnol Biomater 5:185. doi:10.4172/2155-952X.1000185
Copyright: © 2015 Kim Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Focal adhesions (FAs) are multi-protein structures containing integrin that serve as a focal point for the association between the extracellular matrix (ECM) and actin cytoskeleton. After cells adhere to the ECM, the cell membrane forms filopodia and lamellipodia. Cells deficient in FA complexes show reduced lamellipodia dynamics and this deficiency influences cell migration. Lamellipodia dynamics have distinguishable stages of lamellipodia protrusion, retraction, and persistence. Particularly, cells with decreasing or absent FA formation commonly show longer persistence time as analyzed using computer-assisted stroboscopic analysis. These results indicate that after cells adhere to the substratum, the reduced lamellipodia dynamics associated with defective FA formation influences cell motility.
Cell migration plays a central role in many physiological and pathological processes including embryogenesis, inflammatory response, wound healing, and metastasis. Cell migration is a distinctive, integrative, multistep process including formation of the cell adhesion, membrane protrusion at the front area, cell body contraction, and tail detachment . Cell adhesion utilizes focal adhesions (FAs), fibrillary adhesions, and podosomes  to link the extracellular matrix (ECM) and various cytoplasmic proteins. After cells adhere to the ECM, the membrane begins filopodium formation and lamellipodium extension at the front edge of the cell. These are driven by actin polymerization and microtubule dynamics . During the next steps of cell migration, the cell body moves forward in the migration direction and releases the cell-substrate adhesion at the cell rear . At the cell front, the membrane begins as a flat cellular protrusion that is powered by actin polymerization and can be visualized by phase contrast microscopy as dark waves, which are called membrane ruffles [4,5]. Lamellipodia are sheet-like projections formed at the leading edge of many migrating cells, including fibroblasts, immune cells, neural crest cells, and melanoblasts [5,6]. This review will discuss the role of FA and its effect on lamellipodia dynamics in understanding adhesion-dependent cell migration. FAs are highly dynamic structures that form at sites of membrane contact with the ECM and associate with many cellular proteins known as FA complexes, including vinculin, focal adhesion kinase (FAK), Src family kinases (SFKs), paxillin, p130CAS (Crk-associate substrate), and Crk [2,7]. Deficiency of FA complexes in mouse embryo fibroblasts (MEFs) results in severe defects in cell spreading and culminates in embryonic death. For example, FAK-null MEFs show mesodermal defects in the late phase of gastrulation and have a delay in cell migration in vitro [8,9]. Deficiency of p130CAS causes severe defects in cell spreading [9,10]. In SYF cells (deficient for Src, Yes, and Fyn), MEFs show a decreased formation of FAs, which results in severe developmental defects, lethality, and delayed cell migration [11-13]. Additionally, Crknull mice die during a late stage of embryonic development  and present with a defect in cell-spreading in vitro . These results imply that FA-associated proteins play primary roles in cell migration.
Lamellipodia and membrane ruffles form when cell adhesions fail or detach from the substrate and retract toward the cell body . Membrane ruffles appear at the edge of cells moving in culture and disappear at the border between the lamella base and the main cell body [16,17]. To elucidate the mechanisms underlying cell motility, a quantitative analysis of lamella dynamics was introduced. This computer-assisted stroboscopic analysis of cell dynamics (SACED) is used for analyzing lamellipodia and ruffle formation after the cell adheres to the ECM [4,17, 18] (Figure 1). The SACED analysis of lamellipodia dynamics has distinctive stages including lamellipodia protrusion, persistence, and ruffle retraction. Membrane dynamics and ruffling involve many proteins including, α4β1 integrin , Rac1 , Arp2/3 , and others [21,22].
Figure 1: Quantitation of lamellipodia dynamics using computer-assisted stroboscopic analysis of cell dynamics (SACED). After cell adhesion, images are captured and the velocities of lamellipodia protrusion and retraction are calculated using a 10-pixel-wide (1.06 mm) box drawn in the direction of cell protrusion to create montages of images captured every 5 sec for 5 min. dt1:time to protrusion, dt2: time to retraction, dx: protrusion distance.
In the comparative studies on defective FA formation in MEFs, these MEFs commonly show longer lamellipodia persistence time than wild type (wt) MEFs. The SACED analysis of persistence time (sec; mean ± SD) of Crk-null MEFs is 40.30s ± 1.7 and wt Crk is 25.82s ± 0.66, SYF MEF is 90.81s ± 27.79 and wt Fyn (recovered Fyn cDNA in SYF MEF)  is 65.25s ± 38.46, and p130CAS-null MEF is 67.2 s ± 10.1083 and wt p130CAS is 36.5455s ± 6.3303 [13,15,18]. Furthermore, experiments in p130CAS-null MEFs show the lamellipodia protrusion and ruffle retraction velocity decreases compared with wt p130CAS MEFs . The Fyn-null MEFs also show that the lamellipodia protrusion and ruffle retraction velocity decreases compared to the wt Fyn MEFs . These SFK deficient MEFs commonly show less formation of FAs and reduced lamellipodia protrusion and retraction velocity, but membrane persistence times are prolonged compared to wt MEFs. Based on these results, FA-associated proteins modulate membrane dynamics and lamellipodia protrusion, ruffle retraction, and membrane persistence are involved in the cell spreading necessary for cell motility.
Overall, the findings summarized in this article suggest that lamellipodia dynamics control cell migration and membrane persistence time and thus are useful as one criterion of cell migration. However, to fully understand the precise mechanisms in lamellipodia dynamics, it will be necessary to examine the associated cellular factors during lamellipodia protrusion, retraction, and persistence.
This study was supported by research funds from Nambu University, 2014.