Biomedical Engineering Reference
In-Depth Information
In addition to biochemical factors, mechanical forces have been shown to directly
affect cellular activities. For instance, tension resulting from actomyosin contractility
contributes to the formation and maintenance of actin stress fibers (SFs) (Costa et al.
2002
). This has been demonstrated experimentally by partially releasing intracellular
tension to induce fiber disassembly, leading to structural reorganization (Sato et al.
2006
). Indeed, it has been determined that at a certain threshold, mechanical strain can
induce fiber disassembly and reorganization through the release of pretension in the
SFs (Sato et al.
2005
).
In this chapter, we focus on quantitative analysis of mechanical strain in the actin
filament network that constitutes the lamellipodium of a migrating fish keratocytes.
Because of their steady and persistent movement, fish epidermal keratocytes are con-
sidered as suitable models for elucidating the complex phenomena of cell migration
(Lee et al.
1993
). Keratocytes have a large lamellipodium that typically spans ~10 ʼm
from front to back (Laurent et al.
2005
) that can be divided into two regions; lamelli-
podium and lamella in mesenchymal and epithelial cell types (Burnette et al.
2011
).
The lamellipodium extends 3-5 ʼm from the leading edge and consist of highly
dynamic actin network. The lamella is immediately behind the lamellipodium and is
characterized by sparsely bundled actin filaments which is in close association with
focal adhesions.
In this chapter, we examine how to track and map the displacement of the
fluorescent-dye-labeled actin network using a combination of fluorescent speckle
microscopy (FSM) and particle imaging velocimetry (PIV) (Willert and Gharib
1991
).
Next, we explain how to compute the distribution of actin network deformation and
correlate these distributions with that of actin filament network in the lamellipodia.
Finally, we describe the role of mechanical strain in the reorganization of actin network
during cell migration and discuss how it is regulated by coupling interactions among
mechanical and biomechanical factors.
4.2
Approaches to Quantitative Analysis
of Mechanical Factors in Migrating Cells
Mechanical factors such as strain and tension release play important roles in the regu-
lation of actin network dynamics, and therefore cell migration. The most common
approach to analyze the effect of mechanical forces involve application of a mechani-
cal stimuli, for instance, cyclic strain and observing cell response such as stress fiber
realignment. A more quantitative approach for evaluating intracellular mechanical
factors involves using a combination of FSM and PIV to track intracellular dynamics
of the actin structure and map out its deformation. Using this approach, F-actin flow
in the lamellipodial fragments formed from fish keratocytes has been determined and
the flow found to be centripetally oriented with a decreasing flow magnitude from the
front to rear. This flow pattern suggests that the actin network in migrating cells
undergoes compressive deformation during cell migration.
