25.7: Cell Migration
Cell migration, the process by which cells move from one location to another, is essential for the proper development and viability of organisms throughout their life. When cells are not able to migrate properly to their ordained locations, various disorders may occur. For example, disruption in cell migration causes chronic inflammatory diseases such as arthritis.
Generally, cellular migration begins when a cell, such as a fibroblast, responds to an external-polarizing-chemical signal. As a result, one end extends itself as a protrusion called the leading edge, which attaches itself to substrates via secreted adhesive compounds, in its microenvironment. The trailing edge—the area that serves as the back of the cell—also adheres to substrates to anchor the cell. After adhesion, the cell is propelled towards its destination by a sequence of contractions that are generated by cytoskeletal motility structures. Then, the adhesive attachment at the trailing edge gets released. These steps are repeated cyclically until the fibroblast reaches its destination.
There is a diversity in the different types of signaling molecules that initiate cell migration. They illicit two types of responses: chemokinetic and chemotactic. Chemokinesis refers to movement that occurs when signaling molecules either symmetrically or asymmetrically stimulate cell migration without dictating the directionality of the resultant movement. Chemotaxis refers to a movement where a gradient of soluble (chemotactic) or substrate-bound (haptotactic) signaling molecules dictates the directionality of cellular movement.
Membrane receptors such as G-protein coupled receptors (GPCR) and receptor tyrosine kinase receptors (RTK) detect external signaling molecules and cause an accumulation of phosphatidylinositol (3,4,5) triphosphate (PIP3) at the leading edge. The accumulation of PIP3 then leads to the activation of Rho-family Ras-like small proteins called Rac, Cdc42, and Rho. Rac and/or Cdc42 cause cytoskeletal changes such as actin polymerization at the leading edge while Rho causes actin-myosin contractions at the trailing edge. As a result of actin polymerization, protrusions are generated at the leading edge.
Types of Protrusions
Actin serves as a physical scaffold for protrusions. Consequently, the shape of protrusion structures varies depending on how actin is assembled. Two commonly studied types of protrusions are lamellipodia and filopodia. Lamellipodia are broad, sheet-like protrusions that contain a branched network of thin, short actin filaments. When lamellipodia lift away from the substrate and move backward, a notably distinct ruffling movement occurs. Lamellipodia protrusions can be found in cells like fibroblasts, immune cells, and neurons. Filopodia are thin-finger-like protrusions that emanate from cell membranes. They are often observed in cells, such as neurons, working in tandem with lamellipodia during migration.