Adhesion
Dorsal stress fibers in motile cells are formed from actin filament bundles that are initiated and extended from cell-substrate adhesions at the leading edge (aka focal complexes [FXs]) (reviewed in [1]).

Soluble clues or binding of cell surface receptors (e.g., integrin) to the substratum components (e.g., fibronectin) promotes clustering of adhesion receptors and subsequent activation of the Rho family of GTPases (e.g., Rac1). Rac1 activity strengthens the nascent adhesion by promoting the assembly of scaffolding proteins and actin-binding proteins to the adhesion site (figure adapted from [16651381])
Initiation
Initiation is triggered by the activation of adhesion molecules, such as integrin, and the tension-dependent recruitment and activation of the Rho family of GTPases, including Rac1 (reviewed in [9][10]).

Rho GTPases promote signal transduction cascades and interact directly with components of the actin polymerizing module to initiate actin filament assembly. Actin filaments in dorsal stress fibers are primarily initiated by formin-mediated nucleation (e.g., mDia1) (figure adapted from [16651381]).
The short unipolar actin filaments in dorsal stress fibers are polymerized and elongated from focal adhesions via a formin-dependent mechanism (e.g., mDia1/DRF1) [14]. This leaves the barbed ends of the actin filaments directed towards the adhesion (reviewed in [15]). The enzymatic activity of Rac, Rho, and their effectors (e.g., ROCK) is necessary to recruit formins to focal adhesions, where they are thought to play a vital role in creating a stable pool of cortical actin and in maintaining free filament barbed ends [16][17][18].
The elasticity of formins at focal adhesions may be tied to their mechanosensing ability, as suggested by increased force-induced actin polymerization at these adhesive sites [19]. Several studies also show that actin filament bundles in filopodia can serve as precursors of dorsal stress fibers or graded polarity bundles [20][2]. Certain groups that did not observe actin polymerization during stress fiber formation have suggested that stress fibers may form as a result of F-actin stabilization brought about by filament bundling and merging [8].
Assembly
Stress fiber assembly at focal adhesions is thought to occur by a general mechanism irrespective of the cell type [21].

The nascent adhesions continue to grow and exchange components. Nascent unipolar actin filaments are extended and they are stabilized by actin binding proteins (e.g., α-actinin, filamin). The population of filaments are crosslinked into bundles and they steadily exchange actin subunits as they move towards the cell body due to retrograde actin flow and actin treadmilling. The orientation and polarity of the filaments may become mixed as they are transported (figure adapted from [16651381]).
Similar to other cortical actin networks that are composed of actin bundles (e.g., microvilli, stereocilia), the elongation of filaments from the base of the adhesion would presumably contribute to the retrograde flow of actin away from the adhesion and promote the condensation of the dorsal fiber into larger bundles as they are moved towards the cell body [14][23][24]. The filament orientation may also change despite the adhesion site remaining stationary relative to the substrate [4]. Dorsal stress fibers can also be connected to transverse arcs in the cell body where the transverse arcs filaments are suggested to continuously supply the dorsal stress fibers with actin filaments [25]. Precisely how this is achieved, is unknown (reviewed in [26]); however, it likely involves actin binding proteins and myosin motor activity.
Incorporation
Interestingly, the assembly of dorsal stress fibers and transverse arcs appears to be connected: transverse arcs encounter dorsal stress fibers as they are transported towards the cell body via cell-wide actin-myosin contractions [14].

Dorsal stress fibers (DSF) appear to contact transverse arcs at their proximal ends towards the cell body. DSF are thought to ‘feed’ the transverse arcs with filaments of mixed polarity. Although myosin II bundles are generally absent from DSF, occasionally they will displace α-actinin and become incorporated towards the proximal end of DSF (figure adapted from [1869506]).
Although transverse arcs are not connected directly to focal adhesions, the contractile tension generated by transverse arcs presumably can be transmitted to the cell surface and transferred to the substrate through the dorsal stress fibers; whether this predicted force transmission actually leads to extracellular remodeling, is relatively unexplored.
- What is the cytoskeleton?
- What are actin filaments?
- What are microtubules?
- What are intermediate filaments?
- What are stress fibers?
- What is the function of stress fibers?
- What are the steps in the formation of transverse arcs?
- What are the steps in ventral stress fiber formation?
- How is stress fiber assembly regulated?
- How do the mechanical properties of cells change with respect to substrate rigidity?steve2018-02-19T11:09:32+08:30
How do the mechanical properties of cells change with respect to substrate rigidity?
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