What is the first step in focal adhesion assembly?2017-12-26T14:22:02+08:30

What is the first step in focal adhesion assembly?

Focal adhesion formation is initiated upon the binding of adhesion receptors to extracellular matrix (ECM) ligands (e.g. fibronectin, vitronectin, collagen) along the cell periphery usually at the protruding edge of a cell. Both intracellular and extracellular factors can influence the level of matrix binding, in terms of affinity (the strength of interactions, reviewed in [1][2]) and avidity (the number of interactions, such as lateral interactions between independently activated proteins within a focal adhesion). Nascent focal adhesions first appear exclusively in the lamellipodium as submicron-sized puncta that are typically immobile but can at times travel short distances along the direction of the actin retrograde flow [3][4].

As the primary ECM receptor in the focal adhesions, integrins are heterodimeric transmembrane proteins, with large multidomain extracellular portions and small cytoplasmic tails. Although there are many different types of integrins with specificity to different ECM, a major portion of cellular and biophysical studies have focused on fibronectin-binding α5β1 and αvβ3 integrins. β1 integrins have been shown to exhibit catch bond behavior[5][6] and function as the force-bearing component. In this capacity β1 integrins maintain adhesion strength despite fluctuating matrix forces, which can often change quite rapidly. However, it is unclear whether β1 integrins bind matrix molecules at the leading edge and translocate inwards [7][8] or get recruited at later stages [9][10]. The less stable β3 integrins are responsible for initiating mechanotransduction and reinforcing focal adhesion attachments to the ECM, in complex with talin [11]. Existence of a phosphorylation-dependent crosstalk between the two integrin types during migration has also been reported [12].

Upon binding of ligands by integrins and clustering, a number of signal transduction cascades are activated. One key event is Rac1 activation and consequent phosphoinositide production [13][14], which leads to the recruitment of the talin homodimer [15][16]. Recent studies provide evidence that anchoring of talin at focal adhesions also requires F-actin and vinculin [17]. This is followed by integrin recruitment and activation upon fibronectin binding [18]. The talin-mediated linkage to the actin cytoskeleton serves to stabilize the integrin-ECM bonds [7]. As talin is pulled by the moving actin, it either stretches leading to rearward translocation of β1 integrins and unfolding of fibronectin [19] or results in frequent slip bonds of 2 pN on immobilized β3 integrins [20][21]. Thus, a dynamic nanoscale organization of integrins exists inside focal adhesions, determined by their extracellular domains [18]. Further, this study also reports a distinct dynamics of integrins within focal adhesions, where they constantly alternate between ligand-bound activated and unbound inactivated states, which is thought to confer the adaptability to focal adhesions in order to withstand rapid changes in force [22].

Talin is one of the best characterized focal adhesion proteins that play important roles during focal adhesion initiation. In addition to its binding site to integrin cytoplasmic tails that can activate integrin (reviewed in [1]), talin can also bind directly to actin and associate with numerous cytoskeletal and signaling proteins (reviewed in [23]), effectively forming one of the core cell-ECM mechanotransduction units.

With the integrin considered ‘engaged’, the adhesion complexes are capable of exerting low level forces to fibronectin, leading to a cascading series of events:

  1. The rearrangement of ECM ligand domains to adjust the length of talin
  2. The strengthening of the talin-actin slip bond [20]
  3. The slowing down of the actin retrograde flow which also helps to prevent the disintegration of the nascent adhesion complexes [3].

In addition to integrins, syndecan-4 also plays an essential role [24][25], binding different domains of matrix proteins and eliciting cooperative signals (reviewed in [26]). Several membrane proteins are also believed to be interacting with integrins, likely as co-receptors, although their functions in cell migration and mechanotransduction remain not well understood (reviewed in [27]).

It should be noted that nascent adhesions do not uniformly undergo these sequential growth. Rather, shear forces generated by actin retrograde flow result in the disassembly of certain fraction of nascent adhesions, and only a subset of nascent adhesions survive to proceed into later stages.

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