Molecular stretching modulates mechanosensing pathways

Abstract

For individual cells in tissues to create the diverse forms of biological organisms, it is necessary that they must reliably sense and generate the correct forces over the correct distances and directions. There is considerable evidence that the mechanical aspects of the cellular microenvironment provide critical physical parameters to be sensed. How proteins sense forces and cellular geometry to create the correct morphology is not understood in detail but protein unfolding appears to be a major component in force and displacement sensing. Thus, the crystallographic structure of a protein domain provides only a starting point to then analyze what will be the effects of physiological forces through domain unfolding or catch-bond formation. In this review, we will discuss the recent studies of cytoskeletal and adhesion proteins that describe protein domain dynamics. Forces applied to proteins can activate or inhibit enzymes, increase or decrease protein-protein interactions, activate or inhibit protein substrates, induce catch bonds and regulate interactions with membranes or nucleic acids. Further, the dynamics of stretch-relaxation can average forces or movements to reliably regulate morphogenic movements. In the few cases where single molecule mechanics are studied under physiological conditions such as titin and talin, there are rapid cycles of stretch-relaxation that produce mechanosensing signals. Fortunately, the development of new single molecule and super-resolution imaging methods enable the analysis of single molecule mechanics in physiologically relevant conditions. Thus, we feel that stereotypical changes in cell and tissue shape involve mechanosensing that can be analyzed at the nanometer level to determine the molecular mechanisms involved.

Keywords: bioimaging; dSTORM; localization microscopy; mechanobiology; mechanoenzymatics; mechanosensing; molecular forces; protein stretching; single molecule.

Figure 1

(A) Illustration of the domain structure of full‐length talin. The talin head domain contains a FERM domain (50 kDa), followed by a flexible “neck” (10 kDa) which connects the head domain to its C‐terminal rod domain (220 kDa). The rod domain contains 11 cryptic VBS (drawn in blue). The dimerization domain is a single helix that sits at the end of the rod. (B) Schematics of the talin structure and interaction of the talin dimer with vinculin in cells. A. Illustration of the domain structure of full‐length talin. Talin head domain contains a FERM domain (50 kDa), followed by a flexible “neck” (10 kDa), which connects the head domain to its C‐terminal rod domain (220 kDa). The rod domain contains 11 cryptic VBS (drawn in blue). The dimerization domain is a single helix that sits at the end of the rod domain. B. (left) In the initial stage of FA formation, the talin dimer binds to actin and integrin. At this stage, the cryptic VBSs remain buried among the α‐helical bundles. (right) As the actin filament starts to pull on talin, the formerly buried VBS are revealed to allow vinculin binding, and cause more actin filament recruitment.

Figure 2

Force extension curves of 13 helical bundles of talin rod domain. Magnetic tweezers were used to develop a constant rate of force increase (3.8 pN/s) to a full length talin rod domain molecule. The R3 domain unfolds first at ∼5 pN and the rest of the talin rod domains unfold at forces between 10 to 25 pN.