Receptor-binding domain of SARS-CoV-2 is a functional αv-integrin agonist.

Among the novel mutations distinguishing SARS-CoV-2 from similar coronaviruses is a K403R substitution in the receptor-binding domain (RBD) of the viral spike (S) protein within its S1 region. This amino acid substitution occurs near the angiotensin-converting enzyme 2-binding interface and gives rise to a canonical RGD adhesion motif that is often found in native extracellular matrix proteins, including fibronectin. Here, the ability of recombinant S1-RBD to bind to cell surface integrins and trigger downstream signaling pathways was assessed and compared with RGD-containing, integrin-binding fragments of fibronectin. We determined that S1-RBD supported adhesion of fibronectin-null mouse embryonic fibroblasts as well as primary human small airway epithelial cells, while RBD-coated microparticles attached to epithelial monolayers in a cation-dependent manner. Cell adhesion to S1-RBD was RGD dependent and inhibited by blocking antibodies against αv and ß3 but not α5 or ß1 integrins. Similarly, we observed direct binding of S1-RBD to recombinant human αvß3 and αvß6 integrins, but not α5ß1 integrins, using surface plasmon resonance. S1-RBD adhesion initiated cell spreading, focal adhesion formation, and actin stress fiber organization to a similar extent as fibronectin. Moreover, S1-RBD stimulated tyrosine phosphorylation of the adhesion mediators FAK, Src, and paxillin; triggered Akt activation; and supported cell proliferation. Thus, the RGD sequence of S1-RBD can function as an αv-selective integrin agonist. This study provides evidence that cell surface αv-containing integrins can respond functionally to spike protein and raises the possibility that S1-mediated dysregulation of extracellular matrix dynamics may contribute to the pathogenesis and/or post-acute sequelae of SARS-CoV-2 infection.

Receptor binding domain of SARS-CoV-2 is a functional αv-integrin agonist.

Among the novel mutations distinguishing SARS-CoV-2 from similar respiratory coronaviruses is a K403R substitution in the receptor-binding domain (RBD) of the viral spike (S) protein within its S1 region. This amino acid substitution occurs near the angiotensin-converting enzyme 2 (ACE2)-binding interface and gives rise to a canonical RGD adhesion motif that is often found in native extracellular matrix proteins, including fibronectin. In the present study, the ability of recombinant S1-RBD to bind to cell surface integrins and trigger downstream signaling pathways was assessed and compared to RGD-containing, integrin-binding fragments of fibronectin. S1-RBD supported adhesion of both fibronectin-null mouse embryonic fibroblasts as well as primary human small airway epithelial cells. Cell adhesion to S1-RBD was cation- and RGD-dependent, and was inhibited by blocking antibodies against α v and ß 3 , but not α 5 or ß 1 , integrins. Similarly, direct binding of S1-RBD to recombinant human α v ß 3 and α v ß 6 integrins, but not α 5 ß 1 integrins, was observed by surface plasmon resonance. Adhesion to S1-RBD initiated cell spreading, focal adhesion formation, and actin stress fiber organization to a similar extent as fibronectin. Moreover, S1-RBD stimulated tyrosine phosphorylation of the adhesion mediators FAK, Src, and paxillin, Akt activation, and supported cell proliferation. Together, these data demonstrate that the RGD sequence within S1-RBD can function as an α v -selective integrin agonist. This study provides evidence that cell surface α v -containing integrins can respond functionally to spike protein and raise the possibility that S1-mediated dysregulation of ECM dynamics may contribute to the pathogenesis and/or post-acute sequelae of SARS-CoV-2 infection.

Roadmap of molecular, compositional, and functional markers during embryonic tendon development.

Tendon is a specialized connective tissue that connects muscle to bone, thereby enabling musculoskeletal movement. Tendon injury leads to formation of tissue with aberrant functional properties. Current approaches to treat tendon injuries, including surgical repair and tissue engineering, have not achieved normal tendon. A roadmap of markers could help with identifying when mis-steps occur during aberrant tendon formation and providing instructions for normal tendon formation. We propose this roadmap should be based on the embryo-the perfect model of tissue formation. Our prior studies have shown that adult mesenchymal stem cells mimic tendon progenitor cell behavior when treated with tendon developmental cues. Although transcription factors and extracellular matrix molecules are commonly used to assess tendon development, we have shown that these markers do not reliably reflect functional property elaboration. Thus, evaluating tendon formation on the basis of a combination of these molecular, compositional, and functional markers is important. In this review, we highlight various tendon markers with focus on their temporal profiles and roles in tendon development to outline a roadmap that may be useful for informing tendon healing and tissue engineering strategies.

Embryo movements regulate tendon mechanical property development.

Tendons transmit forces from muscles to bones to enable skeletal motility. During development, tendons begin to bear load at the onset of embryo movements. Using the chick embryo model, this study showed that altered embryo movement frequency led to changes in elastic modulus of calcaneal tendon. In particular, paralysis led to decreased modulus, whereas hypermotility led to increased modulus. Paralysis also led to reductions in activity levels of lysyl oxidase (LOX), an enzyme that we previously showed is required for cross-linking-mediated elaboration of tendon mechanical properties. Additionally, inhibition of LOX activity abrogated hypermotility-induced increases in modulus. Taken together, our findings suggest embryo movements are critical for tendon mechanical property development and implicate LOX in this process. These exciting findings expand current knowledge of how functional tendons form during development and could guide future clinical approaches to treat tendon defects associated with abnormal mechanical loading in uteroThis article is part of the Theo Murphy meeting issue 'Mechanics of development'.