Resumen de Harnessing protein nanomechanics to engineer hydrogel viscoelasticity
Biomaterials are dynamic tools with many applications: from the primitive use of bone and wood in the replacement of lost limbs and body parts, to the refined involvement of smart and responsive biomaterials in modern medicine and biomedical sciences. Biomaterial is a very broad term, and it includes every kind of material that can be incorporated into the human body to enhance or replace specific functionalities. Hydrogels constitute a subtype of biomaterials built from water-swollen polymer networks. Their large water content and soft mechanical properties are highly similar to most biological tissues, making them ideal for tissue engineering and biomedical applications. The mechanical properties of hydrogels and their modulation have attracted a lot of attention from the field of mechanobiology, which studies the crosstalk between cells and mechanical forces.
The extracellular matrix (ECM) is a three-dimensional network that works as one of the main sources of mechanical inputs for cells. Due to its composition and water percentage the ECM can be mimicked by hydrogels in vitro; thus, getting rid of its complexity and variability. Mechanical modulation of hydrogels employed as ECM mimics can be achieved through different strategies, including polymer concentration and crosslinking density. Following these approaches, the scientific community has discovered the key role of substrate stiffness in cell biology, and how alterations in this parameter can lead to pathological transformation. However, these studies face two challenges. First, although substrate stiffness is indeed an essential mechanical factor, the behavior of the ECM is more complex and includes non-linear elasticity and viscoelasticity (e.g. energy dissipation). Second, the strategies employed to achieve mechanical regulation are typically coupled to non-mechanical parameters. This concomitant modulation hinders the understanding of the mechanisms underlying purely mechanical crosstalk between cells and the ECM.
In this thesis, we have engineered protein-based hydrogels that overcome both limitations. Due to the presence of folded tertiary structures in proteins, unfolding/refolding transitions efficiently dissipate energy. Therefore, different proteins provide hydrogels with different degrees of viscoelasticity. We have then employed Post-Translational Modifications (PTMs), point mutations, random coil proportion and force anisotropy as strategies aimed at mechanical modulation of hydrogels, built with the well-characterized I91 domain of titin. Change in the proportion of random coils and the pulling geometry applied to protein building blocks effectively modulates hydrogel viscoelasticity, while preserving non-mechanical parameters. These hydrogels can be used in cell mechanobiology studies and we have shown how the activity of the mechanotransductor YAP depends on hydrogel viscoelasticity. We foresee the application of our protein-based hydrogels in tissue and biomedical engineering
