Poly(vinyl alcohol)/κ-Carrageenan-based hydrogels enriched with the adhesive mussel protein Pvfp5β as 3D cell culture scaffold for tissue engineering applications

Abstract

Many marine organisms such as sandcastle worms, barnacles and mussels, produce natural adhesives to attach to wet surfaces in aqueous tidal environments. In mussels, the adhesion is possible through the secretion of a protein-based water-resistant glue, composed of a mixture of proteins called mussel adhesive proteins (MAPs) or mussel foot proteins (mfps), that allow anchoring to almost any kind of surface in wet conditions [1]. The proteins confined to adhesive plaques are mfp-2, -3, -4, -5, and -6. All these proteins contain an atypically high concentration of the catecholic amino acid 3,4- dihydroxy-l-phenylalanine (DOPA), obtained by the post-translational enzymatic hydroxylation of tyrosine (Tyr) [2]. DOPA is the key molecule allowing the underwater mussel adhesion to different surfaces through the formation of irreversible covalent and reversible noncovalent bonds [3]. However, growing evidences show that the presence of DOPA is not a sufficient condition to generate strong underwater adhesion. A synergistic effect of catechols and adjacent lysine in bioadhesion of mussels has been suggested. The pair structure of Tyr/DOPA with basic residues of lysine, rather than the post-translational modification of Tyr to DOPA, should be responsible for the strong binding ability of mussel adhesive proteins [4]. We have recently fine-tuned the expression of the recombinant Pvfp5β protein in Escherichia coli [5]. Furthermore, the structural characterization showed that the produced protein was correctly folded as a β-rich protein with precise pairing of the sulfur bridges. We have also demonstrated that surfaces coated with recombinant Pvfp5β are not toxic, and improve cell adhesion, proliferation and spreading of NIH-3T3 and HeLa cell lines [5]. Thus, adhesive protein/peptide coatings and efficient exposure of bioactive domains might improve the biological properties of scaffold materials. Hydrogels, a unique class of polymeric materials, are three-dimensional porous networks constituted by polymeric chains crosslinked by chemical and/or physical bonds. They exhibit remarkable structure-derived properties, including high surface area, stimuli-responsiveness, inherent flexibility, controllable mechanical strength, and excellent biocompatibility. They are capable of absorbing and retaining large amounts of water in their networks and can extensively swell without dissolution due to the presence of crosslinks, that are at the basis of the network structure, and maintain macroscopic integrity [6]. k-carrageenan (kCar) is a naturally occurring polysaccharide extracted from marine red algae (Rhodophyceae). Chemically it consists of an alternating linear chain of (1->3)-β-D-galactose-4SO3- - (1->4)-3,6, anhydro-α-D-galactose. Due to its half-ester sulphate moieties, it is a strong anionic polymer and resembles the natural glycosaminoglycans (GAGs), which are an important component of the connective tissue. kCar is biocompatible, biodegradable, non-toxic, and gel-forming. Thus, kCar has been investigated for a wide range of biomedical puropses, such as wound dressing [7] and tissue engineering [8] applications. The generally accepted model of the gelling process of carrageenan solutions involves a coil-to-helix transition, followed, in the presence of certain cations, by aggregation of double helices to form a stiff, extended network [9]. Blending kCar with polyvinyl alcohol (PVA) allows obtaining composite scaffolds with higher, interconnected porosity, high swelling degree, improved toughness and degradability. PVA can be chemically crosslinked, with glutaraldehyde, or physically crosslinked by inducing the formation of crystalline domains via freeze-thawing [10]. In the present study, we used the crosslinking-agent free, freeze–thaw approach for the fabrication of PVA/kCar composite hydrogels as 3D scaffolds for cell culture, whose adhesion was enhanced by simple adsorption of