polymers Review Proteins and Peptides as Important Modifiers of the Polymer Scaffolds for Tissue Engineering Applications—A Review Katarzyna Klimek * and Grazyna Ginalska Chair and Department of Biochemistry and Biotechnology, Medical University of Lublin, Chodzki 1 Street, 20-093 Lublin, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-81-448-7028; +48-81-448-7020 Received: 29 January 2020; Accepted: 2 April 2020; Published: 6 April 2020 Abstract: Polymer scaffolds constitute a very interesting strategy for tissue engineering. Even though they are generally non-toxic, in some cases, they may not provide suitable support for cell adhesion, proliferation, and differentiation, which decelerates tissue regeneration. To improve biological properties, scaffolds are frequently enriched with bioactive molecules, inter alia extracellular matrix proteins, adhesive peptides, growth factors, hormones, and cytokines. Although there are many papers describing synthesis and properties of polymer scaffolds enriched with proteins or peptides, few reviews comprehensively summarize these bioactive molecules. Thus, this review presents the current knowledge about the most important proteins and peptides used for modification of polymer scaffolds for tissue engineering. This paper also describes the influence of addition of proteins and peptides on physicochemical, mechanical, and biological properties of polymer scaffolds. Moreover, this article sums up the major applications of some biodegradable natural and synthetic polymer scaffolds modified with proteins and peptides, which have been developed within the past five years. Keywords: bioactive construct; biocompatibility; biomolecules; cytotoxicity; ECM; hydrogels; protein carrier; regenerative medicine; stem cells; tissue repair 1. Introduction: The Role of Proteins and Peptides in TE Tissue engineering (TE) is a multidisciplinary field, which constitutes an alternative and promising approach for grafts, i.e., autografts, allografts, and xenografts [1–3]. Thus, TE focuses on providing appropriate solutions, which enable repair or substitute of damaged skin, cartilage, bone, nerves, bladder, blood vessels, or heart valves [4,5]. The classical TE involves scaffolds, cells, and bioactive molecules alone or the association of these three elements. The combination of scaffold, cells, and bioactive molecules—a bioactive construct—is currently considered as the best option for tissue repair and regeneration [5–8]. The main assumptions of the classical TE are presented in Figure1. Suitable scaffolds play a pivotal role in the classical TE strategy. Scaffolds are three-dimensional (3D) matrices, which mimic native extracellular matrix (ECM) to support cell growth. Therefore, they should be biocompatible, biodegradable at the desired rate, porous, and mechanically stable [6,9–11]. Considering these requirements, the scaffolds based on natural and synthetic polymers alone, as well as composites, exhibit the highest biomedical potential [12–14]. Among known biodegradable natural polymers, collagen, gelatin, chitosan, elastin, hyaluronic acid (HA), and silk fibroin are commonly used for TE purposes [15,16]. In turn, poly-"-caprolactone (PCL), polylactic acid (PLA), poly-glycolic acid (PGA), and poly(lactic-co-glycolic acid) (PLGA) have attracted great attention in the group of biodegradable synthetic polymers [17,18]. Nevertheless, both natural and synthetic polymers have advantages and drawbacks. For instance, natural polymers are usually biocompatible, but exhibit poor Polymers 2020, 12, 844; doi:10.3390/polym12040844 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 37 Polymers 2020, 12, 844 2 of 38 synthetic polymers have advantages and drawbacks. For instance, natural polymers are usually biocompatible, but exhibit poor mechanical properties, whereas synthetic ones are generally mechanicalcharacterized properties, by good whereas mechanical synthetic strength ones are and generally relatively characterized low biocompatibility by good mechanical [1,4,10,11,19]. strength andBiocompatibility relatively low is biocompatibilityone of the most [important1,4,10,11,19 featur]. Biocompatibilityes of scaffolds, isbecause one of it the allows most for important proper featuresinteractions of sca betweenffolds, becausecells and it scaffold. allows for It means proper that interactions biocompatible between biomaterials cells and scaareff non-toxicold. It means and thatthey biocompatibleenable excellent biomaterials cell adhesion, are non-toxicspreading, and proliferation, they enable and excellent differentiation cell adhesion, [10,13,18,19]. spreading, In proliferation,some cases, the and polymer differentiation scaffolds [10 do,13 ,not18, 19exhibit]. In some toxic cases, effect, the but polymer the properties scaffolds of do their not surface exhibit toxic(e.g., etopography,ffect, but the wettability, properties ofcharge) their surfacedo not (e.g.,promote topography, cell adhesion, wettability, proliferation, charge) doand not differentiation. promote cell adhesion,Such biomaterials proliferation, should and be di ffmodifiederentiation. (e.g., Such with biomaterials bioactive shouldmolecules) be modified in order (e.g., to withincrease bioactive their molecules)biocompatibility in order [6,11,20,21]. to increase their biocompatibility [6,11,20,21]. Figure 1.1. TheThe classicalclassical TETE paradigmparadigm includingincluding scascaffolds,ffolds, cells,cells, andand bioactivebioactive molecules.molecules. These three elements may be used alonealone oror inin combination.combination. TheirTheir association, known as a “bioactive construct”, currently makes upup thethe mostmost successfulsuccessful strategystrategy forfor tissuetissue repairrepair andand regeneration.regeneration. The choice ofof appropriate scascaffoldsffolds (e.g., polymer-based),polymer-based), cells (e.g., primarily stem cells) as well asas moleculesmolecules (especially(especially proteinsproteins andand peptides) peptides) is is crucial crucial to to carry carry out out an an auspicious auspicious therapy therapy and and tightly tightly depends depends on futureon future applications. applications. Apart fromfrom sca scaffolds,ffolds, cells cells also also take take an important an important place inplace classical in classical TE paradigm. TE paradigm. To select To adequate select kindadequate of cells, kind practical, of cells, ethical, practical, and ethical, biological and aspects biological need toaspects be included. need to Recently, be included. it is considered Recently, thatit is adultconsidered mesenchymal that adult stem mesenchymal cells (MSCs) stem constitute cells (MSC a “golds) standard”constitute fora “gold regenerative standard” medicine for regenerative [19,22,23]. Givenmedicine available [19,22,23]. sources Given of available MSCs, they sources are mainly of MSCs, isolated they fromare mainly bone marrowisolated (BMSCs)from bone as marrow well as adipose(BMSCs) tissue as well (ASCs) as adipose [24–26]. tissue The MSCs (ASCs) possess [24–26] capacity. The MSCs to self-renew possess and capacity differentiate to self-renew into various and cells,differentiate such as adipocytes,into various chondrocytes, cells, such fibroblasts,as adipocytes, myocytes, chondrocytes, and osteoblasts fibroblasts, [25,26 ].myocytes, In the ex vivoand strategy,osteoblasts including [25,26]. interIn the alia ex fabricationvivo strategy, of bioactive including construct, inter alia the fabrication isolated cellsof bioactive need suitable construct, support the forisolated growth cells (sca needffold) suitable and biological support signalsfor growth (mainly (scaffold) growth and factors), biological which signals promote (mainly their growth,growth proliferation,factors), which and promote differentiation their grow intoth, desired proliferation, tissue type and [25 ,differentiation27–29]. into desired tissue type [25,27–29].The bioactive molecules—constituting the third pillar in classical TE—have a great influence on both scaTheff bioactiveold properties molecules—constituting and cell behavior. From the thethird TE pi pointllar in of classical view, bioactive TE—have molecules a greatare influence biological on factorsboth scaffold or signals properties that improve and propertiescell behavior. of sca Fromffolds, the support TE point cellular of activities,view, bioactive and as amolecules consequence are leadbiological to better factors and or intensive signals that regeneration improve ofproperties specific tissueof scaffolds, [6,18,19 support]. Thus, cellular scaffold activities, enrichment and with as a bioactiveconsequence molecules lead to primarily better and promotes intensive its regeneration biocompatibility, of specific by leading tissue to [6,18,19]. increased Thus, cell adhesion, scaffold proliferation,enrichment with and dibioactivefferentiation. molecules It is worth primarily noting thatpromotes proteins, its especially biocompati growthbility, factors by leading (GFs), areto currentlyincreased onescell adhesion, of the most proliferation, applied bioactive and differentiat moleculesion. in classical It is worth TE [noting9,18,19 that,30– 32proteins,]. Besides especially growth factors,growth ECMfactors proteins, (GFs), are adhesive currently peptides, ones of hormones, the most cytokines,applied bioactive or some molecules enzymes havein classical favorable TE influence on scaffold biocompatibility [6,9,18,19,33]. Polymers 2020, 12, 844 3 of 38 Given the high significance of bioactive molecules in classical