# Recombinant Proteins-Based Strategies in Bone Tissue Engineering
## 8 Recombinant Peptides Targeting Integrins  
Integrins are a family of heterodimeric transmembrane glycoproteins that are known to mediate cell-cell and cell-matrix interactions. The path of integrin research applied to bone tissue regeneration has just begun and is very promising, because the interaction of \(\alpha 2\beta 1\) integrin with collagen I is a crucial signal for osteoblastic differentiation and mineralization. Integrins may mediate the differing interactions of cells of the osteoblast and osteoclast lineages with the matrix of bone [81]. [PERSON] et al. [52] developed a protease-degradable poly(ethylene glycol) (PEG) synthetic hydrogel, functionalized with a triple helical \(\alpha 2\beta 1\) integrin-six aminoacid-peptide (GFOGER) as a BMP-2 delivery scaffold. First, the research engineered and produced a synthetic collagen I-mimetic-containing peptide, GGYGGGP(GPP)\({}_{5}\)GFOGER(GPP)\({}_{5}\)GPC, which recapitulates the triple helical structure of native collagen and binds \(\alpha 2\beta 1\) integrin with high affinity and specificity. Then, rhBMP2 was linked to this construct using GCRDVPMSMRGGDRCG (VPM) as a cross-linker peptide. In a murine non-healing radial bone defect, this composite hydrogel promoted osteoprogenitor cell recruitment to the defect site and produced robust repair and bone bridging. Thus, this type of hydrogel displayed intrinsic osteogenic activity, underwent rapid degradation in vivo, and bridged a critical-sized bone defect at low BMP-2 doses, which is of the utmost importance.  
More advanced studies on recombinant integrins were performed, such as computational simulations for silk-silica-integrin binding, which showed activation of \(\alpha\)V\(\beta 3\) integrin in contact with silica. This integrated experimental and computational research provides novel insights into interactions that regulate osteogenesis towards more efficient biomaterial designs [82]. Recently, these ideas were reinforced by stating that integrin-functionalized hydrogels might modulate the in vitro behavior of hMSCs. Thus, it was demonstrated that integrin-specific hydrogels regulate hMSC adhesion, paracrine signaling, and osteoblastic differentiation in vitro. Furthermore, hydrogels presenting previously named six aminoacid peptide GFOGER prolong hMSC survival and engraftment in a segmental bone defect, resulting in improved bone repair [83]. In a recent work, [PERSON] et al. [84] compared the bioactivity of a cyclic peptide, cRRETAWA, which targets \(\alpha 5\beta 1\)integrins, with those of the widely used RGDS motif that binds to many different integrins in hMSC cultures, and found its high efficiency in inducing hMSC osteogenic differentiation.  
The ligands which integrins recognize are conserved amino acid sequences found in many components within the extracellular matrix. The clustering of integrins has a large impact on the downstream cellular effects, increasing signalling effectiveness, in times independent of the global density of ligand present [85]. The use of recombinant integrins as the basis for new scaffolds is intended to address cellular processes to improve bone tissue regeneration. However, since the in vivo concentrations of ligands that enable cluster-enhanced integrin signaling are variable, much research remains to be done to apply this concept to scaffolds applied to bone tissue regeneration in pre-clinical models [86].