Supramolecular Modification of a Sequence-Controlled Collagen-Mimicking Polymer.

Structurally and functionally well-defined recombinant proteins are an interesting class of sequence-controlled macromolecules to which different crosslinking chemistries can be applied to tune their biological properties. Herein, we take advantage of a 571-residue recombinant peptide based on human collagen type I (RCPhC1), which we functionalized with supramolecular 4-fold hydrogen bonding ureido-pyrimidinone (UPy) moieties. By grafting supramolecular UPy moieties onto the backbone of RCPhC1 (UPy-RCPhC1), increased control over the polymer structure, assembly, gelation, and mechanical properties was achieved. In addition, by increasing the degree of UPy functionalization on RCPhC1, cardiomyocyte progenitor cells were cultured on "soft" (∼26 kPa) versus "stiff" (∼68-190 kPa) UPy-RCPhC1 hydrogels. Interestingly, increased stress fiber formation, focal adhesions, and proliferation were observed on stiffer compared to softer substrates, owing to the formation of stronger cell-material interactions. In conclusion, a bioinspired hydrogel material was designed by a combination of two well-known natural components, i.e., a protein as sequence-controlled polymer and UPy units inspired on nucleobases.

Follistatin Effects in Migration, Vascularization, and Osteogenesis in vitro and Bone Repair in vivo.

The use of biomaterials and signaling molecules to induce bone formation is a promising approach in the field of bone tissue engineering. Follistatin (FST) is a glycoprotein able to bind irreversibly to activin A, a protein that has been reported to inhibit bone formation. We investigated the effect of FST in critical processes for bone repair, such as cell recruitment, osteogenesis and vascularization, and ultimately its use for bone tissue engineering. In vitro, FST promoted mesenchymal stem cell (MSC) and endothelial cell (EC) migration as well as essential steps in the formation and expansion of the vasculature such as EC tube-formation and sprouting. FST did not enhance osteogenic differentiation of MSCs, but increased committed osteoblast mineralization. In vivo, FST was loaded in an in situ gelling formulation made by alginate and recombinant collagen-based peptide microspheres and implanted in a rat calvarial defect model. Two FST variants (FST288 and FST315) with major differences in their affinity to cell-surface proteoglycans, which may influence their effect upon in vivo bone repair, were tested. In vitro, most of the loaded FST315 was released over 4 weeks, contrary to FST288, which was mostly retained in the biomaterial. However, none of the FST variants improved in vivo bone healing compared to control. These results demonstrate that FST enhances crucial processes needed for bone repair. Further studies need to investigate the optimal FST carrier for bone regeneration.

Novel In Situ Gelling Hydrogels Loaded with Recombinant Collagen Peptide Microspheres as a Slow-Release System Induce Ectopic Bone Formation.

New solutions for large bone defect repair are needed. Here, in situ gelling slow release systems for bone induction are assessed. Collagen-I based Recombinant Peptide (RCP) microspheres (MSs) are produced and used as a carrier for bone morphogenetic protein 2 (BMP-2). The RCP-MSs are dispersed in three hydrogels: high mannuronate (SLM) alginate, high guluronate (SLG) alginate, and thermoresponsive hyaluronan derivative (HApN). HApN+RCP-MS forms a gel structure at 32 ºC or above, while SLM+RCP-MS and SLG+RCP-MS respond to shear stress displaying thixotropic behavior. Alginate formulations show sustained release of BMP-2, while there is minimal release from HApN. These formulations are injected subcutaneously in rats. SLM+RCP-MS and SLG+RCP-MS loaded with BMP-2 induce ectopic bone formation as revealed by X-ray tomography and histology, whereas HApN+RCP-MS do not. Vascularization occurs within all the formulations studied and is significantly higher in SLG+MS and HApN+RCP-MS than in SLM+RCP-MS. Inflammation (based on macrophage subset staining) decreases over time in both alginate groups, but increases in the HApN+RCP-MS condition. It is shown that a balance between inflammatory cell infiltration, BMP-2 release, and vascularization, achieved in the SLG+RCP-MS alginate condition, is optimal for the induction of de novo bone formation.

Collagen I derived recombinant protein microspheres as novel delivery vehicles for bone morphogenetic protein-2.

Bone morphogenetic protein-2 (BMP-2) is a powerful osteoinductive protein; however, there is a need for the development of a safe and efficient BMP-2 release system for bone regeneration therapies. Recombinant extracellular matrix proteins are promising next generation biomaterials since the proteins are well-defined, reproducible and can be tailored for specific applications. In this study, we have developed a novel and versatile BMP-2 delivery system using microspheres from a recombinant protein based on human collagen I (RCP). In general, a two-phase release pattern was observed while the majority of BMP-2 was retained in the microspheres for at least two weeks. Among different parameters studied, the crosslinking and the size of the RCP microspheres changed the in vitro BMP-2 release kinetics significantly. Increasing the chemical crosslinking (hexamethylene diisocyanide) degree decreased the amount of initial burst release (24h) from 23% to 17%. Crosslinking by dehydrothermal treatment further decreased the burst release to 11%. Interestingly, the 50 and 72µm-sized spheres showed a significant decrease in the burst release compared to 207-µm sized spheres. Very importantly, using a reporter cell line, the released BMP-2 was shown to be bioactive. SPR data showed that N-terminal sequence of BMP-2 was important for the binding and retention of BMP-2 and suggested the presence of a specific binding epitope on RCP (KD: 1.2nM). This study demonstrated that the presented RCP microspheres are promising versatile BMP-2 delivery vehicles.

Ice-templating of anisotropic structures with high permeability.

Nutrient diffusion and cellular infiltration are important factors for tissue engineering scaffolds. Maximizing both, by optimizing permeability and scaffold architecture, is important to achieve functional recovery. The relationship between scaffold permeability and structure was explored in anisotropic scaffolds from a human collagen I based recombinant peptide (RCP). Using ice-templating, scaffold pore size was controlled (80-600µm) via the freezing protocol and solution composition. The transverse pore size, at each location in the scaffold, was related to the freezing front velocity, via a power law, independent of the freezing protocol. Additives which interact with ice growth, in this case 1wt% ethanol, altered ice crystallization and increased the pore size. Variations in composition which did not affect the freezing, such as 40wt% hydroxyapatite (HA), did not change the scaffold structure, demonstrating the versatility of the technique. By controlling the pore size, scaffold permeability could be tuned from 0.17×10-8 to 7.1×10-8m2, parallel to the aligned pores; this is several orders of magnitude greater than literature values for isotropic scaffolds: 10-9-10-12m2. In addition, permeability was shown to affect the migration of osteoblast-like cells, suggesting that by making permeability a design parameter, tissue engineering scaffolds can promote better tissue integration.

Osteogenesis and mineralization of mesenchymal stem cells in collagen type I-based recombinant peptide scaffolds.

Recombinant peptides have the power to harness the inherent biocompatibility of natural macromolecules, while maintaining a defined chemistry for use in tissue engineering. Creating scaffolds from peptides requires stabilization via crosslinking, a process known to alter both mechanics and density of adhesion ligands. The chemistry and mechanics of linear scaffolds from a recombinant peptide based on human collagen type I (RCP) was investigated after crosslinking. Three treatments were compared: dehydrothermal treatment (DHT), hexamethylene diisocyanate (HMDIC), and genipin. With crosslinking, mechanical properties were not significantly altered, ranging from 1.9 to 2.7 kPa. However, the chemistry of the scaffolds was changed, affecting properties such as water uptake, and initial adhesion of human mesenchymal stem cells (hMSCs). Genipin crosslinking supported the lowest adhesion, especially during osteoblastic differentiation. While significantly altered, RCP scaffold chemistry did not affect osteoblastic differentiation of hMSCs. After four weeks in vitro, all scaffolds showed excellent cellular infiltration, with up-regulated osteogenic markers (RUNX2, Osteocalcin, Collagen type I) and mineralization, regardless of the crosslinker. Thus, it appears that, without significant changes to mechanical properties, crosslinking chemistry did not regulate hMSC differentiation on scaffolds from recombinant peptides, a growing class of materials with the ability to expand the horizons of regenerative medicine. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1856-1866, 2017.

Biomineralization of Recombinant Peptide Scaffolds: Interplay among Chemistry, Architecture, and Mechanics.

Biomineralized scaffolds are an attractive option for bone tissue engineering, being similar to native bone. However, optimization is difficult, due to the complex interplay among architecture, chemistry, and mechanics. Utilizing biomimetic nucleation, linear mineralized scaffolds were created from a collagen type I based recombinant peptide (RCP). Osteoblast mineralization was assessed, in response to changes in scaffold architecture, hydroxyapatite (HA) content, and mechanics. Changes in scaffold pore size (150-450 µm) had little effect on mRNA levels but influenced cell proliferation, achieving a balance between nutrient diffusion and surface area for cell attachment at 300 µm. Increasing the scaffold mechanical strength, from 2.9 to 5.2 kPa, enhanced the expression of osteocalcin, a late marker of mineralization. Further addition of HA, up to 20 wt %, increased osteoblast mineralization, without altering the compressive modulus. Thus, it was shown that architectural cues influence cellular proliferation, while the scaffold chemistry and mechanics independently contribute to gene expression.

Development of recombinant collagen-peptide-based vehicles for delivery of adipose-derived stromal cells.

Stem cell therapy is a promising approach for repair, remodeling and even regenerate tissue of otherwise irreparable damage, such as after myocardial infarction (aMI). A severe limitation of cardiac stem cell therapy is the generally poor retention of administered cells in the target tissue. In tissue repair the main mode of action of adipose tissue-derived stem cells (ADSC) is the production of various growth factors, cytokines, anti-inflammatory and anti-apoptotic factors that together augment repair, remodeling, and regeneration. In this experiment, we used recombinant collagen peptide (RCP) with additional integrin-binding motives and different crosslinkers. Formulated as 50-100 µm microspheres with bound ADSC, we hypothesized that this would improve ADSC retention and function. Crosslinking was performed with chemical crosslinkers (EDC and HMDIC) at high and low concentrations or by thermal treatment (DHT). ADSC adhesion, proliferation, apoptosis/necrosis, and gene expressions in two-dimensional and three-dimensional were analyzed. In addition, the effect of ADSC conditioned medium (ADSC-CM) on proapoptotic/sprouting HUVEC was examined. Our results show that all materials support cell adhesion in short time point, however, EDC-High crosslinker induced ADSC apoptosis/necrosis. Gene expression results revealed lower expression of proinflammatory genes in chemical crosslinked materials, despite EDC-High the proinflammatory genes expressions were similar or higher than TCPS. In addition, cultured ADSC on DHT crosslinked RCP showed a proinflammatory phenotype compared to TCPS. Sprouting assay results confirmed the protective effect of ADSC-CM derived from TCPS and HMDIC-High crosslinked RCP proapoptotic HUVEC. We conclude that ADSC adhere to the materials and maintain their therapeutic profile.

Recombinant gelatin microspheres: novel formulations for tissue repair?

Microspheres (MSs) can function as multifunctional scaffolds in different approaches of tissue repair (TR), as a filler, a slow-release depot for growth factors, or a delivery vehicle for cells. Natural cell adhesion-supporting extracellular matrix components like gelatin are good materials for these purposes. Recombinant production of gelatin allows for on-demand design of gelatins, which is why we aim at developing recombinant gelatin (RG) MSs for TR. Two types of MSs (50 < Ø < 100 microm) were prepared by crosslinking two RGs, Syn-RG, and the arginine-glycine-aspartate-containing Hu-RG. The MSs were characterized, and their tissue reaction and degradation in rats was examined. Histological analysis of the explants after 14 and 28 days in vivo also showed that Syn-RG was degraded slower than Hu-RG, which correlated with the in vitro degradation assay. Hu-RG explants displayed more cellular ingrowth (60% vs. 15% for Syn-RG at day 14), which was associated with extracellular matrix deposition and vascularization. The infiltrating cells consisted of mainly macrophages, part of which fused to giant cells locally, and fibroblasts. No differences were found in matrix metalloproteinase mRNA levels, whereas gelatinase activity was clearly higher in Hu-RG explants. In conclusion, the in vitro and in vivo results of these novel formulations pave the way for cell- and/or factor-driven TR by these RG MSs.