Fibrillar peptide gels in biotechnology and biomedicine

Abstract

Peptides, peptidomimetics, and peptide derivatives that self-assemble into fibrillar gels have received increasing interest as synthetic extracellular matrices for applications in 3D cell culture and regenerative medicine. Recently, several of these fibrillizing molecules have been functionalized with bioactive components and chemical features such as cell-binding ligands, degradable sequences, drug eluting compounds, and cross-linkable groups, thereby producing gels that can reliably display multiple factors simultaneously. This capacity for incorporating precise levels of many different biological and chemical factors is advantageous given the natural complexity of cell-matrix interactions that many current biomaterial strategies seek to mimic. In this review, recent efforts in the area of fibril-forming peptide materials are described, and advantages of biomaterials containing multiple modular elements are outlined. In addition, a few hurdles and open questions surrounding fibrillar peptide gels are discussed, including issues of the materials' structural heterogeneity, challenges in fully characterizing the diversity of their self-assembled structures, and incomplete knowledge of how the materials are processed in vivo.

Figure 1

Negative-stained TEM images of β-rich fibrils from different peptides, PAs, and peptide derivatives. (a) fibrils formed from the peptide Q11; (b) fibrils formed from the peptide RGDS-Q11; (c) fibrils formed from the thiol-presenting peptide Cys-SGSG-Q11; (d) fibrils formed by the β-hairpin peptide MAX1; (e) fibrils formed by mixtures of two aromatic short peptides, Fmoc-FF and Fmoc-RGD; (f) fibrils formed by a PA terminated with a cyclic RGD sequence. (d) is reprinted with permission from Ozbas et al. Macromolecules 2004, 37, 7331–7337; (e) is reprinted with permission from Zhou et al. Biomaterials 2009, 30, 2523–2530; (f) is reprinted with permission from Guler et al. Biomacromolecules 2006, 7, 1855–1863.

Figure 2

Co-assembling peptides for controlling matrix mechanics and enhancing cell adhesion. Self-assembling peptides with N-terminal Cys residues and C-terminal thioesters self-assemble into fibrils capable of undergoing native chemical ligation (a). RGD-bearing peptides are co-assembled within these gels to provide for cell attachment. Both ligation and RGD functionalization significantly and additively improved endothelial cell growth (p<0.01 by ANOVA compared to Q11 (*) or all others (**), n=4, means±SD). Ligand display was also evidenced by streptavidin-colloidal gold labeling of biotinylated RGDS-Q11 fibrils (c). RGDS-Q11 incorporation significantly improved human umbilical vein endothelial cell (HUVEC) growth (d, Q11 gel; e, Q11 gel containing 10% RGDS-Q11, both at day 7 post-plating). Stiffening by native chemical ligation significantly improved CD31 expression in HUVEC cultures (f, Q11 gel; g, ligated CQ11G-thioester; green, CD31; blue, DAPI). (d) and (e) were reprinted with permission from Jung et al. Biomaterials 2009, 30, 2400–2410. (b), (f), and (g) were reprinted with permission from Jung et al. Biomaterials 2008, 29, 2143–2151.

Figure 3

Potential types of heterogeneity within ligand-bearing self-assemblies. Several structural aspects may influence the number and spatial distributions of ligand-integrin binding events, which collectively determine cell behavior. Peptide ligands must be appropriately spaced from surfaces to efficiently deliver the ligand to the binding pockets of integrins (a). Inter-fibril lateral aggregation (b), ligand burial, or ligand adsorption (c) may also remove a subset of ligands from interacting with integrins. These aspects as well as potential phase separation may influence the clustering of the ligands on the fibril (d), which may in turn influence integrin clustering, focal adhesion assembly, and the resultant intracellular signaling. It is also conceivable that the density of the ligand itself could sterically influence the ability of integrins to bind (e). Strategies to define and control these aspects will contribute significantly to the biological precision possible with these materials.

Figure 4

Potential uses of fibrillar peptide gels for clinical applications. Improvement in cell-based therapies (a). Fibrillizing peptides are mixed with cardiomyocytes or undifferentiated stem cells and injected into damaged myocardium for improved transplanted cell survival and wound healing after myocardial infarction.^, Improvement of prosthetics (b). PAs are integrated into the pores of titanium foam to create bioactive composites that induce mineralization and vascularization around and within orthopedic implants.^, Direct application of a therapeutic-releasing gel (c). Following angioplasty, a nitric oxide-releasing PA gel is applied directly to the exterior of the vessel at the site of injury, reducing smooth muscle cell proliferation and increasing endothelialization compared to non-hydrogel treated controls.