Self-assembling peptides for stem cell and tissue engineering

Abstract

Regenerative medicine holds great potential to address many shortcomings in current medical therapies. An emerging avenue of regenerative medicine is the use of self-assembling peptides (SAP) in conjunction with stem cells to improve the repair of damaged tissues. The specific peptide sequence, mechanical properties, and nanotopographical cues vary widely between different SAPs, many of which have been used for the regeneration of similar tissues. To evaluate the potential of SAPs to guide stem cell fate, we extensively reviewed the literature for reports of SAPs and stem cell differentiation. To portray the most accurate summary of these studies, we deliberately discuss both the successes and pitfalls, allowing us to make conclusions that span the breadth of this exciting field. We also expand on these conclusions by relating these findings to the fields of nanotopography, mechanotransduction, and the native composition of the extracellular matrix in specific tissues to identify potential directions for future research.

Figure 1

Peptides self-assembled into nanostructures and their functional principles in the niche of stem cells. A. Graphical illustration of the formation of short peptides assembled into collagen-mimicking nanofiber. Reprinted with permission from ref. . B. Amphiphilic peptides for cylindrical nanostructures (scale bar in the SEM photo: 300 nm). Reprinted with permission from ref. . C. Action mechanisms of SAPs: (a) nanotopographical cues, (b) cell-ECM interactions, (c) transduction of mechanical forces.

Figure 2

Effects of SAPs on in vitro differentiation of neural stem cells (A and B) and in vivo repair of neural tissues (C and D). A. (a) Neuronal density (green) is selectively increased relative to astrocytes (red) by soft (7.3 ± 0.9 kPa storage modulus) vs stiff (22.9 ± 5 kPa) PA substrate (DAPI stain) in hippocampal culture. (b) Comparison of cell densities (n = 3, p<0.001). Reprinted with permission from ref. . B. Left: IKVAV-PA gels enable statistically significant growth of neurons relative to laminin and poly-D-lysine (PDL) controls at one and seven days (P<0.5, P<0.01 in duplicate). Right: Increased concentrations of IKVAV-PA, but not IKVAV peptide, increases one-day neuronal differentiation. IKVAV-PA and EQS-PA (solid line) and EQS-PA combined with varied amounts of soluble IKVAV peptide (dashed line). Reprinted with permission from ref. . C. IKVAV-PA injection reduces the normalized cortical and hippocampal area of amyloid-beta plaques (Green, thioflavin S stain) in a murine model of Alzeheimer’s Disease relative to vehicle control (P<0.05). Reprinted with permission from ref. . D. Left: Neurolucida tracing of regenerating murine BDA-labeled corticospinal tract motor neurons 11 weeks post-SCI. IKVAV-PA presence results in greatly increased migration into the lesion. Right: This regrowth corresponds to statistically significant improvement of motor function after five weeks as quantified by BBB score. Reprinted with permission from ref. .

Figure 3

Influence of SAP on bone and cartilage. A. (Left) schematic representation of an SAP and MSC injection into a rat osteoarthritis model. (Right) our previously published data, which shows a repression of IL-2 secretion with the use of SAPs. Reprinted with permission from ref. and ref. . B. The histology panel on the left highlights the abundance of apoptotic cells cultured in agarose gels compared to RADA SAP gels. The graph to the right shows an increase of GAG accumulation using RADA over agarose, supporting the use of RADA SAPs to direct chondrogenic stem cell fate. Reprinted with permission from ref. . C. The histology panel on the left shows an increase in calcification in a 3D culture over a 2D culture, and further enhancement of tissue calcification of both cultures in the presence of an osteogenic media. The graph on the right compares the effects of normal SAP cultures (white bar) to SAP cultures with an osteogenic media (dashed bar) and SAP cultures with an osteogenic media exposed to heat-shock (gray bar). These data highlight the enhanced osteogenic potential of SAPs for bone tissue engineering, and further optimization using novel stimuli like heat shock. Reprinted with permission from ref. .

Figure 4

Application of SAPs to cardiac tissue repair in the animal model. A. Conceptual illustration of injection of SAP and stem cells into the damaged myocardium. B. (above) Masson trichrome staining of heart sections at four weeks after myocardial infarction, observed under a light microscope. NA: normal area, SA: scar area. (below) Infarct area as determined by percent of total LV area for the no treatment control, stem cells only and cells with scaffold (p<0.001 vs. control, p<0.001 vs. cell alone). Reprinted with permission from ref. . C. (above) Treating with SAP and cSca-1 cells diminishes infarct expansion as seen in Masson trichrome stained heart slices and corresponding infarct size graph (below) 14 days after transplantation. The slices and graph depict; non-treated MI (A), SAP only (B), bone marrow stem cell with SAP (C), skeletal myoblasts with SAP (D), adipose-tissue derived mesenchymal cells with SAP (E) and cardiac progenitor cells with SAP (F). Data shows mean +/− SD. P < 0.05 cSca-1 with SAP vs. BM with SAP and p<0.01 for cSca-1 with SAP vs. non-treated MI, SAP only and skeletal myoblasts with SAP. Reprinted with permission from ref. . D. Mature vessel-like structures are detected in the SAP scaffold 14 days post injection (a) and after 28 days (b) several arteriole-like structures were seen within the microenvironment in all samples. Cells (blue, DAPI) are in the microenvironment positively stained for alpha - smooth muscle actin antibody. Reprinted with permission from ref. .

Figure 5

Strategy to fine control over stem cell fate using SAPS. A. Normalized and processed microarray data were downloaded from GSE1133 from the NIH GEO database. The integrin gene family was isolated and the expression profiles for each tissue in the study are displayed graphically to visualize the integrin expression profiles unique to each tissue type. Reprinted with permission from ref. . B. By adding GLY spacers between the a standard RADA repeat sequence in the SAP amino acid sequence, better expose of the integrin binding motifs was achieved. Reprinted with permission from ref. . C. Computational and modeling of the self-assembling potential of peptide sequences has resulted in the development of an extensive library of theoretical self-assembling peptides that have yet to be investigated. Reprinted with permission from ref. .