Chirality-induced bionic scaffolds in bone defects repair-a review

Due to lack of amino sugar with aging, people will suffer from various epidemic bone diseases called "undead cancer" by the World Health Organization. The key problem in bone tissue engineering that has not been completely resolved is the repair of critical large-scale bone and cartilage defects. The chirality of the extracellular matrix plays a decisive role in the physiological activity of bone cells and the occurrence of bone tissue, but the mechanism of chirality in regulating cell adhesion and growth is still in the early stage of exploration. This paper reviews the application progress of chirality-induced bionic scaffolds in bone defects repair based on "soft" and "hard" scaffolds. The aim is to summarize the effects of different chiral structures (L-shaped and D-shaped) in the process of inducing bionic scaffolds in bone defects repair. In addition, many technologies and methods as well as issues worthy of special consideration for preparing chirality-induced bionic scaffolds are also introduced. We expect that this work can provide inspiring ideas for designing new chirality-induced bionic scaffolds and promote the development of chirality in bone tissue engineering. This article is protected by copyright. All rights reserved.

Revisiting bone morphogenetic protein-2 knuckle epitope and redesigning the epitope-derived peptides

The bone morphogenetic protein-2 (BMP2) plays a crucial role in bone formation, growth and regeneration, which adopts a conformational wrist epitope and a linear knuckle epitope to interact with its type-I (BRI) and type-II (BRII) receptors, respectively. In this study, we systematically examine the BRII-recognition site of BMP2 at structural, energetic and dynamic levels and accurately locate hotspots of the recognition at BMP2-BRII complex interface. It is revealed that the traditional knuckle epitope (BMP2 residue range 73-92) do fully match the identified hotspots; the BMP2-recognition site includes the C-terminal region of traditional knuckle epitope as well as its flanked β-strands. In addition, the protein context of full-length BMP2 is also responsible for the recognition by addressing conformational constraint on the native epitope segment. Therefore, we herein redefine the knuckle epitope to BMP2 residue range 84-102, which has a similar sequence length but is slid along the protein sequence by ~10 residues as compared to traditional knuckle epitope. The redefined one is also a linear epitope that is natively a double-stranded β-sheet with two asymmetric arms as compared to the natively single β-strand of the traditional version, although their sequences are partially overlapped to each other. It is revealed that the redefined epitope-derived peptide LN^84-102 exhibits an improved affinity by >3-fold relative to the traditional epitope-derived peptide KL^73-92 . Even so, the LN^84-102 peptide still cannot fully represent the BMP2 recognition event by BRII that has been reported to have a nanomolar affinity. We further introduce a disulfide bond across the two arms of double-stranded β-sheet to constrain the free LN^84-102 peptide conformation, which mimics the conformational constraint addressed by protein context. Consequently, several cyclic peptides are redesigned, in which the LN^84-102 (cyc89-101) is determined to exhibit a sub-micromolar affinity; this value is ~5-fold higher than its linear counterpart. Structural analysis also reveals that the cyclic peptide can interact with BRII in a similar binding mode with the redefined knuckle epitope region in full-length BMP2 protein.

Analysis of the bone fracture targeting properties of osteotropic ligands

PURPOSE

Although more than 18,000,000 fractures occur each year in the US, methods to promote fracture healing still rely primarily on fracture stabilization, with use of bone anabolic agents to accelerate fracture repair limited to rare occasions when the agent can be applied to the fracture surface. Because management of broken bones could be improved if bone anabolic agents could be continuously applied to a fracture over the entire course of the healing process, we undertook to identify strategies that would allow selective concentration of bone anabolic agents on a fracture surface following systemic administration. Moreover, because hydroxyapatite is uniquely exposed on a broken bone, we searched for molecules that would bind with high affinity and specificity for hydroxyapatite. We envisioned that by conjugating such osteotropic ligands to a bone anabolic agent, we could acquire the ability to continuously stimulate fracture healing.

RESULTS

Although bisphosphonates and tetracyclines were capable of localizing small amounts of peptidic payloads to fracture surfaces 2-fold over healthy bone, their specificities and capacities for drug delivery were significantly inferior to subsequent other ligands, and were therefore considered no further. In contrast, short oligopeptides of acidic amino acids were found to localize a peptide payload to a bone fracture 91.9 times more than the control untargeted peptide payload. Furthermore acidic oligopeptides were observed to be capable of targeting all classes of peptides, including hydrophobic, neutral, cationic, anionic, short oligopeptides, and long polypeptides. We further found that highly specific bone fracture targeting of multiple peptidic cargoes can be achieved by subcutaneous injection of the construct.

CONCLUSIONS

Using similar constructs, we anticipate that healing of bone fractures in humans that have relied on immobilization alone can be greately enhanced by continuous stimulation of bone growth using systemic administration of fracture-targeted bone anabolic agents.

Periosteum Mimetic Coating on Structural Bone Allografts via Electrospray Deposition Enhances Repair and Reconstruction of Segmental Defects

Structural bone allograft transplantation remains one of the common strategies for repair and reconstruction of large bone defects. Due to the loss of periosteum that covers the outer surface of the cortical bone, the healing and incorporation of allografts is extremely slow and limited. To enhance the biological performance of allografts, herein, we report a novel and simple approach for engineering a periosteum mimetic coating on the surface of structural bone allografts via polymer-mediated electrospray deposition. This approach enables the coating on allografts with precisely controlled composition and thickness. In addition, the periosteum mimetic coating can be tailored to achieve desired drug release profiles by making use of an appropriate biodegradable polymer or polymer blend. The efficacy study in a murine segmental femoral bone defect model demonstrates that the allograft coating composed of poly(lactic-co-glycolic acid) and bone morphogenetic protein-2 mimicking peptide significantly improves allograft healing as evidenced by decreased fibrotic tissue formation, increased periosteal bone formation, and enhanced osseointegration. Taken together, this study provides a platform technology for engineering a periosteum mimetic coating which can greatly promote bone allograft healing. This technology could eventually result in an off-the-shelf and multifunctional structural bone allograft for highly effective repair and reconstruction of large segmental bone defects. The technology can also be used to ameliorate the performance of other medical implants by modifying their surfaces.

Morphogenic Peptides in Regeneration of Load Bearing Tissues

Morphogenic proteins due to their short half-life require high doses of growth factors in regeneration of load bearing tissues which leads to undesirable side effects. These side effects include bone overgrowth, tumor formation and immune reaction. An alternative approach to reduce undesirable side effects of proteins in regenerative medicine is to use morphogenic peptides derived from the active domains of morphogenic proteins or soluble and insoluble components of the extracellular matrix of mineralized load bearing tissues to induce differentiation of progenitor cells, mineralization, maturation and bone formation. In that regard, many peptides with osteogenic activity have been discovered. These include peptides derived from bone morphogenic proteins (BMPs), those based on interaction with integrin and heparin-binding receptors, collagen derived peptides, peptides derived from other soluble ECM proteins such as bone sialoprotein and enamel matrix proteins, and those peptides derived from vasculoinductive and neuro-inductive proteins. Although these peptides show significant osteogenic activity in vitro and increase mineralization and bone formation in animal models, they are not widely used in clinical orthopedic applications as an alternative to morphogenic proteins. This is partly due to the limited availability of data on structure and function of morphogenic peptides in physiological medium, particularly in tissue engineered scaffolds. Due to their amphiphilic nature, peptides spontaneously self-assemble and aggregate into micellar structures in physiological medium. Aggregation alters the sequence of amino acids in morphogenic peptides that interact with cell surface receptors thus affecting osteogenic activity of the peptide. Aggregation and micelle formation can dramatically reduce the active concentration of morphogenic peptides with many-fold increase in peptide concentration in physiological medium. Other factors that affect bioactivity are the non-specific interaction of morphogenic peptides with lipid bilayer of the cell membrane, interaction of the peptide with cell surface receptors that do not specifically induce osteogenesis leading to less-than-optimal osteogenic activity of the peptide, and less-than-optimal interaction of the peptide with osteogenic receptors on the cell surface. Covalent attachment or physical interaction with the tissue engineered matrix can also alter the bioactivity of morphogenic peptides and lead to a lower extent of osteogenesis and bone formation. This chapter reviews advances in discovery of morphogenic peptide, their structural characterization, and challenges in using morphogenic peptides in clinical applications as growth factors in tissue engineered devices for regeneration of load bearing tissues.

Peptide microarrays for the discovery of bioactive surfaces that guide cellular processes: a single step azide-alkyne "click" chemistry approach

Cell behavior in vivo is guided by a complex microenvironment containing many different molecules including extracellular matrix (ECM) proteins, growth factors, and proteoglycans. Controlling the interaction between these various components at the cell-material interface will be invaluable in developing new materials for biomedical devices and tissue engineering applications. We report a single step approach to forming mixed peptide conjugated self-assembled monolayers on gold using copper-catalyzed azide-alkyne cycloaddition chemistry to study the combinatorial effects of different peptide ligands on cellular processes. We synthesized ECM adhesion peptides (YIGSR, GRGDS), a bone morphogenetic protein 7 (BMP-7) derived peptide (KPSSAPTQLN), and a heparin binding peptide (KRSR), and arrayed them, alone and in combination, onto gold coated coverslips. SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, and arrayed peptide combinations were seen to differentially bind to adipose derived stem cells (ADSCs) and mouse embryonic fibroblasts (MEFs). We further investigated the osteogenesis of ADSCs on SAMs containing combinations of adhesion peptide and BMP-7 peptide in both standard culture and osteogenic differentiation media. We demonstrate enhanced expression of osteogenic markers Runx2 and osteopontin when ADSCs are adherent to BMP-7 derived peptide alone or in combination with ECM adhesion peptides. The platform presented here enables immobilization of multiple peptides in a single step using a commercially available microarray spotter which will prove useful in fabricating biomolecule interfaces for cell biology studies and biochemical assays.

Peptide decorated nano-hydroxyapatite with enhanced bioactivity and osteogenic differentiation via polydopamine coating

To be better used as implant materials in bone graft substitutes, bioactivity and osteogenesis of nano-hydroxyapatite (nano-HA) need to be further enhanced. Inspired by adhesive proteins in mussels, here we developed a novel bone forming peptide decorated nano-HA material. In this study, nano-HA was coated by one-step pH-induced polymerization of dopamine, and then the peptide was grafted onto polydopamine (pDA) coated nano-HA (HA-pDA) through catechol chemistry. Our results demonstrated that the peptide-conjugated nano-HA crystals could induce the adhesion and proliferation of MG-63 cells. Moreover, the highly alkaline phosphatase activity of the functionalized nano-HA indicated that the grafted peptide could maintain its biological activity after immobilization onto the surface of HA-pDA, especially at the concentration of 100μg/mL. These modified nano-HA crystals with better bioactivity and osteogenic differentiation hold great potential to be applied as bioactive materials in bone repairing, bone regeneration and bio-implant coating applications.

Nanophase hydroxyapatite and poly(lactide-co-glycolide) composites promote human mesenchymal stem cell adhesion and osteogenic differentiation in vitro

Human mesenchymal stem cells (hMSCs) typically range in size from 10 to 50 μm and proteins that mediate hMSC adhesion and differentiation usually have a size of a few nanometers. Nanomaterials with a feature size smaller than 100 nm have demonstrated the unique capability of promoting osteoblast (bone forming cell) adhesion and long-term functions, leading to more effective bone tissue regeneration. For new bone deposition, MSCs have to be recruited to the injury or disease sites and then differentiate into osteoblasts. Therefore, designing novel nanomaterials that are capable of attracting MSCs and directing their differentiation is of great interest to many clinical applications. This in vitro study investigated the effects of nanophase hydroxyapatite (nano-HA), nano-HA/poly(lactide-co-glycolide) (PLGA) composites and a bone morphogenetic protein (BMP-7) derived short peptide on osteogenic differentiation of hMSCs. The short peptide was loaded by physical adsorption to nano-HA or by dispersion in nanocomposites and in PLGA to determine their effects on hMSC adhesion and differentiation. The results showed that the nano-HA/PLGA composites promoted hMSC adhesion as compared to the PLGA controls. Moreover, nano-HA/PLGA composites promoted osteogenic differentiation of hMSCs to a similar extent with or without the presence of osteogenic factors in the media. In the MSC growth media without the osteogenic factors, the nanocomposites supported greater calcium-containing bone mineral deposition by hMSC than the BMP-derived short peptide alone. The nanocomposites provided promising alternatives in controlling the adhesion and differentiation of hMSCs without osteogenic factors from the culture media, and, thus, should be further studied for clinical translation and the development of novel nanocomposite-guided stem cell therapies.