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Cite This: Biomacromolecules XXXX, XXX, XXX−XXX
Angiogenic Self-Assembling Peptide Scaffolds for Functional Tissue Regeneration Biplab Sarkar,† Peter K. Nguyen,† William Gao,† Akhil Dondapati,† Zain Siddiqui,† and Vivek A. Kumar*,†,‡,§ Department of Biomedical Engineering and ‡Department of Chemical and Materials Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States § Rutgers School of Dental Medicine, Newark, New Jersey 07101, United States
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ABSTRACT: Implantation of acellular biomimetic scaffolds with proangiogenic motifs may have exciting clinical utility for the treatment of ischemic pathologies such as myocardial infarction. Although direct delivery of angiogenic proteins is a possible treatment option, smaller synthetic peptide-based nanostructured alternatives are being investigated due to favorable factors, such as sustained efficacy and high-density epitope presentation of functional moieties. These peptides may be implanted in vivo at the site of ischemia, bypassing the first-pass metabolism and enabling long-term retention and sustained efficacy. Mimics of angiogenic proteins show tremendous potential for clinical use. We discuss possible approaches to integrate the functionality of such angiogenic peptide mimics into self-assembled peptide scaffolds for application in functional tissue regeneration.
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INTRODUCTION There is a growing need for regenerative scaffolds for repairing diseased tissues.1 Biodegradable acellular scaffolds are promising candidates for such application. A major requirement of such regenerative acellular scaffolds is adequate vascularization after implantation.2 In this review, we discuss salient aspects of the growth factors involved in physiological vascularization and their synthetic mimics. Next, we discuss a promising strategy to deliver and retain such functionality in situ through conjugation with self-assembling peptide scaffolds, which are inherently biodegradable and responsive to programmed manipulation. This therapeutic avenue is worth exploring in treating various ischemic pathologies.
nonimmunogenic formulations with biological signals embedded in its structure. Implantation of such functionalized acellular scaffolds may induce and direct spatiotemporal regeneration of specific tissue components. Formation of functional niches via angiogenesis, neurogenesis, or osteogenesis may allow approximation of the structure and function of native tissue. A few requirements for these regenerative scaffolds are (a) recruitment, segregation, and differentiation of progenitor cells into different cell types within the scaffold,13−15 (b) a gradient of chemokines or homing signals laid on rationally designed tracks for guiding cells into intended niches,16,17 (c) material multifunctionality for construction of supports for different cell types,18−21 (d) tunable porosity and tortuosity in the different domains of the scaffold,22−24 (e) integration of the different niches within the scaffold such that cells in different locales can interact and communicate,25,26 (f) mechanical robustness and mimicry of native tissue,27−29 (g) ability to support the recruited cells through the supply of oxygen and nutrients,30,31 (h) responsiveness to environmental stimuli,32−35 and (i) controlled biodegradability.36−41 Optional aspects could include the presence of sacrificial components42,43 for spatiotemporal remodeling and a cache of sequestered signals44 activated by rational programming.45 Researchers have made progress toward meeting the physical requirements of such multicomponent scaffolds
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ACELLULAR SCAFFOLDS FOR TISSUE REGENERATION Functional tissue regeneration requires recruitment and integration of various cells with concomitant deposition of nonpathologic (nonscar) extracellular matrix components into multilayered and hierarchical functional niches while maintaining the rheological and material properties required for tissue function.3,4 Xenogeneic and allogeneic tissue transplants for such applications are limited by immunological concerns and batch-to-batch variability.5−7 Synthetic tissue-mimetic scaffolds are promising alternatives to such biologically derived transplants.8,9 However, they often have insufficient mechanical integrity or inadequate biological signaling.10−12 A threedimensional acellular biomimetic scaffold with patterned signals encoded in its overall organization offers a viable compromise, blending together facile, reproducible, and © XXXX American Chemical Society
Received: July 25, 2018 Revised: August 19, 2018 Published: August 22, 2018 A
DOI: 10.1021/acs.biomac.8b01137 Biomacromolecules XXXX, XXX, XXX−XXX
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Figure 1. An acellular scaffold for tissue regeneration: role of angiogenic signals. (a) Tissue contains different layers of extracellular matrices and various cell types that are vascularized by a network of blood vessels. (b) A tissue defect can result from injury or a surgical procedure. (c) A way to regenerate the affected tissue is through the placement of an acellular scaffold in the void containing a gradient of biological signaling molecules either in a soluble form or covalently sequestered within the scaffold. (d) Revascularization of the scaffold due to the angiogenic signals is crucial for the survival and integration of the scaffold.
through the formulation and integration of various biomaterial formats, decellularization of tissues, and 3D printing of biocompatible tissue mimics.6,12,27,34,46−59 The chemical requirements, on the other hand, can be met by either embedding bioactive factors in the scaffolds44 or by modifying the sequence of the constituent system to encode functionality directly60−62 (“the medium is the message”). The design of the required chemical tracks would be aimed at the controlled migration, proliferation, differentiation, and attachment of cells.63,64 Here, we outline the progress and the path for designing a specific aspect of such constructs: encoding a regenerative scaffold with biological signals for biomimetic angiogenesis.61,62,65 In Figure 1, we delineate a general strategy using a multicomponent acellular scaffold with an inbuilt angiogenic signal gradient for functional tissue regeneration. The integration and maturation of the scaffold postimplantation in part depends on the potency of the angiogenic signal to direct vascularization of the scaffold.
Bone healing and postsurgical recovery, especially among the geriatric population, are slow and could benefit from facilitated tissue vascularization.76 Smaller organs and some thin tissues, including cartilage, bladder, and skin, are viable with little or no vasculature, subsisting on diffusion for nutrients, waste, and oxygen transport.77,78 In contrast, thicker and larger organs, such as lungs, muscles, and heart, are structurally organized with cells that are farther away from major blood vessels.77,78 Pathological hypoxia in parts of the such tissues may lead to necrosis.31 Such cellular death and organ degeneration could be overcome with increased vascularization of the tissues.77 Despite continued efforts over the past few decades in enhancing angiogenic processes, clinical applications have been limited.79 Complicating necessary advances,80−82 there is no in vitro model of angiogenesis that truly simulates supporting cells, such as pericytes, fibroblasts, and smooth muscle cells, the extracellular matrix, and circulating blood.83 In vivo angiogenesis models84−86 involve a different set of limitations, and there have been conflicting results due to factors such as interspecies differences and variation in experimental models/ methodology (e.g., compensating for autoregenerative potential).87−89 Efforts to translate the results from laboratory models to clinically useful solutions have been hindered due to biological variation and the inherent complexity of the angiogenic cascade. For the development of efficient methods that promote angiogenesis, it is critical to understand the molecular mechanisms of how new vessels form, grow, and develop into a vascular network.90−93 Our current understanding of vascularization and angiogenic processes is a result of studies in the fields of oncology, cardiovascular disease, embryogenesis, and organ development.94,95
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ROLE OF THERAPEUTIC VASCULARIZATION The physical dimensions of 3D tissue-engineered scaffolds are often dictated by the transport of oxygen, nutrients, and waste products to and from the cells. If cells in the interior of the scaffold cannot access these transport pipelines, necrotic zones form and limit the utility of the implant.66 For example, longterm failure of autologous vascular grafts after implantation is often due to postsurgical necrosis of the tunica media.67,68 Rapid vascularization of these scaffolds post implantation could alleviate ischemic necrosis. Proangiogenic implants could additionally be viable clinical candidates for the treatment of ischemic pathologies such as peripheral vascular disease and tissue reperfusion injuries after stroke and myocardial infarction. For example, such constructs could provide an alternative approach to the current development of acellular vascular grafts.27,46,69−75 B
DOI: 10.1021/acs.biomac.8b01137 Biomacromolecules XXXX, XXX, XXX−XXX
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MECHANISM OF ANGIOGENESIS Vascularization96 is regulated through a set of regulatory proteins, such as vascular endothelial growth factors (VEGFs), angiostatins, endostatins, cytokines, chemokines, hormones, neuropeptides, and lipid mediators.97 The main driver of angiogenesis is hypoxia. Hypoxia-inducible factors (HIFs) play very important roles in the detection of hypoxia in cells and the regulation of angiogenic response.98−103 HIF-1, HIF-2, and HIF-3 are divided into α and β subunits each responsible for specific regulatory functions. HIF-1α and HIF-2α are similar in structure and function and interact with hypoxia response elements to initiate proangiogenic transcriptional activity.104−106 HIF-3α is responsible for the negative regulation of the hypoxia response elements.107,108 Iron-dependent oxygenases hydroxylate HIF-α residues in an oxygen-dependent manner.109−112 In the presence of oxygen, the hydroxylation contributes to HIF inactivation through inhibition of transcriptional activity as well as proteolytic destruction. HIF hydroxylases are 2-oxogluarate-dependent oxygenases that require oxygen to deactivate HIF,113 suggesting a relationship between the regulation of angiogenesis and oxygen availability. Under hypoxic conditions, transcription of VEGF is upregulated by HIF, inducing angiogenesis (Figure 2).114 The earliest therapeutic approaches to peripheral artery disease and cardiovascular disease involved plasmid delivery of HIF1α.115−117
endothelial cells is governed by specialized endothelial tip cells and stalk cells124 (Figure 2), which receive signals from surrounding cells.125 The vascular remodeling processes are crucial for tissue regeneration after injury or infarction and are thus clinically relevant for the treatment of conditions, such as myocardial infarction, critical limb ischemia, and hind-limb ischemia. The growth factors and cytokines involved in these processes are delineated in the following sections, and where applicable, functional mimics of those biomolecules are also discussed.
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ANGIOGENIC FACTORS AND FUNCTIONAL MIMICS VEGF and Mimics. The VEGF family of proteins is the crucial group of proteins initiating and sustaining angiogenesis.126 The process is generally induced through a change in the expression and upregulation of VEGF and its associated receptors.127,128 There are several members of the VEGF family, including VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and placental growth factor (PlGF).129,130 These VEGF members are regulated by tyrosine kinase receptors (VEGFRs). VEGF-A binds to VEGFR-1 and VEGFR-2, VEGF-B binds to VEGFR-1, VEGF-C and VEGF-D bind to VEGFR-2 and VEGFR-3, and VEGF-E binds to VEGFR2.131−133 VEGF-A has a number of functions, which include increasing vascular permeability and stimulating cell migration in macrophages and endothelial cells.134 VEGF-B is required for blood vessel survival.135 VEGF-C is similar in angiogenic effects to VEGF-A, but the outgrowth of vessels stimulated is significantly longer than those stimulated by VEGF-A.136 VEGF-D has larger angiogenic effects compared to VEGF-A when delivered into skeletal muscle via adenoviral vectors.137 PlGF is another member of the VEGF family that acts similarly to VEGF-A and can stimulate angiogenesis in ischemic tissues without certain side effects of VEGF-A-based treatments, such as hypotension and edema.138 For proper development and maintenance of new blood vessels, VEGF must be present for prolonged periods of time for stabilization of the vasculature.139 Although VEGF is an important initiator of angiogenesis, complementary proteins, such as PDGF and angiopoietins, are crucial to stabilize the newly grown vasculature through recruitment and differentiation of smooth muscle cells and pericytes.140,141 In addition to VEGF itself, VEGFR2 plays a large role in angiogenic signaling as it modulates endothelial cell proliferation, tube formation, and migration.142 During the process of wound recovery, there is significant upregulation of VEGFR2 in blood vessels.127 Srinivasan et al. have demonstrated that a molecular chaperone, phosducin-like protein 3 (PDCL3), associates with VEGFR2 in response to VEGF binding and prevents its degradation through the ubiquitin pathway.143 PDCL3 stability appears to play an important role in angiogenesis as its expression is upregulated by hypoxia. PDCL3 keeps VEGFR2 ready for activation by VEGF and is necessary in the VEGF signaling pathway.143 The academic community and pharmaceutical industry have focused efforts on VEGF and its tyrosine kinase receptors (both VEGFR1 and VEGFR2) as potential targets for therapy, especially in areas of oncology144 and neovascular posterior segment diseases, such as diabetic retinopathy145 and macular degeneration.146 A functional peptide domain, QK (sequence consists of the VEGF 17−25 helix region KLTWQELYQLKYKGI), which promotes angiogenesis in vitro, has been
Figure 2. A simplified overview of sprouting angiogenesis. HIF expression leads to release of growth factors such as VEGF, EGF, and FGF. The gradient of growth factors induces sprouting of a new blood vessel from an existing one toward the source of the signal with the help of tip cells and stalk cells.
The angiogenic pathway involves the enlargement of the blood vessel, the formation of transendothelial cell bridges, and the split into capillaries.118 VEGF promotes an increase in vascular permeability through the formation of fenestrations and the redistribution of vascular endothelial-cadherin and platelet endothelial cell adhesion molecule 1 (PECAM-1), resulting in vasodilation. 119 Endothelial cells are then destabilized due to the alleviation of interendothelial cell contacts and periendothelial cell support.120 Angiopoietin-2 is involved in loosening the cell matrix in the presence of VEGF.121,122 Proteases degrade matrix proteins and are involved in the release of sequestered growth factors from the extracellular matrix.123 The endothelial cells, freed from the monolayer in the vessel wall, can proliferate and migrate to other areas of the body. Overall, the reorganization of C
DOI: 10.1021/acs.biomac.8b01137 Biomacromolecules XXXX, XXX, XXX−XXX
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Biomacromolecules found by phage display.60,65,147−153 QK works by binding to VEGF receptors and inducing cellular proliferation, increasing VEGF biological response, and activating the VEGF signaling cascade.147 Other peptides targeting the receptor binding site of VEGFR2 show induction of endothelial cell networks on Matrigel substrates.154 A bifunctional peptide with an ECactivating domain derived from QK and a collagen-binding domain has successfully demonstrated the formation of new microvasculature.155 Angiopoietin-1 and Mimics. Angiopoietin-1 (Ang1) is a member of the angiopoietin growth factor family and binds to the angiopoietin receptor Tie2.156,157 Ang1 is necessary for proper organization and maturation of newly created vasculature and promotes stabilization of already existing mature blood vessels.158,159 It is especially important for embryonic vasculogenesis. For example, engineered Ang1 knockout mice display embryonic mortality by approximately day 12.5.160 These embryos have cardiovascular defects, such as a collapsed atrial endothelial lining, aberrantly rounded and poorly associated endothelial cells, and significantly lower ventricular endocardium complexity compared to those of wild-type embryos.160 Ang1 promotes blood vessel survival via suppression of inflammatory gene expression and a decrease in vascular leakage.156 There are a few different functional mimics for Ang1 including COMP-Ang1,161 Vasculotide (VT),162−164 and QHREDGS (Table 1).165,166 COMP-Ang1 was developed to
demonstrated to be an important autocrine and paracrine modulator of cells involved in vascularization and has a role in the recruitment of pericytes and the maturation, sprouting, and remodeling of blood vessels.173 PDGF enhances new capillary formation by increasing the concentration of a soluble, myofibroblast-derived endothelial cell growth factor and can also activate surrounding connective tissue cells.174 PBA2-1c is a peptide mimic of PDGF and was designed to bind to PDGF receptors and induce the PDGF signaling cascade.173 To test its efficacy, in vitro analysis was performed by incubating C2C12 cells in PDGF-BB or PBA2-1c.173 Results showed that both PDGF-BB and PBA2-1c induced a signaling cascade that increased tyrosine phosphorylation of PDGF receptors and activated AKT as well as ERK-1 and ERK-2 pathways. Additionally, the PBA2-1c cultures stimulated cell proliferation and increased cell migration, although the increased cell migration was still less than with PDGF-BB. Nonetheless, PBA2-1c was demonstrated to be an adequate model of PDGF albeit with limited in vivo data.173 Osteonectin and Mimics. Osteonectin, also known as secreted protein acidic and rich in cysteine (SPARC), is a glycoprotein expressed in many cells including endothelial cells, fibroblasts, and smooth muscle cells.175−177 It has been shown that SPARC is an important regulator of angiogenesis and tissue morphogenesis.177 Animal studies have shown that osteonectin expression is present in new blood vessels, brain capillaries, and capillaries in dermal wounds. They have also revealed that the number of blood vessels in osteonectin knockout mice decreased significantly compared to that of wild type.178 A functional peptide mimic of osteonectin (sequence: GHK), based on its copper(II)-binding domain, promotes angiogenesis and healing, increases extracellular matrix production and remodeling, increases fibroblast production of VEGF and FGF, and recruits capillary cells and macrophages.179 When attached to alginate hydrogels,180 GHK increases production of FGF, VEGF, and other angiogenic factors by mesenchymal stem cells.181 Fibroblast Growth Factor (FGF) and Mimics. FGFs are a family of growth factors that work synergistically with VEGF in promoting angiogenesis via endothelial cell tubular proliferation.182−184 FGFs bind to four receptor tyrosine kinases: FGFR-1, FGFR-2, FGFR-3, and FGFR-4.185−187 FGFR-1 has been shown to maintain and develop vasculature in an embryo, and FGFR-2 increases vascular tone and blood pressure.188 The effect of FGF in inducing vascular sprouting has been demonstrated through ex vivo 3D microfluidic cell culture.189 Using siRNA functional studies, researchers have also shown that FGF-2 and FGF-5 both promote cell sprouting and are angiogenic, but the exact mechanism is not yet fully understood.189 In vivo, FGF-2 has been shown to increase VEGF expression, cell proliferation, and angiogenesis.190 Decellularized extracellular matrix scaffolds derived from aortic adventitia and infused with FGF-2 retain cellular proliferative abilities as well as the ability to induce angiogenesis in vivo.47 In a rat model, it was shown that an FGF-2 mimic, F2A4-KNS, along with demineralized bone matrix, can be used for angiogenesis and osteoinduction.191 Although the mechanism was not completely elucidated, the increased production of a mineralized matrix was hypothesized to be in part due to the anti-inflammatory effect of the peptide.192 Cytokines. Cytokines can be used to attract these cells from other parts of the body to ischemic sites.193 Interleukins are a type of cytokine that regulate immune responses and
Table 1. Angiogenic Peptides That Mimic Angiogenic Factors angiogenic peptide mimic
relevant growth factor
QK COMP-Ang1 VT (vasculotide) QHREDGS PBA2-1c GHK F2A4-K-NS
VEGF angiopoietin-1 angiopoietin-1 angiopoietin-1 PDGF Osteonectin FGF
angiogenic activity in in in in in in in
vivo vivo vivo vivo vitro vivo vivo
prevent Ang1 aggregation and insolubility by modifying its central coiled-coil domain and the N-terminal superclustering domain.161 In an in vivo corneal micropocket assay, COMPAng1 induced corneal angiogenesis comparable with VEGFinduced angiogenesis and better than Ang1-induced angiogenesis.161 VT was designed to have a four-armed polyethylene glycol backbone where each of the arms is attached to a peptide that can bind to Tie2 receptor.167 VT has been applied for treatments of acute skin injury following ionizing radiation and dermatitis167,168 as well as protection of the vascular barrier after acute kidney injury.169 The peptide QHREDGS was based on the integrin-binding domain of Ang1 and can help endothelial cell survival and network formation in tube formation assays.165 The peptide, because of its ability to promote cell survival,170 has been applied to deliver transplanted cardiomyocytes in vivo after myocardial infarction, resulting in enhanced metabolism and reduced apoptosis.166,171 This strategy has been showcased in a delivery platform for QHREDGS based on a collagen−chitosan composite hydrogel.165,171 Platelet-Derived Growth Factor (PDGF) and Mimics. PDGF is released from platelets at sites of vascular injuries and is involved in wound healing and embryogenesis.172 It has been D
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Biomacromolecules hematopoiesis.194 Interleukin-6 is a proangiogenic cytokine that promotes MMP-9 activation and the release of VEGF from endothelial cells, leading to neovascularization and vascular remodeling.195 Interleukin-8,196 which binds to CXC chemokine receptors (CXCR1 and CXCR2) on endothelial cells, has been shown to enhance endothelial cell proliferation, MMP expression, cellular survival, and angiogenesis.197 Peptide mimics of such angiogenic interleukins remain elusive, and invention of such mimics will be a positive development for angiogenic treatment of ischemic diseases. Granulocytecolony stimulating factor (G-CSF) is another cytokine that demonstrates proangiogenic effects through mobilization of bone marrow stem cells and progenitor cells.198,199 In a study of patients with occlusive coronary artery disease, treatment with G-CSF demonstrated overall clinical improvement. However, there was no detectable increase in left ventricular perfusion.200 Clinical trials of G-CSF have been associated with an inconsistent pattern of vascularization.201 So far, the search for peptide agonists of the G-CSF receptor has been unsuccessful.202 Biomimetic Self-Assembling Peptides. Self-assembling peptides are short (∼5−50 aa) polypeptides that can be rationally designed to undergo hierarchical assembly into nanostructures.203 They offer a versatile alternative to biologically sourced materials and synthetic polymeric materials for tissue engineering.204 They can be induced to assemble into different material formats, such as hydrogels, sponges, and sheets.205−207 Secondary structure adopted by these peptides can be as varied as the conformations of native proteins (Table 2).205 The sequences of these peptides can be edited during solid-phase synthesis, enabling their use as multipurpose design platforms.61,65
the former case can be the class of collagen mimetic peptides,210,211 and an example of the latter is the set of elastin mimetic peptides.215 To a first order approximation, hydrogen bonding and ionic interactions predominate in the first case, and hydrophobic interaction is the primary driving force in the latter.209 Self-assembled peptide nanostructures can sequester smallmolecule drugs as well as large biomolecules, allowing for prolonged and controlled release of drugs and growth factors in tissue environments.245 These peptides do not require extensive chemical cross-linking to form a nanofibrous structure, forming robust gels through supramolecular interactions and physical cross-links (similar to natural fibers formed by collagen and silk fibroin). This aspect of higherorder assembly yields superior biocompatibility and ease of fabrication compared to synthetic polymeric materials.246 In addition, the mechanical and chemical properties of scaffolds can be tuned by altering peptide composition, further improving the utility of self-assembled peptides in a variety of applications.247 Angiogenic growth factors can also be incorporated into the hydrogels and released to facilitate vascularization.248 The set of peptide amphiphiles first reported by Hartgerink et al.217 and developed by the Stupp group207,218,219,221,222,224,225,227,233,234 is a notable example of such a platform. Attachment of a hydrophobic chain to a peptide strand imparts amphiphilicity to these hybrid constructs, leading to a nanofibrous micellar assembly in aqueous solution. The water-facing surface of the nanofibers may display bioactive epitopes, enabling interaction with specific cells.61,65,249 Other self-assembled platforms have been built upon αhelical coiled coils,240−243 β-sheets,237−239 triple-helical collagen mimetic peptides,210,211,235,236 and elastin-like peptides215,244 (Table 2). These peptides have been used for drug delivery,245 tissue engineering,204 and other interesting applications (e.g., biomedical semiconductors250). Self-assembly of these peptides is generally mediated by noncovalent forces. Hierarchical self-assembly of the peptides may constitute hydrogels that resemble the extracellular matrix. Representative examples of self-assembling peptide scaffolds are illustrated in Figure 3.62 Self-assembling peptides allow facile sequence modification. Therapeutic molecules can be sequestered in a self-assembled peptide hydrogel for sustained in vivo delivery. In the hydrogel, the therapeutic agent can be attached in a variety of ways: it can travel in the hydrophobic core of a nanofiber, be attached directly via ionic binding, or be delivered via nanofiberliposome hydrogel.251 Drug kinetics can be regulated by changing the affinity of the drug to the hydrogel itself.252 Depending on the desired result, modifications of the peptide sequence can impart flexibility to the rapidity of drug delivery.251,253,254
Table 2. Biomimetic Self-Assembling Peptide Platforms peptide
conformation
nanostructure
physical format
peptide amphiphiles65,207,217−234 collagen mimetic peptides210,211,235,236 β-sheet peptides237−239 coiled-coil peptides240−243
β-sheet
nanofibers
hydrogel
triple helix
hydrogel
elastin-like peptides215,244
random coil
nanofibers, 2D sheets nanofibers nanofibers, nanocages nanofibers
β-sheet α-helix
hydrogel hydrogel mats
Supramolecular polymerization of these peptides may either be predominantly driven by a decrease in enthalpy or by an increase in entropy.208,209 In the former case, the process becomes less favorable at higher temperatures (as the noncovalent interactions, such as ionic interactions and hydrogen bonds, weaken).210,211 However, these noncovalent bonds reform when the temperature is lowered. Such reversibility enables thermal annealing212,213 of the peptide. When the peptide solution is heated to a high temperature and then cooled, the metastable states break down and during reassembly there is enough activation energy to favor thermodynamically optimal assembly.210,211 In contrast, if the self-assembly is driven by the resulting increase of the entropy of the system, the stability of the fibril may increase with temperature (as the TΔS term in ΔG predominates at higher temperature).214 For such self-assembly, a thermal melting transition may not be observed, and in fact, an inverse transition temperature may be detected.215,216 An example of
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PEPTIDES WITH FUNCTIONAL MIMICS These self-assembling peptides can organize into fibrous scaffolds or hydrogels when doped with angiogenic cytokines or growth factors, noncovalently sequestering them.254 In this case, the stored biomolecules diffuse away slowly from the storage cache, creating a natural gradient that could be useful for inviting the target cell types, such as endothelial progenitor cells, to the desired functional niche. However, the peptide primary structure can be edited directly to insert the E
DOI: 10.1021/acs.biomac.8b01137 Biomacromolecules XXXX, XXX, XXX−XXX
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Figure 3. A few representative self-assembling peptide platforms. (A) Peptide amphiphiles undergo a micellar assembly (From ref 217. Reprinted with permission from AAAS. http://science.sciencemag.org/content/294/5547/1684). (B) Placement of hydrophobic residues and charged residues in designated places in the primary structure may afford a precisely controlled self-assembly of an α-helical coiled coil assembly (Reprinted with permission from ref 236. Copyright 2013, American Chemical Society). (C) A β-sheet fibrillizing domain may enable attachment of fusion protein onto a self-assembled nanofiber (Reprinted by permission from Springer Nature, ref 255. Copyright 2014, Macmillan Publishers Limited. http://www.nature.com/nmat/). (D) Mimicry of the hierarchical self-assembly of fibrillar collagen may offer clues to self-assembly of nanofibrous hydrogels from collagen mimetic peptides (Reprinted by permission from Springer Nature, ref 210. Copyright 2011, Macmillan Publishers Limited. http://www.nature.com/nchem/).
Figure 4. Presentation of different functional epitopes on self-assembling peptides may enable the fabrication of acellular scaffolds with specific spatial patterns promoting or inhibiting the infiltration or proliferation of a specific cell type. For example, angiogenic or antiangiogenic mimics on self-assembling platforms may facilitate or inhibit the recruitment of endothelial progenitor cells. When such patterned acellular scaffolds are implanted in vivo, recruitment and integration of autologous cells into the scaffold may lead to spontaneous spatial segregation, an important requirement of functional tissue regeneration.
that can be employed selectively to make a given acellular scaffold appropriate for a given application. A unique advantage of peptide mimics is that they can be modified by a change in the peptide backbone and the
angiogenic mimic sequences discussed in Table 1. In this case, the embedded signals are relatively stationary and are present at a high epitope density; thus, this scenario can lead to sustained efficacy. Both approaches have unique advantages F
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Figure 5. (A) Various stages of atherosclerosis. In a healthy vessel (stage A1), the vessel is patent. Because of age and other risk factors, there is a gradual build-up of fatty plaques between the endothelial cell layer and tunica intima (stage A2), leading to activation of endothelial cells and recruitment of macrophages into the plaques (stage A3). The macrophages phagocytose the lipid build-up to transform into foam cells and in turn signal further recruitment of macrophages, leading to neointimal hyperplasia and pathological stenosis of the blood vessel (stage A4). (B) Replacement of a diseased vessel with an acellular graft is often necessary when the stenosis of the vessel is too severe (stage A4). Surgical excision of the diseased segment (B1) with an acellular scaffold (B2) is a viable therapeutic approach. The surgical approach could be augmented by embedding angiogenic signals in the acellular scaffold to facilitate perianastomotic neoendothelialization (B3) and to prevent platelet-mediated restenosis and medial necrosis.
attached to promoter or inhibitor mimics (e.g., angiogenic or antiangiogenic) can potentially pattern an acellular scaffold to allow or prevent the infiltration of specific cells (e.g., endothelial progenitor cells vs macrophages) (Figure 4A). We can envision making multicomponent scaffolds that induce surrounding stem or stromal cells to spontaneously segregate into function-specific niches based on these rationally designed patterns (Figure 4B). A useful analogy can be made with logic gates if we think of these encoded signals as 1 (promoter) or 0 (inhibitor). We can then direct the flow of specific types of cells from the surrounding niche into the scaffold.271 Such programmed scaffolds may behave as logic gates for cellular migration/differentiation (e.g., AND/OR gates). We propose that programmable acellular scaffolds may represent the next generation of functional tissue-engineered scaffolds. A Specific Example: Tissue-Engineered Acellular Vascular Grafts. A concrete example where establishment of angiogenic niches may lead to marked improvement over the current standard of care is the field of vascular prostheses, especially vascular grafts. In severe cases of atherosclerosis, the patency of a blood vessel is drastically reduced due to build-up of fatty plaques in the vessel wall resulting in loss of flexibility and high blood pressure (Figure 5A). In such cases, surgical replacement of the native blood vessel with a vascular graft becomes necessary (Figure 5B). However, the failure rate for vascular grafts, especially for small-diameter (
