Sulfated glycopeptide nanostructures for multipotent protein activation

Self-assembly in elastin-like recombinamers: a mechanism to mimic natural complexity

The topic of self-assembled structures based on elastin-like recombinamers (ELRs, i.e., elastin-like polymers recombinantly bio-produced) has released a noticeable amount of references in the last few years. Most of them are intended for biomedical applications. In this review, a complete revision of the bibliography is carried out. Initially, the self-assembly (SA) concept is considered from a general point of view, and then ELRs are described and characterized based on their intrinsic disorder. A classification of the different self-assembled ELR-based structures is proposed based on their morphologies, paying special attention to their tentative modeling. The impact of the mechanism of SA on these biomaterials is analyzed. Finally, the implications of ELR SA in biological systems are considered.

Solvent-tunable dipeptide-based nanostructures with enhanced optical-to-electrical transduction

We explore a facile approach to construct solvent-tunable dipeptide-based nanostructures, which exhibit a structure-dependent and remarkably enhanced optoelectronic response.

Autonomous Ultrafast Self-Healing Hydrogels by pH-Responsive Functional Nanofiber Gelators as Cell Matrices

The synthesis of hybrid hydrogels by pH-controlled structural transition with exceptional rheological properties as cellular matrix is reported. "Depsi" peptide sequences are grafted onto a polypeptide backbone that undergo a pH-induced intramolecular O-N-acyl migration at physiological conditions affording peptide nanofibers (PNFs) as supramolecular gelators. The polypeptide-PNF hydrogels are mechanically remarkably robust. They reveal exciting thixotropic behavior with immediate in situ recovery after exposure to various high strains over long periods and self-repair of defects by instantaneous reassembly. High cytocompatibility, convenient functionalization by coassembly, and controlled enzymatic degradation but stability in 2D and 3D cell culture as demonstrated by the encapsulation of primary human umbilical vein endothelial cells and neuronal cells open many attractive opportunities for 3D tissue engineering and other biomedical applications.

A Heparan Sulfate Device for the Regeneration of Osteochondral Defects

IMPACT STATEMENT

Repairing damaged joint cartilage remains a significant challenge. Treatment involving microfracture, tissue grafting, or cell therapy provides some benefit, but seldom regenerates lost articular cartilage. Providing a point-of-care solution that is cell and tissue free has the potential to transform orthopedic treatment for such cases. Glycosaminoglycans such as heparan sulfate (HS) are well suited for this purpose because they provide a matrix that enhances the prochondrogenic activities of growth factors normally found at sites of articular damage. In this study, we show the potential of a novel HS device, which is free of exogenous cells or growth factors, in regenerating osteochondral defects.

Minimalistic supramolecular proteoglycan mimics by co-assembly of aromatic peptide and carbohydrate amphiphiles

A modular two-component supramolecular hydrogel composed of a peptide core and carbohydrate shell as a minimalistic mimic of proteoglycans.

We report the co-assembly of aromatic carbohydrate and dipeptide amphiphiles under physiological conditions as a strategy to generate minimalistic proteoglycan mimics. The resulting nanofibers present a structural, fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) core and a functional carbohydrate (Fmoc-glucosamine-6-sulfate or -phosphate) shell. The size, degree of bundling and mechanical properties of the assembled structures depend on the chemical nature of the carbohydrate amphiphile used. In cell culture medium, these nanofibers can further organize into supramolecular hydrogels. We demonstrate that, similar to proteoglycans, the assembled gels prolong the stability of growth factors and preserve the viability of cultured cells. Our results demonstrate that this approach can be applied to the design of extracellular matrix (ECM) substitutes for future regenerative therapies.

C-Terminal Residue of Ultrashort Peptides Impacts on Molecular Self-Assembly, Hydrogelation, and Interaction with Small-Molecule Drugs

Single molecular changes on a tripeptide can have dramatic effects on their self-assembly and hydrogelation. Herein, we explore C-terminal residue variation on two consistent ultrashort peptide backbones, i.e. acetylated-Leu-Ile-Val-Ala-Gly-Xaa and acetylated-Ile-Val-Xaa (Xaa = His, Arg, Asn). The objective of this study is to identify candidates that can form hydrogels for small-molecule drug (SMD) delivery. Haemolysis and cytotoxicity (with human adipose-derived mesenchymal stem cells) assays showed that the new soluble peptides (Xaa = His, Arg) are cytocompatible. Gelation studies showed that all but acetylated-Ile-Val-Arg could gel under physiological conditions. Longer peptidic backbones drive self-assembly more effectively as reflected in field emission scanning electron microscopy (FESEM) and circular dichroism spectroscopy studies. Rheological studies revealed that the resultant hydrogels have varying stiffness and yield stress, depending on the backbone and C-terminal residue. Visible spectroscopy-based elution studies with SMDs (naltrexone, methotrexate, doxorubicin) showed that besides the C-terminal residue, the shape of the SMD also determines the rate and extent of SMD elution. Based on the elution assays, infrared spectroscopy, and FESEM, we propose models for the peptide fibril-SMD interaction. Our findings highlight the importance of matching the molecular properties of the self-assembling peptide and SMD in order to achieve the desired SMD release profile.

A polycaprolactone-β-tricalcium phosphate-heparan sulphate device for cranioplasty

BACKGROUND

Cranioplasty is a surgical procedure used to treat a bone defect or deformity in the skull. To date, there is little consensus on the standard-of-care for graft materials used in such a procedure. Graft materials must have sufficient mechanical strength to protect the underlying brain as well as the ability to integrate and support new bone growth. Also, the ideal graft material should be individually customized to the contours of the defect to ensure a suitable aesthetic outcome for the patient.

PURPOSE

Customized 3D-printed scaffolds comprising of polycaprolactone-β-tricalcium phosphate (PCL-TCP) have been developed with mechanical properties suitable for cranioplasty. Osteostimulation of PCL-TCP was enhanced through the addition of a bone matrix-mimicking heparan sulphate glycosaminoglycan (HS3) with increased affinity for bone morphogenetic protein-2 (BMP-2). Efficacy of this PCL-TCP/HS3 combination device was assessed in a rat critical-sized calvarial defect model.

METHOD

Critical-sized defects (5 mm) were created in both parietal bones of 19 Sprague Dawley rats (Male, 450-550 g). Each cranial defect was randomly assigned to 1 of 4 treatment groups: (1) A control group consisting of PCL-TCP/Fibrin alone (n = 5); (2) PCL-TCP/Fibrin-HSft (30 μg) (n = 6) (HSft is the flow-through during HS3 isolation that has reduced affinity for BMP-2); (3) PCL-TCP/Fibrin-HS3 (5 μg) (n = 6); (4) PCL-TCP/Fibrin-HS3 (30 μg) (n = 6). Scaffold integration and bone formation was evaluated 12-weeks post implantation by μCT and histology.

RESULTS

Treatment with PCL-TCP/Fibrin alone (control) resulted in 23.7% ± 1.55% (BV/TV) of the calvarial defect being filled with new bone, a result similar to treatment with PCL-TCP/Fibrin scaffolds containing either HSft or HS3 (5 μg). At increased amounts of HS3 (30 μg), enhanced bone formation was evident (BV/TV = 38.6% ± 9.38%), a result 1.6-fold higher than control. Further assessment by 2D μCT and histology confirmed the presence of enhanced bone formation and scaffold integration with surrounding host bone only when scaffolds contained sufficient bone matrix-mimicking HS3.

CONCLUSION

Enhancing the biomimicry of devices using a heparan sulphate with increased affinity to BMP-2 can serve to improve the performance of PCL-TCP scaffolds and provides a suitable treatment for cranioplasty.

Molecular bionics - engineering biomaterials at the molecular level using biological principles

Life and biological units are the result of the supramolecular arrangement of many different types of molecules, all of them combined with exquisite precision to achieve specific functions. Taking inspiration from the design principles of nature allows engineering more efficient and compatible biomaterials. Indeed, bionic (from bion-, unit of life and -ic, like) materials have gained increasing attention in the last decades due to their ability to mimic some of the characteristics of nature systems, such as dynamism, selectivity, or signalling. However, there are still many challenges when it comes to their interaction with the human body, which hinder their further clinical development. Here we review some of the recent progress in the field of molecular bionics with the final aim of providing with design rules to ensure their stability in biological media as well as to engineer novel functionalities which enable navigating the human body.

Peptide supramolecular materials for therapeutics

Supramolecular assembly of peptide-based monomers into nanostructures offers many promising applications in advanced therapies. In this Tutorial Review, we introduce molecular designs to control the structure and potential biological function of supramolecular assemblies. An emphasis is placed on peptide-based supramolecular nanostructures that are intentionally designed to signal cells, either directly through the incorporation of amino acid sequences that activate receptors or indirectly by recruiting native signals such as growth factors. Additionally, we describe the use and future potential of hierarchical structures, such as single molecules that assemble into nanoscale fibers which then align to form macroscopic strings; the strings can then serve as scaffolds for cell growth, proliferation, and differentiation.

Supramolecular Nanostructure Activates TrkB Receptor Signaling of Neuronal Cells by Mimicking Brain-Derived Neurotrophic Factor

Brain-derived neurotrophic factor (BDNF), a neurotrophin that binds specifically to the tyrosine kinase B (TrkB) receptor, has been shown to promote neuronal differentiation, maturation, and synaptic plasticity in the central nervous system (CNS) during development or after injury and onset of disease. Unfortunately, native BDNF protein-based therapies have had little clinical success due to their suboptimal pharmacological properties. In the past 20 years, BDNF mimetic peptides have been designed with the purpose of activating certain cell pathways that mimic the functional activity of native BDNF, but the interaction of mimetic peptides with cells can be limited due to the conformational specificity required for receptor activation. We report here on the incorporation of a BDNF mimetic sequence into a supramolecular peptide amphiphile filamentous nanostructure capable of activating the BDNF receptor TrkB and downstream signaling in primary cortical neurons in vitro. Interestingly, we found that this BDNF mimetic peptide is only active when displayed on a peptide amphiphile supramolecular nanostructure. We confirmed that increased neuronal maturation is linked to TrkB signaling pathways by analyzing the phosphorylation of downstream signaling effectors and tracking electrical activity over time. Furthermore, three-dimensional gels containing the BDNF peptide amphiphile (PA) nanostructures encourage cell infiltration while increasing functional maturation. Our findings suggest that the BDNF mimetic PA nanostructure creates a highly bioactive matrix that could serve as a biomaterial therapy in injured regions of the CNS. This new strategy has the potential to induce endogenous cell infiltration and promote functional neuronal maturation through the presentation of the BDNF mimetic signal.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

Supramolecular Platform Stabilizing Growth Factors

High concentrations of supplemented growth factors can cause oversaturation and adverse effects in in vitro and in vivo studies, though these supraphysiological concentrations are often required due to the low stability of growth factors. Here we demonstrate the stabilization of TGF-β1 and BMP4 using supramolecular polymers. Inspired by heparan sulfate, sulfonated peptides were presented on a supramolecular polymer to allow for noncovalent binding to growth factors in solution. After mixing with excipient molecules, both TGF-β1 and BMP4 were shown to have a prolonged half-life compared to the growth factors free in solution. Moreover, high cellular response was measured by a luciferase assay, indicating that TGF-β1 remained highly active upon binding to the supramolecular assembly. The results demonstrate that significant lower concentrations of growth factors can be used when supramolecular polymers bearing growth factor binding moieties are implemented. This approach can also be exploited in hydrogel systems to control growth factor release.

Peptide-based nanoprobes for molecular imaging and disease diagnostics

Pathological changes in a diseased site are often accompanied by abnormal activities of various biomolecules in and around the involved cells. Identifying the location and expression levels of these biomolecules could enable early-stage diagnosis of the related disease, the design of an appropriate treatment strategy, and the accurate assessment of the treatment outcomes. Over the past two decades, a great diversity of peptide-based nanoprobes (PBNs) have been developed, aiming to improve the in vitro and in vivo performances of water-soluble molecular probes through engineering of their primary chemical structures as well as the physicochemical properties of their resultant assemblies. In this review, we introduce strategies and approaches adopted for the identification of functional peptides in the context of molecular imaging and disease diagnostics, and then focus our discussion on the design and construction of PBNs capable of navigating through physiological barriers for targeted delivery and improved specificity and sensitivity in recognizing target biomolecules. We highlight the biological and structural roles that low-molecular-weight peptides play in PBN design and provide our perspectives on the future development of PBNs for clinical translation.

Glycosaminoglycan-based hybrid hydrogel encapsulated with polyelectrolyte complex nanoparticles for endogenous stem cell regulation in central nervous system regeneration

The poor regenerative capability of stem cell transplantation in the central nervous system limits their therapeutic efficacy in brain injuries. The sustained inflammatory response, lack of structural support, and trophic factors deficiency restrain the integration and long-term survival of stem cells. Instead of exogenous stem cell therapy, here we described the synthesis of nanohybrid hydrogel containing sulfated glycosaminoglycan-based polyelectrolyte complex nanoparticles (PCN) to mimic the brain extracellular matrix and control the delivery of stromal-derived factor-1α (SDF-1α) and basic fibroblast factor (bFGF) in response to matrix metalloproteinase (MMP) for recruiting endogenous neural stem cells (NSC) and regulating their cellular fate. Bioactive factors are delivered by electrostatic sequestration on PCN to amplify the signaling of SDF-1α and bFGF to regulate NSC in vitro. In in vivo ischemic stroke model, the factors promoted neurological behavior recovery by enhancing neurogenesis and angiogenesis. These combined strategies may be applied for other tissue regenerations by regulating endogenous progenitors through the delivery of different kinds of glycosaminoglycan-binding molecules.

Fabrication of polycaprolactone-silanated β-tricalcium phosphate-heparan sulfate scaffolds for spinal fusion applications

BACKGROUND CONTEXT

Interbody spinal fusion relies on the use of external fixation and the placement of a fusion cage filled with graft materials (scaffolds) without regard for their mechanical performance. Stability at the fusion site is instead reliant on fixation hardware combined with a selected cage. Ideally, scaffolds placed into the cage should both support the formation of new bone and contribute to the mechanical stability at the fusion site.

PURPOSE

We recently developed a scaffold consisting of silane-modified PCL-TCP (PCL-siTCP) with mechanical properties that can withstand the higher loads generated in the spine. To ensure the scaffold more closely mimicked the bone matrix, we incorporated collagen (Col) and a heparan sulfate glycosaminoglycan sugar (HS3) with increased affinity for heparin-binding proteins such as bone morphogenetic protein-2 (BMP-2). The osteostimulatory characteristic of this novel device delivering exogenous BMP2 was assessed in vitro and in vivo as a prelude to future spinal fusion studies with this device.

STUDY DESIGN/SETTING

A combination of cell-free assays (BMP2 release), progenitor cell-based assays (BMP2 bioactivity, cell proliferation and differentiation), and rodent ectopic bone formation assays was used to assess the osteostimulatory characteristics of the PCL-siTCP-based scaffolds.

MATERIALS AND METHODS

Freshly prepared rat mesenchymal stem cells were used to determine reparative cell proliferation and differentiation on the PCL-siTCP-based scaffolds over a 28-day period in vitro. The bioactivity of BMP2 released from the scaffolds was assessed on progenitor cells over a 28-day period using ALP activity assays and release kinetics as determined by enzyme-linked immunosorbent assay. For ectopic bone formation, intramuscular placement of scaffolds into Sprague Dawley rats (female, 4 weeks old, 120-150 g) was achieved in five animals, each receiving four treatments randomized for location along the limb. The four groups tested were (1) PCL-siTCP/Col (5-mm diameter×1-mm thickness), PCL-siTCP/Col/BMP2 (5 µg), (3) PCL-siTCP/Col/HS3 (25 µg), and (4) PCL-siTCP/Col/HS3/BMP2 (25 and 5 µg, respectively). Bone formation was evaluated at 8 weeks post implantation by microcomputed tomography (µCT) and histology.

RESULTS

Progenitor cell-based assays (proliferation, mRNA transcripts, and ALP activity) confirmed that BMP2 released from PCL-siTCP/Col/HS3 scaffolds increased ALP expression and mRNA levels of the osteogenic biomarkers Runx2, Col1a2, ALP, and bone gla protein-osteocalcin compared with devices without HS3. When the PCL-siTCP/Col/HS3/BMP2 scaffolds were implanted into rat hamstring muscle, increased bone formation (as determined by two-dimensional and three-dimensional µCTs and histologic analyses) was observed compared with scaffolds lacking BMP2. More consistent increases in the amount of ectopic bone were observed for the PCL-siTCP/Col/HS3/BMP2 implants compared with PCL-siTCP/Col/BMP2. Also, increased mineralizing tissue within the pores of the scaffold was seen with modified-tetrachrome histology, a result confirmed by µCT, and a modest but detectable increase in both the number and the thickness of ectopic bone structures were observed with the PCL-siTCP/Col/HS3/BMP2 implants.

CONCLUSIONS

The combination of PCL-siTCP/Col/HS3/BMP2 thus represents a promising avenue for further development as a bone graft alternative for spinal fusion surgery.

Manipulation of VEGF-induced angiogenesis by 2-N, 6-O-sulfated chitosan

Emerging evidence suggests that vascular endothelial growth facto (VEGF) is important in the treatment of various ischemic and cardiovascular diseases. However, it often suffers from high cost and easy deactivation with a short half-life. Here, we describe a synthetic 2-N, 6-O-sulfated chitosan (26SCS) with a high affinity to VEGF promoting the binding of the signaling protein to its VEGF receptor 2 (VEGFR2), activating receptor phosphorylation and pro-angiogenic related genes expression, and further stimulating downstream VEGF-dependent endothelial cell viability, migration, tube formation and rat aortic rings outgrowth. Interestingly, the obvious recruitment of mural cells were occurred to stabilize the sprouted microvessels. In addition, the pro-angiogenic potential of 26SCS composited VEGF was confirmed in vivo using the chick embryo chorioallantoic membrane (CAM) assay with an extensive perfusable vascular network. A longer monitoring was administered subcutaneously to mice in a biocompatible gelatin sponge and showed that VEGF with 26SCS had the capability to efficiently enhance neovascularization. These findings highlight that 26SCS, the semi-synthetic natural polymer, may be a promising coagent with VEGF for vascular therapy.

STATEMENT OF SIGNIFICANCE

Vascular endothelial growth factor (VEGF) is crucial for facilitating angiogenesis to supply oxygen and nutrient during wound healing and tissue regeneration. However, appropriate use of VEGF is an ongoing challenge due to its rapidly clearance and severe side effects at higher dosage. In this study, we described a synthetic 2-N, 6-O-sulfated chitosan (26SCS) with a high affinity to VEGF, which could significantly promote its binding capacity to VEGF receptor 2 and further stimulate the angiogenic behavior of endothelial cells. We further confirmed that 26SCS was spatially combined with VEGF in a "lying manner", and this spatial arrangement was more conducive to exposure of the receptor binding domain of VEGF. Additionally, it also promoted in vivo angiogenesis in a chicken chorioallantoic membrane assay and mouse subcutaneous implant model. This strategy may afford a new avenue to enhance pro-angiogenic capacity of VEGF.

Supramolecular scaffolds enabling the controlled assembly of functional molecular units

To assemble functional molecular units into a desired structure while controlling positional and orientational order is a key technology for the development of high-performance organic materials that exhibit electronic, optoelectronic, biological and even dynamic functions. For this purpose, we cannot rely simply on the inherent self-assembly properties of the target functional molecular units, since it is difficult to predict, based solely on the molecular structure, what structure will be achieved upon assembly. To address this issue, it would be useful to employ molecular building blocks with self-assembly structures that can be clearly predicted and defined, to make target molecular units assemble into a desired structure. To date, various motifs of molecular assemblies, polymers, discrete and/or three-dimensional metal-organic complexes, nanoparticles and metal/metal oxide substrates have been developed to create materials with particular structures and dimensionalities. In this perspective, we define such assembly motifs as "supramolecular scaffolds". The structure of supramolecular scaffolds can be classified in terms of dimensionality, and they range in size from nano- to macroscopic scales. Functional molecular units, when attached to supramolecular scaffolds either covalently or non-covalently, can be assembled into specific structures, thus enabling the exploration of new properties, which cannot be achieved with the target molecular units alone. Through the classification and overview of reported examples, we shed new light on supramolecular scaffolds for the rational design of organic and polymeric materials.

Prolonged survival of transplanted stem cells after ischaemic injury via the slow release of pro-survival peptides from a collagen matrix

Stem-cell-based therapies hold considerable promise for regenerative medicine. However, acute donor-cell death within several weeks after cell delivery remains a critical hurdle for clinical translation. Co-transplantation of stem cells with pro-survival factors can improve cell engraftment, but this strategy has been hampered by the typically short half-lives of the factors and by the use of Matrigel and other scaffolds that are not chemically defined. Here, we report a collagen-dendrimer biomaterial crosslinked with pro-survival peptide analogues that adheres to the extracellular matrix and slowly releases the peptides, significantly prolonging stem cell survival in mouse models of ischaemic injury. The biomaterial can serve as a generic delivery system to improve functional outcomes in cell-replacement therapy.

Controlling protein activity by dynamic recruitment on a supramolecular polymer platform

Nature uses dynamic molecular platforms for the recruitment of weakly associating proteins into higher-order assemblies to achieve spatiotemporal control of signal transduction. Nanostructures that emulate this dynamic behavior require features such as plasticity, specificity and reversibility. Here we introduce a synthetic protein recruitment platform that combines the dynamics of supramolecular polymers with the programmability offered by DNA-mediated protein recruitment. Assembly of benzene-1,3,5-tricarboxamide (BTA) derivatives functionalized with a 10-nucleotide receptor strand into µm-long supramolecular BTA polymers is remarkably robust, even with high contents of DNA-functionalized BTA monomers and associated proteins. Specific recruitment of DNA-conjugated proteins on the supramolecular polymer results in a 1000-fold increase in protein complex formation, while at the same time enabling their rapid exchange along the BTA polymer. Our results establish supramolecular BTA polymers as a generic protein recruitment platform and demonstrate how assembly of protein complexes along the supramolecular polymer allows efficient and dynamic control of protein activity.

DNA-origami allows the precise recruitment of DNA-protein conjugates but lacks the dynamics found in natural protein assemblies. Here the authors present a synthetic polymer platform that combines the dynamics of supramolecular polymers with the programmability of DNA-mediated protein recruitment.

Tuning carbohydrate density enhances protein binding and inhibition by glycosylated β-sheet peptide nanofibers

The efficacy of glycosylated β-sheet peptide nanofibers for inhibiting carbohydrate-binding proteins can be increased by tuning carbohydrate density to maximize protein binding affinity.

The Components of Bone and What They Can Teach Us about Regeneration

The problem of bone regeneration has engaged both physicians and scientists since the beginning of medicine. Not only can bone heal itself following most injuries, but when it does, the regenerated tissue is often indistinguishable from healthy bone. Problems arise, however, when bone does not heal properly, or when new tissue is needed, such as when two vertebrae are required to fuse to stabilize adjacent spine segments. Despite centuries of research, such procedures still require improved therapeutic methods to be devised. Autologous bone harvesting and grafting is currently still the accepted benchmark, despite drawbacks for clinicians and patients that include limited amounts, donor site morbidity, and variable quality. The necessity for an alternative to this "gold standard" has given rise to a bone-graft and substitute industry, with its central conundrum: what is the best way to regenerate bone? In this review, we dissect bone anatomy to summarize our current understanding of its constituents. We then look at how various components have been employed to improve bone regeneration. Evolving strategies for bone regeneration are then considered.

An in situ Dynamic Continuum of Supramolecular Phosphoglycopeptides Enables Formation of 3D Cell Spheroids

Higher-order assemblies of proteins, with a structural and dynamic continuum, is an important concept in biology, but these insights have yet to be applied in designing biomaterials. Dynamic assemblies of supramolecular phosphoglycopeptides (sPGPs) transform a 2D cell sheet into 3D cell spheroids. A ligand-receptor interaction between a glycopeptide and a phosphopeptide produces sPGPs that form nanoparticles, which transform into nanofibrils upon partial enzymatic dephosphorylation. The assemblies form dynamically and hierarchically in situ on the cell surface, and interact with the extracellular matrix molecules and effectively abolish contact inhibition of locomotion (CIL) of the cells. Integrating molecular recognition, catalysis, and assembly, these active assemblies act as a dynamic continuum to disrupt CIL, thus illustrating a new kind of biomaterial for regulating cell behavior.

Reversible Loading of Nanoscale Elements on a Multicomponent Supramolecular Polymer System by Using DNA Strand Displacement

Nucleic acids are excellent building blocks to enable switchable character in supramolecular polymer materials because of their inherent dynamic character and potential for orthogonal self-assembly. Herein, DNA-grafted squaramide bola-amphiphiles are used in a multicomponent supramolecular polymer system and it is shown that they can be addressed by DNAlabeled gold nanoparticles (5 and 15 nm) through sequence complementarity. These nanoparticles can be selectively erased or rewritten on-demand by means of DNA-strand displacement.

Supramolecular Assembly of Peptide Amphiphiles

Peptide amphiphiles (PAs) are small molecules that contain hydrophobic components covalently conjugated to peptides. In this Account, we describe recent advances involving PAs that consist of a short peptide sequence linked to an aliphatic tail. The peptide sequence can be designed to form β-sheets among the amino acids near the alkyl tail, while the residues farthest from the tail are charged to promote solubility and in some cases contain a bioactive sequence. In water, β-sheet formation and hydrophobic collapse of the aliphatic tails induce assembly of the molecules into supramolecular one-dimensional nanostructures, commonly high-aspect-ratio cylindrical or ribbonlike nanofibers. These nanostructures hold significant promise for biomedical functions due to their ability to display a high density of biological signals on their surface for targeting or to activate pathways, as well as for biocompatibility and biodegradable nature.

Recent studies have shown that supramolecular systems, such as PAs, often become kinetically trapped in local minima along their self-assembly reaction coordinate, not unlike the pathways associated with protein folding. Furthermore, the assembly pathway can influence the shape, internal structure, and dimension of nanostructures and thereby affect their bioactivity. We discuss methods to map the energy landscape of a PA structure as a function of thermal energy and ionic strength and vary these parameters to convert between kinetically trapped and thermodynamically favorable states. We also demonstrate that the pathway-dependent morphology of the PA assembly can determine biological cell adhesion and survival rates.

The dynamics associated with the nanostructures are also critical to their function, and techniques are now available to probe the internal dynamics of these nanostructures. For example, by conjugating radical electron spin labels to PAs, electron paramagnetic resonance spectroscopy can be used to study the rotational diffusion rates within the fiber, showing a liquidlike to solidlike transition through the cross section of the nanofiber. PAs can also be labeled with fluorescent dyes, allowing the use of super-resolution microscopy techniques to study the molecular exchange dynamics between PA fibers. For a weak hydrogen-bonding PA, individual PA molecules or clusters exchange between fibers in time scales as short as minutes. The amount of hydrogen bonding within PAs that dictates the dynamics also plays an important role in biological function. In one case, weak hydrogen bonding within a PA resulted in cell death through disruption of lipid membranes, while in another example reduced hydrogen bonding enhanced growth factor signaling by increasing lipid raft mobility.

PAs are a promising platform for designing advanced hybrid materials. We discuss a covalent polymer with a rigid aromatic imine backbone and alkylated peptide side chains that simultaneously polymerizes and interacts with a supramolecular PA structure with identical chemistry to that of the side chains. The covalent polymerization can be “catalyzed” by noncovalent polymerization of supramolecular monomers, taking advantage of the dynamic nature of supramolecular assemblies. These novel hybrid structures have potential in self-repairing materials and as reusable scaffolds for delivery of drugs or other chemicals. Finally, we highlight recent biomedical applications of PAs and related structures, ranging from bone regeneration to decreasing blood loss during internal bleeding.