Optimising a self-assembling peptide hydrogel as a Matrigel alternative for 3-dimensional mammary epithelial cell culture

Three-dimensional (3D) organoid models have been instrumental in understanding molecular mechanisms responsible for many cellular processes and diseases. However, established organic biomaterial scaffolds used for 3D hydrogel cultures, such as Matrigel, are biochemically complex and display significant batch variability, limiting reproducibility in experiments. Recently, there has been significant progress in the development of synthetic hydrogels for in vitro cell culture that are reproducible, mechanically tuneable, and biocompatible. Self-assembling peptide hydrogels (SAPHs) are synthetic biomaterials that can be engineered to be compatible with 3D cell culture. Here we investigate the ability of PeptiGel® SAPHs to model the mammary epithelial cell (MEC) microenvironment in vitro. The positively charged PeptiGel®Alpha4 supported MEC viability, but did not promote formation of polarised acini. Modifying the stiffness of PeptiGel® Alpha4 stimulated changes in MEC viability and changes in protein expression associated with altered MEC function, but did not fully recapitulate the morphologies of MECs grown in Matrigel. To supply the appropriate biochemical signals for MEC organoids, we supplemented PeptiGels® with laminin. Laminin was found to require negatively charged PeptiGel® Alpha7 for functionality, but was then able to provide appropriate signals for correct MEC polarisation and expression of characteristic proteins. Thus, optimisation of SAPH composition and mechanics allows tuning to support tissue-specific organoids.

Effect of Peptide-Polymer Host-Guest Electrostatic Interactions on Self-Assembling Peptide Hydrogels Structural and Mechanical Properties and Polymer Diffusivity

Peptide-based supramolecular hydrogels are an attractive class of soft materials for biomedical applications when biocompatibility is a key requirement as they exploit the physical self-assembly of short self-assembling peptides avoiding the need for chemical cross-linking. Based on the knowledge developed through our previous work, we designed two novel peptides, E(FKFE)2 and K(FEFK)2, that form transparent hydrogels at pH 7. We characterized the phase behavior of these peptides and showed the clear link that exists between the charge carried by the peptides and the physical state of the samples. We subsequently demonstrate the cytocompatibility of the hydrogel and its suitability for 3D cell culture using 3T3 fibroblasts and human mesenchymal stem cells. We then loaded the hydrogels with two polymers, poly-l-lysine and dextran. When polymer and peptide fibers carry opposite charges, the size of the elemental fibril formed decreases, while the overall level of fiber aggregation and fiber bundle formation increases. This overall network topology change, and increase in cross-link stability and density, leads to an overall increase in the hydrogel mechanical properties and stability, i.e., resistance to swelling when placed in excess media. Finally, we investigate the diffusion of the polymers out of the hydrogels and show how electrostatic interactions can be used to control the release of large molecules. The work clearly shows how polymers can be used to tailor the properties of peptide hydrogels through guided intermolecular interactions and demonstrates the potential of these new soft hydrogels for use in the biomedical field in particular for delivery or large molecular payloads and cells as well as scaffolds for 3D cell culture.

Novel hydrogels: are they poised to transform 3D cell-based assay systems in early drug discovery?

INTRODUCTION

Success in drug discovery remains unpredictable. However, more predictive and relevant disease models are becoming pivotal to demonstrating the clinical benefits of new drugs earlier in the lengthy drug discovery process. Novel hydrogel scaffolds are being developed to transform the relevance of such 3D cell-based in vitro assay systems.

AREAS COVERED

Most traditional hydrogels are still of unknown composition and suffer significant batch-to-batch variations, which lead to technical constraints. This article looks at how a new generation of novel synthetic hydrogels that are based on self-assembling peptides are poised to transform 3D cell-based assay systems by improving their relevance, reproducibility and scalability.

EXPERT OPINION

The emerging advantages of using these novel hydrogels for human 3D screening assays should enable the discovery of more cost-effective drugs, leading to improved patient benefits. Such a disruptive change could also reduce the considerable time lag from obtaining in vitro assay data to initiating clinical trials. There is now a sufficient body of data available in the literature to enable this ambition to become a reality by significantly improving the predictive validity of 3D cell-based assays in early drug discovery. Novel hydrogels are key to unlocking the full potential of these assay systems.

Growth and differentiation of human induced pluripotent stem cell (hiPSC)-derived kidney organoids using fully synthetic peptide hydrogels

Human induced pluripotent stem cell (hiPSC)-derived kidney organoids have prospective applications ranging from basic disease modelling to personalised medicine. However, there remains a necessity to refine the biophysical and biochemical parameters that govern kidney organoid formation. Differentiation within fully-controllable and physiologically relevant 3D growth environments will be critical to improving organoid reproducibility and maturation. Here, we matured hiPSC-derived kidney organoids within fully synthetic self-assembling peptide hydrogels (SAPHs) of variable stiffness (storage modulus, G'). The resulting organoids contained complex structures comparable to those differentiated within the animal-derived matrix, Matrigel. Single-cell RNA sequencing (scRNA-seq) was then used to compare organoids matured within SAPHs to those grown within Matrigel or at the air-liquid interface. A total of 13,179 cells were analysed, revealing 14 distinct clusters. Organoid compositional analysis revealed a larger proportion of nephron cell types within Transwell-derived organoids, while SAPH-derived organoids were enriched for stromal-associated cell populations. Notably, differentiation within a higher G' SAPH generated podocytes with more mature gene expression profiles. Additionally, maturation within a 3D microenvironment significantly reduced the derivation of off-target cell types, which are a known limitation of current kidney organoid protocols. This work demonstrates the utility of synthetic peptide-based hydrogels with a defined stiffness, as a minimally complex microenvironment for the selected differentiation of kidney organoids.

Controlling Doxorubicin Release from a Peptide Hydrogel through Fine-Tuning of Drug-Peptide Fiber Interactions

Hydrogels are versatile materials that have emerged in the last few decades as promising candidates for a range of applications in the biomedical field, from tissue engineering and regenerative medicine to controlled drug delivery. In the drug delivery field, in particular, they have been the subject of significant interest for the spatially and temporally controlled delivery of anticancer drugs and therapeutics. Self-assembling peptide-based hydrogels, in particular, have recently come to the fore as potential candidate vehicles for the delivery of a range of drugs. In order to explore how drug–peptide interactions influence doxorubicin (Dox) release, five β-sheet-forming self-assembling peptides with different physicochemical properties were used for the purpose of this study, namely: FEFKFEFK (F8), FKFEFKFK (FK), FEFEFKFE (FE), FEFKFEFKK (F8K), and KFEFKFEFKK (KF8K) (F: phenylalanine; E: glutamic acid; K: lysine). First, Dox-loaded hydrogels were characterized to ensure that the incorporation of the drug did not significantly affect the hydrogel properties. Subsequently, Dox diffusion out of the hydrogels was investigated using UV absorbance. The amount of drug retained in F8/FE composite hydrogels was found to be directly proportional to the amount of charge carried by the peptide fibers. When cation−π interactions were used, the position and number of end-lysine were found to play a key role in the retention of Dox. In this case, the amount of Dox retained in F8/KF8K composite hydrogels was linked to the amount of end-lysine introduced, and an end-lysine/Dox interaction stoichiometry of 3/1 was obtained. For pure FE and KF8K hydrogels, the maximum amount of Dox retained was also found to be related to the overall concentration of the hydrogels and, therefore, to the overall fiber surface area available for interaction with the drug. For 14 mM hydrogel, ∼170–200 μM Dox could be retained after 24 h. This set of peptides also showed a broad range of susceptibilities to enzymatic degradation opening the prospect of being able to control also the rate of degradation of these hydrogels. Finally, the Dox released from the hydrogel was shown to be active and affect 3T3 mouse fibroblasts viability in vitro. Our study clearly shows the potential of this peptide design as a platform for the formulation of injectable or sprayable hydrogels for controlled drug delivery.

Substrate Stiffness-Driven Membrane Tension Modulates Vesicular Trafficking via Caveolin-1

Liver fibrosis, a condition characterized by extensive deposition and cross-linking of extracellular matrix (ECM) proteins, is idiosyncratic in cases of chronic liver injury. The dysregulation of ECM remodeling by hepatic stellate cells (HSCs), the main mediators of fibrosis, results in an elevated ECM stiffness that drives the development of chronic liver disease such as cirrhosis and hepatocellular carcinoma. Tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) is a key element in the regulation of ECM remodeling, which modulates the degradation and turnover of ECM components. We have previously reported that a rigid, fibrotic-like substrate can impact TIMP-1 expression at the protein level in HSCs without altering its mRNA expression. While HSCs are known to be highly susceptible to mechanical stimuli, the mechanisms through which mechanical cues regulate TIMP-1 at the post-translational level remain unclear. Here, we show a mechanism of regulation of plasma membrane tension by matrix stiffness. We found that this effect is orchestrated by the β1 integrin/RhoA axis and results in elevated exocytosis and secretion of TIMP-1 in a caveolin-1- and dynamin-2-dependent manner. We then show that TIMP-1 and caveolin-1 expression increases in cirrhosis and hepatocellular carcinoma. These conditions are associated with fibrosis, and this effect can be recapitulated in 3D fibrosis models consisting of hepatic stellate cells encapsulated in a self-assembling polypeptide hydrogel. This work positions stiffness-dependent membrane tension as a key regulator of enzyme secretion and function and a potential target for therapeutic strategies that aim at modulating ECM remodeling in chronic liver disease.

TGF-β3-loaded graphene oxide - self-assembling peptide hybrid hydrogels as functional 3D scaffolds for the regeneration of the nucleus pulposus

Intervertebral disc (IVD) degeneration is a process that starts in the central nucleus pulposus (NP) and leads to inflammation, extracellular matrix (ECM) degradation, and progressive loss of disc height. Early treatment of IVD degeneration is critical to the reduction of low back pain and related disability. As such, minimally invasive therapeutic approaches that can halt and reverse NP degeneration at the early stages of the disease are needed. Recently, we developed an injectable graphene oxide (GO) - self-assembling peptide FEFKFEFK (F: phenylalanine; K: lysine; E: glutamic acid) hybrid hydrogels as potential delivery platform for cells and/or drugs in the NP. In this current study, we explored the possibility of using the GO present in these hybrid hydrogels as a vehicle for the sequestration and controlled delivery of transforming growth factor beta-3 (TGF-β3), an anabolic growth factor (GF) known to direct NP cell fate and function. For this purpose, we first investigated the potential of GO to bind and sequestrate TGF-β3. We then cultured bovine NP cells in the new functional scaffolds and investigated their response to the presence of GO and TGF-β3. Our results clearly showed that GO flakes can sequestrate TGF-β3 through strong binding interactions resulting in a slow and prolonged release, with the GF remaining active even when bound to the GO flakes. The adsorption of the GF on the GO flakes to create TGF-β3-loaded GO flakes and their subsequent incorporation in the hydrogels through mixing, [(GO/TGF-β3_Ads)-F8] hydrogel, led to the upregulation of NP-specific genes, accompanied by the production and deposition of an NP-like ECM, rich in aggrecan and collagen II. NP cells actively interacted with TGF-β3-loaded GO flakes and remodeled the scaffolds through endocytosis. This work highlights the potential of using GO as a nanocarrier for the design of functional hybrid peptide-based hydrogels. STATEMENT OF SIGNIFICANCE: Intervertebral disc (IVD) degeneration is a process that starts in the central nucleus pulposus (NP) and leads to inflammation, extracellular matrix (ECM) degradation, and progressive loss of disc height. As such, minimally invasive therapeutic approaches that can halt and reverse NP degeneration at the early stages of the disease are needed. In this current study, we explored the possibility of using peptide - GO hybrid hydrogels as a vehicle for the sequestration and controlled delivery of transforming growth factor beta-3 (TGF-β3), an anabolic growth factor (GF) known to direct NP cell fate and function.

Role of Sheet-Edge Interactions in β-sheet Self-Assembling Peptide Hydrogels

Hydrogels' hydrated fibrillar nature makes them the material of choice for the design and engineering of 3D scaffolds for cell culture, tissue engineering, and drug-delivery applications. One particular class of hydrogels which has been the focus of significant research is self-assembling peptide hydrogels. In the present work, we were interested in exploring how fiber-fiber edge interactions affect the self-assembly and gelation properties of amphipathic peptides. For this purpose, we investigated two β-sheet-forming peptides, FEFKFEFK (F8) and KFEFKFEFKK (KF8K), the latter one having the fiber edges covered by lysine residues. Our results showed that the addition of the two lysine residues did not affect the ability of the peptides to form β-sheet-rich fibers, provided that the overall charge carried by the two peptides was kept constant. However, it did significantly reduce edge-driven hydrophobic fiber-fiber associative interactions, resulting in reduced tendency for KF8K fibers to associate/aggregate laterally and form large fiber bundles and consequently network cross-links. This effect resulted in the formation of hydrogels with lower moduli but faster dynamics. As a result, KF8K fibers could be aligned only under high shear and at high concentration while F8 hydrogel fibers were found to align readily at low shear and low concentration. In addition, F8 hydrogels were found to fragment at high concentration because of the high aggregation state stabilizing the fiber bundles, resulting in fiber breakage rather than disentanglement and alignment.

Sequence-Specific Detection of Unlabeled Nucleic Acid Biomarkers Using a "One-Pot" 3D Molecular Sensor

DNA and RNA biomarkers have not progressed beyond the automated specialized clinic due to failure in the reproducibility necessary to standardize robust and rapid nucleic acid detection at the point of care, where health outcomes can be most improved by early-stage diagnosis and precise monitoring of therapy and disease prognosis. We demonstrate here a new analytical platform to meet this challenge using functional 3D hydrogels engineered from peptide and oligonucleotide building blocks to provide sequence-specific, PCR-free fluorescent detection of unlabeled nucleic acid sequences. We discriminated at picomolar detection limits (<7 pM) "perfect-match" from mismatched sequences, down to a single nucleotide mutation, buried within longer lengths of the target. Detailed characterization by NMR, TEM, mass spectrometry, and rheology provided the structural understanding to design these hybrid peptide-oligonucleotide biomaterials with the desired sequence sensitivity and detection limit. We discuss the generic design, which is based on a highly predictable secondary structure of the oligonucleotide components, as a platform to detect genetic abnormalities and to screen for pathogenic conditions at the level of both DNA (e.g., SNPs) and RNA (messenger, micro, and viral genomic RNA).

Graphene oxide containing self-assembling peptide hybrid hydrogels as a potential 3D injectable cell delivery platform for intervertebral disc repair applications

Cell-based therapies have shown significant promise in tissue engineering with one key challenge being the delivery and retention of cells. As a result, significant efforts have been made in the past decade to design injectable biomaterials to host and deliver cells at injury sites. Intervertebral disc (IVD) degeneration, a major cause of back pain, is a particularly relevant example where a minimally-invasive cellular therapy could bring significant benefits specifically at the early stages of the disease, when a cell-driven process starts in the gelatinous core of the IVD, the nucleus pulposus (NP).

In this present study we explore the use of graphene oxide (GO) as nano-filler for the reinforcement of FEFKFEFK (β-sheet forming self-assembling peptide) hydrogels. Our results confirm the presence of strong interactions between FEFKFEFK and GO flakes with the peptide coating and forming short thin fibrils on the surface of the flakes. These strong interactions were found to affect the bulk properties of hybrid hydrogels. At pH 4 electrostatic interactions between the peptide fibres and the peptide-coated GO flakes are thought to govern the final bulk properties of the hydrogels while at pH 7, after conditioning with cell culture media, electrostatic interactions are removed leaving the hydrophobic interactions to govern hydrogel final properties. The GO-F820 hybrid hydrogel, with mechanical properties similar to the NP, was shown to promote high cell viability and retained cell metabolic activity in 3D over the 7 days of culture and therefore shown to harbour significant potential as an injectable hydrogel scaffold for the in-vivo delivery of NP cells.

Statement of Significance

Short self-assembling peptide hydrogels (SAPHs) have attracted significant interest in recent years as they can mimic the natural extra-cellular matrix, holding significant promise for the ab initio design of cells’ microenvironments. Recently the design of hybrid hydrogels for biomedical applications has been explored through the incorporation of specific nanofillers. In this study we exploited graphene oxide (GO) as nanofiller to design hybrid injectable 3Dscaffolds for the delivery of nucleus pulposus cells (NPCs) for intervertebral disc regeneration. Our work clearly shows the presence of strong interactions between peptide and GO, mimicking the mechanical properties of the NP tissue and promoting high cell viability and metabolic activity. These hybrid hydrogels therefore harbour significant potential as injectable scaffolds for the in vivo delivery of NPCs.

Designing Peptide/Graphene Hybrid Hydrogels through Fine-Tuning of Molecular Interactions

A recent strategy that has emerged for the design of increasingly functional hydrogels is the incorporation of nanofillers in order to exploit their specific properties to either modify the performance of the hydrogel or add functionality. The emergence of carbon nanomaterials in particular has provided great opportunity for the use of graphene derivatives (GDs) in biomedical applications. The key challenge when designing hybrid materials is the understanding of the molecular interactions between the matrix (peptide nanofibers) and the nanofiller (here GDs) and how these affect the final properties of the bulk material. For the purpose of this work, three gelling β-sheet-forming, self-assembling peptides with varying physiochemical properties and five GDs with varying surface chemistries were chosen to formulate novel hybrid hydrogels. First the peptide hydrogels and the GDs were characterized; subsequently, the molecular interaction between peptides nanofibers and GDs were probed before formulating and mechanically characterizing the hybrid hydrogels. We show how the interplay between electrostatic interactions, which can be attractive or repulsive, and hydrophobic (and π-π in the case of peptide containing phenylalanine) interactions, which are always attractive, play a key role on the final properties of the hybrid hydrogels. The shear modulus of the hydrid hydrogels is shown to be related to the strength of fiber adhesion to the flakes, the overall hydrophobicity of the peptides, as well as the type of fibrillar network formed. Finally, the cytotoxicity of the hybrid hydrogel formed at pH 6 was also investigated by encapsulating and culturing human mesemchymal stem cells (hMSC) over 14 days. This work clearly shows how interactions between peptides and GDs can be used to tailor the mechanical properties of the resulting hydrogels, allowing the incorporation of GD nanofillers in a controlled way and opening the possibility to exploit their intrinsic properties to design novel hybrid peptide hydrogels for biomedical applications.

A de novo self-assembling peptide hydrogel biosensor with covalently immobilised DNA-recognising motifs

We report here the first experimental evidence of a self-assembling three-dimensional (3D) peptide hydrogel, with recognition motifs immobilized on the surface of fibres capable of sequence-specific oligonucleotide detection. These systems have the potential to be further developed into diagnostic and prognostic tools in human pathophysiology.

Western blot analysis of cells encapsulated in self-assembling peptide hydrogels

Continuous optimization of in vitro analytical techniques is ever more important, especially given the development of new materials for tissue engineering studies. In particular, isolation of cellular components for downstream applications is often hindered by the presence of biomaterials, presenting a major obstacle in understanding how cell–matrix interactions influence cell behavior. Here, we describe an approach for western blot analysis of cells that have been encapsulated in self-assembling peptide hydrogels (SAPHs), which highlights the need for complete solubilization of the hydrogel construct. We demonstrate that both the choice of buffer and multiple cycles of sonication are vital in obtaining complete solubilization, thereby enabling the detection of proteins otherwise lost to SAP aggregation. Moreover, we show that the presence of self-assembling peptides (SAPs) does not interfere with the standard immunoblotting technique, offering the potential for use in more full-scale proteomic studies.

In vivo characterisation of a therapeutically relevant self-assembling 18 F-labelled β-sheet forming peptide and its hydrogel using positron emission tomography

Positron emission tomography (PET) and fluorescence labelling have been used to assess the pharmacokinetics, biodistribution and eventual fate of a hydrogel‐forming nonapeptide, FEFKFEFKK (F9), in healthy mice, using ¹⁸F‐labelled and fluorescein isothiocyanate (FITC)‐labelled F9 analogues. F9 was site‐specifically radiolabelled with 2‐[¹⁸F]fluoro‐3‐pyridinecarboxaldehyde ([¹⁸F]FPCA) via oxime bond formation. [¹⁸F]FPCA‐F9 in vivo fate was evaluated both as a solution, following intravenous administration, and as a hydrogel when subcutaneously injected. The behaviour of FITC‐F9 hydrogel was assessed following subcutaneous injection. [¹⁸F]FPCA‐F9 demonstrated high plasma stability and primarily renal excretion; [¹⁸F]FPCA‐F9 when in solution and injected into the bloodstream displayed prompt bladder uptake (53.4 ± 16.6 SUV at 20 minutes postinjection) and rapid renal excretion, whereas [¹⁸F]FPCA‐F9 hydrogel, formed by co‐assembly of [¹⁸F]FPCA‐F9 monomer with unfunctionalised F9 peptide and injected subcutaneously, showed gradual bladder accumulation of hydrogel fragments (3.8 ± 0.4 SUV at 20 minutes postinjection), resulting in slower renal excretion. Gradual disaggregation of the F9 hydrogel from the site of injection was monitored using FITC‐F9 hydrogel in healthy mice (60 ± 3 over 96 hours), indicating a biological half‐life between 1 and 4 days. The in vivo characterisation of F9, both as a gel and a solution, highlights its potential as a biomaterial.

Peptide hydrogel in vitro non-inflammatory potential

Peptide‐based hydrogels have attracted significant interest in recent years as these soft, highly hydrated materials can be engineered to mimic the cell niche with significant potential applications in the biomedical field. Their potential use in vivo in particular is dependent on their biocompatibility, including their potential to cause an inflammatory response. In this work, we investigated in vitro the inflammatory potential of a β‐sheet forming peptide (FEFEFKFK; F: phenylalanine, E: glutamic acid; K: lysine) hydrogel by encapsulating murine monocytes within it (3D culture) and using the production of cytokines, IL‐β, IL‐6 and TNFα, as markers of inflammatory response. No statistically significant release of cytokines in our test sample (media + gel + cells) was observed after 48 or 72 h of culture showing that our hydrogels do not incite a pro‐inflammatory response in vitro. These results show the potential biocompatibility of these hydrogels and therefore their potential for in vivo use. The work also highlighted the difference in monocyte behaviour, proliferation and morphology changes when cultured in 2D vs. 3D. © 2016 The Authors Journal of Peptide Science published by European Peptide Society and John Wiley & Sons Ltd.

Osteogenic differentiation of human mesenchymal stem cells promotes mineralization within a biodegradable peptide hydrogel

An attractive strategy for the regeneration of tissues has been the use of extracellular matrix analogous biomaterials. Peptide-based fibrillar hydrogels have been shown to mimic the structure of extracellular matrix offering cells a niche to undertake their physiological functions. In this study, the capability of an ionic-complementary peptide FEFEFKFK (F, E, and K are phenylalanine, glutamic acid, and lysine, respectively) hydrogel to host human mesenchymal stem cells in three dimensions and induce their osteogenic differentiation is demonstrated. Assays showed sustained cell viability and proliferation throughout the hydrogel over 12 days of culture and these human mesenchymal stem cells differentiated into osteoblasts simply upon addition of osteogenic stimulation. Differentiated osteoblasts synthesized key bone proteins, including collagen-1 (Col-1), osteocalcin, and alkaline phosphatase. Moreover, mineralization occurred within the hydrogel. The peptide hydrogel is a naturally biodegradable material as shown by oscillatory rheology and reversed-phase high-performance liquid chromatography, where both viscoelastic properties and the degradation of the hydrogel were monitored over time, respectively. These findings demonstrate that a biodegradable octapeptide hydrogel can host and induce the differentiation of stem cells and has the potential for the regeneration of hard tissues such as alveolar bone.

Modification of β-Sheet Forming Peptide Hydrophobic Face: Effect on Self-Assembly and Gelation

β-Sheet forming peptides have attracted significant interest for the design of hydrogels for biomedical applications. One of the main challenges is the control and understanding of the correlations between peptide molecular structure, the morphology, and topology of the fiber and network formed as well as the macroscopic properties of the hydrogel obtained. In this work, we have investigated the effect that functionalizing these peptides through their hydrophobic face has on their self-assembly and gelation. Our results show that the modification of the hydrophobic face results in a partial loss of the extended β-sheet conformation of the peptide and a significant change in fiber morphology from straight to kinked. As a consequence, the ability of these fibers to associate along their length and form large bundles is reduced. These structural changes (fiber structure and network topology) significantly affect the mechanical properties of the hydrogels (shear modulus and elasticity).

A modular self-assembly approach to functionalised β-sheet peptide hydrogel biomaterials

Two complementary β-sheet-forming decapeptides have been created that form binary self-repairing hydrogels upon combination of the respective free-flowing peptide solutions at pH 7 and >0.28 wt%. The component peptides showed little structure separately but formed extended β-sheet fibres upon mixing, which became entangled to produce stiff hydrogels. Microscopy revealed two major structures; thin fibrils with a twisted or helical appearance and with widths comparable to the predicted lengths of the peptides within a β-sheet, and thicker, longer, interwoven fibres that appear to comprise laterally-packed fibrils. A range of gel stiffnesses (G' from 0.05 to 100 kPa) could be attained in this system by altering the assembly conditions, stiffnesses that cover the rheological properties desirable for cell culture scaffolds. Doping in a RGD-tagged component peptide at 5 mol% improved 3T3 fibroblast attachment and viability compared to hydrogel fibres without RGD functionalisation.

Self-assembling peptide/thermoresponsive polymer composite hydrogels: effect of peptide-polymer interactions on hydrogel properties

We have investigated the effect of doping the self-assembling octapeptide FEFEFKFK (F, phenylalanine; E, glutamic acid; K, lysine) hydrogels with various amounts of thermoresponsive conjugate of FEFEFKFK and poly(N-isopropylacrylamide) (PNIPAAm) in order to create novel hydrogels. The samples were characterized using a range of techniques including microdifferential scanning calorimetry (μDSC), oscillatory rheology, transmission electron microscopy (TEM), atomic force microscopy (AFM), and small angle neutron scattering (SANS). The peptide from the conjugate was shown to be incorporated into the peptide fiber, resulting in the polymer being anchored to the peptide fiber. The conjugation of the polymer to the peptide and its anchoring to the peptide fibers did not affect its lower critical solution temperature (LCST). On the other hand, it did result in a decrease in the LCST enthalpy and a significant increase in the G' of the hydrogels, suggesting the presence of hydrogen bond interactions between the peptide and the polymer. As a result, the polymer was found to adopt a fibrillar arrangement tightly covering the peptide fiber. The polymer was still found to go through a conformational change at the LCST, suggesting that it collapses onto the peptide fiber. On the other hand, the fibrillar network was found to be mainly unaffected by the polymer LCST. These changes at the LCST were also found to be fully reversible. The nature of the interaction between the polymer and the peptide was shown to have a significant effect on the conformation adopted by the polymer around the fibers and the mechanical properties of the hydrogels.

Human osteoblasts within soft peptide hydrogels promote mineralisation in vitro

Biomaterials that provide three-dimensional support networks for the culture of cells are being developed for a wide range of tissue engineering applications including the regeneration of bone. This study explores the potential of the versatile ionic-complementary peptide, FEFEFKFK, for such a purpose as this peptide spontaneously self-assembles into β-sheet-rich fibres that subsequently self-associate to form self-supporting hydrogels. Via simple live/dead cell assays, we demonstrated that 3 wt% hydrogels were optimal for the support of osteoblast cells. We went on to show that these cells are not only viable within the three-dimensional hydrogel but they also proliferate and produce osteogenic key proteins, that is, they behave like in vivo bone cells, over the 14-day period explored here. The gel elasticity increased over time when cells were present – in comparison to a decrease in control samples – indicating the deposition of matrix throughout the peptide scaffold. Moreover, significant quantities of calcium phosphate were deposited. Collectively, these data demonstrate that ionic-complementary octapeptides offer a suitable three-dimensional environment for osteoblastic cell function.

Nanofibrillar Peptide hydrogels for the immobilization of biocatalysts for chemical transformations

Enzymes are attractive, "green" alternatives to chemical catalysts within the industrial sector, but their robustness to environmental conditions needs optimizing. Here, an enzyme is tagged chemically and recombinantly with a self-assembling peptide that allows the conjugate to spontaneously assemble with pure peptide to form β-sheet-rich nanofibers decorated with tethered enzyme. Above a critical concentration, these fibers entangle and form a 3D hydrogel. The immobilized enzyme catalyzes chemical transformations and critically its stability is increased significantly where it retains activity after exposure to high temperatures (90 °C) and long storage times (up to 12 months).

Site-specific, covalent incorporation of Tus, a DNA-binding protein, on ionic-complementary self-assembling peptide hydrogels using transpeptidase Sortase A as a conjugation tool†Dedicated to the memory of Joachim H. G. Steinke.‡Electronic supplementary information (ESI) available: Further experimental data. See DOI: 10.1039/c3sm00131hClick here for additional data file

The site-specific conjugation of DNA-binding protein (Tus) to self-assembling peptide FEFEFKFKK was demonstrated. Rheology studies and TEM of the corresponding hydrogels (including PNIPAAm-containing systems) showed no significant variation in properties and hydrogel morphology compared to FEFEFKFKK. Critically, we demonstrate that Tus is accessible within the gel network displaying DNA-binding properties.

Thermal aggregation of recombinant protective antigen: aggregate morphology and growth rate

The thermal aggregation of the biopharmaceutical protein recombinant protective antigen (rPA) has been explored, and the associated kinetics and thermodynamic parameters have been extracted using optical and environmental scanning electron microscopies (ESEMs) and ultraviolet light scattering spectroscopy (UV-LSS). Visual observations and turbidity measurements provided an overall picture of the aggregation process, suggesting a two-step mechanism. Microscopy was used to examine the structure of aggregates, revealing an open morphology formed by the clustering of the microscopic aggregate particles. UV-LSS was used and developed to elucidate the growth rate of these particles, which formed in the first stage of the aggregation process. Their growth rate is observed to be high initially, before falling to converge on a final size that correlates with the ESEM data. The results suggest that the particle growth rate is limited by rPA monomer concentration, and by obtaining data over a range of incubation temperatures, an approach was developed to model the aggregation kinetics and extract the rate constants and the temperature dependence of aggregation. In doing so, we quantified the susceptibility of rPA aggregation under different temperature and environmental conditions and moreover demonstrated a novel use of UV spectrometry to monitor the particle aggregation quantitatively, in situ, in a nondestructive and time-resolved manner.

Effect of peptide and guest charge on the structural, mechanical and release properties of β-sheet forming peptides

The effect of peptide charge on the self-assembly, gelation behavior, and model drug release profiles has been explored here for three octa-peptides, VEVKVEVK (VEK2), VKVKVEVK (VEK3), and VEVEVKVE (VEK1), that carry a net charge of 0, +2, and -2 at neutral pH, respectively. Transparent, self-supporting hydrogels were found to form above a critical concentration when the peptide charge modulus was >1 and this was independent of the sign of the charge. TEM, SAXS, and shear rheology revealed that there were no differences in hydrogel structure or mechanical properties when the peptides were at the same concentration and carried the same charge modulus. All peptides were found to form dense fibrillar networks formed by β-sheet rich single fibers where lateral aggregation of the fibers occurred and increased with decreasing charge modulus. Such behavior was found to correlate with an increase in hydrogel mechanical properties, demonstrating that fiber lateral aggregation is inextricably linked with the mechanical properties of these hydrogels. Two hydrophilic model drug molecules, namely napthol yellow (NY) and martius yellow (MY), were subsequently incorporated within the VEK1 and VEK3 hydrogels at pH 7 and although they did not effect the self-assembly of the peptide at a molecular level, they did effect the level of lateral fiber aggregation observed and, therefore, the mechanical properties of the hydrogels. The release of each molecule from the hydrogels was monitored over time and shown to be controlled by Fickian diffusion where the diffusion rate, D, was dependent on the ratio between the overall effective charges carried by the peptide, i.e., the fibrillar network, and the overall charges carried by the guest molecules, but independent from the hydrogel concentration and mechanical properties within the ranges investigated. This work highlights the possibility of controlling the rate of release of small drug molecules by manipulating the charges on the guest molecules as well as the charged state of the self-assembling peptide.

Enzymatic catalyzed synthesis and triggered gelation of ionic peptides

We investigate the possibility of using the protease thermolysin to drive the synthesis and gelation of ionic-complementary peptides from nongelling precursors. In this system, short peptide fragments are continuously interconverted to form a dynamic peptide library, which eventually favors synthesis of peptides that are thermodynamically stabilized by molecular self-assembly. Thermolysin was added at a fixed concentration (0.3 mg mL(-1)) to solutions (0-300 mg mL(-1)) of the short tetrapeptide FEFK. Initially, the protease partially hydrolyzed the tetrapeptide into dipeptides in all samples. Subsequently, longer peptide sequences were found to form through reverse-hydrolysis. The stability of the different sequences was found to be dependent on their self-assembling properties. The sequences that self-assembled into antiparallel beta-sheet rich fibers became the stable products for the reverse hydrolysis reaction, while the others formed were unstable and disappeared with increasing incubation time. Ultimately, the main product of the system was octapeptide, which suggests that it represents the thermodynamically favored product of this dynamic library. Its concentration dictated the gelation behavior of the sample, and gels with moduli up to 25 kPa where obtained depending on the initial concentration of tetrapeptide.

Thermo-reversible protein fibrillar hydrogels as cell scaffolds

Hen egg white lysozyme has been exposed to various physical and chemical denaturing environments and the physical properties of the resulting gels have been examined and their potential for use as tissue engineering scaffolds has been explored. Transparent, self-supporting fibrillar hydrogels were obtained when lysozyme was heated at low pH, while opaque, particulate gels were obtained at high pH. No increase in viscosity was observed for lysozyme at pH 7 unless the native state was disrupted by reducing the disulfide bridges. This was achieved by adding 20 mM of the reductant dithiothreitol (DTT). Under these conditions the macroscopic critical gelation concentration, C(gel), was found to be approximately 3.0 mM and mechanical spectra obtained as a function of temperature revealed that the gelling and melting temperatures increased with increasing lysozyme concentration. The mechanical strength of the hydrogel measured as the plateau elastic modulus shows a scaling behavior of G(e) approximately c2.43 for concentrations > or = C(gel), which is in good agreement with the theoretical prediction for densely cross-linked semi-flexible networks. Infrared spectroscopy showed that an alpha-helix to beta-sheet molecular transition occurred during heating resulting in beta-sheet rich fibrils forming through the self-assembly of beta-sheet rich denaturated proteins. Cryo-transmission electron microscopy shows these fibres (6 nm in diameter) exist as single entities at low concentration, and at C(gel) associate to form the junctions of a well defined regular network. Our preliminary cell culture experiments show the gel matrix promotes cell spreading, attachment and proliferation; indicating our lysozyme hydrogels are cytocompatible and they provide a viable support for the cells.

Thermoreversible lysozyme hydrogels: properties and an insight into the gelation pathway

The gelation behaviour of aqueous solutions of hen egg white lysozyme (HEWL) in the presence of 20 mM DTT in the concentration range 0.7 to 4.0 mM has been investigated using microDSC, FTIR, cryoTEM, SANS and oscillatory rheology. The macroscopic critical gelation concentration, Cgel, was found to be ∼ 3.0 mM. The disruption of the disulfide bonds by the DTT and the destabilisation of the protein were found to be a prerequisite for the formation of β-sheet rich fibrils under the mild conditions used in this work. Using our methodology the hydrogels obtained have a pH of 7, hence are suitable for cell culture, and are also thermoreversible. The hydrogel melting temperature was found to increase with increasing concentration and a similar structure was observed across the concentration range investigated. Our results suggest these systems are composed of a well defined regular network where single β-sheet rich fibrils (∼ 3 nm diameter) form initially, then two of these fibrils associate two-by-two to form junctions (∼ 6 nm diameter) and then on cooling further aggregate to form larger bundles of fibres. The network mesh size was found to decrease with increasing concentration. Our results suggest that below Cgel small unconnected gel-like aggregates exist that have a similar structure to the hydrogels obtained above Cgel. Using our data we propose a model for the denaturation and gelation behaviour of our system. During the first heating an α-helix to β-sheet molecular transition for the protein conformation occurs resulting in β-sheet rich fibrils forming through the self-assembly of β-sheet rich denaturated proteins. At high temperature the solution contains β-sheet rich fibrils with dissolved protein. On cooling an increase in the amount of β-sheet was observed via FTIR suggesting that as the temperature is decreased more and more protein forms β-sheet rich fibrils. At the gelation temperature these fibrils associate two-by-two to form the network junctions resulting in the macroscopic gelation of the sample. Our results suggest the network junctions are formed via specific hydrophobic interactions. The hydrogels elastic modulus was found to scale as C2.45 for C > Cgel.

Carbon-centered radicals in isolated soy proteins

Solid-state electron paramagnetic resonance (EPR) spectroscopy of commercial samples of isolated soy proteins (ISP) revealed a symmetrical free-radical signal typical of carbon-centered radicals (g= 2.005) ranging from 2.96 x 10(14) to 6.42 x 10(14) spins/g. The level of free radicals in ISP was 14 times greater than similar radicals in sodium caseinate, 29 times greater than egg albumin, and about 100 times greater levels than casein. Nine soy protein powdered drink mixes contained similar types of free radicals up to 4.10 x 10(15) spins/g of drink mix, or up to 6.4 times greater than the highest free-radical content found in commercial ISP. ISP samples prepared in the laboratory contained trapped radicals similar to the levels in commercial ISP samples. When ISP was hydrated in 2.3 mM sodium erythorbate or 8.3 mM L-cysteine, frozen and dried, the level of trapped free radicals increased by about 17- and 19-fold, respectively. The ESR spectrum of defatted soybean flakes contained overlapping signals from the primary free-radical peak (g= 2.005) and a sextet pattern typical of manganese-II. The manganese signal was reduced in the laboratory ISP and very weak in the commercial ISP.

Irreversible Collapse of Poly(vinyl stearate) Monolayers at the Air-Water Interface

The collapse of Langmuir monolayers of poly(vinyl stearate) (PVS) at the air-water interface has been investigated by combined measurements of the surface pressure-area isotherms and Brewster angle microscopy (BAM). Atomic force microscopy (AFM) has been used to gain out-of-plane structural information on collapsed films transferred onto a solid substrate by a modified version of the inverse Langmuir-Schaefer deposition method. At high areas per monomer repeat unit, BAM imaging revealed that the films are heterogeneous, with large solidlike domains (25-200 mum in diameter) coexisting with liquidlike domains. Upon film compression, the domains coalesced to form a homogeneous monolayer before the film collapsed at constant pressure, forming irreversible three-dimensional (3D) structures. BAM images showed that two 3D structures coexisted: buckles of varying width extending across the surface and perpendicular to the direction of the compression and dotted islandlike structures. Upon expansion, the film fractured and both 3D protrusions persisted, explaining the marked hysteresis recorded in the Langmuir isotherms. Experiments with AFM confirmed the 3D nature of both protrusions and revealed that many buckles contain substructures corresponding to narrow buckles whose heights are a multiple of a single bilayer. Additionally, many multilayer islands with diameters spanning from 0.2 mum to over 3.5 mum were characterized by varying heights between 2 nm and up to over 50 nm. The key to the formation of the irreversible 3D structures is the presence of large inhomogeneities in the PVS monolayer, and a generalized phenomenological model is proposed to explain the collapse observed.

Thermoreversible Protein Hydrogel as Cell Scaffold

A thermoreversible fibrillar hydrogel has been formed from an aqueous lysozyme solution in the presence of dithiothreitol (DTT). Its physical properties and potential as a tissue engineering scaffold have been explored. Hydrogels were prepared by dissolving 3 mM protein in a 20 mM DTT/water mixture, heating to 85 degrees C and cooling at room temperature. No gel was observed for the equivalent sample without DTT. The elastic nature of the gel formed was confirmed by rheology, and the storage modulus of our gel was found to be of the same order of magnitude as for other cross-linked biopolymers. Micro differential scanning calorimetry (microDSC) experiments confirmed that the hydrogel was thermally reversible and that gelation and melting occurs through a solid-liquid-like first-order transition. Infrared spectroscopy of the hydrogel and transmission electron microscopy studies of very dilute samples revealed the presence of beta-sheet-rich fibrils that were approximately 4-6 nm in diameter and 1 mum in length. These fibrils are thought to self-assemble along their long axes to form larger fibers that become physically entangled to form the three-dimensional network observed in both cryo-scanning electron microscopy (cryo-SEM) and small-angle neutron scattering (SANS) studies. The hydrogel was subsequently cultured with 3T3 fibroblasts and cells spread extensively after 7 days and stretched actin filaments formed that were roughly parallel to each other, indicating the development of organized actin filaments in the form of stress fibers in cells.

The formation of spherulites by amyloid fibrils of bovine insulin

Bovine insulin has long been known to self-assemble in vitro into amyloid fibrils. We have observed a further higher-order self-association of the protein into spherical structures, with diameters typically around 50 microm but ranging from 10 to 150 microm. In a polarizing light microscope, these structures exhibit a "Maltese-cross" extinction pattern typical of spherulites. Spherical structures of a similar size distribution can be observed in the environmental scanning electron microscope, which also reveals the presence of significant amounts of water in the structures. The spherulites contain a large quantity of well defined amyloid fibrils, suggesting that they are formed at least in part as a consequence of the self-assembly of preformed fibrils. Similar structures also have been observed in the tissues of patients suffering from amyloid disorders. The ability of amyloid fibrils to form such higher-order assemblies supports the hypothesis that they represent a generic form of polypeptide structure with properties that are analogous to those of classical synthetic polymers.