Enzyme-triggered self-assembly of a small molecule: a supramolecular hydrogel with leaf-like structures and an ultra-low minimum gelation concentration

Solvent selective gelation of cetyltrimethylammonium bromide: structure, phase evolution and thermal characteristics

We report herein the gelation behaviour of cetyltrimethylammonium bromide (CTAB), a cationic surfactant, in a variety of solvent compositions. A turbid gel of CTAB in a binary solvent mixture at a critical composition was observed to be 1 : 3 v/v toluene : water. The molecular structure of the as-formed gel was investigated by X-ray diffraction and microscopic techniques, namely, optical and polarizing microscopy, scanning electron microscopy and small-angle X-ray scattering (SAXS). The phase evolution has been studied using UV-visible transmittance measurements and the thermal characteristics of the gel by differential scanning calorimetry measurements. SAXS studies, in conjunction with molecular modelling, revealed the gel to assemble as lamellae with high interdigitation of bilayer assembly of CTAB molecules with predominant non-covalent interactions, where the gel lamellae were inferred from the interplanar spacings. Rheological studies revealed the viscoelastic nature of the CTAB gels. The ability to form a gel has been evaluated in several polar solvents, such as methanol and chloroform, and non-polar solvents, such as toluene and carbon tetrachloride.

Self-Assembly, Bioactivity, and Nanomaterials Applications of Peptide Conjugates with Bulky Aromatic Terminal Groups

The self-assembly and structural and functional properties of peptide conjugates containing bulky terminal aromatic substituents are reviewed with a particular focus on bioactivity. Terminal moieties include Fmoc [fluorenylmethyloxycarbonyl], naphthalene, pyrene, naproxen, diimides of naphthalene or pyrene, and others. These provide a driving force for self-assembly due to π-stacking and hydrophobic interactions, in addition to the hydrogen bonding, electrostatic, and other forces between short peptides. The balance of these interactions leads to a propensity to self-assembly, even for conjugates to single amino acids. The hybrid molecules often form hydrogels built from a network of β-sheet fibrils. The properties of these as biomaterials to support cell culture, or in the development of molecules that can assemble in cells (in response to cellular enzymes, or otherwise) with a range of fascinating bioactivities such as anticancer or antimicrobial activity, are highlighted. In addition, applications of hydrogels as slow-release drug delivery systems and in catalysis and other applications are discussed. The aromatic nature of the substituents also provides a diversity of interesting optoelectronic properties that have been demonstrated in the literature, and an overview of this is also provided. Also discussed are coassembly and enzyme-instructed self-assembly which enable precise tuning and (stimulus-responsive) functionalization of peptide nanostructures.

In Vitro Self-Assembly of a Modified Diphenylalanine Peptide to Nanofibers Induced by the Eye Absent Enzyme and Alkaline Phosphatase and Its Activity against Breast Cancer Cell Proliferation

Drug-resistant breast cancers such as Triple negative breast cancer (TNBC) do not respond successfully to chemotherapy treatments because they lack the expression of receptor targets. Drug-resistant anti-cancer treatments require innovative approaches to target these cells without relying on the receptors. Intracellular self-assembly of small molecules induced by enzymes is a nanotechnology approach for inhibiting cancer cell growth. In this approach, enzymes will induce the self-assembly of small molecules to nanofibers, which leads to cell death. Here, we investigate the self-assembly of a modified small peptide induced by two different phosphatases: alkaline phosphatase (ALP) and eye absent tyrosine phosphatase (EYA). ALPs are expressed in many adult human tissues and are critical for many cellular functions. EYAs are embryonic enzymes that are over-expressed in drug-resistant breast cancers. We synthesized a small diphenylalanine-based peptide with a tyrosine phosphate end group as the substrate of phosphatase enzymes. Peptides were synthesized with solid phase techniques and were characterized by HPLC and MALDI-TOF. To characterize the self-assembly of peptides exposed to enzymes, different techniques were used such as scattering light intensity, microscopes, and phosphate detection kit. We then determined the toxicity effect of the peptide against normal breast cancer cells, MCF-7, and drug-resistant breast cancer cells, MDA-MB-231. The results showed that the EYA enzyme is able to initiate self-assembly at lower peptide concentration with higher self-assembling intensity compared to ALP. A significant decrease in the TNBC cell number was observed even with a low peptide concentration of 60 μM. These results collectively support the exploration of enzyme self-assembly to treat TNBC.

Prospects of microbial polysaccharides-based hybrid constructs for biomimicking applications

Polysaccharides are biobased polymers obtained from renewable sources. They exhibit various interesting features including biocompatibility, biodegradability, and nontoxicity. Microbial polysaccharides are produced by several microorganisms including yeast, fungi, algae, and bacteria. Microbial polysaccharides have gained high importance in biotechnology due to their novel physiochemical characteristics and composition. Among microbial polysaccharides, xanthan, alginate, gellan, and dextran are the most commonly reported polysaccharides for the development of biomimetic materials for biomedical applications including targeted drug delivery, wound healing, and tissue engineering. Several chemical and physical cross-linking reactions are performed to increase their technological and functional properties. Owning to the broad-scale applications of microbial polysaccharides, this review aims to summarize the characteristics with different ways of physical/chemical crosslinking for polysaccharide regulation. Recently, several biopolymers have gained high importance due to their biologically active properties. This will help in the formation of bioactive nutraceuticals and functional foods. This review provides a perspective on microbial polysaccharides, with special emphasis given to applications in promising biosectors and the subsequent advancement on the discovery and development of new polysaccharides for adding new products.

Hypertoxic self-assembled peptide with dual functions of glutathione depletion and biosynthesis inhibition for selective tumor ferroptosis and pyroptosis

Abundant glutathione (GSH) is a biological characteristic of lots of tumor cells. A growing number of studies are utilizing GSH depletion as an effective adjuvant therapy for tumor. However, due to the compensatory effect of intracellular GSH biosynthesis, GSH is hard to be completely exhausted and the strategy of GSH depletion remains challenging. Herein, we report an l-buthionine-sulfoximine (BSO)-based hypertoxic self-assembled peptide derivative (NSBSO) with dual functions of GSH depletion and biosynthesis inhibition for selective tumor ferroptosis and pyroptosis. The NSBSO consists of a hydrophobic self-assembled peptide motif and a hydrophilic peptide derivative containing BSO that inhibits the synthesis of GSH. NSBSO was cleaved by GSH and thus experienced a morphological transformation from nanoparticles to nanofibers. NSBSO showed GSH-dependent cytotoxicity and depletion of intracellular GSH. In 4T1 cells with medium GSH level, it depleted intracellular GSH and inactivated GSH peroxidase 4 (GPX4) and thus induced efficient ferroptosis. While in B16 cells with high GSH level, it exhausted GSH and triggered indirect increase of intracellular ROS and activation of Caspase 3 and gasdermin E, resulting in severe pyroptosis. These findings demonstrate that GSH depletion- and biosynthesis inhibition-induced ferroptosis and pyroptosis strategy would provide insights in designing GSH-exhausted medicines.

Ultrashort Peptide Bioinks Support Automated Printing of Large-Scale Constructs Assuring Long-Term Survival of Printed Tissue Constructs

We report about rationally designed ultrashort peptide bioinks, overcoming severe limitations in current bioprinting procedures. Bioprinting is increasingly relevant in tissue engineering, regenerative and personalized medicine due to its ability to fabricate complex tissue scaffolds through an automated deposition process. Printing stable large-scale constructs with high shape fidelity and enabling long-term cell survival are major challenges that most existing bioinks are unable to solve. Additionally, they require chemical or UV-cross-linking for the structure-solidifying process which compromises the encapsulated cells, resulting in restricted structure complexity and low cell viability. Using ultrashort peptide bioinks as ideal bodylike but synthetic material, we demonstrate an instant solidifying cell-embedding printing process via a sophisticated extrusion procedure under true physiological conditions and at cost-effective low bioink concentrations. Our printed large-scale cell constructs and the chondrogenic differentiation of printed mesenchymal stem cells point to the strong potential of the peptide bioinks for automated complex tissue fabrication.

Enzymatic Noncovalent Synthesis

Enzymatic reactions and noncovalent (i.e., supramolecular) interactions are two fundamental nongenetic attributes of life. Enzymatic noncovalent synthesis (ENS) refers to a process where enzymatic reactions control intermolecular noncovalent interactions for spatial organization of higher-order molecular assemblies that exhibit emergent properties and functions. Like enzymatic covalent synthesis (ECS), in which an enzyme catalyzes the formation of covalent bonds to generate individual molecules, ENS is a unifying theme for understanding the functions, morphologies, and locations of molecular ensembles in cellular environments. This review intends to provide a summary of the works of ENS within the past decade and emphasize ENS for functions. After comparing ECS and ENS, we describe a few representative examples where nature uses ENS, as a rule of life, to create the ensembles of biomacromolecules for emergent properties/functions in a myriad of cellular processes. Then, we focus on ENS of man-made (synthetic) molecules in cell-free conditions, classified by the types of enzymes. After that, we introduce the exploration of ENS of man-made molecules in the context of cells by discussing intercellular, peri/intracellular, and subcellular ENS for cell morphogenesis, molecular imaging, cancer therapy, and other applications. Finally, we provide a perspective on the promises of ENS for developing molecular assemblies/processes for functions. This review aims to be an updated introduction for researchers who are interested in exploring noncovalent synthesis for developing molecular science and technologies to address societal needs.

Self-Assembly of Aggregation-Induced-Emission Molecules

The last decade has witnessed rapid developments in aggregation-induced emission (AIE). In contrast to traditional aggregation, which causes luminescence quenching (ACQ), AIE is a reverse phenomenon that allows robust luminescence to be retained in aggregated and solid states. This makes it possible to fabricate various highly efficient luminescent materials, which opens new paradigms in a number of fields, such as imaging, sensing, medical therapy, light harvesting, light-emitting devices, and organic electronic devices. Of the various important features of AIE molecules, their self-assembly behavior is very attractive because the formation of a well-defined emissive nanostructure may lead to advanced applications in diverse fields. However, due to the nonplanar topology of AIEgens, it is not easy for them to self-assemble into well-defined structures. To date, some strategies have been proposed to achieve the self-assembly of AIEgens. Herein, we summarize the most recent approaches for the self-assembly of AIE molecules. These approaches can be sorted into two classes: 1) covalent molecular design and 2) noncovalent supramolecular interactions. We hope this will inspire more excellent work in the field of AIE.

Self-assembled GFFYK peptide hydrogel enhances the therapeutic efficacy of mesenchymal stem cells in a mouse hindlimb ischemia model

Mesenchymal stem cell (MSC) transplantation has emerged as a very promising strategy for the treatments of peripheral artery disease (PAD). However, MSC-based therapies are limited by low cell retention and survival rate in the ischemic zone. Small molecular (SM) hydrogels have shown attractive abilities to enhance the therapeutic effects of human MSCs via promoting their proliferation or maintaining their differentiation potential. Here, we designed and synthesized a new bioactive and biocompatible hydrogel, Nap-GFFYK-Thiol, using disulfide bonds as cleavable linkers to control the molecular self-assembly and we hypothesized this hydrogel could enhance the retention and engraftment of human placenta-derived MSCs (hP-MSCs) in a mouse ischemic hindlimb model. In vitro results demonstrated that the Nap-GFFYK-Thiol hydrogel increased cell viability through paracrine effects. Moreover, it enhanced the proangiogenic and anti-apoptotic effects of hP-MSCs. In vivo, Nap-GFFYK-Thiol hydrogel improved the hP-MSC retention in the murine ischemic hindlimb model as visualized by bioluminescence imaging. Furthermore, cotransplantation of hP-MSCs with hydrogel improved blood perfusion, leading to superior limb salvage. These therapeutic effects may attribute to reduced inflammatory cell infiltration, enhanced angiogenesis as well as suppressed collagen deposition. In conclusion, the Nap-GFFYK-Thiol hydrogel fabricated using disulfide bonds as cleavable linkers serves as an artificial niche for promoting hP-MSC survival and proangiogenic factor secretion in PAD therapy and thereby provide an alternative strategy for PAD therapy. STATEMENT OF SIGNIFICANCE: Although several phase I/II clinical trials of MSC-based treatments for critical limb ischemia (CLI) are ongoing, MSC-based therapies are still challenged by the low quality and quantity of cells in the ischemic zone, especially in cases of extensive or irreversible damage. Hydrogels have favorable biocompatibility and safety records in the medical field. In the current study, we engineered a new bioactive and biocompatible hydrogel, Nap-GFFYK-Thiol, using disulfide bonds as cleavable linkers to enhance the therapeutic efficacy of human placenta-derived MSCs (hP-MSCs) in mouse limb ischemia model. Notably, Nap-GFFYK-Thiol hydrogel acts as an artificial niche for promoting hP-MSC survival and proangiogenic factor secretion in PAD therapy, which further promoted the restoration of blood perfusion and regeneration of muscle cells. Considering the proangiogenic effect of Nap-GFFYK-Thiol on hP-MSCs, our results may provide a new strategy for the treatment of PAD.

Applications of self-assembling ultrashort peptides in bionanotechnology

Peptides are intriguing building blocks for a variety of applications in bionanotechnology. Peptides can self-assemble into well-ordered nanostructures. Among the various nanomorphology forms, peptide nanofibers and nanotubes are relevant in biomedical applications. In this review, their applications as tissue engineering scaffolds, drug delivery vehicles, three-dimensional printing bioinks and bioimaging nanoprobes will be illustrated. This review article describes di-, tri-, tetra-peptides because they are cost-effective, simple to prepare, and amenable to production on a large scale.

Ultrashort peptide applications in bionanotechnology.

Enzymatic hydrogelation of self-assembling peptide I4K2and its antibacterial and drug sustained-release activities

I₄K₂hydrogel induced by plasma amine oxidase (PAO) has antibacterial and drug sustained-release properties.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

Biocompatible fluorescent supramolecular nanofibrous hydrogel for long-term cell tracking and tumor imaging applications

Biocompatible peptide-based supramolecular hydrogel has recently emerged as a new and promising system for biomedical applications. In this work, Rhodamine B is employed as a new capping group of self-assembling peptide, which not only provides the driving force for supramolecular nanofibrous hydrogel formation, but also endows the hydrogel with intrinsic fluroescence signal, allowing for various bioimaging applications. The fluorescent peptide nanofibrous hydrogel can be formed via disulfide bond reduction. After dilution of the hydrogel with aqueous solution, the fluorescent nanofiber suspension can be obtained. The resultant nanofibers are able to be internalized by the cancer cells and effectively track the HeLa cells for as long as 7 passages. Using a tumor-bearing mouse model, it is also demonstrated that the fluorescent supramolecular nanofibers can serve as an efficient probe for tumor imaging in a high-contrast manner.

Self-assembling choline mimicks with enhanced binding affinities to C-LytA protein

Streptococcus pneumoniae (pneumococcus) causes multiple illnesses in humans. Exploration of effective inhibitors with multivalent attachment sites for choline-binding modules is of great importance to reduce the pneumococcal virulence. In this work, we successfully developed two self-assembling choline mimicks, Ada-GFFYKKK' and Nap-GFFYKKK', which have the abilities to self-assemble into nanoparticles and nanofibers, respectively, yielding multivalent architectures. Additionally, the best characterized choline-binding module, C-terminal moiety of the pneumococcal cell-wall amidase LytA (C-LytA) was also produced with high purity. The self-assembling Ada-GFFYKKK' and Nap-GFFYKKK' show strong interactions with C-LytA, which possess much higher association constant values to the choline-binding modules as compared to the individual peptide Fmoc-K'. This study thus provides a self-assembly approach to yield inhibitors that are very promising for reducing the pneumococcal virulence.

Using phosphatases to generate self-assembled nanostructures and their applications

SIGNIFICANCE

Self-assembled nanostructures have received significant research interest in the last decade, because they show great promise for drug delivery, diagnostics, tissue engineering, and regenerative medicine. Recently, the development of enzyme-assisted self-assembled nanostructures has become an active area of research because of the attractive characteristics of enzymes, such as ready availability, good biocompatibility, and high selectivity and specificity. Phosphatases, taking part in approximately 30% of intra- and extracellular activities, have been widely employed as triggers for the generation of self-assembled biomaterials, including static, reversible, and dynamic systems.

RECENT ADVANCES

In this review, we highlight the generation of self-assembled systems of synthetic molecules using phosphatases and their potential applications. We first summarize the generation of different kinds of static and dynamic self-assembled structures, including nanofibers and nanoparticles, by the dephosphorylation reaction catalyzed by phosphatases. The antagonistic interactions of phosphatases and kinases make this system one of the most attractive candidates for biotransformation. Diverse biomedical applications of phosphatases/kinases-involved self-assembled systems have been extensively explored in fields such as bacterial growth inhibition, drug delivery, imaging of self-assembly inside live cells, and biomineralization. We then summarize the reversible self-assembled systems controlled by the pair enzymes of phosphatases/kinases, in which different morphologies of self-assembled nanostructures can be achieved and switched by the pair enzymes. These phosphatase-involved self-assembled systems can be used for many applications such as controlled drug delivery, enzyme activity imaging, and cancer cell inhibition.

CRITICAL ISSUES

Phosphatases are over-expressed in several cancer cell lines. Their detection is, therefore, important for cancer diagnostics. Nanomaterials that can respond to abnormal phosphatase activities also have big potential for the delivery of therapeutic agents on demand. The study of reversible self-assembling systems control by the phosphatase/kinase switch may provide useful insights to understand the working principle of this important biological switch.

FUTURE DIRECTIONS

The design principle mentioned in this review may stimulate the generation of smart self-assembled systems by other enzymes or other pairs of enzymes. The combination of environment-sensitive fluorescence property of fluorescent dyes and self-assembling molecules that can respond to enzymes may lead to the development of smart probes to monitor important biological processes.

Self-assembling peptide of D-amino acids boosts selectivity and antitumor efficacy of 10-hydroxycamptothecin

D-peptides, which consist of D-amino acids and can resist the hydrolysis catalyzed by endogenous peptidases, are one of the promising candidates for construction of peptide materials with enhanced biostability in vivo. In this paper, we report on a self-assembling supramolecular nanostructure of D-amino acid-based peptide Nap-G(D)F(D)F(D)YGRGD (D-fiber, (D)F meant D-phenylalanine, (D)Y meant D-tyrosine), which were used as carriers for 10-hydroxycamptothecin (HCPT). Transmission electron microscopy observations demonstrated the filamentous morphology of the HCPT-loaded peptides (d-fiber-HCPT). The better selectivity and antitumor activity of D-fiber-HCPT than L-fiber-HCPT were found in the in vitro and in vivo antitumor studies. These results highlight that this model D-fiber system holds great promise as vehicles of hydrophobic drugs for cancer therapy.

A peptide-based nanofibrous hydrogel as a promising DNA nanovector for optimizing the efficacy of HIV vaccine

This report shows that a nanovector composed of peptide-based nanofibrous hydrogel can condense DNA to result in strong immune responses against HIV. This nanovector can strongly activate both humoral and cellular immune responses to a balanced level rarely reported in previous studies, which is crucial for HIV prevention and therapy. In addition, this nanovector shows good biosafety in vitro and in vivo. Detailed characterizations show that the nanofibrous structure of the hydrogel is critical for the dramatically improved immune responses compared to existing materials. This peptide-based nanofibrous hydrogel shows great potential for efficacious HIV DNA vaccines and can be potentially used for delivering other vaccines and drugs.

BSA-stabilized molecular hydrogels of a hydrophobic compound

We report a protein-stabilized mechanism to understand the formation of molecular hydrogels from hydrophobic compounds.

Supramolecular hydrogels inspired by collagen for tissue engineering

Supramolecular hydrogels are promising biomaterials for cell culture in 2-D and 3-D environments. Inspired by the chemical structure of collagen, which bears the repeating tripeptide of glycine-Xaa-4R-hydroxyproline (GXO; Xaa is any one of the natural amino acids), we designed and synthesized a small library of supramolecular hydrogelators (a total of 6). We found that four of the hydrogels were suitable for NIH 3T3 cell culture in the 2-D environments. Gel 2, the best hydrogel, has properties that are similar to those of collagen for 3T3 cell culture. These findings not only provide more supramolecular hydrogel candidates for tissue engineering, but also offer a new strategy for designing biomaterials that mimic nature.