Artificial Extracellular Matrix Proteins Research Articles

IPTG-independent autoinduction of extracellular matrix proteins using recombinant E. coli as the expression host

At present, isopropyl β-D-thiogalactopyranoside (IPTG) is the universal inducer for expressing recombinant proteins under the lac operator/repressor system. In this study, we propose an autoinduction (IPTG-independent) system for recombinant proteins using E. coli as the expression host. We applied this bacterial host for autoinduction to the expression of recombinant proteins, including green fluorescence protein (GFP) and an artificial extracellular matrix protein (aECM-CS5-ELF). The host harbors a mutant Ala294Gly/Thr251Gly phenylalanyl-tRNA synthetase (PheRS**) with an enlarged binding pocket that is expressed under the control of the T7 promoter. Using this system, we demonstrate marked overexpression of the biosynthesized GFP and aECM-CS5-ELF from a 1-L culture containing glucose (5 g/L) and galactose (20 g/L) as the carbon sources, with GFP and aECM-CS5-ELF yields 2.3- and 8.1-fold higher, respectively, than that from an IPTG-induced culture. This unique trial is intended to stimulate novel overexpression strategies based on autoinduction. We chose an autoinduction (IPTG-independent) system for overexpression of recombinant proteins using E. coli as the expression host. In autoinduction, glucose and lactose are used as main carbon sources for cell growth. When the glucose is almost completely consumed as the first growth of E. coli., the carbon source turns to lactose, accompanied by regioselective chemical transformation of lactose to allolactose, which acts as the trigger for activation of transcription by releasing the repressor. Using this system, we achieved marked overexpression of the biosynthesized GFP and aECM-CS5-ELF.

Adhesion control of human umbilical vein endothelial cells using clickable poly(2-oxazoline)-grafted biosynthesized extracellular matrix protein

A bacterial expression host was used for in vivo incorporation of p-azidophenylalanine (p-N3Phe), a phenylalanine (Phe) analog, into an artificial extracellular matrix protein (aECM-CS5-ELF). The host harbors the mutant phenylalanyl-tRNA synthetase (PheRS) which has an enlarged binding pocket, where the Ala294Gly/Thr251Gly mutant PheRS (PheRS**) was expressed under the control of T7 promoters. Biosynthesized aECM-CS5-ELF containing p-N3Phe (aECM-CS5-ELF-N3) was coupled with alkyne-containing poly(2-oxazoline) prepared via ring-opening polymerization of 2-oxazolines (2-methyl-2-oxazoline, 2-ethyl-2-oxazoline, and 2-iso-propyl-2-oxazoline) using methyl triflate (TfOMe) and propiolic acid as the initiator and terminator, respectively. A copper-catalyzed alkyne-azide click reaction was used to graft the poly(2-oxazoline)s onto the aECM. Examination of the solubility in organic and aqueous media, and the lower critical transition temperature (LCST), revealed a dependence on the incorporation ratio of p-N3Phe and graft chain species; the LCST behavior was markedly altered when poly(2-oxazoline) moieties were present as side chains. Circular dichroism measurements indicate that grafting was not responsible for conformational changes, because the conformation was retained both at below and above the LCST. Specific adhesion and subsequent temperature-sensitive detachment of human umbilical vein endothelial cells (HUVECs) onto the cross-linked aECM-CS5-ELF-N3-graft-poly(2-oxazoline) surfaces was also demonstrated.

In Vitro Colony Assays for Characterizing Tri-potent Progenitor Cells Isolated from the Adult Murine Pancreas.

Stem and progenitor cells from the adult pancreas could be a potential source of therapeutic beta-like cells for treating patients with type 1 diabetes. However, it is still unknown whether stem and progenitor cells exist in the adult pancreas. Research strategies using cre-lox lineage-tracing in adult mice have yielded results that either support or refute the idea that beta cells can be generated from the ducts, the presumed location where adult pancreatic progenitors may reside. These in vivo cre-lox lineage-tracing methods, however, cannot answer the questions of self-renewal and multi-lineage differentiation-two criteria necessary to define a stem cell. To begin addressing this technical gap, we devised 3-dimensional colony assays for pancreatic progenitors. Soon after our initial publication, other laboratories independently developed a similar, but not identical, method called the organoid assay. Compared to the organoid assay, our method employs methylcellulose, which forms viscous solutions that allow the inclusion of extracellular matrix proteins at low concentrations. The methylcellulose-containing assays permit easier detection and analyses of progenitor cells at the single-cell level, which are critical when progenitors constitute a small sub-population, as is the case for many adult organ stem cells. Together, results from several laboratories demonstrate in vitro self-renewal and multi-lineage differentiation of pancreatic progenitor-like cells from mice. The current protocols describe two methylcellulose-based colony assays to characterize mouse pancreatic progenitors; one contains a commercial preparation of murine extracellular matrix proteins and the other an artificial extracellular matrix protein known as a laminin hydrogel. The techniques shown here are 1) dissociation of the pancreas and sorting of CD133(+)Sox9/EGFP(+) ductal cells from adult mice, 2) single cell manipulation of the sorted cells, 3) single colony analyses using microfluidic qRT-PCR and whole-mount immunostaining, and 4) dissociation of primary colonies into single-cell suspensions and re-plating into secondary colony assays to assess self-renewal or differentiation.

Synthesis of Glycopolymer Containing Cell-Penetrating Peptides as Inducers of Recombinant Protein Expression under the Control of Lactose Operator/Repressor Systems.

We recently reported on newly synthesized S-galactosyl oligo(Arg) conjugates to overcome the serious problem of the passage through the E. coli cell membrane. Following in vivo expression of green fluorescent protein (GFP) induced by each of the S-galactosyl (Arg)n constructs (n = 5, 6, 8) at the T5 promoter in E. coli for 18 h, we visually observed that the cultures fluoresced green light when excited with UV light. The fluorescence intensities for these cultures were greater than that found for a control culture, indicating that the peptides had induced GFP expression. In order to accomplish higher expression efficiency, we investigated the cluster effect and structural fine-tuning of new poly(2-oxazoline) containing CysArgArg as the cell-penetrating peptide (CPP) and S-galactosides when acting as inducers of recombinant protein expression under the control of lac operator/repressor systems in this article. Quantitative fluorescence intensities (calculated per molecule) also supported the observations that the cell-penetrating glyco poly(2-oxazoline)s were better inducers of GFP expression than glyco poly(2-oxazoline) containing no CPP or isopropyl β-d-thiogalactoside. Because the level of GFP expression was directly related to the number of sugar residues in each glyco poly(2-oxazoline), we propose that a cluster effect of the S-galactosides attached to the cell-penetrating poly(2-oxazoline) is responsible for how well the galactosides inhibited the lac repressor to activate the protein expression under the control of the lac operator/repressor system. A similar tendency was observed when the T7 promoter was placed upstream of the gene for an artificial extracellular matrix protein and glyco poly(2-oxazoline)s-CPP conjugates were used as inducers. To assess how the glyco poly(2-oxazoline) penetrate the cell membrane, we labeled the glyco poly(2-oxazoline) using 1-amino pyrene and directly observed the penetration process. Furthermore, we could visualize protein expression under the control of a lac promoter/operator/repressor system using transmission electron microscopy combined with energy dispersive X-ray analysis mapping.

Construction of a tissue-specific transcription factor-tethered extracellular matrix protein via coiled-coil helix formation.

Tissue-specific transcription factors are key regulators of cellular differentiation. Previously, we succeeded in introducing basic helix-loop-helix tissue-specific transcription factor proteins into cells to induce cellular differentiation. Based on these results, we decided to focus on the use of tissue-specific transcription factor proteins in the construction of biomaterials. In this proof-of-concept study, we demonstrate the construction of a tissue-specific transcription factor-tethered extracellular matrix protein. Here, the tissue-specific transcription factor Olig2 was tethered to a designed artificial extracellular matrix protein via coiled-coil helix formation. Tethered Olig2 was introduced into mouse embryonic carcinoma P19 cells attached to our designed extracellular matrix protein, and was shown to exhibit the ability to induce neural differentiation.

Design and Construction of Artificial Extracellular Matrix (aECM) Proteins from Escherichia coli for Skin Tissue Engineering.

Recombinant technology is a versatile platform to create novel artificial proteins with tunable properties. For the last decade, many artificial proteins that have incorporated functional domains derived from nature (or created de novo) have been reported. In particular, artificial extracellular matrix (aECM) proteins have been developed; these aECM proteins consist of biological domains taken from fibronectin, laminins and collagens and are combined with structural domains including elastin-like repeats, silk and collagen repeats. To date, aECM proteins have been widely investigated for applications in tissue engineering and wound repair. Recently, Tjin and coworkers developed integrin-specific aECM proteins designed for promoting human skin keratinocyte attachment and propagation. In their work, the aECM proteins incorporate cell binding domains taken from fibronectin, laminin-5 and collagen IV, as well as flanking elastin-like repeats. They demonstrated that the aECM proteins developed in their work were promising candidates for use as substrates in artificial skin. Here, we outline the design and construction of such aECM proteins as well as their purification process using the thermo-responsive characteristics of elastin.

Postnatal Pancreas of Mice Contains Tripotent Progenitors Capable of Giving Rise to Duct, Acinar, and Endocrine Cells In Vitro.

Postnatal pancreas is a potential source for progenitor cells to generate endocrine β-cells for treating type 1 diabetes. However, it remains unclear whether young (1-week-old) pancreas harbors multipotent progenitors capable of differentiating into duct, acinar, and endocrine cells. Laminin is an extracellular matrix (ECM) protein important for β-cells' survival and function. We established an artificial extracellular matrix (aECM) protein that contains the functional IKVAV (Ile-Lys-Val-Ala-Val) sequence derived from laminin (designated aECM-lam). Whether IKVAV is necessary for endocrine differentiation in vitro is unknown. To answer these questions, we cultured single cells from 1-week-old pancreas in semi-solid media supplemented with aECM-lam, aECM-scr (which contains a scrambled sequence instead of IKVAV), or Matrigel. We found that colonies were generated in all materials. Individual colonies were examined by microfluidic reverse transcription-polymerase chain reaction, immunostaining, and electron microscopy analyses. The majority of the colonies expressed markers for endocrine, acinar, and ductal lineages, demonstrating tri-lineage potential of individual colony-forming progenitors. Colonies grown in aECM-lam expressed higher levels of endocrine markers Insulin1, Insulin2, and Glucagon compared with those grown in aECM-scr and Matrigel, indicating that the IKVAV sequence enhances endocrine differentiation. In contrast, Matrigel was inhibitory for endocrine gene expression. Colonies grown in aECM-lam displayed the hallmarks of functional β-cells: mature insulin granules and glucose-stimulated insulin secretion. Colony-forming progenitors were enriched in the CD133(high) fraction and among 230 micro-manipulated single CD133(high) cells, four gave rise to colonies that expressed tri-lineage markers. We conclude that young postnatal pancreas contains multipotent progenitor cells and that aECM-lam promotes differentiation of β-like cells in vitro.

Construction of a bFGF-tethered multi-functional extracellular matrix protein through coiled-coil structures for neurite outgrowth induction

In this study, an artificial multi-functional extracellular matrix (ECM) protein, tethered with a growth factor, was developed for neurite outgrowth induction. The designed ECM protein was comprised of an elastin-like peptide, as a structural unit, as well as the AG73 peptide sequence derived from the laminin and the C3 peptide sequence, which binds to neural cell adhesion molecules (derived from a synthetic peptide library) as functional units. Both AG73 and C3 have been demonstrated to promote cell adhesion and enhance neurite outgrowth. For the tethering of basic fibroblast growth factor (bFGF) to the ECM protein, helical peptides were fused to the ECM protein to form a coiled-coil helical structure with helical peptide-fused bFGF. Neurite outgrowth was induced in the PC12 cells that were cultured on this ECM protein as a result of the tethered-bFGF. Moreover, neurite outgrowth was enhanced by the AG73 and C3 peptides of the ECM protein.

Synthesis of Cell-Penetrating S-Galactosyl-Oligoarginine Peptides as Inducers of Recombinant Protein Expression under the Control of lac Operator/Repressor Systems.

The synthesis of glycodendrimers and glycopoly(oxazoline)s as inducers of recombinant protein expression has recently been reported; however, these compounds induced the expression of only small amounts of the green fluorescence protein (GFP), which was used as the model recombinant protein, because of their poor ability to penetrate the Escherichia coli cell membrane. Therefore, S-galactosyl-oligo(Arg) conjugates have now been synthesized to overcome this problem. Following in vivo expression of GFP induced by each of the S-galactosyl (Arg)n constructs (n=5, 6, 8) at the T5 promoter in E. coli for 18 hours, we visually observed that the cultures fluoresced green light when excited with UV light. The fluorescent intensities for these cultures were greater than that found for a control culture, which indicates that the peptides had induced GFP expression. Quantitative fluorescent measurements also supported the observations that the peptides were better inducers of GFP expression than the galactosyl dendrimers and the poly(oxazoline)s and the natural inducer lactose. Because the level of GFP expression was directly related to the number of arginine moieties in each peptide, we propose that the number of arginine moieties is responsible for how well each peptide passes through the E. coli membrane, which affects the expression level. A similar tendency was observed when the T7 promoter was placed upstream from the gene for an artificial extracellular matrix protein and the S-Gal-oligo(Arg) peptides were used as inducers. To assess how the distance between two galactosyl moieties as well as how the multivalent effect (cluster effect) in an oligo(Arg) inducer affects the expression level of GFP, we synthesized a conjugate of Lys(Arg)8 (Lys=lysine) and two S-galactosyls, which enhanced the expression of GFP in comparison with that obtained for S-Gal(Arg)8 .

Strain propagation in artificial extracellular matrix proteins can accelerate cell spreading and polarization

In recent years the ability of cells to sense the mechanical properties of their environments and deform them locally has become increasingly clear. To better understand the mechanical coupling between cells and their surroundings, we have examined the dynamics of adhesion of Chinese hamster ovary cells cultured on engineered protein substrates with different viscoelastic properties. We find that cell spreading and polarization rates vary two- and five-fold, respectively, for crosslinked and uncrosslinked proteins, despite the fact that the rates of growth of individual adhesion complexes on the different substrates are comparable. A wave of adhesion growth along the cell contour is observed by total internal reflection fluorescence microscopy for cells plated on crosslinked materials, but not on uncrosslinked substrates. We propose a mechanism in which cell-induced strains accumulate in crosslinked materials as a result of adhesion growth. Strain propagation within the material explains the observed adhesion growth patterns and the increased rates of spreading and polarization characteristic of cells cultured on crosslinked substrates. We investigate the proposed mechanism through Brownian dynamics simulation.

Construction of multi-functional extracellular matrix proteins that inhibits migration and tube formation of endothelial cells

Artificial extracellular matrix (ECM) proteins have been designed that have strong cell-adhesive activity and active functional units that inhibit network formation among vascular endothelial cells. A laminin-derived sequence (YIGSR) that blocks migration of vascular endothelial cells was designed to incorporate into an elastin-derived structural unit. The designed ECM fusion protein also had a cell-adhesive RGDN sequence that conferred an intense migration-inhibitory effect. The resultant ECM showed cell-adhesive activity superior to that of the synthetic YIGSR peptide and it blocked the migration of vascular endothelial cells. Furthermore, the designed ECM inhibited the angiogenic activity of a collagen gel. The engineering strategy of designing multi-functional ECM proteins could be applied to support novel tissue engineering techniques.

Artificial Extracellular Matrix Proteins Containing Phenylalanine Analogues Biosynthesized in Bacteria Using T7 Expression System and the PEGylation

In vivo incorporation of phenylalanine (Phe) analogues into an artificial extracellular matrix protein (aECM-CS5-ELF) was accomplished using a bacterial expression host that harbors the mutant phenylalanyl-tRNA synthetase (PheRS) with an enlarged binding pocket. Although the Ala294Gly/Thr251Gly mutant PheRS (PheRS**) under the control of T5 promoter allows incorporation of some Phe analogues into a protein, the T5 system is not suitable for material science studies because the amount of materials produced is not sufficient due to the moderate strength of the T5 promoter. This limitation can be overcome by using a pair of T7 promoter and T7 RNA polymerase instead. In the T7 expression system, it is difficult, however, to achieve a high incorporation level of Phe analogues, due to competition of Phe analogues for incorporation with the residual Phe that is required for synthesis of active T7 RNA polymerase. In this study, we prepared the PheRS** under T7 promoter and optimized culture condition to improve both the incorporation level of recombinant aECM protein and the incorporation level of Phe analogues. Incorporation and expression levels tend to increase in the case of p-azidophenylalanine, p-iodophenylalanine, and p-acetylphenylalanine. We evaluated the lower critical transition temperature, which is dependent on the incorporation ratio and the turbidity decreased when the incorporation level increased. Circular dichromism measurement indicated that this tendency is based on conformational change from random coil to β-turn structure. We demonstrated that polyethylene glycol (PEG) can be conjugated at reaction site of Phe analogues incorporated. We also demonstrated that the increased hydrophilicity of elastin-like sequences in the aECM-CS5-ELF made by PEG conjugation could suppress nonspecific adhesion of human umbilical vein endothelial cells (HUVEC).

Boundary crossing in epithelial wound healing

The processes of wound healing and collective cell migration have been studied for decades. Intensive research has been devoted to understanding the mechanisms involved in wound healing, but the role of cell-substrate interactions is still not thoroughly understood. Here we probe the role of cell-substrate interactions by examining in vitro the healing of monolayers of human corneal epithelial (HCE) cells cultured on artificial extracellular matrix (aECM) proteins. We find that the rate of wound healing is dependent on the concentration of fibronectin-derived (RGD) cell-adhesion ligands in the aECM substrate. The wound closure rate varies nearly sixfold on the substrates examined, despite the fact that the rates of migration and proliferation of individual cells show little sensitivity to the RGD concentration (which varies 40-fold). To explain this apparent contradiction, we study collective migration by means of a dynamic Monte Carlo simulation. The cells in the simulation spread, retract, and proliferate with probabilities obtained from a simple phenomenological model. The results indicate that the overall wound closure rate is determined primarily by the rate at which cells cross the boundary between the aECM protein and the matrix deposited under the cell sheet.

Collective Cell Migration on Artificial Extracellular Matrix Proteins Containing Full‐Length Fibronectin Domains

Protein-based biomaterials that promote rapid wound healing are prepared by expression of artificial genes in bacterial cells. Artificial extracellular matrix proteins containing full-length fibronectin domains 9 and 10 exhibit strong α_5β_1 integrin binding and support rapid spreading, proliferation, and collective migration of fibroblasts.

Promotion of Angiogenesis by an Artificial Extracellular Matrix Protein Containing the Laminin-1-Derived IKVAV Sequence

The laminin-1-derived IKVAV sequence is known for its angiogenic function. We previously developed artificial extracellular matrix (ECM) proteins containing the IKVAV sequence. They were designed to have collagen-binding activity and active functional units that promote network formation of vascular endothelial cells. The resultant fusion protein, called EREI2CBD, was confirmed to bind to collagen type I and promote tubular network formation of endothelial cells cultured in collagen gel in vitro. In this study, EREI2CBD was applied to the chick chorioallantoic membrane (CAM) assay to investigate in vivo angiogenic activity. The CAM assay results showed that EREI2CBD caused the number and area of vascular branches to be increased. The constructed fusion protein and the engineering strategy of designing multifunctional ECM proteins support current tissue engineering techniques.

Gradient lithography of engineered proteins to fabricate 2D and 3D cell culture microenvironments.

Spatial patterning of proteins is a valuable technique for many biological applications and is the prevailing tool for defining microenvironments for cells in culture, a required procedure in developmental biology and tissue engineering research. However, it is still challenging to achieve protein patterns that closely mimic native microenvironments, such as gradient protein distributions with desirable mechanical properties. By combining projection dynamic mask lithography and protein engineering with non-canonical photosensitive amino acids, we demonstrate a simple, scalable strategy to fabricate any user-defined 2D or 3D stable gradient pattern with complex geometries from an artificial extracellular matrix (aECM) protein. We show that the elastic modulus and chemical nature of the gradient profile are biocompatible and allow useful applications in cell biological research.

Cell Response to RGD Density in Cross-Linked Artificial Extracellular Matrix Protein Films

This study examines the adhesion, spreading, and migration of human umbilical vein endothelial cells on cross-linked films of artificial extracellular matrix (aECM) proteins. The aECM proteins described here were designed for application in small-diameter grafts and are composed of elastin-like structural repeats and fibronectin cell-binding domains. aECM-RGD contains the RGD sequence derived from fibronectin; the negative control protein aECM-RDG contains a scrambled cell-binding domain. The covalent attachment of poly(ethylene glycol) (PEG) to aECM substrates reduced nonspecific cell adhesion to aECM-RDG-PEG but did not preclude sequence-specific adhesion of endothelial cells to aECM-RGD-PEG. Variation in ligand density was accomplished by the mixing of aECM-RGD-PEG and aECM-RDG-PEG prior to cross-linking. Increasing the density of RGD domains in cross-linked films resulted in more robust cell adhesion and spreading but did not affect cell migration speed. Control of cell-binding domain density in aECM proteins can thus be used to modulate cell adhesion and spreading and will serve as an important design tool as these materials are further developed for use in surgery, tissue engineering, and regenerative medicine.

Construction of multi-functional extracellular matrix proteins that promote tube formation of endothelial cells

We developed artificial extracellular matrix proteins designed to have collagen-binding activity and active functional units that promote network formation of vascular endothelial cells. We engineered a laminin-derived IKVAV sequence, which stimulates capillary network formation of vascular endothelial cells, to incorporate into an elastin-derived structural unit. The designed fusion protein also had a cell-adhesive RGD sequence and a collagen-binding domain derived from fibronectin. The resultant fusion protein could bind to collagen type I and promote angiogenic activity of collagen gel. The collagen-binding domain also had slight angiogenic activity; however, the designed fusion protein also enhanced cellular migration activity. The engineering strategy of designing multi-functional ECM proteins has a possibility for supporting current tissue engineering techniques.

Mechanically Tunable Thin Films of Photosensitive Artificial Proteins: Preparation and Characterization by Nanoindentation

Thin films of controlled elastic modulus were made by photo-cross-linking artificial extracellular matrix (aECM) proteins containing the photosensitive amino acid p-azidophenylalanine (pN_3Phe). The elastic moduli of the films were calculated from nanoindentation data collected by atomic force microscopy (AFM) using a thin-film Hertz model. The modulus was shown to be tunable in the range 0.3−1.0 MPa either by controlling the irradiation time or by varying the level of pN_3Phe in the protein. Tensile measurements on bulk films of the same proteins and finite-element simulation of the indentation process agreed with the thin-film modulus measurements from AFM. Substrates characterized by spatial variation in elastic modulus were created by local control of the irradiation time.

Artificial extracellular matrix proteins contain heparin‐binding and RGD‐containing domains to improve osteoblast‐like cell attachment and growth

Through the recombinant DNA technology, it is possible to create artificial extracellular matrix (aECM) proteins with domains chosen to modulate cellular behaviors. In this study, we constructed three aECM proteins containing both heparin-binding and RGD-containing domains (387RGDS, Tri-FN10, and TNC-FN3) produced in Escherichia coli. Promotion of MG-63 cell attachment and growth in two-dimensional (2D) cultures (tissue culture plate, polycaprolactone membrane) and 3D cultures (Cytodex 1 and Plastic Plus microcarriers) were demonstrated on these three aECM protein-coated surfaces. These three aECM proteins improved MG-63 cell attachment and growth in the order TNC-FN3 > Tri-FN10 > 387RGDS in both 2D and 3D cultures. This study is the first report of the construction of aECM proteins that contain both heparin-binding and RGD-containing domains used in osteoblast tissue engineering applications.