Engineering a Biomimetic In Vitro Model of Bruch's Membrane Using Hagfish Slime Intermediate Filament Proteins.

Bruch's membrane resides in the subretinal tissue and regulates the flow of nutrients and waste between the retinal pigment epithelial (RPE) and vascular layers of the eye. With age, Bruch's membrane becomes thicker, stiffer, and less permeable, which impedes its function as a boundary layer in the subretina. These changes contribute to pathologies such as age-related macular degeneration (AMD). To better understand how aging in Bruch's membrane affects surrounding tissues and to determine the relationship between aging and disease, an in vitro model of Bruch's membrane is needed. An accurate model of Bruch's membrane must be a proteinaceous, semipermeable, and nonporous biomaterial with similar mechanical properties to in vivo conditions. Additionally, this model must support RPE cell growth. While models of subretinal tissue exist, they typically differ from in vivo Bruch's membrane in one or more of these properties. This study evaluates the capability of membranes created from recombinant hagfish intermediate filament (rHIF) proteins to accurately replicate Bruch's membrane in an in vitro model of the subretinal tissue. The physical characteristics of these rHIF membranes were evaluated using mechanical testing, permeability assays, brightfield microscopy, and scanning electron microscopy. The capacity of the membranes to support RPE cell culture was determined using brightfield and fluorescent microscopy, as well as immunocytochemical staining. This study demonstrates that rHIF protein membranes are an appropriate biomaterial to accurately mimic both healthy and aged Bruch's membrane for in vitro modeling of the subretinal tissue.

Scalable purification of recombinant structural proteins, hagfish intermediate filament α and É£, from inclusion bodies for fiber formation.

The purpose of this study was to determine a method to purify recombinant hagfish intermediate filament proteins, alpha and gamma, in a scalable manner. The study succeeded by having an increase in protein recovery of up to 35% when comparing centrifuge purification and the developed tangential flow purification. The proteins were approximately the same purity of 70% pure but further purification increased the purity of the proteins by 16%, based on ImageJ analysis. The developed tangential flow filtration purification and final purification methods could be easily scaled up to meet industry scale purification needs. The scaled-up processes described in this study did not interfere with fiber production or formation, indicating the methods can produce usable proteins for material development.

The next generation of protein super-fibres: robust recombinant production and recovery of hagfish intermediate filament proteins with fibre spinning and mechanical-structural characterizations.

Native hagfish intermediate filament proteins have impressive mechanical properties. However, using these native fibres for any application is impractical, necessitating their recombinant production. In the only literature report on the proteins (denoted α and É£), heterologous expression levels, using E. coli, were low and no attempts were made to optimize expression, explore wet-spinning, or spin the two proteins individually into fibres. Reported here is the high production (~8 g l-1 of dry protein) of the hagfish intermediate filament proteins, with yields orders of magnitude higher (325-1000×) than previous reports. The proteins were spun into fibres individually and in their native-like 1:1 ratio. For all fibres, the hallmark α-helix to ß-sheet conversion occurred after draw-processing. The native-like 1:1 ratio fibres achieved the highest average tensile strength in this study at nearly 200 MPa with an elastic modulus of 5.7 GPa, representing the highest tensile strength reported for these proteins without chemical cross-linking. Interestingly, the recombinant α protein achieved nearly the same mechanical properties when spun as a homopolymeric fibre. These results suggest that varying the two protein ratios beyond the natural 1:1 ratio will allow a high degree of tunability. With robust heterologous expression and purification established, optimizing fibre spinning will be accelerated compared to difficult to produce proteins such as spider silks.

Large scale production of synthetic spider silk proteins in Escherichia coli.

Spider silk, which has remarkable mechanical properties, is a natural protein fiber produced by spiders. Spiders cannot be farmed because of their cannibalistic and territorial nature. Hence, large amounts of spider silk cannot be produced from spiders. Genetic engineering is an alternative approach to produce large quantities of spider silk. Our group has produced synthetic spider silk proteins in E. coli to study structure/function and to produce biomaterials comparable to the silks produced by orb-weaving spiders. Here we give a detailed description of our cloning, expression, and purification methods of synthetic spider silk proteins ranging from ~30 to ~200 kDa. We have cloned the relevant genes of the spider Nephila clavipes and introduced them into bacteria to produce synthetic spider silk proteins using small and large-scale bioreactors. We have optimized the fermentation process, and we have developed protein purification methods as well. The purified proteins are spun into fibers and are used to make alternative materials like films and adhesives with various possible commercial applications.

Silkworm Silk Fiber Bundles as Improved In Vitro Scaffolds for Skeletal Muscle.

To mimic skeletal muscle tissues in vitro, native and transgenic spider silk/silkworm silks were seeded with C2C12 myoblasts to observe if these three-dimensional substrates are preferable to a traditional two-dimensional polystyrene cell culture surface. Silks were wound around an acrylic chassis to produce a novel, three-dimensional cell culture device with suspended muscle fibers that genetically and morphologically resemble native skeletal muscle tissue. The transgenic spider silk/silkworm silk has never before been studied for this application. Genetic expression verified skeletal muscle lineage and differentiation, while fluorescent imaging verified contractile protein synthesis. Genetic analysis also revealed an increase in expression of the Myh2 contractile protein gene on silkworm silks, particularly on the transgenic silk. Mechanical properties and protein secondary structure content of the silks indicated correlation between substrate properties and Myh2 gene expression. This increase in contractile protein gene expression suggests that biologically derived silk substrates that are suspended may be a preferable substrate for in vitro muscle modeling because of the proteinaceous character and mechanical flexibility of the silk.

Silkworms with Spider Silklike Fibers Using Synthetic Silkworm Chow Containing Calcium Lignosulfonate, Carbon Nanotubes, and Graphene.

Silkworm silk has become increasingly relevant for material applications. However, the industry as a whole is retracting because of problems with mass production. One of the key problems is the inconsistent properties of the silk. A means by which to improve the silk material properties is through enhanced sericulture techniques. One possible technique is altering the feed of the silkworms to include single-wall carbon nanotubes (SWNTs) or graphene (GR). Recently published results have demonstrated substantial improvement in fiber mechanical properties. However, the effect of the surfactant used to incorporate those materials into the feed on the fiber mechanical properties in comparison to normal silkworm silk has not been studied or reported. Thus, the total effect of feeding the SWNT and GR in the presence of surfactants on silkworms is not understood. Our study focuses on the surfactant [calcium lignosulfonate (LGS)] and demonstrates that it alone results in appreciable improvement of mechanical properties in comparison to nontreated silkworm silk. Furthermore, our study demonstrates that mixing the LGS, SWNT, and GR directly into the artificial diet of silkworms yields improved mechanical properties without decline below the control silk at high doses of SWNT or GR. Combined, we present evidence that mixing surfactants, in this case LGS, directly with the diet of silkworms creates a high-quality fiber product that can exceed 1 GPa in tensile strength. With the addition of nanocarbons, either SWNT or GR, the improvement is even greater and consistently surpasses control fibers. However, feeding LGS alone is a more economical and practical choice to consistently improve the mechanical properties of silkworm fiber.

CRISPR/Cas9 Initiated Transgenic Silkworms as a Natural Spinner of Spider Silk.

Using transgenic silkworms with their natural spinning apparatus has proven to be a promising way to spin spider silk-like fibers. The challenges are incorporating native-size spider silk proteins and achieving an inheritable transgenic silkworm strain. In this study, a CRISPR/Cas9 initiated fixed-point strategy was used to successfully incorporate spider silk protein genes into the Bombyx mori genome. Native-size spider silk genes (up to 10 kb) were inserted into an intron of the fibroin heavy or light chain (FibH or FibL) ensuring that any sequence changes induced by the CRISPR/Cas9 would not impact protein production. The resulting fibers are as strong as native spider silks (1.2 GPa tensile strength). The transgenic silkworms have been tracked for several generations with normal inheritance of the transgenes. This strategy demonstrates the feasibility of using silkworms as a natural spider silk spinner for industrial production of high-performance fibers.

Material Formation of Recombinant Spider Silks through Aqueous Solvation using Heat and Pressure.

Many spiders produce seven types of silks. Six of the silks are fiber in form when produced by the spiders. These fibers are not water soluble. In order to reproduce the remarkable mechanical properties of spider silks, they must be produced in heterologous hosts as spiders are both territorial and cannibalistic. The synthetic analogs of spider silk also tend to be insoluble in aqueous solutions. Thus, a large percentage of research in recombinant spider silks rely upon organic solvents that are detrimental to large scale production of materials. Our group's method forces the solvation of these recombinant spider silks into water. Remarkably, when these proteins are prepared using this method of heat and pressure, a wide range of material forms can be prepared from the same solution of recombinant spider silk proteins (rSSp) including: films, fibers, sponge, hydrogel, lyogel, and adhesives. This article demonstrates the production of the solvated rSSp and material forms in a manner that is more easily understood than from written materials and methods alone.

Utilizing Recombinant Spider Silk Proteins To Develop a Synthetic Bruch's Membrane for Modeling the Retinal Pigment Epithelium.

Spider silks are intriguing biomaterials that have a high potential as innovative biomedical processes and devices. The intent of this study was to evaluate the capacity of recombinant spider silk proteins (rSSps) as a synthetic Bruch's membrane. Nonporous silk membranes were prepared with comparable thicknesses (<10 µm) to that of native Bruch's membrane. Biomechanical characterization was performed prior to seeding cells. The ability of RPE cells (ARPE-19) to attach and grow on the membranes was then evaluated with bright-field and electron microscopy, intracellular DNA quantification, and immunocytochemical staining (ZO-1 and F-actin). Controls were cultured on permeable Transwell support membranes and characterized with the same methods. A size-dependent permeability assay, using FITC-dextran, was used to determine cell-membrane barrier function. Compared to Transwell controls, RPE cells cultured on rSSps membranes developed more native-like "cobblestone" morphologies, exhibited higher intracellular DNA content, and expressed key organizational proteins more consistently. Comparisons of the membranes to native structures revealed that the silk membranes exhibited equivalent thicknesses, biomechanical properties, and barrier functions. These findings support the use of recombinant spider silk proteins to model Bruch's membrane and develop more biomimetic retinal models.

Method for the Destruction of Endotoxin in Synthetic Spider Silk Proteins.

Although synthetic spider silk has impressive potential as a biomaterial, endotoxin contamination of the spider silk proteins is a concern, regardless of the production method. The purpose of this research was to establish a standardized method to either remove or destroy the endotoxins present in synthetic spider silk proteins, such that the endotoxin level was consistently equal to or less than 0.25 EU/mL, the FDA limit for similar implant materials. Although dry heat is generally the preferred method for endotoxin destruction, heating the silk proteins to the necessary temperatures led to compromised mechanical properties in the resultant materials. In light of this, other endotoxin destruction methods were investigated, including caustic rinses and autoclaving. It was found that autoclaving synthetic spider silk protein dopes three times in a row consistently decreased the endotoxin level 10-20 fold, achieving levels at or below the desired level of 0.25 EU/mL. Products made from triple autoclaved silk dopes maintained mechanical properties comparable to products from untreated dopes while still maintaining low endotoxin levels. Triple autoclaving is an effective and scalable method for preparing synthetic spider silk proteins with endotoxin levels sufficiently low for use as biomaterials without compromising the mechanical properties of the materials.

Importance of Heat and Pressure for Solubilization of Recombinant Spider Silk Proteins in Aqueous Solution.

The production of recombinant spider silk proteins continues to be a key area of interest for a number of research groups. Several key obstacles exist in their production as well as in their formulation into useable products. The original reported method to solubilize recombinant spider silk proteins (rSSp) in an aqueous solution involved using microwaves to quickly generate heat and pressure inside of a sealed vial containing rSSp and water. Fibers produced from this system are remarkable in their mechanical ability and demonstrate the ability to be stretched and recover 100 times. The microwave method dissolves the rSSPs with dissolution time increasing with higher molecular weight constructs, increasing concentration of rSSPs, protein type, and salt concentration. It has proven successful in solvating a number of different rSSPs including native-like sequences (MaSp1, MaSp2, piriform, and aggregate) as well as chimeric sequences (FlAS) in varied concentrations that have been spun into fibers and formed into films, foams, sponges, gels, coatings, macro and micro spheres and adhesives. The system is effective but inherently unpredictable and difficult to control. Provided that the materials that can be generated from this method of dissolution are impressive, an alternative means of applying heat and pressure that is controllable and predictable has been developed. Results indicate that there are combinations of heat and pressure (135 °C and 140 psi) that result in maximal dissolution without degrading the recombinant MaSp2 protein tested, and that heat and pressure are the key elements to the method of dissolution.

Sticky Situation: An Investigation of Robust Aqueous-Based Recombinant Spider Silk Protein Coatings and Adhesives.

The mechanical properties and biocompatibility of spider silks have made them one of the most sought after and studied natural biomaterials. A biomimetic process has been developed that uses water to solvate purified recombinant spider silk proteins (rSSps) prior to material formation. The absence of harsh organic solvents increases cost effectiveness, safety, and decreases the environmental impact of these materials. This development allows for the investigation of aqueous-based rSSps as coatings and adhesives and their potential applications. In these studies it was determined that fiber-based rSSps in nonfiber formations have the capability to coat and adhere numerous substrates, whether rough, smooth, hydrophobic, or hydrophilic. Further, these materials can be functionalized for a variety of processes. Drug-eluting coatings have been made with the capacity to release a variety of compounds in addition to their inherent ability to prevent blood clotting and biofouling. Additionally, spider silk protein adhesives are strong enough to outperform some conventional glues and still display favorable tissue implantation properties. The physical properties, corresponding capabilities, and potential applications of these nonfibrous materials were characterized in this study. Mechanical properties, ease of manufacturing, biodegradability, biocompatibility, and functionality are the hallmarks of these revolutionary spider silk protein materials.

More than just fibers: an aqueous method for the production of innovative recombinant spider silk protein materials.

Spider silk is a striking and robust natural material that has an unrivaled combination of strength and elasticity. There are two major problems in creating materials from recombinant spider silk proteins (rSSps): expressing sufficient quantities of the large, highly repetitive proteins and solvating the naturally self-assembling proteins once produced. To address the second problem, we have developed a method to rapidly dissolve rSSps in water in lieu of traditional organic solvents and accomplish nearly 100% solvation and recovery of the protein. Our method involves generating pressure and temperature in a sealed vial by using short, repetitive bursts from a conventional microwave. The method is scalable and has been successful with all rSSps used to date. From these easily generated aqueous solutions of rSSps, a wide variety of materials have been produced. Production of fibers, films, hydrogels, lyogels, sponges, and adhesives and studies of their mechanical and structural properties are reported. To our knowledge, ours is the only method that is cost-effective and scalable for mass production. This solvation method allows a choice of the physical form of product to take advantage of spider silks' mechanical properties without using costly and problematic organic solvents.