Spider Silk Proteins – Mechanical Property and Gene Sequence

Secondary Structure Adopted by the Gly-Gly-X Repetitive Regions of Dragline Spider Silk

Solid-state NMR and molecular dynamics (MD) simulations are presented to help elucidate the molecular secondary structure of poly(Gly-Gly-X), which is one of the most common structural repetitive motifs found in orb-weaving dragline spider silk proteins. The combination of NMR and computational experiments provides insight into the molecular secondary structure of poly(Gly-Gly-X) segments and provides further support that these regions are disordered and primarily non-β-sheet. Furthermore, the combination of NMR and MD simulations illustrate the possibility for several secondary structural elements in the poly(Gly-Gly-X) regions of dragline silks, including β-turns, 310-helicies, and coil structures with a negligible population of α-helix observed.

Folding recombinant spider-silk in H2 O: Effect of osmolytes on the solution conformation of a 15-repeat spider-silk mimetic

The folding of a recombinant spider silk protein-polymer in the presence of the tri-methylamine osmolytes TMANO and Betaine in 80% H2 O is reported. Circular dichroism measurements (CD) reveal an increase in α-helical secondary structure with increasing osmolyte concentrations, as determined by an increase in ellipticity at 222 nm. Consistent with this observation, the signal for random coil sampling, observed at 205 nm, is greatly reduced with increasing trimethylamine. Fluorescence spectra of a single tyrosine positioned within the conserved 33-amino acid repeat primary sequence (of the spider-silk mimetic) complements the conformational changes observed by CD. Importantly, there is a correlation between the number of Alkyl-groups (CH3 -) on the amine of the osmolyte and enhanced helicity of the 15-repeat silk-mimetic for the osmolytes tested, ie TMANO, Betaine, Sarcosine and Glycine. These preliminary results are applicable to storing and processing recombinant silk sequences in H2 O, an important mile-stone for widespread use of recombinant silk polymers.

Spider Silk-CBD-Cellulose Nanocrystal Composites: Mechanism of Assembly

The fabrication of cellulose-spider silk bio-nanocomposites comprised of cellulose nanocrystals (CNCs) and recombinant spider silk protein fused to a cellulose binding domain (CBD) is described. Silk-CBD successfully binds cellulose, and unlike recombinant silk alone, silk-CBD self-assembles into microfibrils even in the absence of CNCs. Silk-CBD-CNC composite sponges and films show changes in internal structure and CNC alignment related to the addition of silk-CBD. The silk-CBD sponges exhibit improved thermal and structural characteristics in comparison to control recombinant spider silk sponges. The glass transition temperature (Tg) of the silk-CBD sponge was higher than the control silk sponge and similar to native dragline spider silk fibers. Gel filtration analysis, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (TEM) indicated that silk-CBD, but not the recombinant silk control, formed a nematic liquid crystalline phase similar to that observed in native spider silk during the silk spinning process. Silk-CBD microfibrils spontaneously formed in solution upon ultrasonication. We suggest a model for silk-CBD assembly that implicates CBD in the central role of driving the dimerization of spider silk monomers, a process essential to the molecular assembly of spider-silk nanofibers and silk-CNC composites.

Nanomechanics of silk: the fundamentals of a strong, tough and versatile material

Spider silk is a remarkable material that provides a template for upscaling molecular properties to the macroscale. In this article we review fundamental aspects of the mechanisms behind these behaviors, discuss the molecular makeup, chemical designs, and how these integrate in a complex arrangement to form webs, cocoons and other material architectures. Moreover, this review paper explores the unique ability of silk to tolerate various kinds of defects, in a way enabling this material platform to serve as one of the most resilient materials in nature. We conclude the discussion with a summary of key scaling laws, an attempt model and define hierarchical length-scales, and the translation to synthetic materials.

Silk protein-based hydrogels: Promising advanced materials for biomedical applications

Hydrogels are a class of advanced material forms that closely mimic properties of the soft biological tissues. Several polymers have been explored for preparing hydrogels with structural and functional features resembling that of the extracellular matrix. Favourable material properties, biocompatibility and easy processing of silk protein fibers into several forms make it a suitable material for biomedical applications. Hydrogels made from silk proteins have shown a potential in overcoming limitations of hydrogels prepared from conventional polymers. A great deal of effort has been made to control the properties and to integrate novel topographical and functional characteristics in the hydrogel composed from silk proteins. This review provides overview of the advances in silk protein-based hydrogels with a primary emphasis on hydrogels of fibroin. It describes the approaches used to fabricate fibroin hydrogels. Attempts to improve the existing properties or to incorporate new features in the hydrogels by making composites and by improving fibroin properties by genetic engineering approaches are also described. Applications of the fibroin hydrogels in the realms of tissue engineering and controlled release are reviewed and their future potentials are discussed.

STATEMENT OF SIGNIFICANCE

This review describes the potentiality of silk fibroin hydrogel. Silk Fibroin has been widely recognized as an interesting biomaterial. Due to its properties including high mechanical strength and excellent biocompatibility, it has gained wide attention. Several groups are exploring silk-based materials including films, hydrogels, nanofibers and nanoparticles for different biomedical applications. Although there is a good amount of literature available on general properties and applications of silk based biomaterials, there is an inadequacy of extensive review articles that specifically focus on silk based hydrogels. Silk-based hydrogels have a strong potential to be utilized in biomedical applications. Our work is an effort to highlight the research that has been done in the area of silk-based hydrogels. It aims to provide an overview of the advances that have been made and the future course available. It will provide an overview of the silk-based hydrogels as well as may direct the readers to the specific areas of application.

In Vitro Evaluation of Spider Silk Meshes as a Potential Biomaterial for Bladder Reconstruction

Reconstruction of the bladder by means of both natural and synthetic materials remains a challenge due to severe adverse effects such as mechanical failure. Here we investigate the application of spider major ampullate gland-derived dragline silk from the Nephila edulis spider, a natural biomaterial with outstanding mechanical properties and a slow degradation rate, as a potential scaffold for bladder reconstruction by studying the cellular response of primary bladder cells to this biomaterial. We demonstrate that spider silk without any additional biological coating supports adhesion and growth of primary human urothelial cells (HUCs), which are multipotent bladder cells able to differentiate into the various epithelial layers of the bladder. HUCs cultured on spider silk did not show significant changes in the expression of various epithelial-to-mesenchymal transition and fibrosis associated genes, and demonstrated only slight reduction in the expression of adhesion and cellular differentiation genes. Furthermore, flow cytometric analysis showed that most of the silk-exposed HUCs maintain an undifferentiated immunophenotype. These results demonstrate that spider silk from the Nephila edulis spider supports adhesion, survival and growth of HUCs without significantly altering their cellular properties making this type of material a suitable candidate for being tested in pre-clinical models for bladder reconstruction.

Self-Assembled Proteins and Peptides as Scaffolds for Tissue Regeneration

Self-assembling proteins and peptides are increasingly gaining interest for potential use as scaffolds in tissue engineering applications. They self-organize from basic building blocks under mild conditions into supramolecular structures, mimicking the native extracellular matrix. Their properties can be easily tuned through changes at the sequence level. Moreover, they can be produced in sufficient quantities with chemical synthesis or recombinant technologies to allow them to address homogeneity and standardization issues required for applications. Here. recent advances in self-assembling proteins, peptides, and peptide amphiphiles that form scaffolds suitable for tissue engineering are reviewed. The focus is on a variety of motifs, ranging from minimalistic dipeptides, simplistic ultrashort aliphatic peptides, and peptide amphiphiles to large "recombinamer" proteins. Special emphasis is placed on the rational design of self-assembling motifs and biofunctionalization strategies to influence cell behavior and modulate scaffold stability. Perspectives for combination of these "bottom-up" designer strategies with traditional "top-down" biofabrication techniques for new generations of tissue engineering scaffolds are highlighted.

Silk-Its Mysteries, How It Is Made, and How It Is Used

This article reviews fundamental and applied aspects of silk-one of Nature's most intriguing materials in terms of its strength, toughness, and biological role-in its various forms, from protein molecules to webs and cocoons, in the context of mechanical and biological properties. A central question that will be explored is how the bridging of scales and the emergence of hierarchical structures are critical elements in achieving novel material properties, and how this knowledge can be explored in the design of synthetic materials. We review how the function of a material system at the macroscale can be derived from the interplay of fundamental molecular building blocks. Moreover, guidelines and approaches to current experimental and computational designs in the field of synthetic silklike materials are provided to assist the materials science community in engineering customized finetuned biomaterials for biomedical applications.

Silk-based biomaterials in biomedical textiles and fiber-based implants

Biomedical textiles and fiber-based implants (BTFIs) have been in routine clinical use to facilitate healing for nearly five decades. Amongst the variety of biomaterials used, silk-based biomaterials (SBBs) have been widely used clinically viz. sutures for centuries and are being increasingly recognized as a prospective material for biomedical textiles. The ease of processing, controllable degradability, remarkable mechanical properties and biocompatibility have prompted the use of SBBs for various BTFIs for extracorporeal implants, soft tissue repair, healthcare/hygiene products and related needs. The present Review focuses on BTFIs from the perspective of types and physical and biological properties, and this discussion is followed with an examination of the advantages and limitations of BTFIs from SBBs. The Review covers progress in surface coatings, physical and chemical modifications of SBBs for BTFIs and identifies future needs and opportunities for the further development for BTFIs using SBBs.

Self-assembly of nucleic acids, silk and hybrid materials thereof

Top-down approaches based on etching techniques have almost reached their limits in terms of dimension. Therefore, novel assembly strategies and types of nanomaterials are required to allow technological advances. Self-assembly processes independent of external energy sources and unlimited in dimensional scaling have become a very promising approach. Here,we highlight recent developments in self-assembled DNA-polymer, silk-polymer and silk-DNA hybrids as promising materials with biotic and abiotic moieties for constructing complex hierarchical materials in ‘bottom-up’ approaches. DNA block copolymers assemble into nanostructures typically exposing a DNA corona which allows functionalization, labeling and higher levels of organization due to its specific addressable recognition properties. In contrast, self-assembly of natural silk proteins as well as their recombinant variants yields mechanically stable β-sheet rich nanostructures. The combination of silk with abiotic polymers gains hybrid materials with new functionalities. Together, the precision of DNA hybridization and robustness of silk fibrillar structures combine in novel conjugates enable processing of higher-order structures with nanoscale architecture and programmable functions.

Analysis of transcriptomes of three orb-web spider species reveals gene profiles involved in silk and toxin

As an ancient arthropod with a history of 390 million years, spiders evolved numerous morphological forms resulting from adaptation to different environments. The venom and silk of spiders, which have promising commercial applications in agriculture, medicine and engineering fields, are of special interests to researchers. However, little is known about their genomic components, which hinders not only understanding spider biology but also utilizing their valuable genes. Here we report on deep sequenced and de novo assembled transcriptomes of three orb-web spider species, Gasteracantha arcuata, Nasoonaria sinensis and Gasteracantha hasselti which are distributed in tropical forests of south China. With Illumina paired-end RNA-seq technology, 54 871, 101 855 and 75 455 unigenes for the three spider species were obtained, respectively, among which 9 300, 10 001 and 10 494 unique genes are annotated, respectively. From these annotated unigenes, we comprehensively analyzed silk and toxin gene components and structures for the three spider species. Our study provides valuable transcriptome data for three spider species which previously lacked any genetic/genomic data. The results have laid the first fundamental genomic basis for exploiting gene resources from these spiders.

Multi-tissue transcriptomics of the black widow spider reveals expansions, co-options, and functional processes of the silk gland gene toolkit

Background

Spiders (Order Araneae) are essential predators in every terrestrial ecosystem largely because they have evolved potent arsenals of silk and venom. Spider silks are high performance materials made almost entirely of proteins, and thus represent an ideal system for investigating genome level evolution of novel protein functions. However, genomic level resources remain limited for spiders.

Results

We de novo assembled a transcriptome for the Western black widow (Latrodectus hesperus) from deeply sequenced cDNAs of three tissue types. Our multi-tissue assembly contained ~100,000 unique transcripts, of which > 27,000 were annotated by homology. Comparing transcript abundance among the different tissues, we identified 647 silk gland-specific transcripts, including the few known silk fiber components (e.g. six spider fibroins, spidroins). Silk gland specific transcripts are enriched compared to the entire transcriptome in several functions, including protein degradation, inhibition of protein degradation, and oxidation-reduction. Phylogenetic analyses of 37 gene families containing silk gland specific transcripts demonstrated novel gene expansions within silk glands, and multiple co-options of silk specific expression from paralogs expressed in other tissues.

Conclusions

We propose a transcriptional program for the silk glands that involves regulating gland specific synthesis of silk fiber and glue components followed by protecting and processing these components into functional fibers and glues. Our black widow silk gland gene repertoire provides extensive expansion of resources for biomimetic applications of silk in industry and medicine. Furthermore, our multi-tissue transcriptome facilitates evolutionary analysis of arachnid genomes and adaptive protein systems.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-365) contains supplementary material, which is available to authorized users.

Spider genomes provide insight into composition and evolution of venom and silk

Spiders are ecologically important predators with complex venom and extraordinarily tough silk that enables capture of large prey. Here we present the assembled genome of the social velvet spider and a draft assembly of the tarantula genome that represent two major taxonomic groups of spiders. The spider genomes are large with short exons and long introns, reminiscent of mammalian genomes. Phylogenetic analyses place spiders and ticks as sister groups supporting polyphyly of the Acari. Complex sets of venom and silk genes/proteins are identified. We find that venom genes evolved by sequential duplication, and that the toxic effect of venom is most likely activated by proteases present in the venom. The set of silk genes reveals a highly dynamic gene evolution, new types of silk genes and proteins, and a novel use of aciniform silk. These insights create new opportunities for pharmacological applications of venom and biomaterial applications of silk.

Spiders use self-produced venom and silk for their daily survival. Here, the authors report the assembled genome of the social velvet spider and a draft assembly of the tarantula genome and, together with proteomic data, provide insights into the evolution of genes that affect venom and silk production.

Structure-function-property-design interplay in biopolymers: spider silk

Spider silks have been a focus of research for almost two decades due to their outstanding mechanical and biophysical properties. Recent advances in genetic engineering have led to the synthesis of recombinant spider silks, thus helping to unravel a fundamental understanding of structure-function-property relationships. The relationships between molecular composition, secondary structures and mechanical properties found in different types of spider silks are described, along with a discussion of artificial spinning of these proteins and their bioapplications, including the role of silks in biomineralization and fabrication of biomaterials with controlled properties.

Biosorption of Cu(II) and Pb(II) ions from aqueous solution by natural spider silk

Aside from its excellent mechanical properties, spider silk (SS) would offer an active surface for heavy metal interaction due to its rich protein structure. The present study describes the potential use of natural (SS) as a sorbent of heavy metals from aqueous solutions. Single and multi-species biosorption experiments of heavy metals by natural SS were conducted using batch and column experiments. The biosorption kinetics, in general, was found to follow the second-order rate expression, and the experimental equilibrium biosorption data fitted reasonably well to Freundlich isotherm. From the Freundlich isotherm, the biosorption capacities of Cu(II) and Pb(II) ions onto SS were found as 0.20 and 0.007 mmol g⁻¹, respectively. The results showed a decrease in the extent of metal ion uptake with lowering the pH.

Plasticity in major ampullate silk production in relation to spider phylogeny and ecology

Spider major ampullate silk is a high-performance biomaterial that has received much attention. However, most studies ignore plasticity in silk properties. A better understanding of silk plasticity could clarify the relative importance of chemical composition versus processing of silk dope for silk properties. It could also provide insight into how control of silk properties relates to spider ecology and silk uses. We compared silk plasticity (defined as variation in the properties of silk spun by a spider under different conditions) between three spider clades in relation to their anatomy and silk biochemistry. We found that silk plasticity exists in RTA clade and orbicularian spiders, two clades that differ in their silk biochemistry. Orbiculariae seem less dependent on external spinning conditions. They probably use a valve in their spinning duct to control friction forces and speed during spinning. Our results suggest that plasticity results from different processing of the silk dope in the spinning duct. Orbicularian spiders seem to display better control of silk properties, perhaps in relation to their more complex spinning duct valve.

Heat Capacity of Spider Silk-like Block Copolymers

We synthesized and characterized a new family of di-block copolymers based on the amino acid sequences of Nephila clavipes major ampulate dragline spider silk, having the form HAB_n and HBA_n (n=1-3), comprising an alanine-rich hydrophobic block, A, a glycine-rich hydrophilic block, B, and a histidine tag, H. The reversing heat capacities, C_p(T), for temperatures below and above the glass transition, T_g, were measured by temperature modulated differential scanning calorimetry. For the solid state, we then calculated the heat capacities of our novel block copolymers based on the vibrational motions of the constituent poly(amino acid)s, whose heat capacities are known or can be estimated from the ATHAS Data Bank. For the liquid state, the heat capacity was estimated by using the rotational and translational motions in the polymer chain. Excellent agreement was found between the measured and calculated values of the heat capacity, showing that this method can serve as a standard by which to assess the C_p for other biologically inspired block copolymers. The fraction of beta sheet crystallinity of spider silk block copolymers was also determined by using the predicted C_p, and was verified by wide angle X-ray diffraction and Fourier transform infrared spectroscopy. The glass transition temperatures of spider silk block copolymer were fitted by Kwei's equation and the results indicate that attractive interaction exists between the A-block and B-block.

Spider silk: understanding the structure-function relationship of a natural fiber

Spider silk is of great interest because of its extraordinary physical properties, such as strength and toughness. Here we discuss how these physical properties relate to the way in which spiders have utilized this material in prey capture, forcing its evolution to a high-performance fiber. Female spiders can produce up to seven different types of silk, and all these have different physical properties, which relate to their various functions. The variation in properties are due to underlying differences in the proteins making up these silks. As our understanding of spider silk has increased in the recent years, it has been possible to produce recombinant versions of the respective proteins. Recombinant proteins open up the potential to produce synthetic silk fibers with properties similar to those of the natural spider silk threads.

Interactions between spider silk and cells--NIH/3T3 fibroblasts seeded on miniature weaving frames

Background

Several materials have been used for tissue engineering purposes, since the ideal matrix depends on the desired tissue. Silk biomaterials have come to focus due to their great mechanical properties. As untreated silkworm silk has been found to be quite immunogenic, an alternative could be spider silk. Not only does it own unique mechanical properties, its biocompatibility has been shown already in vivo. In our study, we used native spider dragline silk which is known as the strongest fibre in nature.

Methodology/Principal Findings

Steel frames were originally designed and manufactured and woven with spider silk, harvesting dragline silk directly out of the animal. After sterilization, scaffolds were seeded with fibroblasts to analyse cell proliferation and adhesion. Analysis of cell morphology and actin filament alignment clearly revealed adherence. Proliferation was measured by cell count as well as determination of relative fluorescence each after 1, 2, 3, and 5 days. Cell counts for native spider silk were also compared with those for trypsin-digested spider silk. Spider silk specimens displayed less proliferation than collagen- and fibronectin-coated cover slips, enzymatic treatment reduced adhesion and proliferation rates tendentially though not significantly. Nevertheless, proliferation could be proven with high significance (p<0.01).

Conclusion/Significance

Native spider silk does not require any modification to its application as a biomaterial that can rival any artificial material in terms of cell growth promoting properties. We could show adhesion mechanics on intracellular level. Additionally, proliferation kinetics were higher than in enzymatically digested controls, indicating that spider silk does not require modification. Recent findings concerning reduction of cell proliferation after exposure could not be met. As biotechnological production of the hierarchical composition of native spider silk fibres is still a challenge, our study has a pioneer role in researching cellular mechanics on native spider silk fibres.

Preliminary evaluation of a novel strong/osteoinductive calcium phosphate cement

We developed a novel calcium phosphate cement (CPC) by combining the silk fibroin and osteogenic supplements (β-glycerophosphate, ascorbic acid, and dexamethasone) with α-tricalcium phosphate cement. Mesenchymal stem cells (MSCs) were cultured on the novel CPC scaffold. Results showed that the novel CPC scaffold was biocompatible and favorable for the adhesion, spreading, and proliferation of MSCs. Osteogenic differentiation of MSCs was confirmed by high osteocalcin content and elevated gene expressions of bone markers, such as alkaline phosphatase, collagen type I, and osteocalcin. Therefore, the novel CPC scaffold may be potentially useful for implant fixation and more rapid new bone formation in moderate load-bearing applications.

Molecular diversity of spider venom

Spider venom, a factor that has played a decisive role in the evolution of one of the most successful groups of living organisms, is reviewed. Unique molecular diversity of venom components including substances of variable structure (from simple low molecular weight compounds to large multidomain proteins) with different functions is considered. Special attention is given to the structure, properties, and biosynthesis of toxins of polypeptide nature.

Antheraea pernyi silk fiber: a potential resource for artificially biospinning spider dragline silk

The outstanding properties of spider dragline silk are likely to be determined by a combination of the primary sequences and the secondary structure of the silk proteins. Antheraea pernyi silk has more similar sequences to spider dragline silk than the silk from its domestic counterpart, Bombyx mori. This makes it much potential as a resource for biospinning spider dragline silk. This paper further verified its possibility as the resource from the mechanical properties and the structures of the A. pernyi silks prepared by forcible reeling. It is surprising that the stress-strain curves of the A. pernyi fibers show similar sigmoidal shape to those of spider dragline silk. Under a controlled reeling speed of 95 mm/s, the breaking energy was 1.04 x 10(5) J/kg, the tensile strength was 639 MPa and the initial modulus was 9.9 GPa. It should be noted that this breaking energy of the A. pernyi silk approaches that of spider dragline silk. The tensile properties, the optical orientation and the beta-sheet structure contents of the silk fibers are remarkably increased by raising the spinning speeds up to 95 mm/s.

Containment of extended length polymorphisms in silk proteins

The spider silk gene family to the current date has been developed by gene duplication and homogenization events as well as conservation of crucial sequence parts. These evolutionary processes have created an amazing diversity of silk types each associated with specific properties and functions. In addition, they have led to allelic and gene variants within a species as exemplified by the major ampullate spidroin 1 gene of Nephila clavipes. Due to limited numbers of individuals screened to date little is known about the extent of these heterogeneities and how they are finally manifested in the proteins. Using expanded sample sizes, we show that sequence variations expressed as deletions or insertions of tri-nucleotides lead to different sized and structured repetitive units throughout a silk protein. Moreover, major ampullate spidroins 1 can quite dramatically differ in their overall lengths; however, extreme variants do not spread widely in a spider population. This suggests that a certain size range stabilized by purifying selection is important for spidroin 1 gene integrity and protein function. More than one locus for spidroin 1 genes possibly exist within one individual genome, which are homogenized in size, are differentially expressed and give a spider a certain degree of adaptation on silk's composition and properties. Such mechanisms are shared to a lesser extent by the second major ampullate spidroin gene.

Role of polyalanine domains in beta-sheet formation in spider silk block copolymers

Genetically engineered spider silk-like block copolymers were studied to determine the influence of polyalanine domain size on secondary structure. The role of polyalanine block distribution on beta-sheet formation was explored using FT-IR and WAXS. The number of polyalanine blocks had a direct effect on the formation of crystalline beta-sheets, reflected in the change in crystallinity index as the blocks of polyalanines increased. WAXS analysis confirmed the crystalline nature of the sample with the largest number of polyalanine blocks. This approach provides a platform for further exploration of the role of specific amino acid chemistries in regulating the assembly of beta-sheet secondary structures, leading to options to regulate material properties through manipulation of this key component in spider silks.

Engineering the Salmonella type III secretion system to export spider silk monomers

The type III secretion system (T3SS) exports proteins from the cytoplasm, through both the inner and outer membranes, to the external environment. Here, a system is constructed to harness the T3SS encoded within Salmonella Pathogeneity Island 1 to export proteins of biotechnological interest. The system is composed of an operon containing the target protein fused to an N-terminal secretion tag and its cognate chaperone. Transcription is controlled by a genetic circuit that only turns on when the cell is actively secreting protein. The system is refined using a small human protein (DH domain) and demonstrated by exporting three silk monomers (ADF-1, -2, and -3), representative of different types of spider silk. Synthetic genes encoding silk monomers were designed to enhance genetic stability and codon usage, constructed by automated DNA synthesis, and cloned into the secretion control system. Secretion rates up to 1.8 mg l(-1) h(-1) are demonstrated with up to 14% of expressed protein secreted. This work introduces new parts to control protein secretion in Gram-negative bacteria, which will be broadly applicable to problems in biotechnology.

Biomechanical variation of silk links spinning plasticity to spider web function

Spider silk is renowned for its high tensile strength, extensibility and toughness. However, the variability of these material properties has largely been ignored, especially at the intra-specific level. Yet, this variation could help us understand the function of spider webs. It may also point to the mechanisms used by spiders to control their silk production, which could be exploited to expand the potential range of applications for silk. In this study, we focus on variation of silk properties within different regions of cobwebs spun by the common house spider, Achaearanea tepidariorum. The cobweb is composed of supporting threads that function to maintain the web shape and hold spiders and prey, and of sticky gumfooted threads that adhere to insects during prey capture. Overall, structural properties, especially thread diameter, are more variable than intrinsic material properties, which may reflect past directional selection on certain silk performance. Supporting threads are thicker and able to bear higher loads, both before deforming permanently and before breaking, compared with sticky gumfooted threads. This may facilitate the function of supporting threads through sustained periods of time. In contrast, sticky gumfooted threads are more elastic, which may reduce the forces that prey apply to webs and allow them to contact multiple sticky capture threads. Therefore, our study suggests that spiders actively modify silk material properties during spinning in ways that enhance web function.

High-resolution NMR characterization of a spider-silk mimetic composed of 15 tandem repeats and a CRGD motif

Multidimensional solution NMR spectroscopic techniques have been used to obtain atomic level information about a recombinant spider silk construct in hexafluoro-isopropanol (HFIP). The synthetic 49 kDa silk-like protein mimics authentic silk from Nephila clavipes, with the inclusion of an extracellular matrix recognition motif. 2D (1)H-(15)N HSQC NMR spectroscopy reveals 33 cross peaks, which were assigned to amino acid residues in the semicrystalline repeat units. Signals from the amorphous segments in the primary sequence were weak and broad, suggesting that this region is highly dynamic and undergoing conformational exchange. An analysis of the deviations of the (13)C(alpha), (13)C(beta), and (13)CO chemical shifts relative to the expected random coil values reveals two highly alpha-helical regions from amino acid 12-19 and 26-32, which comprise the polyalanine track and a GGLGSQ sequence. This finding is further supported by phi-value analysis and sequential and medium-range NOE interactions. Pulsed field gradient NMR measurements indicate that the topology of the silk mimetic in HFIP is nonglobular. Moreover, the 3D (15)N-NOESY HSQC spectrum exhibits few long-range NOEs. Similar spectral features have been observed for repeat modules in other polypeptides and are characteristic of an elongated conformation. The results provide a residue-specific description of a silk sequence in nonaqueous solution and may be insightful for understanding the fold and topology of highly concentrated, stable silk before spinning. Additionally, the insights obtained may find application in future design and large-scale production and storage of synthetic silks in organic solvents.

Transgenic silkworms (Bombyx mori) produce recombinant spider dragline silk in cocoons

Spider dragline silk is a unique fibrous protein with a combination of tensile strength and elasticity, but the isolation of large amounts of silk from spiders is not feasible. In this study, we generated germline-transgenic silkworms (Bombyx mori) that spun cocoons containing recombinant spider silk. A piggyBac-based transformation vector was constructed that carried spider dragline silk (MaSp1) cDNA driven by the sericin 1 promoter. Silkworm eggs were injected with the vector, producing transgenic silkworms displaying DsRed fluorescence in their eyes. Genotyping analysis confirmed the integration of the MaSp1 gene into the genome of the transgenic silkworms, and silk protein analysis revealed its expression and secretion in the cocoon. Compared with wild-type silk, the recombinant silk displayed a higher tensile strength and elasticity. The results indicate the potential for producing recombinant spider silk in transgenic B. mori.

Egg sac silk of Theridiosoma gemmosum (Araneae: Theridiosomatidae)

Cocoons of Theridiosoma gemmosum consist of two main parts, the egg sac case and the stalk. The inner space of the egg sac case is filled with nonsticky flocculent silk. Measuring 600-800 nm in diameter, the flocculent threads are never made up of bundles of longitudinally oriented nanofibrils. The egg case wall consists of a lower layer of highly ordered threads and an upper layer of cover silk. The lower, permanently white layer consists of threads in a mesh-like arrangement, the thicker threads being 4-6 microm and the thinner threads being 2-3 microm in diameter. Each thread is a bundle of parallel nanofibrils, with a diameter between 150 and 300 nm. The silk secretions of these fibers, emitted from spigots, are processed by legs. The upper layer of the egg case is applied to the threads of the lower layer by direct rubbing against its surface, i.e. without the use of legs. In the lower and middle part of the egg case, the accumulated secretion forms a virtually compact encrustation, whereas in the upper, conically shaped, part of the egg case where it becomes the stalk, this secretion becomes substantially scarcer. The stalk is a continuation of the egg case, its proximal part made of fibers similar to those forming the inner layer of the egg case wall. The distal part of the stalk continues towards the suspension area either as a compact bundle of parallel fibers, or the stalk forks into two bundles of roughly the same thickness, which continue towards the suspension area separately. On the surface of objects onto which cocoons are attached, the secretion of the piriform glands acts as an adhesive sheet.

Mimicking biopolymers on a molecular scale: nano(bio)technology based on engineered proteins

Proteins are ubiquitous biopolymers that adopt distinct three-dimensional structures and fulfil a multitude of elementary functions in organisms. Recent systematic studies in molecular biology and biotechnology have improved the understanding of basic functional and architectural principles of proteins, making them attractive candidates as concept generators for technological development in material science, particularly in biomedicine and nano(bio)technology. This paper highlights the potential of molecular biomimetics in mimicking high-performance proteins and provides concepts for applications in four case studies, i.e. spider silk, antifreeze proteins, blue mussel adhesive proteins and viral ion channels.

Similarities in the structural organization of major and minor ampullate spider silk

Minor and major ampullate spider silks are studied under varying mechanical stress by static and time-resolved FT-IR spectroscopy. This enables one to trace the external mechanical excitation on a microscopic level and to determine for the different moieties the time dependence of the molecular order parameters and corresponding band shifts. It is concluded that the hierarchical nanostructure of both types of silk is similar, being composed of highly oriented nanocrystals, which are interconnected by amorphous chains that obey the worm-like chain model and have a Gaussian distribution of pre-strain. By that it is possible to describe the mechanical properties of both silks by two adjustable parameters only, the center and width of the distribution. For major ampullate silk, the observed variability is small in pronounced contrast to the findings for minor ampullate.

The elaborate structure of spider silk: structure and function of a natural high performance fiber

Biomaterials, having evolved over millions of years, often exceed man-made materials in their properties. Spider silk is one outstanding fibrous biomaterial which consists almost entirely of large proteins. Silk fibers have tensile strengths comparable to steel and some silks are nearly as elastic as rubber on a weight to weight basis. In combining these two properties, silks reveal a toughness that is two to three times that of synthetic fibers like Nylon or Kevlar. Spider silk is also antimicrobial, hypoallergenic and completely biodegradable. This article focuses on the structure-function relationship of the characterized highly repetitive spider silk spidroins and their conformational conversion from solution into fibers. Such knowedge is of crucial importance to understanding the intrinsic properties of spider silk and to get insight into the sophisticated assembly processes of silk proteins. This review further outlines recent progress in recombinant production of spider silk proteins and their assembly into distinct polymer materials as a basis for novel products.

Skeletal tissue engineering using silk biomaterials

Silks have been proposed as potential scaffold materials for tissue engineering, mainly because of their physical properties. They are stable at physiological temperatures, flexible and resist tensile and compressive forces. Bombyx mori (silkworm) cocoon silk has been used as a suture material for over a century, and has proved to be biocompatible once the immunogenic sericin coating is removed. Spider silks have a similar structure to silkworm silk but do not have a sericin coating. This paper provides a general overview on the use of silk protein in biomaterials, with a focus on skeletal tissue engineering.

Identification and characterization of multiple Spidroin 1 genes encoding major ampullate silk proteins in Nephila clavipes

Spider dragline silk is primarily composed of proteins called major ampullate spidroins (MaSps) that consist of a large repeat array flanked by nonrepetitive N- and C-terminal domains. Until recently, there has been little evidence for more than one gene encoding each of the two major spidroin silk proteins, MaSp1 and MaSp2. Here, we report the deduced N-terminal domain sequences for two distinct MaSp1 genes from Nephila clavipes (MaSp1A and MaSp1B) and for MaSp2. All three MaSp genes are co-expressed in the major ampullate gland. A search of the GenBank database also revealed two distinct MaSp1 C-terminal domain sequences. Sequencing confirmed that both MaSp1 genes are present in all seven Nephila clavipes spiders examined. The presence of nucleotide polymorphisms in these genes confirmed that MaSp1A and MaSp1B are distinct genetic loci and not merely alleles of the same gene. We experimentally determined the transcription start sites for all three MaSp genes and established preliminary pairing between the two MaSp1 N- and C-terminal domains. Phylogenetic analysis of these new sequences and other published MaSp N- and C-terminal domain sequences illustrated that duplications of MaSp genes may be widespread among spider species.

Environmental conditions impinge on dragline silk protein composition

The silk formed in the major ampullate (MA) gland of the orb weaving spider Nephila clavipes is composed of two silk fibroins, which are called major ampullate spidroins 1 (MaSp1) and 2 (MaSp2). Analysis of proteolytic peptides and reactivity to spidroin type specific antibodies indicated that MaSp2 constituted only a minor part in the spinning dope as well as in the spun filaments. Upon starvation, a change in the silk's characteristic features was observed that was concomitant of a decrease in the contribution of MaSp2. The silk became less elastic and stiffer, which will better tailor its usability for the safety line, albeit at the expense of its employment as the web frame threads. In addition, since MaSp2 production requires greater ATP consumption, such a shift in the protein ratio cuts down on the energy costs to produce the silk. From this change in protein composition the spider might therefore benefit twice, by synthesizing 'cheaper' silk that into the bargain has properties that potentially can better support foraging in times of food shortage.

Distinct contributions of model MaSp1 and MaSp2 like peptides to the mechanical properties of synthetic major ampullate silk fibers as revealed in silico

All characterized major ampullate silks from orb-web weaving spiders are composites of primarily two different proteins: MaSp1 and MaSp2. The conserved association of MaSp1 and MaSp2 in these spider species, the highly conserved amino acid motifs, and variable ratios of MaSp1 to MaSp2 demonstrate the importance of both MaSp1 and MaSp2 to the strength and elasticity of the fiber. Computer simulated mechanical tests predicted differing roles for MaSp1 and MaSp2 in the mechanical properties of the fibers. Recombinant MaSp1 and MaSp2 proteins were blended and spun into fibers mimicking the computer-simulated conditions. Mechanical testing verified the differing roles of MaSp1 and MaSp2.

In vivo degradation of three-dimensional silk fibroin scaffolds

Three-dimensional porous scaffolds prepared from regenerated silk fibroin using either an all-aqueous process or a process involving an organic solvent, hexafluoroisopropanol (HFIP), have shown promise in cell culture and tissue engineering applications. However, their biocompatibility and in vivo degradation have not been fully established. The present study was conducted to systematically investigate how processing method (aqueous vs. organic solvent) and processing variables (silk fibroin concentration and pore size) affect the short-term (up to 2 months) and long-term (up to 1 year) in vivo behavior of the protein scaffolds in both nude and Lewis rats. The samples were analyzed by histology for scaffold morphological changes and tissue ingrowth, and by real-time RT-PCR and immunohistochemistry for immune responses. Throughout the period of implantation, all scaffolds were well tolerated by the host animals and immune responses to the implants were mild. Most scaffolds prepared from the all-aqueous process degraded to completion between 2 and 6 months, while those prepared from organic solvent (hexafluoroisopropanol (HFIP)) process persisted beyond 1 year. Due to widespread cellular invasion throughout the scaffold, the degradation of aqueous-derived scaffolds appears to be more homogeneous than that of HFIP-derived scaffolds. In general and especially for the HFIP-derived scaffolds, a higher original silk fibroin concentration (e.g. 17%) and smaller pore size (e.g. 100-200microm) resulted in lower levels of tissue ingrowth and slower degradation. These results demonstrate that the in vivo behavior of the three-dimensional silk fibroin scaffolds is related to the morphological and structural features that resulted from different scaffold preparation processes. The insights gained in this study can serve as a guide for processing scenarios to match desired morphological and structural features and degradation time with tissue-specific applications.

In vivo study of anterior cruciate ligament regeneration using mesenchymal stem cells and silk scaffold

Although most in vitro studies indicate that silk is a suitable biomaterial for ligament tissue engineering, in vivo studies of implanted silk scaffolds for ligament reconstruction are still lacking. The objective of this study is to investigate anterior cruciate ligament (ACL) regeneration using mesenchymal stem cells (MSCs) and silk scaffold. The scaffold was fabricated by incorporating microporous silk sponges into knitted silk mesh, which mimicked the structures of ligament extracellular matrix (ECM). In vitro culture demonstrated that MSCs on scaffolds proliferated vigorously and produced abundant collagen. The transcription levels of ligament-specific genes also increased with time. Then MSCs/scaffold was implanted to regenerate ACL in vivo. After 24 weeks, histology observation showed that MSCs were distributed throughout the regenerated ligament and exhibited fibroblast morphology. The key ligament ECM components including collagen I, collagen III, and tenascin-C were produced prominently. Furthermore, direct ligament-bone insertion with typical four zones (bone, mineralized fibrocartilage, fibrocartilage, ligament) was reconstructed, which resembled the native structure of ACL-bone insertion. The tensile strength of regenerated ligament also met the mechanical requirements. Moreover, its histological grading score was significantly higher than that of control. In conclusion, the results imply that silk scaffold has great potentials in future clinical applications.