The Adhesive Glycoprotein of the Orb Web of Argiope Aurantia (Araneae, Araneidae)

Regionalization of cell types in silk glands of Larinioides sclopetarius suggest that spider silk fibers are complex layered structures

In order to produce artificial silk fibers with properties that match the native spider silk we likely need to closely mimic the spinning process as well as fiber architecture and composition. To increase our understanding of the structure and function of the different silk glands of the orb weaver Larinioides sclopetarius, we used resin sections for detailed morphology, paraffin embedded sections for a variety of different histological stainings, and a histochemical method for localization of carbonic anhydrase activity. Our results show that all silk glands, except the tubuliform glands, are composed of two or more columnar epithelial cell types, some of which have not been described previously. We observed distinct regionalization of the cell types indicating sequential addition of secretory products during silk formation. This means that the major ampullate, minor ampullate, aciniform type II, and piriform silk fibers most likely are layered and that each layer has a specific composition. Furthermore, a substance that stains positive for polysaccharides may be added to the silk in all glands except in the type I aciniform glands. Active carbonic anhydrase was found in all silk glands and/or ducts except in the type I aciniform and tubuliform glands, with the strongest staining in aggregate glands and their ductal nodules. Carbonic anhydrase plays an important role in the generation of a pH gradient in the major ampullate glands, and our results suggest that some other glands may also harbor pH gradients.

Fine structural aspects on the web glue production in the golden orb-web spider Trichonephila clavata

The water-soluble glue substance of the capture threads in Trichonephila clavata is solely produced from two pairs of aggregate silk glands. During the web glue production, secretory vesicles were synthesized via the extensive rough endoplasmic reticulum of epithelial cells. Unlike the clearly described fibrous web production in spiders, the process of aqueous web glue production appears to involve either a condensing or a packaging step by the Golgi complex. In particular, the fine structure of secretory vesicles varies from cell to cell and may represent the secretory cycle. The electron-dense multivesicular bodies were clearly visible as discrete droplets, and the mature secretory product in the glandular epithelium appeared as a spherical vacuole grown by fusion with surrounding small vesicles. Our fine structural observation reveals that the secretion occurs when the release of secreted material involves the loss of part of the cytoplasm. The bleb along the luminal surface of the secretory cells and membrane-bound extracellular vesicles which pinched off from the cell suggests that the secretory product is released by the mechanism of apocrine secretion.

Recombinant Spider Silk: Promises and Bottlenecks

Spider silk threads have exceptional mechanical properties such as toughness, elasticity and low density, which reach maximum values compared to other fibre materials. They are superior even compared to Kevlar and steel. These extraordinary properties stem from long length and specific protein structures. Spider silk proteins can consist of more than 20,000 amino acids. Polypeptide stretches account for more than 90% of the whole protein, and these domains can be repeated more than a hundred times. Each repeat unit has a specific function resulting in the final properties of the silk. These properties make them attractive for innovative material development for medical or technical products as well as cosmetics. However, with livestock breeding of spiders it is not possible to reach high volumes of silk due to the cannibalistic behaviour of these animals. In order to obtain spider silk proteins (spidroins) on a large scale, recombinant production is attempted in various expression systems such as plants, bacteria, yeasts, insects, silkworms, mammalian cells and animals. For viable large-scale production, cost-effective and efficient production systems are needed. This review describes the different types of spider silk, their proteins and structures and discusses the production of these difficult-to-express proteins in different host organisms with an emphasis on plant systems.

Connecting materials, performance and evolution: a case study of the glue of moth-catching spiders (Cyrtarachninae)

Morphological structures and extended phenotypes are made possible by materials that are encoded by the genome. Nearly all biomaterials are viscoelastic, which means that to understand performance, one must understand the strain rate-dependent properties of these materials in relevant ecological interactions, as the behavior of a material can vary dramatically and rapidly. Spider silks are an example of materials whose properties vary substantially intra- and inter-specifically. Here, we focus on aggregate silk, which functions as a biological adhesive. As a case study to understand how a material manifests from genome through organism to ecology, we highlight moth-specialist spiders, the Cyrtarachninae, and their glues as an ideal experimental system to investigate the relationship between genomics and ecologically variable performance of a biological material. There is a clear eco-evolutionary innovation that Cyrtarachne akirai and related species have evolved, a unique trait not found in other spiders, a glue which overcomes the scales of moths. By examining traditional orb-weavers, C. akirai and other subfamily members using biomechanical testing and genomic analysis, we argue that we can track the evolution of this novel bioadhesive and comment on the selection pressures influencing prey specialization. The importance of the ecological context of materials testing is exemplified by the poor performance of C. akirai glue on glass and the exceptional spreading ability and adhesive strength on moths. The genetic basis for these performance properties is experimentally tractable because spider silk genes are minimally pleiotropic and advances in genomic technologies now make possible the discovery of complete silk gene sequences.

Sum frequency generation spectroscopy of the attachment disc of a spider

The pyriform silk of the attachment disc of a spider was studied using infrared-visible vibrational sum frequency generation (SFG) spectroscopy. The spider can attach dragline and radial lines to many kinds of substrates in nature (concrete, alloy, metal, glass, plant branches, leaves, etc.) with the attachment disc. The adhesion can bear the spider's own weight, and resist the wind on its orb web. From our SFG spectroscopy study, the NH group of arginine side chain and/or NH₂ group of arginine and glutamine side chain in the amino acid sequence of the attachment silk proteins are suggested to be oriented in the disc. It was inferred from the observed doublet SFG peaks at around 3300 cm^-1 that the oriented peptide contains two kinds of structures.

Repeat variation resolves a complete aggregate silk sequence of bolas spider Mastophora phrynosoma

Many species of spider use a modified silk adhesive, called aggregate glue, to aid in prey capture. Aggregate spidroins (spider fibroins) are modified members of the spider silk family, however they are not spun into fibers as are their solid silk relatives. The genes that encode for aggregate spidroins are the largest of the known spidroin genes and are similarly highly repetitive. In this study, we used long read sequencing to discover the aggregate spidroin genes of the toad-like bolas spider, Mastophora phrynosoma, which employs the glue in a unique way, using only a single, large droplet to capture moths. While Aggregate Spidroin 1 (AgSp1) remains incomplete, AgSp2 is more than an extraordinary 62 kilobases of coding sequence, 20 kb longer than the longest spidroin on record. The structure of repeats from both aggregate silk proteins follows a similar pattern seen in other species, with the same strict conservation of amino acid residue number for much of the repeats' lengths. Interestingly, AgSp2 lacks the elevated number and groupings of glutamine residues seen in the other reported AgSp2 of a classic orb weaving species. The role of gene length in glue functionality remains a mystery, and thus discovering length differences across species will allow understanding and harnessing of this attribute for the next generation of bio-inspired adhesives.

Properties of orb weaving spider glycoprotein glue change during Argiope trifasciata web construction

An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them. Each droplet’s viscoelastic glycoprotein adhesive core extends to dissipate the forces of prey struggle as it transfers force to stiffer, support line flagelliform fibers. In large orb webs, switchback capture thread turns are placed at the bottom of the web before a continuous capture spiral progresses from the web’s periphery to its interior. To determine if the properties of capture thread droplets change during web spinning, we characterized droplet and glycoprotein volumes and material properties from the bottom, top, middle, and inner regions of webs. Both droplet and glycoprotein volume decreased during web construction, but there was a progressive increase in the glycoprotein’s Young’s modulus and toughness. Increases in the percentage of droplet aqueous material indicated that these increases in material properties are not due to reduced glycoprotein viscosity resulting from lower droplet hygroscopicity. Instead, they may result from changes in aqueous layer compounds that condition the glycoprotein. A 6-fold difference in glycoprotein toughness and a 70-fold difference in Young’s modulus across a web documents the phenotypic plasticity of this natural adhesive and its potential to inspire new materials.

Fine structure of the aggregate silk nodules in the orb-web spider Nephila clavata

Among the triad spinning units for capture thread producing system in the orb-web spiders, aqueous gluey substances are produced from two pairs of aggregate silk glands (ASG). Although biochemical analysis of glue substance is produced by way of their passage through the ASG, its structural modifications in the nodular area have been nearly neglected till now. This paper focused on identifying the fine structural characteristics of the aggregate nodules in the golden orb-web spider Nephila clavata using both of light and transmission electron microscopes. The ASG in N. clavata is composed of a multi-lobed secretory region and a thick excretory duct surrounded by large irregular nodules. Histological analysis of the nodules demonstrates that the nodule forming cells have extensive membrane-bound tubular system that is continuous with the surface membrane. In particular, the nodule forming cells contain numerous mitochondria and glycogen particles within their cytoplasms, and they are surrounded by the same sheath of thin connectives. As previously described, each gluey droplet is formed of a central glycoprotein mass surrounded by an aqueous covering components, the nodular organization in N. clavata indicates that the extensive membrane system is thought to have a function for gluey silk production in spider. The results of this study also strengthen the premise that spider glues are made of glycoproteins, and the aggregate nodule functions as a key component for the spider web glue production.

Hygroscopic compounds in spider aggregate glue remove interfacial water to maintain adhesion in humid conditions

Adhesion in humid environments is fundamentally challenging because of the presence of interfacial bound water. Spiders often hunt in wet habitats and overcome this challenge using sticky aggregate glue droplets whose adhesion is resistant to interfacial failure under humid conditions. The mechanism by which spider aggregate glue avoids interfacial failure in humid environments is still unknown. Here, we investigate the mechanism of aggregate glue adhesion by using interface-sensitive spectroscopy in conjunction with infrared spectroscopy. We demonstrate that glycoproteins act as primary binding agents at the interface. As humidity increases, we observe reversible changes in the interfacial secondary structure of glycoproteins. Surprisingly, we do not observe liquid-like water at the interface, even though liquid-like water increases inside the bulk with increasing humidity. We hypothesize that the hygroscopic compounds in aggregate glue sequester interfacial water. Using hygroscopic compounds to sequester interfacial water provides a novel design principle for developing water-resistant synthetic adhesives.

Spider aggregate glue avoids failure in humid environments but the fundamental mechanism behind it is still unknown. Here, the authors demonstrate that humidity-dependent structural changes of glycoproteins and sequestering of liquid water by low molecular mass compounds prevents adhesion failure of the glue in humid environments.

Tuning orb spider glycoprotein glue performance to habitat humidity

Orb-weaving spiders use adhesive threads to delay the escape of insects from their webs until the spiders can locate and subdue the insects. These viscous threads are spun as paired flagelliform axial fibers coated by a cylinder of solution derived from the aggregate glands. As low molecular mass compounds (LMMCs) in the aggregate solution attract atmospheric moisture, the enlarging cylinder becomes unstable and divides into droplets. Within each droplet an adhesive glycoprotein core condenses. The plasticity and axial line extensibility of the glycoproteins are maintained by hygroscopic LMMCs. These compounds cause droplet volume to track changes in humidity and glycoprotein viscosity to vary approximately 1000-fold over the course of a day. Natural selection has tuned the performance of glycoprotein cores to the humidity of a species' foraging environment by altering the composition of its LMMCs. Thus, species from low-humidity habits have more hygroscopic threads than those from humid forests. However, at their respective foraging humidities, these species' glycoproteins have remarkably similar viscosities, ensuring optimal droplet adhesion by balancing glycoprotein adhesion and cohesion. Optimal viscosity is also essential for integrating the adhesion force of multiple droplets. As force is transferred to a thread's support line, extending droplets draw it into a parabolic configuration, implementing a suspension bridge mechanism that sums the adhesive force generated over the thread span. Thus, viscous capture threads extend an orb spider's phenotype as a highly integrated complex of large proteins and small molecules that function as a self-assembling, highly tuned, environmentally responsive, adhesive biomaterial. Understanding the synergistic role of chemistry and design in spider adhesives, particularly the ability to stick in wet conditions, provides insight in designing synthetic adhesives for biomedical applications.

Molecular Structural Analysis of Spider's Capture Thread and Viscid Droplets Studied by Microscopic FT-IR Spectroscopy

The molecular structural analysis of capture thread, including its viscid droplets of oriental golden orb-web spider Nephila clavata, has been performed by microscopic FT-IR spectroscopy. The obtained spectra of capture threads with and without viscid droplets indicate that the features in the region of 1400 - 1000 cm^-1 will be useful as marker bands for the degree of the dissolving of viscid droplet; further, the bands at 1395 and 1335 cm^-1 are attributable to the components of anchoring granules located at the inner side of viscid droplets. By recrystallization and its infrared measurements, the main chemical component of viscid droplets is assignable to glycosylated proline. It has also been demonstrated that the components of the anchoring granule of a viscid droplet are decomposed by UV irradiation, and that the molecular conformation of silk fiber protein of a capture thread is denatured at over 60°C, whereas the viscid droplets on a capture thread retain their structure.

Temperature mediates the effect of humidity on the viscoelasticity of glycoprotein glue within the droplets of an orb-weaving spider's prey capture threads

Sticky viscous prey capture threads retain insects that strike araneoid orb-webs. The threads' two axial fibers support a series of glue droplets, each featuring a core of adhesive viscoelastic glycoprotein covered by an aqueous solution. After sticking, the glue extends, summing the adhesion of multiple droplets, and dissipates some of the energy of a struggling prey. As a day progresses, threads experience a drop in humidity and an increase in temperature, environmental variables that have the potential to alter thread and web function. We hypothesize that thread droplets respond to these opposing environmental changes in a manner that stabilizes their performance, and test this by examining threads spun by Argiope aurantia, a species that occupies exposed, weedy habitats. We confirmed that decreased humidity increases glycoprotein viscosity and found that increased temperature had the opposite effect. To evaluate the combined effect of temperature and humidity on a droplet's ability to transfer adhesive force and dissipate energy, we extended a droplet and measured both the deflection of the axial line supporting the droplet and the duration of its tensive load. The cumulative product of these two indices, which reflects the energy required to extend a droplet, was greatest under afternoon (hot and dry) conditions, less under morning (cool and humid) conditions, and least under hot and humid afternoon conditions. Although the opposing effects of temperature and humidity tend to stabilize glycoprotein performance, A. aurantia thread droplets appear to function optimally during the afternoon, equipping this species to capture large orthopterans, which are most active at this time.

Environmental response and adaptation of glycoprotein glue within the droplets of viscous prey capture threads from araneoid spider orb-webs

Viscous threads that form the prey capture spiral of araneoid orb-webs retain insects that strike the web, giving a spider more time to locate and subdue them. The viscoelastic glycoprotein glue responsible for this adhesion forms the core of regularly spaced aqueous droplets, which are supported by protein axial fibers. Glycoprotein extensibility both facilitates the recruitment of adhesion from multiple droplets and dissipates the energy generated by insects struggling to free themselves from the web. Compounds in the aqueous material make the droplets hygroscopic, causing an increase in both droplet volume and extensibility as humidity (RH) rises. We characterized these humidity-mediated responses at 20%, 37%, 55%, 72% and 90% RH in two large orb-weavers, Argiope aurantia, which is found in exposed habitats, and Neoscona crucifera, which occupies forests and forest edges. The volume-specific extension of A. aurantia glycoprotein reached a maximum value at 55% RH and then declined, whereas that of N. crucifera increased exponentially through the RH range. As RH increased, the relative stress on droplet filaments at maximum extension, as gauged by axial line deflection, decreased in a linear fashion in A. aurantia, but in N. crucifer increased logarithmically, indicating that N. crucifera threads are better equipped to dissipate energy through droplet elongation. The greater hygroscopicity of A. aurantia threads equips them to function in lower RH environments and during the afternoon when RH drops, but their performance is diminished during the high RH of the morning hours. In contrast, the lower hygroscopicity of N. crucifera threads optimizes their performance for intermediate and high RH environments and during the night and morning. These interspecific differences support the hypothesis that viscous capture threads are adapted to the humidity regime of an orb-weaver's habitat.

Small organic solutes in sticky droplets from orb webs of the spider Zygiella atrica (Araneae; Araneidae): β-alaninamide is a novel and abundant component

In northeastern North America, Zygiella atrica often build their orb webs near the ocean. We analyzed individual field-built Z. atrica webs to determine if organic low-molecular-mass solutes (LMM) in their sticky droplets showed any unusual features not previously seen in orb webs of other species living in less salty environments. While two of the three most abundant organic LMM (putrescine (butane-1,4-diamine) and GABamide (4-aminobutanamide)) are already well-known from webs of inland spiders, the third major LMM, β-alaninamide (3-aminopropanamide), a homolog of GABamide, has not been detected in sticky droplets from any other araneoid spiders (27 species). It remains to be established, however, whether or not use of β-alaninamide is related to proximity to saltwater. We observed variability in organic LMM composition in Z. atrica webs that appeared to be influenced more by an undetermined factor associated with different collecting locations and/or collection dates than by different genders or instars. Shifts in composition when adult females were transferred from the field to the laboratory were also observed. Structural similarities and inverse correlations among β-alaninamide, GABamide, and N-acetylputrescine suggest that they may form a series of LMM fulfilling essentially the same, as yet unknown, role in the webs of those species in which they occur.

The role of capture spiral silk properties in the diversification of orb webs

Among a myriad of spider web geometries, the orb web presents a fascinating, exquisite example in architecture and evolution. Orb webs can be divided into two categories according to the capture silk used in construction: cribellate orb webs (composed of pseudoflagelliform silk) coated with dry cribellate threads and ecribellate orb webs (composed of flagelliform silk fibres) coated by adhesive glue droplets. Cribellate capture silk is generally stronger but less-extensible than viscid capture silk, and a body of phylogenic evidence suggests that cribellate capture silk is more closely related to the ancestral form of capture spiral silk. Here, we use a coarse-grained web model to investigate how the mechanical properties of spiral capture silk affect the behaviour of the whole web, illustrating that more elastic capture spiral silk yields a decrease in web system energy absorption, suggesting that the function of the capture spiral shifted from prey capture to other structural roles. Additionally, we observe that in webs with more extensible capture silk, the effect of thread strength on web performance is reduced, indicating that thread elasticity is a dominant driving factor in web diversification.

Humidity affects the extensibility of an orb-weaving spider's viscous thread droplets

The prey-capture threads found in most spider orb webs rely on viscous droplets for their stickiness. Each droplet is formed of a central mass of viscoelastic glycoprotein glue surrounded by an aqueous covering, both of which incorporate hydrophilic components. We found that the extensibility of droplets on Larinioides cornutus threads increased as humidity increased. However, the deflection of the droplets' supporting axial lines did not change, indicating that atmospheric water uptake increases glycoprotein plasticity, but not glycoprotein adhesion. The extensibility of droplets, along with that of the thread's supporting axial fibers, is responsible for summing the adhesion of multiple thread droplets. Therefore, daily changes in humidity have the potential to significantly alter the performance of viscous threads and orb webs.

Constraints on the adhesion of viscous threads spun by orb-weaving spiders: the tensile strength of glycoprotein glue exceeds its adhesion

In this study we tested the hypothesis that a viscous thread releases its hold on a surface because its glycoprotein glue pulls from the surface and not because its elongating droplets break near their attachment to the surface. We compared the values obtained when three species' viscous threads adhered to four smooth surfaces, which differed in their total surface energy and in the proportions of their dispersion and polar energy components. Although water comprised 43–70% of the volume of these viscous droplets, only the dispersion surface energies of test materials and not their polar surface energies impacted thread adhesion. These results support the droplet pull-off hypothesis and are consistent with a previous finding that capillary force contributes little to thread adhesion. Just as a viscous thread's stickiness is constrained by the tensile strength of its supporting axial fibers, our findings suggest that glycoprotein adhesion is constrained by glycoprotein tensile strength.

Adhesive compatibility of cribellar and viscous prey capture threads and its implication for the evolution of orb-weaving spiders

Evolution of orb-weaving spiders that comprise the Orbiculariae clade involved a transition in the composition of prey capture thread that has been challenging to explain. The primitive cribellar threads spun by members of the Deinopoidea subclade resemble the capture threads of their non-orb-web-weaving ancestors and are formed of thousands of fine, dry, protein cribellar fibrils. In contrast, the derived viscous capture threads spun by members of the Araneoidea subclade have regularly spaced, aqueous adhesive droplets. When second instar deinopoid spiderlings emerge from egg sacs they are unable to spin cribellar threads, and, therefore, do not construct orb-webs; whereas second instar araneoids spin capture threads and construct orb-webs. If, as we hypothesize, viscous material evolved to enable second instar spiderlings to construct orb-webs, early araneoids may have spun composite cribellar-viscous capture threads. To examine the functional feasibility of such intermediate capture threads, we compared the adhesion of cribellar threads, viscous threads, and combined cribellar-viscous threads. The stickiness of these combined threads was greater than that of native cribellar or viscous threads alone. The viscous material of Araneus marmoreus threads exhibited a substantial increase in stickiness when combined with cribellar fibrils and that of Argiope aurantia threads a small increase in stickiness when combined with cribellar fibrils. Thus, if early araneoids retained their ability to spin cribellar threads after having evolved glands that produced viscous material, their composite threads could have formed a functional adhesive system that achieved its stickiness at no loss of material economy.

The role of granules within viscous capture threads of orb-weaving spiders

Sticky viscous prey capture threads form the spiral elements of spider orb-webs and are responsible for retaining insects that strike a web. These threads are formed of regularly spaced aqueous droplets that surround a pair of supporting axial fibers. When a thread is flattened on a microscope slide a small, opaque granule can usually be seen within each droplet. These granules have been thought to be the glycoprotein glue that imparts thread adhesion. Both independent contrast and standard regressions showed that granule size is directly related to droplet volume and indicated that granule volume is about 15% of droplet volume. We attempted to find support for the hypothesized adhesive role of granules by establishing an association between the contact surface area and volume of these granules and the stickiness of the viscous threads of 16 species in the context of a six-variable model that describes thread stickiness. However, we found that granule size made either an insignificant or a small negative contribution to thread stickiness. Consequently, we hypothesize that granules serve to anchor larger, surrounding layers of transparent glycoprotein glue to the axial fibers of the thread, thereby equipping droplets to resist slippage on the axial fibers as these droplets generate adhesion, elongate under a load, and transfer force to the axial fibers.

Adhesive efficiency of spider prey capture threads

Cribellar capture threads are comprised of thousands of fine silk fibrils that are produced by the spigots of a spider's cribellum spinning plate and are supported by larger interior axial fibers. This study examined factors that constrain the stickiness of cribellar threads spun by members of the orb-weaving family Uloboridae in the Deinopoidea clade and compared the material efficiency of these threads with that of viscous capture threads produced by members of their sister clade, the Araneoidea. An independent contrast analysis confirmed the direct relationship between cribellar spigot number and cribellar thread stickiness. A model based on this relationship showed that cribellar thread stickiness is achieved at a rapidly decreasing material efficiency, as measured in terms of stickiness per spigot. Another limitation of cribellar thread was documented when the threads of two uloborid species were measured with contact plates of four widths. Unlike that of viscous threads, the stickiness of cribellar threads did not increase as plate width increased, indicating that only narrow bands along the edges of thread contact contributed to their stickiness. As thread volume increased, the gross material efficiency of cribellar threads decreased much more rapidly than that of viscous threads. However, cribellar threads achieved their stickiness at a much greater gross material efficiency than did viscous threads, making it more challenging to explain the transition from deinopoid to araneoid orb-webs.

The effects of capture spiral composition and orb-web orientation on prey interception

Cribellar prey capture threads found in primitive, horizontal orb-webs reflect more light, including ultraviolet wavelengths, than viscous threads found in more derived, vertical orb-webs. Low web visibility and vertical orientation are each thought to increase prey interception and may represent key innovations that contributed to the greater diversity of modern, araneoid orb-weaving spiders. This study compares prey interception rates of cribellate orb-webs constructed by Uloborus glomosus (Uloboridae) with viscous orb-webs constructed by Leucauge venusta (Tetragnathidae) and Micrathena gracilis (Araneidae). We placed sectors of cribellar and viscous threads side by side in frames that were oriented either horizontally or vertically. The webs of both U. glomosus and L. venusta intercepted more prey when vertically oriented. In each orientation L. venusta webs intercepted more insects than did U. glomosus. Although this is consistent with the greater visibility of cribellar threads, the more closely spaced capture spirals of L. venusta may have contributed to this difference. Micrathena gracilis webs intercepted more prey than did U. glomosus webs, although web orientation did not affect the performance of this araneoid species. The stickier and more closely spaced capture spirals of M. gracilis may have enhanced the interception rates of this species and accounted for the greater number of smaller dipterans retained in its webs. The tendency for these slow, weak flight insects to be blown into both horizontal and vertical webs may account for similar interception rates of horizontal and vertical M. gracilis webs. These observations support the enhanced prey interception of vertically oriented orb-webs, but offer only qualified support for the contributions of lower visibility viscous capture threads.