The secretion process of liquid silk with nanopillar structures from Stenopsyche marmorata (Trichoptera: Stenopsychidae)

Comprehensive analysis of silk proteins and gland compartments in Limnephilus lunatus, a case-making trichopteran

Caddisfly larvae produce silk containing heavy and light fibroins, similar to the silk of Lepidoptera, for the construction of underwater structures. We analyzed the silk of Limnephilus lunatus belonging to the case-forming suborder Integripalpia. We analyzed the transcriptome, mapped the transcripts to a reference genome and identified over 80 proteins using proteomic methods, and checked the specificity of their expression. For comparison, we also analyzed the transcriptome and silk proteome of Limnephilus flavicornis. Our results show that fibroins and adhesives are produced together in the middle and posterior parts of the silk glands, while the anterior part produces enzymes and an unknown protein AT24. The number of silk proteins of L. lunatus far exceeds that of the web-spinning Plectrocnemia conspersa, a previously described species from the suborder Annulipalpia. Our results support the idea of increasing the structural complexity of silk in rigid case builders compared to trap web builders.

The silk gland proteome of Stenopsyche angustata provides insights into the underwater silk secretion

Caddisworms (Trichoptera) spin adhesive silks to construct a variety of underwater composite structures. Many studies have focused on the fibroin heavy chain of caddisworm silk and found that it contains heavy phosphorylation to maintain a stable secondary structure. Besides fibroins, recent studies have also identified some new silk proteins within caddisworm silk. To better understand the silk composition and its secretion process, this study reports the silk gland proteome of a retreat-building caddisworm, Stenopsyche angustata Martynov (Trichoptera, Stenopsychidae). Using liquid chromatography tandem mass spectrometry (LC-MS/MS), 2389 proteins were identified in the silk gland of S. angustata, among which 192 were predicted as secreted silk proteins. Twenty-nine proteins were found to be enriched in the front silk gland, whereas 109 proteins were enriched in the caudal silk gland. The fibroin heavy chain and nine uncharacterized silk proteins were identified as phosphorylated proteins. By analysing the sequence of the fibroin heavy chain, we found that it contains 13 Gly/Thr/Pro-rich regions, 12 Val/Ser/Arg-rich regions and a Gly/Arg/Thr-rich region. Three uncharacterized proteins were identified as sericin-like proteins due to their larger molecular weights, signal peptides and repetitive motifs rich in serine. This study provides valuable information for further clarifying the secretion and adhesion of underwater caddisworm silk.

The structural analysis of secretion in the freshwater mite Limnesia maculata (Acariformes, Limnesiidae) supports the idea of a new form of arthropod silk

The structural characteristics of silk secretion of the freshwater mite Limnesia maculata (O.F. Müller) (Acariformes, Limnesiidae) are described and analyzed for the first time based on light, atomic force and electron-microscopical approaches. The common dermal glands (14 pairs scattered over the body) produce silk mostly during the warm summer season. The process of silk secretion lasts from several hours to several days. The silk may appear like barely recognized clouds of a fine whitish substance. An individual silk thread is an indefinitely long uniform unbranched and non-stretchable tube, hollow or with a vesicular electron-dense residual content. In the silk bundle, threads may be freely interlaced, bent, curved or occasionally broken. The diameter of the tubes is in the range of 0.9-1.5 µm. The width of the tube walls varies greatly from 60 to 300 nm. Chaotically interlaced fine fibrils build the tube walls. On the external surface of the tube wall, these fibrils are loosely organized and frequently rising vertically, whereas on the internal side they are packed more tightly sometimes showing a mesh. The walls may reveal a layered structure or, contrary, are quite thin with through foramens. The revealed organization of silk in the freshwater mites is found to be the simplest among that of other arthropods. We propose a role of the silk in the capture of potential prey in the summer season. Silk in water mites significantly widens the wholesome area for the mites' life and gives them better chances in competition for potential resources.

Conservation of Three-Dimensional Structure of Lepidoptera and Trichoptera L-Fibroins for 290 Million Years

The divergence of sister orders Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) from a silk-spinning ancestor occurred around 290 million years ago. Trichoptera larvae are mainly aquatic, and Lepidoptera larvae are almost entirely terrestrial—distinct habitats that required molecular adaptation of their silk for deployment in water and air, respectively. The major protein components of their silks are heavy chain and light chain fibroins. In an effort to identify molecular changes in L-fibroins that may have contributed to the divergent use of silk in water and air, we used the ColabFold implementation of AlphaFold2 to predict three-dimensional structures of L-fibroins from both orders. A comparison of the structures revealed that despite the ancient divergence, profoundly different habitats, and low sequence conservation, a novel 10-helix core structure was strongly conserved in L-fibroins from both orders. Previously known intra- and intermolecular disulfide linkages were accurately predicted. Structural variations outside of the core may represent molecular changes that contributed to the evolution of insect silks adapted to water or air. The distributions of electrostatic potential, for example, were not conserved and present distinct order-specific surfaces for potential interactions with or modulation by external factors. Additionally, the interactions of L-fibroins with the H-fibroin C-termini are different for these orders; lepidopteran L-fibroins have N-terminal insertions that are not present in trichopteran L-fibroins, which form an unstructured ribbon in isolation but become part of an intermolecular β-sheet when folded with their corresponding H-fibroin C-termini. The results are an example of protein structure prediction from deep sequence data of understudied proteins made possible by AlphaFold2.

Organization of dermal glands and characteristic of secretion in the freshwater mite, Limnesia maculata (O.F. Müller, 1776) (Acariformes, Limnesiidae)

The presence of dermal glands is a synapomorphy in the freshwater mites - the large branch of the hyporder Parasitengonina. Dermal glands of the mite Limnesia maculata (O.F. Müller, 1776) (Acariformes, Limnesiidae) were studied using light and electron microscopy (TEM and SEM) for the first time. Two types of dermal glands were recognized in adult mites - 14 pairs of the uniformly organized common dermal glands scattered throughout the entire body volume, and one pair of the characteristic so-called idiosomal dermal glands centrally located and stretched along the ventral body wall, supposedly corresponding to the epimeroglandularia 4. The common dermal glands are composed of a single large alveolus with the basally located columnar epithelium. An intra-alveolar lumen is positioned above the epithelium and packed with a secretion in the form of long curved electron-dense bands. These bands are directed to the excretory opening and released to the outside in form of strongly coiled bands. The cytoplasm of the common dermal glands is filled with short RER cisterns and shows few characteristic Golgi bodies with round dense secretory granules at the distal pole. In contrast, a large elongated sac represents the idiosomal dermal glands having a relatively thin secretory epithelium on the periphery. The cuboidal epithelial cells contain areas with tightly packed RER cisterns and weakly recognized Golgi bodies. The apical cell surface bears irregular short microvilli. The large intra-alveolar lumen always contains an electron-dense secretion with myriads of small lighter particles. This kind of secretion was never seen released through the opening. The excretory opening of the common and the idiosomal dermal glands shows differences in its organization but basically is represented by a cuticular 'bell' narrowing to the opening and showing a particular kind of valves at the inner side of the flaps of the orifice. This article is protected by copyright. All rights reserved.

Bio-based and bio-inspired adhesives from animals and plants for biomedical applications

With the "many-headed" slime mold Physarum polycelphalum having been voted the unicellular organism of the year 2021 by the German Society of Protozoology, we are reminded that a large part of nature's huge variety of life forms is easily overlooked - both by the general public and researchers alike. Indeed, whereas several animals such as mussels or spiders have already inspired many scientists to create novel materials with glue-like properties, there is much more to discover in the flora and fauna. Here, we provide an overview of naturally occurring slimy substances with adhesive properties and categorize them in terms of the main chemical motifs that convey their stickiness, i.e., carbohydrate-, protein-, and glycoprotein-based biological glues. Furthermore, we highlight selected recent developments in the area of material design and functionalization that aim at making use of such biological compounds for novel applications in medicine - either by conjugating adhesive motifs found in nature to biological or synthetic macromolecules or by synthetically creating (multi-)functional materials, which combine adhesive properties with additional, problem-specific (and sometimes tunable) features.

Exploring the underwater silken architectures of caddisworms: comparative silkomics across two caddisfly suborders

Caddisfly (Trichoptera) larvae assemble a variety of underwater structures using bioadhesive silk. The order is divided into two primary sub-orders distinguished by how the larvae deploy their silk. Foraging Integripalpia larvae construct portable tube cases. Annulipalpia larvae construct stationary retreats, some with suspended nets to capture food. To identify silk molecular adaptations that may have contributed to caddisfly diversification, we report initial characterization of silk from a net-spinner genus, Parapsyche, for comparison with the silk of a tube case-maker genus, Hesperophylax. Overall, general features of silk structure and processing are conserved across the sub-orders despite approximately 200 Ma of divergence: the H-fibroin proteins comprise repeating phosphoserine-rich motifs, naturally spun silk fibres contain approximately 1 : 1 molar ratios of divalent metal ions to phosphate, silk fibre precursors are stored as complex fluids of at least two types of complexes, and silk gland proteins contain only traces of divalent metal ions, suggesting metal ions that solidify the fibres are absorbed from the aqueous environment after silk extrusion. However, the number and arrangement of the repeating phosphoserine blocks differ between genera, suggesting molecular adaptation of H-fibroin through duplication and shuffling of conserved structural modules may correspond with the radiation of caddisflies into diverse environments. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Dual-Mode Cross-Linking Enhances Adhesion of Silk Fibroin Hydrogels to Intestinal Tissue

Compared to conventional wound closure methods like sutures and staples, polymer-based tissue adhesives afford some distinct advantages, such as greater ease of deployment in spatially constrained surgical sites. One way to achieve aqueous adhesion is by introducing catechol functional groups that form coordinate and covalent bonds with a variety of substrates. This approach, inspired by marine organisms, has been applied to biopolymers and synthetic polymers, but one key challenge is that compositions that are soluble in water are often susceptible to high swelling ratios that can result in undesired compression of neighboring tissues. This work sought to synthesize aqueous adhesive gels that are capable of two modes of association: (1) adhesion and covalent cross-linking reactions arising from catechol oxidation and (2) noncovalent cross-linking arising from self-assembly of polymer backbones within the gelled adhesive. The network's self-assembly after gelation was envisioned to afford control over swelling and reinforce its strength. Bombyx mori silk fibroin was selected as the backbone of the adhesive network because it can be processed into an aqueous solution yet later be rendered insoluble in water through the assembly of its hydrophobic protein core. Distinct from a previous approach to functionalize silk directly with catechol groups, this work investigated in situ generation of catechol on silk fibroin by enzymatically modifying phenolic side chains, where it was found that this enzymatic approach led to conjugates with higher degrees of catechol functionalization and aqueous solubility. Silk fibroin was functionalized with tyramine to enrich the protein's phenolic side chains, which were subsequently oxidized into catechol groups using mushroom tyrosinase (MT). The gelation of the silk conjugates with MT was monitored by rheology, and the gels exhibited low water uptake. Phenolic enrichment increased the rate of chemical cross-linking leading to gelation but did not interrupt assembly of silk's secondary structures. Adhesion of the tyramine-silk conjugates to porcine intestine was found to be superior to fibrin sealant, and induction of β sheet secondary structures was found to further enhance adhesive strength through a second mode of cross-linking. Neither the chemical functionalization nor phenol oxidation affected the ability of intestinal epithelial cells (Caco-2) to attach and proliferate. Phenolic functionalization and oxidative cross-linking of silk fibroin was found to afford a new route to water-soluble, catechol-functionalized polymers, which were found to display excellent adhesion to mucosal tissue and whose secondary structure provides an additional mode to control strength and swelling of adhesive gels.

When phylogeny and ecology meet: Modeling the occurrence of Trichoptera with environmental and phylogenetic data

Ecological studies are increasingly considering phylogenetic relationships among species. The phylogeny is used as a proxy or filter to improve statistical tests and retain evolutionary elements, such as niche conservation. We used the phylogenetic topology to improve the model for occurrence of Trichoptera genera in Cerrado (Brazilian Savanna) streams. We tested whether parameters generated by logistic models of occurrence, using phylogenetic signals, are better than models generated without phylogenetic information. We used a model with Bayesian updating to examine the influence of stream water pH and phylogenetic relationship among genera on the occurrence of Trichoptera genera. Then, we compared this model with the logistic model for each Trichoptera genus. The probability of occurrence of most genera increased with water pH, and the phylogeny-based explicit logistic model improved the parameters estimated for observed genera. The inferred relationship between genera occurrence and stream pH improved, indicating that phylogeny adds relevant information when estimating ecological responses of organisms. Water with elevated acidity (low pH values) may be restrictive for the occurrence of Trichoptera larvae, especially if the regional streams exhibit neutral to alkaline water, as is observed in the Cerrado region. Using phylogeny-based modeling to predict species occurrence is a prominent opportunity to extend our current statistical framework based on environmental conditions, as it enables a more precise estimation of ecological parameters.

Strength of silk attachment to Ilex chinensis leaves in the tea bagworm Eumeta minuscula (Lepidoptera, Psychidae)

Silks play an important role in the life of various arthropods. A highly neglected prerequisite to make versatile use of silks is sufficient attachment to substrates. Although there have been some studies on the structure and mechanics of silk anchorages of spiders, for insects only anecdotal reports on attachment-associated spinning behaviour exist. Here, we experimentally studied the silk attachment of the pupae and last instar caterpillars of the tea bagworm Eumeta minuscula (Butler 1881) (Lepidoptera, Psychidae) to the leaves of its host plant Ilex chinensis We found that the bagworms spin attachment discs, which share some structural features with those of spiders, like a plaque consisting of numerous overlaid, looped glue-coated silk fibres and the medially attaching suspension thread. Although the glue, which coats the fibres, cannot spread and adhere very well to the leaf surface, high pull-off forces were measured, yielding a mean safety factor (force divided by the animal weight) of 385.6. Presumably, the bagworms achieve this by removal of the leaf epidermis prior to silk attachment, which exposes the underlying tissue that represents a much better bonding site. This ensures a reliable attachment during the immobile, vulnerable pupal stage. This is the first study on the biomechanics and structure of silk attachments to substrates in insects.

Larval cases of caddisfly (Insecta: Trichoptera) affinity in Early Permian marine environments of Gondwana

Caddisflies (Trichoptera) are small, cosmopolitan insects closely related to the Lepidoptera (moths and butterflies). Most caddisflies construct protective cases during their larval development. Although the earliest recognisable caddisflies date back to the early Mesozoic (Early and Middle Triassic), being particularly numerous and diverse during the Late Jurassic and Early Cretaceous, the first records of their larval case constructions are known exclusively from much younger, Early to Middle Jurassic non-marine deposits in the northern hemisphere. Here we present fossils from the Early Permian (Asselian–Sakmarian) marine deposits of Brazil which have strong morphological and compositional similarity to larval cases of caddisflies. If they are, which is very probable, these finds not only push back the fossil record of true caddisflies, but also indicate that their larvae constructed cases at the very beginning of their evolution in marine environments. Since modern caddisflies that construct larval cases in marine environments are only known from eastern Australia and New Zealand, we suggest that this marine ecology may have first evolved in western Gondwana during the Early Permian and later spread across southern Pangea.