From fibres to adhesives: evolution of spider capture threads from web anchors by radical changes in silk gland function.

Spider webs that serve as snares are one of the most fascinating and abundant type of animal architectures. In many cases they include an adhesive coating of silk lines-so-called viscid silk-for prey capture. The evolutionary switch from silk secretions forming solid fibres to soft aqueous adhesives remains an open question in the understanding of spider silk evolution. Here we functionally and chemically characterized the secretions of two types of silk glands and their behavioural use in the cellar spider, Pholcus phalangioides. Both being derived from the same ancestral gland type that produces fibres with a solidifying glue coat, the two types produce respectively a quickly solidifying glue applied in thread anchorages and prey wraps, or a permanently tacky glue deployed in snares. We found that the latter is characterized by a high concentration of organic salts and reduced spidroin content, showing up a possible pathway for the evolution of viscid properties by hygroscopic-salt-mediated hydration of solidifying adhesives. Understanding the underlying molecular basis for such radical switches in material properties not only helps to better understand the evolutionary origins and versatility of ecologically impactful spider web architectures, but also informs the bioengineering of spider silk-based products with tailored properties.

Changes in the material properties of spider glue droplet proteins accompanied shifts in prey capture biomechanics as cobweb spiders diverged from their orb weaving ancestors.

Although descended from orb weavers, spiders in the family Theridiidae spin cobwebs whose sticky prey capture gumfoot lines extend from a silk tangle to a surface below. When a crawling insect contacts glue droplets at the bottom of a gumfoot line, the line's weak pyriform anchor releases, causing the taut line to contract, pulling the insect from the surface and making its struggles to escape ineffective. To determine if this change in prey capture biomechanics was accompanied by a change in the material properties of theridiid glue, we characterized the elastic modulus and toughness of the glue droplet proteins of four theridiid species at 20-90 % relative humidity and compared their properties with those of 13 orb weaving species in the families Tetragnathidae and Araneidae. Compared to orb weavers, theridiid glue proteins had low extensions per protein volume and low elastic modulus and toughness values. These differences are likely explained by the loss of tension on a gumfoot line when its anchor fails, which may prioritize glue droplet adhesion rather than extension. Similarities in theridiid glue droplet properties did not reflect these species' evolutionary relationships. Instead, they appear associated with differences in web architecture. Two species that had stiffer gumfoot support lines and longer and more closely spaced gumfoot lines also had stiffer glue proteins. These lines may store more energy, and, when their anchors release, require stiffer glue to resist the more forceful upward thrust of a prey. STATEMENT OF SIGNIFICANCE: When a crawling insect contacts glue droplets on a theridiid cobweb's gumfoot line, this taut line's anchor fails and the insect is hoisted upward, rendering its struggles to escape ineffective. This strategy contrasts with that of orb weaving ancestors, which rely on more closely spaced prey capture threads to intercept and retain flying insects. A comparison of the elastic modulus and toughness of gumfoot and orb web glue proteins shows that this change in prey capture biomechanics is associated with reductions in the stiffness and toughness of cobweb glue. Unlike orb web capture threads, whose droplets extend in a coordinated fashion to sum adhesive forces, gumfoot lines become untethered, which prioritizes glue droplet adhesive contact over glue droplet extension.

Effect of Phosphate on the Molecular Properties, Interactions, and Assembly of Engineered Spider Silk Proteins.

Phosphate plays a vital role in spider silk spinning and has been utilized in numerous artificial silk spinning attempts to replicate the remarkable mechanical properties of natural silk fiber. Its application in artificial processes has, however, yielded varying outcomes. It is thus necessary to investigate the origins and mechanisms behind these differences. By using recombinant silk protein SC-ADF3 derived from the garden spider Araneus diadematus, here, we describe its conformational changes under various conditions, elucidating the effect of phosphate on SC-ADF3 silk protein properties and interactions. Our results demonstrate that elevated phosphate levels induce the irreversible conformational conversion of SC-ADF3 from random coils to ß-sheet structures, leading to decreased protein solubility over time. Furthermore, exposure of SC-ADF3 to phosphate stiffens already formed structures and reduces the ability to form new interactions. Our findings offer insights into the underlying mechanism through which phosphate-induced ß-sheet structures in ADF3-related silk proteins impede fiber formation in the subsequent phases. From a broader perspective, our studies emphasize the significance of silk protein conformation for functional material formation, highlighting that the formation of ß-sheet structures at the initial stages of protein assembly will affect the outcome of material forming processes.

Interfacial interactions between spider silk protein and cellulose studied by molecular dynamics simulation.

CONTEXT: Due to their excellent biocompatibility and degradability, cellulose/spider silk protein composites hold a significant value in biomedical applications such as tissue engineering, drug delivery, and medical dressings. The interfacial interactions between cellulose and spider silk protein affect the properties of the composite. Therefore, it is important to understand the interfacial interactions between spider silk protein and cellulose to guide the design and optimization of composites. The study of the adsorption of protein on specific surfaces of cellulose crystal can be very complex using experimental methods. Molecular dynamics simulations allow the exploration of various physical and chemical changes at the atomic level of the material and enable an atomic description of the interactions between cellulose crystal planes and spider silk protein. In this study, molecular dynamics simulations were employed to investigate the interfacial interactions between spider silk protein (NTD) and cellulose surfaces. Findings of RMSD, RMSF, and secondary structure showed that the structure of NTD proteins remained unchanged during the adsorption process. Cellulose contact numbers and hydrogen bonding trends on different crystalline surfaces suggest that van der Waals forces and hydrogen bonding interactions drive the binding of proteins to cellulose. These findings reveal the interaction between cellulose and protein at the molecular level and provide theoretical guidance for the design and synthesis of cellulose/spider silk protein composites. METHODS: MD simulations were all performed using the GROMACS-5.1 software package and run with CHARMM36 carbohydrate force field. Molecular dynamics simulations were performed for 500 ns for the simulated system.

Nanoassembly of spider silk protein mediated by intrinsically disordered regions.

Spider silk is the self-assembling product of silk proteins each containing multiple repeating units. Each repeating unit is entirely intrinsically disordered or contains a small disordered domain. The role of the disordered domain/unit in conferring silk protein storage and self-assembly is not fully understood yet. Here, we used biophysical and biochemical techniques to investigate the self-assembly of a miniature version of a minor ampullate spidroin (denoted as miniMiSp). miniMiSp consists of two identical intrinsically disordered domains, one folded repetitive domain, and two folded terminal domains. Our data indicated that miniMiSp self-assembles into oligomers and further into liquid droplets. The oligomerization is attributed to the aggregation-prone property of both the disordered domains and the folded repetitive domain. Our results support the model of micellar structure for silk proteins at high protein concentrations. The disordered domain is indispensable for liquid droplet formation via liquid-liquid phase separation, and tyrosine residues located in the disordered domain make dominant contributions to stability of the liquid droplets. As the same tyrosine residues are also critical to fibrillation, the liquid droplets are likely an intermediate state between the solution state and the fiber state. Additionally, the terminal domains contribute to the pH- and salt-dependent self-assembly properties.

Establishing superfine nanofibrils for robust polyelectrolyte artificial spider silk and powerful artificial muscles.

Spider silk exhibits an excellent combination of high strength and toughness, which originates from the hierarchical self-assembled structure of spidroin during fiber spinning. In this work, superfine nanofibrils are established in polyelectrolyte artificial spider silk by optimizing the flexibility of polymer chains, which exhibits combination of breaking strength and toughness ranging from 1.83 GPa and 238 MJ m-3 to 0.53 GPa and 700 MJ m-3, respectively. This is achieved by introducing ions to control the dissociation of polymer chains and evaporation-induced self-assembly under external stress. In addition, the artificial spider silk possesses thermally-driven supercontraction ability. This work provides inspiration for the design of high-performance fiber materials.

Modulating Polyalanine Motifs of Synthetic Spidroin for Controllable Preassembly and Strong Fiber Formation.

Spider dragline (major ampullate) silk is one of the toughest known fibers in nature and exhibits an excellent combination of high tensile strength and elasticity. Increasing evidence has indicated that preassembly plays a crucial role in facilitating the proper assembly of silk fibers by bridging the mesoscale gap between spidroin molecules and the final strong fibers. However, it remains challenging to control the preassembly of spidroins and investigate its influence on fiber structural and mechanical properties. In this study, we explored to bridge this gap by modulating the polyalanine (polyA) motifs in repetitive region of spidroins to tune their preassemblies in aqueous dope solutions. Three biomimetic silk proteins with varying numbers of alanine residues in polyA motif and comparable molecular weights were designed and biosynthesized, termed as N16C-5A, N15C-8A, and N13C-12A, respectively. It was found that all three proteins could form nanofibril assemblies in the concentrated aqueous dopes, but the size and structural stability of the fibrils were distinct from each other. The silk protein N15C-8A with 8 alanine residues in polyA motif allowed for the formation of stable nanofibril assemblies with a length of approximately 200 nm, which were not prone to disassemble or aggregate as that of N16C-5A and N13C-12A. More interestingly, the stable fibril assembly of N15C-8A enabled spinning of simultaneously strong (623.3 MPa) and tough (107.1 MJ m-3) synthetic fibers with fine molecular orientation and close interface packing of fibril bundles. This work highlights that modulation of polyA motifs is a feasible way to tune the morphology and stability of the spidroin preassemblies in dope solutions, thus controlling the structural and mechanical properties of the resulting fibers.

In vitro and in vivo investigation of Argiope bruennichi spider silk-based novel biomaterial for medical use.

As a natural and biocompatible material with high strength and flexibility, spider silk is frequently used in biomedical studies. In this study, the availability of Argiope bruennichi spider silk as a surgical suture material was investigated. The effects of spider silk-based and commercial sutures, with and without Aloe vera coating, on wound healing were evaluated by a rat dorsal skin flap model, postoperatively (7th and 14th days). Biochemical, hematological, histological, immunohistochemical, small angle x-ray scattering (SAXS) analyses and mechanical tests were performed. A. bruennichi silk did not show any cytotoxic effect on the L929 cell line according to MTT and LDH assays, in vitro. The silk materials did not cause any allergic reaction, infection, or systemic effect in rats according to hematological and biochemical analyses. A. bruennichi spider silk group showed a similar healing response to commercial sutures. SAXS analysis showed that the 14th-day applications of A. bruennichi spider silk and A. vera coated commercial suture groups have comparable structural results with control group. In conclusion, A. bruennichi spider silk is biocompatible in line with the parameters examined and shows a healing response similar to the commercial sutures commonly used in the skin.

A review on complete silk gene sequencing and de novo assembly of artificial silk.

Silk, especially spider and insect silk, is a highly versatile biomaterial with potential applications in biomedicine, materials science, and biomimetic engineering. The primary structure of silk proteins is the basis for the mechanical properties of silk fibers. Biotechnologies such as single-molecule sequencing have facilitated an increasing number of reports on new silk genes and assembled silk proteins. Therefore, this review aims to provide a comprehensive overview of the recent advances in representative spider and insect silk proteins, focusing on identification methods, sequence characteristics, and de novo design and assembly. The review discusses three identification methods for silk genes: polymerase chain reaction (PCR)-based sequencing, PCR-free cloning and sequencing, and whole-genome sequencing. Moreover, it reveals the main spider and insect silk proteins and their sequences. Subsequent de novo assembly of artificial silk is covered and future research directions in the field of silk proteins, including new silk genes, customizable artificial silk, and the expansion of silk production and applications are discussed. This review provides a basis for the genetic aspects of silk production and the potential applications of artificial silk in material science and biomedical engineering.

Role of ErbB and IL-1 signaling pathways in the dermonecrotic lesion induced by Loxosceles sphingomyelinases D.

Sphingomyelinase D (SMase D), the main toxic component of Loxosceles venom, has a well-documented role on dermonecrotic lesion triggered by envenomation with these species; however, the intracellular mechanisms involved in this event are still poorly known. Through differential transcriptomics of human keratinocytes treated with L. laeta or L. intermedia SMases D, we identified 323 DEGs, common to both treatments, as well as upregulation of molecules involved in the IL-1 and ErbB signaling. Since these pathways are related to inflammation and wound healing, respectively, we investigated the relative expression of some molecules related to these pathways by RT-qPCR and observed different expression profiles over time. Although, after 24 h of treatment, both SMases D induced similar modulation of these pathways in keratinocytes, L. intermedia SMase D induced earlier modulation compared to L. laeta SMase D treatment. Positive expression correlations of the molecules involved in the IL-1 signaling were also observed after SMases D treatment, confirming their inflammatory action. In addition, we detected higher relative expression of the inhibitor of the ErbB signaling pathway, ERRFI1, and positive correlations between this molecule and pro-inflammatory mediators after SMases D treatment. Thus, herein, we describe the cell pathways related to the exacerbation of inflammation and to the failure of the wound healing, highlighting the contribution of the IL-1 signaling pathway and the ERRFI1 for the development of cutaneous loxoscelism.

Disabling spidroin N-terminal homologs' reverse reaction unveils why its intermolecular disulfide bonds have not evolved for 380 million years.

Spiders, ubiquitous predators known for their powerful silks, rely on spidroins that self-assemble from high-concentration solutions stored in silk glands, which are mediated by the NT and CT domains. CT homodimers containing intermolecular disulfide bonds enhance silk performance, promoting spider survival and reproduction. However, no NT capable of forming such disulfide bonds has been identified. Our study reveals that NT homodimers with sulfur substitution can form under alkaline conditions, shedding light on why spiders have not evolved intermolecular disulfide bonds in the NT module during their 380 million years of evolution. This discovery significantly advances our comprehension of spider evolution and silk spinning mechanisms, while also providing novel insights into protein storage, assembly, as well as the mechanisms and therapeutic strategies for neurodegenerative diseases associated with protein aggregation.

Identification of a common epitope in knottins and phospholipases D present in Loxosceles sp venom by a monoclonal antibody.

In the Americas and specially in Brazil, the Loxosceles intermedia, Loxosceles gaucho and Loxosceles laeta are the three most medically relevant brown spider species, and whose bites can lead to the condition known as loxoscelism. Here, we report the development of a tool capable of identifying a common epitope amongst Loxosceles sp. venom's toxins. A murine monoclonal antibody (LmAb12) and its recombinant fragments (scFv12P and diabody12P) have been produced and characterized. This antibody and its recombinant constructs were able to recognize proteins of Loxosceles spider venoms with specificity. The scFv12P variant was also able to detect low concentrations of Loxosceles venom in a competitive ELISA assay, displaying potential as a venom identification tool. The primary antigenic target of LmAb12 is a knottin, a venom neurotoxin, that has a shared identity of 100 % between the L. intermedia and L. gaucho species and high similarity to L. laeta. Furthermore, we observed LmAb12 was able to partially inhibit in vitro hemolysis, a cellular event typically induced by the Loxosceles sp. venoms. Such behavior might be due to LmAb12 cross-reactivity between the antigenic target of LmAb12 and the venom's dermonecrotic toxins, the PLDs, or even the existence of synergism between these two toxins.

Recombinant Production, NMR Solution Structure, and Membrane Interaction of the Phα1ß Toxin, a TRPA1 Modulator from the Brazilian Armed Spider Phoneutria nigriventer.

Phα1ß (PnTx3-6) is a neurotoxin from the spider Phoneutria nigriventer venom, originally identified as an antagonist of two ion channels involved in nociception: N-type voltage-gated calcium channel (CaV2.2) and TRPA1. In animal models, Phα1ß administration reduces both acute and chronic pain. Here, we report the efficient bacterial expression system for the recombinant production of Phα1ß and its 15N-labeled analogue. Spatial structure and dynamics of Phα1ß were determined via NMR spectroscopy. The N-terminal domain (Ala1-Ala40) contains the inhibitor cystine knot (ICK or knottin) motif, which is common to spider neurotoxins. The C-terminal α-helix (Asn41-Cys52) stapled to ICK by two disulfides exhibits the µs-ms time-scale fluctuations. The Phα1ß structure with the disulfide bond patterns Cys1-5, Cys2-7, Cys3-12, Cys4-10, Cys6-11, Cys8-9 is the first spider knottin with six disulfide bridges in one ICK domain, and is a good reference to other toxins from the ctenitoxin family. Phα1ß has a large hydrophobic region on its surface and demonstrates a moderate affinity for partially anionic lipid vesicles at low salt conditions. Surprisingly, 10 µM Phα1ß significantly increases the amplitude of diclofenac-evoked currents and does not affect the allyl isothiocyanate (AITC)-evoked currents through the rat TRPA1 channel expressed in Xenopus oocytes. Targeting several unrelated ion channels, membrane binding, and the modulation of TRPA1 channel activity allow for considering Phα1ß as a gating modifier toxin, probably interacting with S1-S4 gating domains from a membrane-bound state.

Biochemical mechanism involved in the enhancement of the Young's modulus of silk by the SpiCE protein.

Silk fibers are known for their superior mechanical properties, with the strongest possessing over seven times the toughness of kevlar. Recently, low molecular weight non-spidroin protein, spider-silk constituting element (SpiCE), has been reported to enhance the mechanical properties of silk; however, its specific action mechanism has not yet been elucidated. Here, we explored the mechanism by which SpiCE strengthened the mechanical properties of major ampullate spidroin 2 (MaSp2) silk through hydrogen bonds and salt bridges of the silk structure via all-atom molecular dynamics simulations. Tensile pulling simulation on silk fiber with SpiCE protein revealed that the SpiCE protein enhanced the Young's modulus by up to 40% more than that of the wild type. Bond characteristic analysis revealed that SpiCE and MaSp2 formed more hydrogen bonds and salt bridges than the MaSp2 wild-type model. Sequence analysis of MaSp2 silk fiber and SpiCE protein revealed that SpiCE protein contained more amino acids that could act as hydrogen bond acceptors/donors and salt bridge partners. Our results provide insights into the mechanism by which non-spidroin proteins strengthen the properties of silk fibers and lay the groundwork for the development of material selection criteria for the design of de novo artificial silk fibers.

Functional Profiling of the A-Family of Venom Peptides from the Wolf Spider Lycosa shansia.

The venoms of spiders from the RTA (retro-lateral tibia apophysis) clade contain diverse short linear peptides (SLPs) that offer a rich source of therapeutic candidates. Many of these peptides have insecticidal, antimicrobial and/or cytolytic activities, but their biological functions are unclear. Here, we explore the bioactivity of all known members of the A-family of SLPs previously identified in the venom of the Chinese wolf spider (Lycosa shansia). Our broad approach included an in silico analysis of physicochemical properties and bioactivity profiling for cytotoxic, antiviral, insecticidal and antibacterial activities. We found that most members of the A-family can form α-helices and resemble the antibacterial peptides found in frog poison. The peptides we tested showed no cytotoxic, antiviral or insecticidal activities but were able to reduce the growth of bacteria, including clinically relevant strains of Staphylococcus epidermidis and Listeria monocytogenes. The absence of insecticidal activity may suggest that these peptides have no role in prey capture, but their antibacterial activity may help to defend the venom gland against infection.

Reference genome of the long-jawed orb-weaver, Tetragnatha versicolor (Araneae: Tetragnathidae).

Climate-driven changes in hydrological regimes are of global importance and are particularly significant in riparian ecosystems. Riparian ecosystems in California provide refuge to many native and vulnerable species within a xeric landscape. California Tetragnatha spiders play a key role in riparian ecosystems, serving as a link between terrestrial and aquatic elements. Their tight reliance on water paired with the widespread distributions of many species make them ideal candidates to better understand the relative role of waterways versus geographic distance in shaping the population structure of riparian species. To assist in better understanding population structure, we constructed a reference genome assembly for Tetragnatha versicolor using long-read sequencing, scaffolded with proximity ligation Omni-C data. The near-chromosome-level assembly is comprised of 174 scaffolds spanning 1.06 Gb pairs, with a scaffold N50 of 64.1 Mb pairs and BUSCO completeness of 97.6%. This reference genome will facilitate future study of T. versicolor population structure associated with the rapidly changing environment of California.

Methods for Silk Property Analyses across Structural Hierarchies and Scales.

Silk from silkworms and spiders is an exceptionally important natural material, inspiring a range of new products and applications due to its high strength, elasticity, and toughness at low density, as well as its unique conductive and optical properties. Transgenic and recombinant technologies offer great promise for the scaled-up production of new silkworm- and spider-silk-inspired fibres. However, despite considerable effort, producing an artificial silk that recaptures the physico-chemical properties of naturally spun silk has thus far proven elusive. The mechanical, biochemical, and other properties of pre-and post-development fibres accordingly should be determined across scales and structural hierarchies whenever feasible. We have herein reviewed and made recommendations on some of those practices for measuring the bulk fibre properties; skin-core structures; and the primary, secondary, and tertiary structures of silk proteins and the properties of dopes and their proteins. We thereupon examine emerging methodologies and make assessments on how they might be utilized to realize the goal of developing high quality bio-inspired fibres.

Subsidy Quality Affects Common Riparian Web-Building Spiders: Consequences of Aquatic Contamination and Food Resource.

Anthropogenic stressors can affect the emergence of aquatic insects. These insects link aquatic and adjacent terrestrial food webs, serving as high-quality subsidy to terrestrial consumers, such as spiders. While previous studies have demonstrated that changes in the emergence biomass and timing may propagate across ecosystem boundaries, the physiological consequences of altered subsidy quality for spiders are largely unknown. We used a model food chain to study the potential effects of subsidy quality: Tetragnatha spp. were exclusively fed with emergent Chironomus riparius cultured in the absence or presence of either copper (Cu), Bacillus thuringiensis var. israelensis (Bti), or a mixture of synthetic pesticides paired with two basal resources (Spirulina vs. TetraMin®) of differing quality in terms of fatty acid (FA) composition. Basal resources shaped the FA profile of chironomids, whereas their effect on the FA profile of spiders decreased, presumably due to the capacity of both chironomids and spiders to modify (dietary) FA. In contrast, aquatic contaminants had negligible effects on prey FA profiles but reduced the content of physiologically important polyunsaturated FAs, such as 20:4n-6 (arachidonic acid) and 20:5n-3 (eicosapentaenoic acid), in spiders by approximately 30% in Cu and Bti treatments. This may have contributed to the statistically significant decline (40%-50%) in spider growth. The observed effects in spiders are likely related to prey nutritional quality because biomass consumption by spiders was, because of our experimental design, constant. Analyses of additional parameters that describe the nutritional quality for consumers such as proteins, carbohydrates, and the retention of contaminants may shed further light on the underlying mechanisms. Our results highlight that aquatic contaminants can affect the physiology of riparian spiders, likely by altering subsidy quality, with potential implications for terrestrial food webs. Environ Toxicol Chem 2023;42:1346-1358. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Structural features, intrinsic disorder, and modularity of a pyriform spidroin 1 core repetitive domain.

Orb-weaving spiders produce up to seven silk types, each with distinct biological roles, protein compositions, and mechanics. Pyriform (or piriform) silk is composed of pyriform spidroin 1 (PySp1) and is the fibrillar component of attachment discs that attach webs to substrates and to each other. Here, we characterize the 234-residue repeat unit (the "Py unit") from the core repetitive domain of Argiope argentata PySp1. Solution-state nuclear magnetic resonance (NMR) spectroscopy-based backbone chemical shift and dynamics analysis demonstrate a structured core flanked by disordered tails, structuring that is maintained in a tandem protein of two connected Py units, indicative of structural modularity of the Py unit in the context of the repetitive domain. Notably, AlphaFold2 predicts the Py unit structure with low confidence, echoing low confidence and poor agreement to the NMR-derived structure for the Argiope trifasciata aciniform spidroin (AcSp1) repeat unit. Rational truncation, validated through NMR spectroscopy, provided a 144-residue construct retaining the Py unit core fold, enabling near-complete backbone and side chain 1H, 13C, and 15N resonance assignment. A six α-helix globular core is inferred, flanked by regions of intrinsic disorder that would link helical bundles in tandem repeat proteins in a beads-on-a-string architecture.

Mesoscale Confinement in Bagworm Silk: A Hidden Structural Organization.

While silk fibers produced by silkworms and spiders are frequently described as a network of amorphous protein chains reinforced by crystalline ß-sheet nanodomains, the importance of higher-order, self-assembled structures has been recognized for advanced modeling of mechanical properties. General acceptance of hierarchical structural models is, however, currently limited by lack of experimental results. Indeed, X-ray scattering studies of spider's dragline-type fibers have been particularly limited by low crystallinities. Here we are reporting on probing the local structure of exceptionally crystalline bagworm silk fibers by X-ray nanobeam scattering. Probing the comparable thickness of cross sections with an X-ray nanobeam allows removing the strong scattering background from the outer sericin layer and reveals a hidden structural organization due to a radial gradient in diameters of mesoscale nanofibrillar bundles in the fibroin phase. Our results provide direct support for lateral interactions between nanofibrils.