Review: mapping proteins localized in adhesive setae of the tokay gecko and their possible influence on the mechanism of adhesion

Spatio-temporal expression patterns of glycine-rich beta proteins and cysteine-rich beta proteins in setae development of Gekko japonicus

BACKGROUND

Setae on the pad lamellae of the Japanese gecko Gekko japonicus (Schlegel, 1836), a vital epidermal derivative, are primarily composed of cornified beta-proteins (CBPs) and play a pivotal role in adhesion and climbing. The amino acid composition of CBPs might be a determining factor influencing their functional properties. However, the molecular mechanisms governed by CBP genes with diverse amino acid compositions in setae development remain unexplored.

RESULTS

Based on RNA-seq analyses, this study confirmed that all G. japonicus CBPs (GjCBPs) are involved in setae formation. Cysteine-rich CBPs encoding genes (ge-cprp-17 to ge-cprp-26) and glycine-rich CBPs encoding genes (ge-gprp-17 to ge-gprp-22) were haphazardly selected, with quantitative real-time PCR revealing their expression patterns in embryonic pad lamellae and dorsal epidermis. It is inferred that glycine-rich CBPs are integral to the formation of both dorsal scales and lamellar setae, cysteine-rich CBPs are primarily associated with setae development. Additionally, fluorescence in situ hybridization revealed spatiotemporal differences in the expression of a glycine-rich CBP encoding gene (ge-gprp-19) and a cysteine-rich CBP encoding gene (ge-cprp-17) during dorsal scales and/or lamellar development.

CONCLUSIONS

All 66 CBPs are involved in the formation of setae. Glycine-rich CBPs hold a significant role in the development of dorsal scales and lamellar setae, whereas most cysteine-rich CBPs appear to be essential components of G. japonicus setae. Even GjCBPs with similar amino acid compositions may play diverse functions. The clear spatio-temporal expression differences between the glycine-rich and cysteine-rich CBP encoding genes during epidermal scale and/or setae formation were observed. Embryonic developmental stages 39 to 42 emerged as crucial phases for setae development. These findings lay the groundwork for deeper investigation into the function of GjCBPs in the development of G. japonicus setae.

Actin-templated Structures: Nature's Way to Hierarchical Surface Patterns (Gecko's Setae as Case Study)

The hierarchical design of the toe pad surface in geckos and its reversible adhesiveness have inspired material scientists for many years. Micro- and nano-patterned surfaces with impressive adhesive performance have been developed to mimic gecko's properties. While the adhesive performance achieved in some examples has surpassed living counterparts, the durability of the fabricated surfaces is limited and the capability to self-renew and restore function-inherent to biological systems-is unimaginable. Here the morphogenesis of gecko setae using skin samples from the Bibron´s gecko (Chondrodactylus bibronii) is studied. Gecko setae develop as specialized apical differentiation structures at a distinct cell-cell layer interface within the skin epidermis. A primary role for F-actin and microtubules as templating structural elements is necessary for the development of setae's hierarchical morphology, and a stabilization role of keratins and corneus beta proteins is identified. Setae grow from single cells in a bottom layer protruding into four neighboring cells in the upper layer. The resulting multicellular junction can play a role during shedding by facilitating fracture of the cell-cell interface and release of the high aspect ratio setae. The results contribute to the understanding of setae regeneration and may inspire future concepts to bioengineer self-renewable patterned adhesive surfaces.

Scales of non-avian reptiles and their derivatives contain corneous beta proteins coded from genes localized in the Epidermal Differentiation Complex

The evolution of modern reptiles from basic reptilian ancestors gave rise to scaled vertebrates. Scales are of different types, and their corneous layer can shed frequently during the year in lepidosaurians (lizards, snakes), 1-2 times per year in the tuatara and in some freshwater turtle, irregularly in different parts of the body in crocodilians, or simply wore superficially in marine and terrestrial turtles. Lepidosaurians possess tuberculate, non-overlapped or variably overlapped scales with inter-scale (hinge) regions. The latter are hidden underneath the outer scale surface or may be more exposed in specific body areas. Hinge regions allow stretching during growth and movement so that the skin remains mechanically functional. Crocodilian and turtles feature flat and shield scales (scutes) with narrow inter-scale regions for stretching and growth. The epidermis of non-avian reptilian hinge regions is much thinner than the exposed outer surface of scales and is less cornified. Despite the thickness of the epidermis, scales are mainly composed of variably amount of Corneous Beta Proteins (CBPs) that are coded in a gene cluster known as EDC (Epidermal Differentiation Complex). These are small proteins, 100-200 amino acid long of 8-25 kDa, rich in glycine and cysteine but also in serine, proline and valine that participate to the formation of beta-sheets in the internal part of the protein, the beta-region. This region determines the further polymerization of CBPs in filamentous proteins that, together a network of Intermediate Filament Keratins (IFKs) and other minor epidermal proteins from the EDC make the variable pliable or inflexible corneous material of reptilian scales, claws and of turtle beak. The acquisition of scales and skin derivatives with different mechanical and material properties, mainly due to the evolution of reptile CBPs, is essential for the life and different adaptations of these vertebrates.

Bristles formation in adhesive pads and sensilli of the gecko Tarentola mauritanica derive from a massive accumulation of corneous material in Oberhautchen cells of the epidermis

Among lizards, geckos possess special digital scales modified as hairy-like lamellae that allow attachment to vertical substrates for the movement using adhesive nanoscale filaments called setae. The present study shows new ultrastructural details on setae formation in the gecko Tarentula mauritanica. Setae derive from the special differentiation of an epidermal layer termed Oberhauchen and can reach 30-60 µm in length. Oberhautchen cells in the adhesive pad lamellae becomes hypertrophic and rest upon 2 layers of non-corneous and pale cells instead of beta-cells like in the other scales. Only 1-2 beta-layers are formed underneath the pale layer. Setae derive from the accumulation of numerous roundish and heterogenous beta-packets with variable electron-density in Oberhautchen cells, possibly indicating a mixed protein composition. Immunofluorescence and immunogold labeling for CBPs show that beta-packets merge at the base of the growing setae forming long corneous bundles. Pale cells formed underneath the Oberhautchen layer contain small vesicles or tubules with a likely lipid content, sparse keratin filaments and ribosomes. In mature lamellae these cells merge with Oberhautchen and beta-cells forming a thin electron-paler layer located between the Oberhautchen and the thin beta-layer, a variation of the typical sequence of epidermal layers present in other scales. The formation of a softer pale layer and of a thin beta-layer likely determines a flexible corneous support for the adhesive setae. The specific molecular mechanism that stimulates the cellular changes observed during Oberhautchen hypertrophy and the alteration of the typical epidermal stratification in the pad epidermis remains unknown.

The Periodic Replacement of Adhesive Setae in Pad Lamellae of Climbing Lizards Is Driven by Patterns of Corneous Layer Growth

The adhesive digital pads in some gecko and anoline lizards are continuously utilized for movements on vertical surfaces that may determine wear and a decrease of adhesion efficiency. The pads are formed by lamellae bearing adhesive setae that are worn out following frequent usage and are replaced by new inner setae that maintain an efficient adhesion. Whether the extensive usage of adhesive setae determines a higher shedding frequency in the digital pads with respect to other body regions remains unknown. Setae replacement has been analyzed in embryos and adult lizards using autoradiography and 5BrdU-immunohistochemistry. The observation strongly suggests that during development and epidermal renewal in adult lamellae, there is a shifting of the outer setae toward the apex of the lamella. This movement is likely derived from the continuous addition of proteins in the beta- and alpha-layers sustaining the outer setae while the inner setae are forming. Ultrastructural and in situ hybridization studies indicate that the thin outer beta- and alpha-layers still contain mRNAs and ribosomes that may contribute to the continuous production of corneous beta proteins (CBPs) and keratins for the growth of the free margin at the apex of the lamella. This process determines the apical shifting and release of the old setae, while the new inner setae formed underneath becomes the new outer setae.

High Performance Marine and Terrestrial Bioadhesives and the Biomedical Applications They Have Inspired

This study has reviewed the naturally occurring bioadhesives produced in marine and freshwater aqueous environments and in the mucinous exudates of some terrestrial animals which have remarkable properties providing adhesion under difficult environmental conditions. These bioadhesives have inspired the development of medical bioadhesives with impressive properties that provide an effective alternative to suturing surgical wounds improving closure and healing of wounds in technically demanding tissues such as the heart, lung and soft tissues like the brain and intestinal mucosa. The Gecko has developed a dry-adhesive system of exceptional performance and has inspired the development of new generation re-usable tapes applicable to many medical procedures. The silk of spider webs has been equally inspiring to structural engineers and materials scientists and has revealed innovative properties which have led to new generation technologies in photonics, phononics and micro-electronics in the development of wearable biosensors. Man made products designed to emulate the performance of these natural bioadhesive molecules are improving wound closure and healing of problematic lesions such as diabetic foot ulcers which are notoriously painful and have also found application in many other areas in biomedicine. Armed with information on the mechanistic properties of these impressive biomolecules major advances are expected in biomedicine, micro-electronics, photonics, materials science, artificial intelligence and robotics technology.

Evidence that gecko setae are coated with an ordered nanometre-thin lipid film

The fascinating adhesion of gecko to virtually any material has been related to surface interactions of myriads of spatula at the tips of gecko feet. Surprisingly, the molecular details of the surface chemistry of gecko adhesion are still largely unknown. Lipids have been identified within gecko adhesive pads. However, the location of the lipids, the extent to which spatula are coated with lipids, and how the lipids are structured are still open questions. Lipids can modulate adhesion properties and surface hydrophobicity and may play an important role in adhesion. We have therefore studied the molecular structure of lipids at spatula surfaces using near-edge X-ray absorption fine structure imaging. We provide evidence that a nanometre-thin layer of lipids is present at the spatula surfaces of the tokay gecko (Gekko gecko) and that the lipids form ordered, densely packed layers. Such dense, thin lipid layers can effectively protect the spatula proteins from dehydration by forming a barrier against water evaporation. Lipids can also render surfaces hydrophobic and thereby support the gecko adhesive system by enhancement of hydrophobic-hydrophobic interactions with surfaces.

Structure of Keratins in Adhesive Gecko Setae Determined by Near-Edge X-ray Absorption Fine Structure Spectromicroscopy

Geckos have the astonishing ability to climb on vertical surfaces due to the adhesive properties of fibrous setae at the tips of their toe pads. While the adhesion mechanism principle, based on van der Waals interactions of myriads of spatula located at the outermost end of the setal arrays, has been studied extensively, there are still open questions about the chemistry of gecko setae. The gecko adhesive system is based on keratin fibrils assembled to support the entire setal structure. At the same time, the structure and alignment of keratin molecules within the ultrafine spatula tissue, which can support the enormous mechanical strain, still remain unknown. We have studied the molecular structure of gecko spatula using near-edge X-ray absorption fine structure (NEXAFS) imaging. We indeed found that the setae consist of a β-sheet structure aligned with the adhesion direction of the setae. Such alignment may provide mechanical stability to the setae and resistance to wear across different length scales.

Direct evidence of acid-base interactions in gecko adhesion

While it is generally accepted that van der Waals (vdW) forces govern gecko adhesion, several studies indicate contributions from non-vdW forces and highlight the importance of understanding the adhesive contact interface. Previous work hypothesized that the surface of gecko setae is hydrophobic, with nonpolar lipid tails exposed on the surface. However, direct experimental evidence supporting this hypothesis and its implications on the adhesion mechanism is lacking. Here, we investigate the sapphire-setae contact interface using interface-sensitive spectroscopy and provide direct evidence of the involvement of acid-base interactions between polar lipid headgroups exposed on the setal surface and sapphire. During detachment, a layer of unbound lipids is left as a footprint due to cohesive failure within the lipid layer, which, in turn, reduces wear to setae during high stress sliding. The absence of this lipid layer enhances adhesion, despite a small setal-substrate contact area. Our results show that gecko adhesion is not exclusively a vdW-based, residue-free system.

Revisiting the classification of squamate adhesive setae: historical, morphological and functional perspectives

Research on gecko-based adhesion has become a truly interdisciplinary endeavour, encompassing many disciplines within the natural and physical sciences. Gecko adhesion occurs by the induction of van der Waals intermolecular (and possibly other) forces between substrata and integumentary filaments (setae) terminating in at least one spatulate tip. Gecko setae have increasingly been idealized as structures with uniform dimensions and a particular branching pattern. Approaches to developing synthetic simulacra have largely adopted such an idealized form as a foundational template. Observations of entire setal fields of geckos and anoles have, however, revealed extensive, predictable variation in setal form. Some filaments of these fields do not fulfil the morphological criteria that characterize setae and, problematically, recent authors have applied the term 'seta' to structurally simpler and likely non-adhesively competent fibrils. Herein we briefly review the history of the definition of squamate setae and propose a standardized classificatory scheme for epidermal outgrowths based on a combination of whole animal performance and morphology. Our review is by no means comprehensive of the literature regarding the form, function, and development of the adhesive setae of squamates and we do not address significant advances that have been made in many areas (e.g. cell biology of setae) that are largely tangential to their classification and identification. We contend that those who aspire to simulate the form and function of squamate setae will benefit from a fuller appreciation of the diversity of these structures, thereby assisting in the identification of features most relevant to their objectives.

Multiple Mechanical Gradients are Responsible for the Strong Adhesion of Spider Attachment Hair

Wandering spiders climb vertically and walk upside-down on rough and smooth surfaces using a nanostructured attachment system on their feet. The spiders are assumed to adhere by intermolecular van der Waals forces between the adhesive structures and the substrate. The adhesive elements are arranged highly ordered on the hierarchically structured attachment hair (setae). While walking, it has been suggested that the spiders apply a shear force on their legs to increase friction. However, the detailed mechanical behavior of the hair's structures during attachment and detachment remains unknown. Here, gradients of the mechanical properties of the attachment hair on different length scales that have evolved to support attachment, stabilize adhesion in contact, and withstand high stress at detachment, examined by in situ experiments, are shown. Shearing helps to self-align the adhesive elements with the substrate. The study is anticipated to contribute to the development of optimized artificial dry adhesives.

Influence of carbon dioxide plasma treatment on the dry adhesion of vertical aligned carbon nanotube arrays

Vertical aligned carbon nanotube (VACNT) arrays used as dry adhesive materials have broad prospects in the applied fields of space, medicine and electronics. The adhesion of VACNT arrays is believed to be related not only to the nano-array structure and a clean surface, but also to chemical composition. Here, radio-frequency (RF) carbon dioxide (CO₂) plasma treatment is introduced as an effective method for purifying and functionalizing the surface to improve the dry adhesive performance of VACNT arrays. Scanning electron microscopy showed that the VACNT arrays retained the alignment architecture with minimal damage at low power. X-ray photoelectron spectroscopy and contact angle tests revealed that the content of non-polar components (C = C bonds) decreased after treatment, while the content of polar groups (C = O and O-C = O bonds) increased, which changed the surface polarity of the VACNT arrays. Raman analyses and transmission electron microscopy demonstrated that amorphous carbon can be selectively removed with increasing time (0-18 min), but was continuously generated with increasing power (30-90 W). The best adhesive strength of 18 N cm^-2 (increased by 39%) was obtained after CO₂ plasma treating for 10 min at 30 W power, which was attributed to the combined action of purification and polarization.

Immunolocalization of corneous proteins including a serine-tyrosine-rich beta-protein in the adhesive pads in the tokay gecko

Adhesive pads of geckos contain many thousands of nanoscale spatulae for the adhesion and movement along vertical or inverted surfaces. Setae are composed of interlaced corneous bundles made of small cysteine-glycine-rich corneous beta proteins (CBPs, formerly indicated as beta-keratins), embedded in a matrix material composed of cytoskeletal proteins and lipids. Negatively charged intermediate filament keratins (IFKs) and positively charged CBPs likely interact within setae, aside disulphide bonds, giving rise to a flexible and resistant corneous material. Using differernt antibodies against CBPs and IFKs an updated model of the composition of setae and spatulae is presented. Immunofluorescence and ultrastructural immunogold labeling reveal that one type of neutral serine-tyrosine-rich CBP is weakly localized in the setae while it is absent from the spatula. This uncharged protein is mainly present in the thin Oberhautchen layer sustaining the setae, although with a much lower intensity with respect to the cysteine-rich CBPs. These proteins in the spatula likely originate a positively charged or neutral contact surface with the substrate but the influence of lipids and cytoskeletal proteins present in setae on the mechanism of adhesion is not known. In the spatula, protein-lipid complexes may impart the pliability for the attachment and adapt to irregular surfaces. The presence of cysteine-glycine medium rich CBPs and softer IFKs in alpha-layers sustaining the setae forms a flexible base for compliance of the setae to substrate and improved adhesion.

Mussel-inspired bioadhesives in healthcare: design parameters, current trends, and future perspectives

Mussels are well-known for their extraordinary capacity to adhere onto different surfaces in various hydrophillic conditions. Their unique adhesion ability under water or in wet conditions has generated considerable interest towards developing mussel inspired polymeric systems that can mimic the chemical mechanisms used by mussels for their adhesive properties. Catechols like 3,4-dihydroxy phenylalanine (DOPA) and their biochemical interactions have been largely implicated in mussels' strong adhesion to various substrates and have been the centerpoint of research and development efforts towards creating superior tissue adhesives for surgical and tissue engineering applications. In this article, we review bioadhesion and adhesives from an engineering standpoint, specifically the requirements of a good tissue glue, the relevance that DOPA and other catechols have in tissue adhesion, current trends in mussel-inspired bioadhesives, strategies to develop mussel-inspired tissue glues, and perspectives for future development of these materials.

Stick or Slip: Adhesive Performance of Geckos and Gecko-Inspired Synthetics in Wet Environments

The gecko adhesive system has inspired hundreds of synthetic mimics principally focused on replicating the strong, reversible, and versatile properties of the natural system. For geckos native to the tropics, versatility includes the need to remain attached to substrates that become wet from high humidity and frequent rain. Paradoxically, van der Waals forces, the principal mechanism responsible for gecko adhesion, reduce to zero when two contacting surfaces separate even slightly by entrapped water layers. A series of laboratory studies show that instead of slipping, geckos maintain and even improve their adhesive performance in many wet conditions (i.e., on wet hydrophobic substrates, on humid substrates held at low temperatures). The mechanism for this is not fully clarified, and likely ranges in scale from the chemical and material properties of the gecko's contact structures called setae (e.g., setae soften and change surface confirmation when exposed to water), to their locomotor biomechanics and decision-making behavior when encountering water on a substrate in their natural environment (e.g., some geckos tend to run faster and stop more frequently on misted substrates than dry). Current work has also focused on applying results from the natural system to gecko-inspired synthetic adhesives, improving their performance in wet conditions. Gecko-inspired synthetic adhesives have also provided a unique opportunity to test hypotheses about the natural system in semi-natural conditions replicated in the laboratory. Despite many detailed studies focused on the role of water and humidity on gecko and gecko-inspired synthetic adhesion, there remains several outstanding questions: (1) what, if any, role does capillary or capillary-like adhesion play on overall adhesive performance of geckos and gecko-inspired synthetics, (2) how do chemical and material changes at the surface and in the bulk of gecko setae and synthetic fibrils change when exposed to water, and what does this mean for adhesive performance, and (3) how much water do geckos encounter in their native environment, and what is their corresponding behavioral response? This review will detail what we know about gecko adhesion in wet environments, and outline the necessary next steps in biological and synthetic system investigations.

Gecko Adhesion in Space and Time: A Phylogenetic Perspective on the Scansorial Success Story

An evolutionary perspective on gecko adhesion was previously hampered by a lack of an explicit phylogeny for the group and of robust comparative methods to study trait evolution, an underappreciation for the taxonomic and structural diversity of geckos, and a dearth of fossil evidence bearing directly on the origin of the scansorial apparatus. With a multigene dataset as the basis for a comprehensive gekkotan phylogeny, model-based methods have recently been employed to estimate the number of unique derivations of the adhesive system and its role in lineage diversification. Evidence points to a single basal origin of the spinulate oberhautchen layer of the epidermis, which is a necessary precursor for the subsequent elaboration of a functional adhesive mechanism in geckos. However, multiple gains and losses are implicated for the elaborated setae that are necessary for adhesion via van der Waals forces. The well-supported phylogeny of gekkotans has demonstrated that convergence and parallelism in digital design are even more prevalent than previously believed. It also permits the reexamination of previously collected morphological data in an explicitly evolutionary context. Both time-calibrated trees and recently discovered amber fossils that preserve gecko toepads suggest that a fully-functional adhesive apparatus was not only present, but also represented by diverse architectures, by the mid-Cretaceous. Further characterization and phylogenetically-informed analyses of the other components of the adhesive system (muscles, tendons, blood sinuses, etc.) will permit a more comprehensive reconstruction of the evolutionary pathway(s) by which geckos have achieved their structural and taxonomic diversity. A phylogenetic perspective can meaningfully inform functional and performance studies of gecko adhesion and locomotion and can contribute to advances in bioinspired materials.

Duplications in Corneous Beta Protein Genes and the Evolution of Gecko Adhesion

Corneous proteins are an important component of the tetrapod integument. Duplication and diversification of keratins and associated proteins are linked with the origin of most novel integumentary structures like mammalian hair, avian feathers, and scutes covering turtle shells. Accordingly, the loss of integumentary structures often coincides with the loss of genes encoding keratin and associated proteins. For example, many hair keratins in dolphins and whales have become pseudogenes. The adhesive setae of geckos and anoles are composed of both intermediate filament keratins (IF-keratins, formerly known as alpha-keratins) and corneous beta-proteins (CBPs, formerly known as beta-keratins) and recent whole genome assemblies of two gecko species and an anole uncovered duplications in seta-specific CBPs in each of these lineages. While anoles evolved adhesive toepads just once, there are two competing hypotheses about the origin(s) of digital adhesion in geckos involving either a single origin or multiple origins. Using data from three published gecko genomes, I examine CBP gene evolution in geckos and find support for a hypothesis where CBP gene duplications are associated with the repeated evolution of digital adhesion. Although these results are preliminary, I discuss how additional gecko genome assemblies, combined with phylogenies of keratin and associated protein genes and gene duplication models, can provide rigorous tests of several hypotheses related to gecko CBP evolution. This includes a taxon sampling strategy for sequencing and assembly of gecko genomes that could help resolve competing hypotheses surrounding the origin(s) of digital adhesion.

Hierarchical architecture of spider attachment setae reconstructed from scanning nanofocus X-ray diffraction data

When sitting and walking, the feet of wandering spiders reversibly attach to many surfaces without the use of gluey secretions. Responsible for the spiders' dry adhesion are the hairy attachment pads that are built of specially shaped cuticular hairs (setae) equipped with approximately 1 µm wide and 20 nm thick plate-like contact elements (spatulae) facing the substrate. Using synchrotron-based scanning nanofocus X-ray diffraction methods, combining wide-angle X-ray diffraction/scattering and small-angle X-ray scattering, allowed substantial quantitative information to be gained about the structure and materials of these fibrous adhesive structures with 200 nm resolution. The fibre diffraction patterns showed the crystalline chitin chains oriented along the long axis of the attachment setae and increased intensity of the chitin signal dorsally within the seta shaft. The small-angle scattering signals clearly indicated an angular shift by approximately 80° of the microtrich structures that branch off the bulk hair shaft and end as the adhesive contact elements in the tip region of the seta. The results reveal the specific structural arrangement and distribution of the chitin fibres within the attachment hair's cuticle preventing material failure by tensile reinforcement and proper distribution of stresses that arise upon attachment and detachment.