Sticky proteins serve as glue: blue mussel
Despite the constant turbulence of tides, mussels manage to stay attached to their slippery, rocky homes. Their secret is a massively strong, sticky adhesive — now the target of scientists hoping to replicate it.
The bivalves secrete proteins in a goopy form that solidifies into a thread-like water-resistant adhesive. Along its length, the thread is protected by protruding, knobby proteins that are mixed with iron. Proteins poking out of the leading edge of the sticky mix contain a high percentage of a particular kind of amino acid, called dihydroxyphenylalanine, which allows the mix to dry quickly and stick to slippery surfaces. Now, scientists have incorporated this amino acid into existing biomimetic mussel glues, and are testing the materials’ ability to seal fetal membranes and arteries, and stick to places that superglue otherwise doesn’t.
Byssus threads of the blue mussel attach to a wet, solid surface due to catechols on adhesive proteins that overcome the surface’s affinity for water molecules
Blue mussels (Mytilus edulis) – bivalves that attach to rocks in wave-battered intertidal seashores – produce adhesives comparable in strength to human-made glues but without carcinogens such as formaldehyde and which can cure under water.
A key feature of the blue mussel’s unique adhesive chemistry is the presence of the amino acid 3,4-dihydroxyphenylalanine, with its reactive catechol functional group (two hydroxyl groups sticking out from a benzene ring) that forms strong bonds with catechols on adjacent molecules and with metal atoms present in the surface of most natural solid substrates. Another key feature is the ability of catechol chains to overcome a solid surface’s otherwise strong preference for water molecules (which is why conventional adhesives fail on wet surfaces).
Application
New mussel-inspired adhesives, which have wide-ranging applications from surgical to wood composites, currently use soy as an inexpensive, accessible feedstock, and work by blocking certain amino acids in soy proteins that are not present in mussel proteins, such as glutamic acid, so that the resulting compound bears a closer resemblance to that of mussel proteins.
“Pounding waves are no match for the mighty mussel, that produces strong, flexible threads that cling to rocks…mussels secrete a unique amino acid called dihydroxyphenylalanine…Researchers have developed a new group of adhesives for wood products inspired by the ability of mussels to cling to rocks using thread-like tentacles. These threads are proteins that retain powerful adhesive properties even in water.”
There are few adhesives with a greater spatial range of function than the underwater glue of mussels (Mytilus). At the nanoscale, mussel adhesive proteins interact energetically with diverse surface chemistries thereby securing the ends of byssal threads (scale: microns-millimeters) onto rocks, for example. Cohorts of threads collectively provide each mussel with a holdfast or byssus (scale: centimeters). Mussels rely on their byssus to network socially with one another and to build massive reef-like aggregates that stabilize shorelines and establish new ecosystems (scale: meters to kilometers). Unlike the tendons that they resemble, byssal threads are produced in minutes from complex fluids by injection molding and curing. The proteins in these fluids are well-adapted for their roles as tensile filaments, wear resistant coatings or bonding of wet surfaces. Regardless of function, all mussel adhesive proteins contain the modified catecholic amino acid DOPA (3, 4-dihydroxyphenylalanine), which is notoriously prone to oxidation. Despite this, mussels control DOPA reactivity with remarkable precision.
At or near the interface between the adhesive proteins and the substratum, mussels impose a highly reducing local environment (low pH and high thiol content) in order to exploit the superior chemisorption of DOPA to a variety of surfaces. Farther from the interface, mussel proteins are maintained in either Fe3+-rich or oxidizing local environments (high pH and catecholoxidase). The first leads to protein gelation stabilized by multifunctional DOPA- Fe3+-complexation, whereas the latter, to the covalent cross-links between DOPAquinone and reactive amino acids. By adjusting the redox of the local environment, mussels “tune” the optimal level of adhesion (DOPA) or cohesion (quinone and cross-linking) needed in each part of the holdfast. A deeper understanding of mussel adhesive chemistry and its regulation is likely to inspire improvements in adhesive technology especially in wet applications.
Blue Mussels (Mytilus edulis)
The blue mussel (Mytilus edulis), also known as the common mussel, is a medium-sized edible marine bivalve mollusc in the family Mytilidae, the mussels. Blue mussels are subject to commercial use and intensive aquaculture.
Blue mussels are boreo-temperate invertebrates that live in intertidal areas attached to rocks and other hard substrates by strong (and somewhat elastic) thread-like structures called byssal threads, secreted by byssal glands located in the foot of the mussel.
The shape of the shell is triangular and elongate with rounded edges. The shell is smooth with a sculpturing of fine concentric growth lines but no radiating ribs. The shells of this species are purple, blue or sometimes brown in color, occasionally with radial stripes. The outer surface of the shell is covered by the periostracum which as eroded, exposes the colored prismatic calcitic layer. Blue Mussels are semi-sessile, having the ability to detach and reattach to a surface allowing the mollusk to reposition itself relative to the water position.
Development
Once the pediveliger is fully developed, its foot extends and makes contact with substrate. The initial contact with the substrate is loose. If the substrate is suitable, the larva will metamorphoses into the juvenile form, plantigrade, and attach byssus threads. The mussel will remain in that state until reaching 1-1.5mm in length. This attachment is the prerequisite for the foundation for the blue mussel population. In sheltered environments large masses sometimes form beds which offer shelter and food for other invertebrates. Byssal thread are secreted by byssal glands located in the foot of the mussel, and are made up of polyphenolic proteins are proteins which serve as a bioadhesive.
Blue mussels have separate sexes. Sperm and eggs are released simultaneously into the water column. Fertilized eggs develop into swimming larvae (Trochophore Larvae) that reside in the plankton for a period of time and then settle on the bottom in areas generally occupied by other mussels. The young juveniles move around seeking a suitable site and eventually attach themselves to a spot with their byssal threads. (see below).
Blue mussels are held in place by strong byssal threads secreted by a byssal gland near the base of the foot.
Blue mussels (Phylum Mollusca, Class Bivalvia) can be found in all of the zones. Like the great majority of bivalve molluscs they feed by capturing small particles of plankton, organic detritus and bacteria in mucus on the gills as the respiratory current passes through them. The mucus and the food are swept to the lower margin of the gills by special tracts of strong cilia and are then brought anteriorly along the food groove at the gill margin to the palps which then transfer food to the mouth. (above)
Blue mussels occasionally form clumps, or aggregates, of individuals when population density is low.The mussels attach to one another via collagenous protein strands called byssal threads. The aggregates are observed mostly in Mussel fields, which are short-lived populations of Mussels, usually exhibiting a clumped distribution pattern. It is hypothesized that the mussels form these aggregates to increase reproductive success in low density populations. This hypothesis, however, has yet to be conclusively proven. Alternative possible reasons for the behavior include resisting wave action and increasing water flow through the siphons of the mussel. The significance of the behavior is its relation to the formation of mussel beds from mussel fields. Mussel beds are persistent, dense mussel populations. Beds generally form from fields that persist long enough to establish a dense population. Thus in areas where blue mussels are threatened, such as the Wadden Sea, it is of great importance to enhance the survival of mussel fields, of which mussel aggregates are the primary component.
Biomimicry: Copying our way to conservation
The adhesive strategies of marine mussels are key to their survival on wet, wind-swept, and wave-swept surfaces. Given this, mussel tenacity has become a poster child for the wet adhesion needed elsewhere in human technology, particularly in health-care delivery. Mussel adhesion is complex with both chemical and physical underpinnings at multiple length scales. The peculiar catechol-based chemistry of mussel adhesion has inspired a variety of applications ranging from hard and soft tissue repair to drug delivery to magnetic imaging agents. Although the emphasis on new bioinspired materials is inevitable, it should be coupled with the recognition that society is equally well served by the mussel byssus (holdfast) as an indicator of mussel well-being.
Translation of Mussel Adhesion to Beneficial New Concepts and Materials
Byssally interconnected mussel clusters are the basis of mariculture and diverse reef-like intertidal ecologies that resist coastal erosion. Given its exquisite sensitivity to environmental conditions, mussel byssus also serves as an important monitor of pollution and climate change.
Mussel Attachment in Changing Climates: An Ecomaterial Approach
Mussels dominate temperate rocky shores worldwide by forming dense aggregations that are firmly tethered by byssal threads, extracellular collagenous secretions of the mussel foot. Strong byssal attachment ensures mussels can withstand heavy wave action, resist mobile predators and overgrow competitors for limited space. Field studies with Mytilus spp. have shown byssus strength, or tenacity, follows a strong seasonal cycle, rendering both wild and farmed populations prone to “fall-off” in late summer/early fall when increased storm activity coincides with weak attachment. These seasonal cycles in tenacity are not due to changes in rate of thread production, but rather reflect environmentally-induced changes in the material properties of the individual byssal threads mussels produce; threads produced in summer are significantly weaker and less extensible than those produced in spring.
The underlying mechanism by which byssal threads weaken seasonally is unknown. Using custom laboratory mesocosms, we quantified the effects of two common environmental stressors, elevated temperature and pCO2 (= acidification), on the mechanical performance of byssal threads secreted by the bay mussel M. trossulus. Both stressors caused thread weakening and loss of extensibility, but targeted different regions of a thread. Specifically, high water temperature (~25C) weakened the proximal region, while high pCO2 (> 1200 μatm pCO2) reduced the strength of the plaque. Both stressors reduced overall thread extensibility as well. These results suggest multiple environmental stressors, including ocean acidification and warming, can combine to critically compromise the structural integrity of mussels.
Understanding the underlying mechanism by which environment alters the manufacturing process of mussel byssus (e.g. direct physiochemical v. indirect physiologicial effects) will provide insight into developing strategies that promote the manufacture of strong byssus material, in industry and in nature.
Mussel-Inspired Materials for Surgical Repair and Drug Delivery
The adhesive proteins employed by mussels have very specialized amino acid compositions undoubtedly related to the particular challenges of achieving permanent adhesion in the wet marine environment. Mussel adhesive proteins (MAPs) are known to contain high levels of the catecholic amino acid 3,4-dihydroxy-L-alanine (DOPA). Catechols are versatile in a chemical sense, participating in redox reactions, high affinity coordination of metal ions, and strong interfacial activity, leading to important roles for catechols in a variety of biological processes such as pigment formation, neurotransmission, iron sequestration, and in the case of mussel byssal proteins- mechanical adhesion. The rich chemical and biochemical landscape of catechols has led to numerous opportunities for the development biologically inspired materials for a variety of applications. Here described the work on mussel-inspired synthetic catechol materials for use in surgical adhesion/wound closure, and in drug delivery. Synthetic catechol polymer medical sealants and adhesives are being developed, exploiting the same cohesive and adhesive interactions of catechols that are believed to be important in mussel adhesion. Examples of surgical applications where these materials are candidates for use in humans, include vascular wound closure and for sealing of spontaneous or fetal surgery-associated cases of fetal membrane rupture.
In the area of drug delivery, scientists are developing a novel catechol polymer conjugates of the proteasome inhibitor bortezomib (BTZ) for pH-sensitive delivery to cancer cells. One design takes advantage of the facile conjugation of BTZ to catechols to form pH-sensitive drug delivery vehicles that are stable and inactive in the bloodstream but dissociate in the acidic tumor interstitium or intracellular environment to liberate the drug, activating its proteasome inhibiting function and thereby potentially increasing the efficacy and reducing the peripheral toxicity of BTZ.
Biomedical Applications of Ultra-Small Magnetic Nanoparticles
The number of potential applications of nanoparticles in biology and medicine, e.g., for application as contrast agents in medical imaging (diagnostics), for (targeted) drug delivery (therapy) and their combination (theragnostics) is rapidly increasing with emerging technologies to tune and control their bulk and, even more importantly, surface properties. An example are superparamagnetic iron oxide nanoparticles (SPIONs).
Catechols in the form of DOPA are found in high concentrations in mussel adhesive proteins (MAPs) and contribute to the unique ability of MAPs to strongly bind to almost any material surface. We have stabilized sub-10-nm superparamagnetic iron oxide nanoparticles (SPIONs) through catechol-derivative anchor groups, such as nitroDOPA, bound to 5 kDa poly(ethylene glycol) and shown that the dispersed particles possess essentially irreversible binding affinity to iron oxide and thus can optimally disperse superparamagnetic nanoparticles under physiologic conditions1. This not only leads to ultrastable iron oxide nanoparticles but also allows close control over the hydrodynamic diameter and interfacial chemistry. The latter is a crucial aspect for the assembly of functionalized magnetic nanoparticles, e.g., as targeted magnetic resonance contrast agents. Preliminary applications in vitro and in vivo of functionalized SPIONs for Magnetic Resonance Imaging (MRI).
