How Marine Mussels Grip Rocks: Iron Atoms Convey Mussel Fibers With a Robust but Stretchy Covering

We may like to eat mussels steamed in white wine, but we also like to find mussels at the beach. Mostly they are burrowed into the ground or tethered to rocks. But if you look closer you will find a mollusc which has adapted to life and nutrition in a special and fascinating way. Mussels thrive in rocky seashore habitats, in spite of the enormous physical demands present there. This is in no small part due to the evolution of the byssus, which mussels employ to tether themselves to accessible surfaces.

The individual byssal threads that compose the byssus are stiff, but stretchy and are fashioned by the mussel in a process resembling injection molding. Byssal threads are depended upon for dissipating the energy of crashing waves and also for resisting abrasive damage from water-borne debris. To this end, threads are sheathed with a thin and knobby outer cuticle; a biological polymer, which exhibits epoxy-like hardness, while straining up to 100% without cracking.
Incredible hardness and extensibility
Matthew Harrington, a researcher who worked on the project and Humboldt fellow at the Max Planck Institute for Colloids and Interfaces explains the motivation for studying the byssus cuticle: "Protective coatings are important for prolonging the lifetime of materials and devices. However, considering that hardness and extensibility are seldom coupled in engineered polymers or composites, understanding how one protects a flexible substrate becomes quite important." Byssal cuticles have a knobby appearance due to inclusions of submicron-sized granular structures in an apparently continuous matrix. Submicron-sized tears that form in the matrix during stretching of the cuticle are believed to hinder the formation of larger cracks that could lead to material failure.
Central to understanding the peculiar mechanical behaviour of the cuticle are the high concentration of iron ions in the cuticle and the presence of an uncommon modification of the amino acid tyrosine known commonly as dopa. Dopa is found at high concentrations in the main cuticle component, mussel foot protein-1 (mfp-1). Dopa is distinguished from typical amino acids due to its impressive affinity for complexing with transition metal ions, particularly iron. As Admir Masic, a scientist at the Max Planck Institute for Colloids and Interfaces who worked on the project, explains, "when 2-3 dopa residues complex with a single iron ion, they create an incredibly stable complex that can be utilized to cross-link structural proteins." These metal-protein complexes have a high breaking force (nearly half that of covalent bonds), but unlike covalent bonds they are reversibly breakable, making them ideal for creating sacrificial cross-links.
Cuticle is stabilized by dopa-iron complexes
Using a technique known as in situ Raman spectroscopy to probe the chemical composition of the cuticle, the researchers provided the first direct evidence that the cuticle is a protein-based polymeric scaffold stabilized by dopa-iron complexes. Moreover, it was discovered that the distribution of dopa-iron complexes is clustered, with areas of high density coinciding with the granular inclusions and low density with the inter-granular matrix. These observations, coupled with previous mechanical observations suggest that the densely cross-linked granules function as hard inclusions and the less cross-linked matrix functions in a sacrificial manner, allowing bonds to break prior to catastrophic failure.
"Nature has evolved an elegant solution to a problem that engineers are still struggling with; namely, how to combine the properties of abrasion resistance and high extensibility in the same material," says Peter Fratzl, director of the biomaterials department at the Max Planck Institute for Colloids and Interfaces. Apparently, the cuticle achieves this through a careful tailoring of protein-metal chemistry and the submicron organization of cross-link density. "Conceivably, this same strategy could be applied in engineered polymers and composites." Max-Planck-Gesellschaft (2010, March 29). How marine mussels grip rocks: Iron atoms convey mussel fibers with a robust but stretchy covering. ScienceDaily. Retrieved March 30, 2010, from http://www.sciencedaily.com­ /releases/2010/03/100304142234.htm

Mussel-Inspired 'Glue' for Fetal Membrane Repair

A sealant inspired by mussels' ability to stick to surfaces under wet conditions has shown promise in the repair of defects in human fetal membranes, according to a recent Northwestern University study.

During pregnancy, defects -- ruptures or holes -- in the fetal membrane can lead to the leakage of amniotic fluid, resulting in premature labor or termination of the pregnancy. Although some defects do repair themselves naturally, no method currently exists to effectively repair those that don't. One idea is to find a biocompatible material to seal off the opening.

"We tested our mussel-inspired sealant on living fetal tissue and found it was both biocompatible and effective at sealing the tiny holes -- two features essential in such a material," said Phillip B. Messersmith, who was one of the study's leaders. He is professor of biomedical engineering at Northwestern's McCormick School of Engineering and Applied Science.

The findings are published online by the American Journal of Obstetrics & Gynecology.

The fetal membrane is the structure that surrounds the developing fetus. Defects in the membrane result either from incisions during endoscopic fetal surgeries used in the treatment of some birth defects or premature and spontaneous ruptures in the fetal sac.

Messersmith and colleagues from Belgium, Switzerland and Canada punched holes three millimeters wide into human fetal tissue in vitro to replicate the tiny holes found in fetal membrane defects. They then applied their sealant as well as other sealant candidates (such as medical-grade superglues) to the holes and analyzed fetal tissue cell death for each sealant. The mussel-inspired sealant had the best results in both bonding and toxicity.

The injectable sealant is a mixture of two different solutions that, when combined, form a sealant or gel in 10 to 20 seconds. One solution is a simple synthetic polymer containing DOPA, a key amino acid found in the sophisticated proteins that are essential to mussels' ability to adhere to wet surfaces, and the other is a catalyst. (Messersmith first developed the polymer in 2002.)

The foot of the common mussel (Mytilus edulis) produces a sticky glue that keeps the shelled organism anchored to rocks and other objects, allowing them to withstand the extreme pounding of waves. Chemical analysis of this natural, waterproof glue showed that the key to its adhesiveness is a family of unique proteins called mussel adhesive proteins, which contain a high concentration of DOPA (dihydroxyphenylalanine).

Messersmith and his colleagues currently are testing the mechanical qualities of the mussel-inspired sealant and plan to conduct in vivo experiments in animal models.

The National Institutes of Health and a pilot grant from the Northwestern University Clinical and Translational Sciences Institute supported the research.

Barnacles' sticky secret revealed

Barnacles are able to attach themselves to almost anything. They are found clinging to the hulls of ships, the sides of rock pools and even to the skin of whales. Just how they stick so steadfastly whilst underwater has remained a biochemical puzzle for scientists for many years. Now researchers have solved this mystery, showing that barnacle glue binds together exactly the same way as human blood does when it clots. Barnacles are crustaceans that live in shallow ocean environments.We've found homologous enzymes in barnacles and humans .

As larvae they affix to hard substrates, then remain stationary for the rest of their lives. To attach themselves to a surface, the barnacles secrete an adhesive substance. Scientist knew the chemical properties of this glue, but not how these chemicals interact to create a sticky effect. Now researchers reveal all in The Journal of Experimental Biology. Sticking pointActually obtaining some barnacle glue proved an initial hurdle. "No one really knew how to work with barnacle glue before this study," says Dr Gary Dickinson, a member of the research team from Duke University's Marine Laboratory in Durham, North Carolina, US. "Most people try to cut it off the bottom of a barnacle and then dissolve it, but we knew this does not work well, and this approach has limited potential," he explains.

So Dr Dickinson and his colleagues learnt how to gently remove glue from the barnacles (Amphibalanus amphitrite) as they secreted it. They were then able to deconstruct the glue to find out exactly how it works. The team initially compared the glue to another substance which clots in solution; red blood cells. They expected the mechanism by which glue particles bind, and red blood cells bind, to be different. However, they found they are remarkably similar. In blood, a number of enzymes work to create long protein fibres that bind red blood cells, or platelets, together into a clot and create a scab. Using techniques including atomic force microscopy and mass spectrometry, the team found that very similar enzymes, known as trypsin-like serine proteases, are at work in barnacle glue. One of these glue enzymes is remarkably like Factor XIII, an essential blood clotting agent in human blood.The enzymes are highly conserved because they are very effective at what they do

"We've found homologous enzymes in barnacles and humans, which serve the same function of clotting proteins underwater, despite roughly a billion years of evolutionary separation," says Dr Dickinson. However, this surprising result does make evolutionary sense, says team member Professor Dan Rittschof, also from Duke University's Marine Laboratory. "Virtually no biochemical pathway is brand new. Everything is related and really important pathways are used over and over," he explains. "Really key parts of those pathways can't change because if they do, the pathway fails and the animal dies."

Dr Dickinson believes other organisms might also use this glue. "The enzymes are highly conserved because they are very effective at what they do." "There are bound to be a number of other organisms that use the same enzymes for the same purpose," he says. His team hopes that further research might lead to a solution to the problem of marine fouling, where barnacles stick to boat hulls creating drag. Many anti-fouling compounds used to paint the undersides of boats are toxic, so Dr Dickinson's team hopes to find a more environmentally-friendly solution.

BBC