So it’s only right to be scared: ‘I don’t wanna go, who knows what’s waiting for us out there,’ cries the bespectacled spermatozoon Woody Allen in the comedy Everything You Always Wanted to Know About Sex* (* But Were Afraid to Ask). Like a prince attempting to reach the sleeping princess in her overgrown castle, sperm run the gauntlet on their journey towards the egg, advancing like unwelcome foreign bodies first through the acidic environment of the vagina and then through the mucus plug which safeguards access to the womb. This barrier is almost always too thick and dense for pathogens, and sperm too. This changes during ovulation, when a mature ovum is present and can be fertilized; pH is a crucial factor here—the more acidic the environment, the thicker the mucus.
With ovulation, the pH rises. The hydrogel in the cervix softens by absorbing excess water, it thins and becomes more penetrable. Then, and only then, are at least healthy sperm permitted to enter on their quest to find Sleeping Beauty: ‘But by this time the hundred years had just passed, and the day had come when Briar-rose was to awake again. When the king’s son came near to the thorn-hedge, it was nothing but large and beautiful flowers, which parted from each other of their own accord, and let him pass unhurt, then they closed again behind him like a hedge.’ It is a complex mode of selection, but it’s not just the female body that knows how to rig the game. The man’s seminal fluid can also raise the pH of the vagina, creating a more moderate climate for Woody and his cohort, who would otherwise find themselves stuck in the deadly environment outside the gel barricade, surviving only briefly. But what happens when the whole system fails?
Premature birth remains the leading cause of death in newborn babies and can effect severe complications for those who survive. Infections in the womb play a significant role in this. For a long time, however, it was unclear as to how the pathogens found their way into the uterus since access is blocked by the mucus plug, which is supposed to remain impermeable throughout the entire pregnancy. A study done in Ribbeck’s lab showed that the barrier wasn’t thick enough in many women with a high risk of premature birth, instead remaining as porous as it would be on fertile days. Any pathogens present would have an easy job of it unless science finds a way to strengthen the weak barrier and protect the unborn child. Maybe an as yet uninvented synthetic mucus that would have uses with other bodily barriers as well would help.
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Unlike the genital tract, the stomach was thought of as sterile for a long time. What pathogen could possibly survive and settle in the highly acidic environment of an organ whose walls are protected by a double layer of compact slime? It seemed unfeasible, but the bacterium Helicobacter pylori manages to do just that by enzymatically neutralizing the stomach acid in its immediate vicinity. It creates a mellow buffer zone whose altered pH opens up a pathway of thinning mucus in the barrier for the pathogen to swim through right up to the stomach wall. It’s an old trick to use our own biology against us, it seems, since traces of Helicobacter pylori were even found in the mummified Stone Age ice man, Ötzi.
Secreted slimes like the mucus in our stomachs are ubiquitous in animals with a diverse range of properties and functions in the body. They all contain a large proportion of so-called mucins. They are the major components of the hydrogels’ inner three-dimensional networks that bind the water. Mucins are very large, complex and—in evolutionary terms—very old molecules. A recent publication on secreted mucuses states that most higher animals express at least five individual mucin genes but can contain more than twenty; in humans, we know of twenty-two so far.
It’s the structure that makes a molecule a mucin. Whether found in humans, fish, amphibians, molluscs, birds or one of many other organisms, all mucins look alike, a bit like a bottlebrush. A single protein stretches out to form a kind of backbone, with some areas of the molecule carrying a large number of specific building blocks. These domains are characteristic of mucins and mucin-like molecules and they act as designated docking sites. Hundreds of sugar chains attach here in high density, hence the look of bristles on a handle. But these so-called glycans are a bit more complex than thin bristles: they’re long and branched chains made up of different sugar molecules. They stick out from the protein backbone and can in some mucins make up nearly 80 percent of the molecular mass.
How do cells cope with a bunch of giant bottlebrushes that dislike each other in a tight space? The sweet thicket of glycans makes mucins so bulky and unwieldy that the handling of these gigantic molecules is not entirely clear yet. One model focuses on Mucin 2 (MUC2), which builds the framework for mucus in our digestive tract. It is produced and secreted by so-called goblet cells, a name that refers to their uneven shape, with a swollen upper half crammed full of nicely packaged mucins. But how are these molecules parceled up? According to the model they team up in threes. And only in the goblet cells’ special environment will they suspend their molecular antipathy. Once new mucus is needed, the molecules are released from the cell and the shackles of enforced friendship. Now they can’t get away from each other quickly enough, unfurling and extending their dense bristles at an explosive rate—maybe a little like automatic umbrellas. In milliseconds, they can take up water and swell 3,000 times to form a hydrogel which blends seamlessly into the existing mucus.
This hypothesis has yet to be proved conclusively and the glycans’ role as part of mucins is still a long way from being fully appreciated. They’re essential for binding water and forming gels, but their significance goes beyond the realm of slime. These sugars are a central cog in the organism: alongside proteins, lipids and nucleic acids, they are one of the four major building blocks of life, but the least understood of them. For a long time, there were no methods to study these complex structures even though they look deceptively simple at first. Glycans in mammals are based on building blocks of no more than nine simple sugars. They are being combined in multiple ways into long chains—just as about twenty amino acids make up all proteins, or as a set of distinctive beads would be enough to create ever more necklaces with unique patterns. That means that in the human body alone, those few simple sugars create several tens of thousands of different complex glycans.
As if that weren’t variety enough, these chains attach themselves in various and changing combinations to mucins and many other proteins but also lipids and even specific nucleic acids, as a recent publication has shown. About that last association very little is known so far, but what is certain is that protein and lipid partners need their glycans; without them they misfold, lose stability or can’t function. Modern glycobiology might still be in its infancy compared to other disciplines. But thanks to new methods and approaches it is well equipped for the future. It is hoped that it will finally help to explain what exactly these multifaceted sugars do and what role they play in diseases. It’s a matter of deciphering whole glycomes—that is all the sugars in a cell, an organ or organism—both during good health and sickness.
Just one example: cystic fibrosis is a condition where great quantities of exceptionally sticky mucus accumulate in multiple organs. In the lungs it provides an ideal breeding ground for pathogenic bacteria, irreparably damaging the airways. In organs like the gut it impairs the uptake of nutrients, while it might make it hard or impossible for patients to conceive, possibly due to a defective barrier in the genital tract. But the lung is hit particularly hard in these cases and its mucus is insufficiently hydrated.
A mucin involved in this barrier shows a starkly altered and unique glycan pattern. That means, it displays a combination of sugar chains that has only been found in patients with cystic fibrosis so far and might change the molecule’s function, including its ability to bind water. This in turn could prevent the mucus barrier from being fully hydrated. If we understand glycans better, we might be able to help patients with this and other diseases. A deeper understanding could also pave the way for a unifying theory of biology and possibly herald a new era of modern medicine.
As we have seen, MUC2 is secreted by goblet cells to build the framework for extensive gel layers in the digestive tract, as do the majority of our mucins elsewhere in the body, in the airways or genital tract, for instance. But the secreted mucuses that coat our inner surfaces are just one type of hydrogel system in our organism. There are three more, and one of them also contains mucins, if not the gel-forming ones that are secreted by goblet cells. These mucins are not fully released from their cell but remain anchored to its membrane as part of a team of proteins and lipids that stay attached but reach out to the cell’s environment.
Most or even all of them carry glycans, with just a few to tens of thousands of sugary building blocks per chain. They form a dense layer of sugars that covers the cell’s surface. Interwoven with them are additional free glycans and together they are the so-called glycocalyx—literally ‘sweet husk’—of the cell. It is a universal feature, as a recent paper states: ‘Every cell in the human body—endothelial cells, immune cells, muscle cells, blood cells, neurons, and all others—exhibits a glycocalyx,’ writes the researcher Leonhard Möckl at the Max-Planck-Institute for the Science of Light in Erlangen, Germany.
He adds: ‘The latest research has shown that the glycocalyx is an organelle of vital significance, actively involved in and functionally relevant for various cellular processes, that can be directly targeted in therapeutic contexts.’ In his view the sweet husk is a ‘fundamental cellular agent’. But is it also a slime? Only to a certain extent. A robust glycocalyx might rise high from a cell’s surface and resemble thick woods in profile but it is not a regular network. There is some structure and connection, but no three-dimensional framework as in mucus. But still, as Möckl mentions, the first identified function of the glycocalyx was protection, and this is thanks to its function as a physical barrier for any object to enter the cell: ‘The glycocalyx is a dense, gel-like meshwork that surrounds the cell.’
Millions of glycans can make up the unique glycocalyx of each cell. It works a bit like a molecular barcode to help our immune system; for example, to differentiate between friends, like regular body cells, and microbial foe. As with treacherous slime trails, being easily identifiable has a downside too: viral pathogens, for instance, enter host cells that they target via their telltale glycocalyces. In fact, the cellular glycocalyx has received a lot of attention lately. The coronavirus that caused a pandemic in 2020 docks on to human cells that carry the heavily glycan-adorned receptor ACE2. This molecule is found on the surface of many different cells from the heart to the gut, lung, kidney, brain, testis and blood vessels—which makes the Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) so much more dangerous.
How can that be remedied? Blocking ACE2, which is indispensable for different cellular functions, seems impossible. But maybe the virus could be lured from its destructive path before it docks on? In some labs around the world synthetic mimics of the cellular receptor are now in the works, a soluble ACE2-decoy that could potentially be taken up to fool the virus into assuming it had found a target cell while never coming close to any tissue, only to be removed by the body’s defenses. It’s an elegant and presumably old trick: other molecular receptors that are routinely targeted by dangerous viruses when they hit our inner surfaces are thought to be secreted as decoys into the extracellular mucus as well.
Mucins might be among them, but they also fight back in different ways. The membrane-bound variants on inner surfaces such as the digestive tract are thought to be able to release their outer part once a pathogen is attached. And their size seems to matter as well, when the towering mucins shadow underlying receptors and protect them from any pathogen’s grasp. Another stage of tricks and betrayal features Sleeping Beauty and her undaunted prince: sperm are surrounded by a thick coating which protects them on their perilous journey. But identification counts here too, and their sweet gel might prove single sperm cells as suitable candidates. Egg cells wear an elaborate coat of glycans which acts as a last hurdle for any lovelorn spermatozoon, proving him as a healthy candidate from the right species. A real prince, not a frog stuck in disguise.
Excepted from Slime: A Natural History by Susanne Wedlich; published by Melville House, 2023.
