Existing by decorating cells, proteins, lipids, and even RNA, sugars are one of the most common substances in biology. However, these carbohydrates, also known as glycans, receive relatively little attention in research, one reason for which is the lack of antibodies and other reagents for glycans compared to those for proteins and nucleic acids.
Introduction to Glycans
It's widely accepted that the structural complexity of glycans is mind-boggling due to various linkages of monosaccharides, branching structures, and enzymes, for which researchers would rather study neatly encoded biomolecules like DNA, RNA, and proteins than complex glycans. Nevertheless, the influence exerted by glycans in biological activities can't be overlooked.
For instance, studies have proven that glycans can facilitate intercellular communication and immune responses; are altered in cancer cells; and are exploited by pathogens to get a foothold in host cells, all of which could provide significant biological information in related research. Moreover, it's the slight differences of glycans on red blood cells that enable us to define human blood as type A, B, AB, or O. More recently, scientists find that the SARS-CoV-2 spike protein utilizes glycans to enter cells and thwart the host immune response, which provides insights for the development of antibodies and small molecules as well as vaccine design.
Reagents for Glycans Analysis
Most reagents for glycans analysis are less than optimal, though a few do have the glycan-binding affinity. For instance, lectins, the natural proteins from diverse organisms that bind to carbohydrates, have the leading affinity reagents for glycans. Given the important role of protein glycosylation in normal and disease-related processes, antibodies to lectins are often used to recognize and localize protein with lectin activity as biomarkers for disease detection.
However, the downside of lectins is that they are not specific to individual glycans and have a shallow binding interface with glycan targets. Therefore, besides engineered lectins, glycobiologists have long been looking for reagents that enable assays, western blots, cell stainings, and other experiments for glycans, just as those that antibodies have enabled for protein studies. Some scientists are attempting to resort to anti-glycan antibody detection services, and some turn to improve these reagents with downsides. Either way, these new reagents for glycan analysis have the potential to make glycobiology experiments more straightforward than before.
Development of Anti-glycan Antibodies
Theoretically, conventional antibodies could be generated as analytic reagents for glycans, but it turned out that most antibodies rarely bind tightly to their target antigens and may cross-react with other molecules. In some depth, this has discouraged the idea of generating antibodies for carbohydrates. However, later research in 2000 has proven that an abundance of specific and high-affinity antibodies to various carbohydrates could be found in jawless sea lampreys (Petromyzon marinus), a snakelike creature with toothy, round mouths in the sea.
By comparing the immune system of Petromyzon marinus with those of jawed creatures, researchers discovered that the antibodies of jawless sea lampreys have different forms with a crescent-shaped binding site. It indicates that their antibodies can distinguish and more rigidly and specifically bind to intended antigens than mammalian antibodies, even though these targets look the same. This finding provides insights for carbohydrate antibody engineering and shows that making anti-glycan antibodies to distinguish many glycans is possible.
To make the Petromyzon marinus molecules more convenient for laboratory use and enable their further therapeutic applications, researchers have designed hybrid molecules that combine the mammalian antibody base—the vertical section of the Y-shaped structure—with the unique binding domains of Petromyzon marinus antibodies. These novel intelligent anti-glycan reagents, which can distinguish different glycans, can be detected using the secondary antibodies that researchers have already used to identify conventional antibodies.
