Medical Applications of Glycomaterials
Authors: Gregory A. Hudalla
Journal: Advanced Healthcare Materials (2022)

Carbohydrates are the most abundant organic molecules on Earth. Chains of covalently-linked carbohydrates, or glycans, decorate the surfaces of cells found throughout the various kingdoms of life as well as viruses, and are abundant within the extracellular matrices of multicellular organisms. In human biology, glycans participate in nearly all aspects of life, as exemplified by numerous disorders associated with deficient glycan synthesis. In general, glycans can be subdivided into long, often repeated structures, referred to as polysaccharides, or short, complex structures, referred to as oligosaccharides. Polysaccharides are key structural components of plant cell walls and vegetable fibers (cellulose), arthropod exoskeletons (chitin), bacterial biofilms (polysaccharide intercellular adhesin), and mammalian tissues (glycosaminoglycans), among others. For example, the glycosaminoglycan known as hyaluronic acid (HA) is abundant in articular cartilage and synovial fluid, where it acts as a lubricant, shock absorber, regulator of water content, and contact site for cell adhesion. Oligosaccharides, on the other hand, are generally found attached to proteins (i.e., glycoproteins) or lipids (i.e., glycolipids), where they impact the structure, solubility, and stability of the carrier molecule onto which they are attached. Oligosaccharides are also key mediators of cell-to-cell communication through their ability to bind to a broad class of soluble and membrane-anchored proteins known as lectins. The diverse biological activity of oligosaccharides arises from their extraordinary chemical complexity — the 10 different monosaccharides found in humans could give rise to ∼1010 different hexasaccharide glycans by way of linkages formed via the anomeric carbons or various hydroxyls, which leads to different linear and branched structures, as well as the α and β stereochemistry of the glycosidic bond. Additional glycan modifications, such as methylation, sulfation, phosphorylation, and acetylation, broaden this chemical diversity even further. Naturally derived polysaccharides have a long and well-established history as biomaterials for medical applications, while oligosaccharides now receive considerable attention as active components of biopharmaceuticals. A new frontier of glycan-modified biomaterials, or glycomaterials, has emerged through advances in our understanding of glycans, coupled with the development of sophisticated new techniques to identify, characterize, and synthesize them. This special section, Medical Applications of Glycomaterials, presents a collection of articles demonstrating the enormous potential of glycomaterials for lectin detection, studies of cancer cell behavior, and controlling growth factor signaling in stem cells. Lateral flow devices (LFDs) are an effective, low-cost option for point-of-care diagnosis of infection and detection of pregnancy. In typical LFD designs, a clinical sample is flowed over a paper strip that has been functionalized with a molecule that possesses binding affinity for the analyte of interest. Concurrently, a mobile “signal generating substance” phase that also possesses binding affinity for the analyte of interest is flowed over the strip. If analyte is present in the sample, it becomes sandwiched between the mobile and paper-bound stationary phases, leading to detectable accumulation of the signal generating molecule at the location of the stationary phase (i.e., a positive result). Conventional LFDs employ immobilized antibodies in the stationary and mobile phase due to the high specificity and selectivity of antibody-analyte interactions. Alternatively, recent work demonstrates that immobilized lectins can be employed for detection of glycans, or conversely immobilized glycans for detection of lectins, thereby expanding the range of analytes that can be assayed using LFDs. Developing a successful LFD requires insights into the features of the stationary and mobile phases that impact device performance, such as the signal-to-noise ratio and the analyte detection limit. Toward this end, Matthew I. Gibson and colleagues created a mobile phase based on gold nanoparticles modified with glycopolymers and studied the effect of glycan density, polymer length, and nanoparticle diameter on the performance of lectin detection LFDs (2101784). Short polymers increase non-specific binding, while long polymers reduce non-specific binding at the expense of specific binding. Smaller gold nanoparticles are more effective than larger gold nanoparticles, because the latter amplify both specific and non-specific binding. Notably, the optimal polymer length depends on the lectin analyte; a longer glycopolymer is ideal for detection of Ricinis communis agglutinin I, while a shorter glycopolymer is most effective for detection of soybean agglutinin. Optimized LFDs detect lectins at nanomolar concentrations, which is competitive against conventional antibody-based devices. Tools to understand dysregulated signaling events that occur between cancer cells and the tumor extracellular matrix are important for understanding cancer progression and identifying possible therapeutic interventions. Aberrant glycosylation of the cell membrane and the extracellular matrix are hallmarks of many tumor microenvironments. For example, Chaitanya Tondepu and Lohitash Karumbaiah provide a detailed survey of glycosylation in glioblastoma, which outlines known changes in glycan structure and signaling associated with the disease, the impact of these changes on immune response and cell invasiveness, and emerging glycomaterial tools that can be used to study the role of aberrant cell and matrix glycosylation in disease (2101956). The synthesis and degradation of the glycosaminoglycan HA is disrupted in many tumors, including glioblastoma and breast cancer. Dysregulated HA homeostasis leads to the accumulation of oligosaccharide fragments that can alter matrix mechanics and perturb signaling via HA-binding cell surface receptors. Of particular interest in cancer progression is the interaction between dysregulated tumor HA and its receptor CD44, which is commonly overexpressed by cancer cells. Natively, HA is partially immobilized on the cell surface via interactions with hyaluronan-binding proteins or within the extracellular matrix via interactions with other matrix components. Recent work demonstrates that the signaling activity of HA depends on its presentation, with electrostatic interactions, chemical crosslinkers, or natural HA-binding proteins showing promise for maintaining HA bioactivity in synthetic systems. However, conventional cell culture models used to study HA signaling have typically relied on exogenously supplied HA in solution, which starkly contrasts with its natural presentation. To address this gap, Iva Pashkuleva and co-workers created a synthetic culture platform to present immobilized HA gradients to breast cancer cells (2101309). The platform is based on gold nanoparticles modified with HA fragments which are immobilized onto glass coverslips via diffusional deposition. The benefit of this approach is that it allows for analysis of cell-HA interactions using conventional imaging modalities. Using this platform, they observed that increasing HA density increases the adhesion, motility, and CD44 expression of CD44-expressing breast cancer cells relative to cells lacking CD44. Beyond cancer cells, emerging glycomaterial tools are increasingly being used to study signaling events in stem cells. Glycosaminoglycans, such as heparan sulfate, are commonly employed as bioactive components of biomaterials for tissue engineering and regenerative medicine because of their ability to regulate the activity of various growth factors that signal changes in stem cell phenotype and function. However, heparan sulfate chemistry is quite complex — the core structure is a repeated disaccharide consisting of N-acetylglucosamine and either glucuronic acid or its epimer, iduronic acid, any of which can be sulfated to confer specificity and selectivity for binding to different growth factors. Further, extracellular matrix polysaccharides are notoriously difficult to extract or synthesize, although considerably recent progress has been made with regard to the latter. Nonetheless, our understanding of the structure-function relationships that link heparan sulfate chemistry with stem cell behavior lags far behind that of other extracellular matrix features. To address this gap, Kamil Godula and colleagues developed an immobilized glycosaminoglycan array that enables analysis of growth factor signaling during stem cell culture (2101232). Using heparan sulfate variants that are either natively sulfated or selectively 6-O-desulfated, 3-O-desulfated, or N-desulfated they demonstrate that binding of fibroblast growth factor-2 to this synthetic extracellular matrix decreases with desulfation. Further, array spots presenting sulfated heparan sulfate restore fibroblast growth factor-2 signaling in embryonic stem cells that are incapable of synthesizing their own heparan sulfate chains, similar to supplementing the culture with soluble heparan sulfate. This collection of articles highlights the enormous potential of glycomaterials for medical applications, ranging from the development of LFDs to the creation of in vitro culture tools to study stem and cancer cell behavior. Each of these examples was enabled by recent progress in methodologies to identify, characterize and synthesize glycans. The intersection of these methodologies with materials science is providing new capabilities to advance our understanding of the roles of glycans in biology, in turn harboring new opportunities to leverage glycans in therapeutic and diagnostic applications. I hope that this special section inspires researchers in the areas of biomaterials and glycobiology to join forces to drive the continued development of this exciting new research frontier. I would like to thank all of the authors for their tremendous work in preparing the articles for this special section, as well as the reviewers, editors, and Wiley-VCH staff who provided critical feedback and assistance along the way. I would also like to extend my gratitude to Advanced Healthcare Materials Editor-in-Chief, Dr. Uta Göbel, for recognizing the importance of glycomaterials and providing a home for this special section. Gregory A. Hudalla is an associate professor in the J. Crayton Pruitt Family Department of Biomedical Engineering in the Wertheim College of Engineering at the University of Florida. He received his Ph.D. in biomedical engineering from the University of Wisconsin in 2010 and was an NIH NRSA Postdoctoral Fellow at the University of Chicago and Northwestern University from 2010–2013. His research interests include supramolecular biomaterials, lectin-glycan interactions, and molecular engineering.