Progress in Glycobiology: Building on Foundations and Continuing Momentum
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Despite its role in countless biological processes in health and disease, the field of glycobiology remains largely outside the global spotlight. But recent discoveries have continued to highlight the importance of the glycome, revealing the vast potential of glycan identification and modification for the diagnosis and treatment of disease. Fully unraveling the dynamic complexities of the glycome remains challenging for several reasons. Glycosylation is among the most common post-translational modifications of proteins, and glycans have many diverse structures as well as being part of other glycoconjugates like lipids and RNA. Novel approaches have expanded our ability to characterize glycans, understand their mechanistic impact on biological processes, and explore methods to leverage these key biomolecules in novel therapies.
This article walks through some of the foundational developments in the field of glycobiology that have powered the progress made in 2022 and what these advancements signal for the future.
New capabilities bring us closer to embracing the glycome
Novel bioinformatics tools, including computational methods for predicting glycan structure and function, are laying the groundwork for our ability to study the glycome. The growth of multiomics is a valuable development in advancing glycobiology, bringing attention to the glycome amidst more widely studied “-omes”. Glycan structure, function and synthesis are incredibly dynamic and depend on a complex interplay of biological processes, making multiomics an ideal approach for understanding the relationship between changes in the cell and changes in the glycome.
SUGAR-seq (SUrface-protein Glycan And RNA-seq), a multimodal method for single-cell analysis of the transcriptome and surface N-linked glycosylation, was reported in 2021. This approach was the first to tie the transcriptome to a cell’s glycan profile, giving scientists a route to identify mechanistic links between the two. A 2022 study leveraged SUGAR-seq data with a deep-learning model to predict glycan profiles in mouse T lymphocytes. Further analyses identified genes correlated with high or low glycan expression, which also impacted processes regulating T-cell differentiation and function. These findings illustrated the potential of single-cell multiomics for understanding how gene expression patterns influence heterogeneity in glycosylation and, in turn, impact vital biological processes.
Unlocking new approaches to cancer research, diagnosis and treatment
Changes in glycosylation are often a fingerprint of disease states, and cancer is no exception. Researchers have made slow but steady progress in uncovering cancer-associated changes in glycosylation and the role of glycans in tumor initiation and progression. This relationship has formed the backbone for new approaches to diagnosing and treating cancer. For example, changes in serum immunoglobulin (namely IgG) glycosylation patterns present a new class of biomarkers for colorectal, breast, prostate, lung and other cancers. By focusing on these glycan targets, researchers may be able to develop highly specific and non-invasive diagnostic methods for a wide range of cancers.
These aberrant glycosylation patterns are also a viable target for novel cancer therapies such as antibody-drug conjugates, CAR T-cell therapies, and immune checkpoint inhibitors. Immune checkpoint inhibitors such as pembrolizumab (Keytruda) and atezolizumab (Tecentriq) have significantly improved patient outcomes, but new approaches are necessary to achieve long-term benefits for a majority of patients. Studies of therapies targeting glyco-immune checkpoints, glycan binding sites that can regulate immune responses, show promising results in promoting anti-tumor immunity. Additionally, novel antibodies against tumor-specific glycans could improve CAR T-cell therapies. One example is CART-TnMUC1, an investigational therapy targeting a tumor-associated alternate form of the glycoprotein MUC1. A Phase 1 trial of CART-TnMUC1 recently completed enrollment and, while results have yet to be reported, preliminary findings show no evidence of off-tumor toxicity or safety concerns. As researchers gain a better picture of glycobiology in disease states, the field can leverage a range of new targets like TnMUC1 for diagnostics and drugs.
Glycobiology’s pivotal role in immune function
We’ve known for years that the interactions of glycans and lectins, carbohydrate-binding proteins, are key in infection and immunity. Cell surface glycan profiles are a major component of “self” versus “other” determination, as glycan structures are more species-specific than proteins, nucleic acids or other biomolecules. In the mammalian immune system, lectins recognize pathogen-specific glycans and trigger immune responses, and lymphocytes’ glycan profiles are used to direct their migration to sites of inflammation.
The COVID-19 pandemic ignited new interest in the role of glycosylation in viral infection. Glycosites on the SARS-CoV-2 spike (S) protein shield critical areas from immune recognition, influence sensitivity to neutralizing antibodies, and stabilize the receptor-binding domain in an “up” conformation that enables binding to ACE2. In turn, ACE2 receptor glycosylation on host cells can also impact viral attachment and infectivity. These variables offer many avenues for preventing and treating SARS-CoV-2 infection. One 2022 study demonstrated the potential of developing a “pan-coronavirus” inhibiting protein from a surprising source: bananas. The researchers engineered a banana lectin (BanLec) to recognize high-mannose glycans on viral surfaces while minimizing lectin-induced mitogenicity. H84TBanLec demonstrated potent in vitro and in vivo activity against high-mannose viruses, including human immunodeficiency virus (HIV), Ebola virus, MERS-CoV and SARS-CoV-2. While therapeutics employing BanLec are unlikely to hit the market any time soon, this exciting finding underscores the potential of glycobiology-focused interventions in infectious disease.
A glycobiologist in the spotlight
The 2022 Nobel Prize in Chemistry was awarded to three scientists for the development of bioorthogonal chemistry and click chemistry. While this doesn’t sound like an impactful event for glycobiology, among the winners was Dr. Carolyn Bertozzi, a Stanford University professor who has long been a leading expert in the field. In the early 2000s, Bertozzi began to leverage click chemistry in living cells, developing bioorthogonal reactions that can take place in vivo without disrupting cells’ normal biochemistry. This allowed her lab to branch further into methods for exploring and characterizing cellular structure and function, including a dynamic imaging approach for glycans and lipids in living cells. More recent research has built on bioorthogonal reaction-based glycan imaging using bioorthogonal glycoprotein tags to distinguish cell line-specific glycosylation sites.
Bioorthogonal reactions have also garnered attention as a novel approach to developing anti-cancer therapeutics and other targeted drugs. Recent studies have demonstrated how bioorthogonal glycan labeling and click chemistry can be used to visualize tumor-specific glycans across cancer subtypes and precisely deliver therapeutics. Bertozzi’s Nobel win also represents a broader potential for the study of glycobiology, bringing additional awareness of the field through association with her success. The spotlight placed on click chemistry in the context of probing cell surface glycans may inspire an uptick in this application, further empowering researchers to unravel the complexities of the glycome.
Looking to the future of the field
The growing body of glycobiology literature shows that the field is undeniably gaining momentum. We’re beginning to see that, time and again; glycobiology has the potential to be a powerful lever in advancing diagnostics, drug discovery and more. While some in the field believe it’s approaching “critical mass”, there’s still progress to be made in appreciating the importance of glycobiology and developing more advanced tools. It may be helpful for researchers to view glycobiology not as a field of study unto itself but as a critical component of the systems where they already focus. As more tools are developed to study glycosylation, we can truly appreciate the extent of its influence. Locating a treasure trove is only half the battle— without the proper tools to uncover and examine what lies beneath, we can never know the true value of what we’ve found.