Pablo Argüeso

Sugars on the Eye’s Surface

Dr. Pablo Argüeso received his PhD (Biochemistry) in Spain in 1997. After working briefly at the Institute for Cancer Research at The Norwegian Radium Hospital in Oslo, he moved in 1998 to the laboratory of Dr. Ilene Gipson at the Schepens as a postdoctoral fellow. He is currently a faculty member (Assistant Scientist) at the Schepens and Assistant Professor of Ophthalmology at Harvard Medical School. His research focuses on one of the last frontiers of molecular biology—glycobiology—analyzing the sugars that coat the surface of the eye and deciphering their role in protecting the eye against dehydration and pathogen invasion. 

What is glycobiology?

“Glyco” derives from the Greek word "glykos”—meaning “sweet”—and refers to sugars or carbohydrates. The term glycobiology was originally coined in 1988 and recognizes the discipline of biology that studies the structure and function of the carbohydrate chains (also called glycans) present in all living organisms. In humans, there are ten major sugar molecules that combine in multiple conformations among themselves, and then with proteins and fats, to create a vast array of glycoconjugates that help determine the fate of our cells. Unlike the case with the genetic code, there is no a universal code that predicts the structure of each carbohydrate chain and, therefore, the study of glycans has lagged far behind those of other major classes of biological polymers.

Glycobiology is one of the most rapidly expanding fields in the biomedical sciences. Why?

Carbohydrates have been traditionally considered only as sources of energy for the living organism. However, during the last few years, it has become evident that carbohydrates also play important roles in determining cell function. The large number and variety of carbohydrate chains that are located on cell surfaces modulate a wide variety of cell-cell and cell-pathogen interactions. This communication results in a varied spectrum of cellular events, such as secretion of bioactive substances, recruitment of immune cells in areas of cellular damage, and cancer cell metastasis.

Recent progress in the field of glycobiology is being facilitated by the development of new methodologies for the high-throughput analysis of glycans—a type of study known as glycomics. In 2001, the NIH’s National Institute of General Medical Sciences awarded a $34 million grant to form the Consortium for Functional Glycomics, whose goal is to develop and disseminate resources to researchers who are defining the paradigms by which carbohydrates function in cellular communication. As a result, the discipline of glycobiology is growing exponentially, with an increasing number of investigators entering the field as well as biotech and pharmaceutical companies investing in it.

Can glycomics be applied to the study of the surface of the eye?

Similarly to other mucosal surfaces, such as the gastro-intestinal tract or the mouth, the ocular surface contains large amounts of the molecules known as membrane-associated mucins. Due to their extremely large size, they extend far away from the cell surface, forming a protective barrier (see the Figure) that is thought to prevent desiccation and to protect the eye against infection resulting from its continuous exposure to pathogenic microorganisms. Historically, it has been very difficult to analyze these molecules, because they are heavily glycosylated—up to 60% of a mucin molecule’s mass is sugars, the remainder being protein. Glycomics has provided the technology necessary to analyze the fine structure of these molecules and to test different hypotheses about their protective function.

What is known about the glycan composition of mucins on the ocular surface?

Data from our laboratory have shown that the glycan structure of mucins at the ocular surface is different from that at other human mucosal surfaces; this difference may reflect specific requirements of the eye. In this regard, ocular mucins seem to have unique carbohydrate arrangements and modifications at the end of the glycan chain (red spheres in the Figure). These unique structures, which are in close proximity to the external milieu, may represent a defense mechanism, such that pathogens surveying the surface of the eye would encounter these atypical glycans rather than binding receptors that would allow colonization. This is a hypothesis that we are currently testing in the laboratory.

Are carbohydrates altered in ocular surface disease?

Yes. In patients with dry eye—a disease that affects 10 to 15% of middle-aged and older Americans—there is an alteration in the enzymatic machinery used by the cell to add a sugar known as N-acetylgalactosamine to mucins. Since carbohydrates are specialized molecules that retain water, we have hypothesized that their relative absence in dry-eye patients may contribute to the desiccation of the ocular surface. Interestingly, in these patients, there is an initial attempt by the eye to produce more of the required enzyme and, therefore, to add more N-acetylgalactosamine to mucins, but the battle is lost as the levels of enzyme decrease and the ocular surface dries out.

We are currently using high-throughput technology to obtain a more detailed picture of the glycosylation changes that occur with ocular surface disorders. This is being performed using microarrays—provided by the Consortium for Functional Glycomics—that will monitor the expression of approximately 2,000 human genes relevant to carbohydrate biology.

Can diseases be treated with therapies based on carbohydrate technology?

Yes. An example is a pathology known as “congenital disorder of glycosylation type Ib,” in which the defective synthesis of glycans causes gastrointestinal disorders and, in some instances, life-threatening bleeding. This disorder is currently treated with oral administration of mannose, the particular sugar that is lacking in this disease. In another application, pharmaceutical companies are attaching glycans to drugs to improve their efficacy and decrease the likelihood of side effects. We hope that research on the carbohydrate composition of the ocular surface will not only provide new information on ocular surface biology and disease, but will also help to develop new approaches to prevent and cure ocular surface disorders.

Carbohydrates (colored spheres) are attached to the cell surface by different proteins (ribbons). Mucin carbohydrates extend far away from the cell membrane and act as a protective layer that senses the external milieu. Underneath the mucin barrier, and close to the cell membrane, there are smaller glycoconjugates that participate in the normal functioning of the cell.