All living cells in nature display a dense and complex array of sugar chains (glycans) in their cell surface and extracellular compartments. Vertebrate glycans often terminate their outer ends with members of a family of sugars called Sialic Acids (Sias). These Sias are attached to underlying glycans in various linkages, and are modified in multiple ways. Such diversity in Sia linkages and modifications are expressed in specific patterns, which change markedly during normal and abnormal processes. The long-term goal of this grant has been to unravel mechanisms generating this Sia diversity, and to elucidate its significance in health and disease. During the last funding period we directed all attention to the human-specific genetic loss of the common mammalian sialic acid N-glycolylneuraminic acid (Neu5Gc). Given our current exciting and novel findings, we will continue this focus. Thus, the entire proposal is focused on interesting implications of the single oxygen atom that differs between Neu5Gc and its precursor Neu5Ac, some of which we have elucidated. In the current proposal we focus on our surprising discovery that human cells can take up and metabolically incorporate exogenous Neu5Gc, and the relevance of this finding to human dietary intake of Neu5Gc, which is enriched in red meats. The resulting presence of foreign Neu5Gc on the surface of some human cell types in the body is the first known example of such a process. While the extent of incorporation is small, it is relevant because all humans have a variable and complex circulating antibody response to Neu5Gc. Our initial evidence shows that this unusual combination of antigen and antibody in the same individual generates a novel form of chronic inflammation, the first example of such a process. Thus Neu5Gc loss in humans has implications ranging from biochemical, cell biological and gastrointestinal issues involving Neu5Gc uptake, incorporation and metabolism in cells and tissues, to the origins, diversity and significance of anti-Neu5Gc antibodies in humans, to the need to eliminate Neu5Gc from human tissues. Further interest arises from our unexpected finding that the N-glycolyl group derived from Neu5Gc metabolism can enter other metabolic pathways, generating previously unknown glycan structures and potential new targets for autoantibodies. Besides biochemical, cell biological and epidemiological studies of human cells and human samples, we will also use mice with a human-like defect in Neu5Gc production. We propose to complete elucidation of the metabolism of N-glycolyl groups in human and animal cells, and in such mice. We will also explore mechanisms generating the diverse and variable range of anti-Neu5Gc antibodies, including attempts to clone monoclonal human antibodies. In parallel, we will study selected consequences of interactions between anti- Neu5Gc antibodies and Neu5Gc-containing glycans in mice and humans, asking if the resulting inflammation can help explain increased risks of heart disease, cancer and diabetes associated with red meat consumption. In the long run it may become necessary to develop non-toxic ways to eliminate Neu5Gc from the body.

Public Health Relevance

Humans differ from other mammals in that our bodies are deficient in producing a type of cell surface molecule called Neu5Gc. We have made a surprising discovery that the human body incorporates Neu5Gc from dietary intake of red meats, and that antibodies against this foreign molecule generate inflammation, which may help explain the increased risk of heart disease, cancer and diabetes associated with red meat consumption. We propose a range of novel studies ranging from basic biochemical, cell biological and physiological issues, to the origins, diversity and significance of anti-Neu5Gc antibodies, to the ultimate need to eliminate Neu5Gc from the human body.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Intercellular Interactions Study Section (ICI)
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Somers, Scott D
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University of California San Diego
Other Basic Sciences
Schools of Medicine
La Jolla
United States
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Siddiqui, Shoib; Schwarz, Flavio; Springer, Stevan et al. (2017) Studies on the Detection, Expression, Glycosylation, Dimerization, and Ligand Binding Properties of Mouse Siglec-E. J Biol Chem 292:1029-1037
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