Rieko Imae
Research Team for Mechanism of Aging
◆Introduction
Glycans are chains of sugar units, such as glucose and galactose (Fig. 1).
Fig. 1. Different structures of glycans found in the body.
Glycans contain approximately 10 different types of sugar, and their structures are highly diverse owing to differences in the types of sugars involved, mode of linkage, and length. Glycans, along with nucleic acids and proteins, are important biological components, referred to as the third chain molecule, and they often bind to proteins and lipids and alter their properties, thereby regulating their stability and functions. Moreover, some, such as hyaluronic acid, exist as single glycans. Unlike nucleic acids and proteins, glycans do not have templates (blueprints), and therefore, the structures of glycans synthesized within cells vary depending on the environment in the body (such as aging, stress, and the nutritional state). With recent studies, we have begun to understand that the structure and amount of glycans actually change with aging (Miura and Endo 2016; Cindrić et al. 2021). Changes in glycans can lead to altered functions of proteins and lipids, increasing the risk of age-related diseases. However, the overall picture of age-related changes in glycans and the occurrence mechanisms underlying their changes are not well understood.
◆What are nucleotide sugars?
During the synthesis of glycans in the body, sugar units, such as galactose, are added individually by enzymes called glycosyltransferases. However, since glycosyltransferases cannot directly use sugar units, they must utilize "nucleotide sugars," which are formed by a sugar unit linked to a nucleotide (Fig. 2).
Fig. 2. Nucleotide sugars serve as raw materials for synthesizing glycans. (Top) An example of a glycan synthesis reaction. Glycosyltransferases synthesize glycans by linking sugar units one after the other from nucleotide sugars, which act as sugar donors (donor substrates). As an example, the figure shows the reaction in which uridine diphosphate-galactose (UDP-galactose) is used as a nucleotide sugar to link galactose. (Bottom) Structure of UDP-galactose.
Nucleotides are the building blocks of nucleic acids, such as DNA and RNA, and they are composed of a base (guanine, cytosine, uracil, etc.), a pentose sugar (ribose, etc.), and a phosphate group. The nucleotides found in nucleotide sugars include uridine diphosphate (UDP), guanosine diphosphate (GDP), and cytidine monophosphate (CMP). Because the production of nucleotide sugars is closely related to the metabolism of various substances, such as sugars, amino acids, lipids, and nucleic acids (Fig. 3), the amount and ratio of nucleotide sugars produced are likely affected by the environment in which metabolism changes, such as diet, aging, and disease.
Fig. 3. Synthetic pathways for nucleotide sugars. The nucleotide sugars analyzed in this study are enclosed in red boxes.
Changes in the amount of nucleotide sugars, which serve as raw materials, are, of course, expected to affect the synthesis of glycans, altering their structure and the amount produced. Based on this, we investigated the relationship between aging and the amount of nucleotide sugars.
◆Measurement of nucleotide sugars in mouse organs
We examined the amounts of nucleotide sugars present in various organs of mice (Imae et al. 2024). As we have recently discovered new nucleotide sugars in mammals (CDP-ribitol and CDP-glycerol) (Kanagawa et al. 2016; Imae et al. 2021), we developed a new measurement method to analyze all nucleotide sugars, including those. Because nucleotide sugars are present in trace amounts in organs, we developed a method for measuring nucleotide sugars using liquid chromatography-mass spectrometry (LC-MS), which enables highly sensitive and accurate measurements. In LC-MS, substances are separated via liquid chromatography, based on the difference in the interaction between the stationary phase and the mobile phase, and the masses of the substances are then measured using a mass spectrometer. With this approach, we constructed an analytical system capable of measuring 12 different nucleotide sugars (UDP-glucose, UDP-galactose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, UDP-glucuronic acid, UDP-xylose, GDP-mannose, GDP-fucose, CMP-N-acetylneuraminic acid, CMP-N-glycolylneuraminic acid, CDP-ribitol, and CDP-glycerol). Using this technique, we measured the amounts of nucleotide sugars in seven organs (brain, liver, heart, skeletal muscle, kidneys, lungs, and large intestine) collected from 7-month-old (young) and 26-month-old (old) male mice. Our analysis showed that there were differences in the amounts and compositions of nucleotide sugars among these organs, regardless of whether the mice were young or old. For example, we found that UDP-glucuronic acid was abundant in the kidneys, whereas UDP-glucose was abundant in the brain. These differences among the organs likely reflect differences in the types and amounts of glycans needed in each organ.
◆Age-related changes in the amounts of nucleotide sugars in mouse organs
Our analysis revealed the characteristic age-related changes in nucleotide sugars in each of the seven organs mentioned previously herein. In particular, the kidneys showed a significant decrease in UDP-glucuronic acid, and the large intestine exhibited a decreasing trend for UDP-glucuronic acid. Furthermore, the brain showed a decrease in UDP-N-acetylgalactosamine, as well as a decreasing trend for UDP-glucose, UDP-galactose, and UDP-N-acetylglucosamine (Fig. 4).
Fig. 4. Age-related changes in the amounts of nucleotide sugars in the kidneys, large intestine, and brain of mice. The kidneys showed a significant decrease in UDP-glucuronic acid, and a similar trend was observed in the large intestine. The brain showed a decrease in UDP-N-acetylgalactosamine, as well as a decreasing trend for UDP-glucose, UDP-galactose, and UDP-N-acetylglucosamine. n=4, *P<0.05.
As mentioned, UDP-glucuronic acid was abundant in the kidneys, whereas UDP-glucose was abundant in the brain, and interestingly, these nucleotide sugars decreased with aging. Some nucleotide sugars are abundant in certain organs, likely because they allow for the synthesis of the large amounts of glycans needed by the organs. This suggests that a decrease in such nucleotide sugars with aging may prevent the synthesis of the necessary amounts of glycans, leading to a decline in organ functions.
◆Future developments
Our study has revealed that the amounts of nucleotide sugars in various organs change with aging. Such changes in nucleotide sugars may cause age-related changes in glycans. For example, UDP-glucuronic acid is used to synthesize glycans, called glycosaminoglycans (such as hyaluronic acid, heparan sulfate, and chondroitin sulfate), which are components of the extracellular matrix; the amount of heparan sulfate in the kidneys was reported to decrease with aging (Murata and Horiuchi 1978; Vasan et al. 1983), and this may be caused by the decrease in UDP-glucuronic acid in aged kidneys. Additionally, UDP-N-acetylgalactosamine, UDP-glucose, and UDP-galactose levels were found to decrease in the aged brain, and these are used to synthesize gangliosides (sphingolipids containing sialic acid), which are glycolipids found in abundance in the brain; the amount of ganglioside in the brain was reported to decrease with aging (Segler-Stahl et al. 1983; Kracun et al. 1991), and this may be associated with the decrease in UDP-N-acetylgalactosamine, UDP-glucose, and other nucleotide sugars in the aged brain. In the future, we hope to clarify how glycans change as a result of changes in the nucleotide sugars that we discovered and how those changes in glycans are associated with the age-related decline in organ functions. We believe that the elucidation of new molecular mechanisms of aging will lead to the extension of a healthy lifespan.
References
Related article
Press release (Japanese)
The amount of "nucleotide sugars," materials for glycan synthesis, changes in the organs of aged mice
( 加齢マウス臓器では糖鎖合成の材料である "糖ヌクレオチド"の量が変化する )
