The sugar code: Why glycans are so important

The cell surface is the platform for presentation of biochemical signals that are required for intercellular communication. Their profile necessarily needs to be responsive to internal and external factors in a highly dynamic manner. The structural features of the signals must meet the criterion of high-density information coding in a minimum of space. Thus, only biomolecules that can generate many different oligomers ('words') from few building blocks ('letters') qualify to meet this challenge. Examining the respective properties of common biocompounds that form natural oligo- and polymers comparatively, starting with nucleotides and amino acids (the first and second alphabets of life), comes up with sugars as clear frontrunner. The enzymatic machinery for the biosynthesis of sugar chains can indeed link monosaccharides, the letters of the third alphabet of life, in a manner to reach an unsurpassed number of oligomers (complex carbohydrates or glycans). Fittingly, the resulting glycome of a cell can be likened to a fingerprint. Conjugates of glycans with proteins and sphingolipids (glycoproteins and glycolipids) are ubiquitous in Nature. This implies a broad (patho)physiologic significance. By looking at the signals, at the writers and the erasers of this information as well as its readers and ensuing consequences, this review intends to introduce a broad readership to the principles of the concept of the sugar code.

Rapid regulation of sialidase activity in response to neural activity and sialic acid removal during memory processing in rat hippocampus

Sialidase cleaves sialic acids on the extracellular cell surface as well as inside the cell and is necessary for normal long-term potentiation (LTP) at mossy fiber-CA3 pyramidal cell synapses and for hippocampus-dependent spatial memory. Here, we investigated in detail the role of sialidase in memory processing. Sialidase activity measured with 4-methylumbelliferyl-α-d-N-acetylneuraminic acid (4MU-Neu5Ac) or 5-bromo-4-chloroindol-3-yl-α-d-N-acetylneuraminic acid (X-Neu5Ac) and Fast Red Violet LB was increased by high-K⁺-induced membrane depolarization. Sialidase activity was also increased by chemical LTP induction with forskolin and activation of BDNF signaling, non-NMDA receptors, or NMDA receptors. The increase in sialidase activity with neural excitation appears to be caused not by secreted sialidase or by an increase in sialidase expression but by a change in the subcellular localization of sialidase. Astrocytes as well as neurons are also involved in the neural activity-dependent increase in sialidase activity. Sialidase activity visualized with a benzothiazolylphenol-based sialic acid derivative (BTP3-Neu5Ac), a highly sensitive histochemical imaging probe for sialidase activity, at the CA3 stratum lucidum of rat acute hippocampal slices was immediately increased in response to LTP-inducible high-frequency stimulation on a time scale of seconds. To obtain direct evidence for sialic acid removal on the extracellular cell surface during neural excitation, the extracellular free sialic acid level in the hippocampus was monitored using in vivo microdialysis. The free sialic acid level was increased by high-K⁺-induced membrane depolarization. Desialylation also occurred during hippocampus-dependent memory formation in a contextual fear-conditioning paradigm. Our results show that neural activity-dependent desialylation by sialidase may be involved in hippocampal memory processing.

Role of Sialidase in Long-Term Potentiation at Mossy Fiber-CA3 Synapses and Hippocampus-Dependent Spatial Memory

Sialic acid bound to glycans in glycolipids and glycoproteins is essential for synaptic plasticity and memory. Sialidase (EC 3.2.1.18), which has 4 isozymes including Neu1, Neu2, Neu3 and Neu4, regulates the sialylation level of glycans by removing sialic acid from sialylglycoconjugate. In the present study, we investigated the distribution of sialidase activity in rat hippocampus and the role of sialidase in hippocampal memory processing. We previously developed a highly sensitive histochemical imaging probe for sialidase activity, BTP3-Neu5Ac. BTP3-Neu5Ac was cleaved efficiently by rat Neu2 and Neu4 at pH 7.3 and by Neu1 and Neu3 at pH 4.6. When a rat hippocampal acute slice was stained with BTP3-Neu5Ac at pH 7.3, mossy fiber terminal fields showed relatively intense sialidase activity. Thus, the role of sialidase in the synaptic plasticity was investigated at mossy fiber terminal fields. The long-term potentiation (LTP) at mossy fiber-CA3 pyramidal cell synapses was impaired by 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA), a sialidase inhibitor. DANA also failed to decrease paired-pulse facilitation after LTP induction. We also investigated the role of sialidase in hippocampus-dependent spatial memory by using the Morris water maze. The escape latency time to reach the platform was prolonged by DANA injection into the hippocampal CA3 region or by knockdown of Neu4 without affecting motility. The results show that the regulation of sialyl signaling by Neu4 is involved in hippocampal memory processing.

[Multidimensional Analysis of Hippocampal Excitatory Neurotransmission and Development of Analytical Tools for Glycans]

Sialidase removes sialic acid residues from sialoglycoconjugates such as glycoproteins and glycolipids. Since sialic acid plays crucial roles in synaptic plasticity and memory in the hippocampus, the regulation of sialyl signaling by sialidase is also necessary for neural functions. However, since mammalian sialidase activity is remarkably weak, it has been difficult to detect sialidase activity in mammalian tissues. Determination of the distribution of sialidase activity in living mammalian tissues would provide much valuable information for understanding the roles of sialidase in physiological functions. Therefore, we synthesized a novel benzothiazolylphenol-based sialic acid derivative (BTP-Neu5Ac) as a fluorescent sialidase substrate. After cleavage of BTP-Neu5Ac, which is water soluble and shows little fluorescence, with sialidase, the water-insoluble fluorophore benzothiazolylphenol (BTP) released from BTP-Neu5Ac stains tissue and shows bright fluorescence. BTP-Neu5Ac can visualize sialidase activity in brain tissue with high levels of sensitivity and specificity. The sialidase expression level is markedly high in various human cancers such as colon, renal, prostate, and ovarian cancers. BTP-Neu5Ac can detect human colon cancers sensitively. Thus, BTP-Neu5Ac is useful not only for physiological research but also as a cancer probe. BTP-Neu5Ac is now being used in virology research. In this review, methods for histochemical imaging of sialidase activity and the role of sialidase in hippocampal memory are described based on the author's study of multidimensional analysis of hippocampal excitatory neurotransmission and development of analytical tools for glycans, which was awarded a prize by the Tokai branch of the Pharmaceutical Society of Japan.

Sweet complementarity: the functional pairing of glycans with lectins

Carbohydrates establish the third alphabet of life. As part of cellular glycoconjugates, the glycans generate a multitude of signals in a minimum of space. The presence of distinct glycotopes and the glycome diversity are mapped by sugar receptors (antibodies and lectins). Endogenous (tissue) lectins can read the sugar-encoded information and translate it into functional aspects of cell sociology. Illustrated by instructive examples, each glycan has its own ligand properties. Lectins with different folds can converge to target the same epitope, while intrafamily diversification enables functional cooperation and antagonism. The emerging evidence for the concept of a network calls for a detailed fingerprinting. Due to the high degree of plasticity and dynamics of the display of genes for lectins the validity of extrapolations between different organisms of the phylogenetic tree yet is inevitably limited.

Preferential Accumulation of 14C-N-Glycolylneuraminic Acid over 14C-N-Acetylneuraminic Acid in the Rat Brain after Tail Vein Injection

The two main molecular species of sialic acid existing in nature are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Neu5Ac is abundant in mammalian brains and plays crucial roles in many neural functions. In contrast, Neu5Gc is present only at a trace level in vertebrate brains. The brain-specific suppression of Neu5Gc synthesis, which is a common feature in mammals, suggests that Neu5Gc has toxicity against brain functions. However, in vivo kinetics of Neu5Gc in the whole body, especially in the brain, has not been studied in sufficient detail. To determine the in vivo kinetics of Neu5Gc, ¹⁴C-Neu5Gc was enzymatically synthesized and injected into rat tail veins. Although most of ¹⁴C-Neu5Gc was excreted in urine, a small amount of ¹⁴C-Neu5Gc was detected in the brain. Brain autoradiography indicated that ¹⁴C-Neu5Gc was accumulated predominantly in the hippocampus. ¹⁴C-Neu5Gc transferred into the brain was incorporated into gangliosides including GM1, GD1a, GD1b, GT1b and GQ1b. Reduction of ¹⁴C-Neu5Gc after intracerebroventricular infusion was slower than that of ¹⁴C-Neu5Ac in the brain and hippocampus. The results suggest that Neu5Gc is transferred from blood into the brain across the blood brain barrier and accumulates in the brain more preferentially than does Neu5Ac.

Visualization of sialidase activity in Mammalian tissues and cancer detection with a novel fluorescent sialidase substrate

Sialidase removes sialic acid from sialoglycoconjugates and plays crucial roles in many physiological and pathological processes. Various human cancers express an abnormally high level of the plasma membrane-associated sialidase isoform.Visualization of sialidase activity in living mammalian tissues would be useful not only for understanding sialidase functions but also for cancer diagnosis. However, since enzyme activity of mammalian sialidase is remarkably weak compared with that of bacterial and viral sialidases, it has been difficult to detect sialidase activity in mammalian tissues. We synthesized a novel benzothiazolylphenol-based sialic acid derivative (BTP-Neu5Ac) as a fluorescent sialidase substrate. BTP-Neu5Ac can visualize sialidase activities sensitively and selectively in acute rat brain slices. Cancer cells implanted orthotopically in mouse colons and human colon cancers (stages T3-T4) were also clearly detected with BTP-Neu5Ac. The results suggest that BTP-Neu5Ac is useful for histochemical imaging of sialidase activities.