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--acetylneuraminic acid (4MU-Neu5Ac) or 5-bromo-4-chloroindol-3-yl-α-d--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 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.

Sialidase, which removes sialic acid residues in sialylglycoconjugates, is essential for hippocampal memory and synaptic plasticity. Enzyme activity of sialidase is rapidly increased in response to neural excitation. Because sialic acid bound to gangliosides such as the tetra-sialoganglioside GQ1b is crucial for calcium signalling and neurotransmitter release, neural activity-dependent removal of sialic acid may affect hippocampal neurotransmission. In the present study, we found that 2-deoxy-2, 3-didehydro-D-N-acetylneuraminic acid (DANA), a sialidase inhibitor, increased expression of ganglioside GQ1b/GT1a in hippocampal acute slices. Extracellular glutamate level in the rat hippocampus measured by using in vivo microdialysis was increased by the sialidase inhibitor 2, 3-dehydro-2-deoxy-N-glycolylneuraminic acid as well as DANA. Synaptic vesicle exocytosis and intracellular Ca2+ increase evoked by high-K+ were also enhanced by DANA in primary cultured hippocampal neurons. Expression of GQ1b/GT1a was rapidly decreased by depolarization with high-K+, suggesting that the increase in sialidase activity by neural excitation is sufficient for cleavage of sialic acid. Our findings indicate that sialidase down-regulates glutamate release from hippocampal neurons via Ca2+ signalling modulation. Neural activity-dependent desialylation by sialidase may be a negative-feedback factor against presynaptic activity.

Sialidase cleaves sialic acid residues from a sialoglycoconjugate: oligosaccharides, glycolipids and glycoproteins that contain sialic acid. Histochemical imaging of the mouse pancreas using a benzothiazolylphenol-based sialic acid derivative (BTP3-Neu5Ac), a highly sensitive histochemical imaging probe used to assess sialidase activity, showed that pancreatic islets have intense sialidase activity. The sialidase inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA) remarkably enhances glutamate release from hippocampal neurons. Since there are many similar processes between synaptic vesicle exocytosis and secretory granule exocytosis, we investigated the effect of DANA on insulin release from β-cells. Insulin release was induced in INS-1D cells by treatment with 8.3 mM glucose, and the release was enhanced by treatment with DANA. In a mouse intraperitoneal glucose tolerance test, the increase in serum insulin levels was enhanced by intravenous injection with DANA. However, under fasting conditions, insulin release was not enhanced by treatment with DANA. Calcium oscillations induced by 8.3 mM glucose treatment of INS-1D cells were not affected by DANA. Blood insulin levels in sialidase isozyme Neu3-deficient mice were significantly higher than those in WT mice under ad libitum feeding conditions, but the levels were not different under fasting conditions. These results indicate that DANA is a glucose-dependent potentiator of insulin secretion. The sialidase inhibitor may be useful for anti-diabetic treatment with a low risk of hypoglycemia. Sialidase is a hydrolase that removes sialic acid from a sialoglycoconjugate. Mammalian sialidase has four isozymes: Neu1, Neu2, Neu3 and Neu4. These four isozymes have differences in the tissues where they are expressed, their subcellular locations, their substrate specificity and their pH dependency 1. It has been proposed that sialidase contributes to the regulation of insulin signalling and sensitivity. Neu1, a lysosomal sialidase, regulates insulin signalling for energy metabolism and glucose uptake 2. Neu3, a plasma membrane-associated sialidase, is associated with tissue-specific insulin sensitivity and glucose tolerance 3-5. Transgenic mice overexpressing Neu3 mainly in muscles developed severe insulin-resistant diabetes mellitus associated with hyperinsulinemia, islet hyperplasia and increased β-cell mass 5. Hepatic Neu3 overexpression, however, improves insulin sensitivity and glucose tolerance and is associated with changes in ganglioside composition and peroxisome proliferator-activated receptor gamma signalling 4. Recently, we reported that sialidase downregulates glutamate release from neurons 6. Since sialidase activity increases rapidly in response to neural excitation, sialidase is thought to play a role in the negative-feedback mechanism for glutamate release 7. In an effort to understand how sialidase might regulate synaptic transmission,