Increased fucose incorporation into rat hippocampus during learning. A biochemical and microautoradiographic study

Targeting cell surface glycans with lectin-coated fluorescent nanodiamonds

Glycosylation is arguably the most important functional post-translational modification in brain cells and abnormal cell surface glycan expression has been associated with neurological diseases and brain cancers. In this study we developed a novel method for uptake of fluorescent nanodiamonds (FND), carbon-based nanoparticles with low toxicity and easily modifiable surfaces, into brain cell subtypes by targeting their glycan receptors with carbohydrate-binding lectins. Lectins facilitated uptake of 120 nm FND with nitrogen-vacancy centers in three types of brain cells - U87-MG astrocytes, PC12 neurons and BV-2 microglia cells. The nanodiamond/lectin complexes used in this study target glycans that have been described to be altered in brain diseases including sialic acid glycans via wheat (Triticum aestivum) germ agglutinin (WGA), high mannose glycans via tomato (Lycopersicon esculentum) lectin (TL) and core fucosylated glycans via Aleuria aurantia lectin (AAL). The lectin conjugated nanodiamonds were taken up differently by the various brain cell types with fucose binding AAL/FNDs taken up preferentially by glioblastoma phenotype astrocyte cells (U87-MG), sialic acid binding WGA/FNDs by neuronal phenotype cells (PC12) and high mannose binding TL/FNDs by microglial cells (BV-2). With increasing recognition of glycans having a role in many diseases, the lectin bioconjugated nanodiamonds developed here are well suited for further investigation into theranostic applications.

Metabolic flux analysis of the neural cell glycocalyx reveals differential utilization of monosaccharides

Saccharides in our diet are major sources of carbon for the formation of biomass such as proteins, lipids, nucleic acids and glycans. Among the dietary monosaccharides, glucose occupies a central role in metabolism, but human blood contains regulated levels of other monosaccharides as well. Their influence on metabolism and how they are utilized have not been explored thoroughly. Applying metabolic flux analysis on glycan synthesis can reveal the pathways that supply glycosylation precursors and provide a snapshot of the metabolic state of the cell. In this study, we traced the incorporation of six 13C uniformly labeled monosaccharides in the N-glycans, O-glycans and glycosphingolipids of both pluripotent and neural NTERA-2 cells. We gathered detailed isotopologue data for hundreds of glycoconjugates using mass spectrometry methods. The contributions of de novo synthesis and direct incorporation pathways for glucose, mannose, fructose, galactose, N-acetylglucosamine and fucose were determined based on their isotope incorporation. Co-feeding studies revealed that fructose incorporation is drastically decreased by the presence of glucose, while mannose and galactose were much less affected. Furthermore, increased sialylation slowed down the turnover of glycans, but fucosylation attenuated this effect. Our results demonstrated that exogenous monosaccharide utilization can vary markedly depending on the cell differentiation state and monosaccharide availability, and that the incorporation of carbons can also differ among different glycan structures. We contend that the analysis of metabolic isotope labeling of glycans can yield new insights about cell metabolism.

Fucose and fucose-containing sugar epitopes enhance hippocampal long-term potentiation in the freely moving rat

Male Wistar rats were intrahippocampally injected with L-fucose and the sugar epitope 2'-fucosyl-lactose prior to induction of long-term potentiation (LTP). Both substances had only a minimal and short-lasting depressive effect on the monosynaptically evoked field potential recorded in the dorsal blade of the dentate gyrus of freely moving rats upon stimulation of the perforant pathway. However, LTP induced by fractionated tetanization of the perforant pathway, which declined within 24 h in control animals injected with Lactose, remained at the initial level even 48 h after tetanization (difference to the control group significant with P < 0.01). The results support earlier findings which have indicated a participation of fucosylated macromolecules in the maintenance of LTP. Different molecular mechanisms concerning the effect of both substances and the significance of the data in elucidation of the relationship between LTP and memory formation are discussed.

Hippocampal NCAM180 transiently increases sialylation during the acquisition and consolidation of a passive avoidance response in the adult rat

Synaptic connectivity change is a consistent anatomical feature of memory formation and the possibility that this is mediated by a replay of neurodevelopmental events has been investigated by measuring change in neural cell adhesion molecule sialylation state during the acquisition and consolidation of a passive avoidance response in the adult rat. The avoidance response was always generated after two to three trials and the animals remained on the platform for the criterion time of 5 min. In all cases training was complete within 5-8 min. Change in sialylation state was monitored following intraventricular infusion of the 3H-ManNAc precursor at 4 hr prior to the reference point. No task-specific change in general glycoconjugate sialylation was apparent in hippocampal P2 pellets at increasing times following training. Increased sialylation state was observed only in neural cell adhesion molecule (NCAM) immunoprecipitates of hippocampal membrane fractions at 12 and 24 hr after training. Change in hippocampal sialylation state could not be attributed to an increased accumulation of NCAM as detected by an immunoabsorbent assay. Immunoblotting of antibody precipitated NCAM demonstrated the 3H-ManNAc to be incorporated into the synapse-specific, 180 kDa isoform of NCAM and a novel 210 kDa isoform. Immunoprecipitation and immunoblotting procedures with an antibody specific for a2-8-polysialic acid showed the 180 and 210 kDa isoforms to be polysialylated. The role of NCAM180 sialylation as a mechanism for synapse selection in information storage is discussed.

The amnesic substance 2-deoxy-D-galactose suppresses the maintenance of hippocampal LTP

Male Wistar rats were intraventricularly injected with 2-deoxy-D-galactose (do-gal), a substance interfering with the fucosylation of glycomacromolecules and impairing memory consolidation in various learning tasks. Do-gal was found to have no influence on the monosynaptically evoked field potential (MEFP) recorded in the dentate gyrus upon stimulation of the perforant pathway. However, hippocampal long-term potentiation (LTP) induced in do-gal-pretreated animals by fractionated tetanization of the perforant pathway declined to control levels 2 h after tetanization, whereas it remained constant for 24 h in saline-treated rats. Similar effects were observed in the CA1 region of hippocampal slices. The results indicate a participation of fucosylated macromolecules in the maintenance of LTP. The possible significance of processes involved in LTP for memory formation is discussed.

Incorporation of [3H]fucose in rat hippocampal structures after conditioning by perforant path stimulation and after LTP-producing tetanization

The contribution of glycoprotein synthesis to functional synaptic changes and to the formation of memory traces was investigated by autoradiographic determination of the incorporation of [3H]fucose into the hippocampal structures of rats. In the first experiment, the fucose incorporation was measured after induction of post-tetanic long-term potentiation (LTP) in granular cell synapses by repeated tetanization (200 cps) of the perforant path, and after stimulation of this hippocampal input by the same number of impulses with very low frequency (0.2 cps) not producing LTP. In the second experiment, the incorporation of fucose was determined after an active avoidance training using the stimulation of the perforant path by impulse trains of 15 cps as conditioning stimuli, and after a session of corresponding unpaired stimulations of the perforant path. Unstimulated animals were used in both experiments to measure the basal glycosylation. LTP-producing tetanization resulted only in a slight increase of incorporation into the ipsilateral hippocampal structures without significant differences to similar changes after the corresponding control stimulation with single impulses. After a session of unpaired stimulation of the perforant path with impulse trains of 15 cps only slight and inconsistent changes of incorporation occurred in the hippocampus too. However, after conditioning by the corresponding perforant path stimulation as conditioned stimulus, considerable increases of incorporation were observed in all structures of the ipsilateral hippocampus, when compared to the unpaired control stimulation. An enhanced labeling occurred also in some structures of the contralateral hippocampus mainly receiving commissural inputs. The results suggest again, that the activation of one single hippocampal afferent, even if producing LTP, would not be sufficient to induce an increased glycosylation of neuronal proteins. The increase of glycoprotein formation seems to require the convergence of several inputs, which can be assumed to occur during learning. Therefore, LTP of a single synaptic population seems not to represent the complete long-lasting memory trace, but only one of its components, or a preceding transient storage mechanism.

Distribution of hippocampal glycoproteins as demonstrated in rats by lectin binding and autoradiography after intraventricular injections of labelled fucose, N-acetyl-glucosamine and mannose

The present study demonstrates distinct distribution patterns of glycoproteins in rat hippocampus, with respect to synthesis from precursors (autoradiography) and endogenous contents (lectin binding). The autoradiographic analysis performed 1, 2, 8 and 24 h after intraventricular injections of tritium-labelled L-fucose. N-acetyl-D-glucosamine and D-mannose revealed that up to 2 h after application of any of the three precursors, radioactivity occurred in the pyramidal and granular cell layers. Afterwards, however, rapid migration of label proceeded from the cell bodies into the neuropil after application of fucose and acetylglucosamine, while after injection of mannose a considerable amount of radioactivity stayed in the cell body layers, even 24 h after administration of labelled precursor. These findings were consistent with the histochemical visualization of glycoprotein constituents by fluorescent wheat germ lectin (preferentially binding to glucosaminyl residues) and concanavalin A-horseradish peroxidase (preferentially binding to mannosyl residues). These showed a heavy staining predominantly in neuropil and somata, respectively, with concanavalin A-binding giving more distinct patterns than the application of labelled mannose. The usefulness of the three glycoprotein precursors as correlates with functional behavioural changes in discussed.

Effect of L-fucose on brain protein metabolism and retention of a learned behavior in rats

The intraperitoneal or intraventricular application of L-fucose (100 mg/kg or 75 microgram, respectively) prior to training in shuttle box avoidance as well as in shock-motivated brightness discrimination in rats significantly improved the retention of learned behavior 24 hr later. The application of D-fucose was without influence on retention. In naive animals, intraventricularly applied L-fucose (75-200 microgram) caused an increase in the rate of protein synthesis in the hippocampus, resulting in a significant increase in total proteins of this brain structure, mainly attributed to the Tris-insoluble protein fractions. The results are discussed in terms of an activation of glycoprotein formation by increasing supply with L-fucose.

Stimulation of a soluble protein fraction in the hippocampus of rats, subjected to brief training

After a brief training period (2 x 25 min) consisting of reversal of handedness, soluble protein synthesis in the hippocampus was measured by incorporation of [3H]leucine and double labelling with [3H]valine and [14C]valine together with densitometric tracings of polyacrylamide gels. Among the proteins showing increased incorporation values, 3 polypeptides of mol. wt. 13,000, 30,000 and 65,000 were found. Thus, the soluble protein-polypeptide pattern in the hippocampus was found to be altered after a short training period for the establishment of a new behavior.

Subcellular localization of increased incorporation of [3H]fucose following passive avoidance learning in the chick

The effect of training on a passive avoidance test on incorporation of [3H]fucose into subcellular fractions of the anterior forebrain roof of 1-day-old chicks was determined. Isotope was injected intraperitoneally, and birds tested and killed after 3 h. There was an increase in incorporation in trained compared with untrained control birds in the synaptic membrane (19% elevation, P less than 0.02) and the mitochondrial (14% elevation, P less than 0.05) fractions. Following intracranial injection, increased incorporation in trained birds was seen only in the synaptic membrane fraction (39% elevation, P less than 0.01); this increase in incorporation was not limited to any particular electrophoretically separated glycopeptides, but occurred in all 9 detected peaks.

The incorporation of intrastriatally injected [3H]fucose into electrophoretically separated synaptosomal glycoproteins. II. The influence of passive avoidance training

Following intrastriatal injections of [3H]L-fucose, male albino rats served as (a) trained subjects in a step-down passive avoidance task, (b) stress controls receiving inescapable shock or (c) handled controls. At a series of time points after treatment the animals were sacrificed and the P2 fraction of the injected neostriatum was isolated. This tissue was electrophoresed on SDS-polyacrylamide gels and radioactivity profiles were constructed from 1 mm gel slices. The profiles of trained subjects were compared to shocked and handled control subjects from the same time point group. No differences in total [3H]fucose incorporation into neostriatal glycoproteins were detected as a result of the behavioral treatment used, nor was an incorporation into the majority of electrophoresed peaks altered. Three radioactive gel peaks were significantly altered as a function of experience. At the one day time point, trained subjects exhibited a significant increase in the tritium content of a 70,000 dalton fucosyl-glycoprotein peak. At the 5 day time point, increased label was detected in a 180,000 dalton peak in both trained and shocked subjects, while a significant increase in a 140,000 dalton peak was observed only in trained animals. The relation of the present findings to perviously reported training related differences in glycoprotein metabolism are discussed.

Dopamine and macromolecule synthesis in rat hippocampus

In hippocampus slices both dopamine and apomorphine lead to an increased incorporation of (3H)-fucose into total proteins, whereas the incorporation of (14C)-leucine was unchanged or decreased, respectively. Noradrenaline did not alter the incorporation of both precursors, whereas haloperidol partially reduced the dopamine induced increase in incorporation of fucose. Thus, an induction process of the observed macromolecular changes involving dopaminoceptive structures of hippocampus can be assumed.

Hippocampal activation and incorporation of macromolecule precursors

The effect of rhythmic slow wave activity (theta rhythm) on the incorporation of 3H-leucine and 3H-fucose into the total proteins of different hippocampus areas was studied. The theta rhythm was elicited by electrical stimulation of medial septum nuclei. An increase in 3H-leucine incorporation into the total proteins of CA 3 and CA 1 sectors of the hippocampus was observed, whereas the stimulation had no influence on precursor incorporation into the complex CA 4/area dentata. In contrast to these findings 3H-fucose incorporation into hippocampal proteins was not influenced by electrical stimulation of the medial septum. These findings are discussed in comparison to the results obtained in a learning experiment, which revealed an increased incorporation of both leucine and focuse into hippocampal proteins.