Age related differences in binding of concanavalin A to plasma membranes of isolated neurons

The dynamic brain N-glycome

The attachment of carbohydrates to other macromolecules, such as proteins or lipids, is an important regulatory mechanism termed glycosylation. One subtype of protein glycosylation is asparagine-linked glycosylation (N-glycosylation) which plays a key role in the development and normal functioning of the vertebrate brain. To better understand the role of N-glycans in neurobiology, it's imperative we analyse not only the functional roles of individual structures, but also the collective impact of large-scale changes in the brain N-glycome. The systematic study of the brain N-glycome is still in its infancy and data are relatively scarce. Nevertheless, the prevailing view has been that the neuroglycome is inherently restricted with limited capacity for variation. The development of improved methods for N-glycomics analysis of brain tissue has facilitated comprehensive characterisation of the complete brain N-glycome under various experimental conditions on a larger scale. Consequently, accumulating data suggest that it's more dynamic than previously recognised and that, within a general framework, it has a given capacity to change in response to both intrinsic and extrinsic stimuli. Here, we provide an overview of the many factors that can alter the brain N-glycome, including neurodevelopment, ageing, diet, stress, neuroinflammation, injury, and disease. Given this emerging evidence, we propose that the neuroglycome has a hitherto underappreciated plasticity and we discuss the therapeutic implications of this regarding the possible reversal of pathological changes via interventions. We also briefly review the merits and limitations of N-glycomics as an analytical method before reflecting on some of the outstanding questions in the field.

Changes in glycoprotein carbohydrate content in the aging human brain

There is no significant change in the concentration, per gram of fresh tissue, of the total carbohydrate associated with brain glycoproteins as the human brain ages from 25 to 85 years. Nevertheless, notable shifts in the concentration of varied types of oligosaccharides do occur. The concentration and percentage of total glycopeptide-carbohydrate recovered from the whole brain represented by the N-glycosidically linked complex sialooligosaccharides of the tri- and tetraantennary type show a marked decline with age. On the other hand, the N-linked mannose high oligosaccharides of the ovalbumin type, the N-linked biantennary sialoglycopeptides of the transferrin type, and the O-linked oligosaccharides show an increase as the brain ages. Other age related changes that were noted include an increase in the number of sulfate ester groups associated with the N-linked sialoglycopeptides and a decrease in the number of phosphorylated high mannose oligosaccharides as the brain ages.

Surface determinants of low density lipoprotein uptake by endothelial cells

The surface sialic acid content of aortic endothelial cells in vitro was substantially lower in sparse cultures than at confluence. Binding of LDL to endothelial cells did not change at different culture densities and was unaffected by brief pretreatment with neuraminidase to partially remove surface sialic acid residues. In contrast, internalization of LDL declined by a factor of 3 between low density cell cultures and confluent monolayers; neuraminidase pretreatment increased LDL uptake and the effect was most marked (greater than 10-fold) at confluence. Pretreatment with cationised ferritin, which removed most of the surface sialic acid residues as well as glycosaminoglycans, increased LDL internalization by up to 20-fold, again with most effect on confluent monolayers. Thus LDL uptake is inversely correlated with sialic acid content. We conclude that changes in the surface density of sialic acid (and possibly other charged) residues significantly modulate endothelial LDL uptake, and suggest that focal increases in LDL accumulation during atherogenesis may be related to alterations in endothelial endocytic properties at sites of increased cell turnover or damage.

Glycoconjugates of the avian eye: the development and maturation of the neural retina as visualized by lectin binding

The distribution of binding sites for four rhodamine-conjugated lectins has been examined in paraffin-embedded tissue sections of the developing and mature avian neural retina. The method of fixation employed here (95% ethanol:glacial acetic acid) is shown to conserve substantially more in vivo labeled 3H-glycoprotein or 3H-ganglioside than fixation with either 3.7% formaldehyde or 2% glutaraldehyde. Each lectin was shown to be optimally active by direct hemagglutination and inhibitable by the appropriate hapten in the cytochemical assay. Staining with wheat germ agglutinin (leads to N-acetylglucosamine) occurs predominantly to each of the anuclear retinal layers and progressively disappears from each of the nuclear layers. By adulthood, the inner nuclear layer is stained in a gradient which is highest adjacent to the inner nuclear layer while the nuclear layers are virtually unstained. Although similar to wheat germ agglutinin, staining with concanavalin A (leads to mannose) is less intense and persists in the adult nuclear layers. Dolichos biflorus agglutinin (leads to N-acetylgalactosamine) predominantly stains the embryonic ganglion cell layer. With development, this lectin demonstrates profound differences between the major cell types of the inner nuclear layer and the inner (negative) and outer (positive) plexiform layers. Differences between the principal synaptic layers are further accentuated by Ulex europaeus agglutinin I (leads to fucose): Binding sites are virtually absent from the 12th day embryonic retina, but are abundant throughout the retina by the 18th day and persist after hatching. However, in the adult retina staining persists in the outer plexiform layer but is lost from the inner plexiform and nerve fiber layers. In contrast to Dolichos biflorus agglutinin, Ulex europaeus agglutinin I stains the inner nuclear layer uniformly. Temporal differences in the characteristic lectin binding patterns exhibited by each of the major nuclear and anuclear layers of the developing avian neural retina are thus documented.

Age-development changes in susceptibility of erythrocytes to perfringolysin O

Susceptibility to perfringolysin O of erythrocytes from mice of different ages was examined. Erythrocytes of mice younger than 5 weeks' old were more resistant to the toxin than those of young adult and adult mice. Erythrocytes of aged mice were about 3.5 times more susceptible to the toxin than erythrocytes from 4-week-old mice. The membrane cholesterol content of erythrocytes appeared to be maintained at a constant level throughout the ages of mice examined. About 5% of the total membrane cholesterol was supposed to provide receptor sites for the toxin from an experiment in which cholesterol was specifically extracted by liposomes. It was demonstrated in this experiment that susceptibility of erythrocytes to the toxin was lost in proportion to the reduction in the toxin binding. The susceptibility, however, of erythrocytes from young or aged mice was much lower or higher than expected from the changes in toxin binding. Therefore, two possibilities were raised to account for age-related alterations in the susceptibility of erythrocytes; not only expansion of a particular compartment of membrane cholesterol as a toxin receptor but also some activation of intracellular reactions leading to hemolysis might occur in senescence.

Effect of removing sialic acids from endothelium on the adherence of circulating platelets in arteries in vivo

The contribution of the net negative charge excess due to sialic acids on endothelium in preventing adhesion of circulating platelets in vivo was investigated in anaesthetized rabbits. Platelets in the rabbit's circulation were selectively labelled with radioactive 5-hydroxytryptamine in vivo. Segments of carotid arteries temporarily isolated from the circulation were perfused with one or other of two commercial preparations of neuraminidase; the opposite carotid artery was perfused similarly without the enzyme, as control. A neuraminidase preparation from Behringwerke free of proteolytic activity released sialic acid into the perfusate with a peak concentration after 10-15 min which decreased gradually later. A neuraminidase preparation from Sigma that contained demonstrable proteolytic activity released sialic acid similarly during the first hour and thereafter more sialic acid in a second peak. After blood flow through the carotids had been restored the adhesion of labelled platelets in the artery perfused with neuraminidase was compared with that in the artery perfused without the enzyme. The radioactivities were significantly higher in carotids that had been perfused with neuraminidase than in those that had been perfused without the enzyme. Neuraminidase perfusion had no effect on the production of prostacyclin by the carotids. Perfusion with acetylsalicylic acid before neuraminidase increased the adhesion of platelets significantly. It is concluded that diminution in electrostatic repulsion between circulating platelets and vascular endothelium from which the net negative charge excess due to sialic acids has been removed increases the adhesion of circulating platelets, irrespective of the production of prostacyclin by the arterial walls, and that inhibition of prostacyclin production augments this adhesion of platelets.

Variation in cytoskeletal assembly during spreading of progressively subcultivated human embryo fibroblasts (IMR-90)

The cytoskeletons of early and late passage IMR-90 human diploid fibroblasts have been directly imaged in replicas of Triton X-100 extracted cells during spreading following reseeding. All cells from both young and sensescent cultures exhibit a cytoskeletal network of actin microfilaments, intermediate (10 nm) filaments, microtubules, and interconnecting thin filaments (6-8 nm in diameter) which do not interact with heavy meromyosin. Early passage cells assemble linear aggregates of actin filaments within 1 h of spreading. By 4 h of incubation, these bundles establish a structural bond with the cell membrane which results in resistance by the plasmalemma to detergent extraction at these sites. Furthermore, these membrane regions are associated with developing stress fibers of well-spread cells. In contrast, late passage cells exhibit slower spreading which correlates with a retarded assembly of actin bundles. In addition, by 8 h of spreading, cells of older cultures do not exhibit the regions of membrane-actin interaction which impart detergent resistance to the plasmalemma. We conclude that the ability to reassemble actin-actin and actin-membrane association during cell spreading is reduced with increased serial subcultivation of cells.

Developmental-related changes in the lectin binding to cells of the central nervous system

Developmental related changes in Concanavalin A binding to nerve tissue are reported in this paper. ConA binding sites were studied in brains of rats and chicks at different embryological stages using the ConA-Peroxidase-DAB sequence. Developmental related changes in the extent, and pattern of ConA binding sites were observed in the neuronal perikaryon. An increment in the uptake of ConA is parallel to neuronal maturation, and to the development of Golgi apparatus and endoplasmic reticulum. A great density of ConA binding sites is found in radial glial processes in rat brain at day 15 following insemination. The density of ConA binding sites in radial glial processes decreases in the following days. At day 18 ConA binding sites in these glial processes are not found. ConA binding sites in oligodendroglia and myelin coat are not found before birth.

Synaptic atrophy in the senescent hippocampus

Quantitative analyses of electron micrographs have shown a decrease in the number of synapses in the dentate gyrus of the senescent Fischer-344 rat. The loss of synapses, involving both dendritic spines and shafts and axon terminals of more than one population of presynaptic neurons, did not depend upon the antecedent loss of postsynaptic neurons or their dendrites. These findings suggest that the age-related loss of synapses in the dentate gyrus may depend upon an inability of presynaptic elements to maintain the structural integrity of synapses in senescence. It is proposed that a change in the glycoprotein component of presynaptic plasma membranes resulting from a deficiency in axonal transport mechanisms in the septo-hippocampal pathway may underly this presynaptic malfunction. The resulting partial deafferentation of neurons in the dentate gyrus in senescence appears to be associated with a secondary atrophy of dendrites, which results in a loss of postsynaptic membranes before a loss of postsynaptic neurons can be documented.