Incorporation of radioactivity of D-glucosamine-1-14C into heteropolysaccharide chains of glycoproteins in adult and developing rat brain

Brain glycogen serves as a critical glucosamine cache required for protein glycosylation

Glycosylation defects are a hallmark of many nervous system diseases. However, the molecular and metabolic basis for this pathology is not fully understood. In this study, we found that N-linked protein glycosylation in the brain is metabolically channeled to glucosamine metabolism through glycogenolysis. We discovered that glucosamine is an abundant constituent of brain glycogen, which functions as a glucosamine reservoir for multiple glycoconjugates. We demonstrated the enzymatic incorporation of glucosamine into glycogen by glycogen synthase, and the release by glycogen phosphorylase by biochemical and structural methodologies, in primary astrocytes, and in vivo by isotopic tracing and mass spectrometry. Using two mouse models of glycogen storage diseases, we showed that disruption of brain glycogen metabolism causes global decreases in free pools of UDP-N-acetylglucosamine and N-linked protein glycosylation. These findings revealed fundamental biological roles of brain glycogen in protein glycosylation with direct relevance to multiple human diseases of the central nervous system.

The lysosomal sialic acid transporter sialin is required for normal CNS myelination

Salla disease and infantile sialic acid storage disease are autosomal recessive lysosomal storage disorders caused by mutations in the gene encoding sialin, a membrane protein that transports free sialic acid out of the lysosome after it is cleaved from sialoglycoconjugates undergoing degradation. Accumulation of sialic acid in lysosomes defines these disorders, and the clinical phenotype is characterized by neurodevelopmental defects, including severe CNS hypomyelination. In this study, we used a sialin-deficient mouse to address how loss of sialin leads to the defect in myelination. Behavioral analysis of the sialin(-/-) mouse demonstrates poor coordination, seizures, and premature death. Analysis by histology, electron microscopy, and Western blotting reveals a decrease in myelination of the CNS but normal neuronal cytoarchitecture and normal myelination of the PNS. To investigate potential mechanisms underlying CNS hypomyelination, we studied myelination and oligodendrocyte development in optic nerves. We found reduced numbers of myelinated axons in optic nerves from sialin(-/-) mice, but the myelin that was present appeared grossly normal. Migration and density of oligodendrocyte precursor cells were normal; however, a marked decrease in the number of postmitotic oligodendrocytes and an associated increase in the number of apoptotic cells during the later stages of myelinogenesis were observed. These findings suggest that a defect in maturation of cells in the oligodendrocyte lineage leads to increased apoptosis and underlies the myelination defect associated with sialin loss.

Synthesis of [1-11C]D-glucosamine and evaluation of its in vivo distribution in rat with PET

D-Glucosamine is a structural unit of many biologically interesting macromolecules. To investigate the feasibility of using labelled D-glucosamine as a tracer for anabolic processes, a two-step synthetic procedure for specifically labelling D-glucosamine in position 1 with carbon-11 was developed. [11C]Cyanide was reacted with an imine precursor, N-benzyl-D-arabinosylamine, to generate the [1-11]alpha-amino nitrile. Reduction to [1-11C]D-glucosamine was accomplished by catalytic hydrogenation using PdCl2 and the N-benzyl group was simultaneously removed. The total synthesis time from end-of-trapping of [11C]cyanide was 40-45 min and the decay-corrected radiochemical yield was 5-10% after HPLC isolation. The biodistribution of [1-11C]D-glucosamine in rat following i.v. bolus injection was investigated using positron emission tomography and showed that the availability of this substance for CNS anabolism is low with the primary limitation being the intact blood-brain barrier.

Gangliosides and sialoglycoproteins in hypothalamus of normal, postnatal, and pre- and postnatal protein undernourished rats

Total ganglioside and sialoglycoprotein concentrations were determined in the hypothalamus of normal (diet: 25% casein), postnatal undernourished (diet: 8% casein since birth), and pre- and postnatal undernourished rats (diet: 8% casein since pregnancy). Hypothalamic weights for the two low protein diet groups were lower than for the normal diet groups at all ages studied. Total hypothalamic ganglioside and sialoglycoproteins (mumol NANA) of postnatal undernourished rats were lower than control at day 10, while in pre- and postnatal undernourished rats this difference occurred at day 7. The reduction in gangliosides and sialoglycoprotein contents was not solely a consequence of the decrease in hypothalamic weight since, when the data were expressed as nmol NANA/mg tissue, similar reductions were observed principally in the pre- and postnatal protein undernutrition group. These results suggest that the effects of pre- and postnatal undernutrition on hypothalamic gangliosides and sialoglycoproteins are more pronounced than those that occur as a result of postnatal undernutrition.

Turnover of free sialic acid, CMP-sialic acid, and bound sialic acid in rat brain

Adult male rats were injected intraventricularly with N-[3H]acetylmannosamine. After different time intervals the rats were killed and free sialic acid, CMP-sialic acid, lipid- and protein-bound sialic acid were isolated from brain and the specific radioactivities determined. Maximal specific radioactivity was reached after approximately 4 h for CMP-sialic acid, after 10-12 h for free sialic acid and after approximately 42 h for lipid- and protein-bound sialic acid. After some days the specific radioactivities of all four pools were the same and decreased equally, with a calculated turnover rate of approximately 3.5 weeks. The conclusion was that this phenomenon was the result of reutilisation of sialic acid and/or precursors. Therefore, the calculated turnover is not the turnover of bound sialic acid, but merely the rate of leakage of sialic acid and/or precursors out of the brain, so that no real turnover can be measured by this method. The first few hours after injection the specific radioactivity of CMP-sialic acid rose above that of free sialic acid. It is supposed that a compartmentalization exists of free sialic acid. The newly synthesised sialic acid molecules are not secreted into the cytoplasmic pool but are preferentially used for the synthesis of CMP-sialic acid. The results and conclusions are discussed in view of the general problems concerning turnover measurements of glycoconjugates.

Development of synaptic glycoproteins: effect of postnatal age on the synthesis and concentration of synaptic membrane and synaptic junctional fucosyl and sialyl glycoproteins

Synaptic plasma membranes (SPM) and synaptic junctions (SJ) were isolated from the cortices of rats varying in age between 5 and 28 days. Gel electrophoresis of SPM and SJ indicated a marked increase in the concentration of the "PSD protein" (M. W. 52,000) with development. The biosynthesis of glycoproteins was measured following the intracranial injection of [3H]fucose or [3H]N'-acetylmannosamine. The incorporation of [3H]fucose into synaptic fractions decreased two- to threefold between 10 and 28 days whereas little change in the incorporation of [3H]N'-acetylmannosamine occurred over the same period. Gel electrophoretic analyses of labeled synaptic membranes indicated major increases in the relative incorporation of radiolabeled precursors into glycoproteins with apparent molecular weights of 74,000, 65,000, 50,000, and 40,000 with increasing age. Identification of fucosyl and sialyl glycoproteins following reaction with 125I-fucose-binding protein or labeling of sialic acid with NaIO4/NaB[3H4] demonstrated similar increases in the concentrations of these glycoproteins. Synaptic junctions contained three major glycoproteins with apparent molecular weights of 180,000, 130,000 and 110,000. The reaction of these glycoproteins with 125I-fucose-binding protein increased one- to twofold between 10 and 28 days but little variation in their relative distribution or synthesis occurred over this period. The reaction of synaptic junctional glycoproteins GP 180 and GP 110 with 125I-wheat germ agglutinin increased between 10 and 28 days. The results indicate that the molecular composition of the synapse continues to evolve after the initial synaptic contact has been formed.

The effect of lithium on the incorporation of 3H-glucosamine into glycopeptides and the transformation of 3H-glucosamine into sialic acid in rat brain

Rats were fed a lithium-containing diet (40 mmol LiCl/kg of diet) for five weeks. The plasma-lithium concentration was 0.48 mmol Li+/l of plasma. 3H-glucosamine (400 nCi/kg body weight was injected intraperitoneally, and 24 hrs later the brain was divided anatomically into hemisphere, cerebellum, pons and thalamus. The brain parts were defatted and the sialic acid-containing glycopeptides were separated by column chromatography before determination of 3H-glucosamine, 3H-sialic acid and total sialic acid. Neither the total amount nor the specific activity of sialic acid were influenced by the lithium treatment. Also the 3H-content of the sialic acid-free glycopeptides remained unchanged.

Incorporation of radiolabelled sugars into synaptic junctional macromolecules from chick brain

Individual radiolabelled sugars ([1-14C]-L-fucose, [U-14C]-D-galactose, [U-14C]-D-mannose, [U-14C]-D-glucose and [U-14C]-D-glucosamine) were injected intraventricularly into the forebrains of day-old chickens, and the relative incorporation, after 6 hr, into the glycoprotein sugars, aglycosyl protein, glycolipid sugars and aglycosyl lipid assessed. In addition, the relative total and specific activities of the individual glycoprotein sugars was also determined. Exclusive labelling of glycoprotein sugars (and of the fucose within these oligosaccharides) using fucose as precursor was confirmed, while optimal labelling of glycolipid sugars was achieved using the precursor mannose. Galactose, mannose, glucose and glucosamine were converted to varying degrees to other sugars and non-sugar precursors prior to incorporation. Glucosamine gave rise to the most even distribution of radioactivity among the glycoprotein sugar residues. The highest relative specific activities of the glycoprotein oligosaccharides were achieved using fucose and glucosamine as precursors.

Incorporation of intrastriatally injected[3H]fucose into electrophoretically separated synaptosomal glycoproteins. I. Turnover and molecular weight estimations

The radioactivity profiles of electrophoresed neostriatal P2 fraction glycoproteins were examined at a series of times (2.5, 3 and 4 h; 1, 5 and 10 days) following intracranial injections of [3H]fucose into the neostriatum. Ten major fucosylglycoprotein peaks were discerned in these profiles and certain aspects of their metabolism were characterized. The half-life of fucosylglycoproteins in the P2 fraction was estimated to be 9.7 days. The half-lives of the individual glycoprotein peaks ranged from 4.9 to 17.9 days. The apparent molecular weights of the glycoprotein peaks obtained by our procedures ranged from 32,000 to 180,000 daltons. One peak (peak VIII) incorporated radioactivity primarily at short intervals following the injection. The time course of [3H] fucose incorporation into this peak suggests involvement in the transport, activation and/or incorporation of fucose in brain. Since intracranial injections of [3H]fucose are incorporated into proteins in the cell body, synaptosomal fractions from caudate neurons alone are labeled by this technique. This may be useful in separating pre- and postsynaptic glycoprotein biochemistry. Finally, we tentatively propose that the glycoprotein peaks observed in neostriatum may be identical to previously isolated glycoproteins of known function or subcellular location.

Glucosamine incorporation into rat cerebrum: effect of adrenalectomy, corticosterone, exercise, and training

Incorporation of D-[I-14C]glucosamine into various metabolic fractions was studied in an experiment designed to quantify the relative influence of physiological and behavioral factors. Different physiological states were established by sham operation (S), adrenalectomy (A), and adrenalectomy plus corticosterone replacement (H). Within each physiological condition the behavioral state was varied by swim-escape training (E), swimming exercise (X) or nonswimming controls (C). Adrenalectomy caused a generalized increase in label uptake by cerebral cortex and hippocampus, but precursor levels in the blood were elevated also, suggesting a systemic physiological effect. Behavioral state had no effect on overall uptake, but did influence the distribution of label between soluble and membrane-bound glycoproteins. These results indicate that D-[I-14C]glucosamine is an effective glycoprotein and ganglioside precursor in behavioral experiments, provided corrections for the influence of systemic physiological factors are made.

A study of 3H-L-flucose-containing glycoproteins of the crude synaptosomal fraction obtained from rat brain regions at various ages

The frontal cortex showed a marked elevation of incorporation of 3H-L-fucose into glycoproteins of the crude synaptosomal fraction at 9 days of age. The effect was observed at both 8 and 24 hr after intraperitoneal administration of radioactive fucose. The elevation was absent in the 1, 5, 16, and 25-day-old frontal cortices. It was also absent in the hippocampus and cerebellum at all ages studied. Fucose-labeled glycoproteins of high molecular weight in the crude synaptosomal fraction, as separated by polyacrylamide gels,were preferentially labeled in the hippocampus, frontal cortex, and cerebellum at all ages studied. In general, these glycoproteins became less prominent in the oldest animals studied (25 days). However, the relative amount of these glycoproteins varied with age of the animal, time between administration of isotope and sacrifice, and with the brain areas studied.

Brain gangliosides: increased incorporation of (1- 3H) glucosamine during training

The incorporation of (1-3H)glucosamine into biochemical fractions of brain was studied in mice trained in a conditioned avoidance jump-up task, in mice yoked to the trained animals, and in undisturbed mice in their home cages. The (3H)glucosamine was injected subcutaneously 15 min before 15 min of training, and the mice killed after a total pulse time of one hour. There was a consistent and significant increase of about 21 percent of the incorporation of the 3H into the brain ganglioside fraction when trained mice were compared with quiet mice. This increase was not observed in any of the other chemical fractions studied, including the total chloroform-methanol-soluble compounds, the non-ganglioside lipids, and the chloroform-methanol-insoluble compounds (radioactivity principally in glycoprotein). Yoked mice showed an intermediate level of incorporation, exhibiting only a 12 percent increase in the ganglioside fraction. When the individual ganglioside species were analyzed by thin-layer chromatography the changed incorporation was not specific for any ganglioside species.