Glucocorticoid induction of the alpha2,6 sialyltransferase enzyme in a mouse neural cell line

Effect of hydrocortisone on the production and glycosylation of an Fc-fusion protein in CHO cell cultures

Glucocorticoids are known to modulate various cellular functions such as cell proliferation, metabolism, glycosylation, and secretion of many proteins. We tested the effect of hydrocortisone (HC) on cell growth, viability, metabolism, protein production, and glycosylation of an Fc-protein expressing Chinese hamster ovary (CHO) cell culture. HC extended cell viability but impaired cell growth. The inhibitory effect on cell growth was dose-dependent and decreased when the glucocorticoid addition was delayed. When HC was added after 2 or 3 days of culture, an increase in glutamate consumption was observed, which was reversed by the glucocorticoid receptor antagonist mifepristone (Mif). Titer and specific productivity increased in the presence of HC. The increase in titer was only slightly reversed by Mif. On the other hand, Mif by itself induced an increase in titer to a level comparable to or higher than HC. Protein glycosylation was altered by the glucocorticoid in a dose- and time-dependent manner, with a shift to more acidic bands, which correlated with an increase in sialic acid moieties. This increase, which was not linked to a decrease in extracellular sialidase activity in HC-treated cultures, was reversed by Mif. Predictive models based on design of experiments enabled the definition of optimal conditions for process performance in terms of viability and titer and for the quality of the Fc-fusion protein in terms of glycosylation. The data obtained suggest a use of glucocorticoids for commercial production of Fc-fusion proteins expressed in CHO cells.

Sialylation enhancement of CTLA4-Ig fusion protein in Chinese hamster ovary cells by dexamethasone

The importance of glycoprotein sialic acid levels is well known, as increased levels have been shown to increase in vivo serum half-life profiles. Here we demonstrate for the first time that dexamethasone (DEX) was capable of improving the sialylation of a CTLA4-Ig fusion protein produced by Chinese hamster ovary (CHO) cells. DEX was shown to enhance the intracellular addition of sialic acid by sialyltransferases as well as reduce extracellular removal of sialic acid by sialidase cleavage. We illustrated that DEX addition resulted in increased expression of the glycosyltransferases alpha2,3-sialyltransferase (alpha2,3-ST) and beta1,4-galactosyltransferase (beta1,4-GT) in CHO cells. Based upon our previous results showing DEX addition increased culture cell viability, we confirmed here that cultures treated with DEX also resulted in decreased sialidase activity. Addition of the glucocorticoid receptor (GR) antagonist mifepristone (RU-486) was capable of blocking the increase in sialylation by DEX which further supports that DEX affected sialylation as well as provides evidence that the sialylation enhancement effects of DEX on recombinant CHO cells occurred through the GR. Finally, the effects of DEX on increasing sialylation were then confirmed in 5-L controlled bioreactors. Addition of 1 microM DEX to the bioreactors on day 2 resulted in harvests with average increases of 16.2% for total sialic acid content and 15.8% in the protein fraction with N-linked sialylation. DEX was found to be a simple and effective method for increasing sialylation of this CTLA4-Ig fusion protein expressed in CHO cells.

Stress, cognitive impairment and cell adhesion molecules

Stress has profound effects on brain structure and function, but the underlying mechanisms are still poorly understood. Recent studies imply that neuronal cell adhesion molecules of the immunoglobulin superfamily--NCAM and L1--are important mediators of the effects of stress on the brain. Chronic stress regimes that lead to hippocampal atrophy and spatial-learning impairment in rodents simultaneously induce a pattern of changes in cell adhesion molecule expression that fits with a role for these molecules in stress-induced neuronal damage and neuroprotective mechanisms. These findings highlight cell adhesion molecules as potential therapeutic targets to treat stress-related cognitive disturbances.

Repeated restraint stress suppresses neurogenesis and induces biphasic PSA-NCAM expression in the adult rat dentate gyrus

Chronic restraint stress has been shown to induce structural remodelling throughout the interconnected dentate gyrus-CA3 fields. To find out how this stressor affects the rate of adult hippocampal neurogenesis, we subjected rats to acute or chronic restraint stress and assessed the proliferation, survival and differentiation of newly born cells in the dentate gyrus. We also examined polysialylated neural cell adhesion molecule expression, a molecule normally expressed in immature neurons and important for morphological plasticity. The results show that acute restraint stress did not change either the proliferation of dentate gyrus precursor cells or the expression of polysialylated neural cell adhesion molecule, whereas 3 weeks of chronic restraint stress suppressed proliferation by 24% and increased polysialylated neural cell adhesion molecule expression by 40%. The study was extended for an additional 3 weeks to trace the survival and development of the cells born after the initial 3 weeks of restraint. Rats subjected to 6 weeks of daily restraint stress exhibited suppressed cell proliferation and attenuated survival of the recently born cells after the extended time course, resulting in a 47% reduction of granule cell neurogenesis. Furthermore, 6 weeks of chronic stress significantly reduced the total number of granule cells by 13% and the granule cell layer volume by 5%. Expression of polysialylated neural cell adhesion molecule followed a biphasic time course, displaying a significant up-regulation after 3 weeks of daily restraint stress that was lost after 6 weeks of stress. These studies may help us understand the basis for hippocampal shrinkage and raise questions about the ultimate reversibility of the effects of chronic stress.

Retinoic acid induction of sialyltransferase activity in neuroblastoma cells of differing sialylation potentials

In order to determine how glycosylation changes associated with cellular differentiation may be influenced by the basal cellular sialylation potential, the effect of retinoic acid (RA)-induced differentiation was investigated in neuroblastoma cells expressing differing levels (and activities) of the alpha2,6(N) sialyltransferase (ST6N) enzyme. The increase in ST activity was proportional to the basal cellular sialylation potentials with the high activity clones showing the greatest increase. This was paralleled by an up-regulation of the level of overall sialoglycoprotein glycosylation level. An increase in the levels of the polysialic acid (PSA) epitope was associated with a parallel increase in the levels of the neural cell adhesion molecule (NCAM) protein backbone although there was no overall change in the PSA:NCAM ratio following RA treatment.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.

Overexpression of alpha2,3 sialyltransferase in neuroblastoma cells results in an upset in the glycosylation process

Glycosylation is key posttranslational modification for membrane-bound and secreted proteins that can influence both the secondary structure and the function of the protein backbone. In order to investigate the effect of altered cellular glycosylation potential, we have generated a number of clonal cell lines over-expressing the alpha2,3(N) sialyltransferase enzyme (ST3N). In general, there was a decrease in total sialyltransferase (ST) enzyme activity in the clones transfected with the ST3N cDNA, with this decrease being inversely proportional to the quantity of the mRNA coding for the enzyme. The ST3N enzyme was, however, functional and there was an increase in both MAA lectin staining and the expression of polysialic acid, which is attached to the NCAM protein backbone primarily via an alpha2,3 linkage. These results suggest that the overexpression of a sialyltransferase may upset the sialylation potential of the cell.

Opposite effects on NCAM expression in the rat frontal cortex induced by acute vs. chronic corticosterone treatments

The temporal pattern of exposure to glucocorticoids has been reported to be a critical factor in determining the outcome of glucocorticoid actions at the brain. In this work, the effects of different regimes of subcutaneous corticosterone administration (acute-single injection-vs. chronic-daily injection for 21 days) on the expression of the neural cell adhesion molecule (NCAM) were evaluated in different rat brain regions (CA1-CA4, dentate gyrus, frontal cortex, striatum, and hypothalamus). The treatments were selected according to previous studies in which we showed biphasic effects of corticosterone on memory formation, with acute corticosterone effects being facilitating and chronic effects being deleterious. In addition, the chronic treatment was shown by others to result in structural alterations at the hippocampus. NCAM was evaluated given its cell-cell recognition and adhesion properties, and the involvement on synaptic stabilisation subserving long-term memory formation. The results showed a biphasic modulation of NCAM levels at the frontal cortex, with acute corticosterone resulting in enhanced NCAM levels at 8 h and 24 h posttraining, and the chronic treatment decreasing its expression. None of the other brain areas examined showed significant changes in NCAM expression with corticosterone treatments, except for the hypothalamus that showed reduced NCAM levels after the chronic corticosterone regime. These results support the view that NCAM regulation at the frontal cortex might be a mechanism by which corticosterone treatments influence memory formation. They also highlight the hypothalamus as a brain area particularly sensitive to NCAM regulation by prolonged exposure to elevated glucocorticoids.

Stimulation of sialyltransferase by subchronic low-level lead exposure in the developing nervous system. A potential mechanism of teratogen action

Chronic low-level lead exposure has been associated with mental deficits in young children, possibly due to its actions on specific targets in the developing nervous system. Protein glycosylation has been demonstrated to play a critical role during CNS development, and the negatively charged sialic acid group has been particularly associated with the modulation of cell adhesion. In this study, we have used an in vitro model system to examine the effect of subchronic low-level lead on the expression and activity of the sialyltransferase (ST) enzyme family. Subchronic exposure of neuronal cells to low concentrations of lead (10(-6)-10(-16) M) resulted in up to a 3-fold induction of total cellular ST activity, the level of induction being more pronounced in embryonically derived cells compared with postnatally derived cells. The increase was not due to a direct interaction of the metal with the enzyme and was only observed after at least 72 h exposure to the metal. The induction was blocked by the protein synthesis inhibitor, cycloheximide, and could be reversed upon removal of the metal. The increase was due primarily to the induction of the alpha2,3(N) ST enzyme with no effect on the expression of the alpha2,6(N) enzyme. These results suggest that the ST enzyme may serve as a target for the actions of chronic low-level lead in vivo with an alteration in the developmental regulation of protein glycosylation being at least partially responsible for the behavioral deficits associated with toxin exposure.