Glucocorticoid potentiation of lead neurotoxicity in the mouse HN9 hippocampal cell line

The biochemical events underlying neurobehavioural deficits following persistent low-level lead exposure during embryonic and early postnatal development remain unclear. Because glucocorticoids have previously been demonstrated to potentiate the action of certain neurotoxins, their role in low-level lead-mediated neurotoxicity was examined using a glucocorticoid-responsive immortalized hippocampal neural cell line (HN9). Low-level lead (10(-10)m) reduced the cell number by approximately 30% over a 5-day treatment period. This effect was potentiated by treatment with the synthetic glucocorticoid dexamethasone, with a 52% decrease in cell number. Lead toxicity in actively differentiating cells was also potentiated by dexamethasone. However, while neither lead nor dexamethasone alone was toxic for cells that had previously been induced to differentiate, a combination of both drugs significantly reduced cell number. These results suggest that lead is preferentially toxic in actively dividing cells and that glucocorticoids may play a role in the potentiation of the lead-induced neurotoxicity.

The amyloidogenic potential and behavioral correlates of stress

Observations of elevated basal cortisol levels in Alzheimer's disease (AD) patients prompted the hypothesis that stress and glucocorticoids (GC) may contribute to the development and/or maintenance of AD. Consistent with that hypothesis, we show that stress and GC provoke misprocessing of amyloid precursor peptide in the rat hippocampus and prefrontal cortex, resulting in increased levels of the peptide C-terminal fragment 99 (C99), whose further proteolytic cleavage results in the generation of amyloid-beta (Abeta). We also show that exogenous Abeta can reproduce the effects of stress and GC on C99 production and that a history of stress strikingly potentiates the C99-inducing effects of Abeta and GC. Previous work has indicated a role for Abeta in disruption of synaptic function and cognitive behaviors, and AD patients reportedly show signs of heightened anxiety. Here, behavioral analysis revealed that like stress and GC, Abeta administration causes spatial memory deficits that are exacerbated by stress and GC; additionally, Abeta, stress and GC induced a state of hyperanxiety. Given that the intrinsic properties of C99 and Abeta include neuroendangerment and behavioral impairment, our findings suggest a causal role for stress and GC in the etiopathogenesis of AD, and demonstrate that stressful life events and GC therapy can have a cumulative impact on the course of AD development and progression.

The over-expression of the wild type or mutant forms of the presenilin-1 protein alters glycoprotein processing in a human neuroblastoma cell line

Mutations in the presenilin proteins (PS1 and PS2) are responsible for more than 70% of the cases of the familial form of Alzheimer's disease (FAD). The proteins are expressed in the cell at a low level, primarily in the endoplasmic reticulum and cis Golgi, where they have been proposed to play a role in protein processing. As protein glycosylation is a key post-translational event that occurs within the Golgi, we have investigated the effect of altered PS1 expression levels on the protein glycosylation pattern using the SH-SY5Y human neuroblastoma cell line. In cells over-expressing either the wild type or mutant (M146L) PS1-FAD proteins, there was a decrease in the expression levels of protein-bound alpha2,3-linked sialic acid residues at the level of the cell membrane. This was particularly manifest as a significant decrease in the expression of the polysialic acid chain that is linked to the core oligosaccharide of the neural cell adhesion molecule in an alpha2,3 bond. These results suggest that the over-expression of either the wild type or mutant PS1 disturbs glycoprotein processing within the Golgi.

The role of post-translational modification in beta-amyloid precursor protein processing

The beta-amyloid precursor protein (APP) plays a pivotal role in the early stages of neurodegeneration associated with Alzheimer's disease. An alteration in the processing pattern of the protein results in an increase in the generation of the 40-42-amino-acid beta-amyloid (A beta) peptide, which coalesces to form insoluble, extracellular amyloid deposits. A greater understanding of the factors that influence APP processing may assist in the design of effective therapeutic agents to halt progression of Alzheimer's disease. APP is a sialoglycoprotein with two potential N-linked glycosylation sites, one of which may contain a complex oligosaccharide chain. An alteration in the glycosylation state of APP by the generation of oligomannosyl oligosaccharides results in a decrease in the secretion of the neuroprotective, soluble form of the protein and a parallel increase in the deposition of the cellular protein within the perinuclear region of the cell. Conversely, the attachment of additional terminal sialic acid residues on to the oligosaccharide chain results in an increase in secretion of soluble APP (sAPP alpha). One factor that has been widely reported to alter APP processing is the activation of protein kinase C (PKC). This process has been characterized using synaptosomal preparations, which suggests that the PKC action is occurring at the level of the plasma membrane. Furthermore, when cells are transfected with the sialyltransferase enzyme, there is a direct relationship between the sialylation potential of APP and the fold stimulation of sAPP alpha, after PKC activation. These results suggest that the post-translational modification of APP by glycosylation is a key event in determining the processing of the protein.

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.

Activation of phospholipase D by metabotropic glutamate receptor agonists in rat cerebrocortical synaptosomes

The pharmacological profile of metabotropic glutamate receptor (mGluR) activation of phospholipase D (PLD), and the associated signalling pathways, were examined in rat cerebrocortical synaptosomes. The assay was conducted using a transphosphatidylation reaction in synaptosomes which were pre-labelled with either [(3)H]-arachidonic acid or [(32)P]-orthophosphate. The mGluR agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD) and (RS)-3,5-dihydroxyphenylglycine (DHPG), both activated PLD, while phorbol 12,13-dibutyrate (PDBu) treatment caused receptor-independent activation of PLD and had an additive effect on 1S,3R-ACPD induced PLD activity. A protein kinase C (PKC) inhibitor, GF109203X, failed to antagonize mGluR receptor-coupled PLD activity. We could not detect any increase in the products of PI (phosphoinositide)-specific phospholipase C (PI-PLC), inositol(1,4, 5)trisphosphate or diacylglycerol, by 1S, 3R-ACPD at 15 s. However, diacylglycerol increased monophasically in response to mGluR agonists and remained elevated for at least 15 min. Phosphatidic acid phosphohydrolase (PAP) activity, which converts PA to DAG, was present in the synaptosomes. These data suggest that, in rat cerebrocortical synaptosomes, the 1S,3R-ACPD-sensitive mGluR is coupled to PLD through a mechanism that is independent of both PKC and PI-PLC.

Inhibition of N-glycan processing alters axonal transport of synaptic glycoproteins in vivo

Synaptic glycoproteins are synthesized and glycosylated in the neuronal cell body, and conveyed to terminals by fast axonal transport. We used the alpha-mannosidase inhibitor, 2-deoxymannojirimycin (dMan), to investigate the effects of disrupting N-glycan processing on the axonal trafficking of proteins in vivo. dMan significantly reduced rapid axonal transport in retinal ganglion cells to about 34% of control values 4h after metabolic labeling; at 8 h post-labeling the inhibition was reversed. 2-D gel analysis showed that dMan completely inhibited the arrival of radiolabeled L1 and NCAM at axon terminals, and resulted in the appearance of two novel proteins of 230 kDa and 155 kDa. Our results show that disruption of the N-glycosylation pathway has an immediate inhibitory effect on total axonal transport and longer lasting effects on the trafficking of specific glycoproteins to axon terminals in vivo.

Factors influencing the processing and function of the amyloid beta precursor protein--a potential therapeutic target in Alzheimer's disease?

The amyloid beta precursor protein (AbetaPP), which plays a pivotal role in Alzheimer's disease (AD), can exist as either a membrane-bound or soluble protein. The former is cleaved at the level of the plasma membrane to generate the soluble form of the protein (AbetaPP(s)). An alternative pathway exists, however, for the cleavage of AbetaPP to generate a 40-42 amino acid peptide termed amyloid beta (Abeta), either within the lysosomal or the endoplasmic reticulum/Golgi compartments of the cell. In AD, there is an increase in the ratio of the 42 amino acid form of the Abeta peptide (Abeta(42)) to Abeta(40). The Abeta(42) form is the more amyloidogenic form and has an increased potential to form the insoluble amyloid deposits characteristic of AD pathology. Studies on the familial form of the disease, with mutations in AbetaPP or in the presenilin proteins, have confirmed an increase in Abeta(42) generation associated with the early stages of the disease. This review will examine the factors that influence AbetaPP processing, how they may act to modulate the biological effects of AbetaPP(s) and Abeta, and if they provide a viable target for therapeutic intervention to modify the rate of progression of the disease.

Overexpression of the alpha2,6 (N) sialyltransferase enzyme in human and rat neural cell lines is associated with increased expression of the polysialic acid epitope

The function of the neural cell adhesion molecule, NCAM, is modulated by the expression of the N-linked polysialic acid (PSA) oligosaccharide chain, with PSA serving to decrease the adhesive potential of the protein backbone. In this study, we have generated clonal cells of the rat B104 and human SH-SY5Y neuroblastoma cell lines that over-express the alpha2,6(N) sialyltransferase (ST6N) enzyme in order to investigate the role of this enzyme in PSA biosynthesis. The clonal cells exhibited ST enzyme activities of up to 20-times control levels, which remained stable throughout the duration of the study. The increase in enzyme activity paralleled an increase in enzyme protein levels, as determined by Western blot analysis, and immunocytochemical analysis confirmed the Golgi localisation of the enzyme. The induction of PSA-NCAM expression in the cells expressing high levels of ST6N was confirmed both by using anti-PSA antisera and by specific digestion with endo-N-acetylneuraminidase E, whose actions are specific for alpha2, 8-linked PSA chains. These results demonstrate that the cellular ST6N activity serves to positively influence the expression of PSA in neuronal cells.

The effect of corticosteroids on amyloid beta precursor protein/amyloid precursor-like protein expression and processing in vivo

In this study, we have investigated the effect of altered corticosteroid levels on the expression and processing of the amyloid beta precursor protein (A betaPP) and its amyloid precursor-like protein (APLP) homologue in rat brain. Four groups of animals were used in the study: sham operated, adrenalectomised, and adrenalectomised treated with either dexamethasone or aldosterone, with the A betaPP/APLP expression being determined by western blot analysis. While there were no changes in the levels of A betaPP/APLP following adrenalectomy, treatment with dexamethasone, but not aldosterone, resulted in a marked increase in protein expression levels with the level of increase varying between the brain regions examined. Corticosteroids had a more marked effect on the particulate rather than the soluble form of the protein, thus suggesting that elevated glucocorticoids may also be adversely influencing A betaPP/APLP processing.

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.

Modest cholinergic deafferentation fails to alter hippocampal G-proteins

The integrity of hippocampal G-protein mediated signalling following ibotenate induced lesion of the medial septum was examined. The lesion was confined histologically to the septum and induced a 23% reduction in hippocampal choline acetyltransferase (ChAT) activity and G-proteins levels and related enzyme activities were measured in the hippocampus following a 21 day survival period. The relative levels of five G-protein subunits (Gbeta, G(alpha)o, G(alpha)i1, G(alpha)i2, and G(alpha)s-L), basal GTPase, the degree of carbachol- or baclofen-stimulated GTPase activities, and the basal and fluoroaluminate-stimulated adenylate cyclase activities were apparently unaffected. To determine if our assay methodology was sensitive to changes in pre-synaptic signalling, we compared G-protein density in synaptosomes with total hippocampal homogenates. The concentration of G(alpha)q/11, G(alpha)i1, and G(alpha)i2. were significantly lower in synaptosomes, while G(alpha)o, was only marginally reduced. Thus, modest lesions of the medial-septal nucleus fail to alter G-protein signalling. However, our findings that G-protein density is lower in synaptosomal membranes than in total homogenates, indicates that the analysis of signalling events in synaptosomes following deafferentation could clarify adaptive changes which may occur at the presynaptic level.

The role of the protein glycosylation state in the control of cellular transport of the amyloid beta precursor protein

The amyloid beta precursor protein can exist as both a membrane-bound and a secreted protein, with the former having the potential to generate the amyloid beta peptide present in the neuritic plaques which are characteristic of Alzheimer's disease. In this study, we have used a clone of the AtT20 mouse pituitary cell line which expresses high levels of the amyloid beta precursor protein to characterize the glycosylation state of the secreted and membrane-bound forms of the protein and to examine the role of post-translational modifications in protein processing. Lectin blot analysis of immunoprecipitated amyloid beta precursor protein demonstrated that the soluble form of the protein contains significant amounts of sialic acid, with the lectin staining being reduced in the particulate cellular fractions. Treatment of the cells with mannosidase inhibitors to interfere with the formation of complex-type N-linked glycans resulted in a decrease in secreted amyloid beta precursor protein and an increase in the level of the cellular form of the protein. The increase in amyloid beta precursor protein levels in the cellular fraction was accompanied by an increase in perinuclear staining. Furthermore, cells overexpressing the alpha2,6(N)-sialyltransferase enzyme also demonstrated an increase in amyloid beta precursor protein secretion. These results suggest that the presence of terminal sialic acid residues on complex-type N-glycans may be required for the optimal transport of the amyloid beta precursor protein from the Golgi to the cell membrane with the subsequent cleavage to generate the secreted form of the protein.

Protein kinase C activation potentiates the rapid secretion of the amyloid precursor protein from rat cortical synaptosomes

In this study we have used the presynaptic-rich rat cerebrocortical synaptosomal preparation to investigate the proteolytic cleavage of the amyloid precursor protein (AbetaPP) by the alpha-secretase pathway within the betaA4 domain to generate a soluble secreted N-terminal fragment (AbetaPPs). AbetaPP was detected in crude cortical synaptosomal membranes, although at a lower density than that observed in whole-tissue homogenates. Protein kinase C (PKC) activation induced a translocation of the conventional PKC isoform beta1 and novel PKCepsilon from cytosol to membrane fractions, but there was no alteration in the proportion of AbetaPP associated with the Triton-soluble and -insoluble fractions. AbetaPPs was constitutively secreted from cortical synaptosomes, with this secretion being enhanced significantly by the direct activation of PKC with phorbol ester. The PKC-induced secretion of AbetaPPs was only partially blocked by the PKC inhibitor GF109203X (2.5 microM), whereas the phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein was significantly inhibited by GF109203X. The differential sensitivities of the MARCKS phosphorylation and AbetaPPs secretion to GF109203X may imply that different PKC isoforms are involved in these two events in the synaptosomal system. These findings strongly suggest that the alpha-secretase activity leading to the secretion of AbetaPPs can occur at the level of the presynaptic terminal.

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.

The interaction between chronic low-level lead and the amyloid beta precursor protein

Chronic low-level lead exposure is toxic to the developing nervous system. The amyloid beta precursor protein (A beta PP) plays a pivotal role in this developmental process, both as a neurotrophic/neuroprotective factor and as a mediator of cell adhesion. In this study, we have used an in vitro system to examine the interaction between chronic low-level lead and the expression and function of A beta PP. Chronic exposure of the HN9 mouse hippocampal cell line to lead chloride (10(-14) M to 10(-6) M) for 96 hours resulted in a 50% increase in the levels of the particulate form of the protein with a parallel decrease in the soluble form (A beta PP). This effect of lead was reversible following the removal of the toxin. This increase in membrane-bound A beta PP was also paralleled by an increase in cell adhesivity to a fibronectin substrate. In addition, A beta PP also acted to attenuate lead toxicity. Cells which secreted high levels of the protein were resistant to lead toxicity when compared with control cells suggesting that the protein may be acting to chelate the metal and thus attenuating its toxic action within the cell.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

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

Combined sialyltransferase (ST) activities were induced in the HN9 hippocampal cell line following treatment with the synthetic glucocorticoid dexamethasone (dex) for 24 hr. Induction occurred in a dose-dependent manner, with the maximum induction of a 2-fold increase over control enzyme levels occurring at a concentration of 10(-8) M dex. A minimum of 6 hr pretreatment with drug was required before significant induction could be detected and elevated enzyme levels persisted for up to 48 hr post-treatment. The induced form of the enzyme showed an increase in reaction maximum velocity (Vmax) while showing no change in affinity (Km) for the acceptor substrate asialofetuin. The alpha2,6 ST enzyme was demonstrated to be the primary enzyme induced and there was no change in expression of the alpha2,3 ST enzyme. Lectin blot analysis demonstrated an increase in the levels of the alpha2,6-linked cellular sialoglycoproteins and a parallel decrease in the alpha2,3 sialoglycoprotein levels.

The generation and characterization of a rat neural cell line overexpressing the alpha2,6(N) sialyltransferase

In order to examine the effects of altered protein sialylation on neural cell function, B104 rat neuroblastoma cells were stably transfected with the cDNA coding for alpha2,6(N) sialyltransferase (ST(6)N). Lectin blot analysis of the clones demonstrated an increase in staining of the Sambucus nigra lectin, which detects alpha2,6 linked sialic acid, in parallel with enzyme activity. There was a concomitant decrease in staining by the Maackia amurensis lectin which labels alpha2,3-linked sialic acid, indicating that the individual sialyltransferase enzymes may compete for penultimate galactose acceptor sites. While there was an initial increase in protein-bound sialic acid in parallel with enzyme activity, the sialylation of the cells was demonstrated to be saturable. There was an inverse relationship between cell adhesion to a fibronectin substrate and ST(6)N activity suggesting that the negatively charged sugar acts to modulate cell-substrate interaction. These cells will provide an ideal model system with which to further investigate the effect of altered sialic acid on neural cell function.

The effect of corticosteroids on serum sialyltransferase enzyme activities in the rat

Previous studies have demonstrated corticosteroid regulation of sialyltransferase (sialyl-T) enzyme activities in a number of different tissues throughout the body. In this study we examined the regulatory effect of corticosteroids on serum enzyme activity in the rat. The total serum sialyl-T activity was not affected by a decrease in corticosteroid levels following adrenalectomy. However, while there was a significant increase in enzyme activity following dexamethasone treatment, aldosterone had no effect on this parameter. Subsequent examination of individual sialyl-T enzymes demonstrated a slight decrease in alpha2,6 sialyl-T activity following adrenalectomy which was restored to basal levels following dexamethasone treatment. The activity of the alpha2,3 sialyl-T enzyme was not affected by adrenalectomy or dexamethasone treatment, but was stimulated significantly by aldosterone. In general, the levels of serum sialoglycoproteins mirrored well the activities of the appropriate sialyl-T enzymes. These results demonstrate that serum sialyltransferase activity in the rat is under the influence of circulating corticosteroids.

The biochemical consequences of alpha2,6(N) sialyltransferase induction by dexamethasone on sialoglycoprotein expression in the rat H411e hepatoma cell line

Previous studies have demonstrated sialyltransferase (ST) enzyme activity to be induced in hepatic cells by corticosteroids. In this study, we used the H411e rat hepatoma cell line to further characterise this induction with particular reference to the subsequent changes in the pattern of sialoglycoprotein (SGP) expression. The induction of total ST activity by dexamethasone was concentration dependent with maximum induction occurring 12 h subsequent to drug addition. Western blot analysis demonstrated that the induction was associated with an increase in the expression of the alpha2,6(N) ST enzyme with no change in the expression levels of the alpha2,3(N) enzyme. While the induction resulted in an increase in the reaction velocity (Vmax) of the enzyme for both the sugar donor (CMP-Neu5Ac) and the asialofetuin acceptor protein, there was no significant change in the enzyme affinity (Km) for the substrates, suggestive of either an increase in the expression or efficiency of the existing enzyme(s) rather than an induction of novel ST enzymes. Lectin blot analysis of cellular glycoprotein expression demonstrated no change in the expression patterns of either alpha2,3 or alpha2,6-linked SGP following enzyme induction. These results suggest that the available acceptor sites for the terminal sialic acid group(s) may be fully occupied in the control cells and therefore there are no further sites onto which the sialic acid can be transferred following induction of ST enzyme activity. This may be due to the high basal enzyme levels in the control cells already exhausting endogenous acceptor sites.

Individual isoforms of the amyloid beta precursor protein demonstrate differential adhesive potentials to constituents of the extracellular matrix

The amyloid beta precursor protein (AbetaPP) can exist as a membrane-bound glycoprotein which modulates neural cell adhesion. The adhesion of clones of the AtT20 mouse pituitary cell line, transfected with cDNA coding for the 695 (AbetaPP695) and 751 (AbetaPP751) amino acid forms of the protein, to individual components of the extracellular matrix was determined using a centrifugal shear assay. On laminin, poly-L-lysine, fibronectin, and uncoated glass substrata, the cells transfected with AbetaPP695 (6A1 cells) demonstrated a 50% increase in adhesivity over nontransfected cells, while those transfected with AbetaPP751 (7A1 cells) showed a significant decrease in adhesion. There was, however, a significant increase in the adhesive strength of the 7A1 cells to collagen type IV with no change in the adhesivity of the 6A1 cells when compared with control. These changes in adhesivity could be attributed to changes in the levels of the membrane-bound protein and were not due to the interaction of soluble AbetaPP with elements of the extracellular matrix. These studies provide evidence for differential adhesivities of the constituent AbetaPP isoforms and the possible role of the Kunitz protease inhibitor (KPI) domain in influencing the adhesive properties of the protein backbone.

Plasma sialyltransferase levels in psychiatric disorders as a possible indicator of HPA axis function

A dysfunction in the regulation of the hypothalamic-pituitary-adrenal (HPA) axis, possibly attributed to a change in glucocorticoid receptor (GR) functionality, has been implicated in depression. We have measured both lymphocyte GR receptor binding parameters and plasma sialyltransferase activity, as a biochemical marker of GR function, in two groups of patients suffering from depression or schizophrenia and in a group of age- and sex-matched controls. While there was a significant increase in plasma cortisol levels in the depressed group, there were no changes in the lymphocyte GR binding parameters (K(m) and Bmax). There was, however, a significant decrease in the plasma sialyltransferase: cortisol ratio in the depressed group suggesting an inability of the raised cortisol levels to induce enzyme expression and this ratio may provide a useful biochemical marker of cortisol receptor function. Although there was an increase in the plasma activity of the alpha 2,6 sialyltransferase isozyme in the schizophrenic group, no other changes were determined. Therefore, while the total plasma sialyltransferase:cortisol ratio reflects HPA axis function, alterations in specific isozyme activity may also be associated with other CNS disease states.

Evidence for a correlation between ambient cholesterol levels and soluble plasma sialyltransferase enzyme activity

While soluble forms of the sialyltransferase (sialyl-T) enzyme have been detected in significant quantities in serum, the exact source(s) of the enzyme, or the factors controlling its secretion are poorly understood. In this study, we have examined the relationship between ambient plasma cholesterol concentrations and sialyl-T-activities and also levels of constituent plasma sialoglycoproteins (SGP). There was an inverse relationship between levels of the alpha 2, 6 sialyl-T enzyme and both total plasma cholesterol and HDL, although no such relationship was observed for the alpha 2,3 enzyme. While there was no correlation between total cholesterol and the levels of plasma SGPs, there was an inverse relationship between the HDL component and alpha 2,3 SGPs.

Tissue-specific regulation of sialyltransferase activities in the rat by corticosteroids in vivo

In this study we have examined the effects of corticosteroids on both the total sialyltransferase (sialyl-T) activity and on two individual isozymes in neural, hepatic, and renal tissues using an in vivo model system. Rats were adrenalectomised to deplete their endogenous stores of steroid hormones, and some subsequently received steroid replacement with dexamethasone or aldosterone. Adrenalectomy resulted in a significant decrease in total neural sialyl-T activity when compared with sham-operated animals in the four brain regions examined, indicating that total sialyl-T activity is normally under positive corticosteroid control. The subsequent effects of exogenous corticosteroids exhibited regional specificity with the enzyme activities in the cortex, cerebellum, and brainstem being stimulated by both dexamethasone and aldosterone and enzyme activity in the hippocampus being stimulated by aldosterone alone. In general, the changes in total enzyme activity could be attributed to the alpha 2,6 sialyl-T isozyme, although the changes in the cerebellum appeared to coincide with alpha 2,3 sialyl-T activity. In the liver, adrenalectomy resulted in an increase in enzyme activity which was not altered by administration of corticosteroids. There were no changes in total renal sialyl-T activity in any of the four experimental groups although certain changes were observed at the level of individual sialyl-T isozymes. These results demonstrate that sialyl-T activity in certain tissues is under the control of corticosteroids and that this is both a tissue-specific and region-specific effect.

The control of sialyltransferase activity in tumor-cell lines derived from different tissues in multifactorial

The activities of the sialyltransferase enzymes and the resulting expression of sialoglycoproteins were examined in tumor cells derived from different tissues in order to gain a greater understanding of the factors controlling the cell glycosylation state. Cell-cell contact, which is dependent on cell confluency state, was shown to influence glycosylation in the neurally-derived mouse neuro-2A neuroblastoma and the C6 glioma cell lines. Both showed a relatively high level of cell sialyltransferase activity under sub-confluent conditions with activity decreasing upon the formation of cell-cell contacts associated with confluency. A parallel decrease in the expression of sialoglycoproteins, as determined by lectin blot analysis, was observed under these conditions. In contrast, the H411e hepatoma cell line showed an increase in enzyme activity with confluency with the susceptibility of the enzyme in this cell line to glucocorticoid induction only being detected in sub-confluent cell cultures. The number of trypsinisation cycles of the cells was also shown to affect the enzyme activity of the neuro-2A and C6 cells with an increase in enzyme activity coincident with passage number being observed in the neuro-2A cells, and a decrease in the C6 glioma cell line. Trypsinisation had no effect on enzyme activity in the H411e cells. These results demonstrate that the control of sialyltransferase activity in tumor cells is multifactorial with the tissue of origin playing a key role.

A decrease in neural sialyltransferase activity in Alzheimer's disease

The activities of both a particulate and soluble form of the sialyltransferase enzyme have been examined in post-mortem brain samples from Alzheimer's disease patients and age-matched controls. There was a considerable decrease in the activity of both the soluble and membrane-bound forms of the enzyme in the frontal and temporal cortical lobes, although no change was observed in the hippocampus. There was, however, no change in activity of the Golgi marker enzyme thiamine pyrophosphatase. Therefore, it is suggested that the decrease in sialyltransferase enzyme activity may be a specific biochemical event associated with the AD-like neurodegeneration.

Expression of the neural cell adhesion molecule (NCAM) in Alzheimer's disease

Expression of the nerve cell adhesion molecule, NCAM, was examined in post-mortem brain samples from patients with Alzheimer's disease (AD) and from age-matched controls. Four major isoforms of NCAM were identified by Western blot analysis 195, 185, 145 and 120 kDa with no differences being observed in either the intensity of individual band staining or in the expression of the constituent NCAM isoforms in AD when compared with controls. Immunohistochemical staining revealed star-shaped NCAM-positive structures with numerous radiating 'processes' or a more protoplasmic shape with short and thick 'processes'. These structures were not obviously more abundant in AD and there was no NCAM immunoreactivity in neuritic plaques or neurofibrillary tangles. This evidence suggests there is no change in NCAM expression in AD.

The effect of cell confluency state on the expression of neural cell surface glycoconjugates

The expression of neural cell surface glycoconjugates during development reflects the precise control of cellular adhesivity that is required for the exact positioning of the developing cells. We have investigated the effect of cell confluency state on the expression of key cell surface glycoconjugates using the mouse neuroblastoma cell line, neuro-2A. There was an up-regulation of expression of the neural cell adhesion molecule (NCAM) coincident with an increase in cell confluency state and a parallel decrease in the expression of the amyloid precursor protein, beta APP. The expression pattern of cell surface glycoconjugates in neuro-2A cells is therefore similar to that observed during the early embryonic period of development during which cells aggregate to form collectives.

A decrease in serum sialyltransferase levels in Alzheimer's disease

A reliable antemortem serum marker for Alzheimer's disease (AD) would be of great importance for the early detection and subsequent therapeutic management of the disease. We have noted a significant decrease in serum levels of the soluble form of the sialyltransferase enzyme in a group of AD patients when compared with both age-matched elderly (over 60 years) and young (under 60 years) controls. In a population of Down's syndrome patients, who develop AD pathology with increasing age, there was an age-related decrease in serum sialyltransferase activity in patients from 20 to 60 years to approach enzyme levels similar to those observed in the AD group. This significant decrease in serum sialyltransferase levels observed may both prove a useful peripheral early biochemical marker of neurodegeneration and provide an indication of the underlying cellular events that occur during the process of nerve cell death in AD.

Neurofilament expression in human T lymphocytes

The expression of intermediate filaments in normal cells is mainly determined by their embryonal developmental origin. Flow cytometry using monoclonal antibody RT97 demonstrated that neurofilament was detectable in the human HuT 78 T-cell line and on resting T lymphocytes. Expression was greatly increased on lymphocytes activated for 3 days with phorbol ester. Western blotting confirmed the presence of the 200,000 MW form of neurofilament in T lymphocytes. Stimulation of peripheral blood T cells with phorbol myristate acetate (PMA) or with anti-CD3 monoclonal antibodies resulted in a marked increase in detection of phosphorylated neurofilament on Western blotting. Stimulation of HuT 78 cells with anti-LFA-1 resulted in redistribution of neurofilament from a perinuclear spheroid core into dendritic processes. These data indicate that T cells activated through the T-cell receptor associated complex express an intermediate filament usually associated with neurally derived cells. The finding that neurofilament expression and organization are regulated by T-cell surface molecules suggests a role for this intermediate filament in T-cell function.

APP-collagen interaction is mediated by a heparin bridge mechanism

The amyloid precursor protein (APP) is a glycoprotein consisting of at least four isoforms derived from a single gene by a process of alternative splicing. The membrane-bound forms of APP have been suggested to have adhesive properties and to mediate neural cell adhesion. Previous studies have demonstrated the ability of Fab' fragments of antibodies to extracellular domains of APP to inhibit neural cell binding to a collagen substrate, suggesting a physiological role for the collagen-binding properties of APP. The binding of APP has been demonstrated to be specific for type IV collagen, and no binding to other extracellular matrix components, including fibronectin and laminin, was detected. The APP-collagen binding appeared to be mediated by a heparin-bridge mechanism, since the binding was abolished by the addition of excess heparan or heparinase. These results were observed by both a homogenate-collagen binding assay and a cell-surface adhesion assay, thus providing further evidence for the adhesion role of APP. They also pose the question of the possible role of the heparin-binding properties of APP in the genesis of the neuritic plaques characteristic of Alzheimer's disease.

Temporal expression of neurofilament polypeptides in differentiating neuroblastoma cells

NEURO-2A neuroblastoma cells were differentiated by the addition of dibutyryl-cyclic AMP, resulting in an increase in transcription of mRNA coding for the two neurofilament polypeptides NF-L and NF-M. The mRNAs for these two polypeptides appeared to be co-expressed with both being switched on at 24 h after the addition of drug. This was in contrast to NF-H whose induction was only seen at 60 h following addition of drug. These results are in good agreement with the expression time course previously reported in-vivo and suggest that the neurofilament triplet polypeptides can be divided into two subsets which appear to be under different genetic control mechanisms according to their time course of expression.

Beta amyloid precursor protein mediates neuronal cell-cell and cell-surface adhesion

The beta-amyloid precursor protein (APP) is a membrane-bound glycoprotein which has been proposed to play a role both as a growth factor and a mediator of cell adhesion. Using the Neuro-2A neuroblastoma cell line, we have investigated the capacity of APP to mediate neural cell adhesion. The cells express the protein at a high level, the immunohistochemical staining pattern at the level of the membrane having a punctate pattern. Fab' fragments of antibodies to the extracellular portion of the molecule were found to inhibit cell binding to a collagen substrate, but not to laminin, fibronectin, or poly-l-lysine. Fab' fragments of antibodies to the nerve cell adhesion molecule N-CAM also inhibited binding of Neuro-2A cells specifically to collagen. This inhibition of cell-surface binding was accompanied by a repression of neurite outgrowth in differentiating cells in the presence of antibodies. APP antibodies also inhibited neuron-neuron and neuron-glial binding, but not glial-glial cell adhesion. These data suggest that the APP, which is expressed primarily on differentiated neuronal cells, may play a role in the mediation of both cell-cell and cell-substrate adhesion.

Intraventricular infusions of antibodies to amyloid-beta-protein precursor impair the acquisition of a passive avoidance response in the rat

Intraventricular infusions of an antiserum raised against a 14 amino acid residue in the extracellular domain of amyloid-beta-protein precursor significantly decreased stepdown latency, at both 24 h and 48 h recall times, in rats trained to avoid an electroshock by remaining on a platform. The antiserum was effective when infused up to 2.5 h following training and no retention deficit was noted when it was administered at 4h or 6h after training. An antiserum generated against a 17 amino acid residue of the A4 amyloid peptide had no effect on learning. Thus the amyloid precursor protein, which is aberrantly processed in Alzheimer's disease, appears to be directly involved in memory formation.

Cyclic AMP-dependent expression of the heavy neurofilament (NF-H) polypeptide in differentiating neuroblastoma cells

Dibutyryl cyclic AMP (dbcAMP) treatment of Neuro 2A neuroblastoma cells induces cell differentiation and neurite outgrowth. Undifferentiated cells express the heavy neurofilament polypeptide (NF-H) at a low level. Following differentiation, there is a large increase in its expression due to an increase in the corresponding cellular mRNA. A similar increase is seen following forskolin treatment of the cells indicating that the increase in NF-H expression is cyclic AMP-dependent. The increase in mRNA is due to an increase in gene transcription as demonstrated by a nuclear run-off assay.

The developmental regulation of the L2/HNK-1 and L3 carbohydrate epitopes in mouse brain. Evidence for separate control of lipid- and protein-bound epitopes

The carbohydrate epitopes L2/HNK-1 and L3 have previously been identified on various neural cell adhesion molecules and have been suggested to play a role in the mediation of cell-cell adhesion. In this study, the developmental expression of the two epitopes in soluble, membrane-bound and chloroform/methanol-extracted fractions of the constituent mouse brain regions was examined by enzyme-linked immunosorbent assay (ELISA). The protein-bound epitopes were shown to be uniformly developmentally regulated, with levels peaking at postnatal day 20 (P20). The epitopes in a crude chloroform/methanol fraction, however, demonstrated a different pattern, with L2 peaking earlier at postnatal day zero (P0). These results suggest a possible interaction between the control of the two pools of the epitope.

Partial sequence of the rat heavy neurofilament polypeptide (NF-H). Identification of putative phosphorylation sites

A 3 kb cDNA clone has previously been isolated in this laboratory corresponding to the rat heavy neurofilament polypeptide (NF-H). This clone, equivalent to approximately 70% of the total mRNA of the protein has been sequenced and shown to contain the carboxy-terminal region of the message. This contains 51 of the Lys-Ser-Pro repeat triplets which are reported to be the site of neurofilament phosphorylation. The sequence obtained was subsequently compared to those of mouse and human NF-H, showing a homology of approximately 85%. There is, however, one region which is variable between the species, this being the highly phosphorylated region of the protein containing the Lys-Ser-Pro triplet repeat.

Developmental control of N-CAM sialylation state by Golgi sialyltransferase isoforms

A rat brain Golgi sialyltransferase activity capable of the differentiation-dependent control of N-CAM sialylation state is described. The specific activity of Golgi sialyltransferase was found to be developmentally regulated with respect to both endogenous and exogenous protein acceptors, with a particular elevation on postnatal days 10–12 when the heavily sialylated or ‘embryonic’ form of N-CAM is re-expressed. The subsequent developmental decrease in activity was associated with a significant decrease in apparent Km for the CMP-NeuNAc substrate, but not for the asialofetuin exogenous acceptor, which could not be attributed to the temporal expression of an endogenous competitive inhibitor. The apparent Vmax remained constant for CMP-NeuNAc but was significantly reduced for asialofetuin. Sialyltransferase activity, which was optimal at pH 7.0-7.5, was also modulated by various cations. Zinc abolished enzyme function, in contrast to ferric ions which stimulated activity fourfold-sevenfold. The marked activation of the adult form of the enzyme by potassium and magnesium ions, together with the alterations in kinetic constants, suggested this activity to be distinct from that derived from postnatal day-12 tissue. The kinetics of [14C]sialic acid incorporation into immuno-precipitated N-CAM demonstrated the individual polypeptides to be sialylated, possibly by addition of polysialosyl units, in a developmental sequence. The presence of four distinct sialyltransferase activities was demonstrated by non-denaturing gel electrophoresis followed by solid-phase enzyme assay. These isoforms were temporally expressed during development, two being correlated with the postnatal reexpression of the ‘embryonic’ form of N-CAM.

Soluble rat brain sialidase does not influence intracellular glycosylation of Golgi sialyltransferase or its constitutive glycoproteins

Cytosol- and Golgi-enriched fractions were obtained from whole rat brain homogenates by density gradient centrifugation. Using a 4-methylumbelliferyl neuraminic acid substrate a soluble neural sialidase has been identified and characterised. The enzyme had optimal activity at pH 6.0 and a Km of 0.44 +/- 0.18 mM. The specific activity increased during postnatal development and this was in parallel with the described temporal changes in total brain neuraminic acid turnover. The potential of this enzyme to influence the intracellular processing of sialoglycoconjugates was also investigated. Cytosol fractions were incapable of releasing [14C]NeuNAC [( 14C]N-acetylneuramic acid) transferred to the glycoproteins of isolated Golgi membranes by their associated sialyltransferase. Further preincubation of Golgi membranes with soluble sialidase had no effect on their intrinsic sialyltransferase activity. These results demonstrate that no epigenetic regulation of processed sialoglycoconjugates occurs intracellularly and these finding are related to post-translational control of neural cell adhesion molecule (N-CAM) sialylation state.

Lead stimulates Golgi sialyltransferase at times coincident with the embryonic to adult conversion of the neural cell adhesion molecule (N-CAM)

The neural cell adhesion molecule N-CAM is believed to be intimately involved in the structuring of the central nervous system. During post-natal development the molecule exists in 2 forms--a sialic acid-rich form which is preferentially expressed during cell acquisition and fibre outgrowth and a sialic acid-poor form which appears at times coincident with synaptogenesis. The developmental changes between these 2 forms have been demonstrated to be impaired by chronic low-level lead exposure and this is consistent with the reduced synaptic elaboration associated with this action. Here is described the effect of lead on the Golgi-associated sialyltransferase which regulates N-CAM sialylation state. Lead chloride was found to markedly stimulate sialyltransferase with an ED50 of 5 X 10(-7) M in adult Golgi fractions. This was not observed in fractions derived from 12-day old animals. At the concentration of 5 X 10(-5) M lead was found to have a differential effect on the developmental expression of this enzyme. During the early phases of development (days 4-16) sialyltransferase activity was inhibited. However, in coincidence with periods of N-CAM desialylation (days 16-30), it was significantly stimulated. These findings are related to the perturbations of N-CAM function during chronic low-level lead exposure.

Differentiation-dependent sialylation of individual neural cell adhesion molecule polypeptides during postnatal development

The postnatal sialylation of individual neural cell adhesion molecule (N-CAM) polypeptides by a developmentally regulated sialyltransferase in Golgi-enriched fractions isolated from rat brain is described. The 120-kilodalton polypeptide of N-CAM was found to be sialylated at each developmental age examined. This was in contrast to the 140- and 180-kilodalton N-CAM polypeptides which were only sialylated until postnatal day 10 and from postnatal day 12, respectively. Immunoblotting procedures demonstrated that all N-CAM polypeptides were expressed in the Golgi fractions at each developmental stage examined. The heavily sialylated "embryonic" form of N-CAM was found to be reexpressed at postnatal days 10 and 12, a time coincident with extensive fibre outgrowth. The "embryonic" form of N-CAM incorporated similar amounts of [14C]sialic acid into its constituent polypeptides reflecting the difference in sialic acid to protein ratio, as this form of N-CAM was virtually undetectable in the immunoblots of postnatal material.

Postnatal D2-CAM/N-CAM sialylation state is controlled by a developmentally regulated Golgi sialyltransferase

Golgi-enriched fractions have been isolated from rat brain of increasing postnatal age and defined by electron microscopy and distribution of marker enzymes. The expression of sialyltransferase activity associated with these fractions has been demonstrated to developmentally decrease and this appeared to be, in part, dependent on endogenous competitive inhibition. The developmental regulation of this activity paralleled the sialylation state of the neural cell adhesion molecule (D2-CAM/N-CAM) and could be demonstrated to be capable of endogenously sialylating this protein in the isolated Golgi fractions. In 12-day-old animals the majority of the transferred [14C]sialic acid was found to be associated with the high-molecular-weight [greater than 200 kilodaltons (kd)] form of D2-CAM/N-CAM, indicative of the protein having been heavily sialylated. Sialylation of the individual D2-CAM/N-CAM polypeptides was also demonstrated in both 12-day and adult animals and transfer was evident only in the 180-kd and 115-kd components and not in the 140-kd component. In contrast, Golgi-enriched fractions prepared from adult animals showed little capability of heavily sialylating D2-CAM/N-CAM to any significant extent.

Synaptosomal sialyltransferase glycosylates surface proteins that are inaccessible to the action of membrane-bound sialidase

Sialyltransferase has been characterized in P2 pellets derived from animals of increasing age. The enzyme was found to be associated with the plasma membrane and to be developmentally regulated at times coincident with cell migration and fibre outgrowth. This regulation appeared to be due, in part, to an endogenous competitive inhibitor in the P2 pellet but not in the synaptosome. Optimal transfer of [14C]N-acetylneuraminic acid to endogenous synaptosomal acceptors was achieved only in the absence of detergent. Furthermore, the transferred sialic acid was found to be inaccessible to the action of membrane-bound sialidase. The significance of these findings is discussed.

Characterization and cellular localization of a developmentally regulated rat neural sialidase

A developmentally regulated neural sialidase has been identified in particulate, subcellular fractions of rat brain. Enzyme activity, measured using a [3H]sialoganglioside substrate, was linear with time and had a pH optimum of 4.0-4.5. Protein linearity was only observed at low protein concentrations. This appeared to be caused by enzyme access to a lipophilic substrate, as activity was significantly stimulated by membrane-fluidizing agents. Enzyme activity was developmentally expressed in P2 pellets coincident with in vivo synaptogenesis. It was located on the synaptosome and was particularly high in myelin-containing fractions. Its cellular distribution was confined to neuronal cells and centrally derived oligodendrocytes.