Glycosylation of acetylcholinesterase as diagnostic marker for Alzheimer's disease

The Involvement of Post-Translational Modifications in Regulating the Development and Progression of Alzheimer's Disease

Post-translational modifications (PTMs) have been recently reported to be involved in the development and progression of Alzheimer's disease (AD). In detail, PTMs include phosphorylation, glycation, acetylation, sumoylation, ubiquitination, methylation, nitration, and truncation, which are associated with pathological functions of AD-related proteins, such as β-amyloid (Aβ), β-site APP-cleavage enzyme 1 (BACE1), and tau protein. In particular, the roles of aberrant PTMs in the trafficking, cleavage, and degradation of AD-associated proteins, leading to the cognitive decline of the disease, are summarized under AD conditions. By summarizing these research progress, the gaps will be filled between PMTs and AD, which will facilitate the discovery of potential biomarkers, leading to the establishment of novel clinical intervention methods against AD.

Presenilin 1 Modulates Acetylcholinesterase Trafficking and Maturation

In Alzheimer's disease (AD), the reduction in acetylcholinesterase (AChE) enzymatic activity is not paralleled with changes in its protein levels, suggesting the presence of a considerable enzymatically inactive pool in the brain. In the present study, we validated previous findings, and, since inactive forms could result from post-translational modifications, we analyzed the glycosylation of AChE by lectin binding in brain samples from sporadic and familial AD (sAD and fAD). Most of the enzymatically active AChE was bound to lectins Canavalia ensiformis (Con A) and Lens culinaris agglutinin (LCA) that recognize terminal mannoses, whereas Western blot assays showed a very low percentage of AChE protein being recognized by the lectin. This indicates that active and inactive forms of AChE vary in their glycosylation pattern, particularly in the presence of terminal mannoses in active ones. Moreover, sAD subjects showed reduced binding to terminal mannoses compared to non-demented controls, while, for fAD patients that carry mutations in the PSEN1 gene, the binding was higher. The role of presenilin-1 (PS1) in modulating AChE glycosylation was then studied in a cellular model that overexpresses PS1 (CHO-PS1). In CHO-PS1 cells, binding to LCA indicates that AChE displays more terminal mannoses in oligosaccharides with a fucosylated core. Immunocytochemical assays also demonstrated increased presence of AChE in the trans-Golgi. Moreover, AChE enzymatic activity was higher in plasmatic membrane of CHO-PS1 cells. Thus, our results indicate that PS1 modulates trafficking and maturation of AChE in Golgi regions favoring the presence of active forms in the membrane.

Proceedings of workshop: "Neuroglycoproteins in health and disease", INNOGLY cost action

The Cost Action "Innovation with glycans: new frontiers from synthesis to new biological targets" (INNOGLY) hosted the Workshop "Neuroglycoproteins in health and disease", in Alicante, Spain, on March 2022. This event brought together an european group of scientists that presented novel insights into changes in glycosylation in diseases of the central nervous system and cancer, as well as new techniques to study protein glycosylation. Herein we provide the abstracts of all the presentations.

What Can N-glycomics and N-glycoproteomics of Cerebrospinal Fluid Tell Us about Alzheimer Disease?

Proteomics-large-scale studies of proteins-has over the last decade gained an enormous interest for studies aimed at revealing proteins and pathways involved in disease. To fully understand biological and pathological processes it is crucial to also include post-translational modifications in the "omics". To this end, glycomics (identification and quantification of glycans enzymatically or chemically released from proteins) and glycoproteomics (identification and quantification of peptides/proteins with the glycans still attached) is gaining interest. The study of protein glycosylation requires a workflow that involves an array of sample preparation and analysis steps that needs to be carefully considered. Herein, we briefly touch upon important steps such as sample preparation and preconcentration, glycan release, glycan derivatization and quantification and advances in mass spectrometry that today are the work-horse for glycomics and glycoproteomics studies. Several proteins related to Alzheimer disease pathogenesis have altered protein glycosylation, and recent glycomics studies have shown differences in cerebrospinal fluid as well as in brain tissue in Alzheimer disease as compared to controls. In this review, we discuss these techniques and how they have been used to shed light on Alzheimer disease and to find glycan biomarkers in cerebrospinal fluid.

Role of Post-translational Modifications in Alzheimer's Disease

The global burden of Alzheimer's disease (AD) is growing. Valiant efforts to develop clinical candidates for treatment have continuously met with failure. Currently available palliative treatments are temporary and there is a constant need to search for reliable disease pathways, biomarkers and drug targets for developing diagnostic and therapeutic tools to address the unmet medical needs of AD. Challenges in drug-discovery efforts raise further questions about the strategies of current conventional diagnosis; drug design; and understanding of disease pathways, biomarkers and targets. In this context, post-translational modifications (PTMs) regulate protein trafficking, function and degradation, and their in-depth study plays a significant role in the identification of novel biomarkers and drug targets. Aberrant PTMs of disease-relevant proteins could trigger pathological pathways, leading to disease progression. Advancements in proteomics enable the generation of patterns or signatures of such modifications, and thus, provide a versatile platform to develop biomarkers based on PTMs. In addition, understanding and targeting the aberrant PTMs of various proteins provide viable avenues for addressing AD drug-discovery challenges. This review highlights numerous PTMs of proteins relevant to AD and provides an overview of their adverse effects on the protein structure, function and aggregation propensity that contribute to the disease pathology. A critical discussion offers suggestions of methods to develop PTM signatures and interfere with aberrant PTMs to develop viable diagnostic and therapeutic interventions in AD.

HNK-1 Carrier Glycoproteins Are Decreased in the Alzheimer's Disease Brain

The human natural killer-1 (HNK-1), 3-sulfonated glucuronic acid, is a glycoepitope marker of cell adhesion that participates in cell-cell and cell-extracellular matrix interactions and in neurite growth. Very little is known about the regulation of the HNK-1 glycan in neurodegenerative disease, particularly in Alzheimer's disease (AD). In this study, we investigate changes in the levels of HNK-1 carrier glycoproteins in AD. We demonstrate an overall decrease in HNK-1 immunoreactivity in glycoproteins extracted from the frontal cortex of AD subjects, compared with levels from non-demented controls (NDC). Immunoblotting of ventricular post-mortem and lumbar ante-mortem cerebrospinal fluid with HNK-1 antibodies indicate similar levels of carrier glycoproteins in AD and NDC samples. Decrease in HNK-1 carrier glycoproteins were not paralleled by changes in messenger RNA (mRNA) levels of the enzymes involved in the synthesis of the glycoepitope, β-1,4-galactosyltransferase (β4GalT), glucuronyltransferases GlcAT-P and GlcAT-S, or sulfotransferase HNK-1ST. Over-expression of amyloid precursor protein in Tg2576 transgenic mice and in vitro treatment of SH-SY5Y neuroblastoma cells with the amyloidogenic Aβ42 peptide resulted in a decrease in HNK-1 immunoreactivity levels in brain and cellular extracts, whereas the levels of soluble HNK-1 glycoproteins detected in culture media were not affected by Aβ treatment. HNK-1 levels remain unaffected in the brain extracts of Tg-VLW mice, a model of mutant hyperphosphorylated tau, and in SH-SY5Y cells over-expressing hyperphosphorylated wild-type tau. These results provide evidence that cellular levels of HNK-1 carrier glycoforms are decreased in the brain of AD subjects, probably influenced by the β-amyloid protein.

Interaction of acetylcholinesterase with neurexin-1β regulates glutamatergic synaptic stability in hippocampal neurons

Background

Excess expression of acetylcholinesterase (AChE) in the cortex and hippocampus causes a decrease in the number of glutamatergic synapses and alters the expression of neurexin and neuroligin, trans-synaptic proteins that control synaptic stability. The molecular sequence and three-dimensional structure of AChE are homologous to the corresponding aspects of the ectodomain of neuroligin. This study investigated whether excess AChE interacts physically with neurexin to destabilize glutamatergic synapses.

Results

The results showed that AChE clusters colocalized with neurexin assemblies in the neurites of hippocampal neurons and that AChE co-immunoprecipitated with neurexin from the lysate of these neurons. Moreover, when expressed in human embryonic kidney 293 cells, N-glycosylated AChE co-immunoprecipitated with non-O–glycosylated neurexin-1β, with N-glycosylation of the AChE being required for this co-precipitation to occur. Increasing extracellular AChE decreased the association of neurexin with neuroligin and inhibited neuroligin-induced synaptogenesis. The number and activity of excitatory synapses in cultured hippocampal neurons were reduced by extracellular catalytically inactive AChE.

Conclusions

Excessive glycosylated AChE could competitively disrupt a subset of the neurexin–neuroligin junctions consequently impairing the integrity of glutamatergic synapses. This might serve a molecular mechanism of excessive AChE induced neurodegeneration.

The role of protein glycosylation in Alzheimer disease

Glycosylation is one of the most common, and the most complex, forms of post-translational modification of proteins. This review serves to highlight the role of protein glycosylation in Alzheimer disease (AD), a topic that has not been thoroughly investigated, although glycosylation defects have been observed in AD patients. The major pathological hallmarks in AD are neurofibrillary tangles and amyloid plaques. Neurofibrillary tangles are composed of phosphorylated tau, and the plaques are composed of amyloid β-peptide (Aβ), which is generated from amyloid precursor protein (APP). Defects in glycosylation of APP, tau and other proteins have been reported in AD. Another interesting observation is that the two proteases required for the generation of amyloid β-peptide (Aβ), i.e. γ-secretase and β-secretase, also have roles in protein glycosylation. For instance, γ-secretase and β-secretase affect the extent of complex N-glycosylation and sialylation of APP, respectively. These processes may be important in AD pathogenesis, as proper intracellular sorting, processing and export of APP are affected by how it is glycosylated. Furthermore, lack of one of the key components of γ-secretase, presenilin, leads to defective glycosylation of many additional proteins that are related to AD pathogenesis and/or neuronal function, including nicastrin, reelin, butyrylcholinesterase, cholinesterase, neural cell adhesion molecule, v-ATPase, and tyrosine-related kinase B. Improved understanding of the effects of AD on protein glycosylation, and vice versa, may therefore be important for improving the diagnosis and treatment of AD patients.

Integrated approach toward the discovery of glyco-biomarkers of inflammation-related diseases

Glycobiology has contributed tremendously to the discovery and characterization of cancer-related biomarkers containing glycans (i.e., glyco-biomarkers) and a more detailed understanding of cancer biology. It is now recognized that most chronic diseases involve some elements of chronic inflammation; these include cancer, Alzheimer's disease, and metabolic syndrome (including consequential diabetes mellitus and cardiovascular diseases). By extending the knowledge and experience of the glycobiology community regarding cancer biomarker discovery, we should be able to contribute to the discovery of diagnostic/prognostic glyco-biomarkers of other chronic diseases that involve chronic inflammation. Future integration of large-scale "omics"-type data (e.g., genomics, epigenomics, transcriptomics, proteomics, and glycomics) with computational model building, or a systems glycobiology approach, will facilitate such efforts.

Revisiting the Role of Acetylcholinesterase in Alzheimer's Disease: Cross-Talk with P-tau and β-Amyloid

A common feature in the Alzheimer’s disease (AD) brain is the presence of acetylcholinesterase (AChE) which is commonly associated with β-amyloid plaques and neurofibrillary tangles (NFT). Although our understanding of the relationship between AChE and the pathological features of AD is incomplete, increasing evidence suggests that both β-amyloid protein (Aβ) and abnormally hyperphosphorylated tau (P-tau) can influence AChE expression. We also review recent findings which suggest the possible role of AChE in the development of a vicious cycle of Aβ and P-tau dysregulation and discuss the limited and temporary effect of therapeutic intervention with AChE inhibitors.

Human plasma glycome in attention-deficit hyperactivity disorder and autism spectrum disorders

Over a half of all proteins are glycosylated, and their proper glycosylation is essential for normal function. Unfortunately, because of structural complexity of nonlinear branched glycans and the absence of genetic template for their synthesis, the knowledge about glycans is lagging significantly behind the knowledge about proteins or DNA. Using a recently developed quantitative high throughput glycan analysis method we quantified components of the plasma N-glycome in 99 children with attention-deficit hyperactivity disorder (ADHD), 81 child and 5 adults with autism spectrum disorder, and a total of 340 matching healthy controls. No changes in plasma glycome were found to associate with autism spectrum disorder, but several highly significant associations were observed with ADHD. Further structural analysis of plasma glycans revealed that ADHD is associated with increased antennary fucosylation of biantennary glycans and decreased levels of some complex glycans with three or four antennas. The design of this study prevented any functional conclusions about the observed associations, but specific differences in glycosylation appears to be strongly associated with ADHD and warrants further studies in this direction.

Beta-amyloid controls altered Reelin expression and processing in Alzheimer's disease

Reelin is a glycoprotein that modulates synaptic function and plasticity in the mature brain, thereby favouring memory formation. We recently reported altered cerebral Reelin expression in Alzheimer's disease (AD). Here we demonstrate pronounced Reelin changes at protein and mRNA levels in the frontal cortex in adult Down's syndrome (DS), where the extra copy of chromosome 21 leads to overexpression of beta-amyloid. In cortical extracts of fetal DS samples we detected increased levels of the full-length Reelin and the 310-kDa fragment. Overexpression of mutant human amyloid precursor protein also led to an increase in levels of Reelin fragments in Tg2576 transgenic mice for human beta-amyloid. Finally, in vitro Abeta42 treatment of SH-SY5Y neuroblastoma cells led to increased Reelin levels. An altered pattern of Reelin glycosylation was detected in extracts from the frontal cortex of AD patients and in Abeta42-treated SH-SY5Y cells, supporting the notion that beta-amyloid triggers altered Reelin processing. These results provide evidence that Reelin expression and processing is altered in several amyloid conditions.

Association between acetylcholinesterase and beta-amyloid peptide in Alzheimer's cerebrospinal fluid

The altered expression of acetylcholinesterase (AChE) in the brains of patients with Alzheimer's disease (AD) has raised much interest of late. Despite an overall decrease in the AD brain, the activity of AChE increases around beta-amyloid plaques and indeed, the beta-amyloid peptide (Abeta) can influence AChE levels. Such evidence stimulated our interest in the possibility that the levels of AChE and amyloid might vary together in AD. We previously found that the different AChE forms present in both the brain and in the cerebrospinal fluid (CSF) of AD patients varied in conjunction with abnormal glycosylation. Thus, the alterations in glycosylation are correlated with the accumulation of a minor subspecies of AChE monomers. We also recently analysed whether long-term exposure to the cholinesterase inhibitor (ChE-I) donepezil influences the AChE species found in AD CSF. The marked increase in CSF-AChE activity in AD patients following long-term treatment with donepezil was not paralleled by a rise in this subset of light variants. Hence, the correlation with the levels of CSF-Abeta is unique to these AChE species in patients receiving such treatment. The aim of this report is to review the links between AChE and beta-amyloid, and to discuss the significance of the responses of the distinct AChE species to ChE-I during the treatment of AD.

Presenilin 1 interacts with acetylcholinesterase and alters its enzymatic activity and glycosylation

Presenilin 1 (PS1) plays a critical role in the gamma-secretase processing of the amyloid precursor protein to generate the beta-amyloid peptide, which accumulates in plaques in the pathogenesis of Alzheimer's disease (AD). Mutations in PS1 cause early onset AD, and proteins that interact with PS1 are of major functional importance. We report here the coimmunoprecipitation of PS1 and acetylcholinesterase (AChE), an enzyme associated with amyloid plaques. Binding occurs through PS1 N-terminal fragment independent of the peripheral binding site of AChE. Subcellular colocalization of PS1 and AChE in cultured cells and coexpression patterns of PS1 and AChE in brain sections from controls and subjects with sporadic or familial AD indicated that PS1 and AChE are located in the same intracellular compartments, including the perinuclear compartments. A PS1-A246E pathogenic mutation expressed in transgenic mice leads to decreased AChE activity and alteration of AChE glycosylation and the peripheral binding site, which may reflect a shift in protein conformation and disturbed AChE maturation. In both the transgenic mice and humans, mutant PS1 impairs coimmunoprecipitation with AChE. The results indicate that PS1 can interact with AChE and influence its expression, supporting the notion of cholinergic-amyloid interrelationships.

Cerebrospinal fluid acetylcholinesterase changes after treatment with donepezil in patients with Alzheimer's disease

We analyzed whether donepezil differently influences acetylcholinesterase (AChE) variants from cerebrospinal fluid (CSF) in patients with Alzheimer's disease (AD) after long-term treatment. Overall CSF-AChE activity in AD patients before treatment was not different from controls, but the ratio between the major tetrameric form, G(4), and the smaller G(1) and G(2) species was significantly lower. AChE levels at study outset were found to correlate positively with beta-amyloid (1-42) (Abeta42). When patients were re-examined after 12 months treatment with donepezil, there was a remarkable increase in both the G(4) and the lighter species of CSF AChE. As compared with placebo, donepezil caused decreases in the percentage of AChE that failed to bind to the lectin concanavalin A and the antibody AE1. These non-binding species comprised primarily a small subset of G(1) and G(2) forms. In treated patients, these light variants were the only subset of CSF AChE that correlated with CSF-Abeta42 levels. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed that a 77-kDa band, attributed in part to inactive AChE, was lower in AD patients than in controls. Unlike enzyme activity, the intensity of this band did not increase after donepezil treatment. The varying responses of different AChE species to ChE-I treatment suggest different modes of regulation, which may have therapeutic implications.

Altered glycosylation of acetylcholinesterase in Creutzfeldt-Jakob disease

Changes in the glycosylation pattern of brain proteins have been associated with Creutzfeldt-Jakob disease (CJD). We have investigated the glycosylation status of acetylcholinesterase (AChE) by lectin binding assay. Our data show that in lumbar CSF from definite and probable sporadic CJD cases AChE activity is lower compared with that in age-matched controls. We also show, for the first time, that AChE glycosylation is altered in CJD CSF and brain. Unlike Alzheimer's disease, in which an alteration in both the glycosylation and levels of AChE molecular forms is observed, the abnormal glycosylation of AChE in CJD appears to be unrelated to changes in molecular forms of this enzyme. These findings suggest that altered AChE glycosylation in CJD may be a consequence of the general perturbation of the glycosylation machinery that affects prion protein, as well as other proteins. The diagnostic potential of these changes remains to be explored.

Association of a salivary acetylcholinesterase with Alzheimer's disease and response to cholinesterase inhibitors

OBJECTIVES

A decrease in cholinergic activity is a key event in the biochemistry of Alzheimer's disease (AD). The aim of the study was to investigate the expression levels of markers of cholinergic function in saliva, which is a readily accessible body fluid that can be obtained from subjects with minimal distress.

DESIGN AND METHODS

Salivary samples were obtained from people with NINCDS-ARDRA "probable" Alzheimer's disease and age- and sex-matched controls. Salivary acetylcholinesterase enzyme (AChE) activity was determined colorometrically.

RESULTS

Robust AChE catalytic activity was detected in the saliva samples that was stable for up to 6 h at room temperature following the provision of the salivary sample. The activity of the enzyme was significantly lower in people with AD than in age-matched controls. In addition, there were significant differences in activity between those who responded to acetylcholinesterase inhibitor (AChE-I) therapy and those who did not.

CONCLUSIONS

Salivary enzyme activity may therefore prove to be a useful marker of central cholinergic activity.

α7-Nicotinic acetylcholine receptors mediate an Aβ1−42-induced increase in the level of acetylcholinesterase in primary cortical neurones

The beta-amyloid protein (Abeta) is the major protein component of amyloid plaques found in the Alzheimer brain. Although there is a loss of acetylcholinesterase (AChE) from both cholinergic and non-cholinergic neurones in the brain of Alzheimer patients, the level of AChE is increased around amyloid plaques. Previous studies using P19 cells in culture and transgenic mice which overexpress human Abeta have suggested that this increase may be due to a direct action of Abeta on AChE expression in cells adjacent to amyloid plaques. The aim of the present study was to examine the mechanism by which Abeta increases levels of AChE in primary cortical neurones. Abeta1-42 was more potent than Abeta1-40 in its ability to increase AChE in primary cortical neurones. The increase in AChE was unrelated to the toxic effects of the Abeta peptides. The effect of Abeta1-42 on AChE was blocked by inhibitors of alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) as well as by inhibitors of L- or N-type voltage-dependent calcium channels (VDCCs), whereas agonists of alpha7 nAChRs (choline, nicotine) increased the level of AChE. The results demonstrate that the effect of Abeta1-42 on AChE is due to an agonist effect of Abeta1-42 on the alpha7 nAChR.

Glycosylation of acetylcholinesterase and butyrylcholinesterase changes as a function of the duration of Alzheimer's disease

The identification of biochemical markers of Alzheimer's disease (AD) may help in the diagnosis of the disease. Previous studies have shown that Abeta(1-42) is decreased, and tau and phospho-tau are increased in AD cerebrospinal fluid (CSF). Our own studies have identified glycosylated isoforms of acetylcholinesterase (Glyc-AChE) and butyrylcholinesterase (Glyc-BuChE) that are increased in AD CSF. Glyc-AChE is increased in APP (SW) Tg2576 transgenic mice prior to amyloid plaque deposition, which suggests that Glyc-AChE may be an early marker of AD. The aim of this study was to determine whether Glyc-AChE or Glyc-BuChE is increased in CSF at early stages of AD and to compare the levels of these markers with those of Abeta(1-42), tau and phospho-tau. Lumbar CSF was obtained ante mortem from 106 non-AD patients, including 15 patients with mild cognitive impairment (MCI), and 102 patients with probable AD. Glyc-AChE, tau and phospho-tau were significantly increased in the CSF of AD patients compared to non-neurological disease (NND) controls. Abeta(1-42) was lower in the AD patients than in NND controls. A positive correlation was found between the levels of Glyc-AChE or Glyc-BuChE and disease duration. However, there was no clear correlation between the levels of tau, phospho-tau or Abeta(1-42) and disease duration. The results suggest that Glyc-AChE and Glyc-BuChE are unlikely to be early markers of AD, although they may have value as markers of disease progression.

Altered levels of cerebrospinal fluid reelin in frontotemporal dementia and Alzheimer's disease

Reelin is an essential glycoprotein for correct cytoarchitectonic organization during CNS development. Its function in the adult brain is far less well understood, but altered brain and blood reelin levels have been reported in some psychiatric disorders, and the possibility has been considered of an involvement of the reelin signaling pathway in neurodegeneration. Here we report, for the first time, the presence of detectable levels of reelin in rat and human cerebrospinal fluid (CSF) and show evidence for the involvement of a 180-kDa reelin fragment in two neurodegenerative disorders. This fragment was analyzed by Western blotting in CSF samples from 13 healthy control individuals and 14 frontotemporal dementia (FTD) and 20 Alzheimer's disease (AD) patients. Increased CSF 180-kDa reelin was found in FTD (161.7 +/- 6.7 arbitrary units; a.u.) and AD (151.4 +/- 3.8 a.u.) compared with control individuals (141.4 +/- 1.2 a.u., P < 0.05). Our results strongly suggest the involvement of reelin signaling in neurodegenerative pathologies.

Changes in molecular isoform distribution of acetylcholinesterase in rat cortex and cerebrospinal fluid after intracerebroventricular administration of amyloid beta-peptide

Previous studies have shown that an abnormal salt-soluble form of G(1) acetylcholinesterase (AChE) is increased in the Alzheimer's disease (AD) brain. The aim of the present study was to examine changes in AChE activity in an in vivo model of beta-amyloid peptide (A beta) administration. Rats received intracerebroventricular injections of A beta(25-35) (20 microg/day for seven days). Levels of AChE were measured in cerebral cortex and cerebrospinal fluid (CSF) after two months. A beta(25-35) administration did not alter total AChE activity in the cerebral cortex or CSF. However, analysis of salt-extractable AChE isoforms revealed an increase in the proportion of G(1) in both cortex and CSF, similar to that previously observed in AD patients. The results support the view that changes in AChE isoform pattern in the AD brain are a direct consequence of A beta accumulation.

Altered glycosylation of acetylcholinesterase in APP (SW) Tg2576 transgenic mice occurs prior to amyloid plaque deposition

Previous studies have shown that a minor glycoform of acetylcholinesterase (AChE) is increased in Alzheimer's disease brain and cerebrospinal fluid. This glycoform can be distinguished from other AChE species by its lack of binding to concanavalin A (Con A). In this study, the temporal relationship between AChE glycosylation and Abeta deposition was examined in Tg2576 mice. There was a significant (p < 0.05) difference in AChE glycosylation in Tg2576 mice compared with age-matched background strain control mice at 4 months of age. This difference in glycosylation was also observed in 8- and 12-month-old Tg2576 mice. In contrast, Abeta plaques were only seen in the Tg2576 mice at 12 months of age, and were not detected at 4 and 8 months of age. Soluble human-sequence Abeta was detected as early as 4 months of age in the transgenic mice. The altered AChE glycosylation was due to an increase in a minor AChE isoform, which did not bind Con A, similar to that previously observed to be increased in Alzheimer's disease brain and cerebrospinal fluid. The results demonstrate that in transgenic mice altered AChE glycosylation is associated with very early events in the development of AD-like pathology. The study supports the possibility that glycosylation may also be a useful biomarker of AD.

An unusually glycosylated form of acetylcholinesterase is a CSF biomarker for Alzheimer's disease

The identification of a biochemical marker of Alzheimer's disease (AD) is a major research aim of many groups. Abnormal levels of tau and Abeta have been identified in the cerebrospinal fluid (CSF) of AD patients, although the sensitivity and specificity of the changes in these two biomarkers alone is not sufficient to be of diagnostic value. Recently, our group has identified an abnormality in the glycosylation of acetylcholinesterase (AChE). The increase in this glycoform of AChE is very specific for Alzheimer's disease and is not seen in many other neurological diseases including other dementias.

Altered glycosylation of cerebrospinal fluid butyrylcholinesterase in Alzheimer's disease

Our previous studies have shown that a minor isoform of acetylcholinesterase (AChE) is increased in the cerebrospinal fluid (CSF) of Alzheimer's disease (AD) patients. In the present study, the glycosylation of butyrycholinesterase (BuChE) was found to be altered in AD CSF. By combining an analysis of CSF AChE and BuChE glycosylation, it is possible to identify cases of AD with more than 90% sensitivity and specificity.

Molecular isoform distribution and glycosylation of acetylcholinesterase are altered in brain and cerebrospinal fluid of patients with Alzheimer's disease

The glycosylation of acetylcholinesterase (AChE) in CSF was analyzed by lectin binding. AChE from Alzheimer's disease (AD) patients was found to bind differently to two lectins, concanavalin A and wheat germ agglutinin, than AChE from controls. As multiple isoforms of AChE are present in both CSF and brain, we examined whether the abnormal glycosylation of AD AChE was due to changes in a specific molecular isoform. Globular amphiphilic dimeric (G2a) and monomeric (G1a) isoforms of AChE were found to be differentially glycosylated in AD CSF. Glycosylation of AChE was also altered in AD frontal cortex but not in cerebellum and was also associated with an increase in the proportion of light (G2 and G1) isoforms. This study demonstrates that the glycosylation of AChE is altered in the AD brain and that changes in AChE glycosylation in AD CSF may reflect changes in the distribution of brain isoforms. The study also suggests that glycosylation of AChE may be a useful diagnostic marker for AD.

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.