Characterization of an O-glycosylated plaque-associated protein from Alzheimer disease brain

CD33 isoforms in microglia and Alzheimer's disease: Friend and foe

Alzheimer's disease (AD) is the most common form of neurodegenerative disease and is considered the main cause of dementia worldwide. Genome-wide association studies combined with integrated analysis of functional datasets support a critical role for microglia in AD pathogenesis, identifying them as important potential therapeutic targets. The ability of immunomodulatory receptors on microglia to control the response to pathogenic amyloid-β aggregates has gained significant interest. Siglec-3, also known as CD33, is one of these immunomodulatory receptors expressed on microglia that has been identified as an AD susceptibility factor. Here, we review recent advances made in understanding the multifaceted roles that CD33 plays in microglia with emphasis on two human-specific CD33 isoforms that differentially correlate with AD susceptibility. We also describe several different therapeutic approaches for targeting CD33 that have been advanced for the purpose of skewing microglial cell responses.

Neuroinflammation and galectins: a key relationship in neurodegenerative diseases

Neurodegeneration is a pathological condition that is associated with the loss of neuronal function and structure. In neurodegenerative diseases, mounting evidence indicates that neuroinflammation is a common factor that contributes to neuronal damage and neurodegeneration. Neuroinflammation is characterized by the activation of microglia, the neuroimmune cells of the central nervous system (CNS), which have been implicated as active contributors to neuronal damage. Glycan structure modification is defining the outcome of neuroinflammation and neuronal regeneration; moreover, the expression of galectins, a group of lectins that specifically recognize β-galactosides, has been proposed as a key factor in neuronal regeneration and modulation of the inflammatory response. Of the different galectins identified, galectin-1 stimulates the secretion of neurotrophic factors in astrocytes and promotes neuronal regeneration, whereas galectin-3 induces the proliferation of microglial cells and modulates cell apoptosis. Galectin-8 emerged as a neuroprotective factor, which, in addition to its immunosuppressive function, could generate a neuroprotective environment in the brain. This review describes the role of galectins in the activation and modulation of astrocytes and microglia and their anti- and proinflammatory functions within the context of neuroinflammation. Furthermore, it discusses the potential use of galectins as a therapeutic target for the inflammatory response and remodeling in damaged tissues in the central nervous system.

The therapeutic potential of galectin-1 and galectin-3 in the treatment of neurodegenerative diseases

Introduction: Neuroinflammation has been proposed as a common factor and one of the main inducers of neuronal degeneration. Galectins are a group of β-galactoside-binding lectins, that play an important role in the immune response, adhesion, proliferation, differentiation, migration and cell growth. Up to 15 members of the galectin's family have been identified; however, the expression of galectin-1 and galectin-3 has been considered a key factor in neuronal regeneration and modulation of the inflammatory response. Galectin-1 is necessary to stimulate the secretion of neurotrophic factors in astrocytes and promoting neuronal regeneration. In contrast, galectin-3 fosters the proliferation of microglial cells and modulates cellular apoptosis, therefore these proteins are considered a useful alternative for the treatment of degenerative diseases.Areas covered: This review describes the roles of galectin-1 and galectin-3 in the modulation of neuroinflammation and their potential as therapeutic targets in the treatment for neurodegenerative diseases.Expert opinion: Although data in the literature vary, the effects of galectin-1 and galectin-3 on the activation and modulation of astrocytes and microglia has been described. Due to its anti-inflammatory effects, galectin-1 is proposed as a molecule with therapeutic potential, whereas the inhibition of galectin-3 could contribute to reduce the neuroinflammatory response in neurodegenerative diseases.

Early Stage Glycosylation Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) is of great cause for concern in our ageing population, which currently lacks diagnostic tools to permit accurate and timely diagnosis for affected individuals. The development of such tools could enable therapeutic interventions earlier in the disease course and thus potentially reducing the debilitating effects of AD. Glycosylation is a common, and important, post translational modification of proteins implicated in a host of disease states resulting in a complex array of glycans being incorporated into biomolecules. Recent investigations of glycan profiles, in a wide range of conditions, has been made possible due to technological advances in the field enabling accurate glycoanalyses. Amyloid beta (Aβ) peptides, tau protein, and other important proteins involved in AD pathogenesis, have altered glycosylation profiles. Crucially, these abnormalities present early in the disease state, are present in the peripheral blood, and help to distinguish AD from other dementias. This review describes the aberrant glycome in AD, focusing on proteins implicated in development and progression, and elucidates the potential of glycome aberrations as early stage biomarkers of AD.

Neuroinflammation induced by amyloid β25-35 modifies mucin-type O-glycosylation in the rat's hippocampus

Amyloid-β (Aβ) plays a relevant role in the neurodegenerative process of Alzheimer's disease (AD). The 25-35 peptide of amyloid-β (Aβ25-35) induces the inflammatory response in brain experimental models. Mucin-type O-glycosylation has been associated with inflammation of brain tissues in AD, thus in this work, we aimed at identifying changes in the glycosylation profile generated by the injection of Aβ25-35 into the CA1 of the hippocampus of rats, using histochemistry with lectins. Our results indicate that 100μM Aβ25-35 induce increased recognition of the Amaranthus leucocarpus lectin (ALL) (specific for Galβ1,3-GalNAcα1,0-Ser/Thr); whereas concanavalin A (Con A) (specific for α-Man) showed no differences among treated and control groups of rats. Jacalin and peanut agglutinin (Galβ1,3GalNAcα1,0-Ser/Thr) showed no recognition of brain cells of control or treated rats. After 6-h treatment of the tissue with trypsin or with 200mM GalNAc, the interaction with ALL was inhibited. Immunohistochemistry showed positive anti-NeuN and ALL-recognition of neurons; however, anti-GFAP and anti-CD11b showed no co-localization with ALL. The ALL+ neurons revealed the presence of cytochrome C in the cytosol and active caspase 3 in the cytosol and nucleus. Administration of the interleukin-1 receptor antagonist (IL-1RA) to Aβ25-35-treated rats diminished neuroinflammation and ALL recognition. These results suggest a close relationship among over-expression of mucin-type O-glycosylation, the neuroinflammatory process, and neuronal death.

Genome-wide association meta-analysis of neuropathologic features of Alzheimer's disease and related dementias

Alzheimer's disease (AD) and related dementias are a major public health challenge and present a therapeutic imperative for which we need additional insight into molecular pathogenesis. We performed a genome-wide association study and analysis of known genetic risk loci for AD dementia using neuropathologic data from 4,914 brain autopsies. Neuropathologic data were used to define clinico-pathologic AD dementia or controls, assess core neuropathologic features of AD (neuritic plaques, NPs; neurofibrillary tangles, NFTs), and evaluate commonly co-morbid neuropathologic changes: cerebral amyloid angiopathy (CAA), Lewy body disease (LBD), hippocampal sclerosis of the elderly (HS), and vascular brain injury (VBI). Genome-wide significance was observed for clinico-pathologic AD dementia, NPs, NFTs, CAA, and LBD with a number of variants in and around the apolipoprotein E gene (APOE). GalNAc transferase 7 (GALNT7), ATP-Binding Cassette, Sub-Family G (WHITE), Member 1 (ABCG1), and an intergenic region on chromosome 9 were associated with NP score; and Potassium Large Conductance Calcium-Activated Channel, Subfamily M, Beta Member 2 (KCNMB2) was strongly associated with HS. Twelve of the 21 non-APOE genetic risk loci for clinically-defined AD dementia were confirmed in our clinico-pathologic sample: CR1, BIN1, CLU, MS4A6A, PICALM, ABCA7, CD33, PTK2B, SORL1, MEF2C, ZCWPW1, and CASS4 with 9 of these 12 loci showing larger odds ratio in the clinico-pathologic sample. Correlation of effect sizes for risk of AD dementia with effect size for NFTs or NPs showed positive correlation, while those for risk of VBI showed a moderate negative correlation. The other co-morbid neuropathologic features showed only nominal association with the known AD loci. Our results discovered new genetic associations with specific neuropathologic features and aligned known genetic risk for AD dementia with specific neuropathologic changes in the largest brain autopsy study of AD and related dementias.

Alzheimer's disease (AD) and related dementias are a major public health challenge and present a therapeutic imperative for which we need additional insight into molecular pathogenesis. We performed a genome-wide association study (GWAS), as well as an analysis of known genetic risk loci for AD dementia, using data from 4,914 brain autopsies. Genome-wide significance was observed for 7 genes and pathologic features of AD and related diseases. Twelve of the 22 genetic risk loci for clinically-defined AD dementia were confirmed in our pathologic sample. Correlation of effect sizes for risk of AD dementia with effect size for hallmark pathologic features of AD were strongly positive and linear. Our study discovered new genetic associations with specific pathologic features and aligned known genetic risk for AD dementia with specific pathologic changes in a large brain autopsy study of AD and related dementias.

Proteomic profiling of phosphoproteins and glycoproteins responsive to wild-type alpha-synuclein accumulation and aggregation

A tetracycline inducible transfectant cell line (3D5) capable of producing soluble and sarkosyl-insoluble assemblies of wild-type human alpha-synuclein (alpha-Syn) upon differentiation with retinoic acid was used to study the impact of alpha-Syn accumulation on protein phosphorylation and glycosylation. Soluble proteins from 3D5 cells, with or without the induced alpha-Syn expression were analyzed by two-dimensional gel electrophoresis and staining of gels with dyes that bind to proteins (Sypro ruby), phosphoproteins (Pro-Q diamond) and glycoproteins (Pro-Q emerald). Phosphoproteins were further confirmed by binding to immobilized metal ion affinity column. alpha-Syn accumulation caused differential phosphorylation and glycosylation of 16 and 12, proteins, respectively, whose identity was revealed by mass spectrometry. These proteins, including HSP90, have diverse biological functions including protein folding, signal transduction, protein degradation and cytoskeletal regulation. Importantly, cells accumulating alpha-Syn assemblies with different abilities to bind thioflavin S displayed different changes in phosphorylation and glycosylation. Consistent with the cell-based studies, we demonstrated a reduced level of phosphorylated HSP90 alpha/beta in the substantia nigra of subjects with Parkinson's disease as compared to normal controls. Together, the results indicate that alpha-Syn accumulation causes complex cellular responses, which if persist may compromise cell viability.

Proteomic analysis of glial fibrillary acidic protein in Alzheimer's disease and aging brain

Chronic inflammation is known to play an important role in the heterogeneous pathogenesis of Alzheimer's disease (AD). Activated astrocytes expressing glial fibrillary acidic protein (GFAP) are closely associated with AD pathology, such as tangles, neuritic plaques and amyloid depositions. Altogether, 46 soluble isoforms of GFAP were separated and most of them quantified by two-dimensional immunoblotting in frontal cortices of AD patients and age-matched controls. A 60% increase in the amount of more acidic isoforms of GFAP was observed in AD and these isoforms were both phosphorylated and N-glycosylated, while more basic isoforms were O-glycosylated and exhibited no quantitative differences between post-mortem AD and control brains. These data highlight the importance of exploring isoform-specific levels of proteins in pathophysiological conditions since modifications of proteins determine their activity state, localization, turnover and interaction with other molecules. Mechanisms, structures and functional consequences of modification of GFAP isoforms remain to be clarified.

Glycosylation changes in Alzheimer's disease as revealed by a proteomic approach

Glycosylation influences the biological activity of proteins and affects their folding and stability. Because aberrant glycosylation is associated with Alzheimer's disease (AD), we applied proteome analysis together with Pro-Q Emerald 300 glycoprotein staining to investigate changes in glycosylated cytosolic proteins in AD and control brain. Frontal cortex proteins from 10 AD patients and 7 non-demented controls were subjected to separation by two-dimensional gel electrophoresis and subsequently stained with carbohydrate-specific Pro-Q Emerald 300 dye. Changes in glycosylation of separated proteins were quantified, and proteins of interest identified by mass spectrometry. Approximately 30% of all detectable proteins in the human frontal cortex appeared glycosylated, including heat shock cognate 71 stress protein and beta isoform of creatine kinase. The glycosylation of collapsin response mediator protein 2 (CRMP-2) and an unknown protein was reduced in AD, while the glycosylation of glial fibrillary acidic protein was increased. CRMP-2 regulates the assembly and polymerization of microtubules and is associated with neurofibrillary tangles in AD. Aberrant glycosylations in AD may help understand the mechanisms of neurodegenerative diseases.