O-Glycosylation in sprouting neurons in Alzheimer disease, indicating reactive plasticity

New insight into protein glycosylation in the development of Alzheimer's disease

Alzheimer's disease (AD) is a chronic neurodegenerative disease that seriously endangers the physical and mental health of patients, however, there are still no effective drugs or methods to cure this disease up to now. Protein glycosylation is the most common modifications of the translated proteins in eukaryotic cells. Recently many researches disclosed that aberrant glycosylation happens in some important AD-related proteins, such as APP, Tau, Reelin and CRMP-2, etc, suggesting a close link between abnormal protein glycosylation and AD. Because of its complexity and diversity, glycosylation is thus considered a completely new entry point for understanding the precise cause of AD. This review comprehensively summarized the currently discovered changes in protein glycosylation patterns in AD, and especially introduced the latest progress on the mechanism of protein glycosylation affecting the progression of AD and the potential application of protein glycosylation in AD detection and treatment, thereby providing a wide range of opportunities for uncovering the pathogenesis of AD and promoting the translation of glycosylation research into future clinical applications.

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.

PHF-Core Tau as the Potential Initiating Event for Tau Pathology in Alzheimer's Disease

Worldwide, around 50 million people have dementia. Alzheimer's disease (AD) is the most common type of dementia and one of the major causes of disability and dependency among the elderly worldwide. Clinically, AD is characterized by impaired memory accompanied by other deficiencies in the cognitive domain. Neuritic plaques (NPs) and neurofibrillary tangles (NFTs) are histopathological lesions that define brains with AD. NFTs consist of abundant intracellular paired helical filaments (PHFs) whose main constituent is tau protein. Tau undergoes posttranslational changes including hyperphosphorylation and truncation, both of which favor conformational changes in the protein. The sequential pathological processing of tau is illustrated with the following specific markers: pT231, TG3, AT8, AT100, and Alz50. Two proteolysis sites for tau have been described-truncation at glutamate 391 and at aspartate 421-and which can be demonstrated by reactivity with the antibodies 423 and TauC-3, respectively. In this review, we describe the molecular changes in tau protein as pre-NFTs progress to extracellular NFTs and during which the formation of a minimal nucleus of the filament, as the PHF core, occurs. We also analyzed the PHF core as the initiator of PHFs and tau phosphorylation as a protective neuronal mechanism against the assembly of the PHF core.

Roles and Biomedical Applications of Haemolymph Lectin

BACKGROUND

Lectins are class of proteins characterized by their ability to selectively bind carbohydrate moieties of glycoproteins. Many invertebrate lectins, especially derived from hemolymph, are being purified, and yet their functions and medical applications are subjects of major interest.

METHODS

Hemolymph lectins in invertebrates play a major role in protecting against many pathogens and microbes. Further, many hemolymph lectins show anticancer properties towards various cancer cell lines, which expresses globotriaosyl ceramides on their cell surface.

RESULTS

These vast repertoires of hemolymph lectins in recognizing and inhibiting the growth of various harmful microbes and cancerous cells have spurred the biochemist to use them in histochemical and cytochemical studies.

CONCLUSION

The present review will address the biological roles and biomedical applications of hemolymph lectin.

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.

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.

Synaptic degeneration in Alzheimer's disease

Synaptic loss is the major neurobiological substrate of cognitive dysfunction in Alzheimer's disease (AD). Synaptic failure is an early event in the pathogenesis that is clearly detectable already in patients with mild cognitive impairment (MCI), a prodromal state of AD. It progresses during the course of AD and in most early stages involves mechanisms of compensation before reaching a stage of decompensated function. This dynamic process from an initially reversible functionally responsive stage of down-regulation of synaptic function to stages irreversibly associated with degeneration might be related to a disturbance of structural brain self-organization and involves morpho-regulatory molecules such as the amyloid precursor protein. Further, recent evidence suggests a role for diffusible oligomers of amyloid beta in synaptic dysfunction. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. Previously, we formulated the hypothesis that differentiated neurons after having withdrawn from the cell cycle are able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within neurons might put them at risk of erroneously converting signals derived from plastic synaptic changes into the program of cell cycle activation, which subsequently leads to cell death. The molecular mechanisms involved in cell cycle activation might, thus, link aberrant synaptic changes to cell death.

Pathway-based approaches for analysis of genomewide association studies

Published genomewide association (GWA) studies typically analyze and report single-nucleotide polymorphisms (SNPs) and their neighboring genes with the strongest evidence of association (the "most-significant SNPs/genes" approach), while paying little attention to the rest. Borrowing ideas from microarray data analysis, we demonstrate that pathway-based approaches, which jointly consider multiple contributing factors in the same pathway, might complement the most-significant SNPs/genes approach and provide additional insights into interpretation of GWA data on complex diseases.

Altered detection of molecules associated with leukocyte traffic in HUVECs derived from newborns with a strong family history of myocardial infarction

Atherosclerosis is a chronic inflammatory disease. As such, recruitment of immune cells is a significant event. Tightly controlled signaling molecules regulate leukocyte adhesion and migration to the tissues. The aim of this study was to determine if human umbilical vein endothelial cells (HUVECs) derived from healthy newborns with a strong family history of myocardial infarction (FHMI) showed variations in the presence of molecules related with leukocyte traffic and migration, in comparison to control healthy newborns. For this purpose, we evaluated the labeling of sialic acid containing glycoproteins, tight junction claudins and the cytoskeleton, using lectin- and immunocytochemistry in HUVECs from individuals with and without a strong FHMI. Our results show important differences in the labeling of alpha-2,3 or alpha-2,6 sialic acid-containing glycoconjugates, a disarrangement of actin filaments secondary to the absence of cytoplasmic claudin-5 immunopositivity and an increase in the binding of FHMI HUVECs to CD3+ Jurkat cells. It is possible that these differences relate to a predisposition for early appearance of atherosclerotic lesions.

Presenilin-1 C410Y Alzheimer disease plaques contain synaptic proteins

Presenilin-1 (PS-1) mutations are associated with familial Alzheimer's disease (AD). Although beta-amyloid (Abeta) plaques in brain tissue are characteristic of AD patients, space occupying "cotton-wool" plaques (CWPs) lacking dense Abeta cores have also been described in patients with mutations in exon 9 of the PS-1 gene. The composition of CWPs has not been fully described. To better elucidate the composition of these space-occupying plaques, we used immunohistochemistry with antibodies to the synaptic proteins synapsin-1 and synaptophysin, as well as antibodies to tau, Abeta(-42), Abeta(-40), ubiquitin, neurofilament, and glial fibrillary acidic protein. Confocal laser scanning microscopy (CLSM) was utilized to further characterize these plaques. CWPs showed increased synapsin-1 and synaptophysin immunoreactivity relative to the background gray matter. Synaptic protein-containing CWPs occurred in all affected MTL regions, including the granule cell layer of the dentate gyrus, where synaptic terminals are usually sparse. These data suggest that in C410Y PS-1 AD patients, CWPs may constitute a major component of synaptic terminal-specific proteins, and that the C410Y PS-1 mutation may influence either synaptic structure or synaptic protein expression.

Linking cell-cycle dysfunction in Alzheimer's disease to a failure of synaptic plasticity

Higher cerebral functions are based upon a dynamic organization of neuronal networks. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. In other words, synaptic plasticity can only occur on the expense of the ability to proliferate. Previously, we have put forward a hypothesis, coined "Dr. Jekyll and Mr. Hyde concept" that differentiated neurons after having withdrawn from the cell cycle are able to use those molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity [T. Arendt, Synaptic plasticity and cell cycle activation in neurons are alternative effector pathways The Dr. Jekyll and Mr. Hyde Theory of Alzheimer's disease or The yin and yang of Neuroplasticity. Progr. Neurobiol. 71 (2003) 83-248]. The existence of these alternative effector pathways within a neuron might put it on the risk to erroneously convert signals derived from plastic synaptic changes into cell cycle activation which subsequently leads to cell death. Here we add further evidence to this hypothesis demonstrating a tight association of the origin recognition complex (ORC) with neurofibrillar pathology in AD. The ORC is a critical "guard" of DNA replication and point of convergence of numerous functionally redundant signaling pathways involved in cell cycle progression and transcriptional silencing of apoptotic programmes. ORC subunits in the mammalian brain and their homologes in Drosophila, however, have further been implicated in the regulation of structural neuronal plasticity and cognitive function. We propose that the abnormal subcellular distribution and segregation of ORC proteins in AD might compromise their physiological function in gene silencing and plasticity. This might result in cell cycle activation, DNA-replication and de-repression of apoptotic programmes. ORC subunits might, thus, provide a direct molecular link between synaptic plasticity, DNA replication and cell death.

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.

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

In this work we characterized a 90-kDa glycoprotein from Alzheimer disease (9OAzgp) brain extracts that is recognized by the GalNAc-specific lectin from Amaranthus leucocarpus (ALL), as determined through Western blot. The 90Azgp was purified by electro-elution, and its amino acid sequence determined from peptides obtained after trypsin digestion through MALDI-TOF (Matrix-assisted laser desorption ionization-time of flight), and compared with the relative values obtained from the NCBInr (Swiss-Prot 10/01/2001) database. The 90Azgp showed 32% and 42% homology with the KIAA0310 protein from human brain and the human gastric mucin, respectively. Presence of O-glycosidically linked glycans in the proteins recognized by ALL was confirmed by inhibition of the lectin-glycoprotein interaction through hapten-inhibition assays and also by elimination of the O-glycosidically linked glycans after treatment with O-glycanase from Diplococcus pneumoniae. Electron transmission microscopy confirmed that the receptor recognized by the lectin is processed in the Golgi apparatus of AD neurons. Although the specific role of this glycoprotein has not been identified, considering that the presence of this lectin receptor co-localized with neuritic plaques and in AD sprouting neurons, it could suggest that the O-glycosyl-protein identified by the A. leucocarpus lectin participates in the pathogenesis of neurodegenerative diseases.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Sialylation is modulated through maturation in hemocytes from Macrobrachium rosenbergii

In this work we identified in adult and juvenile freshwater prawn, Macrobrachium rosenbergii, three major type of circulating hemocytes: fusiform; rounded; and large ovoid hemocytes. Rounded and large hemocytes represent the first defense line, since this type of cells exerts phagocytic activity as well as lectin synthesis. Considering that glycosylation plays important roles in cell communication and as a target for pathogenic microorganisms, in this report was also described the main glycosidic modifications that occur in the large and rounded hemocytes from the freshwater prawn during maturation as determined with lectins. Neu5Acalpha2,6Gal, was identified homogeneously distributed in the membrane in 90% of hemocytes from juvenile organisms. Maturation of the freshwater prawn induced a decrease or complete loss of Neu5Acalpha2,6Gal residues that were replaced with Neu5Acalpha2,3 molecules in practically all hemocytes from adult organisms. This change was paralleled by a diminution in 9-O-acetyl-neuraminic acid (Neu5,9Ac(2)) expression. T and Tn antigens (Galbetal,3 GalNAcalpha1-0-Ser/Thr or GalNAcalpha1-0-Ser/Thr, respectively), as well as N-glycosidically linked glycans, seem to be highly conserved throughout maturation. Our results show that sialylation of freshwater prawn hemocytes is modulated throughout the maturation process.