Reelin expression and glycosylation patterns are altered in Alzheimer's disease

Impaired structural and functional development of cerebellum following gestational exposure of deltamethrin in rats: role of reelin

Reelin is an extracellular matrix molecule that is involved in the normal development of the cerebellar lamination, Bergmann glial fibres alignment, Purkinje cell monolayer arrangement and granule cell migration. In this study, we have examined the effects of maternal exposure of deltamethrin (DLT), a type II pyrethroid insecticide, on the structural and functional development of rat cerebellum during postnatal life. DLT (0.75 mg/kg body weight, intraperitoneally dissolved in dimethylsulphoxide) was administered in timed pregnant rats during two different gestational time periods, i.e. gestational days of 7-10 and 11-14, respectively. In DLT exposed rats, a significant overexpression of reelin was observed in the cells of the external granule cell layer (EGL) and internal granule cell layer along with an ectopic expression of reelin in the EGL as well as in the migrating granule cells just below the EGL, revealing an arrest of granule cell migration in this zone. Mis-orientation and hypertrophy of the Bergmann glial fibres further hampered the journey of the granule cells to their final destination. Possibly reelin overexpression also caused misalignment of the Purkinje cells and inhibited the neurite growth leading to a significant decrease in the spine density, main dendritic length and width of the dendritic arbour. Thus, it is proposed that the DLT exerts its neurotoxic effects possibly via the intracellular accumulation and low release of reelin leading to an impaired granule cell and Purkinje cell migration, inhibition of neurite outgrowth and reduced spine density. Such impaired cerebellar development leads to motor coordination deficits.

Reelin immunoreactivity in neuritic varicosities in the human hippocampal formation of non-demented subjects and Alzheimer's disease patients

BACKGROUND

Reelin and its downstream signaling members are important modulators of actin and microtubule cytoskeleton dynamics, a fundamental prerequisite for proper neurodevelopment and adult neuronal functions. Reductions in Reelin levels have been suggested to contribute to Alzheimer's disease (AD) pathophysiology. We have previously reported an age-related reduction in Reelin levels and its accumulation in neuritic varicosities along the olfactory-limbic tracts, which correlated with cognitive impairments in aged mice. Here, we aimed to investigate whether a similar Reelin-associated neuropathology is observed in the aged human hippocampus and whether it correlated with dementia status.

RESULTS

Our immunohistochemical stainings revealed the presence of N- and C-terminus-containing Reelin fragments in corpora amylacea (CAm), aging-associated spherical deposits. The density of these deposits was increased in the molecular layer of the subiculum of AD compared to non-demented individuals. Despite the limitation of a small sample size, our evaluation of several neuronal and glial markers indicates that the presence of Reelin in CAm might be related to aging-associated impairments in neuronal transport leading to accumulation of organelles and protein metabolites in neuritic varicosities, as previously suggested by the findings and discussions in rodents and primates.

CONCLUSIONS

Our results indicate that aging- and disease-associated changes in Reelin levels and proteolytic processing might play a role in the formation of CAm by altering cytoskeletal dynamics. However, its presence may also be an indicator of a degenerative state of neuritic compartments.

Developmental reelin expression and time point-specific alterations from lead exposure in zebrafish

Reelin (RELN) is a large glycoprotein that plays critical roles during and after neuronal development. In a previous study utilizing developing zebrafish, the RELN ortholog was in a subset of genes associated with neuronal function that was altered following exposure to 100ppb Pb at 72h post-fertilization (hpf). Although RELN is associated with neurodevelopment and neurological diseases, it represents a previously uncharacterized target for developmental Pb neurotoxicity. To further investigate Pb-induced expression decrease of RELN, a more detailed analysis was done using in situ hybridization and quantitative PCR (qPCR) on developing zebrafish at multiple developmental time points and at different exposure concentrations. Time points included in the analysis were 24, 36, 48, 60, 72 and 96hpf. The four exposure conditions tested were 10, 50 or 100ppb Pb as well as a control. In situ hybridization demonstrated concentrated expression of the gene in the telencephalon and in midbrain, hindbrain and spinal cord. Expression was less spatially restricted early in development but became isolated to specific cell populations in the central nervous system by 48hpf. The telencephalon demonstrated a high degree of expression throughout development. Pb exposure did not affect the observed location of expression at any of the time points tested. However, qPCR analysis demonstrated that reln expression statistically decreased in embryos exposed to the highest concentration compared to controls at 60hpf. No changes in brain morphology as assessed by measuring brain length and no significant increase in the number of apoptotic cells in the brain were observed at the highest exposure concentration. These data demonstrate that a decrease in reln expression occurs following Pb exposure at a concentration below one that results in changes in brain length or in the number of apoptotic cells in the zebrafish model system during a short window and is likely linked to concurrent developmental processes.

The involvement of Reelin in neurodevelopmental disorders

Reelin is a glycoprotein that serves important roles both during development (regulation of neuronal migration and brain lamination) and in adulthood (maintenance of synaptic function). A number of neuropsychiatric disorders including autism, schizophrenia, bipolar disorder, major depression, Alzheimer's disease and lissencephaly share a common feature of abnormal Reelin expression in the brain. Altered Reelin expression has been hypothesized to impair neuronal connectivity and synaptic plasticity, leading ultimately to the cognitive deficits present in these disorders. The mechanisms for abnormal Reelin expression in some of these disorders are currently unknown although possible explanations include early developmental insults, mutations, hypermethylation of the promoter for the Reelin gene (RELN), miRNA silencing of Reelin mRNA, FMRP underexpression and Reelin processing abnormalities. Increasing Reelin expression through pharmacological therapies may help ameliorate symptoms resulting from Reelin deficits. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Decisive role of Reelin signaling during early stages of Alzheimer's disease

Alzheimer's disease (AD) is one of the largest unmet medical concerns of our society. Around 25 million patients worldwide together with their families are still waiting for an effective treatment. We have recently initiated a re-evaluation of our knowledge of the molecular and cellular mechanisms underlying sporadic AD. Based on the existing literature, we have proposed a mechanistic explanation of how the late-onset form of the disease may evolve on the cellular level. Here, we expand this hypothesis by addressing the pathophysiological changes underlying the early and almost invariant appearance of the neurofibrillary tangles, the only reliable correlate of the cognitive status, in distinct brain areas and their consistent "spread" along interconnected neurons as the disease advances. In this review we present and discuss novel evidence that the extracellular signaling protein Reelin, expressed along the olfactory and limbic pathways in the adult brain, might hold a key to understand the earliest steps of the disease, highlighting the olfactory pathway as the brain's Achilles heel involved in the initiation of the pathophysiological characteristic of late-onset AD.

An optimized approach for enrichment of glycoproteins from cell culture lysates using native multi-lectin affinity chromatography

Lectins are capable of recognizing specific glycan structures and serve as invaluable tools for the separation of glycosylated proteins from nonglycosylated proteins in biological samples. We report on the optimization of native multi-lectin affinity chromatography, combining three lectins, namely, concanavalin A, jacalin, and wheat germ agglutinin for fractionation of cellular glycoproteins from MCF-7 breast cancer lysate. We evaluated several conditions for optimum recovery of total proteins and glycoproteins such as low pH and saccharide elution buffers, and the inclusion of detergents in binding and elution buffers. Optimum recovery was observed with overnight incubation of cell lysate with lectins at 4°C, and inclusion of detergent in binding and saccharide elution buffers. Total protein and bound recoveries were 80 and 9%, respectively. Importantly, we found that high saccharide strength elution buffers were not necessary to release bound glycoproteins. This study demonstrates that multi-lectin affinity chromatography can be extended to total cell lysate to investigate the cellular glycoproteome.

Reelin supplementation recovers sensorimotor gating, synaptic plasticity and associative learning deficits in the heterozygous reeler mouse

The lipoprotein receptor ligand Reelin is important for the processes of normal synaptic plasticity, dendritic morphogenesis, and learning and memory. Heterozygous reeler mice (HRM) show many neuroanatomical, biochemical, and behavioral features that are associated with schizophrenia. HRM show subtle morphological defects including reductions in dendritic spine density, altered synaptic plasticity and behavioral deficits in associative learning and memory and pre-pulse inhibition. The present studies test the hypothesis that in vivo elevation of Reelin levels can rescue synaptic and behavioral phenotypes associated with HRM. We demonstrate that a single in vivo injection of Reelin increases GAD67 expression and alters dendritic spine morphology. In parallel we observed enhancement of hippocampal synaptic function and associative learning and memory. Reelin supplementation also increases pre-pulse inhibition. These results suggest that characteristics of HRM, similar to those observed in schizophrenia, are sensitive to Reelin levels and can be modified with Reelin supplementation in male and female adults.

TIMP-1 inhibits the proteolytic processing of Reelin in experimental epilepsy

Temporal lobe epilepsy is frequently associated with granule cell dispersion (GCD), an abnormal widening of the granule cell layer in the dentate gyrus. There is increasing evidence that a loss and the functional inactivation of the positional signal Reelin is involved in GCD formation. Reelin is synthesized and released by Cajal-Retzius cells and interneurons, and its function depends on proteolytic cleavage after secretion. Epileptic conditions impair Reelin processing by inhibition of matrix metalloprotease (MMP) activity and cause the extracellular accumulation of unprocessed Reelin. Here we investigated how epileptic conditions inhibit MMP activity. We used kainate (KA) treatment of organotypic hippocampal slice cultures as an epilepsy model and found a significant increase of tissue inhibitor of metalloproteases 1 (TIMP-1) levels and strongly enhanced TIMP-1 immunolabeling in hippocampal neurons. Functional inhibition of TIMP-1 prevented the KA-induced impairment of Reelin cleavage indicating that TIMP-1 inhibits MMP activity. Moreover, application of recombinant TIMP-1 alone was sufficient to impair Reelin processing and to induce GCD, similar to that observed after KA treatment. In summary, we present evidence that epileptic conditions inhibit MMP activity by up-regulation of endogenous TIMP-1, which in turn leads to extracellular accumulation of uncleaved and inactive Reelin and thereby to GCD.

Recommendations to standardize preanalytical confounding factors in Alzheimer's and Parkinson's disease cerebrospinal fluid biomarkers: an update

Early diagnosis of neurodegenerative disorders such as Alzheimer's (AD) or Parkinson's disease (PD) is needed to slow down or halt the disease at the earliest stage. Cerebrospinal fluid (CSF) biomarkers can be a good tool for early diagnosis. However, their use in clinical practice is challenging due to the high variability found between centers in the concentrations of both AD CSF biomarkers (Aβ42, total tau and phosphorylated tau) and PD CSF biomarker (α-synuclein). Such a variability has been partially attributed to different preanalytical procedures between laboratories, thus highlighting the need to establish standardized operating procedures. Here, we merge two previous consensus guidelines for preanalytical confounding factors in order to achieve one exhaustive guideline updated with new evidence for Aβ42, total tau and phosphorylated tau, and α-synuclein. The proposed standardized operating procedures are applicable not only to novel CSF biomarkers in AD and PD, but also to biomarkers for other neurodegenerative disorders.

Searching for reliable premortem protein biomarkers for prion diseases: progress and challenges to date

Prion diseases are a unique family of fatal neurodegenerative diseases caused by abnormal folding of normal cellular prion proteins in the brain. Due to the high risk of prion disease transmission and the lack of effective treatment to cure or delay the disease progression, prion diseases pose a serious threat to public health. To control and prevent prion diseases, an early diagnosis is urgently needed. Proteomic analysis has emerged as a powerful technology to decipher biological and pathophysiological processes and identify protein biomarkers indicative of disease. In this article, the authors review the use of the latest proteomic technologies for the identification of promising prion disease biomarkers, the challenges that exist in biomarker development pipelines and the new directions for utilizing proteomics for future biomarker discovery in the context of prion disease diagnostics.

Characterization of G protein-coupled receptor 56 protein expression in the mouse developing neocortex

GPR56, one of the adhesion G-protein-coupled receptors (GPCRs), plays an important role in the development of the cerebral cortex. Mutations in GPR56 cause a severe human cortical malformation called bilateral frontoparietal polymicrogyria (BFPP), characterized by a global malformation of the cerebral cortex that most severely affects the frontal and parietal regions. To characterize the expression pattern of GPR56 in the developing cerebral cortex, we developed a mouse monoclonal antibody against mouse GPR56. We revealed that GPR56 is expressed in multiple cell types in the preplate, marginal zone, subventricular zone (SVZ), and ventricular zone (VZ). Most interestingly, the expression of GPR56 in preplate neurons showed an anterior-to-posterior gradient at embryonic day (E) 10.5-11.5. In contrast, the expression pattern of the GPR56 ligand, collagen III, revealed no visible gradient pattern. With the widespread expression of GPR56 in the developing cortex, it is difficult to draw a specific conclusion as to which of the GPR56-expressing cells are critical for human brain development. However, the correlation between GPR56 expression in neurons at E10.5-E11.5 and the anatomic distribution of the cortical malformation in both humans and mice suggests that its function in preplate neurons is indispensible.

Regulated proteolytic processing of Reelin through interplay of tissue plasminogen activator (tPA), ADAMTS-4, ADAMTS-5, and their modulators

The extracellular signaling protein Reelin, indispensable for proper neuronal migration and cortical layering during development, is also expressed in the adult brain where it modulates synaptic functions. It has been shown that proteolytic processing of Reelin decreases its signaling activity and promotes Reelin aggregation in vitro, and that proteolytic processing is affected in various neurological disorders, including Alzheimer's disease (AD). However, neither the pathophysiological significance of dysregulated Reelin cleavage, nor the involved proteases and their modulators are known. Here we identified the serine protease tissue plasminogen activator (tPA) and two matrix metalloproteinases, ADAMTS-4 and ADAMTS-5, as Reelin cleaving enzymes. Moreover, we assessed the influence of several endogenous protease inhibitors, including tissue inhibitors of metalloproteinases (TIMPs), α-2-Macroglobulin, and multiple serpins, as well as matrix metalloproteinase 9 (MMP-9) on Reelin cleavage, and described their complex interplay in the regulation of this process. Finally, we could demonstrate that in the murine hippocampus, the expression levels and localization of Reelin proteases largely overlap with that of Reelin. While this pattern remained stable during normal aging, changes in their protein levels coincided with accelerated Reelin aggregation in a mouse model of AD.

Reelin demarcates a subset of pre-Bötzinger complex neurons in adult rat

Identification of two markers of neurons in the pre-Bötzinger complex (pre-BötC), the neurokinin 1 receptor (NK1R) and somatostatin (Sst) peptide, has been of great utility in understanding the essential role of the pre-BötC in breathing. Recently, the transcription factor dbx1 was identified as a critical, but transient, determinant of glutamatergic pre-BötC neurons. Here, to identify additional markers, we constructed and screened a single-cell subtractive cDNA library from pre-BötC inspiratory neurons. We identified the glycoprotein reelin as a potentially useful marker, because it is expressed in distinct populations of pre-BötC and inspiratory bulbospinal ventral respiratory group (ibsVRG) neurons. Reelin ibsVRG neurons were larger (27.1 ± 3.8 μm in diameter) and located more caudally (>12.8 mm caudal to Bregma) than reelin pre-BötC neurons (15.5 ± 2.4 μm in diameter, <12.8 mm rostral to Bregma). Pre-BötC reelin neurons coexpress NK1R and Sst. Reelin neurons were also found in the parahypoglossal and dorsal parafacial regions, pontine respiratory group, and ventromedial medulla. Reelin-deficient (Reeler) mice exhibited impaired respones to hypoxia compared with littermate controls. We suggest that reelin is a useful molecular marker for pre-BötC neurons in adult rodents and may play a functional role in pre-BötC microcircuits.

Glycans in sera of amyotrophic lateral sclerosis patients and their role in killing neuronal cells

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease caused by degeneration of upper and lower motor neurons. To date, glycosylation patterns of glycoproteins in fluids of ALS patients have not been described. Moreover, the aberrant glycosylation related to the pathogenesis of other neurodegenerative diseases encouraged us to explore the glycome of ALS patient sera. We found high levels of sialylated glycans and low levels of core fucosylated glycans in serum-derived N-glycans of patients with ALS, compared to healthy volunteer sera. Based on these results, we analyzed the IgG Fc N²⁹⁷-glycans, as IgG are major serum glycoproteins affected by sialylation or core fucosylation and are found in the motor cortex of ALS patients. The analyses revealed a distinct glycan, A2BG2, in IgG derived from ALS patient sera (ALS-IgG). This glycan increases the affinity of IgG to CD16 on effector cells, consequently enhancing Antibody-Dependent Cellular Cytotoxicity (ADCC). Therefore, we explore whether the Fc-N²⁹⁷-glycans of IgG may be involved in ALS disease. Immunostaining of brain and spinal cord tissues revealed over-expression of CD16 and co-localization of intact ALS-IgG with CD16 and in brain with activated microglia of G93A-SOD1 mice. Intact ALS-IgG enhanced effector cell activation and ADCC reaction in comparison to sugar-depleted or control IgG. ALS-IgG were localized in the synapse between brain microglia and neurons of G93A-SOD1 mice, manifesting a promising in vivo ADCC reaction. Therefore, glycans of ALS-IgG may serve as a biomarker for the disease and may be involved in neuronal damage.

Neurodegeneration in Alzheimer disease: role of amyloid precursor protein and presenilin 1 intracellular signaling

Alzheimer disease (AD) is a heterogeneous neurodegenerative disorder characterized by (1) progressive loss of synapses and neurons, (2) intracellular neurofibrillary tangles, composed of hyperphosphorylated Tau protein, and (3) amyloid plaques. Genetically, AD is linked to mutations in few proteins amyloid precursor protein (APP) and presenilin 1 and 2 (PS1 and PS2). The molecular mechanisms underlying neurodegeneration in AD as well as the physiological function of APP are not yet known. A recent theory has proposed that APP and PS1 modulate intracellular signals to induce cell-cycle abnormalities responsible for neuronal death and possibly amyloid deposition. This hypothesis is supported by the presence of a complex network of proteins, clearly involved in the regulation of signal transduction mechanisms that interact with both APP and PS1. In this review we discuss the significance of novel finding related to cell-signaling events modulated by APP and PS1 in the development of neurodegeneration.

Casting a net on dendritic spines: the extracellular matrix and its receptors

Dendritic spines are dynamic structures that accommodate the majority of excitatory synapses in the brain and are influenced by extracellular signals from presynaptic neurons, glial cells, and the extracellular matrix (ECM). The ECM surrounds dendritic spines and extends into the synaptic cleft, maintaining synapse integrity as well as mediating trans-synaptic communications between neurons. Several scaffolding proteins and glycans that compose the ECM form a lattice-like network, which serves as an attractive ground for various secreted glycoproteins, lectins, growth factors, and enzymes. ECM components can control dendritic spines through the interactions with their specific receptors or by influencing the functions of other synaptic proteins. In this review, we focus on ECM components and their receptors that regulate dendritic spine development and plasticity in the normal and diseased brain.

Exploring the use of social media to measure journal article impact

Science blogs, Twitter commentary, and comments on journal websites represent an immediate response to journal articles, and may help in identifying relevant publications. However, the use of these media for establishing paper impact is not well studied. Using Wikipedia as a proxy for other social media, we explore the correlation between inclusion of a journal article in Wikipedia, and article impact as measured by citation count. We start by cataloging features of PubMed articles cited in Wikipedia. We find that Wikipedia pages referencing the most journal articles are about disorders and diseases, while the most referenced articles in Wikipedia are about genomics. We note that journal articles in Wikipedia have significantly higher citation counts than an equivalent random article subset. We also observe that articles are included in Wikipedia soon after publication. Our data suggest that social media may represent a largely untapped post-publication review resource for assessing paper impact.

Exercise during pregnancy mitigates Alzheimer-like pathology in mouse offspring

Physical activity protects brain function in healthy individuals and those with Alzheimer's disease (AD). Evidence for beneficial effects of parental exercise on the health status of their progeny is sparse and limited to nondiseased individuals. Here, we questioned whether maternal running interferes with offspring's AD-like pathology and sought to decipher the underlying mechanisms in TgCRND8 mice. Maternal stimulation was provided by voluntary wheel running vs. standard housing during pregnancy. Following 5 mo of standard housing of transgenic and wild-type offspring, their brains were examined for AD-related pathology and/or plasticity changes. Running during pregnancy reduced β-amyloid (Aβ) plaque burden (-35%, P=0.017) and amyloidogenic APP processing in transgenic offspring and further improved the neurovascular function by orchestrating different Aβ transporters and increasing angiogenesis (+29%, P=0.022). This effect was accompanied by diminished inflammation, as indicated by reduced microgliosis (-20%, P=0.002) and down-regulation of other proinflammatory mediators, and resulted in less oxidative stress, as nitrotyrosine levels declined (-28%, P=0.029). Moreover, plasticity changes (in terms of up-regulation of reelin, synaptophysin, and ARC) were found not only in transgenic but also in wild-type offspring. We conclude that exercise during pregnancy provides long-lasting protection from neurodegeneration and improves brain plasticity in the otherwise unstimulated progeny.

Reelin supplementation enhances cognitive ability, synaptic plasticity, and dendritic spine density

Apolipoprotein receptors belong to an evolutionarily conserved surface receptor family that has intimate roles in the modulation of synaptic plasticity and is necessary for proper hippocampal-dependent memory formation. The known lipoprotein receptor ligand Reelin is important for normal synaptic plasticity, dendritic morphology, and cognitive function; however, the in vivo effect of enhanced Reelin signaling on cognitive function and synaptic plasticity in wild-type mice is unknown. The present studies test the hypothesis that in vivo enhancement of Reelin signaling can alter synaptic plasticity and ultimately influence processes of learning and memory. Purified recombinant Reelin was injected bilaterally into the ventricles of wild-type mice. We demonstrate that a single in vivo injection of Reelin increased activation of adaptor protein Disabled-1 and cAMP-response element binding protein after 15 min. These changes correlated with increased dendritic spine density, increased hippocampal CA1 long-term potentiation (LTP), and enhanced performance in associative and spatial learning and memory. The present study suggests that an acute elevation of in vivo Reelin can have long-term effects on synaptic function and cognitive ability in wild-type mice.

A quantitative proteomic approach to prion disease biomarker research: delving into the glycoproteome

Mass spectrometry (MS) -- based proteomic approaches have evolved as powerful tools for the discovery of biomarkers. However, the identification of potential protein biomarkers from biofluid samples is challenging because of the limited dynamic range of detection. Currently there is a lack of sensitive and reliable premortem diagnostic test for prion diseases. Here, we describe the use of a combined MS-based approach for biomarker discovery in prion diseases from mouse plasma samples. To overcome the limited dynamic range of detection and sample complexity of plasma samples, we used lectin affinity chromatography and multidimensional separations to enrich and isolate glycoproteins at low abundance. Relative quantitation of a panel of proteins was obtained by a combination of isotopic labeling and validated by spectral counting. Overall 708 proteins were identified, 53 of which showed more than 2-fold increase in concentration whereas 58 exhibited more than 2-fold decrease. A few of the potential candidate markers were previously associated with prion or other neurodegenerative diseases.

Role for Reelin in neurotransmitter release

The extracellular matrix molecule Reelin is known to control neuronal migration during development. Recent evidence suggests that it also plays a role in the maturation of postsynaptic dendrites and spines as well as in synaptic plasticity. Here, we aimed to address the question whether Reelin plays a role in presynaptic structural organization and function. Quantitative electron microscopic analysis of the number of presynaptic boutons in the stratum radiatum of hippocampal region CA1 did not reveal differences between wild-type animals and Reelin-deficient reeler mutant mice. However, additional detailed analysis showed that the number of presynaptic vesicles was significantly increased in CA1 synapses of reeler mutants. To test the hypothesis that vesicle fusion is altered in reeler, we studied proteins known to control transmitter release. SNAP25, a protein of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, was found to be significantly reduced in reeler mutants, whereas other SNARE complex proteins remained unaltered. Addition of recombinant Reelin to organotypic slice cultures of reeler hippocampi substantially rescued not only SNAP25 protein expression levels but also the number of vesicles per bouton area indicating a role for Reelin in presynaptic functions. Next, we analyzed paired-pulse facilitation, a presynaptic mechanism associated with transmitter release, and observed a significant decrease at CA1 synapses of reeler mutants when compared with wild-type animals. Together, these novel findings suggest a role for Reelin in modulating presynaptic release mechanisms.

Quest for new genomic and proteomic biomarkers in neurology

The possibility of identifying novel biomarkers for neurodegenerative diseases has been greatly enhanced with recent advances in genomics and proteomics. Novel technologies have the potential to hasten the development of new biomarkers useful as predictors of disease etiology and outcome, as well as responsiveness to therapy. Disease-modifying new therapies are very much needed in modern approaches to treatment of neurodegenerative diseases. Current progress in the field encounters a degree of skepticism about the reliability of genomic and proteomic data and its relevance for clinical applications. Standard operating procedures covering sample collection, methodology and statistical analysis need to be fully developed and strictly adhered to in order to assure reproducible and clinically relevant results. Previous studies involving patients with neurodegenerative diseases show promise in using genomic and proteomic approaches for development of new biomarkers. Confirmation of any novel biomarker in multiple independent patient cohorts and correlation of the improvement in biomarker endpoint with clinical improvement in longitudinal patient studies remains crucial for future successful application. We propose that a combination of approaches in biomarker discovery may in the end lead to identification of promising candidates at DNA, RNA, protein and small molecule level.

Review: disruption of the postsynaptic density in Alzheimer's disease and other neurodegenerative dementias

The most common causes of neurodegenerative dementia include Alzheimer's disease (AD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD). We believe that, in all 3, aggregates of pathogenic proteins are pathological substrates which are associated with a loss of synaptic function/plasticity. The synaptic plasticity relies on the normal integration of glutamate receptors at the postsynaptic density (PSD). The PSD organizes synaptic proteins to mediate the functional and structural plasticity of the excitatory synapse and to maintain synaptic homeostasis. Here, we will discuss the relevant disruption of the protein network at the PSD in these dementias and the accumulation of the pathological changes at the PSD years before clinical symptoms. We suggest that the functional and structural plasticity changes of the PSD may contribute to the loss of molecular homeostasis within the synapse (and contribute to early symptoms) in these dementias.

Role for Reelin-induced cofilin phosphorylation in the assembly of sympathetic preganglionic neurons in the murine intermediolateral column

Sympathetic preganglionic neurons (SPNs) are located in the intermediolateral column (IMLC) of the spinal cord. This specific localization results from primary and secondary migratory processes during spinal cord development. Thus, following neurogenesis in the neuroepithelium, SPNs migrate first in a ventrolateral direction and then, in a secondary step, dorsolaterally to reach the IMLC. These migratory processes are controlled, at least in part, by the glycoprotein Reelin, which is known to be important for the development of laminated brain structures. In reeler mutants deficient in Reelin, SPNs initially migrate ventrolaterally as normal. However, most of them then migrate medially to become eventually located near the central canal. Here, we provide evidence that in wild-type animals this aberrant medial migration towards the central canal is prevented by Reelin-induced cytoskeletal stabilization, brought about by phosphorylation of cofilin. Cofilin plays an important role in actin depolymerization, a process required for the changes in cell shape during migration. Phosphorylation of cofilin renders it unable to depolymerize F-actin, thereby stabilizing the cytoskeleton. Using immunostaining for phosphorylated cofilin (p-cofilin), we demonstrate that SPNs in wild-type animals, but not in reeler mutants and other mutants of the Reelin signalling cascade, are immunoreactive for p-cofilin. These findings suggest that Reelin near the central canal induces cofilin phosphorylation in SPNs, thereby preventing them from aberrant migration towards the central canal. The results extend our previous studies on cortical neurons in which Reelin in the marginal zone was found to stabilize the leading processes of migrating neurons and terminate the migration process.

Reelin Signaling, Hippocampal Neurogenesis, and Efficacy of Aspirin Intake & Stem Cell Transplantation in Aging and Alzheimer's disease

Comprehending the mechanisms underlying the pathophysiology of aging and Alzheimer's disease has immense value for developing strategies that promote successful aging and prevent or cure Alzheimer's disease. The first issue of the new journal, "Aging & Disease" comprises articles that discuss the current knowledge pertaining to changes in reelin signaling in normal & pathological forms of aging, memory and neurogenesis in Aging & Alzheimer's disease, the efficacy of a non-steroidal anti-inflammatory drug aspirin in combination with docosahexaenoic acid for reducing the risk for Alzheimer's disease, and the usefulness of stem cell transplantation for improving memory in aging and Alzheimer's disease. The highlights and the importance of the above issues to Aging and Alzheimer's disease are discussed in this commentary.

Reduced Reelin expression accelerates amyloid-beta plaque formation and tau pathology in transgenic Alzheimer's disease mice

In addition to the fundamental role of the extracellular glycoprotein Reelin in neuronal development and adult synaptic plasticity, alterations in Reelin-mediated signaling have been suggested to contribute to neuronal dysfunction associated with Alzheimer's disease (AD). In vitro data revealed a biochemical link between Reelin-mediated signaling, Tau phosphorylation, and amyloid precursor protein (APP) processing. To directly address the role of Reelin in amyloid-beta plaque and Tau pathology in vivo, we crossed heterozygous Reelin knock-out mice (reeler) with transgenic AD mice to investigate the temporal and spatial AD-like neuropathology. We demonstrate that a reduction in Reelin expression results in enhanced amyloidogenic APP processing, as indicated by the precocious production of amyloid-beta peptides, the significant increase in number and size of amyloid-beta plaques, as well as age-related aggravation of plaque pathology in double mutant compared with single AD mutant mice of both sexes. Numerous amyloid-beta plaques accumulate in the hippocampal formation and neocortex of double mutants, precisely in layers with strongest Reelin expression and highest accumulation of Reelin plaques in aged wild-type mice. Moreover, concentric accumulations of phosphorylated Tau-positive neurons around amyloid-beta plaques were evident in 15-month-old double versus single mutant mice. Silver stainings indicated the presence of neurofibrillary tangles, selectively associated with amyloid-beta plaques and dystrophic neurites in the entorhinal cortex and hippocampus. Our findings suggest that age-related Reelin aggregation and concomitant reduction in Reelin-mediated signaling play a proximal role in synaptic dysfunction associated with amyloid-beta deposition, sufficient to enhance Tau phosphorylation and tangle formation in the hippocampal formation in aged Reelin-deficient transgenic AD mice.

Reelin-mediated Signaling during Normal and Pathological Forms of Aging

Reelin is a large extracellular matrix protein essential for mediating proper neuronal positioning during development. Employing the same lipoprotein-mediated signaling cascade, Reelin regulates NMDA receptor homeostasis and modulates synaptic function and plasticity in adult synapses. In line, aging-related reduction in Reelin expression has been shown to contribute to cognitive impairments during normal aging. Although recent experimental evidence suggests an involvement of dysfunctional Reelin in pathological forms of aging, such as late-onset Alzheimer's disease (AD), the molecular mechanisms by which this conserved extracellular glycoprotein contributes to the pathogenesis of AD remains still largely unknown. In the present review, we briefly summarize the role of Reelin in the developing and adult brain and discuss the implication of loss- or gain-of-functions of developmental programs in the adult brain as putative inducing factors of pathological forms of aging. Finally, we will propose some new concepts on the role of inflammatory cytokines in interfering with Reelin-mediated signaling during neurodevelopment and adult synaptic function, and discuss how this could be translated into a novel non-transgenic mouse model of late-onset AD. Thus, the findings presented in this review are aimed to highlight the important role of Reelin-mediated signaling in maintaining a crucial developmental program in the adult brain that is required to prevent the shift from normal to pathological aging.

Role for Reelin in stabilizing cortical architecture

Reelin controls the migration of neurons and layer formation during brain development. However, recent studies have shown that disrupting Reelin function in the adult hippocampus induces repositioning of fully differentiated neurons, suggesting a stabilizing effect of Reelin on mature neuronal circuitry. Indeed, Reelin was recently found to stabilize the actin cytoskeleton by inducing cofilin phosphorylation. When unphosphorylated, cofilin acts as an actin-depolymerizing protein that promotes the disassembly of F-actin. Here, a novel hypothesis is proposed whereby decreased Reelin expression in the mature brain causes destabilization of neurons and their processes, leading to aberrant plasticity and aberrant wiring of brain circuitry. This has implications for brain disorders, such as epilepsy and schizophrenia, in which deficiencies in Reelin expression occur.

Reelin regulates postnatal neurogenesis and enhances spine hypertrophy and long-term potentiation

Reelin, an extracellular protein essential for neural migration and lamination, is also expressed in the adult brain. To unravel the function of this protein in the adult forebrain, we generated transgenic mice that overexpress Reelin under the control of the CaMKIIalpha promoter. Overexpression of Reelin increased adult neurogenesis and impaired the migration and positioning of adult-generated neurons. In the hippocampus, the overexpression of Reelin resulted in an increase in synaptic contacts and hypertrophy of dendritic spines. Induction of long-term potentiation (LTP) in alert-behaving mice showed that Reelin overexpression evokes a dramatic increase in LTP responses. Hippocampal field EPSP during a classical conditioning paradigm was also increased in these mice. Our results indicate that Reelin levels in the adult brain regulate neurogenesis and migration, as well as the structural and functional properties of synapses. These observations suggest that Reelin controls developmental processes that remain active in the adult brain.

Reelin is a platelet protein and functions as a positive regulator of platelet spreading on fibrinogen

Abnormalities of platelet functions have been linked to reelin-impaired neuronal disorders. However, little attention has been given to understanding the interplay between reelin and platelet. In this study, reelin was found to present in the human platelets and megakaryocyte-like leukemic cells. Reelin-binding assays revealed that extracellular reelin can interact with platelets through the receptor belonging to the low density lipoprotein receptor gene family. The reelin-to-platelet interactions enhance platelet spreading on fibrinogen concomitant with the augmentation of lamellipodia formation and F-actin bundling. In contrast, reelin has no effect on integrin alphaIIbbeta3 activation and agonist-induced platelet aggregation. Molecular analysis revealed that the up-regulation of Rac1 activity and the inhibition of protein kinase C delta-Thr505 phosphorylation are important for reelin-mediated enhancement of platelet spreading on fibrinogen. These findings demonstrate for the first time that reelin is present in platelets and the reelin-to-platelet interactions play a novel role in platelet signaling and functions.

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.

C-terminal region-dependent change of antibody-binding to the Eighth Reelin repeat reflects the signaling activity of Reelin

Reelin is a secreted glycoprotein that plays pivotal roles in the development and function of the brain, but how it activates downstream intracellular signaling is not fully understood. We have recently reported that the highly conserved C-terminal region (CTR) of Reelin is required for its full signaling activity, although the underlying mechanism remains unknown. During biochemical study of Reelin, we serendipitously found that one commercially available anti-Reelin antibody G20 can bind to CTR-lacking mutant Reelin proteins, but not wild-type Reelin, on Western blotting. The G20 epitope resides in the last 19 residues of Reelin-repeat 8 (RR8), and neither posttranslational modification nor proteolysis can explain this effect. Furthermore, when an unrelated sequence, such as FLAG-tag, is inserted between RR8 and CTR, the reactivity of the corresponding antibody greatly decreases. These results suggest that RR8 and CTR form a tight structure that makes the surrounding sequence inaccessible to an antibody. Taking advantage of this phenomenon, we show the existence of CTR-lacking Reelin isoform in vivo for the first time and estimate its contribution to the total amount of secreted Reelin. Importantly, the extent to which Reelin mutants react with G20 is inversely correlated with their signaling activity, indicating that the CTR-induced structural change of RR8 is a prerequisite for downstream signaling activation, presumably via binding to a certain neuronal membrane molecule(s).

Differential gene expression analysis of human entorhinal cortex support a possible role of some extracellular matrix proteins in the onset of Alzheimer disease

The onset of Alzheimer's disease (AD) has been associated with the specific vulnerability of neurons in the upper layers of the entorhinal cortex. To define the molecular characteristics of those neurons, we have used microarrays to define the gene expression in that region. In this way, we identified several genes that are expressed distinctly in the upper and lower layers of the entorhinal cortex. These include the genes encoding the matrix Gla protein, collagen type 1alpha2, reelin, semaphorin 3C or the relaxin receptor, all related to the extracellular matrix. Thus, differences in the extracellular matrix components between the upper and lower layers of the entorhinal cortex may in part explain the vulnerability of neurons present in the upper layers of this brain region in disorders like AD.

Neurochemical biomarkers in the differential diagnosis of movement disorders

In recent years, the neurochemical analysis of neuronal proteins in cerebrospinal fluid (CSF) has become increasingly accepted for the diagnosis of neurodegenerative dementia diseases such as Alzheimer's disease and Creutzfeldt-Jakob disease. CSF surrounds the central nervous system, and in the composition of CSF proteins one finds brain-specific proteins that are prioritized from blood-derived proteins. Levels of specific CSF proteins could be very promising biomarkers for central nervous system diseases. We need the development of more easily accessible biomarkers, in the blood. In neurodegenerative diseases with and without dementia, studies on CSF and blood proteins have investigated the usefulness of biomarkers in differential diagnosis. The clinical diagnoses of Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, progressive supranuclear palsy, and corticobasal degeneration still rely mainly on clinical symptoms as defined by international classification criteria. In this article, we review CSF biomarkers in these movement disorders and discuss recent published reports on the neurochemical intra vitam diagnosis of neurodegenerative disorders (including recent CSF alpha-synuclein findings).

Accumulation of reelin-positive plaques is accompanied by a decline in basal forebrain projection neurons during normal aging

Besides its critical role during neurodevelopment, the extracellular glycoprotein reelin is also a pivotal regulator of adult synaptic function and plasticity, and altered reelin-mediated signalling has been suggested to contribute to neuronal dysfunction associated with Alzheimer's disease. We have recently discovered, in aged rodents and non-human primates, a pronounced decline in reelin-positive interneurons and concomitant accumulation of reelin in extracellular amyloid-like deposits, both being associated with episodic-like memory impairments. Here, we report that these age-related neuropathological changes in hippocampus, entorhinal and piriform cortices of aged wild-type mice are accompanied by abnormal axonal varicosities and altered expression profiles of calcium-binding proteins in plaque-dense areas, as well as a significant reduction in the number of parvalbumin-positive gamma-aminobutyric acid (GABA)ergic projection neurons in basal forebrain areas, including medial septum (MS), ventral and horizontal diagonal Band of Broca (VDB/HDB) and substantia innominata (SI), compared with young subjects. In addition, a significant reduction in the number of choline acetyltransferase-positive cholinergic projection neurons was evident in the HDB/SI area but not in the MS of aged compared with young wild-type mice. No reelin-deposits were found in these basal forebrain regions. Our findings suggest that the elevated reelin plaque load in the projection areas of afferent subcortical GABAergic and cholinergic neurons including hippocampus, entorhinal and piriform cortices affects the axonal integrity and survival of these neurons, potentially contributing to the cognitive impairments observed in aged wild-type mice.

Interaction of reelin with amyloid precursor protein promotes neurite outgrowth

The processing of amyloid precursor protein (APP) to Abeta is an important event in the pathogenesis of Alzheimer's disease, but the physiological function of APP is not well understood. Our previous work has shown that APP processing and Abeta production are regulated by the extracellular matrix protein Reelin. In the present study, we examined whether Reelin interacts with APP, and the functional consequences of that interaction in vitro. Using coimmunoprecipitation, we found that Reelin interacted with APP through the central domain of Reelin (repeats 3-6) and the E1 extracellular domain of APP. Reelin increased cell surface levels of APP and decreased endocytosis of APP in hippocampal neurons in vitro. In vivo, Reelin levels were increased in brains of APP knock-out mice and decreased in APP-overexpressing mice. RNA interference knockdown of APP decreased neurite outgrowth in vitro and prevented Reelin from increasing neurite outgrowth. Knock-out of APP or Reelin decreased dendritic arborization in cortical neurons in vivo, and APP overexpression increased dendritic arborization. APP and Reelin have previously been shown to promote neurite outgrowth through interactions with integrins. We confirmed that APP interacted with alpha3beta1 integrin, and alpha3beta1 integrin altered APP trafficking and processing. Addition of an alpha3beta1 integrin antibody prevented APP and Reelin-induced neurite outgrowth. These findings demonstrate that Reelin interacts with APP, potentially having important effects on neurite development.

Molecular regulation of neuronal migration during neocortical development

Neocortex, a distinct six-layered neural structure, is one of the most exquisite nerve tissues in the human body. Proper assembly of neocortex requires precise regulation of neuronal migration and abnormalities can result in severe neurological diseases. Three major types of neuronal migration have been implicated in corticogenesis: radial migration of excitatory neuron precursors and tangential migration of interneurons as well as Cajal-Retzius cells. In the past several years, significant progress has been made in understanding how these parallel events are regulated and coordinated during corticogenesis. New insights have been gained into regulation of radial neuron migration by the well-known Reelin signal. New pathways have also been identified that regulate radial as well as tangential migration. Equally important, better understandings have been obtained on the cellular and molecular mechanics of cell migration by both projection neurons and interneurons. These findings have not only enhanced our understanding of normal neuron migration but also revealed insights into the etiologies of several neurological diseases where these processes go awry.

Quantifying proteins by mass spectrometry: the selectivity of SRM is only part of the problem

Precise and accurate protein quantification is critical to many areas of proteomics. Antibody-based approaches are costly and time-consuming to develop, consequently, there is considerable interest in alternative quantitative methods that are versatile and can be implemented without the considerable delays associated with antibody development and characterization. Approaches based on MS have therefore attracted considerable attention and are now frequently touted as the most practical and powerful of all options. Nevertheless, there are serious limitations associated with quantifying a protein based on tandem mass analysis of one or two peptides generated by either chemical or enzymatic cleavage. In an accompanying Viewpoint article, Molloy and coworkers point out that selectivity is not necessarily guaranteed despite the power of SRM. Here we address an additional concern that can also compromise specificity. In complex mammalian systems, multiple proteins can serve as precursors of a single peptide and consequently, depending on the peptide(s) selected, protein levels may be significantly under- or overestimated.

Chronic psychotropic drug treatment causes differential expression of Reelin signaling system in frontal cortex of rats

Disruption of the Reelin and GABAergic signaling systems have been observed in psychiatric disorders including autism, schizophrenia, bipolar disorder, and major depression. Less is known of therapeutic interventions that may help ameliorate the effects of these disruptions. The current study investigated whether chronic administration of psychotropic medications (clozapine, fluoxetine, haloperidol, lithium, olanzapine, and valproic acid) used in the treatment of psychiatric disorders alters levels of Reelin, its receptor Vldlr, downstream molecules Gsk3 beta, Dab-1, and Gad65/67 in rat prefrontal cortex as measured by qRT-PCR and SDS-PAGE and western blotting. qRT-PCR revealed that mRNAs for Reelin, Vldlr, Dab-1, Gsk3 beta, and Gad65 were each significantly altered by at least one of the drugs tested, and in the case of Reelin, Dab-1, and Gsk3 beta, by multiple drugs. To verify our results, we also performed SDS-PAGE and western blotting experiments. Again, several of the protein products for Reelin, Vldlr, Dab-1, Gsk3 beta, Gad65, and Gad67 were also significantly altered by multiple drugs. The present results suggest that the Reelin signaling and GABAergic systems are affected by commonly used psychotropic medications. These changes may help explain the efficacy of these drugs and provide further support for the investigation of the Reelin and GABAergic signaling systems as therapeutic targets for the treatment of neuropsychiatric diseases.

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The scale-free and small-world network models reflect the functional units of networks. However, when we investigated the network properties of a signaling pathway using these models, no significant differences were found between the original undirected graphs and the graphs in which inactive proteins were eliminated from the gene expression data. We analyzed signaling networks by focusing on those pathways that best reflected cellular function. Therefore, our analysis of pathways started from the ligands and progressed to transcription factors and cytoskeletal proteins. We employed the Python module to assess the target network. This involved comparing the original and restricted signaling cascades as a directed graph using microarray gene expression profiles of late onset Alzheimer's disease. The most commonly used method of shortest-path analysis neglects to consider the influences of alternative pathways that can affect the activation of transcription factors or cytoskeletal proteins. We therefore introduced included k-shortest paths and k-cycles in our network analysis using the Python modules, which allowed us to attain a reasonable computational time and identify k-shortest paths. This technique reflected results found in vivo and identified pathways not found when shortest path or degree analysis was applied. Our module enabled us to comprehensively analyse the characteristics of biomolecular networks and also enabled analysis of the effects of diseases considering the feedback loop and feedforward loop control structures as an alternative path.

Neuroepithelial cells require fucosylated glycans to guide the migration of vagus motor neuron progenitors in the developing zebrafish hindbrain

The molecular mechanisms by which neurons migrate and accumulate to form the neural layers and nuclei remain unclear. The formation of vagus motor nuclei in zebrafish embryos is an ideal model system in which to address this issue because of the transparency of the embryos and the availability of established genetic and molecular biological techniques. To determine the genes required for the formation of the vagus motor nuclei, we performed N-ethyl-N-nitrosourea-based mutant screening using a zebrafish line that expresses green fluorescent protein in the motor neurons. In wild-type embryos, the vagus motor neuron progenitors are born in the ventral ventricular zone, then migrate tangentially in the dorsolateral direction, forming the nuclei. However, in towhead (twd(rw685)) mutant embryos, the vagus motor neuron progenitors stray medially away from the normal migratory pathway and fail to stop in the right location. The twd(rw685) mutant has a defect in the GDP-mannose 4,6 dehydratase (gmds) gene, which encodes a key enzyme in the fucosylation pathway. Levels of fucosylated glycans were markedly and specifically reduced in twd(rw685) mutant embryos. Cell transplantation analysis revealed that GMDS is not essential in the vagus motor neuron progenitors for correct formation of the vagus motor nuclei, but is required in the neuroepithelial cells that surround the progenitors. Together, these findings suggest that fucosylated glycans expressed in neuroepithelial cells are required to guide the migration of vagus motor neuron progenitors.

Neurochemical approaches in the laboratory diagnosis of Parkinson and Parkinson dementia syndromes: a review

The diagnosis of Parkinson disease (PD) is rendered on the basis of clinical parameters, whereby laboratory chemical tests or morphological imaging is only called upon to exclude other neurodegenerative diseases. The differentiation between PD and other diseases of the basal ganglia, especially the postsynaptic Parkinson syndromes multisystem atrophy (MSA) and progressive supranuclear palsy (PSP), is of decisive importance, on the one hand, for the response to an appropriate therapy, and on the other hand, for the respective prognosis of the disease. However, particularly at the onset of symptoms, it is difficult to precisely distinguish these diseases from each other, presenting with an akinetic-rigid syndrome. It is not yet possible to conduct a neurochemical differentiation of Parkinson syndromes. Therefore, a reliable biomarker is still to be found that might predict the development of Parkinson dementia. Since this situation is currently the subject of various different studies, the following synopsis is intended to provide a brief summary of the investigations addressing the field of the early neurochemical differential diagnosis of Parkinson syndromes and the early diagnosis of Parkinson dementia, from direct alpha-synuclein detection to proteomic approaches. In addition, an overview of the tested biomarkers will be given with regard to their possible introduction as a screening method.

Mechanism and significance of specific proteolytic cleavage of Reelin

Reelin is a secreted glycoprotein essential for normal brain development and function. In the extracellular milieu, Reelin is subject to specific cleavage at two (N-t and C-t) sites. The N-t cleavage of Reelin is implicated in psychiatric and Alzheimer's diseases, but the molecular mechanism and physiological significance of this cleavage are not completely understood. Particularly, whether the N-t cleavage affects the signaling activity of Reelin remains controversial. Here, we show that the protease in charge of the N-t cleavage of Reelin requires the activity of certain proprotein convertase family for maturation and has strong affinity for heparin. By taking advantage of these observations, we for the first time succeeded in obtaining "Uncleaved" and "Completely Cleaved" Reelin proteins. The N-t cleavage splits Reelin into two distinct fragments and virtually abolishes its signaling activity. These findings provide an important biochemical basis for the function of Reelin proteolysis in brain development and function.

Comparative glycoproteomics: approaches and applications

Glycosylation plays fundamental roles in controlling various biological processes. Therefore, glycosylation analysis has become an important target for proteomic research and has great potential for clinical applications. With the continuous development and refinement of glycoprotein isolation methods, increasing attention has been directed to the quantitative and comparative aspects. This review describes the mass spectrometry (MS)-based techniques for the comparative analysis of glycoproteins and their applications to answer a wide range of interesting biological questions.