Siglec receptors and hiding plaques in Alzheimer's disease

Dessert or Poison? The Roles of Glycosylation in Alzheimer's, Parkinson's, Huntington's Disease, and Amyotrophic Lateral Sclerosis

Ministry of Education and Key Laboratory of Neurons and glial cells of the central nervous system (CNS) are modified by glycosylation and rely on glycosylation to achieve normal neural function. Neurodegenerative disease is a common disease of the elderly, affecting their healthy life span and quality of life, and no effective treatment is currently available. Recent research implies that various glycosylation traits are altered during neurodegenerative diseases, suggesting a potential implication of glycosylation in disease pathology. Herein, we summarized the current knowledge about glycosylation associated with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic lateral sclerosis (ALS) pathogenesis, focusing on their promising functional avenues. Moreover, we collected research aimed at highlighting the need for such studies to provide a wealth of disease-related glycosylation information that will help us better understand the pathophysiological mechanisms and hopefully specific glycosylation information to provide further diagnostic and therapeutic directions for neurodegenerative diseases.

Which Moiety Drives Gangliosides to Form Nanodomains?

Gangliosides are important glycosphingolipids involved in a multitude of physiological functions. From a physicochemical standpoint, this is related to their ability to self-organize into nanoscopic domains, even at molar concentrations of one per 1000 lipid molecules. Despite recent experimental and theoretical efforts suggesting that a hydrogen bonding network is crucial for nanodomain stability, the specific ganglioside moiety decisive for the development of these nanodomains has not yet been identified. Here, we combine an experimental technique achieving nanometer resolution (Förster resonance energy transfer analyzed by Monte Carlo simulations) with atomistic molecular dynamic simulations to demonstrate that the sialic acid (Sia) residue(s) at the oligosaccharide headgroup dominates the hydrogen bonding network between gangliosides, driving the formation of nanodomains even in the absence of cholesterol or sphingomyelin. Consequently, the clustering pattern of asialoGM₁, a Sia-depleted glycosphingolipid bearing three glyco moieties, is more similar to that of structurally distant sphingomyelin than that of the closely related gangliosides GM₁ and GD_1a with one and two Sia groups, respectively.

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.

TREM2 signalling as a multifaceted player in brain homoeostasis and a potential target for Alzheimer's disease treatment

Triggering receptor expressed on myeloid cells-2 (TREM2) has crucial roles in microglial physiology, differentiation, metabolism and survival. Genome-wide association studies (GWAS) show that genetic mutations of the TREM2 increase the risk of late-onset Alzheimer's disease (AD) by two to four times, disrupting the microglial function in reducing the progression of the disease. Accumulating data show that TREM2 function in AD is related primarily to the clearance of soluble and insoluble amyloid beta (Aβ42) aggregates from the brain. TREM2 also ameliorates the pathological effects of activated microglia on neuronal tau pathology, demonstrating its protective anti-inflammatory effects. However, since the excessive activation of TREM2 signalling can inhibit pro-inflammatory reactions and suppress the role of microglia in immune surveillance, at the late stages of the disease, it might promote immune tolerance, which is detrimental. The contradictory effects of TREM2 mutations on brain amyloidopathy and tauopathy in multiple mouse models, as well as studies revealing various effects of TREM2 overexpression, complicate the understanding of the role that TREM2 plays in AD aetiopathogenesis. In this review, we summarize the latest developments regarding the significance of TREM2 signalling in the stability of microglial pro- and anti-inflammatory activations and propose the mechanisms that should be targeted in the future to treat AD.

Evolution of Human-Specific Alleles Protecting Cognitive Function of Grandmothers

The myelomonocytic receptor CD33 (Siglec-3) inhibits innate immune reactivity by extracellular V-set domain recognition of sialic acid (Sia)-containing "self-associated molecular patterns" (SAMPs). We earlier showed that V-set domain-deficient CD33-variant allele, protective against late-onset Alzheimer's Disease (LOAD), is derived and specific to the hominin lineage. We now report multiple hominin-specific CD33 V-set domain mutations. Due to hominin-specific, fixed loss-of-function mutation in the CMAH gene, humans lack N-glycolylneuraminic acid (Neu5Gc), the preferred Sia-ligand of ancestral CD33. Mutational analysis and molecular dynamics (MD)-simulations indicate that fixed change in amino acid 21 of hominin V-set domain and conformational changes related to His45 corrected for Neu5Gc-loss by switching to N-acetylneuraminic acid (Neu5Ac)-recognition. We show that human-specific pathogens Neisseria gonorrhoeae and Group B Streptococcus selectively bind human CD33 (huCD33) as part of immune-evasive molecular mimicry of host SAMPs and that this binding is significantly impacted by amino acid 21 modification. In addition to LOAD-protective CD33 alleles, humans harbor derived, population-universal, cognition-protective variants at several other loci. Interestingly, 11 of 13 SNPs in these human genes (including CD33) are not shared by genomes of archaic hominins: Neanderthals and Denisovans. We present a plausible evolutionary scenario to compile, correlate, and comprehend existing knowledge about huCD33-evolution and suggest that grandmothering emerged in humans.

New insights into the genetic etiology of Alzheimer's disease and related dementias

Characterization of the genetic landscape of Alzheimer's disease (AD) and related dementias (ADD) provides a unique opportunity for a better understanding of the associated pathophysiological processes. We performed a two-stage genome-wide association study totaling 111,326 clinically diagnosed/'proxy' AD cases and 677,663 controls. We found 75 risk loci, of which 42 were new at the time of analysis. Pathway enrichment analyses confirmed the involvement of amyloid/tau pathways and highlighted microglia implication. Gene prioritization in the new loci identified 31 genes that were suggestive of new genetically associated processes, including the tumor necrosis factor alpha pathway through the linear ubiquitin chain assembly complex. We also built a new genetic risk score associated with the risk of future AD/dementia or progression from mild cognitive impairment to AD/dementia. The improvement in prediction led to a 1.6- to 1.9-fold increase in AD risk from the lowest to the highest decile, in addition to effects of age and the APOE ε4 allele.

Siglecs as Therapeutic Targets in Cancer

Hypersialylation is a common post-translational modification of protein and lipids found on cancer cell surfaces, which participate in cell-cell interactions and in the regulation of immune responses. Sialic acids are a family of nine-carbon α-keto acids found at the outermost ends of glycans attached to cell surfaces. Given their locations on cell surfaces, tumor cells aberrantly overexpress sialic acids, which are recognized by Siglec receptors found on immune cells to mediate broad immunomodulatory signaling. Enhanced sialylation exposed on cancer cell surfaces is exemplified as "self-associated molecular pattern" (SAMP), which tricks Siglec receptors found on leukocytes to greatly down-regulate immune responsiveness, leading to tumor growth. In this review, we focused on all 15 human Siglecs (including Siglec XII), many of which still remain understudied. We also highlighted strategies that disrupt the course of Siglec-sialic acid interactions, such as antibody-based therapies and sialic acid mimetics leading to tumor cell depletion. Herein, we introduced the central roles of Siglecs in mediating pro-tumor immunity and discussed strategies that target these receptors, which could benefit improved cancer immunotherapy.

Hypoxia/ischemia impairs CD33 (Siglec-3)/TREM2 signaling: Potential role in Alzheimer's pathogenesis

Recent genetic and molecular studies have indicated that the innate immune system, especially microglia, have a crucial role in the accumulation of β-amyloid plaques in Alzheimer's disease (AD). In particular, the CD33 receptor, also called Siglec-3, inhibits the TREM2 receptor-induced phagocytic activity of microglia. CD33 receptors recognize the α2,3 and α2,6-linked sialic groups in tissue glycocalyx, especially sialylated gangliosides in human brain. The CD33 receptor triggers cell-type specific responses, e.g., in microglia, CD33 inhibits phagocytosis, whereas in natural killer cells, it inhibits the cytotoxic activity of the NKG2D receptor. Nonetheless, the regulation of the activity of CD33 receptor needs to be clarified. For example, it seems that hypoxia/ischemia, a potential cause of AD pathology, increases the expression of CD33 and its downstream target SHP-1, a tyrosine phosphatase which suppresses the phagocytosis driven by TREM2. Moreover, hypoxia/ischemia increases the deposition of sialylated gangliosides, e.g., GM1, GM2, GM3, and GD1, which are ligands for inhibitory CD33/Siglec-3 receptors. In addition, β-amyloid peptides bind to the sialylated gangliosides in raft-like clusters and subsequently these gangliosides act as seeds for the formation of β-amyloid plaques in AD pathology. It is known that senile plaques contain sialylated GM1, GM2, and GM3 gangliosides, i.e., the same species induced by hypoxia/ischemia treatment. Sialylated gangliosides in plaques might stimulate the CD33/Siglec-3 receptors of microglia and thus impede TREM2-driven phagocytosis. We propose that hypoxia/ischemia, e.g., via the accumulation of sialylated gangliosides, prevents the phagocytosis of β-amyloid deposits by inhibiting CD33/TREM2 signaling.

Microglia and its Genetics in Alzheimer's Disease

Alzheimer's Disease (AD) is the most prevalent form of dementia across the world. While its discovery and pathological manifestations are centered on protein aggregations of amyloid- beta (Aβ) and hyperphosphorylated tau protein, neuroinflammation has emerged in the last decade as a main component of the disease in terms of both pathogenesis and progression. As the main innate immune cell type in the central nervous system (CNS), microglia play a very important role in regulating neuroinflammation, which occurs commonly in neurodegenerative conditions, including AD. Under inflammatory response, microglia undergo morphological changes and status transition from homeostatic to activated forms. Different microglia subtypes displaying distinct genetic profiles have been identified in AD, and these signatures often link to AD risk genes identified from the genome-wide association studies (GWAS), such as APOE and TREM2. Furthermore, many AD risk genes are highly enriched in microglia and specifically influence the functions of microglia in pathogenesis, e.g. releasing inflammatory cytokines and clearing Aβ. Therefore, building up a landscape of these risk genes in microglia, based on current preclinical studies and in the context of their pathogenic or protective effects, would largely help us to understand the complex etiology of AD and provide new insight into the unmet need for effective treatment.

Sialic Acid-Modified Nanoparticles-New Approaches in the Glioma Management-Perspective Review

The cell surface is covered by a dense and complex network of glycans attached to the membrane proteins and lipids. In gliomas, the aberrant sialylation, as the final stage of glycosylation, is an important regulatory mechanism of malignant cell behavior and correlates with worse prognosis. Better understanding of the role of sialylation in cellular and molecular processes opens a new way in the development of therapeutic tools for human brain tumors. According to the recent clinical observation, the cellular heterogeneity, activity of brain cancer stem cells (BCSCs), immune evasion, and function of the blood–brain barrier (BBB) are attractive targets for new therapeutic strategies. In this review, we summarize the importance of sialic acid-modified nanoparticles in brain tumor progression.

Analysis of Genetic Variants Associated with Levels of Immune Modulating Proteins for Impact on Alzheimer's Disease Risk Reveal a Potential Role for SIGLEC14

Genome-wide association studies (GWAS) have identified immune-related genes as risk factors for Alzheimer’s disease (AD), including TREM2 and CD33, frequently passing a stringent false-discovery rate. These genes either encode or signal through immunomodulatory tyrosine-phosphorylated inhibitory motifs (ITIMs) or activation motifs (ITAMs) and govern processes critical to AD pathology, such as inflammation and amyloid phagocytosis. To investigate whether additional ITIM and ITAM-containing family members may contribute to AD risk and be overlooked due to the stringent multiple testing in GWAS, we combined protein quantitative trait loci (pQTL) data from a recent plasma proteomics study with AD associations in a recent GWAS. We found that pQTLs for genes encoding ITIM/ITAM family members were more frequently associated with AD than those for non-ITIM/ITAM genes. Further testing of one family member, SIGLEC14 which encodes an ITAM, uncovered substantial copy number variations, identified an SNP as a proxy for gene deletion, and found that gene expression correlates significantly with gene deletion. We also found that SIGLEC14 deletion increases the expression of SIGLEC5, an ITIM. We conclude that many genes in this ITIM/ITAM family likely impact AD risk, and that complex genetics including copy number variation, opposing function of encoded proteins, and coupled gene expression may mask these AD risk associations at the genome-wide level.

Sialometabolism in Brain Health and Alzheimer's Disease

Sialic acids refer to a unique family of acidic sugars with a 9-carbon backbone that are mostly found as terminal residues in glycan structures of glycoconjugates including both glycoproteins and glycolipids. The highest levels of sialic acids are expressed in the brain where they regulate neuronal sprouting and plasticity, axon myelination and myelin stability, as well as remodeling of mature neuronal connections. Moreover, sialic acids are the sole ligands for microglial Siglecs (sialic acid-binding immunoglobulin-type lectins), and sialic acid-Siglec interactions have been indicated to play a critical role in the regulation of microglial homeostasis in a healthy brain. The recent discovery of CD33, a microglial Siglec, as a novel genetic risk factor for late-onset Alzheimer's disease (AD), highlights the potential role of sialic acids in the development of microglial dysfunction and neuroinflammation in AD. Apart from microglia, sialic acids have been found to be involved in several other major changes associated with AD. Elevated levels of serum sialic acids have been reported in AD patients. Alterations in ganglioside (major sialic acid carrier) metabolism have been demonstrated as an aggravating factor in the formation of amyloid pathology in AD. Polysialic acids are linear homopolymers of sialic acids and have been implicated to be an important regulator of neurogenesis that contributes to neuronal repair and recovery from neurodegeneration such as in AD. In summary, this article reviews current understanding of neural functions of sialic acids and alterations of sialometabolism in aging and AD brains. Furthermore, we discuss the possibility of looking at sialic acids as a promising novel therapeutic target for AD intervention.

The Paired Siglecs in Brain Tumours Therapy: The Immunomodulatory Effect of Dexamethasone and Temozolomide in Human Glioma In Vitro Model

The paired sialic acid-binding immunoglobulin like lectins (Siglecs) are characterized by similar cellular distribution and ligand recognition but opposing signalling functions attributed to different intracellular sequences. Since sialic acid—Siglec axis are known to control immune homeostasis, the imbalance between activatory and inhibitory mechanisms of glycan-dependent immune control is considered to promote pathology. The role of sialylation in cancer is described, however, its importance in immune regulation in gliomas is not fully understood. The experimental and clinical observation suggest that dexamethasone (Dex) and temozolomide (TMZ), used in the glioma management, alter the immunity within the tumour microenvironment. Using glioma-microglia/monocytes transwell co-cultures, we investigated modulatory action of Dex/TMZ on paired Siglecs. Based on real-time PCR and flow cytometry, we found changes in SIGLEC genes and their products. These effects were accompanied by altered cytokine profile and immune cells phenotype switching measured by arginases expression. Additionally, the exposure to Dex or TMZ increased the binding of inhibitory Siglec-5 and Siglec-11 fusion proteins to glioma cells. Our study suggests that the therapy-induced modulation of the interplay between sialoglycans and paired Siglecs, dependently on patient’s phenotype, is of particular signification in the immune surveillance in the glioma management and may be useful in glioma patient’s therapy plan verification.

Deletion of Alzheimer's disease-associated CD33 results in an inflammatory human microglia phenotype

Genome-wide association studies demonstrated that polymorphisms in the CD33/sialic acid-binding immunoglobulin-like lectin 3 gene are associated with late-onset Alzheimer's disease (AD). CD33 is expressed on myeloid immune cells and mediates inhibitory signaling through protein tyrosine phosphatases, but the exact function of CD33 in microglia is still unknown. Here, we analyzed CD33 knockout human THP1 macrophages and human induced pluripotent stem cell-derived microglia for immunoreceptor tyrosine-based activation motif pathway activation, cytokine transcription, phagocytosis, and phagocytosis-associated oxidative burst. Transcriptome analysis of the macrophage lines showed that knockout of CD33 as well as knockdown of the CD33 signaling-associated protein tyrosine phosphatase, nonreceptor type 6 (PTPN6) led to constitutive activation of inflammation-related pathways. Moreover, deletion of CD33 or expression of Exon 2-deleted CD33 (CD33^ΔE2 /CD33m) led to increased phosphorylation of the kinases spleen tyrosine kinase (SYK) and extracellular signal-regulated kinase 1 and 2 (ERK1 and 2). Transcript analysis by quantitative real-time polymerase chain reaction confirmed increased levels of interleukin (IL) 1B, IL8, and IL10 after knockout of CD33 in macrophages and microglia. In addition, upregulation of the gene transcripts of the AD-associated phosphatase INPP5D was observed after knockout of CD33. Functional analysis of macrophages and microglia showed that phagocytosis of aggregated amyloid-β_1-42 and bacterial particles were increased after knockout of CD33 or CD33^ΔE2 expression and knockdown of PTPN6. Furthermore, the phagocytic oxidative burst during uptake of amyloid-β_1-42 or bacterial particles was increased after CD33 knockout but not in CD33^ΔE2 -expressing microglia. In summary, deletion of CD33 or expression of CD33^ΔE2 in human macrophages and microglia resulted in putative beneficial phagocytosis of amyloid β_1-42 , but potentially detrimental oxidative burst and inflammation, which was absent in CD33^ΔE2 -expressing microglia.

Reactive or transgenic increase in microglial TYROBP reveals a TREM2-independent TYROBP-APOE link in wild-type and Alzheimer's-related mice

Introduction

Microglial TYROBP (DAP12) is a network hub and driver in sporadic late‐onset Alzheimer's disease (AD). TYROBP is a cytoplasmic adaptor for TREM2 and other receptors, but little is known about its roles and actions in AD. Herein, we demonstrate that endogenous Tyrobp transcription is specifically increased in recruited microglia.

Methods

Using a novel transgenic mouse overexpressing TYROBP in microglia, we observed a decrease of the amyloid burden and an increase of TAU phosphorylation stoichiometry when crossed with APP/PSEN1 or MAPT^P301S mice, respectively. Characterization of these mice revealed Tyrobp‐related modulation of apolipoprotein E (Apoe) transcription. We also showed that Tyrobp and Apoe mRNAs were increased in Trem2‐null microglia recruited around either amyloid beta deposits or a cortical stab injury. Conversely, microglial Apoe transcription was dramatically diminished when Tyrobp was absent.

Conclusions

Our results provide evidence that TYROBP‐APOE signaling does not require TREM2 and could be an initiating step in establishment of the disease‐associated microglia (DAM) phenotype.

Sialic Acid-Siglec Axis as Molecular Checkpoints Targeting of Immune System: Smart Players in Pathology and Conventional Therapy

The sialic acid-based molecular mimicry in pathogens and malignant cells is a regulatory mechanism that leads to cross-reactivity with host antigens resulting in suppression and tolerance in the immune system. The interplay between sialoglycans and immunoregulatory Siglec receptors promotes foreign antigens hiding and immunosurveillance impairment. Therefore, molecular targeting of immune checkpoints, including sialic acid-Siglec axis, is a promising new field of inflammatory disorders and cancer therapy. However, the conventional drugs used in regular management can interfere with glycome machinery and exert a divergent effect on immune controlling systems. Here, we focus on the known effects of standard therapies on the sialoglycan-Siglec checkpoint and their importance in diagnosis, prediction, and clinical outcomes.

The sialoglycan-Siglec-E checkpoint axis in dexamethasone-induced immune subversion in glioma-microglia transwell co-culture system

Dexamethasone (Dex) is considered as the main steroid routinely used in the standard therapy of brain tumor-induced edema. Strong immunosuppressive effects of Dex on effector systems of the immune system affect the patients' antitumor immunity and may thereby worsen the prognosis. Siglecs and their interacting sialoglycans have been described as a novel glyco-immune checkpoint axis that promotes cancer immune evasion. Despite the aberrant glycosylation in cancer is described, mechanisms involved in regulation of immune checkpoints in gliomas are not fully understood. The aim of this study was to investigate the effect of Dex on the Siglec-sialic acid interplay and determine its significance in immune inversion in monocultured and co-cultured microglia and glioma cells. Both monocultured and co-cultured in transwell system embryonic stem cell-derived microglia (ESdM) and glioma GL261 cells were exposed to Dex. Cell viability, immune inversion markers, and interaction between sialic acid and Siglec-E were detected by flow cytometry. Cell invasion was analyzed by scratch-wound migration assay using inverted phase-contrast microscopy. Exposure to Dex led to significant changes in IL-1β, IL-10, Iba-1, and Siglec-E in co-cultured microglia compared to naïve or monocultured cells. These alterations were accompanied by increased α2.8-sialylation and Siglec-E fusion protein binding to co-cultured glioma cell membranes. This study suggests that the interplay between sialic acids and Siglecs is a sensitive immune checkpoint axis and may be crucial for Dex-induced dampening of antitumor immunity. The targeting of sialic acid-Siglec glyco-immune checkpoint can be a novel therapeutic method in glioma therapy.

Lipid and Lipoprotein Metabolism in Microglia

Microglia, once viewed as static bystanders with limited homeostatic functions, are now considered key players in the development of neuroinflammatory and neurodegenerative diseases. Microglial activation is a salient feature of neuroinflammation involving a dynamic process that generates multitudinous microglial phenotypes that can respond to a variety of situational cues in the central nervous system. Recently, a flurry of single cell RNA-sequencing studies have defined microglial phenotypes in unprecedented detail, and have highlighted robust changes in the expression of genes involved in lipid and lipoprotein metabolism. Increased expression of genes such as Apolipoprotein E (ApoE), Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) and Lipoprotein Lipase (LPL) in microglia during development, damage, and disease, suggest that increased lipid metabolism is needed to fuel protective cellular functions such as phagocytosis. This review describes our current understanding of lipid and lipoprotein metabolism in microglia, and highlights microglial lipid metabolism as a modifiable target for the treatment of neurodegenerative diseases such as Alzheimer's disease and multiple sclerosis.

Spatial Lipidomics Reveals Region and Long Chain Base Specific Accumulations of Monosialogangliosides in Amyloid Plaques in Familial Alzheimer's Disease Mice (5xFAD) Brain

Ganglioside metabolism is significantly altered in Alzheimer's disease (AD), which is a progressive neurodegenerative disease prominently characterized by one of its pathological hallmarks, amyloid deposits or "senile plaques". While the plaques mainly consist of aggregated variants of amyloid-β protein (Aβ), recent studies have revealed a number of lipid species including gangliosides in amyloid plaques along with Aβ peptides. It has been widely suggested that long chain (sphingosine) base (LCBs), C18:1-LCB and C20:1-LCB, containing gangliosides might play different roles in neuronal function in vivo. In order to elucidate region-specific aspects of amyloid-plaque associated C18:1-LCB and C20:1-LCB ganglioside accumulations, high spatial resolution (10 μm per pixel) matrix assisted laser desorption ionization imaging mass spectrometry (MALDI-IMS) of gangliosides in amyloid plaques was performed in hippocampal and adjacent cortical regions of 12 month old 5xFAD mouse coronal brain sections from two different stereotaxic coordinates (bregma points, -2.2 and -2.7 mm). MALDI-IMS uncovered brain-region (2 and 3D) and/or LCB specific accumulations of monosialogangliosides (GMs): GM1, GM2, and GM3 in the hippocampal and cortical amyloid plaques. The results reveal monosialogangliosides to be an important component of amyloid plaques and the accumulation of different gangliosides is region and LCB specific in 12 month old 5xFAD mouse brain. This is discussed in relation to amyloid-associated AD pathogenesis such as lipid related immune changes in amyloid plaques, AD specific ganglioside metabolism, and, notably, AD-associated impaired neurogenesis in the subgranular zone.

Evaluation of CD33 as a genetic risk factor for Alzheimer's disease

In 2011, genome-wide association studies implicated a polymorphism near CD33 as a genetic risk factor for Alzheimer's disease. This finding sparked interest in this member of the sialic acid-binding immunoglobulin-type lectin family which is linked to innate immunity. Subsequent studies found that CD33 is expressed in microglia in the brain and then investigated the molecular mechanism underlying the CD33 genetic association with Alzheimer's disease. The allele that protects from Alzheimer's disease acts predominately to increase a CD33 isoform lacking exon 2 at the expense of the prototypic, full-length CD33 that contains exon 2. Since this exon encodes the sialic acid ligand-binding domain, the finding that the loss of exon 2 was associated with decreased Alzheimer's disease risk was interpreted as meaning that a decrease in functional CD33 and its associated immune suppression was protective from Alzheimer's disease. However, this interpretation may need to be reconsidered given current findings that a genetic deletion which abrogates CD33 is not associated with Alzheimer's disease risk. Therefore, integrating currently available findings leads us to propose a model wherein the CD33 isoform lacking the ligand-binding domain represents a gain of function variant that reduces Alzheimer's disease risk.

The potential importance of myeloid-derived suppressor cells (MDSCs) in the pathogenesis of Alzheimer's disease

The exact cause of Alzheimer's disease (AD) is still unknown, but the deposition of amyloid-β (Aβ) plaques and chronic inflammation indicates that immune disturbances are involved in AD pathogenesis. Recent genetic studies have revealed that many candidate genes are expressed in both microglia and myeloid cells which infiltrate into the AD brains. Invading myeloid cells controls the functions of resident microglia in pathological conditions, such as AD pathology. AD is a neurologic disease with inflammatory component where the immune system is not able to eliminate the perpetrator, while, concurrently, it should prevent neuronal injuries induced by inflammation. Recent studies have indicated that AD brains are an immunosuppressive microenvironment, e.g., microglial cells are hyporesponsive to Aβ deposits and anti-inflammatory cytokines enhance Aβ deposition. Immunosuppression is a common element in pathological disorders involving chronic inflammation. Studies on cancer-associated inflammation have demonstrated that myeloid-derived suppressor cells (MDSCs) have a crucial role in the immune escape of tumor cells. Immunosuppression is not limited to tumors, since MDSCs can be recruited into chronically inflamed tissues where inflammatory mediators enhance the proliferation and activation of MDSCs. AD brains express a range of chemokines and cytokines which could recruit and expand MDSCs in inflamed AD brains and thus generate an immunosuppressive microenvironment. Several neuroinflammatory disorders, e.g., the early phase of AD pathology, have been associated with an increase in the level of circulating MDSCs. We will elucidate the immunosuppressive armament of MDSCs and present evidences in support of the crucial role of MDSCs in the pathogenesis of AD.

Sialylation pattern in lung epithelial cell line and Siglecs expression in monocytic THP-1 cells as cellular indicators of cigarette smoke - induced pathology in vitro

PURPOSE

Cellular response to cigarette smoke (CS) involves activation of recognition receptors resulting in changes in immune status, oxidative stress and cell turnover. We investigated the effects of CS on sialic acid-binding immunoglobulin type lectins (Siglecs) expression and their sialylated ligands in human immune and non-immune cells.

METHODS

Human monocytes (THP-1) and epithelial cells (A549) were cultured in CS-conditioned medium (CSM). Expression of Siglec-8 and Siglec-5/Siglec-14 was analysed in THP-1 cells using flow cytometry. The effects of CS on immune activity was evaluated flow cytometrically in these cells by assessment of phagocytosis and intracellular expression IL-1β and IL-10. Detection and differentiation of sialic acids was analyzed by dot blot, western blot and flow cytometry using plant lectins and antibodies.

RESULTS

Exposure to CS significantly increased expression of Siglec-8 and Siglec-5/Siglec-14 in THP-1 cells. These changes were accompanied by enhanced intracellular level of IL-1β and IL-10 but reduced phagocytic activity. In THP-1 and A549 cells, the level of α2,3-sialic acids, but not α2,6-sialic acid, was significantly increased when compared to naïve cells. The level of α2,8-sialic acids increased significantly in A549 cells, but not in THP-1 cells, after exposure to CS.

CONCLUSION

These results show that cellular response to CS involves changes in expression of Siglec receptors and sialylated ligands functionally associated with immunity.

The Pathogenic Role of Ganglioside Metabolism in Alzheimer's Disease-Cholinergic Neuron-Specific Gangliosides and Neurogenesis

Alzheimer's disease (AD) is the most common type of dementia with clinical symptoms that include deficits in memory, judgment, thinking, and behavior. Gangliosides are present on the outer surface of plasma membranes and are especially abundant in the nervous tissues of vertebrates. Ganglioside metabolism, especially the cholinergic neuron-specific gangliosides, GQ1bα and GT1aα, is altered in mouse model of AD and patients with AD. Thus, alterations in ganglioside metabolism may participate in several events related to the pathogenesis of AD. Increased expressions of GT1aα may reflect cholinergic neurogenesis. Most changes in ganglioside metabolism occur in the specific brain areas and their lipid rafts. Targeting ganglioside metabolism in lipid rafts may represent an underexploited opportunity to design novel therapeutic strategies for AD.

Genetics of Alzheimer's disease: From pathogenesis to clinical usage

Alzheimer's disease (AD) is the most common type of dementia and has caused a major global health concern. Understanding the etiology of AD can be beneficial for the diagnosis and intervention of this disease. Genetics plays a vital role in the pathogenesis of AD. Research methods in genetics such as the linkage analysis, study of candidate genes, genome-wide association study (GWAS), and next-generation sequencing (NGS) technology help us map the genetic information in AD, which can not only provide a new insight into the pathogenesis of AD but also be beneficial for early targeted intervention of AD. This review summarizes the pathogenesis as well as the diagnostic and therapeutic value of genetics in AD.

Network Analysis of a Comprehensive Knowledge Repository Reveals a Dual Role for Ceramide in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common cause of senile dementia. Many inflammatory factors such as amyloid-β and pro-inflammatory cytokines are known to contribute to the inflammatory response in the AD brain. Sphingolipids are widely known to have roles in the pathogenesis of inflammatory diseases, where the precise roles for sphingolipids in inflammation-associated pathogenesis of AD are not well understood. Here we performed a network analysis to clarify the importance of sphingolipids and to model relationships among inflammatory factors and sphingolipids in AD. In this study, we have updated sphingolipid signaling and metabolic cascades in a map of AD signaling networks that we named “AlzPathway,” a comprehensive knowledge repository of signaling pathways in AD. Our network analysis of the updated AlzPathway indicates that the pathways related to ceramide are one of the primary pathways and that ceramide is one of the important players in the pathogenesis of AD. The results of our analysis suggest the following two prospects about inflammation in AD: (1) ceramide could play important roles in both inflammatory and anti-inflammatory pathways of AD, and (2) several factors such as Sphingomyelinase and Siglec-11 may be associated with ceramide related inflammation and anti-inflammation pathways in AD. In this study, network analysis of comprehensive knowledge repository reveals a dual role for ceramide in AD. This result provides a clue to clarify sphingolipids related inflammatory and anti-inflammatory pathways in AD.

Associations Between Genetic Variants in 19p13 and 19q13 Regions and Susceptibility to Alzheimer Disease: A Meta-Analysis

BACKGROUND

Alzheimer disease (AD) has become an epidemic within the growing elderly population and effective therapies of AD have not been discovered. Genetic factors accounted for over 70% of the incidence of AD and the disease-related polymorphisms are located on chromosome 19, which is one of several prominent chromosomes related to the development of AD. Many inconsistent associations between polymorphisms in ABCA7, CD33, and TOMM40 genes and the susceptibility to AD have been suggested by several independent studies.

MATERIAL/METHODS

A comprehensive literature search for studies involving the association between gene polymorphisms and AD was performed, and we finally selected 3 genes (4 polymorphisms) for the meta-analysis: ABCA7 (rs3764650), CD33 (rs3865444), and TOMM40 (rs157580, rs2075650).

RESULTS

A total of 25 articles investigating 3 genes (4 polymorphisms) were included in the meta-analysis. The pooled results of 4 polymorphisms were all significantly associated with the susceptibility to AD. The pooled effect of ABCA7 rs3764605 allele G was significantly associated with an increased the risk of AD (OR=1.20, 95% CI: 1.14-1.26, P value <0.001). Similarly, our evidence suggested that allele A of TOMM40 rs2075650 polymorphism was a risk factor for AD (OR=2.87, 95% CI: 2.46-3.34, P value <0.001). Alleles A of CD33 rs3865444 and A of TOMM40 rs157580 were both protective factors for AD onset (OR=0.94, 95% CI: 0.90-0.98, P value=0.003; OR=0.62, 95% CI: 0.57-0.66, P value <0.001). CONCLUSIONS" Results from the meta-analysis revealed that the pooled ABCA7 rs376465, CD33 rs3865444, TOMM40 rs157580, and rs2075650 variants were significantly associated with the susceptibility to AD. However, the association differed significantly between Asian and Caucasian groups for SNPs of CD33 rs3865444, TOMM40 rs157580, and rs2075650.

Genetics ignite focus on microglial inflammation in Alzheimer's disease

In the past five years, a series of large-scale genetic studies have revealed novel risk factors for Alzheimer’s disease (AD). Analyses of these risk factors have focused attention upon the role of immune processes in AD, specifically microglial function. In this review, we discuss interpretation of genetic studies.  We then focus upon six genes implicated by AD genetics that impact microglial function: TREM2, CD33, CR1, ABCA7, SHIP1, and APOE. We review the literature regarding the biological functions of these six proteins and their putative role in AD pathogenesis. We then present a model for how these factors may interact to modulate microglial function in AD.

Associations of genetic polymorphisms of Siglecs with human diseases

Genetic polymorphism studies in humans provide unique opportunities to understand human biology and the mechanisms of diseases. Correlations between polymorphisms in the genes encoding human Siglecs and various diseases have been reported. Leading examples, such as the CD33 polymorphism associated with late-onset Alzheimer's disease, are well supported by genetic replication and mechanistic studies, while some others (such as SIGLEC8 polymorphism associated with bronchial asthma and SIGLEC14 polymorphism associated with exacerbation of chronic obstructive pulmonary disease) may benefit reinforcement by independent genetic replication or mechanistic studies. In a few cases, such as MAG polymorphism associated with psychological disorder and CD22 polymorphism associated with autoimmune disease, the phenotype associated with a genetic polymorphism of a Siglec gene and that of an enzyme gene involved in the biosynthesis of Siglec ligand show some overlap, providing indirect support for the observed genotype-phenotype association. Although studies using engineered mutant mice have provided invaluable insights into the biological functions and mechanisms of diseases, it is not always possible to develop appropriate mouse model to replicate human situations because of significant species-to-species differences, which can be a major obstacle in understanding the biology of some of human CD33/Siglec-3-related Siglecs. Further studies in genetic polymorphisms of human Siglecs, combined with appropriate functional studies, may reveal unexpected biological roles of human Siglecs, and identify possible targets for prevention and/or treatment of certain diseases.

Pharmacogenomic predictors of citalopram treatment outcome in major depressive disorder

OBJECTIVES

A significant proportion of patients with major depressive disorder (MDD) do not improve following treatment with first-line antidepressants and, currently, there are no objective indicators of predictors of antidepressant response. The aim of this study was to investigate pre-treatment peripheral gene expression differences between future remitters and non-responders to citalopram treatment and identify potential pharmacogenomic predictors of response.

METHODS

We conducted a gene expression study using Affymetrix HG-U133 Plus2 microarrays in peripheral blood samples from untreated individuals with MDD (N = 77), ascertained at a community outpatient clinic, prior to an 8-week treatment with citalopram. Gene expression differences were assessed between remitters and non-responders to treatment. Technical validation of significant probesets was carried out by qRT-PCR.

RESULTS

A total of 434 probesets displayed significant correlation to change in score and 33 probesests were differentially expressed between eventual remitters and non-responders. Probesets for SMAD 7 (SMA- and MAD-related protein 7) and SIGLECP3 (sialic acid-binding immunoglobulin-like lectin, pseudogene 3) were the most significant differentially expressed genes following FDR correction, and both were down-regulated in individuals who responded to treatment.

CONCLUSIONS

These findings point to SMAD7 and SIGLECP3 as candidate predictive biomarkers of antidepressant response.

Wnt1 inducible signaling pathway protein 1 (WISP1) targets PRAS40 to govern β-amyloid apoptotic injury of microglia

Given the present challenges to attain effective treatment for β-amyloid (Aβ) toxicity in neurodegenerative disorders such as Alzheimer's disease, development of novel cytoprotective pathways that can assist immune mediated therapies through the preservation of central nervous system microglia could offer significant promise. We show that the CCN4 protein, Wnt1 inducible signaling pathway protein 1 (WISP1), is initially up-regulated by Aβ and can modulate its endogenous expression for the protection of microglia during Aβ mediated apoptosis. WISP1 activates mTOR and phosphorylates p70S6K and 4EBP1 through the control of the regulatory mTOR component PRAS40. Loss of PRAS40 through gene reduction or inhibition by WISP1 is cytoprotective. WISP1 ultimately governs PRAS40 by sequestering PRAS40 intracellularly through post-translational phosphorylation and binding to protein 14-3-3. Our work identifies WISP1, mTOR signaling, and PRAS40 as targets for new strategies directed against Alzheimer's disease and related disorders.

CD33 in Alzheimer's disease

The amyloid-beta peptide (Aβ) cascade hypothesis posits that Aβ accumulation is the fundamental initiator of Alzheimer's disease (AD), and mounting evidence suggests that impaired Aβ clearance rather than its overproduction is the major pathogenic event for AD. Recent genetic studies have identified cluster of differentiation 33 (CD33) as a strong genetic locus linked to AD. As a type I transmembrane protein, CD33 belongs to the sialic acid-binding immunoglobulin-like lectins, mediating the cell-cell interaction and inhibiting normal functions of immune cells. In the brain, CD33 is mainly expressed on microglial cells. The level of CD33 was found to be increased in the AD brain, which positively correlated with amyloid plaque burden and disease severity. More importantly, CD33 led to the impairment of microglia-mediated clearance of Aβ, which resulted in the formation of amyloid plaques in the brain. In this article, we review the recent epidemiological findings of CD33 that related with AD and discuss the levels and pathogenic roles of CD33 in this disease. Based on the contributing effects of CD33 in AD pathogenesis, targeting CD33 may provide new opportunities for AD therapeutic strategies.

Tuberous sclerosis protein 2 (TSC2) modulates CCN4 cytoprotection during apoptotic amyloid toxicity in microglia

More than 110 million individuals will suffer from cognitive loss worldwide by the year 2050 with a majority of individuals presenting with Alzheimer's disease (AD). Yet, successful treatments for etiologies that involve β.-amyloid (Aβ.) toxicity in AD remain elusive and await novel avenues for drug development. Here we show that Wnt1 inducible signaling pathway protein 1 (WISP1/CCN4) controls the post-translational phosphorylation of Akt1, p70S6K, and AMP activated protein kinase (AMPK) to the extent that tuberous sclerosis complex 2 (TSC2) (Ser1387) phosphorylation, a target of AMPK, is decreased and TSC2 (Thr1462) phosphorylation, a target of Akt1, is increased. The ability of WISP1 to limit TSC2 activity allows WISP1 to increase the activity of p70S6K, since gene silencing of TSC2 further enhances WISP1 phosphorylation of p70S6K. However, a minimal level of TSC2 activity is necessary to modulate WISP1 cytoprotection that may require modulation of mTOR activity, since gene knockdown of TSC2 impairs the ability of WISP1 to protect microglia against apoptotic membrane phosphatidylserine (PS) exposure, nuclear DNA degradation, mitochondrial membrane depolarization, and cytochrome c release during Aβ. exposure.

Prevention of β-amyloid degeneration of microglia by erythropoietin depends on Wnt1, the PI 3-K/mTOR pathway, Bad, and Bcl-xL

Central nervous system microglia promote neuronal regeneration and sequester toxic β-amyloid (Aβ) deposition during Alzheimer's disease. We show that the cytokine erythropoietin (EPO) decreases the toxic effect of Aβ on microgliain vitro. EPO up-regulates the cysteine-rich glycosylated wingless protein Wnt1 and activates the PI 3-K/Akt1/mTOR/ p70S6K pathway. This in turn increases phosphorylation and cytosol trafficking of Bad, reduces the Bad/Bcl-x_L complex and increases the Bcl-x_L/Bax complex, thus preventing caspase 1 and caspase 3 activation and apoptosis. Our data may foster development of novel strategies to use cytoprotectants such as EPO for Alzheimer's disease and other degenerative disorders.

Significance of the cell adhesion molecules and sialic acid in neurodegeneration

The central nervous system (CNS) is a complex and precise mechanism that controls the most highest functions of the body. All of them depend on the cellular and molecular interactions called by neurobiologists "cellular plasticity". The CNS is a flexible structure but its regeneration after damage is strongly limited. Better understanding of cellular and molecular basis of brain repair can open new way in the development of therapeutic tools for neurodegeneration. Among many molecules that participate in the formation of neuronal networks, neural cell adhesion molecule (NCAM) and its sialylated derivative seem to play crucial role in the life of brain. In particular, polysialylated cell adhesion molecule (PSA-NCAM) is proposed to participate in the neuroprotective response in neurodegeneration by reducing of AMPA/NMDA receptors sensitivity to glutamate and facilitating disconnection of cell-cell interactions. These mechanisms protect from excitotoxic damage and promote dendritic/spine re-growth. This review briefly focuses on the expression and role of PSA-NCAM in neurodegenerative diseases and its potential application in therapy.

Microglial activatory (immunoreceptor tyrosine-based activation motif)- and inhibitory (immunoreceptor tyrosine-based inhibition motif)-signaling receptors for recognition of the neuronal glycocalyx

Microglia sense intact or lesioned cells of the central nervous system (CNS) and respond accordingly. To fulfill this task, microglia express a whole set of recognition receptors. Fc receptors and DAP12 (TYROBP)-associated receptors such as microglial triggering receptor expressed on myeloid cells-2 (TREM2) and the complement receptor-3 (CR3, CD11b/CD18) trigger the immunoreceptor tyrosine-based activation motif (ITAM)-signaling cascade, resulting in microglial activation, migration, and phagocytosis. Those receptors are counter-regulated by immunoreceptor tyrosine-based inhibition motif (ITIM)-signaling receptors, such as sialic acid-binding immunoglobulin superfamily lectins (Siglecs). Siglecs recognize the sialic acid cap of healthy neurons thus leading to an ITIM signaling that turns down microglial immune responses and phagocytosis. In contrast, desialylated neuronal processes are phagocytosed by microglial CR3 signaling via an adaptor protein containing an ITAM. Thus, the aberrant terminal glycosylation of neuronal surface glycoproteins and glycolipids could serve as a flag for microglia, which display a multitude of diverse carbohydrate-binding receptors that monitor the neuronal physical condition and respond via their ITIM- or ITAM-signaling cascade accordingly.

The prevalence of CD33 and MS4A6A variant in Chinese Han population with Alzheimer's disease

CD33 and MS4A6A genes play potential key roles in the pathogenesis of Alzheimer's disease (AD). One recent genome-wide association study has revealed that the rs3865444 polymorphism in the CD33 gene and rs610932 polymorphism in the MS4A6A gene are associated with susceptibility to AD in Caucasians. To evaluate the relationship between the polymorphism of the CD33, MS4A6A gene and AD in the ethnic Chinese Han, we conducted a case-control study (n = 383, age > 54) to determine the prevalence of single-nucleotide polymorphism of two genes in patients with AD in Chinese population of Mainland, and clarified whether these polymorphisms are risk factors for AD. The prevalence of the allele (T) in the rs3865444 polymorphism of the CD33 gene and allele (C) in rs610932 polymorphism of the MS4A6A gene was significantly different in AD patients and control subjects (P < 0.001, respectively), and the results were not influenced by age, gender, or APOE status. Our data revealed the allele (T) of the rs3865444 polymorphism of the CD33 gene and the allele (C) of the rs610932 polymorphism of the MS4A6A gene may contribute to AD risk in the Chinese Han population.

Evolution of siglec-11 and siglec-16 genes in hominins

We previously reported a human-specific gene conversion of SIGLEC11 by an adjacent paralogous pseudogene (SIGLEC16P), generating a uniquely human form of the Siglec-11 protein, which is expressed in the human brain. Here, we show that Siglec-11 is expressed exclusively in microglia in all human brains studied-a finding of potential relevance to brain evolution, as microglia modulate neuronal survival, and Siglec-11 recruits SHP-1, a tyrosine phosphatase that modulates microglial biology. Following the recent finding of a functional SIGLEC16 allele in human populations, further analysis of the human SIGLEC11 and SIGLEC16/P sequences revealed an unusual series of gene conversion events between two loci. Two tandem and likely simultaneous gene conversions occurred from SIGLEC16P to SIGLEC11 with a potentially deleterious intervening short segment happening to be excluded. One of the conversion events also changed the 5' untranslated sequence, altering predicted transcription factor binding sites. Both of the gene conversions have been dated to ~1-1.2 Ma, after the emergence of the genus Homo, but prior to the emergence of the common ancestor of Denisovans and modern humans about 800,000 years ago, thus suggesting involvement in later stages of hominin brain evolution. In keeping with this, recombinant soluble Siglec-11 binds ligands in the human brain. We also address a second-round more recent gene conversion from SIGLEC11 to SIGLEC16, with the latter showing an allele frequency of ~0.1-0.3 in a worldwide population study. Initial pseudogenization of SIGLEC16 was estimated to occur at least 3 Ma, which thus preceded the gene conversion of SIGLEC11 by SIGLEC16P. As gene conversion usually disrupts the converted gene, the fact that ORFs of hSIGLEC11 and hSIGLEC16 have been maintained after an unusual series of very complex gene conversion events suggests that these events may have been subject to hominin-specific selection forces.

Human-Specific Changes in Sialic Acid Biology

Sialic acids are components of cell-surface glycans and play important roles in cell–cell communication and host–pathogen interaction. More than 55 genes, encoding receptors, enzymes, and transporters, are known to be involved in sialic acid biology. Nearly 10 years of research have revealed that several of these genes show human-specific changes in genome structure, expression, or function. In this chapter, we introduce these human-specific changes and their possible impact on the human evolution. Also, we give an overview of the evolution of sialic acid biology in primates. The discovery of human-specific changes in sialic acid biology is one step toward explaining the genetic basis of human uniqueness, one of the major activities in primatology, contributing to answering a transdisciplinary question: What makes us human?

Erythropoietin and Wnt1 govern pathways of mTOR, Apaf-1, and XIAP in inflammatory microglia

Inflammatory microglia modulate a host of cellular processes in the central nervous system that include neuronal survival, metabolic fluxes, foreign body exclusion, and cellular regeneration. Elucidation of the pathways that oversee microglial survival and integrity may offer new avenues for the treatment of neurodegenerative disorders. Here we demonstrate that erythropoietin (EPO), an emerging strategy for immune system modulation, prevents microglial early and late apoptotic injury during oxidant stress through Wnt1, a cysteine-rich glycosylated protein that modulates cellular development and survival. Loss of Wnt1 through blockade of Wnt1 signaling or through the gene silencing of Wnt1 eliminates the protective capacity of EPO. Furthermore, endogenous Wnt1 in microglia is vital to preserve microglial survival since loss of Wnt1 alone increases microglial injury during oxidative stress. Cellular protection by EPO and Wnt1 intersects at the level of protein kinase B (Akt1), the mammalian target of rapamycin (mTOR), and p70S6K, which are necessary to foster cytoprotection for microglia. Downstream from these pathways, EPO and Wnt1 control "anti-apoptotic" pathways of microglia through the modulation of mitochondrial membrane permeability, the release of cytochrome c, and the expression of apoptotic protease activating factor-1 (Apaf-1) and X-linked inhibitor of apoptosis protein (XIAP). These studies offer new insights for the development of innovative therapeutic strategies for neurodegenerative disorders that focus upon inflammatory microglia and novel signal transduction pathways.

Alzheimer's disease genetics: lessons to improve disease modelling

In the present review, we look back at the recent history of GWAS (genome-wide association studies) in AD (Alzheimer's disease) and integrate the major findings with current knowledge of biological processes and pathways. These topics are essential for the development of animal models, which will be fundamental to our complete understanding of AD.

Linking lipids to Alzheimer's disease: cholesterol and beyond

Lipid-mediated signalling regulates a plethora of physiological processes, including crucial aspects of brain function. In addition, dysregulation of lipid pathways has been implicated in a growing number of neurodegenerative disorders, such as Alzheimer's disease (AD). Although much attention has been given to the link between cholesterol and AD pathogenesis, growing evidence suggests that other lipids, such as phosphoinositides and phosphatidic acid, have an important role. Regulators of lipid metabolism (for example, statins) are a highly successful class of marketed drugs, and exploration of lipid dysregulation in AD and identification of novel therapeutic agents acting through relevant lipid pathways offers new and effective options for the treatment of this devastating disorder.

Novel avenues of drug discovery and biomarkers for diabetes mellitus

Globally, developed nations spend a significant amount of their resources on health care initiatives that poorly translate into increased population life expectancy. As an example, the United States devotes 16% of its gross domestic product to health care, the highest level in the world, but falls behind other nations that enjoy greater individual life expectancy. These observations point to the need for pioneering avenues of drug discovery to increase life span with controlled costs. In particular, innovative drug development for metabolic disorders such as diabetes mellitus becomes increasingly critical given that the number of diabetic people will increase exponentially over the next 20 years. This article discusses the elucidation and targeting of novel cellular pathways that are intimately tied to oxidative stress in diabetes mellitus for new treatment strategies. Pathways that involve wingless, β-nicotinamide adenine dinucleotide (NAD(+)) precursors, and cytokines govern complex biological pathways that determine both cell survival and longevity during diabetes mellitus and its complications. Furthermore, the role of these entities as biomarkers for disease can further enhance their utility irrespective of their treatment potential. Greater understanding of the intricacies of these unique cellular mechanisms will shape future drug discovery for diabetes mellitus to provide focused clinical care with limited or absent long-term complications.

Changing face of microglia

Microglia are resident brain cells that sense pathological tissue alterations. They can develop into brain macrophages and perform immunological functions. However, expression of immune proteins by microglia is not synonymous with inflammation, because these molecules can have central nervous system (CNS)-specific roles. Through their involvement in pain mechanisms, microglia also respond to external threats. Experimental studies support the idea that microglia have a role in the maintenance of synaptic integrity. Analogous to electricians, they are capable of removing defunct axon terminals, thereby helping neuronal connections to stay intact. Microglia in healthy CNS tissue do not qualify as macrophages, and their specific functions are beginning to be explored.

Wnt1, FoxO3a, and NF-kappaB oversee microglial integrity and activation during oxidant stress

Elucidating the underlying mechanisms that govern microglial activation and survival is essential for the development of new treatment strategies for neurodegenerative disorders, since microglia serve not only as guardian sentries of the nervous system, but also play a significant role in determining neuronal and vascular cell fate. Here we show that endogenous and exogenous Wnt1 in inflammatory microglial cells is necessary for the prevention of apoptotic early membrane phosphatidylserine exposure and later DNA degradation, since blockade of Wnt1 signaling abrogates cell survival during oxidative stress. Wnt1 prevents apoptotic demise through the post-translational phosphorylation and maintenance of FoxO3a in the cytoplasm to inhibit an apoptotic cascade that relies upon the loss of mitochondrial membrane permeability, cytochrome c release, Bad phosphorylation, and activation of caspase 3 and caspase 1 as demonstrated by complimentary gene knockdown studies of FoxO3a. Furthermore, subcellular trafficking and gene knockdown studies of NF-kappaB p65 illustrate that microglial cell survival determined by Wnt1 during oxidative stress requires NF-kappaB p65. Our work highlights Wnt1 and the control of novel downstream transcriptional pathways as critical components for the oversight of nervous system microglial cells.