Repression of phagocytosis by human CD33 is not conserved with mouse CD33

Understanding the Glycosylation Pathways Involved in the Biosynthesis of the Sulfated Glycan Ligands for Siglecs

Carbohydrate sulfation plays a pivotal role in modulating the strength of Siglec-glycan interactions. Recently, new aspects of Siglec binding to sulfated cell surface carbohydrates have been discovered, but the class of glycan presenting these sulfated Siglec ligands has not been fully elucidated. In this study, the contribution of different classes of glycans to cis and trans Siglec ligands was investigated within cells expressing the carbohydrate sulfotransferase 1 (CHST1) or CHST2. For some Siglecs, the glycan class mediating binding was clear, such as O-glycans for Siglec-7 and N-glycans for Siglec-2 and Siglec-9. Both N-glycans and mucin-type O-glycans contributed to ligands for Siglec-3, -5, -8, and -15. However, significant levels of Siglec-3 and -8 ligands remained in CHST1-expressing cells lacking complex N-glycans and mucin-type O-glycans. A combination of genetic, pharmacological, and enzymatic treatment strategies ruled out heparan sulfates and glycoRNA as contributors, although Siglec-8 did exhibit some binding to glycolipids. Genetic disruption of O-mannose glycans within CHST1-expressing cells had a small but significant impact on Siglec-3 and -8 binding, demonstrating that this class of glycans can present sulfated Siglec ligands. We also investigated the ability of sulfated cis ligands to mask Siglec-3 and Siglec-7. For Siglec-7, cis ligands were again found to be mucin-type O-glycans. While N-glycans were the major sulfated trans ligands for Siglec-3, disruption of complex mucin-type O-glycans had the largest impact on Siglec-3 masking. Overall, this study enhances our knowledge of the types of sulfated glycans that can serve as Siglec ligands.

SMEPPI: An indenone derivative that selectively inhibits M1 macrophage activation and enhances phagocytic activity

SMEPPI is a small molecule synthesized as a derivative of KR-62980 that has anti-diabetic and anti-inflammatory activities. Despite the established physiological effects of KR-62980, the effects and benefits of SMEPPI remain largely unexplored. This study investigated the immunomodulatory functions of SMEPPI on macrophages and inflammatory diseases. SMEPPI did not affect the differentiation and maturation of bone marrow-derived monocytes into macrophages, nor did it affect the proliferation of M1 or M2 macrophages. Although SMEPPI did not affect M2 macrophage polarization, it significantly inhibited IL-1β and IL-6 cytokine production in both M1 macrophages and activated RAW264.7 macrophages. Importantly, SMEPPI inhibited the expression and phosphorylation of NF-κB p65 through inhibition of Akt expression, preventing its translocation to the nucleus. It also promoted p65 degradation through the stimulation of the proteasomal degradation pathway by inducing the expression of proteasome-related genes, thereby inhibiting p65 transcriptional activity. SMEPPI also enhanced the expression of various molecules associated with macrophage phagocytosis, including CD68, CD33, and lectins, thereby increasing phagocytic activity. Moreover, SMEPPI mitigated lipopolysaccharides-induced acute lung injury by suppressing IL-1β and IL-6 production in M1 macrophages and reduced mortality related to severe lung injury. These findings indicate that SMEPPI effectively regulates inflammatory diseases by impeding p65-induced cytokine production and enhancement of phagocytosis by M1 macrophages.

Roles for Siglec-glycan interactions in regulating immune cells

Cell surface complex carbohydrates, known as glycans, are positioned to be the first point of contact between two cells. Indeed, interactions between glycans with glycan-binding can modulate cell-cell interactions. This concept is particularly relevant for immune cells, which use an array of glycan-binding proteins to help in the process of differentiating 'self' from 'non-self'. This is exemplified by the sialic acid-binding immunoglobulin-type lectins (Siglecs), which recognize sialic acid. Given that sialic acid is relatively unique to vertebrates, immune cells leverage Siglecs to recognize sialic acid as a marker of 'self'. Siglecs serve many biological roles, with most of these functions regulated through interactions with their sialoglycan ligands. In this review, we provide a comprehensive update on the ligands of Siglecs and how Siglec-sialoglycan interactions help regulate immune cells in the adaptive and innate immune system.

Siglecs-mediated immune regulation in neurological disorders

The surfaces of various immune cells are rich in glycan chains, including the sialic-acid-binding immunoglobulin-like lectins (Siglecs) family. As an emerging glyco-immune checkpoint, Siglecs have the ability to bind and interact with various glycoproteins, thereby eliciting a series of downstream reactions to modulate the immune response. The impact of Siglecs has been extensively studied in tumor immunotherapy. However, research in neurological disorders and neurological diseases is very limited, and therapeutic options involving Siglecs need further exploration. Siglecs play a crucial role in the development, homeostasis, and repair processes of the nervous system, especially in degenerative diseases. This review summarizes studies on the immunomodulatory role mediated by Siglecs expressed on different immune cells in various neurological disorders, elucidates how dysregulated sialic acid contributes to several psychiatric disorders, and discusses the progress and limitations of research on the treatment of neurological disorders.

Unique N-glycosylation signatures in Aβ oligomer-and lipopolysaccharide-activated human iPSC-derived microglia

Microglia are the immune cells in the central nervous system (CNS) and become pro-inflammatory/activated in Alzheimer's disease (AD). Cell surface glycosylation plays an important role in immune cells; however, the N-glycosylation and glycosphingolipid (GSL) signatures of activated microglia are poorly understood. Here, we study comprehensive combined transcriptomic and glycomic profiles using human induced pluripotent stem cells-derived microglia (hiMG). Distinct changes in N-glycosylation patterns in amyloid-β oligomer (AβO) and LPS-treated hiMG were observed. In AβO-treated cells, the relative abundance of bisecting N-acetylglucosamine (GlcNAc) N-glycans decreased, corresponding with a downregulation of MGAT3. The sialylation of N-glycans increased in response to AβO, accompanied by an upregulation of genes involved in N-glycan sialylation (ST3GAL4 and 6). Unlike AβO-induced hiMG, LPS-induced hiMG exhibited a decreased abundance of complex-type N-glycans, aligned with downregulation of mannosidase genes (MAN1A1, MAN2A2, and MAN1C1) and upregulation of ER degradation related-mannosidases (EDEM1-3). Fucosylation increased in LPS-induced hiMG, aligned with upregulated fucosyltransferase 4 (FUT4) and downregulated alpha-L-fucosidase 1 (FUCA1) gene expression, while sialofucosylation decreased, aligned with upregulated neuraminidase 4 (NEU4). Inhibition of sialyation and fucosylation in AβO- and LPS-induced hiMG alleviated pro-inflammatory responses. However, the GSL profile did not exhibit significant changes in response to AβO or LPS activation. AβO- and LPS- specific glycosylation changes could contribute to impaired microglia function, highlighting glycosylation pathways as potential therapeutic targets for AD.

HuM195 and its single-chain variable fragment increase Aβ phagocytosis in microglia via elimination of CD33 inhibitory signaling

CD33 is a transmembrane receptor expressed on cells of myeloid lineage and regulates innate immunity. CD33 is a risk factor for Alzheimer's disease (AD) and targeting CD33 has been a promising strategy drug development. However, the mechanism of CD33's action is poorly understood. Here we investigate the mechanism of anti-CD33 antibody HuM195 (Lintuzumab) and its single-chain variable fragment (scFv) and examine their therapeutic potential. Treatment with HuM195 full-length antibody or its scFv increased phagocytosis of β-amyloid 42 (Aβ42) in human microglia and monocytes. This activation of phagocytosis was driven by internalization and degradation of CD33, thereby downregulating its inhibitory signal. HumM195 transiently induced CD33 phosphorylation and its signaling via receptor dimerization. However, this signaling decayed with degradation of CD33. scFv binding to CD33 leads to a degradation of CD33 without detection of the CD33 dimerization and signaling. Moreover, we found that treatments with either HuM195 or scFv promotes the secretion of IL33, a cytokine implicated in microglia reprogramming. Importantly, recombinant IL33 potentiates the uptake of Aβ42 in monocytes. Collectively, our findings provide unanticipated mechanistic insight into the role of CD33 signaling in both monocytes and microglia and define a molecular basis for the development of CD33-based therapy of AD.

Dissecting the abilities of murine Siglecs to interact with gangliosides

Siglecs are cell surface receptors whose functions are tied to the binding of their sialoglycan ligands. Recently, we developed an optimized liposome formulation and used it to investigate the binding of human Siglecs (hSiglec) against a panel of gangliosides. Animal models, more specifically murine models, are used to understand human biology; however, species-specific differences can complicate the interpretation of the results. Herein, we used our optimized liposome formulation to dissect the interactions between murine Siglecs (mSiglecs) and gangliosides to assess the appropriateness of mSiglecs as a proxy to better understand the biological roles of hSiglec-ganglioside interactions. Using our optimized liposome formulation, we found that ganglioside binding is generally conserved between mice and humans with mSiglec-1, -E, -F, and -15 binding multiple gangliosides like their human counterparts. However, in contrast to the hSiglecs, we observed little to no binding between the mSiglecs and ganglioside GM1a. Detailed analysis of mSiglec-1 interacting with GM1a and its structural isomer, GM1b, suggests that mSiglec-1 preferentially binds α2-3-linked sialic acids presented from the terminal galactose residue. The ability of mSiglecs to interact or not interact with gangliosides, particularly GM1a, has implications for using mice to study neurodegenerative diseases, infections, and cancer, where interactions between Siglecs and glycolipids have been proposed to modulate these human diseases.

Alzheimer's disease associated isoforms of human CD33 distinctively modulate microglial cell responses in 5XFAD mice

Microglia play diverse pathophysiological roles in Alzheimer's disease (AD), with genetic susceptibility factors skewing microglial cell function to influence AD risk. CD33 is an immunomodulatory receptor associated with AD susceptibility through a single nucleotide polymorphism that modulates mRNA splicing, skewing protein expression from a long protein isoform (CD33M) to a short isoform (CD33m). Understanding how human CD33 isoforms differentially impact microglial cell function in vivo has been challenging due to functional divergence of CD33 between mice and humans. We address this challenge by studying transgenic mice expressing either of the human CD33 isoforms crossed with the 5XFAD mouse model of amyloidosis and find that human CD33 isoforms have opposing effects on the response of microglia to amyloid-β (Aβ) deposition. Mice expressing CD33M have increased Aβ levels, more diffuse plaques, fewer disease-associated microglia, and more dystrophic neurites compared to 5XFAD control mice. Conversely, CD33m promotes plaque compaction and microglia-plaque contacts, and minimizes neuritic plaque pathology, highlighting an AD protective role for this isoform. Protective phenotypes driven by CD33m are detected at an earlier timepoint compared to the more aggressive pathology in CD33M mice that appears at a later timepoint, suggesting that CD33m has a more prominent impact on microglia cell function at earlier stages of disease progression. In addition to divergent roles in modulating phagocytosis, scRNAseq and proteomics analyses demonstrate that CD33m+ microglia upregulate nestin, an intermediate filament involved in cell migration, at plaque contact sites. Overall, our work provides new functional insights into how CD33, as a top genetic susceptibility factor for AD, modulates microglial cell function.

Dissecting the Ability of Siglecs To Antagonize Fcγ Receptors

Fcγ receptors (FcγRs) play key roles in the effector function of IgG, but their inappropriate activation plays a role in several disease etiologies. Therefore, it is critical to better understand how FcγRs are regulated. Numerous studies suggest that sialic acid-binding immunoglobulin-type lectins (Siglecs), a family of immunomodulatory receptors, modulate FcγR activity; however, it is unclear of the circumstances in which Siglecs can antagonize FcγRs and which Siglecs have this ability. Using liposomes displaying selective ligands to coengage FcγRs with a specific Siglec, we explore the ability of Siglec-3, Siglec-5, Siglec-7, and Siglec-9 to antagonize signaling downstream of FcγRs. We demonstrate that Siglec-3 and Siglec-9 can fully inhibit FcγR activation in U937 cells when coengaged with FcγRs. Cells expressing Siglec mutants reveal differential roles for the immunomodulatory tyrosine-based inhibitory motif (ITIM) and immunomodulatory tyrosine-based switch motif (ITSM) in this inhibition. Imaging flow cytometry enabled visualization of SHP-1 recruitment to Siglec-3 in an ITIM-dependent manner, while SHP-2 recruitment is more ITSM-dependent. Conversely, both cytosolic motifs of Siglec-9 contribute to SHP-1/2 recruitment. Siglec-7 poorly antagonizes FcγR activation for two reasons: masking by cis ligands and differences in its ITIM and ITSM. A chimera of the Siglec-3 extracellular domains and Siglec-5 cytosolic tail strongly inhibits FcγR when coengaged, providing evidence that Siglec-5 is more like Siglec-3 and Siglec-9 in its ability to antagonize FcγRs. Additionally, Siglec-3 and Siglec-9 inhibited FcγRs when coengaged by cells displaying ligands for both the Siglec and FcγRs. These results suggest a role for Siglecs in mediating FcγR inhibition in the context of an immunological synapse, which has important relevance to the effectiveness of immunotherapies.

Roles in Innate Immunity

Microglia are best known as the resident phagocytes of the central nervous system (CNS). As a resident brain immune cell population, microglia play key roles during the initiation, propagation, and resolution of inflammation. The discovery of resident adaptive immune cells in the CNS has unveiled a relationship between microglia and adaptive immune cells for CNS immune-surveillance during health and disease. The interaction of microglia with elements of the peripheral immune system and other CNS resident cells mediates a fine balance between neuroprotection and tissue damage. In this chapter, we highlight the innate immune properties of microglia, with a focus on how pattern recognition receptors, inflammatory signaling cascades, phagocytosis, and the interaction between microglia and adaptive immune cells regulate events that initiate an inflammatory or neuroprotective response within the CNS that modulates immune-mediated disease exacerbation or resolution.

Angiotensin II modulates THP-1-like macrophage phenotype and inflammatory signatures via angiotensin II type 1 receptor

Angiotensin II (Ang II) is a major component of the renin-angiotensin or renin-angiotensin-aldosterone system, which is the main element found to be involved in cardiopathology. Recently, long-term metabolomics studies have linked high levels of angiotensin plasma to inflammatory conditions such as coronary heart disease, obesity, and type 2 diabetes. Monocyte/macrophage cellular function and phenotype orchestrate the inflammatory response in various pathological conditions, most notably cardiometabolic disease. An activation of the Ang II system is usually associated with inflammation and cardiovascular disease; however, the direct effect on monocyte/macrophages has still not been well elucidated. Herein, we have evaluated the cellular effects of Ang II on THP-1-derived macrophages. Ang II stimulated the expression of markers involved in monocyte/macrophage cell differentiation (e.g., CD116), as well as adhesion, cell-cell interaction, chemotaxis, and phagocytosis (CD15, CD44, CD33, and CD49F). Yet, Ang II increased the expression of proinflammatory markers (HLA-DR, TNF-α, CD64, CD11c, and CD38) and decreased CD206 (mannose receptor), an M2 marker. Moreover, Ang II induced cytosolic calcium overload, increased reactive oxygen species, and arrested cells in the G1 phase. Most of these effects were induced via the angiotensin II type 1 receptor (AT1R). Collectively, our results provide new evidence in support of the effect of Ang II in inflammation associated with cardiometabolic diseases.

Current views on meningeal lymphatics and immunity in aging and Alzheimer's disease

Alzheimer's disease (AD) is an aging-related form of dementia associated with the accumulation of pathological aggregates of amyloid beta and neurofibrillary tangles in the brain. These phenomena are accompanied by exacerbated inflammation and marked neuronal loss, which altogether contribute to accelerated cognitive decline. The multifactorial nature of AD, allied to our still limited knowledge of its etiology and pathophysiology, have lessened our capacity to develop effective treatments for AD patients. Over the last few decades, genome wide association studies and biomarker development, alongside mechanistic experiments involving animal models, have identified different immune components that play key roles in the modulation of brain pathology in AD, affecting its progression and severity. As we will relay in this review, much of the recent efforts have been directed to better understanding the role of brain innate immunity, and particularly of microglia. However, and despite the lack of diversity within brain resident immune cells, the brain border tissues, especially the meninges, harbour a considerable number of different types and subtypes of adaptive and innate immune cells. Alongside microglia, which have taken the centre stage as important players in AD research, there is new and exciting evidence pointing to adaptive immune cells, namely T and B cells found in the brain and its meninges, as important modulators of neuroinflammation and neuronal (dys)function in AD. Importantly, a genuine and functional lymphatic vascular network is present around the brain in the outermost meningeal layer, the dura. The meningeal lymphatics are directly connected to the peripheral lymphatic system in different mammalian species, including humans, and play a crucial role in preserving a "healthy" immune surveillance of the CNS, by shaping immune responses, not only locally at the meninges, but also at the level of the brain tissue. In this review, we will provide a comprehensive view on our current knowledge about the meningeal lymphatic vasculature, emphasizing its described roles in modulating CNS fluid and macromolecule drainage, meningeal and brain immunity, as well as glial and neuronal function in aging and in AD.

CRISPR-Cas9 in Alzheimer's disease: Therapeutic trends, modalities, and challenges

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with no known cure, which has prompted the exploration of novel therapeutic approaches. The clustered regularly interspaced palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) tool has generated significant interest for its potential in AD therapeutics by correcting faulty genes. Our report comprehensively reviews emerging applications for CRISPR-Cas9 in developing in vitro and in vivo models for AD research and therapeutics. We further assess its ability to identify and validate genetic markers and potential therapeutic targets for AD. Moreover, we review the current challenges and delivery strategies for the in vivo application of CRISPR-Cas9 in AD therapeutics.

Microglia in Alzheimer's disease: pathogenesis, mechanisms, and therapeutic potentials

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by protein aggregation in the brain. Recent studies have revealed the critical role of microglia in AD pathogenesis. This review provides a comprehensive summary of the current understanding of microglial involvement in AD, focusing on genetic determinants, phenotypic state, phagocytic capacity, neuroinflammatory response, and impact on synaptic plasticity and neuronal regulation. Furthermore, recent developments in drug discovery targeting microglia in AD are reviewed, highlighting potential avenues for therapeutic intervention. This review emphasizes the essential role of microglia in AD and provides insights into potential treatments.

Siglec-6 mediates the uptake of extracellular vesicles through a noncanonical glycolipid binding pocket

Immunomodulatory Siglecs are controlled by their glycoprotein and glycolipid ligands. Siglec-glycolipid interactions are often studied outside the context of a lipid bilayer, missing the complex behaviors of glycolipids in a membrane. Through optimizing a liposomal formulation to dissect Siglec-glycolipid interactions, it is shown that Siglec-6 can recognize glycolipids independent of its canonical binding pocket, suggesting that Siglec-6 possesses a secondary binding pocket tailored for recognizing glycolipids in a bilayer. A panel of synthetic neoglycolipids is used to probe the specificity of this glycolipid binding pocket on Siglec-6, leading to the development of a neoglycolipid with higher avidity for Siglec-6 compared to natural glycolipids. This neoglycolipid facilitates the delivery of liposomes to Siglec-6 on human mast cells, memory B-cells and placental syncytiotrophoblasts. A physiological relevance for glycolipid recognition by Siglec-6 is revealed for the binding and internalization of extracellular vesicles. These results demonstrate a unique and physiologically relevant ability of Siglec-6 to recognize glycolipids in a membrane.

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.

Human sialoglycan ligands for immune inhibitory Siglecs

Most human Siglecs (sialic acid binding immunoglobulin-like lectins) are expressed on the surfaces of overlapping subsets of immune cells, and most carry immunoreceptor tyrosine-based inhibitory domains on their intracellular motifs. When immune inhibitory Siglecs bind to complementary sialoglycans in their local milieu, engagement results in down-regulation of the immune response. Siglecs have come under scrutiny as potential targets of drugs to modify the course of inflammation (and other immune system responses) and as immune checkpoints in cancer. Human Siglecs bind to endogenous human sialoglycans. The identities of these endogenous human sialoglycan immune regulators are beginning to emerge, along with some general principles that may inform future investigations in this area. Among these principles is the finding that a cell type or tissue may express a ligand for a particular Siglec on a single or a very few of its sialoglycoproteins. The selected protein carrier for a particular Siglec may be unique in a certain tissue, but vary tissue-to-tissue. The binding affinity of endogenous Siglec ligands may surpass that of its binding to synthetic sialoglycan determinants by several orders of magnitude. Since most human Siglecs have evolved rapidly and are distinct from those in most other mammals, this review describes endogenous human Siglec ligands for several human immune inhibitory Siglecs. As the identities of these immune regulatory sialoglycan ligands are defined, additional opportunities to target Siglecs therapeutically may emerge.

Alzheimer's Disease Genetics: A Dampened Microglial Response?

Alzheimer's disease (AD) is a debilitating age-related neurodegenerative condition. Unbiased genetic studies have implicated a central role for microglia, the resident innate immune cells of the central nervous system, in AD pathogenesis. On-going efforts are clarifying the biology underlying these associations and the microglial pathways that are dysfunctional in AD. Several genetic risk factors converge to decrease the function of activating microglial receptors and increase the function of inhibitory receptors, resulting in a seemingly dampened microglial phenotype in AD. Moreover, many of these microglial proteins that are genetically associated with AD appear to interact and share pathways or regulatory mechanisms, presenting several points of convergence that may be strategic targets for therapeutic intervention. Here, we review some of these studies and their implications for microglial participation in AD pathogenesis.

Non-canonical roles of Siglecs: Beyond sialic acid-binding and immune cell modulation

Siglecs (Sialic acid-binding immunoglobulin-type lectins) are I-type lectins that bind with sialic acid ligands (Sia). Most are expressed on the surface of leukocytes and are involved in immune regulation and possess immune tyrosine-based inhibitory motif (ITIM) in the intracellular domain, thus leading to inhibition of the immune response. This signaling is instrumental in maintaining quiescence under physiological conditions and acts as a brake for inflammatory cascades. By contrast, activating Siglecs carry positively charged residues in the transmembrane domain and interact with immune tyrosine-based activating motif (ITAM)-containing proteins, a DNAX-activating protein of 10-12 kDa (DAP10/12), to activate immune cells. There are various characteristics of Siglecs that do not fit within the classification of Siglec receptors as being either inhibitory or activating in nature. This review focuses on elucidating the non-canonical functions and interactions of Siglec receptors, which include Sia-independent interactions such as protein-protein interactions and interactions with lipids or other sugars. This review also summarizes Siglec expression and function on non-immune cells, and non-classical signaling of the receptor. Thus, this review will be beneficial to researchers interested in the field of Siglecs and sialic acid biology.

Roles of Siglecs in neurodegenerative diseases

Microglia are resident myeloid cells in the central nervous system (CNS) with a unique developmental origin, playing essential roles in developing and maintaining the CNS environment. Recent studies have revealed the involvement of microglia in neurodegenerative diseases, such as Alzheimer's disease, through the modulation of neuroinflammation. Several members of the Siglec family of sialic acid recognition proteins are expressed on microglia. Since the discovery of the genetic association between a polymorphism in the CD33 gene and late-onset Alzheimer's disease, significant efforts have been made to elucidate the molecular mechanism underlying the association between the polymorphism and Alzheimer's disease. Furthermore, recent studies have revealed additional potential associations between Siglecs and Alzheimer's disease, implying that the reduced signal from inhibitory Siglec may have an overall protective effect in lowering the disease risk. Evidences suggesting the involvement of Siglecs in other neurodegenerative diseases are also emerging. These findings could help us predict the roles of Siglecs in other neurodegenerative diseases. However, little is known about the functionally relevant Siglec ligands in the brain, which represents a new frontier. Understanding how microglial Siglecs and their ligands in CNS contribute to the regulation of CNS homeostasis and pathogenesis of neurodegenerative diseases may provide us with a new avenue for disease prevention and intervention.

Fractalkine enhances oligodendrocyte regeneration and remyelination in a demyelination mouse model

Demyelinating disorders of the central nervous system (CNS) occur when myelin and oligodendrocytes are damaged or lost. Remyelination and regeneration of oligodendrocytes can be achieved from endogenous oligodendrocyte precursor cells (OPCs) that reside in the adult CNS tissue. Using a cuprizone mouse model of demyelination, we show that infusion of fractalkine (CX3CL1) into the demyelinated murine brain increases de novo oligodendrocyte formation and enhances remyelination in the corpus callosum and cortical gray matter. This is achieved by increased OPC proliferation in the cortical gray matter as well as OPC differentiation and attenuation of microglia/macrophage activation both in corpus callosum and cortical gray matter. Finally, we show that activated OPCs and microglia/macrophages express fractalkine receptor CX3CR1 in vivo, and that in OPC-microglia co-cultures fractalkine increases in vitro oligodendrocyte differentiation by modulating both OPC and microglia biology. Our results demonstrate a novel pro-regenerative role of fractalkine in a demyelinating mouse model.

Alzheimer's Disease-Associated Alternative Splicing of CD33 Is Regulated by the HNRNPA Family Proteins

Genetic variations of CD33 have been implicated as a susceptibility factor of Alzheimer's disease (AD). A polymorphism on exon 2 of CD33, rs12459419, affects the alternative splicing of this exon. The minor allele is associated with a reduced risk of AD and promotes the skipping of exon 2 to produce a shorter CD33 isoform lacking the extracellular ligand-binding domain, leading to decreased suppressive signaling on microglial activity. Therefore, factors that regulate the splicing of exon 2 may alter the disease-associated properties of CD33. Herein, we sought to identify the regulatory proteins of CD33 splicing. Using a panel of RNA-binding proteins and a human CD33 minigene, we found that exon 2 skipping of CD33 was promoted by HNRNPA1. Although the knockdown of HNRNPA1 alone did not reduce exon 2 skipping, simultaneous knockdown of HNRNPA1 together with that of HNRNPA2B1 and HNRNPA3 promoted exon 2 inclusion, suggesting functional redundancy among HNRNPA proteins. Similar redundant regulation by HNRNPA proteins was observed in endogenous CD33 of THP-1 and human microglia-like cells. Although mouse Cd33 showed a unique splicing pattern of exon 2, we confirmed that HNRNPA1 promoted the skipping of this exon. Collectively, our results revealed novel regulatory relationships between CD33 and HNRNPA proteins.

Microglial Activation in Metal Neurotoxicity: Impact in Neurodegenerative Diseases

Neurodegenerative processes encompass a large variety of diseases with different pathological patterns and clinical features, such as Alzheimer's and Parkinson's diseases. Exposure to metals has been hypothesized to increase oxidative stress in brain cells leading to cell death and neurodegeneration. Neurotoxicity of metals has been demonstrated by several in vitro and in vivo experimental studies, and most probably, each metal has its specific pathway to trigger cell death. As a result, exposure to essential metals, such as manganese, iron, copper, zinc, and cobalt, and nonessential metals, including lead, aluminum, and cadmium, perturbs metal homeostasis at the cellular and organism levels leading to neurodegeneration. In this contribution, a comprehensive review of the molecular mechanisms by which metals affect microglia physiology and signaling properties is presented. Furthermore, studies that validate the disruption of microglia activation pathways as an essential mechanism of metal toxicity that can contribute to neurodegenerative disease are also presented and discussed.

CD33/TREM2 Signaling Mediates Sleep Deprivation-Induced Memory Impairment by Regulating Microglial Phagocytosis

Sleep deprivation causes significant memory impairment in healthy adults. Extensive research has focused on identifying the biological mechanisms underlying memory impairment. Microglia-mediated synaptic elimination plays an indispensable role in sleep deprivation. Here, the potential role of the CD33/TREM2 signaling pathway in modulating memory decline during chronic sleep restriction (CSR) was evaluated. In this study, adult male C57BL/6 mice were sleep-restricted using an automated sleep deprivation apparatus for 20 h per day for 7 days. The Y-maze test revealed that spontaneous alternation was significantly reduced in CSR mice compared with control mice. The percentage of exploratory preference for the novel object in CSR mice was significantly decreased compared with that in control mice. These memory deficits correlated with aberrant microglial activation and increased phagocytic ability. Moreover, in CSR mice, the CD33 protein level in hippocampal tissue was significantly downregulated, but the TREM2 protein level was increased. In BV2 microglial cells, downregulation of CD33 increased TREM2 expression and improved microglial phagocytosis. Then, the sialic ligand monosialo-ganglioside 1 (GM1, 20 mg/kg, i.p.) was administered to mice once a day during CSR. Our results further showed that GM1 activated CD33 and consequently disturbed TREM2-mediated microglial phagocytosis. Finally, GM1 reversed CSR-induced synaptic loss and memory impairment via the CD33/TREM2 signaling pathway in the CA1 region of the hippocampus. This study provides novel evidence that activating CD33 and/or inhibiting TREM2 activity represent potential therapies for sleep loss-induced memory deficits through the modulation of microglial phagocytosis.

Amyloid-beta and tau pathologies act synergistically to induce novel disease stage-specific microglia subtypes

BACKGROUND

Amongst risk alleles associated with late-onset Alzheimer's disease (AD), those that converged on the regulation of microglia activity have emerged as central to disease progression. Yet, how canonical amyloid-β (Aβ) and tau pathologies regulate microglia subtypes during the progression of AD remains poorly understood.

METHODS

We use single-cell RNA-sequencing to profile microglia subtypes from mice exhibiting both Aβ and tau pathologies across disease progression. We identify novel microglia subtypes that are induced in response to both Aβ and tau pathologies in a disease-stage-specific manner. To validate the observation in AD mouse models, we also generated a snRNA-Seq dataset from the human superior frontal gyrus (SFG) and entorhinal cortex (ERC) at different Braak stages.

RESULTS

We show that during early-stage disease, interferon signaling induces a subtype of microglia termed Early-stage AD-Associated Microglia (EADAM) in response to both Aβ and tau pathologies. During late-stage disease, a second microglia subtype termed Late-stage AD-Associated Microglia (LADAM) is detected. While similar microglia subtypes are observed in other models of neurodegenerative disease, the magnitude and composition of gene signatures found in EADAM and LADAM are distinct, suggesting the necessity of both Aβ and tau pathologies to elicit their emergence. Importantly, the pattern of EADAM- and LADAM-associated gene expression is observed in microglia from AD brains, during the early (Braak II)- or late (Braak VI/V)- stage of the disease, respectively. Furthermore, we show that several Siglec genes are selectively expressed in either EADAM or LADAM. Siglecg is expressed in white-matter-associated LADAM, and expression of Siglec-10, the human orthologue of Siglecg, is progressively elevated in an AD-stage-dependent manner but not shown in non-AD tauopathy.

CONCLUSIONS

Using scRNA-Seq in mouse models bearing amyloid-β and/or tau pathologies, we identify novel microglia subtypes induced by the combination of Aβ and tau pathologies in a disease stage-specific manner. Our findings suggest that both Aβ and tau pathologies are required for the disease stage-specific induction of EADAM and LADAM. In addition, we revealed Siglecs as biomarkers of AD progression and potential therapeutic targets.

Alzheimer's Disease Risk Variant rs3865444 in the CD33 Gene: A Possible Role in Susceptibility to Multiple Sclerosis

Polymorphisms in genes encoding receptors that modulate the activity of microglia and macrophages are attractive candidates for participation in genetic susceptibility to multiple sclerosis (MS). The aims of the study were to (1) investigate the association between Alzheimer’s disease-linked variant rs3865444:C>A in the CD33 gene and MS risk, (2) assess the effect of the strongest MS risk allele HLA-DRB1*15:01 on this association, and (3) analyze the correlation of rs3865444 with selected clinical phenotypes, i.e., age of onset and disease severity. CD33 rs3865444 was genotyped in a cohort of 579 patients and 1145 controls and its association with MS risk and clinical phenotypes was analyzed by logistic and linear regression analysis, respectively. Statistical evaluation revealed that rs3865444 reduces the risk of MS in the HLA-DRB1*15:01-positive subpopulation but not in the cohort negative for HLA-DRB1*15:01. A significant antagonistic epistasis between rs3865444 A and HLA-DRB1*15:01 alleles in the context of MS risk was detected by the interaction synergy factor analysis. Comparison of allele and genotype distribution between relapsing-remitting MS, secondary progressive MS, and control groups revealed that rs3865444 C to A substitution may also be associated with a decreased risk of transition of MS to its secondary progressive form, irrespective of the HLA-DRB1*15:01 carrier status. On the other hand, no correlation could be found between rs3865444 and the age of disease onset or MS severity score. Future studies are required to shed more light on the role of CD33 in MS pathogenesis.

A Review of CRISPR Cas9 for Alzheimer’s Disease: Treatment Strategies and Could target APOE e4, APP, and PSEN-1 Gene using CRISPR cas9 Prevent the Patient from Alzheimer’s Disease?

   A Review of CRISPR Cas9 for Alzheimer’s Disease: Treatment Strategies and Could target APOE e4, APP, and PSEN-1 Gene using CRISPR cas9 Prevent the Patient from Alzheimer’s Disease? BACKGROUND: Alzheimer’s disease is a neurodegenerative disorder characterized by the formation of β-amyloid plaques and neurofibrillary tangles from hyperphosphorylated tau. Several studies suggest that targeting the deletion of the APOE e4, PSEN-1, and APP will reduce tau phosphorylation and Aβ protein accumulation, a crucial hypothesis for the causation of Alzheimer’s disease. APOE e4, PSEN-1, and APP with genome editing Clustered Regular interspersed Short Palindromic Repeats-CRISPR-related (CRISPR/Cas9) are thought to have therapeutic promise for Alzheimer’s disease.AIM: The purpose of this study was to determine whether targeting APOE e4, PSEN-1, and APP using CRISPR/Cas9 is an effective therapeutic and whether it has a long-term effect on Alzheimer’s disease.METHODS: The method used in this study summarized articles by examining the titles and abstracts of specific specified keywords. In this situation, the author picked the title and abstract that matched PubMed, Google Scholar, Science Direct, Cochrane, and the Frontiers in Neuroscience; this was followed by checking to see whether the paper was available in full-text. Eventually, the researcher will study the entire article to decide if it is valuable and relevant to the issue.RESULTS: CRISPR/Cas9 deletion of APOE e4, PSEN-1, and APP in induced pluripotent stem cells (iPSC’s) and g2576 mice as APP mutant models reduce tau phosphorylation and Aβ protein accumulation from neurofibrillary tangles and prevent cell death, vascular damage, and dementia. Furthermore, CRISPR/Cas9 deletion in APOE e4, PSEN-1, and APP improved neuronal cell resilience to oxidative stress and inflammation.CONCLUSION: APOE e4, PSEN-1, and APP deletion by genome editing CRISPR/Cas9 is effective to reduce tau phosphorylation and Aβ protein accumulation from neurofibrillary tangles, cell death, vascular damage, and dementia. However, further research is needed to determine the side effects and safety of its use.

Human brain sialoglycan ligand for CD33, a microglial inhibitory Siglec implicated in Alzheimer's disease

Alzheimer's disease (AD) is characterized by accumulation of misfolded proteins. Genetic studies implicate microglia, brain-resident phagocytic immune cells, in AD pathogenesis. As positive effectors, microglia clear toxic proteins, whereas as negative effectors, they release proinflammatory mediators. An imbalance of these functions contributes to AD progression. Polymorphisms of human CD33, an inhibitory microglial receptor, are linked to AD susceptibility; higher CD33 expression correlates with increased AD risk. CD33, also called Siglec-3, is a member of the sialic acid-binding immunoglobulin-type lectin (Siglec) family of immune regulatory receptors. Siglec-mediated inhibition is initiated by binding to complementary sialoglycan ligands in the tissue environment. Here, we identify a single sialoglycoprotein in human cerebral cortex that binds CD33 as well as Siglec-8, the most abundant Siglec on human microglia. The ligand, which we term receptor protein tyrosine phosphatase zeta (RPTPζ)S3L, is composed of sialylated keratan sulfate chains carried on a minor isoform/glycoform of RPTPζ (phosphacan) and is found in the extracellular milieu of the human brain parenchyma. Brains from human AD donors had twofold higher levels of RPTPζS3L than age-matched control donors, raising the possibility that RPTPζS3L overexpression limits misfolded protein clearance contributing to AD pathology. Mice express the same structure, a sialylated keratan sulfate RPTPζ isoform, that binds mouse Siglec-F and crossreacts with human CD33 and Siglec-8. Brains from mice engineered to lack RPTPζ, the sialyltransferase St3gal4, or the keratan sulfate sulfotransferase Chst1 lacked Siglec binding, establishing the ligand structure. The unique CD33 and Siglec-8 ligand, RPTPζS3L, may contribute to AD progression.

Neuroproteomics Chip-Based Mass Spectrometry and Other Techniques for Alzheimer´S Disease Biomarkers – Update

Background:

Alzheimer's disease (AD) is a progressive neurodegenerative disease of growing interest given that there is cognitive damage and symptom onset acceleration. Therefore, it is important to find AD biomarkers for early diagnosis, disease progression, and discrimination of AD and other diseases.

Objective:

To update the relevance of mass spectrometry for the identification of peptides and proteins involved in AD useful as discriminating biomarkers.

Methods:

Proteomics and peptidomics technologies that show the highest possible specificity and selectivity for AD biomarkers are analyzed, together with the biological fluids used. In addition to positron emission tomography and magnetic resonance imaging, MALDI-TOF mass spectrometry is widely used to identify proteins and peptides involved in AD. The use of protein chips in SELDI technology and electroblotting chips for peptides makes feasible small amounts (L) of samples for analysis.

Results:

Suitable biomarkers are related to AD pathology, such as intracellular neurofibrillary tangles; extraneuronal senile plaques; neuronal and axonal degeneration; inflammation and oxidative stress. Recently, peptides were added to the candidate list, which are not amyloid-b or tau fragments, but are related to coagulation, brain plasticity, and complement/neuroinflammation systems involving the neurovascular unit.

Conclusion:

The progress made in the application of mass spectrometry and recent chip techniques is promising for discriminating between AD, mild cognitive impairment, and matched healthy controls. The application of this technique to blood samples from patients with AD has shown to be less invasive and fast enough to determine the diagnosis, stage of the disease, prognosis, and follow-up of the therapeutic response.

Transgenic mouse models to study the physiological and pathophysiological roles of human Siglecs

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are important immunomodulatory receptors. Due to differences between human and mouse Siglecs, defining the in vivo roles for human Siglecs (hSiglecs) can be challenging. One solution is the development and use of hSiglec transgenic mice to assess the physiological roles of hSiglecs in health and disease. These transgenic mice can also serve as important models for the pre-clinical testing of immunomodulatory approaches that are based on targeting hSiglecs. Four general methods have been used to create hSiglec-expressing transgenic mice, each with associated advantages and disadvantages. To date, transgenic mouse models expressing hSiglec-2 (CD22), -3 (CD33), -7, -8, -9, -11, and -16 have been created. This review focuses on both the generation of these hSiglec transgenic mice, along with the important findings that have been made through their study. Cumulatively, hSiglec transgenic mouse models are providing a deeper understanding of the differences between human and mice orthologs/paralogs, mechanisms by which Siglecs regulate immune cell signaling, physiological roles of Siglecs in disease, and different paradigms where targeting Siglecs may be therapeutically advantageous.

Dietary fats modulate neuroinflammation in mucin 2 knock out mice model of spontaneous colitis

Specific diets regulate neuroimmune responses and modify risk of inflammatory bowel diseases, including ulcerative colitis. A link between gut and brain inflammation is also emerging. We hypothesized that adjusting dietary fatty acid composition modulates the neuroimmune responses in the mucin 2 knock out mice model of spontaneous colitis. Mice were randomly divided into three groups and fed isocaloric diets that only differed in their fatty acid composition. Diets enriched with anhydrous milk fat, corn oil, or Mediterranean diet fats were used. After nine weeks, brain and serum concentrations of ten inflammatory cytokines were measured. Three of these cytokines, including interleukin (IL)-2, IL-12 p70 and interferon-γ, were differentially expressed in the brains of animals from the three diet groups while there were no differences in the serum concentrations of these cytokines. Since only limited information is available about the functions of IL-2 in the central nervous system, in vitro experiments were performed to assess its effects on microglia. IL-2 had no effect on the secretion of neurotoxins and nitric oxide by microglia-like cells, but it selectively regulated phagocytic activity and reactive oxygen species production by stimulated microglia-like cells. Modulation of microglial reactive oxygen species through altered brain IL-2 concentrations could be one of the mechanisms linking diets with modified risk of neuroimmune disorders including Parkinson's disease.

Application of CRISPR/Cas9 in Alzheimer's Disease

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder clinically characterized by cognitive impairment, abnormal behavior, and social deficits, which is intimately linked with excessive β-amyloid (Aβ) protein deposition along with many other misfolded proteins, neurofibrillary tangles formed by hyperphosphorylated tau protein aggregates, and mitochondrial damage in neurons, leading to neuron loss. Currently, research on the pathological mechanism of AD has been elucidated for decades, still no effective treatment for this complex disease was developed, and the existing therapeutic strategies are extremely erratic, thereby leading to irreversible and progressive cognitive decline in AD patients. Due to gradually mental dyscapacitating of AD patients, AD not only brings serious physical and psychological suffering to patients themselves, but also imposes huge economic burdens on family and society. Accordingly, it is very imperative to recapitulate the progress of gene editing-based precision medicine in the emerging fields. In this review, we will mainly focus on the application of CRISPR/Cas9 technique in the fields of AD research and gene therapy, and summarize the application of CRISPR/Cas9 in the aspects of AD model construction, screening of pathogenic genes, and target therapy. Finally, the development of delivery systems, which is a major challenge that hinders the clinical application of CRISPR/Cas9 technology will also be discussed.

Laurent CD, Khoo KH, Mahal LK, Zandberg WF, Huang X, Klassen JS, Macauley MS. Carbohydrate Sulfation As a Mechanism for Fine-Tuning Siglec Ligands

The immunomodulatory family of Siglecs recognizes sialic acid-containing glycans as "self", which is exploited in cancer for immune evasion. The biochemical nature of Siglec ligands remains incompletely understood, with emerging evidence suggesting the importance of carbohydrate sulfation. Here, we investigate how specific sulfate modifications affect Siglec ligands by overexpressing eight carbohydrate sulfotransferases (CHSTs) in five cell lines. Overexpression of three CHSTs─CHST1, CHST2, or CHST4─significantly enhance the binding of numerous Siglecs. Unexpectedly, two other CHSTs (Gal3ST2 and Gal3ST3) diminish Siglec binding, suggesting a new mode to modulate Siglec ligands via sulfation. Results are cell type dependent, indicating that the context in which sulfated glycans are presented is important. Moreover, a pharmacological blockade of N- and O-glycan maturation reveals a cell-type-specific pattern of importance for either class of glycan. Production of a highly homogeneous Siglec-3 (CD33) fragment enabled a mass-spectrometry-based binding assay to determine ≥8-fold and ≥2-fold enhanced affinity for Neu5Acα2-3(6-O-sulfo)Galβ1-4GlcNAc and Neu5Acα2-3Galβ1-4(6-O-sulfo)GlcNAc, respectively, over Neu5Acα2-3Galβ1-4GlcNAc. CD33 shows significant additivity in affinity (≥28-fold) for the disulfated ligand, Neu5Acα2-3(6-O-sulfo)Galβ1-4(6-O-sulfo)GlcNAc. Moreover, joint overexpression of CHST1 with CHST2 in cells greatly enhanced the binding of CD33 and several other Siglecs. Finally, we reveal that CHST1 is upregulated in numerous cancers, correlating with poorer survival rates and sodium chlorate sensitivity for the binding of Siglecs to cancer cell lines. These results provide new insights into carbohydrate sulfation as a general mechanism for tuning Siglec ligands on cells, including in cancer.

Increasing phagocytosis of microglia by targeting CD33 with liposomes displaying glycan ligands

CD33 is an immunomodulatory receptor expressed by microglia and genetically linked to Alzheimer's disease (AD) susceptibility. While antibodies targeting CD33 have entered clinical trials to treat neurodegeneration, it is unknown whether the glycan-binding properties of CD33 can be exploited to modulate microglia. Here, we use liposomes that multivalently display glycan ligands of CD33 (CD33L liposomes) to engage CD33. We find that CD33L liposomes increase phagocytosis of cultured monocytic cells and microglia in a CD33-dependent manner. Enhanced phagocytosis strongly correlates with loss of CD33 from the cell surface and internalization of liposomes. Increased phagocytosis by treatment with CD33L liposomes is dependent on a key intracellular signaling motif on CD33 as well as the glycan-binding ability of CD33. These effects are specific to trans engagement of CD33 by CD33L liposomes, as cis engagement through insertion of lipid-linked CD33L into cells produces the opposite effect on phagocytosis. Moreover, intracerebroventricular injection of CD33L liposomes into transgenic mice expressing human CD33 in the microglial cell lineage enhances phagocytosis of microglia in a CD33-dependent manner. These results demonstrate that multivalent engagement of CD33 with glycan ligands can modulate microglial cell function.

Pseudogene-Mediated Gene Conversion After CRISPR-Cas9 Editing Demonstrated by Partial CD33 Conversion with SIGLEC22P

Although gene editing workflows typically consider the possibility of off-target editing, pseudogene-directed homology repair has not, to our knowledge, been reported previously. Here, we employed a CRISPR-Cas9 strategy for targeted excision of exon 2 in CD33 in U937 human monocyte cell line. Candidate clonal cell lines were screened by using a clinically relevant antibody known to label the IgV domain encoded by exon 2 (P67.6, gemtuzumab). In addition to the anticipated deletion of exon 2, we also found unexpected P67.6-negative cell lines, which had apparently retained CD33 exon 2. Sequencing revealed that these lines underwent gene conversion from the nearby SIGLEC22P pseudogene during homology repair that resulted in three missense mutations relative to CD33. Ectopic expression studies confirmed that the P67.6 epitope is dependent upon these amino acids. In summation, we report that pseudogene-directed homology repair can lead to aberrant CRISPR gene editing.

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.

Reporter cell assay for human CD33 validated by specific antibodies and human iPSC-derived microglia

CD33/Sialic acid-binding Ig-like lectin 3 (SIGLEC3) is an innate immune receptor expressed on myeloid cells and mediates inhibitory signaling via tyrosine phosphatases. Variants of CD33 are associated with Alzheimer’s disease (AD) suggesting that modulation of CD33 signaling might be beneficial in AD. Hence, there is an urgent need for reliable cellular CD33 reporter systems. Therefore, we generated a CD33 reporter cell line expressing a fusion protein consisting of the extracellular domain of either human full-length CD33 (CD33M) or the AD-protective variant CD33^ΔE2 (D2-CD33/CD33m) linked to TYRO protein tyrosine kinase binding protein (TYROBP/DAP12) to investigate possible ligands and antibodies for modulation of CD33 signaling. Application of the CD33-specific antibodies P67.6 and 1c7/1 to the CD33M-DAP12 reporter cells resulted in increased phosphorylation of the kinase SYK, which is downstream of DAP12. CD33M-DAP12 but not CD33^ΔE2-DAP12 expressing reporter cells showed increased intracellular calcium levels upon treatment with CD33 antibody P67.6 and partially for 1c7/1. Furthermore, stimulation of human induced pluripotent stem cell-derived microglia with the CD33 antibodies P67.6 or 1c7/1 directly counteracted the triggering receptor expressed on myeloid cells 2 (TREM2)-induced phosphorylation of SYK and decreased the phagocytic uptake of bacterial particles. Thus, the developed reporter system confirmed CD33 pathway activation by CD33 antibody clones P67.6 and 1c7/1. In addition, data showed that phosphorylation of SYK by TREM2 activation and phagocytosis of bacterial particles can be directly antagonized by CD33 signaling.

Regulation of microglia population dynamics throughout development, health, and disease

The dynamic expansions and contractions of the microglia population in the central nervous system (CNS) to achieve homeostasis are likely vital for their function. Microglia respond to injury or disease but also help guide neurodevelopment, modulate neural circuitry throughout life, and direct regeneration. Throughout these processes, microglia density changes, as does the volume of area that each microglia surveys. Given that microglia are responsible for sensing subtle alterations to their environment, a change in their density could affect their capacity to mobilize rapidly. In this review, we attempt to synthesize the current literature on the ligands and conditions that promote microglial proliferation across development, adulthood, and neurodegenerative conditions. Microglia display an impressive proliferative capacity during development and in neurodegenerative diseases that is almost completely absent at homeostasis. However, the appropriate function of microglia in each state is critically dependent on density fluctuations that are primarily induced by proliferation. Proliferation is a natural microglial response to insult and often serves neuroprotective functions. In contrast, inappropriate microglial proliferation, whether too much or too little, often precipitates undesirable consequences for nervous system health. Thus, fluctuations in the microglia population are tightly regulated to ensure these immune cells can execute their diverse functions.

Putting the brakes on phagocytosis: "don't-eat-me" signaling in physiology and disease

Timely removal of dying or pathogenic cells by phagocytes is essential to maintaining host homeostasis. Phagocytes execute the clearance process with high fidelity while sparing healthy neighboring cells, and this process is at least partially regulated by the balance of "eat-me" and "don't-eat-me" signals expressed on the surface of host cells. Upon contact, eat-me signals activate "pro-phagocytic" receptors expressed on the phagocyte membrane and signal to promote phagocytosis. Conversely, don't-eat-me signals engage "anti-phagocytic" receptors to suppress phagocytosis. We review the current knowledge of don't-eat-me signaling in normal physiology and disease contexts where aberrant don't-eat-me signaling contributes to pathology.

Siglec Ligands

A dense and diverse array of glycans on glycoproteins and glycolipids decorate all cell surfaces. In vertebrates, many of these carry sialic acid, in a variety of linkages and glycan contexts, as their outermost sugar moiety. Among their functions, glycans engage complementary glycan binding proteins (lectins) to regulate cell physiology. Among the glycan binding proteins are the Siglecs, sialic acid binding immunoglobulin-like lectins. In humans, there are 14 Siglecs, most of which are expressed on overlapping subsets of immune system cells. Each Siglec engages distinct, endogenous sialylated glycans that initiate signaling programs and regulate cellular responses. Here, we explore the emerging science of Siglec ligands, including endogenous sialoglycoproteins and glycolipids and synthetic sialomimetics. Knowledge in this field promises to reveal new molecular pathways controlling cell physiology and new opportunities for therapeutic intervention.

The role of innate immune genes in Alzheimer's disease

PURPOSE OF REVIEW

The aim of this study was to provide an update on the role of the innate immune system and neuroinflammation in the pathogenesis of Alzheimer's disease, with an emphasis on microglial receptors CD33 and TREM2.

RECENT FINDINGS

Genome-wide association studies (GWAS) have identified many Alzheimer's disease risk genes related to immune response and microglia including the phagocytic receptors CD33 and TREM2. Recent GWAS and pathway analyses emphasize the crucial role of the innate immune system and neuroinflammation in the pathogenesis of Alzheimer's disease. Disease-associated microglia have been characterized by TREM2-dependent upregulation of phagocytic and lipid metabolism genes. Impaired microglial phagocytosis results in amyloid beta (Aβ) accumulation leading to neuroinflammation that is the primary cause of neurodegeneration. CD33 and TREM2 modulate neuroinflammation in Alzheimer's disease and have emerged as therapeutic targets in Alzheimer's disease. Progress has been made to inhibit CD33 by gene therapy, small molecules or immunotherapy, and to increase TREM2 activity by immunotherapy. Finally, mAbs against CD33 and TREM2 have entered clinical trials and may reduce neuroinflammation in Alzheimer's disease brain.

SUMMARY

Targeting neuroinflammation via CD33 inhibition and/or TREM2 activation may have important implications for neurodegeneration in Alzheimer's disease and may be an addition to monoclonal anti-Aβ antibody treatments that remove plaques without reducing neuroinflammation.

Laurent CD, McCord KA, Bains A, Sidhu G, Sarkar S, Plemel JR, Macauley MS. The CD33 short isoform is a gain-of-function variant that enhances Aβ1-42 phagocytosis in microglia

BACKGROUND

CD33 is genetically linked to Alzheimer's disease (AD) susceptibility through differential expression of isoforms in microglia. The role of the human CD33 short isoform (hCD33m), preferentially encoded by an AD-protective CD33 allele (rs12459419T), is unknown. Here, we test whether hCD33m represents a loss-of-function or gain-of-function variant.

METHODS

We have developed two models to test the role of hCD33m. The first is a new strain of transgenic mice expressing hCD33m in the microglial cell lineage. The second is U937 cells where the CD33 gene was disrupted by CRISPR/Cas9 and complemented with different variants of hCD33. Primary microglia and U937 cells were tested in phagocytosis assays and single cell RNA sequencing (scRNAseq) was carried out on the primary microglia. Furthermore, a new monoclonal antibody was developed to detect hCD33m more efficiently.

RESULTS

In both primary microglia and U937 cells, we find that hCD33m enhances phagocytosis. This contrasts with the human CD33 long isoform (hCD33M) that represses phagocytosis, as previously demonstrated. As revealed by scRNAseq, hCD33m+ microglia are enriched in a cluster of cells defined by an upregulated expression and gene regulatory network of immediate early genes, which was further validated within microglia in situ. Using a new hCD33m-specific antibody enabled hCD33m expression to be examined, demonstrating a preference for an intracellular location. Moreover, this newly discovered gain-of-function role for hCD33m is dependent on its cytoplasmic signaling motifs, dominant over hCD33M, and not due to loss of glycan ligand binding.

CONCLUSIONS

These results provide strong support that hCD33m represents a gain-of-function isoform and offers insight into what it may take to therapeutically capture the AD-protective CD33 allele.

Function of Macrophages in Disease: Current Understanding on Molecular Mechanisms

Tissue-resident macrophages (TRMs) are heterogeneous populations originating either from monocytes or embryonic progenitors, and distribute in lymphoid and non-lymphoid tissues. TRMs play diverse roles in many physiological processes, including metabolic function, clearance of cellular debris, and tissue remodeling and defense. Macrophages can be polarized to different functional phenotypes depending on their origin and tissue microenvironment. Specific macrophage subpopulations are associated with disease progression. In studies of fate-mapping and single-cell RNA sequencing methodologies, several critical molecules have been identified to induce the change of macrophage function. These molecules are potential markers for diagnosis and selective targets for novel macrophage-mediated treatment. In this review, we discuss some of the recent findings regarding less-known molecules and new functions of well-known molecules. Understanding the mechanisms of these molecules in macrophages has the potential to yield new macrophage-mediated treatments or diagnostic approaches to disease.

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.

Implications of Glycosylation in Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting millions of people worldwide, and no cure is currently available. The major pathological hallmarks of AD are considered to be amyloid beta plaques and neurofibrillary tangles, generated by respectively APP processing and Tau phosphorylation. Recent evidence imply that glycosylation of these proteins, and a number of other AD-related molecules is altered in AD, suggesting a potential implication of this process in disease pathology. In this review we summarize the understanding of glycans in AD pathogenesis, and discuss how glycobiology can contribute to early diagnosis and treatment of AD, serving as potential biomarkers and therapeutic targets. Furthermore, we look into the potential link between the emerging topic neuroinflammation and glycosylation, combining two interesting, and until recent years, understudied topics in the scope of AD. Lastly, we discuss how new model platforms such as induced pluripotent stem cells can be exploited and contribute to a better understanding of a rather unexplored area in AD.

Siglec-E retards atherosclerosis by inhibiting CD36-mediated foam cell formation

BACKGROUND

The accumulation of lipid-laden macrophages, foam cells, within sub-endothelial intima is a key feature of early atherosclerosis. Siglec-E, a mouse orthologue of human Siglec-9, is a sialic acid binding lectin predominantly expressed on the surface of myeloid cells to transduce inhibitory signal via recruitment of SH2-domain containing protein tyrosine phosphatase SHP-1/2 upon binding to its sialoglycan ligands. Whether Siglec-E expression on macrophages impacts foam cell formation and atherosclerosis remains to be established.

METHODS

ApoE-deficient (apoE^-/-) and apoE/Siglec-E-double deficient (apoE^-/-/Siglec-E^-/-) mice were placed on high fat diet for 3 months and their lipid profiles and severities of atherosclerosis were assessed. Modified low-density lipoprotein (LDL) uptake and foam cell formation in wild type (WT) and Siglec-E^-/-- peritoneal macrophages were examined in vitro. Potential Siglec-E-interacting proteins were identified by proximity labeling in conjunction with proteomic analysis and confirmed by coimmunoprecipitation experiment. Impacts of Siglec-E expression and cell surface sialic acid status on oxidized LDL uptake and signaling involved were examined by biochemical assays.

RESULTS

Here we show that genetic deletion of Siglec-E accelerated atherosclerosis without affecting lipid profile in apoE^-/- mice. Siglec-E deficiency promotes foam cell formation by enhancing acetylated and oxidized LDL uptake without affecting cholesterol efflux in macrophages in vitro. By performing proximity labeling and proteomic analysis, we identified scavenger receptor CD36 as a cell surface protein interacting with Siglec-E. Further experiments performed in HEK293T cells transiently overexpressing Siglec-E and CD36 and peritoneal macrophages demonstrated that depletion of cell surface sialic acids by treatment with sialyltransferase inhibitor or sialidase did not affect interaction between Siglec-E and CD36 but retarded Siglec-E-mediated inhibition on oxidized LDL uptake. Subsequent experiments revealed that oxidized LDL induced transient Siglec-E tyrosine phosphorylation and recruitment of SHP-1 phosphatase in macrophages. VAV, a downstream effector implicated in CD36-mediated oxidized LDL uptake, was shown to interact with SHP-1 following oxidized LDL treatment. Moreover, oxidized LDL-induced VAV phosphorylation was substantially lower in WT macrophages comparing to Siglec-E^-/- counterparts.

CONCLUSIONS

These data support the protective role of Siglec-E in atherosclerosis. Mechanistically, Siglec-E interacts with CD36 to suppress downstream VAV signaling involved in modified LDL uptake.

Sialylation acts as a checkpoint for innate immune responses in the central nervous system

Sialic acids are monosaccharides that normally terminate the glycan chains of cell surface glyco-proteins and -lipids in mammals, and are highly enriched in the central nervous tissue. Sialic acids are conjugated to proteins and lipids (termed "sialylation") by specific sialyltransferases, and are removed ("desialylation") by neuraminidases. Cell surface sialic acids are sensed by complement factor H (FH) to inhibit complement activation or by sialic acid-binding immunoglobulin-like lectin (SIGLEC) receptors to inhibit microglial activation, phagocytosis, and oxidative burst. In contrast, desialylation of cells enables binding of the opsonins C1, calreticulin, galectin-3, and collectins, stimulating phagocytosis of such cells. Hypersialylation is used by bacteria and cancers as camouflage to escape immune recognition, while polysialylation of neurons protects synapses and neurogenesis. Insufficient lysosomal cleavage of sialylated molecules can lead to lysosomal accumulation of lipids and aggregated proteins, which if excessive may be expelled into the extracellular space. On the other hand, desialylation of immune receptors can activate them or trigger removal of proteins. Loss of inhibitory SIGLECs or FH triggers reduced clearance of aggregates, oxidative brain damage and complement-mediated retinal damage. Thus, cell surface sialylation recognized by FH, SIGLEC, and other immune-related receptors acts as a major checkpoint inhibitor of innate immune responses in the central nervous system, while excessive cleavage of sialic acid residues and consequently removing this checkpoint inhibitor may trigger lipid accumulation, protein aggregation, inflammation, and neurodegeneration.

Novel PET Biomarkers to Disentangle Molecular Pathways across Age-Related Neurodegenerative Diseases

There is a need to disentangle the etiological puzzle of age-related neurodegenerative diseases, whose clinical phenotypes arise from known, and as yet unknown, pathways that can act distinctly or in concert. Enhanced sub-phenotyping and the identification of in vivo biomarker-driven signature profiles could improve the stratification of patients into clinical trials and, potentially, help to drive the treatment landscape towards the precision medicine paradigm. The rapidly growing field of neuroimaging offers valuable tools to investigate disease pathophysiology and molecular pathways in humans, with the potential to capture the whole disease course starting from preclinical stages. Positron emission tomography (PET) combines the advantages of a versatile imaging technique with the ability to quantify, to nanomolar sensitivity, molecular targets in vivo. This review will discuss current research and available imaging biomarkers evaluating dysregulation of the main molecular pathways across age-related neurodegenerative diseases. The molecular pathways focused on in this review involve mitochondrial dysfunction and energy dysregulation; neuroinflammation; protein misfolding; aggregation and the concepts of pathobiology, synaptic dysfunction, neurotransmitter dysregulation and dysfunction of the glymphatic system. The use of PET imaging to dissect these molecular pathways and the potential to aid sub-phenotyping will be discussed, with a focus on novel PET biomarkers.

Phosphoproteomics identifies microglial Siglec-F inflammatory response during neurodegeneration

Alzheimer's disease (AD) is characterized by the appearance of amyloid-β plaques, neurofibrillary tangles, and inflammation in brain regions involved in memory. Using mass spectrometry, we have quantified the phosphoproteome of the CK-p25, 5XFAD, and Tau P301S mouse models of neurodegeneration. We identified a shared response involving Siglec-F which was upregulated on a subset of reactive microglia. The human paralog Siglec-8 was also upregulated on microglia in AD. Siglec-F and Siglec-8 were upregulated following microglial activation with interferon gamma (IFNγ) in BV-2 cell line and human stem cell-derived microglia models. Siglec-F overexpression activates an endocytic and pyroptotic inflammatory response in BV-2 cells, dependent on its sialic acid substrates and immunoreceptor tyrosine-based inhibition motif (ITIM) phosphorylation sites. Related human Siglecs induced a similar response in BV-2 cells. Collectively, our results point to an important role for mouse Siglec-F and human Siglec-8 in regulating microglial activation during neurodegeneration.