Associations of plasma soluble CD22 levels with brain amyloid burden and cognitive decline in Alzheimer's disease

Systematic review of amyloid-beta clearance proteins from the brain to the periphery: implications for Alzheimer's disease diagnosis and therapeutic targets

Amyloid-beta clearance plays a key role in the pathogenesis of Alzheimer's disease. However, the variation in functional proteins involved in amyloid-beta clearance and their correlation with amyloid-beta levels remain unclear. In this study, we conducted meta-analyses and a systematic review using studies from the PubMed, Embase, Web of Science, and Cochrane Library databases, including journal articles published from inception to June 30, 2023. The inclusion criteria included studies comparing the levels of functional proteins associated with amyloid-beta clearance in the blood, cerebrospinal fluid, and brain of healthy controls, patients with mild cognitive impairment, and patients with Alzheimer's disease. Additionally, we analyzed the correlation between these functional proteins and amyloid-beta levels in patients with Alzheimer's disease. The methodological quality of the studies was assessed via the Newcastle‒Ottawa Scale. Owing to heterogeneity, we utilized either a fixed-effect or random-effect model to assess the 95% confidence interval (CI) of the standard mean difference (SMD) among healthy controls, patients with mild cognitive impairment, and patients with Alzheimer's disease. The findings revealed significant alterations in the levels of insulin-degrading enzymes, neprilysin, matrix metalloproteinase-9, cathepsin D, receptor for advanced glycation end products, and P-glycoprotein in the brains of patients with Alzheimer's disease, patients with mild cognitive impairment, and healthy controls. In cerebrospinal fluid, the levels of triggering receptor expressed on myeloid cells 2 and ubiquitin C-terminal hydrolase L1 are altered, whereas the levels of TREM2, CD40, CD40L, CD14, CD22, cathepsin D, cystatin C, and α2 M in peripheral blood differ. Notably, TREM2 and cathepsin D showed changes in both brain (SMD = 0.31, 95% CI: 0.16-0.47, P < 0.001, I2 = 78.4%; SMD = 1.24, 95% CI: 0.01-2.48, P = 0.048, I2 = 90.1%) and peripheral blood (SMD = 1.01, 95% CI: 0.35-1.66, P = 0.003, I2 = 96.5%; SMD = 7.55, 95% CI: 3.92-11.18, P < 0.001, I2 = 98.2%) samples. Furthermore, correlations were observed between amyloid-beta levels and the levels of TREM2 ( r = 0.16, 95% CI: 0.04-0.28, P = 0.009, I2 = 74.7%), neprilysin ( r = -0.47, 95% CI: -0.80-0.14, P = 0.005, I2 = 76.1%), and P-glycoprotein ( r = -0.31, 95% CI: -0.51-0.11, P = 0.002, I2 = 0.0%) in patients with Alzheimer's disease. These findings suggest that triggering receptor expressed on myeloid cells 2 and cathepsin D could serve as potential diagnostic biomarkers for Alzheimer's disease, whereas triggering receptor expressed on myeloid cells 2, neprilysin, and P-glycoprotein may represent potential therapeutic targets.

Implications of Mucin-Type O-Glycosylation in Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders linked to aging. Major hallmarks of AD pathogenesis include amyloid-β peptide (Aβ) plaques, which are extracellular deposits originating from the processing of the amyloid precursor protein (APP), and neurofibrillary tangles (NFTs), which are intracellular aggregates of tau protein. Recent evidence indicates that disruptions in metal homeostasis and impaired immune recognition of these aggregates trigger neuroinflammation, ultimately driving disease progression. Therefore, a more comprehensive approach is needed to understand the underlying causes of the disease. Patients with AD present abnormal glycan profiles, and most known AD-related molecules are either modified with glycans or involved in glycan regulation. A deeper understanding of how O-glycosylation influences the balance between amyloid-beta peptide production and clearance, as well as microglia's pro- and anti-inflammatory responses, is crucial for deciphering the early pathogenic events of AD. This review aims to provide a comprehensive summary of the extensive research conducted on the role of mucin-type O-glycosylation in the pathogenesis of AD, discussing its role in disease onset and immune recognition.

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.

Peripheral and central neuroimmune mechanisms in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) poses a growing global health challenge as populations age. Recent research highlights the crucial role of peripheral immunity in AD pathogenesis. This review explores how blood-brain barrier disruption allows peripheral immune cells to infiltrate the central nervous system (CNS), worsening neuroinflammation and disease progression. We examine recent findings on interactions between peripheral immune cells and CNS-resident microglia, forming a self-perpetuating inflammatory cycle leading to neuronal dysfunction. Moreover, this review emphasizes recent developments in the dysregulation of immune factors from both the periphery and CNS, and their impact on AD progression. With ongoing research and development of new therapeutic strategies, this review underscores the importance of modulating interactions between the peripheral immune system and CNS in AD therapy.

CD22 modulation alleviates amyloid β-induced neuroinflammation

Neuroinflammation is a crucial driver of multiple neurodegenerative diseases, including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD). Yet, therapeutic targets for neurodegenerative diseases based on neuroinflammation still warrant investigation. CD22 has been implicated in neuroinflammatory diseases, namely AD. Specifically, plasma soluble CD22 (sCD22) level is upregulated in patients with AD. Direct experimental evidence for the role of CD22 in neuroinflammation is needed, as is a better understanding of its impact on microglia activation and therapeutic potential. Here we reported that sCD22 promotes neuroinflammation both in vivo and in vitro. sCD22 activated microglia via both p38 and ERK1/2 signaling pathway for the secretion of TNFα, IL-6 and CCL3. Moreover, sCD22 activated microglia via sialic acid binding domain and 2,6 linked sialic acid glycan on sCD22. The pivotal therapeutic potential of targeting CD22 was demonstrated in Amyloid β (Aβ) induced-neuroinflammation in hCD22 transgenic mice. Suciraslimab improved working memory and resolved neuroinflammation in vivo. Further, membrane CD22 inhibited Amyloid β (Aβ) induced-NFκB signaling pathway and mechanistic study delineated that suciraslimab suppressed Aβ-induced IL-1β secretion in human microglia and PBMC. Suciraslimab also suppressed IL-12 and IL-23 secretion in human PBMC. Moreover, suciraslimab reduced the surface expression of α4 integrin on B cells. Intriguingly, we discovered that CD22 interact with Aβ and suciraslimab enhanced internalization of CD22-Aβ complex in microglia. Our data highlights the importance of sCD22 in driving neuroinflammation and the dual mechanism of targeting CD22 to resolve Aβ-induced inflammation and promote Aβ phagocytosis.

Neuronal cathepsin S increases neuroinflammation and causes cognitive decline via CX3CL1-CX3CR1 axis and JAK2-STAT3 pathway in aging and Alzheimer's disease

Aging is an intricate process involving interactions among multiple factors, which is one of the main risks for chronic diseases, including Alzheimer's disease (AD). As a member of cysteine protease, cathepsin S (CTSS) has been implicated in inflammation across various diseases. Here, we investigated the role of neuronal CTSS in aging and AD started by examining CTSS expression in hippocampus neurons of aging mice and identified a significant increase, which was negatively correlated with recognition abilities. Concurrently, we observed an elevation of CTSS concentration in the serum of elderly people. Transcriptome and fluorescence-activated cell sorting (FACS) results revealed that CTSS overexpression in neurons aggravated brain inflammatory milieu with microglia activation to M1 pro-inflammatory phenotype, activation of chemokine C-X3-C-motif ligand 1 (CX3CL1)-chemokine C-X3-C-motif receptor 1 (CX3CR1) axis and janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) pathway. As CX3CL1 is secreted by neurons and acts on the CX3CR1 in microglia, our results revealed for the first time the role of neuron CTSS in neuron-microglia "crosstalk." Besides, we observed elevated CTSS expression in multiple brain regions of AD patients, including the hippocampus. Utilizing CTSS selective inhibitor, LY3000328, rescued AD-related pathological features in APP/PS1 mice. We further noticed that neuronal CTSS overexpression increased cathepsin B (CTSB) activity, but decreased cathepsin L (CTSL) activity in microglia. Overall, we provide evidence that CTSS can be used as an aging biomarker and plays regulatory roles through modulating neuroinflammation and recognition in aging and AD process.

B-cell immune dysregulation with low soluble CD22 levels in refractory seronegative myasthenia gravis

Myasthenia gravis (MG), primarily caused by acetylcholine receptor (AChR) autoantibodies, is a chronic autoimmune disorder causing severe muscle weakness and fatigability. In particular, seronegative MG constitutes 10%-15% of MG cases and presents diagnostic challenges especially in early-onset female patients who often show severe disease and resistance to immunosuppressive therapy. Furthermore, the immunopathology of seronegative MG remains unclear. Thus, in this study, we aimed to elucidate the pathogenic mechanism of seronegative MG using scRNA-seq analysis and plasma proteome analysis; in particular, we investigated the relationship between immune dysregulation status and disease severity in refractory seronegative MG. Employing single-cell RNA-sequencing and plasma proteome analyses, we analyzed peripheral blood samples from 30 women divided into three groups: 10 healthy controls, 10 early-onset AChR-positive MG, and 10 refractory early-onset seronegative MG patients, both before and after intravenous immunoglobulin treatment. The disease severity was evaluated using the MG-Activities of Daily Living (ADL), MG composite (MGC), and revised 15-item MG-Quality of Life (QOL) scales. We observed numerical abnormalities in multiple immune cells, particularly B cells, in patients with refractory seronegative MG, correlating with disease activity. Notably, severe MG cases had fewer regulatory T cells without functional abnormalities. Memory B cells were found to be enriched in peripheral blood cells compared with naïve B cells. Moreover, plasma proteome analysis indicated significantly lower plasma protein levels of soluble CD22, expressed in the lineage of B-cell maturation (including mature B cells and memory B cells), in refractory seronegative MG patients than in healthy donors or patients with AChR-positive MG. Soluble CD22 levels were correlated with disease severity, B-cell frequency, and RNA expression levels of CD22. In summary, this study elucidates the immunopathology of refractory seronegative MG, highlighting immune disorders centered on B cells and diminished soluble CD22 levels. These insights pave the way for novel MG treatment strategies focused on B-cell biology.

Therapeutic potential to target sialylation and SIGLECs in neurodegenerative and psychiatric diseases

Sialic acids, commonly found as the terminal carbohydrate on the glycocalyx of mammalian cells, are pivotal checkpoint inhibitors of the innate immune system, particularly within the central nervous system (CNS). Sialic acid-binding immunoglobulin-like lectins (SIGLECs) expressed on microglia are key players in maintaining microglial homeostasis by recognizing intact sialylation. The finely balanced sialic acid-SIGLEC system ensures the prevention of excessive and detrimental immune responses in the CNS. However, loss of sialylation and SIGLEC receptor dysfunctions contribute to several chronic CNS diseases. Genetic variants of SIGLEC3/CD33, SIGLEC11, and SIGLEC14 have been associated with neurodegenerative diseases such as Alzheimer's disease, while sialyltransferase ST8SIA2 and SIGLEC4/MAG have been linked to psychiatric diseases such as schizophrenia, bipolar disorders, and autism spectrum disorders. Consequently, immune-modulatory functions of polysialic acids and SIGLEC binding antibodies have been exploited experimentally in animal models of Alzheimer's disease and inflammation-induced CNS tissue damage, including retinal damage. While the potential of these therapeutic approaches is evident, only a few therapies to target either sialylation or SIGLEC receptors have been tested in patient clinical trials. Here, we provide an overview of the critical role played by the sialic acid-SIGLEC axis in shaping microglial activation and function within the context of neurodegeneration and synaptopathies and discuss the current landscape of therapies that target sialylation or SIGLECs.

Development and validation of a 13-gene signature associated with immune function for the detection of Alzheimer's disease

Current knowledge of Alzheimer's disease (AD) etiology and effective therapy remains limited. Thus, the identification of biomarkers is crucial to improve the detection and treatment of patients with AD. Using robust rank aggregation method to analyze the microarray data from Gene Expression Omnibus database, we identified 1138 differentially expressed genes in AD. We then explored 13 hub genes by weighted gene co-expression network analysis, least absolute shrinkage, and selection operator, and logistic regression in the training dataset. The detection model, which composed of CD163, CDC42SE1, CECR6, CSF1R, CYP27A1, EIF4E3, H2AFJ, IFIT2, IL10RA, KIAA1324, PSTPIP1, SLA, and TBC1D2 genes, along with APOE gene, showed that the area under the curve for detecting AD was 0.821 (95% confidence interval [CI] = 0.782-0.861) and the model was validated in ADNI dataset (area under the curve = 0.776; 95%CI = 0.686-0.865). Notably, the 13 genes in the model were highly enriched in immune function. These findings have implications for the detection and therapeutic target of AD.

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.

The Diagnostic Potential of Circulating Autoantibodies to Amyloid-β in Alzheimer's Disease

BACKGROUND

The profile of naturally occurring antibodies to amyloid-β (NAbs-Aβ) is altered in patients with Alzheimer's disease (AD). However, the diagnostic potential of NAbs-Aβ for AD is not clear yet.

OBJECTIVE

This study aims to investigate the diagnostic capacities of NAbs-Aβ for AD.

METHODS

A total of 40 AD patients and 40 cognitively normal (CN) controls were enrolled in this study. Levels of NAbs-Aβ were detected by ELISA. The correlations of NAbs-Aβ levels with cognitive function and AD-associated biomarkers were examined by Spearman correlation analysis. Diagnostic abilities of NAbs-Aβ were evaluated by the receiver operating characteristic (ROC) curve analyses. The integrative diagnostic models were established by logistic regression models.

RESULTS

We found that NAbs-Aβ7-18 had the highest diagnostic capability (AUC = 0.72) among all single NAbs-Aβ. The combined model (NAbs-Aβ7-18, NAbs-Aβ19-30, and NAbs-Aβ25-36) had a noticeable improvement (AUC = 0.84) in the diagnostic capacity compared with each single NAbs-Aβ.

CONCLUSION

NAbs-Aβs are promising in the diagnosis of AD. Further investigations are needed to confirm the translational potential of this diagnostic strategy.

Systems Genetic Identification of Mitochondrion-Associated Alzheimer's Disease Genes and Implications for Disease Risk Prediction

Cumulative evidence has revealed the association between mitochondrial dysfunction and Alzheimer’s disease (AD). Because the number of mitochondrial genes is very limited, the mitochondrial pathogenesis of AD must involve certain nuclear genes. In this study, we employed systems genetic methods to identify mitochondrion-associated nuclear genes that may participate in the pathogenesis of AD. First, we performed a mitochondrial genome-wide association study (MiWAS, n = 809) to identify mitochondrial single-nucleotide polymorphisms (MT-SNPs) associated with AD. Then, epistasis analysis was performed to examine interacting SNPs between the mitochondrial and nuclear genomes. Weighted co-expression network analysis (WGCNA) was applied to transcriptomic data from the same sample (n = 743) to identify AD-related gene modules, which were further enriched by mitochondrion-associated genes. Using hub genes derived from these modules, random forest models were constructed to predict AD risk in four independent datasets (n = 743, n = 542, n = 161, and n = 540). In total, 9 potentially significant MT-SNPs and 14,340 nominally significant MT-nuclear interactive SNPs were identified for AD, which were validated by functional analysis. A total of 6 mitochondrion-related modules involved in AD pathogenesis were found by WGCNA, from which 91 hub genes were screened and used to build AD risk prediction models. For the four independent datasets, these models perform better than those derived from AD genes identified by genome-wide association studies (GWASs) or differential expression analysis (DeLong’s test, p  <  0.05). Overall, through systems genetics analyses, mitochondrion-associated SNPs/genes with potential roles in AD pathogenesis were identified and preliminarily validated, illustrating the power of mitochondrial genetics in AD pathogenesis elucidation and risk prediction.