A single nuclear transcriptomic characterisation of mechanisms responsible for impaired angiogenesis and blood-brain barrier function in Alzheimer's disease

Brain perfusion and blood-brain barrier (BBB) integrity are reduced early in Alzheimer's disease (AD). We performed single nucleus RNA sequencing of vascular cells isolated from AD and non-diseased control brains to characterise pathological transcriptional signatures responsible for this. We show that endothelial cells (EC) are enriched for expression of genes associated with susceptibility to AD. Increased β-amyloid is associated with BBB impairment and a dysfunctional angiogenic response related to a failure of increased pro-angiogenic HIF1A to increased VEGFA signalling to EC. This is associated with vascular inflammatory activation, EC senescence and apoptosis. Our genomic dissection of vascular cell risk gene enrichment provides evidence for a role of EC pathology in AD and suggests that reducing vascular inflammatory activation and restoring effective angiogenesis could reduce vascular dysfunction contributing to the genesis or progression of early AD.

Avoiding false discoveries in single-cell RNA-seq by revisiting the first Alzheimer's disease dataset

Mathys et al. conducted the first single-nucleus RNA-seq (snRNA-seq) study of Alzheimer's disease (AD) (Mathys et al., 2019). With bulk RNA-seq, changes in gene expression across cell types can be lost, potentially masking the differentially expressed genes (DEGs) across different cell types. Through the use of single-cell techniques, the authors benefitted from increased resolution with the potential to uncover cell type-specific DEGs in AD for the first time. However, there were limitations in both their data processing and quality control and their differential expression analysis. Here, we correct these issues and use best-practice approaches to snRNA-seq differential expression, resulting in 549 times fewer DEGs at a false discovery rate of 0.05. Thus, this study highlights the impact of quality control and differential analysis methods on the discovery of disease-associated genes and aims to refocus the AD research field away from spuriously identified genes.

Translocator protein is a marker of activated microglia in rodent models but not human neurodegenerative diseases

Microglial activation plays central roles in neuroinflammatory and neurodegenerative diseases. Positron emission tomography (PET) targeting 18 kDa Translocator Protein (TSPO) is widely used for localising inflammation in vivo, but its quantitative interpretation remains uncertain. We show that TSPO expression increases in activated microglia in mouse brain disease models but does not change in a non-human primate disease model or in common neurodegenerative and neuroinflammatory human diseases. We describe genetic divergence in the TSPO gene promoter, consistent with the hypothesis that the increase in TSPO expression in activated myeloid cells depends on the transcription factor AP1 and is unique to a subset of rodent species within the Muroidea superfamily. Finally, we identify LCP2 and TFEC as potential markers of microglial activation in humans. These data emphasise that TSPO expression in human myeloid cells is related to different phenomena than in mice, and that TSPO-PET signals in humans reflect the density of inflammatory cells rather than activation state.

A70 FECAL-ADHERENT MUCUS IS A NON-INVASIVE SOURCE OF PRIMARY HUMAN MUC2 FOR STRUCTURAL AND FUNCTIONAL CHARACTERIZATION IN HEALTH AND DISEASE

Background

The gel-forming O-glycoprotein Mucin-2 (MUC2) is a key mediator of host-microbe homeostasis in part by forming a barrier to segregate inflammatory microbes from distal colon tissues. While animal models have provided insights into how Muc2 functions, relatively few studies have explored human MUC2. This is due to difficulty in accessing primary MUC2 which has been traditionally seen as firmly adherent to tissues and thus attainable mainly through invasive approaches (e.g. surgery, biopsies, etc), or via transformed cell lines (e.g. LS174T). This highlights a need to find alternative sources of MUC2.

Purpose

The purpose of this study is to develop a non-invasive method to analyze human MUC2 from healthy persons and determine if this can be applied to disease states. Recent studies have shown a significant portion of MUC2 is bound to feces (Bergstrom and Shan et al, 2020). We therefore reasoned this fecal MUC2 was accessible for both purification and structural and functional characterization.

Method

We purified MUC2 from feces via established extraction methods used for tissues. The mucins were resolved by composite urea agarose polyacrylamide gel electrophoresis (UreaAgPAGE) and analyzed by in-gel staining with Alcian Blue (AB), or Western blot for lectins and MUC2. Mucins were subjected to both proteomics to confirm enrichment of MUC2; and O-glycomics via non-reductive ammonia-catalyzed β-elimination followed by capillary electrophoresis (CE) and mass spectrometry in parallel with Type III porcine gastric mucin (PGM) O-glycans for comparison. Purified O-glycans were tested functionally via microbial growth assays with Bacteroides spp, and the ability to be metabolized into short-chain fatty acids (SCFA). Mucus barrier function was also visualized directly on Carnoy's-fixed paraffin-embedded (CFPE) fecal sections followed by dual staining for bacteria by FISH, and MUC2 and/or lectins.

Result(s)

Confocal imaging of CFPE fecal sections revealed a microbiota-encapsulating barrier layer of varying thickness among various healthy human subjects. UreaAgPAGE showed high molecular weight bands (~1 – 2 MDa) by AB staining. Proteomics and western analysis confirmed MUC2 enrichment in fecal mucin preparations. Western analysis via a lectin panel showed human MUC2 bound several lectins but was notably lacking in signal for Sambucus nigra lectin (SNA; α2,6-linked sialic acid) or Ulex europaeus agglutinin I (UEA1; α1,2-linked fucose). O-glycomics revealed extensive sialylation, moderate sulfation, and very little fucosylation vs. PGM. Functionally, the glycans supported growth of Bacteroides thetaiotaomicron as well as B.theta-dependent SCFA production in vitro. Pilot studies with human Ulcerative Colitis (UC) showed intact MUC2 with a differential O-glycosylation profile by glycan "fingerprinting" via CE.

Conclusion(s)

These studies highlight a new way to access primary human MUC2 for downstream functional analyses and pave the way for characterizing MUC2 dysfunction in diseases including IBD.

Disclosure of Interest

None Declared

A71

SIGNIFICANT RACIAL/ETHNIC DIFFERENCES EXIST IN THE RECEIPT OF IBD-RELATED SURGERY

A SYSTEMATIC REVIEW AND META-ANALYSIS

T. Chhibba^*, P. Tandon, N. Natt, G. Brar, G. Malhi, G. Nguyen

Background

Patients with inflammatory bowel disease (IBD) may require surgical intervention for management of their disease. There is a rising incidence of IBD in racial and ethnic minorities but studies regarding healthcare utilization patterns in these populations have yielded variable results.

Purpose

We aimed to examine the differences in surgical rates of ethnic and racial groups compared to White patients with IBD.

Method

Electronic databases were searched through December 20, 2021. Studies that compared ulcerative colitis (UC) or Crohn’s disease (CD) surgery rates between different racial/ethnic groups were included. Both pediatric and adult studies were included. Pooled event rates were generated and p-value < 0.05 was considered statistically significant in generating odds ratios (OR) with 95% confidence interval (CI). We also compared differences in disease location, phenotype, and IBD-medication exposure amongst different groups included.

Result(s)

Forty-one studies stratified rates of IBD-related surgeries by race or ethnicity (n=1,094,693 patients). Black patients were less likely to undergo IBD-related surgeries compared to White patients (pooled OR 0.70, 95% CI, 0.55-0.89, I2=87.0%). Black patients were also less likely compared to White patients to undergo an emergent colectomy with an incidence rate ratio of 0.43 (95% CI, 0.32-0.58). Furthermore, Hispanic patients were less likely to undergo a CD-related surgery (pooled OR 0.57, 95% CI, 0.48-0.68, I2=0%) compared to White patients. Finally, Asian patients had no significant difference in likelihood of CD-related and UC-related surgeries compared to White patients. Black patients were more likely to have perianal disease (pooled OR 1.40, 95% CI, 1.06-1.86), I2=58.2%) but otherwise disease characteristics and phenotypes were similar across all populations compared to Caucasians.

Conclusion(s)

Black and Hispanic patients with IBD are less likely to have surgery, including emergent surgery, for IBD compared to White patients with IBD, despite similar disease phenotype characteristics. Disparities in access to care may be contributory toward these findings and efforts should be made to provide equitable care to all persons living with IBD, regardless of race and ethnicity.

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Nil

Disclosure of Interest

None Declared

Alzheimer's disease-related transcriptional sex differences in myeloid cells

Sex differences have been identified in many diseases associated with dysregulated immune responses, including Alzheimer's disease (AD), for which approximately two-thirds of patients are women. An accumulating body of research indicates that microglia may play a causal role in the pathogenesis of this disease. We hypothesised that sex differences in the transcriptome of human myeloid cells may contribute to the sex difference observed in AD prevalence. To explore this, we assessed bulk and single-nuclear RNA sequencing data sets generated from four human derived myeloid cell populations: post-mortem microglial nuclei, peripheral monocytes, monocyte-derived macrophages (MDMs) and induced pluripotent stem cell derived microglial-like cells (MGLs). We found that expression of AD risk genes, gene signatures associated with the inflammatory response in AD, and genes related to proinflammatory immune responses were enriched in microglial nuclei isolated from aged female donors without ante-mortem neurological disease, relative to those from males. In addition, these inflammation-associated gene sets were found to be enriched in peripheral monocytes isolated from postmenopausal women and in MDMs obtained from premenopausal individuals relative to age-matched males. Expression of these gene sets did not differ in MDMs derived from women whose blood was sampled across the menstrual cycle or in MGLs cultured with 17β-oestradiol. This suggests that the observed gene set enrichments in myeloid cells from women were not being driven by acute hormonal influences. Together, these data support the hypothesis that the increased prevalence of AD in women may be partly explained by a myeloid cell phenotype biased towards expression of biological processes relevant to AD.

Diverse human astrocyte and microglial transcriptional responses to Alzheimer's pathology

To better define roles that astrocytes and microglia play in Alzheimer’s disease (AD), we used single-nuclei RNA-sequencing to comprehensively characterise transcriptomes in astrocyte and microglia nuclei selectively enriched during isolation post-mortem from neuropathologically defined AD and control brains with a range of amyloid-beta and phospho-tau (pTau) pathology. Significant differences in glial gene expression (including AD risk genes expressed in both the astrocytes [CLU, MEF2C, IQCK] and microglia [APOE, MS4A6A, PILRA]) were correlated with tissue amyloid or pTau expression. The differentially expressed genes were distinct between with the two cell types and pathologies, although common (but cell-type specific) gene sets were enriched with both pathologies in each cell type. Astrocytes showed enrichment for proteostatic, inflammatory and metal ion homeostasis pathways. Pathways for phagocytosis, inflammation and proteostasis were enriched in microglia and perivascular macrophages with greater tissue amyloid, but IL1-related pathway enrichment was found specifically in association with pTau. We also found distinguishable sub-clusters in the astrocytes and microglia characterised by transcriptional signatures related to either homeostatic functions or disease pathology. Gene co-expression analyses revealed potential functional associations of soluble biomarkers of AD in astrocytes (CLU) and microglia (GPNMB). Our work highlights responses of both astrocytes and microglia for pathological protein clearance and inflammation, as well as glial transcriptional diversity in AD.