Exploring the Disease-Associated Microglia State in Amyotrophic Lateral Sclerosis

BACKGROUND

Neuroinflammation, and specifically microglia, plays an important but not-yet well-understood role in the pathophysiology of amyotrophic lateral sclerosis (ALS), constituting a potential therapeutic target for the disease. Recent studies have described the involvement of different microglial transcriptional patterns throughout neurodegenerative processes, identifying a new state of microglia: disease-associated microglia (DAM). The aim of this study is to investigate expression patterns of microglial-related genes in ALS spinal cord.

METHODS

We analyzed mRNA expression levels via RT-qPCR of several microglia-related genes in their homeostatic and DAM state in postmortem tissue (anterior horn of the spinal cord) from 20 subjects with ALS-TDP43 and 19 controls donors from the Navarrabiomed Biobank.

RESULTS

The expression levels of TREM2, MS4A, CD33, APOE and TYROBP were found to be elevated in the spinal cord from ALS subjects versus controls (p-value < 0.05). However, no statistically significant gene expression differences were observed for TMEM119, SPP1 and LPL.

CONCLUSIONS

This study suggests that a DAM-mediated inflammatory response is present in ALS, and TREM2 plays a significant role in immune function of microglia. It also supports the role of C33 and MS4A in the physiopathology of ALS.

CD20 is a mammalian odorant receptor expressed in a subset of olfactory sensory neurons that mediates innate avoidance of predators

The mammalian olfactory system detects and discriminates between millions of odorants to elicit appropriate behavioral responses. While much has been learned about how olfactory sensory neurons detect odorants and signal their presence, how specific innate, unlearned behaviors are initiated in response to ethologically relevant odors remains poorly understood. Here, we show that the 4-transmembrane protein CD20, also known as MS4A1, is expressed in a previously uncharacterized subpopulation of olfactory sensory neurons in the main olfactory epithelium of the murine nasal cavity and functions as a mammalian odorant receptor that recognizes compounds produced by mouse predators. While wild-type mice avoid these predator odorants, mice genetically deleted of CD20 do not appropriately respond. Together, this work reveals a novel CD20-mediated odor-sensing mechanism in the mammalian olfactory system that triggers innate behaviors critical for organismal survival.

Renewal and Differentiation of GCD Necklace Olfactory Sensory Neurons

Both canonical olfactory sensory neurons (OSNs) and sensory neurons belonging to the guanylate cyclase D (GCD) "necklace" subsystem are housed in the main olfactory epithelium, which is continuously bombarded by toxins, pathogens, and debris from the outside world. Canonical OSNs address this challenge, in part, by undergoing renewal through neurogenesis; however, it is not clear whether GCD OSNs also continuously regenerate and, if so, whether newborn GCD precursors follow a similar developmental trajectory to that taken by canonical OSNs. Here, we demonstrate that GCD OSNs are born throughout adulthood and can persist in the epithelium for several months. Phosphodiesterase 2A is upregulated early in the differentiation process, followed by the sequential downregulation of β-tubulin and the upregulation of CART protein. The GCD and MS4A receptors that confer sensory responses upon GCD neurons are initially expressed midway through this process but become most highly expressed once CART levels are maximal late in GCD OSN development. GCD OSN maturation is accompanied by a horizontal migration of neurons toward the central, curved portions of the cul-de-sac regions where necklace cells are concentrated. These findings demonstrate that-like their canonical counterparts-GCD OSNs undergo continuous renewal and define a GCD-specific developmental trajectory linking neurogenesis, maturation, and migration.

The FcεRIβ homologue, MS4A4A, promotes FcεRI signal transduction and store-operated Ca2+ entry in human mast cells

Members of the membrane spanning 4A (MS4A) gene family are clustered around 11q12-13, a region linked to allergy and asthma susceptibility. Other than the known functions of FcεRIβ (MS4A2) and CD20 (MS4A1) in mast cell and B cell signaling, respectively, functional studies for the remaining MS4A proteins are lacking. We thus explored whether MS4A4A, a mast cell expressed homologue of FcεRIβ, has related functions to FcεRIβ in FcεRI signaling. We establish in this study that MS4A4A promotes phosphorylation of PLCγ1, calcium flux and degranulation in response to IgE-mediated crosslinking of FcεRI. We previously demonstrated that MS4A4A promotes recruitment of KIT into caveolin-1-enriched microdomains and signaling through PLCγ1. Caveolin-1 itself is an important regulator of IgE-dependent store-operated Ca²⁺ entry (SOCE) and promotes expression of the store-operated Ca²⁺ channel pore-forming unit, Orai1. We thus further report that MS4A4A functions through interaction with caveolin-1 and recruitment of FcεRI and KIT into lipid rafts. In addition to proximal FcεRI signaling, we similarly show that MS4A4A regulates Orai1-mediated calcium entry downstream of calcium release from stores. Both MS4A4A and Orai1 had limited effects with compound 48/80 stimulation, demonstrating some degree of selectivity of both proteins to FcεRI receptor signaling over Mas-related G Protein coupled receptor X2 signaling. Overall, our data are consistent with the conclusion that MS4A4A performs a related function to the homologous FcεRIβ to promote PLCγ1 signaling, SOCE, and degranulation through FcεRI in human mast cells and thus represents a new target in the regulation of IgE-mediated mast cell activation.

APOE-MS4A genetic interactions are associated with executive dysfunction and network abnormality in clinically mild Alzheimer's disease

PURPOSE OF THE RESEARCH

Although single nucleotide polymorphisms of membrane-spanning 4A (MS4A) (rs670139) and several other susceptibility genes have shown interaction effects on the risk of Alzheimer's disease (AD), little is known about the interaction effects of apolipoprotein E (APOE) with MS4A (rs670139) on cognitive performances, and the underlying pathogenesis is unclear. The study aimed to investigate the APOE-MS4A (rs670139) interaction effects on cognitive performances, cortical volumes, and functional connectivity (FC) in brain networks.

PRINCIPAL RESULTS

Cognitive performances were characterized in each genotypic group, and were compared between normal controls and patients in each genotypic group. APOE-MS4A interaction effects on memory and executive function scores, cortical volumes, and FC in brain networks were demonstrated. Significant effects of APOE-MS4A interactions on FC were observed in executive control network (ECN) (T maxima = 4.99, false discovery rate-corrected p < .001), the calculation score (F3, 87 = 6.218; p = .015), and the volume in prefrontal (F3, 87 = 4.374; p = .039) and orbitofrontal cortices (F3, 87 = 6.022; p = .016). The calculation score was correlated with each frontal volume (cc) (ρ = 0.304; p = .004) and genetic interaction-associated FC in ECN (ρ = 0.282; p = .008). Variations in genotypes affected the relationship between the calculation score and each frontal volume (cc).

MAJOR CONCLUSIONS

These findings indicate that the genetic interaction effects on FC in ECN might contribute to pathogenic mechanisms underlying the interaction effects of APOE-MS4A on calculation ability in AD.

Meta-analysis of association between the genetic polymorphisms on chromosome 11q and Alzheimer's disease susceptibility

Alzheimer's disease (AD) is a neurodegenerative disease mostly occurred in the elderly. Genetic mutation is one of well-established risk factors for AD. Several polymorphisms on chromosome 11q were reported to be associated with AD susceptibility. Hence we performed a meta-analysis to systematically assess the association between the most-reported polymorphisms on chromosome 11q (rs10793294, rs7115850, rs7101429, rs4945261, rs2373115, rs670142, rs610932, rs541458 and rs3851179) and AD risk. A comprehensive literature search in the electronic databases was performed to identify all eligible studies. The pooled odds ratios (OR) and 95% confidence intervals (95% CI) were calculated to evaluate the association between 11q variants and AD risk by using the allelic model. Sensitivity analysis was carried out to analyze the influence of single study on the overall results. Begg's funnel plots and Egger's test were used to assess the publication biases among studies. All the statistical analyses were conducted by using STATA 12.0 Software (Stata Corp, College Station, TX, USA). A total of 35 eligible articles were included in our meta-analysis. Our data showed that the polymorphism of rs610932 were significantly associated with lower AD risk with a pooled OR of 0.88 (95% CI: 0.84-0.92, P=0.005). The other SNPs of rs494526 (OR=0.83, 95% CI: 0.65-1.00, P<0.001), rs2373115 (OR=0.85, 95% CI: 0.75-0.95, P<0.001) and rs670139 (OR=1.09, 95% CI: 1.05-1.12, P=0.554) were shown to be correlated with lower AD risk. Subgroup analysis revealed a similar result in Caucasians. But only the rs610932 polymorphism was found to be associated with lower AD risk in Asians. The polymorphism of rs610932 was shown to be a risk factor for AD while the other three genetic variants (rs494526, rs2373115 and rs610932) may act as protective factors against AD.

Association of CD33 and MS4A cluster variants with Alzheimer's disease in East Asian populations

CD33 and MS4A cluster variants have been identified to modulate the risk of Alzheimer's disease (AD) in several recent genome-wide association studies (GWAS) in Caucasians. In the present study, we first conducted a case-control study to investigate the CD33 single nucleotide polymorphisms (SNPs) rs3865444 and rs3826656 and the MS4A cluster SNPs rs610932 and rs670139 in a cohort from eastern China that comprised 126 late-onset Alzheimer's disease (LOAD) patients and 129 healthy controls. The results revealed that the frequency of rs3826656 major (G) allele carriers was higher among the LOAD patients than among the controls [P=0.005; odds ratio (OR), 1.760; 95% confidence interval (CI), 1.185-2.615]. In apolipoprotein E (APOE) ε4 allele carriers, the G allele of the SNP rs3865444 was found to be associated with an increased risk of LOAD (P=0.002; OR, 3.391; 95% CI, 1.512-7.605). Next, we re-evaluated the association between these variants and LOAD by conducting a meta-analysis using data from studies of East Asian populations, including the present case-control study, and confirmed that rs3826656 increased the risk of LOAD. In addition, we identified a significant association between rs610932 and LOAD (P=0.035; OR, 0.79; 95% CI, 0.63-0.98). Note that heterogeneity should be considered during the interpretation of these results; significant heterogeneity was identified among studies on rs3865444, even in a subgroup analysis based on stratification of studies by the country of origin. In summary, our results suggest that CD33 and MS4A cluster variants are associated with LOAD susceptibility in East Asian populations.

Expression of MS4A and TMEM176 Genes in Human B Lymphocytes

The MS4A gene family in humans includes CD20 and at least 15 other genes. CD20 exists as homo-oligomers in the plasma membrane, however different MS4A proteins expressed in the same cell may hetero-oligomerize. Given the importance of CD20 in B-cell function and as a therapeutic target, we sought to explore the potential for CD20 hetero-oligomerization with other MS4A proteins. We investigated expression in primary human B-cells of the four MS4A genes previously shown to be expressed in human B-cell lines (MS4A4A, MS4A6A, MS4A7, MS4A8B), as well as two genes comprising the closely related TMEM176 gene family, with a view to identifying candidates for future investigation at the protein level. TMEM176A and TMEM176B transcripts were either not detected, or were detected at relatively low levels in a minority of donor B-cell samples. MS4A4A and MS4A8B transcripts were not detected in any normal B-cell sample. MS4A6A and MS4A7 transcripts were detected at low levels in most samples, however the corresponding proteins were not at the plasma membrane when expressed as GFP conjugates in BJAB cells. We also examined expression of these genes in chronic lymphocytic leukemia (CLL), and found that it was similar to normal B-cells with two exceptions. First, whereas MS4A4A expression was undetected in normal B-cells, it was expressed in 1/14 CLL samples. Second, compared to expression levels in normal B-cells, MS4A6A transcripts were elevated in 4/14 CLL samples. In summary, none of the MS4A/TMEM176 genes tested was expressed at high levels in normal or in most CLL B-cells. MS4A6A and MS4A7 were expressed at low levels in most B-cell samples, however the corresponding proteins may not be positioned at the plasma membrane. Altogether, these data suggest that CD20 normally does not form hetero-oligomers with other MS4A proteins and that there are unlikely to be other MS4A proteins in CLL that might provide useful alternate therapeutic targets.

Association of GWAS-linked loci with late-onset Alzheimer's disease in a northern Han Chinese population

OBJECTIVE

Five genomewide association studies (GWAS) in white populations have recently identified and confirmed 9 novel Alzheimer's disease (AD) susceptibility loci (CLU, CR1, PICALM, BIN1, ABCA7, MS4A gene cluster, CD2AP, CD33, and EPHA1). These studies have been conducted almost exclusively in white populations and it is unclear whether these observations generalize to populations with different ethnicities.

METHODS

We recruited 1224 unrelated northern Han Chinese subjects comprising 612 patients with a clinical diagnosis of late-onset AD (LOAD) according to the criteria of the National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Disorders Association and 612 healthy age- and sex-matched control subjects. Because of our previous study investigating CLU, CR1, and PICALM in the Han population, we limited the current analysis to BIN1, ABCA7, MS4A gene cluster, CD2AP, CD33, and EPHA1.

RESULTS

In a multivariate analysis, associations of MS4A6A (rs610932; odds ratio = 0.632, Bonferroni corrected P = .019) and CD33 (rs3865444; odds ratio = 1.492, Bonferroni corrected P = .017) with LOAD were replicated successfully. When these data were stratified by apolipoprotein E (APOE) ε4 status, both rs610932 and rs610932 were evident only among subjects without the APOE ε4 allele. For BIN1, assuming a dominant model of inheritance, a positive association for rs7561528 in APOE ε4 carriers was observed. This association, however, did not remain significant after Bonferroni correction. As for ABCA7, CD2AP, and EPHA1 single nucleotide polymorphisms from recent GWAS, despite the similar directional effects, no significant differences in genotype and estimated allele frequency distribution between patients and control subjects were observed.

CONCLUSIONS

This study provides the first independent evidence that MS4A and CD33 loci are associated with the risk of LOAD in northern Han Chinese population. Genotypes at the two loci confer risk predominantly in APOE ε4-negative subjects.