CircNF1-419 improves the gut microbiome structure and function in AD-like mice

Gut microbiota plays an important role in the circRNA sequences in AD-like mice

To understand the network of circRNAs acting on the microbiome-gut-brain axis, the gut microbiota (Supplementary Figure 1A–1B), serum metabolites (Supplementary Figure 2A–2B), and circRNA sequences of brain tissues (Supplementary Table 1) from 8-month-old APP/PS1 mice were subjected to choice-based conjoint analysis. The interactions of gut microbiota and metabolites are shown in Figure 1A–1B, which revealed that Adlercreutzia (Actinobacteria), Streptococcus, Lactobacillus,Ruminococcus and Prevotella may be the key bacteria regulating serum metabolites such as L-acetylcarnitine, 11-beta-hydroxyandrosterone- 3-glucuronide, phosphatidylethanolamine and phosphorus esters. With the gut microbiota and serum metabolite changes, there were 501 differentially expressed circRNAs in the model group (fold change > 1.50, p < 0.05 vs normal C57 mice, Figure 2A) up-regulated, and 624 circRNAs down-regulated, including chr14_84532875_ 84533695_+ (Pcdh17), chr1_140425193_ 140450304_+ (Kcnt2), chr8_94047272_ 94055344_+ (Ogfod1), chr3_158104428_ 158148699_- (Lrrc7), chr10_62633977_ 62643372_- (Ddx50), chr12_69603476_69607412_- (Sos2), chr12_85028770_85030887_+ (Ylpm1), chr10_93208172_93218089_+ (Cdk17), chr11_79546971_79551134_+ (NF1), and chr6_52609145_52630349_- (Hibadh); more details are shown in Supplementary Table 1, (PRJNA553830).

Figure 1. The combined data analysis between gut microbiome and serum metabolome of APP/PS1 mice. (A) The Spearman correlation coefficient for each metabolite type and each bacterial genus, see also Supplementary Figure 1A–1B and Supplementary Figure 2A–2B. (B) Serum metabolome and genera. Red and blue indicate positive and negative correlations, see also Supplementary Figure 1A–1B and Supplementary Figure 2A–2B. Hub nodes with the most connections are highlighted in red. Data are presented as the means of more than 6 independent experiments.

Figure 2. (A) Differentially expressed circRNAs in the APP/PS1 double transgenic mice brain samples (fold change > 1.50, p < 0.05 vs normal C57 mice, n=3). (B) Differentially expressed mRNAs in the APP/PS1 double transgenic mice brain samples (fold change > 1.50, p < 0.05 vs normal C57 mice, n=3). (C) Rarefaction curves based on OTU. The structure at the (D) phylum and (E) genus levels. (F) Histopathological changes in the brain identified using H&E staining and biomarker expression of Tau and Aβ_1-42 using immunohistochemical methods after fecal microbiota from six months APP/PS1 mice transplantated into 9 months old C57 mice, (n≥6).

Given that the function of most circRNAs is unknown, the mRNA sequence was concurrently analyzed to offer an alternative explanation. Using RNA-sequencing, 347 genes were up-regulated and 507 genes were down-regulated (fold change > 1.50, p < 0.05, Figure 2B) in brain tissues dissected from APP/PS1 double transgenic mice. The networks of Ingenuity Canonical Pathways (IPA) data showed that the diseases and functions of cellular movement, cell death and survival might be regulated by molecules in a network including 26s Proteasome, AGT, ALB, BCR (complex), BLNK, CA3, CASP1, CCND2, CD4, CDT1, CENPF, DAPK1, DIO2, DSP, EGR1, Focal adhesion kinase, FST, HGF, IgG, KDELR3, KIF22, LBX1, LY9, MAPK12, MMP19, NEU4, NOD2, P38 and MAPK, PRNP, RIN1, S100A8, S100A9, SDC1 and ST14; cell-to-cell signaling and interaction, nervous system development and function, cellular function and maintenance might be regulated by molecules in a network including Akt, Alp, CAMK2A, CAMK2N1, CCKBR, CHRM3, CLEC7A, CSF2RB, CYBB, EPHA4, ERK, FGF7, FLT3, FOSL2, Growth hormone, IL23, IRX1, KCNA5, KLF10, MAPK, MGLL, MIA, MMD, NPY, PLK2, PPP1R1B, RAG1, RGS4, RORC, SNCA, SPARC, STAT5a/b, TLR2, VEGF, WNT9A mRNAs (Supplementary Figure 3). Additional studies are needed in this area.

GO (gene ontology) enrichment analysis of differentially expressed circRNAs showed that they mainly influenced synaptic transmission, post-synapse, positive regulation of neurogenesis, single-organism behavior, protein serine/threonine kinase activity, histone modification, regulation of GTPase activity, and neuron death. KEGG enrichment analysis showed that they mainly influenced MAPK, Rap1, cAMP, glutamatergic synapse, neurotrophin and Rap1 signaling pathways. Reactome enrichment analysis of differentially expressed circRNAs showed that they mainly influenced developmental biology, signaling by Rho GTPases, neuronal system, signaling by NGF, axon guidance, signaling by EGFR, transmission across chemical synapses, NGF signaling via TRKA from the plasma membrane, organelle biogenesis and maintenance, and signaling by FGFR1~4. Bioinformatics analyses of circRNA and mRNA annotation, functional classification, functional enrichment and cluster analyses showed that the changes of gut microbiota and serum metabolites could influence the circRNA sequences and function. This conclusion requires additional evidence, particularly clinical sample support, and the mechanism remains poorly understood and needs further investigation.

In order to understand the connections between the gut microbiota and brain function, an additional experiment was carried out using fecal microbiota transplantation. Fecal samples were collected from 8-month-old APP/PS1 mice, PBS was added and the bacterial solution was concentrated immediately by centrifugation. C57 mice (9 months old) were treated with the bacterial solution by enema, for 3 weeks, after the mixture of antibiotics according to the method of de Groot PF, et al. [59]. Microbiome analysis of the 16S rRNA genes was performed after 3 months. Gut microbiota from the APP/PS1 mice settled in the normal mice (Figure 2C–2E), influenced the intestinal microenvironment (Figure 2F, H&E of intestine), and up-regulated the expression of Tau and Aβ_1-42 (Figure 2F) in the hippocampus. Together these results and those of our previous study demonstrating that the most highly abundant gut microbiota were correlated with the level of Aβ_1-42 [3], led us to summarize that the gut microbiota have some interactions with the circRNA sequences of the brain in AD-like mice, and targeting the gut microbiota might be a feasible and effective strategy for ameliorating the symptoms or even delaying the progression of AD. Thus the gut microbiota plays an important role in the progression of AD.

CircNF1-419 changes the intestinal physiology and gut microbiota in SAMP8 mice

Recently, a report demonstrated that avian leucosis virus targets circ-Vav3 and then sponges gga-miR-375 to promote epithelial-mesenchymal transition [38], indicating that a microorganism could directly influence the formation, expression and function of circRNAs, and the mysterious of circRNA is suggested in increasing numbers of studies [30, 38, 39]. In order to verify these, the expression level of circRNAs was examined by qRT-PCR in AD-like mice (PCR primers are listed in Table 1). The circRNAs of circ_zfyve1-504, circ_zcchc11-811, circ_zfp652-1147, circ_zfp236-1257, circNF1-419 and circ_zranb1-1575 were differentially expressed (Figure 3A), suggesting that these circRNAs may be related to AD. We then over-expressed circNF1-419 and circ_0001239 in the animal brain using an adeno-associated virus (AAV) system, with the aim of identifying possible connections.

Figure 3. CircRNA NF1-419 influences the gut microbiota composition and cholinergic system. (A) The circRNAs of circ_zfyve1-504, circ_zcchc11-811, circ_zfp652-1147, circ_zfp236-1257, circNF1-419 and circ_zranb1-1575 by reverse transcription polymerase chain reaction in Ganoderma lucidum extractions (LZ) (oral LZ of 50 mg/[kg·d]) and Hericium erinaceus extractions (HE) (oral HE of 50 mg/[kg·d]) treated SAMP8 mice brain samples lasting 24 weeks. (B) Over expression of circNF1-419 in brain could change the gut microbiota at phylum (a) and genus level (b) of SAMP8 mice, see also in Supplementary Figure 8. (C) Expression of the proteins AChE, AMP, CHRNA1 and CHRNB1 in the brain tissues of SAMP8 mice after infection of over-expressing circNF1-419 AAV. (D) Expression of the proteins AChE, AMP, CHRNA1 and CHRNB1 in the brain tissues of 12-month-old mice after infection of over-expressing circNF1-419 AAV. (E) Expression of the proteins AChE, AMP, CHRNA1 and CHRNB1 in the brain tissues of 2-month-old mice after infection of over-expressing circNF1-419 AAV. Data are presented as the means ± SD of more than 8 independent experiments, and more than 3 independent experiments in Western bolting. ^*p <0.05 and ^**p <0.01 vs. the model group by one-way ANOVA, followed by the Holm-Sidak test.

First, we utilized our previous designed AAV viral transduction system with RNA interference (sicircNF1-419) and separately an over-expressing circNF1-419 (sscircNF1-419) (Supplementary Figure 4). Two microliters of the AAV packaging system (virus titer 1 × 10¹²) were injected into the cerebral cortex of SAMP8 mice. Statistical analysis of the 16S rRNA gene sequencing data showed that sscircNF1-419 can significantly change the gut microbiota composition of the SAMP8 mice 7 weeks after infection – the relative abundance of Bacteroidetes, Actinobacteria, Deferribacteria and Cyanobacteria were increased (Figure 3B, p < 0.05 vs SAMP8 group) at the phylum level, and the relative abundance of Bacteroides, Alloprevotella, Lactobacillus, Lachnoclostridium and Ruminiclostridium 9 were changed (Figure 3B, p < 0.05 vs SAMP8 group) at the genus level. Previous studies indicated that people who eat plenty of protein and animal fats have predominantly Bacteroides bacteria, while for those who consume more carbohydrates, Prevotella species dominate, which is due to the fact that the main sources of energy for Bacteroides species in the gut are complex host-derived and plant glycans [40]. Studies have revealed that the presence of Prevotella in the human gastrointestinal tract is inversely correlated with Parkinson’s disease [41, 42], and members of the family Lachnospiraceae could protect against colon cancer in humans by producing butyric acid with the functions of anti-inflammation and immunomodulation [43]. Histopathological observations showed that circNF1-419 could improve the damage of intestinal tissues in SAMP8 mice and 12-month-old mice (Figure 4A), expression of AChE and AMP, CHRNB1 and CHRNA1 in the brain were improved (Figures 4C–4D), and expression of NF-κB p65 was activated (Figure 19B, p < 0.05). This all indicated that circNF1-419 in the brain could improve the central cholinergic system and improve intestinal physiology in AD-like mice.

Figure 4. CircNF1-419 changes the gut microbiota genetic trajectory in newborn KM mice. (A) Intestinal physiology changes after infection of over-expressing circNF1-419 AAV. (B) Heatmap of gut microbiota in fimo at phylum level after infection of over-expressing circNF1-419 AAV in young mice. (C) Heatmap of gut microbiota infimo at genus level after infection of over-expressing circNF1-419 AAV in young mice. Data are presented as the means ± SD of more than 8 independent experiments. ^*p <0.05 and ^**p <0.01 vs. the model group by one-way ANOVA, followed by the Holm-Sidak test.

CircNF1-419 changes the gut microbiota genetic trajectory in newborn KM mice

To elucidate how circNF1-419 in the brain influences the gut microbiota, we hypothesized that new discoveries could be found via monitoring the colonization of gut microbiota in newborn KM mice. Two microliters of the AAV (over-expressing circNF1-419) packaging system (virus titer 1 × 10¹²) were injected into the cerebral cortex of 2-week-old KM mice, whose mother was fed with standard diet or the HSHF diet.

Histopathological observations showed that circNF1-419 could improve the damage of intestinal tissues in mice fed with the HSHF diet (Figure 4A). 16S rRNA gene sequencing revealed that the relative abundance of Bacteroidetes, Actinobacteria, Deferribacteria and Cyanobacteria were changed (p < 0.05 vs normal group, Figure 4B) at the phylum level, and the relative abundance of Bacteroidales S24-7 group, Lachnospiraceae NK4A136 group, Alistipes, Alloprevotella, Lachnospiraceae, Bacteroides, Desulfovibrio, Lactobacillus, Roseburia and Helicobacter were changed (p < 0.05 vs normal group, Figure 4C) at the genus level 8 weeks after infecting with the AAV. The expression of AChE and AMP, CHRNB1 and CHRNA1 were improved (Figure 3E, p < 0.05). Additional findings were that interference of circNF1-419 aggravates some initial bacteria imbalance, including increasing the abundance of Actinobacteria, Deferribacteres, Proteobacteria while decreasing the Firmicutes (Figure 4B–4C), and circNF1-419 could improve these changes. This indicated that the gut microbiota had notable correlation with the host, and this certainly included the circRNAs.

Association analysis of circNF1-419, intestinal transcriptome and gut microbiota

To understand how circNF1-419 influences the gut microbiota, we integrated analysis of the association between gut microbiota (Figure 3B and 4C) and differential expression mRNAs of the small intestine tissue (Supplementary Figure 5A–5B) on the circNF1-419 treated SAMP8 and KM mice. As shown in Figure 3, circNF1-419 in the brain mainly influences the bacteria Candidatus Arthromitus, Lachnospiraceae FCS020 group, Lachnospiraceae UCG-006, and [Eubacterium] xylanophilum group, and the mRNAs of Gal3st1 (galactose-3- O-sulfotransferase 1), Gamt (guanidinoacetate methyltransferase), Gpr62 (G protein-coupled receptor 62), and Opalin (oligodendrocytic myelin paranodal and inner loop protein). The relative abundance of those bacteria shown in Figures 3B and 4C, and the expression of mRNAs shown in Supplementary Figure 5A–5B, indicated that circNF1-419 in the brain plays a role on multiple targets in the intestine, but how circNF1-419 influences these mRNAs and the gut microbiota still needs considerable work on their metabolomics, signaling pathways and electrophysiology.

The mRNAs of brain tissue showed that the expression of GABRA1, GABRA6, St6galnac4, St8sia5, Pde9a, Tmem132a, Tmem163, Tmem62 and Tmem63c (Figure 5A and 5B) were up-regulated after over-expressed circNF1-419 in brain, which indicated that the central immune system were activated, in other words that circNF1-419 could activated central immune to improve the symptoms of AD, but need much more further study. And the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment of 16S functional gene prediction analysis using PICRUSt also showing altered signaling pathways (Figure 5C) and the network (Figure 5D). Although the function and signaling pathway need much more studies.

Figure 5. Association analysis of circNF1-419, intestinal transcriptome and gut microbiota. Influences on the expression of GABRA1, GABRA6, St6galnac4, St8sia5, Pde9a (A), Tmem132a, Tmem163, Tmem62 and Tmem63c (B) in brain tissues. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment of 16S functional gene prediction analysis using PICRUSt also showing altered signaling pathways (C) and the network (D). Data are presented as the means of more than 8 independent experiments, see also in Supplementary Figure 4.

Circular RNA circ_0001239 changes the gut microbiota genetic trajectory in mice

Additionally, we designed another AAV viral transduction system with RNA over-expressing mmu_circ_0001239 (ZCCHC11, Supplementary Figure 6). The function of this circRNA in the brain was unknown but its expression was increased in the APP/PS1 and SAMP8 mice (Figure 4A). Two microliters of the AAV packaging system (virus titer 1 × 10¹²) were injected into the cerebral cortex of 7-day-old mice. After 3 months continuous infection, the gut microbiota composition was detected using 16S rRNA gene sequencing. The results revealed that circ_0001239 can change the alpha diversity (Figure 6A) and beta diversity (Figure 6B) of the gut microbiota. The ternary phase diagram showed that the most of the bacteria were closed to the circ_0001239 group (Figure 6C), and the relative abundance of different bacteria is shown in Figure 6D (LDA < 3.5), for example increases in the Lachnospiraceae NK4B4 group, Lachnospiraceae and Lactobacillus, which indicated that over-expression of circ_0001239 in the brain could change the genetic trajectory of the gut microbiota.

Figure 6. Circ0001239 changes the gut microbiota genetic trajectory in new born mice. (A) The alpha diversity (A) and beta diversity (B) of the gut microbiota when the circ0001239 over expressed in brain, see also in Supplementary Figure 6. (C) The ternary phase diagram showed that the most of the bacteria were closed to the circ_0001239 group. (D) The relative abundance of different bacteria when the circ0001239 over expressed in brain (LDA < 3.5). Data are presented as the means ± SD of 8 independent experiments. ^*p <0.05 and ^**p < 0.01 vs. the model group by one-way ANOVA, followed by the Holm-Sidak test.

The miRNA and mRNA sequences of brain tissues from the circ_0001239-treated mice showed that expression of mmu-miR-452-5p, let-7j and miR-205-5p were down-regulated (Figure 7A, |log2(FC)|≥1, FDR≤0.05) compared to the control, and there were 10 mRNAs up-regulated while seven were down-regulated (Figure 7B). Most previous studies on miR-452-5p [44, 45], let-7 [46, 47] and miR-205-5p [48, 49] focused on cancers. KEGG analysis of the mRNA sequences indicated that over-expression of circ_0001239 in the brain mainly influences the ribosome and proteoglycans in cancer pathways, suggesting that circ_0001239 has the sponge miRNA function and mediates a series of chain reactions in the brain, then influences gut function and microbiome engraftment from their parent.

Figure 7. Effects of over expressing circ_0001239 on KM mice brain. (A) Nile red (red) and DAPI (blue) staining of BV2 cells bearing ss-ctrl or ss-circ0001239. Representative of three independent experiments. (B) MiRNA levels in over expressing circ_0001239 KM mice brain samples. Representative of three independent experiments. (C) Differently expressed mRNA in over expressing circ_0001239 KM mice brain samples. Representative of three independent experiments.

Animals

Male SAMP8 mice (5 months old, mean body weight 20 ± 5 g) were purchased from Beijing HFK Bioscience Co., LTD (Certificate No: SCXK [Jing] 2014-0004), and adult KM mice (18-22 g, 16 female and 8 male, 8 weeks old) were obtained from Center of Laboratory Animal of Guangdong Province, SCXK [Yue] 2008-0020, SYXK [Yue] 2008-0085). All the mice pair-housed in plastic cages in a temperature-controlled (25 ± 2°C) colony room with a 12/12-h light/dark cycle. The animals had free access to food and water. All experimental protocols were approved by the Center of Laboratory Animals of the Guangdong Institute of Microbiology. All efforts were made to minimize the number of animals used. All animals were allowed to acclimate for at least 1 week prior to the initiation of the experiments.

Then, SAMP8 mice were randomly allocated into 3 groups of 8 animals each: SAMP8 model group, sscircNF1-419-OV-AAV infected group, and sicircNF1-419-OV-AAV infected group, two μl of the AAV packaging system (virus titer 1 × 10¹²) were injected into the cerebral cortex.

The female KM mice were randomly allocated into 2 groups of 8 animals each, one fed with high sugar & fat diet, and the other fed with standard diet. A month later, the female mice were randomly re-allocated into groups of 2 animals each and one male mice, keep feeding the original diet to the end of pregnancy, then all the mice fed with standard diet. After births one week, two μl of the AAV packaging system (virus titer 1 × 10¹²) were injected into the brain.

Balb/c mice were obtained from the Center of Laboratory Animals of Guangdong Province of China, (SCXK [Yue] 2008-0020, SYXK [Yue] 2008-0085), conventional breeding to 12 months old, two μl of the AAV packaging system (virus titer 1 × 10¹²) were injected into the cerebral cortex.

Histopathology and immunostaining

The brain, colon tissues were removed and fixed in the 4% paraformaldehyde at pH 7.4 for further pathological observation. These tissue samples were made into paraffin sections after drawing materials, fixation, washing, dehydration, transparency, dipping in wax, and embedding. Obesity related parameters or other related pathologic changes were measured. The brains of animals were dissected. A total of four brains from each group were fixed in 4% paraformaldehyde solution and prepared as paraffin sections. Sections were stained with hematoxylin-eosin (H&E).

Microbiome analysis

Fresh intestinal content samples were collected before the fasting of the rats for 12 hour and stored at -80 °C. Microbial DNA isolated from the samples, with total masses ranging from 1.2 to 20.0 ng, were stored at -20 °C. The microbial 16S rRNA genes were amplified using the forward primer 5′-ACTCCTACG GGAGGCAGCA-3′ and the reverse primer 5′-GGA CTACHVGGGTWTCTAAT-3′, and the rat forward primer 5′-CCTAYGGGRBGCASCAG-3′ and reverse primer 5′-GGACTACNNGGGTATCTAAT-3′. Each amplified product was concentrated via solid-phase reversible immobilization and quantified by electrophoresis using an Agilent 2100 Bioanalyzer (Agilent Technologies, Santa Clara, CA, USA). After NanoDrop quantification of DNA concentration, each sample was diluted to a concentration of 1×10⁹ molecules/μl in TE buffer and pooled. 20μl of the pooled mixture was sequenced on an Illumina MiSeq sequencing system (Illumina Inc, San Diego, CA, USA) according to the manufacturer’s instructions.

Metabolomic analysis

After the water maze testing, the serum was acquired. 80 μL serum were added into 240 μL of cold mixture methanol with acetonitrile (2:1, v/v), and 10 μL internal tagging standard (L-2-chlorine-phenylalanine, 0.3 mg/mL, dissolved in methanol) were added, vortex after 2 min, then extracted using ultrasonic extraction method for 5 min. After 20 min standing at -20 °C, centrifuge for 10 min at low temperature (14000 prm, 4 °C), 200 μL of supernatant loaded into a sample bottle with lining tube for LC - MS analysis (Waters UPLC I-class system equipped with a binary solvent delivery manager and a sample manager, coupled with a Waters VION IMS Q-TOF Mass Spectrometer equipped with an electrospray interface (Waters Corporation, Milford, USA). LC Conditions: Column: Acquity BEH C18 column (100mm × 2.1mm i.d., 1.7μm; Waters, Milford, USA). Data analysis as described previously (Han, et al.).

RNA isolation and sequencing

Total RNA was isolated using Qiazol and miRNeasy Kits, including additional DNase I digestion. Then remove the ribosomal RNA using Ribo-Zero^TM Magnetic Gold Kit, enzymatic degradation of linear RNA using Rnase R enzyme. Add fragmentation buffer to be fragments, synthesize the first chain of cDNA with six bases random hexamers, then the buffer, dNTPs, RNase H and DNA polymerase I to synthesize the second chain of cDNA. Purification uing QiaQuick PCR kit and elution using EB buffer after terminal repairing, processing bases A, add sequencing joint. For next generation sequencing, 0.5 μg ribosomal RNA-depleted RNA was fragmented and primed. The sequencing libraries were constructed using Illumina TruSeq RNA Sample Preparation Kits and were sequenced by Illumina HiSeq^TM 2500 flowcell.

The bioinformatics analysis of RNA annotation, functional classification, functional enrichment and cluster analyses were performed [47, 50, 51].

Computational analysis of circRNAs

The reads were first mapped to the latest UCSC transcript set using Bowtie2 version 2.1.0 [52] and the gene expression level was estimated using RSEM v1.2.15 [53]. For lincRNA expression analysis, we used the transcripts set form Lncipedia (http://www.lncipedia.org). TMM (trimmed mean of M-values) was used to normalize the gene expression. Differentially expressed genes were identified using the edgeR program [54]. Genes showing altered expression with p < 0.05 and more than 1.5 folds changes were considered differentially expressed. The pathway and network analysis were performed using Ingenuity (IPA). IPA computes a score for each network according to the fit of the set of supplied focus genes. These scores indicate the likelihood of focus genes to belong to a network versus those obtained by chance. A score > 2 indicates a <= 99% confidence that a focus gene network was not generated by chance alone. The canonical pathways generated by IPA are the most significant for the uploaded data set. Fischer’s exact test with FDR option was used to calculate the significance of the canonical pathway.

For circRNA expression analysis, the reads were to mapped genome using the STAR [55] and DCC [56] was used to identify the cirRNA and to estimate the circRNA expression. TMM (trimmed mean of M-values) was used to normalize the gene expression. Differentially expressed genes were identified using the edgeR program. miRanda [57] was used to predict the miRNA target of the circRNA. R was used to generate the figure.

CircRNA verification by quantitative real-time polymerase chain reaction (qRT-PCR)

To validate the reliability of high-throughput RNA sequencing and to explore the expression of circRNAs during aging, the expression levels of circRNAs were examined by qRT-PCR. With reference to Memczak’s method [58], two sets of primers for each circRNA were designed using Primer Express software version 5.0 (Table 1): an outward-facing set which was expected to amplify only the circRNA, and an opposite-directed set to amplify the linear form.

Total RNA was extracted (TRIzol^® Reagent, Life technologies), digested using RNase R, and purified. cDNA was synthesized using a Geneseed^® II First Strand cDNA Synthesis Kit (Geneseed, USA). Outward-facing primers were designed to amplify the fragment across the junction from cDNA, then the fragment was sequenced by Sangon Biological Engineering Company (Shanghai China). QRT-PCR was performed using Geneseed^® qPCR SYBR^® Green Master Mix (Geneseed, USA), and PCR-specific amplification was conducted with an ABI 7500 (Applied Biosystems, USA). The expression of circRNAs was defined based on the threshold cycle (Ct), and relative expression levels were calculated via the 2^-ΔΔCt method. GAPDH served as an internal standard control with all reactions performed in triplicate.

Western bloting analysis

Global brain tissue was dissected from treated mice (purchased from the Beijing HFK Bioscience Co., LTD [Certificate No: SCXK (Jing) 2014-0004]) and proteins extracted with radioimmunoprecipitation assay (RIPA) lysis buffer (Thermo Scientific^TM T-PER^TM Tissue Protein Extraction Reagent, 78510). The proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred onto polyvinylidene fluoride membranes. After blocking with 5% nonfat dry milk in Tris-buffered saline (20 mM Tris-HCl, 500 mM NaCl, pH 7.4) with 0.2% Tween-20 (Aladdin, T104863), the membranes were probed with antibodies overnight at 4°C, followed by incubation with a horseradish peroxidase-conjugated goat anti-mouse (Servicebio, G2211-1-A) or goat anti-rabbit (Servicebio, G2210-2-A) IgG secondary antibody (1:2000). The antibodies were as follows; anti-Abeita A4, -APOE, -BACE1, -p53 (abcam, 66064), -p16 (Affinity, AF0228), -p21, -p-Tau-396, -TNF-α (abcam, 9739), -p-Tau-202, obtained from Affinity as well as GAPDH (CST, 2118L) and β-Actin (CST, 4970S). Band intensity was quantified using ImageJ software (NIH).

Statistical analysis

All data are described as the means ± standard deviations (SD) of at least three independent experiments. The significant differences between treatments were analyzed by one-way analysis of variance (ANOVA) test at p < 0.05 using statistical package for the social sciences (SPSS, Abacus Concepts, Berkeley, CA, USA) and Prism 5 (GraphPad, San Diego, CA, USA) software.

Ethics approval

The animal protocols used in this work were approved by the Institutional Animal Care and Use committee of the Center of Laboratory Animals of the Guangdong Institute of Microbiology.

Study limitations

There are several strengths with this study, but there are also limitations. For example, the study lacks a rescue test with single bacteria, so we have not identified the one-on-one contact between circNF1-419 and single bacteria, and therefore could not find the signaling pathway or linking ligament. Using targeted strains in germ-free mice with multi-omics studies to reveal the interaction of circNF1-419 on the microbiome-gut-brain axis, or using knock-out circNF1-419 mice are needed in the future for positive validations.