Microbiota regulate intestinal absorption and metabolism of fatty acids in the zebrafish

Abstract

Regulation of intestinal dietary fat absorption is critical to maintaining energy balance. While intestinal microbiota clearly impact the host's energy balance, their role in intestinal absorption and extraintestinal metabolism of dietary fat is less clear. Using in vivo imaging of fluorescent fatty acid (FA) analogs delivered to gnotobiotic zebrafish hosts, we reveal that microbiota stimulate FA uptake and lipid droplet (LD) formation in the intestinal epithelium and liver. Microbiota increase epithelial LD number in a diet-dependent manner. The presence of food led to the intestinal enrichment of bacteria from the phylum Firmicutes. Diet-enriched Firmicutes and their products were sufficient to increase epithelial LD number, whereas LD size was increased by other bacterial types. Thus, different members of the intestinal microbiota promote FA absorption via distinct mechanisms. Diet-induced alterations in microbiota composition might influence fat absorption, providing mechanistic insight into how microbiota-diet interactions regulate host energy balance.

Figure 1. Fatty acids accumulate in the intestinal epithelium in the presence of microbiota and diet

(A) Schematic of BODIPY-FL delivery assay in gnotobiotic zebrafish. Zebrafish derived germ-free (GF) at 0 days post-fertilization (dpf) were either reared GF (top) or inoculated at 3 dpf with normal microbiota (conventionalized, CONVD; bottom). From 3-6 dpf, fish were either starved, or fed a control (C) or low calorie (LC) diet (see Table S1). At 6 dpf, zebrafish were incubated with BODIPY-FL liposomes for 6 hrs and imaged or fixed for later imaging. (B,C) Representative confocal images of the intestines of live 6 dpf GF and CONVD zebrafish incubated with BODIPY-FL C₅ or C₁₆. Scale bar, 50 μm. (B) The intestinal lumen (Lum) and epithelium (Epi; bounded by dotted lines) of GF and CONVD zebrafish are indicated. The epithelium shows apical (white arrow) and basolateral accumulation of lipid droplets (white arrowhead) labeled with BODIPY-C₅. (C) Incubation with BODIPY-FL C₁₆. (D-G) Quantification of total epithelial (D,F) and luminal (E,G) fluorescence expressed in relative fluorescence units (RFU). Values represent the means ± SEM from 3 independent experiments: *, p<0.05; **, p<0.01.

Figure 2. The microbiota stimulate lipid absorption into intestinal epithelial lipid droplets and extra-intestinal tissues

(A) Representative confocal images of fixed 6 dpf Tg(-4.5fabp2:DsRed) GF and CONVD zebrafish fed a control diet and incubated with BODIPY-FL C₅ for 6 hrs. Scale bar, 20 μm. Intestinal epithelial cells show BODIPY-C₅ accumulation as lipid droplets (LDs) in the epithelium and the lamina propria (white arrow). Large LDs are detected in the epithelium of CONVD zebrafish (black arrowheads). (B,C) Lipid droplet quantification assay was developed using Volocity software (see Figure S1A-F) to determine LD number (B) and size frequency (C) in an epithelial region of interest (7500 μm²). The graphs depict the mean ± SEM of at least two independent experiments (3-15 fish/condition/experiment). Results of statistical significance analysis: a, significant vs. GF fed same diet; b, significant vs. starved in same microbial condition. See Figure S2 for data from a 3 hr timepoint. (D) Representative confocal images of livers in 6 dpf GF and CONVD zebrafish incubated with BODIPY-FL C₅ for 6 hrs. Scale bar, 20 μm. (E) BODIPY-C₅ fluorescence scores in livers of 6 dpf GF and CONVD zebrafish. The graph depicts the mean ± SD of two independent experiments (3-5 fish/condition/experiment) that were scored blindly (score scale 0-5). (F) Non-GI BODIPY-C₅ fluorescence in GF and CONVD C-fed zebrafish. The data represent mean ± SD of two experiments (20-30 carcasses and 9-10 whole larvae/condition/experiment). Significant differences are indicated: *, p<0.05 (E,F).

Figure 3. Lipid droplet clearance is more efficient in the presence of microbiota

(A) Schematic representation of the BODIPY-FL C₅ washout experiment. (B) Representative confocal images of control-fed GF and CONVD zebrafish pre- and post-wash. Scale bar, 10 μm. (C,D) Quantification of lipid droplet (LD) number (C) and relative size frequency (D), shown as the mean ± SEM from two independent experiments (4-14 fish/condition/experiment), and significant differences are identified: ***, p<0.001; a, significant vs. pre-wash in same microbial condition; b, significant vs. same wash in other microbial condition. See also Figure S2.

Figure 4. 16S rRNA gene sequencing reveals distinct bacterial communities in the zebrafish gut and water that are strongly influenced by dietary status

(A,B) UniFrac principal coordinates analysis (PCoA) plots using unweighted (A) and weighted (B) algorithms. Each replicate sample is represented by a single shape, with the solid grey ellipsoid around each shape indicating the confidence interval from 100 jackknife replicates of 500 sequences per sample. Apparent clusters of samples are indicated with open ovals. Samples C-Fed gut 1 (a) and Starved gut 3 (b) are labeled. See also Table S2. (C) Stacked bar graph showing relative abundance (Y-axis) of 16S rRNA gene sequences from different bacterial classes (legend at right) observed in different samples (X-axis). (D) Percentage of 16S rRNA gene sequences classified as Proteobacteria, Firmicutes, and Bacteroidetes, shown as the mean ± SD across different replicate sample groups and the inoculum sample. See also Table S3. Alpha diversity measures of (E) Phylogenetic distance and (F) Chao1 richness are shown as the mean ± SD across different replicate sample groups and the inoculum sample. See also Figure S3 and Table S4.

Figure 5. Monoassociation with individual community members reveals diet-dependent colonization of a representative Firmicutes species

(A-D) Colony forming units (CFU) in the intestine (per dissected gut; n=4-5 per condition) or surrounding water (per mL; n=3 per condition) of 6 dpf zebrafish. The results represent the mean ± SEM of at least two independent experiments (n.d., not detected) with identified significant differences: *, p<0.05; **, p<0.01; ***, p<0.001. (A) Density of the conventional microbiota in CONVD zebrafish. (B-D) Bacterial densities in GF zebrafish monoassociated with (B) Exiguobacterium sp. ZWU0009 (Firmicutes), (C) Chryseobacterium sp. ZOR0023 (Bacteroidetes), or (D) Pseudomonas sp. ZWU0006 (γ-Proteobacteria). See also Table S5.

Figure 6. Monoassociations reveal distinct bacterial mechanisms for inducing fatty acid absorption in the intestinal epithelium

(A-D) Representative confocal images of the intestinal epithelium of 6 dpf C-fed zebrafish raised GF (A) or monoassociated with Exiguobacterium sp. ZWU0009 (B), Chryseobacterium sp. ZOR0023 (C), or Pseudomonas sp. ZWU0006 (D) incubated with BODIPY-FL C₅ for 6 hrs. BODIPY-C₅ accumulation in large epithelial lipid droplets (black arrowheads) and in the lamina propria (white arrow) is indicated. (E) Lipid droplet quantification in the intestinal epithelium of monoassociated zebrafish compared to GF controls. (F) Relative frequency of intestinal LD sizes in monoassociated zebrafish. The data represent the mean ± SEM of at least two independent experiments (5-16 fish/condition/experiment). Significant differences compared to GF controls are identified: *, p<0.05; ***, p<0.001. (G) BODIPY-C₅ fluorescence scores in the livers of monoassociated zebrafish compared to GF controls. The graph represents the mean ± SEM of at least two independent experiments (2-7 fish/condition/experiment) that were scored blindly (score scale 0-5). Significant differences to GF controls are identified: *, p<0.05. (H,I) Intestinal LD number (H) and relative frequency of intestinal LD sizes (I) in GF zebrafish treated with filter-sterilized Exiguobacterium sp., Chryseobacterium sp., or Pseudomonas sp. conditioned media. Results are presented as mean ± SEM of at least two independent experiments (3-7 fish/condition/experiment) with identified significant differences compared to GF control: **, p<0.01.

Figure 7. Model for diet-dependent microbial regulation of intestinal fatty acid absorption

The microbiota promote LD size in intestinal enterocytes independent of diet (dashed violet arrow). The presence of diet promotes LD number in CONVD zebrafish and in those monoassociated with a Firmicutes strain (i.e., Exiguobacterium sp.; dashed brown arrow). Monoassociation with other bacterial strains Chryseobacterium sp. or Pseudomonas sp. promotes LD size. Although the extent to which these findings are generalizable to their respective phyla remains unclear, these data suggest two bacterial mechanisms that promote distinct LD accumulation phenotypes: a Firmicutes-induced increase in LD number and a non-Firmicutes bacterial induction in LD size.