Microplastic effects on mouse colon in normal and colitis conditions: A literature review

Effect of MP on mouse colon in normal conditions (healthy animals)

The experimental conditions in the analyzed articles varied considerably (Table 1). Animal sex: 21 studies were performed on males, four on females, three on animals of both sexes, and six studies did not specify sex. Animal strain: 24 studies were performed on C57BL/6, C57BL/6J, or C57BL/6N mice, six studies on ICR, two on BALB/c, and one each on Kunming and C57BKS/Leprdb. Type of plastic: 26 used polystyrene (PS) particles, four studies used polyethylene (PE), single articles used polypropylene (PP), polyvinyl chloride (PVH), polyethylene terephthalate (PET), and low-density polyethylene (LDPE), two studies used biodegradable plastics: PLGA (poly lactic-co-glycolic acid) and PLA (polylactic acid), and one study did not specify the plastic type. Particle size: six studies used nanoplastics (particle diameter from 20 to 100 nm). MP particle size ranged from 0.14 to 300 µm, with particles with a diameter of 0.5 µm (seven articles) and 5 µm (11 articles) was the most commonly used. Dosages: ranged from 0.002 to 500 mg MP/kg of body weight per day, with dosages from 0.2 to 20 mg/kg/day mainly used (18 articles). Exposure: varied from one day to three months, mostly 4–6 weeks.

Macroscopic changes in the colon of mice exposed to MP were described in six studies. MP caused the colon shortening (Wen et al., 2022; Shaoyong et al., 2023) or the entire intestine (Choi et al., 2021b). According to Xu et al. (2021) colon mass decreased under the MP exposure, while according to Park et al. (2023) it did not change. In article of Sun et al. (2021), MP consumption led to a significant increase in the length and mass of the cecum.

Most of the analyzed studies (22 out of 34) presented the colon sections stained with hematoxylin and eosin histological examination. At a qualitative level, according to three studies (Sun et al., 2021; Zolotova et al., 2023; Zha et al., 2024) MP did not cause obvious pathological or inflammatory changes. According to 11 studies, MP consumption led to colon mucous membrane and submucosa infiltration by immune cells (Li et al., 2020; Shi et al., 2022; Wen et al., 2022; Xie et al., 2022; Jia et al., 2023; Wang et al., 2023b; Wang et al., 2023a; Huang et al., 2023; Zeng et al., 2024; Sun et al., 2024). A number of studies described the increase in the distance between crypts (Li et al., 2020; Choi et al., 2021b; Wen et al., 2022; Huang et al., 2023; Sun et al., 2024) and crypt dysplasia (Xu et al., 2021). Also noted the decrease in the number of goblet cells (Choi et al., 2021a; Shi et al., 2022; Xie et al., 2022; Huang et al., 2023), edema of the submucosa or mucous membrane lamina propria (Li et al., 2020; Shi et al., 2022; Jia et al., 2023), glandular hyperplasia of the mucous membrane (Jia et al., 2023), damage of the mucus layer (Wang et al., 2023c), destruction of the epithelium (Xu et al., 2021). During the morphometric study, same scientists (Djouina et al., 2022; Xie et al., 2022; Xie et al., 2023; Wang et al., 2023c) revealed an increase in the depth of the crypts, while other (Choi et al., 2021b; Choi et al., 2021a; Wen et al., 2022) on the contrary, detected the decrease in the crypts length and the mucous and muscularis externa thickness. In addition, Choi et al. (2021a), Choi et al. (2021b) noted a decrease in the superficial enterocytes height, while Xie et al. (2023) detected the increase in the number of crypts. In four studies, the histopathological score was calculated, assessing semi-quantitatively the severity of inflammatory infiltration, the prevalence and depth of damage, crypt damage, the edema absence or presence. In all experiments, the researchers investigated a statistically significant increase in the Histology score when exposed to MP (Li et al., 2020; Wen et al., 2022; Wang et al., 2023b; Sun et al., 2024).

Much attention was paid to MP effect on the colon epithelium. MP enhanced colon epithelial cells apoptosis and proliferation. The mRNA expression of intestinal stem cell markers increased: Lgr5, Bmi1 and Olfm4, and the volume fraction of proliferation markers c-Myc and Pcna in the colon also increased (Djouina et al., 2022; Xie et al., 2023). MP caused an increase in the number of dying TUNEL⁺ cells, the level of cleaved caspase-9, cleaved caspase-3 and the pro-apoptotic factor Bax (Liang et al., 2021; Jia et al., 2023; Park et al., 2023). The level of the anti-apoptotic factor Bcl-2 in the colon under the MP influence significantly decreased (Jia et al., 2023).

Most often, the morphofunctional state of goblet cells and the mucus they produced was assessed (17 out of 34 articles). According to the majority of authors, MP caused a decrease in the goblet cells number and volume fraction (Choi et al., 2021b; Liang et al., 2021; Sun et al., 2021; Choi et al., 2021a; Chen et al., 2022c; Wen et al., 2022; Xie et al., 2023; Jia et al., 2023; Wang et al., 2023b; Zolotova et al., 2023; Shaoyong et al., 2023; Wang et al., 2023a; Zeng et al., 2024; Sun et al., 2024). However, Djouina et al. (2022) and Yu et al. (2024) reported an increase in the goblet cells volume fraction. Ultrastructural study revealed that after MP exposure, goblet cells were inconsistent in shape and uneven in size. The average number of mucus drops in each goblet cell was significantly increased (Choi et al., 2021b). The main structural component of mucus produced by goblet cells is the mucin Muc2. According to the most studies, the glycoprotein Muc2 level (Shaoyong et al., 2023) and the expression of its mRNA (Choi et al., 2021b; Liang et al., 2021; Chen et al., 2022c; Wen et al., 2022; Xie et al., 2023; Zeng et al., 2024) in the colon decreased under the MP influence. However, Djouina et al. (2022) revealed an increase in the Muc2 mRNA expression. It was also demonstrated that MP could cause a decrease in other mucus components mRNA expression: Tff3 (Trefoil factor 3) (Xie et al., 2023), Klf4 (Kruppel-like factor 4) (Choi et al., 2021b; Chen et al., 2022c), Retnlb (Resistin-like beta) (Chen et al., 2022c). The terminal carbohydrate groups of secretory mucins could be modified by sulfuric or sialic acid residues (acidic mucins) or unmodified (neutral mucins). There were contradictory data on changes in the acidic and neutral mucins in goblet cells level when exposed to MP. Acidic mucins were stained with alcian blue. According to Zolotova et al. (2023) the intensity of Alcian blue staining increased when exposed to MP, according to Park et al. (2023)—it did not change, and according to Choi et al. (2021b)—it decreased. The intensity of the PAS reaction, which detects neutral mucins, did not change when exposed to MP according to Zolotova et al. (2023) and Park et al. (2023)—it increased.

There were few data on MP effect on other types of epithelial cells. According to Liang et al. (2021) the expression of Ecad mRNA, an epithelial cell marker E-cadherin, was reduced in mice treated with MP. Djouina et al. (2022) demonstrated that exposure to MP increased the mRNA expression for the absorptive epithelial cell marker villin 1 (Vil1) and the enteroendocrine cell marker chromogranin A (Chga). We also previously investigated the increase in the chromogranin A⁺ endocrine cells number in the colonic mucosa when exposed to MP (Zolotova et al., 2023). Choi et al. (2021b) reported a decrease in the cholecystokinin level and gastrin hormones in the colon of mice consuming MP.

MP affected the expression of transmembrane mucins that were part of the colon glycocalyx. According to the most studies, the Muc1 mRNA glycoprotein level (Choi et al., 2021b; Liang et al., 2021; Chen et al., 2022c; Jia et al., 2023; Shaoyong et al., 2023; Zeng et al., 2024), mRNA Muc3 (Liang et al., 2021; Chen et al., 2022c) and mRNA Muc13 (Liang et al., 2021) decreased under the MP influence.

Much attention was paid to MP effect on tight junctions in the colonic epithelium. The claudins Cldn2, Cldn3, Cldn4, Cldn7 mRNA expression in the colon increased when exposed to MP (Liang et al., 2021). In contrast, the expression of mRNA and protein level of claudin Cldn1 decreased (Liu et al., 2022a; Wen et al., 2022; Jia et al., 2023; Wang et al., 2023a; Zeng et al., 2024; Sun et al., 2024). According to Liang et al. (2021) and Djouina et al. (2022) the occludin mRNA expression increased, and according to other works (Liu et al., 2022a; Wen et al., 2022; Jia et al., 2023; Shaoyong et al., 2023; Zeng et al., 2024; Sun et al., 2024) both the expression of mRNA and the occludin protein level decreased. The mRNA expression and the ZO-1 protein level decreased (Wen et al., 2022; Jia et al., 2023; Shaoyong et al., 2023; Wang et al., 2023a; Zeng et al., 2024; Sun et al., 2024). According to Djouina et al. (2022) the expression of F11r mRNA (junctional adhesion molecule A) increased, and according to Shaoyong et al. (2023) the β-catenin protein level decreased.

Two studies assessed the membrane transport of intestinal epithelial cells. When exposed to MP Jia et al. (2023) revealed a decrease in the NKCC1 (Na⁺-K⁺-2Cl⁻ cotransporter 1), SLC26A6 (solute carrier family 26 member 6) and CFTR (cystic fibrosis transmembrane conductance regulator), level, and Choi et al. (2021b) revealed a decrease in the level of chloride ions, a decrease in the chloride channels Cftr, Clc2 and aquaporins Aqp3, Aqp8 mRNA expression.

MP led to the increased intestinal permeability. A number of studies demonstrated increased intestinal permeability for fluorochrome-labeled dextran with a molecular weight of 4 kDa and 70 kDa (Liang et al., 2021; Chen et al., 2022c; Wen et al., 2022; Shaoyong et al., 2023; Liang et al., 2024; Jiang et al., 2024; Sun et al., 2024). Increased intestinal permeability with MP consumption was also evidenced by the increased diamine oxidase (DAO), D-Lactate, intestinal fatty acid-binding protein (IFABP), D-Lactate dehydrogenase (Lv et al., 2023) and lipopolysaccharide (LPS) (Liang et al., 2024) blood levels (Shaoyong et al., 2023; Lv et al., 2023; Zeng et al., 2024). In addition, bacterial translocation to the mesenteric lymph nodes, liver, and spleen increased (Shaoyong et al., 2023; Sun et al., 2024).

MP induced oxidative stress. In mice treated with MP, the production of reactive oxygen species (ROS) was increased in the colon (Liang et al., 2021; Shaoyong et al., 2023). RNA seq data also indicated the increase in mitochondrial activity and ROS production (Rawle et al., 2022). The lipid peroxidation marker malondialdehyde (MDA) level increased (Wen et al., 2022; Xie et al., 2022; Jia et al., 2023; Wang et al., 2023c; Zeng et al., 2024) and the tyrosine oxidation product 3-nitrotyrosine (Zeng et al., 2024), and the level of an important intracellular antioxidant, glutathione (GSH) levels decreased (Wen et al., 2022; Jia et al., 2023; Wang et al., 2023c), but the level of its oxidized form GSSG (Glutathione disulfide) increased (Jia et al., 2023). However, in the Xie et al. (2022) study, the GSH level in the colon increased under the MP influence, while in the work of Shaoyong et al. (2023) it was not described as changed, in addition, the total antioxidant capacity did not change. Furthermore, MP affected the production and antioxidant enzymes activity such as superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT), and peroxidase (POD). According to the provided data, the level and SOD activity decreased under the MP influence (Wen et al., 2022; Jia et al., 2023; Shaoyong et al., 2023; Wang et al., 2023c; Zeng et al., 2024), however, in the work of Yu et al. (2024) SOD activity did not change, and in the study of Xie et al. (2022) SOD level increased. Regarding GPx, the obtained data were contradictory: it was reported the increase in GPx mRNA expression, Jia et al. (2023) claimed the decrease in GPx level, and Shaoyong et al. (2023) and Yu et al. (2024) did not reveal changes in the level and activity of this enzyme. The colon CAT level and activity decreased (Jia et al., 2023; Shaoyong et al., 2023; Zeng et al., 2024). One study noted an increase in peroxidase activity (Xie et al., 2022).

As mentioned above, most studies qualitatively noted colon mucosa immune cell infiltration when exposed to MP. In four studies, infiltration was assessed quantitatively. According to our data (Zolotova et al., 2023) MP caused a statistically significant increase in the number of cells in the lamina propria of the mucosa. Djouina et al. (2022) demonstrated the increase in the ratio of polymorphonuclear neutrophils and a decrease in the ratio of anti-inflammatory macrophages. In the work of Huang et al. (2023) an increase in the CD11b⁺Ly6C⁺ proinflammatory monocytes and CD11b⁺Ly6G⁺ neutrophils ratio was observed. Xie et al. (2022) identified the ratio of inflammatory cells in the colon mucosa depending on the type of MP used: PS > PVC > PET > PE > PP > control (when using PS, inflammatory cells accounted for 21.53% of all cells; PP—2.79%, in the control group—1.47%).

MP affected the cytokines in the colon production. According to most studies, MP exposure increased the proinflammatory cytokines TNFα, IL-1β and IL-6 level, as well as their mRNA expression (Choi et al., 2021a; Liu et al., 2022a; Djouina et al., 2022; Wen et al., 2022; Luo et al., 2022; Xie et al., 2023; Jia et al., 2023; Wang et al., 2023b; Shaoyong et al., 2023; Wang et al., 2023a; Wang et al., 2023c; Zeng et al., 2024; Sun et al., 2024). However, one study noted a decrease in Il1b mRNA expression (Sun et al., 2021). Regarding the anti-inflammatory cytokine IL-10, the data were contradictory: some studies demonstrated a decrease in its level (Wen et al., 2022; Jia et al., 2023), while others report—an increase in its mRNA expression (Sun et al., 2021; Choi et al., 2021a). There were isolated data on the IFNγ level increase and the mRNA Ifng, Tgfb, Il8, Il17a, Il22 expression when exposed to MP (Sun et al., 2021; Choi et al., 2021a; Liu et al., 2022a; Djouina et al., 2022; Luo et al., 2022; Zeng et al., 2024).

Several studies investigated MP effect on intracellular signaling pathways. According to the study of Choi et al. (2021a) results MP activated the ASC-inflammasome pathway: in the colon, the level of NLRP3 (NLR family pyrin domain containing 3), ASC (apoptosis-associated speck like protein containing a CARD) and the ratio of uncoupled caspase 1 to the total level of caspase 1 increased. The authors also demonstrated pro-inflammatory NF-κB pathway activation: in the colon, the level of phosphorylated NF-κB and phosphorylated IκB-α increased, as well as Nfκb mRNA expression. Choi et al. (2021b) revealed MP effect on the muscarinic acetylcholine receptor signaling pathway: the level of mAChR M2 and mAChR M3 receptors decreased, but Gα level increased, as well as PKC (protein kinase C) phosphorylation and PI3K (phosphoinasilitol 3 kinase) phosphorylation, which were mAChR downstream signaling molecules. The MAPK/NF-κB signaling pathway also changed: the level of ERK (extracellular-regulated kinase) phosphorylation, p38, NF-κB, and IκB increased. According to Jia et al. (2023) MP activated the TLR4/NF-κB signaling pathway: the TLR4, p50, and phosphorylated p65 levels increased, and the IκB level decreased. According to Li et al. (2020), MP consumption activated the pro-inflammatory pathway via TLR4: the level of TLR4, AP-1 (Activator protein 1) and IRF5 (Interferon regulatory factor 5) in the colon increased. The data from Sun et al. (2021), on the contrary, indicated the TLR4 signaling pathway suppression: Erk1 and Nfκb mRNA expression decreased, and Tlr4 and MyD88 mRNA expression did not change. The authors also assessed the expression of short-chain fatty acid receptors and did not detect changes in the Gpr41 (Ffar3) and Gpr43 (Ffar2) mRNA expression. In the work of Xie et al. (2023) activation of the Notch signaling pathway was observed: mRNA expression of Dll1, Dll4, Jag1 and Hes1 increased. Rawle et al. (2022) used RNA-seq to demonstrate that MP consumption caused pro-inflammatory signals moderate activation, sirtuin signaling pathway suppression, stimulated ribosomal and translational activity, and caused changes in the genes expression associated with the cytoskeleton and interaction with the extracellular matrix.

In 19 of the 34 articles analyzed, gut microbiota was studied using feces 16S rRNA or intestinal content sequencing. Only Rawle et al. (2022) concluded that MP consumption did not have a significant effect on the gut microbiome. The other studies revealed changes in the intestine microbiota composition during MP consumption (Li et al., 2020; Sun et al., 2021; Sun et al., 2024; Zhao et al., 2022; Shi et al., 2022; Chen et al., 2022c; Chen et al., 2022b; Djouina et al., 2022; Wen et al., 2022; Liu et al., 2022b; Xie et al., 2022; Wang et al., 2023a; Wang et al., 2023c; Lv et al., 2023; Yu et al., 2024; Zha et al., 2024; Liang et al., 2024; Jiang et al., 2024). Data on the microbiota diversity impact were contradictory: there were indications of the increase (Li et al., 2020; Liu et al., 2022b; Xie et al., 2022; Yu et al., 2024), and of the decrease (Shi et al., 2022; Liu et al., 2022a; Wen et al., 2022; Wang et al., 2023a; Wang et al., 2023c; Sun et al., 2024), and of zero changes (Sun et al., 2021; Chen et al., 2022c; Zha et al., 2024). There were little data on the increase in the gut microbiota abundance and richness (Li et al., 2020; Chen et al., 2022c; Liu et al., 2022b; Xie et al., 2022). Zha et al. (2024) reported an increase in fungal richness. The predominant bacterial types in mice gut microbiota were Firmicutes and Bacteroidetes. According to the obtained data, the Firmicutes/Bacteroidetes ratio increased when exposed to MP (Li et al., 2020; Zhao et al., 2022; Shi et al., 2022; Wen et al., 2022; Wang et al., 2023c), according to others, it decreased (Sun et al., 2021; Liu et al., 2022b; Xie et al., 2022; Wang et al., 2023a; Yu et al., 2024), and according to Sun et al. (2024) it did not change. The studies revealed many changes in the content of microorganism’s families and genera under the MP influence, but they varied greatly depending on the type of plastic and the timing of the experiment, which made it not possible to compare data from different studies. It should be stated the important observation of Liu et al. (2022b): in their work, under MP influence, the probiotic bacteria ratio decreased and the pathogenic bacteria ratio increased. Thus, MP can cause disturbances in the intestinal microflora composition.

A number of mouse colon and feces samples metabolomic studies were conducted. All data indicated that MP affected the metabolism of intestinal microflora and cells, changed the metabolism of amino acids, carbohydrates, lipids, vitamins and cofactors. However, the severity and direction of specific changes varied significantly in different studies (Sun et al., 2021; Shi et al., 2022; Chen et al., 2022c; Chen et al., 2022b; Liu et al., 2022a; Wen et al., 2022; Wang et al., 2023a; Wang et al., 2023c; Zha et al., 2024; Jiang et al., 2024; Garcia et al., 2024).

Table 2:

MP influence on colitis course.

Reference	Experimental conditions	MP+colitis group versus colitis without MP group
Zolotova et al. (2023)	Animals: adult male C57BL/6 mice MP: PS, size 5 μ m, dose 2.3 mg/kg/day (suspension in drinking bowls 10 mg/ml), 1–42 d.e., Colitis: 1% DSS 36–40 d.e. End of experiment: 7 day from the colitis start	↑ Colon ulcers and inflammation prevalence ↓ Neutral mucins in GC (↓ PAS-reaction intensity in GC)
Xie et al. (2023)	Animals: adult male C57BL/6 mice MP: PS, size 5 μ m, dose 0.01 mg/kg/day (suspension in drinking bowls 100 μ g/l), 1–42 d.e. Colitis: 3% DSS 36–42 d.e. End of experiment: 7 day from the colitis start	↓ Colon length ↑ Diarrhea and bloody stool 1.5x scores on the 3th day of colitis. ↑ Cecum and colon inflammation severity at the macro level ↑ Colon histological score ↑ LPS level in blood ↑ TNF-α, IL-1β and IL-6 levels and mRNA of Il1b and Il6 expression levels in colon ↓ Body weight ↑Il1b and Il6 mRNA expression in the liver ↑ Liver histological damage
Zheng et al. (2021)	Animals: adult male C57 mice MP: PS, size 5 μ m, dose 0.1 mg/kg/day, 1–28 d.e. Colitis: 3% DSS 7 days. End of experiment: 7 day from the colitis start	↑Intestinal permeability (Serum FITC-dextran content) ↑ IL-1β, TNF-α and IFN-γ levels in blood ↑ Liver inflammatory infiltration ↑ Fat vacuoles in the liver ↑ MDA and PPAR-γ (peroxisome proliferator-activated receptor levels Compared to the colitis group, the MP+colitis group exhibited distinct alterations in various metabolic pathways, including alanine, aspartate, and glutamate metabolism, phenylalanine metabolism, D-glutamine and D-glutamine metabolism, and others.
Luo et al. (2022)	1. Animals: adult male C57BL/6J mice MP: PS, size 5 μ m, dose 0.02 and 0.2 mg/kg/day (gastric tube), 1–63 d.e. Colitis: 2% DSS 1–7 d.e. End of experiment: 21 day from the colitis start 2. Animals: adult male C57BL/6J mice MP: PS, size 5 μ m, dose 0,02 mg/kg/day (gastric tube), 1–63 d.e. Colitis: 1–2% DSS 1–7, 22–28, 43–49 d.e. End of experiment: 63 day from the colitis start	21st day of colitis: ↓ Colon length (high dose) ↑ Colon tissue damage ↑ LPS blood level (both doses) ↓ GC volume fraction (AB-PAS stain) ↑ Genes associated with inflammation and immune response, including Tgfb (low dose), Cox2, Il17a (both dose), Il22 (high dose) mRNA expression in colon ↑ TNF-α and IL-10 levels in blood ↑ Liver histological damage 63rd day of colitis: ↑ colon tissue damage ↑ LPS blood level ↑ Genes associated with inflammation and immune response, including Il1b, iNOS, Cox2 mRNA expression level No changes: IL-β, IL-6, TNF-α and IL-10 blood levels ↑ Liver histological damage
Ma et al. (2023)	Animals: adult male C57BL/ 6J mice MP: PS, size 0,1 μ m, dose 1, 5 and 25 mg/kg/day (gastric tube), 6–33 d.e. Colitis: 2,5% DSS 1–5 d.e., 2% DSS 17–21 and 29–33 d.e. End of experiment: 33 day from the colitis start	↓ Colon length (all doses) ↑ Colon pathological score (high dose) ↑Tnfa mRNA expression (high dose) and ↓Il10 mRNA expression (all doses) in colon ↑ MAPK signaling pathway activity in colon: ↑ Erk1/2 phosphorylation level (all doses), JNK and p38 (high dose) ↑ Relative liver mass (high dose) ↑ Liver pathological score (all doses) ↑ Cholesterol and blood glucose (high dose) levels ↑ Oxidative stress in the liver: ↑ MDA (all doses) and ↓ SOD, GSH and T-AOC (high dose) Lipid metabolism in the liver changes (high dose)
Liu et al. (2022a)	Animals: adult male C57BL/6 mice MP: PS, size 5 μ m, dose 0.1 mg/kg/day, 8–35 d.e. Colitis: 3% DSS 1–7 d.e. End of experiment: 35 day from the colitis start	↑ Colon histopathological damage Compared with the colitis group, the MP+ colitis group had different colon microbiome and metabolism
Schwarzfischer et al. (2022)	Animals: female C57BL/6 mice immediately after weaning MP: PS, size 0.05 1 μ m, dose 10 mg/kg/day (suspension in drinking bowls 0.05 mg/ml), 1–178 d.e. Colitis: 1,5% DSS 169–175 d.e. End of experiment: 10 day from the colitis start 2. Animals: female mice C57BL/6 immediately after weaning MP: PS, size 0.05 1 μ m, dose 10 mg/kg/day (suspension in drinking bowls 0,05 mg/ml), 1–166 d.e. Colitis: 1–1,5% DSS 85-91, 102–108, 119–125, 136–142 d.e. End of experiment: 82 day from the colitis start	PS accumulated in the small intestine and organs distant from the gastrointestinal tract, but PS intake did not affect intestinal health and did not worsen colitis. It accumulated in the small intestine, mesenteric lymph nodes, spleen, and liver, but was not detected in the colon. 10th day of colitis: MP exposure did not worsen acute colitis: - weight loss was similar in all DSS-treated groups, regardless of MP particle exposure - according to endoscopy and histology, the inflammation severity was comparable in all DSS groups, regardless of plastic treatment - there were no differences in spleen weight and colon length between the DSS-treated groups - plastic exposure did not alter the proinflammatory cytokines or barrier molecules (Tnfa, Ifng, Il12b, Cdh1, Cldn1 and 2, Ocln) expression 82nd day of colitis: MP exposure did not worsen chronic colitis: - similar weight loss in all DSS groups and equivalent recovery time - no differences between the DSS groups based on colonoscopy, histology, spleen weight, and colon length were observed

DOI: 10.7717/peerj.18880/table-2

Notes:

MP

microplastic

DSS

dextran sulfate sodium

PS

polystyrene

d.e.

day of the experiment

GC

goblet cells

MP influence on the acute and chronic experimental colitis

The experimental conditions varied in the seven identified publications (Table 2). The experiments were performed on C57BL/6, C57BL/6J or C57 mice, usually males were used, and in only one study were used females. All studies used PS microparticles, size 5 µm in five studies, 0.1 µm in one study, and 0.05 and 1 µm in another study. MP ranged from 0.01 to 25 mg/kg/day. MP exposure duration ranged from 3 weeks to 6 months, mainly 4–6 weeks. Acute colitis was induced by replacing drinking water with 1–3% dextran sulfate sodium (DSS) solution for 5–7 days. To induce chronic colitis, DSS cycles with recovery with drinking water for 1–2 weeks were 3–4 times repeated. The colitis induction in five experiments was initiated 5–24 weeks after preliminary MP consumption, in two experiments DSS and MP exposure started simultaneously, and in another two experiments MP started after the DSS course. Animals were withdrawn from the experiment 7, 10 or 21 days after the start of colitis induction (acute and subacute colitis) or 33, 35, 63 or 82 days after the start of colitis induction (chronic colitis) (Fig. 3).

Data from six studies indicated that MP consumption leads to a more pronounced course of both acute and chronic experimental colitis (Zheng et al., 2021; Liu et al., 2022a; Luo et al., 2022; Xie et al., 2023; Zolotova et al., 2023; Ma et al., 2023). On the 3^rd day of colitis, according to Xie et al. (2023) diarrhea and bloody stool scores were 1.5 times higher with MP consumption. On the 7^th day of DSS colitis development, according to Zolotova et al. (2023), mice that received MP exhibited a higher prevalence of ulcers and inflammatory infiltration in their colon compared to mice that did not receive MP. Additionally, they observed lower levels of neutral mucins in goblet cells. According to Xie et al. (2023) at the same colitis time points, MP consumption led to a more pronounced colon shortening, a higher inflammation severity in the cecum and colon at the macroscopic level and histological score of the colon, an increase in the level of proinflammatory cytokines TNF-α, IL-1β and IL-6 and Il1b and Il6 mRNA in the colon and the level of lipopolysaccharide in the blood. Authors also noted more pronounced inflammatory infiltration and dystrophy in the liver and higher Il1β and Il6 mRNA expression in it on the 7th day. Zheng et al. (2021) reported the increased intestinal permeability on the 7th day of colitis under MP influence, as well as higher levels of proinflammatory cytokines IL-1β, TNF-α and IFN-γ in blood and more severe liver damage. In the liver, a more pronounced inflammatory infiltration, a greater number of fat vacuoles, higher levels of MDA and PPAR-γ (peroxisome proliferator-activated receptor γ), alterations in the metabolism of alanine, aspartate, and glutamate, phenylalanine, d-glutamine, and d-glutamine metabolism, and other changes were observed.

On the 21^st day of colitis, mice consuming MP demonstrated more pronounced colon shortening, more pronounced histological changes in the colon, a lower goblet cells volume fraction and higher intestinal permeability (higher LPS level in the blood). The colon demonstrated higher mRNA expression of genes associated with inflammation and immune response, including Tgfb, Cox2, Il17a, Il22, and the blood demonstrated higher TNF-α and IL-10 levels. In addition, more pronounced histological changes were detected in the liver (Luo et al., 2022).

On the 33^rd day of colitis, under MP influence in the colon, there was also a more pronounced colon shortening, a higher Pathological score, higher proinflammatory cytokine Tnfa mRNA expression, and lower anti-inflammatory cytokine Il10 mRNA, as well as higher activity of the MAPK signaling pathway: higher levels of phosphorylation of Erk1/2, JNK and p38. In the blood were revealed higher glucose and cholesterol levels. The relative weight of the liver was higher, it had a higher Pathological score, oxidative stress (higher MDA level and lower SOD, GSH and T-AOC indicators), and changes in lipid metabolism were observed (Ma et al., 2023). On the 35^th day of colitis in mice consuming MP, histological examination of the colon demonstrated more pronounced pathological changes: severe epithelial damage and vacuolation of the colonic mucosa, with disordered goblet cell structure, and a large number of inflammatory cells gathered near the intestinal mucosa. The authors also stated that differences in the intestinal microflora composition and the intestinal metabolome were detected between the groups of animals with colitis that received and did not receive MP (Liu et al., 2022a).

On 63^rd day of colitis with MP consumption, there was a tendency for the histological score to increase in the colon in comparison to colitis without MP, and the LPS level in the blood was higher. In the colon, there was a higher mRNA expression of genes associated with inflammation and immune response, including Il1b, iNOS, Cox2, but the blood level of cytokines IL-β, IL-6, TNF-α and IL-10 did not differ between the groups of animals with colitis receiving and not receiving MP (Luo et al., 2022).

However, according to Schwarzfischer et al. (2022) MP did not aggravate the course of either acute (10^th day) or chronic (84^th day) colitis. The authors found accumulation of MP particles in the small intestine, mesenteric lymph nodes, spleen and liver, but did not detect it in the colon. According to endoscopy and qualitative histological examination, the severity of inflammation in the groups of animals with colitis that received and did not receive MP was comparable. There were also no differences in body weight, spleen weight, colon length, expression of proinflammatory cytokines or barrier molecules (Tnfα, Ifng, Il12b, Cdh1, Cldn1, Cldn2, Ocln).