Selenium (Se) is an essential micronutrient crucial for antioxidant, anticancer, antiviral, and immune responses, primarily through its incorporation into selenoproteins. These selenoproteins, such as glutathione peroxidase (GPx) and thioredoxin reductase (TrxR), maintain redox homeostasis. Se deficiency, a significant nutritional challenge globally, is implicated in various diseases, including cardiomyopathy, Keshan disease, and intestinal disorders. The gastrointestinal tract, constantly exposed to exogenous factors, is highly susceptible to oxidative stress, and maintaining its barrier function relies heavily on redox homeostasis. Mitochondria, key sites for cellular processes and ROS production, are particularly sensitive to oxidative stress. Se's protective effects are linked to its modulation of mitochondrial function and induction of biogenesis. The gut microbiota also plays a vital role in intestinal health and interacts with Se intake. Studies have shown reduced serum Se levels in inflammatory bowel disease (IBD) patients, suggesting a link between Se, gut health, and redox balance. Inorganic Se is highly toxic and poorly bioavailable. Biogenic Se nanoparticles (SeNPs), with low toxicity and high bioavailability, offer a safer and more effective Se delivery system. This study hypothesized that dietary SeNPs benefit intestinal health by regulating redox balance and influencing the gut microbiota, investigating the effects of various SeNP concentrations on intestinal barrier function, mitochondrial function, and gut microbiota in mice, and using fecal microbiota transplantation (FMT) to examine the gut microbiota's role in these effects.

The literature review extensively explores the roles of selenium in maintaining redox balance and overall health, particularly focusing on the consequences of selenium deficiency. Multiple studies highlighted the association of Se deficiency with various diseases, including cardiovascular issues, Keshan disease, and intestinal problems. The importance of an intact intestinal barrier in maintaining health and its susceptibility to oxidative stress was emphasized. The crucial role of mitochondria in cellular processes and its relationship with redox homeostasis were described. Existing research on the impact of Se on mitochondrial function and biogenesis was reviewed, showing Se's ability to mitigate oxidative stress by preserving mitochondrial membrane potential and function. The growing body of evidence concerning the interaction between Se intake, gut microbiota composition, and intestinal health was discussed, pointing to a possible interconnected mechanism. The unique properties and advantages of biogenic SeNPs as a safer and more effective Se supplement were also highlighted, paving the way for this study to investigate the effects of SeNPs on intestinal health in a mouse model.

The study employed two animal experiments. In Experiment I, 72 three-week-old male C57BL/6 mice were divided into three groups (0.0-Se, 0.3-Se, and 0.6-Se mg/kg SeNPs) and fed different diets for 10 weeks. Fecal samples were collected two weeks prior to the end of the experiment for later FMT. Half the mice in each group were then treated with diquat to induce oxidative stress. Experiment II involved 40 six-week-old mice divided into four groups: control, diquat-treated, 0.0-Se-FMT + diquat, and 0.6-Se-FMT + diquat. The latter two groups received a cocktail of antibiotics for 8 weeks, followed by fecal microbiota transplantation (FMT) from Experiment I for 7 weeks, before being treated with diquat. Se content was measured by ICP-MS. mRNA expression was analyzed via real-time PCR. Gut microbiota composition was assessed using 16S rRNA gene sequencing. Intestinal permeability was measured using FITC-dextran. Serum D-LA and DAO were also determined. Intestinal morphology and goblet cell numbers were evaluated by H&E and AB-PAS staining. Antioxidant capacity (T-AOC, SOD, GPx, TrxR, MDA) and immune responses (IL-1β, IL-18, sIgA) were measured using ELISA kits. ROS generation was assessed using DHE staining. Mitochondrial ultrastructure and function (ATP, MMP, 8-OHdG) were analyzed using TEM and relevant kits. mtDNA copy number was determined by qPCR. Western blot analysis was performed to assess protein expression levels of tight junction proteins, Nrf2 signaling pathway components, and NLRP3 inflammasome proteins. Statistical analysis was done using one-way ANOVA or Student's t-test.

Se deficiency resulted in significantly lower body weight, higher food intake, and lower feed conversion rate compared to the Se-supplemented groups. Se deficiency significantly decreased the antioxidant capacity (T-AOC, SOD, GPx, TrxR) and increased lipid peroxidation (MDA) in the jejunum. Diquat exposure further aggravated these effects. The 0.6-Se group showed significantly improved antioxidant capacity compared to the 0.0-Se and 0.3-Se groups. Se deficiency increased pro-inflammatory cytokines (IL-1β, IL-18) and decreased sIgA. The 0.6-Se group showed significantly lower levels of IL-1β and IL-18 and higher sIgA levels. Se deficiency caused mitochondrial dysfunction (swelling, cristae disruption, reduced ATP, MMP), and increased 8-OHdG (DNA damage) levels. The 0.6-Se group exhibited improved mitochondrial morphology and function. Dietary SeNPs supplementation altered gut microbiota composition and diversity, increasing Bacteroidetes and decreasing Verrucomicrobia. The F/B ratio decreased in the 0.6-Se group. Se deficiency increased Desulfovibrio and decreased Bacteroides and Clostridium_XIVa. SeNPs supplementation increased total SCFAs, butyrate, isobutyrate, valerate, and isovalerate. Se deficiency increased NLRP3 inflammasome activation, while SeNPs supplementation upregulated Nrf2 and its downstream proteins (NQO-1, HO-1). FMT experiments showed that the gut microbiota from Se-deficient mice exacerbated diquat-induced intestinal barrier dysfunction. FMT from the 0.6-Se group improved intestinal barrier function, antioxidant capacity, and immune responses. FMT from the 0.6-Se group also protected against diquat-induced mitochondrial dysfunction by activating the Nrf2 pathway and inhibiting NLRP3 inflammasome activation.

This study demonstrates that Se deficiency leads to impaired intestinal barrier function and increased susceptibility to oxidative stress through redox imbalance, mitochondrial dysfunction, and gut dysbiosis. Dietary SeNPs supplementation effectively alleviates these effects by enhancing antioxidant defense, protecting mitochondria, regulating immune response, and modulating the gut microbiota. The Nrf2-mediated NLRP3 signaling pathway is a key mechanism involved in these protective effects. The FMT experiments confirm the crucial role of the gut microbiota in mediating SeNPs' benefits. The study highlights the importance of adequate Se intake for maintaining intestinal health, and the potential of SeNPs as a safe and effective nutritional supplement.

This study definitively shows that Se deficiency weakens the intestinal barrier, leading to increased susceptibility to oxidative stress. Biogenic SeNPs effectively counter these effects by improving antioxidant capacity, protecting mitochondria, and promoting a healthy gut microbiome. The Nrf2-NLRP3 pathway is central to these improvements. Future studies should investigate the specific mechanisms by which SeNPs interact with gut bacteria and explore the clinical applications of SeNPs in treating intestinal diseases.

The study used a mouse model, which might not fully reflect human physiology. The specific mechanisms by which SeNPs modulate gut microbiota require further investigation. Long-term effects of SeNPs supplementation were not examined. Further research is needed to determine the optimal dose and long-term safety of SeNPs in humans.