Effects of Schisandra chinensis Polysaccharide-Conjugated Selenium Nanoparticles on Intestinal Injury in Mice

Simple Summary Animal enteritis caused by Gram-negative bacterial infection is one of the most common and harmful diseases in the livestock and poultry industry. Schisandra chinensis polysaccharide (SCP) has potential effects on the treatment of intestinal injury. Selenium nanoparticle modification can improve the bioactivity of polysaccharides. In this study, SCP-conjugated selenium nanoparticles (SCP-Se NPs) were firstly synthesized, and their effect on intestinal injury was also investigated. The results showed that SCP-Se NPs could more effectively alleviate LPS-induced diarrhea, intestinal tissue injury, and tight junction destruction and decrease the elevated inflammatory cytokine expression levels of TNF-α, IL-1β, and IL-6 compared with SCP in mouse models, suggesting that SCP-Se NPs can serve as a good candidate for preventing and treating enteritis in the livestock and poultry industry. Abstract Schisandra chinensis polysaccharide (SCP) is an experimental therapeutic for the treatment of intestinal injury. Selenium nanoparticle modification can improve the bioactivity of polysaccharides. In this study, SCP was firstly extracted and purified by a DEAE-52 column, then SCP-Selenium nanoparticles (SCP-Se NPs) were prepared, and the procedure was optimized. Thereafter, the obtained SCP-Se NPs were characterized by transmission electron microscope, X-ray diffraction, energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy. The influence of different storage environments on the stability of colloidal SCP-Se NPs was also investigated. Finally, the therapeutic effects of SCP-Se NPs on LPS-induced intestinal inflammatory injuries in mice were evaluated. Results showed that the optimized SCP-Se NPs were amorphous, uniform, spherical particles with a diameter of 121 nm, and the colloidal solution was stable at 4 °C for at least 14 d. Moreover, SCP-Se NPs could more effectively alleviate LPS-induced diarrhea, intestinal tissue injury, and tight junction destruction and decrease the elevated expression levels of TNF-α, IL-1β, and IL-6 compared with SCP. These results demonstrate that SCP-Se NPs may alleviate LPS-induced enteritis through their anti-inflammatory effects, indicating that SCP-Se NPs can serve as a good candidate for preventing and treating enteritis in the livestock and poultry industry.


Introduction
The intestinal tract is the main organ for the digestion and absorption of nutrients [1,2]. Therefore, its health status directly determines the nutritional supply and overall growth of the animal. The intestinal tract is also a crucial barrier for defending against external pathogenic factors. Animal enteritis, caused by Gram-negative bacterial infection, is one of the most common and harmful diseases in the livestock and poultry industry [3,4]. In production practice, antibiotic treatment is the current standard of care for enteritis. However, with the rise of drug residues, antibiotic resistance, and food safety concerns, the development of new safe antibacterial alternatives to alleviate animal enteritis has become an urgent matter for the development of the current livestock and poultry industry.
Traditional Chinese veterinary medicine has a long history of safeguarding the health of livestock and poultry in China and other Asian countries [5,6]. Schisandra chinensis polysaccharide (SCP) is one of the main active components of the traditional Chinese medicine Schisandra chinensis (Turcz.) Baill and has beneficial therapeutic potential for a variety of enteritis diseases [7,8]. For example, SCP has been reported to alleviate antibioticassociated enteritis caused by lincomycin hydrochloride by inhibiting pro-inflammatory factors and increasing the expression of anti-inflammatory factors [9]. Additionally, SCP also has a therapeutic effect on mice with ulcerative colitis by restoring the intestinal structure, regulating cytokine levels, and improving the diversity of intestinal flora [10]. However, due to the macromolecular structure of the polysaccharide, its bioavailability is extremely low, which limits its clinical application.
Selenium is an essential trace element for animals, as shown with its anti-inflammatory, antioxidant, and antiviral properties [11][12][13][14]. Selenium supplementation can alleviate intestinal inflammation, improve the structure of intestinal flora, and enrich the diversity of intestinal flora [15][16][17][18]. Currently, inorganic and organic selenium are important sources of selenium supplements, but the narrow therapeutic window limits its clinical application [19].
As a new form of selenium, nano-selenium has attracted much attention for its higher biological activity and safety compared with traditional inorganic and organic selenium [20,21]. However, due to high surface energy, bare selenium nanoparticles (Se NPs) quickly aggregate into black or gray elemental selenium, reducing their bioactivity, biocompatibility, and bioavailability [22]. It has been reported that polysaccharide molecules can react with the surface of Se NPs to form stable amorphous nanoparticles through intermolecular hydrogen bonds, seleno-oxygen bonds, and selenium-nitrogen bonds [23]. The polysaccharide-Se NPs conjugate preparation using the polysaccharide as a soft template solves the drawback of easy agglomerative inactivation of Se NPs. It also improves the bioavailability and bioactivity of polysaccharides by nanosizing and selenization [24]. Hence, the research on polysaccharide-based Se NPs derivatives has become a hot spot in the field of polysaccharide and nanomedicine in recent years. For example, it was found that the nano-selenium derivative of Ulva polysaccharide significantly enhanced its therapeutic effect on DSS-induced ulcerative colitis in mice by inhibiting the pro-inflammatory response mediated by the NF-κB pathway [25]. However, whether SCP-Se NPs can be synthesized by using SCP as a template and whether the synthesized SCP-Se NPs have an effect on the intestinal injury induced by LPS in mice has not been reported.
In this study, considering the bioactivities of SCP, we firstly extracted and purified SCP and then optimized the synthesis procedure of SCP-Se NPs using the particle diameter, polydispersity index (PDI), and Zeta potential examination analyzed by ultravioletvisible (UV-Vis) spectroscopy and dynamic light scattering (DLS). Transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-Ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectroscopy were used to further characterize the prepared SCP-Se NPs colloidal solution. Additionally, the stability of the colloidal solution in different storage conditions was studied. Finally, the intestinal protective effects of SCP-Se NPs were investigated based on the disease activity index (DAI) scoring analysis, hematoxylin-eosin (HE) staining, immunohistochemical (IHC) staining, and qRT-PCR examination of the pro-inflammation index in vivo.

Optimization of SCP-Se NPs Synthesis
The obtained SCP-Se NPs were characterized by using multiple methods. Since SCP-Se NPs solution has the basic colloidal characteristics, dual-wavelength spectrophotometry was first carried out to monitor the particle size according to previous research [30,31]. In brief, the absorbance of the SCP-Se NPs solution was measured at 410 nm and 490 nm by using a G-9 Series double beam UV-Vis spectrophotometer (G-9S, Nanjing FILA Instrumen, Nanjing, China), and the A410/A490 value was calculated. Meanwhile, the index of particle diameter, PDI, and Zeta potential were also determined by a DLS analyzer (Litesizer 500, Anton Paar, Graz, Austria).

Characterization of SCP-Se NPs
To observe the morphology and distribution of SCP-Se NPs directly, TEM (JEM-2800, JEOL, Akishima, Japan) was applied. Briefly, 20.0 μL of the SCP-Se NPs colloidal solution was placed on the porous carbon film surface of the copper mesh and dried in an oven to avoid contamination by dust and other impurities. After drying, the sample area was observed by TEM.
The crystal form of SCP-Se NPs was measured with an X-ray diffractometer from 5° to 90° at an angle of 2 θ and a speed of 10°/min with the operating current and voltage set at 30 mA and 40 kV, respectively.
The SCP-Se NPs sample was put into the test window, and the energy-dispersive Xray spectroscopy was started to obtain the SCP-Se NP element analysis results.

Optimization of SCP-Se NPs Synthesis
The obtained SCP-Se NPs were characterized by using multiple methods. Since SCP-Se NPs solution has the basic colloidal characteristics, dual-wavelength spectrophotometry was first carried out to monitor the particle size according to previous research [30,31]. In brief, the absorbance of the SCP-Se NPs solution was measured at 410 nm and 490 nm by using a G-9 Series double beam UV-Vis spectrophotometer (G-9S, Nanjing FILA Instrumen, Nanjing, China), and the A 410 /A 490 value was calculated. Meanwhile, the index of particle diameter, PDI, and Zeta potential were also determined by a DLS analyzer (Litesizer 500, Anton Paar, Graz, Austria).

Characterization of SCP-Se NPs
To observe the morphology and distribution of SCP-Se NPs directly, TEM (JEM-2800, JEOL, Akishima, Japan) was applied. Briefly, 20.0 µL of the SCP-Se NPs colloidal solution was placed on the porous carbon film surface of the copper mesh and dried in an oven to avoid contamination by dust and other impurities. After drying, the sample area was observed by TEM.
The crystal form of SCP-Se NPs was measured with an X-ray diffractometer from 5 • to 90 • at an angle of 2 θ and a speed of 10 • /min with the operating current and voltage set at 30 mA and 40 kV, respectively.
The SCP-Se NPs sample was put into the test window, and the energy-dispersive X-ray spectroscopy was started to obtain the SCP-Se NP element analysis results.
The infrared absorption spectra of SCP and SCP-Se NPs were determined by an infrared spectrometer (iS50 FTIR, Thermo Nicolet Corporation, Waltham, MA, USA). Briefly, dried samples were mixed with potassium bromide solid powder (spectrally pure grade) in a ratio of 1/100, thoroughly ground in a mortar, transferred to a mold, and made into sheets for analysis in the spectral range of 4000-400 cm −1 .

Stability of SCP-Se NPs Colloidal Solution at Different Storage Conditions
The stability of the SCP-Se NPs colloidal solution at different storage temperatures was determined by visual changes and the detection of particle diameter size, PDI, and Zeta potential. The SCP-Se NPs colloidal solution samples stored at 4 • C and RT were observed, and the particle size, PDI, and Zeta potentials of the SCP-Se NP samples were measured on days 0, 3, 7, and 14. Twenty-eight SPF male KM mice were obtained from Southwest Medical University Laboratory Animal Center. During the whole experiment, all the mice were housed in the Animal Experimental Center of Southwest University at a temperature of 25 • C, relative humidity of 50%, and a light and dark cycle of 12 h, and they had adaptive feeding for one week before the experiment. Meanwhile, the mental state, activity, diet, urine, and feces of the mice were observed every day. The animal study was conducted in accordance with the guidelines approved by the Southwest University Laboratory Animal Welfare Ethics Committee (Ethics NO.: IACUC-20210915-03).

Effects of SCP-Se NPs on LPS-Induced Enteritis
After adaption, the mice were randomly divided into four groups: blank control group (BC group), LPS model group, SCP group, and SCP-Se NPs group. After 14 days of intragastric administration of the corresponding solutions (SCP group-10.0 mg/kg SCP solution; SCP-Se NPs group-10.0 mg/kg SCP-Se NPs solution [32]; and BC group and LPS group-equal volume of sterile water), mice in the LPS, SCP, and SCP-Se NPs groups were intraperitoneally injected with 20.0 mg/kg LPS solution [33], and the BC group was intraperitoneally injected with the equal volume of sterile saline. Thereafter, body weight, fecal occult blood, and diarrhea of mice within 6 h after intraperitoneal administration were observed and recorded for DAI score analysis [34]. At the end of the experiment (6 h post-LPS injection), the mice in the different groups were sacrificed under anesthesia, serum was collected, part of the jejunum tissue was fixed in 4% paraformaldehyde solution, and the other part was rapidly collected and frozen in liquid nitrogen.

Histopathological Analysis of Jejunum
After being fixed in 4% paraformaldehyde solution for 24 h, the jejunum tissue samples were dehydrated with gradient alcohol solutions, made transparent with xylene, embedded in paraffin, and sliced into sections of 4.00 µm thickness by a microtome (LEICA RM2016, Wetzlar, Germany). Subsequently, the slices were stained with HE-staining solution, and images were taken under an optical microscope (ZEISS Axiocam ERc 5S, Oberkochen, Germany). Meanwhile, the villi height (from the apex of the villi to the junction of the villi crypt) and crypt depth (the depth of the invagination between adjacent villi) were measured with the ZEISS image analysis system. Values are means from 20 complete villi, and only vertically oriented villi and crypts from each slide were measured.

IHC Staining
Paraffin-embedded tissues were prepared as described in Section 2.8.3, cut into 4.00 µm thick sections, dewaxed with xylene, rehydrated with gradient alcohol solutions, and sealed with 5% BSA. Then, sections were incubated with the anti-ZO-1 antibody at 4 • C overnight and with HRP-labeled goat anti-rabbit IgG at RT for 30 min. Finally, the DAB working solution was used for color rendering, and the hematoxylin solution was incubated for 5 min. The images were analyzed after capture using ImageJ and the IHC Profiler plugin, and six different images were counted for each group. The average gray value (staining intensity) and percentage of the positive area (staining area) of positive cells were taken as IHC measurement indicators, and the final scores were given.

Inflammatory Cytokine Analysis
To investigate the effect of different treatments to alleviate intestinal inflammatory injury, the serum cytokine contents and the relative mRNA expression levels of jejunum IL-1β, IL-6, and TNF-α were detected. The serum cytokine contents were tested by ELISA kits according to the manufacturer's instructions. The relative mRNA expression levels of the inflammatory cytokines were detected using qRT-PCR method. Briefly, total RNA was extracted from jejunum tissue homogenate with an RNAiso Plus kit. After the concentration was determined by BioDrop µLITE (BioDrop, UK), cDNA was synthesized using a PrimerScript TM RT Reagent kit with gDNA Eraser. Hereafter, the mRNA expression level of each gene was determined by using a QuantStudio 3 real-time fluorescence quantitative PCR instrument (Thermo Fisher Scientific Co., Ltd. New York), and the relative gene expression level of each target gene was calculated by the 2 -∆∆Ct method. The primer sequences used in this experiment are shown in Table 1. The reaction procedures are as follows: 95 • C for 30 s, followed by 40 cycles at 95 • C for 5 s, and 60 • C for 30 s. All these above operations were strictly in accordance with the instructions.

Statistical Analysis
All results were expressed as "Mean ± SD" from at least three independent experiments. The differences between multiple groups were analyzed by Duncan's multiple range test using SPSS 20.0 analysis software. Differences were considered significant at p < 0.05.

Isolation and Purification of SCP
In this study, SCP was successfully extracted by the water extraction and alcohol precipitation method and purified with DEAE-52 cellulose column chromatography. As shown in Figure 2, two main fractions were obtained, of which the elution curves were single peaks in distilled water and 0.200 M NaCl eluents. Meanwhile, there was no obvious elution peak in the 0.400 M NaCl eluent. Therefore, eluents of tubes 12-18 and 62-68 were collected and combined. After dialysis and lyophilization, the purity of SCP was determined using the carbohydrate content of 89.2%.

Monosaccharide Composition of SCP
The monosaccharide composition results of SCP are shown in Figure 3. SCP was composed of mannose, ribose, rhamnose, glucuronic acid, galacturonic acid, glucose, xylose, and arabinose with the mole ratio of 6.93:1.00:14.

Preparation and Characterization of SCP-Se NPs
To investigate the appropriate SCP concentration for SCP-Se NPs formation, 0 mg/mL, 0.0100 mg/mL, 0.0500 mg/mL, 0.100 mg/mL, 0.500 mg/mL, 1.00 mg/mL, and 2.00 mg/mL of SCP solution were added to the reaction system, respectively. As shown in Figure 4A, Se NPs quickly aggregated and precipitated when there was no SCP in the system and when the Se NPs solution with SCP modification was orange-red and evenly dispersed. Meanwhile, the absorbance values of the solution prepared with different SCP concentrations were also measured, and the values of A 410 /A 490 were calculated ( Figure 4B). When SCP concentration reached 0.100 mg/mL, the value of A 410 /A 490 was the highest and tended to be more stable at higher concentrations. Figure 4C,D shows the particle size distribution, PDI, and median particle size of the Se NPs prepared by different SCP concentrations. The particle size distribution of the bare Se NPs was polydisperse with a PDI of 0.430 and a median particle size of 1453 nm. However, the uniformity of the solution greatly changed when SCP was added to the reaction system. Especially for 0.100 mg/mL SCP, the nanoparticles showed the best uniformity with the PDI of 0.170 and the median particle size of 121 nm. Figure 4E,F illustrates the Zeta potential of SCP and SCP-Se NPs modified with different concentrations of SCP. As shown in Figure 4E, the Zeta potential of SCP was −28.9 mV. With the increase of the SCP concentration, the Zeta potential of the SCP-Se NPs first increased and then decreased ( Figure 4F). Moreover, at the SCP concentration of 0.1 mg/mL, SCP-Se NPs showed the highest potential of −28.9 mV.

Preparation and Characterization of SCP-Se NPs
To investigate the appropriate SCP concentration for SCP-Se NPs formation, 0 mg/mL, 0.0100 mg/mL, 0.0500 mg/mL, 0.100 mg/mL, 0.500 mg/mL, 1.00 mg/mL, and 2.00 mg/mL of SCP solution were added to the reaction system, respectively. As shown in Figure 4A, Se NPs quickly aggregated and precipitated when there was no SCP in the system and when the Se NPs solution with SCP modification was orange-red and evenly dispersed. Meanwhile, the absorbance values of the solution prepared with different SCP concentrations were also measured, and the values of A410/A490 were calculated ( Figure  4B). When SCP concentration reached 0.100 mg/mL, the value of A410/A490 was the highest and tended to be more stable at higher concentrations. Figure 4C,D shows the particle size distribution, PDI, and median particle size of the Se NPs prepared by different SCP concentrations. The particle size distribution of the bare Se NPs was polydisperse with a PDI of 0.430 and a median particle size of 1453 nm. However, the uniformity of the solution greatly changed when SCP was added to the reaction system. Especially for 0.100 mg/mL SCP, the nanoparticles showed the best uniformity with the PDI of 0.170 and the median particle size of 121 nm. Figure 4E,F illustrates the Zeta potential of SCP and SCP-Se NPs modified with different concentrations of SCP. As shown in Figure 4E, the Zeta potential of SCP was −28.9 mV. With the increase of the SCP concentration, the Zeta potential of the SCP-Se NPs first increased and then decreased ( Figure 4F). Moreover, at the SCP concentration of 0.1 mg/mL, SCP-Se NPs showed the highest potential of −28.9 mV.

Characterization of SCP-Se NPs
Based on the above experiments, 0.100 mg/mL SCP was identified as the optimal concentration for SCP-Se NP preparation. TEM images showed that SCP-Se NPs presented a monodisperse uniform spherical structure with a particle diameter of almost 120 nm ( Figure 5A), similar to the above results shown in Figure 4D. The XRD result showed that there was only a broad diffraction peak at lower angles ( Figure 5B), indicating that SCP-Se NPs were amorphous according to the JCPDS database. Furthermore, the elemental composition of SCP-Se NPs was also analyzed by EDX, as shown in Figure 5C-F. It was found that five signals were obtained, in which the proportion of carbon was 65.5%, followed by selenium (18.5%), oxygen (15.3%), calcium (0.400%), and sodium (0.0200%).
NPs solution at different SCP concentrations. (C) The particle size distribution of SCP-Se NPs at different SCP concentrations. (D) The median particle size and PDI of SCP-Se NPs at different SCP concentrations. (E) Zeta potential of SCP solution. (F) Zeta potential of SCP-Se NPs at different SCP concentrations.

Characterization of SCP-Se NPs
Based on the above experiments, 0.100 mg/mL SCP was identified as the optimal concentration for SCP-Se NP preparation. TEM images showed that SCP-Se NPs presented a monodisperse uniform spherical structure with a particle diameter of almost 120 nm (Figure 5A), similar to the above results shown in Figure 4D. The XRD result showed that there was only a broad diffraction peak at lower angles ( Figure 5B), indicating that SCP-Se NPs were amorphous according to the JCPDS database. Furthermore, the elemental composition of SCP-Se NPs was also analyzed by EDX, as shown in Figure 5C-F. It was found that five signals were obtained, in which the proportion of carbon was 65.5%, followed by selenium (18.5%), oxygen (15.3%), calcium (0.400%), and sodium (0.0200%). To clarify the possible banding mechanism between selenium and SCP, FTIR was employed to provide information on their interaction. As shown in Figure   To clarify the possible banding mechanism between selenium and SCP, FTIR was employed to provide information on their interaction. As shown in Figure

Stability of SCP-Se NPs at Different Storage Temperatures
To investigate the influence of the temperature on the stability of the SCP-Se NPs colloidal solution, the well-prepared colloidal solution from the same reaction was divided into two parts. One was kept in a RT environment, and another was stored at 4 • C. During the monitoring period (lasting for 14 d), the visual changes, particle size, PDI, and Zeta potential of the SCP-Se NPs colloidal solution were recorded. As shown in Figure 7A, the solution remained uniformly dispersed for 14 d when it was kept at 4 • C, while visible precipitation occurred on the 14th day of storage at RT ( Figure 7B). Meanwhile, the particle size of SCP-Se NPs at 4 • C was relatively stable, with a particle size of 121 nm on day 0 and 112 nm on day 14, and the PDI remained at a rather low value ( Figure 7C). By contrast, the particle size and the PDI of SCP-Se NPs at RT gradually increased to 516 nm and 0.420 on the 14th day, respectively ( Figure 7C). Zeta potential results are shown in Figure 7D, and there was no obvious difference between these two different storage conditions. Animals 2023, 13, x FOR PEER REVIEW 10 of 19 Figure 6. FTIR spectra of SCP and SCP-Se NPs.

Stability of SCP-Se NPs at Different Storage Temperatures
To investigate the influence of the temperature on the stability of the SCP-Se NPs colloidal solution, the well-prepared colloidal solution from the same reaction was divided into two parts. One was kept in a RT environment, and another was stored at 4 °C. During the monitoring period (lasting for 14 d), the visual changes, particle size, PDI, and Zeta potential of the SCP-Se NPs colloidal solution were recorded. As shown in Figure  7A, the solution remained uniformly dispersed for 14 d when it was kept at 4 °C, while visible precipitation occurred on the 14th day of storage at RT ( Figure 7B). Meanwhile, the particle size of SCP-Se NPs at 4 °C was relatively stable, with a particle size of 121 nm on day 0 and 112 nm on day 14, and the PDI remained at a rather low value ( Figure 7C). By contrast, the particle size and the PDI of SCP-Se NPs at RT gradually increased to 516 nm and 0.420 on the 14th day, respectively ( Figure 7C). Zeta potential results are shown in Figure 7D, and there was no obvious difference between these two different storage conditions.

Effects of SCP-Se NPs on LPS-Induced Enteritis in Mice
The animal experiment flow chart is shown in Figure 8A. During the experiment, mice in the SCP and SCP-Se NPs groups were treated with the corresponding drug solutions for 14 d. As shown in Figure 8B, the body weights of SCP and SCP-Se NPs groups were slightly higher than that of the BC group, but each group was relatively the same amount. After LPS injection, mice in the BC group reacted flexibly and frequently moved with clean and smooth coats and formed stools ( Figure 8C). In contrast, mice in the LPS group were depressed, curled together, had bristled hair, diarrhea, perianal contamination, and were positive for fecal occult blood ( Figure 8C). Furthermore, the enteritis symptoms of mice treated with SCP and SCP-Se NPs were effectively alleviated ( Figure 8C). DAI scoring results are shown in Figure 8D. Among the four groups, the DAI score of the LPS group was the highest, significantly higher than that of the BC group (p < 0.05). Meanwhile, compared with the LPS group, the DAI scores of the SCP group and SCP-Se NPs group decreased significantly, and the difference between it and the SCP-Se NPs group was significant (p < 0.05). For the SCP-Se NPs group, the enteritis symptoms were reduced, and the DAI score was much lower than that of the SCP group, though there was no significant difference between them (p > 0.05).

Effects of SCP-Se NPs on LPS-Induced Enteritis in Mice
The animal experiment flow chart is shown in Figure 8A. During the experiment, mice in the SCP and SCP-Se NPs groups were treated with the corresponding drug solutions for 14 d. As shown in Figure 8B, the body weights of SCP and SCP-Se NPs groups were slightly higher than that of the BC group, but each group was relatively the same amount. After LPS injection, mice in the BC group reacted flexibly and frequently moved with clean and smooth coats and formed stools ( Figure 8C). In contrast, mice in the LPS group were depressed, curled together, had bristled hair, diarrhea, perianal contamination, and were positive for fecal occult blood ( Figure 8C). Furthermore, the enteritis symptoms of mice treated with SCP and SCP-Se NPs were effectively alleviated ( Figure 8C). DAI scoring results are shown in Figure 8D. Among the four groups, the DAI score of the LPS group was the highest, significantly higher than that of the BC group (p < 0.05). Meanwhile, compared with the LPS group, the DAI scores of the SCP group and SCP-Se NPs group decreased significantly, and the difference between it and the SCP-Se NPs group was significant (p < 0.05). For the SCP-Se NPs group, the enteritis symptoms were reduced, and the DAI score was much lower than that of the SCP group, though there was no significant difference between them (p > 0.05).

Effects of SCP-Se NPs on LPS-Induced Intestinal Injuries in Mice
In order to investigate the effects of SCP-Se NPs on LPS-induced intestinal injuries in mice, HE staining was performed on jejunum tissue sections. As shown in Figure 9A, the jejunum villus was complete in structure, orderly arranged, and had no obvious inflammatory cell infiltration in the BC group. Conversely, in the LPS group, a great amount of

Effects of SCP-Se NPs on LPS-Induced Intestinal Injuries in Mice
In order to investigate the effects of SCP-Se NPs on LPS-induced intestinal injuries in mice, HE staining was performed on jejunum tissue sections. As shown in Figure 9A, the jejunum villus was complete in structure, orderly arranged, and had no obvious inflammatory cell infiltration in the BC group. Conversely, in the LPS group, a great amount of inflammatory cell infiltration appeared, the jejunum villus structure was destroyed with epithelial cell exfoliation, the villus was broken and atrophied, villus height significantly decreased (p < 0.05) ( Figure 9B), crypt depth significantly increased (p < 0.05) (Figure 9C), and the ratio of villus height to crypt depth significantly decreased (p < 0.05) ( Figure 9D). Compared with the LPS group, in the SCP and SCP-Se NPs groups, the inflammatory cell infiltration was greatly alleviated, the villus structure was repaired with higher villus height and shallower crypt depth, and the ratio of villus height to crypt depth was significantly elevated (p < 0.05) ( Figure 9B-D).

Influence of SCP-Se NPs on Jejunum ZO-1 Expression
The IHC results of jejunum ZO-1 expression in each group are illustrated in Figure  10A. The yellow-brown points indicate the position of the ZO-1 protein. Compared with the BC group, the jejunum ZO-1 expression area of the LPS group was much smaller, and the color of the yellow-brown points was much lighter. Meanwhile, the quantitative analysis results shown in Figure 10B also revealed the significantly decreased expression level of ZO-1 in the LPS group (p < 0.05). For the SCP and SCP-Se NPs groups, the ZO-1 expression area was obviously expanded, and the yellow-brown color got darker compared with the LPS group. Importantly, the expression level of ZO-1 in the SCP-Se NPs group was significantly increased (p < 0.05), which was close to that of the BC group ( Figure 10B).

Influence of SCP-Se NPs on Jejunum ZO-1 Expression
The IHC results of jejunum ZO-1 expression in each group are illustrated in Figure 10A. The yellow-brown points indicate the position of the ZO-1 protein. Compared with the BC group, the jejunum ZO-1 expression area of the LPS group was much smaller, and the color of the yellow-brown points was much lighter. Meanwhile, the quantitative analysis results shown in Figure 10B also revealed the significantly decreased expression level of ZO-1 in the LPS group (p < 0.05). For the SCP and SCP-Se NPs groups, the ZO-1 expression area was obviously expanded, and the yellow-brown color got darker compared with the LPS group. Importantly, the expression level of ZO-1 in the SCP-Se NPs group was significantly increased (p < 0.05), which was close to that of the BC group ( Figure 10B).

Effects of SCP-Se NPs on the Expression of Inflammatory Cytokines
To investigate the effects of SCP-Se NPs on alleviating LPS-induced intestinal inflammation, the levels of IL-1β, IL-6, and TNF-α were determined. As shown in Figure 11, the serum concentration and the relative mRNA expression levels of IL-1β, IL-6, and TNFα of the LPS group were all significantly elevated compared with those of the BC group (p < 0.05). Administration of SCP and SCP-Se NPs significantly downregulated the expression of IL-1β, IL-6, and TNF-α (p < 0.05). For serum content, all these inflammatory cytokine contents of the SCP-Se NPs group were significantly lower than those of the LPS and SCP groups (p < 0.05). For mRNA expression, the IL-6 mRNA expression level of the SCP-Se NPs group was significantly lower than that of the SCP group (p < 0.05) and all three of these inflammatory cytokines' mRNA expression levels of the SCP-Se NPs group were back to normal, showing no significant difference with those of the BC group (p > 0.05). each group. (C) Jejunum crypt depth in each group. (D) The ratio of jejunum villus height to crypt depth in each group. Different lowercase letters (a-c) indicate significant differences (p < 0.05).

Influence of SCP-Se NPs on Jejunum ZO-1 Expression
The IHC results of jejunum ZO-1 expression in each group are illustrated in Figure  10A. The yellow-brown points indicate the position of the ZO-1 protein. Compared with the BC group, the jejunum ZO-1 expression area of the LPS group was much smaller, and the color of the yellow-brown points was much lighter. Meanwhile, the quantitative analysis results shown in Figure 10B also revealed the significantly decreased expression level of ZO-1 in the LPS group (p < 0.05). For the SCP and SCP-Se NPs groups, the ZO-1 expression area was obviously expanded, and the yellow-brown color got darker compared with the LPS group. Importantly, the expression level of ZO-1 in the SCP-Se NPs group was significantly increased (p < 0.05), which was close to that of the BC group ( Figure 10B).

Effects of SCP-Se NPs on the Expression of Inflammatory Cytokines
To investigate the effects of SCP-Se NPs on alleviating LPS-induced intestinal i mation, the levels of IL-1β, IL-6, and TNF-α were determined. As shown in Figure  serum concentration and the relative mRNA expression levels of IL-1β, IL-6, and of the LPS group were all significantly elevated compared with those of the BC gr < 0.05). Administration of SCP and SCP-Se NPs significantly downregulated the e sion of IL-1β, IL-6, and TNF-α (p < 0.05). For serum content, all these inflammator kine contents of the SCP-Se NPs group were significantly lower than those of the L SCP groups (p < 0.05). For mRNA expression, the IL-6 mRNA expression level of th Se NPs group was significantly lower than that of the SCP group (p < 0.05) and al of these inflammatory cytokines' mRNA expression levels of the SCP-Se NPs grou back to normal, showing no significant difference with those of the BC group (p > 0 Figure 11. Effects of SCP-Se NPs on inflammatory cytokine (IL-1β, IL-6, and TNF-α) express els in LPS-treated mice. (A) Serum inflammatory cytokine (IL-1β, IL-6, and TNF-α) content junum inflammation cytokine (IL-1β, IL-6, and TNF-α) relative expression level. Different low letters (a-c) indicate significant differences (p < 0.05).

Discussion
Enteritis is a common disease in animal husbandry, which seriously affe

Discussion
Enteritis is a common disease in animal husbandry, which seriously affects the healthy development of the animal breeding industry. SCP has demonstrated good therapeutic potential on a variety of enteritis diseases [9,10]. However, its macromolecular structural property causing low bioavailability limits its clinical application. Recent studies found that polysaccharides extracted from herbal medicine could be modified with Se NPs to improve their bioactivity and bioavailability [35]. Therefore, SCP-Se NPs were synthesized for the first time by the sodium selenite reduction method, and their anti-enteritis effects were also investigated ( Figure 1).
In this experiment, purified SCP was firstly obtained by deproteinization and DEAE-52 cellulose column purification ( Figure 2) and then applied to SCP-Se NP synthesis. To investigate the optimal SCP concentration for SCP-Se NP formation, the particle diameter, PDI, and Zeta potential were examined by using dual-wavelength colorimetric and DLS methods. The results showed that bare Se NPs quickly aggregated into black precipitation in the reaction system, the particle diameter and PDI of SCP-Se NPs were the lowest, and the Zeta potential was the highest at the SCP concentration of 0.100 mg/mL ( Figure 4). These results demonstrate that SCP acts as a stabilizer, and 0.100 mg/mL SCP is the optimal concentration for SCP-Se NP synthesis. Meanwhile, TEM, XRD, and EDX were applied to further characterize the SCP-Se NPs ( Figure 5). We found that the SCP-Se NPs were amorphous, sphere-like particles with a particle diameter of almost 120 nm, consistent with the result detected by DLS ( Figure 4C), and the selenium content of SCP-Se was 18.5%, similar to the previous research [36]. These results further confirm the successful synthesis of the SCP-Se NPs. FTIR spectroscopy analysis is an important means to identify the interaction between polysaccharide and Se NPs. Some studies have found that the O-H peak and C-O-H band of some polysaccharide-bound Se NPs did not change significantly, indicating that only a weak interaction between Se and hydroxyl groups occurred without breaking the chemical bond of the hydroxyl group on the Se surface [37]. The red or blue shift could be observed in the O-H peak and C-O-H band of some polysaccharide-bound Se NPs, implying the formation of the new bonds [38,39]. In this study, the hydroxyl peak at 3393 cm −1 in the SCP spectrum was red-shifted to 3375 cm −1 in the SCP-Se NP spectrum, indicating a reduction of free hydroxyl groups after Se linkage attributed to the formation of O-H···Se bonds. The peak at 1093 cm −1 shifted to 1081 cm −1 , indicating the formation of C-O···Se bonds ( Figure 6). This is similar to the combination mode of 1,6-a-D-glucan selenium nanoparticles (CPA-Se NPs) [40]. As shown in Figure 4F, SCP-Se NPs were negatively charged and displayed stability in water. Furthermore, the stability analysis showed that SCP-Se NPs easily aggregated at RT (Figure 7), indicating that the proper storage environment of SCP-Se NPs should be at 4 • C.
Gram-negative bacterial infection is one of the main pathogenic factors of enteritis in livestock and poultry production. LPS is the major cell wall component of the Gramnegative bacteria, commonly used to establish an animal enteritis model of Gram-negative bacterial infection [41,42]. In this study, LPS induced typical symptoms of enteritis, such as diarrhea, weight loss, fecal occult blood, and intestinal tissue damage, indicating the animal intestinal injury model was successfully established (Figures 8 and 9). Interestingly, the symptoms of enteritis and the intestinal pathological injury were obviously attenuated when treated with SCP and SCP-Se NPs, and the effects of SCP-Se NPs were better than SCP (Figures 8 and 9). These results suggest that SCP-Se NPs have better anti-intestinal injury activity than SCP in vivo.
The intestinal barrier plays a key role in preventing pathogen invasion and maintaining intestinal environmental homeostasis. Tight junction proteins (TJP) are the main proteins to maintain the function of the gastrointestinal barrier [43][44][45]. Zonula occludens-1 (ZO-1) is an important member of the TJP family that acts as a scaffold and is the main component of TJPs [46]. Loss of ZO-1 results in a significant increase in the permeability of the colonic mucosa, which allows endotoxin to enter the intestine and leads to marked intestinal inflammation in a mouse model of colitis [47]. In this study, we found that both SCP-Se NPs and SCP treatment effectively alleviated LPS-induced impairment of ZO-1, and the effect of SCP-Se NPs was more pronounced (Figure 10). These results indicate that SCP-Se NPs could more effectively protect the intestinal barrier function than SCP.
When pathogens invade the intestinal cavity of the body, they immediately active the innate immune response by regulating the corresponding signaling pathways, such as JAK-STAT, MAPKs, PI3K-AKT, and NF-κB, to produce a large number of cytokines, such as IL-1β, IL-6, and TNF-α [48]. IL-1, mainly secreted by monocytes, macrophages, and epithelial cells, has strong pro-inflammatory activity and can induce a variety of proinflammatory mediators, such as cytokines and chemokines [49]. IL-6 is mainly produced by macrophages and T helper cells and can exert pro-inflammatory functions by activating a variety of target cells, including antigen-presenting cells (APCs) and T cells [50]. TNF-α is one of the earliest and most important mediators of the inflammatory response. During pathogen invasion, intestinal macrophages recruited from blood monocytes rapidly transform into a pro-inflammatory phenotype and produce large amounts of TNF-α once activated [51]. Although the secretion of these cytokines is important for pathogen elimination, the excessive immune response is a key trigger for the impairment of intestinal barrier function. For example, TNF-α can significantly destroy the expression and distribution of TJPs, and IL-1β and IL-6 can not only regulate the reorganization of cytoskeleton proteins, but also can directly lead to the rearrangement of TJPs and weaken the barrier function [52][53][54]. Furthermore, studies have also found that the reduction of IL-1β content in intestinal tissue can improve colitis [55]. Anti-TNF-α targeting therapies can improve the clinical score and mucosal healing in IBD patients [56]. The present study found that treatment of SCP-Se NPs more effectively reduced LPS-induced elevation of IL-1β, IL-6, and TNF-α relative to SCP (Figure 11), closely correlated with its anti-enteritis effects. These results may suggest that SCP-Se NPs can more effectively alleviate the excessive inflammatory response against LPS-induced intestinal injury than SCP, indicating that SCP-Se NPs may serve as good candidates in preventing and treating enteritis in the livestock and poultry industry.

Conclusions
In conclusion, SCP-Se NPs were synthesized and characterized for the first time and showed a better effect against LPS-induced intestinal injury than SCP, suggesting that SCP-Se NPs may serve as a good candidate for preventing and treating enteritis in the livestock and poultry industry.  Institutional Review Board Statement: The animal study was conducted in accordance with the guidelines approved by the Southwest University Laboratory Animal Welfare Ethics Committee (Ethics NO.: IACUC-20210915-03).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.