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Bachu Mushroom Polysaccharide Alleviates Colonic Injury by Modulating the Gut Microbiota

Objective This study was to define the protective effect of purified Helvella leucopus polysaccharide (p-HLP) against dextran sulfate sodium- (DSS-) induced colitis. Methods The novel p-HLP was isolated from Bachu mushroom through hot water extraction, ethanol precipitation, and column chromatography. Then, we evaluated the potential effects of p-HLP on colonic histopathology, inflammation, and microbiota composition in DSS-induced colitis mice. Results p-HLP was a homopolysaccharide with an average molecular weight of 39.14 × 108 Da. Functionally, p-HLP significantly attenuated DSS-induced body weight loss and colon shortening. The histological score of the colon lesion was significantly decreased upon p-HLP treatment. Also, p-HLP treatment led to decreased expression of proinflammatory cytokines and mediators (IL-6, IL-1β and TNF-α, and COX-2 and iNOS) and increased expression of anti-inflammatory cytokine (IL-10) in the colon tissues. Illumina MiSeq sequencing revealed that p-HLP modulated the composition of the gut microbiota. Conclusion p-HLP is a potent regulator that protects the lesions from DSS-induced colitis.


Introduction
Inflammatory bowel disease (IBD), an umbrella term for chronic and recurrent inflammation in the digestive tract, is characterized by abdominal pain, chronic diarrhea, rectal bleeding fatigue, and weight loss [1]. IBD encompasses ulcerative colitis and Crohn's disease, and this disease has become a noticeable public health burden worldwide with increasing prevalence [2]. Although the precise pathogenic mechanism of IBD is not fully understood, a complex interaction between IBD-related etiologies, including host genetics, environmental risk factors, immune dysfunction, and dysregulation of the gut microbiota, has been associated with the risk of IBD onset and progression [3].
Unbalanced expression of cytokines, such as increased production of proinflammatory cytokines IL-6, IL-1β, and TNF-α and decreased production of anti-inflammatory cytokine IL-10 and IL-4, is coupled to the risk of acute colitis [4]. Such an abnormal expression leads to mucosal injury and consequent epithelial barrier damage, aberrant immune responses, and dysbiosis of gut microbiota associated with functional changes in the microbial metabolome [5]. The changes of gut microbiota may in turn affect the physiology and immunology of the host [6][7][8].
Previously, various treatments including aminosalicylate, corticosteroids, immunomodulators, Janus kinase inhibitors, and biological therapies [9][10][11] have been evidenced as beneficial to IBD; however, these drugs may also have limitations, which are mainly the high recurrences of clinical manifestations, long treatment cycles, and low responses to initial treatment [12]. Therefore, it is desirable to identify healthy and natural compounds with IBD prevention activity. Latterly, novel therapeutic options based on gut microbiota modulation have been suggested [13]. For examples, the in vitro and in vivo anti-inflammatory effects of several natural compounds, including the edible mushroom polysaccharides (MPs), have been critically studied for their nontoxicity and high efficiency [14,15]. The administrations of MPs, as shown in various clinical studies, contribute to the integrity of intestinal mucosa by regulating inflammatory responses and the gut microbiota in IBD patients [16].
Helvella leucopus is distributed in Populus euphratica forests in Tarim Basin, also called Bachu mushroom by local residents in Bachu County (Xinjiang, China). Bachu mushroom is an edible and medicinal wild fungus enriched with various bioactive components including polysaccharides, adenosine, cordycepic acid, and minerals [17]. Traditionally, it is broadly used for tuberculosis, fester, bellyache, stomach ailments, hypercholesterolemia, diabetes, and cerebral and cardiovascular diseases [18]. However, so far, no report has connected Helvella leucopus polysaccharide (HLP) to the anti-inflammatory effect and the regulation of gut microbial ecosystem in colitis.

Materials and Chemicals.
Dried fruit bodies of the Bachu mushroom were obtained from Bachu County in Xinjiang Province, China. All chemical reagents used in highperformance liquid chromatography (HPLC) and ultrahigh performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis were of mass spectrometry grade and purchased from Thermo Fisher Scientific Inc. (Waltham, MA, USA). Dextran sulfate sodium salt (36-50 kDa) was obtained from MP Biomedicals (Santa Ana, CA, USA). Antibodies against TNF-α, IL-6, IL-1β, IL-10, iNOS, COX-2, and GAPDH were purchased from Key-GEN Biotech (Nanjing, China).

Extraction and Purification of Polysaccharides from
Bachu Mushroom. The dried fruiting bodies of Bachu mushroom were mixed with double-distilled water in the ratio of 1 : 30 (w/v) and then boiled for 2 h twice. The collected supernatant was combined and concentrated to 1/4 volume at 60°C by a rotary evaporator (RE-2000A, Yarong Co. Ltd., Shanghai, China). After that, the protein content was removed more than 3 times with chloroform : butanol (4 : 1, v/v) following the Sevag method [19]. The obtained solution was precipitated with 4 volumes of 80% ethanol for 24 h at 4°C. After centrifugation at 12000 r/min for 10 min, the polysaccharides were collected and lyophilized. The residue was dissolved in distilled water (50 mg/mL) and dialyzed with a cellulose membrane (5 kDa) with distilled water at room temperature for 48 h, followed by lyophilization to obtain crude HLP.
The dried aqueous polysaccharide sample (100 mg) was separated by its polarity, was weighed, dissolved in 3 mL of distilled water, and centrifuged at 12000 rpm for 10 min. The supernatant was subjected to purification by using a polysaccharide gel purification system combined with a differential detector (RI-502, Shodex, Tokyo, Japan) for online detection and collection. A symmetrical peak with high polysaccharide content was collected; then, the sample was concentrated by a rotary evaporator at 60°C and lyophilized to obtain purified HLP (p-HLP).

Determination of Monosaccharide Composition.
After dialysis and lyophilization, the monosaccharide composition was analyzed by a Thermo ICS5000+ chromatography system (Thermo Fisher Scientific, MMAS, USA) with a Dio-nex™ CarboPac™ PA200 column as described previously with some modifications [20].

Determination of Glycosidic
Linkages. Reduction of uronic acid was performed according to the protocol reported by Taylor and Conrad [21]. Then, the obtained reduced substances were subjected to methylation analysis as described by Anumula and Taylor [22] with some modifications. In this procedure, 10 mg of polysaccharide was dissolved in DMSO/NaOH and incubated for 30 min. After incubation, methyl iodide solution was added for 1 h reaction, and then, the samples were methylated with CH3I. The methylated products were hydrolyzed with trifluoroacetic acid (TFA, 2 mol/L, 100 μL) at 121°C for 90 min. After reduction, the hydrolysates were converted to partially methylated alditol acetates (PMAAs) and analyzed by electron ionization mass spectrometry (EI-MS, Agilent 7890A/ 5977B, USA).

Animals and Experimental
Design. Male C57BL/6 mice (SPF grade, 6-8 weeks old) were purchased from the Model Animal Research Center of Nanjing University, and five mice were housed per cage under a 12-12 h light/dark cycle at 20-22°C and 55 ± 5% relative humidity for one week before the experiment. For each group of experiments, mice were matched by body weight. All experimental procedures and protocols were approved by the Institutional Animal Care and Use Committee of Nanjing University, Nanjing, China (Approval No. JN. No2018-035210-225A).
After adaption, experimental subjects were randomly assigned to 5 groups (n = 5 each group): the negative control (NC) group, dextran sulfate sodium (DSS) model group, p-HLP group, low-dose p-HLP (DSS+p-HLP(L)) group, high-dose p-HLP (DSS+p-HLP(H)) group, and p-HLP group. NC group mice received drinking water from day 0 to day 7. DSS group mice were allowed access to water containing 3% DSS for 7days to induce ulcerative colitis, and water was replaced every other day. In the DSS+p-HLP(L) group, mice were treated by oral gavage with p-HLP (10 mg/kg BW/d) for 7days combined with DSS induction same as the DSS group. In the DSS+p-HLP(H) group, mice were orally administered with p-HLP (50 mg/kg BW/d) for 7 days combined with DSS induction same as the DSS group. In the p-HLP group, mice received normal drinking water and orally administered with p-HLP (100 mg/kg BW/d) for 7days. As shown in Table S1, the disease activity index (DAI) was recorded daily, which is the combined score of body weight loss, stool consistency, and bleeding [23]. On day 8, all animals were sacrificed to collect colon tissue, spleen, and fecal samples. Then, the colon length and spleen weight were measured.
2.5. Histopathological Analysis. The colon samples were rinsed with cold PBS (phosphate-buffered saline), and then, the distal part of each colon was fixed with 10% paraformaldehyde and embedded in paraffin. Tissue sections (5 μm) were stained with hematoxylin-eosin (HE) for histological evaluation and microscopic analysis. The histological score was also evaluated based on a previously described method [24] for DSS-induced colitis (Table S2).
2.6. RNA Isolation and RT-qPCR. Total RNA was isolated from the colonic tissues using the FastPure Cell/Tissue Total RNA Isolation Kit (Vazyme, Nanjing, Jiangsu, China), and cDNA was obtained using the PrimeScript™ RT Master Mix (TaKaRa, Beijing, China). Real-time quantitative polymerase chain reaction (RT-qPCR) was conducted in triplicate on a QuantStudio 5 (Thermo Fisher Scientific, Shanghai, China) using the PowerUp™ SYBR™ Green Master Mix (Thermo Fisher Scientific, Shanghai, China) according to the manufacturer's protocol. The relative amount of each transcript was normalized to the expression of the housekeeping gene (GAPDH), and data were analyzed according to the 2 -ΔΔCt method. The primer sequences for TNF-α, IL-1β, IL-6, IL-10, iNOS, and COX-2 are shown in Table S3.
2.7. DNA Extraction and PCR Amplification. Microbial DNA was extracted from colonic samples using the E.Z.N.A.® Soil DNA Kit (Omega Bio-Tek, Norcross, GA, U.S.). The V4 region of the bacterial 16S rRNA gene was amplified using the primers 515F (5 ′ -barcode-GTGCCAGCMGCCGCGG-3 ′ ) and 806R (5 ′ -GGACTACNVGGGTWTCTAA-3 ′ ). The PCR products were examined by 2% agarose electrophoresis, purified using the AxyPrep DNA Gel Extraction Kit (Axygen Biosciences, Union City, CA, U.S.), and quantified by Qubit®3.0 (Life Invitrogen) according to the manufacturer's instructions. The amplicons were normalized, pooled, and paired-end sequenced (2 × 250) on an Illumina MiSeq platform (Shanghai Biozeron Co., Ltd.). Merged reads were analyzed with QIIME v2, and low-quality reads were removed. After paired read merging and chimera filtering, the phylogenetic affiliation of each 16S rRNA gene sequence (herein, called RSVs) was analyzed by the RDP Classifier (http:// rdp.cme.msu.edu/) against the SILVA (SSU132) 16S rRNA database using a confidence threshold of 70%. Finally, oper-ational taxonomic units (OTUs) were classified using BLASTn against a curated database derived from Greengenes, RDPII, and NCBI. LEfSe (linear discriminant analysis (LDA) effect size) analysis was performed to determine the size effect of each distinctively abundant taxon [25]. A significance value of <0.05 combined with an effect size threshold of 4 was used for LEfSe analysis.
2.8. Statistical Analysis. Statistical analysis was performed by using two-way analysis of variance (ANOVA) or one-way ANOVA followed by Tukey's multiple comparison with SPSS 22.0 software (IBM, Chicago, IL, USA). Data are presented as the mean ± standard deviation (SD) or as boxand-whisker plots unless otherwise indicated, and P < 0:05 was considered statistically significant.

Characterization of Compositional Features of p-HLP.
The crude p-HLP was initially isolated by water extraction along with alcohol precipitation with approximately 4.25% of original weight of the dry powder of Bachu mushroom. After isolation, the highest yielding independent elution peak fraction was further purified by a polysaccharide gel purification system combined with a differential detector. A single, symmetrical, and narrow peak that indicated as a homogeneous polysaccharide was eluted (Figure 1(a)). The fraction was collected, concentrated, and lyophilized with a yield of approximately 2.8% of crude polysaccharides in weight, and it was designated as p-HLP.

p-HLP Potently Attenuated DSS-Induced Acute Colitis
Symptoms. Next, the function of p-HLP was tested in a colitis mouse model (3% DSS, Methods). As presented in Figure 2, DSS treatment caused a significant body weight loss (Figures 2(a) and 2(b)), whereas the administration of p-HLP, especially at a high dose (50 mg/kg), significantly reversed the body weight loss (P <0.01) (Figures 2(a) and 2(b)). Both low and high doses of p-HLP markedly alleviated the DAI compared with that of the DSS group (Figure 2(c)). Also, the DSS-induced colon shortening was significantly suppressed by both low and high doses of p-HLP (P < 0:05 and P < 0:01, respectively) (Figures 2(d) and 2(e)). Moreover, DSS induced a significant increase in the spleen index (the ratio of spleen weight to body weight), whereas the index returned to near normal levels in both low and high p-HLP-dosed animals (P < 0:01) (Figure 2(f)). No significant difference, it should be noted, was identified between the NC group and the p-HLP alone group in all the tests, suggesting that p-HLP at the tested level (100 mg/kg BW/d) had no detectable effect on mice.

DSS-Induced Gut Lesions
Were Ameliorated with p-HLP Treatment. We further investigated the therapeutic value of p-HLP in DSS-induced colitis by HE staining. As shown in Figures 3(a) and 3(e), the NC group and the p-HLP alone group showed normal morphology of the colons, including a complete mucosal layer accompanied by well-arranged crypts, rich intestinal glands, and goblet cells. In contrast, the administration of DSS induced the severe damage on the colon epithelium, the pronounced decrease in the number of crypts, and the infiltration of inflammatory cells in the muscular layer in the colon tissues (Figure 3(b)). However, p-HLP treatment, especially at a high dose, relieved the histological lesions, resulting in a relatively intact epithelium and glandular structure and causing regenerative crypts, along with less inflammatory cell infiltration and mild submucosal edema (Figures 3(c) and 3(d)). The histological index in the DSS-induced mice was markedly reduced after the high dose of p-HLP (P < 0:001) (Figure 3(f)), although the low dose of p-HLP showed a reduced protective effect in the histological index (Figure 3(f)).

p-HLP Reduced the Expression of Inflammatory Genes in
Colitis Colon. We next measured the mRNA levels of inflammatory cytokines and factors (IL-6, IL-1β, TNF-α, IL-10,  Computational and Mathematical Methods in Medicine iNOS, COX-2) in the colon tissues by RT-qPCR. As expected, DSS treatment resulted in increases in the levels of proinflammatory cytokine IL-6, IL-1β, and TNF-α and proinflammatory factors iNOS and COX-2 (P < 0:01) (Figures 4(a)-4(f)) and decreases in the anti-inflammatory cytokine IL-10 (P < 0:01) (Figure 4(d)). p-HLP administration, specifically at a high dose, markedly downregulated the productions of IL-6, IL-1β, TNF-α, and iNOS and COX-2, and upregulated IL-10, when compared with those of the DSS group (P < 0:01) (Figures 4(a)-4(f)). The mRNA levels of inflammatory cytokines and factors were not changed in the p-HLP alone group when compared to those in the NC group (Figures 4(a)-4(f)).

p-HLP Regulated the Gut Dysbiosis in DSS-Induced
Colitis. To assess the effect of p-HLP on gut microbiota, we conducted a 16S rRNA sequencing analysis with a high sequencing depth. The rarefaction curves of all tested groups have reached their plateaus, indicating that the sequencing depth was adequate ( Figure S1A). The alpha diversities presented as Shannon, Chao1, and Simpson indices were used to estimate the richness/evenness of the gut microbiota in each group. Compared with normal mice, the Shannon and Chao1 indices exhibited a reduction after a high dose of p-HLP treatment (Table S5). A Venn diagram revealed the shared and unique operational taxonomic units (OTUs) across the groups ( Figure S1B). A total of 1632 OTUs were shared by all the groups. Notably, the unique numbers of OTUs in the NC and p-HLP(H) groups were slightly reduced (from 853 to 823), while the changes of the unique detected OTUs in DSS (730) vs. p-HLP(L) (812) and p-HLP(H) (569) were bidirectional. Nevertheless, the high-dose P-HLP treatment reduced the number of unique gut OTUs in both NC and DSS backgrounds (P < 0:05).
The compositions of the gut microbiota at the phylum and genus levels were assessed and presented in stacked histograms ( Figure S1C, D). The top five phyla were actually shared in all groups and were Firmicutes, Bacteroidetes, Proteobacteria, Verrumicrobiota, and Epsilonbacteraeota, accounting for >98% of the total microbes ( Figure S1C, Table 2). In the NC group, the proportion of Firmicutes was greater than that of Bacteroidetes, whereas in the DSS group, the relative abundances of the two were more alike each other (P < 0:05) ( Figure S1C). However, the trends of the changes of Firmicutes and Bacteroidetes abundances in low and high p-HLP-dosed groups were different. In the low-dose p-HLP group, the abundances of the two phyla were as those in the NC group ( Figure S1C), while in the high-dose p-HLP-treated mice, Bacteroidetes was significantly higher than Firmicutes (P < 0:05) ( Figure S1C). Interestingly, compared with the other four groups, the relative abundances of Verrumicrobiota and Proteobacteria in the p-HLP alone group were significantly increased and accounted for approximately 30% of the total OTUs (P < 0:01) ( Figure S1C).
The compositions of the gut microbiota were also analyzed at the genus levels. The genera with relative abundance less than 0.1% were not analyzed. Eventually, 20 genera constituting >78% of the total gut microbiota were selected for analysis ( Figure S1D; Table 3). The results showed the genera Muribaculaceae_norank occupied an obvious leading role in the microbiota composition of the DSS mice with/without p-HLP treatment, while two Lachnospiraceae genera, Lachonospiraceae_NK4A136_group and Lachnospiraceae_unclassified, were increased in DSS animals but reducing along with p-HLP treatment in a dose-sensitive manner ( Figure 5(a)). In contrast, there was an increase in the relative abundance of Lactobacillus in the p-HLP(L) and p-HLP(H) groups compared with that in the DSS group ( Figure 5(a)).
The beta diversity described the structural variability of the gut microbiota as whole between the groups. With the NC group positioned at the central-basal area in the space defined by PC1 (33.26%) and PC2 (23.63%), the DSS group was on the top of the NC group, while the p-HLP, p-HLP(L), and p-HLP(H) groups were distributed either left or right to the NC group ( Figure 5(b)), indicating that the main differences of the microbes between DSS and NC as well as p-HLP-treated mice were in the PCA2.
LEfSe analysis further determined the microorganisms with significant differences among the five groups at different taxonomic levels (Figures 5(c) and 5(d)). Overall, we  To estimate the overall function of gut microbiotas that was altered after DSS and p-HLP treatment, we employed the Tax4Fun2 analysis, an R package for the prediction of habitat-specific functional profiles and functional redundancy based on 16S rRNA gene sequences. The NC, DSS, and p-HLP(H) groups were selected in this section since they had the most distinct phenotypical characteristics. According to analysis, the pathway of amoebiasis was significant in abundance of the three groups (https://www.genome .jp/kegg-bin/show_pathway?ko05146), with a high level of

Discussion
Accumulating evidences have demonstrated that MPs posed favorable therapeutic and health-promoting properties, including anticancer [26], antiviral [27], anti-inflammatory effects [28], and immunomodulatory function [29]. Bachu mushroom is an edible and medicinal mushroom and traditionally used for the treatment of various diseases including digestive diseases, such as stomach ailments [18]. However, Bachu mushroom, especially p-HLP, has not been studied in colitis. In this paper, the polysaccharide from the Bachu mushroom was purified followed by determination of structure features of p-HLP, and the effect of p-HLP in DSS-induced colitis was studied. The p-HLP with the Mw of 93:84 × 10 8 Da was identified, which is different from two previously defined Bachu mushroom polysaccharides, HLP1-1 and HLP2-1, with the Mw of 21,382 Da and 23,038 Da,   Functional assays showed that after p-HLP treatment, mice with DSS-induced colitis had significant improvement in the DAI score, a comprehensive evaluation metric of disease severity based on weight loss, stool consistency, and occult blood. Furthermore, colon length and spleen index, the additional key indicators reflecting the severity of intestinal inflammation, were also markedly ameliorated upon p-HLP treatment. The histological analysis revealed that p-HLP, particularly at a high dosage, reduced the DSSinduced histological score for measuring pathological symp-toms. These results argue that p-HLP treatment effectively protects against DSS-induced colitis.
It has been reported that proinflammatory cytokines play a vital role in intestinal inflammation in IBD [31]. Other proinflammatory factors, such as COX-2 and iNOS, have also been shown to affect the integrity of the colonic mucosa and induce intestinal damage in colitis [32]. However, polysaccharides, especially those isolated from mushrooms, e.g., L-theanine [33], can limit the gut inflammation by reducing the proinflammatory cytokines/ mediators and increasing anti-inflammatory cytokines in the colon tissue in colitis [34]. Similarly, we found a potent effect of p-HLP in relieving DSS-induced inflammation in colon tissue in a dose-dependent manner.
The DSS-induced dysregulation of the gut mucosal immune system is believed to introduce excessive immunological responses towards bacteria, leading to alterations in gut microbial composition [35]. As reported, natural polysaccharides can deliver the health benefit potentially through the regulation of the gut microbiota [36]. Therefore, we performed a high-throughput sequencing of the V4 region of bacterial 16S rRNA using an Illumina MiSeq platform, aiming to investigate the impact of p-HLP on the gut microbiota in DSS-induced colitis mice. The beta diversity by the PCA plot suggested that colonic microbial community at the OTU level was significantly changed in DSS mice upon p-  9 Computational and Mathematical Methods in Medicine HLP treatment. DSS, it should be noted, significantly increased the Lachonospiraceae_NK4A136_group and Lachnospiraceae_unclassified, which is in line with the report that Lachnospiraceae exhibited higher relative abundance in IBD patients and mouse models with more severe colitis [37]. Previous in vitro and in vivo studies have indicate that consumption of polysaccharides from plants or fungi can effectively alleviate the symptoms of IBD by promoting the growth of beneficial bacteria (i.e., Akkermansia, Bifidobacterium, and Lactobacillus) [38][39][40][41]. It is interesting to note that p-HLP also increased the abundance of beneficial bacteria Lactobacillus. In addition, p-HLP in normal controls greatly enhanced the abundance of Akkermansia, but not in DSS mice. It was also observed that a high dose of p-HLP treatment remarkably increased the population of Bacteroidetes, the best carbohydrate-degrading bacteria in the colon, which is consistent with findings in African children consuming high-plant polysaccharide diets [42]. Finally, the result of the prediction of habitat-specific functional profiles suggested an increased suppression of inflam-matory responses upon p-HLP treatment, which is consistent with the findings in the section regarding inflammatory cytokines.

Conclusion
In conclusion, the purified Bachu mushroom polysaccharide has a protective effect on DSS-induced colitis in mice. Such an effect is achieved by its anti-inflammatory activity; specifically, p-HLP can downregulate proinflammatory cytokines (IL-6, IL-1β, and TNF-α) and proinflammatory mediators (COX-2 and iNOS) and upregulate proinflammatory cytokines(IL-10) in a dose-dependent manner. Additionally, the structure and composition of gut microbiota are altered after p-HLP treatment in mice with DSS-induced colitis; p-HLP significantly promotes Lactobacillus and downregulates the abundance of Lachnospiraceae_NK4A136 and Lachnospiraceae_unclassified. Collectively, p-HLP may have the potential to serve as an effective candidate in the treatment of IBD.   Figure 5: Effect of p-HLP on gut microbiota diversity indices in DSS-induced colitis mice. Letters A-E represent NC, DSS, DSS+p-HLP(L), DSS+p-HLP(H), and p-HLP groups, respectively. (a) Relative abundances of the 3 most typical gut bacteria at the genus level. One-way ANOVA followed by Tukey's test was used to evaluate the statistical significance. The different letters (a and b) represent significant differences between different groups (P < 0:05). (b) Beta diversity was measured using PCA plots. Each point represents an individual sample, and the points with different colors belong to different treatments. The distance between pairs of points represent the similarity or difference in the microbial community structures. (c) Cladogram illustrating highly abundant taxa across various treatments. (d) Linear discriminant analysis (LDA) indicates the most differentially abundant bacterial taxa in specific samples.