4.1 WGPA is the Effective Subfraction For the Anti-Colitis Activity of WGP
In this study, the colitis model was induced by adding 5% DSS orally to the drinking water of rats for 7 days and was used to investigate the effects of ginseng polysaccharides and their sub-fractions on colitis rats. The results showed that the Wistar rats exhibited physiological and pathological manifestations of colitis. Compared with the DSS group, WGP and WGPA significantly improved the diarrhea status of rats, including a decrease in the DAI index, and an increase in the colon length, colon weight, reduced the intestinal injury. The recovery effects of WGP and WGPA were better than that of WGPN.
The function of the intestinal mucosal barrier is largely dependent on intercellular tight junctions (TJs) (19). TJs are the most apical intercellular complexes in epithelial cells and are composed of transmembrane barrier proteins (e.g., Claudins, Occludin, and junctional adhesion molecules) and cytoplasmic scaffolding proteins (e.g., the ZO family, AF-6, and Cingulin) (20, 21). The downregulation of TJ proteins leads to an increase in the permeability of the intestinal epithelium, which might induce bacterial translocation, the risk of intestinal infection, and inflammation (22). In this study, the expression of the Occludin and ZO-1 proteins was significantly downregulated in the DSS group, which suggests that DSS destroys the mucosal layer and increases intestinal permeability. In contrast, WGP promoted the expression of ZO-1 and Occludin in the colon significantly, suggesting that ginseng polysaccharides repair the intestinal epithelium after DSS damage, while the WGPN and WGPA treatments only promoted the levels of ZO-1. Therefore, regarding colon structure repair, WGP was more effective than WGPN and WGPA.
4.2 Potential Mechanism of WGP and WGPA on Anti-Colitis Activity
The lamina propria T cells increase the production of Th17-associated cytokines (IL17A and a little IFN-γ), which in turn upregulate the expression of IL-1, IL-6, and IL-8 during the process of colitis (23, 24). The levels of inflammatory cytokines reflect the degree of inflammation of the intestinal mucosa. Here, we chose IL-1β, IL-2, IL-6, and IL-17 as representative cytokine factors, with the aim of detecting changes in the levels of inflammatory molecules in the colon. It was shown that DSS aggravated the inflammation levels, while the three ginseng polysaccharide fractions significantly reduced the expression of these inflammatory cytokines in the colon. Concomitantly, the WGPN and WGPA sub-fractions exerted stronger effects on inflammation than did WGP.
NF-κB pathway is a very important signal pathway in inflammation response. The occurrence of colitis was closely related with NF-κB pathway (6). The results of our study also identified the activation of NF-κB pathway in colitis rats that was induced by DSS. Ginseng polysaccharide could significantly inhibit the activation of the NF-κB pathway via regulation of the TLR4 and MyD88 protein expression, especially WGPA treatment, further inhibiting the production of inflammation factors. Notably, we found that WGNP can also suppress this signaling pathway to a certain level, which was in accordance with the results of cytokine levels. These results suggested that although WGPN did not play a role in colitis treatment, it might have functions in other inflammatory diseases.
Dysbiosis of gut microbiota is an important feature of colitis. The diversity of gut microbiota is reduced in patients with colitis, together with the decrease in SCFA-producing bacteria and the increase in mucus-dissolving bacteria, sulfate-reducing bacteria, and pathogenic bacteria (3). Here, we found that the Simpson and Shannon indexes of fecal microbiota were decreased significantly in the DSS group compared with the Con group, suggested the dysbiosis and the destruction of the structure of the gut microbiota. After treatment with WGP and WGPA, the diversity of fecal microbiota was increased and became similar to that of the Con group, which was indicative of recovery from dysbiosis. WGPN did not exert obvious effects on the diversity of fecal microbiota. These results indicate that the beneficial effects of WGP and WGPA on colitis might be associated with their ability of adjusting the diversity of the gut microbiota. We also found that WGP and WGPA recovered the composition of fecal microbiota at the phylum level. The key microbiota changes afforded by WGP and WGPA that played a role in the DSS treatment were also observed at the genus level. The relative abundance of Ruminococcus was significantly decreased in the DSS group compared with the Con group; however, this abundance was increased to normal levels after treatment with WGP or WGPA, while there was no significant change after treatment with WGPN. Ruminococcus is a common bacterium that produces SCFAs and has beneficial effects on hosts (25). SCFAs are important metabolites in the intestinal microbial environment and are closely related to immune, anti-tumor, and anti-inflammatory activities (26). Some plant polysaccharides that are not digested by the host are fermented by a series of anaerobic probiotics. Because of the different structure and sources of polysaccharides, different kinds of SCFAs are produced (27). WGPA was rich in acidic pectin, which is often referred to as a prebiotic that ferments in the colon and produces beneficial metabolites, especially SCFAs. Since the gut microbiota adjusting ability, especially the increasing effects on Ruminococcus and the SCFAs production, ginseng polysaccharide WGP and WGPA might used as potential prebiotics, which help for against diarrhea, gut microbiota dysbiosis, or other colonic related diseases.
WGPN and WGPA were purified from the ginseng polysaccharide extract, WGP. WGPN is a starch-like glucan that belongs to the group of neutral polysaccharides. In contrast, WGPA is an acidic pectin that is rich in type I rhamnogalacturonan and homogalacturonan (12). The different structural features of these sub-fractions determine their different activities. Of the two purified fractions, WGPA showed more obvious beneficial effects on colitis compared with WGPN, not only regarding the normal status, but also regarding gut microbiota diversity and composition. WGP could protect against colistis through inhibit the NF-κB pathway, adjust the diversity and composition of gut microbiota and recover the intestinal barrier.
4.3 The Comparison of WGP and Other Herbal Polysaccharides on Anti-Colitis Activity
The Chinese herbal medicine has a long history of application in Asia, especially in China, and had the characteristics of natural and non-toxic; certain medicinal food was the primary choice for daily healthcare, but the mechanisms underlying some of these activities were not clear. Polysaccharides from many traditional Chinese medicines can be used to treat and relieve colitis, such as certain non-starch polysaccharides, including glucan from oat bran, mushroom, seaweed, pectin, gum, prebiotics, etc (28). Other types of polysaccharide, such as Dictyophora indusiata polysaccharide could alleviate the severity of colitis by improving the gut epithelial integrity and inflammatory reactions, which was similar to the WGP and its effective fraction WGPA (29). Polysaccharides extracted from Blidingia minima showed an anti-inflammatory effect on DSS-treated colitis by repaired colonic dysfunction, and improved colonic morphology, infiltration, and the expression of tight junction, pro-inflammatory cytokines, as well as the protein levels of NF-κB in colonic tissue (30). A novel alkali-soluble polysaccharide from purple sweet potato could restore the immune organ indices, increased colon length, improved colonic histopathology in colitis mice as well as inhibited the levels of pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) in colonic tissue, ameliorated the compositions and functions of gut microbiota in DSS-induced colitis mice (31). Different from the WGP and WGPA, it could adjust the relative abundance of Parasutterella, Desulfovibrio, Lachnospiraceae, Lactobacillus, Erysipelotrichaceae, and Bacteroidetes. Altogether, it seems that most of the polysacchrides that could work against colitis can alleviate colonic inflammation via blocking pro-inflammatory cytokines, however, not all of them could affect the same type of gut microbiota. Further investigation of the crosstalk between epithelial cells and gut microbiota for evaluating the effects of different types functional polysaccharide will lead to well-designed clinical intervention trials and improved treatment and prevention of colitis.
Although some poisitve results have been obtained on the effects and mechanism of ginseng polysaccharide on colitis in this research, it should be noted that there are still some deficiencies and limitations. For example, the relationship between initial body weight, final body weight, weight increment, food intake, and energy conversion rate has not been compared and discussed, which may also have an impact on the physical condition of rats. In terms of mechanism research, the detail of the signaling pathway was not fully discussed in this research. In the following work, we will focus more on these aspects and further study the therapeutic effect of ginseng polysaccharides on colitis and its specific mechanism, so as to provide data basis for conquer with colitis.