Here we present a protocol to investigate the effect of the nullification of gustation-related genes on immune responses in a dextran sulfate sodium (DSS)-induced inflammatory bowel disease (IBD) mouse model.
Inflammatory bowel disease (IBD) is one of the immune-related gastrointestinal disorders, including ulcerative colitis and Crohn’s disease, that affects the life quality of millions of people worldwide. IBD symptoms include abdominal pain, diarrhea, and rectal bleeding, which may result from the interactions among gut microbiota, food components, intestinal epithelial cells, and immune cells. It is of particular importance to assess how each key gene expressed in intestinal epithelial and immune cells affects inflammation in the colon. G protein-coupled taste receptors, including G protein subunit α-gustducin and other signaling proteins, have been found in the intestines. Here, we use α-gustducin as a representative and describe a dextran sulfate sodium (DSS)-induced IBD model to evaluate the effect of gustatory gene mutations on gut mucosal immunity and inflammation. This method combines gene knockout technology with the chemically induced IBD model, and thus can be applied to assess the outcome of gustatory gene nullification as well as other genes that may exuberate or dampen the DSS-induced immune response in the colon. Mutant mice are administered with DSS for a certain period during which their body weight, stool, and rectal bleeding are monitored and recorded. At different timepoints during administration, some mice are euthanized, then the sizes and weights of their spleens and colons are measured and gut tissues are collected and processed for histological and gene expression analyses. The data show that the α-gustducin knockout results in excessive weight loss, diarrhea, intestinal bleeding, tissue damage, and inflammation vs. wild-type mice. Since the severity of induced inflammation is affected by mouse strains, housing environment, and diet, optimization of DSS concentration and administration duration in a pilot experiment is particularly important. By adjusting these factors, this method can be applied to assess both anti- and pro-inflammatory effects.
The two major forms of inflammatory bowel disease (IBD), Crohn's disease (CD), and ulcerative colitis (UC) are characterized by chronic remittent or progressive inflammatory conditions of the intestine with multifactorial etiology1,2. The development of IBD depends on genetic as well as certain environmental factors such as diet, antibiotic use, and importantly, pathogenic infections. However, the etiology and regulatory molecular mechanisms underlying IBD are still unclear. Hence, numerous chemically induced IBD animal models have been constructed and applied to delineate the pathogenesis and regulatory mechanisms and evaluate the effectiveness of human therapeutics3.
Taste receptors are G protein-coupled receptors (GPCRs) and are classified as two major types: type I (T1Rs), and type II (T2Rs) that detect sweet, umami, and bitter compounds. Taste signaling cascades are initiated by tastant binding to T1Rs or T2Rs, activating the heterotrimeric G proteins consisting of α-gustducin and a Gβγ dimer and leading to release of the Gβγ subunits. The Gβγ moiety in turn stimulates the downstream effector enzyme phospholipase C-β2 (PLC-β2). Activated PLC-β2 then hydrolyzes phosphatidylinositol-4,5-bisphosphate into two intracellular secondary messengers [inositol-1,4,5-trisphosphate (IP3) and diacylglycerol], and IP3 binds to and open its channel-receptor IP3R3, releasing calcium ions from the endoplasmic reticulum. This eventually leads to the opening of transient receptor potential ion channel Trpm5 and release of the neurotransmitter ATP onto the gustatory nerves4,5,6,7. Yet, the signaling pathways of salty and sour tastes are different and independent from sweet, umami, and bitter tastes8. In addition, the components of taste GPCRs and downstream proteins exist in various extra-oral tissues. Recent studies indicated that α-gustducin, the principal component of taste signaling, is found to be expressed in the intestinal mucosa. Further studies are needed to understand the functions of these taste signaling components in extra-oral tissues9,10.
The method described here is used to characterize functions of the gustatory signaling proteins expressed in extra-oral tissues. We combine a transgenic mouse line developed for delineating signaling cascades in taste buds with the chemically induced colitis model. Largely due to its procedural simplicity and pathological similarities to human ulcerative colitis, the dextran sulfate sodium (DSS)-induced IBD model has been most widely used among the various chemically induced colitis models11. In this study, we used α-gustducin-deficient mice as a representative mouse line to reveal novel functions of α-gustducin in gut mucosal immunity and inflammation by 1) analyzing morphological changes in the tissue and 2) assaying differences in the expression of cytokines related to inflammation in the colon. This method can be used to quantitatively and qualitatively determine the contributions of gustatory signaling proteins (and other proteins expressed in the gut) to tissue damage and intestinal inflammation, when genetically modified mouse lines for the genes of interest are available. Advantages of this method are enabling users to obtain integrated data resulting from actions of both the chemical DSS and deficiency of the gene of interest. This method can be further improved to increase its sensitivity and reveal subtle intestinal changes at the cellular and molecular levels.
All experiments involving mice were reviewed and approved by the Institutional Animal Care and Use Committees of Zhejiang University. It is advised to wear appropriate personal protective equipment before performing this protocol.
1. Preparation of Mice and DSS
2. Induction and Evaluation of DSS Colitis in Mice
3. Preparation of Tissue Samples
4. Histological Assessment of the Severity of DSS-induced Colitis
5. Gene Expression Assessment of DSS-induced Colitis
A DSS-induced IBD procedure was established by administrating 3% DSS in drinking water to α-gustducin-knockout (KO) and wild-type (WT) mice. Compared to WT mice, the knockout mice exhibited more severe colitis with excessive weight loss, diarrhea, and intestinal bleeding (Figure 1). After a 7 day DSS administration, the differences in tissue integrity were analyzed using H&E staining as the histological method, and more aggravated tissue damage was found in the proximal, middle, and distal colons of the knockout mice than in WT mice (Figure 2). Furthermore, the excessive immune activation led to colitis with infiltration of various inflammatory cells such as macrophages and neutrophils. Immunohistochemical analyses using a number of markers for the immune cells were carried out to determine whether the α-gustducin deficiency affected immune cell infiltration. Comparative analysis showed that the infiltration of leukocytes, neutrophils, and macrophages was significantly increased in the knockout mice compared to WT control mice (Figure 3). Finally, some cytokine expression levels in the colons of DSS-induced colitis mice were determined using qPCR with gene-specific primers. Results showed that compared to WT mice, the knockout mice had higher expression levels of TNF and IFN-γ but lower expression levels of IL-5, IL-13, and IL-10; however, no difference was seen in the expression level of TGF-β1 between the knockout and WT mice (Figure 4).
Figure 1: DSS administration renders more severe colitis in α-gustducin knockouts (KO). (A) Percentage of body weight loss: KO mice displayed significantly more body weight loss starting from day 3. (B) Colitis disease index based on the severity of diarrhea and rectal bleeding: KO mice showed significantly greater disease indices than WT controls. (C) Colon (upper panel) and spleen (lower panels) from representative WT and KO mice 7 days post-DSS administration: KO mice had significantly shorter colons and larger spleens. Error bars represent SEM (*p < 0.05, **p < 0.005, ***p < 0.0005; ANOVA with post hoc t-tests). Scale bar = 1 cm. This figure has been modified from a previous publication13. Please click here to view a larger version of this figure.
Figure 2: α-gustducin KO mice show more severe tissue damage following DSS treatment. (A) H&E staining of colon tissues from WT and α-gustducin KO mice not treated with DSS (water) or treated with DSS for 7 days (DSS). (B) Tissue injury scores based on histological staining of colon tissues from WT and KO mice 7 days after DSS administration. DSS treatment induced some tissue damage in WT colons, which was much worse in the KO specimen. Error bars represent SEM (*p < 0.05). Scale bar = 50 µm. This figure has been modified from a previous publication13. Please click here to view a larger version of this figure.
Figure 3: α-gustducin KO mice display increased infiltration of immune cells in DSS-induced colitis. (A) Massive immune cell infiltration in the colons of DSS-treated KO mice. (B) Quantification of immune cell numbers: the percentage of immunostained areas divided by the total area of measured tissue based on image analyses. Error bars represent SEM. Scale bar = 50 µm. This figure has been modified from a previous publication13. Please click here to view a larger version of this figure.
Figure 4: α-gustducin KO mice display different expression levels of immune cytokines in DSS-induced colitis. DSS treatment increased expression of TNF and IFN-γ and decreased expression of IL-5, IL-13, and IL-10 in the KO mice colons compared to WT mice. Real-time quantitative RT-PCR was performed using gene-specific primers. β-actin was used as an internal control gene. Error bars represent SEM (*p < 0.05, **p < 0.005). This figure has been modified from a previous publication13. Please click here to view a larger version of this figure.
Table 1: List of primers used.
This method can be employed to quantitively determine the effect of mutations of specific gustatory genes on inflammation in a DSS-induced IBD mouse model. To take full advantage, optimal induction of IBD is a key step. The development of colitis is affected by several factors, including mouse strain, housing environment, intestinal microflora, as well as the genes of interest. It is recommended to perform a pilot experiment with a small number of mice to test different dosages and durations of DSS administration. During the pilot experiment, gross symptoms such as weight loss, diarrhea, intestinal bleeding, and some microscopic changes such as tissue damage, inflammation associated with immune cell infiltration, and expression-level changes of cytokines should be analyzed and compared to control groups that have drinking water without DSS16,17,18,19. Severity of the induced colitis can be monitored by analyzing daily changes in mouse body weight and scores of the collected stool during the DSS-treatment period. After DSS treatment, the tissue damage can be assessed by H&E staining on frozen sections of the colon, whereas infiltration of immune cells and expression levels of cytokines can be identified and quantified using immunohistochemistry and qPCR. DSS dosage, administration duration, and diet can be adjusted to reveal subtle effects of gustatory gene mutations on the colonic immune responses. However, the sensitivity of this method may still limit its application to studies on the minimal effects of some other genes. In this case, some modifications can be adopted; for example, by minimizing the variables from individual-specific intestinal microflora by administrating antibiotics.
The DSS-induced colitis model has been used the most extensively among chemical agent-induced IBD models, largely due to its simplicity, rapidity, reproducibility, controllability, and most importantly its similarities to human ulcerative colitis, which is useful in evaluating the effectiveness of human therapeutics2. One disadvantage researchers must be aware of is that T and B cells are not required for the development of colitis, which is different from development of the disease in humans. However, it may be useful for studying the role of the innate immune system in the development of acute colitis2.
This study has established a DSS-induced IBD model using α-gustducin-deficient mice, which can be used to investigate novel functions of the G protein α subunit in gut mucosal immunity and inflammation. The results show that mice lacking α-gustducin are more susceptible to DSS-induced colitis and are accompanied by more severe symptoms, including tissue damage, excessive inflammatory responses, and altered expression of potent cytokines13. In agreement with recent findings indicating the involvement of taste-like chemosensory pathways in type II immune responses to gut parasites20, α-gustducin and other taste signaling proteins may have novel and important functions in the intestinal immune balance. However, the exact molecular mechanisms underlying these skewed immune responses remain to be uncovered.
Furthermore, this method can be applied to study effects of other gustatory genes that are expressed in the colon (e.g., the transient receptor potential ion channel Trpm5, which is critical to sweet, bitter, and umami tastes and expressed in human colonic cells7). Finally, the same strategy can be used to discover new functions of key proteins and factors in intestinal immunity and help evaluate the effectiveness of novel treatments of certain human diseases.
The authors have nothing to disclose.
This work is supported by grants from the National Natural Sciences Foundation of China (81671016, 31471008, and 31661143030) and National Institutes of Health (DC010012, DC015819) and by the Siyuan Foundation.
Antibody | |||
CD45 | BD Biosciences | 550539 | |
CD3 | BD Biosciences | 555273 | |
B220 | BD Biosciences | 550286 | |
CD11b | BD Biosciences | 550282 | |
Ly6G | BD Biosciences | 551459 | |
Reagent | |||
Dextran Sulfate Sodium Salt (DSS) | MP Biomedicals | 2160110 | |
Streptavidin-HRP complex | BD Pharmingen | 551011 | |
H&E Staining Kit | BBI Life Sciences | E607318 | |
Phosphate Buffered Saline (PBS) | Sangon Biotech | B548117 | |
FastStart Universal SYBR Green Master(ROX) | Roche | 4913850001 | |
MMLV Reverse Transcriptase, GPR | Clontech,TaKaRa | 639574 | |
TaKaRa MiniBEST Universal RNA Extraction Kit | TaKaRa | 9767 | |
BD 10 ml Syringe | BD Biosciences | 309604 | |
Instruments and equipment | |||
balance | |||
scissors | |||
forceps | |||
centrifuge | |||
qPCR machine | |||
staining jars | |||
Software | |||
Imag-Pro Plus | Media Cybernetics, Inc. |