The N-terminal region of serum amyloid A3 protein activates NF-κB and up-regulates MUC2 mucin mRNA expression in mouse colonic epithelial cells

Mucin glycans and their degradation by gut microbiota

The human intestinal tract is inhabited by a tremendous number of microorganisms, which are collectively termed "the gut microbiota". The intestinal epithelium is covered with a dense layer of mucus that prevents penetration of the gut microbiota into underlying tissues of the host. Recent studies have shown that the maturation and function of the mucus layer are strongly influenced by the gut microbiota, and alteration in the structure and function of the gut microbiota is implicated in several diseases. Because the intestinal mucus layer is at a crucial interface between microbes and their host, its breakdown leads to gut bacterial invasion that can eventually cause inflammation and infection. The mucus is composed of mucin, which is rich in glycans, and the various structures of the complex carbohydrates of mucins can select for distinct mucosa-associated bacteria that are able to bind mucin glycans, and sometimes degrade them as a nutrient source. Mucin glycans are diverse molecules, and thus mucin glycan degradation is a complex process that requires a broad range of glycan-degrading enzymes. Because of the increased recognition of the role of mucus-associated microbes in human health, how commensal bacteria degrade and use host mucin glycans has become of increased interest. This review provides an overview of the relationships between the mucin glycan of the host and gut commensal bacteria, with a focus on mucin degradation.

Orchestration of MUC2 - The key regulatory target of gut barrier and homeostasis: A review

The gut mucosa of human is covered by mucus, functioning as a crucial defense line for the intestine against external stimuli and pathogens. Mucin2 (MUC2) is a subtype of secretory mucins generated by goblet cells and is the major macromolecular component of mucus. Currently, there is an increasing interest on the investigations of MUC2, noting that its function is far beyond a maintainer of the mucus barrier. Moreover, numerous gut diseases are associated with dysregulated MUC2 production. Appropriate production level of MUC2 and mucus contributes to gut barrier function and homeostasis. The production of MUC2 is regulated by a series of physiological processes, which are orchestrated by various bioactive molecules, signaling pathways and gut microbiota, etc., forming a complex regulatory network. Incorporating the latest findings, this review provided a comprehensive summary of MUC2, including its structure, significance and secretory process. Furthermore, we also summarized the molecular mechanisms of the regulation of MUC2 production aiming to provide developmental directions for future researches on MUC2, which can act as a potential prognostic indicator and targeted therapeutic manipulation for diseases. Collectively, we elucidated the micro-level mechanisms underlying MUC2-related phenotypes, hoping to offer some constructive guidance for intestinal and overall health of mankind.

Defects in the expression of colonic host defense factors associate with barrier dysfunction induced by a high-fat/high-cholesterol diet

The effect of Western diets in the gastrointestinal system is largely mediated by their ability to promote alterations in the immunity and physiology of the intestinal epithelium, and to affect the composition of the commensal microbiota. To investigate the response of the colonic epithelium to high-fat/high-cholesterol diets (HFHCDs), we evaluated the synthesis of host defense factors involved in the maintenance of the colonic homeostasis. C57BL/6 mice were fed an HFHCD for 3 weeks and their colons were evaluated for histopathology, gene expression, and microbiota composition. In addition, intestinal permeability and susceptibility to Citrobacter rodentium were also studied. HFHCD caused colonic hyperplasia, loss of goblet cells, thinning of the mucus layer, moderate changes in the composition of the intestinal microbiota, and an increase in intestinal permeability. Gene expression analyses revealed significant drops in the transcript levels of Muc1, Muc2, Agr2, Atoh1, Spdef, Ang4, Camp, Tff3, Dmbt1, Fcgbp, Saa3, and Retnlb. The goblet cell granules of HFHCD-fed mice were devoid of Relmβ and Tff3, indicating defective production of those two factors critical for intestinal epithelial defense and homeostasis. In correspondence with these defects, colonic bacteria were in close contact with, and invading the epithelium. Fecal shedding of C. rodentium showed an increased bacterial burden in HFHCD-fed animals accompanied by increased epithelial damage. Collectively, our results show that HFHCD perturbs the synthesis of colonic host defense factors, which associate with alterations in the commensal microbiota, the integrity of the intestinal barrier, and the host's susceptibility to enteric infections.

Inhibition of lncRNA TCONS_00077866 Ameliorates the High Stearic Acid Diet-Induced Mouse Pancreatic β-Cell Inflammatory Response by Increasing miR-297b-5p to Downregulate SAA3 Expression

Long-term consumption of a high-fat diet increases the circulating concentration of stearic acid (SA), which has a potent toxic effect on β-cells, but the underlying molecular mechanisms of this action have not been fully elucidated. Here, we evaluated the role of long noncoding (lnc)RNA TCONS_00077866 (lnc866) in SA-induced β-cell inflammation. lnc866 was selected for study because lncRNA high-throughput sequencing analysis demonstrated it to have the largest fold-difference in expression of five lncRNAs that were affected by SA treatment. Knockdown of lnc866 by virus-mediated shRNA expression in mice or by Smart Silencer in mouse pancreatic β-TC6 cells significantly inhibited the SA-induced reduction in insulin secretion and β-cell inflammation. According to lncRNA-miRNAs-mRNA coexpression network analysis and luciferase reporter assays, lnc866 directly bound to miR-297b-5p, thereby preventing it from reducing the expression of its target serum amyloid A3 (SAA3). Furthermore, overexpression of miR-297b-5p or inhibition of SAA3 also had marked protective effects against the deleterious effects of SA in β-TC6 cells and mouse islets. In conclusion, lnc866 silencing ameliorates SA-induced β-cell inflammation by targeting the miR-297b-5p/SAA3 axis. lnc866 inhibition may represent a new strategy to protect β-cells against the effects of SA during the development of type 2 diabetes.

Induction of Serum Amyloid A3 in Mouse Mammary Epithelial Cells Stimulated with Lipopolysaccharide and Lipoteichoic Acid

Simple Summary

Serum amyloid A (SAA) is an acute phase protein present in mammals and birds. Based on the amino acid sequence, SAA has been classified into isoforms SAA1–4 in mice. Previously, it was reported that after the stimulation with bacterial antigens, the expression of the Saa3 mRNA was induced more strongly than that of the Saa1 mRNA in mouse epithelia, including colonic and alveolar epithelial cells, indicating that SAA3 plays a role in the local response. However, the contribution of SAA3 to the local response in mouse mammary epithelium, where mastitis occurs due to bacterial infection, has not been completely determined yet. In this study, to clarify whether mouse SAA3 has a role in the defense against bacterial infection in mouse mammary epithelium, normal murine mammary gland (NMuMG) epithelial cells were stimulated with lipopolysaccharide (LPS) and lipoteichoic acid (LTA). LPS and LTA significantly enhanced mRNA expression level of the Saa3 gene but not that of Saa1. Furthermore, LPS induced SAA3 protein expression more strongly than LTA. Our data indicate that SAA3 expression in mouse mammary epithelial cells was increased by the stimulation with bacterial antigens, suggesting that SAA3 is involved in the defense against bacterial infection in mouse mammary epithelium.

Abstract

In this study, to establish whether serum amyloid A (SAA) 3 plays a role in the defense against bacterial infection in mouse mammary epithelium, normal murine mammary gland (NMuMG) epithelial cells were stimulated with lipopolysaccharide (LPS) and lipoteichoic acid (LTA). LPS and LTA significantly enhanced mRNA expression level of the Saa3 gene, whereas no significant change was observed in the Saa1 mRNA level. Furthermore, LPS induced SAA3 protein expression more strongly than LTA, whereas neither LPS nor LTA significantly affected SAA1 protein expression. These data indicate that the expression of SAA3 in mouse mammary epithelial cells was increased by the stimulation with bacterial antigens. SAA3 has been reported to stimulate neutrophils in the intestinal epithelium and increase interleukin-22 expression, which induces activation of the innate immune system and production of antibacterial proteins, such as antimicrobial peptides. Therefore, collectively, these data suggest that SAA3 is involved in the defense against bacterial infection in mouse mammary epithelium.

Mucus barrier, mucins and gut microbiota: the expected slimy partners?

The gastrointestinal tract is often considered as a key organ involved in the digestion of food and providing nutrients to the body for proper maintenance. However, this system is composed of organs that are extremely complex. Among the different parts, the intestine is viewed as an incredible surface of contact with the environment and is colonised by hundreds of trillions of gut microbes. The role of the gut barrier has been studied for decades, but the exact mechanisms involved in the protection of the gut barrier are various and complementary. Among them, the integrity of the mucus barrier is one of the first lines of protection of the gastrointestinal tract. In the past, this 'slimy' partner was mostly considered a simple lubricant for facilitating the progression of the food bolus and the stools in the gut. Since then, different researchers have made important progress, and currently, the regulation of this mucus barrier is gaining increasing attention from the scientific community. Among the factors influencing the mucus barrier, the microbiome plays a major role in driving mucus changes. Additionally, our dietary habits (ie, high-fat diet, low-fibre/high-fibre diet, food additives, pre- probiotics) influence the mucus at different levels. Given that the mucus layer has been linked with the appearance of diseases, proper knowledge is highly warranted. Here, we debate different aspects of the mucus layer by focusing on its chemical composition, regulation of synthesis and degradation by the microbiota as well as some characteristics of the mucus layer in both physiological and pathological situations.

Hepatic transcriptome study of Taenia asiatica infection in suckling pigs

Taenia asiatica is a crucial Taenia that is prevalent in East and Southeast Asia. Domestic pigs and wild boars are essential intermediate hosts for Taenia. Cysticercus larvae are mainly parasitic in the liver of domestic pigs. The Taenia asiatica was collected from Liangmu Township, Duyun City, Guizhou Province. Twelve Yorkshire Suckling pigs of 20 days of age were randomly divided into an experimental and control group of 6 pigs each. RNA sequencing (RNA-seq) technology was used to detect the expression differences of the mRNA transcriptomes in the liver of the experimental and control group at different infection times. Differential genes were analyzed by bioinformatics and verified by Real Time-PCR(RT-PCR). On the 15th and 75th days after infection, 152 and 558 differentially expressed genes were detected in the liver of the experimental group, respectively, accounting for 0.85% and 3.12% of all identified transcribed RNA genes, respectively. Through GO and KEGG related bioinformatics analysis, it was found that these differentially expressed genes are involved in the immune response, material metabolism, fibrosis, and tissue proliferation and repair of suckling pig liver, and related to MHC antigen processing and presentation, cytochrome P450, transforming growth factor-beta (TGF-β) signaling pathway and so on. Cysticercus asiatica parasites cause significant differential gene expression in the liver of suckling pigs. Specific differentially expressed genes are involved in biological processes such as liver metabolism, immune response, and tissue repair or regeneration in suckling pigs. The immune evasion is related to the immuno-suppressive response of the intermediate host.

In vitro and ex vivo expression of serum amyloid A3 in mouse lung epithelia

BACKGROUND AND PURPOSE

Serum amyloid A (SAA), an acute-phase protein whose level tracks infection and inflammation, is the precursor protein of amyloid A (AA) fibrils that is thought to cause AA amyloidosis in human and animals. SAA protein has several isoforms based on the difference of amino acid sequence, such as SAA1 to SAA4 in mice. AA fibrils are associated with chronic inflammation and are mainly originated from SAA1 produced in the liver. SAA3 reportedly contributes to the innate immune response in epithelia; however, little is known about its role at the lung epithelia. Therefore, we investigated SAA3 expression in the lung epithelium activated by bacterial antigens.

MATERIALS AND METHODS

The expressions of SAA3 and SAA1 mRNA were investigated using quantitative real-time PCR, in vitro using mouse Clara (Club) cells and ex vivo using surgically removed mouse lungs, after their stimulation by using either lipopolysaccharide (LPS), the major outer membranous antigen of gram-negative bacteria, or lipoteichoic acid (LTA), the major outer membranous antigen of gram-positive bacteria. In addition, SAA3 and SAA1/2 proteins in treated lung samples were detected by immunohistochemistry (IHC).

RESULTS

SAA3 mRNA expression increased in cells and lungs treated with either LPS or LTA. SAA3 mRNA was more sensitively expressed in LPS than LTA treatment. In contrast, SAA1 mRNA expression did not increase by either LPS or LTA treatment. Furthermore, SAA3 mRNA expression increased in a dose-dependent manner in cells treated with tumor necrosis factor-alpha. By IHC, SAA3 protein was highly expressed in the luminal side of the bronchial epithelium, while SAA1/2 was not expressed.

CONCLUSION

These results obtained from in vitro and ex vivo experiments suggest that SAA3 plays an important role in the innate immune response to bacterial infection in the lung epithelia.

Biological Characterization of Commercial Recombinantly Expressed Immunomodulating Proteins Contaminated with Bacterial Products in the Year 2020: The SAA3 Case

The serum amyloid A (SAA) gene family is highly conserved and encodes acute phase proteins that are upregulated in response to inflammatory triggers. Over the years, a considerable amount of literature has been published attributing a wide range of biological effects to SAAs such as leukocyte recruitment, cytokine and chemokine expression and induction of matrix metalloproteinases. Furthermore, SAAs have also been linked to protumorigenic, proatherogenic and anti-inflammatory effects. Here, we investigated the biological effects conveyed by murine SAA3 (mu rSAA3) recombinantly expressed in Escherichia coli. We observed the upregulation of a number of chemokines including CCL2, CCL3, CXCL1, CXCL2, CXCL6 or CXCL8 following stimulation of monocytic, fibroblastoid and peritoneal cells with mu rSAA3. Furthermore, this SAA variant displayed potent in vivo recruitment of neutrophils through the activation of TLR4. However, a major problem associated with proteins derived from recombinant expression in bacteria is potential contamination with various bacterial products, such as lipopolysaccharide, lipoproteins and formylated peptides. This is of particular relevance in the case of SAA as there currently exists a discrepancy in biological activity between SAA derived from recombinant expression and that of an endogenous source, i.e. inflammatory plasma. Therefore, we subjected commercial recombinant mu rSAA3 to purification to homogeneity via reversed-phase high-performance liquid chromatography (RP-HPLC) and re-assessed its biological potential. RP-HPLC-purified mu rSAA3 did not induce chemokines and lacked in vivo neutrophil chemotactic activity, but retained the capacity to synergize with CXCL8 in the activation of neutrophils. In conclusion, experimental results obtained when using proteins recombinantly expressed in bacteria should always be interpreted with care.

Pasteurized Akkermansia muciniphila protects from fat mass gain but not from bone loss

Probiotic bacteria can protect from ovariectomy (ovx)-induced bone loss in mice. Akkermansia muciniphila is considered to have probiotic potential due to its beneficial effect on obesity and insulin resistance. The purpose of the present study was to determine if treatment with pasteurized Akkermansia muciniphila (pAkk) could prevent ovx-induced bone loss. Mice were treated with vehicle or pAkk for 4 wk, starting 3 days before ovx or sham surgery. Treatment with pAkk reduced fat mass accumulation confirming earlier findings. However, treatment with pAkk decreased trabecular and cortical bone mass in femur and vertebra of gonadal intact mice and did not protect from ovx-induced bone loss. Treatment with pAkk increased serum parathyroid hormone (PTH) levels and increased expression of the calcium transporter Trpv5 in kidney suggesting increased reabsorption of calcium in the kidneys. Serum amyloid A 3 (SAA3) can suppress bone formation and mediate the effects of PTH on bone resorption and bone loss in mice and treatment with pAkk increased serum levels of SAA3 and gene expression of Saa3 in colon. Moreover, regulatory T cells can be protective of bone and pAkk-treated mice had decreased number of regulatory T cells in mesenteric lymph nodes and bone marrow. In conclusion, treatment with pAkk protected from ovx-induced fat mass gain but not from bone loss and reduced bone mass in gonadal intact mice. Our findings with pAkk differ from some probiotics that have been shown to protect bone mass, demonstrating that not all prebiotic and probiotic factors have the same effect on bone.

Short communication: Recombinant mammary serum amyloid A3 as a potential strategy for preventing intramammary infections in dairy cows at dryoff

Mammary serum amyloid A3 (M-SAA3) has shown potential in stimulating innate immunity during intramammary infections, at calving and at dryoff. In this study, we produced recombinant caprine M-SAA3 to test its ability to reduce intramammary infections with Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, and Escherichia coli, which are all common mastitis-producing pathogens. Recombinant production of M-SAA3 (followed by lipopolysaccharide removal to avoid lipopolysaccharide-nonspecific stimulation of the immune system) was successfully achieved. Mammary serum amyloid A3 stimulated the expression of IL-8 in a dose-dependent manner in primary mammary cultures. Although a direct killing effect on Staph. aureus by M-SAA3 was not detected, this acute phase protein was able to reduce Staph. aureus, Strep. uberis, and Strep. dysgalactiae infections by up to 50% and induced a reduction in E. coli counts of 67%. In general, the best concentration of caprine M-SAA3 for inhibiting infections was the lowest concentration tested (10 μg/mL), although higher concentrations (up to 160 μg/mL) increased its antimicrobial potential against some pathogens.

Lipopolysaccharide and lipoteichoic acid enhance serum amyloid A3 mRNA expression in murine alveolar epithelial cells

Serum amyloid A (SAA) is an acute-phase protein indicative of inflammation. In murine colonic epithelial cells, lipopolysaccharide (LPS), a gram-negative bacterial antigen, strongly enhanced mRNA expression of SAA3, but not SAA1 or SAA2, suggesting that SAA3 might respond to bacterial infection in other epithelia. We examined SAA1/2 and SAA3 mRNA expression in murine alveolar epithelial cells exposed to LPS or the gram-positive bacterial antigen, lipoteichoic acid (LTA), using real-time PCR. LPS enhanced SAA3 mRNA expression at lower concentrations than did LTA, whereas SAA1/2 mRNA expression was not enhanced by either LPS or LTA. These results suggest that SAA3 expression is enhanced in lung epithelium upon bacterial infection as part of innate immunity, with higher sensitivity to LPS than to LTA.

Matrix Metalloproteinase-9-Generated COOH-, but Not NH2-Terminal Fragments of Serum Amyloid A1 Retain Potentiating Activity in Neutrophil Migration to CXCL8, With Loss of Direct Chemotactic and Cytokine-Inducing Capacity

Serum amyloid A1 (SAA1) is a prototypic acute phase protein, induced to extremely high levels by physical insults, including inflammation and infection. Human SAA and its NH₂-terminal part have been studied extensively in the context of amyloidosis. By contrast, little is known about COOH-terminal fragments of SAA. Intact SAA1 chemoattracts leukocytes via the G protein-coupled receptor formyl peptide receptor like 1/formyl peptide receptor 2 (FPR2). In addition to direct leukocyte activation, SAA1 induces chemokine production by signaling through toll-like receptor 2. We recently discovered that these induced chemokines synergize with intact SAA1 to chemoattract leukocytes in vitro and in vivo. Gelatinase B or matrix metalloproteinase-9 (MMP-9) is also induced by SAA1 during infection and inflammation and processes many substrates in the immune system. We demonstrate here that MMP-9 rapidly cleaves SAA1 at a known consensus sequence that is also present in gelatins. Processing of SAA1 by MMP-9 at an accessible loop between two alpha helices yielded predominantly three COOH-terminal fragments: SAA1(52–104), SAA1(57–104), and SAA1(58–104), with a relative molecular mass of 5,884.4, 5,327.3, and 5,256.3, respectively. To investigate the effect of proteolytic processing on the biological activity of SAA1, we chemically synthesized the COOH-terminal SAA fragments SAA1(52–104) and SAA1(58–104) and the complementary NH₂-terminal peptide SAA1(1–51). In contrast to intact SAA1, the synthesized SAA1 peptides did not induce interleukin-8/CXCL8 in monocytes or fibroblasts. Moreover, these fragments possessed no direct chemotactic activity for neutrophils, as observed for intact SAA1. However, comparable to intact SAA1, SAA1(58–104) cooperated with CXCL8 in neutrophil activation and migration, whereas SAA1(1–51) lacked this potentiating activity. This cooperative interaction between the COOH-terminal SAA1 fragment and CXCL8 in neutrophil chemotaxis was mediated by FPR2. Hence, proteolytic cleavage of SAA1 by MMP-9 fine tunes the inflammatory capacity of this acute phase protein in that only the synergistic interactions with chemokines remain to prolong the duration of inflammation.

Serum amyloid A1 (SAA1) protein in human colostrum

Proteins of the serum amyloid A (SAA) family have been remarkably conserved in evolution. Their biologic function(s) are not fully defined but they are likely to be a part of primordial host defense. We have detected a ∼ 12‐kDa protein reacting with antibodies against serum amyloid A (SAA) in human colostrum by western blotting. Mass spectrometry identified the reactive species as SAA1, previously identified as a prominent member of the acute‐phase response in serum. Our finding SAA1 in human colostrum contrasts with bovine, caprine and ovine colostrum where a species corresponding to putative SAA3 is uniformly present. SAA1 protein in human colostrum presumably contributes to neonatal protection.