Introduction

The intestine is not only critical for the digestion and absorption of nutrients, but also interacts with a complex external milieu1. As the largest component of innate immune system, the intestine employs a number of unique strategies to defend against bacterial-derived endogenous and exogenous harmful agents2. However, the intestinal health status could be hazarded by a lot of factors including infection and inflammation3,4. Inflammation can potentially leads to intestinal injury and dysfunction which can result in decreased animal performance and health5. Therefore, maintaining intestinal health is vital for humans and animals.

In weaned piglets, conditions that compromise the integrity of the small intestine may be related to processes of the production of proinflammatory cytokines such as tumor necrosis factor-α (TNF-α)6, which could be induced by activation of inflammatory signaling pathways such as toll-like receptors (TLRs) and nucleotide-binding oligomerization domain proteins (NODs)7,8. TLRs and NODs are important protein families that participate in recognizing pathogen-associated molecular patterns (PAMPs) and modulating congenital antibacterial and inflammatory responses7,9. In the TLRs family, TLR4 is a critical receptor in transduction of the inflammatory response and plays an important role in intestinal homeostasis10. Meanwhile, NOD1 and NOD2 are prominent members in NODs family, whose roles in innate immunity and inflammatory diseases have been studied11, particularly in intestine12,13. After interacting with their specific PAMPs, activated TLRs or NODs trigger downstream signaling pathways that lead to activation of nuclear factor-κB (NF-κB), which further motivates the expression of pro-inflammatory cytokines, including interleukin (IL)-1β, IL-6 and TNF-α7,8. Therefore, exploring nutrients to improve intestinal integrity through inhibiting intestinal TLR4 and NOD signaling pathways further reducing inflammatory response has been a research hotspot in the human or animal nutrition areas.

Mammalian target of rapamycin (mTOR) is a serine-threonine kinase which is related to several important aspects of mammalian cell function14. The activity of mTOR is regulated by various extracellular and intracellular factors such as amino acids, energy and hormones, in turn, mTOR changes rates of translation, transcription, protein synthesis and degradation, cell signaling, metabolism as well as cytoskeleton dynamics15,16,17. Many researchers have presented that activation of mTOR signaling pathway could improve intestinal health status including enhanced intestinal barrier function in Caco-2 cells18, and improved cell proliferation and protein synthesis in intestinal porcine epithelial cells-J2 (IPEC-J2)15,19. Therefore, exploring a nutritional additive to improve intestinal integrity by activating mTOR signaling is also a critical issue which needs to be solved.

Glutamate (Glu), as an acidic amino acid, is traditionally classified as a non-essential amino acid. As the research developed, some studies have shown that Glu has some important functions in nutrition, metabolism and organ health status. Firstly, Glu is one of the major metabolic fuels for small intestine to maintain function and protect mucosal integrity20. In addition, it is important in stabilizing the protein structure21 and activating taste receptors in the digestive tract21,22. Moreover, Glu is a member of the “arginine family”, which comprises arginine, glutamine, glutamate, proline, aspartate, asparagine, ornithine, and citrulline. These amino acids are interconvertible via complex interorgan metabolism to play important roles in the body23. Some studies showed that dietary supplementation with Glu improved intestinal health status23,24. Rezaei et al. reported that monosodium glutamate supplementation maintained the small-intestinal structure and function in weaned pigs24. Jiao et al. reported that Glu enhanced barrier and antioxidative functions in intestinal porcine epithelial cells25. These studies are mostly focused on the beneficial effect of Glu on intestinal morphology, barrier and antioxidative capacity, however, few reports about the mechanisms underlying the protective effect of Glu are found.

Accordingly, we hypothesized that Glu may ameliorate intestinal injury induced by lipopolysaccharide (LPS) through regulating mTOR, TLR4 as well as NOD signaling pathways. The aim of our experiment was to explore whether Glu could attenuate LPS-induced intestinal injury, and to reveal its molecular mechanism(s).

Results

Intestinal morphology

The intestinal mucosal condition in control piglets was good (Fig. 1). However, the LPS-challenged pigs exhibited intestinal injury. After supplementation with Glu, the intestinal mucosal injury was alleviated. Compared with the pigs in LPS group, supplementation with 2.0% Glu increased villus height/crypt depth ratio (VCR, Table 1) in jejunum (p < 0.05).

Figure 1
figure 1

Histological appearance of jejunal mucosa (hematoxylin and eosin). (a) control. (b) LPS. Arrow represents the damaged intestinal mucosa in the LPS-challenged pigs. (c) LPS + 1.0% Glu. (d) LPS + 2.0% Glu. Original magnification 100×. Scale bars = 82.6 μm.

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Table 1 Effects of dietary supplementation with Glu on intestinal morphology of weanling piglets after 4 h Escherichia coli LPS challenge.
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Intestinal protein, DNA and RNA contents

LPS challenge reduced the ratio of RNA to DNA in ileum compared with the pigs in control group (p < 0.05, Table 2). Among the LPS-challenged pigs, supplementation of 2.0% Glu increased the ratio of RNA to DNA in jejunum (p < 0.05). Supplementation with 1.0% Glu increased the ratio of RNA to DNA in ileum (p < 0.05).

Table 2 Effects of dietary supplementation with Glu on intestinal mucosal protein, DNA and RNA concentrations of weanling piglets after 4 h Escherichia coli LPS challenge.
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Intestinal disaccharidase activities

LPS challenge decreased the activity of maltase in jejunum, as well as the activities of maltase and sucrase in ileum of the piglets (p < 0.05, Table 3). However, supplementation with 1.0% Glu increased the activities of maltase and sucrase in ileum compared with pigs in LPS group (p < 0.05). Moreover, supplementation with 2.0% Glu increased the activities of lactase compared with other groups (p < 0.05).

Table 3 Effects of dietary supplementation with Glu on disaccharidase activities of weanling piglets after 4 h Escherichia coli LPS challenge.
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Intestinal claudin-1 protein expression

No difference was observed in intestinal claudin-1 protein expression between control and LPS-challenged piglets (Table 4). Supplementation with Glu increased the protein expression of claudin-1 both in jejunum and ileum compared with the pigs in LPS group (Fig. 2, p < 0.05).

Table 4 Effects of dietary supplementation with Glu on tight junction protein expression in the intestinal mucosa of weanling piglets after 4 h Escherichia coli LPS challenge.
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Figure 2
figure 2

Western blotting analysis of the expression of clauding-1 in the jejunal and ileal mucosa. n = 6.

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Plasma TNF-α concentration

Compared with control piglets, LPS challenge increased TNF-α concentration in plasma (p < 0.05, Table 5). Supplementation with 2.0% Glu decreased plasma TNF-α concentration in piglets compared with LPS group piglets (p < 0.05).

Table 5 Effects of dietary supplementation with Glu on TNF-α concentration in plasma after 4 h Escherichia coli LPS challenge in weaned pigs.
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Intestinal protein abundances of key signaling molecules in mTOR signaling pathway

The piglets challenged with LPS had decreased phosphorylated mTOR (Ser2448) (p-mTOR, p < 0.05) and phosphorylated 4E-binding protein 1 (Thr70) (p-4EBP1, p < 0.05) protein expression in ileum (Table 6, Fig. 3). The ratio of p-4EBP1/t-4EBP1 protein expression in ileum of LPS-challenged pigs was also reduced compared with pigs in control group (p < 0.05). Compared with the pigs in LPS group, supplementation with 2.0% Glu increased jejunal ratio of p-mTOR/t-mTOR (p < 0.05) and supplementation with 1.0% Glu increased ileal ratio of p-mTOR/t-mTOR (p < 0.05).

Table 6 Effects of dietary supplementation with Glu on protein expression of proteins related to mTOR signaling pathway in the intestinal mucosa of weanling piglets after 4 h Escherichia coli LPS challenge.
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Figure 3
figure 3

Western blotting analysis of the expression of mTOR and 4EBP1 in the jejunal and ileal mucosa. n = 6.

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Intestinal mRNA expression of key signaling molecules in TLR4 and NODs signaling pathways

The LPS-challenged pigs had increased mRNA expression of TLR4, Myeloid differentiation protein 2 (MD2), cluster of differentiation 14 (CD14), myeloid differentiation factor 88 (MyD88), IL-1 receptor-associated kinase 1 (IRAK1), TNF receptor associated factor 6 (TRAF6), NOD2, receptor-interacting serine/threonine-protein kinase 2 (RIPK2), NF-κB in jejunum, and NOD2 and RIPK2 in ileum compared with control piglets (p < 0.05, Table 7). Compared with pigs in LPS group, supplementation with 2.0% Glu reduced mRNA expression of jejunal TLR4, MyD88, IRAK1, TRAF6, and NOD2 (p < 0.05).

Table 7 Effects of dietary supplementation with Glu on mRNA expression of key genes related to TLR4 and NODs signaling pathways in the intestinal mucosa of weanling piglets after 4 h Escherichia coli LPS challenge.
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Intestinal mRNA expression of TLR4 and NODs negative regulators

Compared with the pigs in control group, LPS challenge decreased mRNA expression of toll-interacting protein (Tollip), and Erbb2 interacting protein (ERBB2IP) in ileum (p < 0.05, Table 8). Supplementation with 2.0% Glu enhanced mRNA expression of Tollip in ileum (p < 0.05). However, the mRNA expressions of ERBB2IP in jejunum were decreased with the supplementation with 2.0% Glu (p < 0.05).

Table 8 Effects of dietary supplementation with Glu on mRNA expression of negative regulating factors related to TLR4 and NODs signaling pathways in the intestinal mucosa of weanling piglets after 4 h Escherichia coli LPS challenge.
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Discussion

Glu is one of the most abundant amino acids in alimentary proteins26. The majority of Glu molecule is either oxidized as energy or metabolized into other nonessential amino acids via transamination27. Recently, some studies showed that Glu plays an important role in maintaining physiological function as well as energy level in intestine28. In the present study, Glu improved the intestinal integrity and function via maintaining mTOR, as well as suppressing TLR4 and NOD signaling pathways in the piglets under an inflammation condition.

The intestinal healthy status is often reflected by both structural and functional integrity4. Villus height, crypt depth and VCR have been used to quantify the intestinal morphology29. Mucosal protein content is an important indicator for cell metabolism, and protein deficiency negatively affects the intestinal epithelial barrier of piglets30. RNA/DNA ratio reflects the cell capacity for protein synthesis31. Protein/DNA ratio in mucosa has been employed as an indicator of intestinal growth and repair4. Mucosal disaccharidases, namely lactase, maltase and sucrase, reflect intestinal digestive function32. Claudins are major constituents of tight junctions33. In present study, the piglets challenged with LPS had decreased RNA/DNA ratio, and maltase and sucrase activities, which is similar to the findings of Liu et al. and Wang et al.4,34. These results indicate that injection of LPS caused intestinal injury in weaned pigs. Glu supplementation to the LPS-challenged pigs increased jejunal VCR. Besides, supplementation with Glu increased RNA/DNA ratio as well as protein expression of claudin-1 in jejunum and ileum. Furthermore, the pigs fed Glu diets had enhanced ileal disaccharidase activities. These results indicate that Glu had a significant effect on protecting intestinal mucosa and improving intestinal repair. Similarly, there were several studies that showed results in accordance with our findings. Rezaei et al. reported that monosodium glutamate supplementation increased villus height and DNA content in weaned pigs24. In addition, Lin et al. found that Glu supplementation increased the relative mRNA expression of occludin and zonula occludens protein-1 in jejunal mucosa in weaning piglets35.

mTOR is a serine-threonine kinase which is related to several important aspects of mammalian cell function. The activity of mTOR is modulated through various extracellular and intracellular factors (such as amino acids, energy, hormones), in turn, mTOR changes rates of translation, transcription, protein synthesis degradation, cell signaling, metabolism, and cytoskeleton dynamics14. Specially, mTOR signaling pathway plays a key role in maintaining intestinal health15,18,19. 4EBP1, as one of the most well-known substrates of mTOR, is involved in regulation of the rate of protein synthesis36. Phosphorylation of 4EBP1 by mTOR promotes dissociation of 4EBP1 from Eukaryotic initiation factor 4E (EIF4E), enabling EIF4E to induce protein translation37. Recent studies have shown that amino acids as important signaling regulators for intestinal protein synthesis and cell growth via mTOR, could regulate downstream signal 4EBP115,16. In this experiment, supplementation with Glu increased jejunal and ileal ratio of p-mTOR/t-mTOR in the LPS-challenged pigs. These results indicate that Glu activated mTOR signaling pathway, further to improve protein synthesis, which is consistent with increased intestinal RNA/DNA. We speculated that the protective effect of Glu on intestinal mTOR signaling pathway might be due to the following mechanisms. Firstly, Glu phosphorylated mTOR directly. There were reports showed that Glu activated mTOR as neurotransmitter, or the activation of both ionotropic and metabotropic glutamate receptors (mGluR) induced mTOR14. Glu supplementation increased the relative mRNA expression of the jejunal mucosa mGluR1 and mGluR4 in weaning piglets35. Secondly, Glu could be oxidized directly to produce energy which further affects mTOR. Glu has been demonstrated to be one of the major sources of energy in mammalian enterocytes via mitochondrial oxidation21. In addition, Glu might exert its beneficial effects through transforming into arginine and glutamine23. Abundant evidences have shown that arginine and glutamine participate in regulating mTOR and their downstream signals in intestine15,38. For example, there was study demonstrating that arginine-dependent cell survival and protein synthesis signaling in IPEC-J2 cells were mediated by mTOR15. Xi et al. reported that glutamine could regulate protein synthesis in intestinal cells through the mTOR signaling pathway38.

TLR4 and NOD signaling pathways are activated to defense against pathogens invading via triggering the production of pro-inflammatory cytokines and inflammatory response. In our present study, supplementation with 2.0% Glu reduced mRNA expressions of key genes in TLR4 (jejunal TLR4, MyD88, IRAK1 and TRAF6) and NOD (jejunal NOD2) signaling pathways in the pigs challenged with LPS, which is in consistent with the decreased TNF-α concentration in plasma. However, there are few studies on Glu regulating TLR4, NODs and their downstream signals. In the central nervous system, Glu is the major excitatory neurotransmitter26, and its metabolism is involved with inflammation39. So it is possible that Glu exerted a protective effect on inhibiting TLR4 and NOD signaling pathways as an excitatory neurotransmitter, further reducing inflammation in intestine. In addition, as a member of the “arginine family”, Glu can convert into other amino acids (arginine, glutamine, aspartate, asparagine, proline, ornithine and citrulline) via complex interorgan metabolism in most mammals, including the pig23. Glu might exert its beneficial effects through transforming into arginine, asparagine and aspartate23. Some previous evidence has shown that arginine, aspartate, and asparagine participate in regulating TLRs, NOD and their downstream signals in weaned piglets40,41,42.

The aberrant activation of inflammatory signaling pathways (TLR4 and NOD) elicits collateral host tissue injury43. In order to prevent excessive inflammatory responses, many mechanisms can negatively control inflammatory signaling pathways. So far, several negative regulators of TLR4 (such as SOCS1 and Tollip) and NOD (such as ERBB2IP and ACAP1) signaling pathways have been identified and characterized44,45,46. Our data reflected that the LPS-challenged pigs had reduced mRNA expressions of ileal ERBB2IP and Tollip, which is in agreement with the study of Wang et al.40. Our results indicate that LPS challenge down-regulated the gene expressions of intestinal negative regulators in TLR4 and NOD signaling pathways, which is in consistent with increased mRNA expressions of intestinal key genes in TLR4 and NOD signaling-related genes. Thus, the up-regulation of key genes in TLR4 and NOD signaling-related genes might be related to the decrease of their negative regulators. In the present study, supplementation with 2.0% Glu increased mRNA expression of ileal Tollip, decreased gene expression of jejunal ERBB2IP. Wu et al. demonstrated that Tollip down-regulated the TLR4 signaling pathway via bounding to IL-1 receptor-associated kinase (IRAK) and inhibiting IRAK phosphorylation44. This indicates that supplementation with Glu could inhibit the activity of IRAK through increasing the gene expression of Tollip, leading to impair the signaling from TLR4 to downstream pathways and reduce the synthesis of pro-inflammatory cytokines. Curiously, Glu inhibited mRNA expressions of ERBB2IP. It is possible that Glu inhibited TLR4 and NOD signaling pathways directly or through other non-negative regulatory pathways to reduce excessive release of inflammatory cytokines, which resulted in less mRNA expression of ERBB2IP, but this needs to be further studied.

In conclusion, supplementation with Glu alleviates intestinal damage and improves intestinal repair in LPS-challenged piglets. The protective effects of Glu on the intestine may be associated with maintaining mTOR and inhibiting TLR4 and NOD signaling pathways.

Methods

Animal care and experimental design

The animal experiment was carried out in accordance with the Chinese Guidelines for Animal Welfare and Experimental Protocol, and was approved by the Animal Care and Use Committee of Wuhan Polytechnic University. A total of 24 crossbred pigs (Duroc × Large White × Landrace, initial body weight of 7.02 ± 0.21 kg) were used and randomly allotted into 4 treatments. Every treatment included 6 replicate pens. Piglets were housed in 1.80 × 1.10 m2 pens and had free access to drinking water and their diets individually. The basal diet was formulated according to National Research Council requirements for all nutrients (Table 9)47. Ambient temperature was maintained at 25–27 °C.

Table 9 Ingredient composition of experimental diets (%, as-fed basis).
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The 4 treatments were set as follows: (1) control (basal diet + 0.9% NaCl solution); (2) LPS (basal diet + E. coli LPS (E. coli serotype 055: B5; Sigma Chemical Inc., St Louis, MO, USA); (3) LPS + 1.0% Glu (1.0% Glu diet + LPS); (4) LPS + 2.0% Glu (2.0% Glu diet + LPS).

The doses of Glu (purity > 99%; Amino Acid Bio-Chemical, Wuhan, China) were used according to Rezaei’s studies24. In order to obtain isonitrogenous diets, 1.21, 0.61 and 0% alanine (purity > 99%; Amino Acid Bio-Chemical, Wuhan, China) were added to the control, 1.0% Glu and 2.0% Glu diets, respectively. On day 28, pigs were injected intraperitoneally with either LPS at 100 μg/kg body weight or the same amount of 0.9% (w/v) saline. The LPS was dissolved in sterile 0.9% NaCl solution (500 mg LPS /L saline).

Sample collection

At 4 h post-injection, we collected blood samples into heparinized vacuum tubes from the anterior vena cava and centrifuged (3000 r/min, 15 min, 4 °C) to separate plasma. Plasma was stored at −80 °C until analysis of TNF-α. Following blood collection, piglets were harvested using pentobarbital at 80 mg/kg body weight, and 2 gut samples were taken from the mid-jejunum (3 cm and 10 cm) as well as mid-ileum (3 cm and 10 cm). The collection methods of intestinal samples were referred to Liu et al.4. The 3 cm sections were rinsed gently with 0.9% ice-cold saline, and then kept in 4% paraformaldehyde in PBS waiting to determine intestinal morphology. The 10 cm sections were opened and luminal contents were removed and rinsed. Mucosal samples were collected using a sterile glass slide, then immediately packaged and put in liquid nitrogen to froze, and stored at −80 °C for subsequent analysis.

Intestinal morphology

Intestinal samples were dehydrated, embedded in paraffin, sectioned, and stained with haematoxylin and eosin after fixation in 4% paraformaldehyde for 24 h4. Then jejunal and ileal samples were analyzed on histologic slides by a microscope (Olympus, Japan) at 10× magnification using Image-Pro Plus software. Villus height and crypt depth were measured referred to the previous methods reported by Nunez et al.48. Briefly, a minimum of 10 well-oriented and intact villi were chosen. Villus height was defined as the distance from the villus tip to crypt mouth, and crypt depth was measured from crypt mouth to base.

Concentrations of mucosal protein, DNA and RNA

Intestinal mucosal samples were homogenized, and then centrifuged to harvest the supernatant for determining the concentrations of protein, DNA and RNA. The analysis procedures were performed according to the previous reports4. Protein concentration of mucosal homogenates was determined by a detergent-compatible protein assay (Bio-Rad Laboratories, Hercules, CA, USA) and using bovine serum albumin as standards. Mucosal DNA content was measured by a fluorometric assay. Mucosal RNA content was measured by spectrophotometry with a modified Schmidt-Tannhauser method.

Disaccharidase activities

Disaccharidase activities in the supernatant of intestinal mucosa were determined referred to previous study reported by Hou et al.49 using glucose kits (#A082–1 for lactase, #A082-2 for sucrase and #A082-3 for maltase; Nanjing Jiancheng Bioengineering Institute, Nanjing, China).

Plasma TNF-α concentration

The TNF-α concentration in plasma was measured by using a commercially available porcine ELISA kit (R&D Systems, Minneapolis, MN, USA, REF: PTA00)40.

mRNA abundance analysis

Intestinal total RNA was extracted using RNAiso Plus (TaKaRa, 9108/9109, Dalian, China), and dissolved in RNase-free water. A total of 1 μg of RNA, as templates, was reverse-transcribed into cDNA using the PrimeScript™ RT reagent kit with gDNA Eraser (TaKaRa, RR047A, Dalian, China). The reaction was carried out for 15 min at 37 °C, 5 sec at 85 °C, ∞ at 4 °C. RT-PCR was performed on an ABI 7500 Real Time PCR system (Applied Biosystems, Foster City, CA, USA) using SYBR® Premix Ex Taq™ (TaKaRa, Dalian, China). We used specific primers to measure the mRNA abundance. GAPDH was used as a housekeeping gene to normalize the target gene transcript levels. Primer sequences are listed in Table 10. The relative mRNA expression of genes was calculated as a ratio of the target gene to the control gene depending on the the 2−ΔΔCT method4.

Table 10 Primer sequences used for real-time PCR.
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Protein abundance analysis

The protein abundance of intestinal mucosa was determined according to the procedures of Wang et al.34. Specific primary antibodies included rabbit anti-claudin-1 (1:1000) from Invitrogen (Invitrogen Technology Inc., Danvers, MA, USA), rabbit anti-mTOR (1:1000), rabbit anti-phosphorylated mTOR (Ser2448; 1:1000), rabbit anti-4EBP1 (1:1000), rabbit anti-phosphorylated 4EBP1 (Thr70; 1:1000) from Cell Signaling (Cell Signaling Technology Inc., Danvers, MA, USA), and mouse anti-β-actin (1:10,000) from Sigma Aldrich (Sigma Aldrich Inc., St. Louis, MO, USA). The protein expression of claudin-1 was expressed as the ratio of claudin-1/β-actin. Phosphorylated forms of mTOR and 4EBP1 were normalized against the total protein content of each protein.

Statistical analysis

Data were analyzed by the GLM procedure of SAS (SAS Inst. Inc., Cary, NC, USA). The differences among group means were compared using Duncan Multiple Comparison based on the variance derived from ANOVA. Individual piglet was used as an experimental unit for the data. Results were expressed as means and pooled SEM. Significant differences were considered at p < 0.05.