Glutamate alleviates intestinal injury, maintains mTOR and suppresses TLR4 and NOD signaling pathways in weanling pigs challenged with lipopolysaccharide

This experiment aimed to explore whether glutamate (Glu) had beneficial effects on intestinal injury caused by Escherichia coli LPS challenge via regulating mTOR, TLRs, as well as NODs signaling pathways. Twenty-four piglets were allotted to 4 treatments including: (1) control group; (2) LPS group; (3) LPS + 1.0% Glu group; (4) LPS + 2.0% Glu group. Supplementation with Glu increased jejunal villus height/crypt depth ratio, ileal activities of lactase, maltase and sucrase, and RNA/DNA ratio and protein abundance of claudin-1 in jejunum and ileum. In addition, the piglets fed Glu diets had higher phosphorylated mTOR (Ser2448)/total mTOR ratio in jejunum and ileum. Moreover, Glu decreased TNF-α concentration in plasma. Supplementation with Glu also decreased mRNA abundance of jejunal TLR4, MyD88, IRAK1, TRAF6, NOD2 and increased mRNA abundance of ileal Tollip. These results indicate that Glu supplementation may be closely related to maintaining mTOR and inhibiting TLR4 and NOD signaling pathways, and concomitant improvement of intestinal integrity under an inflammatory condition.

The intestine is not only critical for the digestion and absorption of nutrients, but also interacts with a complex external milieu 1 . 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 agents 2 . However, the intestinal health status could be hazarded by a lot of factors including infection and inflammation 3,4 . Inflammation can potentially leads to intestinal injury and dysfunction which can result in decreased animal performance and health 5 . 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 responses 7,9 . In the TLRs family, TLR4 is a critical receptor in transduction of the inflammatory response and plays an important role in intestinal homeostasis 10 . Meanwhile, NOD1 and NOD2 are prominent members in NODs family, whose roles in innate immunity and inflammatory diseases have been studied 11 , particularly in intestine 12,13 . 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-α 7,8 . 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. 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).
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).
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).

Discussion
Glu is one of the most abundant amino acids in alimentary proteins 26 . The majority of Glu molecule is either oxidized as energy or metabolized into other nonessential amino acids via transamination 27 . Recently, some studies showed that Glu plays an important role in maintaining physiological function as well as energy level in intestine 28 . 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 integrity 4 . Villus height, crypt depth and VCR have been used to quantify the intestinal morphology 29 . Mucosal protein content is an important indicator for cell metabolism, and protein deficiency negatively affects the intestinal epithelial barrier of piglets 30 . RNA/DNA ratio reflects the cell capacity for protein synthesis 31 . Protein/DNA ratio in mucosa has been employed as an indicator of intestinal growth and repair 4 . Mucosal disaccharidases, namely lactase, maltase and sucrase, reflect intestinal digestive function 32 . Claudins are major constituents of tight junctions 33 . 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 pigs 24 . 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 piglets 35 .
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 dynamics 14 . Specially, mTOR signaling pathway plays a key role in maintaining intestinal health 15,18,19 . 4EBP1, as one of the most well-known substrates of mTOR, is involved in regulation of the rate of protein synthesis 36 . Phosphorylation of 4EBP1 by mTOR promotes dissociation of 4EBP1 from Eukaryotic initiation factor 4E (EIF4E), enabling EIF4E to induce protein translation 37 . Recent studies have shown that amino acids as important signaling regulators for intestinal protein synthesis and cell growth via mTOR, could regulate downstream signal 4EBP1 15,16 . 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 mTOR 14 . Glu supplementation increased the relative mRNA expression of the jejunal mucosa mGluR1 and mGluR4 in weaning piglets 35 . 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 oxidation 21 . In addition, Glu might exert its beneficial effects through transforming into arginine and glutamine 23 . Abundant evidences have shown that arginine and glutamine participate in regulating mTOR and their downstream signals in intestine 15,38 . For example, there was study demonstrating that arginine-dependent cell survival and protein synthesis signaling in IPEC-J2 cells were mediated by mTOR 15 .  Xi et al. reported that glutamine could regulate protein synthesis in intestinal cells through the mTOR signaling pathway 38 . 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 neurotransmitter 26 , and its metabolism is involved with inflammation 39 . 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 pig 23 . Glu might exert its beneficial effects through transforming into arginine, asparagine and aspartate 23 . Some previous evidence has shown that arginine, aspartate, and asparagine participate in regulating TLRs, NOD and their downstream signals in weaned piglets [40][41][42] .
The aberrant activation of inflammatory signaling pathways (TLR4 and NOD) elicits collateral host tissue injury 43 . 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 characterized [44][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 phosphorylation 44 . 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 m 2 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.
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 h 4 . 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  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.   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.
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 method 4 .
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.