Rumen-derived lipopolysaccharide induced ruminal epithelium barrier damage in goats fed a high-concentrate diet
Introduction
Ruminants are usually fed a high-concentrate diet to stimulate rapid growth rates or high milk production which will result in subacute rumen acidosis (SARA) [1,2]. SARA is characterized by longer duration of averaged pH value below 5.6 for more than 3 h per day [3]. Decreased ruminal pH can compromise rumen epithelial permeability and disrupt rumen epithelial integrity, increasing the risk of the translocation of free lipopolysaccharides (LPS) in gram-negative bacteria from the digestive tract [4,5], resulting in low dry matter intake, laminitis, liver abscesses, and other diseases [[6], [7], [8]].
Rumen epithelium barrier plays an important role in nutrient absorption and metabolism, and it can counteract decreases in ruminal pH and immune response. Penetration of the ruminal epithelial barrier by endotoxins and antigens is limited in the normal physiologic state [9]. However, it has been reported that SARA induced by a high-grain diet impairs ruminal epithelial barrier function. The low pH and hyperosmolarity caused by SARA prompt the translocation of LPS and other microbial components, affecting animal health and productivity [10,11].
Tight junctions (TJs) play a key role in forming a selective barrier via direct cell-cell interactions. TJs are composed of the integral transmembrane proteins, claudins and occludin, as well as adaptor proteins, zonula occludins (ZO) 1, 2, and 3, which form the scaffold of the cytoplasmic plaque [[12], [13], [14]]. Occludin is the most reliable immunohistochemical marker for TJs [[15], [16], [17]]. Its expression correlates with various junctional functions, and overexpression of occludin increases the transepithelial electrical resistance and barrier function in mammalian epithelial cells [18,19]. In addition, evidence indicates that claudins are also important in the maintenance of junctional ion permeability and barrier function [20,21]. Overexpression of claudin-1 has been shown to enhance barrier function [22], while Zona occludin-1 (ZO-1), the essential TJ component, can interact with claudins and occludin [23].
Expression of these TJ proteins is mainly regulated by the mitogen-activated protein kinase (MAPK) signaling pathways. It has been reported that MAPK signaling pathway activity can disrupt epithelial barrier function through alteration of TJ protein expression in cell models [24,25]. However, little known if LPS can cause ruminal epithelial barrier dysfunction through MAPK signaling pathway when grain-induced SARA occurs. Therefore, we hypothesised that LPS can decrease the expression of TJs through modification and regulation of signaling pathways.
Section snippets
Ethical approval
The collection of experimental samples was performed in strict accordance with the guidelines of Jiangsu Province Animal Regulations (Government Decree No. 45).
Animals, diet, and experimental design
Twelve Boer goats (live weight 31.75 ± 1.35 kg) fitted with a rumen fistula were randomly divided into two groups (n = 6): low-concentrate diet (LC; concentrate:forage = 3:7) and high-concentrate diet (HC; concentrate:forage = 7:3). The ingredients and nutritional composition of the diets are listed in Table 1. The goats were fed these
Rumen pH, LPS content and primary pro-inflammatory cytokines
The mean rumen pH was lower when cows were fed HC compared with LC (P < 0.05, Table 2). The mean pH value in the HC group was lower than 5.6 for more than 3 h per day. The LPS concentration in rumen fluid was 34.2 kEU/mL in the LC group and was significantly higher in the HC group, 89.3 kEU/mL (P < 0.01, Table 2). The plasma concentrations of primary pro-inflammatory cytokines IL-1β (P < 0.05), IL-6 (P < 0.01) and TNF-α (P < 0.01) in the lacteal vein were significantly increased in the HC group
Discussion
An HC diet in ruminants can increase growth and milk production over the short term, but it can have a negative impact on the health of animals as a result of accumulation of volatile fatty acids, which decrease the rumen pH, leading to SARA [27,28]. A decrease in the rumen pH also results in ruminal epithelial barrier dysfunction. The ruminal epithelial barrier is a critical component of the immune system in ruminants [29]. Impairment of ruminal epithelial barrier function can lead to LPS
Conclusion
High concentrate diet feeding for nine weeks impaired ruminal epithelial barrier function. The expression of tight junction proteins was decreased in the ruminal epithelium of goats fed HC diet through MAPK signaling pathway.
Novelty statement
Our research explores that goats fed high concentrate diet for 9 weeks can induce SARA. This in turn, down-regulates transcription of TJs protein and then increases both the expression of MAPK and secretion of proinflammatory cytokines in the rumen epithelium of goats. Furthermore, we demonstrated novelty that the MAPK signaling pathway is activated by LPS in the rumen epithelium of goats during SARA.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgments
This study was supported by Project of Jiang Su Independent Innovation (CX(15)1003) and Postdoctoral Foundation of Jiangsu Province (Grant number 1701031A).
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