Key role of short-chain fatty acids in epithelial barrier failure during ruminal acidosis

Abstract

Subacute ruminal acidosis is induced by high concentrations of short-chain fatty acids (SCFA, mainly acetate, propionate, and butyrate) that release protons to decrease the pH of the ruminal digesta. This low pH, in turn, is thought to damage epithelial barrier function. The present study applied a model of simulated ruminal acidosis ex vivo to investigate if SCFA directly contribute to epithelial barrier failure beyond their role as proton donors. Epithelial tissues from the rumen of slaughtered sheep were mounted in Ussing chambers and incubated under 3 different conditions. Two groups were incubated in the absence of SCFA at mucosal pH 6.1 (control) and pH 5.1, respectively, for 7 h. A third group was first incubated in a mucosal solution containing 100 mM SCFA at pH 5.1 for 2 h and, thereafter, in a mucosal solution without SCFA at pH 6.1 for the remaining 5 h. Transepithelial conductance (G), short-circuit current (I), and fluorescein fluxes were determined. After 7 h of incubation, the expression levels of claudin-1, claudin-4, claudin-7, and occludin were measured by quantitative reverse-transcription PCR and Western blot. Furthermore, the local distribution of these tight junction (TJ) proteins was examined by confocal laser scanning microscopy. A 7-h incubation at pH 5.1 in the absence of SCFA did not influence either G or fluorescein flux rates of ruminal tissues ex vivo compared with the control. In contrast, incubation at pH 5.1 with SCFA for only 2 h induced increases in G and fluorescein flux rates that continued even after tissues were returned back to pH 6.1. Expression analysis showed that pH 5.1 without SCFA for 7 h induced no changes in mRNA expression of claudin-1, claudin-4, claudin-7, and occludin and a selective decrease in protein expression of only claudin-4 compared with the control. However, a 2-h incubation at pH 5.1 in the presence of SCFA decreased the mRNA-expression of claudin-7, as well as the protein expression of claudin-4, claudin-7, and occludin. The decreased expression of these TJ proteins in the group incubated with SCFA was also evident in immunohistochemistry. Immunohistochemistry additionally evidenced a considerable retraction of all tested TJ proteins out of the TJ in that group. We conclude that a low mucosal pH of 5.1 is tolerated well by ruminal epithelia for several hours. However, a low pH in combination with SCFA induces damage to the TJ and disturbs barrier function, which is not immediately reversible upon the removal of the acidotic insult.

Effects of glucagon‐like peptides 1 and 2 and epidermal growth factor on the epithelial barrier of the rumen of adult sheep

“…Alterations in J fluor and G t are commonly used to assess the permeability of the paracellular space to large ( J fluor ) and small ions ( G t ), assuming that the transcellular route has negligible ( J fluor ) or rather constant ( G t ) contribution to these functional barrier readouts. Nevertheless, we have demonstrated in previous studies that fluorescein is also able to use the transcellular route for permeation across RE, for example, after an acid challenge (Greco et al, ; Meissner et al, ). Based on the fact that G t and the protein expression of all tested TJ proteins were unchanged by GLP‐1 and GLP‐2 in the present study, we thus propose that paracellular barrier function of RE was not affected by these hormones during the 7‐hr incubation, despite the fact that J fluor was changed in the last flux period by either of them.…”

“…Nevertheless, we have demonstrated in previous studies that fluorescein is also able to use the transcellular route for permeation across RE, for example, after an acid challenge (Greco et al, 2018;Meissner et al, 2017). Based on the fact that G t and the protein expression of all tested TJ proteins were unchanged by GLP-1 and GLP-2 in the present study, we thus propose that paracellular barrier function of RE was not affected by these hormones during the 7-hr incubation, despite the fact that J fluor was changed in the last flux period by either of them.…”

“…The epithelial barrier (EB) of the rumen provides effective protection against free noxious agents in the ruminal milieu (Aschenbach et al, ; Penner, Steele, Aschenbach, & McBride, ). However, it can become ineffective under pathological conditions, e.g., when luminal accumulation of short‐chain fatty acids and protons challenge key epithelial functions (Aschenbach & Gabel, ; Greco et al, ; Meissner et al, ). Different studies proposed a loss of barrier integrity and selectivity under such circumstances (Liu, Xu, Liu, Zhu, & Mao, ; Steele, AlZahal, Hook, Croom, & McBride, ).…”

“…However, it can become ineffective under pathological conditions, e.g., when luminal accumulation of short-chain fatty acids and protons challenge key epithelial functions (Aschenbach & Gabel, 2000;Greco et al, 2018;Meissner et al, 2017). Different studies proposed a loss of barrier integrity and selectivity under such circumstances (Liu, Xu, Liu, Zhu, & Mao, 2013;Steele, AlZahal, Hook, Croom, & McBride, 2009).…”

Growth of rumen papillae in weaned calves is associated with lower expression of insulin‐like growth factor‐binding proteins 2, 3, and 6

“…These data suggested that higher expression of IGFBP5 in rumen papillae is associated with the changes of the fermentation in rumen. Higher expression of IGFBP5 in rumen papillae might be necessary for adaptation to highly fermentable diet causing disruption of rumen epithelial barrier (Greco et al, 2018;Meissner et al, 2017;Steele et al, 2011). IGFBP5 induced monocyte migration in lung epithelium in mice (Yasuoka, Yamaguchi, & Feghali-Bostwick, 2009).…”