Effects of Acute Ruminal Acidosis on Rumen Epithelial Integrity, Permeability, and Transcriptome in Sheep

Acute ruminal acidosis (ARA) is characterized by a significant decline in rumen pH, a significant increase in lipopolysaccharide (LPS) and lactic acids, and a significant decrease in volatile fatty acids (VFA) in rumen fluid. The objectives of this study were to characterize the structural, functional, and molecular changes in rumen epithelium during ARA and to determine which rumen fluid components drive these changes. Twelve sheep were used in this study, of which six were fasted overnight followed by ad libitum access to a concentrate diet to develop ARA, and six sheep were fed alfalfa hay to serve as controls. Forty-eight hours later, all sheep were euthanized, and rumen tissue samples were collected for histological analysis, ex vivo permeability assessment, and RNA sequencing. Primary rumen epithelial cells were isolated from additional healthy sheep for in vitro experiments. Statistical analyses were performed using Student's t-test or one-way ANOVA followed by Tukey's HSD test. The rumen epithelium from acidotic sheep showed increased permeability and histological damage, including parakeratosis, epithelial lifting, and partial loss of the stratum corneum layer of the rumen epithelium, compared to that from control sheep (P < 0.05). RNA sequencing identified 2,563 differentially expressed genes (adjusted P < 0.05 and |log2 fold change| ≥ 1) in the rumen epithelium between acidotic and control sheep. Functional enrichment analyses revealed that genes upregulated in acidotic rumen were enriched in ribosome biogenesis, translation, and keratinization, whereas genes downregulated in acidotic rumen were associated with immune response, cell adhesion, and tight junction (P < 0.05). Examples of differentially expressed genes were CLDN1, OCLN, and TJP1 (tight-junction genes); MRPL23, NOP53, and RPS6 (ribosome and translation genes); and IL17B, TLR4, and MYD88 (immune genes). To determine which rumen fluid changes in acidotic versus control sheep are directly responsible for differential expression of these genes in the rumen, primary ovine rumen epithelial cells were treated with pH, lipopolysaccharides, L-lactic acid, D-lactic acid, and butyrate at levels approximating those in acidotic or control sheep. Low medium pH (5.0) decreased OCLN and MYD88 expression while increasing TJP1, MRPL23, and TLR4 expression compared to normal medium pH (7.4) (P < 0.05). LPS, L-lactate, and D-lactate at concentrations found in rumen fluid of acidotic sheep did not affect the expression of these genes compared to those found in control sheep (P < 0.05). Butyrate at concentration found in rumen fluid of control sheep increased (P < 0.05) CLDN1, OCLN, and RPS6 expression while having no effect on the other genes, compared with concentration found in rumen fluid of acidotic sheep. In conclusion, ARA is associated with marked structural, functional, and transcriptomic changes in rumen epithelium, and these changes may be partially driven by reduced rumen pH and reduced butyrate concentration in rumen fluid.