Localization of UT‐B Urea Transporter in the Developing Bovine Rumen

Abstract

The rumen ecosystem represents a classic example of a symbiotic relationship between host animals and microbial populations. Urea nitrogen secreted from the blood to the rumen is a crucial factor shaping this relationship. The transport of urea across rumen epithelium is facilitated by urea transport proteins namely urea transporter B (UT‐B) and possibly aquaporin‐3 (AQP3). However, how ruminants regulate nitrogen availability to microbes through these transport proteins remain unclear. Given that calves are the stage at which rumen microbial colonization begins, the developing rumen is an excellent model to observe this regulation. Using rumen epithelium samples of calves aged from birth (Day0, n = 4) to three months old (Day7, Day14, Day21, Day28 and Day96; n = 4–6 each age), immunolocalization was performed to locate UT‐B protein. Results showed that, although varied between the individual animals within each age group, signals were detected at all six stages by a previously characterized UT‐B antibody (n = 25/26). The strongest staining was found in stratum basale cells (or innermost layer for undeveloped papillae), especially those surrounding capillaries (n = 16/26). The staining in stratum spinosum was mainly present in well‐developed papillae at Day96 and was weak (n = 5/6), while stratum granulosum and stratum corneum were mainly negative throughout the six stages. The subcellular staining was clearly seen in the cells of stratum basale at Day96, with the strongest density at the basolateral side of plasma membranes facing large blood vessels (n = 5/6). Given these data, it is reasonable to speculate that UT‐B protein expression started from cells of stratum basale and progressively extended to adjacent cell layers toward the rumen lumen as rumen papillae developed. This localization pattern fits in the functional organization of transporting urea nitrogen from blood to rumen. Further semi‐quantification of staining strength by expression score suggested that UT‐B protein expression differed across ages (p < 0.01, n= 4–6, ANOVA), with the lowest at Day0 and highest at Day96. No difference was found among the stages of Day7, Day14, Day21 and Day28, but as a whole the abundance was higher than Day0 (p < 0.05, ANOVA) and lower than Day96 (p < 0.01, ANOVA). Overall, this study characterized the location pattern of UT‐B protein in calves’ rumen epithelium and confirmed age‐dependent change of UT‐B protein abundance. Its alteration might be associated with interactions with rumen microbial populations and reflect the nitrogen need for both microbes and host animals. These results give insights of how ruminant host animals regulate nitrogen availability to commensal microbial populations and in return benefit from it, through altering the permeability of the rumen wall to the supposed “waste” urea.Support or Funding InformationThis project is supported by China Scholarship Council (China) and University College Dublin (Ireland).