Transcriptome Analysis of Three Sheep Intestinal Regions reveals Key Pathways and Hub Regulatory Genes of Large Intestinal Lipid Metabolism

The large intestine, also known as the hindgut, is an important part of the animal digestive system. Recent studies on digestive system development in ruminants have focused on the rumen and the small intestine, but the molecular mechanisms underlying sheep large intestine metabolism remain poorly understood. To identify genes related to intestinal metabolism and to reveal molecular regulation mechanisms, we sequenced and compared the transcriptomes of mucosal epithelial tissues among the cecum, proximal colon and duodenum. A total of 4,221 transcripts from 3,254 genes were identified as differentially expressed transcripts. Between the large intestine and duodenum, differentially expressed transcripts were found to be significantly enriched in 6 metabolism-related pathways, among which PPAR signaling was identified as a key pathway. Three genes, CPT1A, LPL and PCK1, were identified as higher expression hub genes in the large intestine. Between the cecum and colon, differentially expressed transcripts were significantly enriched in 5 lipid metabolism related pathways, and CEPT1 and MBOAT1 were identified as hub genes. This study provides important information regarding the molecular mechanisms of intestinal metabolism in sheep and may provide a basis for further study.

Utilization of cellulose by pigs and its effects on caecal function

1. Six pigs, four with caecal cannulae, were given diets containing 8% or 26% cellulose. Cannulation did not affect the digestibility of dry matter or cellulose.

2. Digestibility of cellulose, though variable, was higher for the 8%-cellulose diet.

3. Pigs on the 26%-cellulose diet had larger amounts of digesta in the caecum, and lower caecal retention times, than pigs on the 8%-cellulose diet.

4. Measurements of production rates of volatile fatty acids in the caecum indicated that only 2·7% and 1·9% of the apparent digestible energy of the 26%- and 8%-cellulose diets respectively came from the acids, and it was concluded that the caecum played only a small role in the breakdown of feed substances.

The rumen and hindgut as source of ruminant methanogenesis

The advantage of ruminants is their ability to convert fibrous biomass to high quality protein for human nutrition purposes. Rumen fermentation, however, is always associated with the formation of methane - a very effective greenhouse gas. Hindgut fermentation differs from rumen fermentation by a substantially lower methane production and the presence of reductive acetogenesis or dissimilatory sulfate reduction. Sulfate reduction and methanogenesis seem to be mutually exclusive, while methanogenesis and reductive acetogenesis may occur simultaneously in the hindgut. Although acetogenic bacteria have been isolated from the bovine rumen, methanogenesis prevails in the forestomachs. The substitution of acetate for methane as a hydrogen sink in the rumen should increase energy yield for the animal and decrease methane emissions into the environment. Differences in the major hydrogen sinks in both microbial ecosystems are discussed and mainly related to differences in substrate availability and to the absence of protozoa in the hindgut.

Energy disappearance in hindgut of sheep

Digestibility and disappearance of energy in the large intestine of sheep fed 75% concentrates was investigated with Cerium-141 as an unabsorbed reference substance. Cecal fermentation increased acetic acid and decreased propionic acid compared with ruminal fermentation. This situation was associated with cellulose disappearance in the lower gut and accounted for 26% of total cellulose disappearance from the entire alimentary tract. Energy disappearance in the hindgut accounted for 13% of energy disappearance in the gastrointestinal tract.

Studies of the large intestine of sheep. 3. Nitrogen kinetics in sheep given chopped lucerne (medicago sativa) hay

A study was made of nitrogen kinetics in the large intestine of sheep given 800 g chopped lucerne (Medicago sativa) hay/d. Four sheep were continuously infused with (15NH4)2SO4 into the caecum and three other sheep were infused intravenously with [15N]urea. A digesta marker, 51Cr complexed with EDTA (51Cr-EDTA), was infused into the rumen of each sheep to allow estimation of the rates of digesta constituents. Infusions were continued until tracer concentrations reached plateaux in digesta and blood pools, after which the sheep were anaesthetized and slaughtered. Pre-infusion samples and samples on plateau were obtained before slaughter for subsequent analysis to give plasma urea and rumen ammonia-N concentration and enrichment. At slaughter, digesta were obtained from the ileum and segments of the large intestine. These were analysed for 51Cr-EDTA content and concentration and enrichment of ammonia-N, microbial N and non-urea non-ammonia-N (NU-NAN). N flows in segments of the large intestine were calculated and represented in a quantitative eight-pool model. Transfer of plasma urea across the wall of the caecum and proximal colon was negligible but there was an input of 0.8 g endogenous NU-NAN/d. Flow of urea plus ammonia-N in digesta from the ileum into the caecum contributed 1.0 g N/d to the caecal ammonia pool. Proteolysis and deamination produced a further 3.0 g ammonia-N/d in the caecum and proximal colon. The net absorption of N between the ileum and the rectum was 2.8 g N/d but 3.0 g ammonia-N/d was absorbed from the caecum and proximal colon and, in addition, at least 0.9 g ammonia-N/d from the distal colon and rectum. Ammonia-N was incorporated into caecal microbes (0.6 g N/d) and approximately 57% of the NU-NAN in caecal digesta was microbial N. The majority of the microbial N flowing from the caecum was excreted in faeces. The rate of irreversible loss of urea-N from plasma, measured by intravenous infusion of [15N]urea, was 13.6 g/d. On average 83 (SE 6.8)% of the 15NH3 apparently absorbed from the caecum was incorporated into plasma urea; caecal ammonia contributed 9-19% of the N in plasma urea and 0.2-3.1% of the N in rumen ammonia.

Studies of the large intestine of sheep. 1. Fermentation and absorption in sections of the large intestine

1. Fermentation and absorption of constituents of digesta in segments of the large intestine of sheep given different diets were studied by analysis of gut contents obtained at slaughter after a period during which the sheep had been administered a non-absorbable gut marker. 2. In sheep given chopped, dried lucerne (Medicago sativa) there was net absorption of water throughout the large intestine with concomitant increases in the proportion of dry matter (DM) and organic matter (OM). There was net disappearance of 62 g OM, 1.66 g non-urea non-ammonia-nitrogen (NU-NAN) and 0.6 g (urea + NH3(-N in the caecum and proximal colon. There was no significant change in OM and NU-NAN flow through the remainder of the large intestine but there was a net disappearance of 0.3 g NH3-N. There was also net appearance of volatile fatty acids (VFA) in the caecum, most of which was apparently absorbed before the rectum. 3. Metabolism in the caecum was also studied in sheep grazing fresh pasture or consuming one of three sugar cane-bagasse-based diets, or barley pellets. In the lucerne- and pasture-fed sheep there was a net disappearance of approximately 0.5 g NH3-N/d from the caecum, while in sheep fed on bagasse plus urea, 1.4 g NH3-N/d was apparently absorbed from this region. The addition of fish meal to this latter diet resulted in apparent disappearance of 5.3 g NH3-N/d from the caecum and proximal colon. 4. There was apparent loss of NU-NAN from the caecum of sheep on all diets except the barley diet. With the latter diet there was a net gain of 1 g NU-NAN/d which was associated with relatively high VFA concentration and production; taken together these results indicate that microbial fermentation in the caecum was more extensive in the sheep fed on the barley diet than in those fed on the other diets. 5. The proportions of individual VFA in digesta from the rumen and caecum of lucerne-fed and pasture-fed sheep and in digesta from the caecum of sheep given the bagasse-based or barley diets are also reported and discussed. 6. In general the results indicate that the caecum and to a lesser extent the proximal colon were the major regions of fermentation and absorption of the components of the digesta in the large intestine.

Concentrations and metabolism of volatile fatty acids in the fermentative organs of two species of kangaroo and the guinea-pig

1. Contents from the fermentative organs of the kangaroo and guinea-pig were found to have concentrations of total volatile fatty acids ranging from 50 to 140 mM.

2. In each instance acetic was the most abundant acid, followed by propionic, then n-butyric, Trace amounts of isobutyric, n-valeric and isovaleric acids were present.

3. When studiedin vitro, tissue from the wall of each fermentative organ was shown to metabolize butyrate to ketone bodies. Acetoacetate was the major ketone body. The presence of acetate and propionate did not affect ketogenesis from butyrate.

4. In the guinea-pig caecum most of the ketogenic activity resided in the mucosa.

5. The upper colon of the guinea-pig was as active as the caecum in metabolizing butyrate to ketone bodies.

6. For both the guinea-pig caecum and the kangaroo fermentative stomach, incubations with14C-labelled butyrate showed that the proportion oxidized to CO2was considerably less than that metabolized to ketone bodies.