Regulation of short-chain fatty acid transport and metabolism across the rumen epithelium

短链脂肪酸（SCFA）是瘤胃厌氧发酵的产物，是反刍动物的主要能量来源。提高饲料发酵能力可增加SCFA产量和动物生产力（18），SCFA吸收可稳定瘤胃环境（6）。然而，跨瘤胃上皮（RE）的SCFA吸收的调节因素还不清楚。已经报道了游离脂肪酸受体（FFA）2和3（39）以及toll样受体（TLR）2和4（22）在瘤胃中的表达。在结肠中，FFA 2和FFA 3位于肠内分泌细胞中（25）。SCFA对FFA 2和FFA 3的刺激刺激神经内分泌信号传导（37），从而调节SCFA吸收。RE不含肠内分泌细胞，因此，需要进行研究以确定RE中的定位。TLR 2和4在RE中表达（22），据报道，TLR的刺激可降低脂肪细胞中游离脂肪酸的摄取并降低脂质代谢（11）;尚不清楚RE中是否会出现类似的结果。假设是管腔传感机制调节RE SCFA吸收和代谢。目标是：1）表征参与RE对SCFA吸收和代谢的调节的潜在的管腔传感机制，2）阐明管腔传感机制对RE对SCFA吸收和代谢的调节作用，以及3）评估饮食处理是否可以诱导管腔传感机制的表达和活性的变化。为了实现这些目标，将招募两名博士生进行一系列研究，其中一名学生专注于模式识别受体（PRR），如TLR，另一名学生专注于管腔营养受体（LNR）。这些研究将使用PCR和免疫组织化学确定PRR和LNR在整个反刍动物胃肠道中的定位和表达。使用分离的RE细胞进行的体外研究将用于评价PRR和LNR的激动剂和拮抗剂对SCFA代谢的作用，并将进行Ussing室实验以检查PRR和LNR是否调节SCFA吸收。最后，将进行体内研究，以评估PRR和LNR是否（以及如何）调节血流量和SCFA的内脏代谢。这项研究计划将提供有关SCFA吸收代谢调节机制的新信息，并可能导致开发新化合物，通过增加SCFA吸收来提高反刍动物的效率和健康。

Short-chain fatty acids (SCFA), a product of anaerobic fermentation in the rumen, are the primary energy source for ruminants. Increasing diet fermentability increases SCFA production and animal productivity(18), with SCFA absorption stabilizing the ruminal environment(6). However, the regulatory factors for SCFA absorption across the rumen epithelium (RE) are not well understood. Expression of free fatty acid receptors (FFA) 2 and 3(39) and toll-like receptors (TLR) 2 and 4(22) have been reported in the rumen. In the colon, FFA2 and FFA3 have been localized in enteroendocrine cells(25). Stimulation of FFA2 and FFA3 by SCFA, stimulates neuroendocrine signalling(37) thereby modulating SCFA absorption. The RE does not contain enteroendocrine cells and therefore, studies to determine localization in the RE are needed. TLR2 and 4 are expressed in the RE(22) and stimulation of TLR has been reported to decrease free fatty acid uptake and reduce lipid metabolism in adipocytes(11); it is not known if similar results occur in the RE. The hypothesis is that luminal sensing mechanisms regulate RE SCFA absorption and metabolism. The objectives are to; 1) characterize potential luminal-sensing mechanisms involved in the regulation of SCFA absorption and metabolism by the RE, 2) elucidate the regulatory role of luminal-sensing mechanisms on the absorption and metabolism of SCFA by the RE, and 3) evaluate whether dietary treatments can induce changes in the expression and activity of luminal-sensing mechanisms. To address these objectives, two PhD students will be recruited to conduct a series of studies with one student focusing on pattern recognition receptors (PRR) such as TLR and the other focusing on luminal nutrient receptors (LNR). The studies will determine the localization and expression of PRR and LNR throughout the ruminant gastrointestinal tract using PCR and immunohistochemistry. In vitro studies with isolated RE cells will be used to evaluate the role of agonists and antagonists of the PRR and LNR on SCFA metabolism and Ussing chamber experiments will be conducted to examine whether PRR and LNR regulate SCFA absorption. Finally, in vivo studies will be conducted to evaluate whether (and how) PRR and LNR regulate blood flow and splanchnic metabolism of SCFA. This research program will provide novel information on regulatory mechanisms for the absorptive metabolism of SCFA and may lead to the development of novel compounds to improve the efficiency, and health of ruminants by increasing SCFA absorption.