Absorptive transport of amino acids by the rat colon

Abstract

The capacity of the colon to absorb microbially produced amino acids (AAs) and the underlying mechanisms of AA transport are incompletely defined. We measured the profile of 16 fecal AAs along the rat ceco-colonic axis and compared unidirectional absorptive AA fluxes across mucosal tissues isolated from the rat jejunum, cecum, and proximal colon using an Ussing chamber approach, in conjunction with 1H-NMR and ultra-performance liquid chromatography-mass spectrometry chemical analyses. Passage of stool from cecum to midcolon was associated with segment-specific changes in fecal AA composition and a decrease in total AA content. Simultaneous measurement of up to 16 AA fluxes under native luminal conditions, with correction for endogenous AA release, demonstrated absorptive transfer of AAs across the cecum and proximal colon at rates comparable (30–80%) to those across the jejunum, with significant Na+-dependent and H+-stimulated components. Expression profiling of 30 major AA transporter genes by quantitative PCR revealed comparatively high levels of transcripts for 20 AA transporters in the cecum and/or colon, with the levels of 12 exceeding those in the small intestine. Our results suggest a more detailed model of major apical and basolateral AA transporters in rat colonocytes and provide evidence for a previously unappreciated transfer of AAs across the colonic epithelium that could link the prodigious metabolic capacities of the luminal microbiota, the colonocytes, and the body tissues. NEW & NOTEWORTHY This study provides evidence for a previously unappreciated transfer of microbially generated amino acids across the colonic epithelium under physiological conditions that could link the prodigious metabolic capacities of the luminal microbiota, the colonocytes, and the body tissues. The segment-specific expression of at least 20 amino acid transporter genes along the colon provides a detailed mechanistic basis for uniport, heteroexchange, Na+-cotransport, and H+-cotransport components of colonic amino acid absorption.

A three-sodium-to-glycine stoichiometry shapes the structural relationships of ATB0,+ with GlyT2 and GlyT1 in the SLC6 family

“…Located in the apical membrane of colonocytes in the absorptive epithelium ( D’Argenio et al, 2006 ; Gupta et al, 2005 ; Hatanaka et al, 2002 ; Ugawa et al, 2001 ), ATB 0,+ absorbs mixtures of amino acids issues from the bacterial proteolysis taking place in the outer mucus layer and lumen of the colon, and ATB 0,+ clearance of nutrients helps to maintaining an apparent sterility of the adherent inner mucus layer ( Atuma et al, 2001 ; Johansson et al, 2008 ; Jakobsson et al, 2015 ; Li et al, 2015 ; Chen et al, 2020 ).…”

“…As over 90 % of NaCl and water that enter the cecum are reabsorbed in the proximal colon ( Phillips, 1969 ; Thiagarajah and Verkman, 2018 ; Sandle, 1998 ), considerably lower [Na + ] (~15 mM) and [Cl − ] (<10 mM) have been measured at the surface of the rat distal colon ( Talbot and Lytle, 2010 ; Chen et al, 2020 ). If these concentrations correspond to those of the thin mucus layer, then a 3 Na + -coupling is not an option but a requirement for ATB 0,+ in order to provide enough driving force for an uphill transport of amino acids since it would be minimal or even not possible otherwise ( Chen et al, 2020 ).…”

“…Upregulation of ATB 0,+ has been detected in twelve types of solid cancers ( Sikder et al, 2017 , 2020 ) suggesting that its overexpression is the main mechanism to increase the uptake rate of glutamine or glycine, because of the dynamic cis and trans competition when ATB 0,+ is exposed to mixture of amino acids. Furthermore ATB 0,+ overexpression in colon cancer leads to its misrouting to the basolateral side of the epithelium with unrestrained access to the serosal pool of amino acids ( Chen et al, 2020 ).…”

“…ATB 0,+ mediates electrogenic and concentrative uptakes of all neutral and cationic amino acids with Hill coefficients suggesting a 2 Na + and 1 Cl − stoichiometry ( Sloan and Mager, 1999 ; Hatanaka et al, 2001 ; Nakanishi et al, 2001 ; Karunakaran et al, 2008 ). ATB 0,+ is expressed in the lung and distal colon epithelia, and in the mammary and pituitary glands ( Villalobos et al, 1997 ; Sloan and Mager, 1999 ; Ugawa et al, 2001 ; Sloan et al, 2003 ; Chen et al, 2020 ). Mice lacking ATB 0,+ are viable and with normal phenotype ( Babu et al, 2015 ; Ahmadi et al, 2018 ), but genome-wide association studies have identified SNPs in non coding regions of SLC6A14 that are linked to obesity ( Suviolahti et al, 2003 ; Corpeleijn et al, 2010 ; Sivaprakasam et al, 2021 ), male infertility ( Noveski et al, 2014 ), and the phenotypic variation and severity of cystic fibrosis ( Sun et al, 2012 ; Corvol et al, 2015 ).…”

Differential Effects of Sulfur Amino Acid-Restricted and Low-Calorie Diets on Gut Microbiome Profile and Bile Acid Composition in Male C57BL6/J Mice

“…In addition to small intestine, the large intestine, where the bulk of gut microbiome reside, is a significant site of amino acid absorption. 1 H-NMR studies indicate that the rate of transfer of free amino acids across cecum and colon are comparable to the transfer rates across small intestine and that the expression of amino acid transporters is at least as abundant as or higher than that in small intestine ( 38 ). Studies in mice show that elemental diets with free amino acids not only alter the microbiome profile adherent to colonic mucosa and in feces, but also reduce bowel inflammation in a colitis model ( 39 , 40 ).…”