The peptide transporter PEPT2 mediates the cellular uptake of di-and tripeptides and selected drugs by proton-substrate cotransport across the plasma membrane. PEPT2 was functionally identified initially in the apical membrane of renal tubular cells but was later shown to be expressed in other tissues also. To investigate the physiological importance of PEPT2 and for a detailed analysis of the protein expression sites, we generated a Pept2 knockout mouse line in which the Pept2 gene was disrupted by insertion of a ␤-galactosidase gene under the control of the PEPT2 promoter. The Pept2 ؊/؊ mice showed no obvious phenotypic abnormalities but also no adaptive upregulation in the expression level of related genes in the kidney. The importance of PEPT2 in the reabsorption of filtered dipeptides was demonstrated in knockout animals by significantly reduced renal accumulation of a fluorophore-labeled and a radiolabeled dipeptide after in vivo administration of the tracers. This indicates that PEPT2 is the main system responsible for tubular reabsorption of peptide-bound amino acids, although this does not lead to major changes in renal excretion of protein or free amino acids.Cellular uptake of amino acids in peptide-bound form is a biological phenomenon found throughout nature. The membrane proteins responsible for uptake of di-and tripeptides have been grouped into the peptide transporter (PTR) family of proton-dependent peptide transporters (28). A common feature of this family is that the carriers couple substrate movement across the membrane to movement of protons down an inwardly directed electrochemical proton gradient, allowing transport of peptides against a substrate gradient.The mammalian members of the PTR family are divided into two subfamilies, represented by the peptide transporters PEPT1 (SLC15A1) and PEPT2 (SLC15A2) and the peptide/ histidine transporters PHT1 and PHT2. Not much is known about the latter except that they are able to transport di-and tripeptides and the amino acid histidine. PHT1 was exclusively found in brain and eye (30), although another study suggested its presence in other tissues, including the kidney (2). PHT2 has so far only been localized in the lymphatic system (22). Both proteins are presumably lysosomal transporters. Much more information is available on PEPT1 and PEPT2 (for reviews, see references 7, 17, and 20). These two peptide transporters possess the capability for sequence-independent but stereoselective transport of all possible di-and tripeptides, including the differently charged species.Mammalian peptide transporters also have pharmacological importance, based on their ability to transport a large variety of drugs, including angiotensin-converting enzyme inhibitors and ␤-lactam antibiotics (for reviews, see references 13 and 20). PEPT1 is a low-affinity transporter type with apparent affinities in the millimolar range, and PEPT2 is a high-affinity carrier with apparent affinities in the micromolar range for the same substrates. Both are found mainly in apical memb...

The peptide transporter PEPT2 mediates transmembrane uptake of small peptides. So far, its expression has not been evidenced in the gastrointestinal tract. We have investigated peptide transport activity in the neuromuscular layers of the gastrointestinal tract by using the fluorescent tracer-dipeptide beta-Ala-Lys-Nepsilon-7-amino-4-methyl-coumarin-3-acetic acid (Ala-Lys-AMCA). Whole-mount preparations from mouse, rat, and guinea pig stomach and small and large intestine were incubated with Ala-Lys-AMCA in the presence or absence of the uptake-inhibitors L-histidine, D-phenylalanyl-L-alanine (D-Phe-Ala), glycyl-L-sarcosine (Gly-Sar), glycyl-L-glutamine (Gly-Gln), benzylpenicillin, and cefadroxil. Fluorescence microscopy revealed that Ala-Lys-AMCA specifically accumulated in both ganglionic layers of the enteric nervous system (ENS) in all regions and species studied. This could be inhibited by Gly-Sar, D-Phe-Ala, Gly-Gln, and cefadroxil, but not by free histidine and benzylpenicillin, indicating uptake via PEPT2. Accordingly, dipeptide uptake was completely abolished in PEPT2-deficient mice. Reverse transcriptase-polymerase chain reaction analysis detected a PEPT2-specific transcript in extracts from the ganglionic ENS layers of mouse small and large intestine, further proving that enteric dipeptide transport activity is specifically mediated via PEPT2. The cellular site of dipeptide uptake was immunohistochemically localized to enteric glial cells and tissue-resident macrophages. In addition, dipeptide uptake occurred in a neurochemically defined subset of neurons in the guinea pig ENS. Our results constitute the first functional evidence for dipeptide transport activity in the ENS. PEPT2-mediated dipeptide transport in enteric glia could contribute to the clearance of neuropeptides in the ENS. In addition, the fluorophore-coupled dipeptide uptake via PEPT2 is a novel vital marker for glial cells in the ENS.

PEPT2 is an integral membrane protein in the apical membrane of renal epithelial cells that operates as a rheogenic transporter for di- and tripeptides and structurally related drugs. Its prime role is thought to be the reabsorption of filtered di- and tripeptides contributing to amino acid homeostasis. To elucidate the role of PEPT2 in renal amino acid metabolism we submitted kidney tissues of wild-type and a Pept2(-/-) mouse line to a comprehensive transcriptome, proteome and metabolome profiling and analyzed urinary amino acids and dipeptides. cDNA microarray analysis identified 147 differentially expressed transcripts in transporter-deficient animals, and proteome analysis by 2D-PAGE and MALDI-TOF-MS identified 37 differentially expressed proteins. Metabolite profiling by GC-MS revealed predominantly altered concentrations of amino acids and derivatives. Urinary excretion of amino acids demonstrated increased glycine and cysteine/cystine concentrations and dipeptides in urine were assessed by amino acid analysis of urine samples before and after in vitro dipeptidase digestion. Dipeptides constituted a noticeable fraction of urinary amino acids in Pept2(-/-) animals, only, and dipeptide-bound glycine and cystine were selectively increased in Pept2(-/-) urine samples. These findings were confirmed by a drastically increased excretion of cysteinyl-glycine (cys-gly). Urinary loss of cys-gly together with lower concentrations of cysteine, glycine, and oxoproline in kidney tissue and altered expression of mRNA and proteins involved in glutathione (GSH) metabolism suggests that PEPT2 is predominantly a system for reabsorption of cys-gly originating from GSH break-down, thus contributing to resynthesis of GSH.

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Cystinuria is a hereditary disorder caused by a defect in the apical membrane transport system for cystine and dibasic amino acids in renal proximal tubules and intestine, resulting in recurrent urolithiasis. Mutations in SLC3A1 and SLC7A9 genes, that codify for rBAT/b 0,ϩ AT transporter subunits, cause type A and B cystinuria, respectively. In humans, cystinuria treatment is based on the prevention of calculi formation and its dissolution or breakage. Persistent calculi are treated with thiols [i.e., D-penicillamine (DP) and mercaptopropionylglycine (MPG)] for cystine solubilization. We have developed a new protocol with DP to validate our Slc7a9 knockout mouse model for the study of the therapeutic effect of drugs in the treatment of cystine lithiasis. We performed a 5-wk treatment of individually caged lithiasic mutant mice with a previously tested DP dose. To appraise the evolution of lithiasis throughout the treatment a noninvasive indirect method of calculi quantification was developed: calculi mass was quantified by densitometry of X-ray images from cystinuric mice before and after treatment. Urine was collected in metabolic cage experiments to quantify amino acids in DP-treated and nontreated, nonlithiasic mutant mice. We found significant differences between DP-treated and nontreated knockout mice in calculi size and in urinary cystine excretion. Histopathological analysis showed that globally nontreated mutant mice had more severe and diffuse urinary system damage than DP-treated mice. Our results validate the use of this mouse model for testing the efficacy of potential new drugs against cystinuria.