Cloning and functional identification of slc5a12 as a sodium-coupled low-affinity transporter for monocarboxylates (SMCT2)

Abstract

We report in the present paper, on the isolation and functional characterization of slc5a12, the twelfth member of the SLC5 gene family, from mouse kidney. The slc5a12 cDNA codes for a protein of 619 amino acids. Heterologous expression of slc5a12 cDNA in mammalian cells induces Na+-dependent transport of lactate and nicotinate. Several other short-chain monocarboxylates compete with nicotinate for the cDNA-induced transport process. Expression of slc5a12 in Xenopus oocytes induces electrogenic and Na+-dependent transport of lactate, nicotinate, propionate and butyrate. The substrate specificity of slc5a12 is similar to that of slc5a8, an Na+-coupled transporter for monocarboxylates. However, the substrate affinities of slc5a12 were much lower than those of slc5a8. slc5a12 mRNA is expressed in kidney, small intestine and skeletal muscle. In situ hybridization with sagittal sections of mouse kidney showed predominant expression of slc5a12 in the outer cortex. This is in contrast with slc5a8, which is expressed in the cortex as well as in the medulla. The physiological function of slc5a12 in the kidney is likely to mediate the reabsorption of lactate. In the intestinal tract, slc5a12 is expressed in the proximal parts, whereas slc5a8 is expressed in the distal parts. The expression of slc5a12 in the proximal parts of the intestinal tract, where there is minimal bacterial colonization, suggests that the physiological function of slc5a12 is not to mediate the absorption of short-chain monocarboxylates derived from bacterial fermentation but rather to mediate the absorption of diet-derived short-chain monocarboxylates. Based on the functional and structural similarities between slc5a8 and slc5a12, we suggest that the two transporters be designated as SMCT1 (sodium-coupled monocarboxylate transporter 1) and SMCT2 respectively.

<b>Cellular distributions of monocarboxylate transporters: a r</b><b>eview </b>

“…In a sense, the expression patterns of SMCT1 and SMCT2 along the long axis of the gut are complementary; SMCT2 and SMCT1 predominate in the small intestine and large intestine, respectively. Since SMCT2 has a main productive site in the proximal small intestine with minimal bacterial fermentation, the function of SMCT2 may be to absorb acetate, lactate, and other monocarboxylates from drinks and diets such as milk and fermented products (134). The transport capacity of SMCT2 is higher compared with that of SMCT1.…”

“…In 2004, SLC5A8 (SMCT1), a 69-kDa protein, was found to code for a Na + -coupled transporter for SCFAs, lactate, and pyruvate but not for di-carboxylate succinate or tri-carboxylate citrate. Soon afterwards, another type of SMCT2 (SLC5A12) with a 59% identity with SMCT1 at the amino acid level was identified (134). Both of these are abundant in the intestine and kidney and localized in the brush border, being apical types of transporters.…”

The solute carrier (SLC) complement of the human genome: Phylogenetic classification reveals four major families

“…2a. This class constitutes mainly metal ion transporters from nine families (SLC5, 9,11,12,20,24,30,39 and 41 families) but also the sulfate ion transporters from the SLC26 family. In addition, these families include in total 20 orphan transporters, which also are likely to transport some sort of metal ions, which means that the human genome could contain close to 80 transporters for metal ions.…”

“…Three additional families, SLC44-SLC46 containing in total 12 genes, have since been described [6][7][8]. Moreover, the number of members from the other SLC families has continued to increase and new members have been identified from for example the SLC5 [9], SLC6 [10], SLC25 [11], and SLC35 families [12], in total approximately 50 members, although this has not been summarized systematically.…”

Antenatal Dexamethasone Treatment Leads to Changes in Gene Expression in a Murine Late Placenta

“…Aquaporin 5 gene expression was studied by in situ hybridization according to method described by others [26]. For the preparation of antisense and sense (negative control) riboprobes, a 549 bp nucleotide fragment corresponding to nucleotide sequence 5-553 of the aquaporin 5 cDNA (accession number NM_009701) was cloned into pGEM-T vector.…”