Short-chain fatty acids (SCFA) are monocarboxylates produced by bacterial fermentation that play a crucial role in maintaining homeostasis in the large intestine. Two major transporters for SCFA, monocarboxylate transporter (MCT) and slc5a8 (or SMCT), exist in the digestive tract. The present histochemical study using in situ hybridization and immunohistochemistry revealed the distribution and subcellular localization of the MCT family in the digestive tract of mice, rats, and humans, comparing these with that of slc5a8. The expression of mucosal MCT1 in the mouse and rat was most intense in the cecum, followed by the colon, but low in the stomach and small intestine. Among other MCT subtypes, only MCT2 was detected in the parietal cell region of the gastric mucosa. Slc5a8 had predominant expression sites in the distal half of the large bowel and in the most terminal ileum. The mucosal MCT1 was localized in the basolateral membrane of enterocytes, while slc5a8 was restricted to the apical cell membrane, suggesting the involvement of slc5a8 in the uptake of luminal SCFA, and of MCT1 in the effl ux of SCFA and monocarboxylate metabolites towards blood circulation. The large intestine expressed both types of the transporter, but their distribution patterns differed along the longitudinal axis of the intestine and along the perpendicular axis of the mucosa.Plant-derived dietary fi ber and undigested carbohydrates are fermented by bacterial microfl ora in the large intestine and produce acetate, propionate, and butyrate, collectively called short-chain fatty acids (SCFA). These are monocarboxylates which contain less than fi ve carbon atoms: acetate has two carbons, propionate has three, and butyrate has four. The SCFA in the bowel lumen have some effects on the intestinal wall, including the stimulation of colonic blood fl ow and of fl uid and electrolyte uptake (30). Butyrate also functions as an energy source of epithelial cells in the large intestine and promotes differentiation of the cells (2, 21) while suppressing the proliferation of tumor cells by the induction of apoptosis (6). Thus, direct and indirect intraluminal supplementation of butyrate maintains and strengthens the epithelial integrity to suppress mucosal damage such as ulcerative colitis (15,16,19,27). At least 60% of the SCFA uptake in the large intestine occurs by simple diffusion of the unionized form across the cell membrane; the remainder occurs by the active cellular uptake of ionized SCFA involving co-transport of inorganic protons, such as Na + , K + , and H + (7). As a transporter of SCFA, monocarboxylate transporter (MCT)-1 was fi rst identifi ed and localized in intestinal epithelial cells as well as the heart, kidney, and epididymis (9). MCT1 can transport lactate, pyruvate, and SCFA in a H + -dependent manner. The

Gap junctions are specialized cell-cell junctions that directly link the cytoplasm of neighboring cells. They mediate the direct transfer of metabolites and ions from one cell to another. Discoveries of human genetic disorders due to mutations in gap junction protein (connexin [Cx]) genes and experimental data on connexin knockout mice provide direct evidence that gap junctional intercellular communication is essential for tissue functions and organ development, and that its dysfunction causes diseases. Connexin-related signaling also involves extracellular signaling (hemichannels) and non-channel intracellular signaling. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. In recent years, it has become clear that epigenetic processes are also essentially involved in connexin gene expression. In this review, we summarize recent knowledge on regulation of connexin expression by transcription factors and epigenetic mechanisms including histone modifications, DNA methylation, and microRNA. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Lactate plays an important role as an alternative energy substrate, especially in conditions with a decreased utility of glucose. Proton-coupled monocarboxylate transporters (MCTs) are essential for the transport of lactate, ketone bodies, and other monocarboxylates through the plasma membrane and may contribute to the net transport of lactate through the placental barrier. The present study examined the expression profile and subcellular localization of MCTs in the mouse placenta. An in situ hybridization survey of all MCT subtypes detected intense mRNA expressions of MCT1, MCT4, and MCT9 as well as GLUT1 in the placenta from gestational day 11.5.The expression of MCT mRNAs decreased in the intensity at the end of gestation in contrast to a consistently intense expression of GLUT1 mRNA.Immunohistochemically, MCT1 and MCT4 showed a polarized localization on the maternal side and fetal side of the two cell-layered syncytiotrophoblast, respectively.The membrane-oriented localization of MCTs was supported by the coexistence of CD 147 which recruits MCT to the plasma membrane. However, the subcellular arrangement of MCT1 and MCT4 along the trophoblastic cell membrane was completely opposite of that in the human placenta. Although we cannot exactly explain the reversed localization of MCTs between human and murine placentas, it may be related to differences between humans and mice in the origin of lactate and its utilization by fetuses.3

Short-chain fatty acids in the intestinal lumen affect colonic cell proliferation as well as function as an energy source for intestinal epithelial cells. A novel transporter of monocarboxylates, Slc5a8, is expressed abundantly in the colon, where it may participate in the Na + -coupled absorption of short-chain fatty acids produced by bacterial fermentation of dietary fiber. The present study examined the cellular localization of Slc5a8 in the murine gastrointestinal tract and kidney by in situ hybridization and immunohistochemistry. The hybridization signals were recognized in the terminal ileum and whole length of the large intestine, and were especially intense in the distal colon and rectum. The immunoreactivity of Slc5a8 was restricted to the striated border (the brush border) of enterocytes, and was not present in goblet cells, Paneth cells, or lamina propria cells. In the kidney, proximal tubules of both the cortex and the outer stripe of the outer medulla intensely expressed Slc5a8 mRNA, while the distal portions, including the loop of Henle, lacked the signals. The renal Slc5a8 immunoreactivity was localized only in the brush border of proximal tubules, not along the basolateral membrane. Thyroid follicular cells were immunoreactive for Slc5a8, with predominant labeling on the apical membrane. No other organs, including the esophagus, stomach, liver, pancreas, and salivary glands contained any notable signals of Slc5a8. These findings on the cellular and subcellular localization of Slc5a8 under normal conditions are helpful for understanding the physiological and pathological roles of Slc5a8.Short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate are generated abundantly in the large intestine by bacterial fermentation of dietary fiber and unabsorbed carbohydrates, and intestinal epithelia derive 60-70% of their energy supply from SCFAs, particularly butyrate (16,19,20). It has also been shown that SCFAs prevent colonic cell proliferation and reduce the incidence of colon cancer (4,5,21). Slc5A8, a tumor suppressor gene down-regulated in colon cancer, codes for a Na + -coupled transporter for SCFAs and other monocarboxylates such as lactate and pyruvate (6,9,15). Transfection of the Slc5a8 gene into Xenopus laevis oocytes increased the uptake of these monocarboxylates Na + -dependently, the rates of increase being especially high for lactate, pyruvate, and propionate (15). The comparative affinities of SCFAs for Slc5a8 were found to be in the following order: butyrate > propionate >> acetate (15). Similarly, the substrate-induced current in Slc5a8-expressing oocytes was markedly smaller upon superfusion with formic

Peripheral nerves express GLUT1 in both endoneurial blood vessels and the perineurium and utilize glucose as a major energy substrate, as does the brain. However, under conditions of a reduced utilization of glucose, the brain is dependent upon monocarboxylates such as ketone bodies and lactate, being accompanied by an elevated expression of a monocarboxylate transporter (MCT1) in the blood-brain barrier. The present immunohistochemical study aimed to examine the expression of MCT1 in the peripheral nerves of mice. MCT1 immunoreactivity was found in the perineurial sheath and colocalized with GLUT1, while the endoneurial blood vessels expressed GLUT1 only. An intense expression of MCT1 in the perineurium was confirmed by Western blot and in situ hybridization analyses. Ultrastructurally, the MCT1 and GLUT1 immunoreactivities in the thick perineurium showed an intensity gradient decreasing towards the innermost layer. In neonates, the MCT1 immunoreactivity in the perineurium was intense, while the GLUT1 immunoreactivity was faint or absent. These findings suggest that peripheral nerves depend on monocarboxylates as a major energy source and that MCT1 in the perineurium is responsible for the supply of monocarboxylates to nerve fibers and Schwann cells.

SMCT1 (slc5a8) is a sodium-coupled monocarboxylate transporter expressed in the brush border of enterocytes. It regulates the uptake of short-chain fatty acids (SCFAs) produced by bacterial fermentation in the large intestine. Another subtype, SMCT2 (slc5a12), is expressed abundantly in the small intestine, but its precise expression profile remains unknown. The present study using in situ hybridization method, immunohistochemistry, and quantitative PCR analysis examined the distribution and cellular localization of SMCT2 in the digestive tract of mice and compared the expression pattern with those of other transporters for monocarboxylates. While an abundant expression of SMCT2 was found in the jejunum, this was negligible in the duodenum, terminal ileum, and large intestine. In contrast, SMCT1 had predominant expression sites in the large bowel and terminal ileum. Subcellularly, SMCT2 was localized in the brush border of enterocytes in the intestinal villi-as is the case for SMCT1, suggesting its involvement in the uptake of foodderived monocarboxylates such as lactate and acetate. MCT (slc16) is a basolateral type transporter of the gut epithelium and conveys monocarboxylates in an H + -dependent manner. Since among the main subtypes of MCT family only MCT1 was expressed significantly in the small intestine, it is able to function as a counterpart to SMCT2 in this location.

Expression analysis of transporters selective for monocarboxylates such as lactate and ketone bodies in the kidney contributes to understanding the renal energy metabolism. Distribution and expression intensity of a sodium-dependent monocarboxylate transporter (SMCT) and proton-coupled monocarboxylate transporters (MCT) were examined in the mouse kidney. In situ hybridization survey detected significant mRNA expressions of SMCT and MCT-1, 2, 5, 8, 9, 10, and 12. Among these, signals for SMCT, MCT2 and MCT8 were predominant; transcripts of SMCT were restricted to the cortex and the outer stripe of outer medulla, while those of MCT2 and MCT8 gathered in the inner stripe of outer medulla and the cortex, respectively. Immunohistochemically, SMCT was present at the brush border in S2 and S3 of proximal tubules, suggesting the active uptake of luminal monocarboxylates here. MCT1 and MCT2 immunoreactivities were respectively found baso-laterally in S1 and thick ascending limbs of Henle's loop. The cellular localization of transporters suggests the involvement of SMCT in the uptake of filtrated lactate and ketone bodies and that of MCTs in the transport of monocarboxylate metabolites between tubular cells and circulation, but the different distribution patterns do not support the notion of a functional linkage between SMCT and MCT1/MCT2.

Proton-coupled monocarboxylate transporters (MCTs) are essential for the transport of lactate, ketone bodies, and other monocarboxylates through the plasma membrane, but the direction and substrates of transporting in loco remain unclear. The present study examined the expression and subcellular localization of MCTs in lipogenic organs. An in situ hybridization survey of major MCT subtypes detected an intense expression of MCT1 mRNA in the mammary gland, Harderian gland, and sebaceous gland. The MCT1 immunoreactivity was found baso-laterally in acinar cells of the mammary and Harderian glands. Alveolar cells of sebaceous glands in the skin, eyelids, and penis contained the membrane-associated MCT1 immunoreactivity along the entire length of the cell surface at the margin of alveoli. These MCT1-expressing exocrine glands possessed more abundant transcripts of acetyl-CoA carboxylase-1, a key enzyme for lipogenesis, than did representative lipogenetic organs such as the liver. Since the secretions from these glands contain fat as a major product, the cellular localization of MCT1 suggests the involvement of the transporter in the uptake of lactate, acetate, and other monocarboxylates for production of medium- and long-chain fatty acids.