Identity of SMCT1 (SLC5A8) as a neuron-specific Na+-coupled transporter for active uptake of L-lactate and ketone bodies in the brain

SMCT1 is a sodium-coupled (Na(+)-coupled) transporter for l-lactate and short-chain fatty acids. Here, we show that the ketone bodies, beta-d-hydroxybutyrate and acetoacetate, and the branched-chain ketoacid, alpha-ketoisocaproate, are also substrates for the transporter. The transport of these compounds via human SMCT1 is Na(+)-coupled and electrogenic. The Michaelis constant is 1.4 +/- 0.1 mm for beta-d-hydroxybutyrate, 0.21 +/- 0.04 mm for acetoacetate and 0.21 +/- 0.03 mm for alpha-ketoisocaproate. The Na(+) : substrate stoichiometry is 2 : 1. As l-lactate and ketone bodies constitute primary energy substrates for neurons, we investigated the expression pattern of this transporter in the brain. In situ hybridization studies demonstrate widespread expression of SMCT1 mRNA in mouse brain. Immunofluorescence analysis shows that SMCT1 protein is expressed exclusively in neurons. SMCT1 protein co-localizes with MCT2, a neuron-specific Na(+)-independent monocarboxylate transporter. In contrast, there was no overlap of signals for SMCT1 and MCT1, the latter being expressed only in non-neuronal cells. We also demonstrate the neuron-specific expression of SMCT1 in mixed cultures of rat cortical neurons and astrocytes. This represents the first report of an Na(+)-coupled transport system for a major group of energy substrates in neurons. These findings suggest that SMCT1 may play a critical role in the entry of l-lactate and ketone bodies into neurons by a process driven by an electrochemical Na(+) gradient and hence, contribute to the maintenance of the energy status and function of neurons.     

The Fugu tyrp1 promoter directs specific GFP expression in zebrafish: tools to study the RPE and the neural crest‐derived melanophores

In vertebrates, pigment cells account for a small percentage of the total cell population and they intermingle with other cell types. This makes it difficult to isolate them for analyzes of their functions in the context of development. To alleviate such difficulty, we generated two stable transgenic zebrafish lines (pt101 and pt102) that express green fluorescent protein (GFP) in melanophores under the control of the 1 kb Fugu tyrp1 promoter. In pt101, GFP is expressed in both retinal pigment epithelium (RPE) cells and the neural crest‐derived melanophores (NCDM), whereas in pt102, GFP is predominately expressed in the NCDM. Our results indicate that the Fugu tyrp1 promoter can direct transgene expression in a cell‐type‐specific manner in zebrafish. In addition, our findings provide evidence supporting differential regulations of melanin‐synthesizing genes in RPE cells and the NCDM in zebrafish. Utilizing the varying GFP expression levels in these fish, we have isolated melanophores via flow cytometry and revealed the capability of sorting the NCDM from RPE cells as well. Thus, these transgenic lines are useful tools to study melanophores in zebrafish.     

24S-hydroxycholesterol and cholesterol-24S-hydroxylase (CYP46A1) in the retina: from cholesterol homeostasis to pathophysiology of glaucoma

Free cholesterol is the predominant form of cholesterol in the neural retina. The vertebrate neural retina exhibits its own capacity to synthesize cholesterol and meets its demand also by taking it from the circulation. Defects in cholesterol synthesis and trafficking in the neural retina has detrimental consequences on its structure and function, highlighting the crucial importance of maintaining cholesterol homeostasis in the retina. Our purpose was to give a review on the functioning of the retina, the role of cholesterol and cholesterol metabolism therein, with special emphasis on cholesterol-24S-hydroxylase (CYP46A1). Similar to the brain, the retina expresses cholesterol-24S-hydroxylase (CYP46A1) and is enriched in its metabolic product, 24S-hydroxycholesterol. We recently published that one single nucleotide polymorphism in CYP46A1 gene, designated as rs754203, was a risk factor for glaucoma. Glaucoma is the second leading cause of blindness worldwide, affecting more than 60 million people. Glaucoma is characterized by the loss of retinal ganglion cells, which show high CYP46A1 expression. These data suggest the potential involvement of CYP46A1 and 24S-hydroxycholesterol in the pathophysiology of glaucoma.     

Diffusional interaction behavior of NSAIDs in lipid bilayer membrane using molecular dynamics (MD) simulation: Aspirin and Ibuprofen

In this research, for the first time, molecular dynamics (MD) method was used to simulate aspirin and ibuprofen at various concentrations and in neutral and charged states. Effects of the concentration (dosage), charge state, and existence of an integral protein in the membrane on the diffusion rate of drug molecules into lipid bilayer membrane were investigated on 11 systems, for which the parameters indicating diffusion rate and those affecting the rate were evaluated. Considering the diffusion rate, a suitable score was assigned to each system, based on which, analysis of variance (ANOVA) was performed. By calculating the effect size of the indicative parameters and total scores, an optimum system with the highest diffusion rate was determined. Consequently, diffusion rate controlling parameters were obtained: the drug–water hydrogen bond in protein-free systems and protein–drug hydrogen bond in the systems containing protein.     

Sodium-coupled electrogenic transport of pyroglutamate (5-oxoproline) via SLC5A8, a monocarboxylate transporter

Pyroglutamate, also known as 5-oxoproline, is a structural analog of proline. This amino acid derivative is a byproduct of glutathione metabolism, and is reabsorbed efficiently in kidney by Na⁺-coupled transport mechanisms. Previous studies have focused on potential participation of amino acid transport systems in renal reabsorption of this compound. Here we show that it is not the amino acid transport systems but instead the Na⁺-coupled monocarboxylate transporter SLC5A8 that plays a predominant role in this reabsorptive process. Expression of cloned human and mouse SLC5A8 in mammalian cells induces Na⁺-dependent transport of pyroglutamate that is inhibitable by various SLC5A8 substrates. SLC5A8-mediated transport of pyroglutamate is saturable with a Michaelis constant of 0.36 ± 0.04 mM. Na⁺-activation of the transport process exhibits sigmoidal kinetics with a Hill coefficient of 1.8 ± 0.4, indicating involvement of more than one Na⁺ in the activation process. Expression of SLC5A8 in Xenopuslaevis oocytes induces Na⁺-dependent inward currents in the presence of pyroglutamate under voltage-clamp conditions. The concentration of pyroglutamate necessary for induction of half-maximal current is 0.19 ± 0.01 mM. The Na⁺-activation kinetics is sigmoidal with a Hill coefficient of 2.3 ± 0.2. Ibuprofen, a blocker of SLC5A8, suppressed pyroglutamate-induced currents in SLC5A8-expressing oocytes; the concentration of the blocker necessary for causing half-maximal inhibition is 14 ± 1 μM. The involvement of SLC5A8 can be demonstrated in rabbit renal brush border membrane vesicles by showing that the Na⁺-dependent uptake of pyroglutamate in these vesicles is inhibitable by known substrates of SLC5A8. The Na⁺ gradient-driven pyroglutamate uptake was stimulated by an inside-negative K⁺ diffusion potential induced by valinomycin, showing that the uptake process is electrogenic.     

SLC5A8 (SMCT1)-mediated transport of butyrate forms the basis for the tumor suppressive function of the transporter

The identification of SLC5A8 as a tumor suppressor gene in colorectal cancer marks, for the first time, the association of a plasma membrane transporter with tumor suppressive properties. The subsequent establishment of the functional identity of SLC5A8 as a Na+-coupled transporter for short-chain monocarboxylates provides a mechanism for the tumor suppressive function of the transporter. Butyrate, a substrate for the transporter, is a histone deacetylase inhibitor and protective against colorectal cancer. This fatty acid is produced in the colonic lumen by bacterial fermentation of dietary fiber. SLC5A8 mediates the concentrative entry of butyrate from the lumen into colonocytes. Consequently, the transport function of SLC5A8 has the ability to influence the acetylation status of histones and hence gene expression in colonocytes. The ability of SLC5A8 to deliver butyrate into colonic epithelial cells most likely underlies the tumor suppressive role of this transporter.     

RUBCN/rubicon and EGFR regulate lysosomal degradative processes in the retinal pigment epithelium (RPE) of the eye

Macroautophagy/autophagy is an intracellular stress survival and recycling system whereas phagocytosis internalizes material from the extracellular milieu; yet, both pathways utilize lysosomes for cargo degradation. Whereas autophagy occurs in all cells, phagocytosis is performed by cell types such as macrophages and the retinal pigment epithelial (RPE) cells of the eye where it is supported by the noncanonical autophagy process termed LC3-associated phagocytosis (LAP). Autophagy and LAP are distinct pathways that use many of the same mediators and must compete for cellular resources, suggesting that cells may regulate both processes under homeostatic and stress conditions. Our data reveal that RPE cells promote LAP through the expression of RUBCN/Rubicon (RUN domain and cysteine-rich domain containing Beclin 1-interacting protein) and suppress autophagy through the activation of EGFR (epidermal growth factor receptor). In the morning when photoreceptor outer segments (POS) phagocytosis and LAP are highest, RUBCN expression is increased. At the same time, outer segment phagocytosis activates the EGFR resulting in MTOR (mechanistic target of rapamycin [serine/threonine kinase]) stimulation, the accumulation of SQSTM1/p62, and the phosphorylation of BECN1 (Beclin 1, autophagy related) on an inhibitory residue thereby suppressing autophagy. Silencing Rubcn, preventing EGFR activity or directly inducing autophagy in RPE cells by starvation inhibits phagocytic degradation of POS. Thus, RPE cells regulate lysosomal pathways during the critical period of POS phagocytosis to support retinal homeostasis.     

Cloning and functional characterization of human SMCT2 (SLC5A12) and expression pattern of the transporter in kidney

Recently, we cloned two Na⁺-coupled lactate transporters from mouse kidney, a high-affinity transporter (SMCT1 or slc5a8) and a low-affinity transporter (SMCT2 or slc5a12). Here we report on the cloning and functional characterization of human SMCT2 (SLC5A12) and compare the immunolocalization patterns of slc5a12 and slc5a8 in mouse kidney. The human SMCT2 cDNA codes for a protein consisting of 618 amino acids. When expressed in mammalian cells or Xenopus oocytes, human SMCT2 mediates Na⁺-coupled transport of lactate, pyruvate and nicotinate. The affinities of the transporter for these substrates are lower than those reported for human SMCT1. Several non-steroidal anti-inflammatory drugs inhibit human SMCT2-mediated nicotinate transport, suggesting that NSAIDs interact with the transporter as they do with human SMCT1. Immunofluorescence microscopy of mouse kidney sections with an antibody specific for SMCT2 shows that the transporter is expressed predominantly in the cortex. Similar studies with an anti-SMCT1 antibody demonstrate that SMCT1 is also expressed mostly in the cortex. Dual-labeling of SMCT1 and SMCT2 with 4F2hc (CD98), a marker for basolateral membrane of proximal tubular cells in the S1 and S2 segments of the nephron, shows that both SMCT1 and SMCT2 are expressed in the apical membrane of the tubular cells. These studies also show that while SMCT2 is broadly expressed along the entire length of the proximal tubule (S1/S2/S3 segments), the expression of SMCT1 is mostly limited to the S3 segment. These studies suggest that the low-affinity transporter SMCT2 initiates lactate absorption in the early parts of the proximal tubule followed by the participation of the high-affinity transporter SMCT1 in the latter parts of the proximal tubule.     

Ex-vivo models of the Retinal Pigment Epithelium (RPE) in long-term culture faithfully recapitulate key structural and physiological features of native RPE

The Retinal Pigment Epithelium (RPE) forms the primary site of pathology in several blinding retinopathies. RPE cultures are being continuously refined so that dynamic disease processes in this important monolayer can be faithfully studied outside the eye over longer periods. The RPE substrate, which mimics the supportive Bruch’s membrane (BrM), plays a key role in determining how well in-vitro cultures recapitulate native RPE cells. Here, we evaluate how two different types of BrM substrates; (1) a commercially-available polyester transwell membrane, and (2) a novel electrospun scaffold developed in our laboratory, could support the generation of realistic RPE tissues in culture. Our findings reveal that both substrates were capable of supporting long-lasting RPE monolayers with structural and functional specialisations of in-situ RPE cells. These cultures were used to study autofluorescence and barrier formation, as well as activities such as outer-segment internalisation/trafficking and directional secretion of key proteins; the impairment of which underlies retinal disease. Hence, both substrates fulfilled important criteria for generating authentic in-vitro cultures and act as powerful tools to study RPE pathophysiology. However, RPE grown on electrospun scaffolds may be better suited to studying complex RPE-BrM interactions such as the formation of drusen-like deposits associated with early retinal disease.     

A novel deletion mutation in the proton-coupled folate transporter (PCFT; SLC46A1) in a Nicaraguan child with hereditary folate malabsorption

Hereditary folate malabsorption (OMIM 229050) is a rare autosomal recessive disorder caused by loss-of-function mutations in the proton-coupled folate transporter gene (pcft/SLC46A1) resulting in impaired folate transport across the intestine and into the central nervous system. We report a novel, homozygous, deletion mutation in a child of Nicaraguan descent in exon 2 (c.558–588 del, ss778190447) at amino acid position I188 resulting in a frameshift with a premature stop.     

Aspirin upregulates expression of urokinase type plasminogen activator receptor (uPAR) gene in human colon cancer cells through AP1

In this study, the effects of acetylsalicylic acid (aspirin) on the expression of uPAR and the mechanism by which it regulates expression of uPAR was examined in two different colon cancer cell lines HCT116 and GEO, respectively. The study shows that under physiological concentration, aspirin upregulates steady-state level expression of uPAR mRNA as well as expression of uPAR protein. Using a transient transfection assay, a region corresponding to -1 to -398 region of uPAR promoter has been identified which shows maximum responsiveness to aspirin treatment and found that this region is sufficient for the aspirin-induced up-regulation of uPAR. A stable integration of a single copy of this region coupled to luciferase reporter gene into the HCT116 genome also behaved similarly. Using gel mobility shift assays, it is found that the distal AP1 region between -171 and -186 is responsible for the aspirin-induced up-regulation of uPAR. Mutation of this region reduced up-regulation. Supershift assays identify that the bound proteins at this region are c-Jun and Fra-1. Real-time PCR analysis showed more than 4-fold increase in the binding of c-Jun and a 1.6-fold increase in the binding of Fra-1 in this region and this up-regulation corresponds to an increased binding of acetylated histone H4 in this region. Since an increase in the expression of uPAR corresponds to an increase in the migration of the cell, a migration assay was performed and result showed a 3-fold increased migration of HCT116 cells through the vitronectin-coated layer. Thus, an AP1 mediated pathway for aspirin induced up-regulation of uPAR has been identified.     

X-linked creatine transporter (SLC6A8) mutations in about 1% of males with mental retardation of unknown etiology

Mutations in the creatine transporter gene, SLC6A8 (MIM 30036), located in Xq28, have been found in families with X-linked mental retardation (XLMR) as well as in males with idiopathic mental retardation (MR). In order to estimate the frequency of such mutations in the MR population, a screening of 478 males with MR of unknown cause was undertaken. All 13 exons of SLC6A8 were sequenced using genomic DNA. Six novel potentially pathogenic mutations were identified that were not encountered in at least 588 male control chromosomes: two deletions (p.Asn336del, p.Ile347del) and a splice site alteration (c.1016+2C>T) are considered pathogenic based on the nature of the variant. A mutation (p.Arg391Trp) should be considered pathogenic owing to its localization in a highly conserved region. Two other missense variants (p.Lys4Arg, p.Gly26Arg) are not conserved but were not observed in over 300 male control chromosomes. Their pathogenicity is uncertain. A missense variant (p.Val182Met), was classified as a polymorphism based on a normal creatine/creatinine (Cr:Crn) ratio and cerebral creatine signal in proton magnetic resonance spectroscopy (H-MRS) in the patient. Furthermore, we found 14 novel intronic and neutral variants that were not encountered in at least 280 male control chromosomes and should be considered as unclassified variants. Our findings of a minimum of four pathogenic mutations and two potentially pathogenic mutations indicate that about 1% of males with MR of unknown etiology might have a SLC6A8 mutation. Thus, DNA sequence analysis and/or a Cr:Crn urine screen is warranted in any male with MR of unknown cause.Amy J. Clark and Efraim H. Rosenberg have contributed equally to this work.     

A Variant of SLC1A5 Is a Mitochondrial Glutamine Transporter for Metabolic Reprogramming in Cancer Cells

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草地贪夜蛾取食Cry1Ab和Cry1Fa蛋白后中肠转录组及ABC基因表达分析

【目的】为揭示草地贪夜蛾Spodoptera furgiperda幼虫取食Bt蛋白后与中肠上相关ATP结合盒转运子(ATP-binding cassette transporter, ABC)蛋白基因的表达量变化的关系。【方法】分别使用含活化晶体蛋白Cry1Ab (LC₇₀=240.2 μg/g)和Cry1Fa (LC₇₀=270.0 μg/g)蛋白的人工饲料饲喂草地贪夜蛾4龄幼虫48 h,利用高通量测序对中肠进行转录组测序并进行生物信息学分析,筛选处理后差异表达基因;利用RT-qPCR验证差异表达ABC基因的表达量。【结果】与饲喂正常人工饲料的对照相比,饲喂含240.2 μg/g Cry1Ab和270.0 μg/g Cry1Fa的人工饲料后草地贪夜蛾4龄幼虫中肠转录组中分别检测到1 305和1 202个差异表达基因。Cry1Ab和Cry1Fa处理组与对照组之间分别有994和912个差异表达基因被GO功能注释到生物学过程、分子功能和细胞组分三大类。在最终筛选到的9个差异表达的ABC家族基因中,Cry1Ab处理组与对照组之间有4个差异表达ABC基因,3个上调,1个下调; Cry1Fa处理组与对照组之间有5个差异表达ABC基因,2个上调,3个下调;Cry1Ab和Cry1Fa处理组与对照组之间有2个ABC基因(LOC118267200和LOC118267201)表达量均显著上调。RT-qPCR验证结果表明,与对照组相比,Cry1Ab处理组有3个ABC基因表达量极显著上调,2个ABC基因表达量下调;Cry1Fa处理组有5个ABC基因表达量上调,1个ABC基因表达量下调。【结论】Cry1Ab和Cry1Fa蛋白的摄入可以影响草地贪夜蛾幼虫中肠一些ABC家族基因的表达量变化,这些基因的表达量变化与昆虫抗性产生有关。经比对后发现,ABCC家族与ABCG8基因表达量变化显著。本研究为下一步明确草地贪夜蛾体内ABC转运蛋白在Bt蛋白杀虫机制中的作用,以及合理使用Bt蛋白防治草地贪夜蛾及延缓抗性提供了理论依据。     

Increased Lipocalin‐2 in the retinal pigment epithelium of Cryba1 cKO mice is associated with a chronic inflammatory response

Although chronic inflammation is believed to contribute to the pathology of age‐related macular degeneration (AMD), knowledge regarding the events that elicit the change from para‐inflammation to chronic inflammation in the pathogenesis of AMD is lacking. We propose here that lipocalin‐2 (LCN2), a mammalian innate immunity protein that is trafficked to the lysosomes, may contribute to this process. It accumulates significantly with age in retinal pigment epithelial (RPE) cells of Cryba1 conditional knockout (cKO) mice, but not in control mice. We have recently shown that these mice, which lack βA3/A1‐crystallin specifically in RPE, have defective lysosomal clearance. The age‐related increase in LCN2 in the cKO mice is accompanied by increases in chemokine (C‐C motif) ligand 2 (CCL2), reactive gliosis, and immune cell infiltration. LCN2 may contribute to induction of a chronic inflammatory response in this mouse model with AMD‐like pathology.     

Deciphering the mechanisms of intestinal imino (and amino) acid transport: The redemption of SLC36A1

The absorption of zwitterionic imino and amino acids, and related drugs, is an essential function of the small intestinal epithelium. This review focuses on the physiological roles of transporters recently identified at the molecular level, in particular SLC36A1, by identifying how they relate to the classical epithelial imino and amino acid transporters characterised in mammalian small intestine in the 1960s-1990s. SLC36A1 transports a number of d- and l-imino and amino acids, β- and γ-amino acids and orally-active neuromodulatory and antibacterial agents. SLC36A1 (or PAT1) functions as a proton-coupled imino and amino acid symporter in cooperation with the Na⁺/H⁺ exchanger NHE3 (SLC9A3) to produce the imino acid carrier identified in rat small intestine in the 1960s but subsequently ignored because of confusion with the IMINO transporter. However, it is the sodium/imino and amino acid cotransporter SLC6A20 which corresponds to the betaine carrier (identified in hamster, 1960s) and IMINO transporter (identified in rabbit and guinea pig, 1980s). This review summarises evidence for expression of SLC36A1 and SLC6A20 in human small intestine, highlights the differences in functional characteristics of the imino acid carrier and IMINO transporter, and explains the confusion surrounding these two distinct transport systems.     

Hereditary folate malabsorption: A positively charged amino acid at position 113 of the proton-coupled folate transporter (PCFT/SLC46A1) is required for folic acid binding

The proton-coupled folate transporter (PCFT/SLC46A1) mediates intestinal folate uptake at acidic pH. Some loss of folic acid (FA) transport mutations in PCFT from hereditary folate malabsorption (HFM) patients cluster in R113, thereby suggesting a functional role for this residue. Herein, unlike non-conservative substitutions, an R113H mutant displayed 80-fold increase in the FA transport Km while retaining parental Vmax, hence indicating a major fall in folate substrate affinity. Furthermore, consistent with the preservation of 9% of parental transport activity, R113H transfectants displayed a substantial decrease in the FA growth requirement relative to mock transfectants. Homology modeling based on the crystal structures of the Escherichia coli transporter homologues EmrD and glycerol-3-phosphate transporter revealed that the R113H rotamer properly protrudes into the cytoplasmic face of the minor cleft normally occupied by R113. These findings constitute the first demonstration that a basic amino acid at position 113 is required for folate substrate binding.     

A new treatment for human malignant melanoma targeting L-type amino acid transporter 1 (LAT1): A pilot study in a canine model

L-type amino acid transporter 1 (LAT1), an isoform of amino acid transport system L, transports branched or aromatic amino acids essential for fundamental cellular activities such as cellular growth, proliferation and maintenance. This amino acid transporter recently has received attention because of its preferential and up-regulated expression in a variety of human tumors in contrast to its limited distribution and low-level expression in normal tissues. In this study, we explored the feasibility of using LAT1 inhibitor as a new therapeutic agent for human malignant melanomas (MM) using canine spontaneous MM as a model for human MM. A comparative study of LAT expression was performed in 48 normal tissues, 25 MM tissues and five cell lines established from MM. The study observed LAT1 mRNA levels from MM tissues and cell lines that were significantly (P < 0.01) higher than in normal tissues. Additionally, MM with distant metastasis showed a higher expression than those without distant metastasis. Functional analysis of LAT1 was performed on one of the five cell lines, CMeC-1. [³H]l-Leucine uptake and cellular growth activities in CMeC-1 were inhibited in a dose-dependent manner by selective LAT1 inhibitors (2-amino-2-norbornane-carboxylic acid, BCH and melphalan, LPM). Inhibitory growth activities of various conventional anti-cancer drugs, including carboplatin, cyclophosphamide, dacarbazine, doxorubicin, mitoxantrone, nimustine, vinblastine and vincristine, were significantly (P < 0.05) enhanced by combination use with BCH or LPM. These findings suggest that LAT1 could be a new therapeutic target for MM.