The amino acid transport system bo,+and cystinuria

Interaction of mercury species with proteins: towards possible mechanism of mercurial toxicology

The nature of the binding of mercurials (organic and inorganic) and their subsequent transformations in biological systems is a matter of great debate as several different hypotheses have been proposed and none of them has been conclusively proven to explain the characteristics of Hg binding with the proteins. Thus, the chemical nature of Hg-protein binding through the possible transportation mechanism in living tissues is critically reviewed herein. Emphasis is given to the process of transportation, and binding of Hg species with selenol-containing biomolecules that are appealing for toxicological studies as well as the advancement of environmental and biological research.

Heat Stress Alters the Effect of Eimeria maxima Infection on Ileal Amino Acids Digestibility and Transporters Expression in Meat-Type Chickens

Simple Summary

Heat stress (HS) and Eimeria (E.) maxima infection are the most common physical and pathological stressors in chicken houses, and both affect intestinal digestibility and absorption leading to reduction in growth, morbidity, and mortality, causing massive economic losses. This study identifies the impact of each stressor and their combined effects on apparent amino acid digestibility and molecular transporters expression in the ileum of broiler chicken. Heat-stressed chickens showed no change in amino acids digestibility, despite the reduction in feed intake. Combining HS and E. maxima infection modulated the reduction in amino acids digestibility observed in the infected chickens. The expression of the ileal amino acid transporters was severely impacted by E. maxima infection but not by HS. Interestingly, the infected group reared under HS exhibited significantly higher expression levels in all the enterocytic apical and about half of the basolateral amino acid transporters than the infected birds raised in thermoneutral environment. Thus, HS putatively curtailed the maldigestion effects of E. maxima.

Abstract

Eimeria (E.) maxima invades the midgut of chickens and destroys the intestinal mucosa, impacting nutrient digestibility and absorption. Heat stress (HS) commonly affects the broiler chicken and contributes to inflammation and oxidative stress. We examined the independent and combined effects of HS and E. maxima infection on apparent amino acid ileal digestibility (AID) and mRNA expression of amino acid transporters in broiler chickens (Ross 708). There were four treatment groups: thermoneutral-control (TNc) and infected (TNi), heat-stress control (HSc) and infected (HSi), six replicates of 10 birds/treatment. Ileal content and tissue were sampled at 6 d post infection to determine AID and transporters expression. Surprisingly, the HSi chickens exposed to two critical stressors exhibited normal AID. Only the TNi group displayed reduction in AID. Using TNc as control, the HSc group showed upregulated CAT1, LAT4, TAT1, SNAT1, and SNAT7. The HSi group showed upregulated CAT1 and LAT1, and downregulated b^0,+AT, rBAT, SNAT1, and SNAT2. The TNi group showed upregulated CAT1, LAT1, and SNAT1 and downregulated B⁰AT1, b^0,+AT, rBAT, LAT4, and TAT1. The expression of all enterocytic-apical and about half of the basolateral transporters was higher in the HSi group than in the TNi group, indicating that HS can putatively alleviate the E. maxima adverse effect on ileal digestion and absorption.

Enhanced metanephric specification to functional proximal tubule enables toxicity screening and infectious disease modelling in kidney organoids

While pluripotent stem cell-derived kidney organoids are now being used to model renal disease, the proximal nephron remains immature with limited evidence for key functional solute channels. This may reflect early mispatterning of the nephrogenic mesenchyme and/or insufficient maturation. Here we show that enhanced specification to metanephric nephron progenitors results in elongated and radially aligned proximalised nephrons with distinct S1 - S3 proximal tubule cell types. Such PT-enhanced organoids possess improved albumin and organic cation uptake, appropriate KIM-1 upregulation in response to cisplatin, and improved expression of SARS-CoV-2 entry factors resulting in increased viral replication. The striking proximo-distal orientation of nephrons resulted from localized WNT antagonism originating from the organoid stromal core. PT-enhanced organoids represent an improved model to study inherited and acquired proximal tubular disease as well as drug and viral responses.

The zebrafish cationic amino acid transporter/glycoprotein-associated family: sequence and spatiotemporal distribution during development of the transport system b0,+ (slc3a1/slc7a9)

System b^0,+ absorbs lysine, arginine, ornithine, and cystine, as well as some (large) neutral amino acids in the mammalian kidney and intestine. It is a heteromeric amino acid transporter made of the heavy subunit SLC3A1/rBAT and the light subunit SLC7A9/b^0,+AT. Mutations in these two genes can cause cystinuria in mammals. To extend information on this transport system to teleost fish, we focused on the slc3a1 and slc7a9 genes by performing comparative and phylogenetic sequence analysis, investigating gene conservation during evolution (synteny), and defining early expression patterns during zebrafish (Danio rerio) development. Notably, we found that slc3a1 and slc7a9 are non-duplicated in the zebrafish genome. Whole-mount in situ hybridization detected co-localized expression of slc3a1 and slc7a9 in pronephric ducts at 24 h post-fertilization and in the proximal convoluted tubule at 3 days post-fertilization (dpf). Notably, both the genes showed co-localized expression in epithelial cells in the gut primordium at 3 dpf and in the intestine at 5 dpf (onset of exogenous feeding). Taken together, these results highlight the value of slc3a1 and slc7a9 as markers of zebrafish kidney and intestine development and show promise for establishing new zebrafish tools that can aid in the rapid screening(s) of substrates. Importantly, such studies will help clarify the complex interplay between the absorption of dibasic amino acids, cystine, and (large) neutral amino acids and the effect(s) of such nutrients on organismal growth.

Critical transporters of methionine and methionine hydroxyl analogue supplements across the intestine: What we know so far and what can be learned to advance animal nutrition

DL-methionine (DL-Met) and its analogue DL-2-hydroxy-4-(methylthio) butanoic acid (DL-methionine hydroxyl analogue or DL-MHA) have been used as nutritional supplements in the diets of farmed raised animals. Knowledge of the intestinal transport mechanisms involved in these products is important for developing dietary strategies. This review provides updated information of the expression, function, and transport kinetics in the intestine of known Met-linked transporters along with putative MHA-linked transporters. As a neutral amino acid (AA), the transport of DL-Met is facilitated by multiple apical sodium-dependent/-independent high-/low-affinity transporters such as ASCT2, B⁰AT1 and rBAT/b^0,+AT. The basolateral transport largely relies on the rate-limiting uniporter LAT4, while the presence of the basolateral antiporter y⁺LAT1 is probably necessary for exchanging intracellular cationic AAs and Met in the blood. In contrast, the intestinal transport kinetics of DL-MHA have been scarcely studied. DL-MHA transport is generally accepted to be mediated simply by the proton-dependent monocarboxylate transporter MCT1. However, in-depth mechanistic studies have indicated that DL-MHA transport is also achieved through apical sodium monocarboxylate transporters (SMCTs). In any case, reliance on either a proton or sodium gradient would thus require energy input for both Met and MHA transport. This expanding knowledge of the specific transporters involved now allows us to assess the effect of dietary ingredients on the expression and function of these transporters. Potentially, the resulting information could be furthered with selective breeding to reduce overall feed costs.

Gallic acid affects intestinal-epithelial-cell integrity and selected amino-acid uptake in porcine in vitro and ex vivo permeability models

Gallic acid (GA) is widely used as a dietary supplement due to several health-promoting effects, although its effects on intestinal-epithelial-cell integrity and transport remain mostly unknown. The present study aims to clarify the effects of GA on tight junctions and intestinal nutrient uptake through in vitro and ex vivo models. Both intestinal porcine enterocyte cell line-J2 cells and porcine middle-jejunum segments were treated with 5 (T5), 25 (T25) and 50 (T50) µm GA and mounted in Ussing chambers to determine transepithelial resistance (TEER), claudin-1 (CLDN1), occludin (OCLN), zonula occludens-1 (ZO-1) protein (in tissues and cells) and mRNA (in cells) expression. In addition, uptake of l-glutamate (l-Glut), l-arginine (l-Arg), l-lysine (l-Lys) and l-methionine (l-Meth) together with cationic-amino-acid transporter-1 (CAT-1) and excitatory-amino-acid transporter-3 (EAAT3) expression was evaluated. No apoptosis was observed in GA-treated cells, but TEER and CLDN1 protein abundance was lower with T50 compared with untreated cells. l-Arg and l-Lys uptake was greater with T5 than with T25 and T50. Ex vivo, T50 decreased the TEER values and the protein levels of CLDN1, OCLN and ZO-1, whereas T5 and T25 only decreased CLDN1 protein expression compared with untreated tissues. Moreover, T25 increased l-Glut and l-Arg uptake, the latter confirmed by an increased protein expression of CAT-1. GA influences intestinal uptake of the tested cationic amino acids at low concentrations and decreases the intestinal-cell barrier function at high concentrations. Similarities were observed between in vitro and ex vivo, but different treatment times and structures must be considered.

Sulfur metabolism and its contribution to malignancy

Metabolic dysregulation is an appreciated hallmark of cancer and a target for therapeutic intervention. Cellular metabolism involves a series of oxidation/reduction (redox) reactions that yield the energy and biomass required for tumor growth. Cells require diverse molecular species with constituent sulfur atoms to facilitate these processes. For humans, this sulfur is derived from the dietary consumption of the proteinogenic amino acids cysteine and methionine, as only lower organisms (e.g., bacteria, fungi, and plants) can synthesize them de novo. In addition to providing the sulfur required to sustain redox chemistry, the metabolism of these sulfur-containing amino acids yield intermediate metabolites that constitute the cellular antioxidant system, mediate inter- and intracellular signaling, and facilitate the epigenetic regulation of gene expression, all of which contribute to tumorigenesis.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Associating mutations causing cystinuria with disease severity with the aim of providing precision medicine

Background

Cystinuria is an inherited disease that results in the formation of cystine stones in the kidney, which can have serious health complications. Two genes (SLC7A9 and SLC3A1) that form an amino acid transporter are known to be responsible for the disease. Variants that cause the disease disrupt amino acid transport across the cell membrane, leading to the build-up of relatively insoluble cystine, resulting in formation of stones. Assessing the effects of each mutation is critical in order to provide tailored treatment options for patients. We used various computational methods to assess the effects of cystinuria associated mutations, utilising information on protein function, evolutionary conservation and natural population variation of the two genes. We also analysed the ability of some methods to predict the phenotypes of individuals with cystinuria, based on their genotypes, and compared this to clinical data.

Results

Using a literature search, we collated a set of 94 SLC3A1 and 58 SLC7A9 point mutations known to be associated with cystinuria. There are differences in sequence location, evolutionary conservation, allele frequency, and predicted effect on protein function between these mutations and other genetic variants of the same genes that occur in a large population. Structural analysis considered how these mutations might lead to cystinuria. For SLC7A9, many mutations swap hydrophobic amino acids for charged amino acids or vice versa, while others affect known functional sites. For SLC3A1, functional information is currently insufficient to make confident predictions but mutations often result in the loss of hydrogen bonds and largely appear to affect protein stability. Finally, we showed that computational predictions of mutation severity were significantly correlated with the disease phenotypes of patients from a clinical study, despite different methods disagreeing for some of their predictions.

Conclusions

The results of this study are promising and highlight the areas of research which must now be pursued to better understand how mutations in SLC3A1 and SLC7A9 cause cystinuria. The application of our approach to a larger data set is essential, but we have shown that computational methods could play an important role in designing more effective personalised treatment options for patients with cystinuria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3913-1) contains supplementary material, which is available to authorized users.

Increases in circulating amino acids with in-feed antibiotics correlated with gene expression of intestinal amino acid transporters in piglets

In-feed antibiotics have been commonly used to promote the growth performance of piglets. The antibiotics can increase protein utilization, but the underlying mechanism is largely unknown. The present study investigated the effects of in-feed antibiotics on intestinal AA transporters and receptors to test the hypothesis that the alteration of circulating AA profiles may be concomitant with the change of intestinal AA transporters and receptors. Sixteen litters of piglets at day 7 started to receive creep feed with (Antibiotic) or without (Control) antibiotic. Piglets were weaned at day 23 after birth, and fed the same diets until day 42. In-feed antibiotics did not affect the BW of 23-day-old (P = 0.248), or 42-day-old piglets (P = 0.089), but increased the weight gain to feed ratio from day 23 to 42 (P = 0.020). At day 42 after birth, antibiotic treatment increased the concentrations of most AAs in serum (P < 0.05), and decreased the concentrations of most AAs in jejunal and ileal digesta. Antibiotics upregulated (P < 0.05) the mRNA expression levels for jejunal AAs transporters (CAT1, EAAC1, ASCT2, y⁺LAT1), peptide transporters (PepT1), and Na⁺-K⁺-ATPase (ATP1A1), and ileal AA transporters (ASCT2, y⁺LAT1, b^0,+AT, and B⁰AT1), and ATP1A1. The antibiotics also upregulated the mRNA expression of jejunal AAs receptors T1R3 and CaSR, and ileal T1R3. Protein expression levels for jejunal AA transporters (EAAC1, b^0,+AT, and ASCT2) and PepT1 were also upregulated. Correlation analysis revealed that the alterations of AA profiles in serum after the in-feed antibiotics were correlated with the upregulations of mRNA expression levels for key AA transporters and receptors in the small intestine. In conclusion, the in-feed antibiotics increased serum level of most AAs and decreased most AAs in the small intestine. These changes correlated with the upregulations of mRNA expression levels for key AA transporters and receptors in the small intestine. The findings provide further insights into the mechanism of in-feed antibiotics, which may provide new framework for designing alternatives to antibiotics in animal feed in the future.

Compensatory Renal Hypertrophy and the Uptake of Cysteine S-Conjugates of Hg2+ in Isolated S2 Proximal Tubular Segments

Chronic kidney disease is characterized by a progressive and permanent loss of functioning nephrons. In order to compensate for this loss, the remaining functional nephrons undergo significant structural and functional changes. We hypothesize that luminal uptake of inorganic mercury (Hg²⁺), as a conjugate of cysteine (Cys; Cys-S-Hg-S-Cys), is enhanced in S2 segments of proximal tubules from the remnant kidney of uninephrectomized (NPX) rabbits. To test this hypothesis, we measured uptake and accumulation of Cys-S-Hg-S-Cys in isolated perfused S2 segments of proximal tubules from normal (control) and NPX rabbits. The remnant kidney in NPX rabbits undergoes significant hypertrophy during the initial 3 weeks following surgery. Tubules isolated from NPX rabbits were significantly larger in diameter and volume than those from control rabbits. Moreover, real-time PCR analyses of proximal tubules indicated that the expression of selected membrane transporters was greater in kidneys of NPX animals than in kidneys of control animals. When S2 segments from control and NPX rabbits were perfused with cystine or Cys-S-Hg-S-Cys, we found that the rates of luminal disappearance and tubular accumulation of Hg²⁺were greater in tubules from NPX animals. These increases were inhibited by the addition of various amino acids to the perfusate. Taken together, our data suggest that hypertrophic changes in proximal tubules lead to an enhanced ability of these tubules to take up and accumulate Hg².

Mechanisms involved in the transport of mercuric ions in target tissues

Mercury exists in the environment in various forms, all of which pose a risk to human health. Despite guidelines regulating the industrial release of mercury into the environment, humans continue to be exposed regularly to various forms of this metal via inhalation or ingestion. Following exposure, mercuric ions are taken up by and accumulate in numerous organs, including brain, intestine, kidney, liver, and placenta. In order to understand the toxicological effects of exposure to mercury, a thorough understanding of the mechanisms that facilitate entry of mercuric ions into target cells must first be obtained. A number of mechanisms for the transport of mercuric ions into target cells and organs have been proposed in recent years. However, the ability of these mechanisms to transport mercuric ions and the regulatory features of these carriers have not been characterized completely. The purpose of this review is to summarize the current findings related to the mechanisms that may be involved in the transport of inorganic and organic forms of mercury in target tissues and organs. This review will describe mechanisms known to be involved in the transport of mercury and will also propose additional mechanisms that may potentially be involved in the transport of mercuric ions into target cells.

Leucine stimulates ASCT2 amino acid transporter expression in porcine jejunal epithelial cell line (IPEC-J2) through PI3K/Akt/mTOR and ERK signaling pathways

Leucine has been shown to influence intestinal protein metabolism, cell proliferation and migration. Furthermore, our previous study demonstrated that branched-chain amino acids could modulate the intestinal amino acid and peptide transporters in vivo. As the possible mechanisms are still largely unknown, in the present work, we studied the transcriptional and translational regulation of leucine on amino acid transporter production in IPEC-J2 cells and the signaling pathways involved. Treatment of IPEC-J2 cells with 7.5 mM leucine enhanced the mRNA expression of the Na(+)-neutral AA exchanger 2 (ASCT2) and 4F2 heavy chain (4F2hc) and caused an increase in ASCT2 protein expression. Leucine also activated phosphorylation of 4E-BP1 and eIF4E through the phosphorylation of mTOR, Akt and ERK signaling pathways in IPEC-J2 cells. Pre-treatment of IPEC-J2 cells with inhibitors of mTOR and Akt (rapamycin and wortmannin) or an inhibitor of ERK (PD098059) for 30 min before leucine treatment attenuated the positive effect of leucine in enhancing the protein abundance of ASCT2. These results demonstrate that leucine could up-regulate the expression of the amino acid transporters (ASCT2) through transcriptional and translational regulation by ERK and PI3K/Akt/mTOR activation.

Cystinuria: mechanisms and management

Cystinuria is a relatively uncommon cause of pediatric stone disease, but has significant morbidity if not properly controlled because of its significant stone recurrence rate. Cystinuria is caused by the inability of the renal tubules to reabsorb filtered cystine, which is poorly soluble at a typical urine pH <7. Although many advances have been made in the understanding of the genetic and physiological basis of cystinuria, the cornerstones of treatment still involve stone prevention with dietary measures and pharmacological therapy, coupled with surgical interventions for stone removal. Pharmacological treatments can carry significant side effects that must be monitored and can limit therapy as well as impede compliance. Most patients will require surgical intervention for stone removal, although compliance with prevention strategies reduces the need for intervention.

An evolutionary comparative scan for longevity-related oxidative stress resistance mechanisms in homeotherms

Key mechanisms relating oxidative stress to longevity from an interespecies comparative approach are reviewed. Long-lived animal species show low rates of reactive oxygen species (ROS) generation and oxidative damage at their mitochondria. Comparative physiology also shows that the specific compositional pattern of tissue macromolecules (proteins, lipids and nucleic acids) in long-lived animal species gives them an intrinsically high resistance to modification that likely contributes to their superior longevity. This is obtained in the case of lipids by decreasing the degree of fatty acid unsaturation, and in the case of proteins by lowering their methionine content. These findings are also substantiated from a phylogenomic approach. Nutritional or/and pharmacological interventions focused to modify some of these molecular traits were translated with modifications in animal longevity. It is proposed that natural selection tends to decrease the mitochondrial ROS generation and to increase the molecular resistance to the oxidative damage in long-lived species.

Cysteine/cystine redox signaling in cardiovascular disease

Extracellular thiol/disulfide redox environments are highly regulated in healthy individuals. The major thiol/disulfide redox couple in human plasma is cysteine (Cys) and its disulfide form, cystine (CySS). Oxidation of this redox couple, measured as a more positive steady-state redox potential (E(h)), is associated with risk factors for cardiovascular disease (CVD), including aging, smoking, obesity, and alcohol abuse. Rodent and vascular cell studies show that the extracellular redox state of Cys/CySS (E(h)CySS) can play a vital role in controlling CVD through proinflammatory signaling. This inflammatory signaling is regulated by cell-surface protein redox state and involves mitochondrial oxidation, nuclear factor-κB activation, and elevated expression of genes for monocyte recruitment to endothelial cells. Gene array and proteomics studies reveal the global nature of redox effects, and different cell types, e.g., endothelial cells, monocytes, fibroblasts, and epithelial cells, show cell-specific redox responses with different phenotypic traits, e.g., proliferation and apoptosis, which can contribute to CVD. The critical nature of the proinflammatory redox signaling and cell biology associated with E(h)CySS supports the use of plasma levels of Cys, CySS, and E(h)CySS as key indicators of vascular health. Plasma redox-state-based pharmacologic interventions to control or improve E(h)CySS may be effective in preventing CVD onset or progression.

Transport of inorganic mercury and methylmercury in target tissues and organs

Owing to the prevalence of mercury in the environment, the risk of human exposure to this toxic metal continues to increase. Following exposure to mercury, this metal accumulates in numerous organs, including brain, intestine, kidneys, liver, and placenta. Although a number of mechanisms for the transport of mercuric ions into target organs were proposed in recent years, these mechanisms have not been characterized completely. This review summarizes the current literature related to the transport of inorganic and organic forms of mercury in various tissues and organs. This review identifies known mechanisms of mercury transport and provides information on additional mechanisms that may potentially play a role in the transport of mercuric ions into target cells.

Cystinuria

Cystinuria is an inherited disorder characterized by the impaired reabsorption of cystine in the proximal tubule of the nephron and the gastrointestinal epithelium. The only clinically significant manifestation is recurrent nephrolithiasis secondary to the poor solubility of cystine in urine. Although cystinuria is a relatively common disorder, it accounts for no more than 1% of all urinary tract stones. Thus far, mutations in 2 genes, SLC3A1 and SLC7A9, have been identified as being responsible for most cases of cystinuria by encoding defective subunits of the cystine transporter. With the discovery of mutated genes, the classification of patients with cystinuria has been changed from one based on phenotypes (I, II, III) to one based on the affected genes (I and non-type I; or A and B). Most often this classification can be used without gene sequencing by determining whether the affected individual's parents have abnormal urinary cystine excretion. Clinically, insoluble cystine precipitates into hexagonal crystals that can coalesce into larger, recurrent calculi. Prevention of stone formation is the primary goal of management and includes hydration, dietary restriction of salt and animal protein, urinary alkalinization, and cystine-binding thiol drugs.

Inherited epithelial transporter disorders--an overview

In the late 1990s, the identification of transporters and transporter-associated genes progressed substantially due to the development of new cloning approaches such as expression cloning and, subsequently, to the implementation of the human genome project. Since then, the role of many transporter genes in human diseases has been elucidated. In this overview, we focus on inherited disorders of epithelial transporters. In particular, we review genetic defects of the genes encoding glucose transporters (SLC2 and SLC5 families) and amino acid transporters (SLC1, SLC3, SLC6 and SLC7 families).

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

A novel missense mutation of SLC7A9 frequent in Japanese cystinuria cases affecting the C-terminus of the transporter

Cystinuria is caused by the inherited defect of apical membrane transport systems for cystine and dibasic amino acids in renal proximal tubules. Mutations in either SLC7A9 or SLC3A1 gene result in cystinuria. The mutations of SLC7A9 gene have been identified mainly from Italian, Libyan Jewish, North American, and Spanish patients. In the present study, we have analyzed cystinuria cases from oriental population (mostly Japanese). Mutation analyses of SLC7A9 and SLC3A1 genes were performed on 41 cystinuria patients. The uptake of 14C-labeled cystine in COS-7 cells was measured to determine the functional properties of mutants. The protein expression and localization were examined by Western blot and confocal laser-scanning microscopy. Among 41 patients analyzed, 35 were found to possess mutations in SLC7A9. The most frequent one was a novel missense mutation P482L that affects a residue near the C-terminus end of the protein and causes severe loss of function. In MDCK II and HEK293 cells, we found that P482L protein was expressed and sorted to the plasma membrane as well as wild type. The alteration of Pro482 with amino acids with bulky side chains reduced the transport function of b(0,+)AT/BAT1. Interestingly, the mutations of SLC7A9 for Japanese cystinuria patients are different from those reported for European and American population. The results of the present study contribute toward understanding the distribution and frequency of cystinuria-related mutations of SLC7A9.

Kidney stones: pathophysiology and medical management

The formation of stones in the urinary tract stems from a wide range of underlying disorders. That clinicians look for the underlying causes for nephrolithiasis is imperative to direct management. There are many advances in genetics, pathophysiology, diagnostic imaging, medical treatment, medical prevention, and surgical intervention of nephrolithiasis. Here, I provide a brief general background and focus mainly on pathophysiology and medical treatment of kidney stones. Although important advances have been made in understanding nephrolithiasis from single gene defects, the understanding of polygenetic causes of kidney stones is still largely elusive. A substantial proportion of data that resulted in new methods of treatment and prevention, which can be empirical or definitive, has focused on urinary luminal chemical composition of the precipitating solutes. Manipulation of inhibitors and epithelial factors is important and needs further investigation. Advances in the management of nephrolithiasis depend on combined efforts of clinicians and scientists to understand the pathophysiology.

Heterogeneous mutations in the SLC3A1 and SLC7A9 genes in Chinese patients with cystinuria

Cystinuria is a recessively inherited aminoaciduria that leads to recurrent urolithiasis. It is caused by the defective transport of cystine and dibasic amino acids in the proximal renal tubules and intestinal epithelium. Two genes responsible for this, SLC3A1 and SLC7A9, are known. Patients with two SLC3A1 mutations are classified as type A cystinuria, whereas patients with two SLC7A9 mutations are classified as type B cystinuria. Few clinical and molecular data have been reported for Asian cystinuria patients. In this study, we determined the molecular basis of cystinuria in eight unrelated Chinese subjects. Coding exons and flanking introns of the SLC3A1 and SLC7A9 genes were directly sequenced after amplification by polymerase chain reaction. Five different SLC3A1 mutations were found. Two missense mutations, D210G and S547L, were novel. The other three SLC3A1 mutations (IVS6+2T>C, R181Q and R365W) have been described previously. In addition, four novel SLC7A9 mutations, C137R, c.730delG, IVS10+2_3delTG and IVS12+3insT, together with two previously reported mutations (A70V and G195R) were found. All patients except one carried compound heterozygous mutations. IVS12+3insT was detected in patients from two families. This is the first molecular genetic study on Chinese cystinuria patients. Three patients with type A cystinuria, two with type B cystinuria, and three carriers of type B cystinuria were identified. Our results suggest that the molecular basis of cystinuria is heterogeneous in our local population.

Cystine and glutamate transport in renal epithelial cells transfected with human system x(-) (c)

BACKGROUND

System x(-) (c) is a heterodimeric transporter, comprised of a light chain, xCT, and heavy chain, 4F2hc, which mediates the sodium-independent exchange of cystine and glutamate at the plasma membrane. In the current study we tested the hypothesis that stable transfection of Madin-Darby canine kidney (MDCK) cells with human xCT and 4F2hc results in the expression of functional system x(-) (c).

METHODS

MDCK cells were transfected stably with human clones for xCT and 4F2hc. Analyses of time- and temperature-dependence, saturation kinetics, and substrate specificity of l-cystine and l-glutamate transport were carried out in control and xCT-4F2hc-transfected MDCK cells. We also measured the uptake of l-cystine in Xenopus oocytes expressing human xCT and/or 4F2hc or xCT and/or rBAT (a heavy chain homologous to 4F2hc).

RESULTS

All of the different sets of data revealed that transport of l-cystine and l-glutamate increased significantly (twofold to threefold) in the MDCK cells subsequent to transfection with xCT-4F2hc. Moreover, uptake of l-cystine also increased (about tenfold) in Xenopus oocytes expressing hxCT and h4F2hc. Biochemical analyses of l-cystine uptake in oocytes verified our findings in the transfected MDCK cells. Interestingly, in oocytes injected with rBAT with or without xCT, uptake of l-cystine was significantly greater than that in water-injected oocytes.

CONCLUSION

Our findings indicate that stable transfection of MDCK cells with xCT and 4F2hc results in a cell-line expressing a functional system x(-) (c) transporter that can utilize l-cystine and l-glutamate as substrates. This study apparently represents the first stable transfection of a mammalian cell line with system x(-) (c).

Neutral amino acid transport in epithelial cells and its malfunction in Hartnup disorder

Hartnup disorder is an autosomal recessive abnormality of renal and gastrointestinal neutral amino acid transport. A corresponding transport activity has been characterized in kidney and intestinal cells and named system B(0). The failure to resorb amino acids in this disorder is thought to be compensated by a protein-rich diet. However, in combination with a poor diet and other factors, more severe symptoms can develop in Hartnup patients, including a photosensitive pellagra-like skin rash, cerebellar ataxia and other neurological symptoms. Homozygosity mapping in a Japanese family and linkage analysis on six Australian pedigrees placed the Hartnup disorder gene at a locus on chromosome 5p15. This fine mapping facilitated a candidate gene approach within the interval, which resulted in the cloning and characterization of a novel member of the sodium-dependent neurotransmitter transporter family (B(0)AT1, SLC6A19) from mouse and human kidney, which shows all properties of system B(0). Flux experiments and electrophysiological recording showed that the transporter is Na(+) dependent and Cl(-) independent, electrogenic and actively transports most neutral amino acids. In situ hybridization showed strong expression in intestinal villi and in the proximal tubule of the kidney. Expression of B(0)AT1 was restricted to kidney, intestine and skin. A total of ten mutations have been identified in SLC6A19 that co-segregate with disease in the predicted recessive manner, with the majority of affected individuals being compound heterozygotes. These mutations lead to altered neutral amino acid transport function compared to the wild-type allele in vitro. One of the mutations occurs in members of the original Hartnup family described in 1956, thereby defining SLC6A19 as the 'Hartnup'-gene.

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine

Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

Homocysteine, system b0,+ and the renal epithelial transport and toxicity of inorganic mercury

Proximal tubular epithelial cells are major sites of homocysteine (Hcy) metabolism and are the primary sites for the accumulation and intoxication of inorganic mercury (Hg(2+)). Previous in vivo data from our laboratory have demonstrated that mercuric conjugates of Hcy are transported into these cells by unknown mechanisms. Recently, we established that the mercuric conjugate of cysteine [2-amino-3-(2-amino-2-carboxy-ethylsulfanylmercuricsulfanyl)propionic acid; Cys-S-Hg-S-Cys], is transported by the luminal, amino acid transporter, system b(0,+). As Cys-S-Hg-S-Cys and the mercuric conjugate of Hcy (2-amino-4-(3-amino-3-carboxy-propylsulfanylmercuricsulfanyl)butyric acid; Hcy-S-Hg-S-Hcy) are similar structurally, we hypothesized that Hcy-S-Hg-S-Hcy is a substrate for system b(0,+). To test this hypothesis, we analyzed the saturation kinetics, time dependence, temperature dependence, and substrate specificity of Hcy-S-Hg-S-Hcy transport in Madin-Darby canine kidney (MDCK) cells stably transfected with system b(0,+). MDCK cells are good models in which to study this transport because they do not express system b(0,+). Uptake of Hg(2+) was twofold greater in the transfectants than in wild-type cells. Moreover, the transfectants were more susceptible to the toxic effects of Hcy-S-Hg-S-Hcy than wild-type cells. Accordingly, our data indicate that Hcy-S-Hg-S-Hcy is transported by system b(0,+) and that this transporter likely plays a role in the nephropathy induced after exposure to Hg(2+). These data are the first to implicate a specific, luminal membrane transporter in the uptake and toxicity of mercuric conjugates of Hcy in any epithelial cell.

Cystinuria and other noncalcareous calculi

Urinary stone disease is the only clinical presentation in patients with cystinuria. Two genes have been associated with type I (SLC3A1) and non-type I (SLC7A9) cystinuria and multiple mutations of these genes have been identified. The type I form is completely recessive while the non-type I form is incompletely recessive. Clinically, heterozygotes with type I mutations are silent while heterozygotes with non-type I (types II and III) present with a wide range of urinary cystine levels and some even have symptomatic urolithiasis. Although the exact molecular basis for these differences needs additional investigations, the future of medical management of cystinuria is based on molecular and gene therapy. Minimally invasive surgery using percutaneous and ureteroscopic techniques is the cornerstone of surgical management. Both cystine and struvite calculi can form staghorn configuration with propensity for rapid growth and frequent recurrences after surgical treatment. While urinary alkalinization for cystine calculi is an integral part of medical management, the effect of oral alkalinizing agents is limited because of the high pKa (8.3) of cystine. Chelating agents, therefore, are frequently used to decrease cystine solubility and stone recurrences. Similarly, urinary acidification for struvite calculi may dissolve existing stones and prevent recurrences. However, no effective oral agent is available today. A future challenge will be to introduce reliable oral agents for urinary acidification.

Homologues of amino acid permeases: cloning and tissue expression of XAT1 and XAT2

The L-type (LAT) family of amino acid transporters is composed of exchangers for neutral, cationic, and anionic amino acids. They form functional heterodimers with membrane glycoproteins, rBAT or 4F2hc/CD98, to which they are linked by a disulphide bond. We report the molecular cloning and tissue expression of new mouse and human homologues of the LAT family, termed mXAT1, mXAT2 and hXAT2. The latter two proteins may correspond to ortholog genes in mouse and human. The hXAT2 gene is located on chromosome 8q21.3. The cloned X amino acid transporter (XAT) cDNAs are predicted to encode proteins of about 50 kDa. From a phylogenetic point of view, the three XAT proteins cluster together, but sequence comparison and secondary structure prediction show that they are also related to the members of the LAT family. Like these transporters, the XAT proteins show 12 transmembrane domains and a conserved cysteine residue, located in the second extracellular loop. This conserved cysteine is involved in the disulphide bond formed between the known members of the LAT family and 4F2hc or rBAT. The mXAT1 and hXAT2 mRNAs are expressed in the kidney but they are not detectable in a variety of other tissues. The corresponding proteins were efficiently translated following transfection of their cDNAs in Chinese hamster ovary (CHO) cells. However, cDNA transfection in CHO cells did not induce amino acid uptake, even when cotransfected with vectors expressing 4F2hc or rBAT. This could be related to the fact that mXAT1 and hXAT2 did not form detectable disulphide-linked heterodimers with 4F2hc or rBAT when they were co-expressed in CHO cells. Identification of other putative partner(s) of these LAT family-related transporters may be necessary to understand their role in renal physiology.

Expression, regulation and function of carrier proteins for cationic amino acids

Different carrier proteins exhibiting distinct transport properties participate in cationic amino acid transport. There are sodium-independent systems, such as b+, y+, y+L and b0,+, and a sodium-dependent system B0,+, most of which have now been identified at the molecular level. In most non-epithelial cells, members of the cationic amino acid transporter (CAT) family mediating system y+ activity seem to be the major entry pathway for cationic amino acids. CAT proteins underlie complex regulation at the transcriptional, post-transcriptional and activity levels. Recent evidence indicates that individual CAT isoforms are necessary for providing the substrate for nitric oxide synthesis, for example CAT-1 for Ca2+-independent nitric oxide production in endothelial cells and CAT-2B for sustained nitric oxide production in macrophages.

Heteromeric amino acid transporters: biochemistry, genetics, and physiology

The heteromeric amino acid transporters (HATs) are composed of two polypeptides: a heavy subunit (HSHAT) and a light subunit (LSHAT) linked by a disulfide bridge. HSHATs are N-glycosylated type II membrane glycoproteins, whereas LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs have been known since 1992, and the LSHATs have been described in the last three years. HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, x(c)(-), and b(0,+)). Members of the HAT family are the molecular bases of inherited primary aminoacidurias cystinuria and lysinuric protein intolerance. In addition to the role in amino acid transport, one HSHAT [the heavy subunit of the cell-surface antigen 4F2 (also named CD98)] is involved in other cell functions that might be related to integrin activation. This review covers the biochemistry, human genetics, and cell physiology of HATs, including the multifunctional character of CD98.