Renal clearance of amino acids in canine cystinuria

Novel cystine transporter in renal proximal tubule identified as a missing partner of cystinuria-related plasma membrane protein rBAT/SLC3A1

Heterodimeric amino acid transporters play crucial roles in epithelial transport, as well as in cellular nutrition. Among them, the heterodimer of a membrane protein b(0,+)AT/SLC7A9 and its auxiliary subunit rBAT/SLC3A1 is responsible for cystine reabsorption in renal proximal tubules. The mutations in either subunit cause cystinuria, an inherited amino aciduria with impaired renal reabsorption of cystine and dibasic amino acids. However, an unsolved paradox is that rBAT is highly expressed in the S3 segment, the late proximal tubules, whereas b(0,+)AT expression is highest in the S1 segment, the early proximal tubules, so that the presence of an unknown partner of rBAT in the S3 segment has been proposed. In this study, by means of coimmunoprecipitation followed by mass spectrometry, we have found that a membrane protein AGT1/SLC7A13 is the second partner of rBAT. AGT1 is localized in the apical membrane of the S3 segment, where it forms a heterodimer with rBAT. Depletion of rBAT in mice eliminates the expression of AGT1 in the renal apical membrane. We have reconstituted the purified AGT1-rBAT heterodimer into proteoliposomes and showed that AGT1 transports cystine, aspartate, and glutamate. In the apical membrane of the S3 segment, AGT1 is suggested to locate itself in close proximity to sodium-dependent acidic amino acid transporter EAAC1 for efficient functional coupling. EAAC1 is proposed to take up aspartate and glutamate released into luminal fluid by AGT1 due to its countertransport so that preventing the urinary loss of aspartate and glutamate. Taken all together, AGT1 is the long-postulated second cystine transporter in the S3 segment of proximal tubules and a possible candidate to be involved in isolated cystinuria.

Changing paradigms in diagnosis of inherited defects associated with urolithiasis

The way in which veterinary scientists think about and approach the study of genetic disease has not changed, but the tools available to veterinary scientists have and will continue to change, allowing us to study increasingly complex problems and to make more rapid advances in the context of simple problems. To put these advances in perspective, this article first gives a historical perspective on the approaches to studying genetic diseases, particularly in human beings, and then outlines the advances that have become possible with the availability of the dog genome sequence. The article then discusses two inherited defects that are associated with urolithiasis, in particular, those responsible for cystine and purine (uric acid and its salts) stone formation. Together, these two conditions illustrate the contemporary use of a broad range of genetic approaches.

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.

SLC7A9 cDNA cloning and mutational analysis of SLC3A1 and SLC7A9 in canine cystinuria

Cystinuria is a genetic disorder in the domestic dog that leads to recurrent urolith formation. The genetic basis of the disorder is best characterized in humans and is caused by mutations in one of the amino acid transporter genes SLC3A1 or SLC7A9, which results in hyperexcretion of cystine and the dibasic amino acids in the urine and subsequent precipitation of cystine due to its low solubility in urine. In this study we describe the cloning of the canine SLC7A9 cDNA and present a thorough mutation analysis of the coding SLC3A1 and SLC7A9 regions in cystinuric dogs of different breeds. Mutation analysis of the two cystinuria disease genes revealed one SLC7A9 mutation (A217T) and two SLC3A1 mutations (I192V and S698G) in French and English Bulldogs that affect nonconserved amino acid residues, arguing against functional impact on the proteins. The absence of deleterious mutations linked to cystinuria in the remainder of our panel of cystinuric dogs is surprising because SLC3A1 or SLC7A9 mutations explain approximately 70% of all human cystinuria cases studied. The present study, along with previous investigations of canine and human cystinuria, implies that regulatory parts of the SLC3A1 and SLC7A9 genes as well as other unknown genes may harbor mutations causing cystinuria.

Dilated cardiomyopathy in an American cocker spaniel with taurine deficiency

An American Cocker Spaniel with low plasma taurine concentration (< 2 nmol/mL) was presented with dyspnoea associated with pulmonary oedema and a left ventricular shortening fraction of 9%. Emergency therapy with furosemide, dobutamine, nitroglycerine and oxygen supplementation led to a good response. Chronic therapy was started with enalapril, furosemide, digoxin and taurine. Improvement in all echocardiographic indices were noted over a 22 week follow-up, most notably an increase in left ventricular shortening fraction to 20%, a decrease of E-point septal separation from 14 mm to 7 mm and marked left ventricular remodelling. This degree of improvement in myocardial function may represent a direct link between dilated cardiomyopathy in the American Cocker Spaniel and plasma taurine deficiency. Alternatively, this response may reflect a breed-related cardiomyopathy with a natural history and therapeutic response not commonly seen in the more common large breed cardiomyopathy presentations.

Fanconi syndrome in a Labrador retriever

A 2-year-old male Labrador Retriever was presented to the University of Missouri Veterinary Teaching Hospital with the primary complaints of polydipsia, polyuria, and joint or muscle pain. Low blood urea nitrogen concentration, hyperchloremia, and marked proteinuria were the only abnormalities in a serum biochemical profile and urinalysis. Decreased creatinine clearance and increased renal fractional excretion of sodium, potassium, calcium, and phosphorus were detected by renal clearance studies. Increased excretion of most amino acids was found by amino acid analysis of urine, but not all amino acids were lost with equal magnitude. Amino acids with secondary amino groups or basic side chains were lost at increased rates, whereas those with acidic side chains were not. These differences could be related to defects in specific renal amino acid transport mechanisms. Identification of these transport mechanisms may allow for pharmacologic intervention at the point of renal loss to alleviate clinical signs of disease.

Urinary excretion of amino acids in normal and cystinuric dogs

The 24-h urine excretion of 20 amino acids was investigated in 24 cystinuric and 15 normal dogs. The diagnosis of cystinuria was based on infrared spectroscopy of removed uroliths, which in all cases were composed of pure cystine. Seven of 24 cystinuric dogs showed normal cystine excretion compared to normal dogs, and four of 24 dogs showed normal total amino acid excretion. In contrast to earlier investigations, almost half of the cystinuric dogs (46%) showed elevated excretion of five or more amino acids. Isolated cystinuria, or isolated dibasic amino aciduria was not found. Compared to normal dogs, the cystinuric dogs showed a significantly (P < 0.05) increased excretion of cystine, arginine, lysine, cystathionine, glutamic acid, threonine and glutamine. There was a significant correlation (P < 0.05) between the urinary excretion of cystine and 10 other amino acids, with the highest correlation found (P < 0.001) for arginine, lysine, cystathionine, ornithine and 1-methyl-histidine. Three patterns of amino acid excretion could be identified: (1) increased excretion and a significant correlation with cystine for the three dibasic amino acids (lysine, arginine and ornithine), compatible with a common reabsorption mechanism as shown in man. This pattern was also found for cystathionine and glutamic acid, which might indicate a relation in metabolism or transport; (2) increased excretion but no correlation with cystine for glutamine, threonine and citrulline; (3) good correlation with cystine, but no increased excretion for 1-methyl-histidine, phenylalanine, 3-methyl-histidine, leucine and alanine. The great variation in urinary cystine excretion suggests that factors other than the excretion of cystine must be considered as causes of cystine urolith formation. For example, cystinuric dogs were found to have lower diuresis than normal dogs and produced urine with higher cystine concentration thereby increasing the risk of cystine urolith formation.

Pharmacokinetics and bioavailability of 2-mercaptopropionylglycine administered intravenously and orally in dogs

The pharmacokinetic disposition of 2-mercaptopropionylglycine (2-MPG) given as a single intravenous injection and/or as a single oral dose was studied in 9 normal and 13 cystinuric dogs. After intravenous injection of approximately 10 or 20 mg/kg body weight the pharmacokinetics were best described by a three-exponential function. The first phase involved a distribution process apparently including establishment of drug-plasma protein and drug-tissue binding. The second phase involved rapid renal elimination and 60% of the drug was excreted within 3 h of administration. There was also a slow terminal third phase with a long half-life after both intravenous (t1/2 = 23 h) and oral (t1/2 = 22 h) administration. No dose dependency was observed. A deep pool of reversibly tissue-bound 2-MPG was indicated by a Vss of 3.3 +/- 0.9 l/kg body weight and the long terminal elimination phase. Total clearance was estimated as 4.1 +/- 0.9 ml/min/kg body weight. 2-MPG was eliminated mainly by renal excretion, but there was a difference in recovery of dose between normal and cystinuric dogs. During the first 24 h after intravenous and oral administration, 69% and 54%, respectively, of the drug was recovered in the urine of normal dogs. The corresponding figures in cystinuric dogs were 44% and 29%, respectively. The absolute bioavailability (FAUC) was 88 +/- 20% in normal dogs.

Characteristics of lysine uptake by isolated renal cortical tubule fragments from mature and immature dogs

The uptake of L-lysine was examined in isolated renal cortical tubule fragments from adult and 1-week-old dogs. Lysine uptake by adult tubules was initially more rapid than that by the immature tubules. This uptake by mature tubules reached a steady state after 30 min of incubation, while the newborn tubules still had not reached a steady state by 90 min of incubation. Because a steady state of lysine uptake was not attained with the immature tubules, their uptake of lysine exceeded that of the adult after 60 min of incubation. Kinetic studies revealed that lysine was taken up by one saturable transport system with a Km of 0.56 mM and Vmax of 6.18 mmol/liter intercellular fluid per 5 min in the adult and one saturable transport system in the 1-week-old with a Km of 0.38 mM and Vmax of 3.66 mmol/l intracellular fluid per 5 min. Lysine also entered the renal tubule cells in both age groups via a diffusional pathway with a kd of 0.35 min-1 in the adult and 0.30 min-1 in the newborn. Cystine competitively inhibited lysine uptake by adult dog tubules with a Ki of 0.61 mM. The other dibasic amino acids, ornithine and arginine, also inhibited lysine uptake in both the adult and the newborn.

Canine cystine urolithiasis

Cystine uroliths form as a result of a complex metabolic disturbance in amino acid metabolism and transport. The inheritance of this disease is obscure because it does not follow a standard mendelian pattern. Uroliths are a vexing clinical problem because the recurrence rate is high. Accurate diagnosis of uroliths requires use of crystallographic methods. The renal defect leading to excessive excretion of cystine is variable. The only effective method of prevention of recurrence is use of D-penicillamine, which is poorly tolerated by some dogs.

Renal tubule reabsorption of amino acids after lysine loading of cystinuric dogs

The renal reabsorption of cystine, lysine, arginine, and ornithine as well as other amino acids has been determined before and after lysine infusion in four normal and five cystinuric dogs. The large filtered load of lysine caused defective reabsorption of cystine in three of four normals and augmented the basal defect in all of the cystinuric dogs. Two of the cystinuric dogs responded with cystine clearances in excess of the glomerular filtration rate. The magnitude of increase of the reabsorptive defect for cystine observed after lysine infusion into the cystinuric dogs was unrelated to the extent of the basal defect. Two of the normal dogs and four of the five cystinuric dogs demonstrated defective arginine reabsorption after lysine loading, one of the cystinuric dogs having greater arginine excretion than the filtered load. Although normal dogs showed a moderate inability to reabsorb the large filtered load of lysine, three of the cystinuric dogs exhibited a -60 to -70% reabsorption, consistent with lysine secretion. Both normal and cystinuric dogs showed defective glycine absorption after lysine loading, but only cystinuric dogs showed variable defects in threonine, serine, histidine, methionine, and tyrosine when the basic amino acid was infused. Each cystinuric dog responded to lysine infusion in a different way, and the overall pattern of response differed from the normal, with evidence of induced secretion of cystine, lysine, and arginine in the affected dogs.

Transport of L-cystine by rat renal brush border membrane vesicles

Brush border membranes were isolated from rat renal cortex by a divalent cation precipitation method. L-35S-cystine uptake into the vesicles was measured by a rapid filtration method. Covalent incorporation of tracer into membrane proteins was observed after prolonged incubations. At short incubation periods (1 min) binding was small and allowed an analysis of transmembrane transport. To guarantee transport of L-cystine, the experiments were performed in the presence of the oxidant diamide. Sodium stimulated L-cystine uptake specifically. A potassium/valinomycin induced inside negative diffusion potential stimulated sodium dependent L-cystine transport. Thus, transport is potential sensitive in the presence of sodium. At low substrate and inhibitor concentrations, L-cystine transport was inhibited by L-lysine, L-ornithine and L-arginine but not by D-lysine in the presence and absence of sodium. At higher inhibitor concentration, the neutral amino acids L-phenylalanine and L-leucine also inhibited L-cystine uptake, but only the sodium dependent uptake. These inhibition experiments suggest that L-cystine is transported by the brush border membrane by a transport system for basic amino acids not necessarily requiring sodium. In addition, transport of L-cystine can also proceed via sodium dependent transport pathways for neutral amino acids. In the concentration range tested (up to 0.225 mmoles/l), no saturation of L-cystine transport was observed in the presence and absence of sodium.

Developmental changes of glycine transport in the dog

The renal clearance of amino acids was measured in canine pups between 5 days and 12 weeks of age. The reabsorption of glycine was incomplete at 5 and 21 days, indicating a physiologic aminoaciduria of immaturity. An adult pattern of 97-100% reabsorption appeared by 8 weeks of age. The uptake of glycine by isolated renal tubules from 5-day-old, 3-month-old and adult dogs was examined towards an understanding of the events underlying this aminoaciduria. The initial uptake of 0.042 mM glycine by isolated tubules from the newborn was lower than that of the adult, but after 30 min of incubation the newborn surpassed the adult. A steady state of uptake was not achieved by the newborn even after 90 min of incubation, while it was achieved in the adult after 30 min. The uptake by the 3-month-old tubules resembled the adult at the early time points and the newborn at later points. With 1.032 mM glycine, a similar relationship of uptake between adult and newborn tubules was found, except with this concentration, the uptake by both the newborn and adult tubules reached a steady state. The concentration dependence of glycine uptake showed two saturable transport systems with similar apparent Km and Vmax values after 30 min of incubation for all three age groups. Determination of glycine flux by compartmental analysis revealed decreased influx and efflux in the newborn, but with a greater decrease in efflux, compared to adult. These changes of influx and efflux which accompany renal tubule maturation could contribute to the increased intracellular amino acid levels and decreased reabsorption of amino acids seen in the immature dog.

Membrane transport in the proximal tubule and thick ascending limb of Henle's loop: mechanisms and their alterations

Over the past few years, our knowledge on renal tubular transport mechanisms has increased considerably. Due to new technical developments, it is now possible to understand in part transepithelial transport and its pathological and pharmacological alterations at the level of the cell membranes. Different membrane transport mechanisms are discussed in this article, whereby sodium coupled solute transport in the proximal tubule and sodium chloride transport in the thick ascending limb of Henle's loop are taken as examples. It is indicated that an altered function of the kidney can often be equated with an alteration of the membrane transport.

Cystinuria in the maned wolf of South America

Of 42 maned wolves in zoos or live-trapped in Brazil, 34 had excessive cystine in their urine. Renal clearance studies of five of the affected wolves revealed a variable defect for the reabsorption of cystine and dibasic amino acids. The renal tubular handling of other solutes including glucose, phosphate, sodium, potassium, and uric acid was considered normal. Urinary calculi composed of cystine were found in four wolves and proved fatal in three of them. With the exception of the high incidence in this species, this hereditary disease resembles the disorder described in dogs and humans.

Cystine uptake by rat renal brush-border vesicles

Uptake of L-cystine by brush-border membrane vesicles isolated from rat renal-cortical tissue was time-dependent and occurred in the absence of cystine reduction. A significant capacity for vesicular binding of cystine was observed. The amount bound increased with time of incubation and could be displaced by thiol reagents. At early time points, cystine uptake measured the transport of cystine into the intravesicular space. Total cystine uptake was mediated by multiple transport systems, including a low-Km high-affinity component which was shared by lysine, arginine, ornithine and glutamine and on which hetero-exchange diffusion of lysine and cystine was demonstrated.

Inherited metabolic disease in laboratory animals: a review

Research on the screening for and study of animal models of inherited metabolic disease is reviewed. It is emphasized that an animal model, to be of value, must be an inherited deficiency of the same enzyme as the one deficient in the human syndrome. If this criterion is adhered to there is a remarkable identity in aetiology between animal and man. Specific examples of inherited metabolic disease in laboratory animals are described for: amino acid metabolism; lysosomal storage diseases, carbohydrate metabolism, transport disorders and trace element metabolism; the mutants found in mice being the easiest to manipulate biochemically and genetically. There is still a lack of adequate screening programmes for animal homologues of the more serious human inborn errors (such as lysosomal storage diseases) where laboratory studies could provide significant advances in therapy.

Transport interactions of cystine and dibasic amino acids in isolated rat renal tubules

Isolated renal cortical tubules prepared from adult male Sprague-Dawley rats were used to study the nature of cystine entry into tubule cells and its transport interactions with dibastic amino acids. The uptake of cystine over time was progressive, reaching a steady-state after 60 min of incubation. Analysis of the intracellular pool after incubation revealed that a significant fraction of the transported cystine was reduced to cysteine. A kinetic analysis of uptake demonstrated that two systems for cellular entry of cystine existed with a Km1 of 0.012 mM and Km2 of 0.55 mM. Cystine uptake was sodium dependent with an apparent Km for sodium of 36 mEq/liter. Lysine inhibited cystine uptake via the low Km system, but appeared not to inhibit cystine uptake via the high Km system. Ornithine, leucine, and isoleucine each inhibited cystine uptake via the low Km system. Arginine appeared to affect both systems for cystine uptake. Cystine inhibited the uptake of lysine by isolated renal tubules. The data suggest that cystine at physiologic concentrations is transported into renal tubule cells nearly equally by two systems, of which, the low Km system is shared with the dibasic amino acids. A defective low Km system could in part explain human cystinuria.

The fanconi syndrome in Basenji dogs: a new model for renal transport defects

The renal defects resulting in a Fanconi syndrome were seen in eight Basenji dogs by measuring renal clearance and in vitro amino acid and sugar uptake and performing histopathologic evaluations. Renal tubular handling of glucose, phosphate, sodium, potassium, uric acid, and amino acids was abnormal, and in vitro uptake of labeled lysine, glycine, and alpha-methyl-D-glucoside by renal cortical slices was impaired. Histopathology was normal except for enlarged nuclei in some renal tubule cells. These Basenji dogs, which may be genetically affected, represent a likely model for idiopathic Fanconi syndrome in humans.

Spontaneous Fanconi syndrome in the dog

Three dogs with spontaneous renal tubular defects similar to idiopathic Fanconi syndrome are characterized. Renal clearance studies revealed a fractional reabsorption of glucose ranging from 31% to 82%. Abnormal glucose thulium values were present in all dogs. A generalized aminoaciduria occurred in two dogs while one had aminoaciduria characteristic of canine cystinuria. Fractional reabsorption of phosphate ranged from 47% to 79%. In vitro uptake of alpha-methyl-D-glucoside was significantly depressed (p less than 0.001). In vitro uptake of amino isobutyric acid was similar to controls. Renal biopsy revealed nonspecific interstitial change in two dogs and normal histology in the other. These animals represent a useful new model for the study of renal tubular transport defects.

Transport interaction of cystine and dibasic amino acids in renal brush border vesicles

The uptake of cystine by vesicles prepared from rat kidney brush borders occurs by two distinct transport systems. The higher affinity system is inhibited by the dibasic amino acids lysine, arginine, and ornithine. The lower affinity system, unaffected by dibasic amino acids, appears to correspond to that observed by studying uptake of cystine by kidney slices.