Maturation of the proximal tubule in the puppy kidney: a comparison to the adult

Albumin Uptake and Processing by the Proximal Tubule: Physiologic, Pathologic and Therapeutic Implications

For nearly 50 years the proximal tubule (PT) has been known to reabsorb, process, and either catabolize or transcytose albumin from the glomerular filtrate. Innovative techniques and approaches have provided insights into these processes. Several genetic diseases, nonselective PT cell defects, chronic kidney disease (CKD), and acute PT injury lead to significant albuminuria, reaching nephrotic range. Albumin is also known to stimulate PT injury cascades. Thus, the mechanisms of albumin reabsorption, catabolism, and transcytosis are being reexamined with the use of techniques that allow for novel molecular and cellular discoveries. Megalin, a scavenger receptor, cubilin, amnionless, and Dab2 form a nonselective multireceptor complex that mediates albumin binding and uptake and directs proteins for lysosomal degradation after endocytosis. Albumin transcytosis is mediated by a pH-dependent binding affinity to the neonatal Fc receptor (FcRn) in the endosomal compartments. This reclamation pathway rescues albumin from urinary losses and cellular catabolism, extending its serum half-life. Albumin that has been altered by oxidation, glycation, or carbamylation or because of other bound ligands that do not bind to FcRn traffics to the lysosome. This molecular sorting mechanism reclaims physiological albumin and eliminates potentially toxic albumin. The clinical importance of PT albumin metabolism has also increased as albumin is now being used to bind therapeutic agents to extend their half-life and minimize filtration and kidney injury. The purpose of this review is to update and integrate evolving information regarding the reabsorption and processing of albumin by proximal tubule cells including discussion of genetic disorders and therapeutic considerations.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Renal imino acid and glycine transport system ontogeny and involvement in developmental iminoglycinuria

Renal maturation occurs post-natally in many species and reabsorption capacity at birth can vary substantially from the mature kidney. However, little is known regarding the maturation of amino acid transport mechanisms, despite the well-known physiological state of developmental iminoglycinuria. Commonly seen during early infancy, developmental iminoglycinuria is a transient version of the persistent inherited form of the disorder, referred to as iminoglycinuria, and manifests as a urinary hyperexcretion of proline, hydroxyproline and glycine. The transporters involved in developmental iminoglycinuria and their involvement in the improvement of renal reabsorption capacity remain unknown. qPCR (quantitative real-time PCR) and Western blot analysis in developing mouse kidney revealed that the expression of Slc6a18, Slc6a19, Slc6a20a and Slc36a2 was lower at birth (approx. 3.4-, 5.0-, 2.4- and 3.0-fold less than adult kidney by qPCR respectively) and increased during development. Furthermore, immunofluorescence confocal microscopy demonstrated the absence of apical expression of Slc6a18, Slc6a19, Slc6a20a and the auxiliary protein collectrin in kidneys of mice at birth. This correlated with the detection of iminoglycinuria during the first week of life. Iminoglycinuria subsided (proline reduction preceded glycine) in the second week of life, which correlated with an increase in the expression of Slc6a19 and Slc6a20a. Mice achieved an adult imino acid and glycine excretion profile by the fourth week, at which time the expression level of all transporters was comparable with adult mice. In conclusion, these results demonstrate the delayed expression and maturation of Slc6a18, Slc6a19, Slc6a20a and Slc36a2 in neonatal mice and thus the molecular mechanism of developmental iminoglycinuria.

Clinically Relevant Physiology of the Neonate

During the 4- to 6-week neonatal period (birth to weaning), the physiology of canine and feline neonates undergoes dramatic change. Despite the neonate having the appearance of a miniature adult, their unique physiology has a significant impact on physical examination and laboratory test interpretation and may limit diagnostic capabilities and therapeutic options. In this article, the most striking physiologic differences between neonatal and adult body systems are discussed with respect to the resulting clinical implications.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Citrinin produces acute adverse changes in renal function and ultrastructure in pentobarbital-anesthetized dogs without concomitant reductions in [potassium]plasma

Citrinin's nephrotoxicity was examined in pentobarbital-anesthetized dogs under conditions that minimized or avoided significant changes in a number of its actions that could indirectly and adversely affect renal function and ultrastructure, such as, (i) major acute reductions in blood pressure and renal blood flow and, (ii) emesis and diarrhea that could lead to dehydration and electrolyte imbalances, especially hypokalemia. Slow intravenous injection of 20 mumol citrinin/kg to pentobarbital-anesthetized dogs did not induce any alterations in renal tissue ultrastructure or in any of the 23 whole blood, plasma or renal function parameters that were monitored over a 6-h post-citrinin period. On the other hand, 80 mumol citrinin/kg produced significant increases in the hematocrit and in the renal excretion rates of protein and glucose; modest reductions were noted in CIN, RBF and excretion rate of inorganic phosphorus. In addition, 80 mumol citrinin/kg induced ultrastructural lesions in the cells of the S2 proximal tubular segment, the thick ascending limb, the distal convoluted tubule and the collecting ducts. The glomeruli, S1 and S3 cells of the proximal tubule and the thin descending and ascending limbs of Henle's loop were unaffected by both citrinin doses. The location and nature of the adverse ultrastructural lesions were most likely the result of the direct actions of citrinin (or a citrinin metabolite) since the effects of citrinin that could lead to indirect adverse renal effects were totally avoided or greatly minimized.

S-(1,2-dichlorovinyl)-3-mercaptopropionic acid effects on renal function and ultrastructure in pentobarbital-anesthetized dogs: site-specific toxicity and evidence for its toxification via the pathway responsible for beta-oxidation of fatty acids

S-(1,2-dichlorovinyl)-3-mercaptopropionic acid (DCV-3-MPA) was equally nephrotoxic to spontaneously-respiring and mechanically-ventilated, pentobarbital-anesthetized dogs. Its nephrotoxicity was expressed as dose-dependent changes in key renal function parameters, in proximal tubular S1, S2 and S3 cellular architecture and in the ability of the kidneys to respond maximally to ethacrynic acid, an efficacious loop diuretic. The nephrotoxicity associated with DCV-3-MPA was not the result of extrarenal actions such as hypoxemia and subsequent renal tissue hypoxia because mechanical ventilation was not protective. Four lines of evidence suggested that DCV-3-MPA was taken-up by renal proximal tubular cells like a fatty acid and metabolized by the mitochondrial beta-oxidation pathway to a reactive nephrotoxic intermediate: (i) probenecid pretreatment, which reduces the renal uptake of many organic anions but fails to do so with anions of fatty acids, failed to modify the nephrotoxicity of DCV-3-MPA; (ii) the next higher and lower homologues of DCV-3-MPA (i.e., S-(1,2-dichlorovinyl)-4-mercaptobutanoic acid (DCV-4-MBA) and S-(1,2-dichlorovinyl)-mercaptoacetic acid (DCV-MAA)) cannot yield the same reactive intermediate as DCV-3-MPA upon beta-oxidation and neither was nephrotoxic; (iii) DCV-MAA was found in plasma and urine following administration of DCV-4-MBA and (iv) the renal mitochondria were reproducibly damaged by DCV-3-MPA whereas the peroxisomes, which are also capable of performing beta-oxidation of certain fatty acids, were unscathed.

Scanning electron microscopic observations of human fetal kidney maturing in vivo and in serum-free organ culture

A serum-free model has been developed in our laboratory enabling us to maintain human fetal kidney in culture for periods of 5 days or more. In this totally defined system, morphological integrity of these explants was shown to be preserved at both the light and the electron microscopic levels. The present work was undertaken to validate our culture model via scanning electron microscopy, a technique allowing surface observation of micromorphological features overlooked by conventional microscopy. In uncultured kidney, different developmental stages of nephron formation were identified. A sparse population of short microvilli was present on most cell apical membranes. Cell outlines were polygonal and demarcated by longer and densely packed microvilli. In proximal tubules, these microvilli were in the process of forming a brush border. In the majority of cells, one or two cilia with twisted or hooked tips projected into the capsular space or tubule lumen. Microcraters and bleb-like structures characterized the luminal membrane of many cells. The urinary papilla epithelium was composed of some ciliated principal cells but mostly of intercalated cells with either apical microplicae, microvilli, or both. Micro-projections formed zipper-like intercellular junctions. In culture, ultrastructural features, including membrane pits and spherical vesicles, were similar to those in uncultured explants. In summary, these novel observations in cultured fetal kidney indicate that ultrastructural integrity is well preserved in serum-free medium and that the present model is a valuable tool to study human nephrogenesis.

Developmental differences in renal sulfate reabsorption: transport kinetics in brush border membrane vesicles

Renal tubular reabsorption of inorganic sulfate is greater in younger than older guinea pigs. To determine the mechanism of this difference, we studied the transport of inorganic sulfate in renal brush border membrane vesicles (BBMV) obtained from young (< 25 days) and adult guinea pigs (> 60 days). BBMV were obtained by mechanical and osmotic disruption of dissected renal cortices followed by magnesium precipitation and differential centrifugation. After the membranes were incubated for 10 s in solutions containing inorganic sulfate at several concentrations (0.1-10 mM) and trace amounts of 35sulfate, intravesicular uptake was measured. Based on 35sulfur uptake, reabsorption transport kinetics (Vmax and Km) were estimated. BBMV obtained from young guinea pigs demonstrated higher sodium-sulfate cotransport, Vmax (51.79 +/- 4.34 pmol/mg protein per s) than those obtained from adult animals (Vmax = 34.28 +/- 9.17 pmol/mg per s), P < 0.05. Vmax values are represented as means plus or minus standard deviation. No differences in Km were observed. Our results indicate that age-related differences in renal inorganic sulfate reabsorption are due to a higher Vmax for sodium-sulfate cotransport in the younger animals, suggesting a higher density of sodium-sulfate cotransporters or an increased cotransport turnover rate in this age group.

Developmental changes in the renal capacity for sulfate reabsorption in the guinea pig

During growth, immature guinea pigs maintain a positive inorganic sulfate balance. In the present study, renal clearance techniques were used to determine the maximum renal capacity for sulfate reabsorption [TmRsi/glomerular filtration rate (GFR)] in three groups of guinea pigs at different stages of development (10-34 days, 35-80 days and greater than 120 days of age). These ages are comparable to infant, adolescent, and adult guinea pigs. The guinea pigs were weaned at 10 days and then maintained on normal guinea pig chow (0.13% sulfate). The TmRsi/GFR was determined by infusions of increasing concentrations of sulfate (0-16.8 mumol/min). TmRsi/GFR was significantly greater in young (infant and adolescent) than in older (adult) guinea pigs (2.20 +/- 0.26 mumol/ml and 1.80 +/- 0.27 mumol/ml vs 0.942 +/- 0.08 mumol/ml, P less than 0.05). These results demonstrate that the tubular capacity for inorganic sulfate reabsorption per milliliter of glomerular filtrate is enhanced in immature guinea pigs and decreases with age.

Acute effects of carboxyatractyloside and stevioside, inhibitors of mitochondrial ADP/ATP translocation, on renal function and ultrastructure in pentobarbital-anesthetized dogs

To assess the direct renal toxicity of carboxyatractyloside (CATR), it was administered in relatively low intravenous (i.v.) doses (6.5 and 13.0 mumol/kg) to pentobarbital-anesthetized dogs that were being mechanically ventilated in order to circumvent severe extrarenal effects, such as hypoxemia, that could contribute to its nephrotoxicity. Within 2 h post-CATR, site-specific renal damage was noted in S2 and S3 cells of the proximal tubules; characteristic lesions in both cell types included loss of brush border, condensation of mitochondria and proliferation of small vesicles. Other S2 cells exhibited intense staining and reduced cell height. In 3 of 14 CATR-treated dogs, extrarenal effects were of sufficient magnitude to induce cellular swelling and occlusion of tubular lumina in S3 and thick ascending limb segments. Stevioside (STEV), related to CATR in structure and actions on the mitochondrial ADP/ATP translocase, was totally devoid of acute extrarenal or direct renal effects during the 6-h period following intravenous administration of 2.5 times the higher dose of CATR. The ability of CATR to produce renal toxicity via its renal and extrarenal actions emphasizes the importance of minimizing the latter actions of any toxicant when attempting to ascertain the mechanism by which it adversely affects renal function and ultrastructure.

Renal tubular differentiation in mouse and mouse metanephric culture. I. Ultrastructural studies

We studied the morphologic features of renal tubular differentiation in transfilter metanephric culture. Differentiation of portions of the S-shaped loop into proximal convoluted tubule was detected shortly after 72 h of culture by the appearance of microvilli, coated membrane invaginations, and an apical vacuolar-microtubular network. These features developed synchronously, and the microvilli became progressively more numerous and more compact to form a brush border. Computer-assisted morphometric analysis showed only minor differences between proximal tubular cells from 18-day embryos and tubular cells from 7-day cultures of blastema taken from 11-day embryos. Segments of tubule corresponding to distal convoluted tubules were lined with relatively simple cells that contained few differentiating characteristics. Morphometric modeling of the tubular cells indicated a simple shape consistent with an inherent transport function.

Renal reabsorption of phosphate during development: tubular events

Studies performed in our laboratory on the isolated perfused kidney of the guinea pig have demonstrated that the rate of Pi reabsorption is substantially greater in the newborn than in the adult, when appropriate corrections are being made either for differences in glomerular filtration rate (GFR) or in renal tubular mass. In order to determine the location of this enhanced reabsorption along the nephron, micropuncture experiments were performed on euvolemic, non-fasted guinea pigs 5-14 and 42-49 days of age, maintained on standard guinea-pig chow diet (0.76% Pi). Concomitant measurements of overall kidney function were also obtained. The results confirmed that fractional reabsorption of Pi (TRPi%) across the entire kidney was significantly higher (P less than 0.01) in the newborn (89.93 +/- 2.55%) than in the adult (78.25 +/- 2.89%) animals. The difference was also significant (P less than 0.05) when TRPi was expressed in mol/ml GFR (1.87 +/- 0.14 vs 1.53 +/- 0.12, respectively). At comparable locations along the proximal tubule (TF/Pin of 1.90 +/- 0.16 in the newborn, and 1.79 +/- 0.15 in the adult, P greater than 0.70), the fraction of the filtered load of Pi reabsorbed was significantly higher (P less than 0.001) in the immature (76.66 +/- 2.74%) than in the mature (67.21 +/- 2.74%) guinea pigs. Estimates based on the differences between proximal Pi reabsorption and the urinary excretion of Pi indicate that the reabsorption of Pi in tubular segments located beyond the proximal tubule is also enhanced in the newborn when compared with the adult (15.62 +/- 2.11% vs 10.51 +/- 1.83%, respectively, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional development of outer versus inner cortical proximal tubules

The concept of centrifugal renal development is based on renal embryogenesis. It implies a relationship between nephron age and nephron position along a cortical to medullary axis. In common usage, however, it often also implies a relationship between nephron age or position and nephron maturity. We consider here whether the ideas of centrifugal development and centrifugal maturation should in fact be considered as separate and distinct concepts. That is, we consider the possibility that nephron maturity does not necessarily correlate with nephron age. Unfortunately, pertinent reported data give no clear answer. We conclude only that further study will be required before definitive conclusions about renal developmental stages can be stated with certainty.

Scanning electron microscopy of basolateral surfaces of rat renal tubules isolated by sequential digestion

Renal tubular cells and segments isolated by a trypsin, pepsin, pronase E digestion procedure were studied with scanning electron microscopy. The basal and lateral surfaces of S1, S2, S3 proximal tubular (PT) segments, descending and ascending thin limbs of Henle (TL), distal ascending thick limb of Henle, or distal straight tubule (DST) and distal convoluted tubule (DCT) segments, connecting tubules (CNT), and collecting ducts (CD) were identified and characterized. The basal processes of the S1 and S2 PT cells were fan shaped, were oriented in a circumferential direction, and terminated in microvilli at the basement membrane. S3 PT cells had microvillous basal processes mainly on the lateral edges of the cells. The basal processes of DST and DCT were similar to PT in orientation but terminated on the basement membrane with flattened, thin attachments. The long-loop descending TL and the ascending TL exhibited distinctive interdigitating cell processes. TL segments with simple contours were present in smaller numbers and were characteristic of short-loop descending limbs. CNT showed some cells with basal surfaces resembling DCT cells and others resembling CD cells. Both cortical and medullary CD segments exhibited intercalated cells with round basal contours and a sparse pattern of basal infolding clefts. The cortical CD principal cells revealed a much more elaborate mosaic of plicae, clefts, and microvilli than those of the medullary CD. These observations extend the previous knowledge gained from transmission electron microscopy and assist in the interpretation of that knowledge.

Identification of proximal tubule segments in the mouse nephron by simultaneous visualization of alkaline phosphatase and gamma-glutamyl transpeptidase

ALP and gamma-GT are 2 brush border enzymes that can be individually demonstrated on adjacent sections by the histochemical methods of Mayahara (ALP) and Rutenberg (gamma-GT). On the basis of each enzyme activity, it was possible to recognize different categories of tubules in the mouse nephron. In fact, both enzymes were heterogeneously distributed along the proximal tubule, but in opposite gradients. The various staining intensities probably corresponded to proximal segmentation, but were sometimes difficult to evaluate. A technique was perfected to localize both enzymes in the same tissue section. Since each enzyme produced a distinct type of colored precipitates (ALP: black, gamma-GT: red), 4 categories of tubules could be identified, according to staining characteristics: 1. black tubules where ALP activity was predominant, corresponded to S1 segments, 2. black and red tubules where the 2 activities were about equivalent, were considered as parts of S2, 3. red ones where gamma-GT activity was high, were identified as portions of S3, 4. negative tubules where no activity was apparent, represented distal and straight collecting tubules. In addition to economize time and tissue, this simple technique permits to easily estimate variations in enzyme activities that probably correspond to structural and functional differences in the segments of the proximal tubule.

Ultrastructure of the developing vascular system in the puppy kidney

The present study defined the ultrastructural features of peritubular capillary development. Two-day-old beagle puppies and adult dogs were perfused with 2.5% glutaraldehyde and routinely prepared for light and transmission electron microscopy. Some of the fixed tissue was subsequently used to make freeze-fracture replicas. The outer cortex of the puppy kidney possessed large, thick-walled vessels best termed sinusoidal capillaries instead of the small caliber vessels (peritubular capillaries) noted in the adult. These sinusoidal vessels showed extensive overlapping of the endothelium with isolated patches of fenestrae. Their luminal surfaces were irregular, owing to prominent ridges and sporadic bulges of endothelium. The basement membrane of most vessels was not present. Interstitial spaces were filled with mesenchymal cells and cells closely resembling pericytes. The diameter of the fenestrae of vessels throughout the cortex was similar; however, the number of fenestrae per micrometer of endothelium increased significantly from outer to inner cortex. Vessels of the inner cortex were also immature when compared to the adult. From these morphological findings, it was apparent that a true peritubular capillary system does not exist in the two-day-old puppy. Ultrastructural features of these vessels suggested reduced permeability characteristics.