Direct effect of cadmium on citrate uptake by isolated rat renal brush border membrane vesicles

EPR study of lipid phase in renal cortical membrane organelles from intact and cadmium-intoxicated rats

Numerous studies have demonstrated various structure/function correlations at the level of transport proteins in the kidney cell membranes and various intracellular organelles. However, characterization of the lipid phase of these membranes is rare. Here, we report the differences in lipid organization and dynamics of the brush-border membranes (BBM), basolateral membranes (BLM) and endocytotic vesicles (EV), isolated from the kidney cortex of intact rats, studied with the EPR spectroscopy of the spin-labeled membrane lipids. The EPR spectra were analyzed by comparing experimentally observed line shapes with the line shapes calculated according to the theoretical model developed for liquid crystals. In the fitting procedure, three different lipid domains were assumed, which revealed clear differences in the lipid ordering and rotational correlation times, as well as in the lipid partition of these domains in each of the three types of membranes. A similar approach, used to compare the spectroscopic characteristics of BBM from control and cadmium-intoxicated rats, showed significantly changed ordering and increased molecular mobility in the lipid phase of BBM from Cd-treated animals. As tested by an established fluorescence assay, the Cd-induced changes in the lipid mobility co localized with approximately 5-fold higher conductance of BBM for potassium, with unchanged conductance for protons.

Direct effect of CoC12 and NiCl2 on citrate uptake by the rat renal brush border membrane

Co and Ni are essential but relatively rare elements as to organisms. In the mammalian membrane, these metals are transported by the same carrier proteins. The aim of this study was to investigate the direct effects of CoCl2 and NiCl2 on citrate uptake by rat renal brush border membrane vesicles (BBMV). BBMV were prepared by the divalent cation precipitation methods, and citrate uptake was measured by the Millipore rapid membrane filtration technique. The time course of citrate uptake during 120-min of incubation with 1 mM CoCl2 and NiCl2 showed a rapid significant inhibition at the early phase and a slight recover at the late phase. Incubation for 1 min of BBMV with 1, 5 and 25 mM CoCl2 and NiCl2, respectively, significantly inhibited citrate uptake in a concentration-dependent manner compared with that of 0 mM. We discuss these findings from the point of view that Co and Ni are located in Group VIII of the periodic table.

pH-Specific Aqueous Synthetic Chemistry in the Binary Cadmium(II)−Citrate System. Gaining Insight into Cadmium(II)−Citrate Speciation with Relevance to Cadmium Toxicity

The involvement of Cd(II) in toxic manifestations and pathological aberrations in lower and higher organisms entails interactions with low and high molecular mass biological targets. To understand the relevant chemistry in aqueous media, we have launched pH-dependent synthetic efforts targeting Cd(II) with the physiological ligand citric acid. Reactions of Cd(II) with citric acid upon the addition of NaOH at pH 2.5 and pyridine at pH 3 and the addition of ammonia at pH approximately 7 led to the new complexes [Cd3(C6H5O7)2(H2O)5] x H2O (1) and (NH4)[Cd(C6H5O7)(H2O)] x H2O (2), respectively. Complexes 1 and 2 were characterized by elemental analysis, spectroscopy (FT-IR and NMR), and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 18.035(6) A, b = 10.279(4) A, c = 12.565(4) A, beta = 109.02(1) degrees, V = 2202(2) A3, and Z = 4. Complex 2 crystallizes in the monoclinic space group P2(1), with a = 9.686(4) A, b = 8.484(4) A, c = 7.035(3) A, beta = 110.28(1) degrees, V = 542.3(4) A3, and Z = 2. Complex 1 is a trinuclear assembly with the citrate ligand securing a stable metallacyclic ring around one Cd(II), with the terminal carboxylates spanning into the coordination sphere of two nearby Cd(II) ions. Complex 2 contains mononuclear units of Cd(II) bound by citrate in an overall coordination number of 8. In both 1 and 2, the participating citrates exhibit three different modes of coordination, thus projecting a distinct yet variable aqueous structural chemistry of Cd(II) with physiological substrates. The pH-dependent chemistry and its apparent structural diversity validate past solution speciation studies, projecting the existence of mononuclear species such as the one in the anion of 2. The spectroscopic and structural properties of 2 emphasize the significance of the information emerging from synthetic studies that otherwise would not have been revealed through conventional solution studies, while concurrently shedding light onto the linkage of the requisite chemistry with the potential biological toxicity of Cd(II).

Synthesis and spectroscopic and structural studies of a new cadmium(II)-citrate aqueous complex. Potential relevance to cadmium(II)-citrate speciation and links to cadmium toxicity

The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimate toxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) with physiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueous Cd(II)-citrate complex [Cd(C(6)H(6)O(7))(H(2)O)](n)() (1). Compound 1 was characterized by elemental analysis, and spectroscopically by FT-IR and (113)Cd MAS NMR. Compound 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 6.166(2) A, b = 10.508(3) A, c = 13.599(5) A, V = 881.2(5) A(3), and Z = 4. The X-ray structure of 1 reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts as a tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylate group, and the central carboxylate group. In addition, the central carboxylate binds to three Cd(II) ions. Specifically, one of the oxygens of the central carboxylate serves as a bridge to two neighboring Cd(II) ions, while the other oxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). (113)Cd MAS NMR studies project the spectroscopic signature of the nature of the coordination environment around Cd(II) in 1, thus corroborating the X-ray findings. Collectively, the data at hand are in line with past solution studies. The latter predict that other similar low molecular mass Cd(II)-citrate complexes may exist in the acidic pH region, thus influencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, and possibly Cd(II) toxicity.

Cd-MT causes endocytosis of brush-border transporters in rat renal proximal tubules

Nephrotoxicity in humans and experimental animals due to chronic exposure to cadmium (Cd) is manifested by defects in the reabsorptive and secretory functions of proximal tubules (PT). The main symptoms of Cd nephrotoxicity, including polyuria, phosphaturia, aminoaciduria, glucosuria, and proteinuria, suggest that various brush-border membrane (BBM) transporters are the main targets of Cd. Specific transporters may be either directly inhibited by Cd or lost from the BBM after Cd treatment, or both. We have recently proposed that Cd may impair the vesicle-dependent recycling of BBM transporters by inhibiting vacuolar H+-ATPase (V-ATPase) activity and endocytosis in PT cells (Herak-Kramberger CM, Sabolic I, and Brown D. Kidney Int 53: 1713-1726, 1998). The mechanism underlying the Cd effect was further explored in an in vivo model of experimental Cd nephrotoxicity induced by Cd-metallothionein (Cd-MT; 0.4 mg Cd/kg body mass; a single dose sc) in rats. The time-dependent redistribution of various BBM transporters was examined in this model by fluorescence and gold-labeling immunocytochemistry on tissue sections and by immunoblotting of isolated renal cortical BBM. In PT cells of Cd-MT-treated rats, we observed 1) shortening and loss of microvilli; 2) time-dependent loss of megalin, V-ATPase, aquaporin-1 (AQP1), and type 3 Na+/H+ exchanger (NHE3) from the BBM; 3) redistribution of these transporters into vesicles that were randomly scattered throughout the cell cytoplasm; and 4) redistribution of NHE3, but not megalin, into the basolateral plasma membrane. The internalization of BBM transporters was accompanied by fragmentation and loss of microtubules and by an increased abundance of alpha-tubulin monomers in PT cells. Transporter redistribution was detectable as early as 1 h after Cd-MT treatment and increased in magnitude over the next 12 h. We conclude that the early mechanism of Cd toxicity in PT cells may include a colchicine-like depolymerization of microtubules and impaired vesicle-dependent recycling of various BBM proteins. These processes may lead to a time-dependent loss of cell membrane components, resulting in reabsorptive and secretory defects that occur in Cd-induced nephrotoxicity.

Subchronic cadmium treatment affects the abundance and arrangement of cytoskeletal proteins in rat renal proximal tubule cells

Disfunction of proximal tubules (PT) in cadmium (Cd) nephrotoxicity in mammals results from the diminished functional capacity of brush-border membrane (BBM) caused by (a) direct inhibition of BBM transporters by Cd, (b) shortening and loss of microvilli, and (c) loss of specific BBM transporters. The loss of transporters may partially result from impaired intracellular vesicle recycling due to loss or/and inhibition of vacuolar H+-ATPase in the PT cell organelles. Cytoskeleton plays an important role in vesicle-mediated recycling and processing of BBM transporters in PT cells. Experiments in vitro have indicated that Cd may affect the state of polymerization of some cytoskeletal proteins. In this work we studied the in vivo effect of CdCl2-treatment in rats (2 mg Cd/kg b. m., s.c., daily for 14 days) upon abundance and arrangement of actin filaments, actin-bundling protein villin, and microtubules (MT) in PT cells. Cd-treatment elicited a dramatic accumulation of Cd in the kidney cortex (200 microg/g tissue wet mass after 14 days) and a strongly increased abundance of metallothionein in PT cells. As revealed by immunocytochemistry in tissue cryosections, the staining intensity of actin and villin in PT cells of Cd-treated rats was generally decreased, without a marked change in their intracellular distribution, whereas MT became largely irregular, diminished in most cells, and lost in many cells. However, the immunoblots revealed an increased content of villin and alpha-tubulin in cortical tissue homogenates from Cd-treated rats, thus indicating an impaired bundling of actin and greatly depolymerized MT in cells intoxicated with Cd. The partial loss of apical actin and villin in PT cells of Cd-treated rats may reflect (or cause) shortening and loss of microvilli, whereas derangement and depolymerization of MT may contribute to the impairment of intracellular recycling of BBM proteins, and lead to the loss of BBM transporters.

The integrity of renal cortical brush-border and basolateral membrane vesicles is damaged in vitro by nephrotoxic heavy metals

Poisoning of experimental animals with cadmium (Cd), mercury (Hg), lead (Pb) or cis-diamminedichloroplatinum (cis-Pt) causes shortening and focal loss of microvilli in proximal tubule (PT) cells, thus indicating that the reduced reabsorptive surface due to damaged integrity of brush-border membrane (BBM) may contribute to the reabsorptive and secretory defects in these toxic states. In addition, in in vitro studies with isolated renal cortical BBM vesicles (BBMV), heavy metals (HM) inhibit transport of various compounds, and these data were interpreted as being a result of a direct inhibition of the respective membrane transporters. In this work we used a DeltapH-driven acridine orange fluorescence quench assay to test if various divalent cations affect in vitro the integrity of BBMV and basolateral membrane vesicles (BLMV) isolated from the rat renal cortex. In Cd-treated BBMV we found that: (a) the integrity of vesicles decreased with increasing concentrations of Cd; and (b) the loss of sealed vesicles was high at 37 degrees C, intermediate at 25 degrees C, and very low at 0 degrees C. The loss of sealed BBMV was caused also by Hg, Cu, Pb and Zn (Hg>>>Cu=Cd>Pb=Zn). Cis-Pt, Al, Fe, Ba, Mg and Mn had no effect. BLMV were damaged by HM with an efficiency Hg>>>Cd=Pb=Cu, whereas other divalent cations, including Zn, were ineffective. An SH-group protector, dithiothreitol, prevented the loss of sealed vesicles in some (Hg, Pb, Cu) but not in all (Cd, Zn) cases. We conclude that the nephrotoxic HM directly damage the integrity of PT cell plasma membranes; this may cause shortening and loss of microvilli and basolateral invaginations in HM-treated experimental animals in vivo. The data also indicate that caution should be taken when effects of HM on various transports are studied in isolated membrane vesicles in vitro; an impaired transport may result from the loss of vesicle integrity, and not necessarily from the direct inhibition of a transporter.

The adaptation of mussels Crenomytilus grayanus to cadmium accumulation result in alterations in organization of microsomal enzyme-membrane complex (non-specific phosphatase)

The kinetic parameters (V(m), K(m) and slope) of membrane-bound microsomal non-specific phosphatase (NPase, with G6P as the substrate) from the digestive gland of unexposed and cadmium adapted (45 days for 100 µg Cd(2+)/l) mussels were investigated. In vivo and in vitro approaches were used. Adaptation of mussels (Crenomytilus grayanus) to cadmium resulted in a 1.6-fold increase in NPase activity. V(m) was increased by 1.6-fold, but K(m) was the same in terms of enzyme kinetics. This indicates that the total concentration of the enzymes in the digestive gland increased. Cd(2+) (1 mM) did not significantly alter the activity of the membrane-bound enzyme in vitro both for unexposed and for cadmium adapted mussels, meaning that cadmium ions are not a direct inhibitor of the membrane-bound enzyme in this concentration. The microsomal NPase activity in both unexposed and cadmium adapted mussels was inhibited by in vitro solubilization of microsomes with non-ionic detergent (Triton X100, 0.01%). This inhibition was uncompetitive for microsomes of unexposed mussels (K(m) decreased 3.1-fold). The most drastic events were observed in cadmium adapted mussels, where inhibition was mixed (K(m) decreased 7.2-fold). The simultaneous actions of detergent and cadmium ions did not alter NPase activity significantly in comparison with action of the detergent alone. The differences in the types and the extents of inhibition of the enzymes activity by membrane disordering agent (Triton X100) indicated that the enzyme-membrane complex (NPase) has been altered as a result of adaptation of mussels to cadmium accumulation. We conclude that the mussels produced a new enzyme-membrane complex, with the same K(m) as the previous complex, but with other detergent sensitivity and greater amounts. Thus, the adaptation capacity of this enzyme is reduced as result of adaptation of mussels to cadmium accumulation.

Citrate and succinate transport in proximal tubule cells

Urinary citrate, which inhibits calcium nephrolithiasis, is determined by proximal reabsorption via an apical dicarboxylate transporter. Citrate is predominantly trivalent at physiological pH, but citrate(-2) is transported at the apical membrane. We now demonstrate that low-Ca solutions induce transport of citrate(-2) and succinate in opossum kidney cells. With 1.2 mM extracellular Ca, citrate uptake was pH insensitive and not competed by succinate(-2). In contrast, with low extracellular Ca, citrate uptake increased twofold, was inhibited by succinate (and other dicarboxylates), was stimulated by lowering extracellular pH (consistent with citrate(-2) transport), and increased further by lowering extracellular Mg. The effect of Ca was incrementally concentration dependent, between 0 and 1.2 mM. The effect of Ca was not simply complexation with citrate because succinate (which is complexed significantly less) was affected by Ca similarly. Incubation of cells for 48 h in a low-pH media increased citrate transport (studied at control pH) more than twofold, suggesting induction of transporters.