Renal Na-Si cotransporter NaSi-1 is inhibited by heavy metals

Exposure to the Heavy-Metal Lead Induces DNA Copy Number Alterations in Zebrafish Cells

DNA copy number variants are associated with the development of complex neurological diseases and disorders including autism spectrum disorder, schizophrenia, Alzheimer's disease, and Parkinson's disease. Exposure to multiple environmental chemicals including various heavy metals is suggested as a risk factor in these neurological diseases and disorders, but few studies have addressed if heavy-metal exposure can result in de novo DNA copy number changes as a genetic mechanism contributing to these disease outcomes. In this study to further investigate the relationship between heavy-metal exposure and de novo copy number alterations (CNAs), zebrafish fibroblast cells were exposed to the neurotoxicant lead (Pb). A crystal violet assay was first used to determine exposure concentrations with >80% cell confluency. Then a zebrafish-specific array comparative genomic hybridization platform was used to detect CNAs following a 72 h Pb exposure (0.24, 2.4, or 24 μM). The Pb exposure resulted in 72 CNA amplifications ranging in size from 5 to 329 kb. No deletions were detected. CNAs resulted in 15 CNA regions (CNARs), leaving 7 singlet CNAs. Two of the singlets were within high repeat genomic locations. The number of CNAs tended to increase in a concentration-dependent manner. Several CNARs encompassed genes previously reported to have altered expression with Pb exposure, suggesting a mechanistic link. In addition, almost all genes are associated within a molecular network with amyloid precursor protein, a key molecular target associated with the pathophysiology of Alzheimer's disease. Overall, these findings show that Pb exposure results in de novo CNAs that could serve as a mechanism driving adverse health outcomes associated with Pb toxicity including neurological disease pathogenesis for further study.

Characterization of the human renal Na(+)-sulphate cotransporter gene ( NAS1) promoter

Sulphate (SO(4)(2-)) plays an essential role during growth, development, and cellular metabolism. Recently, we have isolated the human renal Na(+)-SO(4)(2-) cotransporter (hNaSi-1) that is implicated in the regulation of serum SO(4)(2-) levels. To gain an insight into hNaSi-1 regulation, our aims were to clone and characterize functionally the hNaSi-1 gene ( NAS1) promoter. We PCR-amplified 3742 bp of the NAS1 5'-flanking region, which is 64% AT-rich and contains numerous putative cis-acting elements. The NAS1 transcription start site was mapped to 25 bp upstream from the translation start site. NAS1 promoter truncations fused to luciferase gene constructs transfected into renal LLC-PK1, MDCK and OK cells allowed us to establish that the first 169 bp of the NAS1 promoter are sufficient for basal transcription. Furthermore, the NAS1 promoter conferred responsiveness to the polycyclic aromatic hydrocarbon 3-methylcholanthrene (3-MC), but not to thyroid hormone (T(3)) or vitamin D [1,25-(OH)(2)D(3)]. Site-directed mutagenesis of the NAS1 promoter identified a functional xenobiotic response element at -2,052, which conferred 3-MC responsiveness. The human NAS1 gene promoter is not responsive to Vitamin D or T(3), unlike the mouse Nas1 promoter with which it shares approximately 40% sequence similarity, but is transactivated by 3-MC, suggesting that the control of renal SO(4)(2-) reabsorption via the regulation of NAS1 transcription may be important for maintaining the sulphation potential for kidney polycyclic aromatic hydrocarbon metabolism.

Transepithelial sulfate transport by avian renal proximal tubule epithelium in primary culture

The mechanisms and control of transepithelial inorganic sulfate (Si) transport by primary cultures of chick renal proximal tubule monolayers in Ussing chambers were determined. The competitive anion, S2 O 3 2- (5 mM), reduced both unidirectional reabsorptive and secretory fluxes and net Si reabsorption with no effect on electrophysiological properties. The carbonic anhydrase (CA) inhibitor ethoxzolamide decreased net Si reabsorption approximately 45%. CAII protein and activity were detected in isolated chick proximal tubules by immunoblots and biochemical assay, respectively. Cortisol reduced net Si reabsorption up to approximately 50% in a concentration-dependent manner. Thyroid hormone increased net Si reabsorption threefold in 24 h, and parathyroid hormone (PTH) acutely stimulated net Si reabsorption approximately 45%. These data indicate that CA participates in avian proximal tubule active transepithelial Si reabsorption, which cortisol directly inhibits and T3 and PTH directly stimulate.

Functional differences between flounder and rat thiazide-sensitive Na-Cl cotransporter

The purpose of the present study was to determine the major functional, pharmacological, and regulatory properties of the flounder thiazide-sensitive Na-Cl cotransporter (flTSC) to make a direct comparison with our recent characterization of the rat TSC (rTSC; Monroy A, Plata C, Hebert SC, and Gamba G. Am J Physiol Renal Physiol 279: F161-F169, 2000). When expressed in Xenopus laevis oocytes, flTSC exhibits lower affinity for Na(+) than for Cl(-), with apparent Michaelis-Menten constant (K(m)) values of 58.2 +/- 7.1 and 22.1 +/- 4.2 mM, respectively. These K(m) values are significantly higher than those observed in rTSC. The Na(+) and Cl(-) affinities decreased when the concentration of the counterion was lowered, suggesting that the binding of one ion increases the affinity of the transporter for the other. The effect of several thiazides on flTSC function was biphasic. Low concentrations of thiazides (10(-9) to 10(-7) M) resulted in activation of the cotransporter, whereas higher concentrations (10(-6) to 10(-4) M) were inhibitory. In rTSC, this biphasic effect was observed only with chlorthalidone. The affinity for thiazides in flTSC was lower than in rTSC, but the affinity in flTSC was not affected by the Na(+) or the Cl(-) concentration in the uptake medium. In addition to thiazides, flTSC and rTSC were inhibited by Hg(2+), with an apparent higher affinity for rTSC. Finally, flTSC function was decreased by activation of protein kinase C with phorbol esters and by hypertonicity. In summary, we have found significant regulatory, kinetic, and pharmacological differences between flTSC and rTSC orthologues.

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.

Functional and molecular characterization of the K-Cl cotransporter of Xenopus laevis oocytes

The K-Cl cotransporters (KCCs) have a broad range of physiological roles, in a number of cells and species. We report here that Xenopus laevis oocytes express a K-Cl cotransporter with significant functional and molecular similarity to mammalian KCCs. Under isotonic conditions, defolliculated oocytes exhibit a Cl(-)-dependent (86)Rb(+) uptake mechanism after activation by the cysteine-reactive compounds N-ethylmaleimide (NEM) and mercuric chloride (HgCl(2)). The activation of this K-Cl cotransporter by cell swelling is prevented by inhibition of protein phosphatase-1 with calyculin A; NEM activation of the transporter was not blocked by phosphatase inhibition. Kinetic characterization reveals apparent values for the Michaelis-Menten constant of 27.7 +/- 3.0 and 15.4 +/- 4.7 mM for Rb(+) and Cl(-), respectively, with an anion selectivity for K(+) transport of Cl(-) = PO(4)(3-) = Br(-) > I(-) > SCN(-) > gluconate. The oocyte K-Cl cotransporter was sensitive to several inhibitors, including loop diuretics, with apparent half-maximal inhibition values of 200 and 500 microM for furosemide and bumetanide, respectively. A partial cDNA encoding the Xenopus K-Cl cotransporter was cloned from oocyte RNA; the corresponding transcript is widely expressed in Xenopus tissues. The predicted COOH-terminal protein fragment exhibited particular homology to the KCC1/KCC3 subgroup of the mammalian KCCs, and the functional characteristics are the most similar to those of KCC1 (Mercado A, Song L, Vazquez N, Mount DB, and Gamba G. J Biol Chem 275: 30326--30334, 2000).

Autism: a novel form of mercury poisoning

Autism is a syndrome characterized by impairments in social relatedness and communication, repetitive behaviors, abnormal movements, and sensory dysfunction. Recent epidemiological studies suggest that autism may affect 1 in 150 US children. Exposure to mercury can cause immune, sensory, neurological, motor, and behavioral dysfunctions similar to traits defining or associated with autism, and the similarities extend to neuroanatomy, neurotransmitters, and biochemistry. Thimerosal, a preservative added to many vaccines, has become a major source of mercury in children who, within their first two years, may have received a quantity of mercury that exceeds safety guidelines. A review of medical literature and US government data suggests that: (i) many cases of idiopathic autism are induced by early mercury exposure from thimerosal; (ii) this type of autism represents an unrecognized mercurial syndrome; and (iii) genetic and non-genetic factors establish a predisposition whereby thimerosal's adverse effects occur only in some children.