Characterization of anion exchangers in an inner medullary collecting duct cell line

Nanoscale Analysis of Randall's Plaques by Electron Energy Loss Spectromicroscopy: Insight in Early Biomineral Formation in Human Kidney

Idiopathic kidney stones originate mainly from calcium phosphate deposits at the tip of renal papillae, known as Randall's plaques (RPs), also detected in most human kidneys without stones. However, little is known about the mechanisms involved in RP formation. The localization and characterization of such nanosized objects in the kidney remain a real challenge, making their study arduous. This study provides a nanoscale analysis of the chemical composition and morphology of incipient RPs, characterizing in particular the interface between the mineral and the surrounding organic compounds. Relying on data gathered from a calculi collection, the morphology and chemical composition of incipient calcifications in renal tissue were determined using spatially resolved electron energy-loss spectroscopy. We detected microcalcifications and individual nanocalcifications found at some distance from the larger ones. Strikingly, concerning the smaller ones, we show that two types of nanocalcifications coexist: calcified organic vesicles and nanometric mineral granules mainly composed of calcium phosphate with carbonate in their core. Interestingly, some of these nanocalcifications present similarities with those reported in physiological bone or pathological cardiovascular biominerals, suggesting possible common formation mechanisms. However, the high diversity of these nanocalcifications suggests that several mechanisms may be involved (nucleation on a carbonate core or on organic compounds). In addition, incipient RPs also appear to present specific features at larger scales, revealing secondary calcified structures embedded in a fibrillar organic material. Our study proves that analogies exist between physiological and pathological biominerals and provides information to understand the physicochemical processes involved in pathological calcification formation.

Taking Peritoneal Dialysis beyond the Year 2000

Over the past 25 years, peritoneal dialysis (PD) has steadily improved so that now its outcomes, in the form of patient survival, are equivalent to, and at times better than, those for hemodialysis. We now have a better understanding of the pathophysiology of peritoneal membrane function and damage and the importance of appropriate prescription to meet agreed-upon targets of solute and fluid removal. In the next millennium, greater emphasis will be put on prescription setting and subsequent monitoring. This will entail an increase in automated PD, especially for lifestyle reasons as well as for patients with a hyperpermeable peritoneal membrane.

To improve outcomes, dialysis should be started earlier than is currently the case. It is easy to do this with PD, where an incremental approach is made easier by the introduction of icodextrin for long-dwell PD. In the future, solutions will be tailored to be more biocompatible and to provide improved nutrition and better cardiovascular outcomes.

Finally, economic considerations favor PD, which is cheaper than in-centre hemodialysis. Thus, for many, PD has become a first-choice therapy, and with further improvements this trend will continue.

Osteogenic Protein-1: Gene Expression and Treatment in the Rat Remnant Kidney Model

Osteogenic Protein-1 (OP-1) is a bone morphogen involved in tissue repair and development. We have shown that OP-1 is downregulated during acute ischemic renal injury. Here we report the use of the rat remnant kidney model (RRKM) to evaluate changes in kidney OP-1 expression during chronic injury, and determine if treatment with recombinant human OP-1 (rhOP-1) aids in recovery from injury. Sprague—Dawley rats were subjected to kidney decapsulation (Cx) or 5/6 nephrectomy (Nx). Serum for BUN and creatinine and tissue for histology and mRNA analysis were collected at: 2, 10, and 12—14 wks post Nx. We show kidney OP-1 mRNA levels were downregulated at 2 and 12—14 wks post Nx. To determine the effect of rhOP-1 in the RRKM, rhOP-1 (0.25, 2.5 or 25 μg/kg) or vehicle (V) was injected in a second set of rats, 2 weeks after 2/3 left Nx for a total of six doses. Nx rats treated with rhOP-1 showed significantly increased tubular regeneration (increased mitotic figures, polyoid infolding, and tubular epithelial hyperplasia) in a dose dependent manner without changes in glomerular or tubular damage. rhOP-1 stimulates tubular epithelial cell regeneration, early in the repair process in a chronic renal failure model, before significant fibrosis is established.

CRIT-LINE: a noninvasive tool to monitor hemoglobin levels in pediatric hemodialysis patients

BACKGROUND

The national average for achieving the KDOQI-recommended hemoglobin (Hgb) target level of 11-12 g/dL is low with the current anemia management protocol of measuring Hgb levels every 2-4 weeks to guide intervention. The objective of this study was to correlate initial Hgb readings from the CRIT-LINE monitor with actual serum Hgb levels in pediatric patients on hemodialysis (HD).

METHODS

Data were collected from pediatric HD patients who had Hgb tests ordered for routine and/or clinical reasons. Hgb concentrations were read with the CRIT-LINE after 0.5 or 1 L of blood had been processed by HD in patients with a body weight of ≤20 or >20 kg, respectively. Ultrafiltration was kept at a minimum until the CRIT-LINE Hgb was read.

RESULTS

In total, 217 Hgb readings from 23 HD patients were analyzed. Results showed a statistically significant correlation between CRIT-LINE readings and laboratory Hgb measurements (r = 0.94, p < 0.0001) using Pearson correlation coefficients for well-distributed data. The mean Hgb levels measured by CRIT-LINE and the laboratory were 11.12 ± 1.63 and 11.31 ± 1.69 g/dL, respectively.

CONCLUSIONS

The CRIT-LINE monitor is an accurate instrument for monitoring Hgb levels in HD patients. Further studies will be needed to evaluate whether using CRIT-LINE Hgb levels to guide anemia management will improve the percentage of children with Hgb levels within target.

Characterization of butyrate transport across the luminal membranes of equine large intestine

The diet of the horse, pasture forage (grass), is fermented by the equine colonic microbiota to short-chain fatty acids, notably acetate, propionate and butyrate. Short-chain fatty acids provide a major source of energy for the horse and contribute to many vital physiological processes. We aimed to determine both the mechanism of butyrate uptake across the luminal membrane of equine colon and the nature of the protein involved. To this end, we isolated equine colonic luminal membrane vesicles. The abundance and activity of cysteine-sensitive alkaline phosphatase and villin, intestinal luminal membrane markers, were significantly enriched in membrane vesicles compared with the original homogenates. In contrast, the abundance of GLUT2 protein and the activity of Na(+)-K(+)-ATPase, known markers of the intestinal basolateral membrane, were hardly detectable. We demonstrated, by immunohistochemistry, that monocarboxylate transporter 1 (MCT1) protein is expressed on the luminal membrane of equine colonocytes. We showed that butyrate transport into luminal membrane vesicles is energized by a pH gradient (out < in) and is not Na(+) dependent. Moreover, butyrate uptake is time and concentration dependent, with a Michaelis-Menten constant of 5.6 ± 0.45 mm and maximal velocity of 614 ± 55 pmol s(-1) (mg protein)(-1). Butyrate transport is significantly inhibited by p-chloromercuribenzoate, phloretin and α-cyano-4-hydroxycinnamic acid, all potent inhibitors of MCT1. Moreover, acetate and propionate, as well as the monocarboxylates pyruvate and lactate, also inhibit butyrate uptake. Data presented here support the conclusion that transport of butyrate across the equine colonic luminal membrane is predominantly accomplished by MCT1.

Loss of Slc4a1b chloride/bicarbonate exchanger function protects mechanosensory hair cells from aminoglycoside damage in the zebrafish mutant persephone

Mechanosensory hair cell death is a leading cause of hearing and balance disorders in the human population. Hair cells are remarkably sensitive to environmental insults such as excessive noise and exposure to some otherwise therapeutic drugs. However, individual responses to damaging agents can vary, in part due to genetic differences. We previously carried out a forward genetic screen using the zebrafish lateral line system to identify mutations that alter the response of larval hair cells to the antibiotic neomycin, one of a class of aminoglycoside compounds that cause hair cell death in humans. The persephone mutation confers resistance to aminoglycosides. 5 dpf homozygous persephone mutants are indistinguishable from wild-type siblings, but differ in their retention of lateral line hair cells upon exposure to neomycin. The mutation in persephone maps to the chloride/bicarbonate exchanger slc4a1b and introduces a single Ser-to-Phe substitution in zSlc4a1b. This mutation prevents delivery of the exchanger to the cell surface and abolishes the ability of the protein to import chloride across the plasma membrane. Loss of function of zSlc4a1b reduces hair cell death caused by exposure to the aminoglycosides neomycin, kanamycin, and gentamicin, and the chemotherapeutic drug cisplatin. Pharmacological block of anion transport with the disulfonic stilbene derivatives DIDS and SITS, or exposure to exogenous bicarbonate, also protects hair cells against damage. Both persephone mutant and DIDS-treated wild-type larvae show reduced uptake of labeled aminoglycosides. persephone mutants also show reduced FM1-43 uptake, indicating a potential impact on mechanotransduction-coupled activity in the mutant. We propose that tight regulation of the ionic environment of sensory hair cells, mediated by zSlc4a1b activity, is critical for their sensitivity to aminoglycoside antibiotics.

Role of SNAREs and H+-ATPase in the targeting of proton pump-coated vesicles to collecting duct cell apical membrane

Recycling of H(+)-ATPase to the apical plasma membrane, mediated by vesicular exocytosis and endocytosis, is an important mechanism for controlling H(+) secretion by the collecting duct. We hypothesized that SNAREs (soluble N-ethylmaleimide-sensitive factor attachment proteins) may be involved in the targeting of H(+)-ATPase-coated vesicles. Using a tissue culture model of collecting duct H(+) secretory cells (inner medullary collecting duct (IMCD) cells), we demonstrated that they express the proteins required for SNARE-mediated exocytosis and form SNARE-fusion complexes upon stimulation of H(+)-ATPase exocytosis. Furthermore, exocytic amplification of apical H(+)-ATPase is sensitive to clostridial toxins that cleave SNAREs and thereby inhibit secretion. Thus, SNAREs are critical for H(+)-ATPase cycling to the plasma membrane. The process in IMCD cells has a feature distinct from that of neuronal cells: the SNARE complex includes and requires the vesicular cargo (H(+)-ATPase) for targeting. Using chimeras and truncations of syntaxin 1, we demonstrated that there is a specific cassette within the syntaxin 1 H3 domain that mediates binding of the SNAREs and a second distinct H3 region that binds H(+)-ATPase. Utilizing point mutations of the B1 subunit of the H(+)-ATPase, we document that this subunit contains specific targeting information for the H(+)-ATPase itself. In addition, we found that Munc-18-2, a regulator of exocytosis, plays a multifunctional role in this system: it regulates SNARE complex formation and the affinity of syntaxin 1 for H(+)-ATPase.

Non-polarized targeting of AE1 causes autosomal dominant distal renal tubular acidosis

Autosomal dominant distal renal tubular acidosis (ddRTA) is caused by mutations in SLC4A1, which encodes the polytopic chloride-bicarbonate exchanger AE1 that is normally expressed at the basolateral surface of alpha-intercalated cells in the distal nephron. Here we report that, in contrast with many disorders in which mutant membrane proteins are retained intracellularly and degraded, ddRTA can result from aberrant targeting of AE1 to the apical surface.

Environmental lead exposure and progression of chronic renal diseases in patients without diabetes

BACKGROUND

Previous research suggests that environmental lead exposure correlates with age-related decreases in renal function.

METHODS

Two hundred two patients with chronic renal insufficiency (indicated by a serum creatinine level between 1.5 mg per deciliter and 3.9 mg per deciliter) who had a normal total-body lead burden and no history of exposure to lead were observed for 24 months. After the observation period, 64 subjects with an elevated body lead burden were randomly assigned to the chelation control groups. For three months, the patients in the chelation group received lead-chelation therapy with calcium disodium EDTA, and the control group received placebo. During the ensuing 24 months, repeated chelation therapy was administered weekly to 32 patients with high-normal body lead burdens (at least 80 microg but less than 600 microg) unless on repeated testing the body lead burden fell below 60 microg; the other 32 patients served as controls and received weekly placebo infusions for 5 weeks every 6 months. The primary end point was an increase in the serum creatinine level to 1.5 times the base-line value during the observation period. A secondary end point was the change in renal function during the intervention period.

RESULTS

The primary end point occurred in 24 patients during the observation period; the serum creatinine levels and body lead burden at base line were the most important risk factors. The glomerular filtration rate improved significantly by the end of the 27-month intervention period in patients receiving chelation therapy: the mean (+/-SD) change in the glomerular filtration rate in the patients in the chelation group was 2.1+/-5.7 ml per minute per 1.73 m2 of body-surface area, as compared with -6.0+/-5.8 ml per minute per 1.73 m2 of body-surface area in the controls (P<0.001). The rate of decline in the glomerular filtration rate in the chelation group was also lower than that in the controls during the 24-month period of repeated chelation therapy or placebo.

CONCLUSIONS

Low-level environmental lead exposure may accelerate progressive renal insufficiency in patients without diabetes who have chronic renal disease. Repeated chelation therapy may improve renal function and slow the progression of renal insufficiency.

Electrolyte and fluid secretion by cultured human inner medullary collecting duct cells

Inner medullary collecting ducts (IMCD) are the final nephron segments through which urine flows. To investigate epithelial ion transport in human IMCD, we established primary cell cultures from initial (hIMCD(i)) and terminal (hIMCD(t)) inner medullary regions of human kidneys. AVP, PGE(2), and forskolin increased cAMP in both hIMCD(i) and hIMCD(t) cells. The effects of AVP and PGE2 were greatest in hIMCD(i); however, forskolin increased cAMP to the same extent in hIMCD(i) and hIMCD(t). Basal short-circuit current (I(SC)) of hIMCD(i) monolayers was 1.4 +/- 0.5 microA/cm2 and was inhibited by benzamil, a Na+ channel blocker. 8-Bromo-cAMP, AVP, PGE(2), and forskolin increased I(SC); the current was reduced by blocking PKA, apical Cl- channels, basolateral NKCC1 (a Na+ - K+ - 2Cl- cotransporter), and basolateral Cl-/HCO(3)(-) exchangers. In fluid transport studies, hIMCD(i) monolayers absorbed fluid in the basal state and forskolin reversed net fluid transport to secretion. In hIMCD(t) monolayers, basal current was not different from zero and cAMP had no effect on I(SC). We conclude that AVP and PGE2 stimulate cAMP-dependent Cl- secretion by hIMCD(i) cells, but not hIMCD(t) cells, in vitro. We suggest that salt secretion at specialized sites along human collecting ducts may be important in the formation of the final urine.

Role of SNAP-23 in trafficking of H+-ATPase in cultured inner medullary collecting duct cells

The trafficking of H+-ATPase vesicles to the apical membrane of inner medullary collecting duct (IMCD) cells utilizes a mechanism similar to that described in neurosecretory cells involving soluble N-ethylmaleimide-sensitive factor attachment protein target receptor (SNARE) proteins. Regulated exocytosis of these vesicles is associated with the formation of SNARE complexes. Clostridial neurotoxins that specifically cleave the target (t-) SNARE, syntaxin-1, or the vesicle SNARE, vesicle-associated membrane protein-2, reduce SNARE complex formation, H+-ATPase translocation to the apical membrane, and inhibit H+ secretion. The purpose of these experiments was to characterize the physiological role of a second t-SNARE, soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP)-23, a homologue of the neuronal SNAP-25, in regulated exocytosis of H+-ATPase vesicles. Our experiments document that 25-50 nM botulinum toxin (Bot) A or E cleaves rat SNAP-23 and thereby reduces immunodetectable and (35)S-labeled SNAP-23 by >60% within 60 min. Addition of 25 nM BotE to IMCD homogenates reduces the amount of the 20 S-like SNARE complex that can be immunoprecipitated from the homogenate. Treatment of intact IMCD monolayers with BotE reduces the amount of H+-ATPase translocated to the apical membrane by 52 +/- 2% of control and reduces the rate of H+ secretion by 77 +/- 3% after acute cell acidification. We conclude that SNAP-23 is a substrate for botulinum toxin proteolysis and has a critical role in the regulation of H+-ATPase exocytosis and H+ secretion in these renal epithelial cells.

Effect of acidification on the location of H+-ATPase in cultured inner medullary collecting duct cells

In previous studies, our laboratory has utilized a cell line derived from the rat inner medullary collecting duct (IMCD) as a model system for mammalian renal epithelial cell acid secretion. We have provided evidence, from a physiological perspective, that acute cellular acidification stimulates apical exocytosis and elicits a rapid increase in proton secretion that is mediated by an H+-ATPase. The purpose of these experiments was to examine the effect of acute cellular acidification on the distribution of the vacuolar H+-ATPase in IMCD cells in vitro. We utilized the 31-kDa subunit of the H+-ATPase as a marker of the complete enzyme. The distribution of this subunit of the H+-ATPase was evaluated by immunohistochemical techniques (confocal and electron microscopy), and we found that there is a redistribution of these pumps from vesicles to the apical membrane. Immunoblot evaluation of isolated apical membrane revealed a 237 +/- 34% (P < 0.05, n = 9) increase in the 31-kDa subunit present in the membrane fraction 20 min after the induction of cellular acidification. Thus our results demonstrate the presence of this pump subunit in the IMCD cell line in vitro and that cell acidification regulates the shuttling of cytosolic vesicles containing the 31-kDa subunit into the apical membrane.