Techniques for isolation of brush-border and basolateral membrane vesicles from dog kidney cortex

SWATH-Based Comprehensive Determination of the Localization of Apical and Basolateral Membrane Proteins Using Mouse Liver as a Model Tissue

The purpose of this study was to develop a method to comprehensively determine the localization of apical and basolateral membrane proteins, using a combination of apical/basolateral membrane separation and accurate SWATH (Sequential Window Acquisition of all THeoretical fragment ion spectra) proteomics. The SWATH analysis of basolateral and apical plasma membrane fractions in mouse liver quantified the protein expression of 1373 proteins. The basolateral/apical ratios of the protein expression levels were compared with the reported immunohistochemical localization for 41 model proteins (23 basolateral, 11 apical and 7 both membrane-localized proteins). Three groups were perfectly distinguished. Border lines to distinguish the apical-, both- and basolateral localizations were determined to be 0.766 and 1.42 based on probability density. The method that was established was then applied to the comprehensive determination of the proteins in mouse liver. The findings indicated that 154 and 125 proteins were localized in the apical and basolateral membranes, respectively. The levels of receptors, CD antigens and integrins, enzymes and Ras-related molecules were much higher in apical membranes than in basolateral membranes. In contrast, the levels of adhesion molecules, scaffold proteins and transporters in basolateral membranes were much higher than in apical membranes.

A lasting symbiosis: how the Hawaiian bobtail squid finds and keeps its bioluminescent bacterial partner

For more than 30 years, the association between the Hawaiian bobtail squid, Euprymna scolopes, and the bioluminescent bacterium Vibrio fischeri has been studied as a model system for understanding the colonization of animal epithelia by symbiotic bacteria. The squid-vibrio light-organ system provides the exquisite resolution only possible with the study of a binary partnership. The impact of this relationship on the partners' biology has been broadly characterized, including their ecology and evolutionary biology as well as the underlying molecular mechanisms of symbiotic dynamics. Much has been learned about the factors that foster initial light-organ colonization, and more recently about the maturation and long-term maintenance of the association. This Review synthesizes the results of recent research on the light-organ association and also describes the development of new horizons for E. scolopes as a model organism that promises to inform biology and biomedicine about the basic nature of host-microorganism interactions.

Urolithiasis and hepatotoxicity are linked to the anion transporter Sat1 in mice

Urolithiasis, a condition in which stones are present in the urinary system, including the kidneys and bladder, is a poorly understood yet common disorder worldwide that leads to significant health care costs, morbidity, and work loss. Acetaminophen-induced liver damage is a major cause of death in patients with acute liver failure. Kidney and urinary stones and liver toxicity are disturbances linked to alterations in oxalate and sulfate homeostasis, respectively. The sulfate anion transporter-1 (Sat1; also known as Slc26a1) mediates epithelial transport of oxalate and sulfate, and its localization in the kidney, liver, and intestine suggests that it may play a role in oxalate and sulfate homeostasis. To determine the physiological roles of Sat1, we created Sat1-/- mice by gene disruption. These mice exhibited hyperoxaluria with hyperoxalemia, nephrocalcinosis, and calcium oxalate stones in their renal tubules and bladder. Sat1-/- mice also displayed hypersulfaturia, hyposulfatemia, and enhanced acetaminophen-induced liver toxicity. These data suggest that Sat1 regulates both oxalate and sulfate homeostasis and may be critical to the development of calcium oxalate urolithiasis and hepatotoxicity.

Hormonal regulation of phosphate homeostasis in goats during transition to rumination

Regulatory processes in phosphorus (P) homeostasis in small ruminants are quite different compared to monogastric animals. Adaptive responses of modulating hormones [parathyroid hormone (PTH) and calcitriol] to feeding variable amounts of P are lacking. Therefore, the aim of this study was to examine the influence of high dietary P intake (control diet: 4 g kg(-1) dry matter; high-P diet: 8 g kg(-1) dry matter) on the expression levels of PTH receptor (PTHR), vitamin D receptor (VDR) and Na+-dependent Pi transporters (NaPi II) in kidney and jejunum of goats starting rumination. After 3 months of feeding, plasma phosphate (Pi) and PTH concentrations were increased in the high-P diet group, whereas calcium and calcitriol were not changed. The intestinal Na+-dependent Pi transport capacity was not influenced by a high-P diet and the expression of jejunal VDR, PTHR and NaPi IIb was not modified. Interestingly, renal Na+-dependent Pi transport capacity was significantly reduced and concomitantly the expression of PTHR and NaPi IIa was decreased. In conclusion, the adaptive response of renal Pi reabsorption in goats, which were in transition from non-ruminant to ruminant stage was comparable to that of monogastric animals. In contrast, the modulation of the intestinal Pi absorption was like in adult ruminants.

Choroid plexus epithelial monolayers--a cell culture model from porcine brain

Background

The goal of the present study was to develop an in vitro choroid plexus (CP) epithelial cell culture model for studying transport of protein-mediated drug secretion from blood to cerebrospinal fluid (CSF) and vice versa.

Methods

Cells were isolated by mechanical and enzymatic treatment of freshly isolated porcine plexus tissue. Epithelial cell monolayers were grown and CSF secretion and transepithelial resistance were determined. The expression of f-actin as well as the choroid plexus marker protein transthyretin (TTR), were assessed. The expression of the export proteins p-glycoprotein (Pgp, Abcb1) and multidrug resistance protein 1 (Mrp1, Abcc1) was studied by RT-PCR, Western-blot and immunofluorescence techniques and their functional activity was assessed by transport and uptake experiments.

Results

Choroid plexus epithelial cells were isolated in high purity and grown to form confluent monolayers. Filter-grown monolayers displayed transendothelial resistance (TEER) values in the range of 100 to 150 Ωcm². Morphologically, the cells showed the typical net work of f-actin and expressed TTR at a high rate. The cultured cells were able to secrete CSF at a rate of 48.2 ± 4.6 μl/cm²/h over 2–3 hours. The ABC-export protein Mrp1 was expressed in the basolateral (blood-facing) membranes of cell monolayers and intact tissue. P-glycoprotein showed only low expression within the apical (CSF directed) membrane but was located more in sub-apical cell compartments. This finding was paralleled by the lack of directed excretion of p-glycoprotein substrates, verapamil and rhodamine 123.

Conclusion

It was demonstrated that CP epithelium can be isolated and cultured, with cells growing into intact monolayers, fully differentiating and with properties resembling the tissue in vivo. Thus, the established primary porcine CP model, allowing investigation of complex transport processes, can be used as a reliable tool for analysis of xenobiotic transport across the blood-cerebrospinal fluid barrier (BCSFB).

Ca2+-independent phospholipase A2 activity in apical plasma membranes from the rat parotid gland

An apical-enriched plasma membrane fraction (A-PM) was prepared from rat parotid gland by Mn2+ precipitation. In this fraction, phosphatidylcholine (PC) labelled at the sn-2 position was mainly decomposed into two labelled compounds (free fatty acid and 1,2-diacylglycerol) under Ca2+-free conditions. Studies using double-labelled PC and 2,3-diphosphoglycerate (as a phospholipase D inhibitor) showed that they were produced through different pathways: free fatty acid was released by phospholipase A2 (PLA2) while 1,2-diacylglycerol may be produced by sequential action of phospholipase D and phosphatidate phosphatase. The PLA2 in A-PM did not require Ca2+ for its activity and was highly activated by Triton X-100 and ATP. The inhibitor of the well-documented Ca2+-independent PLA2, bromoenol lactone, did not inhibit the PLA2 activity in A-PM. Although PLA2 activity was detected in other subcellular fractions, the highest specific activity was in A-PM. Its distribution among various fractions was roughly similar to that of the marker enzyme of apical plasma membranes. These findings suggested that Ca2+-independent PLA2 activity is present in apical plasma membranes from rat parotid gland. In addition, to clarify the involvement of the PLA2 in exocytosis, the fusion of exogenous PLA2-treated membranes with secretory granules was examined by fluorescence dequenching assay. This study clearly demonstrated the facilitation of fusion by PLA2 treatment, which suggests some involvement of apical PLA2 in saliva secretion.

Characterization of Y1, Y2 and Y5 subtypes of the neuropeptide Y (NPY) receptor in rabbit kidney. Sensitivity of ligand binding to guanine nucleotides and phospholipase C inhibitors

The binding of two peptide YY/neuropeptide Y analogues selective for major subtypes of neuropeptide Y (NPY) receptors was compared in particulates from rabbit kidney cortex employing modulators of activity of G-proteins, phospholipase enzymes, and ion channels. The binding of (Leu31,Pro34)human peptide YY resembled the patterns observed previously for the brain tissue Y1 receptor, exhibiting a high sensitivity to monovalent cations, disulfide disruptors, guanosine polyphosphates and phospholipase C inhibitors. However, this binding was bimodal in response to human pancreatic polypeptide and to peptides selective for the Y2 subtype of the NPY receptor, displaying a large component pharmacologically similar to the brain Y5 receptor. This kidney Y5-like binding largely shared the sensitivity to monovalent cations, guanine nucleotides and phospholipase C inhibitors found for either the kidney or the brain Y1 receptor, and also was activated by Ca2+ ion. Both Y1- and Y5-like binding in the kidney displayed a uniformly low reactivity to a nonpeptidic Y1 antagonist, BIBP-3226, and to a receptor peptide mimetic, mastoparan analogue MAS-7. The kidney Y2 binding shared the low sensitivity to ionic environment observed for the brain Y2 subtype, and was only partially sensitive to guanine nucleotides or to MAS-7. The Y2 liganding had a sensitivity to phospholipase C inhibitors similar to the Y1/Y5 binding. This reactivity was retained in the fraction of the Y2 receptor persisting detergent solubilization in a high-affinity form, which, however, was activated rather than inhibited by G-protein agonists.

Competitive inhibition of p-aminohippurate transport by quinapril in rabbit renal basolateral membrane vesicles

The mechanism of quinapril's interaction with the organic anion transporter was characterized by studying its effect on the transport of p-aminohippurate (PAH) in rabbit renal basolateral membrane vesicles (BLMV). Cis-inhibition studies demonstrated that quinapril was a specific and potent inhibitor of PAH. The Ki of quinapril was about 20 microM, a value similar to that of probenecid and eight-times lower than the K(m) value of 165 microM for PAH. Even though quinapril resulted in trans-inhibition of PAH uptake during counterflow studies, kinetic studies revealed that quinapril was a competitive inhibitor of PAH transport. This latter findings suggests that quinapril and PAH share a common binding site on the transporter. Overall, the results indicate that quinapril is a high-affinity inhibitor of the organic anion transporter in renal BLMV, and that drug-drug interactions may occur with other organic anions at the basolateral membrane of proximal cells.

Effects of calcium and calmodulin on the binding of rat parotid secretion granules to the plasma membrane

Since numerous studies suggest that Ca++ and calmodulin may modulate the fusion of secretion granules to the plasma membrane which takes place in exocytosis, we have examined the role of calcium and calmodulin in the binding of isolated parotid secretion granules to plasma membrane vesicles. 125I-labeled inside-out plasma membrane vesicles were incubated with secretion granules, the mixture was layered over 20% sucrose, the gradient was centrifuged, and the amount of 125I in the granule pellet was determined. Addition of Ca++ (20 nM to 10 microM) produced a concentration-dependent increase in the binding of 125I-labeled plasma membrane vesicles to the secretion granules, reaching a maximum value at 10 microM free Ca++; half-maximal binding occurred at 400 nM. Neither right-side-out parotid plasma membrane vesicles nor inside-out pancreatic islet plasma membrane vesicles bound to granules in the presence of 1 microM Ca++. Calmodulin produced a concentration-dependent increase in binding above that of Ca++ alone, and this effect was inhibited by the calmodulin antagonists, trifluoperazine and calmidazolium. Incubation of secretion granules with octadecylrhodamine B (R18)-loaded inside-out plasma membrane vesicles and 2 microM Ca++ caused de-quenching of fluorescence, indicating that the lipids in the granule membrane and the plasma membrane had intermixed. Added calmodulin increased the fluorescence two-fold above that with Ca++ alone. These results suggest that Ca++ and calmodulin may play a role in parotid gland exocytosis by modulating the interaction between the secretion granules and plasma membrane.

Localization of a low Mr GTP-binding protein, rap1 protein, in plasma membranes and secretory granule membranes of rat parotid gland

Subcellular fractions were prepared from rat parotid gland by sequential centrifugation, Percoll gradient centrifugation and divalent-cation precipitation, and the localization of a low Mr GTP-binding protein, rap1 protein (rap1p) was analyzed by immunoblotting using a specific antibody. rap1p was found to be located in apical and basolateral plasma membranes, and secretory granule membranes in rat parotid gland. On the other hand, the beta gamma subunits of heterotrimeric Gt protein was localized in plasma membranes but not in granule membranes.

Molecular evidence for two renal Na+/glucose cotransporters

Previous studies have shown that two kinetically and genetically distinct Na+/glucose cotransporters exist in mammalian kidney. We have recently cloned and sequenced one of the rabbit renal Na+/glucose cotransporters (SGLT1) and have found that it is identical in sequence to the intestinal Na+/glucose cotransporter. Northern blots showed that SGLT1 mRNA was found predominantly in the outer medulla of rabbit kidney. Injection of mRNA from outer medulla and outer cortex into Xenopus oocytes resulted in equal expression of Na(+)-dependent sugar uptake, indicating that the outer cortex sample contained mRNA encoding both SGLT1 and a second Na+/glucose cotransporter. Western blots using antipeptide antibodies against SGLT1 showed that the SGLT1 protein is more abundant in outer medulla than outer cortex. However, brush border membrane vesicles prepared from outer cortex had a greater capacity for Na(+)-dependent glucose transport, indicating the presence of a second transporter in the vesicles from outer cortex. It appears that the cloned renal Na+/glucose cotransporter, SGLT1, is the 'high affinity, low capacity' transporter found predominantly in outer medulla. There is evidence that a second transporter, the 'low affinity, high capacity' transporter, is in outer cortex. Finally, the cDNA and protein sequences of the two renal Na+/glucose cotransporters are predicted to differ by more than 20%.

Incorporation into a planar lipid bilayer of K channels from the luminal membrane of rabbit proximal tubule

The presence of ionic channels at the apical membrane of rabbit proximal tubule cells was investigated by fusion of brush-border membrane vesicles (BBMV) with a planar lipid bilayer (PE/PC, 1:1). The BBMV obtained from native membranes showed poor fusogenic properties. The probability of vesicles fusion with a planar bilayer was, however, enhanced by preincubating the BBMV with liposomes made of azolectin. We report here the presence in BBMV preparations of two K(+)-selective channels of 65 pS and 40 pS, respectively, in asymmetrical 200 parallel 50 mM KCl solutions. The channel of 65 pS appeared highly selective to K+ over Na+ and Cl- ions, while the 40 pS channel discriminated poorly between K+ and Na+ with a permeability ratio PK/PNa = 4. The open probability Po of both channels was found to be voltage-independent within the potential range -60 mV to +60 mV. These K+ channels may be related to channels identified using other methods.

Stimulation of canine kidney BBMV ATPase activity by acidic pH in the presence of Zn2+: an ATPase activity distinct from transport ATPases and alkaline phosphatase that may be an ecto-ATPase

Renal brush border membrane vesicles (BBMV) of the dog possess at least two ATPase activities. In the present study, we have examined the effect of pH, ions, and inhibitors on the activity of ATPase in BBMV. Two different sets of conditions were identified that produced stimulation of ATPase activity. A unique stimulation of BBMV ATPase activity occurred at acidic pH in the presence of 1 mM ZnCl2. In the absence of Zn2+, a second ATPase activity was stimulated by alkaline pH values with peak stimulation occurring between pH 8.5 and 9.0. The results suggest that the alkaline pH-stimulated hydrolysis of ATP probably represents the activity of BBMV alkaline phosphatase. The unique acidic pH + Zn2(+)-stimulated ATPase activity must represent the activity of a second protein other than the alkaline phosphatase, since purified alkaline phosphatase did not show this activity. The biochemical identity and physiological function of this renal BBMV ATPase activity remain to be determined, but it may be an ecto-ATPase.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.