Inducible expression of erythrocyte band 3 protein

Rapid Cl−/HCO3−exchange kinetics of AE1 in HEK293 cells and hereditary stomatocytosis red blood cells

Anion exchanger 1 (AE1) or band 3 is a membrane protein responsible for the rapid exchange of chloride for bicarbonate across the red blood cell membrane. Nine mutations leading to single amino-acid substitutions in the transmembrane domain of AE1 are associated with dominant hereditary stomatocytosis, monovalent cation leaks, and reduced anion exchange activity. We set up a stopped-flow spectrofluorometry assay coupled with flow cytometry to investigate the anion transport and membrane expression characteristics of wild-type recombinant AE1 in HEK293 cells, using an inducible expression system. Likewise, study of three stomatocytosis-associated mutations (R730C, E758K, and G796R), allowed the validation of our method. Measurement of the rapid and specific chloride/bicarbonate exchange by surface expressed AE1 showed that E758K mutant was fully active compared with wild-type (WT) AE1, whereas R730C and G796R mutants were inactive, reinforcing previously reported data on other experimental models. Stopped-flow analysis of AE1 transport activity in red blood cell ghost preparations revealed a 50% reduction of G796R compared with WT AE1 corresponding to a loss of function of the G796R mutated protein, in accordance with the heterozygous status of the AE1 variant patients. In conclusion, stopped-flow led to measurement of rapid transport kinetics using the natural substrate for AE1 and, conjugated with flow cytometry, allowed a reliable correlation of chloride/bicarbonate exchange to surface expression of AE1, both in recombinant cells and ghosts and therefore a fine comparison of function between different stomatocytosis samples. This technical approach thus provides significant improvements in anion exchange analysis in red blood cells.

Collecting duct renin is upregulated in both kidneys of 2-kidney, 1-clip goldblatt hypertensive rats

Renin in collecting duct cells is upregulated in chronic angiotensin II-infused rats via angiotensin II type 1 receptors. To determine whether stimulation of collecting duct renin is a blood pressure-dependent effect; changes in collecting duct renin and associated parameters were assessed in both kidneys of 2-kidney, 1-clip Goldblatt hypertensive (2K1C) rats. Renal medullary tissues were used to avoid the contribution of renin from juxtaglomerular cells. Systolic blood pressure increased to 184+/-9 mm Hg in 2K1C rats (n=19) compared with sham rats (121+/-6 mm Hg; n=12). Although renin immunoreactivity markedly decreased in juxtaglomerular cells of nonclipped kidneys (NCK: 0.2+/-0.0 versus 1.0+/-0.0 relative ratio) and was augmented in clipped kidneys (CK: 1.7+/-1.0 versus 1.0+/-0.0 relative ratio), its immunoreactivity increased in cortical and medullary collecting ducts of both kidneys of 2K1C rats (CK: 2.8+/-1.0 cortex; 2.1+/-1.0 medulla; NCK: 4.6+/-2.0 cortex, 3.2+/-1.0 medulla versus 1.0+/-0.0 in sham kidneys). Renal medullary tissues of 2K1C rats showed greater levels of renin protein (CK: 1.4+/-0.2; NCK: 1.5+/-0.3), renin mRNA (CK: 5.8+/-2.0; NCK: 4.9+/-2.0), angiotensin I (CK: 120+/-18 pg/g; NCK: 129+/-13 pg/g versus sham: 67+/-6 pg/g), angiotensin II (CK: 150+/-32 pg/g; NCK: 123+/-21 pg/g versus sham: 91+/-12 pg/g; P<0.05), and renin activity (CK: 8.6 microg of angiotensin I per microgram of protein; NCK: 8.3 microg of angiotensin I per microgram of protein; sham: 3.4 microg of angiotensin I per microgram of protein) than sham rats. These data indicate that enhanced collecting duct renin in 2K1C rats occurs independently of blood pressure. Upregulation of distal tubular renin helps to explain how sustained intrarenal angiotensin II formation occurs even during juxtaglomerular renin suppression, thus allowing maintained effects on tubular sodium reabsorption that contribute to the hypertension.

Bradykinin-induced chloride conductance in murine proximal tubule epithelial cells

Despite the recognized role of bradykinin (BK)-induced calcium and chloride conductance in regulating salt transport in the kidney, the signaling pathway involved has not been well examined. Patch clamp of murine proximal tubule (TKPTS) cells revealed that BK (10 nM) produced an increase in an outwardly rectifying current from a basal level of 2.9 +/- 0.6 to 13.8 +/- 1.1 pA/pF following addition of BK (n = 8; p < 0.001). The shift in reversal potential seen with BK on changing the intracellular solution to 152 mM chloride and significant inhibition of the current by 100 microM 4,4'-di-isothiocyanato-stilbene-2,2'-disulphonic acid (DIDS) suggested that BK activated a chloride current. BK-induced current was blocked by B2 receptor antagonist but not by B1 antagonist or pertussis toxin indicating that the current was mediated by B2 receptors possibly through Gq activation. TMB-8 completely blocked the BK-calcium rise in fura-2 studies but did not block the BK-chloride response indicating that BK-mediated chloride current is calcium-independent. BK-induced current was dependent on phospholipase C (PLC) since U73122, a PLC-beta blocker (10 microM) blocked it completely. Furthermore, chloride conductance was not modulated by bisindolylmaleimide, an inhibitor of protein kinase C (PKC), but was enhanced by dibutyryl cAMP. We conclude that BK-induced rise in chloride current is mediated by B2 receptors and dependent on PLC activation but not dependent on calcium rise. Furthermore, the current can be modulated by cAMP but not PKC.

Mechanism of bradykinin-induced Ca2+ mobilization in murine proximal tubule epithelial cells

Despite the recognized physiological role of bradykinin (BK) in the kidney in maintaining glomerular and tubule function and its role in pathological states such as endotoxemia, diabetes, and other diseases, relatively little is known about the mechanisms by which BK can impact kidney function. Furthermore, the signaling of BK receptors in the murine nephron has not been fully characterized. The present studies were undertaken to examine BK-stimulated Ca(2+) signaling using Fura-2 in the murine proximal tubule epithelial cell line TKPTS. BK produced a concentration-dependent rise in intracellular Ca(2+) ([Ca(2+)])(i) (pEC(50) = 8.39 +/- 0.04). Selective antagonists showed the rise in [Ca(2+)](i) was mediated through B2 receptors. The rise in [Ca(2+)](i) was rapid and reversible and was maximally stimulated at 1 microM (697 +/- 70 nM above basal level of 115 +/- 6 nM). Studies with thapsigargin and EGTA showed Ca(2+) mobilization was dependent on two events: release and influx. Both U73122 (1-[6-[[17-beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione) [a phospholipase C (PLC) inhibitor] and genistein (a tyrosine kinase inhibitor) partially inhibited BK-stimulated rise in [Ca(2+)](i). When combined, both agents produced a further decrease, suggesting multiple pathways for PLC activation may be involved. The ability of Ni(2+) to inhibit influx indicated the activation of a Ca(2+) release-activated channel (CRAC). Ca(2+) mobilization did not seem to be affected by cyclic nucleotides or protein kinase C. In summary, the TKPTS murine proximal tubule cell line expresses functional B2 receptors linked to Ca(2+) mobilization that is dependent on phospholipase C and activation of CRAC.

Comparison of seven different heterologous protein expression systems for the production of the serotonin transporter

The rat serotonin transporter (rSERT) is an N-glycosylated integral membrane protein with 12 transmembrane regions; the N-glycans improve the ability of the SERT polypeptide chain to fold into a functional transporter, but they are not required for the transmembrane transport of serotonin per se. In order to define the best system for the expression, purification and structural analysis of serotonin transporter (SERT), we expressed SERT in Escherichia coli, Pichia pastoris, the baculovirus expression system and in four different stable mammalian cell lines. Two stable cell lines that constitutively expressed SERT (Imi270 and Coca270) were constructed using episomal plasmids in HEK293 cells expressing the EBNA-1 antigen. SERT expression in the three different inducible stable mammalian cell lines was induced either by a decrease in temperature (cell line pCytTS-SERT), the addition of tetracycline to the growth medium (cell line T-REx-SERT) or by adding DMSO which caused the cells to differentiate (cell line MEL-SERT). All the mammalian cell lines expressed functional SERT, but SERT expressed in E. coli or P. pastoris was nonfunctional as assessed by 5-hydroxytryptamine uptake and inhibitor binding assays. Expression of functional SERT in the mammalian cell lines was assessed by an inhibitor binding assay; the cell lines pCytTS-SERT, Imi270 and Coca270 contained levels of functional SERT similar to that of the standard baculovirus expression system (250,000 copies per cell). The expression of SERT in induced T-REx-SERT cells was 400,000 copies per cell, but in MEL-SERT it was only 80,000 copies per cell. All the mammalian stable cell lines expressed SERT at the plasma membrane as assessed by [3H]-5-hydroxytryptamine uptake into whole cells, but the V(max) for the T-Rex-SERT cell line was 10-fold higher than any of the other cell lines. It was noticeable that the cell lines that constitutively expressed SERT grew extremely poorly, compared to the inducible cell lines whose growth rates were similar to the parental cell lines when not induced. In addition, the cell lines MEL-SERT, Imi270 and T-REx-SERT all expressed fully N-glycosylated SERT and no unglycosylated inactive protein, in contrast to the baculovirus expression system where the vast majority of expressed SERT was unglycosylated and nonfunctional.

Transfection of an inducible trout anion exchanger (AE1) into HEK-EcR cells

A permanent cell line with inducible expression of the trout anion exchanger protein (trAE1) was constructed in a derivative of human embryonic kidney cells (HEK-293). In the absence of the inducer, muristerone A, the new cell line had no detectable trAE1 protein by Western analysis, biotinylation, and (36)Cl(-) flux. The amount of trAE1 protein increased with increasing dose and incubation time with muristerone A. Anion exchange inhibitors significantly inhibited the inducible flux of anions (i.e., (36)chloride and (35)sulfate) and taurine in isotonic media. The transfected cells had the characteristics of trAE1-mediated transport in intact trout erythrocytes: (1) inhibition by anion transport inhibitors, (2) pH independence over the pH range of 6.5-7.5, and (3) activation of (35)sulfate efflux by external anions in the selective order of Cl > Br > I > or = F. These cells, in contrast to trout erythrocytes, were not sensitive to the anion exchange inhibitor, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), suggesting some difference in the properties of the transfected AE1. These results demonstrate the inducible expression of new anion transport membrane protein in HEK-293 cells. This is the first expression of trAE1 in a mammalian system.

Topology of the anion exchange protein AE1: the controversial sidedness of lysine 743

The topology of the band 3 (AE1) polypeptide of the erythrocyte membrane is not fully established despite extensive study. Residues near lysine 743 (K743) have been reported to be extracellular in some studies and cytoplasmic in others. In the work presented here, we have attempted to establish the sidedness of K743 using in situ proteolysis. Trypsin, papain, and proteinase K do not cleave band 3 at or near K743 in intact red cells, even under conditions that cause cleavage on the C-terminal side of the glycosylation site (N642) in extracellular loop 4. In contrast, trypsin sealed inside red cell ghosts cleaves at K743, as does trypsin treatment of inside-out vesicles (IOVs). The transport inhibitor 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate (H(2)DIDS), acting from the extracellular side, blocks trypsin cleavage at K743 in unsealed membranes by inducing a protease-resistant conformation. H(2)DIDS added to IOVs does not prevent cleavage at K743; therefore, trypsin cleavage at K743 in IOVs is not a consequence of cleavage of right-side-out or leaky vesicles. Finally, microsomes were prepared from HEK293 cells expressing the membrane domain of AE1 lacking the normal glycosylation site. This polypeptide does not traffic to the surface membrane; trypsin treatment of microsomes containing this polypeptide produces the 20 kDa fragment, providing further evidence that K743 is exposed at the cytoplasmic surface. Therefore, the actions of trypsin on intact cells, resealed ghosts, unsealed ghosts, inside-out vesicles, and microsomes from HEK293 cells all indicate that K743 is cytoplasmic and not extracellular.

Muristerone A-induced nerve growth factor release from genetically engineered human dermal fibroblasts for peripheral nerve tissue engineering

In this study, human dermal fibroblasts (hDFBs) were genetically modified to release human nerve growth factor (NGF) using an ecdysone-inducible system. NGF cDNA was inserted into the pIND vector and then hDFBs were cotransfected with pIND-NGF and pVgRXR. Muristerone A, an analog of ecdysone, was used as the inducing agent. NGF release from transfected hDFBs was assessed in vitro and in vivo. Transfected hDFBs in the presence of Muristerone A possessed a maximal in vitro release of 8.5 +/- 0.4 pg of NGF/mL per 10(3) cells, demonstrating significantly higher NGF levels compared to control hDFBs. The in vitro release rate curve for transfected hDFBs in the presence of Muristerone A exhibited a maximum of 5.1 +/- 0.2 ng NGF/10(6) cells/day. A PC-12 bioassay demonstrated that the in vitro NGF released is bioactive. When transfected hDFBs in the presence of Muristerone A were placed in vivo in nude rats, NGF levels reach 2074 +/- 257 pg/mL and 1620 +/- 132 pg/mL at 24 and 48 h, respectively. These levels were significantly higher than negative control and wound fluid levels. Results support further in vivo investigation of this molecular "on" switch for peripheral nerve regeneration.

Coexpression of band 3 mutants and Rh polypeptides: differential effects of band 3 on the expression of the Rh complex containing D polypeptide and the Rh complex containing CcEe polypeptide

K562 cells were stably transfected with cDNAs encoding the band 3 found in Southeast Asian ovalocytosis (B3SAO, deletion of residues 400-408), band 3 with a transport-inactivating E681Q point mutation (B3EQ), or normal band 3 (B3). Flow cytometric analysis and quantitative immunoblotting revealed that B3SAO expressed alone was translocated to the plasma membrane, at levels similar to B3 or B3EQ. Nine monoclonal antibodies that reacted with extracellular loops of B3 also reacted with B3SAO, although the affinity of most antibodies for the mutant protein was reduced. Both known Wr(b) epitopes were expressed on K562/B3SAO cells, demonstrating that B3SAO interacts with glycophorin A. The growth rates of K562 clones expressing equivalent amounts of B3 and B3EQ were the same, suggesting that the potentially toxic transport function of band 3 may be regulated in K562 cells. The band 3-mediated enhancement of Rh antigen reactivity and the depression of Rh epitopes on SAO erythrocytes were investigated by comparing the coexpression of B3, B3SAO, or B3EQ in K562 clones expressing exogenous RhcE or RhD polypeptides. The results are consistent with an interaction between band 3 and the Rh polypeptide-Rh glycoprotein (RhAG) complex, which may enhance translocation of the complex or affect its conformation in the plasma membrane. The data suggest that the interaction between band 3 and the RhD-RhAG complex is weaker than it is between band 3 and the RhCcEe-RhAG complex.

Inherited renal tubular acidosis

The past few years have witnessed great progress in elucidating the molecular basis of inherited renal tubular acidosis. Consistent with the physiologically defined importance of multiple gene products in urinary acidification, heritable renal tubular acidosis is genetically heterogeneous. Autosomal dominant distal renal tubular acidosis has been associated with a small number of mutations in the AE1 Cl-/HCO3- exchanger although the pathophysiologic mechanisms behind these mutations remain unclear. Rarely, autosomal recessive distal RTA is caused by homozygosity or compound heterozygosity for the loss-of-function mutation AE1 G701D. A larger proportion, often accompanied by hearing loss, is associated with mutations in the ATP6B1 gene encoding the 58 kDa B1 subunit of the vacuolar H+-ATPase. Mutations in the gene encoding the Na+/HCO3- cotransporter, NBC1, have recently been identified in proximal renal tubular acidosis with corneal calcification.

Involvement of ion channels in human eosinophil respiratory burst

BACKGROUND

Human eosinophils possess a variety of ion channels that play a crucial role in the regulation of cellular activity. During eosinophil respiratory burst, efflux of H(+) ions through H(+) channels provides an efficient mechanism of H(+) extrusion and charge compensation. Interestingly, recent studies suggest that other ion channels may also be involved in this process.

OBJECTIVE

We sought to investigate the role of ion channels in phorbol 12-myristate 13-acetate-induced superoxide (O(2)(*-)) generation by human eosinophils.

METHODS

O(2)(*-) production was measured by using the superoxide dismutase-inhibitable reduction of cytochrome c. Ion channel expression and function were studied by using RT-PCR and the patch clamp technique, respectively.

RESULTS

O(2)(*-) generation was affected by several ion channel blockers, especially 4,4-diisothio-cyanostilbene-2,2'-disulfonic acid. The involvement of Cl(-) channels in this process was confirmed by replacement of Cl(-) with gluconate or other anions. The halide dependence of O(2)(*-) production could be described by the sequence Cl(-)> or =Br(-)>I(-), which is similar to the selectivity sequence of several members of the chloride channel (ClC) family. RT-PCR studies performed with primers for ClC-2, ClC-3, ClC-4, ClC-5, ClC-6, and the cystic fibrosis transmembrane conductance regulator showed only the expression of ClC-3. The presence of phorbol 12-myristate 13-acetate-sensitive Cl(-) channels in human eosinophils with biophysical properties similar to the ClC-3 channel has been studied.

CONCLUSION

Cl(-) channels play an important role in the regulation of O(2)(*-) production by human eosinophils.

Metabolism alters the selectivity of angiotensin-(1-7) receptor ligands for angiotensin receptors

The present study examined whether metabolism of the putative angiotensin-(1-7) receptor agonist and antagonist [angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7), respectively] altered their ability to interact with angiotensin AT(1), AT(2), and AT(4) receptor subtypes. Both angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7) competed with low affinity for (125)I-sarcosine(1), isoleucine(8) angiotensin II binding to AT(1) and AT(2) receptors in rat liver and adrenal medulla membranes, respectively, and competed with low affinity for (125)I-angiotensin IV binding to AT(4) receptors in bovine kidney epithelial cell membranes. In vitro renal metabolism of the angiotensin-(1-7) receptor ligands (incubating peptides with rat cortical tissue homogenates) had minimal influence on low-affinity binding to AT(1) and AT(2) receptors, yet caused a significant and dramatic shift toward high-affinity binding for AT(4) receptors. Low-affinity angiotensin II binding to the AT(4) receptor was also shifted toward high-affinity binding following renal metabolism of the peptide. Conversely, angiotensins with high affinity for the AT(4) receptor (e.g., angiotensin IV) were shifted toward low-affinity binding states following peptide metabolism. Incubation of (125)I-angiotensin-(1-7) with rat cortical tissue generated the high-affinity AT(4) receptor ligand (125)I-angiotensin-(3-7), whereas the renal metabolism of (125)I-angiotensin II generated both (125)I-angiotensin-(3-7) and (125)I-angiotensin IV. These results reveal that renal metabolism of angiotensin-(1-7) receptor ligands and angiotensin II yields products that have high affinity for the AT(4) receptor and could potentially contribute to the biologic actions of the parent peptide in the kidney.

The noncompetitive inhibitor WW781 senses changes in erythrocyte anion exchanger (AE1) transport site conformation and substrate binding

WW781 binds reversibly to red blood cell AE1 and inhibits anion exchange by a two-step mechanism, in which an initial complex (complex 1) is rapidly formed, and then there is a slower equilibration to form a second complex (complex 2) with a lower free energy. According to the ping-pong kinetic model, AE1 can exist in forms with the anion transport site facing either inward or outward, and the transition between these forms is greatly facilitated by binding of a transportable substrate such as Cl(-). Both the rapid initial binding of WW781 and the formation of complex 2 are strongly affected by the conformation of AE1, such that the forms with the transport site facing outward have higher affinity than those with the transport site facing inward. In addition, binding of Cl(-) seems to raise the free energy of complex 2 relative to complex 1, thereby reducing the equilibrium binding affinity, but Cl(-) does not compete directly with WW781. The WW781 binding site, therefore, reveals a part of the AE1 structure that is sensitive to Cl(-) binding and to transport site orientation, in addition to the disulfonic stilbene binding site. The relationship of the inhibitory potency of WW781 under different conditions to the affinities for the different forms of AE1 provides information on the possible asymmetric distributions of unloaded and Cl(-)-loaded transport sites that are consistent with the ping-pong model, and supports the conclusion from flux and nuclear magnetic resonance data that both the unloaded and Cl(-)-loaded sites are very asymmetrically distributed, with far more sites facing the cytoplasm than the outside medium. This asymmetry, together with the ability of WW781 to recruit toward the forms with outward-facing sites, implies that WW781 may be useful for changing the conformation of AE1 in studies of structure-function relationships.