Injection of poly(A)+ RNA from rabbit small intestine into Xenopus laevis oocytes resulted in expression of pH dependent transport of the aminocephalosporin cefadroxil. A cDNA library constructed from a 2.2 to 5 kb fraction was screened for expression of cefadroxil transport after injection of the corresponding cRNA synthetized in vitro from different pools of clones. The single clone identified stimulated uptake of cefadroxil into oocytes about 50-fold at pH 6.5. Kinetic analysis of expressed transport activity revealed a saturable transport system shared by amino beta-lactam antibiotics, dipeptides and selected angiotensin converting enzyme inhibitors. Evidence for rheogenic cefadroxil/H(+)-cotransport was obtained by a) The demonstration that cefadroxil influx increased the inward current in oocytes clamped at a holding potential of -60 mV in sodium-free medium and b) A decrease of intracellular pH in oocytes caused by cefadroxil uptake. Current-voltage relationships in the presence of glycylsarcosine or cefadroxil showed that transport activity is dependent on the membrane potential. Sequencing of the cDNA revealed its identity with the recently cloned peptide transporter from rabbit small intestine designated PepT1.

No abstract

We used the Xenopus laevis oocyte expression system to characterize adenosine 3',5'-cyclic monophosphate (cAMP) activation of the cystic fibrosis transmembrane conductance regulator (CFTR). With conventional two-microelectrode voltage-clamp techniques, we recorded transmembrane conductance (Gm) and membrane current (Im). Using five different sine wave frequencies, we also monitored changes of the plasma membrane surface area by recording continuously membrane capacitance (Cm) under voltage-clamp conditions. Impedance spectra recorded in the frequency range 0.1-500 Hz showed that, at least up to 200 Hz, Cm is independent of the frequency. In control oocytes, cAMP (100 microM) treatment did not affect Gm or Im but evoked a small, slowly occurring increase in Cm, probably mediated by cAMP-stimulated exocytosis. However, in oocytes expressing CFTR, large simultaneous increases of Gm, Im and Cm occurred after stimulation with cAMP. Oocytes injected with the delta F508 CFTR mutant behaved like control oocytes and cAMP had no additional effects on Gm, Im or Cm. In oocytes injected with wild-type CFTR, adenosine 5'-triphosphate (ATP, 100 microM) did not activate the cAMP-induced augmentation of Im, Gm or Cm further. On the other hand, cAMP-induced increases in Cm were reduced significantly by the specific blockers of protein kinase A (PKA) KT5720 and N-[2-(methylamino-9-ethyl]-5-isoquinolinesulphonamide hydrochloride (H8), whereas the increases in Gm and Im were essentially unaffected by these agents. Reducing intracellular Ca2+ by injection of a Ca2+ chelator 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) prevented PKA-dependent exocytosis while activation of Im and Gm of already-inserted CFTR still could be detected. The specific cAMP antagonist adenosine 3',5'-cyclic monophosphothioate Rp diastereomer (RpcAMPS) completely suppressed the effects of cAMP on all parameters. These findings are consistent with the concept of different pathways of CFTR activation by cAMP: already-inserted CFTR Cl- channels are activated directly by cAMP, while traffic of CFTR proteins from an intracellular pool to the plasma membrane and functional insertion into the plasma membrane occurs via cAMP- and Ca(2+)-dependent PKA-mediated exocytosis.

From these data, we conclude that CFTR-mRNA transfection could comprise a novel alternative for gene therapy to restore impaired CFTR function.

In vitro disease models have enabled insights into the pathophysiology of human disease as well as the functional evaluation of new therapies, such as novel genome engineering strategies. In the context of cystic fibrosis (CF), various cellular disease models have been established in recent years, including organoids based on induced pluripotent stem cell technology that allowed for functional readouts of CFTR activity. Yet, many of these in vitro CF models require complex and expensive culturing protocols that are difficult to implement and may not be amenable for high throughput screens. Here, we show that a simple cellular CF disease model based on the bronchial epithelial ΔF508 cell line CFBE41o- can be used to validate functional CFTR correction. We used an engineered nuclease to target the integration of a super-exon, encompassing the sequences of CFTR exons 11 to 27, into exon 11 and re-activated endogenous CFTR expression by treating CFBE41o- cells with a demethylating agent. We demonstrate that the integration of this super-exon resulted in expression of a corrected mRNA from the endogenous CFTR promoter and used short-circuit current measurements in Ussing chambers to corroborate restored ion transport of the repaired CFTR channels. In conclusion, this study proves that the targeted integration of a large super-exon in CFTR exon 11 leads to functional correction of CFTR, suggesting that this strategy can be used to functionally correct all CFTR mutations located downstream of the 5’ end of exon 11.

No abstract

Previously we cloned membrane associated (M(r) 62000-67000) polypeptides from pig (pRS1), rabbit (rbRS1) and man (hRS1) which modified transport activities that were expressed in Xenopus laevis oocytes by the Na(+)-D-glucose cotransporter SGLT1 and/or the organic cation transporter OCT2. These effects were dependent on the species of RS1 and on the target transporters. hRS1 and rbRS1 were shown to be intronless single copy genes which are expressed in various tissues and cell types. Earlier immunohistochemical data with a monoclonal IgM antibody suggested an extracellular membrane association of RS1. In the present paper antibodies against recombinant pRS1 were raised and the distribution and membrane localization of RS1 reevaluated. After subcellular fractionation of renal cortex RS1 was found associated with brush border membranes and an about 1:200 relation between RS1 and SGLT1 protein was estimated. Also after overexpression in X. laevis oocytes RS1 was associated with the plasma membrane, however, at variance to the kidney it was also observed in the cytosol. Labeling experiments with covalently binding lipid-permeable and lipid-impermeable biotin analogues showed that RS1 is localized at the inner side of the plasma membrane. Western blots with plasma membranes from Xenopus oocytes revealed that SGLT1 protein in the plasma membrane was reduced when hRS1 was coexpressed with human SGLT1 which leads to a reduction in V(max) of expressed glucose transport. Measurements of membrane capacitance and electron microscopic inspection showed that the expression of hRS1 leads to a reduction of the oocyte plasma membrane surface. The data suggest that RS1 is an intracellular regulatory protein that associates with the plasma membrane. Overexpression of RS1 may effect the incorporation and/or retrieval of transporters into the plasma membrane.

For ␤-D-glucosylisophosphoramide mustard (␤-D-Glc-IPM), a new alkylating drug in which isophosphoramide mustard is stabilized, a higher selectivity and lower myelotoxicity was observed than for the currently used cytostatic ifosfamide. Because ␤-D-Glc-IPM is hydrophilic and does not diffuse passively through the lipid bilayer, we investigated whether a transporter may be involved in the cellular uptake. A variety of cloned Na ؉ -sugar cotransporters were expressed in Xenopus oocytes, and uptake measurements were performed. By tracer uptake and electrical measurements it was found that ␤-D-Glc-IPM was transported by the lowaffinity Na ؉ -D-glucose cotransporter SAAT1, which had been cloned from pig and is also expressed in humans. At membrane potentials between ؊50 and ؊150 mV, a 10-fold higher substrate affinity (K m Ϸ 0.25 mM) and a 10-fold lower V max value were estimated for ␤-D-Glc-IPM transport than for the transport of D-glucose or methyl-␣-D-glucopyranoside (AMG). Transport of ␤-D-Glc-IPM and glucose by SAAT1 is apparently performed by the same mechanism because similar sodium dependence, dependence on membrane potential, electrogenicity, and phlorizin inhibition were determined for ␤-D-Glc-IPM, D-glucose, and AMG. Transcription of human SAAT1 was demonstrated in various human carcinomas and tumor cell lines. In one of these, the human carcinoma cell line T84, phlorizin inhibitable uptake of ␤-D-Glc-IPM was demonstrated with substrate saturation and an apparent K m of 0.4 mM. The data suggest that the Na ؉ -D-glucose cotransporter SAAT1 transports ␤-D-Glc-IPM into human tumor cells and may accumulate the drug in the cells. They provide an example for drug targeting by employing a plasma membrane transporter.