Quantitative expression patterns of multidrug-resistance P-glycoprotein (MDR1) and differentially spliced cystic-fibrosis transmembrane-conductance regulator mRNA transcripts in human epithelia

Variability of Clinical Presentation in Patients Heterozygous for the F508del Cystic Fibrosis Variant: A Series of Three Cases and a Review of the Literature

Cystic fibrosis (CF) is a genetic disease that affects the lung, pancreas, and other organs caused by the presence of biallelic CF-causing variants in the cystic fibrosis conductance regular gene (CFTR). CFTR variants can also be found in CFTR-related disorders (CFTR-RD), which present milder symptoms. Increasing access to next-generation sequencing has demonstrated that both CF and CFTR-RD have a broader array of genotypes than formerly thought. Here we present three patients who carry the most common CFTR pathogenic variant - F508del - but express a wide array of phenotypes. These cases open discussion on the role of concurrent variants in CFTR, the importance of early diagnosis and treatment, and the contribution of lifestyle factors in CF and CFTR-RD presentation.

Placental ion channels: potential target of chemical exposure

The placenta is an important organ for the exchange of substances between the fetus and the mother, hormone secretion, and fetoplacental immunological defense. Placenta has an organ-specific distribution of ion channels and trophoblasts, and placental vessels express a large number of ion channels. Several placental housekeeping activities and pregnancy complications are at least partly controlled by ion channels, which are playing an important role in regulating hormone secretion, trophoblastic homeostasis, ion transport, and vasomotor activity. The function of several placental ion channels (Na, Ca, and Cl ion channels, cation channel, nicotinic acetylcholine receptors, and aquaporin-1) is known to be influenced by chemical exposure, i.e., their responses to different chemicals have been tested and confirmed in experimental models. Here, we review the possibility that placental ion channels are targets of toxicological concern in terms of placental function, fetal growth, and development.

Vinblastine treatment decreases the undifferentiated cell contamination of human iPSC-derived intestinal epithelial-like cells

Human induced pluripotent stem cell-derived intestinal epithelial cells (hiPSC-IECs) are expected to be utilized in regenerative medicine. To perform a safe transplantation without the risk of tumor formation, residual undifferentiated hiPSCs must be removed from hiPSC-IECs. In this study, we examined whether vinblastine (a multiple drug resistance 1 [MDR1] substrate) could remove residual undifferentiated hiPSCs in hiPSC-IECs and attempted to generate hiPSC-IECs applicable to transplantation medicine. We found that the expression levels of pluripotent markers were largely decreased and those of intestinal markers were increased by vinblastine treatment. The treatment of undifferentiated hiPSCs with vinblastine significantly decreased their viability. These results suggested that undifferentiated hiPSCs can be eliminated from hiPSC-IECs by vinblastine treatment. We hypothesized that MDR1-negative cells (such as undifferentiated hiPSCs) die upon vinblastine treatment because they are unable to excrete vinblastine. As expected, the cell viability of MDR1-knockout hiPSC-IECs was significantly decreased by vinblastine treatment. Furthermore, teratomas were formed by subcutaneous transplantation of hiPSC-IECs mixed with undifferentiated hiPSCs into mice, but they were not observed when the transplanted cells were pre-treated with vinblastine. Vinblastine-treated hiPSC-IECs would be an effective cell source for safe regenerative medicine.

Drug Transporters Expressed in the Human Placenta and Models for Studying Maternal-Fetal Drug Transfer

Tremendous efforts have been directed to investigate the ontogeny of drug transporters in fetuses, neonates, infants, and children based on their importance for understanding drug pharmacokinetics. During development (ie, in the fetus and newborn infant), there is special interest in transporters expressed in the placenta that modulate placental drug transfer. Many of these transporters can decrease or increase drug concentrations in the fetus and at birth, stressing the relevance of elucidating expression in the placenta and potential gestational age-dependent changes therein. Hence, the main objective of this review was to summarize the current knowledge about expression and ontogeny of transporters in the human placenta in healthy pregnant women. In addition, various in vitro, ex vivo, and in silico models that can be used to investigate placental drug transfer, namely, placental cancer cell lines, ex vivo cotyledon perfusion experiments, and physiologically based pharmacokinetic (PBPK) models, are discussed together with their advantages and shortcomings. A particular focus was placed on PBPK models because these models can integrate different types of information, such as expression data, ontogeny information, and observations obtained from the ex vivo cotyledon perfusion experiment. Such a mechanistic modeling framework may leverage the available information and ultimately help to improve knowledge about the adequacy and safety of pharmacotherapy in pregnant women and their fetuses.

Key role of an ADP - ribose - dependent transcriptional regulator of NAD metabolism for fitness and virulence of Pseudomonas aeruginosa

NAD is an essential co-factor of redox reactions and metabolic conversions of NAD-dependent enzymes. NAD biosynthesis in the opportunistic pathogen Pseudomonas aeruginosa has yet not been experimentally explored. The in silico search for orthologs in the P. aeruginosa PAO1 genome identified the operon pncA - pncB1-nadE (PA4918-PA4920) to encode the nicotinamidase, nicotinate phosporibosyltransferase and Nad synthase of salvage pathway I. The functional role of the preceding genes PA4917 and PA4916 was resolved by the characterization of recombinant protein. PA4917 turned out to encode the nicotinate mononucleotide adenylyltransferase NadD2 and PA4916 was determined to encode the transcriptional repressor NrtR that binds to an intergenic sequence between nadD2 and pncA. Complex formation between the catalytically inactive Nudix protein NrtR and its DNA binding site was suppressed by the antirepressor ADP-ribose. NrtR plasposon mutagenesis abrogated virulence of P. aeruginosa TBCF10839 in a murine acute airway infection model and constrained its metabolite profile. When grown together with other isogenic plasposon mutants, the nrtR knock-out was most compromised in competitive fitness to persist in nutrient-rich medium in vitro or murine airways in vivo. This example demonstrates how tightly metabolism and virulence can be intertwined by key elements of metabolic control.

Association of cystic fibrosis transmembrane conductance regulator gene variants with acute lung injury in African American children with pneumonia*

OBJECTIVES

The cystic fibrosis transmembrane conductance regulator regulates fluid balance in alveolar epithelial cells and appears to modulate the inflammatory response. To determine whether more severe lung injury in children who develop community-acquired pneumonia is associated with variations known to affect function in the gene coding for cystic fibrosis transmembrane conductance regulator.

DESIGN

A prospective cohort genetic association study of lung injury in children with community-acquired pneumonia.

SETTING

Three major tertiary care children's hospitals.

SUBJECTS

Caucasian and African American children with community-acquired pneumonia either evaluated in the emergency department or admitted to the hospital.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Caucasian and African American children with pneumonia were genotyped for the most common variants reported to affect cystic fibrosis transmembrane conductance regulator function, the p.508del mutation, the (TG)mTn variable repeat region, and the M470V polymorphism in the cystic fibrosis transmembrane conductance regulator gene. Genotypes and haplotypes were determined, and the association of high-risk alleles or high-risk haplotypes (defined as the presence of at least one variant known to decrease the level of functional cystic fibrosis transmembrane conductance regulator) with the need for mechanical ventilation or the development of acute lung injury was evaluated. Forty-two children in the Caucasian cohort (n = 304) required mechanical ventilation; 32 developed acute lung injury. Forty-three children in the African American cohort (n = 474) required mechanical ventilation; 29 developed acute lung injury. In African American children, high-risk (TG)mTn alleles known to result in decreased levels of functional cystic fibrosis transmembrane conductance regulator were associated with the need for mechanical ventilation (p = .0013) and the development of acute lung injury (p = .0061). Multivariable analysis demonstrated that high-risk (TG)mTn alleles were independently associated with mechanical ventilation (odds ratios = 3.19; 95% confidence interval, 1.63-6.26) and acute lung injury (odds ratios = 3.36; 95% confidence interval, 1.50-7.53) in African American children.

CONCLUSION

Genetic variation in cystic fibrosis transmembrane conductance regulator is associated with acute lung injury in African American children with community-acquired pneumonia.

Pharmacotherapy in pregnancy; effect of ABC and SLC transporters on drug transport across the placenta and fetal drug exposure

Pharmacotherapy during pregnancy is often inevitable for medical treatment of the mother, the fetus or both. The knowledge of drug transport across placenta is, therefore, an important topic to bear in mind when deciding treatment in pregnant women. Several drug transporters of the ABC and SLC families have been discovered in the placenta, such as P-glycoprotein, breast cancer resistance protein, or organic anion/cation transporters. It is thus evident that the passage of drugs across the placenta can no longer be predicted simply on the basis of their physical-chemical properties. Functional expression of placental drug transporters in the trophoblast and the possibility of drug-drug interactions must be considered to optimize pharmacotherapy during pregnancy. In this review we summarize current knowledge on the expression and function of ABC and SLC transporters in the trophoblast. Furthermore, we put this data into context with medical conditions that require maternal and/or fetal treatment during pregnancy, such as gestational diabetes, HIV infection, fetal arrhythmias and epilepsy. Proper understanding of the role of placental transporters should be of great interest not only to clinicians but also to pharmaceutical industry for future drug design and development to control the degree of fetal exposure.

ATP-binding cassette efflux transporters in human placenta

Pregnant women are often complicated with diseases including viral or bacterial infections, epilepsy, hypertension, or pregnancy-induced conditions such as depression and gestational diabetes that require treatment with medication. In addition, substance abuse during pregnancy remains a major public health problem. Many drugs used by pregnant women are off label without the necessary dose, efficacy, and safety data required for rational dosing regimens of these drugs. Thus, a major concern arising from the widespread use of drugs by pregnant women is the transfer of drugs across the placental barrier, leading to potential toxicity to the developing fetus. Knowledge regarding the ATP-binding cassette (ABC) efflux transporters, which play an important role in drug transfer across the placental barrier, is absolutely critical for optimizing the therapeutic strategy to treat the mother while protecting the fetus during pregnancy. Such transporters include P-glycoprotein (P-gp, gene symbol ABCB1), the breast cancer resistance protein (BCRP, gene symbol ABCG2), and the multidrug resistance proteins (MRPs, gene symbol ABCCs). In this review, we summarize the current knowledge with respect to developmental expression and regulation, membrane localization, functional significance, and genetic polymorphisms of these ABC transporters in the placenta and their relevance to fetal drug exposure and toxicity.

Pseudomonas putida KT2440 genome update by cDNA sequencing and microarray transcriptomics

Pseudomonas putida KT2440 is a completely sequenced biosafety strain that has retained its capability to survive and function in the environment. The global mRNA expression profiles of the KT2440 strain grown at 10°C and 30°C were determined by deep cDNA sequencing to refine the genome annotation. Transcriptome sequencing identified 36 yet unknown small non-coding RNAs, 143 novel ORFs in 106 intergenic regions, 42 unclassified genes and eight highly expressed leaderless mRNA transcripts. The genome coordinates of eight genes and the organization of 57 operons were corrected. No overrepresented sequence motifs were detected in the 5'-untranslated regions. The 50 most highly expressed genes made up 60% of the total mRNA pool. Comparison of cDNA sequencing, Affymetrix and Progenika microarray data from the same mRNA preparation revealed a higher sensitivity and specificity of cDNA sequencing, a relatively poor correlation between the normalized cDNA reads and microarray signal intensities, and a systematic signal-dependent bias of microarrays in the detection of differentially regulated genes. The study demonstrates the power of next-generation cDNA sequencing for the quantitation of mRNA transcripts and the refinement of bacterial genome annotation.

Quantification of CFTR transcripts

Quantification and analysis of CFTR transcripts is of crucial importance not only for cystic fibrosis (CF) diagnosis and prognosis, but also in evaluating the efficiency of various therapeutic approaches to CF, including gene therapy. Reverse transcription (RT) followed by quantitative polymerase chain reaction (qPCR) is at present the most sensitive method for transcript abundance measurement. Classical RNA-based methods require significant expression levels in target samples for appropriate analysis, thus PCR-based methods have evolved towards reliable quantification. In this chapter we describe and discuss several protocols for the quantitative analysis of CFTR transcripts, including those variants that result from alternative splicing.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

Multiple roles of Pseudomonas aeruginosa TBCF10839 PilY1 in motility, transport and infection

Polymorphonuclear neutrophils are the most important mammalian host defence cells against infections with Pseudomonas aeruginosa. Screening of a signature tagged mutagenesis library of the non-piliated P. aeruginosa strain TBCF10839 uncovered that transposon inactivation of its pilY1 gene rendered the bacterium more resistant against killing by neutrophils than the wild type and any other of the more than 3000 tested mutants. Inactivation of pilY1 led to the loss of twitching motility in twitching-proficient wild-type PA14 and PAO1 strains, predisposed to autolysis and impaired the secretion of quinolones and pyocyanin, but on the other hand promoted growth in stationary phase and bacterial survival in murine airway infection models. The PilY1 population consisted of a major full-length and a minor shorter PilY1* isoform. PilY1* was detectable in small extracellular quinolone-positive aggregates, but not in the pilus. P. aeruginosa PilY1 is not an adhesin on the pilus tip, but assists in pilus biogenesis, twitching motility, secretion of secondary metabolites and in the control of cell density in the bacterial population.

Expression of ATP-binding cassette multidrug transporters in the giant liver fluke Fasciola gigantica and their possible involvement in the transport of bile salts and anthelmintics

ATP-binding cassette (ABC) transporters belong to one of the largest protein families that either import or export a wide spectrum of different substrates. Certain members of this superfamily have been implicated in multidrug resistance in various types of cancer as well as in pathogenic microorganisms. The role of ABC proteins in parasitic multidrug resistance becomes increasingly evident. However, studies on ABC transporters in helminths have been limited to MDR1 and MRP orthologues. In the present study, we reported, for the first time, the expression and localization of ABC proteins including orthologues of MDR1, MRP1, BCRP, and BSEP in the giant liver fluke Fasciola gigantica. Furthermore, the functional activities of these ABC transporters were characterized in isolated fluke cells using a fluorescent substrate, rhodamine. The results revealed the inhibition of rhodamine efflux by cyclosporin A, a potent inhibitor of ABC transporters. Interestingly, our data suggested that these proteins might play a role in the export of bile salts, in particular, taurocholate. Although, we did not observe any substantial changes in rhodamine transport in the presence of anthelmintics under experimental conditions, however, our findings altogether shed light on the possible involvement of several members of ABC proteins in the mechanism of drug resistance as well as detoxification process in helminths to survive inside their hosts.

Intranasal delivery: physicochemical and therapeutic aspects

Interest in intranasal (IN) administration as a non-invasive route for drug delivery continues to grow rapidly. The nasal mucosa offers numerous benefits as a target issue for drug delivery, such as a large surface area for delivery, rapid drug onset, potential for central nervous system delivery, and no first-pass metabolism. A wide variety of therapeutic compounds can be delivered IN, including relatively large molecules such as peptides and proteins, particularly in the presence of permeation enhancers. The current review provides an in-depth discussion of therapeutic aspects of IN delivery including consideration of the intended indication, regimen, and patient population, as well as physicochemical properties of the drug itself. Case examples are provided to illustrate the utility of IN dosing. It is anticipated that the present review will prove useful for formulation scientists considering IN delivery as a delivery route.

Diversity of the abundant pKLC102/PAGI-2 family of genomic islands in Pseudomonas aeruginosa

The known genomic islands of Pseudomonas aeruginosa clone C strains are integrated into tRNA(Lys) (pKLC102) or tRNA(Gly) (PAGI-2 and PAGI-3) genes and differ from their core genomes by distinctive tetranucleotide usage patterns. pKLC102 and the related island PAPI-1 from P. aeruginosa PA14 were spontaneously mobilized from their host chromosomes at frequencies of 10% and 0.3%, making pKLC102 the most mobile genomic island known with a copy number of 30 episomal circular pKLC102 molecules per cell. The incidence of islands of the pKLC102/PAGI-2 type was investigated in 71 unrelated P. aeruginosa strains from diverse habitats and geographic origins. pKLC102- and PAGI-2-like islands were identified in 50 and 31 strains, respectively, and 15 and 10 subtypes were differentiated by hybridization on pKLC102 and PAGI-2 macroarrays. The diversity of PAGI-2-type islands was mainly caused by one large block of strain-specific genes, whereas the diversity of pKLC102-type islands was primarily generated by subtype-specific combination of gene cassettes. Chromosomal loss of PAGI-2 could be documented in sequential P. aeruginosa isolates from individuals with cystic fibrosis. PAGI-2 was present in most tested Cupriavidus metallidurans and Cupriavidus campinensis isolates from polluted environments, demonstrating the spread of PAGI-2 across habitats and species barriers. The pKLC102/PAGI-2 family is prevalent in numerous beta- and gammaproteobacteria and is characterized by high asymmetry of the cDNA strands. This evolutionarily ancient family of genomic islands retained its oligonucleotide signature during horizontal spread within and among taxa.

Splicing modulation as a modifier of the CFTR function

A significant fraction of CF-causing mutations affects pre-mRNA splicing. These mutations can generate both aberrant and correct transcripts, the level of which varies among different patients. An inverse correlation was found between this level and disease severity, suggesting a role for splicing regulation as a genetic modifier. Subsequent studies showed that overexpression of splicing factors modulated the level of correctly spliced RNA, transcribed from minigenes carrying CF-causing splicing mutations. Overexpression of splicing factors also modulated the level of normal CFTR transcripts, transcribed from the endogenous CFTR allele carrying splicing mutations, in CF-derived epithelial cells. Several of the factors promoted higher level of correct CFTR transcripts. The increased level of normal transcripts led to activation of the CFTR channel and restoration of its function. Restoration was also obtained by sodium butyrate, a histone deacetylase inhibitor, known to up-regulate the expression of splicing factors. These results highlight the role of the splicing machinery as a modifier of disease severity in patients carrying splicing mutations and shed a new light on the therapeutic potential of splicing modulation for genetic diseases caused by splicing mutations.

Reduced splicing efficiency induced by synonymous substitutions may generate a substrate for natural selection of new splicing isoforms: the case of CFTR exon 12

Alternative splicing has been associated with increased evolutionary changes and with recent exon creation or loss. The addition of a new exon can be explained by its inclusion in only a fraction of the transcripts leaving the original form intact and giving to the new form the possibility to evolve independently but the exon loss phenomenon is less clear. To explore the mechanism that could be involved in CFTR exon 12 lower splicing efficiency in primates, we have analyzed the effect of multiple synonymous variations. Random patterns of synonymous variations were created in CFTR exon12 and the majority of them induced exon inclusion, suggesting a suboptimal splicing efficiency of the human gene. In addition, the effect of each single synonymous substitution on splicing is strongly dependent on the exonic context and does not correlate with available in silico exon splicing prediction programs. We propose that casual synonymous substitutions may lead to a reduced splicing efficiency that can result in a variable proportion of exon loss. If this phenomenon happens in in-frame exons and to an extent tolerated by the cells it can have an important evolutionary effect since it may generate a substrate for natural selection of new splicing isoforms.

Loss of Kv and MaxiK currents associated with increased MRP1 expression in small cell lung carcinoma

Regulatory volume decrease and exocrine secretion studies suggest a functional relationship between K+ and organic anion efflux. To test the hypothesis that the expression of K+ channels and MRP1 is reciprocally related, we employed the patch clamp and RT-PCR techniques on weakly (H69) and strongly MRP1-expressing (H69AR) small cell lung cancer cells. H69AR cells do not express the time- and voltage-dependent delayed rectifying K+ current (Kv) reported earlier in H69 cells and confirmed here. About 80% of the Kv current in H69 cells inactivated at 0 mV, allowing us to identify other K+ currents present in these cells. Whole-cell currents from cells dialyzed and bathed in K-gluconate as the major ions exhibited inward rectification in both cell types. Inwardly rectifying (Kir) currents in both H69 and H69AR cells showed time-dependent activation and slow inactivation at large negative potentials. H69 cells also express a threefold larger Ca2+ -stimulated K+ -selective and iberiotoxin-sensitive current relative to H69AR cells. In excised inside-out patches exposed to 145 mM symmetrical K+ solutions, H69 cells expressed a voltage- and Ca2+ -sensitive large conductance (128 +/- 5 pS) K+ channel (MaxiK). MaxiK-like currents were not observed at the whole-cell or single-channel level in H69AR cells. RT-PCR identified MaxiKalpha transcripts in H69 but not H69AR cells. These results indicate that two K+ currents (MaxiK and Kv) and the organic anion transporter MRP1 are reciprocally expressed in H69 and H69AR cells.

P-glycoprotein in the placenta: Expression, localization, regulation and function

Detailed understanding of the mechanisms employed in transfer of drugs across the placenta is essential for optimization of pharmacotherapy during pregnancy. Disclosure of drug efflux transporters as an "active component" of the placental barrier has brought new important insights into the field of transplacental pharmacokinetics. P-glycoprotein (P-gp, MDR1) is the first discovered and so far the best characterized of drug efflux transporters, whose role in the regulation of drug disposition to the fetus has been extensively studied. Expression of P-gp in the placental trophoblast layer was confirmed at the mRNA and protein levels in all phases of pregnancy, and several in vitro and in vivo studies demonstrated functional activity of the transporter in materno-fetal drug transport. P-gp is able to actively pump drugs and other xenobiotics from trophoblast cells back to the maternal circulation, providing thus protection to the fetus. This review summarizes the current knowledge on the expression, localization and function of P-gp in the placenta. In addition, we include the latest data concerning transcriptional regulation of placental P-gp expression and polymorphisms of the MDR1 gene. Clinical significance of placental P-gp and its future perspectives for pharmacotherapy during pregnancy are also discussed.

Transport of anti-allergic drugs across the passage cultured human nasal epithelial cell monolayer

The purpose of this study was to investigate the nasal absorption characteristics of a series of anti-allergic drugs across the human nasal epithelial cell monolayer, which was passage cultured by the liquid-covered culture (LCC) method on Transwell. Characterization of this cell culture model was achieved by bioelectric measurements and morphological studies. The passages 2--4 of cell monolayers exhibited the TEER value of 1731+/-635 Omega cm(2) after 2 days of seeding and maintained high TEER value for 4--6 days. Morphological study by TEM and SEM showed the existence of the tight junctions, and the cuboidal shaped epithelial cells monolayer. A series of anti-allergic drugs, albuterol hemisulfate, albuterol, fexofenadine HCl, dexamethasone, triamcinolon acetonide, and budesonide were selected as model compounds for transport studies. All the drugs were assayed using reversed-phase HPLC under isocratic conditions. Results indicated that within the logP (apparent 1-octanol/water partition coefficient) range from --1.58 (albuterol) to 3.21 (budesonide), there existed 100-fold difference in the apparent permeability coefficients (P(app)). A log-linear relationship was shown between the drug logP and the P(app) across passaged human nasal epithelial monolayers. The amount of fexofenadine HCl and dexamethasone across passaged human nasal cell monolayers was concentration-dependent in the direction of apical to basolateral. The direction dependent transport studies were investigated among all these drugs and no significant difference in the two directions was observed. In conclusion, this LCC passaged human nasal epithelial culture model may be a useful in vitro model for studying the passive transport processes in nasal drug delivery.

Spontaneous rescue from cystic fibrosis in a mouse model

BACKGROUND

From the original CftrTgH(neoim)Hgu mutant mouse model with a divergent genetic background (129P2, C57BL/6, MF1) we have generated two inbred CftrTgH(neoim)Hgu mutant strains named CF/1-CftrTgH(neoim)Hgu and CF/3-CftrTgH(neoim)Hgu, which are fertile and show normal growth and lifespan. Initial genome wide scan analysis with microsatellite markers indicated that the two inbred strains differed on the genetic level. In order to further investigate whether these genetic differences have an impact on the disease phenotype of cystic fibrosis we characterised the phenotype of the two inbred strains.

RESULTS

Reduced amounts, compared to wild type control animals, of correctly spliced Cftr mRNA were detected in the nasal epithelia, lungs and the intestine of both inbred CftrTgH(neoim)Hgu strains, with higher residual amount observed for CF/1-CftrTgH(neoim)Hgu than CF/3-CftrTgH(neoim)Hgu for every investigated tissue. Accordingly the amounts of wild type Cftr protein in the intestine were 9% for CF/1-CftrTgH(neoim)Hgu and 4% for CF/3-CftrTgH(neoim)Hgu. Unlike the apparent strain and/or tissue specific regulation of Cftr mRNA splicing, short circuit current measurements in the respiratory and intestinal epithelium revealed that both strains have ameliorated the basic defect of cystic fibrosis with a presentation of a normal electrophysiology in both tissues.

CONCLUSION

Unlike the outbred CftrTgH(neoim)Hgu insertional mouse model, which displayed the electrophysiological defect in the gastrointestinal and respiratory tracts characteristic of cystic fibrosis, both inbred CftrTgH(neoim)Hgu strains have ameliorated the electrophysiological defect. On the basis of these findings both CF/1-CftrTgH(neoim)Hgu and CF/3-CftrTgH(neoim)Hgu offer an excellent model whereby determination of the minimal levels of protein required for the restoration of the basic defect of cystic fibrosis can be studied, along with the modulating factors which may affect this outcome.

P-glycoprotein expression increases ATP release in respiratory cystic fibrosis cells

P-glycoprotein (Pgp) is a well-defined ATP-binding cassette (ABC) protein and a close relative of cystic fibrosis transmembrane conductance regulator (CFTR), whose dysfunction causes cystic fibrosis (CF). It is postulated that Pgp can complement deficient CFTR functions because of structural and functional homologies. One of the most relevant functions appears to be the regulation of ATP release, which influences mucociliary clearance in respiratory epithelia by nucleotide receptor stimulation. However, mechanisms involved in ATP secretion remain a controversial issue. In the present study, CF epithelial cells (sigmaCFTE29ó) were transduced with the retroviral vector MP1m encoding Pgp, and thus, a stable Pgp-overexpressing CF cell line (sigmaCFTE29óPgp) was established and used for studies of hypothesized CFTR complementation. In addition, overexpression of native Pgp in sigmaCFTE29ó could also be achieved by long-term treatment with colchicine, a drug, which may be of great interest in CF therapy. We confirmed that overexpression of Pgp causes a significant increase in cellular ATP release, which could even be enhanced by stimulation with hypoosmolar medium. A potential clinical benefit is discussed.

Molecular pathology of the CFTR locus in male infertility

Congenital bilateral absence of the vas deferens (CBAVD) is a form of infertility with an autosomal recessive genetic background in otherwise healthy males. CBAVD is caused by cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations on both alleles in approximately 80% of cases. Striking CFTR genotypic differences are observed in cystic fibrosis (CF) and in CBAVD. The 5T allele is a CBAVD mutation with incomplete penetrance. Recent evidence confirmed that a second polymorphic locus exists and is a major CFTR modifier. The development of minigene models have led to results suggesting that CFTR exon 9 is skipped in humans because of unusual suboptimal 5' splice sites. An extremely rare T3 allele has been reported and it has recently been confirmed that the T3 allele dramatically increases exon 9 skipping and should be considered as a 'CF' mutation. Routine testing for the most prevalent mutations in the CF Caucasian population will miss most CFTR gene alterations, which can be detected only through exhaustive scanning of CFTR sequences. Finally, a higher than expected frequency of CFTR mutations and/or polymorphisms is now found in a growing number of monosymptomatic disorders, which creates a dilemma for setting nosologic boundaries between CF and diseases related to CFTR.

P-glycoprotein expression and distribution in the rat placenta during pregnancy

P-glycoprotein (P-gp) is a drug efflux transporter that limits the entry of various potentially toxic drugs and xenobiotics into the fetus and is thus considered a placental protective mechanism. In this study, P-gp expression was investigated in the rat chorioallantoic placenta over the course of pregnancy. Three methods have been employed: real-time RT-PCR, western blotting and immunohistochemistry. The expression of mdr1a and mdr1b genes was demonstrated as early as on the 11th gestation day (gd) and increased with advancing gestation. Western blotting analysis revealed the presence of P-gp in the rat placenta starting from gd 13 onwards. P-gp was localized in the developing labyrinth zone of the placenta on gd 13; from gd 15 up to the term P-gp was seen as a dot like continuous line in the syncytiotrophoblast layers. Our data confirm the presence of P-gp in the rat chorioallantoic placenta starting soon after its development, which may signify the involvement of P-gp in transplacental pharmacokinetics during the whole period of placental maturing.

Quantitative methods for the analysis of CFTR transcripts/splicing variants

In cystic fibrosis (CF), transcript analysis and quantification are important for diagnosis, prognosis and also as surrogate markers for some therapies including gene therapy. Classical RNA-based methods require significant expression levels in target samples for appropriate analysis, thus PCR-based methods are evolving towards reliable quantification. Various protocols for the quantitative analysis of CFTR transcripts (including those resulting from splicing variants) are described and discussed here.

Mutation analysis of the Nijmegen breakage syndrome gene (NBS1) in nineteen patients with acute myeloid leukemia with complex karyotypes

The chromosomal instability disorder Nijmegen Breakage Syndrome (NBS) is caused by germ line mutations in the NBS1 gene. It is associated with immune deficiency, cellular hypersensitivity to ionizing radiation, and high susceptibility to lymphoid malignancies due to a defect in DNA double strand break repair. Since genetic instability has been discussed as a cause in acute myeloid leukemia (AML) with complex chromosomal aberrations, mutations in the NBS1 gene might be found in this AML subgroup. In this study, we analyzed 19 patients with AML and complex chromosomal aberrations for mutations in the NBS1 gene. Tumor DNA was analyzed by dHPLC analysis and all amplicons showing shifts were directly sequenced. One sample was found to be heterozygous for a novel 5 bp deletion in intron 12 (IVS12-53del5). By RT-PCR analysis the expected transcript and an additional faint product with skipped exon 13 was observed, indicative of aberrant splicing. This exon codes for part of the binding site of the NBS1 gene product, nibrin, to MRE11. However, we also found that all controls showed this phenomenon. Thus, the IVS12-53del5 is not responsible for the skipping of exon 13 and most probably represents a rare polymorphism. We found no further NBS1 mutations among the AML samples. Although the number of the analyzed samples is small, our study indicates that NBS1 mutations are not common in AML with a complex karyotype.

Examination of the functional activity of P-glycoprotein in the rat placental barrier using rhodamine 123

Rhodamine 123 (Rho123), a model substrate of P-glycoprotein (P-gp), was used to evaluate the functional activity of P-gp efflux transporter in the rat placental barrier. The dually perfused rat-term placenta method was used. In our experiments, the materno-fetal transplacental passage of Rho123 did not meet the criteria of the first-order pharmacokinetics, suggesting an involvement of transporter-mediated process. Inhibitors of P-gp, such as [3'-keto-Bmt1]-[Val2]-cyclosporine (PSC833), cyclosporine (CsA), quinidine, and chlorpromazine, increased significantly the materno-fetal transplacental passage of Rho123 in the experiments under steady-state conditions. On the other hand, PSC833, CsA, and quinidine decreased the feto-maternal passage of Rho123. Similarly, in the experiments carried out under nonsteady-state conditions, CsA accelerated the passage of Rho123 in the materno-fetal direction and decreased its passage in the opposite direction. Feto-maternal transplacental clearances of Rho123 were found to be considerably higher than those in the materno-fetal course. Potent P-gp inhibitors, such as PSC833 or CsA, partially canceled the asymmetry. Negligible metabolism of Rho123 into its major demethylated metabolite rhodamine 110 was observed in the rat placenta. Expression of P-gp genes was detected using immunohistochemical, Western blotting, and reverse transcription-polymerase chain reaction methods preferentially in the second rat syncytiotrophoblast layer. In conclusion, these data suggest that P-gp limits the entry of Rho123 into fetuses and at the same time it accelerates the feto-maternal elimination of the model compound. Therefore, it seems plausible that pharmacokinetics of xenobiotics in the rat placental barrier could be controlled by P-gp in both directions.

Functional analysis of CFTR chloride channel activity in cells with elevated MDR1 expression

Using the patch-clamp method, we investigated a relationship between MDR1 expression and its effects on the CFTR channel function. Incubation of CaCo-2 cells with increasing concentrations of doxorubicin resulted in a reduction of CFTR chloride channel activity in a dose-dependent manner. This reduction was associated with a decrease of CFTR mRNA and simultaneous up-regulation of MDR1 mRNA in the presence of doxorubicin. Similar alteration of the CFTR function was observed in CaCo-2 cells transiently overexpressing MDR1. No alterations of the cAMP-dependent chloride currents were observed in COS-1 cells transiently co-expressing CFTR and MDR1 from strong CMV promoters. This indicated that repression of CFTR by MDR1 induction requires the presence of the native CFTR promoter.

Can the enhanced renal clearance of antibiotics in cystic fibrosis patients be explained by P-glycoprotein transport?

PURPOSE

To investigate in vitro if P-glycoprotein (P-gp) transport can differentiate between antibiotic drugs exhibiting increased active renal clearance (CL(r)) in cystic fibrosis (CF) patients (i.e., dicloxacillin, trimethoprim) and drugs that do not exhibit this phenomenon (i.e.. cefsulodin, sulfamethoxazole).

METHODS

Transport studies were carried out in MDCK (wild type) and MDR1-MDCK (P-gp overexpressing) cells that were grown to confluence on Transwell inserts. [14C]-mannitol transport and transepithelial electrical resistance (TEER) were measured to validate the integrity of the cells. Drug concentrations were analyzed using HPLC.

RESULTS

Dicloxacillin and trimethoprim are substrates of P-gp (B-->A/A-->B ratios in MDR1-MDCK cells are 32 and 50, respectively). P-gp inhibitors (i.e., GG918, cyclosporine, ketoconazole, vinblastine) decreased the B-->A transport of dicloxacillin and trimethoprim and increased the A-->B transport of trimethoprim while non-P-gp inhibitors (e.g., PAH) had no effect. In contrast, cefsulodin and sulfamethoxazole are not substrates of P-gp (B-->sA/A-->B values in MDCK and MDR1-MDCK cells are -1).

CONCLUSIONS

Our in vitro studies suggest that P-glycoprotein may play a role in increasing renal clearance of drug substrates in CF patients. Dicloxacillin and trimethoprim. which are both substrates of P-gp, show increased active renal clearance in CF patients while cefsulodin and sulfamethoxazole, which are not P-gp substrates, do not show increased active renal clearance in CF patients.

Influence of P-glycoprotein on the transplacental passage of cyclosporine

The transfer kinetics of cyclosporine across the dually perfused rat placenta in the maternal to fetal direction and a possible involvement of P-glycoprotein were investigated. The transplacental clearance of cyclosporine in the materno-fetal direction was found to be dependent on the maternal inflow concentration of cyclosporine. Coadministration of cyclosporine with an excess of quinidine or chlorpromazine into the maternal compartment revealed 1.7- and 1.9-fold increase in cyclosporine concentration in the fetal compartment. In the experiments where quinidine was present both in the maternal and fetal compartments, cyclosporine appeared in the fetal compartment significantly faster, and its amount was three times higher when compared with controls. Conversely, quinidine or chlorpromazine did not affect the transplacental passage of L-[(3)H]-glucose. The interference of quinidine with the metabolism of cyclosporine in the placenta was excluded because only traces of M-1 and M-17 metabolites were found in the fetal solutions. Sodium azide, a mitochondrial respiratory inhibitor, was found to double the rate of cyclosporine, but not L-[(3)H]-glucose, passage across the placenta. Our findings indicate that P-glycoprotein pumps cyclosporine out of the trophoblast cells of the rat placenta in the ATP-dependent manner and restricts the passage of cyclosporine across the placental barrier.

Identification of a splice site mutation (2789 +5 G > A) associated with small amounts of normal CFTR mRNA and mild cystic fibrosis

A splicing mutation was identified at the +5 position of the splice donor site of exon 14b of CFTR in CF patients in a consanguineous family that is remarkable for unusually mild disease. Quantitative studies of nasal epithelial mRNA revealed that homozygotes for the spice site mutation produced approximately 4% of the normal amount of normally-spliced CFTR. We propose that this small amount of normally spliced mRNA is associated with synthesis of some normal CFTR protein, and accounts for the mild phenotype. Further characterization of epithelial function and clinical phenotype in patients bearing this form of mutation, termed a type V mutation, will be useful in determining the level of CFTR associated with amelioration of lung disease.

Multidrug-resistance transporters

P-glycoprotein was initially isolated due to its role in multidrug resistance to cancer chemotherapeutics. Recent work, however, makes it increasingly apparent that this transporter is also involved in the pharmacokinetics of many drugs. P-gp is strategically expressed in the luminal epithelial cells of organs often associated with drug absorption and disposition, for example, hepatocyte canalicular membrane, renal proximal tubules, and the intestinal mucosa. P-gp is also expressed in the endothelial cells comprising the blood-brain barrier. This localization clearly suggests the potential for this protein to serve as a protective mechanism against entry of toxic xenobiotics and also suggests that P-gp is well situated to participate in the removal of therapeutic agents. Numerous investigations with drugs such as digoxin, etoposide, cyclosporine, vinblastine, Taxol, loperamide, dom-peridone, and ondansteron demonstrate that P-gp has an important role in determining the pharmacokinetics of substrate drugs. Pharmacological modulation of P-gp function to increase drug bioavailability, both on a organismal and a cellular level, is one approach currently being explored to enhance therapeutic effectiveness. This approach is not without potential collateral consequences given the wide tissue distribution of P-gp. While animals deficient in P-gp are viable and without obvious abnormalities, the pharmacokinetics and toxic consequences of several compounds are significantly altered in these animals. Thus blockade of the protective P-gp barrier in humans may have adverse effects on substrate drugs. In particular, this situation may arise when several compounds which may be substrates compete for P-gp-mediated transport. Additional multidrug transporters, notably MRP and family members, have been identified and may also determine the fate of pharmaceuticals. Further understanding the physiological role of each of the multidrug transporters is critical for determining their role in pharmacokinetics and for evaluating the consequences of modification of their activities. Such information is also important in the development of novel drugs which may be substrates for these transporters.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Retroviral transfer of human MDR1 gene to hematopoietic cells: effects of drug selection and of transcript splicing on expression of encoded P-glycoprotein

Protection of hematopoietic cells of patients undergoing anticancer chemotherapy by MDR1 gene transfer is currently being studied in clinical trials. From animal studies, it has been suggested that aberrant splicing due to cryptic donor and acceptor sites in the MDR1 cDNA could be a major reason for failure to obtain high-level expression of P-glycoprotein in bone marrow. We investigated effects of drug selection on protein expression levels and on splicing of MDR1 transcripts in murine bone marrow cells (BMCs) in vitro. To this end, retroviruses were generated through an identical plasmid, pHaMDR1/A, introduced into different packaging cells. GP + E86- but not PA317-derived producer cells were found to express truncated in addition to full-length message. In BMCs transduced with GP + E86-derived viruses, both messages were increased after treatment with colchicine or daunomycin. Similar results were obtained with NIH 3T3 fibroblasts. However, transduced and drug-selected BMCs displayed the spliced transcript even if the respective PA317-derived producer cells contained no truncated RNA as detected in transduced NIH 3T3 fibroblasts. Short-term drug selection in BMCs transduced with either ecotropic or amphotropic retroviruses resulted in a striking increase in P-glycoprotein expression. Thus, aberrant splicing failed to abrogate P-glycoprotein expression in BMCs. We also studied a vector in which MDR1 was coexpressed with glucocerebrosidase, using an internal ribosomal entry site. Although chemoprotection was less efficient than with pHaMDR1/A, augmentation of protein expression was observed at low selecting drug concentrations. Our study shows that drug selection can partially compensate for inefficient transduction of hematopoietic cells, and may help to develop strategies by which unstable expression of transduced genes can be overcome.

Sites of mRNA expression of the cystic fibrosis (CF) and multidrug resistance (MDR1) genes in the human placenta of early pregnancy: No evidence for complementary expression

The aims of this study were to establish the sites of mRNA expression of both the cystic fibrosis (CF) and multidrug resistance (MDR1) genes in human placental sections from early pregnancy (first, early and mid-second trimesters). Riboprobes specific for each of these two genes were generated and used for in situ hybridization experiments. The results show parallel mRNA expression for the CF and MDR1 genes, with the signal detected in the syncytiotrophoblast and cytotrophoblast cells of the placental villi. Other cell types within the villous core were negative. Similar results were obtained at all stages of pregnancy studied.

Regulation and expression of multidrug resistance (MDR) transcripts in the intestinal epithelium

A paucity of information exists on the regulation of gene expression in the undifferentiated intestine. The intestinal epithelium is one of the few normal tissues expressing the multidrug resistance (MDR) genes that confer the multidrug resistant phenotype to a variety of tumours. Expression of mdr1a has been observed in the primitive rat intestinal epithelial cell line, IEC-18. It is hypothesized that characterization of MDR gene expression in IEC-18 cells will provide insight into gene regulation in undifferentiated intestinal cells. A series of hamster mdr1a promoter deletion constructs was studied in IEC-18 and a region with 12-13-fold enhancer activity was identified. This region was shown to function in an orientation- and promoter context-independent manner, specifically in IEC-18 cells. Unexpectedly, Northern probing revealed a greater expression of mdr1b than mdr1a in IEC-18 cells. A quantitative reverse transcription polymerase chain reaction assay was used to compare the relative expression of MDR genes in IEC cells, fetal intestine, and in the undifferentiated and differentiated components of adult intestinal epithelium. MDR transcript levels in IEC cells were found to resemble those of fetal intestine and small intestinal crypts, where a conversion from mixed mdr1a/mdr1b to predominantly mdr1a expression occurs as cells mature. This work describes two contributions to the field of gene regulation in the undifferentiated intestine--first, the initial characterization of a putative mdr1a enhancer region with specificity for primitive intestinal cells and secondly, the first report of mdr1b detection in the intestine and its expression in primitive cell types.

The genetics of male infertility in relation to cystic fibrosis

Absence, dysfunction or low levels of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein result in a broad range of clinical manifestations with CF with pancreatic insufficiency at the severe end of the phenotypic spectrum and, at the other end relatively mild clinical conditions, including several forms of male infertility. The condition of congenital bilateral absence of the vas deferens (CBAVD) is in 75-80% of the patients associated with defects in the CFTR gene. In the remaining patients, CBAVD is also associated with urinary tract malformations, and this form of CBAVD is not related to CF. Congenital unilateral absence of the vas deferens also seems to be associated with CF except when associated with renal abnormalities at the ipsilateral side of the absent vas. A possible role of the CFTR protein in the aetiology of infertility due to defects in sperm production and maturation has also been suggested recently. In contrast, Young's syndrome is probably not related to CF. The relation between some conditions of male infertility and CF implies appropriate clinical examination of the patients, CFTR mutation analysis and genetic counselling. Because infertility can now in many cases be treated by artificial reproductive technology couples have an increased risk of having children with CF or infertility if the female partner is also a carrier of a CFTR mutation. Couples should be well informed about these risks and risk prevention including pre-implantation diagnosis. Follow-up studies of children born to these couples are mandatory, whether male infertility is linked to CF or not.

Polymorphic expression of multidrug resistance mRNA in lung parenchyma of nonpregnant and pregnant rats: a comparison to cystic fibrosis mRNA expression

Multidrug resistance (MDR1b) and cystic fibrosis transmembrane conductance regulator (CFTR) proteins are members of the "ATP-binding cassette" superfamily of transporters. They are associated with chloride channel activities and ATP secretion and have complementary patterns of expression in several organs. In the rat uterus, CFTR expression is replaced by MDR1b expression during pregnancy. We have studied whether expression of MDR1b and CFTR also vary in the lung during pregnancy. No variations in MDR1b or CFTR mRNA levels during pregnancy were detected. However, there was an unusual degree of variation in MDR1b mRNA expression in lung parenchyma between animals in both the control group and the pregnant group. If present among humans, polymorphic expression of MDR1 in lung parenchyma may explain part of the differences in lung symptomatology observed in the CF patients carrying the same mutation.

A delta F508 mutation in mouse cystic fibrosis transmembrane conductance regulator results in a temperature-sensitive processing defect in vivo

The most prevalent mutation (delta F508) in cystic fibrosis patients inhibits maturation and transfer to the plasma membrane of the mutant cystic fibrosis transmembrane conductance regulator (CFTR). We have analyzed the properties of a delta F508 CFTR mouse model, which we described recently. We show that the mRNA levels of mutant CFTR are normal in all tissues examined. Therefore the reduced mRNA levels reported in two similar models may be related to their intronic transcription units. Maturation of mutant CFTR was greatly reduced in freshly excised oviduct, compared with normal. Accumulation of mutant CFTR antigen in the apical region of jejunum crypt enterocytes was not observed, in contrast to normal mice. In cultured gallbladder epithelial cells from delta F508 mice, CFTR chloride channel activity could be detected at only two percent of the normal frequency. However, in mutant cells that were grown at reduced temperature the channel frequency increased to over sixteen percent of the normal level at that temperature. The biophysical characteristics of the mutant channel were not significantly different from normal. In homozygous delta F508 mice we did not observe a significant effect of genetic background on the level of residual chloride channel activity, as determined by the size of the forskolin response in Ussing chamber experiments. Our data show that like its human homologue, mouse delta F508-CFTR is a temperature sensitive processing mutant. The delta F508 mouse is therefore a valid in vivo model of human delta F508-CFTR. It may help us to elucidate the processing pathways of complex membrane proteins. Moreover, it may facilitate the discovery of new approaches towards therapy of cystic fibrosis.

The cystic fibrosis transmembrane conductance regulator mediates transepithelial fluid secretion by human autosomal dominant polycystic kidney disease epithelium in vitro

Transepithelial fluid secretion promotes the progressive enlargement of cysts in autosomal dominant polycystic kidney disease (ADPKD). Recent indirect evidence indicated that active chloride transport may drive net fluid secretion in cultures of epithelia derived from ADPKD cysts. We now report that forskolin, which stimulates adenylate cyclase, increased the efflux rate constant for 36Cl in monolayers of ADPKD cells in vitro from 0.23 +/- 0.02 min-1 to 0.44 +/- 0.05 min-1 (N = 4) and that diphenylamine 2-carboxylate (DPC), which blocks chloride channels, eliminated the forskolin-stimulated chloride efflux from these cells. To establish whether the cAMP-regulated chloride transporter, cystic fibrosis transmembrane conductance regulator (CFTR), may potentially be involved in the chloride transport and fluid secretion of ADPKD epithelia, we examined CFTR mRNA and protein in these cultures. Northern blot hybridization using a human (h) CFTR cDNA probe demonstrated the presence of an approximately 6.5 kb transcript in total RNA from polarized cultures of ADPKD, normal human kidney cortex (HKC), and T84 cells. Utilizing several antibodies to hCFTR, immunocytochemistry and confocal fluorescence microscopy localized an immunoreactive protein primarily in the apical region of forskolin-stimulated ADPKD cells grown on permeable supports. This immunoreactivity could be eliminated by preincubation of antibody with immunizing peptide. To determine the effect of CFTR abundance on the magnitude of net fluid secretion, polarized ADPKD cultures were treated with deoxyoligonucleotides that were either complementary (antisense), homologous (sense), or partially complementary (misantisense) to a sequence near the translation initiation site in hCFTR mRNA. Treatment with 5.0 microM antisense oligonucleotide resulted in a 73% reduction in forskolin-stimulated fluid secretion and a comparable reduction in the abundance of CFTR as detected by immunocytochemistry. By contrast, treatment with 5.0 microM sense oligonucleotide reduced fluid secretion by only 34% and had less of an effect on CFTR abundance, while the effects of 5.0 microM misantisense oligonucleotide on both fluid secretion and CFTR abundance were insignificant. On the basis of these results we suggest that CFTR is a major mediator of forskolin-stimulated chloride and fluid secretion by epithelial cells of human polycystic kidneys in vitro.

Chloride and potassium conductances of mouse pancreatic zymogen granules are inversely regulated by a approximately 80-kDa mdr1a gene product

Cl- and cation conductances were characterized in zymogen granules (ZG) isolated from the pancreas of wild-type mice (+/+) or mice with a homozygous disruption of the multidrug resistance P-glycoprotein gene mdr1a (-/-). Cl- conductance of ZG was assayed in isotonic KCl buffer by measuring osmotic lysis, which was induced by maximal permeabilization of ZG membranes (ZGM) for K+ with valinomycin due to influx of K+ through the artificial pathway and of Cl- through endogenous channels. To measure cation conductances, ZG (pHi 6.0-6.5) were suspended in buffered isotonic monovalent cation acetate solutions (pH 7.0). The pH gradient was converted into an outside-directed H+ diffusion potential by maximally increasing H+ conductance of ZGM with carbonyl cyanide m-chlorophenylhydrazone. Osmotic lysis of ZG was induced by H+ diffusion potential-driven influx of monovalent cations through endogenous channels and nonionic diffusion of the counterion acetate. ZGM Cl- conductances were not different in (-/-) and (+/+) mice (2.6 +/- 0.3 h-1 versus 3.1 +/- 0.2 h-1 (relative rate constant)). The nonhydrolyzable ATP analog adenosine 5'-(beta,gamma-methylene)triphosphate (AMP-PCP) (0.5 mM) activated the Cl- conductance both in (+/+) and (-/-) mice. However, activation of Cl- conductance by AMP-PCP was reduced in (-/-) mice as compared with (+/+) mice (5.0 +/- 0.4 h-1 versus 7.6 +/- 0.7 h-1; p < 0. 005). In contrast, ZGM K+ conductance was increased in (-/-) mice as compared with (+/+) mice (14.2 +/- 2.0 h-1 versus 8.5 +/- 1.2 h-1; p < 0.03). In the presence of 0.5 mm AMP-PCP, which completely blocks K+ conductance but leaves a nonselective cation conductance unaffected, there was no difference between (-/-) and (+/+) mice (5.3 +/- 0.7 h-1 versus 3.2 +/- 0.5 h-1). In Western blots of ZGM from wild-type mice, a polyclonal MDR1 specific antibody labeled a protein band of approximately 80 kDa. In mdr1a-deficient mice, the intensity of this band was reduced to 39 +/- 7% of the wild-type signal. This indicates that a mdr1a gene product of approximately 80 kDa enhances the AMP-PCP-activated fraction of mouse ZGM Cl- conductance and reduces AMP-PCP-sensitive K+ conductance.