Cystic fibrosis heterozygote resistance to cholera toxin in the cystic fibrosis mouse model

Ethnicity-related polymorphisms and haplotypes in the human ABCB1 gene

INTRODUCTION

The human multidrug resistance gene ATP-binding cassette B1 (ABCB1) codes for P-glycoprotein (P-gp), an important membrane-bound efflux transporter known to confer anticancer drug resistance as well as affect the pharmacokinetics of many drugs and xenobiotics. A number of single nucleotide polymorphisms (SNPs) have been identified throughout the ABCB1 gene that may have an effect on P-gp expression levels and function. Haplotype as well as genotype analysis of SNPs is becoming increasingly important in identifying genetic variants underlying susceptibility to human disease. Three SNPs, 1236C-->T, 2677G-->T and 3435C-->T, have been repeatedly shown to predict changes in the function of P-gp. The frequencies with which these polymorphisms exist in a population have also been shown to be ethnically related.

METHODS

In this study, 95 individuals representative of the entire ethnic make-up of the USA were compared with 101 individuals from an Ashkenazi-Jewish population. These individuals were analyzed by genomic sequencing and polymerase chain reaction, using restriction fragment length polymorphisms, to calculate their genotype frequencies.

RESULTS

A total of 25 SNPs were located in the exons of the ABCB1 gene. All of the polymorphisms identified were in parts of the ABCB1 gene product predicted to be intracellular, and 16 appear to be novel as compared with those listed by the National Center for Biotechnological Information. Frequencies of the 1236C-->T and 2677G-->T/A/C SNPs were similar for the US and Ashkenazi populations (64.2 and 60.4%, respectively for 1236C-->T [chi2: 0.30; p < or = 1]; 55.8 and 64.4%, respectively for 2677G-->T/A/C [chi2: 1.49; p < or = 1]), but were different for 3435C-->T (24.2% for the US population and 69.3% for the Ashkenazi population [chi2: 39.927; p < or = 0.001]). The 1236T/ 2677T/3435T haplotype occurred in 23.6% (standard error: 0.013) of the Ashkenazi population.

CONCLUSION

The SNP at location 3435C-->T plays a significant role in the ABCB1 gene. The haplotype and genotype analysis from these data may be used as a basis for studies on the relationship between ABCB1 genotypes and drug efficacy, drug toxicity, disease susceptibility or other phenotypes.

Genetic and geographical variability in cystic fibrosis: evolutionary considerations

We have studied cystic fibrosis from an evolutionary perspective by focusing on three main points: the high frequency of cystic fibrosis; the different spectrum of mutations that cause the disease in different areas of Europe; and the age of the delta F508 mutation. Several hypotheses have been suggested to explain the high frequency of cystic fibrosis. We argue that the most plausible is heterozygote resistance to cholera and to other dehydrating intestinal diseases. Different population sizes in the past could explain the higher diversity among cystic fibrosis mutations found in southern Europe. Three microsatellite loci are found close to the delta F508 mutation. The reconstruction of the original microsatellite haplotype in which delta F508 arose, and of the genealogy of the microsatellite haplotypes bearing the mutation, allowed us to date the mutation event at around 50,000 years ago.

Identification of new small molecule inhibitors of cystic fibrosis transmembrane conductance regulator protein: in vitro and in vivo studies

Cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cAMP-regulated chloride channel that has been proposed as a pharmacological target to reduce intestinal fluid loss in cholera. The aim of this study was to identify new CFTR inhibitors by high-throughput screening. Screening of 50,000 drug-like small molecules was performed using a cell-based assay of iodide influx in Fisher rat thyroid (FRT) cells co-expressing human CFTR and halide-sensitive yellow fluorescent protein (YFP-H148Q). Two new CFTR inhibitors, 2-[N-(3-hydroxy-4-carboxyphenyl) amino]-4-(4-methylphenyl)-thiazole (INH 1) and 1-acetyl-5-bromo-2,3-dihydro-N-(1,2,3,4-tetrahydro-1-naphthalenyl)-1H-Indole-7-sulfonamide (INH 2), were identified. They were then determined for potency, reversibility and specificity by electrophysiological methods, and for in vivo efficacy in mouse model of cholera toxin-induced intestinal fluid secretion. INH 1 and INH 2 reversibly inhibited cAMP-activated apical chloride current in FRT cells with Kis of 15 and 20 microM, respectively. Similarly, in short-circuit current analysis in human colonic epithelial cell lines (T84 cells), cAMP-activated chloride secretion was inhibited by INH 1 and INH 2 with Kis of 24.5 and 25.3 microM, respectively. Calcium-activated chloride secretion in the T84 cells was markedly inhibited by 100 microM of INH 1, but was unaffected by 100 microM of INH 2. In vivo studies in mice showed that a single intraperitoneal injection of INH 1 (3 mg/kg) reduced cholera toxin-induced intestinal fluid secretion by 40%, whereas INH 2 produced no effect. Our results indicate that INH 1 could be a new class candidate for a blocker of cholera toxin-induced intestinal fluid secretion as well as a CFTR inhibitor.

How similar are amino acid mutations in human genetic diseases and evolution

Accumulating evidence indicates that some deleterious mutations responsible for genetic diseases may offer benefits for human to prevent other diseases. Therefore, human genetic diseases and evolution were tentatively regarded as the two sides of the same coin, which stimulated our interest to explore how similar are amino acid mutations in human genetic diseases and evolution. Through a large-scale analysis on amino acid mutation patterns of genetic diseases and evolution of Hominidae (Homo sapiens and Pan troglodytes), it was found that there exist significant correlations between two mutation patterns. Besides, there also exist some evident differences between both mutations, especially those associated with four amino acids C, G, R, and L. These findings are of significance to understanding the subtle connections between human genetic diseases and evolution.

Differential expression of intestinal membrane transporters in cholera patients

Vibrio cholerae causes the cholera disease through secretion of cholera toxin (CT), resulting in severe diarrhoea by modulation of membrane transporters in the intestinal epithelium. Genes encoding membrane-spanning transporters identified as being differentially expressed during cholera disease in a microarray screening were studied by real-time PCR, immunohistochemistry and in a CaCo-2 cell model. Two amino acid transporters, SLC7A11 and SLC6A14, were upregulated in acute cholera patients compared to convalescence. Five other transporters were downregulated; aquaporin 10, SLC6A4, TRPM6, SLC23A1 and SLC30A4, which have specificity for water, serotonin (5-HT), magnesium, vitamin C and zinc, respectively. The majority of these changes appear to be attempts of the host to counteract the secretory response. Our results also support the concept that epithelial cells are involved in 5-HT signalling during acute cholera.

Cystic Fibrosis: Lessons from the Sweat Gland

Lessons from the sweat gland on cystic fibrosis (CF) began long before modern medicine became a science. In European folklore, the curse that “a child that taste salty when kissed will soon die” (Alonso y de los Ruyzes de Fonteca J. Diez Previlegios para Mugeres Prenadas. Henares, Spain, 1606) has been taken by many as a direct reference to cystic fibrosis [Busch R. Acta Univ Carol Med (Praha) 36: 13–15, 1990]. The high salt concentration in sweat from patients with CF is now accepted as almost pathognomonic with this fatal genetic disease, but the earliest descriptions of cystic fibrosis as a disease entity did not mention sweat or sweat glands (Andersen DH. Am J Dis Child 56: 344–399, 1938; Andersen DH, Hodges RG. Am J Dis Child 72: 62–80, 1946). Nonetheless, defective sweating soon became an inseparable, and major, component of the constellation of symptoms that diagnose “cystic fibrosis” (Davis PB. Am J Respir Crit Care Med 173: 475–482, 2006). The sweat gland has played a foremost role in diagnosing, defining pathophysiology, debunking misconceptions, and increasing our understanding of the effects of the disease on organs, tissues, cells, and molecules. The sweat gland has taught us much.

Potential role for the common cystic fibrosis DeltaF508 mutation in Crohn's disease

BACKGROUND

Inflammatory bowel disease (IBD) is an epithelial barrier disease that is thought to result from a dysregulated interaction with bacteria in the intestine of genetically predisposed individuals. The cystic fibrosis transmembrane conductance regulator (CFTR), which is mutated in the autosomal recessive disease cystic fibrosis, modulates gut permeability, mucus production, and epithelial interactions with bacteria. The cystic fibrosis DeltaF508 mutation is commonly found in the general population and has been shown to result in a reduced number of CFTR molecules at the surface of epithelial cells. Given the important biological functions of CFTR in the intestine, we tested whether this mutation is of relevance to IBD.

METHODS

Using DNA heteroduplex analysis, we investigated the distribution of DeltaF508 heterozygosity in 2568 subjects from three independent cohorts of Italian, Swedish, and Scottish IBD patients and controls.

RESULTS

In all three cohorts an association between DeltaF508 and Crohn's disease (CD) was observed. Specifically, DeltaF508 heterozygosity was markedly underrepresented in CD patients from Italy and Sweden (P = 0.021 and 0.027 versus controls, respectively), while stratification for disease location revealed an absence of DeltaF508 carriers among Scottish CD patients with right-sided colitis (P = 0.023 versus all other locations).

CONCLUSIONS

DeltaF508 heterozygosity might exert a protective effect in CD.

Nucleoside diphosphate kinase A as a controller of AMP-kinase in airway epithelia

This review integrates recent understanding of a novel role for NDPK-A in two related directions: Firstly, its role in an airway epithelial cell when bound to the luminal (apical) membrane and secondly in the cytosol of many different cells (epithelial and non-epithelial) where an isoform-specific interaction occurs with a regulatory partner, AMPKalpha1. Thus NDPK-A is present in both a membrane and cytosolic environment but in the apical membrane, its roles are not understood in detail; preliminary data suggest that it co-localises with the cystic fibrosis protein (CFTR). In cytosol, we find that NDPK-A is coupled to the catalytic alpha1 isoform of the AMP-activated protein kinase (AMPKalpha subunit), which is part of a heterotrimeric protein complex that responds to cellular energy status by switching off ATP-consuming pathways and switching on ATP-generating pathways when ATP is limiting. We find that ATP is located within this complex and 'fed' from NDPK to AMPK without ever 'seeing' bulk solution. Importantly, the reverse can also happen such that AMPK activity can be made to decline when NDPK-A 'steals' ATP from AMPK. Thus we propose a novel paradigm in NDPK-A function by suggesting that AMP-kinase can be regulated by NDPK-A, independently of AMP.

Evaluating candidate agents of selective pressure for cystic fibrosis

Cystic fibrosis is the most common lethal single-gene mutation in people of European descent, with a carrier frequency upwards of 2%. Based upon molecular research, resistances in the heterozygote to cholera and typhoid fever have been proposed to explain the persistence of the mutation. Using a population genetic model parameterized with historical demographic and epidemiological data, we show that neither cholera nor typhoid fever provided enough historical selective pressure to produce the modern incidence of cystic fibrosis. However, we demonstrate that the European tuberculosis pandemic beginning in the seventeenth century would have provided sufficient historical, geographically appropriate selective pressure under conservative assumptions. Tuberculosis has been underappreciated as a possible selective agent in producing cystic fibrosis but has clinical, molecular and now historical, geographical and epidemiological support. Implications for the future trajectory of cystic fibrosis are discussed. Our result supports the importance of novel investigations into the role of arylsulphatase B deficiency in cystic fibrosis and tuberculosis.

Cystic Fibrosis and Formes Frustes of CFTR-Related Disease

Cystic fibrosis (CF) is the commonest genetic cause of bronchiectasis in the Caucasian population. Since identification of the putative gene in 1989, the molecular basis of the condition has become clearer with characterisation of the unique pathophysiology. The small airways are the primary site of lung disease, with an intense but localised inflammatory picture, dominated by neutrophils. The clinical heterogeneity is explained to some degree by the distinct molecular consequences of the many mutations that have been recognised to affect the CF transmembrane conductance regulator (CFTR) gene; however other genes appear to modify the phenotype as well as environmental exposure. It has become increasingly apparent that certain conditions may result from CFTR dysfunction without fulfilling diagnostic criteria for CF. In some cases this may result in single organ disease for which the term CF (or CFTR)-related disease has been advocated. Congenital bilateral absence of the vas deferens is the most clearly characterised of these. In other cases where a mild CF phenotype is apparent, atypical CF is probably a better term. It remains unclear whether carrier status predisposes to certain conditions such as chronic rhinosinusitis or pancreatitis.

A new model of cystic fibrosis pathology: Lack of transport of glutathione and its thiocyanate conjugates

Many of the symptoms of cystic fibrosis are not explained by the current disease mechanisms. Therefore, the authors conducted an extensive literature review and present a new model of cystic fibrosis pathology, which is the culmination of this research. Understanding that the cystic fibrosis transmembrane conductance regulator (CFTR) is responsible for glutathione (GSH) transport, the authors hypothesize that mutations of the CFTR, which create abnormal GSH transport, will lead to aberrations of GSH levels in both the intracellular as well as the extracellular milieu. These alterations in normal cellular GSH levels affect the redox state of the cell, thereby affecting the intracellular stress protein, metallothionein. The authors describe how this disruption of the redox state caused by excess cellular GSH, will naturally prevent the delivery of zinc as a cofactor for various enzymatic processes, and how these disruptions in normal redox may cause alterations in both humoral and cell-mediated immunity. Moreover, the symptom of thick sticky mucus in these patients might be explained through the understanding that oversulfation of mucus is a direct result of elevated cellular GSH and cysteine. The issues of hyperinflammation, altered pH and the imbalance of fatty acids that are typical in cystic fibrosis are addressed-all of which may also be linked to disruptions in GSH homeostasis. Additionally, this new model of cystic fibrosis pathology, clarifies the relationship between the CFTR and the multi-drug resistance proteins, and the lack of cell-mediated immunity by predicting that the substrate of these proteins is a glutathione adduct of thiocyanate. Finally, a new therapeutic strategy by using isothiocyanates to rectify the GSH imbalance and restore the immune system is suggested for the treatment of cystic fibrosis patients.

Mannose-binding lectin and maladies of the bowel and liver

Mannose-binding lectin (MBL) is a pattern-recognition molecule that binds to characteristic carbohydrate motifs present on the surface of many different pathogens. MBL binding stimulates the immune system via the lectin pathway of complement activation. In certain clinical situations, often characterized by pre-existing immune compromise, MBL deficiency increases the risk of infectious and other disease-specific complications. Many of the key pathogenic processes inherent to common gastroenterological diseases, such as infection, immunological damage, and carcinogenesis, have been linked to MBL. This editorial reviews the biology of MBL, outlines key disease associations to document the breadth of influence of MBL, and finally, highlights the relevance of MBL to both gastroenterological health and disease.

Haemolysin produced by Vibrio mimicus activates two Cl- secretory pathways in cultured intestinal-like Caco-2 cells

Haemolysin (VMH) is a virulent factor produced by Vibrio mimicus, a human pathogen that causes diarrhoea. As intestinal epithelial cells are the primary targets of haemolysin, we investigated its effects on ion transport in human colonic epithelial Caco-2 cells. VMH increased the cellular short circuit current (Isc), used to estimated ion fluxes, and 125I efflux of the cells. The VMH-induced increases in Isc and 125I efflux were suppressed by depleting Ca2+ from the medium or by pretreating the cells with BAPTA-AM or by Rp-adenosin 3',5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS). The Cl- channel inhibitors 4,4'-disothiocyanatostibene-2,2'-disulfonic acid (DIDS), glybenclamide, and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) suppressed the VMH-induced increases in Isc and 125I efflux. Moreover, VMH increased the intracellular concentrations of Ca2+ and cAMP. Thus, VMH stimulates Caco-2 cells to secrete Cl- by activating both Ca2+ -dependent and cAMP-dependent Cl- secretion mechanisms. VMH forms ion-permeable pores in the lipid bilayer that are non-selectively permeable to small ions. However, the ion permeability of these pores was not inhibited by glybenclamide and DIDS, and VMH did not change the cell membrane potential. These observations indicate that the pores formed on the cell membrane by VMH are unlikely to be involved in VMH-induced Cl- secretion. Notably, VMH stimulated fluid accumulation in the iliac loop test that was fully suppressed by a combination of DIDS and glybenclamide. Thus, Ca2+-dependent and cAMP-dependent Cl- secretion may be important therapeutic targets with regard to the diarrhoea that is induced by Vibrio mimicus.

Changes in disease gene frequency over time with differential genotypic fitness and various control strategies

A spreadsheet model was constructed to describe the change in allelic frequency over time for a lethal recessive mutation in an animal population. The model allowed relative fitness to differ between genotypes, between sexes, and over time. Whereas a lethal recessive allele is naturally eliminated very slowly from a population, a small selective disadvantage of the heterozygote results in a large increase in the rate of elimination. With selective advantage of the heterozygote through linkage with a production trait or pleiotropy, the allele is never naturally eliminated but tends toward a stable equilibrium frequency. The model was used to investigate various alternative control programs based on the detection of heterozygotes by genotyping and their exclusion from breeding. The programs (genotyping males only, genotyping males and 50% of females, and genotyping all breeding animals) were modeled for various initial heterozygote frequencies, and the results were described in terms of the number of generations, number of tests, and number of culls required to reduce the heterozygote frequency to a predefined level. The model can be used to compare the feasibility and cost of various control strategies and to illustrate clearly to breeders the expected outcomes, as well as the danger of prematurely terminating a control program when there is a selective advantage of the heterozygote.

CFTR gene analysis in Latin American CF patients: heterogeneous origin and distribution of mutations across the continent

BACKGROUND

Cystic Fibrosis (CF) is the most prevalent Mendelian disorder in European populations. Despite the fact that many Latin American countries have a predominant population of European-descent, CF has remained an unknown entity until recently. Argentina and Brazil have detected the first patients around three decades ago, but in most countries this disease has remained poorly documented. Recently, other countries started publishing their results.

METHODS

We present a compilation and statistical analysis of the data obtained in 10 countries (Argentina, Brazil, Chile, Colombia, Costa Rica, Cuba, Ecuador, Mexico, Uruguay and Venezuela), with a total of 4354 unrelated CF chromosomes studied.

RESULTS

The results show a wide distribution of 89 different mutations, with a maximum coverage of 62.8% of CF chromosomes/alleles in the patient's sample. Most of these mutations are frequent in Spain, Italy, and Portugal, consistent with the origin of the European settlers. A few African mutations are also present in those countries which were part of the slave trade. New mutations were also found, possibly originating in America.

CONCLUSION

The profile of mutations in the CFTR gene, which reflects the heterogeneity of its inhabitants, shows the complexity of the molecular diagnosis of CF mutations in most of the Latin American countries.

Albinism (OCA2) in Amerindians

Homozygosity for a mutation in the P locus mapped to the human chromosome 15q11.2-12 results in tyrosinase-positive albinism (OCA2). This type of albinism has a worldwide distribution, with a prevalence of about 1 in 36,000 among European-Americans in the United States. It has a moderate to relatively high prevalence values (1 in 28 to 1 in 6,500) in various Amerindian populations in the southwestern United States, southern Mexico, eastern Panama, and southern Brazil. The wide distribution of the gene for OCA2 in Amerindian populations, and its relatively high frequency in several of these populations, are enigmatic because of the detrimental nature of OCA2 in the presence of certain environmental conditions. The relative Darwinian fitness of individuals with this inborn error of metabolism would have been reduced in early nomadic hunting-gathering populations because of their poor visual acuity and sensitivity to the sun. Nevertheless, specific situations allowed OCA2 to increase in frequency in certain Amerindian populations at various different times in history. The present objectives are to review the literature on albinism (OCA2) in Amerindians, and propose hypotheses for the variable frequencies of the OCA2 gene in Amerindian populations, which include chance processes (founder effect, bottleneck effect, and genetic drift) in small populations, natural selection, cultural selection, and the interaction of situations that led to the increase of the frequency of the albino gene in some generations. Special emphasis is placed on those Amerindian populations with a relatively high frequency of the OCA2 gene that have been best-studied, namely, the Cuna population of eastern Panama and the Hopi population in the southwestern US. Hypothetical scripts are presented for the present relatively high frequencies of the OCA2 gene in these populations. A hypothetical script is also presented, showing how a mutant gene could have reached a relatively high frequency in a small endogamous early Mayan population and then been spread by migrating groups to other geographical regions, following the rapid increase in size of that population. Comprehensive molecular studies of OCA2 genes in Amerindian populations could yield information on the possible origin of the albino gene present in many of these populations, in addition to the gene flow that occurred among some of them in past generations. The results of these studies could lead to more informed hypotheses concerning the wide distribution of OCA2 in Amerindian populations.

Crossing the line: selection and evolution of virulence traits

The evolution of pathogens presents a paradox. Pathogenic species are often absolutely dependent on their host species for their propagation through evolutionary time, yet the pathogenic lifestyle requires that the host be damaged during this dependence. It is clear that pathogenic strategies are successful in evolutionary terms because a diverse array of pathogens exists in nature. Pathogens also evolve using a broad range of molecular mechanisms to acquire and modulate existing virulence traits in order to achieve this success. Detailing the benefit of enhanced selection derived through virulence and understanding the mechanisms through which virulence evolves are important to understanding the natural world and both have implications for human health.

Calcium-sensing receptor abrogates secretagogue- induced increases in intestinal net fluid secretion by enhancing cyclic nucleotide destruction

The calcium-sensing receptor (CaSR) provides a fundamental mechanism for diverse cells to detect and respond to modulations in the ionic and nutrient compositions of their extracellular milieu. The roles for this receptor are largely unknown in the intestinal tract, where epithelial cells are normally exposed to large variations in extracellular solutes. Here, we show that colonic CaSR signaling stimulates the degradation of cyclic nucleotides by phosphodiesterases and describe the ability of receptor activation to reverse the fluid and electrolyte secretory actions of cAMP- and cGMP-generating secretagogues, including cholera toxin and heat stable Escherichia coli enterotoxin STa. Our results suggest a paradigm for regulation of intestinal fluid transport where fine tuning is accomplished by the counterbalancing effects of solute activation of the CaSR on neuronal and hormonal secretagogue actions. The reversal of cholera toxin- and STa endotoxin-induced fluid secretion by a small-molecule CaSR agonist suggests that these compounds may provide a unique therapy for secretory diarrheas.

Mutual enhancement of virulence by enterotoxigenic and enteropathogenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) and enteropathogenic E. coli (EPEC) are common causes of diarrhea in children in developing countries. Dual infections with both pathogens have been noted fairly frequently in studies of diarrhea around the world. In previous laboratory work, we noted that cholera toxin and forskolin markedly potentiated EPEC-induced ATP release from the host cell, and this potentiated release was found to be mediated by the cystic fibrosis transmembrane conductance regulator. In this study, we examined whether the ETEC heat-labile toxin (LT) or the heat-stable toxin (STa, also known as ST) potentiated EPEC-induced ATP release. We found that crude ETEC culture filtrates, as well as purified ETEC toxins, did potentiate EPEC-induced ATP release in cultured T84 cells. Coinfection of T84 cells with live ETEC plus EPEC bacteria also resulted in enhanced ATP release compared to EPEC alone. In Ussing chamber studies of chloride secretion, adenine nucleotides released from the host by EPEC also significantly enhanced the chloride secretory responses that were triggered by crude ETEC filtrates, purified STa, and the peptide hormone guanylin. In addition, adenosine and LT had additive or synergistic effects in inducing vacuole formation in T84 cells. Therefore, ETEC toxins and EPEC-induced damage to the host cell both enhance the virulence of the other type of E. coli. Our in vitro data demonstrate a molecular basis for a microbial interaction, which could result in increased severity of disease in vivo in individuals who are coinfected with ETEC and EPEC.

Lysophosphatidic acid inhibits cholera toxin-induced secretory diarrhea through CFTR-dependent protein interactions

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells that line the airway, gut, and exocrine glands; it is well established that CFTR plays a pivotal role in cholera toxin (CTX)-induced secretory diarrhea. Lysophosphatidic acid (LPA), a naturally occurring phospholipid present in blood and foods, has been reported to play a vital role in a variety of conditions involving gastrointestinal wound repair, apoptosis, inflammatory bowel disease, and diarrhea. Here we show, for the first time, that type 2 LPA receptors (LPA₂) are expressed at the apical surface of intestinal epithelial cells, where they form a macromolecular complex with Na⁺/H⁺ exchanger regulatory factor–2 and CFTR through a PSD95/Dlg/ZO-1–based interaction. LPA inhibited CFTR-dependent iodide efflux through LPA₂-mediated G_i pathway, and LPA inhibited CFTR-mediated short-circuit currents in a compartmentalized fashion. CFTR-dependent intestinal fluid secretion induced by CTX in mice was reduced substantially by LPA administration; disruption of this complex using a cell-permeant LPA₂-specific peptide reversed LPA₂-mediated inhibition. Thus, LPA-rich foods may represent an alternative method of treating certain forms of diarrhea.

Haplotype block structure study of the CFTR gene. Most variants are associated with the M470 allele in several European populations

An average of about 1700 CFTR (cystic fibrosis transmembrane conductance regulator) alleles from normal individuals from different European populations were extensively screened for DNA sequence variation. A total of 80 variants were observed: 61 coding SNSs (results already published), 13 noncoding SNSs, three STRs, two short deletions, and one nucleotide insertion. Eight DNA variants were classified as non-CF causing due to their high frequency of occurrence. Through this survey the CFTR has become the most exhaustively studied gene for its coding sequence variability and, though to a lesser extent, for its noncoding sequence variability as well. Interestingly, most variation was associated with the M470 allele, while the V470 allele showed an 'extended haplotype homozygosity' (EHH). These findings make us suggest a role for selection acting either on the M470V itself or through an hitchhiking mechanism involving a second site. The possible ancient origin of the V allele in an 'out of Africa' time frame is discussed.

Genetic epidemiology of health disparities in allergy and clinical immunology

The striking racial and ethnic disparities in disease prevalence for common disorders, such as allergic asthma, cannot be explained entirely by environmental, social, cultural, or economic factors, and genetic factors should not be ignored. Unfortunately, genetic studies in underserved minorities are hampered by disagreements over the biologic construct of race and logistic issues, including admixture of different races and ethnicities. Current observations suggest that the frequency of high-risk variants in candidate genes can differ between African Americans, Puerto Ricans, and Mexican Americans, and this might contribute to the differences in disease prevalence. Maintenance of certain allelic variants in the population over time might reflect selective pressures in previous generations. For example, significant associations between markers in certain candidate genes (eg, STAT6, ADRB2, and IFNGR1) for traits such as high total IgE levels observed in resistance to extracellular parasitic disease in one population and atopic asthma in another supports the common disease/common variant model for disease. Herein is a discussion of how genetic variants might explain, at least in part, the marked disparities observed in risk to allergic asthma.

Using epidemiology and archaeology to unearth new drug targets for rheumatoid arthritis therapy

Epidemiological and archaeological evidence suggests that RA could be a consequence of enhanced immunity to Mycobacterium tuberculosis, and that by understanding this connection, new RA drug targets may be uncovered.

Shigatoxin triggers thrombotic thrombocytopenic purpura in genetically susceptible ADAMTS13-deficient mice

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening illness caused by deficiency of the vWF-cleaving protease ADAMTS13. Here we show that ADAMTS13-deficient mice are viable and exhibit normal survival, although vWF-mediated platelet-endothelial interactions are significantly prolonged. Introduction of the genetic background CASA/Rk (a mouse strain with elevated plasma vWF) resulted in the appearance of spontaneous thrombocytopenia in a subset of ADAMTS13-deficient mice and significantly decreased survival. Challenge of these mice with shigatoxin (derived from bacterial pathogens associated with the related human disease hemolytic uremic syndrome) resulted in a striking syndrome closely resembling human TTP. Surprisingly, no correlation was observed between plasma vWF level and severity of TTP, implying the existence of TTP-modifying genes distinct from vWF. These data suggest that microbe-derived toxins (or possibly other sources of endothelial injury), together with additional genetic susceptibility factors, are required to trigger TTP in the setting of ADAMTS13 deficiency.

Luminally active, nonabsorbable CFTR inhibitors as potential therapy to reduce intestinal fluid loss in cholera

Enterotoxin-mediated secretory diarrheas such as cholera involve chloride secretion by enterocytes into the intestinal lumen by the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. We previously identified glycine hydrazide CFTR blockers that by electrophysiological studies appeared to block the CFTR anion pore at its lumen-facing surface. Here, we synthesize highly water-soluble, nonabsorbable malondihydrazides by coupling 2,4-disulfobenzaldehyde, 4-sulfophenylisothiocyante, and polyethylene glycol (PEG) moieties to 2-naphthalenylamino-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] propanedioic acid dihydrazide, and aminoacethydrazides by coupling PEG to [(N-2-naphthalenyl)-2-(2-hydroxyethyl)]-glycine-2-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] hydrazide. Compounds rapidly, fully and reversibly blocked CFTR-mediated chloride current with Ki of 2-8 microM when added to the apical surface of epithelial cell monolayers. Compounds did not pass across Caco-2 monolayers, and were absorbed by <2%/hr in mouse intestine. Luminally added compounds blocked by >90% cholera toxin-induced fluid secretion in mouse intestinal loops, without inhibiting intestinal fluid absorption. These orally administered, nonabsorbable, nontoxic CFTR inhibitors may reduce intestinal fluid losses in cholera.

Differential sensitivity of the cystic fibrosis (CF)-associated mutants G551D and G1349D to potentiators of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel

The genetic disease cystic fibrosis (CF) is caused by loss of function of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. Two CF mutants, G551D and G1349D, affect equivalent residues in the highly conserved LSGGQ motifs that are essential components of the ATP-binding sites of CFTR. Both mutants severely disrupt CFTR channel gating by decreasing mean burst duration (MBD) and prolonging greatly the interburst interval (IBI). To identify small molecules that rescue the gating defects of G551D- and G1349D-CFTR and understand better how these agents work, we used the patch clamp technique to study the effects on G551D- and G1349D-CFTR of phloxine B, pyrophosphate (PP(i)), and 2'-deoxy ATP (2'-dATP), three agents that strongly enhance CFTR channel gating. Phloxine B (5 microm) potentiated robustly G551D-CFTR Cl- channels by altering both MBD and IBI. In contrast, phloxine B (5 microm) decreased the IBI of G1349D-CFTR, but this effect was insufficient to rescue G1349D-CFTR channel gating. PP(i) (5 mm) potentiated weakly G551D-CFTR and was without effect on the G1349D-CFTR Cl- channel. However, by altering both MBD and IBI, albeit with different efficacies, 2'-dATP (1 mm) potentiated both G551D- and G1349D-CFTR Cl- channels. Using the ATP-driven nucleotide-binding domain dimerization model of CFTR channel gating, we suggest that phloxine B, PP(i) and 2'-dATP alter channel gating by distinct mechanisms. We conclude that G551D- and G1349D-CFTR have distinct pharmacological profiles and speculate that drug therapy for CF is likely to be mutation-specific.

A hemolysin of Vibrio mimicus (VMH) stimulates cells to produce ATP and cyclic AMP which appear to be secretory mediators

The hemolysin of Vibrio mimicus(VMH) is a pore-forming toxin with both enterotoxic and hemolytic activity. The hemolysis by VMH is induced by creation of pores in the membrane of erythrocyte; however, the mechanism for the enterotoxic action of VMH has remained unclear. In order to clarify the mechanism, we incubated T84 cells (a human colon carcinoma cell line) with VMH and found that the levels of ATP and cyclic AMP of culture medium increased after exposure of the cells to VMH. Subsequently, we found that the fluid accumulating activity of VMH in a mouse internal loop assay was reduced by administration of glibenclamide, an inhibitor of cyclic AMP-dependent chloride channels, into the intestinal loop. These results suggest that the stimulation of cells to produce nucleotides by VMH is linked to the enterotoxic activity of the toxin.

Susceptibility to typhoid fever is associated with a polymorphism in the cystic fibrosis transmembrane conductance regulator (CFTR)

The cystic fibrosis transmembrane conductance regulator (CFTR) is the affected protein in cystic fibrosis (CF). The high rate of CF carriers has led to speculation that there must be, similar to the sickle cell haemoglobin advantage in malaria, a selective advantage for heterozygotes. Such a selective advantage may be conferred through reduced attachment of Salmonella typhi to intestinal mucosa, thus providing resistance to typhoid fever. We tested this hypothesis by genotyping patients and controls in a typhoid endemic area in Indonesia for two highly polymorphic markers in CFTR and the most common CF mutation. We found an association between genotypes in CFTR and susceptibility to typhoid fever (OR=2.6). These analyses suggest that the role CFTR plays in vitro in S. typhi infection is also important for infection in the human population.

Estrogen-induced abnormally high cystic fibrosis transmembrane conductance regulator expression results in ovarian hyperstimulation syndrome

Ovarian hyperstimulation syndrome (OHSS) remains one of the most life-threatening and potentially fatal complications of assisted reproduction treatments, arising from excessive stimulation of the ovaries by exogenous gonadotropins administrated during in vitro fertilization procedures, which is characterized by massive fluid shift and accumulation in the peritoneal cavity and other organs, including the lungs and the reproductive tract. The pathogenesis of OHSS remains obscure, and no definitive treatments are currently available. Using RT-PCR, Western blot, and electrophysiological techniques we show that cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel expressed in many epithelia, is involved in the pathogenesis of OHSS. Upon ovarian hyperstimulation, rats develop OHSS symptoms, with up-regulated CFTR expression and enhanced CFTR channel activity, which can also be mimicked by administration of estrogen, but not progesterone, alone in ovariectomized rats. Administration of progesterone that suppresses CFTR expression or antiserum against CFTR to OHSS animals results in alleviation of the symptoms. Furthermore, ovarian hyperstimulation does not induce detectable OHSS symptoms in CFTR mutant mice. These findings confirm a critical role of CFTR in the pathogenesis of OHSS and may provide grounds for better assisted reproduction treatment strategy to reduce the risk of OHSS and improve in vitro fertilization outcome.

Macromolecular complexes of cystic fibrosis transmembrane conductance regulator and its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is the product of the gene mutated in patients with cystic fibrosis (CF). CFTR is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells lining the airway, gut, exocrine glands, etc., where it is responsible for transepithelial salt and water transport. CFTR chloride channel belongs to the superfamily of the ATP-binding cassette (ABC) transporters, which bind ATP and use the energy to drive the transport of a wide variety of substrates across extra- and intracellular membranes. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might regulate the activities of other ion channels, receptors, or transporters, in addition to its role as a chloride conductor. The molecular assembly of CFTR with these interacting proteins is of great interest and importance because several human diseases are attributed to altered regulation of CFTR, among which cystic fibrosis is the most serious one. Most interactions primarily occur between the opposing terminal tails (N- or C-) of CFTR and its binding partners, either directly or mediated through various PDZ domain-containing proteins. These dynamic interactions impact the channel function as well as the localization and processing of CFTR protein within cells. This review focuses on the recent developments in defining the assembly of CFTR-containing complexes in the plasma membrane and its interacting proteins.

Important bacterial gastrointestinal pathogens in children: a pathogenesis perspective

This article focuses on the five most common bacterial enteropathogens of the developed world--Helicobacter pylori, Escherichia coli, Shigella, Salmonella, and Campylobacter--from the perspective of how they cause disease and how they relate to each other. Basic and recurring themes of bacterial pathogenesis, including mechanisms of entry, methods of adherence, sites of cellular injury, role of toxins, and how pathogens acquire particular virulence traits (and antimicrobial resistance), are discussed.

A pore-forming toxin produced by Aeromonas sobria activates Ca2+ dependent Cl- secretion

Bacteria produce many types of hemolysin that induce diarrhea by mechanisms that are not completely understood. Aeromonas sobria hemolysin (ASH) is a major virulence factor produced by A. sobria, a human pathogen that causes diarrhea. Since epithelial cells in the intestine are the primary targets of hemolysin, we investigated the effects of ASH on ion transport in human colonic epithelial (Caco-2) cells. ASH increased short-circuit currents (Isc) in a dose-dependent manner, and it also activated a 125I efflux from Caco-2 cells. ASH-induced Isc increases and 125I efflux activations were both suppressed by low Ca2+ levels in the extracellular solution or by pretreatment with the Ca2+ chlelator BAPTA-AM. Intracellular Ca2+ levels were increased by ASH in a biphasic fashion characterized by a rapid sharp increase (peak 1) followed by a sustained low plateau (peak 2). ASH-induced peak 1 was inhibited by pretreatment with pertussis toxin, indicating that Ca2+ was mobilized from intracellular stores, and peak 2 was induced by an influx of extracellular Ca2+. Peak 2 but not peak 1 was related to Cl- secretion. These results indicate that ASH activates Ca2+-dependent Cl- secretion.

Interleukin 8 secretion from monocytes of subjects heterozygous for the deltaF508 cystic fibrosis transmembrane conductance regulator gene mutation is altered

Patients with cystic fibrosis (CF) exhibit an excessive host inflammatory response. The aim of this study was to determine (i) whether interleukin 8 (IL-8) secretion is increased from monocytes from subjects heterozygous as well as homozygous for cystic fibrosis transmembrane conductance regulator (CFTR) mutations and (ii) whether this is due to increased cell surface lipopolysaccharide (LPS) receptors or, alternatively, increased activation of mitogen-activated protein kinases (MAPK). The basal level of IL-8 secretion was higher from monocytes from CF patients than from monocytes from healthy controls (P = 0.02) and obligate heterozygotes (parents of the CF patients). The 50% effective concentrations for LPS-induced IL-8 production for monocytes from both CF patients and obligate heterozygotes were 100-fold lower than those for monocytes from healthy controls (P < 0.05). No differences in the levels of IL-1beta production were seen between these groups. Expression of the LPS surface receptors CD14 and Toll-like receptor 4 were not different between CF patients and healthy controls. In contrast, phosphorylation of the MAPKs p38 and ERK occurred at lower doses of LPS in monocytes from patients heterozygous and homozygous for CFTR mutations. These results indicate that a single allelic CFTR mutation is sufficient to augment IL-8 secretion in response to LPS. This is not a result of increased LPS receptor expression but, rather, is associated with alterations in MAPK signaling.

A pore-forming toxin produced by Aeromonas sobria activates cAMP-dependent Cl- secretory pathways to cause diarrhea

Aeromonas sobria hemolysin (ASH) is one of the major virulence factors produced by A. sobria, a human pathogen that causes diarrhea. We investigated the effects of ASH on Cl(-) transport in human colonic epithelial cells. ASH increased short-circuit currents (Isc) and (125)I efflux from Caco-2 cells, indicating ASH activate Cl(-) secretion. Additions of inhibitors of cyclic AMP dependent Cl(-) channels, glybenclamide and NPPB suppressed the Isc and (125)I efflux increases induced by ASH. And ASH increased the intracellular cyclic AMP concentration. Moreover, ASH stimulated fluid accumulation in the iliac loop test, and glybenclamide and NPPB suppressed this fluid accumulation. Thus, cAMP-dependent Cl(-) secretory pathway could be related with diarrhea induced by A. sobria.

SECRETION AND ABSORPTION BY COLONIC CRYPTS

The intestines play an important role in the absorption and secretion of nutrients. The colon is the final area for recapturing electrolytes and water prior to excretion, and in order to maintain this electrolyte homeostasis, a complex interaction between secretory and absorptive processes is necessary. Until recently it was thought that secretion and absorption were two distinct processes associated with either crypts or surface cells, respectively. Recently it was demonstrated that both the surface and crypt cells can perform secretory and absorptive functions and that, in fact, these functions can be going on simultaneously. This issue is important in the complexities associated with secretory diarrhea and also in attempting to develop treatment strategies for intestinal disorders. Here, we update the model of colonic secretion and absorption, discuss new issues of transporter activation, and identify some important new receptor pathways that are important modulators of the secretory and absorptive functions of the colon.

Reversible silencing of CFTR chloride channels by glutathionylation

The cystic fibrosis transmembrane conductance regulator (CFTR) is a phosphorylation- and ATP-dependent chloride channel that modulates salt and water transport across lung and gut epithelia. The relationship between CFTR and oxidized forms of glutathione is of potential interest because reactive glutathione species are produced in inflamed epithelia where they may be modulators or substrates of CFTR. Here we show that CFTR channel activity in excised membrane patches is markedly inhibited by several oxidized forms of glutathione (i.e., GSSG, GSNO, and glutathione treated with diamide, a strong thiol oxidizer). Three lines of evidence indicate that the likely mechanism for this inhibitory effect is glutathionylation of a CFTR cysteine (i.e., formation of a mixed disulfide with glutathione): (a) channels could be protected from inhibition by pretreating the patch with NEM (a thiol alkylating agent) or by lowering the bath pH; (b) inhibited channels could be rescued by reducing agents (e.g., DTT) or by purified glutaredoxins (Grxs; thiol disulfide oxidoreductases) including a mutant Grx that specifically reduces mixed disulfides between glutathione and cysteines within proteins; and (c) reversible glutathionylation of CFTR polypeptides in microsomes could be detected biochemically under the same conditions. At the single channel level, the primary effect of reactive glutathione species was to markedly inhibit the opening rates of individual CFTR channels. CFTR channel inhibition was not obviously dependent on phosphorylation state but was markedly slowed when channels were first “locked open” by a poorly hydrolyzable ATP analogue (AMP-PNP). Consistent with the latter finding, we show that the major site of inhibition is cys-1344, a poorly conserved cysteine that lies proximal to the signature sequence in the second nucleotide binding domain (NBD2) of human CFTR. This region is predicted to participate in ATP-dependent channel opening and to be occluded in the nucleotide-bound state of the channel based on structural comparisons to related ATP binding cassette transporters. Our results demonstrate that human CFTR channels are reversibly inhibited by reactive glutathione species, and support an important role of the region proximal to the NBD2 signature sequence in ATP-dependent channel opening.

In Vivo Pharmacology and Antidiarrheal Efficacy of a Thiazolidinone CFTR Inhibitor in Rodents

A small-molecule inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone (CFTR(inh)-172), reduces enterotoxin-induced intestinal fluid secretion in rodents. Here, we study CFTR(inh)-172 pharmacology and antidiarrheal efficacy in rodents using (14)C-labeled CFTR(inh)-172, liquid chromatography/mass spectrometry, and a closed intestinal loop model of fluid secretion. CFTR(inh)-172 was cleared primarily by renal glomerular filtration without chemical modification. CFTR(inh)-172 accumulated in liver within 5 min after intravenous infusion in mice, and was concentrated fivefold in bile over blood. At 30-240 min, CFTR(inh)-172 was found mainly in liver, intestine, and kidney, with little detectable in the brain, heart, skeletal muscle, or lung. Pharmacokinetic analysis in rats following intravenous bolus infusion showed a distribution volume of 770 mL with redistribution and elimination half-times of 0.14 h and 10.3 h, respectively. CFTR(inh)-172 was stable in hepatic microsomes. Closed-loop studies in mice indicated that a single intraperitoneal injection of 20 microg CFTR(inh)-172 inhibited fluid accumulation at 6 h after cholera toxin by >90% in duodenum and jejunum, approximately 60% in ileum and <10% in colon. No toxicity was seen after high-dose CFTR(inh)-172 administration (3 mg/kg/day in two daily doses) in mice over the first 6 weeks of life. The metabolic stability, enterohepatic recirculation, slow renal elimination, and intestinal accumulation of CFTR(inh)-172 account for its efficacy as an antidiarrheal.

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.

[Genomic instability and male infertility]

Knowledge of the human genome has opened the genomic era. The genome instability, its causes and the possible consequences especially about fertility start to be understood. This instability can be observed on chromosome structure but also on genes. Different chromosomes rearrangements involved in infertility including translocations and Y chromosome deletions are described. The Y chromosome is a model of instability, and this instability is the source of its evolution. All those rearrangements are the results of illegitimate recombinations between homologous sequences. On genes we find punctual and dynamic mutations, polymorphisms and epigenetic abnormalities. They all are the results of ADN replication mistakes not corrected by the cellular machine. This machinery is the guardian of the genome integrity and in case of abnormality the programmed cellular death is induced. The knowledge of all these instability mechanisms is essential to appreciate the risk for the offspring after intracytoplasmic sperm injection. Indeed we go round physiological barriers without a complete understanding of the mechanisms involved. Thus, this is an important challenge for research teams but also for all assisted reproduction centers, dealing with ART. Genome is unstable - the very basis of its evolution. But this is also the cause of mistakes with pathological consequences like infertility and mental retardation.

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

Choosing mates: good genes versus genes that are a good fit

Female choice for male ornamental traits is widely accepted as a mechanism by which females maximize their reproductive success and/or offspring quality. However, there is an increasing empirical literature that shows a fitness benefit of genetic diversity and a tendency for females to use genetic dissimilarity as a criterion for mate choice. This genetic compatibility hypothesis for female mate choice presents a paradox. How can females use both an absolute criterion, such as male ornamentation, and a relative criterion, such as genetic dissimilarity, to choose their mates? Here, we present potential solutions for this dilemma and the empirical evidence supporting them. The interplay between these two contrasting forms of female mate choice presents an exciting empirical and theoretical challenge for evolutionary ecologists.

Direct block of the cystic fibrosis transmembrane conductance regulator Cl(-) channel by niflumic acid

Niflumic acid is widely used to inhibit Ca(2+) -activated Cl(-) channels. However, the chemical structure of niflumic acid resembles that of diphenylamine-2-carboxylate, a drug that inhibits the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. To investigate how niflumic acid inhibits CFTR Cl(-) channel, we studied recombinant wild-type human CFTR in excised inside-out membrane patches. When added to the intracellular solution, niflumic acid caused a concentration- and voltage-dependent decrease of CFTR Cl(-) current with half-maximal inhibitory concentration (K(i)) of 253 microM and Hill co-efficient of approximately 1, at -50 mV. Niflumic acid inhibition of single CFTR Cl(-) channels was characterized by a very fast, flickery block that decreased dramatically current amplitude without altering open-probability. Consistent with these data, spectral analysis of CFTR Cl(-) currents suggested that channel block by niflumic acid was described by the closed <--> open <--> blocked kinetic scheme with blocker on rate (k(on)) = 13.9 x 10(6) M(-1)s(-1), off rate (k(off))=3348 s(-1) and dissociation constant (K(d)) = 241 microM, at -50 mV. Based on these data, we tested the effects of niflumic acid on transepithelial Cl(-) secretion and cyst growth using type I MDCK epithelial cells. Niflumic acid (200 microM) inhibited cAMP-stimulated, bumetanide-sensitive short-circuit current by 55%. Moreover, the drug potently retarded cyst growth. We conclude that niflumic acid is an open-channel blocker of CFTR that inhibits Cl(-) permeation by plugging the channel pore. It or related agents might be of value in the development of new therapies for autosomal dominant polycystic kidney disease.

A novel extract SB-300 from the stem bark latex of Croton lechleri inhibits CFTR-mediated chloride secretion in human colonic epithelial cells

An oligomeric proanthocyanidin (SP-303) extracted from the bark latex of the tree Croton lechleri (family Euphorbiaceae) is a potent inhibitor of cholera toxin-induced fluid accumulation and chloride secretion. The manufacturing process for SP-303 was optimized and simplified to produce an increased yield of the herbal extract. The novel extract (named SB-300) contained on average 70.6+/-7.2% SP-303 by weight (mean +/- S.D.; n=56 lots). Here, we describe the effectiveness of SB-300 on cAMP-regulated chloride secretion, which is mediated by the cystic fibrosis transmembrane conductance regulator Cl- channel (CFTR) in human colonic T84 cells. Exposure of the apical surface to SB-300 blocked forskolin-stimulated Cl- secretion by 92.2+/-3.0% with a half-maximal inhibition constant (KB) of 4.8+/-0.8 microM. For SP-303, stimulated Cl- currents were decreased by 98.0+/-7.2 % and KB averaged 4.1+/-1.3 microM. There was no significant difference between the blocking kinetics of SP-303 and SB-300. Forskolin-stimulated whole cell Cl- currents were effectively blocked by extracellular addition of SB-300 (63+/-8.5%; n=3) and to a similar extent by SP-303 (83 +/- 0.6%; n=2; at 50 microM each). Both extracts inhibited a time- and voltage-independent Cl- conductance, which indicated the involvement of CFTR Cl- channels. We conclude that both SP-303 (used in Provir) and SB-300 (used in NSF Normal Stool Formula) are novel natural products that target the CFTR Cl- channel. SB-300 is a low cost herbal extract and may present a complementary and alternative medicine approach for the treatment of fluid loss in watery diarrhea.

The "Goldilocks effect" in cystic fibrosis: identification of a lung phenotype in the cftr knockout and heterozygous mouse

BACKGROUND

Cystic Fibrosis is a pleiotropic disease in humans with primary morbidity and mortality associated with a lung disease phenotype. However, knockout in the mouse of cftr, the gene whose mutant alleles are responsible for cystic fibrosis, has previously failed to produce a readily, quantifiable lung phenotype.

RESULTS

Using measurements of pulmonary mechanics, a definitive lung phenotype was demonstrated in the cftr-/- mouse. Lungs showed decreased compliance and increased airway resistance in young animals as compared to cftr+/+ littermates. These changes were noted in animals less than 60 days old, prior to any long term inflammatory effects that might occur, and are consistent with structural differences in the cftr-/- lungs. Surprisingly, the cftr+/- animals exhibited a lung phenotype distinct from either the homozygous normal or knockout genotypes. The heterozygous mice showed increased lung compliance and decreased airway resistance when compared to either homozygous phenotype, suggesting a heterozygous advantage that might explain the high frequency of this mutation in certain populations.

CONCLUSIONS

In the mouse the gene dosage of cftr results in distinct differences in pulmonary mechanics of the adult. Distinct phenotypes were demonstrated in each genotype, cftr-/-, cftr +/-, and cftr+/+. These results are consistent with a developmental role for CFTR in the lung.

Capsaicin potentiates wild-type and mutant cystic fibrosis transmembrane conductance regulator chloride-channel currents

To examine the effects of capsaicin on cystic fibrosis transmembrane conductance regulator (CFTR), we recorded wild-type and mutant CFTR chloride-channel currents using patch-clamp methods. The effects of capsaicin were compared with those of genistein, a well-characterized CFTR activator. In whole-cell experiments, capsaicin potentiates cAMP-stimulated wild-type CFTR currents expressed in NIH 3T3 cells or Chinese hamster ovary cells in a dose-dependent manner with a maximal response approximately 60% of that with genistein and an apparent Kd of 48.4 +/- 6.8 microM. In cell-attached recordings, capsaicin alone fails to activate CFTR in cells that show negligible basal CFTR activity, indicating that capsaicin does not stimulate the cAMP cascade. The magnitude of potentiation with capsaicin depends on the channel activity before drug application; the lower the prestimulated Po, the higher the potentiation. Single-channel kinetic analysis shows that capsaicin potentiates CFTR by increasing the opening rate and decreasing the closing rate of the channel. Capsaicin may act as a partial agonist of genistein because the maximally enhanced wild-type CFTR currents with genistein are partially inhibited by capsaicin. Capsaicin increases DeltaR-CFTR, a protein kinase A (PKA)-independent, constitutively active channel, in cell-attached patches. In excised inside-out patches, capsaicin potentiates the PKA-phosphorylated, ATP-dependent CFTR activity. Both capsaicin and genistein potentiate the cAMP-stimulated G551D-CFTR, DeltaF508-CFTR, and 8SA mutant channel currents. The binding site for capsaicin is probably located at the cytoplasmic domain of CFTR, because pipette application of capsaicin fails to potentiate CFTR activity. In conclusion, capsaicin is a partial agonist of genistein in activation of the CFTR chloride channel. Both compounds affect ATP-dependent gating of CFTR.

Hepatobiliary complications of cystic fibrosis

Cystic fibrosis (CF) is the most common potentially lethal genetic disease in the Caucasian population. The disease results from mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl(-) channel in the apical membrane of most secretory epithelia. In the liver, CFTR is located in biliary epithelial cells or cholangiocytes and gallbladder epithelia, where it appears to play a role in normal bile formation. However, how a defective CFTR protein leads to associated liver and biliary disease in a subset of patients with CF is unknown. Improvements in life expectancy have led to an increasing recognition of hepatobiliary complications from CF. Whereas the biliary tract disease is usually clinically evident, the liver involvement may progress silently, only manifesting as end-stage liver disease and portal hypertension. Unlike the pancreatic involvement in CF, a genotype-phenotype correlation is not apparent in the expression of liver disease, suggesting the presence of as yet unidentifiable "genetic modifiers" influencing disease expression. This review focuses on the pathogenesis, clinical manifestations, screening, diagnosis, and treatment of CF hepatobiliary disease.

Is rheumatoid arthritis a consequence of natural selection for enhanced tuberculosis resistance?

Although the bubonic plague or "Black Death" is notorious for the toll it took on the population of Europe in the middle ages, another epidemic, the "White Death" of tuberculosis is responsible for millions of deaths worldwide over the past 300 years. With one in four deaths due to tuberculosis in Western Europe and the United States in the 19th century, this disease undoubtedly acted as a powerful genetic selective force. The epidemiology of modern day rheumatoid arthritis (RA) is strikingly similar to the epidemiology of tuberculosis 100-200 years ago, suggesting the possibility that genetic factors that enhanced survival in tuberculosis epidemics are now influencing susceptibility to RA. Recent advances in the analysis of genetic polymorphisms associated with disease have identified several genes linked to RA susceptibility that encode proteins involved in the immune response to Mycobacterium tuberculosis infection, including TNF-alpha, NRAMP1, PARP-1, HLA-DRB1, and PADI4. These results suggest that rheumatoid arthritis, and possibly other autoimmune diseases, are modern day manifestations of the genetic selective pressure exerted by tuberculosis epidemics of the recent past.

Identification and molecular characterization of EatA, an autotransporter protein of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) strains remain a formidable cause of diarrheal disease. To identify novel surface proteins of ETEC, we performed TnphoA mutagenesis of prototype ETEC strain H10407 and discovered a secreted protein not previously recognized in ETEC. DNA sequencing of the interrupted locus in mutant TnphoA.977 revealed a candidate 4,095-bp open reading frame without significant homology to commensal E. coli K-12 genomic DNA. Translation of this sequence revealed that it encoded a predicted peptide of 147.7 kDa that bears significant homology to members of the autotransporter family of bacterial virulence factors, particularly the serine protease autotransporters of the Enterobacteriaceae proteins. The gene identified in H10407, eatA (ETEC autotransporter A), encodes a potential serine protease motif (GDSGSP) in the secreted amino-terminal domain, and the predicted peptide shows more than 80% homology with SepA, a virulence protein secreted by Shigella flexneri. DNA hybridization and PCR demonstrated that eatA resides on the 92-kDa pCS1 virulence plasmid of H10407 and that it is present in multiple clinical ETEC strains. Immunoblots with antisera directed against a recombinant EatA passenger protein fragment identified a 110-kDa protein in supernatants purified from H10407 but not from the TnphoA.977 mutant or H10407-P, which lacks pCS1. EatA possesses serine protease activity that is abolished by mutations within a serine protease catalytic triad formed by residues H(134), D(162), and S(267). Finally, interruption of the eatA gene retarded fluid accumulation in the rabbit ileal loop model, suggesting that this autotransporter contributes to the virulence of ETEC.