CFTR induces the expression of DRA along with Cl(-)/HCO(3)(-) exchange activity in tracheal epithelial cells

Identification of seven novel mutations including the first two genomic rearrangements in SLC26A3 mutated in congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder characterized by defective intestinal electrolyte absorption, resulting in voluminous osmotic diarrhea with high chloride content. A variety of mutations in the solute carrier family 26, member 3 gene (SLC26A3, previously known as CLD or DRA) are responsible for the disease. Since the identification of the SLC26A3 gene and the determination of its genomic structure, altogether three founder and 17 private mutations have been characterized within miscellaneous ethnic groups. We screened for mutations in seven unrelated families with CLD. The diagnoses were confirmed by fecal chloride measurements. The combined PCR-SSCP and sequencing analyses revealed altogether seven novel mutations including two missense mutations (S206P, D468V), two splicing defects (IVS12-1G>C, IVS13-2delA), one nonsense mutation (Q436X), one insertion/deletion mutation (2104-2105delGGins29-bp), and an intragenic deletion of SLC26A3 exons 7 and 8. Two previously identified mutations were also found. This is the first report of rearrangement mutations in SLC26A3. Molecular features predisposing SLC26A3 for the two rearrangements may include repetitive elements and palindromic-like sequences. The increasingly wide diversity of SLC26A3 mutations suggests that mutations in the SLC26A3 gene may not be rare events.

Liquid and ion transport by fetal airway and lung epithelia of mice deficient in sodium-potassium-2-chloride transporter

Chloride (Cl(-)) movement across fetal lung epithelia is thought to be mediated by the sodium-potassium-2-Cl(-) cotransporter NKCC1. We studied the role of NKCC1 in Cl(-) and liquid secretion in late-gestation NKCC-null (-/-) and littermate control fetal mouse lung. NKCC -/- mice had decreased lung water compared with littermate controls (wet/dry: control, 8.01 +/- 0.09; NKCC -/-, 7.06 +/- 0.14). Liquid secretion by 17-d NKCC -/- distal lung explants was similar to control explants. Bumetanide inhibited basal liquid secretion in control but not NKCC -/- explants (expansion over 48 h: control, 35 +/- 4%; NKCC -/- 46 +/- 7%). Treatment with 4,4'-diisothiocyanto-stilbene-2,2'-disulfonic acid (DIDS) decreased liquid secretion in both control and NKCC -/- explants. Basal transepithelial potential difference (PD) of control tracheal explants was higher than that of NKCC -/- (control, -13.7 +/- 0.5 mV; NKCC -/-, -11.6 +/- 0.6 mV). Amiloride (10(-)(4) M) inhibited basal PD to the same extent in control and NKCC -/- mice. Terbutaline-stimulated hyperpolarization was less in NKCC -/- than in control tracheas (DeltaPD: control, -10.8 +/- 1.33 mV; NKCC -/-, -6.1 +/- 0.7 mV) and was inhibited by DIDS and acetazolamide in NKCC -/- but not wild-type explants. We conclude that NKCC is rate-limiting for transcellular Cl(-) transport, and that alternative anion transport mechanisms can sustain liquid production at near-normal levels in the fetal NKCC -/- mouse lung.

Regulatory interaction between the cystic fibrosis transmembrane conductance regulator and HCO3- salvage mechanisms in model systems and the mouse pancreatic duct

The pancreatic duct expresses cystic fibrosis transmembrane conductance regulator (CFTR) and HCO3- secretory and salvage mechanisms in the luminal membrane. Although CFTR plays a prominent role in HCO3- secretion, the role of CFTR in HCO3- salvage is not known. In the present work, we used molecular, biochemical, and functional approaches to study the regulatory interaction between CFTR and the HCO3- salvage mechanism Na+/H+ exchanger isoform 3 (NHE3) in heterologous expression systems and in the native pancreatic duct. We found that CFTR regulates NHE3 activity by both acute and chronic mechanisms. In the pancreatic duct, CFTR increases expression of NHE3 in the luminal membrane. Thus, luminal expression of NHE3 was reduced by 53% in ducts of homozygote DeltaF508 mice. Accordingly, luminal Na+-dependent and HOE694- sensitive recovery from an acid load was reduced by 60% in ducts of DeltaF508 mice. CFTR and NHE3 were co-immunoprecipitated from PS120 cells expressing both proteins and the pancreatic duct of wild type mice but not from PS120 cells lacking CFTR or the pancreas of DeltaF508 mice. The interaction between CFTR and NHE3 required the COOH-terminal PDZ binding motif of CFTR, and mutant CFTR proteins lacking the C terminus were not co-immunoprecipitated with NHE3. Furthermore, when expressed in PS120 cells, wild type CFTR, but not CFTR mutants lacking the C-terminal PDZ binding motif, augmented cAMP-dependent inhibition of NHE3 activity by 31%. These findings reveal that CFTR controls overall HCO3- homeostasis by regulating both pancreatic ductal HCO3- secretory and salvage mechanisms.

Pharmacological treatment of the ion transport defect in cystic fibrosis

Cystic fibrosis (CF) is a lethal monogenetic disease characterised by impaired water and ion transport over epithelia. The lung pathology is fatal and causes death in 95% of CF patients. The genetic basis of the disease is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The most common mutation, DeltaF508, results in a protein that cannot properly be folded in the endoplasmic reticulum, is destroyed and hence does not reach the apical cell membrane. This paper will discuss those pharmacological approaches that are directed at correcting the defect in ion transport. At present, no clinically effective drug is available, although research has defined areas in which progress might be made. These are the following: (1) the drug 4-phenylbutyrate (4PBA) increases the expression of DeltaF508-CFTR in the cell membrane, probably by breaking the association between DeltaF508-CFTR and a chaperone; (2) a number of xanthines, in particular 8-cyclopentyl-1, 3-dipropylxanthine (CPX), are effective in activating CFTR, presumably by direct binding and also possibly by correcting the trafficking defect; (3) the isoflavone genistein can activate both wild-type and mutant CFTR, probably through direct binding to the channel; (4) purinergic agonists (ATP and UTP) can stimulate chloride secretion via a Ca(2+)-dependent chloride channel and in this way compensate for the defect in CFTR, but stable analogues will be required before this type of treatment has clinical significance; (5) treatment with inhaled amiloride may correct the excessive absorption of Na(+) ions and water by airway epithelial cells that appears connected to the defect in CFTR; although clinical tests have not been very successful so far, amiloride analogues with a longer half-life may give better results. The role of CFTR in bicarbonate secretion has not yet been established with certainty, but correction of the defect in bicarbonate secretion may be important in clinical treatment of the disease. Currently, major efforts are directed at developing a pharmacological treatment of the ion transport defect in CF, but much basic research remains to be done, in particular, with regard to the mechanism by which defective CFTR is removed in the endoplasmic reticulum by the ubiquitin-proteasome pathway, which is a central pathway in protein production and of significance for several other diseases apart from CF.

Clinical and pharmacological experience with LJP-394

LJP-394 is a synthetic biological with immunomodulatory functions. Composed of four double-stranded oligodeoxynucleotides attached to a central branched platform, the drug acts as an anti-"anti-ds-DNA" B-cell toleragen by rendering specific B-lymphocytes unresponsive to immunogen so they do not produce autoantibodies. Extensive animal studies and Phase II clinical trials suggested that the effects of LJP-394 are effective and safe when used as a weekly dose of 100 mg intravenously. Analysis of a multicentre, international Phase II/III clinical trial showed that patients with lupus nephritis and high affinity IgG antibodies to LJP-394 have clinical benefits. This includes increased time to renal flares, reduced number of renal flares, time to institution of high-dose corticosteroids and/or cyclophosphamide and lower anti-ds-DNA levels. A definitive trial is in progress. LJP-394 appears to be free of serious adverse reactions. Though promising, the role of LJP-394 in patients with active, organ-threatening lupus is still not known.