Mechanism of lonidamine inhibition of the CFTR chloride channel

Lonidamine and domperidone inhibit expansion of transformed cell areas by modulating motility of surrounding nontransformed cells

Cancer cells intrinsically proliferate in an autonomous manner; however, the expansion of cancer cell areas in a tissue is known to be regulated by surrounding nontransformed cells. Whether these nontransformed cells can be targeted to control the spread of cancer cells is not understood. In this study, we established a system to evaluate the cancer-inhibitory activity of surrounding nontransformed cells and screened chemical compounds that could induce this activity. Our findings revealed that lonidamine (LND) and domperidone (DPD) inhibited expansion of oncogenic foci of KRASG12D-expressing transformed cells, whereas they did not inhibit the proliferation of monocultured KRASG12D-expressing cells. Live imaging revealed that LND and DPD suppressed the movement of nontransformed cells away from the attaching cancer cells. Moreover, we determined that LND and DPD promoted stress fiber formation, and the dominant-negative mutant of a small GTPase RhoA relieved the suppression of focus expansion, suggesting that RhoA-mediated stress fiber formation is involved in the inhibition of the movement of nontransformed cells and focus expansion. In conclusion, we suggest that elucidation of the mechanism of action of LND and DPD may lead to the development of a new type of drug that could induce the anticancer activity of surrounding nontransformed cells.

The lonidamine derivative H2-gamendazole reduces cyst formation in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a debilitating renal neoplastic disorder with limited treatment options. It is characterized by the formation of large fluid-filled cysts that develop from kidney tubules through abnormal cell proliferation and cyst-filling fluid secretion driven by cAMP-dependent Cl- secretion. We tested the effectiveness of the indazole carboxylic acid H2-gamendazole (H2-GMZ), a derivative of lonidamine, to inhibit these processes using in vitro and in vivo models of ADPKD. H2-GMZ was effective in rapidly blocking forskolin-induced, Cl--mediated short-circuit currents in human ADPKD cells, and it significantly inhibited both cAMP- and epidermal growth factor-induced proliferation of ADPKD cells. Western blot analysis of H2-GMZ-treated ADPKD cells showed decreased phosphorylated ERK and decreased hyperphosphorylated retinoblastoma levels. H2-GMZ treatment also decreased ErbB2, Akt, and cyclin-dependent kinase 4, consistent with inhibition of heat shock protein 90, and it decreased levels of the cystic fibrosis transmembrane conductance regulator Cl- channel protein. H2-GMZ-treated ADPKD cultures contained a higher proportion of smaller cells with fewer and smaller lamellipodia and decreased cytoplasmic actin staining, and they were unable to accomplish wound closure even at low H2-GMZ concentrations, consistent with an alteration in the actin cytoskeleton and decreased cell motility. Experiments using mouse metanephric organ cultures showed that H2-GMZ inhibited cAMP-stimulated cyst growth and enlargement. In vivo, H2-GMZ was effective in slowing postnatal cyst formation and kidney enlargement in the Pkd1flox/flox: Pkhd1-Cre mouse model. Thus, H2-GMZ treatment decreases Cl- secretion, cell proliferation, cell motility, and cyst growth. These properties, along with its reported low toxicity, suggest that H2-GMZ might be an attractive candidate for treatment of ADPKD.NEW & NOTEWORTHY Autosomal dominant polycystic kidney disease (ADPKD) is a renal neoplastic disorder characterized by the formation of large fluid-filled cysts that develop from kidney tubules through abnormal cell proliferation and cyst-filling fluid secretion driven by cAMP-dependent Cl- secretion. This study shows that the lonidamine derivative H2-GMZ inhibits Cl- secretion, cell proliferation, and cyst growth, suggesting that it might have therapeutic value for the treatment of ADPKD.

On the relationship between anion binding and chloride conductance in the CFTR anion channel

Mutations at many sites within the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel pore region result in changes in chloride conductance. Although chloride binding in the pore - as well as interactions between concurrently bound chloride ions - are thought to be important facets of the chloride permeation mechanism, little is known about the relationship between anion binding and chloride conductance. The present work presents a comprehensive investigation of a number of anion binding properties in different pore mutants with differential effects on chloride conductance. When multiple pore mutants are compared, conductance appears best correlated with the ability of anions to bind to the pore when it is already occupied by chloride ions. In contrast, conductance was not correlated with biophysical measures of anion:anion interactions inside the pore. Although these findings suggest anion binding is required for high conductance, mutations that strengthened anion binding had very little effect on conductance, especially at high chloride concentrations, suggesting that the wild-type CFTR pore is already close to saturated with chloride ions. These results are used to support a revised model of chloride permeation in CFTR in which the overall chloride occupancy of multiple loosely-defined chloride binding sites results in high chloride conductance through the pore.

Antitumor Effect of Lonidamine-Polypeptide-Peptide Nanoparticles in Breast Cancer Models

Biomaterials folded into nanoparticles (NPs) can be utilized as targeted drug delivery systems for cancer therapy. NPs may provide a vehicle for the anticancer drug lonidamine (LND), which inhibits glycolysis but was suspended from use at the clinical trial stage because of its hepatotoxicity due to poor solubility and pharmacokinetic properties. The NPs prepared by coassembly of the anionic polypeptide poly gamma glutamic acid (γ-PGA) and a designed amphiphilic and positively charged peptide (designated as mPoP-NPs) delivered LND to the mitochondria in cell cultures. In this study, we demonstrate that LND-mPoP-NP effective drug concentrations can be increased to reach therapeutically relevant concentrations. The self-assembled NP solution was subjected to snap-freezing and lyophilization and the resultant powder was redissolved in a tenth of the original volume. The NP size and their ability to target the proximity of the mitochondria of breast cancer cells were both maintained in this new formulation, C-LND-mPoP-NPs. Furthermore, these NPs exhibited 40% better cytotoxicity, relative to the nonlyophilized LND-mPoP-NPs and led to tumor growth inhibition with no adverse side effects upon intravenous administration in a xenograft breast cancer murine model.

Structural mechanisms of CFTR function and dysfunction

Hwang et al. integrate new structural insights with prior functional studies to reveal the functional anatomy of CFTR chloride channels.

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel plays a critical role in regulating transepithelial movement of water and electrolyte in exocrine tissues. Malfunction of the channel because of mutations of the cftr gene results in CF, the most prevalent lethal genetic disease among Caucasians. Recently, the publication of atomic structures of CFTR in two distinct conformations provides, for the first time, a clear overview of the protein. However, given the highly dynamic nature of the interactions among CFTR’s various domains, better understanding of the functional significance of these structures requires an integration of these new structural insights with previously established biochemical/biophysical studies, which is the goal of this review.

Loop diuretics are open-channel blockers of the cystic fibrosis transmembrane conductance regulator with distinct kinetics

BACKGROUND AND PURPOSE

Loop diuretics are widely used to inhibit the Na(+), K(+), 2Cl(-) co-transporter, but they also inhibit the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. Here, we investigated the mechanism of CFTR inhibition by loop diuretics and explored the effects of chemical structure on channel blockade.

EXPERIMENTAL APPROACH

Using the patch-clamp technique, we tested the effects of bumetanide, furosemide, piretanide and xipamide on recombinant wild-type human CFTR.

KEY RESULTS

When added to the intracellular solution, loop diuretics inhibited CFTR Cl(-) currents with potency approaching that of glibenclamide, a widely used CFTR blocker with some structural similarity to loop diuretics. To begin to study the kinetics of channel blockade, we examined the time dependence of macroscopic current inhibition following a hyperpolarizing voltage step. Like glibenclamide, piretanide blockade of CFTR was time and voltage dependent. By contrast, furosemide blockade was voltage dependent, but time independent. Consistent with these data, furosemide blocked individual CFTR Cl(-) channels with 'very fast' speed and drug-induced blocking events overlapped brief channel closures, whereas piretanide inhibited individual channels with 'intermediate' speed and drug-induced blocking events were distinct from channel closures.

CONCLUSIONS AND IMPLICATIONS

Structure-activity analysis of the loop diuretics suggests that the phenoxy group present in bumetanide and piretanide, but absent in furosemide and xipamide, might account for the different kinetics of channel block by locking loop diuretics within the intracellular vestibule of the CFTR pore. We conclude that loop diuretics are open-channel blockers of CFTR with distinct kinetics, affected by molecular dimensions and lipophilicity.

Cystic fibrosis transmembrane conductance regulator chloride channel blockers: Pharmacological, biophysical and physiological relevance

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel causes cystic fibrosis, while inappropriate activity of this channel occurs in secretory diarrhea and polycystic kidney disease. Drugs that interact directly with CFTR are therefore of interest in the treatment of a number of disease states. This review focuses on one class of small molecules that interacts directly with CFTR, namely inhibitors that act by directly blocking chloride movement through the open channel pore. In theory such compounds could be of use in the treatment of diarrhea and polycystic kidney disease, however in practice all known substances acting by this mechanism to inhibit CFTR function lack either the potency or specificity for in vivo use. Nevertheless, this theoretical pharmacological usefulness set the scene for the development of more potent, specific CFTR inhibitors. Biophysically, open channel blockers have proven most useful as experimental probes of the structure and function of the CFTR chloride channel pore. Most importantly, the use of these blockers has been fundamental in developing a functional model of the pore that includes a wide inner vestibule that uses positively charged amino acid side chains to attract both permeant and blocking anions from the cell cytoplasm. CFTR channels are also subject to this kind of blocking action by endogenous anions present in the cell cytoplasm, and recently this blocking effect has been suggested to play a role in the physiological control of CFTR channel function, in particular as a novel mechanism linking CFTR function dynamically to the composition of epithelial cell secretions. It has also been suggested that future drugs could target this same pathway as a way of pharmacologically increasing CFTR activity in cystic fibrosis. Studying open channel blockers and their mechanisms of action has resulted in significant advances in our understanding of CFTR as a pharmacological target in disease states, of CFTR channel structure and function, and of how CFTR activity is controlled by its local environment.

Divergent CFTR orthologs respond differently to the channel inhibitors CFTRinh-172, glibenclamide, and GlyH-101

Comparison of diverse orthologs is a powerful tool to study the structure and function of channel proteins. We investigated the response of human, killifish, pig, and shark cystic fibrosis transmembrane conductance regulator (CFTR) to specific inhibitors of the channel: CFTR(inh)-172, glibenclamide, and GlyH-101. In three systems, including organ perfusion of the shark rectal gland, primary cultures of shark rectal gland tubules, and expression studies of each ortholog in cRNA microinjected Xenopus laevis oocytes, we observed fundamental differences in the sensitivity to inhibition by these channel blockers. In organ perfusion studies, shark CFTR was insensitive to inhibition by CFTR(inh)-172. This insensitivity was also seen in short-circuit current experiments with cultured rectal gland tubular epithelial cells (maximum inhibition 4 ± 1.3%). In oocyte expression studies, shark CFTR was again insensitive to CFTR(inh)-172 (maximum inhibition 10.3 ± 2.5% at 25 μM), pig CFTR was insensitive to glibenclamide (maximum inhibition 18.4 ± 4.4% at 250 μM), and all orthologs were sensitive to GlyH-101. The amino acid residues considered responsible by previous site-directed mutagenesis for binding of the three inhibitors are conserved in the four CFTR isoforms studied. These experiments demonstrate a profound difference in the sensitivity of different orthologs of CFTR proteins to inhibition by CFTR blockers that cannot be explained by mutagenesis of single amino acids. We believe that the potency of the inhibitors CFTR(inh)-172, glibenclamide, and GlyH-101 on the CFTR chloride channel protein is likely dictated by the local environment and the three-dimensional structure of additional residues that form the vestibules, the chloride pore, and regulatory regions of the channel.

Regulation of conductance by the number of fixed positive charges in the intracellular vestibule of the CFTR chloride channel pore

Rapid chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is dependent on the presence of fixed positive charges in the permeation pathway. Here, we use site-directed mutagenesis and patch clamp recording to show that the functional role played by one such positive charge (K95) in the inner vestibule of the pore can be "transplanted" to a residue in a different transmembrane (TM) region (S1141). Thus, the mutant channel K95S/S1141K showed Cl(-) conductance and open-channel blocker interactions similar to those of wild-type CFTR, thereby "rescuing" the effects of the charge-neutralizing K95S mutation. Furthermore, the function of K95C/S1141C, but not K95C or S1141C, was inhibited by the oxidizing agent copper(II)-o-phenanthroline, and this inhibition was reversed by the reducing agent dithiothreitol, suggesting disulfide bond formation between these two introduced cysteine side chains. These results suggest that the amino acid side chains of K95 (in TM1) and S1141 (in TM12) are functionally interchangeable and located closely together in the inner vestibule of the pore. This allowed us to investigate the functional effects of increasing the number of fixed positive charges in this vestibule from one (in wild type) to two (in the S1141K mutant). The S1141K mutant had similar Cl(-) conductance as wild type, but increased susceptibility to channel block by cytoplasmic anions including adenosine triphosphate, pyrophosphate, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and Pt(NO(2))(4)(2-) in inside-out membrane patches. Furthermore, in cell-attached patch recordings, apparent voltage-dependent channel block by cytosolic anions was strengthened by the S1141K mutation. Thus, the Cl(-) channel function of CFTR is maximal with a single fixed positive charge in this part of the inner vestibule of the pore, and increasing the number of such charges to two causes a net decrease in overall Cl(-) transport through a combination of failure to increase Cl(-) conductance and increased susceptibility to channel block by cytosolic substances.

The anti-trypanosomal agent lonidamine inhibits Trypanosoma brucei hexokinase 1

Glycolysis is essential to the parasitic protozoan Trypanosoma brucei. The first step in this metabolic pathway is mediated by hexokinase, an enzyme that transfers the gamma-phosphate of ATP to a hexose. The T. brucei genome (TREU927/4 GUTat10.1) encodes two hexokinases (TbHK1 and TbHK2) that are 98% identical at the amino acid level. Our previous efforts have revealed that TbHK2 is an important regulator of TbHK1 in procyclic form parasites. Here, we have found through RNAi that TbHK1 is essential to the bloodstream form parasite. Silencing the gene for 4 days reduces cellular hexokinase approximately 60% and leads to parasite death. Additionally, we have found that the recombinant enzyme is inhibited by lonidamine (IC(50)=850 microM), an anti-cancer drug that targets tumor hexokinases. This agent also inhibits HK activity from whole parasite lysate (IC(50)=965 microM). Last, lonidamine is toxic to cultured bloodstream form parasites (LD(50)=50 microM) and procyclic form parasites (LD(50)=180 microM). Interestingly, overexpression of TbHK1 protects PF parasites from lonidamine. These studies provide genetic evidence that TbHK1 is a valid therapeutic target while identifying a potential molecular target of the anti-trypanosomal agent lonidamine.

Direct and indirect effects of mutations at the outer mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore is thought to contain multiple binding sites for permeant and impermeant anions. Here, we investigate the effects of mutation of different positively charged residues in the pore on current inhibition by impermeant Pt(NO(2)) (4) (2-) and suramin anions. We show that mutations that remove positive charges (K95, R303) influence interactions with intracellular, but not extracellular, Pt(NO(2))(4)(2-) ions, consistent with these residues being situated within the pore inner vestibule. In contrast, mutation of R334, supposedly located in the outer vestibule of the pore, affects block by both extracellular and intracellular Pt(NO(2))(4)(2-). Inhibition by extracellular Pt(NO(2))(4)(2-) requires a positive charge at position 334, consistent with a direct electrostatic interaction resulting in either open channel block or surface charge screening. In contrast, inhibition by intracellular Pt(NO(2))(4)(2-) is weakened in all R334-mutant forms of the channel studied, inconsistent with a direct interaction. Furthermore, mutation of R334 had similar effects on block by intracellular suramin, a large organic molecule that is apparently unable to enter deeply into the channel pore. Mutation of R334 altered interactions between intracellular Pt(NO(2))(4)(2-) and extracellular Cl(-) but not those between intracellular Pt(NO(2))(4)(2-) and extracellular Pt(NO(2))(4)(2-). We propose that while the positive charge of R334 interacts directly with extracellular anions, mutation of this residue also alters interactions with intracellular anions by an indirect mechanism, due to mutation-induced conformational changes in the protein that are propagated some distance from the site of the mutation in the outer mouth of the pore.

CFTR-dependent Cl- secretion in Xenopus laevis lung epithelium

In our present study we used preparations from Xenopus laevis lungs to perform electrophysiological Ussing chamber measurements, unidirectional flux measurements, and employed molecular approaches to elucidate the presence and function of a cystic fibrosis transmembrane conductance regulator (CFTR) homolog in this tissue. Application of different CFTR blockers (NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid), niflumic acid (NFA), glibenclamide, lonidamine, CFTR(inh)-172) to the apical side of the tissues was able to significantly decrease the measured short circuit current (I(SC)) indicating a Cl(-) secretion due to luminal located CFTR channels. This was further supported by a net (36)Cl(-) secretion determined by radioactive tracer flux experiments. Further, Xenopus pulmonary epithelia responded to apical chlorzoxazone exposure - a CFTR activator - and this activated current was inhibited by CFTR(inh)-172. We performed reverse transcription-PCR (RT-PCR) and Western blot analysis and with both approaches we found characteristic signals indicating the presence of a CFTR homolog in Xenopus lung. In addition, we were able to detect CFTR in apical membranes of Xenopus lung slices with immunohistological techniques. We conclude that Xenopus lung epithelium exhibits functional CFTR channels and that this tissue represents a valuable model for the investigation of ion transport properties in pulmonary epithelia.

Identification of a second blocker binding site at the cytoplasmic mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is inhibited by a broad range of substances that bind within a wide inner vestibule in the pore and physically occlude Cl(-) permeation. Binding of many of these so-called open-channel blockers involves electrostatic interactions with a positively charged lysine residue (Lys95) located in the pore. Here, we use site-directed mutagenesis to identify a second blocker binding site located at the cytoplasmic mouth of the pore. Mutagenesis of a positively charged arginine at the cytoplasmic mouth of the pore, Arg303, leads to significant weakening of the blocking effects of suramin, a large negatively charged organic molecule. Apparent suramin affinity is correlated with the side chain charge at this position, consistent with an electrostatic interaction. In contrast, block by suramin is unaffected by mutagenesis of Lys95, suggesting that it does not approach close to this important pore-forming lysine residue. We propose that the CFTR pore inner vestibule contains two distinct blocker binding sites. Relatively small organic anions enter deeply into the pore to interact with Lys95, causing an open-channel block that is sensitive to both the membrane potential and the extracellular Cl(-) concentration. Larger anionic molecules can become lodged in the cytoplasmic mouth of the pore where they interact with Arg303, causing a distinct type of open-channel block that is insensitive to membrane potential or extracellular Cl(-) ions. The pore may narrow significantly between the locations of these two blocker binding sites.

On the origin of asymmetric interactions between permeant anions and the cystic fibrosis transmembrane conductance regulator chloride channel pore

Single channel and macroscopic current recording was used to investigate block of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore by the permeant anion Au(CN)2(-). Block was 1-2 orders of magnitude stronger when Au(CN)2(-) was added to the intracellular versus the extracellular solution, depending on membrane potential. A point mutation within the pore, T-338A, strongly decreased the asymmetry of block, by weakening block by intracellular Au(CN)2(-) and at the same time strengthening block by external Au(CN)2(-). Block of T-338A, but not wild-type, was strongest at the current reversal potential and weakened by either depolarization or hyperpolarization. In contrast to these effects, the T-338A mutation had no impact on block by the impermeant Pt(NO2)4(2-) ion. We suggest that the CFTR pore has at least two anion binding sites at which Au(CN)2(-) and Pt(NO2)4(2-) block Cl- permeation. The T-338A mutation decreases a barrier for Au(CN)2(-) movement between different sites, leading to significant changes in its blocking action. Our finding that apparent blocker binding affinity can be altered by mutagenesis of a residue which does not contribute to a blocker binding site has important implications for interpreting the effects of mutagenesis on channel blocker effects.

Interactions between impermeant blocking ions in the cystic fibrosis transmembrane conductance regulator chloride channel pore: evidence for anion-induced conformational changes

It is well known that extracellular Cl(-) ions can weaken the inhibitory effects of intracellular open channel blockers in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore. This effect is frequently attributed to repulsive ion-ion interactions inside the pore. However, since Cl(-) ions are permeant in CFTR, it is also possible that extracellular Cl(-) ions are directly competing with intracellular blocking ions for a common binding site; thus, this does not provide direct evidence for multiple, independent anion binding sites in the pore. To test for the possible through-space nature of ion-ion interactions inside the CFTR pore, we investigated the interaction between impermeant anions applied to either end of the pore. We found that inclusion of low concentrations of impermeant Pt(NO(2))(4) (2-) ions in the extracellular solution weaken the blocking effects of three different intracellular blockers [Pt(NO(2))(4) (2-), glibenclamide and 5-nitro-2-(3-phenylpropylamino)benzoic acid] without affecting their apparent voltage dependence. However, the effects of extracellular Pt(NO(2))(4) (2-) ions are too strong to be accounted for by simple competitive models of ion binding inside the pore. In addition, extracellular Fe(CN)(6) (3-) ions, which do not appear to enter the pore, also weaken the blocking effects of intracellular Pt(NO(2))(4) (2-) ions. In contrast to previous models that invoked interactions between anions bound concurrently inside the pore, we propose that Pt(NO(2))(4) (2-) and Fe(CN)(6) (3-) binding to an extracellularly accessible site outside of the channel permeation pathway alters the structure of an intracellular anion binding site, leading to weakened binding of intracellular blocking ions.

Human multidrug resistance protein 8 (MRP8/ABCC11), an apical efflux pump for steroid sulfates, is an axonal protein of the CNS and peripheral nervous system

Dehydroepiandrosterone 3-sulfate and other neurosteroids are synthesized in the CNS and peripheral nervous system where they may modulate neuronal excitability by interacting with ligand-gated ion channels. For this modulatory activity, neurosteroids have to be locally released from either neurons or glial cells. We here identify the integral membrane protein ABCC11 (multidrug resistance protein 8) as an ATP-dependent efflux pump for steroid sulfates, including dehydroepiandrosterone 3-sulfate, and localize it to axons of the human CNS and peripheral nervous system. ABCC11 mRNA was detected in human brain by real-time polymerase chain reaction. Antibodies raised against ABCC11 served to detect the protein in brain by immunoblotting and immunofluorescence microscopy. ABCC11 was preferentially found in the white matter of the brain and co-localized with neurofilaments indicating that it is an axonal protein. Additionally, ABCC11 was localized to axons of the peripheral nervous system. For functional studies, ABCC11 was expressed in polarized Madin-Darby canine kidney cells where it was sorted to the apical membrane. This apical sorting is in accordance with the localization of ABCC11 to the axonal membrane of neurons. Inside-out plasma membrane vesicles containing recombinant ABCC11 mediated ATP-dependent transport of dehydroepiandrosterone 3-sulfate with a Km value of 21 microM. This transport function together with the localization of the ABCC11 protein in vicinity to GABAA receptors is consistent with a role of ABCC11 in dehydroepiandrosterone 3-sulfate release from neurons to sites of dehydroepiandrosterone 3-sulfate-mediated receptor modulation. Our findings may provide a basis for the characterization of mutations in the human ABCC11 gene and their linkage with neurological disorders.

Lonidamine transiently affects spermatogenesis in pubertal CD1 mice

Lonidamine (LND) [1-(2,4-dichlorobenzyl)-1H-indazole-3-carboxylic acid], a well-known antispermatogenic drug, was studied for the first time in pubertal mice to assess its possible effects on spermatogenesis. Male CD1 mice were orally treated on Postnatal Day (PND) 28 with a single dose of LND (100 mg/kg body weight) and sacrificed on PND30, PND42, PND74 and PND123. On PND30 (48 h after dosing), severe testicular effects were evidenced in the treated animals: (a) reduction of the testicular sperm head concentration (approximately 50% of the control value); (b) changes in the spermatogenic cell type distribution (mild decrease of the elongated spermatids and S-phase cells fractions); and (c) morphological alterations of the Sertoli cell cytoplasm and germ cell exfoliation. These changes were recovered in adulthood, on PND74 and PND123. However, no effect on sperm chromatin structure was detected on the epididymal sperm of mature mice by sperm chromatin structure assay, suggesting that LND did not interfere with the process of chromatin reorganization and DNA packaging.

Location of a common inhibitor binding site in the cytoplasmic vestibule of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is inhibited by a broad range of organic anions that enter the channel pore from its cytoplasmic end, physically occluding the Cl- permeation pathway. These open channel blocker molecules are presumed to bind within a relatively wide pore inner vestibule that shows little discrimination between different large anions. The present study uses patch clamp recording to identify a pore-lining lysine residue, Lys-95, that acts to attract large blocker molecules into this inner vestibule. Mutations that remove the fixed positive charge associated with this amino acid residue dramatically weaken the blocking effects of five structurally unrelated open channel blockers (glibenclamide, 4,4'-dinitrostilbene-2,2'-disulfonic acid, lonidamine, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and taurolithocholate-3-sulfate) when applied to the cytoplasmic face of the membrane. Mutagenesis of Lys-95 also induced amino acid side chain charge-dependent rectification of the macroscopic current-voltage relationship, consistent with the fixed positive charge on this residue normally acting to attract Cl- ions from the intracellular solution into the pore. These results identify Lys-95 as playing an important role in attracting permeant anions into the channel pore inner vestibule, probably by an electrostatic mechanism. This same electrostatic attraction mechanism also acts to attract larger anionic molecules into the relatively wide inner vestibule, where these substances bind to block Cl- permeation. Thus, structurally diverse open channel blockers of CFTR appear to share a common molecular mechanism of action that involves interaction with a positively charged amino acid side chain located in the inner vestibule of the pore.

Secretin Controls Anion Secretion in the Rat Epididymis in an Autocrine/Paracrine Fashion1

There is growing evidence that secretin, the first hormone discovered in our history, has functions in the brain other than in the gastrointestinal tract. This article reports for the first time that secretin and its receptor mRNAs are produced in distinct cell types within the epididymis. To test if secretin affects electrolyte transport in the epididymis, we measured short-circuit current (Isc) in cultured epididymal epithelia and found secretin dose-dependently stimulated Isc. Ion substitution experiments and use of pharmacological agents inferred that the stimulated Isc is a result of concurrent electrogenic chloride and bicarbonate secretion. It is further shown that secretin and pituitary adenylate cyclase-activating polypeptide (PACAP) function via totally different mechanisms: 1) PACAP works only from the apical side of the epithelium to stimulate chloride and not bicarbonate secretion, while secretin acts on the apical and basolateral sides to stimulate chloride and bicarbonate secretion. 2) the stimulation by PACAP but not secretin requires local prostaglandin synthesis. By immunocytochemical staining, secretin is localized in the principal cells of the initial segment and caput epididymidis, whereas secretin receptor is present in the principal cells of the proximal as well as the distal part of the epididymis. This pattern of distribution appears to be consistent with the idea that secretin is secreted by the proximal epididymis and acts on the proximal and distal epididymis in an autocrine and paracrine fashion. Its function is to control secretion of electrolytes and water.

Maximization of the rate of chloride conduction in the CFTR channel pore by ion–ion interactions

Multi-ion pore behaviour has been identified in many Cl(-) channel types but its biophysical significance is uncertain. Here, we show that mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel that disrupt anion-anion interactions within the pore are associated with drastically reduced single channel conductance. These results are consistent with models suggesting that rapid Cl(-) permeation in CFTR results from repulsive ion-ion interactions between Cl(-) ions bound concurrently inside the pore. Naturally occurring mutations that disrupt these interactions can result in cystic fibrosis.

Mutation-induced blocker permeability and multiion block of the CFTR chloride channel pore

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel is blocked by a broad range of anions that bind tightly within the pore. Here we show that the divalent anion Pt(NO₂)₄²⁻ acts as an impermeant voltage-dependent blocker of the CFTR pore when added to the intracellular face of excised membrane patches. Block was of modest affinity (apparent K_d 556 μM), kinetically fast, and weakened by extracellular Cl⁻ ions. A mutation in the pore region that alters anion selectivity, F337A, but not another mutation at the same site that has no effect on selectivity (F337Y), had a complex effect on channel block by intracellular Pt(NO₂)₄²⁻ ions. Relative to wild-type, block of F337A-CFTR was weakened at depolarized voltages but strengthened at hyperpolarized voltages. Current in the presence of Pt(NO₂)₄²⁻ increased at very negative voltages in F337A but not wild-type or F337Y, apparently due to relief of block by permeation of Pt(NO₂)₄²⁻ ions to the extracellular solution. This “punchthrough” was prevented by extracellular Cl⁻ ions, reminiscent of a “lock-in” effect. Relief of block in F337A by Pt(NO₂)₄²⁻ permeation was only observed for blocker concentrations above 300 μM; as a result, block at very negative voltages showed an anomalous concentration dependence, with an increase in blocker concentration causing a significant weakening of block and an increase in Cl⁻ current. We interpret this effect as reflecting concentration-dependent permeability of Pt(NO₂)₄²⁻ in F337A, an apparent manifestation of an anomalous mole fraction effect. We suggest that the F337A mutation allows intracellular Pt(NO₂)₄²⁻ to enter deeply into the CFTR pore where it interacts with multiple binding sites, and that simultaneous binding of multiple Pt(NO₂)₄²⁻ ions within the pore promotes their permeation to the extracellular solution.

Molecular determinants and role of an anion binding site in the external mouth of the CFTR chloride channel pore

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is blocked by highly lyotropic permeant anions which bind tightly within the pore. Here we show that several different substitutions of a positively charged amino acid residue, arginine R334, in the putative outer mouth of the CFTR pore, greatly reduce the block caused by lyotropic Au(CN)2- ions applied to the intracellular side of the channel. Fixed positive charge at this site appears to play a role in Au(CN)2- binding, as judged by multiple substitutions of differently charged amino acid side chains and also by the pH dependence of block conferred by the R334H mutant. However, non-charge-dependent effects also appear to contribute to Au(CN)2- binding. Mutation of R334 also disrupts the apparent electrostatic interaction between intracellular Au(CN)2- ions and extracellular permeant anions, an interaction which normally acts to relieve channel block. All six mutations studied at R334 significantly weakened this interaction, suggesting that arginine possesses a unique ability to coordinate ion-ion interactions at this site in the pore. Our results suggest that lyotropic anions bind tightly to a site in the outer mouth of the CFTR pore that involves interaction with a fixed positive charge. Binding to this site is also involved in coordination of multiple permeant anions within the pore, suggesting that anion binding in the outer mouth of the pore is an important aspect in the normal anion permeation mechanism.

Coupled movement of permeant and blocking ions in the CFTR chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel pore is blocked in a voltage-dependent manner by a broad range of anionic substances added to the cytoplasmic side of the membrane. Here we investigate the origin of the voltage dependence of block by intracellular Au(CN)2-, a highly permeant lyotropic anion which also acts as a high-affinity blocker of Cl- permeation. Not only the affinity, but also the voltage dependence of block by intracellular Au(CN)2- ions is strongly dependent on extracellular Cl- concentration; following replacement of most extracellular Cl- by glucose or by impermeant anions, block by Au(CN)2- shows greatly weakened voltage dependence. This suggests that coupled movement of Au(CN)2- and Cl- ions within the pore contributes to the voltage dependence of block. This explanation requires that interactions between different anions take place within the pore, implying simultaneous binding of multiple anions to intrapore sites. Other anions are able to substitute for extracellular Cl- and interact with intracellular Au(CN)2- ions. Analysis of the effects of different extracellular anions on the apparent affinity and voltage dependence of block by intracellular Au(CN)2- ions suggests that extracellular anions do not need to permeate through the channel in order to destabilize Au(CN)2- binding within the pore, implying that this destabilizing effect results from binding to an externally accessible site in the permeation pathway. We propose that multiple anions can bind simultaneously within the CFTR channel pore, and that repulsive interactions between bound anions speeds anion exit from the pore.