parallel improvement of sodium and...

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PARALLEL IMPROVEMENT OF SODIUM AND CHLORIDE TRANSPORT

DEFECTS BY MIGLUSTAT IN CYSTIC FIBROSIS EPITHELIAL CELLS

Sabrina Noël, Martina Wilke, Alice G. M. Bot, Hugo R. De Jonge, Frédéric Becq

Institut de Physiologie et Biologie Cellulaires, Université de Poitiers, CNRS, 40 avenue du

recteur Pineau, 86022 Poitiers, France (S.N., F.B.), Department of Biochemistry, Erasmus

University Medical Center, Rotterdam, the Netherlands (M.W., A.G.M.B., H.R.D.J.).

JPET Fast Forward. Published on February 28, 2008 as DOI:10.1124/jpet.107.135582

Copyright 2008 by the American Society for Pharmacology and Experimental Therapeutics.

This article has not been copyedited and formatted. The final version may differ from this version.JPET Fast Forward. Published on February 28, 2008 as DOI: 10.1124/jpet.107.135582

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Running title : ENaC activity in rescued F508del-CFTR cells

Address correspondence to:

Pr Frédéric BECQ

IPBC CNRS UMR 6187,

Université de Poitiers, 40 Avenue du Recteur Pineau 86022 Poitiers, France

Tel : +33-5-49-45-37-29 ; Fax : +33-5-49-45-40-14

E-mail : [email protected]

Text pages : 30

0 table

7 figures

39 refs

Abstract : 195 words

Introduction : 446 words

Discussion : 1445 words

Non standard abbreviations :

CF : cystic fibrosis; CFTR : CF transmembrane conductance regulator; CFTRinh-172 : 3-[(3-

trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone; DMSO :

dimethylsulfoxide, ENaC : Epithelial Sodium Channel ; fsk : forskolin, F508del : deletion of

phenylalanine at 508 position of CFTR protein ; Gst : genistein, NB-DNJ : n-

butyldeoxynojyrimicin ; wt : wild-type.

Recommended section : Cellular and molecular


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Abstract

Cystic Fibrosis, an autosomal recessive disease frequently diagnosed in the Caucasian

population, is characterized by deficient Cl- transport due to mutations in the cystic fibrosis

transmembrane conductance regulator (CFTR) gene. A second major hallmark of the disease

is Na+ hyperabsorption by the airways, mediated by the epithelial Na+ channel (ENaC). Here

we report that in human airway epithelial CF15 cells treated with the CFTR corrector

miglustat, whole-cell patch-clamp experiments showed reduced amiloride-sensitive ENaC

current in parallel with a rescue of defective CFTR Cl- channel activity activated by forskolin

and genistein. Similar results were obtained with cells maintained in culture at 27°C for 24h

prior to electrophysiology experiments. With monolayers of polarized CF15 cells, short-

circuit current (Isc) measurements also show normalization of Na+ and Cl- currents. In excised

nasal epithelium of cftrF508del/F508del mice, like with CF15 cells, we found normalization of

amiloride-sensitive Isc. Moreover, oral administration of miglustat (6 days) decreased the

amiloride-sensitive Isc in cftrF508del/F508del mice but had no effect on cftr-/- mice. Our results

thus show that rescuing the trafficking-deficient F508del-CFTR by miglustat down-regulates

Na+ absorption. A miglustat-based treatment of CF patients may thus have a beneficial effect

both on Cl- and Na+ transports.


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Introduction

The cystic fibrosis disease is the most common lethal genetic disorder in Caucasian

population with also a worldwide distribution. CF is characterized by a defective cAMP-

regulated Cl- conductance due to mutations in the cystic fibrosis transmembrane conductance

regulator (CFTR) gene (Riordan, 1993). One of these mutations, F508del, is the most

common in CF patients leading to abnormal trafficking of CFTR protein which is retained in

the ER (Cheng et al., 1990; Kartner et al., 1991; Pind et al., 1994). This molecular mechanism

makes F508del a prototype of class II (trafficking-deficient) mutations (Welsh & Smith,

1993). CF tissues are also characterized by enhanced Na+ absorption mediated by the

epithelial Na+ channel ENaC (reviewed in Boucher 2007). Abnormal Na+ transport by CF

airway epithelia has been demonstrated by many in vivo and in vitro observations in human

and mice, and an increased amiloride-sensitive transepithelial potential is used as a diagnostic

criterium in CF (Knowles et al., 1981; Knowles et al., 1983; Boucher et al., 1986; Grubb et al.,

1997; Mall et al., 1998). ENaC is expressed in a variety of epithelial tissues including airways,

renal collecting duct, urinary bladder, colon and sweat and salivary glands (Canessa et al.,

1994 ; Barbry et al., 1996).

It is now well documented that CFTR also regulates many transport proteins and

cellular functions due to large and dynamic macromolecular complexes that contain CFTR,

signalling molecules and transport proteins (see for review Kunzelman et al., 2000; Guggino

& Stanton, 2006). In favour of this concept, several studies showed that the functional

interaction between CFTR and ENaC regulates both epithelial Cl- and Na+ conductances

(Guggino & Stanton, 2006). However, it remains unclear why CF tissues display such a high

ENaC activity. In addition, no studies have examined the effect of F508del-CFTR correction


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on Na+ hyperabsorption in native tissues. Finally, other studies argued against a regulation of

ENaC by CFTR after co-expression in Xenopus laevis oocytes (Nagel et al., 2005).

Recently, we showed that miglustat (n-butyldeoxynojyrimicin, NB-DNJ) rescues the

trafficking-deficient F508del-CFTR to the plasma membrane in human airway epithelial cells,

but also in the intestine of F508del-CFTR mice (Norez et al., 2006; Antigny et al., 2007).

Miglustat is now evaluated in CF patients within a pilot phase 2a clinical trial

(http://clinicaltrials.gov/). In this report we addressed the question whether miglustat, by

rescuing F508del-CFTR abnormal trafficking, also down regulates ENaC-dependent sodium

hyperabsorption. To this aim we have studied endogenous CFTR and ENaC channels in

miglustat-corrected human airway epithelial CF15 cells and in cftrF508del/F508del mice. We

demonstrate that the rescue of endogenous F508del-CFTR by miglustat or by low temperature

in human airway and excised nasal epithelium of cftrF508del/F508del mice, is paralleled by a

down-regulation of Na+ absorption.


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Methods

Cell culture. The human nasal epithelial JME/CF15 cell line, derived from a F508del-

CFTR homozygous patient (Jefferson et al., 1990) was grown at 37°C in 5% CO2 under

standard culture conditions, in DMEM-Ham’s F-12 (3:1) nutritive mix supplemented by 10%

FBS, 100 IU/mL penicillin and 100 µg/mL streptomycin, 5 µg/mL insulin, 5 µg/mL

transferrin, 5.5 µM epinephrine, 180 µM adenine, 2 nM T3 (3,3′,5-Triiodo-L-thyronine

sodium salt) and 1.1 µM hydrocortisone (Cao et al., 2005; Norez et al., 2006). All culture

media and antibiotics were from Gibco BRL (Invitrogen, Cergy-Pontoise, France). FBS was

from Perbio (PerbioScience, Brebières, France). Hormones and growth factors were from

Sigma. Cells were seeded in 35-mm plastic dishes for whole-cell patch clamp recordings and

on 12 mm snapwells (diam. 1.13 cm², pores 3 µm, Corning Incorporated life sciences, Acton

MA, USA) for Ussing chamber experiments. Medium was renewed at 2 days-interval.

Patch-clamp experiments. Perforated whole-cell patch clamp experiments were

performed on CF15 cells at room temperature. Currents were recorded with a RK-400 patch-

clamp amplifier (Biologic, Grenoble, France). I-V relationships were built by clamping the

membrane potential to –20 mV and by pulses from –140 mV to +100 mV (20 mV increments).

Pipettes with resistance of 3-4 MΩ were pulled from borosilicate glass capillary tubing

(GC150-TF10, Clark Electromedical Inc., Reading, UK) using a two-step vertical puller

(Narishige, Japan). They were filled with the following solution: 20 mM NaCl, 100 mM L-

Aspartic Acid, 100 mM CsOH, 1 mM MgCl2, 20 mM CsCl, 4 mM EGTA, 10 mM Hepes (pH

7.2). Amphotericin B (100 µg/mL) was dissolved ex temporane. Pipettes were connected to

the head of the patch-clamp amplifier through an Ag-AgCl pellet. Seal resistances ranging

from 6 to 20 GΩ were obtained. Results were analysed with the pClamp 6.0.2 package

software (pClamp, Axon Instruments). The external bath solution contained 150 mM NaCl, 6


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mM CsCl, 1 mM CaCl2, 1 mM MgCl2, 10 mM glucose and 10 mM Hepes (pH 7.4). The

liquid potential was corrected before seal establishment. Pipette capacitances were

electronically compensated in cell-attached mode. To standardize experiments, recordings

were performed only when the input resistance had a value ≤ 15 MΩ. The mean value of

access resistances was 11.7 ± 1.6 MΩ (n=42). Membrane capacitances were measured in the

whole-cell mode by fitting capacitance currents, obtained in response to a hyperpolarisation of

6 mV, with a first-order exponential and by integrating the surface of the capacitance current.

Mean values of membrane capacitance were 30.7 ± 8.2 pF (n=42). For graphic representations,

I-V relationship was normalized to 1 pF, in order to remove variability due to differences in

cell sizes.

Animals. Rotterdam homozygous F508del-CFTR mice (cftrtm1Eur) and their littermate

controls (FVB inbred, 14–17 weeks old, weight between 20 and 30 g) were kept on solid food

in a pathogen-free environment. Cftr-/- mice (cftrtm2Cam) were backcrossed for 12 generations

into the FVB background (Scholte et al., 2004). Animals were anesthetized by an

intraperitoneal injection of a cocktail containing ketamine (10 mg/mL), xylazine (1.5 mg/mL)

and diazepam (0.6 mg/mL). Nasal epithelia were dissected away from mice and mounted in a

mini-Ussing chamber (exposed tissue area 1.13 mm2) (De Jonge et al., 2004).

Short-circuit current measurements. For short-circuit current (Isc) measurements,

we seeded CF15 cells on semipermeable membrane Snapwell inserts (exposed surface area

1.13 cm²). When cells were forming an impermeable monolayer (Transepithelial resistance

(RTE) ≥ 500 Ω.cm-2) short-circuit current recordings were performed. Snapwells were

mounted in a vertical Ussing chamber (Harvard Apparatus, Holliston MA, USA). Cells were

bathed in Meyler buffer (apical and basolateral side) containing : 120 mM NaCl, 1.2 mM

CaCl2, 1.2 mM MgCl2, 0.8 mM K2HPO4, 3.3 mM KH2PO4, 25 mM NaHCO3 and 10 mM D-

Glucose (pH 7.4, gassed with 95% O2-5% CO2 at 37°C). Current magnitude was referred to


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the apical side of the monolayer. Miglustat was added directly to the culture medium (100 µM,

2h, 37°C). Ex vivo short-circuit currents across isolated nasal epithelium were recorded first

under control conditions using continuous oxygenated (95% CO2-5% O2) and temperature-

controlled (37°C) Meyler solution and measurements were repeated on the same tissue

following 2 hours incubation in Meyler solution containing 100 µM miglustat. For other

technical details, see elsewhere (Noël et al., 2006).

Oral administration of miglustat to mice and Isc measurements. To evaluate the

effect of in vivo miglustat treatment on amiloride-sensitive current in nasal epithelium, we

administrated 1200 mg/kg/day miglustat by gavage to cftrF508del/F508del or cftr-/- (FVB) mice.

Control groups received vehicle i.e. PBS solution only. After 6 days, we dissected the nasal

epithelium from mice of the two groups and recorded the amiloride sensitive Isc ex vivo.

Pharmacological agents. The specific CFTR inhibitor 3-[(3-trifluoromethyl)phenyl]-

5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone (CFTRinh-172) (Ma et al., 2002)

was from Calbiochem (VWR international, Fontenay/bois, France). Forskolin was from LC

laboratories (PKC Pharmaceuticals, Inc, Woburn, MA, USA). Miglustat (N-

butyldeoxynojyrimicin) was purchased from Toronto Research Chemicals (Canada). All other

chemicals were from Sigma Aldrich (St Louis, MO, USA). All chemicals were dissolved in

DMSO (final concentration in DMSO<0.1%) except miglustat that was dissolved in water for

all in vitro and ex vivo experiments, and in PBS for oral administration. The currents were not

altered by DMSO alone.

Data analysis. All the data are presented as mean value ± SEM, where n refers to the

number of experiments and N to the number of animals. The unpaired student’s t-test was

used to compare sets of data. All graphs are plotted with GraphPad Prism 4.0 for Windows

(GraphPad Software, San Diego, CA, USA). Values of P<0.05 were considered as


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statistically significant: * P<0.05, ** P<0.01, *** P<0.001. Non significant (ns) difference

was P >0.05.


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Results

Miglustat reduces amiloride-sensitive Na+ current. Perforated whole-cell patch clamp

experiments were performed to measure the impact of miglustat treatment on ENaC and

CFTR currents in human airway epithelial CF cells (Fig. 1, cells maintained 2h at 37°C in a

culture medium containing 100 µM miglustat). JME/CF15 cells derived from the nasal airway

epithelium of a CF patient (homozygous F508del-CFTR; Jefferson et al., 1990) were not

responsive to forskolin/genistein (hereafter noted fsk/gst) stimulation but displayed a

significant amiloride-sensitive Na+ current (Tong et al., 2004; Cao et al., 2005). In the first

series of experiments, we identified and characterized ENaC currents in several bath

conditions (as indicated on the top of each panel) for control CF15 cells cultured at 37°C (left

traces Fig. 1), for temperature (27°C) corrected cells (middle traces Fig.1) and for miglustat-

corrected cells (right traces Fig.1). For each cell for which a perforated whole-cell experiment

was possible, we first recorded basal currents (Fig. 1A) and then added 100 µM amiloride in

the bath solution (Fig. 1B). By substracting the residual current in presence of amiloride from

the basal current, we obtained the mean value for amiloride-sensitive current normalized to

the cell capacitance and calculated the mean current densities (pA/pF, Fig. 2). At +100 mV,

we obtained mean values of 7.8 ± 2.3 pA/pF for control cells (n=12), 0.52 ± 0.35 pA/pF for

low temperature-corrected cells (n=7, P<0.05) and 1.7 ± 0.8 pA/pF for miglustat-corrected

cells (n=10, P<0.05). Then, on the same cell, we added the cocktail containing 10 µM

forskolin plus 30 µM genistein to activate CFTR currents (Fig. 1C). Under this condition we

only observed activation of linear currents with miglustat- (n = 10) and low temperature-

corrected cells (n = 7, Fig. 1C and 2). In presence of fsk/gst in the bath, the corresponding

experimental reversal potential Erev was -33.6 ± 3.4 mV for miglustat and -32.6 ± 2.9 mV for

low temperature corrected cells; both values being near the theoretical Nernst potential for Cl-


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ion (ECl- = -33 mV) showing stimulation of chloride-selective currents. We determined the

current density, after adding amiloride, for the Cl- currents recorded with miglustat-treated

cells (current density at +100 mV: 11.6 ± 3.2 pA/pF, n=10) and with temperature-corrected

CF15 cells (current density at +100 mV: 16.5 ± 4.9 pA/pF, n=7). No activation of Cl- current

was recorded in untreated CF15 cells (current density of 1.75 ± 0.27 pA/pF, n=12). To prove

that the Cl- current activated after amiloride in the presence of fsk/gst was due to F508del-

CFTR, we perfused 10 µM of the specific CFTR inhibitor CFTRinh-172 (Ma et al., 2002).

This maneuver fully inhibited Cl- currents in miglustat- and temperature-corrected cells (Fig.

1D).

Activation of CFTR does not influence ENaC currents in CF15 cells. In a second

series of experiments we wished to evaluate the effect of a pre-activation of CFTR by fsk/gst

on the amiloride-sensitive current on CF15 cells in the same experimental conditions as in

figure 1. To that end, we reversed the protocol for channel activation, i.e. we activated CFTR

first and then measured amiloride-sensitive ENaC currents. Fig. 3A shows spontaneous

control currents in resting CF15 cells. Adding fsk/gst in the experimental chamber activated a

linear Cl- current only in miglustat corrected cells (right traces Fig. 3B, current density at

+100 mV of 13.1 ± 3.1 pA/pF, n=4, data not shown). As expected no Cl- current was activated

in untreated cells (left traces Fig. 3B). The Cl- current density at +100 mV was 6.8 ± 0.9

pA/pF in control condition and 5.8 ± 0.8 pA/pF in presence of fsk/gst in the bath (ns, data not

shown). After adding amiloride to the bath to block the activity of ENaC, the residual current

in miglustat corrected cells (Fig. 3C, right traces), was inhibited by CFTRinh-172 (Fig. 3D,

right traces) indicating that F508del-CFTR channels were active. We measured the

corresponding amiloride-sensitive ENaC current and calculated the amiloride-sensitive

current density at +100 mV (Fig. 4). For miglustat corrected cells, we found no significant

difference between amiloride-sensitive current with fsk/gst in the bath (1.76 ± 0.3 pA/pF at


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+100mV, n=4, ns) or without (1.7 ± 0.8 pA/pF at +100mV, n=10). Similarly, no significant

effect of fsk/gst was noted on the magnitude of the amiloride-sensitive current in untreated

CF15 cells (7.8 ± 2.3 pA/pF in control condition, n = 12, and 6.7 ± 1.7 pA/pF in presence of

fsk/gst, n = 9, ns) (Fig. 4). Also we performed several additional experiments to determine

whether miglustat could by itself activated membrane conductances in CF cells. When

perfusing miglustat in the experimental chamber bathing CF15 cells (cultured at 37°), we

were not able to record any conductances (data not shown). Moreover, using iodide efflux

methods we did not detect any stimulation of efflux in the presence of this agent. Finally with

CF15 cells treated 2h by this corrector, no modulation of either volume- or calcium-

dependent-iodide efflux were noted (not shown, see also Norez et al. 2006). Taken together

these experiments show that the iminosugar miglustat is not a channel activator but rather a

F508del-CFTR corrector.

Miglustat reduces amiloride-sensitive Isc in polarized CF15 cells. We performed

Ussing-chamber experiments on CF15 cells that we seeded on semipermeable snapwell

membrane, in control conditions and after 2 hours incubation in culture medium containing

miglustat (Fig. 5). Mean value of RTE was 531 ± 26 Ω.cm-2 (n=4) for untreated cells

monolayers and increased to 658 ± 39 Ω.cm-2 (n=4, P<0.05) for miglustat-corrected cells

monolayers. Adding amiloride to the apical compartment induced a change of short-circuit

current (Isc), i.e. inhibition of apical Na+ absorption (Fig. 5A). We found ∆Isc of -2.05 ± 1.1

µA.cm-2 for miglustat-corrected CF15 cells monolayers (n = 4) and -7.43 ± 0.7 µA.cm-2 (n = 4)

for control monolayers (Fig. 5B, P<0.01). Then, addition of 10 µM fsk (basolateral side) and

30 µM gst (both sides) stimulated a glibenclamide-sensitive Isc, i.e. activation of apical

F508del-CFTR-dependent Cl- secretion, for miglustat-corrected monolayers (Fig. 5B) but not

for control monolayers (Fig. 5A). The ∆Isc was 8.3 ± 0.6 µA.cm-2 for miglustat-corrected


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monolayers (n = 4) and 0.18 ± 0.09 µA.cm-2 for control monolayers (n = 4, P<0.001) (Fig.

5C).

Miglustat reduces amiloride-sensitive Isc in ex-vivo experiments on nasal

epithelium of cftrF508del/F508del mice. To further explore the potential effect of miglustat on

ENaC, we conducted electrophysiological experiments with nasal epithelium dissected from

cftrF508del/F508del mice (Fig. 6). For each experiment, we recorded amiloride-sensitive ∆Isc

before and after a 2 hours incubation of the tissue with miglustat. In preliminary experiments,

we determined that 2 hours incubation in Meyler buffer without drug did not modify the

amiloride response (data not shown). Compared to untreated condition amiloride-sensitive Isc

was reduced after a 2 hours incubation in miglustat supplemented Meyler buffer. We found

∆Isc of -8.5 ± 1.9 µA.cm-2 in control condition and ∆Isc of -2.8 ± 0.5 µA.cm-2 in miglustat

(Fig. 6, n = 8, P<0.05). With cftr+/+ mice we found no significant difference between

amiloride-sensitive Isc in control condition (-0.6 ± 1.27 µA.cm-2) and after miglustat (-0.77 ±

1.37 µA.cm-2, n = 7, ns, Fig. 6).

Effect of oral administration of miglustat on ex vivo bioelectrics of cftrF508del/F508del

and cftr-/- mice. We administered 1200 mg/kg/day miglustat to cftrF508del/F508del and cftr-/- mice

by gavage for 6 days. This concentration has been applied previously to demonstrate

therapeutic benefits of miglustat in a mouse model of Sandhoff disease (Andersson et al.,

2004). The control group received PBS only. On day 6 (i.e. after 12 applications), we

dissected nasal epithelium from the different mice groups and recorded the amiloride-

sensitive ∆Isc. We found significantly reduced amiloride-sensitive ∆Isc for nasal epithelium

of cftrF508del/F508del mice (∆Isc = -12.3 ± 6.2 µA.cm-2, N = 10 mice) who received miglustat,

compared to the PBS group (∆Isc = -28.2 ± 3.5 µA.cm-2, N = 12 mice) (P<0.05, Fig. 7). To

learn whether the inhibition of ENaC-mediated Na+ absorption in nasal epithelium is a

consequence of the F508del-cftr rescue by miglustat in this tissue or is due to a cftr-


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independent effect, we have repeated the in vivo study with cftr-/- mice. However, no effect

was noted between the two groups of cftr-/- mice (PBS : ∆Isc = -27.3 ± 17.0 µA.cm-2, N = 5

mice; miglustat ; ∆Isc = -24.0 ± 4.3 µA.cm-2, N=5 mice, Fig. 7, ns). Since there is no effect on

KO mice, the effect of miglustat is due to F508del-cftr rescue and therefore this corrector has

no direct effect on ENaC activity.


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Discussion

Nowadays, the majority of clinical treatments of CF targets the secondary manifestations

of the pulmonary disease (inhaled antibiotics and recombinant human DNase). However,

CFTR-directed treatments will probably arise in the near future due to our expanded

knowledge of transepithelial ion transport pharmacology and molecular biology. One

approach for correcting the basic defect in CF (also called protein therapy) aims at creating

conditions to restore cAMP-dependent chloride transport and hence re-hydration, of the

airway surface in priority. However, it remains uncertain whether such a therapy is able to

restore all pleiotropic functions of CFTR (Vankeerberghen et al., 2002). In the present study,

we addressed one particular aspect of this question by analysing the consequence of CFTR

correction for the activity of ENaC of treating airway epithelial cells with miglustat, an agent

that we showed able to restore functional and mature F508del-CFTR in epithelial cells (Norez

et al., 2006; Antigny et al., 2007) and that is now evaluated in a phase 2a clinical trial for

homozygous F508del patients (www.clinicaltrials.gov). The major findings of the present

study are summarized hereafter. In CF15 human airway epithelial cells incubated for 2 h at

37°C in the presence of 100 µM miglustat, a cAMP-dependent and CFTRinh-172-sensitive

F508del-CFTR current was restored in parallel to the reduction in amplitude of the amiloride-

sensitive ENaC Na+ current. In miglustat treated cells the magnitude of the amiloride-

sensitive current was similar in the presence or absence of forskolin/genistein (to open CFTR

channels) arguing that the activation of F508del-CFTR by forskolin/genistein is not required

for down-regulation of ENaC. Finally, a CFTR-dependent normalization of amiloride-

sensitive Na+ absorption in response to miglustat was observed both in human airway

epithelial cells and in nasal epithelia of cftrF508del/F508del mice.


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CFTR is a pleiotropic ion channel, i.e. apart from its ability to transport chloride ions as an

ionic channel, it also regulates many other transport proteins and cellular functions due to

large and dynamic macromolecular complexes that contain CFTR, signalling molecules and

transport proteins (Guggino & Stanton, 2006 ; Vankeerberghen et al., 2002). Abnormal Na+

transport in CF affected airway epithelia has been suggested by many in vivo and in vitro

observations in humans and mice, showing increased amiloride-sensitive transepithelial

potentials in CF (Knowles et al., 1981, Knowles et al., 1983; Boucher et al., 1986; Grubb et

al., 1997; Mall et al, 1998). Although not completely solved, it becomes apparent that

interaction between CFTR and ENaC may involve PDZ-domain proteins and kinases

(Guggino & Stanton, 2006). It is particularly important that the functional and reciprocal

interaction between CFTR and ENaC regulates both epithelial Cl- and Na+ conductances

(Stutts et al., 1995 ; Stutts et al., 1997). However, it remains unclear why CF airway epithelia

display such a high ENaC activity. Despite numerous studies the molecular mechanism (direct

or indirect), is still unknown. A potential direction of investigation to clear these points will

have to address protein-protein interactions between CFTR and ENaC, which have been

studied only in few reports. Recently, Berdiev and colleagues demonstrated a direct physical

interaction between CFTR and the three ENaC subunits, by Fluorescence Resonance Energy

Transfert (FRET) analysis. In the same study, these experiments were confirmed by co-

immunoprecipitation (Berdiev et al, 2007). Nevertheless, these results argue that CFTR and

α- and β-rENaC interact in a complex. But, to our knowledge, no results concerning physical

interactions between ENaC subunits and CFTR mutant proteins, and in particular F508del

mutant, have yet been shown.

We also found that the CFTR activators forskolin/genistein on whole-cell current,

examined under control condition and in presence of amiloride, had no influence on

amiloride-sensitive ENaC currents in CF15 cells. This outcome is at variance with previous


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studies showing that the CFTR regulation of ENaC in human intestinal (Mall et al., 1998) and

colonic epithelial cells (Ecke et al., 1996) required active CFTR channels. The mechanism of

reciprocal regulation of CFTR and ENaC is currently under investigation. It was initially

observed in MDCK cells expressing both ENaC and CFTR and was subsequently

demonstrated in Xenopus laevis (Mall et al., 1996; Stutts et al., 1995). ENaC inhibition by

CFTR was demonstrated when α, β and γ ENaC subunits were co-expressed in oocytes of

Xenopus laevis with wild-type CFTR but not with F508del-CFTR (Mall et al., 1996; Stutts et

al., 1995). In these studies, ENaC was inhibited during stimulation by agonist raising

intracellular cAMP. A few studies showed that ENaC inhibition by CFTR also takes place in

cells expressing both proteins endogenously (Ecke et al., 1996; Letz & Korbmacher, 1997)

and was operational in normal human airways but not in CF patient tissues (Mall et al., 1998).

It is now admitted that these findings may explain the typical enhanced amiloride-sensitive

Na+ conductance and increased reabsorption of electrolytes observed in CF airways, two

phenomenon which are leading to highly viscous mucus and reduced mucociliary clearance

(Zhang et al., 1996).

Interestingly, it has been recently shown that overexpression of βENaC in mice led to a

CF-like phenotype even in the presence of functional CFTR channels (Mall et al., 2004). Thus,

inhibition of ENaC activity alone might already be of therapeutic value in CF. However,

despite the fact that F508del-CFTR mediated chloride secretion can be restored by a number

of physical or pharmacological manoeuvres in vitro and for some, in vivo (reviewed in Becq,

2006 ; MacDonald et al., 2007), a parallel change in amiloride-sensitive Na+ transport has not

been frequently reported. The protein repair agent 4-phenylbutyrate (buphenyl) which has

been clinically evaluated in F508del-homozygous CF patients, partially restored CFTR

function but had no effect on nasal amiloride-sensitive potential (Rubenstein & Zeitlin, 1998).

By contrast, curcumin not only corrected CFTR functions but also affected the amiloride-


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sensitive response in cftrF508del/F508del mice (Egan et al., 2004). Thus these results suggest that a

single pharmacological agent should be, in principle, capable of sufficient correction of the

defects in CF cells to produce clinical benefits. However a preliminary phase 1 clinical trial

with oral curcumin was rather disappointing and did not show correction of CFTR

(http://www.cff.org). Oral administration of miglustat in cftrF508del/F508del mice resulted in

reduction of amiloride-sensitive Isc. Therefore our study demonstrates potential normalization

of cAMP-dependent Cl- secretion and amiloride-sensitive Na+ absorption by a treatment with

a single agent, the CFTR corrector miglustat. Moreover, the persistence of ENaC inhibition in

cftrF508del/F508del animals receiving miglustat, together with a lack of effect on cftr-/- mice, offer

direct proof that the rescue of F508del-cftr and ENaC down-regulation are linked.

How miglustat affects both CFTR and ENaC transports in CF cells? Earlier, we provided

evidence that miglustat prevents, at least in part, the interaction of the mutant channel with the

ER-resident molecular chaperone calnexine (Norez et al., 2006). Preventing the calnexin

interaction with the mutant protein in the ER has been regarded as one of the major

mechanism of rescue (Egan et al., 2004; Norez et al., 2006). During an extensive study to

better understand the mechanism of action of miglustat on epithelial CF cells, we also

observed that this agent has no direct effect either on the Cl- channel activity of CFTR or on

the activity of others non-CFTR Cl- channels. Finally, as far as the literature showed, in

epithelial CF cells the three ENaC subunits are all expressed and located at the apical plasma

membrane. However because F508del-CFTR is retained in the ER, the equilibrium between

Cl- secretion and Na+ absorption is affected due to the absence of negative control of ENaC

by CFTR as proposed by several authors (Stutts et al., 1995; Mall et al., 1996; Letz &

Korbmacher, 1997; Mall et al., 1998; Kunzelmann et al., 2000; Berdiev et al., 2007).

Moreover, the ENaC conductance is not inhibited by F508del-CFTR (Mall et al., 1996; Mall

et al., 1998) and overexpression of the beta-subunit of ENaC produces CF-like lung disease in


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a mouse model (Mall et al., 2004). Therefore we propose that miglustat indirectly affects the

transport of Na+ in CF cells through its effect as a α1,2-glucosidase inhibitor to perturb the

F508del-CFTR/calnexin molecular interaction in the ER. Further studies will be needed to

understand how rescuing F508del-CFTR from its intracellular retention re-establish the

control of ENaC activity and thus normalize the transport of Na+ in CF cells.

In summary, in this report we demonstrate that the rescue of endogenous F508del-CFTR

by miglustat (or by low temperature) in human airway and nasal epithelial cells of

cftrF508del/F508del mice, is accompanied by a down-regulation of Na+ transport. Because the

balance between CFTR-dependent Cl- secretion and ENaC-dependent Na+ reabsorption

regulates the net amount of salt and water in airway periciliary fluid and thus the ability to

clear bacteria and other noxious agents from the lungs, our findings predict that miglustat may

not only ameliorate chloride transport but also sodium hyperabsorption.


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Acknowledgments

The authors thank Nathalie Bizard for cell culture maintenance and James Habrioux for

excellent assistance.


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Footnotes : This work was supported by specific grants from Vaincre La Mucoviscidose and

MucoVie66. S.N. was supported by a studentship from MucoVie66. Part of this work has

already been presented in an abstract form, in Pediatric Pulmonology, supplement 29, 2006.

Contact for reprint request :

Pr Frédéric BECQ

IPBC CNRS UMR 6187,

Université de Poitiers, 40 Avenue du Recteur Pineau

86022 Poitiers, France

Tel : +33-5-49-45-37-29 ; Fax : +33-5-49-45-40-14

E-mail : [email protected]


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Legends for figures

Fig. 1. Perforated whole-cell patch clamp experiments on homozygous F508del/F508del

airway epithelial CF15 cells. Currents were recorded in uncorrected cells (left, noted 37°C),

temperature corrected cells (middle, 27°C) or miglustat corrected cells (right). For each

condition, the example given is the complete sequence of experiments shown for the same cell.

The current was recorded in control condition (A), after addition of amiloride (B), in presence

of amiloride + cocktail composed of forskolin plus genistein (C) and finally after adding

CFTRinh-172 (D). Concentrations used are: amiloride: 100 µM; forskolin: 10 µM; genistein:

30 µM; CFTRinh-172: 10 µM; miglustat: 100 µM.

Fig. 2. Mean voltage-current density relationships of the ENaC Na+ and CFTR Cl- currents on

F508del/F508del airway epithelial CF15 cells. The residual current in presence of amiloride

was substracted from the basal current. The CFTR Cl- current corresponds to Fsk/Gst

activated current. The number of experiments is n = 12 for control 37°C, n = 10 for miglustat

and n = 7 for low temperature. Concentrations used are: forskolin: 10 µM; genistein: 30 µM;

miglustat: 100 µM.

Fig. 3. Effect of forskolin/genistein on ENaC currents in CF15 cells. Shown are representative

current recordings for a single control CF15 cell (37°C, left traces in A-D) and incubated for 2

h with miglustat (right traces in A-D) under control conditions (A), after addition of the

cocktail forskolin/genistein (B), in the presence of amiloride + forskolin/genistein (C), and

after addition of CFTRinh-172 in presence of amiloride + forskolin/genistein (D).

Concentrations used are: amiloride: 100 µM; forskolin: 10 µM; genistein: 30 µM; CFTRinh-

172: 10 µM; miglustat: 100 µM.


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Fig. 4. Effect of forskolin/genistein on the mean voltage-current density relationships of the

ENaC current at +100 mV. The residual current in presence of amiloride was substracted

from the basal current (black bars) or from the Fsk/Gst activated current (open bars). The

number of experiments is indicated on each bar graph. Concentrations used are: forskolin: 10

µM; genistein: 30 µM; miglustat: 100 µM. Non significant (ns) difference was P>0.05.

Fig. 5. Effect of miglustat on Isc recorded in CF15 cells monolayers. CF15 cells were cultured

at 37°C on semipermeable membranes in absence (A) or in presence (B) of miglustat (100

µM during 2h in the culture medium). The effect of apical application of 100 µM amiloride,

basolateral application of 10 µM forskolin and bilateral 30 µM genistein, then bilateral 500

µM glibenclamide was recorded. (C). Mean values of amiloride-sensitive ∆Isc (left) and

fsk/gst-sensitive ∆Isc (right) from control CF15 cells monolayers (open bars) and miglustat-

treated monolayers (black bars) (n = 4 for each condition). ** P<0.01, *** P<0.001.

Fig. 6. Ex vivo effect of miglustat on amiloride-sensitive Isc recorded in nasal epithelium from

F508del-CFTR or wild-type mice. Shown are mean values of amiloride-sensitive ∆Isc

recorded ex vivo in CF tissue under control conditions (open bars) and after incubation during

2h in Meyler solution containing 100 µM miglustat (filled bars). See Method section for

details (n = 7 for each mice type). * P<0.05, ** P<0.01.

Fig. 7. Effect of oral administration of miglustat (6 days) on amiloride-sensitive Isc in mice

nasal epithelium mounted in Ussing chambers. Bars graphs represent the amiloride-sensitive

∆Isc both in cftrF508del/F508del (left) and cftr-/- (right) nasal epithelium dissected away from mice

who received oral administration of miglutat (filled bars) or PBS vehicle only (open bars)


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during 6 days. For cftrF508del/F508del mice : N(PBS) = 12 mice, N(Miglustat) =10 mice. For cftr-/-

mice : N(PBS) = 5 mice; N(Miglustat) = 5 mice. * P<0.05, non significant (ns) difference was

P>0.05.


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