当前位置:文档之家› Regulatory volume increase is associated with p38 kinase-dependent actin cytoskeleton remodeling in

Regulatory volume increase is associated with p38 kinase-dependent actin cytoskeleton remodeling in

Regulatory volume increase is associated with p38 kinase-dependent actin cytoskeleton remodeling in
Regulatory volume increase is associated with p38 kinase-dependent actin cytoskeleton remodeling in

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[PDF] [Full Text] [Abstract], January 1, 2006; 290 (1): C134-C142. Am J Physiol Cell Physiol K. B. E. Gagnon, R. England and E. Delpire

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Regulatory volume increase is associated with p38

kinase-dependent actin cytoskeleton remodeling in rat kidney MTAL

Mauro Bustamante,1Frank Roger,1Marie-Luce Bochaton-Piallat,2

Giulio Gabbiani,2Pierre-Yves Martin,1and Eric Fe ′raille 1

1Division de Ne ′phrologie,Fondation pour Recherches Me ′dicales,

and 2De ′partement de Pathologie,Centre Me ′dical Universitaire,CH-1211Gene `ve 4,Switzerland

Submitted 7January 2003;accepted in ?nal form 22April 2003

Bustamante,Mauro,Frank Roger,Marie-Luce Bo-chaton-Piallat,Giulio Gabbiani,Pierre-Yves Martin,

and Eric Fe ′raille.Regulatory volume increase is associated with p38kinase-dependent actin cytoskeleton remodeling in rat kidney MTAL.Am J Physiol Renal Physiol 285:F336–F347,2003.First published April 29,2003;10.1152/ajprenal.00003.2003.—The kidney medulla is physiologically exposed to variations in extracellular osmolality.In response to hypertonic cell shrinkage,cells of the rat kidney medullary thick ascending limb of Henle’s loop undergo p38kinase-dependent regulatory volume increase (RVI).In the present study,we investigated the role of actin cytoskeleton reorga-nization in this process.Addition of hyperosmotic NaCl or sucrose,which activates MAP kinases and reduces cellular volume,induced a sustained actin polymerization occurring

after 10min and concurrently with RVI.In contrast,hyper-osmotic urea,which does not modify MAP kinase activity and cellular volume,did not induce sustained actin polymeriza-tion.Fluorescence microscopy revealed that hyperosmotic NaCl and sucrose,but not urea,induced the redistribution of F-actin from a dense cortical ring to a diffuse network of actin bundles.Stabilization of actin ?laments by jasplakinolide and inhibition of the generation of new actin ?laments by swinholide A prevented RVI,whereas depolymerization of actin ?laments by latrunculin B attenuated cell shrinkage and enhanced RVI.These actin-interfering drugs did not alter extracellular regulated kinase and p38kinase activa-tion under hypertonic conditions.Similar to swinholide A,

inhibiting p38kinase with SB-203580abolished sustained actin polymerization,actin redistribution,and decreased RVI ef?cacy.We therefore propose that in rat kidney the medul-lary thick ascending limb of Henle’s loop exposed to extracel-lular hypertonicity,p38kinase activation induces depoly-merization of the F-actin cortical ring and polymerization of a dense diffuse F-actin network that both contribute to in-crease RVI ef?cacy.mitogen-activated protein kinase;osmolarity;kidney me-dulla;cell volume DURING DIURESIS AND antidiuresis,the kidney medulla of vertebrates is exposed to large ?uctuations in intersti-tial osmolality (23),which challenges cell volume con-stancy.Under antidiuresis,the countercurrent concen-tration mechanism initiated by active NaCl reabsorp-tion by the medullary thick ascending limb of Henle’s loop (MTAL)leads to NaCl and urea accumulation in the renal medulla.MTAL cells are therefore of special interest because they have developed adaptive mecha-nisms to survive and function in a hypertonic environ-ment.The high interstitial osmolality induces changes in the activity of solute transporters and enzymes involved in solute accumulation and in the expression of genes encoding enzymes required for solute synthe-sis,stress resistance,and yeast cell wall structure (34).After cell shrinkage in response to extracellular hyper-tonicity,MTAL cells progressively recover their initial volume through regulatory volume increase (RVI).This process occurs within minutes and is mediated by stimulation of ion transporters that increase intracel-lular ionic concentrations,which drive water in?ux and restore initial cellular volume (34,43).The rapid increase in ionic concentrations is followed by a slow accumulation of intracellular compatible osmolytes,such as sorbitol,myoinositol,taurine,betaine,and glycerophosphocholine,allowing recovery of normal ionic concentrations (7).This process is a long-term mechanism occurring within hours or days and coun-teracts increased extracellular osmotic pressure.Hypertonic conditions activate MAPK,which is an important signal transducer linking signals from the cell surface to the nucleus.MAPKs are serine/threo-nine kinases activated by a cascade of kinases involv-ing two upstream kinases,MAPKKK and MAPKK (48).In mammalian cells,MAPKs are divided into three families,each responding to distinct extracellu-lar stimuli:ERK 1and 2,JNK (also known as stress-activated protein kinases 1),and p38kinases (or stress-activated protein kinase 2).In our previous study,we showed that cell shrinkage,rather than intracellular hypertonicity,triggers the activation of

ERK and p38kinase in rat MTALs (35).MAPK acti-

vation levels were dependent on the osmolyte used to

increase extracellular osmolality.Hyperosmotic NaCl induced cell shrinkage and activated ERK and p38kinase but not JNK.In comparison,hyperosmotic su-crose induced even greater cell shrinkage and stronger activation of ERK and p38kinase and also activated JNK but to a lesser extent.By contrast,hyperosmotic

Address for reprint requests and other correspondence:E.Fe ′raille,Division de Ne ′phrologie,Fondation pour Recherches Me ′dicales,CH-1211Gene `ve 4,Switzerland (E-mail:Eric.Feraille@medecine.unige.ch).The costs of publication of this article were defrayed in part by the

payment of page charges.The article must therefore be hereby marked ‘‘advertisement ’’in accordance with 18U.S.C.Section 1734solely to indicate this fact.

Am J Physiol Renal Physiol 285:F336–F347,2003.

First published April 29,2003;10.1152/ajprenal.00003.2003.

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urea altered neither cell volume nor MAPK activity.Both hypertonic NaCl and sucrose triggered cellular RVI that restored,almost completely for NaCl and partially for sucrose,the initial cellular volume.Inhi-bition of p38kinase decreased the ef ?ciency of RVI,implying a major role of this kinase in this process,whereas inhibition of ERK did not alter RVI.Modi ?cations of cellular architecture related to hy-pertonicity-induced cell shrinkage are associated with a reorganization of the architecture of the actin cy-toskeleton and with changes in the F-actin-G-actin equilibrium (13,18,19,32,39).Speci ?c cytoskeleton components may sense cell volume decrease and initi-ate signaling cascades leading to RVI.In addition,signal transduction cascades leading to remodeling of the actin cytoskeleton and to MAPK activation share some common elements.For instance,small G proteins of the Rho family,such as Cdc42and Rac 1,are in-volved in both actin cytoskeleton remodeling through ?lipodia and lamellipodia formation (45)and in signal-ing events leading to p38kinase activation (1,51).Activation of p38kinase may,in turn,control actin cytoskeleton dynamics through the activation of down-stream kinases such as MAPKAP kinase 2/3or PRAK,which phosphorylate HSP 25/27(17,28,37),a small heat shock protein that modulates actin polymeriza-tion (27).The actin cytoskeleton may also control the activity of ion transporters,leading to intracellular NaCl uptake and secondary water in ?ux,either di-rectly,through F-actin/G-actin ratio dynamics (9,10),or indirectly,through binding of signaling modules (50)and/or modulation of endocytotic-exocytotic events (42).This study was therefore undertaken to investi-gate the relationship between actin cytoskeleton re-modeling and RVI in rat MTAL.MATERIALS AND METHODS Preparation of single MTALs.Male Wistar rats weighing 150–200g were anesthetized with pentobarbital sodium (5mg/100g body wt ip),and the left kidney was immediately removed after perfusion with ice-cold incubation solution (120mM NaCl,5mM RbCl,4mM NaHCO 3,1mM CaCl 2,1

mM MgSO 4,0.2mM NaH 2PO 4,0.15mM Na 2HPO 4,5mM

glucose,10mM lactate,1mM pyruvate,4mM essential and nonessential amino acids,0.03mM vitamin,20mM HEPES,and 0.1%BSA,pH 7.4)containing 0.18%(wt/vol)collagenase.After incubation at 30°C for 20min in incubation solution (see above)containing 0.05%(wt/vol)collagenase,kidney slices were stored at 4°C,and single MTALs were microdis-sected under stereomicroscopic control in oxygenated (95%O 2-5%CO 2)incubation solution.Preparation of MTAL suspensions.The two kidneys were perfused with ice-cold incubation solution without collage-nase.The inner stripes of the outer medulla were excised,minced on ice,and fragments of medullary tubules were obtained by gentle pressure through nylon ?lters with a pore size decreasing from 150to 100?m.After centrifugation,the pellet was resuspended in ice-cold oxygenated (95%O 2-5%CO 2)incubation solution.As controlled under a stereomicro-scope,MTALs account for ?90%of the tubule fragments in this preparation.Therefore,it will be referred as MTAL suspension.Determination of the Triton X-100-soluble and -insoluble actin fractions.Estimation of actin polymerization level was performed by determining the Triton X (TX)-100-soluble/TX-100-insoluble actin ratio.Indeed,it is largely admitted that F-actin,i.e.,polymerized actin,is contained in the TX-100-insoluble fraction and that G-actin,i.e.,monomeric actin,is contained in the TX-100-soluble fraction (16).After 1-h pre-incubation at 30°C in isotonic incubation solution with or without addition of drugs,MTAL suspensions were incu-

bated at 37°C for 1to 30min under isosmotic or hyperosmotic

(addition of 300mosM/l NaCl,sucrose,or urea)conditions.Incubation was stopped by cooling and centrifugation at 6,000g for 5min at 4°C.The pellet was saved and 20?l of ice-cold lysis buffer (20mM Tris ?HCl,2mM EGTA,2mM EDTA,30mM NaF,30mM Na 4O 7P 2,2mM Na 3VO 4,1mM AEBSF,10?g/ml leupeptin,4?g/ml aprotinin,1%Triton X-100,pH 7.45)were added.After 5min of centrifugation at 12,000g ,the supernatant was saved,the pellet was then mixed with fresh lysis buffer,and after a centrifugation step at 12,000g ,the second supernatant was saved.The ?nal

pellet was then suspended in sample buffer and an equal

volume of sample buffer was added to the pooled superna-tants.The proteins from pooled supernatants and pellet were then separated by 10%SDS-PAGE and transferred to a polyvinylidene di ?uoride (PVDF)membrane (Immobilon-P,Millipore,Waters,MA),and ?-actin was detected by immu-noblot using a monoclonal anti-?-actin antibody (AC-15,Sigma,St.Louis,MO)at 1:40,000dilution (vol/vol).After incubation with anti-mouse IgG coupled to horseradish per-oxidase (Transduction Laboratories,Lexington,UK)at

1:10,000dilution (vol/vol),immunoreactivity was detected by

chemiluminescence using the Super Signal Substrate method (Pierce,Rockford,IL).Results were quanti ?ed under conditions of linearity by integration of the density of total area of each band using a video densitometer and Image-

Quant software (Molecular Dynamics,Sunnyvale,CA).Re-sults are expressed as a percentage of the control optical density (isotonic medium)and are means ?SE.

Determination of the phosphorylation level of ERK and p38

kinase.After 1-h preincubation at 30°C in isotonic incubation

solution with or without addition of drugs,MTAL suspen-sions were incubated at 37°C for 10min under isosmotic or hyperosmotic (addition of 300mosM/l NaCl)conditions.In-cubation was stopped by cooling and centrifugation at 6,000g for 5min at 4°C.After addition of lysis buffer,protein content was measured by the BCA protein assay (Pierce).

Equal amounts of protein (50?g)were separated by 10%

SDS-PAGE and transferred to a PVDF membrane (Immobil-ion-P,Millipore).Phosphorylated ERK and p38kinase were detected using anti-ERK-P and anti-p38-P kinase rabbit polyclonal antibodies (New England Biolabs,Beverly,MA)at 1:10,000dilution (vol/vol).After incubation with anti-rabbit IgG coupled to horseradish peroxidase (Transduction Labo-ratories)at 1:10,000dilution (vol/vol),immunoreactivity was

detected by chemiluminescence,and results were quanti ?ed and expressed as described above.Determination of MTAL cellular volume.A pool of three isolated MTALs was transferred into the concavity of a bac-teriological slide coated with dried BSA.After 1-h preincu-

bation at 30°C in isosmotic incubation solution with or with-out addition of drugs,MTALs were incubated at 37°C for 1to 30min under isosmotic or hyperosmotic (addition of 300mosM/l NaCl)conditions.After preincubation at 30°C in isosmotic incubation solution,tubules were incubated in isos-motic or hyperosmotic incubation solutions with or without drugs.MTALs were visualized with an inverted microscope,and photographs of the same tubules were taken at the end

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of the preincubation period and after incubation.MTAL vol-

ume (V)was calculated from the measured radius (R )and length (L )of the tubules at a 1,000-fold magni ?cation using the formula V ??R 2?L .Because the lumen is collapsed in nonperfused tubules,we assumed that MTAL volume mea-surement is an appropriate estimate of MTAL cellular volume.Results are expressed as a percentage of the control volume (end of the preincubation period)and are means ?SE.Fluorescence microscopy.MTAL suspensions were prein-cubated at 30°C for 1h with or without drugs and then incubated at 37°C under isosmotic or hyperosmotic (addition

of 300mosM/l NaCl or sucrose)conditions.Tubules were then cytocentrifuged on glass slides using a cytospin (70g ,5min in incubation solution supplemented with 1%BSA)and ?xed with 3.7%paraformaldehyde for 10min at room tem-perature.After three washes in PBS,?xed tubules were permeabilized with 0.1%Triton X-100for 1min at room temperature.After a new series of three washes in PBS,specimens were incubated with phalloidin Alexa-488(dilu-tion:1:100in PBS;Molecular Probes,Eugene,OR)for 1h at room temperature.Specimens were observed with a Zeiss Axiophot microscope (Carl Zeiss,Jena,Germany)equipped

with an oil-immersion plan-neo ?uar ?40:1.3objective.Im-ages were acquired with a high-sensitivity,high-resolution color camera (Axiocam,Carl Zeiss).Pictures were printed with a digital pictrography 4000printer (Fuji ?lm,Tokyo,Japan).Statistical analysis.Statistical analysis of variations of TX-100-insoluble/TX-100-soluble actin and cellular volume was done by ANOVA.Statistical analysis of variations of anti-P-ERK and P-p38kinase immunoreactivity was done using the Kruskall-Wallis test.P values ?0.05were consid-ered signi ?cant.RESULTS Actin cytoskeleton remodeling in response to hyperos-motic NaCl,sucrose,or urea.We previously showed that hyperosmotic NaCl and sucrose,but not urea,induced both MAPK activation and cell shrinkage in rat MTAL cells (35).Because cell shrinkage and/or intracellular hypertonicity may induce remodeling of the actin cytoskeleton,we compared the time course of the effects of hyperosmotic NaCl,sucrose,and urea on the F-actin/G-actin ratio and cellular volume in rat MTALs.Figure 1shows that increasing extracellular osmo-larity up to 600mosM/l by addition of NaCl,sucrose,or urea rapidly increased the proportion of TX-100-insol-uble actin,i.e.,F-actin,with a peak observed after 1-to 3-min incubation at 37°C.The TX-100-insoluble/TX-100-soluble actin ratio then returned close to its basal level after 10-min incubation.These rapid variations in cellular TX-100-insoluble actin content were followed by a progressive increase in proportion of TX-100-insoluble actin above the basal levels in samples incu-bated up to 30min in the presence of hyperosmotic NaCl (TX-100-insoluble/TX-100-soluble actin;isos-motic:1.46?0.18;NaCl:2.46?0.30;P ?0.05).This increase in proportion of TX-100-insoluble actin was sustained for at least 60min (data not shown).The progressive increase in proportion of TX-100-insoluble actin was more pronounced after 30min in the pres-ence of hyperosmotic sucrose (TX-100-insoluble/TX-100-soluble actin;isotonic:2.10?0.37;sucrose:5.98?

1.03;P ?0.05)compared with hyperosmotic NaCl (Fig.1,A and B ).In contrast,for incubation periods ranging

from 10to 30min,hyperosmotic urea did not induce signi ?cant variations in the TX-100-insoluble/TX-100-soluble actin ratio (isotonic 30min:1.75?0.11;urea 30min:1.60?0.12;not signi ?cant;Fig.1C ).As shown previously (35),both hyperosmotic NaCl and sucrose rapidly induced cell shrinkage with a maximal de-crease in cellular volume observed after 10min of incubation (%of initial cellular volume;NaCl:68.98?

2.21;sucrose:65.47?0.58).After 30-min incubation in the presence of hyperosmotic NaCl or sucrose,a partial recovery of the initial cellular volume was observed (NaCl:89.14?2.24;sucrose:81.81?5.94;Fig.1,A and B ).In contrast,hyperosmotic urea did not signi ?-cantly alter cellular volume (Fig.1C ).These results show that,in MTAL cells,acute extracellular hyperos-molality induces polyphasic actin cytoskeleton remod-eling re ?ected by the observed changes in the F-actin/G-actin ratio.However,sustained actin polymerization

re ?ected by the progressive increase in cellular F-actin content was only observed in response to osmolytes inducing cell shrinkage and this event occurred con-comitantly with the partial recovery of the initial cel-lular volume.

The effect of extracellular hyperosmolality on actin cytoskeleton organization was assessed by ?uorescence microscopy on isolated rat MTALs incubated at 37°C for 30min under isosmotic or hyperosmotic conditions.As shown by Fig.2A ,rat MTAL cells incubated under isosmotic conditions exhibited a dense cortical F-actin

ring delineating the cell periphery and a sparse diffuse

network of F-actin bundles.After exposure of MTALs to hyperosmotic NaCl,the cortical F-actin ring was thinner and the diffuse F-actin network was more developed compared with tubules incubated under isosmotic conditions (Fig.2B ).This redistribution of F-actin was more pronounced after incubation of tu-bules in the presence of hyperosmotic sucrose (Fig.2C ).In contrast,hyperosmotic urea did not induce any signi ?cant change in F-actin distribution (Fig.2D ).Therefore,the sustained actin polymerization phase observed in response to hyperosmotic NaCl and su-crose was associated with a redistribution of F-actin from the cortical F-actin ring to a diffuse network of F-actin bundles.In contrast,the early actin polymer-ization phase was not associated with apparent F-actin redistribution (data not shown).

Interfering with actin polymerization and remodel-ing altered cellular volume recovery under hypertonic conditions.The actin cytoskeleton is a highly dynamic structure that undergoes constant remodeling,consist-ing of spatially and temporally regulated polymeriza-tion and depolymerization of preexisting ?laments as well as nucleation and branching of new actin ?la-ments.New pharmacological tools derived from marine sponges (40)were used to study the role of actin poly-merization-depolymerization and generation of new ac-tin ?laments on cellular volume recovery under hyper-tonic conditions.Jasplakinolide binds to both ends of

F338HYPERTONICITY-INDUCED ACTIN CYTOSKELETON REMODELING

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actin ?laments preventing their depolymerization and also causes rapid nucleation of actin polymerization (5,40).As expected from its mechanism of action,10?M jasplakinolide (Calbiochem,San Diego,CA)increased the proportion of TX-100-insoluble actin in tubules incubated under hypertonic conditions for 30min (TX-100-insoluble/TX-100-soluble actin;NaCl:2.87?0.49;NaCl ?jasplakinolide:4.02?0.45;P ?0.05;Fig.3A ).In addition,?uorescence microscopy revealed that jas-plakinolide-treated tubules exhibited a very dense F-actin network with diffuse thick and short actin-rich structures,i.e.,actin clumps,throughout the cyto-plasm (Fig.3B ).Therefore,jasplakinolide ef ?ciently increases the actin polymerization in rat MTAL cells.Swinholide A inhibits actin ?lament nucleation and elongation (6,40),thereby preventing stimulus-in-duced actin polymerization without affecting the intact

actin network (46).Measurement of the partition of actin between TX-100-soluble and -insoluble fractions showed that,in agreement with its pharmacological properties,50?M swinholide A (Calbiochem)moder-ately decreased the amounts of TX-100-insoluble actin measured after 30-min incubation under hypertonic conditions (TX-100-insoluble/TX-100-soluble actin;NaCl:2.87?0.49;NaCl ?swinholide:2.17?0.28;not signi ?cant;Fig.3A ).Fluorescence microscopy,how-ever,showed that reorganization of the actin cytoskel-eton induced by hyperosmotic NaCl was largely pre-vented by swinholide A (compare a and c ,Fig.3B ).Indeed,most F-actin remained in the cortical ring

and

Fig.1.Effect of hyperosmotic NaCl,sucrose,and

urea on actin polymerization level and cellular

volume.Medullary thick ascending limb of Hen-

le ’s loop (MTAL)suspensions or microdissected

MTALs were incubated under isosmotic (time 0)or

hyperosmotic (600mosM/l for 1to 30min)condi-

tions.?-Actin sorting between Triton (TX)-100-

insoluble (i)and -soluble (s)fractions was ana-

lyzed by Western blot analysis and the TX-100-

insoluble/soluble actin ratio was calculated after

quanti ?cation by densitometry.Cellular volume

was determined from photographs.Results are

means ?SE from 6to 11independent experi-

ments (*P ?0.05vs.time 0).The graphs show the

time course of TX-100-insoluble/soluble actin ratio

(left axis)and cellular volume (right axis)varia-

tions after addition of hyperosmotic NaCl (A ),hy-

perosmotic sucrose (B ),or hyperosmotic urea (C ).

Insets :representative Western blot analyses illus-

trating actin sorting after 30-min incubation in

the presence of hyperosmotic NaCl,hyperosmotic

sucrose,or hyperosmotic urea.

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the density of the diffuse F-actin network was un-changed compared with MTALs incubated under iso-tonic conditions.Therefore,swinholide A does not sig-ni ?cantly alter actin cytoskeleton organization but pre-vents its hypertonicity-induced remodeling in rat MTAL https://www.doczj.com/doc/bb14495005.html,trunculin B sequesters monomeric ac-tin and decreases G-actin availability,resulting in ac-tin ?lament depolymerization (40,41).Consistent with its actin-depolymerizing properties,100?g/ml latrun-culin B (Calbiochem)induced a large decrease in pro-portion to TX-100-insoluble actin with respect to con-trol after 30-min incubation under hypertonic condi-tions (TX-100-insoluble/TX-100-soluble actin;NaCl:2.87?0.49;NaCl ?latrunculin:0.33?0.09;P ?0.01;Fig.3A ).In addition,?uorescence microscopy revealed that latrunculin B disorganized the actin cytoskeleton.The cortical F-actin ring became irregular and discon-tinuous,and the diffuse F-actin network was almost completely disrupted (compare a and d ,Fig.3B ).Therefore,latrunculin B potently depolymerizes the actin cytoskeleton in rat MTAL cells.The role of actin polymerization or depolymerization and of the generation of new actin ?laments of cellular volume recovery was assessed using actin-interfering drugs.Figure 4A shows that jasplakinolide,which in-duces actin polymerization and prevents actin depoly-merization,almost completely prevented the partial recovery of the initial cellular volume observed after incubation of isolated MTALs for 30min under hyper-tonic conditions but in the absence of drug (%of initial volume;NaCl:88.48?4.71;NaCl ?jasplakinolide:74.65?0.71;P ?0.05).Swinholide A (Fig.4B ),which prevents the generation of new actin ?laments,in-creased cell shrinkage after 5-min incubation under hypertonic conditions (%of initial volume;NaCl:69.61?3.90;NaCl ?swinholide:51.85?4.86;P ?0.05)and decreased the extent of recovery of the initial cellular volume observed after 30min in the presence of hyperosmotic NaCl (%of initial volume;NaCl:88.48?4.71;NaCl ?swinholide:75.90?3.09;P ?0.05;Fig.4B ).Finally,latrunculin B,which depolymer-izes the actin cytoskeleton,largely attenuated cell shrinkage in response to 5-min incubation with hyper-osmotic NaCl (%of initial volume;NaCl:69.61?3.90;NaCl ?latrunculin:80.84? 4.28;P ?0.05)and

allowed a full recovery of cellular volume after 30-min incubation (%of initial volume;NaCl:88.48?4.71;NaCl ?latrunculin:102.54?4.94;P ?0.05;Fig.4C ).Thus,in rat MTALs,inhibition of actin depolymeriza-tion and generation of new actin ?laments decrease the ef ?cacy of RVI,whereas actin depolymerization poten-tiates cell volume recovery after an acute hypertonic challenge.

Interfering with actin polymerization and remodel-ing did not alter MAPK activation by hyperosmotic NaCl.The role of actin cytoskeleton remodeling in MAPKs activation in response to extracellular hyper-tonicty was assessed by measurement of the phosphor-ylation level of ERK and p38kinase in the absence or presence of actin-interfering drugs.Figure 5shows that the increases in phosphorylation levels of ERK and p38kinase observed after incubation at 37°C for 10min under hypertonic conditions in the presence of jasplakinolide,or swinholide A or latrunculin B,

were

Fig. 2.Effect of hyperosmotic NaCl,

sucrose,and urea on actin cytoskeleton

organization.MTAL suspensions were

incubated for 30min under isosmotic

(A )or hyperosmotic conditions (600

mosM/l)with addition of NaCl (B ),su-

crose (C ),or urea (D ).Tubules were

then ?xed with 4%paraformaldehyde,

permeabilized by 0.1%TX-100,and F-

actin was visualized by ?uorescence

microscopy after incubation with phal-

loidin-Alexa 488.Representative en

face views of MTAL epithelium are

shown.A :under isosmotic conditions,

MTAL cells exhibit a dense cortical F-

actin ring (?lled arrow)and a sparse

diffuse network of F-actin bundles

(dashed arrow).Both hyperosmotic

NaCl (B )and hyperosmotic sucrose (C )

induced redistribution of F-actin from

the cortical ring to a dense diffuse net-

work of F-actin bundles,whereas hy-

perosmotic urea (D )did not alter actin

cytoskeleton morphology.

F340HYPERTONICITY-INDUCED ACTIN CYTOSKELETON REMODELING

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similar to those induced by hyperosmotic NaCl alone.Similarly,ERK and p38kinase phosphorylation levels were not altered by actin-interfering drugs in MTALs incubated under isotonic conditions (data not shown).Therefore,activation of ERK and p38kinase in re-sponse to extracellular hypertonicity is independent of actin cytoskeleton remodeling.p38Kinase was involved in cellular volume recovery and actin cytoskeleton reorganization following extra-cellular hypertonic challenge.The following experi-ments were designed to study the role of MAPKs in cellular volume variations and actin cytoskeleton reor-ganization induced by extracellular hypertonicity.In-hibition of the ERK signaling pathway by 4.10?4M PD-98059(Calbiochem)modi ?ed neither cellular vol-ume variation nor TX-100-insoluble/soluble actin ratio pro ?les in response to hyperosmotic NaCl (data not shown).Inhibition of p38kinase by 10?5M SB-203580(Calbiochem)slightly increased the maximal extent of hypertonicity-induced cell shrinkage observed after 10min (%of initial volume;NaCl:77.14?2.82;NaCl ?SB:65.51?2.21?4.94;P ?0.05)and decreased the ef ?cacy of cell volume recovery after 30min (%of initial volume;NaCl:91.28?1.21;NaCl ?SB:78.05?3.11;P ?0.05;Fig.6A ).In addition,SB-203580atten-uated the early increase in proportion to TX-100-insol-uble actin observed after 2min in the presence of hyperosmotic NaCl (TX-100-insoluble/TX-100-soluble actin;NaCl:2.57?0.43;NaCl ?SB:1.79?0.25;P ?0.05)and abolished the sustained increase in amounts of TX-100-insoluble actin observed after 30-min incu-bation with hyperosmotic NaCl

(TX-100-insoluble/TX-

Fig.3.Effect of jasplakinolide,swinholide

A,and latrunculin B on actin polymeriza-

tion level and actin cytoskeleton reorgani-

zation in response to extracellular hyper-

tonicity.MTAL suspensions were incu-

bated under hyperosmotic (600mM NaCl)

conditions without (control;H)or with 10?M jasplakinolide,50?M swinholide A,or

100?g/ml latrunculin B.A :actin sorting between TX-100-insoluble and -soluble

fractions.The TX-100-insoluble/soluble

actin ratio was determined as described in

the legend of Fig.1and results are

means ?SE from 3independent experi-

ments (*P ?0.05vs.H).A representative

Western blot analysis depicting ?-actin

sorting in the absence or presence of drugs

is shown.B :?uorescence imaging of the

actin cytoskeleton.After 30-min incuba-

tion,tubules were ?xed with 4%parafor-

maldehyde,permeabilized by 0.1%

TX-100,and the actin cytoskeleton was

visualized by ?uorescence microscopy af-

ter incubation with phalloidin-Alexa 488.

Representative en face views of MTAL ep-

ithelium are shown.MTAL cells incubated

with hyperosmotic NaCl in the absence of

drugs (a )exhibit a thin cortical F-actin

ring (?lled arrow)and a dense cytoplasmic

network of F-actin bundles (dashed ar-

row).Jasplakinolide (b )induced a clear

densi ?cation of the cellular F-actin net-

work with diffuse punctuated F-actin-rich

structures (actin clumps).MTAL cells in-

cubated with swinholide A (c )exhibited a

dense cortical F-actin ring and a sparse

diffuse F-actin network,similarly to

MTAL cells incubated under isotonic con-

ditions (see Fig.2).Latrunculin B (d )in-

duced disorganization of the actin cy-

toskeleton with partial disruption of the

cortical F-actin ring.

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100-soluble actin;NaCl: 2.46?0.30;NaCl ?SB:

1.50?0.09;P ?0.01;Fig.6B ).Similar results were

obtained in the presence of hyperosmotic sucrose (data

not shown).As previously shown (35),SB-203580did not alter MTAL cellular volume measured under iso-tonic conditions.Thus both cellular volume recovery and sustained actin polymerization phase,which occur concomitantly,are dependent on p38kinase activity.Inhibition of p38kinase activity and swinholide A both prevent F-actin redistribution in response to extra-cellular hypertonicity.SB-203580,which inhibits p38kinase,and swinholide A,which prevents generation of new actin ?laments,both inhibited the sustained actin

polymerization and decreased the ef ?cacy of RVI in

response to extracellular hypertonicity.We therefore

assessed by ?uorescence microscopy the effect of SB-

203580and swinholide A on F-actin redistribution

following hypertonic challenge.As shown by Fig.7,

SB-203580and swinholide A strongly attenuated the

hypertonicity-induced redistribution of F-actin from

the dense cortical F-actin ring to the diffuse network of

F-actin bundles (compare Fig.7,A and B ),

compared

Fig.4.Effect of jasplakinolide,swinholide A,and latrunculin B on

cellular volume in response to extracellular hypertonicity.Microdis-

sected MTALs were incubated under isosmotic (time 0)or hyperos-

motic (600mosM/l for 1to 30min)conditions,without or with

addition of the following actin-interfering drugs:jasplakinolide (A ),

swinholide A (B ),and latrunculin B (C ).Cellular volume was deter-

mined from photographs.Results are expressed as a percentage of

control values and are means ?SE from 5independent experiments

(*P ?0.05vs.

control).

Fig.5.Effect of jasplakinolide (Jas),swinholide A (Swi),and latrun-culin B (Lat)on ERK and p38kinase phosphorylation levels in response to extracellular hypertonicity.MTAL suspensions were incubated under isosmotic (Iso;open bars)or hyperosmotic condi-

tions (600mosM/l,?lled bars)with addition of NaCl in the absence [vehicle (Veh)]or presence of 100?g/ml Lat,10?M Jas,or 50?M

Swi.The phosphorylation levels of p38kinase (A )and ERK (B )were measured by Western blot analysis.After densitometric quanti ?ca-tion,results were expressed as a percentage of control values and are means ?SE from 4independent experiments (*P ?0.05vs.Veh).A

and B ,top :representative Western blot analyses.

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with tubules incubated in the presence of hyperos-motic NaCl alone (compare Fig.7,B -D ).These results suggest that SB-203580and swinholide A share the

same mechanism of inhibition of actin cytoskeleton remodeling.DISCUSSION The present study demonstrates that extracellular hypertonicity induced actin cytoskeleton remodeling in native rat MTAL cells.Hypertonicity-induced remod-eling of the actin cytoskeleton was dependent on p38kinase activity and participated with the RVI.Results suggest that both cortical F-actin depolymerization and build-up of a diffuse F-actin network facilitate

RVI,most likely through modulation of ion transporter

activity.

Rat MTAL cells,which are physiologically exposed to

large variations in interstitial osmolality (23),exhibit a

polyphasic actin polymerization pro ?le in response to

extracellular hyperosmolality (see Fig.1).The initial

rapid actin polymerization and depolymerization

phases were not associated with detectable actin ?la-

ment redistribution and were shared by hyperosmotic

challenges induced by NaCl,sucrose,and urea.Be-

cause urea altered neither cellular volume nor MAPK

activity (35),the transient phases of actin polymeriza-

tion-depolymerization are obviously independent of

cell shrinkage and MAPK activation.In whole organ-

isms,the interstitial osmolality of the kidney medulla

increases progressively under antidiuresis conditions,

whereas under ex vivo experimental conditions used in

this study,extracellular osmolality increased abruptly.

We therefore cannot exclude the possibility that the

observed rapid changes in the levels of actin polymer-

ization observed during the ?rst 10min of incubation

are due to an acute increase in intracellular osmolality.

Exposure of rat MTAL cells to hyperosmotic NaCl and

sucrose,but not urea,induced a progressive actin po-

lymerization phase (from 10-to 30-min incubation)and

redistribution of F-actin from a dense cortical ring to a

diffuse network of F-actin bundles that may rely on cell

shrinkage and subsequent MAPK activation (35).Ac-

tin polymerization and redistribution of F-actin were

more pronounced in response to hyperosmotic sucrose,

which decreases cellular volume and activates MAPKs

to a larger extent than NaCl.In contrast,the sustained

phase of actin polymerization and the redistribution of

F-actin were not observed in the presence of hyperos-motic urea,which does not alter cell volume and MAPK activity.The abolition of sustained actin polymeriza-tion and redistribution of F-actin by a speci ?c p38kinase inhibitor further support this interpretation (see Figs.6and 7).

Our results suggest that actin cytoskeleton remodel-

ing is dependent on p38kinase activation (see Figs.6and 7).The p38kinase-dependent actin cytoskeleton remodeling may be mediated,at least in part,through phosphorylation of HSP25/27,a small heat shock pro-

tein that modulates actin polymerization (27).Phos-

phorylated HSP25/27promotes actin polymerization,whereas its nonphosphorylated form is inhibitory (2,8,25,31,38).In intact cells,activated p38kinase phos-phorylates and activates MAPKAP kinase 2/3,which in turn phosphorylates HSP25/27(17,28,37)and

thereby promotes redistribution of HSP25/27from the

cytoplasm to the actin cytoskeleton (52).However,in

addition to the MAPK pathway,cell shrinkage in-

creases tyrosine phosphorylation of a subset of proteins including nonreceptor tyrosine kinases and cytoskele-ton-associated proteins (20,21,24).Therefore,the ty-rosine kinase pathway may also participate in the actin cytoskeleton remodeling induced by extracellular

hypertonicity.

Fig.6.Role of p38kinase on regulatory volume increase and actin cytoskeleton remodeling in response to extracellular hypertonicity.Microdissected MTALs (A )or MTAL suspensions (B )were incubated

under isosmotic (time 0)or hyperosmotic (600mosM/l for 1to 30min)conditions,without or with addition of 10?5M of SB-203580.A :cellular volume was determined from photographs.Results are ex-pressed as a percentage of control values and are means ?SE from 11independent experiments (*P ?0.05vs.control).B :actin sorting

between TX-100-insoluble and -soluble fractions.The TX-100-insol-uble/soluble actin ratio was determined as in Fig.1and results are means ?SE from 8independent experiments (*P ?0.05vs.control).A representative Western blot analysis depicting ?-actin sorting in the absence or presence of SB-203580is shown.F343

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In addition to native MTAL cells,remodeling of the

actin cytoskeleton in response to extracellular hyper-

osmolality has been observed in yeast (12),Dictyoste-

lium (53),and cultured mammalian nonepithelial cells

(13,18,19,32)as well as in epithelial Madin-Darby

canine kidney cells (39).Results of the present study

and from the literature indicate that actin cytoskeleton

remodeling exhibits some degree of cell speci ?city.In

native rat MTAL cells (see Fig.2)and glial cells (32),

hypertonicity induced redistribution of F-actin from

the cortical ring to a diffuse network of actin bundles,

whereas in ?broblasts and HL60cells,a densi ?cation

of the peripheral actin ring was observed (13,18).

Moreover,the sustained actin polymerization phase

observed in native rat MTAL cells was absent in cul-

tured HL60cells (19).These different patterns of actin

cytoskeleton remodeling are associated with differ-

ences in RVI ef ?cacy.Indeed,in contrast to the major-

ity of cells exhibiting little or no RVI,MTAL cells

undergo robust RVI (35,43).

The temporal relationship between RVI and actin

cytoskeleton reorganization,taken together with re-

sults obtained with actin-interfering drugs,suggests that both cortical F-actin depolymerization and de novo actin polymerization resulting in the generation of a diffuse network of F-actin bundles play an impor-tant role in the RVI of MTAL cells.Results of the present study indicate that whole cell actin depolymer-ization with latrunculin B facilitates RVI,whereas global inhibition of actin depolymerization by

jas-Fig.8.Schematic representation of the sequence of events linking p38kinase activation,actin cytoskeleton,and regulatory volume increase (RVI)in MTAL epithelial cells.Exposure of MTAL epithe-

lial cells to extracellular hypertonicity induces cell shrinkage leading to the activation of p38kinase.Increased p38kinase activity pro-

motes depolymerization of the cortical F-actin ring and polymeriza-tion of new actin ?laments generating a dense diffuse network of F-actin bundles.Both processes may stimulate the activity of ion transporters leading to intracellular ion accumulation,secondary

water in ?ux,and recovery of the initial cellular

volume.

Fig.7.Effect of p38kinase inhibition

and swinholide A on actin cytoskeleton

reorganization induced by extracellu-

lar hypertonicity.MTAL suspensions

were incubated for 30min under isos-

motic (A )or hyperosmotic conditions

(600mosM/l)with addition of NaCl (B ,

C ,and

D ),without or with addition of

10?5M SB-203580(C )or 50?M swin-

holide A (D ).Tubules were then ?xed

with 4%paraformaldehyde,permeabil-

ized by 0.1%TX-100,and the actin

cytoskeleton was visualized by ?uores-

cence microscopy after incubation with

phalloidin-Alexa 488.Representative

en face views of MTAL epithelium are

shown.After incubation under hyper-

tonic conditions (B ),MTAL cells ex-

hibit a thin cortical F-actin ring (?lled

arrow)and a dense diffuse network of

F-actin bundles (dashed arrow).The

hypertonicity-induced redistribution of

F-actin from the cortical ring to the

diffuse network was prevented by SB-

203580(C )and swinholide A (D ).

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plakinolide antagonizes RVI (see Figs.3and 4).These results suggest that depolymerization of F-actin is re-quired for RVI in MTAL epithelial cells.In addition,?uorescence microscopy imaging shows that RVI is associated with reduced cortical F-actin staining (see Fig.2),suggesting that the F-actin depolymerization process involved in RVI speci ?cally takes place at the level of the cortical F-actin ring in MTAL epithelial cells.This result contrasts with those obtained in non-epithelial cells (HL60)that exhibit a densi ?cation of the cortical F-actin ring in response to hypertonicity but which do not undergo RVI (13,18).On the other hand,our results show that swinholide A or SB-203580,an inhibitor of p38kinase,prevented the hypertonicity-induced generation of diffuse F-actin bundles and reduced the ef ?cacy of RVI (see Figs.4and 6).These results suggest that,in addition to cortical F-actin ring depolymerization,the generation of a dense diffuse network of F-actin bundles facilitates RVI.At ?st glance,this ?nding contrasts with the effect of jasplakinolide,which increases the actin poly-merization level and prevents the RVI.It should be noticed,however,that jasplakinolide also increased actin polymerization at the level of the cortical F-actin ring,an effect that most likely antagonizes RVI.It is well established that RVI is associated with ion transporter activation including Na-K-2Cl cotrans-porter,Na/H exchanger,Cl/HCO 3exchanger,and Na-K-ATPase (4,15,29,43,47),which might,at least in part,be dependent on cortical actin polymerization level.For instance,inhibition of F-actin depolymeriza-tion by phalloidin or jasplakinolide impairs the activa-tion of the Na-K-2Cl cotransporter by cAMP in MTAL cells (49).Conversely,depolymerization of F-actin by cytochalasin D stimulates Na-K-2Cl cotransporter in intestinal cells (30).Actin polymerization may control the activity of ion transporters in different ways.A shift in F-actin-G-actin equilibrium toward G-actin may stimulate the activity of speci ?c ion transporters,as shown for Na-K-ATPase (9)and epithelial Na chan-

nels (3,10).On the other hand,depolymerization of the cortical F-actin ring may promote the exocytosis of ion,solute,and water transporters as demonstrated for the Na/H exchanger NHE3(11),volume-sensitive Cl ?

channels (33),the glucose transporter GLUT4(26),and the water channel aquaporin-2(22).The results of the present study indicate that,at the level of the whole cell,the equilibrium between actin polymerization and

depolymerization is shifted toward actin polymeriza-tion during RVI (see Fig.1).This polymerization pro-cess results in the generation of a dense and diffuse network of F-actin bundles (see Fig.2)that may play a functional role in the defense against cell shrinkage.Indeed,inhibition of the sustained actin polymeriza-tion phase and the densi ?cation of the diffuse F-actin network by swinholide A and SB-203580both in-creased the extent of maximal cell shrinkage and re-duced the ef ?cacy of RVI in response to hypertonicity (see Figs.4,6,and 7).This effect might be partly achieved through mechanical constraints exerted on the cell membrane,as described for lamellipodia or ?lipodia formation (44).It occurs,however,most likely indirectly via spacial control of signaling events and/or facilitation of the delivery of ion transporters from intracellular stores to the plasma membrane,as shown for the GLUT4glucose transporter in response to in-sulin (46).

Because activation of MAPKs is mediated by cell shrinkage in rat MTAL cells (35),the hypothesis that the actin cytoskeleton may be part of the osmosensing machinery was considered.Our results,however,sug-gest that actin cytoskeleton remodeling and integrity are not essential for the activation of MAPKs in re-sponse to increased extracellular osmolality.Indeed,interfering with neither the polymerization level of actin nor with the generation of new actin ?laments decreased the extent of ERK and p38kinase activation

in response to extracellular hypertonicity (see Fig.5).Therefore,alternative mechanisms such as an increase in cytoplasmic concentration of macromolecules (34)or aggregation of growth factor and cytokine receptors leading to their ligand-independent activation (36)have to be considered.

In conclusion,we showed that an acute extracellu-lar hypertonic challenge induces actin cytoskeleton remodeling consisting of F-actin redistribution from a cortical ring to a diffuse network of F-actin bundles in native rat MTAL cells.We propose the following working hypothesis summarized by Fig.8.Cell shrinkage induces p38kinase activation,which in turn,promotes cortical F-actin ring depolymeriza-tion and generation of a dense diffuse network of F-actin bundles that both promote RVI most likely through modulation of ion transporter activity.Fur-ther investigation is required to identify the molec-ular players involved in actin cytoskeleton remodel-ing.In addition,the role of actin cytoskeleton remod-eling in the control of the activity and/or abundance of ion transporters involved in RVI remains to be determined.

We thank Dr.C.Chaponnier for helpful discussions and critical reading of the manuscript.

DISCLOSURES

This work was supported in part by Grants 31–50–643.97and 31–56830.99from the Swiss National Foundation to E.Fe ′raille and

by a grant from the Fondation Novartis pour la Recherche en Sci-

ences Me ′dico-biologiques to E.Fe ′raille.

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时间:2018年9月

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