Volume regulation of cultured, transformed, non-pigmented epithelial cells from human ciliary body

Mechanistic Effects of Baicalein on Aqueous Humor Drainage and Intraocular Pressure

Elevated intraocular pressure (IOP) is a major risk factor for glaucoma that results from impeded fluid drainage. The increase in outflow resistance is caused by trabecular meshwork (TM) cell dysfunction and excessive extracellular matrix (ECM) deposition. Baicalein (Ba) is a natural flavonoid and has been shown to regulate cell contraction, fluid secretion, and ECM remodeling in various cell types, suggesting the potential significance of regulating outflow resistance and IOP. We demonstrated that Ba significantly lowered the IOP by about 5 mmHg in living mice. Consistent with that, Ba increased the outflow facility by up to 90% in enucleated mouse eyes. The effects of Ba on cell volume regulation and contractility were examined in primary human TM (hTM) cells. We found that Ba (1–100 µM) had no effect on cell volume under iso-osmotic conditions but inhibited the regulatory volume decrease (RVD) by up to 70% under hypotonic challenge. In addition, Ba relaxed hTM cells via reduced myosin light chain (MLC) phosphorylation. Using iTRAQ-based quantitative proteomics, 47 proteins were significantly regulated in hTM cells after a 3-h Ba treatment. Ba significantly increased the expression of cathepsin B by 1.51-fold and downregulated the expression of D-dopachrome decarboxylase and pre-B-cell leukemia transcription factor-interacting protein 1 with a fold-change of 0.58 and 0.40, respectively. We suggest that a Ba-mediated increase in outflow facility is triggered by cell relaxation via MLC phosphorylation along with inhibiting RVD in hTM cells. The Ba-mediated changes in protein expression support the notion of altered ECM homeostasis, potentially contributing to a reduction of outflow resistance and thereby IOP.

Regulatory Roles of Anoctamin-6 in Human Trabecular Meshwork Cells

Purpose

Trabecular meshwork (TM) cell volume is a determinant of aqueous humor outflow resistance, and thereby IOP. Regulation of TM cell volume depends on chloride ion (Cl-) release through swelling-activated channels (ICl,Swell), whose pore is formed by LRRC8 proteins. Chloride ion release through swelling-activated channels has been reported to be regulated by calcium-activated anoctamins, but this finding is controversial. Particularly uncertain has been the effect of anoctamin Ano6, reported as a Ca2+-activated Cl- (CaCC) or cation channel in other cells. The current study tested whether anoctamin activity modifies volume regulation of primary TM cell cultures and cell lines.

Methods

Gene expression was studied with quantitative PCR, supplemented by reverse-transcriptase PCR and Western immunoblots. Currents were measured by ruptured whole-cell patch clamping and volume by electronic cell sizing.

Results

Primary TM cell cultures and the TM5 and GTM3 cell lines expressed Ano6 3 to 4 orders of magnitude higher than the other anoctamin CaCCs (Ano1 and Ano2). Ionomycin increased cell Ca2+ and activated macroscopic currents conforming to CaCCs in other cells, but displayed significantly more positive mean reversal potentials (+5 to +12 mV) than those displayed by ICl,Swell (-14 to -21 mV) in the same cells. Nonselective CaCC inhibitors (tannic acid>CaCCinh-A01) and transient Ano6 knockdown strongly inhibited ionomycin-activated currents, ICl,Swell and the regulatory volume response to hyposmotic swelling.

Conclusions

Ionomycin activates CaCCs associated with net cation movement in TM cells. These currents, ICl,Swell, and cell volume are regulated by Ano6. The findings suggest a novel clinically-relevant approach for altering cell volume, and thereby outflow resistance, by targeting Ano6.

Effects of cardiotonic steroids on trabecular meshwork cells: search for mediator of ouabain-enhanced outflow facility

Lowering intraocular pressure (IOP) is currently the only strategy documented to slow the onset and progression of glaucomatous blindness. Ouabain, a cardiotonic glycoside inhibitor of Na(+), K(+)-activated ATPase, was recently reported to enhance outflow facility in porcine anterior segments at concentrations as low as 30 nM for ≥4 h, suggesting a novel approach to lowering IOP. The underlying mechanism is unknown, but associated cytoskeletal changes were observed in porcine trabecular meshwork cells. We have previously found that changes in ATP release and subsequent ectoenzymatic conversion to adenosine may play a role in linking cytoskeletal remodeling with modulation of outflow resistance. We now tested whether altered ATP release might also be a mediator of ouabain's effect on outflow facility. ATP release from transformed human TM5 and explant-derived human trabecular meshwork cells was measured by the luciferin-luciferase reaction. Matrix metalloproteinases (MMPs) were studied by zymography, cell Na(+) concentration by SBFI fluorometry, gene expression of ATP-release pathways by real-time PCR, cell volume by electronic cell sorting and cell viability by the LDH and MTT methods. Actin was examined by confocal microscopy of phalloidin-stained cells. Contrary to expectation, ouabain at concentrations ≥10 nM inhibited swelling-triggered ATP release from TM5 cells after ≥4 h of exposure. Inhibition was enhanced by increasing ouabain concentration and exposure time. Similar effects were produced by the reversible cardiac aglycone strophanthidin. Ouabain also inhibited swelling-activated ATP release from explant-derived native human TM cells. Ouabain (4 h, 30 nM and 100 nM) did not alter gene expression of the ATP-release pathways, and cell viability was unchanged by exposure to ouabain (30 nM-1 μM). Preincubation with 30 nM ouabain for 4 h did not detectably change Na(+) level, the regulatory volume decrease (RVD) or the actin cytoskeleton of TM5 cells, but did inhibit hypotonicity-elicited ATP release. Moreover, even when N-methyl-d-glucosamine replaced Na(+) in the extracellular fluid, ouabain still inhibited swelling-initiated ATP release at 100 nM. In the absence of ouabain, extracellular ATP stimulated MMP secretion, which was largely blocked by inhibiting conversion of ATP to adenosine, as expected. In contrast, ouabain reduced ATP release, but did not alter secretion of MMP-2 and MMP-9 from cells pretreated for ≤4 h. The results suggest that: (1) ouabain can trigger enhancement of outflow facility independent of its transport and actin-restructuring effects exerted at higher concentration and longer duration; (2) ouabain exerts parallel independent effects on ATP release and outflow facility; and (3) these effects likely reflect ouabain-induced changes in the scaffolding and/or signaling functions of Na(+), K(+)-activated ATPase.

Cytoskeletal dependence of adenosine triphosphate release by human trabecular meshwork cells

PURPOSE

To test whether adenosine triphosphate (ATP) release links cytoskeletal remodeling with release of matrix metalloproteinases (MMPs), regulators of outflow facility and intraocular pressure.

METHODS

ATP release was measured by luciferin-luciferase. Ecto-ATPases from transformed human trabecular meshwork (TM) cells (TM5) and explant-derived TM cells were identified by RT-PCR. Actin was visualized by phalloidin staining. Cell viability was assayed by lactate dehydrogenase and thiazolyl blue tetrazolium bromide methods and propidium iodide exclusion, gene expression by real-time PCR, and MMP release by zymography. Cell volume was monitored by electronic cell sorting.

RESULTS

Hypotonicity (50%) and mechanical stretch increased ATP release with similar pharmacologic profiles. TM cells expressed ecto-ATPases E-NPP1-3, E-NTPD2, E-NTPD8, and CD73. Prolonged dexamethasone (DEX) exposure (≥ 2 weeks), but not brief exposure (3 days), increased cross-linked actin networks and reduced swelling-triggered ATP release. Cytochalasin D (CCD) exerted opposite effects. Neither DEX nor CCD altered the cell viability, gene expression, or pharmacologic profile of ATP-release pathways. DEX accelerated, and CCD slowed, the regulatory volume decrease after hypotonic exposure. Activating A(1) adenosine receptors (A(1)ARs) increased total MMP-2 and MMP-9 release. DEX reduced total A(1)AR-triggered MMP release, and CCD increased the active form of MMP-2 release. The A(1)AR agonist CHA and the A(1)AR antagonist DPCPX partially reversed the effects of DEX and CCD, respectively.

CONCLUSIONS

Cytoskeletal restructuring modulated swelling-activated ATP release, in part by changing the duration of cell swelling after hypotonic challenge. Modifying ATP release is expected to modulate MMP secretion by altering ecto-enzymatic delivery of adenosine to A(1)ARs, linking cytoskeletal remodeling and MMP-mediated modulation of outflow facility.

A3 adenosine and CB1 receptors activate a PKC-sensitive Cl- current in human nonpigmented ciliary epithelial cells via a G beta gamma-coupled MAPK signaling pathway

(1) We examined A3 adenosine and CB1 cannabinoid receptor-coupled signaling pathways regulating Cl(-) current in a human nonpigmented ciliary epithelial (NPCE) cell line. (2) Whole-cell patch-clamp recordings demonstrated that the A3 receptor agonist, IB-MECA, activates an outwardly rectifying Cl(-)current (I(Cl,Aden)) in NPCE cells, which was inhibited by the adenosine receptor antagonist, CGS-15943 or by the protein kinase C (PKC) activator, phorbol 12,13 dibutyrate (PDBu). (3) Treatment of NPCE cells with pertussis-toxin (PTX), or transfection with the COOH-terminus of beta-adrenergic receptor kinase (ct-betaARK), inhibited I(Cl,Aden). The phosphatidyl inositol 3-kinase (PI3K) inhibitor, wortmannin, had no effect on I(Cl,Aden); however, the mitogen-activated protein kinase kinase (MEK) inhibitor, PD98059, inhibited I(Cl,Aden). (4) Reverse transcription-polymerase chain reaction experiments and immunocytochemistry confirmed mRNA and protein expression for the CB1 receptor in NPCE cells, and the CB1 receptor agonist, Win 55,212-2, activated a PDBu-sensitive Cl(-) current (I(Cl,Win)). (5) Transfection of NPCE cells with the human CB1 (hCB1) receptor, increased I(Cl,Win), consistent with increased receptor expression, and I(Cl,Win) in hCB1 receptor-transfected cells was decreased after application of a CB1 receptor inverse agonist, SR 141716. (6) Constitutive activity for CB1 receptors was not significant in NPCE cells as transfection with hCB1 receptors did not increase basal Cl(-) current, nor was basal current inhibited by SR 141716. (7) I(Cl,Win) was inhibited by PTX preincubation, by transfection with ct-betaARK and by the MEK inhibitor, PD98059, but unaffected by the PI3K inhibitor, wortmannin. (8) We conclude that both A3 and CB1 receptors activate a PKC-sensitive Cl(-) current in human NPCE cells via a G(i/o)/Gbetagamma signaling pathway, in a manner independent of PI3K but involving MAPK.

Renin-angiotensin system expression and secretory function in cultured human ciliary body non-pigmented epithelium

BACKGROUND

Renin-angiotensin system (RAS) components have been identified in human ciliary body and aqueous humour, pointing to a role for the RAS in the regulation of aqueous humour dynamics. Here, the authors examine the functional expression of a RAS and the effects of angiotensin II (AII) on a signal transduction pathway and ion secretion mechanism in cultured human ciliary body non-pigmented epithelium (HNPE).

METHODS

RAS expression was examined in cultured HNPE cells using polymerase chain reaction (PCR) analysis. Secretory function was determined using spectrofluorescence imaging microscopy to measure cell calcium (Ca(2+)(I)) and volume responses. Single channel patch clamp techniques were employed to investigate ion channel activity.

RESULTS

PCR analysis demonstrated the expression of angiotensinogen and the AT(1b) receptor in HNPE cells. A large conductance potassium (BK) channel (mean 190 (SEM 5.6) pS, n = 22 cells), was observed in plasma membrane patches. This channel was calcium sensitive with channel open probability (Po) increasing with increasing Ca(2+)(I) (K(0.5) 10.79 (0.44) microM Ca(2+), Hill coefficient of 1.04 (0.04)). AII (100 nM) increased the number (N) of active BK channels in HNPE cells and also the probability of channel opening (Po). N.P(o) increased from 0.008 (0.002) to 1.38 (0.4) following the addition of AII (p=0.0064). AII also induced a rapid rise in Ca(2+)(I) from resting values of 112 (14) nM to a peak of 992 (106) nM (p<10(-4)). A simultaneous cell volume reduction of 24.70% (3.34%) (p<10(-4)) occurred during this calcium signal. Losartan (1 microM) significantly blocked the AII induced BK channel activation (p=0.0131), the Ca(2+)(I) response (p<10(-4)), and the AII induced volume effect (p=0.0046).

CONCLUSION

It was demonstrated that AII activates a Ca(2+)(I) signalling system which subsequently increases potassium ion channel activity. These effects are accompanied simultaneously by cell volume loss, indicating that AII acts as receptor operated secretagogue in HNPE cells. The ability of an AT(1) receptor antagonist to inhibit these processes may thus offer a new family of pharmaceutical agents to the current armamentarium in the treatment of glaucoma.

Extracellular ATP effects on calcium signaling in cultured human non-pigmented ciliary body epithelium

PURPOSE

To determine the effects of extracellular ATP on calcium signaling in cultured human non-pigmented ciliary body epithelium (HNPE).

METHODS

Intracellular calcium (Ca(2+)(i)) was measured using spectrofluorescence video microscopy in isolated HNPE cells loaded with the fluorescent dye Fura-2.

RESULTS

Nucleotides caused a transient oscillatory increase in Ca(2+)(i) with a potency order of ATP = UTP > ADP > AMP> alpha,beta-methylene-ATP. Treatment with thapsigargin (100 nM), an inhibitor of endoplasmic Ca(2+)-ATPase pumps, produced a sustained increase in Ca(2+)(i). Subsequent exposure to ATP caused a rapid reduction in Ca(2+)(i) and this effect was reduced by pre-exposure to vanadate and to a lesser extent in sodium free solution. Prolonged exposure to ATP in the presence of thapsigargin caused a transient spike increase in Ca(2+)(i) which was prevented by exposure to low extracellular Ca(2+) (1 nmol/l), verapamil, nifedipine or the microfilament disrupting agent, cytochalasin B.

CONCLUSIONS

These results provide evidence for ATP mobilisation of Ca(2+) from intracellular stores via P2Y2 receptor activation in HNPE cells. ATP also primarily activates a vanadate-sensitive Ca(2+ )-ATPase pump, in addition to having a smaller effect on the Na( +)/ Ca(2+) exchanger in terminating the calcium signal. Capacitative calcium entry, possibly via an L-type Ca(2+) channel, is implicated in generating a calcium signal following emptying of intracellular stores and is sensitive to cytoskeleton disruption. ATP can thus regulate a potent intracellular signal for secretion, suggest-ing that purinergic receptors may provide a therapeutic target in glaucoma.

PGE(2), Ca(2+), and cAMP mediate ATP activation of Cl(-) channels in pigmented ciliary epithelial cells

Purines regulate intraocular pressure. Adenosine activates Cl(-) channels of nonpigmented ciliary epithelial cells facing the aqueous humor, enhancing secretion. Tamoxifen and ATP synergistically activate Cl(-) channels of pigmented ciliary epithelial (PE) cells facing the stroma, potentially reducing net secretion. The actions of nucleotides alone on Cl(-) channel activity of bovine PE cells were studied by electronic cell sorting, patch clamping, and luciferin/luciferase ATP assay. Cl(-) channels were activated by ATP > UTP, ADP, and UDP, but not by 2-methylthio-ATP, all at 100 microM. UTP triggered ATP release. The second messengers Ca(2+), prostaglandin (PG)E(2), and cAMP activated Cl(-) channels without enhancing effects of 100 microM ATP. Buffering intracellular Ca(2+) activity with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid or blocking PGE(2) formation with indomethacin inhibited ATP-triggered channel activation. The Rp stereoisomer of 8-bromoadenosine 3',5'-cyclic monophosphothioate inhibited protein kinase A activity but mimicked 8-bromoadenosine 3',5'-cyclic monophosphate. We conclude that nucleotides can act at >1 P2Y receptor to trigger a sequential cascade involving Ca(2+), PGE(2), and cAMP. cAMP acts directly on Cl(-) channels of PE cells, increasing stromal release and potentially reducing net aqueous humor formation and intraocular pressure.

Timolol may inhibit aqueous humor secretion by cAMP-independent action on ciliary epithelial cells

The beta-adrenergic antagonist timolol reduces ciliary epithelial secretion in glaucomatous patients. Whether inhibition is mediated by reducing cAMP is unknown. Elemental composition of rabbit ciliary epithelium was studied by electron probe X-ray microanalysis. Volume of cultured bovine pigmented ciliary epithelial (PE) cells was measured by electronic cell sizing; Ca(2+) activity and pH were monitored with fura 2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. Timolol (10 microM) produced similar K and Cl losses from ciliary epithelia in HCO/CO(2) solution but had no effect in HCO/CO(2)-free solution or in HCO/CO(2) solution containing the carbonic anhydrase inhibitor acetazolamide. Inhibition of Na(+)/H(+) exchange by dimethylamiloride in HCO/CO(2) solution reduced Cl and K comparably to timolol. cAMP did not reverse timolol's effects. Timolol (100 nM, 10 microM) and levobunolol (10 microM) produced cAMP-independent inhibition of the regulatory volume increase (RVI) in PE cells and increased intracellular Ca(2+) and pH. Increasing Ca(2+) with ionomycin also blocked the RVI. The results document a previously unrecognized cAMP-independent transport effect of timolol. Inhibition of Cl(-)/HCO exchange may mediate timolol's inhibition of aqueous humor formation.

Similarity of A(3)-adenosine and swelling-activated Cl(-) channels in nonpigmented ciliary epithelial cells

Chloride release from nonpigmented ciliary epithelial (NPE) cells is a final step in forming aqueous humor, and adenosine stimulates Cl(-) transport by these cells. Whole cell patch clamping of cultured human NPE cells indicated that the A(3)-selective agonist 1-deoxy-1-(6-[([3-iodophenyl]methyl)amino]-9H-purin-9-yl)-N-methyl-be ta-D-ribofuranuronamide (IB-MECA) stimulated currents (I(IB-MECA)) by approximately 90% at +80 mV. Partial replacement of external Cl(-) with aspartate reduced outward currents and shifted the reversal potential (V(rev)) from -23 +/- 2 mV to -0.0 +/- 0.7 mV. Nitrate substitution had little effect. Perfusion with the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid inhibited the currents. Partial Cl(-) replacement with aspartate and NO(3)(-), and perfusion with NPPB, had similar effects on the swelling-activated whole cell currents (I(Swell)). Partial cyclamate substitution for external Cl(-) inhibited inward and outward currents of both I(IB-MECA) and I(Swell). Both sets of currents also showed outward rectification and inactivation at large depolarizing potentials. The results are consistent with the concept that A(3)-subtype adenosine agonists and swelling activate a common population of Cl(-) channels.

Tamoxifen and ATP synergistically activate Cl- release by cultured bovine pigmented ciliary epithelial cells

Purines alter aqueous humour secretion by the bilayered ciliary epithelium. Adenosine but not ATP shrinks non-pigmented ciliary epithelial (NPE) cells by activating Cl- channels. We now report effects of ATP on pigmented ciliary epithelial (PE) cells. Cultured bovine PE cells were studied volumetrically by electronic cell sorting. ATP and tamoxifen acted synergistically to shrink PE cells. Neither ATP nor tamoxifen alone had a consistent effect on cell volume. The tamoxifen, ATP-activated shrinkage required Cl- release since the response was blocked by removing Cl- and was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate and 4,4'-diisothiocyano-2,2'-disulfonic acid. The modulating effect of tamoxifen could have reflected many actions of tamoxifen. Our data do not support the suggestion that tamoxifen inhibits protein kinase C (PKC) or calcium-calmodulin, or that it acts on histamine or carbachol receptors. The shrinkage produced by ATP and tamoxifen was blocked by 17beta-oestradiol, but not 17alpha-oestradiol. The cooperative interaction between tamoxifen and ATP was not mediated by an enhanced rise in [Ca2+]i. The results indicate that tamoxifen interacts synergistically with ATP to activate Cl- release by the PE cells.

Hyposmotically activated chloride channels in cultured rabbit non-pigmented ciliary epithelial cells

1. We used whole-cell patch-clamp recording techniques and noise analysis of whole-cell current to investigate the properties of hyposmotic shock (HOS)-activated Cl- channels in SV40-transformed rabbit non-pigmented ciliary epithelial (NPCE) cells. 2. Under conditions designed to isolate Cl- currents, exposure of cells to hyposmotic external solution reversibly increased the whole-cell conductance. 3. The whole-cell current activated with a slow time course (> 15 min), exhibited outward rectification and was Cl- selective. 4. The disulphonic stilbene derivatives 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 0.5 mM), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS, 0. 5 mM) and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS, 0.5 mM) produced a voltage-sensitive block of HOS-activated Cl- current at depolarized potentials, whereas niflumic acid produced a voltage-independent block of the current. 5. Under Ca2+-free conditions, HOS stimulation still reversibly activated the Cl- current, but the amplitude of current was reduced and the time course of current activation was slower compared with control (P < 0. 05). 6. The non-specific kinase inhibitor H-7 (100 microM), upregulated HOS-activated Cl- current amplitude in all cells tested (P < 0.05). 7. Noise analysis of whole-cell Cl- current indicated that cell swelling activated a high density of small conductance Cl- channels (< 1 pS). 8. We conclude that HOS primarily activates a high density of volume-sensitive small conductance Cl- channels in rabbit NPCE cells, and that Ca2+ and phosphorylation are involved in channel regulation.

Effects of hypotonic swelling on the cellular distribution and expression of pI(Cln) in human nonpigmented ciliary epithelial cells

PURPOSE

It has been proposed that pI(Cln), a highly acidic protein, is a candidate gene product related to the swelling-activated chloride (Cl-) channel Icl.swell in mammalian cells. However, no consensus has been reached as to whether this relationship is direct or indirect. Recently the cDNA for pI(Cln) was isolated from human ciliary epithelial cells. To learn more about the structure-function of pI(Cln) we attempted to: i) overexpress pI(Cln) as a fusion protein in bacteria; ii) carry out its purification; iii) generate polyclonal antibodies to study its expression and cellular localization in the ciliary epithelial cells; and iv) determine whether cell-swelling affects pI(Cln) expression in ciliary epithelial cells.

METHODS

The open reading frame (ORF) of human pI(Cln) was subcloned in the pET-20b(+) plasmid and established as a recombinant vector in E. coli BL21(DE3)pLysS cells. Upon induction with iso-propyl-beta-thio-galactopyranoside (IPTG), pI(Cln) was isolated as a His-Tag fusion protein and purified to homogeneity. Polyclonal antibodies were raised in rabbits after immunization with pI(Cln) purified protein, and its expression and cellular distribution in ciliary epithelial cells determined by Western blot, immunoprecipitation and indirect immunofluorescence respectively. Cell-swelling effect on ciliary epithelial cells was carried out upon treatment of cultured cells with hypotonic solution up to 60 min and pI(Cln) expression measured by Northern and Western blot analysis.

RESULTS

By Western blot analysis or immunoprecipitation, pI(Cln) antisera recognized a main band of 37-kDa in total cell extracts from ciliary body or metabolically labeled ciliary epithelial cells. By indirect immunofluorescence, pI(Cln) antibodies stained the cytoplasm of NPE in the intact tissue, and the perinuclear region of cultured ciliary epithelial cells. When subjected to hypotonic treatment, NPE cells did not induce translocation of pI(Cln) protein from the cytoplasm into the plasma membrane, nor changes in pI(Cln) expression at the protein level, but did down regulate up to 30% the level of pI(Cln) mRNA in continued hypotonic treatment.

CONCLUSIONS

These observations indicate that, contrary to previous suggestions, the pI(Cln) protein is not likely to be in contact with the plasma membrane of ciliary epithelial cells, and its influence on Cl- -channel activity is more likely to be expressed indirectly, (i.e. through cytoskeletal restructuring).

A3 adenosine receptors regulate Cl- channels of nonpigmented ciliary epithelial cells

Adenosine stimulates Cl- channels of the nonpigmented (NPE) cells of the ciliary epithelium. We sought to identify the specific adenosine receptors mediating this action. Cl- channel activity in immortalized human (HCE) NPE cells was determined by monitoring cell volume in isotonic suspensions with the cationic ionophore gramicidin present. The A3-selective agonist N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) triggered shrinkage (apparent Kd = 55 +/- 10 nM). A3-selective antagonists blocked IB-MECA-triggered shrinkage, and A3-antagonists (MRS-1097, MRS-1191, and MRS-1523) also abolished shrinkage produced by 10 microM adenosine when all four known receptor subtypes are occupied. The A1-selective agonist N6-cyclopentyladenosine exerted a small effect at 100 nM but not at higher or lower concentrations. The A2A agonist CGS-21680 triggered shrinkage only at high concentration (3 microM), an effect blocked by MRS-1191. IB-MECA increased intracellular Ca2+ in HCE cells and also stimulated short-circuit current across rabbit ciliary epithelium. A3 message was detected in both HCE cells and rabbit ciliary processes using RT-PCR. We conclude that human HCE cells and rabbit ciliary processes possess A3 receptors and that adenosine can activate Cl- channels in NPE cells by stimulating these A3 receptors.

Adenosine stimulates Cl- channels of nonpigmented ciliary epithelial cells

Ciliary epithelial cells possess multiple purinergic receptors, and occupancy of A1 and A2 adenosine receptors is associated with opposing effects on intraocular pressure. Aqueous adenosine produced increases in short-circuit current across rabbit ciliary epithelium, blocked by removing Cl- and enhanced by aqueous Ba2+. Adenosine's actions were further studied with nonpigmented ciliary epithelial (NPE) cells from continuous human HCE and ODM lines and freshly dissected bovine cells. With gramicidin present, adenosine (> or = 3 microM) triggered isosmotic shrinkage of the human NPE cells, which was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB) and niflumic acid. At 10 microM, the nonmetabolizable analog 2-chloroadenosine and AMP also produced shrinkage, but not inosine, UTP, or ATP. 2-Chloroadenosine (> or = 1 microM) triggered increases of whole cell currents in HCE cells, which were partially reversible, Cl- dependent, and reversibly inhibited by NPPB. Adenosine (> or = 10 microM) also stimulated whole cell currents in bovine NPE cells. We conclude that occupancy of adenosine receptors stimulates Cl- secretion in mammalian NPE cells.

Volume-sensitive chloride current in pigmented ciliary epithelial cells: role of phospholipases

The whole cell recording technique was used to examine an outwardly rectifying chloride current activated by hypotonic shock in bovine pigmented ciliary epithelial (PCE) cells. Removal of internal and external Ca2+ did not affect the activation of these currents, but they were abolished by the phospholipase C inhibitor neomycin. The current was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and 4,4'-disothiocyanostilbene-2,2'-disulfonic acid (DIDS) in a voltage-dependent manner, but tamoxifen, dideoxyforskolin, and quinidine did not affect it. This blocking profile differs from that of the volume-sensitive chloride channel in neighboring nonpigmented ciliary epithelial cells (Wu, J., J. J. Zhang, H. Koppel, and T. J. C. Jacob, J. Physiol, Lond. 491: 743-755, 1996), and this difference implies that the volume responses of the two cell types are mediated by different chloride channels (Jacob, T. J. C., and J. J. Zhang. J. Physiol. Lond. In press). Intracellular administration of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to PCE cells induced a transient, time-independent, outwardly rectifying chloride current that closely resembled the current activated by hypotonic shock. DIDS produced a voltage-dependent block of the GTP gamma S-activated current similar to the block of the hypotonically activated current. Intracellular neomycin completely prevented activation of this current as did incubation of the cells in calphostin C. and inhibitor of protein kinase C (PKC). Removal of Ca2+ did not affect activation of the current by GTP gamma S but extended the duration of the response. Inhibition of phospholipase A2 (PLA2) with p-bromophenacyl bromide prevented the activation of the hypotonically induced current and also inhibited the current once activated by hypotonic solution. The findings imply that the hypotonic response in PCE cells is mediated by both phospholipase C (PLC) and PLA2. Both phospholipases generate arachidonic acid, and, in addition, the PLC pathway regulates the PLA2 pathway via a PKC-dependent phosphorylation of PLA2.

Mechanisms of regulatory volume decrease in nonpigmented human ciliary epithelial cells

To study the net solute and water efflux pathways of the ciliary epithelium we employed a cultured human NPE cell line. Because of the possible relationship between transepithelial ion and water flux and cell volume regulation, the ion efflux pathways mediating regulatory volume decrease (RVD) were investigated. Osmotic swelling of NPE cells was followed by a volume recovery. Volume recovery was K+ dependent and inhibited by K+ channel blockers such as quinine (1 mM). After osmotic swelling, a Cl(-)-dependent membrane depolarization occurred that was inhibited by Cl- channel blockers such as 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) or Ca2+ chelators such as ethylene glycolbis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, 2.0 mM). Cell swelling was also accompanied by an increase in intracellular Ca2+ concentration ([Ca2+]i) of approximately 200 nM. The swelling-induced rise in [Ca2+]i and RVD were diminished in the presence of 10 microM La3+, 50 nM 12-O-tetradecanoylphorbol 13-acetate, and nominally Ca(2+)-free medium. Near total blockage of RVD occurred after pretreatment of NPE cells with Ca(2+)-free EGTA-1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) acetoxymethyl ester-containing solutions. The inhibition of RVD by EGTA-BAPTA treatment was overcome by increasing K+ conductance with gramicidin. The above findings indicate that RVD in NPE cells is mediated by separate K+ and Cl- conductances (channels). These data also show that swelling-induced increases in [Ca2+]i help modulate net ion efflux during regulation.

cDNA from human ocular ciliary epithelium homologous to beta ig-h3 is preferentially expressed as an extracellular protein in the corneal epithelium

The non-pigmented ciliary epithelium is largely responsible for the formation of aqueous humor in the mammalian eye. To provide a basis for studies at the molecular level, a directional expression cDNA library was constructed in Uni-ZAP XR vector from poly A+ RNA of the human non-pigmented ciliary epithelial derived ODM-2 cell line. Fifty-three cDNA clones were isolated from the library and characterized by partial sequence analysis. Approximately 49% of the clones exhibited homology with known genes in the GenBank/EMBL databases. The putative identification of these clones may reflect the transcriptional activity of the ODM-2 cells in culture. One of the identified clones, ODM-42-I, was found to be specific and highly expressed in the corneal epithelium. This clone had an exact match with a recently discovered human gene, beta ig-h3 (Skonier et al., 1992, DNA Cell Biol., 11:511-522), which codes a surface recognition protein, inducible by transforming growth factor beta (TGF-beta), and containing a putative binding site (RDG) for integrins. The ODM-42-I cDNA clone displays a distinctive pattern of expression found in the human eye, expressed almost exclusively in the cornea. Further studies, using sera from a synthetic peptide to the carboxy-terminal region of ODM-42-I, reveal that the protein is heterogeneous in charge and is preferentially expressed on the extracellular surface of corneal epithelial cells, and might share immunologic properties with integrins beta 1.

Ion transport asymmetry and functional coupling in bovine pigmented and nonpigmented ciliary epithelial cells

The solute and water transport properties of the bovine ciliary epithelium were studied using isolated pigmented (PE) and nonpigmented (NPE) cells. It was shown that these cells were functionally coupled by demonstrating dye diffusion between paired PE and NPE cells after microinjection of lucifer yellow. Electronic cell sizing was used to measure cell volume changes of isolated PE and NPE cells in suspension after anisosmotic perturbations and after transport inhibition under isosmotic conditions. The PE cells showed the presence of a regulatory volume increase when subjected to osmotic shrinkage with NaCl, whereas the NPE cells did not demonstrate a regulatory volume increase under these conditions. In contrast, the NPE cells exhibited a regulatory volume decrease when subjected to osmotic swelling, whereas the PE cells did not recover from swelling. The regulatory volume decrease in NPE cells was inhibited by increased bath K or pretreatment with quinine (1 mM). The presence of a bumetanide-sensitive mechanism capable of moving measurable amounts of solute and water, probably Na-K-2Cl cotransport, was demonstrated in the PE cells but absent in the NPE cells. Bumetanide produced a dose-dependent shrinkage of PE cells at concentrations as low as 1 microM. Isosmotically reducing bath Cl, Na, or K concentration caused a rapid shrinkage of PE cells that was bumetanide inhibitable. The asymmetry of transport properties in PE and NPE cells supports a functional syncytium model of aqueous humor formation (39) across the two layers of the ciliary epithelium wherein ion uptake from the blood is carried out by the PE cells and ion extrusion by the NPE cells. Gap-junction coupling between the cells allows the ions taken up by the PE cells to move into the NPE cells. Extrusion of Na by the Na-K pump across the aqueous facing (basolateral) membranes of the NPE cells, most likely accompanied by Cl, determines the formation of the aqueous humor.

Prolonged incubation with elevated glucose inhibits the regulatory response to shrinkage of cultured human retinal pigment epithelial cells

Transport defects by retinal pigment epithelial (RPE) and other cells are observed in experimental models of diabetes mellitus. Recent studies have established that glucose concentration, per se, is the critical risk factor in the pathogenesis of diabetic complications. This study was designed to test whether transport alterations could be produced in the simplest model of diabetes, sustained exposure of cultured cells to a high-glucose environment. The regulatory transport responses to acute changes in cell volume were measured in order to assess the effects of glucose on a range of transport processes. Continuous lines of nontransformed human retinal pigment epithelial (hRPE) cells were grown for two weeks with either 5.6 low glucose (LG) or 26.0 high glucose (HG) mM in paired experiments. The cell volumes of suspended cells were studied in hypo-, iso- and hypertonic solutions containing the same ionic composition. Hypotonic swelling triggered a regulatory volume decrease (RVD), inhibited by reducing the chemical driving force for K+ efflux, or blocking K+ channels (with Ba2+) or Cl- channels (with NPPB). Thus, the RVD of the hRPE cells likely reflects efflux of K+ and Cl- through parallel channels. Shrinkage caused a regulatory volume increase (RVI), which was inhibited by blocking Na+/H+ (with dimethylamiloride) or Cl-/HCO3- exchange (with DIDS). Bumetanide inhibited the RVI significantly only when the K+ concentration was increased above the baseline level. Therefore, the RVI under our baseline conditions likely reflects primarily Na+/H+ and Cl-/HCO3- antiport exchange. Growth in high-glucose medium had no substantial effect on the RVD, but reduced the rate constant of the RVI by approximately 50%. The RVI was unaffected by growth in high-mannitol medium. Stimulation of protein kinase C (PKC) with DiC8 increased the RVI of HG-cells, but not of LG-cells. The DiC8-induced stimulation was bumetanide insensitive and abolished by 1 mM amiloride. Other transport effects of PKC (on the RVD) were unaltered in the HG-cells. We conclude that sustained elevation of extracellular glucose, per se, can downregulate the Na+/H+ antiport of target cells, an effect noted in streptozotocin-treated rats, and that this downregulation does not reflect interruption of the PKC-signaling pathway.

Extrarenal tissue distribution of CHIP28 water channels by in situ hybridization and antibody staining

This study is an extension of in situ hybridization experiments showing expression of mRNA encoding CHIP28 in selected epithelial or endothelia in spleen, colon, lung, and eye (H. Hasegawa, R. Zhang, A. Dohrman, and A. S. Verkman. Am. J. Physiol. 264 (Cell Physiol. 33): C237-C245, 1993). Additional tissues from rat were screened by in situ hybridization, and tissues from rat and humans were stained with a polyclonal anti-CHIP28 antibody. Northern blot showed the 2.8-kilobase mRNA encoding CHIP28 in kidney, lung, and heart. In situ hybridization showed strong hybridization in epithelial cells in choroid plexus, iris, ciliary body, and lens and in epithelial and subepithelial layers of trachea. Except for colonic crypts, specific hybridization was not observed in the gastrointestinal tract, liver, thyroid gland, and muscle. Immunoblot of tissues from exsanguinated rats showed immunoreactive CHIP28 protein in kidney, lung, trachea, and heart. In fixed frozen rat and/or human tissues, the anti-CHIP28 antibody stained epithelial cells in kidney proximal tubule and thin limb of Henle, lung alveolus, bronchial mucosa and glands, choroid plexus, ciliary body, iris, lens surface, colonic crypt, sweat gland, pancreatic acini, gallbladder epithelium, and placental syncytial trophoblast cells. Endothelial cells were stained in many tissues. These studies indicate a wide and selective CHIP28 tissue distribution, suggesting an important role for CHIP28 in fluid transport. The absence of CHIP28 in many nonrenal membranes believed to be water permeable suggests the existence of non-CHIP28 water transporters.

Volume regulation in cultured cells derived from human retinal pigment epithelium

To characterize volume regulatory mechanisms, unidirectional Rb+ efflux and influx, unidirectional Cl- influx, and cell volume were measured in cultured human retinal pigment epithelium (HRPE). The HRPE was found to be capable of both regulatory volume increase (RVI), in response to a hypertonic challenge, and regulatory volume decrease (RVD), in response to a hypotonic challenge. Bumetanide-sensitive Rb+ influx increased almost threefold on incubation in a hypertonic (390 mosmol/kgH2O) medium. Bumetanide-insensitive Rb+ influx was activated by hypotonic (190 mosmol/kgH2O) challenge as well as by treatment with N-ethylmaleimide (NEM). Exposure to hypotonic media also activated unidirectional Cl- influx and unidirectional Rb+ efflux. Both the RVD and hypotonically activated Rb+ efflux were inhibited by the K(+)-channel blocker barium. On the other hand, hypotonically activated Rb+ influx was increased by barium treatment. In sum, the HRPE exhibits volume-sensitive transport mechanisms over a range of volumes from 190 to 390 mosmol/kgH2O. Cultured HRPE possess hypertonically activated Na-K-Cl cotransport, hypotonically activated K-Cl cotransport, and a barium-inhibitable hypotonically activated K+ efflux pathway.

Characterization of stretch-activated ion channels in cultured astrocytes

The presence of a stretch-activated channel in rat cerebellar astrocytes in culture is described. This stretch-sensitive channel is K(+)-selective and its open probability increases with suction following a Boltzmann-like distribution with half activation at 45 mm Hg. Kinetic analysis of the single-channel data indicated that there are two open and two closed states and that the shortest time constants of both open and closed states are the most sensitive to suction. A symmetrical two-barrier-one-site permeation model can quantitatively describe the inward rectification of the single-channel current to voltage relations. It is suggested that this stretch-activated channel plays a role in the regulatory volume response of astrocytes to hyposmotic conditions.

Adrenergic stimulation of bovine non-pigmented ciliary epithelial cells raises cAMP but has no effect on K+ or Cl- currents

Freshly isolated bovine nonpigmented ciliary epithelial cells were studied using the whole-cell voltage-clamp and permeabilized patch techniques. In a study of 148 cells three classes of cell were found; those containing inward currents alone, 31%; those containing outward currents alone, 37% and those containing both inward and outward currents, 32%. The outward currents exhibited slow, delayed activation, were blocked by tetraethylammonium (TEA+) but were not effected by changes in [Cai] suggesting they are K(+)-currents similar to IK(V), the delayed rectifier. The inward currents were reduced by TEA+ and blocked by Cs+ suggesting they are inward rectifier K(+)-currents. Intracellular cAMP levels were increased by isoprenaline with a Ka of 20 nM. However, the resting membrane potential recorded from whole ciliary processes in intracellular microelectrode studies was not effected by adrenergic stimulation, neither were the leak current, the outward current nor the sustained inward current significantly effected by isoprenaline, noradrenaline or dibutyryl-cAMP in whole-cell and permeabilized patch clamp studies.

Whole cell patch clamping of ciliary epithelial cells during anisosmotic swelling

Anisosmotic cell swelling triggers a regulatory volume decrease (RVD) in cell lines derived from human nonpigmented ciliary epithelium. Measurements of cell volume have indicated that the RVD reflects activation of K+ and/or Cl- channels. We have begun to characterize the putative channels by whole cell patch clamping. The results obtained by altering the external K+ and Cl- concentrations and by adding 20-50 microM quinidine or 1 mM Ba2+ indicate that K+ conductances contribute substantially and Cl- conductances contribute very little to the total membrane conductance (GT) under baseline isotonic conditions. Reducing the external osmolality by 20-50% reversibly and reproducibly increased GT by an order of magnitude. Data obtained from ion substitutions and the channel blockers quinidine and 5-nitro-2-(3-phenylpropylamino)-benzoate indicate that most of the hypotonicity-induced conductance reflects stationary Cl(-)-channel activity. The contribution of new K(+)-channel activity was small at intracellular free Ca2+ concentrations of 10 or 200 nM. We conclude that the RVD triggered by bath hypotonicity primarily reflects increased Cl(-)-channel activity.

Regulatory volume decrease by cultured non-pigmented ciliary epithelial cells

Cells (ODM C1-2/SV40) derived from human non-pigmented ciliary epithelial cells were studied by electronic cell sizing. The time course of the cell volume (vc) was monitored after suspending cells in paired experimental and control, isosmotic and hyposmotic solutions of identical ionic composition. Following anisosmotic cell swelling, the cells displayed the regulatory volume decrease (RVD) previously described. The RVD primarily reflects loss of cell KCl since: (1) the K(+)-channel blockers quinidine and Ba2+ both inhibit the RVD; and (2) replacement of external Cl- with gluconate or addition of the Cl- channel blocker NPPB also inhibits the RVD. Bicarbonate has previously been reported to speed the RVD. This action likely reflects pH dependence of the channels since: (1) increasing the external pH speeds the RVD, whether or not HCO3- is present; and (2) DIDS (a blocker of Cl- channels and of Cl-/HCO3- exchange) is an effective inhibitor of the RVD, even after blocking Cl-/HCO3- exchange by removing external HCO3-. The RVD could also be inhibited by reducing the availability of Ca2+, either by omitting Ca2+ from the external medium or by blocking mobilization of intracellular Ca2+ with TMB-8. Furthermore, the RVD was slowed and incomplete in the presence of the calcium/calmodulin blocker trifluoperazine. We conclude that anisosmotic swelling triggers a series of events, mediated at least in part by calcium/calmodulin, leading to the extrusion of KCl through parallel K+ and Cl- channels.

Detection and characterization of Ca(2+)-activated K+ channels in transformed cells of human non-pigmented ciliary epithelium

Cell-attached and excised inside-out membrane patches were used to study single channel currents in a cell line derived from human non-pigmented ciliary epithelium. Most of the patches contained a Ca(2+)-dependent K+ channel with large unitary conductance (200 pS in symmetrical K+ solutions). Single channel current in cell-attached patches exposed to high K+ solution in the pipette showed a null potential of -36 mV. This value, which should yield an approximate estimation of cell membrane potential, was reversibly increased by -30 to -40 mV in the presence of Ca2+ ionophores. Tetraethylammonium up to 10 mM applied at the membrane cytoplasmic face had no effect on the channel. Addition of 1 mM BaCl2 to excised patches caused a voltage-dependent blockade of the channel. In the presence of barium the unit currents were not altered, but the channel remained closed for long periods of time and the open state probability decreased with depolarization. The possibility that this channel participates in regulation of transepithelial ciliary body secretion is discussed.