Mechanism of chloride uptake in rabbit corneal epithelium

Ocular Surface Potential Difference Measured in Human Subjects to Study Ocular Surface Ion Transport

Purpose

The epithelium lining the ocular surface, which includes corneal and conjunctival epithelia, expresses the prosecretory chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) and the proabsorptive epithelial sodium channel (ENaC). Here, methodology was established to measure the millivolt (mV) potential differences at the ocular surface, called ocular surface potential difference (OSPD), in human subjects produced by ion transport.

Methods

OSPD was measured in human subjects in which a fluid-filled measuring electrode contacted a fluid pool created by eversion of the lateral lower eyelid, with a reference electrode placed subcutaneously in the forearm. Through the use of a high-impedance voltmeter, OSPD was measured continuously over 10 to 15 minutes in response to a series of perfusate fluid exchanges.

Results

Baseline OSPD (± SEM) in six normal human subjects was -21.3 ± 3.6 mV. OSPD depolarized by 1.7 ± 0.6 mV following the addition of the ENaC inhibitor amiloride, hyperpolarized by 6.8 ± 1.5 mV with a zero chloride solution, and further hyperpolarized by 15.9 ± 1.6 mV following CFTR activation by isoproterenol. The isoproterenol-induced hyperpolarization was absent in two cystic fibrosis subjects lacking functional CFTR. OSPD measurement produced minimal epithelial injury.

Conclusions

Our results establish the feasibility and safety of OSPD measurement in humans and demonstrate robust CFTR activity, albeit minimal ENaC activity, at the ocular surface. OSPD measurement may be broadly applicable to investigate fluid transport mechanisms and test drug candidates to treat ocular surface disorders.

Translational Relevance

To the best of our knowledge, this is the first measurement of the electrical potential generated by the ocular surface epithelium in human subjects, offering a new approach to study ocular surface function and health.

Pro-Secretory Activity and Pharmacology in Rabbits of an Aminophenyl-1,3,5-Triazine CFTR Activator for Dry Eye Disorders

Purpose

Pharmacological activation of ocular surface cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels is a potential pro-secretory approach to treat dry eye disorders. We previously reported the discovery of aminophenyl-1,3,5-triazines, one of which, N-methyl-N-phenyl-6-(2,2,3,3-tetrafluoropropoxy)-1,3,5-triazine-2,4-diamine (herein called CFTRact-K267), fully activated human wildtype CFTR with EC50 ∼ 30 nM and increased tear volume for 8 hours in mice. Here, functional and pharmacological studies of CFTRact-K267 were done in adult New Zealand white rabbits.

Methods

CFTR chloride conductance was measured in vivo by ocular surface potential differences and in ex vivo conjunctiva by short-circuit current. Tear volume was measured by the Schirmer tear test II and CFTRact-K267 pharmacokinetics and tissue distribution by liquid chromatography/mass spectrometry. Toxicity profile was studied for 28 days with twice-daily topical administration.

Results

Electrophysiological measurements in vivo and in ex vivo conjunctiva demonstrated CFTR activation by CFTRact-K267. A single topical dose of 3 nmol CFTRact-K267 increased tear production by >5 mm for 9 hours by the Schirmer tear test, with predicted therapeutic concentrations maintained in tear fluid. No tachyphylaxis was seen following 28-day twice-daily administration, and changes were not observed in corneal surface integrity or thickness, intraocular pressure, or ocular histology. At day 28, CFTRact-K267 was concentrated in the cornea and conjunctiva and was not detectable in blood or peripheral organs.

Conclusions

These studies support the development of CFTRact-K267 as a pro-secretory therapy for dry eye disorders.

Is the main lacrimal gland indispensable? Contributions of the corneal and conjunctival epithelia

The ocular surface system is responsible for ensuring that the precorneal tear film is sufficient in both quality and quantity to preserve optimal vision. Tear secretion is a complex, multifactorial process, and dysfunction of any component of the ocular surface system can result in tear film instability and hyperosmolarity with resultant dry eye disease. The tear film is primarily composed of lipids, aqueous, and mucins, with aqueous accounting for most of its thickness. The aqueous is produced by the main lacrimal gland, accessory lacrimal glands, and corneal and conjunctival epithelia. Although the main lacrimal gland has long been considered an indispensable source of the aqueous component of tears, there is evidence that adequate tear secretion can exist in the absence of the main lacrimal gland. We review and discuss the basics of tear secretion, the tear secretory capacity of the ocular surface, and emerging treatments for dry eye disease.

1,25(OH)2 vitamin d inhibits foam cell formation and suppresses macrophage cholesterol uptake in patients with type 2 diabetes mellitus

BACKGROUND

Cardiovascular disease is the leading cause of death among those with diabetes mellitus. Vitamin D deficiency is associated with an increased risk of cardiovascular disease in this population. To determine the mechanism by which vitamin D deficiency mediates accelerated cardiovascular disease in patients with diabetes mellitus, we investigated the effects of active vitamin D on macrophage cholesterol deposition.

METHODS AND RESULTS

We obtained macrophages from 76 obese, diabetic, hypertensive patients with vitamin D deficiency (25-hydroxyvitamin D <80 nmol/L; group A) and 4 control groups: obese, diabetic, hypertensive patients with normal vitamin D (group B; n=15); obese, nondiabetic, hypertensive patients with vitamin D deficiency (group C; n=25); and nonobese, nondiabetic, nonhypertensive patients with vitamin D deficiency (group D; n=10) or sufficiency (group E; n=10). Macrophages from the same patients in all groups were cultured in vitamin D-deficient or 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] -supplemented media and exposed to modified low-density lipoprotein cholesterol. 1,25(OH)(2)D(3) suppressed foam cell formation by reducing acetylated or oxidized low-density lipoprotein cholesterol uptake in diabetic subjects only. Conversely, deletion of the vitamin D receptor in macrophages from diabetic patients accelerated foam cell formation induced by modified LDL. 1,25(OH)(2)D(3) downregulation of c-Jun N-terminal kinase activation reduced peroxisome proliferated-activated receptor-gamma expression, suppressed CD36 expression, and prevented oxidized low-density lipoprotein-derived cholesterol uptake. In addition, 1,25(OH)(2)D(3) suppression of macrophage endoplasmic reticulum stress improved insulin signaling, downregulated SR-A1 expression, and prevented oxidized and acetylated low-density lipoprotein-derived cholesterol uptake.

CONCLUSIONS

These results identify reduced vitamin D receptor signaling as a potential mechanism underlying increased foam cell formation and accelerated cardiovascular disease in diabetic subjects.

Characterization of regulatory volume behavior by fluorescence quenching in human corneal epithelial cells

An in-depth understanding of the mechanisms underlying regulatory volume behavior in corneal epithelial cells has been in part hampered by the lack of adequate methodology for characterizing this phenomenon. Accordingly, we developed a novel approach to characterize time-dependent changes in relative cell volume induced by anisosmotic challenges in calcein-loaded SV40-immortalized human corneal epithelial (HCE) cells with a fluorescence microplate analyzer. During a hypertonic challenge, cells shrank rapidly, followed by a temperature-dependent regulatory volume increase (RVI), tau(c) = 19 min. In contrast, a hypotonic challenge induced a rapid (tau(c) = 2.5 min) regulatory volume decrease (RVD). Temperature decline from 37 to 24 degrees C reduced RVI by 59%, but did not affect RVD. Bumetanide (50 microM), ouabain (1 mM), DIDS (1 mM), EIPA (100 microM), or Na(+)-free solution reduced the RVI by 60, 61, 39, 32, and 69%, respectively. K+, Cl- channel and K(+)-Cl(-) cotransporter (KCC) inhibition obtained with either 4-AP (1 mM), DIDS (1 mM), DIOA (100 microM), high K+ (20 mM) or Cl(-)-free solution, suppressed RVD by 42, 47, 34, 52 and 58%, respectively. KCC activity also affects steady-state cell volume, since its inhibition or stimulation induced relative volume alterations under isotonic conditions. Taken together, K+ and Cl- channels in parallel with KCC activity are important mediators of RVD, whereas RVI is temperature-dependent and is essentially mediated by the Na(+)-K(+)-2Cl(-) cotransporter (Na(+)-K(+)-2Cl(-)) and the Na(+)-K(+) pump. Inhibition of K+ and Cl- channels and KCC but not Na(+)-K(+)-2Cl(-) affect steady-state cell volume under isotonic conditions. This is the first report that KCC activity is required for HCE cell volume regulation and maintenance of steady-state cell volume.

Functional and molecular characterization of multiple K-Cl cotransporter isoforms in corneal epithelial cells

The dependence of regulatory volume decrease (RVD) activity on potassium-chloride cotransporter (KCC) isoform expression was characterized in corneal epithelial cells (CEC). During exposure to a 50% hypotonic challenge, the RVD response was larger in SV40-immortalized human CEC (HCEC) than in SV40-immortalized rabbit CEC (RCEC). A KCC inhibitor-[(dihydroindenyl)oxy] alkanoic acid (DIOA)-blocked RVD more in HCEC than RCEC. Under isotonic conditions, N-ethylmaleimide (NEM) produced KCC activation and transient cell shrinkage. Both of these changes were greater in HCEC than in RCEC. Immunoblot analysis of HCEC, RCEC, primary human CEC (pHCEC), and primary bovine CEC (BCEC) plasma membrane enriched fractions revealed KCC1, KCC3, and KCC4 isoform expression, whereas KCC2 was undetectable. During a hypotonic challenge, KCC1 membrane content increased more rapidly in HCEC than in RCEC. Such a challenge induced a larger increase and more transient p44/42MAPK activation in HCEC than RCEC. On the other hand, HCEC and RCEC p38MAPK phosphorylation reached peak activations at 2.5 and 15 min, respectively. Only in HCEC, pharmacological manipulation of KCC activity modified the hypotonicity-induced activation of p44/42MAPK, whereas p38MAPK phosphorylation was insensitive to such procedures in both cell lines. Larger increases in HCEC KCC1 membrane protein content correlated with their ability to undergo faster and more complete RVD. Furthermore, pharmacological activation of KCC increased p44/42MAPK phosphorylation in HCEC but not in RCEC, presumably a reflection of low KCC1 membrane expression in RCEC. These findings suggest that KCC1 plays a role in (i) maintaining isotonic steady-state cell volume homeostasis, (ii) recovery of isotonic cell volume after a hypotonic challenge through RVD, and (iii) regulating hypotonicity-induced activation of the p44/42MAPK signaling pathway required for cell proliferation.

Concentration and pH dependent aggregation of hydrophobic drug molecules and relevance to oral bioavailability

We have examined selected physicochemical properties of compounds from the diaryltriazine/diarylpyrimidine (DATA/DAPY) classes of non-nucleoside reverse transcriptase inhibitors (NNRTIs) and explored possible correlations with their bioavailability. In simple aqueous solutions designed to mimic the gastrointestinal (GI) environment of a fasting individual, all NNRTIs demonstrated formation of aggregates as detected by dynamic light scattering and electron microscopy. Under various conditions mimicking physiological transitions in the GI environment, aggregate size distributions were shown to depend on compound concentration and pH. NNRTIs with good absorption were capable of forming aggregates with hydrodynamic radii of </=100 nm at higher concentrations and over wide ranges of pH, while poorly absorbed inhibitors form aggregates with radii of >/=250 nm at concentrations above 0.01 mM, probably representing precipitate. We propose a model in which the uptake rate into systemic circulation depends on having hydrophobic drug aggregates of appropriate size available for absorption at different locations within the GI tract.

Characterization of active ion transport across primary rabbit corneal epithelial cell layers (RCrECL) cultured at an air-interface

Previously, we reported the development of a primary culture model of tight rabbit corneal epithelial cell layers (RCrECL) characterizing bioelectric parameters, morphology, cytokeratin, and passive permeability. In the present study, we specifically evaluated the active ion transport processes of RCrECL cultured from either pigmented or albino rabbits. Primary cultured RCrECL were grown at an air-interface on Clear-Snapwells precoated with collagen/fibronectin/laminin and mounted in a modified Ussing-type chamber for the evaluation of their active ion transport processes under short-circuited conditions. Contribution of active Na(+) and Cl(-) transport to overall short-circuit current (I(sc)) was evaluated by removing Na(+) and Cl(-), respectively, from bathing fluids of RCrECL and measurements of net fluxes of Na(+) and Cl(-) using (22)Na and (36)Cl, respectively. Amiloride and benzamil were used to determine the role of apical Na(+)-channel activities to net Na(+) fluxes. N-phenylanthranilic acid (NPAA), ouabain, BaCl(2) and bumetanide were used to determine the role of basolateral Na,K-ATPase, apical Cl(-)-channel, and basolateral K(+)-channel and Na(+)(K(+))2Cl(-)-cotransporter activities, respectively, in active ion transport across RCrECL. I(sc) of RCrECL derived from pigmented rabbits was comprised of 64+/-2% and 44+/-5% for active Na(+) and Cl(-) transport, respectively, consistent with net Na(+) absorption and Cl(-) secretion of 0.062+/-0.006 and 0.046+/-0.008 muEq/cm(2)/hr estimated from radionuclide fluxes. Apical amiloride and benzamil inhibited I(sc) by up to approximately 50% with an IC(50) of 1 and 0.1 microm, respectively, consistent with participation of apical epithelial Na(+)-channels to net Na(+) absorption across RCrECL cultured from pigmented rabbits. Addition of ouabain to the basolateral, NPAA to the apical, BaCl(2) to the basolateral and bumetanide to basolateral fluid decreased I(sc) by 86+/-1.5%, 53+/-3%, 18+/-1.8% and 13+/-1.9% in RCrECL cultured from pigmented rabbits, while 85+/-0.7%, 36+/-1.6%, 38+/-1.8% and 15+/-3.5% decreases are observed for RCrECL from albino rabbits, respectively. Air-interface cultured RCrECL from either pigmented or albino rabbits exhibited active ion transport properties similar to those present in excised tissues. This primary culture system may be a reliable in-vitro model for mechanistic characterization of corneal epithelial function and regulation of transport properties.

Electrolyte and fluid transport across corneal, conjunctival and lens epithelia

All ocular epithelia examined to date transport fluid as a consequence of a sufficiently high water permeability bestowed by endogenous water channels (aquaporins) and transepithelial solute movement due to active transport mechanisms. This article provides a synopsis of the current understanding of electrolyte and fluid transport across corneal, conjunctival and lens epithelia.

Fluid transport by cultured corneal epithelial cell layers

BACKGROUND/AIMS

Fluid transport across the in vitro corneal epithelium is short lived, hence difficult to detect and characterise. Since stable rates of fluid transport across several cultured epithelial cell layers have been demonstrated, the behaviour of confluent SV40 transformed rabbit corneal epithelial cells (tRCEC) grown on permeable supports was examined.

METHODS

Fluid transport was determined with a nanoinjector volume clamp; the specific electrical resistance of the layers was 184 (SEM 9) Omega cm(2). tRCEC layers transported fluid (from basal to apical) against a pressure head of 3 cm H(2)O for 2-3 hours.

RESULTS

In the first hour, the rate of fluid transport was 5.2 (0.5) microl/h/cm(-2) (n=23), which is comparable with that found in other epithelia. Fluid transport was completely inhibited in 15-30 minutes by either 100 microM ouabain (n=6), 50 microM bumetanide (n=6), or 1 microM endothelin-1 (ET-1; n=6). Preincubation with 10 microM BQ123 (an ET(A) receptor antagonist) obviated inhibition by ET-1 (n=6). ET-1 also caused a 22% decrease in specific resistance.

CONCLUSIONS

Fluid transport appears to depend on transepithelial Cl(- )transport since (1) their directions are the same (stroma-->tear), and (2) both bumetanide and ouabain inhibit it with similar time course. tRCEC appear useful to investigate aspects of the physiology and pharmacology of fluid transport across this layer, including receptor mediated control of this process.

Mycophenolic acid increases apoptosis, lysosomes and lipid droplets in human lymphoid and monocytic cell lines

BACKGROUND

Mycophenolic acid (MPA), a selective inhibitor of inosine monophosphate dehydrogenase, is the active agent of the immunosuppressive drug, mycophenolate mofetil (MMF). Previous studies have shown that MPA inhibits DNA synthesis in T and B lymphocytes by blocking de novo guanosine synthesis, and that MPA induces monocyte differentiation. MMF is being used for prevention of organ graft rejection and has also shown efficacy in rheumatoid arthritis trials. This study was designed to determine if apoptosis also plays a role in the immunosuppressive and anti-inflammatory effects of MMF.

METHODS

Cultured human T lymphocytic (MOLT-4) and monocytic (THP-1 and U937) cell lines were treated with MPA. Apoptosis, cell viability, DNA content, lipid content, cell volume, and lysosomes were measured by a variety of microscopic, flow cytometric, and biochemical techniques.

RESULTS

MPA inhibits proliferation, arrests cell cycle in S phase, and increases apoptosis in all three cell lines. Exogenous guanosine added within 24 hr of MPA treatment, but not later, partially reversed MPA-induced apoptosis in MOLT-4 cells. MPA increased lipid droplets in all three cell lines and increased both cell volumes and numbers of lysosomes in the monocytic cell lines. In both monocytic cell lines, MPA also reduced the number of nuclei containing nucleoli and greatly increased neutral lipids, primarily triacylglycerols, suggesting that these cells were differentiating.

CONCLUSIONS

Increased apoptosis and terminal differentiation of both lymphocytes and monocytes may promote the antiproliferative, immunosuppressive, and anti-inflammatory effects of MMF seen clinically in transplantation and rheumatoid arthritis.

K+ and Na+ absorption by outer sulcus epithelial cells

Transduction of sound into nerve impulses by hair cells depends on modulation of a current carried primarily by K+ into the cell across apical transduction channels that are permeable to cations. The cochlear function thus depends on active secretion of K+ accompanied by absorption of Na+ by epithelial cells enclosing the cochlear duct. The para-sensory cells which participate in the absorption of Na+ (down to the uniquely low level of 1 mM) were previously unidentified and the existence of a para-sensory pathway which actively absorbs K+ was previously unknown. A relative short circuit current (Isc,probe, measured as the extracellular current density with a vibrating electrode) was directed into the apical side of the outer sulcus epithelium, decreased by ouabain (1 mM), an inhibitor of Na+, K(+)-ATPase, and found to depend on bath Na+ and K+ but on neither Ca2+ nor Cl-. Isc,probe was shown to be an active current by its sensitivity to ouabain. On-cell patch clamp recordings of the apical membrane of outer sulcus cells displayed a channel activity, which carried inward currents under conditions identical to those used to measure Isc,probe. Both Isc,probe and non-selective cation channels (27.4+/-0.6 ps, n = 22) in excised outside-out patches from the apical membrane were inhibited by Gd3+ (1 mM). Ics,prob was also inhibited by 5 mM lidocaine, 1 mM quinine and 500 microM amiloride but not by 10 microM amiloride. These results demonstrate that outer sulcus epithelial cells contribute to the homeostasis of endolymph by actively absorbing Na+ and K+. An entry pathway in the apical membrane was shown to be through non-selective cation channels that were sensitive to Gd3+.

Active chloride transport in the pigmented rabbit conjunctiva

The present study demonstrates, for the first time, that the excised pigmented rabbit conjunctiva is a tight barrier capable of active Cl- transport. The transepithelial potential difference was 17.7 +/- 0.8 mV (tear-side negative), the short-circuit current was 14.5 +/- 0.7 microA/cm2, and the transconjunctival resistance was 1.3 +/- 0.1 k omega.cm2 for n = 45 tissues. Various inhibitors including ouabain (a Na+/K(+)-ATPase inhibitor), amiloride (a Na+ transport blocker), N-phenylanthranilic acid (a chloride transport inhibitor), bumetanide (an inhibitor of Na(+)-(K+)-Cl- cotransport process), and BaCl2 (a K+ channel blocker) were used on the mucosal and serosal sides of the tissue mounted in Ussing chambers to determine the involvement of the respective ion transport processes in the observed short-circuit current across the conjunctiva. The results suggest that a Cl- conductive pathway is present on the mucosal side of the conjunctiva, whereas Na+/K(+)-ATPase, Na(+)-(K+)-Cl- cotransport process, and K+ conductive pathways are present on its serosal side. Amiloride-sensitive Na(+)-conductive pathways do not appear to be present on either side of the pigmented rabbit conjunctiva.

Ionic basis for color changes in the iridescent cornea of the sand goby (Pomatoschistus minutus)

The iridescence from the cornea of the sand goby (Pomatoschistus minutus) occurs because of thin layer interference from the platelet-like cells in the stroma. It is suggested that ionic pumps across the epithelium control the water content in the stroma and thus the spectral reflection. A saline was perfused over goby eyes and simple ion manipulation was carried out to observe any changes in the iridescent characteristics. It was found that removal of Cl- and K+ ions reduced the peak reflected wavelength to the blue end of the spectrum, whereas Na+ had little effect. The removal of K+ also caused a dramatic change to the normal shift in reflected spectral intensity. The iridescence was also found to be sensitive to pH, and the buffer HEPES was detrimental to the cornea compared to controls. These results suggest similarities to amphibian and mammalian corneal hydration control.