Intracellular pH regulation in the embryonic chicken lens epithelium

Effect of hydrogen peroxide on lens transparency, intracellular pH, gap junction coupling, hydrostatic pressure and membrane water permeability

Clouding of the eye lens or cataract is an age-related anomaly that affects middle-aged humans. Exploration of the etiology points to a great extent to oxidative stress due to different forms of reactive oxygen species/metabolites such as Hydrogen peroxide (H2O2) that are generated due to intracellular metabolism and environmental factors like radiation. If accumulated and left unchecked, the imbalance between the production and degradation of H2O2 in the lens could lead to cataracts. Our objective was to explore ex vivo the effects of H2O2 on lens physiology. We investigated transparency, intracellular pH (pHi), intercellular gap junction coupling (GJC), hydrostatic pressure (HP) and membrane water permeability after subjecting two-month-old C57 wild-type (WT) mouse lenses for 3 h or 8 h in lens saline containing 50 μM H2O2; the results were compared with control lenses incubated in the saline without H2O2. There was a significant decrease in lens transparency in H2O2-treated lenses. In control lenses, pHi decreases from ∼7.34 in the surface fiber cells to 6.64 in the center. Experimental lenses exposed to H2O2 for 8 h showed a significant decrease in surface pH (from 7.34 to 6.86) and central pH (from 6.64 to 6.56), compared to the controls. There was a significant increase in GJC resistance in the differentiating (12-fold) and mature (1.4-fold) fiber cells compared to the control. Experimental lenses also showed a significant increase in HP which was ∼2-fold higher at the junction between the differentiating and mature fiber cells and ∼1.5-fold higher at the center compared to these locations in control lenses; HP at the surface was 0 mm Hg in either type lens. Fiber cell membrane water permeability significantly increased in H2O2-exposed lenses compared to controls. Our data demonstrate that elevated levels of lens intracellular H2O2 caused a decrease in intracellular pH and led to acidosis which most likely uncoupled GJs, and increased AQP0-dependent membrane water permeability causing a consequent rise in HP. We infer that an abnormal increase in intracellular H2O2 could induce acidosis, cause oxidative stress, alter lens microcirculation, and lead to the development of accelerated lens opacity and age-related cataracts.

The Appearance of the Warburg Effect in the Developing Avian Eye Characterized In Ovo: How Neurogenesis Can Remodel Neuroenergetics

Purpose

The avian eye is an established model for exploring mechanisms that coordinate morphogenesis and metabolism during embryonic development. Less is known, however, about trafficking of bioenergetic and metabolic signaling molecules that are involved in retinal neurogenesis.

Methods

Here we tested whether the known 3-day delayed neurogenesis occurring in the pigeon compared with the chick was associated with a deferred reshaping of eye metabolism in vivo. Developmental metabolic remodeling was explored using 1H-magnetic resonance spectroscopy of the whole eye and vitreous body, in ovo, in parallel with biochemical and molecular analyses of retinal, vitreous, and lens extracts from bird embryos.

Results

Cross-species comparisons enabled us to show that a major glycolytic switch in the retina is related to neurogenesis rather than to eye growth. We further show that the temporal emergence of an interlocking regulatory cascade controlling retinal oxidative phosphorylation and glycolysis results in the exchange of lactate and citrate between the retina and vitreous.

Conclusions

Our results point to the vitreous as a reservoir and buffer of energy metabolites that provides trophic support to oxidative neurons, such as retinal ganglion cells, in early development. Through its control of key glycolytic regulatory enzymes, citrate, exchanged between extracellular and intracellular compartments between the retina and vitreous, is a key metabolite in the initiation of a glycolytic switch.

UVA light-excited kynurenines oxidize ascorbate and modify lens proteins through the formation of advanced glycation end products: implications for human lens aging and cataract formation

Advanced glycation end products (AGEs) contribute to lens protein pigmentation and cross-linking during aging and cataract formation. In vitro experiments have shown that ascorbate (ASC) oxidation products can form AGEs in proteins. However, the mechanisms of ASC oxidation and AGE formation in the human lens are poorly understood. Kynurenines are tryptophan oxidation products produced from the indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway and are present in the human lens. This study investigated the ability of UVA light-excited kynurenines to photooxidize ASC and to form AGEs in lens proteins. UVA light-excited kynurenines in both free and protein-bound forms rapidly oxidized ASC, and such oxidation occurred even in the absence of oxygen. High levels of GSH inhibited but did not completely block ASC oxidation. Upon UVA irradiation, pigmented proteins from human cataractous lenses also oxidized ASC. When exposed to UVA light (320-400 nm, 100 milliwatts/cm(2), 45 min to 2 h), young human lenses (20-36 years), which contain high levels of free kynurenines, lost a significant portion of their ASC content and accumulated AGEs. A similar formation of AGEs was observed in UVA-irradiated lenses from human IDO/human sodium-dependent vitamin C transporter-2 mice, which contain high levels of kynurenines and ASC. Our data suggest that kynurenine-mediated ASC oxidation followed by AGE formation may be an important mechanism for lens aging and the development of senile cataracts in humans.

Lens gap junctions in growth, differentiation, and homeostasis

The cells of most mammalian organs are connected by groups of cell-to-cell channels called gap junctions. Gap junction channels are made from the connexin (Cx) family of proteins. There are at least 20 isoforms of connexins, and most tissues express more than 1 isoform. The lens is no exception, as it expresses three isoforms: Cx43, Cx46, and Cx50. A common role for all gap junctions, regardless of their Cx composition, is to provide a conduit for ion flow between cells, thus creating a syncytial tissue with regard to intracellular voltage and ion concentrations. Given this rather simple role of gap junctions, a persistent question has been: Why are there so many Cx isoforms and why do tissues express more than one isoform? Recent studies of lens Cx knockout (KO) and knock in (KI) lenses have begun to answer these questions. To understand these roles, one must first understand the physiological requirements of the lens. We therefore first review the development and structure of the lens, its numerous transport systems, how these systems are integrated to generate the lens circulation, the roles of the circulation in lens homeostasis, and finally the roles of lens connexins in growth, development, and the lens circulation.

Regulation of tissue oxygen levels in the mammalian lens

Opacification of the lens nucleus is a major cause of blindness and is thought to result from oxidation of key cellular components. Thus, long-term preservation of lens clarity may depend on the maintenance of hypoxia in the lens nucleus. We mapped the distribution of dissolved oxygen within isolated bovine lenses and also measured the rate of oxygen consumption (QO2) by lenses, or parts thereof. To assess the contribution of mitochondrial metabolism to the lens oxygen budget, we tested the effect of mitochondrial inhibitors on (QO2) and partial pressure of oxygen (PO2). The distribution of mitochondria was mapped in living lenses by 2-photon microscopy. We found that a steep gradient of PO2 was maintained within the tissue, leading to PO2 < 2 mmHg in the core. Mitochondrial respiration accounted for approximately 90% of the oxygen consumed by the lens; however, PO2 gradients extended beyond the boundaries of the mitochondria-containing cell layer, indicating the presence of non-mitochondrial oxygen consumers. Time constants for oxygen consumption in various regions of the lens and an effective oxygen diffusion coefficient were calculated from a diffusion-consumption model. Typical values were 3 x 10(-5) cm(2) s(-1) for the effective diffusion coefficient and a 5 min time constant for oxygen consumption. Surprisingly, the calculated time constants did not differ between differentiating fibres (DF) that contained mitochondria and mature fibres (MF) that did not. Based on these parameters, DF cells were responsible for approximately 88% of lens oxygen consumption. A modest reduction in tissue temperature resulted in a marked decrease in (QO2) and the subsequent flooding of the lens core with oxygen. This phenomenon may be of clinical relevance because cold, oxygen-rich solutions are often infused into the eye during intraocular surgery. Such procedures are associated with a strikingly high incidence of postsurgical nuclear cataract.

Herbimycin A inhibits both dephosphorylation and translocation of cofilin induced by opsonized zymosan in macrophagelike U937 cells

We previously reported that a 21-kDa phosphoprotein may play an important role in superoxide production through dephosphorylation by neutrophillike differentiated HL-60 cells (Suzuki et al., 1995, Biochim Biophys Acta 1266: 261-267). The phosphoprotein was identified as cofilin, an actin-binding protein, and the activation-induced changes in its intracellular distribution have been described elsewhere (Suzuki et al., 1995, J Biol Chem 270:19551-19556). However, the physiologic roles of cofilin in phagocytes remain to be established, and the regulatory mechanisms for dephosphorylation and translocation of cofilin are unknown. In the present study, we investigated the roles of cofilin in the opsonized zymosan (OZ)-activated macrophagelike U937 cells by using herbimycin A, an inhibitor for protein tyrosine kinase. In the individual adherent phagocytes, OZ induced many events: 1) production of superoxide, 2) phagocytosis of the insoluble particles OZ, 3) dephosphorylation of cofilin, 4) translocation of cofilin from cytosol to plasma membrane regions, 5) decrease in intracellular pH from 7.4 to aprroximately 6.8, and 6) rapid and transient increase in filamentous actin at the cell periphery. All of these events were inhibited or reduced significantly by herbimycin A. OZ increased phosphorylation of tyrosine in 110-, 50-, 34-, and 29-kDa proteins, whereas herbimycin A inhibited it. These results suggest that tyrosine kinase plays an essential role upstream of these events through phosphorylation of such proteins. Furthermore, microinjection of anti-cofilin antibody to the differentiated U937 cells caused inhibition of the phagocytosis. These results suggest that cofilin plays critical roles in phagocytic functions through changes in cytoskeletal organization.

Influence of bafilomycin A1 on pHi responses in cultured rabbit nonpigmented ciliary epithelium

Aqueous humor secretion is in part linked to HCO3- transport by nonpigmented ciliary epithelium (NPE) cells. During this process, the cells must maintain stable cytoplasmic pH (pHi). Because a recent report suggests that NPE cells have a plasma membrane-localized vacuolar H(+)-ATPase, the present study was conducted to examine whether vacuolar H(+)-ATPase contributes to pHi regulation in a rabbit NPE cell line. Western blot confirmed vacuolar H(+)-ATPase expression as judged by H(+)-ATPase 31-kDa immunoreactive polypeptide in both cultured NPE and native ciliary epithelium. pHi was measured using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Exposing cultured NPE to K(+)-rich solution caused a pHi increase we interpret as depolarization-induced alkalinization. Alkalinization was also caused by ouabain or BaCl2. Bafilomycin A1 (0.1 microM; an inhibitor of vacuolar H(+)-ATPase) inhibited the pHi increase caused by high K+. The pHi increase was also inhibited by angiotensin II and the metabolic uncoupler carbonyl cyanide m-chlorophenylhydazone but not by ZnCl2, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), omeprazole, low-Cl- medium, HCO3(-)-free medium, or Na(+)-free medium. Bafilomycin A1 slowed the pHi increase after an NH4Cl (10 mM) prepulse. However, no detectable pHi change was observed in cells exposed to bafilomycin A1 under control conditions. These studies suggest that vacuolar H(+)-ATPase is activated by cytoplasmic acidification and by reduction of the proton electrochemical gradient across the plasma membrane. We speculate that the mechanism might contribute to maintenance of acid-base balance in NPE.

Monocarboxylic acids enhance the anesthetic action of procaine by decreasing intracellular pH

Sodium monocarboxylates are known to enhance the anesthetic action of procaine, and also decrease intracellular pH (pHi). We studied the effect of 30 mM Na monocarboxylates (formate, acetate, propionate, butyrate, and salicylate) on the pHi and on the anesthetic action of procaine HCl using giant axons of crayfish (Procambarus clarkii). The pHi was measured using pH sensitive microelectrode method and the anesthetic action was evaluated by the change in the action potential (AP) amplitude. The tested acids except for formate showed apparent decrease in pHi and enhancement of the action of 2 mM procaine. Other organic acids (maleate and benzensulfonate) did not affect pHi and anesthetic action of procaine. In the bicarbonate free solution, pHi increased and the anesthetic action was weakened. The EC25 values (the concentration of procaine which depresses the AP amplitude by 25%) of acetate, propionate, and bicarbonate free solution were coincided with the predicted EC25 values from the simple simulation on intracellular procaine increase according to the pHi change. But the EC25 value of salicylate group was less than half of the predicted. These results suggested that the enhancing action of straight chain monocarboxylic acids is due to pHi decrease, and salicylate has other additional mechanisms.

Intercellular communication between epithelial and fiber cells of the eye lens

Results of electrical, dye-coupling and morphological studies have previously suggested that gap junctions mediate communication between the anterior epithelium of the lens and the underlying lens fiber cells. This connection is believed to permit ‘metabolic cooperation’ between these dissimilar cell types and may be of particular importance to the fiber cells, which are thought incapable of autonomous ionic homeostasis. We reinvestigated the nature of the connection between epithelial and fiber cells of the embryonic chicken lens using fluorescence confocal microscopy and freeze-fracture analysis. In contrast to earlier studies, our data provided no support for gap-junction-mediated transport from the lens epithelium to the fibers. Fluorescent dyes loaded biochemically into the lens epithelium were retained there for more than one hour. There was a decrease in epithelial fluorescence over this period, but this was not accompanied by an increase in fiber cell fluorescence. Diffusional modeling suggested that these data were inconsistent with the presence of extensive epithelium-fiber cell coupling, even if the observed decrease in epithelial fluorescence was attributed exclusively to the diffusion of dye into the fiber mass via gap junctions. Furthermore, the rate of loss of fluorescence from isolated epithelia was indistinguishable from that measured in whole lenses, suggesting that decreased epithelial fluorescence resulted from photobleaching and leakage of dye rather than diffusion, via gap junctions, into the fibers. Analysis of freeze-fracture replicas of plasma membranes at the epithelial-fiber cell interface failed to reveal evidence of gap-junction plaques, although evidence of endocytosis was abundant. These studies were done under conditions where the location of the fracture plane was unambiguous and where gap junctions could be observed in the lateral membranes of neighboring epithelial and fiber cells. Paradoxically, tracer molecules injected into the fiber mass were able to pass into the epithelium via a pathway that was not blocked by incubation at 4 degrees C or by treatment with octanol and which excluded large (approximately 10 kDa) molecular mass tracers. Together with previous measurements of electrical coupling between fiber cells and epithelial cells, these data indicate the presence of a low-resistance pathway connecting these cell types that is not mediated by classical gap junctions.

Evidence for parallel Na(+)-H+ and Na(+)-dependent Cl(-)-HCO3- exchangers in cultured bovine lens cells

BCECF, a cell-entrapable dye with a pH-sensitive fluorescence spectrum, was used to identify transport mechanisms contributing to pH homeostasis of cultured bovine lens epithelial cells. Cells from a spontaneously established lineage were grown on glass coverslips that fit diagonally in a standard curvette and intracellular pH (pHi) was measured. Under perfusion with a CO2-HCO3(-)-free medium (pH 7.45), pHi was 7.19 +/- 0.21 (mean +/- S.D., n = 94 cell preparations). Cell acidifications (pHi to 6.65, n = 8) induced by the 'NH(4+)-loading' method were rapidly followed by a Na(+)-dependent, amiloride-inhibitable pHi recovery. Introduction of a CO2-HCO3(-)-rich medium (pH 7.45) resulted in a small acidification (0.18 +/- 0.04 U, n = 16; P < 0.002) due to rapid CO2 entry and an ensuing slow alkalinization to a pHi near the control CO2-HCO3(-)-free value. Subsequent removal of Cl- resulted in a further alkalinization of 0.18 +/- 0.02 U (n = 13; P < 0.001). This Cl- effect was completely inhibited by the absence of Na+, but was insensitive to amiloride, suggesting the presence of a Na(+)-dependent Cl(-)-HCO3- exchanger. Consistent with this posit, the reintroduction of Na+ to cells perfused in the absence of the cation with a HCO3(-)-containing, amiloride-complemented solution resulted in a gradual recovery from the acidic pHi induced by the baseline conditions (n = 6). The amiloride-insensitive, Na(+)- and HCO3(-)-dependent recovery was completely inhibited in cells pre-incubated with DIDS.(ABSTRACT TRUNCATED AT 250 WORDS)

pH regulation in tissue-cultured bovine lens epithelial cells

The intracellular pH (pHi) of tissue-cultured bovine lens epithelial cells was measured in small groups of 6 to 10 cells using the trapped fluorescent dye 2',7'-bis-(2-,carboxyethyl)-5 (and 6)carboxyfluorescein (BCECF). When perifused at 35 degrees C with artificial aqueous humour solution (AAH) containing 16 mM HCO3- and 5% CO2, pH 7.25, pH(i) was 7.19 +/- 0.02 (SEM, n = 95). On removing HCO3- and CO2 there was an initial transient alkalinization followed by a fall in pH to a steady value of 6.97 +/- 0.03 (SEM, n = 54). Addition of 0.25 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) to AAH containing HCO3- and CO2 led to a rapid and pronounced fall in pH. Exposure to Na(+)-free AAH again led to a marked fall in pH(i), but in this case the addition of DIDS did not produce a further fall. Substitution of the impermeant anion gluconate for Cl- in the presence of HCO3- led to a rise in pHi, while substitution in the absence of HCO3- led to a fall in pHi. The above data indicate a significant role for a sodium-dependent Cl(-)-HCO3- exchange mechanism in the regulation of pHi. Addition of 1 mM amiloride to control AAH in both the presence and absence of HCO3- led to a marked fall in pH(i), indicating that a Na+/H+ exchange mechanism also has a significant role in the regulation of pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion concentrations, fluxes and electrical properties of the embryonic chicken lens

The membrane properties of embryonic chicken lenses were characterized using isotopic and electrical techniques. The lenses had a relatively high water content (80%) and large extracellular space (12.5%). Isotopic uptake measurements indicated that the lens cytoplasm contained 118 mM K+ and 26 mM Cl-. A value for intracellular Na+ of 14 mM was obtained using Na(+)-sensitive microelectrodes. A double-exponential model was used to fit the efflux of 86Rb+, 22Na+, 36Cl- and [3H]mannitol (an extracellular space marker) from the lens. When perfused with artificial aqueous humor (AAH) solution, embryonic lenses exhibited membrane potentials of between -20 and -40 mV. The more negative values were generally observed in lenses from older embryos. A ouabain-sensitive component, contributing -7 mV to the membrane potential, was also identified. The relatively depolarized membrane potentials suggested that the lens membranes were only weakly selective for K+ over Na+. To test this further, lenses were perfused with AAH containing varying concentrations of K+. The resulting changes in potential were interpreted in terms of the Goldman model. The best fit of the Goldman potential equation indicated that, in the presence of ouabain, the chicken lens membranes had a relative permeability to K+, Na+ and Cl- of 1.0, 0.36, 0.51 respectively. Replacing most or all of the Na+ in the AAH caused only a small change in the membrane potential rather than the large hyperpolarization towards the K+ equilibrium potential predicted by the Goldman model. Including the K+ ionophore valinomycin in the low Na(+)-AAH solutions caused a large increase in 86Rb+ efflux but did not result in additional hyperpolarization. This suggested that the insensitivity of the membrane potential to reduced extracellular Na+ was not due to voltage or pH inactivation of lens K+ channels.

Internal acidification modulates membrane and junctional resistance in the isolated lens of the frog Rana pipiens

The normal internal pH (pHi) of the amphibian lens, measured using ion-sensitive microelectrodes, is 7.1 (pHo = 7.4) and the membranes appear to be relatively impermeable to hydrogen ions. Perifusing the lens with 100% CO2 appeared to be the most efficient way of decreasing pHi, which fell to 6.3 after an exposure lasting 30 min. Accompanying this acidification, there was a rapid depolarization of membrane potential (Em), a decrease in membrane resistance (Rm) and increase in internal or bulk resistance (Ri). These changes did not occur if the external pH alone was decreased. All changes were reversible, although the time course of Ri recovery was faster than the others. The decrease in membrane resistance could be prevented if the chloride concentration in the external solution was reduced, suggesting that internal acidification opens chloride channels in the amphibian lens. Since chloride ions are normally close to equilibrium across amphibian lens membranes, it is suggested that the pH-induced depolarization is due to a decrease in potassium conductance. The increase in internal resistance on perifusing with CO2 is most likely due to a closing of gap junctions between the fibre cells. The relationship between internal conductance and pHi was very similar to that obtained in other tissues and could be fitted by the Hill equation with n = 6 and pK = 6.9. Fibre junctional conductance seems sensitive to small changes in hydrogen ion concentration around the resting pH. Two agents, aspirin and cyanate, that are believed to influence cataract development, slowed the recovery of Em, Rm and Ri during recovery from an acid load.(ABSTRACT TRUNCATED AT 250 WORDS)