Raised intracellular free calcium within the lens causes opacification and cellular uncoupling in the frog

Molecular mechanisms underlying enhanced hemichannel function of a cataract-associated Cx50 mutant

Connexin-50 (Cx50) is among the most frequently mutated genes associated with congenital cataracts. Although most of these disease-linked variants cause loss of function because of misfolding or aberrant trafficking, others directly alter channel properties. The mechanistic bases for such functional defects are mostly unknown. We investigated the functional and structural properties of a cataract-linked mutant, Cx50T39R (T39R), in the Xenopus oocyte system. T39R exhibited greatly enhanced hemichannel currents with altered voltage-gating properties compared to Cx50 and induced cell death. Coexpression of mutant T39R with wild-type Cx50 (to mimic the heterozygous state) resulted in hemichannel currents whose properties were indistinguishable from those induced by T39R alone, suggesting that the mutant had a dominant effect. Furthermore, when T39R was coexpressed with Cx46, it produced hemichannels with increased activity, particularly at negative potentials, which could potentially contribute to its pathogenicity in the lens. In contrast, coexpression of wild-type Cx50 with Cx46 was associated with a marked reduction in hemichannel activity, indicating that it may have a protective effect. All-atom molecular dynamics simulations indicate that the R39 substitution can form multiple electrostatic salt-bridge interactions between neighboring subunits that could stabilize the open-state conformation of the N-terminal (NT) domain while also neutralizing the voltage-sensing residue D3 as well as residue E42, which participates in loop gating. Together, these results suggest T39R acts as a dominant gain-of-function mutation that produces leaky hemichannels that may cause cytotoxicity in the lens and lead to development of cataracts.

The effects of GPX-1 knockout on membrane transport and intracellular homeostasis in the lens

Glutathione peroxidase-1 (GPX-1) is an enzyme that protects the lens against H2O2-mediated oxidative damage. The purpose of the present study was to determine the effects of GPX-1 knockout (KO) on lens transport and intracellular homeostasis. To investigate these lenses we used (1) whole lens impedance studies to measure membrane conductance, resting voltage and fiber cell gap junction coupling conductance; (2) osmotic swelling of fiber cell membrane vesicles to determine water permeability; and (3) injection of Fura2 and Na+-binding benzofuran isophthalate (SBFI) into fiber cells to measure [Ca2+]i and [Na+]i, respectively, in intact lenses. These approaches were used to compare wild-type (WT) and GPX-1 KO lenses from mice around 2 months of age. There were no significant differences in clarity, size, resting voltage, membrane conductance or fiber cell membrane water permeability between WT and GPX-1 KO lenses. However, in GPX-1 KO lenses, coupling conductance was 72% of normal in the outer shell of differentiating fibers and 45% of normal in the inner core of mature fibers. Quantitative Western blots showed that GPX-1 KO lenses had about 50% as much labeled Cx46 and Cx50 protein as WT, whereas they had equivalent labeled AQP0 protein as WT. Both Ca2+ and Na+ accumulated significantly in the core of GPX-1 KO lenses. In summary, the major effect on lens transport of GPX-1 KO was a reduction in gap junction coupling conductance. This reduction affected the lens normal circulation by causing [Na+]i and [Ca2+]i to increase, which could increase cataract susceptibility in GPX-1 KO lenses.

Calcium and the physiology of cataract

Calcium has long been known to play a role in cataract formation but techniques have only recently become available for investigating the physiological mechanisms. Previous studies showed that lens membrane permeability alters when the external calcium concentration falls below 1 mM, so it was interesting that values for human aqueous from cataract patients ranged from 0.45 to 2.0 mM. The mean value for the aqueous was one half that for the plasma. The calcium concentration in cataractous lenses ranged from 0.1 to 64 mM and lenses with a high calcium concentration also had a high sodium content. In lenses with near normal sodium content the highest calcium concentrations were associated with highly localized opacities, while nuclear cataracts had a low calcium content. The relationship between calcium and transparency was investigated in a rat lens system using ion-sensitive microelectrodes. The distribution of free calcium in the lens varied with age and was correlated with a change in the sensitivity of the lens to cold cataract and a change in lens birefringence. The highest free calcium levels were obtained from lenses incubated in 10 mM-calcium in the absence of glucose and these lenses showed most light scattering. Ion-sensitive microelectrode techniques applied to human lenses yielded calcium levels of 0.1 microM-2 mM. In lenses with dense, highly localized opacities the calcium distribution was not uniform and was highest in regions that scattered most light. The movement of calcium through individual membrane channels was investigated using patch clamp techniques. Three types of ionic channels have been identified in the lens. The smallest appears to be a calcium channel; the larger current fluctuations are associated with sodium and potassium movements. In organ culture studies of the bovine lens, a marked decrease in protein synthesis and net leakage of proteins was associated more strongly with an increase in calcium than with an increase in sodium. The stability of the lens protein gel thus seems to depend on maintaining a low internal level of calcium ions.

The effect of verapamil in the prevention of radiation-induced cataract

PURPOSE

Cataract is an unavoidable complication when radiation therapy includes the lens, even in small doses. Alterations in the ion content of the lens were considered to play an essential role in cataract formation. In this experimental study, the effect of verapamil on ion concentrations within the irradiated lenses was investigated in rats.

METHODS AND MATERIALS

Forty female Wistar albino rats, each weighing 180-250 g, were divided into three groups: (a) radiation treated (n = 10); (b) no treatment (n = 10); (c) or a combination of radiation and verapamil (n = 20). Both the radiation group and verapamil-treated group received 5 Gy radiation to the cranium in a single fraction, including the eyes, within the irradiation volume. All animals were sacrificed by bleeding, 7.5 weeks posttreatment. Calcium, sodium, and potassium levels were measured in blood and in lens homogenates. However, for technical reasons, magnesium levels could only be studied in lens homogenates.

RESULTS

Potassium and sodium concentrations in lens homogenates did not differ in the control and radiation groups, but both were significantly lower in the verapamil-treated group (p = 0.001, p = 0.009, respectively). Calcium levels were higher in the radiation group and lower in the verapamil-treated group compared to the controls (p < 0.0001); magnesium levels did not differ (p = 0.37).

CONCLUSION

Verapamil effectively decreased the lens calcium concentration, which is accepted as the key element in radiation cataractogenesis. It is therefore concluded that verapamil may reduce the risk of radiation-induced cataract formation.

The relationship between osmotic stress and calcium elevation: in vitro and in vivo rat lens models

Both in vivo and in vitro models were employed in the present study to assess the relative contribution of osmotic stress and increasing calcium levels to the development of sugar cataracts. In galactose cataract obtained from galactosemic weanling rats, the concentration of total calcium increased by nearly 10% at the first sign of visible opacification observed on the fourth day post-galactose feeding. After 7 days of galactose feeding, calcium levels continued to rise, to 0.8 mM. During the first 10 days, loss of lens transparency and calcium elevation was gradual and steady, with precipitous changes occurring on days 11 and 12. In groups of rats where galactose feeding was stopped after 7 days, cataract reversal was followed during the next 5 weeks. During the initial first week of recovery, calcium influx and elevation in the lens continued but began to decline steadily thereafter. After 3 weeks of recovery, lens transparency had returned to almost normal. Calcium levels continued to decline and reached normal levels between day 34 and 42, nearly 4 weeks after removal of the galactose diet. The relationship between osmotic stress and calcium elevation was investigated more directly by culturing normal rat lenses in hypoosmotic medium (280 mOsm) to create osmotic gradients similar to that in galactosemic lenses. The results showed that during the first day of culture (12 hr), osmotically stressed lenses gained 3 mg of water, became opaque and gained excess calcium (7 mM compared to 0.7 mM). Microscopic vacuoles appeared to accompany the process of opacification and contributed to increased light scattering and the loss of lens transparency. Additional experiments were designed to further distinguish between the effects of osmotic stress and calcium elevation on the opacification process. Thus, lenses were incubated in control and high-calcium medium (20 mM) at 300 mOsm. Within 12 hr of incubation, calcium elevation progressed to 1.37 mM, nearly doubling the normal value. Although opacification was observed in these lenses, no sign of vacuoles was evident. Collectively, the findings from this study support the premise that an early influx of calcium is brought about by osmotic stress and is responsible for the observed loss in transparency in osmotic (sugar) cataract.

Spatial variations in membrane properties in the intact rat lens

We have used linear frequency domain techniques to measure impedance at various locations and depths in the intact rat lens. The data are used to obtain best-fit solutions to a new electrical model based on lens structure, allowing us to estimate localized conductances of surface cell membranes (Gs), fiber cell membranes (gm), and gap junctions (Gj) as functions of position. We find that gm is small and fairly uniform throughout the lens (2.02 +/- 0.58 microS/cm2); for the anterior surface-epithelial cells Gs = 1.26 +/- 0.19 mS/cm2; for the posterior surface differentiating fiber cells Gs = 0.46 +/- 0.04 mS/cm2. Thus, Gs varies about the equator in a stepwise fashion. Gj between fiber cells at locations interior to 80% of the radius is fairly uniform (0.75 S/cm2); but in the outer 20% Gj varies smoothly and symmetrically from both poles (0.66 S/cm2) to equator (5.95 S/cm2). This pattern of variation in Gj is similar to the pattern of inward and outward currents reported by Robinson and Patterson (1983. Curr. Eye Res. 2:843-847). We therefore suggest that the nonuniform distribution of functional gap junctions, not the surface cell conductance or Na/K pumps, may be responsible for directing these current flows. Gap junctional uncoupling during exposure to elevated calcium and acidification was also examined. High calcium (20 mM, with the calcium ionophore A23187) produced modest (twofold) irreversible uncoupling along with large, irreversible decreases in membrane potential. We did not pursue this further. Acidification with 20 and 100% CO2-bubbled Tyrode's produced 5- and 15-fold reversible uncoupling, respectively, only in the outer 20% of the lens radius. The remaining inner 80% of the lens gap junctions seemed resistant to the acidification and did not uncouple.

Fresh and cultured human lens epithelial cells: an electrophysiological study of cell coupling and membrane properties

Microelectrode studies of fresh human and rabbit lens epithelia revealed stable membrane potentials [VR (human) = -36 mV; VR (rabbit) = -45 mV] and low input resistances [Ri (human) = 10 M omega; Ri (rabbit) = 20 M omega]. Coupling studies, using two voltage microelectrodes, demonstrated that the low input resistance of the fresh epithelial tissue was due to electrotonic coupling, which was found to be extremely labile and sensitive to perfusion of the apical (fibrefacing) surface of the epithelium. The intercellular coupling could be stabilized by raising the calcium concentration of the perfusate. Studies performed on confluent monolayers of cultured human lens epithelial (HLE) cells demonstrated a membrane potential (VR = -33 mV) and input resistance (Ri = 29 M omega) similar to their fresh counterparts. The intercellular coupling of these cells was found to be much more robust. Ultrastructural studies revealed that the apical junction of cultured HLE cells was less complex than that found in fresh tissue, the latter exhibiting multiple interdigitations and folds. The cultured monolayer was dissociated into single cells by a variety of methods and the membrane properties of individual cells were studied. Single cells were found to have a lower membrane potential (-20 to -25 mV) and an input resistance in the range 110-170 M omega, depending on the method of dissociation. Channel blocking and ion replacement studies revealed significant conductance pathways for potassium, sodium and chloride and a cell-attached patch clamp investigation revealed three distinct channel types. Of the two channels with inward currents at the resting potential, one, with a conductance of 25 pS, is identified as a non-selective cation channel, and the other, with a conductance of 14 pS and reversal potential of - 14 mV, is a possible candidate for a chloride channel but has yet to be characterized. A third channel with an outward current at the resting potential is identified as a potassium channel with a conductance of 49 pS. A link between epithelial uncoupling and certain types of cataract is proposed.

The influence of pH on membrane conductance and intercellular resistance in the rat lens

1. The conductance of the rat lens was measured using a two-internal-microelectrode technique. The voltage response to a step of current consisted of two components arising from bulk and membrane resistance respectively. 2. The potassium permeability was calculated by applying Goldman theory to 86Rb+ efflux data. 3. The internal pH (pHi) and internal free calcium (pCai) were measured directly using single- and double-barrelled ion-sensitive microelectrodes. 4. Lens pHi was 6.9 in control solution (external pH, pHo = 7.3) and was reduced on lowering pHo. The presence of propionate or 100% CO2 in the external solution accentuated this effect. 5. Internal acidification was accompanied by a depolarization of membrane potential, an increase in membrane and cell-to-cell resistance and a decrease in potassium permeability. The acidification had no effect on pCai. 6. The intracellular pH was increased by perifusing with trimethylamine or NH4Cl. Both treatments induced a membrane depolarization with little change in potassium permeability. Subsequent removal of NH4Cl led to a sustained decrease in pHi. 7. In every case where pHi decreased, the changes in membrane potential and conductance could be explained largely on the basis of a decrease in potassium permeability. The concomitant increase in cell-to-cell resistance was less pronounced and probably insufficient to uncouple the lens system.

Calcium-induced opacification and loss of protein in the organ-cultured bovine lens

A long-term system of organ culture for bovine lenses was used to investigate the effect of osmotic stress on lens opacification and crystallin loss. Lenses were pre-incubated in control medium containing L-[U-14C]tyrosine so that labelled crystallins were produced. The fate of these crystallins was studied in relation to two forms of osmotic stress. The addition of either ouabain or EGTA to the medium induced severe osmotic swelling and disturbance of the lens monovalent cation balance, but only the former treatment was followed by an increase in lens calcium. The changes due to osmotic stress were accompanied by loss of transparency and protein only in the lenses with increased calcium. Both opacification and increased calcium were found largely to be confined to the outer cortical fibres. Protein loss increased with time as lens calcium continued to increase. The protein recovered from the incubation medium was characterized by gel filtration and immunological techniques. The first protein detected was beta L-crystallin, and this formed the major part of the lost protein throughout, although alpha- and gamma-crystallins were detected at a later stage. Increased calcium also resulted in a change in the susceptibility of the crystallins to aggregation, since there was an increase in [14C]tyrosine incorporated into the lens high-molecular-weight (HM) fraction after exposure to ouabain, but not after exposure to EGTA. The relevance of these findings to human cataract is discussed.

The role of glial cells in regulation of neurotransmitter amino acids in the external environment. I. Transmembrane electrical and ion gradients and energy parameters in cultured glial-derived cell lines

Studies of energy parameters and intracellular ion concentrations were carried out on two glial cell lines, one derived from an astrocytoma (C6) and the other from an oligodendroglioma (TR33B), to elucidate the mechanism of transport of amino acid neurotransmitters by glial cells. Respiratory rate was 2.7-2.9 nmol/min/mg dry wt.; cytochrome c at 0.035-0.041 nmol/mg dry wt., was 23-29% reduced with a calculated turnover number 4.7-5.1 e-/s at 23 degrees C. ATP levels were high, 5.0-6.5 mM and [CrP]/[Cr] was almost 2. Membrane potentials at [K+]e = 5 mM were approximately -90 mV for C6 cells and -72 mV for TR33B. [K+]i was measured as approximately 100 mM for TR33B and 150 mM for C6 which indicated that the K+ diffusion potential was the major source of the membrane potential. [Na+]i was 5.8 mM for C6 and 20 mM for TR33B cells while free calcium was about 100 nM in both. Near Nernstian relationships were found in both types of cell between [K+]e and membrane potential over a range of 3.5-75 mM for TR33B and 5-110 mM for C6 cells. It is concluded that C6 and TR33B cell lines may be useful models for in vitro studies of some aspects of glial behavior.

Lens junctions are communicating junctions

Lens fibers are electrically coupled with each other and directly exchange dyes and metabolites. In most cells, this form of communication is mediated by gap junctions. Lens fibers lack typical gap junctions. The lens junctions, although morphologically similar to gap junctions, differ from them structurally, chemically and immunologically. Nevertheless, recent evidence suggests that indeed lens junctions are communicating junctions. The lens junction protein, MIP26, displays structural characteristics similar to other channel proteins. Once incorporated into liposomes it forms channels permeable to molecules as heavy as 1.5 kDa. Like other communicating junctions, lens junctions assume crystalline arrays and uncouple with Ca++. The liposome incorporated channels close with Ca++ and H+ in the presence of calmodulin (CaM). Partial loss of gating competency occurs after proteolytic cleavage of the C-terminal arm of MIP26. The need for a unique type of communicating junction in lens is unclear. A possibility is that this tissue has some special cell-to-cell transport requirements, in terms of size and/or charge of permeants, not shared by coupled cells of other tissues.

Characteristics of 45Ca uptake by the rabbit lens

Lens 45Ca uptake was shown to increase when the external calcium concentration was less than 1 mM, suggesting that calcium ions themselves are able to interact with lens membranes in a way that influences the mechanism of calcium permeability. Strontium ions, added to a calcium-deficient bathing solution, were shown to attenuate the increased 45Ca uptake by the lens. There was no evidence for extracellular binding of calcium. Lens membrane permeability to calcium appeared not to be voltage-dependent since 45Ca uptake was not affected when the lens was depolarized by high potassium solutions. Calcium channel blockers were shown not to reduce the increased calcium leak resulting from exposure of the lens to a calcium-deficient medium.

Effects of intracellular calcium on lens membrane permeability

The present investigation was designed to assess whether lens membrane permeability is affected by changes in levels of intracellular calcium. Lanthanum, an inhibitor of Ca-ATPase, affected an increase in the concentration of intracellular calcium (Cai) measured in cortical fiber cells. Preculture of lenses in lanthanum (1.0mM) caused an accumulation of 36Cl during subsequent culture at a rate three-fold higher than control lenses. Changes in calcium levels, however, were not responsible for the observed flux changes because a 40mV depolarization was observed to occur prior to a significant increase in calcium levels. The non-specific effects of lanthanum and other potential inhibitors of calcium transport were avoided by preculturing lenses in an ion-HEPES medium containing 20mM calcium chloride. In lenses with a six-fold increase in calcium levels there resulted only a 10% increase in 36Cl uptake over a 3 hr period. 86Rb efflux was also measured and the rate constant was unchanged compared to control lenses. Calcium accumulation did lead to a small (8mV) depolarization which may account for the small increase in chloride accumulation. By light microscopy, morphology of cortical lens fibers and the epithelium appeared unchanged in the calcium-loaded lens. The results provide little evidence that an increase in Cai leads to acute changes in lens membrane permeability.

Direct measurement of pH in the rat lens by ion-sensitive microelectrodes

The resting pH of the rat lens was determined using H+-sensitive liquid membrane microelectrodes and found to be 6.89 when measured in a perifusing solution of pH 7.20. The pH of the rat vitreous humour was also measured and was found to be 7.25. Attempts were made to perturb the lens pH by varying the pH of the perifusate. In the presence of alkaline solutions, the lens was able to maintain its resting pH and membrane potential but, upon perfusion with a more acidic solution, the lenticular pH equilibrated with the pH of the bathing solution and the potential depolarized. The internal pH could be manipulated independently of the external pH by perfusing the lens with Ringer solution containing 20 mM ammonium chloride. The ammonium chloride induced a rapid alkalinization and the return to control solution caused a fall in pH to below the normal resting level. The biphasic pH response to ammonium chloride was accompanied by changes in lens transparency and membrane potential.

Cytotoxic effects of internal calcium on lens physiology: a review

While calcium is possibly involved in cataractogenesis, it is unquestionably involved in normal lens physiology. Numerous reports have documented the many cellular processes in other tissues affected by alterations in cellular levels of calcium. The homeostasis of the lens is no less dependent on the critical balance of intracellular calcium. With advances being made in calcium-sensitive microelectrodes and pioneering studies progressing in ion channel electrophysiology, interest in calcium metabolism in the lens has been intensified. This report is an attempt to review recent findings that deal solely with biochemical changes resulting from calcium imbalances in the lens interior.

Permeability and gating of lens gap junction channels incorporated into liposomes

The lens junction protein (MIP26), and its trypsin cleavage product (MIP21), isolated from calf fiber cells, are incorporated into liposomes and the permeability and gating of the resulting channels are studied spectrophotometrically by an osmotic swelling assay. Liposomes incorporated with either protein and loaded with Dextran T-10 swell when placed in isotonic or hypertonic KCl, sucrose or polyethyleneglycol (PEG), indicating the presence of channels permeable to molecules as large as MW 1500. In the absence of calmodulin (CaM), the permeability of either MIP26 or MIP21 channels is not altered by Ca++. On the contrary, MIP26-CaM channels reversibly close in the presence of Ca++ (10(-5)M). Preliminary experiments show channel closure with lowered pH (5.5) as well. While MIP26-CaM channels close to all the permeants tested, MIP21-CaM channels close only partially with Ca++, becoming impermeable to large probes (PEG) while remaining permeable to sucrose and KCl. This indicates that the trypsin-cleaved C-terminal arm of MIP26 is the channel gate. Evidence from spectrophotofluorometry and circular dichroism spectroscopy indicates that activated CaM changes the conformation of isolated MIP26, suggesting that channel occlusion could result from a change in protein configuration.

Influence of external calcium and glucose on internal total and ionized calcium in the rat lens

Free calcium in the rat lens, measured by ion-sensitive electrodes, is 1.8 microM while the total, measured by atomic absorption, is of the order of 600 microM. The measured free calcium concentration (pCa) varies with the depth below the surface. It is lowest in the region 100-400 micron below the capsule and again in the nucleus, while the intervening perinuclear cortex has a relatively high free calcium. In young rats (less than 16 weeks) the free calcium in the posterior and anterior cortical regions is the same, while in the older lenses the free calcium is lower in the anterior and the regional variation is greater. Rat lenses incubated in a medium of similar ionic composition to aqueous humour for 15-24 h maintained a low level of free calcium. The maintenance of low internal calcium (both free and total) was dependent on external glucose and on removing glucose the intracellular free calcium increased from 5 to 15 microM while the total calcium increased from 600 to over 1000 microM. Following incubation in high calcium (10 mM), the free and total calcium increased to 40 and 3000 microM respectively. Omitting glucose from the high-calcium solution led to a further increase in both free and total calcium to 400 and 10000 microM respectively. The incubated control lenses maintained their normal sodium and potassium levels and resting potential, while removing glucose gave rise to an increase in sodium, a decrease in potassium and a depolarization of the membrane potential. Increasing external calcium also depolarized the membrane potential, but there was no change in internal sodium and potassium.(ABSTRACT TRUNCATED AT 250 WORDS)