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

Purinergic dysregulation causes hypertensive glaucoma-like optic neuropathy

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs) and visual loss. Although one of the highest risk factors for glaucoma is elevated intraocular pressure (IOP) and reduction in IOP is the only proven treatment, the mechanism of IOP regulation is poorly understood. We report that the P2Y6 receptor is critical for lowering IOP and that ablation of the P2Y6 gene in mice (P2Y6KO) results in hypertensive glaucoma-like optic neuropathy. Topically applied uridine diphosphate, an endogenous selective agonist for the P2Y6 receptor, decreases IOP. The P2Y6 receptor was expressed in nonpigmented epithelial cells of the ciliary body and controlled aqueous humor dynamics. P2Y6KO mice exhibited sustained elevation of IOP, age-dependent damage to the optic nerve, thinning of ganglion cell plus inner plexiform layers, and a reduction of RGC numbers. These changes in P2Y6KO mice were attenuated by an IOP lowering agent. Consistent with RGC damage, visual functions were impaired in middle-aged P2Y6KO mice. We also found that expression and function of P2Y6 receptors in WT mice were significantly reduced by aging, another important risk factor for glaucoma. In summary, our data show that dysfunctional purinergic signaling causes IOP dysregulation, resulting in glaucomatous optic neuropathy.

Effect of preserved and preservative-free timolol eye drops on tear film stability in healthy Africans

Background:

Preserved versus nonpreserved formulations for ophthalmic use have been well described in the literature although not specifically in the African population where beta blockers are frequently used as the first-line therapy due to economic and availability issues. This study sought to determine the effect of preserved and preservative-free Timolol eye drops on tear film stability in healthy black Africans.

Materials and Methods:

Sixty healthy nondry eye subjects aged 19–25 years were randomly assigned into four groups (n = 15) and differently treated with eye drops of phosphate buffered saline (PBS), preservative-free timolol (PFT), benzalkonium chloride (BAK) only, and BAK-preserved timolol (BPT). Noninvasive tear break-up time (NITBUT) was measured using the keratometer at baseline and 30, 60, and 90 min after drop application.

Results:

No significant decline in NITBUT was observed following treatment with PFT and PBS. However, BAK treatment showed a positive time-dependent significant decline in NITBUT (P < 0.001) while a significant decline in the BPT-treated group was only found at 90 min (−3.52 s; P < 0.001). In comparison to the PFT-treated group, treatment with BAK and BPT showed significantly lower NITBUT (P < 0.001).

Conclusion:

BPT is associated with a significant decline in tear film stability in black Africans. This finding has implications in the management of glaucoma in patients with high-risk of dry eyes in this population.

Discovery of Molecular Therapeutics for Glaucoma: Challenges, Successes, and Promising Directions

Glaucoma, a heterogeneous ocular disorder affecting ∼60 million people worldwide, is characterized by painless neurodegeneration of retinal ganglion cells (RGCs), resulting in irreversible vision loss. Available therapies, which decrease the common causal risk factor of elevated intraocular pressure, delay, but cannot prevent, RGC death and blindness. Notably, it is changes in the anterior segment of the eye, particularly in the drainage of aqueous humor fluid, which are believed to bring about changes in pressure. Thus, it is primarily this region whose properties are manipulated in current and emerging therapies for glaucoma. Here, we focus on the challenges associated with developing treatments, review the available experimental methods to evaluate the therapeutic potential of new drugs, describe the development and evaluation of emerging Rho-kinase inhibitors and adenosine receptor ligands that offer the potential to improve aqueous humor outflow and protect RGCs simultaneously, and present new targets and approaches on the horizon.

Effect of DHA on plasma fatty acid availability and oxidative stress during training season and football exercise

The aim was to determine the effects of a diet supplemented with 1.14 g per day of docosahexaenoic acid (DHA) for eight weeks on the plasma oxidative balance and anti-inflammatory markers after training and acute exercise. Fifteen volunteer male football players were randomly assigned to placebo or experimental and supplemented groups. Blood samples were taken under resting conditions at the beginning and after eight weeks of training under resting and post-exercise conditions. The experimental beverage increased the plasma DHA availability in non-esterified fatty acids (NEFAs) and triglyceride fatty acids (TGFAs) and increased the polyunsaturated fatty acid (PUFA) fraction of NEFAs but had no effects on the biomarkers for oxidative balance in plasma. During training, plasma protein markers of oxidative damage, the haemolysis degree and the antioxidant enzyme activities increased, but did not affect lipid oxidative damage. Training season and DHA influenced the circulating levels of prostaglandin E2 (PGE2). Acute exercise did not alter the basal levels of plasma markers for oxidative and nitrosative damage of proteins and lipids, and the antioxidant enzyme activities, although DHA-diet supplementation significantly increased the PGE2 in plasma after acute exercise. In conclusion, the training season and acute exercise, but not the DHA diet supplementation, altered the pattern of plasma oxidative damage, as the antioxidant system proved sufficient to prevent the oxidative damage induced by the acute exercise in well-trained footballers. The DHA-diet supplementation increased the prostaglandin PGE2 plasma evidencing anti-inflammatory effects of DHA to control inflammation after acute exercise.

Melatonin receptors trigger cAMP production and inhibit chloride movements in nonpigmented ciliary epithelial cells

Melatonin and its analog 5-MCA-NAT (5-methylcarboxyamino-N-acetyl tryptamine) are active compounds reducing intraocular pressure (IOP). This action is mediated through MT2 and the putative MT3 melatonin receptor, producing a transient reduction of IOP that lasts for a few hours and has not yet been characterized. The use of melatonin and its analog are causing a decrease in chloride efflux from rabbit nonpigmented epithelial cells (NPE), possibly explaining the decrease in IOP. Melatonin and 5-MCA-NAT inhibited rabbit NPE chloride release in a concentration-dependent manner, whereas the pD2 values were between 4.5 ± 1.2 and 4.4 ± 1.0, respectively. Melatonin hypotensive action was enhanced by the presence of MT2 antagonists, such as DH97 (N-pentanoyl-2-benzyltryptamine) and 4-P-P-DOT (4-phenyl-2-propionamidotetralin) and by the nonselective melatonin receptor antagonist luzindole. Prazosin (1.5 µM) partially reverses the melatonin action by acting as a selective MT3 antagonist. However, at 15 nM it acts as an α-adrenergic receptor antagonist, enhancing the melatonin effect. Regarding the intracellular pathways triggered by melatonin receptors, neither phospholipase C/protein kinase C pathway nor the canonical reduction of intracellular cAMP was responsible for melatonin or 5-MCA-NAT actions. On the contrary, the application of these substances produced a concentration-dependent increase of cAMP, with pD2 values of 4.6 ± 0.2 and 4.9 ± 0.7 for melatonin and 5-MCA-NAT, respectively. In summary, melatonin reduces the release of chloride concomitantly to cAMP generation. The reduction of Cl(-) secretion accounts for a decrease in the water outflow and therefore a decrease in aqueous humor production. This could be one of the main mechanisms responsible for the reduction of IOP after application of melatonin and 5-MCA-NAT.

Temporary sequestration of potassium by mitochondria in astrocytes

Increases in extracellular potassium concentration ([K(+)](o)), which can occur during neuronal activity and under pathological conditions such as ischemia, lead to a variety of potentially detrimental effects on neuronal function. Although astrocytes are known to contribute to the clearance of excess K(+)(o), the mechanisms are not fully understood. We examined the potential role of mitochondria in sequestering K(+) in astrocytes. Astrocytes were loaded with the fluorescent K(+) indicator PBFI and release of K(+) from mitochondria into the cytoplasm was examined after uncoupling the mitochondrial membrane potential with carbonyl cyanide m-chlorophenylhydrazone (CCCP). Under the experimental conditions employed, transient applications of elevated [K(+)](o) led to increases in K(+) within mitochondria, as assessed by increases in the magnitudes of cytoplasmic [K(+)] ([K(+)](i)) transients evoked by brief exposures to CCCP. When mitochondrial K(+) sequestration was impaired by prolonged application of CCCP, there was a robust increase in [K(+)](i) upon exposure to elevated [K(+)](o). Blockade of plasmalemmal K(+) uptake routes by ouabain, Ba(2+), or a mixture of voltage-activated K(+) channel inhibitors reduced K(+) uptake into mitochondria. Also, reductions in mitochondrial K(+) uptake occurred in the presence of mito-K(ATP) channel inhibitors. Rises in [K(+)](i) evoked by brief applications of CCCP following exposure to high [K(+)](o) were also reduced by gap junction blockers and in astrocytes isolated from connexin43-null mice, suggesting that connexins also play a role in K(+) uptake into astrocyte mitochondria. We conclude that mitochondria play a key role in K(+)(o) handling by astrocytes.

Regulation of gap junction coupling in bovine ciliary epithelium

Aqueous humor is formed by fluid transfer from the ciliary stroma sequentially across the pigmented ciliary epithelial (PE) cells, gap junctions, and nonpigmented ciliary epithelial (NPE) cells. Which connexins (Cx) contribute to PE-NPE gap junctional formation appears species specific. We tested whether small interfering RNA (siRNA) against Cx43 (siCx43) affects bovine PE-NPE communication and whether cAMP affects communication. Native bovine ciliary epithelial cells were studied by dual-cell patch clamping, Lucifer Yellow (LY) transfer, quantitative polymerase chain reaction with reverse transcription (qRT-PCR), and Western immunoblot. qRT-PCR revealed at least 100-fold greater expression for Cx43 than Cx40. siCx43 knocked down target mRNA expression by 55 +/- 7% after 24 h, compared with nontargeting control siRNA (NTC1) transfection. After 48 h, siCx43 reduced Cx43 protein expression and LY transfer. The ratio of fluorescence intensity (R(f)) in recipient to donor cell was 0.47 +/- 0.09 (n = 11) 10 min after whole cell patch formation in couplets transfected with NTC1. siCx43 decreased R(f) by approximately 60% to 0.20 +/- 0.07 (n = 13, P < 0.02). Dibutyryl-cAMP (500 microM) also reduced LY dye transfer by approximately 60%, reducing R(f) from 0.41 +/- 0.05 (n = 15) to 0.17 +/- 0.05 (n = 20) after 10 min. Junctional currents were lowered by approximately 50% (n = 6) after 10-min perfusion with 500 microM dibutyryl-cAMP (n = 6); thereafter, heptanol abolished the currents (n = 5). Preincubation with the PKA inhibitor H-89 (2 microM) prevented cAMP-triggered current reduction (n = 6). We conclude that 1) Cx43, but not Cx40, is a major functional component of bovine PE-NPE gap junctions; and 2) under certain conditions, cAMP may act through PKA to inhibit bovine PE-NPE gap junctional communication.

Species variation in biology and physiology of the ciliary epithelium: similarities and differences

Glaucoma is a leading cause of irreversible blindness worldwide. Lowering intraocular pressure (IOP) is the only strategy documented to delay the appearance and retard the progression of vision loss. One major approach for lowering IOP is to slow the rate of aqueous humor formation by the ciliary epithelium. As discussed in the present review, the transport basis for this secretion is largely understood. However, several substantive issues are yet to be resolved, including the integrated regulation of secretion, the functional topography of the ciliary epithelium, and the degree and significance of species variation in aqueous humor inflow. This review discusses species differences in net secretion, particularly of Cl(-) and HCO(3)(-) secretion. Identifying animal models most accurately mimicking aqueous humor formation in the human will facilitate development of future novel initiatives to lower IOP.

Bestrophin-2 is involved in the generation of intraocular pressure

PURPOSE

The bestrophin family of proteins has been demonstrated to generate or regulate Ca2+-activated Cl(-) conductances. Mutations in bestrophin-1 (Best1) cause several blinding eye diseases, but little is known about other bestrophin family members. This study involved disruption of the Best2 gene in mice.

METHODS

The mouse Best2 gene was disrupted by replacing exons 1, 2, and part of exon 3 with a Lac Z. The expression profile of Bestrophin-2 (Best2) was examined using RT-PCR, X-gal staining, and immunohistochemistry. Intraocular pressure (IOP) was measured by anterior chamber cannulation.

RESULTS

RT-PCR of mouse tissues revealed Best2 mRNA in eye, colon, nasal epithelia, trachea, brain, lung, and kidney. X-gal staining, confirmed expression in colon epithelia and in the eye, in the nonpigmented epithelia (NPE). Best2 was not expressed in RPE cells. Best2 protein was observed only in NPE and colon epithelia. The absence of Best2 had no obvious deleterious effect on the mice. However, the Best2-/- mice were found to have significantly (P < 0.02) diminished IOP with respect to the Best2+/+ and Best2+/- littermates. The Best2-/- and Best2+/- mice responded better to the carbonic anhydrase inhibitor brinzolamide than did their Best2+/+ littermates, although the beta-blocker timolol brought IOP to the same level, regardless of genotype.

CONCLUSIONS

Best2 plays a role in the generation of IOP by regulating formation of aqueous humor, and inhibition of Best2 function represents an attractive new avenue for regulating IOP in individuals with glaucoma.

Electron microprobe analysis of rabbit ciliary epithelium indicates enhanced secretion posteriorly and enhanced absorption anteriorly

The rate of aqueous humor formation sequentially across the pigmented (PE) and nonpigmented (NPE) ciliary epithelial cell layers may not be uniform over the epithelial surface. Because of the tissue's small size and complex geometry, this possibility cannot be readily tested by conventional techniques. Rabbit iris-ciliary bodies were divided, incubated, quick-frozen, cryosectioned, and freeze-dried for electron probe X-ray microanalysis of the elemental contents of the PE and NPE cells. We confirmed that preincubation with ouabain to block Na(+),K(+)-ATPase increases Na(+) and decreases K(+) contents far more anteriorly than posteriorly. The anterior and posterior regions were the iridial portion of the primary ciliary processes and the pars plicata, respectively. Following interruption of gap junctions with heptanol, ouabain produced smaller changes in anterior PE cells, possibly reflecting higher Na(+) or K(+) permeability of anterior NPE cells. Inhibiting Na(+) entry selectively with amiloride, benzamil, or dimethylamiloride reduced anterior effects of ouabain by approximately 50%. Regional dependence of net secretion was also assessed with hypotonic stress, which stimulates ciliary epithelial cell regulatory volume decrease (RVD) and net Cl(-) secretion. In contrast to ouabain's actions, the RVD was far more marked posteriorly than anteriorly. These results suggest that 1) enhanced Na(+) reabsorption anteriorly, likely through Na(+) channels and Na(+)/H(+) exchange, mediates the regional dependence of ouabain's actions; and 2) secretion may proceed primarily posteriorly, with secondary processing and reabsorption anteriorly. Stimulation of anterior reabsorption might provide a novel strategy for reducing net secretion.

Basis of chloride transport in ciliary epithelium

The aqueous humor is formed by the bilayered ciliary epithelium. The pigmented ciliary epithelium (PE) faces the stroma and the nonpigmented ciliary epithelium (NPE) contacts the aqueous humor. Cl(-) secretion likely limits the rate of aqueous humor formation. Many transport components underlying Cl(-) secretion are known. Cl(-) is taken up from the stroma into PE cells by electroneutral transporters, diffuses to the NPE cells through gap junctions and is released largely through Cl(-) channels. Recent work suggests that significant Cl(-) recycling occurs at both surfaces of the ciliary epithelium, providing the basis for modulation of net secretion. The PE-NPE cell couplet likely forms the fundamental unit of secretion; gap junctions within the PE and NPE cell layers are inadequate to maintain constancy of ionic composition throughout the epithelium under certain conditions. Although many hormones, drugs and signaling cascades are known to have effects, a persuasive model of the regulation of aqueous humor formation has not yet been developed. cAMP likely plays a central role, potentially both enhancing and reducing secretion by actions at both surfaces of the ciliary epithelium. Among other hormone receptors, A(3) adenosine receptors likely alter intraocular pressure by regulating NPE-cell Cl(-) channel activity. Recently, functional evidence for the regional variation in ciliary epithelial secretion has been demonstrated; the physiologic and pathophysiologic implications of this regional variation remain to be addressed.

The ins and outs of aqueous humour secretion

The intraocular pressure (IOP) reflects a balance between inflow and outflow of aqueous humour. A major strategy in the medical treatment of glaucoma is to reduce inflow and thereby IOP. Understanding the mechanisms and regulation of inflow is thus of clear clinical relevance. Many mechanisms underlying inflow have been identified. The integration and regulation of these mechanisms is less clear. Aqueous humour is secreted across the ciliary epithelium by transferring solute, chiefly NaCl, from the stroma to the posterior chamber of the eye, with water passively following. The epithelium consists of two layers: the pigmented ciliary epithelial (PE) cells abutting the stroma, and the non-pigmented ciliary epithelial (NPE) cells facing the aqueous humour. Gap junctions link adjacent cells within and between these layers. Secretion proceeds in three steps: (1) uptake of NaCl from stroma to PE cells by electroneutral transporters, (2) passage of NaCl from PE to NPE cells through gap junctions, and (3) release of Na+ and Cl- through Na+,K+-activated ATPase and Cl- channels, respectively. Most of our understanding of inflow mechanisms has been obtained by studying in vitro preparations at subcellular, cellular and tissue levels. A particularly productive approach has been the electron probe X-ray microanalysis (EPMA) of the elemental composition of excised ciliary epithelium. This technique permits analysis of adjacent cells within different regions of the ciliary epithelium. EPMA of rabbit preparations has supported the idea that paired activity of Na+/H+ and Cl-/HCO3- antiports can be the dominant mechanism underlying the first step in secretion, stromal NaCl uptake by PE cells. EPMA also indicates that Cl- turnover is faster in the anterior than the posterior region of the epithelium. At the opposite epithelial surface, release of Na+ through Na+,K+-activated ATPase of NPE cells is also greater anteriorly than posteriorly. The accompanying release of Cl- through ion channels is enhanced by agonists of A3 adenosine receptors (ARs). The concepts that paired antiport activity is important in stromal NaCl uptake and that A3ARs modulate NaCl release into the aqueous humour were based on in vitro studies. The potential relevance of these conclusions to in vivo conditions has been tested by measurements of IOP in the living mouse. The results have confirmed the predictions that inhibitors of Na+/H+ antiports lower IOP, and that A3AR agonists and antagonists raise and lower IOP, respectively.

Adenylyl cyclase activity mediated by beta-adrenoceptors in immortalized human trabecular meshwork and non-pigmented ciliary epithelial cells

Non-pigmented ciliary epithelial (NPE) and trabecular meshwork (TM) cells are important in maintaining normal aqueous humor dynamics through the inflow and outflow routes, respectively. The current studies were undertaken to evaluate the ability of several beta-adrenergic receptor agonists to stimulate various antagonists to inhibit cAMP production in cultured immortalized human TM and NPE cells using an automated enzyme immunoassay. Isoproterenol was the most potent agonist in both the NPE and TM cells. The rank order of potency of agonists in NPE and TM cells, respectively, was: isoproterenol [EC50 = 37 and 66 nM] > epinephrine [EC50 = 112 and 526 nM] > albuterol [EC50 = 426 and 785 nM] > norepinephrine [EC50 = 3 and > 10 microM] > phenylephrine [EC50 > 10 microM for both] = dopamine [EC50 > 10 microM for both](n = 3-19). The isoproterenol-induced cAMP production was inhibited by various antagonists with the following rank order of potency in NPE and TM cells, respectively: propranolol [Ki = 0.2 and 0.3 nM] = ICI-118551 [Ki = 0.5 and 0.4 nM] > levobunolol [Ki = 1.1 and 2.1 nM] > levobetaxolol [Ki = 13 and 14 nM] = racemic betaxolol [Ki = 43 and 19 nM] > dextrobetaxolol [Ki = 2,705 and 1,980 nM] > atenolol [Ki > 4,000 nM for both] (n = 3-7). These detailed pharmacological studies using a variety of agonists and antagonists further supported the presence of beta2-adrenergic receptors in immortalized human NPE and TM cells.

Polyunsaturated fatty acyl coenzyme A suppress the glucose-6-phosphatase promoter activity by modulating the DNA binding of hepatocyte nuclear factor 4 alpha

Glucose-6-phosphatase confers on gluconeogenic tissues the capacity to release endogenous glucose in blood. The expression of its gene is modulated by nutritional mechanisms dependent on dietary fatty acids, with specific inhibitory effects of polyunsaturated fatty acids (PUFA). The presence of consensus binding sites of hepatocyte nuclear factor 4 (HNF4) in the -1640/+60 bp region of the rat glucose-6-phosphatase gene has led us to consider the hypothesis that HNF4 alpha could be involved in the regulation of glucose-6-phosphatase gene transcription by long chain fatty acid (LCFA). Our results have shown that the glucose-6-phosphatase promoter activity is specifically inhibited in the presence of PUFA in HepG2 hepatoma cells, whereas saturated LCFA have no effect. In HeLa cells, the glucose-6-phosphatase promoter activity is induced by the co-expression of HNF4 alpha or HNF1 alpha. PUFA repress the promoter activity only in HNF4 alpha-cotransfected HeLa cells, whereas they have no effects on the promoter activity in HNF1 alpha-cotransfected HeLa cells. From gel shift mobility assays, deletion, and mutagenesis experiments, two specific binding sequences have been identified that appear able to account for both transactivation by HNF4 alpha and regulation by LCFA in cells. The binding of HNF4 alpha to its cognate sites is specifically inhibited by polyunsaturated fatty acyl coenzyme A in vitro. These data strongly suggest that the mechanism by which PUFA suppress the glucose-6-phosphatase gene transcription involves an inhibition of the binding of HNF4 alpha to its cognate sites in the presence of polyunsaturated fatty acyl-CoA thioesters.