Effects of diadenosine polyphosphates on tear secretion in New Zealand white rabbits

Efficacy and safety of artificial tears containing Artemia salina extract with dinucleotides for dry eye

CLINICAL RELEVANCE

This clinical trial was conducted as part of the marketing procedures for a medical device comprising artificial tears containing Artemia salina extract with dinucleotides. These molecules previously demonstrated secretagogue properties by enhancing the production of aqueous, mucinous, and lipidic components of the tears.

BACKGROUND

After confirming the efficacy of artificial tears containing Artemia salina extract in an animal model, this study proceeded to evaluate their efficacy and safety on dry eye participants.

METHODS

A randomised controlled clinical trial was performed on 36 dry eye participants (41.6 ± 20.6 years). Half of the participants were treated with saline solution as a placebo for four weeks, while the other half were treated with artificial tears containing Artemia salina, randomly assigned. After a wash-out period of two weeks, the treatments were crossed for another four weeks. Participants were assessed at baseline and after one week, two weeks, and four weeks. Efficacy variables were: eye dryness frequency (primary), eye comfort, visual satisfaction, tear secretion, tear break-up time, corneal staining, conjunctival staining, and conjunctival hyperaemia. Safety variables were: high- and low-contrast visual acuity, intraocular pressure, and eye fundus images analysis.

RESULTS

Compared with the baseline, the saline solution showed no significant changes in any of the studied variables after four weeks of treatment (p ≥ 0.05). However, the topical instillation of the artificial tears with Artemia salina for four weeks significantly improved eye dryness frequency (p = 0.014) and corneal staining (p = 0.010). No systemic or ocular adverse events were reported during the clinical trial.

CONCLUSION

The topical instillation of artificial tears containing Artemia salina in mild to moderate dry eye participants for four weeks slightly improved their symptoms related to eye dryness frequency and reduced corneal damage, with no undesirable side effects observed.

Elucidating acceptance and clinical indications to support the rational design of drug-eluting contact lenses

The advent of drug-eluting contact lenses (DECLs) has opened up new avenues for the treatment of eye diseases. DECLs is expected to partially overcome the shortcomings of eye drops due to single-dose packaging, accurate dosing, prolonged drug elution behavior, and simplified dosing procedures. Currently, a significant proportion of the DECLs design effort has been directed towards enhancing the compatibility of contact lenses with drugs. The appropriate elution time for the drug remains unclear. Additionally, it is ambiguous for which ophthalmic diseases DECLs offers the greatest therapeutic advantage. To rationally design DECLs in practice, it is necessary to understand the acceptance of DECLs by patients and practitioners and to clarify the indications for DECLs. This review will first focus on the acceptance of DECLs by different patients and practitioners and discuss the factors that influence its acceptance. Secondly, this review presents an overview of the current effectiveness of DECLs treatments in animals and in the clinical phase, with a particular focus on the suitability of DECLs for the treatment of ophthalmic diseases. Overall, patients and practitioners expressed positive attitudes towards DECLs. However, this is related to factors such as DECLs' treatment cycle, safety, and price. In addition, DECLs has good application prospects for ocular wound healing, postoperative management, and treatment of contact lenses-related complications. Furthermore, chronic diseases such as glaucoma that necessitate long-term medication and intraocular diseases that require implants or injections represent additional potential applications for DECLs. It is hoped that this review will facilitate a deeper understanding of DECLs acceptance and indications, thereby supporting the rational design of DECLs. At the same time, this review provides a reference for the design of other drug-device combination products.

Targeting purinergic receptors to attenuate inflammation of dry eye

Inflammation is one of the potential factors to cause the damage of ocular surface in dry eye disease (DED). Increasing evidence indicated that purinergic A₁, A_2A, A₃, P2X4, P2X7, P2Y₁, P2Y₂, and P2Y₄ receptors play an important role in the regulation of inflammation in DED: A₁ adenosine receptor (A_1R) is a systemic pro-inflammatory factor; A_2AR is involved in the activation of the MAPK/NF-kB pathway; A_3R combined with inhibition of adenylate cyclase and regulation of the mitogen-activated protein kinase (MAPK) pathway leads to regulation of transcription; P2X4 promotes receptor-associated activation of pro-inflammatory cytokines and inflammatory vesicles; P2X7 promotes inflammasome activation and release of pro-inflammatory cytokines IL-1β and IL-18; P2Y receptors affect the phospholipase C(PLC)/IP3/Ca²⁺ signaling pathway and mucin secretion. These suggested that purinergic receptors would be promising targets to control the inflammation of DED in the future.

Efficacy of Artificial Tears Based on an Extract of Artemia salina Containing Dinucleotides in a Rabbit Dry Eye Model

(1) Background: Artemia salina is a brine shrimp containing high concentrations of dinucleotides, molecules with properties for dry eye treatment. For this reason, the purpose of the study was to evaluate the effect of the artificial tears based on an extract of Artemia salina in a rabbit dry eye model. (2) Methods: A prospective and randomized study was carried out. Twenty rabbits were divided into 4 groups (n = 5, each group): healthy rabbits, dry eye rabbits, dry eye rabbits treated with hypromellose (HPMC), and dry eye rabbits treated with Artemia salina. Dry eye was induced by the topical instillation of 0.2% benzalkonium chloride. The measurements were performed before and after the treatment for 5 consecutive days. (3) Results: The topical instillation of artificial tears containing Artemia salina showed beneficial effects on tear secretion, tear break-up time, corneal staining, the density of Goblet cells, heigh of mucin cloud secreted by these cells, and mRNA levels of IL-1β and MMP9 in conjunctival cells. Compared with the HPMC, there was a statistically significant improvement (p < 0.05) with the Artemia salina in all the variables under study, except for the conjunctival hyperemia, density of Goblet cells, and mRNA levels of IL-6. (4) Conclusions: The potential of artificial tears based on Artemia salina as a secretagogue agent for dry eye treatment was confirmed, opening the door for future clinical trials and studies to extrapolate the findings for dry eye patients.

Preclinical Development of Artificial Tears Based on an Extract of Artemia Salina Containing Dinucleotides in Rabbits

PURPOSE

To evaluate the preclinical efficacy of eye drops based on an extract of Artemia salina on the ocular surface of rabbits. Tear secretion, tear break-up time and corneal staining were measured.

MATERIAL AND METHODS

A preclinical and short-term prospective study was performed. Twenty New Zealand white rabbits were divided into five groups, with four rabbits per group, each receiving a different concentration of Artemia salina. In each rabbit, an extract of Artemia salina (2%, 4%, 6%, 8% or 10%) was randomly instilled in one eye and saline solution (negative control) in the other eye. Tear secretion, tear break-up time and corneal staining were measured before and after the instillation of five drops per eye (one drop per hour) on the same day.

RESULTS

In tear secretion, there was an increase of 43.88 ± 6.73% with 4% Artemia salina in comparison with its baseline measurement (P = .049). The rest of the groups did not show differences (P ≥ 0.05). For tear break-up time, none of the groups showed differences (P ≥ 0.05), while for corneal staining score, there was an improvement of 0.88 ± 0.83 with 4% Artemia salina (P = .038) and a deterioration of 0.50 ± 0.83 with control solution (P = .008).

CONCLUSIONS

Short-term instillation of eye drops with 4% Artemia salina produced both stimulation of tear secretion and a slight improvement of physiological corneal staining. Besides, all the doses of up to 10% Artemia salina did not produce undesirable side effects on the ocular surface. Therefore, these eye drops are presented as a possible new treatment for dry eye due to their secretagogue properties and ocular surface regeneration.

Ligand binding and activation of UTP-activated G protein-coupled P2Y2 and P2Y4 receptors elucidated by mutagenesis, pharmacological and computational studies

The nucleotide receptors P2Y₂ and P2Y₄ are the most closely related G protein-coupled receptors (GPCRs) of the P2Y receptor (P2YR) family. Both subtypes couple to G_q proteins and are activated by the pyrimidine nucleotide UTP, but only P2Y₂R is also activated by the purine nucleotide ATP. Agonists and antagonists of both receptor subtypes have potential as drugs e.g. for neurodegenerative and inflammatory diseases. So far, potent and selective, "drug-like" ligands for both receptors are scarce, but would be required for target validation and as lead structures for drug development. Structural information on the receptors is lacking since no X-ray structures or cryo-electron microscopy images are available. Thus, we performed receptor homology modeling and docking studies combined with mutagenesis experiments on both receptors to address the question how ligand binding selectivity for these closely related P2YR subtypes can be achieved. The orthosteric binding site of P2Y₂R appeared to be more spacious than that of P2Y₄R. Mutation of Y197 to alanine in P2Y₄R resulted in a gain of ATP sensitivity. Anthraquinone-derived antagonists are likely to bind to the orthosteric or an allosteric site depending on their substitution pattern and the nature of the orthosteric binding site of the respective P2YR subtype. These insights into the architecture of P2Y₂- and P2Y₄Rs and their interactions with structurally diverse agonists and antagonist provide a solid basis for the future design of potent and selective ligands.

Tear film lipid layer increase after diquafosol instillation in dry eye patients with meibomian gland dysfunction: a randomized clinical study

Diquafosol promotes secretion of tear fluid and mucin at the ocular surface and is administered for treatment of dry eye (DE). Tear film lipid layer is secreted from meibomian glands and stabilizes the tear film. We recently showed that diquafosol administration increased lipid layer thickness (LLT) for up to 60 min in normal human eyes. We here evaluated tear film lipid layer in DE patients (n = 47) with meibomian gland dysfunction (MGD) before as well as 30, 60, and 90 min after diquafosol administration. One drop of artificial tears or one drop of diquafosol was applied randomly to the eyes of each patient. Diquafosol significantly increased LLT at 30 (P < 0.001) and 60 (P = 0.042) min and noninvasive tear film breakup time for at least 90 min (P < 0.001 at each assessment point). Artificial tears had no such effect. Diquafosol significantly improved the tear interferometric pattern compared with artificial tears (P < 0.001 at each assessment point). A single topical administration of diquafosol thus improved LLT and tear film stability in DE patients with MGD, suggesting that diquafosol is a potential treatment not only for aqueous-deficient DE but also for evaporative DE associated with MGD.

Increased Ap4A levels and ecto-nucleotidase activity in glaucomatous mice retina

The pathogenesis of glaucoma involves numerous intracellular mechanisms including the purinergic system contribution. Furthermore, the presence and release of nucleotides and dinucleotides during the glaucomatous damage and the maintenance of degradation machinery through ecto-nucleotidase activity are participating in the modulation of the suitable extracellular complex balance. The aim of this study was to investigate the levels of diadenosine tetraphosphate (Ap₄A) and the pattern of ecto-nucleotidase activity expression in glaucomatous retinas during the progress the pathology. Ap₄A levels were analyzed by HPLC in glaucomatous retinas from the DBA/2J mice at 3, 9, 15, and 23 months of age. For that, retinas were dissected as flattened whole-mounts and stimulated in Ringer buffer with or without 59 mM KCl. NPP1 expression was analyzed by RT-PCR and western blot and its distribution was assessed by immunohistochemistry studies examined under confocal microscopy. Glaucomatous mice exhibited Ap₄A values, which changed in stimulated retinas as long as the pathology progressed varying from 0.73 ± 0.04 (3 months) to 0.170 ± 0.05 pmol/mg retina (23 months). Concomitantly, NPP1 expression was significantly increased (82.15%) in the DBA/2J mice at 15 months. Furthermore, immunohistochemical studies showed that NPP1 labeling was stronger in OPL and IPL labeling tangentially in the vitreal part of the retina and was upregulated at 15 months of age. Our findings demonstrate that Ap₄A decreased levels may be related with exacerbated activity of NPP1 protein in glaucomatous degeneration and in this way contributing to elucidate different mechanisms involved in retinal impairment in glaucomatous degeneration.

Changes in P2Y Purinergic Receptor Expression in the Ciliary Body in a Murine Model of Glaucoma

Glaucoma is a neuropathology, often accompanied by an elevated intraocular pressure (IOP), which can lead to blindness. Since DBA/2J mice develop glaucoma, several studies of the physiopathology of glaucoma have been reported in this animal model. It is also known that purinergic receptors are involved in the pathology of glaucoma by controlling aqueous humor production and drainage and therefore controlling IOP. There are no studies on purinergic receptors in the DBA/2J model of glaucoma and their relation to the development of the pathology, so the aim of this study was to make an approach to the purinergic mechanisms involved in glaucoma. All the experiments were performed using DBA/2J and C57BL/6J mice and investigating P2Y₁, P2Y₂, and P2Y₆ receptors. IOP measurements were made with a non-invasive rebound tonometer, and animals were instilled with diadenosine tetraphosphate (Ap₄A) and the corresponding purinergic antagonists in order to see their effects on IOP. The expression of mRNA for P2Y₁, P2Y₂, and P2Y₆ purinergic receptors was carried out by quantitative real-time PCR. Additionally, P2Y-receptor expression was performed by immunohistochemical techniques carried out on the ciliary processes. The results showed that IOP decreases when Ap₄A was instilled and that the expressions of the analyzed purinergic receptors were stable throughout all the ages under study in the C57BL/6J mice (control mice). On the other hand, there were significant changes in the purinergic receptor expression in DBA/2J suggesting that elevated IOP in these animals could be related to an increase of P2Y₂ expression and a decrease in P2Y₁ receptors.

Diadenosine tetraphosphate (Ap4A) - an E. coli alarmone or a damage metabolite?

Under stress, metabolism is changing: specific up- or down-regulation of proteins and metabolites occurs as well as side effects. Distinguishing specific stress-signaling metabolites (alarmones) from side products (damage metabolites) is not trivial. One example is diadenosine tetraphosphate (Ap4A) - a side product of aminoacyl-tRNA synthetases found in all domains of life. The earliest observations suggested that Ap4A serves as an alarmone for heat stress in Escherichia coli. However, despite 50 years of research, the signaling mechanisms associated with Ap4A remain unknown. We defined a set of criteria for distinguishing alarmones from damage metabolites to systematically classify Ap4A. In a nutshell, no indications for a signaling cascade that is triggered by Ap4A were found; rather, we found that Ap4A is efficiently removed in a constitutive, nonregulated manner. Several fold perturbations in Ap4A concentrations have no effect, yet accumulation at very high levels is toxic due to disturbance of zinc homeostasis, and also because Ap4A's structural overlap with ATP can result in spurious binding and inactivation of ATP-binding proteins. Overall, Ap4A met all criteria for a damage metabolite. While we do not exclude any role in signaling, our results indicate that the damage metabolite option should be considered as the null hypothesis when examining Ap4A and other metabolites whose levels change upon stress.

Dry Eye Treatment Based on Contact Lens Drug Delivery: A Review

Dry eye disease affects a substantial segment of the word population with increasing frequency. It is a multifactorial disease of the ocular surface and tear film, which causes ocular discomfort, visual disturbances, and tear instability with potential damage to the cornea and conjunctiva. Because of its multifactorial etiology, the use of different pharmacological treatment for dry eye treatment has been proposed, which include anti-inflammatory molecules, lubricants or comfort agents, and secretagogues. However, in some cases these pharmacological approaches only relieve symptoms temporarily, and consequently, eye care professionals continue to have difficulties managing dry eye. To improve pharmacological therapy that allows a more efficient and long-term action, effective ocular drug delivery of the currently available drugs for dry eye treatment is required. Contact lenses are emerging as alternative ophthalmic drugs delivery systems that provide an increased residence time of the drug at the eye, thus leading to enhanced bioavailability and more convenient and efficacious therapy. In this article, we reviewed the different techniques used to prepare contact lens-based drug delivery systems and focused on articles that describe the delivery of compounds for dry eye treatment through contact lenses.

Understanding the Presence and Roles of Ap4A (Diadenosine Tetraphosphate) in the Eye

Diadenosine tetraphosphate abbreviated Ap₄A is a naturally occurring dinucleotide, which is present in most of the ocular fluids. Due to its intrinsic resistance to enzyme degradation compared to mononucleotides, this molecule can exhibit profound actions on ocular tissues, including the ocular surface, ciliary body, trabecular meshwork, and probably the retina. The actions of Ap₄A are mostly carried out by P2Y₂ receptors, but the participation of P2X2 and P2Y₆ in processes such as the regulation of intraocular pressure (IOP), together with the P2Y₂, is pivotal. Beyond the physiological role, this dinucleotide can present on the ocular surface keeping a right production of tear secretion or regulating IOP. It is important to note that exogenous application of Ap₄A to cells or animal models can significantly modify pathophysiological conditions and thus is an attractive therapeutic molecule. The ocular location where Ap₄A actions have not been fully elucidated is in the retina. Although some analogues show interesting actions on pathological situations such as retinal detachment, little is known about the real effect of this dinucleotide, this being one of the challenges that require pursuing in the near future.

Increase in tear film lipid layer thickness after instillation of 3% diquafosol ophthalmic solution in healthy human eyes

PURPOSE

To evaluate the effect of 3% diquafosol ophthalmic solution on tear film lipid layer thickness (LLT) in normal human eyes by tear interferometry.

METHODS

Forty-seven healthy men (mean age of 42.4 years) randomly received one drop of artificial tears in one eye and one drop of 3% diquafosol ophthalmic solution in the other. LLT of each eye was quantified by tear interferometry before and 15, 30, and 60 min after instillation. Ocular symptoms were assessed before and 30 min after instillation. Baseline LLT, tear film breakup time (TBUT), meibomian gland area (meiboscore) of the upper and lower eyelids, and Schirmer test value were evaluated on a different day before treatment.

RESULTS

LLT before and 15, 30, and 60 min after diquafosol instillation was 62.3 ± 31.1, 77.0 ± 39.5, 79.3 ± 40.5, and 77.7 ± 43.6 nm, respectively, with the diquafosol-induced increase in LLT being significant at each time point. Artificial tears did not result in a significant increase in LLT. TBUT (ρ = 0.32, P = 0.026), meiboscore of the lower eyelid (ρ = -0.33, P = 0.022), and the maximum difference in LLT between before and after diquafosol instillation (ΔLLT_max) (ρ = 0.35, P = 0.016) were significantly correlated with baseline LLT in the diquafosol group. Age was not significantly related to baseline LLT or ΔLLT_max. Symptoms did not differ significantly between the two groups.

CONCLUSIONS

Topical instillation of 3% diquafosol ophthalmic solution increased LLT for up to 60 min in normal human eyes regardless of age.

Diadenosine tetraphosphate as a potential therapeutic nucleotide to treat glaucoma

Glaucoma is a neurodegenerative disease that produces blindness. The main factor associated with this disease is an abnormally elevated intraocular pressure (IOP). To date, some attempts have been made to demonstrate the role of nucleotides modulating IOP, but never in a model of glaucoma. The DBA/2J mouse is an animal that develops the pathology spontaneously, starting from the typical rise in IOP at 9 months of age. Using this animal model, together with a control mouse, C57BL/6J, it has been possible to monitor the elevation in IOP in the glaucomatous mice and to check the ability of the dinucleotide diadenosine tetraphosphate AKA Ap₄A to reduce IOP. The topical application of Ap₄A when IOP is maximal (9-12 months) reduced IOP 30.6 ± 6.6% in the DBA/2J and 17.9 ± 4.0% in the C57BL/6J mice. Concentration response curves in both animal strains produced similar pD2 values; these being 4.9 ± 0.5 and 5.1 ± 0.4 for the normotensive C57BL/6J and the glaucomatous DBA/2J respectively. Antagonist studies showed differences between the control and the glaucomatous animals. In particular, the main receptor reducing IOP in the control animal was the P2Y₁ receptor and in the glaucomatous model the P2Y₆, although the participation of other P2 receptors cannot be ruled out. The long-term effect of Ap₄A applied three times a week for 3 months showed a clear stop in the elevation of IOP in the glaucomatous model, thus indicating the possibility of using Ap₄A as an effective compound for the treatment of glaucoma.

Purinergic Signaling in Corneal Wound Healing: A Tale of 2 Receptors

Nucleotide release and purinergic signaling make up the earliest response to corneal injury and are vital for proper wound healing. In this study, we review the importance of nucleotide release in the injury response and focus on the contribution of 2 receptors that mediate purinergic signaling, P2Y2 and P2X7. These receptors mediate the early response to injury and activate downstream signaling to promote cytoskeletal rearrangement and cell migration. The contribution of corneal nerves to the purinergic injury response is also discussed. Finally, we look at implications of altered purinergic signaling in diabetic wound healing and important targets for future research.

The role of dinucleoside polyphosphates on the ocular surface and other eye structures

Dinucleoside polyphosphates comprises a group of dinucleotides formed by two nucleosides linked by a variable number of phosphates, abbreviated NpnN (where n represents the number of phosphates). These compounds are naturally occurring substances present in tears, aqueous humour and in the retina. As the consequence of their presence, these dinucleotides contribute to many ocular physiological processes. On the ocular surface, dinucleoside polyphosphates can stimulate tear secretion, mucin release from goblet cells and they help epithelial wound healing by accelerating cell migration rate. These dinucleotides can also stimulate the presence of proteins known to protect the ocular surface against microorganisms, such as lysozyme and lactoferrin. One of the latest discoveries is the ability of some dinucleotides to facilitate the paracellular way on the cornea, therefore allowing the delivery of compounds, such as antiglaucomatous ones, more easily within the eye. The compound Ap₄A has been described being abnormally elevated in patient's tears suffering of dry eye, Sjogren syndrome, congenital aniridia, or after refractive surgery, suggesting this molecule as biomarker for dry eye condition. At the intraocular level, some diadenosine polyphosphates are abnormally elevated in glaucoma patients, and this can be related to the stimulation of a P2Y₂ receptor that increases the chloride efflux and water movement in the ciliary epithelium. In the retina, the dinucleotide dCp₄U, has been proven to be useful to help in the recovery of retinal detachments. Altogether, dinucleoside polyphosphates are a group of compounds which present relevant physiological actions but which also can perform promising therapeutic benefits.

The influence of rigid gas permeable lens wear on the concentrations of dinucleotides in tears and the effect on dry eye signs and symptoms in keratoconus

PURPOSE

To evaluate the signs and symptoms of dry eye and dinucleotide secretion in tears of keratoconus patients (KC) and the potential effect of rigid gas permeable (RGP) contact lens wear.

METHODS

Twenty-three KC patients and forty control subjects were enrolled in this study. Signs of dry eye including tear volume, tear stability and corneal staining along with symptoms were assessed using the McMonnies questionnaire. Tears were collected using Schirmer strips, and dinucleotide concentrations in collected tears measured using high pressure liquid chromatography. Values obtained in KC and controls were compared. The effect of contact lens wear in KC was also assessed.

RESULTS

KC eyes showed a significantly lower tear volume compared to controls, shorter tear break up time (TBUT), higher corneal staining and higher McMonnies dry eye questionnaire scores (p<0.05). When compared with non-wearers, KC contact lens wearers showed significantly higher symptoms, lower Schirmer and TBUT values (p<0.05). Concentration of Ap4A (0.695±0.304μM vs. 0.185±0.178μM) and Ap5A (0.132±0.128μM vs. 0.045±0.036μM) were higher in KC compared to controls (p<0.001) and only Ap4A was statistically higher in RGP wearers compared to non-wearers (0.794±0.478μM vs. 0.417±0.313μM) (p<0.05).

CONCLUSION

Signs and symptoms of dry eye as well as concentrations of Ap4A and Ap5A were markedly increased in KC patients compared to controls. Moreover, Ap4A and symptoms of dry eye were statistically higher in RGP wearers compared to non-wearers. This seems to indicate that factors such as RGP contact lens wear might exacerbate the clinical condition of dry eye.

Lactoferrin Levels in Tears are Increased by the Topical Application of Diadenosine Tetraphosphate

PURPOSE

This study was undertaken to determine the effect of the topical application of diadenosine tetraphosphate on lactoferrin levels in rabbit tears.

METHODS

Diadenosine tetraphosphate was topically instilled in a single-dose, tear samples were collected by micropipette and lactoferrin was measured by Enzyme-Linked ImmunoSorbent Assay (ELISA).

RESULTS

The concentration of lactoferrin in rabbit tears was significantly increased 1 h after diadenosine tetraphosphate application, remaining elevated for 3 h more. This effect was blocked by P2 receptors antagonists.

CONCLUSIONS

Topical application of diadenosine tetraphosphate stimulates the secretion of lactoferrin in rabbit tears through P2 receptor activation.

Diadenosine polyphosphates in the tears of aniridia patients

PURPOSE

To quantify diadenosine polyphosphate levels in tears of congenital aniridia patients to estimate the ocular surface changes associated with congenital aniridia compared to normal individuals.

METHODS

Fifteen patients diagnosed with congenital aniridia and a control group of forty volunteers were studied. Tears were collected to quantify the levels of diadenosine polyphosphates Ap4 A and Ap5 A by high-performance liquid chromatography (H.P.L.C). Break-up time (BUT), corneal staining, McMonnies questionnaire and the Schirmer I test were applied to both groups.

RESULTS

Dinucleotides in congenital aniridia patients were higher than in control subjects. For the congenital aniridia group, under 15 years old, the values were 0.77 ± 0.01 μm and 0.17 ± 0.02 μm for Ap4 A and Ap5 A, respectively. The group aged from 15 to 40 years old provided concentrations of 4.37 ± 0.97 μm and 0.46 ± 0.05 μm for Ap4 A and Ap5 A, the group over 40 gave concentrations of 11.17 ± 5.53 μm and 0.68 ± 0.17 μm for Ap4 A and Ap5 A. Dinucleotide concentrations increased with age, being statistically significant different among the three age groups (p < 0.05). Congenital aniridia patients showed a normal tear secretion and no dry eye McMonnies scores, except for the group over 40 years old. BUT values decreased and corneal staining increased with age and correlated with the levels of diadenosine polyphosphates (p < 0.05).

CONCLUSIONS

The levels of dinucleotides in tears increase in aniridia patients compared with healthy subjects, and they seem to be related with the progression of corneal disorders in aniridia patients, both of which increase with ageing.

Diadenosine tetraphosphate contributes to carbachol-induced tear secretion

The purpose of this study is to investigate if the cholinergic stimulation by carbachol on tear secretion is a direct process or if it is also mediated by purinergic mechanisms. Experiments were performed in New Zealand male rabbits. The amount of tear secretion was measured with Schirmer's test and then analyzed by a HPLC protocol in order to study the nucleotide levels. Animal eyes were instilled with carbachol (a cholinergic agonist), pirenzepine, gallamine and 4-DAMP (muscarinic antagonists), PPADS, suramin and reactive blue 2 (purinergic antagonists), and a P2Y2 receptor small interfering RNA (siRNA). Tear secretion increased with the instillation of carbachol, approximately 84 % over control values 20 min after the instillation and so did Ap4A and ATP release. When we applied carbachol in the presence of muscarinic antagonists, tear volume only increased to 4 % with atropine, 12 % in the case of pirenzepine, 3 % with gallamine, and 8 % with 4-DAMP. In the presence of carbachol and purinergic antagonists, tear secretion was increased to 12 % (all values compared to basal tear secretion). By analyzing tear secretion induced with carbachol in presence of a P2Y2 receptor siRNA, we found that tear secretion was diminished to 60 %. The inhibition of tear secretion in the presence of carbachol and purinergic antagonists or P2Y2 siRNA occurred with no apparent change in the tear amount of Ap4A. These experiments demonstrated the participation of Ap4A in lacrimal secretion process.

Syntheses of stable, synthetic diadenosine polyphosphate analogues using recombinant histidine-tagged lysyl tRNA synthetase (LysU)

Recombinant Escherichia coli lysyl-tRNA synthase (LysU) has been previously utilised in the production of stabile, synthetic diadenosine polyphosphate (ApnA) analogues. Here we report on the extended use of a new recombinant histidine residue-tagged LysU as a tool for highly controlled phosphatephosphate bond formation between nucleotides, avoiding the need for complex protecting group chemistries. Resulting high yielding tandem LysU-based biosynthetic-synthetic/synthetic-biosynthetic strategies emerge for the preparation of varieties of ApnA analogues directly from inexpensive natural nucleotides and nucleosides. Analogues so formed make a useful small library with which to probe ApnA activities in vitro and in vivo leading to the discovery of new, potentially potent biopharmaceuticals active against chronic pain and other chronic, high-burden disease states.

Diadenosine polyphosphates after laser in situ keratomileusis and photorefractive keratectomy refractive techniques

PURPOSE

To study the concentrations of diadenosine polyphosphates in the ocular surface after PRK and LASIK.

METHODS

Sixty-one patients (30 males and 31 females) with ages ranging from 20 to 63 (34.04 ± 9.13 years) were recruited in Balear Institute of Ophthalmology, Palma de Mallorca, Spain. LASIK was performed in 92 eyes of 46 patients and PRK in 25 eyes of 15 patients. Variations in the levels of diadenosine polyphosphate (Ap4 A and Ap5 A), Schirmer I (Jones test), TBUT, corneal staining together with the Dry Eye Questionnaire to evaluate discomfort and dryness were studied. All tests were performed at the preoperative visit and at 1-day, 2-week, 1-month and 3-month postoperative visits.

RESULTS

Ap4 A showed a 5 and 3.5 fold increase at the 1-day visit for LASIK and PRK, respectively. LASIK patients continued having higher statistically significant concentrations (p = 0.01) all over the follow-up. Ap5 A showed no significant differences at any visit. Tear volume decreased during the 3 months in LASIK. The PRK cases had a normal volume at 1 month. TBUT in LASIK increased at the 1-day visit (p = 0,002) and decreased from the 2 weeks onwards and for the PRK, decreased by a 35% at the 1-day visit and kept reduced for a month. Discomfort only increased at the 1-day visit (p = 0.007). Dryness frequency was similar in all visits.

CONCLUSIONS

Ap4 A levels only are increased in refractive surgery patients during the first day after the surgery. This increasing suggests that Ap4 A may help accelerating the healing process.

P2Y2 receptor agonists for the treatment of dry eye disease: a review

Recent advances in the understanding of dry eye disease (DED) have revealed previously unexplored targets for drug therapy. One of these drugs is diquafosol, a uridine nucleotide analog that is an agonist of the P2Y2 receptor. Several randomized controlled trials have demonstrated that the application of topical diquafosol significantly improves objective markers of DED such as corneal and conjunctival fluorescein staining and, in some studies, tear film break-up time and Schirmer test scores. However, this has been accompanied by only partial improvement in patient symptoms. Although evidence from the literature is still relatively limited, early studies have suggested that diquafosol has a role in the management of DED. Additional studies would be helpful to delineate how different subgroups of DED respond to diquafosol. The therapeutic combination of diquafosol with other topical agents also warrants further investigation.

In vitro and in vivo delivery of the secretagogue diadenosine tetraphosphate from conventional and silicone hydrogel soft contact lenses

Purpose

To evaluate the possible use of soft contact lenses (CL) to improve the secretagogue role of diadenosine tetraphosphate (Ap4A) promoting tear secretion.

Methods

Two conventional hydrogel CL (Omafilcon A and Ocufilcon D) and two silicone hydrogel (SiH) CL (Comfilcon A and Balafilcon A) were used. Ap4A was loaded into the lenses by soaking in a 1 mM Ap4A solution during 12 h. In vitro experiments were performed by placing the lenses in multi-wells during 2 h containing 1 ml of ultrapure water. 100 μl aliquots were taken at time zero and every minute for the first 10 min, and then every 15 min. In vivo experiments were performed in New Zealand rabbits and both the dinucleotide release from SiH and tear secretion were measured by means of Schirmer strips and high-pressure liquid chromatography (HPLC) analysis.

Results

Ap4A in vitro release experiments in hydrogel CL presented a release time 50 (RT50) of 3.9 ± 0.2 min and 3.1 ± 0.1 min for the non-ionic and the ionic CL, respectively. SiH CL released also Ap4A with RT50 values of 5.1 ± 0.1 min for the non-ionic and 2.7 ± 0.1 min for the ionic CL. In vivo experiments with SiH CL showed RT50 values of 9.3 ± 0.2 min and 8.5 ± 0.2 min for the non-ionic and the ionic respectively. The non-ionic lens Ap4A release was able to induce tear secretion above baseline tear levels for almost 360 min.

Conclusion

The delivery of Ap4A is slower and the effect lasts longer with non-ionic lenses than ionic lenses.

Diadenosine polyphosphates release by human corneal epithelium

Diadenosine polyphosphates are a type of dinucleotides that have been detected in rabbit and human tears. However, their origin and their mechanism of release have not been fully elucidated. In this work we investigated whether the dinucleotides Ap4A and Ap5A can be released from human corneal epithelia as a consequence of shear stress stimuli. In in vitro experiments, concentrations of Ap4A and Ap5A before mechanical stimulus of stratified human corneal epithelial cells were 3.18 ± 0.43 nM and 0.81 ± 0.13 nM, respectively. After shear stimulation, concentrations significantly increased to 12.01 ± 2.19 nM for Ap4A and 2.83 ± 0.41 nM for Ap5A. No significant differences in lactate dehydrogenase activity were detected between non-stimulated stratified human corneal epithelial cells and cells exposed to mechanical shear-stress, indicating that the rise of dinucleotide levels was not due to cell lysis. In in vivo experiments, individuals subjected to a rise in blinking frequency showed a significant increase of Ap4A (∼25-fold when experiment was performed without anaesthetic and 75-fold with anaesthetic) and Ap5A concentration in tears (∼50-fold when experiment was performed without anaesthetic and 125-fold with anaesthetic). Shear-stress stimuli induces Ap4A and Ap5A release from human corneal epithelium, thus explaining the origin of these relevant compounds for the ocular surface biochemistry and physiology.

Nucleotides in the eye: focus on functional aspects and therapeutic perspectives

The presence and activity of nucleotides and dinucleotides in the physiology of most, if not all, organisms, from bacteria to humans, have been recognized by the scientific community, and the eye is no exception. Nucleotides in the dynamic fluids interact with many ocular structures, such as the tears and aqueous humor. Moreover, high concentrations of nucleotides in these secretions may reflect disease states such as dry eye and glaucoma. Apart from the nucleotide concentration in these fluids, P2 purinergic receptors have been described on the ocular surface (cornea and conjunctiva), anterior pole (ciliary body, trabecular meshwork), and posterior pole (retina). P2X and P2Y purinergic receptors are essential in maintaining the homeostasis of ocular processes, such as tear secretion, aqueous humor production, or retinal modulation. When they are functioning properly, they allow the eye to do its job (to see), but in some cases, a lack or an excess of nucleotides or a malfunction in the corresponding purinergic receptors leads to disease. This Perspective is focused on the nucleotides and dinucleotides and the P2 purinergic receptors in the eye and how they contribute to normal and disease states. We also emphasize the action of nucleotides and their receptors and antagonists as potential therapeutic agents.

Regional differences in rat conjunctival ion transport activities

Active ion transport and coupled osmotic water flow are essential to maintain ocular surface health. We investigated regional differences in the ion transport activities of the rat conjunctivas and compared these activities with those of cornea and lacrimal gland. The epithelial sodium channel (ENaC), sodium/glucose cotransporter 1 (Slc5a1), transmembrane protein 16 (Tmem16a, b, f, and g), cystic fibrosis transmembrane conductance regulator (Cftr), and mucin (Muc4, 5ac, and 5b) mRNA expression was characterized by RT-PCR. ENaC proteins were measured by Western blot. Prespecified regions (palpebral, fornical, and bulbar) of freshly isolated conjunctival tissues and cell cultures were studied electrophysiologically with Ussing chambers. The transepithelial electrical potential difference (PD) of the ocular surface was also measured in vivo. The effect of amiloride and UTP on the tear volume was evaluated in lacrimal gland excised rats. All selected genes were detected but with different expression patterns. We detected αENaC protein in all tissues, βENaC in palpebral and fornical conjunctiva, and γENaC in all tissues except lacrimal glands. Electrophysiological studies of conjunctival tissues and cell cultures identified functional ENaC, SLC5A1, CFTR, and TMEM16. Fornical conjunctiva exhibited the most active ion transport under basal conditions amongst conjunctival regions. PD measurements confirmed functional ENaC-mediated Na(+) transport on the ocular surface. Amiloride and UTP increased tear volume in lacrimal gland excised rats. This study demonstrated that the different regions of the conjunctiva exhibited a spectrum of ion transport activities. Understanding the specific functions of distinct regions of the conjunctiva may foster a better understanding of the physiology maintaining hydration of the ocular surface.

Effect of diinosine polyphosphates on intraocular pressure in normotensive rabbits

The ability of diinosine polyphosphates, diinosine triphosphate (Ip(3)I), diinosine tetraphosphate (Ip(4)I) and diinosine pentaphosphate (Ip(5)I) to modify intraocular pressure in normotensive New Zealand white rabbits was tested. Ip(5)I produced increase in intraocular pressure, while Ip(3)I and Ip(4)I produced a decrease. Ip(4)I was the most effective reducing intraocular pressure inducing a maximal decrease of intraocular pressure to 74.2 ± 2.5% compared with the control value. Dose-response analysis demonstrated a concentration dependent pattern which presented a pD(2) value of 6.19 ± 0.18, equivalent to an EC(50) of 0.63 μM. Regarding the underlying mechanism used by Ip(4)I to reduce intraocular pressure, studies with agonists and antagonists revealed that Ip(4)I reduces intraocular pressure via P2Y receptors in the eye. We suggest that topical application of Ip(4)I to the cornea has therapeutic potential for lowering intraocular pressure, a major risk factor for glaucoma.

Phospholipase C/Protein Kinase C pathway is essential for corneal re-epithelialization induced by Ap(4)A

PURPOSE

We have previously demonstrated the importance of P2Y(2) receptors in the corneal re-epithelialization effect triggered by diadenosine tetraphosphate (Ap(4)A). In addition, we have also shown the ERK1/2 and ROCK-I activation in Ap(4)A-wound repair response. Phospholipase C/Protein Kinase C (PLC/PKC) pathway activation has been suggested as a molecular mechanism of growth factors-modulated corneal cell migration and P2Y(2) agonists. Hence, the aim of this study is to investigate the role of PLC/PKC cascade in the modification of re-epithelialization rate triggered by Ap(4)A in an established corneal epithelial cell line (Statens Seruminstitut rabbit cornea [SIRC] cells).

METHODS

In wounded confluent SIRC cell monolayers and in the presence or absence of Ap(4)A 100 μM, a group of PLC/PKC inhibitors (U73122 3 μM, Staurosporine 1 nM and Bisindolylmaleimide-I 10 μM) and activator (PDBU 1 μM) were assayed and the migration rate was evaluated. Also, the activation of ERK1/2 and ROCK-I was examined by Western blot assay after treatment with or without Ap(4)A, U73122, Staurosporine, Bisindolylmaleimide-I and PDBU.

RESULTS

Pre-treatment of wounded SIRC cells with PLC/PKC inhibitors significantly diminished the Ap(4)A-stimulated cell migration rate. Furthermore, PLC/PKC inhibitors also reduced ERK1/2 phosphorylation and ROCK-I activation triggered by Ap(4)A.

CONCLUSIONS

The present study shows the involvement of PLC/PKC pathway in the activation of ERK1/2 and ROCK-I downstream signal transduction pathways stimulated by Ap(4)A/P2Y(2) receptor during corneal epithelial wound repair.

Glaucoma patients present increased levels of diadenosine tetraphosphate, Ap(4)A, in the aqueous humour

Previous studies have shown the presence of diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) in the aqueous humour (AH) of different species. When topically applied to the rabbit cornea, Ap(4)A decreased IOP while Ap(5)A increased it. Here we study the presence of dinucleoside polyphosphates in the AH from human patients with or without glaucoma. AH was obtained at the time of cataract surgery from patients with (n=16) or without (n=10) primary open-angle glaucoma. AH (0.1-0.2 ml) was collected at the beginning of surgery through a corneal paracentesis and immediately cooled in liquid nitrogen, kept frozen and protected from light. AH aliquots were analyzed by HPLC for the presence of Ap(4)A and Ap(5)A. Both, Ap(4)A and Ap(5)A were detected in the AH of both experimental groups. No significant differences were found for Ap(5)A. In contrast, Ap(4)A levels were increased by ∼15-fold in the AH from glaucomatous eyes ranging from 19.5±9.2 nM in normal individuals to 286.03±30.9 nM in glaucomatous patients. In conclusion, both Ap(4)A and Ap(5)A were detected for the first time in human AH. Interestingly, glaucomatous eyes presented elevated concentrations of Ap(4)A compared to controls. The role of Ap(4)A needs to be elucidated but it may help to protect the autonomic innervation in the ciliary body/trabecular meshwork. Also, because of its higher levels in glaucoma patients it may be considered as a possible glaucoma biomarker.

Binding and catalysis of Humulus lupulus adenylate isopentenyltransferase for the synthesis of isopentenylated diadenosine polyphosphates

Various plant developmental processes involve phytohormones such as cytokinins. Isopentenyltransferase (IPT) reaction is the key rate-limiting step in cytokinin biosynthesis that transfers the isopentenyl (iP) group from dimethylallyl diphosphate to the N6-amino group of adenine. Here, a series of diadenosine polyphosphates (Ap(n)A) were screened as possible substrates of IPT, among which diadenosine tetraphosphate, diadenosine pentaphosphate and diadenosine hexaphosphate showed higher affinity than did the authentic substrates ADP and ATP. In addition, formation of mono-isopentenyl Ap(n)A and di-isopentenyl Ap(n)A was observed. Judging by the existing biosynthetic and hydrolytic systems for Ap(n)A in plants, Ap(n)A and isopentenyl-Ap(n)A may occur in the plant cells, with functional importance.

Treatment of dry eye syndrome with orally administered CF101: data from a phase 2 clinical trial

OBJECTIVE

To explore the safety and efficacy of CF101, an A(3) adenosine receptor agonist, in patients with moderate to severe dry eye syndrome.

DESIGN

Phase 2, multicenter, randomized, double-masked, placebo-controlled, parallel-group study.

PARTICIPANTS

Sixty-eight patients completed the study, 35 patients in the placebo group and 33 patients in the CF101 group.

INTERVENTION

Patients were treated orally with either 1 mg CF101 pills or matching vehicle-filled placebo pills, given twice daily for 12 weeks, followed by a 2-week posttreatment observation.

MAIN OUTCOME MEASURES

An improvement of more than 25% over baseline at week 12 in one of the following parameters: (1) tear break-up time (BUT); (2) superficial punctate keratitis assessed by fluorescein staining results; and (3) Schirmer tear test 1 results. Clinical laboratory safety tests, ophthalmic examinations, intraocular pressure (IOP) measurements, electrocardiographic evaluations, vital sign measurements, and monitoring of adverse events.

RESULTS

A statistically significant increase in the proportion of patients who achieved more than 25% improvement in the corneal staining and in the clearance of corneal staining was noted between the CF101-treated group and the placebo group. Treatment with CF101 resulted in a statistically significant improvement in the mean change from baseline at week 12 of the corneal staining, BUT, and tear meniscus (TM) height in the CF101-treated group. CF101 was well tolerated and exhibited an excellent safety profile with no serious adverse events. A statistically significant decrease from baseline was observed in the IOP of the CF101-treated group in comparison with the placebo group.

CONCLUSIONS

CF101, given orally, induced a statistically significant improvement in the corneal staining and an improvement in the BUT and TM in patients with moderate to severe dry eye syndrome. The drug was very well tolerated. These data and the anti-inflammatory characteristic of CF101 support further study of the drug as a potential treatment for the signs and symptoms of dry eye syndrome.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

A novel insulin secretagogue based on a dinucleoside polyphosphate scaffold

Dinucleoside polyphosphates exert their physiological effects via P2 receptors (P2Rs). They are attractive drug candidates, as they offer better stability and specificity compared to nucleotides, the most common P2 receptor ligands. The activation of pancreatic P2Y receptors by nucleotides increases insulin secretion. Therefore, in the current study, dinucleoside polyphosphate analogues (di-(2-MeS)-adenosine-5',5''-P(1),P(4),alpha,beta-methylene-tetraphosphate), 8, (di-(2-MeS)-adenosine-5',5''-P(1),P(4),beta,gamma-methylene-tetraphosphate), 9, and di-(2-MeS)-adenosine-5',5''-P(1),P(3),alpha,beta-methylene triphosphate, 10, were developed as potential insulin secretagogues. Analogues 8 and 9 were found to be agonists of the P2Y(1)R with EC(50) values of 0.42 and 0.46 microM, respectively, whereas analogue 10 had no activity. Analogues 8-10 were found to be completely resistant to hydrolysis by alkaline phosphatase over 3 h at 37 degrees C. Analogue 8 also was found to be 2.5-fold more stable in human blood serum than ATP, with a half-life of 12.1 h. Analogue 8 administration in rats caused a decrease in a blood glucose load from 155 mg/dL to ca. 100 mg/dL and increased blood insulin levels 4-fold as compared to basal levels. In addition, analogue 8 reduced a blood glucose load to normal values (80-110 mg/dL), unlike the commonly prescribed glibenclamide, which reduced glucose levels below normal values (60 mg/dL). These findings suggest that analogue 8 may prove to be an effective and safe treatment for type 2 diabetes.

Adenine nucleotide effect on intraocular pressure: Involvement of the parasympathetic nervous system

Nucleotides are present in the aqueous humor possibly exerting physiological effects on intraocular pressure (IOP). To determine the effect of nucleotides such as ATP and its related derivatives on IOP, New Zealand white rabbits were used. IOP was measured in rabbits treated topically either with saline (control) or with a single dose (10 microg/microL) of adenine nucleotides (ATP, 2-meS-ATP, ATP-gamma-S, alpha,beta-meADP, alpha,beta-meATP and beta,gamma-meATP). Those nucleotides reducing IOP (alpha,beta-meATP and beta,gamma-meATP) were then tested in concentrations ranging from 1 to 100 microg/microL to obtain the IC(50) value. Several antagonists for the P2 and adenosine A1 receptors (all at 10 microg/microL) were assayed 30 min before the application of the hypotensive nucleotide beta,gamma-meATP. To see whether the nucleotide was acting directly on the structures involved in aqueous humor dynamics or on the autonomic nerves controlling IOP, animal denervation and sympathetic (yohimbine and ICI-118,551 at 10 microg/microL) and parasympathetic (atropine and hexametonium at 10 microg/microL) receptors' antagonists were used 30 min before the instillation of beta,gamma-meATP. alpha,beta-meATP and beta,gamma-meATP decreased IOP to 60% of control value (basal IOP=23.2+/-1.3 mmHg), with IC(50) of 1.59+/-0.21 microg/microLand 0.56+/-0.62 microg/microL, which corresponds to 3mM and 1mM respectively. Denervation completely abolished the effect of beta,gamma-meATP. Sympathetic antagonists did not modify the hypotensive effect of beta,gamma-meATP, but parasympathetic antagonists were able to abolish it. Among the series of adenine nucleotide tested, alpha,beta-meATP and beta,gamma-meATP presented hypotensive actions on IOP. beta,gamma-meATP seems to stimulate cholinergic terminals being its final effect the IOP reduction. Therefore, these two nucleotides are interesting pharmacological tools for those pathologies related with high intraocular pressure.

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Understanding and analyzing meibomian lipids--a review

PURPOSE

This review is intended to bring to the informed reader the current state of knowledge about meibomian lipids and the art for analyzing them.

METHODS

At the forefront of any endeavor, there are controversies, and these, along with future directions in the field, are brought to the reader's attention.

RESULTS

Function and anatomy of meibomian glands are briefly covered, giving insight into possible mechanisms for secretory controls. Anatomically, some anomalies in meibomian gland distribution of different species, such as whales versus dolphins, are presented, and, for the first time, the structure of the meibomian glands in a selection of marsupials is presented. In attempting to make the literature more accessible, lipid structure and nomenclature are described, and these structures are related to their possible effects on the physicochemical properties of meibomian lipids. The advantages and disadvantages of various collection and storage techniques are described, as well as how gas chromatography and combined HPLC and mass spectrometry coupled with fragmentation are currently enabling us to determine the nature of the lipids in very small samples.

CONCLUSIONS

This review extends to discussing the lipids in tears (as opposed to meibomian gland lipids) and briefly highlights new thoughts about the interactions between proteins of the tear film and meibomian lipids. A model that includes proteins in the outer layer of the tear film is also presented. This model is currently being critically analyzed by the ocular community. It concludes briefly by highlighting possible further areas of research in this area.

Dinucleoside polyphosphates in the eye: from physiology to therapeutics

Diadenosine polyphosphates are a family of dinucleotides with emerging biochemical, physiological, pharmacological and therapeutic properties in the eye and other tissues. These compounds are formed by two adenosine moieties linked by their ribose 5'-ends to a variable number of phosphates. Diadenosine polyphosphates are present as active components of ocular secretions such as tears and aqueous humour and they can activate P2 purinergic receptors present on the ocular surface, anterior segment and retina. Both metabotropic and ionotropic actions mediated by P2Y and P2X receptors, respectively are responsible for the control of processes such as induction of tear secretion, lysozyme production or acceleration of corneal wound healing. Inside the eye the dinucleotide Ap(4)A can reduce intraocular pressure by acting on P2Y(1) receptors present in trabecular meshwork cells and on P2X(2) receptors present on the cholinergic terminals located in the ciliary muscle. In the retina, derivatives of diadenosine polyphosphates can improve the re-absorption of fluids in retinal detachment. Altogether, diadenosine polyphosphates are not only dinucleotides with roles in the physiology of the eye but it is also possible that their properties may serve to help in the treatment of some ocular pathologies.

Effect of diadenosine polyphosphates in achondroplasic chondrocytes: inhibitory effect of Ap4A on FGF9 induced MAPK cascade

Achondroplasia is characterised by a mutation in the gene that encodes for the FGF receptor type 3 (FGFR3), producing a hyperactivation of this receptor and a subsequent increase in MAPK activity. We have tested the ability of nucleotides to decrease the activation of MAPK in chondrocytes with achondroplasic FGFR3 receptor. Diadenosine tetraphosphate, Ap(4)A, reduced the phosphorylation of pERK1/2 triggered by FGF9 (38% reduction). Ap(4)A diminished the expression of achondroplasic FGFR3 receptor (65% reduction), stimulating FGFR3 receptor degradation. The action of Ap(4)A seems to be mediated by a dinucleotide receptor rather than by any other ATP receptor.

Melatonin potentiates tear secretion induced by diadenosine tetraphosphate in the rabbit

Diadenosine tetraphosphate (Ap(4)A, 0.03 nmol) applied topically to the cornea of New Zealand white rabbits, evoked an increase in tear secretion of 9.7 +/- 2.60% (N=7). Melatonin (1 nmol) had no significant effect. Application of Ap(4)A in combination with melatonin, evoked a significantly greater increase in tear secretion of 34.2 +/- 5.8% (N=11). This potentiating effect of melatonin was blocked by pretreating the cornea with a topical application of the melatonin receptor antagonist, luzindole (240 nmol). Melatonin combined with Ap(4)A may be useful for treating dry eye conditions.

Effects of dinucleoside polyphosphates on trabecular meshwork cells and aqueous humor outflow facility

The most important risk factor for the development of glaucoma is elevated intraocular pressure (IOP). Hypotensive drugs decrease IOP, preventing optic nerve damage and further vision loss. The balance between aqueous humor (AH) production and drainage determines IOP, and problems in AH outflow pathways are associated with open-angle glaucoma development. Previous studies have shown the presence of diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) in the AH. Topic application of Ap(4)A to the cornea decreased IOP, whereas Ap(5)A increased it. Because dinucleoside polyphosphates stimulate P2Y purinergic receptors, we studied their presence in trabecular meshwork (TM) cells. Additionally, the effects of diadenosine polyphosphates (Ap(n)As; n = 3-5) and Up(4)U (P(1),P(4)-(diuridine 5')-tetraphosphate; INS365) in outflow facility were tested. P2Y(1), P2Y(2), and P2Y(4) receptors were detected in TM cells by Western blot and immunocytochemistry. In TM cells, Ap(3)A, Ap(4)A, and Ap(5)A induced discrete intracellular calcium concentration ([Ca(2+)](i)) mobilizations compared with higher and more sustained [Ca(2+)](i) mobilizations after Up(4)U application. In bovine ocular anterior segments perfused at constant pressure, 1 microM Ap(3)A or Ap(4)A increased outflow facility, whereas Up(4)U or Ap(5)A did not modify it. 2-MeSADP, a selective P2Y(1) agonist, induced outflow facility increases similar to those obtained after Ap(3)A and Ap(4)A, and these were prevented by addition of the selective P2Y(1) receptor antagonist MRS-2179 (2'-deoxy-N(6)-methyladenosine-3',5'-diphosphate). Our results demonstrate that the hypotensive effect of Ap(4)A and other dinucleotides is mediated, at least in part, by increasing trabecular outflow facility through activation of P2Y(1) receptors. The latter would seem to be an interesting target in the development of antiglaucomatous drugs to selectively increase AH outflow.

Structure-activity relationships of dinucleotides: Potent and selective agonists of P2Y receptors

Dinucleoside polyphosphates act as agonists on purinergic P2Y receptors to mediate a variety of cellular processes. Symmetrical, naturally occurring purine dinucleotides are found in most living cells and their actions are generally known. Unsymmetrical purine dinucleotides and all pyrimidine containing dinucleotides, however, are not as common and therefore their actions are not well understood. To carry out a thorough examination of the activities and specificities of these dinucleotides, a robust method of synthesis was developed to allow manipulation of either nucleoside of the dinucleotide as well as the phosphate chain lengths. Adenosine containing dinucleotides exhibit some level of activity on P2Y₁ while uridine containing dinucleotides have some level of agonist response on P2Y₂ and P2Y₆. The length of the linking phosphate chain determines a different specificity; diphosphates are most accurately mimicked by dinucleoside triphosphates and triphosphates most resemble dinucleoside tetraphosphates. The pharmacological activities and relative metabolic stabilities of these dinucleotides are reported with their potential therapeutic applications being discussed.

Cellular injury induces activation of MAPK via P2Y receptors

Wound healing is a complex process that involves cell communication, migration, proliferation, and changes in gene expression. One of the first events after injury is the rapid release of Ca(2+) that propagates as a wave to neighboring cells (Klepeis et al. [2001]: J. Cell. Sci. 114:4185-4195). Our goal was to examine the signaling events induced by cellular injury and identify extracellular molecules that induce the activation of extracellular signal responsive kinase (ERK) (p42/44). In this study we demonstrated that injury induced ERK1/2 activation occurred within 2 min and was negligible by 15 min. Treatment of unwounded cells with wound media caused activation of ERK that could be inhibited by apyrase III. Stimulation with epidermal growth factor (EGF) did not mimic the injury response and it was not detected in the wound media. To identify the active component, size fractionation was performed and factor(s) less than 3 kDa that induced the release of Ca(2+) and activation of ERK1/2 were identified. Activity was not altered by heat denaturation, incubation with proteinase K but it was lost by treatment with apyrase. Adenosine triphosphate (ATP), uridine triphosphate (UTP), adenosine diphosphate (ADP), and uridine diphosphate (UDP) promoted activation by 2 min with similar profiles as that generated by injury. Preincubation with phospholipase C inhibitor, U73122, inhibited activation that was induced by injury and/or nucleotides. Lack of activation by alpha-beta-methylATP (alpha, beta-MeATP) and beta-gamma-methylATP (beta, gamma-MeATP) to purinergic (P)2X receptors further indicated that activation occurs via P2Y and not P2X purinergic receptors. These results indicate that injury-induced activation of ERK1/2 is mediated by a P2Y signaling pathway.

UTP and diadenosine tetraphosphate accelerate wound healing in the rabbit cornea

Nucleotides are naturally occurring substances present in tear film that can stimulate tear secretion in animals and humans. We investigated whether certain nucleotides can affect the rate of wound healing in the cornea of white rabbits. In the absence of any added compound, the rate of healing was 72.4 +/- 2.2 microm h(-1). Of all the tested nucleotides, UTP and Ap(4)A were the most active ones, maximally increasing the rate of healing to 121.6 +/- 3.7 and 93.7 +/- 3.2 microm h(-1), respectively. Responses to UTP were dose dependent. UTP had a pD(2) value of 8.9 +/- 0.1 (EC(50): 1.25 nM). P2 purinoceptor antagonists such as suramin and reactive blue-2, inhibited the effect of UTP indicating the involvement of P2Y receptors. Mitogen-activated protein kinase (MAPK) cascade inhibitors also abolished the effects of UTP, suggesting that P2Y receptors are coupled to the MAPK cascade, and that this is involved in controlling the rate of epithelial cell migration.

The Nudix hydrolase Ndx1 from Thermus thermophilus HB8 is a diadenosine hexaphosphate hydrolase with a novel activity

The ndx1 gene, which encodes a Nudix protein, was cloned from the extremely thermophilic bacterium Thermus thermophilus HB8. This gene encodes a 126-amino acid protein that includes the characteristic Nudix motif conserved among Nudix proteins. Ndx1 was overexpressed in Escherichia coli and purified. Ndx1 was stable up to 95 degrees C and at extreme pH. Size exclusion chromatography indicated that Ndx1 was monomeric in solution. Ndx1 specifically hydrolyzed (di)adenosine polyphosphates but not ATP or diadenosine triphosphate, and it always generated ATP as the product. Diadenosine hexaphosphate (Ap(6)A), the most preferred substrate, was hydrolyzed to produce two ATP molecules, which is a novel hydrolysis mode for Ap(6)A, with a K(m) of 1.4 microm and a k(cat) of 4.1 s(-1). These results indicate that Ndx1 is a (di)adenosine polyphosphate hydrolase. Ndx1 activity required the presence of the divalent cations Mn(2+), Mg(2+), Zn(2+), and Co(2+), whereas Ca(2+), Ni(2+), and Cu(2+) were not able to activate Ndx1. Fluoride ion inhibited Ndx1 activity via a non-competitive mechanism. Optimal activity for Ap(6)A was observed at around pH 8.0 and about 70 degrees C. We found two important residues with pK(a) values of 6.1 and 9.6 in the free enzyme and pK(a) values of 7.9 and 10.0 in the substrate-enzyme complex. Kinetic studies of proteins with amino acid substitutions suggested that Glu-46 and Glu-50 were conserved residues in the Nudix motif and were involved in catalysis. Trp-26 was likely involved in enzyme-substrate interactions based on fluorescence measurements. Based on these results, the mechanism of substrate recognition and catalysis are discussed.

Diquafosol elicits increases in net Cl- transport through P2Y2 receptor stimulation in rabbit conjunctiva

The purpose of the present study was to understand the mechanisms of action of diquafosol, a stable derivative of uridine 5'-triphosphate, on Cl(-) transport across the isolated rabbit conjunctiva. Rabbit conjunctivas were isolated and mounted in a modified Ussing chamber. Under short-circuit conditions, the effects were determined of mucosal (tear) side diquafosol application on the short-circuit current (Isc). Diquafosol rapidly and dose-dependently increased the Isc at concentrations ranging from 0.1 to 968 microM when added to the mucosal side of the conjunctiva. In the absence of the serosal Cl(-), the Isc induced by 10 microM diquafosol was substantially reduced. On the contrary, in the absence of mucosal side Na(+), the diquafosol-induced increases in Isc were unchanged. Following 45-min preincubation, the P2Y(2) antagonist suramin inhibited the diquafosol-induced increases in the Isc whereas the P2Y(1) antagonist pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid had no effect. These studies suggest that diquafosol stimulates net Cl(-) secretion from the serosal to the mucosal side via stimulation of P2Y(2) receptors in the rabbit conjunctiva.

Diquafosol tetrasodium: a novel dry eye therapy

The ophthalmic formulation of diquafosol tetrasodium (INS365), a P2Y2 receptor agonist, is targeted to treat dry eye disease through rehydration of the ocular surface. Existing pharmacological therapies for dry eye disease are limited, therefore, approval of this medication is anticipated. This review summarises key findings during development and in clinical trials including clinical effectiveness and safety. The relevance of P2Y2 receptor technology to dry eye disease and the disease process is discussed.