Barrier qualities of the mouse eye to topically applied drugs

Apoptosis inhibitor of macrophages/CD5L enhances phagocytosis in the trabecular meshwork cells and regulates ocular hypertension

The trabecular meshwork (TM) cells of the eye are important for controlling intraocular pressure (IOP) and regulating outflow resistance in the aqueous humor. TM cells can remove particles and cellular debris by phagocytosis, decreasing both outflow resistance and IOP. However, the underlying mechanisms remain unclear. Here, we investigate whether apoptosis inhibitor of macrophages (AIM), which mediates the removal of dead cells and debris in renal tubular epithelial cells, regulates the phagocytic capacity of TM cells. In vitro experiments revealed that CD36, the main receptor for AIM, colocalized with AIM in human TM cells; additionally, phagocytosis was stimulated when AIM was provided. Furthermore, in a mouse model with transient IOP elevation induced by laser iridotomy (LI), removal of accumulated iris pigment epithelial cells or debris in the TM and recovery of IOP to baseline levels were delayed in AIM-/- mice, compared with control mice. However, treatment with AIM eyedrops rescued AIM-/- mice from the elevated IOP after LI. Since AIM is a protein known to inhibit macrophage apoptosis, we additionally verified its involvement in macrophage removal of cellular debris and IOP. There were no statistically significant differences in the number of macrophages between control mice and AIM-/- mice in the TM. Additionally, we confirmed the rescue effect of the rAIM eyedrops after macrophages had been removed by clodronate liposomes. Therefore, AIM plays an important role in regulating the phagocytic capacity of TM cells, thereby affecting outflow resistance. Our results suggest that drugs targeting the phagocytic capacity of TM cells via the AIM-CD36 pathway may be used to treat glaucoma.

Adenosine receptors as promising targets for the management of ocular diseases

The ocular drug discovery arena has undergone a significant improvement in the last few years culminating in the FDA approvals of 8 new drugs. However, despite a large number of drugs, generics, and combination products available, it remains an urgent need to find breakthrough strategies and therapies for tackling ocular diseases. Targeting the adenosinergic system may represent an innovative strategy for discovering new ocular therapeutics. This review focused on the recent advance in the field and described the numerous nucleoside and non-nucleoside modulators of the four adenosine receptors (ARs) used as potential tools or clinical drug candidates.

Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A3 Adenosine Receptors: Affinity Enhancement by N6-(2-Phenylethyl) Substitution

Dopamine-derived N6-substituents, compared to N6-(2-phenylethyl), in truncated (N)-methanocarba (bicyclo[3.1.0]hexyl) adenosines favored high A3 adenosine receptor (AR) affinity/selectivity, e.g., C2-phenylethynyl analogue 15 (MRS7591, Ki = 10.9/17.8 nM, at human/mouse A3AR). 15 was a partial agonist in vitro (hA3AR, cAMP inhibition, 31% Emax; mA3AR, [35S]GTP-γ-S binding, 16% Emax) and in vivo and also antagonized hA3AR in vitro. Distal H-bonding substitutions of the N6-(2-phenylethyl) moiety particularly enhanced mA3AR affinity by polar interactions with the extracellular loops, predicted using docking and molecular dynamics simulation with newly constructed mA3AR and hA3AR homology models. These hybrid models were based on an inactive antagonist-bound hA1AR structure for the upper part of TM2 and an agonist-bound hA2AAR structure for the remaining TM portions. These species-independent A3AR-selective nucleosides are low efficacy partial agonists and novel, nuanced modulators of the A3AR, a drug target of growing interest.

Ocular Purine Receptors as Drug Targets in the Eye

Agonists and antagonists of various subtypes of G protein coupled adenosine receptors (ARs), P2Y receptors (P2YRs), and ATP-gated P2X receptor ion channels (P2XRs) are under consideration as agents for the treatment of ocular diseases, including glaucoma and dry eye. Numerous nucleoside and nonnucleoside modulators of the receptors are available as research tools and potential therapeutic molecules. Three of the 4 subtypes of ARs have been exploited with clinical candidate molecules for treatment of the eye: A1, A2A, and A3. An A1AR agonist is in clinical trials for glaucoma, A2AAR reduces neuroinflammation, A3AR protects retinal ganglion cells from apoptosis, and both A3AR agonists and antagonists had been reported to lower intraocular pressure (IOP). Extracellular concentrations of endogenous nucleotides, including dinucleoside polyphosphates, are increased in pathological states, activating P2Y and P2XRs throughout the eye. P2YR agonists, including P2Y2 and P2Y6, lower IOP. Antagonists of the P2X7R prevent the ATP-induced neuronal apoptosis in the retina. Thus, modulators of the purinome in the eye might be a source of new therapies for ocular diseases.

Nucleoside-derived antagonists to A3 adenosine receptors lower mouse intraocular pressure and act across species

The purpose of the study was to determine whether novel, selective antagonists of human A3 adenosine receptors (ARs) derived from the A3-selective agonist Cl-IB-MECA lower intraocular pressure (IOP) and act across species. IOP was measured invasively with a micropipette by the Servo-Null Micropipette System (SNMS) and by non-invasive pneumotonometry during topical drug application. Antagonist efficacy was also assayed by measuring inhibition of adenosine-triggered shrinkage of native bovine nonpigmented ciliary epithelial (NPE) cells. Five agonist-based A3AR antagonists lowered mouse IOP measured with SNMS tonometry by 3-5 mm Hg within minutes of topical application. Of the five agonist derivatives, LJ 1251 was the only antagonist to lower IOP measured by pneumotonometry. No effect was detected pneumotonometrically over 30 min following application of the other four compounds, consonant with slower, smaller responses previously measured non-invasively following topical application of A3AR agonists and the dihydropyridine A3AR antagonist MRS 1191. Latanoprost similarly lowered SNMS-measured IOP, but not IOP measured non-invasively over 30 min. Like MRS 1191, agonist-based A3AR antagonists applied to native bovine NPE cells inhibited adenosine-triggered shrinkage. In summary, the results indicate that antagonists of human A3ARs derived from the potent, selective A3 agonist Cl-IB-MECA display efficacy in mouse and bovine cells, as well. When intraocular delivery was enhanced by measuring mouse IOP invasively, five derivatives of the A3AR agonist Cl-IB-MECA lowered IOP but only one rapidly reduced IOP measured non-invasively after topical application. We conclude that derivatives of the highly-selective A3AR agonist Cl-IB-MECA can reduce IOP upon reaching their intraocular target, and that nucleoside-based derivatives are promising A3 antagonists for study in multiple animal models.

Pupillometry in mice: sex and strain-dependent phenotypes of pupillary functioning

PURPOSE

Pupillometry is used as a phenotyping assay for investigating pupil responses in normal and genetically engineered mice. The mice used most often in contemporary ophthalmic research have not been adequately investigated by pupillometry.

METHODS

An infrared-video camera was used to assess pupillary functioning under standard dark-adapted conditions (30 min acclimatization in a ganzfeld chamber) and 30 min after the installation of either 10 microl tropicamide (1.0%) or 10 microl pilocarpine (5.0%) into the conjunctival cul-de-sac. The experiment was self-controlled using a repeated-measures analysis of variance to analyze 60 mice (30 males, 24 weeks of age) from three strains (C57BL/6, 129SvJ, and F1 hybrid).

RESULTS

The dark-adapted pupillary diameter of mice ranged from 2.3 mm (SD = 0.14) in female C57BL/6 to 2.9 mm (SD = 0.05) in male 129SvJ. Under dark-adapted conditions, all mice examined were sexual dimorphic (F = 19.5, dF = 2, 119, p < 0.001) and strain-dependent differences were observed between male C57BL/6-129SvJ and C57BL/6-F1 and female C57BL/6-F1 (F = 82.32, dF = 1, 119, p < 0.001). The mean pupillary diameter 30 min after the application of tropicamide ranged from 2.5 mm (SD = 0.16) in female F1 to 3.0 mm (SD = 0.13) in male C57BL/6. Tropicamide produced a sexual dimorphism in mydriasis for all mice (F = 56.30, dF = 1, 59, p < 0.001); however, strain-dependent differences were not observed (F = 1.31, dF = 2, 59, p < 0.280). The mean pupillary diameter 30 min after the application of pilocarpine ranged from 2.6 mm (SD = 0.28) in female C57BL/6 to 3.2 mm in both male F1 (SD = 0.22) and 129SvJ (SD = 0.04). Pilocarpine produced a sexual dimorphism in mydriasis for 129SvJ and F1 (F = 106.70, dF = 1, 59, p < 0.001) and strain-dependent differences were observed between female C57BL/6-F1 (F = 17.25, dF = 2, 59, p < 0.001).

CONCLUSIONS

The experiment demonstrates that mice respond idiosyncratically in their pupillary response under standard dark-adapted conditions and to either tropicamide or pilocarpine depending on strain and/or sex of the mouse. The characteristic responses observed are likely due to subtle differences in the genetic expression of phenotype.

Depression of intraocular pressure following inactivation of connexin43 in the nonpigmented epithelium of the ciliary body

PURPOSE

Conditional inactivation of connexin43 (Cx43) in the pigmented epithelium of the mouse eye results in a reduction in aqueous humor production and complete loss of the vitreous chamber. It was proposed that gap junctions between pigmented and nonpigmented epithelia of the ciliary body are critical for the production of the aqueous humor. To form such junctions, Cx43 in the pigmented epithelium must interact with connexin(s) present in the adjacent cells of the nonpigmented epithelium. The importance of Cx43 expression in the nonpigmented epithelium for the establishment of gap junctions and the regulation of intraocular pressure was tested.

METHODS

To inactivate Cx43 in the nonpigmented epithelium of the mouse eye, a mouse line was crossed with a floxed Cx43 locus (Cx43(flox/flox)) and a transgenic mouse line expressing cre recombinase under the control of the Pax6alpha promoter. General eye structure was evaluated by light microscopy, gap junctions were analyzed by electron microscopy, and intraocular pressure was directly assessed with micropipettes.

RESULTS

In Pax6alpha-cre/Cx43(flox/flox) mice, Cx43 was partially inactivated in the nonpigmented epithelium of the ciliary body and iris. Animals developed dilatations between the pigmented and nonpigmented epithelia and displayed a significant reduction in intraocular pressure. However, gap junctions between the ciliary epithelial layers were decreased but not eliminated.

CONCLUSIONS

Cx43 expression in the nonpigmented epithelium of the ciliary body contributes to the formation of gap junctions with the cells of the pigmented epithelium. These gap junctions play a critical role in maintaining the physical integrity of the ciliary body epithelium. Although the partial loss of Cx43 from the nonpigmented epithelium was correlated with a measurable drop in intraocular pressure, possible changes in Cx43 in the aqueous outflow pathway may provide an additional contribution to the observed phenotype.

Telemetric monitoring of 24 h intraocular pressure in conscious and freely moving C57BL/6J and CBA/CaJ mice

PURPOSE

To study change patterns of 24 h intraocular pressure (IOP) in conscious and freely moving mice using telemetry.

METHODS

Adult C57BL/6J and CBA/CaJ mice were entrained to a standard 12 h light and 12 h dark cycle. A telemetric pressure transmitter was implanted subcutaneously on the upper back of each light-dark entrained mouse, and the pressure catheter tip was inserted into the vitreous chamber. Broadcasted IOP data were received at 120 Hz. Means of 2 min IOP were recorded every 5 min for 4-13 days to generate the 24 h IOP pattern in each mouse strain. The pattern of IOP in the C57BL/6J strain was also determined under an acute constant dark condition for 24 h.

RESULTS

There were distinct patterns of 24 h IOP in the C57BL/6J and CBA/CaJ mouse strains. Under the standard light-dark condition, IOP was higher during the dark period than the light period in both strains. Elevation in IOP from the light period to the dark period was significantly smaller in the CBA/CaJ strain (1.6+/-1.7 mmHg, mean+/- standard deviation (SD), n=21) than in the C57BL/6J strain (3.4+/-2.5 mmHg, n=20). The 24 h IOP pattern in the C57BL/6J strain persisted under an acute constant dark condition (n=8).

CONCLUSIONS

Distinct change patterns of 24 h IOP appeared in these two mouse strains. Although mean IOP during the dark period was significantly higher than the light period in both strains, the magnitudes of light-dark IOP elevation differed. The 24 h IOP change pattern can be driven endogenously in the absence of light.