Outflow physiology of the mouse eye: pressure dependence and washout

Measurement of Outflow Facility Using iPerfusion

Elevated intraocular pressure (IOP) is the predominant risk factor for glaucoma, and reducing IOP is the only successful strategy to prevent further glaucomatous vision loss. IOP is determined by the balance between the rates of aqueous humour secretion and outflow, and a pathological reduction in the hydraulic conductance of outflow, known as outflow facility, is responsible for IOP elevation in glaucoma. Mouse models are often used to investigate the mechanisms controlling outflow facility, but the diminutive size of the mouse eye makes measurement of outflow technically challenging. In this study, we present a new approach to measure and analyse outflow facility using iPerfusion^™, which incorporates an actuated pressure reservoir, thermal flow sensor, differential pressure measurement and an automated computerised interface. In enucleated eyes from C57BL/6J mice, the flow-pressure relationship is highly non-linear and is well represented by an empirical power law model that describes the pressure dependence of outflow facility. At zero pressure, the measured flow is indistinguishable from zero, confirming the absence of any significant pressure independent flow in enucleated eyes. Comparison with the commonly used 2-parameter linear outflow model reveals that inappropriate application of a linear fit to a non-linear flow-pressure relationship introduces considerable errors in the estimation of outflow facility and leads to the false impression of pressure-independent outflow. Data from a population of enucleated eyes from C57BL/6J mice show that outflow facility is best described by a lognormal distribution, with 6-fold variability between individuals, but with relatively tight correlation of facility between fellow eyes. iPerfusion represents a platform technology to accurately and robustly characterise the flow-pressure relationship in enucleated mouse eyes for the purpose of glaucoma research and with minor modifications, may be applied in vivo to mice, as well as to eyes from other species or different biofluidic systems.

Dose- and time-dependent effects of actomyosin inhibition on live mouse outflow resistance and aqueous drainage tissues

Actomyosin contractility modulates outflow resistance of the aqueous drainage tissues and intraocular pressure, a key pathogenic factor of glaucoma. We established methodology to reliably analyze the effect of latrunculin-B (Lat-B)-induced actin depolymerization on outflow physiology in live mice. A voltage-controlled microperfusion system for delivering drugs and simultaneously analyzing outflow resistance was tested in live C57BL/6 mice. Flow rate and perfusion pressure were reproducible within a coefficient of variation of 2%. Outflow facility for phosphate-buffered saline (0.0027 ± 0.00036 μL/min/mmHg; mean ± SD) and 0.02% ethanol perfusions (Lat-B vehicle; 0.0023 ± 0.0005 μL/min/mmHg) were similar and stable over 2 hours (p > 0.1 for change), indicating absence of a ‘washout’ artifact seen in larger mammals. Outflow resistance changed in graded fashion, decreasing dose- and time-dependently over 2 hours for Lat-B doses of 2.5 μM (p = 0.29), 5 μM (p = 0.039) and 10 μM (p = 0.001). Resulting outflow resistance was about 10 times lower with 10 μM Lat-B than vehicle control. The filamentous actin network was decreased and structurally altered in the ciliary muscle (46 ± 5.6%) and trabecular meshwork (37 ± 8.3%) of treated eyes relative to vehicle controls (p < 0.005; 5 μM Lat-B). Mouse actomyosin contractile mechanisms are important to modulating aqueous outflow resistance, mirroring mechanisms in primates. We describe approaches to reliably probe these mechanisms in vivo.

System for Rapid, Precise Modulation of Intraocular Pressure, toward Minimally-Invasive In Vivo Measurement of Intracranial Pressure

Pathologic changes in intracranial pressure (ICP) are commonly observed in a variety of medical conditions, including traumatic brain injury, stroke, brain tumors, and glaucoma. However, current ICP measurement techniques are invasive, requiring a lumbar puncture or surgical insertion of a cannula into the cerebrospinal fluid (CSF)-filled ventricles of the brain. A potential alternative approach to ICP measurement leverages the unique anatomy of the central retinal vein, which is exposed to both intraocular pressure (IOP) and ICP as it travels inside the eye and through the optic nerve; manipulating IOP while observing changes in the natural pulsations of the central retinal vein could potentially provide an accurate, indirect measure of ICP. As a step toward implementing this technique, we describe the design, fabrication, and characterization of a system that is capable of manipulating IOP in vivo with <0.1 mmHg resolution and settling times less than 2 seconds. In vitro tests were carried out to characterize system performance. Then, as a proof of concept, we used the system to manipulate IOP in tree shrews (Tupaia belangeri) while video of the retinal vessels was recorded and the caliber of a selected vein was quantified. Modulating IOP using our system elicited a rapid change in the appearance of the retinal vein of interest: IOP was lowered from 10 to 3 mmHg, and retinal vein caliber sharply increased as IOP decreased from 7 to 5 mmHg. Another important feature of this technology is its capability to measure ocular compliance and outflow facility in vivo, as demonstrated in tree shrews. Collectively, these proof-of-concept demonstrations support the utility of this system to manipulate IOP for a variety of useful applications in ocular biomechanics, and provide a framework for further study of the mechanisms of retinal venous pulsation.

Analysis of Circadian Rhythm Gene Expression With Reference to Diurnal Pattern of Intraocular Pressure in Mice

PURPOSE

To determine the expression of circadian rhythm clock genes in the iris-ciliary body complex of mice and their association with the diurnal pattern of intraocular pressure (IOP).

METHODS

Thirty wild-type C57BL/6 mice were acclimated to a 12-hour light-dark cycle. Intraocular pressure was measured with a rebound tonometer at six time points daily (circadian time [CT] 2, 6, 10, 14, 18, and 22 hours) for five consecutive days. On day 6, mice were euthanized at CT 2, 6, 10, 14, 18, and 22. Eyes were flash-frozen or fixed in 4% phosphate-buffered paraformaldehyde. Total RNA was extracted from the iris-ciliary body complex, and RNA expression of circadian rhythm genes Bmal1, Clock, Cry1, Cry2, Per1, and Per2 was assessed by quantitative real-time PCR. Fixed eyes were paraffin embedded, and immunohistochemistry was performed to localize corresponding proteins (BMAL1, CLOCK, CRY1, CRY2, PER1, and PER2). Linear regression analysis was performed to correlate gene expression with IOP.

RESULTS

Intraocular pressure demonstrated a consistent circadian pattern. The clock genes Bmal1, Clock, Cry1, Cry2, Per1, and Per2 showed a circadian pattern of expression in the iris-ciliary body complex of mice. Bmal1, Clock, Cry1, Per1, and Per2 gene expression demonstrated statistically significant correlations with diurnal variations of IOP. BMAL1, CLOCK, CRY1, CRY2, PER1, and PER2 proteins were found to be expressed locally in the nonpigmented epithelium of the ciliary body.

CONCLUSIONS

Expression patterns of candidate circadian rhythm genes correlates with the diurnal pattern variation of IOP in mouse eyes, indicating a possible mechanism of IOP regulation through these genes.

Using genetic mouse models to gain insight into glaucoma: Past results and future possibilities

While all forms of glaucoma are characterized by a specific pattern of retinal ganglion cell death, they are clinically divided into several distinct subclasses, including normal tension glaucoma, primary open angle glaucoma, congenital glaucoma, and secondary glaucoma. For each type of glaucoma there are likely numerous molecular pathways that control susceptibility to the disease. Given this complexity, a single animal model will never precisely model all aspects of all the different types of human glaucoma. Therefore, multiple animal models have been utilized to study glaucoma but more are needed. Because of the powerful genetic tools available to use in the laboratory mouse, it has proven to be a highly useful mammalian system for studying the pathophysiology of human disease. The similarity between human and mouse eyes coupled with the ability to use a combination of advanced cell biological and genetic tools in mice have led to a large increase in the number of studies using mice to model specific glaucoma phenotypes. Over the last decade, numerous new mouse models and genetic tools have emerged, providing important insight into the cell biology and genetics of glaucoma. In this review, we describe available mouse genetic models that can be used to study glaucoma-relevant disease/pathobiology. Furthermore, we discuss how these models have been used to gain insights into ocular hypertension (a major risk factor for glaucoma) and glaucomatous retinal ganglion cell death. Finally, the potential for developing new mouse models and using advanced genetic tools and resources for studying glaucoma are discussed.

Animal models of glucocorticoid-induced glaucoma

Glucocorticoid (GC) therapy is widely used to treat a variety of inflammatory diseases and conditions. While unmatched in their anti-inflammatory and immunosuppressive activities, GC therapy is often associated with the significant ocular side effect of GC-induced ocular hypertension (OHT) and iatrogenic open-angle glaucoma. Investigators have generated GC-induced OHT and glaucoma in at least 8 different species besides man. These models mimic many features of this condition in man and provide morphologic and molecular insights into the pathogenesis of GC-OHT. In addition, there are many clinical, morphological, and molecular similarities between GC-induced glaucoma and primary open-angle glaucoma (POAG), making animals models of GC-induced OHT and glaucoma attractive models in which to study specific aspects of POAG.

Role of nitric oxide in murine conventional outflow physiology

Elevated intraocular pressure (IOP) is the main risk factor for glaucoma. Exogenous nitric oxide (NO) decreases IOP by increasing outflow facility, but whether endogenous NO production contributes to the physiological regulation of outflow facility is unclear. Outflow facility was measured by pressure-controlled perfusion in ex vivo eyes from C57BL/6 wild-type (WT) or transgenic mice expressing human endothelial NO synthase (eNOS) fused to green fluorescent protein (GFP) superimposed on the endogenously expressed murine eNOS (eNOS-GFPtg). In WT mice, exogenous NO delivered by 100 μM S-nitroso-N-acetylpenicillamine (SNAP) increased outflow facility by 62 ± 28% (SD) relative to control eyes perfused with the inactive SNAP analog N-acetyl-d-penicillamine (NAP; n = 5, P = 0.016). In contrast, in eyes from eNOS-GFPtg mice, SNAP had no effect on outflow facility relative to NAP (-9 ± 4%, P = 0.40). In WT mice, the nonselective NOS inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME, 10 μM) decreased outflow facility by 36 ± 13% (n = 5 each, P = 0.012), but 100 μM l-NAME had no detectable effect on outflow facility (-16 ± 5%, P = 0.22). An eNOS-selective inhibitor (cavtratin, 50 μM) decreased outflow facility by 19 ± 12% in WT (P = 0.011) and 39 ± 25% in eNOS-GFPtg (P = 0.014) mice. In the conventional outflow pathway of eNOS-GFPtg mice, eNOS-GFP expression was localized to endothelial cells lining Schlemm's canal and the downstream vessels, with no apparent expression in the trabecular meshwork. These results suggest that endogenous NO production by eNOS within endothelial cells of Schlemm's canal or downstream vessels contributes to the physiological regulation of aqueous humor outflow facility in mice, representing a viable strategy to more successfully lower IOP in glaucoma.

Extracellular matrix in the trabecular meshwork: intraocular pressure regulation and dysregulation in glaucoma

The trabecular meshwork (TM) is located in the anterior segment of the eye and is responsible for regulating the outflow of aqueous humor. Increased resistance to aqueous outflow causes intraocular pressure to increase, which is the primary risk factor for glaucoma. TM cells reside on a series of fenestrated beams and sheets through which the aqueous humor flows to exit the anterior chamber via Schlemm's canal. The outer trabecular cells are phagocytic and are thought to function as a pre-filter. However, most of the outflow resistance is thought to be from the extracellular matrix (ECM) of the juxtacanalicular region, the deepest portion of the TM, and from the inner wall basement membrane of Schlemm's canal. It is becoming increasingly evident that the extracellular milieu is important in maintaining the integrity of the TM. In glaucoma, not only have ultrastructural changes been observed in the ECM of the TM, and a significant number of mutations in ECM genes been noted, but the stiffness of glaucomatous TM appears to be greater than that of normal tissue. Additionally, TGFβ2 has been found to be elevated in the aqueous humor of glaucoma patients and is assumed to be involved in ECM changes deep with the juxtacanalicular region of the TM. This review summarizes the current literature on trabecular ECM as well as the development and function of the TM. Animal models and organ culture models targeting specific ECM molecules to investigate the mechanisms of glaucoma are described. Finally, the growing number of mutations that have been identified in ECM genes and genes that modulate ECM in humans with glaucoma are documented.

High-throughput screening for modulators of cellular contractile force

When cellular contractile forces are central to pathophysiology, these forces comprise a logical target of therapy. Nevertheless, existing high-throughput screens are limited to upstream signalling intermediates with poorly defined relationships to such a physiological endpoint. Using cellular force as the target, here we report a new screening technology and demonstrate its applications using human airway smooth muscle cells in the context of asthma and Schlemm's canal endothelial cells in the context of glaucoma. This approach identified several drug candidates for both asthma and glaucoma. We attained rates of 1000 compounds per screening day, thus establishing a force-based cellular platform for high-throughput drug discovery.

Recent Developments in Understanding the Role of Aqueous Humor Outflow in Normal and Primary Open Angle Glaucoma

Primary open angle glaucoma (POAG) is the second leading cause of blindness in the world's rapidly aging population. POAG is characterized by progressive degeneration of neural structures in the posterior segment, often associated with a concomitant elevation of intraocular pressure. Changes in IOP are believed to be caused by a disruption in the normal outflow of aqueous humor. This article reviews recent research associated with normal and POAG aqueous humor outflow. Novel findings elucidating biochemical and pathological changes in the ocular tissues affected in POAG are presented. Stem cell research, identification of lymphatic markers, and increased use of mouse models give researchers exciting new tools to understand aqueous humor outflow, changes associated with POAG and identify underlying causes of the disease.

Aqueous outflow: segmental and distal flow

The elevated intraocular pressure (IOP) of primary open-angle glaucoma is caused by impaired outflow of aqueous humor through the trabecular meshwork. Within the juxtacanalicular region, alterations of both extracellular matrix homeostasis and the cellular tone of trabecular meshwork endothelial and the inner wall of Schlemm canal cells affect outflow. Newer pharmacologic agents that target trabecular meshwork and Schlemm canal cell cytoskeleton lower IOP. Aqueous drainage occurs nonhomogenously with greater flow going through certain portions of the TM and less going through other portions-a concept known as segmental flow, which is theoretically the result of outflow being dependent on the presence of discrete pores within Schlemm canal. The limited long-term success of trabecular meshwork bypass surgeries implicates the potential impact of resistance in Schlemm canal itself and collector channels. Additionally, others have observed that outflow occurs preferentially near collector channels. These distal structures may be more important to aqueous outflow than previously believed.

FINANCIAL DISCLOSURE

Dr. Rhee is a consultant to Aerie Pharmaceuticals, Alcon Laboratories, Inc., Allegan, Inc., Aquesys, Inc., Glaukos Corp., Ivantis, Inc., Johnson & Johnson, Merck Sharp & Dohme Corp. and Santen, Inc., and has received research funding from Alcon Laboratories, Inc., Merck Sharp & Dohme Corp., and Ivantis, Inc. No other author has a financial or proprietary interest in any material or method mentioned.

Lymphatic regulator PROX1 determines Schlemm's canal integrity and identity

Schlemm's canal (SC) is a specialized vascular structure in the eye that functions to drain aqueous humor from the intraocular chamber into systemic circulation. Dysfunction of SC has been proposed to underlie increased aqueous humor outflow (AHO) resistance, which leads to elevated ocular pressure, a factor for glaucoma development in humans. Here, using lymphatic and blood vasculature reporter mice, we determined that SC, which originates from blood vessels during the postnatal period, acquires lymphatic identity through upregulation of prospero homeobox protein 1 (PROX1), the master regulator of lymphatic development. SC expressed lymphatic valve markers FOXC2 and integrin α9 and exhibited continuous vascular endothelial-cadherin (VE-cadherin) junctions and basement membrane, similar to collecting lymphatics. SC notably lacked luminal valves and expression of the lymphatic endothelial cell markers podoplanin and lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1). Using an ocular puncture model, we determined that reduced AHO altered the fate of SC both during development and under pathologic conditions; however, alteration of VEGF-C/VEGFR3 signaling did not modulate SC integrity and identity. Intriguingly, PROX1 expression levels linearly correlated with SC functionality. For example, PROX1 expression was reduced or undetectable under pathogenic conditions and in deteriorated SCs. Collectively, our data indicate that PROX1 is an accurate and reliable biosensor of SC integrity and identity.

Ultrastructural changes associated with dexamethasone-induced ocular hypertension in mice

PURPOSE

To determine whether dexamethasone (DEX)-induced ocular hypertension (OHT) in mice mimics the hallmarks of steroid-induced glaucoma (SIG) in humans, including reduced conventional outflow facility (C), increased extracellular matrix (ECM), and myofibroblasts within the outflow pathway.

METHODS

Osmotic mini-pumps were implanted subcutaneously into C57BL/6J mice for systemic delivery of DEX (3-4 mg/kg/d, n = 31 mice) or vehicle (n = 28). IOP was measured weekly by rebound tonometry. After 3 to 4 weeks, mice were euthanized and eyes enucleated for ex vivo perfusion to measure C, for electron microscopy to examine the trabecular meshwork (TM) and Schlemm's canal (SC), or for immunohistochemistry to examine type IV collagen and α-smooth muscle actin. The length of basement membrane material (BMM) was measured along the anterior-posterior extent of SC by electron microscopy. Ultrastructural changes in BMM of DEX-treated mice were compared against archived human SIG specimens.

RESULTS

Dexamethasone increased IOP by 2.6 ± 1.6 mm Hg (mean ± SD) over 3 to 4 weeks and decreased C by 52% ± 17% versus controls. Intraocular pressure elevation correlated with decreased C. Dexamethasone treatment led to increased fibrillar material in the TM, plaque-like sheath material surrounding elastic fibers, and myofibroblasts along SC outer wall. The length of BMM underlying SC was significantly increased in mice with DEX and in humans with SIG, and in mice decreased C correlated with increased BMM.

CONCLUSIONS

Dexamethasone-induced OHT in mice mimics hallmarks of human SIG within 4 weeks of DEX treatment. The correlation between reduced C and newly formed ECM motivates further study using DEX-treated mice to investigate the pathogenesis of conventional outflow obstruction in glaucoma.

The structure of the trabecular meshwork, its connections to the ciliary muscle, and the effect of pilocarpine on outflow facility in mice

PURPOSE

To determine the connections between the ciliary muscle (CM), trabecular meshwork (TM), and Schlemm's canal (SC) and their innervations that allows CM contraction (by pilocarpine) to influence conventional outflow in mice.

METHODS

Sequential sections and whole mounts of murine corneoscleral angles were stained for elastin, α-smooth muscle actin (αSMA), vesicular acetylcholine transporter (VAChT), neuronal nitric oxide synthase (nNOS), vasoactive intestinal peptide (VIP), and tyrosine hydroxylase (TH). Elastic (EL) fibers between the CM, TM, and SC were examined in ultrathin, sequential sections from different planes. The effect of pilocarpine (100 μM) on conventional outflow facility was measured by perfusion of enucleated mouse eyes.

RESULTS

The mouse TM contains a three-dimensional (3D) net of EL fibers connecting the inner wall of SC to the cornea anteriorly, the ciliary body (CB) internally and the choroid and CM posteriorly. The CM bifurcates near the posterior TM, extending outer tendons to the juxtacanalicular tissue and inner wall of SC and internal connections to the lamellated TM and CB. Ciliary muscle and lamellated TM cells stain with αSMA and are innervated by VAChT-containing nerve fibers, without TH, VIP, or nNOS. Pilocarpine doubled outflow facility.

CONCLUSIONS

Mouse eyes resemble primate eyes not only by their well developed SC and TM, but also by their 3D EL net tethering together the TM and SC inner wall and by the tendinous insertion of the CM into this net. The increase in outflow facility following cholinergic stimulation in mice, as in primates, supports using mice for studies of aqueous humor dynamics and glaucoma.

Cell senescence reduced the mechanotransduction sensitivity of porcine angular aqueous plexus cells to elevation of pressure

PURPOSE

To investigate the effect of pressure/flow on the barrier function and protein expression of normal and senescent porcine aqueous humor plexus (AAP) cells, which are the porcine equivalent of human Schlemm's canal endothelial cells.

METHODS

AAP cells were grown for 2 weeks in physiological (5% O2) or hyperoxic conditions (40% O2) to model cell senescence. Control and senescent AAP cells were subjected to control and elevated hydrostatic pressure gradient of 10 mm Hg for 72 hours. Hydraulic conductivity (HC) and transendothelial electric resistance (TEER) were measured. The expressions of senescence-associated β-galactosidase and DNA damage marker 8-hydroxy-2'-deoxyguanosine (8-OHdG) were monitored, and the protein expression profile was analyzed by Western blot.

RESULTS

After 14 days of hyperoxia, AAP cells stained positive for 8-OHdG and β-galactosidase. Pressure elevation/flow resulted in significant increase of HC in control cells (from 1.37 ± 0.12 to 1.64 ± 0.18 μL/mm Hg/min/cm(2), P < 0.05), but not in senescent cells (1.15 ± 0.17 and 1.08 ± 0.10 μL μL/mm Hg/min/cm(2)). TEER changes were consistent with the HC results. Western blot analysis showed that the expression level of myosin light chain, claudin-5, and VE-cadherin significantly reduced under pressure elevation in control cells but not in senescent cells.

CONCLUSIONS

AAP cells are mechano-sensitive; however, cell senescence rendered the cells less responsive to mechanical stimulus, which may have pathological consequences.

AMP-activated protein kinase regulates intraocular pressure, extracellular matrix, and cytoskeleton in trabecular meshwork

PURPOSE

In this study, we investigate how adenosine monophosphate-activated protein kinase (AMPK) affects extracellular matrix (ECM) and cellular tone in the trabecular meshwork (TM), and examine how deletion of its catalytic α2 subunit affects IOP and aqueous humor clearance in mice.

METHODS

Human TM tissue was examined for expression of AMPKα1 and AMPKα2, genomically distinct isoforms of the AMPK catalytic subunit. Primary cultured human TM cells were treated for 24 hours with the AMPK activator 5-amino-1-β-Dffff-ribofuranosyl-imidazole-4-carboxamide (AICAR), under basal or TGF-β2 stimulatory conditions. Conditioned media (CM) was probed for secreted protein acidic and rich in cysteine (SPARC), thrombospondin-1 (TSP-1), and ECM proteins, and cells were stained for F-actin. Cells underwent adenoviral infection with a dominant negative AMPKα subunit (ad.DN.AMPKα) and were similarly analyzed. Intraocular pressure, central corneal thickness (CCT), and aqueous clearance were measured in AMPKα2-null and wild-type (WT) mice.

RESULTS

Both AMPKα1 and AMPKα2 are expressed in TM. AICAR activated AMPKα and suppressed the expression of various ECM proteins under basal and TGF-β2 stimulatory conditions. AICAR decreased F-actin staining and increased the phospho-total RhoA ratio (Ser188). Transforming growth factor-β2 transiently dephosphorylated AMPKα. Infection with ad.DN.AMPKα upregulated various ECM proteins, decreased the phospho-total RhoA ratio, and increased F-actin staining. AMPKα2-null mice exhibited 6% higher IOP and decreased aqueous clearance compared with WT mice, without significant differences in CCT or angle morphology.

CONCLUSIONS

Collectively, our data identify AMPK as a critical regulator of ECM homeostasis and cytoskeletal arrangement in the TM. Mice that are AMPKα2-null exhibit higher IOPs and decreased aqueous clearance than their WT counterparts.

Pilocarpine-induced dilation of Schlemm's canal and prevention of lumen collapse at elevated intraocular pressures in living mice visualized by OCT

PURPOSE

The goal was to assess effects of IOP and pilocarpine-induced ciliary muscle contraction on conventional outflow pathway tissues in living anesthetized mice.

METHODS

Intraocular pressure was controlled by intracameral cannulation of mouse eyes while imaging using spectral-domain optical coherence tomography (SD-OCT). Time-lapse sagittal SD-OCT sections through Schlemm's canal (SC) were acquired while changing IOP stepwise between 10 and 45 mm Hg. After topical application of 1% pilocarpine, the series of IOP steps and imaging were repeated. Effects of pilocarpine on IOP and outflow facility in living mice were verified by rebound tonometry and flow measurements at three different IOPs, respectively. In vivo OCT images were compared with eyes analyzed by standard histology.

RESULTS

In living mice imaged by SD-OCT, the lumen of SC progressively collapsed with increasing IOP, reaching near complete closure at 20 mm Hg. Schlemm's canal collapse was reversible, with the lumen opening within minutes after returning IOP from 45 to 10 mm Hg. Pilocarpine-induced ciliary muscle contraction changed SC lumen area by 131.6% ± 21.0% compared with untreated controls at 10 mm Hg, opened the trabecular meshwork, and prevented complete collapse of the SC lumen at higher pressures. Similar results were observed by standard histology. Pilocarpine increased outflow facility 4-fold (P = 0.02) and lowered IOP (16.46 ± 2.23 vs. 11.08 ± 2.28 mm Hg, P = 0.03).

CONCLUSIONS

Spectral-domain OCT was effective at visualizing changes in SC lumen in living mice. Results with pilocarpine are consistent with the concept that a primary role for the ciliary muscle is to prevent collapse of SC.

Feedback-controlled constant-pressure anterior chamber perfusion in live mice

PURPOSE

To describe live mouse, anterior chamber constant-pressure perfusion by an approach using feedback-controlled coupling of pressure and flow to maintain a preset pressure.

METHODS

We established a microperfusion system that maintains a constant preset pressure in the anterior chamber of live mice by automatically regulating the microsyringe pump flow rate with a computer-controlled voltage feedback loop. Perfusion was by single-needle cannulation. We characterized the following in C57BL/6 mice aged 3-4 months in vivo: (i) pressure stability, (ii) pressure and flow rate reproducibility, (iii) total outflow facility, and (iv) anterior segment histology after perfusion.

RESULTS

Twenty live mice underwent perfusion. Constant pressure was quickly attained and stably maintained. The coefficient of pressure variation over time during perfusion at a preset pressure was <0.001. The average coefficient of variation for repeat pressure and flow rate measurements was 0.0005 and 0.127, respectively. The relationship between flow rate and pressure was linear for perfusions between 15 and 35 mmHg. The total outflow facility was 0.0066 µl/min/mmHg. Perfusion system resistance (0.5 mmHg/min/µl) was negligible relative to the ocular outflow resistance (147 mmHg/min/µl) at physiologically relevant perfusion pressures of 15-35 mmHg. No histological disruption of the drainage tissue was seen following perfusion.

CONCLUSIONS

Predetermined pressure was stably maintained during constant-pressure perfusion of live mouse eyes by a method using feedback-controlled coupling of pressure and flow along with single-needle anterior chamber cannulation. Perfusion measurements were reproducible. This approach is potentially useful for exploring aqueous drainage tissue biology, physiology, and pharmacology in live mice.

Intraocular pressure homeostasis: maintaining balance in a high-pressure environment

Although glaucoma is a relatively common blinding disease, most people do not develop glaucoma. A robust intraocular pressure (IOP) homeostatic mechanism keeps ocular pressures within relatively narrow acceptable bounds throughout most peoples' lives. The trabecular meshwork and/or Schlemm's canal inner wall cells respond to sustained IOP elevation and adjust the aqueous humor outflow resistance to restore IOP to acceptable levels. It appears that the cells sense IOP elevations as mechanical stretch or distortion of the actual outflow resistance and respond by initiating a complex extracellular matrix (ECM) turnover process that takes several days to complete. Although considerable information pertinent to this process is available, many aspects of the IOP homeostatic process remain to be elucidated. Components and mechanisms beyond ECM turnover could also be relevant to IOP homeostasis, but will not be addressed in detail here. Known aspects of the IOP homeostasis process as well as possible ways that it might function and impact glaucoma are discussed.

The influence of genetic background on conventional outflow facility in mice

PURPOSE

Intraocular pressure (IOP) varies between genetically distinct strains of mice. The purpose was to test the hypothesis that strain-dependent differences in IOP are attributable to differences in conventional outflow facility (C).

METHODS

The IOP was measured by rebound tonometry in conscious or anesthetized BALB/cJ, C57BL/6J, and CBA/J mice (N = 6-10 per strain). Conventional outflow facility was measured by ex vivo perfusion of enucleated eyes (N = 9-10 per strain).

RESULTS

Conscious IOP varied between strains, being highest in CBA/J (14.5 ± 0.9 mm Hg, mean ± SD), intermediate in C57BL/6J (12.3 ± 1.0 mm Hg), and lowest in BALB/cJ (10.6 ± 1.8 mm Hg) mice. Anesthesia reduced IOP and eliminated any detectable differences between strains. Conventional outflow facility also varied between strains, but, in contrast to IOP, C was lowest in CBA/J (0.0113 ± 0.0031 μL/min/mm Hg) and highest in BALB/cJ (0.0164 ± 0.0059 μL/min/mm Hg). Like IOP, C was intermediate in C57BL/6J (0.0147 ± 0.0029 μL/min/mm Hg). There was a strong correlation between conscious IOP and outflow resistance (1/C) from individual eyes across all three strains, revealing that 70% of the variation in IOP was attributable to variation in outflow resistance.

CONCLUSIONS

Differences in IOP among three genetically distinct murine strains are attributable largely to differences in conventional outflow facility. These results motivate further studies using mice to identify the morphologic and genetic factors that underlie IOP regulation within the conventional outflow pathway.

Pigment epithelium-derived factor decreases outflow facility

PURPOSE

Pigment epithelium-derived factor (PEDF) regulates blood-retinal barrier function. As a constituent of aqueous humor, the role of PEDF in conventional outflow function is unknown. The goals of the study were to examine the effects of PEDF on barrier function of cultured Schlemm's canal (SC) endothelia and outflow facility in mouse eyes in situ.

METHODS

To model the inner wall of SC, transendothelial electrical resistance (TEER) of human SC and porcine angular aqueous plexus (AAP) cells was monitored. To examine an intact conventional outflow pathway, enucleated eyes from culled C57BL/6 mice were perfused with PEDF using a computer-controlled system. Purified PEDF (0.1 and 1 μg/mL) was perfused at four different pressure steps (4, 8, 15, 20 mm Hg), measuring flow to determine outflow facility (slope of flow/pressure relationship).

RESULTS

Pigment epithelium-derived factor increased TEER of porcine AAP cells in a dose-dependent fashion (0.3-3 μg/mL), and 1 μg/mL recombinant PEDF or conditioned media from pigmented retinal pigment epithelial monolayers stabilized TEER of human SC monolayers over time (0-48 hours). In perfusion experiments, we observed a 43.7% decrease in outflow facility (0.016 vs. 0.029 μL/min/mm Hg, P = 4.5 × 10⁻⁵) in eyes treated with 1 μg/mL PEDF compared to vehicle-perfused controls, and a 19.9% decrease (0.021 vs. 0.027 μL/min/mm Hg, P = 0.003) at 100 ng/mL PEDF.

CONCLUSIONS

Pigment epithelium-derived factor increased barrier function in both the in vitro and in situ models of the inner wall of SC. Modification of PEDF signaling in SC cells may be therapeutically exploited to increase outflow facility in people with ocular hypertension or decrease outflow facility in those with hypotony.

Oxidative stress impact on barrier function of porcine angular aqueous plexus cell monolayers

PURPOSE

Our goal was to investigate the effect of chronic oxidative stress on angular aqueous plexus (AAP, functional equivalent to human Schlemm's canal) endothelial cells from porcine eyes.

METHODS

AAP cells were differentially isolated from porcine outflow tissues using puromycin selection. Confluent cultures of porcine AAP cells were grown for 2 weeks in physiological (5% O2) or hyperoxic conditions (40% O2) to model elevated oxidative stress associated with ageing. Cell growth rate, size, transendothelial electrical resistance (TEER), and hydraulic conductivity (HC) were measured. The expression of senescence-associated β-galactosidase and DNA damage marker 8-hydroxy-2'-deoxyguanosine (8-OHdG) was monitored, and the levels of cytoskeletal and cell-cell adhesion proteins such as F-actin, phospho-myosin light chain (phosphor-MLC), occludin, claudin-5, ZO-1, β-catenin, and VE-cadherin were measured by immunofluorescence staining and Western blot analysis.

RESULTS

Data showed that chronic hyperoxia inhibited cell growth rate from day 3 onward, the cell size increased by 18.2%±5.1%, and cells stained positive for β-galactosidase and 8-OHdG. Hyperoxia resulted in a significant 30% increase in TEER compared with the control group (P<0.05, n=6). When perfused in the basal-to-apical direction at 4 mm Hg, HC of AAP cells was 1.97±0.12 and 1.54±0.13 μL/mm Hg/min/cm2 in control and hyperoxia groups, respectively (P<0.05, n=6). Stressed cells expressed a significantly greater abundance of F-actin, phospho-MLC, occludin, claudin-5, β-catenin, and VE-cadherin compared to the control group by both immunofluorescence and Western blot analyses.

CONCLUSIONS

Chronic exposure of AAP cells to oxidative stress decreased cell monolayer permeability and up-regulated cytoskeletal and cell-cell adhesion protein expression; suggesting that, with age and increased oxidative stress, resistance at the level of Schlemm's canal increases.

Optimizing gene transfer to conventional outflow cells in living mouse eyes

The mouse eye has physiological and genetic advantages to study conventional outflow function. However, its small size and shallow anterior chamber presents technical challenges to efficient intracameral delivery of genetic material to conventional outflow cells. The goal of this study was to optimize methods to overcome this technical hurdle, without damaging ocular structures or compromising outflow function. Gene targeting was monitored by immunofluorescence microscopy after transduction of adenovirus encoding green fluorescent protein driven by a CMV promoter. Guided by a micromanipulator and stereomicroscope, virus was delivered intracamerally to anesthetized mice by bolus injection using a 33 gauge needle attached to Hamilton syringe or infusion with glass micropipette connected to syringe pump. The total number of particles introduced remained constant, while volume of injected virus solution (3-10 μl) was varied for each method and time of infusion (3-40 min) tested. Outflow facility and intraocular pressure were monitored invasively using established techniques. Unlike bolus injections or slow infusions, introduction of virus intracamerally during rapid infusions (3 min) at any volume tested preferentially targeted trabecular meshwork and Schlemm's canal cells, with minimal transduction of neighboring cells. While infusions resulted in transient intraocular pressure spikes (commensurate with volume infused, Δ40-70 mmHg), eyes typically recovered within 60 min. Transduced eyes displayed normal outflow facility and tissue morphology 3-6 days after infusions. Taken together, fast infusion of virus solution in small volumes intracamerally is a novel and effective method to selectively deliver agents to conventional outflow cells in living mice.

Endogenous production of extracellular adenosine by trabecular meshwork cells: potential role in outflow regulation

PURPOSE

To investigate the mechanisms for endogenous production of extracellular adenosine in trabecular meshwork (TM) cells and evaluate its physiological relevance to the regulation of aqueous humor outflow facility.

METHODS

Extra-cellular levels of adenosine monophosphate (AMP) and adenosine in porcine trabecular meshwork (PTM) cells treated with adenosine triphosphate (ATP), AMP, cAMP or forskolin with or without specific inhibitors of phosphodiesterases (IBMX) and CD73 (AMPCP) were determined by high-pressure liquid chromatography fluorometry. Extracellular adenosine was also evaluated in cell cultures subjected to cyclic mechanical stress (CMS) (20% stretching; 1 Hz) and after disruption of lipid rafts with methyl-β-cyclodextrin. Expression of CD39 and CD73 in porcine TM cells and tissue were examined by Q-PCR and Western blot. The effect of inhibition of CD73 on outflow facility was evaluated in perfused living mouse eyes.

RESULTS

PTM cells generated extracellular adenosine from extracellular ATP and AMP but not from extracellular cAMP. Increased intracellular cAMP mediated by forskolin led to a significant increase in extracellular adenosine production that was not prevented by IBMX. Inhibition of CD73 resulted, in all cases, in a significant decrease in extracellular adenosine. CMS induced a significant activation of extracellular adenosine production. Inhibition of CD73 activity with AMPCP in living mouse eyes resulted in a significant decrease in outflow facility.

CONCLUSIONS

These results support the concept that the extracellular adenosine pathway might play an important role in the homeostatic regulation of outflow resistance in the TM, and suggest a novel mechanism by which pathologic alteration of the TM, such as increased tissue rigidity, could lead to abnormal elevation of IOP in glaucoma.

Pharmacologic manipulation of conventional outflow facility in ex vivo mouse eyes

PURPOSE

Mouse models are useful for glaucoma research, but it is unclear whether intraocular pressure (IOP) regulation in mice operates through mechanisms similar to those in humans. Our goal was to determine whether pharmacologic compounds that affect conventional outflow facility in human eyes exert similar effects in C57BL/6 mice.

METHODS

A computerized perfusion system was used to measure conventional outflow facility in enucleated mouse eyes ex vivo. Paired eyes were perfused sequentially, either immediately after enucleation or after 3 hours storage at 4°C. Three groups of experiments examined sphingosine 1-phosphate (S1P), S1P with antagonists to S1P(1) and S1P(2) receptors, and the prostanoid EP(4) receptor agonist 3,7-dithia PGE(1). We also examined whether a 24-hour postmortem delay affected the response to 3,7-dithia prostaglandin E(1) (PGE(1)).

RESULTS

S1P decreased facility by 39%, and was blocked almost completely by an S1P(2), but not S1P(1), receptor antagonist. The S1P(2) receptor antagonist alone increased facility nearly 2-fold. 3,7-dithia PGE(1) increased facility by 106% within 3 hours postmortem. By 24 hours postmortem, the facility increase caused by 3,7-dithia PGE(1) was reduced 3-fold, yet remained statistically detectable.

CONCLUSIONS

C57BL/6 mice showed opposing effects of S1P(2) and EP(4) receptor activation on conventional outflow facility, as observed in human eyes. Pharmacologic effects on facility were detectable up to 24 hours postmortem in enucleated mouse eyes. Mice are suitable models to examine the pharmacology of S1P and EP(4) receptor stimulation on IOP regulation as occurs within the conventional outflow pathway of human eyes, and are promising for studying other aspects of aqueous outflow dynamics.

A multiphoton microscope platform for imaging the mouse eye

PURPOSE

To demonstrate the ability of multiphoton microscopy to obtain full three-dimensional high-resolution images of the intact mouse eye anterior chamber without need for enucleation.

METHODS

A custom multiphoton microscope was constructed and optimized for deep tissue imaging. Simultaneous two-photon autofluorescence (2PAF) and second harmonic generation (SHG) imaging were performed. A mouse holder and stereotaxic platform were designed to access different parts of the eye for imaging. A reservoir for keeping the eye moist was used during imaging sessions.

RESULTS

Non-invasive multiphoton images deep inside the anterior chamber of the mouse eye were obtained without the need for enucleation. The iris, corneal epithelium and endothelium, trabecular meshwork region and conjunctiva were visualized by the 2PAF and SHG signals. Identification of the anatomy was achieved by the intrinsic properties of the native tissue without any exogenous labeling. Images as deep as 600 microns into the eye were clearly demonstrated. Full three-dimensional image reconstructions of the entire anterior chamber were performed and analyzed using custom software.

CONCLUSIONS

Multiphoton imaging is a highly promising tool for ophthalmic research. We have demonstrated the ability to image the entire anterior chamber of the mouse eye in its native state. These results provide a foundation for future in vivo studies of the eye.

A gimbal-mounted pressurization chamber for macroscopic and microscopic assessment of ocular tissues

The biomechanical model of glaucoma considers intraocular pressure-related stress and resultant strain on load bearing connective tissues of the optic nerve and surrounding peripapillary sclera as one major causative influence that effects cellular, vascular, and axonal components of the optic nerve. By this reasoning, the quantification of variations in the microstructural architecture and macromechanical response of scleral shells in glaucomatous compared to healthy populations provides an insight into any variations that exist between patient populations. While scleral shells have been tested mechanically in planar and pressure-inflation scenarios the link between the macroscopic biomechanical response and the underlying microstructure has not been determined to date. A potential roadblock to determining how the microstructure changes based on pressure is the ability to mount the spherical scleral shells in a method that does not induce unwanted stresses to the samples (for instance, in the flattening of the spherical specimens), and then capturing macroscopic and microscopic changes under pressure. Often what is done is a macroscopic test followed by sample fixation and then imaging to determine microstructural organization. We introduce a novel device and method, which allows spherical samples to be pressurized and macroscopic and microstructural behavior quantified on fully hydrated ocular specimens. The samples are pressurized and a series of markers on the surface of the sclera imaged from several different perspectives and reconstructed between pressure points to allow for mapping of nonhomogenous strain. Pictures are taken from different perspectives through the use of mounting the pressurization scheme in a gimbal that allows for positioning the sample in several different spherical coordinate system configurations. This ability to move the sclera in space about the center of the globe, coupled with an upright multiphoton microscope, allows for collecting collagen, and elastin signal in a rapid automated fashion so the entire globe can be imaged.

eNOS, a pressure-dependent regulator of intraocular pressure

PURPOSE

Pathology in the primary drainage pathway for aqueous humor in the eye is responsible for ocular hypertension, the only treatable risk factor in patients with glaucoma. Unfortunately, the mechanisms that regulate pressure-dependent drainage of aqueous humor and thus intraocular pressure (IOP) are unknown. To better understand one possible underlying molecular factor that regulates IOP, nitric oxide (NO), pressure-dependent drainage in transgenic mice overexpressing endothelial NO synthase (eNOS) was studied.

METHODS

IOP was measured by rebound tonometry in mice, and pressure versus flow data were measured by ex vivo perfusion at multiple pressures between 8 and 45 mm Hg, using mock AH ±100 μM L-NAME. A subset of eyes was examined histologically using standard techniques or was assayed for fusion protein expression by Western blot analysis.

RESULTS

IOP was lower (9.6 ± 2.7 vs. 11.4 ± 2.5 mm Hg; mean ± SD; P = 0.04) and pressure-dependent drainage was higher (0.0154 ± 0.006 vs. 0.0066 ± 0.0009 μL/min/mm Hg; P = 0.002) in the transgenic mice than in the wild-type animals; however, pressure-independent drainage was unaffected. The NOS inhibitor L-NAME normalized pressure-dependent drainage in transgenic animals. For IOP >35 mm Hg, the slope of the pressure-flow curve in wild-type mice increased to match that seen in transgenic mice. Shear stress in the pressure-dependent pathway at elevated pressures was calculated to be in a range known to affect eNOS expression and activity in vascular endothelia.

CONCLUSIONS

Endothelial NOS overexpression lowers IOP by increasing pressure-dependent drainage in the mouse eye. Data are consistent with NO's having a mechanoregulatory role in aqueous humor dynamics, with eNOS induction at elevated IOPs leading to increased pressure-dependent outflow.

Perfusion-cultured bovine anterior segments as an ex vivo model for studying glucocorticoid-induced ocular hypertension and glaucoma

PURPOSE

To determine whether perfusion-cultured bovine anterior segments would be a suitable model for glaucoma research.

METHODS

Fresh bovine eyes were dissected and sealed on a custom-made acrylic dish with an O-ring. Perfusion medium was infused by a syringe pump at a constant infusion rate of 5 μL/min. After intraocular pressure (IOP) was stable, bovine eyes were perfused with medium containing either a vehicle control (0.1% ethanol [ETH]) or dexamethasone (DEX) for up to 7 days. IOP was recorded by a pressure transducer and a computerized system. Perfusion medium was collected for Western immunoblot analysis of myocilin (MYOC).

RESULTS

The morphology of the bovine trabecular meshwork after perfusion culture was similar to that of freshly dissected, nonperfused bovine eyes. Treatment with DEX elevated IOP in some bovine eyes, whereas others showed little change. The authors analyzed the data from 18 ETH-treated control eyes and defined 2.82 mm Hg as the threshold of ocular hypertension (OHT), which equals mean pressure change + 2× SD. Approximately 40% (12/29) of the bovine eyes were DEX responders, which is very close to the DEX-responsive rates observed in human and monkey eyes. Western blot data showed that DEX treatment induced the expression of the DEX-inducible gene MYOC only in the perfusion-cultured anterior segments with DEX-induced OHT.

CONCLUSIONS

OHT can be induced by DEX in perfusion-cultured bovine anterior segments. This is a fast, convenient, affordable, and reliable model for studying DEX-induced OHT and the mechanisms of trabecular outflow.