Enhanced inflow and outflow rates despite lower IOP in bestrophin-2-deficient mice

Bestrophin-2 and glutamine synthetase form a complex for glutamate release

Bestrophin-2 (BEST2) is a member of the bestrophin family of calcium-activated anion channels that has a critical role in ocular physiology1-4. Here we uncover a directional permeability of BEST2 to glutamate that heavily favours glutamate exit, identify glutamine synthetase (GS) as a binding partner of BEST2 in the ciliary body of the eye, and solve the structure of the BEST2-GS complex. BEST2 reduces cytosolic GS activity by tethering GS to the cell membrane. GS extends the ion conducting pathway of BEST2 through its central cavity and inhibits BEST2 channel function in the absence of intracellular glutamate, but sensitizes BEST2 to intracellular glutamate, which promotes the opening of BEST2 and thus relieves the inhibitory effect of GS. We demonstrate the physiological role of BEST2 in conducting chloride and glutamate and the influence of GS in non-pigmented ciliary epithelial cells. Together, our results reveal a novel mechanism of glutamate release through BEST2-GS.

Quantitative analysis of exosomes in the aqueous humor of Korean patients with pseudoexfoliation glaucoma

We aimed to quantitatively analyze the exosome and its cargo in individual aqueous humor (AH) samples from pseudoexfoliation (PEX) glaucoma patients compared to controls using a novel detection platform. We investigated the size distribution and measured the quantitative exosome particle counts in each AH sample. AH (80–120 µL) was obtained during cataract surgery or glaucoma filtering surgery from 12 Korean subjects (six with PEX glaucoma and six age-matched controls). The mean size of the exosomes was 58.9 ± 18.5 nm measured by a tangential flow filtration system using single-particle interferometric reflectance imaging sensor. Exosome particle count in each CD 63, CD 81, and CD9 spot was significantly greater in PEX glaucoma than in controls in total, CD 63, CD9, syntenin, and scattering(all p < 0.003). The CD63 spot showed a particle count of 8319.1 ± 797.7 in PEX glaucoma patients and 4786.8 ± 1302.1 in controls (p = 1.88E−11). Individual fluorescent capture spot images also revealed denser exosome particles in PEX patients than in controls. Syntenin, indicating exosomal origin, was detected in all AH samples. Exosomes differentially detected in AH suggest the possible role of exosomes in the pathogenesis of PEX glaucoma.

Structures and gating mechanisms of human bestrophin anion channels

Bestrophin-1 (Best1) and bestrophin-2 (Best2) are two members of the bestrophin family of calcium (Ca²⁺)-activated chloride (Cl⁻) channels with critical involvement in ocular physiology and direct pathological relevance. Here, we report cryo-EM structures of wild-type human Best1 and Best2 in various states at up to 1.8 Å resolution. Ca²⁺-bound Best1 structures illustrate partially open conformations at the two Ca²⁺-dependent gates of the channels, in contrast to the fully open conformations observed in Ca²⁺-bound Best2, which is in accord with the significantly smaller currents conducted by Best1 in electrophysiological recordings. Comparison of the closed and open states reveals a C-terminal auto-inhibitory segment (AS), which constricts the channel concentrically by wrapping around the channel periphery in an inter-protomer manner and must be released to allow channel opening. Our results demonstrate that removing the AS from Best1 and Best2 results in truncation mutants with similar activities, while swapping the AS between Best1 and Best2 results in chimeric mutants with swapped activities, underlying a key role of the AS in determining paralog specificity among bestrophins.

Bestrophin channels are critical for physiology of the eye. Here, authors report cryo-EM structures of human bestrophins in various states at up to 1.8 Å resolution, revealing paralog-specific features that underlie molecular mechanisms of permeation.

Measurement of postmortem outflow facility using iPerfusion

The key risk factor for glaucoma is elevation of intraocular pressure (IOP) and alleviating it is the only effective therapeutic approach to inhibit further vision loss. IOP is regulated by the flow of aqueous humour across resistive tissues, and a reduction in outflow facility C, is responsible for the IOP elevation in glaucoma. Measurement of C is therefore important when investigating the pathophysiology of glaucoma and testing candidate treatments for lowering IOP. Due to similar anatomy and response to pharmacological treatments, mouse eyes are a common model of human aqueous humour dynamics. The ex vivo preparation, in which an enucleated mouse eye is mounted in a temperature controlled bath and cannulated, has been well characterised and is widely used. The postmortem in situ model, in which the eyes are perfused within the cadaver, has received relatively little attention. In this study, we investigate the postmortem in situ model using the iPerfusion system, with a particular focus on i) the presence or absence of pressure-independent flow, ii) the effect of evaporation on measured flow rates and iii) the magnitude and pressure dependence of outflow facility and how these properties are affected by postmortem changes. Measurements immediately after cannulation and following multi-pressure facility measurement demonstrated negligible pressure-independent flow in postmortem eyes, in contrast to assumptions made in previous studies. Using a humidity chamber, we investigated whether the humidity of the surrounding air would influence measured flow rates. We found that at room levels of humidity, evaporation of saline droplets on the eye resulted in artefactual flow rates with a magnitude comparable to outflow, which were eliminated by a high relative humidity (>85%) environment. Average postmortem outflow facility was ∼4 nl/min/mmHg, similar to values observed ex vivo, irrespective of whether a postmortem delay was introduced prior to cannulation. The intra-animal variability of measured outflow facility values was also reduced relative to previous ex vivo data. The pressure-dependence of outflow facility was reduced in the postmortem relative to ex vivo model, and practically eliminated when eyes were cannulated >40 min after euthanisation. Overall, our results indicate that the moderately increased technical complexity associated with postmortem perfusion provides reduced variability and reduced pressure-dependence in outflow facility, when experimental conditions are properly controlled.

Structure and Function of the Bestrophin family of calcium-activated chloride channels

Bestrophins are a family of calcium-activated chloride channels (CaCCs) with relevance to human physiology and a myriad of eye diseases termed "bestrophinopathies". Since the identification of bestrophins as CaCCs nearly two decades ago, extensive studies from electrophysiological and structural biology perspectives have sought to define their key channel features including calcium sensing, gating, inactivation, and anion selectivity. The initial X-ray crystallography studies on the prokaryotic homolog of Best1, Klebsiella pneumoniae (KpBest), and the Best1 homolog from Gallus gallus (chicken Best1, cBest1), laid the foundational groundwork for establishing the architecture of Best1. Recent progress utilizing single-particle cryogenic electron microscopy has further elucidated the molecular mechanism of gating in cBest1 and, separately, the structure of Best2 from Bos taurus (bovine Best2, bBest2). Meanwhile, whole-cell patch clamp, planar lipid bilayer, and other electrophysiologic analyses using these models as well as the human Best1 (hBest1) have provided ample evidence describing the functional properties of the bestrophin channels. This review seeks to consolidate these structural and functional results to paint a broad picture of the underlying mechanisms comprising the bestrophin family's structure-function relationship.

Transcriptomic Network Interactions in Human Skin Treated with Topical Glucocorticoid Clobetasol Propionate

Glucocorticoids are the most frequently used anti-inflammatory drugs in dermatology. However, the molecular signature of glucocorticoids and their receptor in human skin is largely unknown. Our validated bioinformatics analysis of human skin transcriptome induced by topical glucocorticoid clobetasol propionate (CBP) in healthy volunteers identified numerous unreported glucocorticoid-responsive genes, including over a thousand noncoding RNAs. We observed sexual and racial dimorphism in the CBP response including a shift toward IFN-α/IFN-γ and IL-6/Jak/Signal transducer and activator of transcription (STAT) 3 signaling in female skin; and a larger response to CBP in African-American skin. Weighted gene coexpression network analysis unveiled a dense skin network of 41 transcription factors including circadian Kruppel-like factor 9 (KLF9), and ∼260 of their target genes enriched for functional pathways representative of the entire CBP transcriptome. Using keratinocytes with Kruppel-like factor 9 knockdown, we revealed a feedforward loop in glucocorticoid receptor signaling, previously unreported. Interestingly, many of the CBP-regulated transcription factors were involved in the control of development, metabolism, circadian clock; and 80% of them were associated with skin aging showing similarities between glucocorticoid-treated and aged skin. Overall, these findings indicate that glucocorticoid receptor acts as an important regulator of gene expression in skin-both at the transcriptional and posttranscriptional level-via multiple mechanisms including regulation of noncoding RNAs and multiple core transcription factors.

Autoantibody profiling in intraocular fluid of patients with uveitis

A high prevalence of serum antiretinal antibodies (ARAs) in patients with uveitis has been previously described, though their clinical role remains elusive. Assessment of intraocular ARAs may provide further insight into the pathogenesis of diverse uveitis entities. In this study we investigate the prevalence of multiple specific anti-ocular antibodies (AOcAs), including ARAs, in intraocular fluid of patients with uveitis. Autoantibody profiling with 188 different ocular antigens was performed by a multiplex immunoassay with intraocular fluid samples of 76 patients with uveitis. Clinical data from uveitis patients were collected and statistical analyses were executed to evaluate associations between intraocular AOcAs and clinical characteristics. Controls consisted of 19 intraocular fluid samples from cataract patients. A spectrum of 22 different AOcAs was present in higher levels in patients with uveitis than in controls (p < 0.05), but in moderately elevated titers (<2x). High elevations of intraocular AOcAs in uveitis (>5x compared to cataract) were observed in varicella zoster virus-induced uveitis, multiple sclerosis-associated uveitis and patients with unexplained uveitis but positive quantiferon test. Presence of macular edema was associated with increased intraocular levels of tyrosinase antibodies. Our results show that patients with uveitis are characterized by the presence of a broad spectrum of moderately elevated levels of intraocular AOcAs, and high intraocular AOcA levels were found in several specific uveitis entities. This study favors secondary production of AOcAs and not their inciting role.

Total Outflow Facility in Live C57BL/6 Mice of Different Age

Purpose

To characterize total outflow facility across the live adult mouse lifespan as a reference for mouse glaucoma studies and the common C57BL/6 background strain.

Methods

Microperfusion was performed by single-needle cannulation and feedback-controlled coupling of pressure and flow to maintain a constant pressure in the anterior chambers of live C57BL/6NCrl mice aged 3-4 months (n = 17), 6-9 months (n = 10), and 23-27 months (n = 12). This mouse age range represented an equivalent human age range of young adult to elderly. We characterized the following across age groups in vivo: (1) outflow facility based on constant pressure perfusion in a pressure range of 15-35 mm Hg, (2) perfusion flow rates, and (3) anterior segment tissue histology after perfusion. Thirty-nine live mice underwent perfusion.

Results

Pressure-flow rate functions were consistently linear for all age groups (all R ² > 0.96). Total outflow facility in mice aged 3-4, 6-9, and 23-27 months was 0.0066, 0.0064, and 0.0077 μL/min/mm Hg, respectively. Facility was not significantly different between age groups (all p > 0.4). The groups had closely overlapping frequency distribution profiles with right-sided tails. Post hoc estimates indicated that group facility differences of at least 50% would have been detectable, with this limit set mainly by inherent variability in the strain. A trend toward higher perfusion flow rates was seen in older mice aged 23-27 months, but this was not significantly different from that of mice aged 3-4 months or 6-9 months (p > 0.2). No histological disruption or difference in iridocorneal angle or drainage tissue structure was seen following perfusion in the different age groups.

Conclusion

We did not find a significant difference in total outflow facility between different age groups across the live C57BL/6 mouse adult lifespan, agreeing with some human studies. The possibility that more subtle differences might exist ought to be judged with respect to the heterogeneity in facility at different ages. Our findings provide reference data for live perfusion studies pertaining to glaucoma involving the C57BL/6 strain.

Impaired angiopoietin/Tie2 signaling compromises Schlemm's canal integrity and induces glaucoma

Primary open-angle glaucoma (POAG) is often caused by elevated intraocular pressure (IOP), which arises due to increased resistance to aqueous humor outflow (AHO). Aqueous humor flows through Schlemm's canal (SC), a lymphatic-like vessel encircling the cornea, and via intercellular spaces of ciliary muscle cells. However, the mechanisms underlying increased AHO resistance are poorly understood. Here, we demonstrate that signaling between angiopoietin (Angpt) and the Angpt receptor Tie2, which is critical for SC formation, is also indispensable for maintaining SC integrity during adulthood. Deletion of Angpt1/Angpt2 or Tie2 in adult mice severely impaired SC integrity and transcytosis, leading to elevated IOP, retinal neuron damage, and impairment of retinal ganglion cell function, all hallmarks of POAG in humans. We found that SC integrity is maintained by interconnected and coordinated functions of Angpt-Tie2 signaling, AHO, and Prox1 activity. These functions diminish in the SC during aging, leading to impaired integrity and transcytosis. Intriguingly, Tie2 reactivation using a Tie2 agonistic antibody rescued the POAG phenotype in Angpt1/Angpt2-deficient mice and rejuvenated the SC in aged mice. These results indicate that the Angpt-Tie2 system is essential for SC integrity. The impairment of this system underlies POAG-associated pathogenesis, supporting the possibility that Tie2 agonists could be a therapeutic option for glaucoma.

Direct measurement of pressure-independent aqueous humour flow using iPerfusion

Reduction of intraocular pressure is the sole therapeutic target for glaucoma. Intraocular pressure is determined by the dynamics of aqueous humour secretion and outflow, which comprise several pressure-dependent and pressure-independent mechanisms. Accurately quantifying the components of aqueous humour dynamics is essential in understanding the pathology of glaucoma and the development of new treatments. To better characterise aqueous humour dynamics, we propose a method to directly measure pressure-independent aqueous humour flow. Using the iPerfusion system, we directly measure the flow into the eye when the pressure drop across the pressure-dependent pathways is eliminated. Using this approach we address i) the magnitude of pressure-independent flow in ex vivo eyes, ii) whether we can accurately measure an artificially imposed pressure-independent flow, and iii) whether the presence of a pressure-independent flow affects our ability to measure outflow facility. These studies are conducted in mice, which are a common animal model for aqueous humour dynamics. In eyes perfused with a single cannula, the average pressure-independent flow was 1 [-3, 5] nl/min (mean [95% confidence interval]) (N = 6). Paired ex vivo eyes were then cannulated with two needles, connecting the eye to both iPerfusion and a syringe pump, which was used to impose a known pressure-independent flow of 120 nl/min into the experimental eye only. The measured pressure-independent flow was then 121 [117, 125] nl/min (N = 7), indicating that the method could measure pressure-independent flow with high accuracy. Finally, we showed that the artificially imposed pressure-independent flow did not affect our ability to measure facility, provided that the pressure-dependence of facility and the true pressure-independent flow were accounted for. The present study provides a robust method for measurement of pressure-independent flow, and demonstrates the importance of accurately quantifying this parameter when investigating pressure-dependent flow or outflow facility.

Anterior chamber perfusion versus posterior chamber perfusion does not influence measurement of aqueous outflow facility in living mice by constant flow infusion

Mice are now routinely utilized in studies of aqueous humor outflow dynamics. In particular, conventional aqueous outflow facility (C) is routinely measured via perfusion of the aqueous chamber by a number of laboratories. However, in mouse eyes perfused ex-vivo, values for C are variable depending upon whether the perfusate is introduced into the posterior chamber (PC) versus the anterior chamber (AC). Perfusion via the AC leads to posterior bowing of the iris, and traction on the iris root/scleral spur, which may increase C. Perfusion via the PC does not yield this effect. But the equivalent situation in living mice has not been investigated. We sought to determine whether AC versus PC perfusion of the living mouse eye may lead to different values for C. All experiments were conducted in C57BL/6J mice (all ♀) between the ages of 20 and 30 weeks. Mice were divided into groups of 3-4 animals each. In all groups, both eyes were perfused. C was measured in groups 1 and 2 by constant flow infusion (from a 50 μL microsyringe) via needle placement in the AC, and in the PC, respectively. To investigate the effect of ciliary muscle (CM) tone on C, groups 3 and 4 were perfused live via the AC or PC with tropicamide (muscarinic receptor antagonist) added to the perfusate at a concentration of 100 μM. To investigate immediate effect of euthanasia, groups 5 and 6 were perfused 15-30 min after death via the AC or PC. To investigate the effect of CM tone on C immediately following euthanasia, groups 7 and 8 were perfused 15-30 min after death via the AC or PC with tropicamide added to the perfusate at a concentration of 100 μM. C in Groups 1 (AC perfusion) and 2 (PC perfusion) was computed to be 19.5 ± 0.8 versus 21.0 ± 2.1 nL/min/mmHg, respectively (mean ± SEM, p > 0.4, not significantly different). In live animals in which tropicamide was present in the perfusate, C in Group 3 (AC perfusion) was significantly greater than C in Group 4 (PC perfusion) (22.0 ± 4.0 versus 14.0 ± 2.0 nL/min/mmHg, respectively, p = 0.0021). In animals immediately following death, C in groups 5 (AC perfusion) and 6 (PC perfusion) was computed to be 21.2 ± 2.0 versus 22.8 ± 1.4 nL/min/mmHg, respectively (mean ± SEM, p = 0.1196, not significantly different). In dead animals in which tropicamide was present in the perfusate, C in group 7 (AC perfusion) was greater than C in group 8 (PC perfusion) (20.6 ± 1.4 versus 14.2 ± 2.6 nL/min/mmHg, respectively, p < 0.0001). C in eyes in situ in living mice or euthanized animals within 15-30 min post mortem is not significantly different when measured via AC perfusion or PC perfusion. In eyes of live or freshly euthanized mice, C is greater when measured via AC versus PC perfusion when tropicamide (a mydriatic and cycloplegic agent) is present in the perfusate.

Unconventional aqueous humor outflow: A review

Aqueous humor flows out of the eye primarily through the conventional outflow pathway that includes the trabecular meshwork and Schlemm's canal. However, a fraction of aqueous humor passes through an alternative or 'unconventional' route that includes the ciliary muscle, supraciliary and suprachoroidal spaces. From there, unconventional outflow may drain through two pathways: a uveoscleral pathway where aqueous drains across the sclera to be resorbed by orbital vessels, and a uveovortex pathway where aqueous humor enters the choroid to drain through the vortex veins. We review the anatomy, physiology and pharmacology of these pathways. We also discuss methods to determine unconventional outflow rate, including direct techniques that use radioactive or fluorescent tracers recovered from tissues in the unconventional pathway and indirect methods that estimate unconventional outflow based on total outflow over a range of pressures. Indirect methods are subject to a number of assumptions and generally give poor agreement with tracer measurements. We review the variety of animal models that have been used to study conventional and unconventional outflow. The mouse appears to be a promising model because it captures several aspects of conventional and unconventional outflow dynamics common to humans, although questions remain regarding the magnitude of unconventional outflow in mice. Finally, we review future directions. There is a clear need to develop improved methods for measuring unconventional outflow in both animals and humans.

Role of cyclic AMP in the eye with glaucoma

Glaucoma is characterized by a slow and progressive degeneration of the optic nerve, including retinal ganglion cell (RGC) axons in the optic nerve head (ONH), leading to visual impairment. Despite its high prevalence, the biological basis of glaucoma pathogenesis still is not yet fully understood, and the factors contributing to its progression are currently not well characterized. Intraocular pressure (IOP) is the only modifiable risk factor, and reduction of IOP is the standard treatment for glaucoma. However, lowering IOP itself is not always effective for preserving visual function in patients with primary open-angle glaucoma. The second messenger cyclic adenosine 3′,5′-monophosphate (cAMP) regulates numerous biological processes in the central nervous system including the retina and the optic nerve. Although recent studies revealed that cAMP generated by adenylyl cyclases (ACs) is important in regulating aqueous humor dynamics in ocular tissues, such as the ciliary body and trabecular meshwork, as well as cell death and growth in the retina and optic nerve, the functional role and significance of cAMP in glaucoma remain to be elucidated. In this review, we will discuss the functional role of cAMP in aqueous humor dynamics and IOP regulation, and review the current medications, which are related to the cAMP signaling pathway, for glaucoma treatment. Also, we will further focus on cAMP signaling in RGC growth and regeneration by soluble AC as well as ONH astrocytes by transmembrane ACs to understand its potential role in the pathogenesis of glaucoma neurodegeneration

Bestrophin 1 and retinal disease

Mutations in the gene BEST1 are causally associated with as many as five clinically distinct retinal degenerative diseases, which are collectively referred to as the "bestrophinopathies". These five associated diseases are: Best vitelliform macular dystrophy, autosomal recessive bestrophinopathy, adult-onset vitelliform macular dystrophy, autosomal dominant vitreoretinochoroidopathy, and retinitis pigmentosa. The most common of these is Best vitelliform macular dystrophy. Bestrophin 1 (Best1), the protein encoded by the gene BEST1, has been the subject of a great deal of research since it was first identified nearly two decades ago. Today we know that Best1 functions as both a pentameric anion channel and a regulator of intracellular Ca²⁺ signaling. Best1 is an integral membrane protein which, within the eye, is uniquely expressed in the retinal pigment epithelium where it predominantly localizes to the basolateral plasma membrane. Within the brain, Best1 expression has been documented in both glial cells and astrocytes where it functions in both tonic GABA release and glutamate transport. The crystal structure of Best1 has revealed critical information about how Best1 functions as an ion channel and how Ca²⁺ regulates that function. Studies using animal models have led to critical insights into the physiological roles of Best1 and advances in stem cell technology have allowed for the development of patient-derived, "disease in a dish" models. In this article we review our knowledge of Best1 and discuss prospects for near-term clinical trials to test therapies for the bestrophinopathies, a currently incurable and untreatable set of diseases.

Model systems for the study of steroid-induced IOP elevation

Steroid-induced IOP elevation affects a significant number of patients. It results from a decrease in outflow facility of the aqueous humor. To understand the pathophysiology of this condition a number of model systems have been created. These include ex-vivo cell and organ cultures as well as in-vivo animal models in organisms ranging from rodents to primates. These model systems can be used to investigate specific aspects of steroid-induced IOP elevation. This brief review summarizes the strengths and limitations of the various model systems and provides examples of where these systems have been successfully used to advance our understanding of steroid-induced IOP elevation.

A C-terminally truncated mouse Best3 splice variant targets and alters the ion balance in lysosome-endosome hybrids and the endoplasmic reticulum

The Bestrophin family has been characterized as Cl(-) channels in mammals and Na(+) channels in bacteria, but their exact physiological roles remian unknown. In this study, a natural C-terminally truncated variant of mouse Bestrophin 3 (Best3V2) expression in myoblasts and muscles is demonstrated. Unlike full-length Best3, Best3V2 targets the two important intracellular Ca stores: the lysosome and the ER. Heterologous overexpression leads to lysosome swelling and renders it less acidic. Best3V2 overexpression also results in compromised Ca(2+) release from the ER. Knocking down endogenous Best3 expression in myoblasts makes these cells more excitable in response to Ca(2+) mobilizing reagents, such as caffeine. We propose that Best3V2 in myoblasts may work as a tuner to control Ca(2+) release from intracellular Ca(2+) stores.

Physical Factors Affecting Outflow Facility Measurements in Mice

PURPOSE

Mice are commonly used to study conventional outflow physiology. This study examined how physical factors (hydration, temperature, and anterior chamber [AC] deepening) influence ocular perfusion measurements in mice.

METHODS

Outflow facility (C) and pressure-independent outflow (Fu) were assessed by multilevel constant pressure perfusion of enucleated eyes from C57BL/6 mice. To examine the effect of hydration, seven eyes were perfused at room temperature, either immersed to the limbus in saline and covered with wet tissue paper or exposed to room air. Temperature effects were examined in 12 eyes immersed in saline at 20 °C or 35 °C. Anterior chamber deepening was examined in 10 eyes with the cannula tip placed in the anterior versus posterior chamber (PC). Posterior bowing of the iris (AC deepening) was visualized by three-dimensional histology in perfusion-fixed C57BL/6 eyes and by spectral-domain optical coherence tomography in living CD1 mice.

RESULTS

Exposure to room air did not significantly affect C, but led to a nonzero Fu that was significantly reduced upon immersion in saline. Increasing temperature from 20 °C to 35 °C increased C by 2.5-fold, more than could be explained by viscosity changes alone (1.4-fold). Perfusion via the AC, but not the PC, led to posterior iris bowing and increased outflow.

CONCLUSIONS

Insufficient hydration contributes to the appearance of pressure-independent outflow in enucleated mouse eyes. Despite the large lens, AC deepening may artifactually increase outflow in mice. Temperature-dependent metabolic processes appear to influence conventional outflow regulation. Physical factors should be carefully controlled in any outflow studies involving mice.

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.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

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.

Non-continuous measurement of intraocular pressure in laboratory animals

Glaucoma is a leading cause of blindness, which is treatable but currently incurable. Numerous animal models therefore have both been and continue to be utilized in the study of numerous aspects of this condition. One important facet associated with the use of such models is the ability to accurately and reproducibly measure (by cannulation) or estimate (by tonometry) intraocular pressure (IOP). At this juncture there are several different approaches to IOP measurement in different experimental animal species, and the list continues to grow. We feel therefore that a review of this subject matter is timely and should prove useful to others who wish to perform similar measurements. The general principles underlying various types of tonometric and non-tonometric techniques for non-continuous determination of IOP are considered. There follows discussion of specific details as to how these techniques are applied to experimental animal species involved in the research of this disease. Specific comments regarding anesthesia, circadian rhythm, and animal handling are also included, especially in the case of rodents. Brief consideration is also given to possible future developments.

Mechanical analysis of rat trabecular meshwork

Stiffness of trabecular meshwork (TM) may play an important role in regulating outflow resistance in healthy and glaucomatous eyes. However, the current techniques for stiffness measurement can only be applied to TM dissected from human donor or large animal eyes. It is a challenge to measure TM stiffness in mouse/rat eyes because of their smaller sizes and the delicate nature of TM dissection. To this end, a new technique was developed to determine the stiffness of rat TM using atomic force microscopy (AFM). In the study, rat eyes were enucleated immediately after death and perfused with a tracer (Evans blue) for 40 min. Then, the anterior segment was dissected and flat-mounted on a Petri dish with TM facing upwards. An AFM probe with a gold-coated colloid tip was used to sequentially indent the corneal, TM, and uveoscleral tissues. Assuming these tissues to be neo-Hookean materials, the indentation data were analyzed with a newly developed mathematical model to calculate the apparent initial Young's moduli (E0)(app). The geometric mean & SE of (E0)(app) were 162 Pa & 1.2 (n = 13) for TM and 6189 Pa & 1.4 (n = 11) for cornea; and the difference was statistically significant (p < 0.01). The technique established in this study allows the use of rat eye as a potential model for investigation of TM stiffness and its influences on outflow resistance. Future studies may also utilize this technique to evaluate mechanisms of TM stiffness change caused by aging, outflow dysfunction, pathogenesis of glaucoma, and drug treatment.

Human aqueous humor exosomes

Aqueous humor (AH) is a dynamic intraocular fluid that supports the vitality of tissues that regulate intraocular pressure. We recently discovered that extracellular nanovesicles called exosomes are a major constituent of AH. Exosomes function in extracellular communication and contain proteins and small RNA. Our goal was to characterize the physical properties of AH exosomes and their exosomal RNA (esRNA) content. We isolated exosomes from human AH collected during cataract surgery from five patients using serial ultracentrifugation. We measured the size and concentration of AH exosomes in solution using nanoparticle tracking analysis. We found a single population of vesicles having a mean size of 121 ± 11 nm in the unprocessed AH. Data show that centrifugation does not significantly affect the mean particle size (121 ± 11 nm versus 124 ± 21 nm), but does impact the final number of exosomes in solution (87% loss from the unprocessed AH; n = 5). We extracted esRNA from the pooled human AH samples using miRCURY RNA isolation kit from Exiqon. The quality of extracted esRNA was evaluated using Agilent Bioanalyzer 2100 and was used to generate a sequencing library for small RNA sequencing with Illumina MiSeq sequencer. More than 10 different miRNAs were identified; abundant species included miR-486-5p, miR-204, and miR-184. We found that the majority of extracellular vesicles in the AH were in the exosome size range, suggesting that miRNAs housed within exosomes may function in communication between AH inflow and outflow tissues.

Soluble adenylyl cyclase in the eye

Adenylyl cyclases (ACs) are a family of enzymes which convert ATP to cAMP, an essential intermediate in many signal transduction pathways. Of the 10 AC genes in man, 9 fall into the category of transmembrane ACs (tmACs), which associate with G-protein coupled receptors (GPCRs) and are activated by forskolin. The 10th AC, termed soluble AC (sAC) is neither activated by forskolin nor does it interact with GPCRs. Rather, sAC can be found in many compartments within the cell and is activated by bicarbonate. As such, sAC is considered a major sensor of bicarbonate in many tissues. The pathways involving sAC vary in different tissues and organ systems, and are as diverse as facilitating sperm capacitation and regulating pressure in the eye. The role of sAC in the eye has only recently begun to receive significant attention. Here we summarize what is known about the roles of sAC in the eye. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

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.

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.

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.

Control of outflow resistance by soluble adenylyl cyclase

Abstract Glaucoma is a leading cause of blindness in the United States affecting as many as 2.2 million Americans. All current glaucoma treatment strategies aim to reduce intraocular pressure, even in patients with normal tension glaucoma. Typically, this is accomplished by reducing the rate of aqueous flow by limiting aqueous production or enhancing drainage using drugs and surgery. Whereas these strategies are effective in diminishing vision loss, some patients continue to lose vision and many discontinue use of their medications because of undesirable side effects. Drugs known to be effective in altering conventional outflow have for the most part been abandoned from modern clinical practice due to undesirable side effects. Identification of new drugs that could enhance conventional outflow, would offer additional options in the treatment of glaucoma and ocular hypertension. To this end, our laboratory has recently uncovered a novel pathway for regulation of conventional outflow by the ciliary body. This pathway is dependent on soluble adenylyl cyclase, an enzyme that catalyzes the generation of cyclic adenosine 3',5' monophosphate (cAMP) in response to bicarbonate.

Connexins form functional hemichannels in porcine ciliary epithelium

The expression of connexins in the ciliary epithelium is consistent with gap junctions between the pigmented (PE) and nonpigmented ciliary epithelium (NPE) that form when connexon hemichannels from adjacent cells pair to form a channel. Here we present evidence that suggests undocked connexons may form functional hemichannels that permit exchange of substances between NPE and the aqueous humor. Intact porcine eyes were perfused via the ciliary artery and propidium iodide (PI) (MW 668) was added to the aqueous humor compartment as a tracer. After calcium-free solution containing PI was introduced into the aqueous humor compartment for 30 min, fluorescence microscopy revealed PI in the NPE cell layer. PI entry into the NPE was inhibited by calcium and by the connexin antagonist 18α-glycyrrhetinic acid (18-AGA). Studies also were carried out with cultured porcine NPE. Under normal conditions, little PI entered the cultured cells but calcium-free medium stimulated PI accumulation and the entry was inhibited by 18-AGA. In cells loaded with calcein (MW 622), calcium-free solution stimulated calcein exit. 18-AGA partially suppressed calcein exit in calcium-free medium. Connexin 43 and connexin 50 proteins were detected by western blot analysis in both native and cultured NPE. In the intact eye, immunolocalization studies revealed connexin 50 at the basolateral, aqueous humor-facing, margin of the NPE. In contrast, connexin 43 was observed at the junction of the PE and NPE layer and on the basolateral membrane of PE. The results point to functional hemichannels at the NPE basolateral surface. It is feasible that hemichannels might contribute to the transfer of substances between the ciliary epithelium cytoplasm and aqueous humor.

Cyp1b1 mediates periostin regulation of trabecular meshwork development by suppression of oxidative stress

Mutation in CYP1B1 has been reported for patients with congenital glaucoma. However, the underlying mechanisms remain unknown. Here we show increased diurnal intraocular pressure (IOP) in Cyp1b1-deficient (Cyp1b1(-/-)) mice. Cyp1b1(-/-) mice presented ultrastructural irregular collagen distribution in their trabecular meshwork (TM) tissue along with increased oxidative stress and decreased levels of periostin (Postn). Increased levels of oxidative stress and decreased levels of Postn were also detected in human glaucomatous TM tissues. Furthermore, Postn-deficient mice exhibited TM tissue ultrastructural abnormalities similar to those of Cyp1b1(-/-) mice. Administration of the antioxidant N-acetylcysteine (NAC) restored structural abnormality of TM tissue in Cyp1b1(-/-) mice. In addition, TM cells prepared from Cyp1b1(-/-) mice exhibited increased oxidative stress, altered adhesion, and decreased levels of Postn. These aberrant cellular responses were reversed in the presence of NAC or by restoration of Cyp1b1 expression. Cyp1b1 knockdown or inhibition of CYP1B1 activity in Cyp1b1(+/+) TM cells resulted in a Cyp1b1(-/-) phenotype. Thus, metabolic activity of CYP1B1 contributes to oxidative homeostasis and ultrastructural organization and function of TM tissue through modulation of Postn expression.

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.

Regulation of anterior chamber drainage by bicarbonate-sensitive soluble adenylyl cyclase in the ciliary body

Glaucoma is a leading cause of blindness affecting as many as 2.2 million Americans. All current glaucoma treatment strategies aim to reduce intraocular pressure (IOP). IOP results from the resistance to drainage of aqueous humor (AH) produced by the ciliary body in a process requiring bicarbonate. Once secreted into the anterior chamber, AH drains from the eye via two pathways: uveoscleral and pressure-dependent or conventional outflow (C(t)). Modulation of "inflow" and "outflow" pathways is thought to occur via distinct, local mechanisms. Mice deficient in the bicarbonate channel bestrophin-2 (Best2), however, exhibit a lower IOP despite an increase in AH production. Best2 is expressed uniquely in nonpigmented ciliary epithelial (NPE) cells providing evidence for a bicarbonate-dependent communicative pathway linking inflow and outflow. Here, we show that bicarbonate-sensitive soluble adenylyl cyclase (sAC) is highly expressed in the ciliary body in NPE cells, but appears to be absent from drainage tissues. Pharmacologic inhibition of sAC in mice causes a significant increase in IOP due to a decrease in C(t) with no effect on inflow. In mice deficient in sAC IOP is elevated, and C(t) is decreased relative to wild-type mice. Pharmacologic inhibition of sAC did not alter IOP or C(t) in sAC-deficient mice. Based on these data we propose that the ciliary body can regulate C(t) and that sAC serves as a critical sensor of bicarbonate in the ciliary body regulating the secretion of substances into the AH that govern outflow facility independent of pressure.

Complex genetic mechanisms in glaucoma: an overview

Glaucomas comprise a group of hereditary optic neuropathies characterized by progressive and irreversible visual field loss and damage to the optic nerve head. It is a complex disease with multiple molecular mechanisms underlying its pathogenesis. Genetic heterogeneity is the hallmark of all glaucomas and multiple chromosomal loci have been linked to the disease, but only a few genes have been characterized, viz. myocilin (MYOC), optineurin (OPTN), WDR36 and neurotrophin-4 (NTF4) in primary open angle glaucoma (POAG) and CYP1B1 and LTBP2 in congenital and developmental glaucomas. Case-control-based association studies on candidate genes involved in different stages of glaucoma pathophysiology have indicated a very limited involvement. The complex mechanisms leading to glaucoma pathogenesis indicate that it could be attributed to multiple genes with varying magnitudes of effect. In this review, we provide an appraisal of the various efforts in unraveling the molecular mystery in glaucoma and also some future directions based on the available scientific knowledge and technological developments.

Outflow physiology of the mouse eye: pressure dependence and washout

PURPOSE

Mice are commonly used in glaucoma research, but relatively little is known about aqueous outflow dynamics in the species. To facilitate future use of the mouse as a model of aqueous humor outflow, several fundamental physiological parameters were measured in the mouse eye.

METHODS

Eyes from adult mice of either sex (C57BL/6 background) were enucleated, cannulated with a 33-gauge needle, and perfused at constant pressure while inflow was continuously measured.

RESULTS

At 8 mm Hg, total outflow facility (C(total)) was 0.022 ± 0.005 μL/min/mm Hg (all values mean ± SD; n = 21). The flow-pressure relationship was linear up to 35 mm Hg. The conventional outflow facility (C(conv)) was 0.0066 ± 0.0009 μL/min/mm Hg, and the unconventional outflow (F(u)) was 0.114 ± 0.019 μL/min, both measured at room temperature. At 8 mm Hg, 66% of the outflow was via the unconventional pathway. In a more than 2-hour-long perfusion at 8 mm Hg, the rate of facility change was 2.4% ± 5.4% (n = 11) of starting facility per hour. The ocular compliance (0.086 ± 0.017 μL/mm Hg; n = 5) was comparable to the compliance of the perfusion system (0.100 ± 0.004 μL/mm Hg).

CONCLUSIONS

Mouse eyes are similar to human eyes, in that they have no detectable washout rate and a linear pressure-flow relationship over a broad range of intraocular pressures. Because of the absence of washout and the apparent presence of a true Schlemm's canal, the mouse is a useful model for studying the physiology of the inner wall of Schlemm's canal and the conventional outflow tissues.

Duration of anesthesia affects intraocular pressure, but not outflow facility in mice

PURPOSE

The study of aqueous humor dynamics (AHD) in mice is becoming more prevalent as more strains with elevated intraocular pressure (IOP) are developed. High IOP is usually associated with reduced outflow facility making this one of the more important AHD parameters to evaluate. Ocular measurements in mice require anesthesia that has profound effects on IOP but unknown effects on outflow facility. This study evaluates the effects of anesthesia duration and latanoprost treatment on outflow facility and IOP in BALB/c mice.

METHODS

IOPs were measured in conscious and anesthetized mice by tonometry. Outflow facility was evaluated in 15-min intervals at three pressure levels over two 45-min periods. Comparisons were made between latanoprost-treated eyes and untreated contralateral eyes. To determine the effect of anesthesia duration on IOP, a microneedle method was used to follow IOP for 120 min in separate mice.

RESULTS

IOP was 9.7 +/- 0.3 mmHg (mean +/- SEM) in conscious mice and 7.1 +/- 0.02 within 10 min of anesthesia initiation (p < 0.01). IOP changed significantly between but not within assessment periods. IOP at 75 min was significantly (p = 0.004) reduced compared to IOP at 15 min after initial anesthesia. In control eyes, outflow facility did not change between the two 45-min assessment periods during the 120 min test (p = 0.80). In latanoprost-treated eyes, outflow facility increased compared with control eyes during both assessment periods (p = 0.03). A test of filters in series with known resistance found that the method was sensitive enough to detect a change in outflow facility of 0.001 microl/min/mmHg.

CONCLUSIONS

Administration of ketamine/xylazine anesthesia for 120 min did not alter outflow facility or lessen the effect of latanoprost on outflow facility in mice as determined by a new analysis system. Accurate IOP measurements must be made within minutes of anesthesia administration but outflow facility measurements can be made with less haste.

Chloride channels: often enigmatic, rarely predictable

Until recently, anion (Cl(-)) channels have received considerably less attention than cation channels. One reason for this may be that many Cl(-) channels perform functions that might be considered cell-biological, like fluid secretion and cell volume regulation, whereas cation channels have historically been associated with cellular excitability, which typically happens more rapidly. In this review, we discuss the recent explosion of interest in Cl(-) channels, with special emphasis on new and often surprising developments over the past five years. This is exemplified by the findings that more than half of the ClC family members are antiporters, and not channels, as was previously thought, and that bestrophins, previously prime candidates for Ca(2+)-activated Cl(-) channels, have been supplanted by the newly discovered anoctamins and now hold a tenuous position in the Cl(-) channel world.

Bestrophin 2 is expressed in human non-pigmented ciliary epithelium but not retinal pigment epithelium

PURPOSE

Mice in which bestrophin 2 (Best2) is disrupted exhibit changes in aqueous flow and drainage, resulting in a reduction in intraocular pressure in comparison to wild-type mice. Best2 encodes a putative anion channel localized uniquely to the basolateral plasma membrane of non-pigmented epithelium cells in mice. In this study, we examine the localization of Best2 in the human eye.

METHODS

Rabbit polyclonal antibodies recognizing human Best2 (hBest2) were generated and characterized for use in western blot, immunoprecipitation, and immunofluorescence assays. The expression of hBest2 using these antibodies was examined using human donor eye tissues.

RESULTS

We could not detect hBest2 in human ciliary bodies or other ocular tissues by western blot. However, when enriched by immunoprecipitation, hBest2 was identified in ciliary bodies, but not in the retinal pigment epithelium. Using immunofluorescence, we located hBest2 in the basolateral plasma membrane of non-pigmented epithelial cells.

CONCLUSIONS

We found expression of hBest2 similar to mice only in NPE cells. These data suggest that Best2 may play a functional role in the regulation of aqueous flow and drainage in humans. We conclude that Best2 represents a new potential target for glaucoma therapy.

Suppression of Ca2+ signaling in a mouse model of Best disease

Mutations in BEST1, encoding bestrophin-1 (Best1), cause Best vitelliform macular dystrophy (BVMD), a dominantly inherited macular degeneration characterized by a diminished electrooculogram light peak (LP), lipofuscin in retinal pigment epithelial cells (RPE), and fluid- and debris-filled retinal detachments. To understand the pathogenesis of BVMD we generated knock-in mice carrying the BVMD-causing mutation W93C in Best1. Both Best1(+/W93C)and Best1(W93C/W93C) mice had normal ERG a- and b-waves, but exhibited an altered LP luminance response reminiscent of that observed in BVMD patients. Morphological analysis identified fluid- and debris-filled retinal detachments in mice as young as 6 months of age. By 18-24 months of age Best1(+/W93C)and Best1(W93C/W93C) mice exhibited enhanced accumulation of lipofuscin in the RPE, and a significant deposition of debris composed of unphagocytosed photoreceptor outer segments and lipofuscin granules in the subretinal space. Although Best1 is thought to function as a Ca(2+)-activated Cl(-) channel, RPE cells from Best1(W93C) mice exhibited normal Cl(-) conductances. We have previously shown that Best1(-/-) mice exhibit increased [Ca(2+)](i) in response to ATP stimulation. However, ATP-stimulated changes in [Ca(2+)](i) in RPE cells from Best1(+/W93C) and Best1(W93C/W93C) mice were suppressed relative to Best1(+/+) littermates. Based on these data we conclude that mice carrying the Best1(W93C) mutation are a valid model for BVMD. Furthermore, these data suggest that BVMD is not because of Best1 deficiency, as the phenotypes of Best1(+/W93C) and Best1(W93C/W93C) mice are distinct from that of Best1(-/-) mice with regard to lipofuscin accumulation, and changes in the LP and ATP Ca(2+) responses.

Outflow facility in mice with a targeted type I collagen mutation

PURPOSE

Transgenic Col1a1(r/r) mice develop elevated intraocular pressure (IOP) with an open angle and progressive optic nerve axon loss. The present study was undertaken to evaluate aqueous outflow facility and its age dependence in these mice.

METHODS

Homozygous B6;129S4-Col1a1(tm1Jae) mice and corresponding wild-type Col1a1(+/+) mice from 12 to 56 weeks of age were anesthetized, and IOP was measured with a microneedle. Outflow facility was determined by a two-level, constant-pressure infusion

METHOD

Type I collagen, subunit alpha1 was assessed in sclera and choroid by Western blot analysis.

RESULTS

The mean IOP in 12- to 36-week-old transgenic Col1a1(r/r) mice was 25.1% higher than in control Col1a1(+/+) mice (P < 0.01), whereas the mean outflow facility was 25.4% lower than in control mice (P < 0.01). After this period, the mean IOP in 42- to 56-week-old transgenic mice returned to normal levels, whereas outflow facility increased by 36.0%. Over the 12- to 56-week study period, IOP and outflow facility in the transgenic mice were inversely correlated (r(2) = -0.702, P < 0.01). Collagen I alpha1 content was greater in 37- and 43-week-old transgenic mice than in age-matched wild-type control mice.

CONCLUSIONS

Outflow facility is reduced in transgenic Col1a1(r/r) mice with IOP elevation. The inverse correlation of IOP elevation to facility reduction indicates that increased resistance in the aqueous outflow pathway contributes to ocular hypertension in Col1a1(r/r) mice. These mice may be useful as a model for open-angle glaucoma, as well as for assessing the relationship between collagen type I metabolism and aqueous outflow.

Best1 is a gene regulated by nerve injury and required for Ca2+-activated Cl- current expression in axotomized sensory neurons

We investigated the molecular determinants of Ca(2+)-activated chloride current (CaCC) expressed in adult sensory neurons after a nerve injury. Dorsal root ganglia express the transcripts of three gene families known to induce CaCCs in heterologous systems: bestrophin, tweety, and TMEM16. We found with quantitative transcriptional analysis and in situ hybridization that nerve injury induced upregulation of solely bestrophin-1 transcripts in sensory neurons. Gene screening with RNA interference in single neurons demonstrated that mouse Best1 is required for the expression of CaCC in injured sensory neurons. Transfecting injured sensory neurons with bestrophin-1 mutants inhibited endogenous CaCC. Exogenous expression of the fusion protein green fluorescent protein-Bestrophin-1 in naive neurons demonstrated a plasma membrane localization of the protein that generates a CaCC with biophysical and pharmacological properties similar to endogenous CaCC. Our data suggest that Best1 belongs to a group of genes upregulated by nerve injury and supports functional CaCC expression in injured sensory neurons.

Calcium-activated chloride currents in olfactory sensory neurons from mice lacking bestrophin-2

Olfactory sensory neurons use a chloride-based signal amplification mechanism to detect odorants. The binding of odorants to receptors in the cilia of olfactory sensory neurons activates a transduction cascade that involves the opening of cyclic nucleotide-gated channels and the entry of Ca(2+) into the cilia. Ca(2+) activates a Cl(-) current that produces an efflux of Cl(-) ions and amplifies the depolarization. The molecular identity of Ca(2+)-activated Cl(-) channels is still elusive, although some bestrophins have been shown to function as Ca(2+)-activated Cl(-) channels when expressed in heterologous systems. In the olfactory epithelium, bestrophin-2 (Best2) has been indicated as a candidate for being a molecular component of the olfactory Ca(2+)-activated Cl(-) channel. In this study, we have analysed mice lacking Best2. We compared the electrophysiological responses of the olfactory epithelium to odorant stimulation, as well as the properties of Ca(2+)-activated Cl(-) currents in wild-type (WT) and knockout (KO) mice for Best2. Our results confirm that Best2 is expressed in the cilia of olfactory sensory neurons, while odorant responses and Ca(2+)-activated Cl(-) currents were not significantly different between WT and KO mice. Thus, Best2 does not appear to be the main molecular component of the olfactory channel. Further studies are required to determine the function of Best2 in the cilia of olfactory sensory neurons.

Functional roles of bestrophins in ocular epithelia

There are four members of the bestrophin family of proteins in the human genome, of which two are known to be expressed in the eye. The gene BEST1 (formerly VMD2) which encodes the protein bestrophin-1 (Best1) was first identified in 1998. Mutations in this gene have now been associated with four clinically distinguishable human eye diseases, collectively referred to as "bestrophinopathies". Over the last decade, laboratories have sought to understand how Best1 mutations could result in eye diseases that range in presentation from macular degeneration to nanophthalmos. The majority of our knowledge comes from studies that have sought to understand how Best1 mutations or dysfunction could induce the classical symptoms of the most common of these diseases: Best vitelliform macular dystrophy (BVMD). BVMD is a dominant trait that is characterized electrophysiologically by a diminished electrooculogram light peak with a normal clinical electroretinogram. This together with the localization of Best1 to the retinal pigment epithelium (RPE) basolateral plasma membrane and data from heterologous expression studies, have led to the proposal that Best1 generates the light peak, and that bestrophins are a family of Ca(2+) activated Cl(-) channels (CaCCs). However, data from Best1 knock-out and knock-in mice, coupled with the recent discovery of a recessive bestrophinopathy suggest that Best1 does not generate the light peak. Recently Best2 was found to be expressed in non-pigmented epithelia in the ciliary body. However, aqueous dynamics in Best2 knock-out mice do not support a role for Best2 as a Cl(-) channel. Thus, the purported CaCC function of the bestrophins and how loss of this function relates to clinical disease needs to be reassessed. In this article, we examine data obtained from tissue-type and animal models and discuss the current state of bestrophin research, what roles Best1 and Best2 may play in ocular epithelia and ocular electrophysiology, and how perturbation of these functions may result in disease.