Intraocular pressure measurement in mice: a comparison between Goldmann and rebound tonometry

Macrophage-induced integrin signaling promotes Schlemm's canal formation to prevent intraocular hypertension and glaucomatous optic neuropathy

Schlemm's canal (SC) functions to maintain proper intraocular pressure (IOP) by draining aqueous humor and has emerged as a promising therapeutic target for glaucoma, the second-leading cause of irreversible blindness worldwide. However, our current understanding of the mechanisms governing SC development and functionality remains limited. Here, we show that vitronectin (VTN) produced by limbal macrophages promotes SC formation and prevents intraocular hypertension by activating integrin αvβ3 signaling. Genetic inactivation of this signaling system inhibited the phosphorylation of AKT and FOXO1 and reduced β-catenin activity and FOXC2 expression, thereby causing impaired Prox1 expression and deteriorated SC morphogenesis. This ultimately led to increased IOP and glaucomatous optic neuropathy. Intriguingly, we found that aged SC displayed downregulated integrin β3 in association with dampened Prox1 expression. Conversely, FOXO1 inhibition rejuvenated the aged SC by inducing Prox1 expression and SC regrowth, highlighting a possible strategy by targeting VTN/integrin αvβ3 signaling to improve SC functionality.

Loss of Sarm1 reduces retinal ganglion cell loss in chronic glaucoma

Glaucoma is one of the leading causes of irreversible blindness worldwide and vision loss in the disease results from the deterioration of retinal ganglion cells (RGC) and their axons. Metabolic dysfunction of RGC plays a significant role in the onset and progression of the disease in both human patients and rodent models, highlighting the need to better define the mechanisms regulating cellular energy metabolism in glaucoma. This study sought to determine if Sarm1, a gene involved in axonal degeneration and NAD+ metabolism, contributes to glaucomatous RGC loss in a mouse model with chronic elevated intraocular pressure (IOP). Our data demonstrate that after 16 weeks of elevated IOP, Sarm1 knockout (KO) mice retain significantly more RGC than control animals. Sarm1 KO mice also performed significantly better when compared to control mice during optomotor testing, indicating that visual function is preserved in this group. Our findings also indicate that Sarm1 KO mice display mild ocular developmental abnormalities, including reduced optic nerve axon diameter and lower visual acuity than controls. Finally, we present data to indicate that SARM1 expression in the optic nerve is most prominently associated with oligodendrocytes. Taken together, these data suggest that attenuating Sarm1 activity through gene therapy, pharmacologic inhibition, or NAD+ supplementation, may be a novel therapeutic approach for patients with glaucoma.

Sevoflurane-induced hypotension causes cognitive dysfunction and hippocampal inflammation in mice

Sevoflurane commonly adopted for anesthetic in clinical practice, however, its influences on cerebral blood flow and cognitive function remain controversial. Herein, the sevoflurane-induced hypotension on arterial blood pressure, cerebral blood flow, cognitive function, and hippocampal inflammation was investigated in mice. A significant decrease in arterial blood pressure and cerebral blood flow was indicated by the sevoflurane anesthesia treatment. Moreover, sevoflurane-induced hypotension was associated with the impaired cognitive function and the increased levels of NLRP3 inflammasome activation and oxidative stress in hippocampus. These findings suggest that sevoflurane-induced hypotension may lead to the cognitive dysfunction and hippocampal inflammation.

Comparison of the TONOVET Plus®, TonoVet®, and Tono-Pen Vet™ tonometers in normal cats and cats with glaucoma

OBJECTIVES

To determine the accuracy, precision, and clinical applicability of the ICare® TONOVET Plus (TVP) in cats.

ANIMALS AND PROCEDURES

IOP readings obtained with the TVP were compared to values obtained concurrently with the original TONOVET (TV01) and Tono-Pen Vet™ (TP) in 12 normal cats (24 eyes) and 8 glaucomatous LTBP2-mutant cats (13 eyes) in vivo. Reproducibility of TVP readings was also assessed for three observers in the above cats. The anterior chambers of five different normal cat eyes were cannulated ex vivo. IOP was measured with the TVP, TV01, and TP at manometric IOPs ranging from 5 to 70 mmHg. Data were analyzed by linear regression, ANOVA and Bland-Altman plots. ANOVA was used to assess reproducibility of TVP readings obtained by different observers and an ANCOVA model controlled for variation of individual cats. p < .05 was considered significant.

RESULTS

TVP values strongly correlated with TV01 values (y = 1.045x + 1.443, R2  = .9667). The TP significantly underestimated IOP relative to the TVP and TV01, particularly at high IOP. IOP values obtained by 1 observer were significantly higher (~1 mmHg average) compared to the other 2 observers via ANCOVA analysis (p = .0006479 and p = .0203). Relative to manometry, the TVP and TV01 were significantly more accurate (p < .0001) and precise (p < .0070) than the TP in ex vivo eyes.

CONCLUSIONS

IOP readings obtained with the TVP and TV01 are broadly interchangeable between models and between observers, but subtle differences may be important in a research context. TP readings vastly underestimate high IOP in feline glaucoma.

Cannabidiol Loaded Topical Ophthalmic Nanoemulsion Lowers Intraocular Pressure in Normotensive Dutch-Belted Rabbits

Cannabidiol (CBD) is the major non-psychoactive and most widely studied of the cannabinoid constituents and has great therapeutic potential in a variety of diseases. However, contradictory reports in the literature with respect to CBD's effect on intraocular pressure (IOP) have raised concerns and halted research exploring its use in ocular therapeutics. Therefore, the current investigation aimed to further evaluate CBD's impact on the IOP in the rabbit model. CBD nanoemulsions, containing Carbopol^® 940 NF as a mucoadhesive agent (CBD-NEC), were prepared using hot-homogenization followed by probe sonication. The stability of the formulations post-moist-heat sterilization, in terms of physical and chemical characteristics, was studied for three different storage conditions. The effect of the formulation on the intraocular pressure (IOP) profile in normotensive Dutch Belted male rabbits was then examined. The lead CBD-NEC formulation (1% w/v CBD) exhibited a globule size of 259 ± 2.0 nm, 0.27 ± 0.01 PDI, and 23.2 ± 0.4 cP viscosity, and was physically and chemically stable for one month (last time point tested) at 4 °C, 25 °C, and 40 °C. CBD-NEC significantly lowered the IOP in the treated eyes for up to 360 min, with a peak drop in IOP of 4.5 mmHg observed at the 150 min time point, post-topical application. The IOP of the contralateral eye (untreated) was also observed to be lowered significantly, but the effect lasted up to the 180 min time point only. Overall, topically administered CBD, formulated in a mucoadhesive nanoemulsion formulation, reduced the IOP in the animal model studied. The results support further exploration of CBD as a therapeutic option for various inflammation-based ocular diseases.

Changes in Intraocular Pressure following Narcosis With Medetomidine, Midazolam, and Fentanyl in Association With Initial Intraocular Pressure in Mice

PURPOSE

This article describes the development of decreased intraocular pressure (IOP) under general anesthesia with medetomidine, midazolam, and fentanyl in mice with normal and elevated IOP.

METHODS

IOP was measured using the iCare Tonolab rebound tonometer. Twelve 3-4 months-old male and female C57BL/6J mice were randomized to a control group with physiological IOP and a high IOP group with experimentally induced ocular hypertension using tarsal injections of dexamethasone-21-acetate. For anesthesia, medetomidine and midazolam were used, subgroups additionally received fentanyl. IOP was measured every 2.5 min for 30 min.

RESULTS

Control group differed with 14.89 mmHg (SEM: 0.58) significantly (p = 0.0002) from the high IOP group with initial 20.44 mmHg (SEM: 0.75). All groups showed a significant (p < 0.05) decrease in IOP under general anesthesia. There was no significant difference in IOP development and decrease between the group additionally receiving fentanyl and the group without fentanyl. The decrease in IOP was highly dependent on the initial value, with the high IOP group showing a greater decrease. After 10 min, no significant difference in IOP could be detected between the high IOP and control group.

CONCLUSIONS

In mice, general anesthesia with medetomidine and midazolam leads to a declining IOP over time. Adding fentanyl to the anesthesia did not alter these effects. The decline is time-dependent and IOP-dependent.

Systemic Treatment with Pioglitazone Reverses Vision Loss in Preclinical Glaucoma Models

Neuroinflammation significantly contributes to the pathophysiology of several neurodegenerative diseases. This is also the case in glaucoma and may be a reason why many patients suffer from progressive vision loss despite maximal reduction in intraocular pressure. Pioglitazone is an agonist of the peroxisome proliferator-activated receptor gamma (PPARγ) whose pleiotrophic activities include modulation of cellular energy metabolism and reduction in inflammation. In this study we employed the DBA2/J mouse model of glaucoma with chronically elevated intraocular pressure to investigate whether oral low-dose pioglitazone treatment preserves retinal ganglion cell (RGC) survival. We then used an inducible glaucoma model in C57BL/6J mice to determine visual function, pattern electroretinographs, and tracking of optokinetic reflex. Our findings demonstrate that pioglitazone treatment does significantly protect RGCs and prevents axonal degeneration in the glaucomatous retina. Furthermore, treatment preserves and partially reverses vision loss in spite of continuously elevated intraocular pressure. These data suggest that pioglitazone may provide treatment benefits for those glaucoma patients experiencing continued vision loss.

Consensus Recommendation for Mouse Models of Ocular Hypertension to Study Aqueous Humor Outflow and Its Mechanisms

Due to their similarities in anatomy, physiology, and pharmacology to humans, mice are a valuable model system to study the generation and mechanisms modulating conventional outflow resistance and thus intraocular pressure. In addition, mouse models are critical for understanding the complex nature of conventional outflow homeostasis and dysfunction that results in ocular hypertension. In this review, we describe a set of minimum acceptable standards for developing, characterizing, and utilizing mouse models of open-angle ocular hypertension. We expect that this set of standard practices will increase scientific rigor when using mouse models and will better enable researchers to replicate and build upon previous findings.

Effects of Hydrostatic Pressure on Electrical Retinal Activity in a Multielectrode Array-Based ex vivo Glaucoma Acute Model

Glaucoma is a heterogeneous eye disease causing atrophy of the optic nerve head (ONH). The optic nerve is formed by the axons of the retinal ganglion cells (RGCs) that transmit visual input to the brain. The progressive RGC loss during glaucoma leads to irreversible vision loss. An elevated intraocular pressure (IOP) is described as main risk factor in glaucoma. In this study, a multielectrode array (MEA)-based ex vivo glaucoma acute model was established and the effects of hydrostatic pressure (10, 30, 60, and 90 mmHg) on the functionality and survival of adult male and female wild-type mouse (C57BL/6) retinae were investigated. Spontaneous activity, response rate to electrical and light stimulation, and bursting behavior of RGCs was analyzed prior, during, and after pressure stress. No pressure related effects on spontaneous firing and on the response rate of the RGCs were observed. Even a high pressure level (90 mmHg for 2 h) did not disturb the RGC functionality. However, the cells’ bursting behavior significantly changed under 90 mmHg. The number of spikes in bursts doubled during pressure application and stayed on a high level after pressure stress. Addition of the amino sulfonic acid taurine (1 mM) showed a counteracting effect. OFF ganglion cells did not reveal an increase in bursts under pressure stress. Live/dead staining after pressure application showed no significant changes in RGC survival. The findings of our ex vivo model suggest that RGCs are tolerant toward high, short-time pressure stress.

Evidence for ceramide induced cytotoxicity in retinal ganglion cells

Ceramides are bioactive compounds that play important roles in regulating cellular responses to extracellular stimuli and stress. Previous studies have shown that ceramides contribute to retinal degeneration associated with ischemic and ocular hypertensive stress. Acid sphingomyelinase (ASMase) is one of the major enzymes responsible for the stress-induced generation of ceramides. The goals of this study are to investigate the effects of ceramides on retinal ganglion cells (RGCs) and of ASMase inhibition in ocular hypertensive mice. Induced pluripotent stem cell (iPSC)-derived RGCs and primary cultures of human optic nerve head astrocytes were used to characterize the response to C2-ceramide. Microbead-induced ocular hypertension in the ASMase heterozygote mouse model was used to confirm the physiological relevance of in vitro studies. In mice, RGC function and morphology were assessed with pattern ERG (pERG) and immunofluorescence. The addition of C2-ceramide to iPSC-derived RGCs produced a significant concentration- and time-dependent reduction in cell numbers when compared to control cultures. While the addition of C2-ceramide to astrocytes did not affect viability, it resulted in a 2.6-fold increase in TNF-α secretion. The addition of TNF-α or conditioned media from C2-ceramide-treated astrocytes to RGC cultures significantly reduced cell numbers by 56.1 ± 8.4% and 24.7 ± 4.8%, respectively. This cytotoxic response to astrocyte-conditioned media was blocked by TNF-α antibody. In ASMase heterozygote mice, functional and morphological analyses of ocular hypertensive eyes reveal significantly less RGC degeneration when compared with hypertensive eyes from wild-type mice. These results provide evidence that ceramides can induce RGC cell death by acting directly, as well as indirectly via the secretion of TNF-α from optic nerve head astrocytes. In vivo studies in mice provide evidence that ceramides derived through the activity of ASMase contribute to ocular hypertensive injury. Together these results support the importance of ceramides in the pathogenesis of ocular hypertensive injury to the retina.

Early Functional Impairment in Experimental Glaucoma Is Accompanied by Disruption of the GABAergic System and Inceptive Neuroinflammation

Glaucoma is a leading cause of irreversible blindness worldwide, and increased intraocular pressure (IOP) is a major risk factor. We aimed to determine if early functional and molecular differences in the glaucomatous retina manifest before significant retinal ganglion cell (RGC) loss is apparent. Adenoviral vectors expressing a pathogenic form of myocilin (Ad5.MYOC) were used to induce IOP elevation in C57BL/6 mice. IOP and pattern electroretinograms (pERG) were recorded, and retinas were prepared for RNA sequencing, immunohistochemistry, or to determine RGC loss. Ocular injection of Ad5.MYOC leads to reliable IOP elevation, resulting in significant loss of RGC after nine weeks. A significant decrease in the pERG amplitude was evident in eyes three weeks after IOP elevation. Retinal gene expression analysis revealed increased expression for 291 genes related to complement cascade, inflammation, and antigen presentation in hypertensive eyes. Decreased expression was found for 378 genes associated with the γ-aminobutyric acid (GABA)ergic and glutamatergic systems and axon guidance. These data suggest that early functional changes in RGC might be due to reduced GABA_A receptor signaling and neuroinflammation that precedes RGC loss in this glaucoma model. These initial changes may offer new targets for early detection of glaucoma and the development of new interventions.

T and B Lymphocyte Deficiency in Rag1-/- Mice Reduces Retinal Ganglion Cell Loss in Experimental Glaucoma

Purpose

We previously demonstrated that passive transfer of lymphocytes from glaucomatous mice induces retinal ganglion cell (RGC) damage in recipient animals, suggesting a role for immune responses in the multifactorial pathophysiology of glaucoma. Here we evaluate whether absence of an adaptive immune response reduces RGC loss in glaucoma.

Methods

Elevated intraocular pressure (IOP) was induced in one eye of C57BL/6J (B6) or T- and B-cell–deficient Rag1−/− knockout mice. After 16 weeks RGC density was determined in both the induced and the normotensive contralateral eyes. Data were compared to mice having received injections of “empty” vector (controls). The number of extravascular CD3+ cells in the retinas was determined using FACS.

Results

Retinas of eyes with elevated IOP contain significantly more extravasated CD3+ cells than control retinas (46.0 vs. 27.1, P = 0.025). After 16 weeks of elevated IOP the average RGC density in B6 mice decreased by 20.7% (P = 1.9 × 10⁻⁴). In contrast, RGC loss in Rag1−/− eyes with elevated IOP was significantly lower (10.3%, P = 0.006 vs. B6). RGC loss was also observed in the contralateral eyes of B6 mice, despite the absence of elevated IOP in those eyes (10.1%; P = 0.008). In RAG1−/− loss in the contralateral eyes was minimal (3.1%) and significantly below that detected in B6 (P = 0.02).

Conclusions

Our findings demonstrate that T Rag1−/− mice are significantly protected from glaucomatous RGC loss. In this model, lymphocyte activity contributes to approximately half of all RGC loss in eyes with elevated IOP and to essentially all loss observed in normotensive contralateral eyes.

A Comparison of Applanation and Rebound Tonometers in Young Chicks

PURPOSE

To assess the validity of and compare applanation and rebound tonometry readings of intraocular pressure in alert normal chicks from ages 3 to 45 days.

METHODS

Intraocular pressures (IOPs) were measured weekly in awake White Leghorn chicks, from ages 3-45 days (n = 22-30 per age group), with both applanation Tono-Pen and rebound TonoLab tonometers. Three repeated measurements on individual eyes were used to derive variance data for both instruments at each age. Calibration curves were also derived for each instrument and each age, weekly from ages 10-45 days (n = 3-4 per age group), from in situ manometry data collected over IOP settings of 0 to 100 mmHg in 5 mmHg steps in cannulated eyes.

RESULTS

The TonoLab showed less within measurement variability, but more variability with age, than the Tono-Pen. The coefficient of variation ranged from 3.8-8.3% for the TonoLab, compared to 11.0-19.7% for the Tono-Pen across all ages. For the youngest, 3 day-old chicks, mean IOPs recorded with the Tono-Pen and TonoLab were not significantly different (17.0 ± 5.6 and 15.2 ± 3.7 mmHg, respectively, P = .27). However, with increasing age, IOP readings significantly increased for the TonoLab (P < .001), whereas Tono-Pen readings did not. Compared to manometry settings, the Tono-Pen tended to underestimate IOPs while the TonoLab overestimated IOPs over the range 20-60 mmHg, saturating thereafter; there were also age-dependent differences for the TonoLab.

CONCLUSIONS

Both the Tono-Pen and TonoLab gave IOP readings that differed from manometry settings in normal young chicks over some or all of the ages tested. These results reinforce the importance of calibrating clinical tonometers in animal studies involving IOP as a key variable.

Validation and comparison of four handheld tonometers in normal ex vivo canine eyes

OBJECTIVES

To determine the accuracy and precision of the Icare^® TONOVET Plus rebound tonometer and the Tono-Pen AVIA Vet™ applanation tonometer for intraocular pressure (IOP) measurement in normal ex vivo canine eyes and comparison to earlier models of these tonometers.

ANIMALS & PROCEDURES

The anterior chambers of six normal dog eyes were cannulated ex vivo. IOP was measured with the TONOVET (TV01), TONOVET Plus, Tono-Pen Vet™, and Tono-Pen AVIA Vet™ at manometric IOPs ranging from 5 to 70 mm Hg. Data were analyzed by linear regression, ANOVA and Bland-Altman plots. A P value ≤ .05 was considered significant.

RESULTS

Intraocular pressure values obtained using the TONOVET Plus and TV01 were significantly more accurate than with the Tono-Pen VET and Tono-Pen AVIA Vet, particularly at higher IOPs (30-70 mm Hg). Accuracy was not significantly different between any of the devices in the low to normal physiologic IOP range (5-25 mm Hg). Level of precision was high for all devices, though the TONOVET Plus was more precise than the Tono-Pen Vet in the 5-25 mmHg range and the TV01 was more precise than the Tono-Pen AVIA Vet over the whole IOP range.

CONCLUSIONS

All devices underestimated IOP, particularly at higher pressures. Rebound tonometers were more accurate over the full range of IOP tested and in the high IOP range; however, there were no significant differences in accuracy among devices in the physiologic IOP range. All tonometers can provide clinically useful IOP readings in dogs, but rebound and applanation tonometers should not be used interchangeably.

A Population Study of Common Ocular Abnormalities in C57BL/6N rd8 Mice

Purpose

The purpose of this study was to quantify the frequency and severity of ocular abnormalities affecting wild-type C57BL/6N mice, the most common strain used worldwide for the creation of single-gene knockouts.

Methods

A total of 2773 animals (5546 eyes) were examined at one colony at UC Davis and in three more colonies at the Institut Clinique de la Souris in Strasbourg, France. Mice were examined at 15 to 16 weeks postnatal age by performing anterior segment biomicroscopy, posterior segment examination by indirect ophthalmoscopy, intraocular pressure measurement, and optical coherence tomography of anterior and posterior segment structures.

Results

Common ocular findings in the C57BL/6N strain included corneal deposits (3%), increased optical density of the anterior lens capsule (67%), punctate nuclear cataracts (98%), vitreous crystalline deposits (61%), hyaloid vascular remnant (6%), and retinal dysplasia attributed to the rd8 mutation (58%). Interestingly, retinal dysplasia was more common in male mice in all four breeding colonies evaluated in this study. The thickness of ocular tissues and compartments were measured by spectral-domain optical coherence tomography, including the central cornea, anterior chamber, vitreous, and retinal layers. Intraocular pressure was measured by rebound tonometry.

Conclusions

Ocular abnormalities are common in anterior and posterior segments of the C57BL/6N mouse, the most common background on which single-gene knockout mice have been made. It is important that vision scientists understand the extent and variability of ocular findings associated with this particular genetic background of mice.

Restoration of Aqueous Humor Outflow Following Transplantation of iPSC-Derived Trabecular Meshwork Cells in a Transgenic Mouse Model of Glaucoma

Purpose

Primary open-angle glaucoma (POAG) is particularly common in older individuals and associated with pathologic degeneration of the trabecular meshwork (TM). We have shown previously that transplantation of induced pluripotent stem cell (iPSC) derived TM cells restores aqueous humor dynamics in young transgenic mice expressing a pathogenic form of human myocilin (Tg-MYOC^Y437H). This study was designed to determine if this approach is feasible in older mice with more pronounced TM dysfunction.

Methods

Mouse iPSC were differentiated toward a TM cell phenotype (iPSC-TM) and injected into the anterior chamber of 6-month-old Tg-MYOC^Y437H or control mice. IOP and aqueous humor outflow facility were recorded for up to 3 months. Transmission electron microscopy, Western blot, and immunohistochemistry were performed to analyze TM morphology, quantify endoplasmic reticulum (ER) stress, and assess TM cellularity.

Results

A 12 weeks after transplantation, IOP in iPSC-TM recipients was statistically lower and outflow facility was significantly improved compared to untreated controls. The number of endogenous TM cells increased significantly in iPSC-TM recipients along with the appearance of TM cells immmunopositive for a marker of cellular division. Morphologically, transplantation of iPSC-TM preserves ER structure 12 weeks after transplantation. However, myocilin and calnexin expression levels remain elevated in transplanted eyes of these 9-month-old Tg-MYOC^Y437H mice, indicating that ER stress persists within the TM.

Conclusions

Transplantation of iPSC-TM can restore IOP and outflow facility in aged Tg-MYOC^Y437H mice. This type of stem cell–based therapy is a promising possibility for restoration of IOP control in some glaucoma patients.

Tonometer validation and intraocular pressure reference values in the normal chinchilla (Chinchilla lanigera)

OBJECTIVES

To determine accuracy and precision of three commonly used tonometers (TonoVet^® and TonoLab^® (ICare Oy, Finland)-rebound tonometers, and Tono-Pen VET™ (Reichert, NY)-applanation tonometer) in normal chinchillas, and to establish a normal intraocular pressure (IOP) reference range in this species.

METHODS

The anterior chambers of three chinchilla eyes were cannulated ex vivo and readings obtained at manometric IOPs from 5 to 80 mmHg, using each of the three tonometers in random order. Data were analyzed by linear regression, ANOVA, and Bland-Altman plots. Tonometry was performed in both eyes of 60 chinchillas (age 8 weeks-16.2 years) using the TonoVet^® and relationship between age and IOP analyzed using linear regression. For all statistical tests, P < 0.05 was significant.

RESULTS

Intraocular pressure values obtained using the Tono-Pen VET™ and TonoVet^® (in dog calibration mode;'d') showed strong linear correlation with manometry within the physiologic and clinically relevant range of IOP (0-50 mmHg). The TonoVet^® 'd' setting displayed significantly greater precision over the full range of IOP evaluated than the Tono-Pen VET™, and both TonoVet and Tono-Pen VET™ were significantly more accurate than the TonoLab^® tonometer. Mean ± SD IOP (TonoVet^® 'd') in chinchillas was 9.7 ± 2.5 mmHg, and the 95% reference interval was 4.7-14.7 mmHg.

CONCLUSIONS

Both the Tono-Pen VET™ and TonoVet^® provided clinically acceptable estimates of IOP in chinchillas. The TonoVet^® provides accurate and precise IOP values, while Tono-Pen VET™ derived measurements showed greater variability. Values obtained either with the TonoLab^® or TonoVet^® used in the 'unspecified' calibration setting were inaccurate in this species.

Intraocular pressure with rebound tonometry and effects of topical intraocular pressure reducing medications in guinea pigs

AIM

To investigate the intraocular pressure (IOP) of adult guinea pig eyes with rebound tonometry (RBT), and assess the effects of four distinctive topical IOP reducing medications including Carteolol, Brimonidine, Brinzolamide and Latanoprost.

METHODS

The IOPs of twenty-four 12-week-old guinea pigs (48 eyes) were measured every two hours in one day with RBT as baselines. All the animals were then divided into four groups (Carteolol, Brimonidine, Brinzolamide and Latanaprost groups, n=6). The IOPs were measured and compared to the baseline 1, 2, 3, 5, 7, 9, 15 and 24h after treatment.

RESULTS

The mean baseline IOP of 24 guinea pigs (48 eyes) was 10.3±0.36 mm Hg (6-13 mm Hg) and no binocular significant differences of IOPs were observed (t=1.76, P>0.05). No significant difference of IOP in Carteolol group at each time point was observed before and after treatment (t=1.48, P>0.05). In Brimonidine group, IOP was 2.2±1.9 mm Hg lower than the baseline after one hour (t=3.856, P=0.003) and lasted for one hour. In Brinzolamide group, IOP was 1.4±1.1 mm Hg lower than the baseline after one hour (t=4.53, P=0.001) and lasted for 7h and the IOP declined most at 3h. In Latanaprost group, IOP was 2.1±1.3 mm Hg lower than the baseline after one hour (t=6.11, P=0.001) and lasted for one hour.

CONCLUSION

The IOP of guinea pig eyes is relatively stable compared to human eyes. In four reducing IOP medications, no significant effect of Carteolol is observed. Brinzolamide has the longest duration, while the Brimonidine has the shortest duration and the maximum level of treatment.

Transgenic TBK1 mice have features of normal tension glaucoma

Duplication of the TBK1 gene is associated with 1-2% of normal tension glaucoma, a common cause of vision loss and blindness that occurs without grossly abnormal intraocular pressure. We generated a transgenic mouse that has one copy of the human TBK1 gene (native promoter and gene structure) incorporated into the mouse genome (Tg-TBK1). Expression of the TBK1 transgene in the retinae of these mice was demonstrated by real-time PCR. Using immunohistochemistry TBK1 protein was predominantly localized to the ganglion cell layer of the retina, the cell type most affected by glaucoma. More intense TBK1 labelling was detected in the retinal ganglion cells (RGCs) of Tg-TBK1 mice than in wild-type littermates. Tg-TBK1 mice exhibit the cardinal sign of glaucoma, a progressive loss of RGCs. Hemizygous Tg-TBK1 mice (with one TBK1 transgene per genome) had a 13% loss of RGCs by 18 months of age (P = 1.5 × 10-8). Homozygous Tg-TBK1 mice had 7.6% fewer RGCs than hemizygous Tg-TBK1 mice and 20% fewer RGCs than wild-type mice (P = 1.9 × 10-5) at 6 months of age. No difference in intraocular pressures was detected between Tg-TBK1 mice and wild-type littermates as they aged (P > 0.05). Tg-TBK1 mice with extra doses of the TBK1 gene recapitulate the phenotype of normal tension glaucoma in human patients with a TBK1 gene duplication. Together, these studies confirm the pathogenicity of the TBK1 gene duplication in human glaucoma and suggest that excess production of TBK1 kinase may have a role in the pathology of glaucoma.

Heterozygous Pitx2 Null Mice Accurately Recapitulate the Ocular Features of Axenfeld-Rieger Syndrome and Congenital Glaucoma

Purpose

The purpose of this analysis was to assess the utility of Pitx2+/- mice as a model for the ocular features of Axenfeld-Rieger Syndrome and for congenital glaucoma.

Methods

Eyes of Pitx2+/- and wild-type littermates were examined clinically using optical coherence tomography (OCT) and fundus photography. Intraocular pressures were measured using a TonoLab rebound tonometer. Eyes were examined histologically to assess PITX2 expression, structural integrity, and optic nerve and ganglion cell content.

Results

PITX2 is present postnatally in the corneal endothelium and stroma, iris stroma, trabecular meshwork, and Schlemm's canal. Reduced central corneal thickness, iris defects, and iridicorneal adhesions are all prevalent in Pitx2+/- eyes. Although optic nerve heads appear normal at postnatal day 7, IOP is elevated and optic nerve head cupping is fully penetrant in Pitx2+/- eyes by 3 weeks of age. Neurodegeneration is present in a significant percentage of optic nerves from Pitx2+/- mice by 3 weeks of age, and is fully penetrant by 2 months of age. Pitx2+/- eyes show significant reductions in specifically ganglion cell density in all four quadrants by 2 months of age.

Conclusions

Pitx2+/- mice model the major ocular features of Axenfeld-Rieger Syndrome and will be an important resource for understanding the molecular mechanisms leading to anterior segment dysgenesis and a high prevalence of glaucoma in this disease. In addition, these mice may provide an efficient new model for assessing the molecular events in glaucoma more generally, and for developing and testing new treatment paradigms for this disease.

Transplantation of iPSC-derived TM cells rescues glaucoma phenotypes in vivo

Glaucoma is a common cause of vision loss or blindness and reduction of intraocular pressure (IOP) has been proven beneficial in a large fraction of glaucoma patients. The IOP is maintained by the trabecular meshwork (TM) and the elevation of IOP in open-angle glaucoma is associated with dysfunction and loss of the postmitotic cells residing within this tissue. To determine if IOP control can be maintained by replacing lost TM cells, we transplanted TM-like cells derived from induced pluripotent stem cells into the anterior chamber of a transgenic mouse model of glaucoma. Transplantation led to significantly reduced IOP and improved aqueous humor outflow facility, which was sustained for at least 9 wk. The ability to maintain normal IOP engendered survival of retinal ganglion cells, whose loss is ultimately the cause for reduced vision in glaucoma. In vivo and in vitro analyses demonstrated higher TM cellularity in treated mice compared with littermate controls and indicated that this increase is primarily because of a proliferative response of endogenous TM cells. Thus, our study provides in vivo demonstration that regeneration of the glaucomatous TM is possible and points toward novel approaches in the treatment of this disease.

Gremlin Induces Ocular Hypertension in Mice Through Smad3-Dependent Signaling

PURPOSE

Transforming growth factor-β2 induces extracellular matrix (ECM) remodeling, which likely contributes to the defective function of the trabecular meshwork (TM) leading to glaucomatous ocular hypertension. Bone morphogenetic proteins (BMPs) inhibit these profibrotic effects of TGFβ2. The BMP antagonist gremlin is elevated in glaucomatous TM cells and increases IOP in an ex vivo perfusion culture model. The purpose of this study was to determine whether gremlin regulates ECM proteins in the TM, signals through the Smad3-dependent pathway, and induces ocular hypertension in mice.

METHODS

Ad5.Gremlin or Ad5.TGFβ2 was injected intravitreally into one eye of each mouse. Intraocular pressure measurements were taken using a TonoLab tonometer. Gremlin, TGFβ2, fibronectin (FN), and collagen-1 (Col-1) expression in the TM was determined by immunofluorescence, Western immunoblot, and quantitative (q)PCR analyses.

RESULTS

Ad5.Gremlin or Ad5.TGFβ2 each caused significant IOP elevation in mice. Immunofluorescence and Western blot analysis demonstrated that gremlin and TGFβ2 reciprocally increased the expression of each other, and both increased FN expression in the TM and surrounding tissues. Ad5.Gremlin elevated IOP and increased Fn and Col-1 gene expression in the TM of Smad3 wild-type (WT) mice, but had no effect in Smad3 HET or Smad3 KO mice.

CONCLUSIONS

Our results demonstrate that intravitreal injections of either Ad5.Gremlin or Ad5.TGFβ2 elevate IOP and upregulate the ECM protein FN in the TM of mice. These data show that gremlin signals through the Smad3-dependent pathway in the TM to elevate IOP. We determined for the first time gremlin's role in inducing ocular hypertension in an in vivo model system.

Adoptive transfer of immune cells from glaucomatous mice provokes retinal ganglion cell loss in recipients

INTRODUCTION

Several studies have indicated that autoimmune and neuroinflammatory processes contribute to the neurodegeneration of retinal ganglion cells in human glaucoma patients and in animal models. To test the involvement of cellular immune processes in the pathophysiology of retinal ganglion cell degeneration in vivo, we carried out adoptive transfer experiments from two independent genetic mouse models of glaucoma into normal recipient mice.

RESULTS

Our findings indicate that transfer results in a progressive loss of retinal ganglion cells and their axons despite normal intraocular pressure in recipient mice. Signs of pan-retinal inflammation were not detected. Similar findings were obtained following transfer of isolated T-lymphocytes, but not after transfer of splenocytes from immune deficient glaucomatous mice. Transferred lymphocytes were detected integrated in the spleen and in the retinal ganglion cell layer of recipient animals, albeit at very low frequencies. Furthermore, we observed cell-cell interaction between transferred T-cells and recipient microglia along with focal microglial activation in recipient eyes.

CONCLUSION

This study demonstrates that the pathophysiology of glaucomatous degeneration in the tested animal models includes T-cell mediated events that are capable of causing loss of healthy retinal ganglion cells.

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.

Reliability of Tonolab measurements in rats

AIM

To assess the repeatability and reproducibility of Tonolab tonometer in rats with high intraocular pressure (IOP) and evaluate its ability to detect IOP changes in rats with general anaesthesia.

METHODS

Left eyes of adult Fischer rats (F344) were photocoagulated by 532 nm diode laser to induce high IOP. Hypertensive eyes of 30 conscious rats were randomly chosen to measure IOP on a single occasion. Two observers independently and alternately undertook IOP measurements consecutively for three times using the same Tonolab tonometer blind to the other observer's IOP measurements. The within subject standard deviation (Sw), coefficient of variation (CVw) (100×Sw/overall mean), and intraclass correlation coefficient (ICC) were calculated to evaluate intra-observer repeatability. Inter-observer difference was analysed by using 95% limits of agreement described by Bland-Altman and paired sample t-test. Also, another 13 normal F344 rats were intraperitoneally administrated with ketamine/xylazine or chloral hydrate, and IOPs of both eyes were measured by a single operator once every 5min until animals came to conscious. IOPs at various time points were compared by using one-way ANOVAs.

RESULTS

Mean IOP was 35.58 mm Hg (range 17.33 to 65.33 mm Hg). For intraobserver repeatability, the Sw, CVw and ICC of high IOP for two observers were 5.20 mm Hg/3.41 mm Hg, 9.98%/8.08% and 0.820/0.928 respectively. The inter-observer difference was 14.76%±19.76% of the mean IOP of two observers, with a 95% limits of agreement -23.97% to 53.50%, and the difference between mean IOP of these two observers was statistically significant (P=0.001). IOPs dropped slightly during the first 15min post-aneathesia, with a IOP change between 0.17 and 1.17 mm Hg. IOPs changed from basline of 11.75±2.05 mm Hg (n=12) to 8.75±1.06 mm Hg 20min post-anesthesia (P=0.001), and this hypotensive condition persisted until 80min post-anesthesia.

CONCLUSION

In this sample of hypertensive rats, Tonolab measurements demonstrated high levels of intraobserver repeatability, however, its interobserver reproducibility was poor. Longitudinal changes of IOP caused by genral anaesthesia can be sensitively detected by Tonolab. So we suggested that measurements of IOP using Tonolab are best measured by a single observer, and it could be included in experimental glaucoma.

Pressure difference between the anterior chamber and the vitreous cavity in eyes with pupillary block

PURPOSE

The purpose of this study was to measure the pressure difference between the anterior chamber (AC) and the vitreous cavity (VC) in eyes with and without pupillary block.

MATERIALS AND METHODS

Seven vitrectomized porcine eyes were used. Infusion pressures of 10-80 mmHg were generated with a vented gas forced infusion system. Measurements of pressure were obtained with digital manometry connected to 25-gauge catheters from the AC and VC simultaneously. After increasing AC pressure to each target pressure, VC pressure was recorded, and vice versa. Inspection was performed with portable slit-lamp biomicroscopy to identify the development of pupillary block at the end of each experiment.

RESULTS

When the AC pressure was increased, the VC pressure obtained was similar to the AC pressure in all cases. When the VC pressure increased, the AC pressure obtained was similar to that at a VC pressure of less than 50 mmHg. When the VC pressure was increased rapidly to 60, 70, and 80 mmHg, the AC pressures obtained were 57.6 ± 1.0, 64.0 ± 0.8, and 69.6 ± 2.4 mmHg, respectively. Thus, the VC pressures obtained were 1.5, 5.9, and 9.1 mmHg higher than pressures obtained from AC with target pressures of 60, 70, and 80 mmHg, respectively (p = 0.027, 0.001, and 0.001, respectively). Pupillary block was observed in cases where the VC pressure was increased to more than 50 mmHg.

CONCLUSIONS

The AC pressure could be significantly lower than the VC pressure in some eyes with pupillary block.

Smad3 is necessary for transforming growth factor-beta2 induced ocular hypertension in mice

TGFβ2 induces extracellular matrix (ECM) remodeling and alters the cytoskeleton by both the canonical Smad and non-canonical signaling pathways. TGFβ2 regulates the expression of ECM proteins in trabecular meshwork (TM) cells, increases intraocular pressure (IOP) in an ex vivo perfusion organ culture model, and induces ocular hypertension in rodent eyes. A necessary step in the canonical Smad signaling pathway is phosphorylation of receptor protein Smad3 by the TGF-β receptor complex. The purpose of this study was to determine whether TGFβ2 signals in vivo through the canonical Smad signaling pathway in the TM using Smad3 knockout (KO) mice. Ad5.hTGFβ2(226/228) (2.5 × 10(7) pfu) was injected intravitreally into one eye of homozygous (WT), heterozygous (HET), and homozygous (KO) 129-Smad3(tm1Par)/J mice (n = 9-10 mice/group), with the uninjected contralateral eye serving as the control. IOP measurements were taken using a rebound tonometer. To test the effect of TGFβ2 signaling on the ECM, fibronectin expression was determined by immunohistochemistry and qPCR analysis. Transduction of the TM with viral vector Ad5.hTGFβ2(226/228) caused a statistically significant difference in IOP exposure between Smad3 genotypes: WT, 187.7 ± 23.9 mmHg*day (n = 9); HET, 95.6 ± 24.5 mmHg*day (n = 9); KO, 52.8 ± 25.2 mmHg*day (n = 10); (p < 0.05 WT versus HET, p < 0.01 WT versus KO). Immunohistochemistry and qPCR analysis showed that Ad5.hTGFβ2(226/228) increased fibronectin expression in the TM of WT mice (2.23 ± 0.24 fold) compared to Smad3 KO mice (0.99 ± 0.19 fold), p < 0.05. These results demonstrate Smad3 is a necessary signaling protein for TGFβ2-induced ocular hypertension and fibronectin deposition in the TM.

Central corneal thickness does not correlate with TonoLab-measured IOP in several mouse strains with single transgenic mutations of matricellular proteins

Accurate and reliable measurement of intraocular pressure (IOP) is crucial in the study of glaucoma using the mouse model. The purpose of this study was to determine the relationship between TonoLab-measured IOP and central corneal thickness (CCT) in mouse strains with single gene mutations of matricellular proteins. Wild-type (WT) and transgenic mouse strains with single gene mutations (KO) of thrombospondin-1 (TSP-1), thrombospondin-2 (TSP-2), osteopontin (OPN), hevin, and secreted protein acidic rich in cysteine (SPARC) were imaged at six weeks using optical coherence tomography (Stratus, Zeiss) to determine CCT. IOP was measured between 11am and 3pm using TonoLab, one week later. For all measurements, mice were anesthetized using intraperitoneal injection ketamine:xylazine. CCT and IOP were measured in 583 mice (TSP-1 n = 71 and 41, TSP-2 n = 60 and 32, OPN n = 81 and 50, hevin n = 59 and 76, SPARC n = 54 and 59, WT and KO, respectively). Mean CCT was 5-6% lower in three KO strains-TSP-1, OPN, and SPARC-compared to their corresponding WT (p = 1.55 × 10(-7), 1.63 × 10(-11), and 1.91 × 10(-7), respectively). The mean IOP was 8.3%, 6.6%, and 15.1% lower in three KO strains-TSP-1, TSP-2, and SPARC-compared to corresponding WT (p = 2.11 × 10(-5), 2.93 × 10(-3), and 3.76 × 10(-9), respectively. Linear regression of IOP versus CCT yielded no statistically significant within-strain correlations for TSP-1 (p = 0.12 and 0.073), TSP-2 (p = 0.473 and 0.92), OPN (p = 0.212 and 0.916), Hevin (p = 0.746 and 0.257), and SPARC (p = 0.080 and 0.056), reported as p-values considering a null hypothesis of zero slope (WT and KO, respectively). Neither C57-derived strains (TSP-1 and OPN) nor 129-derived strains (TSP-2, hevin, SPARC) demonstrated a correlation between mean IOP and mean CCT across different strains (p = 0.75 and p = 0.53, respectively). Taken together, these results indicate that CCT is not required to interpret TonoLab IOP readings in the mice when CCT varies 10% about the mean. This does not exclude the possibility of an IOP-CCT correlation for CCT values outside this range or for inter-strain comparisons where the mean CCT differs more than 10%.

Validation of the TonoVet® rebound tonometer in normal and glaucomatous cats

Objective  To validate intraocular pressure (IOP) readings obtained in cats with the TonoVet(®) tonometer. Animals studied  IOP readings obtained with the TonoVet(®) were compared to IOP readings determined by manometry and by the Tono-Pen XL(™) in 1 normal cat and two glaucomatous cats. TonoVet(®) and Tono-Pen XL(™) readings were also compared in a further six normal and nine glaucomatous cats. Procedures  The anterior chambers of both eyes of three anesthetized cats were cannulated and IOP was varied manometrically, first increasing from 5 to 70 mmHg in 5 mmHg increments, then decreasing from 70 to 10 mmHg in 10 mmHg decrements. At each point, two observers obtained three readings each from both eyes, with both the TonoVet(®) and Tono-Pen XL(™) . IOP was measured weekly for 8 weeks with both tonometers in six normal and nine glaucomatous unsedated cats. Data were analyzed by linear regression. Comparisons between tonometers and observers were made by paired student t-test. Results  The TonoVet(®) was significantly more accurate than the Tono-Pen XL(™) (P = 0.001), correlating much more strongly with manometric IOP. In the clinical setting, the Tono-Pen XL(™) underestimated IOP when compared with the TonoVet(®) . Conclusions  Both the TonoVet(®) and Tono-Pen XL(™) provide reproducible IOP measurements in cats; however, the TonoVet(®) provides readings much closer to the true IOP than the Tono-Pen XL(™) . The TonoVet(®) is superior in accuracy to the Tono-Pen XL(™) for the detection of ocular hypertension and/or glaucoma in cats in a clinical setting.

Reduction of ER stress via a chemical chaperone prevents disease phenotypes in a mouse model of primary open angle glaucoma

Mutations in myocilin (MYOC) are the most common genetic cause of primary open angle glaucoma (POAG), but the mechanisms underlying MYOC-associated glaucoma are not fully understood. Here, we report the development of a transgenic mouse model of POAG caused by the Y437H MYOC mutation; the mice are referred to herein as Tg-MYOC(Y437H) mice. Analysis of adult Tg-MYOC(Y437H) mice, which we showed express human MYOC containing the Y437H mutation within relevant eye tissues, revealed that they display glaucoma phenotypes (i.e., elevated intraocular pressure [IOP], retinal ganglion cell death, and axonal degeneration) closely resembling those seen in patients with POAG caused by the Y437H MYOC mutation. Mutant myocilin was not secreted into the aqueous humor but accumulated in the ER of the trabecular meshwork (TM), thereby inducing ER stress in the TM of Tg-MYOC(Y437H) mice. Furthermore, chronic and persistent ER stress was found to be associated with TM cell death and elevation of IOP in Tg-MYOC(Y437H) mice. Reduction of ER stress with a chemical chaperone, phenylbutyric acid (PBA), prevented glaucoma phenotypes in Tg-MYOC(Y437H) mice by promoting the secretion of mutant myocilin in the aqueous humor and by decreasing intracellular accumulation of myocilin in the ER, thus preventing TM cell death. These results demonstrate that ER stress is linked to the pathogenesis of POAG and may be a target for treatment in human patients.

Calibration of the TonoLab tonometer in mice with spontaneous or experimental glaucoma

PURPOSE

To measure the accuracy of TonoLab (TioLat, Helsinki, Finland) tonometry in mice with spontaneous or induced experimental glaucoma.

METHODS

Chronic intraocular pressure (IOP) elevation was induced in one eye of 32 mice by injection of polystyrene beads and viscoelastic material. Three to 6 weeks later, the eyes were cannulated and manometrically set to 10, 20, 30, 40, or 50 mm Hg. The mice were 8-week and 8-month-old C57BL/6, 8-week-old DBA/2J, and 8-week-old CD1. The TonoLab calibration was also tested on five aged DBA/2J mice with spontaneous glaucoma. The relation of the TonoLab reading to manometric IOP was evaluated in multivariate linear regression models with axial length, IOP history, and mouse strain as independent variables.

RESULTS

The slope of the relationship between TonoLab and manometric IOP in all the mice was 0.998, with an intercept of 2.3 mm Hg (adjusted R in univariate regression = 0.86). Neither the mice with bead-induced glaucoma nor those with spontaneous glaucoma (older DBA/2J mice) differed significantly from the control animals in having an excellent correlation between TonoLab and manometer IOP. Longer and wider mouse eyes had slightly higher tonometrically measured IOP, whether glaucomatous or control (multivariate regression, adjusted R(2) = 0.90, P < 0.0001). There was no difference in tonometric accuracy among the three mouse strains: CD1, C57BL/6, and DBA/2J, nor between 8-week and 8-month-old C57BL/6 mice (multivariate regression, P = 0.32).

CONCLUSIONS

The TonoLab accurately reflects IOP in both normal mice and in eyes of mice with experimental or spontaneous glaucoma, with no detectable effect of age.

Evaluation of rebound tonometry in non-human primates

To determine the accuracy and reproducibility of intraocular pressure (IOP) measurements obtained with the TonoVet® rebound tonometer in cynomolgus macaques and to determine the effects of corneal thickness on measurements obtained by the TonoVet®. The anterior chambers of both eyes of anesthetized monkeys were cannulated with branched 23-G needles; one branch was connected to a vertically adjustable reservoir and the other to a pressure transducer. IOP was increased by 5 mmHg increments and then decreased by 10 mmHg decrements. IOP was measured using the TonoVet® at each increment and decrement by 2 independent observers and at every other increment and every decrement by a single observer using 'minified' Goldmann applanation tonometry. Central corneal thickness was measured with a PachPen(TM) ultrasonic pachymeter. TonoVet® readings correlated well with manometric IOP (slope = 0.972, r(2) coefficient = 0.955). No significant differences were observed when comparing eyes or operators; however there was a non-significant trend for TonoVet® readings taken in right eyes to be closer to manometric IOP than those taken in left eyes. The TonoVet® had a non-significant tendency to underestimate manometric IOP. TonoVet® readings obtained during the decremental phase of the experiment were significantly closer (p < 0.004) to manometric IOP than those obtained during the incremental phase. Central corneal thickness significantly increased (p < 0.0001) over the course of the experiment. The TonoVet® rebound tonometer is a reliable and accurate tool for the measurement of IOP in cynomolgus macaques. This tonometer has several advantages, including portability, ease of use, and brief contact with the corneal surface making topical anesthetics unnecessary.

Rat models for glaucoma research

Rats are becoming an increasingly popular model system for understanding mechanisms of optic nerve injury in primary open-angle glaucoma (POAG). Although the anatomy of the rat optic nerve head (ONH) is different from the human, the ultrastructural relationships between astrocytes and axons are quite similar, making it likely that cellular processes of axonal damage in these models will be relevant to human glaucoma. All of these models rely on elevating intraocular pressure (IOP), a major risk factor for glaucoma. Methods that produce increased resistance to aqueous humor outflow at the anterior chamber angle, specifically hypertonic saline injection of aqueous outflow pathways and laser treatment of the limbal tissues, appear to produce a specific regional pattern of injury that may have a particular relevance to understanding regional injury in human glaucoma. Because increased pressure fluctuations are a characteristic of such models and the rodent ONH appears to have high susceptibility to elevated IOP, special instrumentation and measurement techniques are required to document pressure exposure in these eyes and understand the pressure levels that the eyes and the optic nerve are exposed to. With these techniques, it is possible to obtain an excellent correlation between pressure and the extent of nerve damage. Continued use of these models will lead to a better understanding of cellular mechanisms of pressure-induced optic nerve damage and POAG.

Lyst mutation in mice recapitulates iris defects of human exfoliation syndrome

PURPOSE

Human eyes with exfoliation syndrome (XFS) exhibit a distinctive pattern of iris transillumination defects that are recapitulated in Lyst mutant mice carrying the beige allele. The purpose of this study was to determine the anatomic basis for Lyst-mediated transillumination defects, test whether Lyst mutant mice develop other features of XFS, and describe the molecular basis of the beige mutation.

METHODS

Lyst mutant mice and strain-matched controls were compared by clinical, histologic, immunohistochemical, and molecular genetic analyses.

RESULTS

Slit-lamp examination showed that Lyst mutant mice uniformly exhibit XFS-like transillumination defects. Histologic analysis showed that these defects correlate with a sawtooth morphology of the iris pigment epithelium. Lyst mutant mice also produce an exfoliative-like material and exhibit pronounced pigment dispersion. Despite these insults, Lyst mutation does not cause increased intraocular pressure or optic nerve damage in the C57BL/6J genetic background. Sequence analysis identified that the beige mutation is predicted to delete a single isoleucine from the WD40 domain of the LYST protein, suggesting that this mutation is likely to disrupt a protein-protein interaction.

CONCLUSIONS

Lyst mutant eyes exhibit multiple features of XFS. Recent human genetic association studies have identified changes occurring in the LOXL1 gene as an important risk factor for XFS but also indicated that other factors contributing to risk likely exist. These results demonstrated that mutation of the Lyst gene can produce ocular features of human XFS and suggested that LYST or LYST-interacting genes may contribute to XFS.

Rebound tonometry in conscious, conditioned mice avoids the acute and profound effects of anesthesia on intraocular pressure

AIMS

The aims of this study were to evaluate the accuracy, repeatability, and safety of multiple intraocular pressure (IOP) measurements by a commercially available rebound tonometer in conscious, conditioned mice, and to characterize the acute and profound effects of anesthesia on IOP in mice.

METHODS

To test the accuracy of the tonometer, IOPs of CD-1 mice under ketamine/xylazine anesthesia were experimentally set and monitored with a water manometer/transducer system following transcorneal cannulation while simultaneously performing tonometry. The long- and short-term repeatability of the tonometer was tested in conscious, restrained mice, as measurements were taken once-daily in the afternoon for 4 consecutive days. On day 5, IOPs were measured in the same mice once every 4 min for 32 min. On 2 separate days, mice were administered ketamine/xylazine or 2,2,2-tribromoethanol anesthesia, in a crossover design, and IOPs were measured once every 2 min for 32 min. Rebound tonometry was performed in conscious mice before and 1 hour after 1 drop of timolol maleate (10 microL of 0.5%) application to 1 eye.

RESULTS

IOP measurements by rebound tonometry correlated well with manometry for pressures between 8 and 38 mmHg (y = 0.98x - 0.32, R(2) = 0.94; P < 0.001). The average tonometric IOP was invariant over 4 days (range, 11.7-13.2 mmHg). IOPs dropped significantly ( P < or = 0.05) within 6 min (ketamine/xylazine) or 10 min (2,2,2-tribromoethanol) postadministration of anesthesia but not with conscious restraint. Timolol significantly (P < 0.001) lowered IOP from 12.8 +/- 0.3 (mean +/- standard error of the mean) to 10.1 +/- 0.6 mmHg, as measured by the tonometer.

CONCLUSIONS

Rebound tonometry can be used to obtain accurate IOP measurements in conscious, restrained mice while avoiding the rapid and profound ocular hypotensive effects of general anesthesia. Small changes in IOP with an aqueous-flow suppressant are readily detectable with conscious restraint that may be missed with chemical restraint.