Optometry and Vision Sciences - Research Publications
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ItemNo Preview AvailableReversibility of retinal ganglion cell dysfunction due to chronic IOP elevation.(Association for Research in Vision and Ophthalmology, 2018-07-01)Purpose : To determine the duration of chronic IOP elevation beyond which ganglion cell function can no longer recover using the mouse circumlimbal suture model. Methods : IOP elevation was induced in anaesthetized (isoflurane) adult male C57BL6/J mice by attaching a circumlimbal suture (nylon, 10/0) around the equator of one eye, with the contralateral eye serving as a control. The suture was left in place for 8, 12 and 16 weeks (n=27, 23 and 27), respectively, and animals underwent electroretinography and optical coherence tomography at these time points. In two other groups, the suture was removed after 8 and 12 weeks (n=26 and 28), and the capacity for recovery assessed 4 weeks later. IOP was measured weekly (Tonolab). Retinal ganglion cell (RGC) function (or integrity) was assessed with the positive scotopic threshold response (pSTR) and retinal nerve fibre layer (RNFL) thickness. Data (mean ± SEM) were compared using t-test (control vs. treatment) and one-way ANOVA (within groups). Results : IOP in sutured eyes was higher than control eyes (8wk: 17.1 ± 0.3 vs. 26.8 ± 0.6 mmHg, 12wk: 13.8 ± 0.3 vs. 19.5 ± 0.5 mmHg, 16wk: 17.1 ± 0.2 vs. 27.4 ± 0.6 mmHg; all P<0.001). After suture removal, IOP returned to levels comparable to control eyes (8+4wk: 16.9 ± 0.3 vs. 16.1 ± 0.3 mmHg; P=0.08, 12+4wk: 17.3 ± 0.2 vs. 17.1 ± 0.3 mmHg; P=0.5). With IOP elevation, RGC function declined to 75% ± 8% (8wk), 78% ± 7% (12wk) and 59% ± 4% (16wk, all P<0.001) of control eyes. RNFL thinning was also evident (8wk: 84% ± 4%, 12wk: 83% ± 5%; 16wk: 83% ± 3%; P<0.001) but no change in total retinal thickness was noted (P=0.33). Suture removal at week 8 facilitated full recovery of RGC function (97% ± 7%, P=0.9 vs. baseline) 4 weeks later. However, there was no recovery in RNFL thickness (87% ± 3%, P<0.001 vs. baseline). When the suture was removed at week 12, neither function (79% ± 9%, P<0.05) nor RNFL thickness recovered (89% ± 3%, P<0.01) 4 weeks later. Conclusions : RGC dysfunction can be recovered 4 weeks after an 8-week period of mild IOP elevation, but not after a 12-week period. Beyond 12 weeks, IOP reversal only served to prevent further functional decline. This identifies a critical chronic IOP duration that results in irreversible ganglion cell dysfunction. This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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ItemNo Preview AvailableEvaluating retinal biomarkers in a mouse model of Parkinson's disease(Association for Research in Vision and Ophthalmology, 2019-07-01)Purpose : The retina, an accessible outpouching of the central nervous system, may manifest cortical changes that occur with Parkinson’s disease (PD), lending itself as a potential biomarker. PD is characterised by reduced dopamine levels, a neurotransmitter found in amacrine cells. Human PD patients have also shown structural changes in the outer retina. This work aims to determine if retinal function and structure are altered in a murine model of PD and whether deficits can be ameliorated with L-DOPA treatment. Methods : A PD model was induced in adult C57BL6/J mice using MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 4x i.p. injections, 20mg/kg) and vehicle control and examined at day 21 and 45. Another MPTP group was administered L-DOPA (L-3,4-dihydroxyphenylalanine 0.2 mg/ml) or control in their drinking water and assessed at day 45 (n=12–15/group). In ketamine:xylazine anaesthetised (80:10mg/kg) mice full-field dark- and light-adapted electroretinography (ERG) was assessed to target dopamine-related responses. Optical coherence tomography (OCT) was used to quantify thickness of retinal layers. Retinal and cortical tissue were collected for immunohistochemical assessment of changes in tyrosine hydroxylase (TH)and imaged using confocal microscopy. Data (mean±SEM) were compared using unpaired ANOVA and t-tests as appropriate. Results : At day 21 no retinal changes were found. At day 45 dark and light adapted ERGs showed slower amacrine cell responses (oscillatory potential, p<0.05), a finding which reversed with L-DOPA treatment (p<0.05). Other components of the ERG were unchanged. TH staining showed a trend towards decreased retinal levels in MPTP mice but this did not reach significance (p=0.10). Reduced levels of TH were found in the ventral hippocampus of MPTP mice compared with control (p<0.05). OCT revealed thinning of the outer plexiform layer at day 45, and the L-DOPA group exhibited a thinning of the outer nuclear layer (p<0.05). Conclusions : This study shows for the first time that the MPTP model recapitulates key dopaminergic changes previously reported in humans. In particular, electroretinographic changes that correspond with dopaminergic retinal cells occur in the Parkinson’s model and reverse with therapeutic treatment. Structural thinning of the outer retinal layers also occur, which parallels some human findings. This work paves the way for retinal measures as preclinical screening tools in drug development.
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ItemNo Preview AvailableHow ganglion cell responses to IOP elevation are impacted by blood pressure and intracranial pressure(Association for Research in Vision and Ophthalmology, 2019-07-01)Purpose : The extent to which blood pressure or intracranial pressure modifies ganglion cell responses to acute intraocular pressure (IOP) elevation incompletely understood. Using the electroretinogram (ERG) we measure ganglion cell mediated responses in rat retina, whilst acutely modifying IOP, BP and ICP in a systematic manner. We quantify the relationship between ganglion cell function and ocular perfusion pressure (BP - IOP) at low, normal and high ICP. Methods : Six groups of adult Long-Evans rats (n=7-11 eyes/group, total animals = 25) were anaesthetised (60:5mg/kg ketamine:xylazine) and underwent acute pressure modification. A femoral artery and vein were cannulated for blood pressure measurement and manipulation (sodium nitroprusside to lower and angiotensin II to elevate pressure). ICP was set to -5, 5 or 25 mmHg via a dual cannula (30G infusion needle inside a 23G measurement needle) placed into the lateral ventricle (-1.5mm from bregma, ±2mm from midline) on the ipsilateral side to the cannulated eye (30G, vitreal chamber). At each ICP (-5, 5 or 25 mmHg) and BP setting (normal or high), IOP was raised from 10 to 90 mmHg in 10 mmHg steps (3 min each). At each IOP level ganglion cell function was assessed using the scotopic threshold response (-5 log cd.s/m2, 20 repeats). Data were compared using one- and two-way ANOVA. Results : Average blood pressure at baseline was similar for the normal blood pressure groups (ICP-5 93±3; ICP5 99±5; ICP25 105±3mmHg, p=0.8). There was significant BP elevation in all the high blood pressure groups (ICP-5 160±3; ICP5 157±3; ICP25 157±5mmHg p<0.001). Compared with normal blood pressure groups (32.0±2.0μV), animals with high blood pressure (24.5±1.8μV) had significantly smaller baseline STR amplitudes (p<0.01). There was also a significant ICP effect (p<0.01), with larger baseline amplitudes in the 25mmHg ICP group (34.8±1.6μV) compared with normal (26.4±2.5μV) and low ICP groups (23.9±2.5μV). The ocular perfusion pressure (BP-IOP) relationship fully could not account for difference in ganglion cell function between ICP levels. Conclusions : Ganglion cell function is dependent on ocular perfusion pressure, excessive low or high perfusion attenuates function. Higher intracranial pressure appears to protect against acute ocular perfusion stress.
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ItemNo Preview AvailableA retinal imaging biomarker of Alzheimer's disease(Wiley, 2019-11-01)Background: Amyloid-beta (Ab) deposition in the brain is a diagnostic marker for Alzheimer's disease (AD), but current tests are costly and not widely available. Evidence from transgenic rodent models and post-mortem human tissues suggest that retinal accumulation of Ab may serve as a surrogate marker of brain Ab levels. As Ab has a wavelength-dependent effect on light scatter, we investigated the potential for in vivo retinal hyperspectral imaging to serve as a biomarker of brain Ab. Purpose: To develop and validate a retinal imaging biomarker of Alzheimer's disease. Methods: We performed human retinal hyperspectral imaging on individuals with high Ab burden on brain PET imaging and mild cognitive impairment (cases; n = 15), and age-matched PET-negative controls (n = 20). Image analysis methods were developed and validated on a second group of participants with and with (n = 4) and without (n = 13) moderate-to-high brain Ab burden and on transgenic mice (5xFAD) known to accumulate retinal Ab. Results: We show significant differences in retinal reflectance spectra between cases and controls in both cohorts (AUC ROC = 0.82, P = 0.001, 95% CI: 0.67-0.97). There was a moderate positive linear correlation between retinal imaging scores and brain Abburden (r = 0.46, 95%CI: 0.13-0.69, P = 0.008).The technique also enabled discrimination of AD-model mice from wild-type controls. Conclusion: We have developed a novel retinal imaging method to distinguish people with moderate-high brain Ab load from those without. This approach may have value for the diagnostic confirmation of AD.
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ItemNo Preview AvailableThe effect of TrkB receptor knockdown on mouse retinal ganglion cell function and their response to acute mild intraocular pressure stress(Association for Research in Vision and Ophthalmology, 2019-07-01)Purpose : To examine the effect of tropomyosin receptor kinase B (TrkB) receptor removal on basal retinal ganglion cell (RGC) function and recovery from acute intraocular pressure (IOP) stress following conditional deletion of neuronal-specific TrkB receptors in adult mice. Methods : Conditional TrkB receptor knockout (KO) from Thy-1 positive neurons was induced via daily tamoxifen injections (100 ul i.p. 75mg/kg, 5 days) in 3-month old Thy1-GFP CreERT2+/- TrkBfl/fl transgenic (TrkB KO n=12) and control CreERT2-/- TrkBfl/fl mice (n=12). Four weeks later, one eye was exposed to controlled IOP stress of 50mmHg for 30 minutes, achieved via anterior chamber cannulation (50 μm glass micropipette connected to height-adjustable Hanks balanced salt solution reservoir; 80:10mg/kg ketamine:xylazine). After 7 days of recovery, retinal function (full-field electroretinogram -5.53 – 2.07 log cd.s/m2) and structure (optical coherence tomography) were assessed in sedated mice. Following in vivo assays, eyes were enucleated for immunohistochemical assessment of TrkB receptor KO efficiency using confocal microscopy. Unpaired t-test and two-way ANOVA were used for statistical analysis. Results : TrkB receptor expression was largely confined to the ganglion cell layers and reduced by 81.3±5.8% in TrkB KO retinas compared to controls (P<0.05). Deletion of TrkB receptors significantly reduced RGC-mediated negative scotopic threshold response (nSTR -39.1±13.7% P<0.05, positive STR -38.0±12.1% P=0.05). No changes in photoreceptor (amplitude P>0.05, sensitivity P>0.05) and bipolar cell (amplitude P>0.05, sensitivity P>0.05) function. At day 7 post-IOP stress, photoreceptor and bipolar cell responses recovered back to baseline whilst RGC function did not (pSTR P<0.05; nSTR P<0.05). This effect was similar for both genotypes. TrkB KO did not affect total retinal, retinal nerve fibre, ganglion cell and inner plexiform layer thicknesses compared with control retina (P>0.05). Conclusions : Conditional removal of TrkB receptors in adult mice suggests that TrkB is critical for the ongoing maintenance of ganglion cell function. Specific changes in RGC morphology, synapse expression or intrinsic excitability associated with TrkB deficiency remain to be elucidated. It appears that TrkB receptors do not play an integral role in recovery from a single episode of mild IOP stress.
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ItemEffect of intraocular pressure on vascular autoregulation of the mouse trilaminar network(ASSOC RESEARCH VISION OPHTHALMOLOGY INC, 2019-07-01)PURPOSE. The purpose of this study was to test the hypothesis that the superficial, intermediate, and deep retinal vascular plexus show different responses to intraocular pressure (IOP) elevation. METHODS. Anesthetized adult Long Evans rats (n = 14) were imaged using optical coherence tomography angiography (OCTA; Spectralis) at baseline (IOP 10 mm Hg) and in follow-up mode to examine the vasculature during IOP elevation (10 to 110 mm Hg, 10 mm Hg steps, each step 3 minutes). A 20° × 10° field was imaged. Vessel density within a 2D projection image was determined (%) for the superficial vascular complex (SVC), intermediate capillary plexus (ICP), and deep capillary plexus (DCP). Comparisons were made between layers using 2-way repeated measures ANOVA (layer versus IOP) following normalization to baseline (% relative to 10 mm Hg). RESULTS. The three vascular layers responded differently to IOP elevation. For IOPs between 40 and 60 mm Hg, DCP and ICP capillaries were significantly more resistant to IOP elevation than those in the SVC. When IOP was elevated above 70 mm Hg, all layers showed reduced vessel density. IOP induced change in SVC vessel density closely followed reductions in thickness of the inner retinal layers (nerve fiber, ganglion cell, and inner plexiform layer). This close relationship between reductions in tissue thickness and vessel density was less apparent for the ICP and DCP. CONCLUSIONS. These data show that the intermediate and deep vascular plexus in the rat retina have a greater capacity for autoregulation against mild IOP elevation but are more affected at high IOP.
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ItemNo Preview AvailableTyro3 is a key regulator of myelin structure and retinal function in the central nervous system(SAGE PUBLICATIONS LTD, 2020-03)Background: While it is evident how critical myelin is for neural signalling in the CNS, the biological processes underpinning myelination remain unknown. Previously, we have shown that the receptor tyrosine kinase, Tyro3, regulates developmental myelination and myelin thickness in the CNS. Objectives: The aim of this study was to extend on our previous data by assessing the role of Tyro3 in regulating other myelin structures such as the node of Ranvier; and to assess the effect of Tyro3 loss on axonal conductivity and retinal function. Methods: To investigate node of Ranvier microstructure, we used electron microscopy and manually assessed paranodal loops. We measured nodal and paranodal widths using fluorescent immunohistochemistry staining and ImageJ software. To investigate neural conduction velocities, we measured visual evoked potentials in vivo and compound action potentials in slice cultures. Lastly, full-field electroretinograms and optical coherence tomographies were performed to assess retinal function. All experiments were wild-type to constitutive Tyro3 KO comparisons. Results: We show that Tyro3 receptor loss results in wider nodes of Ranvier and dysregular paranodal loop attachment at nodes. KO mice did not show significant alterations to axonal conduction in visual evoked potentials but did have reduced response amplitudes in compound action potentials. Finally, we show that Tyro3 loss results in a decrease in signal output from photoreceptor, bipolar and particularly retinal ganglion cells. Conclusion: Signalling via Tyro3 is key for the attachment of paranodal loops at nodes of Ranvier. Tyro3 deficient myelin results in a decrease in neural response amplitude. Tyro3 is important for the function of photoreceptor, bipolar and retinal ganglion cells of the retina.