Ionic dysregulatory phenotyping of pathologic retinal thinning with manganese-enhanced MRI

Photoreceptor Cell Calcium Dysregulation and Calpain Activation Promote Pathogenic Photoreceptor Oxidative Stress and Inflammation in Prodromal Diabetic Retinopathy

This study tested the hypothesis that diabetes promotes a greater than normal cytosolic calcium level in rod cells that activates a Ca2+-sensitive protease, calpain, resulting in oxidative stress and inflammation, two pathogenic factors of early diabetic retinopathy. Nondiabetic and 2-month diabetic C57Bl/6J and calpain1 knockout (Capn1-/-) mice were studied; subgroups were treated with a calpain inhibitor (CI). Ca2+ content was measured in photoreceptors using Fura-2. Retinal calpain expression was studied by quantitative RT-PCR and immunohistochemistry. Superoxide and expression of inflammatory proteins were measured using published methods. Proteomic analysis was conducted on photoreceptors isolated from untreated diabetic mice or treated daily with CI for 2 months. Cytosolic Ca2+ content was increased twofold in photoreceptors of diabetic mice as compared with nondiabetic mice. Capn1 expression increased fivefold in photoreceptor outer segments of diabetic mice. Pharmacologic inhibition or genetic deletion of Capn1 significantly suppressed diabetes-induced oxidative stress and expression of proinflammatory proteins in retina. Proteomics identified a protein (WW domain-containing oxidoreductase [WWOX]) whose expression was significantly increased in photoreceptors from mice diabetic for 2 months and was inhibited with CI. Knockdown of Wwox using specific siRNA in vitro inhibited increase in superoxide caused by the high glucose. These results suggest that reducing Ca2+ accumulation, suppressing calpain activation, and/or reducing Wwox up-regulation are novel targets for treating early diabetic retinopathy.

Rod Photoreceptor Neuroprotection in Dark-Reared Pde6brd10 Mice

Purpose

The purpose of this study was to test the hypothesis that anti-oxidant and / or anti-inflammation drugs that suppress rod death in cyclic light-reared Pde6brd10 mice are also effective in dark-reared Pde6brd10 mice.

Methods

In untreated dark-reared Pde6brd10 mice at post-natal (P) days 23 to 24, we measured the outer nuclear layer (ONL) thickness (histology) and dark-light thickness difference in external limiting membrane-retinal pigment epithelium (ELM-RPE) (optical coherence tomography [OCT]), retina layer oxidative stress (QUEnch-assiSTed [QUEST] magnetic resonance imaging [MRI]); and microglia/macrophage-driven inflammation (immunohistology). In dark-reared P50 Pde6brd10 mice, ONL thickness was measured (OCT) in groups given normal chow or chow admixed with methylene blue (MB) + Norgestrel (anti-oxidant, anti-inflammatory), or MB or Norgestrel separately.

Results

P24 Pde6brd10 mice showed no significant dark-light ELM-RPE response in superior and inferior retina consistent with high cGMP levels. Norgestrel did not significantly suppress the oxidative stress of Pde6brd10 mice that is only found in superior central outer retina of males at P23. Overt rod degeneration with microglia/macrophage activation was observed but only in the far peripheral superior retina in male and female P23 Pde6brd10 mice. Significant rod protection was measured in female P50 Pde6brd10 mice given 5 mg/kg/day MB + Norgestrel diet; no significant benefit was seen with MB chow or Norgestrel chow alone, nor in similarly treated male mice.

Conclusions

In early rod degeneration in dark-reared Pde6brd10 mice, little evidence is found in central retina for spatial associations among biomarkers of the PDE6B mutation, oxidative stress, and rod death; neuroprotection at P50 was limited to a combination of anti-oxidant/anti-inflammation treatment in a sex-specific manner.

Electrophysiological and Imaging Calcium Biomarkers of Aging in Male and Female 5×FAD Mice

BACKGROUND

In animal models and tissue preparations, calcium dyshomeostasis is a biomarker of aging and Alzheimer's disease that is associated with synaptic dysfunction, neuritic pruning, and dysregulated cellular processes. It is unclear, however, whether the onset of calcium dysregulation precedes, is concurrent with, or is the product of pathological cellular events (e.g., oxidation, amyloid-β production, and neuroinflammation). Further, neuronal calcium dysregulation is not always present in animal models of amyloidogenesis, questioning its reliability as a disease biomarker.

OBJECTIVE

Here, we directly tested for the presence of calcium dysregulation in dorsal hippocampal neurons in male and female 5×FAD mice on a C57BL/6 genetic background using sharp electrodes coupled with Oregon-green Bapta-1 imaging. We focused on three ages that coincide with the course of amyloid deposition: 1.5, 4, and 10 months old.

METHODS

Outcome variables included measures of the afterhyperpolarization, short-term synaptic plasticity, and calcium kinetics during synaptic activation. Quantitative analyses of spatial learning and memory were also conducted using the Morris water maze. Main effects of sex, age, and genotype were identified on measures of electrophysiology and calcium imaging.

RESULTS

Measures of resting Oregon-green Bapta-1 fluorescence showed significant reductions in the 5×FAD group compared to controls. Deficits in spatial memory, along with increases in Aβ load, were detectable at older ages, allowing us to test for temporal associations with the onset of calcium dysregulation.

CONCLUSION

Our results provide evidence that reduced, rather than elevated, neuronal calcium is identified in this 5×FAD model and suggests that this surprising result may be a novel biomarker of AD.

Manganese-Enhanced Magnetic Resonance Imaging: Application in Central Nervous System Diseases

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on the strong paramagnetism of Mn2+. Mn2+ is a calcium ion analog and can enter excitable cells through voltage-gated calcium channels. Mn2+ can be transported along the axons of neurons via microtubule-based fast axonal transport. Based on these properties, MEMRI is used to describe neuroanatomical structures, monitor neural activity, and evaluate axonal transport rates. The application of MEMRI in preclinical animal models of central nervous system (CNS) diseases can provide more information for the study of disease mechanisms. In this article, we provide a brief review of MEMRI use in CNS diseases ranging from neurodegenerative diseases to brain injury and spinal cord injury.

Applications of Manganese-Enhanced Magnetic Resonance Imaging in Ophthalmology and Visual Neuroscience

Understanding the mechanisms of vision in health and disease requires knowledge of the anatomy and physiology of the eye and the neural pathways relevant to visual perception. As such, development of imaging techniques for the visual system is crucial for unveiling the neural basis of visual function or impairment. Magnetic resonance imaging (MRI) offers non-invasive probing of the structure and function of the neural circuits without depth limitation, and can help identify abnormalities in brain tissues in vivo. Among the advanced MRI techniques, manganese-enhanced MRI (MEMRI) involves the use of active manganese contrast agents that positively enhance brain tissue signals in T1-weighted imaging with respect to the levels of connectivity and activity. Depending on the routes of administration, accumulation of manganese ions in the eye and the visual pathways can be attributed to systemic distribution or their local transport across axons in an anterograde fashion, entering the neurons through voltage-gated calcium channels. The use of the paramagnetic manganese contrast in MRI has a wide range of applications in the visual system from imaging neurodevelopment to assessing and monitoring neurodegeneration, neuroplasticity, neuroprotection, and neuroregeneration. In this review, we present four major domains of scientific inquiry where MEMRI can be put to imperative use — deciphering neuroarchitecture, tracing neuronal tracts, detecting neuronal activity, and identifying or differentiating glial activity. We deliberate upon each category studies that have successfully employed MEMRI to examine the visual system, including the delivery protocols, spatiotemporal characteristics, and biophysical interpretation. Based on this literature, we have identified some critical challenges in the field in terms of toxicity, and sensitivity and specificity of manganese enhancement. We also discuss the pitfalls and alternatives of MEMRI which will provide new avenues to explore in the future.

Manganese Enhanced MRI for Use in Studying Neurodegenerative Diseases

MRI has been extensively used in neurodegenerative disorders, such as Alzheimer’s disease (AD), frontal-temporal dementia (FTD), mild cognitive impairment (MCI), Parkinson’s disease (PD), Huntington’s disease (HD) and amyotrophic lateral sclerosis (ALS). MRI is important for monitoring the neurodegenerative components in other diseases such as epilepsy, stroke and multiple sclerosis (MS). Manganese enhanced MRI (MEMRI) has been used in many preclinical studies to image anatomy and cytoarchitecture, to obtain functional information in areas of the brain and to study neuronal connections. This is due to Mn²⁺ ability to enter excitable cells through voltage gated calcium channels and be actively transported in an anterograde manner along axons and across synapses. The broad range of information obtained from MEMRI has led to the use of Mn²⁺ in many animal models of neurodegeneration which has supplied important insight into brain degeneration in preclinical studies. Here we provide a brief review of MEMRI use in neurodegenerative diseases and in diseases with neurodegenerative components in animal studies and discuss the potential translation of MEMRI to clinical use in the future.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

Adaptive Plasticity in the Retina: Protection Against Acute Injury and Neurodegenerative Disease by Conditioning Stimuli

Although both preclinical and clinical conditioning studies in heart and brain lead the field of conditioning medicine, investigations of retinal conditioning still number more than 100. In this brief review, we highlight findings to date from animal and cell culture models of conditioning that provide demonstrated protection in acute and chronic retinal injury and disease models. The multitude of stimuli used to condition the retina, the signaling mediators and pathways identified, and the injury- and disease-resilient phenotypes documented are discussed herein, along with our recommendations for the kinds of studies needed to continue to advance this promising field. In our view, the robust protection afforded by these adaptive epigenetic responses to conditioning stress provides significant incentives for both furthering our investment in bench research and underwriting clinical trials, so that the full potential of this therapy can be realized.

Catalase therapy corrects oxidative stress-induced pathophysiology in incipient diabetic retinopathy

PURPOSE

Preclinical studies have highlighted retinal oxidative stress in the pathogenesis of diabetic retinopathy. We evaluated whether a treatment designed to enhance cellular catalase reduces oxidative stress in retinal cells cultured in high glucose and in diabetic mice corrects an imaging biomarker responsive to antioxidant therapy (manganese-enhanced magnetic resonance imaging [MEMRI]).

METHODS

Human retinal Müller and pigment epithelial cells were chronically exposed to normal or high glucose levels and treated with a cell-penetrating derivative of the peroxisomal enzyme catalase (called CAT-SKL). Hydrogen peroxide (H2O2) levels were measured using a quantitative fluorescence-based assay. For in vivo studies, streptozotocin (STZ)-induced diabetic C57Bl/6 mice were treated subcutaneously once a week for 3 to 4 months with CAT-SKL; untreated age-matched nondiabetic controls and untreated diabetic mice also were studied. MEMRI was used to analytically assess the efficacy of CAT-SKL treatment on diabetes-evoked oxidative stress-related pathophysiology in vivo. Similar analyses were performed with difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase.

RESULTS

After catalase transduction, high glucose-induced peroxide production was significantly lowered in both human retinal cell lines. In diabetic mice in vivo, subnormal intraretinal uptake of manganese was significantly improved by catalase supplementation. In addition, in the peroxisome-rich liver of treated mice catalase enzyme activity increased and oxidative damage (as measured by lipid peroxidation) declined. On the other hand, DFMO was largely without effect in these in vitro or in vivo assays.

CONCLUSIONS

This proof-of-concept study raises the possibility that augmentation of catalase is a therapy for treating the retinal oxidative stress associated with diabetic retinopathy.

Systemic Retinaldehyde Treatment Corrects Retinal Oxidative Stress, Rod Dysfunction, and Impaired Visual Performance in Diabetic Mice

PURPOSE

Diabetes appears to induce a visual cycle defect because rod dysfunction is correctable with systemic treatment of the visual cycle chromophore 11-cis-retinaldehyde. However, later studies have found no evidence for visual cycle impairment. Here, we further examined whether photoreceptor dysfunction is corrected with 11-cis-retinaldehyde. Because antioxidants correct photoreceptor dysfunction in diabetes, the hypothesis that exogenous visual chromophores have antioxidant activity in the retina of diabetic mice in vivo was tested.

METHODS

Rod function in 2-month-old diabetic mice was evaluated using transretinal electrophysiology in excised retinas and apparent diffusion coefficient (ADC) MRI to measure light-evoked expansion of subretinal space (SRS) in vivo. Optokinetic tracking was used to evaluate cone-based visual performance. Retinal production of superoxide free radicals, generated mostly in rod cells, was biochemically measured with lucigenin. Diabetic mice were systemically treated with a single injection of either 11-cis-retinaldehyde, 9-cis-retinaldehyde (a chromophore surrogate), or all-trans-retinaldehyde (the photoisomerization product of 11-cis-retinaldehyde).

RESULTS

Consistent with previous reports, diabetes significantly reduced (1) dark-adapted rod photo responses (transretinal recording) by ∼18%, (2) rod-dominated light-stimulated SRS expansion (ADC MRI) by ∼21%, and (3) cone-dominated contrast sensitivity (using optokinetic tracking [OKT]) by ∼30%. Both 11-cis-retinaldehyde and 9-cis-retinaldehyde largely corrected these metrics of photoreceptor dysfunction. Higher-than-normal retinal superoxide production in diabetes by ∼55% was also significantly corrected following treatment with 11-cis-retinaldehyde, 9-cis-retinaldehyde, or all-trans-retinaldehyde.

CONCLUSIONS

Collectively, data suggest that retinaldehydes improve photoreceptor dysfunction in diabetic mice, independent of the visual cycle, via an antioxidant mechanism.

Photobiomodulation Mitigates Diabetes-Induced Retinopathy by Direct and Indirect Mechanisms: Evidence from Intervention Studies in Pigmented Mice

Objective

Daily application of far-red light from the onset of diabetes mitigated diabetes-induced abnormalities in retinas of albino rats. Here, we test the hypothesis that photobiomodulation (PBM) is effective in diabetic, pigmented mice, even when delayed until weeks after onset of diabetes. Direct and indirect effects of PBM on the retina also were studied.

Methods

Diabetes was induced in C57Bl/6J mice using streptozotocin. Some diabetics were exposed to PBM therapy (4 min/day; 670 nm) daily. In one study, mice were diabetic for 4 weeks before initiation of PBM for an additional 10 weeks. Retinal oxidative stress, inflammation, and retinal function were measured. In some mice, heads were covered with a lead shield during PBM to prevent direct illumination of the eye, or animals were treated with an inhibitor of heme oxygenase-1. In a second study, PBM was initiated immediately after onset of diabetes, and administered daily for 2 months. These mice were examined using manganese-enhanced MRI to assess effects of PBM on transretinal calcium channel function in vivo.

Results

PBM intervention improved diabetes-induced changes in superoxide generation, leukostasis, expression of ICAM-1, and visual performance. PBM acted in part remotely from the retina because the beneficial effects were achieved even with the head shielded from the light therapy, and because leukocyte-mediated cytotoxicity of retinal endothelial cells was less in diabetics treated with PBM. SnPP+PBM significantly reduced iNOS expression compared to PBM alone, but significantly exacerbated leukostasis. In study 2, PBM largely mitigated diabetes-induced retinal calcium channel dysfunction in all retinal layers.

Conclusions

PBM induces retinal protection against abnormalities induced by diabetes in pigmented animals, and even as an intervention. Beneficial effects on the retina likely are mediated by both direct and indirect mechanisms. PBM is a novel non-pharmacologic treatment strategy to inhibit early changes of diabetic retinopathy.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

Laminar specific detection of APP induced neurodegeneration and recovery using MEMRI in an olfactory based Alzheimer's disease mouse model

Manganese enhanced MRI (MEMRI) was used to detect specific laminar changes in the olfactory bulb (OB) to follow the progression of amyloid precursor protein (APP)-induced neuronal pathology and its recovery in a reversible olfactory based Alzheimer's disease (AD) mouse model. Olfactory dysfunction is an early symptom of AD, which suggests that olfactory sensory neurons (OSNs) may be more sensitive to AD related factors than neurons in other brain areas. Previously a transgenic mouse model was established that causes degeneration of OSNs by overexpressing humanized APP (hAPP), which results in a disruption of the olfactory circuitry with changes in the glomerular structure. In the present work, OB volume and manganese enhancement of the glomerular layer in the OB were decreased in mutant mice. Turning off APP overexpression with doxycycline produced a significant increase in manganese enhancement of the glomerular layer after only 1week, and further recovery after 3weeks, while treatment with Aβ antibody produced modest improvement with MRI measurements. Thus, MEMRI enables a direct tracking of laminar specific neurodegeneration through a non-invasive in vivo measurement. The use of MRI will enable assessment of the ability of different pharmacological reagents to block olfactory neuronal loss and can serve as a unique in vivo screening tool to both identify potential therapeutics and test their efficacy.

Magnetic resonance imaging of the retina: from mice to men

This mini-review provides an overview of magnetic resonance imaging (MRI) applications to study rodent, cat, non-human primate, and human retinas. These techniques include T(1) - and T(2) -weighted anatomical, diffusion, blood flow, blood volume, blood-oxygenation level dependent, manganese-enhanced, physiological, and functional MRI. Applications to study the retinas in diabetic retinopathy, glaucoma, and retinal degeneration are also reviewed. MRI offers some unique advantages compared with existing imaging techniques and has the potential to further our understanding of physiology and function in healthy and diseased retinas.

Confirming a prediction of the calcium hypothesis of photoreceptor aging in mice

Prior work in healthy rats supported a calcium hypothesis of photoreceptor aging, wherein progressive age-related declines in photopic vision are explainable by the extent of earlier escalating d-cis-diltiazem-insensitive increases in photoreceptor L-type calcium channel (LTCC) activity in vivo. Unlike rats, healthy mice have relatively stable photopic vision until after 18 months of age. We therefore hypothesized that photoreceptor LTCC activity in mice would not progressively increase until after 18 months. In 2-5, 10, 18, and 26 months male C57Bl/6J mice, photoreceptor LTCC activity and retinal thickness were evaluated in vivo (manganese-enhanced magnetic resonance imaging) with some groups also treated with d-cis-diltiazem; visual performance was evaluated (optokinetic tracking). Data were calibrated for cone-only responses using mice without rod transducin (GNAT1-/-). Photopic vision was stable until after 18 months without retinal thinning or progressive increases in retinal manganese uptake. We measured an uptake spike at 10 months. This spike, unlike that in the rat, was diltiazem sensitive in the dark and diltiazem insensitive in the light. Between dark and light, uptake in inner retina of older mice was unequal (unlike that in 2-5 months mice); outer retinal uptake was similar to that in 2-5 months mice. Stable murine photopic visual performance and nonescalating photoreceptor LTCC activity before 18 months of age were consistent with a prediction of the calcium hypothesis. Stark differences in the temporal evolution of mouse and rat photoreceptor LTCC activity suggest the need for personalized identification of the retinal mechanisms contributing to declines in photopic vision to ensure success of future treatment efforts.

MRI biomarkers for evaluation of treatment efficacy in preclinical diabetic retinopathy

INTRODUCTION

One sober consequence of the current epidemic of diabetes mellitus is that an increasing number of people world-wide will partially or completely lose their sight to diabetic retinopathy. Clinically, the sight-threatening complications of diabetes are diagnosed and treated based on visible retinal lesions (e.g., dot-blot hemorrhages or retinal neovascularization). However, such anatomical microvascular lesions are slow to respond with treatment. Thus, there remains an urgent need for imaging biomarkers that are abnormal before retinal lesions are visibly apparent and are responsive to treatment.

AREAS COVERED

Here, the development of new MRI methods, such as manganese-enhanced MRI, for evaluating early diabetes-evoked retinal pathophysiology, and its usefulness in guiding new treatments for diabetic retinopathy are reviewed.

EXPERT OPINION

In diabetic retinopathy, not all important diagnostic and prognostic needs are well served by optical methods. In the absence of gross anatomy changes, critical times when drug intervention is most likely to be successful at reducing vision loss are missed by most light-based methods and thus provide little help in guiding diagnosis and treatment. For example, before clinical symptoms, is there an optimal time to intervene with drug therapy? Is a drug reaching its target? How does one assess optimal drug dose, schedule, and routes? How well do current experimental models mimic the clinical condition? As discussed herein, MRI is as an analytical tool for addressing these unmet needs. Future clinical applications of MRI can be envisioned such as in clinical trials to assess drug treatment efficacy, or as an adjunct approach to refine or clarify a difficult clinical case. New MRI-generated hypotheses about the pathogenesis of diabetic retinopathy and its treatment are discussed. In the coming years, a substantial growth in the development and application of MRI is expected to address relevant question in both the basic sciences and in the clinic.

Layer-specific blood-flow MRI of retinitis pigmentosa in RCS rats

The Royal College of Surgeons (RCS) rat is an established animal model of retinitis pigmentosa, a family of inherited retinal diseases which starts with loss of peripheral vision and progresses to eventual blindness. Blood flow (BF), an important physiological parameter, is intricately coupled to metabolic function under normal physiological conditions and is perturbed in many neurological and retinal diseases. This study reports non-invasive high-resolution MRI (44 × 44 × 600 μm) to image quantitative retinal and choroidal BF and layer-specific retinal thicknesses in RCS rat retinas at different stages of retinal degeneration compared with age-matched controls. The unique ability to separate retinal and choroidal BF was made possible by the depth-resolved MRI technique. RBF decreased with progressive retinal degeneration, but ChBF did not change in RCS rats up to post-natal day 90. We concluded that choroidal and retinal circulations have different susceptibility to progressive retinal degeneration in RCS rats. Layer-specific retinal thickness became progressively thinner and was corroborated by histological analysis in the same animals. MRI can detect progressive anatomical and BF changes during retinal degeneration with laminar resolution.

Blood flow and anatomical MRI in a mouse model of retinitis pigmentosa

This study tested the sensitivity of an arterial spin labeling MRI method to image changes in retinal and choroidal blood flow (BF) and anatomical thickness of the retina in the rd10 mouse model of retinitis pigmentosa. High-resolution (42 × 42 μm) MRI was performed on rd10 mice and age-matched controls at 25, 35, and 60 days of age (n = 6 each group) on a 7-T scanner. Anatomical MRI was acquired, and quantitative BF was imaged using arterial spin labeling MRI with a separate cardiac labeling coil. Histology was obtained to confirm thickness changes in the retina. In control mice, the retinal and choroidal vascular layers were quantitatively resolved. In rd10 mice, retinal BF decreased progressively over time, while choroidal BF was unchanged. The rd10 retina became progressively thinner at later time points compared with age-matched controls by anatomical MRI and histology (P < 0.01). BF and anatomical MRI were capable of detecting decreased BF and thickness in the rd10 mouse retina. Because BF is tightly coupled to metabolic function, BF MRI has the potential to noninvasively assess retinal diseases in which metabolism and function are perturbed and to evaluate novel treatments, complementing existing retinal imaging techniques.

Refractive Development in the "ROP Rat"

Although retinopathy of prematurity (ROP) is clinically characterized by abnormal retinal vessels at the posterior pole of the eye, it is also commonly characterized by vascular abnormalities in the anterior segment, visual dysfunction which is based in retinal dysfunction, and, most commonly of all, arrested eye growth and high refractive error, particularly (and paradoxically) myopia. The oxygen-induced retinopathy rat model of ROP presents neurovascular outcomes similar to the human disease, although it is not yet known if the "ROP rat" also models the small-eyed myopia characteristic of ROP. In this study, magnetic resonance images (MRIs) of albino (Sprague-Dawley) and pigmented (Long-Evans) ROP rat eyes, and age- and strain-matched room-air-reared (RAR) controls, were examined. The positions and curvatures of the various optical media were measured and the refractive state (℞) of each eye estimated based on a previously published model. Even in adulthood (postnatal day 50), Sprague-Dawley and Long-Evans ROP rats were significantly myopic compared to strain-matched controls. The myopia in the Long-Evans ROP rats was more severe than in the Sprague-Dawley ROP rats, which also had significantly shorter axial lengths. These data reveal the ROP rat to be a novel and potentially informative approach to investigating physiological mechanisms in myopia in general and the myopia peculiar to ROP in particular.

Intraretinal calcium channels and retinal morbidity in experimental retinopathy of prematurity

PURPOSE

To test the hypothesis that intraretinal calcium channels participate in retinal morbidity in a variable oxygen (VO) model of retinopathy of prematurity.

METHODS

In control and VO Long Evans (LE) rats, either untreated or treated with voltage- or ligand-gated calcium channel antagonists, we measured retinal neovascular (NV) incidence and severity (adenosine diphosphatase staining), and retinal thickness and intraretinal ion channel activity (manganese-enhanced magnetic resonance imaging). Comparisons with the commonly studied Sprague Dawley rats were performed. Visual performance (optokinetic tracking) in untreated VO LE rats was also evaluated.

RESULTS

In control LE rats, specific L-type voltage calcium channel antagonism, but not ligand-gated channel blockers, suppressed retinal manganese accumulation, while the inhibition of L-type channels normalized intraretinal uptake in VO LE rats. VO LE rats developed more severe NV than VO Sprague Dawley rats. Following VO, both strains demonstrated significant and similar degrees of retinal thinning and supernormal intraretinal manganese uptake. However, over time, intraretinal uptake remained elevated only in VO LE rats. Visual performance was subnormal in VO LE rats. L-type voltage-gated calcium channel antagonism reduced NV severity by 28% (p<0.05) in experimental LE rats compared to that in the control group.

CONCLUSIONS

Abnormal intraretinal calcium channel activity is linked with retinal morbidity in experimental retinopathy of prematurity.

Retinal channelrhodopsin-2-mediated activity in vivo evaluated with manganese-enhanced magnetic resonance imaging

PURPOSE

Ectopic expression of light-sensitive proteins, such as channelrhodopsin-2, represent a novel approach for restoring light-detection capabilities to degenerated retina. A noninvasive method that can detect light-mediated activities of such light-sensitive proteins in the retina in vivo would be important for correlating expression patterns and retinal function. In this study, we tested the hypothesis that retinal uptake of manganese, measured noninvasively with manganese-enhanced magnetic resonance imaging (MEMRI), is a biomarker of channelrhodopsin-2-mediated activity in vivo.

METHODS

The eyes of 3-month-old rd1/rd1 mice were either untreated ("uninjected," negative control) or injected intravitreally with either saline ("saline," negative control) or adeno-associated virus carrying a fusion construct of channelopsin-2 (Chop2) and green fluorescent protein (GFP; "Chop2-GFP"). MEMRI examination was performed 2 months later on either dark or continuous bright blue light-exposed mice to assess the distribution and extent of manganese uptake in the retina and optic nerve. In separate experiments, MEMRI was used to map laminar accumulation of manganese vertically through the retina. For comparison, Chop2-GFP expression was evaluated in whole mounts and vertical sections of virus-infected retinas and optic nerve.

RESULTS

In the two control groups (regardless of lighting exposure) and between the control groups and the dark-exposed virus-treated eyes, retinal and optic nerve uptake of manganese did not differ. In light-exposed virus-treated eyes, manganese uptake in the retina and optic nerve was significantly greater relative to the other groups. In a retinal cross-section, manganese accumulation in light-exposed virus-treated eyes was spatially matched with Chop2-GFP expression in the optic nerve and all remaining retinal layers except the inner nuclear layer.

CONCLUSIONS

First-time evidence is presented indicating the usefulness of measuring intraretinal manganese accumulation as a noninvasive biomarker of channelrhodopsin-2-mediated activity in vivo.

Retinal ion regulation in a mouse model of diabetic retinopathy: natural history and the effect of Cu/Zn superoxide dismutase overexpression

PURPOSE

To test the hypotheses that manganese-enhanced MRI (MEMRI) is useful in evaluating intraretinal ion dysregulation in wild-type (WT) and Cu/Zn superoxide dismutase (SOD1) overexpressor mice.

METHODS

Central intraretinal ion activity and retinal thickness were measured from high-resolution data of light- and dark-adapted WT C57BL/6 mice (to gauge MEMRI sensitivity to normal visual processing in mice) and dark-adapted diabetic and nondiabetic WT and Cu/Zn superoxide dismutase overexpressor (SOD1OE) mice. Glycated hemoglobin and retinal vascular histopathology were also determined.

RESULTS

In WT mice, light adaptation reduced outer retinal manganese uptake compared with that in dark adaptation; no effect on inner retinal uptake was found. In diabetic WT mice, intraretinal manganese uptake became subnormal between 1.5 and 4 months of diabetes onset and then relatively increased. Central retinal thickness, as determined with MEMRI, decreased as a function of age in diabetic mice but remained constant in control mice. Nondiabetic SOD1OE mice had normal retinal manganese uptake but subnormal retinal thickness and supernormal acellular capillary density. At 4.2 months of diabetes, SOD1OE mice had normal manganese uptake and no further thinning; acellular capillaries frequency did not increase by 9 to 10 months of diabetes.

CONCLUSIONS

In emerging diabetic retinopathy, MEMRI provided an analytic measure of an ionic dysregulatory pattern that was sensitive to SOD1 overexpression. The potential benefit of SOD1 overexpression to inhibit retinal abnormality in this model is limited by the retinal and vascular degeneration that develops independently of diabetes.

Quantitative mapping of ion channel regulation by visual cycle activity in rodent photoreceptors in vivo

PURPOSE

To test the hypothesis that the extent of outer retina uptake of manganese, measured noninvasively with manganese-enhanced MRI (MEMRI), is a quantitative biomarker of photoreceptor ion channel regulation by visual cycle activity.

METHODS

Four groups of animals were studied: control rats adapted to three different background light intensities, dark-adapted control mice systemically pretreated with retinylamine, and dark-adapted mice with a nonsense mutation in exon 3 of the RPE65 gene (RPE65(rd12)) with and without systemic 11-cis-retinal pretreatment. In all cases, rodents were anesthetized and studied with MEMRI 4 hours after manganese administration IP. Central retinal thickness and intraretinal ion channel regulation were measured from the MEMRI data.

RESULT

No differences (P>0.05) in retinal thickness were noted within any arm of this study. In rats, manganese uptake was inversely proportional to the background light intensity in the outer retina but not in the inner retina. Specific inhibition at the level of RPE65 activity, either acutely with retinylamine or chronically in RPE65(rd12) mice, similarly reduced (P<0.05) outer retinal manganese uptake compared with that in control mice. In RPE65(rd12) mice, outer retinal manganese uptake returned to normal (P>0.05) after 11-cis retinal treatment. Inner retinal uptake was supernormal (P<0.05) in retinylamine-treated mice but normal in untreated or 11-cis treated RPE65(rd12) mice.

CONCLUSIONS

The present data support measuring the extent of manganese uptake in the outer retina as an analytic noninvasive metric of visual cycle regulation of photoreceptor ion channel activity in vivo.