Changes in visual fields and lateral geniculate nucleus in monkey laser-induced high intraocular pressure model

The heterogeneity of astrocytes in glaucoma

Glaucoma is a leading cause of blindness with progressive degeneration of retinal ganglion cells. Aging and increased intraocular pressure (IOP) are major risk factors. Lowering IOP does not always stop the disease progression. Alternative ways of protecting the optic nerve are intensively studied in glaucoma. Astrocytes are macroglia residing in the retina, optic nerve head (ONH), and visual brain, which keep neuronal homeostasis, regulate neuronal activities and are part of the immune responses to the retina and brain insults. In this brief review, we discuss the activation and heterogeneity of astrocytes in the retina, optic nerve head, and visual brain of glaucoma patients and animal models. We also discuss some recent transgenic and gene knockout studies using glaucoma mouse models to clarify the role of astrocytes in the pathogenesis of glaucoma. Astrocytes are heterogeneous and play crucial roles in the pathogenesis of glaucoma, especially in the process of neuroinflammation and mitochondrial dysfunction. In astrocytes, overexpression of Stat3 or knockdown of IκKβ/p65, caspase-8, and mitochondrial uncoupling proteins (Ucp2) can reduce ganglion cell loss in glaucoma mouse models. Based on these studies, therapeutic strategies targeting the heterogeneity of reactive astrocytes by enhancing their beneficial reactivity or suppressing their detrimental reactivity are alternative options for glaucoma treatment in the future.

Degeneration of retina-brain components and connections in glaucoma: Disease causation and treatment options for eyesight preservation

Eyesight is the most important of our sensory systems for optimal daily activities and overall survival. Patients who experience visual impairment due to elevated intraocular pressure (IOP) are often those afflicted with primary open-angle glaucoma (POAG) which slowly robs them of their vision unless treatment is administered soon after diagnosis. The hallmark features of POAG and other forms of glaucoma are damaged optic nerve, retinal ganglion cell (RGC) loss and atrophied RGC axons connecting to various brain regions associated with receipt of visual input from the eyes and eventual decoding and perception of images in the visual cortex. Even though increased IOP is the major risk factor for POAG, the disease is caused by many injurious chemicals and events that progress slowly within all components of the eye-brain visual axis. Lowering of IOP mitigates the damage to some extent with existing drugs, surgical and device implantation therapeutic interventions. However, since multifactorial degenerative processes occur during aging and with glaucomatous optic neuropathy, different forms of neuroprotective, nutraceutical and electroceutical regenerative and revitalizing agents and processes are being considered to combat these eye-brain disorders. These aspects form the basis of this short review article.

Visual cortex damage in a ferret model of ocular hypertension

PURPOSE

We aimed to analyze the changes in the visual cortex of a ferret model of ocular hypertension (OH) using cytochrome oxidase (CO) staining.

STUDY DESIGN

Experimental.

METHODS

OH was induced in 9 ferrets by means of injection of cultured conjunctival cells into the anterior chamber of the right eye. Three ferrets were used as the controls. CO staining was performed to assess the metabolic intensity at the II-III and IVC layers of the visual cortex.

RESULTS

The intensities of CO staining in the right and left II-III layers of the primary visual cortex (V1) in the OH ferrets were 39.8 ± 10.3 and 41.9 ± 9.2 arbitrary units, respectively. In the control ferrets, the intensity was 88.1 ± 8.1 arbitrary units. The intensity of CO staining of the II-III layers obtained from the OH eyes was significantly lower than that from the control eyes (unpaired t test, P < .01). The intensities of CO staining in the right and left IVC layers of V1 in the OH ferrets were 60.3 ± 12.8 and 60.0 ± 13.5 arbitrary units, respectively. In the control ferrets, the intensity was 111.4 ± 9.6 arbitrary units. The CO staining intensity of the IVC layer obtained from the OH eyes was significantly lower than that from the control eyes (unpaired t test, P < .01).

CONCLUSION

The CO staining intensity was reduced in the visual cortex from OH eyes. This study revealed that OH causes metabolic change in the visual cortex.

Therapeutic Drugs and Devices for Tackling Ocular Hypertension and Glaucoma, and Need for Neuroprotection and Cytoprotective Therapies

Damage to the optic nerve and the death of associated retinal ganglion cells (RGCs) by elevated intraocular pressure (IOP), also known as glaucoma, is responsible for visual impairment and blindness in millions of people worldwide. The ocular hypertension (OHT) and the deleterious mechanical forces it exerts at the back of the eye, at the level of the optic nerve head/optic disc and lamina cribosa, is the only modifiable risk factor associated with glaucoma that can be treated. The elevated IOP occurs due to the inability of accumulated aqueous humor (AQH) to egress from the anterior chamber of the eye due to occlusion of the major outflow pathway, the trabecular meshwork (TM) and Schlemm’s canal (SC). Several different classes of pharmaceutical agents, surgical techniques and implantable devices have been developed to lower and control IOP. First-line drugs to promote AQH outflow via the uveoscleral outflow pathway include FP-receptor prostaglandin (PG) agonists (e.g., latanoprost, travoprost and tafluprost) and a novel non-PG EP2-receptor agonist (omidenepag isopropyl, Eybelis^®). TM/SC outflow enhancing drugs are also effective ocular hypotensive agents (e.g., rho kinase inhibitors like ripasudil and netarsudil; and latanoprostene bunod, a conjugate of a nitric oxide donor and latanoprost). One of the most effective anterior chamber AQH microshunt devices is the Preserflo^® microshunt which can lower IOP down to 10–13 mmHg. Other IOP-lowering drugs and devices on the horizon will be also discussed. Additionally, since elevated IOP is only one of many risk factors for development of glaucomatous optic neuropathy, a treatise of the role of inflammatory neurodegeneration of the optic nerve and retinal ganglion cells and appropriate neuroprotective strategies to mitigate this disease will also be reviewed and discussed.

Transneuronal Degeneration in the Brain During Glaucoma

The death of retinal ganglion cells (RGCs) is a key factor in the pathophysiology of all types of glaucoma, but the mechanism of pathogenesis of glaucoma remains unclear. RGCs are a group of central nervous system (CNS) neurons whose soma are in the inner retina. The axons of RGCs form the optic nerve and converge at the optic chiasma; from there, they project to the visual cortex via the lateral geniculate nucleus (LGN). In recent years, there has been increasing interest in the dysfunction and death of CNS and retinal neurons caused by transneuronal degeneration of RGCs, and the view that glaucoma is a widespread neurodegenerative disease involving CNS damage appears more and more frequently in the literature. In this review, we summarize the current knowledge of LGN and visual cortex neuron damage in glaucoma and possible mechanisms behind the damage. This review presents an updated and expanded view of neuronal damage in glaucoma, and reveals new and potential targets for neuroprotection and treatment.

Inflammation in Glaucoma: From the back to the front of the eye, and beyond

The pathophysiology of glaucoma is complex, multifactorial and not completely understood. Elevated intraocular pressure (IOP) and/or impaired retinal blood flow may cause initial optic nerve damage. In addition, age-related oxidative stress in the retina concurrently with chronic mechanical and vascular stress is crucial for the initiation of retinal neurodegeneration. Oxidative stress is closely related to cell senescence, mitochondrial dysfunction, excitotoxicity, and neuroinflammation, which are involved in glaucoma progression. Accumulating evidence from animal glaucoma models and from human ocular samples suggests a dysfunction of the para-inflammation in the retinal ganglion cell layer and the optic nerve head. Moreover, quite similar mechanisms in the anterior chamber could explain the trabecular meshwork dysfunction and the elevated IOP in primary open-angle glaucoma. On the other hand, ocular surface disease due to topical interventions is the most prominent and visible consequence of inflammation in glaucoma, with a negative impact on filtering surgery failure, topical treatment efficacy, and possibly on inflammation in the anterior segment. Consequently, glaucoma appears as an outstanding eye disease where inflammatory changes may be present to various extents and consequences along the eye structure, from the ocular surface to the posterior segment, and the visual pathway. Here we reviewed the inflammatory processes in all ocular structures in glaucoma from the back to the front of the eye and beyond. Our approach was to explain how para-inflammation is necessary to maintain homoeostasis, and to describe abnormal inflammatory findings observed in glaucomatous patients or in animal glaucoma models, supporting the hypothesis of a dysregulation of the inflammatory balance toward a pro-inflammatory phenotype. Possible anti-inflammatory therapeutic approaches in glaucoma are also discussed.

Magnetic Resonance Imaging for Glaucoma Evaluation

The damage caused by glaucoma has been extensively evaluated at the level of the retina and optic nerve head. Many advances have been shown in this field in the last decades. Recent studies have also proved degenerative changes in the brain involving the intracranial optic nerve, lateral geniculate nucleus, and visual cortex. Moreover, these brain abnormalities are also correlated with clinical, optic nerve head, and visual field findings. In this review, we critically evaluate the existing literature studying the use of magnetic resonance imaging in glaucoma, and we discuss issues related to how magnetic resonance imaging results should be incorporated into our clinical practice.

Glaucoma: A Degenerative Optic Neuropathy Related to Neuroinflammation?

Glaucoma is one of the leading causes of irreversible blindness in the world and remains a major public health problem. To date, incomplete knowledge of this disease’s pathophysiology has resulted in current therapies (pharmaceutical or surgical) unfortunately having only a slowing effect on disease progression. Recent research suggests that glaucomatous optic neuropathy is a disease that shares common neuroinflammatory mechanisms with “classical” neurodegenerative pathologies. In addition to the death of retinal ganglion cells (RGCs), neuroinflammation appears to be a key element in the progression and spread of this disease. Indeed, early reactivity of glial cells has been observed in the retina, but also in the central visual pathways of glaucoma patients and in preclinical models of ocular hypertension. Moreover, neuronal lesions are not limited to retinal structure, but also occur in central visual pathways. This review summarizes and puts into perspective the experimental and clinical data obtained to date to highlight the need to develop neuroprotective and immunomodulatory therapies to prevent blindness in glaucoma patients.

Structural Changes and Astrocyte Response of the Lateral Geniculate Nucleus in a Ferret Model of Ocular Hypertension

We investigated structural changes and astrocyte responses of the lateral geniculate nucleus (LGN) in a ferret model of ocular hypertension (OH). In 10 ferrets, OH was induced via the injection of cultured conjunctival cells into the anterior chamber of the right eye; six normal ferrets were used as controls. Anterograde axonal tracing with cholera toxin B revealed that atrophic damage was evident in the LGN layers receiving projections from OH eyes. Immunohistochemical analysis with antibodies against NeuN, glial fibrillary acidic protein (GFAP), and Iba-1 was performed to specifically label neurons, astrocytes, and microglia in the LGN. Significantly decreased NeuN immunoreactivity and increased GFAP and Iba-1 immunoreactivities were observed in the LGN layers receiving projections from OH eyes. Interestingly, the changes in the immunoreactivities were significantly different among the LGN layers. The C layers showed more severe damage than the A and A1 layers. Secondary degenerative changes in the LGN were also observed, including neuronal damage and astrocyte reactions in each LGN layer. These results suggest that our ferret model of OH is valuable for investigating damages during the retina–brain transmission of the visual pathway in glaucoma. The vulnerability of the C layers was revealed for the first time.

Diagnostic utility of central damage determination in glaucoma by magnetic resonance imaging: An observational study

The aim of the present study was to investigate the utility of diffusion tensor imaging (DTI) parameters of the central visual pathway and the size of the lateral geniculate nucleus (LGN) in the diagnosis of primary open-angle glaucoma (POAG). To test this, 24 POAG patients and 24 age- and gender-matched controls underwent 3.0 Tesla magnetic resonance imaging examinations, including DTI and LGN structural imaging. Receiver operating characteristic (ROC) curves were generated and Spearman's correlation coefficients were also determined. The results indicated that regarding the discrimination of POAG patients from healthy controls, the fractional anisotropy (FA) values of the optic tract, at a cutoff of ≤0.412, with an area under the ROC curve of 0.931, exhibited the highest sensitivity (79.2%) and specificity (89.6%). The FA values of the optic tract and the optic radiation were significantly correlated with LGN size, while the mean diffusivity values were not. In conclusion, the FA value of the optic tract may be a sensitive and reliable biomarker for glaucoma evaluation.

Long-term histological changes in the macaque primary visual cortex and the lateral geniculate nucleus after monocular deprivation produced by early restricted retinal lesions and diffuser induced form deprivation

Ocular dominance (OD) plasticity has been extensively studied in various mammalian species. While robust OD shifts are typically observed after monocular eyelid suture, relatively poor OD plasticity is observed for early eye removal or after tetrodotoxin (TTX) injections in mice. Hence, abnormal binocular signal interactions in the visual cortex may play a critical role in eliciting OD plasticity. Here, we examined the histochemical changes in the lateral geniculate nucleus (LGN) and the striate cortex (V1) in macaque monkeys that experienced two different monocular sensory deprivations in the same eye beginning at 3 weeks of age: restricted laser lesions in macular or peripheral retina and form deprivation induced by wearing a diffuser lens during the critical period. The monkeys were subsequently reared for 5 years under a normal visual environment. In the LGN, atrophy of neurons and a dramatic increase of GFAP expression were observed in the lesion projection zones (LPZs). In V1, although no obvious shift of the LPZ border was found, the ocular dominance columns (ODCs) for the lesioned eye shrunk and those for the intact eye expanded over the entirety of V1. This ODC size change was larger in the area outside the LPZ and in the region inside the LPZ near the border compared to that in the LPZ center. These developmental changes may reflect abnormal binocular interactions in V1 during early infancy. Our observations provide insights into the nature of degenerative and plastic changes in the LGN and V1 following early chronic monocular sensory deprivations.

Structural and functional analyses of the optic nerve and lateral geniculate nucleus in glaucoma

Purpose

To analyze the correlation between structural characteristics of intraorbital optic nerve (ION) and lateral geniculate nucleus (LGN) measured by 3-Tesla magnetic resonance imaging (3T MRI), and the severity of glaucomatous damage.

Methods

In this cross-sectional study, 41 glaucoma patients and 12 age- and sex-matched controls underwent standard automated perimetry (SAP) and frequency doubling technology (FDT) as functional evaluation; optic disc stereophotograph, spectral-domain optical coherence tomography (OCT) and confocal scanning laser tomography as ocular structural evaluation; and 3T MRI. Structure-structure and structure-function correlation were performed using bootstrap resampling method for clustered data.

Results

The ION mean diameter and cross-sectional area were different between glaucoma and control groups at 5mm and 10mm (all, p≤0.011) from the globe, but not at 15mm (both, p≥0.067). LGN height was significantly lower in glaucoma group (p = 0.005). OCT rim area and functional parameters (SAP and FDT) correlated significantly with all ION segments, showing stronger correlations at 10 and 15 mm. ION parameters at 10 and 15 mm presented mild-to-moderate correlation with OCT peripapillary nerve fiber layer thickness, and ION at 15mm had mild association with the neuroretinal rim area on stereophotographs. Although LGN height was significantly smaller in glaucoma group (p = 0.005), LGN parameters were not associated with any ocular structural or functional parameter.

Conclusion

Assessment of central and peripheral nervous systems using 3T MRI confirmed that glaucoma patients had smaller ION dimensions and LGN height compared to the control group. In general, ION dimensions presented mild to moderate correlations with functional and ocular structural parameters. Although ION had significant correlations at any distance from the eye, the ION distal locations correlated better with OCT results and functional parameters. However, LGN parameters were not associated with functional or ocular structural parameters.

Microstructural visual pathway abnormalities in patients with primary glaucoma: 3 T diffusion kurtosis imaging study

AIM

To evaluate microstructural visual pathway damage in patients with primary glaucoma (PG) by using 3 T diffusion kurtosis imaging (DKI).

MATERIALS AND METHODS

The study was approved by the ethics committee, and all participants provided written informed consent. Ten patients with PG were examined. Twenty healthy individuals served as control subjects. DKI was performed with a GE Silent 3 T magnetic resonance imaging (MRI) unit. Mean diffusivity (MD), fractional anisotropy (FA), and mean kurtosis (MK) maps were automatically created. Mean MK, MD, and FA values were calculated for each part of the visual pathway.

RESULTS

No abnormalities in the shape and signal intensity were observed along the entire visual pathway in patients and the control group on the conventional MRI. Higher MD, and lower MK and FA were observed in the optic nerves (ON), lateral geniculate nucleus (LGN), optic radiations (OR), and visual cortex (VCx) of PG patients, as compared with control subjects. A significantly higher MD was observed in the ON (p<0.01), and significantly lower FA was observed in OR (p<0.05). Additionally, significantly lower MK was observed in the ON, LGN, and VCx, except for OR (p<0.01). Changes of DKI parameters in the ON were the most distinct.

CONCLUSION

Glaucoma is a complex neurological disease that affects the entire visual pathway. MK derived from DKI would be a better biomarkers than FA and MD in detecting microstructural damage.

Central visual pathways in glaucoma: evidence for distal mechanisms of neuronal self-repair

As in other age-related neurodegenerative diseases, progression of neurodegeneration in glaucoma involves early axonopathy. In glaucoma, this is marked by degradation of active transport along retinal ganglion cell (RGC) axons projecting from the retina to the brain. In experimental systems, transport degradation occurs first in the most distal site in the RGC projection, the superior colliculus (SC) of the midbrain. Even as degradation progresses from one retinotopic sector to the next, important structures in the affected sectors persist, including synapses from RGC axon terminals onto SC neurons. This structural persistence is accompanied by focally increased brain-derived neurotrophic factor in hypertrophic SC astrocyte glia and defines a therapeutic window of opportunity. Thus, central brain structures in glaucoma may respond to disease-relevant stress by induction of mechanisms useful for maintaining retinal signals.

Association of n3 and n6 polyunsaturated fatty acids in red blood cell membrane and plasma with severity of normal tension glaucoma

AIM

To determine whether red blood cell (RBC) membrane and plasma lipids, particularly long-chain polyunsaturated fatty acids such as eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), arachidonic acid (AA) are significantly correlated with severity of normal tension glaucoma (NTG).

METHODS

This study included 35 patients with NTG and 12 healthy normal control subjects, matched for age and sex with the study group. The stage of glaucoma was determined according to the Hodapp-Parrish-Anderson classification. Lipids were extracted from RBC membranes and plasma, and fatty acid methyl esters prepared and analyzed by gas chromatography-mass spectrometry (GC-MS).

RESULTS

When RBC lipids were analyzed, the levels of EPA, the levels of DHA and the ratio of n3 to n6 were positively associated with the Humphrey Perimetry mean deviation (MD) score (r=0.617, P<0.001; r=0.727, P<0.001 and r=0.720, P<0.001, respectively), while the level of AA was negatively associated with the MD score (r=-0.427, P=0.001). When plasma lipids were analyzed, there was a significant positive relationship between the levels of EPA and the MD score (r=0.648, P<0.001), and the levels of AA were inversely correlated with the MD score (r=-0.638, P<0.001).

CONCLUSION

The levels of n3 and n6 polyunsaturated fatty acids in RBC membrane and plasma lipids were associated with severity of NTG.

Progressive degeneration of retinal and superior collicular functions in mice with sustained ocular hypertension

PURPOSE

We investigated the progressive degeneration of retinal and superior collicular functions in a mouse model of sustained ocular hypertension.

METHODS

Focal laser illumination and injection of polystyrene microbeads were used to induce chronic ocular hypertension. Retinal ganglion cell (RGC) loss was characterized by in vivo optical coherence tomography (OCT) and immunohistochemistry. Retinal dysfunction was also monitored by the full-field ERG. Retinal ganglion cell light responses were recorded using a 256-channel multielectrode array (MEA), and RGC subtypes were characterized by noncentered spike-triggered covariance (STC-NC) analysis. Single-unit extracellular recordings from superficial layers of the superior colliculus (SC) were performed to examine the receptive field (RF) properties of SC neurons.

RESULTS

The elevation of intraocular pressure (IOP) lasted 4 months in mice treated with a combination of laser photocoagulation and microbead injection. Progressive RGC loss and functional degeneration were confirmed in ocular hypertensive (OHT) mice. These mice had fewer visually responsive RGCs than controls. Using the STC-NC analysis, we classified RGCs into ON, OFF, and ON-OFF functional subtypes. We showed that ON and OFF RGCs were more susceptible to the IOP elevation than ON-OFF RGCs. Furthermore, SC neurons of OHT mice had weakened responses to visual stimulation and exhibited mismatched ON and OFF subfields and irregular RF structure.

CONCLUSIONS

We demonstrated that the functional degeneration of RGCs is subtype-dependent and that the ON and OFF pathways from the retina to the SC were disrupted. Our study provides a foundation to investigate the mechanisms underlying the progressive vision loss in experimental glaucoma.

Ocular hypertension results in retinotopic alterations in the visual cortex of adult mice

PURPOSE

Glaucoma is a group of optic neuropathies characterized by the loss of retinal ganglion cells (RGCs). Since ocular hypertension (OHT) is a main risk factor, current therapies are predominantly based on lowering eye pressure. However, a subset of treated patients continues to lose vision. More research into pathological mechanisms underlying glaucoma is therefore warranted in order to develop novel therapeutic strategies. In this study we investigated the impact of OHT from eye to brain in mice.

METHODS

Monocular hypertension (mOHT) was induced in CD-1 mice by laser photocoagulation (LP) of the perilimbal and episcleral veins. The impact on the retina and its main direct target area, the superficial superior colliculus (sSC), was examined via immunostainings for Brn3a, VGluT2 and GFAP. Alterations in neuronal activity in V1 and extrastriate areas V2L and V2M were assessed using in situ hybridization for the activity reporter gene zif268.

RESULTS

Transient mOHT resulted in diffuse and sectorial RGC degeneration. In the sSC contralateral to the OHT eye, a decrease in VGluT2 immunopositive synaptic connections was detected one week post LP, which appeared to be retinotopically linked to the sectorial RGC degeneration patterns. In parallel, hypoactivity was discerned in contralateral retinotopic projection zones in V1 and V2. Despite complete cortical reactivation 4 weeks post LP, in the sSC no evidence for recovery of RGC synapse density was found and also the concomitant inflammation was not completely resolved. Nevertheless, sSC neurons appeared healthy upon histological inspection and subsequent analysis of cell density revealed no differences between the ipsi- and contralateral sSC.

CONCLUSION

In addition to RGC death, OHT induces loss of synaptic connections and neuronal activity in the visual pathway and is accompanied by an extensive immune response. Our findings stress the importance of looking beyond the eye and including the whole visual system in glaucoma research.

Establishment of an experimental ferret ocular hypertension model for the analysis of central visual pathway damage

Glaucoma optic neuropathy (GON) is a condition where pathogenic intraocular pressure (IOP) results in axonal damage following retinal ganglion cell (RGC) death, and further results in secondary damage of the lateral geniculate nucleus (LGN). Therapeutic targets for glaucoma thus focus on both the LGN and RGC. However, the temporal and spatial patterns of degeneration and the mechanism of LGN damage have not been fully elucidated. Suitable and convenient ocular hypertension (OH) animal models with binocular vision comparable to that of monkeys are strongly needed. The ferret is relatively small mammal with binocular vision like humans – here we report on its suitability for investigating LGN. We developed a new method to elevate IOP by injection of cultured conjunctival cells into the anterior chamber to obstruct aqueous outflow. Histologically, cultured conjunctival cells successfully proliferated to occlude the angle, and IOP was elevated for 13 weeks after injection. Macroscopically, the size of the eye gradually expanded. Subsequent enlargement of optic nerve head cupping and atrophic damage of LGN projected from the OH eye were clearly observed by anterograde staining with cholera toxin B. We believe the ferret may be a promising OH model to investigate secondary degeneration of central nervous system including LGN.

Refined Data Analysis Provides Clinical Evidence for Central Nervous System Control of Chronic Glaucomatous Neurodegeneration

PURPOSE

Refined data analysis was performed to assess binocular visual field conservation in patients with bilateral glaucomatous damage to determine whether unilateral visual field loss is random, anatomically symmetric, or nonrandom in relation to the fellow eye.

METHODS

This was a case-control study of 47 consecutive patients with bilaterally severe glaucoma; each right eye visual field locus was paired with randomly selected coisopteric left eye loci, with 760,000 (10,000 complete sets of 76 loci) such iterations performed per subject. The potential role of anatomic symmetry in bilateral visual field conservation was also assessed by pairing mirror-image loci of the paired fields. The mean values of the random coisopteric and the symmetric mirror pairings were compared with natural point-for-point pairings of the two eyes by paired t-test.

RESULTS

Mean unilateral thresholds across the entire visual field were 18.9 dB left and 19.9 dB right (average 19.4), 4 dB lower than the better of the naturally paired concomitant loci of 23.4 dB (P < 10^-15). A remarkable natural tendency for conservation of the binocular visual field was confirmed, far stronger than explicable by random chance or anatomic symmetry (P < 0.0001), and reaffirmed by subsequent prospective simultaneous binocular visual field retesting of an arbitrary subset (n = 16) of the study population (P < 0.0001).

CONCLUSIONS

Refined data analysis of paired visual fields confirms the existence of a natural optimization of binocular visual function in severe bilateral glaucoma via interlocking fields that could be created only by central nervous system (CNS) involvement.

TRANSLATIONAL RELEVANCE

Integrated bilateral visual field analysis should better define actual visual disability and more accurately reflect the functional efficacy of current ocular and future CNS-oriented therapeutic approaches to the treatment of glaucoma. Glaucomatous eyes provide a highly accessible paired-organ study model for developing therapeutics to optimize conservation of function in neurodegenerative disorders.

Transplantation with retinal progenitor cells repairs visual function in rats with retinal ischemia-reperfusion injury

The retinal ischemia-reperfusion injury (RIR) is a common pathological process that leads to progressive visual loss and blindness in many retinal diseases such as retinal vascular occlusion disease, diabetic retinopathy, and acute glaucoma. Currently, there has been no effective therapy. The purpose of this study was to investigate the effects of transplantation of retinal progenitor cells (RPCs) into the subretinal space (SRS) and the superior colliculus (SC) in a rat model of RIR injury. We used cultured postnatal day 1 rat RPCs transfected with adeno-associated virus containing the cDNA encoding enhanced green fluorescence protein (EGFP) for transplantation. RIR injury was induced by increases in the intraocular pressure to 110 mmHg for 60 min. The effects of transplantation were evaluated by immunohistochemistry, electroretinography (ERG), and visual evoked potentials (VEP). We found that in rats with RIR injury, RPCs transplanted into the SRS and the SC survived for at least 8 weeks, migrated into surrounding tissues, and improved the ERG and VEP responses. Cells transplanted into the SC improved the VEP response more than those transplanted into the SRS. Our data suggest that transplantation of RPCs into the SRS and the SC may be a possible method for cell replacement therapy for retinal diseases.

Establishment of the ocular hypertension model using the common marmoset

The purpose of this study was to establish an experimental glaucoma model in the common marmoset (Callithrix jacchus). Chronic intraocular pressure (IOP) elevation was induced by laser trabeculoplasty twice at 2-week intervals in the left eyes of 4 common marmosets. IOP was measured before and at 4, 7, 8, 11, 13 weeks after first laser treatment, and ophthalmoscopic examinations were also performed. At 13 weeks after laser treatment, each eye was enucleated, and retinal cross-sections and optic nerve were prepared for histological examination. Mean IOP values measured at the above 5 time points were over 40 mmHg in laser-treated eyes in 3 marmosets, but IOP in one marmoset was transiently increased to 26.6 mmHg at 7 weeks and then declined to the baseline level. In ophthalmoscopy, deepened and enlarged optic disc cupping, depending on the extent of IOP elevation and duration, were observed in laser-treated eyes of 3 marmosets with persistent IOP elevation, but there was no apparent change in the optic disc in the laser-treated eye of one marmoset with transient IOP elevation. Histological examination showed marked atrophy with deepened and enlarged cupping of optic disc, thinning of retinal nerve fiber layer and retinal ganglion loss in the retina, and axonal atrophy and loss in the optic nerve, depending on the extent of IOP elevation and duration. In conclusion, we succeeded in producing an experimental glaucoma model in the common marmoset, and this model may be useful in elucidating the pathophysiological mechanism for glaucoma.

Kinetics of neurodegeneration based on a risk-related biomarker in animal model of glaucoma

Background

Neurodegenerative diseases including Parkinson’s and Alzheimer’s diseases progress slowly and steadily over years or decades. They show significant between-subject variation in progress and clinical symptoms, which makes it difficult to predict the course of long-term disease progression with or without treatments. Recent technical advances in biomarkers have facilitated earlier, preclinical diagnoses of neurodegeneration by measuring or imaging molecules linked to pathogenesis. However, there is no established “biomarker model” by which one can quantitatively predict the progress of neurodegeneration. Here, we show predictability of a model with risk-based kinetics of neurodegeneration, whereby neurodegeneration proceeds as probabilistic events depending on the risk.

Results

We used five experimental glaucomatous animals, known for causality between the increased intraocular pressure (IOP) and neurodegeneration of visual pathways, and repeatedly measured IOP as well as white matter integrity by diffusion tensor imaging (DTI) as a biomarker of axonal degeneration. The IOP in the glaucomatous eye was significantly increased than in normal and was varied across time and animals; thus we tested whether this measurement is useful to predict kinetics of the integrity. Among four kinds of models of neurodegeneration, constant-rate, constant-risk, variable-risk and heterogeneity models, goodness of fit of the model and F-test for model selection showed that the time course of optic nerve integrity was best explained by the variable-risk model, wherein neurodegeneration kinetics is expressed in an exponential function across cumulative risk based on measured IOP. The heterogeneity model with stretched exponential decay function also fit well to the data, but without statistical superiority to the variable-risk model. The variable-risk model also predicted the number of viable axons in the optic nerve, as assessed by immunohistochemistry, which was also confirmed to be correlated with the pre-mortem integrity of the optic nerve. In addition, the variable-risk model identified the disintegrity in the higher-order visual pathways, known to underlie the transsynaptic degeneration in this disease.

Conclusions

These findings indicate that the variable-risk model, using a risk-related biomarker, could predict the spatiotemporal progression of neurodegeneration. This model, virtually equivalent to survival analysis, may allow us to estimate possible effect of neuroprotection in delaying progress of neurodegeneration.

Failure of axonal transport induces a spatially coincident increase in astrocyte BDNF prior to synapse loss in a central target

Failure of anterograde transport to distal targets in the brain is a common feature of neurodegenerative diseases. We have demonstrated in rodent models of glaucoma, the most common optic neuropathy, early loss of anterograde transport along the retinal ganglion cell (RGC) projection to the superior colliculus (SC) is retinotopic and followed by a period of persistence of RGC axon terminals and synapses through unknown molecular pathways. Here we use the DBA/2J mouse model of hereditary glaucoma and an acute rat model to demonstrate that retinotopically focal transport deficits in the SC are accompanied by a spatially coincident increase in brain-derived neurotrophic factor (BDNF), especially in hypertrophic astrocytes. These neurochemical changes occur prior to loss of RGC synapses in the DBA/2J SC. In contrast to BDNF protein, levels of Bdnf mRNA decreased with transport failure, even as mRNA encoding synaptic structures remained unchanged. In situ hybridization signal for Bdnf mRNA was the strongest in SC neurons, and labeling for the immature precursor pro-BDNF was very limited. Subcellular fractionation of SC indicated that membrane-bound BDNF decreased with age in the DBA/2J, while BDNF released from vesicles remained high. These results suggest that in response to diminished axonal function, activated astrocytes in the brain may sequester mature BDNF released from target neurons to counter stressors that otherwise would challenge survival of projection synapses.

Diffusion tensor magnetic resonance imaging reveals visual pathway damage that correlates with clinical severity in glaucoma

BACKGROUND

To investigate nerve fibre damage of the visual pathway in patients with primary open-angle glaucoma using tract-based spatial statistical analysis of diffusion tensor imaging and correlate these measures with the clinical severity of glaucoma.

DESIGN

Cross-sectional study.

PARTICIPANTS

Twenty-five individuals with primary open-angle glaucoma and 24 healthy controls were recruited.

METHODS

All subjects underwent detailed ophthalmological examinations, including the cup-to-disc ratio, retinal nerve fibre layer thickness and visual fields test. Diffusion tensor imaging of the visual pathway was performed using a 3.0-T magnetic resonance scanner.

MAIN OUTCOME MEASURES

Diffusivity changes of the nerve fibres in the visual pathway were calculated through tract-based spatial statistical analysis. The mean diffusivity and fractional anisotropy were assessed and compared with ophthalmological measurements.

RESULTS

Compared with controls, bilateral optic tracts and optic radiations in primary open-angle glaucoma patients showed significantly decreased fractional anisotropy and increased mean diffusivity (P < 0.05). In the glaucoma group, the fractional anisotropy of the optic tracts and optic radiations varied consistently with the cup-to-disc ratio, retinal nerve fibre layer thickness and visual function analysis, respectively (P < 0.05). The mean diffusivity of the optic tracts correlated with these ophthalmological measurements (P < 0.05). However, no significant correlation was observed between the mean diffusivity of the optic radiations and the ophthalmological measurements (P > 0.05).

CONCLUSIONS

The optic tracts and optic radiations of primary open-angle glaucoma patients demonstrated radiological evidence of neurodegeneration. This varied with damage to the optic disc and with the loss of visual function. Tract-based spatial statistical analysis of diffusion tensor imaging is an objective and effective tool for detecting the loss of cortical nerve fibres in primary open-angle glaucoma.

An alteration in the lateral geniculate nucleus of experimental glaucoma monkeys: in vivo positron emission tomography imaging of glial activation

We examined lateral geniculate nucleus (LGN) degeneration as an indicator for possible diagnosis of glaucoma in experimental glaucoma monkeys using positron emission tomography (PET). Chronic intraocular pressure (IOP) elevation was induced by laser trabeculoplasty in the left eyes of 5 cynomolgus monkeys. Glial cell activation was detected by PET imaging with [¹¹C]PK11195, a PET ligand for peripheral-type benzodiazepine receptor (PBR), before and at 4 weeks after laser treatment (moderate glaucoma stage). At mild, moderate, and advanced experimental glaucoma stages (classified by histological changes based on the extent of axonal loss), brains were stained with cresyl violet, or antibodies against PBR, Iba-1 (a microglial marker), and GFAP (an activated astrocyte marker). In laser-treated eyes, IOP was persistently elevated throughout all observation periods. PET imaging showed increased [¹¹C]PK11195 binding potential in the bilateral LGN at 4 weeks after laser treatment; the increase in the ipsilateral LGN was statistically significant (P<0.05, n = 4). Immunostaining showed bilateral activations of microglia and astrocytes in LGN layers receiving input from the laser-treated eye. PBR-positive cells were observed in LGN layers receiving input from laser-treated eye at all experimental glaucoma stages including the mild glaucoma stage and their localization coincided with Iba-1 positive microglia and GFAP-positive astrocytes. These data suggest that glial activation occurs in the LGN at a mild glaucoma stage, and that the LGN degeneration could be detected by a PET imaging with [¹¹C]PK11195 during the moderate experimental glaucoma stage after unilateral ocular hypertension. Therefore, activated glial markers such as PBR in the LGN may be useful in noninvasive molecular imaging for diagnosis of glaucoma.

Astrocytic responses in the lateral geniculate nucleus of monkeys with experimental glaucoma

OBJECTIVE

  To investigate the responses of lateral geniculate nucleus (LGN) astrocytes to experimental glaucoma in monkeys.

ANIMAL STUDIED

  Rhesus monkeys (Macaca mulatta).

PROCEDURES

  Unilateral chronic elevation of intraocular pressure (IOP) was induced in six rhesus monkeys by laser photocoagulation of the trabecular meshwork. Four normal monkeys were used as controls. Immunohistochemistry with antibodies to glial fibrillary acidic protein (GFAP), S100β and parvalbumin was used to specifically label astrocytes and neurons in the LGN. The relative immunointensity (RI) of GFAP was defined as the ratio of intensity between each region of interest to a reference field and compared between the experimental and control groups as a function of percentage optic nerve fiber loss. Ultrastructural changes of LGN astrocytes were examined by transmission electron microscopy.

RESULTS

  An increase in GFAP and S100β immunoreactivity was observed in the LGN layers receiving projections from the experimental glaucoma eyes. Quantitative analysis revealed that the RI of GFAP in both the magnocellular and parvocellular layers connected to the glaucomatous eyes increased in a linear fashion with increasing optic nerve fiber loss. Compared to controls, the RI of GFAP was also moderately elevated in LGN layers connected to the fellow nonglaucomatous eyes. Ultrastructurally, accumulation of glial filaments that occurred throughout the perikaryon and extended into the process in reactive astrocytes was observed in LGN layers of glaucomatous monkeys.

CONCLUSIONS

  Reactive astrogliosis occurs in the magnocellular and parvocellular LGN layers of monkeys with unilateral glaucoma. Astrocytes may play an important role in the regulation of LGN microenvironment in glaucoma.

Retinal nerve fiber layer assessment: area versus thickness measurements from elliptical scans centered on the optic nerve

PURPOSE

An evaluation of the retinal nerve fiber layer (RNFL) provides important information on the health of the optic nerve. Standard measurements of the RNFL consider only thickness, but an accurate assessment should also consider axial length, size of the optic nerve head (ONH), blood vessel contribution, and distance of the scan from the ONH margin. In addition, although most primate ONHs are elliptical, the circular scan centered on the ONH is the mainstay in both clinical and research analyses. The purpose of this study was to evaluate thickness and area measures of RNFL cross sections when axial length and ONH shape are included.

METHODS

Circular, raster, and radial scans of left eye optic nerves were acquired from 40 normal rhesus monkeys (Macaca mulatta) using spectral domain optical coherence tomography. The disc margin was identified by manually selecting the RPE/Bruch's membrane opening and ONH border tissue. With a pixel-to-micrometer conversion computed from a three-surface schematic eye, RNFL scans were interpolated at 300 to 600 μm (50-μm increments) from the edge of the ONH. The thickness and area of the RNFL at each distance were obtained by custom programs. Blood vessels in the RNFL were selected and removed from the overall RNFL measures.

RESULTS

The average RNFL thickness decreased systematically from 149 ± 12.0 μm for scans 300 μm from the disc margin to 113 ± 7.2 μm at an eccentricity of 600 μm (P < 0.05). In contrast, the cross-sectional areas of the RNFL did not vary with scan location from the disc margin (0.85 ± 0.07 mm(2) at 300 μm compared with 0.86 ± 0.06 mm(2) at 600 μm). Blood vessels accounted for 9.3% of total RNFL thickness or area, but varied with retinal location. On average, 17.6% of the superior and 14.2% of the inferior RNFL was vascular, whereas blood vessels accounted for only 2.3% of areas of the temporal and nasal RNFL regions.

CONCLUSIONS

In nonhuman primates, with appropriate transverse scaling and ONH shape analysis, the cross-sectional area of the RNFL is independent of scan distance, up to 600 μm from the rim margin, indicating that the axonal composition changes little over this range. The results suggest that, with incorporation of transverse scaling, the RNFL cross-sectional area, rather than RNFL thickness, provides an accurate assessment of the retinal ganglion cell axonal content within the eye.

Specific amacrine cell changes in an induced mouse model of glaucoma

BACKGROUND

To investigate retinal cell population changes under chronic elevated intraocular pressure in an inducible mouse model of glaucoma.

METHODS

Chronic unilateral ocular hypertension was induced in 40 C57BL6/J mice by ablation of the limbal episcleral veins. After 5, 20, 40 and 60 days of elevated intraocular pressure, specific retinal cell types were identified and/or quantified by immunohistochemistry for protein kinase C α, glial fibrillary acidic protein, parvalbumin and calretinin. Apoptotic cells were identified by TUNEL and cleaved caspase-3 immunohistochemistry.

RESULTS

Elevations in intraocular pressure in the range 22-30 mmHg were developed and sustained in mice for up to 60 days. Protein kinase C α immunoreactivity localized to bipolar cells was unchanged. We observed a rapid increase in glial fibrillary acidic protein expression in Müller cells and a progressive loss of parvalbumin-labelled ganglion cells. After 60 days of elevated intraocular pressure, calretinin-immunoreactive cell counts declined by 55.4% and 46.4% in the inner nuclear and ganglion cell layers, respectively. However, at all time points examined, the markers of cell death were only observed in the ganglion cell layer, not in the inner nuclear layer.

CONCLUSIONS

In addition to ganglion cell death and reactive Müller cell changes, chronic experimental elevation of intraocular pressure alters calcium-binding protein immunohistochemistry in amacrine cells. However, these changes are not indicative of amacrine cell loss but may represent early indicators of cellular distress that precede physiological dysfunction or cell death.

Dendrite plasticity in the lateral geniculate nucleus in primate glaucoma

Neural degeneration in glaucoma involves retinal ganglion cells and neurons of their major target, the lateral geniculate nucleus (LGN). Dendrites of relay LGN neurons projecting to the visual cortex were studied by immunocytochemical and quantitative Sholl analysis in combination with confocal microscopy and 3D-morphometry. In non-human adult primate glaucoma, relay LGN neurons showed reduced dendrite complexity and length, and these changes were modified by NMDA receptor blockade. Dendrite plasticity of LGN relay neurons in adult primate glaucoma has implications for potential disease modification by treatment interventions.