Yeni H. Yücel

Effects of retinal ganglion cell loss on magno-, parvo-, koniocellular pathways in the lateral geniculate nucleus and visual cortex in glaucoma

Glaucoma is a leading cause of world blindness, and retinal ganglion cell death is its pathological hallmark. There is accumulating evidence that glaucomatous damage extends from retinal ganglion cells to vision centers in the brain. In an experimental primate model of unilateral glaucoma, degenerative changes are observed in magnocellular, parvocellular, and koniocellular pathways in the lateral geniculate nucleus, and these changes are presented in relation to intraocular pressure and the severity of optic nerve damage. Neuropathological findings are also present in lateral geniculate nucleus layers driven by the unaffected fellow eye. Finally, there is information on changes in the visual cortex in relation to varying degrees of retinal ganglion cell loss. The implications of these findings for refining concepts regarding the pathobiology of progression, and the detection and treatment of glaucoma, are discussed.

Glaucoma as a neurodegenerative disease

Purpose of review Glaucoma is a leading cause of irreversible world vision loss characterized by progressive retinal ganglion cell death. Elevated eye pressure is a major risk factor for glaucoma; however, despite effective medical and surgical therapies to reduce intraocular pressure, progressive vision loss among glaucoma patients is common. These observations suggest that mechanisms independent of intraocular pressure are also implicated in glaucomatous degeneration. Numerous similarities exist between glaucoma and neurodegenerative diseases such as Alzheimers and Parkinsons diseases. Similarities include the selective loss of neuron populations, transsynaptic degeneration in which disease spreads from injured neurons to connected neurons, and common mechanisms of cell injury and death. Recent findings Glaucomatous injury to retinal ganglion cells has profound effects on target vision structures within the brain, including the lateral geniculate nucleus and visual cortex in experimental primate and human glaucoma. Mechanisms involved in central visual system damage in glaucoma include oxidative injury and glutamate toxicity, as seen in neurodegenerative diseases. Summary Glaucoma as a neurodegenerative disease is a valid working hypothesis to understand neural injury in the visual system. This paradigm may stimulate the discovery of innovative intraocular pressure-independent strategies to help prevent loss of vision in glaucoma patients.

Mania-like behavior induced by genetic dysfunction of the neuron-specific Na+,K+-ATPase α3 sodium pump.

Bipolar disorder is a debilitating psychopathology with unknown etiology. Accumulating evidence suggests the possible involvement of Na+,K+-ATPase dysfunction in the pathophysiology of bipolar disorder. Here we show that Myshkin mice carrying an inactivating mutation in the neuron-specific Na+,K+-ATPase α3 subunit display a behavioral profile remarkably similar to bipolar patients in the manic state. Myshkin mice show increased Ca2+ signaling in cultured cortical neurons and phospho-activation of extracellular signal regulated kinase (ERK) and Akt in the hippocampus. The mood-stabilizing drugs lithium and valproic acid, specific ERK inhibitor SL327, rostafuroxin, and transgenic expression of a functional Na+,K+-ATPase α3 protein rescue the mania-like phenotype of Myshkin mice. These findings establish Myshkin mice as a unique model of mania, reveal an important role for Na+,K+-ATPase α3 in the control of mania-like behavior, and identify Na+,K+-ATPase α3, its physiological regulators and downstream signal transduction pathways as putative targets for the design of new antimanic therapies.

Identification of lymphatics in the ciliary body of the human eye: A novel ''uveolymphatic'' outflow pathway

Impaired aqueous humor flow from the eye may lead to elevated intraocular pressure and glaucoma. Drainage of aqueous fluid from the eye occurs through established routes that include conventional outflow via the trabecular meshwork, and an unconventional or uveoscleral outflow pathway involving the ciliary body. Based on the assumption that the eye lacks a lymphatic circulation, the possible role of lymphatics in the less well defined uveoscleral pathway has been largely ignored. Advances in lymphatic research have identified specific lymphatic markers such as podoplanin, a transmembrane mucin-type glycoprotein, and lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). Lymphatic channels were identified in the human ciliary body using immunofluorescence with D2-40 antibody for podoplanin, and LYVE-1 antibody. In keeping with the criteria for lymphatic vessels in conjunctiva used as positive control, D2-40 and LYVE-1-positive lymphatic channels in the ciliary body had a distinct lumen, were negative for blood vessel endothelial cell marker CD34, and were surrounded by either discontinuous or no collagen IV-positive basement membrane. Cryo-immunogold electron microscopy confirmed the presence D2-40-immunoreactivity in lymphatic endothelium in the human ciliary body. Fluorescent nanospheres injected into the anterior chamber of the sheep eye were detected in LYVE-1-positive channels of the ciliary body 15, 30, and 45 min following injection. Four hours following intracameral injection, Iodine-125 radio-labeled human serum albumin injected into the sheep eye (n = 5) was drained preferentially into cervical, retropharyngeal, submandibular and preauricular lymph nodes in the head and neck region compared to reference popliteal lymph nodes (P < 0.05). These findings collectively indicate the presence of distinct lymphatic channels in the human ciliary body, and that fluid and solutes flow at least partially through this system. The discovery of a uveolymphatic pathway in the eye is novel and highly relevant to studies of glaucoma and other eye diseases.

Object recognition memory and BDNF expression are reduced in young TgCRND8 mice

The TgCRND8 mouse model of Alzheimers disease exhibits progressive cortical and hippocampal β-amyloid accumulation, resulting in plaque pathology and spatial memory impairment by 3 months of age. We tested whether TgCRND8 cognitive function is disrupted prior to the appearance of macroscopic plaques in an object recognition task. We found profound deficits in 8-week-old mice. Animals this age were not impaired on the Morris water maze task. TgCRND8 and littermate controls did not differ in their duration of object exploration or optokinetic responses. Thus, visual and motor dysfunction did not confound the phenotype. Object memory deficits point to the frontal cortex and hippocampus as early targets of functional disruption. Indeed, we observed altered levels of brain-derived neurotrophic factor (BDNF) messenger ribonucleic acid (mRNA) in these brain regions of preplaque TgCRND8 mice. Our findings suggest that object recognition provides an early index of cognitive impairment associated with amyloid exposure and reduced brain-derived neurotrophic factor expression in the TgCRND8 mouse.

Glaucoma of the brain: a disease model for the study of transsynaptic neural degeneration.

The identification of mechanisms precipitating neuronal death and injury is an intense area of investigation requiring reliable models to assess the effects of neuroprotective agents. Most are suboptimal since the effects of initial damage are diffuse and may not be reproducible or easily quantifiable. The ideal laboratory model should have the ability to (a) clearly detect evidence of neuronal injury and recovery, (b) accurately measure morphologically the extent of these changes, and (c) provide functional evidence for damage and recovery. Glaucoma is a disease of visual neurons in the eye and brain. In the visual system, neuroanatomical pathways and retinotopic organization are exquisitely defined, functional modalities are highly characterized and can be dissected physiologically, visual input parameters can be modified, visual functional output can be readily tested and measured, changes in the eye and the visual brain can be directly visualized and imaged, and pathological and compensatory changes in brain centers of vision can be examined and measured specifically. For these reasons, the glaucoma disease model is ideal for the study of response and recovery to injury in the central nervous system due to anterograde and retrograde degeneration from the eye to the brain and the brain to the eye, respectively. The study of this glaucoma model of transsynaptic brain injury may be relevant to understanding more complex pathways and point to new strategies to prevent disease progression in other neurodegenerative diseases.

Retinal tau pathology in human glaucomas

BACKGROUND Tau protein is a microtubule-associated protein critical to neuron structure and integrity. The abnormal hyperphosphorylated tau protein AT8 disrupts microtubules, interferes with axonal transport, and is associated with neuron injury in neurodegenerative diseases such as Alzheimers disease. The purpose of this study was to assess the presence of tau protein and abnormal tau protein AT8 in human glaucomas and to determine whether abnormal tau protein plays a role in glaucomatous neural degeneration. METHODS Sections from 11 surgical eye specimens with glaucoma from elevated intraocular pressure causes and 10 age-matched control eye specimens were immunostained for normal tau protein (BT2) and hyperphosphorylated tau protein (AT8). Postmortem specimens with incidental open-angle glaucoma (n = 6) were compared with controls (n = 3). Measurements of immunofluorescence intensity in glaucoma retinas were compared with those in control retinas. Abnormal tau AT8 and parvalbumin, a horizontal cell-specific marker, were studied with double-immunofluorescence techniques to determine colocalization. RESULTS In surgical glaucoma specimens, normal tau protein was decreased in both the optic nerve and retina compared with age-matched controls. Abnormal tau AT8 was evident within the posterior retina, predominantly at the outer border of the inner nuclear layer in surgical glaucoma specimens, and this was not observed in controls or incidental glaucoma cases. Quantitative immunofluorescence techniques demonstrated significantly increased abnormal tau AT8 in surgical glaucoma specimens compared with controls. Abnormal tau AT8 colocalized with parvalbumin in horizontal cells of the retina. INTERPRETATION Abnormal tau AT8, a marker of injury in various neurological diseases, is present in human glaucomas with uncontrolled intraocular pressure. The finding of abnormal tau protein in retinal horizontal cells may relate to elevated intraocular pressure and (or) neural degeneration in glaucoma. Tau protein abnormality in glaucoma underscores shared pathways with other neurodegenerative diseases.

Attenuation of the Electrophysiological Function of the Corpus Callosum after Fluid Percussion Injury in the Rat

This study describes a new method used to evaluate axonal physiological dysfunction following fluid percussion induced traumatic brain injury (TBI) that may facilitate the study of the mechanisms and novel therapeutic strategies of posttraumatic diffuse axonal injury (DAI). Stimulated compound action potentials (CAP) were recorded extracellularly in the corpus callosum of superfused brain slices at 3 h, and 1, 3, and 7 days following central fluid percussion injury and demonstrated a temporal pattern of functional deterioration. The maximal CAP amplitude (CAPA) covaried with the intensity of impact 1 day following sham, mild (1.0-1.2 atm), and moderate (1.8-2.0 atm) injury (p < 0.05; 1.11 +/- 0.10, 0.82 +/- 0.11, and 0.49 +/- 0.08 mV, respectively). The CAPA in sham animals were approximately 1.1 mV and did not vary with survival interval (3 h, and 1, 3, and 7 days); however, they were significantly decreased at each time point following moderate injury (p < 0.05; 0.51 +/- 0.11, 0.49 +/- 0.08, 0.46 +/- 0.10, and 0.75 +/- 0.13 mV, respectively). The CAPA at 7 days in the injured group were higher than at 3 h, and 1 and 3 days. H&E and amyloid precursor protein (APP) light microscopic analysis confirmed previously reported trauma-induced axonal injury in the corpus callosum seen after fluid percussion injury. Increased APP expression was confirmed using Western blotting showing significant accumulation at 1 day (IOD 913.0 +/- 252.7; n = 3; p = 0.05), 3 days (IOD 753.1 +/- 159.1; n = 3; p = 0.03), and at 7 days (IOD 1093.8 = 105.0; n = 3; p = 0.001) compared to shams (IOD 217.6 +/- 20.4; n = 3). Thus, we report the characterization of white matter axonal dysfunction in the corpus callosum following TBI. This novel method was easily applied, and the results were consistent and reproducible. The electrophysiological changes were sensitive to the early effects of impact intensity, as well as to delayed changes occurring several days following injury. They also indicated a greater degree of attenuation than predicted by APP expression changes alone.

Histomorphometric analysis of optic nerve changes in experimental glaucoma.

PURPOSE To assess relative changes in different tissue components of optic nerve and their relationship to nerve fiber loss in the experimental monkey model of glaucoma. METHODS Chronic intraocular pressure (IOP) elevation was induced by laser trabeculoplasty in the right eye of eight monkeys (Macaca fascicularis). Both experimental right optic nerves and control left optic nerves were studied. Histomorphometric analysis was performed on optic nerve cross-sections using bright field microscopy with camera lucida. Cross-sectional areas of optic nerve tissue components were estimated by point counting. Nerve fiber density was estimated by unbiased random sampling. Nerve fiber number was calculated by multiplying nerve fiber density with neuroglial area. RESULTS Varying degrees of nerve fiber loss were seen in eight optic nerves with chronic IOP elevation. More than 50% nerve fiber loss was noted in four of eight experimental optic nerves. In these severely affected optic nerves, total optic nerve area was significantly decreased compared with control optic nerves. Among the optic nerve tissue components, only the ratio of myelinated fiber area to total optic nerve area was significantly decreased. The ratio of extraaxonal area to total optic nerve area was significantly increased, whereas the ratio of interfascicular septal area to total optic nerve area did not change significantly. For all optic nerves, differences in nerve fiber count between control and experimental optic nerves showed the strongest correlation with differences in myelinated fiber area, followed by differences in extraaxonal area and total optic nerve area. CONCLUSION This histomorphometric study suggests the validity of the experimental monkey model of glaucoma in studying changes occurring in the nonaxonal optic nerve tissue components in human glaucomatous optic neuropathy. Glial scar tissue area was significantly increased in optic nerves with severe glaucomatous damage. Although a decrease in total optic nerve area was observed, among the optic nerve tissue components only myelinated nerve fiber area decreased significantly. Myelinated nerve fiber area also showed the strongest association with nerve fiber loss in experimental glaucoma.

Glaucoma and the brain.

The pathobiologic features of retinal ganglion cell death in glaucoma have been studied extensively at the level of the retina and optic nerve head. Retinal ganglion cell (RGC) axons form the optic nerve, chiasm, and optic tract and convey visual information to multiple nuclei in the brain. Most RGCs terminate in the lateral geniculate nucleus (LGN), the major vision center relaying information from the eye to the visual cortex. In each LGN, the complete contralateral hemifield of vision is represented, and parallel central visual pathways are segregated into anatomically distinct magnocellular, parvocellular, and koniocellular channels. The axons of relay LGN neurons form the optic radiations projecting to eye specific columns in the primary visual cortex. Recent investigations show evidence of extension of glaucomatous injury to these central visual stations and may offer additional insights into the nature of glaucomatous damage and vision loss.