Early Intervention and Lifelong Treatment with GLP1 Receptor Agonist Liraglutide in a Wolfram Syndrome Rat Model with an Emphasis on Visual Neurodegeneration, Sensorineural Hearing Loss and Diabetic Phenotype

Wolfram syndrome (WS), also known as a DIDMOAD (diabetes insipidus, early-onset diabetes mellitus, optic nerve atrophy and deafness) is a rare autosomal disorder caused by mutations in the Wolframin1 (WFS1) gene. Previous studies have revealed that glucagon-like peptide-1 receptor agonist (GLP1 RA) are effective in delaying and restoring blood glucose control in WS animal models and patients. The GLP1 RA liraglutide has also been shown to have neuroprotective properties in aged WS rats. WS is an early-onset, chronic condition. Therefore, early diagnosis and lifelong pharmacological treatment is the best solution to control disease progression. Hence, the aim of this study was to evaluate the efficacy of the long-term liraglutide treatment on the progression of WS symptoms. For this purpose, 2-month-old WS rats were treated with liraglutide up to the age of 18 months and changes in diabetes markers, visual acuity, and hearing sensitivity were monitored over the course of the treatment period. We found that treatment with liraglutide delayed the onset of diabetes and protected against vision loss in a rat model of WS. Therefore, early diagnosis and prophylactic treatment with the liraglutide may also prove to be a promising treatment option for WS patients by increasing the quality of life.

Heterozygous Meg2 Ablation Causes Intraocular Pressure Elevation and Progressive Glaucomatous Neurodegeneration

Glaucomatous neurodegeneration represents one of the major causes of irreversible blindness worldwide. Yet, the detailed molecular mechanisms that initiate optic nerve damage and retinal ganglion cell (RGC) loss are not fully understood. Members of the protein tyrosine phosphatase (PTP) superfamily are key players in numerous neurodegenerative diseases. In order to investigate the potential functional relevance of the PTP megakaryocyte 2 (Meg2) in retinal neurodegeneration, we analyzed Meg2 knockout (KO) and heterozygous (HET)-synonym protein-tyrosine phosphatase non-receptor type 9 (Ptpn9)-mice. Interestingly, via global microarray and quantitative real-time PCR (RT-qPCR) analyses of Meg2 KO and HET retinae, we observed a dysregulation of several candidate genes that are highly associated with retinal degeneration and intraocular pressure (IOP) elevation, the main risk factor for glaucoma. Subsequent IOP measurements in Meg2 HET mice verified progressive age-dependent IOP elevation. Ultrastructural analyses and immunohistochemistry showed severe optic nerve degeneration accompanied by a dramatic loss of RGCs. Additionally, HET mice displayed reactive micro-/macrogliosis and early activation of the classical complement cascade with pronounced deposition of the membrane attack complex (MAC) in the retina and optic nerve. When treated with latanoprost, significant IOP lowering prevented RGC loss and microglial invasion in HET mice. Finally, electroretinogram (ERG) recordings revealed reduced a- and b-wave amplitudes, indicating impaired retinal functionality in Meg2 HET mice. Collectively, our findings indicate that the heterozygous loss of Meg2 in mice is sufficient to cause IOP elevation and glaucomatous neurodegeneration. Thus, Meg2 HET mice may serve as a novel animal model to study the pathomechanism involved in the onset and progression of glaucoma.

Hypothermic neuroprotection during reperfusion following exposure to aglycemia in central white matter is mediated by acidification

Hypoglycemia is a common iatrogenic consequence of type 1 diabetes therapy that can lead to central nervous system injury and even death if untreated. In the absence of clinically effective neuroprotective drugs we sought to quantify the putative neuroprotective effects of imposing hypothermia during the reperfusion phase following aglycemic exposure to central white matter. Mouse optic nerves (MONs), central white matter tracts, were superfused with oxygenated artificial cerebrospinal fluid (aCSF) containing 10 mmol/L glucose at 37°C. The supramaximal compound action potential (CAP) was evoked and axon conduction was assessed as the CAP area. Extracellular lactate was measured using an enzyme biosensor. Exposure to aglycemia, simulated by omitting glucose from the aCSF, resulted in axon injury, quantified by electrophysiological recordings, electron microscopic analysis confirming axon damage, the extent of which was determined by the duration of aglycemia exposure. Hypothermia attenuated injury. Exposing MONs to hypothermia during reperfusion resulted in improved CAP recovery compared with control recovery measured at 37°C, an effect attenuated in alkaline aCSF. Hypothermia decreases pH implying that the hypothermic neuroprotection derives from interstitial acidification. These results have important clinical implications demonstrating that hypothermic intervention during reperfusion can improve recovery in central white matter following aglycemia.

Optic Nerve Hypoplasia in Fetal Alcohol Syndrome: An Update

Optic nerve hypoplasia was detected in up to one half of a group of Swedish children born to alcoholic mothers. Using an experimental model of pre- and postnatal alcohol exposure in rats fed a liquid diet, reduced optic nerve size from gestational day 21 (294 ± 26×102 μm2 vs 502 ± 16×102 μm2; n=6; p≤0.001) to later in development was observed as a result of the daily mean blood alcohol levels achieved in dams and their offspring. Altered glial cells and degenerating and atrophic optic axons, myelin sheaths and ganglion cells were frequent in the alcohol-exposed optic nerves.

Smaller optic nerve (1,918 ± 61×102 μm2 vs 2.195 ± 40×102 μm2; n=4; p≤0.001), reduced gaglion cell and axonal densities, and ultrastructural damage to the macroglial cells and myelin sheaths were also detected in the treated group. All these changes remained in the retina and optic nerve of the oldest rats, as a consequence of the long-lasting effects of prenatal alcohol exposure. In summary, alcohol as a major teratogenic agent may induce dysmorphogenesis and irremediable damage to the retina and optic nerve, which frequently manifests itself as hypoplastic optic nerve.

Treatment with Bifemelane for Optic Nerve Damage following High Intraocular Pressure in Rat Eyes

This study found that pretreatment with 4-(o-benzylphenoxy)-N- methylbuty-lamine hydrochloride (bifemelane hydrochloride, Celeport) reduced ischemia-reperfusion injury in rat eyes. Bifemelane (25 mg/kg) was injected intraperitoneally 30 minutes before an ischemic insult, then acute ischemia of the retina and optic disc was induced by increasing intraocular pressure to 110 mmHg for 45 minutes. After one week, the axonal count of the optic nerve was investigated using electron microscopy. The control group consisted of vehicle-treated eyes which received normal saline. The axon count was 93.4 +/- 7.9 for the bifemelane treated group, and 79.2 +/- 6.4 for the controls. The axon count in the treated group was significantly higher. These results suggest that bifemelane, which prevents cerebral nerve cell damage from ischemia, can reduce ischemic retinal nerve cell injury.

In-vivo effects of intraocular and intracranial pressures on the lamina cribrosa microstructure

There is increasing clinical evidence that the eye is not only affected by intraocular pressure (IOP), but also by intracranial pressure (ICP). Both pressures meet at the optic nerve head of the eye, specifically the lamina cribrosa (LC). The LC is a collagenous meshwork through which all retinal ganglion cell axons pass on their way to the brain. Distortion of the LC causes a biological cascade leading to neuropathy and impaired vision in situations such as glaucoma and idiopathic intracranial hypertension. While the effect of IOP on the LC has been studied extensively, the coupled effects of IOP and ICP on the LC remain poorly understood. We investigated in-vivo the effects of IOP and ICP, controlled via cannulation of the eye and lateral ventricle in the brain, on the LC microstructure of anesthetized rhesus monkeys eyes using the Bioptigen spectral-domain optical coherence tomography (OCT) device (Research Triangle, NC). The animals were imaged with their head upright and the rest of their body lying prone on a surgical table. The LC was imaged at a variety of IOP/ICP combinations, and microstructural parameters, such as the thickness of the LC collagenous beams and diameter of the pores were analyzed. LC microstructure was confirmed by histology. We determined that LC microstructure deformed in response to both IOP and ICP changes, with significant interaction between the two. These findings emphasize the importance of considering both IOP and ICP when assessing optic nerve health.

Metabolic profile of visual cortex in diabetic rats measured with in vivo proton MRS

The purpose of the present study was to characterize the metabolic profile of the visual cortex in streptozotocin-induced Type 1 diabetic rats by means of in vivo proton MRS. Several metabolite concentration ratios in the visual cortex were calculated. In addition, postmortem histologic analyses for retinal ganglion cell (RGC) loss, optic nerve injury and visual cortex alterations were monitored. The results showed that diabetes induced several changes in visual cortex metabolites, such as reduced N-acetylaspartate, glutamate, γ-aminobutyric acid, taurine and choline-containing compound levels. Nevertheless, myo-inositol levels increased significantly as compared with controls. Remarkable RGC loss and optic nerve degeneration were observed by morphological analysis. Moreover, the results showed significant neuronal loss and glial activation in the visual cortex. These findings indicated that, besides vascular abnormalities, neuronal loss and degeneration in the visual pathway were induced due to disrupted glucose homeostasis in diabetes. Metabolic or functional abnormalities were induced in cerebral neurons of the visual cortex by diabetes.

Ultrastructural and histopathologic findings after pars plana vitrectomy with a new hypersonic vitrector system. Qualitative preliminary assessment

Purpose

Preliminary assessment of a new prototype ultrasound-based hypersonic vitrector (HV) by qualitatively examining the histopathological changes in the retina and vitreous body after pars plana vitrectomy (PPV) and its ability to fragment vitreous collagen.

Methods

Fourteen porcine cadaveric eyes, 20 eyes in live swine and six human cadaveric eyes underwent PPV using the HV or a pneumatic guillotine vitrector (GV). An additional 4 porcine crystalline lenses were touched with either the HV or GV for 1 minute. Following PPV, human vitreous was removed and processed for electron microscopy (EM). Eyes and lenses were fixed and sectioned for light microscopy (LM).

Results

There were no macroscopic retinal or optic nerve defects associated with either HV or GV PPVs. Cadaveric retinal specimens showed separation of the inner limiting membrane (ILM) and vacuolization and fragmentation at the nerve fiber layer (NFL) and the ganglion cell layer (GCL). ILM fragmentation and separation were found after PPV in live swine with both vitrectors. Small disruptions of the posterior capsule or structural lens defects were found after HV touch. The EM analysis revealed more fragmentation of human vitreous collagen fibrils after HV compared to GV PPV.

Conclusions

LM and EM analysis of retina, vitreous, and crystalline lens after PPV showed similar morphological changes using the HV or the GV. Vitreous fragmentation appeared more effective with the HV. Overall this study suggests that the HV may be a promising new technology. More work is needed to quantitatively assess its safety and efficacy.

Mammalian-Specific Central Myelin Protein Opalin Is Redundant for Normal Myelination: Structural and Behavioral Assessments

Opalin, a central nervous system-specific myelin protein phylogenetically unique to mammals, has been suggested to play a role in mammalian-specific myelin. To elucidate the role of Opalin in mammalian myelin, we disrupted the Opalin gene in mice and analyzed the impacts on myelination and behavior. Opalin-knockout (Opalin^−/−) mice were born at a Mendelian ratio and had a normal body shape and weight. Interestingly, Opalin^−/− mice had no obvious abnormalities in major myelin protein compositions, expression of oligodendrocyte lineage markers, or domain organization of myelinated axons compared with WT mice (Opalin^+/+) mice. Electron microscopic observation of the optic nerves did not reveal obvious differences between Opalin^+/+ and Opalin^−/− mice in terms of fine structures of paranodal loops, transverse bands, and multi-lamellae of myelinated axons. Moreover, sensory reflex, circadian rhythm, and locomotor activity in the home cage, as well as depression-like behavior, in the Opalin^−/− mice were indistinguishable from the Opalin^+/+ mice. Nevertheless, a subtle but significant impact on exploratory activity became apparent in Opalin^−/− mice exposed to a novel environment. These results suggest that Opalin is not critical for central nervous system myelination or basic sensory and motor activities under conventional breeding conditions, although it might be required for fine-tuning of exploratory behavior.

Age-related ultrastructural and monoamine oxidase changes in the rat optic nerve

The aim of this paper is to study the morphology and the distribution of the monoamine oxidase enzymatic system in the optic nerve of 4 month-old Wistar (young) and 28 month-old Wistar (old) rats. The optic nerve was harvested from 20 young and old rats. The segment of optic nerve was divided longitudinally into two pieces, each 0.1 mm in length. The first piece was used for transmission electron microscopy. The second piece was stained with histochemical reaction for monoamine oxidase. The agerelated changes in the optic nerve of rats include micro-anatomical details, ultrastructure and monoamine oxidase histochemical staining. A strong decrease of the thin nerve fibers and a swelling of the thick ones can be observed in optic nerve fibers of old rats. Increased monoamine oxidase histochemical staining of the optic nerve of aged rats is well demonstrated. The increase of meningeal shealth and the decrease of thin nerve fibers of the optic nerve in old rats are well documented. Morphological, ultrastructural and histochemical changes observed in optic nerve fibers of the old rats show a close relation with aging.

Dynamic Modulation of Myelination in Response to Visual Stimuli Alters Optic Nerve Conduction Velocity

Myelin controls the time required for an action potential to travel from the neuronal soma to the axon terminal, defining the temporal manner in which information is processed within the CNS. The presence of myelin, the internodal length, and the thickness of the myelin sheath are powerful structural factors that control the velocity and fidelity of action potential transmission. Emerging evidence indicates that myelination is sensitive to environmental experience and neuronal activity. Activity-dependent modulation of myelination can dynamically alter action potential conduction properties but direct functional in vivo evidence and characterization of the underlying myelin changes is lacking. We demonstrate that in mice long-term monocular deprivation increases oligodendrogenesis in the retinogeniculate pathway but shortens myelin internode lengths without affecting other structural properties of myelinated fibers. We also demonstrate that genetically attenuating synaptic glutamate neurotransmission from retinal ganglion cells phenocopies the changes observed after monocular deprivation, suggesting that glutamate may constitute a signal for myelin length regulation. Importantly, we demonstrate that visual deprivation and shortened internodes are associated with a significant reduction in nerve conduction velocity in the optic nerve. Our results reveal the importance of sensory input in the building of myelinated fibers and suggest that this activity-dependent alteration of myelination is important for modifying the conductive properties of brain circuits in response to environmental experience.

SIGNIFICANCE STATEMENT

Oligodendrocyte precursor cells differentiate into mature oligodendrocytes and are capable of ensheathing axons with myelin without molecular cues from neurons. However, this default myelination process can be modulated by changes in neuronal activity. Here, we show, for the first time, that experience-dependent activity modifies the length of myelin internodes along axons altering action potential conduction velocity. Such a mechanism would allow for variations in conduction velocities that provide a degree of plasticity in accordance to environmental needs. It will be important in future work to investigate how these changes in myelination and conduction velocity contribute to signal integration in postsynaptic neurons and circuit function.

Myelinated mouse nerves studied by X-ray phase contrast zoom tomography

We have used X-ray phase contrast tomography to resolve the structure of uncut, entire myelinated optic, saphenous and sciatic mouse nerves. Intrinsic electron density contrast suffices to identify axonal structures. Specific myelin labeling by an osmium tetroxide stain enables distinction between axon and surrounding myelin sheath. Utilization of spherical wave illumination enables zooming capabilities which enable imaging of entire sciatic internodes as well as identification of sub-structures such as nodes of Ranvier and Schmidt-Lanterman incisures.

Morphological characteristics of the optic nerve evaluated by confocal laser tomography (HRT3) and laser polarimetry (GDx-VCC) in a normal population from the city of Barcelona

OBJECTIVE

To evaluate morphological parameters of optic disc and retinal nerve fiber layer (RNFL) examined with confocal laser tomography (HRT3) and laser polarimetry (GDx-VCC) in a normal population, and analyze correlations of these parameters with demographic variables.

PATIENTS AND METHODS

Cross-sectional study in the context of a glaucoma screening campaign in the primary care center of Barcelona. The individuals selected were non-hypertensive Mediterranean Caucasians with risk for glaucoma development (individuals≥60 years old or≥40 years old with family history of glaucoma or intraocular pressure or myopia>3diopter). All subjects underwent a complete ophthalmic examination, confocal laser tomography (HRT3) and scanning laser polarimetry (GDX-VCC), subjects with results within normal limits only being included. Structural parameters were analyzed along with age, refraction, and pachymetry based on the Spearman rank correlation test.

RESULTS

A total of 224 subjects included, with a mean age of 63.4±11.1 years. Disc areas, excavation and ring area were 2.14±0.52mm(2), 0.44±0.34mm (2) and 1.69±0.38mm(2), respectively. The mean RNFL (GDX) was 55.9±6.9μm. Age was correlated with lower ring volume, highest rate of cup shape measure, largest mean and maximum cup depth, lower nerve fiber index (NFI) and RNFL (all p-values below .05).

CONCLUSION

The mean values and distribution of several parameters of the papilla and the RNFL in normal Mediterranean Caucasians population are presented. A loss of thickness of the RNFL, ring thinning, and enlarged cup was observed with increased age.

[Morphological evaluation of the optic nerve and retina in the rat model of non-arteritic anterior ischemic optic neuropathy]

OBJECTIVE

To evaluate the morphology of optic nerve and retina in the model of the rat non-arteritic anterior ischemic optic neuropathy (rNAION).

METHODS

Experimental study. Thirty-six SD rats were randomly divided into four groups: 6 for naïve group, 3 for laser group, 3 for Rose Bengal (RB) group and 24 for rNAION group. After rNAION was induced by RB and laser, the retina and optic nerve were observed by Hematoxylin-Eosin (HE) staining and the optic nerve was investigated by transmission electron microscope and toludidine blue staining at different time points.

RESULTS

With these methods, significant changes were only found in the RB-laser induced eyes. The damage only occurred in the retinal ganglion cell (RGC) layer without any loss of the other layers of retina in the rNAION. Toludine blue-stained optic nerve showed that there was a loss of axons centrally and largely intact axons were spared in the peripheral nerve. More and more degenerative axons were observed with the transmission electron microscope as time went on.

CONCLUSIONS

The morphological changes of HE staining, toludine blue staining and transmission electron microscopic finding were consistent with each other. The results made a contribution to further study.

Age-related changes in the optic nerve of Sprague-Dawley rats: an ultrastructural and immunohistochemical study

The optic nerve is a unique part of the central nervous system. It lacks neuronal cell bodies and consists of axons of the retinal ganglion cells together with the supporting neuroglial cells. In the present study, aging of the optic nerve was studied in female Sprague-Dawley rats aged 3, 12, 24 and 30 months old, ultrastructurally, immunohistochemically and morphometrically trying to answer the question why aging is a common risk factor for many ocular diseases especially glaucoma. Additionally, studying the optic nerve aging offered a good opportunity to gain further insight into the effects of aging on white matter. Both nerve fibers and neuroglial cells demonstrated several age related changes which were more profound in 30 months old rats. Optic nerve axons displayed watery degeneration and dark degeneration. Myelin disturbances including widening, whorls, splitting and vacuolations of the myelin lamellae were also observed. Neuroglial cells appeared to be more frequent than in younger rats especially microglia cells and developed dense cytoplasmic inclusions. GFAP-positive astrocytes delineated age-related progressive increase in number, size as well as length and thickness of their processes. CD68 immunohistochemical staining revealed age-related changes in the morphology, location and number of CD68 positive microglia cells.

Aging-related changes of optic nerve of Wistar albino rats

Aging is a biological phenomenon that involves an increase of oxidative stress associated with gradual degradation of the structure and function of the optic nerve. Gender differences and subsequent deterioration of optic nerve are an interesting topic, especially because there is little published work concerning it. One hundred male and female Wistar albino rats' with ages 1, 6, 18, 24, and 30 months (n = 20 equal for male and female) were used. At the time interval, optic nerve was investigated by light and transmission electron microscopy (TEM), assessments of antioxidant enzymes (catalase, superoxide dismustase, and glutathione-S-transferase), caspase 3 and 7, malondialdhyde, flow cytometry of DNA, annexin v, and CD8, immunochemistry of vascular endothelial growth factor (VEGF), CD31, and CD45, and single-strand DNA fragmentation. Light and TEM observations of the older specimens (24 and 30 months) revealed apparent deterioration of optic nerve axons, abundant oligodendrocytes with pyknotic nuclei, swollen astrocytes, angiogenesis, vacuolar degeneration, and mitochondrial damage. Females were highly susceptible to aging processes. Concomitantly, there was a marked reduction of antioxidant's enzymes and an increase of lipid peroxidation and apoptotic markers. Old age exhibited a marked increase of G1 apoptosis, UR and LR of annexin V and CD8 as well as increased immuno-positive reaction with VEGR, CD31 and CD45. We conclude that aging contributed to an increase of oxidative stress resulting from damage of mitochondria in axons, oligodendrocytes, and astrocytes. Age-related loss of optic nerve axons is associated with multifactorial agents including reduction in antioxidant enzymes, disruption of vasculature, astrocyte, and oligodendrocyte, demyelination, and damage of mitochondria, which enhance the liberation of reactive oxygen species as assessed by an increase of apoptotic markers malondialdhyde and caspase 3 and 7.

Quantitative ultrastructural evidence of myelin malformation in optic nerves of rats submitted to a multideficient diet

Pups were subjected to malnutrition by feeding the lactating mothers a multi-deficient (8% protein content) diet, known as regional basic diet (RBD), from birth up to weaning. The weanings were fed the same diet until 60 days of age. Ultrastructure of the optic nerve was analyzed at postnatal (P) day P8, P13, P21, P30 and P60. Electron microscopy revealed that at P8 the process of myelination has not started yet in neither groups. At P 13 different stages of myelination were observed and, in the experimental group, the optic nerve showed non-organized axon bundles and empty spaces. Control optic nerve at P21 exhibited axons with fully developed myelin sheath; whereas malnourished group had many axons being enveloped by myelin with anomalous alteration. These alterations were present in malnourished group at P30 and P60. Quantitative analysis showed statistically significant difference between control and malnourished groups when compared to the percentage of myelinated axons, axons with myelin anomalous alterations (MAA) and non-myelinated axons. Also, the myelin area was significantly smaller in malnourished groups when compared to control group. Finally, a high percentage of large non-myelinated fibers were found in the malnourished group. In conclusion, our results show that early malnutrition by a multideficient diet (RBD) affects permanently the optic nerve organization and myelination, indicating an impairment of nerve transmission and a probable dysfunction in the visual ability.

Evaluation of lateral spread of transgene expression following subretinal AAV-mediated gene delivery in dogs

Dog models with spontaneously occurring mutations in retinal dystrophy genes are an invaluable resource for preclinical development of retinal gene therapy. Adeno-associated virus (AAV) vectors have been most successful; to target the outer retina and RPE they are delivered by subretinal injection, causing a temporary retinal detachment with some potential for retinal morbidity. A recent reporter gene study using an AAV2/8 vector in dogs reported transgene expression beyond the boundary of the subretinal bleb. This could be a desirable feature which increases the area of retina treated while minimizing the retinal detachment and any associated morbidity. We performed a detailed study of the lateral spread of transgene expression beyond the subretinal injection site following subretinally delivered AAV vectors in normal dogs. Vectors expressed green fluorescent protein (GFP) using a small chicken beta-actin promoter. AAV2/2 (quadruple tyrosine to phenylalanine (Y-F) capsid mutant), self-complementary (sc) AAV2/8 (single Y-F capsid mutant) and a scAAV2/5 were used. We found that in all eyes GFP expression involved retina beyond the initial post-injection subretinal bleb boundary. In all eyes there was post-injection spread of the retinal detachment within the first 3 days post procedure and prior to retinal reattachment. In 11/16 eyes this accounted for the entire "lateral spread" of GFP expression while in 5/16 eyes a very slight extension of GFP expression beyond the final boundary of the subretinal bleb could be detected. All 3 AAV constructs induced GFP expression in the nerve fiber layer with spread to the optic nerve. Patients treated by subretinal injection should be monitored for possible expansion of the subretinal injection bleb prior to reattachment. Injections in the para-foveal region may expand to lead to a foveal detachment that may be undesirable. Cell-specific promoters may be required to limit spread of expressed transgene to the brain with these AAV serotypes.

Accumulation of lipid inclusions in astrocytes of aging human optic nerve

We examined age-related changes in the human optic nerve (ON) from 10 postmortem donor eye samples (age: 21- to 94-year-old). In aged ON, many axons showed paucity of cytoskeleton, and possessed disorganized myelin that remained in the extracellular space. Lipid inclusions were detected in glia, as stained by oil red O, and these accumulated with aging. To identify and confirm which glial cell type possessed lipid inclusions, we performed immunohistochemistry (IHC) and transmission electron microscopy (TEM). Comparisons were made from TEM features and size of the glia immunolabeled with glial fibrillary acidic protein and glutamine synthetase (markers for astrocytes) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (a marker for oligodendrocytes). It was found that lipid inclusions were restricted to the astrocytes having larger perikarya than the oligodendrocytes (IHC) and possessing filaments in cytoplasm (TEM). These astrocytes also possessed myelin debris and it is thus likely that those inclusions originated from degenerated myelin of the ON axons. These data indicate that astrocytes play a role in phagocytosis and clearance of disorganized myelin in aging human ON.

Activating transcription factor 3 and reactive astrocytes following optic nerve injury in zebrafish

Nerve regeneration in the central nervous system is restricted in mammals, but fish and amphibians show amazing resiliency following injury to the central nervous system. We have examined the response of zebrafish (Danio rerio) to optic nerve injury to try to understand the differences between fish and mammals that enable fish to regenerate their optic nerves following crushing and severing. In previous work, we have shown that activating transcription factor 3 (atf3) is expressed at higher levels following optic nerve injury. Here we use a polyclonal anti-ATF3 antibody, anti-cytokeratin (KRT-18) and anti-bystin (BYSL) antibodies to show that Atf3 and Bysl colocalize with cytokeratin-expressing astrocytes in the optic nerve following severing. Furthermore, anti-ATF3 antibodies fail to colocalize with GFP in transgenic zebrafish expressing EGFP in astrocytes Tg(gfap:GFP) or oligodendrocytes Tg(olig2:EGFP). Interestingly, labeling of Atf3 was detected in retinal ganglion cell axons in both the nerve fiber layer and the optic nerve on the injured side. Finally, optic nerve astrocytes labeled with anti-bystin antibodies showed evidence of hypertrophy, suggesting that fish astrocytes in the optic nerve raise a bona fide reactive response to injury even though they do not express glial fibrillary acidic protein.

Genetic inactivation of the p66 isoform of ShcA is neuroprotective in a murine model of multiple sclerosis

Although multiple sclerosis (MS) has traditionally been considered to be an inflammatory disease, recent evidence has brought neurodegeneration into the spotlight, suggesting that accumulated damage and loss of axons is critical to disease progression and the associated irreversible disability. Proposed mechanisms of axonal degeneration in MS posit cytosolic and subsequent mitochondrial Ca(2+) overload, accumulation of pathologic reactive oxygen species (ROS), and mitochondrial dysfunction leading to cell death. In this context, the role of the p66 isoform of ShcA protein (p66) may be significant. The ShcA isoform is uniquely targeted to the mitochondrial intermembrane space in response to elevated oxidative stress, and serves as a redox enzyme amplifying ROS generation in a positive feedforward loop that eventually mediates cell death by activation of the mitochondrial permeability transition pore. Consequently, we tested the hypothesis that genetic inactivation of p66 would reduce axonal injury in a murine model of MS, experimental autoimmune encephalomyelitis (EAE). As predicted, the p66-knockout (p66-KO) mice developed typical signs of EAE, but had less severe clinical impairment and paralysis than wild-type (WT) mice. Histologic examination of spinal cords and optic nerves showed significant axonal protection in the p66-KO tissue, despite similar levels of inflammation. Furthermore, cultured p66-KO neurons treated with agents implicated in MS neurodegenerative pathways showed greater viability than WT neurons. These results confirm the critical role of ROS-mediated mitochondrial dysfunction in the axonal loss that accompanies EAE, and identify p66 as a new pharmacologic target for MS neuroprotective therapeutics.

Successful amelioration of mitochondrial optic neuropathy using the yeast NDI1 gene in a rat animal model

Background

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder with point mutations in mitochondrial DNA which result in loss of vision in young adults. The majority of mutations reported to date are within the genes encoding the subunits of the mitochondrial NADH-quinone oxidoreductase, complex I. Establishment of animal models of LHON should help elucidate mechanism of the disease and could be utilized for possible development of therapeutic strategies.

Methodology/Principal Findings

We established a rat model which involves injection of rotenone-loaded microspheres into the optic layer of the rat superior colliculus. The animals exhibited the most common features of LHON. Visual loss was observed within 2 weeks of rotenone administration with no apparent effect on retinal ganglion cells. Death of retinal ganglion cells occurred at a later stage. Using our rat model, we investigated the effect of the yeast alternative NADH dehydrogenase, Ndi1. We were able to achieve efficient expression of the Ndi1 protein in the mitochondria of all regions of retinal ganglion cells and axons by delivering the NDI1 gene into the optical layer of the superior colliculus. Remarkably, even after the vision of the rats was severely impaired, treatment of the animals with the NDI1 gene led to a complete restoration of the vision to the normal level. Control groups that received either empty vector or the GFP gene had no effects.

Conclusions/Significance

The present study reports successful manifestation of LHON-like symptoms in rats and demonstrates the potential of the NDI1 gene therapy on mitochondrial optic neuropathies. Our results indicate a window of opportunity for the gene therapy to be applied successfully after the onset of the disease symptoms.

Number of axons in the right and left optic nerves of right-pawed and left-pawed rats: a stereologic study

OBJECTIVE

To compare the number of axons in the right and left optic nerves of right- and left-pawed rats.

STUDY DESIGN

In this study, optic nerve samples were obtained from right- and left-pawed rats and axon numbers of optic nerves and vice versa were stereologically and histologically evaluated.

RESULTS

In the right-pawed rats, more axons were found in the right optic nerve than in the left optic nerve, and left-pawed rats had more axons in the left optic nerve than in the right optic nerve.

CONCLUSION

The paw preference is associated with eye dominance and the number of axons in the ipsilateral optic nerve.

Constitutively active Akt induces enhanced myelination in the CNS

The serine/threonine kinase Akt regulates multiple cellular functions. The current studies identify a new role for Akt in CNS myelination. In earlier studies on cultured oligodendrocytes, we showed that neuregulin signals through phosphatidylinositol-3'-OH kinase and Akt to enhance survival of oligodendrocytes. However, when transgenic animals were generated that overexpressed constitutively active Akt in oligodendrocytes and their progenitor cells, no enhanced survival of oligodendrocytes or progenitors was found. No alteration in the proliferation or death of progenitors was noted. In contrast, the major impact of Akt overexpression in oligodendrocytes was enhanced myelination. Most interestingly, oligodendrocytes in these mice continued actively myelinating throughout life. Thus, expression of constitutively active Akt in oligodendrocytes and their progenitor cells generated no more oligodendrocytes, but dramatically more myelin. The increased myelination continued as these mice aged, resulting in enlarged optic nerves and white matter areas. In older animals with enlarged white matter areas, the density of oligodendrocytes was reduced, but because of the increased area, the total number of oligodendrocytes remained comparable with wild-type controls. Interestingly, in these animals, overexpression of Akt in Schwann cells did not impact myelination. Thus, in vivo, constitutively active Akt enhances CNS myelination but not PNS myelination and has no impact developmentally on oligodendrocyte number. Understanding the unique aspects of Akt signal transduction in oligodendrocytes that lead to myelination rather than uncontrolled proliferation of oligodendrocyte progenitor cells may have important implications for understanding remyelination in the adult nervous system.

The effect of the rotational angle on MR diffusion indices in nerves: is the rms displacement of the slow-diffusing component a good measure of fiber orientation?

In recent years, much effort has been made to increase our ability to infer nerve fiber direction through the use of diffusion MR. The present study examines the effect of the rotational angle (alpha), i.e. the angle between the diffusion sensitizing gradients and the main axis of the fibers in the nerves, on different NMR indices. The indices examined were the apparent diffusion coefficient (ADC), extracted from low b-values (b(max) approximately 1200 s/mm(2)), and the root mean square (rms) displacement of the fast and the slow-diffusing components extracted from high b-value q-space diffusion MR data. In addition, the effect of both the diffusion time and myelination was evaluated. We found that the most sensitive index to the rotational angle is the rms displacement of the slow-diffusing component extracted from the high b-value q-space diffusion MR experiment. For this component the rms displacement was nearly constant for alpha values ranging from -10 degrees to +80 degrees (where alpha=0 degrees is the z direction), but it changed dramatically when diffusion was measured nearly perpendicular to the nerve fiber direction, i.e., for alpha=90+/-10 degrees. The ADC and the rms displacement of the fast-diffusing component exhibited only gradual changes, with a maximal change at alpha=45+/-15 degrees. The sensitivity of the rms displacement of the slow-diffusing component to the rotational angle was found to be higher at longer diffusion times and in mature fully myelinated nerves. The relevance of these observations for determining the fiber direction is briefly discussed.

Phagocytes of the transected feline optic nerve: an ultrastructural study

Following eye enucleation, there is a spatio-temporal pattern of degeneration along the length of the feline optic nerve, and this depends on the proximity of the segment or zone to the transected end. Thus, the rate of axonal degeneration depends on how close it is to the point of excision of the eye. In the proximal and medial segments, degeneration is rapid and the phagocytes originate from extrinsic haematogenous cells, which have invaded the optic nerve from surrounding meningeal blood vessels and capillaries. These phagocytes appear initially around blood vessels and then progressively spread to the parenchyma. In the distal segments, where Wallerian degeneration has occurred, the corresponding process is relatively slow and the phagocytes originate intrinsically. In contrast, these phagocytes are first seen in the parenchyma and then later spread to the blood vessels. Ultrastructurally, these phagocytes share fine features specifically identified with neuroglial cells such as microglia, oligodendrocytes but not astrocytes. For example, these phagocytes are rich in organelles such as mitochondria, ribosomes, microtubules, and rough and smooth endoplasmic reticulum including the Golgi apparatus. Cytoplasmic inclusions such as dense laminar bodies, lipid droplets, lysosomes and lipofuscin granules are also present. Intermediate filaments, seen only in astrocytes, are not found in these phagocytes. Although the study has established that the phagocytes are intrinsic and neuroglial in origin, their true identity is still unresolved. Secondarily, the study has shown that oligodendrocytes continue to survive and remain viable in an axon-free environment. The question about their new role in Wallerian degeneration remains.

P2X(7) receptor blockade prevents ATP excitotoxicity in oligodendrocytes and ameliorates experimental autoimmune encephalomyelitis

Oligodendrocyte death and demyelination are hallmarks of multiple sclerosis (MS). Here we show that ATP signaling can trigger oligodendrocyte excitotoxicity via activation of calcium-permeable P2X(7) purinergic receptors expressed by these cells. Sustained activation of P2X(7) receptors in vivo causes lesions that are reminiscent of the major features of MS plaques, i.e., demyelination, oligodendrocyte death, and axonal damage. In addition, treatment with P2X(7) antagonists of chronic experimental autoimmune encephalomyelitis (EAE), a model of MS, reduces demyelination and ameliorates the associated neurological symptoms. Together, these results indicate that ATP can kill oligodendrocytes via P2X(7) activation and that this cell death process contributes to EAE. Importantly, P2X(7) expression is elevated in normal-appearing axon tracts in MS patients, suggesting that signaling through this receptor in oligodendrocytes may be enhanced in this disease. Thus, P2X(7) receptor antagonists may be beneficial for the treatment of MS.

Role of eye movements in the retinal code for a size discrimination task

The concerted action of saccades and fixational eye movements are crucial for seeing stationary objects in the visual world. We studied how these eye movements contribute to retinal coding of visual information using the archer fish as a model system. We quantified the animal's ability to distinguish among objects of different sizes and measured its eye movements. We recorded from populations of retinal ganglion cells with a multielectrode array, while presenting visual stimuli matched to the behavioral task. We found that the beginning of fixation, namely the time immediately after the saccade, provided the most visual information about object size, with fixational eye movements, which consist of tremor and drift in the archer fish, yielding only a minor contribution. A simple decoder that combined information from <or=15 ganglion cells could account for the behavior. Our results support the view that saccades impose not just difficulties for the visual system, but also an opportunity for the retina to encode high quality "snapshots" of the environment.

Connective tissue structure of the tree shrew optic nerve and associated ageing changes

PURPOSE

To identify the structure and composition of the tree shrew optic nerve to determine its potential as a model for glaucoma.

METHODS

Tree shrew optic nerves, aged 4 weeks to 5 years, were wax or cryoembedded for analysis of overall morphology and cellular (glial fibrillary acidic protein [GFAP]) and extracellular matrix (collagen types I, III, IV, V, VI; fibronectin; and elastin) immunolocalization studies. In addition, transmission and scanning electron microscopy were performed. In vivo optic disc imaging was performed by HRT2 and fundus camera photography.

RESULTS

The optic nerve of the tree shrew comprised regions comparable to the human prelaminar and lamina cribrosa (LC) in the optic nerve head and the retrolaminar region, immediately posterior. The multilayered connective tissue plates of tree shrew LC stretched across the optic nerve canal at the level of the sclera and consisted of collagen types I, III, IV, V, and VI; elastin; and fibronectin. Significant age-related alterations in connective tissue components were indicated. Connective tissue was present in the central retinal vessel sheaths and was identified as longitudinally oriented collagen fibrils in the retrolaminar optic nerve. GFAP immunofluorescence indicated a high concentration of astrocytic processes in the LC. Myelination of axons was evident in the retrolaminar optic nerve. Ultrastructural studies supported the structural organization and spatial distribution of connective tissue.

CONCLUSIONS

In contrast to many rodent models of glaucoma, since the tree shrew optic nerve resembles that in humans, especially at the LC, the tree shrew offers an ideal opportunity to investigate glaucoma pathophysiology in a subprimate model.

Optical coherence tomography in a patient with tobacco-alcohol amblyopia

AIM

To report optical coherence tomography (OCT) finding in a patient with tobacco-alcohol amblyopia.

METHODS

A 45-year-old man presented with a gradual decrease in vision over 4 years. He had smoked a half to one pack of cigarettes per day and had consumed 350 cc of gin per day for 30 years. A detailed ophthalmologic examination was performed.

RESULTS

His corrected visual acuities were 20/800 OD and 20/200 OS. A Goldmann visual field examination showed ceco-central scotomas in both eyes. OCT using a peripapillary Fast RNFL (retinal nerve fiber layer) programme showed a small decrease in the RNFL thickness of the superotemporal quadrant in the normative diagram of the right eye in spite of a markedly increased RNFL thickness in both eyes.

CONCLUSION

During the phase of visual loss in a patient with tobacco-alcohol amblyopia, visual loss may precede optic disc changes as detected by OCT.

[Characteristics of blood-optic nerve barrier: experiment with rats]

OBJECTIVE

To investigate the difference of endothelial cells in ultrastructure, marker expression, and permeability between blood-brain barrier and blood-optic nerve barrier.

METHODS

The optic nerve, including the prelaminar region, lamina cribrosa, retro-laminar region, intraorbital portion, intracanalicular portion, and intracranial portion, and frontal cortex of 20 male SD rats were obtained to undergo electron microscopy and immunohistochemistry was used to detect the expression of endothelial barrier antigen (EBA) and the extravasation of fibrinogen around the microvessels.

RESULTS

Electron microscopy showed tight junction among the endothelial cells in the microvessels of all portions of optic nerve and cerebral cortex. The number of plasmalemmal vesicles of the prelaminar region was (108.0 +/- 12.0)/microm2, significantly greater than that of the cerebral cortex [(31.8 +/- 2.9)/microm2, P < 0.05]. However, there was no significantly difference in the number of plasmalemmal vesicle among the other portion of optic nerve and cerebral cortex (all P > 0.05). Immunohistochemistry showed that EBA was negative in the endothelial cells in the microvessels of prelaminar region, but strongly positive in the microvessels in the other portions of the optic nerve and in the cerebral cortex. Fibrinogen was present around the microvessels in the prelaminar region of optic nerve in a small amount, however, not present in the other portions of optic nerve and cerebral cortex.

CONCLUSION

There is significant differences in the number of plasmalemmal vesicle, EBA expression, and permeability between the prelaminar region of optic nerve and cerebral cortex, which demonstrates that this region lacks the characteristics of typical blood-brain barrier. However, the other parts of optic nerve possess the properties of classical blood-brain barrier.

The mutant human ND4 subunit of complex I induces optic neuropathy in the mouse

PURPOSE

To produce a mouse model of Leber hereditary optic neuropathy.

METHODS

A mutant ND4 subunit made compatible with the universal genetic code and containing an arginine-to-histidine substitution at residue 340, or a synthetic normal human ND4 gene was delivered to the mouse visual system. The expression and effects of the mutant ND4 gene on the optic nerve and cultured retinal ganglion cells was assessed by magnetic resonance imaging, immunohistochemistry, and light and transmission electron microscopy.

RESULTS

The ATPc mitochondrial targeting sequence directed the allotopically expressed mutant human R340H and wild-type ND4FLAG polypeptides into mitochondria. Expression of normal human ND4 in murine mitochondria posed no ocular toxicity. In contrast, the mutant ND4 disrupted mitochondrial cytoarchitecture, elevated reactive oxygen species, induced swelling of the optic nerve head, and induced apoptosis, with a progressive demise of ganglion cells in the retina and their axons comprising the optic nerve.

CONCLUSIONS

Allotopic expression of the mutant human R340H ND4 subunit of complex I replicated the hallmarks of human mitochondrial disease in the mouse. In contrast, ocular expression of the wild-type human ND4 subunit in lower mammals appears safe, suggesting that it may be useful for treatment of patients with Leber hereditary optic neuropathy.

[Optic neuropathy in diabetic rat and mechanism thereof]

OBJECTIVE

To observe the diabetic optic neuropathy in diabetic rats, and analyze the relation between the diabetic optic neuropathy and change of blood flow and microvascular permeability of the optic nerve.

METHODS

Twenty male Wistar rats were randomly divided into control and diabetic groups. Diabetic model was induced by the intravenous injection of streptozotocin (50 mg/kg). Three months later blood flow of the optic nerve was measured using laser Doppler perfusion imager. 1.5% Evan's blue was injected into the caudal vein, 1 hour later the rats were killed with their bilateral optic nerves taken out. The permeability of optic nerve was tested by spectrophotometer. The optic nerves are observed with light microscopy and transmission electron microscopy.

RESULTS

Atrophy and demyelination of the optic nerve, proliferation of glial cells, neurite swelling, and decrease of organelles in the endothelial cells in optic nerve capillaries were seen. Leukocytes aggregated and adhered to the endothelial cells of pia-mater capillaries. The blood flow of the optic nerve in the diabetic rats was 0.68 v +/- 0.05 v, significantly lower than that of the control rats (1.43 v +/- 0.58 v, P < 0.01). Whereas, the permeability of the optic nerve of the diabetic rats showed a 2.03-fold increase compared with that of the control rats (P < 0.01).

CONCLUSION

Three-month diabetes induced optic neuropathy may be related to the decrease of blood flow and increase of microvascular permeability.

Early development of the optic nerve in the turtle Mauremys leprosa

We show the distribution of the neural and non-neural elements in the early development of the optic nerve in the freshwater turtle, Mauremys leprosa, using light and electron microscopy. The first optic axons invaded the ventral periphery of the optic stalk in close relationship to the radial neuroepithelial processes. Growth cones were thus exclusively located in the ventral margin. As development progressed, growth cones were present in ventral and dorsal regions, including the dorsal periphery, where they intermingled with mature axons. However, growth cones predominated in the ventral part and axonal profiles dorsally, reflecting a dorsal to ventral gradient of maturation. The size and morphology of growth cones depended on the developmental stage and the region of the optic nerve. At early stages, most growth cones were of irregular shape, showing abundant lamellipodia. At the following stages, they tended to be larger and more complex in the ventral third than in intermediate and dorsal portions, suggesting a differential behavior of the growth cones along the ventro-dorsal axis. The arrival of optic axons at the optic stalk involved the progressive transformation of neuroepithelial cells into glial cells. Simultaneously with the fiber invasion, an important number of cells died by apoptosis in the dorsal wall of the optic nerve. These findings are discussed in relation to the results described in the developing optic nerve of other vertebrates.

A standardized method to create optic nerve crush: Yasargil aneurysm clip

It is often difficult to compare results obtained by different investigators on nerve compression injuries, owing to differences in method of pressure application and noncomparable pressure levels. In the present study, we described a new method to crush the optic nerve by using a specially designed and commercially available device. We think that standardization of the compression methods is necessary to compare interlaboratory results.

The postnatal development of the optic nerve of a reptile (Vipera aspis): A quantitative ultrastructural study

The number of axons in the optic nerve of the ovoviviparous reptile Vipera aspis was estimated from electron micrographs taken during the first 5 weeks of postnatal life. One to two days after birth, the optic nerve contains about 170,000 fibres, of which about 9% are myelinated. At the end of the fifth postnatal week, the number of optic fibres has fallen to about 100,000, of which about 42% are myelinated. This fibre loss continues after the fifth postnatal week, since in the adult viper the nerve contains about 60,000 fibres, of which 85% are myelinated; overall, about 65% of the optic nerve fibres present at birth disappear before the number of axons stabilises at the adult level. This study shows, for the first time, that the mode of development of the visual axons of reptiles is not that of anamniote vertebrates but similar to that of birds and mammals.

Subtle myelin defects in PLP-null mice

This study explores subtle defects in the myelin of proteolipid protein (PLP)-null mice that could potentially underlie the functional losses and axon damage known to occur in this mutant and in myelin diseases including multiple sclerosis. We have compared PLP-null central nervous system (CNS) myelin with normal myelin using ultrastructural methods designed to emphasize fine differences. In the PLP-null CNS, axons large enough to be myelinated often lack myelin entirely or are surrounded by abnormally thin sheaths. Short stretches of cytoplasm persist in many myelin lamellae. Most strikingly, compaction is incomplete in this mutant as shown by the widespread presence of patent interlamellar spaces of variable width that can be labeled with ferricyanide, acting as an aqueous extracellular tracer. In thinly myelinated fibers, interlamellar spaces are filled across the full width of the sheaths. In thick myelin sheaths, they appear filled irregularly but diffusely. These patent spaces constitute a spiral pathway through which ions and other extracellular agents may penetrate gradually, possibly contributing to the axon damage known to occur in this mutant, especially in thinly myelinated fibers, where the spiral path length is shortest and most consistently labeled. We show also that the "radial component" of myelin is distorted in the mutant ("diagonal component"), extending across the sheaths at 45 degrees instead of 90 degrees. These observations indicate a direct or indirect role for PLP in maintaining myelin compaction along the external surfaces of the lamellae and to a limited extent, along the cytoplasmic surfaces as well and also in maintaining the normal alignment of the radial component.

Developmental changes in the fibre population of the optic nerve follow an avian/mammalian-like pattern in the turtle Mauremys leprosa

The changes in the axon and growth cone numbers in the optic nerve of the freshwater turtle Mauremys leprosa were studied by electron microscopy from the embryonic day 14 (E14) to E80, when the animals normally hatch, and from the first postnatal day (P0) to adulthood (5 years on). At E16, the first axons appeared in the optic nerve and were added slowly until E21. From E21, the fibre number increased rapidly, peaking at E34 (570,000 fibres). Thereafter, the axon number decreased sharply, and from E47 declined steadily until reaching the mature number (about 330,000). These observations indicated that during development of the retina there was an overproduction and later elimination of retinal ganglion cells. Growth cones were first observed in the optic nerve at as early as E16. Their number increased rapidly until E21 and continued to be high through E23 and E26. After E26, the number declined steeply and by E40 the optic nerve was devoid of growth cones. These results indicated that differentiation of the retinal ganglion cells occurred during the first half of the embryonic life. To examine the correlation between the loss of the fibres from the optic nerve and loss of the parent retinal ganglion cells, retinal sections were processed with the TUNEL technique. Apoptotic nuclei were detected in the ganglion cell layer throughout the period of loss of the optic fibres. Our results showed that the time course of the numbers of the fibres in the developing turtle optic nerve was similar to those found in birds and mammals.

Age-related molecular reorganization at the node of Ranvier

In myelinated axons, action potential conduction is dependent on the discrete clustering of ion channels at specialized regions of the axon, termed nodes of Ranvier. This organization is controlled, at least in part, by the adherence of myelin sheaths to the axolemma in the adjacent region of the paranode. Age-related disruption in the integrity of internodal myelin sheaths is well described and includes splitting of myelin sheaths, redundant myelin, and fluctuations in biochemical constituents of myelin. These changes have been proposed to contribute to age-related cognitive decline; in previous studies of monkeys, myelin changes correlate with cognitive performance. In the present study, we hypothesize that age-dependent myelin breakdown results in concomitant disruption at sites of axoglial contact, in particular at the paranode, and that this disruption alters the molecular organization in this region. In aged monkey and rat optic nerves, immunolabeling for voltage-dependent potassium channels of the Shaker family (Kv1.2), normally localizing in the adjacent juxtaparanode, were mislocalized to the paranode. Similarly, immunolabeling for the paranodal marker caspr reveals irregular caspr-labeled paranodal profiles, suggesting that there may be age-related changes in paranodal structure. Ultrastructural analysis of paranodal segments from optic nerve of aged monkeys shows that, in a subset of myelinated axons with thick sheaths, some paranodal loops fail to contact the axolemma. Thus, age-dependent myelin alterations affect axonal protein localization and may be detrimental to maintenance of axonal conduction.

K+ channel KV3.1 associates with OSP/claudin-11 and regulates oligodendrocyte development

K(+) channels are differentially expressed throughout oligodendrocyte (Olg) development. K(V)1 family voltage-sensitive K(+) channels have been implicated in proliferation and migration of Olg progenitor cell (OPC) stage, and inward rectifier K+ channels (K(IR))4.1 are required for OPC differentiation to myelin-forming Olg. In this report we have identified a Shaw family K(+) channel, K(V)3.1, that is involved in proliferation and migration of OPC and axon myelination. Application of anti-K(V)3.1 antibody or knockout of Kv3.1 gene decreased the sustained K(+) current component of OPC by 50% and 75%, respectively. In functional assays block of K(V)3.1-specific currents or knockout of Kv3.1 gene inhibited proliferation and migration of OPC. Adult Kv3.1 gene-knockout mice had decreased diameter of axons and decreased thickness of myelin in optic nerves compared with age-matched wild-type littermates. Additionally, K(V)3.1 was identified as an associated protein of Olg-specific protein (OSP)/claudin-11 via yeast two-hybrid analysis, which was confirmed by coimmunoprecipitation and coimmunohistochemistry. In summary, the K(V)3.1 K(+) current accounts for a significant component of the total K(+) current in cells of the Olg lineage and, in association with OSP/claudin-11, plays a significant role in OPC proliferation and migration and myelination of axons.

The optic nerve: a new window into cerebrospinal fluid composition?

Cerebrospinal fluid (CSF) pressure and composition are generally thought to be homogeneous within small limits throughout all CSF compartments. CSF sampled during lumbar puncture therefore should be representative for all CSF compartments. On the basis of clinical findings, histology and biochemical markers, we present for the first time strong evidence that the subarachnoid spaces (SAS) of the optic nerve (ON) can become separated from other CSF compartments in certain ON disorders, thus leading to an ON sheath compartment syndrome. This may result in an abnormal concentration gradient of CSF molecular markers determined in locally sampled CSF compared with CSF taken during lumbar puncture.

Optic nerve degeneration in the DBA/2NNia mouse: is the lamina cribrosa important in the development of glaucomatous optic neuropathy?

To study optic nerve (ON) degeneration in the DBA/2NNia (DBA) mouse, a species lacking a lamina cribrosa and a model for secondary angle-closure glaucoma, serial semi- and ultra-thin sectioning of the myelinated ON and of the ON head was performed and sections evaluated qualitatively and quantitatively by light and electron microscopy. Immunohistochemistry was performed using antibodies against collagen type I, III, VI, laminin, and connexin43. The major finding on the myelinated ON was a significant decrease in cross section area during ON degeneration which was paralleled by a loss of axons and an increase in microglia. The number of astrocytes and blood vessels did not change. The major findings on the ON papilla were that ON heads with only mild degeneration showed a pronounced focal degeneration around the central retinal artery. In more severely degenerated ON, newly formed bundles of collagen type VI were located between astrocyte processes within the ON head. In a species that has no lamina cribrosa, DBA mice can develop typical signs of glaucomatous optic neuropathy. The entrance of the central retinal vessels into the ONH seems to be a preferentially vulnerable region for axon loss in this mouse model. In addition, astrocytes in the ON head form extracellular material similar to that found in human glaucomatous eyes.

Synergistic action of brain-derived neurotrophic factor and lens injury promotes retinal ganglion cell survival, but leads to optic nerve dystrophy in vivo

Trauma or disease in the CNS often leads to neuronal death and consequent loss of functional connections. The idea has been put forward that strategies aimed at repairing the injured CNS involve stimulation of both neuronal survival and axon regeneration. We tested this hypothesis in the adult rat retinocollicular system by combining two strategies: (i) exogenous administration of brain-derived neurotrophic factor (BDNF), a potent survival factor for damaged retinal ganglion cells (RGCs) and (ii) lens injury, which promotes robust growth of transected RGC axons. Our results demonstrate that BDNF and lens injury interact synergistically to promote neuronal survival: 71% of RGCs were alive at 2 weeks after optic nerve injury, a time when only approximately 10% of these neurons remain without treatment. Intravitreal injection of BDNF, however, led to regeneration failure following lens injury. The effect of BDNF could not be generalized to other growth factors, as ciliary neurotrophic factor did not cause a significant reduction of lens injury-induced regeneration. Growth arrest in optic nerves treated with BDNF and lens injury correlated with the formation of hypertrophic axonal swellings in the proximal optic nerve. These swellings were filled with numerous vesicular bodies, disorganized neurofilaments and degenerating organelles. Our results demonstrate that: (i) increased neuronal survival does not necessarily lead to enhanced axon regeneration and (ii) activation of survival and growth pathways may produce axonal dystrophy similar to that found in neurodegenerative disorders including glaucoma, Alzheimer's disease and multiple sclerosis. We propose that loss of axonal integrity may limit neuronal recovery in the injured, adult CNS.

[Ultrastructure of optic nerve fibers in rabbits after intravitreal infusion of perfluoroorganic compounds]

Ultrastructure of optic nerve myelinated fibers after intravitreal infusion of perfluorooctylbromide (PFOB) and perfluorodecaline (PFD) for 30 days was studied in rabbits (n = 14). After inraocular administration of PFOB, the ultrastructural changes of optic nerve fibers were mainly reactive in nature. Mitochondrial swelling and partial destruction of cristae were detected in the axons. After PFD infusion, the changes were observed that were not only reactive, but highly degenerative, including destruction of organelles and myelin sheath and axon cytoskeleton disintegration. Thus, the changes associated with PFOB were less pronounced than those found after PFD application. Therefore, PFOB is more promising for the use as an ophthalmologic implant.

Transplanted Schwann cells, not olfactory ensheathing cells, myelinate optic nerve fibres

In a previous study we found that olfactory ensheathing cells transplanted into complete retrobulbar transections of the rat optic nerve mediated regeneration of severed retinal ganglion cell axons through the graft region. Although the regenerating axons were ensheathed by the transplanted cells, none of the regenerating axons became myelinated by either central or peripheral type myelin. In the present study we used the same operative procedure but transplanted Schwann cells instead of olfactory ensheathing cells. As with the olfactory ensheathing cell transplants the Schwann cells transplants also induced regeneration of the severed retinal ganglion cell axons into the graft region. In contrast to the situation with the olfactory ensheathing cell transplants, however, a considerable number of the regenerating axons became myelinated by peripheral type myelin produced by the transplanted Schwann cells. This observation identifies a further distinction between these two cell types which are phenotypically similar in many ways, but which have been shown to have major functional differences with regard to regeneration in spinal cord lesions.

Developmental impairment of compound action potential in the optic nerve of myelin mutant taiep rats

The taiep rat is a myelin mutant with an initial hypomyelination, followed by a progressive demyelination of the CNS. The neurological correlates start with tremor, followed by ataxia, immobility episodes, epilepsy and paralysis. The optic nerve, an easily-isolable central tract fully myelinated by oligodendrocytes, is a suitable preparation to evaluate the developmental impairment of central myelin. We examined the ontogenic development of optic nerve compound action potentials (CAP) throughout the first 6 months of life of control and taiep rats. Control optic nerves (ON) develop CAPs characterized by three waves. Along the first month, the CAPs of taiep rats showed a delayed maturation, with lower amplitudes and longer latencies than controls; at P30, the conduction velocity has only a third of the normal value. Later, as demyelination proceeds, the conduction velocity of taiep ONs begins to decrease and CAPs undergo a gradual temporal dispersion. CAPs of control and taiep showed differences in their pharmacological sensitivity to TEA and 4-AP, two voltage dependent K+ channel-blockers. As compared with TEA, 4-AP induced a significant increase of the amplitudes and a remarkable broadening of CAPs. After P20, unlike controls, the greater sensitivity to 4-AP exhibited by taiep ONs correlates with the detachment and retraction of paranodal loops suggesting that potassium conductances could regulate the excitability as demyelination of CNS axons progresses. It is concluded that the taiep rat, a long-lived mutant, provides a useful model to study the consequences of partial demyelination and the mechanisms by which glial cells regulate the molecular organization and excitability of axonal membranes during development and disease.

Slow, medium, or fast re-warming following post-traumatic hypothermia therapy? An ultrastructural perspective

It was hypothesized that rapid rather than slow re-warming following traumatic brain injury (TBI) and short-term hypothermia results in secondary, ultrastructural pathology. After stretch injury to the right optic nerve, adult guinea pigs were randomly allocated to one of six experimental groups. Either (1) sham (all procedures but not stretch-injured; n = 4); injured and (2) maintained at normal temporalis core temperature (38.5 degrees C) for 8 hours (n = 6); (3) cooled rapidly to 32.5 degrees C (temporalis temperature), maintained for 4 h and re-warmed to 38.5 degrees C at 1 degrees C rise every 10 min (fast; n = 6); (4) cooled and re-warmed at 1 degrees C rise every 20 min (medium; n = 6); (5) cooled and rewarmed at 1 degrees C rise every 40 min (slow; n = 6) before being killed 8 h after injury; and (6) uninjured animals (n = 6) cooled to 32.5 degrees C for 4 h and then re-warmed at 1 degrees C every 10 min before killing 4 h later. Tissue was processed for light immunocytochemistry (beta-APP and RMO-14) and ultrastructural stereology. In both uninjured and injured fast re-warmed animals, there was almost total loss of axonal microtubules (MT) and an increased number of neurofilaments (NF) within the axoplasm. In the former, there was also compaction of NF. The number of MT was reduced to 40% of control values, NFs were increased but were not compacted after medium rate re-warming. Following slow re-warming the axonal cytoskeleton did not differ from that in control animals. It is concluded that re-warming faster than 1 degrees C every 40 min following mild post-traumatic hypothermia induces secondary axonal pathology.

Structure and Stability of Internodal Myelin in Mouse Models of Hereditary Neuropathy

Peripheral neuropathies often result in abnormalities in the structure of internodal myelin, including changes in period and membrane packing, as observed by electron microscopy (EM). Mutations in the gene that encodes the major adhesive structural protein of internodal myelin in the peripheral nervous system of humans and mice--P0 glycoprotein--correlate with these defects. The mechanisms by which P0 mutations interfere with myelin packing and stability are not well understood and cannot be provided by EM studies that give static and qualitative information on fixed material. To gain insights into the pathogenesis of mutant P0, we used x-ray diffraction, which can detect more subtle and dynamic changes in native myelin, to investigate myelin structure in sciatic nerves from murine models of hereditary neuropathies. We used mice with disruption of one or both copies of the P0 gene (models of Charcot-Marie-Tooth-like neuropathy [CMT1B] or Dejerine-Sottas-like neuropathy) and mice with a CMT1B resulting from a transgene encoding P0 with an amino terminal myc-tag. To directly test the structural role of P0, we also examined a mouse that expresses P0 instead of proteolipid protein in central nervous system myelin. To link our findings on unfixed nerves with EM results, we analyzed x-ray patterns from unembedded, aldehyde-fixed nerves and from plastic-embedded nerves. From the x-ray patterns recorded from whole nerves, we assessed the amount of myelin and its quality (i.e. relative thickness and regularity). Among sciatic nerves having different levels of P0, we found that unfixed nerves and, to a lesser extent, fixed but unembedded nerves gave diffraction patterns of sufficient quality to distinguish periods, sometimes differing by a few Angstroms. Certain packing abnormalities were preserved qualitatively by aldehyde fixation, and the relative amount and structural integrity of myelin among nerves could be distinguished. Measurements from the same nerve over time showed that the amount of P0 affected myelin's stability against swelling, thus directly supporting the hypothesis that packing defects underlie instability in "live" or intact myelin. Our findings demonstrate that diffraction can provide a quantitative basis for understanding, at a molecular level, the membrane packing defects that occur in internodal myelin in demyelinating peripheral neuropathies.