Virus-mediated EpoR76E Therapy Slows Optic Nerve Axonopathy in Experimental Glaucoma

Glaucoma, a common cause of blindness, is currently treated by intraocular pressure (IOP)-lowering interventions. However, this approach is insufficient to completely prevent vision loss. Here, we evaluate an IOP-independent gene therapy strategy using a modified erythropoietin, EPO-R76E, which has reduced erythropoietic function. We used two models of glaucoma, the murine microbead occlusion model and the DBA/2J mouse. Systemic recombinant adeno-associated virus-mediated gene delivery of EpoR76E (rAAV.EpoR76E) was performed concurrent with elevation of IOP. Axon structure and active anterograde transport were preserved in both models. Vision, as determined by the flash visual evoked potential, was preserved in the DBA/2J. These results show that systemic EpoR76E gene therapy protects retinal ganglion cells from glaucomatous degeneration in two different models. This suggests that EPO targets a component of the neurodegenerative pathway that is common to both models. The efficacy of rAAV.EpoR76E delivered at onset of IOP elevation supports clinical relevance of this treatment.

Nanosponge-Mediated Drug Delivery Lowers Intraocular Pressure

PURPOSE

We examined the efficacy of an extended-release drug delivery system, nanosponge (NS) encapsulated compounds, administered intravitreally to lower intraocular pressure (IOP) in mice.

METHODS

Bilateral ocular hypertension was induced in mice by injecting microbeads into the anterior chamber. Hypertensive mice received NS loaded with ocular hypotensive drugs via intravitreal injection and IOP was monitored. Retinal deposition and retinal ganglion cell (RGC) uptake of Neuro-DiO were examined following intravitreal injection of Neuro-DiO-NS using confocal microscopy.

RESULTS

Brimonidine-loaded NS lowered IOP 12% to 30% for up to 6 days (P < 0.02), whereas travoprost-NS lowered IOP 19% to 29% for up to 4 days (P < 0.02) compared to saline injection. Three bimatoprost NS were tested: a 400-nm NS and two 700-nm NS with amorphous (A-NS) or amorphous/crystalline (AC-NS) crosslinkers. A single injection of 400 nm NS lowered IOP 24% to 33% for up to 17 days compared to saline, while A-NS and AC-NS lowered IOP 22% to 32% and 18% to 26%, respectively, for up to 32 days (P < 0.046). Over time retinal deposition of Neuro-DiO increased from 19% to 71%; Neuro-DiO released from NS was internalized by RGCs.

CONCLUSIONS

A single injection of NS can effectively deliver ocular hypotensive drugs in a linear and continuous manner for up to 32 days. Also, NS may be effective at targeting RGCs, the neurons that degenerate in glaucoma.

TRANSLATIONAL RELEVANCE

Patient compliance is a major issue in glaucoma. The use of NS to deliver a controlled, sustained release of therapeutics could drastically reduce the number of patients that progress to vision loss in this disease.

Activation of the TRPV1 cation channel contributes to stress-induced astrocyte migration

Astrocytes provide metabolic, structural, and synaptic support to neurons in normal physiology and also contribute widely to pathogenic processes in response to stress or injury. Reactive astrocytes can undergo cytoskeletal reorganization and increase migration through changes in intracellular Ca(2+) mediated by a variety of potential modulators. Here we tested whether migration of isolated retinal astrocytes following mechanical injury (scratch wound) involves the transient receptor potential vanilloid-1 channel (TRPV1), which contributes to Ca(2+)-mediated cytoskeletal rearrangement and migration in other systems. Application of the TRPV1-specific antagonists, capsazepine (CPZ) or 5'-iodoresiniferatoxin (IRTX), slowed migration by as much as 44%, depending on concentration. In contrast, treatment with the TRPV1-specific agonists, capsaicin (CAP) or resiniferatoxin (RTX) produced only a slight acceleration over a range of concentrations. Chelation of extracellular Ca(2+) with EGTA (1 mM) slowed astrocyte migration by 35%. Ratiometric imaging indicated that scratch wound induced a sharp 20% rise in astrocyte Ca(2+) that dissipated with distance from the wound. Treatment with IRTX both slowed and dramatically reduced the scratch-induced Ca(2+) increase. Both CPZ and IRTX influenced astrocyte cytoskeletal organization, especially near the wound edge. Taken together, our results indicate that astrocyte mobilization in response to mechanical stress involves influx of extracellular Ca(2+) and cytoskeletal changes in part mediated by TRPV1 activation.

Secondary neuroprotective effects of hypotensive drugs and potential mechanisms of action

Primary open-angle glaucoma, a long-term degenerative ocular neuropathy, remains a significant cause of vision impairment worldwide. While many risk factors have been correlated with increased risk for primary open-angle glaucoma, intraocular pressure (IOP) remains the only modifiable risk factor and primary therapeutic target. Pharmacologic therapies are administered topically; these include α(2)-agonists, β-antagonists, prostaglandin analogs and carbonic anhydrase inhibitors. Some of these topical medications exhibit secondary neuroprotective effects independent of their effect on IOP. This review covers the possible mechanisms of neuroprotection stimulated by drugs currently marketed for the lowering of IOP, based on known literature. While the neuroprotective properties of many glaucoma pharmaceuticals are promising from an experimental standpoint, key challenges for the development of new clinical practices include unknown systemic side effects, limited methods of drug delivery to the retina and optic nerve, and development of extended-release formulations.

Age-related changes in the visual pathways: blame it on the axon

The aging visual system is marked by a decline in some, but not all, key functions. Some of this decline is attributed to changes in the optics of the eye, but other aspects must have a neural basis. Across mammals, with aging there is remarkable persistence of central structures to which retinal ganglion cell (RGC) axons project with little or no loss of neurons. Similarly, RGC bodies in the retina are subject to variable age-related loss, with most mammals showing none over time. In contrast, the RGC axon itself is highly vulnerable. Across species, the rate of axon loss in the optic nerve is related inversely to the total number of axons at maturity and lifespan. The result of this scaling is approximately a 40% total decline in axon number. Evidence suggests that the consistent vulnerability of RGC axons to aging arises from their high metabolic demand combined with diminishing resources. Thus, therapeutic interventions that conserve bioenergetics may have potential to abate age-related decline in visual function.

Proximal inhibition of p38 MAPK stress signaling prevents distal axonopathy

The p38 mitogen-activated protein kinase (MAPK) isoforms are phosphorylated by a variety of stress stimuli in neurodegenerative disease and act as upstream activators of myriad pathogenic processes. Thus, p38 MAPK inhibitors are of growing interest as possible therapeutic interventions. Axonal dysfunction is an early component of most neurodegenerative disorders, including the most prevalent optic neuropathy, glaucoma. Sensitivity to intraocular pressure at an early stage disrupts anterograde transport along retinal ganglion cell (RGC) axons to projection targets in the brain with subsequent degeneration of the axons themselves; RGC body loss is much later. Here we show that elevated ocular pressure in rats increases p38 MAPK activation in retina, especially in RGC bodies. Topical eye-drop application of a potent and selective inhibitor of the p38 MAPK catalytic domain (Ro3206145) prevented both the degradation of anterograde transport to the brain and degeneration of axons in the optic nerve. Ro3206145 reduced in the retina phosphorylation of tau and heat-shock protein 27, both down-stream targets of p38 MAPK activation implicated in glaucoma, as well as expression of two inflammatory responses. We also observed increased p38 MAPK activation in mouse models. Thus, inhibition of p38 MAPK signaling in the retina may represent a therapeutic target for preventing early pathogenesis in optic neuropathies.

α-Lipoic acid antioxidant treatment limits glaucoma-related retinal ganglion cell death and dysfunction

Oxidative stress has been implicated in neurodegenerative diseases, including glaucoma. However, due to the lack of clinically relevant models and expense of long-term testing, few studies have modeled antioxidant therapy for prevention of neurodegeneration. We investigated the contribution of oxidative stress to the pathogenesis of glaucoma in the DBA/2J mouse model of glaucoma. Similar to other neurodegenerative diseases, we observed lipid peroxidation and upregulation of oxidative stress-related mRNA and protein in DBA/2J retina. To test the role of oxidative stress in disease progression, we chose to deliver the naturally occurring, antioxidant α-lipoic acid (ALA) to DBA/2J mice in their diet. We used two paradigms for ALA delivery: an intervention paradigm in which DBA/2J mice at 6 months of age received ALA in order to intervene in glaucoma development, and a prevention paradigm in which DBA/2J mice were raised on a diet supplemented with ALA, with the goal of preventing glaucoma development. At 10 and 12 months of age (after 4 and 11 months of dietary ALA respectively), we measured changes in genes and proteins related to oxidative stress, retinal ganglion cell (RGC) number, axon transport, and axon number and integrity. Both ALA treatment paradigms showed increased antioxidant gene and protein expression, increased protection of RGCs and improved retrograde transport compared to control. Measures of lipid peroxidation, protein nitrosylation, and DNA oxidation in retina verified decreased oxidative stress in the prevention and intervention paradigms. These data demonstrate the utility of dietary therapy for reducing oxidative stress and improving RGC survival in glaucoma.

High-resolution matrix-assisted laser desorption ionization-imaging mass spectrometry of lipids in rodent optic nerve tissue

PURPOSE

To develop a method for generating high spatial resolution (10 µm) matrix-assisted laser desorption ionization (MALDI) images of lipids in rodent optic nerve tissue.

METHODS

Ice-embedded optic nerve tissue from rats and mice were cryosectioned across the coronal and sagittal axes of the nerve fiber. Sections were thaw mounted on gold-coated MALDI plates and were washed with ammonium acetate to remove biologic salts before being coated in 2,5-dihydroxybenzoic acid by sublimation. MALDI images were generated in positive and negative ion modes at 10 µm spatial resolution. Lipid identification was performed with a high mass resolution Fourier transform ion cyclotron resonance mass spectrometer.

RESULTS

Several lipid species were observed with high signal intensity in MALDI images of optic nerve tissue. Several lipids were localized to specific structures including in the meninges surrounding the optic nerve and in the central neuronal tissue. Specifically, phosphatidylcholine species were observed throughout the nerve tissue in positive ion mode while sulfatide species were observed in high abundance in the meninges surrounding the optic nerve in negative ion mode. Accurate mass measurements and fragmentation using sustained off-resonance irradiation with a high mass resolution Fourier transform ion cyclotron resonance mass spectrometer instrument allowed for identification of lipid species present in the small structure of the optic nerve directly from tissue sections.

CONCLUSIONS

An optimized sample preparation method provides excellent sensitivity for lipid species present within optic nerve tissue. This allowed the laser spot size and fluence to be reduced to obtain a high spatial resolution of 10 µm. This new imaging modality can now be applied to determine spatial and molecular changes in optic nerve tissue with disease.

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.

Early reduction of microglia activation by irradiation in a model of chronic glaucoma

Glaucoma is a neurodegenerative disease that results in the progressive decline and ultimate death of retinal ganglion cells (RGCs). While multiple risk factors are associated with glaucoma, the mechanisms leading to onset and progression of the disease remain unknown. Molecular analysis in various glaucoma models has revealed involvement of non-neuronal cell populations, including astrocytes, Mueller glia and microglia, at early stages of glaucoma. High-dose irradiation was reported to have a significant long-term protective effect in the DBA/2J (D2) mouse model of glaucoma, although the cellular and molecular basis for this effect remains unclear. In particular, the acute effects of irradiation on specific cell populations, including non-neuronal cells, in the D2 retina and nerve have not been assessed. Here we report that irradiation induces transient reduction in proliferating microglia within the optic nerve head and glial lamina within the first week post-irradiation. This was accompanied by reduced microglial activation, with no effect on astrocyte gliosis in those regions. At later stages we confirm that early high-dose irradiation of the mouse head results in improvement of axonal structural integrity and anterograde transport function, without reduction of intraocular pressure. Thus reduced microglial activation induced by irradiation at early stages is associated with reduced optic nerve and retinal neurodegeneration in the D2 mouse model of glaucoma.

Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Glaucoma is a common optic neuropathy with a complex etiology often linked to sensitivity to intraocular pressure. Though the precise mechanisms that mediate or transduce this sensitivity are not clear, the axon of the retinal ganglion cell appears to be vulnerable to disease-relevant stressors early in progression. One reason may be because the axon is generally thin for both its unmyelinated and myelinated segment and much longer than the thicker unmyelinated axons of other excitatory retinal neurons. This difference may predispose the axon to metabolic and oxidative injury, especially at distal sites where pre-synaptic terminals form connections in the brain. This idea is consistent with observations of early loss of anterograde transport at central targets and other signs of distal axonopathy that accompany physiological indicators of progression. Outright degeneration of the optic projection ensues after a critical period and, at least in animal models, is highly sensitive to cumulative exposure to elevated pressure in the eye. Stress emanating from the optic nerve head can induce not only distal axonopathy with aspects of dying back neuropathy, but also Wallerian degeneration of the optic nerve and tract and a proximal program involving synaptic and dendritic pruning in the retina. Balance between progressive and acute mechanisms likely varies with the level of stress placed on the unmyelinated axon as it traverses the nerve head, with more acute insult pushing the system toward quicker disassembly. A constellation of signaling factors likely contribute to the transduction of stress to the axon, so that degenerative events along the length of the optic projection progress in retinotopic fashion. This pattern leads to well-defined sectors of functional depletion, even at distal-most sites in the pathway. While ganglion cell somatic drop-out is later in progression, some evidence suggests that synaptic and dendritic pruning in the retina may be a more dynamic process. Structural persistence both in the retina and in central projection sites offers the possibility that intrinsic self-repair pathways counter pathogenic mechanisms to delay as long as possible outright loss of tissue.

TRPV1: a stress response protein in the central nervous system

The transient receptor potential (TRP) family comprises a diverse group of cation channels that regulate a variety of intracellular signaling pathways. The TRPV1 (vanilloid 1) channel is best known for its role in nociception and sensory transmission. First studied in the dorsal root ganglia as the receptor for capsaicin, TRPV1 is now recognized to have a broader distribution and function within the central nervous system (CNS). Because it can be activated by a range of potentially noxious stimuli, TRPV1's polymodal nature and ability to interact with other receptor pathways make it a candidate for a stress response protein. As a result, TRPV1 is emerging as a key mediator of CNS function through modulation of both glial and neuronal activity. Growing evidence has suggested that TRPV1 can mediate a variety of pathways from glial reactivity and cytokine release to synaptic transmission and plasticity. This review highlights the increasing importance of TRPV1 as a regulator of CNS function in response to stress.

Brimonidine prevents axonal and somatic degeneration of retinal ganglion cell neurons

BACKGROUND

Brimonidine is a common drug for lowering ocular pressure and may directly protect retinal ganglion cells in glaucoma. The disease involves early loss of retinal ganglion cell transport to brain targets followed by axonal and somatic degeneration. We examined whether brimonidine preserves ganglion cell axonal transport and abates degeneration in rats with elevated ocular pressure induced by laser cauterization of the episcleral veins.

RESULTS

Ocular pressure was elevated unilaterally by 90% for a period of 8 weeks post- cauterization. During this time, brimonidine (1mg/kg/day) or vehicle (phosphate-buffered saline) was delivered systemically and continuously via subcutaneous pump. Animals received bilateral intravitreal injections of fluorescent cholera toxin subunit β (CTB) two days before sacrifice to assess anterograde transport. In retinas from the vehicle group, elevated pressure induced a 44% decrease in the fraction of ganglion cells with intact uptake of CTB and a 14-42% reduction in the number of immuno-labelled ganglion cell bodies, with the worst loss occurring nasally. Elevated pressure also caused a 33% loss of ganglion cell axons in vehicle optic nerves and a 70% decrease in CTB transport to the superior colliculus. Each of these components of ganglion cell degeneration was either prevented or significantly reduced in the brimonidine treatment group.

CONCLUSIONS

Continuous and systemic treatment with brimonidine by subcutaneous injection significantly improved retinal ganglion cell survival with exposure to elevated ocular pressure. This effect was most striking in the nasal region of the retina. Brimonidine treatment also preserved ganglion cell axon morphology, sampling density and total number in the optic nerve with elevated pressure. Consistent with improved outcome in the optic projection, brimonidine also significantly reduced the deficits in axonal transport to the superior colliculus associated with elevated ocular pressure. As transport deficits to and from retinal ganglion cell projection targets in the brain are relevant to the progression of glaucoma, the ability of brimonidine to preserve optic nerve axons and active transport suggests its neuroprotective effects are relevant not only at the cell body, but throughout the entire optic projection.

Optic neuropathy due to microbead-induced elevated intraocular pressure in the mouse

PURPOSE

To characterize a glaucoma model of mice, the authors adopted and modified a method of inducing the chronic elevation of intraocular pressure (IOP) by anterior chamber injection of polystyrene microbeads.

METHODS

Chronic elevation of IOP was induced unilaterally in adult C57BL/6J mice by injecting polystyrene microbeads to the anterior chamber. Effectiveness of microbeads of different sizes (small, 10 μm; large, 15 μm) on inducing IOP elevation was compared, and IOP was measured every other day using a tonometer. After maintaining elevated IOP for 2, 4, or 8 weeks, the degree of RGC and axon degeneration was assessed quantitatively using electron microscopy, fluorogold, retrograde labeling, and immunohistochemistry.

RESULTS

Eighty-one of 87 mice that received anterior chamber injection of microbeads exhibited consistent IOP elevation above that of control eyes. Injection of small microbeads induced longer and higher peak value of IOP elevation compared with that of the large microbeads. A single injection of small microbeads resulted in a 4-week elevation of IOP that was maintained to an 8-week period after a second injection of microbeads in week 4. As the duration of IOP elevation increased, RGC bodies and their axons degenerated progressively and reached an approximately 50% loss after an 8-week elevation of IOP.

CONCLUSIONS

Anterior chamber injection of microbeads effectively induced IOP elevation and glaucomatous optic neuropathy in mice. Development of an inducible mouse model of elevated IOP will allow applications of mouse genetic technology to the investigation of the mechanisms and the evaluation of treatment strategies of glaucoma.

Neurodegeneration in glaucoma: progression and calcium-dependent intracellular mechanisms

Glaucoma is an age-related optic neuropathy involving sensitivity to ocular pressure. The disease is now seen increasingly as one of the central nervous system, as powerful new approaches highlight an increasing number of similarities with other age-related neurodegenerations such as Alzheimer's and Parkinson's. While the etiologies of these diseases are diverse, they involve many important common elements including compartmentalized programs of degeneration targeting axons, dendrites and finally cell bodies. Most age-related degenerations display early functional deficits that precede actual loss of neuronal substrate. These are linked to several specific neurochemical cascades that can be linked back to dysregulation of Ca(2+)-dependent processes. We are now in the midst of identifying similar cascades in glaucoma. Here we review recent evidence on the pathological progression of neurodegeneration in glaucoma and some of the Ca(2+)-dependent mechanisms that could underlie these changes. These mechanisms present clear implications for efforts to develop interventions targeting neuronal loss directly and make glaucoma an attractive model for both interrogating and informing other neurodegenerative diseases.

The microbead occlusion model: a paradigm for induced ocular hypertension in rats and mice

PURPOSE

Elevated intraocular pressure (IOP) is an important risk factor for glaucoma. Animal models often involve techniques for IOP elevation that are surgically invasive. Here the authors describe a novel and relatively simple method for inducing a highly consistent, modest, and repeatable elevation in IOP for rats and mice.

METHODS

IOP was elevated unilaterally by injection of polystyrene microbeads into the anterior chamber to occlude aqueous outflow in rats (2.5-7 microL) and mice (1 microL). The fellow eye received an equivalent saline injection as internal control. The authors used tonometry to measure microbead-induced IOP elevations. Optic nerves were processed histologically to determine axon loss.

RESULTS

For rats, a single injection of microbeads raised IOP by 21% to 34%, depending on volume, for approximately 2 weeks, though they were not tracked to full recovery. IOP in the saline-injected eye was constant. An additional injection (5 microL) extended the elevation to 8 weeks. Cumulative pressure exposure for both injections increased linearly. For mice, a single 1-microL injection of microbeads elicited a highly regular 30% elevation in IOP that persisted for more than 3 weeks, with a linear rise in cumulative pressure exposure. For both rats and mice, interanimal variability on a given day was modest, approximately 5% of the mean IOP measurement. Extended elevations (4-5 weeks) induced approximately a 20% loss of axons in both rats and mice.

CONCLUSIONS

These data support a novel and flexible model of modest ocular hypertension with axon loss. The maximal duration of IOP elevation will be further characterized in future studies.

TRPV1: contribution to retinal ganglion cell apoptosis and increased intracellular Ca2+ with exposure to hydrostatic pressure

PURPOSE

Elevated hydrostatic pressure induces retinal ganglion cell (RGC) apoptosis in culture. The authors investigated whether the transient receptor potential vanilloid 1 (TRPV1) channel, which contributes to pressure sensing and Ca(2+)-dependent cell death in other systems, also contributes to pressure-induced RGC death and whether this contribution involves Ca(2+).

METHODS

trpv1 mRNA expression in RGCs was probed with the use of PCR and TRPV1 protein localization through immunocytochemistry. Subunit-specific antagonism (iodo-resiniferatoxin) and agonism (capsaicin) were used to probe how TRPV1 activation affects the survival of isolated RGCs at ambient and elevated hydrostatic pressure (+70 mm Hg). Finally, for RGCs under pressure, the authors tested whether EGTA chelation of Ca(2+) improves survival and whether, with the Ca(2+) dye Fluo-4 AM, TRPV1 contributes to increased intracellular Ca(2+).

RESULTS

RGCs express trpv1 mRNA, with robust TRPV1 protein localization to the cell body and axon. For isolated RGCs under pressure, TRPV1 antagonism increased cell density and reduced apoptosis to ambient levels (P <or= 0.05), whereas for RGCs at ambient pressure, TRPV1 agonism reduced density and increased apoptosis to levels for elevated pressure (P <or= 0.01). Chelation of extracellular Ca(2+) reduced RGC apoptosis at elevated pressure by nearly twofold (P <or= 0.01). Exposure to elevated hydrostatic pressure induced a fourfold increase in RGC intracellular Ca(2+) that was reduced by half with TRPV1 antagonism. Finally, in the DBA/2 mouse model of glaucoma, levels of TRPV1 in RGCs increased with elevated IOP.

CONCLUSIONS

RGC apoptosis induced by elevated hydrostatic pressure arises substantially through TRPV1, likely through the influx of extracellular Ca(2+).

Reduced retina microglial activation and improved optic nerve integrity with minocycline treatment in the DBA/2J mouse model of glaucoma

PURPOSE

In the context of the retinal ganglion cell (RGC) axon degeneration in the optic nerve that occurs in glaucoma, microglia become activated, then phagocytic, and redistribute in the optic nerve head. The authors investigated the potential contribution of retinal microglia activation to glaucoma progression in the DBA/2J chronic mouse glaucoma model.

METHODS

The authors treated 6-week-old DBA/2J mice for 25 weeks with minocycline, a tetracycline derivative known to reduce microglia activation and to improve neuronal survival in other models of neurodegenerative disease. They quantified RGC numbers and characterized microglia activation, gliosis, and both axonal integrity and retrograde tracer transport by RGCs in mice systemically treated with minocycline or vehicle only.

RESULTS

Minocycline reduced microglial activation and improved RGC axonal transport and integrity, yet it had no effect on the characteristic age-related ocular changes that lead to chronically elevated pressure and did not alter Müller or astrocyte gliosis. Specifically, minocycline increased the fraction of microglia with resting ramified morphology and reduced levels of Iba1 mRNA and protein, a microglia-specific calcium ligand linked to activation. The reduction in microglial activation was coupled to significant improvement in RGC axonal transport, as measured by neuronal retrograde tracing from the superior colliculus. Finally, minocycline treatment significantly decoupled RGC axon loss from increased intraocular pressure.

CONCLUSIONS

These observations suggest that in glaucoma, retina and optic nerve head microglia activation may be a factor in the early decline in function of the optic nerve and its subsequent degeneration.

Manganese-enhanced MRI of the DBA/2J mouse model of hereditary glaucoma

PURPOSE

To test the hypothesis that manganese-enhanced magnetic resonance imaging (MEMRI) is a sensitive approach for measuring of age-related ocular changes in experimental pigmentary glaucoma.

METHODS

Four groups of light-adapted mice were studied using MEMRI: young (2-3 months), C57BL/6 (negative controls), and DBA/2J mice and aged (10-11 months) C57BL/6 and DBA/2J mice. In all mice, eye perimeter, optic nerve head width, iridocorneal angle, ciliary body area, and total and inner retinal thickness, and a surrogate of retinal ion regulation (intraretinal uptake of manganese) were assessed from MEMRI data and compared. Axon counts were obtained from optic nerves harvested from MEMRI-assessed eyes.

RESULTS

As the C57BL/6 and DBA/2J mice aged, differential and significant changes in ocular perimeter, retinal thickness, iridocorneal angle, ciliary body area, and optic nerve head width were readily measured from MEMRI data (P < 0.05). In C57BL/6 mice, only inner retinal thickness and perimeter were correlated. In DBA/2J mice, ocular perimeter was correlated with total and inner retinal thickness, ciliary body area, optic nerve head width, and iridocorneal angle. Comparison of young and aged mice revealed a subnormal intraretinal manganese uptake (P < 0.05) in aged DBA/2J mice, but not in aged C57BL/6 mice. Manganese uptake did not correlate with the ocular perimeter. Axon density in the optic nerve correlated with MEMRI-measured optic nerve head width (P < 0.05).

CONCLUSIONS

These studies provide a baseline of noninvasive MEMRI-detectable changes associated with age in a common animal model of hereditary glaucoma that may be useful in the longitudinal evaluation of therapeutic success.

Contribution of TRPV1 to microglia-derived IL-6 and NFkappaB translocation with elevated hydrostatic pressure

PURPOSE

The authors investigated the contributions of the transient receptor potential vanilloid-1 receptor (TRPV1) and Ca(2+) to microglial IL-6 and nuclear factor kappa B (NFkappaB) translocation with elevated hydrostatic pressure.

METHODS

The authors first examined IL-6 colocalization with the microglia marker Iba-1 in the DBA/2 mouse model of glaucoma to establish relevance. They isolated microglia from rat retina and maintained them at ambient or elevated (+70 mm Hg) hydrostatic pressure in vitro and used ELISA and immunocytochemistry to measure changes in the IL-6 concentration and NFkappaB translocation induced by the Ca(2+) chelator EGTA, the broad-spectrum Ca(2+) channel inhibitor ruthenium red, and the TRPV1 antagonist iodo-resiniferatoxin (I-RTX). They applied the Ca(2+) dye Fluo-4 AM to measure changes in intracellular Ca(2+) at elevated pressure induced by I-RTX and confirmed TRPV1 expression in microglia using PCR and immunocytochemistry.

RESULTS

In DBA/2 retina, elevated intraocular pressure increased microglial IL-6 in the ganglion cell layer. Elevated hydrostatic pressure (24 hours) increased microglial IL-6 release, cytosolic NFkappaB, and NFkappaB translocation in vitro. These effects were reduced substantially by EGTA and ruthenium red. Antagonism of TRPV1 in microglia partially inhibited pressure-induced increases in IL-6 release and NFkappaB translocation. Brief elevated pressure (1 hour) induced a significant increase in microglial intracellular Ca(2+) that was partially attenuated by TRPV1 antagonism.

CONCLUSIONS

Elevated pressure induces an influx of extracellular Ca(2+) in retinal microglia that precedes the activation of NFkappaB and the subsequent production and release of IL-6 and is at least partially dependent on the activation of TRPV1 and other ruthenium red-sensitive channels.

Expression of genes encoding glutamate receptors and transporters in rod and cone bipolar cells of the primate retina determined by single-cell polymerase chain reaction

PURPOSE

Light signals from rod and cone photoreceptors traverse distinct types of second-order, bipolar neurons that carry these signals from the outer to inner retina. Anatomic and physiologic studies suggest that the specialization of rod and cone bipolar cells involves the differential expression of proteins involved in glutamatergic signaling. In a previous study, we compared the expression of genes for the AMPA- (GluR1-4) and kainate-sensitive (GluR5-7, KA1-2) ionotropic glutamate receptors, the metabotropic glutamate receptors (mGluR1-8), and five non-vesicular glutamate transporters (EAAT1-5) in full-complement cDNA constructed from fresh and aldehyde-fixed macaque retina using a technique suitable for amplification of a variety of differentially expressed transcripts. Here we apply the same protocol to compare expression of these genes in cDNA constructed from single rod and cone bipolar cells previously-labeled for morphological identification in fixed slices of macaque retina.

METHODS

We used immunocytochemical labeling and unique morphological features in lightly fixed slices of macaque retina to target the rod bipolar or the DB3 cone OFF bipolar cell. Under visual control, we used a micropipette to target and extract labeled cells, and we isolated mRNA from each through enzymatic digestion. Full-length cDNA was synthesized using 3'-end amplification (TPEA) PCR, in which the highly diverse 3' regions were amplified indiscriminately to ensure detection of both high and low abundance genes. We used gene-specific RT-PCR to probe the cDNA of each bipolar cell both for expression of known genes to confirm cell identification as well as expression of genes encoding glutamate receptors GluR1-7, KA1-2, and mGluR1-8 and for transporters EAAT1-5.

RESULTS

Of 27 rod bipolar cells confirmed to express the genes for the a subunit of protein kinase C, mGluR6, and its G protein Galpha(o), 26 expressed at least one AMPA GluR subunit gene, 16 expressed at least two, and nine expressed three or more. Nearly every cell expressed the GluR4 gene (23/27), followed by GluR2 (16/27) and GluR1 (11/27). In addition to mGluR6, 20/27 cells also expressed the mGluR3 gene. Nearly every rod bipolar cell also expressed the genes for the EAAT2 (23/27) and EAAT4 (21/27) transporters. Of 26 DB3 cells confirmed by expression of calbindin D-28 and absence of GAD-65/67, each expressed the gene for the AMPA subunit GluR4, followed by GluR2 (22/26), and GluR1 (15/26), the only kainate subunit gene expressed was GluR6 (18/26). Nearly every DB3 cell also expressed the gene for the EAAT2 transporter (25/26), but no others.

CONCLUSIONS

Rod bipolar cells in the Macaca monkey retina expressed not only the mGluR6 gene, a subunit necessary for transmission of light-ON signals, but also nearly always GluR4 in combination with the glutamate transporter EAAT4 (21/27 cells). The DB3 cell involved in processing light-OFF signals from cones expressed most highly the combination of GluR4 and the transporter EAAT2 (25/26). These results suggest that glutamatergic signaling in rod and cone circuits in the primate retina depends upon complex molecular interactions, involving not only multiple glutamate receptor subunits, but also glutamate transporters. Our data demonstrate a consistent primary pattern for each cell type with subtle variability involving other genes. Thus, like neuronal cell types in other brain regions, morphological and physiologic homogeneity among retinal bipolar cell types does not exclude variations in expression that could serve to adjust the stimulus-response profile of each cell.

Manganese-enhanced MRI studies of alterations of intraretinal ion demand in models of ocular injury

PURPOSE

To provide proof-of-concept that the extent of intraretinal manganese uptake after systemic MnCl(2) injection, detected with manganese-enhanced MRI (MEMRI), assesses alterations in intraretinal ion demand in models of ocular insult.

METHODS

In Sprague-Dawley rats, retinal ion demand and thickness were measured from MEMRI data collected before, 4 hours after, or 1, 3, and 7 days after intraperitoneal injection of MnCl(2). Choroidal contribution or blood-retinal barrier permeability surface area product (BRB PS') was determined using MRI after Gd-DTPA injection. Ocular injury was evaluated 24 hours after intravitreal injection of phosphate-buffered saline (PBS, vehicle) or PBS + ouabain, or after intraperitoneal injection of sodium iodate. Manganese retinal toxicity was assessed by comparing full-field, white-flash electroretinographic (ERG) data obtained before and after systemic MnCl(2) administration. Rat choroidal thickness was measured from cross-sections prepared from paraformaldehyde-perfused adult rats.

RESULTS

Comparing pre- and post-Gd-DTPA images demonstrated minimal choroidal contribution to intraretinal analysis. Intraretinal signal intensity returned to baseline by 7 days after MnCl(2) injection. After ouabain injection, receptor and postreceptor uptake of manganese were subnormal (P < 0.05). After sodium iodate exposure, intraretinal manganese uptake was supernormal (P < 0.05) and did not increase with increasing BRB PS'. ERG data did not show any effect of MnCl(2) on photoreceptor a-wave and postreceptor b-wave relative to baseline at either observation time.

CONCLUSIONS

MEMRI measurements of uptake of systemically administered and nontoxic doses of manganese appear to be a powerful approach for measuring alteration in intraretinal ion demand in models of ocular injury.

Microcircuitry for two types of achromatic ganglion cell in primate fovea

Synaptic circuits in primate fovea have been quantified for midget/parvocellular ganglion cells. Here, based on partial reconstructions from serial electron micrographs, we quantify synaptic circuits for two other types of ganglion cell: the familiar parasol/magnocellular cell and a smaller type, termed "garland." The excitatory circuits both derive from two types of OFF diffuse cone bipolar cell, DB3 and DB2, which collected unselectively from at least 6 +/- 1 cones, including the S type. Cone contacts to DB3 dendrites were usually located between neighboring triads, whereas half of the cone contacts to DB2 were triad associated. Ribbon outputs were as follows: DB3, 69 +/- 5; DB2, 48 +/- 4. A complete parasol cell (30 microm dendritic field diameter) would collect from approximately 50 cones via approximately 120 bipolar and approximately 85 amacrine contacts; a complete garland cell (25 microm dendritic field) would collect from approximately 40 cones via approximately 75 bipolar and approximately 145 amacrine contacts. The bipolar types contributed differently: the parasol cell received most contacts (60%) from DB3, whereas the garland cell received most contacts (67%) from DB2. We hypothesize that DB3 is a transient bipolar cell and that DB2 is sustained. This would be consistent with their relative inputs to the brisk-transient (parasol) ganglion cell. The garland cell, with its high proportion of DB2 inputs plus its high proportion of amacrine synapses (70%) and dense mosaic, might correspond to the local-edge cell in nonprimate retinas, which serves finer acuity at low temporal frequencies. The convergence of S cones onto both types could contribute S-cone input for cortical areas primary visual cortex and the middle temporal area.

An ultrastructural study of the pathology of the retinal pigment epithelium, Bruch's membrane, and the choriocapillaris in the aged Fischer 344 rat

PURPOSE

The neural retinal degeneration in the aging Fischer 344 (F344) rat has been previously characterized. Here we describe the ultrastructural changes that occur in the retinal pigment epithelium (RPE), Bruch's membrane, and choriocapillaris in the periphery of the aged Fischer 344 rat.

METHODS

F344 eyes from 24-month-old animals (n = 4 animals, 8 eyes) were fixed and embedded for ultrastructural study. Serial mid-sagittal sections were taken from the superior peripheral retinas within 300 microm of the ora serrata. Pathology within the RPE, Bruch's membrane, and choriocapillaris was described.

RESULTS

Progressive changes were seen in the RPE/Bruch's/choriocapillaris complex, increasing anteriorly as the ora serrata was approached. Early pathology of the RPE included increased number of basal infoldings, increased number of phagolysosomes and lipofuscin deposits, attenuation, inclusion of vasculature, vesicle formation, and whirling extensions of the basement membrane into the cytoplasm. Bruch's membrane showed spots of considerable thinning, but most prominent was the nodular thickening. The choriocapillaris was found to have severe endothelial degeneration and transformation to fibrous tissue in the most severely affected regions. Lipofuscin was also found in areas of degenerated choriocapillaris.

CONCLUSIONS

Prior work focused on the neural retina, documented photoreceptor cell loss, and showed that Müller cell changes preceded that loss in the periphery of the F344 rat. It is now evident that the pathology in the RPE/Bruch's membrane/choriocapillaris complex may also be a critical component of the overall degenerative process. A possible mechanism for the extensive peripheral retinal degeneration in the F344 is presented.

Pressure-Induced Regulation of IL-6 in Retinal Glial Cells: Involvement of the Ubiquitin/Proteasome Pathway and NFκB

PURPOSE

To investigate how hydrostatic pressure influences regulation of interleukin (IL)-6 by retinal glia and whether this regulation is associated with the ubiquitin/proteasome pathway (UPP) and activation of the transcription factor nuclear factor (NF)kappaB.

METHODS

Astrocytes and microglia isolated from rat retina were maintained in vitro, and the IL-6 concentration in the media at ambient and elevated pressure were compared, with and without the proteasome inhibitor MG132 (10 microM). Immunocytochemistry was used to correlate translocation of NFkappaB with pressure.

RESULTS

Exposure to elevated pressure for 24 hours maximally altered the concentration of media IL-6 of glia cultures, where IL-6 concentrations decreased in astrocyte cultures and increased in microglia cultures. These pressure-induced changes in IL-6 were largely insensitive to MG132 in astrocytes, but were largely MG132-sensitive in microglia. Like IL-6 regulation, pressure-induced activation of NFkappaB also differed between the two glial cell types, where nuclear localization of NFkappaB was transient in astrocytes, but sustained in microglia. Elevated pressure also increased MG132-sensitive expression of IL-6 mRNA by microglia.

CONCLUSIONS

Though pressure-induced regulation of IL-6 by astrocytes is preceded by NFkappaB translocation, it is not altered by MG132 and therefore is not likely to be regulated by NFkappaB or the UPP. In contrast, pressure-induced regulation of IL-6 protein and mRNA by microglia is preceded by NFkappaB translocation and is sensitive to MG132. Together with precedence in the literature, these data suggest that pressure-induced activation of the UPP leads to transcription of IL-6 driven by NFkappaB.

Expression and sequences of genes encoding glutamate receptors and transporters in primate retina determined using 3'-end amplification polymerase chain reaction

PURPOSE

Our long-term goal is to compare how expression of glutamate receptor and non-vesicular transporter subunits differs between single neurons in the primate retina. Here we set out to ascertain general expression in the retina of Macaca fascicularis using a robust technique suitable for both levels of analysis. We constructed full-complement cDNAs from whole retina RNA using a protocol optimized for detection of even low-abundance transcripts and for transcripts of various lengths. We probed these libraries for expression of genes encoding the AMPA- (GluR1-4) and kainate-sensitive (GluR5-7, KA1-2) ionotropic glutamate receptors, the metabotropic glutamate receptors (mGluR1-8), and five non-vesicular glutamate transporters (EAAT1-5) in the macaque retina and brain and determined large portions of coding sequences for each. We also asked whether each gene can be detected in cDNA generated from a limited amount of RNA extracted from an aldehyde-fixed retinal slice, a technique useful for probing gene expression in tissue used for histological studies.

METHODS

We constructed full-length cDNA from RNA harvested from the macaque retina using a modified version of the 3'-end amplification (TPEA) technique of Dixon et al. With this technique, the 3' region is amplified arbitrarily using multiple primers to produce amplified cDNA containing as diverse and complete sample of genes as possible. We probed the cDNA for expression of glutamate receptors and non-vesicular transporters using gene-specific RT-PCR and assembled sequences from the reaction products using a series of overlapping primer pairs. We also used TPEA to compare expression in a small amount of RNA extracted from a fixed retinal slice.

RESULTS

Macaque retinal cDNA created using TPEA contains a high abundance of transcripts of various lengths. Gene-specific PCR using primers designed against human sequences indicates expression for all GluR, mGluR, and non-vesicular transporter subunits for which we probed. Expression patterns were similar between two different macaque retinas, but different in brain. Several differences also exist between macaque and human brain. For some subunits, this is the first demonstration of expression in the macaque retina. The expression pattern obtained probing cDNA libraries generated from fixed tissue RNA with a different primer set was similar to that for fresh tissue. We sequenced between 1865 (mGluR8a) and 3697 (mGluR1) total nucleotides for each macaque gene and obtained complete coding sequences for GluR1-7, mGluR3 and mGluR4. A comparison with the corresponding human sequences reveal that the coding region of macaque GluR6 demonstrates the highest homology with only 26 nucleotide substitutions (99% homology), while mGluR4 demonstrates the lowest with 68 substitutions (97.5%).

CONCLUSIONS

Neural tissue cDNA created using TPEA contains diverse transcripts of varying size, abundance, and homology. We established using TPEA the expression of all nine AMPA- and kainate-sensitive GluRs, all eight mGluRs and all five non-vesicular glutamate transporters and found that expression differs between macaque retina, macaque brain and human brain. The nucleotide sequences for the coding regions differed moderately between the human and macaque genes. We also found a similar expression pattern in a smaller amount of RNA extracted from a fixed retinal slice. Thus, this technique could be useful for comparing gene expression in cells extracted from fixed tissue pre-labeled using specific markers.

Interleukin-6 protects retinal ganglion cells from pressure-induced death

PURPOSE

The response of retinal ganglion cells (RGCs) to ocular pressure in glaucoma likely involves signals from astrocytes and microglia. How glia-derived factors influence RGC survival at ambient and elevated pressure and whether the inflammatory cytokine interleukin-6 (IL-6) is a contributing factor were investigated.

METHODS

Primary cultures of retinal astrocytes, microglia, and RGCs were prepared using immunomagnetic separation. Comparisons were made of RGC survival at ambient and elevated pressure (+70 mm Hg) and with pressure-conditioned medium from glia with, and depleted of, IL-6.

RESULTS

Pressure elevated for 24 to 48 hours reduced RGC density, increased TUNEL labeling, and upregulated several apoptotic genes, including the early immediate genes c-jun and jun-B. Pressure-conditioned medium from astrocytes reduced RGC survival another 38%, while microglia medium returned RGC survival to ambient levels. These effects were unrelated to IL-6 in microglia medium. Neither astrocyte- nor microglia-conditioned medium affected ambient RGC survival unless depleted of IL-6, which induced a 63% and a 18% decrease in RGCs, respectively. Recombinant IL-6 equivalent to levels in glia-conditioned medium doubled RGC survival at elevated pressure.

CONCLUSIONS

For RGCs at ambient pressure, IL-6 secreted from astrocytes and microglia under pressure is adequate to abate other proapoptotic signals from these glia. For RGCs challenged by elevated pressure, decreased IL-6 in astrocyte medium is insufficient to counteract these signals. Increased IL-6 in microglia medium counters not only proapoptotic signals from these cells but also the pressure-induced apoptotic cascade intrinsic to RGCs.

Quantitative correlation of optic nerve pathology with ocular pressure and corneal thickness in the DBA/2 mouse model of glaucoma

PURPOSE

To investigate quantitatively the relationships between elevated intraocular pressure (IOP), axonal loss, and corneal thickness in the DBA/2 mouse model of glaucoma, to understand better how these factors contribute to disease progression.

METHODS

IOP was measured with a handheld tonometer (Tono-Pen; Medtronic Solan, Jacksonville, FL) in 195 to 446 eyes of mice 2 to 10 months of age sampled from a colony of 400 DBA/2 mice. From a group of 24 eyes at 4, 9, and 10 months of age, correlations were determined between the density and number of RGC axons, corneal thickness, and IOP.

RESULTS

Mean IOP levels in the colony were 15 to 16 mm Hg at 2 months of age and rose almost linearly at a rate of 0.9 mm Hg/mo before reaching 22 to 23 mm Hg at 10 months. Both the density and number of axons decreased with increasing average lifetime IOP. IOP variation within age groups strongly correlated with density. Age-matched mice with lower mean IOP had greater preservation of axons in the optic nerve. Elevated IOP was accompanied by increased corneal thickness at the limbus. Surprisingly, corneal thickness was a strong predictor of axonal density (r2 = -0.75), regardless of age.

CONCLUSIONS

IOP increased with age in most, but not all, DBA/2 mice. In age-matched mice, differences in IOP corresponded to differences in axonal density and number. In young mice with elevated IOP, the loss of axons resembled that of older animals with similar IOP. Whether corneal thickness is a byproduct of elevated IOP remains unknown, but it may be useful as an index of optic nerve degeneration.

Microarray analysis of retinal gene expression in the DBA/2J model of glaucoma

PURPOSE

The DBA/2J mouse is a model for secondary angle-closure glaucoma, due to iris atrophy and pigment dispersion, which ultimately lead to increased intraocular pressure (IOP). The study was undertaken to correlate changes in retinal gene expression with IOP elevation by performing microarray analysis of retinal RNA from DBA/2J mice at 3 months before disease onset and at 8 months after IOP elevation.

METHODS

IOP was monitored monthly in DBA/2J animals, and animals with normal (3 months) or elevated IOP (8 months) were identified. RNA was prepared from three individual retinas at each age, and the RNA was amplified and used to generate biotin-labeled probe for high-density mouse gene microarrays (U430.2; Affymetrix, Santa Clara, CA). A subset of genes was selected for confirmation by quantitative RT-PCR, by using independent retina samples from DBA/2J animals at 3, 5, and 8 months of age and compared to retinas from C57BL/6J control animals at 3 and 8 months.

RESULTS

There were changes in expression of 68 genes, with 32 genes increasing and 36 genes decreasing at 8 months versus 3 months. Upregulated genes were associated with immune response, glial activation, signaling, and gene expression, whereas downregulated genes included multiple crystallin genes. Significant changes in nine upregulated genes and two downregulated genes were confirmed by quantitative RT-PCR, with some showing changes in expression by 5 months.

CONCLUSIONS

DBA/2J retina shows evidence of glial activation and an immune-related response after IOP elevation, similar to what has been reported after acute elevation of IOP in other models.

Localization of ionotropic glutamate receptors to invaginating dendrites at the cone synapse in primate retina

The separation of OFF pathways that signal light decrements from ON pathways that signal light increments occurs at the first retinal synapse. The dendrites of OFF bipolar cells abut the cone pedicle at basal positions distal to the site of glutamate release and express ligand-gated or ionotropic glutamate receptors (GluR). The dendrites of ON bipolar cells penetrate narrow invaginations of the cone pedicle proximal to the site of release and express the G-protein-coupled, metabotropic glutamate receptor, mGluR6. However, recent studies demonstrating the expression of GluR subunits in the rodent rod bipolar cell, known to yield an ON response to light, call this basic segregation of receptors into question. The light-microscopic distribution of many glutamate receptors in the primate retina is now well established. We reexamined their ultrastructural localization in the outer retina of Macaca fascicularis to test systematically whether invaginating dendrites at the cone synapse, presumably from ON bipolar cells, also express one or more ionotropic subunits. Using preembedding immunocytochemistry for electron microscopy, we quantified the distribution of the AMPA-sensitive subunits GluR2/3 and GluR4 and of the kainate-sensitive subunits GluR6/7 across 207 labeled dendrites occupying specific morphological loci at the cone pedicle. We report, in agreement with published investigations, that the majority of labeled processes for GluR2/3 (70%) and GluR4 (67%) either occupy basal positions or arise from horizontal cells. For GluR6/7, we find a significantly lower fraction of labeled processes at these positions (47%). We also find a considerable number of labeled dendrites for GluR2/3 (10%), GluR4 (21%), and GluR6/7 (18%) at invaginating positions. Surprisingly, for each subunit, the remainder of labeled processes corresponds to "fingers" of presynaptic cytoplasm within the cone invagination.

Remembering Bob Rodieck: 1937-2003

In this special issue of Visual Neuroscience , we present a series of papers to honor the life and career of Robert William Rodieck, who passed away at his home in Seattle on September 30, 2003. Rodieck held the E.K. Bishop Professorship in Ophthalmology at the University of Washington Medical Center from 1978-1997. Known to everyone as "Bob," he leaves behind an intellectual legacy often admired by his colleagues and friends for its scope, intensity, and empathy for what was beautiful in the object of his studies.

Morphological identification of ganglion cells expressing the alpha subunit of type II calmodulin-dependent protein kinase in the macaque retina

Expression of the alpha subunit of type II calmodulin-dependent protein kinase (alphaCamKII) distinguishes the koniocellular neurons of the primate lateral geniculate nucleus (LGN) from the primary parvo- and magnocellular neurons, but whether the same neurochemical signature distinguishes the retinal ganglion cells providing them input is not known. We find that, in the retina, alphaCamKII expression also differentiates two primary groups of ganglion cell, both characterized by broad, sparsely branching dendritic trees and cell bodies intermediate in size between the parvo- and magnocellular-projecting ganglion cells. Cells in the first group have three or four primary dendrites, a thick axon, and a rounded cell body and likely are made up of multiple types. In contrast, ganglion cells in the second group demonstrate a highly regular morphology, with strictly two primary dendrites and a thinner axon emanating from a smaller, elliptical cell body. This cell resembles the "large sparse" ganglion cell identified by others in retrograde labeling from the LGN and represents about 2% of all ganglion cells. In the optic nerve, alphaCamKII+ axons are also intermediate in size and form a bimodal distribution, correlating with the axonal sizes of the two groups of ganglion cell. For the LGN, we describe a group of alphaCamKII+ axon terminals with morphology consistent with terminals from retinal ganglion cells. These terminals form long, filamentous contacts with alphaCamKII+ relay cells and increase in frequency from the dorsal to the ventral koniocellular regions. Our results indicate that ganglion cells expressing alphaCamKII represent multiple projections to the brain, at least one of which provides input to one or more koniocellular regions of the LGN.

Light scattering from intact cells reports oxidative-stress-induced mitochondrial swelling

Angularly resolved light scattering measurements were performed on suspensions of EMT6 cells and on mitochondria isolated from rabbit liver. Mie theory analysis of the scattering from intact cells indicated that mitochondrial-sized organelles dominated scattering in the range 5-90 degrees . This interpretation was supported by the analysis of scattering from isolated mitochondria. Intact cells were subjected to oxidative stress by photodynamic insult. After 3 h of incubation in the heme precursor aminolevulinic acid hexylester, EMT6 cells accumulated abundant protoporphyrin IX, an endogenous photosensitizer formed in mitochondria. Irradiation of aminolevulinic acid/protoporphyrin IX-sensitized cells with 10 J cm(-2) of 514 nm light led to pronounced changes in angularly resolved light scattering consistent with mitochondrial swelling. Electron microscopy of similarly treated EMT6 cell monolayers showed significant changes in mitochondrial morphology, which included distension of the outer unit membrane and bloating of the internal mitochondrial compartment. Informed by these electron microscopy results, we implemented a coated sphere model to interpret the scattering from intact cells subjected to oxidative stress. The coated sphere interpretation was compatible with the scattering measurements from these cells, whereas simpler Mie theory models based on homogenous swelling were dramatically unsuccessful. Thus, in this system, angularly resolved light scattering reports oxidative-stress-induced changes in mitochondrial morphology.

Optic nerve degeneration in a murine model of juvenile ceroid lipofuscinosis

PURPOSE

To investigate optic nerve degeneration associated with CLN3 deficiency in a murine model of juvenile neuronal ceroid lipofuscinosis (Batten disease).

METHODS

Using light and electron microscopy, the density and diameter of axons and the thickness of myelin in optic nerve were compared between age-matched cln3 knock-out (cln3-/-) and wild-type (129ev/TAC) mice. Western blot analysis was used to assay expression of Cln3 in mouse and primate retina and optic nerve.

RESULTS

Morphologically identified mast cells were present in the meningeal sheaths surrounding the cln3-/- nerve and in the nerve itself. The cln3-/- optic nerve exhibited an overall loss of uniformity and integrity. Axon density in cln3-/- optic nerve was only 64% of that in wild-type optic nerve (P < 0.01). Accounting for differences in axon density, the diameter of axons in cln3-/- optic nerve was 1.2 times greater than in wild-type optic nerve (P < 0.01). Electron micrographs revealed large spaces between axons and 32% thinner myelin surrounding axons in cln3-/- mice than in wild type (P < 0.01). Western blot analysis demonstrated that Cln3 was expressed in retinas and optic nerves of mouse and primate.

CONCLUSIONS

The presence of apparent mast cells in cln3-/- optic nerve suggests compromise of the blood-brain barrier. The absence of Cln3 causes loss of axons, axonal hypertrophy, and a reduction in myelination of retinal ganglion cells. Furthermore, expression of CLN3 in mouse and primate optic nerve links degeneration to loss of Cln3.

Localization of kainate receptors to the presynaptic active zone of the rod photoreceptor in primate retina

Visual information is encoded at the photoreceptor synapse by modulation of the tonic release of glutamate from one or more electron-dense ribbons. This release is highest in the dark, when photoreceptors are depolarized, and decreases in grades when photoreceptors hyperpolarize with increasing light. Functional diversity between neurons postsynaptic at the synaptic ribbon arises in part from differential expression of both metabotropic (G-protein-gated) and ionotropic (ligand-gated) glutamate receptor. In the brain, different subunits also modulate the presynaptic active zone. In hippocampus, ionotropic kainate receptors localize to the presynaptic membrane of glutamatergic axon terminals and facilitate depolarization of the synapse (e.g. Lauri et al., 2001). Such facilitation may be helpful in the retina, where consistent depolarization of the photoreceptor axon terminal is necessary to maintain glutamate release in the dark. We investigated whether such a mechanism could be present in primate retina by using electron microscopy to examine the localization of the kainate subunits GluR6/7 at the rod axon terminal, where only a single ribbon synapse mediates glutamate release. We scored 54 rod axon terminals whose postsynaptic space contained one or more GluR6/7-labeled processes and traced these processes through serial sections to determine their identity. Of 68 labeled processes, 63% originated from narrow "fingers" of cytoplasm extending from the presynaptic axon terminal into the postsynaptic cleft. Each rod terminal typically inserts 4-6 presynaptic fingers, and we scored several instances where multiple fingers contained label. Such consistency suggests that each presynaptic finger expresses GluR6/7. The physiological properties of kainate receptors and the geometry of the rod axon terminal suggest that presynaptic GluR6/7 could provide a steady inward current to maintain consistent depolarization of the rod synapse in the long intervals between photons in the dark.

High-resolution localization of clathrin assembly protein AP180 in the presynaptic terminals of mammalian neurons

Synaptic vesicles (SVs) assemble at the presynaptic compartment through a clathrin-dependent mechanism that involves one or more assembly proteins (APs). The assembly protein AP180 is especially efficient at facilitating clathrin cage formation, but its precise ultrastructural localization in neurons is unknown. Using immunoelectron microscopy, we demonstrate the presynaptic localization of AP180 in axon terminals of rat cerebellar neurons. In contrast, the assembly protein AP2 was associated with both the presynaptic plasma membrane and the cytosolic side of the membrane at postsynaptic and extrasynaptic sites. Furthermore, ultrastructural analysis of primate retina showed that AP180 immunoreactivity was preferentially and highly enriched at ribbon synapses, where glutamate is released tonically at high levels and rapid vesicle turnover is essential. To maintain functional synaptic transmission, neurotransmitter-filled SVs must be readily available, and this requires proper reassembly of new vesicles. The expression of AP180, in addition to AP-2, in the clathrin-mediated endocytic pathway might add another level of control to SV reformation for efficient assembly of clathrin, effectively controlling the size of assembled vesicles and faithfully recovering SV-specific components.

Seeing with S cones

The S cone is highly conserved across mammalian species, sampling the retinal image with less spatial frequency than other cone photoreceptors. In human and monkey retina, the S cone represents typically 5-10% of the cone mosaic and distributes in a quasi-regular fashion over most of the retina. In the fovea, the S cone mosaic recedes from a central "S-free" zone whose size depends on the optics of the eye for a particular primate species: the smaller the eye, the less extreme the blurring of short wavelengths, and the smaller the zone. In the human retina, the density of the S mosaic predicts well the spatial acuity for S-isolating targets across the retina. This acuity is likely supported by a bistratified retinal ganglion cell whose spatial density is about that of the S cone. The dendrites of this cell collect a depolarizing signal from S cones that opposes a summed signal from M and L cones. The source of this depolarizing signal is a specialized circuit that begins with expression of the L-AP4 or mGluR6 glutamate receptor at the S cone-->bipolar cell synapse. The pre-synaptic circuitry of this bistratified ganglion cell is consistent with its S-ON/(M+L)-OFF physiological receptive field and with a role for the ganglion cell in blue/yellow color discrimination. The S cone also provides synapses to other types of retinal circuit that may underlie a contribution to the cortical areas involved with motion discrimination.

Representation of cone signals in the primate retina

Vision begins with specialized retinal circuits that encode diverse types of information. For Old World primates, these circuits sample three submosaics formed by cone photoreceptors sensitive to short, middle, and long wavelengths. For spatial acuity, the photon catch between any two cones is compared for discrimination of patterns as fine as the cone mosaic. For color vision, the photon catch between different cone types is compared for discrimination of fine spectral differences on the basis of hue. The retinal circuits for these two tasks differ at the synaptic level to form distinct representations of signals from the cone mosaic.

Neuronal chemistry and functional organization in the primate visual system

Beginning with the first step of visual processing and proceeding outward from that point, the neurons involved in different aspects of vision are distinct. Stated simply, neurons doing different things look different. They often display distinct morphological features and they usually express different molecules. In addition, neurons that perform a common function usually aggregate together to form recognizable layers or compartments that can be studied in isolation because they are neurochemically distinct. Here is found, then, a junction of two major domains in neuroscience research, as discovery of molecular diversity among neurons is exploited to study organization and function of the primate visual system.

Microcircuitry and mosaic of a blue-yellow ganglion cell in the primate retina

Perception of hue is opponent, involving the antagonistic comparison of signals from different cone types. For blue versus yellow opponency, the antagonism is first evident at a ganglion cell with firing that increases to stimulation of short wavelength-sensitive (S) cones and decreases to stimulation of middle wavelength-sensitive (M) and long wavelength-sensitive (L) cones. This ganglion cell, termed blue-yellow (B-Y), has a distinctive morphology with dendrites in both ON and OFF strata of the inner plexiform layer (Dacey and Lee, 1994). Here we report the synaptic circuitry of the cell and its spatial density. Reconstructing neurons in macaque fovea from electron micrographs of serial sections, we identified six ganglion cells that branch in both strata and have similar circuitry. In the ON stratum each cell collects approximately 33 synapses from bipolar cells traced back exclusively to invaginating contacts from S cones, and in the OFF stratum each cell collects approximately 14 synapses from bipolar cells (types DB2 and DB3) traced to basal synapses from approximately 20 M and L cones. This circuitry predicts that spatially coincident blue-yellow opponency arises at the level of the cone output via expression of different glutamate receptors. S cone stimuli suppress glutamate release onto metabotropic receptors of the S cone bipolar cell dendrite, thereby opening cation channels, whereas M and L cone stimuli suppress glutamate release onto ionotropic glutamate receptors of DB2 and DB3 cell dendrites, thereby closing cation channels. Although the B-Y cell is relatively rare (3% of foveal ganglion cells), its spatial density equals that of the S cone; thus it could support psychophysical discrimination of a blue-yellow grating down to the spatial cutoff of the S cone mosaic.

Foveal cones form basal as well as invaginating junctions with diffuse ON bipolar cells

The response of a mammalian bipolar cell is generally thought to be determined by the location and morphology of synapses from the cone terminal: ON bipolar cells are believed to be depolarized strictly at invaginating contacts and OFF bipolar cells hyperpolarized at basal contacts. This hypothesis was re-investigated in the macaque fovea (1 deg nasal) using electron micrographs of serial sections. We determined the number of invaginating sites available and then identified the contacts to bipolar cells with axons in the ON level of the inner plexiform layer. A cone terminal forms about 20 active zones marked by ribbons. A few active zones house two invaginating dendrites, so there are 22 invaginating sites per cone. A midget ON bipolar cell collects 18 invaginating contacts from one cone, thus only about four invaginating sites remain for diffuse ON bipolar cells. Two diffuse ON cells were reconstructed; each collects about 25 contacts from an estimated 10 cones. Only three or four of these contacts are invaginating; the rest are basal, adjacent to the triad. This suggests that basal contacts can be depolarizing. The distance from the vesicle release site at active zones to an invaginating contact is 140 +/- 40 nm; to a basal contact adjacent to the triad is 500 +/- 160 nm, and to the next nearest basal contact is 950 +/- 370 nm.

Absence of spectrally specific lateral inputs to midget ganglion cells in primate retina

Visual information is conveyed to the brain by the retinal ganglion cells. Midget ganglion cells serve fine spatial vision by summing excitation from a receptive field 'centre', receiving input from a single cone in the central retina, with lateral inhibition from a receptive field 'surround', receiving input from many surrounding cones. Midget ganglion cells are also thought to serve colour opponent vision because the centre excitation is from a cone of one spectral type, while the surround inhibition is from cones of the other type. The two major cone types, middle(M)- and long-(L)wavelength sensitive, are equally numerous and randomly distributed in the primate central retina, so a spectrally homogeneous surround requires that the cells mediating lateral interactions (horizontal or amacrine cells) receive selective input from only one cone type. Horizontal cells cannot do this because they receive input indiscriminately from M and L cones. Here we report that the amacrine cells connected to midget ganglion cells are similarly indiscriminate. The absence of spectral specificity in the inhibitory wiring raises doubt about the involvement of midget ganglion cells in colour vision and suggest that colour opponency may instead be conveyed by a different type of ganglion cell.

M and L cones in macaque fovea connect to midget ganglion cells by different numbers of excitatory synapses

Visual acuity depends on the fine-grained neural image set by the foveal cone mosaic. To preserve this spatial detail, cones transmit through non-divergent pathways: cone-->midget bipolar cell-->midget ganglion cell. Adequate gain must be established along each pathway; crosstalk and sources of variation between pathways must be minimized. These requirements raise fundamental questions regarding the synaptic connections: (1) how many synapses from bipolar to ganglion cell transmit a cone signal and with what degree of crosstalk between adjacent pathways; (2) how accurately these connections are reproduced across the mosaic; and (3) whether the midget circuits for middle (M) and long (L) wavelength sensitive cones are the same. We report here that the midget ganglion cell collects without crosstalk either 28 +/- 4 or 47 +/- 3 midget bipolar synapses. Two cone types are defined by this difference; being about equal in number and distributing randomly in small clusters of like type, they are probably M and L.

Monochromatism determined at a long-wavelength/middle-wavelength cone-antagonistic locus

The foveal increment threshold spectral sensitivity function for a 500 msec raised cosine stimulus without spatial edges exhibits a sharp drop or "notch" in sensitivity that coincides with the wavelength of a long-wavelength adapting field. An appropriate name for this phenomenon is the "Sloan notch", after Louise Sloan, who first observed a notch in a foveal threshold spectrum. We have examined suprathreshold discriminability on both sides of the Sloan notch produced by a 6700 td, 578 nm adapting field. In a temporal two-alternative forced-choice paradigm, a suprathreshold 650 nm low-frequency "standard" stimulus was paired with low-frequency "test" stimuli, of wavelength between 600 and 670 nm and varied intensity; the observer's task was to identify the interval containing the standard. Discriminability of the test and standard typically dropped to chance for some particular test intensity, producing "indiscriminability action spectra", up to 0.7 log units above threshold. Truncated spectra (between about 530 and 560 nm) were also obtained from observers on the middle wavelength side of the Sloan notch, for a 550 nm standard. The indiscriminability action spectra of each observer were identical, up to scaling, with the observer's threshold action spectrum. Analysis of the action spectra shows that the indiscriminable stimuli are rendered equivalent at the input to a neural pathway where L- and M-cone signals converge with opposite sign. We also investigated discriminability in the spectral region containing and immediately surrounding the Sloan notch. Suprathreshold stimuli in the spectral region near the notch produce percepts that are always discriminable from 650 and 550 nm standards (and from one another), and thus we conclude that in this spectral region, perception is mediated in part by a pathway distinct from that which signals the standards. The action spectrum of this latter pathway was estimated with a variant of the discrimination procedure, and found similar to V lambda over the spectral region 575-610 nm.