Astrocytes in the Optic Nerve Are Heterogeneous in Their Reactivity to Glaucomatous Injury

The optic nerve head is thought to be the site of initial injury to retinal ganglion cell injury in glaucoma. In the initial segment of the optic nerve directly behind the globe, the ganglion cell axons are unmyelinated and come into direct contact to astrocytes, suggesting that astrocytes may play a role in the pathology of glaucoma. As in other parts of the CNS, optic nerve head astrocytes respond to injury by characteristic changes in cell morphology and gene expression profile. Using RNA-sequencing of glaucomatous optic nerve heads, single-cell PCR, and an in-vivo assay, we demonstrate that an up-regulation of astrocytic phagocytosis is an early event after the onset of increased intraocular pressure. We also show that astrocytes in the glial lamina of the optic nerve are apparently functionally heterogeneous. At any time, even in naïve nerves, some of the cells show signs of reactivity-process hypertrophy, high phagocytic activity, and expression of genetic markers of reactivity whereas neighboring cells apparently are inactive. A period of increased intraocular pressure moves more astrocytes towards the reactive phenotype; however, some cells remain unreactive even in glaucomatous nerves.

Vitamin C protects retinal ganglion cells via SPP1 in glaucoma and after optic nerve damage

Glaucoma is a common neurodegenerative disorder characterized by retinal ganglion cell death, astrocyte reactivity in the optic nerve, and vision loss. Currently, lowering the intraocular pressure (IOP) is the first-line treatment, but adjuvant neuroprotective approaches would be welcome. Vitamin C possesses neuroprotective activities that are thought to be related to its properties as a co-factor of enzymes and its antioxidant effects. Here, we show that vitamin C promotes a neuroprotective phenotype and increases gene expression related to neurotropic factors, phagocytosis, and mitochondrial ATP production. This effect is dependent on the up-regulation of secreted phosphoprotein 1 (SPP1) in reactive astrocytes via the transcription factor E2F1. SPP1+ astrocytes in turn promote retinal ganglion cell survival in a mouse model of glaucoma. In addition, oral administration of vitamin C lowers the IOP in mice. This study identifies an additional neuroprotective pathway for vitamin C and suggests a potential therapeutic role of vitamin C in neurodegenerative diseases such as glaucoma.

Individual astrocyte morphology in the collagenous lamina cribrosa revealed by multicolor DiOlistic labeling

Astrocytes in the lamina region of the optic nerve head play vital roles in supporting retinal ganglion cell axon health. In glaucoma, these astrocytes are implicated as early responders to stressors, undergoing characteristic changes in cell function as well as cell morphology. Much of what is currently known about individual lamina astrocyte morphology has been learned from rodent models which lack a defining feature of the human optic nerve head, the collagenous lamina cribrosa (LC). Current methods available for evaluation of collagenous LC astrocyte morphology have significant shortcomings. We aimed to evaluate Multicolor DiOlistic labeling (MuDi) as an approach to reveal individual astrocyte morphologies across the collagenous LC. Gold microcarriers were coated with all combinations of three fluorescent cell membrane dyes, DiI, DiD, and DiO, for a total of seven dye combinations. Microcarriers were delivered to 150 μm-thick coronal vibratome slices through the LC of pig, sheep, goat, and monkey eyes via MuDi. Labeled tissues were imaged with confocal and second harmonic generation microscopy to visualize dyed cells and LC collagenous beams, respectively. GFAP labeling of DiOlistically-labeled cells with astrocyte morphologies was used to investigate cell identity. 3D models of astrocytes were created from confocal image stacks for quantification of morphological features. DiOlistic labeling revealed fine details of LC astrocyte morphologies including somas, primary branches, higher-order branches, and end-feet. Labeled cells with astrocyte morphologies were GFAP+. Astrocytes were visible across seven distinct color channels, allowing high labeling density while still distinguishing individual cells from their neighbors. MuDi was capable of revealing tens to hundreds of collagenous LC astrocytes, in situ, with a single application. 3D astrocyte models allowed automated quantification of morphological features including branch number, length, thickness, hierarchy, and straightness as well as Sholl analysis. MuDi labeling provides an opportunity to investigate morphologies of collagenous LC astrocytes, providing both qualitative and quantitative detail, in healthy tissues. This approach may open doors for research of glaucoma, where astrocyte morphological alterations are thought to coincide with key functional changes related to disease progression.

Loss of epigenetic information as a cause of mammalian aging

All living things experience an increase in entropy, manifested as a loss of genetic and epigenetic information. In yeast, epigenetic information is lost over time due to the relocalization of chromatin-modifying proteins to DNA breaks, causing cells to lose their identity, a hallmark of yeast aging. Using a system called "ICE" (inducible changes to the epigenome), we find that the act of faithful DNA repair advances aging at physiological, cognitive, and molecular levels, including erosion of the epigenetic landscape, cellular exdifferentiation, senescence, and advancement of the DNA methylation clock, which can be reversed by OSK-mediated rejuvenation. These data are consistent with the information theory of aging, which states that a loss of epigenetic information is a reversible cause of aging.

Viability of mitochondria-labeled retinal ganglion cells in organotypic retinal explant cultures by two methods

Retinal explant cultures provide a valuable system to study retinal function in vitro. This study established a new retinal explant culture method to prolong the survival of retinal ganglion cells (RGCs). Explants were prepared in two different ways: with or without optic nerve. Retinas from newborn mice that had received an injection of MitoTracker Red into the contralateral superior colliculus to label axonal mitochondria were cultured as organotypic culture for 7 days in vitro. At several time points during the culture, viability of RGCs was assessed by multi-electrode array recording, and morphology by immunohistochemical methods. During the culture, the thickness of the retinal tissue in both groups gradually decreased, however, the structure of the layers of the retina could be identified. Massive apoptosis in the retinal ganglion cell layer (GCL) appeared on the first day of culture, thereafter the number of apoptotic cells decreased. Glial activation was observed throughout the culture, and there was no difference in morphology between the two groups. RGCs loss was exacerbated on 3rdday of culture, and RGCs loss in retinal explants with preserved optic nerve was significantly lower than in retinas that did not preserve the optic nerve. More and longer-lasting mitochondrial signals were observed in the injured area of the optic nerve-preserving explants. Retinal explants provide an invaluable tool for studying retinal function and developing treatments for ocular diseases. The optic nerve-preserving culture helps preserve the integrity of RGCs. The higher number of mitochondria in the nerve-preserving cultures may help maintain viability of RGCs.

Secreted phosphoprotein 1 slows neurodegeneration and rescues visual function in mouse models of aging and glaucoma

Aging causes an irreversible, cumulative decline in neuronal function. Using the visual system as a model, we show that astrocytes play a critical role in maintaining retinal ganglion cell health and that deletion of SPP1 (secreted phosphoprotein 1, or osteopontin) from astrocytes leads to increased vulnerability of ganglion cells to age, elevated intraocular pressure, and traumatic optic nerve damage. Overexpression of SPP1 slows the age-related decline in ganglion cell numbers and is highly protective of visual function in a mouse model of glaucoma. SPP1 acts by promoting phagocytosis and secretion of neurotrophic factors while inhibiting production of neurotoxic and pro-inflammatory factors. SPP1 up-regulates transcription of genes related to oxidative phosphorylation, functionally enhances mitochondrial respiration, and promotes the integrity of mitochondrial microstructure. SPP1 increases intracellular ATP concentration via up-regulation of VDAC1.

Microglia depletion exacerbates retinal ganglion cell loss in a mouse model of glaucoma

To test whether depletion of microglia in the optic nerve head has a beneficial effect on retinal ganglion cell numbers and function, we depleted microglia by oral administration of the CSF1R antagonist PLX5622. Then, ocular hypertension was induced by unilateral injection of magnetic microbeads into the anterior chamber. Visual function was assessed with pattern electroretinography and measurement of the optomotor reflex. Retinal ganglion cell bodies and axons were counted and gene expression patterns in optic nerve head astrocytes were tested on freshly dissociated astrocytes. PLX5622 efficiently depleted microglia in the retina and the optic nerve head, but about 20% of microglia persisted in the myelinated optic nerve proper even after prolonged exposure to the drug. PLX5622 did not affect ganglion cell function by itself. Elevation of the IOP for four weeks led to the expected decrease in visual acuity and pattern ERG amplitude. Microglia ablation did not affect these parameters. Ganglion cell and axon numbers were counted histologically post mortem. Mice in the microglia depletion group showed a moderate but significantly greater loss of ganglion cells than the control group. At four weeks post microbead injection, gene expression patterns in optic nerve head astrocytes are consistent with an A2 (or neuroprotective) pattern. Microglia depletion blunted the up-regulation of A2 genes in astrocytes. In conclusion, microglia depletion is unlikely to protect retinal ganglion cells in early glaucoma.

Lamina cribrosa vessel and collagen beam networks are distinct

Our goal was to analyze the spatial interrelation between vascular and collagen networks in the lamina cribrosa (LC). Specifically, we quantified the percentages of collagen beams with/without vessels and of vessels inside/outside of collagen beams. To do this, the vasculature of six normal monkey eyes was labeled by perfusion post-mortem. After enucleation, coronal cryosections through the LC were imaged using fluorescence and polarized light microscopy to visualize the blood vessels and collagen beams, respectively. The images were registered to form 3D volumes. Beams and vessels were segmented, and their spatial interrelationship was quantified in 3D. We found that 22% of the beams contained a vessel (range 14%-32%), and 21% of vessels were outside beams (13%-36%). Stated differently, 78% of beams did not contain a vessel (68%-86%), and 79% of vessels were inside a beam (64%-87%). Individual monkeys differed significantly in the fraction of vessels outside beams (p < 0.01 by linear mixed effect analysis), but not in the fraction of beams with vessels (p > 0.05). There were no significant differences between contralateral eyes in the percent of beams with vessels and of vessels outside beams (p > 0.05). Our results show that the vascular and collagenous networks of the LC in monkey are clearly distinct, and the historical notions that each LC beam contains a vessel and all vessels are within beams are inaccurate. We postulate that vessels outside beams may be relatively more vulnerable to mechanical compression by elevated IOP than are vessels shielded inside of beams.

Ultrastructural Morphology of the Optic Nerve Head in Aged and Glaucomatous Mice

Purpose

To study age- and intraocular pressure–induced changes in the glial lamina of the murine optic nerve on the ultrastructural level.

Methods

Naïve C57bl/6 mice at various ages spanning the time between early adulthood (3 months) and senescence (30 months) were used in this study. In addition, the intraocular pressure (IOP) was increased in a group of young mice by injection of microbeads into the anterior chamber. The unmyelinated segments of the optic nerve containing the glial lamina were prepared for transmission electron microscopy and imaged at high resolution.

Results

Axon packing density decreased slightly with age. Aging nerves contained higher numbers of enlarged and degenerating axons. Mean axonal diameter and in particular the variance of axonal diameter correlated well with age. Axonal mitochondria also showed age-dependent signs of pathology. The mean diameter of axonal mitochondria increased, and aged axons often contained profiles of mitochondria with very few or no cristae. Astrocytic mitochondria remained normal even in very old nerves. Changes to axons and axonal mitochondria in young glaucomatous nerves were comparable with those of 18- to 30-month-old naïve mice. In addition to axons and mitochondria, aged and glaucomatous nerves showed thickening of the blood vessel basement membranes and increased deposition of basement membrane collagen.

Conclusions

On the ultrastructural level, the effects of age and elevated IOP are quite similar. One month of elevated IOP seems to have as strongly detrimental effects on the nerve as at least 18 months of normal aging.

Astrocytes in the Optic Nerve Head of Glaucomatous Mice Display a Characteristic Reactive Phenotype

Purpose

Optic nerve head astrocytes, a subtype of white-matter astrocytes, become reactive early in the course of glaucoma. It was shown recently that in the DBA/2J mouse model of inherited glaucoma optic nerve astrocytes extend new longitudinal processes into the axon bundles before ganglion cell loss becomes apparent. The present study aims at testing whether this behavior of astrocytes is typical of early glaucomatous damage.

Methods

Mice expressing green fluorescent protein in individual astrocytes were used to evaluate the early response of astrocytes in the glial lamina of the optic nerve head after increasing the IOP using the microbead occlusion method. Tissue sections from the glial lamina were imaged consecutively by confocal and electron microscopy.

Results

Confocal and electron microscope images show that astrocytes close to the myelination transition zone in the hypertensive nerve heads extend new processes that follow the longitudinal axis of the optic nerve and invade axon bundles in the nerve head. Ultrastructurally, the longitudinal processes were largely devoid of subcellular organelles except for degenerating mitochondria.

Conclusions

The longitudinal processes are a common feature of glaucomatous optic nerve astrocytes, whereas they are not observed after traumatic nerve injury. Thus, astrocytes appear to fine-tune their responses to the nature and/or timing of the injury to the neurons that they surround.

Optic nerve astrocyte reactivity protects function in experimental glaucoma and other nerve injuries

Reactive remodeling of optic nerve head astrocytes is consistently observed in glaucoma and other optic nerve injuries. However, it is unknown whether this reactivity is beneficial or harmful for visual function. In this study, we used the Cre recombinase (Cre)-loxP system under regulation of the mouse glial fibrillary acidic protein promoter to knock out the transcription factor signal transducer and activator of transcription 3 (STAT3) from astrocytes and test the effect this has on reactive remodeling, ganglion cell survival, and visual function after experimental glaucoma and nerve crush. After injury, STAT3 knockout mice displayed attenuated astrocyte hypertrophy and reactive remodeling; astrocytes largely maintained their honeycomb organization and glial tubes. These changes were associated with increased loss of ganglion cells and visual function over a 30-day period. Thus, reactive astrocytes play a protective role, preserving visual function. STAT3 signaling is an important mediator of various aspects of the reactive phenotype within optic nerve astrocytes.

Mice Homozygous for a Deletion in the Glaucoma Susceptibility Locus INK4 Show Increased Vulnerability of Retinal Ganglion Cells to Elevated Intraocular Pressure

A genomic region located on chromosome 9p21 is associated with primary open-angle glaucoma and normal tension glaucoma in genome-wide association studies. The genomic region contains the gene for a long noncoding RNA called CDKN2B-AS, two genes that code for cyclin-dependent kinase inhibitors 2A and 2B (CDKN2A/p16(INK4A) and CDKN2B/p15(INK4B)) and an additional protein (p14(ARF)). We used a transgenic mouse model in which 70 kb of murine chromosome 4, syntenic to human chromosome 9p21, are deleted to study whether this deletion leads to a discernible phenotype in ocular structures implicated in glaucoma. Homozygous mice of this strain were previously reported to show persistent hyperplastic primary vitreous. Fundus photography and optical coherence tomography confirmed that finding but showed no abnormalities for heterozygous mice. Optokinetic response, eletroretinogram, and histology indicated that the heterozygous and mutant retinas were normal functionally and morphologically, whereas glial cells were activated in the retina and optic nerve head of mutant eyes. In quantitative PCR, CDKN2B expression was reduced by approximately 50% in the heterozygous mice and by 90% in the homozygous mice, which suggested that the CDKN2B knock down had no deleterious consequences for the retina under normal conditions. However, compared with wild-type and heterozygous animals, the homozygous mice are more vulnerable to retinal ganglion cell loss in response to elevated intraocular pressure.

Synapse Loss and Dendrite Remodeling in a Mouse Model of Glaucoma

It has been hypothesized that synaptic pruning precedes retinal ganglion cell degeneration in glaucoma, causing early dysfunction to retinal ganglion cells. To begin to assess this, we studied the excitatory synaptic inputs to individual ganglion cells in normal mouse retinas and in retinas with ganglion cell degeneration from glaucoma (DBA/2J), or following an optic nerve crush. Excitatory synapses were labeled by AAV2-mediated transfection of ganglion cells with PSD-95-GFP. After both insults the linear density of synaptic inputs to ganglion cells decreased. In parallel, the dendritic arbors lost complexity. We did not observe any cells that had lost dendritic synaptic input while preserving a normal or near-normal morphology. Within the temporal limits of these observations, dendritic remodeling and synapse pruning thus appear to occur near-simultaneously.

Astrocytes in the optic nerve head express putative mechanosensitive channels

PURPOSE

To establish whether optic nerve head astrocytes express candidate molecules to sense tissue stretch.

METHODS

We used conventional PCR, quantitative PCR, and single-cell reverse transcription PCR (RT-PCR) to assess the expression of various members of the transient receptor potential (TRP) channel family and of the recently characterized mechanosensitive channels Piezo1 and 2 in optic nerve head tissue and in single, isolated astrocytes.

RESULTS

Most TRP subfamilies (TRPC, TRPM, TRPV, TRPA, and TRPP) and Piezo1 and 2 were expressed in the optic nerve head of the mouse. Quantitative real-time PCR analysis showed that TRPC1, TRPM7, TRPV2, TRPP2, and Piezo1 are the dominant isoforms in each subfamily. Single-cell RT-PCR revealed that many TRP isoforms, TRPC1-2, TRPC6, TRPV2, TRPV4, TRPM2, TRPM4, TRPM6-7, TRPP1-2, and Piezo1-2, are expressed in astrocytes of the optic nerve head, and that most astrocytes express TRPC1 and TRPP1-2. Comparisons of the TRPP and Piezo expression levels between different tissue regions showed that Piezo2 expression was higher in the optic nerve head and the optic nerve proper than in the brain and the corpus callosum. TRPP2 also showed higher expression in the optic nerve head.

CONCLUSIONS

Astrocytes in the optic nerve head express multiple putative mechanosensitive channels, in particular the recently identified channels Piezo1 and 2. The expression of putative mechanosensitive channels in these cells may contribute to their responsiveness to traumatic or glaucomatous injury.

Isolation of intact astrocytes from the optic nerve head of adult mice

The astrocytes of the optic nerve head are a specialized subtype of white matter astrocytes that form the direct cellular environment of the unmyelinated ganglion cell axons. Due to their potential involvement in glaucoma, these astrocytes have become a target of research. Due to the heterogeneity of the optic nerve tissue, which also contains other cell types, in some cases it may be desirable to conduct gene expression studies on small numbers of well-characterized astrocytes or even individual cells. Here, we describe a simple method to isolate individual astrocytes. This method permits obtaining astrocytes with intact morphology from the adult mouse optic nerve and reduces contamination of the isolated astrocytes by other cell types. Individual astrocytes can be recognized by their morphology and collected under microscopic control. The whole procedure can be completed in 2-3 h. We also discuss downstream applications like multiplex single-cell PCR and quantitative PCR (qPCR).

Differential gene expression in glaucoma

In glaucoma, regardless of its etiology, retinal ganglion cells degenerate and eventually die. Although age and elevated intraocular pressure (IOP) are the main risk factors, there are still many mysteries in the pathogenesis of glaucoma. The advent of genome-wide microarray expression screening together with the availability of animal models of the disease has allowed analysis of differential gene expression in all parts of the eye in glaucoma. This review will outline the findings of recent genome-wide expression studies and discuss their commonalities and differences. A common finding was the differential regulation of genes involved in inflammation and immunity, including the complement system and the cytokines transforming growth factor β (TGFβ) and tumor necrosis factor α (TNFα). Other genes of interest have roles in the extracellular matrix, cell-matrix interactions and adhesion, the cell cycle, and the endothelin system.

The Time Course of Gene Expression during Reactive Gliosis in the Optic Nerve

Reactive gliosis is a complex process that involves changes in gene expression and morphological remodeling. The mouse optic nerve, where astrocytes, microglia and oligodendrocytes interact with retinal ganglion cell axons and each other, is a particularly suitable model for studying the molecular mechanisms of reactive gliosis. We triggered gliosis at the mouse optic nerve head by retro orbital nerve crush. We followed the expression profiles of 14,000 genes from 1 day to 3 months, as the optic nerve formed a glial scar. The transcriptome showed profound changes. These were greatest shortly after injury; the numbers of differentially regulated genes then dropped, returning nearly to resting levels by 3 months. Different genes were modulated with very different time courses, and functionally distinct groups of genes responded in partially overlapping waves. These correspond roughly to two quick waves of inflammation and cell proliferation, a slow wave of tissue remodeling and debris removal, and a final stationary phase that primarily reflects permanent structural changes in the axons. Responses from astrocytes, microglia and oligodendrocytes were distinctively different, both molecularly and morphologically. Comparisons to other models of brain injury and to glaucoma indicated that the glial responses depended on both the tissue and the injury. Attempts to modulate glial function after axonal injuries should consider different mechanistic targets at different times following the insult.

Reversible reactivity by optic nerve astrocytes

Reactive astrocytes are typically studied in models that cause irreversible mechanical damage to axons, neuronal cell bodies, and glia. Here, we evaluated the response of astrocytes in the optic nerve head to a subtle injury induced by a brief, mild elevation of the intraocular pressure. Astrocytes demonstrated reactive remodeling that peaked at three days, showing hypertrophy, process retraction, and simplification of their shape. This was not accompanied by any significant changes in the gene expression profile. At no time was there discernible damage to the optic axons, as evidenced by electron microscopy and normal anterograde and retrograde transport. Remarkably, the morphological remodeling was reversible. These findings underscore the plastic nature of reactivity. They show that reactivity can resolve fully if the insult is removed, and suggest that reactivity per se is not necessarily deleterious to axons. This reaction may represent very early events in the sequence that eventually leads to glial scarring.

Morphology of astrocytes in a glaucomatous optic nerve

PURPOSE

To establish the morphologic changes of astrocytes in the glial lamina of glaucomatous mice.

METHODS

A strain of mice that expresses GFP in individual astrocytes (hGFAPpr-GFP) was crossed into the DBA/2J strain that develops glaucoma. In the resulting strain (D2.hGFAPpr-GFP) we assessed the severity of glaucoma by staining the retina for neurofilaments and counting the neurons of the retinal ganglion cell layer. We observed the morphology of astrocytes in the glial lamina of the optic nerves.

RESULTS

D2.hGFAPpr-GFP mice developed glaucoma in an age-dependent manner. Astrocytes in the glial lamina showed morphologic changes that correlated with the severity of glaucoma. The cells showed thickening of processes from 1.3 ± 0.28 μm in nondiseased animals to 1.71 ± 0.46 μm in eyes with moderate glaucoma and 2.1 ± 0.42 μm in those with severe glaucoma. Their spatial coverage, as determined by their convex polygon area, was reduced in eyes with severe glaucoma. The astrocytes in severely glaucomatous optic nerves also showed simplification of their processes. In 6-month-old mice with no obvious signs of degeneration in the retina, we found astrocytes with appendages growing out of primary astrocyte processes into the axon bundles. This localized hypertrophy of processes was never observed in the hGFAPpr-GFP strain.

CONCLUSIONS

Confirming results after optic nerve crush, astrocytes in glaucomatous optic nerves had thickened and simplified processes, and reduced spatial coverage. We also found evidence of localized sprouting of new processes in early stages of the disease, before detectable changes in ganglion cell number.

Structural remodeling of astrocytes in the injured CNS

Astrocytes respond to all forms of CNS insult and disease by becoming reactive, a nonspecific but highly characteristic response that involves various morphological and molecular changes. Probably the most recognized aspect of reactive astrocytes is the formation of a glial scar that impedes axon regeneration. Although the reactive phenotype was first suggested more than 100 years ago based on morphological changes, the remodeling process is not well understood. We know little about the actual structure of a reactive astrocyte, how an astrocyte remodels during the progression of an insult, and how populations of these cells reorganize to form the glial scar. New methods of labeling astrocytes, along with transgenic mice, allow the complete morphology of reactive astrocytes to be visualized. Recent studies show that reactivity can induce a remarkable change in the shape of a single astrocyte, that not all astrocytes react in the same way, and that there is plasticity in the reactive response.

Regular mosaic of synaptic contacts among three retinal neurons

Retinal bipolar, amacrine, and ganglion cells contact each other within precisely defined synaptic laminae, but the spatial distribution of contacts between the cells is generally treated as random. Here we show that not to be the case. Excitatory inputs to inner retinal neurons were visualized by introduction of a plasmid coding for the postsynaptic protein PSD95-GFP. Our initial finding was that synapses on the dendrites of retinal ganglion cells are regularly spaced, at 2-3-μm intervals, along the dendrites. Thus, the presence of a PSD95 punctum creates a nearby zone from which other inputs appear to be excluded. Despite their great variation in size and different morphologies, the spacing is similar for the arbors of different retinal ganglion cell types. Regular spacing was also observed for the starburst amacrine cells. This regularity is mirrored in the spacing of axonal varicosities of the stratified bipolar cells, which have a regular, nonrandom interval consistent with that of the PSD95 puncta on ganglion cells. Thus, for each level of the inner plexiform layer all three cell types participate in a single 2D mosaic of synaptic contacts. These findings raise a new set of questions: How does the self-avoidance of synaptic sites along an individual dendrite arise and how is it physically maintained? Why is a regular spacing of inputs important for the computational function of the cells? Finally, which of the three players, if any, is developmentally responsible for the initial establishment of the pattern?

The morphology and spatial arrangement of astrocytes in the optic nerve head of the mouse

We evaluated the shapes, numbers, and spatial distribution of astrocytes within the glial lamina, an astrocyte-rich region at the junction of the retina and optic nerve. A primary aim was to determine how the population of astrocytes, collectively, partitions the axonal space in this region. Astrocyte processes labeled with glial fibrillary acidic protein (GFAP) compartmentalize ganglion cell axons into bundles, forming "glial tubes," and giving the glial architecture of the optic nerve head in transverse section a honeycomb appearance. The shapes of individual astrocytes were studied by using transgenic mice that express enhanced green fluorescent protein in isolated astrocytes (hGFAPpr-EGFP). Within the glial lamina the astrocytes were transverse in orientation, with thick, smooth primary processes emanating from a cytoplasmic expansion of the soma. Spaces between the processes of neighboring astrocytes were spatially aligned, to form the apertures through which the bundles of optic axons pass. The processes of individual astrocytes were far-reaching-they could span most of the width of the nerve-and overlapped the anatomical domains of other near and distant astrocytes. Thus, astrocytes in the glial lamina do not tile: each astrocyte participates in ensheathing approximately one-quarter of all of the axon bundles in the nerve, and each glial tube contains the processes of about nine astrocytes. This raises the mechanistic question of how, in glaucoma or other cases of nerve damage, the glial response can be confined to a circumscribed region where damage to axons has occurred.

The spatial distribution of glutamatergic inputs to dendrites of retinal ganglion cells

The spatial pattern of excitatory glutamatergic input was visualized in a large series of ganglion cells of the rabbit retina, by using particle-mediated gene transfer of an expression plasmid for postsynaptic density 95-green fluorescent protein (PSD95-GFP). PSD95-GFP was confirmed as a marker of excitatory input by co-localization with synaptic ribbons (RIBEYE and kinesin II) and glutamate receptor subunits. Despite wide variation in the size, morphology, and functional complexity of the cells, the distribution of excitatory synaptic inputs followed a single set of rules: 1) the linear density of synaptic inputs (PSD95 sites/linear mum) varied surprisingly little and showed little specialization within the arbor; 2) the total density of excitatory inputs across individual arbors peaked in a ring-shaped region surrounding the soma, which is in accord with high-resolution maps of receptive field sensitivity in the rabbit; and 3) the areal density scaled inversely with the total area of the dendritic arbor, so that narrow dendritic arbors receive more synapses per unit area than large ones. To achieve sensitivity comparable to that of large cells, those that report upon a small region of visual space may need to receive a denser synaptic input from within that space.

Axons of retinal ganglion cells are insulted in the optic nerve early in DBA/2J glaucoma

Here, we use a mouse model (DBA/2J) to readdress the location of insult(s) to retinal ganglion cells (RGCs) in glaucoma. We localize an early sign of axon damage to an astrocyte-rich region of the optic nerve just posterior to the retina, analogous to the lamina cribrosa. In this region, a network of astrocytes associates intimately with RGC axons. Using BAX-deficient DBA/2J mice, which retain all of their RGCs, we provide experimental evidence for an insult within or very close to the lamina in the optic nerve. We show that proximal axon segments attached to their cell bodies survive to the proximity of the lamina. In contrast, axon segments in the lamina and behind the eye degenerate. Finally, the Wld(s) allele, which is known to protect against insults to axons, strongly protects against DBA/2J glaucoma and preserves RGC activity as measured by pattern electroretinography. These experiments provide strong evidence for a local insult to axons in the optic nerve.

Expression of mRNA for glutamate receptor subunits distinguishes the major classes of retinal neurons, but is less specific for individual cell types

PURPOSE

To investigate the expression of ionotropic glutamate receptor subunits by retinal neurons, to assess the extent to which different functional types of retinal neurons are characterized by the expression of the receptor subtypes.

METHODS

Rod photoreceptor cells and bipolar cells were identified in retina dissociates. Amacrine cells were identified in dissociates from transgenic mice or by staining with an antibody against the extracellular carbohydrate epitope CD15. Ganglion cells were identified by retrograde axonal transport of FITC-dextran or by green fluorescent protein (GFP) fluorescence in a transgenic strain. We examined the receptors simultaneously using non-quantitative single-cell reverse transcriptase polymerase chain reaction for GluR1-R4 (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors), GluR5-R7, and KA1 and 2 (kainate receptors), delta1 and delta2 subunits, and the N-methyl-D-aspartate (NMDA) receptor subunits NR1, 2a-d, and 3a.

RESULTS

The expression of glutamate receptors on bipolar cells and rod photoreceptors was limited: Neither expressed functional NMDA receptors, and rods were also negative for AMPA receptors. The sample of ganglion cells included examples of many ganglion cell types; these were distinguished morphologically using quantitative parameters defined in a previous cluster analysis. All types of ionotropic glutamate receptors were found to be expressed on ganglion cells. The iGluR subunits GluR4, KA2, delta1, and NR1 were expressed on almost all ganglion cells examined.

CONCLUSIONS

Despite the heterogeneity of ganglion cell types, differences among them in this PCR-based method were minor. Thus, retinal interneurons are characterized by expression of distinctive glutamate receptor types, but functional differences among ganglion cells seem to be reflected instead in the amounts as well as spatial distributions of a widely expressed group of receptors.

Organotypic culture of adult rabbit retina

Organotypic culture systems of functional neural tissues are important tools in neurobiological research. Ideally, such a system should be compatible with imaging techniques, genetic manipulation, and electrophysiological recording. Here we present a simple interphase tissue culture system for adult rabbit retina that requires no specialized equipment and very little maintenance. We demonstrate the dissection and incubation of rabbit retina and particle-mediated gene transfer of plasmids encoding GFP or a variety of subcellular markers into retinal ganglion cells. Rabbit retinas cultured this way can be kept alive for up to 6 days with very little changes of the overall anatomical structure or the morphology of individual ganglion- and amacrine cells.

Organotypic culture of physiologically functional adult mammalian retinas

Background

The adult mammalian retina is an important model in research on the central nervous system. Many experiments require the combined use of genetic manipulation, imaging, and electrophysiological recording, which make it desirable to use an in vitro preparation. Unfortunately, the tissue culture of the adult mammalian retina is difficult, mainly because of the high energy consumption of photoreceptors.

Methods and Findings

We describe an interphase culture system for adult mammalian retina that allows for the expression of genes delivered to retinal neurons by particle-mediated transfer. The retinas retain their morphology and function for up to six days— long enough for the expression of many genes of interest—so that effects upon responses to light and receptive fields could be measured by patch recording or multielectrode array recording. We show that a variety of genes encoding pre- and post-synaptic marker proteins are localized correctly in ganglion and amacrine cells.

Conclusions

In this system the effects on neuronal function of one or several introduced exogenous genes can be studied within intact neural circuitry of adult mammalian retina. This system is flexible enough to be compatible with genetic manipulation, imaging, cell transfection, pharmacological assay, and electrophysiological recordings.

Horizontal slice preparation of the retina

Traditionally the vertical slice and the whole-mount preparation of the retina have been used to study the function of retinal circuits. However, many of retinal neurons, such as amacrine cells, expand their dendrites horizontally, so that the morphology of the cells is supposed to be severely damaged in the vertical slices. In the whole-mount preparation, especially for patch-clamp recordings, retinal neurons in the middle layer are not easily accessible due to the extensive coverage of glial cell (Mueller cell) s endfeets. Here, we describe the novel slicing method to preserve the dendritic morphology of retinal neurons intact. The slice was made horizontally at the inner layer of the retina using a vibratome slicer after the retina was embedded in the low-temperature melting agarose gel. In this horizontal slice preparation of the retina, we studied the function of retinal neurons compared with their morphology, by using patch-clamp recording, calcium imaging technique, immunocytochemistry, and single-cell RT-PCR.

Retinal ganglion cell degeneration is topological but not cell type specific in DBA/2J mice

Using a variety of double and triple labeling techniques, we have reevaluated the death of retinal neurons in a mouse model of hereditary glaucoma. Cell-specific markers and total neuron counts revealed no cell loss in any retinal neurons other than the ganglion cells. Within the limits of our ability to define cell types, no group of ganglion cells was especially vulnerable or resistant to degeneration. Retrograde labeling and neurofilament staining showed that axonal atrophy, dendritic remodeling, and somal shrinkage (at least of the largest cell types) precedes ganglion cell death in this glaucoma model. Regions of cell death or survival radiated from the optic nerve head in fan-shaped sectors. Collectively, the data suggest axon damage at the optic nerve head as an early lesion, and damage to axon bundles would cause this pattern of degeneration. However, the architecture of the mouse eye seems to preclude a commonly postulated source of mechanical damage within the nerve head.

Different functional types of bipolar cells use different gap-junctional proteins

Rod signals are transmitted to ON retinal ganglion cells by means of gap junctions between AII amacrine cells and ON bipolars. The AII amacrine cells are known to express connexin36 (Cx36), but previous studies of Cx36 in ON cone bipolars have been ambiguous. Here, we studied bipolar cells in a transgenic mouse line that expresses high levels of green fluorescent protein (GFP) in one type of ON cone bipolar cell. We found strong Cx36 immunostaining in the axon terminals of the GFP-labeled type 357 bipolar cells in both vertical sections and whole mounts of the retina. This finding was confirmed by single-cell immunostaining and single-cell reverse transcription-PCR (RT-PCR). As reported previously (Maxeiner et al., 2005), Cx45 was found in some ON bipolar cells, but RT-PCR showed Cx36 and not Cx45 to be expressed by the type 357 bipolar cells. Some of the remaining GFP-negative bipolar cells expressed Cx45 but not Cx36. It appears that different types of ON cone bipolar cells express different connexins at their gap junctions with AII amacrine cells.

Inward rectifying currents stabilize the membrane potential in dendrites of mouse amacrine cells: patch-clamp recordings and single-cell RT-PCR

PURPOSE

To explore the possible existence of inward rectifying currents in the distal dendrites of amacrine cells.

METHODS

Patch-clamp recordings were made from amacrine cells in a new horizontal slice preparation of mouse retina. Single-cell RT-PCR studies were performed after the patch-clamp recordings.

RESULTS

In contrast to results from vertical slices or dissociated cells, all amacrine cells tested demonstrated inward rectifying currents, IIR. Within the limits of our sample, this current did not depend on the morphological and physiological type of the amacrine cell. Amacrine cells from which the dendrites had been removed did not possess detectable amounts of IIR. Pharmacological experiments with ZD7288 (100 microM) and single-cell RT-PCR from recorded cells revealed that IIR includes an h-current (I(H)) carried by hyperpolarization-activated cyclic nucleotide gated channels (HCN), HCN1 and/or HCN2 subtypes. In the presence of extracellular Cs+ (5 mM), which greatly suppressed IIR, the resting membrane conductance was reduced. IIR suppressed the generation of oscillatory potentials. Intracellular cAMP (8-cpt-cAMP, 1 mM) activated IIR.

CONCLUSIONS

IIR appears to occur within dendrites of many amacrine cells, where it tends to stabilize the resting membrane potential. HCN1 and/or HCN2 channels contribute to IIR in amacrine cells. Dendritic IIR would be expected to contribute to functional independence of the distal dendrites of amacrine cells that express it.

CD15 immunoreactive amacrine cells in the mouse retina

The mouse retina has become an important model in vision research, mainly because of the wide availability of transgenic animals. In order to study cell function and connectivity in the inner retina, antibodies that differentially stain one cell type, or a small number of cell types, are helpful as markers. Here we characterize the CD15 (3[alpha1-3]-fucosyl-N-acetyl-lactosamine)-positive cells in the mouse retina using immunofluorescence confocal microscopy and reverse-transcription polymerase chain reaction. CD15 immunoreactivity was observed in two distinct types of amacrine cells and, faintly, in some cone bipolar cells. Type I CD15+ amacrine cells are GABAergic wide-field cells that stratify in lamina 3 and 4/5 of the inner plexiform layer. Type II CD15+ amacrine cells are also GABAergic and costratify with the dopaminergic tyrosine hydroxylase-positive cells in lamina 1 of the inner plexiform layer. The densities of types I and II CD15+ amacrine cells in mid-periphery were 258 cells/mm(2) and 274 cells/mm(2). Double labeling with several other markers for amacrine cell types showed that neither type belongs to another previously identified subpopulation of amacrine cells. Single-cell RT-PCR showed that CD15+ amacrine cells coexpress several AMPA receptors - GluR1, GluR2, and GluR4 being the most common combination.

Proinflammatory cytokines inhibit the expression and function of human type I 5'-deiodinase in HepG2 hepatocarcinoma cells

OBJECTIVE

The sick euthyroid syndrome in critically ill patients without primary disease of the thyroid gland is characterised by low serum total triiodothyronine (T3), normal to elevated thyroxine (T4), elevated reverse T3 (rT3) and normal TSH levels. The aim of this work was to clarify if impaired T4 and rT3 5'-deiodination is an underlying mechanism.

DESIGN AND METHODS

We analysed the effect of the human recombinant proinflammatory cytokines interleukin (IL)-6 and IL-1beta, tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) on human type I 5'-iodothyronine deiodinase (5'DI) enzyme activity in the human hepatocarcinoma cell line HepG2, i.e. in a homologous human system. Furthermore, we analysed transcriptional effects of the cytokines by transient transfection assays using the luciferase or chloramphenicol acetyltransferase (CAT) reporter genes under the control of 1480 nucleotides of the human 5'DI promoter.

RESULTS

IL-6 at 500 pg/ml and TNF-alpha at 25 ng/ml had no significant effect, whereas 100 ng/ml IFN-gamma or 10 ng/ml IL-1beta reduced 5'DI enzyme activity to 77.9 and 59.5% of control values. IFN-gamma did not alter, IL-6 and TNF-alpha moderately decreased (in the case of IL-6 only in the CAT system), and IL-1beta (0.01-10 ng/ml) dose-dependently inhibited 5'DI promoter activity to a minimum of 38.1%.

CONCLUSION

IL-1beta inhibited both 5'DI enzyme and promoter activity and, thus, may exert its effect on thyroid hormone metabolism at least partially through direct inhibition of hepatic 5'DI gene transcription.

Reverse transcriptase-polymerase chain reaction analysis of thyrocyte-relevant genes in fine-needle aspiration biopsies of the human thyroid

Currently, fine-needle aspiration cytology is a valuable tool in the routine diagnosis of suspicious thyroid nodules. We present a very sensitive method for the molecular analysis of the expression of several genes important for normal thyroid function in parallel to the cytological diagnosis. We adapted reverse transcriptase polymerase chain reaction (RT-PCR) to amplify thyroid-typical mRNAs in samples of thyroid carcinoma cells as small as those obtained by fine-needle aspiration biopsy (FNAB), ie, 100-1000 cells, and applied this procedure to four routinely taken FNABs. Gene products such as thyroglobulin (Tg), thyroid-stimulating hormone-receptor (TSHr), sodium/iodide-symporter (NIS), type I iodothyronine-5'-deiodinase (DI), and type II iodothyronine-5'-deiodinase (DII) were analyzed. To establish RT-PCR protocols, serial dilutions of follicular thyroid carcinoma cells, FTC-133, which express these genes at low levels, were initially used for RNA isolation. Successful RNA isolation and reverse transcription were checked by the amplification of beta-actin mRNA. We detected the mRNAs coding for Tg in as little as 10 cells, for NIS in 100 cells, and for TSHr, DI, and DII in 10,000 cells. After preparing cytological smears of four routinely taken FNABs, all above-mentioned thyroid-typical mRNAs were observed by using the material remaining in the needle for RNA isolation followed by RT-PCR. This method offers the possibility of obtaining two different types of information from the same routinely taken thyroid FNAB: the cytological diagnosis and the expression pattern of several diagnostically relevant genes. Therefore, a more specific diagnosis could be rendered in the preoperative state, and may lead to more specific therapy.

Functional retinoid and thyroid hormone receptors in human thyroid-carcinoma cell lines and tissues

Thyroid carcinomas no longer accessible to radio-iodide or TSH-suppressive T4 therapy, due to loss of thyroid-specific functions, might be sufficiently re-differentiated by retinoic acid (RA) to be treated by conventional methods again. To help evaluate the feasibility of RA re-differentiation therapy in thyroid carcinomas, we examined the functionality of RA receptors (RARs/RXRs), central RA signal mediators, in human thyroid-carcinoma cell lines as model systems. [3H]-RA binding assays with nuclear extracts from follicular thyroid-carcinoma cell lines FTC-133 and -238 revealed high-affinity binding sites for RA. Electrophoretic mobility shift and super-shift assays using a DR2 ("direct repeat" 2) RA response element demonstrated DNA-binding of RARalpha, RARgamma, RXRalpha and RXRbeta in nuclear extracts of FTC-133 and anaplastic HTh74 cells. Use of a DR5 RA response element revealed no difference in DNA binding. In supershift assays with a DR4 T3 response element, we found DNA-binding by TRalpha1, TRalpha2, and TRbeta. Northern-blot analysis showed low expression of RXRbeta mRNA in FTC-133 and of TRalpha1 mRNA in FTC-133 and FTC-238 cells. Using RT-PCR, we detected mRNA for RARalpha, RARbeta, RARgamma, RXRalpha, and RXRbeta in the 4 cell lines and in human thyroid-carcinoma samples. RARbeta mRNA was reduced in FTC-238 cells and RXRbeta mRNA was decreased in anaplastic C643 cells and 9 of 12 tumor samples. Differential RA regulation of RA-receptor-mRNA expression was observed in the various cell lines. Thus, RA and T3 nuclear receptors are present in thyroid-carcinoma cell lines or tissues, albeit with cell-line and tumor-dependent variations; in the cell lines, they were shown to be functional with respect to DNA and/or ligand binding.

Cloning and characterization of the human selenoprotein P promoter. Response of selenoprotein P expression to cytokines in liver cells

We isolated an 18-kilobase (kb) genomic selenoprotein P clone from a human placenta library and cloned, sequenced, and characterized the 5'-flanking region of the human selenoprotein P gene. Sequence analysis revealed an intron between base pairs (bp) -13 and -14 upstream of the ATG codon and another one between bp 534 and 535 of the coding region. The major transcription start site of selenoprotein P in human HepG2 hepatocarcinoma cells was mapped to bp -70 by 5'-rapid amplification of cDNA ends and by primer extension. 1.8 kb of the 5'-flanking sequence were fused to a luciferase reporter gene. They exhibited functional promoter activity in HepG2 hepatocarcinoma and Caco2 colon carcinoma cells in transient transfection experiments. Treatment of transfected HepG2 cells with the cytokines interleukin 1beta, tumor necrosis factor alpha, and interferon gamma repressed promoter activity. Nuclear extracts of interferon gamma-treated cells bound to a signal transducer and activator of transcription response element of the promoter in gel retardation experiments. By transfection of promoter-deletion constructs, a TATA box and a putative SP1 site were identified to be necessary for selenoprotein P transcription. These data indicate that the human selenoprotein P gene contains a strong promoter that is cytokine responsive. Furthermore, selenoprotein P, secreted by the liver, might react as a negative acute phase protein.

The promoter of the human type I 5'-deiodinase gene--mapping of the transcription start site and identification of a DR+4 thyroid-hormone-responsive element

The selenoenzyme thyroxine 5'-deiodinase type I deiodinates the prohormone thyroxine to the active thyroid hormone 3,3',5-triiodothyronine. It is thus one of the key enzymes involved in the triiodothyronine-mediated control of growth, differentiation and basal metabolism in vertebrates. We report here the identification of the transcription start site and the cloning of 1500 bases of the upstream regulatory region of the human 5'-deiodinase gene. They contain a complex triiodothyronine-responsive element at nucleotides -696 to -673, consisting of an ideal direct repeat (DR) of two AGGTCA half-sites with a spacing of four nucleotides (DR+4) and a third putative AGTTCA half-site with a spacing of another two nucleotides (DR+2). The whole DR+4+2 specifically bound to thyroid hormone receptor and retinoid X receptor in electrophoretic mobility shift assays. The DR+4+2 mediates triiodothyronine-responsiveness in cotransfection experiments of constructs containing the 5'-deiodinase upstream promoter and enhancer region fused to luciferase or chloramphenicol acetyltransferase reporter genes with expression plasmids of thyroid hormone receptor subtypes. Also, an about 2.5-fold induction of the 5'-deiodinase-promoter-luciferase-reporter construct by all-trans retinoic acid was observed in a cotransfection assay with retinoic acid receptors. Point mutation analysis of the DR+4+2 type hormone-responsive element, however, revealed that it does not alone mediate the retinoic acid effect. The transcription start point of the 5'-deiodinase gene was mapped to nucleotides -23 and -24. No CAAT or TATA box is located within the usual distance to the transcription initiation site. Two GC boxes were found at nucleotides -68 to -63 and -39 to -34. Transfection analysis revealed that the proximal 105 nucleotides in the 5'-flanking region of the 5'-deiodinase gene act as a functional core promoter. This data indicates that triiodothyronine, the end product of thyroid hormone synthesis, positively regulates one of the key enzymes in its production.

Structure of the human type I iodothyronine 5'-deiodinase gene and localization to chromosome 1p32-p33

The human type I iodothyronine 5'-deiodinase gene encodes a member of the family of selenocysteine-containing deiodinases. These enzymes catalyze the activation of the prohormone thyroxine to 3,3',5-triiodothyronine or the degradation of thyroxine and triiodothyronine to inactive metabolites. Here we report the isolation of two genomic type I 5'-deiodinase clones from a chromosome 1-specific gridded cosmid library, the localization of the gene to chromosome 1p32-p33 by fluorescence in situ hybridization, and the determination of the complete structure of the 17.5-kb gene.