Glial cells in the bird retina: immunochemical detection

Ocular Lesions in Red-Tailed Hawks (Buteo jamaicensis) With Naturally Acquired West Nile Disease

Ocular lesions are common in red-tailed hawks with West Nile (WN) disease. These lesions consist of pectenitis, choroidal or retinal inflammation, or retinal necrosis, but detailed investigation of the ocular lesions is lacking. Postmortem examination of the eyes of 16 red-tailed hawks with naturally acquired WN disease and 3 red-tailed hawks without WN disease was performed using histopathology, immunohistochemistry for West Nile virus (WNV) antigen, glial fibrillary acid protein, cleaved caspase-3, and the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling method. Retinal lesions were classified as type I or type II lesions. Type I lesions were characterized by lymphoplasmacytic infiltrates in the subjacent choroid with degeneration limited to the outer retina (type Ia lesion) or with degeneration and necrosis of the outer retina or outer and inner retina (type Ib lesion) while retinal collapse, atrophy, and scarring were hallmarks of type II lesions. Type II retinal lesions were associated with a more pronounced choroiditis. Although not statistically significant, WNV antigen tended to be present in larger quantity in type Ib lesions. Type I lesions are considered acute while type II lesions are chronic. The development of retinal lesions was associated with the presence of an inflammatory infiltrate in the choroid. A breakdown of the blood-retina barrier is suspected to be the main route of infection of the retina. Within the retina, virus appeared to spread via both neuronal and Müller cell processes.

Sox10 Expression in Goldfish Retina and Optic Nerve Head in Controls and after the Application of Two Different Lesion Paradigms

The mammalian central nervous system (CNS) is unable to regenerate. In contrast, the CNS of fish, including the visual system, is able to regenerate after damage. Moreover, the fish visual system grows continuously throughout the life of the animal, and it is therefore an excellent model to analyze processes of myelination and re-myelination after an injury. Here we analyze Sox10⁺ oligodendrocytes in the goldfish retina and optic nerve in controls and after two kinds of injuries: cryolesion of the peripheral growing zone and crushing of the optic nerve. We also analyze changes in a major component of myelin, myelin basic protein (MBP), as a marker for myelinated axons. Our results show that Sox10⁺ oligodendrocytes are located in the retinal nerve fiber layer and along the whole length of the optic nerve. MBP was found to occupy a similar location, although its loose appearance in the retina differed from the highly organized MBP⁺ axon bundles in the optic nerve. After optic nerve crushing, the number of Sox10⁺ cells decreased in the crushed area and in the optic nerve head. Consistent with this, myelination was highly reduced in both areas. In contrast, after cryolesion we did not find changes in the Sox10⁺ population, although we did detect some MBP^- degenerating areas. We show that these modifications in Sox10⁺ oligodendrocytes are consistent with their role in oligodendrocyte identity, maintenance and survival, and we propose the optic nerve head as an excellent area for research aimed at better understanding of de- and remyelination processes.

Activation of glucocorticoid receptors in Müller glia is protective to retinal neurons and suppresses microglial reactivity

Reactive microglia and macrophages are prevalent in damaged retinas. Glucocorticoid signaling is known to suppress inflammation and the reactivity of microglia and macrophages. In the vertebrate retina, the glucocorticoid receptor (GCR) is known to be activated and localized to the nuclei of Müller glia (Gallina et al., 2014). Accordingly, we investigated how signaling through GCR influences the survival of neurons using the chick retina in vivo as a model system. We applied intraocular injections of GCR agonist or antagonist, assessed microglial reactivity, and the survival of retinal neurons following different damage paradigms. Microglial reactivity was increased in retinas from eyes that were injected with vehicle, and this reactivity was decreased by GCR-agonist dexamethasone (Dex) and increased by GCR-antagonist RU486. We found that activation of GCR suppresses the reactivity of microglia and inhibited the loss of retinal neurons resulting from excitotoxicity. We provide evidence that the protection-promoting effects of Dex were maintained when the microglia were selectively ablated. Similarly, intraocular injections of Dex protected ganglion cells from colchicine-treatment and protected photoreceptors from damage caused by retinal detachment. We conclude that activation of GCR promotes the survival of ganglion cells in colchicine-damaged retinas, promotes the survival of amacrine and bipolar cells in excitotoxin-damaged retinas, and promotes the survival of photoreceptors in detached retinas. We propose that suppression of microglial reactivity is secondary to activation of GCR in Müller glia, and this mode of signaling is an effective means to lessen the damage and vision loss resulting from different types of retinal damage.

Reactive retinal microglia, neuronal survival, and the formation of retinal folds and detachments

Reactive microglia and macrophages are prevalent in damaged retinas. Accordingly, we investigate how the activation or ablation of microglia/macrophages influences the survival of neurons in the chick retina in vivo. We applied intraocular injections of interleukin 6 (IL6) to stimulate the reactivity of microglia/macrophages and clodronate-liposomes to ablate microglia/macrophages. Activation of the microglia/macrophages with IL6 delays the death of retinal neurons from N-methyl-D-aspartate (NMDA) -induced excitotoxicity. In addition, activation of microglia/macrophages combined with colchicine-mediated retinal damage diminished the survival of ganglion cells. Application of IL6 after an excitotoxic insult greatly exacerbates the damage, and causes widespread retinal detachments and folds, accompanied by accumulation of microglia/macrophages in the subretinal space. Damage-induced retinal folds and detachments were significantly reduced by the ablation of microglia/macrophages. We conclude that microglial reactivity is detrimental to the survival of ganglion cells in colchicine-damaged retinas and detrimental to the survival of photoreceptors in retinal folds. In addition, we conclude that IL6-treatment transiently protects amacrine and bipolar cells against an excitotoxic insult. We propose that suppressing reactivity of microglia/macrophages may be an effective means to lessen the damage and vision loss resulting from damage, in particular during retinal detachment injuries.

Reactive microglia and macrophage facilitate the formation of Müller glia-derived retinal progenitors

In retinas where Müller glia have been stimulated to become progenitor cells, reactive microglia are always present. Thus, we investigated how the activation or ablation of microglia/macrophage influences the formation of Müller glia-derived progenitor cells (MGPCs) in the retina in vivo. Intraocular injections of the Interleukin-6 (IL6) stimulated the reactivity of microglia/macrophage, whereas other types of retinal glia appear largely unaffected. In acutely damaged retinas where all of the retinal microglia/macrophage were ablated, the formation of proliferating MGPCs was greatly diminished. With the microglia ablated in damaged retinas, levels of Notch and related genes were unchanged or increased, whereas levels of ascl1a, TNFα, IL1β, complement component 3 (C3) and C3a receptor were significantly reduced. In the absence of retinal damage, the combination of insulin and Fibroblast growth factor 2 (FGF2) failed to stimulate the formation of MGPCs when the microglia/macrophage were ablated. In addition, intraocular injections of IL6 and FGF2 stimulated the formation of MGPCs in the absence of retinal damage, and this generation of MGPCs was blocked when the microglia/macrophage were absent. We conclude that the activation of microglia and/or infiltrating macrophage contributes to the formation of proliferating MGPCs, and these effects may be mediated by components of the complement system and inflammatory cytokines.

The reactivity, distribution and abundance of Non-astrocytic Inner Retinal Glial (NIRG) cells are regulated by microglia, acute damage, and IGF1

Recent studies have described a novel type of glial cell that is scattered across the inner layers of the avian retina and possibly the retinas of primates. These cells have been termed Non-astrocytic Inner Retinal Glial (NIRG) cells. These cells are stimulated by insulin-like growth factor 1 (IGF1) to proliferate, migrate distally into the retina, and become reactive. These changes in glial activity correlate with increased susceptibility of retinal neurons and Müller glia to excitotoxic damage. The purpose of this study was to further study the NIRG cells in retinas treated with IGF1 or acute damage. In response to IGF1, the reactivity, proliferation and migration of NIRG cells persists through 3 days after treatment. At 7 days after treatment, the numbers and distribution of NIRG cells returns to normal, suggesting that homeostatic mechanisms are in place within the retina to maintain the numbers and distribution of these glial cells. By comparison, IGF1-induced microglial reactivity persists for at least 7 days after treatment. In damaged retinas, we find a transient accumulation of NIRG cells, which parallels the accumulation of reactive microglia, suggesting that the reactivity of NIRG cells and microglia are linked. When the microglia are selectively ablated by the combination of interleukin 6 and clodronate-liposomes, the NIRG cells down-regulate transitin and perish within the following week, suggesting that the survival and phenotype of NIRG cells are somehow linked to the microglia. We conclude that the abundance, reactivity and retinal distribution of NIRG cells can be dynamic, are regulated by homoestatic mechanisms and are tethered to the microglia.

Up-regulation of GLT1 reverses the deficit in cortically evoked striatal ascorbate efflux in the R6/2 mouse model of Huntington's disease

A corticostriatal-dependent deficit in the release of ascorbate (AA), an antioxidant vitamin and neuromodulator, occurs concurrently in striatum with dysfunctional GLT1-dependent uptake of glutamate in the R6/2 mouse model of Huntington's disease (HD), an autosomal dominant condition characterized by overt corticostriatal dysfunction. To determine if deficient striatal AA release into extracellular fluid is related to altered GLT1 activity in HD, symptomatic R6/2 mice between 6 and 9 weeks of age and age-matched wild-type (WT) mice received single daily injections of 200 mg/kg ceftriaxone, a β-lactam antibiotic that elevates the functional expression of GLT1, or saline vehicle for five consecutive days. On the following day, in vivo voltammetry was coupled with corticostriatal afferent stimulation to monitor evoked release of AA into striatum. In saline-treated mice, we found a marked decrease in evoked extracellular AA in striatum of R6/2 relative to WT. Ceftriaxone, in contrast, restored striatal AA in R6/2 mice to WT levels. In addition, intra-striatal infusion of either the GLT1 inhibitor dihydrokainic acid or dl-threo-beta-benzyloxyaspartate blocked evoked striatal AA release. Collectively, our results provide compelling evidence for a link between GLT1 activation and release of AA into the striatal extracellular fluid, and suggest that dysfunction of this system is a key component of HD pathophysiology.

Pax2 is expressed in a subpopulation of Müller cells in the central chick retina

Müller cells in the chick retina are generally thought to be a homogeneous population. We show that the transcription factor Pax2 is expressed by Müller cells in the central chick retina and its expression was first observed at stage 32 (embryonic day [E] 7.5). Birth-dating indicated that the majority of Pax2-positive Müller cells are generated between stage 29 and 33 (E5.5-E8). At stage 42 (E16), several Müller cell markers, such as Sox2 and 2M6, had reached the peripheral retina, while the Pax2 labeling extended approximately half-way. A similar pattern was maintained in the 6-month-old chicken. Neither the Pax2-positive nor the Pax2-negative Müller cells could be specifically associated to proliferative responses in the retina induced by growth factors or N-methyl-D-aspartate. Pax2 was not detected in Müller cells in mouse, rat, guinea-pig, rabbit, or pig retinas; but the zebrafish retina displayed a similar pattern of central Pax2-expressing Müller cells.

Heterogeneity of glia in the retina and optic nerve of birds and mammals

We have recently described a novel type of glial cell that is scattered across the inner layers of the avian retina [1]. These cells are stimulated by insulin-like growth factor 1 (IGF1) to proliferate, migrate distally into the retina, and up-regulate the nestin-related intermediate filament transitin. These changes in glial activity correspond with increased susceptibility of neurons to excitotoxic damage. This novel cell-type has been termed the Non-astrocytic Inner Retinal Glia-like (NIRG) cells. The purpose of the study was to investigate whether the retinas of non-avian species contain cells that resemble NIRG cells. We assayed for NIRG cells by probing for the expression of Sox2, Sox9, Nkx2.2, vimentin and nestin. NIRG cells were distinguished from astrocytes by a lack of expression for Glial Fibrilliary Acidic Protein (GFAP). We examined the retinas of adult mice, guinea pigs, dogs and monkeys (Macaca fasicularis). In the mouse retina and optic nerve head, we identified numerous astrocytes that expressed GFAP, S100β, Sox2 and Sox9; however, we found no evidence for NIRG-like cells that were positive for Nkx2.2, nestin, and negative for GFAP. In the guinea pig retina, we did not find astrocytes or NIRG cells in the retina, whereas we identified astrocytes in the optic nerve. In the eyes of dogs and monkeys, we found astrocytes and NIRG-like cells scattered across inner layers of the retina and within the optic nerve. We conclude that NIRG-like cells are present in the retinas of canines and non-human primates, whereas the retinas of mice and guinea pigs do not contain NIRG cells.

A novel type of glial cell in the retina is stimulated by insulin-like growth factor 1 and may exacerbate damage to neurons and Müller glia

Recent studies have demonstrated that insulin can have profound affects on the survival of neurons within the retina. The purpose of this study was to determine how insulin-like growth factor 1 (IGF1) influences retinal cells; in particular, the glial cells. We identify a novel type of glial cell in the avian retina and provide evidence that these cells can respond to acute damage and IGF1. In normal retinas, we found a distinct cell-type, scattered across the ganglion cell and inner plexiform layers that express Sox2, Sox9, Nkx2.2, vimentin, and transitin, the avian homologue of mammalian nestin. These glial cells have a unique immunohistochemical profile, morphology, and distribution that are distinct among other known types of retinal glia, including microglia, oligodendrocytes, astrocytes, and Muller glia. We termed these cells nonastrocytic inner retinal glia-like (NIRG) cells. We found that the NIRG cells may express the IGF1 receptor and respond to IGF1 by proliferating, migrating distally into the retina, and upregulating transitin. In addition, IGF1 stimulated microglia to become reactive and upregulate lysosomal membrane glycoprotein and CD45. With microglia and NIRG cells stimulated by IGF1 there were elevated levels of cell death and numerous focal detachments across the retina in response to excitotoxic damage. Cell death was prominent within the areas of detachment coinciding with a stark loss of Müller glia and accumulation of NIRG cells. We conclude that NIRG cells are a novel type of retinal glia that is sensitive to IGF1 and whose activity may impact the survival of neurons and Müller glia.

Analysis of retinal cell development in chick embryo by immunohistochemistry and in ovo electroporation techniques

Background

Retinal cell development has been extensively investigated; however, the current knowledge of dynamic morphological and molecular changes is not yet complete.

Results

This study was aimed at revealing the dynamic morphological and molecular changes in retinal cell development during the embryonic stages using a new method of targeted retinal injection, in ovo electroporation, and immunohistochemistry techniques. A plasmid DNA that expresses the green fluorescent protein (GFP) as a marker was delivered into the sub-retinal space to transfect the chick retinal stem/progenitor cells at embryonic day 3 (E3) or E4 with the aid of pulses of electric current. The transfected retinal tissues were analyzed at various stages during chick development from near the start of neurogenesis at E4 to near the end of neurogenesis at E18. The expression of GFP allowed for clear visualization of cell morphologies and retinal laminar locations for the indication of retinal cell identity. Immunohistochemistry using cell type-specific markers (e.g., Visinin, Xap-1, Lim1+2, Pkcα, NeuN, Pax6, Brn3a, Vimentin, etc.) allowed further confirmation of retinal cell types. The composition of retinal cell types was then determined over time by counting the number of GFP-expressing cells observed with morphological characteristics specific to the various retinal cell types.

Conclusion

The new method of retinal injection and electroporation at E3 - E4 allows the visualization of all retinal cell types, including the late-born neurons, e.g., bipolar cells at a level of single cells, which has been difficult with a conventional method with injection and electroporation at E1.5. Based on data collected from analyses of cell morphology, laminar locations in the retina, immunohistochemistry, and cell counts of GFP-expressing cells, the time-line and dynamic morphological and molecular changes of retinal cell development were determined. These data provide more complete information on retinal cell development, and they can serve as a reference for the investigations in normal retinal development and diseases.

Lentoid Bodies in the Avian Retina

In-vitro studies suggest that, in avian retina, lentoid bodies arise from Müller cells or developing neuroretina. This report describes lentoid bodies in adult avian retinas in association with retinal trauma or degeneration. Retinal lentoids were identified in four birds (three owls and one parrot) in the course of routine diagnostic histopathology. Sections were stained with periodic acid-Schiff for the purposes of descriptive histology, and immunolabelled for a Müller cell marker (glial fibrillary acidic protein; GFAP) and a lens-specific marker (crystallin alpha-A). Intraretinal lentoids of varying size were identified, the constituent cells resembling bladder cells similar to those seen in cataracts. The process of lentoid formation followed a consistent pattern, characterized by progressive Müller cell hypertrophy in damaged areas, culminating in lentoid formation. GFAP immunoreactivity was strongest in Müller cells in the early stages of hypertrophy and receded as Müller cell hypertrophy advanced and lentoids developed. In contrast to GFAP immunoreactivity, crystalline alpha-A labelling increased in distribution and intensity as Müller hypertrophy became more prominent and lentoids were formed. This represents the first report of intraretinal lentoids in birds in vivo. The immunohistochemical data suggest that they arise from Müller cells. Association of lentoids with retinal damage supports the assertion that they arise following disruption of normal cell-cell communication.

Peculiar and typical oligodendrocytes are involved in an uneven myelination pattern during the ontogeny of the lizard visual pathway

We studied the myelination of the visual pathway during the ontogeny of the lizard Gallotia galloti using immunohistochemical methods to stain the myelin basic protein (MBP) and proteolipid protein (PLP/DM20), and electron microscopy. The staining pattern for the PLP/DM20 and MBP overlapped during the lizard ontogeny and was first observed at E39 in cell bodies and fibers located in the temporal optic nerve, optic chiasm, middle optic tract, and in the stratum album centrale of the optic tectum (OT). The expression of these proteins extended to the nerve fiber layer (NFL) of the temporal retina and to the outer strata of the OT at E40. From hatching onwards, the labeling became stronger and extended to the entire visual pathway. Our ultrastructural data in postnatal and adult animals revealed the presence of both myelinated and unmyelinated retinal ganglion cell axons in all visual areas, with a tendency for the larger axons to show the thicker myelin sheaths. Moreover, two kinds of oligodendrocytes were described: peculiar oligodendrocytes displaying loose myelin sheaths were only observed in the NFL, whereas typical medium electron-dense oligodendrocytes displaying compact myelin sheaths were observed in the rest of the visual areas. The weakest expression of the PLP/DM20 in the NFL of the retina appears to be linked to the loose appearance of its myelin sheaths. We conclude that typical and peculiar oligodendrocytes are involved in an uneven myelination process, which follows a temporo-nasal and rostro-caudal gradient in the retina and ON, and a ventro-dorsal gradient in the OT.

Retinoblastoma in the eye of a llama (Llama glama)

ANIMAL STUDIED

A 6-year-old, pregnant female llama experienced a 6-month history of epiphora, buphthalmos, and acute loss of vision in the left eye. The condition was unresponsive to topical antimicrobial and anti-inflammatory therapy and progressed to corneal rupture.

PROCEDURES

Transpalpebral enucleation was performed and an intraorbital silicone prosthesis was implanted. The eye was fixed in formalin and processed according to routine paraffin technique. Sections of a mass were immunohistochemically prepared routinely and stained for glial fibrillary acidic protein (GFAP), S-antigen, and rhodopsin.

RESULTS

Gross, histopathologic, and immunohistochemical analysis revealed a retinal tumor consistent with a retinoblastoma. The neoplastic tissue formed Flexner-Wintersteiner and Homer-Wright rosettes, originated from the retina, and demonstrated photoreceptor differentiation with S-antigen and rhodopsin expression. Neoplastic cells were negative for GFAP. Four years after enucleation, the llama showed no signs of recurrent neoplasia.

CONCLUSIONS

This report describes the diagnosis and successful treatment of the first known retinoblastoma in a llama.

Migration and differentiation of Nkx-2.2+ oligodendrocyte progenitors in embryonic chicken retina

During early neural development, the Nkx-2.2 homeobox gene is initially induced in the ventral ventricular zone of the central nervous system (CNS). At later stages, Nkx-2.2+ cells migrate away into the surrounding regions and co-express several oligodendrocyte marker genes. Here we report that Nkx-2.2+ cells also migrate into the avian retina from the optic nerve. These Nkx-2.2+ cells are specifically labeled by the early oligodendrocyte markers PDGFR alpha, O4 and GalC, but not the microglial marker QH1 or neuron-specific marker MAP-2. These observations provide further evidence that Nkx-2.2 is selectively expressed in migratory oligodendrocyte progenitor cells and that retina oligodendrocytes originate from the ventral diencephalon.